Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 We would like to dedicate this book to the memory of Hubert Sainsbury. His lively and enquiring mind and his passion for knowledge and understanding were always an inspiration, and his enthusiasm for this Dictionary was a strong motivating force during its long gestation. The book owes more to him than he would have believed. Copyright  1978, 1987, 2001, 2006 John Wiley & Sons Ltd., The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (+44) 1243 779777 Email (for orders and customer service enquiries): cs-books@wiley.co.uk Visit our Home Page on www.wileyeurope.com or www.wiley.com 1st edition 1978 (reprinted 1980) 2nd edition 1987 (reprinted 1988, 1989, 1991, 1994) Paperback edition 1993 (reprinted 1993, 1994, 1995, 1996 (twice), 1997 (twice), 1999, 2000) 3rd edition 2001 (reprinted 2002) 3rd edition revised 2006 Japanese edition (Asakura, Tokyo) 1997 Chinese edition in preparation 2006 All Rights Reserved. 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Library of Congress Cataloging-in-Publication Data Requested British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN-13 978-0-470-03545-0 (PB) ISBN-10 0-470-03545-5 (PB) Typeset in 10/12pt Times by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire, UK This book is printed on acid-free paper responsibly manufactured from sustainable forestry, in which at least two trees are planted for each one used for paper production. Preface This edition follows the style of previous editions. It has similar aims, and was written with the same enthusiasm and care. It is vital that readers be aware of the type of alphabetization used in the Dictionary. A glance at ‘Notes for the User’ – particularly the first paragraph – is essential. September 2001 v Preface to the Second Edition In writing this new edition of the Dictionary we had several aims in mind. One of these was to provide clear and up-to-date definitions of the numerous terms and phrases which form the currency of communication in modern microbiology and molecular biology. In recent years the rapid advances in these disciplines have thrown up a plethora of new terms and designations which, although widely used in the literature, are seldom defined outside the book or paper in which they first appeared; moreover, ongoing advances in knowledge have frequently demanded changes in the definitions of older terms – a fact which is not always appreciated and which can therefore lead to misunderstanding. Accordingly, we have endeavoured to define all of these terms in a way which reflects their actual usage in current journals and texts, and have also given (where appropriate) former meanings, alternative meanings, and synonyms. A second – but no less important – aim was to encapsulate and integrate, in a single volume, a body of knowledge covering the many and varied aspects of microbiology. Such a reference work would seem to be particularly useful in these days of increasing specialization in which the reader of a paper or review is often expected to have prior knowledge of both the terminology and the overall biological context of a given topic. It was with this in mind that we aimed to assemble a detailed, comprehensive and interlinked body of information ranging from the classical descriptive aspects of microbiology to current developments in related areas of bioenergetics, biochemistry and molecular biology. By using extensive cross-referencing we have been able to indicate many of the natural links which exist between different aspects of a particular topic, and between the diverse parts of the whole subject area of microbiology and molecular biology; hence the reader can extend his knowledge of a given topic in any of various directions by following up relevant cross-references, and in the same way he can come to see the topic in its broader contexts. The dictionary format is ideal for this purpose, offering a flexible, ‘modular’ approach to building up knowledge and updating specific areas of interest. There are other more obvious advantages in a reference work with such a wide coverage. Microbiological data are currently disseminated among numerous books and journals, so that it can be difficult for a reader to know where to turn for information on a term or topic which is completely unfamiliar to him. As a simple example, the name of an unfamiliar genus, if mentioned out of context, might refer to a bacterium, a fungus, an alga or a protozoon, and many books on each of these groups of organisms may have to be consulted merely to establish its identity; the problem can be even more acute if the meaning of an unfamiliar term is required. A reader may therefore be saved many hours of frustrating literature-searching by a single volume to which he can turn for information on any aspect of microbiology. An important new feature of this edition is the inclusion of a large number of references to recent papers, reviews and monographs in microbiology and allied subjects. Some of these references fulfil the conventional role of indicating sources of information, but many of them are intended to permit access to more detailed information on particular or general aspects of a topic – often in mainstream journals, but sometimes in publications to which the average microbiologist may seldom refer. Furthermore, most of the references cited are themselves good sources of references through which the reader can establish the background of, and follow developments in, a given area. While writing this book we were very fortunate in having exceptional and invaluable cooperation from a number of libraries in South-West England. In particular, we would like to acknowledge the generous help of Mr B. P. Jones, B.A., F.L.A., of the Medical Library, University of Bristol, Mrs Jean Mitchell of the Library at Bicton College of Agriculture, Devon, and Maureen Hammett of Exeter Central Library, Devon. Finally, vii Preface to the Second Edition we are grateful to Michael Dixon, Patricia Sharp, and Prue Theaker at John Wiley & Sons, Chichester, for their enthusiastic and efficient cooperation in the production of the book. Paul Singleton & Diana Sainsbury Clyst St Mary, Devon, April 1987 viii Notes for the User 1. Alphabetization. Alphabetization would need no comment if every term consisted of a single word; in practice, however, many terms consist of two or more words and often contain single letters, numbers, symbols etc. Terms consisting of two or more words can be alphabetized in either of two ways: on the basis of the first word, or on the basis that both or all of the words are run together and treated as one; thus, e.g., according to the ‘first-word’ (‘nothing-before-something’) system, red tide comes before redox potential, but according to the second system redox potential comes before red tide. Terms in this Dictionary have been alphabetized by the first-word system; in this system a single letter counts as a word (hence e.g. R plasmid comes before rabies), as does a group of letters (e.g. an abbreviation, or a gene designation). Examples: air sacculitis airlift fermenter AIV process Ajellomyces atoxyl ATP ATP synthase ATPase black stem rust black wart disease black yeasts blackeye cowpea mosaic virus RecA protein recapitulation theory recB gene RecBC pathway When a hyphen connects two complete words, or occurs between a letter (or group of letters) and a word, the hyphen is regarded as a space; however, if a hyphen is used to link a prefix to a word (i.e., if the letters preceding the hyphen form a part-word which cannot stand alone) the term is alphabetized as though it were a single, non-hyphenated word. (In a few cases an entry heading contains words which can be written as separate, hyphenated or non-hyphenated words, or closed up as a single word: e.g. red water fever, red-water fever, redwater fever; in such cases an entry or cross-reference has been included in both possible positions.) Examples: BL-type starter bla gene black beans Black beetle virus M M antigen M-associated protein M bands nonsense mutation nonsense suppressor non-specific immunity non-specific immunization preaxostyle pre-B cell prebuccal area precipitation When a Greek letter forms a significant part of an entry heading it is counted as a word and is alphabetized as spelt (i.e., a as alpha, b as beta, etc: see Appendix VI for the Greek alphabet). A Greek letter is ignored for the purposes of alphabetization if it is a relatively minor qualification: e.g., part of a chemical designation (which can usually be replaced by a number, as in b-hydroxybutyrate, = 3-hydroxybutyrate). Examples: Delhi boil 1 delta agent d antigen MTOC µ Mu mu chain pHisoHex fX phage group Phlebia Phlebotomus polyhedrosis poly-b-hydroxyalkanoate poly-b-hydroxybutyrate Polyhymenophorea A number which forms part of an entry heading affects the position of that entry only if the number immediately follows a letter or word (but cf. chemical names, below). A number which precedes a letter or word is usually ignored, although in the few cases where a number is the first and main part of an entry heading it is alphabetized as spelt. Letter–number combinations come after a letter–space but before letter–letter combinations, as in the illustrative sequence A, A2, A2A, A3, A22, AA, ABA etc. Roman numerals are treated as ordinary numbers (I as 1, II as 2 etc). (The reader should bear in mind that, in an unfamiliar term, ‘I’ could be a letter I or a Roman one, and its location in the Dictionary will be affected accordingly; similarly, ‘V’ could be letter V or Roman five. O and 0 (zero) may also be confused. If in doubt check both possible positions.) Examples: bacteriophage bacteriophage bacteriophage bacteriophage Pf2 fI fII f6 D loop D period 12D process D-type particles Fitz-Hugh–Curtis syndrome five–five–five test five-kingdom classification five–three–two symmetry ix T1 side-chains T-2 toxin T2H test T7 phage group Notes for the User Subscript/superscript numbers and letters are alphabetized as though they were ordinary numbers and letters (except in the case of ion designations: see below). Examples: avoparcin aw axenic axial fibrils B virus B12 coenzymes B663 Babes–Ernst granules C3 convertase C3 cycle C3bina C5 convertase CO2 CO2 -stat CoA coactin Primes, apostrophes and other non-alphabetizable symbols (including e.g. plus, minus and % signs) are ignored. Examples: brown rust Browne’s tubes Brownian movement Brown’s tubes F antigens F+ donor F-duction F factor Gautieriales Gazdar murine sarcoma virus GC% GC type pluronic polyol F127 plus progamone plus strand Pluteaceae In chemical names qualifications such as D-, L-, N -, o-, p-, numbers and Greek letters, as well as hyphens between parts of chemical names, are all ignored for the purposes of alphabetization. Examples: acetyl-CoA synthetase N-acetyl-D-glucosamine acetylmethylcarbinol N-acetylmuramic acid diazomycin A 6-diazo-5-oxo-L-norleucine diazotroph dibromoaplysiatoxin methylmethane sulphonate N-methyl-N
-nitro-N-nitrosoguanidine N-methyl-N-nitrosourea Methylobacterium In entry headings which include an ion designation, the ion is treated as a word, the charge being ignored; thus, H+ is regarded as H, Ca2+ as Ca, etc. Examples: H antigens H+ -ATPase H+ /2e− ratio H-lysin H+ /P ratio H+ -PPase H strand H-1 virus K cells K+ pump K+ transport K virus Na+ -ATPase Na+ -motive force Na+ pump nabam 2. Cross-references. References from one entry to another within the Dictionary are indicated by SMALL CAPITAL letters. In order to effect maximum economy of space, information given in any particular entry is seldom repeated elsewhere, and cross-referencing has been extensively employed to ensure continuity of information. In some cases a complete understanding of an entry, or an appreciation of context, is dependent on a knowledge of information given in other entries; where it is particularly important to follow up a crossreference, the cross-reference is followed by ‘q.v.’. In other cases a cross-reference may be used to link one topic with another of related interest, or to extend the scope of a given topic in one or more directions; in such cases a cross-reference is usually preceded by ‘see also’ or ‘cf.’. (N.B. For a variety of reasons, not every microbiological term or taxon used in the text is cross-referred – even though most of these terms and taxa are defined in the Dictionary; the reader is therefore urged to use the Dictionary for any unfamiliar term or taxon.) When reading an entry for a genus, family or other taxon, it is especially important to follow up, when indicated, a cross-reference to the higher taxon to which it belongs. An entry for a given higher taxon gives the essential features applicable to all members of that taxon, and such features are usually not repeated in the entries for each of the constituent lower-ranked taxa; thus, in failing to follow up such cross-references, the reader will forfeit fundamental information relating to the lower taxon in question. In some cases an entry heading is followed simply by ‘See CROSS-REFERENCE’. This is not intended to indicate that the two terms are synonymous (usually they are not); such referral signifies only that the meaning of the term is given under the heading indicated. When the entry heading and cross-reference are synonymous, this is indicated by Syn., thus: entry heading Syn. CROSS-REFERENCE. 3. External references. References to papers, articles etc in books or journals are given in square brackets. In order to save space, books are referred to by a ‘Book ref.’ number, and journal titles are abbreviated x Notes for the User somewhat more than is usual; keys to book reference numbers and journal title abbreviations can be found at the end of the Dictionary (after the Appendices). A book reference is usually quoted as a source of general background information for the reader, while papers in journals are usually quoted for specific details of current information (or for reviews) and/or for their references to other literature in the field. We should emphasize that the papers we have cited are not necessarily (and are commonly not) those which were the first to report a particular fact, finding or theory; rather, we have chosen, where possible, to cite the most recent references available to us, so that the reader is referred to current information and can, if he wishes, trace the earlier literature via references given in the cited papers. We should also point out that the quoting of a single reference in an entry is not intended to indicate that the entry was written solely from information in that paper or book. In relatively few cases does the information in an entry derive from a single source; in the great majority of entries the information has been derived from, or checked against, a range of sources, but limitations of space have necessarily prevented us from citing all of them. 4. Numbered definitions. In some cases a term is used with different meanings by different authors, or it may have different meanings in different contexts; for such a term the various definitions are indicated by (1), (2), (3), etc. The order in which the numbered definitions occur is not intended to reflect in any way appropriateness or frequency of usage. 5. Taxonomy. See entries ALGAE, BACTERIA, FUNGI, PROTOZOA and VIRUS for some general comments on the taxonomy of each of these groups of microorganisms. Each of these entries (except that on bacteria) provides a starting point from which the reader can, via cross-references, follow through a hierarchical system down to the level of genus and, in many cases, species and below; similarly, the hierarchy can be ascended from genus upwards. 6. The Greek alphabet. See Appendix VI. xi Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 A A (1) Adenine (or the corresponding nucleoside or nucleotide) in a nucleic acid. (2) Alanine (see AMINO ACIDS). Å (Ångström unit) 10−10 m (= 10−1 nm). 2–5A See INTERFERONS. A-DNA See DNA. a-factor See MATING TYPE. A layer An S LAYER associated with virulence in strains of Aeromonas salmonicida. A-protein In TOBACCO MOSAIC VIRUS: a mixture of small oligomers and monomers of coat protein subunits which occur in equilibrium with the larger ‘disc’ aggregates under conditions of physiological pH and ionic strength; coat protein occurs mainly as A-protein under conditions of high pH and low ionic strength. (cf. PROTEIN A.) A site (of a ribosome) See PROTEIN SYNTHESIS. A-tubule (A-subfibre) See FLAGELLUM (b). A-type inclusion body See POXVIRIDAE. A-type particles Intracellular, non-infectious, retrovirus-like particles. Many embryonic and transformed mouse cells contain retrovirus-like ‘intracisternal A-type particles’ (IAPs) which form by budding at the endoplasmic reticulum; these particles have reverse transcriptase activity and an RNA genome coding for the structural protein of the particles. The mouse genome contains ca. 1000 copies (per haploid genome) of DNA sequences homologous to IAP-associated RNA; these sequences appear to be capable of transposition within the mouse genome – probably via an RNA intermediate [Book ref. 113, pp. 273–279], i.e., they may be RETROTRANSPOSONS. Some A-type particles are non-enveloped precursors of B-type particles (see TYPE B ONCOVIRUS GROUP). A23187 An IONOPHORE which transports divalent cations, particularly Ca2+ ; it can effect the transmembrane exchange of 1Ca2+ (or 1Mg2+ ) for 2H+ without causing perturbation in the gradients of other monovalent cations. AAA ATPases ‘ATPases associated with diverse cellular activities’. AAA ATPases occur e.g. in PEROXISOMES and as components of eukaryotic PROTEASOMES. AAA pathway AMINOADIPIC ACID PATHWAY. AAC Aminoglycoside acetyltransferase (see AMINOGLYCOSIDE ANTIBIOTICS). AAD Aminoglycoside adenylyltransferase (see AMINOGLYCOSIDE ANTIBIOTICS). AAS Aminoalkylsilane (3-aminopropyltriethoxy-silane, APES; 3(triethoxysilyl)-propylamine, TESPA): a reagent used for binding a tissue section to the surface of a glass slide (e.g. for in situ hybridization); it reacts with silica glass and provides aminoalkyl groups which bind to aldehyde or ketone groups in the tissue section. aat gene In Escherichia coli: a gene whose product promotes the early degradation of those proteins whose N-terminal amino acid is either arginine or lysine. aat encodes an ‘amino acid transferase’ which catalyses the addition of a leucine or phenylalanine residue to the N-terminus of the protein; this destabilizes the protein, facilitating its degradation. (See also N-END RULE.) AatII See RESTRICTION ENDONUCLEASE (table). Aaterra See ETRIDIAZOLE. AAUAAA locus See MRNA (b). AAV Adeno-associated virus: see DEPENDOVIRUS. ab (immunol.) ANTIBODY. AB-transhydrogenase See TRANSHYDROGENASE. ABA ABSCISIC ACID. abacavir A NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITOR. abacterial pyuria See PYURIA. Abbe condenser A simple two- or three-lens substage CONDENSER which is uncorrected for spherical or chromatic aberrations. ABC (1) (immunol.) ANTIGEN-BINDING CELL. (2) See ABC TRANSPORTER. ABC excinuclease See EXCISION REPAIR. ABC exporter An ABC TRANSPORTER concerned with export/ secretion. These systems are found in both prokaryotic and eukaryotic microorganisms and in higher animals, including man. (The mammalian transporters include P-glycoprotein (‘multidrug-resistance protein’) – a molecular pump by which some types of cancer cell can extrude anti-cancer drugs.) In Saccharomyces cerevisiae, an ABC exporter mediates secretion of a peptide PHEROMONE (the a-factor) which regulates sexual interaction. In bacteria, ABC exporters transport various proteins (including enzymes and antibiotics) and, in some species, the polysaccharide components of the capsule; an exporter may be able to transport various related or similar molecules. [Bacterial ABC exporters: MR (1993) 57 995–1017.] In Escherichia coli the a-haemolysin is secreted via an ABC exporter – a one-step process direct from cytoplasm to environment; this exporter is in the type I class of protein secretory systems in Gram-negative bacteria (see PROTEIN SECRETION). Other proteins secreted by these systems include the cyclolysin of Bordetella pertussis and the alkaline protease of Pseudomonas aeruginosa. In Streptomyces antibioticus an ABC exporter secretes the antibiotic OLEANDOMYCIN. Bacterial proteins secreted by ABC exporters typically lack an N-terminal signal sequence (see SIGNAL HYPOTHESIS) but they have a C-terminal secretion sequence that may interact directly with the ABC protein. Exporters which transport molecules to the periplasm, or outer membrane, as the final destination may have fewer protein components than those exporters which secrete proteins. In Gram-negative bacteria, at least some exporters appear to consist of (i) ABC proteins; (ii) a membrane fusion protein (MFP) (in the periplasm and cytoplasmic membrane); and (iii) an OUTER MEMBRANE component. Assembly seems to occur in a definite sequence which is promoted and/or initiated by the binding of substrate (i.e. the molecule to be secreted) to the ABC protein; in this scheme, substrate–ABC binding is followed by ABC–MFP interaction – MFP then binding to the outer membrane, presumably to complete the secretory channel [EMBO (1996) 15 5804–5811]. ABC immunoperoxidase method An IMMUNOPEROXIDASE METHOD involving the use of a preformed avidin–biotin–peroxidase complex (ABC) which has surplus biotin-binding capacity. Initially, a (‘primary’) antiserum is raised against the required antigen; if the primary antiserum is derived from e.g. a rat, a ‘secondary’ anti-rat antiserum is prepared, and the anti-rat Ig antibodies are BIOTINylated. To locate a specific antigen, the section is treated with primary antiserum, washed, and then treated with secondary antiserum; the subsequent addition of ABC localizes peroxidase at the site of specific antigen (since the 1 ABC protein ABC adheres non-specifically to biotin). Peroxidase (and hence antigen) is detected by incubating the section with e.g. H2 O2 and diaminobenzidine (which results in the antigenic site being stained brown) or H2 O2 and 4-chloro-1-naphthol (resulting in a blue stain). The ABC method can be used for paraffin-embedded sections, frozen sections, and smears. Endogenous (tissue or cell) peroxidase may be quenched e.g. with H2 O2 in methanol. ABC protein See ABC TRANSPORTER. ABC transporter (traffic ATPase) A type of TRANSPORT SYSTEM which, in bacteria, consists typically of a multiprotein complex in the cell envelope, two of the proteins having a specific ATPbinding site (termed the ATP-binding cassette; ABC) on their cytoplasmic surface; a (bacterial) protein with an ABC site has been called an ‘ABC protein’ or an ‘ABC subunit’. In eukaryotes, an ABC transporter generally consists of a single polypeptide chain – which also has two ATP-binding sites. Transport mediated by an ABC transporter is energized by ATP hydrolysis at the ABC sites. [ATP-hydrolysing regions of ABC transporters: FEMS Reviews (1998) 22 1–20.] (See also PROTEIN SECRETION.) A given type of ABC transporter imports or exports/secretes certain type(s) of ion or molecule. Collectively, these transporters import or secrete a wide range of substances, including ions, sugars and proteins; for example, some import nutrients, or ions for OSMOREGULATION, while others secrete antibiotics or protein toxins. The LmrA transporter in Lactococcus lactis mediates an efflux system that extrudes amphiphilic compounds and appears to be functionally identical to the mammalian Pglycoprotein that mediates multidrug-resistance [Nature (1998) 391 291–295]. The AtrB transporter of Aspergillus nidulans mediates energy-dependent efflux of a range of fungicides [Microbiology (2000) 146 1987–1997]. ABC transporters occur e.g. in Gram-positive and Gramnegative bacteria, members of the Archaea, and in higher animals, including man. In man, certain inheritable diseases (e.g. CYSTIC FIBROSIS and adrenoleukodystrophy) result from defective ABC transporters. The bacterial ABC importer is commonly called a BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM (q.v.). (See also ABC EXPORTER.) ABE process An industrial process in which acetone, butanol and ethanol are produced by the fermentation of e.g. molasses by Clostridium acetobutylicum. (See also ACETONE–BUTANOL FERMENTATION.) Abelson murine leukaemia virus (Ab-MuLV) A replicationdefective, v-onc+ MURINE LEUKAEMIA VIRUS isolated from a prednisolone-treated BALB/c mouse inoculated with Moloney murine leukaemia virus (Mo-MuLV). Ab-MuLV apparently arose by recombination between Mo-MuLV and mouse c-abl sequences; the v-abl product has tyrosine kinase activity. (See also ABL.) Ab-MuLV induces B-cell lymphoid leukaemia with a short latent period (3–4 weeks). [Abelson virus–cell interactions: Adv. Imm. (1985) 37 73–98.] abequose (3,6-dideoxy-D-galactose) A sugar, first isolated from Salmonella abortusequi, which occurs in the O-specific chains of the LIPOPOLYSACCHARIDE in certain Salmonella serotypes and which contributes to the specificity of O antigen 4 in group B salmonellae (see KAUFFMANN–WHITE CLASSIFICATION). aberration (chromosomal) See CHROMOSOME ABERRATION. abhymenial Of or pertaining to a region opposite or away from the HYMENIUM. abiogenesis (spontaneous generation) The spontaneous formation of living organisms from non-living material; apart from its application to the evolutionary origin of life, this doctrine has long been abandoned. abiotic Non-living; of non-biological origin. abl An ONCOGENE originally identified as the transforming determinant of ABELSON MURINE LEUKAEMIA VIRUS (Ab-MuLV). The v-abl product has tyrosine kinase activity. In humans, c-abl normally occurs on chromosome 9, but is translocated to chromosome 22q- (the Philadelphia chromosome) in cells from patients with chronic myelogenous leukaemia (CML); in chromosome 22 it forms a chimeric fusion gene, bcr-abl, encoding a tumour-specific tyrosine kinase designated P210. ablastin Antibody which specifically inhibits reproduction of epimastigote forms of Trypanosoma lewisi in the vertebrate host. abomasitis Inflammation of the abomasum. (See also BRAXY; cf. RUMENITIS.) abomasum See RUMEN. aboral Away from, or opposite to, the mouth. abortifacient Able to cause abortion. abortive infection (virol.) A viral infection of (non-permissive) cells which does not result in the formation of infectious progeny virions, even though some viral genes (e.g. early genes) may be expressed. (cf. PERMISSIVE CELL.) abortive transduction See TRANSDUCTION. abortus Bang reaction (abortus Bang ring-probe) Syn. MILK RING TEST. ABR See MILK RING TEST. abrB gene See ENDOSPORE (figure (a) legend). abscess A localized collection of PUS surrounded by inflamed and necrotic tissue; it may subside spontaneously or may rupture and drain before healing. Abscesses may occur in any tissue and may be caused by any of a variety of organisms. Abscesses in internal organs (e.g. liver, kidney, brain) may follow bacteraemia or septicaemia and may be due to staphylococci, streptococci, coliforms, etc. A cold (or chronic) abscess is one with little inflammation, often due to tubercle bacilli. (See also DYSENTERY (b) and QUINSY.) abscisic acid (ABA) A terpenoid PHYTOHORMONE which acts e.g. as a growth inhibitor, as an inhibitor of germination, and as an accelerator of e.g. leaf abscission. ABA is also formed (as a secondary metabolite) e.g. by the fungus Cercospora rosicola. Absidia See MUCORALES. absorption (serol.) The removal or effective removal of particular antibodies, antigens, or other agents from a given sample (e.g. serum) by the addition of particular antigens, antibodies, or agents to that sample; the resulting antigen–antibody (or other) complexes may or may not be physically removed from the sample. Absorption is used e.g. to remove HETEROPHIL ANTIBODIES. absorptive pinocytosis See PINOCYTOSIS. 7-ACA 7-Aminocephalosporanic acid (see CEPHALOSPORINS). Acanthamoeba A genus of amoebae (order AMOEBIDA) in which the pseudopodia each have a broad hyaline zone (see PSEUDOPODIUM) from which arise several to many slender, tapering, flexible, and sometimes forked projections (acanthopodia). Polyhedral or roughly circular cysts with cellulose-containing walls are formed. Species are widespread and common in soil and fresh water, where they prey on e.g. bacteria, yeasts etc. [Adhesion of Acanthamoeba castellanii to bacterial flagella: JGM (1984) 130 1449–1458; bacterial endosymbionts of Acanthamoeba: J. Parasitol. (1985) 71 89–95.] Some strains can cause e.g. eye infections, MENINGOENCEPHALITIS [pathogenicity: RMM (1994) 5 12–20]. (cf. HARTMANNELLA.) 2 Acetobacter Acantharea A class of marine, mostly planktonic protozoa (superclass ACTINOPODA) which have elaborate ‘skeletons’ composed of strontium sulphate; typically, the skeleton consists of 10 spines arranged diametrically in the (more or less spherical) cell, or 20 spines which radiate from the cell centre (where they may or may not be joined at their bases, according to species). In many species the cell contains a central capsule (cf. RADIOLARIA); many species contain zooxanthellae. Five orders are recognized; genera include e.g. Acanthochiasma, Acanthometra, Astrolophus, Gigartacon. Acanthochiasma See ACANTHAREA. Acanthocystis See CENTROHELIDA. Acanthoeca See CHOANOFLAGELLIDA. Acanthometra See ACANTHAREA. acanthopodia See ACANTHAMOEBA. acaricide Any chemical which kills mites and ticks (order Acarina). Acarospora A genus of LICHENS (order LECANORALES). Thallus: crustose, areolate, with prominent areolae. Apothecia are embedded in the areolae; ascospores: very small, many per ascus. All species are saxicolous, some are ENDOLITHIC; A. smaragdula occurs on rocks and slag rich in heavy metals. Acarpomyxea A class of protozoa (superclass RHIZOPODA) with characteristics intermediate between those of the naked amoebae and the plasmodial slime moulds: they form small plasmodia (or large uninucleate plasmodium-like forms) which are usually branched and which sometimes anastomose to form a coarse reticulum. Spores, fruiting bodies and tests are absent; cysts are produced by some species. Orders: Leptomyxida (soil and freshwater organisms, e.g. Leptomyxa [Book ref. 133, pp. 143–144], Rhizamoeba) and Stereomyxida (marine organisms, e.g., Corallomyxa, Stereomyxa). Acaryophrya See GYMNOSTOMATIA. Acaulopage See e.g. NEMATOPHAGOUS FUNGI. Acaulospora See ENDOGONALES. acceptor site (of a ribosome) See PROTEIN SYNTHESIS. acceptor splice site See SPLIT GENE (a). accessory cells (immunol.) Those cells which, together with B LYMPHOCYTES and/or T LYMPHOCYTES, are involved in the expression of humoral and/or cell-mediated immune responses; they include e.g. MACROPHAGES, DENDRITIC CELLS, and LANGERHANS’ CELLS. accessory pigments In PHOTOSYNTHESIS: those pigments contained in LIGHT-HARVESTING COMPLEXES. AcCoA Acetyl-COENZYME A. Ace toxin (Vibrio cholerae) See BACTERIOPHAGE CTX8. acellular (non-cellular) (1) Refers to an organism, usually a protozoon, which consists essentially of a single cell but in which occur functionally specialized regions sometimes regarded as analogous to the organs and tissues of a differentiated multicellular organism. (2) Refers to an organism (e.g. a VIRUS) or structure (e.g. the stalk of ACYTOSTELIUM) which is not CELLULAR in any sense. (3) Not divided into cells (as e.g. in a PLASMODIUM). acellular slime moulds See MYXOMYCETES. acentric (of a chromosome) Having no CENTROMERE. acephaline gregarines See GREGARINASINA. acer tar spot See RHYTISMA. acervulus A flat or saucer-shaped fungal STROMA supporting a mass of typically short and densely-packed conidiophores; acervuli commonly develop subcuticularly or subepidermally in a plant host, becoming erumpent at maturity, i.e., rupturing the overlying plant tissue to allow dispersal of the conidia. Some acervuli bear setae (see SETA). Acetabularia A genus of DASYCLADALEAN ALGAE. The vegetative thallus consists of a single cell in which the CELL WALL contains MANNAN as a major component and is generally more or less heavily calcified; the cell is differentiated into an erect stalk or axis (up to several centimetres tall) anchored to the substratum by a branching rhizoid. The single nucleus is located in one branch of the rhizoid. As the stalk grows, whorls of sterile ‘hairs’ develop around the tip; these hairs are eventually shed, leaving rings of scars around the stalk. When the thallus is mature, gametangia develop as an apical whorl of elongated sac-like structures which, depending on species, may or may not be joined to form a characteristic cap (giving rise to the popular name ‘mermaid’s wine-glass’). Once the gametangial sacs have developed, the primary nucleus in the rhizoid grows to ca. 20 times its original size; it then undergoes meiosis, and numerous small secondary nuclei are formed. These migrate from the rhizoid to the gametangia by cytoplasmic streaming. Within a gametangial sac, each nucleus becomes surrounded by a resistant wall, resulting in the formation of many resistant cysts; the cyst walls contain cellulose rather than mannan, and are often heavily calcified. The cysts are liberated into the sea and then undergo a period of dormancy before liberating numerous biflagellate isogametes; pairs of gametes fuse to form zygotes which then develop into new vegetative thalli. acetate formation See e.g. ACETIFICATION and ACETOGENESIS. acetate thiokinase See METHANOGENESIS. acetate utilization See e.g. METHANOGENESIS and TCA CYCLE. Acetator See VINEGAR. acetic acid bacteria (1) Acetobacter spp. (2) Any bacteria capable of ACETIFICATION, including Acetobacter spp and Gluconobacter sp. aceticlastic Able to catabolize acetate. acetification The aerobic conversion of ethanol to acetic acid by bacteria (usually Acetobacter spp). Ethanol is converted to hydrated acetaldehyde (CH3 CH(OH)2 ) which is then dehydrogenated to give acetic acid. Acetification is an exothermic process. (See also e.g. VINEGAR, BEER SPOILAGE, WINE SPOILAGE.) Acetivibrio A genus of bacteria (family BACTEROIDACEAE) whose natural habitat is unknown. Cells: straight to slightly curved rods, 0.5–0.9 × 1.5–10.0 µm; in motile species the concave side of the cell has either a single flagellum or a number of flagella which arise in a line along the longitudinal axis of the cell. The cells stain Gram-negatively but the cell wall of the type species resembles those of Gram-positive bacteria. The major products of carbohydrate fermentation typically include acetic acid, ethanol, CO2 and H2 ; butyric, lactic, propionic and succinic acids are not formed. GC%: ca. 37–40. Type species: A. cellulolyticus. A. cellulolyticus. Monotrichous. Substrates include cellobiose, cellulose and salicin; aesculin is not hydrolysed. The type strain was isolated from a methanogenic enrichment culture. A. cellulosolvens. A non-motile species (isolated from sewage sludge) which can hydrolyse cellulose, cellobiose, aesculin and salicin; the cells apparently have an outer membrane. [IJSB (1984) 34 419–422.] A. ethanolgignens. Multitrichous. Substrates include fructose, galactose, lactose, maltose, mannitol and mannose – but not cellobiose, cellulose or aesculin. A. ethanolgignens is consistently present in the colons of pigs suffering from SWINE DYSENTERY. Acetobacter A genus of Gram type-negative bacteria of the family ACETOBACTERACEAE; the organisms occur e.g. on certain fruits and flowers, are responsible for some types of BEER SPOILAGE and WINE SPOILAGE, and are used e.g. in the manufacture of VINEGAR. 3 Acetobacteraceae Cells: typically ovoid or rod-shaped, 0.6–0.8 × 1.0–4.0 µm, non-motile or with peritrichous or lateral flagella. Most strains are catalase-positive. Typically, ethanol is oxidized to acetic acid, and acetic acid is oxidized (‘overoxidation’) to CO2 (cf. GLUCONOBACTER). Principal substrates include e.g. ethanol, glycerol and lactate; most strains grow well on glucose–yeast extract–CaCO3 agar (GYC agar), forming round pale colonies. (See also CARR MEDIUM.) Some strains form CELLULOSE (see PELLICLE (1)). Sugars appear to be metabolized primarily via the HEXOSE MONOPHOSPHATE PATHWAY and the TCA CYCLE; phosphofructokinase seems to be absent (cf. Appendix I(a)). The ENTNER–DOUDOROFF PATHWAY appears to occur only in cellulosesynthesizing strains. Growth on HOYER’S MEDIUM appears to involve enzymes of the glyoxylate shunt. Optimum growth temperature: 25–30° C. GC%: ca. 51–65. Type species: A. aceti. A. aceti. Ketogenic with glycerol or sorbitol substrates; 5-ketogluconic acid (but not 2,5-diketogluconic acid) formed from D-glucose. No diffusible brown pigments are formed on GYC agar. Grows on sodium acetate. A. hansenii. Ketogenic with glycerol or sorbitol substrates; 5ketogluconic acid (but not 2,5-diketogluconic acid) is formed by some strains from D-glucose. No growth on sodium acetate. No diffusible brown pigments are formed on GYC agar. (cf. A. xylinum.) A. liquefaciens. Brown diffusible pigments are formed on GYC agar. 2,5-Diketogluconic acid is formed from D-glucose. Ketogenic with glycerol as substrate. A. pasteurianus. Ketogluconic acids are not formed from Dglucose. No brown diffusible pigments are formed on GYC agar. Some strains (formerly called A. peroxydans) are catalasenegative. (cf. A. xylinum.) A. peroxydans. See A. pasteurianus. A. suboxydans. See GLUCONOBACTER. A. xylinum. Cellulose-producing strains formerly classified as a subspecies of A. aceti, then distributed between the two species A. hansenii and A. pasteurianus; A. xylinum has now been accepted as a revived name for cellulose-forming and celluloseless, acetate-oxidizing strains [IJSB (1984) 34 270–271]. [Book ref. 22, pp. 268–274.] Acetobacteraceae A family of aerobic, oxidase-negative, chemoorganotrophic, Gram type-negative bacteria which typically oxidize ethanol to acetic acid. Metabolism: strictly respiratory (oxidative), with O2 as terminal electron acceptor. Growth occurs optimally at ca. pH 5–6. The organisms occur e.g. in acidic, ethanol-containing habitats. GC%: ca. 51–65. Two genera: ACETOBACTER (type genus), GLUCONOBACTER [Book ref. 22, pp. 267–278]. Acetobacterium A genus of Gram-negative, obligately anaerobic bacteria which occur in marine and freshwater sediments [IJSB (1977) 27 355–361]. Cells: polarly flagellated ovoid rods, ca. 1.0 × 2.0 µm, often in pairs. The type species, A. woodii, can carry out a homoacetate fermentation of e.g. fructose, glucose or lactate, or can grow chemolithoautotrophically (see ACETOGENESIS); it contains group B PEPTIDOGLYCAN. Optimum growth temperature: 30° C. GC%: ca. 39. (See also ANAEROBIC DIGESTION.) acetogen (1) Any bacterium (e.g. Acetobacterium woodii, Clostridium aceticum, C. thermoaceticum) which produces acetate – as the main product – from certain sugars (via homoacetate fermentation and reduction of carbon dioxide) and (in some strains) from carbon dioxide and hydrogen (see ACETOGENESIS). (2) (hydrogenogen; proton-reducing acetogen) Any bacterium which can use protons as electron acceptors for the oxidation of certain substrates (e.g. ethanol, lactate, fatty acids) to acetate with concomitant formation of hydrogen. Obligate hydrogenogens include e.g. SYNTROPHOMONAS (see also ANAEROBIC DIGESTION). Some SULPHATE-REDUCING BACTERIA appear to be facultative hydrogenogens. The synthesis of acetate by hydrogenogens is thermodynamically favourable only when the partial pressure of hydrogen is very low – e.g. in the presence of a hydrogen-utilizing methanogen. acetogenesis Acetate formation. A variety of microorganisms can form acetate, as a major or minor product, e.g. via the MIXED ACID FERMENTATION or PROPIONIC ACID FERMENTATION. (cf. ACETIFICATION.) The term is also used more specifically to refer to the particular pathways used by the ACETOGENS (sense 1). These organisms form acetate, as the main product, from e.g. certain hexoses in a process (homoacetate fermentation) in which the hexose is metabolized to pyruvate (via the EMBDEN–MEYERHOF–PARNAS PATHWAY) and thence to acetate and carbon dioxide. Additional acetate is formed as follows. Some of the carbon dioxide is reduced to formate; this formate is bound to tetrahydrofolate (THF) and is further reduced (in an ATP-dependent reaction) to yield 5-methyl-THF. The methyl group is then transferred to coenzyme B12 . The remainder of the carbon dioxide is reduced to carbon monoxide (by CO dehydrogenase). Carbon monoxide reacts with methyl-coenzyme B12 in the presence of coenzyme A and CO dehydrogenase disulphide reductase to yield acetyl-CoA. Acetyl-CoA is converted to acetate and CoASH with concomitant substrate-level phosphorylation to yield ATP. Some acetogens (e.g. A. woodii, C. aceticum, some strains of C. thermoaceticum) can form acetate from carbon dioxide and hydrogen [autotrophic pathways in acetogens: JBC (1986) 261 1609–1615]. This process resembles the latter part of the pathway above: CO is derived from carbon dioxide, 2H+ and 2e− , and 5-methyl-THF from THF, carbon dioxide and hydrogen. acetoin (CH3 .CHOH.CO.CH3 ; acetylmethylcarbinol) See e.g. Appendix III(c); BUTANEDIOL FERMENTATION; VOGES–PROSKAUER TEST. Acetomonas Former name of GLUCONOBACTER. acetone–butanol fermentation (solvent fermentation) A FERMENTATION (sense 1), carried out by certain saccharolytic species of Clostridium (e.g. C. acetobutylicum), in which the products include acetone (or isopropanol) and n-butanol (collectively referred to as ‘solvent’). Glucose is initially metabolized via the BUTYRIC ACID FERMENTATION, but subsequently the pH drops to ca. 4.5–5.0 and acetone and n-butanol are formed as major end products [Appendix III (g)]. This fermentation is carried out on an industrial scale to a limited extent. [Review: AAM (1986) 31 24–33, 61–92.] acetosyringone See CROWN GALL. 3-acetoxyindole See INDOXYL ACETATE. acetylcholine (neurotransmitter) See BOTULINUM TOXIN. acetyl-CoA synthetase See TCA CYCLE. N-acetyl-L-cysteine See MUCOLYTIC AGENT. N-acetyl-D-glucosamine (GlcNAc) N-Acetyl-(2-amino-2-deoxyD-glucose): an amino sugar present in various polysaccharides – see e.g. CHITIN, HYALURONIC ACID, LIPOPOLYSACCHARIDE, PEPTIDOGLYCAN (q.v. for formula), TEICHOIC ACIDS. acetylmethylcarbinol Syn. ACETOIN. N-acetylmuramic acid See PEPTIDOGLYCAN. N-acetylmuramidase Syn. LYSOZYME. N-acetylneuraminic acid See NEURAMINIC ACID. 4 Acinetobacter A-CGT See IMMUNOSORBENT ELECTRON MICROSCOPY. achlorophyllous Syn. ACHLOROTIC. achlorotic (achlorophyllous) Lacking chlorophyll. (cf. APOCHLOROTIC.) Achlya A genus of aquatic fungi (order SAPROLEGNIALES) in which the thallus is characteristically a branched, coenocytic mycelium; the width of the hyphae varies with species. Although Achlya species are typically saprotrophic some have been reported to parasitize rice plants. (See also DIPLANETISM, HETEROTHALLISM and PHEROMONE.) Achnanthes See DIATOMS. Acholeplasma A genus of facultatively anaerobic, ureasenegative bacteria (family ACHOLEPLASMATACEAE) which are associated with various vertebrates (and possibly with invertebrates and plants), and which also occur e.g. in soil and sewage and as contaminants in TISSUE CULTURES. Cells: non-motile cocci (minimum diam. ca. 300 nm) or filaments (typically ca. 2–5 µm in length); carotenoid pigments occur in some species. The organisms resemble Mycoplasma spp in their general properties, but differ e.g. in that their growth is sterol-independent, and in that NADH oxidase occurs in the cytoplasmic membrane rather than in the cytoplasm. Acholeplasma spp are susceptible to various ACHOLEPLASMAVIRUSES. GC%: ca. 26–36. Type species: A. laidlawii; other species: A. axanthum, A. equifetale, A. granularum, A. hippikon, A. modicum, A. morum, A. oculi. [Book ref. 22, pp. 775–781.] Acholeplasmataceae A family of bacteria of the order MYCOPLASMATALES; species of the sole genus, ACHOLEPLASMA, differ from the other members of the order e.g. in that their growth is not sterol-dependent. [Proposal for re-classifying Acholeplasmataceae as the order Acholeplasmatales: IJSB (1984) 34 346–349.] acholeplasmaviruses BACTERIOPHAGES which infect Acholeplasma species: see PLECTROVIRUS, PLASMAVIRIDAE, MV-L3 PHAGE GROUP. achromat (achromatic objective) An objective lens (see MICROSCOPY) in which chromatic aberration has been corrected for two colours (usually red and blue), and spherical aberration has been corrected for one colour (usually yellow–green). (cf. APOCHROMAT.) A FLAT-FIELD OBJECTIVE LENS of this type is called a planachromat. Achromobacter An obsolete bacterial genus. achromogenic Refers to an organism (or e.g. reagent) which does not produce pigment (or colour); used e.g. of non-pigmented strains of normally CHROMOGENIC organisms. achromycin See TETRACYCLINES. aciclovir A spelling used by some authors for the drug ACYCLOVIR. acicular Needle-shaped. Aciculoconidium A genus of fungi (class HYPHOMYCETES) which form budding ovoid or ellipsoidal cells (occurring singly or in short chains or clusters) as well as branched septate hyphae. Conidia are formed terminally and are acicular, rounded at one end and pointed at the other. NO3 − is not assimilated. One species: A. aculeatum (formerly Trichosporon aculeatum), isolated from Drosophila spp. [Book ref. 100, pp. 558–561.] acid dye See DYE. acid-fast organisms Organisms (e.g. Mycobacterium spp) which, once stained with an ACID-FAST STAIN, cannot be decolorized by mineral acids or by mixtures of acid and ethanol. acid-fast stain Any stain used to detect or demonstrate ACID-FAST ORGANISMS – e.g. ZIEHL–NEELSEN’S STAIN, AURAMINE–RHODAMINE STAIN. acid fuchsin See FUCHSIN. acid phosphatase See PHOSPHATASE. Acidaminococcus A genus of Gram-negative bacteria (family VEILLONELLACEAE) which occur e.g. in the intestine in humans and pigs. Cells: typically kidney-shaped cocci, 0.6–1.0 µm diam, occurring in pairs. Amino acids are the main sources of carbon and energy; all strains need e.g. arginine, glutamate, tryptophan and valine, and most need e.g. cysteine and histidine. In general, the organisms metabolize carbohydrates weakly or not at all. Optimum growth temperature: 30–37° C. Optimum pH: 7.0. GC%: ca. 57. Type species: A. fermentans. acidophile An organism which grows optimally under acidic conditions, having an optimum growth pH below 6 (and sometimes as low as 1, or below), and which typically grows poorly, or not at all, at or above pH 7: see e.g. SULFOLOBUS, THERMOPLASMA, THIOBACILLUS. (cf. ALKALOPHILE and NEUTROPHILE; see also LEACHING.) acidophilus milk A sour, medicinal beverage made by fermenting heat-treated, partially skimmed milk with Lactobacillus acidophilus. (Viable L. acidophilus appears to have a therapeutic effect on some intestinal disorders.) The main fermentation product is lactic acid which reaches a level of ca. 1.0%. A more palatable preparation, ‘sweet acidophilus milk’, is made by adding L. acidophilus to milk at ca. 5° C; under these conditions the cells remain viable but lactic acid is not produced. (See also DAIRY PRODUCTS.) acidosis (1) (lactic acidosis) (vet.) A (sometimes fatal) condition which may occur in ruminants fed excessive amounts of readily fermentable carbohydrates (e.g. starch, sugars – found e.g. in grain and beet, respectively) or when the transfer from a roughage to a ‘concentrate’ diet is made too quickly. Under these conditions the rate of acid production in the RUMEN is very high; the resulting fall in pH in the rumen (due mainly to the accumulation of lactic acid) inhibits cellulolytic bacteria and protozoa, and favours the growth of certain LACTIC ACID BACTERIA – so that the pH falls still further. (See also RUMENITIS.) A gradual transition from roughage to concentrate may permit the somewhat more acid-tolerant bacterium Megasphaera elsdenii to metabolize the lactic acid and maintain a normal pH in the rumen. (See also THIOPEPTIN.) (2) (med., vet.) A pathological condition characterized by an abnormally low pH in the blood and tissues. Acidothermus A proposed genus of aerobic, thermophilic (growing at 37–70° C), acidophilic (growing at pH 3.5–7.0), cellulolytic, non-motile, rod-shaped to filamentous bacteria isolated from acidic hot springs; GC%: ca. 60.7. [IJSB (1986) 36 435–443.] aciduric Tolerant of acidic conditions. (cf. ACIDOPHILE.) Acineria See GYMNOSTOMATIA. Acineta See SUCTORIA. Acinetobacter A genus of strictly aerobic, oxidase −ve, catalase +ve Gram-type-negative bacteria of the family MORAXELLACEAE (within the gamma subdivision of PROTEOBACTERIA); the organisms occur e.g. in soil and water and may act as opportunist pathogens in man. (See also MEAT SPOILAGE and SEWAGE TREATMENT.) Cells: short rods, 0.9–1.6 × 1.5–2.5 µm, or coccobacilli (coccoid in stationary-phase cultures); cells often in pairs. Nonmotile, but may exhibit TWITCHING MOTILITY. Non-pigmented. Metabolism is respiratory (oxidative), with oxygen as terminal electron acceptor; no growth occurs anaerobically, with or without nitrate. 5 AcLVs as a chemoattractant for cell aggregation. Acrasins are a diverse group of substances; they include cAMP in Dictyostelium discoideum (q.v.), a pterin in Dictyostelium lacteum [PNAS (1982) 79 6270–6274], and a dipeptide, ‘glorin’, in Polysphondylium violaceum (q.v.). Acrasiomycetes (acrasid cellular slime moulds; acrasids) A class of cellular SLIME MOULDS (division MYXOMYCOTA) in which the vegetative phase consists of amoeboid cells that form lobose pseudopodia; the amoebae aggregate (without streaming) to form a pseudoplasmodium which is not slug-like and does not migrate (cf. DICTYOSTELIOMYCETES). The pseudoplasmodium gives rise to multispored fruiting bodies which may have long or short stalks (but no cellulosic stalk tube) bearing e.g. simple globular sori or branched or unbranched chains of spores. Flagellated cells have been observed in only one species (Pocheina rosea). Sexual processes are unknown. Acrasids occur in various habitats: e.g. dung, tree-bark, dead plant materials, etc. Genera include Acrasis, Copromyxa, Copromyxella, Fonticula, Guttulinopsis, Pocheina (formerly Guttulina). (Zoological taxonomic equivalents of the Acrasiomycetes include the class Acrasea of the MYCETOZOA, and the class Acrasea of the RHIZOPODA.) Acrasis See ACRASIOMYCETES. Acremonium A genus of fungi of the class HYPHOMYCETES; teleomorphs occur in e.g. Emericellopsis and Nectria. The genus includes organisms formerly classified as species of Cephalosporium [for references see MS (1986) 3 169–170]. Acremonium spp form septate mycelium; conidia, often in gelatinous masses, are produced from phialides which develop from simple, single branches of the vegetative hyphae. A. kiliense (= Cephalosporium acremonium) produces cephalosporin C (see CEPHALOSPORINS). (See also MADUROMYCOSIS.) acridine orange (basic orange, or euchrysine; 3,6-bis(dimethylamino)-acridinium chloride) A basic dye and FLUOROCHROME used e.g. in fluorescence MICROSCOPY to distinguish between dsDNA (which fluoresces green) and ss nucleic acids (which fluoresce orange-red). Sublethal concentrations of the dye are used for CURING plasmids. (See also ACRIDINES.) acridines Heterocyclic compounds which include acridine and its derivatives. At low concentrations, aminoacridines (e.g. proflavine (3,6-diaminoacridine), QUINACRINE) appear to bind to dsDNA (or to double-stranded regions of ssDNA) primarily as INTERCALATING AGENTS. At higher concentrations there is also a weaker, secondary type of binding in which the acridine binds to the outside of dsDNA or to ssDNA or ssRNA; the two types of binding may account for the differential staining of DNA and RNA by ACRIDINE ORANGE. [Book ref. 14, pp. 274–306.] Acridines inhibit DNA and RNA synthesis and cause e.g. FRAMESHIFT MUTATIONS. They are used e.g. as antimicrobial agents (see e.g. ACRIFLAVINE), as mutagens, and as fluorescent stains for nucleic acids; they also have potential antitumour activity. (See also CURING (2).) As antimicrobial agents, acridines are active against a wide range of bacteria, but they are not sporicidal; some are active against certain parasitic protozoa (see e.g. QUINACRINE and KINETOPLAST) and inhibit the replication of certain viruses. Activity is not significantly affected by proteinaceous matter. [Acridines as antibacterials (review): JAC (2001) 47 1–13.] As mutagens, acridines may be effective in replicating bacteriophages but are generally not effective in bacteria. However, compounds in which an acridine nucleus is linked to an alkylating side-chain – ICR compounds (ICR = Institute for Cancer Research) – can induce frameshift and other mutations in bacteria. Most strains can grow on a mineral salts medium containing an organic carbon source such as acetate, ethanol or lactate as the sole source of carbon and energy; some can use amino acids (e.g. L-leucine, ornithine) and/or pentoses (e.g. Larabinose, D-xylose), and some are able to degrade e.g. benzoate, n-hexadecane and alicyclic compounds (see HYDROCARBONS). Acinetobacters appear to contain all the enzymes of the TCA CYCLE and the glyoxylate cycle. Many carbohydrates can be used. Most strains in the A. calcoaceticus–A. baumannii complex (and in certain other groups) can form acid from glucose (oxidatively), but many (e.g. most strains designated A. lwoffii ) cannot. The optimal growth temperature is typically 33–35° C. GC%: ∼38–47. Type species: A. calcoaceticus. The taxonomy of Acinetobacter is confused and unsatisfactory. Emended descriptions of the two species A. calcoaceticus and A. lwoffii, and proposals for four new species (A. baumannii, A. haemolyticus, A. johnsonii and A. junii ), were published in 1986 [IJSB (1986) 36 228–240]. Since then, a number of adjustments have been made to the taxonomic structure of the genus. [Taxonomy, and epidemiology of Acinetobacter infections: RMM (1995) 6 186–195.] Acinetobacters have been isolated in a number of hospitalassociated (and other) outbreaks of disease, often as part of a mixed infection; in most cases such infections involve glucolytic strains of the A. calcoaceticus–A. baumannii complex – particularly A. baumannii (also called group 2, or genospecies 2). The most common manifestations of disease include septicaemia and infections of the urinary tract, lower respiratory tract and central nervous system. Transmission may occur by direct contact or may involve the airborne route. Acinetobacters have been reported to survive on dry surfaces for at least as long as e.g. Staphylococcus aureus. One problem associated with the pathogenic role of Acinetobacter is that these organisms appear easily to acquire resistance to antibiotics – so that they have the potential to develop as multiresistant pathogens; currently, for example, acinetobacters are reported to be resistant to most b-lactam antibiotics, particularly penicillins and cephalosporins, and to chloramphenicol and trimethoprim–sulphamethoxazole. [Mechanisms of antimicrobial resistance in A. baumannii: RMM (1998) 9 87–97.] AcLVs AVIAN ACUTE LEUKAEMIA VIRUSES. acne A chronic skin disorder characterized by increased sebum production and the formation of comedones (‘blackheads’ and ‘whiteheads’) which plug the hair follicles. Propionibacterium acnes, present in the pilosebaceous canal (see SKIN MICROFLORA), may play a causal role; it produces a lipase that hydrolyses sebum triglycerides to free fatty acids, and these can cause inflammation and comedones [JPed (1983) 103 849–854]. Treatment: e.g. topical SALICYLIC ACID or benzoyl peroxide; the latter has keratinolytic activity and exerts bactericidal action on P. acnes by releasing free-radical oxygen. Aconchulinida See FILOSEA. aconitase See Appendix II(a) and NITRIC OXIDE. Aconta Algae of the RHODOPHYTA. (cf. CONTOPHORA.) acquired immune deficiency syndrome See AIDS. acquired immunity (1) SPECIFIC IMMUNITY acquired through exposure to a given antigen. (2) PASSIVE IMMUNITY. (3) NONSPECIFIC IMMUNITY acquired through exposure to certain viruses (see e.g. INTERFERONS) or by immunization with BCG. Acrasea See ACRASIOMYCETES. acrasids See ACRASIOMYCETES. acrasin In cellular slime moulds: a generic term for a chemotactic substance which is produced by cells and which serves 6 actinoidin (6) (5) (4) 8 9 1 (7) 7 (8) 6 2 (3) 3 (2) The formation and fate of microfilaments are regulated in vivo e.g. by various proteins. Profilin binds to G-actin, inhibiting polymerization. Gelsolin (in e.g. macrophages), severin (in Dictyostelium), fragmin (in Physarum), and villin (in microvilli) can each cleave F-actin into fragments in a Ca2+ -dependent reaction, thereby e.g. effecting a gel-to-sol transition. Filamin and a-actinin can cross-link microfilaments, promoting gel formation. b-Actinin can act as a CAPPING (sense 2) protein. Vinculin may help to anchor microfilaments to other cell components. [Binding of microfilaments to the cytoplasmic membrane in Dictyostelium discoideum: JCB (1986) 102 2067–2075.] Fimbrin binds together longitudinally adjacent microfilaments to form bundles. Actin polymerization/depolymerization is affected e.g. by agents such as CYTOCHALASINS and by phalloidin (see PHALLOTOXINS). (2) See MACROTETRALIDES. actin-based motility See DYSENTERY (1a) and LISTERIOSIS. Actinichona See HYPOSTOMATIA. a-actinin See ACTIN. b-actinin See ACTIN. actino- Prefix signifying a ray or rays. actinobacillosis Any animal (or human) disease caused by a species of Actinobacillus. A. lignieresii causes granulomatous lesions in and around the mouth – particularly the tongue (‘wooden tongue’) – in cattle; in sheep A. lignieresii is associated with suppurative lesions in the skin and internal organs. A. equuli is pathogenic for horses (see SLEEPY FOAL DISEASE) and pigs; in pigs symptoms may include fever, haemorrhagic or necrotic skin lesions, arthritis and endocarditis. A. suis causes septicaemia and localized lesions in pigs. (See also PERIODONTITIS.) Actinobacillus A genus of Gram-negative bacteria of the PASTEURELLACEAE. Cells: mostly rod-shaped (ca. 0.3–0.5 × 0.6–1.4 µm), but a coccal form often occurs at the end of a rod, giving a characteristic ‘Morse code’ form; filaments may occur in media containing glucose or maltose. Extracellular slime is often produced. Cells stain irregularly. Glucose, fructose, xylose, and (most strains) lactose are fermented (no gas). Growth occurs only on complex media; all species (except A. actinomycetemcomitans) can grow on MacConkey’s agar. Most species are non-haemolytic, but A. suis and some strains of A. equuli exhibit clear haemolysis on sheep blood agar; A. suis causes partial haemolysis on horse blood agar. GC%: 40–43. Type species: A. lignieresii. Actinobacilli occur as commensals in the alimentary, respiratory and/or genital tracts of animals: A. lignieresii in cattle and sheep, A. equuli in horses, A. suis in pigs(?) and horses, A. capsulatus in rabbits(?), A. actinomycetemcomitans in man. All can be opportunist pathogens (see ACTINOBACILLOSIS). (A. muris = Streptobacillus moniliformis; A. mallei = Pseudomonas mallei; A. ureae: see PASTEURELLA.) [Book ref. 22, pp. 570–575; proposal to re-classify A. actinomycetemcomitans as Haemophilus actinomycetemcomitans: IJSB (1985) 35 337–341.] Actinobifida An obsolete genus of actinomycetes which included species with dichotomously-branching sporophores; at least some strains were transferred to THERMOMONOSPORA. Actinobolina A genus of carnivorous ciliates (subclass GYMNOSTOMATIA). Cells: roughly ovoid, with uniform somatic ciliature, an apical cytostome, TOXICYSTS, and retractable tentacles distributed evenly over the body. Actinocephalus See GREGARINASINA. actinoidin See VANCOMYCIN. N 5 10 4 (9) (10) (1) ACRIDINE. The numbering system used in this dictionary is indicated by the numbers which are not in parentheses; an alternative numbering system (numbers in parentheses) is used by some authors. acriflavine (acriflavin; syn. euflavin) 3,6-Diamino-10-methylacridinium chloride or (according to some authors) a mixture of this compound and 3,6-diaminoacridine (proflavine). Acriflavine is soluble in water and in ethanol, and has been used as an ANTISEPTIC. (See also ACRIDINES.) acro- Prefix meaning tip or outermost part. Acrocordia See PYRENULALES. acrolein (CH2 =CH−CHO) An aldehyde used e.g. for preFIXATION; it penetrates tissues more rapidly than GLUTARALDEHYDE. acronematic Refers to a eukaryotic FLAGELLUM which is smooth and tapers to a fine point. acropetal development Development from the base, or point of attachment, towards the tip; e.g., in a chain of acropetally developing spores the first-formed spores occupy positions in the chain nearest the base of the spore-bearing structure, while spores formed later occupy positions in the distal parts of the chain. (cf. BASIPETAL DEVELOPMENT.) acropleurogenous Located both at the tip and on the sides of an elongated structure. Acrosiphonia A genus of branched, filamentous, siphonocladous green algae (division CHLOROPHYTA). Acrospermum See CLAVICIPITALES. acrylate pathway See PROPIONIC ACID FERMENTATION. ActA protein (Listeria monocytogenes) See LISTERIOSIS. actaplanin See VANCOMYCIN. Actidione Syn. CYCLOHEXIMIDE. actin (1) A protein, found in most types of eukaryotic cell, which can polymerize (reversibly) to form non-contractile filaments (microfilaments) that are involved e.g. in maintaining cell shape and structure (see e.g. CYTOSKELETON) and (together with MYOSIN) in CAPPING (sense 3), amoeboid movement (see PSEUDOPODIUM), CYTOPLASMIC STREAMING, PHAGOCYTOSIS, and (in higher animals) muscle contraction. Actins from various sources are similar in structure. The monomeric form (G-actin) is a globular protein (MWt ca. 42000) consisting of ca. 375 amino acid residues; each molecule can bind one molecule of ATP. In most non-muscle cells, G-actin occurs in dynamic equilibrium with the polymerized (filamentous) form, F-actin, which consists of a helical, doublestranded chain of monomers ca. 7 nm thick. Although F-actin is itself non-contractile, its interaction with myosin can cause microfilaments to slide relative to one another – thereby bringing about movements and contractions in structures bound to the microfilaments. During the polymerization of G-actin ATP is hydrolysed; as in the assembly of MICROTUBULES, energy is not essential for – but increases the rate of – polymerization. Polymerization and depolymerization can occur at both ends of a microfilament, but one of the ends may grow (or depolymerize) at a greater rate than the other. (See also CAPPING sense 2.) 7 Actinomadura Actinomadura A genus of bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes) which occur e.g. in soil; some species (A. madurae, A. pelletieri ) can be pathogenic in man (see MADUROMYCOSIS). The organisms form a branching, usually stable, substrate mycelium, but (spore-forming) aerial mycelium may be common or rare according to species; some species contain only trace amounts of madurose, or none at all. GC%: reported to be within the range 65–78. Type species: A. madurae. [Taxonomic studies on Actinomadura and Nocardiopsis: JGM (1983) 129 3433–3446; ecology, isolation and cultivation: Book ref. 46, pp. 2103–2117.] Actinomucor See MUCORALES. Actinomyces A genus of asporogenous bacteria (order ACTINOMYCETALES; wall type varies with species); species occur in warm-blooded animals e.g. as part of the microflora of the mucous membranes (particularly in the mouth) and can act as opportunist pathogens. The organisms occur as rods, branched rods or filaments, or as a rudimentary mycelium. All species can grow anaerobically, or under reduced partial pressure of oxygen; growth in vitro occurs readily on rich media at 37° C, and is typically enhanced if the partial pressure of carbon dioxide is increased. Carbohydrates are fermented anaerogenically – acetic, lactic and succinic acids being the main acidic end products of glucose fermentation in PYG MEDIUM. Most species are catalase-negative; A. viscosus is catalase-positive. GC%: ca. 57–73. Type species: A. bovis. A. bovis (wall type VI) and A. israelii (wall type V) can cause chronic disease in animals and man (see ACTINOMYCOSIS); A. naeslundii and A. viscosus (both wall type V) can cause periodontitis e.g. in rodents. (See also COAGGREGATION.) A. pyogenes (formerly Corynebacterium pyogenes [JGM (1982) 128 901–903]) is the cause of ‘summer mastitis’ in cattle, and is often isolated from pyogenic lesions in cattle, pigs and other animals; A. pyogenes typically occurs as short rods or coryneforms which secrete a soluble haemolysin. A. hordeovulneris [IJSB (1984) 34 439–443] is a causal agent of actinomycosis in dogs. Actinomycetales An order of GRAM TYPE-positive, typically aerobic bacteria; species range from those which occur as cocci and/or rods to those which form a well-developed, branching SUBSTRATE MYCELIUM and/or AERIAL MYCELIUM, and which may form sophisticated structures such as sclerotia, sporangia and synnemata. (cf. ACTINOMYCETE.) Most members of the order have a GC%>55, thus distinguishing them from species of the other major subbranch of Gram-positive bacteria: the Clostridium–Bacillus–Thermoactinomyces line (but cf. CORYNEBACTERIUM, RENIBACTERIUM and THERMOACTINOMYCES). Phylogenetic relationships between actinomycetes are indicated by 16S rRNA oligonucleotide cataloguing and nucleic acid hybridization; within the order, groups of genera can be distinguished on the basis of e.g. the chemical nature of the cell wall and the lipid profiles of the organisms. [The system of classification adopted in the Dictionary is based on the scheme proposed in Book ref. 73, pp. 7–164.] Actinomycetes are widespread in nature, occurring typically in soil, composts (see COMPOSTING) and aquatic habitats; most species are free-living and saprotrophic, but some form symbiotic associations (see e.g. ACTINORRHIZA) and others are pathogenic in man, other animals, and plants (see e.g. ACTINOMYCOSIS, DERMATOPHILOSIS, JOHNE’S DISEASE, POTATO SCAB, and TUBERCULOSIS). The organisms are chemoorganotrophs; collectively they can degrade a wide range of substances which include e.g. agar, cellulose, chitin, keratin, paraffins and rubber. Some species produce important antibiotics (see e.g. STREPTOMYCES). Ultrastructure and staining. The cell structure is that of a Gram-positive prokaryote; most species give an unequivocally positive reaction in the Gram stain (but see e.g. CELLULOMONAS), and some species are acid-fast (see e.g. MYCOBACTERIUM, NOCARDIA, RHODOCOCCUS). Cytoplasmic inclusions observed in at least some species include e.g. granules of poly-b-hydroxybutyrate, polyphosphate, and polysaccharide, and globules of lipid. The cell wall commonly appears to be either uniformly electrondense or three-layered, the electron-density of the middle layer being somewhat less than that of the layer on either side of it. The wall contains PEPTIDOGLYCAN and other polymers, e.g. TEICHOIC ACIDS – although the latter appear not to occur in the NOCARDIOFORM ACTINOMYCETES; the cell wall is commonly surrounded by a layer of diffuse or (in sporoactinomycetes) fibrous material. Depending on the presence of certain amino acids in the peptidoglycan, and the identity of the cell wall sugars, eight wall types (chemotypes I–VIII) of actinomycetes can be distinguished [Book ref. 46, pp. 1915–1922]: I. LL-DAP (LL-diaminopimelic acid), glycine. II. meso-DAP, glycine. III. meso-DAP. IV. meso-DAP, arabinose, galactose. V. Lysine, ornithine. VI. Lysine; aspartic acid and galactose sometimes present. VII. DAB (2,4-diaminobutyric acid), glycine; lysine sometimes present. VIII. Ornithine. A further wall type (IX), characterized by meso-DAP and numerous amino acids, was defined for species of MYCOPLANA. In most species which form non-fragmenting mycelium (e.g. Streptomyces spp) the vegetative hyphae are largely aseptate, although septa (cross-walls) can be present – particularly in the older parts of the mycelium. The septa in non-fragmenting mycelium have been designated type 1 septa; each septum consists of a single layer which develops centripetally from the cell wall. Such septa may contain microplasmodesmata, each 4–10 nm in diameter. In fragmenting mycelium each septum consists of two distinct layers, each layer eventually forming a terminal wall of one of the two neighbouring cells; such septa are designated type 2 septa. Spore formation. Spores are formed by the septation and fragmentation of hyphae, the spore wall being formed, at least in part, from all the wall layers of the sporogenous hypha. Spore-delimiting septa are of various types, and different types may occur even within a given genus; such septa have been designated type I (two layers developing centripetally), type II (two layers which develop centripetally on a single, initially-formed annulus), and type III (a single, thick layer which develops centripetally). Spore chains are reported to develop acropetally (in e.g. Pseudonocardia), basipetally (in e.g. Micropolyspora), randomly (in e.g. Nocardiopsis), or more or less simultaneously (in e.g. Streptomyces). In some actinomycetes the spores are formed within sporangia: see e.g. ACTINOPLANES, AMORPHOSPORANGIUM, AMPULLARIELLA, DACTYLOSPORANGIUM, FRANKIA and PILIMELIA. Genetic aspects. Genetic exchange has been studied in various actinomycetes, particularly Streptomyces spp [Streptomyces genetics: Book ref. 73, pp. 229–286; genetics of nocardioform actinomycetes: Book ref. 73, pp. 201–228]. Actinomycetes are hosts to a number of ACTINOPHAGES, and generalized transduction with phage fSV1 has been recorded in strains of Streptomyces [JGM (1979) 110 479–482]. Actinomycetes can contain various transmissible or non-transmissible plasmids, some of which 8 Actinosphaerium are involved in antibiotic production. Genetic analyses have been carried out by methods involving e.g. conjugation and protoplast fusion. Genera include: ACTINOMADURA, ACTINOMYCES, ACTINOPLANES, ACTINOPOLYSPORA, ACTINOSYNNEMA, AGROMYCES, AMORPHOSPORANGIUM, AMPULLARIELLA, ARACHNIA, ARCANOBACTERIUM, ARTHROBACTER, BREVIBACTERIUM, CASEOBACTER, CELLULOMONAS, CORYNEBACTERIUM, CURTOBACTERIUM, DACTYLOSPORANGIUM, DERMATOPHILUS, EXCELLOSPORA, FRANKIA, GEODERMATOPHILUS, INTRASPORANGIUM, KINEOSPORIA, MICROBACTERIUM, MICROBISPORA, MICROMONOSPORA, MICROPOLYSPORA, MICROTETRASPORA, MYCOBACTERIUM, NOCARDIA, NOCARDIOIDES, NOCARDIOPSIS, OERSKOVIA, PILIMELIA, PLANOBISPORA, PLANOMONOSPORA, PROMICROMONOSPORA, PSEUDONOCARDIA, RENIBACTERIUM, RHODOCOCCUS, ROTHIA, SACCHAROMONOSPORA, SACCHAROPOLYSPORA, SPIRILLOSPORA, SPORICHTHYA, STREPTOALLOTEICHUS, STREPTOMYCES, STREPTOSPORANGIUM, STREPTOVERTICILLIUM, THERMOMONOSPORA. [Ecology, isolation, cultivation etc: Book ref. 46, pp. 1915– 2123.] actinomycete Any member of the order ACTINOMYCETALES; the name is often used to refer specifically to those species which form mycelium, i.e. excluding many members of the NOCARDIOFORM ACTINOMYCETES. actinomycetoma See MADUROMYCOSIS. actinomycin D (actinomycin C1 ) An ANTIBIOTIC from Streptomyces sp; it contains a (red) substituted phenoxazone chromophore linked to two identical pentapeptide lactone rings. All cell types are potentially susceptible, any resistance being due to low permeability of cells to the drug. Actinomycin D specifically inhibits DNA-directed RNA synthesis. It binds specifically to B-DNA as an INTERCALATING AGENT (f ca. 26° ). The phenoxazone chromophore intercalates primarily between two adjacent (antiparallel) GC pairs, while the lactone rings fit into the minor groove [ARB (1981) 50 171–172]. The drug dissociates from DNA only very slowly; it blocks the movement of RNA polymerase along its DNA template. (Since actinomycin D shows little binding to AT-rich PROMOTERS chain initiation is not inhibited.) DNA replication may be insensitive to actinomycin D because strand separation by the replicative apparatus may facilitate dissociation of the antibiotic. actinomycosis (1) Any human or animal disease caused by a species of ACTINOMYCES: A. israelii in man, A. bovis in cattle. Infection is probably endogenous. Dense nodular lesions are formed, mainly around the jaw (‘lumpy jaw’), developing into pus-discharging abscesses. Abscesses may also occur in the lungs, brain or intestine. Lab. diagnosis: the pathogen may be isolated from small yellow granules (‘sulphur granules’) present in the pus. Chemotherapy: e.g. penicillins. (2) Any human or animal disease caused by an ACTINOMYCETE: e.g. actinomycosis (sense 1); MADUROMYCOSIS. (See also LACHRYMAL CANALICULITIS.) actinophage Any BACTERIOPHAGE whose host(s) are member(s) of the ACTINOMYCETALES. Actinophages, which include both temperate and virulent types, can be isolated from e.g. soils and composts; most have a wide host range, but some (e.g. BACTERIOPHAGE fEC, BACTERIOPHAGE VP5) can infect only one or a few species. (See also STYLOVIRIDAE). [Soil actinophages which lyse Streptomyces spp: JGM (1984) 130 2639–2649.] Actinophryida An order of protozoa (class HELIOZOEA) in which the cells have no skeleton and no centroplast (cf. CENTROHELIDA). Some members have flagellated stages. Sexual processes have been observed in some species. Genera include e.g. ACTINOPHRYS, ACTINOSPHAERIUM, Ciliophrys. Actinophrys A genus of heliozoa (order ACTINOPHRYIDA). A. sol is common among vegetation in freshwater ponds and lakes. The cell is ca. 40–50 µm diam., with a highly vacuolated cytoplasm; the distinction between ectoplasm and endoplasm is not clear in living cells (cf. ACTINOSPHAERIUM). The axial filaments of the axopodia (see AXOPODIUM) originate close to the single central nucleus. Reproduction occurs asexually by binary fission. Autogamy occurs when environmental conditions are unfavourable: meiosis follows encystment of the uninucleate cell, 2–4 gametes being formed; fusion of gametes results in the formation of zygotes which can remain dormant in the cyst until conditions improve. Actinoplanes A genus of aerobic, sporogenous bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in soil, plant litter and aquatic habitats. The organisms form a branching surface mycelium, hyphal diameter ca. 0.2–1.5 µm, which may also ramify into the substratum; the mycelium later forms vertical hyphae, each developing, at its tip, a (commonly spherical) desiccation-resistant sporangium containing a number of spherical or oval spores – each bearing a polar tuft of flagella. Colonies may be e.g. yellow, orange, red, blue, brown or purple. Type species: A. philippinensis. [Morphology, ecology, isolation: Book ref. 46, 2004–2010; isolation: JAB (1982) 52 209–218.] Actinopoda A superclass of protozoa (subphylum SARCODINA) which are typically more or less spherical, typically have axopodia (filopodia in some members), and are usually planktonic. Classes: ACANTHAREA, HELIOZOEA, Phaeodarea and Polycystinea (see RADIOLARIA). Actinopolyspora A genus of bacteria (order ACTINOMYCETALES, wall type IV); the sole species, A. halophila, was isolated from a salt-rich bacteriological medium. The organisms form substrate and aerial mycelium, the latter giving rise to chains of spores; at least 10% (w/v) sodium chloride is required for growth, the optimum being ca. 15–20%, and the maximum ca. 30%. GC%: ca. 64. Type species: A. halophila. [Book ref. 73, 122–123.] Actinopycnidium See STREPTOMYCES. actinorrhiza A bacterium–plant root association in which nitrogen-fixing root nodules are formed in certain non-leguminous angiosperms infected (through root hairs) by FRANKIA strains; the plants involved are typically woody pioneers of nutrient-poor soils in cold or temperate regions in the northern hemisphere. There are at least two morphological types of actinorrhizal root nodule. In the Alnus type, formed in Alnus spp (alder) and many other plants, the root nodules are coralloid (i.e., thickened and dichotomously branched). In the Myrica type, formed e.g. in species of Myrica, Casuarina and Rubus, the nodule is clothed with upward-growing (negatively geotropic) rootlets which may aid aeration in boggy habitats. In either type, the endophyte occurs within the cortical parenchyma of the nodule and does not invade vascular or meristematic tissues. In the distal part of the nodule the (young) hyphae spread from cell to cell, perforating the host cell walls. In the proximal part the hyphal tips swell to form vesicles which appear to provide a reducing environment within which NITROGEN FIXATION can occur; rates of nitrogen fixation are comparable to those in leguminous ROOT NODULES. [Book ref. 55, pp. 205–223.] (See also MYCORRHIZA.) Actinosphaerium A genus of heliozoa (order ACTINOPHRYIDA) in which the cells are multinucleate and ca. 200 µm to 1.0 mm in diameter, according to species; the highly vacuolated ectoplasm is clearly distinct from the granular endoplasm (cf. ACTINOPHRYS). Numerous needle-like axopodia radiate from the 9 Actinosporangium acute-phase proteins Various types of protein, found in plasma, formed as a rapid response to infection; they are synthesized in the liver e.g. under stimulation from cytokines produced in a region of INFLAMMATION. These proteins include C-REACTIVE PROTEIN (CRP) and serum amyloid A (SAA), both of which can bind to phospholipids in the microbial cell envelope and act as OPSONINS; additionally, binding by CRP activates COMPLEMENT. CRP and SAA are so-called pentraxin proteins in which the molecule consists of five identical subunits. (See also CD14.) acute-phase serum Serum obtained from a patient during the acute phase of a disease. acute respiratory disease See ARD. ACV ACYCLOVIR. ACVs See VACCINE. acycloguanosine Syn. ACYCLOVIR. acyclovir (ACV; acycloguanosine; Zovirax) An ANTIVIRAL AGENT, 9-(2-hydroxyethoxymethyl)guanine, which is active against alphaherpesviruses. It is phosphorylated by the virus-encoded thymidine kinase to the monophosphate; the monophosphate is converted by host-cell enzymes to the active triphosphate form which inhibits DNA polymerase – the viral polymerase being much more sensitive than the cellular a-polymerase. (cf. BROMOVINYLDEOXYURIDINE.) Uninfected cells do not effectively phosphorylate acyclovir, and the drug is relatively non-toxic to the host. Acyclovir is used topically, systemically or orally in the treatment of e.g. herpes simplex keratitis, primary genital herpes, mucocutaneous herpes simplex in immunocompromised patients, progressive varicella and HERPES ZOSTER. Acyclovir is not equally active against all alphaherpesviruses – its ability to inhibit the replication of varicella-zoster virus is approximately 10-fold lower than its ability to inhibit replication of herpes simplex virus. [Use of acyclovir in the treatment of herpes zoster: RMM (1995) 6 165–174 (167–170).] acylalanine antifungal agents See PHENYLAMIDE ANTIFUNGAL AGENTS. N-acyl-L-homoserine lactone See QUORUM SENSING. Acytostelium A genus of cellular slime moulds (class DICTYOSTELIOMYCETES) in which the sorocarp stalk is acellular, cellulosic, slender, and apparently tubular; no myxamoebae are sacrificed in stalk formation (cf. DICTYOSTELIUM). The stalk bears a single terminal sorus of spores. Four species are recognized [descriptions and key: Book ref. 144, pp. 393–407]. Ad Human adenovirus: see MASTADENOVIRUS. A–D group ALKALESCENS–DISPAR GROUP. ADA deficiency See ADENOSINE DEAMINASE DEFICIENCY. ada gene See ADAPTIVE RESPONSE. adamantanamine See AMANTADINE. adamantane See AMANTADINE. Adansonian taxonomy A method of biological classification, proposed in the 18th century by Michel Adanson, in which relationships between organisms are defined by the number of characteristics which the organisms have in common; the same degree of importance (‘weighting’) is attached to each characteristic. (cf. NUMERICAL TAXONOMY.) adaptation Change(s) in an organism, or population of organisms, by means of which the organism(s) become more suited to prevailing environmental conditions. Genetic adaptation involves e.g. mutation and selection: those (mutant) organisms in a given population which are genetically more suited to the existing environment thrive and become numerically dominant. (See also FLUCTUATION TEST.) Non-genetic (phenotypic) adaptation cell, their axial filaments arising at the junction between ectoplasm and endoplasm. Asexual reproduction involves plasmotomy. Autogamy occurs when environmental conditions become unfavourable: the cell produces a gelatinous covering, and many of its nuclei degenerate; numerous uninucleate daughter cells are produced, and each encysts. Meiosis within the cyst results in two haploid gametes which fuse, and the resulting zygote remains dormant until conditions improve. Actinosporangium See STREPTOMYCES. Actinosporea A class of protozoa (phylum MYXOZOA) which are parasitic in invertebrates (particularly annelid worms). The spores contain 3 polar capsules (each enclosing a single polar filament) and several to many sporoplasms. The spore wall consists of 3 valves which may be smooth (e.g. in Sphaeractinomyxon) or drawn out into long, horn-like processes (as in Triactinomyxon, a parasite of tubificid and sipunculid worms). (cf. WHIRLING DISEASE.) Actinosynnema A genus of bacteria (order ACTINOMYCETALES, wall type III) which occur e.g. on vegetable matter in aquatic habitats. The organisms form a thin, branching, yellow substrate mycelium (hyphae <1 µm diam.) on which develop synnemata (up to ca. 180 µm in height) or ‘dome-like bodies’; aerial hyphae may arise from the substrate mycelium, or from the tips of the synnemata, and give rise to chains of spores which become motile (flagellated) in liquid media. GC%: ca. 71. Type species: A. mirum. [Book ref. 73, pp 116–117.] activated acetic acid pathway See AUTOTROPH. activated sludge See SEWAGE TREATMENT. activation (immunol.) (1) (of lymphocytes) A process which begins with BLAST TRANSFORMATION and continues with proliferation (cell division) and differentiation; some authors use ‘activation’ to refer specifically to stage(s) preceding proliferation. (2) (of complement) See COMPLEMENT FIXATION. (3) See MACROPHAGE. (4) (of spores) See ENDOSPORE and SPORE. activation-induced cytidine deaminase See RNA EDITING. activator (1) Syn. COFACTOR (sense 2). (2) See SPORE. (3) See OPERON and REGULON. active bud See LIPOMYCES. active immunity Specific IMMUNITY (3) afforded by the body’s own immunological defence mechanisms following exposure to antigen. (cf. PASSIVE IMMUNITY.) active immunization See IMMUNIZATION. active transport See TRANSPORT SYSTEMS. actomyosin An actin–myosin complex (see ACTIN and MYOSIN). aculeacin A An antibiotic which is active against yeasts, inhibiting the formation of yeast CELL WALL glucan. Echinocandin B is a structurally related antibiotic with an apparently similar mode of action. (cf. PAPULACANDIN B.) aculeate Slender and sharp-pointed, or bearing narrow spines. acute (med.) Refers to any disease which has a rapid onset and which persists for a relatively short period of time (e.g. days) – terminating in recovery or death. The term is also used to refer to an exceptionally severe or painful condition. (cf. CHRONIC.) acute cardiac beriberi See CITREOVIRIDIN. acute haemorrhagic conjunctivitis (AHC) A highly infectious form of CONJUNCTIVITIS, a worldwide pandemic of which occurred during 1969–1971; it is caused by enterovirus 70 (see ENTEROVIRUS) and is characterized by subconjunctival haemorrhages ranging from petechiae to larger areas covering the bulbar conjunctivae. Recovery is usually complete in ca. 10 days. acute herpetic gingivostomatitis See GINGIVITIS. acute necrotizing ulcerative gingivitis See GINGIVITIS. 10 Adenoviridae Adeleina A suborder of protozoa (order Eucoccida [JP (1964) 11 7–20] or Eucoccidiida [JP (1980) 27 37–58]) equivalent to the ADELEORINA. Adeleorina A suborder of protozoa (order EUCOCCIDIORIDA) in which syzygy characteristically occurs (cf. EIMERIORINA, HAEMOSPORORINA). Genera include Adelea, Haemogregarina, Klossiella. adenine arabinoside Syn. VIDARABINE. adenitis Inflammation of gland(s). adeno-associated viruses See DEPENDOVIRUS. adeno-satellite viruses See DEPENDOVIRUS. adenosine See NUCLEOSIDE and Appendix V(a). adenosine 3
, 5
-cyclic monophosphate See CYCLIC AMP. adenosine deaminase deficiency A congenital lack of the enzyme adenosine deaminase (EC 3.5.4.4), the effects of which include a marked reduction in the numbers of functional B and T lymphocytes. (See SEVERE COMBINED IMMUNODEFICIENCY.) The disease has been treated by GENE THERAPY. adenosine triphosphatase See ATPASE. adenosine 5
-triphosphate See ATP. Adenoviridae (adenovirus family) A family of non-enveloped, icosahedral, linear dsDNA-containing viruses which infect mammals (genus Mastadenovirus) or birds (genus Aviadenovirus). Adenoviruses are generally specific for one or a few closely related host species; infection may be asymptomatic or may result in various diseases (see AVIADENOVIRUS and MASTADENOVIRUS). Many adenoviruses can induce tumours when injected into newborn rodents, but none is known to cause tumours in natural circumstances. In cell cultures, adenoviruses cause characteristic CPE, including the rounding of cells and the formation of grape-like clusters of cells; adenovirus replication and assembly occur in the nucleus, resulting in the formation of intranuclear inclusion bodies. Virions sometimes form paracrystalline arrays. Many adenoviruses can haemagglutinate RBCs from various species. The adenovirus virion consists of an icosahedral CAPSID (ca. 70–90 nm diam.) enclosing a core in which the DNA genome is closely associated with a basic (arginine-rich) viral polypeptide (VP), VII. The capsid is composed of 252 capsomers: 240 hexons (capsomers each surrounded by 6 other capsomers) and 12 pentons (one at each vertex, each surrounded by 5 ‘peripentonal’ hexons). Each penton consists of a penton base (composed of viral polypeptide III) associated – apparently by hydrophobic interactions – with one (in mammalian adenoviruses) or two (in most avian adenoviruses) glycoprotein fibres (viral polypeptide IV); each fibre carries a terminal ‘knob’ ca. 4 nm in diam. The fibres can act as haemagglutinins and are the sites of attachment of the virion to a host cell-surface receptor. The hexons each consist of three molecules of viral polypeptide II; they make up the bulk of the icosahedron. Various other minor viral polypeptides occur in the virion. The adenovirus dsDNA genome (MWt ca. 20–25 × 106 for mammalian strains, ca. 30 × 106 for avian strains) is covalently linked at the 5 end of each strand to a hydrophobic ‘terminal protein’, TP (MWt ca. 55000); the DNA has an inverted terminal repeat (ITR) of different length in different adenoviruses. In most adenoviruses examined, the 5 -terminal residue is dCMP (dGMP in CELO virus). Adenovirus virions are stable and are not inactivated by e.g. lipid solvents or by pancreatic proteases, low pH, or bile salts. Replication cycle. The virion attaches via its fibres to a specific cell-surface receptor, and enters the cell by endocytosis or by direct penetration of the plasma membrane. Most of may involve a change in metabolic activity – e.g., by enzyme induction or repression (see OPERON). (See also CHROMATIC ADAPTATION.) Behavioural adaptation may involve changes in tactic responses (see TAXIS); thus, e.g., if a phototactic organism is subjected to a sudden increase in light intensity followed by steady illumination at the new intensity, the organism initially responds phototactically, but after a period of time it adapts to the new light intensity and resumes its normal pattern of motility. (See also CHEMOTAXIS.) adaptive response A DNA REPAIR system which is induced in cells of Escherichia coli in response to exposure to low concentrations of certain ALKYLATING AGENTS (e.g. MNNG, MNU); the response is independent of the SOS system. Genes involved in the adaptive response include ada, aidB, alkA and alkB. The alkA gene encodes 3-methyladenine-DNA glycosylase II, an enzyme which (despite its name) cleaves various methylated bases (e.g. N 3 - or N 7 -methylpurines and O 2 methylpyrimidines) from alkylated DNA; studies on the crystal structure of AlkA complexed with DNA indicate that the enzyme distorts DNA considerably as it ‘flips out’ (i.e. exposes) the target site [EMBO (2000) 19 758–766]. (Another enzyme of E. coli, DNA glycosylase I (the Tag protein; tag gene product), is synthesized constitutively; the Tag protein cleaves 3methyladenine from DNA.) DNA is susceptible to chemical change owing to the reactivity of its bases [Nature (1993) 362 709–715]. As well as methylation in vitro, DNA is also subject to aberrant, non-enzymatic methylation in vivo via S-adenosylmethionine (normally a legitimate methyl donor) which can give rise to 3-methyladenine and/or 7-methylguanine; these aberrant methylated bases are also cleaved by the glycosylases mentioned above. Cleavage of a chemically aberrant base, to form an AP SITE, is the first stage in the repair process; repair continues via BASE EXCISION REPAIR. A different aspect of the adaptive response involves the ada gene product: a bifunctional methyltransferase which directly reverses the effects of the methylating agent. One function of Ada transfers a methyl group from a major-groove adduct – O 6 methylguanine (a potentially highly mutagenic lesion) or O 4 methylthymine – to a cysteine residue near the C-terminus of the Ada protein itself. The second function of the Ada protein transfers a methyl group from a methyl phosphotriester (formed by methylation of a phosphodiester bond in the DNA) to a cysteine residue in the N-terminal portion of the protein. Once methylated at both sites, the Ada protein is inactivated. The Ada protein acts as a positive regulator of its own expression as well as that of aidB, alkA and alkB; alkylation of the Ada protein from a methyl phosphotriester is apparently the intracellular signal for induction of the adaptive response [Cell (1986) 45 315–324]. adaptor (mol. biol.) A synthetic oligodeoxyribonucleotide which is similar to a linker (see LINKER DNA) but which contains more than one type of restriction site and may also have pre-existing STICKY ENDS. ADCC Antibody-dependent CELL-MEDIATED CYTOTOXICITY: the killing of antibody-coated target cells by a non-phagocytic mechanism in which the effector cell (see e.g. NK CELLS) initially binds to the Fc portion of the (bound) antibodies via specific receptors. (See also CD16.) addition mutation Syn. INSERTION MUTATION. Adelea See ADELEORINA. 11 adenoviruses effects on various metabolic processes – the activity of certain enzymes being regulated by the actual concentration of a given adenine nucleotide or by the ratio of particular nucleotides (e.g. ATP:ADP). Thus, e.g., in certain yeasts the enzyme phosphofructokinase is inhibited by ATP (an effect which is reversed by AMP), while in Escherichia coli the same enzyme is stimulated by ADP; hence, glycolysis tends to be stimulated when the EC is depressed. Energy charge is also a regulatory factor for the degree of supercoiling (superhelical density) in a cell’s DNA; changes in superhelical density can, in turn, influence the activity of various gene promoters. Thus, via energy charge and superhelicity, the environment can modulate the expression of particular genes. (Interestingly, the expression of genes or operons may also be regulated by local changes in superhelicity due to divergent transcription from closely spaced gene promoters [Mol. Microbiol. (2001) 39 1109–1115].) Estimations of EC involve both rapid sampling and precautions to prevent hydrolysis or interconversion of adenine nucleotides. ATP is often measured by techniques which involve CHEMILUMINESCENCE. adenylate kinase An enzyme (EC 2.7.4.3) which catalyses the reversible conversion of two molecules of adenosine 5 -diphosphate (ADP) to one molecule each of ATP and AMP. [Structural and catalytic properties of adenylate kinase from Escherichia coli: JBC (1987) 262 622–629.] adenylylsulphate See APS. ADH ARGININE DIHYDROLASE. adherent cells (immunol.) Cells which adhere to e.g. glass and plastics; they include e.g. MACROPHAGES and DENDRITIC CELLS. adhesin A cell-surface component, or appendage, which mediates ADHESION to other cells or to inanimate surfaces or interfaces; there are many different types of adhesin, and a given organism may have more than one type. (The term adhesin is also used to include certain secreted substances which behave as adhesins – e.g. MUTAN.) Bacterial adhesins. Many bacterial adhesins are FIMBRIAE, and in some pathogenic species fimbrial adhesins are important virulence factors which mediate the initial stage of pathogenesis (adhesion to specific site(s) in the host organism). For example, fimbria-mediated adhesion is important for virulence in ETEC – strains of Escherichia coli whose capacity to cause disease depends on their ability to bind to the intestinal mucosa; among strains of ETEC there are more than 10 different types of fimbrial adhesin (as well as non-fimbrial adhesins) [RMM (1996) 7 165–177]. [Expression of fimbriae by enteric pathogens (review): TIM (1998) 6 282–287.] Fimbrial adhesins are also important in Haemophilus influenzae type b for the initial binding to respiratory tract epithelium [adhesins in Haemophilus, Actinobacillus and Pasteurella: FEMS Reviews (1998) 22 45–59]. (See also UPEC and P FIMBRIAE.) Some fimbrial adhesins additionally function as an INVASIN (see e.g. UPEC) or as a phage receptor (see e.g. BACTERIOPHAGE CTX8). The dimensions and charge characteristics of fimbriae are such that they experience minimal repulsion from a surface bearing charge of the same polarity; thus, fimbrial adhesins can help to bridge the gap between the charged bacterial surface (see ZETA POTENTIAL) and the surface of another cell or substratum which bears a charge similar (in polarity) to that on the bacterium. Non-fimbrial adhesins include the filamentous haemagglutinin (FHA) of Bordetella pertussis, the high-molecular-weight adhesion proteins (HMW1, HMW2) of ‘non-typable’ strains of the capsid proteins are removed in the cytoplasm; the virion core enters the nucleus, where the uncoating is completed to release viral DNA almost free of virion polypeptides. Virus gene expression then begins. The viral dsDNA contains genetic information on both strands. [By convention, the AT-rich half of the DNA molecule is designated the right-hand end, and the strand transcribed from left to right is called the r-strand, the leftward-transcribed strand being called the l-strand; the DNA is divided into 100 ‘map units’ (m.u.): Book ref. 116, pp. 40–42.] Early genes (regions E1a, E1b, E2a, E3, E4) are expressed before the onset of viral DNA replication. Late genes (regions L1, L2, L3, L4 and L5) are expressed only after the initiation of DNA synthesis. Intermediate genes (regions E2b and IVa2 ) are expressed in the presence or absence of DNA synthesis. Region E1a encodes proteins involved in the regulation of expression of other early genes, and is also involved in transformation (see MASTADENOVIRUS). The RNA transcripts are capped (with  m7 G5 ppp5 N) and polyadenylated (see MRNA) in the nucleus before being transferred to the cytoplasm for translation. Viral DNA replication requires the terminal protein, TP, as well as virus-encoded DNA polymerase and other viral and host proteins. TP is synthesized as an 80K (MWt 80000) precursor, pTP, which binds covalently to nascent replicating DNA strands. pTP is cleaved to the mature 55K TP late in virion assembly; possibly at this stage, pTP reacts with a dCTP molecule and becomes covalently bound to a dCMP residue, the 3 -OH of which is believed to act as a primer for the initiation of DNA synthesis (cf. BACTERIOPHAGE f29). Late gene expression, resulting in the synthesis of viral structural proteins, is accompanied by the cessation of cellular protein synthesis, and virus assembly may result in the production of up to 105 virions per cell. [Book ref. 116.] adenoviruses Viruses of the ADENOVIRIDAE. adenylate cyclase An enzyme (EC 4.6.1.1) which catalyses the synthesis of CYCLIC AMP (cAMP) from ATP. (cAMP is degraded to AMP by the enzyme cAMP phosphodiesterase.) In Escherichia coli and other enterobacteria, adenylate cyclase is a single protein (product of the cya gene) whose activity is modified e.g. by the PTS system (see CATABOLITE REPRESSION); it may also be regulated directly by pmf, and its synthesis may be repressed by cAMP–CRP. The EF component of ANTHRAX TOXIN and the CYCLOLYSIN of Bordetella pertussis both have CALMODULIN-stimulated adenylate cyclase activity; the pore-forming activity of the cyclolysin may serve to internalize its adenylate cyclase activity, resulting e.g. in raised levels of cAMP in cells of the immune system. In higher eukaryotes, adenylate cyclase occurs as part of a plasma membrane complex which includes e.g. hormone receptors and GTP-binding regulatory components (so-called G proteins; = N proteins); Gs (= Ns ) stimulates adenylate cyclase, Gi (= Ni ) inhibits it. This system is the target for certain bacterial toxins: see e.g. CHOLERA TOXIN and PERTUSSIS TOXIN. adenylate energy charge (energy charge, EC) A unitless parameter which gives a measure of the total energy associated with the adenylate system, at a given time, within a cell. It is defined as: EC = [ATP] + 1/2 [ADP] [ATP] + [ADP] + [AMP] Cells growing under ideal conditions have an energy charge of ca. 0.8–0.95, while senescent cells may have an EC of ca. 0.6 or less. Changes in the relative proportions of adenine nucleotides in a cell (i.e., changes in EC) have regulatory 12 adiaspore bonding, and van der Waals’ forces. Because some types of cell resemble colloids in their dimensions and electrical characteristics (see ZETA POTENTIAL), the interaction between a cell and a substratum (or between two cells) in an aqueous medium is sometimes considered in the context of classical colloid theory – in particular the theory of Derjaguin, Landau, Verwey and Overbeek (the ‘DLVO’ theory). The DLVO theory supposes that a particle which bears a distributed surface charge of given polarity is surrounded by a layer of ions of opposite charge – forming a so-called ‘double layer’ extending some distance from the surface of the particle; two similarly charged particles will therefore be mutually repulsive through the interaction of their double layers. An increase in the ionic concentration in the medium effectively compresses each double layer – so that the particles can then approach each other more closely; in a number of cases, cell–substratum or cell–cell contact has been shown to be facilitated by raising the concentration of electrolyte. However, a rigid application of the DLVO theory (or any other mathematically based theory) to biological systems is made difficult by a number of factors which include the susceptibility of the cell to physical deformation, the chemical complexity and non-uniformity of the cell surface, and the possibility of ionic flux across juxtaposed surfaces. Microorganisms which are normally attached to a substratum may be dispersed by means of either non-adherent progeny or propagules – or they may be able to detach, temporarily, in order to colonize fresh surfaces. The hydrophobic, benthic cyanobacterium Phormidium J-1 appears to achieve dispersal by forming an emulsifying agent (EMULCYAN) which masks cellsurface hydrophobicity – permitting detachment; the emulcyan is presumed to be washed off the cells at some stage so that attachment is again possible [FEMS Ecol. (1985) 31 3–9]. adhesion site (Bayer’s junction; Bayer’s patch) In Gram-negative bacteria: a localized ‘fusion’ between the OUTER MEMBRANE and CYTOPLASMIC MEMBRANE [Book ref. 101, pp. 167–202]. In electronmicrographs, a plasmolysed cell may show adhesion sites under some experimental conditions but not under others [JB (1984) 160 143–152]. [Cell envelope fraction with apparent adhesion sites: JBC (1986) 261 428–443.] Adhesion sites appear to be osmotically sensitive, physiologically important regions of the cell envelope which serve e.g. as sites for the export of proteins (such as porins), LPS molecules [JB (1982) 149 758–767] and filamentous phages, as sites of infection for certain phages, and as the anchorage sites of e.g. F pili. Certain proteins (e.g. penicillin binding protein 3, THIOREDOXIN [JB (1987) 169 2659–2666]) have been associated with adhesion sites. [Mol. Microbiol. (1994) 14 597–607.] (See also PERISEPTAL ANNULUS, and ‘lysis protein’ in LEVIVIRIDAE.) adiaspiromycosis (adiaspirosis; haplomycosis) A non-infectious MYCOSIS which primarily affects animals, rarely affecting man. It is caused by Chrysosporium parvum var. parvum (formerly Emmonsia parva or Haplosporangium parvum) or by C. parvum var. crescens (formerly E. crescens). Infection occurs by inhalation of conidia (formed e.g. in soil); the conidia enlarge within the lungs to reach diameters of ca. 40 µm (var. parvum) or 400 µm (var. crescens). Granulomas may develop around the adiaspores. adiaspirosis Syn. ADIASPIROMYCOSIS. adiaspore A spore (conidium) which grows in size without dividing – see e.g. ADIASPIROMYCOSIS and CHRYSOSPORIUM. Haemophilus influenzae, and the M PROTEIN of streptococci. (See also DR ADHESINS and OPA PROTEINS.) Proteinaceous ‘capsular’ adhesins include K88 in certain strains of Escherichia coli. Non-proteinaceous adhesins include the capsular carbohydrates of Rhizobium trifolii which bind to TRIFOLIIN A on the root hairs of the clover plant. (See also streptococcal lipoTEICHOIC ACIDS.) Receptors for bacterial adhesins. Binding sites for bacterial adhesins on mammalian tissues include various cell-surface molecules, but a given adhesin typically binds only to a specific site. For example, the type I FIMBRIAE of E. coli bind to mannose residues, while the P fimbriae of uropathogenic E. coli bind to a-D-galactopyranosyl-(1–4)-b-D-galactopyranoside receptors of glycolipids on urinary tract epithelium. Some pathogens (e.g. Bordetella pertussis, Borrelia burgdorferi, Yersinia enterocolitica) bind to specific INTEGRINS. Protein adhesins of Staphylococcus aureus bind to components such as collagen and fibronectin in the mammalian extracellular matrix [TIM (1998) 6 484–488]. Certain LECTINS (q.v.) may promote disease by enhancing attachment of ETEC to the porcine ileum. adhesion Microorganisms often bind specifically or nonspecifically to a substratum or to other cells – adhesion being mediated by specialized microbial components or structures: see e.g. ADHESIN and PROSTHECA. For some pathogens, adhesion to specific host cells or tissues is a prerequisite for disease, so that, in these organisms, adhesins are important virulence factors (see ADHESIN). The adherence of a pathogen to a host cell may be necessary simply promote the host’s uptake of a secreted toxin. However, in some cases, adhesion triggers a more interactive sequence of events. For example, when the FHA adhesin of Bordetella pertussis binds to an INTEGRIN receptor on a monocyte, it generates signals within the monocyte that upregulate the expression of a second type of integrin – one which binds to a different site on the adhesin. Thus, this pathogen ‘manipulates’ the host cell’s internal signalling system in order to secure for itself additional binding sites. In another example, the binding of UPEC (q.v.) to uroepithelium via type I FIMBRIAE promotes uptake (internalization) of the pathogen. In enteropathogenic E. coli, initial adhesion to gut epithelium is followed by complex prokaryote–eukaryote interactions that result in a unique form of colonization by the pathogen (see EPEC). The adhesion of bacteria to inanimate surfaces can be problematical in the context of prosthetic devices. Infection associated with these devices is a serious complication which is often difficult to treat (not least because adherent bacteria are typically less susceptible to antibiotics) [prosthetic device infections: RMM (1998) 9 163–170]. Free-living microorganisms in aquatic habitats often adhere to submerged surfaces on stones, particles of debris, other organisms or man-made structures – sometimes forming BIOFILMS. (See also EPILITHON.) Adhesion may affect the activity of such organisms because the conditions at a submerged surface differ from those in the bulk aqueous phase; for example, surfaces can adsorb nutrients and/or stimulatory or inhibitory ions, so that solid–liquid interfaces may be significantly more advantageous or disadvantageous compared with the liquid phase. Cell–cell interactions may or may not be facilitated in biofilms. The interaction between microorganisms and a surface is governed by various physicochemical forces that may include electrostatic attraction or repulsion, hydrophobic interaction (i.e. mutual attraction between hydrophobic molecules), hydrogen 13 adjuvant In the carrier state the incidence of ATL is low; latency can last for many decades. The disease has been categorized into four clinical subtypes: acute, chronic, smouldering and lymphoma. Acute cases comprise the majority. Chronic and smouldering forms often progress to the acute form. In some cases disease is nodal rather than leukaemic. Various manifestations of ATL may be seen. In many cases there are lesions in liver, spleen and/or lymph nodes, though other sites (including the central nervous system) may be affected; skin lesions may include nodules, ulcers or rashes. Hypercalcaemia may be present, and the undermined immune system may permit infection by opportunist pathogens (e.g. Pneumocystis carinii, cytomegalovirus). Leukaemic cells are monoclonal, originating from a single cell infected with HTLV-I; the cells are larger than normal and may have multilobed or convoluted nuclei. The median survival time for the acute form of ATL is reported to be ∼6 months (∼2 years for the chronic form). Lab. diagnosis. Diagnosis involves e.g. clinical observation, serology (for anti-HTLV-I antibody) and detection of abnormal T cells by microscopy. Chemotherapy. Given the absence of standard therapy, it has been recommended that treatment be limited to acute and lymphoma-type cases; for these patients combination chemotherapy may be successful, but relapses (often involving the CNS) are common. [ATL (epidemiology, leukaemogenesis, clinical features, laboratory findings, diagnosis, treatment, prognosis and prevention): BCH (2000) 13 231–243.] (cf. MYCOSIS FUNGOIDES and SÉZARY SYNDROME.) adventitious septum See SEPTUM (b). aecial cup See UREDINIOMYCETES stage I. aecidioid (cupulate) Refers to a peridiate, cup-shaped to cylindrical aecium of the type formed e.g. by species of Puccinia and Uromyces. aecidiospore See UREDINIOMYCETES stage I. aecidium See UREDINIOMYCETES stage I. aeciospore See UREDINIOMYCETES stage I. aecium See UREDINIOMYCETES stage I. Aedes A genus of mosquitoes (order Diptera, family Culicidae); Aedes spp are vectors of certain diseases: see e.g. CHIKUNGUNYA FEVER, YELLOW FEVER. Aedes aegypti EPV See ENTOMOPOXVIRINAE. Aedes albopictus cell-fusing agent See FLAVIVIRIDAE. Aegyptianella A genus of Gram-negative bacteria of the family ANAPLASMATACEAE. Cells: pleomorphic cocci (diam. ca. 0.3–0.8 µm) which form membrane-limited inclusion bodies (each containing up to 30 cells) within the erythrocytes of the infected host (cf. ANAPLASMA). The sole species, A. pullorum, is parasitic in a range of birds and can cause disease e.g. in chickens; transmission occurs mainly or exclusively via ticks. A. pullorum occurs in southern Europe and the Mediterranean area, in Africa south of the Sahara, and in Asia. aequihymeniiferous See LAMELLA. Aer See AEROTAXIS. aerial mycelium In many actinomycetes: mycelium which projects above the level of the medium; no aerial mycelium is formed e.g. by Arachnia or Intrasporangium, while Sporichthya forms only aerial mycelium. Aerial mycelium and SUBSTRATE MYCELIUM differ e.g. morphologically, structurally and physiologically [Book ref. 73, pp. 169–170]; aerial mycelium is typically less branched and, in at least some species, its surface is adjuvant (1) (immunol.) Any substance which, when administered with or before an antigen, heightens and/or affects qualitatively the immune response in terms of antibody formation and/or the cell-mediated response. (The adjuvant L18-MDP(A) has also been reported to enhance non-specific phagocytosis by polymorphonuclear leucocytes [JGM (1982) 128 2361–2370].) Adjuvants include e.g. BCG, aluminium hydroxide, and waterin-oil emulsions (e.g. FREUND’S ADJUVANT). (2) Any substance which is added to a drug or other chemical (e.g. a disinfectant) to enhance its activity. Adjuvant 65 A water-in-oil emulsion ADJUVANT made by emulsifying peanut oil with mannide monooleate and stabilizing with aluminium monostearate. adk gene (dnaW gene) In Escherichia coli: the gene for adenylate kinase. Adler test A test used for the identification of Leishmania spp. The organisms are cultured in an immune serum; in the presence of homologous antibodies, promastigotes develop in clusters or syncytia. adnate (1) Of the region of lamella–stipe attachment in an agaric: extending for a length equal to much or most of the depth of the lamella. (2) Of flagellar spines: flattened against the flagellum from which they arise. adnexed (mycol.) Of the region of lamella–stipe attachment in an agaric: extending for a length equal to only a small fraction of the depth of the lamella. adonitol Syn. RIBITOL. adoptive immunity Syn. PASSIVE IMMUNITY. adoral ciliary spiral See AZM. adoral zone of membranelles See AZM. ADP Adenosine 5 -diphosphate. (See also ATP and ADENYLATE KINASE.) ADP-ribosylation The transfer of an ADP-ribosyl group from NAD+ to a protein, catalysed by an ADP-ribosyl transferase. In eukaryotic cells, various proteins may be ADP-ribosylated – apparently as a normal regulatory mechanism; certain bacterial toxins act by exerting an ADP-ribosyl transferase function: see e.g. BOTULINUM C2 TOXIN, CHOLERA TOXIN, DIPHTHERIA TOXIN, EXOTOXIN A and PERTUSSIS TOXIN. ADPribosylation also occurs e.g. in cells of Escherichia coli infected with BACTERIOPHAGE T4, the host RNA polymerase undergoing phage-induced ADP-ribosylation. [Review: TIBS (1986) 11 171–175.] adrenalin (epinephrine) A multifunctional hormone, secreted by the adrenal gland, which affects e.g. carbohydrate metabolism and the activity of smooth muscle, particularly that of the cardiovascular and bronchial systems. Adrenalin is used e.g. for the treatment of ANAPHYLACTIC SHOCK – in which it counteracts the effects of histamine, relaxing smooth muscle and reducing vascular permeability. adriamycin See ANTHRACYCLINE ANTIBIOTICS. ADRY reagents Certain substituted thiophenes which can e.g. stimulate the photooxidation of cytochrome b559 in photosystem II (see PHOTOSYNTHESIS). adsorption (serol.) Non-specific adherence of substances (in solution or in suspension) to cells or to other forms of particulate matter. (See e.g. BOYDEN PROCEDURE; cf. ABSORPTION.) adsorption chromatography See CHROMATOGRAPHY. adult T-cell leukaemia (ATL; adult T-cell leukaemia/lymphoma, ATLL) A T-cell LEUKAEMIA (q.v.) which affects adults; the causal agent is an exogenous retrovirus, HTLV-I (see HTLV). ATL is endemic in certain regions of Japan, the Caribbean, Africa and the Americas. 14 aesculin at 5° C. Three subspecies are recognized. Subsp salmonicida is indole −ve, aesculin +ve, and produces a water-soluble brown pigment when grown aerobically on media containing 0.1% tyrosine or phenylalanine. Subsp achromogenes is indole +ve/−ve, aesculin −ve, and does not produce brown pigment. Subsp masoucida is indole +ve, aesculin +ve, and does not produce brown pigment. A. salmonicida is haemolytic on blood agar. The species is strictly parasitic and often pathogenic in fish, causing e.g. FURUNCULOSIS and secondary infections in various other diseases. [Aeromonas bacteriophages: Ann. Vir. (1985) 136E 175–199.] aerosol Minute (colloidal) particles of liquid and/or solid dispersed in a gas (e.g. air), formed e.g. by sneezing, coughing, liquids splashing, bubbles bursting, etc. An aerosol may contain viable microorganisms. aerotaxis A TAXIS in which a (motile) cell migrates along an oxygen concentration gradient to a location where the concentration of oxygen is optimal for that cell. In bacteria, aerotaxis appears to be exhibited by all aerobic and facultatively aerobic motile species. Interestingly, though, organisms which use oxygen as terminal electron acceptor in energy metabolism do not necessarily migrate to positions where oxygen is at atmospheric levels; for example, the respiratorytype bacterium Azospirillum brasiliense is microaerophilic, and this organism migrates towards regions where the oxygen concentration is only 3–5 µM. (This species may have a highly efficient oxidase which enables it to carry out oxygen-based respiratory metabolism in microaerobic conditions.) For some time it has been thought that aerotaxis may be linked to changes in proton motive force (pmf). In support of this idea, it was found that different types of change in pmf (i.e. increases or decreases) are associated with different effects on motility; thus, for example, if the partial pressure of oxygen is kept constant, then an artificially induced increase in pmf encourages smooth, continual swimming, while a decrease in pmf promotes tumbling in peritrichously flagellated cells. More recently it has been shown that, in A. brasiliense, pmf increases when the cell swims towards the preferred concentration of oxygen and decreases when the cell swims in the opposite direction; this has suggested that the change in level of pmf acts as a signal which regulates aerotactic movements [JB (1996) 178 5199–5204]. Aerotaxis apparently cannot occur in a bacterium in which the pmf is at a maximum; under such conditions certain other (pmfindependent) taxes – e.g. CHEMOTAXIS – may occur. In Escherichia coli a sensor protein, Aer, may mediate aerotaxis by responding to redox changes in component(s) of the electron transport chain; the Tsr protein (an MCP in chemotaxis) appears to be another, independent sensor for aerotaxis and may respond to changes in pmf [PNAS (1997) 94 10541–10546]. aerotolerant Refers to an ANAEROBE which, in the presence of air, can either survive (but not grow) or grow at sub-optimal rates. aeruginocin Syn. PYOCIN. aesculin (esculin) The 6-b-D-glucosyl derivative of 6,7-dihydroxycoumarin. Certain bacteria (e.g. most strains of the former group D streptococci, many Bacteroides spp – including B. fragilis) can hydrolyse aesculin to yield 6,7-dihydroxycoumarin which gives a brown coloration with soluble ferric salts. (Hydrolysis may also be detected by the disappearance of aesculin fluorescence under Wood’s lamp.) Various media are used for aesculin hydrolysis tests. Aesculin broth may contain e.g. aesculin (0.1%) and FeCl3 (0.05%) in heart infusion broth. Aesculin agar is aesculin broth gelled with hydrophobic. In some genera the aerial mycelium is the main, or only, part of the organism which bears spores or sporophores. Aerobacter An obsolete bacterial genus (nom. rejic.). Most strains formerly regarded as ‘Aerobacter aerogenes’ are referable (e.g. on the basis of motility) to Klebsiella pneumoniae or to Enterobacter aerogenes. [Book ref. 21, pp. 322, 324, 339–340.] aerobactin See SIDEROPHORES. aerobe An organism which has the ability to grow in the presence of oxygen, i.e., in or on media or substrates which are in contact with air – the term commonly being reserved for those organisms which, in nature, normally grow in aerobic habitats; some aerobes can also grow under anaerobic conditions, i.e., they are FACULTATIVE anaerobes. (cf. ANAEROBE; see also MICROAEROPHILIC and ANAEROBIC RESPIRATION.) aerobic (1) Refers to an environment in which oxygen is present at a partial pressure similar to that in air. (cf. ANAEROBIC, MICROAEROBIC.) (2) Having the characteristic(s) of AEROBE(s). aerobiosis (1) The state or condition in which oxygen is present. (2) Life in the presence of air. Aerococcus A genus of Gram-positive bacteria of the family Streptococcaceae. Cells: non-motile cocci, commonly in pairs and tetrads. Growth occurs optimally under microaerobic conditions; glucose is fermented homofermentatively to L(+)-lactic acid. Some strains form a pseudocatalase. A. viridans (formerly Gaffkya homari and Pediococcus homari ) is the causal agent of GAFFKAEMIA. aerogenic Gas-producing. aeromonad A species or strain of Aeromonas. Aeromonadaceae See AEROMONAS. Aeromonas A genus of Gram-negative bacteria of the VIBRIONACEAE. [Proposal to move Aeromonas from the Vibrionaceae to a new family, Aeromonadaceae: SAAM (1985) 6 171–182; IJSB (1986) 36 473–477.] Cells variable: straight, roundended rods or coccobacilli (0.3–1.0 × 1.0–3.5 µm) – occurring singly, in pairs, or in short chains – or short filaments. The genus is divided into two groups: non-motile psychrotrophs (A. salmonicida) and motile mesophiles (A. hydrophila group). Motile cells usually have a single, polar, unsheathed flagellum (wavelength ca. 1.7 µm); some cells in young cultures on solid media may be peritrichously flagellate. Aeromonas spp are oxidase +ve; a range of sugars and organic acids can be used as carbon sources; acid (± gas) is formed from glucose but not e.g. from inositol; gelatinase, DNase, RNase and Tween 80 esterase (lipase) are formed; NO3 − is reduced to NO2 − . NaCl is not required for growth. Strains are resistant to O/129. Most strains can grow on e.g. nutrient agar or trypticase-soy agar but usually not e.g. on TCBS agar. [Media, identification etc: Book ref. 46, pp. 1288–1294.] GC%: 57–63. Type species: A. hydrophila. The taxonomy of the motile strains remains unsettled. Three species have been recognized: A. hydrophila, A. caviae and A. sobria [Book ref. 22, pp. 545–548]. (‘A. punctata’ is regarded as a later synonym of A. hydrophila, with VP −ve, anaerogenic strains now included in A. caviae; ‘A. proteolytica’ = Vibrio proteolyticus.) Motile strains can grow on many media used for enterobacteria (e.g. DCA, MacConkey’s agar), usually forming colourless (lactose −ve) colonies. Optimum growth temperature: ca. 28° C, maximum usually 38–41° C; some strains can grow at 5° C. Species occur in fresh water, sewage, and associated with aquatic animals; strains can cause disease in fish (see e.g. RED PEST and RED MOUTH), amphibians (e.g. RED LEG), reptiles, and cattle, and in man – causing e.g. septicaemia, meningitis, gastroenteritis (enterotoxigenic strains), etc. A. salmonicida has an optimum growth temperature of 22–25° C, maximum usually ca. 35° C; most strains can grow 15 aethalium nucleotides (a-[33 P]-dATP) that are incorporated into products during amplification [BioTechniques (2000) 28 622–623].) [Taxonomic evaluation of Bacillus anthracis and related species by AFLP fingerprinting: JB (1997) 179 818–824. AFLP fingerprinting used for detecting genetic variation in Xanthomonas: Microbiology (1999) 145 107–114. AFLP-based study of Escherichia coli : JCM (1999) 37 1274–1279. Review of AFLP fingerprinting: JCM (1999) 37 3083–3091.] African farcy Syn. EPIZOOTIC LYMPHANGITIS. African horse sickness An infectious HORSE DISEASE caused by an Orbivirus and transmitted by insects (e.g. Culicoides spp); it occurs in Africa, parts of the Middle East, and the Mediterranean region. The disease may be acute, with an incubation period of ca. 5–7 days, followed by fever, laboured breathing, severe paroxysms of coughing, and a profuse nasal discharge of yellowish, frothy serous fluid; death usually occurs within 4–5 days of onset. Subacute forms of the disease may occur in enzootic areas: an incubation period of up to 3 weeks is followed by oedema of the head region, spreading to the chest; cardiac and pulmonary symptoms and paralysis of the oesophagus may occur, but mortality rates are generally lower than in the acute form. A mild form involving fever and moderate dyspnoea (‘horse sickness fever’) may occur e.g. in partially immune animals. African horse sickness may also cause severe debility in mules and donkeys, but mortality rates are lower than in horses. Control: e.g. vaccination; control of vectors. African swine fever A highly infectious PIG DISEASE which clinically resembles European SWINE FEVER (q.v.); it occurs e.g. in Africa, Spain and Portugal. The causal agent is a virus previously classified in the IRIDOVIRIDAE but currently considered to belong to a separate family. The virion is icosahedral, enveloped, and contains a DNA-dependent RNA polymerase and RNA-modifying enzymes; the genome is dsDNA (MWt ca. 100 × 106 ) in which the strands are covalently joined at each end (cf. POXVIRIDAE), and which contains terminal inverted repeats. African trypanosomiases See TRYPANOSOMIASIS. ag (immunol.) ANTIGEN. Agamococcidiorida An order in the COCCIDIASINA. agamont See ALTERNATION OF GENERATIONS. agar A complex galactan which is widely used (in gel form) as a base for many kinds of solid and semi-solid microbiological MEDIUM; agar (or agarose – see below) is also used e.g. in techniques such as GEL DIFFUSION, GEL FILTRATION and ELECTROPHORESIS, and in industry as a gelling agent in foods, pharmaceuticals etc. Agar is produced by many marine rhodophycean algae and is obtained commercially from e.g. Gelidium and Gracilaria spp; in the alga it is associated with the CELL WALL and intercellular matrix. (The term ‘agar’ derives from the Malay agar-agar which refers to certain edible seaweeds.) Agar consists of two main components: agarose (ca. 70%) and agaropectin (ca. 30%). Agarose is a non-sulphated linear polymer consisting of alternating residues of D-galactose and 3,6anhydro-L-galactose: [-3,6-anhydro-a-L-galactopyranosyl-(1 → 3)-b-D-galactopyranosyl-(1 → 4)-]n ; in agarose from some species, a proportion of the D-galactose residues have 6-O-methyl substituents. Agaropectin is a mixture of sulphated galactans which may also contain e.g. glucuronic acid or pyruvic acid, depending on source. (Agar-like substances from noncommercial seaweeds show various structural differences from commercial agar [Book ref. 38, pp. 291–292].) An agar gel is a translucent or transparent jelly-like substance formed when a mixture of agar and water is heated to >100° C and then cooled; gelling occurs at ca. 40–45° C. agar. Bile–aesculin agar contains e.g. oxgall (4%), aesculin (0.1%) and ferric citrate (0.05%) in nutrient agar. Bacteroides bile–aesculin agar is bile–aesculin agar with the addition of e.g. haemin and gentamicin. The former group D streptococci can be identified by their ability to cause blackening on bile–aesculin agar slants – usually within 48 h; most non-group D streptococci do not cause blackening, although some viridans streptococci may do so. aethalium See MYXOMYCETES. aetiology (etiology) The study of causation. The aetiological agent of an infectious disease is the organism which initiates/causes the disease. aetioporphyrins See PORPHYRINS. AEV Avian erythroblastosis virus: see AVIAN ACUTE LEUKAEMIA VIRUSES. AF-2 (furylfuramide) See NITROFURANS. AFC (immunol.) Antibody-forming cell. AFDW Ash-free dry weight. affinity (immunol.) The strength of binding between a given antibody and a single antigenic determinant or monovalent hapten. Factors which affect affinity include the area of contact between the antibody combining site and the antigenic determinant, the closeness of fit, and the nature of the intermolecular forces involved. (cf. AVIDITY.) affinity chromatography See CHROMATOGRAPHY. affinity labelling A technique used to identify the COMBINING SITE of an antibody. In one method, the antibody is allowed to combine with a homologous HAPTEN (the ‘affinity label’) that carries an azide side-chain; on subsequent irradiation with ultraviolet light the azide forms a highly reactive nitrene group that combines with any of a range of organic groups in the combining site. The nature of the combining site may subsequently be determined e.g. by chemical analysis. affinity maturation See ANTIBODY FORMATION. affinity tail See FUSION PROTEIN. aflatoxicosis A MYCOTOXICOSIS caused by ingestion of AFLATOXINS. aflatoxins A group of MYCOTOXINS, produced by strains of Aspergillus flavus and A. parasiticus, which contain a bifuran moiety fused at the 4,5 ring positions to a substituted coumarin. (cf. STERIGMATOCYSTIN.) Aflatoxins are soluble in organic solvents, and some exhibit fluorescence on UV irradiation; they are extremely heat-stable. Aflatoxins are toxic to a wide range of eukaryotes; they can retard germination and growth in plants, inhibit the germination of several moulds (e.g. Mucor, Neurospora, Penicillium), and are hepatotoxic and hepatocarcinogenic in animals. Severe outbreaks of disease have occurred in domestic animals and poultry fed with aflatoxincontaminated feedstuffs – particularly groundnut and cereal feeds. (See also HEPATITIS X; TURKEY X DISEASE; cf. RUBRATOXINS.) Aflatoxins can affect the immune system, reducing resistance to infection, and may increase the risk of hepatocellular carcinoma due to HEPATITIS B VIRUS infection. The toxins can inhibit mitosis in tissue cultures. Aflatoxins may e.g. cause errors in DNA replication by reacting with guanine bases; toxic effects may differ in different species. [Aflatoxin biosynthesis genes: AEM (2004) 70 1253–1262; (2005) 71 3192–3198.] AFLP fingerprinting A PCR-based TYPING method [original description: NAR (1995) 23 4407–4414] which requires only a small amount of purified genomic DNA. The principle of the method is outlined in the figure. (In the standard protocol, primers are end-labelled; a simplified procedure avoids the need for end-labelling of primers by using a-labelled 16 (a) (b) adaptor restriction fragment NNNG−3′ adaptor 5′−AATTGNNNNNNN−3′ NNNCTTAA−5′ (c) CNNNNNNN−5′ NNNGAATTGNNNNNNN−3′ NNNCTTAACNNNNNNN−5′ (d) NNNGAATTGNNNNNNN−3′ TCTTAACNNNNNNN−5′ primer AFLP FINGERPRINTING (principle, diagrammatic). Chromosomes of the test strain are initially digested with two types of RESTRICTION ENDONUCLEASE, commonly EcoRI (recognition site: G/AATTC) and MseI (recognition site: T/TAA). As a result, the two sticky ends of each fragment may be created either by the same enzyme or by different enzymes. The chromosomal fragments are then mixed with ‘adaptor’ molecules of two types, here designated A and B. Each type of adaptor molecule is a short DNA sequence which has one sticky end that corresponds to the recognition sequence of one of the two restriction enzymes. The reaction mixture contains a ligase, so that covalent binding between fragments and adaptor molecules gives rise to the following sequences: A-fragment-A, A-fragment-B, B-fragment-A and B-fragment-B. Note. In each adaptor molecule a ‘mutant’ nucleotide is incorporated immediately adjacent to the sticky end so that, after ligation to a fragment, the cutting site of the enzyme is not restored; thus, following ligation, the sequence is not susceptible to restriction. The fragments, flanked on each side by (ligated) adaptor molecules, are now subjected to PCR under high-stringency conditions. Each PCR primer is designed to be complementary to one or other of the adaptor molecules, including the restriction site; however, an important feature of each primer is that its 3 end extends for one or a few nucleotides beyond the restriction site – i.e. into the ‘unknown’ fragment. The one (or few) 3 nucleotide(s) of the primer are selective nucleotides, i.e. the primer will be extended only if these nucleotides are paired with complementary nucleotides in the fragment. Hence, while primers may bind to all fragments in the mixture, only a subset of fragments will be amplified, i.e. those fragments containing nucleotides that are complementary to the selective 3 nucleotide(s) of the primer. A primer with one selective nucleotide has a 1-in-4 chance of binding to a complementary nucleotide in the fragment; this type of primer will amplify only about one in four of the fragments to which it binds. The primers of one type are labelled so that, following PCR and gel electrophoresis of the products, a fingerprint of (e.g. ∼50–200) detectable bands is obtained. (a) Each restriction fragment is flanked by (ligated) adaptor molecules. (b) Left. A fragment’s sticky end produced by EcoRI (N = nucleotide). Right. An adaptor molecule with the complementary 5 -AATT overhang; note that, in the overhang strand, the 5 -AATT is followed by G, rather than C. (c) Following base-pairing of the sticky ends in (b), and ligation, the resulting sequence 5 -GAATTG-3 3 -CTTAAC-5 differs from the cutting site of EcoRI and is not susceptible to cleavage by EcoRI. (d) During cycling, a primer binds to one strand of the fragment–adaptor junction region. As this primer’s 3 -terminal (selective) nucleotide is T, the primer will be extended only if the complementary nucleotide (A) occurs at this location in the fragment; extension will not occur on this fragment if T is mis-matched. Reproduced from Figure 7.6, page 193, in DNA Methods in Clinical Microbiology (ISBN 07923-6307-8), Paul Singleton (2000), with kind permission from Kluwer Academic Publishers, Dordrecht, The Netherlands. 17 agar diffusion test Cortinariaceae. Basidiocarp: stipitate, characteristically with a fine, cobweb-like cortina; basidiospores: rust-coloured or some shade of brown, smooth to rough. Genera include Cortinarius, Galerina, Inocybe (see also MUSCARINE). Crepidotaceae. Basidiocarp: non-stipitate, or with a rudimentary lateral stipe; basidiospores: cinnamon-coloured. Genera include CREPIDOTUS. Hygrophoraceae (‘wax caps’). Basidiocarp: stipitate, often brightly coloured, the lamellae being waxy, and the basidia typically elongated; basidiospores: colourless. Genera include Hygrocybe and Hygrophorus. Pluteaceae. Basidiocarp: stipitate with a volva, the lamellae each having a convergent BILATERAL TRAMA; basidiospores: pink. Genera include Volvariella (see also PADI-STRAW MUSHROOM). Strophariaceae. Basidiocarp: stipitate (stipe often elongated), pileus e.g. buff, yellow, ochre, or (in Stropharia aeruginosa) greenish; basidiospores: typically brown to purplish-brown. Genera include Hypholoma, Panaeolus, Pholiota, Psilocybe, Stropharia. (See also HALLUCINOGENIC MUSHROOMS.) Tricholomataceae. Basidiocarp: stipitate, lamellae with nonbilateral trama; basidiospores: white or pink, without a germ pore. Genera include ARMILLARIA, Clitocybe, Collybia, Crinipellis (see also WITCHES’ BROOM), Flammulina (see also ENOKITAKE), LENTINULA, LEPISTA, Marasmius (see also MYCORRHIZA), Mycena (see also BIOLUMINESCENCE), Omphalotus, Oudemansiella, Tricholoma. agaricoid Refers to the type of fruiting body which is characteristic of fungi of the AGARICALES: gymnocarpic, with the hymenium forming a layer on lamellae and giving rise to ballistospores. (cf. GASTEROID.) Agaricus (formerly Psalliota) A genus of fungi (AGARICALES, Agaricaceae), most (not all) species of which are edible. Except in A. brunnescens, the basidia each form four basidiospores; basidiospores are dark brown and commonly ovoid. A. arvensis is the horse mushroom, A. campestris the common or field mushroom, and A. silvicola the wood mushroom (all edible species). A brunnescens (= A. bisporus), a species which forms two-spored basidia, is the cultivated mushroom (see MUSHROOM CULTIVATION). (See also FUNGUS GARDENS.) agarobiose A disaccharide: 3,6-anhydro-4-O-(b-D-galactopyranosyl)-L-galactose, a degradation product of AGAR. agaropectin See AGAR. agarophyte (agarphyte) Any AGAR-producing seaweed. agarose See AGAR. age-dependent polioencephalomyelitis (in mice) See LACTATE DEHYDROGENASE VIRUS. agglutinated test (protozool.) See e.g. FORAMINIFERIDA. agglutination The formation of insoluble aggregates following the combination of antibodies with cells or other particulate antigens (see e.g. WEIL–FELIX TEST) or with soluble antigens bound to cells or other particles (see e.g. LATEX PARTICLE TEST), or following the combination of soluble (or particulate) antigens with cell-bound or particle-bound antibodies (see e.g. PROTEIN A); agglutination may also be mediated by e.g. LECTINS or by fibrinogen (see clumping factor in COAGULASE). Agglutination may be detected macroscopically as suspended aggregates or (subsequently) as sedimented aggregates. (See also PASSIVE AGGLUTINATION and HAEMAGGLUTINATION; cf. PRECIPITATION and FLOCCULATION sense 1.) On sedimentation, agglutinated particles may form a mat over a relatively large area of the bottom of the test-tube; by contrast, non-agglutinated particles generally sediment to form a smaller, dense button in the control tube. Media made from Japanese agars usually contain 1.5–2.0% w/v agar; however, semi-solid agars contain 0.5% or less, and stiff agars contain e.g. 8% w/v. (If New Zealand agars are used, these concentrations should be halved to give gels of similar strengths.) Sterile gels are prepared by autoclaving a suspension of agar in water (see AUTOCLAVE). Growth media are prepared by adding nutrients, selective agents etc to the agar – usually before autoclaving but sometimes after (see e.g. BLOOD AGAR). If the medium is to have a pH of 6.0 or less, the pH must be adjusted after autoclaving, since agar is hydrolysed during heating at low pH. Various refined forms of agar may be used for specific purposes: e.g. ion-free agar may be used in immunoelectrophoresis. (See also ELECTROENDOSMOSIS.) Agar is a useful base for microbiological media in that it gels at moderate temperatures and, once set, the gels are stable at temperatures up to ca. 65° C or higher (although syneresis tends to occur at these temperatures); furthermore, the ability to degrade agar is confined to only a few organisms (including e.g. strains of Streptomyces coelicolor, certain marine pseudomonads, marine species of Cytophaga). (cf. GELATIN.) However, agar shortages, product variability, and rising prices have led to a search for suitable substitutes for agar; substitutes which have found some applications include CARRAGEENAN, Gelrite (see GELLAN GUM), low-methoxy PECTINS, and SILICA GELS. agar diffusion test A DIFFUSION TEST which differs from the DISC DIFFUSION TEST in that, instead of employing antibioticimpregnated discs, a solution of each antibiotic is allowed to diffuse from a separate ‘well’ cut into the agar. agar dilution test See DILUTION TEST. agar disc diffusion test Syn. DISC DIFFUSION TEST. agar gel diffusion See GEL DIFFUSION. agar plate See PLATE. agar-slide method Syn. DIP-SLIDE METHOD. agaric (1) Any fungus of the Agaricaceae. (2) Any fungus of the AGARICALES. (3) Any fungus whose hymenium is borne on lamellae (see LAMELLA). Agaricaceae See AGARICALES. Agaricales An order of terrestrial (typically humicolous or lignicolous), mainly saprotrophic fungi (subclass HOLOBASIDIOMYCETIDAE) most of which form mushroom-shaped, gymnocarpic or semiangiocarpic, fleshy fruiting bodies in which the hymenium is borne on radially arranged ‘gills’ (= lamellae, see LAMELLA) on the underside of the pileus; the pileus and (when present) stipe do not contain sphaerocysts (cf. RUSSULALES). (See also SECOTIOID FUNGI.) The order may be divided into families on the basis of e.g. basidiospore colour, the structure of the trama, and the nature of the cortical layers of the pileus [Book ref. 64, pp. 7–8]; these families include: Agaricaceae. Basidiocarp: stipitate, typically with an annulus when mature; basidiospores: typically dark brown or colourless, but not rust- or cinnamon-coloured. Genera include AGARICUS and LEPIOTA. Amanitaceae. Basidiocarp: stipitate, the lamellae each having a divergent BILATERAL TRAMA; basidiospores: white or pale. Some species form both a PARTIAL VEIL and a UNIVERSAL VEIL. Genera include AMANITA (volva formed), Limacella (volva not formed), and TERMITOMYCES. Bolbitiaceae. Basidiocarp: stipitate; basidiospores: ochre or cinnamon to rust-brown. Genera include Agrocybe and Conocybe. Coprinaceae. Basidiocarp: stipitate, a palisade-like layer of cells occurring in the pellis; basidiospores: dark or black, each usually containing a germ pore. Genera include COPRINUS and Psathyrella. 18 AHG Agrobacterium spp occur in soil, mainly in the rhizosphere. All (except A. radiobacter) can infect a wide range of dicotyledonous plants (and some gymnosperms) and induce the formation of self-proliferating galls or adventitious roots. The species are defined primarily or solely on the basis of their pathogenic characteristics: A. radiobacter is non-pathogenic, A. rhizogenes causes HAIRY ROOT, A. rubi causes CANE GALL, and A. tumefaciens causes CROWN GALL. However, pathogenicity depends on the presence of a plasmid(s) and can readily be altered or lost; hence the currently recognized species do not reflect true taxonomic relationships among the agrobacteria. [Book ref. 22, pp. 244–254.] [Media and culture: Book ref. 45, 842–855.] agrocin 84 See AGROCINS. agrocinopines A class of (sugar phosphodiester) opines found in CROWN GALL. (See also AGROCINS.) agrocins Antibiotics which are produced by certain strains of Agrobacterium and which are active against other strains of the same genus; being non-protein in structure, agrocins are not strictly BACTERIOCINS. Agrocin 84 is produced by a nonpathogenic, nopaline-catabolizing strain of A. radiobacter (strain 84, NCPPB 2407) and is selectively active against agrobacteria which harbour a nopaline Ti plasmid. Strain 84 is used in the BIOLOGICAL CONTROL of CROWN GALL; a cell suspension is used to treat seeds, roots or wounded plant surfaces (e.g. graft wounds), and almost 100% control of nopaline pathogens (responsible for most of the economic damage due to crown gall) can be achieved. Agrocin 84 is an adenine nucleotide derivative containing an N 6 -phosphoramidate substituent (necessary for uptake by sensitive cells) and a 5 -phosphoramidate substituent (necessary for toxicity). Agrocin 84 is taken up by sensitive strains via a high-affinity ‘agrocin permease’, apparently an agrocinopine transport system normally inducible by agrocinopines (sugar phosphodiesters) present in galls caused by nopaline strains. Strain 84 contains at least three plasmids: one (pAgK84, 47.7 kb) coding for agrocin 84 production, another (pAt84b, ca. 200 kb) coding for nopaline catabolism. pAgK84 is self-transmissible only at very low frequencies, but can be mobilized by the conjugative plasmid pAt84b. As transfer of pAgK84 to a crown gall pathogen could threaten the continued use of agrocin 84 in biocontrol, a transfer-deficient mutant strain was prepared. However, strain K84 apparently exerts some activity against agrocin 84-resistant pathogens independently of pAgK84 [AEM (1999) 65 1936–1940]. Agrocybe See AGARICALES (Bolbitiaceae). agroinfection A method for introducing viral DNA (or cDNA) into a plant. Viral DNA is initially incorporated into the T-DNA part of a Ti plasmid. The plasmid is then introduced into the bacterium Agrobacterium tumefaciens–which is used to infect the plant; during infection the viral DNA is transferred to plant cells within the T-DNA (see CROWN GALL). [Example of use: JV (2003) 77 3247–3256.] Agromyces A genus of microaerophilic to anaerobic, catalasenegative, asporogenous bacteria (order ACTINOMYCETALES, wall type VII – see also PEPTIDOGLYCAN). The organisms grow as a branched mycelium which subsequently fragments into coccoid and diphtheriod forms; metabolism: oxidative. A. ramosus, the type species, occurs in large numbers in certain soils; it appears to attack and destroy other species of bacteria [AEM (1983) 46 881–888]. agropine See CROWN GALL and HAIRY ROOT. Agropyron mosaic virus See POTYVIRUSES. AHG (serol.) Anti-human globulin: ANTIGLOBULIN homologous to human globulins. agglutination factor (algol.) See CHLAMYDOMONAS. agglutination test Any test in which reactions between particulate and/or soluble entities (particularly free or particle-bound antibodies and antigens) is detected by AGGLUTINATION. agglutinin (1) Any ANTIBODY involved in an AGGLUTINATION reaction. (2) Any substance which can agglutinate cells or inanimate particles by binding to their surface components. agglutinogen The antigen homologous to an agglutinin. Aggregata See EIMERIORINA. aggregate gold standard See GOLD STANDARD. aggregation substance See PHEROMONE. aggressin Any product or component of a pathogenic microorganism which promotes the invasiveness of that organism – see e.g. HYALURONATE LYASE. (cf. STREPTOKINASE; see also ADHESION and IGA1 PROTEASES.) aglycon The non-sugar portion of a glycoside. agmatine See e.g. DECARBOXYLASE TESTS. Agmenellum A phycological genus of ‘blue-green algae’ currently included in the SYNECHOCOCCUS complex. agnogene See AGNOPROTEIN. agnoprotein A 61-amino acid, highly basic polypeptide encoded by a gene (the ‘agnogene’) in the late leader region of SIMIAN VIRUS 40; it appears to play a role in viral assembly. Agonomycetales (Mycelia Sterilia) An order of fungi (class HYPHOMYCETES) which include species that form neither conidia nor sexual structures, though some species have been found to have teleomorphs in the Ascomycotina or the Basidiomycotina. The order includes lichenized fungi (e.g. LEPRARIA) and plant pathogens (see e.g. RHIZOCTONIA and SCLEROTIUM). agr locus (in Staphylococcus aureus) Accessory gene regulator locus: a chromosomal sequence which encodes, inter alia, (i) the sensor and response regulator of a TWO-COMPONENT REGULATORY SYSTEM (AgrA–AgrC), and (ii) an octapeptide ‘pheromone’ which is secreted by the cell and which, at appropriate concentrations (see QUORUM SENSING), activates the two-component system. When activated, the AgrA–AgrC system upregulates expression of the (agr-encoded) transcript RNA III which, in turn, regulates the expression of genes for exotoxins and certain cell-surface-associated virulence factors. Another two-component system, encoded by the srrAB genes (staphylococcal respiratory response genes), appears to regulate the expression of exotoxins and certain cell-surface virulence factors in accordance with the levels of environmental oxygen, such regulation being exerted, in part, via the agr system [JB (2001) 183 1113–1123]. agranulocyte Any white blood cell which has non-granular cytoplasm, e.g., a LYMPHOCYTE. Agrobacterium A genus of Gram-negative bacteria of the RHIZOBIACEAE. Cells: rods (0.6–1.0 × 1.5–3.0 µm), capsulated. Motile, with between one (often non-polar) and six peritrichous flagella. Optimum growth temperature: 25–28° C. Species can metabolize a wide range of mono- and disaccharides and salts of organic acids; acid (no gas) is formed from glucose (which is metabolized mainly via the ENTNER–DOUDOROFF PATHWAY and the HEXOSE MONOPHOSPHATE PATHWAY). Colonies on carbohydrate-containing media are typically mucilaginous, abundant extracellular slime (including a neutral (1 → 2)-b-glucan) being produced. (See also CURDLAN.) Some strains can use NH4 + and NO3 − as nitrogen sources, while others require amino acids; some strains can carry out NITRATE RESPIRATION. Nitrogen fixation does not occur. GC%: 57–63. Type species: A. tumefaciens. 19 AHL Antibodies (e.g. anti-gp120, anti-p24) are first detectable ∼6–12 weeks after infection; their appearance may follow, or coincide with, the rapid decline in viraemia. During seroconversion some patients experience a seroconversion illness which may include e.g. fever, sore throat, skin rash, generalized lymphadenopathy, pneumonitis, gastrointestinal and/or CNS involvement. A subsequent phase of infection is characterized by persistent generalized lymphadenopathy (PGL) (also called lymphadenopathy syndrome) – swollen lymph nodes reflecting an active immune response to HIV. (In some cases, PGL is the first manifestation of disease following infection, i.e. in patients who do not exhibit an acute phase.) Infection may then become asymptomatic (‘clinically latent’), and this state may continue for months or years (during which time viral replication continues). Patients with clinically latent infection, as well as those with PGL, may pass directly to AIDS. Alternatively, both types of patient may progress to AIDS via a further stage commonly referred to as the AIDS-related complex (ARC) (= category B of the CDC clinical staging system). Category B diseases include bacillary angiomatosis (see BARTONELLA), oropharyngeal candidiasis, HAIRY LEUKOPLAKIA, LISTERIOSIS, PID (pelvic inflammatory disease), herpes zoster (involving at least two distinct episodes, or more than one dermatome), and peripheral neuropathy. In AIDS, the final stage, the CD4+ count is often <200 cells/ml; there is high-level viraemia, and the p24 antigen is again detectable. Note that the AIDS-defining diseases (see above) include those that result from infection by ‘low-grade’ pathogens, i.e. organisms which generally cause disease only in those patients who are severely immunodeficient (including HIV− individuals who may be immunodeficient for other reasons). Conversely, the ‘high-grade’ pathogens (which can cause disease even in the immunocompetent) may cause disease in HIV+ patients whose immune system is only marginally impaired. Immunopathogenesis. In HIV+ individuals, the reduction in numbers of CD4+ T cells may arise in various ways – e.g. (i) productive infection by HIV and subsequent lysis; (ii) killing of HIV-infected cells by HIVspecific CD8+ cytotoxic T cells (see T LYMPHOCYTE) or by NK CELLS; (iii) ADCC (of virus-infected cells). It also appears that non-infected CD4+ T cells may be susceptible to attack e.g. by HIV-specific cytotoxic T cells or by NK CELLS; this may occur when free (isolated) gp120 protein (see HIV) binds to CD4 on virus-free (‘bystander’) cells – such cells then becoming vulnerable to the antiviral immune response. CD4+ cells may also be vulnerable to APOPTOSIS if viral gp120 cross-links CD4 molecules on the cell surface, such cross-linking resulting in an upregulation of the FAS antigen. Following depletion of CD4+ T cells, the virus may persist within tissue macrophages (see under Chemotherapy, below). HIV infection produces (i) a direct effect (death of infected cells), and (ii) an indirect effect (weakening of the overall immune system). The direct effect may include neurological deficits (e.g. encephalopathy) as well as immunological damage; the indirect effect is manifested in the range of ARC and AIDSdefining diseases (including both neoplastic and opportunistic diseases). Depletion of CD4+ T cells is only one of many abnormalities in the immune system of HIV+ /AIDS patients. Other types of cell (including B cells) are reported to display abnormalities, and DELAYED HYPERSENSITIVITY reactions may be either absent or of reduced intensity. AHL See QUORUM SENSING. ahpC gene See ISONIAZID. AID (activation-induced cytidine deaminase) See RNA EDITING. AIDA-I In Escherichia coli: an adhesin that mediates diffuse adherence (to e.g. HeLa cells) – hence the designation adhesin involved in diffuse adherence. AIDA-I is the a domain of an autotransporter (see type IV systems in PROTEIN SECRETION); in wild-type cells the a domain is apparently cleaved (autocatalytically?), but it remains attached (non-covalently) to the cell surface. (See also AUTODISPLAY.) aidB gene See ADAPTIVE RESPONSE. AIDS (acquired immune deficiency syndrome) In an HIV+ individual (see HIV): the stage of disease characterized by (i) counts of CD4+ T LYMPHOCYTES commonly within or below the range 200–500/µl (in adults and adolescents) and (ii) the presence of one or more category C diseases (AIDS-defining diseases specified in the clinical staging system of the Centers for Disease Control (CDC), Atlanta, Georgia, USA); category C diseases include e.g. CANDIDIASIS of the lower respiratory tract; disseminated COCCIDIOIDOMYCOSIS; extrapulmonary CRYPTOCOCCOSIS; retinitis due to cytomegalovirus (BETAHERPESVIRINAE); herpes simplex oesophagitis; HIV-related encephalopathy; extrapulmonary HISTOPLASMOSIS; chronic infection with ISOSPORA; KAPOSI’S SARCOMA; primary lymphoma of the brain; extrapulmonary infection with Mycobacterium tuberculosis (see also MAC); pneumonia caused by PNEUMOCYSTIS CARINII; TOXOPLASMOSIS of the brain. [Infections in AIDS (MICROSPORIDIOSIS, invasive pneumococcal disease, non-typhoid salmonellae): JMM (2000) 49 947–957.] The normal CD4 count in adults/adolescents is ∼1000/µl; in neonates it is much higher (≥2000), so that the adultbased relationship between CD4 counts and susceptibility to opportunist pathogens is not appropriate for very young children. [AIDS Insight (various aspects): Nature (2001) 410 961– 1007.] Transmission/containment of HIV. HIV can be transmitted: (i) by sexual contact (male ↔ female, as well as homosexual); (ii) via transfusion of (infected) blood or blood products (e.g. contaminated Factor VIII formerly given to haemophiliacs); (iii) via the placenta; (iv) by breast-feeding; and (v) through use of contaminated needles by intravenous drug abusers. The highest concentration of virus (free and intracellular) is found in blood; HIV also occurs e.g. in semen, milk and cerebrospinal fluid. Efforts to prevent/limit the spread of HIV have included: (i) education (making clear the basic facts of the disease, including routes of transmission); (ii) discouraging promiscuity; (iii) encouraging the use of condoms; (iv) discouraging needlesharing by drug addicts; (v) screening of blood donors; (vi) treatment of blood products. Clinical manifestations of HIV infection. Infection is commonly followed, in ∼2–6 weeks, by an early ‘acute’ phase which is characterized by high-level viraemia; p24 antigen (the major core protein: see HIV) can often be demonstrated in serum during the viraemic phase. Levels of virus remain high for some weeks – after which there is a sharp decline (and a loss of detectable p24 antigen); this decline in viraemia appears to reflect the activity of antigen-specific CD8+ cytotoxic T cells (see T LYMPHOCYTE) and may also involve nonspecific killing of virus-infected cells by NK CELLS. 20 air The role of CYTOKINES in HIV infection has yet to be clarified, but it seems that, at some stage, the cytokine milieu becomes aberrant, and that increased amounts of e.g. TNF-a and several interleukins (including INTERLEUKIN-4) are formed. It has been suggested that TNF-a may promote viral transcription through activation of the host cell’s nuclear transcription factor, NFkB (see HIV), and that increased levels of IL-4 may bring about a (deleterious) switch to the Th2 subset of T cells (see T LYMPHOCYTE). Diagnosis of HIV infection. In adults and adolescents, laboratory diagnosis of HIV infection is serologically based. The viral antigen p24 (major core protein) is commonly detectable (transiently) in blood during the initial viraemic (acute) phase of infection, i.e. before any antibodies are detectable. However, not all patients exhibit an acute phase of infection; while a positive test for p24 is useful, a negative test is not a reliable indication of the absence of HIV infection. Tests for anti-HIV antibodies (e.g. anti-gp120) may be carried out e.g. by ELISA. A positive result may be confirmed by Western blot analysis of viral proteins. Antibodies are commonly present by ∼6 weeks post-infection, although positive serology is sometimes delayed. (In some cases, patients who are serologically negative have been implicated, epidemiologically, in the transmission of AIDS.) Serology is inappropriate for neonates and very young children: if the mother is HIV+ then, owing to passive transfer of maternal antibodies, the neonate will also contain anti-HIV antibodies, whether infected or not. Culture is a useful approach for diagnosis in this context. The pre-seroconversion ‘window’ (the period, post-infection, before development of antibodies) presents a problem in blood transfusion because donor samples taken in this period are likely to contain virus. To minimize this risk, samples can be subjected to PCR-based tests designed to detect viral nucleic acid. Such tests may involve initial concentration of virions from plasma [Lancet (1999) 353 359–363]. Alternatively, the procedure may test for the integrated (i.e. provirus) form of HIV; in this case, PCR is carried out on DNA extracted from blood leukocytes [PNAS (1999) 96 6394–6399]. (See also BLOOD DNA ISOLATION KITS.) Chemotherapy for HIV infection. The drug zidovudine (see AZT) was the first agent used for the treatment of HIV infection. Although initially successful, the use of AZT was found to be limited by the development of drug-resistant strains of HIV. In recent years, combinations of ANTIRETROVIRAL AGENTS have been used in so-called ‘highly active antiretroviral therapy’ (see HAART). Attempts are being made to extend the range of antiretroviral agents by developing inhibitors of the viral integrase (in order to inhibit integration of HIV provirus into the host cell’s genome). (See also NU-1320.) Even with HAART, however, HIV commonly (or always) emerges after cessation of therapy, and it is generally believed that, during therapy, HIV persists in CD4+ memory T cells. However, using a chimeric virus (simian immunodeficiency virus/HIV-1), it has been shown that tissue macrophages (in lymph nodes, liver, spleen etc.) form the principal reservoir of virus following depletion of CD4+ T cells in rhesus macaques; this has suggested that macrophages may be a major source of HIV in the symptomatic phase of human infection [PNAS (2001) 98 658–663]. The drug nevirapine is reported to inhibit intrauterine transmission of HIV to the fetus in a proportion of cases. One problem associated with chemotherapy is that the combination of drugs used in HAART is often augmented by other drugs (e.g. antibiotics) used for the treatment of opportunistic infections; this provides potential for adverse drug–drug interactions. [Drug interactions in AIDS: JMM (2000) 49 947–967 (957–962).] One suggested approach is to combine chemotherapy (using a range of drugs to discourage the emergence of resistant strains of virus) with a programme of vaccination aimed at maintaining or increasing the population of specific cytotoxic T lymphocytes (CTLs); an appropriate level of CTLs would serve to augment the effects of chemotherapy by helping to prevent the growth of mutant strains of virus [PNAS (2000) 97 8193–8195]. Gene therapy. See GENE THERAPY. Anti-AIDS vaccines. The production of an anti-AIDS vaccine is fraught with difficulties. One of the major problems is the extensive variability of the envelope protein gp120 (see HIV); a vaccine which is active against one isolate of HIV may be less active, or non-active, against many other isolates. Early attempts at a vaccine included the use of a recombinant VACCINIA VIRUS containing the env sequence of HIV [Nature (1987) 326 249–250]. More recently it has been found that a particular, hypervariable part of the gp120 glycoprotein, the V3 loop, is an immunodominant area which influences certain phenotypic features of the virus, including infectivity; within this region is a tetrapeptide subregion, GPGR (see AMINO ACID), which seems to be conserved in a large number of field isolates. Recent studies have examined the effect of variation in the V3 loop on the immunogenic potential of gp120, and it appears that certain changes in composition have considerable influence on the immune response to this glycoprotein [Arch. Virol. (2000) 145 2087–2103]. Considerable efforts are currently being made to find a useful anti-AIDS vaccine, emphasis often being placed on the need for combined antibody- and cell-mediated responses. In one approach, a DNA VACCINE – HIV-derived DNA linked to polylactide co-glycolide – was found significantly to improve both cell-mediated and humoral immunity in monkeys. Use is being made of various vectors in candidate vaccines – including e.g. Salmonella, canarypoxvirus and Semliki Forest virus. [AIDS vaccines (news focus): Science (2001) 291 1686–1688.] AIDS-like diseases have been recognized in certain animals – e.g. cats (see FELINE LEUKAEMIA VIRUS) and monkeys (see SIMIAN AIDS). AIDS-related complex (ARC) See AIDS. AIDS virus See AIDS. Aino virus See AKABANE VIRUS DISEASE and BUNYAVIRUS. air (microbiological aspects) Air normally contains various microorganisms (particularly spores), pollen, and other particulate matter (see also AEROSOL); the microflora varies e.g. with location, general weather conditions, and with particular factors (such as relative humidity), while the viability of the microflora depends e.g. on the extent to which the air has been exposed to ultraviolet radiation. The airborne microflora is sampled e.g. in studies on pneumonitis-type allergies [Book ref. 51, pp. 27–65], environmental microflora [AEM (1983) 45 919–934], the stability of aerosols [AEM (1982) 44 903–908], and organisms of potential meteorological interest [AEM (1982) 44 1059–1063]. Instruments used to sample the airborne microflora include those of the simple gravity-type (e.g. the DURHAM SHELTER, TAUBER TRAP), and e.g. the ALL-GLASS IMPINGER, ANDERSEN SAMPLER, HIRST SPORE TRAP and ROTOROD (see also SLIT SAMPLER). (In the context of air samplers the terms ‘impactor’ and ‘impinger’ are sometimes used indiscriminately; thus, e.g. the Andersen sampler has 21 air bladder apparently functions as an overwintering propagule. Akinetes are formed under various growth-limiting environmental conditions (e.g., nutrient limitation); in e.g. Anabaena spp, akinetes develop adjacent to HETEROCYSTS, while in e.g. Nostoc spp they develop in positions midway between two heterocysts. An akinete is typically larger than a vegetative cell; it has a thickened wall and granular cytoplasm rich in storage materials (cyanophycin, glycogen etc). Rates of photosynthesis, respiration etc are usually much lower in akinetes than in vegetative cells; in e.g. Anabaena doliolum, akinetes appear to be deficient in both photosynthesis and inorganic nitrogen metabolism [JGM (1984) 130 1299–1302]. On germination of an akinete, a single cell or short filament may be released via a pore in – or by rupture of – the akinete wall, depending on species. (2) (algol.) A thick-walled non-motile resting cell produced by certain algae of the CHLOROPHYTA and XANTHOPHYCEAE. (3) (mycol.) A non-motile spore. akinetoplasty Obsolete syn. DYSKINETOPLASTY. AktA (of Actinobacillus actinomycetemcomitans) See RTX TOXINS. AKV A replication-competent MURINE LEUKAEMIA VIRUS which occurs endogenously in various strains of mice (e.g. AKR, BALB/c). It appears to have given rise to the transforming virus ‘AKR mink cell focus-forming virus’ (AKR-MCF) by recombination with one or more endogenous xenotropic viruses. (See also MCF VIRUSES.) alafosfalin (alaphosphin; L-alanyl-L-1-aminoethylphosphonic acid) A synthetic ANTIBIOTIC which is taken up by the LL-dipeptide transport system of a sensitive cell; it is subsequently hydrolysed intracellularly to release the inhibitory component, 1-aminoethylphosphonic acid (ala-P), which itself cannot cross the cytoplasmic membrane. (cf. WARHEAD DELIVERY.) Ala-P acts by competitively inhibiting alanine racemase and – at higher concentrations – by inhibiting the addition of L-alanine to UDPMurNAc during PEPTIDOGLYCAN synthesis. Alafosfalin has a broad spectrum of activity, but is generally more effective against Gram-negative than Gram-positive bacteria. alamethicin A water-soluble peptide antibiotic (MWt ca. 2100) which is produced – often together with a related antibiotic, suzukacillin – by strains of Trichoderma viride. It can act as an IONOPHORE [Book ref. 14, pp. 219–224]. L-alanine biosynthesis See Appendix IV(b) and AMMONIA ASSIMILATION. ala-P (AlaP) See ALAFOSFALIN. alaphosphin Syn. ALAFOSFALIN. Alaria See PHAEOPHYTA. alarmone A low-MWt molecule, synthesis of which serves as a trigger or signal for the redirection of cellular metabolism in response to a particular type of stress; an example is ppGpp in STRINGENT CONTROL (sense 1). alastrim See SMALLPOX. alazopeptin See DON. albamycin Syn. NOVOBIOCIN. Albert’s stain A stain used to demonstrate METACHROMATIC GRANULES. To prepare Albert’s stain: TOLUIDINE BLUE (0.15 g) and MALACHITE GREEN (0.2 g) are dissolved in 95% ethanol (2 ml) and added to 1% acetic acid (100 ml); the whole is filtered after standing for 24 h. A heat-fixed smear is stained with Albert’s stain (3–5 min), washed in tap water, and blotted dry; LUGOL’S IODINE is applied for 1 min and the smear washed and blotted dry. Granules stain black, cytoplasm pale green. albicidin An antibiotic, produced by Xanthomonas albilineans, which inhibits DNA synthesis in Escherichia coli [JGM (1985) 131 1069–1075]. been described as a ‘sieve impinger’ [Book ref. 51, pp. 59–61] and as a ‘cascade impactor’ [Book ref. 57, p. 163].) Individual types of sampler have particular limitations; for example, gravity-type instruments tend to collect the larger particles in preference to smaller ones, the Rotorod can be used only for short periods of time in air containing high concentrations of particulate matter, while some samplers tend to dehydrate the collecting medium and the deposited microorganisms. The air in confined spaces can be disinfected e.g. by ULTRAVIOLET RADIATION, by sprays of e.g. propylene glycol, or by the use of hydrophobic membrane filters (see FILTRATION). air bladder (algol.) Syn. PNEUMATOCYST. air sacculitis A POULTRY DISEASE which affects mainly chickens and turkey poults, particularly birds reared in broiler houses. It is caused by Mycoplasma gallisepticum, M. synoviae or (in turkeys only) M. meleagridis – often in association with Escherichia coli or respiratory virus infection. Symptoms: coughing, nasal discharge, conjunctivitis etc; sinuses below the eyes are characteristically swollen. The air sacs become filled with a thick white or yellowish caseous material. Transmission may occur from parent to offspring via the egg and from bird to bird. airlift fermenter A LOOP FERMENTER in which the circulation of the culture is typically achieved by pumping air in at the bottom of a DRAFT TUBE (or at the base of the annulus) – the air being voided via an opening in the top of the column; the air bubbles lower the hydrostatic pressure of culture in the draft tube so that culture continually flows down the annulus and up into the draft tube. (Reversed flow occurs if air is bubbled into the annulus.) The ICI pressure-cycle fermenter (used for Pruteen production – see SINGLE-CELL PROTEIN) is an example of a tubular loop airlift fermenter. It consists essentially of two tall vertical columns (of different diameters) which communicate at top and bottom, and there is no draft tube; air is pumped into the bottom of the wider column (the ‘riser’) and promotes circulation on the airlift principle. This fermenter achieves a high level of dissolved oxygen since oxygen solubility is increased by the increased hydrostatic pressure at the base of the (tall) riser; additionally, the physical separation of riser and downcomer facilitates the removal of heat by allowing the inclusion of a heat-exchanger in the downcomer. [Construction, behaviour and uses of airlift fermenters: Book ref. 3, pp. 67–95.] AIV process A process for preparing SILAGE by direct acidification of vegetable matter (to ca. pH 3.5) with a mixture of dilute HCl and H2 SO4 . In the Penthesta process HCl and H3 PO4 are used. Ajellomyces A genus of fungi of the GYMNOASCALES. A. dermatitidis = teleomorph of Blastomyces dermatitidis (q.v.); it is heterothallic and produces spherical, 8-spored asci. A. capsulatus = teleomorph of Histoplasma capsulatum (q.v.). Akabane virus disease A CATTLE DISEASE (which can also affect sheep and goats) caused by the Akabane virus (genus BUNYAVIRUS, serogroup Simbu) and transmitted by midges (Culicoides spp) and mosquitoes; it occurs e.g. in Africa, Australia and Japan. Infection of cows early in pregnancy causes malformation of the fetus; deformities depend on the stage of development at the time of infection, but may involve absence of cerebral hemispheres (hydranencephaly) and/or fixation of joints leading to deformities of limbs and spine (arthrogryposis). Abortion or stillbirth may occur. The cow shows no other clinical symptoms. A similar condition is caused by the Aino virus (Bunyavirus, serogroup Simbu). akinete (1) In certain CYANOBACTERIA: a specialized cell which shows some resistance to desiccation and cold and which 22 Alectoria albofungin Syn. KANCHANOMYCIN. albomycin See SIDEROMYCINS. alborixin See MACROTETRALIDES. Albugo A genus of obligately plant-parasitic fungi (order PERONOSPORALES) which are distinguished by their production of basipetally formed chains of zoosporangia. A. candida (economically the most important species) is the causal agent of ‘white rust’ of crucifers (= ‘crucifer white blister’ or ‘white blister disease’). Within the tissues of the host plant, this fungus forms a branching, aseptate, intercellular mycelium which produces rounded haustoria. The chains of zoosporangia develop on short, club-shaped sporangiophores beneath the host’s epidermis, giving rise to smooth, white, blister-like lesions within which the zoosporangia are compacted; subsequently, the epidermis ruptures, and the powdery mass of zoosporangia is dispersed by wind and rain. During sexual reproduction, oogonia and antheridia are produced within the host, and a fertilization tube is formed between them; following fertilization and meiosis, the oosphere gives rise to a thick-walled, warty oospore which later germinates to form zoospores. albumen The white of an egg. (cf. ALBUMINS.) albumins A class of low-MWt proteins which are soluble in dilute salt solutions and (unlike globulins) readily soluble in water. Alcaligenes A genus (incertae sedis) of catalase-positive, oxidase-positive, Gram-negative bacteria which occur e.g. in soil, water, the alimentary tract in vertebrates, and in clinical specimens. Cells: non-pigmented rods (up to ca. 3.0 µm in length), coccobacilli, or cocci (ca. 0.5–1.0 µm diam.), with 1–12 flagella per cell. Metabolism is respiratory (oxidative); all strains can use O2 as terminal electron acceptor, and some can use nitrate (anaerobic respiration). All strains can grow chemoorganotrophically on e.g. amino acids, acetate, fumarate, lactate, malate or succinate – carbohydrates being little used, although some strains can use (and form acid from) glucose and/or xylose; some (H2 -oxidizing) strains can grow chemolithotrophically. The organisms usually grow well on e.g. peptone-containing media and blood agar. Alkali is formed from the salts of certain organic acids and from some amides. GC%: ca. 56–70. Type species: A. faecalis. A. denitrificans. Most strains can reduce both nitrate and nitrite to N2 . Strains of subsp. xylosoxydans are typically able to use glucose and xylose, those of subsp. denitrificans are not. The species includes strains previously named A. ruhlandii (H2 oxidizing organisms which have sheathed, peritrichous flagella), and ‘Achromobacter xylosoxidans’. A. faecalis. Most strains (including those previously named A. odorans) can reduce nitrite but not nitrate. No chemolithotrophic strains have been reported. A. odorans. See A. faecalis. A. ruhlandii. See A. denitrificans. Species incertae sedis [according to Book ref. 22, pp. 370–373] include the peritrichously flagellated, aerobic, H2 -oxidizing bacteria known as A. eutrophus, A. lactus and A. paradoxus, and the non-fermentative, peritrichously flagellated marine bacteria known as A. aestus, A. aquamarinus, A. cupidus, A. pacificus and A. venustus (see DELEYA); these organisms are considered not to belong to the genus Alcaligenes. [Book ref. 22, pp. 361–373.] alcian blue A basic dye used e.g. for staining glycoproteins and polysaccharides. alcohol oxidase (alcohol:oxygen oxidoreductase; EC 1.1.3.13) An enzyme (see ENZYMES) which oxidizes alcohols, giving the corresponding aldehydes and H2 O2 . It is obtained e.g. from Pichia pastoris. (See also LIGNIN.) alcoholic beverages See e.g. BREWING, CIDER, KEFIR, KOUMISS, PULQUE, SAKE, SPIRITS, WINE-MAKING. alcoholic fermentation (ethanol fermentation) A type of FERMENTATION (sense 1), carried out by various yeasts and other fungi (e.g. species of Saccharomyces, Pichia, Aspergillus, Fusarium, Mucor ) and by certain bacteria (e.g. Zymomonas), in which ethanol is formed from D-glucose (or certain other sugars, depending e.g. on organism). In e.g. Saccharomyces, glucose is converted to pyruvate via the EMBDEN–MEYERHOF–PARNAS PATHWAY; pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase and thiamine pyrophosphate, and acetaldehyde is then reduced to ethanol by NAD-dependent alcohol dehydrogenase – thus allowing reoxidation of the NAD reduced during the EMP pathway. Small amounts of side-products are usually formed, e.g., GLYCEROL (see also NEUBERG’S FERMENTATIONS), acetaldehyde, lactic acid, 2,3-butanediol, succinic and acetic acids, and FUSEL OIL; these occur in proportions which depend on organism and conditions. Alcoholic fermentation by Saccharomyces spp is widely exploited commercially: see e.g. BREWING, CIDER, INDUSTRIAL ALCOHOL, SPIRITS, WINE-MAKING. (cf. ZYMOMONAS.) [Physiological function of alcohol dehydrogenases and longchain (C30 ) fatty acids in the alcohol tolerance (∼8% ethanol) of a mutant strain of Thermoanaerobacter ethanolicus: AEM (2002) 68 1914–1918.] alcohols (as antimicrobial agents) Under appropriate conditions certain alcohols can be rapidly lethal to a range of bacteria, fungi and viruses; they have little or no effect on endospores. The mechanism of antimicrobial activity may involve the denaturation of structural proteins or enzymes and/or the solubilization of lipids (e.g. those in the bacterial cytoplasmic membrane, or in the envelope of certain viruses); methanol and ethanol can cause translational errors in protein synthesis. The antimicrobial activity of alcohols increases with molecular weight and with chain length up to ca. C10 ; above this, insolubility becomes important. Activity decreases in the order primary, iso-primary, secondary, tertiary. Methanol (methyl alcohol, CH3 OH) has poor antimicrobial activity. Ethanol (ethyl alcohol, C2 H5 OH) exerts maximum activity as ca. 60–90% (v/v) ethanol/water mixtures. Isopropanol (isopropyl alcohol, (CH3 )2 CHOH) is less volatile and more effective than ethanol, and is used e.g. as a skin antiseptic. Phenylethanol (phenylethyl alcohol, C6 H5 (CH2 )2 OH) is more active against Gram-negative than Gram-positive bacteria, and has been used e.g. as a selective agent in bacteriological media. Phenoxyethanol (phenoxetol, C6 H5 O(CH2 )2 OH) and benzyl alcohol (phenylmethanol, C6 H5 CH2 OH) are used e.g. as preservatives in pharmaceutical preparations; the activity of benzyl alcohol is improved by halogenation: 2,4-dichlorobenzyl alcohol is used e.g. as a skin antiseptic. Ethylene glycol, propylene glycol and trimethylene glycol (dihydric alcohols) have been used, in aerosol form, for the disinfection of air; a relative humidity of ca. 60% is required. Bronopol (2-bromo2-nitropropan-1,3-diol) is an antibacterial and antifungal compound used e.g. as a preservative in pharmaceutical preparations. The trihydric alcohol glycerol is bacteriostatic at concentrations above 50%; it has been used e.g. as a preservative in vaccines. (See also DISINFECTANTS and STERILIZATION.) aldopentose See PENTOSES. Alectoria A genus of LICHENS (order LECANORALES); photobiont: a green alga. The thallus is fruticose, greenish-grey or fuscous 23 Aleppo boil e.g. on damp soil, on ice (‘ice algae’ – see DIATOMS) and snow (‘snow algae’ – see RED SNOW), and on tree-trunks etc. Some algae are photobionts in LICHENS or endosymbionts in various organisms (see e.g. ZOOCHLORELLAE and ZOOXANTHELLAE). [Algal symbioses: Book ref. 129.] A few algae are parasitic or pathogenic (see e.g. CHOREOCOLAX, HOLMSELLA, PROTOTHECA, RED RUST). (See also ALGAL DISEASES.) Certain algae have domestic and/or commercial or industrial uses: see e.g. AGAR, ALGINATE, CARRAGEENAN, DIATOMACEOUS EARTH, FUNORAN, FURCELLARAN, KELP, LAVER, NUNGHAM, SINGLECELL PROTEIN, YAKULT. Algae are classified on the basis of their pigments, types of storage carbohydrate, types and arrangements of flagella, CHLOROPLAST ultrastructure (including arrangement of THYLAKOIDS) and CELL WALL composition. However, there is currently no universally accepted taxonomic scheme which encompasses all the algae; moreover alternative taxonomic schemes coexist even within particular subgroups of algae: see CHLOROMONADS, CHLOROPHYTA, CHRYSOPHYTES, CRYPTOPHYTES, DIATOMS, DINOFLAGELLATES, EUGLENOID FLAGELLATES, PHAEOPHYTA, PRYMNESIOPHYCEAE, RHODOPHYTA, SILICOFLAGELLATES and XANTHOPHYCEAE. According to species, algae range from unicellular organisms of a few micrometres to seaweeds of 50 metres or more in length. (Unicellular organisms occur in most of the main groups of algae – cf. PHAEOPHYTA.) A CELL WALL is present in most algae but is absent in a few unicellular algae (e.g. PORPHYRIDIUM). The multicellular algae exhibit a great diversity of forms which include branched and unbranched filaments or ribbons, sheets of cells etc; in the thalli of some species there is considerable differentiation – e.g. in Laminaria spp the thallus includes structures analogous to root, stem and leaf (holdfast, stipe and blade, respectively) and a system of photosynthate-conducting sieve tubes. (See also PNEUMATOCYST.) (Differentiation occurs also e.g. in the unicellular alga ACETABULARIA.) Meristematic tissue may occur in apical, intercalary and/or diffuse regions depending e.g. on species. Some unicellular algae are motile (see also MOTILITY). Colonial organization is exhibited by certain microalgae (see e.g. COENOBIUM (sense 2) and PALMELLOID PHASE). Although normally photosynthetic, some algae (e.g. species of CHLAMYDOMONAS, CHLORELLA and SCENEDESMUS) can grow chemoorganotrophically, in the dark, on substrates such as glucose or acetate; some algae (e.g. OCHROMONAS) can ingest particulate food by phagocytosis. Sexual reproduction (often oogamous) occurs in many algae, and a number of algae exhibit an ALTERNATION OF GENERATIONS which may be isomorphic (e.g. in Ectocarpus, Ulva) or heteromorphic (e.g. in Laminaria). algal diseases ALGAE are subject to various diseases of microbial aetiology, some of which are of economic importance in seaweeds cultivated for food etc. Thus, e.g., diseases of Laminaria japonica (‘haidai’) include ‘frond twist disease’ caused by a mycoplasma-like organism, and various rots caused by alginatedegrading bacteria; sporelings in culture may be killed by H2 S produced e.g. by sulphate-reducing bacteria [Book ref. 130, pp. 706–708]. Porphyra spp are subject to ‘red wasting disease’ (= red rot disease, Pythium red rot) caused by Pythium spp [Experientia (1979) 35 443–444], and to ‘green spot disease’ caused by localized infection with species of Pseudomonas or Vibrio. Various red algae may be attacked by Petersenia spp: e.g. Petersenia palmariae infects Palmaria mollis [CJB (1985) 63 404–408, 409–418]. Other microorganisms which black, and lacks a whitish central strand (cf. USNEA); ascospores are large, brown when mature, 2–4 per ascus (cf. BRYORIA). Species occur e.g. on trees, rocks etc. Aleppo boil See CUTANEOUS LEISHMANIASIS. aleukia Absence or reduced numbers of leucocytes in the blood. Aleuria A genus of fungi (order PEZIZALES) which form sessile or stipitate, discoid or cup-shaped, minute or conspicuous apothecia in which the hymenium may be red, orange (as in A. aurantia, the ‘orange peel fungus’), or yellow. aleuriospore (mycol.) A term which has been used to refer to various types of spore – including e.g. thick-walled and thin-walled, pigmented and non-pigmented blastoconidia – and which has become meaningless owing to indiscriminate use. Aleutian disease of mink A progressive disease of mink caused by an autonomous PARVOVIRUS; it is a virus-induced immune complex-mediated disease and is characterized by glomerulonephritis, arteritis, plasmacytosis and hypergammaglobulinaemia. Death may occur 2–24 months after infection. Virus strain and host genotype are major determinants of disease development. Mink homozygous for the (recessive) Aleutian coat colour gene are most susceptible; infected non-Aleutian mink may have a slow progressive disease, may not have disease, or may later shed the virus. [3D structure of ADM virus and implications for pathogenicity: JV (1999) 73 6882–6891.] Alexandrium tamarense Syn. Gonyaulax tamarensis. alexin (1) (immunol.) Archaic syn. COMPLEMENT. (2) (plant pathol.) Syn. PHYTOALEXIN. alfalfa mosaic virus (AMV) A multicomponent ssRNA-containing PLANT VIRUS which has a wide host range and is transmitted via seeds (in some plants), by aphids (non-persistently), and mechanically (under experimental conditions). The genome consists of three linear positive-sense ssRNA molecules: RNA1 (MWt ca. 1.1 × 106 ), RNA2 (MWt ca. 0.8 × 106 ) and RNA3 (MWt 0.7 × 106 ); coat protein mRNA (‘RNA4’, MWt ca. 0.3 × 106 ) is also encapsidated. The four RNAs are capped at the 5 end and occur in at least four different types of virus particle, three bacilliform (B particles: 58 × 18 nm; M particles: 48 × 18 nm; Tb particles: 36 × 18 nm) and one ellipsoidal (Ta particles: ca. 28 × 18 nm). B, M and Tb particles contain one molecule of RNA1, RNA2 and RNA3, respectively; Ta particles contain two molecules of RNA4. RNAs 1, 2 and 3, together with coat protein or RNA4, are necessary for infectivity; coat protein from most ILARVIRUSES can also activate the AMV genome. AMV particles accumulate mainly in the host cell cytoplasm and may form whorled aggregates. Alferon N See INTERFERONS. ALG (serol.) See ANTILYMPHOCYTE SERUM. algae A heterogeneous group of unicellular and multicellular eukaryotic photosynthetic organisms. (cf. CYANOBACTERIA, PROCHLOROPHYTES, RHODOSPIRILLALES; see also MICROORGANISMS.) Algae resemble higher plants in that they evolve oxygen during PHOTOSYNTHESIS, and in that their photosynthetic pigments include CHLOROPHYLL a; they differ from vascular plants e.g. in that they typically lack vascular conducting systems – although sieve tubes occur in some of the large brown SEAWEEDS. Algae differ from bryophytes (mosses etc) in that, in most cases, algal reproductive structures (when formed) lack a peripheral envelope of sterile cells (cf. CHAROPHYTES). Some organisms are classified in both algal and protozoal classification schemes: see PHYTOMASTIGOPHOREA. Aquatic algae occur in fresh, brackish and marine waters (according to species) where they are often important in PRIMARY PRODUCTION. (See also PLANKTON.) Terrestrial algae occur 24 alkalophile Alginates have a wide range of uses. Calcium alginate fibres can be made by passing a solution of sodium alginate through a spinneret immersed in a solution of calcium chloride acidified with hydrochloric acid; calcium alginate is precipitated in the form of continuous threads which may be processed (stabilized) to form calcium alginate wool. This material (marketed as e.g. ‘Calgitex’) is used as a COTTON WOOL substitute for making SWABS or absorbent and absorbable surgical dressings. (Calcium alginate swabs have been reported to be inhibitory when used to prepare cultures of herpes simplex virus.) Calcium alginate wool can be sterilized by autoclaving or by dry heat; it can be dissolved e.g. in a 5% solution of sodium citrate or in quarter-strength Ringer’s solution containing 1% sodium hexametaphosphate. Thus, a swab carrying an inoculum can be completely dissolved to release its entire complement of microorganisms. In industry, alginates are used e.g. as emulsifiers and thickeners in foods (alginates are easily digested), cosmetics, pharmaceuticals etc., and as supports for the IMMOBILIZATION of cells or enzymes (for which purpose alginates rich in guluronic acid are preferred as they form stronger gels). alginate lyase (alginase) Any enzyme within the categories EC 4.2.2.3 and EC 4.2.2.11 which can degrade ALGINATE; such enzymes have been isolated from many types of organism. [Alginate lyase (sources, characteristics, structure–function analysis, roles and applications): ARM (2000) 54 289–340.] alginic acid See ALGINATE. algivorous Feeding on algae. algology The study of ALGAE. AlgU (sE ) See ALGINATE. alicyclic hydrocarbons See HYDROCARBONS. alimentary toxic aleukia A severe, usually lethal MYCOTOXICOSIS caused by ingestion of mouldy grain contaminated with certain TRICHOTHECENES – usually T-2 toxin produced by Fusarium tricinctum. Symptoms include extreme leucopenia and multiple haemorrhages. aliphatic hydrocarbons See HYDROCARBONS. alkA gene See ADAPTIVE RESPONSE. Alkalescens–Dispar group Non-motile strains of Escherichia coli in which glucose is fermented anaerogenically and lactose fermentation is delayed or absent. alkaline peptone water See APW. alkaline phosphatase See PHOSPHATASE. alkaline phosphatase test See e.g. PHOSPHATASE TEST (for milk). alkaliphile Syn. ALKALOPHILE. alkalophile (alkaliphile) An organism which grows optimally under alkaline conditions – typically exhibiting one or more growth optima within the pH range 8–11 – and which typically grows slowly, or not at all, at or below pH 7. (cf. ACIDOPHILE.) Alkalophiles include a range of bacteria – e.g. certain Bacillus spp (including B. alcalophilus, B. firmus and B. pasteurii ), Ectothiorhodospira abdelmalekii, Exiguobacterium aurantiacum, species of Natronobacterium and Natronococcus, and Thermomicrobium roseum – and certain fungi; the organisms occur e.g. in natural alkaline lakes and in waters made alkaline by the effluents from certain industrial processes (such as rayon manufacture). (Natural alkaline environments are characterized by high concentrations of free or complexed Na2 CO3 – and usually by high concentrations of NaCl.) A number of alkalophiles have an obligate requirement for Na+ – an ion important e.g. in SYMPORT processes; in at least some flagellated alkalophiles flagellar rotation is driven by SODIUM MOTIVE FORCE. However, in some species capable of can infect algae, but whose pathogenicity is uncertain, include e.g. LABYRINTHULAS, PHAGOMYXA, and members of the PLASMODIOPHOROMYCETES. (See also CHOREOCOLAX, HOLMSELLA and PHYCOVIRUS.) algal rust Syn. RED RUST. algicides Chemical agents which kill algae. Algicides include e.g. copper sulphate (see also BLOOM) and TBTO. Some herbicides (e.g. Diquat, Paraquat, TERBUTRYNE) are also effective against at least some algae, as are various general disinfectants (e.g. chlorine in WATER SUPPLIES). algin Syn. ALGINATE. alginase Syn. ALGINATE LYASE. alginate A salt of alginic acid: a linear polymer consisting of (1 → 4)-b-linked D-mannuronic acid residues and (1 → 4)-alinked L-guluronic acid residues. (Guluronic acid is the C-5 epimer of mannuronic acid.) Alginates occur in the CELL WALL and intercellular mucilage in phaeophycean algae; a similar polymer (differing in that at least some of the mannuronic acid residues are acetylated) occurs as capsular material in certain (‘mucoid’) strains of Pseudomonas and in the resting stage of Azotobacter vinelandii. (See also CAPSULE (bacterial) and CYST (bacterial).) Algal alginic acid is insoluble in water but sodium alginate is soluble; alginate solutions form gels in the presence of Ca2+ . The alginate molecule contains mannuronic acid-rich regions (‘M-blocks’), guluronic acid-rich regions (‘G-blocks’), and regions containing both types of residue (‘MG-blocks’). The binding of calcium ions (and other divalent cations) occurs preferentially at the G-blocks; a calcium alginate gel can therefore be envisaged as a three-dimensional network of long-chain molecules cross-linked (between G-blocks) by calcium ions. In the alga, where alginate is in equilibrium with seawater, alginate is associated mainly with calcium, magnesium and sodium ions. The composition (and hence properties) of the polymer varies with species (e.g. alginate from Laminaria spp is rich in guluronic acid, while that from Ascophyllum and Macrocystis is rich in mannuronic acid) and also varies with environmental conditions; the composition of the polymer may even differ in different parts of the same plant. [Biosynthesis of alginate: Microbiology (1998) 144 1133– 1143.] Alginate in bacteria. Genes for alginate synthesis are common in strains of Pseudomonas aeruginosa, but strains isolated from the general environment typically do not express these genes. In patients with CYSTIC FIBROSIS, conditions in the lung seem to select for mucoid (i.e. alginate-producing) strains; in these patients P. aeruginosa can form a viscous alginate slime associated with a poor prognosis. In P. aeruginosa the conversion of non-mucoid strains to mucoidy may occur if a specific SIGMA FACTOR – AlgU (= sE ) – becomes available for transcription of the alginate genes. Constitutive expression of AlgU may occur as a result of mutation in the muc genes, the activity of AlgU being inhibited by binding of MucA. [Mucoidy of P. aeruginosa in cystic fibrosis: JB (1996) 178 4997–5004. Alginate/biofilms/antibiotic resistance in P. aeruginosa: JB (2001) 183 5395–5401.] AlgU is also required for alginate production in Azotobacter vinelandii, and the activity of the sigma factor is similarly regulated by products of the muc genes; the products of both mucA and mucC have a negative role in alginate production [JB (2000) 182 6550–6556]. [Alginate formation in A. vinelandii in the stationary phase: Microbiology (2001) 147 483–490.] Commercial applications of alginate. 25 alkane metabolism growth at neutral pH, Na+ is required only under non-alkaline conditions. [Book ref. 192; the alkaline saline environment: Book ref. 191, pp. 25–54; genetic engineering of alkalophiles: Book ref. 191, pp. 297–315.] alkane metabolism See HYDROCARBONS. alkB gene See ADAPTIVE RESPONSE. alkene metabolism See HYDROCARBONS. alkylating agents Agents which react with nucleophilic groups (e.g. amino, carboxyl, hydroxyl, phosphate, and/or sulphhydryl groups) in e.g. proteins and nucleic acids, substituting them with alkyl groups. (As commonly used, the term ‘alkylating agent’ is also applied to agents which substitute nucleophilic groups with derivatives of alkyl groups: e.g. hydroxyethyl groups in the case of ETHYLENE OXIDE.) Bifunctional alkylating agents have two reactive groups and can cause cross-linking between nucleophilic groups in proteins and/or nucleic acids (see e.g. NITROGEN MUSTARDS and MITOMYCIN C). (See also SULPHUR MUSTARDS.) Depending e.g. on their reactivities, alkylating agents can be effective antimicrobial agents and/or MUTAGENS. Some react directly with cell components, others (e.g. alkyl-N-nitrosamines: see N-NITROSO COMPOUNDS) require prior metabolic activation (apparently to generate an alkyl carbonium cation). In general, methylating agents are more reactive with DNA than are the corresponding ethylating agents. Alkylating agents form a range of products with DNA. However, only some of the lesions are directly mutagenic: e.g. O 6 -alkylguanine can pair with thymine during subsequent DNA replication, resulting in G·C-to-A·T transitions, and alkylation of the O-4 position of thymine can cause A·T-to-G·C transitions. Most other lesions (e.g. N 7 -alkylguanine, N 3 -alkyladenine) are not directly mutagenic, but they may be lethal unless repaired by the cell (e.g. alkylation of the N-3 position of adenine blocks replication forks); various DNA REPAIR systems can recognize and repair alkylated bases (see e.g. ADAPTIVE RESPONSE). The mutagenic effects of a given alkylating agent depend largely on the nature of the lesions it produces. For example, MNNG (q.v.), EMS (ethylmethane sulphonate) and MNU (Nmethyl-N-nitrosourea: see N-NITROSO COMPOUNDS) produce relatively more directly mutagenic lesions (particularly O 6 alkylguanine), while MMS (methylmethane sulphonate) produces higher proportions of e.g. N 7 -methylguanine and N 3 methyladenine but relatively little O 6 -methylguanine. However, in organisms (such as Escherichia coli ) which have an inducible error-prone repair system (see SOS SYSTEM), MMS can be mutagenic by causing lesions which induce this system; even in the case of directly mutagenic agents such as MNNG, a proportion of the mutations induced may be due to error-prone repair in E. coli [JB (1985) 163 213–220]. alkyldimethylbenzylammonium chloride See QUATERNARY AMMONIUM COMPOUNDS. N-alkylnitrosoureas See N-NITROSO COMPOUNDS. all-glass impinger (bubbler) An instrument used e.g. for sampling the airborne microflora. (See also AIR.) Essentially, it consists of a vertical glass cylinder, containing a volume of liquid, and a longer, narrower glass tube fitted coaxially within the cylinder and partly submerged in the liquid; when suction is applied to the annular space, air is drawn in through the narrow tube and bubbles up through the liquid – during which process particles are transferred from the air to the liquid. allantoid Sausage-shaped; elongated and slightly curved with rounded ends. allele (allelomorph) Any of one or more alternative forms of a given GENE; both (or all) alleles of a given gene are concerned with the same trait or characteristic, but the product or function coded for by a particular allele differs, qualitatively and/or quantitatively, from that coded for by other alleles of that gene. Three or more alleles of a given gene constitute an allelomorphic series. In a diploid cell or organism the members of an allelic pair (i.e., the two alleles of a given gene) occupy corresponding positions (loci) on a pair of homologous chromosomes; if these alleles are genetically identical the cell or organism is said to be homozygous – if genetically different, heterozygous – with respect to the particular gene. A wild-type allele is one which codes for a particular phenotypic characteristic found in the WILD TYPE strain of a given organism. (See also DOMINANCE.) allelomorph Syn. ALLELE. allergen An antigen (or autocoupling HAPTEN – e.g., certain drugs) which can initiate a state of HYPERSENSITIVITY (commonly IMMEDIATE HYPERSENSITIVITY) or which can provoke a hypersensitivity reaction in individuals already sensitized with the allergen. allergic alveolitis See EXTRINSIC ALLERGIC ALVEOLITIS. allergy (1) A condition in which contact with a given allergen provokes a TYPE I REACTION. (See also PRAUSNITZ–KÜSTNER TEST.) (2) A condition in which contact with a given allergen gives rise to any manifestation of HYPERSENSITIVITY (see e.g. EXTRINSIC ALLERGIC ALVEOLITIS). (3) Formerly, the condition of a PRIMED individual. allergy of infection An early name for DELAYED HYPERSENSITIVITY. Allerton disease An African CATTLE DISEASE which involves a mild febrile condition followed by the appearance of skin nodules; it is caused by the bovine mammillitis virus and is apparently similar or identical to pseudo-LUMPY SKIN DISEASE. Allescheria boydii See PSEUDALLESCHERIA. alloantigen Antigen from a genetically different individual of the same species. allochromasy Gradual, spontaneous chemical modification which occurs in the solutions of certain dyes – a single dye becoming a mixture of dyes. (See e.g. polychrome METHYLENE BLUE and NILE BLUE A.) allochthonous Not indigenous to a given environment. (cf. AUTOCHTHONOUS sense 1.) alloenzyme See MULTILOCUS ENZYME ELECTROPHORESIS. allogeneic Derived from a genetically different individual of the same species. (cf. SYNGENEIC; XENOGENEIC.) Allogromia See FORAMINIFERIDA. Allogromiina See FORAMINIFERIDA. allolactose b-D-Galactopyranosyl-(1 → 6)-D-glucopyranose: a minor product of b-galactosidase action on LACTOSE; it is the natural inducer of the LAC OPERON in Escherichia coli. Allomonas A genus of bacteria (family VIBRIONACEAE) which have been isolated from fresh water, sewage and faeces; GC%: ca. 57. Type species: A. enterica [IJSB (1984) 34 150–154]. Allomyces A genus of fungi (order BLASTOCLADIALES) which occur in moist soils, muds, and water. The thallus is a branched, coenocytic mycelium which is attached to the substratum by branching rhizoids; in at least some species the cell wall contains CHITIN. (See also CONCENTRIC BODIES.) Some species exhibit an ALTERNATION OF GENERATIONS (q.v.). In e.g. A. macrogynus, the sporothallus forms both thick-walled resistant sporangia (meiosporangia) and thin-walled sporangia (mitosporangia); the meiosporangia give rise to haploid zoospores (which develop into gametothalli), while the mitosporangia form diploid 26 Alternaria zoospores (which develop into new sporothalli). Terminal branches of a gametothallus give rise to an orange-coloured distal male gametangium and a colourless subterminal female gametangium; the small male gametes fuse with the larger female gametes, and the zygote germinates to form a sporothallus. (See also PHEROMONE.) Species which do not exhibit sexual processes are sometimes placed in the subgenus Brachyallomyces. allopatric Existing in different environments or geographical regions (cf. SYMPATRIC). allophycocyanins See PHYCOBILIPROTEINS. allotype Any one of a range of serologically distinguishable variant forms of an Ig molecule produced as a consequence of allelic variation in the Ig-specifying genes; a given allotype is thus present only in those individuals who have the relevant allele (cf. ISOTYPE). Different allotypes usually differ in amino acid sequence in the ‘constant’ region of their heavy or light chains; sometimes the variable region is involved. Allotypes in man include e.g. the Gm (G1m, G2m etc) allotypes of IgG. allulose phosphate pathway Syn. RMP PATHWAY. allylamines A group of synthetic ANTIFUNGAL AGENTS which are highly active against dermatophytes and show somewhat variable activity against yeasts (e.g. Candida spp), apparently by inhibiting the enzyme squalene epoxidase; they include naftifine and terbinafine. Terbinafine has in vitro activity against Paracoccidioides brasiliensis [JCM (2002) 40 2828–2831]. Alnus root nodule See ACTINORRHIZA. alopecia Loss of hair. a (linking number) See DNA. alpha chain (immunol.) See HEAVY CHAIN. a-factor See MATING TYPE. a-granules CYANOPHYCIN granules. alpha interferon See INTERFERONS. a operon See RIBOSOME (biogenesis). a peptide (‘auto-a’) A peptide (185 amino acids long) which is cleaved from the N-terminus of the (lacZ-encoded) bgalactosidase of Escherichia coli e.g. during autoclaving. The a peptide can restore some b-galactosidase activity to a population of cells which, owing to a deletion mutation in lacZ, produce an inactive enzyme lacking the N-terminal portion. The lacZ sequence corresponding to the a peptide can be used as a marker in a CLONING vector. During cloning, use is made of a restriction endonuclease which cleaves at a site within this sequence; thus any insertion of exogenous DNA will usually result in loss of a peptide synthesis. The DNA is introduced into suitable lacZ deletion mutants, and the cells are plated on Xgal medium (see XGAL). Cells which receive the intact vector form blue colonies (due to complementation between the a peptide and the defective b-galactosidase), while those receiving recombinant DNA usually form white colonies. a1-a2 hypothesis See MATING TYPE. Alphaherpesvirinae (herpes simplex virus group) A subfamily of viruses of the HERPESVIRIDAE (q.v.). Alphaherpesviruses have a short replication cycle (<24 hours); they spread rapidly in cell cultures, causing mass lysis of susceptible cells. The natural host range varies from narrow to wide, according to virus. While, in cell cultures, latent infection with (nondefective) viruses does not occur readily, latent infection often occurs in nerve ganglia within the living host. The subfamily includes at least two genera: Simplexvirus (human herpesvirus 1 group) and Poikilovirus (proposed name) (suid herpesvirus 1 group) [Intervirol. (1986) 25 141–143]. The type species of the genus Simplexvirus is human (alpha) herpesvirus 1 (HERPES SIMPLEX virus type 1, HSV-1). The linear dsDNA genome of HSV-1 (∼152 kbp) contains two regions, designated L (‘long’) and S (‘short’), each region flanked by inverted repeats; the ability of each region (L and S) to invert, independently, means that DNA isolated from virions will include four isomeric forms. The genome also includes several copies of a short sequence (a) – a cis-acting region involved in circularization of the genome. [Molecular epidemiology of herpes simplex virus type 1: RMM (1998) 9 217–224.] Other simplexviruses include human (alpha) herpesvirus 2 (HSV-2), bovine – or bovid – (alpha) herpesvirus 2 (bovine mammillitis virus, causal agent of e.g. BOVINE ULCERATIVE MAMMILLITIS), and probably cercopithecine (or cercopithecid) herpesviruses 1 (B VIRUS) and 2. The type species of the genus ‘Poikilovirus’ is suid (alpha) herpesvirus 1 (AUJESZKY’S DISEASE virus). Other members include human (alpha) herpesvirus 3 (varicella-zoster virus (VZV), causal agent of CHICKENPOX and HERPES ZOSTER [review: AVR (1983) 28 285–356]) and equid (alpha) herpesvirus 1 (equine abortion virus, causal agent of e.g. abortion, respiratory disease and/or neurological disease in horses). (See also DELTA HERPESVIRUS.) Probable members of the Alphaherpesvirinae include equid herpesvirus 3 (EQUINE COITAL EXANTHEMA virus) and felid herpesvirus 1 (FELINE RHINOTRACHEITIS virus). Possible members of the subfamily include canid herpesvirus 1 (canine herpesvirus). Alphavirus (‘arbovirus group A’) A genus of viruses of the family TOGAVIRIDAE (q.v. for replication cycle etc); nearly all members are transmitted by mosquitoes, and many can cause disease (commonly encephalitis or fever with rash and arthralgia) in man and/or animals. Some alphaviruses are grouped into three serologically defined complexes – the complex-specific antigen being associated with the E1 protein (see TOGAVIRIDAE), the species-specific antigen with the E2 protein. The Semliki Forest virus complex includes Bebaru virus, CHIKUNGUNYA FEVER virus, GETAH VIRUS, MAYARO FEVER virus, O’NYONG–NYONG FEVER virus, ROSS RIVER VIRUS, Sagiyama virus, SEMLIKI FOREST VIRUS, and Una virus. The Venezuelan equine encephalomyelitis (= Venezuelan encephalitis) virus complex includes Cabassou virus, Everglades virus, Mucambo virus, Pixuna virus, and VENEZUELAN EQUINE ENCEPHALOMYELITIS virus. The Western equine encephalomyelitis (= Western encephalitis) virus complex includes Aura virus, Fort Morgan virus, Highlands J virus, Kyzylagach virus, SINDBIS VIRUS, WESTERN EQUINE ENCEPHALOMYELITIS virus, and What-aroa virus. Other alphaviruses include Barmah Forest virus [JGV (1986) 67 295–299], originally thought to be a bunyavirus; EASTERN EQUINE ENCEPHALOMYELITIS (= Eastern encephalitis) virus; Middelburg virus; and Ndumu virus. [Clinical aspects: Book ref. 148, pp. 931–953.] ALS ANTILYMPHOCYTE SERUM. Alsever’s solution A solution containing D-glucose (20.5 g), sodium citrate dihydrate (8.0 g) and NaCl (4.2 g) dissolved in distilled water (1.0 litre); the pH is adjusted to 6.1 with citric acid. The solution is sterilized by filtration and used for the preservation of sheep blood; blood is added to Alsever’s solution (1:1 by volume) and stored at 4° C. Alternaria A genus of fungi (class HYPHOMYCETES) which include many plant-pathogenic species – e.g. A. solani (see e.g. EARLY BLIGHT) and A. radicina (causal agent of black rot of carrot seedlings and stored carrots). (See also TIMBER STAINING.) Alternaria spp form septate mycelium and pyriform to elongated, dark-coloured conidia which usually have both transverse and longitudinal septa; conidiogenesis is tretic (see CONIDIUM), the conidia developing singly or in chains. 27 alternaria rot altro-heptulose Syn. SEDOHEPTULOSE. Alu sequences In the human genome: a family of closely related, dispersed sequences, each ca. 300 nt long, many of which contain a common cleavage site for the restriction enzyme AluI; Alu-like sequences occur in the genomes of other mammals and of certain lower eukaryotes. (cf. REP SEQUENCE.) AluI A RESTRICTION ENDONUCLEASE from Arthrobacter luteus; AG/CT. ALV AVIAN LEUKOSIS VIRUS. alveolar membrane (protozool.) See PELLICLE (sense 3). alveolitis Inflammation of the pulmonary alveoli. (cf. PNEUMONIA; see also EXTRINSIC ALLERGIC ALVEOLITIS.) alveolysin See THIOL-ACTIVATED CYTOLYSINS. Alysiella A genus of GLIDING BACTERIA (see CYTOPHAGALES); A. filiformis occurs in the oral cavity in various vertebrates. The organisms occur as flat filaments – each composed of elongated cells (2–3 µm long) arranged side-by-side. Gliding occurs in a direction perpendicular to the long axis of the cells – i.e. along the axis of the filament. Metabolism: chemoorganotrophic. Alzheimer’s disease See CREUTZFELDT–JAKOB DISEASE. am mutant An ‘amber mutant’, i.e., a mutant with an amber NONSENSE MUTATION. amaas See SMALLPOX. amanin See AMATOXINS. Amanita A genus of fungi (AGARICALES, Amanitaceae) which occur in deciduous and/or coniferous woodlands; some species form mycorrhizal associations – e.g. A. muscaria with birch (Betula). According to species, the colour of the pileus may be e.g. white, yellowish, red or brown. A. muscaria (the ‘fly agaric’) forms a bright red pileus to which often adhere scattered white scales (remnants of the universal veil). Some species are highly poisonous; these include e.g. A. muscaria (see also MUSCARINE), A. pantherina, A. phalloides (the ‘death cap fungus’), A. verna, and A. virosa (the ‘destroying angel’). (See also AMATOXINS.) Amanitaceae See AGARICALES. a-amanitin An AMATOXIN which, at low concentrations, specifically inhibits eukaryotic RNA POLYMERASE II; RNA polymerase III is inhibited at high concentrations. amantadine (1-adamantanamine hydrochloride; 1-aminoadamantane hydrochloride) A polycyclic ANTIVIRAL AGENT (adamantane = tricyclodecane, C10 H16 ) which inhibits the replication of certain viruses in tissue culture. It is used for the prophylaxis and early treatment of INFLUENZA caused by type A influenzaviruses; it can be administered orally or by aerosol. At high concentrations, the action of amantadine is nonspecific: it raises the pH in endosomes and thus inhibits membrane fusion following endocytosis of a virus (see ENVELOPE). At lower concentrations the drug selectively inhibits an early stage in the infection of type A influenza viruses, the primary target apparently being the M2 protein (see INFLUENZAVIRUS); the activity of the drug results in failure of the pH-dependent fusion of viral and vesicle membranes. Rimantadine (a-methyl-1-adamantane methylamine hydrochloride) resembles amantadine in its spectrum of activity but apparently causes fewer side-effects. amanullin See AMATOXINS. Amapari virus See ARENAVIRIDAE. amastigote A form assumed by the cells of many species of the TRYPANOSOMATIDAE (q.v.) during certain stages of their life cycles. (See also LEISHMAN–DONOVAN BODIES.) amatoxins Toxic cyclic peptides which occur in some species of Amanita, e.g. A. phalloides, A. verna. In man, small quantities of toxin (e.g. 5 mg) may be lethal; clinical effects are produced in alternaria rot A firm, dark rot produced in various plant hosts by species of Alternaria. alternaric acid A complex compound, produced by Alternaria solani, which contains a diketotetrahydropyran group linked to a long-chain fatty acid; it inhibits germination of the spores of certain fungi, and causes wilting and necrosis in the tissues of higher plants. alternate host (of heteroxenous rust fungi) (1) The secondary host (see UREDINIOMYCETES). (2) Sometimes, loosely: either host of a heteroxenous rust. alternation of generations In the life cycles of some organisms: the alternating formation of one or more generations of mature haploid individuals and one or more generations of mature diploid individuals; in this context ‘mature’ refers to the ability of the organisms to produce reproductive cells (gametes or spores). An alternation of generations occurs e.g. in some fungi (e.g. Allomyces spp), in certain protozoa of the FORAMINIFERIDA, and in many algae (e.g. Laminaria). An individual in the diploid phase is known variously as a sporophyte, sporothallus or agamont. When a sporophyte undergoes meiotic division (sporic meiosis) it gives rise to haploid meiospores. Each meiospore gives rise to a haploid individual known variously as a gametophyte, gametothallus or gamont; individuals in this generation produce gametes. Male and female gametes may be formed on the same gametophyte (as e.g. in Allomyces macrogynus) or on separate male and female gametophytes (as e.g. in Laminaria). The gametes fuse to form a (diploid) zygote from which a sporophyte develops. An isomorphic (= homologous) alternation of generations is one in which the gametophyte and sporophyte are morphologically similar. A heteromorphic (= heterologous) alternation of generations is one in which the gametophyte and sporophyte differ morphologically – and perhaps also in other ways: see e.g. CELL WALL (algal). alternative splicing See SPLIT GENE. Alteromonas A genus (incertae sedis) of aerobic, chemoorganotrophic, Gram-negative bacteria which occur in coastal and marine waters. [Book ref. 22, pp. 343–352.] Cells: straight or curved, round-ended rods, 0.7–1.5 × 1.8–3.0 µm, each having a single, unsheathed polar flagellum. Some species form insoluble pigments: orange and yellow non-carotenoid pigments are formed by A. aurantia and A. citrea, respectively; A. luteoviolacea forms violacein; A. rubra forms prodigiosin; some strains of A. hanedai form soluble brown pigments. ‘A. hanedai ’ (see SHEWANELLA) exhibits BIOLUMINESCENCE. Metabolism is exclusively respiratory (oxidative), with O2 as terminal electron acceptor. All species need Na+ for growth (optimum: 100 mM Na+ ). Utilizable carbon sources vary with species; they include e.g. acetate, alcohols, amino acids, aromatic compounds and sugars. Some species can attack extracellular alginate and/or chitin; several species – including A. communis, A. espejiana, A. haloplanktis, A. macleodii, A. undina and A. vaga – can metabolize D-glucose via an inducible ENTNER–DOUDOROFF PATHWAY. (It has been proposed that A. communis and A. vaga be transferred to a new genus, Marinomonas [JGM (1983) 129 3057–3074].) No species can accumulate PHB intracellularly. All species grow at 20° C. GC%: ca. 38–50. Type species: A. macleodii. (See also BACTERIOPHAGE PM2.) A. nigrifaciens ([IJSB (1984) 34 145–149], formerly ‘Pseudomonas nigrifaciens’) and ‘A. putrefaciens’ (formerly ‘Pseudomonas putrefaciens’, now Shewanella putrefaciens: see SHEWANELLA) can be responsible for e.g. FISH SPOILAGE and/or MEAT SPOILAGE (see also BUTTER and DFD MEAT). 28 amino acids ambruticin An antifungal antibiotic (a cyclopropylpolyenepyran acid) obtained from a strain of Polyangium cellulosum; it shows in vitro activity against e.g. Candida spp, dermatophytes, and other pathogenic fungi. amdinocillin Syn. MECILLINAM. ameba Syn. AMOEBA. American foulbrood A BEE DISEASE which affects the larvae of Apis mellifera – usually after they have spun their cocoons; the causal agent is Bacillus larvae. Infection occurs by ingestion of food contaminated by spores of B. larvae. The spores germinate in the gut, and the bacteria penetrate to the haemolymph and multiply; the larvae die, turn brown, and putrefy. (cf. EUROPEAN FOULBROOD.) amerosporae See SACCARDOAN SYSTEM. Ames test (Mutatest; Salmonella/microsome assay) A test for detecting whether or not a particular agent is mutagenic (and hence possibly carcinogenic) by determining its ability to cause reversion to prototrophy in certain histidine-requiring mutants of Salmonella typhimurium. Various ‘tester strains’ of S. typhimurium may be used, each having a different type of MUTATION (frameshift, missense or nonsense) in the histidine operon. Many of the strains used also contain mutations in uvrB (preventing EXCISION REPAIR) and in rfa (causing LPS deficiency and hence increased permeability to certain chemicals); most contain the plasmid pKM101 which carries genes for error-prone repair (see SOS SYSTEM) and which thus enhances the mutagenic effects of DNA-damaging agents. Since certain chemicals are mutagenic/carcinogenic only after metabolic activation, the test commonly includes a preparation of microsomal enzymes from a liver homogenate (9000 g supernatant, fraction ‘S9’) obtained from rats pre-treated with a carcinogen (to induce the appropriate enzymes). The test may be carried out e.g. as a ‘plate incorporation test’. A culture of a particular tester strain of S. typhimurium, an S9 preparation, and the chemical under test are mixed with soft agar containing a low concentration of histidine, and this is poured onto a minimal agar plate; the whole is then incubated at 37° C for 48 h in the dark. The low level of histidine permits limited growth of the auxotrophic mutant cells, resulting in a background of confluent light growth in the upper layer of agar (‘top agar’); any prototrophic revertants (whose growth is not limited) can be seen as isolated colonies. In scoring revertants, account must be taken of the (known) spontaneous reversion rate for the strain used. (Absence of background growth implies that the agent under test has general antibacterial activity, and any colonies which develop are unlikely to be revertants.) The basic test has been modified in various ways for particular purposes. [Example of use with a carbamic acid derivative: AAC (2005) 49 1160–1168.] (See also SOS CHROMOTEST.) amicyanin A BLUE PROTEIN (MWt ca. 12000) present in certain methylamine-utilizing bacteria (e.g. ‘Pseudomonas AM1’) – see METHYLOTROPHY. Amies transport medium See TRANSPORT MEDIUM. amikacin A semi-synthetic derivative of KANAMYCIN A which contains an a-aminohydroxybutyric acid residue; it is more active than kanamycin against e.g. Pseudomonas aeruginosa and has been used against gentamicin-resistant strains. aminacrine Syn. 9-AMINOACRIDINE. amino acids For principal biosynthetic pathways see Appendix IV; see also e.g. AROMATIC AMINO ACID BIOSYNTHESIS, GLUTAMIC ACID, OPERON (attenuator control). For standard abbreviations for amino acids see table. ca. 8–24 hours after ingestion. Initial symptoms include severe vomiting and diarrhoea; degenerative changes occur in the liver and kidneys, and death may follow within a few days. Amatoxins include a-AMANITIN, b-, g- and e-amanitins, and amanin. A non-toxic compound of similar chemical composition, amanullin, also occurs in A. phalloides. The amatoxins are more toxic than the PHALLOTOXINS found in Amanita spp. (Other Amanita toxins include tryptamines such as bufotenine (see also HALLUCINOGENIC MUSHROOMS) and isoxazole alkaloids such as ibotenic acid.) amber codon See GENETIC CODE. amber mutation See NONSENSE MUTATION. amber suppressor See SUPPRESSOR MUTATION. ambisense RNA A viral ssRNA genome or genome segment which is positive-sense with respect to some genes but negativesense with respect to others (see VIRUS). Ambisense genome segments have been found in the ARENAVIRIDAE and in the genus PHLEBOVIRUS. In these cases the −ve-sense sequences are transcribed directly into (viral-complementary) subgenomic mRNA; however, +ve-sense sequences are expressed only after genome replication, subgenomic mRNA being transcribed from the RNA strand complementary to the genomic strand. [Review: AVR (1986) 31 1–51.] Amblyospora See MICROSPOREA. amboceptor Current usage: syn. HAEMOLYTIC IMMUNE BODY. ambrosia fungi Fungi which grow in the tunnels made by woodboring ambrosia beetles, e.g. Xyleborus spp (Scolytidae). The beetle larvae and adults form tunnels mainly in the sapwood of fallen timber and of dead or weakened standing trees; healthy trees are not normally attacked. The beetles derive nutrients mainly or solely from the fungal growth lining their tunnels (i.e., they are ‘xylomycetophagous’); wood apparently plays little or no direct part in their nutrition. (In some cases the ambrosia fungi also appear to be necessary for reproduction or pupation of the beetles, possibly by supplying an essential sterol.) Each of the many species of ambrosia beetle is associated with one or more particular species of fungus, usually a hyphomycete (e.g. Ambrosiella sp, Fusarium sp) or an ascomycete (e.g. Ambrosiozyma sp, Dipodascus sp). The fungus grows in the tunnels as a palisade-like layer or as separate or confluent sporodochia, frequently bearing chains of conidia or terminal chlamydospores; the mycelium may penetrate the wood to a depth of a few millimetres. Ambrosia fungi appear to use only storage sugars, starch etc in the wood cells, and do not cause significant damage to the structural components of the wood. Fungal propagules (spores or yeast-like cells) are carried to new tunnels in specialized pockets (mycetangia or mycangia) in the exoskeleton of the (usually female) beetle; mycetangia contain an oily secretion and differ in structure and location in different species of beetle. Bark-boring (‘phloeophagous’) beetles of the Scolytidae are also associated with various fungi, including e.g. sap-stain fungi (mostly Ceratocystis spp): see also DUTCH ELM DISEASE. In many cases these associations are fortuitous and non-specific, but some blue-stain fungi may be carried in mycetangium-like structures, such a structure occurring e.g. at the anterior margin of the prothorax in Dendroctonus frontalis. (See also WOODWASP FUNGI.) Ambrosiozyma A genus of fungi (family SACCHAROMYCETACEAE) which form budding yeast cells, pseudomycelium, and true mycelium with dolipore-like septa. Asci are formed on the hyphae; ascospores are bowler-hat-shaped. Species have been isolated e.g. from the tunnels of wood-boring beetles. [Book ref. 100, pp. 106–113.] (cf. AMBROSIA FUNGI.) 29 9-aminoacridine Aminoglycoside antibiotics are less effective under anaerobic conditions and are ineffective against obligate anaerobes. The aminoglycoside antibiotics are widely used therapeutically, sometimes in combination with other drugs. [Clinical roles of aminoglycosides (symposium): Am. J. Med. (1985) 79 (1A) 1–76.] Side-effects (particularly with neomycin and streptomycin) include dose-dependent damage to the 8th cranial nerve (associated with ototoxicity); kidney damage and hypersensitivity reactions are also possible. Aminoglycoside antibiotics bind to the 30S ribosomal subunit in bacteria – some (e.g. streptomycin) bind at a single site while others (e.g. gentamicin, kanamycin, neomycin) apparently bind at multiple sites. They inhibit PROTEIN SYNTHESIS. For example, low levels of streptomycin cause misreading of mRNA (i.e. incorporation of incorrect amino acids) while higher levels completely inhibit protein synthesis – apparently by blocking ribosomes specifically at the start of translation. There are other sites of activity (e.g. the outer membrane in Gram-negative bacteria), but the main effect of these antibiotics appears to result from their influence on protein synthesis. Resistance to aminoglycoside antibiotics can arise by several distinct mechanisms: (i) mutation(s) in ribosomal proteins of the 30S subunit which can affect the binding of these antibiotics (see e.g. STREPTOMYCIN); (ii) modification (inactivation) of the antibiotics by plasmid-encoded or chromosome-encoded bacterial enzymes which carry out O-phosphorylation, Nacetylation or O-adenylation; (iii) reduced uptake. [TIM (1998) 6 323–327.] Mutations giving one-step high-level resistance to aminoglycoside antibiotics are uncommon, but resistance to some (e.g. spectinomycin, streptomycin) can occur in this way; spectinomycin-resistant mutants do not show cross-resistance to streptomycin, and streptomycin-resistant mutants do not show cross-resistance to those antibiotics (e.g. kanamycin) which appear to bind at multiple sites on the ribosome. Aminoglycoside antibiotic-inactivating enzymes include acetyltransferases (AACs), adenylyltransferases (AADs) and phosphotransferases (APHs) [for nomenclature and sites of action see: BMB (1984) 40 28–35]. Some of these enzymes can inactivate only a few aminoglycoside antibiotics while others can inactivate many; some are found only in Gram-negative species or in Gram-positive species, and some are found only in e.g. Pseudomonas or Staphylococcus spp. Some of the enzymes occur in a wide range of species. (See also e.g. Tn5 and Tn21 .) Certain (mutant) bacteria exhibit drug-dependence in respect of certain aminoglycoside antibiotics: see e.g. SPECTINOMYCIN and STREPTOMYCIN. 6-aminopenicillanic acid (6-APA) A derivative of natural PENICILLINS (see b-LACTAM ANTIBIOTICS for structure): a precursor of many semi-synthetic penicillins. 6-APA itself has little or no antibacterial activity. aminopeptidase See PROTEASES. (See also METHIONINE AMINOPEPTIDASE). aminopterin See FOLIC ACID ANTAGONIST. 2-aminopurine See BASE ANALOGUES. aminoquinolines A group of ANTIMALARIAL AGENTS that include 4-aminoquinolines (e.g. chloroquine, mefloquine, amodiaquine) and 8-aminoquinolines (e.g. primaquine). The quinoline-type antimalarials (including quinine) apparently kill the parasite by preventing its detoxification of the ferriprotoporphyrin IX by-product of haemoglobin metabolism (see HAEMOZOIN and QUININE). Chloroquine is an inexpensive synthetic drug which is used orally for treating uncomplicated malaria. It has also been used AMINO ACIDS: standard abbreviations Amino acid alanine arginine asparagine aspartic acid cysteine glutamic acid glutamine glycine histidine isoleucine leucine lysine methionine phenylalanine proline serine threonine tryptophan tyrosine valine Three-letter abbreviation One-letter abbreviation Ala Arg Asn Asp Cys Glu Gln Gly His Ile Leu Lys Met Phe Pro Ser Thr Trp Tyr Val A R N D C E Q G H I L K M F P S T W Y V 9-aminoacridine (aminacrine) A substituted acridine (see ACRIDINES) used e.g. as an ANTISEPTIC for the treatment of wounds, and as a laser dye. aminoacyl-tRNA synthetase See PROTEIN SYNTHESIS. aminoadipic acid pathway (AAA pathway) A pathway for lysine biosynthesis [see Appendix IV(e)] which occurs only in certain lower fungi (Blastocladiales, Chytridiales, Mucorales), ascomycetes (including e.g. Saccharomyces), basidiomycetes, and euglenoid flagellates. [Review: CRM (1985) 12 131–151.] (cf. DIAMINOPIMELIC ACID PATHWAY.) p-aminobenzoic acid (PABA; PAB) A component of FOLIC ACID. PABA is synthesized from chorismate [see Appendix IV(f)]. Certain microorganisms – e.g. Clostridium and Lactobacillus spp – require PABA as a growth factor. (See also PAS and SULPHONAMIDES.) 7-aminocephalosporanic acid See CEPHALOSPORINS. aminoglycoside antibiotics A class of broad-spectum ANTIBIOTICS; a typical aminoglycoside antibiotic contains an aminosugar and either STREPTIDINE or (more commonly) 2-DEOXYSTREPTAMINE. [Book ref. 14, pp 418–442, gives detailed chemical structures.] This group of antibiotics is generally taken to include e.g. AMIKACIN, APRAMYCIN, butirosin, FRAMYCETIN, GENTAMICIN, hygromycin B, KANAMYCIN, KASUGAMYCIN, LIVIDOMYCIN, NEAMINE, NEOMYCIN, netilmicin, PAROMOMYCIN, ribostamycin, SISOMYCIN, SPECTINOMYCIN, STREPTOMYCIN and TOBRAMYCIN. Many aminoglycoside antibiotics are bactericidal but some (e.g. kasugamycin, spectinomycin) are bacteriostatic. Aminoglycoside antibiotics are active against both Gram-positive and Gram-negative bacteria; at least some (e.g. gentamicin, kanamycin, streptomycin) are inactive, or only weakly active, against certain archaeans (methanogens [SAAM (1985) 6 125–131]), while others (e.g. kasugamycin, neomycin, paromomycin, STREPTOMYCIN) are active against both prokaryotic and eukaryotic microorganisms. 30 ammonification mitotic divisions of each nucleus of the dikaryon in the ascus mother cell. (cf. APOMIXIS.) Ammodiscus See FORAMINIFERIDA. Ammonia See FORAMINIFERIDA. ammonia assimilation Ammonia can be used as the sole source of nitrogen by many types of microorganism; it may be obtained from an external source or produced intracellularly – for example, by deamination reactions or by ASSIMILATORY NITRATE REDUCTION. Ammonia can be taken up readily through the cytoplasmic membrane; ammonium ions require a TRANSPORT SYSTEM [transport of ammonium ions in bacteria: FEMS Reviews (1985) 32 87–100]. Within the cell, ammonia can be assimilated in different ways. With high concentrations of ammonia, some bacteria (including Escherichia coli, Klebsiella spp) use ammonia for the reductive amination of 2-oxoglutarate to L-glutamate in a reaction catalysed by NADPH-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2). L-Glutamate can act as an N-donor in transamination reactions in which amino acids are synthesized from 2-oxoacids; E. coli synthesizes three low-specificity transaminases: transaminase A (which preferentially catalyses the synthesis of e.g. L-alanine and L-aspartate), B (aromatic amino acids) and C (branched-chain amino acids). With low concentrations of ammonia (e.g. <ca. 1 mM) many bacteria and other organisms assimilate ammonia via a twostep reaction which is catalysed by (i) glutamine synthetase (GS; EC 6.3.1.2) and (ii) glutamine:2-oxoglutarate aminotransferase (GOGAT; glutamate synthase; EC 1.4.1.13). In this pathway, glutamate is initially aminated to glutamine in an ATPdependent, GS-catalysed reaction which consumes ammonia. Then, in an NADPH-dependent, GOGAT-catalysed amination, one molecule of glutamine and one of 2-oxoglutarate yield two molecules of glutamate (see Appendix IV(a)). Many organisms can use the initial (GS-catalysed) step for glutamine synthesis, glutamine being used as a donor of amino groups in the biosynthesis of e.g. purines, carbamoyl phosphate (a precursor of pyrimidines), histidine and tryptophan. In E. coli, it appears that the GDH-catalysed pathway is used under energy-limiting conditions (the other pathway is ATPdependent). [Pathway choice in glutamate synthesis in E. coli : JB (1998) 180 4571–4575.] Mutants of E. coli lacking both pathways cannot use exogenous ammonia, but can grow on an external source of glutamate. In enterobacteria, the GDH and GS/GOGAT systems are regulated at the level of transcription and (in the case of GS) enzyme activity. High concentrations of ammonia repress the synthesis of GS (and other nitrogen-regulated genes: see NTR GENES) and induce the synthesis of GDH; this situation is reversed when ammonia levels become limiting. GS activity is controlled in response to ammonia levels, the enzyme being inactivated by (reversible) adenylylation in the presence of high levels of ammonia. In e.g. Streptomyces venezuelae the anaerobic assimilation of ammonia involves synthesis of alanine by the reductive amination of pyruvate via NADH-dependent alanine dehydrogenase (EC 1.4.1.1) [CJM (1985) 31 629–634]. In Streptomyces clavuligerus the GS/GOGAT pathway appears to be the only one for ammonia assimilation [JGM (1986) 132 1305–1317]. ammoniacal silver nitrate See SILVER. ammoniated mercury See MERCURY. ammonification The formation of free ammonia (or NH4 + ) during the microbial breakdown of nitrogenous organic matter parenterally to treat severe malaria; however, as resistant strains of the parasite are common, other drugs may be recommended for the treatment of severe disease. Chloroquine is generally the drug of choice for non-falciparum malaria, and is also used (with e.g. proguanil) for prophylaxis. Mefloquine has been used for uncomplicated falciparum malaria and (via nasogastric tube) for the treatment of severe disease. It has also been used prophylactically where, owing to resistance, chloroquine/proguanil may be ineffective. Mefloquine prophylaxis is absolutely contraindicated in the first trimester of pregnancy, and should not be used in the second or third trimesters; its therapeutic use in pregnancy, especially during the first trimester, should take into consideration the potential risk to the fetus. The drug is not recommended e.g. for treating patients with epilepsy or for prophylaxis in those with e.g. severe renal insufficiency or abnormal liver function [mefloquine (evaluation): Drugs (1993) 45 430–475]. The mode of resistance to quinoline-type drugs is poorly understood, but a consistent finding is that resistant strains accumulate lower levels of the drug. Most studies have been carried out on chloroquine, and several mechanisms have been suggested. (i) Weakened proton pump activity in the membrane of the parasite’s food vacuole may result in a higher pH within the vacuole; this would tend to decrease the level of protonation of chloroquine in the vacuole and, hence, militate against its retention within the vacuole (the non-protonated form of the drug can freely diffuse outward across the vacuolar membrane). However, the mechanism of chloroquine accumulation assumed in this model (i.e. uptake by inward diffusion into the vacuole, and retention following protonation) may not account for the high levels of drug achieved in sensitive strains of the parasite. (ii) The vacuolar membrane in P. falciparum includes an ABC TRANSPORTER, designated Pgh1, encoded by gene pfmdr1. When pfmdr1 was expressed in Chinese hamster ovary cells, Pgh1 apparently localized within the membrane of the lysosome and mediated uptake of chloroquine. This may indicate that Pgh1 mediates the uptake of chloroquine into the food vacuole of P. falciparum; this is supported by the observation that ‘deamplification’ of pfmdr1 occurs in mutants selected for chloroquine resistance [EMBO (1992) 11 3067–3075]. Interestingly, when chloroquine-resistant mutants were selected for mefloquine resistance (by mefloquine-limited growth), the mefloquine-resistant mutants were found to over-express Pgh1 and to be less resistant to chloroquine (but more resistant to e.g. quinine). Hence, resistance to chloroquine and mefloquine seem to be inversely related; this is not understood. Moreover, the apparent crossresistance between mefloquine and quinine may mean that resistance to mefloquine in geographical areas not previously exposed to mefloquine may have developed as a result of exposure to quinine [PNAS (1994) 91 1143–1147]. [Mode of action and mechanism of resistance to antimalarial drugs: Acta Tropica (1994) 56 157–171.] p-aminosalicylic acid See PAS. amitosis In certain eukaryotes: an atypical form of MITOSIS (q.v.) in which the chromosomes do not condense, a spindle is not formed, the nuclear membrane persists throughout division, and the nucleus divides by constriction. amixis In certain haploid organisms: a type of reproduction, considered to be a deviant form of the sexual process, in which karyogamy and meiosis do not occur even though morphologically differentiated structures (e.g. spore-containing asci) are formed; thus, e.g., the eight haploid ascospores formed (per ascus) by Podospora arizonensis are said to be derived by 31 ammonium salt sugars amoebostome A sucker-like structure present on free-living cells of Naegleria fowleri; amoebostomes are apparently used to engulf food particles [JP (1985) 32 12–19]. (cf. CYTOSTOME). Amorphosporangium A genus of bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in leaf litter. The organisms resemble Actinoplanes spp but differ e.g. in that the sporangium is highly irregular in shape, and the zoospores are rod-shaped. Type species: A. auranticolor. Amorphotheca resinae See HORMOCONIS. amoxycillin A PENICILLIN; 6-(a-amino-4-hydroxybenzylamido) penicillanic acid. (See also AUGMENTIN.) AMP Adenosine 5 -monophosphate [see Appendix V(a)]. (See also ADENYLATE KINASE; cf. CYCLIC AMP.) Amphiacantha See RUDIMICROSPOREA. amphibiotic (1) Amphibious. (2) Able to behave either as a commensal or as a parasite in a given host. (See also OPPORTUNIST PATHOGENS.) amphibolic pathway A metabolic pathway which can have both catabolic and anabolic functions: e.g. the TCA CYCLE. Amphidinium See DINOFLAGELLATES. amphiesma The wall – including the vesicles, thecal plates (if present), etc. – of a DINOFLAGELLATE. amphigenous Situated or growing on both sides of, or all over, a given structure. amphipathic Having both hydrophilic and hydrophobic regions. amphiphilic Syn. AMPHIPATHIC. amphispore A type of dark, thick-walled uredospore formed e.g. by some species of Hyalospora and Puccinia. Amphistegina See FORAMINIFERIDA. amphithecium (lichenol.) Syn. THALLINE MARGIN. amphitrichous Refers to the presence of a single flagellum (or e.g. fimbria) at each pole of a cell. amphitrophic Refers to an organism which can grow photosynthetically in the light and chemotropically in the dark. ampholyte An electrolyte whose molecule has both acidic and basic groups; the pH of a solution of a given ampholyte depends on the relative strengths of its acidic and basic groups. (See also ISOELECTRIC FOCUSING.) Amphora A genus of pennate DIATOMS in which the cingulum is much wider on one side (the ‘dorsal’ side) than on the other, resulting in the two valves facing in more or less the same direction; the raphes in both valves can thus both make contact with the substratum. Species are aquatic, benthic organisms capable of GLIDING MOTILITY. amphoteric dye See DYE. amphotericin B A MYCOSAMINE-containing heptaene POLYENE ANTIBIOTIC produced by Streptomyces nodosus. It is active against a wide range of fungi and certain pathogenic protozoa; it is administered orally for the treatment of various mycoses, including systemic infections, but its clinical use is limited by its nephrotoxicity. (See also VISCERAL LEISHMANIASIS.) amphotropic Refers to a retrovirus which can replicate both in its host of origin and in ‘foreign’ host cells (e.g. a murine retrovirus which can replicate in both murine and non-murine mammalian cells). Murine amphotropic viruses do not show cross-interference or cross-neutralization with the ecotropic or xenotropic murine retroviruses, i.e., their env glycoproteins are unrelated to those of the ecotropic or xenotropic viruses [Book ref. 114, p. 73]. (cf. DUALTROPIC.) ampicillin See PENICILLINS. amplicon (1) In a target molecule of nucleic acid: the specific sequence of nucleotides which is to be copied (amplified) by a nucleic-acid-amplification technique such as NASBA or PCR. and/or during DISSIMILATORY NITRATE REDUCTION. Ammonia does not normally accumulate under aerobic conditions (being readily assimilated by a wide range of organisms), but may do so under anaerobic conditions. (See also MINERALIZATION; NITROGEN CYCLE; SAPROBITY SYSTEM.) ammonium salt sugars A range of media used for determining which (if any) of a series of carbohydrates are metabolized by a given organism; these media are used e.g. for those organisms which form excess alkaline products from peptone media and which, therefore, cannot be tested in PEPTONE-WATER SUGARS. The medium contains (g/l): KCl (0.2), MgSO4 .7H2 O(0.2), (NH4 )2 HPO4 (1.0), yeast extract (0.2), agar (20) and bromcresol purple (4 ml of a 0.2% solution); the carbohydrate is added after autoclaving (to give a final concentration of 1% w/v), and the medium is poured to form slopes. amoeba (ameba) A type of cell or organism characterized by its ability to alter its shape drastically, generally by the extrusion of one or more pseudopodia (see PSEUDOPODIUM). Such amoeboid cells are characteristic of the SARCODINA, but also occur in other groups – for example, in the genus HISTOMONAS (see also MACROPHAGE). Amoeba A genus of free-living amoebae (order AMOEBIDA). The cell is differentiated into an outer hyaline layer (the ectoplasm) and an inner granular endoplasm. Each cell typically forms several lobopodia, one of which is dominant at a given time, and each of which has a hemispherical tip and (usually) a hyaline cap. Reproduction occurs asexually by binary fission. A. proteus is common in slow-moving or still freshwater habitats – often on the undersides of floating leaves; it feeds mainly on small flagellates, ciliates and bacteria. The cell may reach ca. 600 µm diam. (See also x-BACTERIA.) amoebiasis (1) Any disease of man or animals caused by a protozoon of the AMOEBIDA – see e.g. DYSENTERY (b) and MENINGOENCEPHALITIS. (2) Syn. amoebic DYSENTERY. amoebic dysentery See DYSENTERY (b). amoebicide Strictly: any chemical agent which kills amoebae; in practice, the term is commonly used for any drug which is active against (particularly) Entamoeba histolytica, even when the drug may inhibit, rather than kill, the parasite (i.e., it may act as an amoebistat). (See e.g. ARSENIC; DILOXANIDE; EMETINE; FUMAGILLIN; 8-HYDROXYQUINOLINE; METRONIDAZOLE.) Amoebida An order of naked, typically uninucleate (cf. CHAOS) amoebae (class LOBOSEA) in which mitochondria are typically present (cf. PELOBIONTIDA) and flagellate stages do not occur (cf. SCHIZOPYRENIDA). Most species occur in fresh water and/or moist soil (e.g. ACANTHAMOEBA, AMOEBA, Echinamoeba, MAYORELLA, Rosculus, Saccamoeba, Thecamoeba, TRICHAMOEBA, Vannella, Vexillifera); some are marine (e.g. Flabellula, PARAMOEBA, most Platyamoeba spp), and a few are parasitic (see e.g. ENDAMOEBA, ENDOLIMAX, ENTAMOEBA, IODAMOEBA, MALPIGHAMOEBA, PARAMOEBA). (See also HYDRAMOEBA.) [Descriptions of freshwater and soil species and genera: Book ref. 133.] Amoebidiales See TRICHOMYCETES. Amoebidium See TRICHOMYCETES. amoebistat See AMOEBICIDE. Amoebobacter See CHROMATIACEAE. amoeboflagellate An essentially amoeboid protozoon which can – at least under certain circumstances – form flagella as well as pseudopodia. (See e.g. SCHIZOPYRENIDA; HISTOMONAS.) amoeboid AMOEBA-like. amoeboid movement See PSEUDOPODIUM. amoeboma See DYSENTERY (b). 32 Anabaena (2) A copy of the specific nucleotide sequence which has been amplified by a nucleic-acid-amplification technique such as NASBA or PCR. (3) A defective virus vector [e.g. PNAS (1985) 82 694–698]. amplicon containment See PCR. amplicon inactivation See PCR. amplification (mol. biol.) See GENE AMPLIFICATION. amplification pathway (immunol.) See COMPLEMENT FIXATION (b). amplified Mycobacterium tuberculosis direct test See TMA. amplimer An alternative (though infrequently used) name for a primer in a PCR reaction. AmpliTaq Gold DNA polymerase See HOT-START PCR. AmpliWax See HOT-START PCR. ampoule Syn.VIAL. AMPPD Trade name (Tropix, Bedford, MA, USA) of a 1,2dioxetane substrate which gives rise to CHEMILUMINESCENCE when activated by the enzyme alkaline phosphatase. amprenavir See ANTIRETROVIRAL AGENTS. amprolium A coccidiostatic agent which is structurally similar to thiamine. ampulla (1) (protozool.) In certain ciliates (e.g. some hymenostomes): a channel, or the wider part of a channel, leading into a contractile vacuole. (2) (protozool.) In certain hypostome ciliates: an adhesive organelle. (3) (mycol.) The expanded, terminal region of some types of conidiogenous cell (see CONIDIUM). (4) (mycol.) The expanded, terminal region of some types of conidiophore (see e.g. ASPERGILLUS). Ampullariella A genus of bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in soil and freshwater habitats. The organisms resemble Actinoplanes spp but differ e.g. in that the rod-shaped zoospores are formed in parallel rows within the cylindrical sporangium. Type species: A. regularis. ampullate development See CONIDIUM. ampulliform Flask-shaped. Amsacta moorei EPV See ENTOMOPOXVIRINAE. AMTDT See TMA. AMU ATOMIC MASS UNIT. amv See MYB. AMV Avian myeloblastosis virus: see AVIAN ACUTE LEUKAEMIA VIRUSES. AMV reverse transcriptase See REVERSE TRANSCRIPTASE. amygdaliform Shaped like an almond. amygdalin A b-glycoside, present e.g. in bitter almonds, consisting of GENTIOBIOSE linked to mandelonitrile. On b-glucosidase action (or acid hydrolysis), amygdalin yields hydrogen cyanide, glucose and benzaldehyde. amylases Enzymes which cleave glucosidic linkages in e.g. STARCH or GLYCOGEN. a-Amylases ((1 → 4)-a-D-glucan 4-glucanhydrolases, EC 3.2.1.1) are endoenzymes which have little action on terminal (1 → 4)-a-bonds or on bonds adjacent to (1 → 6)-a branch points. They act on amylopectin and glycogen to form glucose, maltose and branched a-limit DEXTRINS, and on amylose to form first maltose and maltotriose, then slowly on maltotriose to form maltose and glucose. a-Amylases are common among microorganisms. They are obtained commercially mainly from Bacillus spp (see also IMMOBILIZATION (sense 1)) and are used e.g. for processing starch to form glucose syrups: insoluble starch granules are dispersed in water by heating, and the starch is partially hydrolysed with thermostable a-amylases from B. amyloliquefaciens and/or B. licheniformis. Further hydrolysis is achieved using e.g. g-amylases (see below). b-Amylases ((1 → 4)-a-D-glucan maltohydrolases, EC 3.2.1.2) are exoenzymes which cleave alternate bonds from the nonreducing end of a linear (1 → 4)-a-D-glucan; thus, e.g. amylose is degraded to maltose. b-Amylase action is halted at (1 → 6)-a branch points; thus maltose and b-limit dextrins are formed from glycogen and amylopectin. b-Amylases are common in plants and are produced e.g. by Bacillus and Streptomyces spp. They may be used, together with DEBRANCHING ENZYMES, in the manufacture of maltose syrups from starch. b-Amylases are inhibited by SCHARDINGER DEXTRINS. g-Amylases (amyloglucosidases, glucoamylases, (1 → 4)a-D-glucan glucohydrolases, exo-(1 → 4)-a-glucosidases, EC 3.2.1.3) are exoenzymes which cleave (1 → 4)-a-bonds consecutively from the non-reducing end of a (1 → 4)-a-Dglucan to yield b-D-glucose; they can also cleave (1 → 6)-a- and (1 → 3)-a-bonds, although at a much lower rate. g-Amylases are found mainly in fungi and are obtained commercially e.g. from Aspergillus niger and Rhizopus spp. They are used to convert malto-oligosaccharides (e.g. limit dextrins) to D-glucose (see above). (See also BREAD-MAKING and ENZYMES.) amyl-3-cresol See PHENOLS. amyloglucosidases See g-AMYLASES and DEBRANCHING ENZYMES. amyloid (1) Starch-like; staining blue or blue-black with iodine (e.g. MELTZER’S REAGENT). (2) (med.) An abnormal protein deposited, intercellularly, in various tissues in certain pathological conditions. amylolytic Capable of hydrolysing STARCH. amylopectin See STARCH. amyloplast A non-pigmented PLASTID which synthesizes and stores STARCH. amylose See STARCH. Amylostereum See APHYLLOPHORALES (Stereaceae). amylovorin An extracellular polysaccharide which is produced by strains of Erwinia amylovora and which may be involved in the production of wilt symptoms in FIREBLIGHT; it consists essentially of a (1 → 3)- and (1 → 6)-b-linked galactan backbone with side-chains containing residues of galactose, glucuronic acid, glucose and pyruvate. amylum stars Star-shaped aggregates of starch-filled cells which develop from the lower nodes in CHAROPHYTES and function in vegetative propagation. amytal A barbiturate which acts as a mitochondrial RESPIRATORY INHIBITOR: its binding site(s) and mechanism of action appear to be similar to those of ROTENONE. Many bacterial systems are partially or completely insensitive to amytal. Anabaena A genus of filamentous CYANOBACTERIA (section IV) in which the trichomes are made up of spherical, ovoid or cylindrical vegetative cells; heterocysts may be intercalary or terminal. GC%: 38–44. Species include e.g. A. azollae, A. cylindrica, A. flos-aquae and A. variabilis, distinguished e.g. on the basis of cell size and shape, filament form, etc; however, some of these characteristics may be variable in culture. A. flos-aquae contains GAS VACUOLES; it can form extensive BLOOMS in (usually) freshwater habitats, and may produce various toxins (anatoxins) which may be lethal to animals which drink the water. Anatoxin-a is an alkaloid which resembles cocaine; it acts as a potent post-synaptic depolarizing neuromuscular blocking agent, causing stupor, tremors, prostration, convulsions, opisthotonos and death. 33 anabolism be stopped or merely retarded. In some cases oxygen sensitivity is believed to be due to the lack of enzymes such as CATALASE and SUPEROXIDE DISMUTASE – the cells thus lacking protection against the toxic metabolites H2 O2 and O− . Another suggestion 2̇ is that an unacceptably high (positive or low negative) Eh may lead to a high intracellular Eh which, in turn, may e.g. prevent reduction in redox couple(s) which are normally freely reversible within the growing organism, and/or cause irreversible oxidation of vital components. anaerobic (1) Refers to an environment in which oxygen is absent. (cf. AEROBIC.) (2) Having the characteristic(s) of ANAEROBE(s). anaerobic digestion The anaerobic breakdown of complex animal and/or plant materials to simple substances which include a high proportion of gaseous and soluble products; it involves a diverse range of organisms whose metabolic pathways are closely interrelated, and occurs e.g. in benthic muds, in the RUMEN and in certain types of SEWAGE TREATMENT plant (see also TERMITE–MICROBE ASSOCIATIONS). Initially, polysaccharides, lipids and proteins are cleaved, by extracellular enzymes, to (respectively) sugars, (usually) fatty acids and glycerol, and amino acids; most polysaccharides – including cellulose, pectins and starch – are readily cleaved, but LIGNIN is not readily degraded. The sugars and other small molecules are fermented by various organisms (e.g. species of Bacteroides, Clostridium, Selenomonas, Succinivibrio and Veillonella) to products which include acetate, butyrate, ethanol, lactate, propionate and succinate – together with carbon dioxide and hydrogen. Propionate can be metabolized to acetate and/or carbon dioxide by e.g. Desulfobulbus, Desulfonema and Syntrophobacter. [Propionate and succinate degradation: JGM (1985) 131 643–650.] Carbon dioxide and hydrogen are metabolized to acetate by some bacteria (e.g. Acetobacterium woodii, Clostridium aceticum – see ACETOGENESIS) and to methane by certain METHANOGENS. The scavenging of hydrogen by these organisms creates conditions suitable for the growth of the so-called obligate proton reducers or obligate hydrogenogens (see e.g. SYNTROPHOMONAS) – which metabolize ethanol, lactate (and other products of the initial fermentation) to acetate and hydrogen; for these organisms only protons can act as electron acceptors, and the continual scavenging of the resulting hydrogen is essential for their growth. In some anaerobic ecosystems acetate is metabolized to methane by e.g. Methanosarcina and/or oxidized to carbon dioxide by SULPHATE-REDUCING BACTERIA (e.g. Desulfobacter postgatei ) and sulphur-reducing species (e.g. Desulfuromonas acetoxidans); the latter organisms form hydrogen sulphide as a by-product. In the presence of high concentrations of sulphate, the sulphate-reducing bacteria can inhibit METHANOGENESIS by using both acetate and hydrogen. In the rumen, however, much of the acetate is absorbed by the ruminant itself. The process of anaerobic digestion results in an appreciable reduction in the bulk of biodegradable solids – a particularly useful feature in SEWAGE TREATMENT. The conversion of organic carbon to methane forms an important part of the CARBON CYCLE. In sewage treatment (and in the treatment of other organic wastes – such as agricultural and food-industry effluents) anaerobic digestion can yield a rich, relatively odourless agricultural fertilizer (rich in microbial biomass) and a useful fuel gas (biogas, also called marsh gas, sewer gas, sludge gas) which may contain 50% or more of methane; ca. 30–70% of solids may be converted to gases – the actual proportion depending A. azollae occurs in symbiotic association with species of the small floating freshwater fern Azolla (Azollaceae). The nitrogenfixing A. azollae occurs in a specialized cavity at the base of the upper lobe of each leaflet; it has a higher proportion of heterocysts than do free-living Anabaena spp, and grows poorly when isolated from the fern. Azolla grows e.g. in rice fields in the Far East, where it is an important source of nitrogen. The nitrogen becomes available to the rice plants only when the fern dies, sinks and decomposes; this occurs when the temperature rises and coincides with growth and tillering of the rice plants. Azolla has also been used as a forage crop for animals in Asia. [Expression of two kdp operons in Anabaena strain L-31 and relevance to desiccation: JV (2005) 71 5297–5302.] anabolism The metabolic reactions by which cell components, extracellular products etc are built up from organic and/or inorganic precursors. Anabolism requires an input of energy which, in organotrophs, may be derived from CATABOLISM of exogenous organic compounds (cf. LITHOTROPH). (See also ANAPLEROTIC SEQUENCES.) Anacystis A genus of cyanobacteria; those strains designated ‘A. nidulans’ are apparently strains of Synechococcus (restricted sense: see SYNECHOCOCCUS) [Ann. Mic. (1983) 134 B 21–36]. [Cell cycle events in ‘A. nidulans’: JGM (1984) 130 2535– 2542.] anaerobe An organism which has the ability to grow in the absence of oxygen, the term commonly being reserved for those organisms which, in nature, normally grow in – or can grow only in – anaerobic habitats; some anaerobes can also grow under aerobic conditions, i.e., they are FACULTATIVE aerobes. (cf. AEROBE; see also AEROTOLERANT and MICROAEROPHILIC.) Obligately anaerobic organisms occur e.g. in anaerobic river muds and in the RUMEN (see also ANAEROBIC DIGESTION and TERMITE–MICROBE ASSOCIATIONS). The energy-yielding metabolism of an anaerobe may be primarily or solely fermentative (see FERMENTATION sense 1), respiratory (oxidative) (see ANAEROBIC RESPIRATION), or photosynthetic (see anoxygenic PHOTOSYNTHESIS). Culture of obligate anaerobes. A strict anaerobe requires an oxygen-free gaseous phase above the surface of the medium (see ANAEROBIC JAR) and a medium free from dissolved oxygen; even under these conditions, however, many anaerobes will not grow unless the medium has been pre-reduced, i.e., poised at or below a particular REDOX POTENTIAL. The Eh of chemically pre-reduced media is usually −150 mV to −350 mV at pH 7; the precise Eh required by a given anaerobe may depend e.g. on the size of the inoculum – ongoing growth tending to lower the Eh in the surrounding medium. Apart from incubation per se, the handling of a strict anaerobe (i.e., transfer, inoculation etc) is often carried out in an oxygenfree environment (see ROLL-TUBE TECHNIQUE). However, it has been reported that certain strict anaerobes (e.g. Fusobacterium sp) can be handled in air, without significant loss of viability, when use is made of media (containing reducing agents) which have been stored anaerobically prior to inoculation; nevertheless, a significant loss of viability occurs when use is made of cysteine-containing media which have been stored aerobically [CJM (1984) 30 228–235]. Reducing agents containing a sulphhydryl group (e.g., H2 S, cysteine, thioglycollate) have been found to increase the oxygen sensitivity of various sulphatereducing bacteria [FEMS Ecol. (1985) 31 39–45]. (See also BREWER’S THIOGLYCOLLATE MEDIUM; COOKED MEAT MEDIUM; POUR PLATE; SHAKE CULTURE (1).) Oxygen sensitivity of anaerobes. Some anaerobes are killed by exposure to gaseous oxygen, while the growth of others may 34 anchor sequence e.g. on the substrate(s), the initial percentage of solids and the retention time. anaerobic jar A container used for the incubation of materials (e.g. inoculated media) in the absence of oxygen or, in general, under gaseous conditions other than atmospheric. See e.g. BREWER JAR, GASPAK, MCINTOSH AND FILDES’ ANAEROBIC JAR. (See also HOLDING–FLUSH JAR PROCEDURE.) anaerobic respiration RESPIRATION under anaerobic conditions, the terminal electron acceptor being (according to species and/or environmental conditions): fumarate, nitrate, nitrite, nitrous oxide, elemental sulphur, sulphate etc. See e.g. DISSIMILATORY SULPHATE REDUCTION, FUMARATE RESPIRATION, NITRATE RESPIRATION, SULPHUR RESPIRATION and DENITRIFICATION. (See also dissimilatory perchlorate reduction in the entry BIOREMEDIATION.) anaerobiosis (1) The complete absence, or (loosely) the paucity, of gaseous or dissolved elemental oxygen in a given place or environment. In nature, such conditions occur e.g. in the bottom muds of ponds and rivers and in the intestinal tracts of animals (see e.g. RUMEN). (See also ANAEROBIC JAR and ROLL-TUBE TECHNIQUE.) In aerobic habitats, anaerobic microenvironments may be created by the consumption of locally available oxygen e.g. by facultatively anaerobic organisms. (2) Life in the absence of air. Anaerobiospirillum A genus of Gram-negative bacteria (family BACTEROIDACEAE). Cells: round-ended helical rods, 0.6–0.8 × 3.0–8.0 µm, with bipolar tufts of flagella. Various sugars (e.g. glucose, lactose, sucrose) are fermented, the major products of glucose metabolism being succinate and acetate. GC%: ca. 44. Type species: A. succiniciproducens, first isolated from dogs. [In human bacteraemia: JCM (1998) 36 1209–1213. Ileocolitis in cats: JCM (2004) 42 2752–2758.] anaerogenic Refers to an organism which does not produce gas from a given substrate. Anaeroplasma A genus of obligately anaerobic, cell wall-less, sterol-requiring bacteria (class MOLLICUTES) which occur e.g. in the RUMEN. Cells: non-motile, coccoid to pleomorphic; plasmalogens are major components of polar lipids in the cytoplasmic membrane. Some strains are bacteriolytic, causing clearing in agar media containing e.g. Escherichia coli. Carbohydrates are fermented to e.g. acetic, formic and lactic acids (together with propionic or succinic acid in some strains), ethanol and CO2 . GC%: ca. 29–34. Type species: A. abactoclasticum; other species: A. bactoclasticum. [Nucleic acid relationships among anaerobic mycoplasmas: JGM (1985) 131 1223–1227.] Anaerovibrio A genus of Gram-negative bacteria (family BACTEROIDACEAE) which occur e.g. in the RUMEN. Cells: curved rods, ca. 0.5 × 1.2–3.6 µm; monotrichously (polarly) flagellated. Carbon sources include e.g. fructose, lactate and ribose; major fermentation products from these substrates include propionic and acetic acids and CO2 . Fermentation of glycerol yields mainly propionic acid. Lipolytic, producing clearing on linseed oil agar. Type species: A. lipolytica. ana-holomorph (mycol.) A fungus for which no sexual stage or state has been identified. (See also DEUTEROMYCOTINA.) anammox bacteria Bacteria able to oxidize ammonia to dinitrogen, anaerobically, using nitrite as electron acceptor [see e.g. Nature (2003) 422 608–611]. anamnestic response In persons or animals: a heightened response to the second or subsequent administration of a particular antigen given some time after the initial administration; thus, e.g., the concentration of specific antibody increases at a rate much greater than that in response to the first dose of the antigen. (See also BOOSTER.) anamorph (mycol.) The asexual (= imperfect) stage or state of a given fungus. (cf. ANA-HOLOMORPH; HOLOMORPH; SYNANAMORPH; TELEOMORPH.) anamorph-genus Syn. FORM GENUS. anamorph-species Syn. form species. anaphase See MITOSIS and MEIOSIS. anaphylactic shock A manifestation of IMMEDIATE HYPERSENSITIVITY involving a TYPE 1 REACTION. In man, the symptoms of anaphylactic shock include bronchospasm (contraction of bronchial smooth muscle) and cyanosis; in severe cases death may occur, in minutes or hours, from asphyxiation due to bronchiolar contraction and/or laryngeal oedema or from a major fall in blood pressure resulting from vascular permeability. First-line treatment includes administration of ADRENALIN. anaphylatoxins The fragments C3a and C5a formed during COMPLEMENT FIXATION; they induce the release of HISTAMINE from MAST CELLS, and C5a is chemotactic for neutrophils and monocytes. anaphylaxis Syn. TYPE 1 REACTION. Anaplasma A genus of Gram-negative bacteria (family ANAPLASMATACEAE) – once thought to be protozoa of the PIROPLASMEA. Cells: cocci, diam. ca. 0.3 µm. They infect erythrocytes, multiplying by binary fission within membrane-limited vacuoles (formed by invagination of the erythrocyte membrane) to form inclusion bodies – each containing up to ca. 10 cells. The inclusion body typically occupies either a central or a marginal position within the erythrocyte. Anaplasma spp (A. caudatum, A. centrale, A. marginale and A. ovis) occur worldwide; they apparently infect only ruminants (see ANAPLASMOSIS). (cf. PARANAPLASMA.) Anaplasmataceae A family of bacteria (order RICKETTSIALES) that included AEGYPTIANELLA, ANAPLASMA, EPERYTHROZOON and HAEMOBARTONELLA. In a proposed reorganization, changes include transfer of species of EHRLICHIEAE and WOLBACHIEAE to this family; in this edition of the dictionary the lower taxa of this family reflect earlier classifications. [Proposed reorganization of Anaplasmataceae and Rickettsiaceae: International Journal of Systematic and Evolutionary Microbiology (2001) 51 2145–2165.] anaplasmosis Any disease caused by an Anaplasma sp; transmission occurs mainly via ticks. In cattle, A. marginale can cause a severe anaemia with debility and wasting; in sheep and goats infection with A. marginale or A. ovis is usually subclinical. [Book ref. 33, pp. 875–878.] anaplerotic sequences Sequences of reactions which serve to replenish ‘pools’ of intermediates which have been depleted as a consequence of biosynthesis – see e.g. TCA CYCLE. Anaptychia A genus of foliose or fruticose LICHENS (order LECANORALES). Photobiont: a green alga. Apothecia: lecanorine; ascospores: brown, one-septate. Species may occur on e.g. rocks or bark. anatid herpesvirus 1 See DUCK VIRUS ENTERITIS. anatoxin (1) See ANABAENA. (2) Syn. TOXOID. Ancalochloris See CHLOROBIACEAE. Ancalomicrobium A genus of chemoorganotrophic, facultatively anaerobic PROSTHECATE BACTERIA found in aquatic habitats. In high nutrient concentrations the cells are non-motile, rounded to rod-shaped; in low nutrient concentrations they first become ovoid, knobbed, subpolarly flagellate cells, subsequently developing into non-motile forms bearing two to eight cylindrical (occasionally bifurcated) prosthecae. Cell division occurs by budding from the cell body. One species: A. adetum. anchor sequence See SIGNAL HYPOTHESIS. 35 Ancistrocoma animal viruses Viruses (see VIRUS) which can infect and replicate in the cells of animals – including mammals, birds, poikilothermic vertebrates and/or invertebrates. See table for general features. The host range for a given animal virus varies from very narrow (e.g. in nature, rubella and variola viruses apparently infect only man) to very wide (e.g. rabies virus can infect most mammals, while ARBOVIRUSES can replicate in both invertebrates and mammals). A virus may be transmitted from host to host directly (by body contact), via aerosols (DROPLET INFECTION), via food or water (see e.g. FOOD POISONING and POLYHEDRA), via a VECTOR, etc; some viruses can be transmitted transovarially or can cross the placenta and infect the fetus. Probably, many virus infections in animals are asymptomatic, but some can result in diseases ranging from mild and selflimiting (e.g. most cases of the COMMON COLD) to severe or fatal (e.g. AIDS, RABIES, YELLOW FEVER). The virus may remain more or less localized at the site of infection (e.g. in some forms of HERPES SIMPLEX, and in PAPILLOMAS), or it may spread systemically via the bloodstream, lymphatic system or nervous system – often replicating preferentially or only in a specific tissue or organ. (See also TROPISM sense 2 and PERSISTENCE.) Disease symptoms may be due e.g. to disturbances in host cell structure and/or metabolism, host cell lysis, immune responses in the host animal, etc. Animal viruses have been implicated in certain cancers (see DNA TUMOUR VIRUS and ONCOVIRINAE) and in other diseases of unknown causation – e.g. atherosclerosis (see MAREK’S DISEASE), MULTIPLE SCLEROSIS, and even psychiatric disorders (see BORNA DISEASE and RETROVIRIDAE). Identification of animal viruses often cannot be based entirely on clinical manifestations of infection: infection may be asymptomatic, different viruses can sometimes cause similar diseases (see e.g. ENCEPHALITIS), and similar viruses may cause widely differing diseases (see e.g. ENTEROVIRUS). Thus, identification often depends on laboratory procedures: e.g., light MICROSCOPY (for e.g. CYTOPATHIC EFFECT) or ELECTRON MICROSCOPY of clinical specimens; propagation of the virus in TISSUE CULTURE, EMBRYONATED EGGS or living animals, followed by observation of any virus-induced effects; serological tests (e.g. COMPLEMENT-FIXATION TEST, ELISA, HAEMADSORPTION-INHIBITION TEST, HAEMAGGLUTINATION-INHIBITION TEST, IMMUNOFLUORESCENCE, IMMUNOSORBENT ELECTRON MICROSCOPY, NEUTRALIZATION TEST, etc). TYPING (e.g. for epidemiological purposes) may involve e.g. serological tests and/or nucleic acid HYBRIDIZATION techniques. PCR-based examination may be used e.g. to identify drug-resistance mutations [genotyping of HIV-1: Lancet (1999) 353 2195–2199]. In only a few cases can virus diseases in animals be controlled by chemotherapy (see ANTIVIRAL AGENTS); in most cases prevention is the only means of disease control. In humans, this may involve e.g. vaccination, isolation of infected patients, elimination of vectors, etc; SMALLPOX has been effectively eradicated by such methods. Virus diseases of other animals may be controlled by similar methods, and also e.g. by slaughter of infected animals, quarantine of animals for transportation, etc. Viruses which cause INSECT DISEASES are sometimes used in the BIOLOGICAL CONTROL of insect pests. (cf. PLANT VIRUSES.) animalcules (Animaliculae) An archaic term for microscopic organisms, particularly protozoa. anisogamy The union of gametes which differ in form and/or physiology. When the gametes differ in size, the larger (macrogamete) is regarded as female, the smaller (microgamete) as male. (See e.g. OOGAMY; cf. ISOGAMY.) Ancistrocoma See HYPOSTOMATIA. Ancistrum See SCUTICOCILIATIDA. Ancylobacter See MICROCYCLUS (2). Ancylonema A genus of saccoderm DESMIDS. Andersen sampler An instrument used e.g. for sampling the airborne microflora. (See also AIR.) Essentially, the instrument permits air to be drawn, sequentially, over the surface of each of a number of vertically stacked agar plates or adhesive-coated glass or metal discs; a suction pump, connected at the base of the instrument, draws air in through an opening at the top of the stack. Before reaching each plate or disc, the air passes through a radially-arranged pattern of holes in a metal screen – the size of the holes in a given screen being smaller than those in the preceding one; hence, the size range of particles collected by a given plate depends on the position of the plate within the stack. Andes virus See HANTAVIRUS. Andrade’s indicator A PH INDICATOR used e.g. in PEPTONE-WATER SUGARS. To 100 ml 0.5% aqueous acid FUCHSIN is added 15 ml 4% NaOH; the solution is allowed to stand at room temperature for a day, or until its colour changes to a very pale yellow. (A further 1–2 ml NaOH may be added if this has not occurred after 24h.) This solution (before or after autoclaving) is added to the medium (1 ml in 90 ml); the indicator becomes red at ca. pH 5.5. androphage (male-specific phage) Any BACTERIOPHAGE which infects only host cells that contain a conjugative plasmid. Androphages include members of the genus INOVIRUS and of the family LEVIVIRIDAE. (cf. FEMALE-SPECIFIC PHAGE.) anemochoric Dispersed by the wind. anergy (immunol.) A state of unresponsiveness to antigenic stimulus; the term is used to refer e.g. to a B cell or T cell, or to an individual person or animal. The absence of a DELAYED HYPERSENSITIVITY reaction in a PRIMED subject on exposure to the relevant antigen would be one manifestation of anergy; for example, anergy may be exhibited when tuberculous individuals – particularly those with extensive pulmonary tuberculosis – are given a tuberculin test. ‘T cell anergy’ can be induced by a SUPERANTIGEN. aneuploid (anorthoploid) Having one or more complete chromosomes in excess of, or less than, the normal haploid, diploid or polyploid number characteristic of the species. (See also HETEROPLOID.) aneurin Syn. THIAMINE. Angeiocystis See EIMERIORINA. angiocarpic development (angiocarpous development) (mycol.) The mode of development of a fruiting body in which the spores reach maturity within a closed chamber or cavity in the fruiting body. (See e.g. GASTEROMYCETES.) (cf. GYMNOCARPIC DEVELOPMENT.) Angiococcus See MYXOBACTERALES. angiomatosis (bacillary) See BARTONELLA. ang-kak (ang-quac, angkak; red rice) A Chinese product (used mainly as a colouring agent for foods) made by the fermentation of rice by Monascus purpureus strains; the fungus produces two pigments: monascorubrin (red) and monascoflavin (yellow). angolamycin See MACROLIDE ANTIBIOTICS. Ångström unit (Å) 10−10 m (= 10−1 nm). anguibactin See SIDEROPHORES. anguidine Diacetoxyscirpenol: see TRICHOTHECENES. anicteric Without jaundice. AN-IDENT See MICROMETHODS. aniline dyes Dyes derived from aniline (aminobenzene, C6 H5 NH2 ), e.g., some TRIPHENYLMETHANE DYES. 36 anisotropic inhibitor ANIMAL VIRUSES: basic characteristics Genomea dsDNA monopartite, ccc, sc monopartite (partially ds) monopartite, linear monopartite, strands covalently joined at each end monopartite (possibly sometimes bipartite), linear monopartite, linear, with 5 terminal protein monopartite, ccc, sc ssDNA monopartite, linear; −ve-sense, or either −ve- or +ve-sense ccc Nucleocapsid symmetry Virus familyb (host typec in parentheses) + + + +/− helical isometric isometric complex Baculoviridae (I) Hepadnaviridae (V) Herpesviridae (V) Poxviridae (V;I) +/− isometric Iridoviridae (I;V) − isometric Adenoviridae (V) − isometric Papovaviridae (V) − isometric Parvoviridae (V;I) Presence of envelope − isometric porcine circovirus (V) dsRNA (linear) bipartite, with terminal protein multipartite − − isometric isometric Birnaviridae (V;I) Reoviridae (V;I;P) ssRNA (linear) +ve-sense, monopartite +ve-sense, monopartite +ve-sense, ‘diploid’ +ve-sense, monopartite +ve-sense, monopartite +ve-sense, monopartite +ve-sense, monopartite +ve-sense, monopartite +ve-sense, bipartite −ve-sense? monopartite −ve-sense, monopartite −ve-sense, monopartite −ve-sensee , bipartite −ve-sensee , tripartite −ve-sense, multipartite + + + + + − − − − + + + + + + helical helical isometric isometric isometric isometric isometric isometric isometric helical helical helical helical helical helical Toroviridaed (V) Coronaviridae (V) Retroviridae (V;I?) Togaviridae (V;I;P?) Flaviviridae (V;I) Caliciviridae (V) Nudaurelia b virus group (I) Picornaviridae (V;I) Nodaviridae (I) Filoviridaed (V) Paramyxoviridae (V) Rhabdoviridae (V;I;P) Arenaviridae (V) Bunyaviridae (V;I) Orthomyxoviridae (V) ssRNA (circular) ?-sense, monopartite + isometric delta virus (V) a b c d e See entry VIRUS for generalized account. ccc = covalently closed circular; ds = double-stranded; sc = supercoiled; ss = single-stranded. See separate entries for details. I = invertebrate; P = plant; V = vertebrate. Proposed family. Some segments are ambisense. anisotropic inhibitor A type of compound (e.g. monoazide ethidium) which inhibits energy transduction in OXIDATIVE PHOSPHORYLATION by binding to certain negatively charged sites on the outer surface of the mitochondrial inner membrane; the occurrence of redox reactions within the membrane stimulates the binding of anisotropic inhibitors to these sites, and this has been taken to indicate that redox reactions cause conformational changes in membrane proteins [PNAS (1985) 82 1331–1335]. The binding sites of anisotropic inhibitors (hydrophobic proteins designated chargerin I and chargerin II ) may correspond anisokont Syn. HETEROKONT. Anisolpidiaceae See HYPHOCHYTRIOMYCETES. anisomycin (2-[p-methoxybenzyl]-3,4-pyrrolidinediol 3-acetate) An antibiotic which inhibits protein synthesis in eukaryotes, binding to 80S ribosomes and inhibiting the peptidyltransferase reaction. Many members of the ARCHAEA (e.g. species of Halobacterium, Methanobacterium and Thermoplasma) are also sensitive to anisomycin, although Methanococcus and Methanosarcina spp are relatively resistant [Book ref. 157, p. 538]. 37 anisotropic medium to sites in Complexes I, III and IV of the mitochondrial ELECTRON and to the F0 moiety of the PROTON ATPASE. anisotropic medium (optics) See BIREFRINGENCE. Ankistrodesmus A genus of unicellular, non-motile green algae (division CHLOROPHYTA); the cells are generally long and thin and may be straight, curved, helical etc, sometimes occurring in bundles. (See also TBTO.) anlage (pl. anlagen) (ciliate protozool.) Syn. INITIAL(S). annealing (of primers) See PCR. annelate ascus An ASCUS whose wall has a small, annular, apical thickening that is often amyloid or light-refractive; at maturity, the ascospores are released via a pore in the centre of the thickening. annellations See CONIDIUM. annellide See CONIDIUM. annellidic development See CONIDIUM. annulus (microbiol.) Any of various structures which are ringshaped (either actually or in cross-section). Examples include e.g. the outermost layer (= ‘sheath’) of the contractile stalk in some peritrich ciliates, the PERISEPTAL ANNULUS, and the annular remnant of the PARTIAL VEIL. Anodonta See ZOOCHLORELLAE. Anomoeoneis See DIATOMS. anonymous mycobacteria Syn. ATYPICAL MYCOBACTERIA. Anopheles A genus of mosquitoes (order Diptera, family Culicidae); Anopheles spp are vectors of MALARIA. anopheline Refers to mosquitoes of the genus Anopheles. Anoplophrya See ASTOMATIDA. anorthoploid Syn. ANEUPLOID. anoxia A deficiency or lack of oxygen. (Hence adj. anoxic.) anoxygenic Not producing oxygen. anoxygenic photosynthesis See PHOTOSYNTHESIS. Anoxyphotobacteria A proposed class of anoxygenic photosynthetic prokaryotes (division GRACILICUTES): see RHODOSPIRILLALES. ansamycins A group of structurally related ANTIBIOTICS consisting of a macrocyclic ring which comprises an aromatic moiety spanned by an aliphatic bridge. Ansamycins (which include e.g. rifamycins, streptovaricins and tolypomycins) are produced by actinomycetes and appear to share a common mode of antibacterial action (see RIFAMYCINS); at high concentrations they also show activity against certain viruses in cell cultures. anserid herpesvirus 1 See DUCK VIRUS ENTERITIS. antagonism The inhibition of one entity by another of similar kind. Examples: one antibiotic may reduce or abolish the effects of another (e.g. a bacteriostatic drug will counteract the effects of a drug which acts only on growing cells); acid-producing organisms may inhibit or kill acid-intolerant organisms sharing the same environment. (cf. SYNERGISM.) antapical organelle In the bacterium Thiovulum majus: the presumed source of a thin (adhesive?) thread or stalk which projects from the surface of the organism. antenna (in PHOTOSYNTHESIS) See LIGHT-HARVESTING COMPLEX. anterior station (parasitol.) The anterior part of the alimentary canal and/or the salivary glands of an arthropod vector; infective forms of parasites in the anterior station can be transmitted to the vertebrate host by the bite of the vector. (cf. INOCULATIVE INFECTION.) antheraxanthin See CAROTENOIDS. antheridiol See PHEROMONE. antheridium A male gametangium. anthracnose Any of various plant diseases – particularly those caused by fungi of the MELANCONIALES – in which discrete, darkcoloured, necrotic lesions develop on the leaves and/or fruits. For example, anthracnose of beans (Phaseolus spp) is caused by Colletotrichum lindemuthianum and is characterized by brown or black lesions (sometimes bearing masses of pink conidia) on the leaves, pods and seeds. anthracycline antibiotics ANTIBIOTICS which have a tetrahydrotetracenequinone chromophore (containing three flat, coplanar, 6-membered rings). They function as INTERCALATING AGENTS, inhibiting DNA and RNA synthesis. Daunomycin (= rubidomycin, daunorubin), produced by Streptomyces sp, is an anticancer agent which binds to dsDNA (f ca. 12° ) with no apparent base preference; it contains an aminosugar (daunosamine) which projects into the major groove and stabilizes the drug–DNA complex by electrostatic interaction with phosphate in the DNA. Adriamycin is very similar to daunomycin in structure and function. Other anthracyclines: nogalamycin (antitumour agent, f ca. 18° ), cinerubin, rhodomycin. anthramycin A member of the pyrrolo-(1,4)-benzodiazepine group of antibiotics; other members include sibiromycin and tomaymycin. Anthramycin (produced by Streptomyces refuineus) has antibacterial and antitumour activity; it binds covalently to DNA, causing e.g. inhibition of DNA and RNA synthesis. The modes of action of sibiromycin and tomaymycin are apparently similar. anthranilate An intermediate in tryptophan biosynthesis: see Appendix IV(f). anthrax A disease – primarily of animals, but also of man – caused by strains of Bacillus anthracis that are both capsulated and toxinogenic (see ANTHRAX TOXIN, BACILLUS and STERNE STRAIN). In man the disease is usually localized, resulting from infection of skin lesions (anthrax boil, cutaneous anthrax, malignant pustule); sepsis may develop in untreated cases. Rarely, infection occurs via the lungs (pulmonary anthrax, woolsorters’ disease) or gut (intestinal anthrax), leading to sepsis; germination of the endospores of B. anthracis, and expression of toxin genes, have been demonstrated within (murine) alveolar macrophages [Mol. Microbiol. (1999) 31 9–17]. If treated promptly (e.g. with benzylpenicillin or other blactam antibiotic) mortality is low; if untreated, the septicaemic form is usually fatal. In herbivores (e.g. cattle, sheep, horses), the ingestion of B. anthracis spores during grazing can lead to rapidly fatal sepsis (‘splenic fever’). (See also MCFADYEAN’S TEST.) [Anthrax (various aspects): JAM (1999) 87 189–321.] anthrax toxin The TOXIN, produced by Bacillus anthracis, which is responsible for pathogenesis in ANTHRAX. Anthrax toxin comprises three protein components: oedema factor (EF, factor I), ‘protective antigen’ (PA, factor II) and lethal factor (LF, factor III); independently, each component has no biological activity. All three components are encoded by plasmid pOX1. (The capsule of B. anthracis, an essential anti-phagocytic attribute of the pathogen, is encoded by plasmid pOX2.) Activity of the toxin. PA forms a b-barrel structure in the eukaryotic cell membrane; this permits translocation, into the cell, of the two catalytic components of the toxin, LF and EF. LF is a zinc protease. Within the eukaryotic cell, LF cleaves mitogen-activated protein (MAP) kinase, inactivating it. MAP kinase is an essential component in eukaryotic signalling; inhibition of the MAP kinase pathway blocks the development of cells at a specific point in the cell cycle. (LF is also a potent inducer of cytokines, and is thought to be active at a concentration of 10−18 M.) TRANSPORT CHAIN 38 antibiotic the sum of their individual effects; cotrimoxazole is an example of a synergistic combination of antibiotics (trimethoprim + sulphamethoxazole) – two drugs which block different reactions in the same pathway. Antagonism is the converse of synergism. For example, antibiotics which inhibit growth will antagonize other antibiotics (e.g. penicillins) which are active only on growing cells. In another form of antagonism, the presence of certain types of antibiotic will promote the synthesis of inducible enzymes that inactivate other antibiotics; thus, for example, certain b-lactam antibiotics (such as imipenem or cefoxitin) can induce the synthesis of enzymes called b-lactamases which inactivate other b-lactam antibiotics. (For this reason, those b-lactam antibiotics which are strong inducers of b-lactamases are not used in combination with other b-lactam antibiotics.) The antimicrobial activity of an antibiotic in the human (or animal) body does not necessarily correspond to its activity in laboratory tests. For example, under in vivo conditions an antibiotic may bind to plasma proteins so that only a fraction of the administered dose is available for antimicrobial activity. Moreover, some antibiotics are administered in an inactive form that depends on in vivo metabolism for the development of its activity (see e.g. p-N-succinylsulphatriazole in SULPHONAMIDES). The ability of certain antibiotics (e.g. fluoroquinolones, rifampicin) to remain in an active state within phagocytes is an important feature because some bacterial pathogens can survive, or even grow, following uptake by phagocytic cells [RMM (1995) 6 228–235]. Antibiotic action. To be effective an antibiotic must (at least) enter the cell envelope; commonly, antibiotics must pass through the cell envelope in order to reach their target sites. Entry into a cell often occurs via an energy-dependent transport system (see e.g. entries CYCLOSERINE, POLYOXINS, TETRACYCLINES, WARHEAD DELIVERY). Within the pathogen, a given antibiotic acts at specific target site(s). Depending on antibiotic, the target site may be e.g.: (i) enzymes that catalyse the synthesis of a structural polymer in the cell envelope; (ii) the cytoplasmic membrane (certain antibiotics cause leakiness incompatible with cell viability); (iii) enzymes required for the synthesis or supercoiling of nucleic acids; (iv) the ribosome (some antibiotics can inhibit protein synthesis by binding to specific sites on the ribosome); (v) enzymes required for biosynthesis of an essential coenzyme – for example, dihydrofolate reductase (required for the synthesis of tetrahydrofolate) is inhibited by trimethoprim. Antibiotics of the same group have similar or identical target sites, and they all affect cells in the same way. However, antibiotics from different groups may have a similar target site; thus, for example, the target site of oxazolidinones appears to overlap the binding sites of both lincomycin and chloramphenicol [JB (2000) 182 5325–5331]. Some target sites are found in only certain categories of microorganism; for example, the cell wall polymer peptidoglycan occurs only in bacteria – so that antibiotics directed specifically against this target are unlikely to be of use against (e.g.) fungi. Clearly, no antibiotic is effective against all pathogens, and in many cases the activity of a given antibiotic is limited to a particular subset of organisms; in the context of bacterial pathogens, a broad-spectrum antibiotic is one which can be effective against a wide range of species, including both Grampositive and Gram-negative bacteria. Some examples of antibiotics (in various target regions) are listed below; see individual entries for mode of action etc. EF is an ADENYLATE CYCLASE which, within the eukaryotic cell, is stimulated by CALMODULIN, thus causing an increase in the level of cyclic AMP (cAMP). EF is responsible for the oedematous aspect of anthrax (and may also inactivate phagocytic cells). anthroponosis (1) An infectious disease which can be contracted by humans, the pathogen being normally maintained in a reservoir consisting of human population(s). (cf. ZOONOSIS.) (2) An infectious disease which can be contracted by animals, the pathogen being normally maintained in a reservoir consisting of human population(s) [use of term (sense 2): JMM (1997) 46 372–376]. anthropophilic Refers to a parasite or pathogen which preferentially infects man (cf. ZOOPHILIC). antibacterial agents Agents which kill, or inhibit the growth of, bacteria – e.g. certain ANTIBIOTICS, ANTISEPTICS and DISINFECTANTS. antibiogram The pattern of sensitivities of a given microorganism towards a range of antibiotics. antibiosis The antagonism of one organism towards another or others. antibiotic (1) Originally, the term used for any microbial product which, in low concentrations, inhibits or kills susceptible microorganisms; with this meaning, the term could distinguish between an agent such as penicillin (produced by a fungus) and synthetic (i.e. man-made) antimicrobials such as sulphonamides. This use of the term became untenable when penicillins and other natural (i.e. microbially synthesized) antimicrobial products were modified, or synthesized de novo, in the laboratory. (2) Currently, the term antibiotic is used most commonly to refer to any natural, semi-synthetic or wholly synthetic antimicrobial agent effective at low concentrations – although products which are active against viruses are usually referred to as ANTIVIRAL AGENTS rather than antibiotics. This account refers primarily to antibiotics in a medical/veterinary context, i.e. as agents used for the prevention or treatment of infectious diseases in man and other animals. However, it should be noted that natural antibiotics (e.g. BACTERIOCINS) have ecological roles. Moreover, some antibiotics (e.g. NISIN) are used as food preservatives, while others (e.g. AGROCINS) are used in BIOLOGICAL CONTROL. Certain antibiotics have been used in the control of plant diseases (e.g. COCONUT LETHAL YELLOWING). For therapeutic or prophylactic use, antibiotics should, ideally, be toxic only for (the appropriate) pathogens – i.e. they should be harmless to the patient; in other words, antibiotics should exhibit an appropriate level of selective toxicity. Even so, many of the antibiotics currently in use are associated with important side-effects – e.g. streptomycin may cause dose-dependent damage to the 8th cranial nerve, while penicillins may provoke hypersensitivity reactions (ANAPHYLACTIC SHOCK) in certain patients. Some antibiotics are contraindicated for particular categories of patient (e.g. pregnant women). (See also THERAPEUTIC INDEX.) Of the large number of known antibiotics, relatively few are suitable for clinical use; some are too highly toxic for human or animal use, and some would be ineffective as antimicrobial agents at safe doses. An antibiotic may be MICROBICIDAL or MICROBISTATIC; some can be microbicidal or microbistatic according e.g. to concentration. Mixtures of antibiotics may exhibit synergism or antagonism. Synergism is exhibited when two antibiotics, acting simultaneously on an organism, produce an effect which is greater than 39 antibiotic Bacterial cell wall (peptidoglycan). CYCLOSERINE; in a strain of Streptococcus pneumoniae, mutation outside the recognized ‘quinolone resistance-determining region’ has been associated with enhanced resistance to certain of the newer fluoroquinolones [AAC (2001) 45 952–995]. (ii) Inactivation or degradation of antibiotics by enzymes. Enzymes which inactivate or degrade antibiotics are encoded by genes that occur e.g. in plasmids or chromosomes. Such enzymes include the acetyltransferases, adenylyltransferases and phosphotransferases that are active against aminoglycoside antibiotics [TIM (1998) 6 323–327], the enzyme CHLORAMPHENICOL acetyltransferase, and the b-LACTAMASES which inactivate penicillins and related antibiotics. (iii) Efflux mechanisms. See EFFLUX MECHANISM. Some efflux mechanisms involve an ABC TRANSPORTER. [Efflux-mediated resistance to fluoroquinolones in Gram-negative bacteria: AAC (2000) 44 2233–2241.] (iv) Diminished permeability. Any change in the composition of the cell envelope which hinders the uptake of an antibiotic will either increase the MIC of that antibiotic or else give rise to high-level resistance. For example, diminished permeability in Enterobacter aerogenes, associated with an alteration in the outer membrane porins, has been reported to affect this organism’s susceptibility to various antibiotics, including some of the fourthgeneration cephalosporins (e.g. cefepime) [Microbiology (1998) 144 3003–3009]. Interestingly, in Escherichia coli, phenotypic (non-heritable) resistance to some antibiotics (e.g. ampicillin, chloramphenicol) can be induced by the presence of certain chemorepellents – e.g. acetylsalicylate (aspirin) and benzoate – which are detected by the tsr gene product; this phenomenon has been attributed to inhibition of synthesis of the OmpF porin by the chemorepellent [FEMS (1987) 40 233–237]. (v) Increased synthesis of an affected metabolite. Increased production of a given metabolite may overcome competitive inhibition by the antibiotic; for example, production of higher levels of PABA is one form of resistance to sulphonamides. (vi) Acquisition of an antibiotic-insensitive target. One example is the acquisition of a methicillin-resistant penicillin-binding protein by strains of Staphylococcus aureus (see MRSA). [Mechanisms of resistance to b-lactam, aminoglycoside, quinolone and other antibiotics in Acinetobacter baumannii: RMM (1998) 9 87–97.] Induction of resistance to antibiotics. Resistance to certain antibiotics is inducible – the presence of such antibiotics (over a certain minimum concentration) promoting resistance in the cell. For example, in some Gram-positive bacteria (e.g. staphylococci), the enzyme chloramphenicol acetyltransferase may be induced in the presence of its substrate, CHLORAMPHENICOL (see also TRANSLATIONAL ATTENUATION). Also in staphylococci, several distinct modes of resistance to b-lactam antibiotics may be induced by the presence of these antibiotics; thus, resistance may be due to the induction of b-LACTAMASES (leading to mechanism ii, above) or to induction of the specific PENICILLIN-BINDING PROTEIN (PBP) 2a (see MRSA) (mechanism vi, above). In the case of inducible resistance to b-lactam antibiotics, studies on the transmembrane signalling pathway have indicated that induced resistance – through either b-lactamases or the mecA product PBP 2a – may depend on a novel mechanism involving sequential cleavage of several regulatory proteins [Science (2001) 291 1962–1965; commentary: Science (2001) 291 1915–1916]. In this scheme, regulation of mecA expression involves initial binding of a b-lactam antibiotic to a cell-surface sensor–transducer: the MecR1 protein; as a consequence, MecR1 undergoes cleavage, and a (hypothetical) product of such cleavage, MecR2, b-LACTAM ANTIBIOTICS; MOENOMYCIN; VANCOMYCIN. Bacterial cytoplasmic membrane. DEPSIPEPTIDE ANTIBIOTICS; GRAMICIDINS; POLYMYXINS; TYROCIDINS. DNA intercalating agents. ACTINOMYCIN D; ANTHRACYCLINE ANTIBIOTICS; QUINOXALINE ANTIBIOTICS. Folic acid antagonists. CHLORGUANIDE; PYRIMETHAMINE; (See also SULPHONAMIDES.) Fungal cell wall. ACULEACIN A; NIKKOMYCINS; PAPULACANDIN B; POLYOXINS. Fungal cytoplasmic membrane. POLYENE ANTIBIOTICS (sense a). Gyrase. COUMERMYCINS; NOVOBIOCIN; QUINOLONE ANTIBIOTICS. Ribosome (protein synthesis). AMINOGLYCOSIDE ANTIBIOTICS; ANISOMYCIN; CHLORAMPHENICOL; CYCLOHEXIMIDE; FLORFENICOL; FUSIDIC ACID; LINCOSAMIDES; MACROLIDE ANTIBIOTICS; OXAZOLIDINONES; POLYENE ANTIBIOTICS (sense b); PUROMYCIN; SPARSOMYCIN; SPECTINOMYCIN; TETRACYCLINES; THIOSTREPTON; TIAMULIN; VIOMYCIN. RNA polymerase. ANSAMYCINS (e.g. RIFAMYCINS) (see also STREPTOLYDIGIN). Microbial resistance to antibiotics. Resistance to a given antibiotic (‘drug resistance’) is said to be constitutive (= intrinsic) in cells which (i) lack the specific target of that antibiotic, (ii) have an alternative form of the target that is not affected by the antibiotic, and/or (iii) are inherently impermeable to the antibiotic. For example: (i) species of Mycoplasma, which lack cell walls, are intrinsically resistant to penicillins (which inhibit the synthesis of a cell wall polymer); (ii) strains of Staphylococcus aureus that exhibit intrinsic resistance to methicillin contain an alternative form of the target (see MRSA); (iii) many Gram-negative bacteria are intrinsically resistant to e.g. benzylpenicillin because their outer membrane is impermeable to the antibiotic. A cell may acquire heritable resistance to one or more antibiotics in several ways. In bacteria, gene(s) specifying resistance to one or more antibiotics may be transferred to an antibiotic-susceptible cell by CONJUGATION, TRANSDUCTION, TRANSFORMATION or CONJUGATIVE TRANSPOSITION; resistance may also develop as a consequence of MUTATION. The way in which resistance is acquired is important in that it may determine the range of antibiotics to which a cell becomes resistant. Thus, a single point mutation commonly confers resistance to only one type of antibiotic (or to those antibiotics which share the same target site). By contrast, receipt of an R PLASMID or TRANSPOSON may confer resistance to either a single antibiotic or to two or more unrelated antibiotics (multiple drug resistance). Mechanisms of resistance to antibiotics. Six main mechanisms can be distinguished: (i) Mutation. The target site of a given antibiotic may be modified as a result of a MUTATION such that the target (e.g. enzyme, ribosome) functions more or less normally in the presence of otherwise inhibitory concentrations of the antibiotic (see e.g. STREPTOMYCIN). The cell in which such a mutation has occurred can therefore grow and give rise to a population of resistant cells in the presence of the given antibiotic. Examples of mutation-mediated resistance: (i) point mutations affecting 23S rRNA which confer resistance to macrolide antibiotics [AAC (1996) 40 477–480]; (ii) point mutations in the rpoB gene of Mycobacterium tuberculosis (encoding a subunit of RNA polymerase) conferring resistance to rifampicin. Although resistance-conferring mutations are commonly located within known sequences in the genome, resistance may also arise through mutation at novel sites; for example, TRIMETHOPRIM. 40 antibody essential for the synthesis of lipid A (an integral part of the outer membrane in Gram-negative bacteria) (see ENDOTOXIC SHOCK). New approaches to antibiotics may also involve agents such as CcdB (see F PLASMID) and certain bacteriocins. By contrast, peptide deformylase (PDF), which cleaves the Nterminal formyl group from nascent bacterial polypeptides (see PROTEIN SYNTHESIS), seems less than optimal as a candidate drug target; certain derivatives of hydroxamic acid are potent inhibitors of PDF, but resistance to these agents develops readily, and the drugs appear to be only bacteriostatic [AAC (2001) 45 1058–1064]. The ongoing problem of antibiotic-resistant pathogens has prompted consideration of other anti-infection modalities, including bacteriophage therapy [AAC (2001) 45 649–659]. Antibiotics for chemotherapy: factors affecting the choice. Antibiotics may be wasted if they are used without due regard to specificity and pharmacology; moreover, use of inappropriate drugs may forfeit the chance of helping the patient and may also exacerbate unnecessarily the general problem of antibiotic resistance. The kind of factors which affect the choice of antibiotic include: directly or indirectly brings about cleavage of the transcriptional repressor protein, MecI – thus allowing expression of mecA. (See also b-LACTAMASES.) Antibiotic-sensitivity (antibiotic-susceptibility) tests. Laboratory tests are often carried out to determine the susceptibility of a given pathogen to various antibiotics (the pattern of sensitivities being referred to as the pathogen’s antibiogram); the results of such tests may enable the clinician to select optimal antibiotic(s) for chemotherapy. A common form of test is the DISC DIFFUSION TEST (q.v.; see also NEO-SENSITABS); this kind of test is intended primarily for those bacteria which produce visible growth after overnight incubation (rather than slow-growing species such as Mycobacterium tuberculosis). (See also DILUTION TEST.) The E TEST is a diffusion test which can indicate an MIC. Other methods, such as LATEX PARTICLE TESTS, have been assessed for their ability to detect resistance to methicillin in staphylococci [rapid detection of mecA-positive and mecAnegative coagulase-negative staphylococci by an anti-penicillin binding protein 2a slide latex agglutination test: JCM (2000) 38 2051–2054]. Molecular methods (including PCR-based tests) can also be used in certain cases for detecting antibiotic resistance. [Rapid identification of methicillin-resistant Staphylococcus aureus and simultaneous species confirmation using real-time fluorescence PCR: JCM (2000) 38 2429–2433. Multiplex PCR for detection of genes for Staphylococcus aureus enterotoxins, exfoliative toxins, toxic shock syndrome toxin 1 and methicillin resistance: JCM (2000) 38 1032–1035. Simultaneous identification and typing of multidrug-resistant Mycobacterium tuberculosis isolates by analysis of pncA and rpoB: JMM (2000) 49 651–656.] (See also line probe assay in the entry PROBE.) [DNA-based (‘genotypic’) antibiotic susceptibility testing: Book ref. 221, pp 203–228.] Clinical impact of bacterial resistance to antibiotics. Although large numbers of new antibiotics have been identified in the last few decades, bacterial resistance to existing drugs has continued to emerge so that, in some cases, therapeutic options have become very limited; examples of such resistance include: vancomycin-resistant enterococci [EID (2001) 7 183–187]; tetracycline-resistant gonococci; penicillin-resistant pneumococci [Drugs (1996) 51 (supplement 1) 1–5]; multidrug-resistant Vibrio cholerae O1 [Lancet (1997) 349 924]; multidrug-resistant Mycobacterium bovis [Lancet (1997) 350 1738–1742]; and low-level resistance to vancomycin in MRSA [JAC (1997) 40 135–136]. [The crisis in antibiotic resistance: Science (1992) 257 1064–1073.] There is thus an ongoing need to develop new antibiotics and to prevent, or delay, the emergence of resistance to those drugs that are currently useful by avoiding inappropriate usage; given the value of antibiotics in medicine (and our current dependence on them) it is essential that new drugs be used carefully and rationally [quinupristin/dalfopristin and linezolid: where, when, which and whether to use?: JAC (2000) 46 347–350]. Already, clinical resistance to linezolid has been reported in vancomycin-resistant strains of Enterococcus faecium (vancomycin-resistant enterococci, VRE) [Lancet (2001) 357 1179]. [Controlling antimicrobial resistance in hospitals (infection control and use of antibiotics): EID (2001) 7 188–192.] Because of ongoing development of resistance in pathogenic bacteria, new targets for antibiotics are being sought within the bacterial cell. One target in Escherichia coli is de-acetylase (a zinc-requiring enzyme, product of gene lpxC) which is • Effectiveness against relevant pathogens. In ideal circumstances the pathogen’s identity and its sensitivity to antibiotics are known prior to treatment. However, in certain diseases (e.g. meningitis) therapy is often begun before the aetiology has been confirmed. When this happens the choice of antibiotic is based on the probable pathogen; probability reflects factors such as (local) prevalence of particular species/strains of pathogen, reported problems of resistance in the pathogen, and the patient’s age/general condition. [Example: management of bacterial meningitis (paediatric patients): RMM (1997) 8 171–178.] • Prevalence of antibiotic-resistant strains in the locality. Many pathogens are now resistant to antibiotic(s) to which they were once invariably sensitive; moreover, some pathogens are resistant to a range of drugs so that, in some cases, therapeutic options are extremely limited. • Antagonism towards other antibiotics – or even towards other therapeutic agents; for example, rifampicin can potentiate certain mammalian enzymes and, as a consequence, may interfere with oral contraception. • Possible side-effects (e.g. penicillin allergy). • In pregnant patients: possible teratogenicity. • Pharmacokinetics. The physicochemical nature of a drug determines its ability to reach particular sites when administered via a given route. For example, if certain drugs are injected intravenously they do not reach the cerebrospinal fluid (CSF) because they cannot pass the blood–brain barrier; other types of drug can, and some can do so only if the permeability of this barrier has been increased e.g. as a result of inflammation. (See also ANTIFUNGAL AGENTS, ANTIPROTOZOAL AGENTS, ANTIand DISINFECTANTS.) antibiotic-associated colitis See COLITIS. antibody (ab) Any IMMUNOGLOBULIN molecule produced in direct response to an ANTIGEN (or to an autocoupling HAPTEN) and which can combine specifically, non-covalently, and reversibly with the antigen which elicited its formation (see also ANTIBODY FORMATION); specificity is not necessarily absolute: see CROSSREACTING ANTIBODY. (A MYELOMA can produce Ig with antibody function in the absence of antigenic stimulation. See also PASSIVE IMMUNITY.) The term ‘antibody’ is also used to refer to a SEPTICS 41 antibody absorption which promotes activation and clonal proliferation of the T cell and induces it to release cytokines, including INTERLEUKIN-4. However, once activated, T cells produce the cell-surface antigen CTLA-4 (= CD152), a molecule (similar to CD28) to which B7.1 and B7.2 can bind; interaction between B7.1/B7.2 and CTLA-4 may initiate a signal which terminates synthesis of IL-2 in the T cell. (Note that, while IL-2 is not a characteristic product of the Th2 subset of T cells, transient synthesis of this cytokine at an early stage would appear to be necessary as it seems to be an essential growth factor for T cells.) Yet another paired interaction between the B and T cells involves CD40 (q.v.). The mutual (physical) co-operation between the B cell and T cell (described above) is referred to as cognate help or linked recognition. The activated, proliferating B cells form an expanded clone. Some of the cells of this clone become antibody-secreting PLASMA CELLS. Others become MEMORY CELLS; these cells do not secrete antibody but retain the potential to do so, rapidly, if subsequently challenged with the same antigen (giving a heightened secondary response). (See also ANAMNESTIC RESPONSE.) Initially, antibodies secreted by plasma cells are of the IgM class. Subsequently, plasma cells form antibodies of another class – generally IgG – but of the same specificity; such class switching (= isotype switching) is promoted by several stimuli – including the binding of B cell antigen CD40 to the corresponding T cell ligand, and the activity of the T-cell-derived cytokine INTERLEUKIN-4. Class switching occurs in so-called germinal centres within lymph nodes and spleen. Such sites are also involved in a process known as affinity maturation; in this process, mutant cells among the (proliferating) antigen-stimulated B cells are selected for high-affinity binding to the specific antigen – selective pressure presumably being a falling concentration of the given antigen. Within the B cells a process known as somatic hypermutation gives rise to mutations that are concentrated in the V regions of antibody genes; as a consequence, different B cells produce antibodies with different levels of affinity for the given antigen – those producing antibody with the highest affinity being selected. [Hypermutation in antibody genes: PTRSLB (2001) 356 1–125.] (See also RNA EDITING.) In the response to an antigen not previously encountered (a primary response) the numbers of specifically reactive B cells (prior to clonal expansion) are likely to be low. Under these conditions it may be that the presentation of antigen to T cells is carried out not by B cells but by other, more numerous, APCs such as macrophages or dendritic cells. In this scenario, interaction between APC and T cell activates the T cell – which then secretes various cytokines; these cytokines may induce differentiation, proliferation and antibody secretion in B cells that have bound the specific antigen and which are close to (though not necessarily in contact with) the activated T cells [Book ref. 227, pp 207–208]. When produced in response to invasion by pathogens (or their products – e.g. toxins), antibodies commonly have a protective role (see e.g. ADCC and OPSONIZATION). However, under certain conditions, antibodies can promote damage to the host’s tissues (see IMMEDIATE HYPERSENSITIVITY). anticapsin See BACILYSIN. anticlinal Perpendicular to a given surface. (cf. PERICLINAL.) anticoagulant Any agent which inhibits the coagulation (clotting) of blood – e.g. EDTA, HEPARIN, sodium citrate, sodium oxalate. SPS is used in BLOOD CULTURE. homogeneous or heterogeneous population of antibodies (see MONOCLONAL ANTIBODIES and POLYCLONAL ANTISERUM). Antibody–antigen combination occurs by contact between specific antibody COMBINING SITES and antigenic DETERMINANTS, and may promote COMPLEMENT FIXATION. (See also VALENCY, COMPLEMENT-FIXING ANTIBODIES, and OPTIMAL PROPORTIONS.) In vivo, antibody–antigen combination may be beneficial if the antigenic stimulus derives from an invading pathogen (see e.g. OPSONIN), but can be harmful (see e.g. IMMEDIATE HYPERSENSITIVITY). (See also e.g. AGGLUTININ; BLOCKING ANTIBODIES; CYTOPHILIC ANTIBODIES; NATURAL ANTIBODIES; PRECIPITIN.) Antibodies may be identified, assayed etc in vitro by a range of techniques – see e.g. GEL DIFFUSION; IMMUNOASSAY; IMMUNOELECTROPHORESIS; IMMUNOFLUORESCENCE; PRECIPITIN TEST. Detection and quantification of specific antibodies in samples of SERUM are important in the diagnosis of many diseases. antibody absorption See ABSORPTION. antibody-dependent cell-mediated cytotoxicity See ADCC. antibody excess The presence of a high antibody-to-antigen ratio. In an agglutination test gross antibody excess may give rise to a PROZONE – possibly due to the saturation of antigenic determinants by antibodies in the absence of free antigen with which to form links. antibody formation An IMMUNE RESPONSE in which B LYMPHOCYTES synthesize and secrete ANTIBODY of a given antigenic specificity after immunological contact with the corresponding ANTIGEN; for so-called THYMUS-DEPENDENT ANTIGENS, B cells require help from antigen-specific CD4+ T LYMPHOCYTES (‘T helper cells’) and/or the products of such T cells (cf. THYMUSINDEPENDENT ANTIGENS). The response to thymus-independent antigens. Thymus-independent (TI) antigens (such as lipopolysaccharides and bacterial cell wall polysaccharides) induce antibody formation independently of T cells. When activated by TI antigens, certain B cells – CD5+ (‘B1’) cells (see B LYMPHOCYTE) and MARGINAL ZONE B CELLS – proliferate and secrete antibodies; however, the expanded clone of B cells does not give rise to memory cells (see later), and the antibodies formed in response to these antigens are almost exclusively of the IgM class. [T-independent immune response (new aspects of B cell biology): Science (2000) 290 89–92.] The response to thymus-dependent antigens. The process of antibody formation is incompletely understood; the following is a generally accepted scenario. To elicit antibodies, a thymus-dependent (TD) antigen must initially be taken up and processed, intracellularly, by an ANTIGEN-PRESENTING CELL (APC). Processing involves enzymic degradation of the antigen; a particular (peptide) fragment of the antigen is then linked to an MHC class II molecule (synthesized in the APC) and presented, at the surface of the APC, to an antigen-specific T cell. B lymphocytes can act as APCs, so that antigen (plus MHC class II molecule) may be presented at the surface of a B cell to an antigen-specific T cell; T cells of the Th2 subset (see T LYMPHOCYTE) characteristically act as ‘helper’ cells in this context. The (antigen-specific) T cell binds to the antigen with its T CELL RECEPTOR – thus physically linking B and T cells via the antigen; the MHC class II molecule (on the B cell) binds to the CD4 antigen on the T cell. Other ‘paired interactions’ also occur between binding sites on the B and T cells. Thus, certain receptors (B7.1, B7.2) on the B cell bind to antigen CD28 (q.v.) on the T cell; such binding permits the T cell to synthesize INTERLEUKIN-2 – a cytokine 42 antigenic formula The surface of a microorganism typically consists of repeating patterns of antigens, and the classification of some groups of microorganisms is based on differences between the antigens of different strains (see e.g. KAUFFMANN–WHITE CLASSIFICATION). (See also e.g. ALLERGEN; ANTIBODY; ANTIGENIC VARIATION; IDIOTYPE; IMMUNIZATION; THYMUS-DEPENDENT ANTIGENS.) antigen-binding cell (ABC) (1) Any cell which can bind only a specific antigen. (2) Any cell (including e.g. macrophages) which can bind various antigens. antigen deletion In a microorganism: the loss of particular antigenic determinant(s) due e.g. to a mutation or to the loss of a plasmid. (cf. ANTIGEN GAIN.) antigen excess The presence of a high antigen-to-antibody ratio. Under conditions of gross antigen excess precipitation does not occur since, according to the LATTICE HYPOTHESIS, all the combining sites of antibodies are bound to otherwise unattached antigens – leaving no combining sites free to form cross-links and hence larger (insoluble) complexes. (See also TYPE III REACTION.) antigen gain In a microorganism: the formation of new antigenic determinant(s). Antigen gain may be due e.g. to a MUTATION, to the acquisition of a PLASMID, or to BACTERIOPHAGE CONVERSION; thus, e.g., the sex factor in a conjugative plasmid specifies certain donor-specific antigens, while certain cell-surface antigens in Salmonella strains are present only when the cells are lysogenized by a particular phage. (cf. ANTIGEN DELETION.) antigen-presenting cell (APC) Any of a range of cells which bind antigen and present it to an antigen-specific T cell. APCs, which bear MHC class II cell-surface molecules (see HISTOCOMPATIBILITY RESTRICTION), include B LYMPHOCYTES, macrophages, dendritic cells, Langerhans’ cells and Kupffer cells. antigen-specific T helper cell A T LYMPHOCYTE which is specific for, i.e. activated by, a given, specific antigen; such a T cell may give COGNATE HELP to a B cell which has bound the same antigen (see ANTIBODY FORMATION). (See also CARRIER-SPECIFIC T HELPER CELL.) antigenic (1) Syn. immunogenic (see IMMUNOGENICITY). (2) Of or pertaining to an ANTIGEN or antigens. antigenic competition Inhibition of the immune response to a given antigen or DETERMINANT due to exposure to another (different) antigen or determinant; antigenic competition has been regarded as a form of natural immunosuppression. In intermolecular competition, suppression of the response to a given antigen may occur as the result of the previous administration of the other antigen (sequential competition) or of the simultaneous administration of the other antigen. By carefully adjusting (‘balancing’) the relative amounts of the antigens, antigenic competition may be reduced or abolished; this is important e.g. in the preparation of MIXED VACCINES. In intramolecular competition the response to a given determinant is inhibited by the presence of another determinant on the same macromolecule. For example, with IgG as antigen, good titres of anti-Fc may be produced but relatively few anti-Fab antibodies are formed – fewer than would be formed against isolated Fab fragments (i.e., relative to Fab, Fc is immunodominant). antigenic determinant See DETERMINANT. antigenic drift (immunological drift) In a given species, strain or type of microorganism: minor changes in antigenic specificity which occur over an extended period of time (e.g. years) – see e.g. INFLUENZAVIRUS. (cf. ANTIGENIC SHIFT; ANTIGENIC VARIATION.) antigenic formula Any symbolic notation which indicates the antigenic nature of an organism – see e.g. KAUFFMANN–WHITE CLASSIFICATION. anticodon See GENETIC CODE. anticomplementary (immunol.) Refers to any substance (e.g. ZYMOSAN), or to any system (e.g. pre-formed antigen–antibody complexes) other than the specific combination of antibody with antigen, which brings about COMPLEMENT FIXATION. If present in a COMPLEMENT-FIXATION TEST, anticomplementary factors can – by fixing complement – simulate the effects of antigen–antibody combination, giving a false-positive reaction. anti-downstream box See DOWNSTREAM BOX. antifol Syn. FOLIC ACID ANTAGONIST. antifungal agents Chemical agents (including certain ANTIBIOTICS) which either kill or inhibit the growth of fungi (fungicidal and fungistatic action, respectively). (Antifungal agents are often called ‘fungicides’ regardless of whether their action is fungicidal or fungistatic.) Antifungal agents are used e.g. for the treatment or prevention of fungal diseases of man, animals and plants, and as PRESERVATIVES for a wide range of materials. (a) Medical and veterinary antifungal agents (‘antimycotics’) are used for the treatment of local and systemic mycoses (see MYCOSIS); such agents include e.g. ALLYLAMINES, AZOLE ANTIFUNGAL AGENTS, CYCLOHEXIMIDE, FLUCYTOSINE, GRISEOFULVIN, NITROFURANS, POLYENE ANTIBIOTICS, SALICYLIC ACID, TOLNAFTATE. Many general DISINFECTANTS and ANTISEPTICS also have antifungal activity. (b) Agricultural antifungal agents (used against fungal diseases of crop plants) may function as ERADICANTS and/or as PROTECTANTS; they fall into two broad categories: contact (surface or non-systemic) antifungal agents, which adhere to plant surfaces but do not penetrate the plant tissues, and systemic antifungal agents, which are taken up by the plant and translocated within its tissues – usually in the transpiration stream (from roots to leaves). In general, a contact agent typically acts as a general enzyme inhibitor, while a systemic agent usually acts at a specific target site (process or structure) in the fungal cell. Fungi may rapidly acquire resistance to an agent which has a single target site, and continued use of such agents has led in many cases to the emergence and spread of resistant strains of fungal pathogens; the risk of emergence of resistant strains can be reduced by using mixtures containing two or more agents which have different modes of action. Agricultural antifungal agents include e.g. BENZIMIDAZOLES (a), BISDITHIOCARBAMATES, CARBOXIN, CHLORONEB, CHLORONITROBENZENES, copper sulphate (see COPPER), DICARBOXIMIDES, DICHLOFLUANID, DICHLONE, DINITROPHENOLS, DITHIOCARBAMATES, DODINE, ETHAZOLE, GLYODIN, HYDROXYPYRIMIDINE ANTIFUNGAL AGENTS, IMAZALIL, MORPHOLINE ANTIFUNGAL AGENTS, certain ORGANOPHOSPHORUS COMPOUNDS, PHENYLAMIDE ANTIFUNGAL AGENTS, PHTHALIMIDE ANTIFUNGAL AGENTS, PROCHLORAZ, PYRACARBOLID, QUINONE ANTIFUNGAL AGENTS, SULPHUR, THIRAM, TRIFORINE. (See also AUREOFUNGIN, GRISEOFULVIN and POLYOXINS.) antigen (ag) Any agent which can elicit an IMMUNE RESPONSE (cf. HAPTEN); ‘antigen’ may refer to an individual macromolecule or to a homogeneous or heterogeneous population of antigenic macromolecules. A given antigen usually contains more than one DETERMINANT (see also POLYCLONAL ANTISERUM). An antigen may be soluble (e.g. microbial toxins, extracts) or particulate. The most effective antigens are proteins and polysaccharides (see also Z-DNA); in general, large molecules are more antigenic than are smaller ones, and antigenicity is enhanced by the presence of an ADJUVANT. Minute quantities (e.g. a few µg) of antigen may be sufficient to elicit an immune response – the actual amount required depending e.g. on the route of administration. 43 antigenic shift antimalarial agents (‘antimalarials’) Drugs which are effective in the chemotherapy and/or chemoprophylaxis of MALARIA. Most of these drugs are effective against only one stage of the malaria parasite – e.g. chloroquine, mefloquine, quinine and QINGHAOSU are SCHIZONTICIDES. Primaquine and WR238605 are both active against hypnozoites; primaquine is also active against tissue schizonts (i.e. schizonts within liver cells), and WR238605 is also active against gametocytes. (See AMINOQUINOLINES, QINGHAOSU, QUININE, PYRIMETHAMINE and MALARIA for details of antimalarials.) Combinations of antimalarials are often used for treatment; this helps e.g. to limit the emergence of resistant strains of the parasite. antimetabolite A substance which competitively inhibits the utilization, by an organism, of an exogenous substrate or endogenous metabolite. (See e.g. SULPHONAMIDES.) antimony (as an antimicrobial agent) Antimony is a HEAVY METAL whose compounds have long been used as therapeutic agents – though their toxic effects were often greater than the benefits they bestowed. Antimonials have found use in the treatment of certain diseases of protozoal aetiology, e.g. leishmaniasis, and other diseases, e.g. schistosomiasis. It is believed that trivalent antimony is necessary for antimicrobial activity, and that pentavalent compounds owe their activity to in vivo conversion to the trivalent state. Antimonials have been shown to combine with essential sulphhydryl groups in trypanosomal enzymes involved in glucose metabolism; such inhibition may be reversed by thiol compounds, e.g. cysteine. Trivalent antimonials include e.g. potassium antimonyl tartrate (tartar emetic; 2[K(SbO).C4 H4 O6 ].H2 O) and stibophen (sodium antimony bis[pyrocatechol-3,5-disulphonate]). Pentavalent antimonials include e.g. stibanilic acid (p-aminobenzenestibonic acid) and its sodium salt (stibamine), both of which are constituents of a number of widely-used drugs – e.g. ethyl stibamine (neostibosan) which is used for the treatment of espundia and kala-azar. antimutator gene See MUTATOR GENE. antimycin A An antibiotic which acts as a RESPIRATORY INHIBITOR in Complex III of the mitochondrial ELECTRON TRANSPORT CHAIN, apparently blocking electron flow between b-type cytochromes and cyt c1 ; antimycin A and HOQNO appear to act at a common site which may be associated with the b-type cytochromes. antimycotic See ANTIFUNGAL AGENTS (a). antioxidant (food sci.) A substance added to certain foods to inhibit oxidation of the lipids in those foods – thus enhancing stability and prolonging shelf-life. (See also OXIDATIVE RANCIDITY.) Antioxidants currently used include e.g. ascorbic acid, butylated hydroxyanisole, butylated hydroxytoluene, carotene, propylgallate and tertiary butylhydroquinone. Some antioxidants have antimicrobial properties [CRM (1985) 12 153–183]. antiparallel (of DNA strands) See DNA. antiplectic metachrony See METACHRONAL WAVES. antiport (counter transport, exchange diffusion) The transmembrane transport of a solute (e.g. a given ion) coupled, directly, to the transmembrane transport of a different solute in the opposite direction. (cf. SYMPORT, UNIPORT; see also ION TRANSPORT.) antiprotozoal agents Chemical agents which kill or inhibit protozoa; they include e.g. AMINOQUINOLINES; antimonials (see ANTIMONY); arsenicals (see ARSENIC); certain DIAMIDINES; DILOX- antigenic shift In a given species, strain or type of microorganism: major change(s) in antigenic specificity which apparently occur abruptly. For example, antigenic shifts in strains of INFLUENZAVIRUS have been reported to coincide with pandemics of influenza. (cf. ANTIGENIC DRIFT; ANTIGENIC VARIATION.) antigenic variation Successive changes which occur in the surface antigens of certain microorganisms; in some cases, each alternative antigen results from the expression of a specific allele (see PHASE VARIATION), but in other cases (e.g. the fimbriae of Neisseria – see below), successive new antigens are created by repeated recombinational events. Antigenic variation may help a pathogen to evade the host’s immune defence system; thus, e.g. new antigenic forms of the pathogen which arise in the host may not be susceptible to OPSONIZATION by those antibodies which have developed in response to the previous antigenic forms. Antigenic variation occurs e.g. in Borrelia (see RELAPSING FEVER), INFLUENZAVIRUS, visna virus, Leishmania, Plasmodium and Trypanosoma spp (see VSG). The major structural subunit in the fimbriae of Neisseria gonorrhoeae (which are type IV FIMBRIAE) is encoded by the chromosomal gene pilE. The chromosome also contains copies of a gene, pilS, with which pilE can undergo repeated RECOMBINATION; moreover, recombination can also occur between pilE and any homologous DNA acquired through transformation. Recombinational events continually create new versions of the subunit gene, so that the fimbriae of this organism can be synthesized from any one of a very large number of variant forms of the subunit, giving rise to extensive antigenic variation [Mol. Microbiol. (1996) 21 433–440]. These fimbriae are also subject to phase variation (on/off switching). (The Opa adhesins of Neisseria undergo phase variation by DNA re-arrangements that alter reading frames in opa genes [TIM (1998) 6 489–495].) [An update of antigenic variation in African trypanosomes: TIP (2001) 17 338–343.] antigenicity The capacity to function as an ANTIGEN. (cf. IMMUNOGENICITY.) antiglobulin (Coombs’ reagent) Antibody homologous to the antigenic determinants of serum globulins, particularly IMMUNOGLOBULINS; antiglobulin–Ig combination can occur without involvement of the specific COMBINING SITES of the Ig molecule. antiglobulin consumption test (serol.) Any CONSUMPTION TEST in which the presence of cell- or particle-bound Ig is detected by the consumption of added ANTIGLOBULIN. For example, the in vitro combination of autoantibodies with homologous cellsurface antigens can be detected by washing the cells free of uncombined antibody and incubating them with antiglobulin; if autoantibody–antigen combination has occurred, antiglobulin will be consumed and may cause agglutination by the formation of cell–autoantibody–antiglobulin complexes. The amount of antiglobulin consumed can be determined e.g. by titrating the unconsumed antiglobulin against Ig-coated erythrocytes. antiglobulin test (serol.) Any test in which the presence of Ig (including BLOCKING ANTIBODIES, sense 1) is detected by exploiting the ability of ANTIGLOBULIN to agglutinate Ig-bound antigens. anti-idiotype Antiserum to the variable-region domains of an Ig molecule (cf. IDIOTYPE); it contains antibodies homologous to each of the individual determinants (IDIOTOPES), though the different types of antibody may differ quantitatively and in their AFFINITY for their respective determinants. antilymphocyte serum (ALS) Antiserum to the cell-surface antigens of LYMPHOCYTES; it is used e.g. for IMMUNOSUPPRESSION. Antilymphocyte globulin (ALG) is the globulin fraction of ALS. ANIDE; EMETINE; 8-HYDROXYQUINOLINE; METRONIDAZOLE; MONENSIN; NITROFURANS; PYRIMETHAMINE; SULPHONAMIDES; SURAMIN. (See also antimalarial agents under MALARIA.) [Review: Book ref. 153, pp. 95–132.] antiretroviral agents Any of a range of compounds which can inhibit the replication of retroviruses (i.e. viruses of the 44 Aphanizomenon 4° C for several hours (or 37° C for ca. 15 minutes) – to allow antibody–streptolysin O combination – a fixed volume of RBC suspension is added to each dilution, and incubation carried out at 37° C for 30–45 minutes. Complete absence of haemolysis in a given tube indicates that streptolysin O in that tube has been fully neutralized by antibodies. The ASOT titre (often expressed in TODD UNITS) is given by the highest dilution of serum which completely inhibits haemolysis. ASOT can also be carried out as a LATEX PARTICLE TEST using streptolysin O-coated particles. antitermination A process in which, during TRANSCRIPTION, the RNA polymerase fails to recognize a transcription termination signal and therefore continues transcription beyond the terminator. Antitermination may involve various mechanisms and can serve as a means for the regulation of gene expression (see e.g. BACTERIOPHAGE LAMBDA (gpN ) and OPERON (attenuator control)) and for the prevention of erroneous transcription termination (see e.g. RRNA). antitoxin (1) An antibody to a toxin. (2) An antiserum which contains antibodies to one or more particular toxins. antiviral agents Agents which inhibit the replication of viruses in cells, tissues or organisms – either directly (e.g. by inhibiting a specific viral enzyme) or by inducing the synthesis of INTERFERON. (See also TILORONE.) Owing to poor selective toxicity and/or the ready emergence of resistant strains of virus, relatively few compounds which exhibit antiviral activity in cell cultures are suitable for clinical use, and many of these are limited to topical treatment of localized infections. ANTIRETROVIRAL AGENTS (q.v.) are antiviral agents used e.g. in chemotherapy against AIDS. Examples of antiviral agents include: ACYCLOVIR, AMANTADINE, ANSAMYCINS, ARABINOSYL NUCLEOSIDES, ARILDONE, BROMOVINYLDEOXYURIDINE, CYCLARIDINE, DHBG, DHPA, DHPG, DIDEMNINS, ENVIROXIME, EPIPOLYTHIAPIPERAZINEDIONES, FAMCICLOVIR, GANCICLOVIR, IDOXURIDINE, LAMIVUDINE, MONENSIN, PENCICLOVIR, PHOSPHONOACETIC ACID, PHOSPHONOFORMIC ACID, RIBAVIRIN, SINEFUNGIN, THIOSEMICARBAZONES, TRIFLUOROTHYMIDINE, TUNICAMYCIN, VALACICLOVIR, ZANAMIVIR. [See also Book ref. 134.] AOAC Association of Official Analytical Chemists (of the USA). AOAC use-dilution test A CARRIER TEST (q.v.), devised by the AOAC, which determines the USE-DILUTION of a given disinfectant. To each of a number of dilutions of the disinfectant is added a (contaminated) small steel carrier; after immersion for 10 min, each carrier is transferred to broth to detect any surviving microorganisms. Sporicidal activity is determined by exposing the endospores of Bacillus subtilis ATCC 19659 or Clostridium sporogenes ATCC 9081 to the disinfectant and subsequently testing for viability. AP endonuclease See BASE EXCISION REPAIR. AP site An apyrimidinic or apurinic site in DNA (see ADAPTIVE RESPONSE and URACIL-DNA GLYCOSYLASE). 6-APA 6-AMINOPENICILLANIC ACID. ApaI See RESTRICTION ENDONUCLEASE (table). apalcillin A penicillin (see PENICILLINS) which is active against both Gram-positive and Gram-negative bacteria (including Pseudomonas aeruginosa) [AAC (1982) 21 906–911]. Apc APHIDICOLIN. aperture diaphragm See CONDENSER. APES See AAS. APH Aminoglycoside phosphotransferase (see AMINOGLYCOSIDE ANTIBIOTICS). Aphanizomenon A genus of filamentous CYANOBACTERIA; the cells are cylindrical with closely abutting ends, and the filaments RETROVIRIDAE); these agents are used e.g. in chemotherapy against HIV infection and AIDS. [BMJ (2001) 322 1410–1412.] The three main groups of antiretroviral agents are: (i) NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS (q.v. for examples); (ii) NON-NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS (delavirdine, efavirenz and nevirapine) [the emerging role of NNRTIs in antiretroviral therapy: Drugs (2001) 61 19–26]; (iii) PROTEASE INHIBITORS (amprenavir, indinavir, nelfinavir, ritonavir and saquinavir). The target for protease inhibitors is the viral protease (encoded by the pol region in HIV). Single-drug chemotherapy is prone to failure owing to the emergence of drug-resistant mutant viruses. (cf. HAART.) antisense RNA Any RNA that can recognize a specific sense sequence in RNA or DNA and affect the activity/expression of that sequence: see e.g: COLE1 PLASMID, FINOP SYSTEM and R1 PLASMID. MicroRNAs (miRNAs) are ∼20–25 nt RNAs that regulate genes post-transcriptionally by binding to, and blocking, mRNAs; they are cut (by an RNase-III-like enzyme: Dicer) from longer transcripts. Human miRNAs appear to bind preferentially to AT-rich 3 -untranslated regions of mRNAs [PNAS (2005) 102 15557–15562]. ssRNA can form triple-strand structures with dsDNA (cf. TRIPLEX DNA) and can also bind to ssDNA during transcription, in each case affecting transcription. Antisense RNA is also involved in RNA INTERFERENCE. Note that, in contrast to the above, sequence-specific RNAs mediate RdDM (RNA-directed DNA methylation), in plant and human cells, and RNA EDITING. antisense strand The non-CODING STRAND. antisepsis (‘degerming’) Prophylactic or therapeutic treatment of human or animal tissues with ANTISEPTICS. (cf. SEPSIS.) antiseptic paint ANTIBODY-containing secretions which bathe the mucous surfaces of the body. antiseptics Chemical agents used for treating human and animal tissues (particularly skin) with the object of killing or inhibiting pathogens. Antiseptics are typically non-injurious to living tissues (cf. DISINFECTANTS). For examples of antiseptics see under ACRIFLAVINE; ALCOHOLS; AMINOACRIDINE; BISPHENOLS; CHLORHEXIDINE; CHLORINE; IODINE; PHENOLS; QUATERNARY AMMONIUM COMPOUNDS; SALICYLANILIDES; SALICYLIC ACID; TRICLOSAN. antiserum (immune serum) SERUM containing antibodies to one or more particular antigens. anti-sigma factor (anti-s factor) Any molecule which can inhibit the normal activity of a given SIGMA FACTOR by binding to it. Examples of anti-sigma factors include e.g. (i) FlgM (see SIGMA FACTOR); (ii) DnaK, which inhibits s32 (see HEAT-SHOCK PROTEINS); and (iii) AsiA, a protein, encoded by bacteriophage T4, which is active during phage development in Escherichia coli and which can (inter alia) inhibit the E. coli s70 sigma factor. The Rsd protein (‘regulator of sigma D’; rsd gene product) can inhibit s70 in E. coli, and it has been proposed that Rsd is involved in the switching from s70 to sS during the transition from exponential growth to the stationary phase in this organism [JB (1999) 181 3768–3776]. [Anti-sigma factors: ARM (1998) 52 231–286.] antistreptolysin O test (ASLT; ASO test; ASOT) A test used for detecting/quantifying serum antibodies to STREPTOLYSIN o; it is of use e.g. in the differential diagnosis of RHEUMATIC FEVER and rheumatoid arthritis. In one method, INACTIVATED SERUM is serially diluted, and to each dilution is added a fixed volume of streptolysin O solution; after incubation at 45 Aphanoascus Coniophoraceae. Basidiocarp: characteristically resupinate; basidiospores: pigmented, with a cyanophilic spore wall. Genera: e.g. CONIOPHORA, Gyrodontium, and SERPULA. Corticiaceae. Basidiocarp: characteristically resupinate or effused-reflexed, with a hymenium that may be e.g. smooth or wrinkled. Genera: e.g. Botryobasidium, Christiansenia (see also MYCOPARASITE), Merulius, Mycoacia, Peniophora (see also HETEROBASIDION), Phanerochaete (see also WHITE ROT and LIGNIN), and Phlebia. Fistulinaceae. Basidiocarp: annual, pileate with a lateral stipe, fleshy and moist; the hymenophore consists of a layer of closely packed parallel tubules. Fistulina hepatica (the edible ‘beefsteak fungus’) causes BROWN OAK. It forms a fan-shaped or tongue-shaped basidiocarp whose upper surface is rusty brown, and whose lower (hymenial) surface is pale yellow when unbroken; the reddish monomitic context of the pileus exudes a red liquid when cut. Ganodermataceae. Basidiocarp: annual or perennial, typically fan-shaped and horizontal, either non-stipitate or borne on a lateral stipe; the context of the pileus is trimitic, and the underside of the pileus bears a porous hymenophore. Basidiospores: ovoid, typically flattened at one pole; ornamentations on the inner, brown layer of the spore wall penetrate deeply into the outer, hyaline layer of the spore wall. The organisms occur e.g. on felled timber; Ganoderma applanatum causes heart rots in various types of tree, infection occurring via wounds. Hydnaceae. Basidiocarp: typically mushroom-shaped, with a monomitic, non-xanthochroic context; the hymenophore consists of a layer of pendulous tooth-like processes (‘spines’ or ‘teeth’) on the underside of the pileus. Basidiospores: non-pigmented, spheroidal. Hydnum repandum (the ‘hedgehog fungus’) is an edible species which forms a thick, commonly cream-coloured pileus and a short, thick stipe which is often eccentrically located on the pileus; the teeth are often decurrent. Hymenochaetaceae. Basidiocarp: annual or perennial, ranging from resupinate to pileate or clavate according to species; hymenophore: poroid (in e.g. INONOTUS and Phellinus) to smooth (in Hymenochaete). Basidiospores: pigmented or colourless. The organisms are commonly lignicolous. Polyporaceae. Basidiocarp: annual or perennial, ranging (with species) from resupinate to pileate (either stipitate or sessile); the context is non-xanthochroic, and (according to species) may be mono-, di- or trimitic, and e.g. corky, leathery or woody. The hymenophore is typically porous, the hymenium being confined to the walls of the pores. (cf. LENTINUS, PLEUROTUS.) Basidiospores: colourless and usually smooth. The organisms grow on felled timber and on living trees. Genera: e.g. CORIOLUS, DAEDALEA, FOMES, Grifola, HETEROBASIDION, Irpex, Laetiporus, LENTINUS, Lenzites, PIPTOPORUS, PLEUROTUS, POLYPORUS, PORIA, Rigidoporus (see also BUTT ROT), TRAMETES and Tyromyces. Schizophyllaceae. Basidiocarp: fan-shaped or lobed; basidiospores: colourless and smooth. The organisms are characteristically lignicolous. Genera: e.g. SCHIZOPHYLLUM. Sparassidaceae. Basidiocarp: branching, flattened, monomitic lobes, the hymenium being on both sides of the erect lobes. Genera: e.g. SPARASSIS. Stereaceae. Basidiocarp: appressed, effused-reflexed or stipitate, typically dimitic; basidiospores: colourless and smooth. The organisms are characteristically lignicolous. Genera: e.g. Amylostereum (see also WOODWASP FUNGI), Chondrostereum (see also SILVER LEAF), Podoscypha, STEREUM. API system See MICROMETHODS. apical Terminal: at the tip or distal part of a structure. Antonym: basal. aggregate to form raft-like floating colonies. GAS VACUOLES are present, and heterocysts occur but are often sparse. Trichomes commonly terminate in tapering, colourless ‘hair cells’. A. flos-aquae is an important freshwater BLOOM-forming species in temperate regions; it may also occur in brackish waters. The filaments of this species typically form wispy, flakelike colonies, the formation of which, in culture at least, appears to depend on the presence of iron. Strains of A. flos-aquae produce toxins (‘aphantoxin’) which may apparently include e.g. SAXITOXIN and neosaxitoxin; ‘aphantoxin’ acts as a neuromuscular blocking agent, and is toxic for amphibia, fish, etc. Aphanoascus See GYMNOASCALES. Aphanocapsa See SYNECHOCYSTIS. Aphanochaete A genus of filamentous, prostrate, branched green algae (division CHLOROPHYTA) which grow as epiphytes on other freshwater algae. Some of the cells in the filaments bear unicellular ‘hairs’ with bulbous bases. Quadriflagellate zoospores or aplanospores are formed; sexual reproduction is oogamous. Aphanomyces A genus of predominantly aquatic fungi of the order SAPROLEGNIALES (q.v.). A species known as the ‘MG fungus’ has been reported to cause a mycotic granuloma of fish [Book ref. 1, pp. 195–197], while A. astaci causes CRAYFISH PLAGUE. Plant-pathogenic species include A. brassicae, A. camptostylus, A. cochlioides (on sugar-beet), A. euteiches (on peas) and A. raphani (on radish). aphanoplasmodium See MYXOMYCETES. Aphanothece A phycological genus of unicellular ‘blue-green algae’ (Chroococcales) apparently referable to Cyanothece (see SYNECHOCOCCUS). aphantoxin See APHANIZOMENON. aphid (bacteriocytes) See BUCHNERA. aphidicolin (Apc) A tetracyclic diterpenoid, isolated from ‘Cephalosporium aphidicola’, which – in eukaryotic cells – is a potent and specific inhibitor of DNA POLYMERASE a. Apc has no effect on bacterial DNA polymerases, but it inhibits growth and DNA synthesis in certain members of the ARCHAEA: e.g. halobacteria [FEMS (1984) 25 187–190] and methanogens [SAAM (1985) 6 111–118]. (cf. DDTTP.) aphotic zone That part of an aquatic habitat below the PHOTIC ZONE. aphthous fever Syn. FOOT AND MOUTH DISEASE. Aphthovirus A genus of viruses (family PICORNAVIRIDAE) which infect mainly cloven-hooved animals, causing FOOT AND MOUTH DISEASE; there are 7 serotypes and many antigenically distinct strains. Aphthovirus virions are rapidly inactivated at pH below ca. 6–7, but are appreciably resistant to desiccation; stability of the virions is generally increased by high ionic concentrations and low temperatures. Aphyllophorales (Polyporales) An order of terrestrial (typically humicolous or lignicolous), mainly saprotrophic fungi (subclass HOLOBASIDIOMYCETIDAE) which characteristically form tough (rather than fleshy), macroscopic, gymnocarpic fruiting bodies that may be e.g. crust-like, erect and coralloid, bracket-shaped (BRACKET FUNGI) etc; the hymenium may be borne on a smooth, toothed or irregular surface, and it may form the lining of tubular pores (in ‘polypores’) or the surface layer on clavate or coralloid basidiocarps or on lamellae. Constituent families [Book ref. 64, p.20] include e.g.: Clavariaceae. Basidiocarp: erect – simple and clavate (‘club fungi’), branched, or coralloid (‘coral fungi’) – and often brightly coloured (e.g. pink, yellow, violet); basidiospores: white or pale cream, typically non-amyloid. Genera: e.g. Clavaria and Typhula (see also SNOW MOULD). 46 apothecium Some bacteria can induce apoptosis in host cells. Thus, for example, on phagocytosis by a macrophage, Shigella can escape from the vacuole and then induce apoptosis in the phagocyte by means of a plasmid-encoded protein, IpaB. IpaB, secreted by a type III secretory system (see PROTEIN SECRETION), activates the host cell’s IL-1b-converting enzyme (ICE, see INTERLEUKIN1); activated ICE (i) initiates apoptosis in the macrophage, and (ii) activates the intracellular, inactive form of IL-1b. Activated IL-1b, when extracellular, may initiate an acute inflammatory response (involving TNF-a, IL-1, IL-6 and IL-8). In one model of shigellosis, these events have a positive role in promoting a pro-inflammatory response that helps to control the infection [TIM (1997) 5 201–204]. The Shigella example (above) involves an endogenous stimulus (via the IpaB protein). Exogenous stimuli include the binding of certain cytokines, or immune cells, to specific cell-surface receptors on target cells. Thus, apoptosis is one of the possible effects which may follow when tumour necrosis factor binds to its receptor (see TNF). Induction of apoptosis by CD8+ T cells may be possible in at least two different ways. One way involves interaction between cell-surface molecules on the T cell and target cell (see FAS). Alternatively, following contact between T cell and target cell, the T cell may release pore-forming lethal agents (perforins) – and also granzymes which are believed to activate caspases (see below) in the target cell. Whatever the stimulus that triggers apoptosis (in different types of cell under differing conditions), it appears that the mechanism of this process is dependent on certain intracellular cysteine proteases (caspases) that are synthesized as ZYMOGENS and activated specifically during apoptosis; these enzymes, which have a cysteine residue at the active site, cleave specific protein substrates at a site adjacent to an aspartic acid residue. The activity of a caspase on a substrate molecule may lead to either inactivation or (less commonly) activation of the given protein; for example, caspase-mediated activation of a certain nuclease (caspase-activated DNase, CAD) leads to fragmentation of genomic DNA and formation of the characteristic nucleosomal ‘ladder’. (CAD occurs in living cells, in inactive form, complexed with an inhibitory entity referred to as ICAD.) The activation of caspases appears to occur commonly by proteolytic cleavage of the zymogen form, and, in at least some cases, one caspase may be activated by an ‘upstream’ caspase in a caspase cascade. An understanding of the mechanism(s) involved in caspase activation is central to an appreciation of the process of apoptosis, and this area is being actively researched. [Programmed cell death (concept, mechanism and control): Biol. Rev. (1992) 67 287–319. Apoptosis (reviews on biochemistry and other aspects of the process): Nature (2000) 407 770–816.] aporepressor See OPERON. Aporpium See TREMELLALES. Apostomatida See HYPOSTOMATIA. aposymbiotic Refers to an organism (aposymbiont) which has lost symbionts it normally possesses. apothecioid Of an ASCOCARP: having, at maturity, a hymenium more or less open to the environment – as in a typical APOTHECIUM; apothecioid ascocarps may be e.g. cup-shaped, discoid (circular and flat), or lirelliform (see LIRELLA). apothecioid pseudothecium See ASCOSTROMA. apothecium A typically dish- or cup-shaped or discoid, sessile or stipitate ASCOCARP, the inner (or upper) surface of which is lined with a hymenial layer of asci (often interspersed with paraphyses); the main structural tissue of the apothecium is apical complex A collective term for the CONOID, MICRONEMES, POLAR RING and RHOPTRIES – some or all of which are found in members of the APICOMPLEXA, particularly in motile stages. (Some authors include the MICROPORES and/or the subpellicular tubules as part of the apical complex.) apical granule Syn. SPITZENKÖRPER. apical paraphysis See PARAPHYSIS. Apicomplexa A phylum of parasitic PROTOZOA with one type of nucleus, no cilia and no flagella (some have flagellated microgametes); components of the APICAL COMPLEX are present at some stage in the life cycle. Classes: PERKINSASIDA, SPOROZOASIDA. [Cytoskeleton: Microbiology and Molecular Biology Reviews (2002) 66 21–38.] apiculate Having a short, pointed projection (apiculus) at one end. Apiosoma See PERITRICHIA. Aplanochytrium See THRAUSTOCHYTRIDS. aplanogamete A non-motile GAMETE. aplanospore A non-motile spore. aplastic crisis (in parvovirus B19 infection) See ERYTHROVIRUS. aplysiatoxin See LYNGBYA. APO-1 See FAS. apo-activator See OPERON. apochlorotic Having lost chlorophyll – either during evolutionary development or during growth under certain conditions. Apochlorotic organisms may have lost either chloroplasts or chlorophyll; in the latter case a colourless plastid (leucoplast) may be formed. (cf. ACHLOROTIC; see also BLEACHING.) apochromat (apochromatic objective) An objective lens (see MICROSCOPY) in which chromatic aberration has been corrected for three colours (red, green, and blue), and spherical aberration has been corrected for two colours (red and blue). (cf. ACHROMAT.) A FLAT-FIELD OBJECTIVE LENS of this type is called a planapochromat. Apodachlya See LEPTOMITALES. apoenzyme The protein component of a PROSTHETIC GROUPrequiring enzyme. (cf. HOLOENZYME.) apokinetal stomatogenesis See STOMATOGENESIS. apomict See APOMIXIS. apomixis (microbiol.) In certain diploid organisms: a type of reproduction, considered to be a deviant form of the sexual process, in which meiosis, gamete formation and fertilization do not occur even though morphologically differentiated structures (e.g. spore-containing asci) are formed; progeny produced apomictically from a given cell (the apomict) are genetically identical to that cell. Thus, e.g., some apomictic strains of Saccharomyces cerevisiae form asci each containing two uninucleate diploid spores [Yeast (1985) 1 39–47]. (cf. AMIXIS.) apoplastidic Lacking a PLASTID. apoptosis In eukaryotic cells: cell death which occurs in a regulated (‘programmed’) manner: i.e. death resulting from an organized process involving an internal mechanism that characteristically includes fragmentation of the genome. Apoptosis is a natural process. For example, in vertebrates it occurs during embryogenesis, while in invertebrates it occurs during metamorphosis. Apoptosis can also be induced by physical and chemical factors – e.g. heat, radiation and agents such as glucocorticoids. Moreover, apoptosis can be induced by certain pathogens. Thus, e.g. in HIV-infected individuals (see AIDS), apoptosis may occur in CD4+ T cells which have bound viral protein. Again, poliovirus 2A protease is reported to induce apoptosis in human embryonic kidney epithelial cells [MCB (2000) 20 1271–1277]. 47 AP-PCR aprotinin A basic polypeptide which inhibits many serine PROTEASES, including e.g. trypsin, chymotrypsin, and some bacterial proteases. APS Adenosine-5 -phosphosulphate (also called adenosine-5 sulphatophosphate or adenylylsulphate). See ASSIMILATORY SULPHATE REDUCTION and DISSIMILATORY SULPHATE REDUCTION. called the excipulum, and the thin layer of hyphae between the hymenium and the excipulum is termed the hypothecium or subhymenium. Some apothecia are not cup-like or discoid: see e.g. MORCHELLA, PODETIUM and VERPA. Apothecia range from minute to 10 cm or more in diameter; those formed by e.g. species of Aleuria and Scutellinia are brightly coloured. (See also LECANORINE APOTHECIUM.) AP-PCR See ARBITRARILY PRIMED PCR. appertization A method of FOOD PRESERVATION in which certain types of food are exposed to a temperature/time regime which renders them safe for the consumer and microbiologically stable for extended, or indefinite, periods of time on subsequent storage in hermetically sealed containers at temperatures below 40° C; appertized foods are not necessarily sterile, and in certain cases (see CANNING) appertized foods must be kept under refrigeration. For foods which have no intrinsic properties (e.g. acidity) inhibitory to microbial growth and/or toxin production, appertization involves treatment which, at least, destroys the spores of pathogenic bacteria (particularly Clostridium botulinum); acidic foods (below ca. pH 4.5), and those containing appropriate concentrations of e.g. sodium nitrite, are appertized at somewhat lower temperatures. High temperature short time (HTST) appertization involves heat processing at high temperatures for periods ranging from ca. 2 sec to several minutes (cf. HTST PASTEURIZATION). In the ultra-high temperature (UHT) appertization of milk the process involves e.g. maintenance of a minimum temperature of ca. 140° C for at least 2 or 3 sec. (cf. RADAPPERTIZATION.) apple canker A CANKER of apple trees caused by Nectria galligena f. sp. mali; other trees – e.g. pear, poplar, hawthorn – may also be affected. Enlarged, sunken lesions develop on the branches; bark in the centre of a lesion dies and may break away. Infection occurs via wounds or leaf scars, and can be initiated by ascospores and by conidia. apple chlorotic leafspot virus See CLOSTEROVIRUSES. apple diseases See e.g. APPLE CANKER, APPLE SCAB, BROWN ROT (sense 2) and FIREBLIGHT. apple mosaic virus See ILARVIRUSES. apple scab An important disease of apple trees (Malus) caused by Venturia inaequalis (see VENTURIA). In spring, germinating ascospores infect via wounds or by means of appressoria. Small dark spots appear on leaves; on fruits, small dark velvety lesions enlarge to brown, corky scabs. Conidia, formed during late spring and summer, spread the disease. Control: antifungal sprays (see e.g. BENOMYL, DICHLONE, GLYODIN and TRIFORINE). (See also FIREBLIGHT.) apple stem grooving virus See CLOSTEROVIRUSES. App(NH)p (b,g-imido)ATP: a (biologically) non-hydrolysable analogue of ATP. appressed Lying close to, or flattened against, a surface. appressorium A specialized, flattened region of a hypha or germ tube formed by some plant-pathogenic fungi (e.g. Puccinia graminis, Venturia inaequalis); it aids infection by adhering closely to the surface of a host cell and producing a small outgrowth of fungal tissue (infection peg) which penetrates the host cell wall. A plant may respond to an infection peg by depositing a small region of wall-like material (a papilla) near the site of attempted penetration; such a papilla can sometimes prevent infection. apramycin An AMINOGLYCOSIDE ANTIBIOTIC which is resistant to many of the enzymes which inactivate antibiotics of this group; it is inactivated by e.g. chromosome- and plasmid-specified acetyltransferases [JGM (1984) 130 473–482]. NH2 N O O 5′ O N S O − O O P N CH2 O − O N 1′ 4′ H H H H 3′ 2′ OH OH APS (adenosine 5 -phosphosulphate) APT agar An agar-based medium containing tryptone, yeast extract, glucose, Tween 80, citrate, and various inorganic salts. [Recipe: Book ref. 46, p. 1661.] It is used for isolating organisms such as lactobacilli and Brochothrix from e.g. meat and meat products. APT paper BLOTTING paper impregnated with 2-aminophenylthioether; before use, APT is chemically modified to its reactive diazo derivative, and is then able to bind (covalently) ssDNA, RNA and/or protein via the diazonium group. APW Alkaline peptone water: 1% peptone and 1% NaCl in distilled water, pH adjusted to 8.6–9.0, used e.g. for isolating Vibrio cholerae. ApxIA, ApxIIA (of Actinobacillus pleuropneumoniae) See RTX TOXINS. AqpZ See MIP CHANNEL. Aqualinderella See LEPTOMITALES. aquaporin See MIP CHANNEL. Aquaspirillum A phylogenetically heterogeneous genus of Gram-negative, asporogenous bacteria formerly included in the genus SPIRILLUM. Cells: fairly rigid, generally helical (0.2–1.4 × ca. 2 − >30 µm, according to species); A delicatum cells are mainly vibrioid, and those of A. fasciculus are straight rods. S LAYERS, intracellular PHB granules, and POLAR MEMBRANES are commonly present; COCCOID BODIES are commonly formed. Motile, typically with bipolar tufts of flagella (A. polymorphum has a single flagellum at each pole, A. delicatum has one or two flagella at one pole only). Cells swim in a straight line with a characteristic corkscrew-like motion; A fasciculus swims effectively only in media of high viscosity. Metabolism is respiratory; species are usually aerobic or microaerophilic, but some can carry out NITRATE RESPIRATION under anaerobic conditions. Carbon sources: amino acids or salts of organic acids, but usually not carbohydrates; A. autotrophicum can grow autolithotrophically with CO2 , H2 and O2 . Nitrogen source: NH4 + ; A. peregrinum, A. fasciculus and strains of A. itersonii can fix N2 under anaerobic conditions. Aquaspirilla are typically oxidase +ve; catalase +ve; usually phosphatase +ve; indole −ve; arylsulphatase −ve. No growth with 3% NaCl. Optimum growth temperature: usually ca. 30–32° C (ca. 20° C for A. psychrophilum, 41° C for A. bengal ). GC%: 49–66. Type species: A serpens. About 17 48 arbitrarily primed PCR nucleosides probably must be converted to the corresponding nucleoside 5 -triphosphate for activity. (See e.g. ARA-T; CYTARABINE; FIAC; FMAU; VIDARABINE.) arabinoxylans Heteroglycans containing both xylose and arabinose (cf. XYLANS). arabitol A PENTITOL formed e.g. by the reduction of ARABINOSE. It occurs, often with MANNITOL, in various basidiomycetes and ascomycetes. ara-C Syn. CYTARABINE. araC gene See ARA OPERON. arachidonic acid In mammals: an essential C20 straight-chain fatty acid – CH3 (CH2 )3 (CH2 CH=CH)4 (CH2 )3 COOH; it occurs e.g. as a component of cell membrane phospholipids, and (bound to protein) in blood. Free arachidonic acid is formed e.g. during the activation of lymphocytes, and is a precursor of e.g. LEUKOTRIENES, PROSTACYCLIN, PROSTAGLANDINS and thromboxanes. Arachnia A genus of anaerobic (but aerotolerant), catalasenegative, asporogenous bacteria (order ACTINOMYCETALES, wall type I). The organisms grow as a substrate mycelium which fragments into branched filaments and irregularly-shaped rods; aerial hyphae are not formed. Metabolism: fermentative, fermentation of glucose yielding primarily acetic and propionic acids; growth occurs readily in vitro on rich media at 37° C. A. propionica, the type species (GC%: 63–65), is pathogenic in man and other animals; it is a common causal agent of LACHRYMAL CANALICULITIS. arachnoid Resembling a spider’s web. Arachnoidiscus See DIATOMS. Arachnula See GRANULORETICULOSEA. araE gene See ARA OPERON. araFG operon See ARA OPERON. aranotin See EPIPOLYTHIAPIPERAZINEDIONES. ara-T (1-b-D-arabinofuranosylthymine) An ARABINOSYL NUCLEOSIDE isolated from a marine sponge (Cryptoethya crypta). It has antiviral activity against herpes simplex viruses (HSV-1 and -2), varicella-zoster virus and vaccinia virus, and is relatively nontoxic to animal cells. arbitrarily primed PCR Certain TYPING methods in which PCR is used with primers of arbitrary (i.e. random) sequence to copy discrete sequences of chromosomal DNA. Only one type of primer is used (cf. standard PCR protocol), and during the annealing stage the primers bind to various ‘best-fit’ sequences on the strands of DNA. As the primer is of arbitrary sequence it is not possible to predict which chromosomal sequences will be copied; however, results are reproducible if the method is carefully standardized – including not only the primer sequence but also e.g. the use of a specific type of DNA polymerase [NAR (1993) 21 4647–4648] and a particular procedure for preparing the sample DNA [NAR (1994) 22 1921–1922]. Various guidelines have been recommended for promoting reliability and reproducibility in arbitrarily primed PCR [JCM (1997) 35 339–346]. The method is outlined in the figure. Arbitrarily primed PCR (AP-PCR) was first published as a typing procedure in 1990 [NAR (1990) 18 7213–7218; NAR (1990) 18 6531–6535]. Another typing method – RAPD (q.v.) – is based on the same principle; it generally employs shorter primers (∼10-mer) compared with those used in AP-PCR (>20-mer), but, even so, some authors regard the two methods as identical. Typing methods based on this principle have two main advantages. First, the entire chromosome can participate in species are recognized on the basis of morphology, nutrition and GC%. Aquaspirilla occur in a wide range of freshwater habitats – e.g., stagnant waters rich in organic matter; none appears to be pathogenic. [Book ref. 22, pp. 72–90. Media, culture etc: Book ref. 45, pp. 596–608.] (See also MAGNETOTACTIC BACTERIA; cf. AZOSPIRILLUM; OCEANOSPIRILLUM; SPIRILLUM; SPOROSPIRILLUM.) aquifer See WATER SUPPLIES. ara operon (arabinose operon) An OPERON concerned with the metabolism of ARABINOSE; when used without qualification, the term ‘ara operon’ usually refers to the araBAD operon, but in e.g. Escherichia coli three distinct genetic loci are involved in the uptake and metabolism of L-arabinose: araBAD (at ca. 1 min on the chromosome map), araFG (at ca. 45 min) and araE (at ca. 61 min). The araBAD operon encodes enzymes for the conversion of L-arabinose to D-xylulose 5-phosphate; the araA gene encodes L-arabinose ketol-isomerase, araB encodes L-ribulokinase, araD encodes ribulose 5-phosphate 4-epimerase. The araFG operon specifies a high-affinity BINDING PROTEINDEPENDENT TRANSPORT SYSTEM for L-arabinose; araF encodes the binding protein, araG the membrane component. The araE gene specifies a protein involved in an independent low-affinity transport system. All three operons are controlled by the product of the regulator gene araC which is located next to araBAD but transcribed in the opposite direction. (See also REGULON.) In the absence of arabinose, the AraC protein behaves as a repressor of the araBAD operon, binding to the OPERATOR and preventing transcription (negative promoter control: see OPERON). However, when arabinose is present it converts the AraC protein into an activator which binds to an initiator (araI ) to promote araBAD transcription (positive promoter control). (The araC gene is itself subject to autogenous regulation, i.e., the AraC protein represses its own synthesis.) Thus, expression of the araBAD operon depends on the relative amounts of repressor and activator, which in turn depend on the level of arabinose (the co-activator). The ara regulon is also subject to CATABOLITE REPRESSION. ara-A Syn. VIDARABINE. araBAD operon See ARA OPERON. Arabidopsis thaliana A flowering brassica widely used as a model plant for research. [Genome sequence and other data: Nature (2000) 408 791–826.] (See also CAMALEXIN.) arabinogalactans See PECTIC POLYSACCHARIDES. (See also WAX D.) arabinose An aldopentose (see PENTOSES) which occurs e.g. as a component of hemicelluloses (see e.g. XYLANS) and PECTIC POLYSACCHARIDES, and in the cell walls of certain bacteria (e.g. Nocardia spp). Arabinose can be metabolized by various bacteria, including e.g. Escherichia coli and strains of Salmonella, Bacillus, and lactic acid bacteria [Appendix III(d)]. Arabinose is heat-labile; solutions should be sterilized by filtration. (See also ARA OPERON.) arabinose operon See ARA OPERON. arabinosyl nucleosides A group of compounds which inhibit various enzymes – including DNA polymerases and reverse transcriptase – by acting as analogues of biological nucleosides and nucleotides. Arabinosyl nucleosides inhibit DNA synthesis in a variety of eukaryotic cells, and have been used as antitumour and ANTIVIRAL AGENTS; in general, viral replication is more sensitive to these agents than are the host cells. Arabinosyl 49 arboricolous darker with age. The cell contains two nuclei. Slender lobopodia are extended through a ventral aperture and are used chiefly for feeding. A gas-filled vacuole may be formed in the cytoplasm and apparently aids in the orientation of the organism. A. dentata has a test bearing lateral spines, giving it a stellate appearance when viewed from above. Arcella spp occur among vegetation in ponds, in soil, etc. Arcellinida An order of freshwater and soil amoebae (class LOBOSEA) in which the cell is surrounded by a TEST which may be primarily organic or primarily inorganic in composition, and which is perforated by a single aperture through which pseudopodia can be extended. The cells commonly contain two or more nuclei. Reproduction occurs asexually e.g. by binary fission. Genera include e.g. ARCELLA, Centropyxis, COCHLIOPODIUM, DIFFLUGIA. Archaea A DOMAIN of prokaryotic organisms which are evolutionarily distinct from members of the domain BACTERIA [see e.g. JB (1994) 176 1–6]. These organisms were formerly classified in the kingdom Archaebacteria, being distinguished from other prokaryotes (kingdom Eubacteria) on the basis of e.g. nucleotide sequences in 16S rRNA; later, the distinction between these two groups of prokaryotes was deemed to be more fundamental than previously supposed, and each kingdom was elevated to the taxonomic rank of domain. Many archaeans are associated with environments characterized by e.g. high temperatures or high salinity, and such organisms have been referred to as extremophiles. Nevertheless, some archaeans are found in ‘moderate’ environments such as soil [e.g. PNAS (1997) 94 277–282]. [Archaeal dominance in the mesopelagic zone of the Pacific Ocean: Nature (2001) 409 507–510.] The domain Archaea has been divided into the kingdoms Euryarchaeota and Crenarchaeota. Euryarchaeota includes halophiles (e.g. HALOBACTERIUM) and METHANOGENS. Crenarchaeota includes ‘sulphur-dependent’ species such as DESULFUROCOCCUS and SULFOLOBUS. Archaeans differ from bacteria in many ways. For example, gene expression (transcription and translation) appears to be closer to the eukaryotic pattern than the prokaryotic pattern [TIM (1998) 6 222–228]. The archaeal DNA-dependent RNA POLYMERASE resembles more closely the eukaryotic (nuclear) polymerase than the bacterial enzyme. Elongation factors in protein synthesis are (unlike their bacterial counterparts) sensitive to DIPHTHERIA TOXIN. [Assembly of the archaeal signal recognition particle from recombinant components: NAR (2000) 28 1365–1373.] [Replication origin of archaeans: TIBS (2000) 25 521–523.] At least some archaeal enzymes appear to have unique structures [TIM (1998) 6 307–314]. In some archaeans the cell wall may consist mainly or solely of an S LAYER (q.v.) closely associated with the cytoplasmic membrane. The cell wall in some species contains a peptidoglycan-like polymer, PSEUDOMUREIN (which is not cleaved by the enzyme LYSOZYME). The cytoplasmic membrane contains lipids of a type which do not occur in species of Bacteria. In contrast to the esterlinked bacterial lipids, archaeal lipids are characteristically etherlinked molecules that contain e.g. isoprenoid or hydro-isoprenoid components. Certain archaeal and bacterial lipids are structurally analogous – e.g. the di-ether and di-ester lipids, both types of molecule having a single polar end. However, the archaeal lipids also include other types – such as tetra-O-di(biphytanyl) diglycerol – which contain two ether-linked glycerol residues, ARBITRARILY PRIMED PCR (principle, diagrammatic). In the diagram, the two strands of chromosomal DNA (which have been separated by heat) are shown as long parallel lines. During the annealing (primer-binding) stage of PCR, copies of the arbitrary primer bind at a number of ‘best-fit’ sequences, on both strands of DNA, under low-stringency conditions. In some cases (by chance), two primers will bind, on opposite strands, a few hundred bases apart; if strand elongation can occur efficiently from these two primers, and if elongation is time-limited, two short strands of DNA will be produced. In the diagram (right-hand side), two primers (short lines) have bound, close together, on opposite strands; two more primers (left-hand side) have bound, further apart, on opposite strands. Strand elongation from each primer (dashed line) has produced the four fragments shown. (Note that a fragment synthesized on one strand contains a copy of the best-fit sequence of the other strand.) Another cycle of low-stringency PCR produces more copies of the fragments. Subsequently, many cycles of PCR are carried out under higher stringency (using the same primer). Under these conditions, primers bind to best-fit sequences (rather than elsewhere) on the fragments formed by low-stringency PCR – although (due to the higher stringency), primers may not bind to best-fit sequences on all of the fragments, so that only a proportion of the fragments formed under low stringency may be amplified under higher stringency. The length of the fragments formed under higher stringency is shown by the distance between each pair of arrowheads. On electrophoresis, the fragments from a given strain form a characteristic pattern of bands (the fingerprint). Reproduced from Bacteria 5th edition, Figure 16.4(c), page 434, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. the comparison of strains. Second, no prior knowledge of the genome’s sequence is required, so that, potentially, any isolate may be typed. arboricolous Living/growing on trees. arboviruses ‘Arthropod-borne viruses’: a non-taxonomic category of viruses which can replicate in both vertebrate hosts and arthropod vectors; arboviruses include e.g. members of the families ARENAVIRIDAE, BUNYAVIRIDAE, Reoviridae (ORBIVIRUS), Togaviridae (ALPHAVIRUS), and FLAVIVIRIDAE. [Vector competence of mosquitoes for arboviruses: ARE (1983) 28 229–262.] arbuscule See MYCORRHIZA. ARC (AIDS-related complex) See AIDS. Arcanobacterium A genus of asporogenous bacteria (order ACTINOMYCETALES, wall type VI). In culture the organisms initially grow as irregular rods which may later become granular and segmented or may give rise to coryneform cells; growth occurs anaerobically or aerobically and is enhanced by blood or serum or by increased partial pressures of carbon dioxide. Growth is poor on tellurite media. Type species: A. haemolyticum (formerly Corynebacterium haemolyticum [JGM (1982) 128 1279–1281]). Arcella A genus of testate amoebae (order ARCELLINIDA) in which the test is predominantly or entirely organic. In A. vulgaris the test is more or less spherical (ca. 30–100 µm diam.) and is typically somewhat translucent in young cells, often becoming 50 arginine deiminase arenaceous Resembling or (referring e.g. to the test of DIFFLUGIA) incorporating or bearing grains of sand. Arenaviridae (arenavirus group) A family of enveloped ssRNA viruses. One genus: Arenavirus; type species: lymphocytic choriomeningitis virus (LCMV). LCMV has a worldwide distribution; other arenaviruses are restricted to, and named after, particular geographical areas: Lassa virus, Mozambique (= Mopeia) virus (probable member), and the ‘New World’ arenaviruses (the Tacaribe complex) – Amapari (Brazil), Junı́n (Argentina), Latino and Machupo (Bolivia), Parana (Paraguay), Pichinde (Columbia), Tacaribe (Trinidad), and Tamiami (Florida, USA) viruses. In the natural host (usually a single rodent species) arenavirus infection is persistent and silent (see PERSISTENCE); transmission occurs vertically and horizontally, apparently without involving vectors. Only LCMV, Lassa, Junı́n and Machupo viruses cause significant human disease (see LASSA FEVER, LCM VIRUS, VIRAL HAEMORRHAGIC FEVERS). The arenavirus virion is spherical or pleomorphic, diam. ca. 50–300 nm (average: 110–130 nm). It consists of a lipoprotein envelope enclosing a core of viral ribonucleoprotein (RNP) and several host ribosomes (each 20–25 nm diam.). The envelope is derived from the plasma membrane of the host cell and bears club-shaped, apparently hollow surface projections 5–10 nm long [JGV (1983) 64 2157–2167]. There are two types of viral ssRNA, designated L (31–34S, MWt 2.1–3.2 × 106 ) and S (22–25S, MWt 1.1–1.6 × 106 ); the viral RNAs are complexed with the major structural protein of the virion (N protein, ‘p63’) to form a filamentous RNP nucleocapsid which has a beaded appearance in the electron microscope and which forms noncovalently closed, supercoiled circles of various sizes [JGV (1983) 64 833–842]. The viral RNA is usually described as negative-sense, but certain viral proteins are apparently encoded by +ve-sense sequences in the genome – see AMBISENSE RNA; thus, e.g., the viral N protein is encoded by a sequence complementary to the 3 -half of the S RNA, while the viral glycoproteins are synthesized from a sequence corresponding to the 5 -half of the S RNA. A non-glycosylated protein with RNA-dependent RNA polymerase activity has been found in virions of Pichinde virus. Arenavirus virions can be inactivated by temperatures > 56° C, by pH <5.5 or >8.5, and by organic solvents or detergents. Arenaviruses can replicate in a wide range of mammalian cell cultures (e.g. BHK-21, Vero and L cells). Virus replication occurs in the cell cytoplasm; the virus matures by budding through the plasma membrane, when ribosomes become incorporated in the virion. Arenavirus See ARENAVIRIDAE. areolate Divided up into small areas (areolae). (Used e.g. of a lichen thallus: see e.g. RHIZOCARPON.) arg-poly(asp) Syn. CYANOPHYCIN. arg regulon See ARGININE BIOSYNTHESIS. Argentinian haemorrhagic fever A VIRAL HAEMORRHAGIC FEVER caused by the Junı́n virus (see ARENAVIRIDAE). Mortality: usually ca. 3–15%. argentophilic Staining well with SILVER STAINS. L-arginine biosynthesis See Appendix IV(a). In Escherichia coli the genes encoding enzymes for arginine biosynthesis constitute a REGULON (the arg regulon); four of the genes occur in a cluster (argECBH ), the remainder occur singly at loci scattered around the chromosome. [Biosynthesis and metabolism of arginine in bacteria: MR (1986) 50 314–352.] arginine decarboxylase test See DECARBOXYLASE TESTS. arginine deiminase See ARGININE DIHYDROLASE. one at each end of the molecule; such molecules, which have two polar ends, may span the width of the cytoplasmic membrane. [Protein translocation across the archaeal cytoplasmic membrane: FEMS Reviews (2004) 28 3–24.] The archaeal flagellum is markedly different in composition, structure and apparent mode of assembly from that found in bacteria. For example, the subunit, flagellin, is typically glycosylated, and it contains a signal sequence (see SIGNAL HYPOTHESIS) – suggesting passage into/through the cytoplasmic membrane (cf. bacterial flagellin, which passes through the hollow structures of the developing flagellum); this latter feature (as well as certain similarities between archaeal flagellins and bacterial type 4 fimbriae) suggests that archaeal flagella assemble from the base (in contrast to ‘tip growth’ in bacteria) [JB (1996) 178 5057–5065]. The archaeal flagellar filament is typically much thinner than the bacterial filament. Various similar or analogous proteins have been found in archaeans and bacteria. For example, the archaeal RadA protein is apparently analogous to the RecA protein in bacteria [GD (1998) 12 1248–1253]. Again, an FtsZ protein occurs in members of both domains, suggesting that the cell division apparatus was similar in a common ancestor [Mol. Microbiol. (1996) 21 313–319]. [The CELL CYCLE in archaeans: Mol. Microbiol. (2003) 48 599–604.] archaean A member of the domain ARCHAEA. The term is also spelt ‘archaeon’. Archaebacteria A kingdom (now obsolete) which included all those prokaryotes not classified in the kingdom EUBACTERIA. [Phylogeny of the Archaebacteria: SAAM (1985) 6 251–256.] Some archaebacteria were placed in a separate taxon, Eocyta, on the basis of ribosomal characteristics [PNAS (1984) 81 3786–3790]. (See also EOCYTES.) Members of the Archaebacteria are now included in the domain ARCHAEA. Archaeobacteria A proposed class of prokaryotes (see MENDOSICUTES) corresponding to the (later) kingdom ARCHAEBACTERIA (now also obsolete). Archangium See MYXOBACTERALES. archicarp In ascomycetes: the cell(s) which give rise to a fruiting body or to a part of it. archigregarines See GREGARINASINA. Arcobacter A genus of Gram-negative, asporogenous bacteria of the family Campylobacteraceae. Cells: spiral rods with unsheathed flagella. Catalase +ve. Nitrate is reduced. Typically urease −ve. These organisms resemble Campylobacter spp but differ e.g. in their ability to grow in air at 15–25° C. A. butzleri and A. cryophilus have been isolated from patients with diarrhoea, including children in the developing countries; most strains do not hydrolyse hippurate but do hydrolyse indoxyl acetate. arcuate Curved like a bow; arched. Arcyria A genus of slime moulds (class MYXOMYCETES) which form stalked, globose to cylindrical sporangia; the peridium is evanescent, and the spores in masses may be yellow, pinkish, red, etc. A. cinerea is common on dead wood and humus. ARD Acute respiratory disease: a general term for any such disease affecting closed populations of people (e.g. military recruits, school children). Major causal agents of ARDs are adenoviruses (usually Ad3, Ad4, Ad7, Ad14, or Ad21 – see MASTADENOVIRUS); an incubation period of ca. 5–7 days is followed by fever, chills, headache, malaise and coryza, but the disease is usually mild and self-limiting. A live vaccine, administered orally in enteric-coated capsules, is widely used for preventing adenoviral ARD in military recruits. 51 arginine dihydrolase arginine dihydrolase (ADH) An enzyme system which catalyses the catabolism of arginine, with a concomitant substrate-level phosphorylation; the system occurs in a range of bacteria. (i) Arginine deiminase hydrolyses arginine to citrulline and ammonia. (ii) Ornithine carbamoyltransferase catalyses a phosphorolytic cleavage of citrulline to form carbamoyl phosphate and ornithine. (iii) Carbamate kinase transfers the phosphate group of carbamoyl phosphate to ADP to form ATP and carbamic acid, the latter dissociating spontaneously to CO2 and NH3 . Tests for ADH production are widely used in the identification of certain bacteria (e.g. members of the ENTEROBACTERIACEAE). Typically, the organism is grown in a medium containing Larginine and a pH indicator; the presence of ADH is indicated by an alkaline reaction after a few days’ incubation. [Methods: Book ref. 2, pp. 411–412.] arg-poly(asp) Syn. CYANOPHYCIN. argT gene See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. Argyn See SILVER. Argyrol See SILVER. argyrome Syn. SILVER LINE SYSTEM. argyrophilic Syn. ARGENTOPHILIC. arildone (4-[6-(2-chloro-4-methoxyphenoxy)-hexyl]-3,5-heptanedione) An ANTIVIRAL AGENT which is active against various DNA and RNA viruses in vitro; it blocks uncoating in polioviruses and apparently also in herpes simplex viruses. Arildone may be useful clinically e.g. in the topical treatment of HERPES SIMPLEX infections. Arizona See SALMONELLA. arizonosis A POULTRY DISEASE, caused by Salmonella arizonae (= Arizona hinshawii), characterized by malaise, diarrhoea, and often symptoms of CNS involvement. Arkansas bee virus See NODAVIRIDAE. ArlS–ArlR See TWO-COMPONENT REGULATORY SYSTEM. Armillaria (‘Armillariella’) A genus of mainly lignicolous fungi (AGARICALES, Tricholomataceae). A. mellea (the ‘honey fungus’) grows saprotrophically on many types of wood, forms a symbiotic association with certain orchids (see MYCORRHIZA), and is parasitic on a range of deciduous and coniferous trees, on certain shrubs, and on some herbaceous plants. The fruiting body is highly variable in colour and appearance, the pileus being convex to flat (ca. 3–15 cm diam.), ochre to brown, with darker scales particularly near the centre; the upper part of the stipe, which often bears a wide annulus, is initially whitish, later reddish-brown. Basidiospores: ca. 8–9 × 5–6 µm. The fruiting bodies (which exhibit BIOLUMINESCENCE) typically occur in clusters. The organism spreads by means of tough, black RHIZOMORPHS (‘boot laces’) which may be found e.g. under the bark of infected trees or in soil. (See also CARBON DISULPHIDE and THREITOL.) Armillariella See ARMILLARIA. armoured dinoflagellates See DINOFLAGELLATES. aroA gene See STAPHYLOCOCCUS (S. aureus). arogenate See AROMATIC AMINO ACID BIOSYNTHESIS. aroH gene See TRP OPERON. aroma bacteria See DIACETYL. aromatic amino acid biosynthesis Phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp) are synthesized via the shikimate pathway: Appendix IV(f). Although most of the steps in this pathway are apparently more or less universal, the control mechanisms and physical organization of the enzymes (e.g. the formation of multienzyme complexes) vary between species and may be useful taxonomic criteria [CRM (1982) 9 227–252]. The reactions by which prephenate is converted to Phe and Tyr also differ in different species. Thus, e.g., Escherichia coli and Bacillus subtilis synthesize Tyr via 4-hydroxyphenylpyruvate (HPP), and Phe via phenylpyruvate (PPy) [see Appendix IV(f)]. Certain cyanobacteria and ‘coryneforms’ (Corynebacterium glutamicum, Brevibacterium spp) synthesize Phe via PPy but lack prephenate dehydrogenase and synthesize Tyr via arogenate (= ‘pretyrosine’). Pseudomonas diminuta synthesizes Phe via arogenate but Tyr via HPP, while P. aeruginosa can synthesize Phe and Tyr via PPy and HPP, respectively, and also via arogenate. Euglena gracilis synthesizes both Phe and Tyr via arogenate only. (See also TRP OPERON.) aromatic hydrocarbons See HYDROCARBONS. array (DNA) See DNA CHIP. Arrhenatherum blue dwarf virus See FIJIVIRUS. Arrhenius effect See staphylococcal a-HAEMOLYSIN. Arrhenosphaera See ASCOSPHAERALES. ARS Autonomously replicating sequence: a DNA sequence (first described from the yeast Saccharomyces cerevisiae) which, when linked to a non-replicative DNA fragment, promotes the capacity for autonomous intracellular replication. ARSs occur in the yeast genome with a frequency of about one ARS for every ∼40 kilobases. Efficient replication of ARS-containing DNA fragments may require other factors such as the CEN (centromere) element and the minichromosome maintenance protein 1. Many of the ARSs appear to function as origins of replication, although some are so-called silent origins; some of the ARSs (both active and silent) are reported to act as transcription silencers. Most of the studies on ARSs have been carried out on eukaryotes–particularly on yeasts (e.g. Saccharomyces, Hansenula). However, ARSs have also been reported in prokaryotes [see e.g. JB (2003) 185 5959–5966]. arsenate respiration See SELENATE RESPIRATION. arsenic (a) (as an antimicrobial agent) The aromatic compounds of arsenic include some effective antimicrobial agents, some of which have found use in chemotherapy; however, some of these ‘arsenicals’ have been discontinued owing to toxicity. The antimicrobial activity of arsenicals appears to involve their reaction with thiol (−SH) groups within cells – resulting e.g. in the inhibition of many enzymes; LIPOIC ACID is particularly sensitive because arsenic can bridge the two thiol groups in this coenzyme. Apparently, pentavalent arsenic in an arsenical must be converted, in vivo, to the trivalent state before the arsenical can act as an antimicrobial agent. The selective activity of arsenicals is believed to be due to differences in the permeability of different types of cell. Microorganisms can be protected from arsenicals by e.g. thiols, p-aminobenzoic acid or quinoid dyes. Atoxyl (NH2 .C6 H4 .As(OH)2 O; p-aminophenylarsonic acid) was the first arsenical to be used against trypanosomiasis. Salvarsan (3,3 -diamino-4,4 -dihydroxyarsenobenzene) was formerly used e.g. for the treatment of syphilis and trypanosomiasis; it is oxidized in vivo to produce the toxic agent: 3-amino-4hydroxyphenylarsenoxide. This drug has now been superseded. Glycobiarsol (bismuth N-glycolyl-p-arsanilate) has been used for treating amoebic dysentery. Tryparsamide (p-N-phenylglycineamidoarsonic acid) has been used for late-stage trypanosomiasis (African trypanosomiasis: sleeping sickness). Note that arsenicals are not active against Trypanosoma cruzi (Chagas’ disease). 52 Ascetospora brucellosis, gonorrhoea, Haverhill fever, Lyme disease, rickettsioses, syphilis, tuberculosis, yaws; it may also be caused by certain viruses – e.g., parvovirus [Lancet (1985) i 419–421, 422–425], Ross River virus, rubella virus. Septic arthritis is due to infection of the synovial fluid – commonly by staphylococci – e.g. secondarily to OSTEOMYELITIS in an adjacent bone, or as a complication of septicaemia; S. epidermidis may cause chronic septic arthritis of the hip following total hip replacement. (See also RHEUMATOID ARTHRITIS and REITER SYNDROME.) Arthroascus A genus of fungi (family SACCHAROMYCETACEAE) which form yeast cells (which bud, often bipolarly, on a wide base) and true mycelium (which tends to break up into arthrospores). Asci are formed directly after conjugation between two cells. Non-fermentative; NO3 − is not assimilated. Species: A. javanensis, isolated e.g. from soil, fruit, rotting wood. [Book ref. 100, pp. 114–116.] Arthrobacter A genus of obligately aerobic, catalase-positive, asporogenous bacteria (order ACTINOMYCETALES, wall type VI – see also PEPTIDOGLYCAN) which occur in the soil. In culture the organisms initially grow as irregular, ‘lumpy’, pleomorphic rods, with or without primary branching, but stationary-phase cultures consist predominantly of spherical or ovoid cells; on subculture rod-shaped forms develop. Optimum growth temperature: ca. 25° C. Arthrobacter spp have an oxidative-type metabolism; they are not cellulolytic. GC%: ca. 59–66. Type species: A. globiformis. Arthrobotrys A genus of fungi of the HYPHOMYCETES. Species can grow saprotrophically and can trap and digest nematodes (see NEMATOPHAGOUS FUNGI); A. oligospora can also attack and kill other fungi [FEMS Ecol. (1985) 31 283–291]. arthroconidium Syn. ARTHROSPORE. Arthrocystis See EIMERIORINA. Arthroderma A genus of fungi of the GYMNOASCALES (anamorphs: CHRYSOSPORIUM; Trichophyton). arthrospore (arthroconidium) (1) An arthric CONIDIUM. (2) Any CONIDIUM formed by thallic conidiogenesis. Arthuria See UREDINIOMYCETES. Arthus reaction A severe local inflammatory skin reaction which involves a TYPE III REACTION; the reaction becomes maximal 3–12 hours after the intradermal administration of antigen, and involves erythema, oedema, and local haemorrhage and necrosis. Arthus-type reaction Any disorder, other than the classical ARTHUS REACTION, which involves a TYPE III REACTION – see e.g. FARMERS’ LUNG and GLOMERULONEPHRITIS. artichoke curly dwarf virus See POTEXVIRUSES. artichoke mottled crinkle virus See TOMBUSVIRUSES. artifact Syn. ARTEFACT. Artogeia rapae GV See BACULOVIRIDAE. ARV See HIV. arylsulphatase An enzyme which can hydrolyse aromatic sulphate esters at the O–S bond; arylsulphatases occur e.g. in some Aspergillus and Mycobacterium spp. arylsulphatase test A test used in the identification of Mycobacterium spp. Essentially, the test strain is grown in a liquid medium containing tripotassium phenolphthalein disulphate (TPD); ARYLSULPHATASE-positive strains cleave TPD to PHENOLPHTHALEIN which is detected by a colour change on addition of alkali after 10 days (for slow-growing strains) or after 3 or 7 days (for rapidly-growing strains). ascarylose 3,6-Dideoxy-b-L-mannopyranose: a sugar found e.g. in the LIPOPOLYSACCHARIDE of certain strains of Yersinia pseudotuberculosis (and in the eggs of Ascaris worms). Ascetospora A phylum of PROTOZOA [JP (1980) 27 37–58] which form spores containing one or more sporoplasms but Melarsoprol (a derivative of benzenearsenous acid – a trivalent arsenical) is effective against both West and East African strains of the causal agent of sleeping sickness (Trypanosoma brucei gambiense and T. brucei rhodesiense, respectively). The specific target of the drug is trypanothione (N 1 ,N 8 bis[glutathionyl]spermidine) [PNAS (1989) 86 2607–2611]. Melarsoprol reacts with trypanothione to form an adduct (Mel T) which inhibits trypanothione disulphide reductase – an enzyme essential for regulating the parasite’s thiol/disulphide balance. Melarsoprol crosses the blood–brain barrier less readily than eflornithine, but it is still regarded as the most effective trypanocidal drug available for treating sleeping sickness [AP (1994) 33 1–47]. Trypanosomes exposed sub-lethally to arsenicals develop resistance quite readily, possibly owing to decreased uptake. (b)(in energy metabolism) Some bacteria use arsenate (or selenate) as electron acceptor in anaerobic respiration, electron donors including e.g. acetate, lactate and ethanol (according to species); cell-envelope reductases have been found [FEMS Reviews (1999) 23 615–627]. Arsenite is used as electron donor in autotrophic CARBON DIOXIDE fixation and as a respiratory electron acceptor [FEMS Ecol. (2004) 48 15–27; JB (2004) 186 1614–1619]. ART Automated reagin test: a qualitative or quantitative STANDARD TEST FOR SYPHILIS similar in principle to the RPR TEST. arteannuin Syn. QINGHAOSU. artefact (artifact) Any feature which does not occur in a specimen under natural conditions, but which may be seen in that specimen during experimentation. Artefacts are due to the disturbance introduced by the process of experimentation or observation; they may occur e.g. as a result of FIXATION. artemether See QINGHAOSU. artemisinine Syn. QINGHAOSU. arteritis Inflammation of an artery or arteries. Arterivirus A genus of viruses (family TOGAVIRIDAE); the arterivirus group currently includes LACTATE DEHYDROGENASE VIRUS, SIMIAN HAEMORRHAGIC FEVER VIRUS, the ‘Lelystad’ virus (see BLUE-EARED PIG DISEASE) and equine arteritis virus. In equines, equine arteritis virus causes necrosis of small arteries with various clinical manifestations – for example, rhinitis, oedema, enteritis, bronchopneumonia. Infection of pregnant mares commonly results in abortion. Transmission occurs both horizontally and vertically; vectors are unknown. The virus can infect a range of vertebrate cells in vitro. The viruses in this group are similar in their morphology and genomic organization; moreover, all exhibit a predilection for macrophages, and all tend to cause a lengthy period of viraemia. artesunate See QINGHAOSU. Arthonia See ARTHONIALES. Arthoniales An order of fungi of the ASCOMYCOTINA; members include crustose LICHENS (photobiont green, often trentepohlioid) and lichenicolous and saprotrophic fungi. Ascocarps are APOTHECIOID and may be lirelliform, irregular, etc; they contain paraphysoids. Asci are bitunicate, usually clavate. Genera: e.g. Arthonia. Arthopyrenia A genus of crustose LICHENS of the order DOTHIDEALES; ascospores have one to several septa. A. halodytes (photobiont ‘Hyella’) is a marine species which grows endolithically in calcareous rocks and on the shells of limpets, barnacles etc in the intertidal zone; it occurs in Europe and N. America. arthralgia Joint pain. arthric conidium See CONIDIUM. arthritis Inflammation of one or more joints. It may occur as a symptom or complication of various infectious diseases – e.g. 53 Aschaffenburg–Mullen phosphatase test no extrusion apparatus (no polar capsules or polar filaments). (cf. MICROSPORA; MYXOZOA.) The organisms are parasitic in invertebrates. Classes: PARAMYXEA and STELLATOSPOREA. Aschaffenburg–Mullen phosphatase test Syn. PHOSPHATASE TEST (for milk). Aschersonia A genus of fungi (order SPHAEROPSIDALES) which include parasites of scale insects and whiteflies. Strains of Aschersonia have been used e.g. in Florida for the biological control of scale insects. asci See ASCUS. ascigerous Bearing, or giving rise to, asci (see ASCUS). ascites The condition in which fluid (ascitic fluid) accumulates in the peritoneal cavity during certain pathological conditions. (See also INFECTIOUS DROPSY (of carp).) Ascobolus See PEZIZALES. ascocarp (ascoma) A structure at the surface of which, or within which, asci (see ASCUS) develop; the main forms of ascocarp are the APOTHECIUM, ASCOSTROMA, CLEISTOTHECIUM and PERITHECIUM. (Ascocarps are not produced e.g. by Saccharomyces spp and related yeasts.) In ascohymenial species ascocarp development appears to follow the sexual stimulus, i.e., plasmogamy (fertilization) precedes ascocarp development; in ascolocular species the initiation of ascocarp development (i.e., formation of a stroma) occurs before plasmogamy. In general, ascohymenial species form unitunicate asci, and ascolocular species form bitunicate asci. Ascochyta A genus of fungi of the order SPHAEROPSIDALES; A. pisi is the causal agent of leaf spot disease of the pea plant. Conidiophores are borne in dark, thick-walled, non-setose, unilocular, ostiolate pycnidia that are immersed in the host tissue. Ascocoryne See HELOTIALES. ascogenous ASCUS-forming. ascogone Syn. ASCOGONIUM. ascogonium (ascogone) In ascomycetes: the female GAMETANGIUM; it may be unicellular or multicellular, simple or complex in form, and it may or may not bear a TRICHOGYNE (according to species). ascohymenial See ASCOCARP. Ascoideaceae See ENDOMYCETALES. ascolichen An ascomycetous LICHEN. Ascoli’s thermoprecipitin test A serological PRECIPITIN TEST used to detect antigens of Bacillus anthracis (causal agent of ANTHRAX) in various animal products (e.g. hides). The material is extracted with e.g. boiling saline, and the extract is layered over a known positive antiserum in a RING TEST. ascolocular See ASCOCARP. ascoma Syn. ASCOCARP. ascomycetes Fungi of the subdivision ASCOMYCOTINA. (In some taxonomic schemes these fungi form the class Ascomycetes.) Ascomycotina (the ‘ascomycetes’) A subdivision of fungi (division EUMYCOTA) characterized by the formation of sexually derived spores (ASCOSPORES) in asci (see ASCUS). The ascomycetes are typically terrestrial saprotrophs or parasites; they include e.g. most YEASTS, the edible morels and TRUFFLES, the cup fungi, the POWDERY MILDEWS, BLACK MILDEWS and SOOTY MOULDS, and organisms which are better known in their asexual (‘deuteromycete’) states: the common ‘blue moulds’ and ‘green moulds’. Marine ascomycetes include members of the SPATHULOSPORALES. In most species the thallus is a well-developed, septate, branching mycelium in which the CELL WALL contains CHITIN (see also SEPTUM); however, some ascomycetes are unicellular organisms, and some are DIMORPHIC FUNGI. In the typical life cycle, the germination of an ascospore leads to the development of a septate mycelium consisting of uninucleate, haploid cells. Many (but not all) ascomycetes then exhibit a conidial (= asexual, imperfect or anamorphic) phase in which conidia (see CONIDIUM) are formed; although the anamorph is part of the HOLOMORPH, anamorphs are commonly (for convenience) classified in the DEUTEROMYCOTINA. At some stage the thallus enters DIKARYOPHASE – e.g. by GAMETANGIAL CONTACT, SOMATOGAMY or SPERMATIZATION. Subsequently, karyogamy and MEIOSIS occur in the developing ASCOCARP – which commonly gives rise to asci which each contain 8 (haploid) ascospores. In the typical life cycle diplophase is thus of limited duration; however, in some ascomycetes (e.g. many yeasts) diplophase is dominant – plasmogamy and karyogamy quickly following ascospore formation, so that the vegetative cells are commonly diploid. In some taxonomic schemes [see e.g. Book ref. 64] classes are not recognized; instead, the ascomycetes are divided into 37 orders: ARTHONIALES; ASCOSPHAERALES; CALICIALES; CLAVICIPITALES; Coryneliales; Cyttariales; DIAPORTHALES; Diatrypales; DOTHIDEALES; ELAPHOMYCETALES; ENDOMYCETALES; ERYSIPHALES; EUROTIALES; GRAPHIDALES; GYALECTALES; GYMNOASCALES; HELOTIALES; HYPOCREALES; LABOULBENIALES; LECANIDIALES; LECANORALES; MICROASCALES; OPEGRAPHALES; OPHIOSTOMATALES; OSTROPALES; PELTIGERALES; PERTUSARIALES; PEZIZALES; POLYSTIGMATALES; PYRENULALES; RHYTISMATALES; SORDARIALES; SPATHULOSPORALES; SPHAERIALES; TAPHRINALES; TELOSCHISTALES; VERRUCARIALES. Ascophyllum See PHAEOPHYTA. L-ascorbic acid (vitamin C; L-threo-2,3,4,5,6-pentahydroxy-2-hexenoic acid-4-lactone) A VITAMIN, essential to man but not normally to microorganisms, found e.g. in fresh fruit and vegetables; it can be synthesized e.g. by certain algae and strains of Aspergillus niger. It is a strong reducing agent (oxidized form = dehydroascorbic acid) used e.g. as a poising agent in media for anaerobes (see REDOX POTENTIAL). Commercial manufacture (the Reichstein process) involves the hydrogenation of glucose to D-sorbitol followed by the SORBOSE FERMENTATION; L-sorbose is converted chemically to ascorbic acid. Ascoseira See PHAEOPHYTA. Ascosphaera See ASCOSPHAERALES. Ascosphaerales An order of fungi (subdivision ASCOMYCOTINA) which are associated with bees and pollen. Genera: Ascosphaera (see CHALKBROOD), Arrhenosphaera and BETTSIA. ascospore A SPORE formed within an ASCUS. According to species, ascospores may be septate or aseptate, and may be any of a variety of shapes, sizes and colours. On germination, the ascospores of most species form germ tube(s), while those of yeasts characteristically give rise to budding cells. Ascospore discharge. In most ascomycetes the ascospores are forcibly ejected from the ascus; forcible ejection can occur either from a UNITUNICATE ASCUS or from a BITUNICATE ASCUS. The mechanism of ascospore ejection appears to be unknown; however, ejection probably depends on the development of pressure within the ascus by the absorption of water – e.g. as a result of the enzymic breakdown of a polysaccharide with consequent increase in osmotic pressure. Different mechanisms may be involved in different ascomycetes. In apothecial ascomycetes (‘discomycetes’) the ascospores are often released simultaneously by a large proportion of the asci; this results in a visible cloud of ascospores and is called puffing. In cleistothecial ascomycetes the mature asci appear to swell up and rupture the cleistothecial wall. In some species the 54 asepsis Following plasmogamy, the events leading to ascus formation vary according to species. Ascus formation in ascogenous yeasts. After plasmogamy (which may occur e.g. by the fusion of somatic cells or ascospores), karyogamy commonly follows without delay. The zygote may undergo MEIOSIS immediately (as e.g. in Schizosaccharomyces octosporus) so that the ascus develops directly from the zygote; alternatively, meiosis may be delayed for one or more mitotic divisions (e.g. as in the diploid budding phase of Saccharomyces cerevisiae) – in which case the ascus develops following meiosis in one of the diploid descendents of the zygote. Following meiosis, the four (haploid) nuclei may develop directly into four ascospores (as in e.g. S. cerevisiae) or there may be subsequent mitotic division with the eventual formation of e.g. eight ascospores (as in e.g. S. octosporus). The process by which a nucleus gives rise to an ascospore is called free cell formation (see below). Ascus formation in other ascomycetes. (The following is necessarily a generalized account since details of the process vary from species to species.) After plasmogamy, karyogamy is typically delayed. One or more hyphae (ascogenous hyphae) arise from the ASCOGONIUM, and the (haploid) male and female nuclei migrate into these hyphae. The tip of each hypha then curves to form a crook (crozier ) such that two nuclei (one from each parent) occur in the curved upper portion of the crozier. These nuclei then undergo mitosis, simultaneously, with their mitotic spindles arranged parallel to the long axis of the ascogenous hypha; the subsequent formation of a septum creates an apical cell (the ascus mother cell ) which contains one daughter nucleus from each of the two original nuclei. Karyogamy now occurs. The zygote undergoes meiosis followed by one or more mitotic divisions to produce the number of haploid nuclei (ascospore initials) characteristic of the species. The ascospores develop by free cell formation. In this process, each nucleus (with a portion of cytoplasm) becomes enclosed within an envelope (the spore-delimiting membrane, SDM) composed of two unit-type membranes. In some ascomycetes (e.g. many members of the Endomycetales) the nucleus may be enveloped directly by vesicles (derived from the GOLGI APPARATUS or Golgi-equivalent?) which develop in association with the SPINDLE POLE BODY. In other species, e.g., Taphrina deformans and some species of Tuber, each nucleus becomes enveloped by membrane derived from invaginations of the ascus plasma membrane. However, in most ‘euascomycetes’ the SDMs derive from a system of nuclear and/or endoplasmic reticular membranes which, initially, form a discontinuous layer around the periphery of the ascus cytoplasm (the peripheral membrane cylinder ). That portion of the ascus cytoplasm which is not incorporated into ascospores is termed the epiplasm. Ascospore wall material is subsequently laid down between the two layers of the SDM. [Ascospore development: Book ref. 175, pp. 107–129.] Types of ascus. There are at least nine morphologically and/or functionally distinct types of ascus [Bot. J. Lin. Soc. (1981) 82 15–34 (29–33)]: see e.g. ANNELATE ASCUS, BITUNICATE ASCUS, OPERCULATE ASCUS, OSTROPALEAN ASCUS and PROTOTUNICATE ASCUS. (See also UNITUNICATE ASCUS and BILABIATE.) ascus mother cell See ASCUS. Asellariales See TRICHOMYCETES. asepsis (1) The state in which potentially harmful microorganisms (e.g. pathogens in a medical context, spoilage organisms in an industrial context) are absent from particular tissues, materials or environments; in this sense asepsis does not necessarily protruding (or totally freed) asci release their ascospores by violent disintegration. In perithecial ascomycetes various strategies have evolved to ensure that the ascospores do not remain trapped within the perithecium. Thus, e.g. in Sordaria fimicola each of the hymenial asci, in turn, elongates, discharges its ascospores, and collapses. In ascomycetes which form long-necked perithecia the asci may become detached from the hymenium and are subsequently blown through the ostiole as a result of pressure in the perithecial cavity; however, in species of Ceratocystis the asci are evanescent, and the mature ascospores are slowly extruded in a slimy mass through the perithecial neck. In the ascostromal ascomycete Myriangium, ascospore discharge is necessarily preceded by disintegration of the stroma – which occurs as a result of weathering. Once exposed to the environment the (bitunicate) asci forcibly eject their ascospores. In e.g. Tuber spp (and other hypogean ascomycetes) the ascospores are not forcibly discharged. In such species ascospore dispersal is mediated e.g. by burrowing animals. Ascospore discharge can be strongly influenced by environmental conditions (e.g. light, humidity), and a distinct periodicity of spore discharge (‘endogenous rhythm’) has been detected in a number of species. In ascomycetes which form deeply concave (cupulate) apothecia the asci are commonly positively phototropic; asci which line the near-vertical walls of the apothecium thus direct their spores outwards (into the environment) rather than towards the opposite wall of the apothecium. In some fungi, e.g. Daldinia concentrica, ascospore discharge occurs mainly or exclusively in the dark. ascostroma An ASCOCARP consisting of a STROMA (sense 1) containing one or more cavities (locules), each locule containing one or more asci (see ASCUS). In each locule the CENTRUM is bounded only by the stromatic tissue; this contrasts with the ascocarp structure of those ascomycetes (e.g. Xylaria spp) which form perithecia immersed in a stroma: in such species a distinct perithecial wall occurs within the locule. A uniloculate ascostroma (i.e., one containing a single locule) may contain a hymenial layer of asci, and may thus resemble a perithecium; such a structure is referred to as a perithecioid pseudothecium or pseudoperithecium. (In e.g. Rhytidhysteron the hymenium forms an apothecium-like structure which is called an apothecioid pseudothecium.) In some ascomycetes (e.g. Myriangium) the asci develop in uniascal locules which are scattered irregularly in the stroma. An ascostroma may develop on the surface of, or within, the substratum. Ascotricha See SPHAERIALES. ascus (pl. asci) A microscopic, sac-like structure within which are formed the sexually derived (or, exceptionally, parthenogenically derived) spores (ASCOSPORES) of fungi of the subdivision ASCOMYCOTINA. Asci are commonly cylindrical or clavate, but can be e.g. cup-shaped, globose or dumb-bell-shaped, according to species; a mature ascus typically contains eight ascospores, but in some species it contains e.g. two or four ascospores, or (in e.g. Kluyveromyces and Lipomyces) more than eight ascospores. Asci may be formed within or at the surface of an ASCOCARP, and may occur singly, in groups, or in a closely packed layer (hymenium) which, according to species, may be interspersed with sterile structures (see e.g. PARAPHYSIS). In some species the asci are evanescent (see also PROTOTUNICATE ASCUS). Ascus formation is usually preceded by plasmogamy – which, depending on species, occurs by GAMETANGIAL CONTACT, GAMETANGIAL COPULATION, SOMATOGAMY or SPERMATIZATION. Many ascomycetes exhibit HETEROTHALLISM. (See also MATING TYPE.) 55 aseptate involve sterility (cf. STERILE (sense 1)). (2) A state of sterility. (See also ANTISEPSIS and STERILIZATION.) aseptate Lacking septa – see SEPTUM. aseptic (adj.) Refers to ASEPSIS (sense 1 or 2). aseptic meningitis See MENINGITIS (b). aseptic technique Precautionary measures taken to prevent the contamination of cultures, sterile media etc and/or the infection of persons, animals or plants by extraneous microorganisms. Thus, e.g. all vessels for media etc must initially be STERILE (presterilized disposable petri dishes, syringes etc are often used). The working surfaces of instruments (forceps, LOOPS etc), and the rims of bottles used for dispensing sterile (non-flammable) materials, are sterilized by FLAMING. Before use, sterile material should not be exposed to non-sterile material or conditions. The risk of contamination is reduced by treating bench surfaces etc with DISINFECTANTS and/or with ultraviolet radiation (see STERILIZATION). Procedures (e.g. INOCULATION) are carried out in such a way that exposure to the atmosphere is reduced to a minimum, or is eliminated (see e.g. SAFETY CABINET). Ashbya See METSCHNIKOWIACEAE and STIGMATOMYCOSIS; see also RIBOFLAVIN. AsiA See ANTI-SIGMA FACTOR. Asian flu See INFLUENZAVIRUS. Asiatic cholera Syn. CHOLERA. ASLT ANTISTREPTOLYSIN O TEST. ASO test ANTISTREPTOLYSIN O TEST. ASOT ANTISTREPTOLYSIN O TEST. L-asparaginase (L-asparagine aminohydrolase; EC 3.5.1.1) An enzyme which hydrolyses L-asparagine to L-aspartate and NH3 . L-Asparaginase (e.g. from Escherichia coli or Citrobacter sp) is used as an anticancer agent – e.g. for treating acute lymphocytic leukaemia in which the cancer cells require exogenous asparagine. (See also ENZYMES.) L-asparagine biosynthesis See Appendix IV(d). asparenomycins CARBAPENEM antibiotics. aspartame L-Aspartyl-L-phenylalanine methyl ester; it is used as a sweetener in certain foods and beverages. aspartase See ASPARTATE BIOSYNTHESIS. aspartate ammonia-lyase See ASPARTATE BIOSYNTHESIS. L-aspartate biosynthesis For the main pathway of aspartate biosynthesis see Appendix IV(d). In many microorganisms, aspartate can also be produced from fumarate and ammonia by the enzyme aspartase (L-aspartate ammonia-lyase, EC 4.3.1.1), but this (reversible) reaction is probably more important in the deamination of aspartate than in its synthesis. (cf. IMMOBILIZATION sense 1.) aspartokinase See Appendix IV(d). aspergillic acid An antibiotic (a pyrazine derivative) formed by Aspergillus flavus. aspergilloma See ASPERGILLOSIS. aspergillosis Any disease of man or animals in which the causal agent is a species of ASPERGILLUS. In man, the causal agent is usually A. fumigatus, although A. flavus, A. niger or A. terreus may be involved. Infection usually occurs by inhalation of air-borne conidia, but may occur via wounds or by ingestion. The disease may be noninvasive, the fungus colonizing a pre-existing lung cavity (e.g. a lung cyst or healed tuberculosis lesion) and forming compact mycelial masses called aspergillomas (cf. MYCETOMA sense 2); symptoms may be absent or may include chronic productive cough and haemoptysis. Less commonly, the disease may become invasive, disseminating to other organs (particularly in immunocompromised patients); this form is commonly fatal. In some (atopic) individuals inhalation of Aspergillus conidia can lead to allergic bronchopulmonary aspergillosis (‘extrinsic bronchial asthma’); the conidia may germinate in the bronchi and sputum plugs, but the hyphae do not normally invade the tissues. Other forms of aspergillosis include e.g. infections of the ear or paranasal sinuses. Lab. diagnosis: cultural or microscopical (or histological) examination of e.g. biopsy material; repeated isolation of Aspergillus (e.g. from sputum) is presumptive evidence of infection. In some forms of aspergillosis specific antibodies may be demonstrated by gel diffusion techniques. Aspergillosis can affect a wide range of animals, causing e.g. pneumonia, gastroenteritis (e.g. in calves), or placentitis leading to abortion (e.g. in cattle, sheep and horses). Birds (including poultry) are particularly susceptible; almost any organ can be affected, and the disease may be acute and (usually) fatal (as in ‘brooder pneumonia’ of baby chicks) or chronic (in adult birds). (See also AFLATOXINS.) Aspergillus A genus of fungi of the class HYPHOMYCETES: some species are known to have ascomycetous teleomorphs (see later). The organisms are widespread in nature, and are characteristically saprotrophic; they can use a wide range of substrates as nutrients. Some species can be pathogenic (see e.g. ASPERGILLOSIS, MADUROMYCOSIS, STONEBROOD), and many produce toxins (see e.g. AFLATOXINS, ASTELTOXIN, CYCLOPIAZONIC ACID, FESCUE FOOT, GLIOTOXIN, OCHRATOXINS, PATULIN, STERIGMATOCYSTIN). Some species can cause deterioration in various types of material (see e.g. BREAD SPOILAGE, CHEESE SPOILAGE, COAL BIODEGRADATION, GLASS, LEATHER SPOILAGE, PAINT SPOILAGE, PAPER SPOILAGE, PETROLEUM), while certain species are used in the manufacture of particular enzymes, foods or other commodities (see e.g. AMYLASES, BRINASE, CATALASE, CITRIC ACID, COFFEE, GLUCOSE OXIDASE, HAMANATTO, KOJI, MISO, PECTIC ENZYMES, SAKE, TAKADIASTASE). Aspergillus spp form a well-developed, septate mycelium. (See also NIGERAN and PSEUDONIGERAN.) Each conidiophore, which develops from a FOOT CELL (sense 2), consists of an erect hypha which has a more or less spherical terminal swelling (the ampulla or vesicle). In some species the ampulla is partly or completely covered by a layer of phialides (‘primary sterigmata’) – see CONIDIUM; in most species, however, the ampulla is covered by a layer of short, finger-like extensions (‘primary sterigmata’ or metulae) which give rise to the phialides (‘secondary sterigmata’) at their distal ends. Each phialide produces a basipetally formed chain of spherical, pigmented, aseptate conidia which, according to species, may be e.g. yellow, green or black. (Spore colour can also vary according to available trace elements in the medium; thus, e.g. A. niger may form yellow (instead of the normal black) conidia if levels of copper are low.) The ampulla, together with its metulae and/or phialides and associated chains of conidia, is called a conidial head. A PARASEXUAL PROCESS occurs e.g. in A. nidulans. Species include e.g. A. flavus, A. fumigatus, A. glaucus (teleomorph: EUROTIUM), A. nidulans (teleomorph: EMERICELLA), A. niger, A. oryzae, A. parasiticus, A. phialiseptus, A. terreus and A. versicolor. A. fumigatus (teleomorph: SARTORYA) is a major cause of aspergillosis in man. In the conidial head, the ampulla may appear domed (rather than spherical) owing to the gradual widening of the distal end of the conidiophore, and it bears a layer of phialides over the distal half to three-quarters of its surface; the proximally situated phialides are longer, and are inclined such that their free ends tend to be parallel with the more distal phialides – the chains of grey-green, spherical conidia thus 56 Astrephomene forming a parallel-sided columnar spore mass which may reach ca. 1 mm in length. A. fumigatus has been reported to grow at 50° C. (See also FUMAGILLIN.) A. phialiseptus is morphologically similar to A. fumigatus, but it forms septate phialides which are longer than those of A. fumigatus. Aspidisca A genus of ciliate protozoa (order HYPOTRICHIDA) related to Euplotes but differing e.g. in typically having a relatively reduced oral ciliature; species occur in freshwater and marine habitats, and e.g. in some SEWAGE TREATMENT plants. aspirin See PROSTAGLANDINS. asporogenous Not capable of forming spores. assay host (plant virol.) Syn. LOCAL LESION host. assimilatory nitrate reduction The (typically aerobic) reduction of nitrate to ammonia, the ammonia being assimilated as a source of nitrogen (see AMMONIA ASSIMILATION); nitrate can be used as the sole source of nitrogen by many bacteria, various fungi, and most algae and plants. Nitrate is initially reduced to nitrite – typically by a soluble, oxygen-insensitive enzyme (assimilatory nitrate reductase) in a reaction in which the electron donor may be a reduced pyridine nucleotide or a reduced ferredoxin; in fungi the electron donor is often (sometimes specifically) NADPH, but in e.g. Candida nitratophila it can be either NADH or NADPH [JGM (1986) 132 1997–2003]. The assimilatory reduction of nitrate to nitrite does not generate proton motive force – cf. DISSIMILATORY NITRATE REDUCTION. Nitrite is reduced to ammonia by an assimilatory nitrite reductase, reducing power being derived (according to organism) from NADPH, NADH or reduced ferredoxin – commonly the latter in algae and higher plants. The synthesis/activity of enzymes involved in assimilatory nitrate reduction is regulated at least partly by the extracellular concentrations of ammonia and nitrate. assimilatory sulphate reduction Sulphate can be used as the sole source of sulphur by most microorganisms. The major assimilatory pathway in bacteria and fungi is shown in the figure. (cf. DISSIMILATORY SULPHATE REDUCTION; see also SULPHUR CYCLE.) Astasia See EUGLENOID FLAGELLATES. astaxanthin See PHAFFIA. asteltoxin A polyene MYCOTOXIN isolated from toxic maize meal cultures of Aspergillus stellatus (= A. variecolor, Emericella variecolor ). In experimental animals asteltoxin can cause e.g. paralysis of hind-limbs and respiratory impairment. aster (cell biol.) See MITOSIS. aster yellows See YELLOWS. Asterionella A genus of freshwater and marine planktonic pennate DIATOMS. A. formosa occurs e.g. in lakes and reservoirs, forming blooms in spring and, to a lesser extent, in autumn. In this and other species each individual vegetative cell is long and narrow with slightly flared ends; a variable number of cells (ca. 8 under optimum conditions) form a star-shaped colony in which each corner of one end of a given cell is joined to one corner of a neighbouring cell. Asticcacaulis A genus of chemoorganotrophic, strictly aerobic PROSTHECATE BACTERIA; habitat and life-cycle are similar to those of CAULOBACTER, but one or more prosthecae arise subpolarly and/or laterally and do not have an adhesive role – adhesive material occurring on the cell surface. Astomatida An order of protozoa (subclass HYMENOSTOMATIA) in which the cells are typically large or long, are uniformly ciliated, contain a number of contractile vacuoles, lack a cytoproct, and are invariably mouthless; some species have an elaborate holdfast organelle. Asexual reproduction may involve budding SO4 2− ATP sulphate adenylyltransferase PPi APS ATP APS kinase ADP PAPS + NADPH + H FAD thiol PAPS reductase + NADP PAP 2− SO3 + 3NADPH + 3H sulphite reductase + 3NADP 3H2O S2− ASSIMILATORY SULPHATE REDUCTION in bacteria and fungi. APS = adenosine-5 -phosphosulphate (see APS); PAPS = 3 -phosphoadenosine-5 -phosphosulphate; PAP = 3 -phosphoadenosine5 -phosphate. The pathway for the assimilation of sulphide depends on organism. In e.g. Escherichia coli sulphide is incorporated into O-acetylserine to form cysteine [Appendix IV(c)]. In e.g. Yarrowia (Saccharomycopsis) lipolytica sulphide may be incorporated into O-acetylserine (to form cysteine) or into O-acetylhomoserine to form homocysteine; homocysteine may be converted to methionine [Appendix IV(d)] or (via cystathionine) to cysteine [see e.g. MGG (1979) 174 33–38]. and (in e.g. Intoshellina) the formation of chain-like colonies. The organisms are endoparasites of e.g. annelids, molluscs and amphibians in freshwater and marine habitats. Genera include e.g. Anoplophrya, Cepedietta, Jirovecella, and Radiophrya. astomatous Refers to any structure or cell which does not have a pore, opening, or mouth. astome A member of the ASTOMATIDA. Astracantha See RADIOLARIA. Astraeus See SCLERODERMATALES. Astrephomene A genus of VOLVOX-like green algae which form spherical coenobia of 16–128 cells. 57 Astrolophus atovaquone See MALARIA. Atoxoplasma A genus of protozoa (suborder EIMERIORINA) similar to LANKESTERELLA but parasitic in birds. [Book ref. 18, p.13.] atoxyl See ARSENIC. ATP Adenosine 5 -triphosphate (see figure). (See also NUCLEOTIDE.) In both prokaryotic and eukaryotic cells ATP actively participates in a range of processes and reactions which involve the conversion or expenditure of energy (see also ATPASE); energy derived from chemotrophic or phototrophic metabolism holds the mass–action ratio of the reaction (ATP ↔ ADP + Pi) at values such that ATP hydrolysis is thermodynamically highly favourable, i.e., it can yield free energy which the cell can use for specific purposes. (See also ADENYLATE ENERGY CHARGE.) Astrolophus See ACANTHAREA. astropyle See RADIOLARIA. astroviruses Spherical, ether-resistant viruses, 29–30 nm diam., which have a characteristic five- or six-pointed star-shaped surface pattern. Astroviruses have been observed in normal and diarrhoeic faeces and may cause gastroenteritis in infants, calves, lambs and piglets. Lamb astrovirus apparently contains ssRNA [JGV (1981) 53 47–55]. ASW Artificial seawater. [Recipes: Book ref. 2, p. 436.] asymmetric PCR A variant form of PCR in which one of the two types of primer in the reaction mixture is used at a much lower concentration (e.g. 1:50) – so that this primer will be used up quickly when cycling begins; a normal concentration of the other primer ensures that one strand of the amplicon will be significantly amplified. Asymmetric PCR can be used for preparing single-stranded DNA products suitable e.g. for DNA SEQUENCING. An alternative procedure for obtaining ssDNA products from PCR involves tagging one of the two types of primer with BIOTIN, both primers being used at normal concentrations [NAR (1996) 24 3645–3646]. After cycling, STREPTAVIDIN is added to the reaction mixture; streptavidin binds to the biotinylated products, i.e. it binds to one of the two types of ssDNA product (but not to the complementary product). Gel electrophoresis in a denaturing gel (which inhibits hybridization between the complementary ssDNA products) separates the two types of product because the electrophoretic mobility of the streptavidin-bound product is greatly reduced. If a particular product strand is required, the primer which forms the other strand is biotinylated. asymmetric unit membrane See UROPLAKIN. asymptomatic Without symptoms. AT type See BASE RATIO. ATCC American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland 20852, USA. ateline herpesvirus See e.g. GAMMAHERPESVIRINAE (‘Herpesvirus ateles’). Athalamida See GRANULORETICULOSEA. atherosclerosis See MAREK’S DISEASE. Athiorhodaceae See RHODOSPIRILLACEAE. athlete’s foot (tinea pedis) RINGWORM of the foot, characterized by itching (often with cracking and scaling) of the skin – particularly that between the toes. Causal agents: usually Epidermophyton floccosum or Trichophyton spp; Candida albicans (see CANDIDIASIS) can cause a similar condition. athymic Lacking a thymus. Atkinsiella A genus of fungi (order SAPROLEGNIALES) which include organisms parasitic on e.g. the eggs of marine crustacea. Species include A. dubia, A. entomophaga and A. hamanaensis [Book ref. 1, pp. 201–202]. ATL ADULT T-CELL LEUKAEMIA. Atmungsferment Cytochrome aa3 . atomic mass unit (AMU; u) A unit of atomic mass: one-twelfth of the mass of a neutral 12 C atom; it is ca. 1.6605655 × 10−24 gram. (cf. DALTON; RELATIVE MOLECULAR MASS.) Atopobium A genus of asporogenous, obligately anaerobic, catalase-negative Gram-positive bacteria; cells: short rods or cocci which occur singly, in pairs or short chains. Non-motile. Glucose is metabolized to lactic, acetic and formic acids. A. parvulus was formerly classified as Streptococcus parvulus. GC%: 35–46. Type species: A. minutum. [Proposal for the genus Atopobium: FEMS (1992) 95 235–240.] atopy An inherited tendency to develop immunological HYPERSENSITIVITY states. NH2 N N O O− g P O− O O b P O− O O P a O− 5′ O CH2 4′ H N O H H 3′ 2′ OH OH N 1′ H ATP (adenosine 5 -triphosphate) The phosphorylation of ADP to ATP commonly occurs by OXIDATIVE PHOSPHORYLATION, photophosphorylation or SUBSTRATE-LEVEL PHOSPHORYLATION; in some organisms PYROPHOSPHATE hydrolysis by a PROTON PPASE can be coupled to ADP phosphorylation via proton motive force (see CHEMIOSMOSIS). In some energy-requiring reactions nucleotides other than ATP may be used; thus, e.g., guanosine 5 -triphosphate (GTP) is involved in certain stages in PROTEIN SYNTHESIS. Small amounts of ATP can be detected e.g. by techniques which involve CHEMILUMINESCENCE. ATP-binding cassette See ABC TRANSPORTER. ATP synthase See ATPASE. ATPase Adenosine 5 -triphosphatase: any of a wide range of structurally and functionally distinct enzymes and enzyme complexes capable of hydrolysing a phosphate bond in ATP (q.v.) – cf. KINASE; many types of ATPase (sometimes called ATP synthases) can catalyse the synthesis of ATP (see e.g. OXIDATIVE PHOSPHORYLATION) as well as the hydrolysis of ATP. Most ATPases appear to be g-phosphohydrolases, i.e., they catalyse the dephosphorylation of ATP to adenosine 5 -diphosphate (ADP) and inorganic phosphate (Pi), and/or the converse reaction. Energy derived from the hydrolysis of ATP by a given ATPase may be used, directly, in one or other of two main types of energy-requiring activity: (a) energy conversion and/or the transmembrane translocation of ions (see e.g. PROTON ATPASE and ION TRANSPORT); (b) mechanical work (see e.g. PRIMOSOME and eukaryotic FLAGELLUM). By generating or augmenting proton motive force (see CHEMIOSMOSIS), ATP hydrolysis can supply energy for some functions (e.g. rotation of the bacterial FLAGELLUM) that cannot use ATP as a direct source of energy. Note on nomenclature. The activity of some proton-translocating ATPases is enhanced by specific ions – e.g. the cytoplasmic membrane of Escherichia coli contains a ‘Ca2+ , 58 aurintricarboxylic acid ALPHAHERPESVIRINAE). Infection probably occurs mainly via the upper respiratory tract; the virus then invades the nervous system. In piglets, symptoms may include vomiting or diarrhoea, fever, trembling, incoordination, convulsions, and prostration; mortality rates may reach 100% in neonates. In adult pigs infection may be asymptomatic or may result in e.g. fever, respiratory symptoms, anorexia, and abortion; recovery usually occurs in a few days. A carrier state is recognized. In cattle, dogs and cats, the disease may resemble the active form of RABIES, with signs of intense itching, salivation, convulsions, and death. [Book ref. 27.] Aulacantha See RADIOLARIA. Aulosira A phycological genus of filamentous ‘blue-green algae’ (Nostocales) in which both heterocysts and akinetes are formed (cf. CYANOBACTERIA section IV). AUM See UROPLAKIN. Aura virus See ALPHAVIRUS. auramine O A basic, yellow, substituted diphenylmethane FLUOROCHROME; fluorescence: yellow. auramine–rhodamine stain A fluorescent ACID-FAST STAIN. The stain is made by mixing auramine O (1.5 g) and rhodamine B (0.75 g) with distilled water (25 ml) and melted phenol (10 ml); this mixture is added to distilled water (25 ml) and glycerol (75 ml), well mixed, and filtered through glass wool. A heat-fixed smear is stained for 15–20 min at 37° C and rinsed in distilled water. Decolorization, for 2–3 min, is attempted with 70% ethanol containing 0.5% v/v conc. HCl. The smear is washed in distilled water and then flooded with ‘counterstain’ (0.5% aqueous potassium permanganate) for ca. 2 min to suppress non-specific fluorescence of tissue debris etc. (Prolonged counterstaining would mask the fluorescence of acid-fast organisms in the smear.) The smear is then washed in distilled water, blotted dry, and examined by fluorescence MICROSCOPY. Aureobasidium (Pullularia) See HYPHOMYCETES; see also BLACK YEASTS; PAINT SPOILAGE; PULLULAN; SAP-STAIN. aureofungin An antifungal antibiotic of the heptaene group of POLYENE ANTIBIOTICS produced by Streptoverticillium cinnamomeum var. terricolum; it is a golden-yellow powder which is insoluble in water but soluble in dilute alkali or ethanol. Aureofungin is active against a wide range of plant-pathogenic fungi, is translocated within the plant, and has been used e.g. for crop spraying and for the prevention of seed-borne disease. [Book ref. 121, pp. 137–148.] aureomycin See TETRACYCLINES. Auricularia A genus of fungi of the order AURICULARIALES. A. auricula-judae (the ‘Jew’s ear fungus’) can be parasitic e.g. on elder (Sambucus). An edible species, A. polytricha, is cultivated on oak (Quercus) in China. (See also FUNGUS GARDENS.) Auriculariales An order of fungi (subclass PHRAGMOBASIDIOMYCETIDAE) which typically form elongated, transversely septate basidia in gymnocarpous or semiangiocarpous fruiting bodies which may be e.g. stalked or sessile, and which are characteristically gelatinous or waxy; the basidiocarp may be e.g. cup-shaped or ear-shaped. Some species are saprotrophs; others are parasitic e.g. on mosses and on other fungi. Genera include e.g. AURICULARIA, Helicobasidium and Phleogena. aurintricarboxylic acid (ATA) A TRIPHENYLMETHANE DYE which inhibits PROTEIN SYNTHESIS in prokaryote and eukaryote cell-free extracts; it appears to inhibit the binding of mRNA to ribosomes and, at higher concentrations, can inhibit chain elongation. ATA strongly chelates metal ions. Mg2+ -stimulated ATPase’; confusingly, this terminology is also used for some ATPases which translocate ions other than protons: see e.g. ION TRANSPORT. ATP-gS Adenosine-5 -(g-thio)triphosphate: a (biologically) nonhydrolysable ATP analogue. atractyloside A glucoside, produced by the Mediterranean thistle (Atractylis gummitera), which competitively inhibits the ADP/ATP exchange carrier system in the mitochondrial inner membrane. (cf. BONGKREKIC ACID; WEDELOSIDE.) AtrB An ABC TRANSPORTER in Aspergillus nidulans. atrichous (1) Hairless. (2) Lacking flagella. (3) Lacking any filamentous appendages (flagella, fimbriae etc). atrium (ciliate protozool.) A shallow concavity in the cytostomal region – particularly in certain hypostomes. att site A DNA sequence at which site-specific recombination occurs during integration of the genome of a temperate bacteriophage into the chromosome of its host; the site on the phage genome is designated attP, that on the host chromosome attB. (See e.g. BACTERIOPHAGE l.) attaching and effacing lesions See EPEC. attB site See ATT SITE. attenuated (1) Of e.g. a structure: becoming narrow; tapered. (2) Having a reduced virulence – see ATTENUATION (1) and (2). attenuated vaccine A VACCINE containing live pathogens whose virulence for a given host species has been reduced or abolished by ATTENUATION. attenuation (1) (immunol.) Any procedure in which the virulence of a given (live) pathogen for a particular host species is reduced or abolished without altering its immunogenicity; attenuation may be achieved by SERIAL PASSAGE and may be used to prepare VACCINES (e.g., the SABIN VACCINE). (2) A reduction in the virulence of a pathogen under natural conditions. (3) See SPIRITS. (4) (mol. biol.) See OPERON. attenuator See OPERON. attP site See ATT SITE. atypical interstitial pneumonia (of cattle) Any of a clinically distinct group of acute or chronic respiratory diseases which typically involve e.g. pulmonary emphysema and oedema, hyperplasia of alveolar epithelium and interstitial cells, and an unresponsiveness to treatment [Book ref. 33, pp. 1255–1261]. (cf. BOVINE RESPIRATORY DISEASE.) In no naturally occurring case has the causal agent been established with certainty; causes may include the effects of noxious gases, metabolic products (see FOG FEVER), dusts (see BOVINE FARMERS’ LUNG), a ketone constituent of the weed Perilla frutescens, or IPOMEANOL. (See also PNEUMONIA.) atypical mycobacteria (anonymous mycobacteria) Species of MYCOBACTERIUM which can cause disease in man but which (unlike M. tuberculosis and M. bovis) do not cause fatal, disseminated disease when inoculated into guinea pigs. Atypical mycobacteria often give a negative NIACIN TEST. Au antigen See HEPATITIS B VIRUS. Audouinella See RHODOPHYTA. Aufwuchs (periphyton community) Organisms (including certain algae and sessilinids) which colonize and form a coating on submerged objects (stones, plants, etc) in aquatic habitats. Augmentin A mixture of AMOXYCILLIN and CLAVULANIC ACID used e.g. in the treatment of urinary tract infections. Augusta disease (of tulips) See TOBACCO NECROSIS VIRUS. Aujeszky’s disease (pseudorabies; mad itch; infectious bulbar paralysis) A PIG DISEASE which may also affect e.g. cattle, dogs, cats and rats; it occurs in North America, Europe, S. E. Asia, and the UK. The causal agent is a herpesvirus (see 59 aurodox aurodox See POLYENE ANTIBIOTICS (b). aurovertins A group of polyene, a-pyrone-containing MYCOTOXINS isolated from cultures of Calcarisporium arbuscula. [Biosynthesis: PAC (1986) 58 239–256.] Aurovertins bind noncovalently to, and inhibit, (F0 F1 )-type PROTON ATPASES. Australia antigen HBsAg: see HEPATITIS B VIRUS. Australia X disease Syn. MURRAY VALLEY FEVER. Australian bat lyssavirus See LYSSAVIRUS. auto-agglutination (saline agglutination) Spontaneous agglutination which may occur when bacterial cells (or other particulate materials) are suspended in e.g. saline. (See also VW ANTIGENS.) auto-a Syn. ALPHA PEPTIDE. autoantibody An ANTIBODY produced by an individual against one of its own antigens. (See e.g. TYPE V REACTION.) autoantigen An antigen homologous to an AUTOANTIBODY. Autobiocounter M 4000 See HACCP. autocatalytic splicing See SPLIT GENE (b, c and e). autochthonous (1) Indigenous to a given environment. (cf. ALLOCHTHONOUS.) In a given environment the autochthonous microorganisms maintain more or less constant numbers or biomass – reflecting more or less constant (albeit typically low) levels of nutrients. (cf. ZYMOGENOUS.) (2) (immunol.) Syn. AUTOLOGOUS. autoclave An apparatus within which objects or materials can be heat-sterilized by (air-free) saturated steam, under pressure, at temperatures usually in the range 115–134° C. (The pressure itself plays little or no part in the STERILIZATION process – which depends on the combined effects of heat and water vapour.) The simplest (‘bench-type’) autoclaves resemble the domestic pressure cooker in both principle and appearance. Material to be autoclaved is placed on a rack in the chamber and the lid clamped securely in position with the valve open; water in the bottom of the chamber is boiled (by means of an internal electric element, or by external heating) and the steam allowed to escape, via a valve in the lid, until all the air has been displaced from the chamber. The valve is then closed; on further heating, water continues to vaporize, and the pressure and temperature of the steam rise to levels determined by the setting of the steam exhaust valve – which then opens to maintain the required pressure and temperature of steam in the chamber. For a given B D C A E chamber F D F steam inlet 275-550 kPa (40-80 lb/inch2) C H G J K steam/condensate outlet AUTOCLAVE. Simplified, diagrammatic representation of a Series 225 steam–mains autoclave (courtesy of Baird and Tatlock (London) Ltd). A. Steam inlet valve. The valve is opened/closed electromagnetically – its operation being governed by the pressure controller/indicator. (Control by temperature – via the thermocouple – is an available option.) B. Pressure controller/indicator. C. Door microswitch – a safety device; electrical power is connected to the autoclave only when the microswitch has been actuated, i.e., when the door has been fully closed. Additionally, this device automatically opens the steam exit valve if any attempt is made to open the door while the chamber is under pressure. D. Door closure bolts which operate microswitches. E. Safety valve set to open at 275 kPa. F. Filters which prevent blockage of steam discharge lines and steam trap. G. Thermocouple pocket: temperature control/indicator/recording option. H. Manually-operated by-pass valve – can be operated in case of electrical power failure. (Under such conditions valve J could not be opened.) The valve contains a built-in bleed which permits a small but continual flow of steam from the chamber to the exterior. J. Electrically-operated steam exit valve. K. Steam trap. Operational sequence. 1. Insert load. 2. Secure door. 3. Set controls for required pressure/temperature and sterilizing time. 4. Switch on power. Steam enters the chamber through A, and the downwardly-displaced air is purged, via filters F, through (a) the steam trap K and (b) the permanent bleed H; J is closed, and when steam leaves the chamber, K closes. A closes when the chamber pressure reaches the selected value. Steam continues to leave the chamber via the permanent bleed H – permitting continual temperature monitoring by the thermocouple G; since G is located at the lowest (and hence coolest) part of the system, it measures the ‘worst case’ temperature – the chamber temperature always being higher than that registered by G. Sterilizing conditions within the chamber are maintained (against heat losses and against the continual steam bleed) by periodically admitting a pulse of steam via A. At the end of the sterilization cycle steam is discharged from the chamber via J; when atmospheric pressure has been re-established in the chamber the door can be opened and the load withdrawn. Automatic timing of the sterilization cycle commences when the chamber pressure (or, optionally, temperature – measured by G) reaches the desired value. 60 autogamy the entry of contaminated air is prevented by the presence of a filter in the air intake line. In vacuum autoclaves steam can be removed, and materials dried, by connecting a vacuum line to the chamber. Regular monitoring of an autoclave’s performance is important; monitoring devices should be used to check the temperatures reached at a number of different locations within the chamber. Monitoring devices may be physical (e.g. thermocouples), chemical (e.g. BROWNE’S TUBES), or biological (test envelopes containing the endospores of e.g. Bacillus stearothermophilus). In biological monitoring the autoclaved spores are tested for viability by attempting to culture them; although such a process is directly related to the aim of autoclaving, it involves a builtin delay (i.e. the incubation time). (See also AUTOCLAVE TAPE; BOWIE–DICK TEST.) Some materials (e.g. petroleum jelly, liquid paraffin) cannot be sterilized by autoclaving owing to their impermeability to steam; such materials (and e.g. clean glassware) are usually sterilized in a HOT-AIR OVEN. (See also CHEMICLAVE; LOW-TEMPERATURE STEAM DISINFECTION; STEAMER; STERILIZER.) autoclave tape Paper tape (usually self-adhesive) which changes colour (or exhibits other visible changes) when exposed to sterilizing conditions in an AUTOCLAVE. (See also BOWIE–DICK TEST.) autocolony A colony (or coenobium) which develops within one cell of a parent colony and which, when released, resembles the parent colony (see e.g. SCENEDESMUS). (cf. AUTOSPORE.) autocompartmentalization See PROTEASOME. autocoupling hapten See HAPTEN. autodigestion (of lamellae) See BASIDIOSPORES. autodisplay (biotechnol.) Autotransporter-mediated display of recombinant proteins at the cell surface of Escherichia coli. In this method, the a domain of the AIDA-I autotransporter (see type IV systems in PROTEIN SECRETION) is replaced by a recombinant protein; this is done by preparing a fusion protein consisting of the sequence encoding the b domain and that encoding the (heterologous) recombinant protein. In one study, the a domain was replaced e.g. by the cholera toxin B subunit; cells with the (normal) outer membrane protease OmpT cleaved the (recombinant) a domain, which was released from the cell, but in mutant (ompT − ) cells, which lack a functional OmpT protease, the subunit was not cleaved, but displayed at the cell surface. [Autodisplay: JB (1997) 179 794–804.] autoecious Syn. HOMOXENOUS. autofluorescence See FLUORESCENCE. autogamy (self fertilization) In certain protozoa: a sequence of events which culminates in the fusion of haploid nuclei or gametes derived from a single cell; autogamy occurs e.g. in certain ciliates (e.g. Paramecium – but apparently not in e.g. Tetrahymena) and in some sarcodines. In Paramecium aurelia each of the two micronuclei undergoes meiosis; of the eight haploid pronuclei produced all except one disintegrate. The surviving pronucleus divides mitotically to form a pair of gametic nuclei; these nuclei subsequently fuse to form the (homozygous) zygotic nucleus (= synkaryon). Two mitotic divisions then occur; two of the resulting (diploid) nuclei become new micronuclei, the other two become macronuclei – the original macronucleus having disintegrated during the preceding events. During the next binary fission one macronucleus passes to each daughter cell; both micronuclei divide, mitotically, and a pair of micronuclei passes to each daughter cell – thus re-establishing the normal nuclear constitution. steam temperature the time required for sterilization depends on the nature of the load – the rate of steam penetration and the heat capacity of the load being important considerations. Sterilizing temperature/time combinations used with bench-type or larger autoclaves are commonly 115° C (10lb/inch2 ; ca. 69 kPa) for 30–35 min; 121° C (15lb/inch2 ; ca. 103 kPa) for 15–20 min; 134° C (30 lb/inch2 ; ca. 207 kPa) for 4 min. These times may be varied according to e.g. the nature of the load and the nature and degree of contamination. Large autoclaves, used e.g. in hospitals and industry, often use steam piped direct from a boiler, and frequently incorporate automatic controls for timing etc (see Figure). (See also STEAM TRAP.) Steam quality is important for effective sterilization. If air is present in an operating autoclave, the temperature corresponding to a given pressure is lower than it would be in the complete absence of air; thus, air must be completely purged from the autoclave chamber, and all free space within the chamber must be filled with saturated steam at the required temperature and pressure. Saturated steam is steam which holds the maximum amount of water vapour for its temperature and pressure. (Saturated steam effects optimum heat transfer since it delivers up a large amount of latent heat when condensing on objects in the chamber during the initial (pre-sterilization) heating-up period.) In a bench-type autoclave the steam is necessarily saturated since it remains in contact with water in the chamber. In a larger autoclave, steam entering the chamber from a boiler should, for maximum efficiency, be dry as well as saturated, i.e., it should contain no water in the liquid phase. (Wet saturated steam contains droplets of water.) Superheated steam is an unsaturated vapour formed when the pressure of dry saturated steam (at constant temperature) is lowered, or when the temperature of dry saturated steam (at constant pressure) is raised in the absence of free water. Superheated steam tends to behave as a gas, and must be at temperatures higher than those of saturated steam to achieve sterilization; it may be formed e.g. when steam is piped from a boiler to a large autoclave – particularly at a pressure-reduction valve. In most large autoclaves steam enters near the top of the chamber, and air is purged by downward displacement; this may permit pockets of air to become trapped at the bottom of deep, empty vessels – e.g. bottles with small openings or long, narrow necks – and such regions may not be adequately exposed to sterilizing conditions. (Air may also be trapped in items of apparatus which are totally enclosed in a wrapping of e.g. aluminium foil.) In a vacuum autoclave air is purged from the chamber by connecting it briefly to a vacuum line; when steam subsequently enters the chamber it can penetrate more easily into flasks and between the fibres of surgical dressings etc so that the overall sterilizing time is reduced. At the end of the sterilization cycle in a large autoclave the steam inlet valve is closed (or closes automatically); the autoclave may be left to cool slowly, or steam within the chamber may be allowed to escape via the steam exit valve until pressure in the chamber is equal to that of the atmosphere – when the chamber can be safely opened. When bottles of liquid have been autoclaved the temperature and pressure in the chamber should be allowed to decrease slowly to prevent boiling and the consequent loss of contents and/or damage to bottles. (Screwcaps on bottles should always be loosened prior to autoclaving.) Sometimes a partial vacuum is allowed to form in the chamber as the steam condenses; this can be advantageous since it tends to dry materials (e.g. dressings) in the chamber. When ‘breaking the vacuum’, i.e. allowing ingress of air to the chamber, 61 autogenous regulation Since only the original complement of genes is involved, the potential for genetic change in autogamy is less than that in CONJUGATION; nevertheless, new combinations of alleles may be formed during the meiotic divisions. (See also ACTINOPHRYS and ACTINOSPHAERIUM.) In some organisms autogamy occurs spontaneously; in others it may be induced by starvation, ageing, radiation etc. autogenous regulation (mol. biol.) The regulation of expression of a gene or operon by its own product(s). (See e.g. RIBOSOME (biogenesis).) autogenous vaccine A VACCINE prepared by the culture and inactivation of pathogen(s) isolated from a patient and subsequently used to inoculate that patient. Autographa californica NPV See NUCLEAR POLYHEDROSIS VIRUSES. autoimmune disease Any disease directly attributable to AUTOIMMUNITY. In some autoimmune diseases (e.g. acquired haemolytic anaemias) antibodies play a significant role, but many autoimmune diseases are cell-mediated. (See also e.g. MULTIPLE SCLEROSIS.) autoimmunity An expression of IMMUNITY (1) in which the body directs immunological mechanism(s) against one or more of its own components. (See also TYPE V REACTION.) autoinducer See AUTOINDUCIBLE ENZYME and QUORUM SENSING. autoinducible enzyme A (repressible) enzyme whose induction in a given organism is brought about by the presence of a specific compound (autoinducer) that is produced by the organism itself (cf. enzyme induction in e.g. the LAC OPERON). An example is bacterial luciferase (see BIOLUMINESCENCE). In this system autoinducer is secreted by the bacteria and can accumulate to the critical, effective concentration (sufficient to bring about derepression) only when the organisms are in a confined environment (e.g. in the luminous organs of certain marine fish). autoinfection Infection resulting from the transfer of a pathogen from one site to another in the same individual (cf. RE-INFECTION). autologous (immunol.) Present in or derived from an individual’s own tissues. autologous blood transfusion See TRANSFUSION-TRANSMITTED INFECTION. autolysate The products of AUTOLYSIS. (See e.g. YEAST EXTRACT.) autolysin (autolytic enzyme) Any of a range of endogenous enzymes which, by degradation of certain structural cell components (e.g. PEPTIDOGLYCAN in bacteria), can bring about AUTOLYSIS or AUTOPHAGY; autolysins are apparently involved in normal processes of growth and development (e.g., spore germination; cell wall polymer extension and cell separation during normal growth). Mutant bacteria defective in autolysin activity may show e.g. abnormal growth forms (e.g. filaments) or resistance to antibiotics active against the wild-type. In prokaryotes, autolysins occur in the cell wall or (in Gram-negative species) in the periplasmic space, often as inactive precursors which are activated e.g. by proteases (see also BILE SOLUBILITY); in eukaryotes autolysins are sequestered in LYSOSOMES. autolysis The lysis of a cell due to the action of its AUTOLYSINS – usually following the death of the cell or tissue. autolytic enzyme Syn. AUTOLYSIN. automatic volumetric spore trap Syn. HIRST SPORE TRAP. autonomously replicating sequence See ARS. autophagy The digestion, by a eukaryotic cell, of some of its own internal components – e.g., during periods of starvation. (See FOOD VACUOLE.) autoplaque A plaque (in a bacterial lawn plate) which resembles that caused by a lytic bacteriophage, but in which no phage can be demonstrated. Autoplaques occur e.g. in cultures of certain strains of Rhizobium leguminosarum biotype trifolii and seem to result from autolysis. autoplast A PROTOPLAST or SPHAEROPLAST which results from the action of an organism’s own autolytic enzymes. autoradiography (radioautography) The use of a photographic process to locate and/or quantify a radioactively-labelled substance previously incorporated in living cells or tissues; autoradiography can be used e.g. to study sites of biosynthesis, turnover rates, and transport. Initially, a radioactive substrate is taken up by the living cells, and the label becomes incorporated in structures, metabolites etc as would be the non-labelled analogue. (The labelled substrate may be introduced using a PULSE–CHASE TECHNIQUE; since radioactive labels can be injurious to cells, dose and duration of the pulse may have to be limited.) The cells/tissues are then prepared for microscopy (e.g., fixed, dehydrated, embedded, sectioned, stained). For ELECTRON MICROSCOPY the section (e.g. on a formvar-coated grid) is stained with e.g. lead citrate or uranyl acetate, and is coated with a thin (ca. 5 nm) layer of carbon; the carbon layer is then overlaid with a thin layer of photographic emulsion, and the whole preparation is left in the dark at e.g. 4° C for a period of the order of days or weeks. Each point source of radioactivity in the section will affect the overlying area of emulsion, giving rise (on developing and fixing the emulsion) to a local deposit of silver which, under the EM, commonly appears as a tangled filament. Under the light microscope each silver deposit appears as a dot or fleck. Since the silver deposits are superimposed on the tissue section, the cellular location of the radioactive components can be determined. For optimum resolution, the section, carbon layer, and emulsion layer must each be very thin; given a choice of suitable radioactive sources, it is preferable to use the one with the least emission energy. Low-energy radiators, e.g. 3 H, tend to affect a small area of the emulsion immediately above each point source; with radiators of high emission energy, e.g. 32 P, each point source gives a wide-angled cone of effective radiation which affects a larger area of emulsion. Radioactive precursors used for studying particular cell components include e.g. [3 H]thymidine (DNA), [3 H]glucose (polysaccharides), and [3 H]proline (proteins). [Book ref. 4, pp. 235–277.] autosome Any chromosome other than a sex chromosome. autospore (algol.) A type of spore, formed asexually, which is an exact morphological replica of the parent cell. The term is usually reserved for aplanospores produced by Chlorella and related algae, but is sometimes extended to include e.g. the planospores of e.g. BRACHIOMONAS. autotransporter See PROTEIN SECRETION (type IV systems). autotroph An organism which uses CARBON DIOXIDE for most or all of its carbon requirements (cf. HETEROTROPH; LITHOTROPH); all obligate autotrophs appear to be either chemolithotrophs or photolithotrophs, and ‘autotroph’ is often used with the implication of lithoautotrophy (see also CHEMOTROPH). In many autotrophs CO2 fixation occurs via the CALVIN CYCLE or via the REDUCTIVE TRICARBOXYLIC ACID CYCLE. Some ACETOGENS fix CO2 via a different pathway (sometimes called the ‘activated acetic acid pathway’) in which acetyl-CoA can be synthesized by the reduction of two molecules of CO2 (see ACETOGENESIS); a similar pathway occurs in Desulfovibrio baarsii in which the methyl and carboxyl groups of acetyl-CoA are derived from formate and CO2 respectively [FEMS (1985) 28 311–315]. An analogous pathway occurs in METHANOGENS. autoxidation Spontaneous oxidation by atmospheric oxygen: see e.g. OXIDATIVE RANCIDITY and SULPHUR CYCLE. 62 avermectins which catalyse particular stage(s) in the synthesis of essential growth factor(s); such a block may involve (a) the complete absence of enzyme; (b) the presence of normal enzyme in subnormal amounts; (c) the presence of abnormal enzyme which is devoid of or has lowered enzymic activity. (See also SYNTROPHISM.) Apparent auxotrophy may result e.g. from a defective TRANSPORT SYSTEM; thus, e.g. an organism may appear to be an auxotroph if it is unable to take up a substrate which is necessary for the synthesis of an essential growth factor. (See also CRYPTIC MUTANT.) Isolation of auxotrophic mutants. Since auxotrophs have nutritional requirements in excess of those of the corresponding (prototrophic) wild-type strains, they cannot be isolated from mixed auxotroph–prototroph populations by common selective culture methods. In the limited enrichment method, a dilute suspension of mutagenized cells is inoculated onto a minimal medium which has been enriched with limiting amounts of nutrients; the dilution is chosen such that isolated colonies are obtained following incubation. Any auxotrophic cells which may be present form colonies which quickly exhaust the nutrient supply at their locations – so that colony size is restricted; a colony of prototrophic cells, which is not restricted by nutrient supply, attains a greater size. Thus, small colonies may be presumed to be those of auxotrophs. In the delayed enrichment method, the mutagenized preparation of cells is first inoculated onto a minimal medium. A small quantity of molten minimal agar is then layered onto the surface of the inoculated medium and allowed to set. The plate is then incubated. Colonies are formed only by prototrophs, and the positions occupied by these colonies are recorded. Finally, complete medium is layered onto the surface of the plate and allowed to set; the plate is then re-incubated. As nutrients diffuse into the minimal agar the auxotrophic cells begin to grow and form colonies. Auxotrophic mutants may also be isolated by a REPLICA PLATING process. Another method is based on the differing effects of PENICILLIN on growing and non-growing cells of certain (penicillinsensitive) organisms. If a well-washed mixture of auxotrophic and prototrophic cells is suspended in a minimal medium with an appropriate concentration of penicillin, the prototrophs are lysed while the auxotrophs, being unable to grow, remain viable. The suspension is then washed and is plated on a complete medium to recover the auxotrophs. In this method it is essential that the mutagenized cells be grown in a complete medium for several cell-division cycles prior to penicillin treatment. This is essential because the newly mutated cells contain the full complement of enzymes etc found in the prototroph; only after several rounds of cell division do the progeny cells exhibit a truly auxotrophic phenotype. Only low concentrations of cells should be used in this method since substances from the lysed cells may be used as nutrients by the auxotrophs – thereby rendering the latter susceptible to lysis by penicillin. (cf. STREPTOZOTOCIN.) auxotype See AUXANOGRAPHIC TECHNIQUE. avenacin A fluorescent polycyclic SAPONIN formed in the roots of oat plants (Avena spp). Avenacin confers resistance, in Avena spp, to many strains of Gaeumannomyces graminis (see TAKEALL); however, G. graminis var. avenae forms an extracellular glycosidase which detoxifies the compound. avermectins Macrolide-like antihelmintic agents, obtained from Streptomyces avermitilis, which are active against various human and animal parasites [AEM (2003) 69 1263–1269]. auxanogram See AUXANOGRAPHIC TECHNIQUE. auxanographic technique A procedure used e.g. for identifying the substances which a given organism can use as carbon sources, or for identifying the growth factor(s) required by an AUXOTROPH. To identify usable carbon sources, a plate of medium which lacks a carbon source is heavily inoculated with the test organism; a small quantity of one of each of a number of different carbon sources is then placed at a separate location on the plate. Following incubation, usable carbon sources are indicated by growth at their locations on the plate. The pattern of usable carbon sources (or e.g. nitrogen sources) is termed an auxanogram; closely related organisms which are characterized by different auxanograms are said to be different auxotypes (= auxanographic types). An analogous procedure is used for examining a suspected auxotroph; a solid MINIMAL MEDIUM is inoculated with the test organism, and to the plate are then added small, discrete amounts of (a) a mixture of amino acids, (b) a mixture of vitamins, and (c) one or more possible requirements for nucleic acid synthesis. Following incubation, growth at one or other location on the plate indicates the identity of the growth factor(s) required. If, for example, growth occurs at the location of the mixed amino acids, the analysis is continued by repeating the procedure with separate, discrete inoculations of individual amino acids until the specific requirement(s) is/are known. auxanographic type See AUXANOGRAPHIC TECHNIQUE. auxins PHYTOHORMONES which promote stem elongation and, in conjunction with CYTOKININS and/or GIBBERELLINS, play important roles in many plant processes; auxins, which are derivatives of tryptophan, are produced mainly at stem apices and in young leaves. Similar or identical compounds are also formed by certain microorganisms (including a number of plant pathogens). In many plants the main auxin appears to be indole 3-acetic acid (IAA, also known as ‘auxin’ or ‘heteroauxin’), a compound synthesized via the precursor indole 3-acetonitrile (IAN). In vivo, IAA can e.g. stimulate ETHYLENE production, but the mechanisms by which IAA functions in the overall regulation of growth are not well understood. Regulation of the in vivo level of IAA appears to involve an enzyme, ‘IAA-oxidase’, which oxidizes IAA to products which include 3-methylene-oxindole and indolealdehyde; this enzymic oxidation is stimulated e.g. by certain monophenols (e.g. p-coumaric acid) and by Mn2+ , and is inhibited by certain diphenol derivatives (e.g. CAFFEIC ACID, CHLOROGENIC ACID, QUERCETIN and SCOPOLETIN). Certain plant diseases of microbial causation are characterized by the presence of atypical levels of auxins in the diseased plants. Hyperauxiny (abnormally high levels of auxins) occurs in many diseases and can arise in various ways – see e.g. CLUBROOT, CROWN GALL, OLIVE KNOT and SCOPOLETIN. The nature of the relationship between hyperauxiny and disease development is not well understood. auxochrome In a DYE molecule: any ionizable group by means of which the CHROMOPHORE can bind to target molecule(s). auxospore See DIATOMS. auxotroph A strain of microorganism which lacks the ability to synthesize one or more essential growth factors; an auxotroph arises by the occurrence of one or more mutations in a PROTOTROPH. A medium used to culture a given auxotroph must contain any factors which the organism cannot synthesize: see COMPLETE MEDIUM and MINIMAL MEDIUM. Auxotrophy results from a cell’s genetically determined inability to produce (normal amounts of) functional enzyme(s) 63 aversion zone avian myeloblastosis virus See AVIAN ACUTE LEUKAEMIA VIRUSES and MYB. avian myeloblastosis virus reverse transcriptase See REVERSE TRANSCRIPTASE. avian myelocytomatosis virus See AVIAN ACUTE LEUKAEMIA VIRUSES. avian pneumoencephalitis Syn. NEWCASTLE DISEASE. avian reticuloendotheliosis viruses (REVs) A group of type C avian retroviruses of the ONCOVIRINAE. The group includes the replication-competent viruses duck infectious anaemia virus, Trager duck spleen necrosis virus and chicken syncytial virus, and the replication-defective, v-onc+ (v-rel + ) strain T virus (REV-T) and its associated helper virus (REAV, = REV-A). REVs are pathogenic in poultry (chickens, ducks, turkeys), causing a range of (usually rapidly lethal) diseases: e.g. anaemia, visceral reticuloendotheliosis, enlargement and necrosis of the spleen, lymphomas, and infiltrative nerve lesions. REV-T and its helper (REV-A) can be lethal in chickens within ca. 7–14 days of infection; the REV-A component has been reported to induce or activate a splenic suppressor cell population which inhibits the proliferation of cytotoxic cells capable of lysing REV-T-induced tumour cells [MS (1984) 1 107–112]. avian sarcoma viruses (ASVs) A group of v-onc+ type C retroviruses (subfamily ONCOVIRINAE) which, after a short latent period, cause tumours in fowl and (sometimes) other animals. The group includes ROUS SARCOMA VIRUS, Fujinami sarcoma virus (which carries v-fps), and Yamaguichi-73 sarcoma virus (which carries v-yes). (cf. FES.) avian tubercle bacillus Mycobacterium avium. avianized vaccine Any vaccine containing microorganisms whose virulence for a given host has been attenuated by adaptation in live chicks and/or serial passage through chick embryos (eggs). Attenuated organisms may or may not be inactivated prior to use in a vaccine. The FLURY VIRUS is an avianized strain. Avicel A commercial preparation consisting of ground microcrystalline (insoluble) CELLULOSE (average DP ca. 200); it is used e.g. for determining the ability of an organism or enzyme to degrade microcrystalline cellulose. (cf. CM-CELLULOSE.) avidin A protein (MWt ca. 68000) present e.g. in the white of raw hens’ eggs; the chicken avidin molecule consists of four identical subunits, and it can bind – non-covalently, but very strongly – four molecules of BIOTIN. (See also ABC IMMUNOPEROXIDASE METHOD; cf. STREPTAVIDIN.) avidity (immunol.) The stability of the antibody–antigen complex formed when multivalent antigen and homologous antiserum are mixed. Avidity depends not only on the AFFINITY of the individual determinant-combining site bonds but also on the number of satisfied valencies of the antigens and antibodies since, e.g., a complex in which two antigen molecules are linked by two antibody molecules is disproportionately more stable than a complex in which the two antigen molecules are linked by a single antibody molecule. Aviemore model See RECOMBINATION (figure 2). Avipoxvirus (fowlpox subgroup) A genus of viruses of the CHORDOPOXVIRINAE which infect birds (see e.g. FOWL POX). Avipoxviruses are commonly transmitted (mechanically) by arthropod vectors. Infected cells form lipid-rich A-type inclusion bodies; haemagglutinin is not formed. Infectivity is ether-resistant. Members are closely related serologically. Type species: fowlpox virus; other members include canarypox, juncopox, pigeonpox, quailpox, sparrowpox, starlingpox and turkeypox viruses. (See also POXVIRIDAE.) avirulence gene (avr gene) (plant pathol.) In a plant-pathogenic microorganism: a gene which interacts, functionally, with a aversion zone (mycol.) A zone of growth inhibition separating two fungal colonies; such zones are formed e.g. when strains of Phycomyces blakesleeanus of similar mating type are cultured on the same plate. Aviadenovirus (avian adenoviruses) A genus of adenoviruses (family ADENOVIRIDAE) which infect birds; type species: fowl adenovirus type 1 (= chick embryo lethal orphan (CELO) virus, = gal-1). Diseases caused by aviadenoviruses include e.g. EGGDROP SYNDROME 1976, HAEMORRHAGIC ENTERITIS OF TURKEYS, INCLUSION BODY HEPATITIS, and MARBLE SPLEEN DISEASE. Several avian adenoviruses (including CELO virus) can induce tumours in newborn rodents (cf. MASTADENOVIRUS). avian acute leukaemia viruses (AcLVs) A group of replicationdefective, v-onc+ type C retroviruses (subfamily ONCOVIRINAE) which cause acute erythroid and myeloid leukaemias in chickens; they may also induce carcinomas, endotheliomas and sarcomas. AcLVs include avian myeloblastosis virus, AMV (which carries v-myb); avian erythroblastosis virus, AEV (which carries v-erb); and avian myelocytomatosis virus, MC29 (which carries v-myc). (See also ERB, MYB and MYC.) AcLV replication requires the presence of a replication-competent helper virus (see e.g. AVIAN LEUKOSIS VIRUSES). [Molecular biology of AcLVs: Book ref. 105, pp. 38–63.] avian encephalomyelitis (epidemic tremor) A POULTRY DISEASE caused by an ENTEROVIRUS; infection in adult birds is usually asymptomatic, but in young chicks symptoms may include ataxia, tremors and somnolence. Infection occurs e.g. by ingestion of food contaminated with faeces from infected birds; transmission can also occur via the egg. avian erythroblastosis virus See AVIAN ACUTE LEUKAEMIA VIRUSES. avian infectious bronchitis An acute, highly infectious POULTRY DISEASE caused by a coronavirus (IBV – see CORONAVIRIDAE). The symptoms include gasping, coughing, nasal discharge, etc; mortality rates may be high. Secondary bacterial infection may be common. [Experimentally-produced disease with mixed IBV/Escherichia coli infection: JGV (1985) 66 777–786.] Live vaccines are available. A strain of the avian infectious bronchitis virus (the ‘T’ strain) may cause avian kidney disease (Cummings’ disease). avian infectious laryngotracheitis An acute POULTRY DISEASE, affecting mainly chickens and pheasants, caused by gallid herpesvirus 3. Mortality rates may be high. avian leukosis complex A group of POULTRY DISEASES which includes e.g. LYMPHOID LEUKOSIS and MAREK’S DISEASE. avian leukosis viruses (ALVs; lymphatic leukosis viruses; lymphatic leukaemia viruses) A group of avian type C retroviruses (subfamily ONCOVIRINAE) which usually induce neoplastic disease only after a long latent period (several months or more). The most common neoplasm induced is LYMPHOID LEUKOSIS. (See also OSTEOPETROSIS.) ALVs are replication-competent and v-onc− (see RETROVIRIDAE). In some cases induction of neoplastic disease may involve the insertion of a viral LTR adjacent to and upstream of a c-onc (usually c-myc) sequence, the LTR acting as a promoter for c-onc expression; alternatively, or perhaps additionally, an LTR may have a promoter-unrelated transcription-enhancing function [Book ref. 105, pp. 64–68]. Neoplastic transformation may require the activation of a second cellular gene, B-lym, in addition to c-myc. ALVs often occur in association with replication-defective vonc+ viruses (e.g. AVIAN ACUTE LEUKAEMIA VIRUSES) as ‘helper’ or ‘associated’ viruses; such ALVs include Rous-associated viruses (RAVs) and myeloblastosis-associated viruses (MAVs). 64 azole antifungal agents (but see HELIOZOEA). When an axopodium comes into contact with a prey organism, it adheres to it and may – at least in some cases – immobilize it; the axopodia of some heliozoa have extrusomes (kinetocysts) which may function in immobilizing prey. Ingestion may involve the retraction of the axopodium with the prey, and/or a co-operative engulfing action of a number of pseudopodia in the vicinity. axostyle A rod-like endoskeletal structure which occurs in some protozoa (e.g. Giardia, Hexamita, Trichomonas); in Trichomonas it extends from the anterior end of the cell and projects a short distance beyond the posterior end. (cf. COSTA; see also OXYMONADIDA.) azaserine (CO2 H.CHNH2 .CH2 .O.CO.CH=NH+ =N− ) An ANTIBIOTIC and antitumour agent obtained from Streptomyces sp. It is an analogue of glutamine and blocks a number of reactions in which glutamine acts as an NH2 donor; in particular, it binds to and inactivates phosphoribosylformylglycinamidine synthetase, thus preventing purine (and hence nucleotide) synthesis (see Appendix V(a)). (cf. DON; HADACIDIN.) azathioprine See IMMUNOSUPPRESSION. azdimycin See POLYENE ANTIBIOTICS (b). azide (N3 − ) Azide acts e.g. as a RESPIRATORY INHIBITOR by combining with, and preventing the reduction of, oxidized CYTOCHROME OXIDASES of the aa3 -type. Sodium azide (NaN3 ) is an antimicrobial agent used e.g. as a preservative in some laboratory reagents (0.1% final concentration NaN3 ). It is also used e.g. in certain selective media (0.025% w/v NaN3 ) for the isolation of enterococci from samples of sewage-polluted water; at this concentration coliforms and many other Gram-negative bacteria are inhibited. 3
-azido-3
-deoxythymidine See AZT. azithromycin A MACROLIDE ANTIBIOTIC used e.g. as an alternative to clarithromycin in the prophylaxis and treatment of infections involving members of the M. avium complex (see MAC). The drug tends to concentrate in macrophages and tissue cells. Strains resistant to azithromycin are also resistant to clarithromycin. Side-effects are uncommon; they include nausea, diarrhoea, headaches, dizziness, deafness. (See also QUORUM SENSING.) azlocillin See PENICILLINS. AZM Adoral zone of membranelles: a number of MEMBRANELLES serially arranged in a definite and taxonomically important pattern along the left-hand side of the oral area in many members of the OLIGOHYMENOPHOREA and POLYHYMENOPHOREA; the corresponding ciliature in peritrichs and spirotrichs is sometimes called the adoral ciliary spiral. The AZM is primarily concerned with feeding (i.e., the production of water currents directed towards the cytostome) but is sometimes used also for locomotion. (cf. PARORAL MEMBRANE.) Azoarcus See SPLIT GENE (e). azofer See NITROGENASE. azofermo See NITROGENASE. azoferredoxin See NITROGENASE. azole antifungal agents A group of synthetic, broad-spectrum ANTIFUNGAL AGENTS, many of which are useful in the treatment of mycoses in man and animals while others are effective against many fungal diseases of plants. The group contains two main categories: the imidazole derivatives (e.g. the agriculturally useful BENZIMIDAZOLES and the medical and veterinary drugs bifonazole, butoconazole, CLOTRIMAZOLE, econazole, fenticonazole, isoconazole, KETOCONAZOLE, MICONAZOLE, oxiconazole, sulconazole, TIOCONAZOLE, and zinconazole) and the triazole derivatives (e.g. the agricultural antifungals FLUTRIAFOL, PROPICONAZOLE, TRIADIMEFON, and TRIADIMENOL, and the medical drugs ITRACONAZOLE, terconazole and vibunazole). Most of the medically specific ‘resistance gene’ in certain strain(s) of the host species, eliciting a defensive response (HYPERSENSITIVITY) which results in resistance to that particular strain of the pathogen. The products of such genes appear to be delivered to the interior of plant cells via a type III PROTEIN SECRETION system (designated Hrp: hypersensitivity response and pathogenicity) [see e.g. JB (1997) 179 5655–5662]; under in vitro conditions, the Hrp system in Pseudomonas syringae pv. tomato has been found to include a filamentous pilus, 6–8 nm in diameter, in which HrpA is a major structural protein [PNAS (1997) 94 3459–3464]. (See also HOP PROTEINS.) Avirulence genes form part of the ‘gene-for-gene’ concept that relates to interaction between plants and their pathogens [ARPpath. (1971) 9 275–296]. [Gene-for-gene complementarity in plant–pathogen interactions: ARG (1990) 24 447–463.] Direct interaction between the products of a resistance gene and an avirulence gene has been reported to occur, for example, when strains of rice expressing the Pi-ta resistance gene are challenged with strains of the rice pathogen Magnaporthe grisea (= Pyricularia oryzae) that express AVR-Pita; against this challenge, the plant failed to succumb to rice BLAST DISEASE [EMBO (2000) 19 4004–4014]. Avirulence genes from Pseudomonas syringae commonly have a GC% of ∼40–50, i.e. markedly below the chromosomal values (59–61) typical of the species; such a difference in GC% – also commonly seen between PATHOGENICITY ISLANDS and their host chromosomes – is consistent with the concept of horizontal transfer of these genes. Also consistent with horizontal transfer is the finding of highly conserved sequences flanking avirulence genes in P. syringae pv. pisi [Microbiology (2001) 147 1171–1182]. (See also HARPIN.) avirulent Not exhibiting VIRULENCE. avocado sunblotch viroid See VIROID. avoiding reaction Syn. PHOBIC RESPONSE. avoparcin A glycopeptide antibiotic, produced by Streptomyces candidus, which inhibits Gram-positive bacteria by interfering with peptidoglycan synthesis (see VANCOMYCIN); it is a wellestablished FEED ADDITIVE for pigs and poultry, and has been found to have growth-promoting properties for ruminants. In sheep, avoparcin appears to shift the balance of cellulolytic bacteria from ruminococci to Bacteroides succinogenes [JGM (1985) 131 427–435]. avr gene (plant pathol.) See AVIRULENCE GENE. aw Symbol for WATER ACTIVITY. axenic Of a culture: containing or comprising cells of a single species. Thus, e.g., an axenic bacterial culture is a pure (uncontaminated) culture of a single species or strain, while an axenic tissue culture is one which contains cells (of one or more types) from one species (or individual) with no microbial contaminants, intracellular parasites, viruses etc. Certain microorganisms – e.g. obligately intracellular bacteria, obligately predatory protozoa – cannot be grown axenically. axial fibrils See SPIROCHAETALES. axial filament (1) See ENDOSPORE (bacterial). (2) See SPIROCHAETALES. (3) See AXOPODIUM. axile Situated in the centre or on the axis. axoneme (1) See FLAGELLUM (b). (2) See AXOPODIUM. axopodium A fine, rod-like, often tapering, relatively rigid type of PSEUDOPODIUM which has an axial core of microtubules (the axial filament, axial rod or axoneme). Axopodia occur e.g. in HELIOZOEA and RADIOLARIA in which they typically emanate radially from the cell body. They function mainly in feeding 65 Azolla wavelength) are also formed during growth on solid media at 30° C. Enlarged, ovoid or pleomorphic, non-motile, capsulated forms (‘C forms’) may develop under certain conditions (e.g. in old cultures). Metabolism is mainly respiratory, with either oxygen or NO3 − acting as terminal electron acceptor; DENITRIFICATION occurs under microaerobic conditions. Glucose or fructose may be fermented weakly. Disaccharides are not metabolized. Oxidase +ve; catalase variable; phosphatase +ve; indole −ve; weakly pectinolytic. Optimum growth temperature: 35–37° C. Colonies on potato agar are typically pink, often wrinkled, not slimy. NITROGEN FIXATION occurs under microaerobic conditions; an ‘uptake hydrogenase’ (see NITROGENASE) is present, and A. lipoferum (but not A. brasilense) can grow as an H2 -dependent lithoautotroph. Aerobic growth can also occur in the presence of fixed nitrogen (e.g. NH4 + , NO3 − ). GC%: 69–71. Type species: A. lipoferum (formerly Spirillum lipoferum). [Book ref. 22, pp. 94–104.] Azospirilla occur in soil, both free-living and in association with the roots of grasses (including cereals) and tuberous plants; the bacteria occur on the root surface, in the mucigel, and also within the tissues of the root (outer and inner cortex and stele). The plant appears to benefit from the nitrogen fixed by the bacteria; nodule formation does not occur (cf. ROOT NODULES). There is some degree of host specificity: ‘C4 plants’ (e.g. maize) are infected by A. lipoferum, ‘C3 plants’ (e.g. barley, oats, rice, rye, wheat) by strains of A. brasilense. Azotobacter A genus of Gram-negative, CYST-forming bacteria (family AZOTOBACTERACEAE) which occur e.g. in fertile soils of near-neutral pH. The cells are peritrichously flagellated or nonmotile, and are commonly short rods or coccobacilli – though A. paspali regularly forms both rods and long filaments; the organisms appear to contain many copies of the chromosome per cell [JGM (1984) 130 1603–1612]. Carbon sources used by all species include e.g. glucose, fructose, sucrose, acetate, fumarate, gluconate, and ethanol; most species can use nitrate and/or ammonium salts as a source of nitrogen. In the presence of combined nitrogen growth can occur within the pH range ca. 5–8.5; NITROGEN FIXATION occurs optimally within the pH range 7–7.5. In laboratory cultures nitrogen-fixing ability declines with age, and is very poor immediately prior to encystment. (Cyst-formation occurs maximally in old cultures on nitrogenfree media containing 0.2% butanol.) The optimum growth temperature varies with species, and is ca. 30–37° C. GC%: ca. 63–68. Type species: A. chroococcum. A. armeniacus. Peritrichously flagellated. Can use e.g. caprylate and mannitol but not e.g. caproate; some strains can use propionate. A. beijerinckii. Non-motile. Can use e.g. malonate and propionate but not caproate or rhamnose. A. chroococcum. Peritrichously flaggellated. Can use e.g. caproate, mannitol and propionate, but not e.g. caprylate. A. nigricans. Non-motile. Cannot use caproate, propionate or rhamnose. Some strains form e.g. a yellow or dark non-diffusible pigment or a dark diffusible pigment. A. paspali. Peritrichously flagellated. Cannot use caproate, caprylate, malonate, mannitol, propionate or rhamnose, but can use oxaloacetate, and some strains can use propan-1-ol. Appears to occur only on (or within?) the root cortex of the grass Paspalum notatum. A. vinelandii. Peritrichously flagellated; non-motile strains have been reported. Can use caproate, caprylate, malonate, useful azoles are administered topically, being poorly absorbed from the gut and/or too toxic for systemic use; however, a few (e.g. ketoconazole, itraconazole and vibunazole) can be given orally. With the exception of the BENZIMIDAZOLES, the azoles generally appear to have the same primary mode of action. At minimal fungistatic concentrations they interfere with the permeability and function of the CYTOPLASMIC MEMBRANE by inhibiting the biosynthesis of ergosterol. In the normal biosynthetic pathway ergosterol is synthesized from its precursor lanosterol by a 14a-demethylation reaction which involves a cytochrome P-450dependent monooxygenation step; the azole compound binds (in place of oxygen) to the 6th coordination position of the P-450 haem iron, thus preventing the monooxygenation step. As a result, 14a-methylated sterols accumulate and ergosterol (and cholesterol) levels fall; this not only alters membrane permeability: the altered lipid environment of the membrane also interferes with the activity and/or control of other enzyme systems (e.g. chitin synthase). Miconazole – in addition to its effect on the cytoplasmic membrane – also exerts an apparently direct inhibitory effect on the mitochondrial ATPase in yeasts such as Candida albicans and Saccharomyces cerevisiae [Eur. J. Bioch. (1984) 143 273–276]. Actively growing fungi exposed to minimal fungistatic concentrations of azoles (e.g. 10−8 –10−7 M for miconazole) show characteristic structural changes, including the formation of dense membrane-derived vesicles at the cell periphery. At somewhat higher fungistatic concentrations (e.g. 10−6 M miconazole) changes in the cell vacuole also occur, the vacuole becoming filled with vesicles and granular material. At still higher concentrations (e.g. 10−5 M miconazole) the azoles become fungicidal, causing degeneration of organelles (mitochondria and nuclei). [Review of medically useful azoles: Book ref. 153, pp. 133– 153.] Azolla See ANABAENA. Azomonas A genus of Gram-negative or (A. macrocytogenes) Gram-variable, motile bacteria (family AZOTOBACTERACEAE) which occur e.g. in soil and water. Cysts are not formed (cf. AZOTOBACTER). Most strains form water-soluble pigments, but none forms insoluble pigments; a fluorescent pigment is formed by some strains in iron-deficient media. Carbon sources used by all species include e.g. fructose, glucose, acetate, fumarate, lactate, and ethanol; all species can fix nitrogen and can use ammonium salts as the sole source of nitrogen. Optimum growth temperature: 30–37° C, according to species. GC%: ca. 52–59. Type species: A. agilis. A. agilis. Peritrichously flagellated. Can use e.g. malonate, but not mannitol. Growth can occur at 32° C and 37° C. A. insignis. Lophotrichously flagellated. Can use e.g. malonate, but not mannitol. Growth can occur at 32° C but not at 37° C. A. macrocytogenes. Monotrichously flagellated (rarely biflagellated at one pole). Can use mannitol and maltose, but not malonate. [Book ref. 22, pp. 230–234.] azomycin See NITROIMIDAZOLES. Azorhizobium caulinodans See NITROGEN FIXATION. Azospirillum A genus of Gram-negative or Gram-variable, asporogenous, nitrogen-fixing bacteria. Cells: curved and straight rods, often with pointed ends, ca. 1.0 × 2.1–3.8 µm. Motile by a single polar flagellum; numerous lateral flagella (of shorter 66 azygospore drugs (e.g. PROBENECID) that inhibit glucuronidation may therefore affect the plasma concentration of AZT. The use of AZT is associated with megaloblastic changes; the drug may induce a dose-dependent macrocytic anaemia (less often a severe normocytic anaemia) and is also able to induce neutropenia [haematological aspects of HIV infection: BCH (2000) 13 215–230]. The value of AZT in anti-AIDS combination drug therapy (against HIV-1) was indicated by an in vitro study in which those nucleoside reverse transcriptase inhibitors which lacked the 3 -azido moiety were less active against a particular mutant form of the reverse transcriptase [AAC (2005) 49 1139–1144]. azthreonam Syn. AZTREONAM. aztreonam (azthreonam) A MONOBACTAM derivative in which R = an aminothiazoleoxime group, R = H, R = CH3 , R = SO3 − K+ (see b-LACTAM ANTIBIOTICS). It is active only against aerobic Gram-negative bacteria, having a high degree of specificity for PBP3 of Gram-negative bacteria (see PENICILLINBINDING PROTEINS); it is resistant to most b-lactamases. [Action, use: Drugs (1986) 31 96–130.] Azuki bean mosaic virus See POTYVIRUSES. azure See polychrome METHYLENE BLUE. azurin (1) A BLUE PROTEIN which occurs in certain bacteria (e.g. Alcaligenes denitrificans, Bordetella pertussis, Paracoccus denitrificans, Pseudomonas aeruginosa); depending on source, the MWt ranges from ca. 12000 to ca. 15000, and the Em appears to be in the approximate range 230–330 mV. (2) A solution of CuSO4 and NH4 OH used as an agricultural antifungal agent. azygospore A parthenogenically derived spore, similar to a ZYGOSPORE, formed e.g. by many species of the Mucorales. mannitol, propionate and rhamnose. A yellowish-green watersoluble fluorescent pigment is formed on iron-deficient media. [Book ref. 22, pp. 220–229.] Azotobacteraceae A family of Gram-negative (or Gramvariable), aerobic, chemoorganotrophic bacteria which occur in soil, in the rhizosphere, and in aquatic habitats; the organisms are capable of NITROGEN FIXATION – typically under free-living conditions, but sometimes in association with higher plants. Cells: ovoid, or round-ended rods (sometimes filaments), 1.5–2.0 µm or more in width, which may be motile (polarly or peritrichously flagellated) or non-motile; some species form cysts. Pigments, some fluorescent, are formed by some species. PHB can be accumulated. Catalase +ve. Oxidase +ve (most strains of most species). GC%: ca. 52–68. Two genera: AZOMONAS, AZOTOBACTER. [Book ref. 22, pp. 219–234.] azotoflavin See FLAVODOXINS. AZT (3 -azido-3 -deoxythymidine; zidovudine) A thymidine analogue ANTIRETROVIRAL AGENT used in the treatment of AIDS. AZT is converted by cellular enzymes to the triphosphate derivative which is then incorporated – instead of thymidine – into (provirus) DNA by the viral REVERSE TRANSCRIPTASE (see RETROVIRIDAE); the presence of the 3 -azido group inhibits further chain elongation by preventing the formation of a phosphodiester bond at the 3 position. Cellular DNA polymerase a is much less sensitive than reverse transcriptase to AZT triphosphate. AZT competes for intracellular phosphorylation with another NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITOR, stavudine. As high-level resistance develops readily if AZT is used alone, it is used in combination with other drugs. Unlike other nucleoside reverse transcriptase inhibitors, AZT undergoes significant metabolism (glucuronidation) in the liver; 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 67 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 B No further development of the B cell occurs unless it encounters specific antigen e.g. within so-called germinal centres in the spleen or lymph nodes. In the event of contact with specific antigen, the outcome depends e.g. on the type of antigen involved and the contribution of T cells: see ANTIBODY FORMATION. When a B cell is appropriately stimulated it prepares for the role of antibody formation by initially undergoing BLAST TRANSFORMATION and then proliferating to form a clone of cells of identical antigenic specificity (= clonal expansion); some of these cells develop as PLASMA CELLS while others (commonly) become MEMORY CELLS. B cells which do not bind specific antigen remain viable for a limited period of time and subsequently undergo APOPTOSIS. B-tubule (B-subfibre) See FLAGELLUM (b). B-type inclusion body See POXVIRIDAE. B-type particles See TYPE B ONCOVIRUS GROUP. B-type starter See LACTIC ACID STARTERS. B virus (cercopithecine herpesvirus 1; cercopithecid herpesvirus 1; herpesvirus B; Herpesvirus simiae) A herpesvirus (subfamily ALPHAHERPESVIRINAE) which naturally infects monkeys of the genus Macaca. In rhesus monkeys (M. mulatta) infection may be asymptomatic, or vesicular lesions (which may ulcerate) may develop in the mouth and sometimes on the skin and conjunctivae. B virus may be transmitted to humans e.g. by monkey bites or scratches, and can cause in humans a severe (usually fatal) encephalomyelitis (‘monkey-bite encephalomyelitis’). B12 coenzymes See VITAMIN B12 . B19 parvovirus See ERYTHROVIRUS. B663 CLOFAZIMINE. Babes–Ernst granules METACHROMATIC GRANULES observed in bacteria. Babesia A genus of protozoa (subclass PIROPLASMASINA) parasitic in invertebrates and in the erythrocytes (RBCs) of vertebrates (cf. THEILERIA); at least some species (including B. equi and B. microti ) develop in the lymphocytes (as well as in the RBCs) of the vertebrate host, and it has been suggested that such species be transferred to other genera (e.g. Nicollia, Nuttallia). Some species (e.g. B. bigemina, B. bovis) can cause tick-borne disease in domestic animals (see REDWATER FEVER), and some (e.g. B. divergens, B. microti ) can cause disease in man [ARE (1981) 26 90–92]. The cells of Babesia are rounded or pyriform, ca. 1–6 µm. In the typical life cycle, sporozoites of Babesia are injected into the vertebrate host by the tick vector, and each sporozoite enters an RBC and undergoes schizogony to form two or four merozoites – which infect fresh RBCs when the host cell ruptures. Following the tick’s meal of infected blood, gametes are formed in the tick’s gut, and these fuse and give rise to a (motile) kinete which passes, via various tissues, to the salivary glands – there undergoing sporogony and forming many sporozoites. Unlike Theileria, Babesia can invade the ovary and egg of the tick, and can be transmitted transovarially to the tick’s offspring. [Life cycles of Babesia and Theileria: AP (1984) 23 37–103.] babesiosis Any disease of man or animals caused by a species of BABESIA (e.g. REDWATER FEVER). Bacillaria See DIATOMS. Bacillariophyta See DIATOMS. bacillary angiomatosis See BARTONELLA. B cell (immunol.) Syn. B LYMPHOCYTE. B cell differentiation factor See LYMPHOKINES. B cell growth factor See LYMPHOKINES. B cell superantigen See SUPERANTIGEN. B-DNA See DNA. B lymphocyte (B cell) A type of LYMPHOCYTE concerned primarily with ANTIBODY FORMATION (cf. T LYMPHOCYTE). In mammals, B cells are formed initially in the fetal liver, but later they develop from haemopoietic stem cells in the bone marrow. (In birds, B cells appear to develop in the BURSA OF FABRICIUS.) B cells occur e.g. in blood, in lymph and in the spleen. In an adult, the B cell population consists of >106 clones, the cells of each clone being potential sources of antibodies with a unique and highly specific antigen-binding capacity. In mammals, B cells mature in a series of stages. Stem cells give rise to pro-B cells which, in turn, develop as pre-B cells. Pre-B cells differ from mature B cells e.g. in that they express no surface immunoglobulin (Ig), i.e. they are sIg− , although they contain mu HEAVY CHAINS (associated with so-called surrogate light chains) which may be located at the cell surface. The next stage, the immature B cell, is characterized by the presence of monomeric IgM at the cell surface; interaction with specific antigen at this stage may lead to inactivation of the cell. The mature B cell displays IgM and IgD at the cell surface (with antigen-binding sites facing outwards); the IgM and IgD have identical antigenic specificity. The B cell reaches maturity, in an antigen-independent way, within the bone marrow. In addition to antibodies, the surface of a mature B cell displays various types of molecule involved in the recognition of exogenous factors and the initiation of intracellular signalling. These molecules include: (i) a receptor that mediates isotype switching (= class switching – e.g. the IgM → IgG switch following antigenic stimulation) (see CD40); (ii) MHC class II molecules (required for presenting antigen to a T cell: see ANTIBODY FORMATION); (iii) co-stimulatory molecules (designated B7.1 and B7.2) which, during B cell–T cell contact, bind to a specific T cell receptor and initiate signals that activate the T cell (see CD28); (iv) CD32: a low-affinity receptor for the Fc portion of IgG (the binding of antigen-bound antibody to CD32 may suppress the ongoing production of specific antibodies); (v) CD21 (involved in B cell activation, and a binding site for certain components of COMPLEMENT); (vi) receptors for CYTOKINES (including INTERLEUKIN-1; INTERLEUKIN-4, which e.g. induces isotype switching and promotes expression of MHC class II molecules on B cells; and INTERLEUKIN-6). The body’s B cell population can be divided into three subsets on the basis of the CD5 and CD23 antigens. B cells in the general circulation are mainly CD5− , CD23+ ; they have a relatively short existence and are continually renewed from the bone marrow stem cells. Two other subsets, one CD5+ , CD23− , and the other CD5− , CD23− , are found mainly in mucosal sites (e.g. the peritoneal cavity) and are self-renewing (albeit with precursor cells in the bone marrow); these cells can rapidly produce IgM antibodies (of a limited range of specificities) in response to bacterial polysaccharides, and can do so without help from T cells. These two populations of B cells are viewed as a first line of defence against bacterial incursion in sites which are particularly vulnerable to infection. 68 Bacillus bacillary dysentery See DYSENTERY (a). bacillary white diarrhoea Syn. PULLORUM DISEASE. bacille Calmette–Guérin See BCG. bacillin Syn. BACILYSIN. bacillus (1) A member of the genus BACILLUS. (2) Any rodshaped bacterial cell, i.e., a cell whose length is ca. two or more times greater than its width. (cf. COCCOBACILLUS and FILAMENT.) A bacillus may be straight or curved (cf. VIBRIO sense 2), with rounded, truncated or tapered ends (cf. FUSIFORM), and may occur singly, in groups, pairs, chains, etc. (See also PALISADE.) Bacillus A genus of GRAM TYPE-positive, strictly aerobic or facultatively anaerobic, typically catalase-positive, rod-shaped, ENDOSPORE-forming bacteria. The organisms typically occur as saprotrophs in soil and water, but certain species can be pathogenic in man and other mammals (see e.g. B. anthracis and B. cereus, below) and some species are entomopathogenic (see e.g. B. moritai, B. popilliae and B. thuringiensis). [Insecticidal species: Book ref. 171, pp. 185–209; MR (1986) 50 1–24.] Bacillus spp can cause biodeterioration (see e.g. FLAT SOUR, LEATHER SPOILAGE, SWELL), but some species are used commercially as sources of antibiotics (see e.g. BACITRACIN, POLYMYXINS) or other products (e.g. DEBRANCHING ENZYMES, GLUCOSE ISOMERASE, GLYCEROKINASE, SUBTILISINS). Cells: typically motile rods, commonly ca. 0.5–1.5 × 2–6 µm, often in chains; some strains form a CAPSULE, and some are pigmented. (See also S LAYER and TEICHOIC ACIDS.) Metabolism may be respiratory (see RESPIRATION) – some strains being capable of NITRATE RESPIRATION – or facultatively fermentative (see FERMENTATION); a few species (e.g. B. macerans, B. polymyxa) can carry out NITROGEN FIXATION. Most species are chemoorganoheterotrophs; B. schlegelii can grow chemolithoautotrophically (see CARBOXYDOBACTERIA). Utilizable substrates for organoheterotrophic species range from simple sugars and other carbohydrates to e.g. uric acid (see B. fastidiosus) and proteinaceous materials; PENTOSES are metabolized via the HEXOSE MONOPHOSPHATE PATHWAY. Many species can grow on NUTRIENT AGAR. Storage compounds include e.g. POLY-b-HYDROXYBUTYRATE. The genus includes psychrotrophs (e.g. B. globisporus), thermophiles (e.g. B. schlegelii, B. stearothermophilus) and alkalophiles (e.g. B. alcalophilus, B. firmus). GC%: ca. 30–70. Type species: B. subtilis. B. alcalophilus. An ALKALOPHILE (q.v.): optimum growth pH ca. 9–10; no growth below pH 7. B. alvei. See EUROPEAN FOULBROOD. B. amyloliquefaciens. Similar or identical to B. subtilis. (See also AMYLASES.) B. amylolyticus. Nom. rev. [IJSB (1984) 34 224–226]. B. aneurinolyticus. A species, related to B. brevis, which produces a thiaminase. (See also BACTERIOPHAGE fBA1.) B. anthracis. The causal agent of ANTHRAX. Cells: nonmotile, ca. 1.5 × 3–6 µm, typically square-ended and usually in chains. Virulent strains form a PLASMID-encoded toxin (see ANTHRAX TOXIN) and a plasmid-encoded poly-D-glutamic acid CAPSULE. [Capsule-encoding plasmid: see e.g. Inf. Immun. (1985) 49 291–297.] (cf. STERNE STRAIN.) Growth occurs on nutrient agar. In biochemical tests B. anthracis gives results very similar to those of B. cereus. Lecithinase activity is weak or absent. Haemolysis on sheep-blood agar is weak or absent. B. anthracis is susceptible to the ‘g phage’ (B. cereus is not). On agar containing benzylpenicillin (0.05–0.5 unit/ml) B. anthracis gives rise to chains of enlarged, spherical cells (‘string of pearls’ effect). B. azotofixans. A proposed (nitrogen-fixing) species from Brazilian soil [IJSB (1984) 34 451–456]. B. badius. Similar to B. brevis. Growth is not inhibited by 5% NaCl. The (ellipsoidal) spore does not distend the cell. B. brevis. Typically forms no acid or gas from carbohydrates. Growth occurs at 50° C. Spore: ellipsoidal, distending the cell. (See also TYROCIDINS.) ‘B. caldolyticus group’. Strains (including B. caldotenax and B. caldovelox ) closely related to B. stearothermophilus. B. cereus. A saprotroph or opportunist pathogen which can cause FOOD POISONING (q.v.) or, rarely, e.g. meningitis. [Meningitis due to B. cereus, and review of Bacillus infections other than anthrax: Israel J. Med. Sci. (1983) 19 546–551.] Cells: typically motile rods (mean width ca. 1.5 µm), sometimes pigmented. Spore: ellipsoidal, not distending the cell. Acid is formed in anaerobic glucose broth; no acid is formed from arabinose, mannitol or xylose. No gas is formed from carbohydrates. Most strains are VP +ve. Starch and casein are hydrolysed, and lecithinases are produced. Nitrate is reduced by most strains. No growth at 50° C. B. circulans. Similar to B. polymyxa but ferments carbohydrates anaerogenically; typically, a range of carbohydrates is fermented. (See also MOTILE COLONIES.) B. coagulans. Forms lactic acid as the major product of glucose fermentation; 2,3-butanediol is also produced. No gas is formed from carbohydrates. VP +ve. (See also GLUCOSE ISOMERASE.) B. euloomarahae. See MILKY DISEASE. B. fastidiosus. Obligately aerobic. Uses uric acid as a source of carbon, nitrogen and energy. B. firmus. An ALKALOPHILE. Forms acid from glucose but typically from few other carbohydrates. B. fribourgensis. See MILKY DISEASE. B. globisporus. A PSYCHROTROPH (maximum growth temperature ca. 25–30° C). Urease +ve. Spore: round, distending the cell. B. larvae. See AMERICAN FOULBROOD. B. lautus. Nom. rev. [IJSB (1984) 34 224–226]. B. lentimorbus. See MILKY DISEASE. B. licheniformis. Morphologically and biochemically similar to B. subtilis, but growth readily occurs anaerobically (see also DENITRIFYING BACTERIA). B. macerans. Biochemically very similar to B. polymyxa. Forms SCHARDINGER DEXTRINS. B. megaterium. Cell width commonly ca. 1.5 µm but may reach ca. 3.0 µm in carbohydrate-containing media. Spore: ellipsoidal, not distending the cell. Glucose and many other carbohydrates are utilized anaerogenically. B. moritai. A species pathogenic e.g. in house-flies. The addition of B. moritai spores to faeces has been found to reduce the emergence of adult house-flies by up to ca. 90%. B. mycoides. Morphologically and biochemically very similar to B. cereus. Often regarded as a non-motile variety of B. cereus. B. pabuli. Nom. rev. [IJSB (1984) 34 224–226]. B. pasteurii. An ALKALOPHILE (optimum growth at ca. pH 9). Cells: typically slender (ca. 0.5–1.0 µm wide). Spore: spherical, typically distending the cell. Carbohydrates are attacked either very weakly or not at all. Urease +ve. NH3 is needed for growth. ‘B. piliformis’. See TYZZER’S DISEASE. B. polymyxa. Cells: typically slender (ca. 0.5–1.0 µm wide); usually motile. Spore: ellipsoidal, distending the cell. Grows well anaerobically, and produces acid and gas from carbohydrates; typically, glucose, mannitol, and many other sugars and sugar alcohols are utilized. Glucose is fermented via the BUTANEDIOL FERMENTATION. Most strains attack e.g. casein, gelatin, starch 69 bacillus Calmette–Guérin and pectins, and most can carry out NITROGEN FIXATION. (See also POLYMYXINS.) B. popilliae. See MILKY DISEASE. B. psychrophilus. A proposed (psychrophilic) species [IJSB (1984) 34 121–123]. B. pulvifaciens. Proposed species (isolated from a diseased bee) [IJSB (1984) 34 410–413]. B. pumilus. Cells: typically slender (0.5–1.0 µm wide); usually motile. Spore: elongated, not distending the cell. Metabolically similar to B. subtilis, but starch is not utilized. Usually VP +ve. B. schlegelii. See CARBOXYDOBACTERIA and HYDROGEN-OXIDIZING BACTERIA. B. sphaericus. Cells: slender (ca. 0.5–1.0 µm wide); usually motile. Spore: spherical, distending the cell. Typically, carbohydrates are not utilized. Can cause LEATHER SPOILAGE, and can be insecticidal to e.g. mosquito larvae – toxicity apparently being due to an uncharacterized toxin and not associated with sporulation. [Field evaluation of a B. sphaericus strain as a mosquito ‘biocide’: J. Inv. Path. (1986) 48 133–138.] B. stearothermophilus. Thermophilic; can grow at e.g. 65° C. Cells: ca. 1.0 µm wide; spore: ellipsoidal, often distending the cell. The spores are highly resistant to heat, and are sometimes used to monitor AUTOCLAVE performance. (See also FLAT SOUR.) Typically, a range of carbohydrates can be metabolized anaerobically and anaerogenically; the main product of carbohydrate fermentation commonly appears to be lactic acid. (See also GLYCEROKINASE.) B. subtilis. Cells: slender (typically ca. 0.8 µm wide), usually motile; chains are uncommon. (See also CERULENIN and MACROFIBRE.) Spore: ellipsoidal, usually not distending the cell. Metabolism appears to be primarily respiratory. Growth does not occur in anaerobic glucose broth. Glucose, various other sugars and sugar alcohols, and starch, are metabolized anaerogenically. VP +ve. Nitrate is reduced. Casein and gelatin are hydrolysed. Phages which infect B. subtilis include e.g. BACTERIOPHAGE PBS1, BACTERIOPHAGE f105 and BACTERIOPHAGE SPO1. B. thuringiensis. An entomopathogenic species which is morphologically and biochemically very similar to B. cereus, differing primarily in its formation of DELTA ENDOTOXIN (q.v.). B. validus. Nom. rev. [IJSB (1984) 34 224–226]. Other species include e.g. B. insolitus, B. laterosporus, B. lentus, and B. pantothenticus. Note on the identification of species. Some mutually contradictory tables of biochemical test reactions have been published. For example, the VP test reactions (obtained using traditional methods) given in Book ref. 46, p. 1731 differ significantly, in respect of a number of species, from those (obtained using the API system) given in JGM (1984) 130 1871–1882. (Species designated as ‘VP +ve’ above are confirmed as such in many sources.) bacillus Calmette–Guérin See BCG. bacilysin (‘bacillin’; ‘tetaine’) A dipeptide ANTIBIOTIC which is active against a wide range of Gram-positive and Gramnegative bacteria and e.g. against Candida albicans. It consists of a C-terminal epoxy-L-amino acid (‘anticapsin’) and an Nterminal L-alanine residue; it is taken up by peptide transport systems in sensitive cells, and is subsequently hydrolysed by cellular enzymes to release the anticapsin – an inhibitor of glucosamine (and hence e.g. PEPTIDOGLYCAN) synthesis. (cf. WARHEAD DELIVERY.) bacitracin A cyclic dodecapeptide ANTIBIOTIC produced by strains of Bacillus spp. It is active against many Grampositive and certain Gram-negative bacteria (e.g. Neisseria spp, Haemophilus spp). In the presence of divalent cations (particularly Zn2+ ) bacitracin binds to bactoprenol pyrophosphate, inhibiting the regeneration of bactoprenol monophosphate during e.g. PEPTIDOGLYCAN biosynthesis; it can also inhibit other processes involving bactoprenol pyrophosphate (e.g. O-specific chain formation in LIPOPOLYSACCHARIDE biosynthesis) and can affect membrane permeability. Bacitracin is used clinically e.g. for the topical treatment of local infections, and as a FEED ADDITIVE for ruminants to decrease methane production in the RUMEN. back focal plane Of a convex lens: the focal plane furthest from the light source. back mutation (reverse mutation) (1) A MUTATION which reverses the effects of a FORWARD MUTATION by restoring the original nucleotide sequence. (2) Either a mutation as in sense 1 above, or an intragenic SUPPRESSOR MUTATION. background mutation Syn. SPONTANEOUS MUTATION. bacon spoilage See MEAT SPOILAGE. BACTEC culture systems Liquid media (marketed by Becton Dickinson) which can be used e.g. for the growth of Mycobacterium tuberculosis – growth being detectable more rapidly (∼1–2 weeks) than is usually possible on solid media (∼6 weeks). These media may be used for (i) detecting M. tuberculosis in clinical specimens, and (ii) examining an isolate of the pathogen for susceptibility to antibiotics; for the latter purpose an isolate is tested for growth (or lack of growth) in a medium containing a known amount of the given antibiotic. Earlier BACTEC systems were radiometric, i.e. they detected growth by detecting radioactive carbon dioxide produced from a radioactive substrate in the medium. A more recent system, the BACTEC MGIT 960 (MGIT = mycobacteria growth indicator tube), monitors growth by means of a fluorescent sensor system which detects the consumption of oxygen. [Evaluation of BACTEC MGIT 960: JCM (1999) 37 748–752.] bacteraemia (bacteremia) The condition in which viable bacteria are present in the bloodstream. (cf. PYAEMIA; SEPTICAEMIA.) bacteremia Syn. BACTERAEMIA. bacteria (singular: bacterium) (1) Formerly: a term which referred, collectively, to all prokaryotic microorganisms (see PROKARYOTE); the term was also used to refer to a population of specific prokaryote(s) of any given type. (2) All, or particular, members of the domain Bacteria (see next entry). Note. In literature prior to the 1980s ‘bacteria’ was always used with the meaning given in sense 1. With the establishment of the domain ARCHAEA (q.v.), appropriate usage is that given in sense 2. Bacteria A taxon (DOMAIN) comprising one of the two fundamentally distinct groups of prokaryotic microorganisms (see PROKARYOTE and ARCHAEA). (See also previous entry.) The bacteria are a diverse group of (usually) single-celled organisms. Most are free-living, occurring e.g. in soil, on plants, in various aquatic habitats, and even in antarctic snow [AEM (2000) 66 4514–4517]; some are important in the cycles of matter (see CARBON CYCLE, NITROGEN CYCLE, SULPHUR CYCLE). Bacteria are also found as symbionts in plants, animals and certain microorganisms (see CAEDIBACTER, MYCETOCYTE, ROOT NODULES, RUMEN). A minority of bacteria occur as intracellular or extracellular parasites or pathogens in man and/or other animals. In man, bacteria can cause a number of major and minor diseases – which include e.g. ANTHRAX, BOTULISM, BRUCELLOSIS, CHOLERA, DIPHTHERIA, DYSENTERY, ERYSIPELAS, GONORRHOEA, LEGIONNAIRE’S DISEASE, LEPROSY, LYME DISEASE, MENINGITIS, PLAGUE, PSEUDOMEMBRANOUS COLITIS, Q FEVER, SCARLET 70 Bacteria Bacteria Archaea CYANOBACTERIA GRAM POSITIVE EURYARCHAEOTA Anabaena sp Bacillus subtilis Clostridium perfringens Corynebacterium xerosis Enterococcus faecalis Lactobacillus delbrueckii Listeria monocytogenes Mycobacterium tuberculosis Propionibacterium acnes Streptomyces coelicolor Halobacterium halobium Desulfurococcus mobilis Methanobacterium bryantii Methanococcus jannaschii Pyrodictium occultum Gloeobacter sp Oscillatoria sp Prochloron didemni Thermoplasma acidophilum CRENARCHAEOTA Sulfolobus solfatarius Thermoproteus tenax PROTEOBACTERIA (Alpha) (Beta) (Gamma) (Epsilon) Agrobacterium tumefasciens Alcaligenes faecalis Coxiella burnetii Helicobacter pylori Brucella abortus Bordetella pertussis Escherichia coli Rickettsia spp Neisseria gonorrhoeae Neisseria meningitidis Haemophilus influenzae Legionella pneumophila Spirillum natans Proteus vulgaris Pseudomonas aeruginosa Vibrio parahaemolyticus BACTERIA: examples of species in some of the groups in a current taxonomic scheme for the Bacteria (some species of the domain Archaea are also shown); the lines are not intended to reflect evolutionary distances between the organisms. A number of groups not shown in the figure are outlined in the text; note, for example, that the Gram-positive species are divided into ‘high GC%’ and ‘low GC%’. Reproduced from Bacteria, 5th edition, Figure 16.5, page 436, Paul Singleton (1999) copyright John Wiley & Sons Ltd (UK) (ISBN 0471-98880-4) with permission from the publisher. FEVER, SYPHILIS, TETANUS, TOXIC SHOCK SYNDROME, TRACHOMA, bacterial morphology: Microbiology (1998) 144 2803–2808]. (See also e.g. COCCOBACILLUS, FILAMENT, L FORM, MYCELIUM, PLEOMORPHISM (sense 1), SPIROCHAETALES and VIBRIO (sense 2).) In general, the shape of a bacterium is determined primarily by its species. (cf. PROTOPLAST.) According to species, a bacterial cell may have certain appendages: see e.g. FIMBRIAE, FLAGELLUM, PILI, PROSTHECA AND SPINA. (See also CAPSULE.) Many types of bacteria are motile (see MOTILITY); motile species commonly exhibit CHEMOTAXIS. Cells may occur singly or in pairs, chains, clusters, PACKETS, PALISADES etc.; some bacteria of the order ACTINOMYCETALES form a mycelium. (See also COENOCYTE and CONSORTIUM.) Some species form EXOSPORES; some form ENDOSPORES. Members of the MYXOBACTERALES form fruiting bodies. A bacterial cell lacks the sophisticated physical compartmentalization of eukaryotic cells. However, bacteria exhibit a ‘functional compartmentalization’ [FEMS Reviews (1993) 104 327–346]; this refers to various molecular strategies – such as the self-assembly of protein components of composite enzyme systems (see e.g. PROTEASOME). Moreover, the localization of proteins in a bacterium now appears to be much more organized than was previously supposed [Science (1997) 276 712–718]. [Dynamic spatial regulation in the bacterial cell: Cell (2000) 100 89–98.] TUBERCULOSIS, TULARAEMIA, TYPHOID, TYPHUS FEVERS, WHIPPLE’S and WHOOPING COUGH. Bacteria cause economically important diseases in livestock as well as infections in wild animals – see e.g. CATTLE DISEASES, FISH DISEASES, HORSE DISEASES, PIG DISEASES, SHEEP DISEASES. (See also INSECT DISEASES.) Relatively few bacteria cause disease in plants (but see e.g. ERWINIA, SPIROPLASMA, XANTHOMONAS). Predatory/bacteriolytic bacteria include species of ANAEROPLASMA, BDELLOVIBRIO, MYXOBACTERALES and VAMPIROVIBRIO. Bacteria are used in a number of commercial/manufacturing processes: see e.g. BIOPOL, DAIRY PRODUCTS, GLUTAMIC ACID, LEACHING, PICKLING, RETTING, SAUERKRAUT, SOY SAUCE, SUBTILISINS, VINEGAR, VITAMIN B12. Certain antibiotics (e.g. POLYMYXINS, STREPTOMYCIN and some b-LACTAM ANTIBIOTICS) can be synthesized by bacteria. (See also BIOASSAY, BIOFUEL CELL and BIOLOGICAL CONTROL.) In size, most bacteria are between 1 and 10 µm (maximum dimension). The smallest range from <1 µm (e.g. Chlamydia, Francisella, Rickettsia). One of the largest (>600 µm) is Epulopiscium fishelsoni [Nature (1993) 362 239–241]. The basic shapes of bacteria are BACILLUS (sense 2) (also called ‘rod’), COCCUS and SPIRILLUM (sense 1); the coccus seems likely to be a degenerate form of the rod [evolution of DISEASE 71 bacterial blotch of mushrooms • Gram-positive bacteria (high GC%). Species of Arthrobacter, Bifidobacterium, Corynebacterium, Faenia, Frankia, Gardnerella, Mycobacterium and Streptomyces. • Gram-positive bacteria (low GC%). Species of e.g. Bacillus, Clostridium, Desulfotomaculum, Enterococcus, Erysipelothrix, Gemella, Lactobacillus, Leuconostoc, Listeria and Pediococcus. • Green non-sulphur bacteria. Species of e.g. Chloroflexus. • Planctomyces/Chlamydia group. Species of e.g. Chlamydia, Isosphaera, Planctomyces. • Proteobacteria (purple bacteria). See PROTEOBACTERIA. • Spirochaetes. Species of e.g. Borrelia, Leptonema, Spirochaeta, Treponema. • Thermotogales. Species of e.g. Fervidobacterium, Geotoga, Thermotoga. In most species there is a characteristic type of CELL WALL in which PEPTIDOGLYCAN is a common constituent (cf. ARCHAEA); the mycoplasmas are atypical in being wall-less. Ester-linked lipids occur in the bacterial CYTOPLASMIC MEMBRANE (cf. ARCHAEA). In common with e.g. higher animals and plants, the cytoplasmic membrane in many (not all) bacteria (and in some archaeans) contains aquaporins and/or glycerol facilitators: see MIP CHANNELS. (See also OSMOREGULATION.) Various TRANSPORT SYSTEMS are associated with the cell envelope (see e.g. PROTEIN SECRETION and PTS). TWO-COMPONENT REGULATORY SYSTEMS regulate responses to various environmental stimuli. Energy may be obtained by FERMENTATION (sense 1), by RESPIRATION, and/or (in e.g. CYANOBACTERIA) by PHOTOSYNTHESIS. (See also PURPLE MEMBRANE.) According to species, bacteria may be obligate or facultative AEROBES or ANAEROBES. They may be CHEMOTROPHS and/or PHOTOTROPHS, HETEROTROPHS or AUTOTROPHS, and some are CHEMOLITHOAUTOTROPHS. The bacterial genome commonly consists of covalentlyclosed circular DNA; however, in some bacteria (e.g. Borrelia burgdorferi and species of Streptomyces) the DNA is linear. The number of chromosomes per cell depends e.g. on species and on the growth rate. In Vibrio cholerae the genome consists of two circular chromosomes (chromosome 1 = ca. 2961 kb; chromosome 2 = ca. 1073 kb) [Nature (2000) 406 477–483]. For some species, the complete sequence of nucleotides in the genome has been determined – e.g. Borrelia burgdorferi [Nature (1997) 390 580–586], Buchnera [Nature (2000) 407 81–86], Escherichia coli [Science (1997) 277 1453–1474], E. coli O157:H7 [Nature (2001) 409 529–533; erratum: Nature (2001) 410 240], Helicobacter pylori [Nature (1997) 388 539–547], Mycobacterium tuberculosis [Nature (1998) 393 537–544] and Neisseria meningitidis (serogroup A strain) [Nature (2000) 404 502–506]. In many bacteria the genome is supplemented by one or more plasmids (see PLASMID); bacterial plasmids are commonly circular, but some are linear. Certain bacteria are normally plasmidfree; they include species of Anaplasma, Bartonella, Brucella and Rickettsia. (See also BACTERIOPHAGE and LYSOGENY.) Reproduction occurs asexually, usually by BINARY FISSION but sometimes by BUDDING or TERNARY FISSION. (See also CELL CYCLE.) A developmental cycle occurs e.g. in CAULOBACTER, CHLAMYDIA and RHODOMICROBIUM. Despite the lack of sexual reproduction, gene transfer between bacteria can occur by CONJUGATION, CONJUGATIVE TRANSPOSITION, TRANSDUCTION or TRANSFORMATION. Taxonomy. Until the 1980s, bacterial TAXONOMY was based primarily on criteria such as staining reaction, morphology and substrate requirements. Although some of the earlier taxa (e.g. ENTEROBACTERIACEAE) are still recognized, the modern approach is based mainly on molecular criteria, i.e. base sequences in nucleic acids. Using molecular criteria, bacteria have been divided into a number of groups which are believed to reflect evolutionary affinities; some of these groups are shown in the figure on page 71. An outline of the scheme is as follows. Bacteria from ancient sources. It has been reported that an organism resembling Bacillus has been cultured from a brine inclusion located within a salt crystal believed to be ∼250 million years old [Nature (2000) 407 897–900; discussion 844–845]. [Bacteria (general text): Book ref. 223.] bacterial blotch of mushrooms See BROWN BLOTCH and GINGER BLOTCH. bacterial endocarditis ENDOCARDITIS caused by bacteria. bacterial fin rot A common FISH DISEASE in which infection e.g. by species of Flexibacter, Aeromonas or Pseudomonas leads to progressive necrosis of fins and tail; stress is an important factor in disease development. Secondary SAPROLEGNIASIS is common. bacterial gill disease A FISH DISEASE affecting salmonids. Filamentous bacteria (genus uncertain) cover the gills, and death by asphyxia may result. The primary cause is unknown, but overcrowding is a predisposing factor. bacterial kidney disease See KIDNEY DISEASE. bacterial leaching (of ores) See LEACHING. bacterial overgrowth syndrome See GASTROINTESTINAL TRACT FLORA. bacterial vaginosis (non-specific vaginitis) A syndrome characterized by a malodorous vaginal discharge and an increase, in the vagina, in the numbers of certain bacteria – e.g. species of Bacteroides and GARDNERELLA (see also MOBILUNCUS); under these conditions the vaginal pH is usually >4.5, the Eh of the vaginal epithelial surface is ca. +71 mV to −257 mV (normal values ca. +322 mV to +137 mV) [JID (1985) 152 379–382], and ‘clue cells’ (vaginal epithelial cells coated with small Gramnegative rods) can usually be seen in vaginal smears. In cases of bacterial vaginosis, vaginal secretions typically give a fishy, amine-like odour when treated with 10% KOH. [Diagnostic criteria: Am. J. Med. (1983) 74 14–22; JCM (1985) 22 686–687.] Bacteriastrum See DIATOMS. bactericidal (bacteriocidal) Able to kill at least some types of bacteria. (cf. BACTERIOSTATIC.) bactericidal/permeability-increasing protein See BPI PROTEIN. bactericidin (1) An antibody which, under appropriate conditions, can act as a bactericidal agent. (2) A non-specific bactericidal plasma factor (see e.g. COMPLEMENT FIXATION). (3) An antibacterial protein produced by an invertebrate – see e.g. SARCOTOXINS. bacterin A VACCINE containing killed bacterial cells. bacteriochlorophylls See CHLOROPHYLLS. bacteriocidal Syn. BACTERICIDAL. bacteriocin (bacteriocine) Any of a wide variety of (usually) protein or peptide ANTIBIOTICS, produced by certain strains of • Cyanobacteria (see CYANOBACTERIA). • Cytophaga/Flexibacter/Bacteroides group. As well as the named organisms, the group includes species of e.g. Flavobacterium, Microscilla, Saprospira, Sphingobacterium, Spirosoma and Sporocytophaga. • Fibrobacteria. Fibrobacter spp. • Fusobacteria. Fusobacterium, Leptotrichia spp. 72 bacteriocin studies on pediocin PA-1 (a class IIa bacteriocin produced by Pediococcus spp) reported binding to lipsomes in the absence of a protein receptor [AEM (1997) 63 524–531] – suggesting that a protein receptor may not be an absolute requirement for these bacteriocins; it may be that these (cationic) bacteriocins bind to anionic phospholipid groups in the membrane. A given bacteriocin acts on a susceptible target cell in a characteristic way. Some bacteriocins (e.g. some colicins, lantibiotics) form pores in the cytoplasmic membrane; some (e.g. microcin B17) inhibit DNA gyrase; some (e.g. LYSOSTAPHIN) disrupt PEPTIDOGLYCAN; and some (e.g. CLOACIN DF13) cleave 16S rRNA. It appears that, in some cases, a single bacteriocin molecule can be lethal for a sensitive cell. (Although bacteriocins are active primarily against bacteria, certain colicins and VIBRIOCINS have been reported to affect some types of eukaryotic cell [JAC (1980) 6 424–427].) Various mechanisms provide a cell with immunity to the bacteriocin(s) it encodes. For example, while cells actively secreting colicins are damaged by their lysis proteins, neighbouring cells of the same strain, repressed for colicin synthesis, are protected by immunity proteins (see COLICINS). Cells producing LYSOSTAPHIN achieve immunity by modifying their own peptidoglycan. Cells producing the lantibiotic EPIDERMIN achieve immunity by operating an ATP-dependent pump (transport system) which actively secretes any molecules of the agent that are taken up. Biotechnological applications of bacteriocins. The great diversity of bacteriocins, and the ability of many of them to kill/inhibit certain pathogenic bacteria, has prompted much research into the possibility of using particular bacteriocins as antibiotics and/or as food preservatives/additives (see e.g. LACTICIN 3147). Particularly useful bacteriocins are produced by certain LACTIC ACID BACTERIA (e.g. species of Carnobacterium, Enterococcus, Lactobacillus and Lactococcus), some of which are highly active against important food-borne pathogens (such as Listeria monocytogenes). According to structural and other characteristics, bacteriocins of the lactic acid bacteria have been classified into four classes [FEMS Reviews (1993) 12 39–86]: Class I. LANTIBIOTICS: small, typically pore-forming peptides containing unusual constituents (such as lanthionine) – e.g. EPIDERMIN and NISIN. Class II. Small peptides (<10 kDa) which lack unusual constituents (such as lanthionine) and which have a specific form of processing site in the precursor molecule; they exhibit stability at e.g. 100–120° C. This category is divided into three subgroups. Class IIa bacteriocins contain a specific N-terminal sequence, and all are highly active against Listeria spp. They include PEDIOCIN PA-1 and SAKACIN A. [Biosynthesis, structure and activity of class IIa bacteriocins: FEMS Reviews (2000) 24 85–106.] Class IIb bacteriocins are two-component, poreforming agents. They include e.g. lactacin F and lactococcin G. Class IIc bacteriocins are thiol-activated peptides which require reduced cysteine residues for activity. Class III. Large (>30 kDa) heat-labile proteins; they include helveticin J, lactacin A and lactacin B. Class IV. Bacteriocins which consist of a protein moiety plus at least one non-protein (e.g. lipid) constituent which is necessary for activity. They include lactocin 27, leuconocin S and plantaricin S. Other bacteriocins. See also PESTICIN I, PYOCINS, STAPHYLOCOCCIN and ULCERACIN 378. Bacteriocins are also produced by Bacillus megaterium (megacins), Klebsiella spp (klebicins), Listeria monocytogenes (monocins [Zbl. Bakt. Hyg. A (1986) 261 Gram-positive and Gram-negative bacteria, which are bacteriostatic or bactericidal – often specifically to organisms that are closely related to the bacteriocin-producing strain. Bacteriocins include e.g. COLICINS, MICROCINS and LANTIBIOTICS. Agents analogous to bacteriocins are produced by members of the Archaea; these agents (e.g. the halocins produced by halobacteria) are apparently not homologous to any bacteriocin in terms of amino acid sequences. In size and structure, bacteriocins range from a simple modified amino acid (microcin A15), through short peptides and high-MWt colicins, to phage-like PYOCINS. (See also BACTERIOPHAGE fBA1.) Some bacteriocins (lantibiotics) undergo distinctive post-translational modification which is necessary for their activity. The lantibiotic LACTICIN 3147 (produced by Lactobacillus lactis subsp lactis) is a two-component bacteriocin – both peptides being necessary for activity; moreover, each of the two peptides needs modification by a separate enzyme [Microbiology (2000) 146 2147–2154]. Evolutionary relationships are evident within some groups of bacteriocins. [Molecular mechanisms of bacteriocin evolution: ARG (1998) 32 255–278.] Bacteriocins are commonly encoded by PLASMIDS (see e.g. COLICIN PLASMID); bacteriocin-encoding plasmids include both conjugative and non-conjugative types. Examples of chromosomally encoded bacteriocins include ‘bacteriocin 28b’ (a colicin encoded by Serratia marcescens) and some of the class IIa bacteriocins produced by lactic acid bacteria (see later). The mode of regulation of bacteriocin synthesis varies among the different groups. For example, synthesis of colicins is promoted by those conditions which trigger the SOS SYSTEM (see COLICIN PLASMID). By contrast, many bacteriocins of the lactic acid bacteria appear to be regulated by a three-component system which includes a histidine protein kinase, a response regulator, and an induction factor. (cf. TWO-COMPONENT REGULATORY SYSTEM). Induction factors (IFs) are small heat-stable peptides produced by the bacteriocinogenic cell. The mechanism by which IFs promote synthesis of bacteriocin is unknown, but it has been suggested that they may trigger synthesis as a result of their gradual intracellular accumulation or that their influence may reflect environmental factors. Synthesis of the bacteriocin sakacin A (produced by Lactobacillus sake) is reported to be regulated by a three-component system (which includes a 23-amino-acid cationic peptide) in a temperature-sensitive way [Microbiology (2000) 146 2155–2160]. The bacteriocin divercin V41, produced by Carnobacterium divergens strain V41, may be regulated by a two-component system [Microbiology (1998) 144 2837–2844]. The different types of bacteriocin are released in different ways from the cells which synthesize them. For example, release of colicins depends on a lysis protein which causes a (non-specific) increase in the permeability of the cell envelope – allowing dispersal of the bacteriocin (but with concomitant adverse effects on the producing cell). By contrast, certain bacteriocins (e.g. ENTEROCIN P) are secreted via a sec-dependent pathway, while some class IIa bacteriocins of the lactic acid bacteria are reported to be secreted by ABC TRANSPORTERS. The import of some types of bacteriocin (e.g. colicins) into sensitive cells depends on the binding of the bacteriocin to a specific cell-surface receptor. In most or all cases, uptake of colicins appears to be an energy-dependent process (pmf being needed for transport across the outer membrane, and ATP hydrolysis being involved in transport across the cytoplasmic membrane); the route of translocation through the cell envelope depends on the given colicin (see groups A and B COLICINS). By contrast, 73 bacteriocin 28b bacteriocyte In certain invertebrates, particularly insects: a specialized cell which contains intracellular bacterial symbionts. (cf. MYCETOCYTE.) Bacteriocytes occur e.g. in the cockroach (see BLATTABACTERIUM), in some marine sponges (containing ‘Aphanocapsa’ endosymbionts) and in aphids (see BUCHNERA). bacteriolysis The lysis (rupture) of bacterial cells. In nature, various microorganisms produce enzymes or antibiotics which lyse bacteria – either to provide nutrients or (presumably) for competitive advantage. (See e.g. ANAEROPLASMA, ENSIFER, LYSOSTAPHIN, MUSHROOM CULTIVATION, MYXOBACTERALES.) Most BACTERIOPHAGES eventually lyse their host cells to release their progeny. In the laboratory bacteria may be lysed mechanically or enzymically (see CELL DISRUPTION). bacterio-opsin See OPSIN. bacteriophaeophytin A PHAEOPHYTIN derivative of a bacteriochlorophyll. Bacteriophaeophytins occur e.g. in the REACTION CENTRES in a number of ‘purple’ photosynthetic bacteria and in the cytoplasmic membrane of some ‘green’ photosynthetic bacteria. bacteriophage (phage) Any VIRUS whose host is a bacterium. (Viruses which infect cyanobacteria are conventionally called CYANOPHAGES.) Most – probably all – bacteria can be infected by particular phages; commonly, a given phage can infect only one or a few strains or species of bacteria. The consequences of phage infection depend on phage and host, and to some extent on 12–28]) and by Clostridium botulinum (boticins), C. butyricum (butyricins) and C. perfringens (perfringocins). (cf. AGROCINS and KILLER FACTOR.) bacteriocin 28b See COLICINS. bacteriocin typing A form of TYPING in which strains of bacteria are distinguished on the basis of the BACTERIOCIN(s) they produce or the bacteriocin(s) to which they are susceptible. In one common form of bacteriocin typing (used e.g. for colicins and pyocins) the strain under test is inoculated in a diametrical strip on a blood agar plate which is incubated for ca. 12–24 hours; growth is scraped from the plate and discarded, and any cells remaining on the plate are killed by exposure to chloroform. The chloroform is allowed to evaporate, leaving a band of bacteriocin-impregnated agar. Indicator strains are then inoculated onto the agar in lines perpendicular to, and passing through, the bacteriocin-containing band; on incubation, the growth of sensitive strains is inhibited inside the band. In this procedure the test organism is typed by the range of strains susceptible to its bacteriocin(s). [Revised method for pyocin typing: JCM (1984) 20 47–50.] bacteriocinogenic Able to produce a BACTERIOCIN. bacteriocinogenic factor The earlier name for any PLASMID that encodes a BACTERIOCIN. bacteriocuprein A bacterial CuZnSOD (see SUPEROXIDE DISMUTASE). BACTERIOPHAGES: some representative examplesa Bacteriophage (or phage group) Virion morphology Principal host(s) isometric head + long non-contractile tail isometric head + long contractile tail isometric head + short tail isometric head + short non-contractile tail isometric head + long contractile tail isometric head + long contractile tail isometric head + minimal tail elongated head + short non-contractile tail isometric head + long contractile tail elongated head + long contractile tail isometric head + short non-contractile tail isometric with internal lipid membrane (no tail) Escherichia coli enterobacteria Acholeplasma laidlawii Escherichia coli Escherichia coli Escherichia coli Salmonella Bacillus subtilis Bacillus subtilis Escherichia coli enterobacteria various pleomorphic; envelope enclosing a DNA–protein complex icosahedral with internal lipid membrane Acholeplasma laidlawii Genome: ccc ssDNAb Inoviridae (e.g. f1, MV-L51) Microviridae (e.g. fX174) filamentous or rod-shaped icosahedral various enterobacteria Genome: dsRNAb f6 enveloped nucleocapsid Pseudomonas syringae pv. phaseolicola icosahedral various dsDNAb Genome: linear l Mu MV-L3 N4 P1 P2 P22 f29 SPO1 T4 T7 Tectiviridae (e.g. PRD1, AP50) Genome: ccc dsDNAb MV-L2 PM2 Alteromonas espejiana ssRNAb Genome: Leviviridae (e.g. MS2, Qb) a b See separate entries for details. In the virion. ccc = covalently closed circular; ds = double-stranded; ss = single-stranded. 74 bacteriophage conversion have indicated that, on attaching to the host cell, the phage ejects proteins that may assemble to form a channel across the bacterial cell envelope through which the genome is translocated; it has been speculated that two of the ejected proteins may form the components of a ‘motor’ that rachets phage DNA into the host cell [Mol. Microbiol. (2001) 40 1–8]. For phage development to proceed, the phage nucleic acid must escape degradation by the host’s RESTRICTION ENDONUCLEASE system. For a general account of viral replication strategies see VIRUS; for detailed accounts of particular phages see following entries. Virion assembly may occur by the spontaneous aggregation of the various phage components, but the more complex virions require the participation of non-structural phagecoded proteins (see SCAFFOLDING PROTEIN). Host cell lysis may be induced by phage-coded enzymes or by activation of host cell autolysins. (See also LYSIS FROM WITHOUT, LYSIS PROTEIN and ONE-STEP GROWTH EXPERIMENT.) Bacteriophages can cause considerable economic problems in certain biotechnological processes such as the manufacture of dairy products, antibiotics, etc. A few phages are medically important in that they encode certain toxins (see BACTERIOPHAGE CONVERSION). In microbiology, phages are useful e.g. for PHAGE TYPING, for achieving gene transfer between bacteria (see TRANSDUCTION), for probing the molecular biology of bacteria, etc. Phages may be cultivated and/or assayed e.g. by inoculation at low multiplicity of infection on a LAWN PLATE of susceptible bacteria (see PLAQUE and PLAQUE ASSAY). They may also be cultivated in broth cultures of host bacteria, the activity of virulent phages being indicated by the progressive decrease in turbidity of the culture as the cells lyse; a suspension of phages may be prepared from the lysate e.g. by CENTRIFUGATION or by membrane FILTRATION to remove bacterial debris. bacteriophage 7-7-1 See FLAGELLOTROPIC PHAGE. bacteriophage 21 See LAMBDOID PHAGES. bacteriophage 82 See LAMBDOID PHAGES. bacteriophage 434 See LAMBDOID PHAGES. bacteriophage 1307 See PLASMAVIRIDAE. bacteriophage a3 See MICROVIRIDAE and BACTERIOPHAGE G4. (Also: a dsDNA-containing phage of ‘Achromobacter sp 2’ [JGV (1981) 53 275–281].) bacteriophage a15 See LEVIVIRIDAE. bacteriophage AP50 See TECTIVIRIDAE. bacteriophage Bam35 See TECTIVIRIDAE. bacteriophage b See LEVIVIRIDAE. (cf. Corynephage b – see DIPHTHERIA TOXIN.) bacteriophage BF23 A bacteriophage which infects Escherichia coli and which resembles phage T5. (See also BTUB PROTEIN.) bacteriophage BPB1 See STYLOVIRIDAE. bacteriophage Cf See INOVIRUS. bacteriophage c See STYLOVIRIDAE. bacteriophage conversion (phage conversion) In a bacterium: the acquisition, loss or modification of one or more phenotypic characteristics as a result of infection by a BACTERIOPHAGE – typically a temperate phage (lysogenic conversion); it may result e.g. from the expression of phage genes or from inactivation of one or more bacterial genes due to insertion of a prophage into the bacterial chromosome. (Phenotypic alteration in a recipient cell due to TRANSDUCTION of bacterial DNA is not regarded as phage conversion.) Phage conversion is responsible for toxigenicity in a number of pathogenic bacteria. For example, phages encode CHOLERA TOXIN, DIPHTHERIA TOXIN and the shiga-like toxins of enterohaemorrhagic strains of Escherichia coli (including O157:H7) conditions. Some (virulent ) phages always induce a LYTIC CYCLE in the host cell, while other (temperate) phages can establish a stable, non-lytic relationship (LYSOGENY) with the host. Some phages can replicate and produce progeny virions within the host cell without killing or lysing it (see INOVIRIDAE and INOVIRUS). Phages of the MV-L3 PHAGE GROUP apparently kill their host cells without actually lysing them. The antibacterial activity of phages has been exploited (primarily in Eastern Europe and the former Soviet Union) for both therapeutic and prophylactic use against various bacterial pathogens (including strains of Escherichia, Klebsiella, Proteus, Pseudomonas, Salmonella, Shigella, Staphylococcus and Streptococcus) [AAC (2001) 45 649–659]. Bacteriophages are a highly diverse group of viruses. Of the many hundreds known, relatively few have been thoroughly characterized [guidelines for phage characterization: AVR (1978) 23 1–24]. Some phages have been included in the overall taxonomic scheme for the viruses [Book ref. 23], and these are classified in the families CORTICOVIRIDAE, CYSTOVIRIDAE, INOVIRIDAE, LEVIVIRIDAE, MICROVIRIDAE, MYOVIRIDAE, PLASMAVIRIDAE, PODOVIRIDAE, STYLOVIRIDAE and TECTIVIRIDAE; however, many phages remain unclassified. Morphologically, phage virions may be small and icosahedral, ca. 23–32 nm diam. (e.g. Qb and fX174); filamentous, ca. 760–1950 × 6 nm (INOVIRUS); pleomorphic (e.g. BACTERIOPHAGE MV-L2); or – in many phages – complex in structure with a polyhedral ‘head’ (the CAPSID) linked (often via a more or less complex connecting structure) to a long or short, simple or complex, contractile or non-contractile ‘tail’ (see e.g. PODOVIRIDAE, STYLOVIRIDAE, T-EVEN PHAGES). While in many phages the virions consist only of protein and the nucleic acid genome, some have a significant content of lipid – either as an external ENVELOPE (e.g. in f6 and MV-L2) or as an internal layer (in BACTERIOPHAGE PM2 and phages of the TECTIVIRIDAE); the T-even phages contain e.g. dihydropteroyl hexaglutamate, phages of the MV-L3 group contain fucose, and MS2 contains spermidine. The genome of a phage may be linear dsDNA, ccc dsDNA, ccc ssDNA, linear dsRNA or linear ssRNA (see Table). In some of the DNA phages the DNA contains unusual bases: e.g. PBS1 contains deoxyuracil instead of thymine, fW-14 contains a-putrescinylthymine, T-even phages contain hydroxymethylcytosine instead of cytosine, SP8 and SPO1 contain 5hydroxymethyluracil instead of thymine. (See also BACTERIOPHAGE MU and RESTRICTION–MODIFICATION SYSTEM.) The phage replication cycle begins when the virion adsorbs to the host at specific cell-surface sites: e.g., particular components of the cell wall (see e.g. BTUB PROTEIN and LAMB PROTEIN), the flagellum (see FLAGELLOTROPIC PHAGE), a sex pilus (see ANDROPHAGES), etc. Some phages adsorb only in the presence of appropriate concentrations of certain ions or of certain organic ‘adsorption cofactors’ (e.g. tryptophan for BACTERIOPHAGE T4). In Gram-negative bacteria penetration is believed to occur – at least in some phages – at ADHESION SITES. Either the entire virion, or the phage genome with or without certain phage proteins, enters the host cell: see e.g. INOVIRUS, LEVIVIRIDAE, and entries for bacteriophages N4, f6 and T4 for examples. For at least some of the phages with contractile tails (see MYOVIRIDAE) – e.g. BACTERIOPHAGE T4 (q.v.) – the genome is understood to be injected (by a syringe-like action) into the periplasmic region of the bacterium. However, many phages have non-contractile tails, and some are entirely tail-less; hitherto, little information has been available on the way in which the genome of such phages is translocated into the host cell. Studies on phage T7 75 bacteriophage CTX8 bacteriophage fr See LEVIVIRIDAE. bacteriophage G4 An isometric SSDNA PHAGE of the MICROVIRIDAE. On penetrating the host cell, the ss cccDNA genome is coated with host SSB protein; a GC-rich region between genes F and G probably remains uncoated. Stage I (see SSDNA PHAGE) is independent of host dnaB function (as it is in microviruses St-1, a3, fK and fXtB – cf. BACTERIOPHAGE fX174). The F –G intergenic region seems to fold into a secondary structure [JV (1986) 58 450–458] which is capable of direct (dnaB-independent) recognition by host PRIMASE (cf. PRIMOSOME and DNAB GENE). Primase synthesizes a short RNA primer at a unique site (ori ) in the intergenic region. The c strand is completed as in other SSDNA PHAGES. Stage II in G4 requires dnaB function, presumably for initiation of n strand synthesis. (Phages St-1, fK and a3 do not require dnaB at any stage.) G4 n strand synthesis begins at the n strand origin (Ov ) in gene A; the c strand origin (Oc ) in the F –G intergenic region is on the opposite side of the molecule. n strand synthesis seems to proceed by the displacement of the old n strand as a closed loop (D LOOP) at the Ov site, and hence is independent of a gpA-induced nick (cf. BACTERIOPHAGE fX174). Synthesis of n strand by DNA polymerase III holoenzyme and Rep protein proceeds unidirectionally. When the replication fork passes Oc , primer synthesis can begin on the displaced loop, and c strand synthesis can proceed in the opposite direction. The parental n and c circles may separate before replication of either is complete. G4 seems to resemble fX174 (q.v.) in stage III and morphogenesis. bacteriophage G6 See MICROVIRIDAE and BACTERIOPHAGE fX174. bacteriophage G13 See MICROVIRIDAE and BACTERIOPHAGE fX174. bacteriophage G14 See MICROVIRIDAE and BACTERIOPHAGE fX174. bacteriophage HM3 See MYOVIRIDAE. bacteriophage I3 See MYCOBACTERIOPHAGES. bacteriophage If1 An INOVIRUS which adsorbs specifically to I-type pili of enterobacterial hosts; phage If2 is very similar. bacteriophage If2 See BACTERIOPHAGE IF1. bacteriophage IKe An INOVIRUS specific for enterobacteria which contain an IncN plasmid; the phage can apparently establish a pseudolysogenic infection of its host [CJM (1978) 24 1595–1601]. [Nucleotide sequence and genetic organization of IKe genome: JMB (1985) 181 27–39.] bacteriophage L1 group See PLECTROVIRUS. bacteriophage L2 group See PLASMAVIRIDAE. bacteriophage L3 group See MV-L3 PHAGE GROUP. bacteriophage L4 A BACTERIOPHAGE closely related to BACTERIOPHAGE P22 but defective in maintenance of lysogeny; it is commonly used as a transduction vector in Salmonella typhimurium. bacteriophage L17 See TECTIVIRIDAE. bacteriophage L34a See BACTERIOPHAGE CONVERSION. bacteriophage l A temperate BACTERIOPHAGE (family STYLOVIRIDAE) which infects Escherichia coli. Head: icosahedral, ca. 55 nm diam., composed of two major structural proteins (gpE and gpD) and several minor proteins (gpB, gpC, gpFII ). Tail: long (ca. 150 × 10 nm), flexible, tubular, non-contractile, joined to the head via a ‘neck’ or ‘connector’ region, and terminating at the distal end in a small basal structure to which is attached a single fibre (the adsorption organelle). (See also LAMB PROTEIN.) Genome: linear dsDNA (48514 bp long [nucleotide sequence: JMB (1982) 162 729–773]) with single-stranded 12-base 5 STICKY ENDS (designated cos). On infection of a host cell, phage DNA is transferred to the host through the tail and is immediately circularized by base-pairing between the sticky ends followed by host DNA ligase action. Subsequent events may eventually lead either to host cell lysis (involving the production and release of (see SHIGA TOXIN). The enterotoxin A of Staphylococcus aureus (see ENTEROTOXIN) is phage-encoded in at least some strains. (See also BETACIN.) In Staphylococcus aureus, lipase activity is lost on infection with phage L54a owing to inactivation of the lipase structural gene (into which the prophage inserts) [JB (1986) 166 385–391]. Lysogenic conversion can also be manifested by a change in cell-surface antigens, i.e. a change in serotype. For example, in group E salmonellae, phage e15 converts antigens 3,10 to 3,15, and the LPS of the latter serotype can act as a receptor for phage e34 which, in turn, converts the antigen type to 3,15,34 (for the meaning of underlined numbers see KAUFFMANN–WHITE CLASSIFICATION). Phage-mediated modification of serotype is important e.g. when developing vaccine strains of a pathogen [see e.g. TIM (2000) 8 17–23]. (See also PSEUDOLYSOGENY.) bacteriophage CTX8 A temperate, filamentous ssDNA BACTERIOPHAGE which infects Vibrio cholerae and encodes CHOLERA TOXIN (as well as the Ace and Zot toxins). The double-stranded form of the genome can integrate into the bacterial chromosome in a site-specific, RecA-independent process. Isolates of lysogenic E1 Tor and O139 strains of V. cholerae typically contain tandem arrays of prophage DNA at a single locus within the large chromosome, and it appears that the presence of multiple copies of the prophage is necessary for the production of virions; it also seems that the genomes of progeny virions are formed by a process in which replicative forms of the phage develop through hybridization between adjacent prophages (or between a prophage and a (phage-related) RS1 element in the chromosome) [PNAS (2000) 97 8572–8577]. The cell-surface receptor for CTX8 occurs on so-called toxin coregulated pili (TCP) (encoded by a chromosomal PATHOGENICITY ISLAND); genes encoding TCP and the cholera toxin are jointly, and positively, regulated by transcriptional regulator proteins (ToxR, ToxS, ToxT) which apparently become active on receipt of appropriate signals within the gut. Lysogenic conversion of (non-lysogenic) cells of V. cholerae can occur in the mammalian gut in the presence of phage-donating strains; this apparently reflects infection by phage following induction of TCP receptors by the gut-derived signals. [Science (1996) 272 1910–1914; Cell (1996) 87 795–798; Mol. Microbiol. (1997) 24 917–926; TIM (1998) 6 295–297.] Infection of V. cholerae by CTX8 is dependent on the tolQRA gene products [JB (2000) 182 1739–1747]. In the classical biotype of V. cholerae, the prophages are present either singly or as two fused prophages, and integration occurs at two sites; this biotype produces cholera toxin but, apparently owing to deficiencies in the structure of arrays (and the absence of RS1), does not produce virions [JB (2000) 182 6992–6998]. bacteriophage D108 A mutator bacteriophage which is very similar to BACTERIOPHAGE MU; the phage D108 genome shows ca. 90% homology with that of phage Mu, differing mainly in the early-gene region at the left end [EMBO (1985) 4 3031–3037]. bacteriophages e15 , e34 See BACTERIOPHAGE CONVERSION. bacteriophage f1 See INOVIRUS. bacteriophage f2 See LEVIVIRIDAE. bacteriophage F116 A temperate, transducing phage (family STYLOVIRIDAE) which infects Pseudomonas aeruginosa; in lysogenic cells the prophage appears to have an extrachromosomal location [JV (1977) 22 844–847]. bacteriophage fd See INOVIRUS. 76 bacteriophage l head and tail genes and morphogenesis cos A J b att recombination int xis red DNA replication regulation gam cIII pI N pL cI cro pR cII pRE O P lysis Q ori S R cos pR′ pRM Nu3 Nu1AWB C D E F I F II Z U V G T HM L K I J cIII N nutL cI oL pL pRM cro nutR cII oR PRE pR BACTERIOPHAGE l. Simplified map of the virion genome (not drawn to scale). See entry for explanation. progeny phages) or to LYSOGENY (during which l DNA becomes integrated into the host chromosome); the initial sequence of events is common to both pathways. Lytic cycle. Transcription of l DNA by the host RNA polymerase proceeds leftwards from promoter pL and rightwards from promoter pR , stopping at rho-dependent terminators shortly beyond genes N and cro (‘immediate early’ genes), respectively (see figure). gpN functions as an antiterminator which is specific for transcription initiated at the early promoters pL and pR ; gpN recognizes special sites (nutL and nutR) – regions of hyphenated dyad symmetry – downstream from pL and pR , and when the host RNA polymerase traverses these sites it is modified by gpN (in the presence of the host nusA gene product) such that it can read through subsequent terminators. As a result, genes to the left of N (cIII . . . att) and to the right of cro (cII . . . Q) – delayed early genes – are expressed. Expression of cro is necessary for the continuation of the lytic cycle (see later). gpN is unstable (half-life ca. 5 min), so that continued N expression is necessary for ongoing transcription of the delayed early genes. l DNA REPLICATION is initiated at a site (ori ) in the O gene and requires gpO, gpP , and certain host proteins, including DnaB protein, primase (DnaG protein), RNA polymerase, and components of the DNA polymerase III holoenzyme (but not e.g. dnaA, dnaC or dnaI functions). l gpO binds both to a 19-bp tandem repeat in the ori region and to l gpP ; gpP interacts with DnaB protein and appears to act as an analogue of E. coli DnaC protein (see PRIMOSOME). Initially, the l cccDNA is replicated by the CAIRNS MECHANISM, but later replication proceeds by a ROLLING CIRCLE MECHANISM to yield doublestranded concatemers. Phage structural components (and e.g. lysozyme necessary for host cell lysis) are encoded by late phage genes (S, R, A . . . J – juxtaposed by circularization of the genome); transcription of these genes is turned on by the delayed early gene product gpQ, which functions as an antiterminator of late mRNA initiated at pR . Phage morphogenesis is complex, involving host proteins as well as many phage-coded functions. In essence, gpB and gpC appear to form a complex with gpNu3, and gpE is then incorporated; these components undergo modification (fusion/cleavage reactions), and the scaffolding protein gpNu3 is cleaved during or after its elimination from the head precursor. Concatemeric l DNA is cut (by a gpNu1–gpA complex) at a cos site; a left cos site is inserted into the capsid precursor, and DNA insertion continues until the next cos site is reached, when the DNA is cleaved. (cf. COSMID.) The resulting head structure is stabilized by the addition of gpD, gpW and gpFII. The tail is assembled separately and interacts spontaneously with the completed head to form the mature infectious virion. Lysogeny and the lysis/lysogeny decision. During the early period of phage transcription, the system is committed neither to lysis nor to lysogeny, expression of immediate early and delayed early genes being necessary for both pathways. Expression of late genes results in lysis, while establishment of lysogeny requires the repression of most of the l genes by a repressor protein (gpcI ); gpcI prevents both leftward and rightward transcription by binding to operator regions (oL and oR ) that overlap pL and pR , respectively. When a host cell is first infected, cI cannot be expressed. However, transcription of N allows transcription of cII and cIII, and both gpcII and gpcIII are necessary for repressor synthesis. gpcII functions as a positive regulator for the initiation of repressor synthesis from pRE (= pE ), being necessary for recognition of this promoter by RNA polymerase. gpcII is unstable in vivo owing to proteolysis by host proteins (products of the hflA and hflB genes being involved); this proteolysis is apparently inhibited by gpcIII and by cAMP–CAP. The establishment of lysogeny is favoured by certain environmental conditions, e.g., by starvation of cells prior to infection, and by a high multiplicity of infection (MOI). It has been suggested that starvation may increase cAMP–CAP levels, while high MOI increases levels of gpcIII (a gene dosage effect); in either case the result is stabilization of gpcII thus favouring lysogeny. Once synthesized, the repressor maintains its own synthesis by activating cI expression from an alternative promoter, pRM (= pM ); transcription from pRM can occur only when repressor is bound to oR . Translation of the pRM -initiated mRNA is much less efficient than that of the pRE -initiated mRNA. Thus, synthesis of repressor occurs at high levels by the pRE -dependent ‘establishment circuit’ and at lower levels by the pRM -dependent ‘maintenance circuit’. The repressor is antagonized by the product of the immediate early gene cro: gpcro (‘antirepressor’) prevents transcription of cI from pRM , and reduces (but does not eliminate) transcription of early genes from pL and pR [cro-operator and repressor–operator interaction: ARB (1984) 53 293–321]; thus gpcro prevents ‘maintenance synthesis’ of repressor and also (by reducing gpcII formation) ‘establishment synthesis’ from pRE . Sufficient early gene transcription persists to allow expression of Q and hence of the late genes, so that the lytic cycle can be completed. The lysis/lysogeny decision thus depends on whether gpcI or gpcro occupies the operators oR and oL ; this in turn depends largely on the level of gpcII (which also affects 77 bacteriophage M12 events of the lytic cycle. In contrast to pL -initiated transcription in the lytic cycle, pL -initiated transcription in the prophage results in the expression of both int and xis (necessary for excision). This is possible because, owing to the permuted gene order in the prophage, the sib-containing b region is no longer immediately downstream from int; the pL -initiated transcript is thus not recognized by RNase III, and both int and xis can be expressed. [General review: Book ref. 79; l–host interactions: MR (1984) 48 299–325.] bacteriophage M12 See LEVIVIRIDAE. bacteriophage M13 See INOVIRUS. bacteriophage MS2 See LEVIVIRIDAE. bacteriophage Mu A temperate bacteriophage which can infect various enterobacteria (e.g. Escherichia coli, Citrobacter freundii, Erwinia spp, and certain mutant strains of Salmonella typhimurium). The phage particle has an icosahedral head (ca. 60 nm diam.) and a contractile tail (ca. 100 nm long when extended, 60 nm when contracted) to which is attached a base plate with spikes and tail fibres; the phage tail adsorbs to LPS in the outer membrane of a host cell. Phage particles contain linear dsDNA ca. 39 kb long. An early event after phage infection – regardless of whether infection will eventually result in a lytic cycle or the lysogenic state – is the integration of Mu DNA into the host chromosome at a more or less randomly selected location. Integration often results in the inactivation of the gene in which it occurs; ca. 2–3% of bacteria in a population acquire a recognizable mutation on lysogenization by Mu, a frequency much greater than the spontaneous mutation rate in the absence of Mu – i.e., Mu acts as a mutator phage (Mu = abbr. for mutator). The initial integration event may be conservative (i.e., both strands of the Mu DNA may be inserted) and appears to be a simple (nonreplicative) insertion; it requires the product of Mu gene A which can recognize and bind to the ends of Mu DNA [Cell (1984) 39 387–394]. Synthesis of a repressor (the c gene product) results in lysogeny and immunity to superinfection by Mu. During the lytic cycle, Mu DNA replication occurs entirely by repeated replicative transposition of the integrated DNA (which thus functions as a giant TRANSPOSON, causing chromosomal deletions, inversions, duplications etc); Mu DNA insertion results in a 5-bp duplication of the target DNA (cf. Tn3 ). Transposition occurs at very high frequency (ca. 100 transpositions per cell in ca. 30 min), copies of Mu DNA being inserted at random locations in the chromosome; this results in the death of the host cell, and ca. 100 progeny phages are released ca. 60 min after infection. Transposition is controlled by the products of phage genes A and B and requires continual synthesis of gpA – which apparently acts stoichiometrically; gpA functions as a transposase, gpB apparently functions in the modification of gpA activity. [Role of DNA topology in Mu transposition: Cell (1986) 45 793–800.] Phage Mu DNA contains three regions, designated a (ca. 33 kb), G (ca. 3 kb) and b (ca. 1.7 kb), flanked by random sequences of bacterial DNA (see below). The a region contains most of the phage genes, including e.g. the repressor (c) gene, all the early functions, all the head functions, and most of the tail functions. The G region carries the remainder of the tail functions and specifies the host range of the phage. The G segment is flanked by short inverted repeats, and can invert by reciprocal recombination between these sequences; when in one orientation, designated G(+), it specifies a host range which includes E. coli K12 strains, but in the alternative integration – see later) and hence is subject to influence by the host cell e.g. in response to environmental conditions. In Escherichia coli lysogeny involves integration of the phage cccDNA with the host chromosome. This occurs by SITE-SPECIFIC RECOMBINATION, i.e., by reciprocal recombination between specific ‘attachment sites’ – one on the phage genome (attP ) and one on the host genome (attB, also called att l , situated between genes gal and bio); each att sequence consists of a central ‘core’ sequence (O) which is common to both attB and attP, flanked by ‘arm’ sequences designated B and B for attB, P and P for attP. Integration requires the int gene product (integrase) and certain host proteins, including DNA gyrase and INTEGRATION HOST FACTOR, IHF. (IHF consists of two subunits, a and b: products of genes himA and himD (= hip), respectively.) Integrase recognizes attB and attP, while IHF recognizes only attP. Interaction between these recombination proteins, bound to their att sites, brings about recombination by the formation of a staggered break in the O region followed by strand exchange and ligation. The integrated prophage is thus flanked by hybrid att regions, attL (BOP ) and attR (POB ). The integration reaction is reversible, but while integration involves recognition between attP and attB, excision – the reverse reaction – requires recognition between attL and attR; excision requires an additional phage protein, the xis gene product (excisionase), which can bind to a specific region in the P arm of attP and attR [PNAS (1985) 82 997–1001]. The regulation of the reaction such that integration occurs preferentially under conditions which favour lysogeny, while excision occurs only on induction of a lysogen, is achieved by control of the amounts of integrase and excisionase available. The int gene can be transcribed from either of two promoters: pL and pI ; however, integrase is produced only during establishment of lysogeny and not during the lytic cycle. Under conditions which favour lysogeny, gpcII not only activates cI expression (hence preventing transcription from pL ), it also activates int expression from pI . (Since pI overlaps xis, transcription from this promoter allows expression of int without that of xis, thus favouring integration.) However, under lytic conditions – during which transcription proceeds from pL – int gene expression is prevented by RETROREGULATION: gpN-modified, pL -initiated transcription extends beyond int, through att, and into the b region; the b region contains a sequence (sib) which causes the 3 end of the corresponding mRNA to adopt a secondary structure which is recognized and cleaved by RNase III (q.v.). This cleavage is followed by degradation of the transcript in the 3 -to-5 direction, thus preventing int expression. (Under conditions which favour lysogeny, the pI -initiated transcript, which terminates before the sib region, is a poor substrate for RNase III, and hence int expression can occur.) Induction of l may occur spontaneously (ca. 1 per 102 –105 lysogenic cells per generation); however, high levels of induction may be brought about by agents which damage DNA (e.g., UV irradiation, mitomycin C). Damage to DNA results in the ‘activation’ of the host RecA protein (see RECA PROTEIN), resulting in cleavage and inactivation of the repressor. (Levels of DNA damage necessary for l induction are higher than those required to induce the SOS SYSTEM; presumably gpcI becomes susceptible to RecA-mediated cleavage only when host cell viability is threatened.) Since the repressor functions as a positive regulator for its own synthesis, its inactivation also prevents the synthesis of replacement repressor molecules. Inactivation of the repressor allows transcription from pL and pR , followed by excision of the prophage and completion of the 78 bacteriophage P1 bacteriophage MV-L51 See PLECTROVIRUS. bacteriophage 06N 58P See CORTICOVIRIDAE. bacteriophage N4 A virulent coliphage of the PODOVIRIDAE. Genome: linear dsDNA (ca. 71 kb) containing terminal direct repeats 400–450 bp long and a 7-base 3 overhang at one end. The phage particle contains a rifampicin-resistant DNAdependent RNA polymerase which is injected into the host cell with the DNA and which is necessary for transcription of N4 early genes. Transcription of intermediate (middle) genes requires the synthesis and activity of three N4 early proteins, two of which are components of a second rifampicin-resistant RNA polymerase; transcription of late genes requires E. coli RNA polymerase activity. Host DNA replication is blocked by an N4 early gene product, but host RNA and protein synthesis continues (except in the case of cAMP-dependent operons). The N4 genome codes for most functions required for its replication, including a DNA polymerase and DNA-binding protein; the only host functions it appears to require are gyrase, DNA polymerase 5 → 3 exonuclease activity, DNA ligase, and nrdA gene function [Book ref. 69, pp. 245–254]. bacteriophage P1 A temperate BACTERIOPHAGE (family MYOVIRIDAE) which infects Escherichia coli and Shigella spp. Virion: icosahedral head (ca. 90 nm diam.) with a long, complex contractile tail bearing a base-plate and tail fibres. Genome: dsDNA (MWt ca. 66 × 106 ), circularly permuted and terminally redundant. On infection of a host cell, the genome circularizes (with loss of redundancy) and may enter a LYTIC CYCLE or establish LYSOGENY. Relatively little is known about the lytic cycle; both q- and s-type modes of DNA replication occur early in infection, but later only s-type replication can be observed. DNA packaging occurs by a ‘headful’ mechanism, starting at a unique site (pac); cleavage of DNA at a pac site apparently occurs early in infection, before phage heads are formed, and the enzyme responsible may remain bound to the DNA and subsequently interact with a prohead to initiate packaging. (cf. BACTERIOPHAGE P22.) In the lysogenic state, the P1 prophage does not normally integrate into the host chromosome, but persists as an autonomous, circular PLASMID (P1 plasmid) which is maintained at one or two copies per host chromosome. The replication control system of the P1 plasmid resembles that of the F PLASMID (q.v.) e.g. in that it involves a plasmid-encoded protein (designated RepA in the P1 plasmid) which is essential for replication and which autoregulates its own synthesis. Moreover, the repA gene is flanked on each side by multiple 19-bp repeat sequences, and it appears that one of these multiple repeats (incA) may be involved in the control of copy number in that it appears to titrate the RepA protein. [P1 replication: JBC (1986) 261 3548–3555.] The P1 plasmids are segregated (see PARTITION) with great accuracy at each host cell division, and spontaneous loss of the plasmid is extremely rare. This high degree of accuracy involves several mechanisms. For example, P1 encodes a SITE-SPECIFIC RECOMBINATION (SSR) system which facilitates accurate partition by efficiently resolving plasmid dimers into monomers. (Dimers are formed by homologous recombination between the two plasmids resulting from replication.) This system involves a site (loxP ) on the plasmid at which SSR occurs, and a gene (cre) encoding the recombinase. (The lox-Cre system also catalyses cyclization of the phage genome on infection of the host – at least in RecA− hosts; it can also mediate the rare integration of the P1 plasmid into the host chromosome at a site designated loxB.) [Regulation of the cre gene: JMB (1986) 187 197–212.] Partition per se involves a sequence (par ) in the plasmid; a gene (parA) G(−) orientation it specifies different tail fibres specific for a different host range. [Host cell receptors for Mu G(+) and G(−) types: FEMS (1985) 28 307–310.] (See also G LOOP and BACTERIOPHAGE P1.) A gene designated gin, in the b region adjacent to the G region, is essential for G inversion; gin encodes a site-specific recombination enzyme which is stimulated by a small, heat-stable host protein [BBA (1986) 866 170–177] and which apparently recognizes the inverted repeats flanking the G segment. In the lysogenic state, the G segment undergoes inversion at a slow but steady rate due to a low level of gin expression. The level of gin expression is thought to be about the same during the lytic cycle, but G inversion rarely has time to occur before the lytic cycle is complete; hence G loops are rarely observed in denatured/renatured Mu DNA in lysates from lytic infections. (See also RECOMBINATIONAL REGULATION.) In addition to gin, the b region also contains a gene (mom) which encodes a DNA modification function. The targets for this modification system are adenine residues in pentanucleotide sequences 5 . . . (C/G)A(C/G)NPy . . . 3 ; ca. 15% of the adenine residues in the Mu DNA are modified, the modified bases being N 6 -(1-acetamido)-2-deoxyadenosine. The modified DNA is resistant not only to the host restriction system in vivo but also to in vitro cleavage by a range of restriction endonucleases. The mom gene is apparently repressed in the prophage but is strongly expressed on induction; the host Dam methylase (product of the DAM GENE) is necessary for mom gene expression [mechanism: EMBO (1986) 5 2719–2728]. During phage assembly, Mu DNA is packaged by a ‘headful’ mechanism: packaging begins near the ‘left’ (c) end of the Mu DNA and continues until ca. 39 kb of DNA have been incorporated into the phage head. The Mu DNA itself is ca. 37.5 kb long; thus, the packaging mechanism results in a variable length of bacterial DNA at each end of the phage genome (on average ca. 50–100 bp at the c end, ca. 1700 bp at the ‘right’ or S end). These end sequences are random because of the random insertion of Mu DNA into the host chromosome; they appear as ‘split ends’ in phage DNA which has been denatured and reannealed. The ‘split ends’ are lost when the phage DNA integrates into a new host chromosome. (See also MINI-MU.) Mu can function as a generalized transducing phage, mediating gene transfer at frequencies of ca. 10−7 . Transduction by Mu can be detected only in rec+ recipient cells. The transducing particles, which contain only host DNA, probably arise by the rare packaging into Mu heads of host DNA instead of phage DNA. [Review: Book ref. 20, pp. 105–158; behaviour of Mu in Salmonella typhi : JGM (1986) 132 83–89.] bacteriophage m2 See LEVIVIRIDAE. bacteriophage MV-L1 See PLECTROVIRUS. bacteriophage MV-L2 (MVL2, L2 strain L2) A species of the PLASMAVIRIDAE. Host: Acholeplasma laidlawii ; plaques small, turbid. Genome: negatively supercoiled ds cccDNA (MWt ca. 7.8 × 106 ). Virion: roughly spherical, somewhat pleomorphic, (50−)80(−125) nm in diameter, sensitive to detergents, organic solvents, and heat (e.g. 60° C for 5 min). There is apparently no rigid helical or icosahedral nucleocapsid. The virion seems to consist of DNA condensed with protein and enclosed within a flexible, lipid-containing, ‘unit-type’ membrane apparently derived from the host cell membrane. Infection leads to lysogeny, the phage DNA becoming integrated with the host’s chromosome. Progeny virions are released by budding through the host cell membrane; lysis does not occur, and the host remains viable. bacteriophage MV-L3 See MV-L3 PHAGE GROUP. 79 bacteriophage P2 consists of an icosahedral head (ca. 60 nm diam.) attached via a short tubular ‘neck’ to a ‘tail’ which is essentially no more than a hexagonal base-plate bearing short spikes. The ‘tail’ region has endorhamnosidase activity. Genome: dsDNA (MWt ca. 26 × 106 ), terminally redundant, and circularly permuted in certain regions of the genome. On infection of a host cell, the linear DNA is circularized either by the host RecA system or by a general recombinase encoded by the P22 erf gene. In the lytic cycle, the DNA may undergo some replication in the circular form; subsequently, concatemers many unit genomes in length are produced. During virion assembly, a prohead consisting mainly of coat protein (gp5) and a SCAFFOLDING PROTEIN (gp8) interacts with a concatemeric DNA molecule; DNA is encapsidated, beginning at a specific site (pac) in the concatemer and proceeding sequentially until the head is full, and the scaffolding protein is eliminated intact (and is recycled). It appears that once the first packaging event has been completed, the remaining DNA of the concatemer is encapsidated by other proheads – i.e., the leading end of the DNA which enters the second and subsequent proheads is determined not by the pac site but by the length of DNA packaged by previous prohead(s). (The mechanism of this is uncertain; the initial cleavage at the pac site can apparently occur in the absence of encapsidation [JMB (1982) 154 565–579].) (cf. BACTERIOPHAGE P1.) Maturation of the virion requires the addition of certain minor phage proteins. (See also TRANSDUCTION.) The lysogenic pathway in P22 is essentially similar to that in l (see LAMBDOID PHAGES). The prophage inserts in the Salmonella chromosome at a site between proA and proC ; insertion is catalysed by a P22-specified integrase encoded by an int gene located close to the att site. bacteriophage PA2 See STYLOVIRIDAE. bacteriophage PBS1 A large, morphologically complex FLAGELLOTROPIC PHAGE (family MYOVIRIDAE) which infects Bacillus subtilis; its DNA contains deoxyuridine instead of thymidine. PBS1 is a pseudotemperate phage, establishing a carrier state in its host (see PSEUDOLYSOGENY); it can mediate transduction. bacteriophage PBSX A defective bacteriophage, the prophage of which is normally present in the chromosome of its host, Bacillus subtilis. When induced by mitomycin C, PBSX packages 13-kb lengths of DNA which may include sequences from any region of the host chromosome, packaging probably occurring by a ‘headful’ mechanism [JV (1985) 54 773–780]. bacteriophage Pf1 See INOVIRUS. bacteriophage Pf2 See INOVIRUS. bacteriophage fI See T7 PHAGE GROUP. bacteriophage fII See T7 PHAGE GROUP. bacteriophage f6 The sole member of the family Cystoviridae. Host: Pseudomonas syringae pv. phaseolicola; plaques clear, diam. 1–3 mm. Some strains of P. pseudoalcaligenes can be infected. The virion (diam. ca. 75 nm) has a segmented genome of three pieces of dsRNA designated L, M and S (MWt ca. 5.0, 3.1 and 2.3 × 106 , respectively). [Nucleotide sequence of, and translational control in, the S segment: JV (1986) 58 142–151.] The RNA occurs in a polyhedral inner particle (composed of proteins P1, P2, P4 and P7) which is covered with a layer of protein P8 – the whole forming the nucleocapsid (diam. ca. 60 nm). The nucleocapsid is surrounded by a phospholipid- and protein-containing envelope (P3, P5, P6, P9, ?P10) which can be removed (with loss of infectivity) by treatment with detergents. An RNA polymerase (P1, P2) is associated with the capsid. The virion also contains an enzyme (P5, ?P10) active against the host cell wall; this enzyme seems necessary both for penetration of the host and for the release of phage progeny. within this sequence encodes a protein which may promote partition by binding to the plasmid at a particular site (incB ) and to (probably) the host cell envelope [JMB (1985) 185 261–272]. P1 also has a second SSR system which is responsible for the inversion of a 4.2-kb segment (the C segment) of the P1 genome. The C segment was first identified as a ‘C loop’ analogous to the G LOOP of bacteriophage Mu. The C segment of P1 and the G segment of Mu are homologous (each specifies different host ranges in different orientations), and their controlling elements (encoded by cin in P1, gin in Mu) can complement each other (see also RECOMBINATIONAL REGULATION). However, the inverted repeats at each end of the C segment are not homologous with those of the G segment. (Efficient inversion of the C segment apparently requires an additional cis-acting sequence, distinct from the cross-over sites cixL and cixR [PNAS (1985) 82 3776–3780].) [The P1 genome: JB (2004) 186 7032–7068.] bacteriophage P2 A temperate BACTERIOPHAGE (family MYOVIRIDAE) which infects Escherichia coli. The phage virion consists of an isometric head attached to a contractile tail. Genome: linear dsDNA (MWt ca. 22 × 106 ) which has 19-base STICKY ENDS. On infection of a host cell, the DNA forms closed circular molecules which, in lysogenic infections, can integrate into the host chromosome at a preferred site near the his operon; the phage ‘attachment site’ (attP ) occurs between genes int (encoding the P2 integrase) and ogr (see below). (cf. BACTERIOPHAGE LAMBDA.) P2 DNA replication occurs unidirectionally from a single origin near the right-hand end of the genome. Replication requires P2 genes A and B and (at least) host genes dnaB, polC and rep; P2 gpA is a (preferentially cis-acting) DNAbinding protein. The products of DNA replication are cccDNA monomers (i.e., concatemers are not formed). Expression of P2 late genes (which encode phage structural components) depends on the expression of the ogr gene, which in turn depends on P2 DNA replication. (cf. BACTERIOPHAGE P4.) During phage assembly, the DNA monomer undergoes staggered cleavage at a unique site (cos) to form the linear molecule with sticky ends characteristic of the P2 virion. bacteriophage P4 A satellite BACTERIOPHAGE (see SATELLITE VIRUS) which infects Escherichia coli and which requires all of the late genes of a helper phage such as BACTERIOPHAGE P2 to complete its replication cycle. The P4 genome is linear dsDNA (ca. 11.4 kb) with 19-nucleotide sticky ends which allow the DNA to circularize in the host cell. P4 DNA replication occurs bidirectionally from a single origin [JMB (1985) 182 519–527]; an early P4 gene product, gpa, is required for P4 DNA replication and is apparently a (rifamycin-resistant) primase [JMB (1985) 182 509–517]. In the absence of a helper phage, P4 can replicate its DNA and can lysogenize its host, but progeny virions can be formed only in the presence of a helper. In cells co-infected with P2 and P4, P4 capsids predominate and are assembled largely from P2 head proteins – although the P4 capsid is smaller than that of P2. P4 alters the regulation of transcription of P2 head proteins; the product of the P4 d gene is apparently a trans-acting accessory transcription factor which can substitute for P2 gpogr and thus circumvent the dependence of late P2 genes on P2 DNA replication [Book ref. 118, pp. 108–110]. bacteriophage P7 A BACTERIOPHAGE which is closely related to BACTERIOPHAGE P1; however, P1 and P7 are heteroimmune and differ in plasmid maintenance functions. bacteriophage P22 A temperate BACTERIOPHAGE (family PODOVIRIDAE) which infects Salmonella (smooth strains). The virion 80 bacteriophage fX174 (1998) 94 147–150.] In a current model, a number of pRNA molecules form a (static) ring-shaped structure around a hole in the phage head through which the genome is inserted; the connector (a dodecamer of gp10, 75 Å in length, with an axial channel) rotates, driven by ATP hydrolysis, and the ‘threaded’ (i.e. helical) DNA molecule enters the phage head like a bolt drawn through a nut when the nut is rotated [Nature (2000) 408 745–750]. Host cell lysis involves a phage-encoded peptidoglycandegrading enzyme; however, lysis also requires the product of phage gene 14 which is apparently a LYSIS PROTEIN [JB (1993) 175 1038–1042]. bacteriophage f80 See STYLOVIRIDAE and LAMBDOID PHAGES. bacteriophage f105 A temperate BACTERIOPHAGE which infects Bacillus subtilis (see STYLOVIRIDAE). The virion consists of an isometric head (ca. 52 nm diam.) attached to a flexible, noncontractile tail (ca. 220 nm long) which bears a baseplate with six appendages. Genome: dsDNA (MWt ca. 26 × 106 ), nonpermuted, apparently with sticky ends. During lysogeny, the phage DNA integrates at a specific attachment site in the host chromosome but, unlike e.g. BACTERIOPHAGE l, it appears not to circularize prior to integration, and the prophage is not circularly permuted with respect to the virion DNA; the mechanism of integration is not understood. On induction, f105 DNA apparently replicates in situ, together with adjacent regions of host DNA, and the phage DNA is subsequently excised from the newly synthesized molecules. LFT (but not HFT) lysates have been described; transduction appears to require that the recipient be a f105 lysogen. [Book ref. 170, pp. 251–256, 273–275.] bacteriophage fA See MICROVIRIDAE. bacteriophage fB See MICROVIRIDAE. bacteriophage fBA1 A temperate, tailed, dsDNA-containing BACTERIOPHAGE isolated from Bacillus aneurinolyticus. The virions apparently have a BACTERIOCIN-like killing activity against some strains of B. aneurinolyticus; this activity is independent of phage gene expression (phage DNA is rapidly degraded in the sensitive cells), and may be due to a proteinaceous component of the intact virion [JV (1986) 59 103–111]. (cf. BETACIN.) bacteriophage fC (1) See MICROVIRIDAE. (2) See STYLOVIRIDAE. bacteriophage fC31 A temperate bacteriophage originally isolated from Streptomyces coelicolor ; it can infect a wide range of Streptomyces spp. Genome: dsDNA (ca. 41.5 kb) with sticky ends. The prophage integrates in the host chromosome at a specific attachment site. bacteriophage fCbK See STYLOVIRIDAE. bacteriophage fD328 See STYLOVIRIDAE. bacteriophage fEC A tailed, temperate ACTINOPHAGE whose hosts are species of Rhodococcus; genome: linear dsDNA. fEC is inactivated by various lipid solvents. [Book ref. 73, pp. 216–218.] bacteriophage fK See MICROVIRIDAE and BACTERIOPHAGE G4. bacteriophage fNS11 See TECTIVIRIDAE. bacteriophage fR See MICROVIRIDAE. bacteriophage fW-14 A bacteriophage (family MYOVIRIDAE) which infects and lyses certain strains of Pseudomonas acidovorans. In fW-14 DNA ca. 50% of the thymine residues are hypermodified, occurring as a-putrescinylthymine; the hypermodified bases are apparently essential for the production of viable progeny phages – probably being required for DNA packaging [Virol. (1983) 124 152–160]. bacteriophage fX174 The type species of the MICROVIRIDAE (q.v. for morphology etc). After adsorption of fX174 to a host cell, the ss cccDNA phage genome penetrates the cell and is The virion adsorbs (P3, P6) to the sides of the subpolar host pili; the f6 envelope fuses with the host outer membrane, and the nucleocapsid minus P8 enters the host cell. Inside the cell the phage RNA polymerase catalyses RNA synthesis; an ss mRNA, designated l (MWt ca. 2.2 × 106 ), is transcribed early and codes for P1, ?P2, P4 and P7. These four proteins form a 120S procapsid or previrion I which incorporates progeny dsRNA to form previrion II. (The synthesis of dsRNA is not understood, but it occurs within a subvirion structure. A model for the initiation of replication and transcription in bacteriophage f6 has been proposed [Nature (2001) 410 235–240].) Previrion II synthesizes ss mRNAs m and s (MWt ca. 1.4 and 1.1 × 106 , respectively); s codes for P8 and P9, m for P3, P6 and P10 (and possibly also P4 and P7.) Previrion II incorporates P8 to form the complete nucleocapsid. Finally, the lipoprotein envelope is added – apparently mediated by the (non-structural) phage protein P12; although the lipid is apparently derived from host phospholipid, the membrane is formed in the cytoplasm and not in contact with the cell membrane. Phage release occurs by host cell lysis. (Note: the name bacteriophage f6 has also been used for a pilus-dependent dsDNA phage of Caulobacter [JV (1980) 35 949–954].) bacteriophage f29 A BACTERIOPHAGE (family PODOVIRIDAE) which infects strains of Bacillus subtilis and those of certain other species of Bacillus. The virion has a complex morphology: an elongated head (ca. 32 × 42 nm), bearing protein fibres at each end, joined via a connector region to a short tail. Genome: linear dsDNA (ca. 18 kb) with 6-bp terminal inverted repeats. The 5 end of each strand in the DNA genome is formed by a dAMP residue; each dAMP residue is linked covalently (via a phosphodiester bond) to a serine residue in a phage-encoded protein (gp3) – a so-called terminal protein (TP). The terminal proteins are essential for initiation of replication of the f29 genome. Replication of the linear f29 DNA is initiated at both ends of the molecule (not simultaneously); it involves a phageencoded DNA polymerase (gp2) which has exonuclease activity [NAR (1985) 12 1239–1249]. The polymerase binds to a free molecule of the TP, and this complex localizes at the 3 end of each strand – perhaps by interacting with TP at the adjacent 5 end. The polymerase then mediates a dATPdependent reaction in which dAMP is covalently linked to the complexed TP; this dAMP residue serves as a 3 -OH primer. The polymerase begins strand synthesis (5 -to-3 ), and after insertion of nucleotide 10 the polymerase and TP dissociate from one another – the TP remaining covalently bound at the 5 end of the new strand while the polymerase continues to extend the (DNA) primer. The polymerase is able to displace the (5 -to3 ) strand complementary to the template, and to achieve good processivity, without help from a helicase or from other factors [JBC (1996) 271 8509–8512]. Replication of the f29 genome is an example of so-called protein priming – only one example of the strategies used for replicating a linear genome. [Protein priming of DNA replication in phage f29: EMBO (1997) 16 2519–2527.] (See also ADENOVIRIDAE.) During phage assembly, the major coat protein (gp8) is assembled with the aid of a (re-cyclable) scaffolding protein (gp7) which is displaced as the DNA is incorporated. [Morphogenesis: JV (1985) 53 856–861.] The packaging of DNA into the phage head requires energy from ATP hydrolysis. It also requires certain phage-encoded RNA molecules designated pRNA. [DNA packaging: Cell 81 bacteriophage fXahb bacteriophage PR5 See TECTIVIRIDAE. bacteriophage PR772 See TECTIVIRIDAE. bacteriophage PRD1 See TECTIVIRIDAE. bacteriophage Qb See LEVIVIRIDAE. bacteriophage R1 See STYLOVIRIDAE. bacteriophage R2 See STYLOVIRIDAE. bacteriophage R17 See LEVIVIRIDAE. bacteriophage R23 See LEVIVIRIDAE. bacteriophage 7S See LEVIVIRIDAE. bacteriophage S13 See MICROVIRIDAE and BACTERIOPHAGE fX174. bacteriophage SP3 See MYOVIRIDAE. bacteriophage SP6 A BACTERIOPHAGE which infects Salmonella typhimurium; it has a linear dsDNA genome (ca. 43.5 kb) and appears to resemble BACTERIOPHAGE T7 in its morphology and development. SP6 encodes a rifamycin-resistant RNA polymerase which is synthesized shortly after infection of a host cell. [Nucleotide sequence and expression of the cloned SP6 RNA polymerase gene: NAR (1987) 15 2653–2664.] bacteriophage SP8 See MYOVIRIDAE. bacteriophage SP50 See MYOVIRIDAE. bacteriophage SPb See STYLOVIRIDAE and BETACIN. bacteriophage SPO1 A virulent BACTERIOPHAGE which infects Bacillus subtilis. The virion consists of a large isometric head, containing >7 different polypeptides, joined via a complex connector structure to a contractile tail which terminates in a complex base-plate. The genome is a single molecule of linear dsDNA, MWt ca. 108 , which contains hydroxymethyluracil. [Review: Book ref. 170, pp. 218–245.] SPO1 is widely used in studies of transcriptional regulation in Gram-positive bacteria. Phage genes are expressed in three phases: early, middle and late. Early gene promoters are of the same type as those of host genes involved in vegetative functions, allowing transcription of early phage genes by the (unmodified) host RNA POLYMERASE. One early gene, gene 28, encodes a novel SIGMA FACTOR (sgp28 ) which replaces the host s43 (formerly known as s55 ), resulting in a holoenzyme which can recognize only phage middle promoters. Middle genes include genes 33 and 34, the expression of which is necessary for late gene transcription; gp33 and gp34 together replace the sgp28 in the RNA polymerase, resulting in an enzyme which – in the presence of the delta factor (see RNA POLYMERASE) – recognizes only late phage promoters, thus allowing the transcription of late genes (which encode phage structural components). (See also ENDOSPORE sense 1.) bacteriophage SPO2 A temperate BACTERIOPHAGE which infects Bacillus subtilis. Virion: an isometric head (ca. 50 nm diam.) attached to a tail ca. 180 nm long. Genome: dsDNA, MWt ca. 26 × 106 . [Book ref. 170, pp. 247–268 (256–259).] bacteriophage SPP1 See STYLOVIRIDAE. bacteriophage St-1 See MICROVIRIDAE and BACTERIOPHAGE G4. bacteriophage SV-C1 See SPIROPLASMAVIRUSES. bacteriophage SV-C2 See SPIROPLASMAVIRUSES. bacteriophage SV-C3 See SPIROPLASMAVIRUSES. bacteriophage T1 See STYLOVIRIDAE. bacteriophage T2 See T-EVEN PHAGES and MYOVIRIDAE. bacteriophage T3 A bacteriophage (family PODOVIRIDAE) which is very closely related to BACTERIOPHAGE T7. bacteriophage T4 A virulent enterobacterial (linear dsDNAcontaining) BACTERIOPHAGE – the best-known representative of the T-even phages. (See entry T-EVEN PHAGES for phage structure, nature of genome etc; see also MYOVIRIDAE.) The T4 infection cycle is initiated when the phage attaches, via the distal tips of its long tail fibres, to specific sites on the coated with host SSB protein; a region between genes F and G (which can form hairpin loops) remains uncoated. Stage I (see SSDNA PHAGES) is dependent on host dnaB function (as it is in microviruses S13, G6, G13, G14 and fXahb – cf. BACTERIOPHAGE G4). A PRIMOSOME assembles at a specific n recognition sequence in the F–G intergenic region. The primosome migrates (5 to 3 ) along the n strand (displacing the SSB protein?), synthesizing primers at several sites. The c strand is completed as in other SSDNA PHAGES to form the ds RF (parental RF). It seems that most components of the primosome remain bound to the parental RF throughout this and subsequent stages, and that this obviates the need for supercoiling of the RF by gyrase – a step previously thought to be essential for initiation of stage II [PNAS (1981) 78 1436–1440]. Stage II: fX174 gpA nicks the n strand of the parental RF at a specific site in gene A and becomes covalently bound to the 5 end of the nicked strand. The 3 end is elongated by host DNA polymerase III holoenzyme in a ROLLING CIRCLE MECHANISM; ahead of holoenzyme action, the two RF strands are processively unwound by a complex of gpA and Rep protein. (The presence of gpA allows Rep protein to begin unwinding from the nick – not normally possible for HELICASES.) As synthesis proceeds, the displaced n strand (attached by its 5 end to gpA at the replication fork) forms a growing loop which can function as template for the discontinuous synthesis of a c strand (primed by the conserved parental RF primosome components). When one genome length of n DNA has been displaced, it is excised and circularized by gpA and the c strand is completed to form the progeny RF. Progeny RFs are supercoiled by GYRASE and seem at this stage to function primarily as templates for transcription of phage genes. Stage III is closely coupled to phage morphogenesis. RFI (see RF) is cleaved by gpA, as in stage II, to form an RFII–gpA complex which, in the presence of gpC, associates with a procapsid (consisting of gpB, gpD, gpF , gpG and gpH ) to form a 50S particle. n strand synthesis and displacement seem to occur by a rolling circle mechanism; as the n strand is displaced, it is packaged within the procapsid. Packaging (but not phage DNA synthesis) requires gpJ , a component of the phage capsid [JV (1985) 54 345–350]. When a complete n strand has been displaced it is cleaved and circularized by gpA, releasing the RFII–gpA complex (for further rounds of n strand synthesis) and an SS cccDNA-containing immature phage particle; gpB and gpD are subsequently lost to form the mature 114S fX174 virion. bacteriophage fXahb See BACTERIOPHAGE fX174. bacteriophage fXtB See MICROVIRIDAE. bacteriophage PM2 The type species of the CORTICOVIRIDAE; host: Alteromonas espejiana BAL-31 (= Pseudomonas BAL31). The virion is icosahedral (diam. ca. 60 nm) and is sensitive to organic solvents and detergents; it comprises four major proteins, 12–14% by weight phospholipid, and supercoiled ds cccDNA (MWt ca. 6 × 106 ). The outer layer of the capsid is an icosahedral shell of protein PII (= sp27) with spikes of protein PI (= sp43) at the vertices. Within this is a lipid bilayer membrane whose composition reflects that of the host cell membrane at the time of phage assembly. The membrane seems to be associated with one or both of the remaining two proteins; these proteins may also be associated with the DNA, and at least one has transcriptase activity. Virions adsorb to the host cell wall. Maturation occurs at the cell periphery. The progeny are released by cell lysis. bacteriophage PP7 See LEVIVIRIDAE. bacteriophage PR3 See TECTIVIRIDAE. bacteriophage PR4 See TECTIVIRIDAE. 82 bacteriophage T7 completed polynucleotide strands; HMC substitution occurs at the level of precursor synthesis, involving hydroxymethylation of dCTP. Phage components are encoded by late phage genes. Heads, tail fibres and base-plates are each assembled by separate pathways. Head assembly involves at least 20 genes. Initially, a prohead is assembled on the host cytoplasmic membrane; the prohead consists of a shell surrounding a central core, the latter acting as scaffolding and controlling the assembly, size and shape of the prohead. When this structure is complete, nearly all of its constituent proteins undergo proteolytic cleavage, resulting in the removal of the core and the formation of a ‘cleaved prohead’. The cleaved prohead detaches from the membrane, and the main shell protein (gp23) undergoes extensive conformational changes which result in expansion of the prohead; this step is probably normally accompanied by DNA packaging. DNA is packaged by the ‘headful’ mechanism: one end of a linear concatemeric DNA molecule is inserted into the prohead, and the DNA continues to be incorporated until the head is full, when the DNA is cleaved (thus yielding the circularly permuted genome characteristic of T4). Base-plate assembly occurs by separate assembly of the hub and wedges (see T-EVEN PHAGES), followed by binding of the six wedges around the hub. The tail is then polymerized (inner tube first) on the base-plate. The completed tail structure associates spontaneously with the DNA-containing head, and finally the tail fibres and collar ‘whiskers’ are added. Progeny phages are released by lysis of the host cell. The mechanism of lysis is still not fully understood, but requires T4-coded LYSOZYME (gpe), the function of gene t (see LYSIS PROTEIN), and possibly gp5 (the base-plate hub lysozyme – see T-EVEN PHAGES). [Reviews on all aspects of T4: Book ref. 99.] bacteriophage T5 See STYLOVIRIDAE. bacteriophage T6 See T-EVEN PHAGES. bacteriophage T7 A virulent bacteriophage of the PODOVIRIDAE which infects Escherichia coli and other enterobacteria. Head: isometric, ca. 60 nm diam. [capsid architecture: Virol. (1983) 124 109–120]. Tail: ca. 17 nm long, with six short fibres; the distal end of the tail adsorbs to LPS in the outer membrane of a host cell. The T7 genome is a linear, non-permuted dsDNA molecule (ca. 40 kb) with terminally redundant ends (160 bp). T7 early genes constitute a single operon near the left end of the genome; these genes are transcribed by the host RNA polymerase and are concerned mainly with switching off host gene transcription and switching on transcription of T7 intermediate and late genes. (The large early-gene transcript is cleaved by host RNase III (q.v.) to give mRNAs.) Early gene products include a protein kinase (gp0.7), which phosphorylates and inactivates the host RNA polymerase (thus preventing further early gene transcription), and an RNA polymerase (gp1) which transcribes the T7 intermediate genes (concerned mainly with T7 DNA replication) and late genes (concerned with phage assembly and release of phage progeny by host cell lysis). Another phage product, gp2, also binds to and inactivates host RNA polymerase. The phage RNA polymerase is highly specific for T7 promoters; thus, by inhibiting host RNA polymerase, all transcriptional activity in the cell is switched to T7 DNA. [Organization and expression of T7 DNA: CSHSQB (1983) 47 999–1007.] Replication of T7 DNA (which remains linear throughout) is achieved mainly by proteins encoded by T7 DNA; it occurs in three stages: (i) Initiation occurs at a single site near the ‘left’ end of the genome. (Secondary sites can be used, host cell surface (see T-EVEN PHAGES). The tail fibres may occur in either of two states: extended or ‘retracted’ (i.e., folded back along the tail sheath and head); infection can occur only when the fibres are extended. Transition between the two states is affected by conditions: retraction (and hence loss of infectivity) is promoted by e.g. low pH, low temperature, low ionic strength, and in some strains by the absence of the adsorption cofactor tryptophan. Binding is initially reversible, but is followed by irreversible binding during which the base-plate undergoes a conformational change from a hexagonal form to an expanded, star-shaped structure with a central opening. This in turn triggers the contraction (by conformational change) of the tail sheath and the penetration of the host cell envelope by the inner tail tube. (Penetration may occur at ADHESION SITES.) The phage DNA is then transferred through the tail inner tube to the host cell. Since the inner tube tip appears to penetrate only as far as the outer surface of the host cytoplasmic membrane, DNA apparently does not enter the cytoplasm directly; uptake of the DNA by the host cell appears to require a membrane potential and can be inhibited by ionophores. Infection of the host cell is followed by transcription of the T4 DNA. This occurs in three main phases: early (immediate early and delayed early), middle, and late; each phase occurs at a distinct time after infection, is initiated at a distinct class of promoters, and requires a different transcription apparatus. The host RNA POLYMERASE is apparently used throughout, but undergoes a series of phage-induced alterations (e.g. ADPribosylation of first one, then both, of its a subunits) which affect its activity and affinities (e.g. for sigma factor); T4 gp55 is a SIGMA FACTOR which is necessary for the transcription of T4 late genes. Early effects on the host cell include the cessation of host DNA, RNA and protein synthesis (within 2–5 min of infection), unfolding of the host nucleoid, and degradation of host DNA by phage-coded nucleases specific for cytosine-containing DNA. The nucleotides thus released are used for the synthesis of progeny phage DNA. Late transcription is coupled to T4 DNA replication, which begins ca. 5 min after infection. Replication can be initiated at multiple origins [locations: JV (1985) 54 271–277] and occurs bidirectionally. It appears to be catalysed by a complex of phagecoded proteins (a ‘replisome’ – see DNA REPLICATION), including gp43 (DNA polymerase), gp32 (an SSB protein), gp44/45/62 (DNA polymerase accessory proteins which e.g. increase polymerase processivity and affinity for DNA), gpdda (a 5 -to3 helicase), and gp41 and gp61 (involved in the priming of Okazaki fragments). Leading strand synthesis is probably primed initially by host RNA polymerase. Okazaki fragments in lagging strand synthesis are primed by the formation of pentaribonucleotide primers (pppApCpNpNpN, where N = any of the 4 nucleotides) by gp41 and gp61; gp41 may be analogous to the DnaB protein of Escherichia coli (see DNAB GENE), while gp61 may be a PRIMASE. When elongation is complete, the 3 end of the template for each lagging strand remains singlestranded; these single-stranded regions can ‘invade’ homologous regions of other phage DNA molecules, forming recombinational forks (see RECOMBINATION) and resulting in the formation of complex, branching, concatemeric intermediates. Resolution of the branches appears to require gp49 (T4 endonuclease VII). Later rounds of DNA replication may be initiated at recombinational forks, allowing replication to become independent of the (altered) host RNA polymerase. (Alteration of the RNA polymerase may prevent its recognition of origin promoters.) Modification of the DNA (glucosylation and methylation) occurs on the 83 bacteriophage Tb is involved in the (energy-generating) light-dependent transmembrane translocation of protons. The protein part of the bacteriorhodopsin molecule (apoprotein, bacterio-opsin) consists of seven membrane-spanning regions; the pigment – RETINAL – occurs within the space bounded by these seven regions, being linked to one of them via a lysine residue. Bacteriorhodopsin occurs in hexagonal arrays in the plane of the membrane. On illumination, bacteriorhodopsin undergoes a cyclical series of changes with a periodicity of ca. 5–10 msec, and this photocycling is associated with the pumping of protons from the inner (cytoplasmic) to the outer side of the membrane, i.e. generation of proton motive force (pmf). During photocycling at least five photointermediates appear to be formed; these are designated: K590 , L550 , M412 , N530 and O640 (the numbers being wavelengths, in nm, of maximum absorption). bR568 is the lightadapted ground state. Recent studies have clarified the mechanism, and route, of vectorial energy-dependent transport of protons across the membrane. Light induces the isomerization of (protonated) retinal from all-trans to 13-cis, isomerization resulting in deprotonation of the retinal – the proton being transferred to the asp-85 residue of the protein. Deprotonation of the retinal causes it to change conformation, and this, in turn, modifies the protein’s conformation – opening up a channel which permits entry of a proton from the cytoplasmic side of the membrane; this proton reprotonates residue asp-96 – the proton from asp-96 having been donated to reprotonate retinal. The proton on asp85 is transferred to the exterior of the membrane; this transfer appears to coincide with the N530 photointermediate. When incorporated into LIPOSOMES, bacteriorhodopsin tends (except at low pH) to adopt an orientation opposite to that in the cytoplasmic membrane – and hence to mediate inward pumping of protons. [Bacteriorhodopsin (overview): Nature (2000) 406 569–570. Structural changes in bacteriorhodopsin coupled to proton transport: Nature (2000) 406 645–648. Structural alterations in the M state: Nature (2000) 406 649–652. Molecular mechanism of vectorial proton translocation: Nature (2000) 406 653–657.] bacterioruberins C50 CAROTENOID pigments which occur in members of the HALOBACTERIACEAE. bacteriostatic (bacteristatic) Able to inhibit the growth and reproduction of at least some types of bacteria. (cf. BACTERICIDAL.) bacteristatic Syn. BACTERIOSTATIC. bacterium See BACTERIA. Bacterium A bacterial genus which has been obsolete for decades; various bacteria, e.g. Escherichia coli, Clostridium chauvoei etc., were once referred to as B. coli, B. chauvoei etc. bacterium-associated haemophagocytic syndrome (BAHS) See HAEMOPHAGOCYTIC SYNDROME. bacteriuria The presence of bacteria in the urine. bacterivore Any organism (e.g. a ciliate or amoeba) which ingests bacteria as its main or sole source of nutrients. (cf. e.g. DETRITIVORE.) bacterization The process of coating seeds, tubers etc. with certain bacteria (e.g. Azotobacter), prior to planting, with the object of improving plant growth. bacteroid A bacterium-like cell or a modified bacterial cell – see e.g. ROOT NODULES. (In an ACTINORRHIZA, ‘bacteroid’ may refer e.g. to a senescent hyphal fragment.) Bacteroidaceae A family of (typically) Gram-negative, anaerobic, chemoorganotrophic, asporogenous, motile and non-motile, although less efficiently, if the primary site is deleted.) The T7 gp1 RNA polymerase is necessary for initiation, apparently synthesizing a short primer from promoters in the initiation region. (ii) Bidirectional elongation (from the RNA primers) catalysed mainly by two phage-coded enzymes: gp5 and gp4. The protein gp5 combines with host THIOREDOXIN to form a DNA polymerase which synthesizes DNA on the RNA primers in the 5 -to-3 direction, thus forming the leading strands. The protein gp4 is a multifunctional enzyme with primase, helicase and NTPase activity; it binds to unwound ssDNA regions of parental DNA and translocates unidirectionally in the 5 -to-3 direction (driven by NTP hydrolysis), unwinding the parental strands ahead of the replication forks. As it encounters specific primase recognition sites in the DNA, gp4 synthesizes tetraribonucleotide primers (pppApCpCpC or pppApCpCpA) which provide the 3 -OH ends necessary for initiation of lagging strand synthesis by the T7 DNA polymerase. [Book ref. 69, pp. 135–151.] Nucleotides for T7 DNA synthesis are provided by degradation of host DNA by T7 gp3 (an endonuclease) and gp6 (an exonuclease); gp6 (and/or host DNA polymerase I) may be involved in the removal of RNA primers. (iii) The newly synthesized daughter duplexes undergo recombination, probably mainly end-to-end, to form concatemers several times the length of a T7 genome [Virol. (1982) 123 474–479]; these concatemers undergo further rounds of replication. Each concatemer apparently contains only a single copy of the 160-bp terminal repeat sequence at the junctions between the genomes; the terminal repetition of the mature DNA is generated during maturation and packaging of the DNA into preformed prohead structures, when the concatemers are cleaved (possibly by gp3) into genome lengths. Mature progeny phages are released by host cell lysis (gp3.5 has lysozyme activity). Studies on the rate of internalization of the phage genome during infection have led to new insight into the mechanism by which the DNA of phage T7 is transferred to the bacterial cell. It appears that, following attachment, the phage ejects certain proteins which may form a channel across the bacterial cell envelope through which DNA is transferred; it has been speculated that two of the ejected proteins may be components of a ‘motor’ which rachets DNA into the host cell [Mol. Microbiol. (2001) 40 1–8]. bacteriophage Tb Syn. TBILISI PHAGE. bacteriophage typing See PHAGE TYPING. bacteriophage U3 See MICROVIRIDAE. bacteriophage v1 See PLASMAVIRIDAE. bacteriophage v2 See PLASMAVIRIDAE. bacteriophage v4 See PLASMAVIRIDAE. bacteriophage v5 See PLASMAVIRIDAE. bacteriophage v6 See INOVIRUS. bacteriophage v7 See PLASMAVIRIDAE. bacteriophage VP5 An ACTINOPHAGE of the STYLOVIRIDAE which infects Streptomyces coelicolor. bacteriophage W31 See T7 PHAGE GROUP. bacteriophage Xf See INOVIRUS. bacteriophage Xf2 See INOVIRUS. bacteriophage Z See BETACIN. bacteriophage ZJ/2 See INOVIRUS. bacteriophages See BACTERIOPHAGE. bacteriophagous Refers to organisms (e.g. certain protozoa) which consume bacteria. bacteriorhodopsin A hydrophobic, pigment-containing protein (MWt ca. 27000) which is the major protein constituent of the PURPLE MEMBRANE in Halobacterium salinarium; it 84 Baculoviridae typically rod-shaped or filamentous bacteria, some of which are pathogenic in man and other animals. Genera: ACETIVIBRIO, ANAEROBIOSPIRILLUM, ANAEROVIBRIO, BACTEROIDES, BUTYRIVIBRIO, FUSOBACTERIUM, LACHNOSPIRA, LEPTOTRICHIA, PECTINATUS, SELENOMONAS, SUCCINIMONAS, SUCCINIVIBRIO and WOLINELLA. In some of these organisms (e.g. Acetivibrio, Butyrivibrio, Lachnospira) the CELL WALL appears to resemble the Gram-positive (rather than Gram-negative) type of wall; since these organisms give a negative or variable reaction in the Gram stain (at least under some conditions) they have been grouped with the frankly Gram-negative genera [Book ref. 22, pp. 602–662]. Bacteroides A genus of Gram-negative bacteria (family BACTEROIDACEAE) which occur e.g. in the RUMEN and in the mouth and intestinal tract in man and other animals; some species are opportunist pathogens, being isolated from abscesses and other types of lesion (see e.g. FOOT-ROT; PNEUMONIA (f); SCALD). (See also ANAEROBIC DIGESTION; TERMITE–MICROBE ASSOCIATIONS; WETWOOD.) Cells: typically rods or filaments, nonmotile or peritrichously flagellated, sometimes with central or terminal swellings or vacuoles. Some species form a dark or black pigment. [Black-pigmented strains from animals: JAB (1983) 55 247–252.] Metabolism is typically fermentative, though some strains can carry out FUMARATE RESPIRATION in media containing haemin – electron donors including formate and H2 . Most species (‘fermentative’, ‘saccharoclastic’ or ‘saccharolytic’ species) typically attack a range of sugars, producing a mixture of products which may include acetic, formic, lactic, propionic and succinic acids; such species usually use or need CO2 – which is assimilated by the reductive carboxylation of succinate to 2-oxoglutarate. Other species attack sugars weakly, or not at all, but utilize peptones with the formation of mixtures of either small amounts of e.g. acetic, formic, lactic and succinic acids, or mixtures of larger amounts of e.g. acetic, butyric, propionic and succinic acids – isobutyric and isovaleric acids being formed whenever butyric acid is formed. Primary isolation may be carried out on media containing e.g. peptone, yeast extract, haemin and menaquinone under at least 5% CO2 . GC%: ca. 28–61. Type species: B. fragilis. B. endodontalis. A non-saccharolytic, black-pigmented species isolated from the dental root canal [IJSB (1984) 34 118–120]. B. fragilis (formerly Fusiformis fragilis). Cells (in glucose broth): non-motile round-ended rods, 0.8–1.3 × 1.6–8.0 µm. Catalase +ve (most other species are catalase −ve). Saccharolytic; in haemin-containing media some strains form a b-type cytochrome and can carry out fumarate respiration. On horse- or rabbit-blood agar colonies are typically smooth, round, entire, low convex, grey and non-haemolytic; some strains produce clear (‘b’) haemolysis. Optimum growth temperature: 37° C. B. fragilis may be sensitive to e.g. CEFOXITIN and clindamycin (see LINCOSAMIDES). (The ‘B. fragilis group’ commonly includes former subspecies of B. fragilis which are currently classified as species: B. distasonis, B. ovatus, B. thetaiotaomicron and B. vulgatus.) B. melaninogenicus. A saccharolytic species which forms a black pigment (a haem derivative) and which has SOD activity; it occurs e.g. in the human mouth. B. nodosus. A non-motile, non-saccharolytic species which causes foot-rot in sheep. [Surface structure and virulence of B. nodosus: JGM (1983) 129 225–234.] B. ochraceus. See CAPNOCYTOPHAGA. B. pneumosintes (formerly Dialister pneumosintes). A nonsaccharolytic species which occurs e.g. in the human nasopharynx and which may be a secondary invader in upper respiratory tract infections. Cells: typically 0.2–0.4 × 0.3–0.6 µm. B. ruminicola. A saccharolytic species which ferments a wide range of carbohydrates, including e.g. cellobiose and starch; it occurs in the RUMEN. B. succinogenes. A saccharolytic rumen species which attacks relatively few sugars but which can utilize cellobiose and cellulose; unlike B. ruminicola, this species does not digest gelatin or hydrolyse aesculin. B. termitidis (formerly Sphaerophorus siccus var. termitidis). A saccharolytic species which occurs in the intestinal tract in termites. Other species: B. amylophilus, B. asaccharolyticus, B. bivius, B. buccae, B. capillosus, B. coagulans, B. corporis, B. denticola, B. disiens, B. distasonis, B. eggerthii, B. furcosus, B. gingivalis, B. gracilis, B. hypermegas, B. intermedius, B. levii, B. loescheii, B. macacae, B. microfusus, B. multiacidus, B. oralis, B. oris, B. ovatus, B. praeacutus (cf. TISSIERELLA), B. putredinis, B. splanchnicus, B. thetaiotaomicron, B. uniformis, B. ureolyticus, B. vulgatus and B. zoogleoformans. [Book ref. 22, pp. 604–631; Bacteroides of the human lower intestinal tract: ARM (1984) 38 293–314.] Bacto The designation of products of Difco Laboratories Inc., Detroit, USA. bactoprenol (undecaprenol) In bacteria: a lipid-soluble, membrane-bound polyprenol consisting of a linear chain of 11 unsaturated isoprene units: H−[CH2 .C(CH3 )=CH.CH2 ]11 −OH Bactoprenol phosphate acts as a carrier in the synthesis of a number of cell envelope and extracellular polymers: e.g., PEPTIDOGLYCAN, LIPOPOLYSACCHARIDE, TEICHOIC ACIDS and certain CAPSULE polymers; it apparently facilitates the transfer of lipophobic sugar residues across the cytoplasmic membrane. (cf. DOLICHOL; see also TUNICAMYCIN.) Baculoviridae A family of ccc dsDNA-containing VIRUSes which infect arthropods (particularly insects of the Diptera, Hymenoptera and Lepidoptera). Baculovirus virions are structurally complex, consisting of one or more nucleocapsids within an envelope; virions containing one nucleocapsid per envelope are rod-shaped, ca. 200–400 × 40–60 nm. The genome is nonsegmented circular supercoiled dsDNA, MWt ca. 58–110 × 106 . The family is divided (on morphological criteria) into three subgroups: A (the NUCLEAR POLYHEDROSIS VIRUSES, NPVs), B (the GRANULOSIS VIRUSES, GVs), and C. (cf. POLYDNAVIRIDAE.) The NPVs and GVs are characterized by the formation of intracellular crystalline protein occlusion bodies within which virions are embedded; the NPVs form polyhedral intranuclear occlusion bodies (called POLYHEDRA), each containing many virions, while the GVs form ellipsoidal or round-ended rod-shaped occlusion bodies (called ‘granules’ or ‘capsules’), each containing only one (rarely two) virions. The matrix proteins of both types of occlusion body (called polyhedrin, or polyhedrin in NPVs and granulin in GVs) are closely related in structure and function [review: JGV (1986) 67 1499–1513], but are apparently unrelated to the polyhedrins of the CYTOPLASMIC POLYHEDROSIS VIRUS GROUP. In contrast to the NPVs and GVs, baculoviruses of subgroup C do not form occlusion bodies; viruses of this subgroup have been observed in various arthropods (insects, arachnids, crustacea), the type species of the subgroup being Oryctes rhinoceros virus (from the coconut palm rhinoceros beetle). 85 baculoviruses An insect becomes infected with an NPV or GV when it ingests an occlusion body; the polyhedrin dissolves in the alkaline contents of the insect’s midgut, releasing virions which then infect cells of the midgut. Non-occluded virions produced in these cells bud through the plasma membrane of the midgut cells and are disseminated via the haemolymph, subsequently infecting cells of a wide range of tissues. The virion-containing occlusion bodies are formed late in infection, and are eventually released by the death and dissolution of the insect host. Baculoviruses have been extensively used for the BIOLOGICAL CONTROL of insect pests; they are particularly suitable for this purpose since they have no apparent relationship with any other known animal or plant viruses (and are therefore deemed unlikely to infect mammals or plants), and they can be lethal for – and spread rapidly among – insects in nature. Thus, e.g., NPVs have been used to control the European pine sawfly (Neodiprion sertifer ) and European spruce sawfly (Gilpinia hercyniae), GVs have been used against the Small White butterfly (Artogeia (= Pieris) rapae, a pest of crucifers), and Oryctes rhinoceros virus has been used successfully to control the coconut palm rhinoceros beetle. Since NPVs and GVs produce large quantities of polyhedrin/granulin in infected cells, the genomes of these viruses are of interest as vectors for the introduction and high-level expression of foreign genes in insects or insect cell cultures; e.g., a recombinant baculovirus containing a human a-interferon gene linked to the polyhedrin promoter yielded large quantities of a-interferon in silkworms [Nature (1985) 315 592–594]. baculoviruses Viruses of the BACULOVIRIDAE. Badhamia See MYXOMYCETES. BaeI See RESTRICTION ENDONUCLEASE (table). baeocyte A small, spherical reproductive cell (diam. ca. 2–3 µm) formed by members of section II of the CYANOBACTERIA. Baeocytes (formerly termed ‘endospores’) are produced by multiple fission of a vegetative cell which is enclosed in a fibrous layer external to the outer membrane of the cell wall; numerous baeocytes are released on rupture of the fibrous layer. A baeocyte enlarges, without division, until it reaches the size of a vegetative cell, this growth being accompanied by a progressive thickening of the fibrous outer layer. Gliding motility may be exhibited initially by those baeocytes which lack a fibrous outer layer at the time of their release. Baeomyces A genus of LICHENS (order LECANORALES); photobiont: e.g. Myrmecia. Thallus: crustose to squamulose, often sorediate. Fruiting bodies are non-lichenized, stalked, convex hymenial discs resembling miniature mushrooms (cf. PODETIUM). Species occur on soil, rocks etc. BAF See SEWAGE TREATMENT. bagassosis An EXTRINSIC ALLERGIC ALVEOLITIS associated with inhalation of the dust of sugar cane waste (bagasse); certain bacteria (e.g. Thermoactinomyces sacchari ) have been implicated. BAHS (bacterium-associated haemophagocytic syndrome) See HAEMOPHAGOCYTIC SYNDROME. bakanae disease (‘foolish seedling disease’; ‘foolish rice’) A disease of rice caused by Gibberella fujikuroi (Fusarium moniliforme) and characterized by conspicuous elongation of the plant stems (internodes) followed by wilting. The symptoms are believed to be due to the effects of GIBBERELLINS produced by the pathogen. (See also CEREAL DISEASES.) baker’s yeast A specialized strain of Saccharomyces cerevisiae which is capable of rapid fermentative activity in dough (see BREAD-MAKING), i.e., under conditions of low oxygen tension, low water activity, and high osmotic pressure. The yeast is manufactured by batch culture (in a medium containing molasses, vitamins, minerals and a nitrogen source) for 12–18 hours at ca. 30° C with full aeration (to suppress fermentation). The yeast is harvested by centrifugation, washed, and concentrated by pressing or filtration. ‘Fresh yeast’ is blended, extruded, cut into cakes and wrapped; it can be stored for only a few days. ‘Active dry yeast’ is usually dried in a stream of warm air and sold as granules (water content ca. 8.0%); it can be stored for up to ca. 18 months in sealed containers. baking See BREAD-MAKING. BAL British anti-Lewisite: 2,3-dimercaptopropanol; in the presence of oxygen BAL acts as a mitochondrial RESPIRATORY INHIBITOR, apparently blocking electron flow between cytochromes b and c1 in Complex III of the ELECTRON TRANSPORT CHAIN. Balamuthia mandrillaris A species of amoeba which can cause ENCEPHALITIS in the young, old and immunocompromised. Trophozoites, 15–60 µm diam., have a round nucleus and a large, strongly staining nucleolus; cysts are roughly spherical, ca. 6–30 µm diam. [B. mandrillaris infection: JMM (2001) 50 205–207.] balanced growth See GROWTH. balanced salt solution (BSS) Any of several solutions used (with or without supplement) e.g. in TISSUE CULTURE to provide satisfactory ionic, pH and osmotic conditions for the maintenance and/or growth of cells (see e.g. EARLE’S BSS and HANKS’ BSS). Antibiotic(s) may be added to suppress microbial growth. (See also TRANSPORT MEDIUM; DULBECCO’S PBS; EAGLE’S MEDIUM.) Balanosporida See STELLATOSPOREA. Balansia See CLAVICIPITALES. balantidiasis See DYSENTERY (c). Balantidioides See HETEROTRICHIDA. Balantidium A genus of parasitic ciliate protozoa of the order TRICHOSTOMATIDA. B. coli can cause dysentery (see DYSENTERY (c)) in man, and is the only ciliate known to be pathogenic for man; it normally occurs e.g. as a harmless parasite in the intestinal tract in pigs. The trophozoite is ovoid with a tapering anterior end, ca. 40–70 µm in length, and is covered by spirally arranged longitudinal rows of cilia; the organism exhibits a rotatory movement. At the anterior end of the cell there is a deep invagination (the vestibulum) leading to the cytostome, and a smaller invagination (the cytopyge) occurs at the posterior end. The cell contains a large, kidney-shaped macronucleus (generally visible only in stained preparations) and a small micronucleus situated close to it; contractile vacuoles may be present. Reproduction occurs by binary fission; conjugation also occurs. Encystment occurs in the gut; the spherical cyst (ca. 50 µm diam.) contains a large macronucleus which can be seen in unstained preparations or in preparations stained with a solution of methyl green in dilute acetic acid. Trophozoites can be cultured in media which support the growth of parasitic intestinal amoebae. BALB/c mice An inbred mouse strain predisposed to MYELOMA formation on intraperitoneal injection of e.g. mineral oil. [The BALB/c mouse – genetics and immunology: CTMI (1985) 122 1–253.] Ballerup–Bethesda group (1) Strains of Citrobacter freundii which ferment lactose slowly. (2) Citrobacter freundii. ballistoconidium See BALLISTOSPORE. ballistospore A spore which, at maturity, is forcibly projected from its site of attachment on the sporophore; ballistospores are formed by many basidiomycetes (cf. BASIDIOSPORE), by some imperfect fungi (e.g. Sporobolomyces) and by slime moulds of the PROTOSTELIOMYCETES. (cf. STATISMOSPORE and GASTEROID 86 Bartonella barophiles) die at a temperature-dependent rate. Barophilic bacteria in seas and oceans occur both in the nutrient-poor seawater and in the relatively nutrient-rich guts of invertebrates (e.g. holothurians – sea cucumbers); those occurring in cold regions (<4° C) are heterotrophic bacteria (cf. HYDROTHERMAL VENT). [Reviews: ARM (1984) 38 487–514; MS (1986) 3 205–211.] (See also BAROTOLERANT.) barophilic Having the characteristics of a BAROPHILE. barosensitive See BAROTOLERANT. barotolerant (baroduric) Refers to an organism which can grow at elevated pressures (up to ca. 400–600 atmospheres; 1 atm = 101.325 kPa) but which grows optimally at atmospheric pressure (cf. BAROPHILE); many bacteria (including e.g. Escherichia coli and Pseudomonas aeruginosa) are barotolerant, but some (e.g. Vibrio parahaemolyticus) cannot survive for long even under relatively low pressures (e.g. ca. 200 atm) and are termed barosensitive. The inhibitory effect on barotolerant organisms of pressures greater than ca. 400–600 atm appears to be due, at least partly, to inhibition of certain aspects of PROTEIN SYNTHESIS – specifically, the binding of amino acyl-tRNAs and translocation. In a number of organisms the susceptibility of protein synthesis to pressure has been linked with the 30S ribosomal subunit; interestingly, increased barotolerance occurs in those strains of E. coli which have become resistant to streptomycin owing to a mutation affecting the 30S subunit. barren kinetosome See CILIFEROUS. barrier filter See MICROSCOPY (e). Barritt’s method See VOGES–PROSKAUER TEST. Barrouxia See EIMERIORINA. Bartonella A genus of Gram-negative, oxidase-negative bacteria (family BARTONELLACEAE) within the alpha subgroup of Proteobacteria; the cells are small, typically capnophilic, fastidious rods which can usually be grown aerobically on blood-agar in the presence of 5% carbon dioxide [guidelines for the practical identification of isolates: JCM (1993) 31 2381–2386]. The genus now includes several species which have been imported from the (former) genus Rochalimaea [IJSB (1993) 43 777–786]. B. bacilliformis (the type species) is the causal agent of BARTONELLOSIS in man (the only natural host); the bacterium is a small bacillus or pleomorphic coccobacillus, commonly ca. 1–3 µm in length, which, in culture, bears one or more flagella at one pole and divides by binary fission; it is transmitted by sandflies, and occurs in or on erythrocytes, and in endothelial cells, in infected persons. B. clarridgeiae has been associated with cat scratch disease [JCM (1997) 35 1813–1818]. B. elizabethae (formerly Rochalimaea elizabethae) has been isolated from a case of endocarditis [JCM (1993) 31 872–881]. B. henselae (formerly Rochalimaea henselae) has been associated with various types of disease, including e.g. cutaneous bacillary angiomatosis; this disease (which occurs mainly in the immunocompromised) involves reddish-coloured, typically nodular lesions which may ulcerate. (R. quintana can also give rise to the disease.) B. quintana (formerly Rochalimaea quintana) is the causal agent of TRENCH FEVER; the cells are small rods, and the organism is aerobic and capnophilic. B. vinsonii (formerly Rochalimaea vinsonii ) is parasitic in voles (the ‘Canadian vole agent’); it is not capnophilic. [Diseases caused by Bartonella spp: RMM (1995) 6 155–164.] BASIDIOSPORE.) An asexually-derived ballistospore is sometimes referred to as a ballistoconidium. The ballistospore of a basidiomycete is formed terminally and asymmetrically on a sterigma; its surface is readily wettable. Shortly before discharge, a drop of liquid appears on the surface of the spore in a region near the attachment site (e.g. the HILAR APPENDIX); the drop grows in size (for ca. 5–40 sec) until, suddenly, both spore and drop are projected from the sterigma. [Proposed mechanisms for ballistospore discharge in Itersonilia perplexans: TBMS (1984) 82 13–29.] ballotini Small glass beads, obtainable in a range of sizes, used e.g. for the ballistic disintegration of cells; for this purpose, cells and Ballotini are shaken together in e.g. a BRAUN MSK TISSUE DISINTEGRATOR. Baltimore classification An early system of virus classification, proposed by Baltimore [Bact. Rev. (1971) 35 235–241], based on the nature of the genome and the strategy for viral gene expression (see VIRUS). Six classes (groups) were distinguished. I. dsDNA viruses. II. ssDNA viruses (only positive-sense ssDNA were known at the time). III. dsRNA viruses. IV. Positive-sense ssRNA viruses. V. Negative-sense ssRNA viruses. VI. RNA viruses in which DNA is synthesized (by reverse transcription) from the genome, and mRNA is transcribed from the DNA. (More classes would now be required to accommodate current knowledge of viral strategies.) bamboo mosaic virus See POTEXVIRUSES. BamHI A RESTRICTION ENDONUCLEASE from Bacillus amyloliquefaciens; G/GATCC. banana bunchy top virus See LUTEOVIRUSES. banana leaf spot (sigatoka disease) A disease of the banana plant (Musa) characterized by leaf lesions and imperfect development of the fruit; causal agent: Mycosphaerella musicola. The disease has been controlled by spraying with copper compounds or with mineral oil. band centrifugation See CENTRIFUGATION. Bangia See RHODOPHYTA. Bang’s disease See BRUCELLOSIS. bank (gene bank) Syn. LIBRARY. Banzi virus See FLAVIVIRIDAE. bar A unit of pressure equal to 100 kPa (cf. PASCAL). barban 4-Chlorobut-2-ynyl-3-chlorophenylcarbamate: a compound used e.g. for the control of wild oats; it also acts as a NITRIFICATION INHIBITOR. barbital (barbitone) 5,5 -Diethylbarbituric acid. (Barbituric acid = malonylurea.) barbone Syn. HAEMORRHAGIC SEPTICAEMIA (2). Barbour Stoenner Kelly medium See BORRELIA (B. burgdorferi ). Barbulanympha See HYPERMASTIGIDA. bark-boring beetles See SCOLYTIDAE. bark necrosis See CANKER. barley B-1 virus See POTEXVIRUSES. barley diseases See CEREAL DISEASES. barley stripe mosaic virus See HORDEIVIRUSES. barley yellow dwarf virus See LUTEOVIRUSES. barley yellow mosaic virus See POTYVIRUSES. barley yellow striate mosaic virus See RHABDOVIRIDAE. Barmah Forest virus See ALPHAVIRUS. baroduric Syn. BAROTOLERANT. barophile An organism which grows optimally or obligately under elevated hydrostatic pressure; under atmospheric pressure (101.325 kPa) some barophiles (e.g. Shewanella benthica) continue to grow (less vigorously) but others (obligate 87 Bartonellaceae to each other. (Other combinations (A–A, G–T, A–C, G–G, A–G) can occur under certain circumstances [Nature (1976) 263 285–289] but these are less stable than the classical pairs.) Classical base-pairing is responsible for holding together the strands in dsDNA (see DNA) or in dsRNA (e.g. in certain viruses); it is also involved e.g. in ordering the sequence of nucleotides in DNA REPLICATION and TRANSCRIPTION. base ratio (dissymmetry ratio) Of microbial DNA: the ratio (A + T)/(G + C) in which A, T, G and C represent relative molar amounts of adenine, thymine, guanine and cytosine, respectively. (cf. GC%.) Species with a base ratio >1 may be called AT types; GC types have a base ratio <1. basic dye See DYE. basic fuchsin See FUCHSIN. basic orange Syn. ACRIDINE ORANGE. basic replicon (1) The smallest part of a replicon which encodes all the functions necessary for replication. (2) As (1), above, with the added constraint that replication maintains the wild-type COPY NUMBER [Book ref. 161, p. 202]. Basidiobolus See ENTOMOPHTHORALES. basidiocarp A fruiting body of a basidiomycete. basidiole (1) An immature BASIDIUM prior to the appearance of the sterigmata. (2) An aborted basidium. (3) A sterile cell which is similar in appearance to an immature basidium. basidiolichen A lichen (see LICHENS) in which the mycobiont is a basidiomycete. basidiomycetes Fungi of the BASIDIOMYCOTINA. Basidiomycotina (the ‘basidiomycetes’) A subdivision of fungi (division EUMYCOTA) characterized by the formation of sexually derived spores (BASIDIOSPORES) on basidia (see BASIDIUM). The basidiomycetes are typically terrestrial saprotrophs or parasites; they include e.g. mushrooms, puffballs, stinkhorns, and the (parasitic) rust and smut fungi. A few basidiomycetes are marine organisms (see e.g. NIA). In most species the thallus is a well-developed septate mycelium which often has CLAMP CONNECTIONS (see also DOLIPORE SEPTUM); some basidiomycetes can occur in a yeast-like (unicellular) phase (see FILOBASIDIACEAE) – as can certain fungi which appear to be the anamorphs of basidiomycetes (see e.g. SPOROBOLOMYCETACEAE). In the typical life cycle, the germination of a basidiospore leads to the development of a septate mycelium consisting of uninucleate, haploid cells. At some stage the thallus enters DIKARYOPHASE (see also HYPHA), and eventually karyogamy and MEIOSIS occur in the developing basidium – which typically gives rise to four (haploid) basidiospores. Classes: GASTEROMYCETES, HYMENOMYCETES, UREDINIOMYCETES, USTILAGINOMYCETES. Basidiophora A genus of fungi (order PERONOSPORALES) in which the sporangiophores resemble basidia, and the sporangia are borne on short sterigmata; the organisms include parasites of the ornamental aster and of members of the Compositae. basidiospores Sexually-derived, typically uninucleate and haploid SPORES formed on basidia (see BASIDIUM) by fungi of the BASIDIOMYCOTINA. According to species, basidiospores may be any of various shapes and sizes, and either pigmented or non-pigmented. In many basidiomycetes the basidiospores are BALLISTOSPORES, while in some (e.g. smut fungi) they are STATISMOSPORES. (cf. GASTEROID BASIDIOSPORE.) On germination, a basidiospore commonly gives rise to a germ tube, but in some cases GERMINATION BY REPETITION may occur. Dispersal of basidiospores. In agarics with aequihymeniiferous lamellae (see LAMELLA), the ballistospores are projected more or less horizontally for ca. 0.1–0.2 mm before gravity Bartonellaceae A family of bacteria which are parasitic or pathogenic in or on erythrocytes (red blood cells), and, in some cases, other cells, in various mammals, including man; the organisms can be grown in cell-free media. Genera: BARTONELLA and GRAHAMELLA. bartonellosis (Carrión’s disease) A human disease, occurring in parts of South America, caused by Bartonella bacilliformis; it is transmitted by the bites of sandflies of the genus Phlebotomus. There are two forms of the disease which usually occur in sequence: the first stage (Oroya fever) is characterized by fever and anaemia, the second (verruga peruana) by wart-like, ulcerating skin lesions. basal body (a) (prokaryotic) See FLAGELLUM (a). (b) (eukaryotic) (basal granule; blepharoplast; kinetosome; mastigosome) In some (ciliated or flagellated) cells: an intracellular microtubular structure which resembles a CENTRIOLE and which can serve e.g. as a MICROTUBULE-ORGANIZING CENTRE for the formation of the axoneme of a CILIUM or FLAGELLUM; unlike the prokaryotic basal body, a eukaryotic basal body plays no role in flagellar beating per se: see FLAGELLUM (b). A basal body can develop from a centriole or it can be formed de novo (e.g. during the transition of Naegleria gruberi from the non-flagellated to the flagellated form). In at least some cases a basal body can give rise to a centriole. A basal body differs from a centriole e.g. in that it is generally longer, and in that it has a system of microtubular and fibrillar filaments (rootlets) which arise from the side of the basal body and which may pass from one basal body to another. (See also GRAIN CONVENTION; PITELKA CONVENTION.) basal granule Obsolete term for BASAL BODY (b). basal medium See MEDIUM. basauxic development See CONIDIUM. base analogues Reagents which are analogues of – and which can substitute for – normal bases in DNA; they can act as MUTAGENS since, during subsequent replication of the DNA, the base analogue may pair with the wrong normal base to generate a POINT MUTATION. For example, 2-aminopurine (AP) may be incorporated into DNA in place of adenine; during subsequent replication AP can pair with cytosine which, on further replication, pairs with guanine (an A·T-to-G·C transition). (See also 5-BROMOURACIL.) (Some base analogues in DNA may be recognized by the cell as ‘damage’ and may induce mutation by error-prone repair: see SOS SYSTEM.) (cf. BUDR and DDTTP.) base composition (of DNA) See BASE RATIO and GC%. base excision repair A form of EXCISION REPAIR that follows removal of a base from DNA (see ADAPTIVE RESPONSE and URACIL-DNA GLYCOSYLASE). After removal of the base, the phosphodiester bond on one side of the AP SITE is cleaved by an AP endonuclease. The main AP endonuclease in Escherichia coli is endonuclease III ; this is a multifunctional enzyme whose structure suggests that cleavage of phosphodiester bonds involves a nucleophilic attack on the P–3 O link [Nature (1995) 374 381–386]. Once the phosphodiester bond has been cleaved, a DNA polymerase can replace the damaged nucleotide. As well as the damaged nucleotide, several adjacent nucleotides may also be replaced [NAR (1998) 26 1282–1287]. The repair is completed by a ligase. base pair A pair of bases, each in a separate NUCLEOTIDE, in which each base is hydrogen-bonded to the other. A ‘classical’ (Watson–Crick) base pair always contains one purine and one pyrimidine: adenine pairs specifically with thymine (A–T), guanine with cytosine (G–C), uracil with adenine (U–A); the two bases in a classical base pair are said to be complementary 88 batch culture four basidiospores per basidium; in many smut fungi the basidium may give rise to an indefinite number of basidiospores following mitotic divisions within the metabasidium. [Review: BBMS (1983) 17 82–94.] basionym (basonym) The name of the species whose specific epithet is included in a new combination (see COMB. NOV.). basipetal development Development from the tip towards the base or point of attachment; e.g., in a chain of basipetally developing spores the first-formed spores occupy positions in the terminal or distal parts of the chain, while spores formed later occupy the more proximal positions. (cf. ACROPETAL DEVELOPMENT.) basonym Syn. BASIONYM. basophil A PMN (q.v.) which can respond to certain stimuli e.g. by rapidly secreting vasoactive products; it is primarily a circulatory cell, and is important e.g. in immediate-type hypersensitivity reactions. As in the MAST CELL, with which it shares many characteristics, the basophil surface contains e.g. many high-affinity receptor sites for the Fc portion of IgE antibodies, and its (basophilic) cytoplasmic granules also contain substances such as HISTAMINE and SEROTONIN; stimuli which cause activation and degranulation lead to the secretion of various products and to the formation of e.g. SUPEROXIDE and H2 O2 . (See also JONES–MOTE SENSITIVITY.) batch culture (closed culture) A form of CULTURE (sense 2) in which a given volume of liquid medium is inoculated with cells (e.g. bacteria) capable of growth in that medium, and the inoculated medium is incubated for an appropriate period of time; cells growing under these conditions are exposed to a continually changing environment due e.g. to the gradual consumption of nutrients and the accumulation of metabolic wastes (cf. CONTINUOUS CULTURE and FED BATCH CULTURE). The growth curve (see GROWTH) obtained by monitoring a batch culture commonly exhibits a sequence of four main phases of growth. In the lag phase the growth rate – i.e., the rate of increase in cell numbers (or biomass) – is initially minimal but subsequently rises to a value dictated e.g. by the prevailing conditions (e.g. temperature, concentration of nutrients etc). The length of the lag phase is influenced by the cultural history of the cells in the inoculum. For example, if slowly dividing cells from a nutrient-poor environment are transferred to a nutrientrich medium which can support a higher rate of growth, there is usually a relatively long lag phase during which time the cells become adapted to the new environment; during this period of adaptation the cells exhibit unbalanced GROWTH. Subsequently, growth occurs at a new, higher rate permitted by the higher levels of nutrients. A long lag phase may also occur e.g. if the carbon source in the new medium differs from that previously used by the cells. (cf. DIAUXIE.) When actively dividing cells are transferred to a medium which offers conditions similar to those under which the cells were previously growing, a lag phase is not observed. At the end of the lag phase the cells enter the exponential (= logarithmic or log) phase of growth in which, for a given organism, the growth rate is both constant and maximal for the particular growth conditions. In this phase there is an exponential increase in cell numbers (and biomass); this type of growth is referred to as balanced GROWTH. (See also TROPHOPHASE.) In the stationary phase the growth rate declines and eventually reaches zero. (See also IDIOPHASE.) In the death phase the number of viable cells in the culture (maximal in the stationary phase) declines. initiates a vertical fall; the spores on a given basidium are commonly discharged in rapid succession (within seconds or minutes of one another). In these agarics the shape of the lamella facilitates spore dispersal – spores from the more proximal parts of the lamella being able to fall vertically even when the lamellae are orientated several degrees from the vertical. In e.g. Coprinus spp the inaequihymeniiferous lamellae occur close together on the pileus, and the shape of the lamellae does not facilitate dispersal of the ballistospores. However, in some species cystidia (see CYSTIDIUM) may serve to hold apart adjacent hymenial surfaces. In other species the process of autodigestion is an aid to spore dispersal; autodigestion involves the progressive liquefaction of the lamellae, beginning at the free edge of the lamella and progressing towards the pileus as fresh crops of basidia become mature. Thus, spores discharged by a given basidium need fall for only a short distance between adjacent lamellae before becoming free of the fruiting body. See also e.g. CYATHUS, LYCOPERDALES, SPHAEROBOLUS. basidium (pl. basidia) A microscopic, unicellular or multicellular structure which gives rise to the sexually-derived spores (BASIDIOSPORES) in fungi of the BASIDIOMYCOTINA; basidiospores are formed externally, i.e., as extrusions, on the basidium (but see TETRAGONIOMYCES). In some basidiomycetes (e.g. HYMENOMYCETES) the basidia are formed in a layer on specialized fertile tissue (see HYMENIUM), while in others (e.g. UREDINIOMYCETES) they are formed individually from germinating spores. In many species the basidium is a simple, cylindrical or clavate structure which gives rise to (usually) four apically and symmetrically situated basidiospores, each basidiospore commonly being borne on a small, narrow protrusion (STERIGMA). In other species the basidium may be apically bifurcate, each fork (sterigma) bearing a single terminal basidiospore (as in members of the DACRYMYCETALES), or it may be divided, apically, into four substantial finger-like protrusions (sterigmata), each bearing a terminal basidiospore (as in members of the TREMELLALES). A basidium which is not divided, internally, by septa is called a holobasidium; such basidia are formed by members of the HOLOBASIDIOMYCETIDAE. A basidium which is divided, internally, by transverse or longitudinal septa (as e.g. in Auricularia and Tremella, respectively) is called a phragmobasidium; such basidia are formed by members of the PHRAGMOBASIDIOMYCETIDAE and of the UREDINIOMYCETES and USTILAGINOMYCETES. Basidium formation. A simple holobasidium typically develops from a terminal dikaryotic hyphal cell within a hymenium; the boundary between this cell and the parent hypha is often marked by a CLAMP CONNECTION. Within the developing basidium, karyogamy and MEIOSIS commonly give rise to four haploid nuclei. (See also METABASIDIUM and PROBASIDIUM.) Four small protuberances (sterigmata) then form at the apical (free) end of the developing basidium, and the terminal part of each sterigma swells to form a basidiospore initial; one nucleus migrates into each initial, and the basidiospores are subsequently delimited. In some species mitotic division occurs within the developing basidiospores; in such cases either binucleate basidiospores are formed, or uninucleate basidiospores are formed after one daughter nucleus from each pair has returned to the basidium. In rust and smut fungi, the basidium typically develops from a short germ tube which arises from the teliospore. In some cases a diploid nucleus passes from the teliospore into the germ tube and undergoes meiosis; the structure which develops (the metabasidium, or ‘promycelium’) is septate in many species and (in rusts, and in many smut fungi) it gives rise to basidiospores both apically and laterally. Rust fungi characteristically form 89 batch retort batch retort In CANNING: a vessel within which filled, sealed cans (or other containers) undergo heat treatment in a batchtype process (cf. COOKER–COOLER). The cans are subjected to saturated steam under pressure or to an air–steam mixture, or (in an overpressure retort) are submerged in heated water under an air pressure of up to ca. 250 kPa. In some batch retorts (e.g. the Konservomat) the load is agitated to facilitate heat penetration. bating (of hides) See PROTEASES. Batrachospermum See RHODOPHYTA. Battarrea See GASTEROMYCETES (Tulostomatales). Battey bacillus Strain(s) of Mycobacterium intracellulare or, loosely, strain(s) of related species (including M. avium). Bayer’s junction (Bayer’s patch) See ADHESION SITE. Bayleton See TRIADIMEFON. BB-transhydrogenase See TRANSHYDROGENASE. BCDF (immunol.) See LYMPHOKINES. BCF Bioconcentration factor: a measure of the degree to which a compound (commonly a xenobiotic), present in an aquatic environment, is accumulated in the biomass of organisms (e.g. algae) living in that environment. BCF = Co /Cw where Co = concentration of the compound in the organisms, and Cw is the concentration of the compound in the water. (See e.g. TBTO.) BCG Bacille (bacillus) Calmette–Guérin: an attenuated strain of Mycobacterium bovis used e.g. as a vaccine against TUBERCULOSIS. The vaccine often gives protection against e.g. tubercular meningitis in childhood – but in a number of studies it has failed to protect against pulmonary tuberculosis [Lancet (1990) 335 1016–1020]; moreover, it may produce disseminated infections in the immunocompromised (e.g. AIDS patients). In some instances, BCG has provided some protection against LEPROSY. [Mycolic acids in BCG substrains: JGM (1984) 130 2733–2736.] (See also ESAT-6.) BCGF (immunol.) See LYMPHOKINES. Bchl Bacteriochlorophyll: see CHLOROPHYLLS. BclI A RESTRICTION ENDONUCLEASE from Bacillus caldolyticus; T/GATCA. BCNU See N-NITROSO COMPOUNDS. Bd (buoyant density) See CENTRIFUGATION and GC%. BD-type starter See LACTIC ACID STARTERS. bdellocyst See BDELLOVIBRIO. bdelloplast See BDELLOVIBRIO. Bdellospora See ZOOPAGALES. Bdellovibrio A genus of aerobic Gram-negative bacteria which are characteristically predatory on other bacteria (cf. VAMPIROVIBRIO); they occur worldwide in soil and sewage, and in freshwater and marine habitats. On primary isolation all strains are obligately parasitic on certain Gram-negative bacteria, including e.g. Aquaspirillum serpens, Escherichia coli, Pseudomonas spp, and Rhodospirillum rubrum – the range of prey species varying with the strain of Bdellovibrio; prey-independent, cultivable strains can develop from strictly predatory strains, and some strains are facultatively predatory. The predatory strains exhibit a biphasic life cycle: the predatory phase (in which the non-growing cells search for prey) alternating with a phase of growth and reproduction in the periplasmic space of a host cell. Predatory cells of Bdellovibrio are vibrioid, 0.2–0.5 × 0.5–1.4 µm, each having a single polar sheathed flagellum (the sheath being continuous with the outer membrane); cells of prey-independent strains may be larger and non-motile, and may form a yellowish pigment. All strains are chemoorganotrophs. Growth occurs optimally at 28–30° C, and metabolism is respiratory (oxidative). Three species are recognized [Book ref. 22, pp. 118–124]: B. bacteriovorus, B. starrii and B. stolpii ; the genus is divided by phage typing into six groups: groups I–IV consist of strains of B. bacteriovorus, group V contains B. stolpii strains, and group VI consists of strains of B. starrii (for which no phages are currently known). GC%: ca. 33–51. Type species: B. bacteriovorus. Predation. A predatory Bdellovibrio cell initially attaches, via a ‘holdfast’ region at its non-flagellar end, to a prey cell. (‘Bdello’ derives from the Greek for ‘leech’.) The predatory cell secretes enzymes (e.g. lysozyme) and enters the periplasmic space of the prey cell via the pore produced by enzymic action. (Bdellovibrio can penetrate cells of E. coli even when they have a thick capsule [Microbiology (1997) 143 749–754].) The prey cell quickly dies and subsequently rounds up to become a bdelloplast; during this process the peptidoglycan sacculus of the prey cell becomes insensitive to lysozyme, thus inhibiting predation by other Bdellovibrio cells. Within the periplasmic space of the bdelloplast, the predatory cell grows and develops into a long, spiral, non-motile filament; the filament subsequently fragments to form motile predatory cells which, on release, can initiate a new cycle. Bdellovibrio strain W can form a resting cell (bdellocyst) within the bdelloplast; a bdellocyst has a thickened peptidoglycan wall and has increased resistance to elevated temperatures and to desiccation. Isolation. Bdellovibrio may be isolated from soil infusions etc. e.g. by membrane filtration (membrane pore size ca. 1.2 µm) – the filtrate being inoculated onto a lawn plate prepared from a sensitive strain of bacteria, or onto a soft agar which incorporates suitable prey cells; on incubation, viable cells of Bdellovibrio give rise to macroscopic plaques. bDNA assay (branched DNA assay) A procedure, based on signal amplification, which is used for quantifying a specific single-stranded DNA or RNA target sequence in a sample; essentially, signal amplification involves (i) the use of a targetspecific PROBE for immobilizing the target nucleic acid on a solid support, and (ii) the use of other types of probe for labelling each bound target sequence with an enzyme system that generates (from a suitable substrate) an amplified chemiluminescent signal – the intensity of which is related to the number of target molecules in the sample. Signal amplification avoids an important problem associated with target amplification methods (such as PCR): the problem of potential contamination of apparatus and environment with copies of the (highly amplified) target nucleic acid. It has the disadvantage that, by failing to provide copies of the target, further investigation of the target molecule (e.g. by sequencing) is precluded. bDNA-based commercial kits (Quantiplex ; Bayer Diagnostics, Leverkusen, Germany) have been used e.g. for quantifying hepatitis viruses B and C and the human immunodeficiency virus (HIV). In a given assay, a divalent probe is used initially to bind target molecules to the surface of a microtitre plate. Another divalent probe is then used to link each bound target molecule to a synthetic multi-branched DNA molecule (bDNA; the ‘amplifier’); subsequently, excess enzyme-labelled probes are allowed to bind to (multiple) probe-binding sites on the bDNA. After washing, addition of the enzyme’s substrate (a dioxetane) generates the chemiluminescent signal. A bDNA assay may be used for detection, as well as quantification, of a given sequence; for example, this method has been used for detecting the mecA gene in blood culture bottles [JCM (1999) 37 4192–4193]. BDProbe Tec ET See SDA. beak (mycol.) The tubular neck of a perithecium or pycnidium. 90 benign foot-rot beet cryptic viruses See CRYPTIC VIRUSES. beet curly top virus See GEMINIVIRUSES. beet leaf curl virus See RHABDOVIRIDAE. beet mild yellowing virus See LUTEOVIRUSES. beet mosaic virus See POTYVIRUSES. beet necrotic yellow vein virus (BNYVV) A virus which can cause severe disease (‘rhizomania’) in sugar beet (Beta vulgaris); it is transmitted by the ‘fungus’ Polymyxa betae – which is also responsible for the persistence of the virus in the soil. BNYVV may belong to the ‘furovirus’ group – see SOIL-BORNE WHEAT MOSAIC VIRUS. (cf. TOBAMOVIRUSES.) [BNYVV RNA: JGV (1985) 66 345–350.] beet western yellows virus See LUTEOVIRUSES. beet yellow net virus See LUTEOVIRUSES. beet yellow stunt virus See CLOSTEROVIRUSES. beet yellows virus group Syn. CLOSTEROVIRUSES. Beggiatoa A genus of GLIDING BACTERIA (see CYTOPHAGALES); species are widespread e.g. in freshwater, estuarine and marine waters and sediments, in the rice rhizosphere, and in activated sludge. The organisms are non-sheathed, non-pigmented, multicellular filaments (trichomes) which form necridia and hormogonia; according to species, the trichomes are 1 to >50 µm in diameter. The organisms appear to be chemoorganotrophic heterotrophs, although at least some strains may be able to grow mixotrophically using sulphide as an electron donor; refractile ‘intracellular’ granules of sulphur (located between cytoplasmic membrane and cell wall) are formed in the presence of sulphide. Metabolism is respiratory, and a complete TCA cycle is present. Most strains can use acetate and/or lactate as the sole source of carbon; none can use hexoses. The main storage product is poly-b-hydroxybutyrate; volutin is also formed. [Review: ARM (1983) 37 341–354.] Beggiatoaceae See CYTOPHAGALES. Beijerinckia A genus (incertae sedis) of Gram-negative, aerobic, catalase-positive, chemoorganotrophic, asporogenous bacteria which occur e.g. in the phyllosphere and in soils (particularly in the tropics). Cells: typically round-ended rods, ca. 0.5–1.5 × 1.7–4.5 µm, peritrichously flagellated or non-motile; division involves constriction. Each cell characteristically contains two PHB-containing polar bodies. In some species each cell may be encysted, or several cells may occur within a single capsule. Metabolism is respiratory (oxidative), with O2 as terminal electron acceptor. NITROGEN FIXATION is carried out under both atmospheric and microaerobic conditions; in many strains N2 is used in preference to NO3 − . All strains can utilize glucose, fructose and sucrose as carbon sources; peptone does not support growth. Slime (sometimes elastic) is commonly formed, particularly on agar under N2 -fixing conditions. Colonies are often some shade of pink, or orange to pale brown. No pellicle is formed on liquid media. Optimum growth temperature: 20–30° C; no growth at 37° C. Growth can occur within the pH range ca. 3–10. GC%: ca. 55–61. Type species: B. indica; other species: B. derxii, B. fluminensis and B. mobilis. [Book ref. 22, pp. 311–321.] bell morels The fruiting bodies of VERPA spp. belladonna mottle virus See TYMOVIRUSES. benalaxyl See PHENYLAMIDE ANTIFUNGAL AGENTS. Beneckea An obsolete bacterial genus, members of which are now included in the genus VIBRIO. benign (med.) (1) (of disease) Mild; self-limiting; not recurrent. (2) (of tumours) Not MALIGNANT (sense 2). benign enzootic paresis Syn. TALFAN DISEASE. benign foot-rot (of sheep) See SCALD. bean common mosaic virus See POTYVIRUSES. bean golden mosaic virus (BGMV) A member of the GEMINIVIRUSES. BGMV can cause chlorosis in beans (Phaseolus vulgaris); it is transmitted by the whitefly Bemisia tabaci, and occurs in tropical America. bean paste Syn. MISO. bean summer death virus See GEMINIVIRUSES. bean yellow mosaic virus See POTYVIRUSES. beard-lichens (beard-moss) Lichens of the genera ALECTORIA, BRYORIA, and/or USNEA. Beauveria A genus of fungi of the class HYPHOMYCETES. B. bassiana forms a septate mycelium; spheroidal conidia are borne on the sympodially branched, narrow, apical region of each flask-shaped conidiophore. B. bassiana causes the SILKWORM DISEASE muscardine, and has been used (e.g. in the former USSR) for the BIOLOGICAL CONTROL of certain crop pests – e.g. Colorado beetle (Leptinotarsa decemlineata); for biological control the fungus has been used e.g. in the form of a powder, Boverin: a dried mixture of conidia and kaolin. B. bassiana forms a depsipeptide toxin, beauvericin (see ENNIATINS). beauvericin See ENNIATINS. Bebaru virus See ALPHAVIRUS. becampicillin See PENICILLINS. Bedsonia See CHLAMYDIA. bee acute paralysis virus See PICORNAVIRIDAE. bee diseases ‘Domestic’ hive bees – Apis mellifera (the European honey-bee) and A. cerana (the Eastern honey-bee) – are subject to a range of diseases of microbial aetiology. In general, larvae are affected more often than adults. Spread of disease is often kept under control by the activity of worker bees which remove diseased and dead larvae from the hive. Bacterial diseases include e.g. AMERICAN FOULBROOD and EUROPEAN FOULBROOD (see also SPIROPLASMA). Fungal diseases include CHALK-BROOD and STONE-BROOD. Protozoal parasites and pathogens of bees include e.g. Nosema apis, which develops in the midgut epithelial cells in adult honey-bees (infected bees may show no obvious symptoms, although they have a shortened lifespan), and Malpighamoeba mellificae (Vahlkampfia mellificae), which infects the lumen of the Malpighian tubules in adult bees [SEM of developmental stages: JP (1985) 32 139–144]. Virus diseases include several caused by small unclassified RNA viruses (see PICORNAVIRIDAE) and some caused by iridescent and filamentous viruses. [Book ref. 66.] (See also INSECT DISEASES.) bee slow paralysis virus See PICORNAVIRIDAE. bee virus X See PICORNAVIRIDAE. beech bark disease A European and North American disease of beech trees (Fagus spp) caused by Nectria coccinea. Infection commonly follows infestation of the tree by beech scale insects (Cryptococcus fagisuga). Symptoms: phloem and cambium cells in the bark die in patches, allowing leakage and blackening of sap, and there may be yellowing of the leaves and die-back of the crown; infected trees may die. Beechey ground squirrel hepatitis virus Syn. GROUND SQUIRREL HEPATITIS VIRUS. beef-steak fungus Fistulina hepatica (see APHYLLOPHORALES). beer-making See BREWING. beer spoilage Off-flavours in beer may be caused by contaminating organisms – e.g. Acetobacter spp can produce acetic acid and cause ROPINESS; heterofermentative lactobacilli (which preferentially ferment maltose) can cause off-flavours (e.g. diacetyl), turbidity and/or ropiness. Sarcina sickness results from the production of diacetyl by Pediococcus cerevisiae. Other spoilage organisms include Candida and Pichia spp. 91 benign tertian malaria 2-benzyl-4-chlorophenol See PHENOLS. benzylpenicillin (penicillin G) One of the first of the natural PENICILLINS to be produced (from e.g. P. chrysogenum); R = C6 H5 .CH2 .CO (see b-LACTAM ANTIBIOTICS). It is active against many Gram-positive bacteria which do not produce bLACTAMASES, but is poorly active against Gram-negative species. It is administered parenterally. Berenil A trypanocidal DIAMIDINE which binds to AT-rich regions of dsDNA (apparently in the same way as does NETROPSIN). beriberi (acute cardiac) See CITREOVIRIDIN. Berkefeld candle See FILTRATION. Berne virus An enveloped RNA virus (proposed family: TOROVIRIDAE) isolated from a horse. The virion is pleomorphic (ca. 120–140 nm diam.) and consists of an elongated, curved nucleocapsid enclosed by a peplomer-bearing envelope. The genome is a single molecule of positive-sense ssRNA (MWt ca. 6 × 106 ); polyadenylated subgenomic mRNAs are apparently synthesized in infected cells. Maturation apparently involves the budding of preformed tubular nucleocapsids into Golgi vesicles and intracytoplasmic cisternae; the nucleocapsid appears to undergo a morphological change during budding [JGV (1986) 67 1305–1314]. Berry–Dedrick phenomenon See NON-GENETIC REACTIVATION. Besnoitia A genus of coccidian protozoa (suborder EIMERIORINA); members are classified by some authors [AP (1982) 20 403–406] in the genus ISOSPORA. The organisms are parasitic in a range of animals – e.g. cattle, horses, sheep, cats, rodents; transmission may occur e.g. via blood-sucking flies or by ingestion of infected tissues. Disporic, tetrazoic oocysts are formed. b (duplex winding number) See DNA. b-barrel pore See PROTEIN SECRETION (type IV systems). b-exotoxin Syn. THURINGIENSIN. beta interferon See INTERFERONS. beta-lactam antibiotics See b-LACTAM ANTIBIOTICS. beta-lactamases See b-LACTAMASES. b operon See RIBOSOME (biogenesis). beta-rays See IONIZING RADIATION. Betabacterium See LACTOBACILLUS. betacin A BACTERIOCIN-like, heat-resistant, proteinaceous substance produced by bacteriophage SPb-containing (or bacteriophage Z-containing) lysogens of Bacillus subtilis when growing on agar (but apparently not in broth cultures). Betacin inhibits the growth of non-lysogens but not that of lysogens containing SPb (or the related phage Z). [Book ref. 170, p. 280.] (cf. BACTERIOPHAGE fBA1.) Betacoccus Obsolete name for LEUCONOSTOC. Betadine See IODINE (a). Betaferon See INTERFERONS. Betaherpesvirinae (‘cytomegalovirus group’) A subfamily of viruses of the HERPESVIRIDAE (q.v.). The Betaherpesvirinae formerly consisted of only (i) the human cytomegalovirus group – type species human (beta) herpesvirus 5 (= human herpesvirus 5, HHV5; human cytomegalovirus, HCMV) and (ii) murine cytomegalovirus group – type species murid (beta) herpesvirus 1 (= murid herpesvirus 1; mouse cytomegalovirus, mouse CMV) [Book ref. 23, pp 49–50]; possible members included suid herpesvirus 2 (pig CMV, causal agent of INCLUSION BODY RHINITIS), murid herpesvirus 2 (rat CMV) and caviid herpesvirus 1 (guinea-pig CMV). Later, certain bovine herpesviruses were classified as ‘bovine cytomegaloviruses’ [JGV (1984) 65 697–706] (see also MALIGNANT CATARRHAL FEVER). Characteristically, the replication cycle of these viruses is appreciably longer than 24 hours (in contrast to the cycles benign tertian malaria See MALARIA. Benlate See BENOMYL. benomyl (methyl-(1-n-butylcarbamoyl)-benzimidazole-2-carbamate; trade name: e.g. Benlate) An agricultural ANTIFUNGAL AGENT (see BENZIMIDAZOLES (a)). Benomyl is a systemic antifungal agent which has both eradicant and protectant properties against a wide range of plant diseases (e.g. many powdery mildews, apple scab, botrytis, eyespot of wheat and barley, black spot of roses); when used as a seed dressing it protects against seed-borne diseases such as smuts and bunts of cereals, and when mixed with a dithiocarbamate such as maneb or mancozeb it gives some control against glume blotch in wheat and against some cereal rusts. It is also used to prevent storage rots of fruit and vegetables (see e.g. GANGRENE sense 2). Benomyl owes its activity to its degradation, in solution, to the fungitoxic substances MBC (see BENZIMIDAZOLES) and butyl isocyanate; it also acts as a cutinase inhibitor, preventing penetration of the plant cuticle by the pathogen (see CUTIN) [Book ref. 58, pp. 94–95]. Benomyl-resistant strains of many fungal pathogens have emerged, reducing the usefulness of the fungicide in agriculture. [Bacterial degradation of benomyl: AvL (1978) 44 283–292, 293–309.] benquinox (1,2-benzoquinone-N  -benzoylhydrazone-4-oxime) A QUINONE ANTIFUNGAL AGENT which is used mainly as a seed dressing. Benson–Calvin–Bassham cycle Syn. CALVIN CYCLE. benthic Refers to the mud, sand etc, and/or to the indigenous organisms, at the bottom of a lake, sea or other body of water. (cf. PELAGIC; see also LITTORAL.) benzalkonium chloride See QUATERNARY AMMONIUM COMPOUNDS and e.g. KLEINSCHMIDT MONOLAYER TECHNIQUE. benzene Aerobic degradation: see HYDROCARBONS. Degradation in an anoxic environment: see BIOREMEDIATION. benzethonium chloride See entry QUATERNARY AMMONIUM COMPOUNDS. benzidine test A test used to detect bacterial CYTOCHROMES. Colonies of the test strain are flooded with a reagent consisting of benzidine dihydrochloride dissolved in a mixture of acetic acid and ethanol; on addition of an equal volume of H2 O2 (5%), cells which contain cytochromes develop a green or blue–green coloration. (cf. OXIDASE TEST.) benzimidazoles (a) (as antifungal agents) An important group of agricultural systemic ANTIFUNGAL AGENTS; they are often called ‘MBC fungicides’ because many of them decompose in aqueous solution (or in the plant) to form methyl benzimidazol-2-yl carbamate (MBC, CARBENDAZIM) – apparently the primary fungitoxic agent. MBC appears to act primarily in the nucleus where it disrupts the formation of spindle microtubules, inhibiting or disrupting mitosis. Single-step resistance to benzimidazoles tends to emerge readily. The group includes BENOMYL, CARBENDAZIM, FUBERIDAZOLE, THIABENDAZOLE and THIOPHANATE-METHYL. (b) (as antiviral agents) See e.g. ENVIROXIME and HBB. benzoic acid (C6 H5 COOH) An antibacterial and antifungal ORGANIC ACID used (as the free acid or as benzoate) as a PRESERVATIVE (e.g. in fruit juices, cordials, sauces, acidic foods, pharmaceuticals) and, with other agents, for the topical treatment of certain fungal infections; it is maximally effective below ca. pH 5, activity decreasing sharply with increase in pH above this value. Benzoic acid can be inactivated by certain colloids (e.g. kaolin). (cf. PARABENZOATES.) benzoquinones See QUINONES. benzoyl peroxide See ACNE. benzyl alcohol See ALCOHOLS. 92 BIAcore of other herpesviruses). In culture they typically give rise to enlarged, rounded cells (cytomegalic cells) and slowly spreading foci of cell lysis. DNA-containing inclusion bodies may be seen in the nuclei and sometimes in the cytoplasm of infected cells. Typically, the host range for a given virus is narrow and is often restricted to a single species. Latent infection can be established in cell cultures and in vivo. HCMV (often truncated to CMV) is a large herpesvirus with a linear dsDNA genome of MWt ca. 1.5 × 108 containing terminal and internal repeated sequences; the genome encodes a number of structural and regulatory proteins. Each virion consists of the DNA–protein core, a capsid, and an external lipoprotein envelope bearing surface projections, the region between envelope and capsid (the ‘tegument’) containing several virus-encoded proteins. In the replication cycle of HCMV, the virion initially fuses with the cell membrane of the target cell, and the (unenveloped) virus migrates towards the nucleus. Viral DNA is replicated in the nucleus, and virus-encoded proteins are synthesized in the cytoplasm; the nucleocapsid is assembled in the nucleus. HCMV genes are transcribed in strict temporal sequence. The ‘immediate–early’ genes (expressed within 2–4 hours of infection) encode regulatory proteins that are needed for synthesis of ‘early’ and ‘late’ genes. The ‘early’ gene products include a DNA polymerase. ‘Late’ gene products include capsid proteins. Culture of human CMV is carried out in human fibroblasts. (See also SHELL VIAL ASSAY.) In vivo, replication appears to occur primarily in epithelial cells (e.g. in salivary glands, kidneys); HCMV is also known to infect peripheral blood leukocytes and haematopoietic progenitor cells. Infection with HCMV is endemic in human populations world-wide, but in most cases it is asymptomatic; primary infection may be followed by viral persistence which may take the form of low-level replication or latent infection. (The site of latency is currently unknown.) However, the virus is e.g. a common cause of congenital viral infections, and sometimes causes a disease resembling INFECTIOUS MONONUCLEOSIS. HCMV-mediated disease appears to occur most commonly among the immunocompromised (e.g. immunosuppressed transplant patients and AIDS patients) and in those with an immature immune system (e.g. neonates). Typically, HCMV-mediated disease is associated with conditions such as fever, hepatitis and leukopenia. (See also CYTOMEGALIC INCLUSION DISEASE.) [Epidemiology and transmission of HCMV: JID (1985) 152 243–248. Molecular biology and immunology of HCMV (review): Bioch. J. (1987) 241 313–324. Occupational risk of HCMV: RMM (1994) 5 33–38. HCMV in haematological disease: BCH (1995) 8 149–163.] The laboratory diagnosis of HCMV can be achieved in <24 hours e.g. by SHELL VIAL ASSAY or by serological or nucleic acidbased techniques [diagnosis of HCMV by detection of antigen and DNA: RMM (1994) 5 265–276]. A NASBA-based procedure has been used for detecting the transcript of a late HCMV gene in order to monitor transplant patients at risk from HCMVrelated disease [JCM (1998) 36 1341–1346]. In recent years, two more herpesviruses have been added to the Betaherpesvirinae: human (beta) herpesvirus 6 (human herpesvirus 6, HHV6) and human (beta) herpesvirus 7 (human herpesvirus 7, HHV7). HHV6 has been identified as a causal agent of EXANTHEM SUBITUM, and is able to cause severe morbidity in immunocompromised patients; it is also a possible causal agent of e.g. lymphoma and leukaemia. HHV6 can infect different types of cell – although replication may not occur in all of them; for example, latent or poorly replicative viruses have been detected in macrophages and peripheral blood mononuclear cells. HHV6 has a tropism for lymphoid cells; in vitro it replicates well in CD4+ cells. Saliva appears to be a good source of the virus in infected persons. Interestingly, the HHV6 genome includes two genes with homology to chemokine genes, and a gene resembling that of a chemokine receptor. (Certain other herpesviruses, including HCMV, encode soluble chemokine receptors.) In cultures of peripheral blood mononuclear cells, HHV6 induces the cytokines IL-1b and TNF-a. On the basis of (i) restriction patterns (see RFLP), (ii) binding of specific monoclonal antibodies, and (iii) cultural properties, HHV6 has been divided into two variant forms (A and B). Variant A appears to be observed more frequently in HIVinfected patients. [HHV6 (review): BCH (1995) 8 201–223; Lancet (1997) 349 558–563.] HHV7 (as well as HHV6) is widespread in the human population; unlike HHV6 it has not been firmly linked to human disease. HHV7 replicates well in CD4+ cells, and may (unlike HHV6) use the CD4 antigen as a receptor. In culture, HHV7 replicates more slowly than HHV6. [HHV7 (review): Lancet (1997) 349 558–563.] Another recently isolated herpesvirus, HHV8, has been placed in the GAMMAHERPESVIRINAE. betaine N-Trimethylglycine: (CH3 )3 N+ CH2 COO− . (See also OSMOREGULATION and STICKLAND REACTION.) Betaseron See INTERFERONS. Bethell process A method of TIMBER PRESERVATION in which the timber is first subjected to a vacuum (to withdraw air) and then exposed to a PRESERVATIVE under pressure. (cf. BOUCHERIE PROCESS; SAUG–KAPPE PROCESS.) Bethesda–Ballerup group Syn. BALLERUP–BETHESDA GROUP. Bettsia A genus of fungi of the ASCOSPHAERALES. B. alvei (‘pollen mould’) apparently occurs only in stored pollen in the combs of bee-hives. BF0 F1 ATPase A bacterial F0 F1 -type PROTON ATPASE. BFNU See N-NITROSO COMPOUNDS. BFP See PATHOGENICITY ISLAND. BFPR (BFP) BIOLOGICAL FALSE POSITIVE REACTION. BGAV Blue-green algal virus (CYANOPHAGE). BGG Bovine g-globulin. bgl operon (in Escherichia coli) See CATABOLITE REPRESSION. BglF (in Escherichia coli) See CATABOLITE REPRESSION. BglG (in Escherichia coli) See CATABOLITE REPRESSION. BglI A RESTRICTION ENDONUCLEASE from Bacillus globigii; the recognition sequence is: GCCNNNN/NGGC. BglII A RESTRICTION ENDONUCLEASE from Bacillus globigii ; A/GATCT. BGMV BEAN GOLDEN MOSAIC VIRUS. BHIA Brain–heart infusion agar. BHIB Brain–heart infusion broth. BHK-21 An ESTABLISHED CELL LINE derived from hamster kidney; the cells are heteroploid and fibroblast-like. BIAcore An analytical system in which surface plasmon resonance (SPR) is used to study the kinetics, affinities and binding positions in antigen–antibody and other molecular interactions. Specific molecules, in solution, are allowed to bind to an immobilized ligand at a hydrophilic sensor surface; binding affects the refractive index in the vicinity of the sensor surface. The refractive index (and, hence, molecular interaction at the surface) can 93 biased random walk hepatotoxic and may promote liver disease associated with biliary stasis. Bile acid salts and amides inhibit many non-enteric bacteria and are useful (e.g. in MACCONKEY’S AGAR) for selecting enteric species. Bile salts promote multidrug-efflux in Campylobacter jejuni [JB (2005) 187 7417–7424]. bile–aesculin agar See AESCULIN. bile solubility The ‘solubility’ (i.e., tendency to lyse) shown by most (freshly isolated) strains of Streptococcus pneumoniae when exposed (under alkaline conditions) to bile, sodium deoxycholate, sodium taurocholate, or e.g. lauryl sulphate; other similar streptococci do not lyse under these conditions. Lysis appears to result from the activation/stimulation by the bile of an AUTOLYSIN, N-acetylmuramyl-L-alanine amidase, which disrupts the cell wall peptidoglycan. Heat-killed pneumococci, and pneumococci in which choline has been replaced by ethanolamine in the wall teichoic acid (by culture in a defined medium), do not lyse with bile. Bile solubility test (one method): 2 drops of 10% sodium deoxycholate solution are added to ca. 1 ml of a 24-hour broth culture or saline suspension of the test organism, the pH is adjusted to 7.4–7.6, and incubation is carried out (10–15 min /37° C). If the organism is soluble in bile, the turbid suspension becomes clear (positive test). [Methods: Book ref. 53, pp. 1437–1438.] bilin See PHYCOBILIPROTEINS. biliproteins Syn. PHYCOBILIPROTEINS. binapacryl See DINITROPHENOLS. binary fission FISSION (q.v.) in which two cells – usually of similar size and shape – are formed by the growth and division of one cell. (cf. MULTIPLE FISSION.) In binary fission cytoplasmic division is preceded by DNA REPLICATION in prokaryotes, and e.g. MITOSIS in eukaryotes. (See also CELL CYCLE.) If a cell divides across its longitudinal axis the process is termed transverse binary fission; if division occurs along the longitudinal axis it is termed longitudinal binary fission. Binary fission is a common mode of asexual reproduction in many types of microorganism. In bacteria, binary fission is typically of the transverse type, and it usually results in the formation of two morphologically and functionally similar daughter cells; asymmetrical binary fission occurs e.g. in Caulobacter. Following binary fission the daughter cells may separate; alternatively, the progeny cells of sequential binary fissions may remain together to form chains, PACKETS, PALISADES etc. (See also BUDDING; GROWTH (a); SEPTUM (a); TERNARY FISSION.) In protozoa, binary fission in ciliates is typically HOMOTHETOGENIC, and in flagellates it is typically SYMMETROGENIC. (See also STOMATOGENESIS.) In yeasts, binary fission occurs e.g. in Schizosaccharomyces spp; in many yeasts (including e.g. Saccharomyces) cell division involves BUDDING (cf. BUD-FISSION). binding hyphae See HYPHA. binding protein (1) (periplasmic binding protein) In e.g. bacteria of the Enterobacteriaceae: a soluble protein, located in the periplasmic region, which binds to a specific substrate and which is involved in a BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM for that substrate; periplasmic binding proteins can also function as chemoreceptors for CHEMOTAXIS. (2) Any protein which binds, more or less specifically, to other molecules: see e.g. DNA BINDING PROTEIN; SINGLE-STRAND BINDING PROTEIN; PENICILLIN-BINDING PROTEINS. binding protein-dependent transport system (ABC importer; periplasmic transport system; periplasmic permease; osmotic be monitored by a beam of polarized light incident on the sensor surface. [Example of use: NAR (2000) 28 1935–1940.] biased random walk See RANDOM WALK. Bicine A zwitterionic pH buffer: N,N-bis(2-hydroxyethyl)glycine. Unlike HEPES, Bicine binds divalent cations. At 25° C the pKa of Bicine is ca. 8.3. Biddulphia See DIATOMS. bifactorial heterothallism See HETEROTHALLISM. bifermentolysin See THIOL-ACTIVATED CYTOLYSINS. bifid (1) (adj.) Forked. (2) (noun) A bacterium forked at one or both ends. Bifidobacterium A genus of Gram-positive, asporogenous, anaerobic bacteria which occur e.g. among the GASTROINTESTINAL TRACT FLORA and the VAGINA MICROFLORA, and which carry out a HETEROLACTIC FERMENTATION in which the main products of e.g. glucose fermentation are typically acetic and lactic acids in the ratio 3:2; the key enzyme in the pathway, fructose 6phosphate phosphoketolase [see Appendix III(b)], is a diagnostic feature of the genus. Cells: non-motile (often pleomorphic) rods and bifids. GC%: ca. 57–66. Type species: B. bifidum (formerly e.g. Lactobacillus bifidus); other species include e.g. B. adolescentis, B. asteroides (from bees), B. breve, B. infantis, B. suis, B. thermophilum. (See also INDICATOR ORGANISM.) [Isolation and media: Book ref. 46, pp. 1951–1961.] bifidus pathway See HETEROLACTIC FERMENTATION. bifonazole See AZOLE ANTIFUNGAL AGENTS. big bone disease See OSTEOPETROSIS. big liver disease See LYMPHOID LEUKOSIS. big T antigen (large T antigen) See POLYOMAVIRUS. biguanides Syn. DIGUANIDES. biguttulate (mycol.) Containing two GUTTULES. bijou A glass, screw-cap, 5–7 ml bottle. bikaverin A benzoxanthone quinone produced by Fusarium spp (e.g. F. moniliforme); it is a wine-red pigment which shows some anti-protozoal activity [Book ref. 131, pp. 223–225]. Biken test A precipitin test, based on the ELEK PLATE method, used for detecting the production of heat-labile enterotoxin by ETEC (q.v.) [JCM (1981) 13 1–5]. bilabiate (mycol.) Refers to those asci in which spore discharge is preceded by the development of a vertical slit in the apex of the ascus. bilateral trama A type of trama in which the hyphae have a bilateral arrangement. In e.g. members of the Amanitaceae, the hyphae diverge from the central plane of the lamella, passing downwards and outwards towards the lamellar surfaces (divergent trama). In e.g. members of the Pluteaceae, the hyphae converge towards the central plane (convergent trama). bile acids Steroid carboxylic acids – typically hydroxylated derivatives of 5b-cholan-24-oic acid: cholic acid has a-hydroxyl groups at positions 3, 7 and 12, deoxycholic acid at positions 3 and 12, chenodeoxycholic acid at positions 3 and 7, and lithocholic acid at position 3. These compounds occur – usually conjugated via an amide bond with glycine (glycocholic acids) or taurine (taurocholic acids) – in bile, the relative proportions depending on species. Cholates (the salts of bile acids) are surfactants which emulsify dietary fats in the intestine. Primary bile acids are synthesized in the liver. Secondary bile acids are produced from primary acids by the action of intestinal bacteria; they are absorbed from the intestine and returned to the liver for secretion. In mammals, primary bile acids include cholic acid and small amounts of chenodeoxycholic acid. (Chenodeoxycholic acid is the main bile acid in fowl.) Lithocholic acid and deoxycholic acid are secondary bile acids; lithocholic acid, formed from chenodeoxycholic acid, is 94 biofilm time, and a standard curve is constructed by plotting the amount of growth against the concentration of pantothenic acid; the standard curve can then be used to quantify an unknown concentration of pantothenic acid. Clearly, a medium must not contain substance(s) which allow the test organism to bypass the restriction on growth imposed by the limiting concentration of the assayed substance; in the case of BIOTIN bioassay, for example, the presence of pimelic acid in the medium may circumvent the requirement for biotin and permit growth of the test organism. (See also FOLIC ACID; METHANOBREVIBACTER; NICOTINIC ACID; VITAMIN B12 ; cf. BIOAUTOGRAPHY.) Bioassay is also used e.g. for measuring concentrations of antimicrobial agents, and for assaying the pathogenicity of entomopathogenic microorganisms by determining their effect on susceptible insect populations. bioautography A method for detecting a trace amount of a substance (e.g. a vitamin) in a complex mixture, the given substance being an essential growth requirement for the test organism used. The components of the mixture are initially separated by CHROMATOGRAPHY. The presence of a given substance, in a given chromatographic band, is then determined by extracting the band with an appropriate solvent and adding the extract to a medium which lacks a particular growth factor; the medium is then inoculated with an organism which fails to grow in the absence of the growth factor. On incubation, growth of the test organism indicates the presence of the growth requirement in the original mixture. (cf. BIOASSAY.) Biobor An organoboron preservative used e.g. in various liquid hydrocarbon fuels. bioburden In an industrial sterilization process: the number of contaminating organisms per product unit before sterilization. biochemical oxygen demand See BOD. biocide (1) Syn. STERILANT. (2) Sometimes used to refer to a DISINFECTANT or PRESERVATIVE. bioconcentration factor See BCF. biocontrol Syn. BIOLOGICAL CONTROL. bioconversion (1) The conversion of a substrate to particular product(s) by cells or by isolated enzymes. (2) The conversion of substrate(s) to cell biomass. biocytin e-N-Biotinyl-lysine (see BIOTIN). biodegradation The degradation of a substance as a result of biological (usually microbial) activity. In ecological terms, biodegradation is vital for the recycling of matter (see e.g. CARBON CYCLE). In human terms, biodegradation may be useful e.g. in the disposal of waste materials (see e.g. SEWAGE TREATMENT) and xenobiotics (see BIOREMEDIATION), and in processes such as COFFEE production and RETTING; it can also be a nuisance – see BIODETERIORATION. biodeterioration The deterioration (spoilage) of an object or material as a result of biological – usually microbial – activity. See e.g. CATHODIC DEPOLARIZATION THEORY (of iron corrosion), COAL BIODEGRADATION, FOOD SPOILAGE, GLASS, LEATHER SPOILAGE, PAINT SPOILAGE, PAPER SPOILAGE, PETROLEUM (spoilage of fuels and oils), RUBBER SPOILAGE, TEXTILE SPOILAGE, TIMBER SPOILAGE, WOOL SPOILAGE. (See also PRESERVATIVES; cf. BIODEGRADATION.) bioemulsifiers See BIOSURFACTANTS. biofilm An adherent layer of microbial cells embedded in a polymer matrix secreted by the cells. Biofilms can develop on living tissues (e.g. oral cavity) on catheters, in water pipes [FEMS Ecol. (1997) 22 265–279], and on ships’ hulls; pathogens in biofilms (e.g. on prostheses) may be resistant to antibiotics (and to the immune system), but biofilms can be useful e.g. in waste-water treatment systems [TIBtech.(2000) 18 312–320]. (See also EPILITHON.) shock-sensitive permease) In Gram-negative bacteria: an ABC in which an essential stage is the binding of (imported) substrate by a soluble periplasmic binding protein which transfers the substrate to a membrane complex. (Some binding proteins also function in chemosensing.) Binding proteins can be released by OSMOTIC SHOCK; as membrane-bound components cannot function without their binding proteins, this type of transport system is characterized by its sensitivity to osmotic shock. In Escherichia coli and Salmonella typhimurium (for example) various sugars, ions, amino acids, organic acids, oligopeptides etc. (almost half of all imported material) can be transported by binding protein-dependent systems. (See also VITAMIN B12 .) One example is the maltose uptake system in E. coli which consists of (i) membrane-associated proteins: MalF, MalG and the two (MalK) ABC proteins, and (ii) the periplasmic binding protein, MalE; uptake of maltose via the outer membrane involves a specific PORIN, the LAMB PROTEIN. Another importer, the E. coli ProU system, mediates uptake of the osmoregulatory molecule glycine betaine [FEMS Reviews (1994) 14 3–20]. A further example is the high-affinity histidine uptake system (histidine permease) in Salmonella typhimurium [e.g. JBC (1997) 272 859–866] – one of a number of distinct histidine transporters in this organism. This importer consists of (i) HisM, HisQ and two copies of the ABC protein, HisP (all membrane-associated), and (ii) the periplasmic binding protein, HisJ. (Genes encoding the J, Q, M and P proteins constitute a single OPERON.) On binding to histidine, HisJ undergoes a conformational change and interacts with the membrane complex; histidine is translocated concomitantly with ATP hydrolysis. [HisP (crystal structure): Nature (1998) 396 703–707.] In some cases, binding proteins specific for different substrates can interact with the same membrane complex. For example, in Salmonella typhimurium the binding protein for histidine and that for lysine, arginine and ornithine (the LAO binding protein: argT gene product) both interact with the same membrane-bound complex. In E. coli, two binding proteins for branched amino acids – LIV-I (for L-leucine, L-isoleucine and L-valine) and LS (for D- and L-leucine) – also have common membrane-bound components. binomial A designation consisting of two names. For example, a SPECIES is designated by a Latin binomial which consists of the name of the genus to which it belongs (the generic name) followed by the name (specific epithet ) which distinguishes that species from all others in the genus. Thus, e.g. in Bacillus cereus ‘Bacillus’ is the generic name and ‘cereus’ is the specific epithet. bioadsorbents See LEACHING. bioassay Any quantitative procedure in which a given organism is used for assay purposes. Bioassay is used most often to measure trace amounts of a given substance (e.g. a vitamin) in a sample; this is achieved by using an organism for which the given substance is an essential growth factor, and determining the total amount of growth of that organism in the sample. For example, for the bioassay of PANTOTHENIC ACID, Tetrahymena pyriformis is inoculated into a medium containing adequate concentrations of all growth requirements except pantothenic acid – which is present at a known, suboptimal concentration; no pantothenic acid should be carried over in the inoculum. After incubation (under standard conditions) the amount of growth is measured (e.g. in mg dry weight); the amount of growth is proportional to the amount of pantothenic acid in the medium. The procedure is repeated, using different concentrations of pantothenic acid each TRANSPORTER 95 biofilm fluidized-bed reactor ICPs applied to crops are readily degraded – so that repeat application is necessary. To avoid this problem, B. thuringiensis has been genetically modified so that toxin accumulates within the cell; this (partly-protected) toxin is still highly active [Biotechnology (1995) 13 67–71]. Biological control has also been directed against mosquitoes; strains of B. thuringiensis, B. sphaericus and Clostridium bifermentans produce toxins which can kill mosquitoes, and considerable effort is being made to develop a product suitable for controlling mosquito-borne diseases such as MALARIA, filariasis and YELLOW FEVER [MR (1993) 57 838–861]. [Biological control by Gram-positive bacteria: FEMS (1999) 171 1–19.] In a ‘competitive’ form of biological control, Pseudomonas fluorescens reduces frost injury in certain crops by using nutrients which would otherwise be available to ICE-NUCLEATION BACTERIA. Other biological control agents include e.g. strains of Agrobacterium (see AGROCINS), Bacillus popilliae (see MILKY DISEASE), fungi (see e.g. ASCHERSONIA, BEAUVERIA, HETEROBASIDION, METARHIZIUM, NOMURAEA), protozoa (see e.g. NOSEMA) and viruses (see e.g. BACULOVIRIDAE, CYTOPLASMIC POLYHEDROSIS VIRUS GROUP, MYXOMATOSIS). biological false positive reaction (BFPR) In a serological test: a positive reaction obtained as a result of the occurrence, in the patient, of disease(s) or condition(s) other than those being tested for (see e.g. STANDARD TESTS FOR SYPHILIS). biological filter See SEWAGE TREATMENT. biological oxygen demand See BOD. biological safety cabinet Syn. SAFETY CABINET. biological transmission Transmission of a parasite by a biological VECTOR. biological vector See VECTOR (1). biological warfare Any form of warfare which exploits the harmful potential of bioactive substance(s) and/or of particular microorganism(s) – use of the latter sometimes being termed germ warfare. The topic is discussed in a number of books – e.g. Superterrorism by Schweitzer & Dorsch (1998) published by Kluwer Academic Publishers/Plenum. [Bioterrorism as a public health threat: EID (1998) 4 488–494. Bioterrorism: EID (1999) 5 (special issue) 491–565.] There are plans to develop a former Russian bioweapons laboratory into an international centre for studying emerging and re-emerging diseases [see Science (2001) 291 2288–2289]. biological washing powders See SUBTILISINS. biologicals (med.) Certain biologically derived therapeutic agents which include e.g. infliximab (see TNF). bioluminescence CHEMILUMINESCENCE generated by certain microorganisms and (e.g.) by fireflies and by some crustaceans and jellyfish. (cf. PHOSPHORESCENCE.) In all cases, bioluminescence requires the presence of free oxygen – although only very low levels may be necessary. However, bioluminescence in different organisms may require different sources of energy (e.g. ATP in fireflies, NAD and FMN in bacteria); moreover, different organisms may produce light of different wavelengths (e.g. 560 nm in fireflies, ∼475–505 nm in bacteria). In some organisms the light-emitting entity has been identified as a particular compound; for example, in the firefly (Photinus pyralis) it is 4,5-dihydro-2-(6-hydroxy-2benzothiazolyl)-4-thiazole carboxylic acid. The various lightemitting compounds/systems from different organisms are referred to by the generic term luciferin. At least some biofilms contain fluid-filled channels which are kept open by rhamnolipid biosurfactants [JB (2003) 185 699–700; 185 1027–1036]. Biofilm development may be controlled by QUORUM SENSING [Microbiology (2001) 147 2517–2528; JB (2004) 186 1838–1850] and (in Pseudomonas aeruginosa) may involve three-component regulation–a sensor and two response regulators (cf. TWO-COMPONENT REGULATORY SYSTEM) [JB (2005) 187 1441–1454]. [Symposium/review: JB (2004) 186 4427–4440.] biofilm fluidized-bed reactor A reactor in which the liquid to be treated is pumped upwards through a column (‘bed’) of small, biofilm-coated particles (e.g. sand grains of 0.5 mm diam.) at a flow rate sufficient to cause fluidization of the bed, i.e. a state in which the particles, though retained within the reactor, are able to move relative to one another in the liquid – rather than being sedimented and immobile. biofuel cell A device in which certain redox reactions, catalysed by isolated enzymes or by living cells, are coupled to metal electrodes – thereby generating an electromotive force (emf) between the electrodes. In a biofuel cell a given redox system may be coupled to an electrode via a mediator substance which facilitates electron flow between the redox system and electrode; for example, 4,4 -dipyridyl can facilitate electron flow between oxidized/reduced cytochrome c and a gold electrode [Nature (1980) 285 673–674]. In one type of biofuel cell energy is derived from the metabolism of glucose by Proteus vulgaris [JGM (1985) 131 1393–1401]. In another device, energy is derived from the oxidation of carbon monoxide by an isolated CO OXIDASE using e.g. ferrocene monocarboxylic acid ([carboxycyclopentadienyl]cyclopentadienyl-iron) as a mediator [MS (1986) 3 149–153]. More recently, improved results in a microbial fuel cell have been obtained by using neutral red as an electron mediator [AEM (2000) 66 1292–1297]. biogas See ANAEROBIC DIGESTION. bioinsecticide See BIOLOGICAL CONTROL. bioleaching Microbial LEACHING. biological aerated filter See SEWAGE TREATMENT. biological assay Syn. BIOASSAY. biological clock See CIRCADIAN RHYTHMS. biological control The exploitation, by man, of one species or strain of an organism (or a product of that organism) to control the numbers and/or activities of another organism. Biological control is used on a commercial scale e.g. in agriculture, forestry and horticulture; it often involves introducing into the environment a microbial pathogen of insects (a microbial insecticide, bioinsecticide or biopesticide) which can infect and kill, or disable, particular insect pests of certain crop plants. Similarly, weeds may be controlled by the use of plant-pathogenic fungi (mycoherbicides). (See also TRICHODERMA.) Certain strains of Bacillus thuringiensis can synthesize a (typically plasmid-encoded) insecticidal crystal protein (ICP); there are several types of ICP – designated Cry types I–IV (and subtypes) – and these important bioinsecticides are widely used against insect pests on a range of crops. Most of the ICPs are formed as a parasporal crystal (see DELTA ENDOTOXIN), i.e. they are synthesized only in sporulating cells; synthesis and sporulation are linked because transcription of the relevant ICP genes depends on sporulation-specific sigma factors such as sE . ICPs of the Cry III type differ in that they are formed during vegetative growth; overexpression of these ICPs occurs in mutant cells blocked in the phosphorelay (see ENDOSPORE). [ICP production: Mol. Microbiol. (1995) 18 1–12.] 96 bioreactor All bioluminescent organisms contain a species-specific thiolcontaining oxidoreductase which catalyses oxidation of the light-emitting entity – giving rise to an energized form which produces light as it returns to the unexcited ground state. The generic term for this oxidoreductase is luciferase. In Photobacterium fischeri, luciferase (EC 1.14.14.3) is a protein of MWt ∼80000. (The firefly luciferase (EC 1.13.12.7) is a protein of MWt ∼62000.) Bioluminescence in bacteria. The luminescent bacteria include marine and coastal species, e.g. Alteromonas hanedai and species of PHOTOBACTERIUM and XENORHABDUS. All the luminescent bacteria appear to generate light by similar reactions, involving the same types of component, and light is emitted continuously (i.e. not in pulses) under appropriate conditions. Bacterial luciferase is an AUTOINDUCIBLE ENZYME. The autoinducer in e.g. P. fischeri is N-(b-ketocaproyl)homoserine lactone (see QUORUM SENSING); above a certain concentration, autoinducer switches on the lux operon that encodes luciferase. Bacterial bioluminescence apparently involves the initial reduction of FMN by NADH, and the formation of a luciferase–FMNH2 complex. In the presence of (i) free oxygen, and (ii) an aliphatic aldehyde (containing at least seven or eight carbon atoms), a larger complex appears to develop: luciferase–FMNH2 –O2 –RCHO; this complex emits light and yields FMN, luciferase, water and a carboxylic acid corresponding to the aldehyde. (The aldehyde may be re-cycled after reduction of the carboxylic acid by NADH.) Bacterial bioluminescence competes with the respiratory chain for reduced NAD; luminescence is rapidly enhanced following the inhibition of respiration by cyanide. Bioluminescence in fungi. The fruiting bodies and/or mycelium of some agarics (e.g. Armillaria mellea, Mycena spp) produce a continuous (non-pulsing) light which, in some species, exhibits a diurnal fluctuation in intensity; it is dependent on NAD(P)H and appears to involve two types of enzyme. The emitted light may be bluish-green or green (wavelength ca. 530 nm). Bioluminescence in dinoflagellates. On appropriate stimulation (e.g. acidification, agitation), the cells of some dinoflagellates (e.g. species of Gonyaulax, Noctiluca, Pyrocystis, Pyrodinium) emit bluish light (wavelength ca. 480 nm) in flashes of ca. 0.1 second duration. Gonyaulax has a system of subcellular, guanine-containing, crystal-like fluorescent structures referred to as ‘scintillons’; the luciferin occurs (together with at least some luciferase) only in the scintillons, although some luciferase may also occur in other parts of the cell [JCB (1985) 100 1435–1446]. Structures similar to scintillons have been observed in non-luminescent dinoflagellates. Bioluminescence in G. polyedra (and probably in many other dinoflagellates) exhibits a CIRCADIAN RHYTHM. In Noctiluca, luminescence is associated with subcellular organelles, the microsources. Bioluminescence is also exhibited by a number of radiolarians. The function of microbial bioluminescence, if any, is unknown. It has been suggested that it is the vestige of a system once used for removing low levels of environmental oxygen. Applications. Bioluminescent bacteria have been used for detecting very low concentrations of dissolved oxygen. The ATP-dependent firefly luciferase–luciferin system is used for detecting/quantifying ATP (e.g. in PYROSEQUENCING, in which pyrophosphate is first converted to ATP). A luciferase reporter system is used e.g. for studying promoter function [AEM (2005) 71 1356–1363] and the efficacy of antisense oligomers [AAC (2005) 49 249–255]. Three different luciferases have monitored the simultaneous expression of three genes [BioTechniques (2005) 38 891–894]. biomass The dry weight, or other (usually quantitative) estimation of organisms (commonly microorganisms), in a given habitat (soil, water, culture medium, etc). biomimetic technology Biologically-based systems which can achieve results similar to those hitherto dependent on abiotic electrical/mechanical/chemical methodology. One example involves the ability of a strain of Pseudomonas stutzeri to form (in its periplasm) crystalline particles of silver in a nanometre range of sizes; heat treatment (400° C) of a layer of particlecontaining cells can form a carbonaceous matrix containing the small, homogeneously embedded particles of silver – a structure which may have applications as a coating for the absorption of solar energy. Silver–carbon films from P. stutzeri may also be useful (as a consequence of their high porosity) as electrodes in lithium-ion and other batteries. In general, certain microorganisms are capable of producing intracellular or extracellular cermet (ceramic–metal) or orgmet (organic–metal) structures, or composites, which are organized on a nanometre scale and which potentially have a range of applications in materials science. [Metal-accumulating bacteria and their potential for materials science: TIBtech. (2001) 19 15–20.] biomineralization The deposition of minerals due to biological activity – e.g. the formation of magnetosomes (see MAGNETOTACTIC BACTERIA) and the intracellular or extracellular deposition of sulphur by photosynthetic bacteria. (cf. MINERALIZATION.) biomining See LEACHING. Biomyxa See GRANULORETICULOSEA. biont Syn. SYMBIONT. biopesticide See BIOLOGICAL CONTROL. biophore A ‘host’ cell in genetic engineering. bioplastics See BIOPOL. Biopol The trade name (Zeneca, Great Britain) for a range of biodegradable thermoplastics based on POLY-bHYDROXYBUTYRATE (PHB). The homopolymer (PHB) is formed under suitable conditions when Alcaligenes eutrophus uses glucose as the sole source of carbon. When the growth medium is appropriately supplemented, A. eutrophus forms co-polymers of hydroxybutyrate and hydroxyvalerate; the proportion of hydroxyvalerate (controlled by adjusting the growth medium) determines the properties of these co-polymers. The intracellular granules of polymer (either PHB or co-polymer) are harvested and purified to a fine, white powder. Biopol can be used for making containers, mouldings, fibres, films and coatings, and can be worked by blow-moulding and injection-moulding processes. While stable in normal use, Biopol is fully degradable after suitable disposal. To reduce costs, attempts are now being made to develop transgenic plants for the commercial production of bioplastics. [Review on bioplastics: BAB (1996) 49 1–14.] biopsy (1) The removal of tissue from the living body for examination, culture etc. (2) The tissue specimen referred to in (1). biopterin 2-Amino-4-hydroxy-6-(1 ,2 -dihydroxypropyl)-pteridine. Biopterin is believed to act as a coenzyme in certain hydroxylation reactions, and is required as a growth factor e.g. by Crithidia fasciculata and Trypanosoma platydactyli (formerly Leishmania tarentolae); the requirement for biopterin may be partially or totally overcome by high concentrations of FOLIC ACID. bioreactor (reactor) A FERMENTER or other apparatus used for BIOCONVERSIONS. 97 bioremediation Gram-positive bacteria. Genes encoding the relevant degradative activity for biphenyls/polychlorinated biphenyls form the bph operon. [Transcription of the bph operon in Pseudomonas pseudoalcaligenes strain KF707: JBC (2000) 275 31016–31023.] The pesticide pentachlorophenol (PCP) can be degraded by strains of the bacterium Sphingomonas chlorophenolica – the metabolic potential for such degradation probably having developed during the last few decades [TIBS (2000) 25 261–265]. (See also POLYURETHANASE and TOL PLASMID.) Wastewater containing terephthalic acid (used e.g. in the manufacture of plastics) can be treated anaerobically (see SEWAGE TREATMENT). [Heavy metals: FEMS Reviews (2002) 26 327–338.] The development of genetically engineered bacteria for degrading specific compounds (particularly XENOBIOTICS) is likely to be assisted by information on the way in which signals from various environmental substrates are integrated at gene promoters [EMBO (2001) 20 1–11]. Bios test A test used for assessing the integrity of the seams in a can (see CANNING). Essentially, cans are filled with a nutrient medium and are sealed and processed in the normal way – cooling being carried out with water containing a gasproducing organism; the cans are then incubated and examined for the presence of SWELLS. An analogous test is used for testing the integrity of RETORT POUCHES. biostratigraphy The study of the age and structure of stratified rocks in terms of the fossils – e.g. FOSSIL MICROORGANISMS – they contain. [Book ref. 136.] biosurfactants Biological molecules which function as surfactants, i.e., they lower the surface tension of water; however, as commonly used, the term includes bioemulsifiers: substances which act as emulsifying agents but which do not necessarily have a significant effect on surface tension. Most biosurfactants/bioemulsifiers are amphipathic cell components such as fatty acids, phospholipids, lipopolysaccharides, lipoteichoic acids, etc. Certain microorganisms produce extracellular biosurfactants or bioemulsifiers which may play a role e.g. in the adhesion of cells to – and/or their detachment from – surfaces (see e.g. EMULCYAN), or in the utilization of hydrophobic substrates such as sulphur and hydrocarbons. For example, organisms growing on HYDROCARBONS may produce extracellular fatty acids, glycolipids (e.g. RHAMNOLIPIDS, SOPHOROLIPIDS), highMWt polymers (e.g. EMULSAN), etc (see also LIPOSAN); such substances may serve e.g. to emulsify the substrate and/or to allow cells to adhere to substrate–water interfaces. (See also SAPONINS and SURFACTIN.) [Potential use of biosurfactants in industry: MS (1986) 3 145–149.] biotechnology In a broad sense: collectively, the various forms of technology that exploit biological sources – usually microorganisms and/or their products and components (for examples see INDUSTRIAL MICROBIOLOGY). (See also BIOMIMETIC TECHNOLOGY and IMMOBILIZATION.) However, the term ‘biotechnology’ refers more commonly to those forms of technology which depend on the use of modern techniques in molecular biology/genetic engineering to construct novel organisms and/or products for industrial, medical or other purposes. bioterrorism See BIOLOGICAL WARFARE. biotic Living; of biological origin. biotin (vitamin H; coenzyme R) A VITAMIN which acts as a cofactor in carboxylation and transcarboxylation reactions, acting as a carrier of CO2 ; biotin is bound via its carboxyl group (see figure) to the e-amino group of a lysine residue in the apoenzyme. (cf. BIOCYTIN.) bioremediation Biotechnological (microbe-based) clean-up of pollutants (chemical contaminants) in the environment (aqueous and terrestrial). As a group, microorganisms are metabolically highly diverse, so that, in theory, it should be possible to degrade any of a wide range of pollutants given a suitable choice of microorganisms(s). Moreover, microbial degradation of pollutants may give rise to simple products – such as carbon dioxide and water – whereas alternative approaches (e.g. physical decontamination) may simply transfer the problem from one location to another. Nevertheless, while physical methods are typically rapid, often with a predictable outcome, biological methods frequently have unknown, unpredictable or unquantifiable effects in the environment; for example, the bioavailability of a pollutant (its accessibility to microorganisms) may be reduced by adsorption to soil particles, and this may limit or preclude efficient bioremediation. To be more widely accepted and used, bioremediation must be shown to be both effective and reliable in the environment. The efficacy of a bioremediation process may be gauged e.g. by (i) assaying pollutants of all levels of biodegradability at the polluted site; (ii) assaying the long-lived breakdown products of pollutants; (iii) assessing the levels/states of normal constituents of the environment; and (iv) quantifying specific microbial genes involved in the catabolism of pollutants [see e.g. AEM (1996) 62 2381–2386]. [Bioremediation: towards a credible technology (review): Microbiology (1998) 144 599–608.] Certain types of pollutant are susceptible to anaerobic degradation, and it has been suggested that, in the future, anaerobic microbial processes may have the potential for widespread application in bioremediation technology. Anaerobic degradation of the perchlorate ion can be carried out by various facultatively anaerobic or microaerophilic bacteria which are capable of dissimilatory perchlorate reduction (a form of ANAEROBIC RESPIRATION); these bacteria include Wolinella succinogenes and (particularly) species belonging to two new genera: Dechloromonas and Dechlorosoma (both members of the beta subclass of Proteobacteria). In dissimilatory perchlorate reduction, the initial (energy-yielding) stage produces chlorite; the chlorite is split (enzyme: chlorite dismutase) to chloride and molecular oxygen. [Ubiquity and diversity of (per)chloratereducing bacteria: AEM (1999) 65 5234–5241.] Currently (2000), biological methods for perchlorate degradation are based mainly on the use of bioreactors, and there is at present no technique for treating perchlorate contamination in the environment. Anaerobic degradation of certain hydrocarbons (including BTEX) would be very useful when these pollutants occur e.g. in fuel-contaminated aquifers under anoxic conditions – but BENZENE (for example) typically resists anaerobic degradation. A novel approach to the anaerobic degradation of benzene involves the addition of low levels of chlorite – so that the (ubiquitous) perchlorate-reducers (see above) can convert the chlorite to chloride and (free) oxygen; the oxygen is then available for certain aerobic organisms (e.g. Pseudomonas) which are able to degrade cyclic aromatic hydrocarbons [see Nature (1998) 396 730]. [Emerging techniques for anaerobic bioremediation: TIBtech. (2000) 18 408–412.] Degradation of biphenyls and polychlorinated biphenyls (PCBs) – an important category of environmental pollutants – can be carried out by various types of Gram-negative and 98 black beans HN1′ 2′ 4 5 N 3′NH NH 3 1 S (a) or −S− (dihydroxydiphenylsulphides). For maximum antimicrobial activity, each phenolic residue must carry an o(2-)hydroxyl. Halogenation increases activity; two di- or trihalogenated phenolic groups generally confer greater activity against Gram-positive than Gram-negative species. Bacteriostatic at low concentrations, bisphenols may be bactericidal at higher concentrations; they appear to affect the cell membrane – e.g. hexachlorophane affects membrane potentials and, at higher concentrations, may (like various antibacterial agents) cause coagulation of the cytoplasm. Bisphenols are poorly soluble or virtually insoluble in water but are soluble in dilute alkali and/or organic solvents; they are inactivated by non-ionic surfactants. Since bisphenols retain antimicrobial activity in the presence of soaps and are relatively non-toxic to man, they are widely used as antiseptics and preservatives. Bithionol (2,2 -dihydroxy3,5,3 ,5 -tetrachlorodiphenylsulphide) is an antibacterial and antifungal agent used e.g. in antiseptic soaps. Bromochlorophane (3,3 -dibromo-5,5 -dichloro-2,2 -dihydroxydiphenylmethane) is active against Gram-positive bacteria and has been used e.g. in deodorants and toothpastes. Dichlorophane (dichlorophene) (5,5 -dichloro-2,2 -dihydroxydiphenylmethane) is active against fungi, bacteria and algae; it is used e.g. as a preservative for paper and textiles. Fentichlor (2,2 -dihydroxy-5,5 dichlorodiphenylsulphide) is active primarily against Grampositive bacteria (e.g. staphylococci) and fungi (particularly dermatophytes); it has been used to treat infections involving dermatophytes. Hexachlorophane (hexachlorophene) (2,2 dihydroxy-3,5,6,3 ,5 ,6 -hexachlorodiphenylmethane) is much more active against Gram-positive than Gram-negative bacteria; it has been used in antiseptic soaps, liquid soaps (e.g. Ster-zac) and lotions (e.g. pHisoHex ). (cf. TRICLOSAN; see also PHENOLS.) bisterigmate Having two sterigmata. bis(tri-n-butyltin) oxide See TBTO. bisulphite (as a MUTAGEN) Bisulphite (HSO3 − ) undergoes various reactions with nucleic acids, and can also cause cross-linking between proteins and nucleic acids. The reaction believed to be responsible for mutagenesis is the deamination of cytosine and (at a slower rate) of 5-methylcytosine to give, respectively, uracil and thymine (resulting in G·C-to-A·T transition in either case); bisulphite can apparently act on these bases only in ssDNA. (See also SITE-SPECIFIC MUTAGENESIS.) bithionol See BISPHENOLS. Bittner virus Syn. MOUSE MAMMARY TUMOUR VIRUS. bitty cream See MILK SPOILAGE. bitunicate ascus (fissitunicate ascus) An ASCUS whose wall consists of a thin, more or less rigid outer envelope (ectotunica), and a thicker, extensible inner envelope (endotunica); at maturity, the outer envelope ruptures and the inner envelope elongates – the ascospores being discharged via an apical pore in the inner envelope. (cf. UNITUNICATE ASCUS.) Lecanoralean asci release their ascospores in a similar way; they differ from other bitunicate asci e.g. in that the endotunica protrudes only to a limited extent during ascospore discharge. biuret test A test based on the fact that peptide bond-containing substances (e.g. peptides and proteins) give a violet/pink colour when treated with a dilute, alkaline solution of copper sulphate. bivalent (noun) See MEIOSIS. Bivalvulida See MYXOSPOREA. BK virus See POLYOMAVIRUS. BL-type starter See LACTIC ACID STARTERS. bla gene A gene encoding b-LACTAMASE. (See also e.g. Tn3.) black beans Syn. HAMANATTO. O− O C O 2 S (CH2)4 COOH (b) (CH2)4 C O N H protein BIOTIN: (a) free, and (b) bound. Biotin is required as a growth factor e.g. by many fungi (e.g. Saccharomyces cerevisiae, Candida spp) and bacteria (e.g. species of Clostridium, Lactobacillus, Leuconostoc). Some organisms can synthesize biotin from cysteine, pimelic acid and carbamoyl phosphate. (See also AVIDIN and ABC IMMUNOPEROXIDASE METHOD.) biotin enzyme Any enzyme whose active centre is associated with a BIOTIN residue – e.g. PYRUVATE CARBOXYLASE and oxaloacetate decarboxylase. biotope (1) The environment occupied by an organism or organisms. (2) (med., vet.) The location of a particular parasite or pathogen within the body. (3) The spatial distribution of the biomass in a cross-section of a river, lake, etc. biotransformation Syn. BIOCONVERSION (1). biotroph An organism which derives nutrients from the living tissues of another (‘host’) organism. (cf. NECROTROPH.) biotype (biovar) Any VARIETY distinguished by metabolic and/or physiological properties. biovar Syn. BIOTYPE. biparental See PROTOPLAST FUSION. biphenyls (degradation) See BIOREMEDIATION. biplicity See FLAGELLUM (a). bipolar heterothallism See HETEROTHALLISM. Birbeck granule See LANGERHANS’ CELLS. bird flu An outbreak of disease (Hong Kong, 1997) caused by an avian strain (H5N1) of influenza virus type A (see INFLUENZAVIRUS). bird’s nest fungi See NIDULARIALES. birefringence (double refraction) The phenomenon in which a beam of light, on entering certain (optically anisotropic) materials (e.g. a calcite crystal), gives rise to two beams of polarized light which travel at different velocities in mutually perpendicular planes and which often (depending on conditions) follow divergent paths. Birnaviridae See IPN VIRUS. birth scar (mycol.) See SCAR. bisdithiocarbamates (as antimicrobial agents) Metal ethylenebisdithiocarbamates (MS.CS.NH.CH2 CH2 NH.CS.SM, in which M is a metal ion) are used as agricultural ANTIFUNGAL AGENTS – see e.g. MANCOZEB, MANEB, NABAM and ZINEB; they are believed to be converted, under field conditions, to diisothiocyanate derivatives. Bismarck brown A brown basic diazo DYE used e.g. for VITAL STAINING. bismuth sulphite agar See WILSON AND BLAIR’S AGAR. bisphenols Compounds that contain two phenolic residues/ molecule; in bisphenols which are effective ANTISEPTICS or DISINFECTANTS the residues are linked directly (i.e. carbon–carbon) or e.g. via −CH2 − (dihydroxydiphenylmethanes) 99 Black beetle virus growth develop on the meat surface. Species involved include Cladosporium cladosporioides, C. herbarum and Penicillium verrucosum var. corymbiferum (Penicillium hirsutum). [JAB (1982) 52 245–250.] (2) (vet.) Syn. BLACK POX. (3) (plant pathol.) Any of various plant diseases characterized by the formation of black spots e.g. on the leaves and/or fruit. Black spot of roses is caused by Diplocarpon rosae; irregular dark-brown or black patches appear on both surfaces of the leaves, and leaves turn yellow and fall prematurely. Bacterial black spot of mangoes, a serious necrotic disease present in most mango-growing regions, is caused by Xanthomonas campestris pv. mangiferaeindicae [IJSB (1984) 34 77–79]. black stem rust A CEREAL DISEASE, caused by Puccinia graminis, which can affect wheat (causing serious economic losses e.g. in the USA) and other cereals; see UREDINIOMYCETES for details of the disease in wheat. black wart disease (of potato) See WART DISEASE. black yeasts Fungi which can form dark, yeast-like cells (e.g. species of Aureobasidium and Cladosporium). blackeye cowpea mosaic virus See POTYVIRUSES. blackfellow’s bread The SCLEROTIUM of Polyporus mylittae which is used as food by Australian aboriginals. blackhead (1) (vet.) (histomoniasis; infectious enterohepatitis) A disease which affects mainly turkeys but also e.g. chickens and pheasants; it is caused by Histomonas meleagridis. Transmission occurs mainly via the caecal nematode Heterakis gallinae, in which H. meleagridis is transmitted transovarially; the bird is infected on ingestion of nematode eggs passed with the faeces of an infected bird. Symptoms in turkeys: anorexia, lethargy, passage of characteristic yellow faeces; cyanosis or blackening of the head may occur. Initially, small haemorrhagic ulcers occur in the caecum; later the caecal tubes are greatly enlarged. Characteristic circular, yellowish, necrotic lesions occur in the liver, sometimes coalescing to form large areas of necrosis. In the acute form the disease is often rapidly fatal, but it may be chronic in older birds. In chickens, the disease is usually mild (although severe outbreaks may occur under intensive rearing conditions). Chickens are commonly parasitized by H. gallinae and can act as a reservoir of H. meleagridis; they should therefore not be reared alongside turkeys. Lab. diagnosis: H. meleagridis may be observed in scrapings from caecal lesions. Furazolidone and dimetridazole are used for prevention/treatment. (See also POULTRY DISEASES.) (2) (med.) See ACNE. blackleg (1) (blackquarter) (vet.) In cattle (and occasionally in other animals): an acute (usually) fatal disease, characterized by severe toxaemia and inflammation of leg (and/or other) muscles, caused by Clostridium chauvoei ; the pathogen is presumed to reach the affected site via the intestine and bloodstream, but trauma to muscles may be a predisposing factor. Treatment: parenteral administration of antibiotics. (cf. MALIGNANT OEDEMA.) (2) (plant pathol.) Any of various bacterial or fungal diseases of plants in which the plant stem characteristically becomes blackened and often rots. In potatoes blackleg is caused by Erwinia carotovora subsp. atroseptica; symptoms: yellowing of foliage, rotting of stems (which become black and slimy), slimy lesions in the tubers. In cruciferous plants ‘blackleg’ may refer to any of several diseases. That caused by Xanthomonas campestris involves leaf chlorosis and blackening of the veins; infection is seed-borne or occurs via stomata or wounds. Leptosphaeria maculans can be an important cause of blackleg in oil-seed rape and other brassicas. Black beetle virus See NODAVIRIDAE. black bread mould See BREAD SPOILAGE. black cat/white cat principle The principle that different mechanisms for regulating a given catabolic system are equally efficient if they both (or all) ensure responsiveness to the substrate in the context of the organism’s physiological state [EMBO (2001) 20 1–11]. Black Creek Canal virus See HANTAVIRUS. Black Death See PLAGUE. black disease (infectious necrotic hepatitis) An acute, toxaemic disease, which affects mainly sheep and cattle (sometimes pigs), caused by infection of the liver by Clostridium novyi ; the disease is commonly precipitated by invasion of the liver by the liver fluke (Fasciola hepatica). Death usually occurs rapidly. black fluids Commercial DISINFECTANTS consisting of certain phenolic coal tar fractions solubilized with soaps; their activity is decreased by electrolytes. (cf. WHITE FLUIDS.) black gill disease A disease of prawns and shrimps (Pennaeus spp) caused by Fusarium solani (cf. BURNED SPOT DISEASE); the gills bear many black spots and may undergo necrosis or collapse. black light Radiation of ca. 300–420 nm. black mat syndrome A disease of the tanner or snow crab (Chionoecetes bairdi ) caused by the ascomycete Trichomaris invadens [CJB (1981) 59 2121–2128]. The fungus forms a dense black nodular encrusting subiculum (bearing partly embedded perithecia) on the crab exoskeleton; non-pigmented hyphae penetrate the tissues beneath [AEM (1983) 46 499–500]. (See also CRUSTACEAN DISEASES.) black mildews (dark mildews) Plant-parasitic fungi of the family Meliolaceae (order DOTHIDEALES); the organisms form dark mycelium which, in many species, is setose (see also HYPHOPODIUM). Genera: e.g. Meliola. (cf. POWDERY MILDEWS and SOOTY MOULDS.) black piedra A human MYCOSIS in which hairs (e.g. of the scalp) are infected by Piedraia hortae. Hard black nodules adhere firmly to infected hairs (mainly on the distal portions). The nodules are composed of masses of dark hyphae which form a stroma containing many uniascal locules; asci are spherical and contain 8 fusiform ascospores. (cf. WHITE PIEDRA.) black pod disease A CACAO DISEASE caused by species of PHYTOPHTHORA (e.g. P. palmivora, P. megakarya) and characterized by blackening of the (normally green) pods. Pulp prepared from the beans of infected pods contains subnormal amounts of sugar so that the fermentation process (see COCOA) may be affected; such beans also contain excessive amounts of free fatty acid. The disease is usually controlled by antifungal sprays. [Black pod disease in Nigeria: Phytopathol. Paper number 25 (April, 1981).] black pox (black spot) (vet.) Ulcerative lesions, each with a black centre, which occur on the teats, particularly at the orifice, in e.g. cattle. Staphylococcus aureus may be isolable from the lesions. Treatment: antimicrobial ointments or teat dips. (See also MASTITIS.) black queen cell virus See PICORNAVIRIDAE. black rot (plant pathol.) A general term for any of various unrelated plant diseases. For example, brassica black rot, caused by Xanthomonas campestris pv. campestris, is characterized by blackening of leaf-margins and often of leaf-veins, yellowing of leaves, stunting etc. (Carrot black rot: see ALTERNARIA.) black scours Syn. WINTER DYSENTERY. black smoker See HYDROTHERMAL VENT. black spot (1) (food microbiol.) A type of MEAT SPOILAGE which affects red meats at sub-zero temperatures; black spots of fungal 100 bleeding canker present in large numbers [Ped. Inf. Dis. (1985) 4 556–557]. Three morphological forms (amoebic, granular and vacuolated) have been described, but the vacuolated form is the most common in human faecal specimens; these cells are non-motile and spherical, oval or ellipsoid, (5−)8–10(−30) µm diam., and may be mistaken for cysts of certain amoebae. Staining of the vacuolated forms with e.g. iodine reveals a thin peripheral layer of cytoplasm containing one nucleus (sometimes 2–4 nuclei) and surrounding a large central or slightly eccentric body or vacuole. Blastodinium See DINOFLAGELLATES. Blastomyces A genus of fungi of the HYPHOMYCETES. B. dermatitidis (= B. zymonema, teleomorph Ajellomyces dermatitidis) is a dimorphic fungus which causes BLASTOMYCOSIS; in mammalian cells and in culture at 37° C it grows in the form of yeast-like, spherical, thick-walled cells (usually ca. 8–15 µm diam.). The cells reproduce by budding, a (usually) single bud being attached to the mother cell by a broad base. When cultured at <30° C the organism grows as a septate mycelium, producing smooth, spherical to ovoid conidia (3–5 µm diam.) on short, straight conidiophores. Blastomycetes A class of fungi (subdivision DEUTEROMYCOTINA) which include the anamorphic yeasts; the class is sometimes divided into two families: CRYPTOCOCCACEAE (species do not form ballistospores) and SPOROBOLOMYCETACEAE (species form ballistospores). In some taxonomic schemes [Book ref. 64, p. 54] the organisms are included in the HYPHOMYCETES. blastomycosis (North American blastomycosis; Gilchrist’s disease) An acute or chronic MYCOSIS, affecting man and animals (e.g. dogs), caused by Blastomyces dermatitidis; it occurs e.g. in North America, Africa and Israel. Infection apparently occurs by inhalation of spores from the (presumably saprotrophic) fungus, although B. dermatitidis has proved difficult to isolate from environmental habitats. The disease occurs in two clinically distinct but variable forms: the cutaneous form (a chronic disease in which granulomatous, suppurative lesions are confined mainly to the skin, although the pathogen is disseminated from a primary pulmonary site), and the pulmonary form (in which lesions develop primarily in the lungs). Pulmonary blastomycosis may be subclinical, may (occasionally) manifest as an acute, self-limiting, influenza- or pneumonia-like disease, or may be chronic and progressive with dissemination to other regions of the body (e.g. bones, genitourinary tract, CNS). Mortality rates associated with untreated systemic blastomycosis may be >80%. Chemotherapy: e.g. amphotericin B; 2-hydroxystilbamidine. [Review of human blastomycosis: CRM (1982) 9 139–164.] (cf. PARACOCCIDIOIDOMYCOSIS.) blastospore See CONIDIUM. Blattabacterium A genus of Gram negative-type bacteria which occur as intracellular endosymbionts of the cockroach (Blatta spp). Cells: non-motile rods, ca. 1.0 × 1.6–9.0 µm, which may give a Gram-positive or Gram-variable reaction; they occur within the bacteriocytes of the cockroach – and within the oocytes, thus ensuring their transmission to the larvae. GC%: ca. 26–28. Type species: B. cuenoti. [Book ref. 22, pp. 830–831.] bleach See HYPOCHLORITES. bleaching Loss of the chromophore from a biological pigment, or loss of pigment(s) from an organism – e.g. loss of chlorophyll from a photosynthetic organism. bleeding canker A CANKER which affects e.g. horse chestnut, lime and apple trees; it is caused by Phytophthora citricola. A brownish or reddish gum oozes from the regions of dying bark and dries to form a hard black crust; dieback may occur, and the lesions may be infected with other wood-decaying fungi. In beet plants, blackleg may be caused by Pleospora (Phoma) bjoerlingii ; in seedlings, the stem blackens and shrivels, while in mature plants brown leaf spots and blackening of root tissues occur. Infection is seed-borne. Pelargonium blackleg is caused by a Pythium sp; stems blacken from the base upwards, foliage wilts, etc. blackpatch disease (of clover) See SLAFRAMINE. blackquarter Syn. BLACKLEG (1). blackwater fever A complication of falciparum MALARIA characterized by profound haemoglobinuria. bladder (algol.) Syn. PNEUMATOCYST. bladder infection See CYSTITIS and UPEC. bladder wrack See FUCUS. blade (algol.) The flat or crinkled, essentially sheet-like part of the thallus in certain algae (e.g. Fucus, Laminaria). Blakeslea See MUCORALES. blanket weed See CLADOPHORA. BLAST Basic local alignment search tool, a program for comparing sequences: http://www.ncbi.nlm.nih.gov/BLAST/ blast disease (of rice) A disease of the rice plant caused by Pyricularia oryzae (see PYRICULARIA). Leaf lesions are often e.g. bluish or grey, each having a dark brown margin; bluish lesions may occur in the neck of the culm (i.e., just below the flowering head) – often leading to breakage – and small brown lesions may form on the glumes. The disease is favoured e.g. by high levels of soil nitrogen. (See also HINOZAN, KASUGAMYCIN, KITAZIN and THIOPHANATE-METHYL.) blast transformation Morphological and metabolic changes which occur in B LYMPHOCYTES and T LYMPHOCYTES on exposure to specific antigen or to a MITOGEN; in at least some cases, blast transformation appears to be triggered by the cross-linking of receptors on the surface of the lymphocyte. In B cells, within ∼12–24 hours of stimulation, the cell begins to enlarge and develops a basophilic, ribosome-rich cytoplasm, an enlarged nucleolus, and a large, pale-staining nucleus which contains an increased amount of interchromatinic protein; the transformed cell is known as a blast cell (= blast), and its formation appears to represent a shift from the G0 to the G1 phase of the cell cycle. For ANTIBODY FORMATION, a blast proliferates and forms PLASMA CELLS. blastic conidiogenesis See CONIDIUM. Blastobacter A genus of rod-shaped to spherical, Gram-negative budding bacteria which occur e.g. in ponds and lakes; some strains are flagellated, and some are pigmented. Blastocaulis A subjective SYNONYM of PLANCTOMYCES. Blastocladiales An order of fungi (class CHYTRIDIOMYCETES) in which the thallus consists typically of rhizoid-bearing hyphae or mycelium, and in which sexual reproduction involves the fusion of iso- or anisoplanogametes; most species live saprotrophically in water or soil, but Coelomomyces spp (which form wall-less, coenocytic hyphae lacking rhizoids) are obligate parasites of certain insects (e.g. mosquito larvae). [Coelomomyces: Book ref. 190.] Other genera include ALLOMYCES, Blastocladiella, and Catenaria (see also NEMATOPHAGOUS FUNGI). Blastocladiella See BLASTOCLADIALES and GAMMA PARTICLE (2). blastoconidium See CONIDIUM. Blastocrithidia A genus of homoxenous parasitic protozoa (family TRYPANOSOMATIDAE) which occur in the gut of bugs and mosquitoes; epimastigotes, approx. 10–70 µm. B. triatomae forms desiccation-resistant cysts. Blastocystis A genus of organisms currently regarded as protozoa (formerly as fungi of the Entomophthorales). B. hominis occurs in the intestinal tract in man and other animals; it is generally non-pathogenic, but may be associated with diarrhoea when 101 blending combine with allergens and which prevent those allergens from initiating an immediate hypersensitivity reaction. blood agar An agar-based medium containing defibrinated (or whole citrated or oxalated) blood. It is prepared by mixing 5–10% blood (drawn aseptically) with a pre-autoclaved molten agar base (e.g. TRYPTICASE–SOY AGAR) at ca. 50° C; the medium is dispensed e.g. to Petri dishes and allowed to set. (cf. CHOCOLATE AGAR.) Blood agar is used e.g. for the culture of fastidious bacterial parasites of man and animals, and for the detection of HAEMOLYSINS; since a given bacterial haemolysin may be active against the RBCs of only some animals (e.g. horse, sheep), the choice of blood is important. blood clotting See FIBRIN; COAGULASE; ANTICOAGULANT. blood culture A procedure for detecting the presence of viable bacteria in blood. Since blood-borne bacteria may be present only in very small numbers, isolation is attempted only after a blood sample has been incubated in a suitable medium to allow the bacteria to multiply. Blood (5–10 ml), taken aseptically, is immediately added (aseptically) to 50–100 ml of medium (e.g. trypticase soy broth) which usually contains an anticoagulant (e.g. SPS) and sometimes e.g. penicillinase (to inactivate any penicillin in the blood) and/or PABA (to counteract any sulphonamide drugs). (The medium is commonly contained in a plain bottle whose opening is sealed by a rubber disc held in place by a screw-cap with a central hole; blood is injected into the bottle through the rubber disc.) The medium is then incubated. Periodically (e.g. daily for 7 days) the medium is subcultured to a suitable solid medium which is then incubated and examined for bacterial growth. (See also CASTAÑEDA’S METHOD; CLOT CULTURE.) [ARM (1982) 36 467–493.] blood DNA isolation kits Commercial kits used for preparing genomic DNA from samples of blood. In the DNA Stat system (marketed by Stratagene), the blood sample (25–200 µl) is added to a hypotonic solution in a spin cup resting in a 2-ml collection tube; erythrocytes (red blood cells) are lysed in the hypotonic solution. On centrifugation, plasma proteins and lysed RBCs pass through the spin cup into the collection tube; leukocytes are retained within the spin cup. A chaotropic solution is then added to the spin cup; this lyses the leukocytes and releases genomic DNA. DNA binds to the spin cup, and contaminants are removed by two washing steps. DNA can then be eluted into buffer. The resulting DNA is suitable for use as a template in PCR. blood parasites See e.g. ANAPLASMA, BABESIA, ENDOTRYPANUM, LANKESTERELLA, PLASMODIUM and THEILERIA. blood poisoning Syn. SEPTICAEMIA. blood transfusion (infection by) See TRANSFUSION-TRANSMITTED INFECTION. bloom (water bloom) An abundant (easily visible and often conspicuous) growth of e.g. planktonic algae (including phytoflagellates), cyanobacteria (particularly GAS VACUOLE-forming species), or occasionally protozoa (e.g. Stentor ) at or near the surface of a body of water (reservoir, lake, fishpond, ocean, etc). Blooms are often seasonal and may be favoured by particular set(s) of environmental conditions, including e.g. thermal stratification (in lakes and reservoirs), an excess of particular nutrients (e.g. leached nitrogen-rich agricultural fertilizers), an abundance of organic matter, etc; blooms of Chrysochromulina breviturrita may be triggered by the deposition of selenium (apparently an essential growth factor for this species) derived from coal-fired power plants [CJFAS (1985) 42 1783–1788]. Blooms may recur cyclically: e.g. when dinoflagellates in a bloom exhaust their nutrient supply and die, their decomposition releases nutrients blending Vigorous agitation involving high shear forces. bleomycin An anticancer antibiotic produced by Streptomyces verticillus; it is structurally complex, consisting of peptide and disaccharide moieties. Bleomycin binds to DNA and – in the presence of Fe2+ and O2 – causes release of free bases and the degradation of the DNA by single- and double-stranded cleavages. Blepharisma A genus of ciliates (order HETEROTRICHIDA). Cells: ovoid to elongate, non-contractile, with uniform somatic ciliature and an oral region extending about half the length of the cell; usually pink to red. blepharismin See GAMONE. blepharismone See GAMONE. blepharmone See GAMONE. Blepharocorythina See TRICHOSTOMATIDA. blepharoplast (protozool.) Syn. BASAL BODY (b). blewit The edible fruiting body of Lepista saeva (syn. Tricholoma saevum, T. personatum) (= field blewit), or of Lepista nuda (syn. Tricholoma nudum) (= wood blewit). blight (1) (plant pathol.) A general term applied to any of a wide range of unrelated plant diseases: see e.g. CANE BLIGHT, CHESTNUT BLIGHT, FIREBLIGHT, FUSARIUM, HALO BLIGHT, LATE BLIGHT. (2) (vet.) Syn. INFECTIOUS KERATITIS. blister blight A TEA DISEASE which occurs throughout the Asian tea-growing regions; it is caused by Exobasidium vexans. Translucent (sometimes pinkish), circular, concave lesions commonly occur on the upper surfaces of leaves; the convex underside of each lesion (on the lower side of the leaf) becomes white and downy with spores. Only young leaves are susceptible to infection. Control: e.g. copper sprays. blister rust (white pine blister rust) An economically important disease of the western white pine (Pinus monticola), and of the Weymouth pine (P. strobus), caused by Cronartium ribicola (an apparently heterothallic rust [CJB (1985) 63 1086–1088]). The disease develops over a period of years; symptoms include the formation of large orange blisters, and of cankers which may spread until the tree dies. (See also PINE DISEASES.) bloat (ruminal tympany) In ruminants: a (sometimes fatal) condition in which the gaseous products of fermentation become trapped (either as free gas or as a stable foam) in the RUMEN – which thus becomes distended; the cause of bloat is not always clear, but the pasture (or legume) bloat which may occur in cattle grazing legume-rich pasture may be due to the foampromoting properties of soluble leaf proteins or of lipids present in the feedstuff. Feedlot (or grain) bloat appears to be due to certain factor(s) of microbial origin – e.g. extracellular polysaccharides or slimes. One factor which may be important in both pasture and feedlot bloats is the ability of some bacteria (e.g. Butyrivibrio fibrisolvens, Megasphaera elsdenii, Streptococcus bovis) to digest the salivary mucins which act as antifoaming agents. bloater damage (in cucumbers) See PICKING. Blochman bodies Intracellular, prokaryotic symbionts which occur in mycetomes in insects. blocked reading frame See OPEN READING FRAME. blocking antibodies (1) (non-agglutinating antibodies) Antibodies which fail to agglutinate homologous antigens even though antigen–antibody combination may occur; lack of agglutination may be due e.g. to an insufficiency of effective combining sites on the antibody molecules. By combining with antigen, blocking antibodies prevent (‘block’) agglutination by homologous agglutinating antibodies. Blocking antibodies can be detected e.g. by an ANTIGLOBULIN CONSUMPTION TEST. (2) IgG antibodies which 102 blunt-ended DNA blue bodies See EAST COAST FEVER. blue cheese See CHEESE-MAKING. blue-eared pig disease Porcine reproductive and respiratory syndrome (PRRS): a disease of pigs, reported first in the USA and later in Europe, associated with a variety of symptoms, including abortion/stillbirth, agalactia, respiratory distress (particularly in new-born animals) and cyanosis of the ears; some strains can cause encephalitis. The disease may be mild, or may be severe with heavy losses, depending e.g. on the virulence of the causal agent and existing health of infected pigs. Secondary infections may occur. The causal agent is a virus (genus ARTERIVIRUS), first identified in The Netherlands (the ‘Lelystad’ virus); it is also referred to as the swine infertility and respiratory syndrome (SIRS) virus. Infection may occur via the airborne route. The incubation period may be a few days to a week, or much longer. Following experimental infection via the oronasal route, the virus can be detected within 12 hours in alveolar macrophages, in lung lymph nodes within 24 hours, and in spleen within 3–4 days. A reduction in numbers of alveolar macrophages (the major target cell of the virus) may be important in predisposing to secondary infections. Virus is shed in the oronasal secretions, urine and faeces of acutely infected animals. [PRRS: RMM (1995) 6 119–125.] blue-green algae CYANOBACTERIA. blue-light fluorescence See FLUORESCENCE. blue mould (1) (of tobacco) See PERONOSPORA. (2) Any blue-spored species of Penicillium. blue proteins Copper-containing proteins which characteristically absorb strongly at or near 600 nm; at least some blue proteins are known to have roles in electron transport, and these proteins have been referred to as ‘cupredoxins’. Blue proteins occur in bacteria, e.g. AMICYANIN, AZURIN (sense 1), RUSTICYANIN; in fungi, e.g. LACCASE; in higher plants, e.g. cucumber basic blue protein, stellacyanin, umecyanin; in higher plants, algae and cyanobacteria, e.g. PLASTOCYANIN; and in animals, e.g. ascorbate oxidase (EC 1.10.3.3) and ceruloplasmin (found in mammalian blood plasma). blue pus Pus formed in suppurative infections involving Pseudomonas aeruginosa; the pus has a bluish tinge owing to the presence of pyocyanin. blue slime disease See COSTIASIS. blue-stain A form of SAP-STAIN in which timber becomes bluishgrey owing to the deep penetration of fungal hyphae; the bluish-grey ‘stain’ is due to refraction of light by the hyphae. The commonest causal agents of blue-stain in temperate zones are species of Ceratocystis, e.g. C. pilifera; Diplodia spp are important in the tropics. Blue-stain may reduce the value of timber, but it does not significantly affect its mechanical strength since the cellulose and lignin components of the wood are little affected. blueberry shoestring virus See SOBEMOVIRUSES. bluecomb disease virus See CORONAVIRIDAE. bluetongue An acute SHEEP DISEASE caused by an ORBIVIRUS; other ruminants (e.g. cattle, deer) may be affected, but in these the disease is usually mild or subclinical. Bluetongue occurs mainly in Africa but has occurred e.g. in Europe, Asia, USA. Transmission occurs mainly via a biological vector (especially Culicoides spp). Incubation period: 2 days to 2 weeks. Symptoms: fever; inflammation and cyanosis of the mucous membranes of the mouth, nose, and alimentary tract. The lungs and feet may be involved. Mortality (in sheep): e.g. 5–30%. blunt-ended DNA Linear dsDNA with flush ends, i.e., with no 3 or 5 single-stranded extensions. (cf. e.g. STICKY ENDS.) which encourage bacterial growth; the bacteria in turn produce VITAMIN B12 (an essential growth factor for dinoflagellates), thus triggering another dinoflagellate bloom. (See also RED TIDE.) Blooms can cause various deleterious effects on the habitat in which they occur, as well as causing problems for man. The death and sudden decomposition of the massive microbial populations can cause a severe depletion of dissolved oxygen, often resulting in the asphyxiation of fish and other aquatic animals. Many bloom-forming species produce potent PHYCOTOXINS which may cause illness or death in animals or humans drinking the water or consuming aquatic animals from affected waters. (See also RED TIDE.) Certain species can impart unpleasant tastes and odours to the water: e.g. Chrysochromulina breviturrita can cause a ‘rotten cabbage’ odour, while Synura petersenii can cause ‘fishy’ tastes and odours. (See also GEOSMIN and METHYLISOBORNEOL.) Excessive algal growth in a reservoir may, in addition to the above problems, block water filters (see WATER SUPPLIES). The formation of blooms may be discouraged or prevented by e.g. pumping to discourage thermal stratification, adding certain algicides, preventing contamination of waters with fertilizers, etc. ALGICIDES commonly used in this context include e.g. copper sulphate (a few parts per million), although this may actually encourage some smaller, more troublesome species by decimating larger, more susceptible species. Phenanthraquinone and DICHLONE have been reported to be effective against bloomforming cyanobacteria. bloomed lens (coated lens) A lens coated with a thin layer of e.g. magnesium fluoride to reduce the amount of incident light reflected back from the lens surface; the thickness of the coating is calculated such that destructive interference occurs between rays reflected from the bloomed surface and rays reflected at the fluoride–glass interface. In practice, blooming eliminates only that reflected light (usually green or yellow) whose wavelength is about four times the thickness of the fluoride coating. Blooming of a microscope eyepiece lens reduces stray background light in the tube and improves image sharpness. blot (mol. biol.) See BLOTTING. blotch disease Syn. BROWN BLOTCH. blotting Following gel ELECTROPHORESIS: the transfer of nucleic acid and/or protein molecules from a gel strip to a specialized, chemically reactive paper, or other matrix, to which the transferred molecules may bind in a pattern similar to that present in the original gel. Transfer may be effected by capillary action, in which case the matrix (e.g. nitrocellulose, DEAE paper, APT PAPER) is sandwiched between the gel and a highly absorptive pad. Alternatively, in electroblotting, transfer is effected by electrophoresis. In the earliest (capillary) blotting, DNA was transferred to nitrocellulose in the eponymous Southern blot technique (see SOUTHERN BLOT; see also SOUTHERN HYBRIDIZATION); later, blotting of RNA (Northern blot) and protein (Western blot) was carried out. After blotting, a particular target molecule on the matrix may be identified or assayed e.g. by labelled probes or by enzyme immunoassay techniques. In immunoblotting, the transfer of proteins to a matrix is followed by exposure of the matrix to labelled antibodies; the binding of specific antibodies to a particular protein on the matrix may indicate the corresponding antigen. (See also SOUTHWESTERN BLOTTING.) blower Syn. SWELL. blown can Syn. SWELL. 103 blusher blusher Amanita rubescens. BLV Bovine leukosis virus: see BOVINE VIRAL LEUKOSIS. BOD (biochemical oxygen demand; biological oxygen demand) The amount of dissolved oxygen needed for microbial oxidation of soluble, biodegradable matter in an aquatic environment. For example, when discharged to rivers etc., sewage effluents may consume large amounts of dissolved oxygen, and if dilution is inadequate they may give rise, locally, to anaerobic or microaerobic conditions (i.e. exert a high BOD); in such cases there is a risk of asphyxiation for fish and other organisms dependent on appropriate levels of dissolved oxygen. The BOD test measures the amount of oxygen (mg) consumed per litre of (e.g.) sewage, or a known dilution of it, in 5 days at 20° C. (See also SEWAGE TREATMENT.) BOD test See BOD. Bodanella See PHAEOPHYTA. Bodo A genus of protozoa (suborder BODONINA); species occur e.g. in organically polluted waters (see SAPROBITY SYSTEM). The cells are ovoid or tapered, sometimes flattened, ca. 5–20 µm in length; two flagella arise at the anterior end (near the cytostome), one being directed posteriorly. Bodonina A suborder of biflagellate protozoa (order KINETOPLASTIDA); some members are free-living, others are parasitic e.g. in the blood or gut of fish. Genera include BODO, Cryptobia, Rhynchomonas. body microflora Each of the various regions of the body typically harbours a characteristic MICROFLORA (sense 1); for microflora of the human body see e.g. EAR MICROFLORA, EYE MICROFLORA, GASTROINTESTINAL TRACT FLORA, GENITOURINARY TRACT FLORA, MOUTH MICROFLORA, RESPIRATORY TRACT MICROFLORA, SKIN MICROFLORA. body odour See SKIN MICROFLORA. boil (furuncle) A skin lesion due to infection of a hair follicle, sebaceous gland or skin lesion by (usually) Staphylococcus aureus. A tender red nodule becomes pus-filled and may either subside or break at the skin surface and drain. (cf. CARBUNCLE; FURUNCULOSIS (2).) boil disease (in fish) See MYXOBOLUS. Boivin antigens Syn. O ANTIGENS. Bolbitiaceae See AGARICALES. Boletales An order of fungi (subclass HOLOBASIDIOMYCETIDAE) that typically form mushroom-shaped basidiocarps in which the hymenophore lines a mass of vertically orientated tubules on the underside of the pileus (porous basidiocarp) or forms a layer on lamellae; the porous basidiocarps are fleshy (cf. APHYLLOPHORALES). Genera include e.g. Boletus (basidiocarp porous, stipe central), Paxillus (lamellate), and Phylloporus (lamellate). Some species are edible – e.g. Boletus edulis (cèpe or penny bun), some form mycorrhizal associations with trees, and one species, Boletus parasiticus, is parasitic on Scleroderma. [Classification and spore structure: TBMS (1981) 76 103–146.] Boletus See BOLETALES. Boletus virus See POTEXVIRUSES. Bolivian haemorrhagic fever A VIRAL HAEMORRHAGIC FEVER caused by the Machupo virus (see ARENAVIRIDAE). Mortality: usually ca. 5–30%. Bombyx mori NPV See NUCLEAR POLYHEDROSIS VIRUSES. bongkrek See TEMPEH. bongkrekic acid A toxic, substituted heptaenedioic acid produced by ‘Pseudomonas cocovenenans’ in spoiled bongkrek (see TEMPEH); it inhibits (uncompetitively) the ADP/ATP exchange carrier system which operates across the inner mitochondrial membrane. (cf. ATRACTYLOSIDE.) booster (immunol.) A second or subsequent dose of a VACCINE administered with the object of eliciting a higher rate of antibody formation (see ANAMNESTIC RESPONSE). boot laces See ARMILLARIA. boracic acid Syn. BORIC ACID. Bordeaux mixture (plant pathol.) An antifungal preparation first used in Bordeaux in the late 19th century for the treatment of downy mildew of vines; it has since been widely used as an agricultural antifungal agent against various blights, mildews, rusts and leaf-spot diseases, but is now used less frequently owing to its phytotoxicity. It is prepared by mixing a solution of copper sulphate with an aqueous suspension of calcium hydroxide, and appears to contain a mixture of basic copper sulphates (CuSO4 . xCu(OH)2 ). The freshly-prepared mixture, sprayed onto leaves, stems etc, forms an insoluble deposit which gradually releases COPPER ions as a result of the influence of e.g. atmospheric carbon dioxide and/or plant or fungal secretions. border disease (hairy shaker disease) A congenitally acquired SHEEP DISEASE characterized by an abnormally hairy birth-coat, tremors, and poor growth; it is caused by a PESTIVIRUS. [Review: Adv. Vet. Med. (1982) 36 (90 pp.).] Bordet–Gengou agar An agar-based medium, used for the primary isolation of Bordetella spp (especially B. pertussis), containing glycerol, soluble starch, and 20–30% fresh horse or sheep blood; peptone (1%) may be added, but certain peptones inhibit the growth of Bordetella spp. Penicillin may be added to suppress growth of the nasopharyngeal flora. [Recipes: e.g. Book refs. 2, p. 132; 19, p. 145.] Bordetella A genus of aerobic, chemoorganotrophic, Gramnegative bacteria classified within the beta group of Proteobacteria; the organisms occur as parasites and pathogens in mammals. Cells: non-motile or peritrichously flagellated coccobacilli, ca. 0.2–0.5 × 0.5–2.0 µm. Metabolism: respiratory (oxidative). Carbon sources include e.g. various amino acids; carbohydrates are not used. Growth factors required include organic forms of nitrogen and sulphur. Optimum growth at 35–37° C. GC% ca. 66–70. Type species: B. pertussis. B. avium. Proposed name for bacteria causing coryza (rhinotracheitis) in turkey poults [IJSB (1984) 34 56–70]. B. bronchiseptica. Cells: motile. Growth can occur on peptoneagar, and may be visible after overnight incubation. Oxidase +ve. Catalase +ve. Urease +ve. This species occurs e.g. in dogs and other animals, and is sometimes a cause of respiratory disease in man and animals. B. parapertussis. Cells: non-motile. Growth on peptone-agar is characterized by the formation of a brown, soluble pigment. Improved growth may be obtained on Moredun medium (see WHOOPING COUGH). Oxidase −ve. Catalase +ve. Urease +ve. B. parapertussis is isolated from some cases of WHOOPING COUGH (q.v.). B. pertussis. The primary cause of WHOOPING COUGH (q.v.). Initial isolation is usually carried out by plating a pernasal swab on BORDET–GENGOU AGAR or (better) CHARCOAL BLOOD AGAR. The typical colony (ca. 1 mm after 3–6 days) is smooth and domed with a metallic (‘mercury drop’) appearance and a butyrous consistency. Growth is inhibited by unsaturated fatty acids or colloidal sulphur. Cells: non-motile. Oxidase +ve. Some strains are catalase +ve. Urease −ve. Citrate is not utilized. (For toxin production see WHOOPING COUGH.) (See also BVG GENES.) B. pertussis and B. parapertussis can be detected, simultaneously, by a duplex PCR [JCM (1999) 37 606–610]. boric acid (boracic acid) (as an antimicrobial agent) Boric acid (H3 BO3 ) has mild antimicrobial properties, and is widely used 104 botulinum toxin botches disease Synonym for freshwater RED PEST. bothrosome See LABYRINTHULAS and THRAUSTOCHYTRIDS. boticins See BACTERIOCIN. Botrydium A genus of siphonaceous algae (class XANTHOPHYCEAE) in which the mature vegetative organism consists of a globose to elongate (typically pear-shaped) coenocytic vesicle, up to ca. 2 mm diam., anchored to the substratum (mud, damp soil, etc) by colourless branched rhizoids; inside the vesicle is a thin peripheral layer of cytoplasm (containing the chloroplasts and nuclei) surrounding a large central vacuole. Asexual reproduction occurs by the formation of zoospores or aplanospores; resistant cysts are formed under dry conditions. Botryobasidium See APHYLLOPHORALES (Corticiaceae). botryomycosis (‘bacterial pseudomycosis’) A chronic localized infection of the skin and subcutaneous tissues in man and animals. The pathogen (commonly e.g. Pseudomonas aeruginosa, Staphylococcus aureus, or Escherichia coli ) forms one or more granules in the centre of an abscess which is surrounded by fibrous tissue. (cf. MADUROMYCOSIS.) Botryotinia See HELOTIALES. botrytis (plant pathol.) Syn. GREY MOULD. Botrytis A genus of fungi of the class HYPHOMYCETES; many species are known to have teleomorphs in the genus Botryotinia. The organisms include many plant-pathogenic species – e.g. B. aclada (= B. allii ) (see ONION ROT), B. cinerea (see GREY MOULD, NOBLE ROT), and B. fabae (see CHOCOLATE SPOT). (See also SOFT ROT sense 2.) Antifungal agents used against Botrytis infections include e.g. BENOMYL, DICARBOXIMIDES, DICHLOFLUANID, DICLORAN, THIRAM and ZINEB. Botrytis spp form septate mycelium; dark, ovoid, non-septate conidia develop in a layer on the swollen, terminal region of each of the small branches of a conidiophore. Sclerotia are formed e.g. by B. aclada, B. cinerea and B. tulipae. [Book ref. 183.] bottom-fermenting yeast See BREWING. bottromycins A group of peptide antibiotics, produced by Streptomyces spp, which inhibit bacterial protein synthesis. botulinolysin See THIOL-ACTIVATED CYTOLYSINS. (cf. BOTULINUM TOXIN.) botulinum C2 toxin A powerful toxin, produced by certain strains of Clostridium botulinum, which apparently has no neurotoxic activity (cf. BOTULINUM TOXIN) – but which causes e.g. increased intestinal secretion. The C2 toxin consists of two components, the larger of which appears to be involved in cellsurface binding and (possibly) uptake of the smaller component. The smaller component effects ADP-RIBOSYLATION of an ATPbinding ACTIN molecule, G actin, preventing its polymerization to F actin; this disrupts the cell’s microfilament cytoskeleton and leads e.g. to rounding of the cell. botulinum cook In the food processing industry: COOKING equivalent to a 12D PROCESS in respect of the endospores of Clostridium botulinum. The D121.1 value (see D VALUE) of C. botulinum endospores is generally taken to be ca. 0.21 min. (See also CANNING.) botulinum toxin Any of a range of related, powerful protein toxins (MWt ca. 150000) produced mainly by strains of Clostridium botulinum. Botulinum toxins are differentiated antigenically into types A–G, with type C being divided into subtypes C1 and C2. Toxins of types A–F – with the exception of BOTULINUM C2 TOXIN – are powerful neurotoxins which cause BOTULISM in man and animals. The (plasmid-encoded) botulinum toxin G has been isolated e.g. from autopsy material but has not been linked definitively with disease. The botulinum C and D toxins are encoded by genes from two different bacteriophages. as an ANTISEPTIC for delicate tissues (e.g. eyes) and e.g. as a bacteriostatic agent in urine samples. It is sometimes used, in combination with borax (Na2 B4 O7 ), as a wood preservative. Borna disease A disease of horses (and rarely sheep) caused by the BORNA DISEASE VIRUS. Symptoms in the horse include fever, pharyngeal paralysis and hyperaesthesia; death usually occurs in 1–3 weeks. Borna disease virus (BDV) can cause behavioural abnormalities (with aggressive and passive phases) in certain animals, and it has been suggested that BDV could be responsible for certain psychiatric disorders in humans [Arch. Gen. Psych. (1985) 42 1093–1096]; the involvement of BDV in psychiatric disease remains controversial, but immunological and PCR-based studies for BDV in psychiatric patients and blood donors in Japan have suggested that certain individuals are infected with BDV or with a BDV-related virus [JCM (2001) 39 419–429]. Borna disease virus (BDV) A neurotropic virus (family Bornaviridae) which is the causal agent of BORNA DISEASE; BDV has a non-segmented, negative-sense ssRNA genome and appears to be a member of the MONONEGAVIRALES. [The coding srategy of BDV: Virol. (1995) 210 1–8.] Bornholm disease (epidemic myalgia; devil’s grip) A self-limiting, epidemic disease usually caused by a group B coxsackievirus; other enteroviruses (e.g. coxsackieviruses A4, A6, A9, A10, and echoviruses 1 and 6) have sometimes been implicated. Onset is abrupt, with spasmodic, often severe chest pain (pleurodynia) and a brief period of high fever. Borrelia A genus of Gram-negative bacteria (family SPIROCHAETACEAE) which occur as parasites or pathogens in man and other animals (see e.g. RELAPSING FEVER). The cells are motile, helical (3 to 10 loose coils), ca. 0.2–0.5 × 3.0–20 µm, each containing up to 20 periplasmic flagella; they stain well with conventional dyes, and are visible by light microscopy. Some species (including B. recurrentis) have been grown in vitro; cultured species require complex media and are microaerophilic. Species were differentated by their arthropod vectors: e.g. human-louse-borne borreliae B. recurrentis, tick-borne borreliae according to tick species. Type species: B. anserina. The 19 species recognized in Book 22 (pages 57–62) included B. anserina, B. brasiliensis, B. duttonii, B. graingeri, B. harveyi, B. hermsii, B. parkeri, B. persica, B. recurrentis, B. theileri and B. venezuelensis. B. burgdorferi (sensu lato) comprises B. burgdorferi (sensu stricto) and the species afzelii, andersonii, bissettii, garinii, japonica, lusitaniae, tanukii, turdi and valaisiana [Molecular typing of B. burgdorferi sensu lato. Clinical Microbiology Reviews (1999) 12 633–653]. B. burgdorferi [IJSB (1984) 34 496–497; genome: Nature (1997) 390 580–586] causes LYME DISEASE. This organism can be grown in BSK (Barbour–Stoenner–Kelly) II medium [recipe: Book. ref. 219, pp. 570–571], although diagnosis is more often serological (confirmation of a clinical diagnosis by detection of a rising titre of specific IgM during the 3rd–6th week following first symptoms e.g. by an indirect immunofluorescent antibody test). Quantification of B. burgdorferi in tissue samples has been achieved with a PCR-based method [JCM (1999) 37 1958–1963]. The outer surface protein C (OspC) of B. burgdorferi is possibly involved in the binding of the pathogen to host ligands, and this protein has been examined in some detail [crystal structure of OspC: EMBO (2001) 20 971–978]. borreliosis Any disease of man or animals caused by a Borrelia sp: see e.g. RELAPSING FEVER and LYME DISEASE. Bostrychia See RHODOPHYTA. 105 botulism Wound botulism is a rare form of the disease resulting from contamination of a wound by a toxigenic strain of the pathogen. Incubation period ∼4–14 days. Symptoms are similar to those of food-borne botulism, sometimes with the addition of fever. Infant botulism can follow ingestion of spores of the pathogen which germinate and form toxin in the gut (= toxicoinfection). The disease affects mainly infants <6 months of age, and varies from subclinical to rapidly fatal; typical symptoms: e.g. constipation, lethargy, dysphagia and weakness. One source of infection was reported to be honey contaminated with C. botulinum spores [JPed (1979) 94 331–338]. Infant botulism may be responsible for some cases of SUDDEN INFANT DEATH SYNDROME; sudden death may be due to weakening or flaccid paralysis of muscles of the tongue and pharynx, leading to obstruction of the airway and asphyxia. Botulism of undetermined classification includes those cases in individuals, older than 1 year, in which ‘a specific food cause cannot be implicated’. Botulism in an adult may be due to the synthesis of toxin within the intestine during a gut infection with the pathogen [NEJM (1986) 315 239–241; Lancet (1987) i 357–360; (C. barati ) JCM (2002) 40 2260–2262]. Lab. diagnosis (all forms): assay of serum, faeces and implicated foods for botulinum toxin and organisms. Treatment. Any unabsorbed toxin should be removed from the gut. Polyvalent antitoxin may be administered (avoiding possible hypersensitivity reactions to heterologous components). Guanidine, given orally, may counteract the effects of toxin at the nerve–muscle junction. Supportive care will be needed. (A prophylactic botulinum toxoid vaccine has been given to laboratory personnel working with C. botulinum, but such a vaccine is unlikely to be useful for treatment owing to the time required to develop an immune response.) [Botulism (epidemiology, diagnosis, treatment): RMM (1995) 6 58–62.] (b) Botulism in animals. In cattle, the disease may be caused by C. botulinum types B, C or D (see e.g. LAMZIEKTE and MIDLAND CATTLE DISEASE). In horses, types B or C may be involved (see e.g. FORAGE POISONING). Birds may be affected by types A or C (see e.g. LIMBERNECK and WESTERN DUCK DISEASE). In farmed salmonid fish the disease has been caused by type E [J. Fish Dis. (1982) 5 393–399]. In animals, the disease is characterized by progressive paralysis without loss of sensibility and without fever. Boucherie process A method of TIMBER PRESERVATION in which the sap of newly felled trees is replaced by a preservative (under pressure) in order to prevent subsequent fungal decay of the timber. (cf. SAUG–KAPPE PROCESS; BETHELL PROCESS.) Bouin’s fluid A FIXATIVE: a mixture of picric acid (75 ml saturated, aqueous), formalin (25 ml), and glacial acetic acid (5 ml). bound coagulase See COAGULASE. bourbon See SPIRITS. boutonneuse In man: a mild disease caused by Rickettsia conorii and transmitted by tick bite. After an incubation period of 1–2 weeks a characteristic lesion (the tâche noir ) develops at the location of the bite; this lesion (up to 0.5 cm diam.) consists of a black necrotic centre and a reddened margin. The regional lymph nodes may be affected. Fever, headache and muscular pains are typically followed by a maculopapular rash. Diagnosis may include the WEIL–FELIX TEST. Boveria See SCUTICOCILIATIDA. Boverin See BEAUVERIA. bovid herpesvirus See under bovine herpesvirus. All of these toxins can be produced by strains of C. botulinum. Botulinum toxin type F has also been linked with an organism resembling Clostridium barati, associated with infant botulism [JCM (1985) 21 654–655]. [See also JCM (2002) 40 2260–2262.] A toxin similar to the botulinum type E toxin is produced by strains of C. butyricum isolated from cases of infant botulism. The ability to form type C toxin can apparently be transferred, by phage, from C. botulinum type C to C. novyi. All the botulism-causing toxins are similar in structure and mode of action. Proteolytic cleavage into two fragments – a heavy (H) chain and a light (L) chain (connected by a disulphide bond) – activates the toxin; cleavage is mediated by a bacterial protease or an exogenous protease. The activated toxin binds, via the H chain, to the cell membrane of motor neurones at the nerve–muscle junction; toxin is internalized within a vesicle that remains at the nerve–muscle junction. The active part of the toxin (L chain) is translocated from the vesicle to the neurone’s cytoplasm. Within the cytoplasm, the toxin acts as a zinc-endopeptidase in a way similar to that of TETANOSPASMIN (q.v. for essential details); in this case, however, the toxin blocks neuroexocytosis of vesicles containing the neurotransmitter acetylcholine – i.e. it prevents nervous stimulation of the muscle, leading to a flaccid paralysis. (Compare the site and mode of action of TETANOSPASMIN.) Botulinum toxins differ in their targets: the A and E toxins cleave (inactivate) SNAP-25 (see TETANOSPASMIN); the C toxin cleaves syntaxin; B, D, F and G toxins cleave VAMP/synaptobrevin. [Mechanism of action of botulinum neurotoxins: Mol. Microbiol. (1994) 13 1–8.] Purified botulinum toxin is heat-labile, being inactivated e.g. by heating at 100° C/10 minutes. Botulinum toxin has been used as an alternative to surgery for treating some muscular disorders of the eye (strabismus; = ‘squint’) [Br. J. Ophth. (1985) 69 718–724, 891–896]. [Uses of botulinum and other microbial neurotoxins: MR (1992) 56 80–99. Therapy with botulinum toxin: Book ref. 220. Botulinum toxin for spastic gastrointestinal disorders: BCG (1999) 13 131–143.] botulism A disease of man and animals caused by any strain of Clostridium (usually a strain of C botulinum) which forms a (neurotoxic) BOTULINUM TOXIN (q.v.). (a) Botulism in man. The disease is commonly caused by C. botulinum types A, B or E, but may be caused by any neurotoxigenic strain. ‘Classical’ botulism is a food-borne intoxication which follows ingestion of foods contaminated with pre-formed toxin – although additional toxin may be formed in the gut if viable cells of the pathogen are ingested. In the mouse, the LD50 of botulinum toxin is reported to be of the order of 0.1–1.0 ng/kg. Foods of a low-acid, low-salt type are often implicated. C. botulinum type A may be associated with e.g. improperly canned vegetables, fish and meat products (Latin botulus = sausage), types B and F with e.g. meat products, and type E with e.g. uncooked sea-foods. Toxin is absorbed from the gut and disseminated via the circulation. The incubation period is typically 12–36 hours, but may be ∼2–8 days. The symptoms may include nausea, blurred/double vision (due to effects of toxin on the oculomotor muscles), dysphagia, dysphonia etc.; there is a progressive flaccid paralysis, and death may follow from asphyxia or cardiac failure. 106 Boyden procedure BSE prion converts human PrP to the prion form in a cell-free system [Nature (1997) 388 285–288]. [Review of BSE: RMM (1998) 9 119–127.] [Cumulative incidence of BSE in the United Kingdom during the period July 1986–June 1997, animal-associated risk factors influencing age of onset of clinical signs, and effectiveness of control measures: VR (2000) 147 349–354.] (See also CATTLE DISEASES.) bovine syncytial virus See SPUMAVIRINAE. bovine tubercle bacillus Mycobacterium bovis. bovine typhus Syn. RINDERPEST. bovine ulcerative mammillitis (bovine herpes mammillitis) A CATTLE DISEASE caused by bovine (alpha) herpesvirus 2 (bovine mammillitis virus – see ALPHAHERPESVIRINAE). The disease involves severe inflammation and ulceration of the skin of the udder and teats; stomatitis and facial infection may also occur, particularly in calves. (See also LUMPY SKIN DISEASE and MASTITIS.) bovine viral leukosis (enzootic bovine leukaemia) A chronic transmissible CATTLE DISEASE caused by the bovine leukosis virus (BLV), an exogenous v-onc− retrovirus which is closely related to HTLV-I and HTLV-II (see HTLV). [Nucleotide sequence of the BVL genome and its relation to other retroviruses: PNAS (1985) 82 677–681.] Natural infection occurs e.g. in cattle (Bos taurus and Bos indicus), sheep, and water buffalo. The disease involves systemic malignant neoplasia of the reticuloendothelial system; in a small proportion of infected animals, malignant lymphomas (mainly of the B cell type) may develop in almost any organ, resulting in a variety of clinical manifestations. Transmission occurs horizontally by transfer of infected lymphocytes – most commonly via surgical instruments and needles (e.g. during herd vaccinations, blood-sampling, etc); natural transmission may occur via milk or colostrum or, in tropical countries, possibly via insects or parasites. [Reviews: Book refs 105, pp. 229–260, and 110, pp. 197–216.] bovine virus diarrhoea (BVD) A CATTLE DISEASE caused by the BVD-MD virus (see PESTIVIRUS). BVD may affect cattle (and other ruminants) at any age, and follows infection by ingestion or inhalation; the condition is usually mild and self-limiting. (cf. MUCOSAL DISEASE.) Bovista See LYCOPERDALES. bowel oedema Syn. OEDEMA DISEASE. Bowie–Dick test A test used to monitor the performance of an AUTOCLAVE. Essentially, two pieces of AUTOCLAVE TAPE are fixed in the form of an ‘X’ to a piece of paper which is inserted into the middle of a stack of cotton towels; the towels are autoclaved and the tape is examined. A satisfactory sterilizing regime is deemed to have been achieved only if all parts of the tape indicate exposure to sterilizing conditions. box-like bacteria Variously shaped, angular bacteria which are found in hypersaline environments and which contain bacteriorhodopsin-like pigments [JB (1982) 151 1532–1542]. (cf. SQUARE BACTERIA.) Boyden chamber A chamber, divided into two sections by a membrane filter, used e.g. to study macrophage migration; the filter initially separates activated T lymphocytes (or their supernatant) and macrophages – the latter subsequently migrating through the filter in response to macrophage chemotactic factor. Boyden procedure Treatment of erythrocytes with tannic acid (see TANNINS) to enable them to adsorb soluble antigens; such tanned cells are used e.g. in PASSIVE AGGLUTINATION. (Chromium salts or e.g. bis-diazobenzidine have been used in place of tannic acid.) bovine cytomegaloviruses See BETAHERPESVIRINAE. bovine diseases See CATTLE DISEASES. bovine ephemeral fever An infectious CATTLE DISEASE caused by a virus (a probable member of the Rhabdoviridae) and transmitted by insects; it occurs e.g. in Africa and Asia. Incubation period: ca. 2–10 days. Symptoms: e.g. fever, tremors, stiffness, and enlargement of the peripheral lymph nodes; mortality rates are low, but lactation is severely depressed. bovine farcy See FARCY (2). bovine farmers’ lung (vet.) A chronic ATYPICAL INTERSTITIAL PNEUMONIA of cattle; it occurs, infrequently, in housed adult cattle exposed to mouldy or dusty feedstuffs. Symptoms: coughing, dyspnoea, nasal discharge. (cf. FARMERS’ LUNG.) bovine herpesvirus 1 See HERPESVIRIDAE. bovine herpesvirus 2 See ALPHAHERPESVIRINAE. bovine leukosis virus See BOVINE VIRAL LEUKOSIS. bovine mammillitis virus See BOVINE ULCERATIVE MAMMILLITIS. bovine mastitis See MASTITIS. bovine papilloma virus See PAPILLOMAVIRUS. bovine parvovirus See PARVOVIRUS. bovine pustular stomatitis virus See PARAPOXVIRUS. bovine respiratory disease (BRD) A collective term which includes a range of acute respiratory diseases of diverse aetiology in which the symptoms may include e.g. anorexia, coughing, dyspnoea, nasal discharge, pneumonia; cattle of all ages may be affected, but young animals are generally more susceptible. Treatment generally includes broad-spectrum chemotherapy to combat primary or secondary bacterial pathogens. Specific BRDs include: pneumonic pasteurellosis (shipping fever), a disease which is spread by droplet infection, and which can occur with high incidence in young stressed cattle; symptoms: fever, acute bronchopneumonia and toxaemia. The causal agent is usually Pasteurella haemolytica, sometimes P. multocida. There is usually a good response to early chemotherapy, with recovery in 1–2 days. Bovine respiratory syncytial virus infection is an acute bronchiolitis and alveolitis which occurs in (usually) young animals and involves e.g. fever, cough and dyspnoea; the disease can be fatal. (See also ATYPICAL INTERSTITIAL PNEUMONIA; CALF PNEUMONIA; CONTAGIOUS BOVINE PLEUROPNEUMONIA; INFECTIOUS BOVINE RHINOTRACHEITIS.) bovine rhinotracheitis See INFECTIOUS BOVINE RHINOTRACHEITIS. bovine spongiform encephalopathy (BSE; mad cow disease) A TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHY of bovines caused by a PRION; it was reported in Great Britain in 1986. Clinical signs include e.g. nervousness, kicking and locomotor difficulty [clinical picture: VR (1992) 130 197–201]. In Great Britain there were 3.9 confirmed cases per 1000 adult animals up to the end of 1989. Epidemiological studies implicated the meat and bone meal in concentrate feeds as a source of the causal agent; dairy herds were found to be affected more than beef suckler herds, due apparently to different feeding practices. [Epidemiological features: VR (1992) 130 90–94; TIM (1997) 5 421–424.] Transmission of the BSE prion to humans, via the food chain, is now believed to cause nvCJD (variant, or new variant CREUTZFELDT–JAKOB DISEASE). This aetiological link is supported by various experimental data which have accumulated over the last few years. For example: intracerebral injection of monkeys with brain homogenates from affected cows yields clinically and pathologically similar CJD [Nature (1996) 381 743–744]; transgenic mice which express human PrP proteins react with identical clinical, biochemical and histological changes when exposed to BSE or nvCJD material [Nature (1997) 389 448–450]; the 107 bp brain fungus Sparassis crispa. brain–heart infusion medium A medium containing infusions of calf-brain and beef-heart, proteose, D-glucose, NaCl, and Na2 HPO4 ; it is available commercially in dehydrated form. The medium may be used as a broth or solidified with agar; it may be supplemented with e.g. haemin, menadione, yeast extract etc. It is used for the culture of a range of bacteria and medically important fungi. [Recipe: Book ref. 47, pp. 644–645.] branch migration (mol. biol.) See RECOMBINATION (figures 1 and 2). branched DNA assay See BDNA ASSAY. branching enzyme An enzyme which introduces branches into a linear polysaccharide (see e.g. GLYCOGEN and Q ENZYME). (cf. DEBRANCHING ENZYMES.) Branchiomyces A ‘genus’ of fungi of uncertain taxonomic affinity; strains of Branchiomyces have features in common with members of the SAPROLEGNIALES, but it is unknown whether they form heterokont zoospores or aplanospores [Book ref. 1, pp. 215–216]. The organisms have been isolated only from the gill tissues of fish (see GILL ROT). branchiomycosis Syn. GILL ROT. brand spores See USTILAGINALES. Brandenfleckenkrankheit Syn. BURNED SPOT DISEASE. brandy See SPIRITS. Branhamella See MORAXELLA. brassica black rot See BLACK ROT. brassica diseases See CRUCIFER DISEASES. Braun lipoprotein (murein lipoprotein) A small lipoprotein (MWt ca. 7500) which occurs e.g. in Escherichia coli (encoded by gene lpp) and in other enterobacteria; each cell contains ca. 105 –106 copies of the lipoprotein – about one-third of which form links (in association with OmpA) between the peptidoglycan layer and the OUTER MEMBRANE (thus stabilizing the cell envelope). Braun lipoprotein is synthesized as a precursor with a signal sequence (see SIGNAL HYPOTHESIS). The cysteine residue immediately adjacent to the signal peptide cleavage site is modified by substitution with a diacylglycerol, permitting cleavage by the signal peptidase; following cleavage, a fatty acid residue is added to the N-terminal cysteine. Such post-translational modification apparently targets the protein to the outer membrane. [Cell (1990) 61 739–741.] The activity of the signal peptidase is inhibited by the peptide antibiotic globomycin (a structural analogue of the cleavage site). Braun MSK tissue disintegrator An apparatus for CELL DISRUPTION. The sample, together with glass or plastic beads, is placed in a cooled container which is shaken at several thousand oscillations per minute for ca. 5 minutes; cells are broken by shear forces. The apparatus can break e.g. Gram-positive bacteria and endospores. braxy (bradsot) An acute, usually fatal disease of lambs and young sheep; symptoms: fever, anorexia, inflammation of the wall of the abomasum, and toxaemia. Braxy occurs only in winter and appears to be due to the invasion of the abomasum by Clostridium septicum – the ingestion of frozen grass being a possible predisposing factor. Death occurs within hours. BRD BOVINE RESPIRATORY DISEASE. BrdU Syn. BUDR. bread-making Bread is prepared by mixing flour (usually wheat flour), water, BAKERS’ YEAST and salt to form a dough; other ingredients (e.g. milk, sugar, shortening etc) may be added. After thorough mixing the dough is left to ‘prove’ at ca. 25° C; bp BASE PAIR. Bph BACTERIOPHAEOPHYTIN. bph operon See BIOREMEDIATION BPI protein (bactericidal/permeability-increasing protein) A component of the azurophil granules in human and rabbit NEUTROPHILs; it is bactericidal for some Gram-negative bacteria – in which it increases the permeability of the OUTER MEMBRANE – but it is not active against e.g. POLYMYXIN-resistant enteric bacteria (such as Proteus) or Gram-positive bacteria [MR (1992) 56 399]. BPI binds to LPS and can apparently neutralize endotoxicity, and it may also attenuate the inflammatory response [see e.g. RMM (1995) 6 101–108]. BPL b-PROPIOLACTONE. BPV Bovine papilloma virus: see PAPILLOMAVIRUS. Brabant mastitis test See CALIFORNIA MASTITIS TEST. Brachiomonas A genus of unicellular flagellated green algae which are closely related to CHLAMYDOMONAS. The vegetative cells have a distinctive morphology: the posterior end is pointed, and four backward-directed pointed projections extend laterally from the cell body. Motile ‘autospores’ are formed (the daughter cells acquiring the distinctive adult-cell morphology before being released). Brachonella See HETEROTRICHIDA. Brachyallomyces See ALLOMYCES. Brachyarcus A proposed genus of gas-vacuolated, curved rodshaped bacteria which occur, in groups (coenobia), in microaerobic or anaerobic sulphide-containing aquatic habitats. [Book ref. 22, pp. 137–138.] Brachybasidiales An order of plant-endoparasitic fungi (subclass HOLOBASIDIOMYCETIDAE) which form bisterigmate basidia either in definite hymenia or in tuft-like fascicles. Brachybasidium forms non-septate basidiospores on tufts of basidia which emerge from the host plant’s stomata. Members of the other genera form true hymenia – which emerge through the host’s epidermis and stomata – and basidiospores which sometimes become septate. In Dicellomyces, the basidiocarp is gelatinous (when fresh) and the hymenium is of uniform thickness. In Ceraceosorus and Proliferobasidium the basidiocarp is waxy, and the hymenium is not of uniform thickness; Ceraceosorus forms intracellular hyphae, Proliferobasidium forms intercellular hyphae [Mycol. (1976) 68 640–654]. (cf. EXOBASIDIALES.) Brachybasidium See BRACHYBASIDIALES. bracket fungi (shelf fungi) Those fungi of the APHYLLOPHORALES (e.g. Piptoporus betulinus) whose fruiting bodies jut out like brackets or shelves from infected trees or rotting logs etc. bradsot Syn. BRAXY. Bradyrhizobium A genus of Gram-negative bacteria of the RHIZOBIACEAE; strains were formerly included in the genus RHIZOBIUM as ‘slow-growing’ strains (colonies ≤1 mm diam. in 5–7 days on e.g. yeast extract–mannitol–mineral salts agar). Bradyrhizobium spp produce an alkaline reaction in media containing carbohydrate (e.g. mannitol) and mineral salts. Extracellular slime includes a neutral (1 → 2)-b-glucan. Some strains can grow chemolithotrophically with H2 , CO2 and low levels of O2 ; some can fix nitrogen under microaerobic conditions in media which contain e.g. a source of fixed nitrogen. Species induce ROOT NODULE formation in certain leguminous plants (mostly tropical, some temperate) – e.g. soybeans, cowpea, siratro, certain Lotus spp. One non-leguminous plant, Parasponia (Ulmaceae), is also nodulated by a Bradyrhizobium strain. GC%: 61–65. Type species: B. japonicum (formerly Rhizobium japonicum). [Book ref. 22, pp. 242–244.] bradyzoite Syn. CYSTOZOITE. 108 brewing brevetoxins Polycyclic toxins produced by the dinoflagellate Ptychodiscus brevis (see RED TIDE). Brevetoxins A, B and C each consist of 11 contiguous ether rings; they are highly lipidsoluble and are potent neurotoxins, causing depolarization of nerve membranes. [PAC (1986) 58 339–350 (346–347).] Brevibacterium A genus of obligately aerobic, catalase-positive, asporogenous bacteria (order ACTINOMYCETALES, wall type III – see also PEPTIDOGLYCAN) which occur e.g. on certain cheeses. In culture the organisms initially grow as irregular, ‘lumpy’ rods, but coccoid forms predominate in the stationary phase; colonies vary from yellow to orange-red, though some strains produce pigment only in the light. Growth occurs at ca. 20–25° C e.g. on trypticase–soy agar containing 4% sodium chloride; metabolism is oxidative. GC%: ca. 60–64. Type species: B. linens. Brewer jar An ANAEROBIC JAR which consists of a cylindrical glass chamber and a flat, gas-tight lid which is secured by means of a screw clamp. In use, the jar is filled with e.g. a mixture of N2 (85%), CO2 (5%) and H2 (10%), and any residual O2 is eliminated by means of an electrically-heated platinized catalyst which is fitted inside the lid and which is separated from the interior of the jar by a gauze safety screen. Alternatively, the jar may be used for the evacuation–replacement method as described for the MCINTOSH AND FILDES’ ANAEROBIC JAR. Anaerobiosis within the jar is monitored e.g. with a tube containing a METHYLENE BLUE solution. Brewer’s thioglycollate medium A liquid medium (pH 7.0–7.2) used e.g. for the culture of ANAEROBES; it includes e.g. glucose, tryptone, agar (ca. 0.1%), sodium thioglycollate (a poising agent – see REDOX POTENTIAL), and a redox indicator dye. [Recipe: Book ref. 53, pp. 1409–1410.] The medium is particularly useful for testing the sterility of products which contain mercurial preservatives – the antimicrobial activity of the latter being neutralized by the thioglycollate. brewers’ yeast Any of various strains of Saccharomyces cerevisiae used for BREWING (q.v.); ‘bottom-fermenting’ strains (formerly called Saccharomyces uvarum or S. carlsbergensis: see SACCHAROMYCES) are usually used in the manufacture of lagers, while ‘top-fermenting’ strains are usually used for ales. (Topand bottom-fermenting strains can generally be distinguished by the ability of bottom yeasts to produce an extracellular melibiase (a-galactosidase) and thus to hydrolyse raffinose to glucose, galactose and fructose, whereas top yeasts – which do not produce melibiase – hydrolyse raffinose to melibiose and fructose.) Particular strains of yeast may be selected for particular properties (e.g. flavour production), but all brewers’ yeasts must be able to ferment the sugars (particularly maltose and maltotriose) present in wort, and must be able to tolerate the initially high levels of solutes in wort and produce – and tolerate – high levels of ethanol under brewing conditions; the yeast must also be readily separable from the fermented wort (see BREWING) and should retain high viability for re-use in subsequent fermentations. [Selection and modification of brewers’ yeast strains: Food Mic. (1984) 1 289–302.] brewing The preparation of beer or lager by the fermentation of an aqueous extract of malted barley containing essential oils and bitter resins of the dried female flowers (‘cones’) of the hop (Humulus lupulus). During malting the barley is allowed to germinate, and the grain starch and protein are partially degraded by endogenous enzymes. Germination is halted by kilning: drying (e.g. to stop respiration, preserve enzymes and substrates etc), followed by heating (e.g. to ca. 80° C). The dried malt is ground (milled) to expose the starchy endosperm; the during this time the yeast metabolizes sugars (released from the flour starches by AMYLASES present in the flour) and produces bubbles of CO2 which become trapped in an elastic complex of flour proteins (gluten) – lightening the texture of the dough. (Fungal amylases may be added to increase the availability of fermentable sugars (see also PROTEASES).) In addition to its leavening (lightening) action, the yeast contributes flavour to the product and helps to modify the gluten structure. When the leavened dough is baked the yeast is usually killed and any alcohol produced by fermentation evaporates. Sourdough breads are made by using a starter consisting of flour (usually rye) and water inoculated with dough from a previous batch. In the starter, heterofermentative lactic acid bacteria (Lactobacillus sanfrancisco) ferment maltose (released from the starch by amylase action) and produce lactic and acetic acids, ethanol and CO2 ; the pH falls to e.g. ca. 3.8. The acid-tolerant yeast Saccharomyces exiguus, which does not use maltose, uses other sugars (glucose, fructose etc) to leaven the bread. The starter may then be mixed with flour, water and salt to make a dough as with ordinary bread. [Gluten biochemistry: Biotechnology (1995) 13 1185–1190.] bread mould See BREAD SPOILAGE. bread spoilage Bread and similar products are very susceptible to spoilage by the growth of moulds, especially Rhizopus nigricans (black bread mould), Neurospora sitophila (red bread mould), and species of Penicillium and Aspergillus. Preservatives which may be incorporated in the dough include e.g. propionates, sorbic acid, and potassium sorbate. (See also ROPINESS; ERGOTISM.) breakpoint chlorination See WATER SUPPLIES. breakage-and-reunion model See RECOMBINATION. breakbone fever Syn. DENGUE. breaking (plant pathol.) Syn. COLOUR-BREAKING. Breda virus An RNA virus which causes diarrhoea in young calves; it is apparently related to the BERNE VIRUS (see TOROVIRIDAE). [Morphological study of Breda virus replication: JGV (1986) 67 1293–1304.] brefeldin A A fungal toxin which inhibits the activity of the Golgi apparatus and which, as a consequence, can block certain processes in various types of cell; for example, brefeldin A can block protein secretion and inhibit the development of viruses. Breinl strain A virulent strain of Rickettsia prowazekii ; unlike the attenuated E strain it can be cultivated in human macrophage cultures. Bremia A genus of fungi (order PERONOSPORALES) which cause DOWNY MILDEWS on members of the Compositae (e.g. B. lactucae on lettuce). [B. lactucae –lettuce interaction: Book ref. 174, pp. 116–118.] Bremiella See PERONOSPORALES. Brettanomyces A genus of yeasts (class HYPHOMYCETES) which form single spheroidal, cylindrical or elongated budding cells, pseudomycelium, or branched non-septate mycelium. Growth occurs slowly e.g. on malt agar; species may be isolated on cycloheximide-containing malt agar. Most strains can ferment glucose; fermentation is usually stimulated by O2 and acetoin (‘Custers effect’, ‘negative Pasteur effect’). Under aerobic conditions acetic acid is produced from glucose and from ethanol. Only some species can assimilate NO3 − . Species: B. abstinens, B. anomalus, B. bruxellensis (teleomorph: Dekkera bruxellensis), B. claussenii, B. custersianus, B. custersii, B. intermedius (teleomorph: Dekkera intermedia), B. lambicus, B. naardenensis. (‘B. sphaericus’ = Candida etchellsii; ‘B. versatilis’ = Candida versatilis.) Species occur in beers, wines (see WINE SPOILAGE), grape must etc. [Book ref. 100, pp. 562–576.] 109 brick cheese resulting grist is made into a mash by mixing with water. Other carbohydrate sources (adjuncts) may be added at this stage. The mash (pH 5.2–5.5) is subjected to controlled (variable) temperature–time cycles during which starch is broken down to fermentable sugars (glucose, maltose, maltotriose) and nonfermentable dextrins, proteins are degraded to amino acids and peptides, and phytin is hydrolysed to inorganic phosphate (which acts as a buffer) and inositol (an essential yeast growth factor). The liquor (wort) is separated from the spent grain and is boiled (with hops) to inactivate enzymes, coagulate proteins, destroy spoilage organisms etc. The hopped wort is cooled, aerated, and inoculated (seeded or pitched ) with a pure culture of yeast: a strain of Saccharomyces cerevisiae (see BREWERS’ YEAST). A lag period of ca. 12 h is followed by growth of the yeast. As oxygen is used up, metabolism switches from respiratory to fermentative, and sugars in the wort are converted to ethanol and CO2 primarily via the EMBDEN–MEYERHOF–PARNAS PATHWAY. (Fermentation is exothermic, so the fermentation vessel must be cooled.) The formation of bubbles of CO2 keeps the wort agitated and the yeast cells in suspension. At the end of the fermentation the yeast cells clump together – see FLOCCULATION (sense 2); in ‘top-fermenting’ strains of S. cerevisiae the clumps rise to the surface of the liquor, while those of ‘bottomfermenting’ strains sediment. Most of the yeast is removed (e.g. by surface skimming of top yeasts or collection of sediment for bottom yeasts, and/or by centrifugation), and the beer is matured at low temperature (e.g. −1° C); residual yeast cells (a) carry out a ‘secondary fermentation’ of remaining maltotriose, and (b) reduce levels of diacetyl. H2 S and acetaldehyde may be removed by purging with CO2 . Finally, the beer is filtered, bottled or canned, and pasteurized. (See also BEER SPOILAGE.) brick cheese See CHEESE-MAKING. Brie cheese See CHEESE-MAKING. bright-field microscopy See MICROSCOPY (a). brightener See FLUORESCENT BRIGHTENER. Brill–Zinsser disease See TYPHUS FEVERS. brilliant green A yellowish-green TRIPHENYLMETHANE DYE which contains ethyl-substituted amino groups. It has antibacterial activity, inhibiting aerobic spore-formers more than anaerobic spore-formers. brilliant green agar A selective medium used for salmonellae which have a broad host range (e.g. S. typhimurium); it contains e.g. lactose, phenol red, and brilliant green (selective agent). Lactose +ve colonies are green, lactose −ve colonies are pink. brinase A fibrinolysin-like PROTEASE, obtained from Aspergillus oryzae, which can hydrolyse fibrin and fibrinogen and can lyse blood clots. (cf. STREPTOKINASE.) British anti-Lewisite See BAL. brittleworts See CHAROPHYTES. broad bean mottle virus See BROMOVIRUSES. broad bean wilt virus A virus which may be a member of the COMOVIRUSES but which has a wide host range and is transmitted by aphids. broad-range primers Primers, used in PCR, which bind to highly conserved sequences found in the 16S rRNA gene in all bacteria; such primers can therefore be used to detect any species of bacterium whose 16S rRNA gene is accessible. Broad-range primers are also referred to as universal primers. Broad-range primers are used e.g. to study bacterial diversity in environmental samples; the products are sequenced, and variable regions within the amplicon (which include speciesspecific sequences) are checked against the 16S rRNA database of known species in order to attempt identification of bacteria within the sample. Some PCR-based studies use broad-range primers (simultaneously with other primers) as an amplification control [see e.g. JMM (1997) 46 773–778; JCM (1999) 37 2090–2092]. broad-spectrum antibiotics (1) ANTIBIOTICS active against a wide range of bacteria – including both Gram-positive and Gram-negative species. (2) Antibiotics active against a wide range of bacteria which may include Gram-positive species, Gram-negative species, or both. broccoli necrotic yellows virus See RHABDOVIRIDAE. Brochothrix A genus of Gram-positive, facultatively anaerobic bacteria; type species: B. thermosphacta (formerly Microbacterium thermosphactum) [IJSB (1976) 26 102–104]. B. thermosphacta is common e.g. in meat and meat products (see MEAT SPOILAGE). In young cultures cells are rod-shaped (occurring in chains), in older cultures they are small coccobacilli. Growth conditions: 0–30° C (optimum ca. 23–25° C); pH 5–9 (optimum ca. 7.0); all strains can grow in the presence of 6.5% NaCl, many in 10% NaCl. Tests: MR +ve; VP +ve; indole −ve; H2 S −ve; nitrate is not reduced, catalase +ve e.g. on APT agar at 20° C, usually catalase −ve at 30° C. During aerobic growth B. thermosphacta forms cytochromes and can use glycerol or a range of carbohydrates as energy sources. Anaerobically, cytochromes are not formed, and glucose is fermented primarily to L-lactic acid, acetic acid, formic acid, and ethanol in proportions that vary with growth conditions [AEM (1983) 45 84–90]. In meat under aerobic conditions B. thermosphacta uses glucose and, when glucose is depleted, glutamate; under anaerobic conditions only glucose can be used. (See also STAA MEDIUM.) [Review: Book ref. 29, pp. 139–173.] broken cream See MILK SPOILAGE. bromatia See FUNGUS GARDENS. bromcresol green A PH INDICATOR: pH 3.8–5.4 (yellow to green); pKa 4.7. bromcresol purple A PH INDICATOR: pH 5.2–6.8 (yellow to purple); pKa 6.3. brome mosaic virus See BROMOVIRUSES. bromelain A basic glycoprotein thiol PROTEASE (EC 3.4.22.4) obtained from the pineapple plant. bromoaplysiatoxin See LYNGBYA. 5-bromo-4-chloro-3-indolyl-b-D-galactoside See XGAL. 5-bromo-4-chloro-3-indolylphosphate-p-toluidine See XP. bromochlorophane See BISPHENOLS. bromocresol green Syn. BROMCRESOL GREEN. bromocresol purple Syn. BROMCRESOL PURPLE. 5-bromodeoxyuridine See BUDR. bromophenol blue Syn. BROMPHENOL BLUE. bromothymol blue Syn. BROMTHYMOL BLUE. 5-bromouracil (BU) A BASE ANALOGUE which, in the keto tautomeric form (which predominates), pairs with adenine (A) but in the enol form pairs with cytosine (C). Thus, during DNA synthesis, keto-BU may be incorporated into DNA in place of thymine (T); if it then remains in the keto form, BU will continue to mimic T and no mutation will occur, but if it switches to the enol form just prior to further DNA replication it will pair with guanine (G) instead of A, leading to an A·T-to-G·C transition. Conversely, enol-BU may be incorporated into DNA in place of C, and a subsequent switch to the keto form will result in a G·C-to-A·T transition on subsequent DNA replication. bromovinyldeoxyuridine (BVDU; (E)-5-(2-bromovinyl)-2 -deoxyuridine) An ANTIVIRAL AGENT which is highly active against varicella-zoster virus; it is also active against herpes simplex virus (HSV) type 1 (but less so against HSV type 2) and 110 Browne’s tubes with wet lesions which become dark chocolate-brown. (cf. GINGER BLOTCH.) [Disease control by disinfection: JAB (1985) 58 259–281.] brown bodies See LEPROSY. Brown–Boveri UV-C lamp A source of high-intensity ULTRAVIOLET RADIATION; the lamp gives a radiation intensity of up to ca. 1W/cm2 . (Conventional sources give up to ca. 10 µW/cm2 .) Brown–Hopps modification (of the GRAM STAIN) A procedure used for staining bacteria in deparaffinized, hydrated tissue sections. Essentially, sections are stained with crystal violet; washed; treated with Gram’s iodine; washed; differentiated in acetone; washed; stained with basic fuchsin; washed; treated with Gallego’s solution (an aqueous solution of formalin and acetic acid); washed; dipped in acetone, picric acid–acetone, and acetone; and finally transferred, via xylene, to the mountant. Gram-positive bacteria stain blue; Gram-negative bacteria stain red. [AJCP (1973) 60 234–240.] brown oak Timber from an oak tree (Quercus sp) which has been infected by Fistulina hepatica; such infection causes the heartwood of the tree gradually to develop a rich brown colour. The colour appears to derive partly from pigment within the fungal hyphae and partly from extracellular fungal products. Brown oak is somewhat weaker than the normal wood, but it is valued e.g. for furniture-making. brown rot (1) (of timber) A form of fungal TIMBER SPOILAGE in which the CELLULOSE and HEMICELLULOSES of the wood are decomposed while the LIGNIN remains virtually intact (although it may be modified), leaving the wood brown, soft and friable, and subject to cross-grain cracking. The hyphae of brown-rot fungi (e.g. Piptoporus betulinus, Poria placenta) penetrate the wood cell lumen and lie in contact with the inner surface of the cell wall; a generalized thinning of the wall occurs, suggesting a diffusion of degradative enzymes from the hyphae (cf. WHITE ROT). A novel mechanism has been proposed for the disruption of native cellulose by brown-rot fungi (which do not produce exoglucanases – see CELLULASES): H2 O2 , generated by the oxidation of hexoses by glucose oxidase, is believed to act in combination with Fe2+ (present in the wood) to oxidize glucopyranosyl residues, causing rupture of the cellulose chains; this could disrupt microfibrils sufficiently to allow access for endoglucanases [Book ref. 26, pp. 28–29; Book ref. 39, pp. 56–59]. (2) (of fruits) A rot which affects a wide variety of fruits – particularly apples, pears and stone fruits (cherries, plums etc) – both on the tree and in storage. Soft brown lesions develop in the fruits, initially beneath an intact epidermis; subsequently, whitish cottony pustules may develop, often in a concentric arrangement, and the fruit eventually shrivels and dries. Brown rots are caused by Sclerotinia spp: S. fructigena in e.g. apples, pears and almonds, S. laxa or S. fructigena in cherries, plums, peaches etc. brown rust A disease of wheat and rye caused by Puccinia recondita; orange-brown pustules, later becoming dark or black, develop mainly on the upper (adaxial) leaf surface. On barley, a similar disease is caused by Puccinia hordei. (See also CEREAL DISEASES.) Browne’s tubes Sealed glass tubes containing a fluid which changes (irreversibly) from red to green when exposed to certain temperature/time combinations; they are used to monitor conditions at various positions within an operating AUTOCLAVE or hot-air oven. Type I and type II tubes are used in conventional autoclaves and high-vacuum autoclaves, respectively; type III is used in hot-air ovens, and type IV for monitoring exposures of ca. 180° C/12 min. (cf. BROWN’S TUBES.) e.g. against pseudorabies virus, infectious bovine rhinotracheitis virus, and herpesvirus simiae. It can be administered topically or orally. BVDU is apparently converted to the 5 -monophosphate by the viral thymidine kinase; the monophosphate can be converted to the diphosphate by HSV type 1 thymidine kinase (cf. ACYCLOVIR). bromoviruses (brome mosaic virus group) A group of tripartite ssRNA-containing PLANT VIRUSES, each of which infects a narrow range of plants; mechanical transmission can occur readily, and some members are transmitted by beetles. Type member: brome mosaic virus; other members: broad bean mottle virus; cowpea chlorotic mottle virus. Virion: icosahedral, ca. 26 nm diam., containing a single species of coat protein. Genome: three molecules of linear, positive-sense ssRNA – RNA1 (MWt ca. 1.1 × 106 ), RNA2 (MWt ca. 1.0 × 106 ), and RNA3 (MWt ca. 0.7 × 106 ); ‘RNA4’ (MWt ca. 0.3 × 106 ), the coat protein mRNA, is also encapsidated. A given bromovirus particle may contain a molecule of RNA1, a molecule of RNA2, or one molecule each of RNA3 and RNA4; these different particles all have about the same S20 W and all three are necessary for infectivity. The RNAs each have a capped 5 end and a tRNA-like structure (which accepts tyrosine) at the 3 end. Virus assembly occurs in the cytoplasm; crystalline arrays of virus particles and granular cytoplasmic inclusions may be formed. bromphenol blue A PH INDICATOR: pH 3.0–4.6 (yellow to blue); pKa 4.0. bromthymol blue A PH INDICATOR: pH 6.0–7.6 (yellow to blue); pKa 7.0. bromthymol blue agar See BTB AGAR. bronchiolitis Inflammation of the bronchioles. The disease occurs mainly in infants and is often due to human respiratory syncytial virus. Spread occurs by droplet transmission. The disease may be self-limiting or may lead to cyanosis, apnoea and death. bronchitis Inflammation of the bronchi; the condition may be acute or chronic and often follows a (usually viral) upper respiratory tract infection. Bronchitis may be caused by bacteria (e.g. Haemophilus influenzae, Mycoplasma pneumoniae, Staphylococcus aureus, Streptococcus pneumoniae), by viruses, or by physical or chemical irritants. Symptoms: chest pain, cough with mucoid to purulent sputum, fever etc. bronopol See ALCOHOLS. bronze leaf wilt Syn. HARTROT. brood pouch (marsupium) In certain ciliates: a cavity, formed by invagination of the pellicle, within which ‘larval’ cells are formed by budding; in suctorians the brood pouch is a temporary structure, but in chonotrichs (in which it is also called a crypt) it is permanent. In some species the larval cells are released into the brood pouch, while in others the larval cells detach from the parent cell only after the pouch has everted. brooder pneumonia See ASPERGILLOSIS. brosse (ciliate protozool.) A tuft of cilia, of unknown function, which occurs on some gymnostomes. broth In microbiology: any of a variety of liquid media – especially NUTRIENT BROTH or any liquid medium based on nutrient broth and/or hydrolysed protein. broth culture See CULTURE. broth dilution test See DILUTION TEST. broth sugars See PEPTONE-WATER SUGARS. brown algae Algae of the PHAEOPHYTA. brown blotch A MUSHROOM DISEASE caused by Pseudomonas tolaasii. Caps and stalks of infected mushrooms are pitted 111 Brownian movement biovar 2 to fuchsin. Biovars 4, 5 and 9 do not agglutinate in A antiserum; biovars 1, 2, 3 and 6 do not agglutinate in M antiserum. B. abortus ‘strain 19’ is similar to biovar 1 but has reduced virulence; it is used as a live vaccine for cattle. B. canis. Primarily a pathogen of the dog, causing e.g. epididymitis, orchitis, prostatitis and the development of local granulomas; man is occasionally infected. Only rough or mucoid strains occur; these are agglutinated by R antiserum but not by A or M antisera. Insensitive to thionin; most strains sensitive to fuchsin. No biovars. B. melitensis. Typically pathogenic in goats and sheep, but also in bovines, man and pigs. Insensitive to both fuchsin and thionin; biovar 1 is agglutinated by M antiserum, biovar 2 by A antiserum, and biovar 3 by both A and M antisera. B. neotomae. A non-pathogenic or weakly pathogenic species. Acid (no gas) is formed from arabinose, galactose, glucose and xylose. Sensitive to both fuchsin and thionin, and agglutinated by A antiserum. No biovars. B. ovis. A pathogen of sheep, causing epididymitis/orchitis, or abortion in pregnant ewes. Insensitive to thionin; most strains sensitive to fuchsin. No biovars. Smooth strains unknown; agglutination by R antiserum only. B. suis. Primarily a pathogen of pigs, but most or all strains can also infect man; other hosts include e.g. dogs and hares. Insensitive to thionin; most strains sensitive to fuchsin, but biovar 3 is insensitive. Biovars 1–4 are agglutinated by A antiserum; 4 is also agglutinated by M antiserum, and 5 is agglutinated only by M antiserum. An early proposal to include all strains of Brucella in a single species (B. melitensis) [IJSB (1985) 35 292–295] has not been accepted. Brucella ring test Syn. MILK RING TEST. brucellin An antigenic material derived from cultures of Brucella spp. brucellin test A SKIN TEST, analogous to the TUBERCULIN TEST, used for detecting Brucella-induced hypersensitivity (cf. BRUCELLOSIS). brucellosis Any human or animal disease caused by Brucella spp. In animals the reproductive organs are generally affected, often resulting in abortion (contagious abortion). B. abortus is the usual causal agent in cattle, B. suis in pigs, B. melitensis in goats, B. ovis in sheep, and B. canis in dogs, but host specificity is not absolute. Contagious abortion in cattle (Bang’s disease) may follow infection via the mouth, vagina or wounds; the pathogen has a predilection for the udder (resulting in its presence in the milk) and for fetal and placental tissues (its growth apparently being stimulated by erythritol present in these tissues). In man brucellosis (undulant fever, Malta fever ) is an acute or chronic systemic disease contracted from animals via unpasteurized milk, infected carcases, etc; B. abortus, B. melitensis or B. suis is most commonly involved. Infection occurs via the mouth, conjunctivae or wounds. Incubation period: days, weeks or months. Symptoms: variable, but typically include headache, malaise, and intermittent periods of fever. Untreated cases may persist for years. (In individuals frequently exposed to Brucella spp disease may be due to hypersensitivity to Brucella antigens rather than to true infection.) Lab. diagnosis: (i) Blood culture (usually positive if blood is taken at the height of the fever). (ii) Culture of bone marrow or lymph node aspirates (may be useful in chronic cases). (iii) Serological assay for specific serum agglutinins. (N.B. Agglutinins against cholera vibrios cross-react Brownian movement Random movements made by small (ca. 1 µm) particles, or organisms, freely suspended in a fluid medium; the movements are due to bombardment of the particles by molecules of the medium. Brownian movement causes nonmotile organisms to oscillate about more or less fixed positions; by contrast, motile organisms change their relative positions. Brown’s tubes A set of sealed glass tubes containing aqueous suspensions of barium sulphate in increasing concentrations; the tubes thus exhibit a range of optical densities which vary from transparent to turbid. Brown’s tubes are used for standardizing microbial suspensions: the opacity of a given bacterial suspension is matched (visually) with that of a particular tube, and the suspension described e.g. as a Brown’s number 2 or a Brown’s number 5 suspension etc. (Higher numbers correspond to greater opacities.) The unknown suspension should be examined in a tube of size and thickness equivalent to those containing the standard suspensions. Brucella A genus of Gram-negative, aerobic, chemoorganotrophic, catalase-positive bacteria which occur, typically as intracellular parasites or pathogens, in man and other animals (see BRUCELLOSIS). The cells are non-motile bacilli, cocci or coccobacilli, ca. 0.5–0.7 × 0.6–1.5 µm, which stain well with conventional dyes (see also KOSTER’S STAIN). Metabolism is respiratory (oxidative). Growth requirements generally include certain amino acids, nicotinamide and thiamine; 5–10% CO2 may be essential, particularly in primary isolation. Primary culture may be carried out on e.g. serum–glucose agar or chocolate agar; on primary isolation, 48-hour colonies on serum–glucose agar are typically 0.5–1.0 mm in diameter, shiny, convex and entire. Optimum growth temperature: 37° C. With the exception of B. neotomae, no species forms acid from carbohydrates in peptone-water sugars. MR −ve; VP −ve; most strains are oxidase +ve; most strains are urease +ve, but all strains of B. ovis are urease −ve (at 24 hours). The organisms are normally killed by pasteurization, are generally sensitive to tetracyclines, and are often sensitive e.g. to chloramphenicol, erythromycin and streptomycin. GC% ca. 55–58. Type species: B. melitensis. Brucellae can be divided into smooth, rough and mucoid strains on the basis of their reaction in ACRIFLAVINE solution (0.1% aqueous): smooth strains are easily emulsifiable, rough strains agglutinate, and mucoid strains form stringy threads. The brucellae are susceptible to a range of tailed, polyhedral DNA phages which have been divided into five groups that are used in phage typing (see e.g. TBILISI PHAGE, WEYBRIDGE PHAGE); group 5 phages include those which lyse non-smooth brucellae. (Brucella phages are sensitive to e.g. cationic detergents and oxidizing agents.) The sensitivity of some brucellae to certain dyes, e.g. basic FUCHSIN and THIONIN, has been exploited for between-species and within-species differentiation; these dyes inhibit the growth of sensitive strains when incorporated in media at concentrations of ca. 1:50000 (w/v). Differentiation between species and biovars is also achieved by the use of monospecific antisera prepared against each of the two main somatic antigens in the brucellae: the A and M antigens; an antiserum against rough strains (R) is used for B. canis and B. ovis. B. abortus. Primarily a pathogen of bovines, but man and other animals can be infected; infection of pregnant animals commonly results in placentitis and abortion. All strains of B. abortus are lysed by Tbilisi phage at RTD – a characteristic which distinguishes this species from other brucellae. There are 9 biovars, biovars 1, 2 and 4 being sensitive to thionin, 112 bulgaricus milk buccokinetal stomatogenesis See STOMATOGENESIS. Buchnera A genus of intracellular bacteria found in the BACTERIOCYTES of aphids; aphids and Buchnera are mutually dependent – neither can reproduce without the other. The chromosome of Buchnera sp APS is one of the smallest to be sequenced so far in bacteria (a total of 640681 base pairs) [Nature (2000) 407 81–86]. bud-fission In certain fungi (e.g. Saccharomycodes ludwigii ): a form of BUDDING in which the septum between bud and parent cell is almost as wide as the widest part of the parent cell. bud scar (mycol.) See SCAR. budding (1) (bacteriol., mycol.) In certain bacteria and fungi: a process of cell division in which a daughter cell (or spore) develops from the mother cell as a localised outgrowth or protrusion (bud ), the cell wall of the mature daughter cell apparently being composed mainly of newly synthesized material; mother and daughter cells are often physically, and sometimes functionally, distinguishable. (cf. FISSION.) In the BUDDING BACTERIA, budding often – but not always – occurs at one of the poles of the mother cell. In some fungi (e.g. Saccharomyces cerevisiae) buds may arise at any of a number of sites on the parent cell (multipolar budding); in e.g. Kloeckera, buds can develop at both cell poles (bipolar budding), while in Malassezia they can arise only at one pole (monopolar budding). (cf. TRIGONOPSIS; see also BUDFISSION, SCAR and SPROUT MYCELIUM.) (2) (protozool.) In some protozoa (e.g. members of the Astomatida, Peritrichia and Suctoria): a reproductive process in which a cell divides unequally to form a large adult and small progeny cells (external or exogenous budding), or in which progeny develop within the cytoplasm of the adult cell (interior or endogenous budding). (3) (virol.) See ENVELOPE (sense 1). budding bacteria A diverse, non-taxonomic group of bacteria all of which undergo BUDDING; they include e.g. species of BLASTOBACTER, CHAMAESIPHON, ENSIFER, GEMMIGER, GEODERMATOPHILUS, Hyphomicrobium, HYPHOMONAS, NITROBACTER, PASTEURIA, PLANCTOMYCES, RHODOMICROBIUM, RHODOPSEUDOMONAS and SELIBERIA. budgerigar fledgling disease virus A virus which causes a commercially important acute, fatal disease in fledgling budgerigars (Melopsittacus undulatus) and may also be responsible for a milder, chronic disease (‘French moult’) in older birds. The virus is apparently a POLYOMAVIRUS – the first non-mammalian polyomavirus to be recorded. [Virol. (1986) 151 362–370.] BUdR (BrdU) 5-Bromo-2 -deoxyuridine: the deoxynucleoside corresponding to 5-BROMOURACIL. It can be phosphorylated by THYMIDINE KINASE. (cf. IDOXURIDINE.) Buellia A genus of crustose LICHENS (order LECANORALES). Apothecia: lecideine, black; ascospores brown, one-septate. (cf. DIPLOICIA; see also ENDOLITHIC.) buffalopox virus See ORTHOPOXVIRUS. buffy coat The thin layer of white cells which forms at the surface of the packed red cells when unclotted blood is centrifuged. bufotenine See AMATOXINS. bulbil (algol.) See CHAROPHYTES. Bulbochaete A genus of freshwater filamentous green algae related to OEDOGONIUM. The filaments are branched and bear long, colourless ‘hairs’, each with a bulbous base. Bulgaria See HELOTIALES. Bulgarian buttermilk Syn. BULGARICUS MILK. bulgaricus milk (Bulgarian buttermilk) A beverage made by fermenting milk with Lactobacillus bulgaricus. with those against Brucella spp.) Chemotherapy: e.g. tetracyclines with streptomycin. (See also BRUCELLIN TEST and MILK RING TEST.) Bryopsis A genus of siphonaceous green seaweeds (division CHLOROPHYTA) which exhibit a heteromorphic alternation of generations (cf. DERBESIA). Bryoria A genus of fruticose LICHENS (order LECANORALES) separated from ALECTORIA e.g. on the basis of the smaller colourless ascospores (8 per ascus). BSA Bovine serum albumin. BS-C-1 An ESTABLISHED CELL LINE derived from the kidney of an African green monkey (Cercopithecus aethiops); the cells are heteroploid and epithelioid. BSE BOVINE SPONGIFORM ENCEPHALOPATHY. BSS BALANCED SALT SOLUTION. BssHII See RESTRICTION ENDONUCLEASE (table). BstEII A RESTRICTION ENDONUCLEASE from Bacillus stearothermophilus; G/GTNACC. BTB agar (bromthymol blue agar) A medium (used e.g. for enterobacteria) containing e.g. peptone, lactose, glucose, yeast extract, MARANIL, thiosulphate, and bromthymol blue; pH: 7.7–7.8. [Recipe: Book ref. 22, p. 421.] BTEX Benzene, toluene, ethyl-benzene and xylene – a group of compounds which are sometimes present as contaminants in aquifers. (See also BIOREMEDIATION.) BtuB protein In Escherichia coli : an OUTER MEMBRANE protein encoded by the btuB gene; it acts as a receptor for bacteriophage BF23 and for various colicins, and is involved in the uptake of VITAMIN B12 . BtuC protein See VITAMIN B12 . BU 5-BROMOURACIL. bubble column fermenter A FERMENTER in which mixing and circulation of the culture is induced by a stream of bubbles introduced at the base of the column (which lacks a draft tube – cf. LOOP FERMENTER); it may be used e.g. with lowviscosity cultures when efficient mixing is not essential. An increased gas–liquid interface can be achieved by placing mesh sieve plates at intervals in the column. bubble diseases MUSHROOM DISEASES caused by Verticillium fungicola (dry bubble) or Mycogone perniciosa (wet bubble). [Control by PROCHLORAZ and other antifungals: PP (1983) 32 123–131.] bubble point test A test used to determine the physical integrity of a membrane filter (see FILTRATION). In a wetted membrane, water is retained in the pores by the forces of surface tension, and a considerable pressure of air is needed to dislodge this water and to force air through the pores; the minimum pressure which can do this (the bubble point pressure) increases with decrease in pore size, and is about 55 lb/inch2 (379 kPa) for a membrane of pore size 0.22 µm. Using the relationship between bubble point pressure and pore size, the integrity of a membrane of given pore size can be tested by determining its bubble point pressure. (cf. DIFFUSION TEST.) bubbler Syn. ALL-GLASS IMPINGER. bubo An enlarged, inflamed lymph node (particularly in the axilla or groin) formed e.g. in bubonic PLAGUE. bubonic plague See PLAGUE. buccal cavity In some ciliates: a cavity, chamber or surface depression which is typically open to the environment and which contains, at its base, the CYTOSTOME; the ciliature of the buccal cavity is typically readily distinguishable from the somatic ciliature. (See also e.g. PARORAL MEMBRANE.) buccal overture The entrance (outer or distal region) of the BUCCAL CAVITY. 113 bulgecin arthropod vector (if any) etc.; the genera include BUNYAVIRUS; HANTAVIRUS; NAIROVIRUS; PHLEBOVIRUS and UUKUVIRUS. Bunyavirus (‘Bunyamwera supergroup’) A genus of viruses of the BUNYAVIRIDAE. Host range: various vertebrates; vectors: mainly mosquitoes. MWts of L, M and S RNAs: ca. 2.7–3.1, 1.8–2.3, and 0.28–0.50 × 106 , respectively. MWts of proteins L, G1, G2 and N: ca. 145–200, 108–120, 29–41 and 19–25 × 103 , respectively. The genus comprises at least 16 serological groups, including the Bunyamwera group (e.g. Bunyamwera virus (type species of the genus), Cache Valley virus), the California group (e.g. CALIFORNIA ENCEPHALITIS virus, Inkoo virus, La Crosse virus, snowshoe hare virus), and the Simbu group (e.g. Aino virus, Akabane virus – see AKABANE VIRUS DISEASE). bunyaviruses (1) Viruses of the BUNYAVIRIDAE. (2) Viruses of the genus BUNYAVIRUS. buoyant density See CENTRIFUGATION. burdock stunt viroid See VIROID. burdock yellows virus See CLOSTEROVIRUSES. Burdon’s stain A stain for bacterial intracellular lipid. A heatfixed smear is stained with SUDAN-BLACK B for 5–10 min, drained, and blotted dry. The smear is washed in xylene, blotted dry, and counterstained for 5–10 sec in 0.5% aqueous safranin. Lipid stains black, cytoplasm red. (See also NILE BLUE A.) Burgundy mixture (plant pathol.) An agricultural antifungal preparation which consists of a mixture of copper sulphate and sodium carbonate; it is used in aqueous solution as a spray. Active constituent: the COPPER ion. Burgundy truffle See TRUFFLES. Burkard trap See HIRST SPORE TRAP. Burkea See MICROSPOREA. Burkholderia A genus of Gram-negative bacteria initially proposed to accommodate seven (group II) species from the genus PSEUDOMONAS, with Burkholderia (formerly Pseudomonas) cepacia as the type species [Microbiology and Immunology (Tokyo) (1992) 36 1251–1275]; the other species in this proposal were: P. mallei, P. pseudomallei, P. caryophilli, P. gladioli, P. pickettii and P. solanacearum. Two other former species of the genus Pseudomonas (P. plantarii and P. glumae), and a new species, B. vandii, have since been added to the genus [IJSB (1994) 44 235–245]. (See also CYSTIC FIBROSIS.) [B. cepacia (surface chemistry and typing methods): RMM (1995) 6 1–9. B. cepacia (pathogenicity and resistance to antibiotics): RMM (1995) 6 10–16.] Burkitt’s lymphoma (BL) A malignant monoclonal B-cell lymphoma which is endemic in parts of Africa where it is the most common tumour among children. The tumours commonly develop in the mandibular and maxillary bones, but may also develop in the ovaries, thyroid, kidneys, heart, stomach, and spinal column. BL is associated with EPSTEIN–BARR VIRUS (EBV) infection, but other factors – including the occurrence of malaria – are also important in pathogenesis. BL cells contain multiple copies of the EBV genome, and also show characteristic chromosomal rearrangements involving reciprocal translocations between chromosome 8 and chromosome 14 or, less frequently, chromosome 2 or 22. These translocations apparently result in the juxtaposition of a cellular ONCOGENE (c-myc, or possibly cmos) and the immunoglobulin (Ig) genes on chromosomes 14, 2 or 22; c-myc may be activated by the Ig gene promoter, resulting in oncogenesis. These chromosomal rearrangements can occur in the absence of EBV; EBV may predispose to oncogenesis by its mitogenic effect on B cells, providing large numbers of transformed cells and thereby increasing the chances bulgecin A low-MWt glycopeptide which, in combination with a b-lactam antibiotic, causes bulge formation in susceptible bacterial cells [AAM (1986) 31 181–205 (200–201)]. bullate With bubble- or blister-like swellings. Buller phenomenon A form of DIKARYOTIZATION in which a dikaryotic cell or mycelium donates a nucleus to a monokaryotic (haploid) cell or mycelium. Bullera A genus of fungi (see SPOROBOLOMYCETACEAE) which form spheroidal, ovoid or elongate, budding vegetative cells, and which may form pseudomycelium or true mycelium; the cell walls contain xylose. On malt agar, the growth is cream-coloured to slightly yellowish or brownish (cf. SPOROBOLOMYCES). Ballistospores are formed on sterigmata of varying lengths and are rotationally symmetrical, being spheroidal, obovoid, turbinate, apiculate or ampulliform. Metabolism is strictly respiratory. NO3 − is assimilated by B. piricola and B. tsugae, but not by B. alba, B. dendrophila or B. singularis. Species have been isolated from insect frass and plant material. [Book ref. 100, pp. 577–584.] bullous impetigo See IMPETIGO. bumblefoot An arthritis of poultry due to infection via wounds – particularly in the feet – by Staphylococcus aureus. (See also POULTRY DISEASES.) bumper primer See SDA. bundle-forming pili See PATHOGENICITY ISLAND. bundlin The major subunit of bundle-forming pili (see EPEC) – a 19.5 kDa protein encoded by the ‘EPEC adherence factor plasmid’ (EAF plasmid) found in strains of EPEC. (See also PATHOGENICITY ISLAND.) bunts (stinking smuts) See e.g. COMMON BUNT and KARNAL BUNT. Bunyamwera supergroup See BUNYAVIRUS. Bunyaviridae A family of enveloped, ssRNA VIRUSES which infect warm- and cold-blooded vertebrates, arthropods and plants. Some members are important pathogens of man and animals, causing e.g. VIRAL HAEMORRHAGIC FEVERS or pulmonary disease; transmission may occur e.g. via ticks, mosquitoes or sandflies (Phlebotomus spp), while some bunyaviruses are transmitted in aerosols. (See also TOMATO SPOTTED WILT VIRUS; cf. ARBOVIRUSES.) Virion. Roughly spherical, enveloped, ca. 90–100 nm diam., containing three major protein species (nucleocapsid protein N, and glycoproteins G1 and G2), one large but minor protein (L), and three molecules/segments of negative sense (or ambisense) ssRNA designated L, M and S (MWts ca. 2–5, 1–2.3 and 0.3–0.8 × 106 respectively); the envelope contains ca. 20–30% by weight of lipid. Virion structure. The lipoprotein envelope, which contains G1 and G2, encloses three circular (but not covalently closed) helical strands of ribonucleoprotein (comprising RNA and N protein); the haemagglutinating activity and neutralizing antigenic determinants are associated with the glycoproteins. Virions can be inactivated by e.g. lipid solvents and detergents. Transcriptase activity is associated with the virions, and viral replication occurs in the cytoplasm of the host cell. In the genus Bunyavirus, all three genome segments are negative-sense. The S RNA segment encodes (in overlapping reading frames) the N protein and a non-structural protein (NSs ). The M RNA segment encodes G1, G2 and at least one nonstructural protein (NSm ). The L RNA appears to encode the L protein. (cf. PHLEBOVIRUS.) During maturation, the virions bud into smooth-surfaced vesicles in or near the Golgi region. In culture, some bunyaviruses can induce cell fusion. The family is divided into genera on the basis of e.g. size of RNA segments, size and nature of viral proteins, nature of 114 Butyrivibrio of one of the three chromosomal rearrangements occurring. Chronic malaria depresses EBV-specific cell-mediated immunity and stimulates B cell proliferation, thus potentiating the effects of EBV. ‘Burkitt-like’ lymphomas occur rarely and sporadically outside the endemic areas, and only ca. 20% of these are associated with EBV infection. burned spot disease (shell disease; Brandenfleckenkrankheit) A marine CRUSTACEAN DISEASE which affects the exoskeletons of crabs, lobsters, crayfish etc; it is caused by chitinolytic bacteria and/or fungi which form brown erosive lesions in the calcified chitin of the exoskeleton following damage to the epicuticle (e.g. by abrasion). Once initiated, the lesions may support a varied microbial flora. The disease may be fatal, but recovery may occur if the animal survives to ecdysis. Fusarium solani has been associated with the disease in lobsters; the fungus may penetrate the muscles and occasionally the foregut of the lobster [TBMS (1981) 76 25–27]. (cf. BLACK GILL DISEASE.) Burnet’s clonal selection theory See CLONAL SELECTION THEORY. bursa of Fabricius In birds, a sac-like organ which develops from the hindgut in the early embryonic stage; within the bursa, precursors of B LYMPHOCYTES appear to undergo maturation. (See also INFECTIOUS BURSAL DISEASE VIRUS.) Bursaria See HETEROTRICHIDA. bursattee Syn. EQUINE PHYCOMYCOSIS. burst size The average number of virus particles released per infected cell following the lytic infection of a population of sensitive cells; it may be determined e.g. by the ONE-STEP GROWTH EXPERIMENT (by comparing the latent period plaque count with the plateau plaque count) or by the SINGLE BURST EXPERIMENT. burst test A test used for assessing the strength of the seal in a RETORT POUCH; in the test, the pouch is either inflated or is filled with water and then compressed. Bursulla See MYXOMYCETES. Buruli ulcer A chronic, progressive, granulomatous skin lesion caused by Mycobacterium ulcerans; infection occurs via wounds. Buschke–Löwenstein tumour (giant condyloma acuminatum) A large, cauliflower-like, malignant but non-metastasizing genital tumour of humans. The genomes of HPV-6 or HPV-11 (see PAPILLOMAVIRUS) have been identified in Buschke–Löwenstein tumour cells [see e.g. JV (1986) 58 963–966]. (cf. PAPILLOMA.) Busse–Buschke disease Syn. CRYPTOCOCCOSIS. Bussuquara virus See FLAVIVIRIDAE. butanediol fermentation (butylene glycol fermentation) A FERMENTATION (sense 1) carried out e.g. by certain enterobacteria, including species of Enterobacter, Erwinia, Klebsiella and Serratia. The main products of glucose fermentation include ethanol, 2,3-butanediol and formic acid or CO2 and H2 [Appendix III(f)]; small amounts of DIACETYL (CH3 .CO.CO.CH3 ) may also be formed from acetolactate. Organisms which form diacetyl or acetoin give a positive VOGES–PROSKAUER TEST. The amount of acid produced in the butanediol fermentation is generally insufficient to give a positive methyl red test (cf. MIXED ACID FERMENTATION). butanol fermentation Syn. ACETONE–BUTANOL FERMENTATION. butirosin An AMINOGLYCOSIDE ANTIBIOTIC. butoconazole See AZOLE ANTIFUNGAL AGENTS. Bütschli’s granules METACHROMATIC GRANULES observed in diatoms. butt (microbiol.) The thickest part of a SLOPE. The butt of a freshly-prepared slope is a microaerobic or anaerobic environment. butt rot A TREE DISEASE in which the base (butt) of the trunk rots. In e.g. elms (Ulmus spp) a butt rot may be caused by Rigidoporus ulmarius, while in oaks (Quercus spp) it may be caused by Inonotus dryadeus (Polyporus dryadeus). butter A food consisting mainly of the fatty components of milk; it contains at least 80% fat, up to 16% water, and usually ca. 2% added salt. Sweet cream butter is made without a bacterial starter. Cream is pasteurized, cooled, and churned at ca. 8–12° C to destabilize the fat globules in the cream. The butter is then strained from the aqueous phase (buttermilk) and salted; it may be stored for a year or more at ca. −23 to −29° C. Ripened cream butter (cultured creamery butter ) is made from pasteurized cream seeded with a B- or BD-type LACTIC ACID STARTER. The main function of the bacteria is to contribute flavour (LACTIC ACID and DIACETYL). After several hours’ incubation, during which the pH falls to ca. 4.6, the cream is cooled, churned etc. Cultured butters are more common in e.g. Europe than in the USA; they have better flavour than sweet cream butter, but they also undergo chemical deterioration faster – the rate of deterioration being higher if salt is added. (See also DAIRY PRODUCTS.) Butter is generally resistant to microbial spoilage owing to its low water content. However, proteolytic and/or lipolytic psychrotrophs (e.g. Pseudomonas fragi, Ps. fluorescens) can cause putrid or rancid flavours in refrigerated butter. A cheese-like offflavour (‘surface taint’) may be caused by ‘Ps. putrefaciens’, and ‘Ps. nigrifaciens’ can cause a black surface discoloration. buttermilk Originally: the liquid residue formed during BUTTERmaking. Currently, the term usually refers to ‘cultured buttermilk’: a beverage made by fermenting skim milk with lactic acid bacteria; a BD-type LACTIC ACID STARTER is usually used. [Book ref. 8, pp. 140–143.] (See also DAIRY PRODUCTS.) button stage The (early) stage of an agaric fruiting body in which the lamellae are not yet exposed, and the pileus is roughly domeshaped. butyl alcohol fermentation Syn. ACETONE–BUTANOL FERMENTATION. butylene glycol fermentation Syn. BUTANEDIOL FERMENTATION. butyric acid fermentation A FERMENTATION (sense 1) carried out e.g. by Butyrivibrio fibrisolvens, certain species of CLOSTRIDIUM (e.g. C. acetobutylicum, C. butyricum, C. pasteurianum, C. perfringens) and Fusobacterium nucleatum. The products of glucose fermentation include acetic and butyric acids, CO2 and H2 [Appendix III(g)]; some species (e.g. C. perfringens) form lactic acid and/or ethanol in addition to the above products – lactic acid becoming a major product under conditions of iron deficiency. (See also ACETONE–BUTANOL FERMENTATION.) butyricins See BACTERIOCIN. Butyrivibrio A genus of bacteria (family BACTEROIDACEAE) which occur e.g. in the RUMEN and in non-ruminant faeces. Cells: motile or (infrequently) non-motile curved rods, 0.3–0.8 × 1.0–5.0 µm (occurring singly or in chains), or filaments which are sometimes helical; the cells stain Gram-negatively, although the cell wall of B. fibrisolvens is of the Gram-positive type (see also TEICHOIC ACIDS). [Heterogeneity in cell envelopes of Butyrivibrio strains: JUR (1985) 90 286–293.] Fermentation of e.g. glucose or maltose typically yields butyric acid as the main product (although lactic acid may be the major product under certain conditions). Some strains are cellulolytic, pectinolytic and/or amylolytic. GC%: ca. 36–41. Type species: B. fibrisolvens. B. crossotus. Cells are lophotrichously flagellated. Maltose is fermented, but cellobiose, sucrose and xylose are not. B. fibrisolvens. The cells each have one polar or subpolar flagellum. Glucose, maltose, cellobiose, sucrose and xylose are fermented. 115 butyrolactone antifungal agents symptoms of airway obstruction; aetiology may involve e.g. the endotoxins and/or allergens from contaminating Gram-negative bacteria. [AEM (1982) 44 355–362; CIA (1984) 4 45–47.] Byssochlamys A genus of fungi (order EUROTIALES) found e.g. in soil; anamorphs occur in the genus PAECILOMYCES. The mature ascocarp consists of a group of asci surrounded by a few wisps of hyphae. B. fulva can cause spoilage of canned foods (see CANNING); the organism forms extracellular pectinases, and its ascospores can withstand ca. 85° C for 30 minutes, and higher temperatures for shorter periods. (See also PATULIN.) byssoid Composed of slender fibrils. bystander help (immunol.) Non-specific help; for example, an antigen-stimulated T cell may provide help (in the form of cytokines) for another cell which has been stimulated by a different antigen. bystander lysis Syn. REACTIVE LYSIS. butyrolactone antifungal agents See PHENYLAMIDE ANTIFUNGAL AGENTS. butyrous Butter-like in consistency. BVD BOVINE VIRUS DIARRHOEA. BVD-MD virus See PESTIVIRUS. BVDU BROMOVINYLDEOXYURIDINE. bvg genes In Bordetella spp: genes of a TWO-COMPONENT REGULATORY SYSTEM controlling the synthesis of various virulence factors (adhesins, toxins); bvgS encodes the histidine kinase, while bvgA encodes the response regulator. It is not known which signal(s) activate the system in vivo; in vitro it can be regulated e.g. by temperature. BWD See PULLORUM DISEASE. BWYV Beet western yellows virus (see LUTEOVIRUSES). BYDV Barley yellow dwarf virus (see LUTEOVIRUSES). byssinosis A human pulmonary disease, associated with the handling of cotton, flax, hemp or sisal, characterized by acute 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 116 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 C C-type particles See TYPE C ONCOVIRUS GROUP. C value (C-value; c̄) The amount of DNA in a haploid genome; it can be measured e.g. in picograms or in terms of the molecular weight or the number of constituent kilobase-pairs. C0 In the food processing industry: heating (COOKING) for 1 min at 100° C or an equivalent amount of heat processing. C1 cellulase See CELLULASES. C1 compounds (1-C compounds; one-carbon compounds) Carbon compounds which contain no carbon–carbon bonds and which are more reduced than is CO2 – e.g. CO, methane, methanol, formaldehyde, formamide and formate; C1 compounds may have more than one carbon atom: e.g. dimethyl sulphide [(CH3 )2 S], dimethyl sulphoxide [(CH3 )2 SO], dimethyl sulphone [(CH3 )SO2 ], trimethylamine-N-oxide [(CH3 )3 NO]. (See also METHYLOTROPHY.) C1 esterase inhibitor See COMPLEMENT FIXATION (a). C1EI See COMPLEMENT FIXATION (a). C1INH Syn. C1EI – see COMPLEMENT FIXATION (a). C2 carbon oxidation cycle See PHOTORESPIRATION. C2a pathway See COMPLEMENT FIXATION (d). C3 convertase See COMPLEMENT FIXATION. C3 cycle Syn. CALVIN CYCLE. C3bina (C3bINA) Syn. FACTOR I. C5 convertase See COMPLEMENT FIXATION. C27 organisms See PLESIOMONAS. C55 lipid carrier Syn. BACTOPRENOL. Ca2+ -ATPase See ION TRANSPORT. Ca2+ , Mg2+ -stimulated ATPase See ATPASE. Ca2+ transport See ION TRANSPORT. Cabassou virus See ALPHAVIRUS. cabbage B virus Syn. CAULIFLOWER MOSAIC VIRUS. cabbage diseases See CRUCIFER DISEASES. cabinet (safety cabinet) See SAFETY CABINET. cacao diseases For diseases of the cacao plant (Theobroma cacao) see e.g. BLACK POD DISEASE, FUSARIUM (F. decemcellulare), POD ROT, RED RUST, VASCULAR-STREAK DISEASE and WITCHES’ BROOM. cacao yellow mosaic virus See TYMOVIRUSES. Cache Valley virus See BUNYAVIRUS. cachectin An early name for tumour necrosis factor-a (TNF-a). cacodylate Dimethylarsinate. cactus virus 2 See CARLAVIRUSES. cactus virus X See POTEXVIRUSES. CAD See APOPTOSIS. cAD1 See PHEROMONE. cadang-cadang See COCONUT CADANG-CADANG VIROID. cadaverine See e.g. DECARBOXYLASE TESTS. cadherins A family of CELL ADHESION MOLECULES. The cadherins are large, transmembrane glycoproteins that mediate calciumdependent homophilic cell–cell adhesion (i.e. cadherin binds to cadherin). The intracellular domain of the cadherin molecule binds to proteins called catenins which, in turn, are linked to the cell’s actin cytoskeleton. E-cadherin occurs mainly in epithelial tissue (and in the early stage of mammalian embryogenesis); E-cadherin is used as a receptor for the adhesin internalin A of Listeria monocytogenes during invasion of the intestinal epithelium. P-cadherin occurs in the placenta and epidermis, and N-cadherin is found e.g. in nerve and heart cells. C (1) (immunol.) COMPLEMENT. (2) (mol. biol.) Cytosine (or the corresponding nucleoside or nucleotide) in a nucleic acid. (3) Cysteine (see AMINO ACIDS). c See C VALUE. C
(immunol.) COMPLEMENT. 1-C compounds See C1 COMPOUNDS. C forms See AZOSPIRILLUM. C loop See BACTERIOPHAGE P1. c-onc See ONCOGENE. C period See HELMSTETTER–COOPER MODEL. C-reactive protein (CRP) An ACUTE-PHASE PROTEIN, increased amounts of which circulate in the body e.g. during inflammatory processes. CRP is precipitated by so-called C-reactive substances which occur in various microorganisms – e.g. Streptococcus pneumoniae (the basis of the C-reactive protein diagnostic test) and Aspergillus fumigatus. [Binding of CRP to C-reactive substance of Aspergillus fumigatus: JMM (1986) 21 173–177.] Changes in CRP titre may help to distinguish between bacterial and viral infections: a rise in the serum level of CRP is more likely to reflect a bacterial infection because viral infections usually do not produce such a change. C-reactive substance See C-REACTIVE PROTEIN. (cf. C SUBSTANCES.) C region (of Ig) See IMMUNOGLOBULINS. C ring See FLAGELLUM (a). c strand (C strand) Complementary strand, e.g., the strand complementary to the genomic (viral, v) strand in the RF of an ssDNA virus. (cf. CDNA.) C substances (of streptococci) A range of serologically distinct carbohydrates – typically only one of which occurs in a given strain of Streptococcus; C substances are used in LANCEFIELD’S STREPTOCOCCAL GROUPING TEST. In the majority of Lancefield groups the C substance is a CELL WALL component. In Lancefield group A streptococci the C substance is a rhamnose polymer with a terminal residue of N-acetylglucosamine, the latter being immunodominant; in strains of group B the main determinant appears to be rhamnose, while in those of group C it is a terminal residue of N-acetylgalactosamine. [Structure of the C substance in group B streptococci: Biochem. (1987) 26 476–486.] The C substance of group D strains, a glycerol TEICHOIC ACID, occurs in the periplasmic region. C substances may be extracted from cells by various methods. In the hot-HCl method (Lancefield extraction method), packed (centrifuged) cells of the strain under test are resuspended in ca. 0.3 ml of a solution of sodium chloride (0.85%) containing HCl (at a final concentration of 0.2 N) and one drop of the pH indicator metacresol purple (0.04%); the pH should be ca. 2 (pink coloration). The tube containing the suspension is placed in boiling water for ca. 10 min with periodic shaking. The tube is then centrifuged, and the supernatant is decanted into a clean vessel; NaOH (0.2 N) is added dropwise until the pH is ca. 7.5 (weak purple). In the simpler autoclave extraction method, packed cells are re-suspended in 0.5 ml of NaCl solution (0.85%), autoclaved (121° C/15 min), centrifuged, and the supernatant is used for the test. [Methods of preparing antigen extracts for Lancefield grouping: Book ref. 120, pp. 170–172.] 117 cadmium calcium transport See ION TRANSPORT. calcofluor white A FLUORESCENT BRIGHTENER which binds by hydrogen-bonding to b-linked fibrillar polysaccharides such as cellulose and chitin; it can be used as a fluorescent marker to locate these polymers in cells (see MICROSCOPY (e)), and can disrupt CELLULOSE and chitin microfibril formation in growing cells (e.g. yeasts: [JGM (1983) 129 1577–1582]). caldoactive Refers to an extreme THERMOPHILE. The term is used more specifically by some authors to refer to an organism with a maximum growth temperature >70° C, an optimum growth temperature >65° C, and a minimum growth temperature >40° C [Sci. Prog. (1975) 62 373–393]. caldopentamine See POLYAMINES. calf diphtheria See NECROBACILLOSIS. calf pneumonia (enzootic pneumonia of calves; viral pneumonia of calves) A non-specific calf disease, the causal agent(s) being one or more viruses (e.g. adenoviruses, parainfluenza virus type 3, respiratory syncytial virus) frequently in association with bacteria (e.g. Chlamydia, Mycoplasma spp); typical symptoms: fever with constipation, followed by a mucopurulent nasal discharge, PNEUMONIA and diarrhoea. Poor hygiene and housing conditions (e.g. poor ventilation) and overcrowding appear to predispose towards the disease. Mortality rates can be high e.g. in acute RSV pneumonia. Control involves improvements in housing etc. and chemotherapy. (See also danofloxacin in entry QUINOLONE ANTIBIOTICS.) calf scours Scouring (see SCOURS) in young calves due to any of a range of microbial agents, e.g. enterotoxigenic strains of Escherichia coli, coronaviruses etc; aetiology may be complex (e.g. mixed bacterial and viral causal agents) and the occurrence and severity of the disease may involve both immunological and environmental factors. Calgitex See ALGINATE. Caliciales A heterogeneous order of (mostly LICHEN-forming) fungi of the ASCOMYCOTINA. Three families [Book ref. 64]: Caliciaceae. Mostly lichen-forming (photobiont: a green alga). Thallus thin crustose or immersed in the substratum (usually wood or bark). Apothecia (mazaedia) stalked or sessile. Lichenforming genera include e.g. Calicium, Chaenotheca, Coniocybe, Cyphelium. Mycocaliciaceae. Lichenicolous or fungicolous (possibly also lichen-forming) fungi in which the thallus is immersed in the substratum or absent. Apothecia stalked, asci thick-walled (mazaedia are not formed). Genera: e.g. Stenocybe. Sphaerophoraceae. All lichen-forming (photobiont: a green alga). Thallus foliose or fruticose. Apothecia (mazaedia) often globose, marginal or terminal. Genera: e.g. Sphaerophorus. Calicium See CALICIALES. Caliciviridae A family of VIRUSES which infect vertebrates; caliciviruses were formerly classified as a genus in the family Picornaviridae. The virions are non-enveloped, roughly spherical, ca. 35–39 nm diam., and have 32 characteristic cupshaped surface indentations – arranged with icosahedral symmetry – observable by electron microscopy of negatively stained preparations. The capsid contains a single major polypeptide species (MWt ca. 60000–71000) and a minor polypeptide (MWt ca. 15000). Genome: one molecule of positive-sense ssRNA (MWt ca. 2.6–2.8 × 106 ) which is apparently polyadenylated at the 3
end but not capped at the 5
end; the RNA is covalently linked (possibly at its 5
end) to a protein (MWt ca. 10000–15000) which is necessary for infectivity. Replication occurs in the cytoplasm; dsRNAs (presumably replicative intermediates), genome-sized ssRNAs, and subgenomic ssRNAs can cadmium See HEAVY METALS. caducous Tending to be shed, or to fall, early. Caedibacter A genus of Gram-negative bacteria which are obligate endosymbionts in strains of Paramecium aurelia. Cells: non-motile coccobacilli or rods, ca. 0.4–1.0 µm across and 1.0–4.0 µm in length, which stain well with toluidine blue; some (not all) cells of Caedibacter contain an R BODY (or, rarely, two R bodies). Cell division is inhibited in bacterial cells which contain an R body. The possession of R body-containing cells of Caedibacter appears to confer on the host Paramecium the ability to behave as a killer, or as a MATE KILLER, towards sensitive paramecia (i.e. paramecia which lack endosymbiotic Caedibacter ); in the ‘killer’ phenomenon, sensitive paramecia appear to ingest R body-containing bacterial cells (and/or isolated R bodies or toxins?) released by killer paramecia, and are killed as a consequence. The ability of killer paramecia to remain viable while harbouring Caedibacter is dependent on certain dominant allele(s). GC%: ca. 40–44. Type species: C. taeniospiralis. The cells of C. pseudomutans, C. taeniospiralis and C. varicaedens – which occur in the cytoplasm of their hosts, and which confer the killer characteristic – are sometimes called kappa particles. C. paraconjugatus (see also MU PARTICLE) is associated with mate killing. Another endosymbiont, which occurs in the macronucleus of P. caudatum – and which confers the killer characteristic – may belong to this genus. (See also HOLOSPORA, LYTICUM, PSEUDOCAEDIBACTER, SPIN KILLING.) [Book ref. 22, pp. 803–806; protein synthesis in kappa particles: JGM (1984) 130 1517–1523.] caeomatoid (caeomoid) Refers to an irregularly-shaped aecium which lacks a distinct peridium. caesium chloride See CENTRIFUGATION. caffeic acid 3-(3
,4
-Dihydroxyphenyl)-propenoic acid, a compound produced by higher plants (apparently as a precursor of LIGNIN). Caffeic acid may be a factor in the resistance of certain plants to certain diseases; thus, e.g. potatoes resistant to LATE BLIGHT have been found to accumulate caffeic acid on infection with Phytophthora infestans. (Caffeic acid inhibits P. infestans in vitro.) (See also AUXINS and CHLOROGENIC ACID.) caffeine 1,3,7-Trimethylxanthine, a purine derivative present e.g. in tea leaves and coffee beans. In e.g. certain bacteria caffeine can act as an inhibitor of EXCISION REPAIR of DNA. cag See PATHOGENICITY ISLAND. Cairns’ mechanism (q replication) A mechanism for the replication of a ds cccDNA molecule: replication is initiated at a fixed point, progresses (unidirectionally or bidirectionally) around the circle – both strands being replicated more or less simultaneously – and ends at a defined terminus. (The partly replicated intermediate thus has a ‘q’-like structure.) Bidirectional replication occurs e.g. in the chromosome of Escherichia coli (see DNA REPLICATION); unidirectional replication occurs e.g. in the COLE1 PLASMID. (cf. ROLLING CIRCLE MECHANISM.) Calberla’s solution A mixture of glycerol (5 ml), 95% ethanol (10 ml) and distilled water (15 ml) used as a microscopical mounting medium for the ROTOROD. Calcarisporium See HYPHOMYCETES; see also AUROVERTINS and CONTACT BIOTROPHIC MYCOPARASITE. calcicolous Growing preferentially in a calcium-rich environment. calcium alginate wool See ALGINATE. calcium nutrient agar (CNA) NUTRIENT AGAR containing CaCl2 ; a lawn plate prepared from CNA may be used e.g. for the cultivation of certain bacteriophages. 118 CAMP test bacterial growth (apparently by withholding zinc ions), and this may antagonize certain antibiotics (e.g. b-lactams) used for chemotherapy. [Zinc in antimicrobial defence: RMM (1997) 8 217–224.] Calvatia See LYCOPERDALES. Calvin cycle (Calvin–Benson cycle, or Benson–Calvin–Bassham cycle; reductive pentose phosphate cycle; C3 cycle) A cyclic pathway used for the fixation of CARBON DIOXIDE by a wide range of AUTOTROPHS (q.v.). (See also METHYLOTROPHY.) The key enzymes in the Calvin cycle are RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE–OXYGENASE (RuBisCO), responsible for the CO2 fixing reaction, and phosphoribulokinase (ribulose 5-phosphate kinase, EC 2.7.1.19), responsible for regenerating the CO2 acceptor: ribulose 1,5-bisphosphate (RuBP). For every three molecules of CO2 fixed, six molecules of 3-phosphoglycerate are formed; of these, five are required for the regeneration of RuBP, while the remaining molecule is available as a source of carbon for biosynthesis (see figure on page 120). The ATP and reduced NAD(P) required to drive the cycle are supplied by the light reactions of PHOTOSYNTHESIS in phototrophs, or by the oxidation of reduced inorganic compounds in CHEMOLITHOAUTOTROPHS. [Regulation of the Calvin cycle in bacteria: AvL (1984) 50 473–487.] (See also PHOTORESPIRATION.) calyciform Cup-shaped. calymma See RADIOLARIA. Calymmatobacterium A genus (incertae sedis) of Gramnegative bacteria which occur as pathogens in man (see GRANULOMA INGUINALE). Cells: non-motile, pleomorphic, capsulated, round-ended rods, 0.5–1.5 × 1.0–2.0 µm. In exudates from diseased tissues the cells are usually seen in the cytoplasm of large mononuclear phagocytes. The organisms can be cultured in the chick embryo yolk sac or on specialized egg yolk-containing media. Optimum growth temperature: 37° C. Type (only) species: C. granulomatis. [Book ref. 22, pp. 585–587.] Calyptralegnia See SAPROLEGNIALES. CAM (1) Chorioallantoic membrane: see EMBRYONATED EGG. (2) CELL ADHESION MOLECULE. cam gene See METHANOGENESIS. CAM plasmid An IncP-2 Pseudomonas PLASMID (ca. 500 kb) which encodes the capacity for metabolization of camphor (compare TOL PLASMID). cAM373 See PHEROMONE. camalexin A PHYTOALEXIN (a substituted indole) produced by Arabidopsis. Camarops See SPHAERIALES. Camarosporium A genus of fungi of the class COELOMYCETES. (See also LEPTOSPHAERIA.) camelpox virus See ORTHOPOXVIRUS. Camembert cheese See CHEESE-MAKING. cAMP CYCLIC AMP. cAMP–CAP See CATABOLITE REPRESSION. CAMP factor See CAMP TEST. cAMP phosphodiesterase See ADENYLATE CYCLASE. cAMP receptor protein See CATABOLITE REPRESSION. CAMP test (Christie–Atkins–Munch-Petersen test) A test used for the presumptive identification of group B streptococci. (Some group A streptococci may also give a positive test, particularly under anaerobic conditions [Book ref. 46, pp. 1590–1591].) The organism under test is inoculated in a fine streak on the surface of ox- or sheep-blood agar; a second streak – perpendicular to the first but separated from it by a few millimetres – is made with a culture of a b-HAEMOLYSIN-producing strain of Staphylococcus aureus. The plate is then incubated (aerobically) for ca. 12 hours be detected in infected cells. Mature viruses are released by host cell lysis. Caliciviruses are not sensitive to lipid solvents or to mild detergents, but are inactivated at pH 3–5. The family includes VESICULAR EXANTHEMA of swine virus (VESV), FELINE CALICIVIRUS (FCV) and SAN MIGUEL SEA LION VIRUS (SMSV); these three viruses are classified in the genus Calicivirus (type species: VESV serotype A). VESV, SMSV and FCV are readily propagated in cell culture and do not cause gastroenteritis (although they are pathogenic); morphologically similar viruses which have not been cultivated and which cause gastroenteritis in calves, piglets and humans (especially young children) are included as ‘possible members’ of the family [Book ref. 23, pp 133–134] (but see SMALL ROUND STRUCTURED VIRUSES). Calicivirus See CALICIVIRIDAE. California encephalitis (CE) An acute, usually mild, viral ENCEPHALITIS affecting mainly children below ca. 15 years of age; it occurs sporadically in forested areas in parts of North America. The CE virus (genus BUNYAVIRUS) occurs in small mammals and is transmitted by mosquitoes (mainly Aedes spp). California mastitis test (CMT; CM test) (vet.) A test, used for the indirect detection of MASTITIS, in which the number of white blood cells in milk is estimated. A reagent is added to the milk, causing a degree of gelation which corresponds to the white cell count; test results designated ‘negative’, ‘trace’, 1, 2 and 3 correspond to cell counts of ca. <105 , 3 × 105 , 9 × 105 , 27 × 105 , and 81 × 105 , respectively. The Brabant and Wisconsin mastitis tests also depend on a gelation principle. callose A linear (1 → 3)-b-D-glucan which occurs e.g. associated with sieve-plates and sieve-tubes in higher plants and in algae of the Laminariales (PHAEOPHYTA). Callose may be deposited in plant cell walls e.g. in response to wounding or invasion by pathogens. calmodulin A 16.7 kDa, heat-stable, acid-stable Ca2+ -binding protein found in eukaryotic cells, including those of certain protozoa and slime moulds. The complex of calmodulin with calcium ions stimulates a number of enzymes (e.g. cyclic nucleotide phosphodiesterase) and activates certain toxins (e.g. CYCLOLYSIN and the EF component of ANTHRAX TOXIN). Calocera A genus of fungi (order DACRYMYCETALES). C. cornea forms an erect, columnar, pointed, usually unbranched, yellow, gelatinous basidiocarp, ca. 1 cm high, on various types of wood. C. viscosa forms an erect, antler-like, yellow or orange, gelatinous basidiocarp, commonly 3–5 cm high, on coniferous wood. calomel See MERCURY. Calonectria See HYPOCREALES. calonectrin See TRICHOTHECENES. Caloplaca A genus of LICHENS (order TELOSCHISTALES) in which the thallus is crustose or placodioid, usually yellow, orange or red (containing PARIETIN) – but grey in some species. Apothecia: usually yellowish or reddish; ascospores: polarilocular in most species. Species grow on rocks and/or on wood. Caloramator See CLOSTRIDIUM. Calostoma See GASTEROMYCETES. Calothrix A genus of filamentous CYANOBACTERIA (section IV) in which the mature trichomes taper from base to apex and are composed of disc-shaped, isodiametric or cylindrical vegetative cells; a terminal heterocyst occurs at the base of the trichome. Hormogonia are uniform in width, and young trichomes from hormogonia each have a heterocyst at one end only. GC%: 40–44. The genus is taken to include members of the family RIVULARIACEAE. Species are common e.g. on coastal rocks in the littoral zone. calprotectin A zinc-chelating protein released from dying NEUTROPHILS at sites of inflammation (e.g. abscesses); it inhibits 119 Campanella 6NAD (P)H + 6H 6ATP COOH 3[CO2 ] 6 CH2O P + 6NAD (P) COO P 3-phosphoglycerate kinase 6 CH2O 3-phosphoglycerate + 6Pi BIOSYNTHESIS HCOH triose phos P p h at ed 1,3-bisphosphoglycerate ehy dr og Co Bis Ru HCOH 6ADP en as e CH2O P 3 t ke ns tra HCOH HCOH CO 1 CHO 1 CH2O sedoheptulose 7-phosphate P CH2OH ribose phosphate isomerase HOCH fructose 1,6-bisphosphatase H2O 1 sedoheptulose HCOH CO HCOH HOCH 1,7-bisphosphate aldolase 1 CH2O P ribulose 5-phosphate CHO HCOH HCOH 1 H 2O sedoheptulose 1,7-bisphosphate erythrose 4-phosphate CH2OH CO 1 CH2OH HOCH HCOH HCOH CO 2 fructose 1,6-bisphosphatase Pi HCOH CH2O P CH2O P phosphoketopentose epimerase CH2O P fructose 1,6-bisphosphate HCOH HCOH HCOH HCOH CH2O P CO 3 CH2O P CO Pi ribose 5-phosphate CH2O P fructose 1,6-bisphosphate aldolase CH2O P HCOH HCOH P dihydroxyacetone phosphate HCOH HCOH 6 glyceraldehyde 3-phosphate CO CH2O HCOH HCOH 3ADP 2 HCOH trios om te is ha osp e ph CH2OH HOCH ribulose 1,5-bisphosphate 3ATP CHO e eras CH2OH CH2O P phosphoribulokinase e as ol CO CH2O P HOCH HCOH tra n CH2O P fructose 6-phosphate sketola s e xylulose 5-phosphate CALVIN CYCLE. For simplicity, the intermediate withdrawn for biosynthesis is shown as glyceraldehyde 3-phosphate. In reality, other intermediates may be withdrawn. For example, 3-phosphoglycerate is a precursor for the synthesis of various amino acids, fatty acids, purines and pyrimidines; withdrawal of 3-phosphoglycerate rather than glyceraldehyde 3-phosphate reduces the immediate energy cost of the cycle by 1 ATP and 1 NAD(P)H per 3 CO2 fixed. or more at 37° . In a positive test, a typical arrowhead- or flame-shaped area of clear haemolysis – due to synergy between a streptococcal extracellular polypeptide (‘CAMP factor’) and staphylococcal b-HAEMOLYSIN – occurs between the two lines of bacterial growth. [Conditions affecting CAMP factor-induced haemolysis: JGM (1985) 131 817–820.] The CAMP test is used e.g. for the detection of Streptococcus agalactiae (a causal agent of bovine MASTITIS) in milk; milk which is moderately or heavily contaminated with S. agalactiae may itself give a positive CAMP test if streaked onto the test plate in place of an isolate of the organism, as may a culture filtrate of S. agalactiae. A ‘reverse CAMP test’, using a known group B streptococcus, can be used for the identification of b-haemolysin-producing staphylococci. Campanella A genus of sedentary ciliate protozoa (subclass PERITRICHIA) found in freshwater and brackish habitats. Campanella resembles VORTICELLA (q.v.) but its zooid is larger and it is a colonial organism, one zooid occurring on each branch of a non-contractile stalk. Campbell model A model, originally proposed by Campbell [Adv. Gen. (1962) 11 101–145], in which a circular plasmid or phage genome integrates into a bacterial chromosome by a single cross-over between the two circular molecules. (See e.g. BACTERIOPHAGE l; cf. BACTERIOPHAGE f105.) Campylobacter A genus of Gram-negative, asporogenous bacteria of the family CAMPYLOBACTERACEAE. Cells: slender, spirally curved rods, usually ca. 0.2–0.5 × 1.0–5.0 µm; POLAR MEMBRANES are present. (In old cultures, spherical or COCCOID BODIES may occur.) Motile; a single unsheathed flagellum occurs at one or both poles of the cell. Microaerophilic; growth is optimal in 5% oxygen, 10% carbon dioxide and 85% nitrogen. Metabolism: respiratory. Carbon sources: amino acids or TCA cycle intermediates, but not carbohydrates. Oxidase +ve. Indole −ve. Nitrate is reduced. Lipase −ve. MR −ve. VP −ve. GC%: 30–38. Type species: C. fetus. Campylobacter species occur in the reproductive and intestinal tracts of man and animals. Some species are pathogenic (see CAMPYLOBACTERIOSIS.) C. coli. Closely resembles C. jejuni, but e.g. does not hydrolyse hippurate. C. concisus. Cells: small, curved. Catalase −ve. Hydrogen sulphide (TSI) +ve. No growth at 25° C. Hippurate and indoxyl acetate are not hydrolysed. Occurs in the gingival crevice flora 120 Candida in humans with gingivitis, periodontitis etc., but apparently nonpathogenic. C. fetus. Cells appear comma- or S-shaped or ‘gull-winged’. (Loosely wound spiral filaments (up to 8 µm in length) occur in old cultures.) Catalase +ve. Hydrogen sulphide (TSI) −ve. Growth occurs at 25° C but not at 42° C. Hippurate and indoxyl acetate are not hydrolysed. Pathogenic. C. jejuni. Cells: small, tightly coiled spirals. On exposure of cultures to air, coccoid bodies form rapidly. SWARMING occurs on moist agar (the DIENES PHENOMENON does not occur). Catalase +ve. Hydrogen sulphide (TSI) −ve. Growth occurs at 42° C but not at 25° C. Hippurate and INDOXYL ACETATE are hydrolysed. Pathogenic. [PCR-RFLP typing of C. jejuni : LAM (1996) 23 163–166; PGFE typing of Penner HS11 strains of C. jejuni : JMM (1998) 47 353–357.] C. pylori. Re-classified: see HELICOBACTER. C. sputorum. Cells: slender, curved rods, appearing commashaped, ‘gull-winged’ or occasionally filamentous. Catalase −ve. Hydrogen sulphide (TSI) +ve. Growth occurs at 42° C (except for subsp bubulus) but not at 25° C (except for some strains of subsp bubulus). Hippurate and indoxyl acetate are not hydrolysed. Subsp bubulus and sputorum can grow anaerobically with fumarate; subsp mucosalis requires hydrogen or formate (as well as fumarate) for anaerobic growth. Subsp bubulus occurs in the genital tract in cattle and sheep, apparently as a commensal. Subsp mucosalis is found in pigs, apparently as a commensal in the mouth and as a pathogen, causing enteric disease. Subsp sputorum occurs in the gingival crevice flora of man but is apparently non-pathogenic. (See also ARCOBACTER and HELICOBACTER.) [Book ref. 22, pp 111–118. Culture, media etc.: Book ref. 219, pp 444–446. Campylobacter spp (genotyping: minireview): AEM (2000) 66 1–9.] Campylobacteraceae A family comprising the genera ARCOBACTER and CAMPYLOBACTER. campylobacteriosis (med., vet.) Any human or animal disease caused by a strain of CAMPYLOBACTER. In man, disease is commonly food-borne, and is usually caused by C. jejuni or C. coli ; less frequently isolated pathogens include C. hyointestinalis and C. upsaliensis. Typically, disease involves diarrhoea/enterocolitis, but disseminated infection can occur in the immunocompromised patient, and meningitis may develop in neonates. The usual sources of infection are inadequately cooked meats, especially poultry, unpasteurized milk and untreated (or contaminated) water; cross-contamination of food, particularly from raw (uncooked) poultry, is an important factor. The incubation period (∼1–7 days) may be followed by several days of watery diarrhoea, often with abdominal pain. In severe cases the diarrhoea may be bloody; in some cases, symptoms resemble those of acute ulcerative colitis. In a small number of cases patients subsequently develop reactive arthritis; very rarely they develop the GUILLAIN–BARRÉ SYNDROME. Chemotherapy. For persistent cases: erythromycin or e.g. ciprofloxacin. [Campylobacter infections (reviews): RMM (1997) 8 113– 124; Microbiology (1997) 143 5–21.] In animals, campylobacters are commonly found as commensals in the gut. However, C. fetus can cause abortion in sheep (and less often in cattle). The venerealis subspecies of C. fetus causes abortion and infertility in cattle, while C. sputorum subsp mucosalis causes enteric disease in pigs. (See also WINTER DYSENTERY.) CaMV CAULIFLOWER MOSAIC VIRUS. Canada balsam A clear, ethanol-insoluble resin, obtained from fir trees (Abies sp), used (dissolved in xylene or benzene) as a MOUNTANT. It takes weeks to dry (harden) at room temperature; the refractive index of the dry resin is ca. 1.53. Canada balsam becomes yellowish with age owing to oxidation. (See also DPX.) Canadian vole agent See BARTONELLA. canarypox virus See AVIPOXVIRUS. L-canavanine 2-Amino-4-guanidinohydroxybutyric acid. (See also CGB AGAR.) cancellous (cancellate) Spongy; reticulated. cancer Any disease of man or animals in which abnormal, unregulated proliferation of cells results in the formation of a malignant tumour (e.g. a CARCINOMA or SARCOMA); as the tumour grows, it tends to invade and destroy adjacent tissues and may shed cancer cells which can then disseminate (metastasize) to other areas of the body, resulting in the formation of secondary tumours. The causes of cancer are complex and appear to be varied and multifactorial; in general, the detailed mechanisms are not fully understood. Oncogenesis appears commonly to involve an irreversible change in the genome of the cell, such a change ranging from a single point mutation to chromosomal rearrangements. (See also ONCOGENE.) In only relatively few cases have microorganisms been implicated as direct or indirect causes of cancer. Certain viruses are known to induce tumours and e.g. LEUKAEMIAS in animals and man (see DNA TUMOUR VIRUSES and ONCOVIRINAE); others may play an indirect role in predisposing to cancer (e.g. by suppressing the normal host immunological defence mechanisms). (cf. PARVOVIRUS.) [Viruses and cancer: Book ref. 110.] Certain bacteria and fungi may play an indirect role in causing cancer by producing cancer-promoting substances (CARCINOGENS); e.g., certain MYCOTOXINS (e.g. AFLATOXINS) can be oncogenic, and certain bacteria of the human GASTROINTESTINAL TRACT FLORA may be responsible for the formation of substances which may play a role in the induction of bowel cancer. However, some microbial products have anticancer activity: e.g. ANTHRACYCLINE ANTIBIOTICS, ANTHRAMYCIN, BLEOMYCIN, etc. See also e.g. IMMUNOTOXINS and NK CELLS; cf. GALLS. Candelariella A genus of LICHENS (order LECANORALES); photobiont: a green alga. Thallus: crustose, yellow, lacking parietin. Apothecia: lecanorine. candicidin B A heptaene POLYENE ANTIBIOTIC produced by Streptomyces griseus; its spectrum of activity is similar to that of AMPHOTERICIN B, and it is particularly effective against Candida albicans. Candida (formerly e.g. Monilia) A large and heterogeneous genus of yeast-like or dimorphic imperfect fungi (class HYPHOMYCETES). (Although Candida has been traditionally classified in the Fungi Imperfecti, analysis of the genome of C. albicans has indicated a genetic repertoire that could support a sexual cycle [PNAS (2001) 98 3249–3253].) Some species occur as commensals and/or as opportunist pathogens in man and animals (see CANDIDIASIS), and some can be isolated from plants, soil, foods etc. (See also e.g. CIDER, GARI, MEAT SPOILAGE.) The yeast-like cells are e.g. spherical, ovoid, cylindrical or elongate, and reproduce by (usually multilateral) budding. Pseudomycelium may be absent, rudimentary, or well developed, according to species and/or conditions; some species can form a true, septate mycelium. Blastospores and chlamydospores may be formed. Carotenoid pigments are not produced. Many species can ferment glucose and other sugars. Inositol and NO3 − may or may not be assimilated; inositol-assimilating strains form 121 candidate virus of whitish mucoid plaques on the mucous membranes. (See also NON-GONOCOCCAL URETHRITIS.) Cutaneous candidiasis tends to occur on skin areas constantly exposed to moisture, regions commonly affected including e.g. the groin and axillae (see also PARONYCHIA); infected skin is swollen, red and pruritic. Chronic mucocutaneous candidiasis is a severe condition which occurs in immunocompromised or otherwise abnormal individuals; the skin and mucous membranes of the entire body may be affected, with chronic, granulomatous, inflammatory reactions in the underlying tissues. Other forms of candidiasis include bronchocandidiasis, pulmonary candidiasis (often secondary to other diseases), and systemic candidiasis (involving e.g. fungaemia, endocarditis, nephritis etc – cf. TORULOPSOSIS). Lab. diagnosis: microscopical and cultural examination of material from lesions etc (see CANDIDA). In animals, candidiasis may involve the intestinal tract, skin, and/or various internal organs (see also MASTITIS); in calves and piglets the intestine is usually involved, and infection can cause watery diarrhoea, anorexia, dehydration, and even death. In birds, candidiasis commonly involves the digestive tract – particularly the crop. [Book ref. 103.] candidosis Syn. CANDIDIASIS. candle (verb) To carry out CANDLING. candle filter See FILTRATION. candle jar A container within which cultures etc can be incubated under an increased partial pressure of CO2 ; the CO2 is provided by placing a lighted candle in the container prior to closure. candle-snuff fungus See XYLARIA. candling (virol.) A procedure for observing the contents of an intact egg by transmitted light. An EMBRYONATED EGG is placed over an aperture in a box containing an electric lamp; internal details of the egg – such as the positions of the embryo and blood vessels – can be seen, thus facilitating the inoculation of a particular membrane or cavity of the egg. Candling is best performed in a darkened room, and should be carried out rapidly to avoid overheating the egg. cane blight In raspberry canes: a disease caused by Leptosphaeria coniothyrium. Infection occurs mainly via wounds; leaves shrivel and die, dark patches appear on the stem bases, and bark associated with these patches cracks open. cane-cutters’ disease LEPTOSPIROSIS caused by L. interrogans serotype australis A; the disease is common among cane-cutters in N. Queensland, Australia. cane gall A disease of Rubus spp (raspberry, blackberry etc) in which small spherical galls or elongated ridges develop on the stems. Causal agent: Agrobacterium rubi. cane spot (of raspberry) See ELSINOË. canicola fever LEPTOSPIROSIS caused by L. interrogans serotype canicola. canid herpesvirus 1 See ALPHAHERPESVIRINAE. canine distemper An acute, infectious disease of dogs and other animals (e.g. foxes, mink, raccoons, wolves) caused by a virus of the genus MORBILLIVIRUS. Infection occurs mainly by droplet inhalation, also e.g. by ingestion. Incubation period: ca. 3–30 days. Symptoms initially include fever, nasal and ocular discharge, and listlessness. The fever subsides, then recurs with vomiting, diarrhoea, and often pneumonia; signs of CNS involvement (e.g. fits) may occur. (cf. HARD-PAD.) The disease affects mainly young animals. However, persistent infection may result in ‘old dog encephalitis’ – a condition characterized by progressive neurological degeneration. The distemper virus pseudomycelium. The species [196 are described (with keys) in Book ref. 100, pp. 585–844] are distinguished e.g. on the basis of metabolic tests, growth requirements, etc. C. albicans (a species which incorporates e.g. ‘C. stelloidea’) occurs as a commensal e.g. in the mouth and in the genital and intestinal tracts in man; C. albicans is the commonest cause of CANDIDIASIS. Cells: e.g. ca. 3–6 × 5–10 µm. On e.g. cornmeal agar, pseudomycelium may be abundant, typically bearing grapelike clusters of blastospores. True mycelium may be formed, and thick-walled chlamydospores are formed e.g. on cornmeal agar at 20° C. C. albicans can ferment glucose and maltose, but not e.g. lactose. The ability to form GERM TUBES after incubation in serum for 2 hours at 37° C is presumptive evidence of C. albicans. It has been demonstrated that C. albicans can switch between at least seven different phenotypes (identified by colony morphology on agar media); the switching is heritable, reversible, and occurs at high frequency (increased further by low doses of UV radiation) [Science (1985) 230 666–669]. Such switching may be important in the role of C. albicans as an opportunist pathogen (cf. ANTIGENIC VARIATION), but its mechanism is unknown. C. tropicalis can also cause candidiasis. On e.g. cornmeal agar, pseudomycelium is abundant and consists of long, branched pseudohyphae bearing blastospores singly or in short chains and clusters. True mycelium and chlamydospores may also be formed. C. utilis (= Torulopsis utilis; teleomorph: Hansenula jadinii ) is an important food yeast (see SINGLE-CELL PROTEIN, TORULA YEAST, YEAST EXTRACT). On cornmeal agar an abundant pseudomycelium is formed, consisting of branched chains of short, coarse pseudohyphae and oval cells. True mycelium is not formed. A few Candida spp have basidiomycetous affinities: e.g. C. frigida, C. gelida, C. nivalis and C. scottii have teleomorphs in the genus Leucosporidium, C. japonica in FILOBASIDIUM. Many species have teleomorphs among the ascomycetes: e.g., C. ciferrii (STEPHANOASCUS), C. colliculosa (TORULASPORA), C. dattila (KLUYVEROMYCES), C. domercqii (WICKERHAMIELLA), C. globosa (CITEROMYCES), C. guilliermondii (PICHIA), C. holmii (SACCHAROMYCES), C. kefyr (= C. pseudotropicalis) (KLUYVEROMYCES), C. krusei (ISSATCHENKIA), C. lipolytica (Yarrowia lipolytica – see SACCHAROMYCOPSIS), C. lusitaniae (CLAVISPORA), C. pelliculosa (HANSENULA), C. pintolopesii (SACCHAROMYCES), C. pulcherrima (METSCHNIKOWIA), C. reukaufii (METSCHNIKOWIA), C. sorbosa (ISSATCHENKIA), C. sphaerica (KLUYVEROMYCES), C. utilis (HANSENULA), C. valida (PICHIA), C. variabilis (PICHIA). (C. hellenica, C. inositophila, C. steatolytica: see ZYGOASCUS.) candidate virus (1) A virus which is of uncertain taxonomic affinity but which may belong to (i.e., is a ‘candidate’ for) a particular taxonomic group; e.g. a ‘candidate calicivirus’ is a virus which may belong to the family Caliciviridae. (2) A virus which might be the causal agent of a particular disease. candidiasis (candidosis; moniliasis) A MYCOSIS, affecting man and animals, caused by a species of Candida – usually C. albicans (see CANDIDA); other pathogenic species include e.g. C. parapsilosis and C. tropicalis. Infection is probably endogenous in most cases; predisposing factors include e.g. suppression of the normal body microflora (e.g. by antibiotic therapy), impaired immune responses, etc. In man, candidiasis commonly occurs as a localized infection of the mouth (oral thrush) or vagina (vaginal thrush), and involves the formation 122 Cantharellales has also been associated (rarely) with a demyelinating disease in man. Effective anti-distemper vaccines are available. canine herpesvirus See ALPHAHERPESVIRINAE. canine parvovirus A subspecies of the feline parvovirus (genus PARVOVIRUS) which appeared initially in Europe and then spread rapidly in 1978, causing worldwide outbreaks of disease in dogs. Infection may occur via the nasopharynx or via lymphoid tissues, including the tonsils. Viraemia leads to generalized infection of lymphoid tissues. Levels of erythrocytes (red blood cells) are not affected, and panleukopenia is infrequent in the dog (cf. FELINE PANLEUKOPENIA VIRUS). Infection of dividing epithelial cells in the ileum and jejunum impairs osmotic regulation and leads to diarrhoea (often containing blood/mucus); large numbers of virions are shed during the diarrhoeal phase. The disease can be associated with high mortality rates, but the severity varies in individual animals – some infections being mild/subclinical. In neonatal puppies there is no manifestation of enteritis. In these animals infection may lead to death from myocarditis, often at 3–8 weeks of age (but sometimes up to 4 months of age). Onset of disease is rapid, typically with cardiac arrhythmia, pulmonary oedema and dyspnoea; the myocardium suffers multifocal necrosis. Infrequently, infection of the neonatal animal gives rise to lesions in a variety of tissues. Canine parvovirus is apparently distinct from the ‘minute virus of canines’ (sometimes also called ‘canine parvovirus’) – a virus which is widespread in canine populations and which can be isolated from the faeces of asymptomatic dogs. [Pathogenesis of canine parvovirus: BCH (1995) 8 57–71.] canker (plant pathol.) An imprecise term usually used for a plant disease which is characterized (in woody plants) by the death of cambium tissue and resulting loss and/or malformation of bark, or (in non-woody plants) by the formation of sharply delineated, dry, necrotic, localized lesions on the stem. (The term ‘canker’ may also be used to refer to the lesion itself, particularly in woody plants.) Cankers may be caused by bacteria (e.g. bacterial canker of cherry and plum is caused by Pseudomonas syringae pv. morsprunorum) or fungi (see e.g. APPLE CANKER, BEECH BARK DISEASE, BLEEDING CANKER, PITCH CANKER). canning A form of APPERTIZATION in which, typically, suitably prepared foods are put into metal containers (‘cans’, ‘tins’) which are then exhausted, hermetically sealed, and heated; as an alternative, the food may be heat-treated before being placed aseptically in pre-sterilized cans which are then sealed under aseptic conditions. Most cans are made of tinplate (tin-plated steel) – see also TIN – but aluminium or other materials (e.g. lacquered tin-free steel) may be used. (See also RETORT POUCH.) (a) The canning process. Conventional canning involves the following stages. Food preparation. According to the type of food, this may involve grading, cutting, peeling, blanching (see also INDIVIDUAL QUICK BLANCH), washing etc; foods for canning must be as free as possible from microbial contamination because the efficacy of heat treatment is related to the level of contamination. Filling the cans. This process must take into account the fact that the efficiency of subsequent processes depends on the correct volume of free space (headspace) between the surface of the food and the top of the can. Exhausting the cans. The removal of air reduces the strain on the can during subsequent heating and inhibits long-term internal corrosion. Exhausting may be carried out by heating immediately prior to sealing, by sealing cold food into the can under vacuum (vacuumizing), or by injecting steam into the headspace during the sealing process. Sealing. The can end is added by a double-seaming process in which the edge of the can end is wrapped around the body flange to form a seam of five thicknesses of metal – seven at the side seam, if present. (See also BIOS TEST.) (Concentric rings (expansion rings) on the can ends facilitate double-seaming and help to relieve strain on the can seams during the heating process.) Heat processing. Cans may be heated in a BATCH RETORT or a COOKER–COOLER (see also STERIFLAMME PROCESS); in some cases cans are agitated to facilitate heat penetration, and in such cases an adequate headspace is essential. The temperature–time regime used in a particular case depends e.g. on the type and pH of the food. Following heat treatment, the food (i) must not contain Clostridium botulinum capable of growth and toxin production under the conditions of storage; (ii) should not contain organisms capable of causing spoilage; and (iii) should retain, as far as possible, its organoleptic properties. For non-acid foods (e.g. potatoes, mushrooms) – except foods containing curing salts – heat processing is usually to a minimum F0 VALUE of 3.0 in order to destroy the spores of C. botulinum (see also BOTULINUM COOK); for foods with a pH below 4.5, an F0 of 0.7 may be satisfactory because growth and toxin production by C. botulinum is generally considered not to occur at or below this pH (cf. CLOSTRIDIUM). Large cans of ham and other cured meats (pH above 5) are frequently processed at quite low temperatures (e.g. a centre temperature of ca. 70° C for a few minutes); such cans are necessarily stored under refrigeration – consumer safety and preservative action both relying on the combined effects of the curing salts and low temperature. Smaller cans of cured meats generally undergo a higher level of heat treatment and are shelf-stable, without refrigeration, owing to the ability of the curing salts to inhibit growth and toxin production by (heat-injured) pathogens [Book ref. 35]. Some canned foods (e.g. sweetened condensed milk) are not heated after can sealing; this product contains sufficient sugar to lower its WATER ACTIVITY to a level which inhibits most species of bacteria, and pre-PASTEURIZATION of the milk helps to prevent spoilage by fungi. Sweetened condensed milk (like many other – particularly acidic – canned foods) is usually non-sterile (cf. COMMERCIAL STERILITY) and often contains e.g. Bacillus spp and micrococci. Cooling. Cans are cooled as rapidly as possible to avoid overcooking; cooling may be carried out under pressure to prevent unnecessary strain on the can seams and possible leakage. Water used for cooling should be bacteriologically clean and generally of potable standard. (b) Spoilage of canned foods. Spoilage of the contents of a can may result from (i) mechanical damage to the can or leakage of imperfectly sealed cans (permitting the ingress of microorganisms), or (ii) the activities of thermoduric organisms or heat-stable enzymes in foods subjected to inadequate heat treatment. The type of spoilage depends on the type of food and the nature of the spoilage organism(s). (See also FLAT SOUR and SWELL.) Organisms which may survive the canning process include e.g. endospore-forming bacteria and certain fungi – e.g. Byssochlamys spp can cause pectinolytic softening in certain types of canned fruit (the ascospores of B. fulva can survive 87.7° C for 10 min). Canned fruit and vegetables may also be spoiled by heat-stable endopolygalacturonases (see PECTIC ENZYMES) produced by fungal contaminants prior to the canning process. Cantharanthus alkaloids Syn. VINCA ALKALOIDS. Cantharellales An order of fungi (subclass HOLOBASIDIOMYCETIDAE) in which the basidiocarp may be e.g. funnel-shaped (as 123 Cantharellus by a motile cell on contact with a surface. In Dictyostelium discoideum, capping appears to involve interaction between ACTIN microfilaments (which are linked, through the cytoplasmic membrane, to patches of ligand) and cortical MYOSIN [JCB (1985) 100 1884–1893]. (See also DYSENTERY (b).) caprine arthritis–encephalitis (CAE) A disease of goats caused by a retrovirus of the LENTIVIRINAE; symptoms include interstitial pneumonia, leucoencephalitis, inflammatory lesions in the CNS, and a progressive arthritis. caprinized vaccine A VACCINE containing live organisms whose virulence for a given host species has been attenuated by SERIAL PASSAGE through goats. Capripoxvirus (sheep-pox subgroup) A genus of viruses of the CHORDOPOXVIRINAE which cause disease in ungulates (see e.g. GOAT POX, LUMPY SKIN DISEASE and SHEEP POX); capripoxviruses can be transmitted mechanically by arthropod vectors. The virions are longer and narrower than those of orthopoxviruses; haemagglutinin is not produced, and infectivity can be destroyed e.g. by ether. The viruses in this group are very closely related serologically [JGV (1986) 67 139–148]. capsduction A mode of gene transfer in strains of Rhodopseudomonas capsulata: DNA transfer from donor to recipient is mediated by a phage-like particle known as the gene transfer agent (GTA). Although the GTA morphologically resembles a tailed bacteriophage (head: apparently icosahedral, ca. 30 nm diam., with short spikes; tail: variable in length, bearing tail fibres) no phage-specific DNA has been detected; the linear dsDNA (MWt ca. 3 × 106 ) carried by the GTA is derived entirely from the host cell (chromosome or plasmid), and any region of the host genome can be packaged. Strains of R. capsulata which do not naturally produce GTA can act as recipients in capsduction but do not acquire the ability to produce GTA in consequence. Capsduction has been useful for mapping and genetic analysis in R. capsulata. (cf. TRANSDUCTION.) capsid In a virion: a protein coat or shell which surrounds either the nucleic acid genome or a nucleoprotein CORE; it commonly exhibits ICOSAHEDRAL SYMMETRY. (See e.g. ADENOVIRIDAE.) In some viruses the capsid is itself surrounded by an ENVELOPE (see e.g. TOGAVIRIDAE). (cf. NUCLEOCAPSID.) capsidiol A PHYTOALEXIN (a derivative of 5-epiaristolochene) produced e.g. by the tobacco and green pepper plants. capsomer (capsomere) See ICOSAHEDRAL SYMMETRY. capsule (1)(a) In prokaryotes, a layer of material external to but contiguous with the CELL WALL; the term thus includes any polysaccharide and/or protein surface layer(s) (including the bacterial S LAYER) but excludes those S layers which constitute the cell wall proper in members of the Archaea. (cf. GLYCOCALYX.) In some bacteria (e.g. Beijerinckia) a capsule may enclose more than one cell. Capsules are commonly divided into three categories. (i) Macrocapsules or ‘true’ capsules are sufficiently thick to be easily visible (with negative staining – see CAPSULE STAIN) on light microscopy. (ii) Microcapsules cannot be observed by light microscopy, but their presence may be revealed by electron microscopy or by serological techniques (see e.g. QUELLUNG PHENOMENON); some bacteria which have lost the ability to form a macrocapsule may form a serologically identical microcapsule. (iii) Slime layers are diffuse secretions which may adhere loosely to the cell surface; slime layers commonly diffuse into the medium when the organism is grown in liquid culture, and may be too permeable to stains to be observable by microscopy. (Since capsules normally contain a high proportion of water, they may not be detectable in specimens dehydrated prior to microscopy.) e.g. in Cantharellus infundibuliformis and Craterellus cornucopioides) or stalked and pileate; the hymenophore may be smooth, wrinkled, or lamella-like. Cantharellus cibarius (the ‘chanterelle’) is an edible species which forms a deep yellow, flat or centrally concave pileus on a stipe which tapers towards the base; the ‘gills’ are yellow, forked and decurrent, and the basidiospores are ochre, ellipsoidal, ca. 8–10 × 5 µm. Cantharellus see CANTHARELLALES. cap (1) (mycol.) The PILEUS of a mushroom-shaped fruiting body. (2) (mol. biol.) See MRNA (b). (3) See CAPPING sense 3. CAP Catabolite activator protein: see CATABOLITE REPRESSION. cap-binding protein See MRNA (b). capacity test A type of test used to determine the capacity of a DISINFECTANT to retain activity in the presence of increasing numbers of bacteria. Essentially, aliquots of a bacterial suspension are periodically added to a fixed volume of disinfectant solution which is subcultured (to detect viable bacteria) at fixed intervals of time after the addition of each aliquot. (See e.g. KELSEY–SYKES TEST.) capillitium A system of (non-cellular) threads or filaments which ramify through the spore mass in the fruiting bodies of certain slime moulds of the MYXOMYCETES and fungi of the GASTEROMYCETES. In myxomycetes, the capillitial threads may be solid or hollow, free or interconnected, smooth or ornamented, and in certain species they may bear nodes or encrustations of lime (see e.g. FULIGO); the nature of the capillitium is an important taxonomic feature. At least some types of capillitium may aid in spore dispersal; some are elastic, helping to expose spores when the peridium ruptures, while others are hygroscopic, their movements in response to changes in humidity helping to dislodge and release the spores. capneic Syn. CAPNOPHILIC. Capnocytophaga A genus of Gram-negative, capnophilic GLIDING BACTERIA found in the human oral cavity and associated with e.g. PERIODONTITIS, abnormalities in neutrophil function, and – in compromised patients – systemic disease. The organisms are flexible fusiform rods, ca. 0.3–0.5 × 3–8 µm. Metabolism: fermentative (glucose being fermented to succinic and acetic acids) or respiratory (with O2 as terminal electron acceptor). Although CO2 may be required for initial isolation, high levels of CO2 are not necessarily required for fermentation or oxidation of glucose [IJSB (1985) 35 369–370]. Agar is not liquefied. Optimum growth temperature: 37° C. Species: C. gingivalis, C. ochracea (formerly Bacteroides ochraceus), and C. sputigena. Capnodium See SOOTY MOULDS. capnophilic Refers to any organism whose growth either requires, or is stimulated by, higher-than-atmospheric levels of carbon dioxide. capped mRNA See MRNA (b). capping (1) See MRNA (eukaryotic). (2) The binding of a specific protein or agent to the end of an ACTIN microfilament or a microtubule (see MICROTUBULES), resulting e.g. in cessation of polymerization/assembly at that end. (3) In some types of eukaryotic cell: a process which is initiated when cell-surface molecules are cross-linked by multivalent ligands (e.g. antibodies, lectins). The cross-linked molecules from different regions of the cell surface initially cluster into groups or ‘patches’ (patching); subsequently, all the patches undergo an ATP-dependent translocation to one pole of the cell, forming a ‘cap’ (capping). Capping appears to occur only in cells capable of motility, and it has been suggested that it is the expression of a response similar to that given 124 carbomycin capsule stain Bacterial CAPSULES may be demonstrated e.g. by negative STAINING: bacterial growth is mixed with a loopful of e.g. India ink or aqueous NIGROSIN on a clean slide and overlaid with a cover-glass; under the high-dry or oil-immersion objective the capsule appears as a clear zone between the cell wall and the dark background. capsule swelling reaction Syn. QUELLUNG PHENOMENON. captafol (difolatan; N-(1,1,2,2-tetrachloroethylthio)-cyclohex-4ene-1,2-dicarboximide) An agricultural antifungal agent related to CAPTAN; it is used as a PROTECTANT e.g. against late blight of potato. captan An antibacterial and ANTIFUNGAL AGENT (N-(trichloromethylthio)-cyclohex-4-ene-1,2-dicarboximide) widely used as an agricultural PROTECTANT; it is effective against a number of fungal plant pathogens, and is used e.g. for the prevention of apple scab and leaf-spot diseases (such as black spot of roses), and as a component of seed dressings. The activity of captan has been attributed to the −SCCl3 group which may react with essential −SH groups within the fungal cell. Captan is almost insoluble in water and is often formulated as a WETTABLE POWDER. (cf. CAPTAFOL.) carate Syn. PINTA. carbamate kinase See e.g. ARGININE DIHYDROLASE. carbamic acid derivatives (as antimicrobial agents) Antimicrobial derivatives of carbamic acid (NH2 COOH) include UREA, derivatives of thiocarbamic acid, NH2 .CS.OH (e.g. TOLNAFTATE), and DITHIOCARBAMATES. (See also BENOMYL.) carbamide Syn. UREA. carbanilides (as antimicrobial agents) Carbanilide (diphenyl
urea, (C6 H5 NH)2 CO) and substituted carbanilides (e.g. 3,4,4 trichlorocarbanilide, TCC, = triclocarban) are active mainly against Gram-positive bacteria; they appear to affect the functioning of the cytoplasmic membrane. TCC is also active against certain fungi (e.g. Trichophyton spp). carbapenems A class of b-LACTAM ANTIBIOTICS (q.v. for structure) produced (extracellularly) by Streptomyces spp; in addition to possessing antibacterial activity, many carbapenems are inhibitory to b-LACTAMASES. (cf. CLAVULANIC ACID.) Carbapenems are stable in aqueous solution near neutral pH. They include e.g. asparenomycins, epithienamycins, olivanic acids, pluracidomycins, PS-5, and THIENAMYCIN. carbendazim (MBC) An agricultural antifungal agent – the active derivative of many of the BENZIMIDAZOLES. Carbendazim is used e.g. in the treatment of eyespot of wheat and barley and (as a seed treatment) as a protectant against damping off caused by Rhizoctonia solani. It is often mixed with other antifungal agents – e.g. PROPICONAZOLE or PROCHLORAZ – for use as a broad-spectrum antifungal preparation. carbenicillin See PENICILLINS. carbofuran A broad-spectrum carbamate insecticide used as a soil treatment for the control of insect pests on a range of crop plants. The persistence of carbofuran in the field is limited owing to the wide range of soil microorganisms which can degrade the compound [FEMS Ecol. (2000) 34 173–180]. carbolfuchsin A red dye used e.g. in the GRAM STAIN and in ZIEHL–NEELSEN’S STAIN. Concentrated carbolfuchsin is made by adding 10% basic FUCHSIN in 95% ethanol (10 ml) to 5% aqueous phenol (100 ml); the solution is filtered after standing overnight. carbolic acid Syn. phenol (see PHENOLS). carbolic soap See SOAPS. Carbomyces See PEZIZALES. carbomycin See MACROLIDE ANTIBIOTICS. Capsule composition varies with species, growth conditions etc; serologically distinguishable capsules may be formed even by different strains of the same species (e.g. strains of Streptococcus pneumoniae can be distinguished on this basis). Most capsules consist of polysaccharide – either homopolysaccharide (see e.g. CELLULOSE; COLOMINIC ACID; DEXTRAN; LEVAN) or heteropolysaccharide (see e.g. ALGINATE; COLANIC ACID; HYALURONIC ACID). In virulent strains of B. anthracis the capsule consists of a homopolymer of g-linked poly-D-glutamic acid. Xanthobacter spp can form both an a-polyglutamine capsule and copious polysaccharide slime. (See also M PROTEIN (sense 1).) The nature of the binding between the capsule and cell wall is largely unknown, but may involve ionic and/or covalent bonding. For example, capsular material covalently bound to peptidoglycan has been isolated from B. anthracis; in Azotobacter vinelandii and some mucoid strains of Pseudomonas aeruginosa retention of the alginate capsule depends on the level of Ca2+ in the growth medium, alginate being released into the medium at low Ca2+ levels. Biosynthesis of polysaccharide capsules generally appears to involve transglycosylation from sugar–nucleotide precursors; the subunits are commonly transferred to the exterior of the cell via phosphorylated isoprenoid lipid carriers in the cell membrane. In nature, bacterial capsules have various functions: they may act as permeability barriers (protecting e.g. against heavy metal toxicity) and/or as ion exchange systems; they may prevent desiccation, phagocytosis by protozoa, or phage infection (by covering receptor sites and/or by immobilization of the phage); they may promote bacterial adhesion to surfaces (see e.g. DENTAL PLAQUE) and/or play a role in recognition systems in interactions with other organisms (see e.g. ROOT NODULES); and they may act as a nutrient reserve (see e.g. XANTHOBACTER). Among pathogens, capsule formation often correlates with pathogenicity; in an animal host, a capsule may allow the pathogen to evade the host’s defence mechanisms (see e.g. PHAGOCYTOSIS) [review: JID (1986) 153 407–415], while in a plant host capsular material may be involved in the infection process (see e.g. CROWN GALL). (See also e.g. VI ANTIGEN.) In vitro, colonies of capsulated bacteria on solid media are typically mucoid. Capsules may be lost on prolonged in vitro culture, but may be retained if a selective agent (e.g. sublethal levels of a surfactant or antibiotic) is incorporated in the medium. A number of extracellular bacterial polysaccharides have important commercial applications: see e.g. CURDLAN; DEXTRAN; GELLAN GUM; XANTHAN GUM. (b) (mycol.) Structures analogous to bacterial capsules occur in many fungi: gel-like or mucilaginous layers may surround hyphae or yeast cells and may sometimes diffuse into the surrounding medium. For example, in Schizophyllum commune a water-soluble glucan is loosely associated with the hyphal walls and may be released into the medium; the glucan consists of a (1 → 3)-b-D-linked chain with single glucose residues attached by (1 → 6)-b-D-linkages at (on average) every third residue [Book ref. 62, p. 139]. Cells of Cryptococcus spp form capsules of an amylose-like polymer e.g. when cultured in media of pH <3.0, or (C. neoformans) in the tissues of an infected host animal. In certain plant-pathogenic fungi, capsular material may be involved in the recognition of and/or attachment to a suitable host plant [e.g. Drechslera (Helminthosporium) oryzae/rice: Phytopath. (1982) 72 285–292]. (2) (entomopathol.) See BACULOVIRIDAE. (3) (protozool.) See e.g. RADIOLARIA and DESMOTHORACIDA. 125 carbon cycle an imbalance in carbon cycling may mean that periods of net autotrophy (when autotrophic fixation of carbon dioxide exceeds bacterial respiratory output) may occur at different times of the year to compensate for periods of net heterotrophy [Nature (1997) 385 148–151]. Carbon cycling is relevant to the GREENHOUSE EFFECT. Moreover, the evolution of the carbon cycle is an interesting topic in itself [FEMS Reviews (1992) 103 347–354]. carbon dioxide (CO2 ) (a) CO2 as a food preservative. CO2 (5–50%) inhibits the growth of many types of microorganism and is used in FOOD PRESERVATION – e.g. for meats (see MEAT SPOILAGE), fish, fruit and vegetables (in which CO2 also extends storage life e.g. by delaying ripening), and for carbonated beverages. In general, CO2 inhibits moulds more than yeasts, Gram-negative bacteria more than Gram-positive species. CO2 dissolves in water to form carbonic acid, H2 CO3 , which dissociates to form H+ and HCO3 − (pK 6.37); some inhibition may thus be due to a pH effect. However, CO2 also exerts a direct antimicrobial effect which is largely independent of either pH or oxygen concentration. The mechanism of this effect is unknown, but direct inhibitory effects of CO2 on certain enzymes have been reported; it has also been suggested that CO2 dissolves tio n CO2 photo auto n of al p me nutrition microbial biomass animals thy l o tr oph s tr i (h er h/ n de ti o ca y nu et algae bacteria plants nutrition excretion death /decay ms CH4 a n is d an la nt s o rg e s ga en respiration em ic ph ro ot ut oa sp ch ti o re ic, i ra og op hic ph sp h an het ero tr tr o re ti on m er ,f ta en met ira carbon cycle In nature: the cyclical interconversion of carbon compounds (see figure); carbon in its elemental form appears to play no significant role in biological carbon metabolism (cf. NITROGEN CYCLE and SULPHUR CYCLE). A major metabolite in the carbon cycle is carbon dioxide – see CARBON DIOXIDE (b). In AUTOTROPHs, carbon dioxide can be ‘fixed’ (i.e. incorporated into biomass) via several distinct pathways; the energy required for carbon dioxide fixation may be light energy (see PHOTOSYNTHESIS) or energy derived from chemical reactions. Carbon dioxide can also be used in a dissimilatory manner: see METHANOGENESIS. The pool of carbon dioxide is replenished from organic matter by MINERALIZATION, a process in which both oxidative (respiratory) and fermentative forms of metabolism are involved. (See also SAPROTROPH.) In aquatic environments, at least some dissolved organic carbon derives from phytoPLANKTON; in the oceans almost all dissolved carbon derives from this source. (See also PRIMARY PRODUCTION.) Some of the dissolved carbon is used for bacterial growth and respiration, and it appears that, at certain times and locations, the amount of carbon dioxide produced by bacterial respiration may exceed that fixed by the phytoplankton; such ot ro de ph at s) organic carbon pool CARBON CYCLE: simplified scheme showing some of the major interconversions of carbon in nature (see entry). Bacteria have significant roles, as both autotrophs and heterotrophs, and certain members of the Archaea have unique roles as methanogens; the methylotrophs include methane-utilizing bacteria. Various microorganisms, including bacteria and fungi, are responsible for the essential conversion of ‘dead’ organic carbon to biomass and carbon dioxide; without this process the cycle would stop. Reproduced from Bacteria 5th edition, Figure 10.1, page 256, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 126 carcinogen aspects: MS (1986) 3 149–153. Biochemistry and physiology: FEMS Reviews (1986) 39 161–179.] The oxidation of carbon monoxide to carbon dioxide is catalysed by an enzyme sometimes referred to as ‘CO dehydrogenase’ (a CO OXIDASE); the cytochrome oxidase of one branch of the respiratory chain is insensitive to inhibition by carbon monoxide. The carbon dioxide produced is assimilated via the CALVIN CYCLE. In at least some cases it appears that the CO oxidase is functional only when attached to a membrane-bound electron acceptor (cytochrome b561 ) – the ability of the cells to oxidize carbon monoxide being lower during the stationary phase when some of the enzyme is cytoplasmic [FEMS Reviews (1993) 104 332–336]. Many or all of the carboxydobacteria are able to obtain energy by the oxidation of hydrogen, and can grow more rapidly with hydrogen/carbon monoxide/oxygen mixtures than with mixtures of carbon monoxide and oxygen. At least some species are facultatively heterotrophic. The organisms occur e.g. in soil and polluted waters, sewage etc. Species include e.g. Bacillus schlegelii, Pseudomonas carboxydohydrogena, P. carboxydovorans and P. thermocarboxydovorans. The carboxydobacteria were once classified as methylotrophs (users of C1 compounds) but are now excluded from this category as they do not produce/assimilate formaldehyde. carboxymethylcellulose See CM-CELLULOSE. carboxypeptidase See PROTEASES. carboxysomes Intracellular ‘polyhedral bodies’ (typically ca. 90–500 nm diam.) found in many autotrophic prokaryotes: e.g., in Thiobacillus intermedius, T. neapolitanus, T. thiooxidans and T. thioparus (but not in T. novellus or T. versutus); in Nitrobacter winogradskyi and a marine species of Nitrosomonas (but not in Nitrospina gracilis or Nitrosococus mobilis); apparently in Rhodomicrobium vannielii (but not in green bacteria or purple non-sulphur bacteria); in most or all cyanobacteria (in vegetative cells and akinetes but not in heterocysts); in Prochloron; and apparently in many cyanelles. Carboxysomes from T. neapolitanus each consist of a number of doughnut-shaped particles (ca. 10 nm diam.) enclosed within a single-layered membrane or shell; these particles are molecules of RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE–OXYGENASE (RuBisCO). Carboxysomes from other organisms appear to resemble those of T. neapolitanus, and RuBisCO has been found in all carboxysomes in which it has been sought (e.g. in those from Anabaena cylindrica, Chlorogloeopsis fritschii, Nitrobacter sp, Nitrosomonas sp). The function of carboxysomes is unknown. [Biol. Rev. (1984) 59 389–422.] carbuncle A cluster of interconnected BOILS in which PUS is eventually discharged via several openings. It is usually caused by Staphylococcus aureus. Carchesium A genus of sedentary ciliate protozoa (subclass PERITRICHIA). Carchesium occurs in various freshwater and marine habitats, and some species (e.g. C. polypinum) are common in SEWAGE TREATMENT plants. (See also SAPROBITY SYSTEM and SEWAGE FUNGUS.) In size and shape Carchesium resembles VORTICELLA – e.g. it has a bell-shaped zooid: however, Carchesium is a colonial peritrich – the mature organism consisting of a number of zooids carried on a branching stalk, each zooid and its individual branch of the stalk being capable of independent contraction. carcinogen (1) Any agent which can cause CANCER. In general, carcinogens appear to exert their effect by interacting with the DNA of the target cell; many carcinogens are also mutagens. in the cytoplasmic membrane and increases its fluidity, thereby impairing membrane function [Book ref. 29, pp. 335–343]. CO2 forms complexes with amino groups in proteins, a reaction which is slowly reversible at low temperatures and under low partial pressures of CO2 ; this may account for the observed residual effect in CO2 -treated meats subsequently exposed to air [Book ref. 29, pp. 369–375]. (b) CO2 as a carbon source. CO2 is assimilated via several different pathways in AUTOTROPHS (q.v.); in some algae, CO2 concentrating mechanisms operate at low CO2 levels, preventing (energy-wasting) PHOTORESPIRATION. In heterotrophs, CO2 may be fixed in various carboxylation reactions (see e.g. Appendix II (b) and BACTEROIDES). (See also BIOTIN.) carbon disulphide (CS2 ) CS2 (b.p. 46° C) is sometimes used as a soil fumigant for the control of Armillaria mellea root infections. It also acts as a NITRIFICATION INHIBITOR. carbon monoxide (CO) (a) (as a metabolite) See CARBOXYDOBACTERIA, ACETOGENESIS and METHYLOTROPHY. (b) (as a RESPIRATORY INHIBITOR) Carbon monoxide can bind to the reduced a3 component of eukaryotic cytochrome oxidase (see ELECTRON TRANSPORT CHAIN), and to many bacterial terminal oxidases, inhibiting the oxidation of the oxidase; a CO-insensitive respiratory chain occurs in the CARBOXYDOBACTERIA. At least some bacterial CO–cytochrome oxidase complexes can be dissociated by strong illumination (photodissociation) with light of wavelengths absorbed by the complex – with concomitant release of inhibition. carbon monoxide dehydrogenase See ACETOGENESIS and METHANOGENESIS. carbon monoxide oxidase. See CO OXIDASE. carbonic anhydrase A zinc-containing ENZYME (EC 4.2.1.1) which catalyses the reaction: CO2 + H2 O ←−−→ HCO3 − + H+ Carbonic anhydrases are found in eukaryotes and in members of the domains Bacteria and Archaea [PNAS (1999) 96 15184–15189]. (See also acetate in the figure legend of the entry METHANOGENESIS.) The carbonic anhydrases are categorized in classes a, b and g; members of different classes do not share sequence homology. Tests for the ability to produce carbonic anhydrase have been used e.g. to distinguish between species of MORAXELLA and NEISSERIA. carbonylcyanide-m-chlorophenylhydrazone See PROTON TRANSLOCATORS. carbonylcyanide-p-trifluoromethoxyphenylhydrazone See PROTON TRANSLOCATORS. Carbowax See POLYETHYLENE GLYCOL. Carboxide See ETHYLENE OXIDE. carboxin (2,3-dihydro-6-methyl-5-phenylcarbamoyl-1,4-oxathiin) An agricultural systemic ANTIFUNGAL AGENT which is active against basidiomycetes, apparently by inhibiting mitochondrial succinate dehydrogenase. Carboxin is used e.g. for the control of Rhizoctonia solani and of smuts and rusts in cereals. Oxycarboxin, the corresponding sulphone, has also been used against cereal rusts. carboxydismutase Syn. RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE–OXYGENASE. carboxydobacteria A (non-taxonomic) category of bacteria characterized by the ability to use CARBON MONOXIDE, aerobically, as the sole source of carbon and energy [ARM (1983) 37 277–310]. [Isolation and culture: Book ref. 45, pp. 899–902. Metabolism: Book ref. 132, pp. 131–151. Biotechnological 127 carcinoma carnation latent virus See CARLAVIRUSES. carnation mottle virus See TOMBUSVIRUSES. carnation necrotic fleck virus See CLOSTEROVIRUSES. carnation ringspot virus See DIANTHOVIRUSES. carnation vein mottle virus See POTYVIRUSES. carnidazole See NITROIMIDAZOLES. carnivorous Refers to protozoa which feed on other protozoa, crustacea, etc. (cf. HERBIVOROUS.) Carnobacterium A genus of heterofermentative LACTIC ACID BACTERIA found e.g. on vacuum-stored chilled meats and on meats stored in a carbon-dioxide-rich environment. Carnobacterium spp have been studied e.g. as sources of BACTERIOCINS. [Bacteriocin production by C. piscicola: JBC (1994) 269 12204–12211. Divercin V41, a new bacteriocin from C. divergens V41: Microbiology (1998) 144 2837–2844.] Carnoy’s fluid A FIXATIVE: either (a) ethanol (95% or 100%) and glacial acetic acid in the ratio 3:1, or (b) ethanol (95% or 100%), chloroform, and glacial acetic acid (6:3:1). carotenes See CAROTENOIDS. carotenoid band shift A light-induced change in the absorption spectrum of a photosynthetic membrane due, mainly, to a change (of a few nanometres) in the absorption spectra of the carotenoids in the membrane; this change results from the effect on the carotenoids of the light-induced transmembrane pmf (see PHOTOSYNTHESIS) and is sometimes referred to as an ‘electrochromic effect’. carotenoids A class of polyene (isoprenoid), aliphatic or alicyclic, commonly yellow or orange pigments; the class includes the carotenes (hydrocarbons with usually nine conjugated double bonds) and oxygen-containing derivatives of carotenes (xanthophylls) – see figure. Carotenoids are widely distributed in microorganisms, plants and animals; they are present in probably all photosynthetic organisms, in which they function e.g. in LIGHT-HARVESTING COMPLEXES and in protecting CHLOROPHYLLS from photodegradation. (See also SINGLET OXYGEN and ANTIOXIDANTS.) Carotenoids may also be involved in phototaxis (see e.g. EYESPOT), and a carotenoid derivative, RETINAL, occurs in certain bacterial energy-transducing systems (see PURPLE MEMBRANE). The presence of one or more particular carotenoids in a given organism may be an important taxonomic feature, especially in algae. Carotenes include e.g. b-carotene (R
= R

= b-ionone ring, see figure), widely distributed in algae and higher plants, and present in certain bacteria; a-carotene (an isomer of b-carotene), also widely distributed; g-carotene (R
= bionone ring, R

= 2,6-dimethyl-1,5-heptadienyl), present e.g. in some bacteria of the Chlorobiaceae; lycopene (R
= R

= 2,6dimethyl-1,5-heptadienyl), found e.g. in fruits. Xanthophylls include e.g. antheraxanthin (found e.g. in green algae (Chlorophyta), in chloromonads, and in plants); astaxanthin (see PHAFFIA); BACTERIORUBERINS; diatoxanthin (in cryptophytes); fucoxanthin (in brown algae (Phaeophyta), chrysophytes, diatoms and prymnesiophytes – see figure); lutein (3,3
-dihydroxy-a-carotene, common in plants and green algae); myxoxanthophyll (a rhamnosyl-containing xanthophyll found in cyanobacteria); neoxanthin (common in green algae and higher plants); peridinin (in dinoflagellates); siphonein and siphonoxanthin (in siphonaceous green algae); spirilloxanthins (in anoxygenic photosynthetic bacteria); STAPHYLOXANTHIN; torularhodin (in Rhodotorula); violaxanthin (in green algae and higher plants); zeaxanthin (3,3
-dihydroxy-b-carotene, an isomer of lutein, found e.g. in green algae and higher plants). (See also MUTAGENICITY TEST.) Some apparent carcinogens actually require enzymic conversion within the body to an active form, the ‘ultimate carcinogen’, before they can cause neoplastic transformation. (2) Syn. oncogen sensu lato (see ONCOGENIC). carcinoma Any malignant tumour which arises from epithelial tissue: see e.g. NASOPHARYNGEAL CARCINOMA and HEPATOCELLULAR CARCINOMA. (See also CANCER.) cardiac beriberi See CITREOVIRIDIN. Cardiobacterium A genus of oxidase-positive, catalase-negative, Gram-negative bacteria which occur e.g. in the upper respiratory tract (see also HACEK) and which have been isolated from the blood in cases of bacterial endocarditis. Cells: small, pleomorphic, non-motile, round-ended rods (∼0.5–0.75 × 1–3 µm), or filaments. Chemoorganotrophic. Aerobic and facultatively anaerobic; apparently capnophilic. On anaerobic blood agar, with 5% carbon dioxide, colonies are smooth and circular, 1–2 mm diam., after 48 hours. Strains are urease −ve but may be weakly indole +ve. Nitrates are not reduced. Optimum growth temperature 30–35° C. Media which have been used for primary isolation include e.g. casein-soy broth and thioglycollate broth (each containing SPS). No growth occurs on MacConkey agar. Acid (but no gas) is formed from the fermentation of fructose, glucose, mannose, sorbitol or sucrose; lactic acid is the main product of glucose fermentation. The organism is typically sensitive to b-lactam and other antibiotics. GC%: 59–62. Type species: C. hominis. cardioid condenser See MICROSCOPY (b). cardiolipin Diphosphatidylglycerol. Such lipids occur e.g. in bacterial CYTOPLASMIC MEMBRANES. Cardiolipin prepared by alcoholic extraction of beef heart is used in STANDARD TESTS FOR SYPHILLIS. Cardiovirus A genus of viruses (family PICORNAVIRIDAE) isolated mainly from rodents; the true natural host is uncertain. Cardiovirus virions are generally stable at pH 3–9, but are labile at pH 4.5–6.4 in the presence of 0.1 M halide ions. There is only one serotype, and the various named members of the genus – e.g. Columbia SK virus, mouse (or Maus) Elberfeld virus, mengovirus, MM virus – are regarded as strains of murine encephalomyocarditis virus (EMCV). EMCV causes encephalitis and myocarditis in rodents, and a variant can cause a diabetes-like syndrome in certain strains of mice [JGV (1985) 66 727–732]. carfecillin See PENICILLINS. caries (dental) See DENTAL CARIES. cariogenic Refers to any factor, substance or organism which promotes DENTAL CARIES. carlaviruses (carnation latent virus group) A group of PLANT VIRUSES in which the virions are slightly flexuous filaments (ca. 600–700 × 13 nm) containing one molecule of linear, positivesense ssRNA (MWt ca. 2.7 × 106 ). Each member has a fairly narrow host range, and some can cause disease in crop plants. Transmission occurs (non-persistently) via aphids in at least some members; mechanical transmission can be achieved under experimental conditions. Type member: carnation latent virus. Other members include e.g. cactus virus 2, hop latent virus, HOP MOSAIC VIRUS, pea streak virus, potato virus M (= potato paracrinkle virus), potato virus S, red clover vein mosaic virus. Possible members include e.g. chicory blotch virus, eggplant mild mottle virus, and groundnut crinkle virus. CARNA See CUCUMOVIRUSES. carnation etched ring virus See CAULIMOVIRUSES. carnation Italian ringspot virus See TOMBUSVIRUSES. 128 carrier-specific T helper cell (a) R′′ R′ (b) O HO O C CH3 (c) C O C O HO CAROTENOIDS. (a) General structure of a typical carotene. (b) b-Carotene (= b,b-carotene). (c) Fucoxanthin (a xanthophyll). it is widely used in industry e.g. as a gelling and thickening agent in foods, pharmaceuticals etc, and in supports for IMMOBILIZATION of cells and enzymes. It can also be used as an AGAR substitute in certain bacteriological media [Book ref. 2, pp. 144–145]. The KCl-soluble, non-gelling fraction of carrageenan consists of various polymers in which B = e.g. D-galactose 6-sulphate, D-galactose 2,6-disulphate, 3,6-anhydroD-galactose 2-sulphate, etc. (The term l-carrageenan sometimes refers to the whole KCl-soluble fraction, sometimes specifically to the component in which A = D-galactose 2-sulphate, B = Dgalactose 2,6-disulphate.) carragheen moss See CHONDRUS. Carrel flask A squat cylindrical glass vessel which opens, at the side, via a straight, upward-sloping tubular neck. carrier (1) (med., vet.) An individual who harbours a particular pathogen but shows no clinical signs of disease; such an individual can transmit the pathogen to others. Carriers are important reservoirs of infection in certain diseases – e.g. DIPHTHERIA and TYPHOID FEVER (cf. SMALLPOX). [The carrier state (involving bacteria): JAC (1986) 18 Supplement A 1–81.] (2) (immunol.) See HAPTEN. (3) See CARRIER TEST. (4) (virol.) See PSEUDOLYSOGENY and CARRIER CULTURE. carrier culture (virol.) A cell culture which is persistently infected with a virus (see PERSISTENCE) but in which only a small proportion of the cells in the population is actually infected by the virus. The infected cells may release virus (e.g. by lysis) but only a small proportion of the remaining cells is then infected, and so on. The culture can be ‘cured’ of the virus e.g. by treatment with virus-neutralizing antiserum or by serial subculture using small inocula. In e.g. mammalian cell cultures a carrier culture may arise e.g. because INTERFERON production may render most of the cells resistant to infection. (See also PSEUDOLYSOGENY; cf. SEMIPERMISSIVE CELLS.) carrier gene See FUSION PROTEIN. carrier-specific T helper cell (CTh) A T LYMPHOCYTE whose TCR recognizes a specific epitope in the ‘carrier’ part of an antigen – another epitope (the ‘hapten’) on the same antigen being recognized by a B cell. (See COGNATE HELP.) If an antigen carrying a particular hapten–carrier pair is used for primary Some bacterial polyene pigments (e.g. FLEXIRUBINS, XANresemble carotenoids but lack the methyl groups on the hydrocarbon chain. (See also SPOROPOLLENIN.) carp erythrodermatitis (Polish INFECTIOUS DROPSY) A FISH DISEASE affecting carp and certain other fish (e.g. goldfish, bream); aetiology: unknown. The disease is subacute to chronic, with variable mortality. Inflamed skin lesions become necrotic and ulcerative, and generalized oedema follows. Secondary infection with Aeromonas and Pseudomonas spp is common. carp pox (epithelioma papulosum) A benign FISH DISEASE affecting the common carp (Cyprinus carpio) and other cyprinids. Multiple transient hyperplastic nodules (up to several centimetres across) develop on the skin and fins, and occasionally on the eyes and gills; the lesions do not become necrotic. Carp pox is apparently caused by a herpesvirus (see HERPESVIRIDAE). carpomycetes Fungi which form fruiting bodies – particularly the ascomycetes and basidiomycetes. Carr medium A medium used e.g. to distinguish between strains of ACETOBACTER and GLUCONOBACTER; it contains yeast extract (3%), agar (2%), BROMCRESOL GREEN (0.002%) and ethanol (2% v/v). Gluconobacter strains (which typically oxidize ethanol to acetic acid) produce colonies each surrounded by a yellow halo; Acetobacter strains (which typically carry out ‘overoxidation’) initially form similar colonies but, on further incubation, the acetic acid is metabolized and the pH indicator reverts to its original colour (i.e. the halo is lost). carrageenan (carrageenin; carragheen) A mixture of sulphated galactans associated with the CELL WALLS of many rhodophycean algae (e.g. Chondrus crispus, Gigartina spp); they appear to prevent desiccation when the alga is exposed to the atmosphere. Carrageenans have the general structure: -b-A-(1 → 4)-a-B-(1 → 3)-b-A-, where A = D-galactose 4-sulphate or Dgalactose 2-sulphate, and B is any of a range of sulphated galactose or 3,6-anhydrogalactose residues. One component of the carrageenan mixture, k-carrageenan, is precipitated by KCl; in k-carrageenan A = D-galactose 4-sulphate, B = 3,6-anhydroD-galactose. k-Carrageenan forms gels with e.g. K+ , NH4 + , Ca2+ , Mg2+ , Fe3+ , and with water-miscible organic solvents; THOMONADINS) 129 carrier test forms in stationary-phase culture; on yeast extract–glucose agar the colonies are grey-white, pink or red. Up to 12% salt (NaCl) can be tolerated. GC%: 65–67. Type species: C. polymorphus. caseous necrosis Syn. CASEATION. caspases See APOPTOSIS. cassava common mosaic virus See POTEXVIRUSES. cassava latent virus See GEMINIVIRUSES. cassava vein mosaic virus See CAULIMOVIRUSES. cassette mechanism (mol. biol.) A system for gene regulation in which a particular gene is removed from one site (‘active slot’), in which it can be expressed, to another (‘inactive slot’) in which it cannot be expressed – another gene being inserted into the ‘active slot’. (See e.g. MATING TYPE.) Castañeda’s method A form of BLOOD CULTURE in which the culture bottle contains an agar slope which projects above the level of the liquid medium when the bottle is standing vertically. The liquid medium is inoculated in the usual way, and the bottle is incubated. After 24/48 h the bottle is tilted so that the medium washes over the exposed agar; the bottle is then incubated, vertically, and subsequently examined for bacterial growth on the agar. Tilting etc may be repeated at intervals of one or two days. By eliminating subculturing this method avoids a source of contamination. cat (parvovirus infection) See FELINE PANLEUKOPENIA VIRUS. cat bite fever A disease caused by Pasteurella multocida and transmitted by cat-bites; an abscess develops at the site of the bite. (cf. CAT SCRATCH DISEASE.) cat diseases See under ‘feline’. cat flu virus Syn. FELINE CALICIVIRUS. cat gene A gene encoding chloramphenicol acetyltransferase: see CHLORAMPHENICOL and Tn9. cat scratch disease (cat scratch fever) A (usually benign) human disease (more common in children) associated with the scratch or bite of a cat or, occasionally, with injuries from thorns, splinters etc. A local inflammatory skin lesion develops after 2–3 days, and this is followed, in days/weeks, by enlargement of regional lymph nodes – which suppurate in some cases. There is often a general malaise with low-grade fever. The condition usually resolves spontaneously within weeks; however, in some cases there are complications – which may involve e.g. liver, brain or eyes. Various organisms have been proposed as the causal agent of cat scratch disease, including e.g. Bartonella clarridgeiae [JCM (1997) 35 1813–1818]. (See also CAT BITE FEVER.) catabolism The metabolic reactions by which exogenous or endogenous organic compounds are degraded to simpler organic or inorganic compounds. (cf. ANABOLISM; AMPHIBOLIC PATHWAY.) catabolite activator protein See CATABOLITE REPRESSION. catabolite control protein A See CATABOLITE REPRESSION. catabolite inhibition See INDUCER EXCLUSION. catabolite repression The repression of certain inducible enzyme systems by the presence of a ‘preferred’ carbon source (e.g. glucose in enterobacteria and Saccharomyces cerevisiae, certain TCA cycle intermediates in Pseudomonas spp). For example, in Escherichia coli, various operons concerned with the degradation of substrates other than glucose (e.g. ARA OPERON, LAC OPERON, TNA OPERON) are repressed in the presence of glucose, even when their respective inducers are present; thus, when E. coli is grown in a medium containing both glucose and lactose, the lac operon remains repressed until all the glucose has been used – at which time the lac operon becomes derepressed. (See also DIAUXIE, CRABTREE EFFECT.) immunization, then the hapten must be paired with the same carrier for an efficient response during secondary immunization. carrier test A type of test used to determine the ability of a given disinfectant to disinfect an object (‘carrier’) which has been artificially contaminated with microorganisms; the carrier may be e.g. a piece of cloth or an instrument. (See e.g. AOAC USE-DILUTION TEST.) [Evaluation of carriers used in AOAC test methods: AEM (1986) 51 91–94.] Carrión’s disease Syn. BARTONELLOSIS. carrobiose A disaccharide: 3,6-anhydro-4-O-(b-D-galactopyranosyl)-D-galactose, a degradation product of k-carrageenan. carrot black rot See ALTERNARIA. carrot latent virus See RHABDOVIRIDAE. carrot mosaic virus See POTYVIRUSES. carrot mottle virus See TOGAVIRIDAE. carrot red leaf virus See LUTEOVIRUSES. carrot soft rot See SOFT ROT (sense 2). carrot thin leaf virus See POTYVIRUSES. carrot yellow leaf virus See CLOSTEROVIRUSES. Carr’s medium See CARR MEDIUM. Carteria A genus of green algae which closely resemble CHLAMYDOMONAS but which have four flagella instead of two. carvacrol See PHENOLS. Cary–Blair transport medium See TRANSPORT MEDIUM. caryonide Syn. KARYONIDE. Caryophanon A genus of Gram-positive, asporogenous, obligately aerobic, chemoorganotrophic bacteria which form motile (peritrichously flagellate) TRICHOMES of diameter ca. 1.5 µm (C. tenue) or ca. 2.5–3.2 µm (C. latum). Both species have been isolated from cattle manure, but their true habitat is unknown. [Book ref. 46, pp. 1701–1707.] Caryospora A genus of protozoa (suborder EIMERIORINA) which form oocysts each enclosing one sporocyst containing 8 sporozoites. Caryotropha See EIMERIORINA. casamino acids Acid-hydrolysed casein. caseation (med., vet.) A type of necrosis in which diseased tissue breaks down into a dry, amorphous, cheese-like mass (cf. TUBERCULOSIS). casein A phosphoprotein which occurs in MILK mainly in the form of micelles, each micelle consisting of aggregated molecules of the 3 main types of casein: a-, b- and k-casein; the (Ca2+ -precipitable) a- and b-caseins in the micelles are stabilized by the k-casein. k-Casein can be cleaved by various enzymes – e.g. by pepsin (EC 3.4.23.1), or by rennin (= chymosin; EC 3.4.23.4), an enzyme derived from the abomasum (see RUMEN) of suckling ruminants; these enzymes cleave kcasein into an insoluble N-terminal fragment, p-k-casein, and a soluble C-terminal fragment, ‘glycomacropeptide’. Cleavage of micellar k-casein in milk destablizes micelles and leads to the precipitation of a- and b-caseins and p-k-casein (i.e., the milk coagulates). (The proteolytic enzymes of some psychrotrophic bacteria can cleave k-casein and cause ‘gelation’ (coagulation) in stored UHT-treated milk.) Enzyme-independent coagulation (‘clotting’ – see e.g. LITMUS MILK) occurs when the pH is lowered to ca. 4.6 (i.e., at or near the isoelectric points of the caseins). g-Casein is a product of the enzymic cleavage of b-casein by a trypsin-like enzyme which occurs in milk in small amounts. Caseobacter A genus of aerobic, non-motile, asporogenous bacteria (order ACTINOMYCETALES, wall type IV) which occur e.g. on certain cheeses. The organisms are pleomorphic or coryneform rods, ca. 1 × 2–4 µm, which give rise to coccoid 130 catalase test Among the bacteria there are several distinct mechanisms for catabolite repression. Thus, e.g. one mechanism is characteristic of E. coli and other enterobacteria, while others are typical of AT-rich Gram-positive species (e.g. species of Clostridium, Lactobacillus and Staphylococcus); each of these mechanisms is regulated by signals from components of the PTS (q.v.). Models outlining these mechanisms are described below. (a) Catabolite repression in enterobacteria. In these organisms the key regulatory molecule is the IIA component of glucose permease in the PTS; note that, in this particular permease, IIA is a cytoplasmic (i.e. soluble) protein. When glucose is absent, phosphorylated IIA (IIA∼P) does not undergo dephosphorylation via IIB, and under these conditions IIA∼P activates the enzyme ADENYLATE CYCLASE – thus stimulating the synthesis of CYCLIC AMP (cAMP). cAMP binds to, and activates, the so-called cAMP-receptor protein (CRP) – also called catabolite activator protein, CAP, or catabolite gene activator protein, CGA protein. (CRP is encoded by the crp gene.) The cAMP–CRP complex acts as a transcriptional activator by binding in the vicinity of promoters of certain operons (e.g. lac), allowing them to be expressed in the presence of their respective inducers. This explains why the inducibility of the repressed operons can be at least partly restored by the addition of cAMP. The precise way in which cAMP–CRP promotes transcription is still unknown. One early model for the lac operon supposed that the binding of cAMP–CRP displaced RNA polymerase from a weak promoter, P2, to the primary promoter, P1; that such a mechanism is unlikely to contribute significantly to the transcription of the lac operon has been inferred from the failure of inactivation of P2 to result in activation of P1 [Mol. Microbiol. (1992) 6 2419–2422]. Studies on e.g. the lac operon suggest that the presence of cAMP–CRP is not needed after the establishment of a productive open complex, and that only transient interaction is required between cAMP–CRP and RNA polymerase [EMBO (1998) 17 1759–1767]. When glucose (or other rapidly metabolizable substrate) is present, unphosphorylated IIA binds to the permeases of various sugars (e.g. lactose), inhibiting uptake of the corresponding sugars; as these sugars are involved in the induction of their respective operons, this phenomenon is called INDUCER EXCLUSION. Note that, in some cases, cAMP–CRP may repress rather than activate gene expression. For example, in Rhizobium meliloti cAMP–CRP blocks transcription of the dctA gene whose expression is necessary for symbiotic nitrogen fixation [EMBO (1998) 17 786–796]. (In fact, CRP – originally associated with activation of catabolic operons – is involved in a much wider range of gene-regulatory activities.) (b) Catabolite repression in the AT-rich Gram-positive bacteria. In these bacteria, transcription of some catabolic operons is at least partly regulated by operon-specific regulator proteins, each regulator protein having two phosphorylation sites; each of these two sites is called a PRD (= PTS regulation domain). PRDs can be phosphorylated by certain intermediates in the PTS (IIB∼P and HPr∼P); one of the PRDs can be phosphorylated by IIB∼P, the other by HPr∼P. It seems that phosphorylation by IIB∼P is inhibitory, causing the regulator protein to inhibit transcription from relevant operons, while phosphorylation by HPr∼P promotes transcription from those operons. The activity of a given regulator protein thus depends on the mode of its phosphorylation; thus, when phosphorylated only by HPr, the regulator protein promotes transcription from all relevant operons, but phosphorylation by HPr and IIB, or a lack of phosphorylation in both PRDs (i.e. in the presence of both inducer and glucose), causes the regulator protein to function as a repressor of transcription. Induction of operon(s) therefore occurs only under appropriate conditions, i.e. when (i) inducer is present and (ii) glucose is absent. [PTS-dependent control of PRD-containing regulators in induction and repression of catabolic operons: Mol. Microbiol. (1998) 28 865–874.] A PRD-containing regulator protein in E. coli, BglG, is involved in the control of the bgl operon (a b-glucoside catabolic system concerned e.g. with the metabolism of salicin); in the absence of b-glucoside, BglG is phosphorylated by the IIB component of BglF (the b-glucoside permease) – thus inhibiting transcription of the bgl operon. In AT-rich Gram-positive bacteria, catabolite repression involving HPr and the enzyme HPr kinase occurs by ATPdependent phosphorylation of HPr at a site (Ser-46) which differs from that (His-15) phosphorylated by enzyme I in the PTS; when HPr is phosphorylated at Ser-46 it can form a complex with catabolite control protein A (CcpA), and this complex can bind to a regulatory site in target operons, inhibiting transcription. (In vitro, phosphorylation of HPr at the Ser-46 site decreases the ability of HPr to accept phosphate from the ‘routine’ donor, enzyme I; thus, phosphorylation of HPr at this site is likely to inhibit uptake of sugars at the PTS permeases.) HPr kinase is a bifunctional enzyme (HPr kinase/phosphatase), and its role as a kinase in phosphorylating HPr under appropriate conditions of carbon availability is not fully understood; it is possible that the kinase function of the enzyme is stimulated by fructose 1,6-bisphosphate (an intermediate in glucose catabolism). Inactivation of the HPr kinase gene (hprK ) in Staphylococcus xylosus resulted in abolition of repression in the three catabolic enzyme activities examined [JB (2000) 182 1895–1902]. catalase (H2 O2 :H2 O2 oxidoreductase; EC 1.11.1.6) A tetrameric ENZYME, consisting of four ferriprotoporphyrin IX-containing subunits, which catalyses the reaction 2H2 O2 → 2H2 O + O2 ; with low concentrations of H2 O2 the enzyme can act as a PEROXIDASE – but this ability may not be common to catalases from all sources [JGM (1983) 129 997–1004]. Catalase plays a vital role in detoxifying HYDROGEN PEROXIDE produced during aerobic metabolism; it occurs in the majority of aerobic organisms and is absent from most obligate anaerobes (though present e.g. in Bacteroides fragilis). The enzyme is inhibited by e.g. H2 S, HCN and N3 − , and is inactivated by SUPEROXIDE. Catalase is obtained commercially from e.g. Aspergillus niger and Penicillium spp; it is used e.g. in the food industry for removing excess H2 O2 (used for cold disinfection) from milk and dairy products and from irradiated foods. (See also CATALASE TEST; METHYLOTROPHY; PEROXISOMES; cf. PSEUDOCATALASE.) catalase test A test commonly used with the object of determining whether or not a given bacterial strain produces CATALASE. One drop of H2 O2 is added to a bacterial colony, or to a bacterial emulsion; if the cells contain catalase, bubbles of gas (oxygen) appear immediately or within one or two seconds. Such a (positive) reaction may indicate the presence of catalase or (e.g. in some strains of Enterococcus faecalis) the presence of a PSEUDOCATALASE; certain bacteria can synthesize catalase only if they are provided with haem. If, for the test, bacterial growth is obtained from a blood-containing medium, care should be taken to prevent contact between H2 O2 and the medium: erythrocytes, which contain catalase, can give rise to a false-positive reaction. In carrying out the catalase test care should be taken to avoid exposure to the AEROSOL produced by bursting bubbles; in one method the test is carried out in a non-vented Petri dish: a drop of H2 O2 and a speck of bacterial growth are placed at opposite 131 cataloguing DISEASE; AUJESZKY’S DISEASE; BOVINE EPHEMERAL FEVER; BOVINE ULCERATIVE MAMMILLITIS; BOVINE VIRAL LEUKOSIS; BOVINE VIRUS DIARRHOEA; CALF PNEUMONIA; FOOT AND MOUTH DISEASE; INFECTIOUS BOVINE RHINOTRACHEITIS; JEMBRANA DISEASE; LUMPY SKIN DISEASE; MALIGNANT CATARRHAL FEVER; MUCOSAL DISEASE; PSEUDOCOWPOX; RABIES; RIFT VALLEY FEVER; RINDERPEST; ROTAVIRUS; VESICULAR STOMATITIS. (e) Diseases of unknown/variable aetiology: ATYPICAL INTERSTITIAL PNEUMONIA; BOVINE RESPIRATORY DISEASE; CALF SCOURS; WIMMERA GRASS POISONING; WINTER DYSENTERY. (See also BOVINE SPONGIFORM ENCEPHALOPATHY.) cattle plague Syn. RINDERPEST. cattle tick fever Syn. REDWATER FEVER. sides of the dish which, with the lid in place, is tilted so that the H2 O2 runs onto the growth. (See also SUPEROXOL TEST.) cataloguing (of 16S rRNA) See RRNA OLIGONUCLEOTIDE CATALOGUING. Catapyrenium A genus of LICHENS (order VERRUCARIALES) which differ from DERMATOCARPON spp in having a squamulose thallus attached to the substratum by rhizines. The genus includes e.g. C. lachneum (formerly called e.g. Dermatocarpon hepaticum). catarrh Inflammation of a mucous membrane, with excessive production of mucus. catecholamides (in iron uptake) See SIDEROPHORES. catenane A complex of two or more circular nucleic acid molecules interlocked like links in a chain. (cf. CATENATE; CONCATEMER; see also GYRASE.) Catenaria See BLASTOCLADIALES. catenate (1) (verb) To interlock circular nucleic acid molecules to form a CATENANE (hence noun catenation). (2) (adj.) Of e.g. spores: formed in chains. catenins See CADHERINS. catheter-associated infection INFECTION (sense 3) associated with catheterization of the blood vascular system or of the urinary tract; in general, the risk of such infection increases with the duration of catheterization. Catheter-related infections account for a significant proportion of nosocomial disease. In catheter-associated bacteraemia, the source of infection can be investigated either with or without withdrawal of the catheter [current methods for the diagnosis of catheter-related bacteraemia: RMM (1997) 8 189–195]. In catheter-associated infection of the urinary tract, a significant reduction in incidence during short-term (<3 weeks) catheterization may be obtained by using catheters that have anti-infection surface activity; such catheters may be either (i) impregnated with e.g. nitrofurazone, or with minocycline and rifampicin, or (ii) coated with a silver alloy–hydrogel [EID (2001) 7 342–347]. cathodic depolarization theory A theory proposed to account for the microbial corrosion of iron. In this theory, the spontaneous solubilization of iron (by ionization: Fe → Fe2+ + 2e− ) is promoted by (a) hydrogen formation, the electrons from iron combining with protons derived from water ionization, and (b) utilization of the hydrogen by SULPHATE-REDUCING BACTERIA. However, it has been reported that the primary cause of such corrosion may involve an extracellular (probably phosphorus-containing) product of sulphate-reducing bacteria [Book ref. 155, pp. 619–641]. cattle diseases Cattle may be affected by a wide range of diseases, some of which are specific to bovines; different species and breeds of bovines may show differences in susceptibility to a given disease (see e.g. FOOT-ROT). (a) Bacterial diseases: see e.g. ACTINOMYCOSIS; ANAPLASMOSIS; ANTHRAX; BLACK DISEASE; BLACKLEG; BRUCELLOSIS; CAMPYLOBACTERIOSIS; CONTAGIOUS BOVINE PLEUROPNEUMONIA; EPERYTHROZOONOSIS; FARCY (sense 2); FOOT-ROT; HAEMORRHAGIC SEPTICEMIA (sense 2); INFECTIOUS KERATITIS; JOHNE’S DISEASE; LEPTOSPIROSIS; LISTERIOSIS; MASTITIS; NECROBACILLOSIS; PARATYPHOID FEVER; RED WATER (of calves); TUBERCULOSIS; TULARAEMIA; ULCERATIVE LYMPHANGITIS; WHITE SCOURS. (See also COWDRIA; LAMZIEKTE; MIDLAND CATTLE DISEASE.) (b) Fungal diseases: see e.g. ASPERGILLOSIS; ZYGOMYCOSIS. (See also FESCUE FOOT; 4-IPOMEANOL; PASPALUM STAGGERS; PENITREMS; SPORIDESMINS.) (c) Protozoal diseases: see e.g. DALMENY DISEASE; EAST COAST FEVER; NAGANA; REDWATER FEVER. (d) Viral diseases: see e.g. AKABANE VIRUS DISEASE; ALLERTON Caulerpa A genus of siphonaceous, tropical and subtropical green seaweeds related to HALIMEDA (q.v.). The thallus consists of a horizontal green rhizome with colourless rhizoids and erect fronds. (See also ELYSIA.) caulescent Becoming stalked or tailed. cauliflower fungus Sparassis crispa. cauliflower mosaic virus (CaMV; cabbage B virus) A member of the CAULIMOVIRUSES which infects various brassicas. In e.g. cauliflower (Brassica oleracea var. botrytis) symptoms of infection typically include vein-banding and/or vein-clearing; enations are occasionally formed. Transmission: chiefly styletborne via aphids. A CaMV virion consists of a capsid (composed of a single type of protein) and the dsDNA genome. In the virion, the dsDNA is circular with three discontinuities at specific sites in the molecule: two in one strand (the b strand), one in the other (a strand); these discontinuities are neither nicks nor gaps, but are sites of triple-stranded DNA in which the 3
and 5
ends overlap by ca. 8–20 nucleotides [FEBS (1981) 134 67–70]. In the host plant, CaMV DNA appears to occur in the form of covalently closed circular, supercoiled, histone-associated (chromatin-like) ‘mini-chromosomes’ which are apparently active in transcription but not in replication. Only the a-strand is transcribed. Viral RNAs (which are capped and polyadenylated) include a large (8.2 kb, 35S) transcript corresponding to the length of the entire genome with an additional 180-nucleotide terminal repeat; this transcript may be an intermediate in DNA replication: a molecule of plant tRNAmet binds to the 35S transcript at a 14-bp region of homology, and this tRNA may act as a primer for cDNA synthesis by reverse transcription (see RETROID VIRUSES). [Evidence for replicative recombination in CaMV: Virol. (1986) 150 463–468.] Viral transcription occurs in the plant cell nucleus, while virus particle assembly occurs in rounded, amorphous, electron-dense, cytoplasmic viroplasms; the site of DNA replication is unknown. CaMV is widely studied as a model for transcription, DNA replication etc in higher plants, and as a potential (nonintegrating) vector for gene transfer in plants. [Book ref. 80, pp. 17–48.] caulimoviruses (cauliflower mosaic virus group) A group of PLANT VIRUSES characterized by a dsDNA genome (ca. 8000 bp long). The virions are isometric, ca. 50 nm diam.; the capsid is composed of a single type of polypeptide (MWt ca. 42 × 103 ). Type member: CAULIFLOWER MOSAIC VIRUS; other members: carnation etched ring virus, dahlia mosaic virus, figwort mosaic virus, Mirabilis mosaic virus, strawberry vein-banding virus; possible members: cassava vein mosaic virus, Petunia veinclearing virus, Plantago virus, thistle mottle virus. (See also NON-CIRCULATIVE TRANSMISSION sense 1.) Caulobacter A genus of chemoorganotrophic, strictly aerobic, PROSTHECATE BACTERIA found e.g. in oligotrophic waters and in 132 CD8 soils poor in organic matter. Type species: C. vibrioides. GC%: 64–67. Most of the studies on this genus have been carried out on C. crescentus – to which the following account applies. The non-motile, mature (‘mother’) cell is a straight or curved rod, ca. 1–2 µm, which typically has a single polar stalk (PROSTHECA) – the distal end of which is adhesive; cells may adhere, singly, to the substratum – or may occur in groups (‘rosettes’), each group consisting of a number of cells radiating from a central mass of adhesive material. Many strains contain carotenoid pigments. The mother cell gives rise, by asymmetric binary fission, to a daughter cell (swarmer ) which bears a single polar flagellum. The swarmer subsequently loses its flagellum, develops a prostheca, and can then function as a mother cell. In stained mother cells the prostheca may show one or more cross-striations (crossbands), each of which is thought to mark one turn of the cell cycle. The molecular mechanisms involved in the process of differentiation and asymmetric cell division (essential features of the cell cycle) are beginning to be understood through studies on the intracellular localization of proteins using techniques such as gene fusion. In some cases the targeting, or activation, of a specific protein has been linked to an essential aspect of differentiation or cell division. Cell cycle regulation involves various TWO-COMPONENT REGULATORY SYSTEMS. One response regulator, CtrA, has an important role in the control of initiation of DNA replication and in the expression of many genes [PNAS (2002) 99 4632–4637]. Following initiation of DNA replication in the mother cell, levels of the phosphorylated form of CtrA (CtrA∼P) increase, CtrA∼P (and/or CtrA) then promoting transcription of certain genes – including ‘early’ flagellar genes and those regulatory genes needed for expression of the later flagellar genes. The regulation of flagellar genes may be more complex than was previously supposed [PNAS (2002) 99 4632–4637]. The FliF protein is targeted to a specific polar site in the developing daughter (swarmer) cell, i.e. the MS ring in the swarmer cell develops prior to septation; the mechanism responsible for directing FliF to the appropriate location is unknown. After formation of the septum, the FlbD protein is activated by phosphorylation – the kinase, FlbE, occurring near the septum in the daughter cell; the activated FlbD protein (FlbD∼P) promotes transcription of ‘late’ flagellar genes – resulting in the development of a polar flagellum in the daughter cell. In the daughter cell, CtrA∼P binds to the chromosomal origin, thus inhibiting replication; hence, unlike the mother (stalked) cell, the swarmer does not divide until it matures. Under normal circumstances, the role of CtrA (i.e. the unphosphorylated form) in the mother cell includes activation of the late cell division genes ftsQ and ftsA – whose expression is necessary for cell division. If DNA replication is inhibited, then CtrA is not synthesized (thus blocking cell division) – lack of synthesis under these conditions reflecting failure of activation of ctrA; as activation of ctrA is normally dependent on CtrA∼P, it has been suggested that inhibition of DNA replication may inhibit phosphorylation of CtrA – constituting a cell cycle ‘checkpoint’ which blocks cell division in the absence of DNA replication [EMBO (2000) 19 4503–4512]. [Control of differentiation in C. crescentus: Science (1997) 276 712–718. Signal transduction mechanisms in C. crescentus development and cell cycle control: FEMS Reviews (2000) 24 177–191.] Caulococcus See KUSNEZOVIA. caviid herpesvirus 1 See BETAHERPESVIRINAE. cavitation (1) (med.) The formation of an (abnormal) cavity or space in the body (as e.g. in certain types of pneumonia). (2) See ULTRASONICATION. cavity slide A slide (somewhat thicker than a normal microscope SLIDE) which has a shallow, circular depression about 1 cm across in the centre of one face. (cf. RING SLIDE.) Cavostelium See PROTOSTELIOMYCETES. CBP Cap-binding protein: see MRNA (b). CBS Centraalbureau voor Schimmelcultures, Oosterstraat 1, Baarn, The Netherlands; yeast division: CBS, Julianalaan 67A, Delft, The Netherlands. cca gene See TRNA. cccDNA Covalently-closed circular DNA. CCCP See PROTON TRANSLOCATORS. CCCV COCONUT CADANG-CADANG VIROID. ccd mechanism See e.g. F PLASMID. CcdB toxin See F PLASMID. CCFAS (compact colony-forming active substance) A heat-stable, acid-labile carbohydrate, present in the cell walls of certain staphylococci, responsible for the compact morphology of colonies growing in serum soft agar. (Strains without CCFAS form diffuse colonies in such media.) CCFAS exerts its effect by interacting with fibrinogen in the serum agar, promoting the formation of fibrin. It is distinct from, but has been confused with, clumping factor (see COAGULASE). [Book ref. 44, pp. 551–552.] CCM Czechoslovak Collection of Microorganisms, J. E. Purkyne University of Brno, Tr. Obrancu Miru 10, 66243 Brno, Czechoslovakia. CCNU See N-NITROSO COMPOUNDS. CcpA See CATABOLITE REPRESSION. CCPs Critical control points: see HACCP. CCY medium A medium containing casein hydrolysate and yeast extract; cultures on this medium are commonly incubated under a CO2 -enriched atmosphere. CD (meaning of) See CELL ADHESION MOLECULE. CD2 See IMMUNOGLOBULIN SUPERFAMILY. CD3 A T-cell-receptor-associated antigen consisting of three transmembrane polypeptides (g, d and e) which occurs in all T cells (both ab and gd types) and which is part of the so-called Tcell receptor complex. CD3 is not covalently linked to the T cell receptor, and it does not bind antigen; however, when antigen binds to the T cell receptor, serine and tyrosine residues on the three polypeptides of CD3 undergo phosphorylation, and this initiates part of the intracellular signalling in the T cell which follows the binding of antigen. CD4 A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY which occurs on some ab and gd T cells (see also CD8). CD4 binds to MHC class II ligands on antigen-presenting cells (APCs) when antigen is bound by the T cell receptor; such binding stabilizes T cell–APC contact, and causes CD4 to act as a signal-transducing molecule – this involving phosphorylation of its intracellular domain by a specific kinase. CD4 also acts as a receptor for the human immunodeficiency virus (HIV), allowing infection of CD4+ cells. CD5 See B LYMPHOCYTE. CD8 A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY which occurs e.g. on some ab and some gd T cells (see also CD4). CD8 binds to MHC class I ligands; like CD4, bound CD8 acts as a signal-transducing molecule. 133 CD11a/CD18 C4b components of complement, thus protecting the cell from complement-mediated lysis. The Sla antigen of the CR1 glycoprotein (an antigen of the Knops blood group system) is associated with the phenomenon of ‘rosetting’ in falciparum MALARIA. CD36 See MALARIA (vaccines). CD40 A cell-surface molecule which occurs on B lymphocytes. During recognition of a thymus-dependent (TD) antigen by antigen-specific B and T cells, CD40 binds to the T cell’s CD40L (gp39) antigen; such binding generates part of the stimulus which results in clonal proliferation of the B cell and class switching of antibody (IgM → IgG etc.). (The physical interaction between T and B cells also generates signals that activate clonal proliferation in the T cell: see CD28.) (See also RNA EDITING.) CD41b See INTEGRINS. CD44 (syn. homing-associated cell adhesion molecule, HCAM) A CELL ADHESION MOLECULE found e.g. on monocytes, neutrophils, fibroblasts and memory T cells; it is apparently unrelated to other CAMs. CD44 binds to e.g. collagen and hyaluronate, and is apparently involved in ‘lymphocyte homing’ (i.e. translocation of lymphocytes between blood and lymph vessels). CD46 (membrane co-factor protein, MCP) A cell-surface molecule on macrophages which acts as a receptor for COMPLEMENT component C3b. (CD46 can also act as a receptor for the measles virus; the binding of measles virus to CD46 inhibits the ability of the macrophage to form INTERLEUKIN-12.) CD48 A macrophage cell-surface glycoprotein which can bind the type I FIMBRIAE of Escherichia coli. In the absence of opsonizing factors, cells of E. coli that have type I fimbriae can bind to CD48, initiating uptake into a phagosome which is not acidified in the normal way and in which E. coli can survive. CD50 See IMMUNOGLOBULIN SUPERFAMILY. CD54 (syn. ICAM-1) A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY which occurs e.g. on endothelial and epithelial cells, monocytes and fibroblasts. CD54 binds to certain integrins (e.g. LFA-1, Mac-1). (See also INFLAMMATION.) CD55 Decay-accelerating factor (DAF): a glycoprotein present in the membranes of blood cells (red and white), endothelial cells, and epithelial cells of the urinary and gastrointestinal tracts; DAF accelerates decay of the C3 and C5 convertases in the classical and alternative pathways of COMPLEMENT FIXATION, thus helping to protect cells from complement-mediated lysis. The DAF glycoprotein bears antigens of the Cromer blood group system, and also acts as a receptor for Escherichia coli and enteroviruses. CD56 (syn. NCAM) See IMMUNOGLOBULIN SUPERFAMILY. CD58 See IMMUNOGLOBULIN SUPERFAMILY. CD61 See INTEGRINS. CD62E (syn. E-selectin; also known as endothelium–leukocyte adhesion molecule-1, ELAM-1) A CELL ADHESION MOLECULE of the SELECTIN family which is inducible, on endothelial cells, by certain cytokines (e.g. IL-1b, TNF-a); it binds to sialylated carbohydrate ligands on e.g. monocytes, neutrophils and CD4+ T cells. CD62E is involved in the tethering stage of INFLAMMATION. CD62L See SELECTINS. CD62P See SELECTINS. CD66 A category of cell adhesion molecules (of the IMMUNOGLOBULIN SUPERFAMILY), some of which occur on polymorphonuclear leukocytes (PMNs) and/or e.g. epithelial cells. Specific molecule(s) – CD66a, c, d and/or e (not b) – can CD11a/CD18 (syn. LFA-1) A CELL ADHESION MOLECULE of the INTEGRIN family (aL b2 ) found on most leukocytes; when activated, it binds strongly to ICAM ligands (see e.g. INFLAMMATION). (CD11a refers to the a subunit, CD18 to the b subunit.) CD11b/CD18 (syn. Mac-1) See INTEGRINS. CD11c/CD18 The CR4 receptor on macrophages; it binds e.g. the C3bi fragment of complement. CD14 A protein which occurs either in plasma or membraneassociated on macrophages and fibroblasts. Cell-bound CD14 binds exogenous LIPOPOLYSACCHARIDES (LPS); such binding (indirectly) stimulates the synthesis of cytokines – including pro-inflammatory (e.g. IL-1, TNF) and anti-inflammatory (e.g. IL-6, IL-10) molecules. The activity of LPS is greatly increased by the ACUTE-PHASE PROTEIN lipopolysaccharide-binding protein (LBP) which promotes the transfer of LPS to CD14. CD15 (sialyl-Lewis x) A carbohydrate CELL ADHESION MOLECULE, found on leukocytes, which binds to ligands of the SELECTIN family (see e.g. INFLAMMATION). CD16 (syn. FcgRIII) A CELL ADHESION MOLECULE, found on e.g. PMNs and NK cells, which binds to the Fc portion of antigenbound antibody and is involved e.g. in ADCC. CD18 See CELL ADHESION MOLECULE and INTEGRINS. CD21 A CELL ADHESION MOLECULE which occurs e.g. on epithelial cells of the oropharynx; it acts as a receptor for Epstein–Barr virus. CD21 (= CR2) is also found on B lymphocytes, and is involved e.g. in antigen-dependent activation. CD23 See EPSTEIN–BARR VIRUS and B LYMPHOCYTE. CD28 A cell-surface molecule which occurs on T lymphocytes. During presentation of antigen by an ANTIGEN-PRESENTING CELL to a naive CD4+ T cell, so-called co-stimulatory signals – which promote clonal proliferation of the T cell and secretion of cytokines – are generated when CD28 binds to certain molecules on the APC – e.g. B7.1 (also written B7-1; = CD80) or B7.2 (B7-2; CD86). Co-stimulatory signals generated by such binding (which are necessary for T cell development) lead e.g. to the synthesis of INTERLEUKIN-2 by the T cell. During presentation of antigen to an activated T cell, CD80/CD86 binds preferentially to the cell-surface molecule CD152 (found only on activated T cells) – the affinity of CD80/CD86 for CD152 being greater than that for CD28. (CD152 is also called cytokine T-lymphocyte-associated antigen4, or CTLA-4.) Unlike the binding between CD80/CD86 and CD28, binding between CD80/CD86 and CD152 generates a signal that is inhibitory to the T cell’s response, e.g. it inhibits the synthesis of IL-2 in the lymphocyte. (See also CD40.) CD31 (syn. PECAM-1) A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY which occurs e.g. on endothelial cells, monocytes and platelets. Binding, apparently homophilic (i.e. CD31–CD31), is believed to be necessary for extravasation in INFLAMMATION. CD32 See B LYMPHOCYTE. CD34 A marker present on haemopoietic stem cells but not on mature haemopoietic cells. CD35 Complement receptor 1 (CR1): a glycoprotein present e.g. in the membranes of erythrocytes (red blood cells, RBCs) and of various nucleated blood cells, including B cells and some T cells; plasma contains low levels of a soluble form of CR1 (sCR1). CR1 belongs to a family of COMPLEMENT control proteins. The molecules of CR1 on RBCs bind immune complexes, and these complexes are removed as the RBCs circulate through the liver and spleen; CR1 promotes cleavage of the C3b and 134 celery yellow spot virus bind neisserial OPA PROTEINS and, in at least some cases, mediate uptake of bacteria by the eukaryotic cell. CD69 An antigen detectable on immature T lymphocytes (thymocytes) during the process of maturation in vivo. CD77 antigen A B-lymphocyte differentiation antigen which is structurally equivalent to the globotriosylceramide (Gb3 ) receptor for SHIGA TOXIN and the shiga-like toxins. These toxins selectively kill B cells expressing IgG or IgA – so that the immune response to infection may be actively suppressed by toxin. CD80 See CD28. CD81 A protein of the tetraspanin family which is found in the cytoplasmic membrane of cells in many human tissues; CD81 acts as a receptor for the E2 envelope glycoprotein of the hepatitis C virus. [Three-dimesional structure of the extracellular domain of CD81: EMBO (2001) 20 12–18.] CD86 See CD28. CD95 See FAS. CD102 (ICAM-2) A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY found e.g. on endothelial cells, monocytes and some lymphocytes; it binds to the integrin LFA1, and may be involved in lymphocyte homing. CD106 (VCAM-1, vascular cell adhesion molecule-1) A CELL ADHESION MOLECULE of the IMMUNOGLOBULIN SUPERFAMILY found e.g. on endothelial cells, macrophages and fibroblasts; it binds to ligands of the integrin family, and is involved e.g. in the adhesion of leukocytes to the endothelium during the flattening stage of INFLAMMATION. CD120a (p55) See TNF. CD120b (p75) See TNF. CD152 See CD28. CDC Centers for Disease Control, a division of the US Public Health Service. cdc mutant See CELL CYCLE. cdl gene See COLICIN PLASMID. cDNA (complementary DNA; copy DNA) A DNA molecule obtained by reverse transcription of an RNA molecule (commonly an mRNA molecule). cDNAs prepared from mature mRNAs are useful e.g. for CLONING and expressing intron-containing eukaryotic genes; if the genes themselves were cloned in bacteria, the presence of intron(s) would prevent their expression. (Introns are removed during the formation of mature mRNA – see SPLIT GENE.) Most eukaryotic mRNAs [and some bacterial mRNAs: Microbiology (1996) 142 3125–3133] have a polyadenylate (poly(A)) tail at the 3
end (see MRNA). To synthesize a cDNA molecule, a short oligodeoxythymidine (oligo(dT)) strand is hybridized to the poly(A) tail of the mRNA, and a REVERSE TRANSCRIPTASE, together with a mixture of dNTPs, is then added; using oligo(dT) as a primer, the reverse transcriptase synthesizes a DNA strand on the mRNA template. The mRNA strand can then be removed e.g. by alkaline hydrolysis or by RNase H. The DNA strand synthesized by reverse transcriptase often folds over at the 3
end to form a HAIRPIN structure, thus providing a primer for the synthesis of a complementary DNA strand by an appropriate DNA polymerase; following such synthesis, the hairpin structure can be opened by endonuclease S1 to form a linear dsDNA molecule which is suitable for cloning. CDP Cytidine 5
-diphosphate. CDR factor See METHANOGENESIS. CDSC Communicable Diseases Surveillance Centre, Public Health Laboratory Service, Colindale, London, UK. CDTA A chelating agent, trans-1,2-diamino-cyclohexane-N,N, N
,N
-tetraacetic acid. CDTA complexes a range of cations (e.g. Ba2+ , Ca2+ , Mg2+ , Zn2+ ); complexes formed with either Ca2+ or Mg2+ are more stable than those formed by EDTA or EGTA with these cations. cea gene See COLICIN PLASMID. cecidia See GALLS. cecidization GALL-formation. cecropins A family of PEPTIDE ANTIBIOTICS; cecropins appear in the haemolymph of the Cecropia moth (Hyalophora cecropia), and certain other insects, following exposure to bacteria. (cf. SARCOTOXINS.) cedar apples See GYMNOSPORANGIUM. Cedecea A genus of bacteria (family ENTEROBACTERIACEAE) isolated from human clinical specimens (pathogenicity unknown). [Book ref. 22, pp. 514–515.] cefamandole See CEPHALOSPORINS. cefazolin See CEPHALOSPORINS. cefepime A fourth-generation CEPHALOSPORIN. Resistance to cefepime has been associated with diminished permeability of the outer membrane in Enterobacter aerogenes [Microbiology (1998) 144 3003–3009] and with hyperproduction of the blactamase SHV-5 in Klebsiella pneumoniae [JCM (1998) 36 266–268]. cefixime A third-generation CEPHALOSPORIN. It has been used e.g. (together with potassium tellurite) as a supplement to SMAC medium (see EHEC); this supplement has been found to increase the rate of isolation of Escherichia coli strain O157 by suppressing the growth of other non-sorbitol-fermenters (e.g. strains of Proteus). cefmenoxime See CEPHALOSPORINS. cefonicid See CEPHALOSPORINS. ceforanide See CEPHALOSPORINS. cefotaxime See CEPHALOSPORINS. cefoxitin A CEPHAMYCIN which, in e.g. Escherichia coli, binds preferentially to PBP-1A (see PENICILLIN-BINDING PROTEINS) and moderately well to other PBPs (but not to PBP-2). Cefoxitin is active against a range of Gram-negative and Gram-positive bacteria, including anaerobes such as Clostridium and members of the Bacteroides fragilis group. It is resistant to many b-LACTAMASES, including extended-spectrum b-lactamases, but may be sensitive to metalloenzyme blactamases produced by certain strains of anaerobic bacteria. Cefoxitin is an inducer of b-lactamases, so that it should not be used in combination with another b-lactam antibiotic. Cefoxitin has been useful e.g. as a systemic prophylactic agent in both gastrointestinal and gynaecological surgery; when combined with doxycycline it has been used for the treatment of pelvic inflammatory disease, while a combination of cefoxitin and an aminoglycoside antibiotic has been useful for treating post-operative infections. [Cefoxitin (review): RMM (1995) 6 146–153.] cefpirome See CEPHALOSPORINS. cefsulodin See CEPHALOSPORINS. cefsulodin–Irgasan–novobiocin agar See CIN AGAR. ceftazidime See CEPHALOSPORINS. ceftizoxime See CEPHALOSPORINS. ceftriaxone See CEPHALOSPORINS. cefuroxime See CEPHALOSPORINS. celecoxib See PROSTAGLANDINS. celery crown rot See CROWN ROT. celery mosaic virus See POTYVIRUSES. celery yellow mosaic virus See POTYVIRUSES. celery yellow spot virus See LUTEOVIRUSES. 135 celery yellows critical level – the concentration of initiator (per origin) then falling sharply owing to the abrupt doubling of numbers of available origins at the start of replication. (It follows from this model that a cell can govern its rate of division by regulating the rate of synthesis of the initiator.) In both models, initiation of DNA replication is a key event which occurs late in an empirically defined phase that follows cell division and precedes the next round of DNA replication. Once DNA replication has begun the cell is normally committed to continue replication to completion; the point at which a cell becomes committed to DNA replication has been called the restriction point (= R point, or R) or, simply, start. (a) The eukaryotic cell cycle. The typical cell cycle in eukaryotes consists of the following sequence of phases: S phase. DNA replication. G2 phase. Gap phase 2: the period between the end of DNA replication and the start of mitosis. M phase. MITOSIS and cell division. G1 phase. Gap phase 1: the period between cell division and the start of DNA replication. The total period (G1 + S + G2 ) is called interphase. (In some lower eukaryotes, e.g. Physarum, the cell cycle apparently lacks a G1 phase.) Non-cycling (‘resting’) cells are said to be in the G0 phase. A given phase in the cell cycle is characterized e.g. by the cell’s PLOIDY; thus, in a ‘diploid’ cell, the ploidy is 2n in G1 , 2n–4n in S, and 4n in G2 . Typically, when variation occurs in the rate of cell growth and division (due e.g. to nutritional factors) it is the length of G1 which varies, i.e. the other two phases remain relatively constant in length over a wide range of growth rates. Regulation of the eukaryotic cell cycle. The cell cycle can be regulated by signals from outside the cell; thus, cells exposed to e.g. certain growth-inhibitory factors or sex pheromones become arrested in the G1 phase. (Those cells which, at the time of exposure, are in the S, G2 or M phase complete their cycling to the G1 phase.) To investigate regulatory mechanisms within the cell, studies have been carried out e.g. on the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Particularly useful have been the cell-division-cycle mutants (cdc mutants) in which the cell cycle is arrested at specific stage(s). The products of some cdc genes have been found to influence more than one point in the cycle; for example, the cdc2 gene product of S. pombe (Cdc2) affects the cycle in late G1 and also in late G2 . What kind of internal regulatory mechanisms are involved? At certain points in the cell cycle there are ‘gates’ or ‘checkpoints’ at which the cell makes a decision to either continue through the cycle or stop cycling; one such checkpoint is start (mentioned earlier). Continuation through start depends e.g. on the nutrient status of the cell’s environment, i.e. the cycle is arrested at the start checkpoint if nutrient levels are inadequate. The effector molecules involved in regulating ‘continue/stop’ decisions are certain types of protein. They include PROTEIN KINASES and PROTEIN PHOSPHATASES; a cyclical element is contributed by the CYCLINS (q.v.) which accumulate intracellularly, prior to checkpoints, and are cleaved on passage through the checkpoints. (b) The prokaryotic cell cycle. In prokaryotes, most studies on the cell cycle have been carried out on bacteria – particularly Escherichia coli ; the following account is based primarily on the cell cycle in E. coli. [Cell cycle in the Archaea: Mol. Microbiol. (1998) 29 955–961.] celery yellows A YELLOWS disease of celery which may be caused e.g. by an MLO or by the fungus Fusarium oxysporum; in the latter case the main symptoms (stunting and yellowing) are due to vascular occlusion by the pathogen. celesticetin See LINCOSAMIDES. cell adhesion molecule (CAM) In man and other animals: a membrane-associated molecule characteristically involved in cell–cell or cell–matrix adhesion and (often) in the signalling between a cell and its environment. In the latter role, incoming signals modulate various signalling pathways within the cell; signals transmitted via CAMs influence various aspects of cell physiology, including the cell cycle. Most CAMs are proteins which consist of (i) a hydrophobic transmembrane domain; (ii) an extracellular domain – which can bind to a counter-receptor (a different type of molecule on another cell) or to another extracellular ligand; and (iii) an intracellular domain (often linked to the cytoskeleton of the cell). A CAM may be normally present in an inactive form; under appropriate conditions, activation may confer the ability e.g. to bind a counter-receptor. The extracellular domain of a CAM may act e.g. as a receptor for invasive bacterial pathogens (see CADHERINS, INTEGRINS), or as a means of cell migration during INFLAMMATION. In a given cell, the type and number of CAMs depend on factors such as stage of development, function (or change in function), and the presence or absence of viral infection. A CAM may be referred to by a ‘family’ name (see crossreferences, below) or by a CD (‘cluster of differentiation’) number. A given CD number (e.g. CD54) signifies a specific antigen which has been identified by various batches of monoclonal antibodies from different laboratories. An integrin molecule may have two CD numbers (e.g. LFA-1 = CD11a/CD18); one number refers to the a subunit, the other to the b subunit. See: CADHERINS, IMMUNOGLOBULIN SUPERFAMILY, INTEGRINS, SELECTINS and CD44; see also separate entries under ‘CD’. cell-bound antibodies Antibodies bound by their combining sites to homologous cell-surface antigens, or bound by other sites (see e.g. CYTOPHILIC ANTIBODIES) to cell-surface receptors. cell culture See TISSUE CULTURE. cell cycle In prokaryotic or eukaryotic microorganisms: the sequence of events which occur in a growing cell during the period between the cell’s formation from its parent cell and the division of the cell itself into daughter cells; this period of time includes (i) initiation of DNA replication from at least one chromosomal origin, (ii) the replication of chromosome(s), (iii) the segregation of chromosomes, (iv) the formation of a septum or intercell partition, and (v) cell division. These events always occur strictly in the same order. The events which constitute the cell cycle are regulated by both extracellular (environmental) and cell-associated factors. One event regarded as an important juncture in the cell cycle is the initiation of DNA replication; earlier workers proposed two main hypotheses to account for the regulation of this stage of the cell cycle. In the ‘G1 event model’ (devised primarily for eukaryotic cells), triggering of DNA replication, and subsequent cell division, is dependent on event(s) which occur only during a specific part of the cell cycle (the G1 phase – see below). In the ‘continuum model’ (postulated to refer to both prokaryotic and eukaryotic cells), a hypothetical initiator of DNA replication is produced by the cell during all phases of the cell cycle; initiation of DNA replication occurs when the concentration of initiator, per origin of replication in the cell, reaches a certain 136 cell cycle In bacteria, the cell cycle typically involves a period of growth followed by division into two similar or identical daughter cells (BINARY FISSION). (An atypical cell cycle occurs e.g. in CAULOBACTER (q.v.).) The daughter cells receive equal numbers of chromosome(s). Chromosome replication and cell division are related in a manner described by the HELMSTETTER–COOPER MODEL (q.v.). The cell’s dimensions during growth. Cells that are growing rapidly are larger (in both mass and volume) than those growing more slowly. If the doubling time is less than ∼1 hour, the C and D periods (see HELMSTETTER–COOPER MODEL) are more or less constant; at the beginning of each C period the cell’s mass (= initiation mass) is also virtually constant for a given rate of growth. Under these conditions, it follows that the faster a cell grows the greater will be the increase in cell mass and volume during the (constant) C + D interval prior to cell division. Cells of E. coli growing at a constant rate increase in size mainly by elongation (the diameter of the cells remaining largely unchanged). If the growth rate is increased, newly formed cells are larger than those formed at the lower rate. During upregulation of the growth rate, the cell’s diameter increases slowly, but the increase in cell length occurs more rapidly and, in fact, initially ‘overshoots’ – cells being longer than appropriate for the new, higher rate of growth; during ongoing growth at the new rate, the cell diameter increases to its new value while the cell length decreases to its final value (which is still higher than that at the lower rate of growth). If growth rate falls, the cell’s diameter, as before, adjusts more slowly than the cell’s length – the latter exhibiting an initial ‘undershoot’ [JGM (1993) 139 2711–2714]. In Bacillus subtilis, determination of the rod shape appears to involve helical, actin-like filaments within the cell. Filaments composed of MreB protein may influence the cell’s width. Those of Mbl protein influence longitudinal growth; they seem to guide the cell-wall-synthesizing machinery. These types of protein are found in many species, including Escherichia coli. Some bacteria (e.g. Corynebacterium glutamicum) lack these proteins; in these bacteria the rod shape may be maintained through polar growth. [Bacterial morphogenesis: Cell (2003) 113 767–776.] Synthesis of the cell envelope. During steady growth, cell envelope material is presumably being synthesized more or less continually throughout the cell cycle. Studies on the turnover of PEPTIDOGLYCAN in growing cells of the (Gram-negative) bacillus E. coli suggest that the lateral wall of the cell contains a monolayer of peptidoglycan and that multilayered peptidoglycan may occur at the poles [JB (1993) 175 7–11]. In Gram-positive bacteria, wall growth is believed to follow the INSIDE-TO-OUTSIDE MODEL. Höltje [Microbiology (1996) 142 1911–1918] described a model for the mode of incorporation of newly synthesized peptidoglycan in the growing sacculus: see THREE-FOR-ONE MODEL. It has been assumed that synthesis of the CYTOPLASMIC MEMBRANE proceeds passively (secondarily to the growing peptidoglycan sacculus). However, it has been reported that peptidoglycan synthesis is dependent on the synthesis of membrane phospholipids – a finding which suggests a novel mechanism for the regulation of growth of the cell envelope [Microbiology (1996) 142 2871–2877]. DNA replication. As mentioned, co-ordination between chromosome (DNA) replication and cell division is described by the HELMSTETTER–COOPER MODEL. Regulation of the initiation of a round of DNA replication is not understood – although a link between the timing of initiation and growth rate can be seen in the Helmstetter–Cooper model by comparing the start of the C periods in slow-growing and rapidly growing cells. Although the precise mechanism of initiation is unknown, several factors appear to be important. [Some similarities among the initiators of DNA replication in Bacteria, Archaea and Eucarya: FEMS Reviews (2003) 26 533–554.] One factor is the intracellular concentration of DnaA protein (or possibly an activated form of it); thus, replication is dependent on the initial binding of a number of DnaA molecules at specific sites (DnaA boxes) within the oriC region of the chromosome – such binding being followed by localized ‘melting’ (i.e. strand separation) in the DNA, a prerequisite for replication. The concentration of DnaA molecules within the cell is therefore critical because replication is delayed until the DnaA boxes have been occupied. Studies with in vitro replication systems have identified mutant DnaA proteins defective in opening the duplex at oriC [Mol. Microbiol. (2000) 35 454–462]. The link between initiation and growth rate, mentioned earlier, possibly involves guanosine 5
-diphosphate 3
-diphosphate (= ‘guanosine tetraphosphate’; ppGpp) – a small molecule whose concentration is inversely proportional to growth rate; the high levels of ppGpp found during slow growth can e.g. inhibit synthesis of DnaA protein (at the level of transcription), and this may result in delayed initiation. Another factor apparently involved in initiation is the Fis protein (fis gene product). Like DnaA, Fis has binding sites in the oriC region; moreover, it appears to regulate genes which include those encoding DnaA and the b subunit of DNA polymerase III [JB (1996) 178 6006–6012]. Fis binds at oriC in a cell-cycle-specific way [EMBO (1995) 14 5833–5841], forming an ‘initiation-preventive’ complex [NAR (1996) 24 3527–3532]. Nevertheless, the way in which Fis influences the timing of initiation is unknown. Initiation may also be influenced by the degree of methylation in the oriC region – a certain level of methylation apparently being necessary for a functional oriC. It has been suggested that, after a round of replication, the oriC region in a daughter cell may be left temporarily hemi-methylated (i.e. methylated in only the template strand), and that the time needed to achieve a functional level of methylation may affect the timing of initiation. Decatenation and partition of chromosomes. Newly synthesized chromosomes are catenated (i.e. interlocked, like the links of a chain); clearly, prior to partition, the chromosomes have to be separated, and this process is carried out by TOPOISOMERASES. (If decatenation is prevented, e.g. by mutation in an essential topoisomerase gene, the result may include the formation of filamentous cells, containing interlocked chromosomes, as well as anucleate cells which have no chromosome; this can occur e.g. in cells of E. coli which are mutant in the parC gene which encodes a subunit of topoisomerase IV.) On separation, each chromosome forms a compact nucleoid; this may involve the binding of many molecules of HU protein [Mol. Microbiol. (1993) 7 343–350]. The way in which nucleoids are partitioned to daughter cells is unknown. Various mechanisms have been suggested: (i) attachment to the cytoplasmic membrane – the nucleoids being segregated mechanically; (ii) mutual repulsion between nucleoids; (iii) the extrusion–capture model, in which extrusion of DNA from the ‘replication factory’ (replisome) provides the energy for partition [GD (2001) 15 2031–2041]. 137 cell disruption cycle, proteins are synthesized at the right times and targeted to correct locations within the cell. Control of the cell cycle. The occurrence of a given event in the cell cycle could depend directly on the occurrence of the previous event in the sequence. An alternative view is that the cell cycle is a sequence of independent events that are coordinated. Co-ordination was suggested [Mol. Microbiol. (1991) 5 769–774] for reasons that include e.g. (i) The inhibition of certain cell cycle events does not necessarily inhibit others. Thus, cell division can occur, for example, without proper partitioning of the nucleoids – i.e. division can proceed in the absence of a ‘signal’ from the event that normally precedes division. (ii) There is no evidence for direct coupling between chromosome replication and cell division. However, there is evidence for indirect co-ordination: damage to DNA (which may inhibit chromosome replication) triggers the so-called SOS SYSTEM which, among other effects, inhibits cell division. More recent studies on e.g. CAULOBACTER show that the cell cycle has checkpoints: stages at which, normally, a given event must be completed before the next can begin [see JB (2003) 185 1128–1146 (1135–1136)]. (See also essential TWO-COMPONENT REGULATORY SYSTEMS.) cell disruption The breaking of cells – usually for analysis of their components. Methods include: (a) Exposure to physical forces (see e.g. BRAUN MSK TISSUE DISINTEGRATOR, FRENCH PRESS, HUGHES PRESS, ULTRASONICATION). (b) Enzymic digestion of specific cell wall components – e.g. the digestion of PEPTIDOGLYCAN by LYSOZYME or LYSOSTAPHIN; cells thus weakened can be lysed osmotically. The morphology and structure of a cell influence the ease with which it is broken. Cells most resistant to mechanical disruption include yeasts, bacterial endospores, and Gram-positive cocci; Gram-negative bacteria are less resistant, and the least resistant include protozoa and other animal cells. Once cells are broken, their components become susceptible to the action of e.g. AUTOLYSINS. (See also CELL FRACTIONATION.) cell envelope (bacteriol.) The CYTOPLASMIC MEMBRANE together with all layers external to it – including the CELL WALL and/or (where applicable) an S LAYER. cell fractionation The separation of cell components. Fractionation is preceded by some form of CELL DISRUPTION. Methods of fractionation include various forms of CENTRIFUGATION, CHROMATOGRAPHY and ELECTROPHORESIS. cell-free ice nuclei See ICE NUCLEATION BACTERIA. cell fusion See e.g. HYBRIDOMA; see also PROTOPLAST FUSION. cell line (tissue culture) The heterogeneous population of cells resulting from the first subculture of a PRIMARY CULTURE, or from subsequent serial passaging of the cells. (cf. CLONED LINE and ESTABLISHED CELL LINE.) cell-mediated cytotoxicity Lysis of specific or non-specific target cells by certain types of lymphoid cell. Lysis by CD8+ cytotoxic T cells is triggered by specific antigen present at the surface of the target cell, is subject to HISTOCOMPATIBILITY RESTRICTION (involving MHC class I antigens), and is independent of both antibody and complement (see T LYMPHOCYTE). Cytotoxicity mediated by CD8+ T cells is important e.g. as a defence against viral infections (antigen-specific T cells can lyse virus-infected cells which bear virus-specific cell-surface antigens). Lysis of the target cell requires a single collision with a functional cytotoxic T cell – which can kill repeatedly. For other types of cell-mediated cytotoxicity see ADCC and NK CELLS. All types of cell-mediated cytotoxicity appear to require divalent cations (Ca2+ , Mg2+ ). In Bacillus, the oriC regions may be drawn apart actively [JB (1998) 180 547–555]. In B. subtilis the Smc protein seems to contribute to nucleoid structure – hence promoting normal segregation. [Segregation (primarily in B. subtilis): Microbiology (2001) 147 519–526.] After the report of ‘mitosis-like’ partitioning of plasmid R1 in E. coli [EMBO (2002) 21 3119–3127] it was suggested that actin-like filaments of MreB protein (a ‘cytoskeletal’ element associated with the maintenance of cell shape in rod-shaped bacteria) may mediate a similar process in the segregation of nucleoids; dysfunctional MreB inhibits chromosome segregation in E. coli [EMBO (2003) 22 5283–5292]. Septum formation. In E. coli, a cross-wall (septum) eventually divides the growing cell into two daughter cells. This process starts with the formation of a ring-shaped structure composed of FtsZ molecules (encoded by ftsZ ) circumferentially on the inner surface of the cytoplasmic membrane – mid-way along the length of the cell; this FtsZ ring (‘Z ring’) marks the plane of the forthcoming septum. (Expression of FtsZ may be influenced by ppGpp [JB (1998) 180 1053–1062].) Binding between FtsZ and ZipA (a cytoplasmic membrane protein) is reported to be necessary for cell division [Cell (1997) 88 175–185]. [Interaction between ZipA and a C-terminal fragment of FtsZ (X-ray crystallography): EMBO (2000) 19 3179–3191.] Assembly of the Z ring apparently begins at a nucleation site and proceeds, bidirectionally, around the circumference of the cell; development of nucleation sites themselves may involve the Era protein (product of the era gene; a GTPase of the Ras superfamily) which seems to be a key factor in the control of cell division [Mol. Microbiol. (1998) 29 19–26]. During cell division the septum must form mid-way along the cell – not at alternative polar sites; the Z ring is guided by the min gene products. In one model for this guidance system, MinE molecules form a circumferential, annular band at mid-cell. This MinE ring oscillates, i.e. it moves alternately towards one pole and then the other; this causes the MinCD proteins to oscillate between opposite poles of the cell – so that the (inhibitory) influence of MinC on FtsZ polymerization is minimal at the mid-cell location, permitting assembly of the FtsZ ring at the mid-cell position [Science (2002) 298 1942–1946]. (During oscillation of MinCD proteins, arrival/departure at the cell’s poles involves binding/hydrolysis of ATP.) One report suggested that the oscillation of MinD may involve ATPdependent polymerization of MinD into fibres [PNAS (2002) 99 16776–16781]. A fluorescent image of the FtsZ ring has been obtained by means of gene fusion between ftsZ and the gene for green fluorescent protein [PNAS (1996) 93 12998–13003]. An FtsZ ring has been reported in anucleate cells, indicating that division sites can develop even in the absence of a chromosome [Mol. Microbiol. (1998) 29 491–503]. The Z ring also occurs in members of the Archaea; this suggests that the apparatus for cell division was similar in an ancestral prokaryote [Mol. Microbiol. (1996) 21 313–319]. In the plane of the Z ring, peptidoglycan grows inwards to form the septum; this begins very early in the development of the division site [JB (2003) 185 1125–1127]. Cell division. The septum is split into two layers, each of which forms the new pole (end) of a daughter cell; this process is carried out by the EnvA protein. The OUTER MEMBRANE grows inwards to complete the cell envelope. The morphogenes encode those products, such as EnvA, FtsZ and the MinCDE proteins, which ensure that, during the cell 138 cell wall cell-mediated hypersensitivity Syn. DELAYED HYPERSENSITIVITY. cell-mediated immunity (CMI) Any form of immunity which can be transferred to a non-immune individual by the transfer of cells (including e.g. LYMPHOCYTES) but not by (cell-free) serum or plasma (see DELAYED HYPERSENSITIVITY), or in which cells are directly involved as effectors (see CELL-MEDIATED CYTOTOXICITY). (cf. HUMORAL IMMUNITY; see also AIDS.) cell membrane Syn. CYTOPLASMIC MEMBRANE. cell sap (1) The fluid in a plant cell vacuole. (2) Syn. CYTOSOL. cell sorter See e.g. FACS. cell wall In most algae, archaeans, bacteria and fungi: the structure which forms a (usually rigid) layer external to the CYTOPLASMIC MEMBRANE and which is responsible for the shape of the organism; it protects the protoplast from mechanical damage, osmotic lysis etc., and it may also serve as a permeability barrier to ANTIBIOTICS and other substances. Microbial cell walls differ greatly in structure and composition, according to type and species. (a) Algal cell walls. Although a few algae (e.g. DUNALIELLA, PORPHYRIDIUM) lack cell walls, the majority have a cell wall composed of one or more layers of microfibrillar polysaccharide embedded in a matrix of amorphous polysaccharide (cf. CHLAMYDOMONAS); in many species a layer of mucilaginous polysaccharide also occurs external to the cell wall. The most common microfibrillar component in algal cell walls is CELLULOSE, which occurs e.g. in members of the Phaeophyta and in many members of the Chlorophyta (e.g. Chaetomorpha, Chlorella, Scenedesmus), Rhodophyta and Xanthophyceae (e.g. Botrydium, Vaucheria). The proportion of cellulose varies from species to species but is often small compared with that of the matrix polymers. In some green algae (e.g. Bryopsis (gametothallus), Caulerpa, Halimeda, Penicillus, Udotea) and red algae (e.g. Palmaria palmata, Porphyra umbilicalis) XYLANS replace cellulose as the main microfibrillar component of the wall. A (1 → 4)-b-linked MANNAN is the main structural polysaccharide in the cell walls of e.g. Acetabularia and Codium spp; a mannan also occurs as an external layer (cuticle) in the red algae Porphyra umbilicalis and Bangia fuscopurpurea. Xylomannan is a major component of Prasiola japonica. Matrix polymers in algal cell walls are many and varied; they include e.g. AGAR, ALGINATE, CARRAGEENAN and FUCOIDIN. The walls of some algae contain inorganic materials such as silica (see e.g. DIATOMS) and calcium carbonate (e.g. as a surface encrustation in the CHAROPHYTES); calcium carbonate occurs as calcite in some species, aragonite in others (e.g. HALIMEDA). Many unicellular algae of the CHRYSOPHYTES, PRYMNESIOPHYCEAE and ‘PRASINOPHYCEAE’ have a cell covering composed of scales, each consisting of an organic base in which silica or calcite may be deposited. In the Prymnesiophyceae and ‘Prasinophyceae’ these scales are synthesized in the Golgi apparatus and are released to the exterior by fusion of the Golgi vesicles with the cytoplasmic membrane. (See also THECA.) In those algae which have a heteromorphic alternation of generations, the different phases may have different cell wall structures/compositions; for example, in Derbesia marina/Halicystis ovalis, the Derbesia phase has cell walls containing mannan, while the Halicystis phase has walls containing cellulose and glucoxylan. Gametes and spores may lack cell walls. Many algal walls may be stained for light microscopy with e.g. ALCIAN BLUE (for acidic polysaccharides), PAS, or TOLUIDINE BLUE (which gives a pink colour with carboxyl and sulphate groups). [Review: Book ref. 38, pp 278–332.] (b) Bacterial cell walls. Although some species are wall-less (see e.g. MYCOPLASMA), a cell wall is present in the majority of bacteria; moreover, there may be layer(s) external to the cell wall proper – see e.g. CAPSULE, M PROTEIN, S LAYER. Traditionally, bacterial cell walls have been classified into two major types: the Gram-positive type and the Gram-negative type (see also GRAM TYPE). However, while generally classified as Gram-negative organisms, the cyanobacteria commonly have cell walls that exhibit features of both Gram-negative and Grampositive species [JB (2000) 182 1191–1199]. The bacterial cell wall is a target for certain ANTIBIOTICS (see e.g. b-LACTAM ANTIBIOTICS and VANCOMYCIN). Gram-positive-type walls are relatively thick (ca. 30–50 nm) and appear to be largely homogeneous by electron microscopy. The wall consists mainly of PEPTIDOGLYCAN (up to ∼80% dry weight), and this polymer accounts for ∼20–40 nm of the wall’s thickness. The backbone chains of peptidoglycan run parallel to the cell surface; they are orientated perpendicular to the long axis of the cell. During wall growth, new glycan chains appear to be intercalated between existing chains (see THREE-FOR-ONE MODEL). Various acidic and/or neutral polymers (e.g. TEICHOIC ACIDS, TEICHURONIC ACIDS) are covalently linked (via a phosphodiester bridge) to the C-6 of N -acetylmuramic acid residues – either directly or via ‘linkage units’. In some genera the cell wall contains lipids (see e.g. MYCOBACTERIUM) or carbohydrates (e.g. C SUBSTANCE). Some compounds (e.g. lipoteichoic acids) are closely associated with, but not covalently linked to, the wall. A hydrophilic trans-wall channel occurs in STREPTOMYCES and in a species of Corynebacterium [JB (2003) 185 4779–4786]. The precise composition of the Gram-positive type cell wall may vary with growth conditions and with the degree of maturity of the wall; thus, e.g. in Bacillus spp the presence of teichoic and/or teichuronic acids in the cell wall is influenced by environmental levels of phosphate, while (in various genera) the degree of cross-linking is greater in mature than in newly synthesized peptidoglycan. (See also INSIDE-TO-OUTSIDE MODEL.) Gram-negative-type walls are thinner than the walls in Grampositive species and they have a distinctly layered structure; they are ∼20–25 nm in thickness, of which the peptidoglycan layer (the innermost layer of the cell wall) may contribute less than ∼6 nm (being less than ∼10% dry weight of the cell wall). (It has been suggested that the lateral wall in Escherichia coli may contain a monolayer of peptidoglycan [JB (1993) 175 7–11].) External to the peptidoglycan layer is the OUTER MEMBRANE; some of the BRAUN LIPOPROTEIN molecules of the outer membrane are covalently linked to peptidoglycan; in e.g. E. coli the 6amino group of the C-terminal lysine residue of the lipoprotein is linked by a peptide bond to a DAP residue in peptidoglycan. At a number of sites, the outer membrane apparently fuses with the cytoplasmic membrane to form ADHESION SITES. (See also CELL CYCLE; L FORMS; PERIPLASMIC REGION; SPHAEROPLAST; ZETA POTENTIAL.) (c) Fungal cell walls. Cell walls are present in most fungi (absent in Coelomomyces) and they consist chiefly of polysaccharides which vary e.g. according to taxonomic group. The architecture of the fungal cell wall seems to be basically similar in many cases: one or more alkali-insoluble, possibly microfibrillar polysaccharides (e.g. CHITIN, CELLULOSE, other b-glucans) form an innermost ‘structural’ layer, while other (commonly alkali-soluble) components occur throughout the wall – forming an outer layer and also permeating the structural layer. The various wall components may be more or less extensively bonded together by hydrogen, ionic and/or covalent 139 cellar fungus chitosan but no melanin, while spore walls of this fungus contain much less chitosan, more glucan, and up to ca. 10% by dry weight of MELANIN. Wall structure also differs between mycelial and yeast forms of DIMORPHIC FUNGI (sense 1); for example, in Mucor rouxii the transition from mycelial form to yeast form is accompanied by the appearance of mannan in the walls. In Paracoccidioides brasiliensis the yeast-form walls contain (1 → 3)-a-D-glucan as the sole glucan, while mycelial walls contain (1 → 3)-b-Dglucan/(1 → 6)-b-D-glucan in addition to the a-glucan. In addition to their role as structural components, some fungal wall polymers can be used as endogenous sources of nutrient (for example, during carbon starvation or fruiting body formation – see e.g. PSEUDONIGERAN). Certain antifungal antibiotics function by interfering with cell wall biosynthesis: see e.g. ACULEACIN A; NIKKOMYCINS; PAPULACANDIN B; POLYOXINS. (See also GROWTH (fungal).) (d) Archaeal cell walls. Distinctive types of cell wall occur in members of the Archaea; there are also wall-less species (e.g. Thermoplasma). Archaeal walls include those which consist mainly or solely of an S LAYER that is closely associated with the cytoplasmic membrane; this type of wall occurs e.g. in species of Desulfurococcus, Halobacterium, Methanococcus and Thermoproteus. Heteropolysaccharides occur in the cell walls of Halococcus and Methanosarcina, and PSEUDOMUREIN occurs in the cell wall of Methanobacterium and Methanobrevibacter. cellar fungus Coniophora puteana. (cf. WET ROT.) cellobiase See CELLULASES. cellobiohydrolase Exoglucanase: see CELLULASES. cellobiose A reducing disaccharide: b-D-glucopyranosyl-(1 → 4)-D-glucopyranose. (See also CELLULASES and LIGNIN.) cellobiose:quinone oxidoreductase See LIGNIN. cellodextrins Oligomers of (1 → 4)-b-D-linked glucopyranosyl residues formed by the action of CELLULASES on CELLULOSE. cellular (1) Of or pertaining to a cell or cells. (2) Having the form and characteristics of a (single) cell. (3) Composed of two or more cells. (cf. ACELLULAR; see also UNICELLULAR and MULTICELLULAR.) cellular immunity (1) CELL-MEDIATED IMMUNITY. (2) Aspects of NON-SPECIFIC IMMUNITY mediated e.g. by macrophages and other phagocytic cells. cellular microbiology A branch of microbiology concerned with the interactions between microorganisms and eukaryotic cells at the molecular level. [Science (1996) 271 315–316; Book ref. 218.] cellular slime moulds SLIME MOULDS in which the vegetative phase consists of uninucleate amoeboid cells (myxamoebae) which aggregate to form a multicellular pseudoplasmodium from which the fruiting bodies arise. Classes: ACRASIOMYCETES and DICTYOSTELIOMYCETES. (cf. MYXOMYCETES.) cellulases Enzymes which degrade at least some forms of CELLULOSE. Endoglucanase (endo-(1 → 4)-b-D-glucanase, endocellulase, (1 → 4)-b-D-glucan 4-glucanhydrolase, EC 3.2.1.4) attacks more or less randomly at sites within (1 → 4)-b-D-glucan chains in amorphous regions of cellulose or at the surfaces of microfibrils. Exoglucanase (exo-(1 → 4)-b-D-glucanase, exocellulase, (1 → 4)-b-D-glucan cellobiohydrolase, EC 3.2.1.91) releases cellobiose (and, in some cases, glucose) from non-reducing ends of b-D-glucan chains. b-Glucosidase (cellobiase, b-D-glucoside glucohydrolase, EC 3.2.1.21), hydrolyses cellobiose and watersoluble cellodextrins to glucose. Many cellulolytic organisms produce a number of isoenzymes. bonds, and the wall may be surrounded by a gel-like or mucilaginous CAPSULE. [Survey of fungal cell walls: Book ref. 38, pp 352–394.] In (mycelial) basidiomycetes, ascomycetes and chytridiomycetes the main structural components are typically chitin and b-D-glucans (usually containing (1 → 3)- and (1 → 6)linkages in varying proportions); these polymers are closely associated (possibly covalently linked) to form an alkaliinsoluble complex. (This association may prevent the formation of crystalline microfibrils of chitin in the native cell wall [Book ref. 38, pp 355–357]; microfibrils of a-chitin readily form on chemical extraction of the b-glucan.) Alkali-soluble components of these walls include glycoproteins (usually containing mannose and/or galactose in ascomycetes, xylose and mannose in basidiomycetes) and (1 → 3)-a-D-glucans. In zygomycetes the cell wall typically contains CHITOSAN closely associated with poly-anionic substances such as polyphosphate and polyglucuronate (cf. MUCORAN); glucans appear to be absent. In oomycetes the main structural component is cellulose, occurring with (1 → 3)-b-D-glucans and/or (1 → 6)-b-Dglucans; in some oomycetes (e.g. certain members of the Leptomitales, Peronosporales, Saprolegniales), and in some members of the Hyphochytriomycetes, the walls contain both cellulose and chitin. In yeasts the cell wall tends to reflect taxonomic affinities. In Saccharomyces cerevisiae (an ascomycetous yeast) the wall consists mainly of alkali-insoluble b-glucans and mannan (as phosphomannoprotein) in roughly equal amounts; chitin is only a minor component, occurring mainly in the primary septa and bud scars but also dispersed throughout the wall. The glucan forms the innermost layer which comprises two components. The major component (which apparently provides the rigid framework of the wall) is a b-glucan containing mostly (1 → 3)-linkages with some (1 → 6)-linkages, while the minor component contains mainly (1 → 6)-b-linkages with some (1 → 3)-b-linkages. The mannan component in S. cerevisiae is an extensively branched polymer (containing (1 → 6)-a-, (1 → 2)-a- and(1 → 3)-a-linkages) covalently bound to protein. This mannoprotein forms a continuous layer at the wall surface and penetrates the inner glucan layer to some depth; it may have a role in determining the porosity of the wall, and it can e.g. protect the glucan from the action of b-glucanases such as the Z-glucanase ZYMOLYASE [JB (1984) 159 1018–1026]. The mannoprotein molecules are linked covalently (e.g. by disulphide bonds) and by hydrogen and ionic bonds to the glucan and to each other. (Mannoproteins with specific functions (e.g. INVERTASE, acid phosphatase, molecules involved in sexual recognition and agglutination) also occur in the cell wall and/or periplasmic space.) The walls of other yeasts may differ from those of S. cerevisiae. For example, while the walls of Candida albicans yeast cells are essentially similar to those of S. cerevisiae, the alkaliinsoluble b-glucans are highly branched, having a higher proportion of (1 → 6)-b-linkages [JGM (1984) 130 3295–3301]. Schizosaccharomyces pombe contains no chitin, and little or no mannan; instead, it contains (1 → 3)-a-D-glucan (PSEUDONIGERAN) and (1 → 3)-b-D-glucan. The yeast Sporobolomyces (a basidiomycete) contains mainly chitin and galactomannan in its walls. Different morphological forms of a given fungus may show differences in wall composition and structure; thus, for example, in Mucor rouxii the hyphal wall contains a high proportion of 140 cellulose Isolated microbial cellulases have relatively low specific activities and are costly to obtain; they currently have only limited commercial use. Pilot schemes include the use of cellulases or cellulolytic organisms for the saccharification of the cellulose in waste materials such as straw, sugarcane bagasse, sawdust, newspaper etc (see e.g. INDUSTRIAL ALCOHOL and SINGLE-CELL PROTEIN). (See also ENZYMES.) (See also PAPER SPOILAGE.) cellulin granules Granules, composed of CHITIN and (1 → 3)-band (1 → 6)-b-linked glucan, found in the cells of fungi of the LEPTOMITALES; the granules are composed of alternating concentric layers of chitin and glucan. (cf. CONCENTRIC BODIES.) cellulitis A diffuse, spreading, inflammatory condition affecting (usually) subcutaneous tissues and characterized by oedema, redness and pain. Common causal agents are streptococci and staphylococci. Orbital cellulitis (inflammation of the eye-socket) may be caused by Haemophilus influenzae. (See also ERYSIPELAS and GAS GANGRENE.) cellulolytic Able to degrade CELLULOSE. Cellulomonas A genus of aerobic or facultatively anaerobic, asporogenous bacteria (order ACTINOMYCETALES, wall type VIII) which occur e.g. in soil. In young cultures the organisms are slender, irregular rods (diam. ca. 0.5–0.6 µm) and/or filaments, with or without primary branching, but shorter rods and/or coccoid forms occur in the stationary phase; the cells are Gram-positive, but easily decolorized, and cells which are apparently Gram-negative may predominate. The organisms are non-motile, or motile by one to several polar flagella [IJSB (1984) 34 218–219]. Growth occurs e.g. on peptone meat extract at 30° C; all strains require biotin and thiamine for growth. Typically, a yellow non-diffusible pigment is formed when growth occurs on nutrient agar. Glucose is metabolized to acid both oxidatively and fermentatively. Cellulomonas spp are amylolytic and cellulolytic (see CELLULASES). Species include C. biazotea, C. cellasea, C. flavigena, C. fimi, C. gelida, and C. uda. GC%: 71–77. Type species: C. flavigena. cellulose A linear (1 → 4)-b-D-glucan. Cellulose occurs in plants (it is the main structural component of the cell walls in most plants), in the CELL WALLS of most algae and certain fungi, in certain cellular slime moulds (e.g. in the sheath surrounding the sorocarp stalk, in the spore walls, and in the slime of migrating slugs of Dictyostelium discoideum), and in the cyst walls of Acanthamoeba spp; extracellular cellulose is produced by certain bacteria: e.g. Acetobacter xylinum (see PELLICLE sense 1), Sarcina ventriculi (most strains of which form a cellulose microcapsule apparently responsible for the formation of the characteristic packets of cells), strains of Agrobacterium and Rhizobium (in which extracellular cellulose is involved in the adhesion of bacteria to plants), and various floc-forming bacteria in activated sludge. Native cellulose occurs largely in the form of (crystalline) microfibrils (‘a-cellulose’) each composed of stacked sheets of parallel, hydrogen-bonded glucan chains. Each chain is in extended form (held rigid by intrachain hydrogen bonds) and contains many glucosyl residues (e.g. 2000–15000, depending on source). In plant cell walls the microfibrils are embedded in a matrix of HEMICELLULOSES, LIGNIN and PECTIC POLYSACCHARIDES; regions of amorphous cellulose, infiltrated with hemicelluloses, also occur. [Plant cell wall structure: Book ref. 38, pp. 9–46.] Cellulose may be stained for microscopy e.g. with CONGO RED or CALCOFLUOR WHITE – both of which also inhibit microfibril formation during cellulose synthesis. [Biosynthesis of cellulose in microorganisms: Book ref. 62, pp. 85–127.] Many fungi, several bacteria, and a few protozoa (e.g. Trichonympha) can degrade cellulose, but their activities depend e.g. on the degree of polymerization and crystallinity of the cellulose and its association with hemicelluloses and LIGNIN (see CELLULOSE). Crystalline cellulose is highly resistant to enzymic attack; most of the glucan chains in microfibrils are inaccessible to enzymes, and any (surface) bonds cleaved by endoglucanase action can readily be re-formed owing to the stable orientation of the (hydrogen-bonded) glucan chains. Degradation of crystalline cellulose requires the synergistic action of both endoglucanase and exoglucanase; the exoglucanase rapidly removes cellobiose units from the newly created ends formed by endoglucanase action, thus preventing the re-formation of glucosidic bonds. The two enzymes may act consecutively or in concert. Alternative pathways probably also occur – e.g. the progressive release of cellobiose and/or glucose by exoglucanase action only. [PTRSLB (1983) 300 283–291.] Both exo- and endoglucanases are inhibited by cellobiose, and b-glucosidase action (which eliminates cellobiose) is often the rate-limiting step in cellulose degradation. (Cellobiose may also be degraded by cellobiose phosphorylase, EC 2.4.1.20, or oxidized by cellobiose oxidase or by cellobiose:quinone oxidoreductase (see LIGNIN).) Cellulases are subject to induction (in the presence of cellulose) and to catabolite repression; SOPHOROSE (formed e.g. from cellobiose by transglycosylation reactions) is an effective inducer in many (but not all) cellulolytic organisms and may be a natural inducer – at least in some fungi. Repressors include e.g. glucose and other readily utilizable substrates. Cellobiose can act as inducer or repressor, according to concentration. (Reference is still sometimes made to ‘C1 ’ and ‘Cx ’ cellulase activities. These terms derive from the early C1 –Cx concept [JB (1950) 59 485–497] but have subsequently been used in different ways by different authors [Book ref. 31, 290–292]. The terms are thus ambiguous and are best avoided.) Cellulolytic bacteria include e.g. many actinomycetes (e.g. Cellulomonas spp) and RUMEN bacteria, Clostridium thermocellum, Cytophaga spp, Polyangium cellulosum, Pseudomonas spp and Sporocytophaga myxococcoides. In general, bacteria produce endoglucanases and b-glucosidases but not exoglucanases; they thus have little activity on highly crystalline native cellulose, but are highly active against amorphous cellulose or soluble cellulose derivatives (e.g. CM-CELLULOSE). The endoglucanases may be cell-bound, extracellular, or both. In e.g. Cytophaga and Sporocytophaga, which grow in close contact with cellulose fibres, the glucanases are associated with the outer membrane. (See also CELLULOSOME.) In rumen bacteria cellulases are retained in the GLYCOCALYX surrounding the cells, and the products of their action may also be so retained prior to their uptake by the cell [CRM (1981) 8 303–338]. Bacterial b-glucosidases are typically cell-associated (e.g. periplasmic) or intracellular. Cellulolytic fungi include many basidiomycetes (e.g. Coniophora puteana), ascomycetes (e.g. Chaetomium spp, Stachybotrys atra) and deuteromycetes (e.g. Trichoderma spp). (See also NEOCALLIMASTIX.) In general, the brownrot basidiomycetes produce only endoglucanases and bglucosidases, while ascomycetes, deuteromycetes and the whiterot basidiomycetes produce (extracellular) endoglucanases, exoglucanases, and b-glucosidases (see BROWN ROT and WHITE ROT). Cellulases are not produced by animals; herbivores must therefore rely on symbiotic cellulolytic microorganisms to digest their dietary cellulose (see e.g. RUMEN, TERMITE–MICROBE ASSOCIATIONS). 141 cellulosome The strength of a centrifugal field (at a given point) is given by: 4p2 (revs min−1 )2 r G= 3600 Cellulose is a vast and constantly renewable potential source of fermentable sugars which would be valuable feedstock for various biotechnological processes. However, the lignin–hemicellulose–cellulose (LHC) complex is very stable, and crystalline cellulose is very resistant to enzymic attack (see CELLULASES). Thus, before it can be used as feedstock, cellulose must be released from the LHC complex and its microfibrillar structure disrupted by various (costly) mechanical, physical or chemical procedures – e.g. grinding or treatment with steam, mineral acids or alkalis (cf. IOTECH PROCESS); this has placed a constraint on its extensive use in industry. [Lignocellulose hydrolysis: PTRSLB (1983) 300 305–322.] cellulosome In certain cellulolytic bacteria: a CELLULOSEbinding, multiCELLULASE-containing cell-surface organelle. The cellulosome appears to be composed of parallel rows of catalytic units; it has been suggested that these may act together, cleaving cellulose into pieces as small as cellobiose – which can be taken up by the cell [FEMS Reviews (1993) 104 340]. cellulytic Able to lyse cells. CELO virus See AVIADENOVIRUS. centiMorgan (cM) A unit for indicating the distance between markers on a chromosome; the number of centiMorgans between two given markers is the (statistically corrected) RECOMBINATION FREQUENCY exhibited by the markers. Thus, e.g., a corrected RF of 10% gives a distance of 10 cM. (cf. MAP UNIT.) Centipeda A genus of Gram-negative, anaerobic, rod-shaped bacteria; C. periodontii, which has HELICOTRICHOUS flagellation, occurs e.g. in human periodontal lesions. Centraalbureau voor Schimmelcultures See CBS. central capsule (protozool.) See RADIOLARIA. central dogma (mol. biol.) The dogma that genetic information can be transferred only in the direction DNA → protein. Originally, the dogma specified that information could flow only in the direction DNA → RNA → protein, but with the discovery of REVERSE TRANSCRIPTASE this was modified to DNA ↔ RNA → protein. Central European encephalitis virus See FLAVIVIRIDAE. centric (1) (of a chromosome) Having a CENTROMERE. (cf. HOLOCENTRIC and MONOCENTRIC sense 2.) (2) Refers to that type of nuclear division in which CENTRIOLES are involved. (3) See DIATOMS. centrifugation The use of a centrifugal field for the sedimentation of fine particulate matter – or macromolecules (see ultracentrifugation, below) – in a liquid medium, or for separating different types of particulate matter, or macromolecules, within a given suspension or solution. (See also CELL FRACTIONATION and MAECT.) In the most widely used form of centrifugation, the sample is placed in a suitable container at the end of one arm of a spider-shaped motor-driven metal rotor which has a central, vertical shaft; rotation of the shaft at high speed subjects the sample to a centrifugal field which causes particulate matter in the sample to move in an outward (radial) direction, i.e., towards the bottom of the container. (In practice one or more additional samples of equal weight must be carried by the rotor in order to achieve a balanced distribution of weight.) The apparatus described above, suitably enclosed for safety, constitutes a simple centrifuge. Specialized forms of centrifuge are used e.g. for refrigerated centrifugation and for continuous centrifugation; in continuous centrifugation, which is used for dealing with large volumes of liquid, provision is made for the continual inflow of sample (e.g. through a hollow rotor) and the continual discharge of supernatant (and of sediment in some types of centrifuge). in which ‘revs min−1 ’ refers to the number of revolutions per minute (r.p.m.) made by the rotor, and r is the distance (cm) between the given point in the sample and the axis of rotation. G is given in terms of acceleration (cm sec−2 ); since (for a given mass) acceleration is proportional to the force producing it, G gives a measure of the force acting on a body subjected to a given centrifugal field. The centrifugal field may be increased by (a) increasing the number of r.p.m. and/or (b) increasing r. Since the acceleration due to gravity (g) is ca. 980 cm sec−2 , G/980 gives the number of times by which the given centrifugal field is greater than the gravitational field; G/980 is termed the relative centrifugal field (= relative centrifugal force), RCF. (Since, for a given value of r.p.m., the centrifugal field varies with r – i.e., may vary from one centrifuge to another – centrifugation data should be reported in the form g/time, e.g., 1000 g/10 minutes.) G gives the acceleration which a given centrifugal field would impart to a particle in a vacuum. Since particles subjected to centrifugation are suspended in a liquid, factors in addition to the applied field must be taken into account when considering the effect of the field. These factors, which greatly affect the rates of movement of particles, are the size, shape and density of the particles, and the viscosity and density of the liquid in which the particles are suspended. For example, the rate of movement of a spherical particle in a given centrifugal field is proportional to: Gr2p (rp − r)/viscosity of medium where rp is the radius of the particle, and rp and r are the densities of the particle and the suspending medium, respectively. Thus, given a mixed population of particles which differ e.g. in size and/or density, separation into homogeneous populations may be achieved on the basis of their differing sedimentation rates in a centrifugal field (differential centrifugation). If particle density (rp ) is equal to that of the suspending medium (r), the rate of movement in the centrifugal field is zero; if rp is less than r, centrifugally accelerated flotation occurs – an effect which has been used e.g. for the purification of GAS VACUOLES. In general, the time taken for a spherical particle to move a given distance in a centrifugal field is given by: t= h Ra 9 loge 2 !2 r2p (rp − r) Rb in which ! is the angular velocity (radians/sec); h is the viscosity of the medium; rp is the radius of the particle; rp and r are the densities of the particle and medium, respectively; Ra and Rb are the distances from the centre of rotation to the positions where the particle started (a) and stopped (b); and t is the time (sec). Density gradient centrifugation. In some types of centrifugation (e.g. isopycnic and rate-zonal centrifugation) the sample is initially layered onto a column of liquid containing a solute whose concentration increases with increasing distance down the column; solutes used to prepare such density gradients (which may be linear or stepped) include e.g. caesium chloride (CsCl), sucrose, and IODINATED DENSITY-GRADIENT MEDIA. An important practical advantage of a density gradient is that it militates against convection – which can disrupt a liquid column in the absence of a gradient. Isopycnic centrifugation (also called equal density, equilibrium density gradient, or equilibrium zonal centrifugation). The 142 cephalodium sample is layered onto a density gradient column, and centrifugation is continued until equilibrium is reached, i.e., each particle has reached that part of the gradient which corresponds to its own density (its isopycnic or equal density position). Particles in a heterogeneous sample are thus separated on the basis of their individual densities: particles of different density can be separated even if they are similar in size and shape. Rate-zonal centrifugation (also called band, gradient differential, s zonal, zonal, or zone centrifugation). The sample is layered onto a density gradient column, and centrifugation is continued until the various types of particle in the sample form layers or bands at different levels within the column – at which time centrifugation is stopped. The separation of particles into bands is thus achieved on the basis of the differing sedimentation rates of the particles. Ultracentrifugation is used e.g. for the sedimentation of macromolecules, for the determination of molecular weights, for purifying plasmid DNA, for characterizing viruses, and as an indirect method for estimating the GC% of bacterial DNA. The principles involved are similar to those given earlier in the entry; however, while the common laboratory centrifuge can develop a maximum field of ca. 5000 to 10000 g, the ultracentrifuge can achieve fields of the order of 500000 g. In an ultracentrifuge the rotor is housed in a refrigerated and evacuated chamber, and the instrument incorporates various optical systems (e.g. the SCHLIEREN SYSTEM) and photographic systems so that the progress of sedimentation can be followed and recorded at all times. (See also SVEDBERG UNIT.) If a species of DNA is subjected to isopycnic ultracentrifugation in a caesium chloride gradient, the DNA forms a band at a region of the gradient corresponding to its buoyant density (Bd); the Bd of e.g. chromosomal DNA from Escherichia coli is ca. 1.7 g/cm3 . Since there is a linear relationship between Bd and GC%, the value of Bd can be used for estimating GC% (q.v.). Ultracentrifugation can also be used to separate cccDNA (e.g. plasmid DNA) from linear DNA or from circular, nicked DNA (e.g. nicked chromosomal DNA). cccDNA can incorporate smaller amounts of certain INTERCALATING AGENTS (e.g. ETHIDIUM BROMIDE) than can equivalent molecules of the other forms of DNA. The incorporation of e.g. ethidium bromide decreases the Bd of the DNA molecule. Thus, if e.g. a plasmid-containing bacterial lysate is centrifuged in a caesium chloride–ethidium bromide gradient, cccDNA may form a distinct band of higher Bd; such a procedure has been called dye–buoyant density centrifugation. centriolar plaque Syn. SPINDLE POLE BODY. centriole In many types of eukaryotic cell: an intracellular microtubular structure (see MICROTUBULES) which is involved in some types of MITOSIS and which, in some cells, can develop into a BASAL BODY; centrioles do not occur in e.g. some types of fungi. Essentially, a centriole is a hollow cylinder, ca. 300–400 nm in length, whose wall is formed of 9 longitudinally arranged triplets of microtubules; one end of the cylinder contains a dense intracentriolar material. The structure appears to lack DNA, but RNA may be present. A centriole is often surrounded by a number of dense pericentriolar structures which, collectively, are sometimes referred to as a centrosome and which can act as a MICROTUBULE-ORGANIZING CENTRE. (‘Centrosome’ is also used to refer to the entire structure, i.e., centriole(s) plus pericentriolar structures.) Centrioles are often formed from preexisting centrioles, but they can also be formed de novo. Centrohelida An order of protozoa (class HELIOZOEA) in which the cell typically has an eccentric nucleus and a central body (the centroplast) from which the axopodial axonemes appear to arise. Some species lack a centroplast but have a large eccentric nucleus. Typically, a centrohelid has an outer ‘skeleton’ of siliceous plates and/or spines embedded in a gelatinous outer covering; some species have organic spicules. Genera include e.g. Acanthocystis, Gymnosphaera, Heterophrys, RAPHIDIOPHRYS. centromere The region of a replicated eukaryotic chromosome where the two CHROMATIDS are joined together; during MITOSIS a KINETOCHORE is formed on both sides of the centromere. [Molecular structure of centromeres: ARB (1984) 53 163–194; chromatin conformation of yeast centromeres: JCB (1984) 99 1559–1568.] (cf. HOLOCENTRIC and MONOCENTRIC (sense 2).) centroplast See CENTROHELIDA. Centropyxis See ARCELLINIDA. centrosome See CENTRIOLE. centrotype In NUMERICAL TAXONOMY: a strain regarded as that which typifies a given species cluster on the basis of its position within that cluster. centrum (mycol.) The totality of fertile and sterile structures within a CLEISTOTHECIUM, PERITHECIUM, or locule in an ASCOSTROMA. cep Syn. cèpe (see BOLETALES). cèpe (cep) See BOLETALES. Cepedea See OPALINATA. Cepedietta See ASTOMATIDA. Cephaleuros A genus of obligately epiphytic (and/or parasitic) algae (division CHLOROPHYTA) closely related to TRENTEPOHLIA. The heterotrichous thallus is composed of a discoid system of prostrate, compacted, branched filaments and an erect system of sterile, multicellular, uniseriate filaments and specialized zoosporangiophores; the prostrate system grows between the cuticle and epidermis in the leaves, fruits and/or young stems of a host plant, while the erect filaments and gametangia protrude through the host cuticle. Although the alga may cause considerable damage to the host plant (see RED RUST), there is apparently no evidence that it is nutritionally dependent on the plant. (Cephaleuros also occurs as the photobiont in certain obligately foliicolous lichens: e.g. Strigula spp [Book ref. 129, pp. 190–191].) cephalexin See CEPHALOSPORINS. cephaline gregarines See GREGARINASINA. Cephaloascus See ENDOMYCETALES. cephalodium (lichenol.) A discrete group of cyanobacteria occuring in or on the thallus in certain lichens whose main photobiont is a green alga. Cephalodia appear generally to arise by entrapment of free-living cyanobacteria by the mycobiont – usually via the lower cortex of the thallus. The cyanobacteria (usually NOSTOC) carry out NITROGEN FIXATION, fixed nitrogen being transferred to the mycobiont. Internal cephalodia occur e.g. in species of Lobaria, Nephroma and Solorina; they typically consist of colonies of cyanobacteria which lie in the medulla and may be apparent as swellings on the surface of the thallus. External cephalodia occur e.g. in species of Peltigera and Stereocaulon; they may occur (e.g. as warty protuberances) on the upper or lower surface of the thallus, depending on species. Some lichens (e.g. species of Lobaria, Peltigera and Sticta) form well-differentiated, fruticose or subfoliose, cyanobacteriumcontaining structures which arise (usually) from the upper surface of the thallus and which are capable of independent growth. These structures were originally thought to be epiphytes and were assigned to distinct genera (e.g. ‘Dendriscocaulon’ ), but are now commonly believed to be elaborate cephalodia 143 cephaloridine or different morphotypes (‘phycosymbiodemes’) of the same mycobiont. [Chimeroid associations in Peltigera: Lichenol. (1978) 10 157–170.] cephaloridine See CEPHALOSPORINS. cephalosporin C See CEPHALOSPORINS. cephalosporin N Syn. PENICILLIN N. cephalosporin P1 A steroid antibiotic structurally related to FUSIDIC ACID. cephalosporinase A b-LACTAMASE active mainly or solely against CEPHALOSPORINS. cephalosporins A class of antibiotics each characterized by a molecular structure which includes a b-lactam ring fused to a dihydrothiazine ring (see b-LACTAM ANTIBIOTICS for structure and mode of action). (cf. CEPHALOSPORIN P1 and PENICILLIN N.) Cephalosporin C (the first to be prepared) is produced e.g. by Acremonium kiliense (= Cephalosporium acremonium); this antibiotic has poor antibacterial activity. A range of semi-synthetic cephalosporins can be prepared from the 7-aminocephalosporanic acid (7-ACA) derivative of cephalosporin C. When introduced, the earlier cephalosporins (collectively) had useful activity against a range of bacteria, and were typically (particularly cefotaxime and cefuroxime) less susceptible than PENICILLINS to b-LACTAMASES; moreover, compared to penicillins, cephalosporins were found to be less frequently allergenic. When introduced, the first- to third-generation cephalosporins were characterized by antibacterial spectra such as: mainly Gram-positive (cephalexin, cephaloridine); good activity against Gram-positive and moderate activity against b-lactamasenegative Gram-negative species (cephalothin); Gram-positive species and e.g. Neisseria gonorrhoeae and Haemophilus influenzae (cefuroxime); limited spectrum, but some activity against Pseudomonas aeruginosa (cefsulodin); Gram-negative species, including Haemophilus influenzae (cefotaxime). (Note that cephalosporins (and e.g. the cephamycin CEFOXITIN) have little or no activity against enterococci.) The earlier cephalosporins (and some of the newer ones) are commonly susceptible to extended-spectrum b-LACTAMASES (q.v.). (See also CEPHAMYCINS.) Some cephalosporins can be administered orally, while others (e.g. cefuroxime, cefotaxime) are administered parenterally. First-generation cephalosporins include e.g. cefazolin, cephalexin, cephaloridine and cephalothin. Second-generation cephalosporins include e.g. cefamandole, cefonicid [Drugs (1986) 32 222–259], ceforanide, cefsulodin and cefuroxime. Third-generation cephalosporins include e.g. cefixime, cefmenoxime [Am. J. Med. (1984) 77 (supplement 6A)], cefotaxime, ceftazidime [Am. J. Med. (1985) 79 (supplement 2A)], ceftizoxime and ceftriaxone. Fourth-generation cephalosporins include e.g. CEFEPIME and cefpirome. (Note. In the literature, the names of some cephalosporins are spelt with either ‘f’ or ‘ph’.) Cephalosporium See ACREMONIUM. (For C. lecanii see VERTICILLIUM.) Cephalothamnium A genus of colonial CHRYSOPHYTES in which a cluster of biflagellate, non-pigmented cells extends from the tip of a single stalk. cephalothin See CEPHALOSPORINS. cephamycins Antibiotics (7-a-methoxyCEPHALOSPORINS) produced by Streptomyces spp or prepared from cephalosporins. Compared with those cephalosporins which lack the 7-amethoxy group, cephamycins (e.g. CEFOXITIN) tend to have decreased antibacterial activity but increased resistance to bLACTAMASES and improved penetrability of the Gram-negative outer membrane. cephems Compounds which contain the cephalosporin nucleus (see b-LACTAM ANTIBIOTICS). (cf. OXACEPHEMS.) cephradine See CEPHALOSPORINS. Ceraceosorus See BRACHYBASIDIALES. ceramide See SPHINGOMYELIN. Ceramium See RHODOPHYTA Ceratiomyxa A genus of SLIME MOULDS formerly classified with the acellular slime moulds (MYXOMYCETES); currently, the genus is placed e.g. in a separate class, Ceratiomyxomycetes, of the MYXOMYCOTA, or – in zoological schemes – is classified with the protostelids e.g. in the subclass Protosteliia (see EUMYCETOZOEA). C. fruitculosa is a widespread and common species found e.g. on bark and rotting wood. The vegetative PLASMODIUM is almost hyaline. Prior to fruiting, it produces copious amounts of a mucoid matrix, and plasmodium and matrix become extended to form upright, usually branched white columns (1–10 mm tall) which solidify on drying; sporocarps develop on the surface of these columns, each sporocarp consisting of a long slender stalk bearing a single round or cylindrical spore. The spores are deciduous. On germination of a spore, a quadrinucleate ‘thread stage’ is produced, and this eventually gives rise to 4, then 8, flagellated haploid cells which can apparently function as gametes; a diploid vegetative plasmodium develops from the zygote formed by fusion of two of these flagellated cells. Ceratiomyxella See PROTOSTELIOMYCETES. Ceratiomyxomycetes See CERATIOMYXA. Ceratium A genus of DINOFLAGELLATES in which the cells typically bear 3 or 4 horn-like thecal extensions: one arising from the epicone, 2 or 3 from the hypocone. Ceratobasidiaceae See TULASNELLALES. Ceratobasidium See TULASNELLALES. Ceratocystis A genus of fungi (order OPHIOSTOMATALES) which include various plant pathogens – see e.g. DUTCH ELM DISEASE and OAK WILT. (See also BLUE STAIN, PINE DISEASES, and IPOMEAMARONE.) C. ulmi forms erumpent perithecia (see also ASCOSPORE), and in the imperfect (Pesotum) state can form individual conidia on simple conidiophores or chains of conidia in a drop of mucus at the tip of a dark, bristle-like synnema (ca. 1 mm high). C. coerulescens forms superficial perithecia, and in the Chalara state can form cylindrical or ellipsoidal conidia. Ceratomyces See LABOULBENIALES. Ceratomyxa See MYXOSPOREA. Cercophora See SORDARIALES. cercopithecine herpesvirus 1 See B VIRUS. Cercospora A genus of fungi (class HYPHOMYCETES) which include many plant-pathogenic species – e.g. C. beticola (causal agent of leaf spot of beet), and C. apii (e.g. on celery). The organisms form septate mycelium; conidia are characteristically thread-like and multiseptate. cereal chlorotic mottle virus See RHABDOVIRIDAE. cereal diseases Diseases of members of the Gramineae cultivated for their edible grain. Some of the common diseases are listed below. Although some diseases are specific to particular types of cereal, others can also affect certain wild grasses; thus, e.g. the causal agents of HALO SPOT and LEAF BLOTCH can both grow on cocksfoot (Dactylis glomerata) and Timothy grass (Phleum pratense). (See also GRASS DISEASES.) Barley. See e.g. BROWN RUST, ERGOT, EYESPOT (sense 2), GLUME BLOTCH, HALO SPOT, LEAF BLOTCH (‘rhynchosporium’), NET 144 Chagas’ disease H+ -ATPase BLOTCH, POWDERY MILDEW, SMUTS (sense 2), TAKE-ALL, YELLOW (barley yellow dwarf virus) and HORDEIVIRUSES (barley stripe mosaic virus). Maize. See e.g. CORN STUNT DISEASE, MAIZE WALLABY EAR DISEASE, SMUTS (sense 2); see also MAIZE STREAK VIRUS. Oats. See e.g. CROWN RUST, ERGOT, SMUTS (sense 2), TAKE-ALL, VICTORIA BLIGHT. Rice. See e.g. BAKANAE DISEASE, BLAST DISEASE, SUFFOCATION DISEASE; see also FIJIVIRUS, RICE DWARF VIRUS, RICE GALL DWARF VIRUS, RICE RAGGED STUNT VIRUS, RICE STRIPE VIRUS GROUP. (cf. YELLOW RICE.) [Engineered resistance to Rhizoctonia solani (sheath blight): Biotechnology (1995) 13 686–691.] Rye. See e.g. BROWN RUST, COMMON BUNT, ERGOT, HALO SPOT, LEAF BLOTCH (‘rhynchosporium’), SNOW MOULD, TAKE-ALL. Wheat. See e.g. BLACK STEM RUST, BROWN RUST, COMMON BUNT, ERGOT, EYESPOT (sense 2), GLUME BLOTCH, HALO SPOT, KARNAL BUNT, POWDERY MILDEW, SMUTS (sense 2), SNOW MOULD, TAKEALL, WHEAT SCAB, YELLOW RUST; see also SOIL-BORNE WHEAT MOSAIC VIRUS. cereal striate virus See RHABDOVIRIDAE. cereal tillering disease virus See FIJIVIRUS. cerebriform Brain-like in appearance; convoluted. cerebroside See SPHINGOLIPID. cereolysin See THIOL-ACTIVATED CYTOLYSINS. Cerinomyces See DACRYMYCETALES. cermet See BIOMIMETIC TECHNOLOGY. cerulenin An antibiotic – [(3R)-2,3-epoxy-4-oxo-7,10-dodecadienoyl amide] – obtained from ‘Cephalosporium caerulens’; it has antifungal and antibacterial activity, but is too unstable for clinical use. Cerulenin inhibits fatty acid and sterol biosynthesis, inhibiting e.g. the condensation reaction between acyl and malonyl thioesters and thus preventing fatty acid chain elongation. It also inhibits lipoteichoic acid synthesis in e.g. Enterococcus faecalis, prevents the synthesis and secretion of certain extracellular proteins in some bacteria (e.g. glucosyltransferase in oral streptococci [JGM (1983) 129 3293–3302]), and alters morphogenesis and autolytic activity in Bacillus subtilis [JGM (1985) 131 591–599]. ceruloplasmin See BLUE PROTEINS. cervical cancer (cancer of the cervix) See PAPILLOMA. cesspool A watertight container for the temporary storage of sewage. (cf. SEPTIC TANK.) Cetavlon See QUATERNARY AMMONIUM COMPOUNDS. Cetraria A genus of LICHENS (order LECANORALES); photobiont: a green alga. Thallus: either fruticose and erect or foliose with upturned margins, growing on soil, bark, wood or rocks. Apothecia have reddish-brown discs and are marginal in foliose species. C. islandica (‘Iceland moss’) has an erect, branched thallus (ca. 2–7 cm in height); the strap-like branches are curved to appear more or less tubular, the inside surface being greenishgrey to dark brown, the outermost surface being paler with a scattering of whitish pseudocyphellae. The thallus margins bear short black spines. This species forms cushions on the ground in heathland, moorland, and acid woodland; it has been used as food e.g. in Scandinavian countries, and serves as fodder for e.g. reindeer and caribou. Cetrimide See QUATERNARY AMMONIUM COMPOUNDS. cetylpyridinium chloride See QUATERNARY AMMONIUM COMPOUNDS. cetyltrimethylammonium bromide See QUATERNARY AMMONIUM COMPOUNDS. CF (1) COMPLEMENT FIXATION. (2) CYSTIC FIBROSIS. RUST; see also CF0 F1 See PROTON ATPASE. CFA/I, CFA/II See ETEC. CFP-10 See ESAT-6. CFT COMPLEMENT-FIXATION TEST. cfu Colony-forming unit: see COUNTING METHODS. CGA protein See CATABOLITE REPRESSION. CGB agar Canavanine-glycine-bromthymol blue agar: a medium used to distinguish between certain strains of CRYPTOCOCCUS. On this medium, C. neoformans var. gattii hydrolyses glycine, raising the pH and causing the BROMTHYMOL BLUE to turn blue; the few strains of C. neoformans var. neoformans which can hydrolyse glycine are inhibited by the L-canavanine. [Recipe and method: Book ref. 100, p. 95.] CH region See IMMUNOGLOBULINS. CH50 See HD50 Chaenotheca See CALICIALES. Chaetoceros A large genus of planktonic centric DIATOMS in which the cells are often joined together by long setae. Chaetocladium See MUCORALES. Chaetomella See SPHAEROPSIDALES. Chaetomium A genus of fungi (order SORDARIALES) which form evanescent asci in perithecia which bear long, curved, coiled and/or branched hairs or bristles; the unicellular, commonly dark ascospores are discharged from the perithecium in a gelatinous mass. Most or all species are strongly cellulolytic (see CELLULASES, PAPER SPOILAGE, SOFT ROT (sense 1), TEXTILE SPOILAGE). (See also COMPOSTING and OLIVE-GREEN MOULD.) Chaetomorpha A genus of unbranched, filamentous, siphonocladous green algae (division CHLOROPHYTA). Chaetophora A genus of uniseriate, filamentous (heterotrichous) green algae (division CHLOROPHYTA) which occur in freshwater habitats, usually attached to rocks, plants etc. Long colourless multicellular ‘hairs’ extend from the ends of the (branched) filaments. Quadriflagellate zoospores and biflagellate isogametes are formed. Related genera include e.g. Draparnaldia, Fritschiella, STIGEOCLONIUM and URONEMA. chaga fungus The sterile fruiting body of Inonotus obliquus. Chagas’ disease An acute or chronic, often fatal, systemic TRYPANOSOMIASIS of man which occurs in Central and South America. The causal agent, Trypanosoma (SCHIZOTRYPANUM) cruzi, is typically transmitted contaminatively by blood-sucking TRIATOMINE BUGS (family Reduviidae): metacyclic forms in the bug’s faeces infect via wounds, abrasions and mucosal surfaces. (The pathogen can also be transmitted by blood transfusion and, via the milk, to breast-fed infants. Transplacental transmission occurs in over 1% of cases in endemic areas.) Reservoirs of bugtransmissible pathogens exist in domestic animals (e.g. dogs) and wild animals (e.g. anteaters, armadillos, opossums, certain rats and primates). [Book ref. 72, pp 161–182.] In the patient, T. (S.) cruzi occurs as non-dividing trypomastigotes in the blood, and in the reproductive amastigote and epimastigote forms in pseudocysts within host cells. Symptoms may include cardiomyopathy, grossly enlarged colon (‘megacolon’) and oesophagus, and low levels of certain hormones resulting from damage to endocrine glands. Autoantibodies to heart and skeletal muscles, and to nerve tissue, have been detected. Lab. diagnosis: microscopical examination of thick blood films; XENODIAGNOSIS (using e.g. Dipetalogaster maxima); a CFT and/or e.g. immunofluorescence tests. Treatment: chemotherapeutic agents, e.g. Lampit (a nitrofuran) and benznidazole (a 2-nitroimidazole), are commonly used but are associated with significant side-effects; moreover, this treatment may or may not eliminate the intracellular forms of T. (S.) cruzi. LUTEOVIRUSES 145 chain-termination method a nascent protein either co-translationally [JBC (1997) 272 32715–32718] or following translation. Although most chaperones are proteins, a membrane lipid acts as a chaperone for the LacY protein in Escherichia coli [EMBO (1998) 17 5255–5264]. Chara See CHAROPHYTES. Charales See CHAROPHYTES. charcoal blood agar A medium used for the primary isolation of Bordetella pertussis; it is reported to yield better growth than that obtained with the traditional BORDET–GENGOU AGAR. Beef extract, starch, peptone, NaCl, nicotinic acid and agar are dissolved in water by heating; charcoal is added before autoclaving. Sterile, defibrinated horse blood is added to the molten agar at ∼45° C. [Recipe: Book ref. 219, pp 470, 471.] Charcot–Leyden crystals Microscopic, colourless crystals often present in the stools of patients suffering from certain intestinal diseases (e.g. amoebic dysentery) or in the sputum of patients with asthma or other chest conditions. The crystals are diamondshaped or shaped like short, double-pointed needles; they stain with iodine. charge-shift immunoelectrophoresis IMMUNOELECTROPHORESIS used e.g. to distinguish amphiphilic from hydrophilic proteins. Differentiation is based on the fact that when amphiphilic proteins bind non-ionic detergent at their hydrophobic sites, their electrophoretic mobility is changed by the addition of (positively or negatively charged) ionic detergent; hydrophilic proteins are unaffected. chargerin See ANISOTROPIC INHIBITOR. Charon A type of CLONING vector derived from the genome of bacteriophage l; typically, a Charon is a REPLACEMENT VECTOR. charonin An ATP-dependent protease which may also function as a molecular chaperone. Charophyceae See CHLOROPHYTA. Charophyta See CHAROPHYTES. charophytes A group of macroscopic green algae variously regarded as (a) a distinct division (Charophyta) [e.g. Book ref. 123, pp. 303–330]; (b) a class (Charophyceae) within the division Chlorophyta [e.g. Book ref. 130, pp. 36–132]; or (c) an order (Charales) within a class, Charophyceae, which is broader than that in (b): see CHLOROPHYTA. Charophytes (e.g. Chara and Nitella) have several features in common with bryophytes (mosses and liverworts): e.g., the plant is structurally complex, being differentiated into root-, shoot- and leaf-like structures; the shoots are divided into nodes (each node bearing a whorl of branches) and internodes; sexual reproduction is oogamous, and an envelope of sterile cells (shield cells) surrounds the antheridia and oogonia; asexual reproduction may occur e.g. by the formation of bulbils (‘plantlets’) on the rhizoids, or of AMYLUM STARS on the lower nodes, but apparently never by the formation of zoospores. Charophytes are generally heavily calcified (hence the popular names ‘stoneworts’ or ‘brittleworts’); they occur mainly in fresh water (lakes, ponds, etc). (See also GYROGONITES.) chasmoendolith See ENDOLITHIC. Chattonella See CHLOROMONADS. chauveolysin See THIOL-ACTIVATED CYTOLYSINS. che gene In e.g. enterobacteria: a gene involved in CHEMOTAXIS. checker colony See DRAUGHTSMAN COLONY. Cheddar cheese See CHEESE-MAKING. cheese-making Cheese is made by the coagulation and fermentation of MILK; differences between the various types of cheese reflect differences in e.g. mode of manufacture, microorganisms employed, and type of milk used (e.g. cows’, goats’ sheep’s). Dinitroaniline herbicides such as trifluralin (a,a,a-trifluoro2,6-dinitro-N,N-dipropyltoluidine), which inhibit certain protozoan parasites by affecting tubulin polymerization, have been examined for their potential as candidate antimicrobials against T. cruzi [TIP (2001) 17 136–141]. [The disease in Mexico: TIP (2001) 17 372–376.] chain-termination method (of DNA sequencing) See DNA SEQUENCING. Chainia See STREPTOMYCES. Chalara See HYPHOMYCETES and CERATOCYSTIS; see also MURAMIDASE. chalcomycin See MACROLIDE ANTIBIOTICS. chalk-brood A BEE DISEASE caused by Ascosphaera apis. Infected honey-bee larvae become fluffy and swollen, and later shrink and harden; death usually occurs after the larvae have been sealed in their cells. Some of the dead larvae remain chalky-white, others become dark-coloured. Infection occurs by ingestion of spores of A. apis. The spores germinate in the gut; mycelium develops in the gut lumen and penetrates the gut wall. Fruiting bodies may form on the outside of the larvae. Ascosphaera spp can also cause chalk-brood in wild bees – e.g. A. aggregata in the alfalfa leaf-cutting bee Megachile rotundata [life cycle of A. aggregata: Mycol. (1984) 76 830–842]. challenge (immunol.) The exposure of an animal (or other living system) to a pathogen (or other agent) – usually in order to determine the state of immunity or resistance to the pathogen or agent; for example, an animal may be challenged with a pathogen to test the efficacy of a previously administered vaccine. challenge virus See INTERFERENCE (1). chalybeate (of water) Containing iron. Chamaesiphon A genus of unicellular CYANOBACTERIA (section I) in which the cells are ovoid and reproduce by repeated budding at the apical end; the buds were formerly known as ‘exospores’. GC%: ca. 47. Chamaesiphonales See CYANOBACTERIA. Chamberland candle See FILTRATION. chamois contagious ecthyma virus See PARAPOXVIRUS. champagne See WINE-MAKING. chancre A papular lesion formed at the initial site of infection in certain diseases (e.g. SYPHILIS, SLEEPING SICKNESS). chancroid (soft chancre) A VENEREAL DISEASE, caused by Haemophilus ducreyi, characterized by one or more ulcerative lesions which are usually confined to the genitalia. Incubation period: 1–14 (usually 4–5) days. Changuinola subgroup See ORBIVIRUS. channel catfish virus disease (CCVD) A FISH DISEASE which affects cultivated channel catfish (Ictalurus punctatus) under 4 months of age. Symptoms include e.g. abnormal swimming movements, haemorrhages of gills, skin and internal organs, and necrotic lesions in liver, spleen etc. The causal agent is a herpesvirus (see HERPESVIRIDAE). chanterelle See CANTHARELLALES. Chaos A genus of large, multinucleate, freshwater amoebae of the AMOEBIDA [Book ref. 133, p. 145]. In C. carolinense polypodial forms may be 0.7–2.0 mm in diam., monopodial forms up to 3 mm diam.; C. illinoisense is generally smaller (ca. 0.5–1.5 mm). chaperone (molecular chaperone) In most cases: a protein that is specialized for folding/stabilization or direction of a (typically newly synthesized) protein. (See also HEAT-SHOCK PROTEINS, FIMBRIAE (type I) and VIRULON.) A chaperone may bind to 146 chemiluminescence has been made of a starter culture containing organisms that produce the BACTERIOCIN lacticin 3147 – which inhibits Listeria monocytogenes [JAM (1999) 86 251–256]. Cream cheese (for example, Neufchatel ) is made in a similar way using cream instead of skim milk; it thus has a higher fat and lower moisture content. (f) Mozzarella is a soft, usually unripened, Italian cheese formerly made from buffalo milk (but now generally made from cows’ milk). Its manufacture is similar to that of Cheddar but involves higher temperatures (50–55° C) as well as thermophilic streptococci and lactobacilli. (g) Parmesan is a hard, low-moisture Italian cheese; its manufacture is similar to that of Swiss cheeses (see below), but e.g. propionibacteria are not used. (h) Swiss cheeses (e.g. Emmentaler, Gruyère) are manufactured in a way basically similar to that used for Cheddar; however, the process involves higher cooking temperatures (50–55° C) and uses thermophilic bacteria (Streptococcus thermophilus, Lactobacillus spp). Strains of Propionibacterium are used in the inoculum; during ripening, these bacteria (i) contribute flavour components (e.g. propionic acid), and (ii) produce carbon dioxide – which collects in pockets, thus forming the characteristic ‘eyes’ in the cheese. (See also CHEESE SPOILAGE and DAIRY PRODUCTS.) cheese spoilage High-moisture cheeses (e.g. cottage cheese) are very susceptible to spoilage by a wide range of bacteria and moulds, while low-moisture cheeses (e.g. parmesan) are much more resistant. Surface spoilage of cheeses is commonly due to the growth of moulds – e.g. species of Aspergillus, Cladosporium and Penicillium, and (in high-moisture cheeses) Geotrichum spp. Bacteria may produce off-flavours – e.g. H2 S (produced e.g. by clostridia), rancidity (due e.g. to lipolytic pseudomonads); undesirable holes (‘eyes’) may be formed during the ripening process by gas-producing bacteria (e.g. coliforms, Bacillus spp). cheese-washers’ lung An EXTRINSIC ALLERGIC ALVEOLITIS associated with the inhalation of allergens from Penicillium spp – e.g. P. verrucosum (‘P. casei ’). CHEF See PFGE. chelating agents See e.g. CDTA, EDTA, EGTA, NTA. chemical shift See NUCLEAR MAGNETIC RESONANCE. Chemiclave An apparatus used e.g. for sterilizing surgical instruments etc; essentially, it consists of a chamber within which instruments etc are exposed to a mixture of the vapours of alcohol, formaldehyde and water at 132° C, 20 lb/inch2 (ca. 137 kPa). (cf. AUTOCLAVE.) chemiluminescence The production of light as a result of a chemical reaction. The term is commonly used to refer e.g. to the emission of light by phagocytes and other specialized mammalian cells under appropriate conditions, and to light emission in cell-free experimental systems, while BIOLUMINESCENCE usually refers to the emission of light from whole, living organisms; in all cases the emission of light is O2 -dependent. In those cases where the mechanism is known, light emission involves the oxidation of a particular type of organic molecule – the resulting unstable, excited molecule spontaneously returning to the (unexcited) ground state with concomitant emission of light. (cf. PHOSPHORESCENCE.) Chemiluminescence in phagocytes is associated with the respiratory burst which occurs in e.g. NEUTROPHILS during PHAGOCYTOSIS. Experimental chemiluminescence systems employ lightemitting substances such as lucigenin (bis-N-methylacridinium nitrate), lophine (2,4,5-triphenylimidazole) and LUMINOL. Initially, the milk may undergo various treatments: e.g. homogenization (i.e. reduction in size of fat globules), adjustment of fat content. The milk protein is then coagulated to form a solid curd in which milk fat and soluble components are trapped. Coagulation is usually achieved primarily by adding rennet (rennin) or microbial PROTEASES; it is aided by stirring, heating (‘cooking’), and the addition of cultures of selected strain(s) of LACTIC ACID BACTERIA – which lower the pH by fermenting lactose to lactic acid. Strains of the mesophilic bacterium Lactococcus lactis are commonly used, but thermophilic strains (e.g. Streptococcus thermophilus, Lactobacillus bulgaricus, L. helveticus) are used for cheeses cooked at higher temperatures. The bacteria primarily carry out HOMOLACTIC FERMENTATION under the conditions of manufacture, but they also form small amounts of other important flavour components (acetic and propionic acids, ketones, esters etc.). The curd is cut into pieces, and the liquid (whey) is removed by pressing and/or straining; the curd may then be treated to optimize pH, reduce moisture, increase salt content etc., according to the type of cheese. Finally, the cheese is ripened by a controlled storage during which flavour and texture develop due mainly to the action of microorganisms and milk enzymes. (a) Blue cheeses (e.g. Gorgonzola, Roquefort, Stilton). As well as strains of Lactococcus lactis, Penicillium roqueforti is mixed with the curd before removal of the whey; during ripening, the mould grows through the cheese, giving the characteristic flavour and blue ‘veins’. (b) Brick cheese is an American soft cheese. The early stages of manufacture resemble those for Cheddar (see below); subsequently the cheese is surface-ripened : the relative humidity is held at 95% to allow a succession of microorganisms to develop on the surface of the cheese. First, yeasts metabolize the lactic acid, raising the pH and synthesizing vitamins etc.; this stimulates the growth of micrococci and Brevibacterium linens. The microbial growth is washed off before packaging and a further period of ripening. Bel paese, Limburger, Port Salut and Tilsit are made similarly. (c) Camembert is a high-moisture soft, French cheese. In addition to lactic acid bacteria, the mould Penicillium camemberti is added either to the milk or to the separated curd. The curd is placed on racks to maximize surface exposure and facilitate growth of the mould – which forms a cottony white mat over the surface of the cheese. Yeasts (e.g. species of Saccharomyces, Kluyveromyces, Candida), and sometimes Brevibacterium linens, are also present in the surface flora. Brie is made in a similar manner. (d) Cheddar is a hard English cheese made from raw or pasteurized whole milk. The milk is clotted enzymically, inoculated with (usually) strains of Lactococcus lactis, and cooked at temperatures of up to ca. 40° C. (A genetically engineered starter culture has been used to control the pathogen Listeria monocytogenes in Cheddar cheese [AEM (1998) 64 4842–4845].) The curd is cut into small cubes and allowed to settle in the whey. The whey is drained off, and the curd mass is cut into blocks which are piled and turned (a process called ‘cheddaring’). The curd is then cut into smaller pieces, salted, pressed into the desired shape, packaged, and ripened at 2–7° C. Edam and Gouda are semi-soft Dutch cheeses made in a similar manner. (e) Cottage cheese is a high-moisture, unripened soft cheese made from skim milk. The milk is clotted with a B- or BDtype LACTIC ACID STARTER. The final cooking temperature is ca. 55° C (cf. QUARG). To improve the safety of cottage cheese, use PASTEURIZATION, 147 chemiluminogenic probe the matrix side of the inner membrane which becomes relatively alkaline and electrically negative. By contrast, in the chloroplast thylakoid membrane, protons are pumped inwards during PHOTOSYNTHESIS, so that electrochemical gradients develop in the opposite orientation. Respiring cells of Escherichia coli develop a pmf of the order of 200 mV; thus, e.g. 1y may be ca. 100 mV and 1pH ca. −1.75 (1p = 205 mV). These cells maintain an internal pH of ca. 7.4–7.8 when the extracellular pH ranges from 5.5 to 9.0, i.e., the ‘bulk-phase’ transmembrane 1pH is highly dependent on the extracellular pH. (By contrast, in Clostridium pasteurianum a decrease in extracellular pH from 7.1 to 5.1 corresponds to a decrease in internal pH from 7.5 to 5.9 [Book ref. 82, pp. 137–138].) In respiring E. coli, the 1p drops sharply if the external pH rises above ca. 7.8, though 1p can be augmented under these conditions by increasing the extracellular concentration of K+ or Na+ [JB (1985) 163 423–429]. In chloroplasts, the contribution of 1pH to 1p is typically much greater than it is in bacteria such as E. coli. In a respiring bacterial cell there is a characteristic proton circuit: protons are extruded across the cytoplasmic membrane by the primary pmf generator (electron transport chain) and they re-cross the membrane via an H+ -ATPase during ATP synthesis. In such a cell the pmf acts as a thermodynamic back-pressure which opposes further extrusion of protons. However, proton extrusion can occur not only through the operation of an electron transport chain: it can also occur as a result of the tendency of the intracellular reaction (ATP ↔ ADP + Pi) to reach equilibrium; in a respiring cell, the energy derived from metabolism holds this reaction far to the left of its equilibrium position, so that the tendency to reach equilibrium (by ATP hydrolysis) is associated with a large amount of Gibbs free energy (1GATP ; 1Gp ; phosphate potential ; phosphorylation potential ). 1GATP tends to promote ATP hydrolysis (at the H+ -ATPase on the inner side of the membrane) and, hence, to promote proton extrusion in opposition to the pmf. 1GATP and pmf may tend to maintain a dynamic balance; however, if pmf is below a certain value ATPases tend to become (reversibly) inactivated: see PROTON ATPASE. Pmf is the primary factor responsible for RESPIRATORY CONTROL. Experimental manipulation of 1p, 1y or 1pH . Various IONOPHORES can be used to collapse or modify pmf or either of its components. Thus, e.g., VALINOMYCIN can alter 1y, without altering 1pH, by permitting K+ influx into the bacterial cytoplasm sufficient to reduce or eliminate 1y. By contrast, NIGERICIN can effect an electroneutral exchange of K+ for H+ , thus altering 1pH without disturbing 1y. UNCOUPLING AGENTS can abolish 1p (or prevent its development) and (as a consequence) can abolish respiratory control. Other agents (e.g. AUROVERTINS, DCCD, efrapeptin, OLIGOMYCIN, QUERCETIN, TENTOXIN and tributyltin chloride) can inhibit (F0 F1 )-type H+ ATPases. Measurement of 1p. The components of 1p (1y and 1pH) are estimated separately and added. 1y is often estimated by allowing an indicator ion to distribute across the membrane so as to reach electrochemical equilibrium with the membrane potential; at equilibrium, a measurement is made of the concentration of the indicator ion on either side of the membrane, and 1y is estimated from the Nernst equation (see equation 3 in ION TRANSPORT). The indicator ion must be permeable only by electrical uniport (so that its equilibrium distribution reflects 1y); it should not be bound or metabolized; it should disturb the membrane Luminol emits light in an alkaline solution of H2 O2 in the presence of a suitable catalyst; catalysts include e.g. cytochromes, horseradish peroxidase, MICROPEROXIDASE, and even Mn2+ or Ni2+ ions. Other agents – e.g. lophine, lucigenin – emit light in alkaline H2 O2 in the absence of a catalyst. Chemiluminescence involving the (ATP-requiring) firefly luciferin–luciferase system (see BIOLUMINESCENCE) has been used to detect small amounts of ATP; e.g., very small numbers of cells can be quantified by measuring their extractable ATP [e.g. in studies on bacterial adhesion: JGM (1983) 129 621–632]. LUMINOL-dependent chemiluminescence can be triggered in mouse spleen cells by interaction between spleen cell plasma membrane and the HN and (particularly) F glycoproteins of the Sendai virus; such chemiluminescence is apparently associated with the action of unstable oxygen species (e.g., HYDROGEN PEROXIDE, HYDROXYL RADICAL, SUPEROXIDE) on the luminol probe [Book ref. 87, pp. 451–458]. Immunoassay systems using chemiluminescent labels may offer a useful alternative to RADIOIMMUNOASSAY systems [PHLS Digest (1984) 1 58–62]. [Reviews on applications: Book ref. 96; TIBS (1986) 11 104–108.] chemiluminogenic probe See e.g. LUMINOL. chemiosmosis The phenomenon in which an energy-dependent transfer of protons or electrons across an ENERGY-TRANSDUCING MEMBRANE generates, or augments, a transmembrane proton gradient whose inherent energy (proton motive force, pmf ) can be used for chemical, osmotic or mechanical work. A pmf can be generated e.g. by the operation of an ELECTRON TRANSPORT CHAIN; by the illumination of a PURPLE MEMBRANE; by the hydrolysis of ATP by a PROTON ATPASE; by the hydrolysis of pyrophosphate by a PROTON PPASE; by END-PRODUCT EFFLUX; or by EXTRACYTOPLASMIC OXIDATION. A pmf can be used e.g. for ATP (or PYROPHOSPHATE) synthesis (see OXIDATIVE PHOSPHORYLATION); for the concentration of solutes against a concentration gradient (see e.g. ION TRANSPORT); for the regulation of TRANSHYDROGENASE activity; for REVERSE ELECTRON TRANSPORT; and (in flagellated bacteria) for the rotation of the FLAGELLUM. (See also AEROTAXIS and PHOTOTAXIS; cf. SODIUM MOTIVE FORCE.) Proton motive force is usually represented as the sum of (a) the ‘electrical’ energy, and (b) the ‘chemical’ energy associated with protons by virtue of their distribution across a transmembrane proton gradient – such a gradient being, simultaneously, both an electrical (i.e., charge) gradient and a chemical (i.e., concentration) gradient. The ‘electrical’ component of pmf (= membrane potential, transmembrane electrical potential difference, 1y) is generally given in millivolts (mV). The ‘chemical’ component of pmf corresponds to the transmembrane pH differential, 1pH; 1pH can be converted to millivolts by multiplying by the factor 2.3(RT/F) in which R is the gas constant, T is the absolute temperature, and F is the Faraday constant. The factor 2.3(RT/F) is commonly indicated by the letter Z. The expression for pmf (symbol: 1m̃H+ or 1p) is thus: 1p = 1y − Z1pH (1) Z has a value of ca. 60 at 30° C. In this equation the minus sign is necessary for the correct summation of energies: 1pH is itself commonly a negative quantity, being obtained by the subtraction of a higher pH from a lower one. (The equation above refers specifically to proton gradients.) In bacteria, the outward pumping of protons during respiratory pmf generation causes the inner (cytoplasmic) side of the cytoplasmic membrane to become both alkaline and electrically negative relative to the outer (periplasmic) side; in mitochondria it is 148 chemotaxis attractant for e.g. fibroblasts) and IP10 (produced by monocytes and endothelial cells, attractant for e.g. NK cells). (The two latter chemokines are reported to have anti-angiogenic activity, and they may inhibit tumour formation.) Chemokines of the CC family include eotaxin (produced by macrophages and endothelial cells, attractant primarily for eosinophils) and RANTES (‘regulated on activation, normal T cell expressed and secreted’: produced by T cells, monocytes and epithelial cells, attractant for e.g. T cells and NK cells). Chemokines of the C family include lymphotactin: a (murine) protein produced e.g. by activated T cells and a potent attractant for T cells. chemoklinokinesis Klinokinesis (see KINESIS sense 2) in which the stimulus is a change in concentration of a chemical. chemolithoautotroph A CHEMOTROPH with properties of a LITHOTROPH and an AUTOTROPH. The ability to grow chemolithoautotrophically is confined to a relatively small number of prokaryotes; these organisms make important contributions to the cycles of matter, and some are involved in LEACHING processes. Plasmid-mediated anaerobic chemolithoautotrophic growth has been reported for a strain of Sulfolobus ambivalens containing the plasmid pSL10. Aerobically, this organism derives energy from the oxidation of elemental sulphur (with oxygen as the terminal electron acceptor), yielding sulphuric acid; anaerobically it obtains energy from the oxidation of hydrogen (with elemental sulphur as terminal electron acceptor), yielding hydrogen sulphide [Nature (1985) 313 789–791]. chemolithoheterotroph A CHEMOTROPH with the properties of a LITHOTROPH and a HETEROTROPH. (See also MIXOTROPHY.) chemolithotroph See CHEMOTROPH. chemoorganoheterotroph A CHEMOTROPH with the properties of an ORGANOTROPH and a HETEROTROPH. The majority of bacteria and fungi are chemoorganoheterotrophs (cf. CHEMOLITHOAUTOTROPH); some such bacteria are facultative chemolithoautotrophs. Taken together, chemoorganoheterotrophs can use a very wide range of organic substrates, though particular species may be able to use only a very limited number of compounds. Because most – if not all – chemoorganotrophs are heterotrophic, the term ‘heterotroph’ is sometimes used as a synonym of ‘chemoorganoheterotroph’. chemoorganotroph See CHEMOTROPH. chemoprophylaxis The use of drug(s) for PROPHYLAXIS. chemoreceptor See CHEMOTAXIS. chemorepellent See CHEMOTAXIS. chemorespiration See PHOTORESPIRATION. chemostat See CONTINUOUS CULTURE. chemosynthetic primary production See PRIMARY PRODUCTION. chemotaxigen Any agent which promotes the formation of a CYTOTAXIN. chemotaxis A TAXIS in which the stimulus is a concentration gradient of a given chemical (a chemoeffector). A cell may make a net movement towards higher concentrations of some chemoeffectors (chemoattractants) but away from others (chemorepellents); a chemoattractant may or may not be a nutrient. Cells may exhibit chemotaxis when they occupy a physiologically suboptimal position within the concentration gradient. A cell capable of chemotaxis must be able to (i) detect (‘sense’) changes in the concentration of a chemoeffector, (ii) transmit this information to locomotory organelle(s), and (iii) make the appropriate response; it must also be able to adapt by resuming the unbiased mode of motility when in a uniform concentration of chemoeffector, regardless of the actual concentration. potential only minimally; and it should be measurable with ease. Ions used include the synthetic, lipophilic ‘Skulachev’ cations (e.g. triphenylmethylphosphonium, TPMP+ ) and anions (e.g. tetraphenylborate, TPB− ) which disturb the gradient minimally since they can be used in very low concentrations. Methods used for measuring equilibrium concentrations of indicator ions include e.g. continual monitoring of the given ion in the incubation medium. 1pH is usually estimated from the equilibrium distribution of a weak acid (e.g. 5,5-dimethyl-4-oxazolidinedione, DMO) or a weak base which permeates the membrane electroneutrally. One of the unresolved problems in chemiosmosis concerns the precise mode of transmembrane proton translocation; some workers favour the concept of VECTORIAL GROUP TRANSLOCATION, while others favour a model in which proton translocation is associated with conformational changes in the membrane proteins (see ELECTRON TRANSPORT CHAIN). Another problem concerns the extent to which the proton circuit is delocalized into the bulk phases on either side of the membrane; some evidence suggests that the proton circuit is more or less localized at the membrane surface (in accordance with the ‘semi-localized’ hypothesis) [Nature (1986) 322 756–758]. (See also LUNDEGARDH-TYPE RESPIRATION.) chemoattractant See CHEMOTAXIS. chemoeffector See CHEMOTAXIS. chemokines A category of CYTOKINES which function primarily as chemoattractants for particular type(s) of leukocyte – and which are important in the recruitment of leukocytes to regions of infection and INFLAMMATION. Chemokines are typically small (8–10 kDa) secreted proteins which are produced by a range of cells that include monocytes, macrophages, fibroblasts, T cells, epithelial cells and endothelial cells. Chemokines are divided into several families on the basis of e.g. composition and function. Every cytokine molecule contains several conserved cysteine (C) residues, and the sequence of amino acids containing these residues differs in different families. Thus, in the CXC family (a-chemokines) the two initial cysteine residues are separated by one amino acid residue (X). In the CC family (b-chemokines) the two initial cysteine residues are adjacent. Members of the C family (g-chemokines) lack some of the cysteine residues present in the a- and b-chemokines. Yet another family, CX3C, includes the membrane-bound chemokine fractalkine. The two main families (CXC and CC) differ in the principal types of cell which they attract; thus, typically (though not exclusively), the CXC chemokines attract neutrophils while the CC chemokines attract monocytes. The cell-surface receptors for chemokines (in most or all cases) are transmembrane molecules of the rhodopsin superfamily. The intracellular domain of such receptors is coupled to G PROTEINS which initiate intracellular signalling when chemokines bind to their cognate receptors; in at least some cases, the intracellular signalling pathway downstream of the G proteins includes protein kinase C (PKC). Some receptors can bind only one type of chemokine (e.g. the receptor CXC-CKR2 is specific for the CXC-type chemokine IL8) but other receptors can bind more than one type of chemokine. A receptor on erythrocytes (red blood cells) can bind both CXC and CC chemokines; such binding is non-functional, but it may serve as a ‘sink’ to prevent inappropriate stimulation of leukocytes. Chemokines of the CXC family include INTERLEUKIN-8, PF4 (platelet factor 4: produced by the a granules of platelets, 149 chemotaxis of chemoattractant it will exhibit a greater tendency to tumble (clockwise flagellar rotation) – thus increasing the probability of changing to a more favourable direction. ‘Sensing’ the concentration gradient is achieved by means of specific receptors (in the cytoplasmic membrane) to which the chemoeffector binds; the binding of chemoeffector to a receptor results in the transmission of a signal that promotes either counterclockwise or clockwise flagellar rotation – that is, either smooth swimming or tumbling, respectively. (a) In Escherichia coli (and other enterobacteria) FLAGELLAR within a uniform concentration of chemoeffector(s) results in a pattern of movements referred to as a RANDOM WALK. If, however, a cell be placed in a concentration gradient of e.g. chemoattractant, the cell will subsequently exhibit a modified pattern of movements; thus, if the cell were initially swimming towards the higher concentration of chemoattractant it will maintain a smooth mode of swimming (counterclockwise flagellar rotation), but if swimming away from the higher concentration MOTILITY chemoeffector periplasm (a) MCP CM cytoplasm signal chemoeffector (b) MCP ATP +CH3 −CH3 CheW CheA ADP P CheR CheB P CheY- P CW P CheZ low (c) high high Cattr Cattr low low MCP M M M CCW (smooth) M M M CW (tumble) 150 M chemotaxis maltose complex binds to the Tar protein, and the galactose and ribose complexes bind to the Trg protein. (Uptake and metabolism of these sugars are not necessary for a chemotactic response.) Cells can also respond chemotactically to substrates taken up by PTS (q.v.). The mechanism is not understood, but one suggestion is that particular PTS component(s) interact – via unknown intermediate(s) – with the CheY protein (see figure), thus influencing flagellar rotation. PTS-mediated chemotaxis differs from the MCP system in several ways – e.g. (i) substrates must be transported, not simply bound; (ii) adaptation does not involve methylation; and (iii) the system does not respond to repellents [e.g. JCB (1993) 51 1–6]. The mechanism by which flagellar rotation is switched between counterclockwise and clockwise modes appears to be independent of pmf, although pmf is necessary for driving the flagellar motors (cf. AEROTAXIS and PHOTOTAXIS). Interestingly, although SWARMING in E. coli involves components of the chemotaxis system, the substances that promote swarming appear to differ from the known chemoeffectors in chemotaxis [PNAS (1998) 95 2568–2573]. When a cell is swimming towards a higher concentration of chemoattractant, the receptors will be more likely, within a given period of time, to bind molecules of the chemoattractant than if the cell were moving in another direction; when a molecule of chemoattractant binds to a receptor it induces a signal that promotes smooth swimming. Conversely, if the cell is swimming down the concentration gradient, there is likely to be (over a period of time) a net loss of chemoattractant molecules bound to receptors; when a molecule of chemoattractant leaves a receptor it stimulates a signal that promotes tumbling. The diagram (see accompanying figure) gives an outline of the mechanism of receptor-mediated chemotaxis in Escherichia coli. The receptors are designated methyl-accepting chemotaxis proteins (MCPs). In E. coli, MCPs include the products of genes tap, tar, trg and tsr. Some chemoeffectors bind directly to their MCPs. Thus, e.g. serine and aspartate bind to the Tsr and Tar proteins, respectively. Other chemoeffectors, such as galactose, maltose and ribose, initially form a complex with certain periplasmic binding proteins before binding to their respective MCPs (see BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM); thus, e.g. the CHEMOTAXIS in Escherichia coli: a model of receptor-mediated chemotaxis involving signal transduction. (a) The receptors are called methyl-accepting chemotaxis proteins (MCPs), and they occur in the cytoplasmic membrane (CM). MCPs have functional regions in both the periplasm and cytoplasm. There are different types of MCP, each type recognizing its own range of chemoeffectors, and there are several hundred molecules of each type of MCP in a given cell. (In E. coli the MCPs are encoded by genes tap, tar, trg and tsr.) Some chemoeffectors (e.g. aspartate, serine) bind directly to their MCPs, but others (e.g. some sugars, including ribose) first form a complex with certain periplasmic proteins before binding. The binding/release of chemoeffectors by an MCP regulates an intracellular signal that influences the frequency of tumbling. (b) The intracellular signal originates at the protein CheA (encoded by gene cheA). CheA is a HISTIDINE KINASE. In the diagram, CheA is bound to the MCP; CheW is a coupling protein. CheA can transfer phosphate from ATP to a regulator protein, CheY; the phosphorylated (activated) form of CheY (CheY∼P) enhances clockwise (CW) flagellar rotation (and, hence, increases the frequency of tumbling) by interacting with the C ring of the flagellar motor (apparently with the FliM component). [Role of CheY: TIM (1999) 7 16–22.] The basic signal thus consists of the transfer of phosphate from CheA to CheY. This signal is modulated (i.e. regulated) by (i) the binding/release of chemoeffector at the MCP, and (ii) the degree of methylation of the MCP; these two factors are considered below. (Although phosphorylation of CheY has been regarded as the essential mode of signalling in chemotaxis, there is some evidence that, in at least some cases, acetylation of CheY may be involved in the response to repellents [reported in JB (2000) 182 1459–1471 (1466)].) The binding of a chemoattractant inhibits the kinase activity of CheA, thus inhibiting the transfer of phosphate to CheY. (On binding of chemoattractant, it appears that subunits of the MCP move relative to one another, and that such movement is involved in the inhibition of CheA kinase activity [see e.g. JB (2000) 182 1459–1471 (1465–1466)].) Conversely, the release of chemoattractant from an MCP stimulates CheA and promotes the transfer of phosphate to CheY, thus encouraging tumbling. The cytoplasmic side of an MCP is subject to ongoing methylation by a methyltransferase, CheR, which uses S-adenosylmethionine as methyl donor. Opposing this, a methylesterase, CheB, removes the methyl groups; the activity of CheB is enhanced when it is phosphorylated by CheA. The degree of methylation of an MCP thus depends on the activities of both CheR and CheB; increased methylation tends to enhance the activity of CheA. (c) In a low, uniform concentration of chemoattractant (Cattr ), tumbling occurs at a given rate. When the cell swims up a concentration gradient (i.e. towards higher concentrations), there is an increase in the number of molecules of Cattr bound to MCPs; this results in the inhibition of CheA which, in turn, inhibits phosphorylation of CheY – and thus promotes counterclockwise (CCW) flagellar rotation, i.e. smooth swimming in the same direction. Inhibition of CheA also inhibits CheB, allowing increased methylation of the MCP (as CheR methylates continually); hence, inhibition of CheA, due to binding of Cattr , is subsequently offset by the stimulatory effect of increased methylation of the MCP – so that the frequency of tumbling returns to its original value. The cell has thus adapted to the new, higher, concentration of Cattr (MCP in the centre). On swimming down the concentration gradient (i.e. away from higher concentrations), there is a decrease in the number of molecules of Cattr bound to the MCPs. Release of Cattr stimulates CheA and promotes clockwise (CW) flagellar rotation (more frequent tumbling); at the same time, CheB is stimulated, leading to de-methylation of the MCP until CheA activity has been reduced to its original level – i.e. adaptation to the new, low, uniform concentration of Cattr (MCP at right-hand side). Such adaptation depends on an appropriate rate of dephosphorylation of CheY∼P and CheB∼P; CheB∼P undergoes autodephosphorylation, while CheY∼P levels are regulated by the CheZ protein. (It appears that about 30% of the intracellular pool of CheY molecules are in the phosphorylated state in fully adapted cells [EMBO (1998) 17 4238–4248].) Note that the degree of methylation of an MCP in an adapted cell reflects the extracellular concentration of the chemoattractant (compare first, third and fifth MCPs in the diagram). Reproduced from Bacteria, 5th edition, Figure 7.13, pages 152–153, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 151 chemotaxonomy can be transmitted e.g. by hyphal anastomosis; no capsid has been detected (cf. MYCOVIRUS), but the dsRNA appears to be closely associated with an RNA polymerase and may be packaged within vesicles formed by the host fungus [JGV (1985) 66 2605–2614]. [Potential of the hypovirulence factor for biological control of E. parasitica: ARPpath. (1982) 20 349–362; Science (1982) 215 466–471.] (See also DUTCH ELM DISEASE.) chi site (chi sequence) See RECBC PATHWAY. chi structure See RECOMBINATION. c1776 A ‘disabled’ strain of Escherichia coli K12 which e.g. requires various amino acids (including diaminopimelic acid) and is highly sensitive to bile salts and other detergents. It was constructed for use in CLONING experiments to reduce the risk of propagation of potentially dangerous clones should they escape from the laboratory. However, c1776 is difficult to grow and to transform, and is currently rarely used. chiasma The visible manifestation of CROSSING OVER – e.g. between non-sister chromatids of homologous chromosomes during MEIOSIS. chiasma interference See INTERFERENCE (sense 2). chiastobasidial Refers to a transverse arrangement of nuclear spindles in a developing BASIDIUM. (cf. STICHOBASIDIAL.) chick embryo lethal orphan virus See AVIADENOVIRUS. Chick–Martin test A SUSPENSION TEST used to determine the PHENOL COEFFICIENT of a (phenolic) disinfectant; it assesses the efficacy of a disinfectant in the presence of organic matter, i.e., under conditions more closely approximating those in practical use (cf. RIDEAL–WALKER TEST). (Many disinfectants are inactivated by organic matter.) In the original test the organic matter used was 3% sterile dry faeces; a modified form of the test uses a sterilized suspension of yeast (5% dry wt/vol.). To each of several serial dilutions of the disinfectant under test are added the yeast and an inoculum of Salmonella typhi. After incubation for 30 min, an inoculum from each dilution is added to each of two tubes of broth which, after incubation at 37° C for 2 days, are examined for growth. The same procedure is carried out with phenol. For the phenol test two dilutions are identified: (i) that containing the highest concentration of phenol which permitted growth in both tubes inoculated from it, and (ii) that containing the lowest concentration of phenol which did not permit growth in either tube inoculated from it; the mean of these two concentrations is then calculated (Pmean ). For the disinfectant under test Dmean is calculated in the same way. The phenol coefficient of the disinfectant is given by Pmean /Dmean . chicken diseases See POULTRY DISEASES. chicken syncytial virus See AVIAN RETICULOENDOTHELOSIS VIRUSES. chickenpox (varicella) An acute, highly infectious disease which usually occurs in epidemics affecting mainly young children. The causal agent is human (alpha) herpesvirus 3 (varicellazoster virus) – see ALPHAHERPESVIRINAE. Infection occurs by direct contact or by droplet inhalation. The incubation period is ca. 2–3 weeks. Symptoms: fever and a rash of erythematous macules which rapidly develop into thin-walled vesicles, finally becoming dry and crusty; successive crops of lesions occur. The disease is usually mild and self-limiting; complications include e.g. pneumonia (usually in adults), encephalitis, and secondary infection of skin lesions with staphylococci and streptococci. In immunosuppressed or CMI-defective patients the disease is much more severe, with extensive rash and severe systemic symptoms (progressive varicella) which may be fatal; vidarabine and acyclovir have been used in treatment. A live vaccine has been developed [MS (1985) 2 249–254]. Lab. diagnosis: viruses Experimental modification of the concentration of various components of the chemotaxis system (e.g. CheR) indicates that, whereas variation can occur in factors such as the time taken to adapt (e.g. a >20-fold variation with variation in CheR concentration), the precision of adaptation (i.e. the capacity to return to the exact pre-stimulus level of tumbling) is preserved [Nature (1999) 397 168–171]. [Control of chemotaxis: Mol. Microbiol. (1996) 20 903–910. Aspects of chemosensing (including non-enteric bacteria): JB (1998) 180 1009–1022. Signalling components in bacterial locomotion and sensory reception: JB (2000) 182 1459–1471. Signalling in bacterial chemotaxis (review): JB (2000) 182 6865–6873.] (b) In the immune system (see e.g. INFLAMMATION), chemotaxis refers to the migration of certain types of leukocyte (e.g. MACROPHAGE, NEUTROPHIL) in the direction of higher concentrations of chemoeffectors (= cytotaxins); the cytotaxins include certain CYTOKINES and the ANAPHYLATOXIN C5a. (c) In DICTYOSTELIUM (q.v.) and other cellular slime moulds, chemotaxis is involved e.g. in cell aggregation. chemotaxonomy TAXONOMY based on chemical criteria such as DNA base composition, cell wall lipids, 16S rRNA structure etc. chemotherapeutic agent Any drug used for chemotherapy – e.g. an ANTIBIOTIC. chemotherapeutic index Syn. THERAPEUTIC INDEX. chemotherapy The use of drug(s) (e.g. ANTIBIOTICS) for the treatment of disease. chemotroph An organism whose energy is derived from endogenous, light-independent chemical reactions (a mode of metabolism termed chemotrophy). (cf. PHOTOTROPH.) A chemotroph which obtains energy by the metabolism of inorganic substrate(s) – i.e. a LITHOTROPH – is called a chemolithotroph; one which obtains energy by metabolizing organic substrate(s) – i.e. an ORGANOTROPH – is called a chemoorganotroph. According to the type of compound from which carbon is assimilated, a chemolithotroph may be an AUTOTROPH (i.e. a CHEMOLITHOAUTOTROPH) or a HETEROTROPH (i.e. a chemolithoheterotroph). (See also MIXOTROPHY.). A heterotrophic chemoorganotroph is called a CHEMOORGANOHETEROTROPH. The mode of metabolism may vary according to growth conditions: e.g., Thiobacillus strain A2 can exhibit chemolithoautotrophic, mixotrophic or chemoorganoheterotrophic growth under appropriate conditions [JGM (1980) 121 127–138]. chemotrophy A mode of metabolism in which energy is derived from endogenous, light-independent chemical reactions (cf. PHOTOTROPHY). (See also CHEMOTROPH.) chemotropism See TROPISM (sense 1). chemotype Any VARIETY distinguished by structural properties – e.g. a rough mutant. chenodeoxycholic acid See BILE ACIDS. cherry leaf roll virus See NEPOVIRUSES. cherry rasp leaf virus See NEPOVIRUSES. Cheshunt compound An agricultural antifungal preparation which consists of a mixture of copper sulphate (see COPPER) and ammonium carbonate; it is used, in aqueous solution, as a protectant against DAMPING OFF. chestnut blight (chestnut canker) A lethal disease of the American chestnut (Castanea dentata) caused by invasion of the vascular cambium by Endothia parasitica. The disease has virtually eliminated the American chestnut from parts of America; however, in Italy the disease has been ameliorated by the appearance of hypovirulent strains of the pathogen capable of transferring the HYPOVIRULENCE phenotype to virulent strains (‘transmissible hypovirulence’). Hypovirulent strains contain dsRNA which 152 Chl in arthropod exoskeletons, in the CELL WALLS of most fungi, in certain algae (e.g. in the spines of many planktonic centric diatoms and in the lorica of ‘Poterioochromonas stipitata’ – see OCHROMONAS), and in certain protozoa (e.g. in the cyst walls of ciliates and amoebae); it is apparently never formed by bacteria. Native chitin occurs in the form of microfibrils in which extended glucan chains are hydrogen-bonded together (antiparallel in a-chitin, parallel in b-chitin) to form sheets which, in turn, associate (by hydrogen-bonding in a-chitin but not in b-chitin) to form microfibrils. Biosynthesis of chitin involves the enzyme chitin synthase (EC 2.4.1.16) which catalyses the polymerization of Nacetylglucosamine from UDP-N-acetylglucosamine precursors. The enzyme requires Mg2+ or Mn2+ and phospholipids for activity, and is inhibited e.g. by POLYOXINS and NIKKOMYCINS. The location of chitin synthase in fungal cells is controversial; most reports suggest that it occurs in the cytoplasmic membrane [Book ref. 38, pp. 403–405], while evidence has been presented that it is mainly cytoplasmic in Mucor rouxii [JGM (1984) 130 1193–1199]. (See also CHITOSOME and GAMMA PARTICLE (sense 2).) Chitin synthase occurs in zymogen and active forms; its activity in the cell may be controlled by regulation of the proteolytic activation of the zymogen at sites of chitin synthesis. [Chitin biosynthesis in microorganisms: Book ref. 62, pp. 85–127.] (See also GROWTH (fungal).) (See also CHITINASE.) chitinase (poly-[1,4-b-(2-acetamido-2-deoxy-D-glucoside)] glycanohydrolase; EC 3.2.1.14) An enzyme which degrades CHITIN to chitobiose [JGM (1984) 130 1857–1861]; chitobiose can be hydrolysed to N-acetylglucosamine by chitobiase (N-acetylb-D-glucosaminidase, EC 3.2.1.30). Chitinases are produced by a wide range of organisms: e.g., chitin-containing fungi; microorganisms parasitic on chitin-containing organisms (e.g. entomopathogens – see also BURNED SPOT DISEASE); insectivorous animals (e.g. spiders); and plant tissues (where they may play a role in resistance to fungal diseases [Nature (1986) 324 365–367]). chitinolytic (chitinoclastic) CHITIN-degrading: see CHITINASE. chitobiase See CHITINASE. chitobiose A disaccharide consisting of two (1 → 4)-b-linked Nacetyl-D-glucosamine residues. chitosan (mycosin) A polymer of non-acetylated (1 → 4)-blinked D-glucosamine residues (cf. CHITIN). It occurs in nature as a CELL WALL constituent in the hyphae of certain zygomycetes (e.g. Mucor spp) and as a minor component of the spores of Fusarium solani (in which it is a strong elicitor of PHYTOALEXIN production by pea plants [Book ref. 58, pp. 270–271]). [Coordination of chitosan and chitin synthesis: JGM (1984) 130 2095–2102.] chitosome A type of vesicular particle (40–70 nm diam.) comprising a membrane-like shell containing chitin synthase; chitosomes can be isolated from a wide range of fungi and can synthesize CHITIN microfibrils from UDP-N-acetylglucosamine in vitro. Initially, coiled microfibrils develop within the chitosome – which then apparently breaks open to release extended microfibrils. It is uncertain whether chitosomes are organelles (serving e.g. in the transport of chitin synthase from its site of synthesis to its site of action) or small vesiculated fragments of cytoplasmic membrane formed during the isolation procedure. Chitin synthase can be released from chitosomes by treatment with digitonin. Chl Chlorophyll: see CHLOROPHYLLS. can be isolated from vesicular fluid during the first 3 days of the rash; scrapings from the base of a vesicle may be stained and examined for typical varicella GIANT CELLS and eosinophilic intranuclear inclusion bodies. (See also HERPES ZOSTER.) chiclero’s ear A chronic, ulcerating lesion on the ear-lobe, caused by Leishmania mexicana. It occurs in Panama, Honduras, and the Amazon region. chicory blotch virus See CARLAVIRUSES. chicory yellow mottle virus See NEPOVIRUSES. chikungunya fever An acute infectious human disease caused by an ALPHAVIRUS and transmitted by mosquitoes (Aedes spp); epidemics occur in Africa, India and S. E. Asia. The disease is characterized by severe joint and back pains with fever; rarely, a severe (often fatal) haemorrhagic form occurs. Wild primates may act as a reservoir of infection. childbed fever Syn. PUERPERAL FEVER. Chilo iridescent virus See IRIDOVIRUS. Chilodochona A genus of marine protozoa (subclass HYPOSTOMATIA). Cells: vase-shaped, i.e., the body is narrow at the point of attachment, broadening out before constricting and finally becoming flared and convoluted at the distal end. Chilodonella A genus of ciliates (subclass HYPOSTOMATIA). The cells are flattened both ventrally and in the dorsal anterior region, and are rounded in the dorsal posterior region; there is an anteriolateral point (‘break’), and the cytostome is located subterminally in the anterioventral surface. Species occur e.g. as gill parasites of freshwater fish. and are common in stagnant ponds etc and in some SEWAGE TREATMENT plants. Chilomastix A genus of flagellate protozoa of the order RETORTAMONADIDA. C. mesnili is a non-pathogen which occurs e.g. in the caecum and colon in man and other animals (e.g. pigs); it is a pear-shaped organism, ca. 10–15 µm in length, with three anteriorly directed flagella arising from the blunt anterior end, and a fourth flagellum within the conspicuous cytostome. The single nucleus is located at the anterior end of the cell. The cysts are lemon-shaped, ca. 6–10 µm; the nucleus and cytostome are usually visible in stained cysts. Chilomonas See CRYPTOPHYTES. chimeric plasmid (1) Any recombinant PLASMID formed from nucleic acids derived from organisms which do not normally exchange genetic information (e.g. Escherichia coli and Saccharomyces cerevisiae). (See e.g. SHUTTLE VECTOR; cf. HYBRID PLASMID sense 2.) (2) Syn. HYBRID PLASMID sense 1. Chinese letter arrangement An arrangement of the cells of certain bacteria (e.g. Corynebacterium spp) as seen in microscopic preparations: the cells are arranged in groups which, to the occidental eye at least, resemble Chinese characters or ‘letters’. chiniofon See 8-HYDROXYQUINOLINE. Chionosphaera A genus of fungi (see FILOBASIDIACEAE). Basidia occur in a hymenium covering the enlarged terminal portion of a small (<1 mm), stalked fruiting body. One species: C. apobasidiales, isolated from dead tree-limbs. [Book ref. 100, pp. 470–471.] chip (DNA) See DNA CHIP. Chironomus luridus EPV See ENTOMOPOXVIRINAE. Chironomus plumosus iridescent virus See IRIDOVIRUS. chitan CHITIN in which all of the glucosamine residues are Nacetylated. In nature, chitan may occur only in diatom spines. (cf. CHITOSAN.) chitin A linear polysaccharide consisting of (1 → 4)-b-linked D-glucosamine residues, most of which are N-acetylated (cf. CHITAN; CHITOSAN). Chitin is abundant in nature, occurring e.g. 153 Chlamydia infection the cycle is complete – each infected host cell containing ca. 10–1000 EBs in an inclusion which may occupy a major part of the cell. On release, EBs infect fresh host cells. Antibiotic resistance. Chlamydiae are typically sensitive to e.g. tetracyclines. Penicillins may inhibit the development of EBs from RBs, although the mechanism of such inhibition is unknown. Most strains of C. trachomatis are sensitive to sulphonamides. C. pecorum. A species from ruminants (including sheep and cattle) which can cause e.g. respiratory, intestinal and neurological disease. [Description of C. pecorum: IJSB (1992) 42 306–308.] C. pneumoniae. A species first isolated from a suspected case of trachoma in Taiwan (strain TW-183) and subsequently from a case of acute respiratory infection (strain AR-39). These organisms, initially thought to be strains of C. psittaci, were referred to as TWAR strains [see Book ref. 193, pp 321–340] but later classified in a new species, C. pneumoniae [description of C. pneumoniae: IJSB (1989) 39 88–90]. [Phylogenetic relationship of C. pneumoniae to C. psittaci and C. trachomatis: IJSB (1993) 43 610–612.] C. pneumoniae is associated with e.g. epidemics of subacute respiratory disease; re-infection, or chronic infection, can lead to severe disease, apparently involving immunologically-based pathogenesis. C. psittaci. Hosts: commonly birds and mammals other than man (but see PSITTACOSIS); C. psittaci appears to have played a significant role in an epizootic of urogenital and respiratorytract disease and keratoconjunctivitis in the koala (Phascolarctos cinereus) [AJEBMS (1985) 63 283–286]. Folic acid and glycogen are not formed. C. trachomatis. Natural hosts: man and mouse; no strains have been isolated from birds. Folic acid and glycogen are formed. Three biovars: trachoma (typically non-invasive, confined almost exclusively to squamocolumnar epithelial cells), lymphogranuloma venereum (LGV) (typically invasive, infects e.g. lymph nodes), and mouse (isolated only from mice; has an affinity for the lungs). Biovars trachoma and LGV can be distinguished (by means of microimmunofluorescence tests) by virtue of their type-specific antigens: A–K and L1 –L3 , respectively. [Complete genome sequence: Science (1998) 282 754–759.] Chlamydiaceae See CHLAMYDIALES. Chlamydiales An order of Gram-negative bacteria which are obligate intracellular parasites in eukaryotic cells; the organisms multiply within cytoplasmic vacuoles and exhibit a developmental cycle which includes small, rigid-walled infectious forms (elementary bodies), and larger, non-infectious forms (reticulate bodies, sometimes called initial bodies) which have flexible walls and which divide by fission. One family, Chlamydiaceae, which comprises a single genus, CHLAMYDIA. Chlamydodon See HYPOSTOMATIA. Chlamydomonas A large genus (>500 species) of unicellular, usually flagellated green algae (division CHLOROPHYTA – cf. PHYTOMASTIGOPHOREA) which are widespread and common in a range of freshwater habitats and e.g. on damp soils. (See also RED SNOW.) The vegetative cells are usually pyriform or ovoid, ca. 5–20 µm in length, uninucleate, haploid, and usually motile by means of two similar flagella which project from the anterior end (cf. PALMELLOID PHASE). (See also FLAGELLUM (b) and FLAGELLAR MOTILITY (b).) The cell wall is thin and contains no cellulose, being composed of a complex of hydroxyproline-rich glycoproteins [wall structure: JCB (1985) 101 1550–1568, 1599–1607]. The cell contains a single large CHLOROPLAST which may be cup-shaped, stellate, H-shaped, lobed, etc, according to species. Chlamydia (formerly e.g. Bedsonia, Miyagawanella; cf. TRIC agents) A genus of Gram-negative bacteria (order CHLAMYDIALES) which occur as obligate intracellular parasites in man and other animals. Strains of Chlamydia can cause various types of disease – e.g. CURTIS–FITZ-HUGH SYNDROME, INCLUSION CONJUCTVITIS, KERATOCONJUNCTIVITIS, LYMPHOGRANULOMA VENEREUM and TRACHOMA in man, PSITTACOSIS in man and animals, diarrhoea in new-born calves, lambs and piglets, and abortion (see e.g. ENZOOTIC ABORTION) and eye infections in various animals. [Human chlamydial infections: Book ref. 193. Clinical conditions associated with positive COMPLEMENT-FIXATION TESTS for Chlamydia: Epidem. Inf. (1987) 98 101–108.] The organisms undergo a development cycle, in only part of which are they infectious for fresh host cells. The cells are non-motile, coccoid and pleomorphic, 0.2–1.5 µm diam., size varying with the phase of development. The chlamydial cell envelope contains little or no muramic acid, but includes a Gram-negative-type OUTER MEMBRANE containing LPS; a heatstable, genus-specific, KDO-containing antigen occurs in all strains throughout the development cycle and is responsible for positive FREI TEST reactions. The outer membrane contains a major protein species (MWt ca. 40000) with which is associated species-specific and type-specific antigens. The inner surface of the outer membrane consists of a monolayer of hexagonally arranged protein units [EM studies of the cell wall: JMB (1982) 161 579–590]; in the elementary body (see below) the outer surface has a number of hemispherical projections [JB (1985) 164 344–349]. The organisms have been cultured in laboratory animals, in the yolk sacs of chick embryos, and in cell cultures. GC%: ca. 41–44 (Tm ). Type species: C. trachomatis. Development cycle. The form infectious for host cells is the elementary body (EB): a non-dividing, osmotically-stable disseminative cell, ca. 0.2–0.4 µm diam., which has few ribosomes, and in which the condensed nuclear material occupies a major part of the cell; the cell wall proteins are extensively crosslinked by disulphide bonds [JB (1984) 157 13–20]. EBs are metabolically limited: glucose is not metabolized, although glucose 6-phosphate and certain other substrates may yield CO2 if ATP, NADP and other cofactors are also supplied. The development cycle begins when EBs adhere to unidentified receptor sites on a host cell and undergo cytochalasin D-sensitive endocytosis. [Mechanisms of endocytosis of C. trachomatis: JGM (1984) 130 1765–1780.] Phagosomes within which EBs are internalized do not fuse with lysosomes (cf. PHAGOCYTOSIS). The EBs subsequently differentiate, becoming reticulate bodies (RBs): osmotically-labile, trypsin-sensitive cells, ca. 0.6–1.5 µm diam., which contain increased numbers of ribosomes and in which the nuclear material is more diffusely distributed; the disulphide inter-protein cross links in the cell wall are reduced. RBs are not infectious for fresh host cells; those of C. trachomatis may be capable of glycolysis and of ATP synthesis [Mol. Microbiol. (1999) 33 177–187]. Some chlamydiae synthesize e.g. folic acid and glycogen, and all strains must produce the prokaryotic-type fatty acids and lipids which cannot be supplied by the host cell. [Protein synthesis in host-free RBs: JB (1985) 162 938–942.] The RBs undergo binary fission ca. 8–10 hours after the initial infection of the host cell, and continue to divide for up to ca. 20 hours after infection. By about 12–15 hours after infection, the membrane-bounded population of progeny RBs (called an inclusion or microcolony) is large enough to be visible by light microscopy; the inclusions of glycogen-forming strains can be detected by staining with iodine. From ca. 20 hours after infection RBs begin to give rise to EBs, and by ca. 48–72 hours after 154 chlorguanide and also made synthetically. Chloramphenicol is a broadspectrum bacteriostatic agent active against a wide range of bacteria; it also inhibits vegetative growth and sporulation in certain fungi [JGM (1983) 129 3401–3410]. Chloramphenicol binds to the 50S subunit of prokaryotic and mitochondrial RIBOSOMES (but apparently not to the ribosomes in at least some archaeans [SAAM (1985) 6 125–131]); it inhibits peptidyltransferase and, hence, PROTEIN SYNTHESIS. Resistance to chloramphenicol in bacteria is commonly due to a plasmid-specified enzyme – chloramphenicol acetyltransferase (CAT; also called chloramphenicol transacetylase) – which catalyses an acetyl-CoA-dependent acetylation of chloramphenicol at the C-3 hydroxy group to form a compound with little or no antibiotic activity. Plasmid-specified CAT is usually or always synthesized constitutively in Gram-negative bacteria but inducibly (see also TRANSLATIONAL ATTENUATION) in e.g. staphylococci and streptococci. (See also Tn9.) CAT-producing bacteria can be detected by rosanilin dyes [JB (1982) 150 1375–1382]. Non-enzymic resistance to chloramphenicol may be due e.g. to (mutant) ribosomes which fail to bind the antibiotic. Pseudomonas aeruginosa is innately resistant, being able normally to tolerate high levels of the drug. Chloramphenicol antagonizes those antibiotics (e.g. penicillins) which inhibit only actively growing and dividing bacteria. Its clinical usage is limited by its toxicity. chloranil (tetrachloro-p-benzoquinone) A QUINONE ANTIFUNGAL AGENT used e.g. as a seed dressing for the prevention of damping off, seed rots etc; it decomposes in the presence of light. chlorazol black E A black amphoteric triazo DYE used e.g. for the VITAL STAINING of Mycoplasma spp and L-forms [JB (1969) 99 1–7]. chlorbutanol (chlorbutol) Trichloro-t-butanol; it is used e.g. as a PRESERVATIVE (at ca. 0.5% w/v) in ophthalmic solutions. At room temperatures chlorbutanol is unstable at alkaline pH, and it can be inactivated by colloids such as bentonite and magnesium trisilicate. chlorbutol Syn. CHLORBUTANOL. Chlorella A genus of non-motile, unicellular green algae (division CHLOROPHYTA) which occur in fresh water, on soils, bark etc, and in association with various other organisms (see ZOOCHLORELLAE). (cf. VAMPIROVIBRIO.) The cells are spherical (ca. 2–15 µm diam.); each contains a single haploid nucleus and a large, usually cup-shaped chloroplast with or without a pyrenoid. The CELL.WALL contains cellulose and e.g. SPOROPOLLENIN. Asexual reproduction occurs by AUTOSPORE formation: the protoplast divides to form 2, 4, 8 or 16 daughter protoplasts which develop cell walls before being released by rupture of the parent wall. Motile cells are never formed. (cf. CHLOROCOCCUM.) The paucity of morphological features in Chlorella spp has led to an increased use of chemotaxonomy in this genus [Book ref. 123, pp. 391–407]. Although primarily photosynthetic, some Chlorella spp can ferment glucose to lactate under anaerobic conditions. Most species can reduce NO3 − , and a few can produce extracellular gelatin- and/or starch-hydrolysing enzymes. (cf. PROTOTHECA; see also SINGLE-CELL PROTEIN and YAKULT.) chlorguanide (Paludrine; proguanil) N 1 -p-chlorophenyl-N 5 -isopropyldiguanide: an antimalarial drug (see MALARIA) which, itself, has little activity but which is converted in vivo to a highly active cyclic form (‘cycloguanil’ – a substituted dihydrotriazine). Cycloguanil binds competitively to the enzyme dihydrofolate reductase (DHFR), inhibiting the synthesis of tetrahydrofolate. The chloroplast contains an EYESPOT and generally one or more PYRENOIDS. Two (or more) CONTRACTILE VACUOLES are typically present at the anterior end of the cell, and a number of elongated, branched mitochondria occur near the flagellar bases. Asexual reproduction occurs by endodyogeny or endopolygeny: typically, a cell comes to rest, losing or withdrawing its flagella, and the protoplast undergoes one or more longitudinal cleavages within the parental wall. The 2, 4, 8 or 16 daughter protoplasts develop cell walls and flagella, and are then released by enzymic dissolution of the parent cell wall; the enzymes responsible for wall dissolution (‘sporangium wall autolysins’) are apparently species-specific [Book ref. 123, pp. 409–418]. [Cell cycle: New Phyt. (1985) 99 1–40, 41–56.] Sexual reproduction is isogamous in most species, anisogamous in some. Some species are heterothallic. Vegetative cells may develop into gametes e.g. in response to nitrogen starvation (in C. reinhardtii ) or to general nutrient stress (in C. eugametos [JGM (1985) 131 1553–1560]); gametes differ from vegetative cells in that, when cells of opposite mating types are mixed, they rapidly clump together by agglutination of their flagella. Agglutination results from the presence on the flagella of a glycoprotein (the ‘agglutination factor’) which is formed by the cell during gametogenesis. Cells within the clumps become sorted into pairs, their flagella separate, and the cells in each pair become joined at their anterior ends (apical papillae) to form a quadriflagellate, motile zygote; this is followed – often after a period of motility – by plasmogamy and karyogamy. The zygote then develops a thick, ornamented wall, and becomes dormant. On germination, meiosis occurs, and four or eight (occasionally 16 or 32) motile cells are released by dissolution of the zygote wall. The mating process may differ in some respects from one species to another. For example, in C. reinhardtii a ‘fertilization tubule’ extends from the apical papilla of one (mt+ ) gamete, and plasmogamy is initiated when the tip of the tubule contacts a dome-shaped region near the flagellar base of the other (mt− ) gamete; by contrast, in C. eugametos there are no morphological differences between mt+ and mt− gametes, or between gametes and vegetative cells. In C. reinhardtii the gametes shed their cell walls following flagellar contact, while in C. eugametos the walls are shed after zygote formation. [Form and location of cell wall lytic enzyme in vegetative cells and gametes of C. reinhardtii : JCB (1987) 104 321–329; review of cell–cell interactions in Chlamydomonas: ARPphys. (1985) 36 287–315.] Chlamydomonas spp are easily maintained in culture and have been used extensively in research e.g. on ultrastructure, physiology, etc. Mutants of C. reinhardtii which are defective in chlorophyll synthesis have been obtained, and these can often grow in the dark on media supplemented with acetate, forming yellow colonies. Such mutants are commonly killed by light, although a mutant has been isolated which, in the presence of acetate, can form yellow colonies both in the light and in the dark [JGM (1983) 129 159–165]. See also e.g. BRACHIOMONAS; CARTERIA; DUNALIELLA; TETRASPORA; VOLVOX. chlamydospore A loosely used term usually applied to a thickwalled, asexually-produced resting spore formed by certain fungi; the term is sometimes used to refer specifically to the teliospores of smut fungi: see USTILAGINALES. (cf. THALLOSPORE.) chloramine T See CHLORINE. Chloramoeba See XANTHOPHYCEAE. chloramphenicol (chloromycetin; NO2 .C6 H4 .CHOH.CH(CH2 OH).NH.CO.CHCl2 ) An ANTIBIOTIC (the D-threo form of four possible stereoisomers) produced by Streptomyces venezuelae 155 chlorhexidine larvae – and purified virus pellets – exhibit a yellow-green iridescence. Virions (ca. 180 nm diam.) are ether-resistant. Type species: mosquito iridescent virus; other members include insect iridescent viruses 3–5, 7, 8 and 11–15 [Book ref. 23, p. 57]. Chloris striate mosaic virus See GEMINIVIRUSES. chlorite dismutase See BIOREMEDIATION. b-chloroalanine A synthetic ANTIBIOTIC which acts e.g. as an inhibitor of alanine racemase. chlorobactene See LIGHT-HARVESTING COMPLEX. Chlorobiaceae A family of non-motile photosynthetic bacteria (suborder CHLOROBIINEAE); species occur e.g. in anaerobic, sulphide-rich freshwater and estuarine muds. The family includes cocci and rods (some species – e.g. Prosthecochloris aestuarii – are prosthecate); GAS VACUOLES occur in Clathrochloris and PELODICTYON. In green-coloured species the predominant pigments are Bchl c or d and the carotenoid chlorobactene; in brown-coloured species Bchl e and isorenieratene predominate. The organisms are obligate anaerobic phototrophs; they are primarily photolithotrophic autotrophs (electron donors: e.g. sulphur, sulphide – thiosulphate in some strains) which fix CO2 by the REDUCTIVE TRICARBOXYLIC ACID CYCLE, but many strains require vitamin B12 , and all can use simple organic molecules (e.g. acetate) given a suitable electron donor; elemental sulphur, when produced, is deposited extracellularly. Genera: Ancalochloris, CHLOROBIUM (type genus), Clathrochloris, PELODICTYON, Prosthecochloris. (cf. CHLOROHERPETON; CHLOROPSEUDOMONAS ETHYLICA; CHLOROCHROMATIUM AGGREGATUM.) [Evidence for bacteria of the Chlorobiaceae in Palaeozoic seas: Nature (1986) 319 763–765.] Chlorobiineae A suborder of photosynthetic bacteria (order RHODOSPIRILLALES); the organisms contain bacterioCHLOROPHYLL c, d or e in CHLOROSOMES, and their REACTION CENTRES contain Bchl a. Families: CHLOROBIACEAE and CHLOROFLEXACEAE. The suborder appears to be taxonomically unsound; thus, on the basis of RRNA OLIGONUCLEOTIDE CATALOGUING, the species Chlorobium limicola, C. vibrioforme, Prosthecochloris aestuarii and Chloroherpeton thalassium constitute a moderately tight phylogenetic group which has no specific relationship with another, somewhat looser group comprising e.g. Chloroflexus aurantiacus, Herpetosiphon aurantiacus and Thermomicrobium roseum [SAAM (1985) 6 152–156]. Chlorobium A genus of photosynthetic bacteria (family CHLOROBIACEAE). The cells are rods or vibrios, according to species, ca. 1–2.5 µm long; division occurs by binary fission. The genus includes both green species (C. limicola, C. vibrioforme) and brown species (C. phaeobacteroides, C. phaeovibrioides). chlorobium chlorophylls See CHLOROPHYLLS. chlorobium vesicles Syn. CHLOROSOMES. Chlorochromatium aggregatum A consortium consisting of an aggregate of cells: a central, colourless, chemoorganotrophic, motile bacterium covered by green bacteria of the family Chlorobiaceae [microanatomy and ecology: JGM (1984) 130 2717–2723]. Similar consortia, which involve species of the Chlorobiaceae, are called Chlorochromatium glebulum, Cylindrogloea bacterifera, and Pelochromatium roseo-viride. Chlorochromatium glebulum See CHLOROCHROMATIUM AGGREGATUM. Chlorociboria See HELOTIALES. Chlorococcum A genus of unicellular green algae (division CHLOROPHYTA) which differ from CHLORELLA spp e.g. in producing biflagellate zoospores as well as autospores. Species occur e.g. on damp soil, brick-work, etc. The drug has been used mainly for prophylaxis. However, it exhibits different levels of efficacy in different human populations, so that its usefulness for some groups has been questioned [BJCP (1994) 37 67–70]. Resistance to cycloguanil develops readily. For example, a mutant form of DHFR (presumably with lower affinity for cycloguanil) arises by a serine-to-threonine change at residue 108 and an alanine-to-valine change at residue 16 of the DHFR molecule [PNAS (1990) 87 3014–3017]. chlorhexidine (1,6-di-(4
-chlorophenyldiguanido)-hexane) An ANTISEPTIC which is bacteriostatic or bactericidal (depending on concentration) for a wide range of Gram-positive and Gram-negative bacteria; it is not active against bacterial endospores, some fungal spores, or viruses. Chlorhexidine has been reported to inhibit bacterial ATPase activity and damage the bacterial cell membrane; higher concentrations coagulate the cytoplasm. Antimicrobial activity decreases with increasing acidity, becoming very low at ca. pH 5; activity is also decreased in the presence of soaps and anions such as carbonate and chloride. An alcoholic solution of chlorhexidine is used as a skin disinfectant; water-soluble derivatives include the acetate and gluconate (e.g. Savlon, used as a skin antiseptic and in throat lozenges, is a mixture of chlorhexidine gluconate (Hibitane) and Cetrimide). [Review: Book ref. 65, pp. 251–270.] chlorin A 7,8-dihydroporphyrin (see PORPHYRINS), derivatives of which occur in e.g. CHLOROPHYLLS, certain CYTOCHROMES, and sirohaem (see DESULFOVIRIDIN). chlorinated soda solution Syn. DAKIN’S SOLUTION. chlorinator See WATER SUPPLIES. chlorine (as an antimicrobial agent) Chlorine is an effective microbicidal agent used e.g. for the disinfection of WATER SUPPLIES, swimming pools etc. Chlorine is a strong OXIDIZING AGENT; it can also react directly with certain groups in cells and viruses, and it reacts with water to form the strongly antimicrobial compound hypochlorous acid (see HYPOCHLORITES). (N.B. The word ‘chlorine’ is often used to refer to an aqueous solution containing Cl2 , hypochlorous acid and other active chlorine compounds – rather than to elemental chlorine; ‘available chlorine’ is the oxidizing capacity of a chlorine compound expressed in terms of the equivalent amount of elemental chlorine.) The antimicrobial action of chlorine is decreased by the presence of organic matter and other substances with which it can react. Chloramines exert antimicrobial activity by decomposing slowly to release chlorine; their activity is thus much less rapid than that of hypochlorites. Monochloramine (NH2 Cl) has been used e.g. in the treatment of water supplies. The organic chloramines are less toxic and less irritating to animal tissues and have been used e.g. as antiseptics and/or disinfectants; they include chloramine T (sodium ptoluenesulphonchloramide) – a white, crystalline, water-soluble solid; dichloramine T (p-toluenesulphondichloramide) – waterinsoluble and less rapidly effective than chloramine T; halazone (p-carboxy-N,N-dichlorobenzenesulphonamide) – used for the disinfection of small quantities of drinking water. (See also CHLORINE DIOXIDE and IODINE.) chlorine demand See WATER SUPPLIES. chlorine dioxide (ClO2 ) A microbicidal agent active against e.g. bacteria (including endospores), fungi and viruses; it has been used e.g. for the disinfection of swimming-pool water, and as a disinfectant in water-immersion chilling systems for poultry carcasses. Chloriridovirus (large iridescent insect virus group) A genus of viruses (family IRIDOVIRIDAE) which infect insects; infected 156 Chlorophyta two flagella: a tinsel flagellum directed anteriorly, and a whiplash flagellum directed posteriorly; a PALMELLOID PHASE commonly occurs e.g. in Vacuolaria. The numerous yellow-green chloroplasts contain e.g. chlorophylls a and c, b-carotene, and antheraxanthin. Chloromonads occur mainly in freshwater habitats. chloromycetin Syn. CHLORAMPHENICOL. Chloromyxum See MYXOSPOREA. chloroneb (1,4-dichloro-2,5-dimethoxybenzene) An agricultural systemic ANTIFUNGAL AGENT which is taken up by plant roots and is concentrated in the roots and lower stem; it is highly fungistatic for Rhizoctonia spp, moderately fungistatic for Pythium spp, but shows little activity against Fusarium spp. It is used as a seed and soil treatment for the control of damping off etc. Chloronema A genus of filamentous, mesophilic bacteria (family CHLOROFLEXACEAE [ARM (1981) 35 339–364]) which occur e.g. in lakes. In Chloronema bacterioCHLOROPHYLL d occurs in chlorosomes. chloronitrobenzenes (as antifungal agents) These compounds include several useful agricultural ANTIFUNGAL AGENTS: see e.g. DICLORAN, QUINTOZENE and TECNAZENE. (See also DINITROPHENOLS.) chloropeptide (cyclochlorotine) A hepatotoxic MYCOTOXIN produced by Penicillium islandicum (see YELLOW RICE). It appears to bind to actin within cells, modifying the cytoskeleton. chlorophenol red Syn. CHLORPHENOL RED. Chlorophyceae See CHLOROPHYTA. chlorophycean starch See STARCH. chlorophylls A class of pigments involved in PHOTOSYNTHESIS; they may be regarded as derivatives of protoporphyrin IX (see PORPHYRINS) complexed with magnesium (see figure on page 158). (cf. PHAEOPHYTIN.) Chlorophylls in algae and cyanobacteria. Chlorophyll a (Chl a) – see figure – occurs in all organisms which carry out oxygenic PHOTOSYNTHESIS. Chl b (see figure) occurs in euglenoid flagellates and green algae (Chlorophyta), as well as in most higher plants. Chls c1 and c2 occur together in brown algae (Phaeophyta), chrysophytes, diatoms, and prymnesiophytes, while Chl c2 without Chl c1 occurs in cryptophytes and dinoflagellates. Chl d occurs in some red algae (Rhodophyta). Bacteriochlorophylls occur in anoxygenic photosynthetic bacteria (Rhodospirillales). Bchl a is the sole bacteriochlorophyll in most purple bacteria (Rhodospirillineae); those purple bacteria which lack Bchl a (Ectothiorhodospira abdelmalekii, E. halochoris, Rhodopseudomonas sulfoviridis, R. viridis, Thiocapsa pfennigii ) have instead Bchl b. Probably all green bacteria (Chlorobiineae) contain Bchl a – usually together with Bchls c (= ‘chlorobium chlorophyll 660 series’) or Bchls d (‘chlorobium chlorophyll 650 series’). Bchl e is the major bacteriochlorophyll in Chlorobium phaeobacteroides and C. phaeovibrioides, occurring together with small amounts of Bchl a. Bchl g has been identified in Heliobacterium chlorum (q.v.). (See also ERYTHROBACTER.) Chlorophylls generally occur in photosynthetic cells within specialized membranes or organelles: see CHLOROPLAST, CHLOROSOME, CHROMATOPHORE (sense 2), THYLAKOID. Purified chlorophylls are unstable in the presence of light and oxygen. Chlorophyta (green algae) A division of the ALGAE, members of which are characterized by a combination of features: e.g., they contain chlorophylls a and b, b-carotene, and xanthophylls such as antheraxanthin, lutein, neoxanthin, violaxanthin and zeaxanthin; the chloroplasts are enveloped by a double membrane but lack an additional envelope of endoplasmic reticulum; pyrenoids, chlorocresol See PHENOLS. Chlorocystis A genus of unicellular coccoid green algae (division CHLOROPHYTA). Chloroflexaceae A family of filamentous, photosynthetic GLIDING BACTERIA (suborder CHLOROBIINEAE) which includes the genera CHLOROFLEXUS, CHLORONEMA and Oscillochloris [ARM (1981) 35 339–364]. Chloroflexis See CHLOROFLEXUS. Chloroflexus (formerly Chloroflexis) A genus of bacteria (family CHLOROFLEXACEAE). C. aurantiacus occurs in hot, neutral to alkaline (pH ca. 8) springs (often in mats in association with cyanobacteria) and grows optimally at ca. 50–55° C. Under anaerobic conditions in the light, C. aurantiacus can grow as a photoorganotrophic heterotroph or as a photolithotrophic autotroph (electron donor: e.g. sulphide); sulphur produced by sulphide oxidation is deposited extracellularly. The organisms contain bacterioCHLOROPHYLLS a and c. (See also CHLOROSOMES.) Aerobically, C. aurantiacus does not produce chlorophylls and grows chemoorganotrophically. The filaments of C. aurantiacus (ca. 0.45–1.0 µm in width) are bright orange under most conditions but are green when growing photolithotrophically with high sulphide levels or low light intensity. [Photochemistry in C. aurantiacus: PNAS (1982) 79 6532–6536.] chlorogenic acid 1,3,4,5-Tetrahydroxycyclohexane-1-carboxylic acid (quinic acid) esterified at the 3-hydroxy position with CAFFEIC ACID. Chlorogenic acid is an antifungal metabolite found in certain higher plants; it may be involved e.g. in the resistance of certain strains of apple to scab (Venturia inaequalis). Since chlorogenic acid tends to accumulate around damaged tissues at sites of infection it is sometimes regarded as a PHYTOALEXIN. (See also AUXINS.) Chlorogloeopsis (Chlorogloea) A genus of filamentous CYANOBACTERIA (section V) in which the cells divide in more than one plane, producing a multicellular trichome which readily breaks up into irregular Gloeocapsa-like aggregates, each surrounded by a sheath (containing heteropolysaccharides and ca. 20% protein [JGM (1982) 128 267–272]). Hormogonia are formed by rapid division of some cells in one plane, and are composed initially of small cylindrical cells which enlarge and become spherical. Heterocysts develop in terminal and intercalary positions, and akinetes are produced. GC%: 42–43. One species: C. fritschii. The genus currently includes the thermophilic organism known as ‘high-temperature-form (HTF) Mastigocladus’. This organism has an upper temperature limit for growth of 63–64° C and is apparently capable of nitrogen fixation; it occurs in hot springs, and is very sensitive to sulphide. Chlorogonium A genus of unicellular, biflagellated green algae which are closely related to CHLAMYDOMONAS spp. (See also FLAGELLUM (b).) Chloroherpeton A genus of green, photosynthetic, nonfilamentous gliding bacteria; one species: C. thalassium [Arch. Micro. (1984) 138 96–101], which is apparently closely related to Chlorobium limicola and C. vibrioforme [SAAM (1985) 6 152–156]. Chloromonadida See CHLOROMONADS. Chloromonadophyceae See CHLOROMONADS. chloromonads A group of phytoflagellates (including genera such as Chattonella, Gonyostomum, Vacuolaria) which are variously classified as algae of the class Raphidophyceae (= Rhaphidophyceae or Chloromonadophyceae), as algae within the class XANTHOPHYCEAE, or as protozoa of the order Chloromonadida (see PHYTOMASTIGOPHOREA). The vegetative cell typically has 157 Chlorophyta when present, occur within the chloroplast; the main storage polymer is starch which is formed within the chloroplast; motile cells, when formed, bear flagella (commonly 2 or 4) which have a characteristic structure (see FLAGELLUM (b)). The ‘green algae’ (which are not necessarily green in colour) are a diverse group of organisms which, in vegetative form, may be unicellular and non-motile (e.g. ANKISTRODESMUS, CHLORELLA, CHLOROCOCCUM, COCCOMYXA, TREBOUXIA; cf. ACETABULARIA and DESMIDS), unicellular and flagellated (e.g. CHLAMYDOMONAS, MICROMONADOPHYCEAE, TETRASELMIS), sarcinoid (e.g. PSEUDOTREBOUXIA), coenobial (e.g. SCENEDESMUS, VOLVOX), filamentous (e.g. APHANOCHAETE, OEDOGONIUM, SPIROGYRA, STIGEOCLONIUM, TRENTEPOHLIA, ULOTHRIX; see also SIPHONOCLADOUS), siphonaceous (e.g. CODIUM, DERBESIA, HALIMEDA, OSTREOBIUM), or thallose/parenchymatous (e.g. ENTEROMORPHA, ULVA) – reaching a relatively high degree of structural complexity and differentiation (CHAROPHYTES). Asexual reproduction commonly occurs by the formation of flagellated zoospores, but some species may produce e.g. aplanospores (e.g. Aphanochaete, Stigeoclonium), AUTOSPORES, or bulbils (charophytes), or may reproduce by fragmentation of the vegetative thallus (e.g. Spirogyra, Stichococcus). Sexual reproduction may be isogamous (e.g. CH2 H CH3 C a 3 2 H3C 1 N N III IV g H 10 H C O CH3 9 O C phytyl 5 6 7 CH2 CH2 O N N 8 H3C b Mg d H CH2CH3 4 II I O OCH3 CHLOROPHYLLS: Chlorophyll a (phytyl = C20 H39 ). Other chlorophylls differ from chlorophyll a as shown below. Position Chlorophylls Chl b Chl c1 Chl c2 Chl d Bacteriochlorophylls Bchl a Bchl b Bchls c Bchls d Bchls e Bchl g Substituent −CHO −CH=CH.COOH double bond −CH=CH.COOH double bond −CH=CH2 −CHO 3 7 7,8 7 7,8 4 2 −CO.CH3 no double bond phytyl sometimes replaced by geranylgeraniol 2 −CO.CH3 3,4 no double bond =CH.CH3 4 2 −CHOH.CH3 4 −C2 H5 , −C3 H7 , or −C4 H9 5 −C2 H5 or −CH3 7 phytyl replaced by farnesyl or stearyl 10 −H replaces −CO.OCH3 d-methene −CH3 2 −CHOH.CH3 4 −C2 H5 , −C3 H7 , −C4 H9 , or −C5 H11 5 −C2 H5 or −CH3 7 phytyl replaced by farnesyl 10 −H replaces −CO.OCH3 As for Bchls c, but with −CHO at position 3, and phytyl replaced by farnesyl 3,4 no double bond =CH.CH3 4 7 phytyl replaced by geranylgeraniol 2 3,4 7 158 chlortetracycline contains multiple DNA molecules. In at least some higher plants and algae, ctDNA encodes e.g. the large subunit of RuBisCO, various subunits of proton ATPase, several cytochromes, and various rRNAs and ribosomal proteins. Chloroplast development. The following is a provisional summary. Most chloroplast polypeptides are encoded by nuclear genes and synthesized on cytoplasmic ribosomes. Such polypeptides are transported into the chloroplast by a post-translational (rather than co-translational) mechanism. Chloroplast genes, which may encode up to ca. 100 polypeptides, are essential for chloroplast development. Chloroplast-encoded polypeptides and RNAs function within the chloroplast. Light is essential for chlorophyll synthesis in angiosperms but not in most algae, mosses, ferns or gymnosperms. [Book ref. 156, pp. 1–9.] PROTEIN SYNTHESIS in chloroplasts resembles that in bacteria: 70S-type ribosomes are used, N-formylmethionine is the first amino acid incorporated, and the process is inhibited e.g. by chloramphenicol. Synthesis of galactolipids, the major polar lipids in the chloroplast, appears to occur in the chloroplast envelope [Book ref. 156, pp. 193–224]. Origin of chloroplasts. Chloroplasts appear to arise either by the division of existing chloroplasts or by the development of relatively undifferentiated plastids (proplastids) which lack e.g. thylakoids and specialized pigments. (See also ETIOPLAST.) The mechanism of coordination of the expression of nuclear and chloroplast genes is not fully understood. [Chloroplasts of giant-celled and coenocytic algae: Bot. Rev. (1984) 50 267–307.] chloroplast endoplasmic reticulum See CHLOROPLAST. Chloropseudomonas ethylica A species designation [Book ref. 21] which now appears to refer to a syntrophic mixture of green sulphide-oxidizing bacteria (Chlorobium limicola and/or Prosthecochloris aestuarii ) and the sulphur-reducing species Desulfuromonas acetoxidans. chloroquine See AMINOQUINOLINES and MALARIA. Chloros See HYPOCHLORITES. chlorosis (plant pathol.) The loss of chlorophyll from the tissues of a plant – due e.g. to microbial infection, to the action of certain phytotoxins, to lack of light, to magnesium or iron deficiency, etc. Chlorotic tissues commonly appear yellowish (cf. YELLOWS). chlorosomes (chlorobium vesicles) Elongated, intracellular, membranous vesicles formed in all members of the CHLOROBIINEAE grown under appropriate conditions; the vesicles, which contain most of the light-harvesting (‘antenna’) pigments involved in PHOTOSYNTHESIS, are attached to, but differ in structure from, the cytoplasmic membrane. In members of the Chlorobiaceae the chlorosomes contain bacterioCHLOROPHYLL c, d or e and various CAROTENOIDS (e.g. chlorobactene in green cells, isorenieratene in brown cells), while in the Chloroflexaceae they contain bacteriochlorophyll c and g- and b-carotene; a small amount of ‘antenna’ Bchl a is present in the chlorosomes of Chloroflexus aurantiacus and appears to occur also in the chlorosomes of members of the Chlorobiaceae. In the Chlorobiineae the cytoplasmic membrane contains the reaction centre Bchl a together with a small amount of antenna Bchl a. [Biochemistry of lightharvesting systems: ARB (1983) 52 125–157.] Chlorosplenium See HELOTIALES. chloroxylenol See PHENOLS. chlorphenol red A PH INDICATOR: pH 4.8–6.4 (yellow to red); pKa 6.0. chlortetracycline See TETRACYLINES. in Acetabularia, Chaetophora, most Chlamydomonas spp, Klebsormidium, Ulothrix ), anisogamous (e.g. in Codium and Derbesia), or oogamous (e.g. in Aphanochaete, charophytes, Coleochaete, Oedogonium); sexual reproduction is unknown in e.g. Chlorella. An isomorphic ALTERNATION OF GENERATIONS occurs e.g. in CLADOPHORA and ULVA, a heteromorphic alternation of generations occurs e.g. in DERBESIA and MONOSTROMA. The majority of green algae occur in freshwater habitats, but some are found in the sea (e.g. Acetabularia, Codium, Enteromorpha, Halimeda, Ulva), some occur in hypersaline waters (e.g. DUNALIELLA), and some are terrestrial – growing on soil (e.g. Chlorella spp, Chlorococcum, Friedmannia), rocks (e.g. some desmids), snow (see e.g. RED SNOW), treebark (e.g. Chlorella spp, ‘Pleurococcus’, Trentepohlia), plant leaves (e.g. CEPHALEUROS), etc. Certain green algae form close stable associations with other organisms: see e.g. LICHEN and ZOOCHLORELLAE. The classification of the green algae has been, and remains, controversial. Some schemes [e.g. Book ref. 130, pp. 86–132] divide the Chlorophyta into two classes: Chlorophyceae (divided into ca. 16 orders containing the vast majority of green algae) and Charophyceae (containing the charophytes). In another scheme [Book ref. 123, pp. 29–72] – based on comparative cytological data such as the structure of flagellated cells, the nature of cell division, etc. – the division Chlorophyta is divided into 5 classes: MICROMONADOPHYCEAE, Charophyceae (containing, in addition to the charophytes, genera such as Coleochaete, Klebsormidium, Spirogyra and the desmids), Ulvophyceae (containing genera such as Monostroma, Ulothrix, Ulva, and the siphonocladous, siphonaceous and dasycladalean algae [Book ref. 123, pp. 121–156]), Pleurastrophyceae (see TETRASELMIS and TREBOUXIA), and Chlorophyceae (containing genera such as Chaetophora, Chlamydomonas, Oedogonium, Volvox, etc). chloropicrin Trichloronitromethane, Cl3 C(NO3 ); a soil fumigant with fungicidal, herbicidal and insecticidal properties. chloroplast A semi-autonomous PLASTID, one or more of which occur in the cells of eukaryotic photosynthetic organisms; it is the site of PHOTOSYNTHESIS and of the biosynthesis of e.g. CHLOROPHYLLS and certain proteins and lipids. According to species, algal chloroplasts may be cup-shaped, discoid, lobate, spiral, stellate or irregularly shaped, and are commonly 2–20 µm in size. Chloroplast structure. The chloroplast consists essentially of a membrane-bounded matrix (stroma) containing a number of flattened, membranous sacs (THYLAKOIDS) which may occur in one or more stacks (grana – singular: granum); the grana are interconnected by membranous bridges (intergranal frets). The stroma contains e.g. DNA (see also CYTOPLASMIC GENES), RIBOSOMES, PLASTOGLOBULI, and a range of proteins – e.g. ‘fraction I protein’ (= RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE–OXYGENASE) and other CALVIN CYCLE enzymes. (See also EYESPOT and PYRENOID.) The thylakoids contain the photosynthetic apparatus (e.g. chlorophylls, electron transport components), PROTON ATPASES, and (membrane-bound) ribosomes. The membrane which encloses the stroma is a double membrane (the chloroplast envelope). In some algae, one or more layers of endoplasmic reticulum (chloroplast endoplasmic reticulum) occur external to the envelope: one layer surrounds the chloroplast in DINOFLAGELLATES and EUGLENOID FLAGELLATES, while two layers occur in members of the PHAEOPHYTA and XANTHOPHYCEAE; none occur in members of the CHLOROPHYTA. Chloroplast DNA (ctDNA) occurs as covalently-closed circular molecules which lack histones; typically, each chloroplast 159 Choanephora bacterial surface (see also BACTERIOPHAGE CTX8). The symptoms of cholera follow secretion and uptake of CHOLERA TOXIN (q.v.). Lab. diagnosis. A stool specimen is incubated in alkaline peptone water for up to 6 hours and the culture used to inoculate TCBS AGAR. Colonies of interest on TCBS agar should be used to inoculate (heavily) a non-selective agar medium; growth on this medium (after ∼6 hours) may be used for a slide agglutination test with appropriate antisera. For O139, a rapid screening test has been developed with monoclonal antibodies [JCM (1998) 36 3595–3600]. Treatment. Oral or intravenous replacement of fluid and electrolyte (see e.g. ORAL REHYDRATION SOLUTION). Antibiotics (e.g. tetracyclines) may help to reduce the period of shedding and limit the risk of cross-infection (although multidrug-resistant strains of V. cholerae have been recorded). Vaccines. A successful live, oral VACCINE against O1 strains was reported in 1995; however, immunity to O1 does not protect against O139. Earlier, live vaccines against O139 were being developed by deletion of certain virulence genes [JID (1994) 170 278–283]. (See also NICED; NON-CHOLERA VIBRIOS; NON-VIBRIO CHOLERA.) cholera toxin (choleragen) A protein ENTEROTOXIN produced by toxigenic strains of Vibrio cholerae and responsible for the symptoms of CHOLERA. Cholera toxin (CT) is encoded by genes ctxA and ctxB of BACTERIOPHAGE CTX8 (toxigenic strains of V. cholerae are thus lysogenic for this phage). Structurally, CT resembles PERTUSSIS TOXIN in having the AB5 arrangement of subunits: a pentameric ring of B subunits with the central A subunit to one side of the plane of the ring. (A variant of V. cholerae produces B subunits (‘choleragenoid’) but no A subunits [PNAS (1979) 76 2052–2056].) The B subunits bind to glycolipid receptors (GM1 gangliosides) in the brush border membranes of cells lining the small intestine. Internalization of the toxin occurs via a non-clathrincoated vesicle. Within the target cell the A subunit is proteolytically cleaved, but the two fragments remain attached, temporarily, by a disulphide bond. When the bond is reduced, the active (catalytic) fragment of the A subunit (responsible for CT activity) acts as an ADP-ribosyl transferase: it ADP-ribosylates a particular G protein, Gs , which normally stimulates the activity of ADENYLATE CYCLASE (AC). As a result, AC is converted to a permanently active form – thus raising the level of cAMP within the target cell (cf. PERTUSSIS TOXIN). CT activity thus leads to an efflux of water and electrolyte (sodium and chloride ions) into the gut lumen, and the profuse watery diarrhoea characteristic of cholera. choleragen Syn. CHOLERA TOXIN. choleragenoid See CHOLERA TOXIN. cholesterol oxidase An enzyme (EC 1.1.3.6) which oxidizes the 3b-hydroxy group of cholesterol. The enzyme is obtained e.g. from various actinomycetes (e.g. Nocardia and Streptomyces spp) and from Schizophyllum commune; it is used in the detection of cholesterol in body fluids. (See also ENZYMES.) cholic acid See BILE ACIDS. choline See e.g. LECITHIN. chondrioids Syn. MESOSOMES. chondriosome Syn. MITOCHONDRION. Chondrococcus (1) A genus of algae of the RHODOPHYTA. (2) An obsolete bacterial genus; C. columnaris is now Flexibacter columnaris. Chondromyces See MYXOBACTERALES. Choanephora See MUCORALES. Choanephoraceae See MUCORALES. choanoflagellate A member of the CHOANOFLAGELLIDA. Choanoflagellida An order of free-living, motile or sessile, flagellated protozoa (class ZOOMASTIGOPHOREA) characterized by a ring (‘collar’) of tentacles (microvilli) surrounding the proximal part of the single, anteriorly directed flagellum; the cell may have an outer membranous sheath (periplast) or a lorica composed of a basket-like arrangement of siliceous costae. Genera: e.g. Acanthoeca, Codonosiga, Diplotheca, Monosiga, Pleurasiga, Savillea, Stephanoeca. [Ultrastructure and deposition of silica in loricate species: Book ref. 137, pp. 295–322.] choanomastigote A form assumed by Crithidia spp: see TRYPANOSOMATIDAE. chocolate agar A medium used for the isolation and culture of certain fastidious bacteria (e.g. Haemophilus spp, Neisseria gonorrhoeae). A medium such as blood agar base or brain-heart infusion agar is autoclaved and cooled to ca. 80° C; defibrinated horse or bovine blood (10%) is added and the medium shaken gently until chocolate-brown in colour. It may be dispensed into plates, slopes etc. [Recipe: Book ref. 46, p. 1375.] Alternatively, a chocolate agar plate may be prepared by heating a set bloodagar plate to ca. 80° C. chocolate spot A disease of beans (particularly broad beans, Vicia faba) caused by Botrytis fabae. Chocolate-brown, rounded spots develop on all parts of the plant; in severe cases entire organs or plants may be blackened and killed. Infection is usually initiated by conidia from over-wintering plant debris, but may be seed-borne. (See also WYERONE.) choke A GRASS DISEASE (affecting e.g. Agrostis and Festuca spp) caused by Epichloë typhina (see EPICHLOË). Just before the host plant comes into flower, the sheath of the flag-leaf becomes encircled at its apex by a yellow to orange stroma; conidia are produced on the stromal surface, and perithecia develop within the stroma with their ostiolar pores at the surface. Flowering is partly or completely suppressed. cholecystitis Inflammation of the gall-bladder. Inflammation may be due to bacterial infection (e.g. by bacteria from the bowel) secondary to stagnation of the bile caused e.g. by gallstones. cholera An acute, infectious human disease involving profuse dehydrating diarrhoea; cholera is caused by certain strains of Vibrio cholerae (see VIBRIO) which secrete CHOLERA TOXIN. Until 1992, choleragenic strains of V. cholerae were associated only with serogroup O1. In 1992, non-O1 choleragenic strains were isolated in India; these strains were assigned to a new serogroup, O139, and were designated O139 Bengal [V. cholerae O139 Bengal: RMM (1996) 7 43–51]. Infection occurs by the faecal–oral route, usually via contaminated water. The severity (or occurrence) of disease is influenced e.g. by the number of organisms ingested; low stomach acidity is a predisposing factor. Incubation period: 1–5 (usually 2–3) days. Initially the stools contain faecal material, but they rapidly become pale-grey and watery (‘rice-water stools’); 1–30 litres of fluid may be passed per day, resulting in severe dehydration and loss of electrolyte. In untreated cases mortality rates may be high (e.g. 50–75%); death may occur within hours, but usually within 1–2 days. Mild cases also occur, particularly in endemic areas. Diarrhoeal disease may be caused by O1 strains which do not form cholera toxin; such disease may be due e.g. to the Ace and/or Zot toxins (encoded by bacteriophage CTX8). V. cholerae adheres to the small intestine by ‘toxin coregulated pili’ – filamentous appendages (encoded by a chromosomal PATHOGENICITY ISLAND) which form a bundle on the 160 Chromatium chondrosarcoma See SARCOMA. Chondrostereum See APHYLLOPHORALES (Stereaceae). Chondrus A genus of marine algae (division RHODOPHYTA). The thallus consists typically of dichotomously branched fronds which attach to rocks etc by means of a holdfast. C. crispus (‘Irish moss’, ‘carragheen moss’) is used as a source of CARRAGEENAN and, in some countries, as food. [Culture of C. crispus: CJFAS (1986) 43 263–268.] Chonotrichida See HYPOSTOMATIA. chopped meat medium See COOKED MEAT MEDIUM. Chorda See PHAEOPHYTA. Chordaria See PHAEOPHYTA. Chordopoxvirinae A subfamily of viruses, family POXVIRIDAE, which infect vertebrates; most members share a common antigen. Six genera are recognized: AVIPOXVIRUS, CAPRIPOXVIRUS, LEPORIPOXVIRUS, ORTHOPOXVIRUS, PARAPOXVIRUS, SUIPOXVIRUS. The members of a given genus are closely related serologically; genetic recombination can occur between members of a given genus, and NON-GENETIC REACTIVATION can occur between members of the same genus and of different genera. Choreocolax A genus of algae (division RHODOPHYTA). C. polysiphoniae is a parasite of another red alga, Polysiphonia lanosa; the nuclei of C. polysiphoniae apparently enter the cytoplasm of the host via secondary PIT CONNECTIONS [PNAS (1984) 81 5420–5424]. chorioallantoic membrane See EMBRYONATED EGG. chorismate A central intermediate in the biosynthesis of aromatic compounds: see Appendix IV(f). Choristoneura EPVs See ENTOMOPOXVIRINAE. Christensen’s urea agar See UREASES. Christie–Atkins–Munch-Petersen test CAMP TEST. Chromatiaceae (purple sulphur bacteria; Thiorhodaceae) A family of photosynthetic bacteria (suborder RHODOSPIRILLINEAE) which occur typically in sulphide-rich aquatic and terrestrial habitats (for example, muddy soils, riverine and estuarine muds); most species contain Bchl a (see CHLOROPHYLLS) – the photosynthetic pigments typically occurring in vesicular intracytoplasmic membrane systems. The organisms are primarily photolithotrophic autotrophs which can use e.g. sulphur, sulphide or hydrogen as electron donors, but at least some simple organic compounds (e.g. acetate) can be used by all species under appropriate conditions; sulphur and its compounds are ultimately oxidized to sulphate, though elemental sulphur can be deposited intracellularly. Motile species are polarly flagellate (monotrichous or lophotrichous). GAS VACUOLES occur in some species. Genera: Amoebobacter, CHROMATIUM (type genus), LAMPROCYSTIS, THIOCAPSA, Thiocystis, Thiodictyon, Thiopedia, Thiosarcina, THIOSPIRILLUM; cf. ECTOTHIORHODOSPIRA. chromatic adaptation In e.g. certain cyanobacteria: adaptation to changes in the wavelength of incident light by changes in the composition of the light-harvesting pigments. For example, ‘Fremyella diplosiphon’ produces PHYCOBILIPROTEINS allophycocyanin and phycocyanin when grown in red light (ca. 625 nm), but when grown in green light (ca. 550 nm) the cells also produce phycoerythrin [Book ref. 75, pp. 119–126]. chromatid Either of the two longitudinally adjacent threads formed when a (eukaryotic) CHROMOSOME replicates prior to MITOSIS; the chromatids are held together at the CENTROMERE. Sister chromatids are derived from the same chromosome. ChromaTide See PROBE. chromatin The fibrous nucleoprotein complex, containing genomic DNA, HISTONES, NON-HISTONE PROTEINS and RNA, present in the nucleus of most eukaryotic cells. (The term ‘chromatin’ is sometimes used in a more restrictive sense to refer specifically to the dispersed nucleoprotein material in the interphase nucleus, as distinct from the condensed, discrete CHROMOSOMES evident during mitosis and meiosis.) Two types of chromatin are distinguishable in the interphase nucleus: heterochromatin, in which the nucleoprotein fibres are very densely packed, and euchromatin, in which the fibres are less densely packed. Heterochromatin may be condensed permanently, i.e., with little variation through the cell cycle (constitutive heterochromatin), or temporarily (facultative heterochromatin). Euchromatin represents the regions within which gene expression can occur. Chromatin released from cells appears under certain in vitro conditions as a mesh of fibres (ca. 10 nm diam.) which have a regular beaded appearance in the electron microscope; the beads are called nucleosomes. Treatment of these fibres with an endonuclease results in the separation of the nucleosomes, each of which then forms a structure called a chromatosome; a chromatosome contains ca. 165 bp (on average) of DNA, one molecule of histone H1, and a histone octamer comprising two molecules each of histones H2A, H2B, H3 and H4. Further endonuclease action gives rise to a core particle, a disc-shaped structure (ca. 11 nm in diam., 5.7 nm thick) consisting of the histone octamer around which a 146-bp sequence of B-DNA is wrapped in a left-handed superhelix. In the chromatosome, the DNA is believed to form two complete superhelical turns around the histone octamer, the turns being secured by histone H1 which binds to the sequences of DNA entering and leaving the core. In the intact 10-nm chromatin fibre the chromatosome structures are connected to one another via linker DNA which may vary from a few (or no) bp to >80 bp, but which is typically ca. 30–40 bp long. In vivo, the linear chain of nucleosomes (the 10-nm fibre) is itself folded (with consequent further compaction of the DNA) into a 30-nm fibre in which the nucleosomes are packed together – possibly in a helical arrangement, although the details of the structure are uncertain. The 30-nm fibre is apparently the basic structure both in interphase chromatin and in mitotic chromosomes. The H1 histone is essential for the formation of the 30-nm fibre. For processes such as replication, transcription and repair, the highly compacted genome must undergo disruption (socalled ‘remodelling’) in order to expose the DNA to enzymes and binding proteins etc. Various mechanisms are involved. Histone properties (e.g. binding strength) may be modified by post-translational acetylation, phosphorylation, ubiquitylation etc. ATP-dependent remodelling complexes can slide nucleosomes along the DNA and replace histones [COGD (2004) 14 165–173]. Constitutive variant histones (which, unlike other histones, are expressed throughout the cell cycle) can e.g. modify nucleosome properties by replacing existing histones [GD (2005) 19 295–316]. Chromatium A genus of photosynthetic bacteria (family CHROMATIACEAE). The cells are rod-shaped and may be large (up to 15 µm long) or small (ca. 2–6 µm) according to species; they contain Bchl a and various carotenoids (cell suspensions are typically some shade of brown, red or purple). Gas vacuoles are absent. Motility, when it occurs, is mediated by polar flagella. Small-celled species include C. vinosum. The largecelled species (C. buderi, C. okenii, C. warmingii, C. weissei ) are nutritionally fastidious strict anaerobes for which sulphide and vitamin B12 are essential; other species can grow without sulphide under anaerobic reducing conditions. 161 chromatography for example, be stained in situ on the stationary phase – e.g. using specific stains which allow identification of particular types of molecule. chromatoid bodies (protozool.) Ribonucleoprotein bodies (commonly rod-shaped) which occur within the young cysts of certain amoebae (see e.g. ENTAMOEBA). Chromatoid bodies stain well with iron-haematoxylin (they do not stain with iodine); one, two or more bodies per cyst may be seen. The nucleoprotein tends to disperse in the cytoplasm as the cyst ages. chromatophore (1) One of a number of small membranous vesicles obtained by the experimental disruption of photosynthetic membranes from bacteria of the Rhodospirillineae; the vesicles are used e.g. for in vitro studies on photophosphorylation. (2) A system of vesicular, tubular or lamellar membranous structures which occurs in bacteria of the Rhodospirillineae; chromatophores, which contain bacterioCHLOROPHYLL, CAROTENOIDS, and the electron carriers involved in PHOTOSYNTHESIS, are continuous with the cytoplasmic membrane. (cf. CHLOROSOME.) (3) Syn. CHLOROPLAST. (4) Syn. CHROMOPLAST. chromatosome See CHROMATIN. Chromidina See HYPOSTOMATIA. Chromobacterium A genus (incertae sedis) of chemoorganotrophic, Gram-negative bacteria which occur e.g. in soil and water, and as opportunist pathogens in man and other animals. Cells: round-ended, pigmented (VIOLACEIN-containing) rods, ca. 0.6–0.9 × 1.5–3.5 µm, with one polar and one or more lateral and/or subpolar flagella. Growth occurs on unenriched nutrient media. Most strains attack glucose fermentatively (forming acid but usually no gas); some strains attack glucose oxidatively. Nitrate and nitrite are usually reduced. Growth is inhibited by 6% NaCl. Catalase +ve. Usually oxidase +ve. VP −ve. GC%: ca. 50–68. Type species: C. violaceum. C. fluviatile. Optimum growth temperature ca. 25° C, maximum ca. 30° C. GC%: ca. 50–52. C. lividum. See JANTHINOBACTERIUM. C. violaceum. Optimum growth temperature ca. 30–35° C, maximum ca. 40–44° C. GC%: ca. 65–68. (See also MONOBACTAMS and QUORUM SENSING.) [Book ref. 22, pp. 580–582.] chromoblastomycosis (dermatitis verrucosa; verrucose dermatitis) A chronic human MYCOSIS, occurring mainly in tropical and subtropical areas of South America and Africa, caused by any of several black moulds (e.g. Cladosporium carrionii, Phialophora spp, Rhinocladiella spp) which are normally saprotrophic e.g. in soil. Infection occurs via wounds, and warty, spreading, often ulcerating lesions develop in the skin and underlying tissue. In the tissues, the fungi occur as thick-walled, dark-brown muriform cells, 5–12 µm (‘sclerotic bodies’) which multiply by cell separation (not budding); in culture (e.g. on Sabouraud’s dextrose agar) they form slow-growing, velvety, dark-brown to greenish-black colonies. Pathogenicity may involve the body’s reaction to fungal components: e.g., lipids extracted from the fungi can induce granulomatous reactions on intravenous injection into mice [JGM (1985) 131 187–194]. chromogenic Pigment-producing or colour-producing. (cf. ACHROMOGENIC.) chromomycin (chromomycin A3 ) An ANTIBIOTIC and antitumour agent containing an aromatic three-ring chromophore linked to five different sugar residues. Olivomycin and mithramycin are very similar to chromomycin; all three dyes bind non-covalently and non-intercalatively to dsDNA, inhibiting synthesis of RNA and DNA. Binding requires stoichiometric amounts of Mg2+ ions, seems to involve the 2-amino group of guanine, and chromatography The separation of molecules (or ions) in a mixture by any technique which exploits the differing extents to which the different molecules (or ions) become distributed between a stationary phase and a contiguous mobile phase, one of the phases being either a thin film or a surface. Phases may be solid, liquid, or gaseous, according to the type of chromatography. Substances are separated primarily on the basis of their adsorption and/or solubility characteristics (cf. GEL FILTRATION); unlike ELECTROPHORESIS, chromatography does not involve the use of an electrical potential difference. Chromatography is used e.g. for the isolation, purification, and identification of molecules or ions. In adsorption chromatography substances are separated on the basis of quantitative differences in their adsorption onto the surface of a stationary solid phase from a mobile liquid phase. For example, affinity chromatography exploits the specificity of binding which occurs between e.g. antibody and antigen, or enzyme and substrate. Essentially, one component (e.g. antigen) is immobilized on an inert support; this adsorbs the other component (antibody) from the liquid phase which flows over it. [Industrial uses: Book ref. 3, pp. 31–66.] Ionexchange chromatography exploits the preferential adsorption of a particular charged species (e.g. ion, nucleic acid) to an insoluble ion-exchange material under particular conditions. (Ion-exchange chromatography is sometimes excluded from the category ‘adsorption chromatography’ on a mechanistic basis.) Partition chromatography exploits the tendency of different substances to become differently distributed between two fluids in contact. In liquid–liquid chromatography one of the liquids is typically present as a coating on inert particles, or on an inert matrix, over which the mobile liquid phase is passed. For example, in paper partition chromatography the stationary phase is typically water which coats the fibres of a paper strip; the mobile phase, usually an organic solvent, flows along the strip by capillary action. If a spot of sample (mixture of substances) is initially applied to one end of the strip, the substances become separated – according to their different partition coefficients – as the solvent flows along the strip. In gas–liquid chromatography (GLC) the sample is vaporized in a carrier gas (helium, hydrogen, or nitrogen) which is pumped along a heated tube (up to ca. 350° C) containing inert particles coated with a substance that is liquid at the temperature used. Substances in the sample become partitioned between the gas and liquid; a given substance moves along the tube, in the direction of gas flow, at a characteristic rate which depends on its partition coefficient, and its exit from the tube is recorded e.g. by a thermal conductivity detector. High-pressure liquid chromatography (high-performance liquid chromatography; HPLC) uses very high pressures to force a liquid sample through a tightly-packed column of minute particles; separations occur at the surfaces of the particles on the basis of e.g. adsorption processes. The small size of the particles increases the efficiency of interaction between solid and liquid phases; high pressures are necessary to ensure a reasonable rate of flow through the column. Thin-layer chromatography (TLC) employs a thin layer of e.g. silica gel on a flat glass support; the (liquid) mobile phase rises up the thin layer by capillary action. When the substances in a sample have been separated, each may be removed (eluted ) from the stationary phase by means of a suitable solvent or solution (the eluent ); for example, in ionexchange chromatography a suitable eluent may be a solution of a particular electrolyte or a buffer of particular pH. (See also GRADIENT ELUTION.) Alternatively, the separated substances may, 162 Chroomonas results in a shift in the absorption spectrum of the dye to longer wavelengths. The dyes are used in cytochemistry as GCspecific fluorochromes, e.g., they produce specific fluorescent banding patterns in metaphase chromosomes [Science (1977) 195 400–402]. chromomycosis May refer either to CHROMOBLASTOMYCOSIS or to PHAEOHYPHOMYCOSIS. chromophore That part of a dye molecule which is responsible for the colour of the dye. chromoplast (1) A PLASTID in which the main or sole pigments are CAROTENOIDS. (2) (chromatophore) Any pigmented plastid, e.g. a CHLOROPLAST or a carotenoid-rich plastid in ripening fruit. chromosomal fingerprinting See DNA FINGERPRINTING. chromosome In a prokaryotic cell, or in the nucleus of a eukaryotic cell: a structure consisting of or containing DNA which carries genetic information essential to the cell; the term is also commonly applied to DNA in a mitochondrion or chloroplast and to the genome of a DNA virus, but generally not to the genomic RNA of an RNA virus. (cf. GENOME sense 1 and PLASMID.) (a) Bacterial chromosomes. In most bacteria the chromosome is a ccc dsDNA molecule; in some species (e.g. Borrelia burgdorferi [Nature (1997) 390 580–586], species of Streptomyces) the DNA is linear. Bacterial chromosomes are not enclosed by a membrane (see PROKARYOTE) but they are apparently linked to the cell’s cytoplasmic membrane. Typically, a cell contains only one type of chromosome; the chromosome may be present in one, several or many copies per cell – depending e.g. on species and on growth conditions; for example, in Escherichia coli there may be one or more copies per cell according to the growth rate (see HELMSTETTER–COOPER MODEL). Some species of bacteria appear normally to have multiple copies of the chromosome – e.g. Azotobacter vinelandii may have an average of 20–40 ‘genome equivalents’ (apparently separate chromosomes) per cell, A. chroococcum contains 20–25, Deinococcus radiodurans has four (in resting cells) to ca. 10, while Desulfovibrio gigas has ca. 9–17 (in growing cells) [see e.g. JGM (1984) 130 1597–1601, 1603–1612]. The size of the chromosome (in base-pairs) varies considerably among species. One of the smallest is that of Buchnera, an obligate intracellular parasite of aphids (∼640 kb [Nature (2000) 407 81–86]); larger chromosomes occur e.g. in Helicobacter pylori (∼1.7 × 106 bp) [Nature (1997) 388 539–547] and Mycobacterium tuberculosis (∼4.4 × 106 bp [Nature (1998) 393 537–544]). In Vibrio cholerae the genome consists of two dissimilar circular chromosomes – chromosome 1 (∼2961 kb) and chromosome 2 (∼1073 kb) [Nature (2000) 406 477–483]. In E. coli the chromosome contains ca. 4.7 × 106 bp; if opened out, it would be ca. 1.3 mm in length. Within the cell it is extensively folded to form a compact structure variously called a nucleoid, nucleoid body, nuclear area, ‘nucleus’ etc. The nucleoid consists mainly of DNA associated with certain types of protein. The DNA is extensively supercoiled (see DNA), the degree of supercoiling being determined by the joint activities of certain enzymes (see TOPOISOMERASE). The supercoiling is segregated into many topologically distinct domains; a nick in one domain relaxes the DNA in that domain only. The nucleoid can be unfolded in vitro by treatment with RNase. (See also CELL CYCLE and DNA REPLICATION.) [The chromosome of E. coli : Book ref. 122, pp. 161–197, 199–227.] (b) Eukaryotic chromosomes. The eukaryotic NUCLEUS typically contains one or more sets of different chromosomes (see also PLOIDY; each chromosome is believed to contain a single linear dsDNA molecule complexed with protein (see CHROMATIN). Between nuclear divisions (interphase) the chromosomes are dispersed as a mass of 30-nm chromatin fibres occupying much of the volume of the nucleus; during MITOSIS (and MEIOSIS) each chromosome becomes highly condensed to form a discrete structure with a characteristic morphology (see also KARYOTYPE) and structurally and functionally distinct regions (see e.g. CENTROMERE and TELOMERE). Chromosome replication (and transcription) occurs during interphase (see CELL CYCLE), so that at mitosis each chromosome consists of two sister CHROMATIDS. The 30-nm chromatin fibre in a mitotic chromosome appears to be organized into a series of loops (each ca. 30–90 kb), the ends of the loops being secured by a central proteinaceous scaffold ; when the chromosome is treated in vitro to remove most of the histones and non-histone proteins, the central scaffold – similar in shape to the sister chromatids – can be seen surrounded by a halo of naked DNA loops. chromosome aberration An abnormality in the number or structure of chromosomes in a cell. Thus, e.g., a HETEROPLOID cell may have the diploid number of chromosomes minus one chromosome (monosomic cell) or plus one chromosome (trisomic cell). A chromosome may lose a segment (deletion); a segment may move from one site to another on the same or on a different chromosome (translocation); a segment may take up the reverse orientation in the same site (inversion); or a segment may be duplicated. chromosome walking A technique with which an unknown region of a chromosome can be explored. The starting point is a LIBRARY in which the cloned DNA constitutes a series of overlapping fragments. A fragment containing a known gene is selected and used as a probe to identify (e.g. by COLONY HYBRIDIZATION) other, overlapping fragments which contain the same gene. The nucleotide sequences of these fragments are then characterized – thus delimiting an overall segment of the chromosome which comprises the span of the overlapping sequences. A fragment at one end of this segment is selected, and the terminal part of this fragment is cleaved and used as a probe to identify a neighbouring set of overlapping fragments, and so on. chromotrope See METACHROMASY. Chromulina A genus of unicellular CHRYSOPHYTES in which each cell has a long tinsel flagellum but only a very short second flagellum. Cysts resemble narrow-necked flasks. chronic (med.) Refers to a disease which persists for a relatively long period of time (e.g. months, years) – terminating in recovery or death. Examples: LEPROSY; TUBERCULOSIS. (cf. ACUTE.) chronic bacillary diarrhoea Syn. JOHNE’S DISEASE. chronic mucocutaneous candidiasis See CANDIDIASIS. chronic myelogenous leukaemia See ABL. chronic wasting disease See TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES. Chroococcales See CYANOBACTERIA. Chroococcidiopsis A genus of unicellular CYANOBACTERIA (section II) in which growth and binary fission lead to the formation of cubical aggregates of cells; baeocytes are nonmotile and have a fibrous outer cell wall layer at the time of their release. GC%: 40–46. Chroococcus A phycological genus of ‘blue-green algae’ included in the genus GLOEOCAPSA [JGM (1979) 111 1–61 (p. 9)]. Chroodactylon See RHODOPHYTA. Chroomonas See CRYPTOPHYTES. 163 chrysanthemum chlorotic mottle viroid chrysanthemum chlorotic mottle viroid See VIROID. chrysanthemum stunt viroid See VIROID. chrysanthemum white rust A serious disease of chrysanthemums caused by Puccinia horiana; typical early symptoms include pale spots on the upper surfaces of leaves, and raised, buff-coloured pustules on corresponding parts of the lower surfaces – the pustules subsequently becoming white. Common chrysanthemum rust, a less serious disease caused by P. chrysanthemi, is characterized by brown powdery masses on the undersides of leaves. Chryseomonas A genus of yellow-pigmented, aerobic, catalasepositive, Gram-negative bacteria. C. polytricha was isolated from clinical specimens; cells are round-ended rods, motile by lophotrichous flagella. The organisms are chemoorganotrophs and are reported to be oxidatively saccharolytic but oxidasenegative. GC%: 54–56. [IJSB (1986) 36 161–165.] Chrysochromulina See PRYMNESIOPHYCEAE. Chrysococcus A genus of unicellular CHRYSOPHYTES in which young cells each have a mucilaginous lorica which, as the cell ages, becomes impregnated with mineral crystals. Cytoplasmic projections extend through pores in the lorica. chrysolaminarin (leucosin; chrysose) A reserve polysaccharide produced by CHRYSOPHYTES, DIATOMS, and members of the PRYMNESIOPHYCEAE. It is a (1 → 3)-b-D-glucan with some branching via (1 → 6)-b-linkages. Chrysomonadida See PHYTOMASTIGOPHOREA. chrysomonads Syn. CHRYSOPHYTES. chrysophytes (chrysomonads) A group of eukaryotic, mainly freshwater organisms regarded as algae (‘golden, golden-brown or yellow-brown algae’, class Chrysophyceae or division Chrysophyta) or as protozoa (see PHYTOMASTIGOPHOREA). Species are either free-swimming (or occasionally sessile) unicellular organisms (e.g. Chromulina, Mallomonas, Ochromonas, Oikomonas), or they may form sessile colonies (e.g. CEPHALOTHAMNIUM, DINOBRYON) or motile colonies (e.g. SYNURA, UROGLENA). Most species have two flagella, one of which is directed anteriorly and bears two opposite rows of mastigonemes (see FLAGELLUM (b)), while the other is either smooth and directed posteriorly or is reduced to a short stub (e.g. CHROMULINA) or basal body. FLAGELLAR MOTILITY is achieved primarily by the tinsel flagellum. According to species, the chrysophyte cell may be naked, may be enclosed by a LORICA (see e.g. CHRYSOCOCCUS, DINOBRYON, OCHROMONAS), or may bear scales – adherent to the outside of the plasmalemma – which may be silicified (e.g. in MALLOMONAS, PARAPHYSOMONAS, SYNURA) or organic (e.g. in Chromulina placentula); the scales are produced in special membranous vesicles (SILICA DEPOSITION VESICLES), not in the Golgi apparatus (cf. PRYMNESIOPHYCEAE). [Scale formation in Synura, Mallomonas and Paraphysomonas: JUR (1982) 18 13–26.] Pseudopodium-like cytoplasmic projections are present in many species, and many chrysophytes (e.g. OCHROMONAS) can ingest particulate matter. The majority of chrysophytes are photosynthetic, each cell typically having a few (commonly two) golden-brown chloroplasts containing chlorophylls a, c1 and c2 , b-carotene, and fucoxanthin. Mannitol is a major primary product of photosynthesis. Storage compounds include CHRYSOLAMINARIN. Reproduction occurs by longitudinal binary fission. Palmelloid stages are common in some species. Spherical or bottle-shaped cysts (statospores, resting spores) are commonly formed; each cyst develops within a vegetative cell and has a silica wall which may be smooth or ornamented with e.g. spines. [Book ref. 197. Silica in chrysophytes: Book ref. 137, pp. 201–230.] See also SILICOFLAGELLATES. chrysose Syn. CHRYSOLAMINARIN. Chrysosporium A genus of fungi of the HYPHOMYCETES; most have teleomorphs in the Gymnoascaceae. Hyphae: hyaline, septate. Conidia: non-septate, subglobose to pyriform, 2–2.5 × 4–4.5 µm in culture (cf. ADIASPIROMYCOSIS), typically borne (singly or in pairs) on short, peg-like conidiophores. C. pruinosum, a saprotroph, produces adiaspores at high temperatures in vitro. chvE gene See CROWN GALL. chymosin See CASEIN. chymotrypsin A pancreatic serine PROTEASE. chytrid A member of the CHYTRIDIALES. Chytridiales (the ‘chytrids’) An order of fungi (class CHYTRIDIOMYCETES) which characteristically do not form true mycelium (though some species form a RHIZOMYCELIUM) and in which sexual reproduction, where known, may involve e.g. the fusion of isogametes or gametangial copulation; the organisms include parasites and pathogens of terrestrial plants, aquatic saprotrophs, parasites of algae, and MYCOPARASITES. Genera include Chytriomyces, NOWAKOWSKIELLA, OLPIDIUM, Polyphagus, RHIZOPHYDIUM, Rozella and SYNCHYTRIUM. Chytridiomycetes A class of terrestrial and aquatic, saprotrophic and parasitic fungi (subdivision MASTIGOMYCOTINA) which form sporangiospores having one posteriorly-directed whiplash flagellum. (cf. NEOCALLIMASTIX.) According to species, the thallus ranges from a simple, sac-like, holocarpic structure to a well-developed, branched mycelium; in many species the cell wall contains CHITIN, and in some species the mycelium contains pseudosepta – see SEPTUM (b). Orders: BLASTOCLADIALES, CHYTRIDIALES, Harpochytriales (tropical freshwater saprotrophs), MONOBLEPHARIDALES. Chytridiopsis See MICROSPOREA. Chytriomyces See CHYTRIDIALES. Ci CURIE (q.v.). CI See COLOUR INDEX. Ciboria See HELOTIALES. CID CYTOMEGALIC INCLUSION DISEASE. cider (American: hard cider) A beverage made by the ALCOHOLIC FERMENTATION of apple juice. (In the USA ‘cider’ refers to unfermented apple juice.) Cider apples are rich in tannin and malic acid (contributing astringent and sharp flavours, respectively). Fermentation may be carried out by the natural apple flora – including e.g. bacteria, yeasts (e.g. Candida, Kloeckera, Pichia, Saccharomyces) and other fungi (e.g. Aspergillus, Penicillium). Kloeckera spp begin the fermentation, suppressing both Candida and Pichia; later, Saccharomyces spp predominate. In modern processes the natural flora is usually suppressed (with SULPHUR DIOXIDE) or reduced (by pasteurization, filtration etc) and the juice inoculated with cider yeast – usually ‘Saccharomyces uvarum’ (a strain of S. cerevisiae); this yeast can degrade de-esterified PECTINS to galacturonic acid. After fermentation is complete, acidity may be reduced by the MALOLACTIC FERMENTATION. The cider may then be carbonated and/or pasteurized and/or filtered. Cider spoilage. The most common microbial spoilage is ACETIFICATION by contaminating acetic acid bacteria. Cider sickness is a condition caused by Zymomonas mobilis which produces acetaldehyde; this reacts with tannins in the cider to form a milky-white haze and a banana-like off-flavour. ‘Oiliness’ and ROPINESS may be caused by various lactobacilli. cider sickness See CIDER. Cidex An alkaline GLUTARALDEHYDE solution. 164 circadian rhythms cells – e.g. ctenophores (‘comb jellies’) and certain cells in the mammalian respiratory tract.) The structure and ultrastructure of a cilium are similar to those of the eukaryotic FLAGELLUM (q.v.); however, cilia are usually shorter (often 7–10 µm) and they frequently occur in longitudinal rows (see KINETY) or in specialized groups (see COMPOUND CILIATURE). The characteristic rhythmical bending of cilia is achieved by the sliding of the ciliary microtubules relative to one another. The basic ciliary movement begins with an effective stroke, i.e., a rapid swing through a wide angle in a straight, relatively rigid state due to simultaneous activity of the dynein arms on one side of the axoneme; this is followed by a recovery stroke in which the cilium returns to its datum position as a result of the propagation of a wave of dynein activity along the other side of the axoneme, from base to tip. Energy is therefore required for both the effective and the recovery strokes. The rhythmical beating of cilia generates currents in the ambient fluid – thus causing the cell to move or, e.g., propelling food particles towards the cytostome. (See also METACHRONAL WAVES.) Many variations of ciliary movement are found among ciliates – including e.g. the ability to change the direction of the ciliary beating. (See also CLAVATE CILIUM; cf. FLAGELLAR MOTILITY (b).) [Mechanism and controls of microtubule sliding in cilia: SEBS (1982) 35 179–201; production of different ciliary beat patterns: SEBS (1982) 35 139–157; regulation of ciliary motility by membrane potential via cAMP in Paramecium: Cell Mot. (1986) 6 256–272.] (2) (lichenol.) A vegetative appendage which structurally resembles a RHIZINE but which arises from the margin of a thallus or apothecium or, in some lichens, from the upper surface of the thallus. CIN agar Cefsulodin–Irgasan–novobiocin agar, a medium developed for the isolation of Yersinia enterocolitica. At least one strain of Y. enterocolitica (biotype 3B, serotype O3) and strains of Y. pseudotuberculosis are inhibited on this medium [JCM (1986) 24 116–120]. cin gene See BACTERIOPHAGE P1. cinchona alkaloids Alkaloids obtained from the bark of certain tropical trees (especially Cinchona succirubra); the alkaloids include e.g. cinchonine and QUININE. cinerubin See ANTHRACYCLINE ANTIBIOTICS. cingulum See e.g. DIATOMS and DINOFLAGELLATES. cinoxacin See QUINOLONE ANTIBIOTICS. CIP Collection de l’Institut Pasteur, Institut Pasteur, 28 Rue du Docteur Roux, 75724 Paris, Cedex 15, France. ciprofloxacin See QUINOLONE ANTIBIOTICS. circadian rhythms In certain parameters of a cell or organism: rhythmical changes which occur with a periodicity of ca. 24 hours and which may persist for extended periods of time (e.g. days, weeks) even when the cell or organism has been isolated from the natural diurnal fluctuations of the environment; such innate rhythms are often referred to as expressions of a ‘biological clock’. In eukaryotes, parameters that exhibit circadian rhythms include e.g. (according to organism): BIOLUMINESCENCE, cell division, chloroplast ultrastructure, mating type, motility and photosynthetic activity; thus, for example, peak photosynthetic activity in Acetabularia occurs with a 24-hour periodicity even when the organism is kept in continual light. (cf. INFRADIAN RHYTHMS and ULTRADIAN RHYTHMS.) Once believed to occur only in eukaryotes, circadian rhythms have also been detected in some prokaryotes. For example, in the unicellular cyanobacterium Synechococcus, gene psbAI (which CIDS CIRCULAR INTENSITY DIFFERENTIAL SCATTERING. CIE COUNTERCURRENT IMMUNOELECTROPHORESIS. ciguatera A form of toxic FOOD POISONING, involving gastrointestinal and neurological symptoms, which results from the consumption of certain tropical or subtropical fish whose flesh has accumulated certain algal toxin(s); the fish may have fed directly on the algae, or may have consumed other algivorous fish. The dinoflagellate Gambierdiscus toxicus has been identified as the toxin producer in cases in the Gambier Islands of Polynesia; the main toxin (ciguatoxin) produced by this species is a highly potent neurotoxin [PAC (1986) 58 339–350 (347–348)]. (cf. Paralytic SHELLFISH POISONING.) ciguatoxin See CIGUATERA. cilia See CILIUM. ciliary corpuscle Syn. KINETID. ciliate (1) (adj.) Bearing cilia (see CILIUM). (2) (noun) A protozoon of the CILIOPHORA. ciliature (ciliate protozool.) Collectively, those cilia which occur on a ciliate or on specific region(s) of a ciliate; thus, e.g. ‘somatic ciliature’ refers to those cilia which occur on the body of the cell – as opposed to those which occur in the oral (buccal) cavity. ciliferous Bearing cilia; a ciliferous kinetosome has an associated cilium, while a ‘barren’ kinetosome does not. Ciliophora A phylum of PROTOZOA [JP (1980) 27 37–58] in which the organisms are characteristically ciliated (see CILIUM) during at least some stage in their life cycles, and possess two types of nucleus (see MACRONUCLEUS and MICRONUCLEUS). (cf. PHALACROCLEPTES and STEPHANOPOGON.) Many of the 7000 or so known species are free-living in freshwater, brackish or marine habitats; some occur in the soil (e.g. Grossglockneria acuta [SBB (1985) 17 871–875]), in the RUMEN, or in SEWAGE TREATMENT plants. Some species are pathogenic (see e.g. BALANTIDIUM and ICHTHYOPHTHIRIUS) and some (e.g. members of the ASTOMATIDA) are parasitic. In size, ciliates range from ca. 10 µm to several millimetres. The cells generally have a substantial PELLICLE, one or more CONTRACTILE VACUOLES, and most have a CYTOSTOME; some have EXTRUSOMES. CYSTS are formed by some species. Ciliates are typically colourless, but some (e.g. BLEPHARISMA, STENTOR) are pigmented. Some species have endosymbionts (see e.g. PARAMECIUM). Although typically motile, the ciliates include some sedentary species (see e.g. PERITRICHIA). Ciliates commonly feed holozoically, i.e., by PHAGOCYTOSIS. Reproduction typically occurs by transverse, perkinetal binary fission; budding or multiple fission occurs in some species. Sexual processes (see AUTOGAMY and CONJUGATION) occur in some species. In ciliates the GENETIC CODE differs from that previously regarded as the ‘universal’ code, and huge evolutionary distances are believed to separate ciliates from other organisms [MS (1986) 3 36–40]. Currently, ciliate taxonomy involves criteria such as the nature of STOMATOGENESIS and of the PARORAL MEMBRANE. Three classes: KINETOFRAGMINOPHOREA (regarded as the most primitive); OLIGOHYMENOPHOREA; POLYHYMENOPHOREA. (See also SILVER LINE SYSTEM.) [Book ref. 135.] Ciliophrys See ACTINOPHRYIDA. cilium (pl. cilia) A thread-like appendage, numbers of which extend from the cells of most members of the CILIOPHORA; the coordinated movements of cilia are involved in locomotion and/or feeding. (Cilia also occur on other organisms and 165 circannual rhythms encodes a component of photosystem II) is expressed least at dawn and maximally at dusk; certain other genes are regulated in the opposite phase. Under conditions of constant light, the timing of cell division is influenced by the endogenous ∼24hour biological clock, even when average doubling time is only 10.5 hours [TIM (1998) 6 407–410]. [Molecular bases for circadian clocks (review): Cell (1999) 96 271–290. Circadian programmes in cyanobacteria: ARM (1999) 53 389–409.] circannual rhythms In certain parameters of a cell or organism: rhythmical changes which occur with a periodicity of ca. one year (see e.g. RED TIDE). CIRCE See HEAT-SHOCK PROTEINS. Circinoviridae See TT VIRUS. circular intensity differential scattering (CIDS) A phenomenon in which left and right circularly polarized light is scattered in a characteristic way by a given type of cell, virus, or molecule. CIDS has been used for the rapid characterization of microorganisms [AEM (1982) 44 1081–1085]. circularly permuted (mol. biol.) Refers to nucleic acid molecules in which the bases or genes are arranged in the same sequence but with different starting points: ABCDEFG, BCDEFGA, CDEFGAB, etc; such molecules can arise e.g. by cleavage at different positions in a given type of circular parental molecule. (See e.g. BACTERIOPHAGE T4.) circulative transmission (of viruses) (plant pathol.) A mode of transmission in which viruses taken up by an insect VECTOR pass from the alimentary canal to the haemolymph and subsequently to the salivary glands and saliva of the insect; viruses which are transmitted in this way are called circulative or persistent viruses. Insects involved in circulative transmission typically carry plant-infective virions for long periods, sometimes for life. Circulative transmission characteristically involves a latent period (several hours to a number of days) during which the vector, having acquired virus from an infected plant, is unable to transmit the virus to a healthy plant. (cf. NON-CIRCULATIVE TRANSMISSION.) Some viruses (e.g. potato yellow dwarf virus, wound tumour virus) replicate in their insect vectors; circulative transmission of such propagative viruses is termed propagative transmission. Other viruses (e.g. barley yellow dwarf virus) apparently do not replicate in their insect vectors; circulative transmission of such non-propagative viruses is accordingly referred to as nonpropagative transmission. circulative viruses See CIRCULATIVE TRANSMISSION. circulins See POLYMYXINS. circumsporozoite protein See PLASMODIUM. cirramycin See MACROLIDE ANTIBIOTICS. cirri See CIRRUS. cirrus (pl. cirri) A discrete group of somatic cilia (several to over 100) which act primarily as a unified locomotive organelle; the typical cirrus is conical. Cirri are characteristic of the HYPOTRICHIDA. (See also COMPOUND CILIATURE.) cis-acting See CIS-DOMINANCE. cis-dominance The phenomenon in which a genetic element (a gene or a non-coding sequence such as a promoter) affects only the DNA molecule in which it occurs; in a COMPLEMENTATION TEST, a mutant form of such a genetic element (which is called a cis-acting element) cannot be complemented by (i.e., is dominant over) the wild-type form of the element present in the same cell. The converse of a cis-acting element is a trans-acting element, an element which encodes a product that can diffuse through the cytoplasm and act on other DNA molecules. For example, in the LAC OPERON, a mutation in the lacI gene may result in an inactive lac repressor and hence in the constitutive expression of the lac genes; the presence of a wild-type lacI gene (e.g. on a plasmid) in the same cell can re-establish repression of the lac genes, i.e., the lacI − allele is recessive to the wild-type lac+ allele, and the lac repressor is said to be trans-acting. A mutation in the lac operator which renders the operator unable to bind the repressor also results in constitutive expression of the lac genes; however, in this case the presence of a functional lac operator on a plasmid in the same cell does not alter the mutant (constitutive) phenotype, i.e., the mutant operator is dominant over the wildtype form, complementation cannot occur between mutant and wild-type forms, and the operator is said to be cis-acting. Although in most cases a cis-acting element is non-coding (e.g. a promoter, an operator, etc), certain proteins are cis-acting; such a protein can function only on the DNA molecule encoding it. For example, certain transposases can act efficiently only on the ends of an appropriate transposable element in the same DNA molecule as the gene which encoded the transposase (see e.g. Tn5 ). Cis-acting proteins are generally strongly basic; it has been suggested that, as soon as it is synthesized (perhaps even before translation is complete), a cis-acting protein may bind to the nearby DNA template and may subsequently move along the DNA until it reaches an appropriate site of action (e.g. the ends of a transposon in the case of a cis-acting transposase). cis–trans test A form of COMPLEMENTATION TEST which actually comprises two separate tests: a cis test and a trans test. The terms cis and trans refer to the arrangement of mutations in the two genomes which are brought together in the same cell to test for complementation; in the trans arrangement one mutation occurs in each genome, while in the cis arrangement both mutations are located in the same genome, the other genome being wild-type. (The doubly-mutant genome used in the cis test is constructed, by recombination, from the two singly-mutant genomes used in the trans test, so that the mutations in the cis arrangement occupy the same loci as those in the trans arrangement.) Positive results in both cis and trans tests (i.e., the wild-type phenotype is restored in both tests) indicate that the two mutations are in different genes and hence that the phenotypic characteristic under study is determined by more than one gene. A negative trans test with a positive cis test indicates that the two mutations are in the same gene (organisms in the cis test having one doubly-mutant copy and one wild-type copy of the gene, those in the trans test having only two mutant copies of the gene). However, if both cis and trans tests are negative, at least one of the mutations must be dominant (see COMPLEMENTATION TEST), i.e., the mutant phenotype is expressed in the cis test organisms despite the presence of a wild-type copy of the gene. The cis test thus provides a control to prevent the possible misinterpretation of a negative (trans) complementation test as indicative of monogenic control when in fact it could be the result of a dominant mutation in one gene together with a second (dominant or recessive) mutation in another gene. cistron (1) A GENE as defined in terms of the CIS–TRANS TEST: i.e., in a diploid cell or merozygote, either of two homologous sequences in a genetic nucleic acid in which two mutations in trans fail to exhibit complete complementation (see COMPLEMENTATION TEST). (2) Syn. GENE. Cit plasmid An enterobacterial plasmid encoding a citrate transport system; the presence of a Cit plasmid in e.g. Escherichia coli confers on the (usually citrate-negative) cells the ability to utilize citrate as sole source of carbon and energy (see CITRATE TEST). 166 Cladosporium C. freundii. Indole −ve; malonate (usually) −ve; H2 S + ve; growth in KCN media. Nitrogen fixation has been reported in some strains. Serotypes may be distinguished on the basis of O and H antigens, some of which are common to serotypes of Escherichia and Salmonella; C. freundii H antigens are monophasic. Certain strains are reported to have a Vi antigen serologically identical to that of Salmonella typhi (see VI ANTIGEN), but its presence is not related to virulence. (See also PHASE VARIATION.) Citrobacter spp may be isolated from human or animal faeces, from various clinical specimens, and from food, water, sewage, soil etc; they may be opportunist pathogens. C. freundii and C. diversus have been associated with cases of human diarrhoea. [Book refs. 22, pp. 458–461, and 46, pp. 1140–1147.] citrovorum factor See FOLIC ACID. L-citrulline See e.g. Appendix IV (a). Citrus exocortis viroid See VIROID. Citrus leaf rugose virus See ILARVIRUSES. Citrus leprosis virus See RHABDOVIRIDAE. Citrus tristeza virus A member of the CLOSTEROVIRUSES which causes ‘tristeza disease’ of citrus trees; the virus has killed millions of orange trees in Brazil. Citrus variegation virus See ILARVIRUSES. CJD CREUTZFELDT–JAKOB DISEASE. CL region See IMMUNOGLOBULINS. Cladobotryum See HYPHOMYCETES and HYPOMYCES. cladogram Any dendrogram which expresses phylogenetic relationships. (cf. PHENOGRAM.) Cladonia A large genus of LICHENS (order LECANORALES); photobiont: a green alga. Species tend to be variable in appearance. There is a granular or squamulose primary thallus (often evanescent, but predominant in some species) from which may arise hollow podetia (see PODETIUM); according to species, podetia may be awl-shaped, branched and shrubby, or funnel-, cup- or wine-glass-shaped, and may bear brown or red hymenial discs (‘apothecia’). Species occur on the ground (particularly on peaty soils in heaths and moors), on walls, on decaying logs, etc. Some species – e.g. C. portentosa (formerly C. impexa), C. rangiferina, C. stellaris – form cushions of extensively branched podetia (‘reindeer moss’) on the ground in mountainous regions of Europe and in Arctic regions, where they may serve as an important source of winter fodder for reindeer and caribou. Cladophora A genus of filamentous, branched, siphonocladous green algae (division CHLOROPHYTA); the filaments are composed of cylindrical, multinucleate cells, the walls of which contain parallel arrays of cellulose microfibrils. An isomorphic alternation of generations occurs; sporophytes produce quadriflagellate, uninucleate zoospores, gametophytes produce biflagellate isogametes. Species occur in freshwater and marine habitats, sometimes free-floating, but usually attached to a substratum by rhizoids; some species often form extensive tangled skeins up to several metres in length (‘blanket weed’). Cladosiphon See PHAEOPHYTA. cladosporiosis Any human or animal disease caused by a species of CLADOSPORIUM. Cladosporium A genus of fungi (class HYPHOMYCETES) which form septate mycelium and dark-coloured, aseptate or septate, ellipsoidal conidia. The organisms include saprotrophs and pathogens of plants and animals. Species include e.g. C. carrionii (see e.g. CHROMOBLASTOMYCOSIS), C. cucumerinum (see e.g. GUMMOSIS), C. herbarum (teleomorph: Mycosphaerella tassiana) (see e.g. BLACK SPOT (sense 1), SAP-STAIN, TEXTILE Citeromyces A genus of yeasts (family SACCHAROMYCETACEAE). Cells reproduce by multilateral budding; neither pseudomycelium nor true mycelium is formed. Asci are persistent and contain one (occasionally two) rough-surfaced spheroidal spores. Glucose is fermented; NO3 − is assimilated. One species: C. matritensis (anamorph: Candida globosa), isolated from e.g. fruit preserved in syrup, tree exudates etc. [Book ref. 100, pp. 117–119.] Citifluor A photofading retardant and mountant used e.g. for epifluorescence microscopy of soil [SBB (1985) 17 739–746]. citrate metabolism See e.g. TCA CYCLE, CIT PLASMID, DIACETYL; see also CITRIC ACID. citrate synthase See Appendix II(a). citrate test An IMVIC TEST which determines the ability of a bacterial strain to use citrate as the sole source of carbon (see e.g. TCA CYCLE). A saline suspension of the test organism is prepared from growth on a solid medium. An inoculum from this suspension is transferred by means of a STRAIGHT WIRE to the test medium (e.g. KOSER’S CITRATE MEDIUM or SIMMONS’ CITRATE AGAR); the wire is dipped (ca. 1 cm) into the suspension and then dipped into, or streaked across, the test medium. Such a small inoculum minimizes carry-over of nutrients and avoids causing turbidity in Koser’s medium. The inoculated medium is incubated and examined daily for evidence of growth (turbidity in Koser’s medium, blue coloration in Simmons’ medium); growth is a positive reaction. citreoviridin A yellow pigment and neurotoxic MYCOTOXIN produced by Penicillium citreoviride growing e.g. on polished rice. (See also YELLOW RICE.) The molecule consists of a hydrofuran ring linked, via a conjugated polyene, to an a-pyrone chromophore; it emits a brilliant yellow fluorescence on UV irradiation. Consumption, by man, of rice contaminated with citreoviridin may be responsible for acute cardiac beriberi : a fatal disease characterized by ascending paralysis, convulsions, and respiratory arrest; these symptoms can be reproduced in animals by feeding them with citreoviridin. In vitro, citreoviridin inhibits PROTON ATPASE isolated e.g. from mitochondrial membranes. citric acid A tricarboxylic acid present e.g. in many fruits (especially citrus fruits) and an important metabolic intermediate in many organisms (see e.g. TCA CYCLE). Citric acid is obtained commercially mainly from strains of Aspergillus niger grown e.g. on molasses or starch hydrolysates; the accumulation of citric acid is apparently due to abnormal functioning of the TCA cycle. The citric acid is separated from the fermentation liquor by precipitation as the Ca2+ salt. Citric acid has many commercial uses – e.g. as an acidifying agent in various foods, beverages and pharmaceuticals. [Production methods etc: Book ref. 8, pp. 709–747.] citric acid cycle Syn. TCA CYCLE. citrinin A yellow pigment and MYCOTOXIN produced by Penicillium spp in mouldy cereals etc. It is nephrotoxic e.g. in pigs. Citrobacter A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Cells: ca. 1 × 2–6 µm, usually motile. MR +ve; VP −ve citrate +ve; acid and gas from glucose; lactose +ve or −ve, or fermented slowly. Growth usually occurs on/in selective media such as DCA, SS agar, tetrathionate broth and selenite broth. GC%: 50–52. Type species: C. freundii. C. amalonaticus (formerly e.g. C. intermedius biotype a; Levinea amalonatica). Indole +ve; malonate −ve; H2 S −ve; growth in KCN media. C. diversus (= C. koseri ; formerly e.g. C. intermedius biotype b; Levinea malonatica). Indole +ve, malonate +ve and H2 S −ve; no growth in KCN media. 167 Cladostephus clavems A class of antibiotics; a clavem contains a clavam-like nucleus in which a double bond occurs between C-2 and C-3 (see b-LACTAM ANTIBIOTICS, Figure (f)). Clavibacter A genus of Gram-positive, asporogenous, non-acidfast, non-motile, obligately aerobic, phytopathogenic coryneform bacteria; C. xyli subsp. xyli causes ratoon stunting disease of sugarcane, C. xyli subsp. cynodontis causes Bermuda-grass stunting disease. [IJSB (1984) 34 107–117.] Claviceps A genus of fungi (order CLAVICIPITALES) which are typically parasitic on grasses (see e.g. ERGOT; see also PASPALUM STAGGERS). C. purpurea. The cycle of infection begins when ascospores germinate and infect the ovaries of a susceptible grass. The fungus soon produces small, colourless conidia (the Sphacelia stage) which become suspended in a HONEYDEW secretion (possibly produced by the plant in response to infection); insects, attracted by the honeydew, disperse the conidia to the ovaries of uninfected grasses. Later, the fungus produces a SCLEROTIUM (q.v.) (the ergot, the overwintering stage) in the position normally occupied by the grain. In spring or early summer the sclerotia germinate, each giving rise to several erect, drum-stick-like stromata; perithecia develop in the surface layer of each stromal head, their ostiolar pores level with the stromal surface. Each ascus contains 8 thread-like ascospores. Clavicipitales An order of fungi (subdivision ASCOMYCOTINA) which include entomogenous species, and species parasitic on other fungi or on grasses. Ascocarp: perithecioid, superficial or immersed in a stroma. Asci: unitunicate, cylindrical, with a thick apical cap. Ascospores: elongated to filiform, often multiseptate, sometimes fragmenting. Genera: e.g. Acrospermum, Balansia, CLAVICEPS, CORDYCEPS, EPICHLOË, Hypocrella, HYPOMYCES. claviformin Syn. PATULIN. clavine alkaloids See ERGOT ALKALOIDS. Clavispora A genus of yeasts (family SACCHAROMYCETACEAE) which reproduce by multilateral budding; pseudomycelium may be formed. Asci are formed following conjugation between cells of different mating type. Ascospores: clavate, minutely warty, 1–4 per ascus. Glucose is fermented; NO3 − is not assimilated. One species: C. lusitaniae (anamorph: Candida lusitaniae), isolated e.g. from pigs and from human skin and sputum. [Book ref. 100, pp. 120–122.] clavulanic acid A clavam (see b-LACTAM ANTIBIOTICS for structure). Clavulanic acid, produced e.g. by Streptomyces clavuligerus (ATCC 27064), binds to the PENICILLIN-BINDING PROTEINS of Escherichia coli (preferentially to PBP-2) but has only weak antibacterial activity; however, it is a potent inhibitor of many b-LACTAMASES from Gram-positive and Gram-negative bacteria and may thus be used in combination with b-lactamase-sensitive antibiotics (see e.g. AUGMENTIN and TIMENTIN). Clavulanic acid is unstable as the free acid and is isolated from aqueous solutions as a salt. CLB (coccidian-like body) An organism linked with prolonged diarrhoea (duration: weeks) in humans; it appears to be waterborne. [Epidemiology: Lancet (1993) 341 1175–1179.] cleared lysate The supernatant obtained when bacteria in suspension are lysed and centrifuged for ca. 10–20 min at ca. 20000 g; it may contain plasmids (if present in the cells) but is generally free of particulate cell debris and chromosomal DNA. clearing In preparing a specimen for microscopy: the stage in which the dehydrating agent (usually ethanol) is replaced by a solvent (clearing agent ) such as benzene, xylene, or terpineol; if the cleared specimen is to be mounted in a MOUNTANT or impregnated with paraffin wax, the clearing agent must be SPOILAGE), and C. oxysporum (a pathogen of homopteran insects). (See also BLACK YEASTS, MEAT SPOILAGE, PAINT SPOILAGE, PAPER SPOILAGE.) For C. bantianum see XYLOHYPHA. For C. fulvum see FULVIA. For C. resinae see HORMOCONIS. Cladostephus See PHAEOPHYTA and PHEROMONE. clamp connection (clamp; clamp cell) In the dikaryotic mycelia of many (not all) basidiomycetes: a hyphal structure formed during vegetative cell division for ensuring that the daughter nuclei of a given nucleus are segregated to different cells. Prior to MITOSIS a dikaryotic cell develops a short, angled, lateral branch (the incipient clamp connection) into which one of the nuclei migrates. Both nuclei then undergo simultaneous mitotic division (conjugate division) – the spindles being orientated such that, subsequently, one end of the cell contains one daughter nucleus from each of the two parental nuclei; this pair of nuclei is then isolated from the rest of the cell by a septum (which thus delimits a heterokaryotic binucleate cell), while a second septum isolates the single daughter nucleus in the developing clamp connection. The clamp hypha now grows towards – and fuses with – the mononucleate compartment of the original (parental) cell, the clamp connection thus being completed, and a second heterokaryotic binucleate cell being concomitantly formed. The clamp connection persists as a hyphal bulge in the region of septation. A medallion clamp (formed e.g. by species of Lentinus, Lenzites, Poria and Polyporus) develops when only the terminal part of the clamp hypha fuses with the mononucleate cell, a small space or eyelet remaining between the clamp hypha and the parent hypha. Parallels have been drawn between clamp connections and the croziers formed during ASCUS development. clarithromycin A MACROLIDE ANTIBIOTIC used e.g. as part of a combination of antibiotics for treating disease caused by Helicobacter pylori ; this macrolide is more acid-stable (and is absorbed more readily) than e.g. erythromycin. In H. pylori, resistance to macrolide antibiotics has been associated with point mutations in 23S rRNA [AAC (1996) 40 477–480]. class A taxonomic rank (see TAXONOMY and NOMENCLATURE). class I/class II antigens See MAJOR HISTOCOMPATIBILITY COMPLEX. class I/class II introns See SPLIT GENE (b) and (c). class switching See ANTIBODY FORMATION. clathrin See PINOCYTOSIS. Clathrochloris See CHLOROBIACEAE. Clathrulina A genus of freshwater protozoa (order DESMOTHORACIDA). The mature organism is attached to e.g. vegetation by a tubular stalk-like extension (ca. 200–350 µm long) of the capsule; the capsule (ca. 60–90 µm diam.) has circular to polygonal perforations through which axopodia protrude. The cell reproduces by the formation of biflagellate zoospores which lack both stalk and capsule. These cells initially form filopodia (rather than true axopodia); subsequently a stalk, a capsule, and finally axopodia, are produced. Clathrus See PHALLALES. clavacin Syn. PATULIN. clavams A class of b-LACTAM ANTIBIOTICS which includes CLAVULANIC ACID. Clavaria See APHYLLOPHORALES (Clavariaceae). Clavariaceae See APHYLLOPHORALES. clavate Club-shaped, i.e. thicker at one end. clavate cilium (stereocilium) A short cilium whose axoneme lacks the central pair of microtubules; it does not flex. Clavate cilia occur e.g. in Didinium and in the scopula of some peritrichs. clavatin Syn. PATULIN. 168 cloning or for synthesis of the product encoded by the sequence (see EXPRESSION VECTOR). Initially, the required gene (or other sequence) is inserted (‘spliced’) into a REPLICON – often a PLASMID or a phage genome. The replicon functions as a vector (= cloning vector ), i.e. a molecule which, on being replicated in a cell, brings about the replication of the given gene (as the latter is an integral part of the vector). The insertion of a gene into a plasmid vector is carried out in a procedure involving a RESTRICTION ENDONUCLEASE and a ligase (see figure). Vector molecules, each incorporating the given gene, are introduced into cells in which they can replicate; plasmid vectors are introduced into bacteria by TRANSFORMATION or ELECTROPORATION. The cells, containing vectors, are then cultured; because a bacterial population can reach very high numbers in culture, and because each cell contains one or more copies of the vector, large numbers of vector molecules (each containing the given gene) are obtained – i.e. the gene has been cloned. To harvest the cloned gene, the cells are lysed and the vector molecules are isolated e.g. by isopycnic centrifugation (cf. QIAGEN PLASMID KIT). The cloned gene is then cut from each vector molecule (by means of the original restriction enzyme), and copies of the cloned gene are separated from vector DNA e.g. by gel electrophoresis. Cloning with plasmid vectors in bacteria. During the initial stage (see figure), one potential problem is that some plasmid molecules may re-circularize without incorporating a copy of the gene; such plasmids may then be taken up by transformation but, lacking the gene insert, they would not contribute to the cloning process. This difficulty may be addressed by pre-treating the cleaved (linearized) plasmids with a phosphatase to remove the 5
-phosphate groups which are necessary for ligase action and re-circularization. When the gene (target) fragments are added, their 5
-phosphate groups can be ligated to 3
-OH ends in the plasmids, resulting in stable, circular hydrid plasmids which contain single-stranded nicks (at the 5
-OH ends). These hybrid plasmids can then be used for transformation – the nick sites being repaired in vivo to yield ccc dsDNA molecules. Sometimes the target fragment lacks restriction sites, or has incompatible ones. This problem may be addressed as follows. Sticky ends, if present, are changed to blunt ends (see BLUNT-ENDED DNA); this may be achieved by either removing single-stranded overhangs (e.g with ENDONUCLEASE S1) or by synthesizing a complementary strand on each 5
overhang (thus converting 5
overhangs to dsDNA). To each blunt end can now be ligated a short, blunt-ended dsDNA molecule – a linker – which contains a suitable restriction site; one example of a linker is: 5
-GGAATTCC-3
3
-CCTTAAGG-5
miscible with both the dehydrating agent and the mountant or wax solvent. The refractive index of clearing agents is higher than that of ethanol, so that the process of clearing increases the transparency of the specimen. CLED medium Cystine-lactose-electrolyte-deficient medium: a medium used for the culture of organisms in the absence of electrolyte (or NaCl). With added colistin, it may be used for the primary isolation of members of the PROTEEAE, since swarming is inhibited in the absence of NaCl. cleistocarp Syn. CLEISTOTHECIUM. cleistothecium (cleistocarp) A hollow, closed, typically spherical or spheroid ASCOCARP in which develop one or more asci (see ASCUS and ASCOSPORE). In some species the cleistothecial wall (peridium) is a more or less rigid pseudoparenchymatous structure; in others the wall may be little more than an envelope of loosely woven hyphae. In e.g. Erysiphe spp the cleistothecia are ca. 100–200 µm in diameter. clindamycin See LINCOSAMIDES. Clitocybe See AGARICALES (Tricholomataceae). cloacin DF13 A group A colicin (see COLICINS) (MWt ca. 56000) produced by strains of Enterobacter cloacae and Escherichia coli which harbour the CloDF13 plasmid. [Proposed role for protein H (the lysis protein) in cloacin DF13 export: JGM (1986) 132 825–834.] In E. coli the receptor for cloacin DF13 is the IutA protein (which also serves as a receptor for iron–aerobactin complexes – see SIDEROPHORES); translocation across the cell envelope of the target cell is reported to require the TolAQR proteins. Within a sensitive cell, cloacin DF13 acts as an RNase, specifically cleaving 16S rRNA – thereby inhibiting ribosome function and protein synthesis. CloDF13 plasmid See CLOACIN DF13 and COLE1 PLASMID. clofazimine (B663; Lamprene) A lipid-soluble substituted phenazine used e.g. as an anti-LEPROSY drug; in Mycobacterium leprae it binds to guanine residues in DNA and may inhibit transcription. clonal abortion (immunol.) See IMMUNOLOGICAL TOLERANCE. clonal deletion (immunol.) See IMMUNOLOGICAL TOLERANCE. clonal expansion See e.g. T LYMPHOCYTE. (Clonal expansion also occurs in B cells when they react to specific antigen.) clonal selection theory A currently accepted theory of ANTIBODY FORMATION. Essentially, it proposes that particular cells (within a cell population) respond to a specific antigen by binding the antigen on pre-existing specific cell-surface receptors (antibodies); binding promotes proliferation of these cells to form an enlarged clone of cells which are responsive to that antigen – some of the cells secreting antibodies and others acting as ‘memory’ cells which can produce antibodies on later exposure to the antigen. IMMUNOLOGICAL TOLERANCE in respect of certain antigens is explainable as the loss of particular clones due to exposure to self-antigens, or external antigens, at an early stage in the development of the immune system. (cf. INSTRUCTIVE THEORIES OF ANTIBODY FORMATION.) clone (1) (noun) A population of cells or organisms derived asexually from a single progenitor. A clone is generally assumed to be genetically homogeneous. (2) (verb) To carry out CLONING (of DNA). cloned line (tissue culture) A population of cells descended directly from a single CLONE. (cf. CELL LINE.) cloning (of DNA) A procedure (also called gene cloning or molecular cloning) used for obtaining millions of copies of a given gene (or other piece of DNA); copies of the (cloned) DNA may be used e.g. for sequence analysis (see DNA SEQUENCING) which contains the EcoRI restriction site (see table in RESTRICBecause a linker is a PALINDROMIC sequence is the same in both strands) either end can be ligated. Blunted-ended ligation requires a higher concentration of enzyme, a lower temperature and a longer time. (See also ADAPTOR.) Following transformation, only cells which have actually received a plasmid can contribute to the cloning process. These cells can be selected for if the plasmid vector contains an antibiotic-resistance gene; thus, if the transformed cells are TION ENDONUCLEASE). SEQUENCE (the 5
-to-3
169 GA AT C C (a) plasmid C T T A G G T GA A T AA TT CT A TC AA TT G C A TA 'foreign' DNA G restriction endonuclease Eco RI restriction endonuclease Eco RI G T CT G A AT T C (b) A AT AA TC G G C TT G AA AA C T T G CT TAA (c) G A AT T C C TTA A G ligase (d) hybrid plasmid CLONING: insertion of a gene (or other ‘foreign’ DNA) into a plasmid vector (diagrammatic). (a) A plasmid and a piece of ‘foreign’ DNA. Both molecules contain the nucleotide sequence GAATTC which is recognized by the RESTRICTION ENDONUCLEASE EcoRI; in the foreign DNA, the sequence to be cloned lies between the two restriction sites. EcoRI cuts between the guanosine (G) and adenosine (A) nucleotides, as shown by the arrows. (b) As a result of EcoRI activity, the plasmid has been linearized, and both types of molecule now have ‘sticky ends’, i.e. terminal, single-stranded complementary sequences of nucleotides (5
-AATT). (c) The foreign DNA has integrated with the plasmid vector molecule through base-pairing between complementary nucleotides in the sticky ends. (d) An enzyme, DNA ligase, has catalysed a phosphodiester bond between the sugar residues of each G and A nucleotide, forming a hybrid plasmid (recombinant plasmid; also referred to as hybrid DNA, or hDNA). Note that, unless prevented, at least some of the vector molecules may simply re-circularize by base-pairing between their sticky ends; such re-circularization, which would reduce the efficiency of the procedure, may be prevented by treating the linearized plasmids with phosphatase (see entry). A population of hybrid plasmids can be inserted into bacteria by transformation or electroporation for cloning. Note that, if required, the (cloned) fragment can be cut from the vector by using the same restriction enzyme. Reproduced from Bacteria 5th edition, Figure 8.6, page 182, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 170 Clostridium plated on a medium containing the relevant antibiotic, only the plasmid-containing cells (which are resistant to the antibiotic) will form colonies. Some vectors are particularly useful in that, following transformation, they can signal the uptake of both vector and DNA insert. For example, in plasmid pBR322 (see PBR322), the tetracycline-resistance gene contains a BamHI restriction site, and the ampicillin-resistance gene contains a PstI restriction site; if, for example, PstI is used to insert the gene (analogous to EcoRI in the figure), then any transformant which is (i) sensitive to ampicillin but (ii) resistant to tetracycline may be presumed to contain the complete hybrid plasmid. A different approach to selection is provided by REPRESSOR TITRATION. If a plasmid vector replicates under relaxed control (see PLASMID), the number of plasmids per cell can be greatly increased by treating the cells with chloramphenicol; in this way, the yield of cloned product can be maximized. Cloning with phage vectors in bacteria. In some phages (e.g. BACTERIOPHAGE LAMBDA) the genome contains a fairly high proportion of ‘non-essential’ DNA; thus, a sequence may be excised from the genome, using restriction endonucleases, and replaced by the fragment to be cloned. Such a vector is called a REPLACEMENT VECTOR; using the genome of phage lambda as a replacement vector it is possible to clone a fragment of about 20 kb (compared with <10 kb in plasmid pBR322). Even larger fragments, up to ∼40 kb, can be cloned in a COSMID. Phage genomes can be used as vectors only if the recombinant genome (i.e. genome plus insert or fragment) is not too large to fit into the phage head, and only if it contains, where necessary, the appropriate packaging signals. Once incorporated into a phage particle, the recombinant genome can be injected into bacterial cells and will replicate like a plasmid. (Note that, during construction of a phage vector, any sequences in the phage genome that promote integration into the bacterial chromosome should be removed.) ssDNA phages may also be used as vectors; filamentous phages have the advantage that their virions can accomodate a wide range of sizes of target DNA. Another option for cloning is a hybrid vector which includes sequences from a phage and a plasmid: see PHAGEMID. Characterization of clones. Sometimes, clones are prepared from a mixture of DNA fragments or from a LIBRARY, i.e. different cells within the culture will contain vectors that incorporate different fragments of DNA. In such cases one can screen for cells that contain a particular gene or sequence. Screening methods include: COMPLEMENTATION analysis (in which cells are selected when their particular fragment of cloned DNA complements a given chromosomal defect); probing for a specific gene product (e.g. by using labelled antibodies); or hybridization techniques (see e.g. COLONY HYDRIDIZATION). Linguistic note. The above account of cloning should make it clear that a gene (or other sequence) is inserted into a vector, and that a vector (and its insert) is cloned in a population of cells. Unfortunately, the expression ‘cloned into.. . .’ has become widely used in the literature; this piece of gibberish has been promulgated by mindless repetition. A given sequence of DNA may be cloned in a particular vector molecule or cloned in a particular type of cell. Clonothrix A genus of IRON BACTERIA which occur as sheathed, tapering filaments. Type species: C. fusca. clorobiocin An ANTIBIOTIC, obtained from Streptomyces spp, closely related to NOVOBIOCIN and COUMERMYCINS – which it resembles in its action. closed complex See TRANSCRIPTION. closed culture Syn. BATCH CULTURE. closed mitosis See MITOSIS. Closterium A large, morphologically diverse genus of placoderm DESMIDS. The cells are commonly more or less crescent-shaped; the cell is not constricted (i.e. there is no sinus), but the semicells are separated by a median suture line. closteroviruses (beet yellows virus group) A group of PLANT VIRUSES in which the virions are highly flexuous filaments, typically 1200–2000 × 12 nm (some possible members are ca. 600–800 × 12 nm) containing one type of coat protein and one molecule of linear positive-sense ssRNA (MWt ca. 2.2–4.7 × 106 ). In general, the viruses have a fairly wide host range, and some are important pathogens of crop plants. At least some members are transmitted semipersistently by aphids; mechanical transmission can be achieved, often with difficulty, under experimental conditions. In the host plant the virions often aggregate in cross-banded masses in phloem cells. Type member: SUGAR BEET YELLOWS VIRUS (SBYV). Other members include e.g. beet yellow stunt virus, burdock yellows virus, carnation necrotic fleck virus, carrot yellow leaf virus, CITRUS TRISTEZA VIRUS, clover yellows virus, Festuca necrosis virus, wheat yellow leaf virus. Possible members include e.g. apple chlorotic leafspot virus, apple stem grooving virus, and POTATO VIRUS T. Clostridium A genus of ENDOSPORE-forming, chemoorganotrophic, obligately anaerobic (or, in a few cases, aerotolerant) bacteria which cannot carry out dissimilatory sulphate reduction (cf. DESULFOTOMACULUM). Species occur widely e.g. in soil and mud, and in the intestines of man and other animals; some species are saprotrophic, others are opportunistic pathogens. Cells: typically rod-shaped, ca. 0.3–1.9 × 2–10 µm, but may vary from coccoid to filamentous or (in C. cocleatum and C. spiroforme) helical; the Gram reaction is typically positive (though it may become negative in old cultures) but some species (e.g. C. clostridiiforme, C. ramosum) are commonly Gram-negative even in overnight cultures. Most species are peritrichously flagellate; some (e.g. C. innocuum, C. perfringens, C. ramosum) are non-motile. The endospores are usually spherical to ovoid/elongate; they typically distend the sporangium, and may be located terminally or subterminally. Except in rare cases, clostridia are catalase-negative. Many species grow poorly on basal media and require e.g. paminobenzoic acid (PABA), biotin and/or pantothenate. Media used for the culture of clostridia include e.g. blood agar, cooked meat medium, and egg-yolk agar. (See also MCCLUNG TOABE EGGYOLK AGAR and REINFORCED CLOSTRIDIAL MEDIUM.) SWARMING is common in some strains. Metabolism is generally described as ‘fermentative’; however, some strains (e.g. of C. perfringens) are capable of NITRATE RESPIRATION. (See also STICKLAND REACTION.) The growth of saccharolytic clostridia is improved by, or is dependent on, the presence of a fermentable carbohydrate; most saccharolytic species form acetic acid and/or butyric acid as a major product (see also BUTYRIC ACID FERMENTATION), although some form, instead, products such as formic and lactic acids and/or ethanol. [Genetics and biochemistry of Clostridium relevant to fermentations: AAM (1986) 31 1–60.] The proteolytic clostridia digest cooked meat, casein and/or other protein, and generally grow well in e.g. nutrient broth without the addition of carbohydrate; at least some of these species can derive 171 Clostridium energy by the STICKLAND REACTION, and in some species this is the preferred mode of energy metabolism. Certain species (e.g. C. perfringens, C. sordellii ) are both proteolytic and saccharolytic, while others are restricted to specific substrates (see e.g. C. cochlearium, C. cylindrosporum and C. kluyveri ). Some species can reduce nitrate. Some clostridia are thermophilic, others are psychrophilic, but most are mesophilic. GC% range for the genus: ca. 22–55. Type species: C. butyricum. Currently, >80 species (some listed below) – but see end of entry. C. aceticum. Saccharolytic; diazotrophic; acetic acid is formed from CO2 and H2 . (See also ANAEROBIC DIGESTION.) C. acetobutylicum. Proteolytic and saccharolytic; growth requirements appear to include PABA and biotin. (See also ACETONE–BUTANOL FERMENTATION.) C. acidiurici. A species metabolically similar to C. cylindrosporum (q.v.) C. barati. Incorporates the former species C. perenne and C. paraperfringens [IJSB (1982) 32 77–81]. (See also BOTULINUM TOXIN.) LIPASE −ve, LECITHINASE +ve. C. barkeri. Saccharolytic; butyric and lactic acids, CO2 and H2 are formed from carbohydrates. C. beijerinckii. Saccharolytic: butyric and acetic acids and butanol are formed from glucose. C. bifermentans. Proteolytic and saccharolytic; requires e.g. biotin, nicotinamide, pantothenate, pyridoxine and various amino acids. Spores: ovoid, subterminal. Indole-positive. The organisms form a lecithinase similar to the a-toxin of C. perfringens. C. botulinum. The causal agent of BOTULISM (see also BOTULINUM TOXIN). Cells: motile, ca. 0.3–1.9 × 3.4–9.4 µm; spores: ovoid, subterminal. All strains can ferment glucose (none can use lactose, mannitol or sucrose), and all form metabolic products which include acetic and butyric acids. All strains can hydrolyse gelatin and produce lipases. The organisms are divided into seven types (designated A, B, C, D, E, F and G), strains of a given type producing a BOTULINUM TOXIN which is antigenically distinct from those of the other types; in some cases the strains of a given type are metabolically heterogeneous: e.g., type B strains include some which are saccharolytic as well as some which are both proteolytic and saccharolytic. Minimum growth temperatures: often ca. 10° C, but ca. 3.3° in saccharolytic type B strains and in type E strains. Minimum pH for growth is sometimes reported to be ca. 4.6 for types A and B, and ca. 5.0 for type E; under certain conditions growth and toxin formation may occur at pH values down to ca. 4.0 [Nature (1979) 281 398–399]. The heat-resistance of endospores is strain-dependent; thus, e.g., for types A and B the D112.8 (see D VALUE) is ca. 0.15–1.32 min, for type C the D100 is ca. 0.12 min (terrestrial strains) or ca. 0.4–0.9 min (marine strains), and for type E the D80 is ca. 0.33–3.3 min (in media) or ca. 1.6–4.3 min (in fish) [Book ref. 30, p. 327]. C. botulinum type A strains are proteolytic and saccharolytic; they cause botulism in man and other animals (see also LIMBERNECK). C. botulinum type B strains are either saccharolytic or proteolytic and saccharolytic; they cause botulism in man and other animals. C. botulinum type C strains are proteolytic and saccharolytic; they cause botulism in animals: see e.g. FORAGE POISONING, MIDLAND CATTLE DISEASE, WESTERN DUCK DISEASE. C. botulinum type D strains are proteolytic and saccharolytic; they cause botulism in animals (see LAMZIEKTE). C. botulinum type E strains are saccharolytic; they are often associated with fish and fish products, and they cause botulism in man. [Effect of oxygen and REDOX POTENTIAL on growth of C. botulinum type E: Food Mic. (1984) 1 277–287.] C. botulinum type F strains are saccharolytic, or proteolytic and saccharolytic. A comparatively rare cause of botulism in man. C. botulinum type G strains are either proteolytic or neither proteolytic nor saccharolytic; they have been isolated e.g. from autopsy material. Strains of C. botulinum are also divided into three metabolic groups. Group I includes proteolytic strains of types A, B, F and G. Group II includes all strains of types C and D. Group III includes non-proteolytic strains of types B, E and F. C. butyricum. Saccharolytic, requiring biotin for growth. Ferments many carbohydrates, including starch, producing acetic and butyric acids, CO2 and H2 . Gelatin is not hydrolysed. No extracellular lecithinases or lipases are formed. Cells: motile. Spores: ovoid, subterminal. C. cadaveris. Saccharolytic, acetogenic; some strains are also proteolytic. Indole-positive. Spores: ovoid, terminal. C. carnis. Saccharolytic; aerotolerant. C. cellulolyticum. Cellulolytic; mesophilic; isolated from decayed grass. Spores: spherical, terminal. [IJSB (1984) 34 155–159.] C. chauvoei. Saccharolytic, forming acetic and butyric acids; some strains hydrolyse gelatin. Spores: ovoid, subterminal. Toxins: e.g. a-toxin (dermonecrotizing, haemolytic); b-toxin (DNase); g-toxin (hyaluronidase); d-toxin (a THIOL-ACTIVATED CYTOLYSIN). (See also BLACKLEG and MALIGNANT OEDEMA.) C. clostridiiforme. Saccharolytic, forming acetic and lactic (but not butyric) acids, CO2 and H2 . Some strains form indole. Cells typically stain Gram-negatively; spores (ovoid, subterminal) are seldom formed. C. cochlearium. Non-saccharolytic; glutamate, glutamine or histidine are metabolized to acetic and butyric acids, CO2 and H2 . C. cocleatum. Saccharolytic; carbohydrates are fermented to acetic (but not butyric) acid. Cells: helical. C. colinum. See QUAIL DISEASE. C. cylindrosporum. Substrates include xanthine and other purines, but not sugars or amino acids; metabolic products include acetic and formic acids and glycine. C. difficile. Saccharolytic; ferments glucose or mannitol, forming e.g. acetic, butyric, isobutyric and isocaproic acids. Indole-negative. No extracellular lecithinases or lipases are formed. Some strains hydrolyse gelatin. Cells: Gram-positive or Gram-variable; on cycloserine–cefoxitin–egg-yolk–fructose agar (used for primary isolation from faeces) colonies exhibit a yellow-green fluorescence under light of wavelength 360 nm. Some strains are non-motile. Ovoid spores are formed subterminally; spores are reported to occur at both ends of the rod to give a ‘dumb-bell’ appearance [Book ref. 107, pp. 327–346]. Toxinogenic strains can cause COLITIS and PSEUDOMEMBRANOUS COLITIS. [PCR ribotyping of C. difficile in hospitals in England and Wales: RMM (1997) 8 (supplement 1) S55–S56.] C. histolyticum. Proteolytic; gelatin is hydrolysed, and milk is digested. Indole-negative. Some strains can grow aerobically on blood-agar plates. Spores: ovoid, subterminal. Toxins include the a-toxin (dermonecrotizing); b-toxin (collagenase); d-toxin (a non-thiol-activated ELASTASE); and e-toxin (a THIOL-ACTIVATED CYTOLYSIN). (See also CLOSTRIPAIN.) C. innocuum. Saccharolytic; ferments glucose, mannitol and salicin (and sucrose in some strains), forming acetic and butyric acids. Cells non-motile. Spores: ovoid, terminal. 172 Clostridium C. propionicum. See PROPIONIC ACID FERMENTATION. C. ramosum (formerly e.g. Ramibacterium ramosum). Saccharolytic; ferments glucose, maltose, lactose, sucrose and salicin, forming acetic acid as a main product. Cells frequently stain Gram-negatively and are non-motile; spores: spherical to ovoid, terminal, but seldom observed. C. septicum. Proteolytic and saccharolytic; glucose, maltose, lactose and salicin are fermented with the formation of acetic and butyric acids, and gelatin is hydrolysed. Spores: ovoid, subterminal. Toxins formed: a-toxin (dermonecrotizing, haemolytic), b-toxin (a DNase), g-toxin (a hyaluronidase) and d-toxin (a THIOL-ACTIVATED CYTOLYSIN). A causal agent in GAS GANGRENE and in BRAXY and MALIGNANT OEDEMA. (See also NEUTROPENIC ENTEROCOLITIS.) C. sordellii. Proteolytic and saccharolytic; glucose and maltose are fermented to e.g. acetic acid. Gelatin is hydrolysed, and milk is digested. Indole-positive. Urease-positive. Some strains form a lecithinase (a-toxin) and e.g. a THIOL-ACTIVATED CYTOLYSIN. [Taxonomy of lecithinase-negative strains: JGM (1985) 131 1697–1703.] Spores: ovoid, subterminal. (See also MALIGNANT OEDEMA.) C. sphenoides. Saccharolytic; glucose, maltose, salicin and mannitol are fermented to e.g. acetic acid. Reported to be capable of growth on citrate. Some strains hydrolyse gelatin and some produce indole. Cells typically stain Gram-negatively. Spores: spherical, subterminal to terminal. C. spiroforme. Saccharolytic; carbohydrates are fermented to acetic (not butyric) acid. Cells: helical. C. sporogenes. Proteolytic and saccharolytic; glucose and maltose are fermented to e.g. acetic and butyric acids. Gelatin is hydrolysed, and milk is digested. Spores: ovoid, subterminal. C. subterminale. Proteolytic; gelatin is hydrolysed, and milk is digested. Lysine is fermented to e.g. acetic and butyric acids. Spores: ovoid, subterminal. C. tertium. Saccharolytic; glucose, maltose, lactose, sucrose, salicin and mannitol are fermented to acetic and butyric acids. Spores: ovoid, terminal. Growth occurs aerobically on blood agar. C. tetani. The causal agent of TETANUS. (See also TETANOSPASMIN and THIOL-ACTIVATED CYTOLYSIN.) Proteolytic; gelatin is hydrolysed, and acetic and butyric acids are formed from the fermentation of peptones. No carbohydrates – not even glucose – are fermented. Some strains form indole. Spores: spherical and terminal. C. thermoaceticum. See ACETOGENESIS. C. thermocellum. Saccharolytic; cellobiose, CELLULOSE and hemicellulosic materials are degraded and fermented, the main products being ethanol, acetic acid, CO2 , and H2 . (See also CELLULASES and CELLULOSOME.) [Regulation of cellulase formation: JGM (1985) 131 2303–2308.] Pentoses are apparently not utilized. Optimum growth temperature: 55–60° C. C. thermolacticum. Saccharolytic; thermophilic. A wide range of carbohydrates (including xylan) can be fermented, the main product being L(+)lactate. Some strains are cellulolytic. [SAAM (1985) 6 196–202.] C. thermosaccharolyticum. Saccharolytic; many carbohydrates (including e.g. GLYCOGEN, PECTINS and STARCH) are fermented to e.g. acetic, butyric and lactic acids, CO2 , and H2 . Optimum growth temperature: 55–60° C. Spores may have a D VALUE of ca. D120 = 4 min; the organisms are sometimes responsible for hard SWELL spoilage of canned foods. (See also CANNING.) C. thermosulfurogenes. Saccharolytic. Cells are peritrichous, Gram −ve; filaments may occur. Spores: spherical. Opt. growth C. kluyveri. Non-saccharolytic, non-proteolytic; a mixture of ethanol, acetic acid and bicarbonate is metabolized to butyric and caproic acids and H2 . Biotin and PABA are required for growth. C. limosum. Proteolytic; gelatin is hydrolysed, and milk is digested. An extracellular lecithinase is formed. Spores: ovoid, subterminal. C. nigrificans. See DESULFOTOMACULUM. C. novyi (= C. oedematiens). Proteolytic and saccharolytic. Types A, B, C and D are distinguished on the basis of the toxins they produce; types A and B are involved in human diseases (see e.g. GAS GANGRENE). (See also BLACK DISEASE and MALIGNANT OEDEMA.) Both A and B types ferment glucose, hydrolyse gelatin, and form the dermonecrotizing a-toxin. Type A also forms the g-toxin (a phospholipase C) and e-toxin (a lipase), and type B forms the b-toxin (a lethal dermonecrotizing phospholipase C), the z-toxin (a haemolysin), and the h-toxin (a tropomyosinase). Type D forms e.g. the b- and h-toxins. Some strains are non-motile. Spores: ovoid, subterminal. C. oedematiens. See C. novyi. C. paraperfringens. See C. barati. C. paraputrificum. Saccharolytic; ferments glucose, lactose, maltose or salicin, forming acetic and butyric acids. Spores: ovoid, terminal. C. pasteurianum. Saccharolytic; many carbohydrates are fermented to acetic and butyric acids, CO2 and H2 . Requires e.g. PABA and biotin. Diazotrophic (see NITROGEN FIXATION). C. perenne. See C. barati. C. perfringens (formerly C. welchii ). Proteolytic and saccharolytic; gelatin is hydrolysed, milk is digested, and glucose, maltose, lactose and sucrose are fermented to acetic and butyric acids. Some strains can use nitrate as electron acceptor with consequent increase in growth yield and the production of acetic rather than butyric acid. Indole-negative. Cells non-motile; on blood agar (made with human, rabbit or sheep blood) the colonies of haemolytic strains are each surrounded by a double zone of haemolysis: an inner zone of complete haemolysis, and an outer zone of incomplete haemolysis. Spores: ovoid, subterminal, but rarely formed; the spores have been reported to have a D VALUE (D90 ) of 15–145 min for non-haemolytic strains, and 6–7 min for haemolytic strains [Book ref. 30, p. 327]. Strains of C. perfringens collectively form a range of toxins which include: the a-toxin (PHOSPHOLIPASE C: a LECITHINASE; see also HOT–COLD LYSIS); the b-, e- and i-toxins (all dermonecrotizing toxins); the d-toxin (a lethal, haemolytic toxin); the g- and htoxins (both lethal, non-haemolytic toxins); the q-toxin (a THIOLACTIVATED CYTOLYSIN: see also STREPTOLYSIN O); the m-toxin (a HYALURONATE LYASE); the k-toxin (a COLLAGENASE). On the basis of toxin production, strains of C. perfringens are divided into five types, designated A, B, C, D and E: C. perfringens type A strains form the a-toxin; they are the commonest cause of clostridial FOOD POISONING, are often present in the lesions of GAS GANGRENE, and can cause haemorrhagic enteritis in young piglets. C. perfringens type B strains form the a-, b- and e-toxins, and cause e.g. LAMB DYSENTERY. C. perfringens type C strains form the a- and b-toxins, and cause e.g. ENTERITIS NECROTICANS in man, STRUCK in sheep, and a fatal necrotic and haemorrhagic enteritis in young piglets. C. perfringens type D strains form the a- and e-toxins, and cause e.g. PULPY KIDNEY in ruminants. C. perfringens type E strains form the a- and i-toxins; they are associated with diseases in ruminants. 173 clostripain at >60° C. In microbial mats from hot volcanic springs. [JGM (1983) 129 1149.] C. villosum. Isolated from subcutaneous abscesses in cats [IJSB (1979) 29 241–244]. C. welchii. See C. perfringens. Other species include e.g. C. aurantibutyricum, C. celatum, C. cellobioparum, C. coccoides, C. durum, C. fallax, C. felsineum, C. formicoaceticum, C. glycolicum, C. haemolyticum, C. indolis, C. lituseburense, C. oceanicum, C. putrefaciens, C. putrificum, and C. sticklandii. The genus Clostridium is heterogeneous, and proposals for rationalization have included the creation of a number of new genera (Caloramator, Filifactor, Moorella, Oxobacter and Oxalophagus) [IJSB (1994) 44 812–826]. clostripain A thiol PROTEASE (MWt ca. 50000; EC 3.4.22.8), obtained from Clostridium histolyticum, which acts specifically at basic amino acyl (arginyl, lysyl) residues. It also has amidase and esterase activity, cleaving amides and esters of amino acids. clot culture BLOOD CULTURE in which clotted blood, minus serum, is liquefied (e.g. with STREPTOKINASE) and the resulting fluid used as the inoculum. Apparently not widely used – due e.g. to the risk of contamination prior to the inoculation stage. clotrimazole (bis-phenyl-(2-chlorophenyl)-1-imidazole methane) One of the earliest of the clinically useful imidazole antifungal agents (see AZOLE ANTIFUNGAL AGENTS); it is used topically in the treatment of superficial mycoses (e.g. candidiasis, ringworm, pityriasis versicolor). clotting (of blood plasma) See FIBRIN; COAGULASE; ANTICOAGULANT. clover blackpatch disease See SLAFRAMINE. clover rot (clover sickness; sclerotinia crown and stem rot, SCSR) A disease of clover and other forage legumes caused by Sclerotinia spp (particularly S. trifoliorum). Leaves and petioles of infected plants turn olive-brown and subsequently rot; the disease progresses from the petioles to the stem and root, leading to the death of the plant. Control by repeated applications of e.g. benomyl or quintozene is effective but economically impracticable. [Review: Bot. Rev. (1984) 50 491–504.] clover wound tumour virus Syn. WOUND TUMOUR VIRUS. clover yellow mosaic virus See POTEXVIRUSES. clover yellows virus See CLOSTEROVIRUSES. cloverleaf structure (of tRNA) See TRNA. cloxacillin See PENICILLINS. club fungi See APHYLLOPHORALES (Clavariaceae). clubroot A disease of cruciferous plants (cabbages, cauliflowers, turnips etc) caused by Plasmodiophora brassicae (see PLASMODIOPHOROMYCETES). The roots of an infected plant become swollen and distorted, developing a single large gall (the ‘club’ symptom) or clusters of smaller galls (‘finger-and-toe disease’). The aerial parts of the plant may be stunted and may wilt in hot weather; the foliage often acquires a reddish tinge. Secondary infection of the galls by SOFT ROT bacteria commonly occurs. P. brassicae causes both hyperplasia and hypertrophy of infected tissues, apparently by inducing hyperauxiny (see AUXINS). Healthy plant tissue normally contains, in separate compartments, an indole compound, glucobrassicin, and an enzyme (glucosinolase) which hydrolyses glucobrassicin to indoleacetonitrile (IAN), a precursor of indoleacetic acid (IAA). P. brassicae apparently interferes with the compartmentalization of glucobrassicin and glucosinolase, resulting in increased levels of IAN (and hence IAA) in infected tissues. The cysts of P. brassicae can remain viable for years in the soil, and it is difficult to eradicate the disease from contaminated land. Soil ‘sterilants’ such as dazomet may reduce the numbers of cysts. The disease can be controlled to some extent by dipping the roots of plants into a suspension of calomel, thiophanatemethyl or benomyl before planting. clue cells See BACTERIAL VAGINOSIS. clumping factor See COAGULASE. clumping-inducing agent See PHEROMONE. cluster cup See UREDINIOMYCETES stage I. cluster gene A gene encoding a multifunctional protein. (cf. GENE CLUSTER.) cluster of differentiation See CELL ADHESION MOLECULE. CLV Cassava latent virus (see GEMINIVIRUSES). cM CENTIMORGAN. CM COMPLETE MEDIUM. CM-cellulose (carboxymethylcellulose; CMC) A soluble derivative of CELLULOSE which is readily hydrolysed by most or all cellulolytic organisms. It is used e.g. to determine the ability of an organism or enzyme to degrade non-crystalline cellulose; endproduct formation or changes in the viscosity of CMC solutions may be measured. (cf. AVICEL; HE-CELLULOSE; FP-CELLULOSE.) CM test CALIFORNIA MASTITIS TEST. CMC (1) CM-CELLULOSE. (2) Chronic mucocutaneous CANDIDIASIS. CMI (1) CELL-MEDIATED IMMUNITY. (2) Commonwealth Mycological Institute, Kew, Surrey, UK. CML Chronic myelogenous leukaemia (see ABL). CMT CALIFORNIA MASTITIS TEST. CMV Cytomegalovirus (see BETAHERPESVIRINAE). CNA CALCIUM NUTRIENT AGAR. cnidosporans A group of protozoa formerly of the subphylum Cnidospora [JP (1964) 11 7–20], currently included in the phyla MICROSPORA and MYXOZOA. CNS (1) Central nervous system. (2) Coagulase-negative staphylococci. CO See CARBON MONOXIDE. 60 Co See IONIZING RADIATION. CO dehydrogenase A name used (confusingly) for (i) an enzyme which reduces carbon dioxide to carbon monoxide (see e.g. ACETOGENESIS), and (ii) an enzyme (cf. CO OXIDASE) which oxidizes carbon monoxide to carbon dioxide (see e.g. CARBOXYDOBACTERIA, METHANOGENESIS). CO dehydrogenase disulphide reductase See ACETOGENESIS. CO difference spectrum (of a cytochrome) The absorption spectrum of a given reduced, CARBON MONOXIDE-complexed CYTOCHROME minus the absorption spectrum of the same reduced, but non-complexed, cytochrome. CO oxidase (CO:acceptor oxidoreductase) An inducible enzyme which occurs in CARBOXYDOBACTERIA; it catalyses the oxidation of carbon monoxide to carbon dioxide in a reaction in which oxygen is derived from water rather than from air: CO + H2 O −−→ CO2 + 2H+ + 2e− In at least some carboxydobacteria the enzyme contains a noncovalently bound molybdenopterin moiety. [Properties of CO oxidases: MS (1986) 3 149–153.] CO2 See CARBON DIOXIDE. CO2 -stat See CONTINUOUS CULTURE. CoA COENZYME A. coa gene See STAPHYLOCOCCUS (S. aureus). Coactin Syn. MECILLINAM. co-activator See OPERON. co-agglutination (1) See PROTEIN A. (2) Joint agglutination (analogous to CO-PRECIPITATION). (3) Agglutination of different strains 174 cobra venom factor any organism tested for coagulase formation should be unable to metabolize the anticoagulant: citrated plasma, for example, may be clotted by coagulase-negative citrate-utilizing bacteria. Preservatives (e.g. thiomersal) may inhibit staphylocoagulase. Purified fibrinogen, or freeze-dried plasma, may be used instead of fresh plasma. Staphylocoagulases from strains pathogenic for man exhibit maximum activity in human or rabbit plasma. Strains which produce large amounts of fibrinolytic enzymes (see FIBRINOLYSIS) may not form clots, or may lyse any clot formed; such strains are more commonly found among isolates from human infections than among isolates from animals. (See also PSEUDOCOAGULASE.) The slide test detects clumping factor (see COAGULASE). A thick, saline suspension is prepared (on a slide) from a colony of the test strain grown on a non-selective medium; a loopful of citrated or oxalated human or rabbit plasma (not sheep or guineapig plasma) is stirred into the suspension. (Fibrinogen may be used instead of plasma e.g. to avoid any antibody-mediated agglutination of the bacteria.) Clumping of cells within ca. 5 sec indicates the presence of clumping factor. (A false-negative result may be obtained with capsulated strains [Book ref. 44, p. 505].) Controls should be used (cf. AUTO-AGGLUTINATION). (See also STAPHYLOSLIDE.) There is a high, though not total, correlation between positive results in the tube and slide tests; however, a positive slide test should not be regarded as evidence of staphylocoagulase production. A plate method may be used to detect staphylocoagulasepositive organisms. The test organism is inoculated onto the surface of an agar growth medium containing citrated plasma or purified fibrinogen and prothrombin. After overnight incubation, coagulase-positive colonies are each surrounded by a dense zone of precipitated fibrin. This method is useful for estimating numbers of coagulase-positive staphylococci in e.g. foods, but is too slow and prone to misleading results for use in routine clinical diagnosis. coal biodegradation Certain fungi are capable of growing on coal – particularly lignite (‘brown coal’, a form which is relatively low in carbon and rich in volatile compounds) – as the sole source of nutrients; such fungi include e.g. the basidiomycetes Polyporus versicolor and Poria monticola [AEM (1982) 44 23–27] and various other species (e.g. Aspergillus terreus and species of Candida, Mucor, Paecilomyces and Penicillium [SAAM (1985) 6 236–238]). Growth generally appears to be restricted to the surface of the coal; the nature of the substrates utilized is unknown. Coal products, such as benzene and toluene (derived from coal tar), may be degraded e.g. by combinations of bacteria [Nature (1998) 396 730]. [Aerobic and anaerobic degradation of toluene by Thauera: AEM (2004) 70 1385–1392.] (See also LEACHING.) CoASH (CoA-SH) Uncombined COENZYME A. coated lens See BLOOMED LENS. coated pit See PINOCYTOSIS. coated vesicle See PINOCYTOSIS. cobalamin See VITAMIN B12 . cobamide coenzymes See VITAMIN B12 . cobinamide See VITAMIN B12 . cobra venom factor (CVF) Either of the two proteins, derived from cobra venom, which affect the alternative pathway of COMPLEMENT FIXATION. CVFs mimic C3b. CVF from Naja naja binds Factor B and promotes its cleavage (by Factor D) to form a C3 convertase; it can also give rise to a C5 convertase. CVF or species of microorganism by a given antiserum (see e.g. WEIL–FELIX TEST). coaggregation Cell-to-cell adhesion in which the cells of one species adhere more or less specifically to those of a different species; e.g. Actinomyces viscosus and Streptococcus sanguis coaggregate by LECTIN –carbohydrate interaction [JGM (1984) 130 1351–1357]. coagulase Any bacterial component or product which causes coagulation (clotting) or PARACOAGULATION in PLASMA containing an anticoagulant such as citrate, heparin or oxalate. Coagulases are produced e.g. by certain staphylococci and by Yersinia pestis. Staphylococci produce two structurally and functionally distinct types of coagulase: free coagulase (staphylocoagulase, a protein which is released into the medium) and bound coagulase (clumping factor, a protein component of the cell wall). Staphylocoagulase causes a true clotting of plasma (from certain species only); it interacts with a ‘coagulase reacting factor’, CRF, in the plasma (believed to be prothrombin or a form of it) to form a complex (‘staphylothrombin’) which acts on plasma fibrinogen, converting it into insoluble fibrin – apparently by limited proteolytic action similar or identical to that of normal thrombin. The resulting clot differs from a normal clot in that e.g. it is not stabilized by Factor XIII (a normal blood-clotting factor) and it is much more resistant to fibrinolysin than is a normal clot. Several antigenically distinct staphylocoagulases are produced by different staphylococcal strains. The property of staphylocoagulase production is often used to distinguish between pathogenic and non-pathogenic strains, since there is a high degree of correlation between staphylocoagulase production and virulence (although coagulase-negative staphylococci are not necessarily harmless). However, although it can apparently exert its effect freely in vivo, the role of staphylocoagulase – if any – in pathogenesis is unknown. In vitro, the activity of staphylocoagulase is inhibited by various chemicals, particularly oxidizing agents, and by certain antibiotics (e.g. some penicillins). Staphylococcal clumping factor does not form a true clot in plasma; rather, it induces clumping of the cells (paracoagulation) in the presence of fibrinogen. When mixed with plasma, cells with clumping factor bind fibrinogen near the Cterminal ends of its g-chains – this apparently being responsible for the clumping of the cells [Biochem. (1982) 21 1407–1413, 1414–1420]. The role of clumping factor in pathogenesis (if any) is unknown. (cf. CCFAS.) [Isolation of clumping factor from S. aureus: Inf. Immun. (1985) 49 700–708.] Staphylocoagulase and clumping factor may be detected by different forms of the COAGULASE TEST. [Review of staphylococcal coagulases: Book ref. 44, pp. 525–557.] coagulase test Any procedure used to determine whether or not a given bacterial strain produces a COAGULASE. The tests described below are used for staphylococci. The tube test (which detects staphylocoagulase) simply involves incubating the test organism with a suitable plasma sample. In one of many methods, 0.5–1.0 ml of plasma is mixed with an equal (or smaller) volume of an 18–24-hour broth culture of the organism, and the tube is incubated at 35–37° C; the tube is examined at hourly intervals, and after overnight incubation, for clot formation – a firm clot which does not move when the tube is shaken being regarded as a positive result [Book ref. 44, pp. 526–527]. Known coagulase-positive and coagulasenegative strains should be used as controls. Plasma containing anticoagulants such as citrate, oxalate, EDTA, or heparin may be used, although heparin has been reported to delay clot formation; 175 cobweb disease tract and/or various other tissues; they may be mild to fatal. Coccidioses of domestic animals and poultry are often of economic importance. (a) Poultry. In chickens, Eimeria necatrix causes severe haemorrhage in the small intestine, and E. tenella causes haemorrhage and necrosis in the caecum; other pathogens include e.g. E. acervulina and E. brunetti. In ducks, Tyzzeria perniciosa causes haemorrhagic intestinal disease. In turkeys, severe inflammation of the intestine is caused by e.g. E. adenoeides and E. meleagrimitis. (b) Domestic animals. In cattle, haemorrhagic lesions in the intestine are caused e.g. by E. bovis and E. zürnii (E. zuernii ); an increased incidence of disease may occur during cold weather (‘winter coccidiosis’). In sheep, intestinal disease may be caused e.g. by E. ahsata, E. ovina or E. ovinoidalis. In pigs, enteritis, severe diarrhoea and emaciation are caused e.g. by E. scabra. In rabbits, intestinal disease (often fatal) is caused e.g. by E. magna and E. irresidua; E. stiedae (E. stiedai) causes diarrhoea, enlargement of the liver and bile ducts, and emaciation. In cats, intestinal symptoms may be due to infection by Isospora spp (which may also infect dogs) or Toxoplasma (see TOXOPLASMOSIS). (cf. SARCOSPORIDIOSIS; see also EQUINE PROTOZOAL MYELOENCEPHALITIS.) (c) Man. Infections involving Isospora belli and I. hominis are believed to be commonly asymptomatic, but intestinal symptoms may occur. (See also CRYPTOSPORIDIOSIS, SARCOSPORIDIOSIS and TOXOPLASMOSIS.) Diagnosis of animal coccidioses involves e.g. SPORULATION of oocysts concentrated from fresh faeces. [Identification of Eimeria spp: Book ref. 7, pp. 7–30. Identification of coccidia: Book ref. 18, pp. 80–91. Coccidian pathogenicity: Book ref. 18, pp. 287–327. Pathology, lab. diagnosis, and therapy in cryptosporidiosis, isosporiasis, sarcosporidiosis and toxoplasmosis: Book ref. 118, pp. 211–236.] coccidium An individual, or a particular strain, of an organism of the suborder EIMERIORINA. coccobacillus A bacterial cell intermediate in morphology between a COCCUS and a BACILLUS. Coccodiscus See RADIOLARIA. coccoid More or less spherical: cf. COCCUS. coccoid bodies Thin-walled coccoid forms which develop from the helical or vibrioid cells in old cultures of e.g. Aquaspirillum, Campylobacter and Oceanospirillum spp; their formation is enhanced by treatment with mitomycin or UV light. Coccoid bodies resemble sphaeroplasts but are resistant to osmotic lysis; the majority are apparently viable, ‘germinating’ to form normal cells when transferred to a fresh medium. (cf. MICROCYST.) coccolith A calcified scale, generally measuring a few micrometres or less in diam., present on the cell surface in certain cells of the COCCOLITHOPHORIDS. Coccoliths contain crystals of calcite – either of a single type (in holococcolithis) or, more commonly, of different shapes and sizes (in heterococcoliths); coccoliths are often elliptical in shape (but may be round, polygonal, etc) and they commonly exhibit intricate patterns and ornamentations which are species-specific. The coccoliths of present-day coccolithophorids form a significant proportion of some deep-sea oozes; however, coccolithophorids were even more abundant in previous geological ages, particularly in the Cretaceous, and the coccoliths of these organisms are major constituents of Mesozoic (Jurassic and Cretaceous) and Cenozoic chalks and marls. Coccolith morphology apparently changed rapidly over the ages, making fossil coccoliths useful markers in the biostratigraphy of sedimentary rocks [Book ref. 136, from Naja haje can also give rise to a C3 convertase but not to a C5 convertase. Unlike C3b, CVF is not inactivated by Factors I and H; it therefore forms a C3 convertase which is not subject to regulation. cobweb disease A MUSHROOM DISEASE caused by Hypomyces rosellus; it can be controlled e.g. by BENOMYL or PROCHLORAZ [PP (1983) 32 123–131]. cocarboxylase See THIAMINE. coccal Pertaining to a COCCUS. cocci See COCCUS. coccidia The common name for protozoa of the suborder EIMERIORINA. Coccidia (1) A subclass of protozoa (class Telosporea) later incorporated, with the TOXOPLASMEA, in the subclass COCCIDIASINA. (2) A subclass of protozoa (class Sporozoea [JP (1980) 27 37–58]) equivalent to the COCCIDIASINA. Coccidiascus A genus of yeasts (family METSCHNIKOWIACEAE). Cells: spherical to ovoid (5–15 µm diam./length), with a large vacuole and nucleus; neither pseudomycelium nor mycelium is formed. C. legeri (sole species) occurs as a parasite in intestinal epithelial cells of Drosophila spp; it has not been cultured in vitro. [Book ref. 100, pp. 123–124.] Coccidiasina A subclass of protozoa (class SPOROZOASIDA) parasitic mainly in vertebrates but also in invertebrates. In members of this subclass the mature gametocytes typically occur intracellularly in the host (cf. GREGARINASINA). Orders: Agamococcidiorida, EUCOCCIDIORIDA, Protococcidiorida. coccidiasis Any subclinical or very mild infection with a coccidian parasite – as distinct from clinical COCCIDIOSIS. Coccidioides A genus of fungi (class HYPHOMYCETES); C. immitis is a saprotroph in desert soils in hot, arid regions of the American continent, and is the causal agent of COCCIDIOIDOMYCOSIS. In mammalian tissues it occurs mainly as multinucleate, thickwalled, spherical (ca. 20–80 µm) cells (spherules or sporangia). The spherule protoplasm divides into multinucleate protospores and then into uninucleate sporangiospores (endospores) which are released on rupture of the spherule wall and develop into new spherules. C. immitis grows readily on various types of media, forming a mycelium which fragments into barrelshaped arthroconidia which are highly infective and very easily dispersed by air currents. coccidioidin Any antigenic preparation derived from Coccidioides immitis and used in a diagnostic SKIN TEST for COCCIDIOIDOMYCOSIS. coccidioidomycosis (San Joaquin Valley fever; desert fever) A disease of man and animals caused by Coccidioides immitis; it occurs in hot, arid regions of the American continent. Infection usually occurs by inhalation of wind-borne spores (especially arthroconidia); person-to-person transmission does not occur. Infection may be asymptomatic or may result in an acute, self-limiting respiratory disease resembling a common cold or influenza – often with severe chest pain. Rarely, mycetomas (‘fungus balls’) may develop in the lungs. Less common symptoms include joint pains (‘desert rheumatism’) and/or skin lesions. The primary infection occasionally leads to progressive, often fatal, disseminated disease (coccidioidal granuloma) with lesions in e.g. skin, joints, meninges. Lab. diagnosis: serological tests; SKIN TESTS using COCCIDIOIDIN or SPHERULIN; demonstration of the fungus by histology, culture and/or animal inoculation. Treatment: ketoconazole, amphotericin B. [Book ref. 25.] coccidiosis Any disease of man and other animals caused by protozoa of the suborder EIMERIORINA. Coccidioses are typically contracted via the oral route and may involve the intestinal 176 codon bias as four different RNA species. Two RNAs – designated RNA1 small or RNA-1 fast (246 nucleotides) and RNA-2 small or RNA-2 fast (492 nucleotides) – occur in infected palms during the early stages of cadang-cadang disease; later, two additional RNAs – RNA-1 large or RNA-1 slow (287 nucleotides) and RNA-2 large or RNA-2 slow (574 nucleotides) – appear and eventually predominate. The RNA-1 small is regarded as the unit CCCV RNA; the other three RNAs are apparently derived from RNA-1 small by sequence duplication. [Nature (1982) 299 316–321.] coconut hartrot See HARTROT. coconut lethal yellowing A YELLOWS disease of the coconut palm caused by an MLO. The disease is controlled, commercially, by the use of oxytetracycline hydrochloride. coconut palm rhinoceros beetle (biological control) See BACULOVIRIDAE. co-conversion (mol. biol.) See RECOMBINATION. codecarboxylase See PYRIDOXINE. coding strand A term which (like non-coding strand) is frequently used with opposite meanings by different authors. Originally, the term was generally used to refer to that strand of a gene which acts as the template on which mRNA is synthesized during transcription – the strand whose sequence is complementary to that of mRNA. Currently, there appears to be a consensus for the opposite meaning, i.e. that strand of a gene which is not transcribed (i.e. not used as a template) and whose sequence is homologous to that of mRNA. (Even so, given the confusion present in the literature, it may be wise to define the term, in relation to mRNA, whenever it is used.) The coding strand is also called the sense strand or the plus (+) strand. The non-coding strand of a gene (which is complementary to the coding strand) is thus the template strand; it is also called the antisense strand or the minus strand. Interestingly, a certain gene in the fruitfly (Drosophila) has been shown to contain protein-encoding information in both of the strands [Nature (2001) 409 1000]. Codium A genus of siphonaceous green seaweeds (division CHLOROPHYTA). The thallus is terete and dichotomously branched; it is composed of a colourless central medulla of interwoven filaments from which arises a surrounding layer of inflated green branchlets (‘utricles’). Anisogametes are produced in the utricles; these are initially non-motile, later becoming biflagellate. (See also CELL WALL; ELYSIA; NITROGEN FIXATION.) codominant gene One of two (or more) genes which, when present together, specify a phenotype unlike that specified by either (or any) of the genes individually. codon See GENETIC CODE. codon bias In the expression of a heterologous (i.e. ‘foreign’) gene (e.g. a mammalian gene in Escherichia coli): inefficient translation due to the presence of certain codon(s) for which the host cell has insufficient numbers of the corresponding tRNA(s); codon bias can be a problem e.g. when high-level expression in E. coli is required from a heterologous gene containing a high frequency of codons such as the proline codon CCC and/or the arginine codon AGG – codons which occur rarely in homologous (E. coli ) genes. Attempts at high-level expression of genes containing relatively high frequencies of codons that are ‘rare’ for the host cell may lead e.g. to slowing or termination of translation and the degradation of mRNA. One solution to the problem is to insert into the host cell extra (plasmid-borne) copies of the relevant tRNA-encoding gene(s). Codon bias may also arise as a result of a same-sense mutation which replaces a common or ‘routine’ codon with a synonymous but infrequent codon (see SILENT MUTATION). pp. 329–426 (Mesozoic coccoliths), pp. 427–554 (Cenozoic coccoliths)]. (See also FORAMINIFERIDA.) coccolithophorids Those algae of the PRYMNESIOPHYCEAE in which the cell bears a covering of one to several layers of COCCOLITHS during at least some stage of the life cycle. For example, Emiliana huxleyi – the commonest living coccolithophorid – has a life cycle in which two distinct types of cell are formed: the motile ‘S cell’, which is biflagellated and covered with one or several layers of organic scales, and the non-motile ‘C cell’, which lacks flagella and organic scales but is enclosed by a shell-like coccosphere consisting of one or several layers of coccoliths. Life cycles involving motile and non-motile phases also occur in other genera (e.g. Coccolithus, Cricosphaera, Hymenomonas). Coccolithophorids are planktonic marine organisms which are particularly common in tropical waters. Coccolithus See COCCOLITHOPHORIDS. Coccomyces See RHYTISMATALES. Coccomyxa A genus of unicellular, non-motile green algae (division CHLOROPHYTA) which occur as photobionts in certain lichens (see e.g. NEPHROMA, PELTIGERA, SOLORINA). The cells are small, ovoid, and contain a single parietal chloroplast with no pyrenoid; flagellated stages and sexual reproduction have never been observed. (See also RIBITOL and SPOROPOLLENIN.) Cocconeis See DIATOMS. coccosphere See COCCOLITHOPHORIDS. coccus (pl. cocci) A spherical (or near-spherical) bacterial cell. According to species, cocci may occur singly, in pairs (see DIPLOCOCCUS), in regular groups of four or more (see TETRAD and PACKET), in chains, or in irregular clusters. (cf. BACILLUS sense 2.) Cochliobolus See DOTHIDEALES. Cochliopodium A genus of amoebae (order ARCELLINIDA) which have a type of test known as a tectum: a more or less flexible, closely adherent covering of small scales (distinguishable only by electron microscopy); in some species the scales bear spines which may be visible by light microscopy, while in others sand grains and other debris may adhere to the scales. Species occur in fresh water, activated sludge, etc. [Book ref. 133, pp. 76–82.] Cochlonema See ZOOPAGALES. cocksfoot mottle virus See SOBEMOVIRUSES. cocksfoot streak virus See POTYVIRUSES. cocoa Cocoa and chocolate are made from the seeds (beans) of the cacao plant Theobroma cacao. The cacao fruit is a pod containing up to 50 beans covered by a white mucilage. After harvesting, the pods are opened and the beans are extracted, piled up (e.g. in perforated boxes or on plantain leaves) and covered (e.g. with leaves). A fermentation then occurs: initially, various yeasts carry out an ALCOHOLIC FERMENTATION, and this is rapidly followed by ACETIFICATION by acetic acid bacteria – the production of acetic acid and heat inhibiting further yeast action. Other bacteria (e.g. lactic acid bacteria) and moulds are also present. During the process, which lasts ca. 6 days, the beans darken in colour (owing to oxidation) and the mucilage gradually disappears. The beans are then dried. The characteristic cocoa flavour develops only after the beans are roasted. [Book ref. 5, pp. 275–292.] (cf. COFFEE; see also CACAO DISEASES.) cocoa necrosis virus See NEPOVIRUSES. coconut cadang-cadang viroid (CCCV) A VIROID which infects coconut palms; infected palms may be killed, and the viroid has caused severe economic losses e.g. in the Philippines (cf. TINANGAJA DISEASE). The natural mode of transmission is unknown. CCCV is unique among known viroids in that it exists 177 Codonella for catalysis of redox reactions. (2) Any low-MWt, nonprotein organic molecule – whether freely dissociable or firmly bound – necessary for the activity of a given enzyme. coenzyme F420 See METHANOGENESIS. coenzyme F430 See METHANOGENESIS. coenzyme I NAD (q.v.). coenzyme II NADP: see NAD. coenzyme A (CoA) A coenzyme, derived from PANTOTHENIC ACID (see figure), which functions as a carrier of acyl groups – with which it forms thioesters (CoA.S.CO.R). (Uncombined coenzyme A may be represented as CoA-SH or CoASH.) Acylcoenzyme A thioesters have a high free energy of hydrolysis and are commonly involved in SUBSTRATE-LEVEL PHOSPHORYLATIONS. coenzyme B See METHANOGENESIS. coenzyme B12 See VITAMIN B12 coenzyme F See FOLIC ACID. coenzyme M See METHANOGENESIS. coenzyme Q See QUINONES. coenzyme R Syn. BIOTIN. cofactor (1) Any low-MWt, non-protein (organic or inorganic) factor which is necessary for the activity of a given enzyme (cf. COENZYME; PROSTHETIC GROUP). (2) (Syn. activator) Any inorganic component (e.g. metal ion) necessary for the activity of a given enzyme. (3) Any organic or inorganic factor necessary for the activity of an enzyme or enzyme complex. coffee The commercial preparation of coffee from ripe coffee fruits (‘cherries’) requires the removal of the sticky mucilaginous mesocarp which surrounds the two beans in each fruit. This may be achieved mechanically or chemically, but the preferred method is by fermentation – which also improves the quality and appearance of the beans. The coffee fruits are pulped to disrupt the skins and then allowed to ferment either under water or ‘dry’. The mucilage is degraded both by enzymes in the fruit itself and by microbial extracellular enzymes. (Commercial preparations of mould enzymes have also been used.) Various bacteria, yeasts and moulds are associated with the fermenting coffee, the most important probably being pectinolytic species of e.g. Bacillus, Erwinia, Aspergillus, Fusarium and Penicillium. After the fermentation, the beans are washed, dried, blended, roasted and ground before use. (cf. COCOA.) coffee berry disease A disease of the coffee plant characterized by sunken, anthracnose lesions on the berries; causal agent: Colletotrichum coffeanum. [Review: Phytopathol. Paper number 20, March 1977.] Codonella See TINTINNINA. Codonosiga See CHOANOFLAGELLIDA. Coe virus Coxsackievirus A21: see ENTEROVIRUS. coelichelin A tripeptide SIDEROPHORE, encoded by Streptomyces coelicolor, which is synthesized by a non-ribosomal synthetase [see e.g. FEMS (2000) 187 111–114]. Coelomomyces See BLASTOCLADIALES. Coelomycetes A class of fungi (subdivision DEUTEROMYCOTINA) which form septate mycelium and in which the conidiophores line a discrete saucer-shaped, cupulate, flask-shaped or loculate conidioma – e.g. an ACERVULUS or a PYCNIDIUM. (cf. HYPHOMYCETES.) Orders [Book ref. 64, pp. 87–88]: MELANCONIALES, PYCNOTHYRIALES and SPHAEROPSIDALES. Coeloseira See RHODOPHYTA. Coelosphaerium A phycological genus of unicellular ‘bluegreen algae’ (Chroococcaceae) in which the cells occur in a single layer at the periphery of a spherical colony; the cells are ellipsoidal, divide longitudinally, and contain GAS VACUOLES. The organisms are planktonic e.g. in freshwater lakes. (See CYANOBACTERIA.) coelozoic Refers to a parasite which lives within its host in body fluids – e.g. in the gall bladder, bloodstream (outside blood cells), urinary tract, etc. (cf. HISTOZOIC.) Coemansia See KICKXELLALES. coenobium (1) (bacteriol.) A MICROCOLONY (sense 1) in which, following cell division, the cells of a clone form a regular array. Thus, e.g. Thiopedia typically forms a coenobium which consists of a flat sheet of 16 contiguous cells in a ‘4 × 4’ arrangement – but coenobia containing up to ca. 64 cells may be formed. In Brachyarcus the coenobia consist of symmetrically arranged cells embedded in a matrix. (2) (algol.) In certain types of algae: a colonial form which consists of a number of cells arranged in a specific way; the number of cells in a coenobium is usually a stable feature of a given species or genus. Coenobia are formed e.g. by Scenedesmus and Volvox. coenocyte (microbiol.) A multinucleate cell, structure, or organism, formed by the division of an existing multinucleate entity or by nuclear division without the formation of dividing walls or septa. (cf. SYNCYTIUM.) Coenocytes occur e.g. in SIPHONACEOUS and SIPHONOCLADOUS algae; the hyphae of many fungi may be regarded as coenocytic. coenzyme (1) Any low-MWt, non-protein, freely dissociable organic molecule which is necessary for the activity of a given enzyme (cf. PROSTHETIC GROUP); e.g., dehydrogenases require electron acceptors such as NAD+ or NADP+ NH2 N N O N CH2 O 1′ H O CH3 OH O H C N O H C N 5′ N 4′ H H OH P O O − O O − O CH2 C C CH2 CH2 CH2 CH2 SH CH3 H pantothenic acid O − P H 3′ 2′ O P O − O adenosine 3′,5′-bisphosphate pantetheine 4′-phosphate COENZYME A (CoA-SH) 178 2-mercaptoethylamine (= cysteamine) cold-shock response colchicine An alkaloid found e.g. in the meadow saffron (Colchicum autumnale); the colchicine molecule consists of a tricyclic skeleton which includes an aromatic ring (substituted with three methoxy groups) and a tropolone ring. At low concentrations colchicine can e.g. bind to TUBULIN and prevent the in vitro or in vivo assembly of tubulin into MICROTUBULES; colchicine can thus e.g. inhibit MITOSIS in mammalian, plant, and other cells. (However, some complex microtubular structures – e.g. CENTRIOLES, and the ciliar and flagellar axonemes – are normally resistant to colchicine.) The in vitro effect of colchicine on microtubule assembly can be abolished e.g. by ultraviolet radiation (which converts colchicine to lumicolchicine). coffee diseases For diseases of the coffee plant (Coffea spp) see e.g. COFFEE BERRY DISEASE, COFFEE RUST, FUSARIUM WILT, PHLOEM NECROSIS, RED RUST. coffee ringspot virus See RHABDOVIRIDAE. coffee rust A COFFEE DISEASE caused by Hemileia vastatrix. Yellowish spots, which become orange, appear on the undersides of the leaves, corresponding dark patches appearing on the upper surfaces; infected leaves are shed, and yields of berries are greatly reduced. cognac See SPIRITS. cognate In (e.g.) immunology, a word commonly used to mean ‘corresponding’ or ‘matching’. cognate help (linked recognition) (immunol.) The binding of a helper T cell to an antigen which is also bound by a B cell; the B and T cells are thus physically linked via the antigen. Such joint binding of antigen is accompanied by contact between various receptors and ligands on the two cells; this leads to activation and proliferation of both types of cell and formation of antibodies by the B cell. (See also ANTIBODY FORMATION.) The determinant (epitope) bound by the B cell is sometimes referred to as the ‘hapten’, that bound by the T cell as the ‘carrier’ – terminology that derives from earlier hapten–carrier studies. cohesive ends Syn. STICKY ENDS. cointegrate The (circular) product of fusion between two circular replicons (e.g. two plasmids, or a plasmid and a bacterial chromosome) mediated by a TRANSPOSABLE ELEMENT (TE). Cointegration may occur as a result of a transposition event between a replicon containing a TE and one containing a target sequence for that TE. The transposition is accompanied by duplication of both the TE and a sequence at the target site: see entry TRANSPOSABLE ELEMENT. An alternative mechanism of cointegrate formation involves recA-dependent reciprocal recombination between homologous TEs in each of two replicons. (Some authors reserve the term ‘cointegration’ for the transposition event only.) (See also Tn3.) (The term ‘cointegrate’ has also been used for any product of fusion between two replicons, regardless of the mechanism by which fusion occurred.) coital exanthema See EQUINE COITAL EXANTHEMA. col factor See COLICIN PLASMID. col plasmid See COLICIN PLASMID. Colacium A genus of EUGLENOID FLAGELLATES. The organisms differ from other euglenids in that they generally form dendroid or palmelloid colonies; flagellated cells, when formed, resemble those of EUGLENA. C. libellae overwinters in association with damselfly nymphs, becoming established in the rectum of the insect. colanic acid A capsular heteropolysaccharide (the ‘M antigen’) produced e.g. by strains of Escherichia coli and Salmonella. K12 strains of E. coli produce colanic acid and form mucoid colonies at 30° C but not at 37° C; certain mutants (lon or capR) produce colanic acid and appear mucoid at 37° C. Colanic acid consists of repeating trisaccharide units of (-glucose-fucose-fucose-), the central fucose residue of each unit carrying a trisaccharide branch (galactose-glucuronic acidgalactose-). Non-carbohydrate substituents (e.g. acetyl and pyruvyl groups) may be present, the nature and position of such groups varying e.g. with strain. [Organization of the E. coli K12 gene cluster responsible for production of colanic acid: JB (1996) 178 4885–4893.] Colcemid (trade name) DEMECOLCINE. CH3O NHCOCH3 CH3O CH3O O OCH3 COLCHICINE cold See COMMON COLD. cold abscess See ABSCESS. cold agglutinins Agglutinins which combine maximally with homologous or cross-reacting antigens at low temperatures (e.g. 4° C) but not at e.g. 37° C. Cold agglutinins may be detectable e.g. in the serum of patients suffering from PRIMARY ATYPICAL PNEUMONIA (sense 2), and they can be assayed by their ability to agglutinate human group O RBCs. Cold agglutinins occur also in certain other diseases – e.g. pneumonia caused by adenoviruses. cold enrichment ENRICHMENT of psychrophilic or psychrotrophic organisms by incubation at low temperatures (see e.g. Yersinia enterocolitica). ColD plasmid See COLICIN D and COLICIN PLASMID. cold shock See OSMOTIC SHOCK. cold-shock response In prokaryotes and eukaryotes: an apparently adaptive response to a down-shift in temperature (e.g. 37 → 10° C or, in general, a down-shift of at least 13° C) characterized by (i) initial cessation of growth, with resumption after a lag period; (ii) induction or increased synthesis of cold-shock proteins; (iii) repression of heat-shock proteins; (iv) generalized inhibition of protein synthesis, but ongoing synthesis of certain proteins involved in transcription and translation. In Escherichia coli, the cold-shock proteins include CspA, RecA, IF-2 (initiation factor 2, which mediates binding of Nformylmethionine-charged tRNA to the 30S ribosomal subunit), NusA (involved in the last stage of transcription), and the asubunit of DNA gyrase. So far, no cold-inducible sigma factor has been identified. CspA is a small (70 amino-acid) protein which is inducible immediately (at the level of transcription) on temperature downshift. It appears to interact with nucleic acids. Suggested functions of CspA include: (i) a low-temperature activator of translation; (ii) an ‘RNA chaperone’ that inhibits secondary structures in RNA and which may be important e.g. for efficient translation of mRNA at low temperatures [JBC (1997) 272 196–202]; (iii) an agent which inhibits translation from specific mRNAs. (It has been reported that CspA is normally produced 179 cold sore during early exponential growth at 37° C in E. coli – i.e. under non-stress conditions [EMBO (1999) 18 1653–1659].) The cold-shock response can also be triggered by certain inhibitors of translation – for example, the antibiotics chloramphenicol, erythromycin and tetracycline; this has suggested that a decrease in translational capacity may be an important factor in the induction of the cold-shock response [Mol. Microbiol. (1994) 11 811–818]. cold sore (fever blister) (1) The lesion associated with labial herpes (see HERPES SIMPLEX). (2) The recurrent thin-walled vesicular mucocutaneous lesion associated with any of the various forms of HERPES SIMPLEX. cold stability factor See MICROTUBULE-ASSOCIATED PROTEINS. cold water disease (peduncle disease; low temperature disease) A FISH DISEASE affecting young salmonids below 10° C. The tail and peduncle undergo slow progressive necrosis. Causal agent: Flexibacter psychrophila (‘Cytophaga psychrophila’). ColE1 plasmid A small (MWt ca. 4.2 × 106 ), non-conjugative, ccc dsDNA COLICIN PLASMID which is normally maintained in the host cell at a copy number of ca. 10–30. In ColE1, DNA REPLICATION occurs by the CAIRNS MECHANISM. Replication is initiated from a fixed origin and (in contrast to chromosomal replication in Escherichia coli ) proceeds unidirectionally from the origin; it is not dependent on plasmid-encoded proteins but requires the cell’s RNA polymerase, DNA polymerases I and III and components of the PRIMOSOME. The first several hundred nucleotides of the leading strand are synthesized by DNA polymerase I from an RNA primer synthesized by RNA polymerase; subsequent DNA chain elongation is carried out by DNA polymerase III. Initiation of plasmid replication is regulated at the level of primer formation. During the initiation process, two RNA transcripts are synthesized, one on each DNA strand, at the same region ca. 500 nt upstream from the origin (ori). The transcript on the H strand (RNA II) can – unless prevented (see later) – extend to the origin and give rise to a primer for synthesis of the leading strand; the transcript on the L strand (RNA I, 108 nt long) is complementary to the 5
terminal region of RNA II and behaves as a trans-acting repressor of plasmid replication. According to a current model, the 5
end of RNA II is synthesized as a free (single-stranded) transcript, but as the RNA polymerase approaches ori the transcript and template form a DNA/RNA hybrid; the RNA strand in this hybrid is cleaved by RNASE H to form a free hydroxyl (−OH) terminal suitable for extension by DNA polymerase, i.e. the cleaved end of the RNA transcript, at ori, functions as a primer. (Although RNase H is essential for replication of ColE1 in vitro, its role in vivo has been questioned [JB (1986) 166 143–147].) Whether or not a functional primer is formed from the RNA II transcript, as described above, is influenced by several factors – factors which, by regulating the initiation of replication, determine the COPY NUMBER of the plasmid. One regulatory factor is RNA I. RNA I and RNA II each form several stem-and-loop structures which, because the two transcripts are complementary, can undergo mutual base-pairing; when this happens, it inhibits development of a functional primer at the RNA/DNA hybrid in the ori region. (In vitro, RNA I can inhibit primer formation only if it binds to RNA II some time before the complete RNA II–DNA hybrid has been formed; once the nascent RNA II transcript has reached a length of ∼360 nt it becomes resistant to RNA I.) The hybridization of RNA I with RNA II is facilitated/accelerated by a small protein (63 amino acids) encoded by the plasmid gene rop (= rom); the Rop/Rom protein (which acts as a rigid dimer exhibiting exact two-fold symmetry) thus enhances the inhibitory effect of RNA I on primer formation. The ability of RNA I to inhibit primer formation depends on its intracellular concentration. (This, in turn, can be influenced e.g. by the host cell’s RNASE E which can cleave and inactivate RNA I.) It appears that only when the copy number of ColE1 has reached its maximum is the concentration of RNA I high enough to inhibit further replication. Interestingly, the copy number of ColE1 is affected by mutations in the pcnB gene (see MRNA (a)). A similar mechanism for the regulation of plasmid replication occurs e.g. in CloDF13, p15A, pBR322, pMB1 and RSF1030 plasmids. In pBR322, RNA I is synthesized five times more often than RNA II, and it has been estimated that only 1 in 20 pre-priming event results in plasmid replication [JB (1987) 169 1217–1222]. Coleochaete A genus of freshwater epiphytic green algae (division CHLOROPHYTA). Some species are filamentous and dichotomously branched, others are discoid; cell division is confined to apical/marginal cells. Characteristic of the genus is the formation by certain cells of long, fine, unbranched, sheathed ‘hairs’ (setae). Motile cells are scaly. Zoospores are biflagellate, one being formed per cell. Sexual reproduction is oogamous; an envelope of sterile cells may surround the reproductive cells. Plants may be homothallic or heterothallic. Coleosporium See UREDINIOMYCETES. Coleps A genus of freshwater and marine ciliates (subclass GYMNOSTOMATIA). Cells: typically barrel-shaped, ca. 50–100 µm in length, with an anterior (apical) cytostome; a semirigid endoskeleton (formed by the deposition of calcium phosphocarbonate in the pellicular alveoli) gives the appearance of a regular series of platelets forming a grid-like pattern over the entire body surface. Posteriorly directed spines may occur at the posterior edge of the cell. The organisms feed on other protozoa and algae. colibacillosis (vet.) Any of certain diseases, caused by strains of Escherichia coli, which occur primarily in very young animals and which typically involve septicaemia and/or mild to severe diarrhoea. (See also ETEC; cf. WHITE SCOURS; OEDEMA DISEASE.) colicin A A pore-forming colicin (see COLICINS) (MWt ca. 63000) which binds to the OmpF/BtuB protein [mechanism of action: JMB (1986) 187 449–459]. colicin B A pore-forming colicin (see COLICINS) (MWt ca. 80000–90000) which binds to the FEPA PROTEIN. colicin D A colicin (see COLICINS) (MWt ca. 87000) which binds to the FEPA PROTEIN and subsequently inhibits protein synthesis in sensitive cells. [Physical and genetic analysis of the ColD plasmid: JB (1986) 166 15–19.] colicin E A category of COLICINS (MWt ca. 64000–66000) each of which binds to the BtuB protein. E colicins are divided into nine immunity groups (E1. . .E9); colicins of a given group (e.g. E1) are not active against cells which contain a (repressed) plasmid encoding the same colicin and which are synthesizing the corresponding immunity protein. In a sensitive cell, colE1 forms pores; colE2, colE7, colE8 and colE9 are DNases; and colE3 and colE6 are RNases which cleave 16S rRNA. colicin I A category of COLICINS (MWt ca. 80000); colIa and colIb are group B pore-forming colicins. colicin K A group A, pore-forming colicin (see COLICINS) (MWt ca. 70000) which binds to the TSX PROTEIN; uptake across the cell envelope of the target cell is reported to require OmpFA and the TolABQR proteins. 180 colicins colicin L See COLICINS. colicin M A colicin (see COLICINS) (MWt ca. 23000) which binds to the TONA PROTEIN (= FhuA); it inhibits the synthesis of PEPTIDOGLYCAN. colicin N A pore-forming colicin (see COLICINS) MWt ca. 39000) which binds to the OmpF protein. colicin plasmid (col factor, or col plasmid) A PLASMID which encodes one or more COLICINS. Some col plasmids (type 1) are small, MULTICOPY PLASMIDS, while others (type 2) are large, low-copy-number plasmids. The type 1 col plasmids are non-conjugative; type 2 col plasmids are often conjugative (see CONJUGATIVE PLASMID). Conjugative col plasmids occur in various INCOMPATIBILITY (Inc) groups (e.g. ColV is an IncFI plasmid, ColB-K98 is an IncFIII plasmid, and ColIb-P9 is an IncIa plasmid). Some non-conjugative plasmids have been found to exhibit incompatibility [e.g. incompatibility between ColE plasmids: JGM (1986) 132 1859–1862]. Col plasmids contain three essential genes: (i) the colicin structural gene (= ‘activity’ gene); (ii) a gene encoding the immunity protein (see COLICINS); and (iii) a gene encoding the lysis protein. These genes are designated cdl (colicin D lysis gene), cea (colicin E activity gene), cui (colicin U immunity gene) etc. In a population of colicinogenic bacteria only a small proportion of the cells is de-repressed for colicin synthesis; however, the immunity gene is usually expressed constitutively so that cells with repressed genes for the colicin and lysis protein are protected from the given colicin produced by their derepressed neighbours. A col plasmid typically encodes only that type of immunity protein which protects the host cell against the particular colicin encoded by the plasmid. However, ColE9-J encodes two types of immunity protein: one protecting against colE9 and one protecting against colE5 [JGM (1986) 132 61–71]. Transcription of col plasmids is repressed by the LexA protein so that synthesis of colicins is promoted by those agents which trigger the SOS SYSTEM. colicin typing BACTERIOCIN TYPING with COLICINS. colicin U A pore-forming colicin (see COLICINS) (MWt ca. 66300) produced by Shigella boydii ; colicin U binds to sensitive cells via the OmpA and OmpF proteins, and uptake occurs via the TolABQR proteins [JB (1997) 179 4919–4928]. colicin V The designation of a bacteriocin (MWt 6000) which acts by disrupting the membrane potential of sensitive cells [JB (1984) 158 757–759]; it is not inducible by conditions which trigger the SOS system (cf. COLICINS), and at least some authors now classify this bacteriocin as a MICROCIN [e.g. TIM (1998) 6 66–71]. colicin X The designation of a small bacteriocin subsequently classified as a MICROCIN. colicin Y A recently isolated pore-forming colicin (see COLICINS) encoded by plasmid pCol-Let [complete coding sequence: Microbiology (2000) 146 1671–1677]. colicinogenic Able to produce COLICINS. colicinogenic plasmid Syn. COLICIN PLASMID. colicins A category of high-MWt (∼25–90 kDa) BACTERIOCINS produced by colicinogenic bacteria of the family Enterobacteriaceae (e.g. Escherichia coli, Shigella boydii ) and active against (sensitive) strains of the same family; most colicins are encoded by plasmid genes (see COLICIN PLASMID), one exception being the chromosomally encoded ‘bacteriocin 28b’ – a bacteriocin (similar to colicin L) produced by strains of Serratia marcescens. A given colicin is associated with a cluster of three genes: (i) the structural gene for the colicin; (ii) the immunity gene, encoding a protein which protects the cell against molecules of the same colicin produced by other cells in the vicinity; and (iii) a lysis gene, encoding a protein involved in the release of the colicin. Under normal conditions the colicin and lysis genes are not expressed, but the immunity gene is expressed constitutively. The product of the immunity gene (the immunity protein) inhibits the activity (but not the adsorption) of any molecules of the given colicin that bind to the cell surface. In a population of colicinogenic cells, synthesis of the colicin and lysis protein is promoted under conditions which trigger the SOS SYSTEM (see COLICIN PLASMID). However, in a colicinogenic population a small proportion of the cells (at any given time) may be derepressed for colicin synthesis, and these cells will be able to produce and externalize colicins. Colicins synthesized in a given cell leave the cell following the activity of the lysis protein. The lysis protein localizes in the cell envelope; earlier studies indicated that this causes a non-specific increase in the permeability of the cell envelope – allowing release not only of the colicin molecules but also of various other constituent of the cell’s cytoplasm [EMBO (1987) 6 2463–2468]. During the externalization of colicin molecules, a cell may not appear to be undergoing extensive disintegration, but it becomes leaky (releasing various ions and molecules) and it fails to take up essential substrates. Synthesis and externalization of colicins is characteristically linked to the death of the colicin-producing cell. Cells in the vicinity of a colicin-producing cell are insensitive (‘immune’) to the given colicin if they contain the same (repressed) colicin system and a functional immunity protein. In some cases, however, such immunity may be lost (immunity breakdown) in the presence of high concentrations of the given colicin; this presumably occurs because there are too many molecules of colicin for the cell’s immunity system to deal with. Externalized colicin molecules bind to specific receptors on the surface of sensitive cells, and it appears that internalization of the colicin molecule is usually or always an energy-dependent process; in at least some cases (e.g. colicin A), the colicin unfolds on binding to the receptor, such unfolding being essential for translocation through the cell envelope. The specific receptors used by colicins include the BtuB, FepA, FhuA, IutA and Tsx proteins and the OmpF porin. After binding, group A colicins (A, E1–E9, K, L, N, cloacin DF13 and bacteriocin 28b) are internalized via a complex of envelope proteins that include TolA, TolB, TolQ and TolR. After binding, group B colicins (B, D, Ia, Ib, M, 5 and 10) are internalized via a complex of proteins that include the TONB PROTEIN, ExbB and ExbD. [Colicin import into Escherichia coli cells: JB (1998) 180 4993–5002.] Within the target cell, a given colicin may act as a lethal agent by (i) forming pores in the cytoplasmic membrane, thereby collapsing proton motive force and allowing leakage of cytoplasmic constituents; (ii) functioning as a non-specific DNase; (iii) functioning as an RNase specifically against 16S rRNA; or (iv) inhibiting the synthesis of PEPTIDOGLYCAN. A given colicin is associated with a particular mode of lethal activity. Thus, e.g. pore-formers include the group A colicins E1, A, N and K and the group B colicins B, Ia, Ib, 5 and 10; RNases include group A colicins DF13 and E6; DNases include the group A colicins E2 and E8; inhibitors of peptidoglycan synthesis include the group B colicin M. (See also separate entries for colicin A, B, D, E, I, K, M, N, U, V, X and Y, and CLOACIN DF13.) 181 coliform coliform In general: any Gram-negative, non-sporing, facultatively anaerobic bacillus which can ferment lactose, with acid and gas formation, within 48 hours at 37° C. For water bacteriologists (in the United Kingdom), a coliform is defined as any member of the family Enterobacteriaceae which grows at 37° C and which normally encodes b-galactosidase [for further information see Report 71 (1994) HMSO, London (ISBN 0 11 753010 7)]. Escherichia coli is a typical coliform. coliform mastitis See MASTITIS (b). coliform test A test used to detect the presence of COLIFORM organisms in e.g. water (see INDICATOR ORGANISM and WATER SUPPLIES); the test was originally designed to reveal faecal contamination (hence the designation ‘faecal coliform test’). In the test, aliquots of sample are used to inoculate a number of tubes (see MULTIPLE-TUBE METHOD), each of which contains e.g. MACCONKEY’S BROTH and a DURHAM TUBE; incubation is carried out at 37° C. A ‘positive’ test (i.e. lactose fermentation) is shown by acidification (pH indicator) and gas production (Durham tube). The ‘most probable number’ of coliforms in the original sample can be calculated from the number of positive tubes and the use of statistical tables. The count thus obtained is actually a presumptive coliform count because, in any given tube, a ‘positive’ result could be due to certain endosporeforming bacteria (which also ferment lactose and form gas); this possibility is particularly important when testing chlorinated water samples because spore-forming bacteria are more resistant than coliforms to chlorine. Confirmation that each ‘positive’ result is, in fact, due to thermotolerant (= ‘faecal ’) coliforms involves two further 24-hour tests at 44° C: the INDOLE TEST and the EIJKMAN TEST. coli-granuloma Syn. HJÄRRE’s DISEASE. coliphage A BACTERIOPHAGE that infects Escherichia coli. (For ‘coliphage l’, ‘coliphage T4’ etc see BACTERIOPHAGE l, BACTERIOPHAGE T4, etc.) colisepticaemia A respiratory and septicaemic disease of poultry caused by strains of Escherichia coli. Incidence of the disease has increased greatly with the advent of modern intensive rearing methods, and often occurs secondarily to e.g. poor ventilation, vaccination of birds with live vaccines (e.g. live Newcastle disease vaccines), or various other respiratory diseases (e.g. AIR SACCULITIS, AVIAN INFECTIOUS BRONCHITIS, NEWCASTLE DISEASE). Infection occurs mainly via the respiratory tract; characteristic post-mortem findings include fibrinous pericarditis and perihepatitis. colistins See POLYMYXINS. colitis Inflammation of the colon. (cf. ENTERITIS.) Colitis occurs e.g. in certain infectious diseases (e.g. amoebic and bacillary DYSENTERY; see also SHIGA TOXIN). Antibiotic-associated colitis sometimes occurs when the normal bowel flora is suppressed by broad-spectrum antibiotic therapy; this condition ranges from mild diarrhoea to PSEUDOMEMBRANOUS COLITIS, and is commonly due to growth and toxin formation by Clostridium difficile (an organism normally rare in adults, but common in the bowel in infants). C. difficile produces at least two distinct hydrophobic protein toxins, A (an enterotoxin) and B (a cytotoxin) [JMM (1984) 18 385–391]; toxin A causes fluid accumulation in the intestinal lumen (but not by activating adenylate cyclase – cf. e.g. CHOLERA TOXIN), and the B toxin disaggregates filamentous actin in tissue culture cells [GE (1984) 86 1212 (abstr.)]. [Purification and properties of cytotoxin B: JBC (1986) 261 1316–1321.] Chemotherapy: e.g. vancomycin. colitose (3,6-dideoxy-L-galactose) A sugar, first isolated from Escherichia coli, which occurs in the O-specific chains of the LIPOPOLYSACCHARIDE in certain Salmonella serotypes (contributing to O antigen 35 in group O salmonellae – see KAUFFMANN–WHITE CLASSIFICATION) and in the LPS of E. coli serotypes of the O111 group. collagen The main protein component of bone, cartilage and other connective tissues; it is a microfibrillar protein whose subunit, tropocollagen, consists of three polypeptide chains in which the amino acid sequences are essentially (glycine-X-Y)n where X and Y are often proline and hydroxyproline, respectively. Collagen is generally resistant to enzymic degradation (cf. COLLAGENASE). Collagenolytic organisms may cause LEATHER SPOILAGE. (See also GELATIN.) collagenase A COLLAGEN-degrading enzyme; collagenases are produced e.g. by Clostridium histolyticum (the b-toxin) and C. perfringens. The collagenase of C. histolyticum appears to have maximum activity at the peptide bond preceding a glycineproline- sequence. collarette See CONIDIUM. Collema A genus of LICHENS (order LECANORALES); photobiont: Nostoc. The thallus lacks a cortex and is homoiomerous, foliose to fruticose, brownish- to greenish-black, pulpy and gelatinous when wet, firm and cartilaginous when dry. The apothecia, when formed, generally have reddish-brown discs. According to species, globular, cylindrical, coralloid or scale-like isidia may be formed. Species occur on bark, rocks, calcareous soils, etc. Colletotrichum A genus of fungi (order MELANCONIALES) which include some important plant pathogens (see e.g. ANTHRACNOSE, COFFEE BERRY DISEASE, RED ROT and SMUDGE). Conidiophores characteristically develop on setose acervuli (see SETA); the conidia, which lack appendages, are typically elongated, roundended and colourless. Collosphaera See RADIOLARIA. Collybia See AGARICALES (Tricholomataceae). colominic acid The capsular polysaccharide of Escherichia coli K1 strains; it is a linear polymer of N-acetylneuraminic acid residues linked by (2 → 8)-a-ketosidic bonds. [Biosynthesis: JB (1984) 159 321–328.] An apparently identical polysaccharide occurs in strains of Neisseria meningitidis (groups B and C). Colominic acid is resistant to most neuraminidases; however, it can be hydrolysed by an enzyme from Clostridium perfringens, and a neuraminidase associated with a bacteriophage (coliphage E) specifically depolymerizes colominic acid [JV (1985) 55 374–378]. colon microflora See GASTROINTESTINAL TRACT FLORA. colonization factor antigens See ETEC. colony (microbiol.) A number of cells or organisms (of a given species) which, during their growth, have developed as a discrete aggregate or group in which there is commonly direct contact or continuity between the cells. Algal colonies. Some algal colonies are of more or less stable size and form (see COENOBIUM sense 2; cf. PALMELLOID PHASE). In some species the colony is the only form in which the organism occurs. See e.g. SCENEDESMUS and VOLVOX. Bacterial colonies are usually formed only on, or within, a solid MEDIUM (such as an agar gel); each usually consists of a compact mass of individual cells, although certain bacteria (e.g. Streptomyces spp) form mycelial colonies. A discrete colony commonly comprises the progeny of a single cell (see also STREAKING). Many species of bacteria can form macroscopic colonies on appropriate media under suitable conditions; on a given medium, a colony’s shape, colour, consistency, surface appearance and size (for a given incubation time) are often highly characteristic, and these features are often of use in the 182 columella macrophages which, among other functions, stimulates the development of macrophages from precursor forms such as monoblasts and monocytes. Colorado tick fever An acute human disease caused by a virus and transmitted by the tick Dermacentor andersoni ; it occurs in the Rocky Mountain regions of the USA. Symptoms include fever, leucopenia, headache and myalgia, but no rash; CNS complications occasionally occur in young children. Wild rodents (e.g. ground squirrels) may act as a reservoir of infection. The causal agent has been regarded as an ORBIVIRUS, but has been shown to contain 12 dsRNA molecules [Virol. (1981) 112 361–364] and may thus represent a distinct taxonomic group. colostrum A secretion of the mammary gland produced before lactation proper; it contains e.g. immunoglobulins (mainly IgA and IgG). In man, the immunoglobulin content of colostrum is relatively low, but in some animals – e.g., ruminants (in which immunoglobulins do not cross the placenta) – large amounts are normally present. (See also PASSIVE IMMUNITY.) colour-breaking (breaking) The development of VARIEGATION in flower petals; colour-breaking may occur as a result of virus infection (e.g., in tulips infected with tulip breaking virus, a member of the POTYVIRUSES). Flowers affected in this way are often prized for their attractive appearance. Colour Index (CI) A comprehensive reference publication on dyes and pigments published by The Society of Dyers and Colourists, Bradford, Yorkshire BD1 2JB, England. It consists of a number of volumes which are periodically updated. coloured field illumination Syn. RHEINBERG ILLUMINATION. colourless sulphur bacteria Those non-photosynthetic SULPHUR BACTERIA which obtain energy by the oxidation of sulphur and/or reduced inorganic sulphur compounds (e.g. sulphide); although referred to as ‘colourless’, some strains can exhibit pigmentation owing to their content of cytochromes. The colourless sulphur bacteria include e.g. species of Beggiatoa and Thiobacillus. [Ecology of colourless sulphur bacteria: Book ref. 115, pp. 211–240.] Colpidium A genus of ciliates (order HYMENOSTOMATIDA) which occur e.g. in freshwater habitats containing decomposing organic matter. Cells: ovoid or reniform, ca. 50–150 µm, with uniform somatic ciliature and a single macronucleus, micronucleus and contractile vacuole. In C. colpoda (at least) asexual reproduction occurs within a cyst. Colpoda A genus of soil and freshwater ciliates (order COLPODIDA). Cells: typically reniform, ca. 40–120 µm in length, with uniform somatic ciliation; the cytostome is lateral. C. steini is a small species, ca. 20–60 µm, with a conspicuous tuft of cilia (the ‘beard’) projecting from the region of the vestibulum. Food consists of other protozoa, algae and bacteria. Asexual reproduction occurs within CYSTS – in which four or more individuals may be formed. Colpodida An order of protozoa (subclass VESTIBULIFERIA) in which the vestibular ciliature tends to be highly organized. Cysts are common. The organisms are typically free-living, sometimes associated with molluscs. Genera: e.g. COLPODA, Woodruffia. Colpomenia See PHAEOPHYTA. Colsargen See SILVER. Columbia SK virus See CARDIOVIRUS. columella An axial or central, unicellular or multicellular structure within a fruiting body in certain fungi and in certain slime moulds of the MYXOMYCETES; in some cases it is simply an extension of the sporangiophore into the cavity of a sporangium, while in others it may be a spherical, conical or cylindrical structure extending upwards from the base of a sporangium. Although identification of particular bacterial species. The full description of a colony can be very detailed. Thus, e.g. the elevation of a (surface) colony may be flat, low convex, domed, umbonate etc; its edge may be e.g. entire (i.e., circular and unbroken), crenate (scalloped), lobed or fimbriate; its texture may be butyrous, friable or mucoid; its surface may be matt or glossy; it may be whitish or pigmented, or it may contain a dye taken up from the medium. When the colonies of certain species develop on blood agar, each colony is surrounded by a zone in which the blood has been lysed or greened (see HAEMOLYSIS). (See also DAISY HEAD COLONIES, DRAUGHTSMAN COLONY, fried egg colony in MYCOPLASMA, MOTILE COLONIES, SMOOTH–ROUGH VARIATION.) Although a bacterial colony has been traditionally regarded as an aggregate of independent cells, some studies have provided evidence that the behaviour of a given cell in a colony is influenced by its position within the colony, i.e., the behaviour of a cell can be subject to ‘multicellular’ regulatory mechanisms [see Book ref. 198, pp. 27–69]. Other types of bacterial colony include e.g. the rosettes of CAULOBACTER spp, the arrays of Hyphomicrobium and RHODOMICROBIUM spp, and the networks of PELODICTYON. Fungal colonies. Some fungi (e.g. certain yeasts) form colonies which resemble those typical of bacteria. Mycelial fungi often form circular ‘fluffy’ colonies, each consisting of a mass of MYCELIUM which may or may not include reproductive structures (see e.g. PENICILLIUM). (See also PELLET sense 2; PETITE MUTANT.) Protozoal colonies. In the (few) colonial protozoa, the colony typically consists of SESSILE (sense 1) organisms which form a group of variable size (see e.g. CARCHESIUM). Chain-like colonies are formed by some members of the ASTOMATIDA. colony-forming unit See COUNTING METHODS. colony hybridization (modified Grunstein–Hogness procedure) A method for screening bacterial colonies for the presence of a specific sequence of DNA (e.g. among colonies containing a gene LIBRARY). As in a REPLICA PLATING procedure, cells are transferred from colonies on the master plate to a nitrocellulose (or other) filter. The cells on the filter are lysed with alkali – the alkali also serving to denature the DNA (i.e. to separate the strands of the duplex); the alkali is then neutralized, any protein is digested e.g. with proteinase K, and the filter is baked at 70–80° C under vacuum to bind the (single-stranded) DNA to the filter. The filter is then exposed to labelled probes (see PROBE) which are complementary to the sequence of interest. After washing away unhybridized probes, the given sequence, if present, is indicated by the label on bound (hybridized) probes; any probe-positive sites on the filter are then used to identify the corresponding (positive) colonies on the master plate. A probe labelled with a radioactive isotope is detected by AUTORADIOGRAPHY. In the original procedure [Book ref. 177, pp. 172–174], cells transferred from the master plate were allowed to grow and form colonies on the filter (which was overlaid on a plate of nutrient medium); subsequent steps were similar to those described. This technique can also be used for yeast cells, but these cells must be converted to sphaeroplasts (with e.g. ZYMOLYASE) before lysis. colony-stimulating factors (CSFs) CYTOKINES involved in the maturation of LEUCOCYTES. CSFs include e.g. granulocyte–macrophage colony-stimulating factor (GM-CSF), an agent which stimulates proliferation of progenitors of various blood cells (including granulocytes and macrophages), and macrophage colony-stimulating factor (M-CSF; also referred to as CSF-1), an agent formed mainly by monocytes and 183 column fermenter (T. brevifaciens); affected plants show severe stunting and chlorotic flecks on the leaves. (cf. KARNAL BUNT; see also CEREAL DISEASES.) common cold An acute disease of the upper respiratory tract in man; the causal agent may be any of a range of viruses – usually a rhinovirus, coronavirus, influenza virus (type A, B or C), parainfluenza virus (types 1–4) or human respiratory syncytial virus. Incubation period: usually ca. 48–72 hours. Symptoms typically include coryza and nasal obstruction, sore throat, cough, sneezing, but little or no fever. Direct contact may be an important mode of transmission of rhinoviruses [AIM (1978) 88 463]; the other viruses are probably transmitted mainly by aerosols (see DROPLET INFECTION). Transmission of rhinoviruses can be interrupted (experimentally) by the use of virucidal (citric acid-treated) paper handkerchiefs [JID (1986) 153 352–356]. Colds are usually self-limiting, but occasional complications include e.g. OTITIS MEDIA, PNEUMONIA or SINUSITIS. (cf. INFLUENZA.) [Review: Lancet (2003) 361 51–59.] common fimbriae Type 1 FIMBRIAE. common pili (1) Syn. FIMBRIAE. (2) Syn. Type 1 fimbriae. common scab (of potato) See POTATO SCAB. communicable disease Syn. INFECTIOUS DISEASE. comoviruses (cowpea mosaic virus group) A group of PLANT VIRUSES in which the bipartite positive-sense ssRNA genome is encapsidated in (separate) icosahedral particles; both parts are necessary for infection. Most comoviruses cause systemic mosaic or mottling and stunting – occasionally with wilting, necrotic ring formation, etc. – in various plants, including e.g. cowpea (Vigna unguiculata), beans, red clover etc; the host range of individual members is narrow. Most comoviruses are transmitted by leaf-feeding beetles, particularly of the Chrysomelidae (e.g. Cerotoma and Diabrotica spp); seed transmission occurs in some comoviruses, and mechanical transmission can occur readily under experimental conditions. Type member: COWPEA MOSAIC VIRUS (SB isolate). Other members: e.g. cowpea severe mosaic virus, quail pea mosaic virus, red clover mottle virus, squash mosaic virus. (cf. BROAD BEAN WILT VIRUS.) ComP See QUORUM SENSING. compact colony-forming active substance See CCFAS. comparative single intradermal tuberculin test (comparative test; single intradermal comparative tuberculin test) (vet.) A form of TUBERCULIN TEST, applied to cattle, in which two intradermal injections are given (on a single occasion – cf. STORMONT TEST) at different sites on the same side of the neck; PPD derived from Mycobacterium bovis is injected at one site, PPD derived from M. avium is injected at the other site. After 72 hours any increase in skin thickness at the injection sites is measured. If the response to PPD from M. bovis (increase in skin thickness) is greater than that to PPD from M. avium by a certain minimum amount then the animal is designated a ‘reactor’. A positive reaction to PPD from M. avium together with a negative reaction to PPD from M. bovis suggests contact with M. avium and/or mycobacteria other than M. bovis. The use of a defined antigen (ESAT-6), compared with PPD, has been associated with lower sensitivity but higher specificity [VR (2000) 146 659–665]. comparative test (vet.) Syn. COMPARATIVE SINGLE INTRADERMAL TUBERCULIN TEST. comparator See WATER SUPPLIES. compatibility (1) (of plasmids) See PLASMID. (2) (in fungal sexuality) The ability, or otherwise, of a given cell or thallus to interact sexually with itself and/or with other cells or thalli of the same species. In general, the genetic constraints on mating increase, both in number and complexity, from the lower columellae are commonly described as STERILE (sense 2), the columellae of Mucor piriformis have been reported to germinate and give rise to secondary sporangia under certain cultural conditions [Mycol. (1985) 77 353–357]. (cf. PSEUDOCOLUMELLA.) column fermenter See FERMENTER. columnaris disease (cotton wool disease; ‘mouth fungus’) An acute, often fatal freshwater FISH DISEASE caused by Flexibacter columnaris. Necrotic lesions occur on the skin and gills; cottony tufts of epithelium and bacteria appear around the mouth. ColV plasmid Any of a heterogeneous group of IncFI COLICIN PLASMIDS which encode COLICIN V; many also encode e.g. the aerobactin iron-uptake system (see SIDEROPHORES). ColV, I-K94 plasmid An IncFI COLICIN PLASMID which encodes COLICINS V and Ia; it occurs e.g. in Escherichia coli and is derepressed for conjugal transfer. ColX plasmid See COLICIN PLASMID. ComA See QUORUM SENSING. Comatricha See MYXOMYCETES. comb. nov. (combinatio nova; new combination) In NOMENCLATURE: the name of a species which has been moved from one genus to another but which has retained the specific epithet (suitably modified, when necessary, to agree with the form of the new genus name). combinatio nova See COMB. NOV. combined gold standard See GOLD STANDARD. combined residuals See WATER SUPPLIES. combining site (immunol.) That part of an ANTIBODY (or a FAB PORTION) which can combine with a determinant of the homologous antigen or hapten; it is located at the variable (Nterminal) ends of the heavy chain and light chain in an Ig molecule (see IMMUNOGLOBULINS and HYPERVARIABLE REGION). Monomeric antibodies have a VALENCY of two. (See also CYTOPHILIC ANTIBODIES and AFFINITY LABELLING.) come-up time In the CANNING process: the time needed for the correct processing temperature to be reached in the retort. comedone See ACNE. co-metabolism The phenomenon in which a substrate (the cosubstrate) which does not support the growth of a given microorganism can nevertheless be modified or degraded by that organism in the presence of a second, growth-supporting substrate. Co-metabolism is believed to result from the action of an enzyme of relatively low substrate specificity which has a different physiological function in the cell; for example, METHANE MONOOXYGENASE can oxidize certain short-chain alkanes and alkenes as well as methane, and such hydrocarbons can be oxidized by methane-grown cells (co-oxidation) which may be unable to use the oxidation products. In nature, co-metabolism may be important in the degradation of XENOBIOTICS. commensal See COMMENSALISM. commensalism SYMBIOSIS (sense 1) in which one symbiont (the commensal ) derives benefit from the association, and the other (sometimes called the host) derives neither benefit nor harm. (cf. MUTUALISM.) commercial sterility A term used when referring to the condition of a substance following APPERTIZATION. common bunt A seed-borne wheat disease caused by Tilletia caries or T. foetida (T. laevis). (Rye and certain other grasses may also be affected.) Symptoms include retarded growth and the formation of long, narrow ears which have a bluish tinge; grains are filled with grey or black powdery masses of teliospores which, when released during harvesting, give off a characteristic ‘rotten fish’ odour (trimethylamine). Dwarf bunt is a similar (soil- and seed-borne) disease caused by Tilletia contraversa 184 complement fixation Complement fixation can be triggered in various ways, the type of trigger determining which pathway is followed; four pathways are shown in the figure. (a) Classical pathway. This pathway is activated e.g. when antigens bind to COMPLEMENT-FIXING ANTIBODIES in the presence of complement. When such binding occurs, sequentially activated components may bind at or near the antigen–antibody complex; according to the antigen involved, this may result in e.g. phagocytosis of the antigen–antibody complex or, if a cellsurface antigen is involved, it may lead to cell lysis (see later). The cascade of reactions which occur in the (antigen-triggered) classical pathway is as follows. The sequence is triggered when two or more subunits of C1q (in the complement C1 molecule) bind to the FC PORTION of antigen-bound IgG or IgM antibodies. Such binding (dependent on calcium ions) activates the C1r component of C1 which, in turn, activates the C1s component. C1s has serine protease and esterase activity. (Note that antibody-independent activation of C1 can be promoted by certain non-specific activators – e.g. C-reactive protein (an ACUTE-PHASE PROTEIN) and various proteases.) Activated C1s cleaves C4, forming C4a and C4b; as one molecule of C1s can cleave many molecules of C4, this reaction provides initial amplification. C4b molecules can bind to bacterial or other surfaces; like C3b (see later), C4b is an OPSONIN: C4b-coated cells or particles bind (via C4b) to the CR1 receptor on the surface of phagocytes. Surface-bound C4b may also bind C2. C2 is cleaved (in a process involving C1s) to C2a and C2b; the complex formed between the larger fragment, C2a (referred to as C2b in some sources), and C4b (C4b2a) is a C3 convertase: an enzyme which cleaves C3 to C3a and C3b (this being a major phase of amplification in the complement cascade). (A low level of C3 cleavage normally occurs spontaneously (‘C3 tickover’) but the resulting fragments are rapidly degraded.) C3a is an ANAPHYLATOXIN. C3b is an important OPSONIN, promoting e.g. the phagocytosis of pathogens (IMMUNE ADHERENCE); C3b-bound antigens bind (via C3b) to CR1 receptors on phagocytes. C3b can also form a complex with C3 convertase: C4b2a3b; this complex is a C5 convertase, i.e. it cleaves C5 into the small fragment C5a and a larger fragment, C5b. C5a is an ANAPHYLATOXIN and a chemoattractant for phagocytes (e.g. neutrophils). C5b, C6 and C7 form a complex (C5b67) which binds to cell membranes – typically at the site of the initial triggering event. Component C8 binds to the complex; this promotes polymerization of a number of C9 molecules within the membrane – the C9 molecules forming a pore or channel which, in some types of cell (or even LIPOSOMES), can lead to lysis (immune cytolysis). C5b678 and the C9 molecules collectively form the so-called membrane attack complex, MAC. (Even in the absence of C9, the complex C5b678 can bring about slow lysis e.g. in erythrocytes.) For certain pathogens (e.g. trypanosomes), MAC-inflicted damage can be lethal. In Gram-negative bacteria, pores formed by the MAC in the OUTER MEMBRANE can promote cell lysis by giving LYSOZYME access to cell-wall peptidoglycan; thus, e.g. a Gram-negative bacterium on the conjunctiva would be at risk of lysis because the tear film normally contains both complement and lysozyme. Sometimes, C5b67 binds to a cell other than that which triggered the cascade; MAC-inflicted lysis in such a cell is called fungi, through the ascomycetes, to the basidiomycetes. HOMOTHis common in all the major groups of fungi. Bipolar HETEROTHALLISM occurs in some lower fungi (e.g. species of Mucor and Rhizopus), in many ascomycetes (including e.g. Saccharomyces), and in many of the rust and smut fungi. Tetrapolar heterothallism occurs in a number of basidiomycetes – including e.g. Schizophyllum commune and Ustilago maydis. (See also MATING TYPE and PARASEXUAL PROCESSES.) compatible (plant pathol.) Refers to a plant–pathogen interaction which results in the development of disease in the plant. (cf. INCOMPATIBLE.) compatible solute Any low-MWt compound which, when present intracellularly in high concentration, is compatible with metabolism and growth. Compatible solutes involved in OSMOREGULATION include: L-glutamate, GLYCEROL, ISOFLORIDOSIDE, MANNITOL, L-proline, glycine betaine [in Escherichia coli : FEMS Reviews (1994) 14 3–20; in Staphylococcus aureus: Microbiology (1994) 140 3131–3138], and certain sugars. compensation level See PHOTIC ZONE. competence See TRANSFORMATION (1). complement (C or C
; historical syn. alexin or alexine) (immunol.) A group of functionally related proteins present in normal plasma, tissue fluids and the (freshly isolated) serum of vertebrates. (In serum, the activity of complement is abolished by exposure to room temperature for a few days, by heating to 56° C for 30 minutes, or by the action of chelating agents such as EGTA or oxalate.) When activated, the complement system is responsible for a number of important physiological activities (see e.g. COMPLEMENT FIXATION). Before activation, many of the components exist as proenzymes and/or inactive forms (cf. FACTOR D) which, on activation, complex with, or act on, other components to form physiologically active entities. In man, complement consists of nine numbered components (C1–C9, MWts ca. 80000–200000) together with various proteins involved e.g. in stabilization or control functions: see e.g. FACTOR B; FACTOR D; FACTOR H; FACTOR I; PROPERDIN and S PROTEIN. Component C1 is a complex macromolecule which consists of three parts: C1q, C1r and C1s; C1q itself consists of a bundle of six elongated subunits, and the macromolecule as a whole depends on Ca2+ for its structural integrity. Gene-based deficiencies in the complement system are associated with various adverse effects – which depend e.g. on the particular component(s) involved. One important role of complement is the removal of immune complexes (via the liver), and deficiencies in C1, C4 and C2 may give rise to forms of disease reflecting the accumulation of such complexes. A deficiency in C3 (with reduced capacity for opsonization) may result in recurrent infections. A deficiency in MBL (lectin pathway) is associated with increased susceptibility to infection in children. Deficiencies in C5–C8 generally cause increased susceptibility to infection – particularly with Neisseria. complement activation Syn. COMPLEMENT FIXATION. complement fixation (complement activation) Activation of the COMPLEMENT system, i.e. promotion of the sequence (‘cascade’) of reactions in which various components of complement are activated in turn. In vivo, complement fixation is involved e.g. in certain immune responses and in INFLAMMATION; in vitro it provides a sensitive system for various COMPLEMENTFIXATION TESTS. (Complement fixation is sometimes referred to as complement inactivation – an allusion to the fact that components of complement are ‘used up’ during fixation.) ALLISM 185 complement fixation CLASSICAL LECTIN ALTERNATIVE C4 C4, C2 MBL MASPs C3b LPS etc. C1−ag−ab C4b C3bB C2 C4b2a amplification loop factor B factor D properdin PC3bBb (C3 CONVERTASE) C2a factor I (in plasma) (C3 CONVERTASE) C3 C3 C3b C3b C3b C3bi C4b2a3b P(C3b)2Bb (C5 CONVERTASE) C5 (C5 CONVERTASE) (as in classical pathway) C5b C6 C5b6 C7, 8, 9 C5b6789 (MEMBRANE ATTACK COMPLEX) COMPLEMENT FIXATION: four pathways (simplified scheme; see text). C1, C2 etc. denote particular components of the complement system; ‘a’ and ‘b’ denote fragments of components produced by enzymic cleavage during the activation process. For clarity, the diagram does not show all the cleavage products; for example, C4 is cleaved to C4a and C4b, but only the C4b fragment is considered here. Dotted lines indicate those cases in which a given complex or component acts enzymically to cleave certain components of the system. Reproduced from Bacteria 5th edition, Figure 11.1, page 292, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. C1INH), which inhibits initial activation of the classical pathway, and FACTOR I, which cleaves C3b to an inactive form, C3bi. (C3bi, although inactive in the cascade, can nevertheless behave as an opsonin – binding e.g. to the CR4 receptor on macrophages.) Other regulatory factors are involved in the control of the alternative pathway (see later). (b) Alternative pathway. In evolutionary terms, this pathway is apparently older than the classical pathway. In man, fixation via the alternative pathway is initiated by e.g. bacterial LIPOPOLYSACCHARIDES, rabbit (but not sheep) erythrocytes, aggregates of bystander lysis or reactive lysis. Reactive lysis may thus damage cells adjacent to a site of infection, and to limit such damage the plasma contains factors inhibitory to the C5b67 complex (see e.g. S PROTEIN). Because complement fixation creates highly potent physiological agents, and as it includes various stages of amplification, regulatory mechanisms are essential to prevent the system becoming an uncontrolled avalanche of physiological destruction. In fact, there are specific inhibitors at key stages in the process; these include C1EI (C1 esterase inhibitor; also called 186 complementation test lysed indicates the amount of free (unfixed) complement remaining. Practice: to detect or quantify e.g. a given antibody in a sample of serum, SERIAL DILUTIONS of the INACTIVATED SERUM are prepared, and known, fixed quantities of COMPLEMENT and antigen are added to each dilution; the quantity of complement added (determined by titration) should be sufficient to cause lysis of ca. 90% of the erythrocytes in the haemolytic system when no complement-fixation has occurred. Incubation is carried out for ca. 18 hours at ca. 4° C. (Low-temperature incubation is used because some components of complement are labile at room temperature.) A fixed volume of the haemolytic system is then added to each dilution, and the whole is incubated for 15–30 min at 37° C; the test dilutions are then left at ca. 4° C to permit non-lysed erythrocytes to settle. In any given dilution, the extent of haemolysis will be inversely proportional to the extent of complement fixation in that dilution. If maximum haemolysis is observed in the lowest serum dilution (i.e. the highest serum concentration) the test is negative – i.e. the serum contains no detectable antibody. In a positive test, no haemolysis occurs in (at least) the lowest serum dilution; a test may be regarded as positive either (i) when no haemolysis has occurred in a specified number of serum dilutions (e.g. all dilutions up to 1:8 or 1:16 initial serum dilution), or (ii) when an increase in the titre of complement-fixing antibodies is recorded in a second serum sample taken some time after the first. In all CFTs, controls are essential e.g. to check that the test system is functioning correctly and to preclude false-positive interpretations based on the effects of ANTICOMPLEMENTARY substances. complement-fixing antibodies Antibodies which ‘fix’ or ‘activate’ COMPLEMENT when they combine with their homologous antigens. IgG (subclasses 1 and 3) and IgM can bring about complement fixation via the classical pathway; aggregates of IgA (subclasses 1 and 2) can activate the alternative pathway. (See also FC PORTION.) complement receptor 1 See CD35. complementarity-determining regions (immunol.) Those HYPERVARIABLE REGIONS which occur at the surface of the COMBINING SITE of an antibody. complementary bases See BASE PAIR. complementation The ability of a gene (or protein) to compensate for (i.e., to ‘complement’) a functional defect in a homologous gene (or protein) when present in the same organism or cell. For example, a wild-type gene necessary for the synthesis of a given amino acid may, when introduced into a mutant organism auxotrophic for that amino acid, complement the mutant gene (by supplying an active form of the defective gene product) and restore the wild-type (prototrophic) phenotype. (See also COMPLEMENTATION TEST; IN VITRO COMPLEMENTATION ASSAY; NON-GENETIC REACTIVATION.) complementation test A test used to determine whether or not COMPLEMENTATION will occur in a cell with a given mutant phenotype when another mutant genome, encoding the same mutant phenotype, is introduced into that cell. The test involves bringing together, into the same cell, the two (haploid) mutant genomes (or a genome and part-genome – see MEROZYGOTE) and determining the resulting phenotype. The expression of a wildtype phenotype (a positive complementation test) indicates that the mutations are in different genes in the two genomes, i.e., one wild-type form of each gene can be supplied by each of the mutant genomes (intergenic or intercistronic complementation); this indicates that at least two genes are involved in the expression of that phenotype. (If the mutations were in the human myeloma proteins, inulin and ZYMOSAN. (In the human complement system, this pathway can be activated by sheep erythrocytes if the cell-surface sialic acids have been removed.) The sequence of reactions in this pathway is as follows. On a suitable surface (e.g. OUTER MEMBRANE LPS), component C3b (‘tickover’ C3b – see above) binds FACTOR B (in the presence of Mg2+ ) to form C3bB. The bound factor B is then cleaved by FACTOR D; this creates a C3 convertase (C3bBb) which is stabilized by binding to PROPERDIN (P). The C3 convertase (PC3bBb) cleaves C3, forming more C3b and giving rise to an amplification loop (see figure). The C3 convertase can bind a molecule of C3b to form a C5 convertase (P(C3b)2 Bb); the cascade then continues as in the classical pathway. Regulators of the alternative pathway include the plasma protein FACTOR I and the cell-bound decay-accelerating factor (DAF) (see CD55). Other cell-surface-associated inhibitory proteins include CR1, which can displace Bb from surface-bound C3bBb, and the membrane co-factor protein (MCP), which facilitates cleavage of C3b by factor I; collectively, these cell-surface factors help to protect host cells from the ongoing complement cascade (including the MAC) by inhibiting the cascade at the C3 convertase/C3b level. For experimental work on the alternative pathway, COBRA VENOM FACTOR is used as an activator, while COMPLESTATIN and HEPARIN are used as inhibitors. (See also SURAMIN.) When the classical pathway is stimulated, some of the C3b can bind Factor B and give rise to an alternative-pathway C3 convertase; in this way the alternative pathway can augment the classical pathway, and this sequence of reactions is sometimes called the amplification pathway. The alternative pathway can be studied independently of the classical pathway by using the Ca2+ chelator EGTA (the latter pathway being dependent on the presence of calcium ions). (c) The lectin pathway. This pathway effectively bypasses the C1 stage of the classical pathway: when the lectin pathway is triggered, a C3 convertase (C4b2a) is formed directly from C4 and C2. The lectin pathway involves several components of normal plasma: mannose-binding lectin (MBL) and the MBL-associated serine proteases (MASPs) (cf. component C1s in the classical pathway); the system is triggered when MBL binds to certain groups on the microbial cell envelope. [Mannose-binding lectin: IT (1996) 17 532–540.] The lectin pathway may be particularly useful in the young during the ‘window’ period of ∼12–18 months between loss of maternal antibody cover (IgG) and the development of an effective immune system. However, triggering of this pathway by a bacterial pathogen may be greatly inhibited if the pathogen has a capsule (e.g. Neisseria meningitidis). (d) The C2a pathway (= ‘salvage pathway’). This pathway, reported in 1997, may be important in the context of pathogenic mycobacteria. In the C2a pathway, C2a acts as a C3 convertase when it binds to the mycobacterial surface. [See TIM (1998) 6 47–49; TIM (1998) 6 49–50.] complement-fixation test (CFT) Any of a range of sensitive, in vitro tests in which COMPLEMENT FIXATION is used to indicate the presence or absence (or quantity) of a specific antigen or COMPLEMENT-FIXING ANTIBODY. Principle: antigen and antibody are allowed to interact in the presence of complement, and the amount of complement used up (a measure of the amount of antigen–antibody interaction) is estimated, indirectly, by the addition of a HAEMOLYTIC SYSTEM; the proportion of erythrocytes 187 complementing diploids and the heap may be turned and mixed at intervals. The pH may rise substantially (e.g. to 8–9), due e.g. to ammonia production, but later falls to ca. 6–7. After several days the temperature may fall to ca. 30–50° C and may remain at this level for several weeks; during this stage fungi become dominant. Initially, thermophilic members of the Mucorales (e.g. Rhizomucor pusillus) develop; these are then replaced by thermophilic hyphomycetes (e.g. Humicola spp, Thermomyces lanuginosus) and ascomycetes (e.g. Chaetomium thermophilum). Finally, basidiomycetes such as Coprinus cinereus (often accompanied by the zygomycete Mortierella wolfii ) become dominant [Book ref. 39, pp. 263–305]. During composting, most or all of the free soluble substrates, much of the CELLULOSE, HEMICELLULOSES and PECTIC POLYSACCHARIDES, and some of the LIGNIN, are degraded, and the loss in dry weight may reach 50% or more. compound II See SIDEROPHORES. compound ciliature Any form of somatic or oral ciliature which involves a discrete group of cilia (several to many) acting as a functional unit; examples: the CIRRUS, MEMBRANELLE, PARORAL MEMBRANE and SYNCILIUM. compound trichocyst Syn. FIBROCYST. Compsopogon See RHODOPHYTA. co-mutagenesis The generation of two or more mutations at closely linked loci: see e.g. MNNG. ComX See QUORUM SENSING. con gene See OMP. concanavalin A (con A; jack bean lectin) A mitogenic LECTIN (tetrameric form: MWt 102000) from the jack bean, Canavalia ensiformis; it can stimulate T cells and, if cross-linked (e.g. by anti-con A antibody), B cells. Con A binds e.g. to terminal a-D-mannopyranosyl and a-D-glucopyranosyl residues (binding requires Ca2+ and Mg2+ ), and can thus agglutinate e.g. bacteria whose cell wall TEICHOIC ACIDS contain a-linked glucosyl substituents. concatemer Two or more identical linear nucleic acid molecules (e.g. copies of a viral genome) in tandem, i.e., covalently linked end to end in the same orientation. (cf. CATENANE.) concatenate (1) (verb) To form a concatenate (sense 2). (2) (noun) Syn. CATENANE or CONCATEMER. (3) (adj.) Of e.g. spores: formed in chains. concentration exponent (of a disinfectant) See DILUTION COEFFICIENT. concentric bodies (mycol.) Rounded or ellipsoidal intracellular structures (ca. 300 nm diam.) which have a transparent core surrounded by concentric layers of varying optical density; they occur in almost all lichenized ascomycetes and in a few nonlichenized ascomycetes. Their origin and function are unknown. Similar structures (‘concentric granules’) are found in Allomyces and related genera; these may play a role as septal pore plugs [Mycol. (1987) 79 44–54]. (cf. CELLULIN GRANULES.) conceptacle (mycol.) A hollow structure within which spores are formed – e.g. a locule (see ASCOSTROMA). conchate Having the shape of one-half of a bivalve shell. Conchocelis stage See e.g. PORPHYRA. concolorous Similarly coloured. concomitant immunity Syn. PREMUNITION. condenser (substage condenser) In a microscope: a system of lenses which concentrates light and directs it onto the specimen (see MICROSCOPY, Fig. 1). Correct adjustment of the condenser is important for optimum image formation: see e.g. KÖHLER ILLUMINATION. The numerical aperture of a condenser (see RESOLVING POWER) should be at least as great as that of the objective lens with which it is used; an oil-immersion same gene in each genome, no wild-type form of that gene would be present and the mutant phenotype would continue to be expressed; if such a negative result is consistently obtained with a range of mutants of the same phenotype, then the phenotypic characteristic under study may be presumed to be under monogenic control.) Interpretation of the results of a complementation test can be complicated by several factors. (a) In some cases two mutant copies of a gene – each by itself encoding an inactive polypeptide – may be able to complement one another when present together in the same organism. This may occur e.g. when the normal active gene product is a monomer of an oligomeric protein, and the two types of defective monomer encoded by the two mutant alleles can somehow interact to give an oligomer with some activity (intragenic, intracistronic or interallelic complementation). The resulting phenotype usually has characteristics qualitatively or quantitatively intermediate between the wild-type and mutant phenotypes. (b) Certain mutant genes are dominant over their wild-type alleles: e.g., an inactive product of a mutant gene may bind to the active product of the wild-type gene to produce an inactive hybrid oligomer (negative complementation: see e.g. LAC OPERON); such dominant mutations can be detected by a CIS–TRANS TEST. (c) Recombination between two mutant copies of a gene may result in the restoration of a functional gene; this possibility can be minimized by using recombination-deficient strains (e.g. recA− bacteria). (d) Complementation can occur only if the genes under study produce diffusible products; it cannot occur e.g. if the mutation is in a control region (e.g. a promoter) which can affect the expression only of adjacent genes in the same genomic molecule. (See also CIS-DOMINANCE.) (e) A POLAR MUTATION can give a misleading result since a single mutation can inactivate two or more adjacent genes. (See also CISTRON.) complementing diploids See PROTOPLAST FUSION. complestatin A substance, produced by Streptomyces lavendulae, which inhibits the alternative pathway of COMPLEMENT FIXATION by combining with Factor B and preventing its binding to C3b. complete Freund’s adjuvant See FREUND’S ADJUVANT. complete medium (CM) A type of culture medium which contains nutrients sufficient to support the growth of both PROTOTROPHS and AUXOTROPHS. (cf. MINIMAL MEDIUM.) complete transduction See TRANSDUCTION. complex flagellum See FLAGELLUM (a) and FLAGELLIN. complex-mediated hypersensitivity Syn. TYPE III REACTION. composting A process involving the aerobic biological degradation of waste plant matter (e.g. straw, cotton waste, corn-cobs etc) or other organic waste (e.g. paper, dewatered sewage sludge, municipal refuse); composting reduces the bulk of the waste material and converts it to an innocuous form (‘compost’) which can be used e.g. as a soil conditioner. (See also MUSHROOM CULTIVATION.) Composting involves the activities of a succession of microorganisms whose nature depends to some extent on the nature of the material being composted. In the composting of e.g. wheat-straw: the damp straw is piled loosely (to allow free circulation of air); inorganic nitrogen and phosphorus – or animal or poultry manure – may be added. The activities of mesophilic bacteria generate heat, and the temperature in the centre of the heap may rise to ca. 60° C or more within a few days, creating conditions in which thermophilic actinomycetes predominate [ARM (1983) 37 198–200]. Air circulation is maintained by convection, 188 conidium conidiogenous cell See CONIDIUM. conidioma Any plectenchymatous structure which bears conidia – e.g. a pycnidium, sporodochium or synnema. conidiophore A hypha which bears one or more conidiogenous cells (see CONIDIUM). In e.g. some species of Colletotrichum, setae (see SETA) can function as conidiophores. conidiospore Syn. CONIDIUM. conidium (mycol.) An asexually-derived, non-motile SPORE formed in a blastic or thallic mode (see below) from a specialized conidiogenous (i.e., conidium-producing) cell (cf. SPORANGIOSPORE). Conidia are formed e.g. by many fungi of the DEUTEROMYCOTINA and by a few lower fungi (e.g. species of Bremia and Peronospora). The two basic modes of conidiogenesis (conidium formation) are as follows. Blastic conidiogenesis. An apical or lateral part of a conidiogenous cell expands and develops into a mature conidium, becoming delimited from the parent cell by a septum. Such a blastic conidium (= blastoconidium or blastospore) secedes (i.e., breaks away) from the conidiogenous cell by centripetal splitting of the septum (schizolysis) – half the septum becoming the base of the conidium, the other half forming the apex of the conidiogenous cell. If the conidiogenous cell subsequently elongates from a basal growing region it is said to exhibit basauxic development. (Cessation of hyphal tip growth – see GROWTH (b) – preceding blastic conidiogenesis may depend on changes in ion gradients across the envelope of the conidiogenous cell.) Thallic conidiogenesis. A septum-delimited part of a hypha becomes converted to a single, terminal or intercalary, conidium (a holothallic conidium) or to a chain of conidia (arthric conidia). (cf. ARTHROSPORE.) Secession of a terminal, holothallic conidium commonly occurs by rhexolysis: the circumferential splitting of the wall of the penultimate cell (which ruptures) a little below the septum of the conidium; arthric conidia may separate by rhexolysis (alternate cells being sacrificed) or by schizolysis. [Formation and germination of fungal arthroconidia: CRM (1986) 12 271–292.] There are various ways in which the conidiogenous cell can produce a succession of conidia or give rise to a crop of simultaneously formed conidia; some examples are given below. (a) Ampullate development. The apex of the conidiogenous cell becomes globose (‘ampullate’) and gives rise, simultaneously, to a number of blastoconidia. Ampullate development occurs e.g. in Gonatobotrys. (b) Annellidic development. After the first, terminal blastoconidium has been delimited by a septum (and either before or after its secession) a new wall is laid down on the inner surface of the old wall in the distal region of the conidiogenous cell; this new, annular wall, in conjunction with the apical septum, forms an inverted cup-like structure at the free end of the conidiogenous cell. (The formation of the new wall is referred to as enteroblastic proliferation of the conidiogenous cell.) Subsequently, the cup-like structure expands to form the next conidium; such a process, in which the new wall becomes continuous with the conidial wall, is referred to as holoblastic conidial development. Septum formation then delimits the second blastoconidium – which (sooner or later) secedes by schizolysis; the position of this septum is such that, following secession of the blastoconidium, part of the new wall remains in the conidiogenous cell, extending a little beyond the rim of the old wall. Thus, after a number of blastoconidia have been produced, the conidiogenous cell (annellide) exhibits a succession of bands (annellations) at the apical end, each annellation being the remnant of a layer of wall formed inside the previous layer. condenser should be used with an oil-immersion objective. Before entering the condenser, the light passes through an iris diaphragm (aperture diaphragm, substage diaphragm) which, by controlling the amount of light entering the condenser, controls the apical angle of the cone of light which illuminates the specimen. Hence, the aperture diaphragm should not be used as a means of reducing the intensity of illumination because this reduces the NA of the condenser (see RESOLVING POWER). (See also ABBE CONDENSER; CRITICAL ILLUMINATION.) condensing enzyme Syn. citrate synthase (EC 4.1.3.7) [see Appendix II(a)]. conditional lethal mutant A mutant organism (e.g. a bacterium or virus) in which the mutation does not significantly affect the phenotype under one set of conditions (permissive conditions) but is lethal or inhibitory to the organism under another set of conditions (restrictive or non-permissive conditions). See e.g. TEMPERATURE-SENSITIVE MUTANT and HOST-DEPENDENT MUTANT. conduction (mol. biol.) See CONJUGATION (1b)(i). condyloma (pl. condylomas or condylomata) An elevated, wartlike lesion of the skin. Condyloma acuminatum = genital wart (see PAPILLOMA); condyloma latum = a wide, flat condyloma occurring chiefly in the anogenital region in secondary SYPHILIS. Condylostoma See HETEROTRICHIDA. confluent growth See CULTURE. confocal scanning light microscopy (CSLM) A form of light MICROSCOPY in which linear resolution is superior to that obtainable with conventional light microscopes. Essentially, rays from a laser-illuminated pin-hole are focused in the plane of the specimen and collected by a lens system which rapidly scans the specimen in synchrony with the illuminating source. As the optical system scans the specimen the rays entering the collector lens are continually modulated by the specimen; the modulated rays are focused onto a detector plate – generating a signal which modulates a cathode ray tube and forms a visual display of the image. congeneric Of the same genus. congenital Present at birth. conglutinated ‘Glued together’ – used e.g. of hyphae in certain prosenchymatous fungal or lichen tissues. conglutination See CONGLUTININ. conglutinin A protein, present in normal bovine serum, which can bind to a cleavage product of complement component C3b (see COMPLEMENT FIXATION); conglutinin can agglutinate particles/cells to which the cleavage product has adhered – a phenomenon termed conglutination. conglutinogen Potential activity associated with complement component C3b; cleavage of C3b by FACTOR I exposes a site which can bind CONGLUTININ. conglutinogen-activating factor Syn. FACTOR I. Congo–Crimean haemorrhagic fever See VIRAL HAEMORRHAGIC FEVERS. Congo red A red acid diazo DYE used e.g. as a PH INDICATOR (pH 3.0–4.5, blue to red) and for staining b-linked fibrillar polysaccharides (e.g. cellulose); Congo red disrupts CELLULOSE and CHITIN microfibril formation in growing cells (cf. CALCOFLUOR WHITE). conidia Plural of CONIDIUM. conidial head (mycol.) The expanded terminal portion of the conidiophore in certain fungi – see e.g. ASPERGILLUS. conidiation The phenomenon in which conidia function as gametes. Conidiobolus See ENTOMOPHTHORALES. conidiogenesis See CONIDIUM. 189 Coniocybe TRANSPOSON-mediated conjugation is considered separately in the entry CONJUGATIVE TRANSPOSITION. Any of various plasmid-borne genes (and, in some cases, chromosomal genes) may be transferred during conjugation; such genes include those conferring resistance to antibiotic(s). Conjugation occurs in both Gram-positive and Gram-negative bacteria; however, as the process differs in Gram-positive and Gram-negative species it is discussed under separate headings for the two categories. (i) Conjugation in Gram-negative bacteria. Much of the information on this topic has derived from studies on the transfer of the F PLASMID (q.v.), and other IncF plasmids, between strains of Escherichia coli ; the following account is based largely on this information. It is important to note that the general features of the F plasmid–E. coli transfer system are not common to all transfer systems found in Gram-negative bacteria. The stages of conjugation are outlined below, in sequence, on the basis of current models and data. Initial cell–cell contact. The presence of a (plasmid-encoded) pilus (see PILI) is believed to be essential for the donor phenotype. Moreover, different types of pilus mediate conjugation under different physical conditions, and they may function in different ways; for example, some types of pilus mediate conjugation only on moist (but not submerged) solid surfaces (e.g. agar plates), while others mediate conjugation either within the liquid phase or on a solid surface (see PILI) [see also Book ref. 177, pp 33–59 (42–45)]. In the F plasmid–E. coli conjugal system, initial donor–recipient contact is believed to occur when the tip of the pilus binds to a site on the surface of the recipient; the pilus is then believed to retract so that the cells achieve wall-to-wall contact. Stable cell–cell contact, involving specific plasmid-encoded proteins, is established after an initial period of unstable contact (during which donor and recipient are easily separated by mild shearing forces). (Mutant E. coli recipients defective in the OmpA protein (see OMP) form unstable contacts with donor cells, although this deficiency may be overcome by mating on a moist solid surface.) (See also SURFACE EXCLUSION.) An understanding of the initial donor–recipient interaction would require a detailed knowledge of the structure and mode of action of the pilus – from initial contact to the putative retraction. This information is currently unavailable; indeed, the conjugal transfer of DNA, even in the well-studied F plasmid–E. coli system, and particularly in the initial stages of cell–cell contact, has been described as a ‘black box’ [Mol. Microbiol. (1997) 23 423–429]. Some authors report that the region of contact between donor and recipient is characterized by a specific, electron-dense conjugational junction [JSB (1991) 107 146–156; JB (2000) 182 2709–2715]. The composition of this electron-dense layer is unknown but one suggestion is that, in F plasmid-mediated matings, it may consist of, or contain, F-pilin (the subunit of the F pilus) [Mol. Microbiol. (1997) 23 423–429]; however, results from recent studies on RP4-mediated matings indicated that conjugational junctions are not composed of pilin [JB (2000) 182 2709–2715]. On establishment of effective cell–cell contact, a ‘mating signal’ (nature unknown) promotes the mobilization of donor DNA. Mobilization and transfer of DNA. Mobilization is the process in which (in at least some cases, including the F plasmid) a single, specific strand of donor DNA is prepared for transfer to the recipient. Annellidic development occurs e.g. in Scopulariopsis. (See also PERCURRENT PROLIFERATION.) (c) Phialidic development is very similar to annellidic development, the essential difference being that, in phialidic development, successive septa are formed at or near the level of the rim of the old wall; annellations are therefore not formed, although the edges of successive wall layers in the conidiogenous cell (phialide) may be visible just within the apical rim (collarette) formed by the original wall of the conidiogenous cell. Phialidic development occurs e.g. in Phialophora and Trichoderma. (d) Retrogressive development is similar to annellidic and phialidic development but differs in that each blastoconidium formed takes with it, on secession, a part of the original cell wall of the conidiogenous cell; the conidiogenous cell thus progressively shortens. It occurs e.g. in Cladobotryum. (e) Tretic (porogenous) development. The wall of the conidiogenous cell contains a narrow pore or channel; materials for conidial growth pass through this channel during the formation of the blastoconidia (‘poroconidia’ or ‘porospores’) – which arise either solitarily or in acropetally formed chains. Tretic development occurs e.g. in Alternaria. [Reviews: MS (1984) 1 86–89 and MR (1986) 50 95–132.] Conidiophores may arise individually or in discrete masses: see e.g. ACERVULUS, COREMIUM, PYCNIDIUM and SYNNEMA. Coniocybe See CALICIALES. Coniophora A genus of fungi of the APHYLLOPHORALES (family Coniophoraceae). C. puteana (= C. cerebella, ‘cellar fungus’), a cause of WET ROT, forms a thin, resupinate fruiting body which is initially cream-coloured but later olivaceous; the basidiospores are brownish. (See also CELLULASES.) Coniophoraceae See APHYLLOPHORALES. coniosporiosis Syn. MAPLE BARK STRIPPERS’ DISEASE. Coniothyrium See SPHAEROPSIDALES. conjugate (1) (verb) Carry out CONJUGATION (sense 1). (2) (noun) See CONJUGATION (sense 2). conjugate division See CLAMP CONNECTION. conjugate vaccine A VACCINE containing a polysaccharide antigen linked (conjugated) to a protein; such vaccines are useful for young children whose immune system does not respond adequately to (unconjugated) polysaccharide antigens. Some diseases are caused by pathogens with polysaccharide capsules, and conjugate vaccines containing such polysaccharides can elicit protective antibodies in young children. For example, a conjugate vaccine is used against diseases (epiglottitis, meningitis etc.) caused by Haemophilus influenzae type b [RMM (1996) 7 231–241]. [Responsiveness of infants to capsular polysaccharides: RMM (1996) 7 3–12.] [Vaccine against group B streptococci IV and VII: JID (2002) 186 123–126. Meningococcal C vaccine (in teenagers): Lancet (2003) 361 675–676.] conjugation (1) (‘mating’) Any of various processes in which gene transfer (or, exceptionally, complete fusion) follows the establishment of direct contact between two or more microbial cells (which may exhibit little or no morphological differentiation in comparison to vegetative cells). (a) (algol.) Conjugation occurs in several groups of algae (e.g. DESMIDS, certain DIATOMS, SPIROGYRA) and generally involves cell–cell contact followed by protoplast fusion. (b) (bacteriol.) In bacterial conjugation, DNA is transferred from a ‘male’ (donor ) bacterium to a ‘female’ (recipient) bacterium while the cells are in physical contact; a recipient which has received DNA from a donor is called a transconjugant. (cf. RETROTRANSFER.) The donor phenotype is commonly conferred on a cell by the intracellular presence of a CONJUGATIVE PLASMID (sense 1); 190 conjugation In the F plasmid, an essential prerequisite for mobilization is the formation of a NICK at a specific site (designated nic), in a specific strand, within the origin of transfer (oriT); nicking occurs in that strand which is to be transferred to the recipient (the T-strand). Nicking is mediated by an endonuclease (a relaxase) that forms part of a so-called relaxosome (previously called a relaxation complex). The relaxosome associated with the F plasmid is a nucleoprotein complex consisting of the transfer origin (oriT), the relaxase, and certain other proteins. (Relaxases are encoded e.g. by the F plasmid traI gene, the traI gene of plasmid RP4, and the nikA gene of Salmonella plasmid R64.) As well as endonuclease activity, the TraI protein of the F plasmid also has (ATP-dependent) helicase activity and is referred to as helicase I. Components of the relaxosome are assembled in a specific order: TraI binds after the INTEGRATION HOST FACTOR and the TraY protein [JBC (1995) 270 28381–28386]. Recent studies confirm that TraY is required for in vivo nicking [JB (2000) 182 4022–4027]. The relaxase is believed to mediate an ongoing cycle of nicking and ligation at the nic site, i.e. nicking appears not to be triggered by donor–recipient contact. Following the mating signal in F plasmid–E. coli systems, it appears that the nicked strand enters the recipient in the 5
-to
3 direction. (Single-strand transfer in the 5
-to-3
direction is believed to occur in at least some other cases.) Transfer of the nicked strand from donor to recipient requires that the strand be unwound from its complementary strand, and this is thought to involve helicase I (TraI); if helicase I is involved, and if it is immobilized in relation to the cell envelope, then the unwinding of the transferred strand may provide energy for strand transfer. One early study indicated that the transferred strand could pass through an extended pilus to the recipient [JB (1990) 172 7263–7264]. A later study – which used video-enhanced lightmicroscopy and electron microscopy – concluded that DNA is transferred while donor and recipient are in close wall-to-wall contact [JSB (1991) 107 146–156]. The precise mode of transfer of DNA from donor to recipient is not known. For F plasmid-mediated systems, it has been postulated that the relaxosome may be coupled, via TraD, to a transport apparatus (transferosome) consisting of a protein complex located in the cell envelope at the base of the pilus. In RP4-mediated mating, the transfer apparatus in the donor may include a structure that bridges the cytoplasmic and outer membranes [JB (2000) 182 1564–1574], although this was not detected in another study on RP4-mediated mating [JB (2000) 182 2709–2715]. DNA synthesis in the recipient cell. When a cell receives the transmitted strand of an F plasmid, a complementary strand is synthesized to form a circularized dsDNA molecule; this recA-independent process is called repliconation. Complementary strand synthesis apparently involves the recipient’s DNA polymerase III and other host-encoded enzymes. For certain IncI and IncP plasmids (e.g. ColIb, RP4), synthesis of DNA in the recipient is initiated by a donor-plasmid-encoded PRIMASE, i.e. during conjugation, primase, as well as ssDNA, is transferred from donor to recipient. In such cases, DNA synthesis is initiated from primers which are formed at various locations on the transferred strand. [Plasmid DNA primases in conjugation: Book ref. 161, pp 585–603.] Matings with ColIb-P9 involve the transfer, from donor to recipient, of molecules which include the product of the sog gene, which has primase activity [EMBO (1986) 5 3007–3012]. When chromosomal genes have been conjugally transferred (following mobilization of the chromosome), recombination between donor and recipient DNA appears to involve the RECBC PATHWAY. DNA synthesis in the donor cell. In the donor cell a complementary strand is synthesized on the non-transmitted strand in order to reconstitute the dsDNA plasmid; this synthesis, carried out by DNA polymerase III, is referred to as donor conjugal DNA synthesis (DCDS). DCDS is generally believed to be carried out by the ROLLING CIRCLE MECHANISM (although earlier claims that primers are needed for DCDS had made this uncertain). More recently, a study using recombinant plasmids containing two tandemly repeated R64 oriT sequences (one initiation-proficient but termination-deficient, the other initiation-deficient but termination-proficient) has provided evidence of rolling circle synthesis [JB (2000) 182 3191–3196]. [DNA processing reactions in bacterial conjugation: ARB (1995) 64 141–169.] Mobilization of non-conjugative plasmids. The term ‘mobilization’ commonly includes instances in which (a) a nonconjugative plasmid (or part of the bacterial chromosome) is conjugally transferred by becoming covalently linked to a conjugative plasmid (transfer being initiated at oriT in the conjugative plasmid), or (b) a non-conjugative plasmid is conjugally transferred independently of (that is, without covalent linkage to) a conjugative plasmid by means of cell–cell contacts established by the conjugative plasmid; mobilization of the latter type does not necessarily involve co-transfer (i.e. concomitant transfer) of the conjugative plasmid. (Some authors [ARG (1979) 13 99–125 (106)] have referred to (a) and (b) as conduction and donation, respectively.) (See also HFR DONOR and INTERRUPTED MATING.) The F plasmid can mobilize certain non-conjugative plasmids, e.g. the COLE1 PLASMID, in a way that does not require covalent linkage. Plasmid ColE1 is associated with a relaxosome which can be activated to promote mobilization and transfer of plasmid DNA; the in vivo mechanism of activation is unknown. During F-mediated mobilization and transfer of ColE1, one of the proteins of the relaxosome apparently remains covalently bound to the 5
end of the transferred strand, and it has been suggested that this protein may act as a ‘pilot protein’, interacting with membrane protein(s) and guiding the 5
terminal into the recipient. Mobilization of the non-conjugative plasmid R1162 involves at least four trans-acting plasmid-encoded products and a cisacting site (oriT), oriT being distinct from oriV [JB (1986) 167 703–710]. Effects of physicochemical parameters on conjugation. Relatively few studies have been carried out on the effects of variation in the physicochemical conditions under which cell–cell contact occurs. However, a number of early studies were conducted on the effects of variation in basic parameters such as pH, temperature, electrolyte concentration and osmotic pressure; one purpose of these studies was to characterize the response of the conjugal process to changes in ‘external’ conditions, thus obtaining data potentially useful e.g. for optimizing transfer in the laboratory. These studies found e.g. that transfer of the IncFII plasmid R1drd19 could be greatly stimulated by higher levels of electrolyte [FEMS (1983) 20 151–153]; that the plasmid could be transferred between 37 and 17° C [AEM (1981) 42 789–791]; that the effects of non-optimal pH and non-optimal temperature are synergistic [AEM (1983) 46 291–292]; that transfer is subject to osmotic constraint [FEMS (1984) 25 37–39]; and 191 conjugational junction pronucleus depends on its position within the cytoplasm; the nucleus which survives is located in the cytostomal region [JCS (1985) 79 237–246].) The surviving pronucleus (= gonal nucleus) divides mitotically to form a pair of gametic nuclei, one of which passes to the conjugal partner and fuses with the stationary nucleus to form a zygotic nucleus (= synkaryon). Subsequently, mitotic divisions of each synkaryon lead to the formation of four diploid nuclei in each conjugant – the conjugants by now having separated; two of the nuclei give rise to two new micronuclei, while the other two form two new macronuclei. During the first binary fission after conjugation, the micronuclei (but not the macronuclei) undergo mitotic division; each daughter cell receives one macronucleus and two micronuclei – thus restoring the normal nuclear constitution of the species. In P. aurelia conjugation takes ca. 12–18 hours. [DNA synthesis, methylation and degradation during conjugation in Tetrahymena thermophila: NAR (1985) 13 73–87.] In peritrichs, two morphologically dissimilar conjugants are formed: a female macroconjugant and a smaller, motile, ciliated microconjugant which swims in search of, and fuses with, a macroconjugant. (See also AUTOGAMY and CYTOGAMY.) (d) (mycol.) See e.g. MATING TYPE and PHEROMONE. (2) (immunol.) The process of covalently linking two or more species of molecule to form a hybrid molecule (a conjugate); for example, a dye may be linked to a protein (as in fluoresceinconjugated antiglobulin), or glutaraldehyde may be used to link two different antibodies to form a hybrid antibody [SAB (1984) 22 73–78]. (See also CONJUGATE VACCINE.) conjugational junction See CONJUGATION (1b)(i). conjugative pili See PILI. conjugative plasmid (1) (self-transmissible plasmid) As commonly used: a PLASMID which encodes all the functions needed for its own intercellular transmission by CONJUGATION (sense 1b); in e.g. the F PLASMID (q.v.) these functions include pilus formation (see PILI) and the formation of proteins involved in the initial preparation of plasmid DNA for transfer. Such a plasmid may also bring about the mobilization of a (mobilizable) NON-CONJUGATIVE PLASMID or the mobilization of the donor’s chromosome. (cf. SEX FACTOR; see also ANDROPHAGE and PROMISCUOUS PLASMIDS.) (2) According to some authors [ARG (1979) 13 99–125 (105–106)]: a plasmid which encodes e.g. pilus formation but which may or may not be capable of mobilization; only if such a plasmid is mobilizable is it self-transmissible. conjugative transposition In bacteria: CONJUGATION mediated by a conjugative transposon, i.e. independently of a conjugative plasmid, the transposon being transferred from the donor cell to the recipient cell. First reported in Gram-positive bacteria, it is now well established in Gram-negatives (e.g. Bacteroides); indeed, it may permit gene transfer between Gram-positives and Gram-negatives [e.g. AEM (2001) 67 561–568; AEM (2003) 69 4595–4603]. A model for Gram-positive conjugative transposition was as follows. Contact between donor and recipient triggers excision of the transposon in the donor, this involving a transposon-encoded recombinase (product of gene int). Each end of the excised transposon carries single-stranded host DNA called a coupling sequence. The excised transposon then circularizes through basepairing (albeit mis-matched) between coupling sequences. A single strand may be transferred to the recipient, the complementary strand being synthesized in the recipient prior to insertion. Insertion of the (double-stranded) transposon into the target site is neither that transfer is inhibited by particles of colloidal clay [AEM (1983) 46 756–757]. Transfer of the IncN plasmid R269N1, which mediates surface-obligatory mating (see PILI), appears to need, or be assisted by, surface tension [FEMS (1983) 19 179–182; JGM (1983) 129 3697–3699]. Some early experiments [e.g. JGM (1987) 133 3099–3107] purported to demonstrate the possibility of plasmid transfer in natural aquatic environments but were flawed by extreme artificiality. (ii) Conjugation in Gram-positive bacteria. Conjugal transfer is known to occur in organisms from various genera, including Bacillus, Enterococcus, Lactococcus, Staphylococcus and Streptomyces. As in conjugation in Gram-negative bacteria, some plasmids can be transferred across a broad host range; for example, plasmid pAMb1 , which encodes resistance to erythromycin, can be transferred from streptococci to e.g. Bacillus subtilis and Lactobacillus casei. There are two main types of plasmid-mediated conjugation in Gram-positive species: PHEROMONE-mediated and (apparently) pheromone-independent mating; pili have not been demonstrated in either category. Pheromone-mediated conjugation in e.g. strains of Enterococcus faecalis (see PHEROMONE) occurs in liquid (broth) cultures, and the plasmids involved commonly contain genes conferring resistance to antibiotics. Some of these plasmids also encode a peptide which antagonizes the corresponding pheromone; such a peptide may have the function of ensuring that a mating response in the donor cell is not triggered unless the relevant pheromone is in a sufficiently high concentration, i.e. conditions under which there is a good chance of a random collision between donor and recipient cells. [Review: JB (1995) 177 871–876.] Pheromone-independent conjugation requires that the donor and recipient be present on a solid surface (e.g. a nitrocellulose filter) – rather than in a liquid medium. Conjugation of this type occurs e.g. in strains of Staphylococcus and Streptococcus. The plasmids, which encode e.g. resistance to antibiotics, are commonly >15 kb in size. In Bacillus subtilis, recombinants may be obtained by coincubation of two apparently plasmid-less parent strains for extended periods of time (e.g. 20 hours); in such crosses most exconjugants contain the entire genome of both parent strains, and the process is believed to resemble a form of temporary cell fusion [Book ref. 199, pp 25–39]. (cf. PROTOPLAST FUSION.) As in Gram-negative bacteria, certain plasmids may be mobilized by other plasmids, and some of the small multicopy plasmids encode function(s) which specifically promote their own mobilization. Mobilization of the streptococcal plasmid pMV158 has been studied in Escherichia coli using R388 or RP4 as the auxiliary plasmid [Microbiology (2000) 146 2259–2265]. (c) (ciliate protozool.) Conjugation occurs in many ciliates; it involves either the temporary association of two individual cells (as e.g. in Paramecium or Tetrahymena) or the complete fusion of conjugal partners (as e.g. in peritrichs). In some ciliates (e.g. Blepharisma) conjugation is promoted by soluble substances (see GAMONE) which are released by the cells; in other ciliates (e.g. Paramecium) the chemical signals remain bound to the cell. In Paramecium aurelia, two individuals of appropriate mating type (see SYNGEN) pair with their ventral surfaces in contact, and an intercellular cytoplasmic bridge forms in the cytostomal region. In each conjugant (i.e. partner) the MACRONUCLEUS disintegrates, and both of the micronuclei (see MICRONUCLEUS) undergo meiosis to form eight haploid pronuclei; seven pronuclei disintegrate. (In Paramecium caudatum the fate of a given 192 contagious equine metritis consortium A more or less stable physical association between the cells of two or more types of microorganism – advantageous to at least one of the organisms; SYNTROPHISM seems to characterize many (perhaps all) consortia. One example is CHLOROCHROMATIUM AGGREGATUM. [Review: ME (1996) 31 225–247.] conspecific Of the same species. constant region (of Ig) See IMMUNOGLOBULINS. constitutive Refers either to a gene which is expressed constantly, or to the product of such a gene. The level of expression of a constitutive gene depends largely on the efficiency of its PROMOTER. constitutive heterochromatin See CHROMATIN. consumption Pulmonary TUBERCULOSIS. consumption test (serol.) Any test in which a measurement is made of the amount of antibody or antigen which is used up (consumed) by antigen–antibody combination in a test system; the amount consumed is determined by titrating the residual (unconsumed) antibody or antigen and comparing it with the quantity initially present. (See e.g. ANTIGLOBULIN CONSUMPTION TEST.) contact biotrophic mycoparasite A fungus which parasitizes other fungi by means of specialized cells which make contact with – but do not penetrate or cause obvious damage to – mycelium of the (living) host; most of the known species (including e.g. Calcarisporium parasiticum) require biotin, thiamine, and an unidentified substance, ‘mycotrophein’, for growth. (See also MYCOPARASITE.) contact-dependent secretion See type III systems in PROTEIN SECRETION. contact dermatitis Syn. CONTACT SENSITIVITY. contact inhibition See transformation in TISSUE CULTURE. contact sensitivity (contact dermatitis) A form of DELAYED HYPERSENSITIVITY in which the subject is primed by cutaneous exposure to certain protein-binding substances – e.g. dinitrofluorobenzene, certain metals (e.g. nickel) and antibiotics (e.g. neomycin) – which bind skin proteins to form neoantigens; subsequent challenge, after ca. 1 week, with the sensitizing substance results in a typical lesion in 24–48 hours. (See also LANGERHANS’ CELLS.) contagious abortion See BRUCELLOSIS. contagious acne Syn. CONTAGIOUS PUSTULAR DERMATITIS (2). contagious bovine pleuropneumonia In cattle: acute lobar PNEUMONIA, with pleuritis, caused by a strain of Mycoplasma mycoides; infection occurs mainly by droplet inhalation. Incubation period: 3–6 weeks (sometimes longer). Onset is sudden, with fever and anorexia; subsequently there is a deep cough and dyspnoea. Death from anoxia may occur within days or weeks (mortality rate up to ca. 50%); survivors may become asymptomatic carriers. Treatment: antibiotic and/or other therapy is generally given only where the disease is enzootic. (See also BOVINE RESPIRATORY DISEASE.) contagious disease (1) Syn. INFECTIOUS DISEASE. (2) A disease normally transmissible only by direct physical contact between infected and uninfected individuals – as e.g. in many VENEREAL DISEASES. contagious ecthyma Syn. CONTAGIOUS PUSTULAR DERMATITIS (1). contagious equine metritis (CEM) A sexually transmitted HORSE DISEASE (first recognized in Suffolk, UK, in 1977) characterized by endometritis with mucopurulent vulval discharge and temporary infertility; clinical symptoms usually resolve rapidly. A symptomless carrier state occurs in both mares and stallions. The causal agent is a previously unknown bacterial species. It is a Gram-negative coccobacillus (occasionally filamentous); catalase +ve; oxidase +ve; asaccharolytic; gives −ve reactions in site-specific nor random; each target site seems to contain an A-rich sequence of nucleotides and a T-rich sequence, the two sequences being separated by about six nucleotides. In Bacteroides (Gram-negative) excision and transfer of the conjugative transposon CTnDOT (which is found in many strains of Bacteroides) is governed by an OPERON that may be regulated by a TRANSLATIONAL ATTENUATION mechanism [JB (2004) 186 2548–2557]. A recipient which has received a conjugative transposon is a transconjugant; receipt confers the donor phenotype. (Interestingly, strains of Lactobacillus lactis can receive conjugative transposons but, apparently through lack of particular host function(s), transposons cannot be excised – so that L. lactis cannot act as a donor.) Characteristically, conjugative transposons are resistant to restriction. It may be relevant that the orf18 gene in Tn916 has been found to encode a product similar to the antirestriction proteins encoded by some plasmids. All conjugative transposons from pathogenic bacteria carry the tetracycline-resistance tet(M) gene [mechanism of Tet(M): JB (1993) 175 7209–7215], and some carry other types of antibiotic-resistance gene(s). These transposons are mobile among a wide range of host species, and are often responsible for antibiotic resistance in plasmid-less Gram-positive pathogens; they have been identified in e.g. Enterococcus faecalis, E. faecium and Streptococcus pneumoniae. Conjugative transposons occur in e.g. plasmids and chromosomes, and range in size from about 18 kb (Tn916 ) to >50 kb; some of the larger ones (e.g. Tn5253 ) apparently consist of a Tn916-like entity within another transposon (each being able to transpose independently). Conjugative transposons include Tn916, Tn925, Tn1545, Tn3710 and Tn5253. They differ from the ‘classical’ transposons in several ways: (i) they mediate conjugation; (ii) when they insert, they do not duplicate the target site; (iii) when excised, the transposon forms a cccDNA intermediate; (iv) the recipient duplex is characteristically in another cell. conjunctivitis Inflammation of the conjunctivae. It may be due e.g. to physical or chemical irritation, allergic reaction, or to infection by bacteria (e.g. Haemophilus influenzae, Moraxella lacunata) or viruses (e.g. adenoviruses). (See e.g. ACUTE HAEMORRHAGIC CONJUNCTIVITIS; EPIDEMIC KERATOCONJUNCTIVITIS; GONORRHOEA; INCLUSION CONJUNCTIVITIS; PHARYNGOCONJUNCTIVAL FEVER; PINK-EYE; TRACHOMA.) conk A colloquial name for the fruiting body of a wood-rotting basidiomycete (particularly a polypore). Conocybe See AGARICALES (Bolbitiaceae). conocyst An EXTRUSOME of the gymnostome Loxophyllum. conoid A hollow cone of spirally arranged fibrils or tubules, open at the apex, found at the extreme anterior end in certain members of the APICOMPLEXA; it may assist in the penetration of the host cell. consensus sequence (mol. biol.) Of the variant forms of a given type of genetic element (e.g. a PROMOTER): a theoretical ‘representative’ nucleotide sequence in which each nucleotide is the one which occurs most often at that site in the various forms of the genetic element which occur in nature. The phrase is also used to refer to an actual sequence which approximates the theoretical consensus. conserved name In NOMENCLATURE: a name retained (by a recognized taxonomic body) even though it may contravene rule(s) of the relevant code. consolidation (med.) See e.g. PNEUMONIA (a). 193 contagious hypovirulence in a chemostat tend to be unstable at or near the maximum growth rate. In the turbidostat a culture normally grows at or near the maximum growth rate, and all substrates are usually present in excess; within certain limits, the concentration of biomass in the turbidostat can be selected by the operator, and a steady state is achieved by adjusting the dilution rate such that cell growth is matched by the rate of loss of cells from the culture vessel. Control of culture density (i.e. number of cells per unit volume) may be achieved e.g. by a photosensitive device which monitors the opacity of the culture and automatically adjusts the flow rate; however, this method may suffer from inaccuracies due e.g. to foaming and to ‘wall growth’ (see later). Alternatively, culture density can be controlled by monitoring other parameters which are closely linked to specific growth rate – e.g. pH (in a ‘pH-stat’), CO2 evolution (in a ‘CO2 -stat’), or oxygen uptake. In continuous culture, deviations from ideal (predictable) operation are common. Such deviations may arise e.g. as a result of the inability to achieve perfect (100%, instantaneous) mixing of the inflowing medium with the culture fluid; imperfect mixing may be indicated by a stable dilution rate which exceeds Dc . Changes in the rate of agitation can affect the mean cell volume of the cultured cells [JGM (1985) 131 725–736]. Wall growth (adherence to, and growth of, the cultured cells on the walls of the culture vessel) may give an effect similar to (but usually greater than) that of imperfect mixing. Another type of deviation may occur when cells are cultured at low rates of growth; for example, under carbon-limited conditions a major part of the carbon source may be used to provide MAINTENANCE ENERGY (thus giving a lower-than-predicted yield of biomass), while under carbon-excess conditions the yield of biomass may be greater than predicted owing to the accumulation of intracellular reserve polymers (e.g. PHB). A different type of problem involves the emergence of mutants during the (extended) periods of culture – an effect which can change the character of the biomass if particular mutant(s) can outgrow the parent strain; nevertheless, it is possible to take advantage of this effect – e.g. when it is desired to isolate mutants which synthesize more (or more effective) enzyme(s) or which can take up the growthlimiting substrate more efficiently than can the parent strain. continuous-flow culture Syn. CONTINUOUS CULTURE. continuum model See CELL CYCLE. Contophora Algae except members of the RHODOPHYTA. (cf. ACONTA.) contour-clamped homogeneous electric field electrophoresis (CHEF) See PFGE. contractile vacuole An osmoregulatory organelle, one or more of which may occur in the cytoplasm in most freshwater (and some saltwater) eukaryotic microorganisms (cf. PUSULE); basically, it is a membrane-limited region which, under normal environmental conditions, alternately fills with fluid (diastole) and then discharges the fluid to the exterior (systole). During diastole, the contractile vacuole may be fed by a surrounding layer of small vesicles which appear to coalesce with the main vacuole (as in amoebae), and/or – depending on species – it may be fed by a system of collecting tubules which ramify in the cytoplasm (forming the spongiome, = spongioplasm, = ‘nephridial network’) and apparently drain areas remote from the vacuole (a system characteristic of ciliates). In some protozoa (e.g. Paramecium, Tetrahymena) discharge occurs at a fixed site (pore) in the pellicle. The mechanism of contractile vacuole activity is not known. Systole may involve cytoplasmic turgor pressure; at least in most biochemical tests; grows slowly e.g. on chocolate agar at 37° C (optimum temperature); GC%: 36.1. [Book ref. 181, pp. 49–96.] contagious hypovirulence See HYPOVIRULENCE. contagious pustular dermatitis (1) (orf; contagious ecthyma; scabby mouth) A disease which affects mainly sheep (especially lambs) – in which pustular, scab-forming lesions form on the lips and face and in the mouth; the causal agent, a PARAPOXVIRUS, is presumed to infect via abrasions. Morbidity may be high, but mortality rates are generally very low. In cattle, lesions may be formed on the teats. In man, the disease is localized and selflimiting, involving the formation of lesions e.g. on the hands, arms or eyelids. (2) (syn. contagious acne) A HORSE DISEASE characterized by the formation of pustules, particularly where the skin is in contact with harness; causal agent: Corynebacterium pseudotuberculosis. contaminative infection (1) (parasitol.) Infection by pathogens derived from the POSTERIOR STATION of the vector. (cf. INOCULATIVE INFECTION.) (2) Infection brought about by the ingestion of material containing pathogens. context (mycol.) In basidiomycetes: the sterile (i.e., nongenerative) structural tissue which forms the main bulk of a fruiting body, particularly that of the pileus. (cf. TRAMA.) continuous cell line Syn. ESTABLISHED CELL LINE. continuous centrifugation See CENTRIFUGATION. continuous cooker–cooler See COOKER–COOLER. continuous cultivation Syn. CONTINUOUS CULTURE. continuous culture (continuous-flow culture; continuous cultivation; open culture; open-system culture) The culture of microorganisms, in a liquid medium, such that the organisms can exhibit continual exponential or near-exponential growth (balanced GROWTH) for an extended period of time. (cf. BATCH CULTURE.) Essentially, the procedure involves a continual inflow of fresh medium to the culture (with which it is well mixed) and the simultaneous outflow of an equal volume of fluid consisting of a mixture of old and fresh medium and a proportion of biomass; under ‘steady-state’ conditions the concentration of biomass in the culture vessel remains constant. In continuous culture, growth can occur, ideally, under defined and effectively invariant conditions; for this reason continuous culture is used e.g. for studies of microbial metabolism: data from continuous culture are generally more reproducible than are those from batch culture. The commonest forms of apparatus used in continuous culture are the chemostat and the turbidostat (see also GRADOSTAT). In the chemostat a culture normally grows at a sub-maximal growth rate, and a steady state (at a given growth rate) is achieved by controlling the concentration (always at growthlimiting levels) of an essential substrate in the incoming medium (and, hence, in the culture vessel). (By using very low concentrations of a given essential substrate it is possible to achieve very low rates of growth – similar to those believed to occur e.g. in certain aquatic habitats.) Under steady-state conditions, the growth rate and the concentration of the growth-limiting substrate often have the relationship predicted by the Monod equation (see SPECIFIC GROWTH RATE), and the specific growth rate is numerically equal to the DILUTION RATE. Under steadystate conditions the concentration of biomass in the chemostat is governed by the rate at which the growth-limiting substrate is supplied to the culture, and (for a given rate of supply of that substrate) the concentration of biomass cannot be varied by the operator. (See also YIELD COEFFICIENT.) The conditions 194 copy number However, in effective concentrations, copper and certain of its compounds are useful antimicrobial agents; toxicity to microorganisms appears to reside in the cupric ion. Copper sulphate has been used as an anti-algal agent in e.g. swimming pools and as a constituent of various agricultural antifungal agents (see e.g. BORDEAUX MIXTURE, BURGUNDY MIXTURE, CHESHUNT COMPOUND). Copper naphthenate is a greenish, waxy solid, soluble in various organic solvents, which is used as an antifungal preservative for e.g. cellulosic textiles. Copper soaps are copper salts of certain fatty or oleoresinous acids (e.g. copper stearate, copper tallate) which have antifungal activity similar to that of copper naphthenate. Cuprammonium hydroxide is used in aqueous solution for rot-proofing fabrics. Cupric ions are complexed by 8-HYDROXYQUINOLINE (one atom of copper to two molecules of 8-hydroxyquinoline) to form copper 8-quinolinolate (copper-oxine; copper oxinate) which is a highly effective antifungal agent used e.g. in agriculture and as a preservative for textiles. (Copper compounds are not used in rubber products since they catalyse the oxidation of rubber.) copper 8-quinolinolate See COPPER. copper soaps See COPPER. co-precipitation (serol.) The precipitation of otherwise nonprecipitating molecules etc. as part of, or enmeshed with, an immune complex. Coprinaceae See AGARICALES. Coprinus A genus of humicolous, lignicolous or coprophilous fungi (AGARICALES, Coprinaceae) in which, in most species, the lamellae undergo rapid autodigestion at maturity (see LAMELLA and BASIDIOSPORE). (See also INK-CAP FUNGI.) coproantibodies Antibodies produced by the intestinal mucosa and found in the lumen of the gut; they are mainly of the secretory IgA class. Coprococcus A genus of Gram-positive, asporogenous, anaerobic bacteria which occur e.g. in the human gut; the organisms ferment carbohydrates with the production of butyric and acetic acids. Cells: cocci, occuring in pairs or chains. GC%: ca. 39–42. coprogen See SIDEROPHORES. Copromyxa See ACRASIOMYCETES. Copromyxella See ACRASIOMYCETES. coprophilic (coprophilous) Refers to an organism which grows preferentially or exclusively on or in animal faeces. For example, certain fungi (e.g. species of Coprinus, Pilobolus and Sordaria) grow more or less specifically on dung – particularly that of herbivorous animals. co-protease (coprotease) A type of molecule which, acting nonenzymically, can promote autocatalytic cleavage of a protein; an example is RecA∗ in the SOS SYSTEM. coprozoic Refers to e.g. protozoa which are COPROPHILIC. copy-choice model See RECOMBINATION. copy-mutant (copy-number mutant; cop mutant) A mutant PLASMID whose COPY NUMBER differs from that of the wild-type plasmid. In copy mutants the copy number is usually higher than that of the wild-type plasmid, but in some cases it is lower. copy number (1) (of a bacterial plasmid) The number of copies of a given PLASMID, per chromosome, in a cell; copy number depends on the replication control system encoded by the plasmid, on the strain or species of cell in which the plasmid occurs, and on growth conditions. In plasmid-containing bacteria in the exponential phase of growth, a given plasmid occurs with a characteristic copy number; any perturbation in copy number tends to be corrected by a rise or fall in the frequency of plasmid replication. Mutations in a plasmid (and/or chromosome) may give rise to a COPY MUTANT. ciliates, the vacuole wall may be inherently contractile. In general, the length of the diastole–systole cycle (seconds, minutes or hours) appears to depend on temperature, on the size of the cell (being shorter in smaller cells), and on the osmolarity of the medium. [Review: Biol. Rev. (1980) 55 1–46.] convalescent serum SERUM from a patient in the convalescent stage of a disease; in some cases it may be used to provide PASSIVE IMMUNITY. convergent trama See BILATERAL TRAMA. Convoluta A genus of flatworms (platyhelminths), found on sandy sea shores in the intertidal zone, which form a symbiotic association with certain unicellular green algae. On hatching, the young flatworms ingest cells of TETRASELMIS; the algal cells lose their flagella, theca and eyespot, and reside between the cells of the subepidermal tissues in the flatworm where they continue to photosynthesize. The flatworm eventually ceases to feed, its digestive organs degenerate, and it becomes completely dependent on the photosynthate produced by Tetraselmis. Eventually, the algal cells themselves are digested, and the animal then dies. (cf. ELYSIA; see also ZOOCHLORELLAE.) cooked meat medium (chopped meat medium; Robertson’s cooked meat medium) Any of a range of media containing boiled, de-fatted, minced lean beef, and used e.g. for the growth of ANAEROBES, for the sporulation of clostridia, and as maintenance media for e.g. clostridia and streptococci; in some formulations the medium incorporates a reducing agent (e.g. L-cysteine or thioglycollate) and e.g. haemin, vitamin K1 and yeast extract. The medium is sterilized by autoclaving, and the final pH is about 7.4; it is stored in screw-cap bottles, and is sometimes equilibrated under O2 -free conditions at room temperature before the cap is tightened. [Recipe: (e.g.) Book Ref. 53, pp. 1423–1424.] cooker–cooler (continuous cooker–cooler) In CANNING: a vessel within which filled, sealed cans (or other containers) undergo heat treatment in a continuous-type process (cf. BATCH RETORT). In rotary-type cooker–coolers, the cans enter and leave, through self-sealing valves, a chamber containing steam under pressure. In hydrostatic-type cooker–coolers, pressure in the steam chamber supports vertical columns of water through which the cans enter and leave the chamber. cooking (industrial microbiol.) (1) In the food processing industry: heat treatment used for APPERTIZATION. (See also c0 .) (2) A stage in CHEESE-MAKING. Coomassie brilliant blue A sensitive stain for proteins. The red (anionic) form of the dye becomes blue when bound to protein amino groups. Coombs’ reagent (serol.) Syn. ANTIGLOBULIN. Coombs’ test (serol.) Syn. ANTIGLOBULIN TEST. Cooper–Helmstetter model Syn. HELMSTETTER–COOPER MODEL. cooperative binding A mode of binding (e.g. of protein molecules to a DNA strand) in which the binding of one molecule facilitates binding of the next. co-oxidation See CO-METABOLISM. cop gene A gene involved in the control of the COPY NUMBER of a plasmid. (See e.g. R1 PLASMID.) cophenetic correlation coefficient In NUMERICAL TAXONOMY: a measure of the accuracy with which a phenogram represents a given S matrix. copiotroph Any organism which grows only in the presence of high concentrations of nutrients. (cf. OLIGOTROPH.) copper (as an antimicrobial agent) Copper is a HEAVY METAL which is essential (in trace amounts) for the activity of a number of microbial proteins (e.g. cytochrome aa3 , tyrosinase). 195 CoQ velvety, exhibiting a number of dark bands concentric about the region of attachment, while the lower surface is pale; basidiospores: cream-coloured, ca. 6 × 2 µm. (See also WHITE ROT and LIGNIN.) corn oil test (for lipase) See LIPASE. corn stunt disease A leafhopper-transmitted YELLOWS disease of maize, characterized by stunting and leaf striping/discoloration, caused by the MAIZE CHLOROTIC DWARF VIRUS or by certain strains of Spiroplasma citri. [Proposal to regard the corn stunt spiroplasmas as a distinct species, S. kunkelii : IJSB (1986) 36 170–178.] cornmeal agar A mycological medium. Essentially, cornmeal (4% w/v) is heated in distilled water for 1 hour at 60–65° C; agar (1.5% w/v) is added to the filtrate, and the medium is autoclaved. Cornmeal agar is used e.g. to demonstrate chlamydospore formation in Candida albicans – which is reported to be encouraged by the addition to the medium of Tween 80. cornute Syn. ROESTELIOID. Coronaviridae A family of pleomorphic, enveloped, nonsegmented ssRNA viruses which infect man, animals or birds. One genus (Coronavirus) is currently recognized. Members include AVIAN INFECTIOUS BRONCHITIS virus (IBV; type species), human coronavirus (a causal agent of the COMMON COLD), murine hepatitis virus (MHV), porcine haemagglutinating encephalitis virus (see VOMITING AND WASTING DISEASE), and porcine TRANSMISSIBLE GASTROENTERITIS virus; probable members include e.g. coronavirus enteritis of turkeys virus (= turkey bluecomb disease virus), and neonatal calf diarrhoea coronavirus. ‘Possible members’ include feline infectious peritonitis virus (= feline coronavirus) and human enteric coronavirus (a possible causal agent of human FOOD POISONING). Virion: ca. 75–160 nm diam., consisting of a helical nucleocapsid surrounded by the envelope; characteristic club-shaped glycoprotein projections (peplomers, ‘spikes’) 12–24 nm long extend from the outer surface of the envelope. Genome: positivesense ssRNA, MWt typically ca. 6–7 × 106 (8 × 106 has been reported for IBV). The RNA is polyadenylated at the 3
end. Proteins associated with the virion include (at least) S (‘spike’), M (membrane) and N (nucleocapsid). The virions are inactivated e.g. by lipid solvents and by detergents. Virus replication occurs in the host cell cytoplasm. In IBV, 6 subgenomic mRNAs (A–F) are formed, all of which are 3
coterminal with the genomic RNA (a so-called ‘nested set’ of mRNAs); each 5
end has a common leader sequence (ca. 70 bases long) derived from the 5
end of genomic RNA [possible mechanism: JGV (1986) 67 221–228]. (MHV forms a ‘nested set’ of 7 subgenomic mRNAs, designated RNA1–RNA7.) The virions mature by budding through the endoplasmic reticulum into vesicles. [Molecular biology of coronaviruses: AVR (1983) 28 35– 112.] Coronavirus See CORONAVIRIDAE. Coronie wilt Syn. HARTROT. correction collar A part of a microscope objective lens which is used to adjust the lens to work with cover-glasses of various thicknesses. corrinoids See VITAMIN B12 . Corriparta subgroup See ORBIVIRUS. corrosive sublimate See MERCURY. cortex (1) Any of various external or outer layer(s) of a structure or organism – e.g. the outer differentiated tissue of certain LICHENS; in ciliates ‘cortex’ refers to the PELLICLE together with the INFRACILIATURE. (2) See ENDOSPORE. Copy number is determined primarily by the regulation of initiation of plasmid replication; thus, e.g. a particular mutant of pBR322 (a plasmid related to the COLE1 PLASMID – q.v.) which encodes an altered (less effective) negative regulatory molecule (RNA I) exhibits an 8-fold increase in copy number [JGM (1986) 132 1021–1026]. Other factors which govern copy number include the PARTITION system (if any) of the plasmid. In some plasmids the ccd system contributes to the stability of plasmid inheritance (see F PLASMID). (See also INCOMPATIBILITY and MULTICOPY PLASMID.) (2) The number of copies of e.g. a given gene product per copy of that gene (or per cell), or the number of copies of a given gene per cell etc. CoQ Coenzyme Q: see QUINONES. coral fungi See APHYLLOPHORALES (Clavariaceae). coral spot See NECTRIA. coral symbiosis See ZOOXANTHELLAE. Corallina See RHODOPHYTA. Corallococcus See MYXOBACTERALES. Corallomyxa See ACARPOMYXEA. cord factor Any of the MYCOLIC ACID diesters of TREHALOSE found in the cell walls of Mycobacterium spp and in other mycolic acid-containing species. The name ‘cord factor’ was originally given to a glycolipid (6,6
-dimycolyl-a,a
-D-trehalose) derived from tubercle bacilli characterized by growth in the form of ‘cords’. Certain mycolic acid esters (e.g. 6,6
-dimycolyla,a
-D-trehalose, or 5-mycolyldiarabinoside) from the walls of M. tuberculosis can potentially act as ENDOTOXINS (sense 2) by uncoupling mitochondrial electron transport and oxidative phosphorylation [Ann. Mic. (1983) 134 B 233–239]. cordycepin (3
-deoxyadenosine) An inhibitor of RNA synthesis; it is obtained e.g. from Cordyceps militaris. Cordyceps A genus of fungi (order CLAVICIPITALES) which include parasites of insects and of other fungi. At least some entomogenous species can form CHITINASES, and infection appears to be initiated via the cuticle and haemolymph rather than via the gut; the fungus subsequently forms, on the dead insect, an erect, cylindrical or clavate stroma which contains perithecia in the surface layer. The dead host is resistant to decay – apparently due to the presence of CORDYCEPIN. C. militaris (‘scarlet caterpillar fungus’) forms orange stromata (ca. 2–5 cm high) on the larvae and pupae of butterflies and moths. C. canadensis is parasitic on Elaphomyces spp. core (1) (virol.) Any of various internal structures within a virion – commonly a protein–nucleic acid complex; the core may be enclosed within a CAPSID (see e.g. HERPESVIRIDAE), a membrane (see e.g. POXVIRIDAE) or an ENVELOPE (see e.g. RETROVIRIDAE). (cf. NUCLEOCAPSID.) (2) (bacteriol.) See ENDOSPORE sense 1(a). core oligosaccharide See LIPOPOLYSACCHARIDE. core particle (of chromatin) See CHROMATIN. coremium In some fungi (e.g. Penicillium expansum): an erect bundle of spore-bearing hyphae, the hyphae being associated more loosely than those in a SYNNEMA. (For some authors the terms coremium and synnema are synonymous.) corepressor See OPERON. coriaceous Leathery in texture. Coriolus A genus of fungi of the APHYLLOPHORALES (family Polyporaceae) which form non-stipitate, typically semicircular, bracket-type basidiocarps on deciduous wood; the context is trimitic, usually leathery, rubbery or woody, and the hymenophore is porous. In C. versicolor (sometimes called Trametes versicolor ) the upper surface of the basidiocarp is often 196 coryza typically form shiny, greyish-black, entire colonies, 1–2 mm at 24 hours, on blood–tellurite media; haemolysis is very common. Each cell typically contains several metachromatic granules. The ulcerans variety [Book ref. 46, p. 1831] (= C. ulcerans) produces urease and gelatinase, and ferments starch and trehalose as well as glucose, maltose and mannose; haemolysis is common. Pyrazinamide is not hydrolysed. (See also ULCERACIN 378.) C. equi. See RHODOCOCCUS. C. fascians. See RHODOCOCCUS. (See also FASCIATION.) C. flaccumfaciens. See CURTOBACTERIUM. C. glutamicum. A species which occurs e.g. in soil and vegetable matter; certain strains are used for the industrial production of GLUTAMIC ACID. C. glutamicum is nutritionally fastidious; biotin is necessary for growth, and some strains are reported to need B vitamins. C. haemolyticum. See ARCANOBACTERIUM. C. hydrocarboclastus. See CORYNECINS. C. minutissimum. A species [incertae sedis: Book ref. 21, p. 605] which is the causal agent of ERYTHRASMA; the organisms ferment glucose, maltose, mannose, and usually sucrose [Book ref. 46, p. 1831]. C. oortii. See CURTOBACTERIUM. C. ovis. See C. pseudotuberculosis. C. parvum. See PROPIONIBACTERIUM. C. poinsettiae. See CURTOBACTERIUM. C. pseudotuberculosis (formerly C. ovis). The causal agent of pyogenic infections in e.g. cattle, horses and sheep (see e.g. CONTAGIOUS PUSTULAR DERMATITIS (2), and ULCERATIVE LYMPHANGITIS). The organisms typically form yellow colonies; acid is produced from glucose, maltose and mannose, and urease is formed. Some strains are haemolytic. Pyrazinamide is not hydrolysed. C. pyogenes. See ACTINOMYCES. C. renale. A species isolated e.g. from urinary tract infections in cattle. Colonies are typically cream to yellow; glucose and mannose (but not maltose) are fermented, and urease is formed. Pyrazinamidase and caseinase are formed. C. salmoninus. See RENIBACTERIUM. C. ulcerans. See C. diphtheriae. C. xerosis. A species which occurs e.g. on the conjunctivae and in the throat in man; usually non-pathogenic, but cases of endocarditis following aortic valve replacement have been reported. Colonies are greyish; typically, glucose, maltose, mannose and sucrose are fermented, and pyrazinamidase is formed. Urease-negative. corynecins Compounds analogous to chloramphenicol in which the N-dichloroacetyl substituent is replaced with nonhalogenated acyl groups; corynecins have been isolated from ‘Corynebacterium hydrocarboclastus’. (A type strain of C. hydrocarboclastus has been shown to have the properties of a species of Rhodococcus [JGM (1982) 128 2503–2509].) coryneform (1) (‘club-shaped’) Essentially rod-shaped with one end thickened or bulbous. (2) A name applied, loosely, to any Gram-positive, asporogenous, pleomorphic rod-shaped bacterium; as such it covers bacteria from a range of genera. (cf. DIPHTHEROID.) Coryneliales See ASCOMYCOTINA. corynephage A BACTERIOPHAGE which infects Corynebacterium spp. (See also DIPHTHERIA TOXIN.) corynomycolic acids MYCOLIC ACIDS of Corynebacterium spp. coryza Discharge from the nasal mucous membranes – a symptom of e.g. the COMMON COLD and the prodromal phase of MEASLES. corticate Having a CORTEX. Corticiaceae See APHYLLOPHORALES. corticicolous Syn. CORTICOLOUS. corticolous (corticicolous) Growing on and/or in bark. (cf. EPIPHLOEODAL and ENDOPHLOEODAL.) corticosteroid synthesis See STEROID BIOCONVERSIONS. corticotype (ciliate protozool.) The morphological features of the CORTEX determined by staining (particularly silver staining) techniques. (See also SILVER LINE SYSTEM.) Corticoviridae (PM2 phage group) A family of icosahedral, lipid-containing, non-enveloped BACTERIOPHAGES which contain supercoiled ds cccDNA. Type species: BACTERIOPHAGE PM2. Possible member: bacteriophage 06N 58P (host: Vibrio). Corticovirus Currently the sole genus of the CORTICOVIRIDAE. cortina A remnant of the PARTIAL VEIL (q.v.) in a mature fruiting body. (‘Cortina’ is also used to refer to an intact partial veil.) Cortinariaceae See AGARICALES. Cortinarius See AGARICALES (Cortinariaceae) and MYCETISM. cortisol synthesis See STEROID BIOCONVERSIONS. Corynebacterium A genus of Gram-positive, aerobic, facultatively anaerobic, chemoorganotrophic, non-acid-fast, non-motile, asporogenous bacteria (order ACTINOMYCETALES, wall type IV). The genus contains saprotrophic species found e.g. in soil and vegetable matter (including e.g. some species previously classified in the genera Arthrobacter, Brevibacterium and Microbacterium), and a number of species which are parasitic or pathogenic in man and other animals; all the plant-pathogenic species have been reclassified in other genera (e.g. CURTOBACTERIUM, RHODOCOCCUS). Corynebacterium spp exhibit certain features common to all nocardioform actinomycetes – e.g. the presence of cell wall MYCOLIC ACIDS. The organisms are straight or curved, pleomorphic, often coryneform rods. Metabolism can be oxidative or fermentative. Catalase-positive. GC%: ca. 51–59. Type species: C. diphtheriae. C. autotrophicum. See XANTHOBACTER. C. betae. See CURTOBACTERIUM. C. bovis. Occurs e.g. associated with the cow’s udder. Unlike other Corynebacterium spp it contains tuberculostearic acid. C. diphtheriae. Cells rod-shaped, 0.3–0.8 × 0.8–8.0 µm. Toxinogenic strains of C. diphtheriae are the causal agents of DIPHTHERIA (see also DIPHTHERIA TOXIN and CROUP); the organisms may be isolated e.g. on tellurite–blood agar (see TELLURITE MEDIA) and subcultured to Loeffler’s serum. (Growth from Loeffler’s serum is used for the determination of cell morphology; cells grown on tellurite media have an atypical morphology.) The optimum growth temperature is 37° C. C. diphtheriae occurs in three main varieties, gravis, intermedius and mitis, which can be distinguished by cell and colonial morphology and by biochemical activity. All three varieties are urease-negative, and none can hydrolyse pyrazinamide; all produce acid (but no gas) from glucose, maltose and mannose, and some mitis strains can ferment sucrose [Book ref. 46, p. 1831]. Gravis strains typically form DAISY HEAD COLONIES (ca. 3 mm diam. at 24 hours/37° C) on blood–tellurite media; haemolysis is uncommon. Typically, starch is fermented in 24–48 hours. Very few cells contain METACHROMATIC GRANULES. Intermedius strains typically form small colonies (<1 mm diam. at 24 hours/37° C) on blood–tellurite media, the colonies usually being dull greyish-black with a slightly crenated edge; haemolysis does not occur. The cells rarely contain metachromic granules; stained cells exhibit a characteristic ‘barred’ appearance due to uptake of the stain (polychrome methylene blue) in a series of transverse bands. Mitis strains 197 cos cos See BACTERIOPHAGE LAMBDA; BACTERIOPHAGE P2; COSMID. Coscinodiscus See DIATOMS. Cosmarium A large genus of placoderm DESMIDS. The cells are elliptical or angular in outline, with a deep sinus; the semicells may be ornamented but lack prominent projections or spines (cf. XANTHIDIUM). (See also SINGLE-CELL PROTEIN.) cosmid A PLASMID into which has been inserted the cos site of BACTERIOPHAGE LAMBDA. Cosmids can be used as CLONING vectors for large fragments of DNA (e.g. ca. 25–40 kb), and have been useful e.g. in the construction of genomic libraries. The use of a cosmid is shown diagrammatically in the figure. (See also PHASMID.) costa (1) Any rib-like structure. (2) A deeply-staining, curved, endoskeletal rod in trichomonads; its location coincides with the region where the undulating membrane adjoins the cell. (cf. AXOSTYLE.) Costaria See PHAEOPHYTA. Costia A genus of flagellated protozoa. C. necatrix is parasitic in fish (see COSTIASIS); the cell is dorsoventrally flattened and bears two long and two short flagella. The longer flagella are used for locomotion, the shorter flagella for attachment of the organism to a host. costiasis (blue slime disease) A freshwater FISH DISEASE caused by Costia necatrix. The skin and gills are affected, and large COSMID (diagrammatic). The cosmid (top) is a plasmid containing (i) a cos site (solid black square) from bacteriophage lambda, and (ii) the cutting site (short vertical bar) of a RESTRICTION ENDONUCLEASE. Cosmids are cut at the single restriction site, yielding linearized molecules with an internal cos site (top, left). Linearized cosmids are mixed with DNA fragments (dashed lines) which have been cut by the same restriction enzyme. Cosmids and fragments bind randomly via sticky ends, and in some cases a fragment will be flanked by two cosmids (centre). If, in such a molecule, the distance cos-to-cos is approximately 40–50 kb, then the cos–cos sequence is the right size for packaging into a phage lambda head (the phage lambda genome is ∼48 kb). Enzymic cleavage at the cos sites (arrowheads) creates sticky ends, and packaging of the recombinant molecule occurs, in vitro, in the presence of phage components. The completed virions can inject their recombinant molecules into suitable bacteria. Within a bacterium the DNA circularizes, via the cos sites, and replicates as a plasmid; its presence in the cell can be detected e.g. by the expression of plasmid-encoded antibiotic-resistance genes. Screening for specific sequences in cosmids is carried out in a way similar to that used for plasmid vectors. Reproduced from Bacteria 5th edition, Figure 8.15, page 197, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 198 counting methods amounts of slime are secreted over the body. Mortality may be massive in fish hatcheries. co-substrate See CO-METABOLISM. cot death Syn. SUDDEN INFANT DEATH SYNDROME. cotransduction The TRANSDUCTION of two or more donor genes in the same transducing virion. co-transfer (of plasmids) See CONJUGATION (1b) (i). co-transport Syn. SYMPORT. cotrimoxazole See FOLIC ACID ANTAGONIST. cottage cheese See CHEESE-MAKING. cotton anthocyanosis virus See LUTEOVIRUSES. cotton blue Syn. METHYL BLUE. cotton wilt Any VASCULAR WILT of cotton plants (Gossypium spp). Common causal agents include Fusarium oxysporum f. sp. vasinfectum (‘Frenching disease’) and Verticillium dahliae. cotton wool A fibrous, cellulosic material from seed coats of the cotton plant (Gossypium). Non-absorbent cotton wool contains substances (e.g. unsaturated fatty acids) inhibitory to certain organisms; absorbent cotton wool has been treated with solvents and bleached so that the inhibitory substances have been largely or completely removed. Plugs of non-absorbent cotton wool are often used as stoppers in tubes of bacteriological media. (See also SWAB.) (cf. cotton wool substitute in the entry ALGINATE.) cotton wool disease See COLUMNARIS DISEASE. cottontail rabbit papilloma virus See PAPILLOMAVIRUS. cotype Syn. SYNTYPE. cou gene See GYRASE. cough plate A method for isolating Bordetella pertussis from a suspected case of WHOOPING COUGH. A plate of BORDET–GENGOU AGAR is exposed to droplet inoculation from the patient’s cough and is then incubated and examined for growth of B. pertussis. Many clinicians now use a pernasal swab (optimally, with an ALGINATE tip) in preference to the cough plate method. Coulter counter An instrument used for counting particles (cells, spores etc) in a suspension. Essentially, it consists of two chambers separated by a partition of (electrically) insulating material which is perforated by a single hole of size similar to that of the particles to be counted. Each chamber contains an electrode. The sample is placed in one chamber and forced, under pressure, into the other; during this transfer the suspension acts as a conducting path (via the hole) between the two electrodes. As each cell passes through the hole, it momentarily decreases the electrical conductivity between the electrodes; this momentary change in conductivity is recorded as a pulse by an electronic counting circuit controlled by the two electrodes. The cell count is indicated on a digital display. (See also COUNTING METHODS.) coumermycins A group of compounds structurally related to NOVOBIOCIN. Coumermycin A1 inhibits ATP-dependent GYRASE activity in the same way as NOVOBIOCIN, but is much more effective both in vivo and in vitro. Councilman body A cluster of hyaline, necrotic, eosinophilic cells in the liver of an individual suffering from YELLOW FEVER. counter-receptor (to a CAM) See CELL ADHESION MOLECULE. counter transport Syn. ANTIPORT. countercurrent immunoelectrophoresis (CIE) IMMUNOELECTROPHORESIS (used for only certain types of antigen) in which antibodies and antigens (in separate wells in a gel strip) move towards each other when subjected to the same electric field; the antibodies move towards the cathode (see ELECTROENDOSMOSIS) while the antigens – because of their size, charge, and other physico-chemical characteristics – move towards the anode. counterstain (1) A second, contrasting, stain used on a microscopical preparation with the object of staining those features which have not taken up the first stain. (2) In the GRAM STAIN: a stain used to stain those organisms or tissues which have been decolorized. (3) In the AURAMINE–RHODAMINE STAIN: the solution of potassium permanganate used to eliminate background fluorescence. counting chamber (haemocytometer) An instrument used for determining the total cell count, viable cell count, or spore count etc. of a suspension of cells or spores (see COUNTING METHODS). A typical counting chamber is illustrated on p. 200. A grid, etched on the central plateau of the glass block, typically consists of a square of side 1.0 mm divided into 400 squares, each 0.0025 mm2 ; the distance between grid and cover-slip may be 0.1 mm (as in the Thoma chamber) or 0.02 mm (as in the Helber chamber). When the cover-slip is correctly positioned, Newton’s rings should be visible through those parts of the cover-slip in contact with the glass shoulders – indicating close contact. The suspension is introduced (with a Pasteur pipette) into the space between cover-slip and central plateau; the troughs should remain empty. The cells are allowed to settle and are counted under the microscope; since the volume between grid and coverslip is known, the count per unit volume can be calculated. For viable cell counts, cells are subjected to VITAL STAINING. (See also FACS.) counting methods Methods for counting microorganisms may be divided into two categories according to the type of organism considered. (a) Organisms other than viruses, viroids etc. The total number of living and dead cells in a given volume (or on a given area) of a sample is termed the total cell count; usually, total cell counts refer to bacteria, spores or yeasts (single-celled organisms). A total cell count may be determined by direct (visual) counting, i.e., by MICROSCOPY (see also COUNTING CHAMBER), or by the use of electronic or other instruments (see e.g. COULTER COUNTER, NEPHELOMETRY, TURBIDIMETRY); both direct and electronic methods may permit the counting of specific types of cell – e.g. by IMMUNOFLUORESCENCE microscopy and FACS (q.v.), respectively. Liquid samples containing small numbers of cells can be membrane-filtered (see FILTRATION) and the (stained) cells counted in situ. (See also DEFT.) Cell numbers in a given suspension may be estimated by comparison with standard suspensions (see BROWN’S TUBES). Viable cell count refers to the number of living cells (per volume or area) in a given sample; viable cell counts usually refer to single-celled organisms, and they can obviously include only those organisms which are detectable by the particular method used. A viable cell count may be determined in several ways. (i) Direct examination, in a counting chamber, of a sample stained with a vital stain (see VITAL STAINING) – living and dead cells being distinguished by their different reactions to the dye. (ii) Statistical methods: see e.g. MULTIPLE-TUBE METHOD. (iii) Colony counts: the number of viable cells in a sample (or aliquot) is assessed from the number of colonies which develop on incubation of a solid medium which has been inoculated with the sample or aliquot; in such methods it is assumed that each colony developed from a single cell. The selectivity of the medium and the conditions of incubation may significantly affect the number of viable cells which give rise to colonies. For colony-counting methods see e.g. DIP-SLIDE METHOD, MILES AND MISRA’S METHOD, POUR PLATE, ROLL-TUBE TECHNIQUE and SPREAD PLATE (sense 1). Some types of cell tend to form clumps or to grow in chains or filaments or in microcolonies. An accurate viable count of such organisms cannot be obtained by the above methods, and 199 (a) (b) (c) COUNTING CHAMBER (Thoma chamber). The instrument – seen from one side at (a) – consists of a rectangular glass block in which the central plateau lies precisely 0.1 mm below the level of the shoulders on either side. The central plateau is separated from each shoulder by a trough, and is itself divided into two parts by a shallow trough (seen at (b)). On the surface of each part of the central plateau is an etched grid (c) consisting of a square which is divided into 400 small squares, each 1/400 mm2 . A thin glass cover-slip is positioned as shown at (b) and pressed firmly onto the shoulders of the chamber; to achieve proper contact it is necessary, while pressing, to move the cover-slip (slightly) against the shoulders. Proper contact is indicated by the appearance of a pattern of coloured lines (Newton’s rings), shown at (b). Using the chamber. A small volume of a bacterial suspension is picked up in a Pasteur pipette by capillary attraction; the thread of liquid in the pipette should not be more than 10 mm. The pipette is then placed as shown at (b), i.e., with the opening of the pipette in contact with the central plateau, and the side of the pipette against the cover-slip. With the pipette in this position, liquid is automatically drawn by capillary attraction into the space bounded by the cover-slip and one-half of the central plateau; the liquid should not overflow into the trough. (It is sometimes necessary to tap the pipette, lightly, against the central plateau to encourage the liquid to enter the chamber.) A second sample can be examined, if required, in the other half of the counting chamber. The chamber is left for 30 min to allow the cells to settle, and counting is then carried out under a high power of the microscope – which is focused on the grid of the chamber. Since the volume between grid and cover-slip is accurately known, the count of cells per unit volume can be calculated. A worked example. Each small square in the grid is 1/400 mm2 . As the distance between grid and cover-slip is 1/10 mm, the volume of liquid over each small square is 1/4000 mm3 , i.e., (for all practical purposes) 1/4,000,000 ml. Suppose, for example, that on scanning all 400 small squares 500 cells were counted; this would give an average of 500 ÷ 400(= 1.25) cells per small square. Thus, since 1.25 cells occur in 1/4,000,000 ml, the sample must contain 1.25 × 4,000,000 cells per ml, = 5,000,000 cells per ml. If the sample was diluted before examination in the counting chamber, the count obtained must be multiplied by the dilution factor: e.g., if the sample was diluted 1 in 10, the count must be multiplied by 10. 200 cozymase and expressed in cowpea protoplasts in the absence of RNA2, but requires RNA-2 for encapsidation and for cell-to-cell spread; expression of RNA-2 is completely dependent on the expression of RNA-1. Both RNAs appear to be translated (in the cytoplasm) into large POLYPROTEINS which are cleaved to form functional proteins. RNA-1 encodes at least 6 proteins, including VPg and a protease apparently involved in cleavage of the polyprotein [EMBO (1987) 6 549–554]; RNA-2 carries genes for the capsid proteins. Cells infected with CPMV contain characteristic inclusion bodies: membranous, vesicular structures, usually adjacent to the nucleus, which contain dsRNA corresponding to RNA-1 and RNA-2, complementary (minus-strand) RNA, and replication complexes containing RNADEPENDENT RNA POLYMERASE molecules. (Although uninfected cowpea cells contain an RNA-dependent RNA polymerase, the activity of which increases by 10-fold or more on infection with CPMV, this enzyme appears not to be involved in CPMV replication [Book ref. 80, pp. 120–125].) Progeny CPMV particles accumulate in the host cell cytoplasm, sometimes forming crystalline arrays. cowpea ringspot virus See CUCUMOVIRUSES. cowpea severe mosaic virus See COMOVIRUSES. cowpox A mild disease which primarily affects cows but which is transmissible (by direct contact) to human handlers; it is caused by the COWPOX VIRUS. In cows, lesions (which progress from papules through vesicles and pustules to crusts – cf. SMALLPOX) develop mainly on the udder and teats; other animals (e.g. cats [VR (1985) 117 231–233]) may be affected. In man the lesions occur mainly on the hands and arms and may be accompanied by local oedema and lymphangitis. cowpox virus A virus of the genus ORTHOPOXVIRUS; it is the causal agent of COWPOX. The virus has a relatively broad host range, including e.g. cattle, man, and many other animals; it has been recovered from wild rodents, and rodents may be the natural reservoir host. Cowpox virus forms large, haemorrhagic pocks on the CAM, and is highly lethal for chick embryos. Both A- and B-type inclusion bodies are formed in infected cells. DNA MWt: ca. 145 × 106 . coxibs See PROSTAGLANDINS. Coxiella A genus of Gram-negative bacteria of the RICKETTSIEAE. Cells: highly pleomorphic, non-motile rods, ca. 0.2–0.4 × 0.4–1.0 µm, which stain well with the GIMÉNEZ STAIN; C. burnetii, the sole species, undergoes a developmental cycle in which ENDOSPORES are formed. Glucose, glutamate and pyruvate are metabolized (more readily at low pH). C. burnetii is not invariably killed at 63° C/30 min or 90° C for a few sec (cf. PASTEURIZATION). GC%: ca. 43. C. burnetii is an obligate intracellular parasite in vertebrate and arthropod hosts, and is the causal agent of Q FEVER in man; in man, growth occurs e.g. in phagolysosomes (see PHAGOCYTOSIS) within mononuclear phagocytes (e.g. macrophages). Lab. culture is carried out (at ca. 35° C) e.g. in chicken embryos (by inoculating the yolk sac in hen’s eggs), or in monolayer tissue cultures prepared from chicken embryos. On repeated passage, the organisms gradually change from phase I (= phase 1) – i.e., ‘smooth’, hydrophilic cells which are stable in suspension and are not phagocytosed in the absence of specific antibodies – to phase II (= phase 2) – i.e., ‘rough’, hydrophobic, autoagglutinable, readily phagocytosed cells; rapid reversion to phase I occurs when phase II cells are injected into lab. animals. [Book ref. 22, pp. 701–704.] coxsackieviruses See ENTEROVIRUS. cozymase NAD (q.v.). in these cases counts may be given as the number of colonyforming units (cfu) per unit volume or area of the sample; a cfu can be any entity – e.g. a single cell or a chain of cells – capable of giving rise to a single colony. The above methods are generally applicable e.g. to most bacteria and spores, and some of the methods may be used for certain algae, fungi (e.g. yeasts) and protozoa. The counting of rapidly moving microorganisms can be facilitated by the use of a viscous suspending medium – e.g. FICOLL, methylcellulose (2–5% aqueous) or poly(ethylene oxide). (b) Viruses. Viruses may be counted by electron microscopy, and the largest viruses (e.g. some entomopoxviruses) may be counted by light microscopy after suitable staining; however, such methods are seldom used since e.g. they do not distinguish between infective and non-infective virions. In the plaque method of assay, serial dilutions of a viral suspension are initially prepared, and a known volume of each dilution is examined for its content of infectious, plaque-forming virions: see PLAQUE (1) and PLAQUE ASSAY. An alternative procedure is the END-POINT DILUTION ASSAY. Plant viruses may be assayed on LOCAL LESION hosts. coupled cell division See F PLASMID. coupling sequences See CONJUGATIVE TRANSPOSITION. cover-glass (cover-slip) A small round or rectangular sheet of thin glass used primarily to cover a specimen on a microscope SLIDE. When a slide is viewed with a HIGH-DRY OBJECTIVE lens, the image quality is optimum only when the cover-glass is of a particular thickness (specified by the lens manufacturer); thicker or thinner cover-glasses effectively increase spherical aberration. Cover-glass thickness is relatively unimportant with low-power objectives and (usually) with oil-immersion objectives. Coverglasses are made in thickness ranges designated No. 0 (ca. 0.1 mm), No. 1 (ca. 0.15 mm), No. 1 1/2 (ca. 0.175 mm), No. 2 (ca. 0.2 mm), and No. 3 (ca. 0.3 mm). Most objectives are designed for use with No. 1 1/2 cover-glasses. (See also CORRECTION COLLAR.) cover-slip Syn. COVER-GLASS. covered smut See SMUTS (sense 2). covirus Syn. MULTICOMPONENT VIRUS. Cowan I, II & III strains Strains of Staphylococcus aureus used e.g. for the production of standard TYPING antisera. Cowdria A genus of Gram-negative bacteria of the tribe EHRLICHIEAE. Cells: coccoid to rod-shaped, pleomorphic, nonmotile, maximum dimension ca. 0.5 µm. Growth occurs intracytoplasmically in the vascular endothelial cells of ruminants, but does not occur in cell-free media. Type species (sole species): C. ruminantium, the causal agent of ‘heartwater’ (a tick-borne, septicaemic disease of ruminants which occurs in Africa); C. ruminantium is transmitted trans-stadially, but not transovarially, in the tick vector. [Book ref. 22, pp. 709–710.] cowpea aphid-borne mosaic virus See POTYVIRUSES. cowpea chlorotic mottle virus See BROMOVIRUSES. cowpea mosaic virus (CPMV) The type member of the COMOVIRUSES. Three types of particle can be distinguished and are designated T, M and B (S20 w 58, 98 and 118, respectively); each is icosahedral, ca. 28 nm diam., the capsid consisting of 60 subunits of each of two structural proteins (MWts 22000 and 42000). T particles appear to consist only of protein. B particles contain a single molecule of RNA-1 (B-RNA, MWt ca. 2 × 106 ) while the M particles contain one molecule of RNA-2 (M-RNA, MWt ca. 1.2 × 106 ). Both RNAs are polyadenylated at the 3
end and have a small polypeptide (VPg, MWt ca. 4000) covalently bound to the 5
end. RNA-1 can be replicated 201 cpd gene pyridine, quinoline, anthracene and naphthalene. (See also cpd gene In e.g. Escherichia coli : the gene encoding cAMP phosphodiesterase (see CYCLIC AMP). CPE CYTOPATHIC EFFECT. CPV group CYTOPLASMIC POLYHEDROSIS VIRUS GROUP. CR1 See CD35. CR2 (on B lymphocytes) See CD21. Crabtree effect In certain yeasts: the occurrence of high levels of fermentative metabolism under aerobic conditions in the presence of high concentrations of a fermentable sugar (e.g. glucose in the case of Saccharomyces cerevisiae). When growing in batch culture with high concentrations of glucose under aerobic conditions, S. cerevisiae carries out a predominantly fermentative metabolism, converting glucose to ethanol (see ALCOHOLIC FERMENTATION); when the levels of glucose have been depleted, the ethanol can be used as a substrate for respiratory metabolism. Studies with continuous cultures have suggested that cells exposed to high concentrations of glucose under aerobic conditions respond by ‘short-term’ and ‘long-term’ regulatory processes. In the short term, respiratory enzymes are apparently not repressed, but ethanol is nevertheless formed; this is apparently because the cells have only a limited respiratory capacity, i.e., more pyruvate is formed than can be oxidized, and the excess pyruvate is decarboxylated to form ethanol. Long-term regulation is a process of metabolic adaptation involving repression of the synthesis of e.g. certain TCA cycle enzymes, cytochromes, and gluconeogenetic enzymes, together with proteolytic inactivation of enzymes. [JGM (1985) 131 47–52.] (See also CATABOLITE REPRESSION.) Craigie’s tube method A method used e.g. for separating motile from non-motile organisms, and for isolating strains of Salmonella in the alternative flagellar phase. A piece of glass tubing (internal diam. ca. 3 mm) is placed in a test-tube of semisolid nutrient agar such that the tubing projects above the level of the medium; the lower end of the tubing is cut obliquely, thus permitting free communication between the agar in the tubing and that in the remainder of the test-tube. If the medium inside the tubing is inoculated with motile and non-motile organisms, motile organisms may migrate into the medium in the testtube – from which they may be subsequently isolated. For the isolation of alternative-phase strains of Salmonella (see PHASE VARIATION) the medium is supplemented with antiserum to the flagella antigens of the currently predominant phase; the antiserum inhibits motility in strains of the predominant phase, but alternative-phase strains can swim out into the testtube from where they can be subsequently isolated. Craterellus See CANTHARELLALES. crayfish plague (Krebspest) A disease of European crayfish (e.g. Astacus astacus) caused by Aphanomyces astaci ; the American crayfish (Pacifastacus leniusculus) is normally resistant but may be able to carry the pathogen. The disease involves mainly the exoskeleton and CNS; mortality rates are very high. (See also CRUSTACEAN DISEASES.) cre gene See BACTERIOPHAGE P1. cream cheese See CHEESE-MAKING. Credé procedure See GONORRHOEA. Crenarchaeota See ARCHAEA. crenate Scalloped. Crenothrix A genus of IRON BACTERIA (q.v.) which occur as sheathed, sometimes terminally swollen filaments. Type species: C. polyspora. creosote A highly effective antifungal wood preservative for outdoor use. It consists of a complex mixture of coal tar distillation products which include phenol, cresols, xylenols, LENTINUS.) creosote fungus See HORMOCONIS. Crepidotaceae See AGARICALES. Crepidotus A genus of fungi (AGARICALES, Crepidotaceae) which grow e.g. on fallen twigs and leaves, forming typically shellshaped or lobed fruiting bodies up to 2 or 3 cm across. crescent (protozool.) The mature, crescent-shaped gametocyte formed by certain species of Plasmodium (e.g. P. falciparum). cresol red A PH INDICATOR: pH 7.2–8.8 (yellow to red). cresols (as disinfectants) See PHENOLS. cresylic acid A mixture of methyl-, dimethyl- and ethyl-phenols used e.g. as an industrial biocide. Creutzfeldt–Jakob disease (CJD) A human TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHY in which the symptoms at onset typically include dementia (but which, in some patients, are limited to ataxia or to e.g. psychiatric disorders); death commonly occurs within 1 year (sometimes <6 months) of onset, but may not occur for up to 4 years in a minority of patients. Some cases involve inherited CJD – in which the PRNP gene (encoding PrP) contains particular mutations; brain extracts from such patients can transmit the disease to mice (indicating that, in these cases, the causal prion is endogenous, i.e. produced within host cells). CJD can be transmitted e.g. via contaminated neurosurgical instruments (despite ‘sterilization’ with 70% ethanol and formaldehyde vapour), via transplanted corneas from infected donors, and by the administration of contaminated human growth hormone. Several variant forms of CJD are distinguished on the basis of (i) the nucleotide sequence of the PRNP gene, and (ii) the particular strain of prion protein involved. Thus, cases of ‘sporadic’ CJD can be placed into four groups – cases in a given group differing from those in other groups e.g. by differences in initial symptoms and the rapidity with which the disease progresses. Most or all of the strains of CJD prion can be transmitted to chimpanzees and other primates, but only some strains can be transmitted to other animals (e.g. cats, guinea pigs, mice). Goats (but not sheep) inoculated with brain tissue containing CJD prions develop a disease indistinguishable from scrapie. A new variant form of CJD (designated nvCJD) was reported in the United Kingdom [Lancet (1996) 347 921–925]. Cases of nvCJD have some unique features. For example, patients have died in their 2nd–5th decades, i.e. earlier than typical CJD patients. Clinical features that distinguish nvCJD from the typical CJD pattern also include psychiatric conditions, sensory abnormalities and pain preceding the more usual signs of ataxia, myoclonus and cognitive impairment; the electroencephalogram is unlike that obtained in most other cases of CJD. Moreover, histopathology includes multiple ‘florid’ amyloid plaques (rare in other cases of CJD) – as well as the changes that are typical of prion diseases; each of these plaques is encircled by a ring of spongiform vacuoles. It is now believed that nvCJD is caused by a prion derived from meat or meat products from animals with BOVINE SPONGIFORM ENCEPHALOPATHY (q.v.). It has been suggested that – before the appearance of nvCJD – at least some cases of ‘sporadic’ CJD were caused by a prion derived from cattle with a rare spongiform disease, and that the BSE prion is a novel variant of this agent which, in humans, gives rise to nvCJD [RMM (1998) 9 119–127]. Studies with transgenic mice have provided evidence in favour of the transmission of bovine prions to humans [PNAS 202 cross-reacting antibody schedule, the water in a fixed and washed specimen is initially replaced by an ‘intermediate fluid’ (e.g. ethanol or acetone), and the specimen, totally immersed in the intermediate fluid, is placed in a strong metal pressure chamber. The chamber is then closed, cooled to ca. 15–18° C, and the intermediate fluid is replaced by the ‘transitional fluid’ (usually liquid CO2 ) by means of a valve system; the specimen remains below the liquid–gas boundary throughout. The liquid CO2 is then partially drained and fresh liquid CO2 is added; this is repeated several times to dilute out the intermediate fluid, the specimen always remaining fully covered by liquid CO2 . When the last traces of intermediate fluid have been replaced by liquid CO2 the temperature of the (closed) chamber is slowly raised (e.g. 1° C/ min) above 31° C: the critical point above which CO2 cannot exist as a liquid. The gas is then bled off slowly (to avoid e.g. turbulence in the chamber). In an alternative schedule, various halocarbons (e.g. certain Freons) may be used to displace the ethanol or acetone. Thus e.g. ethanol may be displaced by two different halocarbons, used sequentially: the first acts as an additional intermediate fluid, while the second acts as the transitional fluid. (cf. FREEZEDRYING.) critical temperature zone See FREEZING. CRM Cross-reacting material: protein which cross-reacts serologically with a given biologically active protein; CRM may lack the biological activity of the given protein, e.g., it may be a product of a mutant gene or a precursor form of the given protein. cro gene See BACTERIOPHAGE l. Crohn’s disease A (typically) chronic human disease of the intestinal tract which is characterized by granulomatous/ulcerative lesions; diarrhoea and intestinal pain are common. The acute form may resemble appendicitis. The aetiology is not known. Possibly, mutations affecting certain intracellular molecules in the intestinal epithelium (e.g. Nod2) may affect the normal protective response to bacterial infection [JBC (2003) 278 5509–5512, 8869–8872]. cromoglycate A therapeutic agent which stabilizes sensitized MAST CELLS, inhibiting the release of histamine; it is administered before the patient is exposed to antigen/allergen in order to prevent anaphylactic reactions. Cronartium A genus of rust fungi (class UREDINIOMYCETES) which include some important parasites of pine trees (Pinus spp): see e.g. BLISTER RUST. (See also PERIDERMIOID.) cross-feeding (1) Syn. SYNTROPHISM. (2) A particular type of SYNTROPHISM in which each organism derives essential growth factors from the other(s). cross-linking quantitative assay A type of assay which has been used for detecting and quantifying hepatitis B virus (HBV) in serum. The assay, carried out in the solution phase, involves probes which incorporate a derivative of 7-hydroxycoumarin – a cross-linking agent that can be photo-activated by ultraviolet radiation such that each probe can be covalently bound to its target sequence in HBV DNA. Essentially, probes complementary to sequences in the major subtypes of HBV are allowed to hybridize to target sequences in the sample and are then photo-cross-linked to their targets; by means of biotin tags, the cross-linked probes are captured on streptavidin-coated DYNABEADS and subsequently labelled with alkaline phosphatase. The addition of a suitable substrate for the enzyme gives rise to a fluorescent signal. [Cross-linking assay for HBV DNA: JCM (1999) 37 161–164.] cross-reacting antibody Antibody which can combine with nonhomologous antigen (i.e. antigen not identical to that which (1999) 96 15137–15142] (see BOVINE SPONGIFORM ENCEPHALOfor further supporting evidence). Diagnosis may be facilitated by immunohistochemistry and/or Western blot analysis of tonsil biopsies [Lancet (1999) 353 183–189]. At present, there is no chemotherapy for CJD. Various chemical agents have been examined for their ability e.g. to increase the incubation period of the disease in animals [therapeutic strategies (review): RMM (1998) 9 135–151]. The causative prion of BSE has been shown to be transmissible from a sheep in the initial symptom-free phase of experimental infection to another sheep via a whole-blood transfusion; it has been suggested that symptom-free pre-nvCJD individuals may represent a risk of spread to other humans via blood transfusion [Lancet (2000) 356 999–1000.] [UK distribution of CJD 1994–2000: Lancet (2001) 357 1002–1007; nvCJD (UK deaths): Lancet (2003) 361 751–752.] Some similarities between CJD and Alzheimer’s disease have lead to attempts to demonstrate a transmissible agent in the latter disease; however, to date, there is no evidence of such an agent. crevicular fluid A serum-like fluid which enters the mouth via the gingival crevice (= gingival sulcus) – i.e., the shallow crevice at the tooth–gum margin. It contains e.g. IMMUNOGLOBULINS (including IgA, IgG and IgM), COMPLEMENT, and various cells (e.g. LEUCOCYTES). CRF COAGULASE reacting factor. crib death Syn. SUDDEN INFANT DEATH SYNDROME. Cribraria See MYXOMYCETES. Cribrostomum See FORAMINIFERIDA. cricket paralysis virus See PICORNAVIRIDAE. Cricosphaera See COCCOLITHOPHORIDS. Crimean–Congo haemorrhagic fever See VIRAL HAEMORRHAGIC FEVERS. Crinipellis See AGARICALES (Tricholomataceae) and WITCHES’ BROOM. crista (1) See CRISTISPIRA. (2) See MITOCHONDRION. Cristispira A genus of bacteria (family SPIROCHAETACEAE) which occur e.g. in the digestive tract and/or crystalline style in freshwater and marine molluscs, and in other aquatic organisms. Cells are motile, helical, ca. 0.5–3.0 × 30–180 µm, and each cell has 100 or more periplasmic flagella which may distend the outer sheath – giving rise to a longitudinal ridge, the crista; cells divide by transverse fission. Type species: C. pectinis. [Book ref. 22, pp. 46–49.] Crithidia (Strigomonas) A genus of homoxenous parasitic protozoa (family TRYPANOSOMATIDAE) in which the choanomastigote form occurs; species typically occur in the gut of insects (e.g. C. fasciculata occurs worldwide in Anopheles and Culex mosquitoes). crithidial form Obsolete syn. EPIMASTIGOTE. critical control points See HACCP. critical dilution rate See DILUTION RATE. critical illumination (Nelsonian illumination) In MICROSCOPY: illumination in which the light source (e.g. lamp filament) is focused by the CONDENSER to form an image in the plane of the specimen. With electric coiled filaments critical illumination tends to give uneven illumination. (cf. KÖHLER ILLUMINATION.) critical initiation mass See CELL CYCLE. critical point drying A method used e.g. for dehydrating and drying specimens prior to their examination by ELECTRON MICROSCOPY; in this method the specimen is not exposed to a liquid–gas boundary and is thus not subjected to the damage caused by surface tension in air-dried specimens. In a typical PATHY 203 cross-reacting material elicited the antibody) as well as with homologous antigen. Such cross-reaction requires that the homologous and nonhomologous antigens have a certain degree of stereochemical similarity. Cross-reacting antibodies often have a much lower affinity for the non-homologous antigen (cf. HETEROCLICITY.) (See also MONOCLONAL ANTIBODIES.) Antibodies raised against a particular microorganism may combine with related (or even unrelated) species (see e.g. WEIL–FELIX TEST); such organisms may share a common antigenic determinant or may have CROSS-REACTIVE ANTIGENS. cross-reacting material See CRM. cross-reactivation See REACTIVATION. cross-reactive antigens Antigens which differ from one another but which can combine with antibodies raised against either (or any) one of them. In type 1 cross-reactivity, different crossreactive ligands bind with different affinities to a given site on a given antibody molecule; in type 2 cross-reactivity a cross-reactive ligand reacts with some of the antibodies in a POLYCLONAL ANTISERUM raised against the homologous ligand [Mol. Immunol. (1981) 18 751–763]. cross resistance The phenomenon in which the resistance of an organism to one antibiotic correlates with resistance to one or more other antibiotics. cross-wall Syn. SEPTUM. crossband (in prosthecae) See CAULOBACTER. crossed immunoelectrophoresis See TWO-DIMENSIONAL ELECTROPHORESIS. crossing over A RECOMBINATION process in which, e.g., exchange of double-stranded sequences occurs between two linear duplexes. In effect, crossing over involves a break in each of the two strands in both (linear) duplexes, exchange of parts of the duplexes, and ligation to form two recombinant duplexes (see RECOMBINATION). Thus, if the two duplexes carry alleles ABCD and abcd, crossing over results in recombinant molecules Abcd and aBCD, ABcd and abCD, or ABCd and abcD, depending on the site of the cross-over. In e.g. MEIOSIS, crossing over occurs between non-sister chromatids in homologous chromosomes (cf. PARASEXUAL PROCESSES). If crossing over occurs between two circular molecules the result is a single larger circular molecule; conversely, crossing over between regions within a single circular molecule results in two smaller circular molecules. (See also CAMPBELL MODEL.) crotocin See TRICHOTHECENES. croup A type of laryngotracheitis with hoarseness, resonant cough, stridor, and respiratory obstruction. Croup occurs mainly in young children and may be of non-microbial causation (e.g. allergy) or due to infection by e.g. human respiratory syncytial virus, parainfluenza viruses, Corynebacterium diphtheriae. crown gall A plant neoplasm, sometimes bearing root-like, stem-like and/or leaf-like structures, which is formed at the crown (stem–root junction) – or, less commonly, on stems or roots – in a wide range of gymnosperms and dicotyledonous angiosperms (although in few monocotyledonous angiosperms); crown gall follows infection of a wound with virulent strains of certain species of Agrobacterium, commonly A. tumefaciens (see AGROBACTERIUM). Infected plants may be stunted or even killed; the disease can cause important economic losses (for example, in stone-fruit trees and vines) e.g. in parts of Australia, Europe and the USA. Crown gall of grape is caused by strains of Agrobacterium which have been assigned to a new species, A. vitis [crown gall of grape (biology and disease management): ARPpath. (1999) 37 53–80]. (See also GALLS.) A. tumefaciens attaches to the host plant at the site of a wound, attachment being reported to occur via the bacterial lipopolysaccharides and via epicellular cellulose microfibrils formed by the bacteria [Book ref. 55, pp 33–54]. Virulence in A. tumefaciens (and in A. vitis) depends on the presence of a large CONJUGATIVE PLASMID called the Ti (tumourinducing) plasmid (ca. 150–230 kb). Genes responsible for tumorigenesis are located in a small subregion of the Ti plasmid termed the T-region or T-DNA (ca. 15–24 kb); in the infection process, single-stranded T-DNA is excised from the plasmid and transferred to the plant. Transfer of T-DNA to the plant requires certain products of the virulence (vir ) regulon – ∼25 genes in regions of the Ti plasmid outside the T-DNA; transfer is regulated by a TWO-COMPONENT REGULATORY SYSTEM consisting of the proteins VirA (sensor) and VirG (response regulator), both expressed constitutively. [Twocomponent regulatory system in A. tumefaciens: FEMS Reviews (1998) 21 291–319 (301–303).] VirA appears to respond (directly or indirectly) to certain types of phenolic compound (e.g. acetosyringone) released by plant wounds; the activity of phenolics is enhanced by monosaccharides such as glucose, and it has been suggested that such enhancement results from the binding, to VirA, of a complex consisting of monosaccharide and the product of the bacterial virulence gene chvE. (As well as being a sensor protein, VirA may also function as a hostrange determinant [EMBO (1987) 6 849–856]; thus, different (variant) forms of VirA may differ in their response to particular plant signals.) On activation, VirG promotes transcription in the vir regulon. The virD operon encodes a site-specific endonuclease complex which makes nicks in one strand of the plasmid at unique sites within a pair of 24-bp DIRECT REPEATS that flank the TDNA; the (single-stranded) copy of T-DNA, with protein VirD2 covalently attached to the 5
end, is then transferred to the plant cell nucleus. (So-called nuclear localization signals, associated with one of the proteins accompanying the T-DNA, appear to be required for targeting.) [Molecular signals in the interaction between plants and microbes: Cell (1992) 71 191–199.] Pili appear to be necessary for transfer of T-DNA to plants [Science (1996) 273 1107–1109]. Within a plant cell the processed T-DNA can apparently insert at multiple sites in the nuclear DNA, sometimes forming tandem repeats. While T-DNA is transcribed only weakly in the bacterium, in the plant cell it is transcribed into both polyadenylated and non-polyadenylated RNA [Plasmid (1981) 6 17–29]. Genes within T-DNA encode enzymes involved in the formation of CYTOKININS [PNAS (1984) 81 4776–4780, 5994–5998] and IAA (see AUXINS), resulting in persistently raised levels of these phytohormones which are apparently responsible for tumorigenesis. T-DNA also carries genes which specify the production, by tumour cells, of a number of tumour-specific substances called opines. A given Ti plasmid may encode opines of the OCTOPINE family (e.g. octopine, histopine, lysopine, octopinic acid) or of the NOPALINE family (e.g. nopaline, nopalinic acid). Octopine Ti plasmids may also encode agropine; agropine (formerly ‘null type’) Ti plasmids and nopaline Ti plasmids also encode AGROCINOPINES. The different plasmids may specify tumours of different types. (cf. HAIRY ROOT; see also AGROCINS.) A Ti plasmid carries genes for the degradation of those opine(s) synthesized by the tumour induced by its T-DNA; moreover, the pathogen – but not plant cells or other microorganisms – can use opines as the sole source of carbon and nitrogen. The availability of nutrients (opines) promotes growth and development of a population of the pathogen. Conjugal transfer 204 Cryptococcus + − cryosubstitution FREEZE-SUBSTITUTION. cryotome See FREEZE-SECTIONING. Cryoxicide A STERILANT containing ETHYLENE OXIDE (11%), trichlorofluoromethane (79%), and dichlorodifluoromethane (10%). crypt (ciliate protozool.) See BROOD POUCH. cryptic mutant A cell which lacks one or more components of a TRANSPORT SYSTEM so that a particular substrate, or substrates, cannot enter the cell – and hence cannot be utilized, even though the cell may possess all the enzymes necessary for its utilization. For example, the failure of a cell to produce enzyme I (see PTS) – as a result of a (pleiotropic) mutation – would prevent that cell (a cryptic mutant) from taking up and metabolizing a range of sugars. cryptic plasmid A PLASMID which has no apparent effect on the phenotype of the cell in which it occurs. cryptic viruses Virus-like isometric particles which have been detected in a wide range of symptomless plants; the particles are generally ca. 30 nm in diameter, are present in low concentrations, and appear to be seed-borne. Some have been shown to contain dsRNA [e.g. beet cryptic viruses: JGV (1986) 67 363–366]. cryptidin See DEFENSINS. Cryptobia See BODONINA. cryptobiosis See DORMANCY. Cryptocaryon A genus of cyst-forming ciliates related to ICHTHYOPHTHIRIUS; the sole species, C. irritans, causes WHITE SPOT in marine fish. Cryptococcaceae A family of fungi of the class BLASTOMYCETES. The genera include CANDIDA, CRYPTOCOCCUS, KLOECKERA, RHODOTORULA and TRICHOSPORON. cryptococcosis (torulosis; also European blastomycosis or Busse– Buschke disease) A disease of man and animals caused by Cryptococcus neoformans (q.v.); infection occurs on inhalation of dust contaminated with the fungus. Pulmonary infection may be mild or inapparent. However, particularly in individuals with e.g. defective CMI or certain leukaemias, the disease may become disseminated to almost any tissue (liver, bones, skin etc) but especially to the meninges (cryptococcal MENINGITIS – commonly fatal). Diagnosis: demonstration of the fungus by culture or microscopy; latex particle test for cryptococcal capsular polysaccharide. Chemotherapy: e.g. amphotericin B, flucytosine, possibly miconazole or ketoconazole. [Book ref. 17, pp. 180–186.] Cryptococcus A genus of non-fermentative imperfect yeasts (class HYPHOMYCETES). The cells are spheroidal, ovoid, elongate, or polymorphic, and reproduce by multilateral budding. Pseudomycelium is rudimentary or absent. The vegetative cells usually have capsules, the composition of the capsule and the abundance of capsular material depending on growth conditions; under certain conditions the capsule contains starchlike polysaccharides which may be released into the medium. Colonies on e.g. malt agar are typically mucoid and are commonly cream- to tan-coloured, or red in some species (e.g. C. hungaricus and C. macerans). Inositol can be assimilated as sole carbon source. NO3 − is assimilated by some species. Most Cryptococcus species have basidiomycetous affinities: C. albidus var. albidus and C. uniguttulatus are anamorphs of FILOBASIDIUM spp, C. neoformans of FILOBASIDIELLA sp. Three species have ascomycetous affinities: C. cereanus and C. lactativorus, which are anamorphs of SPOROPACHYDERMIA, and C. melibiosum (teleomorph unknown). [Book ref. 100, pp. 845–872.] of the Ti plasmid from Ti cells to Ti agrobacteria involves initial transcriptional activation of the regulatory gene traR (present in the Ti plasmid) – apparently by a gall-derived opine; a second signal, derived from a QUORUM SENSING system (based on the density of donor cells), is also needed to initiate the transfer process. [Tra system in A. tumefaciens: FEMS Reviews (1998) 21 291–319 (299–301).] Ti plasmids may be used as vectors for the insertion of DNA sequences into the genome of dicotyledonous plants [EMBO (1985) 4 277–284]. (See also AGROINFECTION.) Non-oncogenic Ti plasmids and plant protoplasts may be used so that hormone levels are not disturbed, and whole plants can be regenerated from the protoplasts using standard plant cell culture techniques. Transformed protoplasts can be detected by screening for opine synthesis. crown rot Any plant disease in which the ‘crown’ (i.e., the region between the root and stem) rots as a result of microbial infection; causal agents include e.g. Erwinia rhapontici in rhubarb, Mycocentrospora acerina in carrots and celery, Phytophthora cactorum in strawberries. (cf. FOOT-ROT (2).) crown rust A disease of members of the Gramineae. Crown rust of oats (Avena spp) is an economically important disease caused by Puccinia coronata avenae; orange, typically elongated, uredial pustules occur on leaves and sometimes on culms. (See also CEREAL DISEASES.) crozier See ASCUS. CRP (1) C-REACTIVE PROTEIN. (2) cAMP receptor protein: see CATABOLITE REPRESSION. crp gene See CATABOLITE REPRESSION. Crucibulum See NIDULARIALES. crucifer diseases See e.g. ALBUGO (for white rusts), BLACKLEG (sense 2), CLUBROOT and FUSARIUM WILT. crucifer white blister See ALBUGO. cruciform (1) (adj.) Cross-shaped. (2) (noun) In a nucleic acid molecule: two HAIRPINS (or STEM-AND-LOOP STRUCTURES), opposite one another, forming a cross-shaped structure. A cruciform can be formed in a double-stranded molecule or in a double-stranded region of a single-stranded molecule. (See PALINDROMIC SEQUENCE.) crude oil See PETROLEUM. crustacean diseases See e.g. BLACK GILL DISEASE; BLACK MAT SYNDROME; BURNED SPOT DISEASE; CRAYFISH PLAGUE; GAFFKAEMIA; PARAMOEBA. (See also LAGENIDIALES and SAPROLEGNIALES (ATKINSIELLA and Leptolegnia baltica).) (See also INVERTEBRATE DISEASES.) crustaceous Syn. CRUSTOSE. crustose (crustaceous) Forming or resembling a crust. Of a lichen: having a thallus which generally lacks a lower cortex, being attached to the substratum (over the whole of its lower surface) by hyphae of the medulla. A crustose thallus may be e.g. AREOLATE, PLACODIOID, etc. (cf. FOLIOSE; FRUTICOSE.) cryo- Prefix meaning cold. cryoelectron microscopy See ELECTRON MICROSCOPY (d). cryofixation The FREEZING of living cells preparatory to e.g. FREEZE-ETCHING; the aim is similar to that of FIXATION but cells may or may not be killed during the process. [Cryofixation techniques for soil fungi: MS (1985) 2 225–230.] cryogen Any agent which is used for FREEZING a specimen – e.g. liquid nitrogen. (See also FREON.) cryopreservation See FREEZING. cryoprotectant See FREEZING. cryosectioning Syn. FREEZE-SECTIONING. cryostat Syn. cryotome (q.v.). 205 cryptoendolith C. neoformans is the causal agent of CRYPTOCOCCOSIS and may also be associated with e.g. bovine mastitis. Cells (e.g. in malt extract after 3 days at 25° C) are spherical, ca. 3–8 µm diam., occurring singly, in pairs, or in small groups. Pseudomycelium is not formed, and extracellular polysaccharide may be abundant. C. neoformans is the only Cryptococcus sp which can grow well at 37° C (certain other species or strains – e.g. C. melibiosum – may be capable of weak growth at 37° C). C. neoformans has two varieties, var. neoformans and var. gattii (= ‘C. bacillisporus’), distinguished e.g. on the basis of the ability of var. gattii (but not var. neoformans) to give a blue colour on CGB AGAR within 2–5 days. Other Cryptococcus spp (not mentioned above) include e.g. C. ater, C. dimennae, C. elinovii, C. flavus, C. gastricus, C. heveanensis, C. infirmo-miniatus, C. kuetzingii, C. laurentii, C. luteolus, C. magnus, C. skinneri, C. terreus. Strains have been isolated from a wide range of habitats, including e.g. soil, plants, dew-retted flax, seawater, clinical specimens, foods and beverages, etc. Another species, C. friedmannii, has been isolated from rock fragments containing cryptoendolithic lichens from a dry desert area of the Antarctic [Mycol. (1985) 77 149–153]. cryptoendolith See ENDOLITHIC. cryptogam A plant which does not produce true flowers or seeds; cryptogams traditionally include the ALGAE, FUNGI, bryophytes (e.g. mosses) and pteridophytes (e.g. ferns). cryptogram (virol.) A descriptive code used to summarize, in symbolic form, certain basic properties of a given virus. Cryptomonadida See PHYTOMASTIGOPHOREA. cryptomonads Syn. CRYPTOPHYTES. Cryptomonas See CRYPTOPHYTES. cryptophytes (cryptomonads) A category of unicellular, biflagellated, marine or freshwater organisms variously regarded as algae (class Cryptophyceae) or as protozoa (see PHYTOMASTIGOPHOREA); most cryptophytes are photosynthetic (including e.g. Chroomonas and Cryptomonas), but some (e.g. Chilomonas, Cyathomonas) are colourless and heterotrophic. The cryptophyte cell is typically flattened and dorsiventral, lacks a cell wall (having instead a plasma membrane with an internal lining of proteinaceous plates), and possesses EJECTOSOMES. In photosynthetic species each cell has a single bilobed chloroplast containing a pyrenoid, paired thylakoids, chlorophylls a and c2 , a-carotene, diatoxanthin, and PHYCOBILIPROTEINS. Storage products are starch and lipid. Reproduction occurs by longitudinal binary fission; sexual reproduction is unknown. [Taxonomic aspects (including genus descriptions): New Phyt. (1984) 98 627–646.] cryptosporidiosis A COCCIDIOSIS (q.v.) of man and other animals caused by a species of CRYPTOSPORIDIUM; the pathogen is commonly transmitted (as sporulated oocysts) via the direct or indirect faecal–oral route. The disease in man is typically mild and self-limiting; it may involve diarrhoea, abdominal cramps, low-grade fever and headache. In the severely immunocompromised it is a lifethreatening disease: usually severe, persistent watery diarrhoea; other sites (e.g. respiratory, biliary) may be affected. No effective chemotherapy is available [IJP (1995) 25 139–195]. [The treatment dilemma: JMM (2000) 49 207–208.] Cryptosporidium is a cause of diarrhoea, pneumonia and disseminated infections in calves foals, lambs and piglets; it can also cause diarrhoea, inappetance and weight loss in the domestic cat [VR (1985) 116 73–74]. [Cryptosporidiosis in animals and humans: MR (1983) 47 84–96.] Outbreaks of disease can be transmitted via water as thickwalled oocysts of Cryptosporidium are resistant to the level of chlorination normally used in WATER SUPPLIES. The viability of C. parvum oocysts in water has been examined by staining them with fluorescent dyes (SYTO-9, SYTO-59); oocysts which stained above a certain intensity could not establish infection in mice, but those which displayed little or no fluorescence could readily establish infection [AEM (2000) 66 406–412]. Cryptosporidium A genus of parasitic (homoxenous) protozoa (suborder EIMERIORINA – q.v.) which are pathogenic for man and other animals (see CRYPTOSPORIDIOSIS); the organisms typically develop in the intestinal microvilli. The oocysts are very small (ca. 4–6 µm); following ENDOSPORULATION, each oocyst contains four banana-shaped sporozoites. Two types of oocyst are formed; thin-walled oocysts which release sporozoites into the host’s intestine, causing re-infection (‘autoinfection’) of the host, and acid-fast thick-walled oocysts (ca. 80% of the total) which are voided in the faeces. [Complete development of Cryptosporidium in cell cultures: Science (1984) 224 603–605.] The taxonomy of the genus is unsettled. It has been suggested that only 4 species should be recognized e.g. on the basis of host range: C. crotali (infecting reptiles), C. meleagridis (infecting birds), C. muris (infecting mammals) and C. nasorum (infecting tropical fish) [JP (1984) 31 94–98]. However, ‘C. parvum’, the organism which appears to be the cause of most cases of cryptosporidiosis, is apparently distinct from C. muris [J. Parasitol. (1985) 71 625–629]. [C. baileyi from chickens: JP (1986) 33 289–296.] Cryptostroma See HYPHOMYCETES; see also MAPLE BARK STRIPPERS’ DISEASE and SOOTY BARK. crystal toxin (of Bacillus thuringiensis) See DELTA-ENDOTOXIN. crystal violet A violet basic TRIPHENYLMETHANE DYE used e.g. in the GRAM STAIN. CS protein See PLASMODIUM. CS1, CS2, CS3 See ETEC. CSF (1) Cerebrospinal fluid. (2) COLONY-STIMULATING FACTOR. (3) Cold stability factor (see MICROTUBULE-ASSOCIATED PROTEINS). (4) A pheromone of Bacillus subtilis (see QUORUM SENSING). CSLM CONFOCAL SCANNING LIGHT MICROSCOPY. CSP reaction Shedding of CS proteins by the sporozoites of Plasmodium spp (see PLASMODIUM) following cross-linking of the CS proteins by anti-CS antibodies. (cf. Capping phenomenon in DYSENTERY (b).) CspA See COLD-SHOCK RESPONSE. CSPD Trade name (Tropix, Bedford, MA, USA) of a 1,2dioxetane substrate which gives rise to CHEMILUMINESCENCE when activated by the enzyme alkaline phosphatase. (See also AMPPD.) CSSD Central sterile supply department. CTAB See QUATERNARY AMMONIUM COMPOUNDS. ctDNA Chloroplast DNA (see CHLOROPLAST). CTEM Conventional transmission ELECTRON MICROSCOPY. CTh CARRIER-SPECIFIC T HELPER CELL. CTLA-4 See CD28. CTP Cytidine 5
-triphosphate [see Appendix V(b)]. ctxA, ctxB genes See CHOLERA TOXIN. cucumber 4 virus See TOBAMOVIRUSES. cucumber green mottle mosaic virus See TOBAMOVIRUSES. cucumber mosaic virus See CUCUMOVIRUSES. cucumber necrosis virus See TOBACCO NECROSIS VIRUS. cucumber pale fruit viroid See VIROID. 206 curdlan cucumbers (pickled) See PICKLING. cucumoviruses (cucumber mosaic virus group) A category of ssRNA-containing PLANT VIRUSES which infect e.g. cucurbits and solanaceous plants; transmission occurs (non-persistently) via aphids and, in some hosts, via seeds, and can also occur mechanically. Type member: cucumber mosaic virus, CMV (S isolate). Other members: peanut stunt virus (PSV) and tomato aspermy virus (TAV); possible member: cowpea ringspot virus. Virion: icosahedral, ca. 29 nm diam., containing a single type of coat protein and linear, positive-sense ssRNA. Three types of virion exist, differing in the nature of the RNA they contain; a particle may contain one molecule of RNA-1 (MWt ca. 1.27 × 106 ), one molecule of RNA-2 (MWt ca. 1.13 × 106 ), or one molecule each of RNA-3 (MWt ca. 0.82 × 106 ) and RNA-4 (coat protein mRNA). Some isolates of CMV and PSV also contain SATELLITE RNAS designated (respectively) CARNA (CMV-associated RNA) and PARNA (P SV-associated RNA). CARNA 5, the best-known example, consists of 335 nucleotides and is capped at the 5
end; it can code for two polypeptides. The presence of a CARNA attenuates the symptoms of CMV infection in many plants, but may enhance disease severity in others. For example, in tomato plants infection with CMV (only) may cause leaf chlorosis with some leaf distortion, but infection with both CMV and CARNA 5 causes a lethal necrosis. ‘White leaf’ of tomatoes and ‘brilliant yellowing’ of tobacco are also examples of diseases caused by CARNA strains. The presence of a CARNA in a host plant also reduces the yield of CMV – apparently as a result of competition for virus replication machinery. Cucurbitaria See DOTHIDEALES. cud See RUMEN. cudbear A purple dye obtained from Ochrolechia tartarea. Culex A genus of mosquitoes (order Diptera, family Culicidae); Culex spp are vectors of certain diseases: see e.g. MURRAY VALLEY FEVER. Culicinomyces A genus of fungi of the HYPHOMYCETES; C. clavisporus is pathogenic in mosquito larvae [Mycol. (1984) 76 614–625]. Culicoides A genus of biting midges (order Diptera, family Ceratopogonidae); Culicoides spp are vectors of certain diseases: e.g. AFRICAN HORSE SICKNESS and BLUETONGUE. culmination See DICTYOSTELIOMYCETES. cultivar (cv.) A commercial or cultivated ‘variety’ of a given species of plant or fungus. culture (1) (noun) A liquid or solid MEDIUM on or within which has grown a population of particular type(s) of microorganism or cell as a result of the prior INOCULATION and INCUBATION of that medium. The following refers primarily to microbial cultures – cf. TISSUE CULTURE. On a solid medium, microorganisms may grow as a continuous layer or film of surface growth (confluent growth) or as discrete (individual) colonies (see COLONY) – depending e.g. on the method of inoculation (cf. LAWN PLATE and STREAKING). Cultures in liquid media may be turbid or clear, and may or may not have a PELLICLE. Aerobic culture. A culture prepared under AEROBIC (sense 1) conditions. Anaerobic culture. One prepared under ANAEROBIC (sense 1) conditions. (See also ANAEROBE.) Axenic culture. Pure culture (see below). Batch culture. See BATCH CULTURE. Broth culture. One prepared with NUTRIENT BROTH or with any similar liquid medium. Closed culture. Syn. BATCH CULTURE. Contaminated culture. One which has been exposed (usually unintentionally) to non-STERILE conditions and which has become contaminated with extraneous organisms. Continuous culture. See CONTINUOUS CULTURE. Fed batch culture. See FED BATCH CULTURE. Mixed culture. One containing two or more species or strains of organism. Old culture. One which has been incubated or stored for an excessive period of time and in which, as a consequence, degenerative changes may have occurred – see e.g. INVOLUTION FORMS. Open culture. Syn. CONTINUOUS CULTURE. Plate culture (‘plate’). One prepared using a solid medium (e.g. nutrient agar) in a PETRI DISH. Primary culture. See PRIMARY CULTURE. Pure culture (AXENIC culture). One in which all the organisms are of the same species or strain. Shake culture. See SHAKE CULTURE. Slant culture. Slope culture (see below). Slope culture (slant culture). One prepared using a SLOPE. Stab culture. One produced by deep inoculation of a solid medium (e.g. the butt of an agar or gelatin slope) with a STRAIGHT WIRE; the wire is pushed vertically into the medium so that the INOCULUM (on the tip of the wire) is distributed along the length of the stab. Subculture. See SUBCULTURE. Synchronous culture. See SYNCHRONOUS CULTURE. (2) (noun) The process of preparing a culture (sense 1). (3) (verb) To encourage the growth of particular type(s) of microorganism under controlled conditions. Cultures of bacteria or fungi are usually started by seeding (inoculating) a medium with viable cells or spores from another culture, or by inoculating the medium with material expected to contain particular type(s) of viable organism; the type of MEDIUM and the incubation conditions required differ widely among the different types of microorganism. Certain organisms (e.g. Rickettsia spp, Treponema pallidum) can be cultured only in living systems. (See also ASEPTIC TECHNIQUE; ENRICHMENT; LOOP.) culture collections Collections of characterized microorganisms. General collections: see e.g. ATCC, CBS, CCM, CIP, DSM, JCM, LMD, and NCTC; agricultural (including dairy and phytopathological organisms): see e.g. ICPB, NCDO, NCPPB, NRRL and UWO; algae: see e.g. SAG; fungi (including yeasts): see e.g. CBS and CMI; industrial and applied; see e.g. IAM, IFO and NCIB; marine bacteria; NCMB; medical : see e.g. CDC. cultured (of bacteria etc.) Grown. cultured butter See BUTTER. cultured buttermilk See BUTTERMILK. Cummings’ disease See AVIAN INFECTIOUS BRONCHITIS. Cunninghamella See MUCORALES. Cunninghamellaceae See MUCORALES. cup fungi Ascomycetes which form cup-like fruiting bodies, particularly members of the PEZIZALES. cup lichens Species of CLADONIA whose podetia have conspicuous scyphi. cuprammonium hydroxide See COPPER. cupredoxin See BLUE PROTEINS. cupulate Cup-Shaped. (See also AECIDIOID.) curdlan A class of extracellular, water-insoluble, linear (1 → 3)b-glucans formed e.g. by Alcaligenes faecalis ‘var. myxogenes’ and by strains of Agrobacterium and Rhizobium. When aqueous suspensions of curdlans (e.g. 2% w/v) are heated to >90° C and then cooled slowly, they form very elastic, resilient gels. 207 curie cut-and-paste mechanism (of transposition) See e.g. Tn10. cutaneous basophil hypersensitivity Syn. JONES–MOTE SENSITIVITY. cutaneous herpes See HERPES SIMPLEX. cutaneous leishmaniasis LEISHMANIASIS of man and other animals in which the pathogen primarily infects the skin and gives rise to self-healing or metastasizing lesions; the causal agents (which are transmitted by sandflies), and the patterns of disease, vary with geographical location. Old World cutaneous leishmaniases (mainly in Africa and Asia) are typically caused by Leishmania aethiopica, L. major or L. tropica. In man, a papule develops (after several weeks) at the site of a sandfly bite, becoming necrotic at the centre and forming a moist, ulcerative lesion (L. major) or a dry lesion (L. aethiopica, L. tropica) which eventually heals. (These lesions have been called e.g. Aleppo boil, Delhi boil, oriental sore.) L. aethiopica and L. major infections are zoonotic, animal reservoirs including populations of e.g. the great gerbil, Rhombomys opimus (for L. major), and the rock hyrax, Procavia capensis (for L. aethiopica); L. tropica infections appear to be anthroponotic. New World cutaneous leishmaniases (in South America) are caused by L. braziliensis or L. mexicana; they are zoonotic diseases in man, animal reservoirs including rodents and sloths. The clinical picture in man may resemble the Old World pattern; however, following an apparent cure the disease may re-appear at a site remote from the original lesion, and in some cases the infection may metastasize to the oral/nasal mucous membranes (see MUCOCUTANEOUS LEISHMANIASIS). (See also CHICLERO’S EAR and PIAN BOIS.) Diagnosis of cutaneous leishmaniasis may involve e.g. microscopical examination of exudate from lesions, culture of lesion material (in e.g. NNN MEDIUM), and the MONTENEGRO TEST; treatment may involve e.g. pentavalent ANTIMONY compounds and/or antibiotics. (cf. VISCERAL LEISHMANIASIS.) cutaneous T-cell lymphoma See MYCOSIS FUNGOIDES and SEZARY SYNDROME; see also ADULT T-CELL LEUKAEMIA. cuticle (mycol.) Syn. PELLIS. cutin An insoluble polymer which, embedded in waxes, forms the cuticle which covers the epidermal cell walls in the aerial parts of higher plants. (cf. SUBERIN.) Cutin is a polyester formed from C16 and C18 hydroxy fatty acids. It functions as a physical barrier, protecting against water loss, entry of pathogens, etc. Many plant-pathogenic fungi produce enzymes (cutinases) which can breach the cutin barrier prior to infection. [Book ref. 58, pp. 79–100; enzymic penetration of the plant cuticle by fungal pathogens: ARPpath. (1985) 23 223–250.] cutinase A CUTIN-degrading enzyme. cutis (mycol.) Syn. PELLIS. Cutleria See PHAEOPHYTA. cv. CULTIVAR. CVF COBRA VENOM FACTOR. Cx cellulase See CELLULASES. CY agar A medium which includes pancreatic digest of casein, yeast autolysate, and (sometimes) CaCl2 . cya gene See ADENYLATE CYCLASE. cyaA gene See CYCLOLYSIN. cyanelles Endosymbiotic CYANOBACTERIA which occur in various eukaryotes: e.g. in the testate amoeba PAULINELLA, in the ‘algae’ Cyanophora paradoxa and Glaucocystis spp, and in certain sponges; many contain CARBOXYSOMES. A number of authors consider that cyanelles were the evolutionary precursors of CHLOROPLASTS. (See also GLAUCOPHYTA.) Curdlans have potential applications in the food industry (e.g. as non-caloric gelling, thickening or stabilizing agents) and in other industries (e.g. in the formation of films and fibres for use e.g. as molecular sieves and/or as supports for immobilized enzymes). [Book ref. 62, pp. 201–229.] curie (abbr. Ci) A unit of activity of a radioactive source equal to 3.7 × 1010 disintegrations per sec. Curie point pyrolysis PYROLYSIS in which a ferromagnetic wire (coated with the sample) is heated inductively by means of a high-frequency alternating magnetic field; the temperature of the wire increases (within e.g. 0.1 sec) to the Curie point: a temperature which the inducing field maintains – but cannot increase owing to the wire’s thermally promoted paramagnetic state. The Curie point depends on the chemical composition of the wire; iron-nickel wires with a Curie point of 510° C are often used. curing (1) (of meats) A method of FOOD PRESERVATION in which meat (particularly pig meat) is permeated with a solution typically containing NaCl, NaNO2 and NaNO3 at a temperature of e.g. 4° C. Preservation is due mainly to the NaCl (which e.g. lowers the WATER ACTIVITY) and to the nitrite which, under appropriate conditions of e.g. aw , pH (ca. 5.6) and temperature, inhibits the growth of vegetative bacteria and the germination and/or outgrowth of bacterial endospores. Nitrite also enhances the colour and flavour of cured meats. Brines used for curing hams contain a characteristic microflora which reduces nitrate to nitrite, lowers the pH by producing acid from carbohydrates, and (possibly) produces flavour components. The presence of these bacteria (principally halophilic, psychrotolerant Vibrio spp) reduces the spoilage of the cured hams; the organisms may be added to the brine as a starter culture or may derive from the meat itself. (2) (of plasmids in bacteria) Elimination of a PLASMID without loss of bacterial viability. Sublethal doses of e.g. certain INTERCALATING AGENTS (e.g. ACRIDINES) or NOVOBIOCIN can cause preferential inhibition of plasmid replication; hence, a growing, plasmid-containing bacterial population which is exposed to these agents will give rise to an increasing number of plasmidfree progeny cells. MITOMYCIN C is often used for curing plasmids in Pseudomonas spp. Other curing agents include rifampicin (see RIFAMYCINS). curling factor See GRISEOFULVIN. Curtis–Fitz-Hugh syndrome A syndrome, observed most commonly in women, which includes acute PERIHEPATITIS in association with genital tract infection; it can be due e.g. to Neisseria gonorrhoeae or Chlamydia trachomatis. [Perisplenitis and perinephritis in the Curtis–Fitz-Hugh syndrome: Br. J. Surg. (1987) 74 110–112.] Curtobacterium A genus of aerobic, asporogenous bacteria (order ACTINOMYCETALES, wall type VIII – see also PEPTIDOGLYCAN) which occur in soil and plant litter; some species are phytopathogenic. In culture the organisms are short rods, coccobacilli or coccoid forms; they are non-motile, or motile with a few lateral flagella. Growth occurs optimally at ca. 25° C on e.g. media containing peptone, yeast extract, and glucose; the organisms are non-cellulolytic. GC%: ca. 66–73. Type species: C. citreum. [Re-classification of Corynebacterium betae, C. flaccumfaciens, C. oortii and C. poinsettiae (collectively) as Curtobacterium flaccumfaciens: JGM (1983) 129 3545–3548.] Curvularia See HYPHOMYCETES. Custers effect (negative Pasteur effect) A phenomenon in which, apparently, the fermentative metabolism of a substrate is stimulated by the presence of O2 . (See e.g. BRETTANOMYCES; cf. PASTEUR EFFECT.) 208 cyanobacteria − can also catabolize endogenous polysaccharide under anaerobic conditions – either by anaerobic respiration using sulphur as terminal electron acceptor (resulting in sulphide formation) or by a fermentative pathway in which lactic acid is produced. Some cyanobacteria can grow on exogenous organic compounds – either photoheterotrophically or chemoorganotrophically. [Carbon metabolism: Ann. Mic. (1983) 134 B 93–113.] Certain species (e.g. Cyanothece halophytica, Dactylococcopsis salina, Oscillatoria limnetica) can, in the presence of sulphide, carry out facultative anoxygenic PHOTOSYNTHESIS using photosystem I with H2 S as electron donor; in O. limnetica the resulting sulphur accumulates extracellularly. NITROGEN FIXATION occurs in many species, usually in specialized cells called HETEROCYSTS (cf. GLOEOTHECE and TRICHODESMIUM): see also NIFGENES. In some species (e.g. Anabaena variabilis) diazotrophy can occur in the dark under heterotrophic conditions [energy requirement: JGM (1983) 129 2633–2640]. [General metabolic aspects: Book ref. 76.] Cyanobacteria occur in a wide range of habitats, from tropical to polar regions; however, they are rare or absent in habitats of pH < ca. 5. Depending on species, they may be found in fresh, brackish, marine and hypersaline waters, where they may be planktonic, benthic, epiphytic etc (see also BLOOM and OSMOREGULATION); in hot springs; on soils (including hot arid desert soils), muds, sediments and salt-marshes (where they may form extensive and conspicuous mats); on rocks (where they may be endolithic or epilithic); etc. (See also STROMATOLITE.) In many such environments (including rice fields) free-living cyanobacteria may be important providers of fixed nitrogen, and certain nitrogen-fixing cyanobacteria form symbiotic associations with eukaryotic organisms (see e.g. ANABAENA and NOSTOC). Heterocystous cyanobacteria of several genera (Anabaena, Cylindrospermum, ‘Gloeotrichia’, Nostoc) occur in colonies attached to the lower epidermis or in the reproductive pockets in the leaves of floating duckweed plants (Lemnaceae), possibly supplying much of their nitrogen requirement [CJM (1985) 31 327–330]. (See also CYANELLE; RHIZOSOLENIA; RHOPALODIA.) No species appears to be directly pathogenic in any organism, but some freshwater bloom-forming strains produce potent toxins which may cause illness or death in animals drinking the water (see e.g. ANABAENA, APHANIZOMENON, MICROCYSTIS, NODULARIA; cf. LYNGBYA), and cyanobacterial blooms in reservoirs may cause tainting of the WATER SUPPLIES (see e.g. GEOSMIN). [Toxins and health: RMM (1994) 5 256–264.] Cyanobacterial toxins (= cyanotoxins) include e.g. (i) neurotoxins such as the anatoxins (see ANABAENA) and SAXITOXIN; (ii) hepatotoxins such as microcystins (produced by strains of MICROCYSTIS and of e.g. Anabaena, Nostoc and Oscillatoria); (iii) dermatotoxins such as aplysiatoxin (produced by strains of LYNGBYA and of e.g. Oscillatoria). [Ecological and molecular investigations of cyanotoxin production (review): FEMS Ecol. (2001) 35 1–9.] Fossil organisms resembling cyanobacteria are among the oldest fossils known; from the fossil record and other data it is believed that cyanobacteria may have dominated the Earth’s biota during the middle to late Precambrian (Proterozoic, ca. 2500–570 million years ago), and were probably responsible for the generation of oxygen in the atmosphere. [Book ref. 76, pp. 543–564.] The taxonomy of the cyanobacteria is confused. Until quite recently they were regarded as algae and were therefore subject to the Botanical Code of nomenclature. Thus, herbarium specimens, or descriptions and illustrations, were used as type materials (living cultures not being recognized as valid types cyanide (CN ) (1) (as a RESPIRATORY INHIBITOR) Cyanide binds to and inhibits both the oxidized and reduced forms of CYTOCHROME OXIDASES of the aa3 -type. Cytochrome oxidases of the o-type are also generally inhibited, but those of the d-type are insensitive to cyanide. (2) (as a metabolite) Cyanide is formed from glycine by certain microorganisms, e.g. a basidiomycetous fungus [JGM (1980) 116 9–16] and various bacteria, including Chromobacterium violaceum [JGM (1984) 130 521–525]; at least some of these organisms can degrade cyanide – e.g. to CO2 or to b-cyanoalanine. A strain of Pseudomonas fluorescens has been reported to utilize cyanide as a source of nitrogen for growth, converting cyanide to ammonia and CO2 in an oxygendependent reaction [FEMS (1983) 20 337–341]. (3) (as an enzyme inhibitor) Cyanide inhibits e.g. CO dehydrogenase (see METHANOGENESIS). (See also AMYGDALIN and LEACHING.) cyanide broth See KCN BROTH. cyanide test (KCN test) A test used to determine the ability of an organism to grow in the presence of cyanide; it is used in the identification of certain species of bacteria, particularly members of the Enterobacteriaceae. A suitable medium, e.g. KCN BROTH, is inoculated with the strain under test, and the medium is examined for growth after 24 and 48 hours; growth constitutes a positive reaction. Cyanidium See RHODOPHYTA. cyanobacteria (‘blue-green algae’) A large and varied group of prokaryotic, photosynthetic organisms which differ from other bacteria in possessing chlorophyll a (but not bacteriochlorophylls) and in carrying out oxygenic PHOTOSYNTHESIS. (cf. RHODOSPIRILLALES and ALGAE; see also OXYPHOTOBACTERIA and CYANOBACTERIALES.) The organisms are unicellular (cells occurring singly or in aggregates) or filamentous (usually uniseriate and unbranched, sometimes multiseriate and branched); they may form more or less well-defined and distinctive colonies in nature (see e.g. COELOSPHAERIUM, GLOEOTRICHIA, NOSTOC, TRICHODESMIUM). The essentially Gram-negative-type CELL WALL frequently has features typical of the Gram-positive-type wall [JB (2000) 182 1191–1199]; one or more layers external to the outer membrane may be present, forming a firm or diffluent mucilaginous sheath (which may be calcified in some species) or a fibrous outer layer (see section II, below). No cyanobacterium has flagella at any stage, but many strains are capable of GLIDING MOTILITY; a novel type of motility has been reported in SYNECHOCOCCUS (q.v.). Within the cell, the photosynthetic pigments (CHLOROPHYLL a, CAROTENOIDS and PHYCOBILIPROTEINS) are typically attached to THYLAKOIDS (cf. GLOEOBACTER); the relative proportions of these pigments determine the colour of the organisms (blue-green, olive-green, yellowgreen, red, purple, or almost black – depending on organism and environmental conditions: see also CHROMATIC ADAPTATION). Cells may also contain e.g. CARBOXYSOMES, GAS VACUOLES, and granules of CYANOPHYCEAN STARCH, CYANOPHYCIN, (occasionally) POLY-b-HYDROXYBUTYRATE, and/or POLYPHOSPHATE. In many species, cells or filaments (trichomes) may undergo differentiation in response to particular environmental conditions: see e.g. AKINETE, HETEROCYST, HORMOGONIUM. In general, cyanobacteria are primarily aerobic, oxygenic photoautotrophs which fix CO2 via the CALVIN CYCLE; during photosynthesis, cells accumulate storage polysaccharide which, during periods of darkness, is catabolized (via the HEXOSE MONOPHOSPHATE PATHWAY and an aerobic respiratory electron transport chain) to provide maintenance energy. Oscillatoria limnetica 209 Cyanobacteriales Many genera have yet to be incorporated in this scheme: see e.g. APHANIZOMENON, COELOSPHAERIUM, DACTYLOCOCCOPSIS, MICROCYSTIS, STARRIA, TRICHODESMIUM. [Isolation, media etc: Book ref. 45, pp. 212–246.] Cyanobacteriales An order proposed for the CYANOBACTERIA [IJSB (1978) 28 1–6] but not accepted as legitimate owing to the lack of a genus named Cyanobacterium; however, the proposal of a new genus Cyanobacterium [Ann. Mic. (1983) 134 B 21–36] paves the way for legitimization of the order. Cyanobacterium See SYNECHOCOCCUS and CYANOBACTERIALES. cyanobiont A cyanobacterial symbiont: see e.g. ANABAENA and NOSTOC. (cf. PHYCOBIONT.) Cyanobium See SYNECHOCOCCUS. cyanocobalamin See VITAMIN B12 . Cyanocyta See GLAUCOPHYTA. cyanogen bromide (CNBr) A reagent used e.g. for the IMMOBILIZATION of a protein on the surface of a support; CNBr binds to the support (forming a ‘CNBr-activated’ support) and then binds spontaneously to primary amino groups on the protein. The instability of the isourea bond (between the activated support and protein) may lead to some loss of the immobilized protein; this may be avoided e.g. by cross-linking the protein with glutaraldehyde. cyanogenic Having the capacity to produce cyanide (see also CYANIDE sense 2). cyanoguanidine Dicyandiamide, a NITRIFICATION INHIBITOR. cyanomycin An antibiotic produced by ‘Streptomyces cyanoflavus’ and reported to be identical to PYOCYANIN [J. Antibiot. (1969) 22 49–54 cited in JB (1980) 141 156–163]. cyanophage Any VIRUS whose host is a cyanobacterium (‘bluegreen alga’). (cf. PHYCOVIRUS.) Cyanophages may be virulent or temperate, and have been isolated from filamentous and unicellular species – including members of the LPP group (cyanophages LPP-1, LPP-2), Nostoc (N1, N2), and Anabaena (A1, A2); lysis of infected intercalary cells in filamentous species results in progressive fragmentation of the filament. [Ann. Mic. (1983) 134 B 43–59; classification and nomenclature: Intervirol. (1983) 19 61–66.] cyanophilic Having an affinity for blue dyes such as LACTOPHENOL COTTON BLUE. Cyanophora See GLAUCOPHYTA. cyanophycean starch A glycogen-like storage polysaccharide found in cyanobacteria; the polysaccharide occurs as granules or rods located between the thylakoids. cyanophycin (multi-L-arginyl-poly(L-aspartic acid); arg-poly(asp)) A high-MWt polymer consisting of equal amounts of L-aspartic acid and L-arginine; the L-aspartic acid residues occur in a linear chain, and each residue is linked via its free carboxyl group to the a-amino group of an arginine residue. Granules of cyanophycin (a granules, ‘structured granules’) occur in most cyanobacteria. The polymer is synthesized independently of ribosomes and serves primarily as a nitrogen reserve in both vegetative cells and heterocysts. Under CO2 -limiting conditions, cyanophycin may also serve as a source of carbon and energy by an ARGININE DIHYDROLASE pathway, at least in some species. [Review: ARM (1984) 38 13–16.] cyanophytes CYANOBACTERIA. cyanosis (med., vet.) A bluish discoloration or darkening of the skin and mucous membranes due to inadequate oxygenation of the blood. cyanosomes Cyanobacterial PHYCOBILISOMES. Cyanothece See SYNECHOCOCCUS. cyanotoxin Any toxin produced by a member of the CYANOBACTERIA (q.v. for examples). according to the Botanical Code), and emphasis was given to ‘field’ characteristics (e.g. colony form, pattern and frequency of false branching, presence, absence and/or consistency of sheath material) in organisms growing in nature. On this basis >150 genera and >1000 species were described. However, many of these ‘field’ characteristics were found to be indeterminate in culture: e.g., colony forms may be lost, mucilage production and false branching may be variable, etc. Hence, a new scheme for generic assignments, based on properties of the organisms in pure culture, was proposed [JGM (1979) 111 1–61] and widely accepted. The genera were grouped into five sections: Section I. Unicellular; cells occur singly or in colonies. Reproduction occurs by equal binary fission (GLOEOBACTER, GLOEOCAPSA, GLOEOTHECE, SYNECHOCOCCUS, SYNECHOCYSTIS) or by budding (CHAMAESIPHON). This section includes organisms formerly of the Chroococcales, with Chamaesiphon from the Chamaesiphonales. Section II. Unicellular; cells always enclosed by a fibrous layer (F layer) external to the outer membrane. Reproduction occurs by multiple fission only (DERMOCARPA, XENOCOCCUS) or by multiple fission and binary fission (CHROOCOCCIDIOPSIS, DERMOCARPELLA, MYXOSARCINA, PLEUROCAPSA GROUP). Multiple fission occurs by rapid repeated binary fissions (unaccompanied by growth) within the F layer, resulting in the formation of BAEOCYTES which are released by rupture of the F layer. In species capable of binary fission, a series of divisions produces an aggregate of vegetative cells cemented together by their F layers; some or all of the cells in the aggregate eventually undergo multiple fission to form baeocytes. Motility, if it occurs, is restricted to baeocytes prior to the development of an F layer. [Developmental patterns: Book ref. 34, pp. 203–226.] This section includes the Chamaesiphonales (except Chamaesiphon) and the Pleurocapsales. Section III. Filamentous; growth occurs by intercalary cell division in one plane only (at 90° to the long axis of the trichome), giving rise to uniseriate, unbranched trichomes (cf. FALSE BRANCHING) composed only of vegetative cells (i.e., heterocysts and akinetes are not formed). The trichomes may be helical (SPIRULINA) or straight (LPP GROUP, OSCILLATORIA, PSEUDANABAENA). This section includes some genera of the former order Nostocales. Section IV. Filamentous; growth occurs by intercalary cell division in one plane only (at 90° to the long axis of the trichome), giving rise to uniseriate, unbranched trichomes (cf. FALSE BRANCHING); heterocysts are formed in the absence of combined nitrogen, and akinetes are produced by some members. Reproduction occurs by random breakage of trichomes, or by germination of akinetes if formed; hormogonia are produced by CALOTHRIX, NOSTOC and SCYTONEMA, but not by ANABAENA, CYLINDROSPERMUM or NODULARIA. Section IV includes those genera of the Nostocales not included in section III. Section V. Filamentous; growth occurs by intercalary cell division which may occur in more than one plane, i.e., some cells in the mature trichome may divide in a plane parallel to the long axis of the trichome. Thus, the mature trichome may be partly multiseriate with uniseriate lateral (true) branches (FISCHERELLA), but may readily break up into cell aggregates (CHLOROGLOEOPSIS). Heterocysts are formed in the absence of combined nitrogen; akinetes are formed by some members. Reproduction occurs by random breakage of trichomes, by the formation of hormogonia, and (in some species) by the formation and germination of akinetes. Section V is taxonomically equivalent to the former order Stigonematales. 210 cyclolysin more essential stages of its life cycle in the vector. (See e.g. and TRYPANOSOMA.) Cyclidium A genus of ciliates (order SCUTICOCILIATIDA) which occur e.g. in soil, fresh water (including hot springs) and marine habitats. Cells: typically ovoid and generally small, ca. 15–40 µm; the cell surface is typically not densely ciliated, but those cilia which are present tend to be long, and there is often one or more longer caudal cilia. cyclin A type of protein involved in the regulation of the eukaryotic CELL CYCLE. During the cycle, molecules of cyclin accumulate intracellularly at specific stages and form complexes with certain PROTEIN KINASES. Phosphorylation of the kinase part of the complex – and subsequent (partial) dephosphorylation at an appropriate point in the cycle – leads to activation of the kinase. The activated kinase phosphorylates certain proteins which then promote onward cycling; it can also bring about UBIQUITIN-dependent degradation of the cyclin – so that the intracellular concentration of cyclin falls sharply on passage through the checkpoint. There are various types of cyclin, and different types may form complexes with protein kinases at different cell-cycle checkpoints; for example, different types of cyclin form complexes with the yeast protein kinase Cdc2 in order to mediate passage through (a) the start checkpoint, and (b) the G2 → M checkpoint. cyclitol antibiotics ANTIBIOTICS which contain a cyclic alcohol – e.g. AMINOGLYCOSIDE ANTIBIOTICS (which contain a substituted INOSITOL). cycloalkanes See HYDROCARBONS. cycloamyloses Syn. SCHARDINGER DEXTRINS. cyclochlorotine Syn. CHLOROPEPTIDE. cyclodextrins Syn. SCHARDINGER DEXTRINS. cycloguanil See CHLORGUANIDE. cyclohexane metabolism See HYDROCARBONS. cycloheximide (Actidione; b-[2-(3,5-dimethyl-2-oxocyclohexyl)2-hydroxyethyl]-glutarimide) An ANTIBIOTIC produced by certain strains of e.g. Streptomyces griseus; it is a by-product of streptomycin manufacture. Cycloheximide is active against many fungi and other eukaryotes (bacteria are not affected), acting primarily by inhibiting PROTEIN SYNTHESIS; e.g., it prevents translocation by binding to the 60S subunit of 80S ribosomes. Ribosomes from different organisms may differ in sensitivity. Only the yeast-like forms of certain dimorphic fungi (e.g. Blastomyces dermatitidis, Histoplasma capsulatum) are susceptible to cycloheximide. Certain stereoisomers of cycloheximide (e.g. naramycin B) have been isolated from Streptomyces cultures; these are generally less active than cycloheximide against fungi. Streptovitacins are monohydroxyl-substituted cycloheximides produced by S. griseus; streptovitacin A appears to resemble cycloheximide in its mode of action. cyclolysin An RTX TOXIN produced by species of Bordetella. The ∼177 kDa protein is encoded by gene cyaA; activity of the toxin depends on post-translational modification in which a long-chain fatty acid is linked covalently at a specific lysine residue. The toxin is secreted by an ABC EXPORTER. Cyclolysin has ADENYLATE CYCLASE, haemolysin and poreforming activity. A range of eukaryotic cells are susceptible; within the target cell cyclolysin is activated by CALMODULIN. The principal role of cyclolysin in WHOOPING COUGH may involve inhibition of the activity of phagocytic cells (e.g. macrophages) by raising their intracellular levels of cAMP. Moreover, by suppressing anti-bacterial activity in phagocytes, Cyathomonas See CRYPTOPHYTES. Cyathus A genus of fungi (order NIDULARIALES) in which the basidiocarp is funnel-shaped (ca. 1 cm across), the opening of the funnel being closed by a membrane (epiphragm) in the immature fruiting body. Each of the flattened, discoid peridioles is attached to the lower, inner surface of the peridium (i.e., funnel) by means of a complex structure which consists essentially of three components joined end to end. Attached directly to the peridium is the sheath (a short cord of aggregated hyphae) to which is joined the middle piece (a narrow extension of the sheath); the middle piece is attached to an elongated sac (the purse) to the outside of which is attached the peridiole. The purse contains a long thread of coiled hyphae (the funiculus or funicular cord ), one end of which is firmly attached to the peridiole while the other (unattached) end carries a strongly adhesive body (the hapteron). When drops of rain fall into the funnel-shaped peridium (or ‘splash cup’) the peridioles are ejected by the strong upthrust of water on their undersurfaces; each peridiole flies through the air, trailing its funiculus, until the hapteron adheres to e.g. a blade of grass. cycad symbioses See NOSTOC. cyclaridine An ANTIVIRAL AGENT – the carbocyclic analogue of vidarabine; its activity resembles that of VIDARABINE in cell cultures, but it is resistant to deamination by adenosine deaminase. cycles of matter See CARBON CYCLE, NITROGEN CYCLE and SULPHUR CYCLE. cyclic AMP (cAMP) Adenosine 3
,5
-cyclic monophosphate: a cyclic nucleotide synthesized from ATP by ADENYLATE CYCLASE; cAMP is degraded to AMP by cAMP phosphodiesterase (EC 3.1.4.17). PLASMODIUM adenine 5′ O CH2 O P H H HO O 3′ O 1′ H 2′ H OH CYCLIC AMP (cAMP) cAMP is an important regulatory molecule in various types of cell. In prokaryotes, cAMP is involved in CATABOLITE REPRESSION and e.g. in stimulating fruiting in the MYXOBACTERALES. [cAMP in prokaryotes: MR (1992) 56 100–122.] In eukaryotes, cAMP is involved in various regulatory and developmental processes (see e.g. DICTYOSTELIUM); it apparently functions mainly by regulating the activity of PROTEIN KINASES. cAMP-dependent protein kinase A (PKA) consist of two types of subunit: the catalytic (C) and regulatory (R) subunits; the inactive form of PKA consists of a dimer of R subunits to which are bound two C subunits. cAMP activates PKA by binding to the R subunits; this causes the release of both C subunits – which (separately) are then able to carry out their catalytic roles. The active C subunits have a wide range of functions in both cytoplasm and nucleus – including e.g. regulation of certain transcriptional events in the nucleus. cyclic octadepsipeptide antibiotics QUINOXALINE ANTIBIOTICS. cyclical transmission A mode of transmission of a parasite by a VECTOR (sense 1) in which the parasite undergoes one or 211 cyclo-oxygenase cyprofuram See PHENYLAMIDE ANTIFUNGAL AGENTS. Cyrtophorida See HYPOSTOMATIA. cyrtos (cytopharyngeal basket; nasse; pharyngeal basket) A (frequently curved) type of CYTOPHARYNGEAL APPARATUS whose walls are strengthened by longitudinal nematodesmata (which arise at apical kinetosomes) and are lined with extensions of POSTCILIARY MICROTUBULES; toxicysts are absent. The cyrtos is characteristic of the HYPOSTOMATIA. (cf. RHABDOS.) cyst A specialized microbial cell produced either in response to adverse environmental conditions or as a normal part of the life cycle. During cyst formation (encystment), an organism produces a thick or thin wall within which it becomes totally enclosed. Cysts are usually resistant to desiccation, and may be resistant to e.g. ultraviolet radiation and/or heat. (a) (bacteriol.) Cyst formation is uncommon among bacteria; most studies on bacterial cyst formation have been carried out on species of AZOTOBACTER, particularly A. vinelandii. In Azotobacter spp, encystment can be promoted in vitro by the provision of substrates such as b-hydroxybutyrate, and it has been generally supposed that the intracellular accumulation of PHB is a necessary pre-requisite for cyst formation; however, it has been proposed that the stimulus for encystment may be related to the extracellular levels of both carbon and nitrogen sources [SBB (1986) 18 23–28]. During encystment, cells lose their flagella and become spheroidal; these heavily encapsulated cells (‘precysts’) typically accumulate PHB. Membranous blebs develop at the cell surface, and these subsequently detach and coalesce – forming the fragmented outer layer (exine) of the cyst wall. Later, the cyst wall develops an electron-transparent inner layer (intine). The cyst wall contains ALGINATE together with protein and lipid; the exine is particularly rich in polyguluronic acid (and Ca2+ ), while the intine is richer in polymannuronic acid. Azotobacter cysts are metabolically dormant, and they can remain viable in dry soil for many years; they are more resistant than the vegetative cells to desiccation, sonication and ultraviolet radiation, but they are not significantly more resistant to heat. Azotobacter-like ‘lipid cysts’ are formed by some members of the Methylococcaceae. (See also MICROCYST.) (b) (protozool.) Cysts are formed e.g. by some amoebae, ciliates and phytoflagellates. During encystment, structures such as cilia or flagella are lost or resorbed, and considerable reorganization of internal structures may occur; the cell becomes enclosed by a thin or thick, commonly multilayered wall, the outer-most layer(s) being termed the ectocyst or exocyst, the innermost layer(s) the endocyst, and intermediate layer(s) the mesocyst. Cyst walls vary in composition, according to species; they commonly contain a high proportion of protein, while e.g. cellulose occurs in the endocyst of Acanthamoeba spp, chitin apparently occurs in Entamoeba cysts [BBRC (1982) 108 815–821], and sulphated glycosaminoglycans occur in the mesocyst in Histriculus similis [JGM (1983) 129 829–832]. Cysts of CHRYSOPHYTES have silica walls. (In certain loricate or testate organisms – e.g. Euglypha spp – a cyst is formed when the organism withdraws into its lorica/test and plugs the aperture.) Excystment (the release of one or more vegetative cells from the cyst) may involve rupture of the cyst wall (possibly as a result of the intake of water) and/or enzymic dissolution of the wall; in e.g. Naegleria gruberi the cell leaves the cyst via a pore which becomes unplugged, while in Acanthamoeba a lid-like operculum is removed from an exit aperture in the cyst wall. In many protozoa the cyst serves a protective function, allowing the organism to survive e.g. the absence of food, the pathogen may be able to establish an intracellular carrier state. The raised levels of cAMP in epithelial cells may account for the secretion of fluid/mucus. cyclo-oxygenase See PROSTAGLANDINS. cycloparaffins See HYDROCARBONS. cyclophosphamide See IMMUNOSUPPRESSION. cyclopiazonic acid A MYCOTOXIN produced by species of Aspergillus and Penicillium (e.g. A. flavus, A. oryzae, P. verrucosum (‘P. cyclopium’) and P. griseofulvum (‘P. patulum’)). It is toxic e.g. for chickens [AEM (1983) 46 698–703], rats and calves. D-cycloserine (D-4-amino-3-isoxazolidone) A broad-spectrum ANTIBIOTIC obtained from strains of Streptomyces spp or synthesized chemically. It acts as an analogue of D-alanine, competitively inhibiting the two enzymes (alanine racemase and D-alanyl-Dalanine synthetase) involved in the biosynthesis of PEPTIDOGLYCAN. D-Cycloserine is actively taken up by the D-alanine/glycine transport system of the cell, and a mutation which alters this transport system can result in resistance to the antibiotic. (Methanococcus vannielii lacks peptidoglycan but is nevertheless susceptible to D-cycloserine [Book ref. 157, p. 529].) cyclosis Syn. CYTOPLASMIC STREAMING. Cyclospora A genus of protozoa (suborder EIMERIORINA) which form disporic, dizoic oocysts. [C. cayetanensis as a diarrhoeal agent: RMM (1996) 7 143–150.] cyclosporin A A hydrophobic peptide obtained from certain hyphomycetes (Cylindrocarpon lucidum, Tolypocladium inflatum); it is a powerful immunosuppressive drug (see IMMUNOSUPPRESSION) which appears to act mainly on T cells. Cyclotella See DIATOMS. Cydia pomonella GV See GRANULOSIS VIRUSES. Cylindrocarpon A genus of fungi (class HYPHOMYCETES) which include organisms previously classified as species of Fusidium. Some species have teleomorphs in the genus Nectria. (See also CYCLOSPORIN A and FUSIDIC ACID.) Cylindrocystis A genus of saccoderm DESMIDS. Cylindrogloea bacterifera See CHLOROCHROMATIUM AGGREGATUM. Cylindrospermum A genus of filamentous CYANOBACTERIA (section IV) in which the heterocysts occur only at the ends of the trichomes; akinetes are always adjacent to heterocysts. Hormogonia are formed by fragmentation of whole trichomes. GC%: 42–47. Cylindrosporium See MELANCONIALES. Cylindrotheca See DIATOMS. Cymbella See DIATOMS. Cymbidium mosiac virus See POTEXVIRUSES. Cymbidium ringspot virus See TOMBUSVIRUSES. Cymbomonas See MICROMONADOPHYCEAE. Cyniclomyces A genus of fungi (family SACCHAROMYCETACEAE) which form budding yeast cells and pseudomycelium; elevated levels of CO2 are needed for growth. One species, C. guttulatus, isolated from the faeces and stomach of rabbits. [Book ref. 100, pp. 125–129.] Cyphelium See CALICIALES. cyphella (lichenol.) A round depression or pore – visible as a small white pit ca. 0.5–2.0 mm diam. – in the lower surface of the thallus in Sticta spp. The lower cortex forms a protruding rim around the edge of the cyphella, a medullary layer of rounded fungal cells lining the depression. Cyphellae are believed to facilitate gas exchange. [New Phyt. (1981) 88 421–426]. (cf. PSEUDOCYPHELLA.) Cypovirus Syn. CYTOPLASMIC POLYHEDROSIS VIRUS GROUP. 212 cytochrome oxidase some way beyond the tips of the basidia. Their function is largely unknown; however, those which arise from the surfaces of the lamellae of certain species of Coprinus appear to function as ‘spacers’ – maintaining a small but significant distance between adjacent lamellae, thus facilitating spore dispersal. cystine–lactose–electrolyte-deficient medium See CLED MEDIUM. cystine–tellurite–blood agar See TELLURITE MEDIA. cystitis Inflammation of the urinary bladder. Symptoms: frequent, painful micturition, sometimes with haematuria, fever etc. Cystitis may result from the spread of an infection upwards from the urethra or downwards from the kidney. The former is more common, with Escherichia coli (see UPEC) and Proteus spp being the most common causal agents; other causal agents include enterococci and Staphylococcus saprophyticus. Treatment may involve e.g. copious fluid intake and therapy with broad-spectrum antibiotics. Certain adenoviruses (e.g. type 11) have been associated with haemorrhagic cystitis (mainly in children). (See also URINARY TRACT INFECTION.) Cystobacter See MYXOBACTERALES. Cystodinium A genus of DINOFLAGELLATES in which the vegetative cells are unicellular and non-motile, and reproduce by the formation of zoospores. Cystomyces See UREDINIOMYCETES. Cystoseira See PHAEOPHYTA. cystosorus A cluster of cysts: see e.g. PLASMODIOPHOROMYCETES. Cystotheca See ERYSIPHALES. Cystoviridae (f6 phage group) A family of BACTERIOPHAGES containing one genus (Cystovirus) and one member: BACTERIOPHAGE f6. Cystovirus See CYSTOVIRIDAE. cystozoite (bradyzoite) In certain coccidia: a stage within host tissues (see e.g. TOXOPLASMOSIS). cytarabine (1-b-D-arabinofuranosylcytosine; ara-C; cytosine arabinoside) An ARABINOSYL NUCLEOSIDE originally investigated as a possible systemic ANTIVIRAL AGENT for disseminated herpesvirus infections; toxicity has limited its clinical use. cytidine See NUCLEOSIDE and Appendix V(b). cytidine deaminase See RNA EDITING. cytoadherence See MALARIA. cytoadhesin subfamily A category within the INTEGRIN family which includes CD41b/CD61. cytobiosis SYMBIOSIS in which one symbiont occurs within the cells of the other. cytochalasins A family of fungal secondary metabolites (POLYKETIDE derivatives) – cytochalasins A, B, C etc. – which are formed e.g. by species of Aspergillus, Helminthosporium and Phomopsis. Cytochalasins bind to one end of an ACTIN microfilament (the fast-growing or ‘barbed’ end) and inhibit the addition of further monomers – thus inhibiting e.g. phagocytosis, amoeboid movement, the intracellular development of certain viruses, and other microfilament-dependent functions. (Cytochalasin B apparently slows, but does not prevent, the addition of actin monomers [JCB (1986) 102 282–288].) Cytochalasins A and E enhance branching in certain fungi (see GROWTH (fungal)). cytochrome See CYTOCHROMES. cytochrome oxidase In an ELECTRON TRANSPORT CHAIN a (terminal) cytochrome which transfers electrons to molecular oxygen, reducing it to water. Cytochrome oxidases include cyts aa3 , a1 , d and o (see CYTOCHROMES). Bacteria typically have more than one type of cytochrome oxidase. The electron transport chain in the animal MITOCHONDRION appears to have only one type desiccation, and/or unfavourable temperatures. Such ‘resistant cysts’ are generally dormant and may remain viable for years under appropriate conditions. Many such cysts (e.g. those of some euglenoid flagellates) are highly resistant to desiccation; however, those of e.g. Didinium and Euplotes cannot withstand drying, but can remain viable in water for long periods. Resistant cysts may allow dissemination of the organism e.g. by wind or by birds or animals. In parasitic protozoa – e.g. ENTAMOEBA, coccidia, GIARDIA – cysts are the form in which the parasite is transmitted from one host to another. In some protozoa reproductive cysts are formed as a normal part of the life cycle; these may not be more resistant than the vegetative cell to adverse environmental conditions. For example, Colpoda spp form thin-walled cysts within which asexual fission occurs, and can also form thick-walled resistant cysts under adverse conditions. (See also e.g. ICHTHYOPHTHIRIUS.) Some protozoal cysts may serve both reproductive and protective (and disseminative) functions (e.g. the oocysts of the EIMERIORINA). (See also ACTINOPHRYS and ACTINOSPHAERIA.) cystathione See e.g. Appendix IV(d). cysteamine 2-Mercaptoethylamine (= b-aminoethanthiol): see e.g. PANTOTHENIC ACID. L-cysteine biosynthesis See Appendix IV(c). cysteine protease (1) See APOPTOSIS. (2) Syn. thiol PROTEASE. cystibiotics Class IIa BACTERIOCINS (q.v.). cystic fibrosis (CF) An inheritable disease [Review: Lancet (1998) 351 277–282] usually involving defective transmembrane transport of chloride by an ABC TRANSPORTER. Typically, the lungs are congested with a viscid dehydrated mucus (containing e.g. raised levels of calcium and magnesium ions) which may become infected with organisms such as Burkholderia (formerly Pseudomonas) cepacia [review: RMM (1995) 6 1–16], mycobacteria and Pseudomonas aeruginosa; such infections often persist. Patients with CF are frequently given prolonged antibiotic therapy, but this may not prevent colonization of the lungs with e.g. mucoid (ALGINATE-forming) strains of Pseudomonas aeruginosa [mucoidy of P. aeruginosa in CF: JB (1996) 178 4997–5004]; this is associated with a poor prognosis because the alginate may protect the bacteria from phagocytosis, pulmonary surfactant and antibiotics. It has been reported that high levels of salt (NaCl) on CF airway epithelia may permit bacterial colonization by inhibiting anti-bacterial activity normally associated with this habitat [Cell (1996) 85 229–236]. (See also QUORUM SENSING.) Results of studies on the molecular epidemiology of P. aeruginosa in a CF outpatient clinic indicate that crossinfection is not common within the clinic, and that acquisition of infection from a common source is unlikely [JMM (2001) 50 261–267]. It was suggested that long-term colonization of the lungs of CF patients with Stenotrophomonas maltophila (formerly Xanthomonas maltophila) may be a significant factor in prognosis [RMM (1997) 8 15–19]. This environmentally common, Gramnegative, non-glucose-fermenting bacillus [description: IJSB (1993) 43 606–609] is now frequently isolated from the respiratory specimens of CF patients (and is also associated with disease in immunocompromised individuals, cancer patients and recipients of transplants). [Identification/detection of S. maltophila by a PCR-based approach: JCM (2000) 38 4305–4309.] [Genetic basis, treatment etc. of CF: Book ref. 214.] cystidium (mycol.) A large, elongated, sterile cell which occurs among the basidia in the hymenia of certain basidiomycetes. Cystidia vary greatly in size and form, but they usually project 213 cytochrome oxidase test of cytochrome oxidase, cyt aa3 , which resembles the bacterial cyt aa3 in containing two haem a molecules and two copper atoms; in at least some plant mitochondria there is an additional CYANIDE-resistant cytochrome oxidase. (See also RESPIRATORY INHIBITORS.) cytochrome oxidase test See OXIDASE TEST. cytochromes A class of haemoproteins in which the HAEM (sense 1) is usually linked to the protein via the 5th and 6th coordinate positions of the haem iron; cytochromes participate in many types of electron transfer reaction (see e.g. ELECTRON TRANSPORT CHAIN and PHOTOSYNTHESIS), such reactions involving the alternate oxidation and reduction of the haem iron. The Em.7 (see REDOX POTENTIAL) of most cytochromes falls within the approximate range −100 to +500 mV. Cytochrome nomenclature. Cytochromes are classified primarily by the nature of their haem group(s); individual cytochromes within a given class may be distinguished by subscripts (e.g. c1 , c2 , c3 etc) or – see later – they may be designated by certain of their absorption characteristics. Cytochromes a (cyts a) contain haem a (which has a formyl group at the C-8 position, and a long hydrophobic chain at the C-2 position of the PORPHYRIN). Cytochromes b contain haem b (= protohaem: see HAEM). Cytochromes c contain haem c (a mesohaem), the haem being linked to the protein via thioether bonds (between the ethyl substituents of the haem and cysteine residues in the protein) as well as via the coordinate bonds. Some c-type cytochromes (e.g. cyts c2 , c555 ) have a single haem attached near the Nterminal of the protein, while others have a single haem attached near the C-terminal; the latter, which include the c
cytochromes (= cytochromoids), differ chemically and spectroscopically from other c-type cytochromes. Cytochromes d contain a CHLORIN. Certain cytochromes have more than one haem group, while others may contain e.g. a flavin group. Some cytochromes are often referred to by their original names, even though such names do not reflect current classification; thus e.g. cyt a2 is now cyt d, and cyt o is a b-type cytochrome. Identification of cytochromes. Cytochromes may be identified/quantified by various methods, often e.g. by spectrophotometry – in which a given type of cytochrome may be detected, in situ, by its characteristic absorption pattern in the visible regions of the spectrum. Commonly a difference spectrum is determined: e.g., the absorption spectrum of a given cytochrome in the reduced state minus its absorption spectrum in the oxidized state (cf. CO DIFFERENCE SPECTRUM); such a spectrum often exhibits a maximum absorption peak in each of three bands: the a-band (ca. 540–650 nm), the b-band (ca. 510–530 nm) and the g-band (= Soret band ) (ca. 400–450 nm). In some cases a cytochrome may be designated by its absorption peak (in the reduced state) within the a-band – e.g., cyt b-557 (= cyt b557 ). Spectroscopy can sometimes lead to an incorrect in situ identification of a cytochrome [see e.g. JGM (1984) 130 3055–3058]. In some cases it may be necessary to extract the haem, bind to it (coordinately) two pyridine molecules per molecule, and examine by spectroscopy the resulting dipyridine ferrohaemochrome (= pyridine haemochrome) – reduced e.g. with sodium dithionite – in an alkaline pyridine solution. For each class of cytochromes, the corresponding dipyridine ferrohaemochrome a-absorption peak falls within a narrow waveband: cyts a 580–590 nm; cyts b 556–558 nm; cyts c 549–551 nm; cyts d 600–620 nm. [Analysis of cytochromes: Book ref. 138, pp. 285–328.] Cytochrome–ligand binding. See e.g. AZIDE, CARBON MONOXIDE and CYANIDE. Cytochromes in eukaryotes. Cytochromes occur e.g. in the mitochondrial ELECTRON TRANSPORT CHAIN (q.v.) and in photosynthetic systems (see PHOTOSYNTHESIS). Many cells also contain e.g. certain b-type cytochromes – such as cyt b5 and a cyt P-450 – associated with the endoplasmic reticulum. The mammalian cyt P-450 – the cytochrome–carbon monoxide complex absorbs at 450 nm – occurs in liver cells and mediates in hydroxylation reactions involving sterols and other lipidsoluble substances; an analogous cyt P-450 occurs e.g. in many fungi (see e.g. AZOLE ANTIFUNGAL AGENTS and HYDROCARBONS). Cytochromes appear to be absent in e.g. certain parasitic protozoa – including bloodstream forms of trypanosomes and intracellular stages of Leishmania donovani. Cytochromes in prokaryotes. Cytochromes occur in both aerobic and anaerobic respiratory chains in the CYTOPLASMIC MEMBRANE; they also occur in the cytoplasm and/or periplasmic region in some cells, and appear to occur in certain photosynthetic REACTION CENTRES. In bacteria, cytochromes vary – qualitatively and quantitatively – from one species to another, and even in a given species under different growth conditions. [Cytochromes in Escherichia coli under different growth conditions: JGM (1982) 128 1685–1696.] Various cytochromes may act as CYTOCHROME OXIDASES, and it is usual for a given bacterium to have more than one type of cytochrome oxidase. Generalized patterns of cytochromes occur in the various categories of bacteria. Gram-positive aerobic chemoheterotrophs typically have a pattern of the bcaa3 o type, i.e., including two types of cytochrome oxidase; exceptions include e.g. Brochothrix thermosphacta which has only one cytochrome oxidase (aa3 ). Gram-negative chemoheterotrophs (including e.g. enterobacteria and pseudomonads) are, by comparison, more heterogeneous in their cytochrome patterns under aerobic conditions; typically, cyt aa3 is not formed (though it is in e.g. some pseudomonads, many methylotrophs, and Rhizobium spp), the cytochrome pattern frequently being of the bcoa1 d type, but often without cyt c. Cyts a and d are not formed e.g. by Pseudomonas fluorescens, or by certain other pseudomonads, so that these organisms may have patterns of the bco or bo type. Cytochrome oxidases are often absent or greatly decreased during anaerobic respiration. In at least some pseudomonads (and e.g. Thiobacillus denitrificans) which carry out DENITRIFICATION, cyt d1 c (= cd1 ) acts as a nitrite reductase. Chemolithotrophs often have cytochrome patterns of the bcaa3 o type. All members of the RHODOSPIRILLALES appear to contain c-type cytochromes, and most or all appear to have b-type cytochromes. Specialized cytochromes or cytochrome patterns occur in some bacteria; thus, e.g. Pseudomonas putida contains a soluble cyt P-450 which, together with an iron–sulphur protein and an FAD-containing reductase, effects hydroxylation of e.g. camphor in a cyclic reaction sequence. [Structure and chemistry of cyt P450 from P. putida: Book ref. 146, pp. 157–206.] (See also METHYLOTROPHY and RHP.) Cytochromes appear to be absent from some facultative and obligate anaerobes. [Structure, function and evolution of cytochromes (review): Prog. Biophys. Mol. Biol. (1985) 45 1–56.] (See also BENZIDINE TEST and OXIDASE TEST.) cytochromoids See CYTOCHROMES. cytocidal (adj.) Able to kill cells. cytoductant See CYTODUCTION. cytoduction A cross in which two cells undergo cytoplasmic fusion but do not undergo nuclear fusion (karyogamy) owing to a chromosomal mutation; each haploid progeny cell (cytoductant, 214 cytokines a given type of STAT relays the signal for only some type(s) of cytokine – thus permitting different cytokines to initiate different signals in the same cell. Phosphorylated STATs form homo- or heterodimers that enter the nucleus and promote transcription of particular gene(s). [STATs: TIBS (2000) 25 496–502.] As mentioned, a given cytokine may cause diverse effects. For example, the binding of tumour necrosis factor (TNF) to its receptor can result in activation of caspases (and APOPTOSIS) – or e.g. transcription of specific genes through activation of a major transcription factor: nuclear factor-kB (NF-kB). In the latter pathway, the binding of TNF to its receptor promotes phosphorylation (and consequent degradation) of a certain protein (IkBa) which, by binding to NF-kB, inactivates it; that is, phosphorylation (degradation) of IkBa results in the activation of NF-kB. NF-kB is important e.g. in the development of an inflammatory response; it promotes transcription of a range of genes – including those encoding TNF-a, IL-1b and IL-8. In general, the binding of cytokines to their receptors may initiate cellular responses that range from secretion (of cytokines etc.), differentiation or proliferation to chemotaxis or apoptosis; currently, many of the signalling pathways in eukaryotic cells are incompletely understood. [Signalling mechanisms in prokaryotes and eukaryotes: Book ref. 218, pp 89–162; Book ref. 226, pp 111–140.] Note regarding nomenclature of cytokines. The name of a cytokine does not necessarily correlate with its primary function(s) in vivo. For example, tumour necrosis factor was initially identified as an anti-tumour agent but is now known to be a central mediator in host defence and inflammation. Again, some interleukins, initially thought to mediate only leukocyte–leukocyte interactions, are now known to involve other types of cell; thus, e.g. interleukin-8 is secreted by endothelial cells and is classified as a chemokine. Cytokines as factors in health and (infectious) diseases. Various roles have been attributed to cytokines in normal physiology and development. However, the contribution of individual cytokines in vivo has been difficult to establish experimentally – not least because of the complexity of the system and the existence of biochemical redundancy among cytokines; hence, in some cases, the function(s) of a cytokine have been inferred from the effects attributed to inherited deficiencies in the synthesis or activity of that cytokine (and/or its receptor). In other cases, the role(s) of cytokines have been inferred from studies on knockout mice in which genes of particular cytokine(s) have been rendered non-functional. In diseases of microbial aetiology, cytokines typically have protective roles – e.g. in processes such as ANTIBODY FORMATION and INFLAMMATION. However, dysregulation of cytokines (enhanced production, imbalance, inhibition) may be a major factor in pathogenesis. In some cases, the severity of, or susceptibility to, disease has been found to vary in different individuals according to the nature of the TNF-a gene promoter – particular polymorphisms in the promoter region of the gene being associated with enhanced production of the cytokine; this has been reported in cerebral MALARIA [Nature (1994) 371 508–510] and in ENDOTOXIC SHOCK [JAMA (1999) 282 561–568]. Again, polymorphisms in the gene encoding IL-1b have been associated with an increased risk of gastric cancer from Helicobacter pylori infection [Nature (2000) 404 398–402]. In pyelonephritis caused by Escherichia coli, the cytokines IL6 and IL-8 appear to be prominent – levels of IL-6 correlating heteroplasmon) contains the nucleus of one parent cell but may contain cytoplasmic elements from both parent cells. cytofluorometry FLOW CYTOMETRY which involves the detection of specific fluorescence or FLUOROCHROME markers. cytogamy (selfing) (ciliate protozool.) The occurrence of AUTOGAMY (instead of conjugation) in each of two ciliates which have paired. cytohet (cytoplasmic heterozygote) A eukaryotic cell which is HETEROZYGOUS for one or more CYTOPLASMIC GENES. cytokines In the human and animal body: a heterogeneous population of (glyco)proteins which form a dynamic network of intercellular messenger molecules that regulate various aspects of physiology, including the immune response to infection. (This describes, but does not define, cytokines; workers in the field have not yet indicated the essential difference(s) between those molecules which are currently regarded as cytokines and certain other regulatory molecules – thus precluding a formal definition at the present time.) Cytokines may be distinguished from the (protein) hormones in that (i) a given cytokine may be synthesized by different types of cell and/or may act on different types of cell; (ii) cytokines typically act on target cells near the source of cytokine – although if secreted into the circulatory system they may act on distant cells; (iii) cytokines may cause diverse effects (e.g. when acting on different cells under different conditions); (iv) different types of cytokine may give rise to the same physiological effect; and (v) some cytokines can act as mitogens. Two sources [Book refs 218 and 226] recognize the following as cytokines: COLONY-STIMULATING FACTORS, INTERFERONS, INTERLEUKINS, cytotoxic agents such as tumour necrosis factor (see TNF), growth factors and CHEMOKINES. Cytokines are synthesized mainly by leukocytes (white blood cells); some cytokines are synthesized by stationary cells (e.g. endothelial cells). Although most cytokines are soluble (secreted) products, some are, or can be, membrane-associated. In general, transcription of cytokine-encoding genes is inducible by appropriate exogenous or endogenous stimuli. An example of an exogenous stimulus is the binding of lipopolysaccharide to CD14 receptors on macrophages (see CD14); endogenous stimuli can arise e.g. during viral infection. On release, cytokines bind to specific receptors on target cells. Cytokine receptors are divided into a number of families which differ e.g. in structure. (Some of the receptors found on macrophages and T lymphocytes can act as receptors for human immunodeficiency virus (HIV). Cytokine receptors have also been reported to act as binding sites for certain other viruses, including human (alpha) herpesvirus 1.) Interestingly, receptor molecules can be shed from cells; such isolated receptors can e.g. bind to (and antagonize) the corresponding cytokine, or they can bind to other cells – which may then be stimulated by the given cytokine. The binding of a cytokine to its cognate receptor initiates an intracellular signal, the nature of which depends e.g. on the type of cell and cytokine and on the environmental and intracellular conditions under which binding takes place. Cytokine–receptor binding characteristically results (either directly or indirectly) in the activation of kinase(s) at certain stage(s) within the signalling pathway. In one intracellular signalling pathway (triggered by many of the interleukins), the binding of cytokine leads to activation of a tyrosine kinase of the JAK (Janus kinase) family which, when activated, transmits the signal by phosphorylating a molecule of the so-called ‘signal transducers and activators of transcription’ (STATs); STATs exist in different forms, and 215 cytokinesis to be down-regulated in naive CD4+ cells [PNAS (1999) 96 3023–3028].) Studies carried out in vitro and in vivo suggest that the cytokines secreted by (Th1 or Th2) cells are responsible for various aspects of the immune response observed during infections. Thus, ‘inflammatory’ Th1 cells secrete pro-inflammatory cytokines that are associated with important roles in cellmediated immunity. For example, IFN-g activates macrophages which may then (i) exhibit enhanced antimicrobial activity in phagosomes, (ii) secrete IL-8, attracting immune cells to the site of infection, and (iii) secrete IL-12, promoting further development of the Th1 subset. Moreover, IFN-g can promote class switching to complement-fixing, opsonizing antibodies (human IgG1, IgG3; murine IgG2a). Th1 cytokines can e.g. upregulate expression of E selectins (see INFLAMMATION) and they can also upregulate MHC class II antigens. DELAYED HYPERSENSITIVITY is one manifestation of the characteristically cell-mediated Th1type immune response. Th2 cells are typically T-helper cells in ANTIBODY FORMATION. Th2 (anti-inflammatory) cytokines down-regulate macrophages and promote B cell activation, thus being important in the ‘humoral’ (antibody-mediated) immune response to infection by e.g. extracellular bacteria and helminths etc. IL-4 promotes class switching to non-complement-fixing IgG (human IgG4; murine IgG1) as well as to IgE in both man and mice. IL-5 promotes eosinophilia which may give activity against e.g. helminths and other parasites. IL-6 may contribute to antimicrobial activity by stimulating B cell proliferation and/or inducing ACUTE-PHASE PROTEINS. Therapeutic uses of cytokines. Because cytokines regulate so many aspects of the immune defence system they are attractive as candidate therapeutic agents; for example, INTERFERONS have been useful in various contexts. However, some cytokines (e.g. TNF), though potentially useful, may be unsuitable for therapy owing e.g. to instability or toxicity in vivo; interestingly, a non-toxic TNF-mimetic peptide has been found to prevent recrudescence of Mycobacterium bovis (BCG) infection in CD4+ T cell-depleted mice [JLB (2000) 68 538–544]. Inhibition of TNF-a can be achieved by monoclonal antibodies or by soluble TNF-a receptor molecules. [New perspectives on the design of cytokines and growth factors: TIBtech. (2000) 18 455–461.] [Molecular biology of the cytokines: Book ref. 226.] cytokinesis Those events, excluding nuclear division, which occur during the division of a eukaryotic cell into progeny cells; they include the apportionment of the cytoplasm and organelles, and may include e.g. synthesis of new material for the cell wall of each progeny cell. cytokinins (kinins, phytokinins) PHYTOHORMONES which stimulate metabolism and cell division; the cytokinins are 6-Nsubstituted adenines which are synthesized mainly at the root apex and translocated via the xylem. (Cytokinin-like activity is exhibited by certain urea derivatives, e.g. diphenylurea (found in coconut milk); it is believed that such compounds act by promoting the 6-N-substitution of endogenous adenine.) The mechanism by which cytokinins promote cell division is unknown; one suggestion is that cell division is encouraged by an increase in the level of endogenous cAMP brought about by the inhibition of cAMP phosphodiesterase by cytokinins. Compounds similar or identical to cytokinins are produced by certain microorganisms; such compounds may account for the formation of ROOT NODULES and for the development of symptoms in e.g. FASCIATION. with the severity of disease [e.g. PNAS (2000) 97 8829–8835 (8833–8834)]. During infection with Yersinia, the pathogen’s secreted YopP/YopJ (see VIRULON) down-regulates the pro-inflammatory response – e.g. inhibiting the formation of both TNF-a and IL-8; the mechanism involves inhibition of the kinase that phosphorylates (and inactivates) IkBa, thus inhibiting transcription of NF-kB-dependent genes [PNAS (2000) 97 8778–8783 (8781–8782)]. Viruses may inhibit cytokines in various ways; for example, soluble forms of TNF and IL-8 receptors are encoded by Shope fibroma virus and CMV, respectively. (See also INTERLEUKIN-18.) (See also DISSEMINATED INTRAVASCULAR COAGULATION; DNA VACCINE; HAEMOPHAGOCYTIC SYNDROME; JARISCH–HERXHEIMER REACTION; LEPROSY; MODULIN; NITRIC OXIDE; P FIMBRIAE; SUPERANTIGEN; TOXIC SHOCK SYNDROME.) Many infections elicit an immunological response in which a particular subset of T LYMPHOCYTES – either Th1 or Th2 – is dominant. For example, Th1-type responses are typical in certain bacterial and protozoan infections, while Th2-type responses are common e.g. when infection involves helminths [IT (1996) 17 138–146]; moreover, in at least some cases, an ‘inappropriate’ response (e.g. Th2 instead of Th1) is associated with increased susceptibility to the infection. Naive CD4+ T cells (i.e. those not previously exposed to antigen) may develop as either Th1 or Th2 cells on exposure to antigen; the mechanism that selects one or other subset of T cells is unknown, but the decision to develop one way or the other is influenced by the microenvironment of cytokines which are present during activation of the T cell by antigen – e.g. IL-12 promotes the Th1 response, while IL-4 promotes differentiation to Th2 cells. (A newly reported cytokine receptor, designated TCCR, appears to be necessary for the Th1-type immune response in mice [Nature (2000) 407 916–920].) Interestingly, glucocorticoids from stress metabolism may suppress IL-12 (the main inducer of the Th1-type response); that is, stress-derived glucocorticoids may shift the balance of the immune response from Th1-type toward Th2-type [BCEM (1999) 13 583–595]. One important difference between Th1- and Th2-type responses is that Th1 and Th2 cells secrete different types of cytokine (see table) and so have correspondingly dissimilar physiological roles. (In this context, it is interesting to note that when the molecule ICAM-1 was co-expressed with antigen (on the antigen-presenting cell) the Th2 cytokine IL-4 was found CYTOKINES: some of the cytokines secreted by Th1 and Th2 subsets of T lymphocytes Cytokine Th1 Interleukin-2 Interleukin-3 Interleukin-4 Interleukin-5 Interleukin-6 Interleukin-10 Interleukin-13 Interferon-g TNF-a TNF-b + + a + + + Th2 + + + + + + +a Secretion from Th2 cells reported to be lower than that from Th1 cells. 216 cytoplasmic membrane the oral cavity in vertebrates (Simonsiellaceae); non-pigmented filaments, attached (at one end) to other filaments or to the substratum, which form gliding gonidia (Leucotrichaceae). Genera include ALYSIELLA, BEGGIATOA, CYTOPHAGA, FLEXIBACTER, HERPETOSIPHON, LEUCOTHRIX, SAPROSPIRA, SIMONSIELLA, SPOROCYTOPHAGA, THIOPLOCA, THIOTHRIX and VITREOSCILLA. In another taxonomic scheme [ARM (1981) 35 339–364 – cf. FLEXIBACTERIAE], of the above genera only Cytophaga, Flexibacter and Sporocytophaga are included in the Cytophagales; the other genera are classified as apochlorotic cyanobacteria. cytopharyngeal apparatus (ciliate protozool.) Skeletal and certain other elements of the CYTOPHARYNX. Two main types of cytopharyngeal apparatus have been distinguished: the CYRTOS and the RHABDOS; this taxonomically important distinction is believed to reflect evolutionary differences in the different ciliate groups – those having a rhabdos-type structure being regarded as the more primitive. cytopharyngeal basket Syn. CYRTOS. cytopharynx (ciliate protozool.) An invagination of the cytoplasmic membrane which is supported, within the cytoplasm, by a tubelike system of microtubules and/or microfibrils. (See also CYTOPHARYNGEAL APPARATUS and CYTOSTOME.) Food particles which pass into the (non-ciliated) cytopharynx are taken up by PHAGOCYTOSIS through the cytoplasmic membrane at its inner end. cytophilic antibodies Antibodies which can bind to a cell surface without involving their COMBINING SITES; the bound antibody can therefore still bind homologous antigen. (See e.g. REAGINIC ANTIBODIES.) cytoplasmic genes (extrachromosomal genes; extranuclear genes) Genes which are not located on a bacterial CHROMOSOME or in a (eukaryotic) NUCLEUS; in eukaryotic organisms the term commonly refers to the genes in a CHLOROPLAST or a MITOCHONDRION, while in prokaryotes it has been used to refer to the genes carried by a PLASMID. (See also CYTOPLASMIC INHERITANCE.) cytoplasmic heterozygote See CYTOHET. cytoplasmic inheritance (extrachromosomal inheritance; nonMendelian inheritance) Inheritance governed or influenced by CYTOPLASMIC GENES – which are not subject to the Mendelian laws of segregation and assortment; see e.g. MATERNAL INHERITANCE. cytoplasmic membrane (CM; cell membrane; plasma membrane; plasmalemma; protoplast membrane) The lipid- and proteincontaining, selectively permeable membrane which encloses the cytoplasm in prokaryotic and eukaryotic cells; in most types of microbial cell the CM is bordered externally by the CELL WALL. In microbial cells the precise composition of the CM may depend on growth conditions and on the age of the cell. The structure of all biological membranes appears to conform to the basic fluid mosaic model. In this model the lipid molecules form a bilayer within which the protein molecules are partly or wholly embedded – some spanning the entire width of the bilayer; the lipid molecules are orientated such that their polar groups form the outer, hydrophilic surfaces of the bilayer while their hydrocarbon chains form the hydrophobic interior of the bilayer. (cf. UNIT MEMBRANE.) The membrane proteins are sometimes categorized as either peripheral (= extrinsic) proteins (bound to the membrane e.g. by electrostatic forces, and easily removable by electrolytes or chelating agents) or integral (= intrinsic) proteins (bound more strongly by hydrophobic bonds and extractable, with difficulty, by detergents or organic solvents). Cytoplasmic membranes are characteristically asymmetrical, i.e. the components of the outer (externally facing) Interestingly, cytokinins are incorporated in specificitydetermining positions in a small proportion of tRNA molecules. (See also KINETIN and ZEATIN.) cytolysin A toxin which lyses (usually eukaryotic) cells. cytolytic Able to lyse cells. cytolytic T cell See T LYMPHOCYTE. cytomegalic inclusion disease (CID) A disease caused by human cytomegalovirus (CMV) – see BETAHERPESVIRINAE. CMV can infect almost any tissue; infected cells are characteristically enlarged with large intranuclear inclusion bodies. Virus transmission occurs e.g. by direct contact, by transfusion of blood from an infected donor, or via the placenta. Congenital CID is commonly asymptomatic, but can be a severe condition with e.g. hepatosplenomegaly, encephalitis with irreversible CNS damage, and increased incidence of congenital deformities (see also TORCH DISEASES). Postnatal and adult infections are probably largely asymptomatic; in some cases the disease resembles INFECTIOUS MONONUCLEOSIS (‘CMV mononucleosis’, Paul–Bunnell test negative). In immunodeficient patients CMV infection may lead to e.g. pneumonia, transplant rejection, and/or death; infection may be primary or may result from reactivation of latent CMV. Patients at risk from CMV-related disease may be monitored by a NASBA-based assay of cytomegalovirus late gene UL65 (which encodes the pp67 protein) [JCM (1998) 36 1341–1346]; a commercial form of the assay for CMV was introduced by Organon Teknika. Known antiviral agents are not effective against CID. cytomegaloviruses See BETAHERPESVIRINAE. cytomegaly See CYTOPATHIC EFFECT. cytomembranes Syn. INTRACYTOPLASMIC MEMBRANES. cytomere A multinucleate structure which is formed by fragmentation of a schizont and which subsequently gives rise to merozoites. cytopathic effect (CPE) (virol.) A change or abnormality in cells due to virus infection. CPEs may include e.g. INCLUSION BODY formation, cytoplasmic vacuolation (see e.g. SPUMAVIRINAE), cell enlargement (cytomegaly) (see e.g. BETAHERPESVIRINAE), SYNCYTIUM formation (see e.g. PARAMYXOVIRUS), the formation of discrete foci of cell proliferation (see e.g. FRIEND VIRUS and MCF VIRUSES), cell death (see also PLAQUE), etc. The CPEs produced by a given virus in a given cell system are often characteristic and may be useful in the identification of the virus; some viruses characteristically fail to give rise to CPE even when viral replication occurs (see e.g. RUBIVIRUS). Cytophaga A genus of GLIDING BACTERIA of the CYTOPHAGALES; species occur in soil and in freshwater, estuarine and marine habitats. The organisms are rods or filaments, up to ca. 50 µm long, containing carotenoid pigments ranging from yellow to red; they are chemoorganotrophs and may be aerobic (respiratory metabolism using oxygen – or, in at least one strain, nitrate – as electron acceptor) or facultatively anaerobic (facultatively fermentative). Typically, Cytophaga spp can attack polysaccharides such as AGAR, ALGINATE, CELLULOSE and CHITIN. GC%: ca. 28–39. Cytophagaceae See CYTOPHAGALES. Cytophagales An order of GLIDING BACTERIA which do not form fruiting bodies (cf. MYXOBACTERALES); constituent species, all of which are Gram-negative, occur e.g. in soil and in freshwater, estuarine and marine habitats. In an early taxonomic scheme [Book ref. 21, pp. 99–119] four families were distinguished: motile rods or filaments containing carotenoid pigments (Cytophagaceae); motile filaments lacking carotenoid pigments (Beggiatoaceae); flat, non-pigmented, motile filaments found in 217 cytoplasmic membrane Lipid components of bacterial CMs are mainly phospholipids; in many or all cases, these are synthesized within the membrane itself. Phosphatidylglycerols (PG) appear to occur in all bacteria, while phosphatidylethanolamine (PE) is more common and more abundant in Gram-negative species; phosphatidylcholine is absent in Gram-positive cells and is rare in Gram-negative bacteria. Phosphatidylinositol occurs in some bacteria (e.g. Mycobacterium spp). PLASMALOGENS are found in the cytoplasmic membrane in some anaerobes. In Escherichia coli the main phospholipid is PE – PG and diphosphatidylglycerol (DPG, cardiolipin) being relatively minor components. Glycolipids are common in small quantities; in some Grampositive bacteria molecules of glycolipid may be covalently linked to glycerol TEICHOIC ACIDS, forming lipoteichoic acid. Sphingolipids are rare in bacterial membranes. Sterols are absent in most species (cf. MYCOPLASMATACEAE). Hopanes (triterpene derivatives which resemble sterols in size, rigidity and amphiphilicity) are present in some bacteria [JGM (1985) 131 1363–1367]. Lipoamino acids (O-aminoacylphosphatidylglycerols) occur in the membranes of some Gram-positive bacteria (e.g. Bacillus spp, Clostridium spp, Staphylococcus aureus). These are esters of PG and basic amino acids such as arginine, lysine or ornithine; the proportion of lipoamino acids varies according to growth phase and to the pH of the medium. The CM fatty acids may be straight-chained or branched (the latter more common in Gram-positive species), saturated or unsaturated; some contain a cyclopropane ring (which is formed by methylation at a double bond). Phospholipids often contain one saturated and one unsaturated fatty acid residue per molecule. In E. coli the main saturated fatty acids are hexadecanoic (palmitic) and tetradecanoic (myristic) acids, minor components including e.g. octadecanoic (stearic) and dodecanoic (lauric) acids; the main unsaturated fatty acids (all of which are cismonoenes) include e.g. cis-19 -hexadecenoic (palmitoleic) acid. In general, the CM varies in its fatty acid composition according to growth conditions (e.g. temperature, pH). Thus, in some species the proportion of unsaturated fatty acids increases when the growth temperature decreases – e.g. in E. coli the proportion of unsaturated fatty acids increases from 16% to 49% when the growth temperature falls from 36° C to 25° C; such a change appears to be a compensatory response which helps to maintain optimum membrane fluidity. In contrast, almost no increase in monoenoic fatty acids occurs in the CM of Staphylococcus aureus when the growth temperature drops from 37° C to 25° C [Book ref. 44, p 402]. In some bacteria, adaptation to lower growth temperatures involves an increase in the length of fatty acid chains rather than an increase in the degree of unsaturation [JGM (1985) 131 2293–2302]. The lipids of the CM clearly contribute to the essential feature of selective permeability. However, the lipids also have other functions; for example, in E. coli phosphatidylethanolamine can behave as a ‘molecular chaperone’, being required for the correct folding (maturation) of the membrane protein LacY [EMBO (1998) 17 5255–5264]. Proteins in the bacterial CM include a variety of enzymes (involved e.g. in the synthesis of phospholipids and cell wall components); for example, penicillin-binding proteins (involved in the synthesis of PEPTIDOGLYCAN) may form part of a protein complex in the CM [FEMS Reviews (1994) 13 1–12]. There are also components of TRANSPORT SYSTEMS, energy-converting side of the membrane are not identical to those of the inner (cytoplasmic) side. Membrane ‘fluidity’ primarily involves lateral and rotational motion of whole lipid molecules as well as motion of the hydrocarbon chains of the lipid molecules (rather than movement of whole lipid molecules from one layer of the membrane to the other layer). The hydrocarbon chains may be disordered and flexible (the a-conformation) or ordered, rigid and perpendicular to the plane of the bilayer (the b-conformation). Fluidity greater than a certain level is essential for the normal physiological role of the CM. The degree of fluidity is governed by temperature and e.g. by the length and structure of the hydrocarbon chains; membranes containing unsaturated chains are generally more ‘fluid’ than are membranes containing saturated chains of the same length. Previously, it was thought that only rarely do lipid molecules move from one side of the bilayer to the other. It is now known that such transmembrane movement (flip–flop) occurs with a much greater frequency than was originally supposed; in at least one case evidence has been obtained for a ‘flippase’ which may facilitate such translocation [Cell (1985) 42 51–60]. Although it is believed that the membrane lipids are normally present as a bilayer, results of nuclear magnetic resonance (NMR) and other studies have indicated the presence of transient, temperature-dependent, localized non-bilayer lipid phases within biological membranes. In one of these phases, termed hexagonal II (or HII ), the lipid molecules form an array of fine (∼20 Å diam.) water-filled cylinders whose walls are composed of the polar heads of lipid molecules; the array of cylinders is hexagonal in cross-section, the space between the cylinders containing the hydrocarbon chains of the lipid molecules. [Lipid structure of biological membranes: TIBS (1985) 10 418–421.] The CM has various functions, one of which is to regulate the cytoplasmic milieu by controlling the inward and outward passage of ions and molecules. Some uncharged and/or lipophilic molecules can pass relatively freely through the CM; these include e.g. water, carbon dioxide, oxygen, ammonia (but not ammonium ions), acetic acid (undissociated form) and ethanol. However, ions and most molecules cannot pass freely through the CM, so that their transmembrane translocation requires more or less specific TRANSPORT SYSTEMS. (Specific mechanisms also exist for OSMOREGULATION.) Transport commonly occurs at the expense of metabolic energy, and some systems depend on the presence of a transmembrane electrochemical gradient – e.g. proton motive force (pmf: see CHEMIOSMOSIS) or SODIUM MOTIVE FORCE – such gradients being a general feature of CMs. According to species, the CM is also the site of RESPIRATION (and, in some organisms, photosynthesis). The CM is involved in the synthesis of external structures, such as the cell wall and capsule, and also in the synthesis of components of the CM. (a) Bacterial cytoplasmic membranes. In transmission electron micrographs the CM, ca. 7–8 nm thick, typically appears as a trilaminar structure: an electron-translucent layer sandwiched between two electron-dense layers. In Gram-negative bacteria, localized regions of the CM may be involved in ADHESION SITES. In many species of bacteria it has been demonstrated that the inner (cytoplasmic) face of the CM bears minute, spherical, ‘stalked’ particles (see PROTON ATPASE) which are involved in energy conversion. In many (not all) species, CYTOCHROMES and other components of an ELECTRON TRANSPORT CHAIN occur in the CM. (See also PURPLE MEMBRANE.) 218 cytoskeleton systems (see e.g. has caused significant economic losses in the silk industry. A CPV is commercially available in Japan for the BIOLOGICAL CONTROL of the pine caterpillar Dendrolimus spectabilis. (cf. BACULOVIRIDAE; see also INSECT DISEASES.) CPV infection is usually chronic and is generally restricted to the columnar epithelial cells of the midgut. The mature virions occur embedded in POLYHEDRA in the cytoplasm of infected cells (cf. NUCLEAR POLYHEDROSIS VIRUS). The polyhedra may be polyhedral, cuboid, triangular etc, depending on virus and host. The infected insect shows symptoms of starvation due to reduced feeding and a reduction in the absorptive capacity of the gut cells; diarrhoea is common, with large numbers of polyhedra in the faeces. Infected larvae commonly reach adulthood, but the adults are generally small and often malformed, with greatly diminished reproductive capacity. CPVs are apparently transmitted to the larvae via the surface of the egg. [Review: Book ref. 83, pp. 425–504.] cytoplasmic streaming (cyclosis) The intracellular flow of protoplasm which occurs in various types of eukaryotic cell; such a flow ensures e.g. that intracellular reactions are not dependent on simple diffusion. In e.g. the green alga Chara, cytoplasmic streaming appears to result from the interaction between peripheral ACTIN microfilaments and MYOSIN, the relative movement between actin and myosin being reflected in the flow of cytosol and organelles. In amoebae, the characteristic amoeboid movement may involve local actin–myosin-mediated contractions in the cell cortex accompanied by cytoplasmic streaming due to gel–sol interconversions in the microfibrillar networks. cytoplast A eukaryotic cell from which the nucleus has been removed (i.e., an ENUCLEATED CELL). (cf. KARYOPLAST.) cytoproct (cytopyge) In some protozoa: a permanent pore through which is voided particulate non-digestible material. cytopyge Syn. CYTOPROCT. cytosegresome An intracellular, membrane-limited vacuole within which a cell has enclosed some of its own constituents. Cytosegresomes are formed during AUTOPHAGY. cytosine arabinoside Syn. CYTARABINE. cytoskeleton In a eukaryotic cell: the framework, composed primarily of proteinaceous tubules and fibrils, which ramifies throughout the cytoplasm (binding e.g. to the cytoplasmic membrane and to various organelles) and which is responsible e.g. for the shape and internal organization of the cell, the intracellular transport of vesicles and organelles, and (where applicable) cell motility; in a living cell the cytoskeleton probably undergoes continual modification – being disassembled in some regions and reassembled in others, according to the needs of the cell. (cf. NUCLEOSKELETON.) The main structural components of the cytoskeleton are microfilaments (see ACTIN), MICROTUBULES, and INTERMEDIATE FILAMENTS. Microtubules contribute e.g. to cell shape, to the correct location and orientation of intracellular structures and organelles, to certain types of motility (see e.g. axoneme in FLAGELLUM (b)), and to the formation of the spindle in MITOSIS; they can also serve as a temporary scaffolding e.g. during intracellular rearrangements. [Role of microtubules during the cell cycle in Stephanopyxis turris: JCB (1986) 102 1688–1698.] Microfilaments are involved e.g. in those parts of the cytoskeleton concerned with intracellular movements (see e.g. CYTOPLASMIC STREAMING), amoeboid movement (see PSEUDOPODIUM), PHAGOCYTOSIS, and CAPPING (sense 3). Intermediate filaments may e.g. fulfil a strengthening role. There is some evidence that, in multicellular organisms and tissues, cytoskeletal elements may extend from one cell to another by penetrating cytoplasmic membranes and the intercellular matrix. and EXTRACYTOand sensing systems (see e.g. CHEMOTAXIS). CM proteins also include the molecular water channels called aquaporins (see MIP CHANNEL). (See also MECHANOSENSITIVE CHANNEL.) [Structural dynamics of the CM of E. coli : Book ref. 122, pp 121–160.] The bacterial CM is the target for a variety of ANTISEPTICS and DISINFECTANTS (see e.g. QUATERNARY AMMONIUM COMPOUNDS) and ANTIBIOTICS (see e.g. DEPSIPEPTIDE ANTIBIOTICS, GRAMICIDINS, POLYMYXINS and TYROCIDINS). (b) Fungal cytoplasmic membranes. The CMs in fungi (and in other eukaryotes) differ significantly from those in bacteria. In fungal CMs the major lipids typically include phosphatidylcholine and phosphatidylethanolamine (with smaller amounts of phosphatidylinositol and phosphatidylserine) together with SPHINGOLIPIDS; as a rough generalization, the fatty acids of phospholipids in higher fungi tend to contain even numbers of carbon atoms and to be saturated or mono-unsaturated, while those of lower fungi tend to have odd numbers of carbon atoms and to be polyunsaturated. (Changes in the degree of fatty acid saturation can have interesting physiological effects; for example, when Saccharomyces cerevisiae strain Y185 becomes de-repressed for general amino acid permease, a higher degree of fatty acid unsaturation in the CM correlates with a more rapid expression of the permease [JGM (1985) 131 57–65].) Most fungal membranes also contain sterols (e.g. ergosterol), so that fungi are generally susceptible to POLYENE ANTIBIOTICS (see also AZOLE ANTIFUNGAL AGENTS); vegetative cells of members of the Pythiaceae have been reported to lack sterols, although sterols are required during the reproductive phase. Proteins in fungal CMs include those involved in transport processes, components of ATPase complexes, and various enzymes – such as those needed for the synthesis of walls and membranes. In addition to lipids and proteins, fungal CMs contain small amounts of carbohydrate. In e.g. some slime moulds, certain CM carbohydrates are important LECTIN receptors and are involved in cell–cell recognition and/or adhesion. (c) Archaeal cytoplasmic membranes. In members of the ARCHAEA the CM contains lipids of a kind which do not occur in bacteria. Unlike the ester-linked glycerol–fatty acid bacterial lipids, archaeal lipids are characteristically ether-linked molecules that contain e.g. isoprenoid or hydro-isoprenoid components. Some of the archaeal lipids are structurally analogous to those of bacteria; for example, the di-ether and di-ester lipids both have a single polar end. However, some archaeal lipids (e.g. tetra-O-di(biphytanyl) diglycerol) contain one ether-linked glycerol residue at each end of the molecule; having two polar ends, such molecules may span the width of the CM. Given the exteme habitats typically occupied by these organisms, it seems likely that some or all of the archaeans will be found to contain membrane components whose characteristics reflect an adaptation to the environment. Some archaeans contain MIP CHANNELS. cytoplasmic petite See PETITE MUTANT. cytoplasmic polyhedrosis virus group (CPV group; Cypovirus) A genus of entomopathogenic viruses of the REOVIRIDAE. Genome: 10 dsRNA molecules which may be linked together by protein in the virion. CPVs are pathogenic in a wide range of insects of the Diptera, Hymenoptera and Lepidoptera, and can also infect certain crustaceans (Simocephalus expinosus); the type member, a CPV from the silkworm (Bombyx mori ), ELECTRON TRANSPORT CHAIN PLASMIC OXIDATION) 219 cytosol [Molecular biology of the cytoskeleton: Book ref. 166; in vitro translocation of organelles along microtubules: Cell (1985) 40 729–730; reviews: JCS (1986) Supplement 5; the cytoskeleton in protists: Int. Rev. Cytol. (1986) 104 153–249.] cytosol The fluid (non-particulate) fraction of cytoplasm. (cf. CELL SAP; HYALOPLASM.) cytostome In certain protozoa (including many ciliates and some others, e.g. Noctiluca, Peranema): the cell mouth, a specific region of the cell through which particulate food is ingested. (See also AMOEBOSTOME.) In ciliates the cytostome is regarded as a two-dimensional aperture at the level of the PELLICLE; in this region the pellicle lacks both cilia and pellicular alveoli, and is typically invaginated within a supportive collar or tube-like structure of microtubules and/or microfibrils in the cytoplasm (see also CYTOPHARYNX and CYTOPHARYNGEAL APPARATUS). In advanced ciliates (e.g. Paramecium) that part of the pellicle which includes the cytostome typically occurs at the base of a concavity in the body (BUCCAL CAVITY); in the more primitive ciliates (e.g. the gymnostomes) the cytostome-containing region of the pellicle is typically not recessed in this way, and is characteristically situated apically or subapically in the cell. In some ciliates the ciliature surrounding the cytostome is readily distinguishable from the somatic ciliature (see e.g. AZM and PARORAL MEMBRANE). cytotaxin Any agent chemotactic for cells. cytotoxic hypersensitivity Syn. TYPE II REACTION. cytotoxic T cell See T LYMPHOCYTE. cytotropism See TROPISM (sense 2). cytozoic Living (parasitic) within cells. Cyttariales See ASCOMYCOTINA. Czapek–Dox medium (Czapek’s medium) A medium containing sucrose, NaNO3 , K2 HPO4 , MgSO4 , KCl and FeSO4 ; it may be solidified with agar. It is used for culturing e.g. saprotrophic fungi, soil bacteria etc. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 220 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 D D (1) Dihydrouridine (see TRNA). (2) Aspartic acid (see carotenoid pigments. Genera include CALOCERA, Cerinomyces, DACRYMYCES, Dacryopinax, Guepiniopsis. Dacryopinax See DACRYMYCETALES. dactinomycin Syn. ACTINOMYCIN D. Dactylaria See HYPHOMYCETES; see also NEMATOPHAGOUS FUNGI and PHAEOHYPHOMYCOSIS. Dactylella See HYPHOMYCETES and NEMATOPHAGOUS FUNGI. Dactylococcopsis A genus of unicellular CYANOBACTERIA in which the cells are elongate with tapering ends, sometimes occurring in short chains. D. salina is a gas-vacuolate species which is blue-green when grown at low light intensities but deep orange under high light intensities; it is planktonic in hypersaline environments. Although sensitive to concentrations of H2 S > ca. 10 µM, at lower concentrations D. salina is capable of limited anoxygenic photosynthesis using H2 S as electron donor. [Ecophysiology of D. salina: JGM (1983) 129 1849–1856.] Dactylosoma See PIROPLASMASINA. Dactylosporangium A genus of mycelial bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in soil. The organisms form finger-like sporangia each containing a single row of three or more zoospores; non-motile spores are also formed, singly, on the substrate mycelium. Aerial mycelium is not formed. Type species: D. aurantiacum. Dactylostoma See SUCTORIA. DAEC See ENTEROADHERENT E. COLI. Daedalea A genus of fungi of the APHYLLOPHORALES (family Polyporaceae) which occur as perthotrophs (PERTHOTROPH sense 2) and saprotrophs on various types of wood. Basidiocarp: non-stipitate, often hoof-like, commonly corky, the upper surface brown to grey-brown; hymenophore: labyrinthiform (‘mazelike’), consisting of elongated pores with thick dissepiments. D. quercina, which grows e.g. on oak, may form imbricated basidiocarps with a trimitic context; basidiospores: ellipsoidal, hyaline, ca. 6 × 3 µm. (See also WOOD WASP FUNGI.) DAF See CD55. dahlia mosaic virus See CAULIMOVIRUSES. dairy products Many dairy products are made by a LACTIC ACID FERMENTATION of milk using selected strains of lactic acid bacteria (see e.g. LACTIC ACID STARTERS): see ACIDOPHILUS MILK; BUTTER; BUTTERMILK; CHEESE-MAKING; KEFIR; KOUMISS; LEBEN; QUARG; SOUR CREAM; TAETTE; VILIA; YOGHURT. (See also MILK.) daisy head colonies Colonies of the type commonly formed by Corynebacterium diphtheriae (gravis type) on blood–tellurite agar; after 24 hours at 37° C, each colony is 2–3 mm in diameter, dull, with a grey to black centre, translucent margin, crenated edge, and radial striations. Dakin’s solution (chlorinated soda solution) A solution of sodium hypochlorite and sodium bicarbonate used e.g. as an antiseptic for the cleansing of wounds. Daldinia A genus of fungi (order SPHAERIALES). D. concentrica grows on dead or fallen branches of certain trees, particularly ash (Fraxinus), forming hemispherical or subglobose, superficial stromata ca. 2–10 cm across; when young, each stroma has a surface layer of branched conidiophores bearing brown conidia, but the mature stroma is a hard, brittle, shiny black structure which, in vertical cross-section, exhibits alternating light and dark bands concentric about the centre of the base of the stroma. Perithecia develop in the superficial layer of the stroma with their AMINO ACIDS). D (in continuous culture) DILUTION RATE. d-factor See DUTCH ELM DISEASE. D loop (mol. biol.) (1) (displacement loop) A single-stranded loop formed when a short ssDNA molecule pairs with a complementary region of one strand of a dsDNA molecule, displacing the corresponding region of the homologous strand (the D loop). For example, under certain (non-physiological) conditions in vitro a negatively supercoiled ccc dsDNA molecule can spontaneously take up a short complementary single (linear) strand to give rise to a D loop in a reaction driven by the energy of supercoiling; the resulting ‘joint molecule’ is more relaxed than the original supercoiled DNA. D-loop formation is promoted e.g. by the RECA PROTEIN; it may occur e.g. during RECOMBINATION (Fig. 2), and may be involved in the priming of certain types of DNA replication (see e.g. BACTERIOPHAGE G4). (See also SITE-SPECIFIC MUTAGENESIS; cf. R LOOP.) (2) The loop of the ‘D arm’ in a tRNA molecule (see TRNA). D period See HELMSTETTER–COOPER MODEL. 12D process In food processing: heating, at a given temperature, for a period equal to twelve times the D VALUE at that temperature. (See also BOTULINUM COOK.) D-type particles See TYPE D RETROVIRUS GROUP. D-type starter See LACTIC ACID STARTERS. D value (1) (D10 value; decimal reduction time) The time required, at a given temperature, to reduce the number of viable cells or spores of a given microorganism to 10% of the initial number; it is usually quoted in minutes. The temperature (° C) at which the D value is determined may be indicated by a subscript, e.g., D112 . (In the synonym of D value, D10 value, the subscript refers to the 10% survival value.) (cf. F0 VALUE; Z VALUE; see also 12D PROCESS.) (2) (of animal feed) The percentage of digestible organic matter in the ‘dry matter’ (DM) – DM (in g/kg) being determined by oven-drying of feed samples and correction for e.g. loss of volatile fatty acids. (See also SILAGE.) D10 value Syn. D VALUE (1). Da DALTON. dacA gene See PENICILLIN-BINDING PROTEINS. dacB gene See PENICILLIN-BINDING PROTEINS. dacC gene See PENICILLIN-BINDING PROTEINS. Dacrymyces A genus of fungi (order DACRYMYCETALES). D. deliquescens forms bright orange, gelatinous, globular asexual fruiting bodies (each up to ca. 5 mm diam.) on decaying wood; each fruiting body consists of a mass of radiating hyphae which, at the surface, break up into oidia. Subsequently, basidia develop at the surface of the same fruiting body, which concurrently becomes yellow. Dacrymycetales An order of typically lignicolous fungi (subclass HOLOBASIDIOMYCETIDAE) which form the so-called ‘tuning fork’ type of basidium – i.e., a two-spored basidium which is apically bifurcate (forked) and Y-shaped, the two arms of the ‘Y’ (the sterigmata) curving inwards to become more or less parallel. The basidiocarp may be e.g. crust-like, pulvinate, cuplike and stipitate, or columnar and branched or unbranched; it may be gelatinous or waxy, and in many species it is brightly coloured (e.g. yellow, orange) due, apparently, to 221 Dalmau plate technique an equivalent amount of antitoxin in one stage (i.e., all at once) the resulting mixture is non-toxic; however, if the same quantity of toxin is added in two halves (the second ca. 30 min after the first), the resulting mixture contains free (non-neutralized) toxin. In the second method of admixture, the first portion of toxin combines with more than its equivalent of antitoxin – leaving insufficient free antitoxin to neutralize the second portion of toxin. DAP pathway DIAMINOPIMELIC ACID PATHWAY. DAPI 4
,6-Diamidino-2-phenylindole: a FLUOROCHROME used e.g. as a DNA-specific stain. [Limn. Ocean. (1980) 25 948–951.] (See also DIAMIDINES.) dapsone (DDS) A drug used e.g. in the treatment of LEPROSY and MALARIA: 4,4
-diaminodiphenylSULPHONE; DDS may inhibit FOLIC ACID metabolism. [Book ref. 54, p. 422.] (cf. CLOFAZIMINE.) daptomycin A lipopeptide antibiotic which, in vitro, is bactericidal for a range of clinically important Gram-positive bacteria, including e.g. MRSA and vancomycin-resistant enterococci (VRE); daptomycin apparently interferes with the function of the bacterial cytoplasmic membrane. [Resistance studies with daptomycin: AAC (2001) 45 1799– 1802.] dark-field microscopy See MICROSCOPY (b). dark-ground microscopy See MICROSCOPY (b). dark mildews Syn. BLACK MILDEWS. dark reaction See PHOTOSYNTHESIS. dark repair (1) Light-independent DNA REPAIR: e.g. EXCISION REPAIR or RECOMBINATION REPAIR (as opposed to PHOTOREACTIVATION). (2) Syn. EXCISION REPAIR. Darling’s disease Syn. HISTOPLASMOSIS. Darmbrand ENTERITIS NECROTICANS. dasycladalean algae A small group of tropical and subtropical marine green algae (division CHLOROPHYTA) which typically grow in shallow waters. The vegetative thallus is uninucleate and is radially symmetrical, composed of an erect axis with whorls of branches and a rhizoid-like holdfast which contains the nucleus; the thallus becomes multinucleate during gametangial development. Fossil dasyclads are known from the Lower Palaeozoic onwards [Book ref. 123, pp. 297–302]; modern genera include ACETABULARIA. Dasyscyphus See HELOTIALES. Dasyspora See UREDINIOMYCETES. Dasytricha A genus of ciliates (order TRICHOSTOMATIDA) which occur e.g. in the RUMEN. Cells: ovoid, with dense, uniform somatic ciliature and a cytostome located posteriorly. DAT (serol.) Differential agglutination test: any in vitro diagnostic test in which the result consists of two agglutination titres – that of the test proper and that of the control; the titres may be quoted in full or expressed as a ratio. daughter-strand gap repair See RECOMBINATION REPAIR. daunomycin See ANTHRACYCLINE ANTIBIOTICS. daunorubin See ANTHRACYCLINE ANTIBIOTICS. Davson–Danielli model See UNIT MEMBRANE. dazomet A soil fumigant (3,5-dimethyltetrahydro-2H-1,3,5thiadiazine-2-thione) used as a fungicide and nematocide; in soil it breaks down to form e.g. formaldehyde and N-methylisothiocyanate. (cf. METHAM SODIUM.) DBMIB An inhibitor of PHOTOSYNTHESIS. Dc Critical DILUTION RATE. DCA (deoxycholate–citrate agar) An agar medium used for the primary isolation of e.g. Salmonella and Shigella; most strains of Escherichia and of the Proteeae (cf. PROVIDENCIA) fail to grow on DCA. DCA generally contains meat extract, peptone, lactose, ostiolar pores at the surface. The bulk of the stromal tissue may act as a reservoir for water. (See also ASCOSPORE.) Dalmau plate technique A technique used e.g. for studying the formation of pseudomycelium or true mycelium by a yeast. A streak and two point-inoculations of the test strain are made at well-separated locations on a surface-dried plate of e.g. CORNMEAL AGAR; the centre of the streak and one of the pointinoculations are then each overlaid with a sterile cover-glass. After incubation at 25° C for ca. one week the preparation is examined under the microscope. Dalmeny disease A disease of cattle which results in e.g. abortion or death; the causal agent is believed to be a species of SARCOCYSTIS. dalton (Da) Syn. ATOMIC MASS UNIT. Dalyellia viridis See ZOOCHLORELLAE. Dam-directed mismatch repair See MISMATCH REPAIR. dam gene In e.g. Escherichia coli : a gene which encodes a DNA adenine methylase (Dam methylase), i.e. an enzyme that methylates DNA at the N-6 position of adenine in the nucleotide sequence 5
-GATC-3
(‘Dam site’); a strand of DNA is methylated soon after its synthesis (see DNA METHYLATION). Dam methylation is involved e.g. in MISMATCH REPAIR in E. coli. It is also involved in regulating certain genes. For example, transposition of the transposon Tn10 (q.v.) is inhibited by Dam methylation of a sequence in the transposase gene promoter (reducing transposase synthesis) and of a sequence in IS10 (reducing transposase activity); Tn10 transposition is thus apparently coupled to DNA replication: it occurs just after the replication fork has passed but before Dam methylation has occurred. Other genes which contain Dam sites in their promoters, and whose expression is sensitive to Dam methylation, include e.g. the trpR gene of E. coli, the cre gene of BACTERIOPHAGE P1, and the mom gene of BACTERIOPHAGE MU. The chromosomal origin, oriC, in E. coli contains more than 10 Dam sites which may be important in regulating the initiation of DNA replication (see CELL CYCLE (b)). [Minireview: JB (1985) 164 490–493.] Dam methylation apparently occurs in various other Gramnegative bacteria (including certain cyanobacteria) and in some members of the Archaea. Dam methylation See DAM GENE. damping off (plant pathol.) A microbial disease of seedlings in which e.g. roots may rot, or the hypocotyl (lower stem) may either collapse or become wiry (‘wire stem’); seedlings may die before or after they emerge from the soil (pre-emergence and post-emergence damping off, respectively). Common causal agents include species of PYTHIUM and RHIZOCTONIA. Aerial parts are often secondarily affected by GREY MOULD. Damping off may be controlled e.g. by CARBENDAZIM, CHESHUNT COMPOUND, CHLORONEB or ETRIDIAZOLE. (cf. FOOT-ROT (sense 2) and BLACKLEG (sense 2).) Dane particle See HEPATITIS B. Danielli–Davson model See UNIT MEMBRANE. Danish agar Syn. FURCELLARAN. danofloxacin See QUINOLONE ANTIBIOTICS. dansyl chloride A reagent, 5-(dimethylamino)naphthalene-lsulphonyl chloride, which reacts with amino acids and proteins to form derivatives that exhibit an intense yellow fluorescence under UV irradiation. Danysz’ phenomenon The phenomenon in which the residual toxicity in a mixture of diphtheria toxin and antitoxin varies according to the method of admixture. Thus, if toxin is added to 222 defective interfering particle sodium citrate, ferric ammonium citrate, sodium deoxycholate, and neutral red; pH ca. 7.3. DCA should not be sterilized by autoclaving. DCCA See DICHLOROISOCYANURATE. DCCD N,N
-Dicyclohexylcarbodiimide; under weakly alkaline conditions DCCD binds covalently to the ‘DCCD-binding protein’ of F0 in (F0 F1 )-type H+ -ATPases and inhibits both the synthesis and hydrolysis of ATP, while under weakly acidic conditions it binds to the b subunit of F1 , inhibiting the hydrolysis of ATP. DCDS See CONJUGATION (1b) (i). DCMU (Diuron) N-(3,4-dichlorophenyl)-N
-dimethylurea; this agent acts as a mitochondrial RESPIRATORY INHIBITOR, blocking electron flow between cytochromes b and c1 in Complex III of the ELECTRON TRANSPORT CHAIN (cf. BAL), and as an inhibitor of electron flow between photosystems I and II (see PHOTOSYNTHESIS). dda gene (in bacteriophage T4) See HELICASES. DDMR See MISMATCH REPAIR. DDS DAPSONE. ddTTP 2
,3
-Dideoxythymidine triphosphate: an analogue of thymidine triphosphate; it inhibits DNA synthesis since, when incorporated in a growing DNA strand in place of thymidine, it provides no 3
-OH for further polymerization. In mammalian cells it specifically inhibits DNA polymerase b action; the a enzyme is apparently unable to incorporate the analogue in a growing DNA strand. (cf. APHIDICOLIN.) de-acetylase See ANTIBIOTIC. dead man’s fingers See XYLARIA. Dean and Webb titration A serological titration in which a constant volume of a given antiserum is added to each of a number of serial dilutions of the homologous antigen; precipitation occurs most rapidly in that dilution in which antibody and antigen are present in OPTIMAL PROPORTIONS. death cap fungus See AMANITA. death phase See BATCH CULTURE. death rate constant See STERILIZATION (b). Debaryomyces A genus of yeasts (family SACCHAROMYCETACEAE) which reproduce by multilateral budding; pseudomycelium may be formed. Ascus formation is generally preceded by conjugation between bud and mother cell; conjugation between separate cells also occurs. Ascospores: spherical or oval, minutely warty or ridged, usually 1 or 2 (up to 4 in some species) per ascus. Fermentation occurs (weakly) in some species. NO3 − is not assimilated. Species (D. castellii, D. coudertii, D. hansenii, D. marama, D. melissophilus, D. polymorphus, D. pseudopolymorphus, D. tamarii, D. vanriji ) have been isolated from soil, foods etc. [Book ref. 100, pp. 130–145.] debranching enzyme (1) (amylo-1,6-glucosidase) An enzyme which hydrolyses the (1 → 6)-a branch points in e.g. amylopectin, glycogen, and/or related polysaccharides. There are two types: pullulanase (‘limit dextrinase’, pullulan 6-glucanhydrolase, EC 3.2.1.41), which can degrade PULLULAN, and isoamylase (glycogen 6-glucanhydrolase, EC 3.2.1.68), which has no action on pullulan. Pullulanases are obtained commercially from e.g. Klebsiella aerogenes (‘Aerobacter aerogenes’) and Bacillus cereus var. mycoides; the enzyme from K. aerogenes can debranch amylopectin and its b-limit dextrin (see AMYLASES) but has little or no activity on native glycogen, while that from B. cereus var. mycoides has little or no activity on amylopectin but can degrade its b-limit dextrin [Book ref. 31, pp. 133–137]. Isoamylases are obtained e.g. from Pseudomonas sp. (cf. BRANCHING ENZYME.) (2) (mol. biol.) An enzyme which ‘debranches’ the lariat form of an excised intron (see SPLIT GENE). débridement The removal of necrotic/infected tissue and/or foreign material from a wound. debromoaplysiatoxin See LYNGBYA. decarboxylase tests Tests used to determine the ability of a given bacterial strain to decarboxylate arginine, lysine and/or ornithine. Three tubes of MØLLER’S DECARBOXYLASE BROTH, each containing one of the three amino acids, are inoculated with the test organism; each broth is overlaid with a layer of sterile paraffin, incubated at 37° C, and examined daily for 4 days. Initially, in both positive and negative tests, glucose is metabolized to acidic products causing the pH indicator bromcresol purple (in the medium) to turn yellow. In a positive test the amino acid is cleaved by a specific decarboxylase in a reaction requiring pyridoxal phosphate (see PYRIDOXINE); the product is a polyamine (see POLYAMINES) which raises the pH of the medium, causing bromcresol purple to turn purple. Arginine, lysine and ornithine are decarboxylated to agmatine, cadaverine and putrescine, respectively. decay-accelerating factor See CD55. Dechloromonas See BIOREMEDIATION. Dechlorosoma See BIOREMEDIATION. decimal code (for growth stages of cereals) See ZADOKS’ CODE. decimal reduction time Syn. D VALUE (1). declomycin See TETRACYCLINES. decoy receptor See e.g. INTERLEUKIN-1. decoyinine See PSICOFURANINE. decurrent (mycol.) Refers to hymenium-bearing structures (e.g. lamellae in agarics, the layer of ‘teeth’ in Hydnum) whose region of attachment to the fruiting body extends at least partly down the stipe. Dee disease See KIDNEY DISEASE. deep (noun) (1) Any solid medium present (in a tube) at a depth sufficient to permit stab inoculation. (2) A SHAKE CULTURE (sense 1). deep rough mutant See SMOOTH–ROUGH VARIATION. defaunation The removal of the fauna (animal life) from a given environment – especially the removal of protozoa from a mixed microbial population (e.g. that of the RUMEN). defective interfering particle (DI particle; defective interfering virus) A DEFECTIVE VIRUS which typically arises as a result of mutation (usually deletion mutation) in an originally nondefective virus (the standard virus); a DI particle can replicate only in the presence of the standard virus, and its presence reduces the yield of infectious standard virus (i.e., it exhibits INTERFERENCE). (The preferential production of DI particles at the expense of standard virus is known as enrichment.) The generation of DI particles and their ability to cause interference depend, at least to some extent, on the nature of the host cell. The mechanisms of DI particle generation, enrichment and interference are poorly understood. [Primary structure of poliovirus DI particle genomes and possible mechanism for their generation: JMB (1986) 192 473–487.] DI particles commonly accumulate in stocks of animal viruses which have been passaged at high multiplicity of infection (moi). (Passage at low moi minimizes the chances of a cell being coinfected with both DI and standard viruses, so that any DI particles which arise are unlikely to be able to replicate.) DI particles can attenuate the pathological effects of a virulent standard virus (and in some cases of other closely related viruses) both in cell cultures and in animals under experimental conditions; however, the presence of DI particles may also allow 223 defective virus an S LAYER. Carbohydrate chains originating at the outer membrane pass through the S layer and form the outermost surface of the cell. The S layer – also called the HPI (= hexagonally packed intermediate) layer – is composed of a single polypeptide species. [Three-dimensional structure of the HPI layer: JMB (1986) 187 241–253.] D. radiodurans does not contain phosphatidylglycerol or phospholipids derived from it. [Polar lipid profiles of Deinococcus: IJSB (1986) 36 202–206.] The genome is toroidal [Science (2003) 299 254–256]. Dekkera A genus of yeasts (family SACCHAROMYCETACEAE) which reproduce by budding; pseudomycelium is usually formed. Asci are evanescent and are formed directly from (diploid) vegetative cells; ascospores are more or less bowlerhat-shaped, 1–4 per ascus. NO3 − may or may not be assimilated. Two species: D. bruxellensis (anamorph: Brettanomyces bruxellensis) and D. intermedia (anamorph: B. intermedius), found in beers, wines etc. (See also BRETTANOMYCES.) [Book ref. 100, pp. 146–150.] delavirdine See ANTIRETROVIRAL AGENTS. delayed enrichment method See AUXOTROPH. delayed hypersensitivity (DH; delayed-type hypersensitivity; cellmediated hypersensitivity; type IV reaction) (immunol.) In a PRIMED individual: a HYPERSENSITIVITY (sense 1) response which (i) follows the second (or subsequent) exposure to the given antigen, (ii) is mediated by antigen-specific T LYMPHOCYTES, and (iii) involves a local inflammatory reaction that reaches maximum intensity ∼24–48 hours after antigenic challenge. (cf. IMMEDIATE HYPERSENSITIVITY.) Priming and subsequent antigenic challenge may occur in the skin or elsewhere. Initial priming (or ‘sensitization’), in which the individual is initially exposed to the given antigen, involves interaction between an ANTIGEN-PRESENTING CELL (e.g. a DENDRITIC CELL) and an antigen-specific CD4+ T cell. The T cell forms a Th1 clone: when activated by antigen it proliferates and secretes CYTOKINES that include INTERLEUKIN-2. Hence, a primed individual contains an expanded clone of specifically reactive T cells (referred to as TDH , TDTH or Tdth). Sensitization may require a period of ∼1–2 weeks. Following sensitization, a DH reaction can be elicited by further exposure to specific antigen; as before, antigen is presented by an APC, but in this instance the (primed) individual contains an expanded clone of specifically reactive T cells. The secreted cytokines include e.g. interleukin-2, INTERLEUKIN-12 and IFN-g (see INTERFERONS); the cytokines (i) promote expansion of the Th1 subset of T cells (which secrete more pro-inflammatory cytokines) and (ii) bring about the accumulation of activated MACROPHAGES at the site of antigenic challenge. In the skin, the result is typically an indurated (hard), erythematous (red) nodule which develops in 24–48 hours of antigenic challenge. The lesion commonly resolves slowly. However, when there is a failure to eliminate the antigen (or pathogen) the ongoing presence of a concentration of activated macrophages can result in damage to the host’s tissues. In some diseases aggregations of macrophages give rise to epithelioid cells and, subsequently, GIANT CELLS within lesions called GRANULOMAS; tubercles are granulomas formed in tuberculosis. DH to a given antigen can be transferred to a non-primed subject only by the transfer of cells, not serum or plasma alone. (cf. JONES–MOTE SENSITIVITY.) DH reactions can be modified by various drugs; for example, glucocorticosteroids can inhibit DH reactions, and certain antitumour agents, such as mitomycin C, have been found to stimulate DH reactions in mice. the establishment of a persistent infection of cells or animals by a virus which does not normally exhibit PERSISTENCE [e.g. Semliki Forest virus in mice: JGV (1986) 67 1189–1194]. The role of DI particles – if any – in attenuating disease or in establishing persistent infections under natural conditions is unknown. defective virus A virus which – inherently or e.g. as a result of mutation – lacks one or more genetic functions necessary for its replication; such a virus can produce progeny in the presence of another (non-defective) HELPER VIRUS which can provide the missing functions. (See also e.g. DEFECTIVE INTERFERING PARTICLE; RETROVIRIDAE; SATELLITE VIRUS; TRANSDUCTION.) defensins PEPTIDE ANTIBIOTICS, produced e.g. by mucosal epithelial cells and macrophages, which are active against a range of bacteria, including both Gram-positive and Gram-negative species; examples: the tracheal antimicrobial peptide found in bovines, cryptdin in mice, and human neutrophil peptide. [See e.g. Nature (2003) 422 522–526.] defined medium See MEDIUM. definitive host Syn. FINAL HOST. DEFT Direct epifluorescent filter technique: a technique used e.g. for the rapid detection and/or quantification of microorganisms in water, milk etc. Essentially, the sample is passed through a membrane filter (see FILTRATION), and the cells retained on the filter are counted by epifluorescence MICROSCOPY – the cells having been stained with a fluorescent dye either before or after filtration. For the examination of MILK, the sample is pretreated with a protease (e.g. trypsin) and a detergent (e.g. Triton X100) to disrupt, respectively, the casein micelles and fat globules which would otherwise block the membrane filter. degeneracy (mol. biol.) See GENETIC CODE. degerming Syn. ANTISEPSIS. DegP protease See FIMBRIAE (P fimbriae). degradosome In Escherichia coli : a multicomponent complex which includes RNASE E and certain other proteins involved in the degradation/processing of RNA molecules; degradosomes are associated with the cytoplasmic membrane of the cell via the N-terminal part of RNase E. Proteins which have been identified in degradosomes include polynucleotide phosphorylase, enolase, RhlB (RNA helicase), polynucleotide phosphate kinase, DnaK and GroEL. [RNA degradosomes in vivo: PNAS (2001) 98 63–68.] (See also PROTEASOME.) degranulation (immunol.) See MAST CELL. degron Any feature of a protein which acts as a signal for that protein’s in vivo degradation. One example is the identity of the protein’s N-terminal amino acid residue: see N-END RULE. dehiscence Opening on maturity. dehydration (1) (of specimens) The replacement of water, in a specimen, by a non-aqueous medium, e.g. ethanol. (See also ELECTRON MICROSCOPY; CRITICAL POINT DRYING.) (2) (of foods) See FOOD PRESERVATION (c). (3) (of microorganisms) See DESICCATION. dehydroemetine See EMETINE. dehydrogenase An OXIDOREDUCTASE which catalyses the removal of hydrogen atom(s) from a substrate – the hydrogen being donated to an acceptor other than molecular oxygen; hydrogen acceptors include pyridine nucleotides and flavin nucleotides. (cf. OXIDASE.) Deinococcus A genus of GRAM TYPE-negative (GRAM REACTIONpositive) bacteria. D. radiodurans (formerly Micrococcus radiodurans) is highly resistant to IONIZING RADIATIONS; the cells are red-pigmented cocci in which the outer layers of the cell wall (external to the peptidoglycan) include an OUTER MEMBRANE and 224 dendritic cells to develop stable and heritable resistance to B. thuringiensis crystals within a few generations [Science (1985) 229 193–195]. (cf. THURINGIENSIN.) [Reviews: Book ref. 171, pp. 185–209 (commercial aspects), pp. 211–249 (genetics of B. thuringiensis); MR (1986) 50 1–24.] (cf. MILKY DISEASE.) d-factor See RNA POLYMERASE. delta hepatitis virus See DELTA VIRUS. delta herpesvirus A herpesvirus which is antigenically related to human varicella-zoster virus (see ALPHAHERPESVIRINAE) and which causes a varicella-like disease in non-human primates. delta particles See TECTIBACTER. delta plasmid See DELTA. d sequence See TY ELEMENT. delta transfer factor See DELTA. delta virus (delta agent; hepatitis D virus; hepatitis delta virus) A defective RNA virus which can replicate only in the presence of a helper virus of the HEPADNAVIRIDAE – usually HEPATITIS B VIRUS (HBV), although under experimental conditions the delta virus can replicate in woodchucks infected with WOODCHUCK HEPATITIS VIRUS. The delta virus virion (as derived from the serum of an HBV-infected patient) is ca. 35–37 nm in diam.; it consists of a core containing the delta virus RNA genome (MWt ca. 5.5 × 105 ) and a delta virus-encoded protein antigen (HDAg) surrounded by an (HBV-encoded) HBsAg-containing envelope. The genome is circular ssRNA [PNAS (1986) 83 8774–8778]. [Protein composition of the delta virus virion: JV (1986) 58 945–950.] In humans, the delta virus can infect an individual either simultaneously with HBV or subsequent to the establishment of an HBV infection; delta virus infection may itself be acute or chronic. The presence of the delta virus may exacerbate the symptoms of HBV infection, in some cases inducing an acute, severe or fulminant hepatitis – particularly when an acute delta virus infection is superimposed on a pre-existing chronic HBV infection. HDAg can be detected in the serum of the patient only very early in infection, but anti-HDAg antibodies apparently persist indefinitely in chronically infected patients; evidence of active delta virus replication requires immunohistological staining for HDAg in the hepatocytes. 1GATP See CHEMIOSMOSIS. 1Gp See CHEMIOSMOSIS. 1m̃H+ See CHEMIOSMOSIS. 1p See CHEMIOSMOSIS. 1pH See CHEMIOSMOSIS. 1y See CHEMIOSMOSIS dematiaceous (mycol.) Darkly pigmented. demeclocycline See TETRACYCLINES. demecolcine Deacetyl-N-methyl-COLCHICINE. Demerec convention A widely-followed convention for bacterial genetic nomenclature [Genetics (1966) 54 61–76]. demethylchlortetracycline See TETRACYCLINES. demicyclic rusts Those rust fungi (see UREDINIOMYCETES) which form all types of spore except uredospores; they include e.g. most species of Gymnosporangium. demyelination (demyelinization) The removal or destruction of the myelin sheath of one or more nerve fibres. (See also MULTIPLE SCLEROSIS; PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY; VISNA.) denaturing gradient gel electrophoresis See DGGE. Dendriscocaulon See CEPHALODIUM. dendritic cells Motile, non-phagocytic ADHERENT CELLS, derived from the bone marrow, which are present e.g. in spleen and DH reactions form the basis of certain diagnostic SKIN TESTS. Thus, an individual suffering from a given microbial disease may be primed in respect of certain antigens of the pathogen; hence, if the patient is given a cutaneous injection of specific antigen, the primed state may be revealed by a DH inflammatory response (a positive skin test) at the site of the injection. (cf. ANERGY.) (See also CONTACT SENSITIVITY.) delayed-type hypersensitivity Syn. DELAYED HYPERSENSITIVITY. deletion mutation A type of MUTATION in which one or more nucleotides are lost from the genome; if the number of nucleotides lost is not divisible by 3, the mutation will be a FRAMESHIFT MUTATION. (cf. INSERTION MUTATION.) Deleya A genus which accommodates the marine species Alcaligenes aestus, A. pacificus, A. cupidus and A. venustus, and Pseudomonas marina [IJSB (1983) 33 793–802]; another species, D. halophila, was isolated from hypersaline soils [IJSB (1984) 34 287–292]. Delhi boil See CUTANEOUS LEISHMANIASIS. 1 (delta plasmid; also: delta transfer factor) An IncIa, low-COPYNUMBER enterobacterial CONJUGATIVE PLASMID. [Structure of the delta plasmid: JGM (1986) 132 3261–3268.] delta agent Syn. DELTA VIRUS. d antigen HDAg: see DELTA VIRUS. delta chain (immunol.) See HEAVY CHAIN. d-endotoxin A glycoprotein entomotoxin produced by Bacillus thuringiensis. Synthesis of the toxin is typically associated with sporulation, the toxin appearing as a (commonly bipyramidal) crystal, the parasporal crystal, near the spore within a sporulating cell; in B. thuringienis subsp. yunnanensis crystals are apparently produced only in asporogenous cells [J. Inv. Path. (1986) 48 254–256]. (Genes concerned with toxin synthesis are apparently plasmid-borne.) The crystal is composed of subunits of a protoxin, the subunits generally being linked by disulphide bonds (which can be disrupted by reducing conditions or by alkaline pH). When crystals and spores are ingested by an insect larva, the crystals dissolve in the alkaline contents of the insect’s midgut to release the protoxin subunits; the subunits are converted by midgut proteases to one or more toxic components which cause paralysis of the gut and degeneration of the midgut epithelium. This may kill the insect directly, or may allow B. thuringiensis to invade the insect and cause a lethal septicaemia. Larvae may be killed within hours of ingesting the toxin. Different strains of B. thuringiensis are effective against different host ranges: those of ‘Group A’ – e.g. B. thuringiensis subsp. kurstaki – are pathogenic primarily in lepidopteran larvae (caterpillars of butterflies and moths), while those in ‘Group B’ – subsp. israelensis – are pathogenic e.g. in mosquito larvae (Diptera); subsp. tenebrionis is pathogenic in some coleopteran (beetle) larvae [d-endotoxin in subsp. tenebrionis: FEMS (1986) 33 261–265]. Strains of B. thuringiensis have been used, worldwide, as ‘microbial insecticides’ for the BIOLOGICAL CONTROL of various lepidopteran pests. Preparations containing spores and crystals (prepared by deep-tank culture of B. thuringiensis) are applied e.g. to crops; repeated applications are necessary because, e.g., both spores and (to a lesser extent) crystals tend to be inactivated by the ultraviolet component of sunlight. (To avoid the need for repeated applications, B. thuringiensis has been genetically modified so that toxin accumulates within the toxin-producing cells; the partly protected (intracellular) toxin is still highly active [Biotechnology (1995) 13 67–71].) Plodia interpunctella, an important lepidopteran pest of stored grain, has been reported 225 dendrodochiotoxicosis (See also NITROGEN CYCLE). denitrifying bacteria Those bacteria capable of DENITRIFICATION; they occur e.g. in soil and in marine and freshwater environments. (Some species can be useful in the elimination of nitrate from waste water.) Examples include e.g. Bacillus licheniformis, Hyphomicrobium sp, Paracoccus denitrificans, Pseudomonas stutzeri and Thiobacillus denitrificans. density gradient centrifugation See CENTRIFUGATION. densonucleosis viruses See DENSOVIRUS. Densovirus (insect parvovirus group; also: densonucleosis viruses, DNVs) A genus of autonomous (helper-independent) viruses (family PARVOVIRIDAE) which infect insects of the Lepidoptera (and probably of the Diptera and Orthoptera). Both positive- and negative-strand ssDNA molecules are encapsidated (in separate virions). Replication occurs in most tissues of host larvae, nymphs and adults; hypertrophy of the nucleus occurs in infected cells, and virions accumulate in dense intranuclear inclusions. Infected insects may die. Type species: densovirus of Galleria mellonella (the greater wax moth, whose larvae – known as waxworms – feed on pollen and wax in beehives). Other members include densovirus of Junonia (a genus of butterflies); similar viruses have been detected in e.g. Aedes (mosquitoes), Bombyx (silk-moth), and butterflies of the Nymphalidae. dental caries Tooth decay: typically, a process in which acids (particularly lactic acid), formed by plaque microorganisms, cause localized breakdown (demineralization) of tooth enamel, thus exposing the dentine; an important factor in the development of caries is the formation of water-insoluble extracellular microbial glucans which promote adhesion of cariogenic bacteria to the tooth surfaces (see DENTAL PLAQUE). Various bacteria, including e.g. lactobacilli and Streptococcus mutans, can be cariogenic in man; the development of carries may depend on factors such as the nature of the plaque microflora, diet, and the level of immunity in the host (see also CREVICULAR FLUID). Fluoride is a potent anti-caries agent. It converts the apatite in tooth enamel to the more stable (acid-resistant) fluorapatite, and this is believed to be the primary mode of fluoride anticaries activity. Fluoride can also inhibit bacterial metabolism e.g. by blocking the uptake of sugars by the PEP-dependent phosphotransferase system (see FLUORIDES). However, in some oral streptococci (including S. mutans) there is an alternative, fluoride-insensitive, pmf-dependent uptake system for glucose; this is a low-affinity uptake system compared to the high-affinity PTS system (the latter operating as a scavenger system under conditions of substrate limitation). At low pH (e.g. 5.5) the fluoride-insensitive system appears to be more important than the PTS system for the uptake of glucose. dental plaque On the surfaces of teeth: a thin film (up to ca. 0.5 mm thick) consisting of a mixed microbial flora embedded in a matrix composed largely of extracellular (capsular) bacterial polysaccharides and salivary polymers; it is a predisposing factor in DENTAL CARIES and PERIODONTITIS. Early colonizers of a clean enamel surface (in man) include species of Neisseria and Streptococcus (particularly S. sanguis). If plaque is allowed to build up, the nature of the microflora changes with time; although streptococci tend to remain dominant, anaerobes and filamentous bacteria begin to appear in significant numbers after several days or a week. Organisms present in well-established plaque include e.g. species of ACTINOMYCES (particularly A. viscosus), BACTEROIDES and VEILLONELLA; there is a general correlation between the numbers of Streptococcus mutans (see STREPTOCOCCUS) and the dietary intake of sucrose. The nature of the plaque microflora varies e.g. from one individual to another, and from one site to another on a given tooth. lymph nodes. Dendritic cells are irregularly shaped, smoothsurfaced cells which contain pulsatile nuclei and many spherical mitochondria; they bear class II MHC antigens (cf. LANGERHANS’ CELLS) and can act as ANTIGEN-PRESENTING CELLS (see e.g. DELAYED HYPERSENSITIVITY) and as accessory cells for cytotoxic T cells. [Minireviews: Cell (2001) 106 255–274.] dendrodochiotoxicosis (myrotheciotoxicosis) A MYCOTOXICOSIS, affecting domestic animals and man, caused by roridin and verrucarin TRICHOTHECENES produced by Myrothecium spp. Dendrophoma See SPHAEROPSIDALES. Dendrosoma See SUCTORIA. dengue (breakbone fever) An acute, tropical and subtropical human disease caused by any of four serotypes of dengue virus (family FLAVIVIRIDAE) and transmitted by mosquitoes (usually Aedes aegypti ). (The genetic diversity of dengue virus is reported to be increasing, and strains of the virus may now, or in the future, exhibit differences in virulence [TIM (2000) 8 74–77].) Replication of the virus may occur chiefly in macrophages. In most cases the disease is benign and self-limiting (dengue fever ). After an incubation period of ∼5–10 days, typical symptoms include fever, headache, joint and muscular pains, and (often) a rash; bleeding (e.g. haematuria or gingival bleeding) is present in some cases. Mortality rates for this ‘classical’ form of the disease are very low. Less often, a more severe form of disease may develop: dengue haemorrhagic fever (DHF). The manifestations of DHF include high fever, haemorrhagic phenomena, hepatomegaly and thrombocytopenia. The severity of disease is influenced by the extent of plasma leakage (as indicated by the rise in haematocrit reading). The most severe forms of DHF (World Health Organization grades III and IV) are designated dengue shock syndrome (DSS). DSS develops suddenly, after several days’ fever, with signs of circulatory failure/hypovolaemic shock (rapid, weak pulse, narrowed pulse pressure or hypotension, and cold clammy skin); death may occur in 12–24 hours without suitable treatment, but recovery may be rapid following volumereplacement therapy. [Dengue haemorrhagic fever (review): RMM (1995) 6 39–48.] In common with many other diseases caused by arthropodborne viruses, various manifestations of dengue infection (including those in which there is little or no haemorrhage) are characterized by a transient leukopenia [dengue-induced suppression of bone marrow: BCH (1995) 8 249–270]. [Haematology of dengue fever and dengue haemorrhagic fever: BCH (2000) 13 261–276.] Detection of dengue virus in samples of serum can be achieved by rtPCR; in primary infection, detection of viral RNA seems to be optimal prior to seroconversion (soon after onset of symptoms) [JCM (1999) 37 2543–2547]. denitrification Energy-yielding (respiratory) metabolism in which nitrate or nitrite (as terminal electron acceptor) is sequentially reduced to gaseous products, mainly dinitrogen and/or nitrous oxide (cf. DISSIMILATORY NITRATE REDUCTION). It is generally stated that denitrification occurs only under anaerobic or microaerobic conditions; however, aerobic denitrification has been reported [AvL (1984) 50 525–544; denitrification in Rhizobium: SBB (1985) 17 1–9]. In some DENITRIFYING BACTERIA (e.g. Paracoccus denitrificans) electrons from e.g. NADH are transferred to NO3 − via a btype cytochrome, and to NO2 − and N2 O via c-type cytochromes; electron flow to NO3 − , NO2 − and N2 O apparently generates proton motive force. Nitrate reductase is a MOLYBDOENZYME; in some species cytochrome d1 c acts as a nitrite reductase. 226 Dermatocarpon Bacterial polymers typically present in plaque include MUTAN and LEVANS. The presence of plaque can be detected by rinsing the mouth with a solution of a dye (‘disclosing agent’) such as erythrosin. Dentalina See FORAMINIFERIDA. deodorants (skin) See SKIN MICROFLORA. 5
-deoxyadenosylcobalamin See VITAMIN B12 . deoxycholate A salt of deoxycholic acid (see BILE ACIDS). deoxycholate–citrate agar See DCA. deoxycholic acid See BILE ACIDS. deoxycoformycin Syn. pentostatin (see VIDARABINE). 2
-deoxy-2
-fluoro-5-methyl-1-b-D-arabinosyluracil See FMAU. deoxyhemigossypol A terpenoid PHYTOALEXIN produced by the cotton plant. [Role of terpenoid phytoalexins in the resistance of a cultivar of Gossypium barbadense to Verticillium dahliaeinduced wilt: PPP (1985) 26 209–218.] deoxynivalenol See TRICHOTHECENES. deoxyribonuclease (DNase; DNAase) An enzyme which depolymerizes DNA. A DNase is designated DNase I (EC 3.1.21.1) or DNase II (EC 3.1.22.1) according to whether the mono- or oligonucleotide products of its action have (terminal) 5
- or 3
phosphate groups, respectively. DNases are produced e.g. by the pancreas and by many microorganisms. In staphylococci, production of a thermostable, Ca2+ requiring, extracellular DNase (‘thermonuclease’: a 5
phosphodiesterase with both endo- and exonuclease activity, yielding 3
-nucleotides) correlates closely with the production of COAGULASE; thermonuclease also has Ca2+ -dependent RNase activity. (Many coagulase-negative staphylococci produce a DNase which is generally heat-labile.) DNase activity may be detected/assayed e.g. by measuring changes in turbidity, or increasing levels of acid-soluble nucleotides (measured by spectrophotometry at 260 nm), induced in a suspension of DNA. For the detection of DNase production on solid media, an agar medium (e.g. trypticase–soy agar) containing DNA and a calcium salt is inoculated with the test strain and incubated for 18–24 hours. The plate is then flooded with 1 N HCl to precipitate DNA. A clear zone (in which the DNA has been hydrolysed) surrounds each DNase-producing colony. Certain dyes – e.g. methyl green (which combines only with highly polymerized DNA) – may be used instead of HCl, in which case cell viability may be preserved. [Book ref. 44, pp. 757–768.] Certain Clostridium spp produce DNases: e.g. the b-toxins of C. chauvoei and C. septicum. See also STREPTODORNASE. deoxyribonucleic acid See DNA. deoxyribonucleoside See NUCLEOSIDE. deoxyribophage A BACTERIOPHAGE with a DNA genome. deoxyribovirus A VIRUS with a DNA genome. 2-deoxystreptamine 2-Deoxy-1,3-diaminoinositol, a component of many AMINOGLYCOSIDE ANTIBIOTICS. DEPC DIETHYLPYROCARBONATE. Dependovirus (adeno-associated viruses, AAVs; adeno-satellite viruses) A genus of defective viruses (family PARVOVIRIDAE) which are totally dependent on functions of a co-infecting helper adenovirus (or herpesvirus) for their replication. Dependoviruses can infect a wide range of vertebrate hosts. Type species: AAV-1 (AAV type 1), which infects mainly monkeys; other members: AAV-2 and AAV-3 (which can infect man), AAV-4 (common e.g. in African green monkeys), avian AAV, bovine AAV and canine AAV. (Equine AAV and ovine AAV are probable members of the genus.) In vitro, a given AAV seems able to replicate in any type of cell culture which can support the replication of a suitable helper virus. Genome: linear ssDNA, 4675 bases long in AAV-2 [sequence in AAV-2: JV (1983) 45 555–564]. Both positive and negative strands are encapsidated (in separate virions). The DNA has terminal INVERTED REPEATS 145 bases long. Within each repeat, the first 125 bases include a PALINDROMIC SEQUENCE in which bases 1–41 are complementary to bases 125–85; between bases 41 and 85 are two shorter palindromic sequences (bases 42–62 and 64–84). Folding which permits maximum base-pairing thus results in a T-shaped structure. (cf. PARVOVIRUS.) Initial stages of AAV infection are independent of helper virus; the virions adsorb to a host cell and subsequently enter the nucleus, where the DNA is uncoated. In the presence of a coinfecting helper virus, the DNA is replicated and transcribed, its mRNA is translated, and infectious virions are formed. DNA replication appears to be initiated at the hairpin structures and is independent of RNA primers. In the absence of a helper virus, the AAV establishes a latent state by integrating into the host DNA as a provirus. Subsequent infection of the latently infected cell by a helper virus ‘rescues’ the AAV DNA, allowing the formation of infectious AAV virions. AAVs appear not to cause disease in their hosts. In vitro, they may have a strong inhibitory effect on the replication of their helper viruses, but in vivo they seem not to ameliorate symptoms of adenovirus infection to any significant extent (although AAVs have been shown to reduce the oncogenic potential of adenoviruses in hamsters and of herpes simplex virus in cell cultures). AAV-2 infecting cell cultures co-infected with human cytomegalovirus (HCMV) as helper apparently exacerbates HCMV-induced CPE [Virol. (1985) 147 217–222]. [Reviews in Book ref. 97.] depside An ester formed by the condensation of two or more phenolcarboxylic acids; e.g., COOH.ArO.CO.ArOH is a generalized didepside where Ar = a phenolic group. Depsides occur e.g. as ‘lichen substances’ (see LICHEN) and as components of TANNINS (e.g. digallic acid). depsipeptide antibiotics ANTIBIOTICS which are derivatives of depsipeptides (molecules containing both ester and peptide bonds). They include the cyclic octadepsipeptide intercalating agents (QUINOXALINE ANTIBIOTICS) and the cyclic depsipeptide ionophores (see e.g. ENNIATINS and VALINOMYCIN). (See also DIDEMNINS.) Derbesia A genus of siphonaceous, tropical or temperate, epilithic or epiphytic green seaweeds (division CHLOROPHYTA) which exhibit a heteromorphic alternation of generations. The sporophyte and gametophyte are so distinct morphologically that they have been described as separate plants: e.g. Halicystis, with a bulbous vesicular thallus, is the gametophyte of a filamentous Derbesia sporophyte; some species of Bryopsis have also been shown to have Derbesia sporophyte stages. Derbesia sporophytes produce stephanokont zoospores, while gametophytes produce biflagellate anisogametes. (See also CELL WALL.) derepressed (mol. biol.) Refers to the state of a repressible gene or operon when it is not repressed, i.e., when the gene or operon is expressed. Derepression may be brought about e.g. by the presence of a specific inducer: see OPERON. (See also DRD PLASMID.) dermatitis Inflammation of the skin. It is commonly due to mechanical or chemical irritants or to allergic reactions, but may occur in certain infectious diseases (e.g. RINGWORM.) (cf. ECZEMA.) dermatitis verrucosa Syn. CHROMOBLASTOMYCOSIS. Dermatocarpon A genus of foliose LICHENS (order VERRUCARIALES); photobiont: a green alga. Thallus: small, more or less 227 dermatomycosis capitis, FAVUS). Zoophilic dermatophytes commonly show some degree of host specificity: e.g., M. canis (e.g. cats and dogs), T. verrucosum (e.g. cattle, horses, pigs, goats), T. equi (horses); most or all zoophilic species can also infect man. Certain species (e.g. M. gypseum, T. ajelloi ) are said to be ‘geophilic’: they apparently grow as saprotrophs (e.g. in soil, birds’ nests), but can also infect man and animals under certain conditions. dermatophytosis Syn. RINGWORM. Dermocarpa A genus of unicellular CYANOBACTERIA (section II) in which the vegetative cells are rounded and variable in size; baeocytes initially lack a fibrous outer cell wall layer and are motile (by gliding). GC%: ca. 38–44. Dermocarpales Syn. Chamaesiphonales (see CYANOBACTERIA). Dermocarpella A genus of unicellular CYANOBACTERIA (section II) in which growth and binary fission result in pear-shaped aggregates consisting of one or two basal cells and a larger apical cell; multiple fission of the apical cell yields baeocytes which initially lack a fibrous outer cell wall layer and are motile (by gliding). GC%: ca. 45. Dermocystidium See PERKINSUS. dermonecrotic Causing necrosis of the skin. dermonecrotic toxin A TOXIN produced by species of Bordetella; its role (if any) in the pathogenesis of WHOOPING COUGH has not been determined, but it is known to cause e.g. re-arrangement of actin filaments within eukaryotic cells. dermotropism See TROPISM (sense 2). Derrick’s method See IMMUNOSORBENT ELECTRON MICROSCOPY. Derxia A genus (incertae sedis) of Gram-negative, aerobic, catalase-negative, chemoorganotrophic, facultatively chemolithoautotrophic bacteria which occur e.g. in acidic tropical soils (e.g. in Brazil, China, Indonesia, West Bengal). Cells: typically round-ended pleomorphic rods, ca. 1.0–1.2 × 3.0–6.0 µm, with a single flagellum at one pole (or sometimes at each pole). Metabolism is respiratory (oxidative), with O2 as terminal electron acceptor. Carbon sources include glucose, fructose, ethanol and mannitol; little or no growth occurs on acetate, lactate, malate or succinate. NITROGEN FIXATION can occur under atmospheric conditions. Chemolithoautotrophic growth can occur with mixtures of H2 , CO2 and O2 (CO2 being fixed via the Calvin cycle), N2 or NH4 + serving as a source of nitrogen; growth on CH4 or CH3 OH (see METHYLOTROPHY) has been reported. A pellicle is formed in liquid media; broth cultures become gelatinous. Colonies are slimy, becoming raised, wrinkled, and subsequently dark brown. Optimum growth temperature: 25–35° C. Growth occurs within the pH range 5.5ca. 9.0. GC%: ca. 69–73. Type species: D. gummosa. [Book ref. 22, pp. 321–325.] desensitization (syn. hyposensitization) (immunol.) The repeated administration of known quantities of allergen to an individual in whom it usually provokes an immediate or delayed HYPERSENSITIVITY reaction; in some cases this treatment temporarily reduces or abolishes hypersensitivity to the given allergen. In one method used for treating allergy to mould(s), an aqueous extract of the allergen(s) – up to 100-fold weaker than that required to produce an intradermal hypersensitivity reaction – is administered subcutaneously; the dose, initially given weekly, is gradually increased and administered after longer intervals of time for a total period of several years [Book ref. 51, pp. 187–201]. Little is known about the mechanism(s) of desensitization. Possible mechanisms include e.g. (i) the formation of BLOCKING ANTIBODIES (sense 2), and (ii) the stimulation of suppressor T cells. lobed, attached to the substratum (rocks) by a central holdfast (umbilicus – cf. UMBILICARIA). Pseudothecia occur immersed in the thallus, their ostioles visible as black dots in the thallus surface. Some species – e.g. D. rivulorum, D. weberi (formerly D. fluviatile) – occur on rocks in or adjacent to unpolluted streams or lakes. (cf. CATAPYRENIUM.) dermatomycosis Any MYCOSIS affecting the skin – see e.g. CANDIDIASIS, PITYRIASIS VERSICOLOR, RINGWORM. dermatophilosis A disease of animals, transmissible (by direct contact) to man, caused by Dermatophilus congolensis (see e.g. LUMPY WOOL and STRAWBERRY FOOT-ROT). In man, self-limited pustular lesions develop on the skin. (cf. PITTED KERATOLYSIS.) Dermatophilus A genus of aerobic or facultatively anaerobic, catalase-positive bacteria (order ACTINOMYCETALES, wall type III) which occur as pathogens of man and other animals (see DERMATOPHILOSIS). In culture the organisms initially form filaments (<1 µm in diameter) which become branched and undergo transverse and longitudinal septation to form long, tapering hyphae, up to 5 µm in diameter, which bear a tough gelatinous capsule. Further septation gives rise to coccoid forms which develop into zoospores (up to ca. 1 µm in diameter), each having a single tuft of flagella. Growth occurs at 37° C on enriched media (e.g. brain–heart infusion medium), and is enhanced by increased partial pressures of CO2 ; the colonies are initially dry and greyish-white but later become mucoid and yellowish-orange. Clear haemolysis occurs in media containing sheep blood but not in media containing horse blood. Casein, gelatin and starch are metabolized. GC%: ca. 57–59. Type species: D. congolensis. [Morphology, ecology, isolation: Book ref. 46, pp. 2011–2015; differentiation between Dermatophilus and Geodermatophilus by 16S rRNA oligonucleotide cataloguing: JGM (1983) 129 1831–1838.] dermatophytes A group of fungi which can infect and degrade keratinized tissues (skin, nails, hair etc) in living animals (including man), causing RINGWORM. Dermatophytes belong to the HYPHOMYCETES, and many have teleomorphs in the GYMNOASCALES; the three main genera are Epidermophyton, Microsporum (teleomorph Nannizzia), and Trichophyton (teleomorph Arthroderma). The organisms form a hyaline, septate mycelium which in many species tends to fragment to form arthrospores. (See also RACQUET MYCELIUM.) When infecting hair, some dermatophytes (e.g. T. tonsurans and T. violaceum) form both hyphae and arthrospores largely or wholly within the hair shaft (endothrix infection); other species (e.g. T. mentagrophytes, Microsporum spp) may also form hyphae within the hair shaft, but arthrospores develop only on the surface of the hair (ectothrix infection). In culture, most dermatophytes produce both single-celled microconidia and septate macroconidia. (Epidermophyton spp do not form microconidia.) Microconidia are generally oval or pyriform, borne singly or in grape-like clusters along the sides of the hyphae. Characteristics of the macroconidia are used in the classification of genera and species. In Epidermophyton spp, macroconidia are produced abundantly and are smooth-surfaced, 2–4-septate, ca. 6–8 × 15–40 µm, and borne in groups of 2 or 3. In Microsporum spp, macroconidia are usually numerous (but rare in M. audouinii ) and are rough-surfaced, usually thick-walled, 3–15-septate, ca. 3–8 × 40–120 µm, and borne singly. In Trichophyton spp, macroconidia are usually sparse and are smooth-surfaced, usually thin-walled, 2–8-septate, ca. 4–6 × 10–50 µm, borne singly. Some (anthropophilic) dermatophytes occur mainly or only as human pathogens; these include e.g. E. floccosum (ATHLETE’S FOOT), M. audouinii (tinea capitis), and T. schoenleinii (tinea 228 Desulfobulbus desert fever Syn. COCCIDIOIDOMYCOSIS. Desert Shield virus See SMALL ROUND STRUCTURED VIRUSES. desferrioxamine A hydroxamate SIDEROPHORE formed e.g. by Streptomyces pilosus. It is used e.g. in the treatment of patients with iron overload. desiccation (of microorganisms) The removal of water: a process used e.g. for the PRESERVATION of viable populations of certain types of microorganism (including enterobacteria and sporeforming bacteria). However, the method described below is lethal for many types of microorganism, including various pathogens (e.g. Neisseria gonorrhoeae, Treponema pallidum) and many aquatic organisms; some organisms that are killed by this procedure can be preserved by FREEZE-DRYING. Initially, the organisms are suspended in an appropriate medium (e.g. broth, serum, or 5–10% nutrient gelatin). Drops of the suspension are then placed, singly, onto a sterile filter paper (or a piece of waxed paper) which is inserted into a glass ampoule. The ampoule is placed in a desiccator which contains phosphorus pentoxide (as drying agent) and which is subsequently evacuated. The use of a vacuum is essential because it reduces the time for which cells are exposed to the deleterious effects of high concentrations of solute. When the sample is completely desiccated the ampoule is sealed. Under optimal conditions the organisms remain viable for years; spores may remain viable for decades. In general, desiccation is tolerated to different degrees by different organisms; many do not survive for long in the airdried state. [Desiccation tolerance of prokaryotes: MR (1994) 58 755–805.] desmarestene See PHEROMONE. Desmarestia See PHAEOPHYTA. Desmidium A genus of placoderm DESMIDS. desmids A group of freshwater, basically unicellular green algae (division CHLOROPHYTA) which reproduce sexually by conjugation and which never form flagellated cells of any kind. In most species the cells are solitary, although in a few they are joined end-to-end to form filaments (see e.g. HYALOTHECA). The cells are generally more or less bilaterally symmetrical and may be cylindrical, fusiform, crescent-shaped, etc, sometimes bearing elaborate ornamentations. Many species are highly variable, occurring in a range of growth forms – a feature which has caused considerable confusion in speciation and nomenclature [Book ref. 123, pp. 251–269]. In placoderm desmids each cell is divided into two ‘semicells’ either by a median suture line (e.g. in Closterium and Penium) or, more commonly, by an equatorial constriction (sinus) – the narrow region joining the semicells being called the isthmus. The cell wall is perforated by pores through which mucilage may be secreted; the mucilage may allow the desmid to adhere to substrata, and is apparently necessary for the GLIDING MOTILITY often exhibited by these cells. Genera of placoderm desmids include e.g. CLOSTERIUM, COSMARIUM, Desmidium, Euastrum, HYALOTHECA, Micrasterias, PENIUM, STAURASTRUM, Staurodesmus, and XANTHIDIUM. In saccoderm desmids the cells do not have a median suture line or sinus and have smooth walls without pores. They nevertheless produce mucilage, often in copious amounts, and some can exhibit gliding motility. Genera include e.g. Ancylonema (short cylindrical cells joined end-to-end to form short filaments), Cylindrocystis (cells cylindrical, round-ended, containing a stellate chloroplast at each end), and Mesotaenium (cells short, cylindrical, solitary, containing a single chloroplast). Sexual reproduction in desmids involves a process of conjugation analogous to that in SPIROGYRA. Essentially, cells come together in pairs, a conjugation tube is formed between them, and the protoplasts function as amoeboid gametes, fusing to form a zygote which then develops a thick, resistant wall. Desmids occur in various freshwater habitats, particularly in acidic, unpolluted pools, bogs etc; some (e.g. Cylindrocystis, Mesotaenium) may occur in mucilaginous, amorphous colonies on damp soil, wet rocks etc. [Book ref. 128.] Desmococcus See PLEUROCOCCUS. desmodexy See RULE OF DESMODEXY. desmosome One of a number of differentiated regions which occur e.g. where the cytoplasmic membranes of adjacent epithelial cells are closely apposed; it consists of a circular region (ca. 0.5 µm diam.) of each membrane together with associated intracellular microfilaments and an intercellular material which may include e.g. mucopolysaccharides. Desmosomes can be disrupted e.g. by hyaluronidase and Ca2+ chelators. (See also TRYPANOSOMA.) Desmothoracida An order of protozoa (class HELIOZOEA) in which the cell is enclosed in a reticulate, basket-like capsule which is usually spherical and generally composed of an organic matrix often impregnated with silica; axopodia project through perforations in the capsule. In some species a stalk from the capsule attaches the organism to the substratum. There is no centroplast (cf. CENTROHELIDA). Uniflagellate or biflagellate zoospores may be formed. Genera include e.g. CLATHRULINA. desmotubule In plant cells: an extension of the endoplasmic reticulum which is continuous from one cell to the next via the PLASMODESMA. desoxy- A prefix equivalent to ‘deoxy-’; thus, e.g., for desoxycholate see DEOXYCHOLATE. desquamation The shedding of skin, cuticle etc in sheets or flakes. destroying angel See AMANITA. destruxins See METARHIZIUM. Desulfobacter A genus of Gram-negative SULPHATE-REDUCING BACTERIA which occur e.g. in aquatic habitats (primarily brackish and marine). (See also ANAEROBIC DIGESTION.) Cells: non-motile or motile (polarly monoflagellate), rod-shaped to ellipsoidal, ca. 1.0–2.0 × 1.7–3.5 µm. Chemoorganotrophic. Acetate can be used as a source of carbon and energy, and is oxidized to CO2 using sulphate (or another oxidized sulphur compound) as terminal electron acceptor (see DISSIMILATORY SULPHATE REDUCTION and TCA CYCLE). The organisms contain b- and ctype cytochromes; they lack desulfoviridin. Growth is inhibited by elemental sulphur. Typically, at least 0.5% NaCl and 0.1% MgCl2 are required for growth. Optimum growth temperature: 28–32° C. Optimum pH: 7.3. GC%: ca. 46. Type species: D. postgatei. [Book ref. 22, pp. 674–676.] Desulfobulbus A genus of Gram-negative SULPHATE-REDUCING BACTERIA which occur e.g. in freshwater, brackish and marine habitats, and in the RUMEN. Cells: ellipsoidal, ca. 1.0–1.3 × 1.5–2.0 µm; in many strains there is a single polar flagellum. Chemoorganotrophic. Lactate, propionate, pyruvate or certain alcohols are used as sources of carbon and energy, and are oxidized to e.g. acetate using sulphate (or another oxidized sulphur compound) as terminal electron acceptor (see DISSIMILATORY SULPHATE REDUCTION); the organisms contain b- and c-type cytochromes. Lactate or pyruvate can be fermented in the absence of an external electron acceptor. Optimum growth temperature: 28–39° C. GC%: ca. 60. Type species: D. propionicus. [Book ref. 22, pp. 676–677.] 229 Desulfococcus Desulfovibrio A genus of Gram-negative SULPHATE-REDUCING which occur e.g. in mud and sediments in freshwater, brackish and marine habitats. Cells: motile, typically curved (sometimes S-shaped or helical) rods, ca. 0.5–1.5 × 2.5–10.0 µm, each having one polar flagellum or a tuft of polar flagella. Metabolism is chemoorganotrophic or mixotrophic, primarily respiratory, and the cells contain c-type (and usually b-type) cytochromes; sulphate (or another oxidized sulphur compound) is used as a terminal electron acceptor and is reduced to H2 S. (See also DISSIMILATORY SULPHATE REDUCTION; cf. FERMENTATION sense 1.) Most species give a positive DESULFOVIRIDIN TEST. In the absence of oxidized sulphur compounds some strains can use fumarate and/or nitrate as terminal electron acceptor. Substrates used by most species include one or more of the following: lactate, malate, pyruvate and glycerol; these substrates are typically oxidized to acetate and CO2 , the acetate not being oxidized further. (D. baarsii can oxidize e.g. acetate, butyrate or formate to CO2 ; it can also grow as an AUTOTROPH (q.v.).) [Use of amino acids by marine strains of Desulfovibrio: FEMS Ecol. (1985) 31 11–15.] In the absence of a reducible sulphur compound some species can gain energy from e.g. pyruvate (see PHOSPHOROCLASTIC SPLIT). Most strains contain HYDROGENASE, and some can gain energy by the oxidation of H2 (using SO4 2− as terminal electron acceptor) while using acetate and CO2 as carbon sources. (See also EXTRACYTOPLASMIC OXIDATION.) [Hydrogenase, electron-transfer proteins and energy coupling in Desulfovibrio: ARM (1984) 38 551–592.] Some strains can carry out NITROGEN FIXATION. [Diazotrophy in Desulfovibrio: JGM (1985) 131 2119–2122.] GC%: ca. 46–61. Type species: D. desulfuricans; other species: D. africanus, D. baculatus, D. gigas, D. salexigens, D. sapovorans, D. thermophilus, D. vulgaris. [Book ref. 22, pp. 666–672.] (For D. sulfodismutans see FERMENTATION sense 1.) desulfoviridin A (green) sulphite reductase which occurs e.g. in the cytoplasm in species of Desulfovibrio and which is involved in DISSIMILATORY SULPHATE REDUCTION. The enzyme (MWt ca. 200000) contains several 4Fe–4S clusters and two sirohaem groups per molecule (each sirohaem consisting of an ironcontaining porphyrin-like molecule, sirohydrochlorin). In vitro the products of sulphite reduction include trithionate (S3 O6 2− ) and sulphide (S2− ). (cf. DESULFORUBIDIN and P582.) desulfoviridin test A test used to detect DESULFOVIRIDIN in certain SULPHATE-REDUCING BACTERIA. The pellet from a centrifuged culture (15 ml) is resuspended in growth medium, one drop of NaOH (2 M) is added, and the whole is examined under ultraviolet light (wavelength 365 nm); in a positive test the suspension gives a red fluorescence due to the presence of the free sirohydrochlorin chromophore of desulfoviridin. Desulfurococcus A genus of chemolithoheterotrophic archaeans (order THERMOPROTEALES) which occur e.g. in Icelandic solfataras; they metabolize peptides and obtain energy by SULPHUR RESPIRATION, although at least one species can apparently survive by means of a (sulphur-independent) fermentation. The cells are cocci, ca. 1 µm diam.; D. mobilis is motile (flagellated), D. mucosus is not. GC%: 51. Desulfuromonas A genus of Gram-negative, obligately anaerobic bacteria which occur e.g. in freshwater, brackish and marine sediments or muds. Cells: motile, round-ended rods, ca. 0.4–0.9 × 1.0–4.0 µm, which typically have a single lateral or subpolar flagellum. The organisms are chemoorganotrophs; acetate, L-malate and other substrates (e.g. alcohols in some strains) can be utilized. Metabolism is primarily respiratory, and Desulfococcus A genus of Gram-negative SULPHATE-REDUCING BACTERIA which occur e.g. in aquatic habitats (freshwater, brackish and marine) and in anaerobic sewage digesters. Cells: cocci, ca. 1.5–2.2 µm in diameter. Chemoorganotrophic. Benzoate, other aromatic compounds, acetate, lactate, pyruvate and alcohols etc are used as sources of carbon and energy; these substrates are oxidized to CO2 using sulphate (or another oxidized sulphur compound) as terminal electron acceptor (see DISSIMILATORY SULPHATE REDUCTION). The organisms contain b- and c-type cytochromes. Lactate or pyruvate can be fermented in the absence of an external electron acceptor. GC%: ca. 57. Type species: D. multivorans. [Book ref. 22, pp. 673–674.] Desulfomonas A genus of Gram-negative SULPHATE-REDUCING BACTERIA which occur e.g. in the human intestinal tract. Cells: non-motile rods. The organisms, which carry out DISSIMILATORY SULPHATE REDUCTION, contain both b- and c-type cytochromes and desulfoviridin; pyruvate is oxidized to acetate and CO2 . GC%: ca. 66–67. Type species: D. pigra. [Book ref. 22, pp. 672–673.] Desulfonema A genus of SULPHATE-REDUCING BACTERIA which occur e.g. in marine sediments. Cells: gliding filaments, ca. 2.0–7.0 µm in width, several millimetres in length, each being uniseriately multicellular; the organisms stain Gram-positively. Acetate, benzoate and other substrates are oxidized to CO2 . The organisms carry out DISSIMILATORY SULPHATE REDUCTION and contain b- and c-type cytochromes; desulfoviridin occurs in strains of D. limicola but not in those of D. magnum. At least some strains have been reported to grow autotrophically. GC%: ca. 34–42. desulforubidin A (red) sirohaem-containing sulphite reductase which occurs e.g. in Desulfovibrio desulfuricans strain Norway 4. (cf. DESULFOVIRIDIN.) Desulfosarcina A genus of Gram-negative SULPHATE-REDUCING BACTERIA which occur e.g. in brackish and marine waters. Cells: irregularly shaped and sarciniform, or coccoid to ellipsoidal single cells, ca. 1.0–1.5 × 1.5–2.5 µm, occasionally motile by means of a single polar flagellum. Chemoorganotrophic, and reported to be facultatively chemolithoautotrophic. Carbon and energy sources include e.g. acetate, lactate, pyruvate, formate, propionate, and various alcohols and fatty acids; these substrates can be oxidized to CO2 using sulphate (or another oxidized sulphur compound) as terminal electron acceptor (see DISSIMILATORY SULPHATE REDUCTION). Lactate or pyruvate can be fermented in the absence of an external electron acceptor. Growth requires at least 1% NaCl and 0.2% MgCl2 .6H2 O. Optimum growth temperature: 28–33° C. GC%: ca. 51. Type species: D. variabilis. [Book ref. 22, pp. 677–679.] Desulfotomaculum A genus of Gram-negative, endosporeforming SULPHATE-REDUCING BACTERIA which occur e.g. in soil and in the RUMEN. Cells: motile (peritrichously flagellate) rods up to ca. 9 µm in length. Chemoorganotrophic. For most species the carbon and energy sources include lactate and pyruvate; D. acetoxidans can oxidize acetate. The organisms carry out DISSIMILATORY SULPHATE REDUCTION and contain b-type cytochromes; no species contains desulfoviridin. In addition to electron transport-coupled phosphorylation, Desulfotomaculum spp can carry out a substrate-level phosphorylation in which PPi (see PYROPHOSPHATE) reacts reversibly with acetate (enzyme: acetate:PPi phosphotransferase) to form acetyl phosphate and Pi; acetyl phosphate and ADP can then yield acetate and ATP in the presence of acetokinase. GC%: ca. 37–49. Type species: D. nigrificans (formerly Clostridium nigrificans); other species: D. acetoxidans, D. antarcticum, D. orientis, D. ruminis. BACTERIA 230 DGGE deuterosome In a eukaryotic cell: a dense region in the cytoplasm which can act as a MICROTUBULE-ORGANIZING CENTRE for the de novo assembly of a BASAL BODY. deutomerite In a cephaline gregarine: the posterior of the two main regions of the (septate) cell (cf. PROTOMERITE); it usually contains the nucleus. DEV Duck embryo vaccine (see RABIES). Devarda’s alloy An alloy containing aluminium (45%), copper (50%) and zinc (5%); it is used e.g. to reduce nitrite and/or nitrate to ammonia. devil’s grip Syn. BORNHOLM DISEASE. dew retting See RETTING. dexioplectic metachrony See METACHRONAL WAVES. dextrans D-Glucans in which the glucose residues are linked mainly by (1 → 6)-a-glucosidic bonds; branches are formed by occasional (1 → 4)-a- and, less frequently, (1 → 3)-a-linkages. The size of the molecule and the nature and extent of branching depend on the source of the dextran. Extracellular dextrans are produced by a range of microorganisms, sometimes in copious amounts (see e.g. ROPINESS); they are obtained commercially from strains of Leuconostoc mesenteroides grown anaerobically on sucrose-containing media [Book ref. 62, pp. 1–44]. Dextrans of MWt ca. 75000 are used as plasma volume extenders for blood transfusions; they may be obtained by acid hydrolysis of higher-MWt dextrans. Artificially cross-linked dextrans (e.g. ‘Sephadex’) are used in GEL FILTRATION. Dextrans are relatively inert and can withstand autoclaving. dextrins Products of the partial degradation of STARCH or GLYCOGEN by heat, acid hydrolysis, or enzyme action. Limit dextrins are those formed by enzymes that are unable to effect complete hydrolysis (see AMYLASES). (See also SCHARDINGER DEXTRINS.) dextrose Dextrorotatory glucose (D-glucose). DFD meat Dark, firm, dry meat: red meat from animals stressed before slaughter. DFD meat contains less glucose and has a higher pH (>ca. 6.0) than normal meat; during stress, muscle glycogen in the living animal is converted to lactic acid (cf. MEAT SPOILAGE) which is subsequently lost when the animal is bled. DFD meat is more susceptible than normal meat to spoilage. Increased susceptibility to aerobic spoilage appears to be due to the deficiency of glucose rather than to the high pH; since little or no glucose is available, spoilage organisms attack amino acids earlier than in normal meat and, hence, produce off-odours and off-flavours after a shorter period of storage [Book ref. 30, pp. 240–244]. Anaerobic spoilage (e.g. in vacuum packs) may be due to organisms (e.g. Alteromonas putrefaciens, Serratia liquefaciens) which are inhibited by the low pH of normal meat. S. liquefaciens produces off-odours. A. putrefaciens forms H2 S which reacts with e.g. myoglobin to produce a green compound: sulphmyoglobin (‘greening’) – cf. MEAT SPOILAGE (b); since A putrefaciens is inhibited at pH below 6.0, greening can be prevented by treating the meat with citrate buffer. Vacuum-packed DFD meat may also support the growth of large populations of Yersinia enterocolitica, although these seem not to cause significant spoilage (cf. FOOD POISONING). DFMO (DL-a-difluoromethylornithine) See SLEEPING SICKNESS. DFP Diisopropylfluorophosphate – see PROTEASES. DGGE Denaturing gradient gel electrophoresis: a method for comparing samples of related double-stranded DNA (generated e.g. by PCR or restriction) by two-dimensional electrophoresis (cf. SSCP). In DGGE [Biotechnology (1995) 13 137–139], fragments are separated by size in the first phase of electrophoresis. Then, in the same gel, the fragments are moved electrophoretically at right-angles to their original path. In this second phase, the organisms contain c-type cytochromes; elemental sulphur, which acts as terminal electron acceptor, is reduced to H2 S (dissimilatory sulphur reduction), and acetate (or other substrate) is completely oxidized to CO2 . (See also ANAEROBIC DIGESTION.) (Sulphate, sulphite or thiosulphate cannot be used as terminal electron acceptor.) Fumarate or L-malate can be fermented to acetate and succinate. Optimum growth temperature: 30° C. Optimum pH: 7.2–7.5. Colonies are pink or peach-coloured, translucent to opaque. GC%: ca. 50–63. Type species: D. acetoxidans. Strains which are ovoid, with a polar or subpolar flagellum, and which do not use alcohols as substrates have been referred to as ‘D. acetexigens’. [Book ref. 22, pp. 664–666.] determinant (1) (antigenic determinant; determinant group; epitope) (immunol.) Of an antigenic macromolecule: any region of the macromolecule with the ability or potential to elicit, and combine with, specific antibody. Determinants exposed on the surface of the macromolecule are likely to be immunodominant, i.e. more immunogenic than other (immunorecessive) determinants which are less exposed, while some (e.g. those within the molecule) are non-immunogenic (immunosilent ). (See also ANTIGEN.) (2) (genetics) A gene or functional gene group. detritivore Any organism (e.g. earthworm, lugworm, bivalve mollusc) which feeds by ingesting detritus (such as soil particles), removing and digesting e.g. adherent microorganisms, and voiding the residue. Detroit-6 An ESTABLISHED CELL LINE derived from human sternal bone marrow; the cells are heteroploid and epithelioid. Dettol See PHENOLS. Dettol chelate A disinfectant which contains chloroxylenol (see PHENOLS) and EDTA; EDTA potentiates the action of chloroxylenol on Gram-negative bacteria by increasing cell permeability. It is active against e.g. many strains of Pseudomonas aeruginosa. deuteromycetes See DEUTEROMYCOTINA. Deuteromycotina (deuteromycetes; Fungi Imperfecti) A nonphylogenetic category originally created for fungi with no known sexual stage; the category still includes fungi with no known sexual stage (and some fungi which form neither conidia nor sexual structures: see AGONOMYCETALES), but it also includes the asexual (= anamorphic, conidial or imperfect) stages of various fungi which are now known to have a sexual (= teleomorphic or perfect) stage in the Ascomycotina or the Basidiomycotina. For convenience, ‘Deuteromycotina’ is generally treated as a subdivision within the EUMYCOTA. The conidium-forming deuteromycetes are arranged into form genera (see FORM GENUS) primarily on the basis of the characteristics of their conidia and their modes of conidiogenesis (see CONIDIUM). (See also SACCARDOAN SYSTEM.) The inclusion of a number of form species in a given form genus means only that those fungi have similar asexual stages; such fungi are not necessarily related (and are often unrelated) in an evolutionary sense (as determined by the sexual characteristics of the organisms, when known). Thus, a given form genus may contain e.g. anamorphs corresponding to the teleomorphs of different genera together with ANAHOLOMORPHS. (For convenience, the word ‘form’ is generally omitted when referring to a form genus, form species etc.) Classification of the deuteromycetes on the basis of their asexual stages facilitates the identification of those members in which the asexual stage is that which is most commonly encountered in nature (and which may form the sexual stage only rarely). Two classes [Book ref. 64, p. 112]: COELOMYCETES and HYPHOMYCETES. 231 DHBG fragments move through an increasing concentration of DNAdenaturing agents (e.g. formamide + urea) so that, at certain levels in the gradient, localized sequence-dependent melting (i.e. strand separation) occurs within part(s) of the fragments (base-pairing being stronger in GC-rich sections); such localized melting affects the electrophoretic speed of those fragments in which it occurs and allows separation of fragments in the gel. [Example of use of DGGE (differentiation of isolates of Escherichia coli by analysis of the 16S–23S intergenic spacer region): FEMS Ecol. (2001) 35 313–321.] Other uses of DGGE include e.g. characterization of organisms by comparing PCR-amplified sequences of their 16S rRNA genes [AEM (1996) 62 340–346] and detection of Rhizobium spp [LAM (1999) 28 137–141]. Another method, based on the same principle, replaces the chemical denaturing gradient of DGGE with an ongoing increase in temperature during the second phase of electrophoresis; thus, at certain levels of temperature, localized sequence-dependent melting occurs in specific part(s) of the fragments – affecting electrophoretic mobility and allowing separation within the gel. This method is referred to as temporal temperature gradient gel electrophoresis (TTGE). [Comparison of DGGE with TTGE: LAM (2000) 30 427– 431.] DHBG (9-(3,4-dihydroxybutyl)-guanine) An ANTIVIRAL AGENT which is active, both in cell cultures and in vivo, against herpes simplex viruses (HSV-1 and -2); the (R)-enantiomer is more effective than the (S )-enantiomer. The mechanism of action resembles that of ACYCLOVIR, but DHBG has higher affinity for HSV thymidine kinase. DHBV DUCK HEPATITIS B VIRUS. Dhori virus An unclassified virus (see ORTHOMYXOVIRIDAE) which has been isolated from ticks; antibodies to the virus have been found in man and domestic animals. DHPA ((S )-DHPA; (S )-9-(2,3-dihydroxypropyl)-adenine) An ANTIVIRAL AGENT which acts as an analogue of adenosine; it is a potent inhibitor of 5-adenosyl homocysteine hydrolase. DHPA is active e.g. against vaccinia, herpes simplex, varicella-zoster and measles viruses; it acts synergistically with VIDARABINE. (The (R)-enantiomer is inactive.) DHPG (9-(1,3-dihydroxy-2-propoxymethyl)-guanine; 2
-nor-2
-deoxyguanosine, 2
-NDG) An ANTIVIRAL AGENT which has potent activity against herpes simplex viruses (HSV-1 and -2); it is not effective against e.g. Epstein–Barr virus, varicella-zoster virus or cytomegaloviruses. The mechanism of action resembles that of ACYCLOVIR. The isomer (S )-9-(2,3-dihydroxy-1-propoxymethyl)guanine is also active against HSV-1 and -2. DI particle DEFECTIVE INTERFERING PARTICLE. diacetoxyscirpenol See TRICHOTHECENES. diacetyl (dimethylglyoxal; CH3 .CO.CO.CH3 ) A water-soluble compound formed e.g. by certain lactic acid bacteria; it may also be derived by oxidation of acetoin in the BUTANEDIOL FERMENTATION. (See also VOGES–PROSKAUER TEST.) Diacetyl produced by certain lactic acid bacteria is responsible for the characteristic ‘buttery’ flavour and aroma in many fermented DAIRY PRODUCTS. The diacetyl is formed from pyruvate – see Appendix III(c) – but levels of diacetyl are low if the pyruvate is derived from hexoses only. Additional pyruvate (and hence diacetyl) can be produced from citrate by certain strains of lactic acid bacteria (‘aroma bacteria’, ‘flavour bacteria’) – e.g. Lactococcus lactis and Leuconostoc cremoris (see LACTIC ACID STARTERS). (Citrate is naturally present in cows’ milk at levels of ca. 0.2%, with seasonal fluctuations; it may also be added to the milk to enhance diacetyl production.) L. lactis produces both lactic acid and diacetyl in milk. However, Leuconostoc cremoris by itself grows poorly in milk, producing little lactic acid; it can take up citrate only at pH below ca. 6.0, so that diacetyl production by this organism requires the activities of a lactic acid producer such as L. lactis. The ability to produce diacetyl from citrate is apparently plasmid-linked in at least certain strains of L. (‘Streptococcus’ ) lactis [AvL (1983) 49 265–266]. The ratio of flavourful products (e.g. diacetyl) to flavourless products (e.g. acetoin) depends e.g. on the redox potential of the system. Diacetyl has some antimicrobial activity [AEM (1982) 44 525–532], being more effective against Gram-negative bacteria, yeasts and moulds than against Gram-positive bacteria. diacridine A compound containing two ACRIDINE moieties (linked e.g. by a hydrocarbon bridge); it may act as a bis INTERCALATING AGENT. diagnosis (1) In NOMENCLATURE: a list of descriptive characteristics which distinguish a proposed new taxon from other taxa. (2) (med.) In general, a statement of the identity of a given disease, deduced from symptoms etc., or the procedure used for arriving at such a statement. diagnostic window See TRANSFUSION-TRANSMITTED INFECTION. diakinesis See MEIOSIS. Dialister pneumosintes See BACTEROIDES (B. pneumosintes). dialkylnitrosamines See N-NITROSO COMPOUNDS. diamidines Compounds with two amidine [NH2 .C(=NH)−] groups. Aromatic diamidines bind to DNA, and a number of them are trypanocidal agents which bind to kinetoplast DNA and cause structural changes in the kinetoplast; however, there is evidence that at least part of the trypanocidal effect in vivo is due to inhibition of ornithine decarboxylase, leading to depletion of putrescine. (See e.g. BERENIL; DAPI; 2-HYDROXYSTILBAMIDINE; PENTAMIDINE ISETHIONATE.) diaminopimelic acid pathway (DAP pathway) A pathway for lysine biosynthesis [Appendix IV(e)] which occurs in bacteria, certain lower fungi (Hyphochytriales, Leptomitales, Saprolegniales) and e.g. in green plants. (cf. AMINOADIPIC ACID PATHWAY.) diaminopyrimidine drugs See FOLIC ACID ANTAGONIST. diamond skin disease Syn. SWINE ERYSIPELAS. dianemycin See MACROTETRALIDES. o-dianisidine (fast blue B) A (carcinogenic) substance, 3,3
-dimethoxybenzidine, used e.g. in assays for HYDROGEN PEROXIDE: the peroxidase-dependent oxidation of o-dianisidine causes increased absorbance at 500 nm. dianthoviruses (carnation ringspot virus group) A category of PLANT VIRUSES which have a wide host range and are transmitted mechanically and via the soil. Virion: icosahedral, ca. 31–34 nm diam., containing two molecules of positive-sense ssRNA (MWts ca. 1.5 and 0.5 × 106 ) and one type of coat protein (MWt 40000). Virus replication appears to occur in the cytoplasm. Type member: carnation ringspot virus; other members: red clover necrotic mosaic virus, sweet clover necrotic mosaic virus. diapedesis See INFLAMMATION. diaphorase Any enzyme which can catalyse the oxidation of reduced NAD or NADP by an artificial electron acceptor. diaphoromixis Bipolar or tetrapolar multi allele HETEROTHALLISM. diaplectic metachrony See METACHRONAL WAVES. Diaporthales An order of fungi (subdivision ASCOMYCOTINA) which include saprotrophic and plant-parasitic species. Ascocarp: perithecioid, often immersed; hamathecium: absent or evanescent. Asci: unitunicate, evanescent. Genera: e.g. Diaporthe, ENDOTHIA, GAEUMANNOMYCES, Gnomonia (see also ZYTHIA). 232 diatoms etc which are usually arranged in symmetrical, species-specific patterns. The degree of cell wall silicification may vary to some extent with environmental conditions, and exceptionally the wall may be predominantly or entirely organic (e.g. in Phaeodactylum tricornutum – see PHAEODACTYLUM – and in Cylindrotheca fusiformis). [Details of wall structure: Book ref. 137, pp. 129–156.] Two main groups of diatoms are commonly recognized: centric diatoms, in which the valve is radially symmetrical, and pennate diatoms, in which the valve is bilaterally symmetrical. (Some diatoms do not fit easily into either category: e.g. Triceratium spp may have 3-, 4- or 5-fold rotational symmetry; see also HEMIDISCUS.) Centric diatoms are mostly marine and planktonic, are nonmotile, and commonly exhibit oogamous sexual reproduction in which the male gametes each bear a single tinsel flagellum. Genera include e.g. Arachnoidiscus, Bacteriastrum, Biddulphia, CHAETOCEROS, Coscinodiscus, Cyclotella, Hemiaulus, MELOSIRA, Skeletonema, Stephanodiscus, Stephanopyxis, Thalassiosira, Zygoceros. Pennate diatoms occur in various habitats; many are capable of gliding motility, and many exhibit isogamous sexual reproduction involving cell–cell contact (conjugation). All species which are capable of GLIDING MOTILITY (q.v.) have a raphe: basically, a slit which (usually) runs the length of the valve, interrupted by a central nodule. Such ‘raphid’ species are generally benthic or occur attached to a solid substratum. In a few genera (e.g. Achnanthes, Cocconeis) a raphe occurs in only one of the two valves, but in most a raphe occurs in both valves. Pennate diatoms which lack a raphe (‘araphid’ diatoms: e.g. Fragilaria, Striatella, Tabellaria) cannot glide; these organisms may have a central line, ridge, or unornamented central area called a pseudoraphe. (Other pennate diatoms include e.g. AMPHORA, Anomoeoneis, Bacillaria, Cylindrotheca, Cymbella, Diatoma, GOMPHONEMA, Gyrosigma, NAVICULA, Nitzschia, PHAEODACTYLUM, Pinnularia, Pleurosigma, Stauroneis, Surirella.) In many centric and pennate diatoms the cells adhere to one another to form filaments or characteristic groupings (see e.g. ASTERIONELLA and MELOSIRA); the cells may be held together by mucilage secreted via tubular projections in the frustule and/or by the interlocking of spines present on the margins of the valves. Secreted mucilage may also serve to attach some forms to the substratum, and apparently serves an essential role in GLIDING MOTILITY. Asexual reproduction occurs by binary fission. The nucleus (which is diploid in vegetative cells) moves from its position near the epivalve to the cell centre, and the protoplast expands, pushing the two valves apart. New girdle bands are formed, and mitosis occurs. The original parent hypovalve becomes the epivalve of one daughter cell, and both daughter cells synthesize a new hypovalve. Thus, the epivalve is always the older of the two valves in a cell, and – in species in which the epivalve is larger than the hypovalve – one of the daughter cells must be smaller than the parent cell. When division is complete, the nucleus in each cell returns to its position near the epivalve face. The new siliceous structures (valves and cingulum) are synthesized in SILICA DEPOSITION VESICLES beneath the plasmalemma, reaching the exterior by fusion between the silicalemma and plasmalemma. [Diatom wall formation: Book ref. 137, pp. 157–200; silicon ‘biomineral’ synthesis in diatoms: TIBS (1987) 12 151–154.] Since, in many species, the average size of cells in a population progressively decreases with increasing numbers of Diaporthe See DIAPORTHALES. diarrhoea (American: diarrhea) Frequent passage of fluid stools: a symptom of many types of illness. (cf. DYSENTERY; see also SCOURS.) Diarrhoea may be due to e.g. (i) the effect on the gut mucosa of particular microbial toxins which cause hypersecretion of fluid into the lumen; (ii) failure of the gut to absorb small molecules (peptides, sugars – especially lactose) due e.g. to damage to, or non-functioning of, the mucosa, leading to an osmotic effect in the gut lumen; (iii) abnormal activity of the smooth musculature of the gut wall. Severe diarrhoea may lead to dehydration and loss of electrolyte; treatment includes rehydration with solutions containing Na+ , Cl− , Ca2+ , bicarbonate ions and glucose. Some causes of persistent diarrhoea include e.g. CLB and EAGGEC. (See also CRYPTOSPORIDIOSIS and ETEC.) Diaspora See EIMERIORINA. diaspore Syn. PROPAGULE. diastase Syn. AMYLASE. diastatic Able to metabolize starch. diastole See CONTRACTILE VACUOLE. Diatoma See DIATOMS. diatomaceous earth (diatomite; kieselguhr) A siliceous material composed largely of fossil diatom frustules (see DIATOMS), large natural deposits of which are mined in various parts of the world (particularly Lompoc, California). Diatomaceous earth is used e.g. as a mild abrasive (in toothpastes, polishes etc), as an absorbent, as a heat-insulating material, and in various types of filter (including certain microbiological filters: see FILTRATION). diatomite Syn. DIATOMACEOUS EARTH. diatoms A large group (>10000 species) of ALGAE (often regarded as a distinct division – Bacillariophyta) which are essentially unicellular (some are colonial, some form filaments) and which have a characteristic type of CELL WALL consisting typically of a siliceous structure (the frustule) encased in an organic layer. Most cells have brownish chloroplasts containing chlorophylls a, c1 and c2 and e.g. fucoxanthin; storage products include CHRYSOLAMINARIN. Many species are facultatively heterotrophic in the dark; a few marine benthic species are obligately heterotrophic. Diatoms are unusual, if not unique, in having an absolute requirement for silicon – not only for the construction of their walls but also for general metabolism (e.g. in Cylindrotheca fusiformis silicon affects gene expression both directly and indirectly [JGM (1985) 131 1735–1744]). Species occur in aquatic environments, including fresh, brackish and marine waters (where they may be benthic, planktonic, epiphytic etc. – see also AUFWUCHS), and in terrestrial environments such as soil, damp rocks, and even dry rocks and desert sands. Achnanthes exigua is a thermophile (optimum temperature for photosynthesis: 42° C) found in hot springs. Psychrophilic species form a major component of the ‘ice algae’ present on the lower surfaces of ice in polar seas [JGM (1983) 129 1019–1023]. Diatoms occur as endosymbionts e.g. in members of the FORAMINIFERIDA. (See also RHIZOSOLENIA and RHOPALODIA.) The diatom frustule consists of two valves – which, according to species, may each be saucer-shaped, bowl-shaped, boat-shaped etc. – held together by two or more siliceous bands (girdle bands) which, collectively, form the cingulum. In many species one valve (the epivalve or epitheca) is slightly larger than the other (hypovalve or hypotheca); the cingulum, which is attached to the edges of the valves, may overlap the hypovalve to a greater or lesser extent. The valves are characteristically elaborately ornamented with pores, slits, ribs, tubes, projections, 233 diatoxanthin Dick test A SKIN TEST used to determine whether or not an individual is susceptible to SCARLET FEVER. The test procedure (similar to that used in the SCHICK TEST) involves an intradermal injection of the erythrogenic toxin of Streptococcus pyogenes. A positive Dick test (indicating susceptibility) consists of an inflammatory response (at the site of the injection) which becomes evident within ca. 12 hours and reaches peak intensity within ca. 24 hours of the injection. In immune individuals the toxin is neutralized by antitoxin, thus giving a negative test. dicloran An agricultural antifungal agent (2,6-dichloro-4nitroaniline) which is used against various Botrytis infections. It is water-insoluble – hence very persistent – and is usually formulated as a dust; it has very low phytotoxicity. (cf. CHLORONITROBENZENES.) dicloxacillin See PENICILLINS. Dictyocha See SILICOFLAGELLATES. Dictyoglomus A genus of bacteria. One species: D. thermophilum, an anaerobic, caldoactive, asporogenous chemoorganotroph; cells: rod-shaped, occurring in large spherical bodies consisting of a few to many separate cells. [IJSB (1985) 35 253–259.] Dictyosiphon See PHAEOPHYTA. dictyosome (1) One of the stacks of membranous vesicles which form a GOLGI APPARATUS. (2) Syn. Golgi apparatus. dictyosporae See SACCARDOAN SYSTEM. Dictyostelia See EUMYCETOZOEA. dictyostelids See DICTYOSTELIOMYCETES. Dictyosteliia See EUMYCETOZOEA. Dictyosteliomycetes (dictyostelid cellular slime moulds; dictyostelids) A class of cellular SLIME MOULDS (division MYXOMYCOTA). (cf. EUMYCETOZOEA.) The organisms occur mainly in soil, humus and dung, particularly in tropical regions. The vegetative phase consists of myxamoebae which form filose subpseudopodia; flagellated cells are not formed. The life cycle involves a feeding stage, during which myxamoebae ingest food (mainly bacteria) and divide mitotically. When the food supply is exhausted, the amoebae stop growing and, after a period of starvation (pre-aggregation phase or ‘interphase’), the aggregation phase begins: numerous myxamoebae converge (chemotactically – see ACRASIN) and join together to form one to many multicellular, macroscopic pseudoplasmodia. In most species, the pseudoplasmodium can migrate over the surface of the substratum and can show tactic responses. Eventually, the pseudoplasmodium stops moving and differentiates to form a multicellular, multispored, stalked fruiting body, the sorocarp (a process known as culmination); the sorocarp stalk consists of a cellulosic ‘stalk tube’ which, when mature, is either hollow or filled with dead cells. The spores are dispersed by wind, rain, etc; on germination, each spore releases a myxamoeba, thus initiating a new cycle. (In some species myxamoebae may not always pass through this life cycle, forming instead MICROCYSTS and/or MACROCYSTS.) Genera: e.g. ACYTOSTELIUM, DICTYOSTELIUM, POLYSPHONDYLIUM. [Dictyostelids – natural history, life cycles, cultivation: Book ref. 144.] Dictyostelium A genus of slime moulds (class DICTYOSTELIOMYCETES) in which the sorocarp stalk tube is filled with a mesh of empty cell walls (cf. ACYTOSTELIUM) and bears a spherical sorus of spores at its apex. In e.g. D. discoideum the sorocarp consists of a single unbranched stalk that tapers from base to tip; the proximal end is flared into a basal attachment disc, and the distal end bears a single, more or less spherical, white to yellowish mass (sorus) of ellipsoidal to reniform spores (each typically ca. 2.5–3.5 × 6.0–9.0 µm). In some species the sorocarp may divisions, maximum cell size must eventually be re-established; this may be achieved by the formation of resting spores or auxospores. Resting spores generally have thick ornamented walls; on germination, the protoplast expands to maximum cell size prior to wall formation. Auxospores are usually or always formed as a result of fusion of gametes, after which the zygote protoplast escapes from the parent wall, expands, and then synthesizes a wall which is initially organic; a siliceous wall is then synthesized. (See also DIATOMACEOUS EARTH.) diatoxanthin See CAROTENOIDS. diatretyne nitrile See POLYACETYLENES. Diatrypales See ASCOMYCOTINA. diauxie The phenomenon in which, when provided with two sources of carbon, an organism preferentially metabolizes one source (completely) before starting to metabolize the other; the two phases of growth are commonly separated by a lag phase in which the organism produces enzyme(s) necessary for the utilization of the second source of carbon. (See also CATABOLITE REPRESSION.) diauxy Syn. DIAUXIE. diazaborines A range of antibacterial agents [JB (1989) 171 6555–6565] which apparently inhibit fatty acid biosynthesis by inhibiting the enzyme enoyl-acyl carrier protein reductase (ENR). diazomycin A See DON. 6-diazo-5-oxo-L-norleucine See DON. diazotroph Any organism capable of NITROGEN FIXATION. dibromoaplysiatoxin See LYNGBYA. dibromomethylisopropyl-p-benzoquinone An inhibitor of PHOTOSYNTHESIS. dibromopropamidine An aromatic diamidine used as an antiseptic; it is active mainly against asporogenous Gram-positive bacteria. DIC DISSEMINATED INTRAVASCULAR COAGULATION. dicarboximides A group of agricultural ANTIFUNGAL AGENTS which are effective against Botrytis cinerea; they are used on a wide range of crops, functioning primarily as surface protectants. Dicarboximides include e.g. iprodione and vinclozolin. Dice–Leraas diagrams Diagrams that are used for identifying and describing trypanosomes on the basis of their measurements (e.g. length, distances between organelles). Dicellomyces See BRACHYBASIDIALES. dicentric (of a eukaryotic chromosome) Having two CENTROMERES. dichlofluanid A sulphur-containing ANTIFUNGAL AGENT used e.g. for the control of Botrytis cinerea in plants, and in antifungal washes for the prevention of mould in buildings. dichlone (2,3-dichloro-1,4-naphthoquinone) A QUINONE ANTIFUNGAL AGENT used as a seed dressing (e.g., for the seeds of legumes or cotton) and as a foliar spray; it is effective against a range of diseases, including apple scab, damping off, etc. Dichlone is toxic to certain plants, and is also effective in controlling BLOOMS of certain cyanobacteria. dichloramine T See CHLORINE. dichloroisocyanurate (DCCA) (as an antimicrobial agent) Sodium or potassium dichloroisocyanurate, available in powder or tablet form, hydrolyses in water to form hypochlorous acid (see HYPOCHLORITES). When dry, DCCA salts are stable at room temperature. dichlorophane (dichlorophene) See BISPHENOLS. 3-(3,4-dichlorophenyl)-1,1-dimethylurea See DCMU. Dichothrix See RIVULARIACEAE. 234 Didinium substratum, exhibiting e.g. aerotaxis, phototaxis and thermotaxis. The slime sheath does not move relative to the substratum, and may provide traction against which the cells within can move; thus, as the slug moves forwards, a slime trail of collapsed sheath material remains behind. New sheath material is synthesized along the length of the slug, so that the sheath is thinnest at the anterior end, becoming progressively thicker towards the posterior. A migrating slug can split into two smaller slugs, or two slugs can merge to form a larger slug; the size (but not the proportions) of the sorocarp which eventually develops depends on the size of the slug at culmination. The fates of the cells during culmination are apparently determined at the slug migration stage (i.e., before culmination begins): the cells making up the anterior third of the slug are prestalk cells, destined to form the sorocarp stalk, while the cells of the posterior two-thirds are prespore cells. [Patterns of cell differentiation within the slug: Book ref. 67, pp. 255–274.] The culmination process begins when the slug stops moving and reorientates such that the leading (anterior) tip is raised vertically above the rest of the cell mass, forming a nipple-like apical projection. A short cellulosic ‘stalk tube initial’ is secreted by cells near the apex; the entire cell mass then flattens, bringing the tube down through the cell mass to the substratum. Prestalk cells within the lower end of the tube enlarge, become vacuolated and compacted, and eventually die; the stalk lengthens upwards – the remaining prestalk cells progressively undergoing vacuolation and death to result eventually in the characteristic mesh of cellulose cell walls which fills the cellulose stalk tube. Meanwhile, the mass of prespore cells is gradually elevated on the growing stalk, becoming progressively differentiated into spores from the periphery to the centre of the mass. Differentiation is complete when all the cells have become either stalk cells or spores. The sorocarp is usually orientated such that the sorus is at the maximum distance from the substratum or from adjacent objects (including other sorocarps); this orientation is believed to be due to a tropism regulated by an (unidentified) gas or vapour produced by the developing sorocarp. The spores have thick cellulosic walls and are resistant to e.g. desiccation. Germination generally occurs only in the presence of an adequate supply of amino acids. Spores in masses fail to germinate owing to the presence of an autoinhibitor apparently produced during culmination. In D. discoideum macrocysts may be formed, but microcysts have not been observed. Dictyota See PHAEOPHYTA. Dictyuchus See SAPROLEGNIALES. dicyandiamide (DCD; Didin) Cyanoguanidine, a NITRIFICATION INHIBITOR. [Mineralization of DCD in acid soils: SBB (1985) 17 253–254.] N,N
-dicyclohexylcarbodiimide See DCCD. didanosine See NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS. didemnins Cyclic depsipeptide ANTIVIRAL AGENTS isolated from Caribbean tunicates (sea-squirts); they are probably too cytotoxic for therapeutic use. dideoxy fingerprinting See TUBERCULOSIS (antibiotic resistance testing). dideoxy sequencing See DNA SEQUENCING. dideoxyribonucleotide See NUCLEOTIDE (figure legend). dideoxythymidine triphosphate See DDTTP. Didesmis See GYMNOSTOMATIA. Didin See DICYANDIAMIDE. Didinium A genus of freshwater ciliates (subclass GYMNOSTOMATIA). Cells: radially symmetrical, ovoid or barrel-shaped, ca. be sparingly and irregularly branched (cf. POLYSPHONDYLIUM); in D. polycephalum several sorocarps occur in a coremium-like cluster in which the stalks are fused over much of their length, becoming free at their distal, spore-bearing ends. Sorocarps of Dictyostelium spp vary in size from e.g. ca. 0.2–0.6 mm in D. deminutivum to 30 mm or more in D. giganteum; those of D. discoideum may reach a maximum height of ca. 4.0–4.5 mm, but are usually smaller. [Species descriptions and key: Book ref. 144, pp. 246–367.] During the feeding stage of the life cycle, Dictyostelium myxamoebae feed mainly on bacteria, but sometimes ingest other myxamoebae of the same species (‘cannibalism’); D. caveatum is unusual in that it feeds extensively on the myxamoebae of other slime moulds as well as on bacteria. [Self/non-self recognition in D. caveatum: JCB (1986) 102 298–305.] D. discoideum is the best-known species, being widely used in studies on differentiation and cell-cell interactions. It is easily cultured in the laboratory (e.g. on hay infusion agar or SM MEDIUM, with Escherichia coli or Klebsiella pneumoniae as food); some strains can be grown in cell-free media. (The main natural habitat of D. discoideum is apparently deciduous forest soils and leaf-litter.) The vegetative amoebae are commonly ca. 13–16 × 9–11 µm; they are haploid and uninucleate, and contain one or more contractile vacuoles, food vacuoles, etc. They are attracted to their prey by CHEMOTAXIS, chemoattractants apparently including e.g. folic acid released by the bacteria. When food is exhausted, cells enter the pre-aggregation phase (see DICTYOSTELIOMYCETES) during which the myxamoebae discharge their food vacuoles, become smaller, show altered staining properties, and lose the ability to respond chemotactically to folic acid but acquire the ability to be attracted by cyclic AMP (cAMP). Each cell contains six chromosomes, the largest being chromosome 2 (about 25% of the genome). [Sequence and analysis of chromosome 2: Nature (2002) 418 79–85.] In D. discoideum the aggregation phase begins when a subset of myxamoebae begins to secrete cAMP (an ACRASIN) in slow, rhythmic pulses. The cAMP diffuses out from these cells and binds to cell-surface cAMP receptors on nearby myxamoebae. Binding of cAMP to the receptors triggers several responses in the cell: e.g., it (transiently) stimulates adenylate cyclase activity within the cell, resulting in greatly increased production and release of cAMP from the cell, and it triggers migration of the cell towards the source of the cAMP. Thus, the pulses of cAMP are progressively amplified and relayed from the initial subset of cAMP-producing cells throughout the population, resulting in the formation of converging streams of cells which move in regular steps with intervening ‘rest’ periods. In order to maintain the cAMP gradient and to prevent swamping of the receptors by excess cAMP, the concentration of extracellular cAMP is strictly controlled by regulation of the relative proportions of (at least) two proteins: a cAMP phosphodiesterase (which occurs in both soluble extracellular and membrane-bound forms, and which hydrolyses cAMP), and a glycoprotein phosphodiesterase inhibitor (which binds to and inactivates the soluble – but not the bound – form of the enzyme). The aggregated cells adhere to one another, eventually forming a pseudoplasmodium. The multicellular, elongated pseudoplasmodium (called a ‘slug’ or ‘grex’) consists of a mass of cells enclosed within a slime sheath which is composed of cellulose microfibrils embedded in a protein- and carbohydrate-containing matrix. The slug varies in size (e.g. ca. 0.5–2.0 mm long) depending on the number of cells it contains. It migrates over the surface of the 235 Didymella site, the 3
-AAA . . . tail on polyadenylated mRNA molecules. The task is made manageable by converting only some of the (many) mRNAs from each population; selectivity is achieved with a primer such as 5
-TT . . . TTGG-3
which permits conversion of only those mRNAs in which a cytosine (C) residue occurs in the appropriate position after the 3
-AAA . . . tail. cDNAs from each of the populations are then fingerprinted by subjecting them to ARBITRARILY PRIMED PCR and separating the products by gel electrophoresis. When fingerprints from different populations are compared, any band(s) of interest – such as band(s) present in one fingerprint but not in other(s) – can be further examined by removal from the gel and amplification by the same arbitrary primer; the amplified fragments may be e.g. sequenced and/or used to probe a library. [Differential display methodology: NAR (1998) 26 5537– 5543; review: Biotechniques (2002) 33 338–346.] differential host (plant pathol.) A plant host which, on the basis of disease symptoms, serves to distinguish between various strains or races of a given plant pathogen. differential interference-contrast microscopy See MICROSCOPY (d). differential medium See MEDIUM. diffluent Readily dissolving or breaking up in water. Difflugia A genus of testate amoebae (order ARCELLINIDA) in which the test is reinforced with sand grains, diatom frustules, sponge spicules etc which are initially ingested by the cell. D. urceolata is multinucleate and has a rounded test (ca. 200–230 × 150–200 µm) with a pointed top and a rim around the ventral aperture through which several narrow, round-ended pseudopodia emerge. Difflugia spp occur in ponds, swamps, bogs, soil etc. diffusely adherent E. coli See ENTEROADHERENT E. COLI. diffusion-coupled gradostat See GRADOSTAT. diffusion test (1) In ANTIBIOTIC-sensitivity testing: any test which involves the diffusion of antibiotic(s) through agar. See e.g. DISC DIFFUSION TEST and AGAR DIFFUSION TEST. (2) (for membrane filters) A test used to determine the physical integrity of a membrane filter (see FILTRATION). If a wetted membrane is subjected to a pressure lower than its bubble point pressure (see BUBBLE POINT TEST), some air can pass through the pores by simple diffusion; the amount of air transmitted in this way can be significant in filters of large surface area. In the diffusion test a wetted filter is subjected to pressure at ca. 80% of its bubble point pressure, and the volume of air transmitted is determined; the integrity of the filter is assessed by comparing this volume with that expected from an intact filter. DL-a-difluoromethylornithine See SLEEPING SICKNESS. difolatan Syn. CAPTAFOL. DIG Abbreviation for DIGOXIGENIN. digenetic Refers to a parasite which carries out part of its life cycle in each of two different host species. (cf. HETEROXENOUS.) di George syndrome (congenital thymic aplasia) The condition, resulting from an undeveloped thymus gland, which is characterized by a lack of T LYMPHOCYTES; those with this syndrome are susceptible to a range of infections, especially by intracellular pathogens. The condition has been treated by transplantation of fetal thymus gland (using tissue from a fetus at <14 weeks’ gestation). digitonin A mixture of steroid SAPONINS found in the seeds of the purple foxglove (Digitalis purpurea). digoxigenin (DIG) A (poisonous) steroid obtained from species of the plant Digitalis. Digoxigenin is used e.g. as a ‘tag’ for non-radioactive labelling of oligonucleotide probes (both RNA and DNA) – see e.g. DOT-BLOT. 60–200 µm in length; the flattened apical region has a prominent cone-shaped ‘proboscis’ at the apex of which is the cytostome. The cytopharyngeal region is reinforced with nematodesmata. The cell may be encircled by two pectinellae: one somewhat posterior of the midline, the other surrounding the base of the proboscis. The macronucleus is U-shaped. Didinium feeds primarily or exclusively on Paramecium; in the absence of food Didinium encysts. (See also TOXICYST.) Didymella A genus of saprotrophic and plant-parasitic fungi of the order DOTHIDEALES; anamorphs occur in the genera ASCOCHYTA and PHOMA. [Taxonomy: CJB (1981) 59 2016–2042.] Didymium See MYXOMYCETES. didymosporae See SACCARDOAN SYSTEM. Dienes phenomenon (Dienes reaction) The failure of two swarms of different strains of Proteus growing on the same nutrient agar plate to penetrate each other, with the result that a sharp line of demarcation always occurs between them; the phenomenon does not occur between two swarms of the same strain. The Dienes phenomenon is used to distinguish between strains of Proteus in epidemiological studies – the presence of a demarcation line (a positive reaction) being regarded as evidence for strain difference. (The results of a negative Dienes reaction may not be entirely reliable [Book ref. 22, p. 491].) The Dienes phenomenon appears to result from the production of BACTERIOCINS. Dientamoeba A genus of protozoa of the TRICHOMONADIDA. D. fragilis occurs in the large intestine in man; it is nonpathogenic. The trophozoites (5–20 µm diam.) contain numerous vacuoles and (usually) two nuclei, each nucleus lacking peripheral chromatin and containing (usually) four closely grouped karyosomes. Pseudopodia are blunt and are formed slowly. Cysts are not formed. Dieterle silver stain (Dieterle silver impregnation stain) A complex, non-specific, silver-impregnation staining procedure used e.g. for staining spirochaetes, cells of Legionella in tissue sections, etc.; stained bacteria appear brown or black. [Method: Book ref. 53, p. 1388.] N,N-diethyl-p-phenylenediamine (DPD) See WATER SUPPLIES. diethylpyrocarbonate (DEPC; C2 H5 .O.CO.O.CO.C2 H5 ) DEPC is an effective inhibitor of yeasts and moulds once used as a preservative in wines etc.; owing to toxicity (and suspected carcinogenicity) it is no longer a permitted preservative in the USA and UK. In molecular biology, DEPC is useful as a reagent for the carbethoxylation of exposed N-7 groups of (unpaired) adenine residues in DNA or RNA. It also (covalently) inactivates RNases, and is used for treating water, solutions, glassware etc. to prevent degradation of sample RNA by extraneous RNases; whenever possible, water or solutions should be treated with DEPC (0.1%) for a minimum of 1 hour at 37° C and subsequently autoclaved to eliminate any remaining DEPC. Dieudonné alkaline blood agar A medium, containing defibrinated ox blood and sodium hydroxide, formerly used for the primary isolation of Vibrio cholerae. (cf. MONSUR MEDIUM.) difference spectrum (of cytochromes) See CYTOCHROMES and CO DIFFERENCE SPECTRUM. differential centrifugation See CENTRIFUGATION. differential display A technique for detecting those (primarily eukaryotic) genes which are expressed only under specific conditions; it involves isolation, and comparison, of mRNAs from two or more populations, each population having been exposed to different conditions. Initially, mRNAs from each population are converted to cDNAs; this is done by using, as a primer-binding 236 dimixis than those with lower coefficients – e.g. chlorhexidine (h = 2), QACs (h = 1). dilution end-point assay See END-POINT DILUTION ASSAY. dilution rate (D) In CONTINUOUS CULTURE: the parameter F/V in which F is the rate at which medium enters (and leaves) the culture vessel, and V is the volume of liquid in the vessel. Dilution rate has the same dimensions as SPECIFIC GROWTH RATE (i.e., 1/time) and its unit is hour−1 . Under steady-state conditions dilution rate is numerically equal to specific growth rate. The critical dilution rate (Dc ) is the dilution rate which corresponds to mmax ; if the dilution rate exceeds Dc the culture is eventually diluted to extinction and is said to have undergone ‘wash-out’. dilution test In ANTIBIOTIC-sensitivity testing: a procedure in which an organism is tested for its ability to grow in the presence of certain fixed concentrations of a given antibiotic. (cf. DIFFUSION TEST.) In the agar dilution test (= plate dilution test) a series of agar plates containing progressively lower concentrations of a given antibiotic (and an antibiotic-free control plate) are each surfaceinoculated with the test organism and incubated; the MIC (q.v.) is indicated by the lowest concentration of antibiotic at which growth does not occur. The broth dilution test (= tube dilution test) is essentially similar (in principle and interpretation) to the agar dilution test except that growth of the test organism is attempted in each of a series of broths containing progressively lower concentrations of antibiotic. If each broth is ca. 0.2 ml or less the process may be called a microdilution broth test; otherwise it is a macrodilution broth test. Dimargaritales See ZYGOMYCETES. Dimastigamoeba gruberi A former name of Naegleria gruberi. dimension (1) (serol.) In GEL DIFFUSION procedures: the mode in which antibody and antigen come together in order to form precipitate. In single dimension tests, antigen and/or antibody diffuse through the gel in a single direction – i.e., parallel to the long axis of a gel column. In double dimension tests, a given reactant (antigen and/or antibody) diffuses radially outwards from a hole or ‘well’ cut into the gel. (See also SINGLE DIFFUSION and DOUBLE DIFFUSION.) (2) Either stage of TWO-DIMENSIONAL ELECTROPHORESIS. Dimerella See GYALECTALES. dimethirimol (5-n-butyl-2-dimethylamino-4-hydroxy-6-methylpyrimidine) A HYDROXYPYRIMIDINE ANTIFUNGAL AGENT which is active against powdery mildews of chrysanthemums, cucurbits and sugar beet. dimethyldithiocarbamate See DMDC. dimethylglyoxal Syn. DIACETYL. dimethyloxazolidinedione (DMO) See CHEMIOSMOSIS. dimethylsulphoxide (DMSO; (CH3 )2 SO) A non-ionized polar solvent used e.g. as a cryoprotectant in FREEZING. Both hydrophilic and lipophilic substances are soluble in DMSO. dimetridazole (Emtryl) 1,2-Dimethyl-5-nitro-imidazole: a NITROIMIDAZOLE antimicrobial agent used in veterinary medicine e.g. for the prevention and treatment of BLACKHEAD in turkeys and of SWINE DYSENTERY. dimictic Of, or pertaining to, DIMIXIS. dimidiate (1) Split into two. (2) Lacking – or appearing to lack – one half (applied e.g. to a semicircular, non-stipitate fungal fruiting body). dimitic See HYPHA. dimixis (1) Morphological HETEROTHALLISM. (2) One-locus twoallele physiological heterothallism. Incorporation of digoxigenin into RNA probes can be carried out by in vitro transcription on a DNA template of the target sequence using DIG-UTP in the reaction mixture; the DNA template itself may be prepared e.g. as a cloned (then linearized) target sequence carrying a promoter at the 3
end. DIG-dUTP is incorporated into DNA probes during synthesis; this tagged nucleotide can be used as a substrate by a range of polymerases, including e.g. the KLENOW FRAGMENT, Taq DNA polymerase, phage T4 DNA polymerase, and at least some reverse transcriptases. DIG-dUTP can also be covalently bound to the 3
end of DNA strands using the enzyme terminal nucleotidyltransferase. diguanides (as antimicrobial agents) See e.g. CHLORGUANIDE and CHLORHEXIDINE. dihydrofolate reductase See FOLIC ACID. dihydrofolate reductase inhibitor See FOLIC ACID ANTAGONIST. dihydromocimycin See POLYENE ANTIBIOTICS (b). dihydrowyerol See WYERONE. dihydroxyacetone fermentation A commercial aerobic FERMENTATION (sense 2) in which GLYCEROL is oxidized to dihydroxyacetone by strains of Gluconobacter oxydans (‘Acetobacter suboxydans’). The product is used in the pharmaceutical industry and e.g. as a sun-tanning agent. dihydroxyacetone pathway Syn. XMP PATHWAY. 9-(3,4-dihydroxybutyl)-guanine See DHBG. 9-(1,3-dihydroxy-2-propoxymethyl)-guanine See DHPG. (S)-9-(2,3-dihydroxy-1-propoxymethyl)-guanine See DHPG. (S)-9-(2,3-dihydroxypropyl)-adenine See DHPA. diiodohydroxyquin See 8-HYDROXYQUINOLINE. diisopropylfluorophosphate See PROTEASES. dikaryon (dicaryon) (1) A binucleate cell, or a mycelium composed of binucleate cells, in which the two nuclei are haploid and are genetically dissimilar. (See also secondary HYPHA.) (2) A pair of (usually genetically dissimilar) nuclei. dikaryophase In the life cycles of some fungi: a phase characterized by dikaryotic mycelium (see DIKARYON sense 1). dikaryotization Any process which leads to the formation of a DIKARYON (sense 1); it typically involves SOMATOGAMY or SPERMATIZATION in which one mononucleate cell donates its nucleus to another. (See also BULLER PHENOMENON.) Dileptus A genus of carnivorous, cyst-forming ciliates (subclass GYMNOSTOMATIA). Cells: elongate, up to ca. 500 µm in length, the anterior end having a long TOXICYST-bearing ‘neck’ region at the base of which is the cytostome and its associated nematodesmata; the posterior end is pointed. diloxanide A synthetic amoebicide which, as e.g. diloxanide furoate (‘furamide’), is effective in the treatment of some forms of amoebiasis. dilution coefficient (concentration exponent; h) Of a given DISINFECTANT: a number which indicates the effect of dilution on the rate of disinfection under standardized conditions. The concentration and killing rate of a disinfectant are related by the equation: tch = k where t is the time required for 100% kill of cells in a test suspension, c is the concentration of the disinfectant, h is the dilution coefficient, and k is a constant. For example, if a given undiluted disinfectant with an h-value of 4 is diluted by a factor of 2, the expression ch becomes (1/2)4 , i.e. 1/16; thus t must increase 16-fold in order for tch to remain numerically constant. Disinfectants with high dilution coefficients – e.g. ethanol (h = 10) and phenol (h = 6) – lose activity with dilution more 237 dimorphic fungi perpendicularly from the girdle into the hypocone. One of the two flagella (the transverse flagellum) lies in the girdle and encircles the cell, while the other (longitudinal ) flagellum arises from the sulcus and may project beyond the cell. The transverse flagellum [form and function: JP (1985) 32 290–296] is longer than the longitudinal one, is helical in shape, contains a PARAXIAL ROD, and bears a single row of fine hairs; the longitudinal flagellum may be smooth or may bear two rows of stiffer hairs. (cf. e.g. PROROCENTRUM; see also PUSULE.) Dinoflagellate cells contain contractile, non-actin filaments which are apparently responsible for cell contraction and shape changes in at least some species (Kofoidinium and related genera [Cell Mot. (1985) 5 1–15]). The dinoflagellate nucleus contains chromosomes which are unique among eukaryote chromosomes in lacking centromeres and containing little or no protein; dinoflagellate nuclear organization has been considered to be intermediate between prokaryotic and eukaryotic, and has been termed ‘mesokaryotic’ or ‘dinokaryotic’. During MITOSIS the nuclear membrane remains intact; the spindle is typically extranuclear – spindle MTs passing through the nucleus via channels lined with nuclear membrane – but in Oxyrrhis marina the spindle is wholly intranuclear [JCS (1986) 85 161–175]. The majority of dinoflagellates are photosynthetic, typically possessing several chloroplasts per cell and containing chlorophylls a and c2 , b-carotene, and peridinin (replaced by fucoxanthin in Glenodinium foliaceum). Other dinoflagellates (e.g. NOCTILUCA) are colourless and heterotrophic. Both heterotrophic and photosynthetic species can apparently ingest particulate matter in the region of the sulcus. Resistant ‘resting spores’ (cysts, hypnospores) are produced in some species. [Cyst formation in Gonyaulax tamarensis: JEMBE (1985) 86 1–13.] (cf. HYSTRICHOSPHAERIDS.) Sexual reproduction has been observed in some species. The majority of dinoflagellates are free-living in marine, brackish or freshwater habitats, sometimes forming extensive blooms (see e.g. RED TIDE). Some dinoflagellates (e.g. species of Gonyaulax, Noctiluca, Pyrocystis, Pyrodinium) exhibit BIOLUMINESCENCE, and some produce potent toxins (see e.g. SAXITOXIN). Dinoflagellates also occur in association with other organisms: e.g. Blastodinium is parasitic in the intestines of copepods, Oodinium is a parasite of fish (see VELVET DISEASE), and ‘Symbiodinium’ is an endosymbiont in various invertebrates (see ZOOXANTHELLAE). Dinoflagellida See PHYTOMASTIGOPHOREA. dinokaryotic See DINOFLAGELLATES. Dinophyceae See DINOFLAGELLATES. Dinothrix A genus of marine DINOFLAGELLATES in which the cells are non-motile and are joined together to form branching filaments. Diodoquin See 8-HYDROXYQUINOLINE. dioecious Refers to an organism in which the male and female reproductive structures occur in different individuals. (cf. MONOECIOUS.) dioecism Morphological HETEROTHALLISM. 1,2-dioxetane A type of substrate which gives rise to CHEMILUMINESCENCE when activated by certain enzymes (examples: AMPPD and CSPD). dioxygenase See OXYGENASE. dip-slide method (agar-slide method) A method for detecting or estimating microbial populations on solid surfaces and in various fluids (e.g. machine-tool cooling fluids, urine samples). The dipslide, a plastic slide coated with a sterile nutrient agar, is attached dimorphic fungi (1) Those fungi (e.g. species of Blastomyces and Candida) which – according to environmental conditions – can grow in either of two distinct vegetative states: a unicellular (‘yeast-phase’) state or a mycelial state. (See GROWTH (fungal); MORPH.) (See also QUORUM SENSING.) (2) Fungi which exhibit DIPLANETISM. (3) Syn. DIOECIOUS fungi. din genes In Escherichia coli, DNA damage-inducible genes: genes inducible in the SOS SYSTEM. For dinB see DNA POLYMERASE IV. dinactin See MACROTETRALIDES. dinitrogen fixation Syn. NITROGEN FIXATION. dinitrogenase Syn. NITROGENASE. 2,4-dinitrophenol See PROTON TRANSLOCATORS. dinitrophenols Dinitrophenols exhibit antifungal, insecticidal and herbicidal properties. As ANTIFUNGAL AGENTS they are used mainly for the treatment of powdery mildews; they include binapacryl (2,4-dinitro-6-sec-butyl-phenyl-3-methylcrotonate), which is also an acaricide, dinobuton (2,4-dinitro-6-sec-butylphenylisopropylcarbonate), and dinocap (a mixture of three isomers of each of the compounds 2,4-dinitro-6-sec-octylphenol and 2,6-dinitro-4-sec-octylphenol in which the octyl group may be 1-methylheptyl, 1-ethylhexyl or 1-propylpentyl). Dinocap is generally used as the crotonate; it functions as an antifungal ERADICANT and PROTECTANT. (See also PROTON TRANSLOCATORS.) Dinobryon A genus of loricate, generally colonial CHRYSOPHYTES. Each cell has two flagella of unequal length, and sits in a stalked, vase-shaped LORICA composed of helical bands of cellulose microfibrils which are laid down by the slowly rotating cell within. Following cell division, one daughter cell becomes attached to the rim of the parent lorica and secretes a new lorica around itself, so that eventually a branching, fan-shaped colony is formed. dinobuton See DINITROPHENOLS. dinocap See DINITROPHENOLS. dinoflagellates A large group of photosynthetic and/or heterotrophic organisms regarded either as algae (e.g. class Dinophyceae, division Dinophyta) or as protozoa (see PHYTOMASTIGOPHOREA). Most dinoflagellates are unicellular and biflagellated (but cf. e.g. CYSTODINIUM, DINOTHRIX and GLOEODINIUM). The cell typically has a theca composed essentially of a layer of flattened membranous vesicles beneath the plasmalemma; these vesicles may be empty (in ‘unarmoured’ dinoflagellates such as Gymnodinium, Gyrodinium, and Oxyrrhis) or may each contain a cellulosic plate varying from very thin (e.g. in Katodinium and Woloszynskia) to thick (in ‘armoured’ dinoflagellates such as Ceratium and Peridinium). The edges of thick plates are commonly bevelled, allowing the plates to move freely relative to each other. The number of plates per cell ranges from two (e.g. in PROROCENTRUM) to several hundred, the number, form and arrangement of plates being characteristic for a given dinoflagellate. (See also e.g. CERATIUM; ORNITHOCERCUS.) On cell division, the thecal plates may be shared between the daughter cells, each cell then synthesizing new plates to replace those taken by the other; in e.g. Gonyaulax shedding of the theca (‘ecdysis’) by the parent cell occurs prior to cell division. In the majority of dinoflagellates (including e.g. Amphidinium, Ceratium, Glenodinium, Gonyaulax, Gymnodinium, Oxyrrhis, Peridinium and Woloszynskia) the vegetative cell is divided by a transverse groove (the girdle, cingulum or annulus) into two parts: the epicone and the hypocone. (The corresponding thecal regions in armoured dinoflagellates are termed the epitheca and hypotheca.) A longitudinal groove, the sulcus, extends 238 diplococcus to the inside of the cap in a screwcap cylindrical tube; for use, the cap is unscrewed and the slide is dipped into, or pressed against, the sample and it is subsequently incubated. Microbial numbers can be estimated from the number of colonies which develop on the slide. (See also COUNTING METHODS.) diphasic medium Any MEDIUM, e.g. NNN MEDIUM, in which a solid (usually agar-based) component is at least partly covered by a liquid ‘overlay’. diphasic serotypes See H ANTIGEN (Salmonella). diphenyliodonium chloride See IODONIUM COMPOUNDS. diphenylthiourea See THIOUREA DERIVATIVES. diphenylurea See CARBANILIDES. diphosphopyridine nucleotide See NAD. diphthamide See DIPHTHERIA TOXIN. diphtheria (membranous croup) An acute infectious human disease, usually affecting children, caused by toxinogenic strains of Corynebacterium diphtheriae. Infection occurs by droplet inhalation or ingestion of contaminated food, milk etc. Asymptomatic carriers are often sources of infection. Incubation period: 1–10 (usually 2–5) days. Symptoms: fever, sore throat, headache, and the development of a characteristic whitish or grey membrane at the site of infection – usually the tonsils but sometimes e.g. the nasopharynx, larynx, or nose; the membrane and swelling of adjacent tissues can cause respiratory obstruction and difficulty in swallowing. Other sites occasionally affected include skin lesions or wounds (cutaneous diphtheria) and the genitals. The membrane results from the local effects of the DIPHTHERIA TOXIN which causes cell death and subsequent deposition of fibrin, blood cells, cellular debris, and C. diphtheriae cells. Local secondary infection with e.g. Streptococcus pyogenes (septic diphtheria) may occur. C. diphtheriae normally remains localized at the membrane, but the toxin is disseminated via the blood and lymph and can cause severe, often fatal, systemic effects – including myocarditis and nerve demyelination. Treatment : primarily with antitoxin. Penicillin G or erythromycin may be used in treatment and for eradication of C. diphtheriae from carriers. Lab. diagnosis: culture of C. diphtheriae (see CORYNEBACTERIUM) from swabs, and/or identification of the toxin (e.g. by gel diffusion tests). [Review: RMM (1996) 7 31–42.] diphtheria toxin A heat-labile protein exotoxin (MWt ca. 60000) produced by certain lysogenic strains of Corynebacterium diphtheriae and responsible for the symptoms of DIPHTHERIA; it is lethal for many animal species and is also cytotoxic in some types of cell culture – e.g. Vero cells are highly susceptible, though murine cells are almost totally resistant. (cf. EXOTOXIN A.) Diphtheria toxin (DT) is encoded by tox, a gene carried by various temperate CORYNEPHAGES (e.g. b, !, g) [tox + corynephages (DNA relationships): Inf. Immun. (1985) 49 679–684]. The toxin is synthesized as a single polypeptide chain. The Yshaped molecule consists of three domains: translocation domain, receptor-binding domain (RBD) and catalytic domain (CD). Uptake of toxin involves initial binding of the RBD to a receptor protein, and internalization of the toxin within a vesicle via a clathrin-coated pit (PINOCYTOSIS). In the vesicle, CD is proteolytically cleaved but remains attached via a disulphide bond; the translocation domain undergoes an acid-induced conformational change in which it inserts into the membrane of the vesicle – apparently forming a pore through which CD is translocated into the cytoplasm of the target cell. The disulphide link undergoes reduction (scission) – releasing CD into the target cell’s cytoplasm. CD is a heat-stable fragment with resistance to cellular proteolysis. It has ADP-ribosyltransferase activity, and, in the cell, inhibits PROTEIN SYNTHESIS by catalysing the ADP-RIBOSYLATION (inactivation) of elongation factor EF-2. (One molecule of CD can kill a sensitive cell.) In EF-2, the target for ADP-ribosylation is a rare amino acid, ‘diphthamide’, (2-[3-carboxyamido-3(trimethylammonio)propyl]-histidine); diphthamide appears to occur only in EF-2 – a single residue lying within a sequence which is highly conserved in eukaryotes. [Review: TIBS (1987) 12 28–31.] Elongation factors in members of the Archaea (formerly Archaebacteria) appear also to contain a single diphthamide residue, and are characteristically sensitive to diphtheria toxin [Book ref. 157, pp. 379–410]. Bacteria are not susceptible to the toxin. Synthesis of DT is repressed while the intracellular concentration of iron remains above a certain level. Repression of tox is mediated by a repressor protein, DtxR, which, when activated by transition metal ions, binds to the operator of the tox gene and blocks transcription [see Nature (1998) 394 502–506]. diphtheroid (1) Any non-pathogenic strain of Corynebacterium, particularly one isolated from the skin or mucous membranes. (2) Any strain of Corynebacterium other than those of C. diphtheriae. (3) Syn. CORYNEFORM (2). diphtheroid stomatitis A disease of adult chickens, apparently caused by ‘Spirillum pulli ’ (see SPIRILLUM). Yellowish-white, firm lesions (ca. 2–20 mm), adherent to the underlying tissues, occur in the mouth, larynx and pharynx. (See also POULTRY DISEASES.) dipicolinic acid (DPA) Pyridine-2,6-dicarboxylic acid; DPA occurs in bacterial ENDOSPORES, and has been reported e.g. in Penicillium citreoviride. In bacteria, dipicolinic acid is synthesized via a branch of the diaminopimelic acid pathway [see Appendix IV(e)]. [Characterization of spores of (mutant) Bacillus subtilis which lack dipicolinic acid: JB (2000) 182 5505–5512.] diplanetism In certain OOMYCETES: the phenomenon in which two types of morphologically distinct zoospore are formed at separate stages (swarm periods) in the asexual life cycle. Primary zoospores are formed in the first swarm period; in e.g. Achlya and Saprolegnia, primary zoospores are pyriform with two flagella arising at the apex. After release from the sporangium, primary zoospores may encyst directly (e.g. in Achlya) or may swim for some time before encysting (e.g. in Saprolegnia). A cyst subsequently gives rise to a secondary zoospore; in e.g. Achlya and Saprolegnia, secondary zoospores are reniform with two flagella arising from the concavity. After a period of swarming, secondary zoospores encyst and subsequently germinate to form a vegetative thallus. (cf. MONOPLANETISM and POLYPLANETISM.) Diplocalyx See SPIROCHAETALES. Diplocarpon A genus of fungi of the order HELOTIALES; anamorphs occur in the genera Marssonina and Entomosporium. D. earlianum causes strawberry leaf scorch. D. rosae causes BLACK SPOT (sense 3) of roses; the blackish acervuli of the Marssonina stage, bearing uniseptate conidia (the main disseminative spore), develop on leaves and/or on overwintering lesions on the shoots. The ascigerous stage can occur in overwintering leaves. Diplocarpon rosae virus See MYCOVIRUS. diplococcin An antibiotic, structurally related to NISIN, produced by Lactococcus sp. diplococcus A pair of cocci (see COCCUS) – the form commonly assumed by e.g. Enterococcus faecalis and Streptococcus pneumoniae and by species of Neisseria. 239 Diplococcus pneumoniae Diplococcus pneumoniae Incorrect name for Streptococcus pneumoniae. Diplodia See SPHAEROPSIDALES. Diplodinium A genus of ciliates (order ENTODINIOMORPHIDA) which occur e.g. in the RUMEN. The cytostome is close to the anterior pole of the cell; somatic ciliature is limited, cilia occurring in tufts in the oral and anteriodorsal regions. (See also DZM.) The pellicle may be drawn out posteriorly into one or more spines. Diploicia A genus of placodioid LICHENS (order LECANORALES). Apothecia: lecideine, black; ascospores: brown, one-septate. D. canescens (formerly Buellia canescens) has a whitish thallus bearing whitish-grey farinose soralia; this species is common in Atlantic regions of Europe, growing on rocks, wood, bark etc. – particularly in light shade. diploid Having a PLOIDY of two. (cf. HAPLOID and POLYPLOID.) In a prokaryotic cell, partial diploid or merodiploid refers to the possession of extra copies of only some of the chromosomal genes – see MEROZYGOTE. Diplomastigomycotina See OOMYCETES. Diplomonadida An order of protozoa (class ZOOMASTIGOPHOREA); cells contain either one (suborder Enteromonadina) or two (suborder Diplomonadina) KARYOMASTIGONTS, those containing two exhibiting twofold rotational symmetry or (in Giardia) bilateral symmetry. Each karyomastigont has 1–4 flagella. Mitochondria and Golgi apparatus are absent. Members of the Enteromonadina are parasitic; cysts are formed in at least one genus. Genera: e.g. Enteromonas, Trimitus. In the Diplomonadina each karyomastigont has four flagella, one of which is recurrent. Cysts are formed by at least some species. The suborder includes free-living and parasitic species. Genera: e.g. GIARDIA, HEXAMITA, Trepomonas. Diplomonadina See DIPLOMONADIDA. diplont (1) (noun) An organism in whose life cycle only the gametes are haploid. (cf. HAPLONT.) (2) The diploid form of an organism, e.g. a sporophyte (see ALTERNATION OF GENERATIONS). (3) (diplontic) (adj.) Refers to either (1) or (2) above. diplophage A bacteriophage which infects Streptococcus (‘Diplococcus’) pneumoniae. diplophase In organisms which reproduce sexually: the diploid phase of the life cycle (between syngamy and MEIOSIS). Diplornaviridae A family proposed for all dsRNA-containing viruses; the family has not been accepted since differences in genome structure, replication and expression among dsRNA viruses are too great to be encompassed in a single family. Diploschistes See GRAPHIDALES. diplostichomonad membrane See PARORAL MEMBRANE. diplotene stage See MEIOSIS. Diplotheca See CHOANOFLAGELLIDA. Dipodascaceae See ENDOMYCETALES. Dipodascus See ENDOMYCETALES and GEOTRICHUM; see also AMBROSIA FUNGI. dipyridine ferrohaemochrome See CYTOCHROMES. direct epifluorescent filter technique See DEFT. direct immunofluorescence See IMMUNOFLUORESCENCE. direct repeat (DR) Either of two (or more) regions of a nucleic acid molecule in which the sequences of bases or base pairs are similar or identical and have the same polarity and orientation, e.g.: direct repeat (DR) locus (in Mycobacterium tuberculosis) See SPOLIGOTYPING. directly observed therapy, short-course (DOTS) See TUBERCULOSIS. disc (lichenol.) The hymenial layer of an apothecium; it may be circular (flat or convex), elongated (lirelliform), or irregular in shape. disc diffusion test (also: agar disc diffusion test) In ANTIBIOTICsensitivity testing: a DIFFUSION TEST in which the entire surface of an agar plate is inoculated with the organism under test, and several small absorbant paper discs – each impregnated with a different antibiotic – are placed at different locations on the agar surface; on subsequent incubation, antibiotics diffuse from the discs, and a zone of growth inhibition develops around each disc which contains an antibiotic to which the organism is susceptible. Precise details of the test (e.g. inoculum size, medium) commonly conform to the so-called ‘Kirby–Bauer technique’ [AJCP (1966) 45 493–496]. Strains of Staphylococcus aureus containing an inducible bLACTAMASE commonly form zones of inhibition around discs containing an antibiotic sensitive to the enzyme; these zones are typically bordered by relatively heavy growth containing discrete colonies. (Zones formed by sensitive bacteria generally exhibit a gradual or abrupt transition between regions of growth and non-growth.) Cells close to the disc are killed before adequate amounts of b-lactamase can be synthesized. Cells at the periphery of the zone are exposed to gradually increasing concentrations of the (outwardly diffusing) antibiotic; such cells can synthesize sufficient b-lactamase to permit survival, and they give rise to relatively large colonies by using nutrients forfeited by inactivated cells in their immediate vicinity. (cf. AGAR DIFFUSION TEST.) disclosing agent See DENTAL PLAQUE. discocarp Syn. APOTHECIUM. discoidins See LECTINS. Discomycetes A class of ascomycetes in which the ascocarp is typically an apothecium (though some members, e.g. Tuber spp, form closed, hypogean ascocarps). The class is not recognized in most modern taxonomic schemes. Discorbis See FORAMINIFERIDA. disease A term used loosely for any state of an individual (human, animal or plant) characterized by a deviation from the condition regarded as normal or average for members of the species or type, and which is usually wholly detrimental; exceptionally, a disease may be associated with some benefit: see e.g. sickle-cell anaemia in the entry MALARIA. A disease may be due e.g. to a metabolic or genetic defect, to infection by a PATHOGEN, or to the ingestion of a TOXIN or other poison – but not (in conventional use) to physical wounds. By custom, the term ‘disease’ is generally applied only to multicellular organisms – including certain multicellular microorganisms (such as mushrooms); however, analogous states occur in unicellular organisms: e.g., a bacterium may suffer metabolic or genetic lesion(s), it may be infected e.g. by a virulent bacteriophage, or it may be affected adversely by an antibiotic, a bacteriocin, or some other toxic substance. disinfectant Any chemical agent used for DISINFECTION; disinfectants which are non-injurious to human or animal tissues may also be used as ANTISEPTICS. (cf. PRESERVATIVES.) Disinfectants for general use should be active against a wide range of common pathogens and should be microbicidal rather than microbistatic at the concentrations used; microbicidal disinfectants may become microbistatic when diluted, and at low concentrations some disinfectants can be metabolized by certain 5
. . . GGCT . . . GGCT . . . 3
3
. . . CCGA . . . CCGA . . . 5
The repeated sequences may or may not be contiguous. (cf. INVERTED REPEAT.) 240 dissimilatory sulphate reduction Cytokine activity appears to promote coagulation and also to inhibit anticoagulant activity. Additionally, cytokines upregulate CELL ADHESION MOLECULES on the endothelial membrane; adhesion molecules contribute to coagulopathy by binding monocytes and platelets. Activated platelets are degranulated, releasing various products (including fibrinogen); moreover, the platelet membrane gives rise to the potent vasoconstrictor thromboxane A2 . Initiation of coagulation apparently involves cytokine-mediated upregulation of an endothelial membrane protein, tissue factor (TF), which is believed to play a key role in triggering the coagulation cascade. In an animal model, anti-TF antibodies were able to limit coagulopathy in the presence of Escherichia coli sepsis. In vivo, the inhibitor of TF is a protein synthesized in the endothelium: tissue factor pathway inhibitor (TFPI); in at least some studies, the level of TFPI appears to be reduced in cases of DIC. [Disseminated intravascular coagulation: BCH (2000) 13 179–197.] dissepiment In a polypore: the tissue which lies between, and supports, the pores. (cf. TRAMA.) dissimilatory nitrate reduction Energy-yielding (respiratory) metabolism in which nitrate is reduced to e.g. nitrite, nitrous oxide, dinitrogen or ammonia, the product(s) typically being excreted: see DENITRIFICATION and NITRATE RESPIRATION. (cf. ASSIMILATORY NITRATE REDUCTION.) dissimilatory perchlorate reduction See BIOREMEDIATION. dissimilatory reduction of nitrate to ammonia A respiratory process carried out by certain bacteria (e.g. species of Enterobacter and Vibrio) and reported to occur in habitats such as marine sediments. The process appears to occur more readily when the concentration of nitrate is low – higher concentrations of nitrate being reduced to nitrite and correspondingly smaller amounts of ammonia [FEMS Ecol. (1996) 19 27–38]. (cf. DISSIMILATORY NITRATE REDUCTION.) dissimilatory sulphate reduction (sulphate respiration) The use of sulphate (or other oxidized compound of sulphur) as terminal electron acceptor in the anaerobic respiratory metabolism of the SULPHATE-REDUCING BACTERIA. Initially, sulphate is ‘activated’, i.e., converted to adenosine-5
-phosphosulphate (APS) in an energy-requiring reaction (enzyme: sulphate adenylyl transferase): SO4 2− + ATP −−→ APS + PPi organisms – e.g. Pseudomonas spp can metabolize phenol. A given disinfectant may be effective against only one particular category of organisms (e.g. Gram-negative bacteria), but is not necessarily effective against all species in that category. Most disinfectants are primarily antibacterial, but some are also (or primarily) antifungal or antiviral; most disinfectants have little or no activity against bacterial ENDOSPORES. (See also ACRIDINES; ALCOHOLS; BISPHENOLS; CHLORINE; FORMALDEHYDE; HEAVY METALS; HYPOCHLORITES; IODINE; OXIDIZING AGENTS; PHENOLS; PINE DISINFECTANTS; QUATERNARY AMMONIUM COMPOUNDS; SALICYLIC ACID.) Different disinfectants may require different conditions for maximum effectiveness. Important factors include e.g. pH (cf. CHLORHEXIDINE and HYPOCHLORITES), the presence or absence of organic matter (cf. CHLORINE and ACRIDINES), and humidity (see e.g. FORMALDEHYDE). The detergent properties of QUATERNARY AMMONIUM COMPOUNDS permit them to penetrate grease barriers which may protect organisms from non-detergent disinfectants. In general, the activity of a disinfectant rises with temperature (see TEMPERATURE COEFFICIENT) and concentration (see DILUTION COEFFICIENT). The action of some disinfectants is reversible; thus e.g. endospores inactivated by mercuric ions may germinate after exposure to hydrogen sulphide (which combines with mercuric ions). The efficacy of a disinfectant may be estimated by certain tests – see e.g. CAPACITY TEST; CARRIER TEST; CHICK–MARTIN TEST; KELSEY–SYKES TEST; PHENOL COEFFICIENT; RIDEAL–WALKER TEST; SUSPENSION TEST; USE-DILUTION TEST. disinfection The destruction, inactivation or removal of those microorganisms likely to cause infection or other undesirable effects (e.g. spoilage); disinfection does not normally involve STERILIZATION. The term ‘disinfection’ commonly refers to the use of chemical agents (DISINFECTANTS) for the treatment of inanimate objects and surfaces; however, it is sometimes also used to refer to ANTISEPSIS. Disinfection can also be achieved by physical proceduces such as PASTEURIZATION and TYNDALLIZATION. (See also ULTRAVIOLET RADIATION and LOW-TEMPERATURE STEAM DISINFECTION.) A PULSED ELECTRIC FIELD technique may have applications e.g. in the food industry; this approach has been used e.g. to reduce significantly the numbers of viable cells of Mycobacterium paratuberculosis in milk [AEM (2001) 67 2833–2836]. [Disinfection kinetics: JAM (2001) 91 351–363.] disjunctor cells Cells which occur between spores in a chain of spores; their disintegration may aid spore release. (cf. NECRIDIUM.) disk See DISC. dislodgement (mol. biol.) The phenomenon (mechanism unknown) in which the introduction of a second, compatible, plasmid into a cell sometimes causes the elimination of the resident plasmid. disodium phosphonoacetate See PHOSPHONOACETIC ACID. displacement loop See e.g. D LOOP and R LOOP. displacement vector Syn. REPLACEMENT VECTOR. disporic Of a coccidian oocyst: containing two sporocysts. disseminated intravascular coagulation (DIC) A condition characterized by an imbalance between pro- and anticoagulant activities within the blood vascular system, resulting in the formation of disseminated clots; DIC is usually a result of sepsis, but other causes include tissue necrosis, hypothermia and some types of snake bite. DIC is incited when bacterial components (such as LIPOPOLYSACCHARIDE) bind to specific receptors on endothelium and leukocytes; this stimulates the release of a number of CYTOKINES – including the pro-inflammatory TNF-a, IL-1 and IL-6. This reaction is encouraged by the hydrolysis of PPi; the pyrophosphatase involved is inactivated under aerobic conditions, so that ATP is conserved when growth cannot occur. The reduction of APS is catalysed by adenylyl sulphate reductase in the reaction: APS −−→ AMP + SO3 2− Sulphite is reduced to sulphide by an enzyme complex which, in many species of sulphate-reducing bacteria, includes e.g. DESULFOVIRIDIN. Different intermediates (e.g. thiosulphate, trithionate) have been reported from in vitro studies on sulphite reduction; these intermediates may or may not occur in vivo – or may occur in only some species. A small amount of reduced sulphur is assimilated, but nearly all is eliminated as H2 S (cf. ASSIMILATORY SULPHATE REDUCTION). The reduction of APS and sulphite involves electron transport via e.g. c- and/or b-type cytochromes, and is accompanied by OXIDATIVE PHOSPHORYLATION. In many species electrons are 241 dissimilatory sulphur metabolism DMO 5,5-Dimethyl-4-oxazolidinedione: see CHEMIOSMOSIS. DMSO DIMETHYLSULPHOXIDE. DNA Deoxyribonucleic acid: a NUCLEIC ACID consisting of deoxyriboNUCLEOTIDES, each of which typically contains one of the bases adenine, guanine, cytosine and thymine (cf. e.g. BACTERIOPHAGE and DNA METHYLATION). (See also RNA.) DNA is the repository of genetic information in all cells and in many VIRUSES (see GENETIC CODE). In eukaryotes DNA occurs chiefly in CHROMOSOMES but also in organelles: see MITOCHONDRION (mtDNA), CHLOROPLAST (ctDNA) and KINETOPLAST; in prokaryotes DNA constitutes the CHROMOSOME. (See also PLASMID.) DNA can be detected in situ by specific STAINING (see e.g. ACRIDINE ORANGE, DAPI, FEULGEN REACTION). A single chain of deoxyribonucleotides (single-stranded DNA, = ssDNA) occurs in some viruses, usually as a covalently closed ‘circle’ (ss cccDNA) (see e.g. SSDNA PHAGE; cf. PARVOVIRIDAE). In cells and in many viruses DNA is normally double-stranded (dsDNA, duplex DNA): the two strands are held together by Watson–Crick BASE PAIRing between bases on opposite strands; thus, the base sequence of each strand is complementary to that of the other. The two strands in dsDNA are antiparallel, i.e., orientated ‘head-to-tail’ (5
. . . 3
adjacent to 3
. . . 5
). Under physiological conditions the two strands are usually twisted around a common axis to form a double helix. In its commonest form (the Watson–Crick double helix, or B-DNA) the helix is right-handed (i.e., when viewed from one end it winds away from the viewer in a clockwise direction) with on average ca. 10.4 base pairs (bp) per turn, each turn being ca. 3.4 nm long and ca. 2 nm in diameter. The (planar) bases are stacked roughly perpendicular to the helix axis but with a propeller twist of ca. 11.5–17.3° between the bases in each pair. Two spiral grooves are discernible in B-DNA: the minor groove, located between the two strands, and the major groove, which runs between the turns of the helix. Substituents on the bases protrude into these grooves and are thus accessible e.g. for recognition by enzymes. B-DNA is now thought to have local variations in conformation which seem to reflect the nature of the local base sequences; this may further assist specific recognition of particular base sequences by proteins. In vivo the double helix is stabilized by hydrophobic interactions between the stacked bases, and by neutralization of the charges on the phosphate groups by e.g. divalent metal ions or (possibly) POLYAMINES in bacteria, or by HISTONES in eukaryotes. dsDNA may adopt different helical conformations under different conditions [review: ARB (1982) 51 395–427]. For example, A-DNA is a right-handed helix with ca. 11 bp/turn and bases tilted at an angle to the helix axis; the bases are relatively inaccessible, the sugar–phosphate backbone being the most prominent part of the molecule. A-DNA occurs in vitro e.g. at low (75%) relative humidity in the presence of Na+ , K+ or Cs+ as counterion; in vivo a similar conformation occurs e.g. in DNA–RNA hybrid duplexes and probably also in dsRNA. (cf. Z-DNA; see also HAIRPIN; CRUCIFORM.) In most bacterial chromosomes and plasmids, certain viruses, mitochondria and chloroplasts, the DNA is double-stranded and covalently closed ‘circular’ (ds cccDNA), i.e., it has no free 3
or 5
ends. Such DNA can occur in distinct topoisomeric forms (see TOPOISOMER). Relaxed cccDNA can be regarded as linear B-DNA with covalently joined ends, i.e., the molecule can theoretically lie in a plane, without strain, with a pitch (p) of ca. 10.4 bp/turn. The number of times one strand winds around the other in any ds cccDNA molecule lying in a plane derived from the oxidation of certain organic substrates to e.g. acetate; in e.g. Desulfobacter postgatei and Desulfovibrio baarsii electrons can be derived from the oxidation of acetate to CO2 . Some sulphate-reducing bacteria can grow mixotrophically, gaining energy from the oxidation of H2 (at the expense of sulphate) and using acetate and CO2 as sources of carbon. In vitro inhibition of sulphate reduction can be achieved e.g. with chromate, molybdate, monofluorophosphate or selenate ions; molybdate appears to form an analogue of APS, thus depleting the cell of ATP. (See also SULPHUR CYCLE.) dissimilatory sulphur metabolism See DISSIMILATORY SULPHATE REDUCTION, SULPHUR-OXIDIZING BACTERIA, SULPHUR-REDUCING BACTERIA and THIOBACILLUS. dissociation (bacteriol.) A phenotypic change in a given strain (see e.g. SMOOTH–ROUGH VARIATION). (cf. SALTATION.) dissymmetry ratio See BASE RATIO. distamycin A See NETROPSIN. distemper (vet.) (1) Syn. CANINE DISTEMPER. (2) Syn. STRANGLES. Distigma See EUGLENOID FLAGELLATES. distoseptum See SEPTUM (b). Distrene-80 A high-MWt polystyrene (see DPX). distromatic Refers to a structure which is two cells thick (see e.g. ULVA). (cf. POLYSTROMATIC.) ditalimfos See ORGANOPHOSPHORUS COMPOUNDS. dithanes See MANEB (dithane M-22), NABAM (dithane D-14) and ZINEB (dithane Z-78). dithianon (2,3-dicyano-1,4-dihydro-1,4-dithia-anthraquinone) A QUINONE ANTIFUNGAL AGENT used e.g. for the control of apple scab and other fungal diseases of apples and related fruits; it is not effective against apple mildew. dithiocarbamates Dithiocarbamates (salts of dithiocarbamic acid derivatives: R2 N.CS.SH) include several agricultural ANTIFUNGAL AGENTS (e.g. METHAM SODIUM) and preservatives for paper and board; the antifungal activity of at least some dithiocarbamates appears to depend on their conversion to substances which are more highly fungitoxic – methylisothiocyanate in the case of metham sodium. (See also BISDITHIOCARBAMATES; DMDC; THIURAM DISULPHIDES.) dithiothreitol See MUCOLYTIC AGENT. Diuron Syn. DCMU. divercin V41 A 43-amino-acid BACTERIOCIN produced by Carnobacterium divergens strain V41 [Microbiology (1998) 144 2837–2844]. divergent trama See BILATERAL TRAMA. divergent transcription Transcription of different genes (on the same DNA molecule) in opposite directions. When divergent transcription occurs from closely spaced promoters, the (transcription-dependent) generation of negative supercoiling behind each advancing polymerase causes an increase in negative superhelicity in the region between the active polymerases; this increase in superhelicity may affect the expression of genes or operons [Mol. Microbiol. (2001) 39 1109–1115]. division A taxonomic rank (see TAXONOMY and NOMENCLATURE). dizoic Of a coccidian oocyst: containing two sporozoites per sporocyst. DLVO theory See ADHESION. DMDC DimethylDITHIOCARBAMATE; DMDCs have antifungal (and antibacterial) activity and are used in agriculture as broadspectrum antifungal agents – e.g. for the treatment of fruit and vegetables and as a constituent of seed and turf dressings. DMDCs include ferbam, [(CH3 )2 .N.CS.S]3 Fe, and ziram, [(CH3 )2 .N.CS.S]2 Zn. Oxidation of dimethyldithiocarbamic acid gives THIRAM. 242 DNA chip of Z-DNA) and also the unwinding of DNA strands e.g. during DNA REPLICATION, RECOMBINATION, TRANSCRIPTION, etc. The degree of supercoiling of DNA in vivo is controlled by enzymes called TOPOISOMERASES. Normal cellular function requires that the degree of supercoiling be maintained within certain limits. In some circumstances – e.g. during the transition from aerobic growth to anaerobic growth – there is a fall in the cell’s energy charge and a corresponding, transient fall in ‘global’ (i.e. overall) superhelical density; such changes in superhelicity alter the expression of a range of genes (e.g. by affecting the activity of their promoters). Local increases in superhelicity (i.e. in specific regions of the DNA molecule) can be brought about by divergent transcription from closely spaced promoters; such increased negative superhelicity may affect the expression of particular genes/operons [Mol. Microbiol. (2001) 39 1109–1115]. Pure linear dsDNA clearly cannot be supercoiled. However, linear dsDNA (and discrete regions or domains within linear or ccc dsDNA) can maintain a superhelical conformation if there is a constraint on the rotation of the double helix to prevent unwinding of the supercoils (see CHROMATIN and CHROMOSOME (bacterial)). Certain drugs exert their toxic effects by binding to DNA; the drug may bind e.g. to the major or minor grooves of dsDNA (see e.g. NETROPSIN) or may intercalate between the base-pairs (see INTERCALATING AGENTS). [DNA structure and its perturbation by drug binding (review): Bioch. J. (1987) 243 1–13.] See also GC%; DNA HOMOLOGY; THERMAL MELTING PROFILE. DNA I Supercoiled ds cccDNA (see DNA). DNA I
Syn. DNA IV (q.v.). DNA II Nicked relaxed circular dsDNA formed by an ss ‘nick’ in DNA I. DNA III Linear dsDNA derived from DNA I or DNA II by strand breakage. DNA IV (DNA I
) Relaxed (intact) ds cccDNA (see DNA). DNA base composition See e.g. BASE RATIO and GC%. (See also DNA HOMOLOGY.) DNA binding protein (1) Syn. SINGLE-STRAND BINDING PROTEIN. (2) In general: any non-enzymic protein which binds to ssDNA or dsDNA – e.g. a repressor protein, or a single-strand binding protein. DNA blotting See BLOTTING. DNA chip A microarray of short, single-stranded DNA molecules – having diverse (but known) sequences – immobilized on a small piece (‘chip’) of glass, silicon or plastic. The immobilized DNA molecules commonly function as PROBES. A chip can be used e.g. to determine the sequence of an unknown, labelled, single-stranded fragment of nucleic acid by testing the ability of the fragment to hybridize with a particular probe in the array, hybridization being detected by the fragment’s label – using e.g. confocal epifluorescence microscopy. A large microarray of probes can cover all possible combinations of nucleotides for a strand of given length. For example, a complete set (48 ) of nucleotide combinations for an 8-nucleotide probe – 65536 different probes – can be synthesized on a 1 cm2 glass chip. Chips are useful e.g. for detecting mutations, including those that confer resistant to antibiotics. Thus, that section of a genome in which mutations confer resistance to particular antibiotic(s) can be amplified (e.g. by PCR using fluorophorelabelled primers), and the amplicons tested against a microarray which includes all possible mutations (as well as the wildtype sequence). For example, the protease gene in clade B is called the linking number (= topological winding number) of that molecule and is designated Lk (or a); by convention, linking numbers are designated positive in right-handed helices. In a relaxed molecule, the linking number is the same as that of an equivalent B-DNA molecule (ca. N /10.4 where N = the number of base pairs in the molecule) and is called the duplex winding number, Lk0 (or b). The linking number of any cccDNA molecule is a topological property of that molecule; it cannot change without strand breakage. If a relaxed molecule is broken and the broken ends are twisted (in either direction) before rejoining, the linking number of the resulting molecule will be greater or less than Lk0 . Such a molecule is under strain (envisaged as a tendency to restore the stable pitch of ca. 10.4 bp/turn); a nick in one strand would allow rotation of the intact strand at the site of the nick and, hence, relaxation of the molecule. Unlike a relaxed molecule, the strained molecule can lie in a plane only if forced to do so, such constraint necessarily causing a deviation in pitch from 10.4 bp/turn. When such a molecule is unconstrained, the pitch of the helix is kept as nearly as possible to 10.4 bp/turn by contortion of the molecule such that the axis of the double helix itself becomes helical; this contortion is termed writhe, and the molecule is said to be supercoiled or superhelical. (See also DNA I, DNA II, etc.) Supercoiled DNA is much more compact than equivalent relaxed or linear DNA and thus has a much higher sedimentation coefficient and lower viscosity. When all the strain of supercoiling is accommodated by writhe, the amount of writhe (writhing number, Wr – i.e., the number of superhelical turns) in a given cccDNA molecule is indicated by the superhelix winding number, t, given by the difference between the linking number of the supercoiled molecule and that of the equivalent relaxed molecule: t = Lk − Lk0 . If the molecule has fewer turns than the equivalent relaxed molecule (i.e., the molecule is underwound ), Lk < Lk0 , and t is negative (i.e., the molecule is negatively supercoiled). If the molecule is overwound, Lk > Lk0 , t is positive. (When Lk = Lk0 , t = 0, and the molecule is relaxed.) In general, naturally occurring supercoiled DNA is negatively supercoiled (cf. REVERSE GYRASE). The superhelical density, s, of a given molecule is the number of superhelical turns per turn of the helix in relaxed DNA, i.e., s = 10.4t/N. The equation t = Lk − Lk0 assumes that the pitch of the double helix, and hence the number of turns in the helix, remains constant (at the value for the relaxed molecule, Lk0 ), so that any supercoils are accommodated entirely by writhe. However, the strain of supercoiling may force a change in pitch, and hence a change in the number of turns in the double helix – the actual number of turns, the twist (Tw ), being related to the pitch (p) of the helix by the expression T w = N/p; by convention, Tw is positive for a right-handed helix. Pitch, and hence twist, can also be altered by the insertion of an INTERCALATING AGENT. Twist differs from the linking number in that, in a given cccDNA molecule, the amount of twist can vary depending on the amount of writhe; the linking number is invariable and is the sum of twist and writhe. Thus, W r = Lk − T w, and, since Lk is constant for a given molecule, any change in writhe must be accompanied by an equal and opposite change in twist, and vice versa; when T w = Lk0 , W r = t, and when W r = 0, T w = Lk. [PNAS (1976) 73 2639–2643.] The positive free energy associated with negative supercoiling has a number of important consequences for the biological behaviour of DNA. It facilitates the formation of certain secondary structures (e.g. HAIRPINS, CRUCIFORMS, localized regions 243 DNA cloning interpret. One solution to the problem is to use a ‘rare-cutting’ RE (see table in RESTRICTION ENDONUCLEASE); this gives fewer, larger fragments that can be analysed e.g. by PFGE. Alternatively, the original procedure can be carried out with additional use of a labelled probe that reveals only those (few) fragments to which the probe binds; an example of this approach is RIBOTYPING (q.v.). [Evidence from DNA fingerprinting of endoscopic crossinfection in post-endoscopic acute gastritis (Helicobacter pylori ): JCM (2000) 38 2381–2382.] (See also FINGERPRINTING.) dna genes In Escherichia coli : those genes whose products are involved in at least some types of DNA REPLICATION. See separate entries for genes dnaA, dnaB, dnaC (= dnaD), nrdA (= dnaF), dnaI, dnaJ, dnaK, dnaP and dnaT. For dnaE, dnaN, dnaQ, dnaX and dnaZ (= dnaH) see DNA POLYMERASE. For dnaG see PRIMASE. For dnaW see adk gene. DNA glycosylase See ADAPTIVE RESPONSE and URACIL-DNA GLYCOSYLASE. DNA gyrase Syn. GYRASE. DNA helicases Syn. HELICASES. DNA homology The degree of similarity (‘relatedness’) between base sequences in different DNA molecules (or in different parts of the same DNA molecule); two DNA molecules which are 100% homologous have identical sequences of nucleotides. In microbial TAXONOMY the degree of homology between samples of chromosomal DNA from different organisms can be used to indicate the taxonomic relationship of the organisms. The homology between two samples of dsDNA can be assessed by various indirect methods (as well as by sequencing each sample and making a direct comparison of the nucleotides); two approaches are outlined below. In DNA–DNA hybridization, single-stranded (heat-denatured) chromosomal DNA from one test strain (strain A) is bound to a membrane (e.g. nitrocellulose). Chromosomal DNA from another test strain (strain B) is fragmented (for example, by ULTRASONICATION) and the fragments denatured by heat to singlestranded pieces. Similar, single-stranded fragments are also made from labelled DNA of strain A. Under appropriate conditions, the unlabelled, single-stranded DNA from strain A (which is bound to the membrane) is then exposed to the singlestranded fragments from strains A and B; the concentration of B fragments is very much higher than that of the A fragments. This allows the fragments to hybridize, by base-pairing, with complementary sequences in the bound DNA. The fragments from strains A and B compete for sequences on the bound DNA. However, the concentration of B fragments is very much higher than that of A fragments; consequently, if strains A and B are very similar, few (if any) of the (labelled ) A fragments will hybridize. On the other hand, if strains A and B are very different many of the A fragments will hybridize. Thus, the similarity between strains A and B is indicated by the number of A fragments which hybridize; this can be determined by measuring the label on the A fragments after all unbound fragments have been washed away. Such an assay requires controls; in one control, the assay is carried out with only labelled A fragments – this indicating the maximum amount of binding by A fragments. Results are meaningful only if the A fragments hybridize stably; the stability of binding can be assessed by monitoring the dissociation of A fragments from the bound DNA as the temperature is gradually raised. Homology may also be assessed by comparing the thermal stability of HETERODUPLEXes formed from two samples of DNA isolates of the human immunodeficiency virus (HIV-1) has been investigated in this way with the object of linking particular mutations with resistance to protease inhibitors [Nature Medicine (1996) 2 753–759]. Chip technology can also be used for examining the differential expression of genes: with an array of immobilized, singlestranded cDNAs (which bind mRNAs of expressed genes), fluorophore-labelled probes (with binding sites elsewhere on the mRNA molecules) can be used to detect those genes which are active under specific conditions. Chips can be made by (i) immobilizing pre-existing molecules, or (ii) ‘on-chip synthesis’ – in which high-speed robotic devices can synthesize a vast range of nucleotide combinations on a single chip. One problem with microarrays is that (single-stranded) DNA probes have a tendency for intra-strand base-pairing – which may obscure target sequences. A possible solution may be the use of microarrays containing PNA probes because PNA–DNA hybridization can occur in the absence of salts (that is, under conditions which inhibit intra-strand base-pairing in ssDNA). Among the growing number of published uses, high-density arrays have been used to identify species of Mycobacterium and (simultaneously) to test for resistance to rifampicin [JCM (1999) 37 49–55]. The need to include a sufficiently comprehensive range of probes for a given investigation has been demonstrated in another study in which non-clade-B strains of HIV-1 were examined for resistance to antiretroviral agents using an assay system designed for clade B isolates of HIV-1; the assay was unsatisfactory for non-clade-B isolates – indicating the need to ensure that future assays can accomodate newly emergent strains of HIV-1 [JCM (1999) 37 2533–2537]. [DNA chips (reviews): Nature Biotech. (1998) 16 27–31 & 40–44; TIBtech. (1999) 17 127–134. Biomedical discovery with DNA arrays (review): Cell (2000) 102 9–15.] DNA cloning See CLONING. DNA delay mutant A mutant of BACTERIOPHAGE T4 which, on infection of a host cell, shows delayed synthesis of T4 DNA (particularly in Escherichia coli B strains) and a reduced burst size. The mutation occurs in genes 39, 52 or 60 – genes encoding subunits of the T4 TOPOISOMERASE; a DNA delay mutant requires a functional host GYRASE for replication, the gyrase presumably replacing T4 topoisomerase in an essential function in the initiation of early DNA replication. DNA-dependent RNA polymerase See RNA POLYMERASE. DNA–DNA hybridization See DNA HOMOLOGY. DNA fingerprinting (chromosomal fingerprinting; also: restriction enzyme analysis, restriction endonuclease analysis, REA) A method used for TYPING. Essentially, chromosomal DNA, isolated from a culture of the given strain, is first cleaved by a RESTRICTION ENDONUCLEASE; the fragments (of many different lengths) are then separated by gel electrophoresis into a series of bands – which are stained. The pattern of bands, reflecting the cutting sites of the given RE in the chromosome, is the fingerprint of the strain. Strains which differ e.g. through loss or gain of a restriction site (perhaps through a point mutation) will give different fingerprints; similarly, insertion or deletion mutations (or e.g. insertion/excision of phage DNA) will alter the length of particular fragments and will also give rise to changes in the fingerprint. Hence, strains can be typed on the basis of their fingerprints. One problem with this method is that it may yield too many fragments – giving a complex fingerprint which is difficult to 244 DNA polymerase this capacity and requires longer incubation times compared with a reverse transcriptase [EMBO (1993) 12 387–396]. Polymerase II (polB product) is induced in the SOS SYSTEM (q.v.). Polymerase III is the main replicative enzyme in E. coli and is responsible for replication of the chromosome and of phage DNA etc. ‘DNA polymerase III’ refers to a core enzyme consisting of three different subunits: (i) subunit a (product of dnaE /polC ), which is involved in polymerization; (ii) subunit e (product of dnaQ/mutD), which has 3
-to-5
exonuclease activity and is apparently involved in proof-reading [PNAS (1983) 80 7085–7089] (see also MUTATOR GENE); and (iii) q (of unknown function). The core enzyme of polymerase III can catalyse limited DNA synthesis on single-stranded gaps in dsDNA, but for full efficiency, accuracy and processivity on longer templates it requires a number of additional subunits which include: (i) the b subunit (dnaN product; ?‘Factor I’), which apparently contributes to processivity; (ii) the g subunit (dnaZ product), which apparently also contributes to processivity; (iii) the d subunit (dnaX product; ?‘Factor III’), which apparently enhances processivity; and (iv) the t subunit (product of the dnaX-Z region), which acts as a link between two core enzymes. The core enzyme together with the full range of subunits is called DNA polymerase III holoenzyme. (DNA polymerase III∗ refers to the holoenzyme minus the b subunit. DNA polymerase III
refers to a complex of core enzyme plus the t subunit.) In a current model of DNA replication, the holoenzyme functions as a dimer – the enzymes being physically linked via their t subunits; during replication, one enzyme forms the leading strand while the other forms the lagging strand, i.e. they work in opposite directions. In other bacteria, DNA polymerases appear to be essentially similar to those in E. coli. In the Archaea, DNA polymerases may be significantly different (see e.g. APHIDICOLIN). In some prokaryotes (those that inhabit high-temperature environments), the DNA polymerases are thermostable, and this feature is exploited in various techniques used for the amplification of nucleic acids in vitro (e.g. PCR); these enzymes include: (a) Taq DNA polymerase. An enzyme (from the bacterium Thermus aquaticus) which has been widely used in PCR (particularly during the early years); this enzyme lacks 3
-to5
exonuclease activity and (therefore) has no proof-reading capacity. (b) Stoffel fragment. A modified, recombinant form of the Taq polymerase (the C-terminal region of the protein) which is more thermostable than the parent enzyme; it lacks 5
-to-3
(in addition to 3
-to-5
) exonuclease activity. This enzyme is active in a wide range of concentrations of magnesium ions, and has been found useful e.g. in those forms of PCR which use arbitrary primers. (c) UlTma DNA polymerase (Perkin-Elmer). This recombinant form of the DNA polymerase of Thermotoga maritima has 3
-to-5
exonuclease activity and can repair 3
mismatches during PCR – such proof-reading ability making the enzyme useful for high-fidelity copying. (d) Pfu DNA polymerase (Stratagene). This highly thermostable enzyme from Pyrococcus furiosus retains 95% activity after 1 hour at 98° C; it has excellent proof-reading ability. Eukaryotic DNA polymerases. Eukaryotes also have a number of DNA polymerases, including (in most eukaryotic cells) polymerases a, b and g. Each type of polymerase appears to with the thermal stability of the corresponding homoduplexes. This involves determining the mid-point temperature [Tm ] of the thermal melting curve of both the homoduplexes and heteroduplexes (see THERMAL MELTING PROFILE). The homology of DNAs from strains A and B are then assessed by comparing the Tm of the A homoduplexes with that of A–B heteroduplexes – the difference between the two values of Tm [= 1Tm ] indicating the degree of base pair mis-matching in the heteroduplex. [Methods for determining the homology of nucleic acids: Book ref. 138, pp 33–74.] DNA ligase A LIGASE which can make a phosphodiester bond between the 3
-OH end of one ssDNA strand and the (physically juxtaposed) 5
-phosphate end of the same or another ssDNA strand; thus, e.g., a DNA ligase can repair a NICK in one strand of a dsDNA molecule, the ends being held in position by basepairing with the intact strand. (See also e.g. DNA REPLICATION.) DNA looping (transcriptional control) See OPERATOR. DNA methylation Methylation of certain bases in DNA: a normal process which occurs in many or most organisms. In prokaryotes, methylation typically occurs at the N-6 position of adenine and/or at the C-5 position of cytosine, while in most eukaryotes methylation occurs mainly at the C-5 position of cytosine; the bases which are methylated generally occur in a specific sequence of nucleotides – e.g. the second cytosine residue in the sequence CC(A/T)GG is methylated in Escherichia coli strain C. (See also DAM GENE and FROG VIRUS 3.) Methylation serves various functions: see e.g. RESTRICTION–MODIFICATION SYSTEM. It also promotes a certain type of SPONTANEOUS MUTATION. (See also MISMATCH REPAIR.) [DNA methylation: molecular biology and biological significance: Book ref. 222.] DNA modification A physiological process in which bases in DNA are substituted with methyl or other groups – commonly shortly after synthesis of the DNA (see e.g. DNA METHYLATION; cf. BACTERIOPHAGE MU and BACTERIOPHAGE T4). Modification may occur as part of a RESTRICTION–MODIFICATION SYSTEM or may be involved e.g. in the regulation of gene expression (cf. DAM GENE). DNA photolyase See PHOTOREACTIVATION. DNA polymerase A type of enzyme which forms a polymer of deoxyriboNUCLEOTIDES by condensing deoxyribonucleoside triphosphates (dNTPs) with the elimination of pyrophosphate. A DNA polymerase adds nucleotides to the 3
-OH end of a preexisting strand of DNA or an RNA primer; the order in which the nucleotides are added is dictated by the nucleotide sequence of a template strand of DNA (see DNA REPLICATION) or, in some cases, RNA (see REVERSE TRANSCRIPTASE). DNA polymerases function processively (see PROCESSIVE ENZYME). Prokaryotic DNA polymerases. The bacterium Escherichia coli encodes three main DNA polymerases which are designated I, II and III (see also DNA POLYMERASE IV and DNA POLYMERASE V). All three main polymerases have exonuclease activity as well as polymerase activity; all can remove nucleotides sequentially from the 3
end of a strand, and polymerases I and III can also cleave nucleotides from the 5
end. (See also KLENOW FRAGMENT.) The 3
-to-5
exonuclease activity of a DNA polymerase is required for proof-reading during DNA REPLICATION. Polymerase I (Kornberg enzyme, product of gene polA) is active in DNA repair (see EXCISION REPAIR); it is also involved in the removal of RNA primers during DNA replication, and is needed for the replication of certain plasmids (e.g. ColE1). Polymerase I can act as an RNA-dependent DNA polymerase (i.e. a reverse transcriptase) but it exhibits poor processivity in 245 DNA polymerase I be associated with a particular function – for example, the g polymerase carries out DNA replication in mitochondria. Some viruses (e.g. bacteriophage T4) encode their own DNA polymerases; others (e.g. phages fd, fX174, G4) use host enzymes. DNA polymerase I (Kornberg enzyme) In Escherichia coli : an enzyme associated e.g. with EXCISION REPAIR and with the removal of RNA primers during DNA replication. (See also DNA POLYMERASE.) DNA polymerase II In Escherichia coli : a polymerase induced in the SOS SYSTEM. (See also DNA POLYMERASE.) DNA polymerase III In Escherichia coli : the major DNA polymerase; it is involved in chromosomal replication and e.g. synthesis of phage DNA. For further details see entry DNA POLYMERASE. DNA polymerase IV In Escherichia coli : the product of gene dinB (induced in the SOS SYSTEM), a protein which has DNA polymerase activity and which appears to be involved in mutagenesis; DinB has no proof-reading activity. DNA polymerase V In Escherichia coli : a protein complex, consisting of two UmuD
proteins and one UmuC protein, which has DNA polymerase activity and which may carry out some instances of DNA repair (translesion synthesis) during induction of the SOS SYSTEM. DNA polymorphism See POLYMORPHISM (sense 3). DNA primase Syn. PRIMASE. DNA repair Any physiological process in which damaged DNA within a cell is recognized and repaired; damage in this context generally means any alteration or distortion of the normal double-helical structure of dsDNA resulting e.g. from the presence of abnormal bases or base pairs, nicks or gaps in one strand, covalent linkages between bases etc. (but not from replacement of one normal base pair with another: cf. MUTATION). Repair may involve e.g. direct reversal of the damage (e.g. Ada protein in ADAPTIVE RESPONSE) or removal of the damaged region followed by its replacement with normal DNA (e.g. EXCISION REPAIR). Repair systems may be constitutive or inducible. See e.g. ADAPTIVE RESPONSE, BASE EXCISION REPAIR, EXCISION REPAIR, MISMATCH REPAIR, PHOTOREACTIVATION, RECOMBINATION REPAIR and SOS SYSTEM. DNA replication The process in which a copy (i.e. a replica) of a double-stranded DNA molecule is synthesized. In vivo, circular, dsDNA molecules may be replicated by the CAIRNS’ MECHANISMS or the ROLLING CIRCLE MECHANISM; the following refers mainly to the replication of circular dsDNA molecules by the Cairns’ mechanism (replication of linear dsDNA is also considered briefly). (For replication of DNA in vitro see e.g. PCR and SDA.) During replication, a new strand of DNA is synthesized on each of the two pre-existing strands, each pre-existing strand being used as a pattern or template; a template dictates the sequence of nucleotides in a new strand, thus conserving the genetic information encoded in the template strand. Essentially, molecules of deoxyribonucleoside triphosphate (dNTPs) basepair with complementary nucleotides in each template strand and are sequentially polymerized (with elimination of pyrophosphate) by a DNA POLYMERASE. Note that each newly synthesized strand is complementary to – not a copy of – the template strand on which it is synthesized; thus, each of the two new daughter duplexes produced from a given parent duplex consists of one parent strand and one newly synthesized strand. (This type of replication is called semiconservative replication because only one strand in each daughter duplex has been newly synthesized.) Initiation of DNA replication occurs at one or more special sites, called origins (designated ori ); any DNA molecule which lacks an origin cannot be replicated in vivo. (See also REPLICON.) Initiation of replication involves the recognition of an ori site by various initiation factors and enzymes. An essential prerequisite for replication is separation of the two strands of dsDNA (so-called ‘melting’) in the ori region (see CELL CYCLE (b)). As no known DNA polymerase can initiate synthesis of a new strand of DNA on a template, initiation of DNA replication (by the Cairns’ mechanism) requires the provision of a ‘starter’ sequence of nucleotides (a primer ) which the DNA polymerase can elongate (in the 5
-to-3
direction) to form a new strand of DNA; DNA synthesis is usually primed by a short strand of RNA which is transcribed on the DNA template strand in the ori region. Chain elongation (i.e. ongoing strand synthesis) involves ongoing separation (unwinding) of the two strands of the parental duplex. Termination of DNA synthesis usually occurs when the entire duplex has been replicated (cf. ROLLING CIRCLE MECHANISM); it involves dissocation of the replicating machinery, joining of the two ends of each daughter strand (in the case of circular dsDNA molecules), and separation of the two circular daughter molecules (decatenation). Newly synthesized strands usually undergo modification before the next round of replication can begin (see RESTRICTION–MODIFICATION SYSTEM). Replication of the chromosome in Escherichia coli. The way in which a round of replication is initiated (i.e. triggered) is not known, but various factors appear to be relevant; regulation of the initiation of DNA replication in E. coli is discussed in the entry CELL CYCLE. Once triggered, replication of the ccc dsDNA CHROMOSOME of E. coli occurs by the Cairns’ mechanism: replication begins at the origin – oriC (see ORIC) – and proceeds bidirectionally, both template strands being replicated more or less simultaneously. Thus, two migrating, Y-shaped replication forks (in which the stem of the Y is the parental duplex, and the arms the daughter duplexes) move in opposite directions around the chromosome (see figure). Because the strands of the parent duplex are antiparallel, and because both new strands are synthesized more or less simultaneously in the direction of movement of a replication fork, one daughter strand is synthesized in the 5
-to-3
direction while the other is synthesized from the 3
direction. The strand synthesized in the 5
-to-3
direction (the leading strand – see figure) is synthesized continually, but the other strand (lagging strand) cannot be synthesized continually because no known DNA polymerase can synthesize DNA in the 3
-to-5
direction. The lagging strand is actually synthesized as a series of fragments (Okazaki fragments – see figure), each fragment being synthesized in the 5
-to-3
direction from a separate RNA primer. Primers are subsequently excised and replaced with DNA (by extension from the preceding fragment) and the ends are joined together by a DNA LIGASE. Chromosomal replication is a complex process requiring a number of different proteins. These proteins include the dnaA gene product (see DNAA GENE); a dimer of DNA polymerase III (see DNA POLYMERASE); GYRASE; topoisomerase I (see TOPOISOMERASE); HU PROTEIN; the DNAB GENE product (a helicase); PRIMASE and RNA POLYMERASE. At least some of these proteins are components of an initiation complex (earlier name: 246 DNA replication 3′ 5′ primer 'leading' strand DNA duplex 3′ 5′ 3′ 2nd primer 5′ Okazaki fragment 1st primer 3′ 5′ 3′ 5′ DNA REPLICATION (Cairns-type): disposition of template and newly synthesized strands at a replication fork (diagrammatic; see entry). In a circular DNA duplex the strands have separated, locally, in the ori region. Only one half of this region is shown; in this half, the strands of the duplex will continue to separate in a left → right direction (i.e. the replication fork is moving to the right). On one template strand (top) an RNA primer (zigzag line) has been synthesized (see text); ongoing addition of deoxyribonucleotides to the 3
end of this primer will form the ‘leading’ strand of DNA. Synthesis of the leading strand is continuous in the 5
-to-3
direction. On the other template strand (below), the 1st primer has been synthesized (by a primase), and this (RNA) primer has been extended as a strand of DNA by sequential addition of deoxyribonucleotides from the 3
end. This new strand of DNA may form the first Okazaki fragment of the lagging strand; alternatively, it may act as a primer for leading strand synthesis in the other replication fork (not shown). Following synthesis of the 1st primer, the replication fork has moved further to the right – opening up the duplex to the extent shown in the diagram. The 2nd primer has been synthesized and has been extended as a strand of DNA (Okazaki fragment) as far as the 5
end of the first primer. As the replication fork moves even further to the right additional template will become available, and the 3rd, 4th, 5th . . . etc. primers synthesized on this template will (like the 2nd primer) be extended as Okazaki fragments; the sequence of Okazaki fragments will be joined together (forming the lagging strand) when the (RNA) primers are replaced by DNA. Modified from Bacteria, 5th edition, Figure 7.8(a), page 116, Paul Singleton (1999) copyright John Wiley & Sons Ltd, Chichester, UK (ISBN 0471-98880-4) with permission from the publisher. In a current model for DNA replication, a processive complex of proteins (‘replisome’) is associated with each replication fork. The complex includes a DNA polymerase III dimer (the two enzymes physically linked via their t subunits and orientated in opposite directions) – one enzyme synthesizing the leading strand while the other synthesizes the lagging strand. The complex also contains a PRIMOSOME which, in turn, includes a helicase and the DnaG protein (a primase); the helicase is described as a hexamer (a ring-shaped structure of six DnaB proteins) which unwinds the duplex [Cell (1996) 86 177–180]. In this model, a section of lagging-strand template (liberated by ongoing leading-strand synthesis) forms a loop at the replication fork. This allows primer synthesis for the lagging strand (by primase) followed by extension of the primer by DNA polymerase III; when a newly synthesized fragment approaches the 5
-end of the previous fragment, the new (doublestranded) section is released by the complex, and the process is repeated on the next region of exposed lagging-strand template. Thus, the complex synthesizes both leading and lagging strands concurrently as it moves along with the replication fork. ‘orisome’). (During strand synthesis, parts of the parent DNA molecule – around the replication forks – necessarily exist in a single-stranded state, and these ssDNA sequences are stabilized by SINGLE-STRAND BINDING PROTEINS.) A short RNA primer, synthesized on one of the template strands, is elongated by DNA polymerase III in the 5
-to-3
direction to form the leading strand (see figure); synthesis of the primer may be mediated by RNA polymerase (RNA polymerase is needed for initiation of DNA replication). Exposure of the other template strand permits synthesis of the lagging strand. Primers for the lagging strand are synthesized by a PRIMASE (dnaG gene product) that forms part of a PRIMOSOME. Each primer is elongated in the 5
-to-3
direction to form an Okazaki fragment (∼1000–2000 nt in length) that extends to the 5
end of the previous primer. (Primers are subsequently removed and replaced by DNA; this involves the enzyme DNA polymerase I.) As the first replication fork moves away, the first fragment of the lagging strand may act as a primer for leading strand synthesis in the opposite direction. At 37° C a replication fork moves at about 1000 nt per second. 247 DNA restriction most or all cases at least some of the components involved in host chromosomal replication are required. In plasmid ColE1, replication is of the Cairns’ type – but it occurs unidirectionally from the origin; in the F PLASMID replication is bidirectional (as in the E. coli chromosome). Many of the small circular plasmids in Gram-positive bacteria replicate by means of a ROLLING CIRCLE MECHANISM [replication control in rolling circle plasmids: TIM (1997) 5 440–446]. Plasmid replication, and the partitioning to daughter cells, seems likely to involve some kind of association between the plasmid and the cytoplasmic membrane of the host cell [Mol. Microbiol. (1997) 23 1–10]. Replication of the (linear) genome of bacteriophage f29 was mentioned above. In certain phages the rolling circle mechanism is involved in replication (see e.g. SSDNA PHAGE, BACTERIOPHAGE l, BACTERIOPHAGE fX174). [The enzymology of DNA replication (historical perspective): JB (2000) 182 3613–3618.] DNA restriction See RESTRICTION–MODIFICATION SYSTEM. DNA sequencing Any procedure for determining the sequence of nucleotides in a sample of DNA. Sequencing is important in taxonomy, identification and characterization. In the ‘chemical’ (Maxam–Gilbert) method, end-labelled copies of an unknown sequence are cleaved by base-specific reagents; analysis follows electrophoresis of the fragments. The dideoxy (Sanger’s) method is described in the figure on page 249. (See also PYROSEQUENCING.) Automated DNA sequencing is often based on the dideoxy method. Primers are labelled with fluorescent dyes, a different dye being used for each of the four reactions. The products from all four reactions are combined and then subjected to electrophoresis from a single well in a polyacrylamide gel. On excitation by an argon laser, fragments from each reaction mixture can be distinguished from those of other reaction mixtures by their dye-specific emission characteristics; emissions from the fragments of all four reaction mixtures are detected automatically and stored e.g. in digital form. Sequencing provides definitive information on a given sample of nucleic acid. For duplex DNA, greater accuracy is obtained if both strands of the duplex are sequenced. RNA can be retrotranscribed into cDNA; the (single-stranded) cDNA is then amplified, and the amplified product is sequenced. The sequence of nucleotides in the RNA sample is then deduced. Amplified DNA copies of an RNA sample may be obtained by reverse-transcriptase PCR (rtPCR). DNA splicing See CLONING (of DNA). DNA Stat See BLOOD DNA ISOLATION KITS. DNA synthesis See DNA REPLICATION. DNA topoisomerase Syn. TOPOISOMERASE. DNA toroid A highly compacted genomic DNA, found e.g. in Deinococcus radiodurans and ENDOSPORES, which may account at least partly for resistance to DNA-damaging agents (e.g. radiation). Toroidal structure may facilitate repair of doublestranded breaks in a RecA- and template-independent way [JB (2004) 186 5973–5977]. DNA tumour viruses Oncogenic DNA-containing animal viruses; they include members of the ADENOVIRIDAE, Hepadnaviridae (see e.g. HEPATITIS B VIRUS), Herpesviridae (subfamily GAMMAHERPESVIRINAE), and PAPOVAVIRIDAE. DNA unwinding protein Any protein capable of unwinding and separating the strands of double-helical DNA. (See HELICASES; cf. SINGLE-STRAND BINDING PROTEIN.) The physical link between the two enzymes in the DNA polymerase III dimer is apparently needed for co-ordination of synthesis of the leading and lagging strands. It appears that coupling is also needed between the helicase and the polymerases – loss of such coupling being associated with a drastic reduction in efficiency of the helicase [Cell (1996) 84 643–650]. Moving in opposite directions, the two replications forks meet at the ter site on the chromosome, 180° from oriC. During replication, the DNA polymerase appears to recognize only the overall shape of a base-pair, i.e. the same active site of the enzyme must serve for all four possible basepair combinations. The accuracy of DNA synthesis therefore depends initially on base-pairing specificity between incoming nucleotides and template DNA. This might be expected to result in a relatively high frequency of errors because tautomerism in the bases can interfere with the accuracy of base-pairing. However, the overall error rate in E. coli DNA replication has been estimated to be ca. one mistake in 108 –1010 nucleotides polymerized – a much higher level of accuracy than can be accounted for by base-pairing and polymerase specificities. This level of accuracy is achieved by ‘proof-reading’ and ‘editing’ systems which recognize and correct errors in a newly synthesized strand. The 3
-to-5
nuclease activity of DNA polymerase III holoenzyme serves a proof-reading function, removing mispaired nucleotides from the growing (3
) end of the chain before continuing chain elongation. Errors which escape this system may be detected and corrected subsequently by the post-replicative MISMATCH REPAIR system. In growing cells, chromosomal replication must be coordinated with cell division in order that daughter cells each receive the full genetic complement; the rate of DNA replication in E. coli is increased during faster growth by the initiation of additional rounds of synthesis (see HELMSTETTER–COOPER MODEL). Conditions which inhibit DNA replication may cause inhibition of cell division (see SOS SYSTEM). Replication of linear genomes. In some bacteria a linear chromosome is replicated bidirectionally from an internal origin of replication. This raises the question of how the replication machinery avoids leaving the 3
end of the template strand (in each daughter duplex) as single-stranded DNA. In one of several models, DNA synthesized on the new strand (at the fully duplexed end) displaces the 5
terminal nucleotide sequence of the template strand – which is then transferred to (complementary) single-stranded DNA on the other daughter duplex (thus filling the single-stranded gap). [Replication of linear DNA molecules: TIG (1996) 12 192–196.] In some viruses, replication of the linear dsDNA genome involves so-called protein priming – in which a nucleotide, bound covalently to a (virus-encoded) ‘terminal protein’, forms the 5
terminus of a new strand; this system is found e.g. in BACTERIOPHAGE f29 [protein priming in phage f29: EMBO (1997) 16 2519–2527] and in members of the ADENOVIRIDAE. Chromosomal replication in eukaryotes. Replication of eukaryotic CHROMOSOMES occurs during the S phase of the cell cycle. DNA synthesis is initiated at many separate sites along the chromosome, and replication forks proceed bidirectionally until they meet and fuse. Each DNA molecule is replicated only once in a cell cycle. [Eukaryotic DNA replication (minireview): Cell (1987) 48 7–8.] Plasmid and phage replication. Mechanisms of replication in PLASMIDS and (DNA) phages may differ significantly from chromosome replication in the corresponding host cell, but in 248 5′ TCTGACGTAAGC ddG ddC ddT ddA C G A T 5′ T C T G A C G T A A G C DNA SEQUENCING: determining the nucleotide sequence of a cloned DNA fragment by the dideoxy method (Sanger’s chain-termination method) (diagrammatic). Initially, the fragment to be sequenced is obtained in single-stranded form (e.g. by cloning in a phage M13 vector). Regardless of the cloning method used, the unknown sequence will be flanked on its 3
side by single-stranded DNA of known sequence derived from the vector molecule; this permits the design of a labelled primer which can bind at a site next to the unknown sequence such that the first nucleotide to be added to the primer will pair with the first 3
nucleotide of the unknown sequence. In the diagram, the ‘unknown sequence’ is TCTGACGTAAGC; a primer (short line), which carries a label (black disc), has bound at a site flanking the 3
end of the unknown sequence. DNA synthesis in vitro is normally carried out with a reaction mixture that includes (i) templates (in this case, single-stranded fragments containing the unknown sequence); (ii) primers; (iii) the four types of deoxyribonucleoside triphosphate (dATP, dCTP, dGTP and dTTP); and (iv) DNA polymerase. When base-paired to the template strand, the primer is extended (5
-to-3
) by the sequential addition of nucleotides as dictated by the template. In sequencing, there are four separate reaction mixtures (G, C, T, A), each containing all the constituents mentioned above (including millions of copies of the unknown sequence, and of the primer). In addition, each mixture contains a given dideoxyribonucleoside triphosphate (ddNTP); thus, the G mixture contains dideoxyguanosine triphosphate (ddG), the C mixture ddC, the T mixture ddT, and the A mixture ddA. When a dideoxyribonucleotide is added to a growing strand of DNA it prevents addition of the subsequent nucleotide because dideoxyribonucleotides lack the 3
-OH group necessary for making the next phosphodiester bond. Thus, primer extension will stop (= chain termination) at any location where a dideoxyribonucleotide has been incorporated. In each reaction mixture the concentration of ddNTP is such that, in most growing strands, synthesis will be stopped, at some stage, by the incorporation of a dideoxyribonucleotide; because a ddNTP may pair with any complementary base in the template, chain termination can occur at different sites on different copies of the template strand – so that product strands of different lengths will be formed. For example, with ddG (see diagram) the three products are of different lengths because, during extension of the primers, ddG has paired with cytosine residues at three different locations in the template; note that, in this case, the length of a given product strand is related to the location of a particular cytosine residue in the unknown sequence. Analogous comments apply to reaction mixtures containing the other ddNTPs. At the end of the reaction, product strands are separated from templates by formamide. Each reaction mixture is then subjected to electrophoresis in a separate lane of a polyacrylamide gel. Short product strands move further than longer ones, in a given time; products that differ in length by only one nucleotide can be distinguished, the shorter product moving just a little further. The gel contains urea; this inhibits base-pairing between product strands and templates. It also inhibits intra-strand base-pairing in the product strands. This is essential: to deduce the sites of a particular base in the template it is necessary to compare the lengths of all product strands in the given reaction, and this requires proportionality between strand length and electrophoretic mobility; were intra-strand base-pairing to occur it could alter electrophoretic mobility – in which case a product’s length would not necessarily be indicated by the position of its band in the gel. (Continued on page 250.) 249 DNA uptake site challenges of producing pure plasmid DNA: TlBtech. (2000) 18 296–305.] [Plasmids for Therapy and Vaccination: Book ref. 225.] (See also GENE GUN.) DnaA boxes See CELL CYCLE (b). dnaA gene A DNA GENE whose product is necessary for the initiation of DNA REPLICATION in bacteria; in archaeans the Orc1 and CdC6 proteins carry out the same function. DnaA proteins bind to 9-bp sequences (‘DnaA boxes’) in the oriC region during initiation (see CELL CYCLE (b)) and this is followed by localized strand separation. Defective (mutant) DnaA proteins have been reported [Mol. Microbiol. (2000) 35 454–462]. DnaA was earlier reported to be needed for the replication of certain plasmids (e.g. pSC101) but is now seen to be specific to the replication of chromosomal DNA. In eukaryotes, the DnaA function is carried out by six proteins (Orc1–6). [Initiators of DNA replication: FEMS Reviews (2003) 26 533–554.] DNAase DEOXYRIBONUCLEASE. dnaB gene A DNA GENE whose product is necessary for many cases of primer formation; in a current model of DNA REPLICATION in Escherichia coli, a hexamer of DnaB proteins functions as a HELICASE – forming part of the PRIMOSOME and unwinding the parent duplex as the primosome migrates with the replication fork. It appears that ATP binding is necessary for the binding of DnaB to ssDNA, while ATP hydrolysis permits its release. In the absence of single-strand binding protein (SSBP), DnaB–ATP can bind to many sites on ssDNA, and interaction with primase leads to the formation of a short RNA primer at each site (general priming); general priming is inhibited by SSBP. Specific priming occurs in the presence of SSBP and depends on the presence of other proteins of the PRIMOSOME. dnaC gene (dnaD gene) A DNA GENE whose product is required for PRIMOSOME formation. dnaD gene Syn. dnaC gene (q.v.). dnaE gene Syn. polC gene (see DNA POLYMERASE). dnaF gene Syn. nrdA gene (q.v.). dnaG gene See PRIMASE. dnaH gene Syn. dnaZ gene (see DNA POLYMERASE). dnaI gene A DNA GENE whose function is required for initiation of DNA REPLICATION at the replication origin. dnaJ gene A DNA GENE which is closely linked to the dnaK gene (q.v.) and which, like dnaK, is necessary for viability of Escherichia coli at high temperatures and apparently for phage l DNA replication. dnaK gene A DNA GENE whose function is essential for the viability of Escherichia coli at high temperatures, and which is also reported to be required for phage l DNA replication. The product of dnaK was identified as protein B66.0, a HEAT-SHOCK PROTEIN. [Properties of DnaK: JBC (1984) 259 8820–8825.] DNA uptake site (in Haemophilus) See TRANSFORMATION (1). DNA vaccine Any parenterally administered VACCINE consisting of DNA. A DNA vaccine consists e.g. of copies of a PLASMID that encodes specific antigen(s); the encoded antigen(s) are synthesized in vivo (i.e. within the recipient of the vaccine) and may then induce both humoral (antibody) and cell-mediated responses. A DNA vaccine may have prophylactic and/or therapeutic uses. The possibility of using DNA vaccines against the major diseases (malaria, tuberculosis etc.) has attracted much interest. It is known that a DNA vaccine encoding e.g. the 65 kDa heat-shock protein (Hsp65) of Mycobacterium tuberculosis can protect mice against infection by this pathogen; the vaccine appears to stimulate an antigen-specific subset of Th1 T cells which (i) have cytotoxic activity against infected cells, and (ii) secrete interferon-g (IFN-g). More recently, it has been shown that a DNA vaccine encoding Hsp65 can act therapeutically in mice during an experimentally established infection with Mycobacterium tuberculosis [Nature (1999) 400 269–271]. In this study, virulent cells of M. tuberculosis H37Rv were injected intravenously and the infection allowed to develop for 8 weeks. DNA vaccine was then administered in four intramuscular doses at 2-week intervals; each dose of vaccine consisted of 50 µg plasmid DNA in 50 µl saline. Different types of vaccine were used in different groups of animals; the vaccine encoding Hsp65 was found to be significantly more effective than other vaccines encoding different mycobacterial antigens (Hsp70 and ESAT-6). The therapeutic effect of the Hsp65 vaccine (manifested by a marked decrease in the numbers of live tubercle bacilli in lung and spleen) was associated with a switch from a Th-2-dominated to a Th-1-dominated response in the mice. Interestingly, non-methylated CpG regions in the DNA itself can induce secretion of INTERLEUKIN-12 (IL-12) from antigenpresenting cells (such as macrophages). IL-12 is known to promote the development of the Th-1 phenotype in T cells exposed to antigen. Because IL-12 is one of the factors essential for effective immunity to M. tuberculosis in mice, a plasmid encoding IL-12 (rather than a mycobacterial antigen) was tested in the same study [Nature (1999) 400 269–271]; this vaccine was found to cause the maximum reduction in tubercle bacilli by 11 weeks, i.e. its performance exceeded that of the mycobacterial antigens. A DNA vaccine has also been used against Ebola virus: see FILOVIRIDAE. Any future large-scale development/use of DNA vaccines is dependent on appropriate technology for preparing pure plasmid DNA (e.g. free of contaminating chromosomal DNA) in sufficient quantity. [Biochemical engineering approaches to the DNA SEQUENCING (continued) Bands in the gel are revealed e.g. by staining or chemiluminescence; if the primers carry a radioactive label, the gel is examined by autoradiography following electrophoresis. The locations of bands indicate the relative lengths of product strands – shorter products having moved further down the gel (from top to bottom in the diagram). Note that the first unknown 3
nucleotide (C) is identified by (i) the shortest product strand (which has moved the furthest), and (ii) the fact that this product is from the ddG mixture – indicating a base that pairs with G, i.e. C. Similarly, the next unknown (G) is the next shortest product strand which came from the ddC mixture, thus indicating G. The entire unknown sequence can be deduced in this way. The figure also shows part of an autoradiograph of a sequencing gel (courtesy of Joop Gaken, Molecular Medicine Unit, King’s College, London). Figure reproduced from Bacteria, 5th edition, Figure 8.23, pages 222–223, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 250 dot-blot Domestos See HYPOCHLORITES. dominance (genetics) In a diploid (or merodiploid) heterozygous cell or organism: the tendency of certain (dominant) alleles to be expressed in preference to their corresponding (recessive) alleles. (cf. EPISTASIS.) The complete dominance of one allele over another occurs in only some cases; in such cases the phenotype of the heterozygote Aa is identical to that of the homozygote, AA (A being dominant). When partial dominance is exhibited the Aa phenotype is intermediate between the AA and aa phenotypes. dominant allele See DOMINANCE. Domiphen bromide See QUATERNARY AMMONIUM COMPOUNDS. DON (1) (6-diazo-5-oxo-L-norleucine; CO2 H.CHNH2 .(CH2 )2 . CO.CH=N+ =N− ) An ANTIBIOTIC and antitumour agent obtained from Streptomyces sp; its mode of action resembles that of AZASERINE. Other antitumour DON derivatives are produced by Streptomyces spp: e.g. diazomycin A (N-acetyl-DON) and alazopeptin (apparently comprising one alanine and two DON residues). (cf. HADACIDIN.) (2) Deoxynivalenol – see TRICHOTHECENES. donation (mol. biol.) See CONJUGATION (1b) (i). donor Any cell or organism which donates genetic information to another cell or organism (the recipient): see CONJUGATION, TRANSDUCTION and TRANSFORMATION. donor conjugal DNA synthesis See CONJUGATION (1b) (i). donor site (of a ribosome) See PROTEIN SYNTHESIS. donor splice site See SPLIT GENE (a). donor-suicide model (for transposition) See e.g. Tn10. Donovan bodies See GRANULOMA INGUINALE. donovanosis Syn. GRANULOMA INGUINALE. L-dopa See MELANIN. Dorisa See EIMERIORINA. Dorisella See EIMERIORINA. dormancy (hypobiosis) The state of an organism or spore which exhibits minimal physical and chemical change over an extended period of time; when no physical or chemical change can be detected the state is sometimes referred to as cryptobiosis. (See also ENDOSPORE (sense 1(a)) and SPORE.) dorsal zone of membranelles See DZM. Dorset’s egg A MEDIUM used e.g. for the maintenance of Mycobacterium spp; it consists of a saline suspension of homogenized whole hens’ eggs which has been inspissated in sloped bijou or universal bottles. dorsiventral Having two unlike (upper and lower) surfaces. dosa A traditional fermented food in India; it is prepared from milled rice and black gram (cf. IDLI). [Microbiology of dosa: Food Mic. (1986) 3 45–53.] dosage (of genes) See GENE DOSAGE. dot-blot A PROBE-based method for detecting or quantifying a specific sequence of nucleotides in a given sample. A drop of the sample is added to a suitable membrane and treated so that any nucleic acids present in the sample bind to the membrane in single-stranded form. The membrane is then exposed to probes (which are complementary to the target sequence) under conditions permitting probe–target hybridization; when unbound probes are removed by washing, the presence of bound probes indicates the presence of the required target sequence. If, for example, the probe had been tagged with DIGOXIGENIN, the bound probe is detected by treating the membrane with a conjugate consisting of anti-digoxigenin (antibody to digoxigenin) covalently linked to an enzyme (such as alkaline phosphatase); the binding of conjugate to digoxigenin is determined by adding a colourgenerating substrate for the enzyme after all unbound conjugate has been removed by washing. dnaL gene An uncharacterized DNA GENE. dnaM gene An uncharacterized DNA GENE. dnaN gene See DNA POLYMERASE. dnaP gene A DNA GENE whose function is required for initiation of DNA REPLICATION at the replication origin. dnaQ gene See DNA POLYMERASE. DNase DEOXYRIBONUCLEASE. dnaT gene A DNA GENE whose product may interact with DnaC protein in vivo; dnaT mutants cannot destabilize the replication complex at chain termination. dnaW gene Syn. adk gene (q.v.). dnaX gene See DNA POLYMERASE. dnaZ gene See DNA POLYMERASE. dNTP DeoxyriboNUCLEOSIDE-5
-triphosphate. Dobiella See EIMERIORINA. Dobrava-Belgrade virus See HANTAVIRUS. DOC Dissolved organic carbon. (cf. POC.) docking protein See SIGNAL HYPOTHESIS. Döderlein’s bacilli Aciduric, Gram-positive bacilli, probably Lactobacillus sp(p), observed by Döderlein (1892) in human vaginal secretions. dodine (dodine acetate; n-dodecylguanidine acetate) An agricultural (non-systemic) ANTIFUNGAL AGENT used e.g. for the control of apple scab – against which it has eradicant as well as protective action. Dodine has the properties of a cationic surfactant. dog (parvovirus infection) See CANINE PARVOVIRUS. dog lichen Peltigera canina. dolichol A polyprenol consisting of 14–24 a-saturated isoprene units; phosphorylated dolichols function in eukaryotes as membrane-bound sugar carriers, assisting the transfer of hydrophilic sugar residues from soluble nucleotide precursors through membranes (e.g. of the rough endoplasmic reticulum and Golgi apparatus) to polymers such as polysaccharides, glycoproteins etc. (see e.g. PALADE PATHWAY). (cf. BACTOPRENOL.) Dolichomastix See MICROMONADOPHYCEAE. doliform (doliiform) Barrel-shaped. dolipore septum A type of SEPTUM present in the hyphae of most basidiomycetes (cf. AMBROSIOZYMA). The central portion of the septum is thickened and barrel-shaped, the septal pore forming a small tunnel (ca. 0.1 µm diam.) through this thickened region. A curved double membrane (the septal pore cap or parenthesome) typically occurs on each side of the septum, these structures appearing (in cross section) as brackets (parentheses) around the septa. The parenthesomes appear to be formed from the endoplasmic reticulum and may be perforate or imperforate; they may serve as screens, regulating the passage of cellular structures between adjacent cells. Parenthesomes are lacking e.g. in Filobasidiella. dollar spot A disease of turf grasses (particularly red fescue, Festuca rubra) caused by Sclerotinia homeocarpa; yellowishbrown patches, up to ca. 8 cm across, develop on the turf in mild, wet weather. domain (immunol.) Any one of the several compact, globular sections of a heavy chain or a light chain in an IMMUNOGLOBULIN molecule; each domain consists of a part of the polypeptide chain which forms a loop (closed by an intra-chain disulphide bond) and which is extensively folded. In IgG the heavy chain domains, in sequence, are designated VH (variable region), and CH 1, CH 2 and CH 3 (constant region); the light chain domains are designated VL and CL . (Domains are also written as e.g. CH1, CH2 etc, or as CH1 , CH2 etc.) domain (taxon.) A category ranked above kingdom [origin of term: PNAS (1990) 87 4576–4579] – see e.g. ARCHAEA and BACTERIA. 251 dot genes downstream box In certain prokaryotic and phage genes: a sequence of nucleotides, downstream from the start site, which has been regarded as a translation enhancer. The ribosomal 16S rRNA may contain a complementary region (called the anti-downstream box ), although suggestions that this may bind to the downstream box have been disputed [JB (2001) 183 3499–3505]. downstream promoter element In some RNA polymerase II PROMOTERS: a region downstream of the start site with a role apparently similar to that of the TATA box. downy mildews Plant diseases caused by certain members of the PERONOSPORALES (e.g. BREMIA, PERONOSPORA, PLASMOPARA); downy mildews are characterized by the formation of superficial hyphal growth in which, typically, individual spore-bearing structures can be distinguished under low magnification. (cf. POWDERY MILDEWS.) Downy mildews can be controlled e.g. with copper-based antifungals or ZINEB. doxycycline See TETRACYCLINES. Dp Pattern difference. In the comparison of two strains by NUMERICAL TAXONOMY: a coefficient which indicates the degree of dissimilarity corrected for any difference(s) due solely to inter-strain differences in metabolic vigour. (cf. entry Sp .) DP Degree of polymerization: the number of monomeric units per molecule of a polymer. DPD N,N-diethyl-p-phenylenediamine (see WATER SUPPLIES). DPN Diphosphopyridine nucleotide: see NAD. DPT A MIXED VACCINE containing diphtheria toxoid, pertussis vaccine and tetanus toxoid. DPX A neutral, synthetic MOUNTANT of refractive index ca. 1.53 when dry; it consists of Distrene and a plasticizer (tritolyl phosphate) dissolved in x ylol. DR DIRECT REPEAT. Dr adhesins A category that includes both fimbrial and nonfimbrial adhesins, members of which are found e.g. on UPEC strains of Escherichia coli; the Dr adhesins bind to CD55 (at a site corresponding to the Dra blood group antigen). draft tube In a LOOP FERMENTER: a wide, vertical tube typically situated coaxially within the column so as to leave an annular space between itself and the wall of the fermenter; when the fermenter is operating, the draft tube is completely submerged in the culture. Culture is made to flow up (or down) the draft tube in order to generate a circulatory flow in the column – culture flowing down (or up) the annulus. DraI See RESTRICTION ENDONUCLEASE (table). Draparnaldia See CHAETOPHORA. draughtsman colony (checker colony) The form of COLONY typically (though not invariably) produced by Streptococcus pneumoniae growing on blood agar: round and raised, with steep sides and a flat top, i.e., resembling one of the pieces used in a game of draughts (= checkers); after overnight growth under a raised partial pressure of CO2 the colonies are each ca. 1 mm in diameter and surrounded by a zone of a-haemolysis. Very young colonies of S. pneumoniae are typically convex (domed); the flattened colony form apparently results from autolysis of the constituent cells on continued incubation, and further incubation may result in the development of a concavity in the upper surface of the colony. draw tube In some microscopes: a tube coaxial with the body tube (see MICROSCOPY) used to adjust the tube length. drd plasmid A mutant CONJUGATIVE PLASMID in which the TRANSFER OPERON is permanently DEREPRESSED. Drechslera A genus of fungi of the HYPHOMYCETES; teleomorphs occur in the genera Cochliobolus and Pyrenophora. Alternatively, a radioactive probe can be used. For quantification, serial dilutions of the sample are examined as described above; the quantity of target present in the sample can then be assessed by noting the strength of label (i.e. colour, radioactivity etc.) in comparison with that of controls of known concentration. dot genes (of Legionella pneumophila) See end of section (a) in PHAGOCYTOSIS. Dothidea See DOTHIDEALES. Dothideales The largest order of fungi in the subdivision ASCOMYCOTINA; the organisms include saprotrophs, plant parasites and lichenized fungi. Ascocarp: ascolocular (see ASCOCARP and ASCOSTROMA), ostiolate or closed. Asci: bitunicate, cylindrical to spheroidal. Some species exhibit one or more asexual (conidial) stages. Genera: e.g. ARTHOPYRENIA, Cochliobolus (see also DRECHSLERA), Cucurbitaria (syn. Phialospora), DIDYMELLA, Dothidea, ELSINOË, LEPTOSPHAERIA, MYCOSPHAERELLA, Myriangium (see also ASCOSPORE and ASCOSTROMA), Ophiobolus, PIEDRAIA, Pleospora (see also BLACKLEG sense 2), Pyrenophora (see also DRECHSLERA and NET BLOTCH), Sphaerulina (see also NORMANDINA), VENTURIA. (See also BLACK MILDEWS and SOOTY MOULDS.) DOTS Directly observed therapy, short-course: see TUBERCULOSIS. double diffusion (serol.) GEL DIFFUSION in which antigen and antibody both diffuse through the gel (cf. SINGLE DIFFUSION). In one procedure, this is achieved by placing antibody and antigen at opposite ends of a gel column; in the Oakley–Fulthorpe test (double diffusion, single DIMENSION) antibody is incorporated in a layer of gel, and this is separated by a layer of plain gel from the aqueous solution of antigen which is placed on top. (cf. OUCHTERLONY TEST.) double dimension (serol.) See DIMENSION. double fixation See ELECTRON MICROSCOPY (a). double helix See DNA. double lysogen See e.g. TRANSDUCTION. double-negative T cells See T LYMPHOCYTE. double recessive HOMOZYGOUS for a given recessive allele, or alleles. double refraction Syn. BIREFRINGENCE. double-strand break–repair model See RECOMBINATION Fig. 2. (See also RECOMBINATION REPAIR.) double thymidine blockade See SYNCHRONOUS CULTURE. double-vial See FREEZE-DRYING. doubling dilutions See SERIAL DILUTIONS. doubling time See GROWTH (a). Doulton filter See FILTRATION. dourine (mal du coit) A chronic or subacute, often fatal, sexually transmitted disease of equines caused by Trypanosoma equiperdum; it occurs in parts of Africa, America, Asia and Europe. Initial symptoms include oedematous, inflamed lesions of the genitalia, and transient skin lesions (plaques) up to 5 cm in diameter; later symptoms include anaemia and neurological involvement (e.g. paralysis). Quinapyramine and suramin have been used therapeutically. A carrier state occurs. down mutation (down-promoter mutation) A MUTATION in a PROMOTER which results in a decreased level of transcription from that promoter. (cf. UP MUTATION.) downcomer In a LOOP FERMENTER: the descending column of liquid or that part of the fermenter which contains it. (cf. RISER.) downstream (mol. biol.) In, or in relation to, the direction in which a nucleic acid strand or a polypeptide chain is synthesized; the converse of ‘downstream’ is referred to as ‘upstream’. (See e.g. PROMOTER.) 252 Dunaliella The organisms form septate mycelium and pigmented, septate, commonly cylindrical conidia; conidiogenesis is tretic (see CONIDIUM). Drechslera spp are typically graminicolous (cf. e.g. PHAEOHYPHOMYCOSIS). Species which have been transferred from HELMINTHOSPORIUM to this genus include e.g. D. gramineae, D. maydis, D. oryzae (causal agent of brown spot disease of rice), and D. victoriae (see also VICTORIN). Dreyer’s tube A small, conically-based glass test-tube with a flared rim and an internal diameter of ca. 5 mm; it is used in serology for the clear observation of reactions involving agglutination or precipitation. drift See GENETIC DRIFT and ANTIGENIC DRIFT. DRNA DISSIMILATORY REDUCTION OF NITRATE TO AMMONIA. drop plate method Syn. MILES AND MISRA’S METHOD. droplet infection INFECTION (sense 1) by inhalation of an AEROSOL of saliva, mucus etc. (contaminated with pathogens) from an infected individual. dropsy Syn. OEDEMA. (See also INFECTIOUS DROPSY.) Drosophila A virus See PICORNAVIRIDAE. Drosophila C virus See PICORNAVIRIDAE. Drosophila P virus See PICORNAVIRIDAE. Drosophila X virus (DXV) A virus which causes anoxia sensitivity and death in Drosophila melanogaster and which can be cultivated in Drosophila cell lines [JGV (1979) 42 241–254]. DXV closely resembles IPN VIRUS in morphology, genome etc. (See also SIGMA VIRUS.) drug resistance Syn. ANTIBIOTIC resistance. dry bubble See BUBBLE DISEASES. dry rot (1) (of timber) A BROWN ROT of structural (and other) timbers caused by the cellulolytic fungus Serpula lacrymans; only wood having a moisture content greater than about 20% (see TIMBER SPOILAGE) can be attacked initially, but water produced during metabolism can be transported via moisture-conducting rhizomorphs, enabling the fungus to spread to drier regions of wood (or across brickwork etc). Infected timbers often exhibit longitudinal and cross-grain cracking and a surface growth of whitish mycelium containing yellow and/or lilac patches; the leathery fruiting bodies bear a mass of rust-coloured spores. Control of dry rot involves removal of decayed wood and disinfection of remaining timbers by heat and/or the application of fungicides (see TIMBER PRESERVATION). Infections similar to dry rot may be caused e.g. by Poria spp. (cf. WET ROT.) (2) A storage rot of potato tubers caused e.g. by Fusarium coeruleum ( = F. solani var. coeruleum) and F. sulphureum. The tubers develop dark, sunken lesions which develop into mycelium-filled cavities; affected tubers lose water and shrink, becoming wrinkled and eventually drying into a hard mass. Under humid conditions, however, infection by SOFT ROT organisms may result in rapid decomposition of the tubers. (See also TECNAZENE.) dryad’s saddle Polyporus squamosus (q.v.). drying (1) (of foods) See FOOD PRESERVATION (c). (2) (of plates) See PLATE. (3) (of microorganisms) See DESICCATION. ds (of DNA or RNA) Double-stranded. dsb genes See PROTEIN SYNTHESIS (protein folding). ds(c)DNA Double-stranded circular DNA. DSI stain See DIETERLE SILVER STAIN. DSM Deutsche Sammlung von Mikroorganismen (culture collection of microorganisms), Grisebachstr. 8, D-3400 Göttingen, Germany. DTH Delayed-type hypersensitivity: see DELAYED HYPERSENSITIVITY. dTMP Deoxythymidine 5
-monophosphate [see Appendix V(b)]. DTP Syn. DPT. dualtropic (polytropic) Refers to a recombinant retrovirus derived from an ecotropic virus and a xenotropic virus; the recombinant virus has an extended host range and can, like AMPHOTROPIC viruses, replicate in both homologous and heterologous cells. However, a dualtropic murine virus differs from an amphotropic murine virus in that it possesses env glycoprotein antigenic determinants in common with those of the parental ecotropic and xenotropic murine viruses, and may thus be neutralized by antiserum to the env glycoprotein of either or both of these viruses. [Book ref. 114, p. 73.] (See also MCF VIRUSES.) duck diseases See POULTRY DISEASES. duck embryo vaccine (DEV) See RABIES. duck hepatitis B virus (DHBV) A virus of the HEPADNAVIRIDAE which infects Pekin ducks (Anas domesticus) and other domestic ducks; strains of DHBV have also been detected in wild mallard [JGV (1986) 67 537–547]. DHBV can be transmitted vertically via the egg. Infected ducks commonly exhibit persistent viraemia and may develop chronic liver disease; integrated DHBV DNA has been detected in a hepatocellular carcinoma in a domestic duck [PNAS (1985) 82 5180–5184] (cf. HEPATITIS B VIRUS). duck infectious anaemia virus See AVIAN RETICULOENDOTHELIOSIS VIRUSES. duck plague Syn. DUCK VIRUS ENTERITIS. duck virus enteritis (duck plague) An acute, highly infectious disease of ducks, geese and swans; it is caused by anatid (or anserid) herpesvirus 1 (see HERPESVIRIDAE). Symptoms: loss of appetite, thirst, nasal and ocular discharge, neurological signs; multiple haemorrhages occur in internal organs. Transmission is believed to occur mainly via pond-water. duck virus hepatitis (DVH) An acute disease of young ducklings, caused by an ENTEROVIRUS; older ducks and other birds are not susceptible. Infection occurs by ingestion of food contaminated with faeces of infected birds. Incubation period: 24–48 hours. The disease progresses rapidly and death may occur within one or a few hours. On postmortem examination the liver is found to be enlarged and haemorrhagic. Vaccines are available for prevention; immune serum has been used in treatment. Dulbecco’s PBS Phosphate-buffered saline originally formulated to contain (g/l): NaCl (8.0), KCl (0.2), Na2 HPO4 (anhydrous) (1.15), CaCl2 .2H2 O (0.132), KH2 PO4 (0.2), and MgCl2 .6H2 O (0.1). dulcitol Syn. GALACTITOL. dulse See PALMARIA. Dunaliella A genus of unicellular, naked (wall-less), halotolerant, biflagellated green algae (division CHLOROPHYTA) which closely resemble CHLAMYDOMONAS spp except for their lack of cell wall. Dunaliella spp can grow in aquatic environments of a wide range of salinities, up to and including saturated brines, and are common e.g. in hypersaline environments (salt lakes etc) throughout the world – often forming extensive red or green blooms. The organisms have an exceptional osmoregulatory capacity, using glycerol as an osmoregulator (see OSMOREGULATION); the intracellular concentration of glycerol can be varied rapidly to compensate for changes in the osmotic strength of the surrounding medium. The glycerol can be formed either from stored starch or as a direct product of photosynthetic carbon fixation; a reduction in glycerol concentration apparently occurs by glycerol degradation. When certain species are subjected to hypo-osmotic shock, the cells swell, change shape (from elongated to spherical), 253 Duncan disease 10 distinct dsRNA segments whereas healthy isolates contain 0–4 dsRNA segments, suggesting that specific dsRNA segments may be associated with the development of disease [PP (1986) 35 277–287]. (See also CHESTNUT BLIGHT and MYCOVIRUS.) Duttonella A subgenus of TRYPANOSOMA within the SALIVARIA; species include parasites and pathogens of a range of wild and domestic animals (see e.g. NAGANA). The trypomastigote form typically has a large, terminal kinetoplast and a free flagellum. T. (D.) vivax occurs in the vertebrate in the trypomastigote form (ca. 20–30 µm in length) which divides by longitudinal binary fission; it is transmitted cyclically by Glossina spp – in which epimastigotes and metacyclic forms develop solely in the proboscis. Mechanical transmission by other biting flies (e.g. Tabanus) occurs outside the tsetse belt. T. (D.) uniforme is similar to T. (D.) vivax, though somewhat smaller. Duval’s bacillus Shigella sonnei. Duvenhage virus See LYSSAVIRUS. DVH DUCK VIRUS HEPATITIS. dw Dry weight. dwarf bunt See COMMON BUNT. DWELL See ETRIDIAZOLE. dyad symmetry See e.g. PALINDROMIC SEQUENCE. dye A water-soluble, aromatic compound which has coloured anions and/or cations that can bind to particular substance(s); binding may be primarily ionic but may involve e.g. hydrogenbonding. (See also AUXOCHROME; CHROMOPHORE; LEUCO COMPOUND.) (In microbiology ‘dye’ is often used interchangeably with ‘stain’ to refer not only to dyes, as defined, but also to other substances – e.g. LYSOCHROMES – used for STAINING; ‘stain’ may also refer to a mixture of dyes and/or to the process of using a dye or stain.) An acid dye has a coloured anion which combines with cationic groups. A basic dye has a coloured cation which combines with anionic groups. A neutral dye is a compound of acid and basic dyes in which each ion contains a chromophore. (The pH of a solution of such dyes does not necessarily indicate whether the dye is acid, basic or neutral.) An amphoteric dye is either acidic or basic according to pH. Although dyes are commonly used for STAINING, some have additional uses: see e.g. ACRIDINES, ETHIDIUM BROMIDE, METHYLENE BLUE, TRIPHENYLMETHANE DYES; see also PH INDICATORS, RESAZURIN TEST. Dyes are classified according to the nature of their chromophores. Those based on a (para- or ortho-) quinonoid ring include the TRIPHENYLMETHANE DYES, xanthene dyes (e.g. EOSIN, pyronin), azine dyes (e.g. JANUS GREEN, NEUTRAL RED, NIGROSIN), azine-related dyes (oxazines, e.g. NILE BLUE A; thiazines, e.g. METHYLENE BLUE, thionin, toluidine blue), and HAEMATEIN. Dyes with a chromophore of one or more azo groups (−N=N−) include BISMARCK BROWN, CHLORAZOL BLACK E, CONGO RED and TRYPAN BLUE; Janus green also contains an azo group. Dyes with a nitro group as chromophore include 2,4,6-trinitrophenol (picric acid). dye-buoyant density centrifugation See ultracentrifugation in the entry CENTRIFUGATION. dye test Syn. TOXOPLASMA DYE TEST. Dynabeads Microscopic beads used in magnetic separation: a technique in which a particular type of cell or molecule is separated from a mixture (in the liquid phase) by allowing the required cells or molecules to bind to beads coated with a specific ligand – the beads then being segregated by use of a magnetic field. Dynabeads (Dynal, Skøyen, Oslo, Norway) are microscopic spheres that contain a mixture of iron oxides; they are superparamagnetic, i.e. they do not exhibit magnetic properties until and lose motility. (In D. salina expansion of the cytoplasmic membrane during swelling is apparently achieved by fusion of the membrane with numerous small membranous vesicles present in the cytoplasm [JCB (1986) 102 289–297].) Some of the shocked cells lose buoyancy and sink; they may then continue to reproduce slowly or remain dormant, but they recover their motility when transferred to a medium of more favourable salinity. It has been suggested that this is a survival strategy whereby the organism can escape from the upper layers of (potentially damaging) low salinity resulting e.g. from rain or land drainage [JEMBE (1985) 91 183–197]. (See also SINGLE-CELL PROTEIN.) Duncan disease Syn. XLP SYNDROME. Duovirus Syn. ROTAVIRUS. duplex DNA Double-stranded DNA. duplex PCR See MULTIPLEX PCR. duplex winding number See DNA. Durham shelter (gravity slide sampler) An instrument used for sampling airborne particles, particularly pollen. (See also AIR). Essentially it is an adhesive-coated microscope slide mounted horizontally inside a ‘shelter’ consisting of two parallel metal discs joined together by three equidistant 10-cm struts; the upper disc acts as a rain shield. Particle deposition is greatly affected by e.g. the speed and direction of the wind and by turbulence, and the volume of air sampled is unknown. Durham tube A transparent glass tube (2–4 cm long, internal diam. ca. 3–4 mm), closed at one end, used to detect gas production by microorganisms during growth in a liquid medium. An inverted Durham tube is placed in a test-tube of liquid medium before sterilization; when the medium is autoclaved all the air is driven from the Durham tube which thus sinks to the bottom of the test-tube. On inoculation and incubation, any gas formed is trapped in the Durham tube. Durvillaea See PHAEOPHYTA. Dutch elm disease A disease of elm trees (Ulmus spp) caused by Ceratocystis ulmi. In the 1970s/1980s a new aggressive strain of C. ulmi caused massive epiphytotics, destroying millions of elms throughout Europe, SW Asia and North America. The disease is spread by bark beetles of the Scolytidae (commonly Scolytus spp) in a complex series of interactions [review: Book ref. 77, pp. 271–306]. Essentially, adult beetles, carrying C. ulmi propagules, emerge during the spring and summer and feed in the tops of healthy trees; the beetles feed in the crotches of young twigs, and some of the resulting wounds become contaminated with C. ulmi. The pathogen develops and spreads within the xylem vessels (‘pathogenic phase’) and infected twigs wilt and show characteristic streaks or spots; TYLOSES are formed, and the whole tree may die within months. The inner bark (phloem) of dying and dead trees becomes a suitable breeding ground for the scolytid beetles. Female beetles excavate extensive breeding galleries in the bark. C. ulmi colonizes these galleries (‘saprotrophic phase’) and sporulates abundantly (forming conidia, including synnematospores, and ascospores) during the autumn and winter when the larvae are developing. The new generation of adult beetles, carrying spores of C. ulmi, emerges the following year to feed in the twigs of healthy trees, thus completing the cycle. Various ‘diseased’ strains of C. ulmi, characterized by abnormal or reduced growth and impaired reproductive fitness, have been isolated. The disease is apparently caused by an infectious agent, called a ‘d factor’ (for disease factor), which is spread between mycelia by hyphal fusion during the bark stage of the fungus; it has been found that certain diseased isolates contain 254 dysentery placed in a magnetic field. The beads can be coated with any of a variety of ligands – the particular type used depending on the required cell or molecule. For example, if beads are coated with the oligonucleotide T-T-T-T-T-T-T they will bind the poly-A tails of mRNA molecules; this approach can be used to separate mRNAs from total RNA. Essentially, the beads are well dispersed within the sample by thorough mixing; the required cells or molecules bind to the beads – which are then drawn to one side of the vessel by a magnetic field. Unwanted material is discarded, and the required cells/molecules can be washed etc. prior to use. (Note that permanently magnetic beads would not be suitable: they would clump together rather than disperse within the sample.) Beads coated with specific antibodies are useful for isolating particular pathogens – E. coli O157, Salmonella spp, Listeria monocytogenes etc. – from food or other specimens; the bead–pathogen complex can be plated on a suitable medium. This approach can also be used to isolate a specific type of eukaryotic cell from a mixture of cells. This method is called immunomagnetic separation (IMS). Examples of use include: enrichment of Mycobacterium paratuberculosis in milk [AEM (1998) 64 3153–3158]; detection of E. coli O157 in meat [LAM (1998) 26 199–204]; preparation of human glomerular endothelial cells for use in tests with E. coli verocytotoxin [Kidney International (1997) 51 1245–1256]; detection of specific mycobacterial DNA prior to PCR (‘sequence capture’ PCR) [JCM (1996) 34 1209–1256]. dynein See FLAGELLUM (b). dysentery A disease characterized by inflammation of the intestine (particularly the colon), abdominal pain, and frequent passage of fluid stools that may contain blood, mucus and/or pus. ‘Dysentery’ (rather than DIARRHOEA) is the term generally used to refer to a diarrhoeal condition in which (i) blood/mucus is typically present in the stool, and (ii) the intestinal epithelium is actively invaded by the causative organism (as is the case with e.g. Shigella dysenteriae, enteroinvasive E. coli and Entamoeba histolytica). (1) (med.) (a) Bacillary dysentery. The classical (severe) form of dysentery is caused by Shigella dysenteriae serotype 1 (found mainly in tropical countries); somewhat less severe disease is caused by S. sonnei (a common causal agent in temperate zones), S. boydii (mostly tropical) and S. flexneri. As well as the shigellosis, caused by Shigella spp, bacillary dysentery can be caused e.g. by EIEC. Dysentery caused by S. dysenteriae type 1 is transmitted via the faecal–oral route – commonly via food or water contaminated with faeces from patients with dysentery (or from a carrier of the pathogen). Incubation period: 1–6 days. Onset is abrupt, with fever followed by diarrhoea and abdominal cramps. S. dysenteriae proliferates in the gut lumen and invades the epithelium of the terminal ileum and colon. While dysentery due to S. sonnei and S. boydii is typically self-limiting (lasting days), that caused by S. dysenteriae may cause ulceration of the intestinal mucosa and may last for weeks, leading to exhaustion and anaemia; mortality may be high in untreated cases. In animal studies, Shigella is taken up via the so-called ‘M cells’ in Peyer’s patches [M cells in infection (review): TIM (1998) 6 359–365]; from this location the bacteria can spread from cell to cell within the epithelial layer. Such spreading (an essential feature of dysentery) depends on the presence of a (plasmid-encoded) outer membrane protein: IcsA (= VirG). (IcsA is the a-domain of an autotransporter: see type IV secretory systems in PROTEIN SECRETION. An analogous protein occurs in EIEC.) IcsA promotes actin-based motility: i.e. it induces polymerization of actin filaments at one pole of the cell – forming a growing ‘tail’ of actin which propels the bacterium (through the host cell cytoplasm) in a direction opposite to that of the developing tail. The tail remains stationary within the host cell: ongoing deposition of actin causes bacterial motility. The motile bacterium pushes into the wall of the adjacent cell, either forming a pocket or becoming enclosed within a vacuole (bounded by a double membrane); Shigella lyses the membranes, thus entering the adjacent cell. Mutants lacking a functional IcsA do not spread, and do not cause dysentery. Shigella taken up by M cells may be engulfed by macrophages which occur beneath the M cells. Shigella can kill macrophages (see APOPTOSIS), and in the resulting inflammatory response PMNs may migrate between epithelial cells to reach the gut lumen; such migration is likely to expose basolateral surfaces of epithelial cells to invasion by Shigella [TIM (1997) 5 201–204]. Shigella can induce uptake by the so-called ‘trigger mechanism’, invasion apparently occurring via the basolateral surfaces of epithelial cells; the bacterial adhesin(s) are unknown, but epithelial cell receptors may include integrins (which occur on basolateral surfaces). Invasion involves a type III secretory system (see PROTEIN SECRETION) encoded by the plasmid-borne mxi-spa genes. The same plasmid also encodes IpaA; this protein is translocated to the epithelial cell where it induces certain changes (e.g. actin polymerization) needed for uptake. During uptake (macropinocytosis), the bacterium is engulfed by extrusions of host cell membrane which are supported by actin microfilaments. The bacterium thus induces its own uptake; moreover, the host cell’s GTPase ‘Rho’ also appears to be required. Following uptake (into a vacuole), Shigella lyses the membrane and escapes into the host cell’s cytoplasm; such lysis may involve the (plasmid-encoded) IpaB protein. Cell-to-cell translocation within the epithelial layer can then occur as described above. [Invasion of host cells by Shigella and subsequent inflammation: FEMS Reviews (2001) 25 3–14.] Shigella dysenteriae serotype 1 produces SHIGA TOXIN (q.v.); the role of this toxin in dysentery in not fully understood, but it may be at least partly responsible for the vascular damage and bloody stools. Lab. diagnosis. Culture from stool, or rectal swab, followed by serological typing; colicin and/or phage typing may also be useful. Treatment. Antiobiotics (when indicated, and in accordance with results from susceptibility tests); fluid and electrolyte replacement. (See also ORAL REHYDRATION SOLUTION.) (b) Amoebic dysentery (amoebiasis) is caused by Entamoeba histolytica (see ENTAMOEBA). Infection follows ingestion of cysts from the faeces of patients or carriers. Following excystment, the organisms localize in the large intestine and attack the mucosa; establishment of infection apparently requires the presence of normal gut bacteria (see GASTOINTESTINAL TRACT FLORA). Symptoms range from mild, intermittent diarrhoea to severe, and occasionally fatal, dysentery with blood and mucus in the stools. (Trophozoites produce cyto/enterotoxin(s) [Inf. Immun. (1985) 48 211–218].) Chronic relapsing cases, particularly in endemic regions, may be associated with the development of tumour-like granulomatous masses (called amoebomas) in the large intestine. Severe amoebic dysentery may be associated with amoebic hepatitis (apparently not due to amoebic invasion of the liver). The parasite may also cause abscesses, usually in the liver but also in the lungs, brain etc.; the presence of viable 255 dysgonic (2) (vet.) See e.g. LAMB DYSENTERY, SWINE DYSENTERY, WINTER (cf. SCOURS.) dysgonic See EUGONIC. dyskinetoplasty In e.g. certain naturally-occurring strains of Trypanosoma evansi : the absence of a normal KINETOPLAST – the mitochondrion containing, instead, a small, spherical, electrondense body, the ‘kinetoplast remnant’, which is not visible in Giemsa-stained preparations. Dyskinetoplastic strains of T. evansi and other trypanosomes can be obtained by exposure to e.g. Berenil. dysphotic zone See PHOTIC ZONE. dyspnoea (dyspnea) Difficult or laboured breathing. Dysteria See HYPOSTOMATIA. DZM Dorsal zone of membranelles: a SYNCILIUM (or syncilia) occurring in the anteriodorsal region of some ciliates, e.g. Diplodinium, Epidinium. amoebae in patients with high titres of antibodies may be at least partly explained by the ability of amoebae to shed aggregates of absorbed antibodies (‘caps’ – see CAPPING sense 3) [JID (1986) 153 927–932]. Lab. diagnosis involves e.g. demonstration of trophozoites and/or cysts in stools or lesions. (See also FLOTATION.) Chemotherapy: see AMOEBICIDE. (c) Balantidial dysentery (balantidiasis) is a severe, sometimes fatal, dysentery caused by Balantidium coli (see BALANTIDIUM). Infection follows ingestion of cysts in food or water contaminated with infected swine faeces. Excystment occurs in the gut; trophozoites cause extensive ulceration of the colon mucosa. Lab. diagnosis: identification of trophozoites or cysts in faeces. Chemotherapy: e.g. diiodoHYDROXYQUINOLINE. DYSENTERY. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 256 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 E Corynebacterium, Mycobacterium and various yeasts. (See also BODY MICROFLORA.) Earle’s BSS Earle’s BALANCED SALT SOLUTION; it contains (g/l): NaCl (6.8), KCl (0.4), MgSO4 .7H2 O (0.2), NaH2 PO4 .2H2 O (0.158), CaCl2 .2H2 O (0.264), NaHCO3 (2.2), glucose (1.0), and phenol red (0.01); pH: 7.6–7.8. early blight (of potato) A POTATO DISEASE caused by Alternaria solani. Small dark spots appear on leaves, each spot having concentric rings of necrotic tissue which give a characteristic ‘target’ appearance; tubers may exhibit a superficial brown, dry, corky rot. early genes See VIRUS. earth balls See SCLERODERMATALES. earth stars See LYCOPERDALES. earth tongues Fruiting bodies of species of e.g. Geoglossum or Trichoglossum. East Coast fever An East African tick-borne disease of cattle caused by species of THEILERIA; T. parva and T. lawrencei cause a febrile disease which is usually fatal, while T. mutans typically causes non-febrile anaemia which may be fatal. After the tick vector has ingested blood from an infected bovine host, sexual stages of Theileria appear in the tick’s gut; zygotes develop in the gut, and each forms a motile kinete which migrates to the tick’s salivary glands and undergoes schizogony to form infective sporozoites. The tick can then infect a fresh bovine host. In the new (vertebrate) host the parasite grows in fixed and/or circulating lymphoid cells and forms macroschizonts (‘Koch’s blue bodies’; ‘blue bodies’): schizonts, 10–20 µm in size, whose cytoplasm stains blue with Giemsa’s stain. Subsequently, intra-erythrocytic forms (‘piroplasms’) of T. parva (or T. mutans) become common, but may be scanty/undetectable in T. lawrencei. T. parva and T. lawrencei infections usually kill after 15 days. [Life cycles of Babesia and Theileria: AP (1984) 23 37–103. Anti-T. parva vaccine: Annals New York Acad Sci (2000) 916 464–473.] EAST 1 A heat-stable toxin secreted e.g. by EAGGEC, many strains of EHEC O157:H7, and by Yersinia enterocolitica; it is an analogue of the endocrine hormone guanylin (which stimulates fluid outflow from the intestinal mucosa by activating guanylyl cyclase). eastern equine encephalomyelitis (EEE; eastern equine encephalitis; eastern encephalitis) An acute ENCEPHALITIS (or encephalomyelitis) of man and horses, caused by an ALPHAVIRUS; it occurs in eastern North America, the Caribbean, and parts of Central and South America. EEE virus can also cause disease in domestic birds (pigeons, pheasants). The virus occurs in wild birds, in which transmission occurs mainly via Culiseta melanura; other mosquitoes (e.g. Aedes spp) may be responsible for the sporadic transmission of EEE to animals and man. In man, the mortality rate may be ca. 25–70% or higher. Epizootics in horses may be controlled by vaccination. Eaton’s agent Mycoplasma pneumoniae. EB ELEMENTARY BODY. EB virus EPSTEIN–BARR VIRUS. EBERs (EBV) See EPSTEIN–BARR VIRUS. ebna genes (EBV) See EPSTEIN–BARR VIRUS. Ebola virus See FILOVIRIDAE. EBV EPSTEIN–BARR VIRUS. E (1) See REDOX POTENTIAL. (2) Glutamic acid (see AMINO ACIDS). E-cadherin See CADHERINS. E. coli An abbreviation which usually refers to the bacterium Escherichia coli (see entry ESCHERICHIA) but which is also used e.g. for the protozoan Entamoeba coli. (This illustrates the need to ensure that the name of a genus is spelt out in full – on first usage – before using an abbreviated form.) ∗ E precursor cells (immunol.) Cells on whose surface COMPLEMENT FIXATION has occurred, at 4° C, but which do not lyse unless the temperature is raised; lysis can be further inhibited, even at higher temperatures, by EDTA or zinc or uranyl salts. E protein See F PLASMID. E-selectin Syn. CD62E. E site (of a ribosome) See PROTEIN SYNTHESIS. E test A diffusion test for determining the MIC of a given bacterial strain with respect to particular antibiotic(s). One side of a plastic strip (placed in contact with the inoculated plate) carries a given antibiotic, the concentration of which decreases uniformly from one end of the strip; the other side of the strip is graduated with the concentration of antibiotic. Following incubation, the MIC is read by noting the lowest concentration of antibiotic (on the scale) which corresponds to inhibition of growth. Several strips, each with a different antibiotic, can be used simultaneously on a standard-sized plate. The E test has been used for various bacteria, including Pseudomonas aeruginosa [JCM (1991) 29 533–538] and [Helicobacter pylori [JCM (1997) 35 1842–1846]. [Mycobacterium tuberculosis: JCM (2002) 40 2282–2284.] E0 (E0 ) See REDOX POTENTIAL. E0 See REDOX POTENTIAL. (E1 E2 )-type H+ -ATPase See PROTON ATPASE. e14 element See RECOMBINATIONAL REGULATION. E5531 See ENDOTOXIC SHOCK. EA Erythrocyte–antibody (see e.g. HAEMOLYTIC SYSTEM). (cf. EAC.) EAC Erythrocyte–antibody–complement. EAC 142 . . . indicates that COMPLEMENT components 142 . . . have been fixed. (cf. EA.) eae gene See PATHOGENICITY ISLAND. EAEC (1) ENTEROADHERENT E. COLI [RMM (1995) 6 196–206]. (2) Enteroaggregative E. coli (see EAGGEC) [MR (1996) 60 167–215 (p. 186)]. EAF plasmid See PATHOGENICITY ISLAND and BUNDLIN. EAggEC Enteroaggregative Escherichia coli: strains of E. coli associated with persistent diarrhoea – usually (though not exclusively) in children in the developing countries. (See also ENTEROADHERENT E. COLI.) The organisms form toxins and adhesins, but the mechanism of pathogenesis is unknown. EAggEC may be particularly important as pathogens for undernourished/immunosuppressed patients. [Enteroaggregative and diffusely adherent E. coli: RMM (1995) 6 196–206.] Strains of O111:H12 in Brazil were reported to have the properties of EAggEC [FEMS (1997) 146 123–128]. (See also EAEC and EAST 1.) Eagle’s medium Any of a number of growth or maintenance media (used in TISSUE CULTURE) consisting basically of EARLE’S BSS or HANKS’ BSS supplemented with e.g. amino acid(s), vitamin(s), antibiotic(s), serum etc. ear microflora The microflora of the human ear (external auditory canal) commonly includes e.g. strains of Staphylococcus, 257 EC with a salt concentration of ca. 3–30%, sulphide being oxidized to elemental sulphur which is deposited extracellularly. Species include e.g. E. abdelmalekii, E. halochloris, E. halophila and E. vacuolata. ectothrix infection See DERMATOPHYTES. ectotrophic mycorrhiza See MYCORRHIZA. ectotunica See BITUNICATE ASCUS. Ectrogella See SAPROLEGNIALES. ectromelia virus (mousepox virus) A virus (genus ORTHOPOXVIRUS) which infects mice, causing a disease (mousepox) in which internal organs (mainly liver and spleen) are involved, and characteristic lesions develop on the skin. Ectromelia virus replicates slowly on the CAM, forming small, white, irregularly shaped pocks. eczema An inflammatory condition of the skin involving redness, itching, development of papules and vesicles etc; it is usually an allergic reaction. Eczema herpeticum and eczema vaccinatum are severe, sometimes fatal, generalized conditions resulting from disseminated infection with herpes simplex virus and vaccinia virus, respectively; they occur most commonly in patients with atopic dermatitis or certain immunodeficiency syndromes. (See also PROBIOTIC) ED pathway ENTNER–DOUDOROFF PATHWAY. ED50 Effective dose (50%): that dose of a given agent which, when given or applied to each of a number of experimental test animals or systems, affects 50% of those animals or systems under given conditions. Edam cheese See CHEESE-MAKING. edaphic Of or pertaining to terrestrial habitats – particularly soil, but also e.g. leaf litter and the surfaces of living plants. EDDA An iron chelator: ethylenediamine-di(o-hydroxyphenylacetic acid). edeines A group of basic linear peptide antibiotics produced e.g. by Bacillus brevis; they are effective against a broad spectrum of organisms, including Gram-positive and Gram-negative bacteria, fungi, and some mammalian cells. At low concentrations edeines are bacteriostatic, selectively and reversibly inhibiting DNA synthesis; at higher concentrations they may be bactericidal, inhibiting the synthesis of proteins but apparently not of RNA. Edelfäule (German) syn. NOBLE ROT. edema Syn. OEDEMA. edifenphos See HINOZAN. editing (RNA) See RNA EDITING. EDTA Ethylenediaminetetraacetic acid: (CH2 .N(CH2 COOH)2 )2 . EDTA strongly chelates divalent metal ions (e.g. Zn2+ , Ca2+ , Mg2+ ); it is used e.g. as an ANTICOAGULANT (preventing clotting by complexing plasma Ca2+ ) and as a potentiating agent in certain antibacterial preparations (see e.g. DETTOL CHELATE). EDTA causes a non-specific increase in the permeability of Gramnegative bacteria, and under certain conditions it can inhibit or lyse many Gram-negative species – Pseudomonas aeruginosa being particularly susceptible; the antibacterial activity of EDTA may involve one or more of several phenomena: e.g., loss of structural integrity of the OUTER MEMBRANE due to chelation of membrane-stabilizing divalent cations, and potentiation of peptidoglycan-degrading autolysins. EDTA has been used in the preparation of SPHAEROPLASTS of Gram-negative bacteria (see also OSMOTIC SHOCK). (The susceptibility of P. aeruginosa to EDTA can be decreased by growth in Mg2+ -limited media owing to changes in the outer membrane proteins [JGM (1983) 129 509–517].) (cf. CDTA; EGTA; NTA.) Edwardsiella A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Cells: ca. 1 × 2–3 µm, motile (except EC (1) Energy charge (See ADENYLATE ENERGY CHARGE). (2) Enzyme Commission (see ENZYME). ECA ENTEROBACTERIAL COMMON ANTIGEN. ecad A distinct population, within a given species, which has adapted phenotypically to its environment. (cf. ECOTYPE.) ecchymosis See PURPURA. Eccrinales See TRICHOMYCETES. ecdysis The shedding of an outer layer: e.g., the exoskeleton of an invertebrate, or the theca in certain DINOFLAGELLATES. Echinamoeba See AMOEBIDA. echinocandin B See ACULEACIN A. echinomycin See QUINOXALINE ANTIBIOTICS. Echinosporangium See MUCORALES. Echinosteliales See MYXOMYCETES. Echinosteliopsidales See MYXOMYCETES. Echinosteliopsis See MYXOMYCETES. Echinostelium A genus of slime moulds (class MYXOMYCETES) which form protoplasmodia and small, stalked sporangia (<0.5 mm tall). E. lunatum is the smallest known myxomycete, forming stalked sporangia <50 µm tall and bearing only 4–8(–14) spores. [Encystment of myxamoebae and protoplasmodia in E. minutum: Mycol. (1985) 77 253–258.] echinulate Covered with small spines. echoviruses See ENTEROVIRUS. eclipse complex See TRANSFORMATION (1). eclipse period See ONE-STEP GROWTH EXPERIMENT. EcoB A type I RESTRICTION ENDONUCLEASE. EcoK A type I RESTRICTION ENDONUCLEASE. econazole See AZOLE ANTIFUNGAL AGENTS. Economo’s encephalitis Syn. VON ECONOMO’S ENCEPHALITIS. EcoPI A type III RESTRICTION ENDONUCLEASE. EcoRI A RESTRICTION ENDONUCLEASE from Escherichia coli ; G/AATTC. EcoRV See RESTRICTION ENDONUCLEASE (table). ecotropic Refers to a (endogenous or exogenous) retrovirus which can replicate only in hosts of the species in which it originated. (cf. XENOTROPIC, AMPHOTROPIC.) ecotype A distinct population, within a given species, which has adapted genetically to its environment. (cf. ECAD.) ectendomycorrhiza See MYCORRHIZA. ectocarpene See PHEROMONE. Ectocarpus See PHAEOPHYTA. ectocyst See CYST (b). ectomycorrhiza See MYCORRHIZA. ectoparasite See PARASITE. ectoplasm See e.g. SARCODINA. ectoplasmic net See LABYRINTHULAS and THRAUSTOCHYTRIDS. ectosymbiont An organism which lives on another organism in a symbiotic association (SYMBIOSIS sense 1); an ectosymbiont remains external to the cells and tissues of a host, although it may live e.g. within the gut cavity of the host. (cf. ENDOSYMBIONT.) Ectothiorhodospira A genus of photosynthetic bacteria originally placed in the family CHROMATIACEAE but later proposed as type genus of a new family, Ectothiorhodospiraceae [IJSB (1984) 34 338–339]. Cells: spiral, vibrioid or rod-shaped; they contain Bchl a or Bchl b (according to species – see CHLOROPHYLLS) and various CAROTENOIDS (cell suspensions are typically brown to brown-red). The pigments occur in lamellar intracytoplasmic membrane systems which are continuous with the cytoplasmic membrane. Motility is mediated by a single polar flagellum or by a polar tuft of flagella. Gas vacuoles occur in some strains. The organisms occur typically in alkaline sulphidecontaining saline habitats; they grow optimally at pH 7.5–9.1 258 EHEC E. ictaluri ). VP −ve; acid (commonly with gas) from glucose; lactose −ve; phenylalanine deaminase −ve; arginine dihydrolase −ve; lysine decarboxylase +ve. Vitamins and amino acids are required for growth. Optimum temperature: 37° C (ca. 25° C for E. ictaluri ). The organisms are usually resistant to colistin but sensitive to other antibiotics (including penicillins); R plasmids are rare. GC%: 53–59. Type species: E. tarda. E. anguillimortifera. Nomen dubium [Book ref. 22, pp. 488–489]. E. hoshinae. Indole usually −ve; MR +ve; H2 S −ve (on TSI); acid from e.g. sucrose, D-mannitol and trehalose. E. hoshinae occurs in the intestines of animals (including mammals, fish and reptiles). E. ictaluri. Non-motile. Indole −ve; MR −ve; H2 S −ve (on TSI); no acid from sucrose, D-mannitol or trehalose. Causal agent of ENTERIC SEPTICAEMIA in channel catfish. E. tarda. Indole +ve; MR +ve; H2 S +ve (on TSI); usually no acid from sucrose, D-mannitol or trehalose. A few strains (‘biogroup 1’) produce acid from e.g. sucrose and D-mannitol but not from trehalose. E. tarda occurs in the intestines of animals (including mammals, fish and reptiles). It can be an opportunist pathogen in man, and may be a cause of diarrhoea; it can also cause localized and generalized sepsis in fish. [Book ref. 22, pp. 486–491.] EEE EASTERN EQUINE ENCEPHALOMYELITIS. Eel Virus European See IPN VIRUS. EES Ethylethane sulphonate: a mutagenic ALKYLATING AGENT. EF (1) Elongation factor: see PROTEIN SYNTHESIS. (2) Oedema (edema) factor: see ANTHRAX TOXIN. efavirenz See ANTIRETROVIRAL AGENTS. EF0 F1 H+ -ATPase See PROTON ATPASE. effective publication See NOMENCLATURE. efficiency of plating See EOP. efflux mechanism (of antibiotic resistance) In some microorganisms: any of various systems (usually or always energydependent) which can mediate resistance to an ANTIBIOTIC (or to more than one antibiotic) by extruding such agent(s) through the cytoplasmic membrane or the cell envelope; efflux involves a TRANSPORT SYSTEM that often consists of a complex of proteins but which, in some cases, comprises a single protein. Efflux transporters are classified according to their overall structure and e.g. the homology of amino acid sequences in component proteins; some efflux systems are ABC TRANSPORTERS, while others are found e.g. in the major facilitator superfamily (MFS) or resistance–nodulation–division (RND) superfamily. (See also MATE.) For examples of efflux-dependent antibiotic resistance (and/or references) see e.g. entries ABC TRANSPORTER, MACROLIDE ANTIBIOTICS, QUINOLONE ANTIBIOTICS, TETRACYCLINES. effused-reflexed (mycol.) Refers to a sheet-like fruiting body which lies flat against the substratum except at the edge(s), the latter growing outwards from the substratum. EF-G See PROTEIN SYNTHESIS (elongation). eflornithine See SLEEPING SICKNESS. efrapeptin A lipophilic polypeptide which binds non-covalently to and inhibits (F0 F1 )-type PROTON ATPASES. efrotomycin See POLYENE ANTIBIOTICS (b). EF-Ts, EF-Tu See PROTEIN SYNTHESIS (elongation). EGF Epidermal growth factor. (See e.g. ONCOGENE.) egg (for cultivating viruses) See EMBRYONATED EGG. egg-drop syndrome In general: any fall in egg production in poultry, due e.g. to poor housing, incorrect nutrition, various POULTRY DISEASES, etc. ‘Egg-drop syndrome 1976’ is a specific syndrome which came into prominence in the UK and Ireland in 1976; it is caused by an adenovirus which can agglutinate fowl RBCs, and which may be indigenous in duck populations [Book ref. 116, pp. 535–536]. In this syndrome egg-drop is preceded by egg abnormalities – e.g. eggs may be soft-shelled or shellless. egg wrack Ascophyllum nodosum. egg-yolk agar (EYA; lecithovitellin (LV) agar) An agar-based medium incorporating the yolks of hens’ eggs (which contain the lipoprotein lecithovitellin). It is used e.g. to test for bacterial extracellular LECITHINASE activity; on EYA the colony of a lecithinase-producing strain is surrounded by a zone of opacity due to the deposition of insoluble diglycerides resulting from lecithin cleavage. (See also NAGLER’S REACTION.) On EYA, strains which produce LIPASES form colonies which are each surrounded by a pearly or iridescent zone due to a thin film of free fatty acids. eggplant mild mottle virus See CARLAVIRUSES. eggplant mosaic virus See TYMOVIRUSES. eggplant mottled crinkle virus See TOMBUSVIRUSES. eggplant mottled dwarf virus See RHABDOVIRIDAE. EGME Ethylene glycol monomethyl ether; it is used e.g. as a BIOCIDE (and antifreeze) in aircraft fuel. EGS (mol. biol.) External guide sequence: see SPLIT GENE (a). EGTA A chelating agent, ethyleneglycol-bis(b-aminoethylether)N,N,N
,N
-tetraacetic acid. EGTA complexes a range of cations (e.g. Ca2+ , Mg2+ , Zn2+ ); Mg2+ is complexed much less strongly than is Ca2+ . Egtved disease (viral haemorrhagic septicaemia; haemorrhagic septicaemia of trout) A FISH DISEASE affecting mainly rainbow trout (Salmo gairdneri ); the causal agent is a virus (a probable member of the RHABDOVIRIDAE). Symptoms may include skin darkening, bulging of the eyes, and abdominal swelling, or there may be few external symptoms; mortality may be high. The disease was first observed in Egtved, Denmark. eguttulate Lacking GUTTULES. Eh See REDOX POTENTIAL. EHC See OPTOCHIN. EHEC Enterohaemorrhagic Escherichia coli: pathogenic strains of E. coli which secrete (at least) one or both shiga-like toxins (see SHIGA TOXIN). Shiga-like toxins are also called verocytotoxins (VT1, VT2) because they are toxic for Vero cells (kidney cells of the African green monkey); hence, EHEC is sometimes referred to as verocytotoxic E. coli (VTEC). A minority of authors refer to EHEC as shiga-toxin-producing E. coli (STEC). EHEC is not an invasive, intracellular pathogen (cf. EIEC; see also EAGGEC, EPEC, ETEC). EHEC is typically a food-borne pathogen which produces symptoms ranging from mild diarrhoea to severe bloody diarrhoea (haemorrhagic colitis); particularly (though not exclusively) in children, infection can give rise to HAEMOLYTIC URAEMIC SYNDROME. Vascular damage has been associated with the shiga-like toxins. Strains of EHEC typically contain the LEE PATHOGENICITY ISLAND, and many secrete EAST 1; these factors may be relevant to the inflammatory and secretory symptoms of EHEC-mediated disease [MR (1996) 60 167–215 (pp 186–188)]. It has been reported that the expression of operons on the LEE pathogenicity island is regulated by QUORUM SENSING (q.v.). Any of >25 serotypes of EHEC may cause disease, but, world-wide, O157:H7 is the serotype isolated most often in haemorrhagic colitis; O157:H− is also quite common. [Genome sequence of O157:H7: Nature (2001) 409 529–533; erratum: 259 Ehrlichia ehrlichiosis Any human or animal disease caused by a species of EHRLICHIA. Ehrlich’s reagent An INDOLE TEST reagent: p-dimethylaminobenzaldehyde (1 g) is dissolved in 95–100% ethanol (95 ml), and conc. HCl (20 ml) is added; stored at 4° C in the dark. EIA (serol.) (1) ENZYME IMMUNOASSAY. (2) ERYTHROIMMUNOASSAY. EIAV Equine infectious anaemia virus: see SWAMP FEVER and LENTIVIRINAE. eicosanoids Polyunsaturated fatty acids which contain 20 carbon atoms; examples include ARACHIDONIC ACID and LEUKOTRIENES. EIEC Enteroinvasive Escherichia coli: strains of E. coli which can invade and destroy epithelial cells in the ileum/colon, causing watery diarrhoea or (bacillary) DYSENTERY. Invasiveness is dependent on virulence genes carried by a plasmid (pInv ). EIEC encodes a cell-surface protein analogous to the IcsA protein of Shigella (which promotes actin-based motility within epithelial cells: see DYSENTERY); the presence of this protein (in EIEC and Shigella) is associated with the absence of a specific cell-surface protease (OmpT in EIEC and SopA in Shigella) [Mol. Microbiol. (1993) 9 459–468]. EIEC produces at least one enterotoxin: ShET2 (named after Shigella enterotoxin 2: a similar toxin produced by S. flexneri ); this toxin is encoded by the sen gene – a (plasmid) gene formerly referred to as set2. [Toxins produced by EIEC: MR (1996) 60 167–215 (p. 188).] At least part of the secretory (diarrhoeal) effect brought about by invasive organisms (such as EIEC and Shigella dysenteriae) is likely to be due to their ability to upregulate cyclooxygenase activity in the intestinal mucosa – this leading to increased expression of PROSTAGLANDINS (such as PGE2 ) with resulting increase in fluid secretion. EIEC causes disease in both children and adults – outbreaks occurring e.g. in schools, hospitals and institutions. EIEC typically resembles Shigella – often being non-motile, and giving negative results in tests for lactose utilization and lysine decarboxylase. Eijkman test An IMVEC TEST which determines the ability of an organism to produce gas from lactose at 44° C ± 0.2° C; the test is often used to confirm the presence of coliforms in a water sample (see COLIFORM TEST) – for which purpose use is made of a medium such as lauryl tryptose lactose broth that includes an agent which inhibits endospore-forming bacteria. Eikenella A genus (incertae sedis) of anaerobic, facultatively aerobic, oxidase-positive, catalase-negative, chemoorganotrophic, Gram-negative bacteria which occur e.g. in the human mouth and intestine, and which can be opportunist pathogens. Cells: non-motile, round-ended rods (0.3–0.4 × 1.5–4.0 µm) or short filaments; rods may exhibit TWITCHING MOTILITY. Growth occurs e.g. on blood agar; haemin is required for aerobic growth of freshly-isolated strains. Plate cultures have a ‘bleach-like’ odour. Optimum growth temperature: 35–37° C. No acid is formed from carbohydrates. Nitrate is reduced to nitrite. Urease −ve. GC%: ca. 56–58. Type species: E. corrodens. [Book ref. 22, pp. 591–597.] Eimeria A genus of parasitic, homoxenous protozoa of the suborder EIMERIORINA (q.v. for life cycle). Eimeria spp form tetrasporic, dizoic oocysts (cf. e.g. ISOSPORA). No species is known to be pathogenic in man, but many cause disease e.g. in domestic animals (see COCCIDIOSIS). eimeriid Of the suborder EIMERIORINA. Eimeriina (1) A suborder of protozoa (order Eucoccida) later incorporated – together with organisms of the TOXOPLASMEA – in the suborder EIMERIORINA. (2) A suborder of protozoa (order Nature (2001) 410 240.] EHEC occurs e.g. in undercooked meat (particularly minced beef), milk and dairy products, and water (both drinking and recreational); the minimum infective dose is apparently <100 cells. In the UK (at least), farm animals (cattle, sheep) appear to constitute the main reservoir of infection. [EHEC (VTEC) O157 and other strains in England and Wales, 1995–1998 (epidemiology): JMM (2001) 50 135–142.] Under starvation conditions, strains of E. coli O157:H7 have been found to lose antigenicity corresponding to the O157 antigen – but to retain the ability to produce shiga-like toxins; this has suggested that immunological methods based on O157 antigenicity may be unsuitable for detecting toxin-producing strains which have been under starvation conditions for a long time [AEM (2000) 66 5540–5543]. Lab. diagnosis. Culture on sorbitol–MacConkey agar (SMAC medium) is based on the inability of (most) O157 strains to ferment sorbitol; these strains form pale colonies, while most strains of E. coli (which ferment sorbitol) form pink colonies. Increased sensitivity of detection may be achieved by pre-culture enrichment of samples with DYNABEADS [detection of E. coli O157 in food [LAM (1998) 26 199–204]. [Sorbitol-fermenting EHEC: JMM (2002) 51 713–714.] [Lab diagnosis: JCM (2002) 40 2711–2715.] Molecular methods include a PCR-based assay for the rfb gene whose product is essential for synthesis of the O157 lipopolysaccharide [JCM (1998) 36 1801–1804]. This method would not distinguish non-toxigenic strains of O157, but it could form part of a multiplex PCR which also assayed for the toxin gene. A LATEX PARTICLE TEST for the O157 serotype is available from Oxoid (Basingstoke, UK; product number DR620). Detection of ‘free’ toxin in patients’ faeces may be achieved by a ‘reversed passive latex agglutination test’ (see LATEX PARTICLE TEST); this test involves observation of latex particles (conjugated with anti-toxin antibodies) in the presence of various dilutions of sample (faecal extracts) [LAM (2001) 32 370–374]. Serotypes other than O157 may be identified by serotyping; probe-based tests can be used to detect toxigenicity. (See also STARFISH and SYNSORB PK.) Ehrlichia A genus of Gram-negative bacteria of the tribe EHRLICHIEAE. Cells: coccoid, often pleomorphic, non-motile, ca. 0.5–1.0 µm. Growth occurs intracellularly in circulating leucocytes; the bacteria are commonly seen as a group of cells within a membrane-limited inclusion body (‘morula’). Growth does not occur in cell-free media. Type species: E. canis. E. canis is the causal agent of canine ehrlichiosis: a disease which occurs worldwide and which is characterized by fever, nasal discharge and anorexia; a severe haemorrhagic form of the disease (‘tropical canine pancytopenia’) occurs in some dogs – particular breeds (e.g. German shepherd) being more susceptible than others. E. phagocytophila causes ehrlichiosis in cattle and sheep; E. equi causes equine ehrlichiosis; E. risticii causes POTOMAC HORSE FEVER (and can be transmitted experimentally to cattle [VR (2001) 148 86–87]); E. sennetsu (formerly Rickettsia sennetsu) causes ‘sennetsu rickettsiosis’ in man: a disease which, in the severe form, involves fever, anorexia and lymphadenopathy; the disease, which occurs in Japan, has been called ‘glandular fever’, ‘Hyuga fever’, ‘infectious mononucleosis’ and ‘Kagami fever’. (cf. INFECTIOUS MONONUCLEOSIS.) Ehrlichieae A tribe of bacteria of the family RICKETTSIACEAE; species are primarily pathogens of animals (including cattle, dogs, goats and sheep), though Ehrlichia sennetsu is pathogenic in man. The genera: COWDRIA, EHRLICHIA, NEORICKETTSIA. 260 electron microscopy Eucoccidiida [JP (1980) 27 37–58]) equivalent to the the region of the anterior depression (when present) and around the cell periphery; each consists of two interconnected, tightly coiled ribbon-like structures enclosed by a single membrane. On appropriate stimulation of the organism ejectosomes are discharged, the coiled structures of each being ejected to form a long tube. Ejectosomes appear not be toxic; their function is unknown. EI Tor vibrio See VIBRIO (V. cholerae). EL1–EL34 proteins See RIBOSOME. elaioplast A lipid-storing PLASTID. ELAM-1 Syn. CD62E. Elaphomyces See ELAPHOMYCETALES. Elaphomycetales An order of ectomycorrhizal, hypogean fungi of the ASCOMYCOTINA. The dark, globose ascocarp lacks a hamathecium. Asci: globose and unitunicate; evanescent. The ascocarp of Elaphomyces can be parasitized e.g. by Cordyceps spp. Elasmomycetaceae See RUSSULALES. elastase A protease which cleaves the protein elastin (the main component of elastic fibres in e.g. arterial walls and ligaments). An elastase is formed e.g. by Clostridium histolyticum (the dtoxin) and by Pseudomonas aeruginosa; the enzyme formed by P. aeruginosa (a zinc metalloenzyme) can also degrade human IgG in vitro (i.e., it can act as an ‘IgG protease’), and it appears capable of degrading mouse IgG in vivo [CJM (1984) 30 1118–1124]. elasticotaxis The tendency, observed in many species of GLIDING BACTERIA, to become orientated parallel to stress lines induced in an agar medium. electrical transport A term used by some authors to include both ELECTROGENIC TRANSPORT and ELECTROPHORETIC TRANSPORT. electricity generation See BIOFUEL CELL. electroblotting See BLOTTING. electrochromic effect See CAROTENOID BAND SHIFT. electroendosmosis (electro-osmosis) The movement of a charged fluid, relative to a fixed medium carrying the opposite charge, under the influence of an electrical gradient. Electroendosmosis occurs e.g. during the IMMUNOELECTROPHORESIS of serum in agar gels. In this procedure (often carried out at pH 8.6) some negatively charged proteins (e.g. g-globulins) migrate towards the negative pole (cathode). This is due to the ionization of the AGAR; the agar carries a net negative charge, and the fluid contains an excess of positive ions. The fluid thus moves towards the cathode – the flow being sufficiently strong to overcome the attraction of the anode for some negatively charged proteins and to carry them towards the cathode. electrofocusing Syn. ISOELECTRIC FOCUSING. electrofusion A wholly electrical method for bringing about the fusion of microbial or plant protoplasts, or animal cells (cf. HYBRIDOMA); essentially, protoplasts or cells are aggregated by a high-frequency alternating electric field and then fused by a short pulse (e.g. 1 ms) of direct current. (cf. ELECTRO-TRANSFECTION.) electrogenic transport Transport across an energy-transducing membrane resulting in the generation of, or change in, a transmembrane electrical potential difference. (cf. ELECTRICAL TRANSPORT and ELECTRONEUTRAL TRANSPORT.) electroimmunoassay (1) Syn. ROCKET IMMUNOELECTROPHORESIS. (2) Loosely, any form of IMMUNOELECTROPHORESIS. electrokinetic potential Syn. ZETA POTENTIAL. electron microscopy MICROSCOPY in which an electron beam interacts with a specimen and subsequently contributes, directly or indirectly, to the formation of an image of the specimen. (The specimen observed may be the original biological specimen EIMERI- ORINA. Eimeriorina (‘the coccidia’) A suborder of parasitic protozoa (order EUCOCCIDIORIDA) in which syzygy generally does not occur (cf. ADELEORINA) and in which motile zygotes are not formed (cf. HAEMOSPORORINA); sporozoites are typically enclosed within SPOROCYST(s). Genera include e.g. BESNOITIA, CRYPTOSPORIDIUM, EIMERIA, FRENKELIA, ISOSPORA, LANKESTERELLA, SARCOCYSTIS, TOXOPLASMA and TYZZERIA. (Other genera include Aggregata, Angeiocystis, Arthrocystis, Atoxoplasma, Barrouxia, Caryospora, Caryotropha, Cyclospora, Diaspora, Dobiella, Dorisa, Dorisella, Grasseella, Hoarella, Mantonella, Merocystis, Octosporella, Ovivora, Pfeifferinella, Pseudoklossia, Pythonella, Schellackia, Selenococcidium, Selysina, Sivatoshella, Skrjabinella, Wenyonella.) Generalized life cycles are given below. (a) Homoxenous species (e.g. Eimeria spp). The disseminative forms (oocysts) are ingested by a new host, and excystation occurs in the intestine: the oocyst breaks down and liberates a number of banana-shaped, non-flagellated (but motile by flexion) sporozoites ca. 5–20 µm in length. Sporozoites penetrate and usually grow within cells of the intestinal mucosa, each sporozoite rounding up and becoming a feeding cell (trophozoite). Each trophozoite undergoes SCHIZOGONY and subsequently ruptures its host cell to release many merozoites (small cells resembling sporozoites). Merozoites penetrate fresh host cells, become trophozoites, and schizogony is repeated; several schizogonous cycles may occur. Later, intracellular merozoites develop into gametocytes. Some gametocytes become large, uninucleate female macrogametes which remain in situ; others undergo repeated division and produce small, uninucleate, flagellated cells, the male microgametes, which escape from the (ruptured) host cell. Each macrogamete is fertilized in situ, and the zygote encysts to form an oocyst. Oocysts are often spherical or oval, ca. 10–40 µm according to species; a MICROPYLE may be present. The oocyst is liberated from the host cell and voided in the faeces. Outside the host the oocyst undergoes SPORULATION (cf. ENDOSPORULATION) in which the zygote gives rise to a number of (haploid) sporozoites. In many species the sporozoites are formed within one or more sacs (sporocysts) within the oocyst; the number of sporozoites per sporocyst, and the number of sporocysts per oocyst, are taxonomically important features. The sporulated oocysts are able to infect fresh hosts and complete the life cycle. (b) Heteroxenous species. In these species the asexual and sexual phases of the life cycle typically occur in different host species – referred to as the intermediate host and final (definitive) host, respectively; some species (e.g. Toxoplasma gondii ) are facultatively heteroxenous, others (e.g. most or all species of Sarcocystis) are obligately heteroxenous. These coccidia often parasitize extra-intestinal tissues – particularly in the intermediate host. In Sarcocystis spp the intermediate host becomes infected on ingestion of oocysts and/or sporocysts, and the parasite typically undergoes extra-intestinal schizogony, forming cysts in the muscles (see SARCOCYSTIS); when sarcocysts (or sporocysts) are ingested by the final host, sexual reproduction (only) occurs in the intestinal tissues and sporulated oocysts, or sporocysts, are shed in the faeces. (See also TOXOPLASMOSIS.) [See also Book ref. 18, pp. 102–165 (ultrastructure), pp. 229–285 (growth in tissue culture and avian embryos).] (See also COCCIDIOSIS.) ejectosome (‘protrichocyst’) A type of EXTRUSOME present in CRYPTOPHYTES. Ejectosomes occur just below the cell surface in 261 electron microscopy electron gun anode gun alignment coils gun airlock 1st condenser lens 2nd condenser lens beam tilt coils condenser 2 aperture objective lens access for specimen diffraction aperture diffraction lens intermediate lens 1st projector lens 2nd projector lens binocular 12x column vacuum block 35 mm roll film camera focusing screen plate camera 16 cm main screen ELECTRON MICROSCOPY. Cross section of a transmission electron microscope (the Philips EM400). (Courtesy of N. V. Philips’ Gloeilampenfabrieken, Eindhoven, Holland.) or a replica of it.) Electron microscopy can give magnifications within the range ca. 10× to 1000000×. Resolving power depends e.g. on the nature of the specimen and on the type of instrument used. The best resolving power is obtained with the transmission electron microscope (see (a) below) which has resolved a structure of ca. 1 nm periodicity in the purple membrane of Halobacterium salinarium; for non-periodic biological specimens the best resolution may be ca. 3–5 nm. Electron microscopy is used e.g. for examining viruses, macromolecules, and the ultrastructure of cells, and for determining the intracellular locations of e.g. specific enzymes and ions (see also IMMUNOELECTRON MICROSCOPY). Although living material has been examined in the electron microscope (EM), it is usual to examine non-living specimens which have undergone various pre-treatments; treatments vary according to e.g. the nature of the specimen and the purpose of the investigation. (a) Transmission electron microscopy. In the transmission electron microscope (TEM) an electron beam passes through the specimen which deflects or scatters some of the electrons; electrons whose paths are slightly deviated by the specimen are focused onto a fluorescent screen to form an image of the specimen. In general, electrons undergo little or no deflection when they pass through thin layers of biological material; hence, unless specially treated, such materials do not yield images in 262 electron microscopy the electron microscope. However, biological materials can be ‘stained’ with certain heavy metal compounds which become localized in specific regions of the specimen; this enhances the electron-deflecting properties of those regions and/or increases contrast between the specimen and its background. Preparation of thin sections for the TEM. Typically, a tissue or cell initially undergoes double fixation, e.g. pre-FIXATION with GLUTARALDEHYDE and post-fixation with OSMIUM TETROXIDE. The fixed specimen is dehydrated, e.g. by equilibration in increasing concentrations of ethanol (or acetone) in water. From 100% ethanol the specimen is commonly transferred to, and equilibrated with, propylene oxide before being embedded, i.e., impregnated with an epoxy resin (e.g. SPURR’S MEDIUM) or a methacrylate – substances which polymerize and harden without significant change in volume. (Equilibration of the specimen with propylene oxide between dehydration and embedding is advantageous since propylene oxide is more miscible than ethanol with epoxy resins.) The embedded specimen is then sectioned (see MICROTOME) to obtain ultrathin sections of maximum thickness ca. 100 nm. A section is placed on a specimen holder (grid ): a metal (usually copper) disc, 2–3 mm in diameter, perforated by rows of square holes each ca. 0.1 × 0.1 mm. Before receiving the section the surface of the grid may be coated with a support film: a sheet of e.g. FORMVAR or nitrocellulose 20–50 nm thick; such materials have a low, uniform electron-scattering power. (A support film is sometimes omitted when the section is resin-embedded; however, it is essential for a liquid specimen – see e.g. negative staining below.) The support film itself may be coated with a film of carbon (ca. 20 nm thick) which stabilizes it and improves resolution at high magnifications. The section, on the prepared grid, is now stained, i.e., immersed in a solution of a heavy metal compound (e.g. uranyl acetate, lead acetate, or a salt of phosphotungstic acid) which combines or complexes with particular components (e.g. lipids, proteins) – a type of staining called positive staining; images subsequently formed in the EM thus reflect the distribution of heavy metal atoms and the groups with which they combine. (Staining is sometimes carried out before the embedding stage by subjecting the specimen to prolonged exposure to e.g. osmium tetroxide or uranyl acetate; this is called ‘in-block’ staining.) The section is allowed to dry and is then ready for examination in the TEM. Negative staining (negative contrast) is used to reveal the outlines and surface contours of particulate specimens (e.g. macromolecules, viruses, bacteria). Essentially, the particles are immersed in a thin film of electron-opaque stain on a prepared grid; the stain penetrates the interstices of each particle and forms a thin background layer but usually does not permeate the particles themselves. Electrons are more readily transmitted through those regions occupied by the (relatively) electrontransparent particles, so that in the image these regions appear bright against a dark background; the presence of stain in surface hollows and grooves indicates surface topography. For example, the TOBACCO MOSAIC VIRUS virion appears as a light rod in which a dark, central, axial line indicates the presence of stain in the hollow core. To examine bacterial morphology, one drop of (fixed or unfixed) bacterial suspension is left on a prepared grid for one or more minutes (determined by trial and error) so that cells adhere to the surface; most of the drop is then drawn off with the edge of a filter paper. One drop of stain (e.g. 2% aqueous phosphotungstic acid or 2% aqueous ammonium molybdate) is then placed on the grid; after 30–60 seconds this is drawn off, as before. The fine film on the grid is left to dry and is examined in the TEM. (Alternatively, stain and suspension can be mixed, and the stained suspension applied to the grid.) Shadowing (shadow-casting) is used e.g. to study the morphology and/or dimensions of e.g. viruses and bacteria, and is also involved in the KLEINSCHMIDT MONOLAYER TECHNIQUE for examining nucleic acids. Essentially, the particles (e.g. viruses) on a prepared grid are exposed to a point source of the vapour of a heavy metal; the point source is so placed that metal is deposited on only one side of each particle – leaving a nonmetallized (electron-transparent) ‘shadow’ on the remainder of the particle and on that part of the support film shielded (by the particle) from the vapour. The image of a shadowed particle can give information on the shape and size of the particle; its size can be calculated from the length of the shadow and the angle at which the metal vapour was incident on the particle. Replicas of e.g. a freeze-etched specimen (see FREEZEETCHING) may be examined in the TEM. Essentially, this involves shadowing the surface of the frozen specimen with a heavy metal under vacuum; commonly, shadowing is effected at an angle of ca. 30° with a mixture of platinum and carbon vapours. Shadowing throws into relief the features of an irregular surface. Areas protected from the vapour remain nonmetallized and are therefore structurally weak; the primary replica is therefore strengthened by deposition of a layer of carbon from a vapour source placed directly above the specimen. The replica is cleaned by digesting away the specimen with strong oxidizing and hydrolysing agents (e.g. bleach, sulphuric acid, chromic acid). The replica is mounted on a grid and examined in the TEM. The transmission electron microscope (TEM ). A typical TEM is shown in the figure. The electron beam originates at a heated metal filament in the electron gun. The electrons are accelerated through a hole in the anode and pass down through the axis of the instrument; acceleration is caused by a large potential difference (‘accelerating voltage’) maintained between the anode (at ‘earth’ or zero potential) and the filament (commonly −100 kV to −500 kV). The accelerating voltage determines the wavelength of the electron beam which, in turn, determines the resolving power; in general, higher accelerating voltages give better resolving power. (Some specimens can be damaged more than others by high accelerating voltages; resolving power is therefore partly determined by the nature of the specimen.) The various magnetic lenses each consist basically of a soft-iron reel on which are wound many turns of copper wire; current passing through the wire generates a magnetic field which controls the electron beam passing axially through the lens. The specimen, on a grid, is placed in the path of the electron beam; electrons which pass through the specimen are focused to form an image on a screen consisting of a metal plate coated with a fluorescent material. During operation, the interior of the tube-like body (‘column’) of the microscope must be evacuated to prevent e.g. deflection and scattering of the electrons by molecules of gas; pressure in the column is usually less than ca. 6.5 × 10−5 Pa (5 × 10−7 torr; 0.5 nm Hg). (b) Scanning electron microscopy is used to examine the surface of a specimen. The specimen is scanned by an electron beam which moves in a zig-zag raster; a second electron beam synchronously scans a cathode ray tube (CRT) with a similar raster. Electrons which are secondarily emitted from the specimen are collected and used to control the intensity of the beam in the CRT. Since the secondarily emitted electrons vary in quantity from point to point on the specimen’s surface (according to surface topography) the electron beam in the CRT will be continually modulated as the specimen is scanned; hence, the CRT screen will display a pattern of bright and less-bright spots corresponding to the surface topography of the specimen. 263 electron paramagnetic resonance systems the presence of the spin label at high concentration can cause significant perturbation of the membrane characteristics.) In ESR the exciting electromagnetic radiation may involve frequencies in the X-band (ca. 9.3 GHz) or the Q-band (ca. 35 GHz). (1 GHz = 109 cycles per second.) [Book ref. 151.] electron transport chain (ETC; electron transfer chain) A sequence of redox agents along which electrons can be translocated with concomitant conversion of energy; an ETC occurs within an ENERGY-TRANSDUCING MEMBRANE, the redox agents forming a series of redox couples which, collectively, span a REDOX POTENTIAL gradient. In many cases an ETC operates as a proton pump: energy yielded during the flow of electrons towards a less negative (or more positive) redox potential is used to extrude protons across the membrane; the transmembrane proton gradient thus formed can be used as a source of energy (see CHEMIOSMOSIS). (In some organisms an ETC acts as an Na+ pump: see SODIUM MOTIVE FORCE.) An ETC which is used in the respiratory metabolism of a substrate (see RESPIRATION) is often called a respiratory chain; ETCs are also involved in PHOTOSYNTHESIS. Components of ETCs typically include flavoproteins (see RIBOFLAVIN), CYTOCHROMES, IRON–SULPHUR PROTEINS, QUINONES, and copper-containing proteins (e.g. PLASTOCYANIN); atypical ETCs include e.g. those of Mycoplasma spp which lack both cytochromes and quinones. The best-studied ETCs include those which occur in the inner membrane of the MITOCHONDRION – particularly the mitochondria of certain types of mammalian cell (e.g. bovine heart, rat liver). Although the mitochondrial ETCs in many other types of cell are often presumed to be essentially similar to those in mammalian cells, those in e.g. plant mitochondria characteristically exhibit some important differences (see later). Mitochondrial ETCs. The mammalian mitochondrial ETC consists of four groups of functionally interrelated components (Complexes I–IV) arranged in a sequence (see figure). Complex I (NADH–UQ oxidoreductase) contains a flavoprotein which is believed to transfer electrons from NADH (formed e.g. in the TCA CYCLE) to the Fe−S centres (see IRON–SULPHUR PROTEINS) and thence to UQ; electron flow between NADH and UQ can be inhibited by e.g. AMYTAL, PIERICIDIN and ROTENONE. Complex II (succinate–UQ oxidoreductase) contains succinate dehydrogenase (an FAD-containing enzyme), Fe−S centres, and a btype cytochrome (e.g. b560 in bovine heart, b559 in Neurospora sp) of unknown function. In reconstituted ETCs, electron flow between Complex II and UQ is inhibited by TTFA. Complex III (UQ–cytochrome c oxidoreductase) contains (at least) two btype haems (which appear to be bound to a single polypeptide): b562 (formerly bK ) and b566 (formerly bT ); an Fe−S protein (a RIESKE PROTEIN); cytochrome c1 ; and, in at least some species, semiquinones (believed to be ubisemiquinones). Electron flow through Complex III is inhibited by e.g. ANTIMYCIN A, BAL, DCMU and HYDROXYQUINOLINE-N-OXIDES (HOQNOs). (See also XANTHOMEGNIN.) Complex IV (cytochrome oxidase; cytochrome aa3 ; ferrocytochrome c –O2 oxidoreductase) contains two haems and two protein-bound copper atoms per functional unit; the haems (a, a3 ) are chemically identical but have different spectroscopical and other properties owing to differences in the way in which they are bound to protein. Complex IV catalyses the irreversible reduction of oxygen (the terminal electron acceptor) to H2 O; it is inhibited by e.g. AZIDE, CARBON MONOXIDE, sulphide and CYANIDE. Transmembrane translocation of protons can occur at three ‘sites’ which correspond to Complexes I, III and IV. The The scanning electron microscope (SEM) can give magnifications of ca. 10× to 100000×; for biological specimens it is commonly used at magnifications below ca. 10000×. Resolution is usually ca. 10–20 nm. The SEM provides a great depth of field, thus giving valuable three-dimensional images. Preparation of specimens for the SEM. Completely untreated specimens have been examined in the SEM. However, specimens (particularly plant material) are often given the following sequence of treatments. (i) Fixation and dehydration – as for TEM thin sections described above. (ii) Drying (removal of all liquid); this is sometimes achieved by FREEZE-DRYING (simultaneous dehydration and drying) but more usually by CRITICAL POINT DRYING. (iii) Attachment of the specimen to a metal (often aluminium) specimen holder (stub) by means of a ‘glue’ (e.g. SILVER DAG) which conducts both heat and electricity. (iv) Coating of the specimen, under vacuum, with carbon and/or metal – usually by SPUTTER-COATING; this prevents thermal and electrical buildup on the specimen when it is placed in the electron beam (thus avoiding image distortion and damage to the specimen) and enhances secondary electron emission. (As an alternative to coating, the specimen may be thoroughly impregnated, before dehydration, with e.g. osmium tetroxide.) The specimen is then ready for the SEM. Replicas of a specimen may be made e.g. if repeated observations are to be made of the same specimen. Essentially, fluid plastic or latex is poured over the specimen and allowed to harden. This gives a negative replica which is peeled off and lined with a suitable material that hardens to form the positive replica; the latter is sputter-coated and examined in the SEM. (c) Scanning transmission electron microscopy. In the scanning transmission electron microscope (STEM) electrons pass through the specimen, and the energies of the emergent electrons are measured in an analyser. The pattern of energies of the emergent electrons is used to construct an image of the specimen. The resolving power of the STEM can be similar to that of the TEM. [Introduction to STEM: JUR (1984) 88 94–104; imaging biological structures: JUR (1984) 88 105–120, 177–206.] (d) Low temperature electron microscopy (cryoelectron microscopy). A type of electron microscopy in which very small specimens are cooled rapidly to very low temperatures, without prior dehydration, such that the water in the specimen forms a ‘glass-like’ solid without undergoing crystallization – a phenomenon called vitrification [review: TIBS (1985) 10 143–146]. (e) Three-dimensional electron microscopy. A procedure intended for three-dimensional examination of biological structures at atomic resolution. [ARBB (1981) 10 563–592.] electron paramagnetic resonance Syn. ELECTRON SPIN RESONANCE. electron spin resonance (ESR; electron paramagnetic resonance, EPR) The absorption and consequent emission of electromagnetic radiation by unpaired electrons in certain types of atom in the presence of a strong magnetic field; in general principle ESR resembles NUCLEAR MAGNETIC RESONANCE (q.v.). Molecules in which atom(s) contain unpaired, ESR-detectable electrons are paramagnetic and are commonly called ‘free radicals’. In biological materials free radicals are produced during many types of metabolic process but they are typically shortlived; however, a stable, paramagnetic probe (spin label ) can be introduced into an experimental system – e.g. by biosynthetic incorporation or by covalent linking to specific compounds. Most spin labels contain the nitroxide (NO) group; they include derivatives of fatty acids, of the steroids androstane and cholestane, and of 2,2,6,6-tetramethylpiperidine. (In some studies on membrane 264 electron transport phosphorylation succinate fumarate + Complex II NAD fumarate FAD Fe-S centres b -type cyt Complex I NADH UQ b562, b566, Fe-S centre, c 1 site 1 + Complex IV Complex III FMN, Fe-S centres c a a3 Cu Cu site 2 NAD 1 2 O2 + 2H+ H2O site 3 proton motive force ∼ ∆mH+ + NAD + NADPH ion transport transhydrogenase + NADH + NADP proton ATPase 'Complex V' proton PPase ATP + H2O PPi + H2O ADP + Pi 2Pi ELECTRON TRANSPORT CHAIN. A scheme for electron transport in an animal-type mitochondrion, showing electron transfer from NADH and/or succinate to the terminal electron acceptor, oxygen. (Electron transport is believed to be similar in the mitochondria of many types of microorganism.) Sites 1, 2 and 3 indicate sites of transmembrane proton translocation during electron flow. Thick lines indicate energy conversion (or regulation in the case of transhydrogenation). FMN = flavin mononucleotide; Fe−S centre = iron–sulphur centre; FAD = flavin adenine dinucleotide; b562 , c1 etc = cytochromes; UQ = ubiquinone; proton PPase = proton pyrophosphatase. mechanism of proton translocation is not understood; moreover, even the stoichiometry of proton translocation has not yet been settled: in Complex I the H+ /2e− ratio (q.v.) is believed, by various authors, to be 2, 3 or 4. Proton translocation is the subject of two main types of model: the LOOP MODEL and the conformational pump model; in the latter model, proton translocation involves changes in the pK of ionizable groups on certain proton-binding ETC components brought about by electron-dependent redox and conformation changes in those components. (See also ANISOTROPIC INHIBITOR and Q-CYCLE.) [Mitochondrial electron transport: ARB (1985) 54 1015–1069.] Plant mitochondrial ETCs are characteristically branched at the ubiquinone, one branch terminating with a conventional cytochrome oxidase, the other terminating with a cyanideresistant oxidase; the latter branch is apparently non-protontranslocating, so that respiration via this branch is less efficient in terms of energy transduction. Bacterial ETCs are generally located in the CYTOPLASMIC MEMBRANE; in at least some cyanobacteria, however, respiratory electron transport pathways occur in both the cytoplasmic membrane and the thylakoid membranes [Book ref. 75, pp. 199–218]. The composition of bacterial ETCs exhibits wide variability among the various species – and even in a given species under different growth conditions or in different physiological states (e.g. vegetative and sporulating cells of Bacillus cereus [JB (1984) 160 473–477]). Moreover, many bacteria have branched ETCs and may use different pathways for different terminal electron acceptors. Although the components of ETCs in aerobically grown bacteria may resemble those in mitochondrial ETCs, the order in which they occur is typically unlike that in mitochondria. In a given species of bacterium more than one type of cytochrome may be used as a terminal oxidase. For example, in Escherichia coli the major terminal oxidase under aerobic conditions is a b-type cytochrome, cyt o, while under oxygenlimited conditions it is cyt d. During NITRATE RESPIRATION, E. coli uses an inducible cytochrome which functions specifically as part of the nitrate reductase enzyme system; this cytochrome NO − is designated nr b or b5563 . [Respiratory chains in E. coli : MR (1984) 48 222–271.] Cytochromes specific for different terminal electron acceptors may coexist in the same cell [JB (1983) 154 708–727] and, under certain conditions, two terminal electron acceptors may be used simultaneously [JGM (1979) 112 379–383]. electron-transport particle See ETP. electron transport phosphorylation The phosphorylation of ADP to ATP, or the formation of PYROPHOSPHATE from inorganic phosphate, in a reaction driven by the energy of a transmembrane gradient of ions generated by means of an ELECTRON TRANSPORT CHAIN. (In all known cases of electron transport phosphorylation, the ETC generates a transmembrane gradient of protons (proton motive force – see CHEMIOSMOSIS); although some respiratory chains generate a SODIUM MOTIVE FORCE, as yet there have been no reports of an Na+ -motive respiratory chain being used for the synthesis of ATP.) Electron transport phosphorylation can occur in respiratory metabolism (see OXIDATIVE PHOSPHORYLATION), in PHOTOSYNTHESIS, and in certain types of FERMENTATION (sense 1) (q.v.). ‘Electron transport phosphorylation’ excludes not only SUBSTRATE-LEVEL PHOSPHORYLATION but also those modes of 265 electroneutral transport electrophoretic transport Transport of a charged species across an energy-transducing membrane in response to a pre-existing transmembrane electrical potential difference. (cf. ELECTRICAL TRANSPORT.) electroporation An in vitro method for increasing the efficiency of TRANSFORMATION. Essentially, a mixture of cells and e.g. plasmids is exposed to an electric field (up to ∼16 kV/cm) for a fraction of a second; the mechanism of uptake is unknown, but the frequency of transformation varies linearly with the concentration of DNA over a wide range of values, and the efficiency of transformation varies with cell concentration. [Electroporation in Helicobacter pylori: AEM (1997) 63 4866–4871; in Escherichia coli: NAR (1999) 27 910–911.] ‘Electrocompetent’ (electroporation-competent) cells with high-level transformation efficiency are available commercially. The electrocompetent cells marketed by Stratagene (ElectroTenBlue ) are electroporated at 1.7 kV and 600 ohms resistance. ElectroTen-Blue See ELECTROPORATION. electro-transfection Electrically-assisted TRANSFECTION; thus, e.g. tobacco protoplasts have been transfected with RNA from the tobacco mosaic virus under the influence of 50-µsec pulses of DC current at 550–800 V/cm [JGV (1986) 67 2037–2042]. (cf. ELECTROFUSION.) Elek plate An in vitro method for detecting the formation of a diffusible toxin by a given strain of microorganism, e.g. Corynebacterium diphtheriae. A plate of suitable medium is inoculated, in a straight line, with the strain under test; the streak is then overlaid, perpendicularly, with a strip of paper which has been impregnated with relevant antitoxin. After incubation (24–48 hours), toxin production is indicated by the development of lines of whitish precipitate which approximately bisect each of the four right angles formed by the paper strip and the line of microbial growth; the precipitate results from the combination of toxin and antitoxin following their diffusion from the line of growth and paper strip, respectively. Non-specific lines of precipitate may be formed, so that known negative and positive strains must be used as controls. Several unknown strains, and the controls, may be examined on a standard-size Petri dish. In a modified form of the test, an antitoxin-impregnated disc is placed at the centre of the plate, and the test strain(s), together with positive and negative controls, are inoculated in a circular pattern at a fixed radius; this form of test can be read after 16 hours [JCM (1997) 35 495–498]. Diphtheria toxin can also be detected – directly from clinical specimens – by a PCR-based method which detects the A and B subunits of the toxin; a result can be available within hours of collecting the specimen. This assay is used in the Diphtheria Laboratory at the Centers for Disease Control in the USA [JCM (1997) 35 1651–1655]. elementary body (EB) (1) See CHLAMYDIA. (2) An archaic name for a virion. ELISA (enzyme-linked immunosorbent assay) A highly sensitive IMMUNOASSAY for specific antibodies or antigens. For the assay of specific antibodies, the antiserum (or other test material) is allowed to react with homologous antigens which have been adsorbed e.g. to the inner surface of a plastic tube; uncombined antibodies are removed by washing. Antibodies which have combined with the immobilized antigens are then detected by treating the test system with a conjugate (see CONJUGATION sense 2) consisting of anti-immunoglobulin antibodies each covalently linked to an enzyme of a particular type; the test system is washed free of any uncombined conjugate and examined for bound enzyme phosphorylation dependent on a transmembrane ion gradient that is not produced by electron transport; such ion gradients can be derived e.g. by direct proton pumping (as in the PURPLE MEMBRANE) or by END-PRODUCT EFFLUX. With the exception of substrate-level phosphorylation, these forms of phosphorylation resemble electron transport phosphorylation in that they are dependent on ion-motive force and involve a membrane-bound ATP- or PPi-synthesizing enzyme system; all forms of phosphorylation which have these two characteristics may be referred to as ion-motive force phosphorylation (imf phosphorylation). electroneutral transport Transport across an energy-transducing membrane of charged or uncharged species resulting in no net transfer of charge across the membrane; it occurs when an uncharged species is carried by a UNIPORTer, during the SYMPORT of two ions of opposite charge, or during the ANTIPORT of two ions of the same charge. (cf. ELECTROGENIC TRANSPORT.) electro-osmosis Syn. ELECTROENDOSMOSIS. electropherotype (virol.) A virus type (see TYPING) recognized on the basis of ELECTROPHORESIS of e.g. its genome. (See also OLIGONUCLEOTIDE FINGERPRINTING.) electrophoresis Any procedure by means of which the members of a heterogeneous population of charged particles can be translocated or separated (or, exceptionally, brought together: see COUNTERCURRENT IMMUNOELECTROPHORESIS) by virtue of their mobility in an electric field. Electrophoresis is used e.g. for identifying or quantifying proteins, nucleic acids etc., for molecular weight determinations (e.g. SDS-PAGE), for preparing fingerprints in various forms of TYPING, and for identifying/confirming products in nucleicacid-amplification methods. (See also CHROMATOGRAPHY and ISOELECTRIC FOCUSING.) In free electrophoresis the molecules to be separated are present in a liquid medium (usually water). In one form (movingboundary electrophoresis) a mixed population of molecules is subjected to electrophoresis in a U-tube, each limb of which contains a column of buffer above the test suspension; the electric field is applied by placing a positive electrode in one limb and a negative electrode in the other. During electrophoresis the different types of molecule begin to separate into distinct zones, boundaries between the homogeneous and heterogeneous populations moving as the process continues. In surface electrophoresis the molecules in a sample move through a thin film of buffer on the surface of a strip of paper or e.g. cellulose acetate; the mobility of a given molecule may therefore by influenced by interaction with the solid phase. Essentially, a droplet of sample is applied near the centre of a strip which has equilibrated with the vapour of an appropriate buffer solution in a closed chamber; an electrical potential difference is then applied between the two ends of the strip. After electrophoresis, the strip may be stained to reveal discrete zones, bands or spots corresponding to the various components in the sample. In gel electrophoresis the molecules in a sample move through a gel composed, usually, of agarose (see AGAR) or polyacrylamide. Essentially, the sample is placed in a well cut into one end of a gel strip, and an electrical potential difference is applied between the two ends of the strip; after electrophoresis, the separated components may be e.g. stained in situ, extracted for further study, or transferred to another medium (see BLOTTING and SOUTHERN HYBRIDIZATION). (See also ELECTROENDOSMOSIS, IMMUNOELECTROPHORESIS, PFGE, SDS-PAGE, TWO-DIMENSIONAL ELECTROPHORESIS, ZYMOGRAM.) 266 embryonated egg K2 HPO4 (0.2%); eosin Y (0.04%); methylene blue (0.0065%); agar (ca. 1.5%). Final pH: ca. 7.2. Colonies of Escherichia coli usually have dark centres and a greenish metallic sheen, those of Enterobacter aerogenes are usually mucoid, brownish in the centre, and only occasionally show a metallic sheen, while those of Salmonella and Shigella are usually colourless and translucent. Embden–Meyerhof–Parnas pathway (EMP pathway; EmbdenMeyerhof pathway; hexose bisphosphate pathway; glycolysis) A sequence of reactions in which glucose is broken down to pyruvate∗ . The EMP pathway occurs in a wide range of organisms, both prokaryotic and eukaryotic, and can operate under both aerobic and anaerobic conditions. By this pathway one molecule of glucose yields two molecules each of pyruvate and NADH and two (net) of ATP (from SUBSTRATE-LEVEL PHOSPHORYLATION); if glucose 6-phosphate is derived from GLYCOGEN, the net yield of ATP is 3 molecules. [See Appendix I(a).] In respiratory modes of metabolism (see RESPIRATION) the pyruvate is commonly converted to acetyl-CoA which enters the TCA CYCLE; NADH is oxidized via a respiratory chain (see ELECTRON TRANSPORT CHAIN). In fermentative modes (see FERMENTATION) the fate of pyruvate depends on species and/or conditions, but must always involve regeneration of NAD+ from NADH by reduction of an endogenous substrate (see e.g. ALCOHOLIC FERMENTATION, HOMOLACTIC FERMENTATION, and Appendix III). The initial step of the EMP pathway, the formation of glucose 6-phosphate from glucose, is catalysed by hexokinase in e.g. yeasts and some bacteria, while in other bacteria (e.g. enterobacteria, streptococci) glucose is phosphorylated during its uptake by a PTS (q.v.). Phosphofructokinase is a key enzyme in the EMP pathway, and the presence of this enzyme in an organism is generally taken as evidence that the EMP pathway occurs in that organism. In yeasts (and e.g. mammals) phosphofructokinase is inhibited by high levels of ATP and citrate, and is stimulated by high levels of AMP; in e.g. Escherichia coli phosphofructokinase is inhibited by phosphoenolpyruvate (but not by citrate), and is stimulated by ADP and GDP (but not by AMP). (See also ADENYLATE ENERGY CHARGE.) (In certain organisms fructose 6-phosphate is phosphorylated by a pyrophosphate:phosphofructose dikinase – see PYROPHOSPHATE.) The EMP pathway functions not only in energy-yielding metabolism but also in supplying intermediates for biosynthesis: e.g., dihydroxyacetone phosphate can be reduced to glycerol phosphate for use in the biosynthesis of lipids; phosphoenolpyruvate may be drawn off for aromatic amino acid biosynthesis [Appendix IV(f)], 3-phosphoglycerate for cysteine, glycine and serine biosynthesis [Appendix IV(c)], and pyruvate for alanine, leucine and valine biosynthesis [Appendix IV(b)]. (See also GLUCONEOGENESIS.) (See also ENTNER–DOUDOROFF PATHWAY; GLYCOSOMES; HEXOSE MONOPHOSPHATE PATHWAY; METHYLGLYOXAL BYPASS.) ∗ The terms ‘EMP pathway’ and ‘glycolysis’ are often used for the complete pathway for glucose degradation: e.g. glucose → lactate (i.e., HOMOLACTIC FERMENTATION) or glucose → ethanol (i.e., ALCOHOLIC FERMENTATION). embedding See MICROTOME and ELECTRON MICROSCOPY. embryonated egg (embryonating egg) An egg (usually a hen’s or duck’s egg) which contains a live, developing embryo. Such eggs are used in virology e.g. for the identification, culture, and/or assay of certain viruses (see e.g. POCK sense 2), and as a source of cells for TISSUE CULTURE. An embryonated egg is prepared by incubating a fertile egg in a humid atmosphere for ca. 5–14 days at e.g. 37–39° C – the by incubating with an appropriate substrate and assaying for the products of enzymic cleavage. Antigen may be detected or quantified e.g. by a procedure similar to that used for antibody except that the antibody is adsorbed, and enzyme is conjugated to anti-antigen antibody. In an alternative procedure, antigen is detected/quantified by its ability to sequester added antibody – the amount of antibody sequestered (an indirect measure of antigen) being assessed by measuring the amount of free antibody remaining in the test mixture; free antibody is measured by allowing it to bind to immobilized antigen, and detecting the bound antibody with enzyme-linked anti-Ig antibodies. [Developments in ELISA and other solid-phase immunoassay systems: JIM (1986) 87 1–125.] ELISPOT assay (enzyme-linked immunospot assay) A type of assay, based on the ELISA principle, for detecting and quantifying specific molecules or cells. For example, blood has been assayed to quantify those T cells which respond to the tuberculosis-associated antigen ESAT-6 (q.v.) by secreting the cytokine interferon-gamma (IFN-g; see INTERFERONS). Any ESAT-6-specific T cells (present in a blood sample in the well of an assay dish) react to (added) ESAT-6 antigen by secreting IFN-g. Such IFN-g (which is present in high concentration in the immediate vicinity of a secreting T cell) is detected by adding a conjugate consisting of an enzyme linked to a molecule that binds to IFN-g; enzymic activity (on an appropriate substrate) then leads to the formation of a visible ‘spot’ on the floor of the well in the location of a secreting T cell. Hence, each spot corresponds to an ESAT-6-specific T cell. Enumeration of ESAT-6-specific T cells has been used in epidemiological studies (contact tracing) in the context of tuberculosis [Lancet (2001) 357 2017–2021]. Ellobiophrya See PERITRICHIA. elm butt rot See BUTT ROT. Elokomin fluke fever See NEORICKETTSIA. Elphidium See FORAMINIFERIDA. elsinan A water-soluble, essentially linear glucan, consisting of blocks of (1 → 4)-a-linked maltotriose units connected by (1 → 3)-a-bonds, isolated from culture fluids of Elsinoë leucospila. Elsinoë A genus of fungi (order DOTHIDEALES) which include some important plant pathogens – e.g. E. veneta, which causes purple-bordered, sunken grey lesions (‘cane spot’) on the canes of e.g. raspberry (Rubus sp). Elsinoë spp form uniascal locules at various levels in a pseudoparenchymatous ASCOSTROMA (the overwintering stage); the ascospores are septate, colourless or pale yellow. Anamorphs occur in the genus Sphaceloma. eltor vibrio See VIBRIO (V. cholerae). elution (serol., virol.) The dissociation of an adsorbed particle. Elysia A genus of marine molluscs which feed on siphonaceous green algae (e.g. CAULERPA, CODIUM) by piercing the cells and sucking out the cytoplasm; the algal chloroplasts are retained intact within the cells of the digestive gland (hepatopancreas) where they continue to photosynthesize for a time. The chloroplasts do not divide in the animal, and are sooner or later digested. [Book ref. 129, pp. 97–107.] (cf. CONVOLUTA.) Elytrosporangium See STREPTOMYCES. EM Electron microscope. Em,7 See REDOX POTENTIAL. emarginate (mycol.) Syn. SINUATE (sense 2). EMB agar (eosin–methylene blue agar) An agar-based medium used e.g. for the primary isolation of – and for differentiating between – enterobacteria. The medium contains (w/v): peptone (1%); lactose and sucrose (each at 0.5%) or lactose only (at 1%); 267 EMCV growth is transferred to, and mixed with, a drop of the liquid on a slide. (2) The formation of an emulsion, i.e., a suspension of droplets of one liquid in another with which it is non-miscible (e.g. oil in water). Substances which stabilize emulsions are called emulsifying agents. (See also BIOSURFACTANTS.) enanthem A rash on a mucous membrane (cf. EXANTHEM). enation (plant pathol.) A localized proliferation of leaf tissue caused e.g. by viral infection. encapsidation During virion assembly: the process in which nucleic acid becomes incorporated in the viral capsid or – e.g. in certain bacteriophages – in a head/capsid precursor. encephalitis Inflammation of the brain; it is often accompanied by inflammation of the brain meninges (encephalomeningitis, MENINGOENCEPHALITIS) or the spinal cord (encephalomyelitis) or both (meningoencephalomyelitis). Symptoms range from mild (fever, headache, malaise, myalgia, stiffness of the neck and back) to severe (irritability or somnolence, coma, death). Causes of encephalitis are many and varied. Toxic encephalitis results from severe toxaemia early in the course of an acute infectious disease. Post-infective encephalitis occurs after the onset of certain diseases (e.g. CHICKENPOX, MEASLES, MUMPS, WHOOPING COUGH). Post-vaccinal encephalitis may follow active immunization against e.g. rabies, smallpox, whooping cough, yellow fever. Viral encephalitis may be caused by any of a range of viruses: see e.g. B VIRUS; CALIFORNIA ENCEPHALITIS; EASTERN EQUINE ENCEPHALOMYELITIS; HERPES SIMPLEX; JAPANESE B ENCEPHALITIS; POWASSAN ENCEPHALITIS; RUSSIAN SPRING–SUMMER ENCEPHALITIS; ST LOUIS ENCEPHALITIS; SUBACUTE SCLEROSING PANENCEPHALITIS; VENEZUELAN EQUINE ENCEPHALOMYELITIS; WESTERN EQUINE ENCEPHALOMYELITIS. Amoebic encephalitis may involve e.g. certain species of ACANTHAMOEBA or NAEGLERIA – or BALAMUTHIA MANDRILLARIS. (See also MENINGITIS, TALFAN DISEASE, TESCHEN DISEASE; TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES.) Encephalitozoon A genus of protozoa (class MICROSPOREA). E. cuniculi (= Nosema cuniculi) is parasitic (but generally nonpathogenic) in rodents and other animals, and has been recorded as the cause of a case of human encephalitis; it has also been isolated from eye, intestine, kidney, liver and peritoneum. [Mechanism of resistance to E. cuniculi in mice: J. Imm. (1984) 133 2712–2719.] The spores of E. cuniculi (ca. 2.5 × 1.5 µm) can be isolated from the urine of infected animals. E. intestinalis can cause travellers’ diarrhoea [PCR-based identification of E. intestinalis: JCM (1998) 36 37–40]. encephalomeningitis See ENCEPHALITIS. encephalomyelitis See ENCEPHALITIS. encephalomyocarditis virus (murine) See CARDIOVIRUS. encystment The formation of a CYST. end-point dilution assay (virol.) A method of quantifying a given type of virus in a given sample, i.e., measuring the ‘infectivity’ of that virus in the sample. In this method, infectivity is measured by determining the extent to which the sample must be diluted in order that a given volume of the dilution corresponds to the LD50 , TCID50 or (more generally) the ED50 of the virus. Initially, log (or semi-log) dilutions of the sample are prepared. Each dilution is then examined, individually, by inoculating each of a number of similar test units with a fixed volume (dose) of that dilution; test units may be e.g. susceptible laboratory animals or tissue cultures which undergo detectable forms of degeneration when infected with the virus. After incubation, each test unit is examined. Animals or cultures inoculated with a dose from the lower dilutions (i.e. higher concentrations period and temperature of incubation depending e.g. on the region of the egg to be inoculated, the virus to be used, etc. Essentially, an egg is candled (see CANDLING) and inoculated (aseptically) with the virus via a small hole drilled in the shell; inoculations may be made into the yolk sac (egg preincubated ca. 5–8 days), the chorioallantoic membrane (CAM) (pre-incubation 11–13 days), the allantoic cavity (pre-incubation 9–12 days), or the amniotic cavity (pre-incubation 10–14 days). Following inoculation, the hole is sealed and the egg is incubated (at an appropriate temperature and humidity) for e.g. 1–7 days; the egg can then be opened and material removed for examination etc. EMCV See CARDIOVIRUS. Emericella A genus of fungi (order EUROTIALES) which include the teleomorph of Aspergillus nidulans. The ascocarp is a globose, red or violet cleistothecium containing spherical or ovoid evanescent asci; it is surrounded by a layer of hülle cells: thick-walled cells which develop from the hyphae that envelope the cleistothecium. The ascospores are typically reddish or violet. Emericellopsis See EUROTIALES. Emerson enhancement effect See ENHANCEMENT EFFECT. emetine A toxic alkaloid, obtained from ipecacuanha root, used in the treatment of amoebic DYSENTERY. Emetine inhibits eukaryotic protein synthesis [Book ref. 14, pp. 453–454]; at clinical doses it is effective against trophozoites but not cysts. 2-Dehydroemetine is said to cause fewer clinical side-effects. Emiliana See COCCOLITHOPHORIDS. Emmentaler cheese See CHEESE-MAKING. Emmonsia See ADIASPIROMYCOSIS. Emmonsiella Syn. AJELLOMYCES. EMP pathway EMBDEN–MEYERHOF–PARNAS PATHWAY. empty magnification See MAGNIFICATION. empyema Accumulation of pus in a body cavity (usually the lungs). EMRSA Epidemic MRSA clone. Each distinct EMRSA is given a number (e.g. EMRSA-1, EMRSA-2). EMS Ethylmethane sulphonate: see ALKYLATING AGENTS. EMTA A pH buffer based on 3,6-endomethylene-1,2,3,6-tetrahydrophthalic acid. Emtryl See DIMETRIDAZOLE. emulcyan An extracellular amphipathic polymer produced by stationary-phase filaments of a strain of Phormidium – a benthic organism which normally adheres to surfaces by virtue of hydrophobic interactions, and which does not produce hormogonia. In the presence of cations, emulcyan acts as an emulsifying agent; it is believed to mask cell-surface hydrophobicity, thus serving to detach filaments and allow their dispersal. [FEMS Ecol. (1985) 31 3–9.] (See also BIOSURFACTANTS.) emulsan An extracellular, macromolecular bioemulsifier (see BIOSURFACTANTS) produced by Acinetobacter calcoaceticus RAG-1 during growth on e.g. hexadecane, ethanol or acetate. Emulsan (MWt ca. 106 ) consists of a polysaccharide (containing N-acetyl-D-galactosamine and an N-acetylhexosaminuronic acid) esterified with fatty acids [FEBS (1979) 101 175–178]. Emulsan initially accumulates on the surface of the growing cells, forming a ‘minicapsule’, and is released into the medium e.g. as the cells approach the stationary phase; only the free extracellular form has emulsifying activity. An esterase which interacts with emulsan has been detected on the cell surface and free in the medium; this enzyme may play a role in the release of emulsan from the cell surface [JB (1985) 161 1176–1181]. emulsification (1) (bacteriol.) The process of preparing a suspension of bacteria in water, saline etc; using a loop, bacterial 268 endocytosis as the highest dilution which exhibits agglutination in the test system. (See also TITRE.) end-product efflux The outflow, from a growing cell, of the end products of metabolism; in some cases such efflux involves end-product–proton SYMPORT (e.g. lactate–proton symport) capable of generating, or augmenting, a proton motive force (see CHEMIOSMOSIS). Such a mechanism for energy conversion may be particularly important e.g. in at least some obligately fermentative bacteria. In these organisms (which lack a respiratory chain) the pmf required e.g. for certain TRANSPORT SYSTEMS can be generated by ATP hydrolysis at a PROTON ATPASE – though this process is likely to consume much of the ATP generated by substrate-level phosphorylation; pmf generation by end-product efflux has been reported e.g. in Lactococcus lactis (formerly Streptococcus cremoris) [AvL (1984) 50 545–555; JB (1985) 162 383–390]. Endamoeba A genus of cyst-forming amoebae (order AMOEBIDA) parasitic in the gut of invertebrates (e.g. E. blattae in cockroaches). The nucleus lacks a karyosome. endemic (1) Of a disease: commonly or constantly present in a given location or geographical region; typically, a relatively low proportion of the local population exhibits clinical signs of the disease. (2) Constantly present in a given location or geographical region. Endemosarca A genus of endoparasitic, plasmodium-forming eukaryotic organisms which are parasitic in ciliates (Colpoda spp); they appear to be related to members of the PLASMODIOPHOROMYCETES. [Book ref. 147, pp. 203–206.] endergonic reaction Any reaction requiring an uptake of energy. (cf. EXERGONIC REACTION.) endobiotic Living within a host organism. endocarditis Inflammation of the endocardium, i.e. the tissue lining the cavities of the heart. Infective (infectious) endocarditis may be due to infection by any of a range of microorganisms – e.g. Haemophilus sp, Staphylococcus aureus, S. epidermidis (especially in patients with prosthetic heart valves), Enterococcus faecalis, viridans streptococci (see also NUTRITIONALLY VARIANT STREPTOCOCCI), Neisseria gonorrhoeae, Candida, Aspergillus, Histoplasma. Infrequently, the causal agent is one of the more recently described species of coagulase-negative staphylococci: S. lugdunensis [RMM (1995) 6 94–100]. (See also CARDIOBACTERIUM.) Infective endocarditis may be acute or subacute; S. aureus may cause either acute or subacute endocarditis. Infection occurs via the bloodstream. Organisms may gain access to the bloodstream e.g. during dental treatment (e.g. oral viridans streptococci), catheterization or insertion of an intrauterine contraceptive device (e.g. faecal streptococci), intravenous injection (e.g. staphylococci) etc. Symptoms: e.g. fever, malaise, heart murmurs, weight loss, clubbing of fingertips, embolism; late symptoms of subacute bacterial endocarditis (SBE) include vasculitis, petechial rash, and (almost pathognomonic) small, tender, often bluish swellings on the skin (Osler’s nodes). Damage to heart valves may lead to heart failure. Endocarditis may occur as a complication of other infectious diseases – e.g. Q FEVER, SYPHILIS, TUBERCULOSIS. (See also RHEUMATIC FEVER.) Endocarpon See VERRUCARIALES. endocellulase See CELLULASES. endocyst See CYST (b). endocytobiosis Syn. CYTOBIOSIS. endocytosis The ingestion of materials, by a cell, by PHAGOCYTOSIS and/or PINOCYTOSIS. of virus) may react positively in every case; of those inoculated with a dose from the higher dilutions some may react positively while others are unaffected. The important dilution is that which corresponds to the ED50 ; such a ‘50% end point’ is not always obtained experimentally, but it may be calculated from the experimental data – provided that some test units have given a negative reaction. The 50% end-point dilution may be calculated by the Reed–Muench method or the Kärber method; however, these methods can be used only if the dilutions are in a regular logarithmic sequence, and only if a similar number of test units is used for each dilution. Examples of the use of these methods are given below with reference to the specimen data in the accompanying table. END-POINT DILUTION ASSAY: specimen data Column A: Column B: Column C: Column D: Column E: dilution of mortalities in cumulative cumulative C/(C + D)% virus 5 test animals mortalities survivors 10−6 10−5 10−4 10−3 0/5 2/5 4/5 5/5 0 2 6 11 9 4 1 0 0 33.3 85.7 100 Reed–Muench method. The logarithm (to base 10) of the 50% end-point dilution lies between −5 and −4, and is obtained by linear interpolation – appropriate values from column E being substituted in the following equation: log10 50% end point = −5 + 50 − 33.3 85.7 − 33.3 The logarithm of the 50% end point is thus 10−4.68 ; since the antilog of 4.68 is ca. 48000 a dose from the 1/48000 dilution of the sample will contain the LD50 . Kärber method. Substitutions are made in the following equation: log10 50% end point = L1 − L(S − 0.5) in which L1 = logarithm of lowest dilution tested; L = log interval between dilutions; S = sum of the proportion of positive reactions at each dilution; 0.5 is a constant. In the accompanying table, S = 0 + 0.4 + 0.8 + 1.0 = 2.2; hence, the required value is 10−4.7 . An estimate of the number of infectious virions in the original (undiluted) sample can be derived from the expression: n = − loge (1 − p) in which n = average number of infectious virions per dose; p = proportion of positive reactions in the test units inoculated from a given dilution; e = 2.7183. The value of n (calculated for one of the dilutions) multiplied by the dilution factor gives an estimate of the viral titre of the sample; such an estimate is less reliable than that made from a PLAQUE ASSAY. end-point titration (serol.) A method of assaying the concentration of a specific antibody or antigen, in a given sample, by determining the highest dilution of the sample which gives a reaction regarded as positive under the conditions of the test. For example, in an agglutination test the end point may be regarded 269 endodyogeny endolysin is to lyse the host cell’s envelope to release phage progeny. In many or all cases, endolysins (which lack a ‘signal sequence’) are enabled to cross the cytoplasmic membrane (to reach peptidoglycan) by means of a LYSIS PROTEIN. endometritis Inflammation of the endometrium. In humans and animals it may result from infection of the endometrium by various opportunist pathogens (e.g. Klebsiella pneumoniae, Pseudomonas aeruginosa, group A streptococci). (See also CONTAGIOUS EQUINE METRITIS.) endomitosis Chromosome replication without nuclear division. (cf. MITOSIS.) Endomyces See ENDOMYCETALES. Endomycetaceae See ENDOMYCETALES. Endomycetales An order of mainly saprotrophic fungi of the ASCOMYCOTINA; the organisms include both unicellular and dimorphic species. Ascocarps are not formed. Asci (which develop from individual cells, or from cells within hyphae): prototunicate, cylindrical to spheroidal. Families: Dipodascaceae (genera: e.g. Dipodascus, Phialoascus), Endomycetaceae (formerly Ascoideaceae; genera: e.g. Cephaloascus, Endomyces), METSCHNIKOWIACEAE, SACCHAROMYCETACEAE. Endomycopsis An obsolete fungal genus; species are now included e.g. in SACCHAROMYCOPSIS. endomycorrhiza See MYCORRHIZA. endonuclease A NUCLEASE which cleaves phosphodiester bonds within a nucleic acid strand. See also RESTRICTION ENDONUCLEASE; cf. EXONUCLEASE. endonuclease III See BASE EXCISION REPAIR. endonuclease S1 (nuclease S1 , endonuclease S1, etc) A zinccontaining ENDONUCLEASE, first obtained from Aspergillus oryzae, which specifically cleaves ssDNA and ssRNA to yield mainly 5
-nucleoside monophosphates. It can cleave local regions of ssDNA in a dsDNA molecule. endonucleobiosis SYMBIOSIS in which one symbiont occurs within the nucleus of the other (see e.g. HOLOSPORA). [Endonucleobiosis in ciliates: Int. Rev. Cytol. (1986) 102 169–213.] endoparasite See PARASITE. endoparasitic slime moulds See PLASMODIOPHOROMYCETES. endopectate lyase See PECTIC ENZYMES. endopectin lyase See PECTIC ENZYMES. endopeptidase See PROTEASES. endoperidium See LYCOPERDALES. endoperoxide synthase Syn. cyclo-oxygenase (see PROSTAGLANDINS). endophloeodal (endophloedal; hypophloeodal) Growing within bark (cf. CORTICOLOUS). Endophyllum See UREDINIOMYCETES. endophyte (1) An organism parasitic partly or wholly within a plant. (2) Any of certain fungi parasitic in grasses; such fungi include e.g. Epichloë typhina. Infection with certain endophytes can have beneficial and harmful effects on the growth and utilization of pasture grasses. [Artificial infection of grasses with endophytes: Ann. Appl. Biol. (1985) 107 17–24.] endoplasm See e.g. SARCODINA. endoplasmic reticulum (ER) Within a eukaryotic cell: a system of membranes which ramifies through the cytoplasmic region and forms the limiting boundaries of compartments and channels whose lumina are completely isolated from the cytoplasm; the ER membrane is a protein-containing lipid bilayer. In some regions the outer surface of the ER membrane (i.e., that surface in contact with the cytoplasm) bears many ribosomes; such regions are called rough ER (RER), while those regions endodyogeny Asexual reproduction (found e.g. in Chlamydomonas, Sarcocystis, Toxoplasma) in which two daughter cells form within the parent cell (which is destroyed during the process). endoenzyme An ENZYME which cleaves bonds within a polymer chain: see e.g. a-AMYLASES. (cf. EXOENZYME.) endoflagellum See SPIROCHAETALES. endogenote See MEROZYGOTE. endogenous infection Infection by an organism which is part of the normal body microflora. (See e.g. CANDIDIASIS; cf. OPPORTUNIST PATHOGENS.) endogenous retrovirus A retrovirus (see RETROVIRIDAE) which occurs, in provirus form, in the DNA of all normal cells of the host species or strain, and which is transmitted genetically via the germ line from one generation to the next. Endogenous retroviruses have been detected in a very wide range of vertebrates (including man [see e.g. JV (1986) 58 955–959]); they are believed to have arisen by exogenous infection of germ line tissue in the (recent or possibly ancient) ancestors of the host. Many endogenous retroviruses are related to horizontally transmissible EXOGENOUS RETROVIRUSES. Most endogenous proviruses are transcriptionally silent. Many appear to be defective, but some can be activated under certain conditions (e.g. by host hormones, or by exposure to radiation or certain chemicals – e.g. BUdR), when expression may range from transcription of selected virus genes to production of complete virus particles which may be infectious or non-infectious, ECOTROPIC or XENOTROPIC, etc. (See also MOUSE MAMMARY TUMOUR VIRUS.) endoglucanase See e.g. CELLULASES. Endogonales An order of fungi (class ZYGOMYCETES) which form zygospores and in which asexual reproduction involves the formation of chlamydospores, azygospores or sporangiospores. Many species are involved in MYCORRHIZA formation (see also RHIZOSPHERE). Genera include Acaulospora, Endogone, Gigaspora, Glomus and Sclerocystis. Endogone See ENDOGONALES. Endolimax A genus of protozoa (order AMOEBIDA) parasitic in vertebrates and invertebrates. E. nana occurs in the human large intestine; it is non-pathogenic. Trophozoites of E. nana (ca. 6–12 µm diam.) contain numerous vacuoles and form blunt pseudopodia slowly; the single nucleus lacks peripheral chromatin but contains a large, eccentrically located karyosome. Cysts are usually ovoid, 4–8 × 8–16 µm, and contain four nuclei when mature; chromatoid bodies are usually absent. endolithic Growing within rock, stone etc (cf. SAXICOLOUS). For example, certain cyanobacteria, lichens and microfungi can grow endolithically in environments too extreme (e.g. too hot or too cold) to allow normal EPILITHIC growth. Endolithic organisms (endoliths) have been categorized into three principal groups: chasmoendoliths, which colonize fissures and cracks in the rock; cryptoendoliths, which colonize structural cavities within porous rocks; and euendoliths, which actively penetrate the rock (e.g. by releasing substances which dissolve the rock). [J. Sed. Pet. (1981) 51 475–478.] In e.g. hot desert regions, cyanobacteria and other bacteria are generally the dominant cryptoendoliths, while in the dry desert valleys of the Antarctic chasmoendolithic and cryptoendolithic lichens (mainly Acarospora, Buellia, Lecanora and Lecidea spp) are dominant [Science (1982) 215 1045–1053]. These organisms generally inhabit a narrow zone below the rock surface, any fruiting structures being formed at the surface. (See also e.g. ARTHOPYRENIA; LECANORA; OSTREOBIUM.) endolysin A PEPTIDOGLYCAN-degrading enzyme (e.g. LYSOZYME) encoded by many types of bacteriophage; the function of an 270 endospore stains are themselves regulated at the level of transcription by factors dependent on the cell’s physiological state. Thus, whether or not sporulation occurs appears to depend on the opposing influences of kinases (promoting sporulation) and phosphatases (inhibiting sporulation) [TIM (1998) 6 366–370]. During the process of sporulation, gene expression is regulated, at least partly, at the level of transcription – relevant genes being transcribed in a specific temporal sequence; such regulation involves certain SIGMA FACTORS (see figure). Elucidation of the mechanism of sporulation has been assisted by the isolation of various mutants, each blocked at a specific stage of the process. Such spo mutants are designated according to the stage beyond which endospore development does not progress in the mutant cell; mutations which affect the same stage but which occur at different loci are designated A, B etc. Various mutations at spo0 loci prevent the development of the asymmetrical septum. No stage I mutants have been detected. In general, spo mutations have little or no effect on vegetative growth. The figure (part b) outlines the stages of development of an endospore. Dormancy. Freshly formed endospores usually enter a period of DORMANCY in which little or no metabolic activity can be detected. Activation. This is a commonly reversible process in which a dormant endospore prepares for subsequent germination and differentiation into a vegetative cell; it appears to involve changes in the configuration of the endospore’s macromolecules – presumably early steps in the mobilization of metabolic potential. Activation may be brought about e.g. by a short period of sublethal heating, by ‘ageing’, or by subjecting the endospores to low pH or to certain chemicals. The mode of activation in nature is apparently unknown. Germination is an irreversible process in which an endospore becomes metabolically active. The process is largely degradative; it involves e.g. hydrolysis and de-polymerization of certain constituents of the endospore, and is characterized by the release of dipicolinic acid, calcium ions, and the breakdown products of cortical peptidoglycan. Concurrently there is a loss of resistance to heat, and a fall in optical density. Germination requires, or is promoted by, certain substances (germinants) such as L-alanine, L-proline, certain sugars (e.g. glucose), ions (e.g. potassium, calcium, manganese, strontium), and surfactants; a given germinant may be effective for the endospores of some species but not for those of others. Germination is also affected by factors such as pH, and may be promoted by mechanical damage to the spore wall. Certain substances (e.g. D-alanine, sodium bicarbonate) inhibit germination of the endospores of certain species. Outgrowth is the process in which a vegetative cell develops from a germinated endospore. (See also MICROCYCLE SPORULATION.) (b) (cyanobacteriol.) See BAEOCYTE. (c) (mycol.) See e.g. COCCIDIOIDES, OOSPORIDIUM, RHINOSPORIDIUM, SARCINOSPORON, TRICHOSPORON. (2) (mycol.) (syn. endosporium) The innermost layer of a fungal spore wall. endospore stains Bacterial endospores stain poorly (or not at all) with simple staining procedures in the cold. They can be stained e.g. by a modified form of ZIEHL–NEELSEN’S STAIN in which ethanol (only) is used for decolorization. In another method (Schaeffer–Fulton stain) an air-dried, heat-fixed smear is flooded with 0.5% (w/v) aqueous malachite green and heated to nearboiling for 5 min; the smear is then washed in running water, which lack ribosomes are called smooth ER (SER). A given compartment or channel may be bounded partly by rough ER and partly by smooth ER. Within the lumina of the ER occur e.g. the early stage(s) of glycosylation of proteins in the PALADE PATHWAY – typically the addition of mannose and/or glucosamine residues. (See also GOLGI APPARATUS; MICROSOME.) endopolygalacturonase See PECTIC ENZYMES. endopolygalacturonate lyase Syn. endopectate lyase (see PECTIC ENZYMES). endopolygeny Asexual reproduction (found e.g. in Chlamydomonas and Toxoplasma) in which many daughter cells form within the parent cell (which is destroyed during the process). endoral membrane (in Paramecium) See PARORAL MEMBRANE. endoribonuclease See RNASE. endosome (1) A phagosome or pinosome. (2) (ciliate protozool.) A Feulgen-positive body which occurs within the paramere of a heteromerous MACRONUCLEUS in some hypostomes. (3) In euglenoid flagellates: a type of NUCLEOLUS which persists, and divides, during mitosis. endospore (1) A type of SPORE formed intracellularly by the parent cell or hypha. (cf. EXOSPORE.) (a) (bacteriol.) Endospores are formed under conditions of nutrient limitation by species of e.g. Bacillus, Clostridium, Coxiella, Desulfotomaculum, Sporolactobacillus, Sporomusa and Thermoactinomyces (see also SPOROSPIRILLUM). In general, endospores are considerably more resistant than vegetative cells to heat, desiccation, antimicrobial agents and radiation; their irreversible inactivation can be ensured only by the procedures used for STERILIZATION (sense 1). Endospores can remain dormant for long periods of time: the age of some specimens may be measurable on a geological scale [Microbiology (1994) 140 2513–2529]. Studies on the formation and germination of endospores have been carried out mainly with Bacillus subtilis. Sporulation in Bacillus subtilis. When growth becomes limited through depletion of nutrients, the cell re-organizes its metabolism. This involves the induction and repression of certain genes – although there is no abrupt switch from active growth to active sporulation; during this transition state the decision is made to either (i) maintain the vegetative condition, with low-level metabolism but without cell division, or (ii) initiate sporulation. [Transition state in B. subtilis: PNARMB (1993) 46 121–153.] The cell’s decision whether to sporulate or not is influenced by various environmental and intracellular signals which must be integrated and interpreted. For example, sporulation is inhibited if calcium levels drop to 2 µM; this reflects the need for calcium during development of the endospore, and may be seen as a mechanism that prevents sporulation from aborting at an intermediate stage. The (unknown) signals that initiate sporulation appear to stimulate at least two types of sensor molecule which are designated kinase A and kinase B (KinA and KinB). On receipt of appropriate signals, these kinases undergo ATP-dependent autophosphorylation and initiate a so-called phosphorelay (see figure, part (a)); Spo0F appears to act as the main junction through which are channelled various external and internal signals. The key factor which determines the decision between ongoing vegetative growth and sporulation appears to be the intracellular level of Spo0A∼P. Spo0A∼P is de-phosphorylated (i.e. negatively regulated) by the phosphatase Spo0E (see figure). Indirect regulation of Spo0A∼P involves other phosphatases, the RapA and RapB proteins, which de-phosphorylate Spo0F∼P; RapA and RapB 271 Signal Signal KinA KinB P P ATP ATP ADP ADP Spo0F P spollD Spo0B spollG (σ E) P spo0A + + spollA (σ F) Spo0A~P spo0F spollE − Spo0E − − − spo0H (σ H ) abrB + + hpr − sinl − − sinR ENDOSPORE (a): simplified scheme for the regulation of gene expression during the initiation of sporulation in Bacillus subtilis. The mechanism for initiating sporulation appears to recognize both external (environmental) and internal (intracellular) signals. These signals are believed to promote ATP-dependent autophosphorylation of certain kinases – shown in the diagram as KinA and KinB. The kinases transfer P ) to a sequence of proteins: Spo0F → Spo0B → Spo0A (the so-called phosphorelay) [COGD (1993) 3 203–212; phosphate (symbol  JCBiochem. (1993) 51 55–61]. The intracellular concentration of the phosphorylated form of Spo0A (Spo0A∼P) appears to be the key factor in initiating sporulation; this protein is a transcription factor which promotes the expression of certain sporulation-specific genes (dashed box, centre right) and also regulates genes shown in the two other dashed boxes. − ; + indicates that the expression of a given gene is promoted. Control In the diagram, inhibitory influences are indicated by the symbol  is commonly exercised at the level of transcription. Spo0A∼P promotes the expression of genes spo0F and spo0A (dashed box, left) by a positive feedback loop [Mol. Microbiol. (1993) 7 967–974]. Also, by repressing abrB (lower dashed box), Spo0A∼P promotes the expression of spo0H – whose product (the SIGMA FACTOR sH ) is required e.g. for the transcription of spo0F and spo0A. sH is produced at low level during vegetative growth, but its production is greatly enhanced following the initiation of sporulation. During sporulation, sH is needed for transcription of the ftsZ gene (see FtsZ in CELL CYCLE) from a special promoter, p2, which is distinct from the ftsZ promoter used during exponential growth; FtsZ is required for the formation of the asymmetrical septum. Spo0E is a negative regulator of the phosphorelay [PNAS (1994) 91 1756–1760]; it is an enzyme which de-phosphorylates (and thus inactivates) Spo0A∼P and which may therefore help to prevent the initiation of sporulation until the cumulative effect of the various (extracellular and intracellular) signals dictates that sporulation is necessary. Development of the asymmetrical septum (i.e. at a polar rather than a mid-cell location) initially involves the formation of a mid-cell Z ring (see CELL CYCLE) – i.e. as occurs during vegetative cell division. Later, this Z ring develops into a helical structure that extends into both poles of the cell and eventually forms two separate polar Z rings; these developments require the SpollE protein. Only one polar site is used – initiation of septation at one site blocks the other site. [Science (2002) 298 1942–1946.] During vegetative growth, SinR (encoded by sinR in the lower dashed box) represses expression of the spoII genes (dashed box, centre right). During sporulation, Spo0A∼P represses abrB (lower dashed box) – thus (indirectly) inhibiting expression of SinR and helping to promote expression of the spoII genes; SinR is repressed at the protein–protein level, i.e. inhibited by the SinI protein. Note that Spo0A∼P also directly promotes the transcription of sinI. (Continued on page 273.) 272 ENDOSPORE (a) (continued) The product of spoIIG (dashed box, centre right) is a sigma factor, sE (= s29 ), which is needed for transcription of (i) the spoIID gene, and (ii) certain genes in the mother cell. SpoIID is one of several products (others are SpoIIM, SpoIIP and SpoIIB) which are involved in the degradation of peptidoglycan in the asymmetric septum – a necessary preliminary to the engulfment of the prespore by the mother cell (early stage II). The product of spoIIE localizes on the prespore side of the asymmetrical septum where, through its phosphatase activity, it can activate sF [GD (1998) 12 1371–1380]. The product of spollA (sF ) is needed for transcription in the forespore. Figure reproduced from Bacteria, 5th edition, Figure 7.14, page 154, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. stage 0 stage I stage II ii stage II iii stage II i stage III exosporium spore coat cortex cytoplasm cytoplasmic core membrane stage IV stages V−VI stage VII ENDOSPORE (b): stages in the development of an endospore (diagrammatic). Stage 0. Prior to sporulation, a (vegetative) cell is said to be in stage 0. Stage I. The cell contains an axial filament composed of two chromosomes. (Some authors consider that sporulation proper begins at stage II.) Stage II. Initially, an asymmetric (polar) septum divides the cell into two protoplasts. The smaller protoplast (= prespore, forespore), which contains a single chromosome, is the precursor of the endospore; the other protoplast is referred to as the mother cell. Before further development, peptidoglycan in the asymmetric septum must be degraded; this involves SpoIID and other proteins (see legend of figure (a)). The cytoplasmic membrane of the mother cell then begins to engulf the smaller protoplast so that the latter subsequently becomes bounded by a double membrane; the final stages of this process (in Bacillus subtilis), in which the mother cell membrane fuses to complete the outermost membrane, may involve the SpoIIIE protein [PNAS (1999) 96 14553–14558]. Stage III. Completion of development of the double membrane. Stage IV. Modified PEPTIDOGLYCAN is laid down between the two membranes to form a rigid layer called the cortex. The peptidoglycan differs from that in vegetative cell walls in that e.g. it contains fewer cross-links, and in that the lactyl groups of some muramic acid residues form lactam rings by displacing the N-acetyl group. A loose protein envelope called the exosporium may begin to develop at about this time. Stages V–VI. A multilayered protein spore coat is deposited outside the outermost membrane (stage V). The spore coat is rich in e.g. cystine residues, and it is strongly hydrophobic. The spore matures (stages V–VI) to develop its characteristic resistance to heat and its bright, refractile appearance under the light microscope; during this period, calcium dipicolinate (see DIPICOLINIC ACID) accumulates in the protoplast (‘core’). The endospore reaches maturity. Stage VII. The endospore is released by lysis of the mother cell. (In some species the mature endospore carries a loose outer membrane (exosporium) external to the spore coat; the origin and composition of the exosporium appear to be unknown.) Figure reproduced from Bacteria, 5th edition, Figure 4.2, page 61, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 273 endosporium (See also ENDOTOXIC SHOCK.) endotrophic mycorrhiza See MYCORRHIZA. Endotrypanum A genus of protozoa (family TRYPANOSOMATIDAE) parasitic in the erythrocytes of sloths (in Central and South America) and in sandflies (Lutzomyia spp) – which transmit the protozoon to sloths. One species (E. schaudinni ) forms intraerythrocytic epimastigotes, the other (E. monterogei ) forms intraerythrocytic trypomastigotes; in culture both species form promastigotes. endotunica See BITUNICATE ASCUS. endoxylanase See XYLANASES. endozoite (tachyzoite) In certain coccidia: a stage within host tissues (see e.g. TOXOPLASMOSIS). energy charge (EC) See ADENYLATE ENERGY CHARGE. energy-transducing membrane Any biological membrane which contains components that are involved in the conversion of one form of energy (e.g. light, ATP) to another (metabolically usable) form of energy. Such membranes are involved e.g. in RESPIRATION and PHOTOSYNTHESIS; they include the bacterial CYTOPLASMIC MEMBRANE (and the CHLOROSOME and CHROMATOPHORE membranes), the inner membrane of the MITOCHONDRION, the THYLAKOID membranes of cyanobacteria and CHLOROPLASTS, and the PURPLE MEMBRANE in certain halobacteria. (In cyanobacteria, respiratory electron transport occurs in the cytoplasmic membrane and, in at least some species, may occur also (in addition to photosynthetic electron transport) in the thylakoids [Book ref. 75, pp. 199–218].) (See also CHEMIOSMOSIS.) enFeLV viruses See FELINE LEUKAEMIA VIRUS. enhancement effect (Emerson enhancement effect) In algae and green plants illuminated with light of wavelength above ca. 680 nm: an increase in PHOTOSYNTHESIS which occurs when the illumination is augmented with an additional beam of light of shorter wavelength. (cf. RED DROP.) enhancer A cis-acting DNA sequence, present in the genomes of higher eukaryotes and of various animal viruses, which can greatly increase transcription; an enhancer can generally function in either orientation and at various distances (upstream or downstream) from a given promoter. (cf. UAS.) The enhancer of e.g. simian virus 40 (SV40) can function in a wide variety of animal cells, but enhancers of certain other viruses and of cellular genes show more or less strict specificity for particular cell or tissue types. Enhancer function requires the participation of trans-acting protein factors; for example, the SV40 enhancer apparently consists of multiple elements, each of which is recognized and bound by specific transcription factors [Book ref. 189, pp. 19–26]. Interactions between protein factors bound to enhancers and to other genetic elements (e.g. PROMOTER) may be important; in yeast, DNA looping between enhancer and promoter suggests that apposition of these elements is involved in enhancer action [NAR (2005) 33 3743–3750]. enhancer (translational) See DOWNSTREAM BOX. enhancer-dependent sigma factor (s54 ) See SIGMA FACTOR. enniatins Cyclic DEPSIPEPTIDE ANTIBIOTICS, obtained e.g. from Fusarium spp, which behave as IONOPHORES (for monovalent cations) and which are active against certain Gram-positive bacteria, e.g. Mycobacterium tuberculosis. The enniatin molecule includes three residues of D-a-hydroxyisovaleric acid, each occurring between two residues of N-methyl-L-isoleucine (enniatin A), N-methyl-L-valine (enniatin B ), N-methylleucine (enniatin C ), or N-methylphenylalanine (beauvericin, produced by Beauveria bassiana); the residues are linked by alternating peptide and ester bonds. enokitake (winter mushroom) Flammulina velutipes, a mushroom cultivated for food e.g. in Japan. Enokitake is grown on counterstained with safranin (0.5% aqueous) for ca. 30 sec, washed again with water, and blotted dry. Endospores stain green, cells red-brown. endosporium Syn. ENDOSPORE (sense 2). endosporulation In coccidia: SPORULATION which occurs within the host. Endosporulation occurs e.g. in all species of Sarcocystis and in Cryptosporidium, and it has been reported to occur in certain coccidia parasitic in fish (e.g. Eimeria carpelli ). ENDO-STAPH See TEICHOIC ACIDS. Endostelium See PROTOSTELIOMYCETES. endosymbiont An organism which lives within the cells or tissues of another organism in a symbiotic association (SYMBIOSIS sense 1). (cf. ECTOSYMBIONT.) endothelial cells Cells which form a lining (one-cell thick) in the blood vessels, heart and lymphatic system. endothelium–leukocyte adhesion molecule Syn. CD62E. Endothia A genus of fungi of the order DIAPORTHALES. E. parasitica [ultrastructure: CJB (1982) 61 389–399] causes CHESTNUT BLIGHT (q.v.). (See also PROTEASES.) endothrix infection See DERMATOPHYTES. endotoxic shock (septic shock) An often-fatal condition (mortality rate ≥50%) associated with blood-borne ENDOTOXINS of Gram-negative bacteria (Gram-positive bacteria are reported to provoke a similar or identical condition); in the USA alone, the number of deaths from endotoxic shock is over 100000 per year. Microbial components act on e.g. monocytes/macrophages and promote the secretion of CYTOKINES such as IL-1b and (particularly) TNF-a; these agents can recruit others, leading to symptoms such as fever, a marked fall in blood pressure and occlusion of blood vessels by white cells. Death may result from a progressive failure of organs. Individuals with the TNF2 allele appear to be particularly susceptible to endotoxic shock; this allele, which contains a variant form of the TNF-a promoter, correlates with enhanced production of TNF-a, spontaneously or under stimulation, both in vitro and in vivo. [Association of TNF2, a TNF-a promoter polymorphism, with septic shock susceptibility and mortality: JAMA (1999) 282 561–568.] Experimental vaccination against endotoxic shock with antiendotoxin monoclonal antibodies has been tried unsuccessfully [RMM (1994) 5 183–190]. Some recent candidate antibiotics – for example compound L-573,655 [Science (1996) 274 980–982] – are directed against lipid A biosynthesis, and it seems possible that such antibiotics may reduce the risk of endotoxic shock because their activity decreases the levels of lipid A. A possible treatment for endotoxic shock follows the discovery of various molecules with endotoxin antagonist activity; one of these, E5531 (a synthetic analogue of lipid A from Rhodobacter capsulatus), has been found to block endotoxin challenge in human trials. [Antibacterial and anti-inflammatory agents that target endotoxin: TIM (1998) 6 154–159.] endotoxin (1) Traditionally, the LIPOPOLYSACCHARIDES (LPS) of Gram-negative bacteria; the toxic component of LPS is lipid A – although the polysaccharide moiety may contribute to toxicity by conferring water-solubility. (2) Although many authors [e.g. RMM (1994) 5 183–190] equate endotoxin with LPS, others [Book ref. 218, p 26] have used the term to refer to a complex that consists of LPS and outer-membrane protein(s); using this nomenclature, the protein associated with LPS is designated either lipid A-associated protein (LAP) or endotoxin-associated protein (EAP). (3) Any microbial toxin which is released only on cell lysis. (See also DELTA ENDOTOXIN.) 274 Enteridium sawdust supplemented with rice bran; the mycelium is grown at 20–25° C, and fruiting is induced by lowering the temperature to 10–12° C. The cultivated fruiting bodies differ from those which develop in nature, the mushroom being whitish or cream-coloured with a long slender stipe. [Mycol. (1983) 75 351–360.] enoxacin See QUINOLONE ANTIBIOTICS. enoyl-acyl carrier protein reductase (ENR) See DIAZABORINES and TRICLOSAN. enriched medium Any (liquid or solid) basal MEDIUM which has been supplemented (enriched) with serum or blood etc – or with any of a range of nutrients or growth factors – in order to enable it to support or enhance the growth of particular type(s) of organism. Examples: BLOOD AGAR, serum sugars (see PEPTONEWATER SUGARS). (cf. ENRICHMENT MEDIUM.) enrichment Any process which increases the proportion of a given microorganism in a mixed population. For example, faeces from a typhoid case can be cultured in an enrichment medium (e.g. SELENITE BROTH) which suppresses growth of the normal faecal flora while permitting growth of the pathogen, Salmonella typhi ; S. typhi is therefore ‘enriched’ so that its detection is facilitated. (See also DEFECTIVE INTERFERING PARTICLE.) enrichment medium Any medium used for ENRICHMENT; it may contain substance(s) which encourage growth of the required organism and/or inhibit the growth of other type(s) of organism. (See e.g. SELENITE BROTH and TETRATHIONATE BROTH; cf. ENRICHED MEDIUM.) Ensifer A genus of Gram-negative, aerobic, chemoorganotrophic and facultatively predatory, soil-inhabiting BUDDING BACTERIA; cells: rod-shaped (ca. 0.7–1.1 × 1.0–1.9 µm), motile by means of a subterminal tuft of 3–5 flagella. The cells of E. adhaerens (the type species) attach, end on, to a suitable prey bacterium (e.g. Micrococcus luteus), forming a palisade arrangement; under certain conditions the prey cell may be lysed. [IJSB (1982) 32 339–345.] ensiform Sword-like; narrow and pointed. ensilage (1) The process of making SILAGE. (2) The silage itself. Ent plasmid Any plasmid carrying genes for the ST and/or LT enterotoxins of ETEC (q.v.). entA gene (Staphylococcus aureus) See ENTEROTOXIN. entA–entG genes (in Escherichia coli ) See SIDEROPHORES. Entamoeba A genus of protozoa (order AMOEBIDA) characterized by the presence of beaded chromatin on the inner surface of the nuclear membrane. Most species (not E. gingivalis) form cysts which may contain CHROMATOID BODIES and glycogencontaining vacuoles. All species except E. moshkovskii occur as parasites/commensals in the alimentary tract in animals. E. coli. A non-pathogen found in human faeces. Trophozoites are ca. 10–50 µm diam., contain numerous vacuoles, and slowly form blunt, granular pseudopodia; the single karyosome is located eccentrically, and the peripheral chromatin is coarsely and irregularly beaded. Mature cysts contain 8 nuclei. E. gingivalis. A non-pathogenic, non-cyst-forming species found in the mouth in man and other animals. Trophozoites: ca. 5–35 µm diam., morphologically similar to those of E. histolytica. E. hartmanni. A non-pathogenic species found in human faeces. It resembles E. histolytica, but both trophozoites (5–10 µm diam.) and cysts (<10 µm diam.) are smaller. E. histolytica. A parasite and potential pathogen in the human intestine; it can cause amoebic DYSENTERY. Non-invasive trophozoites living in the gut lumen are ca. 20 µm in diam.; invasive forms which penetrate and develop in the gut mucosa are usually 20–50 µm diam. The cells contain few vacuoles, but may contain ingested red blood cells (an almost pathognomonic feature); finger-shaped, hyaline pseudopodia are produced rapidly. The (single) nucleus contains a small central karyosome, and the peripheral chromatin is finely and regularly beaded. In the gut lumen, some trophozoites expel their food particles and shrink to become rounded, non-motile precysts; a precyst secretes a resistant wall and becomes a spherical CYST (usually >10 µm and <20 µm diam.). A cyst initially contains a single nucleus, but two nuclear divisions result in four nuclei in the mature cyst; each nucleus contains a central karyosome. Young cysts may contain round-ended chromatoid bodies in the cytoplasm. Trophozoites, precysts and cysts may all be found in the faeces of the host, but only the cysts are infective for another host. Cysts are resistant to various environmental conditions – and to normal chlorination levels in water supplies – but are sensitive to temperatures >40° C and < −5° C and to desiccation. Cysts may be stained with iodine (cytoplasm yellow, glycogen vacuoles reddish-brown); trophozoites stain well with e.g. iron–haematoxylin after fixation in PVA (cytoplasm grey or pale violet, nuclear structures black). E. histolytica may be cultured anaerobically on media containing e.g. inspissated horse serum, rice starch, or egg albumen, together with bacteria or flagellate protozoa; it has also been cultured in sterile media. Strains have been classified on the basis of growth characteristics (‘classical’ strains grow best at 37° C, while atypical, less pathogenic ‘Laredo-type’ strains grow well at 37° C and at 25° C), or by the presence or absence of the enzyme phosphoglucomutase (present in organisms from patients with clinical amoebiasis, but not in those from asymptomatic carriers). (See also PYROPHOSPHATE.) E. moshkovskii. A non-pathogen found in sewage; it resembles E. histolytica morphologically, but apparently occurs only in the free-living state. E. paulista. A hyperparasite found in the cytoplasm of Opalina ranarum in frogs. Other species include e.g. E. bovis (from cattle), E. invadens (pathogenic in reptiles), E. muris (from rats and mice), and E. polecki (from pigs). [Biochemistry and functional morphology of Entamoeba: JP (1985) 32 221–240.] enteric Of or relating to the intestine (particularly the small intestine). ‘Enteric bacteria’ may refer to any or all bacteria normally found in the (small or large) intestine, or may refer specifically to members of the ENTEROBACTERIACEAE. enteric adenoviruses See MASTADENOVIRUS. enteric fever Any typhoid-like human salmonellosis – caused e.g. by Salmonella paratyphi (cf. PARATYPHOID FEVER), S. typhimurium etc. – as distinct from Salmonella FOOD POISONING. Some authors include TYPHOID FEVER among the enteric fevers. enteric redmouth (red vent disease) A disease of rainbow trout (Salmo gairdneri ) caused by Yersinia ruckeri. Symptoms include e.g. haemorrhagic lesions in and around the mouth and vent. (cf. RED MOUTH.) enteric septicaemia (of fish) A disease of the channel catfish (Ictalurus punctatus) caused by Edwardsiella ictaluri ; the disease has become an important problem in the intensive cultivation of catfish. Naturally infected fish often have a characteristic erosive lesion on the head (‘hole-in-the-head’); chronic cases may involve enteritis, hepatitis, meningoencephalitis, etc. Primary infection can apparently occur via the gut or via the nares. [Pathogenesis: CJFAS (1986) 43 36–42.] (See also FISH DISEASES.) Enteridium See MYXOMYCETES. 275 enteritis Enterobacteriaceae A family of Gram-negative, asporogenous, facultatively anaerobic bacteria. Cells: rod-shaped, typically ca. 0.3–1.0 × 1–6 µm (cf. OBESUMBACTERIUM), motile (peritrichously flagellate – cf. TATUMELLA) or non-motile. Metabolism is chemoorganotrophic and may be respiratory or fermentative, depending e.g. on conditions. Most species can reduce nitrate to nitrite. All species are oxidase −ve and (except for Shigella dysenteriae serotype 1 and Xenorhabdus nematophilus) catalase +ve. Reproduction occurs by binary fission. Type genus: Escherichia. Enterobacteria occur e.g. as parasites, pathogens or commensals in man and/or animals, as saprotrophs or pathogens in plants (see ERWINIA), or as saprotrophs (or faecal contaminants) in soil and water. Many are important opportunist causes of nosocomial infections. Enterobacteria typically grow readily on a range of basal media (e.g. NUTRIENT AGAR). Media selective for enterobacteria commonly contain a sugar (often lactose), bile salts (as selective agents), and a pH indicator (see e.g. MACCONKEY’S AGAR). Genera and species are traditionally differentiated on the basis of biochemical tests – e.g. the IMVIC TESTS, the ability to produce certain enzymes (e.g. UREASE, ARGININE DIHYDROLASE), to utilize particular sugars (mono- and disaccharides, polyols etc), to decarboxylate or deaminate certain amino acids (see e.g. DECARBOXYLASE TEST), to produce HYDROGEN SULPHIDE from thiosulphate (see e.g. TSI AGAR), etc. [Identification tests: Book ref. 184.] Reactions to such tests may be affected e.g. by mutation or by the presence of a metabolic PLASMID, and in some cases are strongly dependent on the reaction conditions (see e.g. YERSINIA). DNA HOMOLOGY (‘DNA relatedness’) studies have been widely used to indicate degrees of relatedness between strains, species and genera. However, the genera Escherichia and Shigella, for example, continue to be recognized even though they have been shown to be sufficiently closely related genetically to be regarded as a single genus (or even as a single species). [Book ref. 46, pp. 1108–1112.] Many enterobacteria can be serotyped (see TYPING) on the basis of O ANTIGENS, H ANTIGENS, K ANTIGENS and/or VI ANTIGENS; cross-reactions between members from different genera are quite common (see e.g. CITROBACTER). All enterobacteria (except Erwinia chrysanthemi ) possess the ENTEROBACTERIAL COMMON ANTIGEN. Colicin, bacteriophage and antibiotic TYPING may be used for particular genera. [Serology/serotyping in enterobacteria: Book ref. 68.] Genera: CEDECEA; CITROBACTER; EDWARDSIELLA; ENTEROBACTER; ERWINIA; ESCHERICHIA; EWINGELLA; HAFNIA; KLEBSIELLA; KLUYVERA; KOSERELLA; LEMINORELLA; MOELLERELLA; MORGANELLA; OBESUMBACTERIUM; PROTEUS; PROVIDENCIA; RAHNELLA; SALMONELLA; SERRATIA; SHIGELLA; TATUMELLA; XENORHABDUS; YERSINIA. (See also SODALIS.) enterobacterial common antigen (ECA; Kunin antigen) A heteropolysaccharide antigen found in the outer membrane in all wild-type strains of the Enterobacteriaceae (excluding Erwinia chrysanthemi ) and e.g. in strains of PLESIOMONAS. It consists of alternating residues of (1 → 4)-linked N-acetyl-D-glucosamine and N-acetyl-D-mannosaminuronic acid which may be esterified with small amounts of e.g. palmitic and acetic acids. ECA may be linked to phospholipid (when it is poorly immunogenic), or it may be bound e.g. to the core region of LPS (when it is highly immunogenic); it is readily accessible to homologous antibodies in non-capsulated rough strains, but may be inaccessible e.g. in smooth, capsulated strains [JGM (1982) 128 1577–1583]. enterobacterial repetitive intergenic consensus sequence See ERIC SEQUENCE. enteritis Inflammation of the lining of any part of the intestine – particularly the small intestine; it may result from the same causes as GASTROENTERITIS. (cf. COLITIS.) enteritis necroticans (necrotizing enteritis; ‘pig bel’; Darmbrand) A rare, severe (often fatal), haemorrhagic enteric disease caused by ingestion of food (commonly pig meat) contaminated with Clostridium perfringens type C. (cf. FOOD POISONING.) The organisms adhere to the intestinal mucosa and produce the necrotizing b-toxin, causing necrosis of the mucosa. Diet influences disease development: the b-toxin is normally readily inactivated by digestive proteases; however, high-carbohydrate, low-protein diets reduce the secretion of proteases, and e.g. sweet potatoes contain a trypsin inhibitor which further lowers proteolytic activity – factors which allow the b-toxin to persist. enteroadherent E. coli Diarrhoeagenic strains of Escherichia coli characterized by their ability to adhere to (mammalian) HEp-2 cells; strains can be differentiated on the basis of their patterns of adherence: ‘diffusely adherent E. coli ’ (DAEC) adheres primarily as isolated, individual cells in a diffuse manner, while enteroaggregative E. coli (EAggEC: see EAGGEC) forms a pattern of scattered aggregates in which most of the cells are obviously grouped with other cells. (EPEC adheres to HEp-2 in a localized manner, forming microcolonies.) The category ‘enteroadherent’ has been discontinued: strains of HEp2-adherent E. coli (excluding EPEC) are now divided into the two main groups EAggEC and DAEC (each of which, though still heterogeneous, is associated with distinct virulence factors and epidemiology) [RMM (1995) 6 196–206]. (See also EAEC.) enteroaggregative E. coli See EAGGEC. Enterobacter A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Cells: ca. 0.6–1.0 × 1.2–3.0 µm, motile (generally 4–6 peritrichous flagella). Most strains possess fimbrial haemagglutinins [JGM (1983) 129 2175–2180]. Typical reactions: indole −ve; MR −ve; VP +ve; citrate +ve; H2 S −ve; acid and gas from glucose at 37° C (no gas at 44.5° C); lactose +ve. Optimum growth temperature: ca. 30° C. GC%: 52–60. Type species: E. cloacae. Enterobacter spp occur e.g. in the intestines of man and animals, in soil, water and sewage, in foods (e.g. dairy products), on plants etc. Most species can be opportunist pathogens, causing nosocomial infections (e.g. meningitis, pneumonia, septicaemia, UTI). E. aerogenes. Lysine and ornithine decarboxylases +ve; arginine dihydrolase −ve; D-sorbitol +ve; non-pigmented. (It has been proposed that E. aerogenes be transferred to the genus Klebsiella as ‘K. mobilis’ [see Book ref. 22, p. 463, 466 and 469].) (cf. AEROBACTER.) E. agglomerans. A heterogeneous species comprising organisms alternatively included in Erwinia herbicola (see ERWINIA). Lysine and ornithine decarboxylases −ve; arginine dihydrolase −ve; lactose −ve. Some strains are yellow-pigmented. E. cloacae. Lysine decarboxylase −ve; ornithine decarboxylase +ve; arginine dihydrolase +ve; D-sorbitol +ve; DNase −ve at 25° C. Non-pigmented. Naturally resistant to ampicillin. E. sakazakii includes yellow-pigmented strains previously included in E. cloacae. D-Sorbitol −ve; DNase delayed +ve at 25° C. Other species: E. amnigenus, E. gergoviae, E. intermedium. (E. hafniae and E. alvei : see HAFNIA; E. liquefaciens: see SERRATIA.) [Book refs. 46, pp. 1173–1180; and 22, pp. 465–469.] enterobacteria Bacteria of the ENTEROBACTERIACEAE. 276 Enterovirus enterobactin See SIDEROPHORES. enteroblastic proliferation (mycol.) See CONIDIUM. enterochelin See SIDEROPHORES. enterochromaffin cells Cells in the mammalian gut epithelium which secrete certain hormones – e.g. SEROTONIN (a stimulator of intestinal secretion). enterocin P A broad-spectrum BACTERIOCIN produced by Enterococcus faecium strain P13 and secreted by a sec-dependent pathway; it inhibits strains of e.g. Listeria monocytogenes, Staphylococcus aureus, Clostridium perfringens and C. botulinum. [Biochemical and genetic characterization of enterocin P: AEM (1997) 63 4321–4330.] enterococci (1) Members of the genus ENTEROCOCCUS. (2) A term which has been used very loosely to refer to some or all of the organisms referable to the genus ENTEROCOCCUS. Enterococcus A genus of Gram-positive, asporogenous, chemoorganotrophic, facultatively anaerobic, coccoid bacteria; the genus was originally proposed to accomodate organisms previously classified as Streptococcus faecalis and Streptococcus faecium [proposal to transfer S. faecalis and S. faecium to the genus Enterococcus as E. faecalis and E. faecium respectively: IJSB (1984) 34 31–34]. [Proposal to transfer other streptococci to this genus: S. avium (as Enterococcus avium), S. casseliflavus (E. casseliflavus), S. durans (E. durans), S. faecalis subsp malodoratus (E. malodoratus) and S. gallinarum (E. gallinarum): IJSB (1984) 34 220–223.] Members of the genus occur in the animal and human intestine; they can usually grow at 10° C and 45° C, and can grow in 6.5% sodium chloride and at pH 9.6. Metabolism: typically fermentative, but at least some strains can carry out respiratory metabolism when provided with haemin under aerobic conditions. (See also PYR.) GC% 37–45. Type species: E. faecalis. E. faecalis. Organisms formerly classified as Streptococcus faecalis (Lancefield group D). Cells: ovoid, occurring singly or in pairs or short chains; motile strains are rare. Non-haemolytic or, rarely, b-haemolytic. E. faecalis can survive 60° C/30 min. Various sugars are fermented (e.g. D-tagatose, but not e.g. D- or L-arabinose), and acid is formed from glycerol both aerobically and anaerobically. Energy can be obtained from pyruvate, citrate and malate. Growth can occur in the presence of 0.02% sodium AZIDE, 0.04% tellurite, or 0.01% TETRAZOLIUM. Some strains form a PSEUDOCATALASE. Some strains of E. faecalis can transmit/receive plasmids that confer resistance to antibiotic(s) (see CONJUGATION). A current problem is the acquisition of (plasmid-borne) resistance to VANCOMYCIN; the appearance of vancomycin-resistant enterococci (VRE) is important because enterococci are frequent causal agents of nosocomial bacteraemia. It has been estimated that ∼15% of enterococci are resistant to vancomycin [EID (1998) 4 239–249]. Vancomycin resistance may be conferred by one or more plasmid-borne van genes; vanA and vanB both confer resistance to vancomycin, while vanA also confers resistance to teichoplanin. (These two genes have been reported in both E. faecalis and E. faecium.) Multiplex PCR has been used for rapidly detecting vanA and vanB in presumptive colonies of VRE [JCM (1999) 37 2090–2092]. [Vancomycin-resistant E. faecium with a VanB phenotype in a Warsaw hospital: JCM (2001) 39 1781–1787.] E. faecium. Organisms formerly classified as Streptococcus faecium (Lancefield group D). The organisms resemble E. faecalis in morphology and general characteristics, including resistance to heat, high pH and salinity; some strains are ahaemolytic. Acid is formed from arabinose but not from tagatose. Unlike E. faecalis, E. faecium cannot use citrate, malate or pyruvate as sources of energy. Hydrogen peroxide may accumulate under aerobic conditions. Enterocytozoon A genus of protozoa (phylum MICROSPORA); E. bieneusi has been reported from the gut of a human AIDS patient [JP (1985) 32 250–254]. enterodiol See LIGNANS. enterohaemorrhagic E. coli See EHEC. enteroinvasive Refers to pathogenic organisms which invade the intestinal mucosa (see e.g. EIEC). enteroinvasive E. coli See EIEC. enterolactone See LIGNANS. Enteromonadina See DIPLOMONADIDA. Enteromonas See DIPLOMONADIDA. Enteromorpha A genus of macroscopic green algae, closely related to ULVA spp, in which the thallus consists of hollow tubes one cell thick; species occur in coastal waters and on estuarine mudflats, often flourishing in conditions which also favour Ulva lactuca. enteropathogenic Pathogenic for the intestine. (See also FOOD POISONING; for enteropathogenic Escherichia coli see EPEC.) Enteropogon See TRICHOMYCETES. enterotoxigenic (enterotoxinogenic) Refers to an organism that produces one or more ENTEROTOXINS. (See also ETEC; cf. ENTEROINVASIVE and ENTEROPATHOGENIC.) enterotoxigenic E. coli See ETEC. enterotoxin A TOXIN which acts on the intestine, either when ingested or when produced by an organism within the intestine (see e.g. CHOLERA TOXIN and ETEC). [Enteric bacterial toxins (review): MR (1996) 60 167–215.] The enterotoxins of Staphylococcus aureus (A, B, C1–C3, D and E) are SUPERANTIGENS, and their ability to stimulate the production of cytokines may account for at least some of the observed symptoms of staphylococcal FOOD POISONING; gut epithelial cells express MHC class II molecules and may present enterotoxin to submucosal T cells [MR (1996) 60 195]. [See also Clinical Microbiology Reviews (2000) 13 16–34.] Staphylococcal enterotoxins are small proteins (∼25–30 kDa) with similar biological activities; they may be encoded by chromosomal, plasmid-borne or phage-borne genes – in at least some strains, the gene encoding enterotoxin A (entA) is phageborne. Each toxin is a single polypeptide chain which is resistant to many proteolytic enzymes, and can generally withstand boiling for up to 30 minutes. The toxins are produced during active growth. (See also TSST-1 under TOXIC SHOCK SYNDROME.) Enterotube II See MICROMETHODS. Enterovioform See 8-HYDROXYQUINOLINE. Enterovirus A genus of viruses (family PICORNAVIRIDAE), most of which replicate primarily in the mammalian intestinal tract – although subsequently other tissues may become infected. The virions are stable at pH 3–9 and at or below room temperature, and are resistant to e.g. ethanol (70%), Lysol (5%), quaternary ammonium compounds, detergents and bile salts; they are generally inactivated by heat (e.g. 50° C), formaldehyde (3%), HCl (0.1 N), or chlorine (ca. 0.3–0.5 ppm free residual Cl2 ). They can be protected against heat inactivation by 1 M MgCl2 . Many enteroviruses can be propagated in primary cultures of human or monkey kidney cells and in some cell lines (e.g. HeLa, Vero, WI-38). Most human enteroviruses are classified into one of three groups (see below) on the basis of e.g. pathogenicity and host range; enterovirus infections are commonly or usually 277 Entner–Doudoroff pathway taken into the cell by specific transport systems and then phosphorylated to form 6-phosphogluconate (which can then be metabolized via the ED pathway and/or the HMP pathway). [ED pathway (history, physiology, molecular biology): FEMS Reviews (1992) 103 1–28. ED pathway in E. coli: JB (1998) 180 3495–3502.] Entodiniomorphida An order of protozoa (subclass VESTIBULIFERIA) in which the cells are naked except for tufts or bands of oral and (often) somatic syncilia. STOMATOGENESIS is apparently apokinetal. In some species the pellicle is drawn out into terminal spikes or spines. The organisms occur e.g. in the alimentary canal in herbivorous mammals. Genera: e.g. DIPLODINIUM, ENTODINIUM, Epidinium, Metadinium. Entodinium A genus of ciliates (order ENTODINIOMORPHIDA) related to DIPLODINIUM. Typically, the anterior pole of the cell is flattened and bears a conspicuous tuft of cilia; there is no DZM, and the posterior end of the cell may or may not be drawn out into spines. entomo- Prefix denoting insect–e.g. entomopathology: the study of INSECT DISEASES; entomopathogenic: pathogenic for insects. entomogenous Growing in or on insects. entomopathogenic Pathogenic for insects (see INSECT DISEASES). Entomophaga See ENTOMOPHTHORALES. Entomophthora A genus of fungi (order ENTOMOPHTHORALES) which include saprotrophs and parasites of insects. The vegetative thallus is commonly an aseptate or septate mycelium with a tendency to fragment; the ‘conidia’ are forcibly discharged. AZYGOSPORES are formed by some species. [Key to Entomophthora (sens. lat.): BBMS (1982) 16 113–143; infection of houseflies by E. muscae: TBMS (1983) 80 1–8; ultrastructural studies of primary spore formation and discharge in Entomophthora: J. Inv. Path. (1986) 48 318–324.] Entomophthorales An order of fungi (class ZYGOMYCETES) which include a number of parasites of insects and mammals. Asexually derived spores – often called ‘conidia’ – are forcibly discharged in members of the families Basidiobolaceae and Entomophthoraceae. (Members of the ZOOPAGALES are sometimes included as a third family in this order.) Genera include Basidiobolus (see also ZYGOMYCOSIS and EQUINE PHYCOMYCOSIS) and (in the family Entomophthoraceae) Conidiobolus (see also ZYGOMYCOSIS), Entomophaga, ENTOMOPHTHORA, ERYNIA, Gonimochaete (see NEMATOPHAGOUS FUNGI), and Zoophthora. (See also LOBOA and HYPHAL BODY; cf. BLASTOCYSTIS.) entomophthoromycosis See ZYGOMYCOSIS. Entomopoxvirinae (entomopoxviruses, EPVs) A subfamily of viruses, family POXVIRIDAE, which infect insects – including members of the Coleoptera (beetles etc), Diptera (flies etc), Lepidoptera (butterflies and moths), and Orthoptera (locusts, crickets, grasshoppers etc). EPV virions are brick-shaped or ovoid, ca. 300–450 × 170–250 nm, with a characteristic ‘mulberry-like’ appearance in negatively stained preparations – the globular surface ‘units’ apparently being folds of the outer membrane. The viruses multiply in the cytoplasm of (mainly) leucocytes and adipose cells of the host. Late in the replication cycle infected cells may contain one or two types of inclusion body: spheroids (= ‘spherules’, each consisting of mature virions occluded within a matrix of protein, spheroidin) and spindles (virus-free, spindleshaped structures composed of a protein which is apparently distinct from spheroidin). A new host is infected when it ingests a spheroid and the virions are released in the alkaline contents of the gut. EPVs apparently vary in their pathogenicity, but at least some can cause diseases (‘spheroidoses’) in their hosts; they are currently little used for biological control of insect pests. subclinical, but may result in various diseases ranging from mild to severe or fatal [review: Book ref. 148, pp. 739–794]. Coxsackieviruses (first isolated in Coxsackie, New York) are generally pathogenic for newborn mice: group A coxsackieviruses cause a widespread inflammation of skeletal muscles (myositis) resulting in flaccid paralysis, group B coxsackieviruses cause a focal myositis with necrotizing inflammation of fatty tissues and e.g. encephalitis, pancreatitis, etc. In humans, coxsackieviruses can cause e.g. aseptic MENINGITIS; BORNHOLM DISEASE; HAND, FOOT AND MOUTH DISEASE; HERPANGINA; paralysis (very rare); pericarditis and myocarditis (mainly coxsackieviruses B1–B5) which may result in chronic cardiovascular disease; PHARYNGITIS; PNEUMONIA; URTIs (e.g. A21, = ‘Coe virus’, can cause a disease resembling the COMMON COLD). (The causal agent of SWINE VESICULAR DISEASE is apparently closely related to human coxsackievirus B5.) Echoviruses (‘enteric cytopathogenic human orphan viruses’) are generally non-pathogenic in newborn mice; in humans they can cause e.g. aseptic MENINGITIS, encephalitis, paralysis, URTIs, etc. Polioviruses are the causal agents of POLIOMYELITIS in man. There are three serotypes: Brunhilde (type 1), Lansing (type 2), and Leon (type 3). [Molecular biology of polioviruses: Book ref. 162; antigenic structure of polioviruses: JGV (1986) 67 1283–1291.] Unclassified human enteroviruses may cause e.g. bronchiolitis (type 68); ACUTE HAEMORRHAGIC CONJUNCTIVITIS (type 70); MENINGITIS and paralysis (types 70 and 71), and hepatitis (type 72, = HEPATITIS A virus). Porcine enteroviruses cause e.g. SWINE VESICULAR DISEASE and TESCHEN DISEASE. In breeding animals porcine enteroviruses (‘SMEDI viruses’) can cause stillbirth, mummification of the fetus, embryonic d eath, and i nfertility, depending e.g. on the period of gestation at which infection occurs. Other animal enteroviruses include bovine enteroviruses, simian enteroviruses, THEILER’S MURINE ENCEPHALOMYELITIS VIRUS, and the causal agents of AVIAN ENCEPHALOMYELITIS and DUCK VIRUS HEPATITIS. Entner–Doudoroff pathway (ED pathway) A metabolic pathway [Appendix I(c)] for the degradation of glucose in a wide range of bacteria (e.g. species of Acetobacter, Agrobacterium, Pseudomonas, Rhizobium, Serratia, Xanthobacter, Xanthomonas and Zymomonas); some bacteria (e.g. Escherichia coli ) which metabolize glucose mainly via the EMBDEN–MEYERHOF–PARNAS PATHWAY can metabolize gluconate and other aldonates via reactions of the ED pathway. The fates of pyruvate and glyceraldehyde 3-phosphate (GAP) produced in the ED pathway depend on organism and conditions. In some bacteria GAP may be converted to pyruvate via phosphoenolpyruvate, as in the latter part of the EMP pathway [Appendix I(a)]; in pseudomonads, GAP is recycled (via fructose 1,6-bisphosphate, fructose 6phosphate and glucose 6-phosphate – see GLUCONEOGENESIS) to 6-phosphogluconate (which can enter the ED pathway). Some of the 6-phosphogluconate may be diverted into the HEXOSE MONOPHOSPHATE PATHWAY. In aerobic bacteria, pyruvate can generally be metabolized via acetyl-CoA and the TCA CYCLE. In (fermentative) ZYMOMONAS species, pyruvate is largely decarboxylated to acetaldehyde – which is then reduced to ethanol. In addition to the (phosphorylative) pathway for glucose metabolism, many pseudomonads can oxidize glucose directly (i.e. without phosphorylation) to gluconate – see EXTRACYTOPLASMIC OXIDATION. A proportion of the resulting gluconate is 278 enzyme Such viruses are taken up (intact) by the host cell by receptormediated endocytosis, and only when the contents of the endosome become sufficiently acidic does fusion occur between the viral envelope and the endosomal membrane – resulting in the release of the nucleocapsid into the cytoplasm. [Review of endocytosis of enveloped viruses: Bioch. J. (1984) 218 1–10.] In many enveloped viruses (e.g. influenzaviruses) the individual virions do not share a common structure or composition; for example, the envelope glycoproteins in influenzaviruses do not occur in a fixed number in all the virions. By contrast, a defined structure is found in the virions of alphaviruses and flaviviruses [see e.g. Cell (2001) 105 5–8]. In most cases a virus acquires its envelope (during virion assembly) by a process of budding (cf. FLAVIVIRIDAE). The virusencoded envelope proteins each contain an anchor peptide (see SIGNAL HYPOTHESIS) and become anchored in the appropriate cell membrane (e.g. endoplasmic reticulum, plasmalemma, nuclear membrane – according to virus); these proteins then interact with proteins in the preformed nucleocapsid, causing the associated region of membrane to wrap around the nucleocapsid. This region of membrane eventually becomes detached from the host membrane, completing the assembly process. Enveloped virions are typically readily inactivated by lipiddisrupting reagents and procedures: e.g. organic solvents such as ether and chloroform, detergents, heat, etc. [Membranes in animal viruses: Book ref. 148, pp. 45–67.] (2) (bacteriol.) Syn. CELL ENVELOPE. environmental clean-up See BIOREMEDIATION. environmental pollution See e.g. BIOREMEDIATION, GREENHOUSE EFFECT, SEWAGE TREATMENT, WATER SUPPLIES and XENOBIOTIC. (See also AEROSOL and AIR.) enviroxime (2-amino-1-(isopropylsulphonyl)-6-benzimidazolylphenyl ketone oxide) An ANTIVIRAL AGENT which specifically inhibits rhinovirus replication in cell cultures; it does not appear to be effective in preventing or treating human rhinovirus infections in vivo. EnvZ See OSMOREGULATION. enzootic Refers to an ENDEMIC animal disease. enzootic abortion (enzootic ovine abortion) Abortion in sheep caused by Chlamydia psittaci. Laboratory diagnosis may involve e.g. detection by microscopy of elementary bodies in stained placental smears, or culture of placental material [VR (1983) 113 413–414]. (cf. INFECTIOUS ABORTION.) enzootic bovine leukaemia Syn. BOVINE VIRAL LEUKOSIS. enzootic ovine abortion See ENZOOTIC ABORTION. enzootic pneumonia of calves See CALF PNEUMONIA. enzyme A protein which acts as a highly efficient and specific biological catalyst. (Certain RNA molecules are capable of catalytic activity in the absence of protein, and these are sometimes called enzymes (cf. RIBOZYME); the account below refers specifically to protein catalysts.) An enzyme increases the rate of a (thermodynamically feasible) reaction by decreasing the activation energy; it cannot alter the equilibrium constant of a reaction. Many enzymes require non-protein COENZYMES, PROSTHETIC GROUPS, or metal ions/atoms for activity. (See also ZYMOGEN.) Most enzymes have trivial names which usually indicate the nature of the substrate(s) and/or the reaction catalysed (e.g. urease splits urea, alcohol dehydrogenase dehydrogenates alcohol). However, a systematic scheme for the classification and precise nomenclature of enzymes has been established by the IUB Commission on Enzymes. The classification involves the division of all known enzymes into six general classes Three genera of EPVs have been proposed on the basis of virion morphology, host range, and genome MWt [Book ref. 23, pp. 44–46]; members of different genera show no serological relationship either to each other or to other poxviruses. Genus A (coleopteran EPVs). Type species: Melolontha melolontha (cockchafer) EPV. Virion: ovoid, ca. 450 × 250 nm, containing a kidney-shaped (unilaterally concave) core with one lateral body (cf. POXVIRIDAE) and bearing surface globular units ca. 22 nm diam.; DNA MWt: ca. 170–240 × 106 . Similar viruses have been isolated from other coleopteran insects – e.g. Phyllopertha horticola (the garden chafer). Spindles are formed by most of these viruses. Genus B (lepidopteran and orthopteran EPVs). Type species: Amsacta moorei EPV. Virion: ovoid, ca. 350 × 250 nm, containing a cylindrical core and a ‘sleeve-shaped’ lateral body, and bearing surface globular units ca. 40 nm diam.; DNA MWt: ca. 132–142 × 106 . Similar viruses have been isolated from various lepidopteran insects – e.g. Choristoneura biennis (the eastern spruce budworm) and other Choristoneura spp – and from the orthopteran Melanoplus sanguinipes (a short-horned grasshopper). Spindles are formed by most of these viruses. Genus C (dipteran EPVs). Type species: Chironomus luridus EPV. Virion: brick-shaped, ca. 320 × 230 × 110 nm, with a biconcave core and two lateral bodies; DNA MWt: ca. 165–250 × 106 . Similar viruses have been isolated from other dipteran insects, including other Chironomus spp (non-biting midges) and Aedes aegypti (see AEDES). Spindles are not formed in cells infected with any of the viruses in this genus. [Review: AVR (1984) 29 195–213.] entomopoxviruses See ENTOMOPOXVIRINAE. Entomosporium See MELANCONIALES. entomotoxin A TOXIN effective against insects. Entorrhiza A genus of smut fungi (order USTILAGINALES) which are parasitic on the roots of rushes and sedges. Entosiphon A genus of EUGLENOID FLAGELLATES closely related to PERANEMA. entry exclusion Syn. SURFACE EXCLUSION. Entyloma A genus of smut fungi (order USTILAGINALES) which form pale-coloured spores; the host plants range from rice to spinach. enucleated cell A CYTOPLAST, or a (eukaryotic) cell in which the nucleus has been inactivated by a ‘chemical enucleator’. enumeration of microorganisms See COUNTING METHODS. env gene See RETROVIRIDAE. EnvA See CELL CYCLE (b). envelope (1) (peplos) (virol.) A lipoprotein membrane which forms the outermost layer of the virion in certain ANIMAL VIRUSES, PLANT VIRUSES and BACTERIOPHAGES. Animal virus envelopes have been the most extensively studied, and the account below refers to them. In general, the lipid components of the viral envelope are derived from host cell membranes, while the protein components (typically glycoproteins) are virus-encoded; many of the proteins project from the surface as ‘spikes’ or peplomers. Envelope proteins play important roles in the infection process; they are responsible for the attachment of the virion to receptor sites on the host cell surface, and they bring about the release of the nucleocapsid into the host cell cytoplasm by triggering fusion between the envelope and host membranes. In most enveloped animal viruses (but not in e.g. PARAMYXOVIRUSes) the activity of the membrane-fusing envelope proteins requires a low pH (ca. 5–6), and hence in these viruses fusion generally cannot occur between the viral envelope and the host plasmalemma in vivo. 279 enzyme I, II and III (numbered 1–6): 1 = OXIDOREDUCTASES; 2 = TRANSFERASES; 3 = HYDROLASES; 4 = LYASES; 5 = ISOMERASES; 6 = LIGASES. According to the nature of the reaction catalysed, each class is divided into (numbered) subclasses and sub-subclasses; within a given sub-subclass, each individual enzyme is given a specific (arbitrarily assigned) serial number. Thus, any enzyme has a unique classification number (EC number) consisting of four figures. For example, pyruvate kinase has the number EC 2.7.1.40, where ‘EC’ = Enzyme Commission, ‘2’ means that the enzyme is a transferase, ‘7’ that a phosphate group is transferred, and ‘1’ that an alcohol group accepts the phosphate; ‘40’ is the arbitrarily assigned serial number for pyruvate kinase. The systematic nomenclature is intended to reflect the nature of the substrate(s), the reaction catalysed, and any coenzyme involved. Thus, e.g., ‘alcohol dehydrogenase’ (EC 1.1.1.1) is alcohol:NAD oxidoreductase, ‘pyruvate kinase’ is ATP: pyruvate 2-O-phosphotransferase. Microbial enzymes have a number of commercial applications, although their use is still restricted in many cases by e.g. economic factors, stringent safety regulations, and problems of finding suitable enzymes which are stable under the required conditions. Enzymes have a number of advantages over purely chemical processes: they can, under mild conditions, efficiently catalyse reactions which might otherwise require extreme conditions of e.g. temperature, pressure, pH etc; they are usually highly specific (and stereospecific); and they can (at least in theory) be extensively re-used. Either whole microbial cells or purified enzymes may be used. Purified enzymes may be costly, but they generally convert a higher proportion of the substrate (living cells may use some for maintenance energy and biomass production), and fewer side-reactions generally occur; however, isolated enzymes are often unstable. Enzymes from thermophiles are considerably more stable than those from mesophiles and are preferred when available. (See also IMMOBILIZATION sense 1.) For examples of commercial applications of microbial enzymes see AMYLASES, ASPARAGINASE, CATALASE, CELLULASES, b-GALACTOSIDASE, GLUCOSE ISOMERASE, GLUCOSE OXIDASE, GLYCEROKINASE, INVERTASE, LIPASE, PECTIC ENZYMES, PROTEASES, STEROID BIOCONVERSIONS. [Industrial and diagnostic enzymes: PTRSLB (1983) 300 237–434.] Enzymes from ‘extremophiles’ are reported to have unique structures and/or to be stabilized by factors such as thermoprotectants; these enzymes are valuable for studies on enzyme stability, and they have potential applications in biotechnology [TIM (1998) 6 307–314]. In certain pathogenic microorganisms enzymes may play an important role in pathogenesis – see e.g. HYALURONATE LYASE, LECITHINASE, PECTIC ENZYMES, SPHINGOMYELINASE, STREPTOKINASE. enzyme I, II and III See PTS. enzyme-catalysed interesterification See INTERESTERIFICATION. enzyme-coupled probes Small, biotinylated probes labelled with a STREPTAVIDIN –alkaline phosphatase conjugate prior to hydridization [NAR (1999) 27 703–705]. enzyme immunoassay (EIA) Any IMMUNOASSAY in which an enzyme (e.g. a peroxidase) is used (as a marker) to indicate the presence of specific antigens, antibodies etc: see e.g. ELISA and IMMUNOPEROXIDASE METHOD. enzyme-linked immunosorbent assay See ELISA. Eocyta See ARCHAEBACTERIA. Eocytes A sub-category of PROKARYOTES which share with the eukaryotes a similar sequence of 11 amino acids in the (protein synthesis) elongation factor EF-1a, suggesting that the eocytes and eukaryotes are sister taxa with a common ancestor [Science (1992) 257 74–76]. EOP Efficiency of plating: the PLAQUE TITRE of a given type of virus expressed as a proportion of the total number of viruses of that type, per unit volume, in a given suspension. Under ideal conditions the EOP of some bacteriophages may exceed 90%; the EOP of animal viruses is often less than 10%. eosin (eosin Y) TetrabromoFLUORESCEIN: a water-soluble red acidic FLUOROCHROME used e.g. as a cytoplasmic stain; it fluoresces greenish-yellow. (See also PHOTODYNAMIC EFFECT.) eosin–methylene blue agar See EMB AGAR. eosinophil A PMN (q.v.) in which cytoplasmic granules contain basic polypeptides that stain strongly with acidic dyes (e.g. eosin). Eosinophilia (an increase in the number of eosinophils in the blood) may be seen e.g. in coccidioidomycosis and allergic pulmonary aspergillosis (sometimes tuberculosis), but, in general, acute infections of bacterial, fungal or viral aetiology tend to cause a fall in the number of blood eosinophils (i.e. eosinopenia). Protozoan pathogens (e.g. Entamoeba histolytica) generally do not give rise to eosinophilia, but eosinophilia is commonly associated with metazoan parasites (such as flatworms and flukes) [eosinophilia and helminthic infections: BCH (2000) 13 301–317]. Eosinophils can participate in ADCC (see also HEPARIN); they can also be phagocytic, but this is not their main function. Eosinophils have cell-surface receptor sites for e.g. IgG, IgE and certain components of COMPLEMENT (e.g. C3b) – sites which may be involved in cell stimulation and release of (cytotoxic) granule contents. Eosinophils are stimulated e.g. by interleukins 3 and (particularly) 5. eotaxin (chemokine) See CHEMOKINES. Epalxella See ODONTOSTOMATIDA. Epalxis See ODONTOSTOMATIDA. EPEC Enteropathogenic Escherichia coli: a food-borne and water-borne pathogen which causes diarrhoea primarily in infants (sometimes severe outbreaks: ‘infantile gastroenteritis’) – particularly under conditions of poor hygiene. EPEC is a major cause of infant mortality in developing countries. Strains of EPEC characteristically do not invade the gut epithelium (cf. EIEC), do not form ST or LT toxins (cf. ETEC), and do not form shiga-like toxins (cf. EHEC). They attach to the intestinal mucosa and produce characteristic lesions termed attaching and effacing (A/E) lesions. Each lesion involves an initial loss of local microvilli and the subsequent development of a ‘pedestal’: an actin-based, columnar protrusion from the eukaryotic host cell which extends into the gut lumen for several micrometres; the distal end of a pedestal may carry one EPEC cell or a small microcolony of such cells. (An early account noted that strains of EPEC destroy the brush border microvilli “. . . fashioning for themselves in the process a cup-like pedestal of bare plasma membrane in which individual bacteria sit like eggs in egg-cups” [ADC (1984) 59 395–396].) The ability of EPEC to produce A/E lesions is encoded by genes on the 35 kb chromosomal LEE PATHOGENICITY ISLAND (q.v.). (Note. Many strains of EHEC can produce EPEC-like lesions in vitro; at least some strains of E. coli O157:H7, and other EHEC, are known to contain the LEE pathogenicity island. A/E lesions are also produced by certain other Gram-negative bacteria, including Hafnia alvei.) The expression of operons on the LEE pathogenicity island is reported to be regulated by QUORUM SENSING (q.v.). 280 epidemiology a stroma, the asci each containing eight filiform ascospores which often fragment in the ascus. E. typhina causes CHOKE. (See also FESCUE FOOT and ENDOPHYTE.) epicone See DINOFLAGELLATES. epidemic In a human population within a given geographical region: an outbreak of an infectious disease in which, for a limited period of time, a high proportion of individuals in the population exhibits overt symptoms of the disease (cf. ENDEMIC); epidemics are characterized by a sudden onset. (An analogous outbreak of disease among animals or plants is called an epizootic or epiphytotic, respectively.) (See also PANDEMIC and EPIDEMIOLOGY.) Currently, the term ‘epidemic’ is often used in a broader sense to include the occurrence of any disease – either infectious or non-infectious – whose incidence in a given population clearly exceeds that common in other populations. epidemic encephalitis Syn. VON ECONOMO’S ENCEPHALITIS. epidemic haemorrhagic fever Syn. KOREAN HAEMORRHAGIC FEVER. epidemic hepatitis Syn. HEPATITIS A. epidemic keratoconjunctivitis (‘shipyard eye’) A highly infectious KERATOCONJUNCTIVITIS caused by an adenovirus (commonly Ad8: see MASTADENOVIRUS). Infection appears to require direct inoculation of the eye and is typically acquired from inadequately disinfected ophthalmological instruments, contaminated eye ointments etc. (The virus is resistant to organic solvents such as alcohol.) Conjunctivitis with oedema, pain, photophobia and a watery discharge may be followed by erosion of the cornea – sometimes resulting in permanent visual impairment. epidemic meningitis Meningococcal MENINGITIS. epidemic myalgia Syn. BORNHOLM DISEASE. epidemic spread (infectious spread) (of plasmids) An effect which occurs when a population of conjugal donor cells (see CONJUGATION sense 1b), carrying a plasmid that is repressed for transfer, is mixed with a population of recipient cells. Initially, a small proportion of the donor cells – becoming spontaneously DEREPRESSED for transfer – donate their plasmids to recipient cells (low frequency transfer ; LFT). In each of these recipient cells, the newly acquired plasmid remains derepressed owing to the absence of repressor molecule(s); such a cell, and/or progeny derived from it, can transmit the plasmid to a fresh recipient. This effect is cumulative, resulting in the rapid (‘epidemic’) spread of the plasmid throughout the population (high frequency transfer ; HFT). Cells in the HFT state gradually (over several generations) become repressed for transfer as the intracellular concentration of repressor molecule(s) builds up. epidemic tremor Syn. AVIAN ENCEPHALOMYELITIS. epidemic vomiting disease See FOOD POISONING (i). epidemiology The study of the interrelationships between a given pathogen, the environment, and groups or populations of the relevant hosts; the object is to investigate the factors and mechanisms which govern the spread of disease within a community or population. Three main factors are involved. (a) The virulence of the pathogen. Since the observed virulence of a pathogen is related to host susceptibility (a variable) it is difficult to define virulence in absolute terms. (b) Herd immunity. This may be regarded as the collective immunity or resistance to a given disease exhibited by a community or population (human or animal) in the setting of its own environment. (Immunity in individual members of a population may result from e.g. VACCINATION or recovery from the disease.) An important feature of herd immunity is the ratio of immunes (individuals resistant to the disease) to susceptibles (those Initial binding of EPEC to the intestinal epithelium was thought, by some authors [TIM (1997) 5 109–114; Book ref. 218], to be mediated by plasmid-encoded bacterial bundleforming pili (BFP; type IV fimbriae). However, in one study, BFP did not adhere to cultures of (paediatric) small intestine – even though they adhered to (mammalian) HeLa cells. In another study, adherence to HeLa cells by (LEE-containing) EHEC apparently depended on filamentous bacterial appendages containing the (LEE-encoded) EspA protein [Mol. Microbiol. (1998) 30 147–161]. One model [TIM (1998) 6 169–172] of EPEC pathogenesis is outlined below. When EPEC comes into contact with an intestinal cell, products of the (LEE-encoded) esc genes, and of the espA gene, together form a type III secretory system (see PROTEIN SECRETION) – the EspA proteins forming short, filamentous links between the two cells. The Tir and EspB proteins (at least) are secreted directly into the eukaryotic cytoplasm. (Energy requirements may derive from ATP hydrolysis at a putative ATPase, EscN, associated with the type III assembly.) Within the eukaryotic cell, Tir is inserted into the cell membrane – where it acts as a receptor for the bacterial cell-surface adhesin intimin. (Intimin may also bind to other host receptors, e.g. b1 integrins.) Phosphorylation of Tir (in the eukaryotic cell) stimulates the polymerization of cytoskeletal elements (e.g. actin) in preparation for pedestal development. (EPEC cells subsequently adherent at the free end of each pedestal may bind to other EPEC, via BFP, to form microcolonies.) EspB also has regulatory roles in the epithelial cell; it activates various enzymes, resulting e.g. in increased secretion of chloride ions into the gut lumen. The activation of myosin light-chain kinase leads to an opening of ‘tight junctions’ between epithelial cells (increasing the permeability of the intestinal mucosa). Diarrhoea may result from e.g. (i) stimulation of C1− secretion (through several mechanisms); (ii) increased permeability of the epithelium (due to opening of tight junctions); and/or (iii) reduced absorptive capacity due to loss of microvilli. EPEC strains commonly belong to serogroups O26, O55, O86, O114, O119, O125, O126, O127, O128 and O142. O111 is also often listed in this context; however, in Brazil, strains of O111:H12 were reported to have the properties of EAggEC [FEMS (1997) 146 123–128]. EPEC adherence factor plasmid See PATHOGENICITY ISLAND. eperezolid See OXAZOLIDINONES. Eperythrozoon A genus of Gram-negative bacteria within the family ANAPLASMATACEAE. Cells: pleomorphic cocci (diam. ca. 0.5–1.0 µm) which occur in the blood of the infected host – either free or loosely attached to the outer surface of erythrocytes. Eperythrozoon spp (E. coccoides, E. ovis, E. parvum, E. suis and E. wenyonii ) have a wide host range (including ruminants and non-ruminants) and occur worldwide. (See also EPERYTHROZOONOSIS.) eperythrozoonosis Any disease caused by an Eperythrozoon sp; the pathogen is transmitted e.g. by biting insects. In stressed feeder pigs, E. suis can cause acute icteroanaemia which involves e.g. mild fever, paleness of the mucosae, wasting, and (often) jaundice; E. suis can also cause anaemia in newborn pigs. Similar types of disease can occur in cattle and sheep (caused by E. wenyonii and E. ovis, respectively). [Book ref. 33, pp. 878–879.] ephemeral fever See BOVINE EPHEMERAL FEVER. epicellular At or on the surface of a cell. Epichloë A genus of fungi (order CLAVICIPITALES) which include parasites and pathogens of grasses. Perithecia are formed within 281 epidermin is unknown; neither is it known why toxin-producing strains produce localized lesions (impetigo) in some individuals and extensive, spreading ‘scalding’ lesions in others. [Book ref. 44, pp. 599–617.] Epidermophyton See DERMATOPHYTES. Epidinium A genus of ciliates related to DIPLODINIUM. epifluorescence microscopy See MICROSCOPY (e). epigean Occurring on the surface of the ground (cf. HYPOGEAN). epiglottitis Inflammation of the epiglottis. Acute epiglottitis is a life-threatening condition caused primarily by Haemophilus influenzae; it occurs mainly in small children. Fever and sore throat are followed rapidly by inspiratory stridor and dysphagia; the epiglottis becomes oedematous and cherry-red, and may cause complete respiratory obstruction. epilimnion The surface layer of a lake which, due to thermal effects and wind, is well-mixed and aerobic and approximately uniform in temperature. (cf. METALIMNION.) epilithic Growing on the surface of rock or stone etc. (cf. SAXICOLOUS.) epilithon The community of sessile organisms which grow on the surfaces of underwater stones. [Composition of epilithon: Oikos (1984) 42 10–22.] epimastigote A form assumed by the cells of species of Blastocrithidia, Endotrypanum and Trypanosoma during at least certain stages of their life cycles: see TRYPANOSOMATIDAE. epimerite In cephaline gregarines: a differentiated region of the PROTOMERITE, delineated by a septum, by means of which the developing parasite remains attached to the host cell (cf. MUCRON). epimicroscopy See MICROSCOPY (g). epinasty (plant pathol.) The downward curvature of a plant structure (particularly a leaf petiole) due to the faster growth of the dorsal side of the structure relative to the ventral side. Epinasty can be induced e.g. by ETHYLENE, and is an early symptom in certain plant diseases. epinephrine Syn. ADRENALIN (q.v.). epineuston See NEUSTON. epipelic Growing on mud. epiphloeodal (epiphloedal) Growing on the surface of bark (cf. CORTICOLOUS). epiphragm See CYATHUS. epiphyte A plant or ‘plant-like’ organism (e.g. a lichen) which grows upon another plant but does not derive nutrients from it – i.e., an epiphyte is non-parasitic. (cf. ENDOPHYTE.) epiphytotic See EPIDEMIC. epiplan objective A FLAT-FIELD OBJECTIVE LENS used for brightfield epimicroscopy (see MICROSCOPY (g)). epiplasm (1) (protozool.) See PELLICLE. (2) (mycol.) That cytoplasm in an ASCUS which is not incorporated into ascospores. epipodophyllotoxins See PODOPHYLLOTOXIN. epipolythiapiperazinediones A group of fungal ANTIBIOTICS. Some (e.g. aranotin, GLIOTOXIN, hyalodendrins, sirodesmins) inhibit the replication of certain RNA viruses – apparently by binding to and inhibiting viral RNA-dependent RNA polymerase; however, they are too toxic for therapeutic use. episome (1) A PLASMID which can exist either in an autonomous (extrachromosomal) state or integrated in the host cell’s chromosome. (2) Syn. PLASMID. episporium See EXOSPORIUM (sense 2). epistasis The dominance of one (epistatic) allele over a nonallelic (hypostatic) allele. (cf. DOMINANCE.) Epistylis See PERITRICHIA. able to contract the disease); a high proportion of immunes reduces the probability of contact between an infectious case and a susceptible, thus impeding the spread of disease. The proportion of susceptibles may often be drastically reduced by vaccination, and the risk to remaining susceptibles may be decreased by placing in quarantine any individuals who have contracted the disease; under such conditions CARRIERS (sense 1) may be important factors in the persistence of disease within the community. (c) Environmental conditions (extrinsic factors) play an important part in the containment or spread of disease. Particularly important are those features of society which favour or impede the transmissibility of the pathogen – e.g., standards of personal hygiene, overcrowding, the quality of communal WATER SUPPLIES, and the general state of sanitation (see e.g. SEWAGE TREATMENT); for a disease transmitted by a VECTOR, the prevalence of the vector and the measures taken to reduce its numbers are important factors. Because, in the present context, a population and its environment are inseparable, these extrinsic factors determine, in part, the observed herd immunity of a given population. The totality of factors which determines the transmissibility of disease and the distribution of immunes and susceptibles within a population has been termed the herd structure of that population. An increase in the proportion of susceptibles in a population may lead from an ENDEMIC to an EPIDEMIC situation; an epidemic may also be initiated when a population immune to certain strains of a given pathogen is exposed to a different strain of that pathogen (see also ANTIGENIC DRIFT). In a given outbreak of disease, the causal organism isolated from each patient may be examined by TYPING procedures in order to detect any variation which may help to establish the source(s) of infection and route(s) of transmission. epidermin A 21-amino-acid, pore-forming LANTIBIOTIC secreted by Staphylococcus epidermidis and active against a range of Gram-positive bacteria. epidermodysplasia verruciformis (EV) A rare, chronic, human skin disease involving the formation of flat, wart-like lesions on the face, neck, hands and feet, and sometimes also on the body. The lesions have a tendency to become malignant, ca. 30–50% of EV patients developing malignant squamous cell carcinoma; exposure to sunlight is believed to be a potentiating factor in the conversion to malignancy. The disease depends on genetic and immunological factors together with infection by certain human papilloma viruses (HPVs – see PAPILLOMAVIRUS). Many HPVs have been detected in EV lesions, only some of which are specific to EV. HPV-5 DNA has been detected in EV carcinoma cells, and HPV-5 is believed to be involved in carcinogenesis; HPV-17 has also been implicated in EV carcinogenesis [Virol. (1985) 144 295–298]. epidermolytic toxin (ET; exfoliative toxin; exfoliatin) A protein toxin produced by strains of Staphylococcus aureus responsible for staphylococcal SCALDED SKIN SYNDROME (SSSS), and also e.g. by many of the strains which cause IMPETIGO. There are (at least) two serologically distinct ETs, variously designated ETA and ETB, TA and DI, type i and type ii, etc. ETA appears to be specified chromosomally, ETB is apparently plasmid-specified. ET-producing staphylococci may produce either or both types of toxin. Both toxins have the same histological effect: neither is cytolytic, but both cause separation of cells of the stratum granulosum from underlying tissue in the skin of only certain species (including humans, mice and monkeys, but not e.g. rats), resulting in a positive NIKOLSKY SIGN. ET apparently plays a role in the pathogenesis of SSSS, but its mechanism of action 282 Epstein–Barr virus within the nucleus. EBNA-1 also appears to be required for replication of the plasmid form of EBV. (See also ZEBRA.) The ebna-1 gene is only one of nine or ten viral genes (out of ∼80) which may be expressed during viral latency. The ebna-2 gene is reported to be necessary for transactivation of lmp-1 and lmp-2 whose products (the latency membrane proteins 1 and 2) are incorporated into the B cell membrane. The products of ebna-2, ebna-3A and ebna-3C may also be expressed at the cell surface. In addition to proteins, latent EBV gives rise to two small polyadenylated RNA molecules called EBV-encoded RNAs (EBER-1 and EBER-2); these molecules are expressed at high levels, but their function is unknown. The EBERs have been used as targets for detection of EBV. Certain genes in latent EBV are also involved in the transactivation of some of the B cell genes; these B cell genes encode e.g. ICAM-1 (see CD54), LFA-1 (see INTEGRINS) and LFA3 (see IMMUNOGLOBULIN SUPERFAMILY). CD23, a B-cell-encoded activation-related marker, is also produced; this molecule is reported to act as an autocrine B-cell growth factor when shed from the cell surface. In vivo, proliferating B cells (with cell-surface molecules of EBNA-2, EBNA-3A, EBNA-3C and LMP-1) are normally targeted by a subset of EBV-specific cytotoxic T cells; moreover, these virus-infected immortalized B cells are targets for NK (natural killer) cells, and are also susceptible to interferons. Thus, the number of proliferating B cells is normally controlled in immunocompetent individuals. Cells containing latent EBV do not necessarily express all of the proteins referred to above; in some cases, however, all of these proteins (and others) may be expressed. In fact, several different types of EBV latency have been distinguished. In type I latency only EBNA-1 is produced. Cells which express only EBNA-1 are found e.g. in some gastric carcinomas; as these cells lack the targets for EBV-specific cytotoxic T cells (see above) they escape this form of immune control. In type II latency the cell expresses EBNA-1 and LMPs 1 and 2; this pattern is found e.g. in some T-cell lymphomas and in nasopharyngeal carcinoma. In type III latency the cell expresses the full complement of these proteins; this pattern is found e.g. in B cells from the acute phase of infectious mononucleosis. (EBERs appear to be expressed in all forms of latency.) EBV-associated diseases include: BURKITT’S LYMPHOMA; HAIRY LEUKOPLAKIA; INFECTIOUS MONONUCLEOSIS; NASOPHARYNGEAL CARCINOMA; XLP SYNDROME, gastric carcinoma and diffuse polyclonal B-cell lymphomas. In both Burkitt’s lymphoma and nasopharyngeal carcinoma, pathogenesis seems to depend on proliferation of host cells latently infected with EBV, whereas in hairy leukoplakia the main pathogenic mechanism appears to involve viral replication; in infectious mononucleosis, the immune response of the host appears to be a major factor in pathogenesis. Diagnosis of EBV-associated diseases involves laboratory tests (see e.g. INFECTIOUS MONONUCLEOSIS) and clinical observation. Levels of positivity of anti-EBV antibodies have been well documented, so that serology may help to distinguish e.g. between a primary (initial) infection (or a previous infection) and reactivation – in the latter case higher levels of antibodies are formed against viral capsid antigen (VCA), early antigen (EA) and EBNA. The EBERs may be detected e.g. by in situ hybridization or by RNA-amplification techniques. Treatments for various EBV-associated diseases include antivirals (such as ACYCLOVIR), immunosuppressants (e.g. epitheca (1) See DINOFLAGELLATES. (2) See DIATOMS. epithecium In the hymenium of some ascomycetes: a layer of prosenchymatous tissue which covers the surface of the layer of asci; it consists of the interwoven, branched tips of paraphyses. (cf. PSEUDOEPITHECIUM.) epithelioma papulosum Syn. CARP POX. epitheliotropism See TROPISM (sense 2). epithienamycins CARBAPENEM antibiotics. epitope Syn. antigenic DETERMINANT. epitunica Syn. EXOSPORIUM (sense 2). epivalve See DIATOMS. epixylous Growing on wood. (cf. CORTICOLOUS.) epizootic See EPIDEMIC. epizootic haemorrhagic disease subgroup See ORBIVIRUS. epizootic lymphangitis (pseudoglanders; equine histoplasmosis; equine blastomycosis; African farcy) A chronic cutaneous and subcutaneous disease of horses and other equines, caused by Histoplasma farciminosum (cf. HISTOPLASMOSIS); it occurs in Asia, Africa and Mediterranean countries. Nodules and suppurating ulcers develop on the skin – commonly on the shoulders, neck and limbs – and spread along lymph vessels; associated lymph nodes become inflamed. Small yeast-like cells can be detected – either free or within macrophages – in the pus. EPR See ELECTRON SPIN RESONANCE. EPS Extracellular polysaccharide. epsilon chain (immunol.) See HEAVY CHAIN. Epstein–Barr virus (EBV) Human (gamma) herpesvirus 4, the prototype of the subfamily GAMMAHERPESVIRINAE (q.v.). [DNA sequence and expression of the type strain (B95-8) of Epstein–Barr virus: Nature (1984) 310 207–211.] EBV is ubiquitous in human populations; it is transmitted horizontally, mainly via saliva (e.g. by kissing). (Large numbers of infectious virions can occur in saliva.) In developing countries, primary (i.e. initial) infection generally occurs in early childhood, and in almost all cases these infections are subclinical. In developed countries, primary infection generally does not occur until adolescence, and a significant proportion of infected individuals develop INFECTIOUS MONONUCLEOSIS. The reasons for the difference in impact of EBV in developing and developed countries are not known. EBV infection results in a life-long carrier state in which the virus occurs in latent form within circulating B cells. In some individuals a productive infection results in virus shedding, thus permitting horizontal transmission of EBV. Although the target cells of EBV were believed to be limited to B lymphocytes and epithelial cells, they are now known to include e.g. T lymphocytes, monocytes, macrophages and endothelial cells. This extended range of host cells may help to explain the fact that EBV is associated with a wide spectrum of diseases. Infection of epithelial cells characteristically leads to viral replication and the release of infectious virions. By contrast, infection of B cells is usually followed by viral latency and the development of a permanent proliferative state in the B cells. The B cell receptor for EBV is CD21. In cultured cells, a protein, EBV nuclear antigen-1 (EBNA-1), is detectable within several hours of viral infection. Subsequently, DNA synthesis occurs in the B cells – which then proliferate continually (i.e. so-called ‘immortalization’). In proliferating B cells, the viral genome exists within the cell nucleus in a circular, ‘plasmid’ form; EBNA-1 binds to oriP (in the viral origin of replication), such binding apparently being necessary to keep the plasmid 283 Epulopiscium fishelsoni cyclophosphamide), anti-B cell monoclonal antibodies and inferon-gamma (IFN-g). [Haematological associations of EBV: BCH (2000) 13 199– 214.] Epulopiscium fishelsoni See BACTERIA. EPV Entomopoxvirus: see ENTOMOPOXVIRINAE. equal density centrifugation See CENTRIFUGATION. equid (alpha) herpesvirus 1 See ALPHAHERPESVIRINAE. equid herpesvirus 3 Syn. EQUINE COITAL EXANTHEMA virus. equilibrium density gradient centrifugation See CENTRIFUGATION. equilibrium zonal centrifugation See CENTRIFUGATION. equine abortion virus See ALPHAHERPESVIRINAE. equine arteritis virus See ARTERIVIRUS. equine blastomycosis Syn. EPIZOOTIC LYMPHANGITIS. equine coital exanthema An acute, usually mild, usually venereally transmitted HORSE DISEASE caused by equid herpesvirus 3 (see ALPHAHERPESVIRINAE). Typically, vesicular or papular lesions develop on the external genitalia (sometimes also on the conjunctivae, lips or nasal mucosa); the lesions progress rapidly to pustules (which may ulcerate), and generally heal in ca. 2 weeks. equine diseases See HORSE DISEASES. equine encephalosis subgroup See ORBIVIRUS. equine herpesvirus 2 See GAMMAHERPESVIRINAE. equine histoplasmosis Syn. EPIZOOTIC LYMPHANGITIS. equine infectious anaemia Syn. SWAMP FEVER. equine monocytic ehrlichiosis Syn. POTOMAC HORSE FEVER. equine phycomycosis (hyphomycosis destruens; swamp cancer; bursattee; Florida horse leech) A tropical and subtropical HORSE DISEASE characterized by ulcerating, spreading, granulomatous lesions on the skin and mucous membranes; the lesions contain masses of yellow-grey necrotic tissue (‘grains’, ‘leeches’) which may calcify. The causal agent is ‘Hyphomyces destruens’, a Pythium sp. In Australia, outbreaks are associated with grazing in areas of permanent standing water [TBMS (1983) 80 13–18]. Clinically similar conditions are caused e.g. by Basidiobolus haptosporus and by certain helminthic parasites. (See also ZYGOMYCOSIS.) equine protozoal myeloencephalitis (equine protozoal myelitis) A HORSE DISEASE which affects young and old horses of both sexes; it occurs in certain areas of the USA. The symptoms are those typical of CNS involvement; they include stumbling and falling, sometimes with partial paralysis of the face and/or tongue, muscle atrophy, and lameness. The causal agent is believed to be a member of the coccidia (suborder EIMERIORINA) [IJP (1987) 17 615–620]. equine rhinoviruses See PICORNAVIRIDAE. equivalence (serol.) Syn. OPTIMAL PROPORTIONS. ER ENDOPLASMIC RETICULUM. Era protein See CELL CYCLE (b). eradicant (plant pathol.) Any chemical agent which eliminates particular pathogen(s) from diseased plants treated with that agent. (cf. PROTECTANT.) erb An ONCOGENE originally identified as the transforming determinant in avian erythroblastosis virus (AEV: see AVIAN ACUTE LEUKAEMIA VIRUSES). In AEV, v-erb consists of two contiguous genes, v-erb-A and v-erb-B; the cellular homologues (c-erb-A and c-erb-B) are not linked in the chicken genome. The cellular homologue of v-erb-A (c-erb-A) encodes the thyroid hormone receptor. This protein lacks tyrosine kinase activity and is apparently not tumorigenic on its own. The cellular homologue of v-erb-B (c-erb-B) encodes a membrane protein that functions as the epidermal growth factor receptor (EGF receptor); unlike the v-erb-A product, this protein does have tyrosine kinase activity. The oncogenic activity of the virus appears to depend on more than one gene. Thus, the product with which the avian erythroblastosis virus transforms cells is apparently a fusion protein that includes the erb-B product. Eremothecium See METSCHNIKOWIACEAE; see also RIBOFLAVIN. erf gene See BACTERIOPHAGE P22. ergine See ERGOT ALKALOIDS. ergobasine See ERGOT ALKALOIDS. ergochromes (secalonic acids) Pigments present in ERGOT sclerotia and also produced e.g. by Penicillium oxalicum; they are mostly weakly acidic, yellow xanthone derivatives, and at least some (e.g. secalonic acid D) are MYCOTOXINS. ergocristine See ERGOT ALKALOIDS. ergocryptine See ERGOT ALKALOIDS. ergolines Syn. ERGOT ALKALOIDS. ergometrine See ERGOT ALKALOIDS. ergonovine See ERGOT ALKALOIDS. ergopeptines See ERGOT ALKALOIDS. ergosome Syn. polyribosome: see PROTEIN SYNTHESIS. ergot A CEREAL DISEASE, caused by Claviceps purpurea (see CLAVICEPS), which can affect e.g. rye, wheat, barley and oats; it affects only the flowering parts. The earliest infections in the year are initiated by ascospores; conidia initiate later infections. Elongated, hard, black sclerotia (‘ergots’) become conspicuous in the ripening ears. The sclerotia overwinter and give rise in the following year to stromata and ascospores. (See also ERGOT ALKALOIDS and ERGOTISM.) ergot alkaloids (ergolines) Alkaloids obtained primarily from Claviceps spp; they are also produced by other fungi (e.g. Aspergillus, Penicillium, Rhizopus spp) and by certain plants. Natural ergot alkaloids stimulate smooth muscle, especially that of peripheral blood vessels (causing vasoconstriction) and of the pregnant uterus at term; certain hydrogenated derivatives have the reverse effect, causing vasodilation. Ergot alkaloids have several important clinical uses (cf. ERGOTISM). There are two classes of ergot alkaloids: the clavine alkaloids and the lysergic acid derivatives; the clinically useful alkaloids belong to the latter class, and may be simple amides or cyclic tripeptide derivatives. The amides include e.g. ergine (lysergic acid amide) and ergometrine (also called ergonovine or ergobasine: lysergic acid L-2-propanolamide); ergometrine is used to control postpartum bleeding. Cyclic tripeptide derivatives (ergopeptines) include ergotamine, used (as ergotamine tartrate) in the treatment of migraine, and ergotoxine (a mixture containing ergocristine, a-ergocryptine and ergocornine); hydrogenated derivatives of ergotoxine are used for the treatment of peripheral and cerebral vascular disorders and of essential hypertension. Synthetic derivatives of ergot alkaloids include e.g. the hallucinogen lysergic acid diethylamide (LSD). Commercial production of ergot alkaloids originally involved extraction from sclerotia of C. purpurea in naturally infected rye; later, fields of rye were inoculated with selected strains of C. purpurea. Currently, many lysergic acid derivatives are obtained from C. paspali grown in submerged culture. ergotamine See ERGOT ALKALOIDS. ergotism (St Anthony’s fire) A MYCOTOXICOSIS caused by the ingestion of ERGOT ALKALOIDS present e.g. in bread made from ergotized grain; outbreaks of ergotism have been responsible for numerous deaths in Europe – especially during the Middle Ages and the 18th century. Mild cases may involve a prickling 284 Erysipelothrix E. chrysanthemi. Pectinolytic. Causes vascular wilts in a wide range of plants. Some strains produce a blue bipyridyl pigment, indigoidine, on YGC agar. E. herbicola. Common as saprotrophs on the surfaces of land plants; some strains can fix nitrogen. May act as secondary invaders in plant lesions. (See also HERBICOLIN A.) Some strains can cause GALL formation. Certain strains are opportunist parasites or pathogens in man and animals; such strains are regarded by clinical microbiologists as Enterobacter agglomerans. [Attempted classification of strains of the Herbicola–Agglomerans group: IJSB (1984) 34 45–55.] E. nigrifluens. Causes bark necrosis of Persian walnut (Juglans regia). E. rhapontici. Causes e.g. crown rot of rhubarb (Rheum rhaponticum) and pink grain of wheat; it can also grow saprotrophically in plant lesions. A pink diffusible pigment is formed on sucrose–peptone agar. E. rubrifaciens. Pectinolytic. Causes phloem necrosis (bark canker) of Persian walnut (Juglans regia). A pink pigment is produced on YGC agar. E. salicis. Pectinolytic. Causes WATERMARK DISEASE in willows (Salix spp). Produces yellow pigment on autoclaved potato tissue. E. stewartii. Non-motile. Causes vascular wilt of e.g. maize (Zea mays); overwinters primarily in the flea beetle Chaetocnema pulicaria. E. tracheiphila. Causes vascular wilt in Cucurbita spp; overwinters in cucumber beetles (Diabrotica spp). E. uredovora. A parasite of the uredia and uredospores of Puccinia graminis. Produces yellow pigment on nutrient agar. Other recognized species: E. ananas (causes pineapple rot), E. cypripedii, E. mallotivora, E. quercina. [Taxonomy: Book ref. 22, pp. 469–476. Habitats, culture etc: Book ref. 46, pp. 1260–1271.] See also COFFEE; PECTIC ENZYMES. Erynia A genus of fungi (order ENTOMOPHTHORALES) which include parasites of insects; E. neophidis is a pathogen of aphids. [Persistence of infectious spores of E. neophidis: Ann. Appl. Biol. (1985) 107 365–376.] erysipelas Typically, well-demarcated lesions of erythema – often affecting the face, although other areas may be involved; fever, prostration and sepsis may occur. The causal agent is usually a strain of Streptococcus pyogenes, and infection generally occurs via wounds or abrasions. (cf. ERYSIPELOID; SWINE ERYSIPELAS.) erysipeloid A disease involving localized inflammation or CELLULITIS of skin (usually on the fingers and hands); it is caused by Erysipelothrix rhusiopathiae and is contracted mainly by handlers of fish and meat products. Infection occurs via wounds etc. Rarely, septicaemia or endocarditis may occur. Erysipelothrix A genus of Gram-positive, asporogenous, nonmotile bacteria; sole species (type species): E. rhusiopathiae (formerly E. insidiosa). Cells may be slightly curved, roundended rods (ca. 0.2–0.4 × 0.8–2.0 µm) which form smooth, (initially) transparent colonies (S-form), or occur mainly as filaments (up to 60 µm long) which form rough, opaque colonies (R-form). S-forms show a strong tendency to become R-forms. (See also PEPTIDOGLYCAN.) The organism is microaerophilic (facultatively aerobic or anaerobic). Growth occurs at 15–42° C (optimum 37° C). Indole −ve; catalase −ve; urease −ve; VP −ve; MR −ve; H2 S +ve; aesculin not hydrolysed. In gelatin stab cultures S-forms produce a typical ‘pipe-cleaner’ (becoming a ‘test-tube brush’) form of growth, with little or no gelatin liquefaction. GC%: 38–40. sensation in the skin, with numbness, fatigue, and e.g. diarrhoea and vomiting. Severe ergotism may take one of two forms, depending on the relative proportions of the ergot alkaloids ingested. The convulsive type is characterized by neurological symptoms, with spasms, epilepsy-like convulsions, and severe pain. In the gangrenous type the fingers and toes – or whole limbs – become blackened and gangrenous and are eventually lost. Severe cases are usually fatal, and in non-fatal cases complete recovery is rare. In animals, ergotism may result e.g. from grazing any of a wide range of fodder grasses infected with Claviceps spp. (See also PASPALUM STAGGERS.) ergotoxine See ERGOT ALKALOIDS. ERIC-PCR See REP-PCR. ERIC sequence Enterobacterial repetitive intergenic consensus sequence (also called intergenic repeat unit): one of the repeated intergenic sequences of nucleotides in the genomic DNA of enteric and other bacteria; an ERIC sequence contains a highly conserved PALINDROMIC SEQUENCE. ERIC sequences are distinct from REP SEQUENCES (they do not share significant homology). [ERIC sequences: Mol. Microbiol. (1991) 5 825–834.] ERIC sequences have been exploited in typing [see e.g. JCM (1999) 37 103–109; JCM (1999) 37 2473–2478; JCM (1999) 37 2772–2776]. ericoid mycorrhiza See MYCORRHIZA. Erlenmeyer flask A conical flask. error-prone repair See SOS SYSTEM. ertapenem A broad-spectrum 1-b-methylCARBAPENEM antibiotic with a long serum half-life (up to 4.5 hours). [In vitro activity of ertapenem against a range of bacteria from clinical specimens: AAC (2001) 45 1860–1867.] erumpent Bursting through. Erwinia A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Cells: 0.5–1.0 × 1–3 µm, motile (except E. stewartii ). Growth occurs e.g. on nutrient agar or YGC AGAR; optimum temperature: 27–30° C. Acid (little or no gas) is produced from glucose, fructose, galactose, sucrose, and bmethylglucoside. GC%: 50–58. Type species: E. amylovora. Erwinia spp typically occur as saprotrophs or pathogens in or on plants; some may also occur in insects, and certain strains are opportunist pathogens in man and animals. The genus is commonly divided into three groups. The Amylovora group includes species (e.g. E. amylovora) which require organic nitrogen for growth and cause dry-necrotic or VASCULAR WILT diseases in plants. The Carotovora group (sometimes regarded as a separate genus, Pectobacterium) includes strongly pectinolytic species (e.g. E. carotovora) which reduce nitrate to nitrite and cause SOFT ROT (sense 2) in a variety of plants. The Herbicola group includes species (e.g. E. herbicola) which typically form yellow (carotenoid) pigments and are not normally primary pathogens. E. amylovora. Non-pectinolytic. Causes FIREBLIGHT. Colonies on sucrose nutrient agar are mucoid (due to levan production). Anaerobic growth is weak. E. amylovora transfers virulence proteins into target cells by means of a type III PROTEIN SECRETION system; it has been shown that secreted proteins are guided along the Hrp pilus assembly that forms between the pathogen and the eukaryotic target cell [Mol. Microbiol. (2001) 40 1129–1139]. E. carotovora. Pectinolytic. Subspecies atroseptica causes BLACKLEG (sense 2) of potato plants and storage rot of potato tubers. Subspecies carotovora causes SOFT ROT (sense 2) in a wide range of plants. Subspecies betavasculorum causes soft rot of sugar-beet. 285 Erysiphaceae E. rhusiopathiae is parasitic in man, animals, birds and fish, and can be found e.g. in animal faeces, abattoir effluents etc. Some strains are pathogenic (see e.g. ERYSIPELOID, JOINT-ILL, SWINE ERYSIPELAS). The organism is resistant e.g. to salting, pickling, smoking, and low concentrations of phenol; in dried form (e.g. in fish-meal, soil) it can remain viable for years. [Book ref. 46, pp. 1688–1700.] Erysiphaceae See ERYSIPHALES. Erysiphales An order of homothallic and heterothallic plantparasitic fungi of the subdivision ASCOMYCOTINA (see also POWDERY MILDEWS); all species are placed in the family Erysiphaceae. The organisms form at least partially superficial, septate mycelium which is typically colourless and which commonly exhibits numerous haustoria. (Dark mycelium is formed e.g. by Sphaerotheca lanestris; cf. BLACK MILDEWS.) Ascocarp (ca. 50–350 µm diam.): cleistothecioid with hamathecium lacking, often dark and rounded, and often with surface appendages; uniascal or containing a small number of asci. Asci: bitunicate, clavate or pyriform. Ascospores: aseptate, ellipsoidal, colourless. Anamorphs occur e.g. in the genera Oidiopsis, Oidium and Ovulariopsis; conidia are often formed in abundance, basipetally, on straight, unbranched conidiophores. Genera: e.g. Cystotheca, ERYSIPHE, Leveillula, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, Uncinula. [Book ref. 187.] Erysiphe A genus of homothallic and heterothallic fungi (order ERYSIPHALES) which include parasites and pathogens of grasses and other plants (e.g. E. betae on beet crops, E. graminis on cereals). Erysiphe spp form superficial mycelium on host plants; most species form globose haustoria, but those of E. graminis are digitate (see also TRIFORINE). The organisms give rise to darkcoloured cleistothecia. The cleistothecial surface appendages (when present) are hypha-like, not dichotomously branched or terminally coiled; each cleistothecium contains more than one ascus. The ovoid to elongated conidia of E. graminis and E. polygoni are highly vacuolated; the conidiophores of E. graminis are distinctive in that in each the basal cell is slightly swollen. Conidia germinate by a germ tube which forms an appressorium. erythema Redness of the skin due e.g. to INFLAMMATION or infection. erythema infectiosum (fifth disease; slapped cheek syndrome) A benign, infectious disease, affecting mainly children, characterized by exanthem which begins on the face and gives a ‘slapped cheek’ appearance. The causal agent is parvovirus B19. (cf. EXANTHEM SUBITUM.) erythema nodosum leprosum See LEPROSY. erythrasma A chronic skin infection caused by Corynebacterium minutissimum. The reddish-brown lesions occur in the groin and armpits and fluoresce coral-pink under Wood’s lamp. erythritol The POLYOL corresponding to erythrose. It occurs e.g. in certain fungi, particularly in the Ustilaginales and Agaricales (where it apparently functions as a storage carbohydrate), and as a major photosynthetic product in certain unicellular and filamentous green algae (e.g. Phycopeltis, Trentepohlia). It can be metabolized e.g. by many yeasts. Erythrobacter A genus of Gram-negative, aerobic, halophilic, ovoid to rod-shaped, orange- or pink-pigmented bacteria which are motile by subpolar flagella. The cells contain bacteriochlorophyll a which is synthesized only under aerobic conditions (cf. RHODOSPIRILLALES). Metabolism appears to be chemoorganotrophic (mainly respiratory); autotrophic growth does not occur. Strains have been isolated from seaweeds growing near high-tide levels (e.g. Enteromorpha, Porphyra). Type species: E. longus. [IJSB (1982) 32 211–217.] erythroblastosis The presence of erythroblasts (nucleated precursors of erythrocytes) in the blood. erythrocyte A red blood cell. erythroimmunoassay (EIA) A sensitive IMMUNOASSAY for detecting and quantifying specific antigens or antibodies. For the detection of antibodies, the inner surface of a well (or tube) is coated with homologous antigen, and the patient’s serum – preabsorbed with sheep erythrocytes (see ABSORPTION) – is added to the well; a hybrid antibody conjugate (see CONJUGATION sense 2) is then added – each hybrid antibody having combining sites for both human immunoglobulins and sheep erythrocytes. Hybrid antibodies bind to any antigen–antibody complexes on the surface of the well, and the presence of bound hybrid antibodies is detected by the addition of sheep erythrocytes. [SAB (1984) 22 73–78.] erythromycin A MACROLIDE ANTIBIOTIC (q.v. for mode of action etc.); it is produced by Streptomyces erythreus. (In some strains of group A streptococci, resistance to erythromycin has been associated with the ability to invade, and survive within, epithelial cells of the human respiratory tract [Lancet (2001) 358 30–33].) (See also POLYKETIDE.) erythrose An aldotetrose; erythrose 4-phosphate is an important intermediate e.g. in the CALVIN CYCLE and HEXOSE MONOPHOSPHATE PATHWAY, and is a precursor for the synthesis of aromatic amino acids [Appendix IV(f)]. Erythrovirus A genus of autonomous viruses (family PARVOVIRIDAE) which infect primates. The genus was originally created for the only parvovirus known to be pathogenic for man, i.e. parvovirus B19; some authors also include the simian parvovirus [Virol. (1995) 210 314–322]. The following details refer to B19 parvovirus. (B19 was formerly referred to as ‘human parvovirus’ (abbreviated to HPV), but this terminology has been abandoned owing to confusion with human papillomavirus. The designation ‘B19’ derives from the laboratory number of the sample in which the virus was first detected.) The B19 virion is 22–26 nm, icosahedral, and contains a single copy of the linear, ssDNA genome (∼5.5 kb); identical PALINDROMIC SEQUENCES (383 nt) occur at the two ends of the genome. In a given population of B19 virions, some capsids enclose a negative-sense strand while others enclose a positivesense strand. B19 virions are stable at 56° C/60 minutes, and (in high concentration) are reported to be stable at 80° C/72 hours); lacking an envelope, they are insensitive to lipid solvents. The virions can be inactivated by e.g. oxidizing agents, formalin and g-radiation. For viral DNA replication see entry PARVOVIRUS. Transcription of the genome yields a number of overlapping RNA transcripts, all of which originate from a single, strong promoter, P6; the function of some of the encoded products is unknown. Proteins encoded by the genome include capsid proteins VP1 and (the smaller) VP2; VP2 is the major capsid protein, accounting for >95% of the capsid. (Recombinant VP2 proteins self-assemble into capsids within various mammalian and insect cell lines.) The major non-structural protein, NS1, can be detected in the nucleus of infected cells; it may be associated with the cytotoxicity of B19. One hypothesis supposes that NS1 acts as a positive regulator of viral DNA replication during the early stage of infection and that, subsequently, high concentrations of NS1 bring about cell death and lysis. 286 Escherichia B19 has been cultured in the erythroid cells from human bone marrow and fetal liver; in vitro propagation of the virus depends on the presence of the hormone erythropoietin (which is specific to erythroid cells). The cell-surface receptor for B19 is the red cell P antigen (globoside – a tetrahexose ceramide), the virus binding with high affinity to the carbohydrate moiety. P antigen is expressed e.g. on erythroid progenitors, mature red cells, megakaryocytes, endothelial cells and heart cells. (Individuals with the rare p phenotype, who do not express P antigen, are not susceptible to B19.) Viral replication is dependent on mitotically active cells, the virus being highly tropic for human erythroid cells in the late S phase of the cell cycle. B19 DNA can be demonstrated in the nasal secretions of individuals with B19 viraemia, and the respiratory route is believed to be important in virus transmission. Infection can also be transmitted via blood and blood products. Clinical manifestations of infection appear to depend primarily on host factors such as immunocompetence. In immunocompetent persons with a normal erythroid turnover, infection may lead to a short-term (∼4–8 days) halt in red cell production without development of anaemia. However, given a high turnover of red cells, even a temporary loss of red cell production may bring about an aplastic crisis. In immunocompromised patients infection may lead to chromic anaemia. In the fetus, infection may result e.g. in HYDROPS FETALIS. Other manifestations of B19 infection include arthralgia and rash. (See also ERYTHEMA INFECTIOSUM.) Among adults, seroprevalence of B19 IgG antibody is high, and the proportion of seropositives increases with age; IgG antibody develops within ∼2 weeks of infection and persists for life. [Persistent B19 infection: RMM (1995) 6 246–256. Molecular epidemiology of parvovirus B19: RMM (1997) 8 21–31. Haematological consequences of parvovirus B19 infection: BCH (2000) 13 245–259.] ES1–ES21 proteins See RIBOSOME. ESAT-6 A 6 kDa secreted protein (early secreted antigenic target) encoded by Mycobacterium tuberculosis and by M. bovis and M. africanum (members of the ‘M. tuberculosis complex’) but apparently not encoded by any of the (vaccine) strains of M. bovis BCG. (The operon encoding ESAT-6 is also reported to encode a novel, low-molecular-weight protein which has been designated CFP-10 [Microbiology (1998) 144 3195–3203].) Yet another secreted protein, MPT64, appears to be encoded by all strains of M. tuberculosis and BCG Tokyo but not by strains of BCG Danish 1331 [BCID (1997) 4 157–172 (167–168)]. Antigens such as ESAT-6 may be useful e.g. in SKIN TESTS for distinguishing between sensitization due to (i) tubercle bacilli, (ii) vaccination strains of BCG, or (iii) ‘environmental’ mycobacteria (cf. TUBERCULIN TEST). A test using ESAT-6 has been compared with a PPDbased test for the diagnosis of bovine tuberculosis; ESAT6 was associated with a lower sensitivity but with increased specificity – useful e.g. for testing in areas which have a low incidence of tuberculosis [VR (2000) 146 659–665]. The specificity of ESAT-6 for the M. tuberculosis complex has been exploited (by means of an ex-vivo ELISPOT ASSAY) for the enhanced tracing of contacts in epidemiological studies on human tuberculosis [Lancet (2001) 357 2017–2021]. ESBLs Extended-spectrum b-LACTAMASES. esc genes See PATHOGENICITY ISLAND. eschar Dead tissue which separates (sloughs) from the underlying living tissue. An eschar is formed e.g. in SCRUB TYPHUS. Escherichia A genus of Gram-negative bacteria of the family ENTEROBACTERIACEAE (q.v. for general features); in molecular taxonomy, Escherichia is placed in the gamma group of the Proteobacteria. GC% ∼50. Type species: E. coli. E. coli has long been the principal ‘guinea pig’ of the microbiologist, and has been studied more extensively than any other species. (Nevertheless, E. coli should not be regarded as ‘the typical bacterium’; the great diversity of bacteria makes such a concept meaningless.) The following refers specifically to E. coli (other species are mentioned at the end of the entry). Cells are typically straight, round-ended rods, ca. 0.5–0.8 × 1–4 µm, normally occurring singly or in pairs; they stain well with ANILINE DYES. The cell has a typical Gram-negative-type CELL WALL consisting of an OUTER MEMBRANE and a layer of PEPTIDOGLYCAN. The CYTOPLASMIC MEMBRANE contains e.g. various TRANSPORT SYSTEMS, energy-converting systems and chemosensors as well as components of osmoregulatory systems (see OSMOREGULATION); the latter include MECHANOSENSITIVE CHANNELS [mechanosensitive channels in E. coli : ARP (1997) 59 633–657], aquaporins and glycerol facilitators (see MIP CHANNELS). Cells are typically motile (peritichously flagellate) [second flagellar system in E. coli. JB (2005) 187 1430–1440]. FIMBRIAE are usually present. Type I fimbriae (the most common type) may be suppressed by P FIMBRIAE (q.v.). Some cells have a CAPSULE or microcapsule (see e.g. COLANIC ACID, COLOMINIC ACID; cf. TEICHOIC ACIDS). Numerous strains of E. coli are distinguished serologically (on the basis of their O ANTIGENS, H ANTIGENS and K ANTIGENS), or by PHAGE TYPING, colicin typing, antibiotic-resistance patterns etc. E. coli exhibits CHEMOTAXIS; the Tsr protein (an MCP in chemotaxis) may also function as a pmf-dependent sensor in aerotaxis [PNAS (1997) 94 10541–10546]. [Ecological role of energy taxis: FEMS Reviews (2004) 28 113–126.] The chromosome (cccDNA) is about 4.7 million base-pairs in length. [Complete sequence: Science (1997) 277 1453–1474.] Replication occurs by the Cairns-type mechanism (bidirectionally from the origin, oriC ). [Localization of chromosomal segments during the cell cycle: GD (2000) 14 212–223.] There are several systems for DNA REPAIR. (See also SOS SYSTEM.) Many of the genes occur within OPERONS which may encode e.g. metabolic enzymes and/or components of TRANSPORT SYSTEMS (e.g. LAC OPERON). TRANSCRIPTION of most genes involves the SIGMA FACTOR s70 ; other sigma factors include s32 (see HEAT-SHOCK PROTEINS). A proportion of mRNA transcripts are polyadenylated at the 3
end [characterization of the E. coli poly(A)polymerase: NAR (2000) 28 1139–1144]. Gene transfer can occur in vivo by CONJUGATION or TRANSDUCTION. Competence in TRANSFORMATION can be induced in the laboratory in ice-cold solutions of calcium chloride [possible mechanism: JB (1995) 177 486–490]. High-efficiency transformation can be achieved by ELECTROPORATION [NAR (1999) 27 910–911]. Metabolism is respiratory under aerobic conditions (see RESPIRATION). FERMENTATION (sense 1) or ANAEROBIC RESPIRATION occurs anaerobically. Glucose, mannitol and various other substrates are taken up by the PTS (q.v.). Lactose is taken up by proton–lactose symport, and melibiose by Na+ /melibiose symport (see SODIUM MOTIVE FORCE). Glucose is fermented (usually aerogenically) by a MIXED ACID FERMENTATION (cf. ALKALESCENSDISPAR GROUP). 287 escN gene estB gene See ETEC. ET EPIDERMOLYTIC TOXIN. h-value (of a disinfectant) See DILUTION COEFFICIENT. etamycin Syn. VIRIDOGRISEIN. etanercept See TNF. ETC ELECTRON TRANSPORT CHAIN. ETEC Enterotoxigenic Escherichia coli: strains of E. coli which adhere to the small intestine in man and/or animals and form ‘heat-labile’ ENTEROTOXIN (LT-I/LT-II: inactivated at ∼60° C/30 min) and/or ‘heat-stable’ enterotoxin (ST-I/ST-II: not inactivated at 100° C/30 min). Strains of ETEC cause mild to severe (cholera-like) diarrhoea in humans of all ages and/or similar disease in animals. (See also COLIBACILLOSIS and FOOD POISONING; cf. EAGGEC, EIEC, EHEC and EPEC.) The ability to adhere to (i.e. colonize) the small intestine – crucial for virulence – involves various plasmid-encoded FIMBRIAE. Strains from humans have non-fimbrial and fimbrial colonization factors, e.g. CS6 and ‘colonization factor antigens’ CFAI (F2) and CFAII (F3); CFAII has three components: CS1, CS2 and CS3 (CS = ‘coli surface’). Strains of ETEC from animals have e.g. K88 or K99 FIMBRIAE (see also F6 FIMBRIAE). [Fimbriae of human strains of ETEC and their possible use in vaccines: RMM (1996) 7 165–177.] LT-I closely resembles the CHOLERA TOXIN in structure: both toxins have the AB5 arrangement of subunits (found also e.g. in SHIGA TOXIN), and there is an ∼80% homology in amino acids. LT-I and CT cross-react antigenically, and they also have similar binding sites and a common mode of action; however, for unknown reasons, disease produced by LT-I is generally less severe, and of shorter duration, than that due to CT. LT-II occurs e.g. in pigs, rarely in humans. The A subunit is <60% homologous to that of LT-I, and the B subunits are essentially non-homologous (so that the binding site differs); the basic mode of action is similar to that of LT-I [MR (1996) 60 167–215 (pp 191, 192)]. Genes encoding the LT toxins are carried on ‘Ent plasmids’. ST-I (= STa) is a (methanol-soluble) peptide containing 18 or 19 amino acids produced from a 72-amino-acid precursor; following cleavage, three disulphide bonds are catalysed in the peptide by the protein DsbA prior to secretion. ST-I binds to receptors in the brush border membrane; the receptors have guanylate cyclase activity, and binding by ST-I activates the cyclase – leading to increased levels of intracellular cyclic GMP that apparently stimulate secretion of chloride and/or inhibit the absorption of NaCl. The net result is believed to be secretion of fluid into the gut. An alternative view of the mode of action of this toxin involves interruption of the luminal acidification mechanism (apparently through inhibition of the Na+ /H+ exchanger in the brush border membrane) leading to a reduction in the absorption of fluid [JAM (2001) 90 7–26]. Interestingly, the receptor sites for ST-I are the same as those used by guanylin – an endocrine hormone (a peptide of 15 amino acid residues) whose normal function apparently involves regulation of NaCl and water homeostasis in the gut. (The number of receptor sites has been reported to decrease with age – infants having a large number of receptors; this may give a reason for the increased severity of diarrhoea in infants and young children with ST-I infection.) [MR (1996) 60 167–215 (pp 190, 191); MP (1998) 24 123–131.] ST-II (STb) is a (methanol-insoluble, trypsin-sensitive) polypeptide containing 48 amino acid residues (derived from a precursor of 71 amino acid residues). The toxin does not cause secretion in the small intestine of mice or rats owing to its Sulphur requirements may be obtained from sulphate; sulphate is reduced, intracellularly, to sulphide – which is incorporated into O-acetylserine, forming cysteine. Nitrogen may be taken up as ammonia (see AMMONIA ASSIMILATION). Typical results of ‘biochemical’ tests: MR +ve; VP −ve; indole +ve; citrate −ve; lactose +ve; malonate −ve; urease −ve; LDC +ve; hydrogen sulphide (on TSI) −ve; no growth with KCN; gelatin hydrolysis −ve. Some strains give atypical reactions which, in at least some cases, are plasmid-mediated; for example: citrate utilization (Cit plasmid), hydrogen sulphide production (Hys plasmid [FEMS (1983) 20 7–11]), HAEMOLYSIN production (Hly plasmid, often associated with Ent and K88 plasmids in ETEC strains), urease production etc. Growth occurs on a range of media (e.g. NUTRIENT AGAR, EMB AGAR, MACCONKEY’S AGAR) but (usually) not on e.g. DCA. The optimum growth temperature is 37° C, and the minimum doubling time is ∼20 minutes. Colonies of E. coli on nutrient agar (37° C/24 hours) are 1–3 mm in diameter and may be smooth, entire, low-convex, greyish-translucent, butyrous and easily emulsifiable – or rough, dry, and not easily emulsifiable (see SMOOTH–ROUGH VARIATION). Mucoid and slime-forming strains also occur. Identification can be aided by the MUG TEST. Strains of E. coli have various modes of PROTEIN SECRETION (q.v.). E. coli and its components (e.g. RESTRICTION ENDONUCLEASES) are widely used in biotechnology (see e.g. OVERPRODUCTION). E. coli is a member of the GASTROINTESTINAL TRACT FLORA in man and animals, and may occur in soil and water as a result of faecal contamination. (It is often regarded as an INDICATOR ORGANISM for faecal pollution.) Pathogenic strains of E. coli include EAGGEC, EIEC, EPEC, ETEC and UPEC (for E. coli O157:H7 see EHEC). (See also COLIBACILLOSIS; COLISEPTICAEMIA; CYSTITIS; HAEMOLYTIC URAEMIC SYNDROME; HJÄRRE’S DISEASE; MENINGITIS; OEDEMA DISEASE; OSTEOMYELITIS; PNEUMONIA; URINARY TRACT INFECTION.) [PCR-based detection of E. coli O157: JCM (1998) 36 1801–1804.] [Target genes for virulence assessment of Escherichia coli isolates from food, water and the environment: FEMS Reviews (2000) 24 107–117.] Other species within the genus. E. blattae is an apparently harmless commensal in the hindgut of the cockroach (Blatta orientalis). Test reactions: indole −ve; malonate +ve; lactose −ve. Other proposed species include E. fergusonii [JCM (1985) 21 77–81], E. hermanii [JCM (1982) 15 703–713] and E. vulneris [JCM (1982) 15 1133–1140]. escN gene See PATHOGENICITY ISLAND. esculin Syn. AESCULIN. Es An RNA POLYMERASE holoenzyme. espA gene See PATHOGENICITY ISLAND. espB gene See PATHOGENICITY ISLAND. EspB protein See PROTEIN SECRETION (type III systems). espE gene See PATHOGENICITY ISLAND. EspE protein See PROTEIN SECRETION (type III systems). espundia See MUCOCUTANEOUS LEISHMANIASIS. ESR ELECTRON SPIN RESONANCE. estA gene See ETEC. established cell line (continuous cell line) A population of cells – derived either from a tumour, or from a TISSUE CULTURE following transformation – which appears to be capable of unlimited in vitro propagation. See e.g. BHK-21, BS-C-1, DETROIT-6, HELA, HEP-2, VERO and WISH. 288 Eubacterium inactivation by trypsin. It is apparently without effect on human intestine under in vitro (USSING CHAMBER) conditions; strains secreting ST-II are infrequently involved in human disease. The mode of action is unknown but apparently does not involve cyclic nucleotides (i.e. the mode of action differs from that of ST-I). [ST-II: Microbiology (1997) 143 1783–1795.] ST-I is encoded by estA – a plasmid-borne gene located within a transposon (Tn1681 ). ST-II is encoded by the plasmidborne gene estB. [Heat-stable enterotoxin associated with resistance to colon cancer: PNAS (2003) 100 2695–2699.] ETEC strains commonly belong to serogroups O6, O8, O15, O25, O27, O63, O78, O115, O148, O153 and O159. ethambutol (Myambutol) A synthetic drug, d-(2,2
-(ethylene diimino)-di-1-butanol), used (e.g. in combination with ISONIAZID) in the treatment of TUBERCULOSIS; it inhibits growing cells of Mycobacterium tuberculosis and M. bovis. Major adverse effects include optic neuritis and a loss of the ability to discriminate red/green (particularly in patients on a high-dose regimen) [BCID (1997) 4 53–54]. ethanol (as an antimicrobial agent) See ALCOHOLS. ethanol fermentation See ALCOHOLIC FERMENTATION and INDUSTRIAL ALCOHOL. ethazole (5-ethoxy-3-trichloromethyl-1,2,4-thiadiazole) An agricultural systemic ANTIFUNGAL AGENT used e.g. as a seed dressing for protection against Pythium, Phytophthora and Rhizoctonia. ethidium bromide (2,7-diamino-10-ethyl-9-phenylphenanthridinium bromide) A trypanocidal, bacteriostatic dye. It is an INTERCALATING AGENT (f = 26° ) which inhibits transcription and DNA replication; it binds specifically to dsDNA (or to base-paired regions of ssDNA) with no apparent preference for particular base sequences. In eukaryotes it is highly selective for mtDNA and kinetoplast DNA. Ethidium bromide can induce nuclease attack and (non-enzymic) photochemical nicking of DNA. Owing to their limited capacity to unwind, small cccDNA molecules can bind less ethidium bromide than can equivalent linear or nicked DNAs, and hence show a smaller reduction in density in the presence of ethidium bromide; this is exploited in methods for separating ccc from linear/nicked DNA by isopycnic ultraCENTRIFUGATION. ethirimol (5-n-butyl-2-ethylamino-4-hydroxy-6-methylpyrimidine; trade names: e.g. Milstem, Milgo) A HYDROXYPYRIMIDINE ANTIFUNGAL AGENT which is used as a seed dressing and as a foliar spray for the control of powdery mildews in cereals; it also controls powdery mildew in e.g. sugar beet. ethyl stibamine See ANTIMONY. ethylene (plant pathol.) A PHYTOHORMONE which, in higher plants, can induce e.g. germination, flowering, fruit ripening, senescence – depending on the plant and its stage of development. Ethylene is also produced by plant tissues in response to stress, damage, or infection by certain pathogens, and large quantities are produced by plant tissues exhibiting HYPERSENSITIVITY; it may give rise to certain symptoms of disease–e.g. EPINASTY, premature ripening of fruit or leaf-fall. ethylene glycol See ALCOHOLS. ethylene oxide A colourless, highly reactive, water-soluble cyclic ether (b.p. 10.8° C) used (as a vapour) for the DISINFECTION or STERILIZATION of e.g. medical equipment and certain heat-labile materials. The vapour forms explosive mixtures with air in a wide range of proportions and is commonly diluted with carbon dioxide, nitrogen or fluorohydrocarbons: e.g. Carboxide contains 10% ethylene oxide, 90% CO2 (see also e.g. CRYOXICIDE, PENNOXIDE). Ethylene oxide is an ALKYLATING AGENT which reacts with e.g. proteins and nucleic acids. Antimicrobial activity depends e.g. on temperature, concentration, duration of action, water content of the target organisms (dried spores etc can be highly resistant), and relative humidity. The use of ethylene oxide as a STERILANT thus requires careful control of conditions. For reliable sterilization the optimum relative humidity is variously reported as 35–60% [Book ref. 2, pp. 481–483] and ca. 100% [Book ref. 12, pp. 548–568]. The vapour has moderate powers of penetration, and sterilization usually necessitates exposure for at least several hours. Ethylene oxide is often used in an apparatus resembling a vacuum autoclave to allow conditions to be controlled and to reduce sterilization time. Ethylene oxide is absorbed by, or reacts with, substances such as hessian, rubber, and certain plastics; this may significantly reduce the effective concentration of the vapour. Ethylene oxide is readily hydrolysed to ethylene glycol; the toxic substance ethylene chlorohydrin (b.p. 129° C) is formed by hydrolysis in the presence of chloride ions. (N.B. Ethylene oxide is irritant and may be carcinogenic.) The sterilizing activity of ethylene oxide at a given location can be monitored by a Royce indicator sachet: a small plastic bag containing MgCl2 , HCl and the pH indicator BROMPHENOL BLUE; under sterilizing conditions ethylene oxide penetrates the sachet and is hydrolysed to ethylene chlorohydrin, causing a rise in pH. Bacterial endospores may be used as biological monitors [see e.g. JAB (1983) 55 39–48]. ethylenediaminetetraacetic acid See EDTA. ethylethane sulphonate A mutagenic ALKYLATING AGENT. ethylhydrocupreine hydrochloride See OPTOCHIN. ethylmethane sulphonate See ALKYLATING AGENTS. etiology See AETIOLOGY. etioplast An achlorophyllous PLASTID formed e.g. in dark-grown (etiolated) seedlings of most types of angiosperm; most algae, mosses and ferns can synthesize chlorophylls in the dark. (See also PROLAMELLAR BODY.) etioporphyrins See PORPHYRINS. ETP (electron-transport particle; sub-mitochondrial particle, SMP) A type of vesicle formed by the ultrasonic disruption of mitochondria (see MITOCHONDRION and ULTRASONICATION). An ETP consists of inverted mitochondrial membranes containing electron-transport and H+ -ATPase functions (although the actual composition varies considerably depending e.g. on the mode of preparation); ATP synthesis in an ETP thus correlates with the outward passage of protons. (cf. KABACKOSOME.) etridiazole (trade names: Aaterra, DWELL, Pansoil, Terrazole) 5-Ethoxy-3-trichloromethyl-1,2,4-thiadiazole, a NITRIFICATION INHIBITOR which is also used as a seed and soil fungicide – e.g. for the control of damping off caused by Phytophthora and Pythium. [Review: Book ref. 121, pp. 45–62.] etruscomycin A MYCOSAMINE-containing tetraene POLYENE ANTIBIOTIC structurally very similar to PIMARICIN. Euascomycetes A class of fungi comprising most or all ascomycetes other than the HEMIASCOMYCETES. The class is not recognized in modern taxonomic schemes. Euastrum A genus of placoderm DESMIDS. Eubacteria (taxon.) A kingdom (now obsolete) which included all those prokaryotes which were not classified in the kingdom ARCHAEBACTERIA. [The major eubacterial taxa defined: SAAM (1985) 6 143–151.] Members of the Eubacteria are now included in the domain BACTERIA (q.v.). Eubacterium An ill-defined genus of Gram-positive, asporogenous, anaerobic, rod-shaped bacteria consisting of those species 289 Eubenangee subgroup euglenoid flagellates (euglenids; euglenoids) A group of unicellular organisms regarded either as algae (class Euglenophyceae, division Euglenophyta) or as protozoa (see PHYTOMASTIGOPHOREA). The vegetative cell has no cell wall, but has a pellicle consisting of spirally arranged, interlocking, proteinaceous strips beneath the plasmalemma. The pellicle may be flexible (in e.g. Astasia, Distigma and EUGLENA) or rigid (in e.g. Menoidium, PHACUS and Rhabdomonas). Mucilage-secreting muciferous bodies occur in helical rows beneath the pellicle. In some genera (e.g. Klebsiella, TRACHELOMONAS) the cell is partly or totally enclosed in a mineralized LORICA. At the anterior end of the cell is an invagination, the reservoir (also called the ‘gullet’ although it does not function in feeding), which opens to the exterior via a narrow canal. A contractile vacuole commonly empties into the reservoir. In most genera two flagella arise from basal bodies at the base of the reservoir, but in some genera (e.g. Astasia, Colacium, Euglena, Phacus) only one flagellum (containing a PARAXIAL ROD) actually emerges from the cell via the canal; in e.g. Distigma, Eutreptia, PERANEMA and HETERONEMA two flagella are emergent, one of which is directed anteriorly, the other laterally or posteriorly. (Parasitic euglenids may have 3 or more emergent flagella.) Many euglenids are photosynthetic, possessing green chloroplasts containing chlorophylls a and b and various carotenoids. However, photosynthetic species can apparently also grow heterotrophically on e.g. acetate or ethanol in the dark, and many genera (e.g. Astasia, Khawkinea, Rhabdomonas) are colourless and obligately heterotrophic; PERANEMA and related genera are holozoic. A storage compound characteristic of both photosynthetic and heterotrophic euglenids is PARAMYLON. Euglenids reproduce chiefly by longitudinal binary fission; MITOSIS occurs within the nuclear membrane and does not involve a spindle. Palmelloid phases are common in some species. (cf. COLACIUM.) Desiccation-resistant cysts are formed in e.g. Distigma and Euglena under adverse conditions. Sexual reproduction is virtually unknown. Euglenoid flagellates are mainly free-living aquatic organisms, occurring in fresh water (e.g. most Euglena spp, Phacus) or in brackish or sea water (e.g. some Euglena spp, Eutreptia, Klebsiella). Some species are parasitic (e.g. Euglenamorpha in the gut of tadpoles). euglenoid movement See EUGLENA. Euglenophyceae See EUGLENOID FLAGELLATES. Euglypha A genus of testate amoebae (order Gromiida, class FILOSEA) in which the test (ca. 30–160 µm diam.) is composed of siliceous scales which are synthesized within the cytoplasm before incorporation into the test. Pseudopodia are generally narrow, filamentous, and sometimes anastomosing. Species occur in fresh water, activated sludge, anaerobic regions in acidic bogs, etc. (cf. PAULINELLA.) eugonic Refers to a strain of e.g. Mycobacterium which, on culture, produces growth which is more luxuriant than that produced by other (dysgonic) strains; the growth itself may also be described as eugonic (or dysgonic). eugregarines See GREGARINASINA. eukaryote (eucaryote) A type of CELLULAR (sense 2 or 3) organism in which the CHROMOSOMES are separated from the cytoplasm by a membrane (see NUCLEUS) and usually contain HISTONES (cf. DINOFLAGELLATES); the CYTOPLASMIC MEMBRANE contains sterols; mitochondria are commonly present; chloroplasts occur in photosynthetic species; RIBOSOMES in the cytoplasm are of the 80S type; the CELL WALL (when present) contains e.g. CELLULOSE or CHITIN; storage compounds apparently never include poly-bhydroxybutyrate; flagella or cilia (when present) are structurally which are excluded from the genera ACTINOMYCES, ARACHNIA, BIFIDOBACTERIUM, LACTOBACILLUS and PROPIONIBACTERIUM on the basis of the nature of the products of fermentation. Eubacterium spp typically form butyric and other fatty acids; some species produce acetic and formic acids (and ethanol), and some do not form acids. The organisms occur e.g. in the human gut and in the RUMEN. [Book ref. 46, pp. 1903–1911.] Eubenangee subgroup See ORBIVIRUS. Eucapsis A phycological genus of unicellular ‘blue-green algae’ (Chroococcales) in which the cells occur (in nature) in cubical packets. See SYNECHOCYSTIS. eucarpic Refers to those organisms in which only part of the thallus is involved in the formation of reproductive organs. (cf. HOLOCARPIC.) eucaryote Syn. EUKARYOTE. Eucheuma A genus of tropical marine algae (division RHODOPHYTA). The organisms are used as a source of CARRAGEENAN. euchromatin See CHROMATIN. euchrysine Syn. ACRIDINE ORANGE. Eucoccida An order of protozoa (subclass Coccidia) later incorporated – together with organisms of the TOXOPLASMEA – in the order EUCOCCIDIORIDA. Eucoccidiida An order of protozoa [JP (1980) 27 37–58] equivalent to the EUCOCCIDIORIDA. Eucoccidiorida An order of protozoa (subclass COCCIDIASINA), parasitic in vertebrates and invertebrates, in which both schizogony and sexual reproduction occur. Suborders: ADELEORINA, EIMERIORINA, HAEMOSPORORINA. Eucoronis See RADIOLARIA. euendolith See ENDOLITHIC. euflavin Syn. ACRIFLAVINE. eu-form rusts Syn. MACROCYCLIC RUSTS. Euglena A genus of photosynthetic EUGLENOID FLAGELLATES (q.v. for general features). Most species are green, but some (e.g. E. rubra) contain a bright red pigment which may mask the green colour under certain conditions. The vegetative cell is typically elongated and fusiform (ca. 15–400 µm long, according to species) but can vary its shape extensively. There is only one emergent flagellum; this bears a single helical row of fine hairs along its length, as well as a coating of shorter hairs. A lateral swelling (the paraflagellar body) is present on the long flagellum in the region within the reservoir, and this swelling lies opposite an EYESPOT present in the cytoplasm. A contractile vacuole empties into the reservoir. Other cell contents include the chloroplasts, a single large nucleus, paramylon granules etc. Euglena spp exhibit two distinct types of motility: a spiral swimming motion (see FLAGELLAR MOTILITY (b)) and ‘euglenoid movement’ (metaboly) – a type of crawling motility involving progressive waves of swelling and constriction of the cell body; metaboly is the main type of motility in E. deses. [Regulation of cell shape in E. gracilis – localization of actin and myosin in the region of the pellicle: JCS (1985) 77 197–208.] Most Euglena spp (e.g. E. gracilis, E. rubra, E. spirogyra) occur in freshwater habitats, particularly those rich in organic matter (e.g. farm ponds) where they may form dense green or red blooms. (See also SAPROBITY SYSTEM.) E. cyclopicola frequently occurs attached by its anterior end to microscopic crustacea (Cyclops, Daphnia etc), when it resorbs its flagellum. Some species occur in brackish waters (e.g. in estuarine mudflats), a few occur in the sea. E. deses is common in soil. Euglenamorpha See EUGLENOID FLAGELLATES. Euglenida See PHYTOMASTIGOPHOREA. euglenids Syn. EUGLENOID FLAGELLATES. 290 excipulum relatively complex (see FLAGELLUM (b) and CILIUM). Eukaryotic MICROORGANISMS include ALGAE, FUNGI, LICHENS and PROTOZOA. Eukaryotes may have arisen some 2–3.5 billion years ago [Science (1996) 271 470–477] from an energy-based symbiotic relationship between cells of the ARCHAEA and BACTERIA [the hydrogen hypothesis: Nature (1998) 392 37–41]. (cf. PROKARYOTE.) eumycetoma See MADUROMYCOSIS. Eumycetozoa See EUMYCETOZOEA. Eumycetozoea According to one protozoal classification scheme [JP (1980) 27 37–58]: a class of SLIME MOULDS of the RHIZOPODA. Members of the class typically form myxamoebae with filose pseudopodia or subpseudopodia; flagella, when formed, lack mastigonemes. Aerial fruiting bodies are produced. Subclasses: Protosteliia (equivalent to the PROTOSTELIOMYCETES together with CERATIOMYXA); Dictyosteliia (equivalent to the DICTYOSTELIOMYCETES); Myxogastria (equivalent to the MYXOMYCETES). According to another classification scheme [Book ref. 147, pp. 4–5], equivalent taxa are the class Eumycetozoa (subphylum MYCETOZOA) and its constituent subclasses Protostelia, Dictyostelia and Myxogastria. Eumycota In some mycological classification schemes [e.g. Book ref. 64]: a division which includes the ‘true fungi’ (subdivisions ASCOMYCOTINA, BASIDIOMYCOTINA, DEUTEROMYCOTINA, MASTIGOMYCOTINA and ZYGOMYCOTINA). (cf. MYXOMYCOTA.) Euphorbia mosaic virus See GEMINIVIRUSES. euphotic zone See PHOTIC ZONE. euploid (1) Having the basic haploid number of chromosomes or any exact multiple of the haploid number. (cf. ANEUPLOID.) (2) Having a chromosome complement characteristic of the species. (cf. HETEROPLOID.) Euplotes A genus of highly evolved ciliate protozoa (order HYPOTRICHIDA); species occur in freshwater, brackish and marine habitats, and some (e.g. E. moebiusi ) are common in some SEWAGE TREATMENT plants. Cells: roughly ovoid, ca. 50–150 µm in length, with a flattened ventral side and a dorsal side which is often markedly ridged; the cytostome is anterior and ventral, the distal part of the AZM forming a conspicuous, protruding fringe. The ventral surface carries a number of cirri which occur in localized groups and on which the organism is capable of ‘creeping’ or ‘walking’ movements; other forms of somatic ciliature are absent. The macronucleus is typically band-like and curved, the micronucleus small and spherical. [Macronuclear development in E. crassus: JCB (1985) 101 79–84.] Some strains contain endosymbionts (see OMICRON). A contractile vacuole is present. Asexual reproduction occurs by binary fission. CONJUGATION occurs (see SYNGEN). TRANSLATIONAL FRAME-SHIFTING seems common [Cell (2002) 111 763–766]. Food: algae, bacteria, other protozoa. European bat lyssavirus See LYSSAVIRUS. European blastomycosis Syn. CRYPTOCOCCOSIS. European foulbrood A BEE DISEASE which affects larvae (ca. 4–5 days old) of Apis mellifera and A. cerana; infected larvae die, turn brown and putrefy. The causal agent is Melissococcus pluton (formerly called Streptococcus pluton) [JAB (1983) 55 65–69]; infection occurs by ingestion, and the bacteria multiply within – and sometimes fill – the midgut. Secondary invaders may include e.g. Enterococcus faecalis and Bacillus alvei. (cf. AMERICAN FOULBROOD.) European swine fever Syn. SWINE FEVER. Eurotiales An order of mainly saprotrophic fungi of the subdivision ASCOMYCOTINA; anamorphs occur e.g. in the genera ASPERGILLUS and PENICILLIUM. Ascocarp: cleistothecioid, sometimes (e.g. in Talaromyces) very loosely constructed of interwoven hyphae, with no appendages; the development of the ascocarp may involve intertwining of the ascogonium and antheridium. Asci: unitunicate, thin-walled, formed at various levels in the ascocarp. Ascospores: unicellular, dark or colourless. Genera: e.g. BYSSOCHLAMYS, EMERICELLA, Emericellopsis (see also ACREMONIUM), EUROTIUM, Pleuroascus, SARTORYA, TALAROMYCES, THERMOASCUS. Eurotium A genus of fungi (order EUROTIALES) which include the teleomorph of Aspergillus glaucus. The ascocarp is a globose, commonly yellow, cleistothecium containing globose to pyriform, evanescent asci. eury- A prefix signifying ‘wide’ or ‘extensive’ – e.g. a euryhaline organism is one able to tolerate a wide range of salt concentrations. (cf. STENO-.) Euryarchaeota See ARCHAEA. euryxenous Having a broad host range. euseptum See SEPTUM (b). Eutreptia See EUGLENOID FLAGELLATES. eutrophic Of lakes rivers etc: rich in nutrients, particularly those which support the growth of aerobic photosynthetic organisms. (cf. OLIGOTROPHIC; see also EUTROPHICATION.) eutrophication The enrichment of a habitat or environment with inorganic materials – in particular those substances (e.g. nitrogen and phosphorus compounds) which encourage the growth of plants and algae. In rivers and lakes, eutrophication may occur as a result of SELF PURIFICATION. evanescent Having a transient existence. EVE See IPN VIRUS. Everglades virus See ALPHAVIRUS. Evernia A genus of LICHENS (order LECANORALES); photobiont: a green alga. The thallus is strap-like, dichotomously branched, typically hanging in festoons from trees; that of E. prunastri (oak-moss), a species used commercially in the perfume industry, is greenish-grey above, whitish below. evernic acid A depside LICHEN substance which has antibacterial properties; it occurs e.g. in Evernia prunastri. evernimicin (SCH 27899) An antibiotic with good in vitro activity against various Gram-positive bacteria [JAC (2001) 47 15–25]. Ewingella A genus of bacteria of the ENTEROBACTERIACEAE. E. americana (formerly ‘enteric group 40’) produces acid but no gas from glucose, is VP +ve, DNase −ve, lipase −ve, LDC and ODC −ve. [Ann. Mic. (1983) 134A 39–52.] exanthem A skin rash, or a disease characterized by a skin rash (cf. ENANTHEM). exanthem subitum (roseola infantum; sixth disease; pseudorubella) A benign disease, of infants and young children, in which high fever (for 3–4 days) is followed by a rubelliform rash on the trunk, spreading to the limbs and face. Human (beta) herpesvirus 6 has been recognized as a causal agent. (cf. ERYTHEMA INFECTIOSUM.) exbB gene See TONB PROTEIN. exbD gene See TONB PROTEIN. Excellospora A genus of thermophilic soil bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes). The organisms form both substrate and aerial mycelium, both of which give rise to spores. Type species: E. viridilutea. [Ecology, isolation, cultivation: Book ref. 46, pp. 2103–2117.] exchange diffusion Syn. ANTIPORT. exciple Syn. EXCIPULUM. excipulum (1) See APOTHECIUM. (2) The peridium (wall) of a PERITHECIUM. 291 excipulum proprium excipulum proprium See PROPER MARGIN. excipulum thallinum See THALLINE MARGIN. excision repair Any DNA REPAIR process which can repair lesions affecting only a single strand of dsDNA (not, for example, interstrand cross-linking) and which involves removal and replacement of one or more nucleotides. Excision repair in Escherichia coli can be considered under three main headings: (i) BASE EXCISION REPAIR, i.e. repair of an AP SITE after preliminary cleavage of a base (see ADAPTIVE RESPONSE and URACIL-DNA GLYCOSYLASE). (ii) MISMATCH REPAIR, used mainly for errors that escape proofreading during DNA replication. (iii) UvrABC-mediated repair (nucleotide excision repair; short patch repair). This common excision repair mechanism recognizes and repairs DNA which has been damaged/distorted by ultraviolet radiation or by certain other causes; damage may consist e.g. of a thymine dimer or a Pyr(6-4)Pyo photoproduct (see ULTRAVIOLET RADIATION). The ATP-dependent enzyme system UvrABC (also called ‘ABC excinuclease’) consists of three proteins encoded by genes uvrA, uvrB and uvrC. Initially a UvrA dimer binds to the damaged DNA. UvrB then binds to the dimer, and the complex UvrA2 B translocates to the precise site of damage; such translocation is apparently ATPdependent (the zinc-binding protein UvrA being an ATPase). UvrC displaces UvrA, and the UvrBC complex nicks the damaged strand on either side of the lesion, the 3
nick being made first. UvrD (= helicase II) then displaces UvrC together with the short sequence of nucleotides containing the site of damage. DNA polymerase I fills the single-stranded gap, displacing UvrB, and the repair is completed by a ligase. In E. coli only about 10 nucleotides in and around the damaged site are excised (thus accounting for the name ‘short patch repair’). (See also PHOTOLYASE.) The UvrABC system is enhanced during the SOS RESPONSE. Another repair system, the so-called ‘error-prone repair’ system (involving translesion synthesis of DNA), appears to operate only during the SOS RESPONSE (q.v.). excisionase See e.g. BACTERIOPHAGE l. exciter filter See MICROSCOPY (e). exclusion limit See PORIN. exclusion zone See MICROTUBULES. exconjugant A cell which has separated from another following CONJUGATION. excystation Syn. EXCYSTMENT. excystment (excystation) The release of one or more vegetative (or other) cells from a CYST. exergonic reaction Any reaction which is accompanied by the liberation of energy. (cf. ENDERGONIC REACTION.) exflagellation See PLASMODIUM. exfoliatin Syn. EPIDERMOLYTIC TOXIN. Exidia See TREMELLALES. Exiguobacterium A genus of Gram-positive, asporogenous, facultatively anaerobic, rod-shaped or coccoid, motile bacteria [JGM (1983) 129 2037–2042]. E. aurantiacum is an ALKALOPHILE which has been isolated from potato processing effluent. exine See CYST (a). Exo Abbr. for exonuclease; for example, Exo III is EXONUCLEASE III. Exobasidiales An order of plant-endoparasitic, characteristically GALL-inciting fungi (subclass HOLOBASIDIOMYCETIDAE) which form hymenia that emerge through the epidermis in parasitized plants. Genera include Exobasidium (see also BLISTER BLIGHT) and Kordyana. (cf. BRACHYBASIDIALES.) Exobasidium See EXOBASIDIALES and BLISTER BLIGHT. exobiology Extraterrestrial biology. exocellular Syn. EXTRACELLULAR (sense 1 or 2). exocellulase See CELLULASES. exochelins Low MWt (<ca. 1000) iron-solubilizing peptides secreted by Mycobacterium spp – particularly in iron-deficient media. ‘MB-type’ exochelins (formed e.g. by M. bovis BCG and by M. tuberculosis) are compounds which can be extracted by chloroform after they have chelated ferric iron. ‘MS-type’ exochelins (formed e.g. by M. smegmatis) are not extractable by organic solvents. exocyst See CYST (b). exocytosis The secretion of material by a cell in a process which resembles ENDOCYTOSIS in reverse. (See also TETANOSPASMIN.) exoenzyme (1) An ENZYME which sequentially removes units from one end of a polymer. (See e.g. b-AMYLASES; cf. ENDOENZYME.) (2) Sometimes used as a synonym of ‘extracellular enzyme’. exoenzyme S An enzyme produced by strains of Pseudomonas aeruginosa; it appears to be capable of ADP-ribosylation of eukaryotic proteins (cf. EXOTOXIN A), but its role in virulence – if any – is unknown. exoerythrocytic schizogony (in Plasmodium) See PLASMODIUM. exogenote See MEROZYGOTE. exogenous retrovirus A retrovirus (see RETROVIRIDAE) which infects a cell from a source outside that cell – e.g. a retrovirus transmitted horizontally from another animal, or one which has spread from another site in the same animal. Such a virus is absent from normal host cells (cf. ENDOGENOUS RETROVIRUS). exoglucanase See CELLULASES. exon See SPLIT GENE. exonuclease A NUCLEASE which sequentially removes nucleotides from one end of a strand of NUCLEIC ACID. (cf. ENDONUCLEASE.) exonuclease I See RECF PATHWAY. exonuclease III An enzyme of Escherichia coli (product of the xthA gene) which degrades either strand of a dsDNA molecule from the 3
end, leaving single-stranded 5
ends. exonuclease V See RECBC PATHWAY. exonuclease VII An enzyme of Escherichia coli (product of the xseA gene) which degrades ssDNA processively from either end but leaves dsDNA intact. exonuclease VIII See RECF PATHWAY. exopectate lyase See PECTIC ENZYMES. exopeptidase See PROTEASES. exoperidium See LYCOPERDALES. Exophiala See HYPHOMYCETES. exopolygalacturonase See PECTIC ENZYMES. exoribonuclease See RNASE. exospore (1) A type of SPORE formed from the parent organism by budding or by septum formation and fission (cf. ENDOSPORE sense 1); see e.g. ACTINOMYCETALES and RHODOMICROBIUM. (See also CHAMAESIPHON and METHYLOCOCCACEAE.) (2) Syn. EXOSPORIUM (sense 2). exosporium (1) (bacteriol.) See ENDOSPORE (sense 1(a)). (2) (mycol.) (exospore; epitunica; trachytectum) The layer external to the two innermost layers (the endosporium – see ENDOSPORE sense 2 – and episporium) of a fungal spore wall. exotoxin An extracellular TOXIN, i.e., a toxin which is secreted by a living cell. (cf. ENDOTOXIN sense 2.) exotoxin A A protein exotoxin (MWt ca. 66000) produced by strains of Pseudomonas aeruginosa; it is believed to be an important virulence factor in P. aeruginosa infections in patients suffering from extensive burns, immunosuppressive therapy, etc. 292 extrusome extracytoplasmic oxidation In certain bacteria: the oxidation of a substrate at the external face of the cytoplasmic membrane, or within the periplasmic region, giving rise to (or augmenting) a proton motive force (pmf: see CHEMIOSMOSIS). During such oxidations, protons formed at the extracytoplasmic site contribute to pmf, while electrons (derived from the substrate) pass through the cytoplasmic membrane (a process sometimes called ‘vectorial electron transfer’) and are involved in electronand proton-consuming reactions. [Early review: MR (1985) 49 140–157.] Substrates oxidized extracytoplasmically include e.g. (depending on species): hydrogen, formate, methanol, glucose and various ions – particularly ferrous ions. Intracellular terminal electron acceptors include e.g. oxygen, fumarate, sulphate ions and nitrate ions. Oxidations carried out extracytoplasmically include e.g. the oxidation of hydrogen by Desulfovibrio spp, Escherichia coli and by those HYDROGEN-OXIDIZING BACTERIA containing an NADindependent membrane-bound HYDROGENASE. In E. coli the hydrogenase is located on the external surface of the cytoplasmic membrane, while in Desulfovibrio spp a soluble hydrogenase is located in the periplasmic region. In Beggiatoa, sulphide appears to be oxidized extracytoplasmically to elemental sulphur. It has been proposed that Thiobacillus thiooxidans can oxidize insoluble (colloidal) sulphur extracytoplasmically. In Thiobacillus ferrooxidans, the oxidation of ferrous to ferric ions occurs extracytoplasmically; it appears to involve e.g. an oxidase, c-type and a-type cytochromes and RUSTICYANIN [see e.g. MR (1994) 58 39–55 (47–48)]. In some bacteria, ions such as Mn2+ or U4+ may be oxidized extracytoplasmically. Some bacteria (e.g. Pseudomonas aeruginosa) can generate pmf by extracytoplasmic oxidation of glucose to gluconate by glucose dehydrogenase (a QUINOPROTEIN) [JB (1985) 163 493–499]; in these organisms the enzyme glucose dehydrogenase, with its cofactor pyrroloquinoline quinone (PQQ), occurs on the outer surface of the cytoplasmic membrane. The gluconate formed can then be transported across the membrane, phosphorylated to 6-phosphogluconate, and fed into the ENTNER–DOUDOROFF PATHWAY. E. coli (and some related bacteria) normally form a membrane-bound glucose dehydrogenase which lacks the cofactor (PQQ); such organisms can carry out extracytoplasmic oxidation only if they are supplied with exogenous PQQ [IJSB (1989) 39 61–67]. However, a mutant strain of E. coli with a non-functional PTS system is able to synthesize PQQ and to carry out extracytoplasmic oxidation; this has suggested that PQQencoding genes are normally present but not expressed [JGM (1991) 137 1775–1782]. extranuclear genes Syn. CYTOPLASMIC GENES. extravasation See INFLAMMATION. extremophile See ARCHAEA. extrinsic allergic alveolitis (EAA; hypersensitivity pneumonitis) A form of pneumonitis, which appears to involve an ARTHUS-TYPE REACTION and a DELAYED HYPERSENSITIVITY (type IV) reaction, caused by the inhalation of certain allergen(s). Examples of EAA include e.g. BAGASSOSIS, CHEESE-WASHERS’ LUNG, FARMERS’ LUNG, MAPLE BARK STRIPPERS’ DISEASE, SUBEROSIS. [CIA (1984) 4 173–192.] extrusome A generic term for an extrusile organelle (function often unknown), various types of which occur near the cell surface in many ciliates and in some other organisms. Examples: The mode of action of exotoxin A appears to be identical with that of DIPHTHERIA TOXIN, although the two proteins differ in other respects: they show no immunological cross-reactivity; they are toxic for different cell lines (e.g. Vero cells are resistant to exotoxin A, murine cells are sensitive); they bind to different cell-surface receptors; they appear to be taken up by sensitive cells via different mechanisms; fragment A of exotoxin A is heat-labile. [Review: MR (1984) 48 199–221; structure: PNAS (1986) 83 1320–1324.] (See also IMMUNOTOXIN.) exotoxin A (streptococcal) See SUPERANTIGEN. exoxylanase See XYLANASES. exp-1 protein See PLASMODIUM. expanded gold standard See GOLD STANDARD. exponential growth rate constant See GROWTH (a). exponential phase (of growth) See BATCH CULTURE. exponential silencing Downregulation of activity at a given gene promoter as a consequence of exponential growth in a rich medium [see e.g. EMBO (2001) 20 1–11 (5)]. export (of a protein) The transmembrane translocation of a protein, towards the cell’s exterior, without eventual release to the external environment. (cf. SECRETION.) export apparatus See FLAGELLUM (a). expression vector (mol. biol.) A CLONING vector that encodes functions for the transcription/translation of an inserted fragment of DNA. In addition to an origin of replication and a marker gene (e.g. an antibiotic-resistance gene), an expression vector includes e.g. (i) a promoter, commonly a strong, hybrid promoter (e.g. the TAC PROMOTER); (ii) a ribosome-binding site to ensure that, following transcription of the insert, the mRNA contains a Shine–Dalgarno sequence (needed for binding to the ribosome); (iii) a multiple cloning site (MCS) – a POLYLINKER with a range of restriction sites that permit flexibility in the preparation of the insert; (iv) a transcription terminator to inhibit unwanted readthrough. Using an expression vector, a given insert can be initially cloned and then expressed. The insert is expressed by initiating transcription, and this can be achieved in various ways – for example, by a specific change in temperature. In an alternative mode of control, the vector incorporates a lac operator (see LAC OPERON) which regulates transcription of the insert and which is itself regulated by the host cell’s LacI protein; activity of the promoter can be switched on e.g. by adding IPTG to the medium. (For cells which do not synthesize LacI, use can be made of a vector which incorporates a lacI gene.) If required, a gene can be cloned in bacteria and then expressed in eukaryotic cells, a single vector carrying a prokaryotic origin of replication and control functions suitable for expression in eukaryotes. extein See INTEIN. extended batch culture See FED BATCH CULTURE. extended-spectrum b-lactamases See b-LACTAMASES. external guide sequence See SPLIT GENE (a). extracellular (1) Refers to enzymes or structures etc which are external to the cells that produced them – whether or not they remain attached to, or form part of, the surface of the cell. (2) Refers to enzymes or structures etc which are released by the cell into the surrounding medium. (cf. EPICELLULAR.) extrachromosomal genes Syn. CYTOPLASMIC GENES. extrachromosomal inheritance Syn. CYTOPLASMIC INHERITANCE. extract broth See NUTRIENT BROTH. extracytoplasmic Of a bacterial enzyme or redox carrier: located in the periplasmic region, either free or attached to the outer face of the cytoplasmic membrane [MR (1985) 49 140–157 (141)]. 293 exudative dermatitis haptocyst, kinetocyst (see rhabdocyst, TOXICYST, TRICHOCYST (cf. polar filament in MICROSPOREA and MYXOSPOREA). exudative dermatitis Syn. GREASY PIG DISEASE. EYA See EGG-YOLK AGAR. eye microflora The surface of the eye and the conjunctival membranes normally bear only a sparse microflora owing to the constant flushing action of the secretions of the lacrimal glands; these secretions contain certain antimicrobial agents – e.g. LACTOFERRIN, LYSOZYME, and immunoglobulins (see IGA). Organisms which may be isolated from the eye surfaces include certain members of the SKIN MICROFLORA, e.g. staphylococci. (See also BODY MICROFLORA and CONJUNCTIVITIS.) eyepiece micrometer See MICROMETER. eyespot (1) (stigma) In certain algae: a body consisting of a group, layer or layers of closely-packed lipid globules which, according to species, occurs within the CHLOROPLAST (e.g. in members of the Chlorophyta) or outside it (e.g. in euglenoid flagellates). In most or all cases an eyespot contains orange or red CAROTENOID pigments. In certain dinoflagellates (e.g. conocyst, EJECTOSOME, FIBROCYST, Nematodinium armatum) there is a large complex eyespot (termed an ocellus) which incorporates a lens and cup-shaped structure containing one or more rows of pigment granules. In at least some algae the eyespot is believed to be involved in PHOTOTAXIS. [The eyespot in green algae: Book ref. 167, pp. 193–268.] (2) A CEREAL DISEASE which affects mainly winter wheat and barley; causal agent: Pseudocercosporella herpotrichoides (which can overwinter in stubble). (cf. HALO SPOT.) Pale oval spots, each with a diffuse dark margin, develop in the spring on leaf sheaths and stalks of the tillers – usually close to soil level; each eyespot lesion has a greyish centre and may contain one or more black dots. A grey mycelium develops within the stem; lodging may occur. Control: e.g. use of resistant varieties, or early foliar spray with e.g. BENOMYL or THIOPHANATE-METHYL. In sharp eyespot (caused by Rhizoctonia solani ) the eyespot lesions have sharper margins, and they are more numerous – and develop further up the stem – than those of ‘true’ eyespot; mycelium does not develop within the stem. AXOPODIUM), PEXICYST, 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 294 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 F f. See FORMA SPECIALIS. F Phenylalanine (see AMINO ACIDS). F-actin See ACTIN. F antigens Fimbrial antigens – see FIMBRIAE. (cf. K ANTIGENS.) F+ donor See F PLASMID. F-duction See SEXDUCTION. F factor See F PLASMID. F layer See CYANOBACTERIA (section II). F-like pili See PILI. F pili See PILI. F plasmid (formerly F factor, ‘fertility factor’) An IncFI PLASMID which occurs e.g. in Escherichia coli with a COPY NUMBER of usually 1 or 2; it consists of a ccc dsDNA molecule (ca. 95 kilobases) which includes two copies of the INSERTION SEQUENCE IS3, one each of IS2 and IS1000, and sequences relating to replication, incompatibility and CONJUGATION (sense 1b). An F plasmid confers on the host cell the characteristics of a conjugal donor, and it can also affect the cell’s phenotype in other ways: see e.g. FEMALE-SPECIFIC PHAGE. (See also PIF.) Within the host cell, an F plasmid exists either in the autonomous (extrachromosomal) state, i.e. as a free, circular plasmid, or integrated in the host’s chromosome. (cf. EPISOME.) Integration occurs by homologous RECOMBINATION between an insertion sequence in the plasmid and one in the chromosome (see CAMPBELL MODEL); copies of insertion sequences IS2 and IS3 are common in the chromosome of E. coli. An integrated F plasmid may undergo aberrant excision from the chromosome: see PRIME PLASMID. Conjugation mediated by the F plasmid. The F plasmid can mediate conjugation either in the autonomous state or in the integrated state. In the autonomous (extrachromosomal) state an F plasmid can efficiently promote its own intercellular transfer by CONJUGATION (sense 1b). A cell containing an autonomous F plasmid is called an F+ donor, while a cell lacking an F plasmid (but able to receive one by conjugation) is called an F− recipient. (Such cells are sometimes referred to as ‘male’ and ‘female’ cells, respectively.) In a mixed population of F+ and F− cells, a copy of the F plasmid is transferred at high frequency, i.e. all, or nearly all, of the F− cells receive a copy (and hence become F+ donors themselves); in such F+ × F− crosses, chromosomal genes are usually not transferred. Integration of an F plasmid into the chromosome gives rise to an HFR DONOR (q.v.) – in which the chromosome is mobilized for transfer by the integrated plasmid (see CONJUGATION). Whether conjugation is mediated by an autonomous or integrated F plasmid, the mechanism of transfer depends on an extensive (ca. 33 kilobase) region of the plasmid – the transfer region – containing over 30 genes. This region is commonly referred to as ‘the transfer operon’, although it contains more than one promoter; however, most of the tra genes are found in a single large operon (traY–I ). The traJ and traM genes (both located immediately downstream of oriT ) are outside the traY–I operon; traJ has one promoter, traM has two. traY, the first gene of the traY–I operon, is adjacent to traJ. The traY–I operon is transcribed by the Es70 holoenzyme; activation of the traY promoter is dependent on TraJ, but other factors appear to be needed for full activation. Apparently some genes within the traY–I operon (e.g. traS, traT ) have their own promoters, and their expression seems to be independent of TraJ. About 15 genes are needed for synthesis and assembly of the F pilus. The traA gene encodes the precursor molecule that is processed to form the subunit, pilin, from which PILI are assembled; the initial product of traA (MWt ca. 13000) is processed to a final size of MWt ca. 7000. Genes traQ and traX are reported to be involved in processing the traA gene product. Products of the traG and traN genes have been associated with stabilization of donor–recipient mating contacts. The traI gene product is a HELICASE which is needed for siteand strand-specific nicking at oriT (the origin of transfer) and for unwinding DNA during conjugal transfer [JBC (1991) 266 16232–16237]. The traM gene product has binding sites in the oriT region (one of which, sbmC, is associated with DNA transfer); it is also linked with the cytoplasmic membrane, possibly via TraD. The role of TraM in conjugation has not been clarified, but it is apparently essential for transfer of DNA. In a related IncF plasmid (R1), TraM stimulated nicking at oriT by TraI [JMB (1998) 275 81–94]; in the F plasmid, TraY and the INTEGRATION HOST FACTOR were reported to stimulate nicking at oriT in vitro [JBC (1995) 270 28374–28380]. TraM may have a role in relaying the (currently unknown) ‘mating signal’ on establishment of stable cell–cell contact. The products of traS and traT are involved in SURFACE EXCLUSION. traM is autoregulated but needs TraY for full expression; expression is affected by external factors, and is negligible in stationary phase cells. Most genes in the traY–I operon are positively regulated by TraJ, and in most IncF plasmids, expression of traJ is under negative control of the FINOP SYSTEM (q.v.) so that the transfer genes are normally repressed. The F plasmid lacks a functional finO gene, i.e. in this particular plasmid, traJ is not under negative control; consequently, the transfer genes are constitutively derepressed (that is, permanently ‘switched on’). (See also FERTILITY INHIBITION.) [Control of transfer genes in F-like transfer systems: FEMS Reviews (1998) 21 291–319 (291–295).] Replication of the F plasmid. In wild-type F plasmids replication occurs bidirectionally from an origin and is under stringent control (see PLASMID). The F plasmid has two origins of replication: oriV (= ori 1) and oriS (= ori 2); in wild-type F plasmids, a PRIMOSOME assembly site is present near ori 2 [PNAS (1983) 80 7132–7136]. (In MINI-F PLASMIDS containing both origins, replication occurs preferentially, and bidirectionally, from ori1.) Host factors needed for F plasmid replication include the products of genes dnaB, dnaC and dnaE ; replication of the plasmid is independent of the dnaA product. (See also INTEGRATIVE SUPPRESSION.) Control of replication involves the (plasmid-encoded) RepE protein (= E protein). Molecules of the E protein (MWt ca. 29000) initiate replication by binding to two sequences of 19-bp DIRECT REPEATS (ITERONS), designated incB and incC, located either side of the E-encoding gene (close to the origin of replication); the bound molecules of E protein contribute to a nucleoprotein complex which is a pre-requisite for replication. Control of replication (and, hence, of COPY NUMBER) is achieved by controlling the intracellular level of E protein to that 295 F
plasmid F430 See METHANOGENESIS. Fab portion (Fab fragment) Either of two identical portions of an IMMUNOGLOBULIN monomer which can be released by certain enzymes (e.g. PAPAIN); the remainder of the molecule is termed the Fc portion (q.v.). Each Fab portion consists of a light chain linked via a disulphide bond to the N-terminal part of a heavy chain, i.e., it is one of the two limbs of the Y-shaped Ig molecule. Each Fab portion of an antibody contains a single COMBINING SITE and behaves as a non-precipitating univalent antibody. Under acid conditions, MERCAPTOETHANOL splits the Fab portion into the heavy chain segment (the Fd portion) and the light chain. F(ab
)2 portion Part of an IMMUNOGLOBULIN monomer released by the proteolytic action of PEPSIN (which also degrades at least part of the remainder of the Ig molecule); it includes both FAB PORTIONS connected by the HINGE REGION. The F(ab
)2 portion of an antibody behaves as a bivalent, precipitating, noncomplement-fixing antibody. facial eczema (vet.) See SPORIDESMINS. facilitated diffusion An energy-independent TRANSPORT SYSTEM in which the diffusion of a substance across a membrane is facilitated by a membrane-bound protein carrier system; in facilitated diffusion a substance necessarily passes down a concentration gradient. Unlike simple diffusion, facilitated diffusion exhibits Michaelis–Menten saturation kinetics, it is relatively substrate-specific, and it is subject to competitive inhibition – the uptake of one substance being inhibited by the presence of another, structurally similar, substance. In e.g. Saccharomyces cerevisiae various sugars, amino acids and vitamins can be taken up by facilitated diffusion – although, depending on conditions, the same substances may also (or alternatively) be taken up by other transport mechanisms. In Escherichia coli, glycerol can cross the cytoplasmic membrane by facilitated diffusion, the membrane containing a specific facilitator protein (see also MIP CHANNEL); once inside the cell the glycerol is phosphorylated. FACS Fluorescence-activated cell sorter: an instrument used for analysing and/or sorting cell populations by FLOW MICROFLUOROMETRY. Essentially, the cells (in suspension) move down a thin, open-ended tube and, in doing so, each cell passes momentarily through a laser beam which crosses the liquid column at right angles. As each cell passes through the beam it gives rise to a scatter signal (scattered laser light), and also – if labelled with a FLUOROCHROME (e.g. TEXAS RED) – a FLUORESCENCE signal. By using antibody-conjugated fluorochromes it is possible e.g. to determine the presence or number of homologous antigens or receptor sites on the cell surface, since the strength of the fluorescence signal is proportional to the amount of fluorochrome bound to the cell surface. In the ‘dual parameter’ mode, cells may be pre-treated, simultaneously, with antibodies of two different specificities – each being conjugated with a different fluorochrome; each cell passes through two separate laser beams (of wavelengths dependent on the fluorochromes chosen) and the two fluorescence signals are interpreted separately. The FACS can also be used e.g. to monitor the proportion of proliferating lymphocytes in a population, measurement being based on the increased content of DNA in proliferating cells; cells are initially permeated with an agent (e.g. mithramycin) which can interact with intracellular DNA to form a complex which fluoresces on irradiation. Cell sorting involves the physical separation of cells into subpopulations. Essentially, ultrasonic energy is applied to the nozzle through which the stream of liquid (i.e. cell suspension) passes, thus breaking up the stream into a series of very which will permit only one (sometimes two) rounds of plasmid replication prior to division of the bacterial cell. When a plasmid enters a daughter cell, subsequent binding of E protein molecules by the iterons ‘titrates’ the E protein, tending to lower its intracellular concentration; this promotes synthesis of E protein and restoration of an appropriate intracellular level. Lowering the concentration of E protein stimulates its synthesis because the gene encoding E protein is autorepressible at the level of transcription, i.e. the E protein represses its own transcription by binding to a sequence in, or overlapping, the gene’s promoter. Interestingly, monomers of E protein bind to iterons whereas dimers are involved in the autorepression of its own gene. Incompatibility. The incB and incC sequences are determinants of INCOMPATIBILITY in IncFI plasmids. Partition. As in other low-copy-number plasmids, an efficient system for PARTITION must be present during cell division to ensure stable maintenance of the F plasmid. To ensure stability of inheritance in bacterial populations, the F plasmid also encodes another system (ccd – coupled cell division) involving the genes ccdA and ccdB (formerly letA and letB, respectively); this system results in the death of any cell which does not contain a plasmid following cell division. CcdB, a lethal toxin for the host cell, is a stable molecule. CcdA neutralizes CcdB, but this antidote is gradually degraded by the host’s Lon protease – so that its protective role is available only while it is being synthesized from the (plasmid-encoded) ccdA gene, i.e. only in those cells containing the plasmid. Following cell division, a plasmid-less cell will be killed by CcdB as there is no ccdA gene from which CcdA can be synthesized. CcdB induces the SOS SYSTEM, and it kills cells by inhibiting DNA gyrase. Interestingly, CcdB has been considered for development as a possible therapeutic agent [TIM (1998) 6 269–275]. F
plasmid See PRIME PLASMID. F-prime plasmid See PRIME PLASMID. F− recipient See F PLASMID. f. sp. See FORMA SPECIALIS. F0 component (of ATPase) See PROTON ATPASE. F0 value The time (in minutes) required, at 121.1° C, to inactivate a population of cells or spores of a given species calculated from the time required for inactivation at another temperature, assuming a Z VALUE of 10° C. The F0 value, sometimes written F10 121.1 , is one particular value in a range of F values; thus, e.g., if the inactivation time at 121.1° C is calculated assuming a z value of 9° C, the relevant F value is indicated by F9121.1 . (See also D VALUE.) (F0 F1 )-type H+ -ATPase See PROTON ATPASE. F1 antigen See YERSINIA (Y. pestis). F1 component (of ATPase) See PROTON ATPASE. F1 fimbriae Type 1 FIMBRIAE. F2 fimbriae Syn. CFA I (see ETEC). F-2 toxin Syn. ZEARALENONE. F3 fimbriae Syn. CFA II (see ETEC). F3 T TRIFLUOROTHYMIDINE. F4 fimbriae K88 FIMBRIAE. F5 fimbriae K99 FIMBRIAE. F6 fimbriae (987P fimbriae) FIMBRIAE which occur on certain strains of ETEC which are pathogenic for newborn piglets. [Organization and expression of genes involved in biosynthesis of 987P fimbriae: MGG (1986) 204 75–81.] F41 fimbriae FIMBRIAE which occur on some strains of ETEC which are pathogenic for piglets; they promote MRHA. F420 See METHANOGENESIS. 296 fatty acid methyl ester famciclovir An oral ANTIVIRAL AGENT that resembles ACYCLOVIR in mode of action and antiviral activity; famciclovir is the 6deoxy diacetyl ester prodrug of the guanine nucleoside analogue penciclovir. After absorption in the intestine, famciclovir is rapidly converted to penciclovir; within cells, the half-life of the active form (penciclovir triphosphate) is much longer than that of acyclovir triphosphate. Famciclovir is used e.g. for the treatment of herpes zoster. FAME Fatty acid methyl ester: a methylated fatty acid derivative prepared for use in a microbial identification scheme; in such a scheme, identification is based on differences in the fatty acid content of whole cells of different species and strains of microorganism. Cells (grown under standard conditions) are harvested, and their fatty acids are initially saponified e.g. by heating in a solution of NaOH containing methanol. The product is then methylated (to increase volatility) e.g. by heating to ca. 80° C with an HCl–methanol mixture. FAMEs are then extracted with organic solvent(s); the resulting solution is washed by agitation with aqueous NaOH, and its FAME content is then analysed by gas chromatography. family A taxonomic rank (see TAXONOMY and NOMENCLATURE). Fansidar See MALARIA (chemotherapy). farcy (vet.) (1) (in equines) See GLANDERS. (2) (in cattle) Bovine farcy is a chronic condition involving purulent lymphangitis and lymphadenitis, and the development of hard, subcutaneous lesions with thickening of the associated lymph nodes and ducts; the causal agent can be either Mycobacterium farcinogenes or M. senegalense [Book ref. 54, pp. 508–509]. The disease occurs mainly in the tropics. (cf. EPIZOOTIC LYMPHANGITIS.) farcy buds, farcy pipes See GLANDERS. farmers’ lung An EXTRINSIC ALLERGIC ALVEOLITIS associated with inhalation of the spores of certain actinomycetes (e.g. Micropolyspora faeni ) and/or other bacteria (e.g. Thermoactinomyces vulgaris) present e.g. in mouldy hay; 4–8 hours after inhalation of these spores there is severe pulmonary dysfunction due to an intrapulmonary TYPE III REACTION. [CIA (1984) 4 175–184.] Farr technique A RADIOIMMUNOASSAY used for quantifying antibody. Essentially, the antiserum under test is added to excess radiolabelled antigen, and the antigen–antibody complex is separated from uncombined antigen by precipitation with ca. 50% ammonium sulphate; the antibody is measured (indirectly) by determining the amount of radiolabelled antigen in the precipitate. (The method can be used only for antigens which remain soluble in the ammonium sulphate.) Fas (CD95, APO-1) On mammalian cells: the cell-surface receptor for Fas ligand (Fas L). Fas ligand is a membrane-anchored cytokine which occurs on CD8+ T cells. The effect of binding between Fas ligand and its receptor depends on the specific signals triggered; in some cases, binding activates caspases and leads to APOPTOSIS. fasciation An abnormal type of plant growth resembling a flattened bundle of coalesced shoots – a symptom of certain plant diseases. Fasciation may be caused in certain plants (e.g. sweet peas) by Rhodococcus fascians, and may result from cytokinin production by the pathogen. fast blue B Syn. DIANISIDINE. fast death factor See MICROCYSTIS. FAT FLUORESCENT ANTIBODY TECHNIQUE. fat spoilage See e.g. RANCIDITY. fat stain See e.g. BURDON’S STAIN. fatal familial insomnia See TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES. fatty acid methyl ester See FAME. small droplets – each droplet (ideally) containing a single cell. Immediately before each droplet is discharged from the nozzle, the fluorescence (or scatter) signal received from the cell is interpreted (electronically) and the droplet is charged positively or negatively according to the nature of the signal received. As each (charged) droplet falls under gravity it passes between a pair of deflector plates – one positively charged, the other negatively charged; each droplet is therefore deflected to the right or left and subsequently falls into one of two collecting vessels. factor 420 See METHANOGENESIS. factor 430 See METHANOGENESIS. Factor B (immunol.) A b-globulin (MWt ca. 93000) found in normal plasma; during COMPLEMENT FIXATION by the alternative pathway, Factor B is cleaved to fragments Ba and Bb. Factor D (immunol.) A glycoprotein (MWt ca. 23000) found in very low concentrations in normal plasma; it circulates as an active enzyme whose substrate is C3b-bound FACTOR B. Factor H (immunol.) A protein (MWt ca. 150000) present in normal serum; it regulates (inhibits) the C3 convertase in the alternative pathway of COMPLEMENT FIXATION, and also acts as an essential cofactor for FACTOR I. Factor I (KAF; C3bina; C3bINA; conglutinogen-activating factor; C3b/C4b inactivator) A protein (a b-globulin, MWt 90000), present in normal plasma, involved in regulating the sequence of reactions in COMPLEMENT FIXATION. Together with an essential co-factor, FACTOR H, Factor I cleaves (inactivates) C3b and C4b. A cleavage product of C3b is involved in conglutination: see CONGLUTININ. factory area (virol.) Syn. VIROPLASM. factumycin See POLYENE ANTIBIOTICS (b). facultative ‘Optional’, referring to the ability of an organism to adopt an alternative life style, mode of nutrition etc; thus, e.g. an AEROBE may be a facultative ANAEROBE. The word ‘facultative’ is often followed by the mode not normally adopted; thus, e.g. ‘facultative anaerobe’ often refers to an organism which normally grows aerobically but which can grow anaerobically. (cf. OBLIGATE.) facultative heterochromatin See CHROMATIN. FAD See RIBOFLAVIN. faecal coliform test See COLIFORM TEST. faecal streptococci A general term for certain Gram-positive cocci which occur primarily in human and animal intestines and faeces; they include Enterococcus faecalis and E. faecium (both formerly classified in the genus Streptococcus) and species such as Streptococcus bovis and S. equinus. faecapentaenes Syn. FECAPENTAENES. faeces See e.g. GASTROINTESTINAL TRACT FLORA; INDICATOR ORGANISMS; SEWAGE TREATMENT. Faenia See MICROPOLYSPORA. FAIDS See FELINE LEUKAEMIA VIRUS. falcarindiol A polyacetylenic PHYTOALEXIN produced by tomato plants. false branching In certain sheathed filamentous bacteria (particularly CYANOBACTERIA, sections III–V): apparent branching which results from the protrusion through the sheath of the broken end(s) of a trichome, or of a loop of trichome which subsequently breaks to generate a pair of false branches. false neisseriae Bacteria regarded either as species incertae sedis in the genus Neisseria (N. caviae, N. cuniculi, N. ovis) or as members of the subgenus Branhamella (see MORAXELLA). false tinder fungus Fomes igniarius. false truffle See TRUFFLES. falx A specialized pellicular region in opalinids. 297 Faulschlamm feedlot bloat See BLOAT. Feekes’ scale (plant pathol.) A scale of numbers which indicate the various growth stages of wheat and barley: 1–5, shoot emergence and tillering; 6–10, stem elongation; 10.1–10.5, heading (ear development); 11.1–11.4, grain ripening. Feekes’ scale does not allow as detailed a designation of plant development as does ZADOKS’ CODE, and is limited to wheat and barley. felid herpesvirus 1 Syn. FELINE RHINOTRACHEITIS virus. feline AIDS See FELINE LEUKAEMIA VIRUS. feline calicivirus (FCV; cat flu virus; ‘feline rhinotracheitis virus’; ‘feline picornavirus’) A virus (family CALICIVIRIDAE) which infects the respiratory tract (occasionally also the gastrointestinal tract) of cats, causing e.g. rhinitis, conjunctivitis, oral ulceration, and pneumonia; the disease ranges from mild to fatal. An attenuated strain of FCV is used as a vaccine. FCV can be propagated in feline cell cultures. (cf. FELINE RHINOTRACHEITIS.) feline coronavirus See CORONAVIRIDAE. feline immunodeficiency virus (FIV) A retrovirus of the subfamily LENTIVIRINAE, originally isolated as a ‘T-lymphotropic virus’ from domestic cats with an immunodeficiency-like syndrome [Science (1987) 235 790–793]; the virus can give rise to illness resembling AIDS in humans and FAIDS (see FELINE LEUKAEMIA VIRUS) in cats. The FIV virion, ∼120–150 nm in size, contains a characteristic lentivirus-type nucleocapsid. The genome – ∼9.5 kb – contains some open reading frames (as well as gag, pol and env regions) within terminal LTRs; the ORFs apparently encode regulatory function(s). Horizontal transmission is believed to occur typically as a result of fighting and biting (cf. FeLV). Vertical transmission by asymptomatic, chronically infected mothers has not been reported, but the virus can be transmitted via milk following experimental infection. [Haematological disorders associated with feline retrovirus infections (FeLV and FIV): BCH (1995) 8 73–112.] feline infectious peritonitis virus See CORONAVIRIDAE. feline leukaemia virus (FeLV) A retrovirus (subfamily ONCOVIRINAE, type C), strains of which cause important diseases in cats; there are many exogenous and some endogenous strains of the virus. The FeLV virion is roughly spherical and is ∼110–120 nm in diameter. The proviral DNA (of replication-competent strains) – approximately 8.4 kb in length – includes the usual retroviral gag, pol and env regions between 5
and 3
LTRs; the expression of proviral genes is under the regulation of cis- acting promoter and enhancer regions located in the U3 sequence of the LTR. FeLV is transmitted horizontally, apparently via the oronasal route, by repeated social contact; transmission also occurs vertically, by intrauterine infection of the developing embryo. Animals which develop chronic viraemia are sources of hightitre infectious FeLV virions (which occur in plasma and in nasal secretions and saliva). In over one half of the cats exposed to FeLV, the humoral and cell-mediated defence systems succeed in abolishing detectable virus in plasma. Nevertheless, in at least a proportion of animals the virus can remain latent for a period of time in myelomonocytic precursors in the bone marrow and in stromal fibroblasts; however, it is thought that spontaneous reactivation of latent virus in nature occurs only rarely. FeLV causes neoplastic, degenerative and immunosuppressive forms of disease, the latter including a syndrome resembling human HIV-AIDS (feline AIDS or FAIDS ); other conditions Faulschlamm Syn. SAPROPEL. favus (tinea favosa) A severe form of RINGWORM (usually tinea capitis) caused by Trichophyton schoenleinii. It is characterized by a spreading alopecia with pathognomonic lesions called scutula (sing. scutulum): round or oval, cup-shaped, sulphuryellow crusts which develop from infected hair follicles. Hyphae, but few arthrospores, occur within the hair shaft, as may fine tubular canals left by the disintegration of hyphae. A bluishwhite fluorescence may be observed in infected hairs under Wood’s lamp. The term favus also refers to a POULTRY DISEASE caused by Trichophyton gallinae. White powdery spots appear on the unfeathered parts of the head and develop to form wrinkled crusts or scabs; in severe cases feathered parts of the body are affected, with resulting loss of feathers. FBJ osteosarcoma virus Finkel–Biskis–Jinkins murine sarcoma virus: a retrovirus complex consisting of a replicationcompetent murine leukaemia virus (FBJ-MuLV) and a replication-defective transforming murine sarcoma virus (FBJMSV); FBJ-MSV carries the oncogene v-fos and induces osteosarcomas in mice after a latent period of ca. 3 weeks. The v-fos product does not have tyrosine kinase activity. Homologues of c-fos have been identified in the genomes of various vertebrates as well as in Drosophila. [Organization and expression of fos: Book ref. 113, pp. 309–321.] FBP Ferrous sulphate–sodium metabisulphite–sodium pyruvate; FBP destroys H2 O2 and superoxide anions and, when added to culture media (0.025%), can increase aerotolerance in certain microaerophilic and anaerobic organisms. Fc portion (Fc fragment) A portion of an IMMUNOGLOBULIN monomer which can be released by certain enzymes (e.g. PAPAIN). (cf. FAB PORTION.) The Fc portion corresponds to the stem of the Y-shaped Ig molecule and consists of the Cterminal sections of the two heavy chains linked by one or more disulphide bonds; it is e.g. the site of complement fixation in COMPLEMENT-FIXING ANTIBODIES. (See also PROTEIN A.) FCA Freund’s complete adjuvant. FCCP See PROTON TRANSLOCATORS. FcgRIII Syn. CD16. Fd FERREDOXIN. Fd portion (Fd fragment) See FAB PORTION. Fe protein See NITROGENASE. fecapentaenes A group of mutagens, each having a highly unsaturated conjugated enol ether structure, produced by certain bacteria. It has been proposed that they act as alkylating agents as a result of the formation of carbocations [Science (1984) 225 521–523]. feces See FAECES. fed batch culture A modified form of BATCH CULTURE in which a solution of nutrient(s) is added at specific time(s) in order e.g. to optimize the production of particular metabolite(s). If volumes of culture are periodically removed to permit repeated feeding, the process is called extended batch culture. feed additives Substances, such as certain antibiotics, that are added to the feedstuffs of domestic animals (including poultry, pigs and/or ruminants) in order to increase growth rates and/or increase feed conversion efficiency. Ideally, feed additives should not be absorbed from the alimentary tract at the dosages used (e.g. ca. 10–60 mg/kg dry-weight feedingstuff, according to additive); absorption may necessitate a ‘withdrawal period’ prior to slaughter in order to allow elimination of the additive. Feed additives include e.g. AVOPARCIN, LASALOCID, MONENSIN, NARASIN, SALINOMYCIN and VIRGINIAMYCINS. [Antibiotics as feed additives for ruminants: Book ref. 121, pp. 331–347.] 298 fentin include myeloid leukaemias, anaemia, marrow aplasia, and lymphosarcomas. (A FAIDS-like syndrome can also be caused by the FELINE IMMUNODEFICIENCY VIRUS.) Typically, chronic infection with FeLV involves a gradual decrease in the numbers of lymphocytes and a reduction in the function of both T and B cells, thus leading to clinical immunodeficiency. Certain strains of FeLV, however, cause an acute immunodeficiency syndrome (FAIDS). Isolates of FeLV are often replication-competent and vonc− , but ONCOGENE-containing sarcoma-inducing recombinant viruses (‘feline sarcoma viruses’) have been isolated from FeLVassociated tumours. Several oncogenes (e.g. FES, abl, sis) have been reported to confer on FeLV the ability to induce sarcomas. Insertional mutagenesis by FeLV of the host cell’s c-myc oncogene is found in a significant proportion of spontaneous and experimentally induced lymphomas. In some experimentally induced lymphomas, the location at which the provirus integrates in the cell’s genome suggests that enhancer activity associated with the proviral LTR region may stimulate expression of c-myc and, in this way, contribute to pathogenesis. The exogenous, replication-competent FeLVs are divided into subgroups A, B and C on the basis of (i) differences in the viral envelope glycoprotein, (ii) the interference pattern in vitro, and (iii) susceptibility of the viruses to neutralization by specific antibodies; differences in the envelope glycoprotein are associated with differences in the host cell range of the various strains of FeLV. Subgroup A strains of FeLV are widespread and readily transmissible – although apparently not highly pathogenic. Subgroup B strains may arise through in vivo recombination and may be more pathogenic than the A strains; in infected animals, B strains are reported to be invariably accompanied by A strains. C strains are said to be comparatively rare, and are associated with pure red cell aplasia. Various germ-line-transmissible endogenous retroviruses may be detectable in the genome of uninfected animals. In some cases the endogenous virus contains sequences homologous to FeLV (enFeLV strains); in other cases (RD-114 viruses) there is no apparent genetic relationship with FeLV. Although enFeLV proviral DNA may not give rise to infectious virions, it may nevertheless contribute to pathogenicity; thus, e.g. expression of enFeLV genes has been detected in both FeLV-positive and FeLV-negative lymphomas. Moreover, enFeLV-encoded product(s) may represent the entity referred to as FOCMA. Diagnosis of persistent infection by FeLV. In cells that harbour a replication-competent strain of FeLV, viral proteins and virions are commonly produced at a high rate. One particular protein, the Pr65 gag precursor protein, is produced in excess and can be easily detected in the blood of infected cats by immunoassay techniques. [Haematological disorders associated with feline retrovirus infections (FeLV and feline immunodeficiency virus): BCH (1995) 8 73–112.] feline panleukopenia virus (FPV) A subspecies of feline parvovirus (genus PARVOVIRUS). Like CANINE PARVOVIRUS (CPV), the ssDNA genome (∼5100 nt) encodes two structural proteins (VP1, VP2: the mRNAs transcribed from overlapping sequences) and two non-structural proteins. DNA from FPV and CPV isolates is almost identical; the host range of these viruses depends on small differences in the composition of the capsid: less than 10 amino acids determine whether a given virus can relicate in dogs or cats. The virion retains infectivity for days/weeks in the environment. Feline panleukopenia virus causes disease in cats, raccoons and some of their relatives. The main targets for viral replication include dividing cells in lymphoid tissues and in the intestinal epithelium. Infection probably occurs via the nasopharyngeal route and/or via lymphoid tissues. In the adult animal, generalized infection of lymphoid tissues follows viraemia. Levels of erythrocytes (red blood cells) are not affected. Often, infection leads to PANLEUKOPENIA, the total leukocyte count sometimes falling to ∼1000 per cubic millimetre, with neutrophils <200 per cubic millimetre; the fall in lymphocyte numbers is less marked, and there is minimal change in numbers of eosinophils, basophils and monocytes. Intestinal infection (in adult animals) involves viral replication in the rapidly dividing epithelial cells of the ileum and jejunum – with consequent diarrhoea (often with blood/mucus). In neonatal kittens there is no manifestation of enteritis. Infection characteristically affects the cerebellum, leading to cerebellar hypoplasia and, typically, to ataxia in surviving animals. [Pathogenesis of FPV: BCH (1995) 8 57–71.] feline parvovirus See PARVOVIRUS. feline picornavirus Syn. FELINE CALICIVIRUS. feline rhinotracheitis (feline viral rhinotracheitis) A severe upper respiratory tract disease of cats caused e.g. by felid herpesvirus 1 (see ALPHAHERPESVIRINAE). This virus replicates mainly in the oral and nasal mucous membranes; symptoms include fever and discharge from the eyes and nose. Latent infection generally follows recovery from the acute disease; latently infected animals may periodically shed virus either spontaneously or after stress. The virus has been isolated from the trigeminal ganglia of latently infected cats [JGV (1985) 66 391–394]. (cf. FELINE CALICIVIRUS.) feline rhinotracheitis virus FELINE CALICIVIRUS or felid herpesvirus 1 (see FELINE RHINOTRACHEITIS). feline sarcoma viruses See FELINE LEUKAEMIA VIRUS. feline syncytial virus See SPUMAVIRINAE. feline T-lymphotropic lentivirus See FELINE IMMUNODEFICIENCY VIRUS. felon Syn. WHITLOW. FeLV FELINE LEUKAEMIA VIRUS. fem genes See MRSA. female-specific phage Any phage whose EOP on bacteria containing certain plasmid(s) is significantly lower than that on plasmid-free strains of the same bacteria. For example, phages of the T7 group (e.g. T7, fI, W31) are inhibited in cells of Escherichia coli K12 which contain the F plasmid; in the case of T7, the presence of the plasmid inhibits translation of the intermediate and late phage proteins. In some cases phage DNA is the target of plasmid-encoded restriction endonucleases; e.g., phages F116 and G101 are restricted in cells of Pseudomonas aeruginosa containing the plasmid pMG7 [Plasmid (1977) 1 115–116]. (cf. ANDROPHAGE.) FeMo protein See NITROGENASE. FeMoco The FeMo cofactor of NITROGENASE. fenpropimorph A MORPHOLINE ANTIFUNGAL AGENT used e.g. to control yellow rust, brown rust, and powdery mildew on wheat and barley. Fentichlor See BISPHENOLS. fenticonazole See AZOLE ANTIFUNGAL AGENTS. fentin Triphenyltin; two derivatives, fentin acetate ((C6 H5 )3 SnO.CO.CH3 ) and fentin hydroxide ((C6 H5 )3 SnOH), are used (sometimes in conjunction with MANEB) as antifungal agents e.g. for the control of late blight of potato and powdery mildew of sugar-beet. (See also TIN.) 299 FepA protein fermenter (fermentor) A vessel in which an aerobic or anaerobic FERMENTATION (sense 2) can be carried out either in BATCH CULTURE or in CONTINUOUS CULTURE. (cf. BIOREACTOR.) Fermenters are typically vertical, essentially closed, cylindrical steel vessels. The traditional STIRRED TANK REACTOR is a squat vessel, but other types are ‘column’ (= ‘tower’) fermenters in which the vessel (the ‘column’) has a height-to-diameter ratio of up to ca. 10 (e.g. the typical AIRLIFT FERMENTER, BUBBLE COLUMN FERMENTER, JET LOOP FERMENTER, and PROPELLER FERMENTER). Important features of fermenter design include provision for: (i) Adequate heat transfer from the culture. (The heat produced from a given substrate can be estimated from the value of 110 kcal/mole O2 taken up – a value obtained empirically using various substrates and organisms [Book ref. 11, p. 32].) (ii) Efficient mixing of the culture. (iii) Efficient oxygen-to-liquid transfer. (iv) Adaptability to a range of operating conditions. (v) Ease of scale-up from the laboratory or pilot stage to industrial use. These factors largely determine the economics of a fermentation and the type of fermenter required for a particular purpose; thus, e.g., an STR – but not a bubble column or airlift fermenter – would generally be considered suitable for the fermentation of a (viscous) mycelial culture since only the STR (which has a greater input energy) would be able to ensure adequate mixing. Fernández–Morán particles The stalked particles (the F1 parts of (F0 F1 )-type PROTON ATPASES) on the inner surface of the mitochondrial inner membrane. Fernandez reaction See LEPROMIN TEST. ferredoxins A category of simple IRON–SULPHUR PROTEINS which are involved only in electron transfer processes; a given ferredoxin may contain one or more iron–sulphur centres of the type [2Fe–2S], [3Fe–3S] and/or [4Fe–4S]. Ferredoxins typically have low-potential (i.e., highly negative) Em values; thus, e.g. a [4Fe–4S] ferredoxin (Fd I) in Desulfovibrio gigas has an Em of −455 mV, and a (2[4Fe–4S]) ferredoxin in Clostridium pasteurianum has an Em of −400 mV. However, a [2Fe–2S] ferredoxin in Pseudomonas putida (putidaredoxin) has an Em of −240 mV. (See also HIPIP; cf. FLAVODOXINS.) ferrichrome See SIDEROPHORES. ferrimycins See SIDEROMYCINS. ferrioxamines See SIDEROPHORES. ferritin An iron-storage protein which occurs e.g. within various mammalian, plant and fungal cells; iron is stored within the large, hollow ferritin molecule. (cf. SIDEROPHILINS; see also IMMUNOELECTRON MICROSCOPY.) ferrocene monocarboxylic acid See BIOFUEL CELL. ferrochelatase See HAEM. ferruginous Rust-coloured. fertility factor Syn. F PLASMID. fertility inhibition (of the F plasmid) Inhibition of expression of the traJ gene (and, hence, inhibition of the TRANSFER OPERON) FepA protein In Escherichia coli : an OUTER MEMBRANE protein encoded by the fepA gene; it acts as a receptor for B and D colicins, and is involved in the uptake of Fe3+ -enterobactin (see SIDEROPHORES). ferbam See DMDC. FERM Fermentation Research Institute of the Agency of Industrial Science and Technology, Tsukuba, Japan. ferment (1) (noun) An archaic name for an enzyme. (2) (verb) To carry out FERMENTATION. fermentation (1) Energy-yielding metabolism in which an energy substrate is metabolized without the involvement of an exogenous electron acceptor; fermentation characteristically occurs under anaerobic or microaerobic conditions (cf. e.g. CRABTREE EFFECT). The absence of an exogenous electron acceptor (= oxidizing agent, ‘electron sink’) necessarily means that the products of fermentation – collectively – have the same oxidation state as that of the substrate, the oxidation of any intermediate in the fermentation pathway being balanced by equivalent reduction of other intermediate(s) in the pathway; since the substrate undergoes no net oxidation, energy derived from the fermentation of a given substrate is less than that obtainable by the RESPIRATION of that substrate. Until the 1980s it was believed that all fermentable substrates were organic compounds; it is now known that inorganic sulphur compounds (such as sulphite) can be fermented e.g. by Desulfovibrio sulfodismutans and by some other SULPHATE-REDUCING BACTERIA [Nature (1987) 326 891–892]. Although the synthesis of ATP during fermentative metabolism commonly involves SUBSTRATELEVEL PHOSPHORYLATION, ATP synthesis by ELECTRON TRANSPORT PHOSPHORYLATION can occur (e.g. when lactate is fermented via the succinate–propionate pathway of the PROPIONIC ACID FERMENTATION); in some organisms, proton motive force can also be generated by excreting the end products of fermentation: see END-PRODUCT EFFLUX. (See also e.g. ACETONE–BUTANOL FERMENTATION; ALCOHOLIC FERMENTATION; BUTANEDIOL FERMENTATION; BUTYRIC ACID FERMENTATION; homoacetate fermentation (in ACETOGENESIS); LACTIC ACID FERMENTATION; MIXED ACID FERMENTATION; PROPIONIC ACID FERMENTATION.) (2) As commonly used in industrial microbiology: any of a wide range of processes carried out by microorganisms, regardless of whether fermentative or respiratory metabolism is involved. (See e.g. DIHYDROXYACETONE FERMENTATION; flor fermentation in WINE-MAKING; MALOLACTIC FERMENTATION; SORBOSE FERMENTATION. See also FERMENTER.) fermentative nitrate reduction See NITRATE RESPIRATION. fermented foods See e.g. BREAD-MAKING, DAIRY PRODUCTS, DOSA, GARI, IDLI, MISO, NATTO, OGI, ONCOM, PICKLING, SALAMI, SAUERKRAUT, SOY SAUCE, TEMPEH, TOFU. (cf. ANG-KAK, COFFEE, COCOA, FOOD PRESERVATION (e).) (See also SILAGE.) FERMENTATION (diagrammatic): the fermentation of an organic substrate showing intermediates undergoing mutual oxidation and reduction. 300 Fijivirus by the intracellular presence of an IncF plasmid which encodes a functional finO product (see FINOP SYSTEM); a plasmid which encodes such a finO product is designated fi+ (fertility inhibition positive) or fin+ . Compatible plasmids which do not encode a functional finO product are designated fi− . ferulate 3-Methoxy-4-hydroxycinnamate. Fervidobacterium See BACTERIA (taxonomy). fes An ONCOGENE present in the Snyder–Theilen and Gardner–Arnstein strains of feline sarcoma virus (see FELINE LEUKAEMIA VIRUS); v-fes is closely related to v-fps (present in Fujinami AVIAN SARCOMA VIRUS) and may be derived from a related c-onc sequence. The products of v-fes and v-fps have tyrosine-specific protein kinase activity. Fe–S protein (Fe/S protein) See IRON–SULPHUR PROTEINS. fescue foot (fescue toxicity syndrome) A condition which affects animals (mainly cattle) grazing pastures dominated by tall fescue grass (Festuca arundinacea); symptoms: lameness, followed by dry gangrene of the extremities (tail tip, hooves, ears). It appears to be caused by a vasoconstrictive mycotoxin produced by fungi parasitic or pathogenic in the grasses; fungi which have been implicated include Aspergillus terreus and biotypes of Epichloë typhina [AEM (1977) 34 576–581]. Festuca necrosis virus See CLOSTEROVIRUSES. Feulgen reaction A specific staining reaction for DNA in situ. Mild acid hydrolysis of DNA (e.g. with 1 N HCl) removes purine bases and makes available the aldehyde group of the deoxyribose; aldehyde groups react with SCHIFF’S REAGENT to give a purple coloration. fever blister Syn. COLD SORE. Ff phages F-specific filamentous phages (e.g. fd, f1, M13, ZJ/2): see INOVIRUS. ff. sp. See FORMA SPECIALIS. Ffh See PROTEIN SECRETION. FH4 TetrahydroFOLIC ACID. FHA See WHOOPING COUGH. FhuA protein Syn. TONA PROTEIN. fhuB–fhuD genes See SIDEROPHORES. FhuE See IRON. fi− plasmid See FERTILITY INHIBITION. fi+ plasmid See FERTILITY INHIBITION. FIAC (2
-deoxy-2
-fluoro-5-iodo-1-b-D-arabinosylcytosine) An ARABINOSYL NUCLEOSIDE which has antiviral activity against herpes simplex viruses 1 and 2, varicella-zoster virus and cytomegaloviruses in cell cultures; its mode of action resembles that of ACYCLOVIR. In trials, FIAC gave better control of progressive herpes zoster than did vidarabine; it can be administered orally. fibrillae See FIMBRIAE. fibrillum See FIMBRIAE. fibrin A fibrous, insoluble protein present e.g. in blood clots; during normal blood-clotting, fibrin is formed from the plasma protein FIBRINOGEN by the action of thrombin in the presence of Ca2+ . (See also COAGULASE.) fibrinogen The soluble glycoprotein precursor of FIBRIN. It consists of two identical monomers – each comprising three chains designated a, b and g; the chains and monomers are held together by disulphide bridges. [Review of fibrin and fibrinogen: ARB (1984) 53 195–229.] fibrinolysin (plasmin) An enzyme which, in mammals, is responsible for dissolving blood clots by the proteolytic degradation of fibrin. Fibrinolysin is formed from an inactive precursor (profibrinolysin, plasminogen) normally present in the blood. The term fibrinolysin has also been applied to various microbial enzymes capable of direct or indirect FIBRINOLYSIS. fibrinolysis The degradation of FIBRIN, and, hence, the lysis of blood clots (see FIBRINOLYSIN). Various microbial enzymes have fibrinolytic activity; such activity may be (i) direct, due to proteolytic activity on fibrin itself (see e.g. BRINASE), or (ii) indirect, due to the activation of plasminogen (profibrinolysin) (see e.g. STREPTOKINASE). The production of plasminogen activators by certain bacteria has been linked to invasiveness [TIM (1997) 5 466–467]. fibrinolytic Capable of FIBRINOLYSIS. Fibrobacter See BACTERIA (taxonomy). fibrocyst (compound trichocyst) (protozool.) A type of TRICHOCYST formed by certain hypostome ciliates; the filament carries an umbrella-like tip. fibroma A benign tumour of connective tissue. (cf. SARCOMA.) fibronectin A heterodimeric glycoprotein, produced e.g. by fibroblasts and macrophages, which is found e.g. in extracellular matrix material and in plasma; it binds e.g. to various INTEGRINS, and its major roles include that of an adhesive molecule. fibrosarcoma A SARCOMA arising from collagen-producing fibroblasts. Ficoll A synthetic, water-soluble, non-ionic co-polymer of sucrose and epichlorhydrin, used e.g. in the preparation of density gradients for CENTRIFUGATION. It is also used for increasing the viscosity of a medium in order to slow down rapidly-motile organisms – a technique useful in certain COUNTING METHODS and in studies of ciliar and flagellar motility; in this context Ficoll has been reported to be superior to e.g. methylcellulose in that Ficoll causes less perturbation of hydrodynamic characteristics [Nature (1979) 278 349–351]. FID Free induction decay: see NUCLEAR MAGNETIC RESONANCE. fide In a literature citation: an indication that a given reference has not been read by the author citing that reference. field blewit See BLEWIT. field diaphragm (in MICROSCOPY) (1) See KÖHLER ILLUMINATION. (2) (Syn. field stop) A metal annulus attached to the inside of an eyepiece at the focal plane of the eyepiece lens. field mushroom See AGARICUS. Field’s stain A water-based staining procedure for detecting e.g. Plasmodium spp and trypanosomes in thick blood smears. The smear is immersed in a solution of azure, rinsed, immersed in a solution of eosin, rinsed, and allowed to dry in air; the water used for staining and rinsing should have a pH of 7.0–7.2. Any parasites present will not be obscured by erythrocytes since the latter are lysed during the procedure; lysis occurs because a methanol fixation stage is omitted. fièvre boutonneuse Syn. BOUTONNEUSE. fifth disease Syn. ERYTHEMA INFECTIOSUM. figwort mosaic virus See CAULIMOVIRUSES. Fiji disease A SUGARCANE DISEASE, caused by a FIJIVIRUS and transmitted by planthoppers (Perkinsiella spp), which is characterized by stunting and the formation of GALLS on the underside of leaves. Pseudo-Fiji diseases of sugarcane include one similar to MAIZE WALLABY EAR DISEASE. Fijivirus (plant reovirus subgroup 2) A genus of plant viruses of the REOVIRIDAE. Genome: 10 dsRNA molecules. Members infect plants of the Gramineae, and some can cause important diseases of crop plants; symptoms typically include stunting, excessive tillering, enations, and unnaturally dark-green leaves. Transmission occurs propagatively via plant-hoppers (Perkinsiella spp) in which transovarial transmission may occur. Type member: Fiji disease virus. Three serotypes of fijiviruses are recognized (host and geographical range in parentheses): Group I: cereal tillering disease virus (CTDV) (barley, oats, maize, wheat; Sweden); maize rough dwarf virus (MRDV) 301 filament terminal sessile basidiospores; blastospores may be formed, teliospores are not. Members have a yeast-like vegetative phase, with globose, oval, apiculate or elongate cells which reproduce by budding. Conjugation between two haploid cells is followed by the formation of a dikaryotic mycelium on which the basidia develop. Most strains are heterothallic. The family includes the genera CHIONOSPHAERA, FILOBASIDIELLA, and FILOBASIDIUM. (See also WALLEMIA.) The taxonomic position of the family is unsettled. It has been included e.g. in the Ustilaginales, and in a separate order (Filobasidiales) [discussion and refs: Book ref. 100, pp. 468–469]; some authors [Book ref. 64] include the above three genera in the SPORIDIALES. Filobasidiella A genus of fungi (see FILOBASIDIACEAE). The yeast-like vegetative cells have CAPSULES, the size of the capsule depending on environmental conditions. The dikaryotic hyphae have clamp connections and dolipore septa which lack parenthesomes. Basidiospores are formed in long chains by basipetal budding from 4 sites on the apex of the basidium. The sole species, F. neoformans, has two varieties – F. neoformans var. neoformans (anamorph: Cryptococcus neoformans var. neoformans) and F. neoformans var. bacillispora (anamorph: Cryptococcus neoformans var. gattii ) – and four serotypes (A–D). Strains have been isolated from pigeon excreta, soil, and humans (see CRYPTOCOCCOSIS). [Book ref. 100, pp. 472–482.] Filobasidium A genus of fungi (see FILOBASIDIACEAE). The dikaryotic hyphae have clamp connections and dolipore septa with or without parenthesomes. Basidia are produced laterally and terminally on the hyphae, and sessile basidiospores are produced on a whorl at the apex of the metabasidium. Species: F. capsuligenum (anamorph: Candida japonica), isolated e.g. from cider and saké; F. floriforme (anamorph: possibly Cryptococcus albidus var. albidus on the basis of physiological characteristics), isolated from plants; and F. uniguttulatum (anamorph: Cryptococcus uniguttulatus), isolated from clinical specimens. [Book ref. 100, pp. 483–491.] filoplasmodium See LABYRINTHULAS. filopodium See PSEUDOPODIUM. filose Thread-like. Filosea A class of amoebae (superclass RHIZOPODA) which form long, tapering, hyaline filopodia that often branch and sometimes anastomose, and that are used for trapping prey. Spores and flagellated stages are not formed. Orders: Aconchulinida (cell naked, i.e., non-testate: e.g. NUCLEARIA, Vampyrella) and Gromiida (cell enclosed by a siliceous or proteinaceous and membrane-like test: e.g. EUGLYPHA, Gromia). Filoviridae A family of viruses originally proposed to accomodate Marburg virus and Ebola virus [Intervirol. (1982) 18 24–32], causal agents of clinically similar VIRAL HAEMORRHAGIC FEVERS in man (see MARBURG FEVER). The viruses occur in various parts of Africa; man is presumed to be infected only incidentally. Monkeys and rodents can be infected experimentally, infection usually resulting in fatal disease. Marburg and Ebola viruses are morphologically similar and genetically related but serologically distinct. The virions are pleomorphic filaments of uniform width (∼80 nm) but variable in length (up to ∼14000 nm); the unit length (i.e. the shortest length with maximum infectivity) is ∼790 nm for Marburg virus, ∼970 nm for Ebola virus. Ring-shaped or branched structures are sometimes seen. The virion consists of a helical nucleocapsid surrounded by an envelope bearing surface spikes. [Ebola virus proteins: Virol. (1985) 147 169–176]. The genome is one molecule of negativesense ssRNA. Virions are stable at room temperature but can (maize, oats, wheat and other grasses; Europe); pangola stunt virus (PSV) (‘pangola grass’ – Digitaria decumbens – and other Digitaria spp; S. America, Taiwan etc); rice black-streaked dwarf virus (RBSDV) (rice, oats, maize, wheat; Japan, China). Group II: Fiji disease virus (FDV) (sugarcane – Saccharum officinarum; Australia, Fiji, etc). Group III: Arrhenatherum blue dwarf virus (ABDV) (Arrhenatherum elatius; Germany); Lolium enation virus (LEV) (ryegrass – Lolium spp; Germany); oat sterile dwarf virus (OSDV) (oats, Lolium spp; Arrhenatherum elatius etc; Europe). MRDV, CTDV, RBSDV and PSV may be strains of the same virus or closely related viruses, as may OSDV, ABDV and LEV. (cf. RICE RAGGED STUNT VIRUS; LEAFHOPPER A VIRUS.) [Review: Book ref. 83, pp. 505–563.] filament (1) (bacteriol.) An elongated bacterial cell, i.e., one in which the length exceeds the width by ca. 10 times or more. (2) (bacteriol.) A (sheathed or unsheathed) chain of cells. (3) Syn. TRICHOME. filament pyrolyser See PYROLYSIS. filamentous haemagglutinin See WHOOPING COUGH. filamentous phages Syn. INOVIRUS. filamin See ACTIN. filar micrometer See MICROMETER. Fildes’ enrichment agar (Fildes’ digest agar) A medium which contains the X FACTOR and the V FACTOR; it is used for isolating Haemophilus spp. A peptic digest of blood is made by mixing 1 volume of defibrinated sheep blood with 3 volumes of physiological saline (0.87% NaCl) and adding conc. HCl (3 ml/100 ml) and granular pepsin (0.5 g/100 ml); after incubation (56° C/ca. 4 hours, with occasional shaking), the pH is adjusted to 7 with conc. NaOH. The digest is stored at 4° C; chloroform (0.25% v/v) may be added as preservative. Before use, any chloroform present should be driven off by heating (56° C/30 min); 2–5% (v/v) of the digest is then added to e.g. molten nutrient agar which has been autoclaved and cooled to 56° C. Fildes’ peptic digest of blood See FILDES’ ENRICHMENT AGAR. filia Syn. VILIA. Filibacter A genus of Gram-negative, strictly aerobic, filamentous gliding bacteria; F. limicola has been isolated from the sediment of a eutrophic lake [JGM (1984) 130 2943–2959]. Filifactor See CLOSTRIDIUM. filiform Thread-like. filipin A pentaene POLYENE ANTIBIOTIC produced by Streptomyces filipinensis. It has a higher affinity for cholesterol than for ergosterol, and is more toxic to mammalian cells than to fungi; it appears not to form pores in cytoplasmic membranes but can cause disruption of membranes in sensitive cells. filli Syn. VILIA. film See SMEAR. film and spots A test used for differentiating species of Mycoplasma. The organisms are grown on a medium containing egg yolk or ca. 20% horse serum. Species which give a positive reaction (e.g. M. gallinarum, M. synoviae) give rise to a wrinkled, pearly surface film, and to minute dark or black spots in the medium under and around the colonies; these cultural features indicate lipolytic activity: the film contains cholesterol and phospholipids, and the spots are small deposits of Ca2+ and Mg2+ salts of fatty acids liberated by lipolysis. A negative test is given e.g. by M. meleagridis, M. mycoides and M. pneumoniae. (See also LIPASE.) Filobasidiaceae A family of basidiomycetous fungi characterized by the formation of long, slender, non-septate basidia with 302 fim operon be inactivated by heat (e.g. 60° C/30 min), ultraviolet radiation and gamma radiation, organic solvents, phenolic disinfectants, hypochlorite and b-propiolactone. Virus replication occurs in the host cell’s cytoplasm; maturation involves budding of preformed nucleocapsids through the plasma membrane. Marburg and Ebola viruses can be grown in various types of cell culture (e.g. Vero cells). Infected cells exhibit CPEs such as intracytoplasmic vesiculation, swelling of mitochondria and degeneration of organelles. [Book ref. 148, pp 1111–1118.] Attempts to elicit protective immunity to Ebola virus in the traditional way have not been successful. However, an approach termed genetic vaccination (= genetic immunization), in which plasmid DNA (encoding viral proteins) is injected into the host, has achieved some success in an animal model: when vaccinated in this way, guinea pigs were protected against a lethal challenge of Ebola virus, protection correlating with both antibody titre and the antibody-specific T cell response [Nature Medicine (1998) 4 37–42]. (See also DNA VACCINES.) The Ebola virus glycoprotein is reported to be the main viral determinant of vascular cell cytotoxicity and damage [Nature Medicine (2000) 6 886–889]. The synthesis of increased amounts of glycoprotein by a mutant virus was associated with a higher level of cytotoxicity [Science (2001) 291 1965–1969]. A combination of ‘DNA immunization’ and boosting with an adenoviral vector (encoding relevant antigenic proteins) has been successful in generating protective immunity against the Ebola virus in non-human primates (cynomolgus macaques) [Nature (2000) 408 605–609]. [Asymptomatic Ebola infection (in contacts of symptomatic patients): Lancet (2000) 355 2210–2215. On the trail of Ebola and Marburg viruses: Science (2000) 290 923–925.] filter-paper cellulose See FP-CELLULOSE. filterable virus An obsolete term originally applied to any infectious agent which could pass through the early microbiological filters (e.g. the Berkefeld candle). Such agents included viruses but also e.g. Chlamydia and mycoplasmas. filtration Filtration can be used to separate microorganisms from a liquid or gas in which they are dispersed; it is used e.g. for the following purposes. (a) The sterilization of e.g. heatlabile liquids (known to be free of small viruses or viroids) by passing them through a filter capable of retaining all other microorganisms. (b) The preparation of cell-free culture filtrates for the recovery of extracellular enzymes, antibiotics etc. (c) The counting of small numbers of bacteria in a large volume of liquid – used e.g. in water bacteriology. A measured volume of the sample is passed through a membrane filter (pore size e.g. 0.45 µm – see below) and the membrane is subsequently incubated, face upward, on a pad of absorbent material which has been saturated with a suitable liquid culture medium. The number of bacteria per unit volume is calculated from the number of colonies which develop on the membrane. Membranes overprinted with a grid facilitate colony counting. (See also DEFT and COUNTING METHODS.) (d) Estimation of particle size in a monodisperse system. The suspension is filtered through a series of membranes of progressively smaller pore size; assuming minimal adsorption, particle size corresponds approximately to the pore size of the first membrane to effect significant retention of the particles. (See also sand filter in WATER SUPPLIES.) Many different types of filtering apparatus have been devised. In the majority of these, liquid is drawn through the filter as a result of reduced pressure in the receiving vessel. The candle filter type consists essentially of a thick-walled test tube-shaped structure made either of unglazed earthenware (e.g. the Chamberland candle or Doulton filter ) or wholly or partly of DIATOMACEOUS EARTH (e.g. the Berkefeld candle or Mandler filter); when assembled, the cavity of a candle filter communicates with the receiving vessel, and the liquid sample is contained in a jacket which surrounds the exterior of the candle. Such filters may be used repeatedly, but should be cleaned and sterilized after each use. The Seitz filter consists of a flat pad of asbestos (or asbestos and cellulose) suitably mounted between the sample container and the receiving vessel; the filter is used once and then discarded. In sintered glass filters the filter proper consists of a layer of minute glass particles which have been fused together (‘sintered’, ‘fritted’) to form a porous mass; such filters may be re-used after having been cleaned (e.g. in acid and/or hypochlorite solutions), washed, dried, and sterilized e.g. in a HOT-AIR OVEN. Membrane filters are thin films of e.g. cellulose nitrate (collodion), cellulose acetate, nylon, polycarbonate, polytetrafluoroethylene (PTFE, Teflon), or polyvinylidene; they are manufactured in a range of pore sizes – commonly used membrane filters having a mean pore diameter of 0.45 µm or 0.22 µm. (Filtration through membranes with such small pores is often referred to as ultrafiltration.) Pores may be tortuous (as in Gelman- or Millipore-type membranes) or cylindrical (as in Nuclepore membranes). In order to avoid blocking the fine pores of a membrane filter with coarse particulate matter it may be necessary to use one or more stages of prefiltration; this may involve e.g. passing the sample through a Seitz filter or through pads of matted glass-fibre before filtering it through the membrane. Hydrophobic membranes (made e.g. of PTFE) are used e.g. for the filtration of air in the brewing industry. Membranes with hydrophobic rims can filter aqueous fluids in the central region and air in the annular region; these membranes can prevent air-locks forming upstream of the filter. The physical integrity of a membrane filter can be monitored e.g. by the BUBBLE POINT TEST or the DIFFUSION TEST; the presence of surfactants in a membrane can affect the results of the former test. Although filtration depends largely on a mechanical sieving action, suspended particles, organisms and macromolecules may also be adsorbed to the material of the filter as a result of electrostatic or other forces. [Adsorption of poliovirus to membrane filters: AEM (1983) 45 526–531.] Some filters (e.g. the Seitz filter) are highly adsorptive, while others (such as the modern porcelain filters) are relatively non-adsorptive. The charge-modified filters such as Zetapor and Zeta Plus, which carry a net positive charge, combine a sieving action with electrostatic adsorption; such filters are used e.g. for the concentration of viruses from water samples [AEM (1983) 45 232–237]. Some membranes are markedly anisotropic in that the two faces of the membrane have different pore characteristics; an effect of this is that flow rate is higher when filtration is carried out in one particular direction. In some cases substances can be leached from a filter during the filtration process: e.g., Seitz filters tend to enrich the filtrate with magnesium ions. In the manufacture of some types of membrane, additives such as glycerol and surfactants are incorporated to improve flexibility and wettability, respectively; unless such substances are leached out prior to use (e.g. during sterilization) they can subsequently contaminate the filtrate. (See also HEMMING FILTER and HEPA FILTER.) fim operon See FIMBRIAE. 303 fimbriae Classification of fimbriae. The different types of fimbria on Gram-negative bacteria can be grouped into categories on the basis of e.g. (i) homology in the amino acid sequences in major subunit proteins; (ii) physical, antigenic and adhesive characteristics; and (iii) the mechanism of secretion and assembly. One group comprises the type I fimbriae (also called type 1 fimbriae); these include: (i) the ‘common’ type I fimbriae of Escherichia coli, (ii) the P FIMBRIAE and (iii) the K88 and K99 ‘fibrillae’. Another well-studied category of fimbriae, the type IV (= type 4) fimbriae, are found on a range of pathogenic bacteria; these fimbriae occur e.g. on strains of EPEC and ETEC, on Neisseria spp and Pseudomonas aeruginosa, and on Vibrio cholerae (‘toxin co-regulated pili’ (TCP) which constitute an important virulence factor in CHOLERA). Type I fimbriae (type 1 fimbriae; F1 fimbriae; ‘common pili’). These fimbriae are chromosomally encoded and are common e.g. on Escherichia coli and other enterobacteria. Each fimbria consists primarily of the main rod-shaped region, about 7 nm in diam., which comprises several thousand copies of the major pilin subunit, FimA (encoded by the fimA gene). The distal end of the fimbria consists of a short (∼16 nm) and narrow (2–3 nm diam.) flexible structure – the fibrillum (plural: fibrillae) – which is composed of the minor pilin subunits FimF, FimG and FimH; FimH is a mannose-binding ADHESIN which appears to be the most distal pilin in the fibrillum. Type I fimbriae can adhere to various cells and surfaces, including guinea-pig and horse erythrocytes – causing HAEMAGGLUTINATION; such adhesion is inhibited by D-mannose, or by methyl-a-D-mannoside, i.e. binding is ‘mannose-sensitive’, and type I fimbriae are said to cause ‘mannose-sensitive haemagglutination’ (MSHA). Fimbriae whose adhesion is not inhibited in this way are said to cause ‘mannose-resistant haemagglutination’ (MRHA). Type I fimbriae can also bind to UROPLAKINS (see also UPEC), such adhesion being a preliminary step in the development of many URINARY TRACT INFECTIONS (such as CYSTITIS). Moreover, type I fimbriae can promote the invasion of human urinary tract epithelium [EMBO (2000) 19 2803–2812]. Genetics of type I fimbriae. In Escherichia coli, type I fimbriae are encoded by the fimA–fimH operon; fimA and fimFGH are structural genes (see above), while the fimCD products are involved in assembly (see later). The fimBE genes have a regulatory role, being involved in PHASE VARIATION: the on/off switching of fimbrial synthesis. Switching involves inversion of a 314-bp sequence that contains the promoter of the first structural gene in the operon, fimA. When the control sequence is in one orientation the promoter is located correctly for transcription of fimA and of the other genes (so that fimbriae are synthesized); when the control sequence is in the opposite orientation, fimA is promoter-less (so that fimbriae are not synthesized) (cf. flagellar PHASE VARIATION in Salmonella). The products of the fimBE genes are recombinases which mediate inversion of the control sequence; FimB can bring about inversion in either direction (i.e. it can switch synthesis from on to off and from off to on), whereas FimE can mediate only the on-to-off switch. (Under in vivo conditions, switching also requires other (intracellular) factors, including INTEGRATION HOST FACTOR and the H–NS PROTEIN.) Interestingly, there is evidence of regulatory cross-talk between the fim operon and the pap operon (which encodes P FIMBRIAE) – this resulting in inhibition of expression of type I fimbriae. In this cross-talk, PapB, a pap regulatory protein, is fimbriae (singular : fimbria) Thin, proteinaceous filaments which extend from the surface of some types of microbial cell; fimbriae may be sparse, or they may occur in large numbers on a given cell. More than one type of fimbria may be present on a given cell. Fimbriae are functionally distinct from both flagella (see FLAGELLUM) and PILI. Note. A number of authors use the two terms fimbriae and pili, interchangeably, to refer to the appendages described in this entry. By contrast, appendages described in the entry PILI (which, unlike fimbriae, are concerned with the transfer of DNA between cells) are referred to consistently by all authors as ‘pili’. A logical, scientific use of terminology would require that different entities be referred to by different terms; for this reason, the terms ‘fimbriae’ and ‘pili’ are used here with distinct and mutually exclusive meanings. Fimbriae are quite common on Gram-negative bacteria (including cyanobacteria) but less common on Gram-positive species; they occur on various fungi – including the yeasts Candida albicans and Saccharomyces cerevisiae [fimbriae of yeasts and yeast-like organisms: Bot. Gaz. (1982) 143 534–541] and some SMUTS. The account below refers specifically to fimbriae of Gramnegative bacteria. Fimbriae may be distributed over the entire surface of a given cell or they may occur only in particular region(s) of the cell surface. Fimbriae may promote cell-to-cell adhesion, or adhesion between the cell and substratum. The adhesive fimbriae of some pathogenic bacteria are important virulence factors which play a critical role in the process of infection. Some polar fimbriae – i.e. fimbriae located at the pole (end) of a rod-shaped bacterium – are associated with TWITCHING MOTILITY. A fimbria is essentially a rod-like structure with a uniform width (typically 2–8 nm) and a length between ∼100 nm and several micrometres; although it may contain more than one type of component, the main structure is composed of a single type of protein subunit. In at least some cases (type I fimbriae of Escherichia coli – see below), the individual fimbria has been seen as a tightly coiled helical filament under the electron microscope, the filament itself comprising a linear sequence of protein subunits. Short fimbriae (e.g. K88) may be seen as a tangled mass on the cell surface, appearing as an amorphous layer under lowpower magnification in the electron microscope; such fimbriae have been referred to as ‘capsular material’ or fibrillae (but see later for an alternative use of ‘fibrillae’). It has been suggested that such short fimbriae may in fact represent broken fimbriae [FEMS Reviews (1996) 19 25–52 (35)]. A fimbria is constructed essentially of linearly repeating molecules of the main protein subunit. Protein subunits have been termed fimbrillins or pilins, the latter term being used more commonly. (The subunits have also been called fimbrins, but this may cause confusion because ‘fimbrins’ are involved in the formation of actin bundles in eukaryotic cells.) (See also PSEUDOPILINS.) Pilins frequently contain a high proportion of non-polar amino acids, so that fimbriate cells are commonly (not always) more hydrophobic than afimbriate cells. (In this context hydrophobicity may be indicated by the tendency of cells to congregate at an air–water or oil–water interface.) The occurrence, number and composition of fimbriae on a cell depend on various factors which include e.g. genetic switching mechanisms (see later) and temperature; for example, K88 and K99 fimbriae are not formed below ∼18° C. 304 final host Type IV fimbriae (type 4 fimbriae). All the fimbriae placed in this category are secreted and assembled in a similar way. These fimbriae include important (adhesive) virulence factors in a range of Gram-negative pathogens such as Neisseria gonorrhoeae and N. meningitidis, Pseudomonas aeruginosa, Vibrio cholerae and certain strains of Escherichia coli (e.g. the ‘bundle-forming pili’ of EPEC). (See also LONGUS.) The type IV fimbriae of e.g. N. gonorrhoeae are subject to PHASE VARIATION (on/off switching) as well as to extensive ANTIGENIC VARIATION. Most type IV fimbriae are encoded chromosomally (see also PIL GENES). The ‘bundle-forming pili’ of EPEC are an exception, being encoded by a virulence plasmid (see also PATHOGENICITY ISLAND). Type IV fimbriae are typically flexible, rod-like appendages, ∼5–6 nm in diam. and 1–2 µm in length; constituent pilins are apparently organized in a helical fashion. In a recent model for the assembly of type IV fimbriae in N. gonorrhoeae [FEMS Reviews (2000) 24 21–44 (31–35)], the assembly of these appendages (unlike the assembly of type I fimbriae) begins at a protein complex in the cytoplasmic membrane; that is, the base of the fimbria is linked to the cytoplasmic membrane – a column of assembled subunits passing from this origin, through the periplasm, through the outer membrane (see later), and into the extracellular environment. The adhesive properties of type IV fimbriae are due, at least partly, to the major (structural) pilins – in contrast to type I fimbriae, in which there are minor pilins specialized for adhesion in the distal fibrillum. Exceptionally, tip adhesins (encoded by the pilC gene) have been indentified in the two major pathogenic species of Neisseria [Nature (1995) 373 357–359; Mol. Microbiol. (1997) 23 879–892]. Secretion and assembly of type IV fimbriae. In the model for assembly of type IV fimbriae in N. gonorrhoeae, the major pilin subunit, PilE, is translocated across the cytoplasmic membrane but initially remains attached to the membrane by an N-terminal hydrophobic sequence; this sequence is subsequently cleaved, so that the PilE subunits can be incorporated, sequentially, into the nascent fimbria – incorporation being mediated by a complex of proteins (including PilFGT) located within the cytoplasmic membrane. A SECRETIN, PilQ, forms a pore (∼5.5 nm diam.) in the outer membrane through which the developing fimbria extends to the exterior; the pore is stabilized by PilP. In N. gonorrhoeae (and N. meningitidis), the distal end of the fimbria has an adhesive subunit, PilC (which is synthesized with a sec-dependent signal sequence); the incorporation of PilC is one of the initial steps in the assembly process. [Type IV fimbriae (review): ARM (1993) 47 565–596.] PSA fimbriae (‘PSA pili’) occur as a polar tuft in many strains of Pseudomonas aeruginosa; they are apparently chromosomally encoded, retractable filaments, ca. 6 nm in diam. and <1 to several micrometres in length. PSA fimbriae act as receptors for a range of bacteriophages. [Secretion and assembly of regular surface structures (fimbriae, flagella and S layers) in Gram-negative bacteria: FEMS Reviews (2000) 24 21–44.] fimbriate (1) Having FIMBRIAE. (2) Having a fringed edge. fimbrillin See FIMBRIAE. fimbrin See ACTIN. fin+ plasmid See FERTILITY INHIBITION. fin rot See BACTERIAL FIN ROT. final host (definitive host) In heteroxenous coccidia: the host in which the sexual phase occurs (see EIMERIORINA). reported to (i) increase the expression of FimE (thus promoting the on-to-off switch), and (ii) inhibit the activity of FimB (thus blocking an off-to-on reversal of FimE activity) [EMBO (2000) 19 1450–1457]. Secretion and assembly of type I fimbriae. The individual protein subunits are synthesized in the cytoplasm; each protein has a signal sequence (see SIGNAL HYPOTHESIS) and is transported across the cytoplasmic membrane in a sec-dependent manner (see type II PROTEIN SECRETION). In the periplasm, each protein is bound by a ‘chaperone’, FimC. FimC folds the protein and prevents it from forming abortive contacts with other subunits; the chaperone also appears to target its subunit to the outer membrane. In the OUTER MEMBRANE, the usher protein, FimD, forms an oligomeric ring structure with a central pore (of ∼2 nm diam.) through which the oligomerized subunits of the nascent fimbria are translocated as a linear (not helical) filament. The first subunits to be externalized are those of the fibrillum; FimG is thought to have a critical role in ‘nucleating’ the fibrillum [PNAS (2000) 97 9240–9245]. Subsequently, the FimA subunits, bound together in linear sequence, are translocated through the usher pore and added to the base of the growing fimbria; only when the FimA subunits have reached the extracellular side of the usher pore do they adopt a helical form. The order in which the various types of subunit are externalized via the usher pore may depend on the affinity with which each type of subunit binds to the pore structure; thus, e.g., under in vitro conditions, the FimH adhesin has the highest affinity for the usher. [Secretion and assembly of type I fimbriae: FEMS Reviews (2000) 24 21–44 (27–31).] P fimbriae. The 11 genes encoding P FIMBRIAE occur in the (chromosomal) pap operon, which is quite similar to the fim operon. PapA is the major subunit; PapC is the usher protein; PapD is the chaperone; PapKEFG form the fibrillum, PapG being the adhesin. PapH binds at the base of the fimbria, forming an anchor and signalling the end of fimbrial growth. (In mutant cells lacking the PapD chaperone, the subunits are degraded by a periplasmic protease, DegP; in papD degP mutants, subunits accumulate in the periplasm.) K88 (F4 ) fimbriae. These (plasmid-encoded) fimbriae occur on strains of E. coli pathogenic for piglets. The major fimbrial subunit, FaeG, also functions as an adhesin; the binding sites for K88 fimbriae contain Gal-a(1–3)Gal. K99 (F5 ) fimbriae. These (plasmid-encoded) fimbriae occur on strains of E. coli pathogenic for calves and lambs. The major subunit, FanC, has adhesive properties. [Chromatographic purification of K99 fimbriae: JGM (1983) 129 1975–1982. Production of K88, K99 and F41 fimbriae in relation to growth phase: FEMS (1985) 26 15–19. K88, K99 and 987P fimbrial antigens used in a vaccine against porcine colibacillosis: VR (1985) 117 408–413.] Type II fimbriae (type 2 fimbriae). These fimbriae occur on some strains of Salmonella and other enterobacteria; they lack the adhesive and haemagglutinating properties of type I fimbriae but are otherwise similar. Type III fimbriae (type 3 fimbriae) occur on various members of the Enterobacteriaceae [JMM (1985) 20 113–121]; their adhesive properties are not inhibited by mannose. These fimbriae promote haemagglutination of ox erythrocytes only when the latter have been treated with tannic acid (the so-called ‘tanned ox’ haemagglutination). [Comparative study of type 3 fimbriae in Klebsiella spp: JMM (1985) 20 203–214.] 305 finger-and-toe disease finger-and-toe disease See CLUBROOT. finger millet mosaic virus See RHABDOVIRIDAE. fingerprinting Any procedure which gives a strain-specific pattern of products when a given organism is examined; the products commonly consist of fragments of DNA (or RNA) distributed electrophoretically in a gel. A fingerprint may be generated e.g. by restriction of chromosomes followed by ELECTROPHORESIS of the products (see e.g. DNA FINGERPRINTING) or by electrophoresis of the products formed in TYPING procedures that involve nucleic acid amplification (e.g. AFLP). Finkel–Biskis–Jinkins murine sarcoma virus See FBJ OSTEOSARCOMA VIRUS. FinOP system In most IncF plasmids: a system in which the plasmid’s finO and finP products act, jointly, to inhibit translation of the traJ gene (the gene which exerts a positive regulatory influence on the transfer genes); as a consequence of this activity, the transfer function of such plasmids is repressed. The FinO products of most IncF plasmids are interchangeable but the FinP products of different plasmids are commonly not interchangeable. [Nucleotide sequences of finP alleles of five IncF plasmids: JB (1986) 167 754–757.] The finP product is an antisense RNA molecule which binds to the 5
end of traJ mRNA (including the ribosomebinding sequence), thus inhibiting translation; the finO product (a polypeptide) binds to, and promotes the stability of, the FinP–mRNA duplex – resulting in repression of the traJ gene and (therefore) repression of transfer (so-called fertility inhibition). (The FinOP binding site was formerly designated fisO, OJ or traO.) Mutant (defective) finO and/or finP products may fail to inhibit the expression of traJ – in which case the negative regulation of the transfer genes is abolished, and the transfer function is expressed constitutively. Plasmids containing mutant finO genes include R1drd 19 and R100-1. It was originally thought that the (constitutively derepressed) F PLASMID lacked a finO locus [Book ref. 161, p 605]. finO is now known to be present but is inactivated by an INSERTION SEQUENCE (IS3 ) located within the coding region of the gene. (See also FERTILITY INHIBITION.) fireblight A disease which can affect many members of the Rosaceae, e.g. apple and pear trees, cotoneaster and hawthorn; it is characterized by wilting and by browning and necrosis of blossoms, fruit, leaves and twigs. The causal agent, Erwinia amylovora, infects e.g. via blossoms and/or wounds; virulent strains of E. amylovora form AMYLOVORIN. [Review: Book ref. 58, pp. 45–63.] firefly luciferin See BIOLUMINESCENCE. (See also CHEMILUMINESCENCE.) Firmibacteria A proposed class of bacteria comprising all organisms of the FIRMICUTES other than the THALLOBACTERIA. Firmicutes A proposed division of the PROCARYOTAE comprising bacteria in which the CELL WALL is of the Gram-positive type; classes: FIRMIBACTERIA; THALLOBACTERIA. Fis protein See CELL CYCLE (b). Fischerella A genus of filamentous, thermophilic CYANOBACTERIA (section V) in which some of the cells in a mature trichome divide in more than one plane, resulting in a partly multiseriate trichome with lateral uniseriate branches. Hormogonia are formed from the ends of the trichomes or from lateral branches, and are composed of small cylindrical cells which enlarge and become rounded; heterocysts initially develop mainly in intercalary positions, and are mainly intercalary or lateral in mature trichomes. Akinetes may be formed. GC%: 42–49. The genus includes ‘Mastigocladus laminosus’, an organism found in hot springs; it has an upper temperature limit for growth of ca. 58° C (cf. CHLOROGLOEOPSIS) and is capable of nitrogen fixation. [Cell division and branching in M. laminosus: JGM (1984) 130 2079–2088.] FISH See PROBE. fish diseases Marine and freshwater fish are susceptible to a wide range of infectious diseases, the severity and spread of which tend to be enhanced under high-density fish farming conditions. (a) Bacterial diseases include e.g. BACTERIAL FIN ROT, BACTERIAL GILL DISEASE, COLD WATER DISEASE, COLUMNARIS DISEASE, ENTERIC REDMOUTH, ENTERIC SEPTICAEMIA, FURUNCULOSIS, KIDNEY DISEASE, RED MOUTH, RED PEST, tuberculosis, ULCER DISEASE, vibriosis (see VIBRIO). [Book ref. 9; control of bacterial diseases by antimi- crobial compounds: Book ref. 121, pp. 255–268.] (b) Fungal diseases include e.g. GILL ROT, ICHTHYOPHONOSIS, SAPROLEGNIASIS. (See also APHANOMYCES.) [Book ref. 10.] (c) Protozoal diseases include e.g. COSTIASIS, ICHTHYOPHTHIRIASIS, PROLIFERATIVE KIDNEY DISEASE, VELVET DISEASE, WHIRLING DISEASE; see also GLUGEA, HENNEGUYA and MYXOBOLUS. (d) Viral diseases include e.g. CARP POX, CHANNEL CATFISH VIRUS DISEASE, EGTVED DISEASE, INFECTIOUS HAEMATOPOIETIC NECROSIS, INFECTIOUS PANCREATIC NECROSIS, LYMPHOCYSTIS, SPINNING DISEASE, SPRING VIRAEMIA OF CARP. Fish may also be affected or killed by algal or cyanobacterial BLOOMS due to the production of toxins (see e.g. RED TIDE) and/or to oxygen depletion resulting from the sudden decomposition of vast numbers of algae or cyanobacteria. fish spoilage Freshly caught, gutted, iced fish and crustacea (shrimps etc) are susceptible to spoilage by a mixed psychrotrophic flora derived from the fish itself and/or from equipment, handlers etc [Book ref. 29, pp. 283–294]. Common spoilage organisms include Photobacterium spp (from the fish’s intestine) and Alteromonas putrefaciens. These organisms are facultative anaerobes which can reduce trimethylamine-N-oxide (TMAO) – present in the flesh of the fish – to the strongly malodorous substance trimethylamine (TMA); TMAO may function as the terminal electron acceptor for anaerobic electron transport [JGM (1983) 129 3689–3696]. Other malodorous products of bacterial spoilage include ammonia and hypoxanthine. In certain types of fish (e.g. mackerel) dangerously high – but often organoleptically undetectable – levels of HISTAMINE may accumulate, during storage, as a result of the decarboxylation of histidine by contaminating bacteria (e.g. Clostridium spp, Escherichia coli, Proteeae); consumption of such fish causes ‘scombroid poisoning’ [Food Mic. (1984) 1 263–267]. Fish spoilage may be delayed by the addition of preservatives (usually benzoic acid) and/or by irradiation. Off-flavours and taints in fish may be due to substances produced by certain actinomycetes and cyanobacteria present in the aquatic environment (see e.g. GEOSMIN and 2-METHYLISOBORNEOL). fisheye spoilage (of olives) See PICKLING. fisO locus See FINOP SYSTEM. fission Cell division in which overall (i.e., not localized) cell growth is followed by septum formation which typically divides the fully grown cell into two similar or identical cells. (See also BINARY FISSION; cf. BUDDING.) fissitunicate ascus Syn. BITUNICATE ASCUS. Fistulina See APHYLLOPHORALES (Fistulinaceae). Fistulinaceae See APHYLLOPHORALES. FITC Fluorescein isothiocyanate: a reactive derivative of FLUORESCEIN used e.g. for labelling proteins in IMMUNOFLUORESCENCE techniques. FITC fluoresces greenish-yellow. 306 flagellar motility together at one pole of the cell – this propelling the cell forward with the opposite pole leading. Within a uniform medium, such ‘smooth swimming’ is interrupted at intervals of ca. 1 second by tumbling: a random, chaotic movement (lasting ca. 0.1 second) resulting from a switch from CCW to clockwise (CW) in some of the bunched flagella; when this happens the flagellar bundle is disrupted, causing the random movement. (The timing of the switch from CCW to CW in a given flagellum is apparently not related to the timing of the switch in the other flagella.) Smooth swimming is resumed when the majority of the flagella are again rotating CCW. The pattern of alternate smooth swimming and tumbling results in a three-dimensional RANDOM WALK. Certain environmental stimuli can influence the pattern of motility – leading to a TAXIS (see e.g. CHEMOTAXIS). Peritrichously flagellated bacteria generally reach speeds of ca. 10–30 µm/second – although peritrichously flagellated cells of Thiovulum majus have been reported to swim at ∼600 µm/second [Microbiology (1994) 140 3109–3116]. Speeds reported in monotrichously flagellated bacteria include 70 µm/second (Pseudomonas aeruginosa) and 100 µm/second (Bdellovibrio). In these organisms, flagellar rotation may occur for approximately equal periods of time in the CCW and CW modes; during rotational reversal, conformational changes in the filament and/or hook may bring about randomization of direction (equivalent to tumbling in peritrichous cells) – thus permitting responses to environmental stimuli. In amphitrichously flagellate bacteria the two flagella rotate in opposite directions. In bacteria of the order SPIROCHAETALES, motility is associated with rotation of the periplasmic flagella – this generating a helical waveform in the outer membrane which enables the cells to ‘screw’ their way through the medium. Thus, CCW flagellar rotation in Leptonema illini causes the anterior (i.e. forwardly directed) end of the cell to assume a helical shape and the cell to rotate clockwise about its longitudinal axis – moving forward with a screw-like action. Spirochaetes can reverse direction and flex – the latter movement perhaps being a means of randomizing direction. The cells can swim through media which are too viscous to permit motility of cells which rely on external flagella. (‘Creeping’ or ‘crawling’ movements have also been recorded for spirochaetes in contact with solid surfaces.) [Structure/motility of spirochaetes: JB (1996) 178 6539–6545.] (b) (eukaryotic) The eukaryotic FLAGELLUM commonly beats in an undulatory manner (usually in one plane, but sometimes – e.g. in Euglena – in a spiral) or in an oar-like manner (as in Chlamydomonas). Undulations commonly originate at the base of the flagellum and travel towards the tip. The beating of a smooth flagellum generates forces which act on the surrounding medium in the direction of the propagated waves, so that the cell moves in the opposite direction (i.e. the flagellum ‘pushes’ the cell from behind). Conversely, if waves travel from the tip to the base of the flagellum (as e.g. in Trypanosoma), the cell swims with the flagellum leading. In tinsel flagella, the mastigonemes appear to act as rigid, passive structures which lie in the plane of the beat and generate forces which act on the surrounding medium in a direction opposite to that of the propagated waves; thus, a tinsel flagellum undulating from base to tip will ‘pull’ the cell from the front (as e.g. in Ochromonas, and in zoospores of Phytophthora). In some species the pattern of flagellar beating can be changed e.g. in response to environmental conditions. For example, in Trypanosoma the flagellum normally propagates waves from tip Fite–Faraco stain An acid-fast stain used for detecting Mycobacterium leprae in tissue sections; the procedure resembles that for the ZIEHL–NEELSEN STAIN but haematoxylin is used as counterstain. Fitz-Hugh–Curtis syndrome See CURTIS–FITZ-HUGH SYNDROME. FIV FELINE IMMUNODEFICIENCY VIRUS. five-five-five test (5-5-5 test) A quantitative SUSPENSION TEST in which a test organism is exposed for 5 min to each of a range of dilutions of the test disinfectant; after neutralization of the disinfectant, surviving bacteria are counted by subculturing to pour plates. To be considered effective, a disinfectant must give a minimum 5-log (105 -fold) reduction in cell numbers. Sporicidal activity is assessed by using a suspension of endospores of Bacillus cereus ATCC 9139 preheated at 80° C for 60 sec; an ‘effective’ sporicidal disinfectant must produce a minimum 1-log decrease in viable spores. five-kingdom classification See KINGDOM. five-three-two symmetry (5-3-2 symmetry) See ICOSAHEDRAL SYMMETRY. fixation The process of killing cells while preserving their structure and organization in as life-like a condition as possible (cf. CRYOFIXATION). Fixation involves the inactivation of enzymes, particularly AUTOLYSINS; ideally, it also hardens cellular structures and renders cell components compatible with particular stains. Most or all methods of fixation create ARTEFACTS. (a) Heat fixation can be used for the fixation of smears of bacteria prior to staining. The SMEAR is passed rapidly through a flame two or three times; such heat-fixed smears are suitable e.g. for the GRAM STAIN but are not suitable for observations on cell structure or morphology. (b) Chemical fixation. A chemical fixative permeates cells and stabilizes their components by binding to them or denaturing them. Some fixatives (e.g. ethanol, mercuric chloride, picric acid) precipitate proteins and thus considerably disturb cellular organization. Others (e.g. FORMALDEHYDE, GLUTARALDEHYDE, OSMIUM TETROXIDE, potassium dichromate) tend to preserve proteins in situ. Osmium tetroxide and potassium dichromate are also important lipid fixatives. For ELECTRON MICROSCOPY osmium tetroxide and glutaraldehyde are often used in e.g. alkaline phosphate buffer or cacodylate buffer. Glutaraldehyde may be used first to fix proteins (pre-fixation), and osmium tetroxide used subsequently for the post-fixation of lipids. (See also BOUIN’S FLUID, CARNOY’S FLUID, POLYVINYL ALCOHOL FIXATIVE, SCHAUDINN’S FLUID, ZENKER’S FLUID.) fixative A substance used for FIXATION. fixed virus A strain of rabies virus attenuated by serial passage e.g. through rabbits. (cf. FLURY VIRUS; STREET VIRUS; SEMPLE VACCINE.) FkpA See PROTEIN SYNTHESIS (protein folding). fla genes In e.g. enterobacteria: genes involved in the synthesis and function of bacterial flagella. [ARBB (1984) 13 51–83.] flabelliform (flabellate) Fan-shaped. Flabellula See AMOEBIDA. flacherie An economically important disease of silkworms caused by a small RNA virus resembling a picornavirus (see PICORNAVIRIDAE). flagella Plural of FLAGELLUM. flagellar motility (a) (bacterial) MOTILITY produced by the rotation of a FLAGELLUM, or the rotation of a number of flagella. In peritrichously flagellated bacteria (such as Escherichia coli and Salmonella typhimurium), the flagella rotate independently of one another [Cell (1983) 32 109–117]. For most (95%) of the time, each flagellum rotates counterclockwise (CCW). When the majority of flagella are rotating CCW they bunch 307 flagellar motor to base, but during an ‘avoiding reaction’ (PHOBIC RESPONSE) the direction of wave propagation can be reversed. Chlamydomonas normally swims with an oar-like ‘breast stroke’ (‘ciliary-type’ action), but in an avoiding reaction the two flagella extend forwards and propagate waves from base to tip (‘flagellar-type’ action). In many species the flagellum is apparently capable of only one type of movement; in these organisms changes in direction may be achieved by a temporary cessation of beating, the flagellum remaining bent during this period. [Dynamics of eukaryotic flagellar movement: SEBS (1982) 35 289–312.] Flagellar movements appear to be regulated by the levels of Ca2+ in the flagellum; some environmental stimuli appear to cause changes in the permeability of the flagellar membrane to calcium ions, and consequent changes in motility patterns. In at least some species, levels of calcium ions within the flagellum may be regulated by a Ca2+ -ATPase located in the flagellar membrane. In Chlamydomonas reinhardtii the photophobic response is apparently initiated by Ca2+ -stimulated phosphorylation of certain axonemal proteins [JCB (1985) 101 1702–1712]. flagellar motor See FLAGELLUM (a). flagellar phase variation See PHASE VARIATION. flagellar stain See FLAGELLUM (a). flagellate (1) (adj.) Bearing one or more flagella (see FLAGELLUM). (2) (noun) A protozoon of the MASTIGOPHORA. flagellin The protein subunit of the filament of a bacterial FLAGELLUM. The composition of flagellin (MWt ca. 30000–55000) varies with species, and determines e.g. the shape of the flagellum, its antigenic specificity (see H ANTIGEN and PHASE VARIATION), and susceptibility to FLAGELLOTROPIC PHAGES. Typically, flagellins are relatively rich in glutamate and aspartate but poor in aromatic amino acids; cysteine is usually absent. Flagellins from the alkalophile Bacillus firmus have a lower content of basic amino acids [JGM (1983) 129 3239–3242]; the ‘complex’ flagella of Rhizobium spp have a higher content of hydrophobic amino acids. A flagellar filament may be dissociated into its flagellin subunits e.g. by treatment with detergents or acid; under appropriate conditions, and given a primer (e.g. a small fragment of flagellum), the flagellin subunits can re-assemble spontaneously. flagellotropic phage Any BACTERIOPHAGE which adsorbs to a host cell flagellum and hence is specific for flagellated host cells (see also FLAGELLIN). Flagellotropic phages include e.g. bacteriophage c (see STYLOVIRIDAE) in enterobacteria, BACTERIOPHAGE PBS1 (q.v.) in Bacillus subtilis, and bacteriophage 7-7-1 in ‘Rhizobium lupini ’. In e.g. phage c, the tip of the phage tail bears a fibre which appears to wrap around the flagellum, and the phage then appears to move down to the base of the flagellum to infect the cell. flagellum (plural : flagella) (a) (bacterial) A thread-like appendage which (commonly) projects from the surface of the cell and which is responsible for MOTILITY in most types of motile bacteria; such flagella may occur e.g. singly, in groups or tufts, or distributed over the surface of the cell (see AMPHITRICHOUS, HELICOTRICHOUS, LOPHOTRICHOUS, MONOTRICHOUS and PERITRICHOUS). By contrast, in bacteria of the SPIROCHAETALES the flagella occur between layers of the cell envelope. This entry refers primarily to flagella which project from the cell surface – although the structural and mechanistic features of spirochaete flagella are apparently similar. Flagella can be detected by dark-field MICROSCOPY – but not by bright-field microscopy unless suitably stained (see e.g. LEIFSON’S FLAGELLA STAIN). [Simple method for staining bacterial flagella: JCM (1989) 27 2612–2615.] The bacterial flagellum consists essentially of three parts: (i) a protein filament which projects from the cell surface and which, by rotating, provides the thrust for FLAGELLAR MOTILITY; (ii) a curved protein structure, the hook, contiguous with the proximal end of the filament, and (iii) the flagellar motor (basal body) – a structure which anchors the hook in the cell envelope and incorporates the energy-converting apparatus. The filament (ca. 5–10 µm × 20 nm) is a semi-rigid, lefthanded helix (of periodicity ∼2 µm) composed of eleven longitudinally arranged fibrils, each fibril being a chain of FLAGELLIN subunits; the fibrils are closely associated (like the strands of a rope) to form a structure with an axial channel. (In some species the cells may have two types of flagellum which have different periodicities, a phenomenon known as biplicity – see e.g. VIBRIO.) In some Gram-negative species the filament is sheathed, i.e. covered by an extension of the OUTER MEMBRANE. Yet further variation in the filament occurs e.g. in strains of Rhizobium which have ‘complex’ flagella that differ e.g. in their fine structure [JMB (1986) 190 569–576] and in their greater fragility. The hook is a tightly coiled right-handed helix, with an axial channel, which is curved through ∼90° ; its protein subunits are distinct from flagellin. The flagellar motor (= basal body) is a cylindrical structure which spans the width of the cell envelope; its composition varies with species. In Escherichia coli the components include a stack of three protein rings and a hollow, axial protein rod (shaft) – together with a fourth protein ring which projects into the cytoplasm (see figure). The L (for ‘lipopolysaccharide’) ring is coplanar with the OUTER MEMBRANE, the P (for ‘peptidoglycan’) ring is coplanar with the PEPTIDOGLYCAN layer of the cell envelope, and the MS ring (once believed to be two distinct rings – the M ring and the S ring) lies within the cytoplasmic membrane; the C (for ‘cytoplasm’) ring, which is associated with the MS ring, carries the components of a switching mechanism which allows the direction of rotation to be reversed. A different number of rings has been reported in other bacteria – e.g. two in Bacillus subtilis and five in Caulobacter crescentus. In Campylobacter a ‘cartwheel structure’ with eleven ‘spokes’ appears to be associated with the basal body [JGM (1984) 130 1307–1310]. In a current model of flagellar rotation (in e.g. Escherichia coli, Salmonella typhimurium and related species) the MS ring and the C ring jointly rotate relative to the stationary L and P rings; this rotation causes the axial shaft (and hence the filament) to rotate, the filament converting torque into thrust. Flagellar rotation needs energy in the form of an ion gradient across the cytoplasmic membrane. In E. coli and S. typhimurium energy is supplied by proton motive force (pmf) (see CHEMIOSMOSIS), and rotation is driven by an inward flow of protons through the flagellar motor (perhaps via a channel in the MotA/MotB protein assembly); torque generation appears to involve interaction between the rotor (MS ring) and the stator (MotA/MotB assembly) – specifically, between the FliG protein, associated with the MS ring, and MotA. [Models for the flagellar motor (theoretical considerations): PTRSLB (2000) 355 491–501.] The C ring (apparently the FliM component of the C ring – see later) plays a pivotal role in the switching mechanism in CHEMOTAXIS. The speed of rotation of a flagellum varies linearly with the level of the cell’s pmf. The minimum (threshold) value of pmf 308 flagellum FLAGELLUM: the origin of a flagellum in the cell envelope of Escherichia coli, Salmonella typhimurium and related Gram-negative bacteria (diagrammatic, not drawn to scale; see entry). (a) An earlier model. (b) A current view based on several sources [e.g. JB (1996) 178 4582–4589; JMB (1996) 255 458–475]. As in the earlier model, the L and P rings are associated with the outer membrane and peptidoglycan layer, respectively. The MS ring – previously regarded as two distinct structures (the M and S rings) – lies in the cytoplasmic membrane. The C ring, which includes proteins FliM and FliN (see table, text), is in the cytoplasm; it is associated with the MS ring and is believed to rotate with the MS ring when the flagellar motor is running. The C ring is involved in switching between counterclockwise and clockwise rotation of the flagellum in chemotaxis. The annular region of the cytoplasmic membrane which encircles the MS ring contains the MotA and MotB proteins; this region acts as the stator of the flagellar motor. It appears that MotA and MotB form a complex, with MotB bound to peptidoglycan, and MotA interacting with molecules of the FliG protein (which are bound to the periphery of the MS ring); the latter interaction (details currently unknown) generates torque and, hence, flagellar rotation. The flagellar motor (= basal body) includes the L, P and MS rings, the axial rod (shaft) and the MotA/MotB complex. The C ring is included by some authors; other authors include the C ring as part of an ‘extended’ basal body. The dotted line enclosed by the C ring represents a structure which has been termed the export apparatus (see entry) – and which now appears to be a form of type III PROTEIN SECRETION system. Reproduced from Bacteria 5th edition, Figure 2.8, page 28, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. (below which rotation does not occur) appears to vary with species; it has been reported to be ∼30 mV for E. coli and Bacillus subtilis but less for e.g. streptococci. Although flagellar rotation is commonly driven by pmf, SODIUM MOTIVE FORCE is used in some species – e.g. smf is used to drive the polar flagellum in Vibrio cholerae [JB (1999) 181 1927–1930]. Assembly of the flagellum. Assembly is a highly organized process in which the components are added in strict sequence. The following refers to flagellar assembly in E. coli, S. typhimurium and related organisms. First, the MS ring – consisting of about 25 molecules of the fliF gene product (FliF protein) – is inserted into the cytoplasmic membrane. Then the FliG, FliM and FliN proteins are incorporated; it appears that molecules of FliG bind around the periphery of the MS ring (on the cytoplasmic side), while FliM and FliN jointly form the C ring, with FliN being more distal to the membrane [JB (1997) 179 813–817]. At this stage an export apparatus assembles within the C ring (see figure). The purpose of the export apparatus is to permit the operation of a functional channel through the MS ring without compromising the cell’s structural integrity; this channel allows 309 flagellum FLAGELLUM (bacterial): some of the genes and gene products involved in the assembly of the flagellum in Escherichia coli, Salmonella typhimurium and related species specific components subsequently to pass through the MS ring when required. The export apparatus is a multiprotein structure that includes FlhA, FlhB, FliH, FliI, FliO, FliP, FliQ and FliR. Interestingly, some of the proteins of the basal body exhibit homology with proteins of type III secretion systems (see PROTEIN SECRETION), and the group of proteins involved in transport of the flagellar components has been referred to as a type III secretion system. In addition to these proteins, FliJ is needed for the export of components – and may be part of the transport apparatus; this protein also appears to act as a ‘chaperone’, preventing intracellular aggregation of various export substrates [JB (2000) 182 4207–4215]. FlgB, FlgC and FlgF are translocated via the export apparatus and form the proximal part of the hollow rod (shaft); FlgG then forms the distal part of the rod. The L and P rings are formed by molecules of FlgH and FlgI, respectively; these proteins, which have a signal sequence (see SIGNAL HYPOTHESIS), are secreted through the cytoplasmic membrane into the periplasmic space, i.e. unlike rod components, they do not pass through the MS ring. Molecules of FlgE exit via the MS ring and give rise to the hook. When the hook has reached an appropriate length, annuli of hook-associated protiens FlgK and FlgL, and a terminal ‘cap’ of FliD protein, are added, in that order, at the distal end. (The mechanism which regulates the length of the hook is unknown, but it appears to involve FliK.) FliD forms a permanent distal cap while the flagellin subunits (FliC protein) pass through the axial channel of the rod and hook and are polymerized at the FlgL–FliD junction to form the growing filament. Incorporation of FliC may be promoted by rotation of the flagellar cap [Science (2000) 290 2148–2152]. Growth of the filament thus occurs at the distal end (‘tip growth’ – compare archaeal flagella in (c), below); during growth, the FliC subunits are incorporated by self-assembly. Genetic regulation of flagellar assembly. The first genes to be expressed – class I genes – are those in the flhCD operon; these genes encode transcriptional activators, and their expression is essential for the expression of all other flagellar genes. (The flhCD genes are expressed at peak levels in synchrony with the cell cycle [JB (1997) 179 5602–5604].) FlhC and FlhD promote the expression of class II genes, including: fliF (MS ring), fliG (switch), fliMN (switch/C ring), flgE (hook) and genes from two operons, flhAB and fliHIOPQR, whose products are required for the export apparatus. Another class II gene, fliA, encodes a SIGMA FACTOR (s28 ) which activates class III genes, but FliA is temporarily inactivated by the binding of FlgM (FlgM is an ANTI-SIGMA FACTOR); however, completion of the hook serves as a signal for the release of FlgM (which is externalized via the axial canal) – thus allowing FliA to mediate expression of the class III genes. The class III genes include fliC (the flagellin subunit of the filament) and the motAB genes (which encode components of the stator). Some of the genes and gene products involved in the assembly of the flagellum are listed in the table. [Genetics and assembly of flagella in Gram-negative bacteria: FEMS Reviews (2000) 24 21–44 (22–27).] (b) (eukaryotic) A thread-like appendage (ca. 0.15–0.3 µm in diam., up to ∼40 µm in length), one or more of which extend from vegetative and/or reproductive cells of certain algae, fungi and protozoa. Eukaryotic flagella typically act as locomotory organelles but may be involved e.g. in mating (see Gene (product) Function of gene product Class I genes flhC (FlhC) flhD (FlhD) Transcriptional activation of class II genes Transcriptional activation of class II genes Class II genes flgB (FlgB) flgC (FlgC) flgE (FlgE) flgF (FlgF) flgG (FlgG) flgH (FlgH) flgI (FlgI) flgK (FlgK) flgL (FlgL) flgM (FlgM) flhA (FlhA) flhB (FlhB) fliA (FliA) fliD (FliD) fliF (FliF) fliG (FliG) fliH (FliH) fliI (FliI) fliJ (FliJ) fliK (FliK) fliM (FliM) fliN (FliN) fliO (FliO) fliP (FliP) fliQ (FliQ) fliR (FliR) Rod (proximal) Rod (proximal) Hook Rod (proximal) Rod (distal) L ring P ring Hook (distal end) Hook (distal end) Anti-sigma factor (delays FliA activity) Export apparatus Export apparatus Sigma factor (s28 ) for class III genes Filament cap MS ring Switch/C ring?/torque generation Export apparatus Export apparatus Export apparatus/chaperone Regulation of hook length C ring/switch C ring/switch Export apparatus Export apparatus Export apparatus Export apparatus Class III genes fliC (FliC) motA (MotA) motB (MotB) Flagellin (protein subunit of filament) Torque generation Torque generation e.g. CHLAMYDOMONAS) or attachment (see e.g. TRYPANOSOMA) (cf. HAPTONEMA). The eukaryotic flagellum is completely different from the bacterial flagellum in both structure and mechanism of action. The main structural component of the eukaryotic flagellum is the axoneme: a system of longitudinally arranged MICROTUBULES (each ca. 25 nm diam.) which extend the length of the flagellum. These microtubules are arranged in a characteristic ‘9 + 2’ pattern: two adjacent tubules (enclosed by a membranous sheath in some species) run through the centre of the axoneme and are surrounded by a circular arrangement of nine pairs of tubules (doublets). Each doublet consists of an A-tubule (= ‘A-subfibre’) and a B-tubule (‘B-subfibre’); the A-tubules are complete microtubules, but the B-tubules are incomplete, sharing part of the wall of the A-tubule (giving the B-tubules a somewhat D-shaped appearance in cross-section). The A-tubule in each doublet bears a series of paired (inner and outer) ‘arms’ composed of high-MWt, Ca2+ - and Mg2+ -activated ATPase called dynein. The A-tubules are linked to the central 310 Flaviviridae flaming Brief exposure of an object (or part of an object) to the hottest part of a flame – usually for the STERILIZATION of the surface thus treated. Flaming is part of the ASEPTIC TECHNIQUE. Objects such the LOOP and STRAIGHT WIRE are flamed for ca. 3–5 sec; somewhat shorter exposures are used for the rims of e.g. bottles from which sterile (non-flammable!) substances are to be poured. Flaming for very short times may be used for the heat FIXATION of bacterial smears. Flammulina See AGARICALES (Tricholomataceae) and ENOKITAKE. flash-freezing Syn. QUENCH-FREEZING. flash pasteurization See PASTEURIZATION. flask fungi PERITHECIUM-forming fungi. flat-field objective lens An objective lens (see MICROSCOPY) which can give an image in which all parts of the field are simultaneously in focus. Flat-field objectives are denoted by the prefix plan- or plano- (see e.g. ACHROMAT). flat sour A can (see CANNING) which appears normal (i.e. the ends are flat or slightly concave – cf. SWELL) but whose contents have been soured as a result of microbial growth; Bacillus stearothermophilus is one of several common causal agents in low-acid foods. (See also SOFT CORE HAM.) flavacol A compound produced by Aspergillus flavus: 3-hydroxy2,5-diisobutylpyrazine. flavin adenine dinucleotide See RIBOFLAVIN. flavin mononucleotide See RIBOFLAVIN. Flaviviridae (‘arbovirus group B’) A family of enveloped ssRNAcontaining VIRUSES [Intervirol. (1985) 24 183–192] formerly regarded as a genus within the family TOGAVIRIDAE. Flaviviruses infect a wide range of vertebrates, and many also infect invertebrates (mosquitoes or ticks) which act as VECTORS. Some flaviviruses are important pathogens of man or animals. The family contains a single genus, Flavivirus; type species: yellow fever virus. The flavivirus virion is more or less spherical (ca. 40–50 nm diam.); it consists of a core (comprising a single type of core protein, C, associated with the genome), a ‘membranelike’ protein, M, which surrounds the C–RNA complex, and a lipoprotein envelope containing one type of virus-specific glycoprotein, E, which acts as a haemagglutinin (e.g. with chick RBCs) and is the target of neutralizing antibodies. The genome is a single linear positive-sense ssRNA molecule (MWt ca. 4 × 106 ) which is capped at the 5
end but is apparently not polyadenylated at the 3
end. Subgenomic mRNA is not produced; the genome-length RNA functions as the sole messenger and contains only one long open reading frame – at least in West Nile virus [Virol. (1986) 149 10–26] and in yellow fever virus [Science (1985) 229 726–733] – suggesting that viral proteins are formed by post-translational cleavage of a polyprotein. The genes encoding structural proteins are located at the 5
end (5
-C–M–E. . .); several non-structural proteins are also produced. A region at the 3
end is not translated and has the potential for forming a stem-and-loop secondary structure [JGV (1986) 67 1183–1188]. Viral RNA synthesis (which involves the formation of both replicative intermediates and replicative forms) apparently occurs in the perinuclear region; maturation may involve a process of ‘condensation’ rather than budding, and appears to occur within cisternae of the endoplasmic reticulum. Flaviviruses can replicate in various types of vertebrate cells in culture, often causing CPE and plaque-formation; infected cells typically exhibit extensive proliferation of the endoplasmic reticulum. Host protein synthesis is not significantly inhibited. Invertebrate cells may also be infected, with or without CPE. tubules (or to the sheath surrounding them) by radial ‘spokes’; protein (nexin) links also occur between the peripheral doublets. [Axoneme substructure: JBC (1985) 100 2008–2018.] The axoneme is embedded in a flagellar matrix, the whole being enclosed by a sheath-like flagellar membrane which is apparently continuous with the cytoplasmic membrane. The proximal end of the flagellum is connected to a BASAL BODY via a more or less distinct region, the transition region (cf. CILIUM). Although most eukaryotic flagella conform to the basic structure described above, there are differences in detail among the species. For example, many eukaryotic flagella bear characteristic surface structures attached to the flagellar membrane – structures such as minute scales (see PRASINOPHYCEAE); a fine tomentum of minute hairs; or larger, more distinct hairs called mastigonemes (or flimmer ) which are commonly arranged in two longitudinal rows, one on each side of the flagellum. (See also TRENTEPOHLIA.) (A distinction is sometimes made between the terms ‘mastigoneme’ and ‘flimmer’: mastigonemes are shorter and more rigid, while flimmer are longer, finer, and more flexible.) A flagellum which lacks obvious scales or mastigonemes is said to be of the smooth, simple, whiplash or peitschengeissel type, while a flagellum bearing mastigonemes is said to be of the tinsel, flimmergeissel or hispid type. The nature and number of a cell’s flagella, and the sites and orientation of the flagella, are important taxonomic features. (See also HETEROKONT, ISOKONT and STEPHANOKONT.) Ultrastructural details of flagella may also be of taxonomic value: e.g. in flagellated cells of the green algae (CHLOROPHYTA) the A-tubule of one of the nine peripheral doublets in the axoneme lacks an outer dynein arm – a characteristic apparently universal in, and unique to, green algae. The flagellum of green algae also has a microfibrillar structure in the transition region which gives a characteristic stellate pattern in cross-section. [Review of flagellar ultrastructure in green algal classification: Book ref. 123, pp 73–120.] (See also PARAXIAL ROD.) The eukaryotic flagellum can move independently of the basal body: flagella which have been severed from the cell are capable of independent movement, and in e.g. Chlorogonium the cells remain motile when the basal bodies have detached from the flagella prior to cell division [JCB (1985) 100 297–309]. Movement is thus a property of the flagellar shaft itself – cf. bacterial flagellum (a), above. The movement involves sliding of axonemal microtubules relative to one another, the energy being supplied by dynein-catalysed hydrolysis of ATP; movement is apparently regulated by Ca2+ : see FLAGELLAR MOTILITY (b). (c) (archaeal) The archaeal flagellum differs from the bacterial flagellum in composition, structure and apparent mode of assembly. Thus, the subunits (flagellin) of the archaeal filament are typically glycosylated proteins, and the filament itself is typically much thinner than the bacterial filament. Moreover, archaeal flagellins have a signal sequence – suggesting passage through the cytoplasmic membrane rather than through an export apparatus; this feature, together with certain similarities between archaeal flagellins and bacterial type 4 fimbriae, have suggested that the archaeal flagellum is assembled from the base – in contrast to the ‘tip growth’ of the bacterial flagellum [JB (1996) 178 5057–5064]. Interestingly, genes homologous to those which encode bacterial flagellar structures have not been detected in the genomes of archaeans, even in those whose complete genome sequence is known; this has suggested that the archaeal flagellum is a unique form of organelle [FEMS Reviews (2001) 25 147–174]. Flagyl See METRONIDAZOLE. 311 Flavivirus uredia on both sides of the leaf and, later, dark-coloured telia on the stems. flax wilt See FUSARIUM WILT. Flectobacillus A genus of pink-pigmented bacteria (family SPIROSOMACEAE). The cells are typically C-shaped, 0.3–1.0 × 1.5–5.0 µm, but spiral forms occur – as do filaments up to 50 µm; ‘ring-shaped’ structures may be apparent due to overlap of the ends of curved cells. Acid is formed from many carbohydrates. GC%: 34–40. Type species: F. major. Flexibacter A genus of GLIDING BACTERIA (see CYTOPHAGALES) which occur in aquatic habitats; some species are pathogenic for fish (see e.g. COLD WATER DISEASE and COLUMNARIS DISEASE). The cells are short, non-motile rods in old cultures but are thin, thread-like and motile in young cultures; typically, they contain pigments ranging from yellow to red. Metabolism: chemoorganotrophic. Flexibacteriae A class of non-photosynthetic GLIDING BACTERIA comprising the orders CYTOPHAGALES and MYXOBACTERALES [ARM (1981) 35 339–364]. flexible pouch Syn. RETORT POUCH. flexirubins Polyene pigments found e.g. in members of the CYTOPHAGALES. flexuous hyphae (receptive hyphae) Hyphae which project from a pycnium and which function as female (receptive) structures. (cf. TRICHOGYNE.) flgM gene See SIGMA FACTOR. fliA gene See SIGMA FACTOR. flimmer See FLAGELLUM (b). flimmergeissel flagellum See FLAGELLUM (b). flip–flop (of membrane lipids) See CYTOPLASMIC MEMBRANE. flippase See CYTOPLASMIC MEMBRANE. flipper A can (see CANNING) which appears normal but in which one end ‘flips out’ (i.e. becomes convex) if the can is struck (cf. SWELL). floSt gene See FLORFENICOL. floc blanket clarifier See WATER SUPPLIES. flocculation (1) (serol.) A term used with different meanings by different authors and for different phenomena: e.g., (i) the development of a flaky or fluffy precipitate in certain precipitin tests; (ii) any form of precipitate; (iii) any form of agglutination, but particularly the agglutination of bacteria by antiflagellar antibodies. (2) The aggregation of (initially separate) cells to form flocs. Spontaneous flocculation occurs e.g. in certain sewage bacteria (see e.g. ZOOGLOEA and CELLULOSE) and in strains of BREWERS’ YEAST during the stationary phase of batch culture (e.g. at the end of the fermentation stage of BREWING). Flocculation of brewers’ yeast aids in the separation of the yeast cells from the fermented wort. (Yeast cells which remain dispersed in all phases of batch culture are called ‘powdery yeasts’.) Prior to flocculation, the cells are presumed to remain dispersed because of their mutually repulsive (usually negative) surface charge (see ZETA POTENTIAL); flocculation is thought to be initiated by a change in the surface structure of the cells, and Ca2+ appears to play a role by bridging the negative charges on adjacent cells [review: Book ref. 108, pp. 323–335]. (cf. COAGGREGATION.) flood plate A PLATE in which the surface has been flooded with liquid inoculum and the excess drawn off with a sterile Pasteur pipette. (See also LAWN PLATE.) flor sherry, flor yeast See WINE-MAKING. florfenicol A synthetic, broad-spectrum antibiotic which has been used e.g. against Mycoplasma bovis in the treatment of calf pneumonia. In vitro tests on field isolates of M. bovis Most flaviviruses can be classified in 7 serologically defined complexes (subgroups); most of those of greatest medical and veterinary importance belong to one of three such subgroups. (YELLOW FEVER virus is serologically distinct from other flaviviruses.) The dengue virus subgroup includes DENGUE viruses types 1–4. The West Nile virus subgroup includes e.g. Kunjin virus, JAPANESE B ENCEPHALITIS virus, MURRAY VALLEY FEVER virus, Rocio virus, ST LOUIS ENCEPHALITIS virus, and West Nile virus. The tick-borne encephalitis virus subgroup includes e.g. KYASANUR FOREST DISEASE virus, LOUPING-ILL virus, Negishi virus, Omsk haemorrhagic fever virus, POWASSAN ENCEPHALITIS virus, and tick-borne encephalitis (TBE) virus – including the eastern subtype (RUSSIAN SPRING–SUMMER ENCEPHALITIS virus) and the European subtype (Central European encephalitis virus). Other flaviviruses which are occasionally responsible for human disease include e.g. Banzi virus, Bussuquara virus, Ilheus virus, Rio Bravo virus, Sepik virus, Spondweni virus, Wesselsbron virus and Zika virus. Possible members of the Flaviviridae include Aedes albopictus cell-fusing agent and SIMIAN HAEMORRHAGIC FEVER VIRUS. [Medical aspects: Book ref. 148, pp. 955–1004.] Flavivirus See FLAVIVIRIDAE. Flavobacterium A genus (incertae sedis) of aerobic, chemoorganotrophic, Gram-negative bacteria which occur e.g. in soil, water, raw meats, milk and in some clinical specimens (F. meningosepticum is a causal agent of neonatal MENINGITIS). Cells: non-motile, round-ended rods, ca. 0.5 × 1.0–3.0 µm, which typically contain orange or yellow non-fluorescent, waterinsoluble pigments; in F. breve and F. odoratum (at least) the pigments may be of the flexirubin type. F. meningosepticum is typically weakly pigmented or non-pigmented. Metabolism is respiratory (oxidative). Acid (but no gas) is formed from carbohydrates in media which have a low content of peptone. Most species (not F. aquatile) grow well in nutrient broth and on nutrient agar; F. aquatile requires media enriched with e.g. casein digest, glucose, and yeast extract. F. meningosepticum is non-haemolytic on blood agar. Catalase +ve. Oxidase +ve. GC%: ca. 31–42. Type species: F. aquatile; other species: F. breve, F. balustinum, F. meningosepticum, F. multivorum, F. odoratum, F. spiritivorum [Book ref. 22, pp. 353–361], and F. gleum [IJSB (1984) 34 21–25]. (See also SPHINGOBACTERIUM.) flavodoxins A category of flavoproteins which have low (i.e. highly negative) Em values which can replace FERREDOXINS as electron carriers in various reactions – though their efficiency as electron carriers is less than that of ferredoxins. Flavodoxins contain 1 FMN per molecule but do not contain metal or labile sulphur. In many organisms (e.g. Clostridium spp, certain algae) flavodoxins are synthesized in place of ferredoxins in response to iron deficiency. Algal flavodoxin is also known as phytoflavin. In Azotobacter vinelandii a flavodoxin, azotoflavin, acts as electron carrier in NITROGEN FIXATION. Flavomycin Syn. MOENOMYCIN. flavoproteins See RIBOFLAVIN. flavour microorganisms Microorganisms which form metabolic products (‘flavour compounds’) that contribute to the flavour and/or aroma of certain foods: see e.g. DIACETYL. (See also GLUTAMIC ACID and YEAST EXTRACT.) [Review: AAM (1983) 29 29–51.] flax retting See RETTING. flax rust (linseed rust) A disease of the flax plant (Linum usitatissimum) caused by the macrocyclic, homoxenous rust Melampsora lini. Symptoms include the presence of orange 312 fluorescence indicate that significant levels of resistance have developed against florfenicol [VR (2000) 146 745–747]. In Salmonella typhimurium DT104 (an emergent multidrug-resistant pathogen), mutation in gene f loSt can result in resistance to both florfenicol and chloramphenicol [JCM (1999) 37 1348–1351]. Florida horse leech Syn. EQUINE PHYCOMYCOSIS. floridean starch An amylopectin-like D-glucan: the main storage carbohydrate in members of the Rhodophyta. [Book ref. 37, pp. 475–478.] floridoside 2-O-Glycerol-a-D-galactopyranoside: a glycoside found in members of the Rhodophyta; it serves as a reserve carbohydrate and as an osmoregulatory compound. (cf. ISOFLORIDOSIDE.) flotation A method for concentrating e.g. protozoan cysts in a sample of faeces; the sample is dispersed in a concentrated solution of e.g. ZnSO4 or NaCl such that any cysts present in the sample float to the surface. (cf. FORMALIN–ETHER METHOD.) Zinc sulphate flotation. The faecal sample (ca. 1 g) is mixed with about ten times its volume of water, centrifuged (ca. 350 g/3 min), and all the supernatant discarded. The sediment is re-suspended in ca. 10 ml of a solution of ZnSO4 of S.G. 1.18 (made by adding water to 331 g ZnSO4 .7H2 O to give a final volume of 1 litre) and filtered through a double layer of wet gauze; the filtrate is centrifuged (ca. 350 g/3 min) – the centrifuge being allowed to stop naturally to avoid turbulence within the liquid. A loopful of liquid from the surface is transferred to a drop of iodine solution on a slide; a cover-glass is added, and the preparation is examined under the microscope. Salt flotation. A solution of NaCl (70–100% saturated) is often used for the concentration of coccidian cysts in faecal samples. The sample is subjected to several cycles of centrifugation in water (each ca. 350 g/2–3 min), the supernatant being discarded each time. The sediment is then dispersed in salt solution and centrifuged at ca. 200 g/5 min. The surface layer of supernatant is transferred, by pipette, to ten times its volume of water; this suspension is centrifuged and the sediment collected for study. Prolonged contact with saturated salt solution causes plasmolysis; however, oocysts generally remain viable, and they regain their original appearance on transfer to water. flotation form (of an amoeba) See PSEUDOPODIUM. flow cytometry Any procedure in which individual cells in a suspension are counted, sorted, or otherwise analysed by the use of an apparatus in which the cells pass, individually, through a small hole or tube; the term is commonly used to refer specifically to those techniques in which cell populations are analysed by detecting fluorescence or FLUOROCHROME-labelled antigens or other structures on/in the cell (see e.g. FACS and FLOW MICROFLUOROMETRY; cf. COULTER COUNTER). flow microfluorometry Any procedure in which populations of cells, or of other particles, are counted, sorted, or otherwise analysed by passing them individually through a small hole or tube and detecting specific fluorescence or FLUOROCHROME labels. (See e.g. FACS; cf. FLOW CYTOMETRY.) flowers of tan See FULIGO. FLP gene See TWO-MICRON DNA PLASMID. flu Syn. INFLUENZA. flucloxacillin See PENICILLINS. fluctuation test A test devised by Luria and Delbrück in 1943 for investigating the mode in which bacterial populations respond to changes in the environment. At that time two hypotheses coexisted. (i) Genetic changes occur adaptively, i.e., as a result of environmental influences on the cells; those cells which adapt to the environment survive and proliferate. (ii) Genetic changes occur spontaneously, i.e., independently of the environment; those cells which have become particularly well suited to certain environmental conditions proliferate (preferentially) if such conditions arise. If bacteria adapted to a new environment, a similar proportion of adapted cells should arise in each of a number of independent cultures. If, however, mutants arose spontaneously and randomly, the number of mutant cells should vary considerably from culture to culture. In the original test, a measurement was made of the statistical variance among the numbers of bacteriophage-resistant cells in samples from each of a number of separate cultures of phagesensitive bacteria; the very high variance could not be attributed to sampling error since a very much lower variance was obtained for the numbers of resistant cells in each of a number of samples taken from a single (bulk) culture. (Phage-resistant cells were counted by plating each sample on a separate plate, subjecting the plate to virulent phage, and incubating; resistant cells formed colonies.) The wide fluctuation (high variance) in mutant numbers among the separate cultures indicated that mutant clones had been initiated in some cultures much earlier than in others – i.e., resistant cells had appeared at different times; this indicated that mutant (resistant) cells had arisen before the cells had been exposed to phage – thus supporting hypothesis (ii). flucytosine (5-fluorocytosine) An ANTIFUNGAL AGENT which is active mainly against yeasts and yeast-like fungi; it is used (usually with AMPHOTERICIN B) in the treatment of e.g. candidiasis and cryptococcosis. It functions as a pyrimidine analogue. After uptake by the yeast cell, it is deaminated to 5-fluorouracil by cytosine deaminase; 5-fluorouracil may then be phosphorylated and incorporated into RNA, or converted to 5fluorodeoxyuridine monophosphate which inhibits thymidylate synthase (see Appendix V(b)), thereby inhibiting DNA synthesis. fluid mosaic model See CYTOPLASMIC MEMBRANE. fluid-phase pinocytosis See PINOCYTOSIS. fluidized-bed fermenter A FERMENTER in which a nutrient solution is pumped upwards through a vertical column of small particles (e.g. stainless steel balls) coated with cells. The upward flow of liquid causes the particles to exhibit small, rapid movements which (a) facilitate interaction between the adherent cells and the medium, and (b) result in the removal (by friction) of a proportion of the adherent cells; a steady state is achieved when the loss of cells is balanced by cell growth. flunidazole See NITROIMIDAZOLES. fluor Syn. FLUOROCHROME. FluoReporter See PROBE. fluorescein A pale-yellow FLUOROCHROME which fluoresces greenish-yellow; it is a substituted xanthene used e.g. in IMMUNOFLUORESCENCE techniques. (See also FITC.) Solubility in water is very low; in ethanol it is ca. 2% w/v. (cf. PYOVERDIN.) fluorescence The emission of light which occurs when certain substances absorb radiation; emission occurs as electrons excited to the singlet state (S1 ) are returning to the (unexcited) ground state (S0 ). The light emitted is of wavelength longer than that of the exciting radiation, and emission typically lasts for ca. 10−9 sec (cf. PHOSPHORESCENCE). In fluorescence microscopy (see MICROSCOPY (e)) the exciting radiation is usually UV light, but blue light is used in some cases (‘blue-light fluorescence’). Primary fluorescence (= autofluorescence) is that produced by a substance which is inherently fluorescent – e.g. chlorophyll fluoresces red under UV radiation. Secondary fluorescence is 313 fluorescence-activated cell sorter FMAU (2
-deoxy-2
fluoro-5-methyl-1-b-D-arabinosyluracil) An ARABINOSYL NUCLEOSIDE which has antiherpesvirus and antileukaemia activity; its mode of antiviral action resembles that of ACYCLOVIR. FMDV FOOT AND MOUTH DISEASE virus. FMN See RIBOFLAVIN. fms An ONCOGENE present in the McDonough strain of feline sarcoma virus (see FELINE LEUKAEMIA VIRUS); the v-fms product appears not to have tyrosine kinase activity. The c-fms gene probably encodes the receptor for the macrophage-specific colony-stimulating factor 1 (CSF-1). foamy viruses See SPUMAVIRINAE. FOCMA Feline oncornavirus-associated cell-membrane antigen, an antigen on the surface of feline lymphosarcoma cells which reacts with sera from cats exposed to FELINE LEUKAEMIA VIRUS (or from cats having recovered from feline sarcoma virus infection). Anti-FOCMA antibodies do not neutralize FeLV, but may protect against the development of lymphosarcoma by exerting a cytotoxic effect on feline leukaemia cells in the presence of complement. focus-forming viruses Viruses which form foci of morphologically transformed cells in cell cultures or in tissues – see e.g. FRIEND VIRUS and MCF VIRUSES. Foettingeria See HYPOSTOMATIA. fog fever (acute bovine pulmonary emphysema and oedema) An acute ATYPICAL INTERSTITIAL PNEUMONIA in adult cattle, characterized by a sudden onset within ca. 1 week of being moved to new, lush green pasture; death may occur quickly, without warning, or animals may exhibit laboured breathing and fail to graze. Movement or exertion may cause death within minutes. The cause of fog fever is unknown. One hypothesis is that fog fever may result from an increased intake of tryptophan; in the rumen, tryptophan is metabolized to 3-methylindole – a substance which, when administered experimentally, produces a syndrome clinically indistinguishable from fog fever. The conversion of tryptophan to 3-methylindole in the rumen is inhibited e.g. by MONENSIN. foliaceous Syn. FOLIOSE. folic acid (pteroylglutamic acid) A water-soluble, photolabile VITAMIN. (See figure for structure.) Some microorganisms form pteroylpolyglutamic acid derivatives (typically containing 3–7 glutamic acid residues), certain of which may be specifically required for particular reactions. The coenzyme form of folic acid (coenzyme F) is 5,6,7,8tetrahydrofolic acid (THF, FH4 ), formed by the reduction of folic acid (via 7,8-dihydrofolic acid) by dihydrofolate reductase and NADPH. THF functions as a carrier of one-carbon units (e.g. formyl, methyl, methylene or hydroxymethyl groups); such groups are carried at the N 5 or N 10 positions (e.g. N 10 -formyl-THF) or bridge these two positions (e.g. N 5 ,N 10 methylene-THF). THF is involved e.g. in glycine–serine interconversion [see Appendix IV (c)], in methionine biosynthesis [Appendix IV (d)], in the biosynthesis of purines and deoxythymidine [Appendix V], in the catabolism of e.g. histidine, in ACETOGENESIS, etc. An exogenous source of folic acid is necessary for the growth of certain microorganisms: e.g. Enterococcus faecalis and Lactobacillus casei (both used for folic acid BIOASSAY), Crithidia fasciculata, Tetrahymena spp, and Trypanosoma platydactyli (formerly Leishmania tarentolae); most fungi can synthesize folic acid, although a few require the precursor p-AMINOBENZOIC ACID. Leuconostoc cremoris (formerly L. citrovorum) requires N 5 -formyl-THF (= citrovorum factor, folinic acid, leucovorin). that produced by a non-fluorescent substance which has been dyed with a FLUOROCHROME. fluorescence-activated cell sorter See FACS. fluorescence in situ hybridization (FISH) See PROBE. fluorescence microscopy See MICROSCOPY (e). fluorescence resonance energy transfer See PCR. fluorescent antibody technique (FAT) Any procedure in which IMMUNOFLUORESCENCE is used (e.g. for clinical lab. diagnosis, for counting or detecting specific microorganisms in water samples, etc). fluorescent brightener (brightener; optical bleach; optical brightener) A compound which exhibits FLUORESCENCE under ultraviolet radiation, and which may be used e.g. to treat (white) paper or textiles etc in order to enhance their ‘whiteness’ in normal daylight (by effectively enhancing the blue end of the visible spectrum); fluorescent brighteners which are absorbed by – and are non-toxic (or selectively toxic) for – microorganisms have also been used e.g. for STAINING microorganisms, the presence of the brightener being demonstrated by fluorescence MICROSCOPY. Type A brighteners (e.g. CALCOFLUOR WHITE) are (anionic) derivatives of 4,4
-diamino-2,2
-stilbene disulphonic acid. Type N brighteners (e.g. TINOPAL AN) are (cationic) oxacyanine compounds which tend to be more inhibitory than type A brighteners for microorganisms. fluorescent dye Any dye capable of FLUORESCENCE. Fluoribacter See LEGIONELLACEAE. fluorides The fluoride ion is inhibitory to a wide range of organisms, inhibiting many enzymes; it inhibits e.g. the fermentation of sugars by streptococci – apparently chiefly by (competitively) inhibiting enolase and thereby preventing the formation of phosphoenolpyruvate (necessary for uptake of PTS sugars) from 2-phosphoglycerate. (See also PEP POTENTIAL.) The inhibitory effect of fluoride is greater at lower pH; the cytoplasmic membrane may be impermeable to F− , and fluoride may enter the cell as HF. Fluorides are used e.g. in the prevention of DENTAL CARIES; they act primarily by becoming incorporated into the tooth enamel, thereby decreasing its susceptibility to acid demineralization; oral bacteria are apparently relatively insensitive to fluoride in the presence of saliva [Inf. Immun. (1986) 51 119–124]. Sodium and ammonium fluorides have also been used e.g. as antifungal wood PRESERVATIVES; their efficacy is said to be greater than that of zinc or copper compounds. fluorite objective (semiapochromatic objective) An objective lens (see MICROSCOPY) in which chromatic and spherical aberrations are each corrected for two colours (cf. ACHROMAT, APOCHROMAT). A FLAT-FIELD OBJECTIVE LENS of this type is called a planofluorite. fluorochrome Any fluorescent dye – e.g. ACRIDINE ORANGE, EOSIN, FITC, FLUORESCEIN. 5-fluorocytosine Syn. FLUCYTOSINE. fluoroquinolone antibiotics See QUINOLONE ANTIBIOTICS. 5-fluorouracil See e.g. PHENOTYPIC SUPPRESSION. Flury virus An avianized strain of the rabies virus used e.g. for anti-rabies vaccination in animals. The Flury LEP strain has been passaged (see SERIAL PASSAGE) in chick embryos ca. 40–60 times; the Flury HEP strain has been passaged >180 times. flutriafol An agricultural AZOLE ANTIFUNGAL AGENT which is active against certain cereal diseases: e.g. powdery mildew, leaf blotch, septoria, net blotch, rusts, and MBC-sensitive and -resistant eyespot. fly agaric See AMANITA and MUSCARINE. 314 food poisoning FOLIC ACID (pteroylglutamic acid) Drugs which interfere with folic acid metabolism include e.g. (2) Fomes is sometimes used as a form genus to include all polypores which form hard (corky or woody) perennial fruiting bodies. (cf. POLYPORUS sense 2.) fomites Objects or materials which have been associated with infected persons or animals and which potentially harbour pathogenic microorganisms. Fonsecaea See HYPHOMYCETES. Fontana’s stain (for spirochaetes) An air-dried smear on a clean, grease-free slide is fixed for 2 min in 2% aqueous formalin containing 1% acetic acid, and washed for 3 min in absolute ethanol. The smear is then flooded with 5% aqueous tannic acid containing 1% phenol, and heated so that the solution steams (not boils) for 30 sec. The smear is washed with distilled water and flooded with a solution prepared by adding 10% w/v aqueous ammonium hydroxide, drop by drop, to 0.5% w/v aqueous silver nitrate until the precipitate which forms just dissolves; the solution is made to steam (not boil) until the smear becomes brown. The slide is washed in distilled water and dried. Spirochaetes appear dark brown. Fonticula See ACRASIOMYCETES. food cup (in Amoeba) See PSEUDOPODIUM. food microbiology For examples of the use of microorganisms as food or in the manufacture of foods see ALCOHOLIC BEVERAGES; COCOA; COFFEE; DAIRY PRODUCTS; FERMENTED FOODS; LAVER; MUSHROOM (senses 3 and 4); SINGLE-CELL PROTEIN; UNDARIA; VINEGAR; YAKULT. Microbial products used as food additives include e.g. ALGINATE; CARRAGEENAN; CITRIC ACID; GLUTAMIC ACID; NISIN; XANTHAN GUM; YEAST EXTRACT. (See also FOOD PRESERVATION and FOOD SPOILAGE.) food poisoning ‘Food poisoning’ generally refers to acute GASTROENTERITIS caused by ingestion of food contaminated with certain pathogenic bacteria and/or their toxins, or contaminated with certain viruses; clinically distinct conditions that may be food-borne (e.g. CHOLERA) are generally excluded, although, traditionally, BOTULISM has been included. (cf. e.g. GIARDIASIS, MYCOTOXICOSIS.) Bacterial food poisoning may involve infection of the intestine (‘food-borne infection’) and/or the action of bacterial toxin(s) produced either in the food, prior to ingestion, or in the intestine (‘food-borne intoxication’, ‘toxic food poisoning’). Food that is contaminated with pathogens often shows no organoleptic signs of deterioration. (See also FOOD SPOILAGE and FISH SPOILAGE.) Some of the common types of food poisoning are described below (under causal agent). (a) Bacillus cereus can cause two distinct syndromes. (i) Diarrhoeal form. After an incubation period of ∼8–16 hours: diarrhoea, abdominal pain and nausea, but little or no vomiting. The (heat-labile) diarrhoeagenic enterotoxin causes a watery FOLIC ACID ANTAGONISTS, PAS and SULPHONAMIDES. (cf. BIOPTERIN and tetrahydromethanopterin under METHANOGENESIS.) folic acid antagonist (antifol) A synthetic drug (typically a diaminopyrimidine derivative) which acts as a structural analogue of FOLIC ACID and which binds to and inhibits the enzyme dihydrofolate reductase (DHFR) – preventing the reduction of dihydrofolate (or folic acid) to tetrahydrofolate (THF) and thereby inhibiting THF-dependent reactions (e.g. synthesis of deoxythymidine and hence of DNA). The folic acid antagonists aminopterin and methotrexate have high affinities for mammalian DHFRs, and are used e.g. as anti-cancer and antileukaemia drugs. (See also HYBRIDOMA and IMMUNOSUPPRESSION.) PYRIMETHAMINE and TRIMETHOPRIM are more effective against microbial DHFRs, and are used in the treatment of certain infectious diseases. A microorganism may become resistant to a folic acid antagonist e.g. by acquiring a plasmid encoding a drug-resistant DHFR (see e.g. O/129), or by a mutation resulting in the overproduction of (normal) DHFR. The action of a folic acid antagonist is often synergistic – at least in laboratory tests – with that of a SULPHONAMIDE, each type of drug inhibiting folic acid-dependent reactions but by different mechanisms; an antifol–sulphonamide mixture (e.g. cotrimoxazole: a mixture of trimethoprim and sulphamethoxazole) is often microbicidal, even though each drug is microbistatic when acting independently. However, sulphonamide–diaminopyrimidine combinations have a wide variety of toxic effects in humans, and their continued clinical use is open to question [JAC (1986) 17 694–696]. foliicolous Living or growing on leaves. folinic acid See FOLIC ACID. foliose (foliaceous) Leaf-like or leafy. Of a LICHEN: having a thallus which is ‘leaf-like’ (i.e., flattened and dorsiventral) and either lobed, with part or most of its lower surface being more or less firmly attached to the substratum (e.g. by RHIZINES), or disc-like, attached to the substratum by a holdfast in the centre of the lower surface (see e.g. UMBILICARIA). (cf. CRUSTOSE and FRUTICOSE.) folpet (N-(trichloromethylthio)phthalimide) An agricultural ANTIFUNGAL AGENT closely related to CAPTAN – with which it shares a similar antifungal spectrum. Fomes (1) A genus of lignicolous fungi of the APHYLLOPHORALES (family Polyporaceae) which form woody, hoof-like, perennial fruiting bodies (with porous, stratified hymenophores) on both wood and living trees; the context is trimitic and has clamp connections. Some species have been transferred to other genera – e.g. F. annosus to HETEROBASIDION. 315 food poisoning own uptake – which occurs by the ‘trigger mechanism’. Effector molecules (secreted via the type III system) are translocated into the epithelial cell where they bring about certain changes (e.g. actin polymerization). During uptake, extrusions of the host cell membrane (supported e.g. by actin elements) engulf the bacterium – which is thus internalized in a vacuole containing extracellular fluid. Unlike Shigella (see DYSENTERY), Salmonella invades via the apical (lumen-facing) surface of gut epithelial cells. Salmonella multiplies within the vacuole. Although much is now known about the way in which salmonellae invade the epithelium, less is known about the mechanism(s) by which infection induces intestinal secretion. Two mechanisms have been considered: (i) the action of toxin(s), and (ii) the effect of invasion per se. S. typhimurium is known to produce at least one toxin: Stn, a 25 kDa protein, encoded by gene stn, which is predicted to function as an ADP-ribosylating agent; however, only low levels of this toxin are formed, and its significance in pathogenesis is uncertain. It has been suggested that invasion may stimulate certain host cell secretory pathways, although invasion and secretion are not always correlated. (In a veterinary context, the cattle pathogen Salmonella dublin has been reported to secrete an effector protein, SopB, via a type III secretory system encoded by the inv-spa genes; SopB is believed to mediate enteropathogenicity by being translocated into epithelial cells and inducing both inflammation and fluid secretion [Mol. Microbiol. (1997) 25 903–912].) Recovery from Salmonella food poisoning commonly begins within a few days, but a carrier state may persist. Complications include sepsis and abscess formation in the meninges (in small children), arteries (leading to arteritis and e.g. rupture of the artery), bones and joints (cf. OSTEOMYELITIS). (f) Shigella. See DYSENTERY. (g) Staphylococcus. Staphylococcal food poisoning usually follows ingestion of food containing one or more ENTEROTOXINS (q.v.) formed by toxigenic, coagulase-positive strains of S. aureus. Commonly implicated are cooked foods eaten cold (e.g. hams, poultry), and prepared foods such as sandwiches, custards etc. The source of staphylococcal contamination is man (typically a food-handler); the organisms derive from the nares or skin, or from skin lesions (e.g. boils). [Detection of staphylococcal enterotoxins: RMM (1994) 5 56–64.] Incubation period: 1–6 hours. Onset is abrupt, with nausea, vomiting, abdominal pain and prostration, sometimes with diarrhoea. Symptoms may last for about 12 hours, but weakness and malaise may persist. Vomiting may result from stimulation, by the toxin, of neural receptors in the upper intestinal tract – this being suggested by abolition of the emetic reflex (in monkeys) following vagotomy and sympathectomy. The superantigenic nature of the enterotoxins suggests that cytokines may play an important role in generating at least some of the symptoms of staphylococcal food poisoning. (h) Vibrio parahaemolyticus. Gastroenteritis due to V. parahaemolyticus (see VIBRIO) is associated with the consumption of raw or inadequately cooked seafoods (fish, shellfish etc.). Most outbreaks occur during warm months. The pathogen may be concentrated in the gut of filter-feeding shellfish (cf. SHELLFISH POISONING) and may proliferate in contaminated seafoods stored without refrigeration. Incubation period: often 12–24 hours, but may be less than 2 hours (a shorter period typically correlating with more severe symptoms). Gastroenteritis: diarrhoea commonly watery, diarrhoea by stimulating adenylate cyclase (cf. CHOLERA TOXIN). Foods implicated: unrefrigerated meat and vegetable dishes, contaminated spices. (ii) Vomiting form. The incubation period (∼1–7 hours) is followed by nausea, vomiting and abdominal pain, but little or no diarrhoea. The pathogen may produce a (heat-stable) emetic toxin in association with sporulation. Foods commonly implicated in this type of illness include cooked rice which has been stored unrefrigerated. (Other species of Bacillus, e.g. B. subtilis and B. megaterium, have been associated with food poisoning.) (b) Campylobacter. Strains of C. jejuni or C. coli are usually involved: see CAMPYLOBACTERIOSIS; these organisms do not grow at 25° C but survive refrigeration and freezing – although they are killed by moderate heating (e.g. PASTEURIZATION). (c) Clostridium. Strains of C. perfringens type A are the commonest causal agents of clostridial food poisoning. Most frequently isolated are type A2 strains (non-haemolytic/weakly haemolytic on horse-blood agar; endospores especially resistant to heat); isolated less often are type A1 strains (b-haemolytic; endospores relatively sensitive to heat). After an incubation period of ∼8–24 hours: abdominal pain and diarrhoea, usually with little vomiting and no fever; symptoms last ∼12–24 hours. Food (especially meat and poultry) may be contaminated with spores present in dust or in human or animal faeces etc.; the spores may survive cooking and, if the food is allowed to cool slowly or is stored without refrigeration, may germinate and grow rapidly. A large number of cells must be ingested in order to initiate infection as many are killed by stomach acidity. In the gut, the surviving organisms multiply and sporulate, releasing an enterotoxin which is distinct from the four major clostridial toxins (a, b, e, i) formed by C. perfringens; this toxin damages the tips of villi of the epithelial cells, inhibiting absorption of glucose and causing an efflux of sodium ions, chloride ions and water. (See also ENTERITIS NECROTICANS.) (d) Escherichia. Certain strains of E. coli can cause gastroenteritis after ingestion of e.g. raw or improperly cooked foods, unpasteurized milk or water which has been contaminated from a faecal source. See e.g. ETEC (travellers’ diarrhoea) and EPEC. (e) Salmonella. Food-poisoning strains include a range of serotypes – e.g. S. typhimurium, S. hadar, S. virchow, S. agona. Infection commonly occurs by consumption of contaminated foods of animal origin: undercooked meat/poultry, meat products, eggs, raw milk etc. Foods may become faecally contaminated during preparation – e.g. via the hands of the cook – but in some cases (e.g. eggs) contamination may be present at source. Infection from pets has been reported. In meat, salmonellae compete poorly with the normal meat flora (cf. MEAT SPOILAGE); the meat flora is destroyed by cooking, so that salmonellae subsequently introduced are likely to thrive. Salmonellae can survive deep freezing but are killed by adequate cooking. After an incubation period of ∼12–48 hours there is a sudden onset with diarrhoea, abdominal pain, vomiting and fever; the stools may contain blood/mucus. The pathogen multiplies in the gut, adheres to microvilli, and then invades the epithelial cells. Invasiveness in salmonellae depends on a type III secretory system (see PROTEIN SECRETION) encoded by the inv-spa complex of genes located on a chromosomal PATHOGENICITY ISLAND. The initial attachment of a bacterial cell may be mediated by type I fimbriae; attachment appears to promote the formation of invasomes: bacterial appendages which resemble very short, thick flagella and which are believed to be essential for invasion. Salmonella induces its 316 food preservation sometimes explosive and dysenteric; other symptoms include nausea, abdominal cramps, vomiting, fever and chills. The illness may last ∼3–6 days. The main diarrhoeagenic factor of the pathogen is a 23 kDa protein, TDH; TDH is the haemolysin responsible for the KANAGAWA PHENOMENON. The binding site for this haemolysin (in the rabbit) is probably a trisialoganglioside designated GT1b ; in vitro studies indicate that the toxin promotes the secretion of chloride. V. parahaemolyticus is reported to be usually sensitive to tetracycline, chloramphenicol, nalidixic acid and gentamicin but resistant to a number of b-lactam antibiotics, including ampicillin. [V. parahaemolyticus in humans (review): RMM (1995) 6 137–145.] (i) Viruses. Strains of ROTAVIRUS are common causal agents of viral gastroenteritis in infants; they infect the mucosa of the jejunum and ileum, causing desquamation and villous atrophy. Infection occurs by ingestion of faecally contaminated food or water. Incubation period: ∼2–3 days. Onset is abrupt, with vomiting and diarrhoea (often associated with an upper respiratory tract infection). Recovery may occur in ∼2–3 days, or the disease may be fatal. Infection in adults seems to be largely asymptomatic. The SMALL ROUND STRUCTURED VIRUSES (also called e.g. ‘parvo-like viruses’ or ‘parvovirus-like agents’) are major causes of gastroenteritis in both children and adults; these infections have been called e.g. ‘epidemic vomiting disease’, while outbreaks during the winter months have been designated ‘winter vomiting disease’. Immunity following infection may be short-lived. Infection is thought to occur by consumption of faecally contaminated food (see e.g. SHELLFISH POISONING) or water. Incubation period: ∼1–2 days. Onset is sudden, with nausea and vomiting, sometimes with mild diarrhoea, fever, myalgia, malaise etc. (j) Yersinia enterocolitica is associated with gastroenteritis mainly in children. The condition is often mild, with fever, a watery diarrhoea, vomiting and abdominal pain, usually resolving in 1–2 days. However, severe cases may involve ileitis – often in association with mesenteric lymphadenitis; in these cases the severe abdominal pain may mimic acute appendicitis (‘pseudoappendicitis’). Various foods have been implicated as vehicles for the pathogen (e.g. meat, especially pork, and milk); large numbers of cells of Y. enterocolitica have been reported in certain vacuumpacked meats (see DFD MEAT). The pathogen can grow at, and below, 4° C (i.e. even under refrigeration). Strains of the pathogen secrete a heat-stable enterotoxin (designated Yst) apparently similar to the ST-I of ETEC in both structure and activity; this toxin may be responsible for the diarrhoeal symptoms. Y. enterocolitica invades the intestinal mucosa – studies in animals indicating that initial entry occurs (as in Shigella) via the M cells of Peyer’s patches. [Invasion by Y. enterocolitica: e.g. Book ref. 218, pp 225–228.] In one model, cells of the pathogen pass through the M cells – so that they gain access to the basolateral surfaces of adjacent epithelial cells on which are found the receptors (integrins) of several bacterial adhesins. Adhesins of Y. enterocolitica include the outer membrane protein invasin (chromosomally encoded by the inv gene), and the YadA protein (encoded by plasmid pYV) which forms a cell-surface fibrillar matrix. Invasion via the basolateral surfaces of epithelial cells is likely to resemble the process seen in vitro. In vitro, the bacterial adhesin invasin binds to a5 b1 integrins, such binding inducing the development of further integrin molecules at the binding site – leading to multiple points of attachment between bacterium and epithelial cell (zippering); this induces uptake in a close-fitting vacuole (within which the pathogen apparently survives). As well as consolidating bacterial attachment (zippering), the greater concentration of integrins also induces tyrosine kinase activity within the epithelial cell; this is necessary for the invasion process, but the precise details are not known. Various invasive pathogens – including e.g. Shigella dysenteriae and EIEC – induce secretion of certain CYTOKINES by the gut epithelial cells; one of these, IL-8 (a chemoattractant for inflammatory leukocytes), probably represents a host response aimed at overcoming invasion. However, virulent strains of Y. enterocolitica have been found to induce significantly less IL-8 compared with non-virulent strains. Virulent strains of Y. enterocolitica also encode various effector molecules (‘Yops’) and an associated type III secretory system (see PROTEIN SECRETION) [the Yersinia Yop virulon: Mol. Microbiol. (1997) 23 861–867]; the genes for this system are located on plasmid pYV. An important function of the Yop system is to avoid phagocytosis by disrupting the phagocyte’s uptake mechanism. [Infections with enteropathogenic Yersinia species (review): RMM (1998) 9 191–205.] (k) Various other organisms have been linked with gastroenteritis/diarrhoea – e.g. species of Aeromonas [RMM (1997) 8 61–72], Citrobacter, Enterobacter, Klebsiella, Proteus, Pseudomonas and Streptococcus. food preservation The various methods of food PRESERVATION aim to prevent or delay microbial and other forms of FOOD SPOILAGE, and to guard against FOOD POISONING; such methods therefore help to retain the nutritive value of the product, extend its shelf-life, and keep it safe for consumption. It is generally considered preferable to use physical methods of preservation (e.g. refrigeration) when possible. (a) Heat treatment. See e.g. APPERTIZATION; CANNING; PASTEURIZATION. (b) Low-temperature preservation. Low temperatures delay or prevent spoilage e.g. by reducing the metabolic activities of contaminating organisms and/or the activity of endogenous enzymes in the food; spoilage by PSYCHROTROPHS may nevertheless still occur. (i) Above 0° C. Refrigeration between 0° C and 5° C is often used for the short-term storage of foods; however, spoilage may occur at these temperatures (see e.g. MEAT SPOILAGE). (ii) Below 0° C. FREEZING may kill a proportion of contaminating organisms, and it also lowers the food’s WATER ACTIVITY – thus making it a less hospitable environment for surviving contaminants. Nevertheless, spoilage may still occur (see e.g. BLACK SPOT (1)); care must therefore be taken to exclude or reduce contamination before freezing. (c) Dehydration. If the WATER ACTIVITY of a food is progressively decreased, a point is reached at which the food will not support the growth of contaminating organisms. The aw of foods may be reduced by evaporation through heating (e.g. dried milk) or e.g. by the addition of sodium chloride (salting – e.g. fish products) or sugar syrups (e.g. fruits). FREEZE-DRYING has been used e.g. for coffee. (See also CURING (1) and SMOKING.) (d) Acidification – see PICKLING. (e) Fermentation. Fermentation of certain types of raw food not only enhances the keeping qualities of the food, it may also improve the flavour and (often) the nutritive value of the food, and avoids the need for expensive heat treatment, 317 food spoilage such structure(s) are initially enclosed within membranous vesicles to form food vacuoles. food yeast See SINGLE-CELL PROTEIN. foolish seedling disease Syn. BAKANAE DISEASE. foot (mycol.) In fungi of the Laboulbeniales: a dark, basal cell which anchors the thallus to the host organism. (cf. FOOT CELL.) foot and mouth disease (aphthous fever) An acute, highly infectious disease which can affect all cloven-hoofed animals – e.g. cattle, goats, pigs, sheep – and e.g. hedgehogs and rodents; horses are immune, and man is affected only rarely. The causal agent is an APHTHOVIRUS. Infection may occur by ingestion of contaminated food or inhalation of virus-bearing aerosols; the incubation period is usually several days to a week, but may be up to ca. 20 days. Symptoms include fever and the formation of vesicles (fluid-filled blister-like lesions) on the mucous membranes of the mouth and on the feet – particularly where the horny material and skin are contiguous; the copious saliva contains viruses derived from the ruptured vesicles. Vesicles may also appear on the teats, and viruses may also be found in the milk, blood, urine and faeces. In adult animals the disease is usually not fatal; however, it causes serious economic losses in terms of meat and milk production. Control : in countries where outbreaks occur only sporadically, control may be effected by slaughter of infected and suspect animals; vaccination has been used in some cases. [Protection of cattle using a synthetic peptide vaccine: Science (1986) 232 639–641.] A carrier state occurs in cattle, goats and sheep (but not pigs). Disinfection. The virus may persist for months in infected premises, stability being promoted e.g. by low temperatures. The virus may be destroyed e.g. by extremes of pH – citric acid solution or a 1% solution of sodium hydroxide (caustic soda) being effective disinfectants. Other disinfectants which have been used include iodophors (see IODINE) and hypochlorites; phenolics are apparently effective only if used in concentrated solutions (e.g. 1:6, 1:8). Laboratory diagnosis: e.g. culture of the virus (from pharyngeal swabs etc) and/or a CFT. In February, 2001, the first cases of a major outbreak of foot and mouth disease were confirmed in the United Kingdom; during the period February–May the number of reported cases was particularly high in Cumbria, Dumfries and Galloway, County Durham and Devon. (A flare-up in Northumberland occurred in late August.) [Epidemiology (diagram of links between confirmed cases as at March 14th, 2001): VR (2001) 148 322–325.] Government policy involved timely slaughter of both infected and suspect animals, including those on adjacent farms (the ‘contiguous cull’). Large numbers of carcases were burnt or buried, but burial was considered inappropriate for cattle over a certain age owing to the presumed risk of environmental contamination with prions of BOVINE SPONGIFORM ENCEPHALOPATHY. Despite the availability of vaccines, vaccination was resisted; an important factor in this decision was the desire to maintain the UK’s ‘disease-free status’ (thus protecting the export trade). In this outbreak, some veterinary inspections revealed lesions of unknown aetiology which could complicate the diagnosis of foot and mouth disease [photographs of oral lesions in sheep and cattle in Dumfries and Galloway that could complicate the diagnosis of foot and mouth disease: VR (2001) 148 720–723]. Currently, work is ongoing to improve the performance of ELISA-based serological tests that distinguish between (i) antibodies to whole virus particles (such as those in inactivated vaccines), and (ii) antibodies to viral proteins exposed only during replication of the virus; tests that could (reliably) distinguish freezing etc. The preservative effect of fermentation may be due e.g. to (i) the lowering of pH, usually by the formation of lactic or acetic acids (see e.g. PICKLING); (ii) production of antimicrobial substances – e.g. ethanol (as in ALCOHOLIC BEVERAGES), CARBON DIOXIDE, antibiotics (e.g. NISIN, DIPLOCOCCIN in DAIRY PRODUCTS), H2 O2 (see e.g. LACTIC ACID BACTERIA; cf. LACTOPEROXIDASE – THIOCYANATE – HYDROGEN PEROXIDE SYSTEM); (iii) the metabolism of readily utilizable substrates (e.g. simple sugars) which would otherwise be available to potential spoilage organisms or pathogens. (See also FERMENTED FOODS.) (f) Preservatives. The addition of PRESERVATIVES to particular foods is generally subject to governmental control; a preservative permissible in one country may be banned in another. A preservative should ideally be innocuous to the consumer and should not be an allergen; it should be microbicidal, rather than microbistatic, to a range of possible contaminants; it should not be inactivated by constituents of the food or by the products of microbial contaminants. Food preservatives include e.g. BENZOIC ACID; nitrites (see e.g. CURING (1)); PARABENZOATES; SORBIC ACID; SULPHUR DIOXIDE (including e.g. metabisulphite e.g. in SAUSAGES). Antibiotics, particularly those which have a medical use, have limited application in food preservation (cf. NISIN and PIMARICIN). (g) IONIZING RADIATION has been used in some countries e.g. for chicken and for fish fillets, strawberries, onions and spices. Toxicological testing of foods irradiated with doses <10 kGy is regarded as unnecessary, and this may lead to an increase in the use of this form of preservation [Book ref. 50, pp. 145–171]. (h) A PULSED ELECTRIC FIELD technique has been able e.g. to reduce significantly the concentration of viable cells of Mycobacterium paratuberculosis in milk [AEM (2001) 67 2833–2836]. food spoilage Any change in the condition of food which causes it to become less palatable (and sometimes toxic); such changes may involve alterations in taste, smell, appearance or texture. (Foods contaminated with pathogens and/or microbial toxins (see e.g. FOOD POISONING and MYCOTOXINS) do not necessarily exhibit obvious deterioration.) Spoilage organisms include a variety of bacteria and fungi – although particular foods may be susceptible to only a limited range of species; the nature of the spoilage flora is determined e.g. by the types of nutrient present in the food, its WATER ACTIVITY and pH, the conditions of storage (particularly temperature and accessibility of air), and the presence or absence of PRESERVATIVES (see also FOOD PRESERVATION). For examples see BEER SPOILAGE; BREAD SPOILAGE; BUTTER; CANNING; CHEESE SPOILAGE; CIDER; FISH SPOILAGE; MEAT SPOILAGE; MILK SPOILAGE; PICKLING; RANCIDITY; ROPINESS; WINE SPOILAGE. (See also BIODETERIORATION.) food vacuole (gastriole) A closed, membrane-limited intracellular sac which contains particle(s) of food; food vacuoles are formed e.g. by certain protozoa as a consequence of PHAGOCYTOSIS. One or more food vacuoles may be present in a given cell at any given time. Soon after its formation, a food vacuole coalesces with a LYSOSOME, and its contents become acidic and enzyme-rich; the food is digested by lysosomal enzymes (which include e.g. acid phosphatases, nucleases, amylases and esterases) and the digested food appears to enter the cytoplasm by pinocytosis – pinocytotic vesicles developing in the vacuolar membrane. Undigested food is voided when the vacuole subsequently coalesces with the cytoplasmic membrane at the cytoproct. Under certain conditions, e.g. starvation, a cell may digest some of its own internal structures (a process called autophagy); 318 Foraminiferida between infected, vaccinated and ‘carrier’ animals could lead to the possibility of vaccination compatible with ‘disease-free status’ [see Nature (2001) 410 1012]. In man, the disease involves fever, malaise, and the formation of vesicles in the mouth and on the lips, hands and feet; infection may occur via wounds or by ingestion of contaminated dairy products. Complete recovery usually occurs. (cf. HAND, FOOT AND MOUTH DISEASE.) (See also SWINE VESICULAR DISEASE; VESICULAR EXANTHEMA; VESICULAR STOMATITIS.) foot cell (1) (mycol.) A foot-shaped terminal cell in a macroconidium of FUSARIUM. (2) (mycol.) A hyphal cell which gives rise to a conidiophore. (cf. FOOT.) foot-rot (foot rot; footrot) (1) (vet.) In cattle and sheep: a disease involving infection and inflammation of the skin–horn junction and adjacent sensitive areas of the foot, frequently resulting in severe lameness; in sheep the causal agent is Bacteroides nodosus (cf. SCALD), while in cattle the condition may be caused by Fusobacterium necrophorum and/or Bacteroides (or other) species. Foot-rot occurs typically in warm, wet weather. (Some breeds are more susceptible than others. Bos taurus cattle appear to be much more susceptible than Bos indicus (Zebu-type, Brahman) breeds, and the Australasian Merino sheep tend to be more susceptible than British breeds.) Symptoms: the foot is tender and inflamed and has a characteristic necrotic odour, but pus is generally not formed in large amounts. Fever may be present. Treatment: parenteral administration of antibiotics and/or topical application (to the cleaned, prepared foot) of e.g. a 5% w/v CuSO4 solution; foot-baths of e.g. 5% CuSO4 , 5% formalin, or 10% ZnSO4 are useful for control. In pigs, foot-rot is not unlike that found in cattle and sheep, but the condition appears to be more obviously linked to mechanical damage to the feet, and the causal agent(s) appear to include a wider range of species (e.g. F. necrophorum, staphylococci); a dietary deficiency of biotin predisposes towards foot lesions in the pig. Topical treatment and foot-baths can be effective. (2) (plant pathol.) Any of various plant diseases in which the base of the stem rots as a result of microbial infection; causal agents include species of e.g. FUSARIUM, Pythium, Rhizoctonia, Sclerotium, Thielaviopsis, etc. These fungi commonly soften the stem tissue by producing pectinolytic – and possibly cellulolytic – enzymes, causing the plant to collapse. (cf. DAMPING OFF.) footprinting In the context of protein–DNA interactions: any of various methods used to determine the particular sequence of DNA to which a given protein binds; essentially, the binding site of the protein is determined by identifying that region of the DNA molecule in which the presence of bound protein protects (‘shields’) the DNA from cleavage by a nuclease (e.g. DNase I) or by certain chemical agents. In one approach, two sets of homologous, end-labelled DNA fragments – one set carrying the bound protein – are exposed to a particular nuclease, or a chemical agent, under conditions in which, ideally, each fragment is cut at only one of a number of potential cleavage sites. The result is a number of labelled sub-fragments in a range of different sizes – reflecting cleavage at different sites in different fragments. If, in protein-bound fragments, the presence of protein obscures one or more cleavage sites, then no sub-fragment will terminate at such protected site(s). Consequently, if the two sets of sub-fragments are compared by gel electrophoresis, the absence of certain size(s) of sub-fragment from the protein-bound set will be seen as a gap in the gel; this gap, resulting from shielding of corresponding cleavage site(s) by the protein, is the protein’s footprint. The use of DNase I as the DNA-cleavage agent commonly gives a large, clear footprint because it tends not to cut at sites close to the bound protein. The binding site of the protein may be determined with greater resolution by employing chemically mediated cleavage e.g. with in vitro-generated hydroxyl radical; this agent can cut at all unprotected phosphodiester bonds, so that the electrophoretic pattern in this case includes a band for each unshielded position in the sequence – a relatively narrow range of missing bands in the gel indicating a protein-binding site. [Example of DNase I-mediated footprinting to investigate protein binding to the regulatory region of the gal operon in Escherichia coli : NAR (1994) 22 4375–4380.] An entirely unrelated procedure has been termed GENETIC FOOTPRINTING (q.v.). forage poisoning A form of BOTULISM in horses caused by C. botulinum type Cb (cf. MIDLAND CATTLE DISEASE); it may occur in epizootics. foramen (pl. foramina) A perforation or aperture in a structure: see e.g. FORAMINIFERIDA. foramina See FORAMEN. Foraminiferida An order of free-living (mainly marine), testate, amoeboid protozoa (class GRANULORETICULOSEA). The nature of the test (<0.2 mm to >5 mm diam., depending on species) is an important taxonomic criterion. In the suborder Allogromiina (genera: e.g. Allogromia, Iridia, Myxotheca) the test is predominantly organic. In the suborder Fusulinina (e.g. Fusulina, Schwagerina) the test may be composed of microgranular calcite or may consist of two or more differentiated layers. Members of the suborder Miliolina (e.g. Quinqueloculina, Triloculina) form smooth, shiny, porcellanous (porcelain-like) tests which may or may not be perforated. In the suborder Rotaliina (e.g. Ammonia, Elphidium, Rosalina) the test is hyaline and calcareous. Members of the suborder Textularina (e.g. Saccammina, Textularia) form ‘agglutinated’ tests, i.e., tests composed of foreign matter (sand, small shells etc) incorporated into a (commonly calcareous) matrix. [JP (1980) 27 37–58.] In a few foraminifera (e.g. Allogromia) the test is unilocular, i.e., it consists of a single chamber (locule), but in the majority the test is multiloculate, new locules being added sequentially to the first-formed chamber (the proloculus) as the organism grows. The arrangement of locules in multiloculate species may be e.g. rectilinear and uniserial (i.e., arranged in a single row, as e.g. in Dentalina, Nodosaria and Orthocerina), rectilinear and biserial (i.e., in two adjacent rows, as in e.g. Cribrostomum and Textularia), planispiral (i.e., in a flat coil, as in e.g. Ammodiscus and Discorbis) or conical (i.e., in a cone-shaped spiral, as in e.g. Elphidium and Glomospira). Adjacent locules communicate via an opening (the foramen) between them, and the reticulopodium extends from the distal opening (aperture) of the most recently formed locule. In some genera reticulopodia also emerge from perforations in the test, and in some a thin layer of cytoplasm covers the outer surface of the test. The life cycle in foraminifera is unique among protozoa in that it generally involves an ALTERNATION OF GENERATIONS. In e.g. Elphidium spp, meiosis occurs in the mature, multiloculate, diploid organism (agamont) and haploid, amoeboid cells (young gamonts) are liberated; each cell secretes a test and grows to form a mature, multiloculate, haploid gamont which eventually releases numerous biflagellate isogametes. Following fertilization, each zygote (young agamont) secretes a test and grows to form a mature agamont. In e.g. Allogromia the mature gamonts and agamonts are morphologically indistinguishable, while in e.g. Elphidium crispum the proloculus in the agamont test is 319 foreign body giant cell rank in which the taxa are distinguished on a physiological basis – particularly on the basis of adaptation to (or pathogenicity for) one or more specific hosts. In mycology, forma specialis is a taxonomic rank lower than (in ascending order) form, subvariety, VARIETY and SUBSPECIES, and higher than PHYSIOLOGICAL RACE [Book ref. 64, p. 83]. formaldehyde (HCHO; methanal) (a) (as an antimicrobial agent) A colourless, water-soluble and ethanol-soluble, pungent gaseous aldehyde which functions as an ALKYLATING AGENT, substituting amino and other groups (in proteins and nucleic acids) with hydroxymethyl groups (cf. GLUTARALDEHYDE); it can be mutagenic and is a presumed carcinogen. Formaldehyde is used e.g. for FIXATION, as a DISINFECTANT, as a biocide in a lowtemperature STERILIZATION process, and as an inactivating agent in certain INACTIVATED VACCINES. It is active against a wide range of bacteria and fungi and some viruses; activity against bacterial endospores is very slow and can involve reversible inhibition of germination. For maximum antimicrobial activity, gaseous formaldehyde requires a high relative humidity (>60%); the gas has poor powers of penetration and is used e.g. as an indoor surface disinfectant or sterilant (cf. FUMIGATION). An aqueous solution of formaldehyde (formalin) is used e.g. as a preservative for biological specimens and as a general disinfectant; alcoholic solutions have been used e.g. for disinfecting surgical instruments. Antimicrobial activity is inhibited by organic matter. Formaldehyde-releasing agents include e.g. HEXAMINE, NOXYTHIOLIN, PARAFORMALDEHYDE, TAUROLIN. (See also LOWTEMPERATURE STEAM–FORMALDEHYDE STERILIZATION.) (b) (as a metabolite) See METHYLOTROPHY. formalin See FORMALDEHYDE. formalin–ether method (formol–ether method) A method for concentrating protozoan cysts (and e.g. parasite larvae) in a sample of faeces. The sample (e.g. 1–2 g faeces) is emulsified in ca. 10 ml physiological saline containing 10% formalin, and left for 30 min; the suspension is then filtered through a double layer of wet gauze and the filtrate is centrifuged (ca. 350 g/2–3 min). The supernatant is discarded, and the sediment is shaken in a stoppered tube with 10 ml formalin–saline and 3 ml diethyl ether. The tube is then centrifuged (ca. 350 g/2–3 min). Cysts, if present, are found in the sediment. This method is unsuitable for the concentration of coccidian cysts if the latter are subsequently to be sporulated. (cf. FLOTATION.) formate hydrogen lyase An enzyme system which splits formic acid into carbon dioxide and hydrogen (see e.g. MIXED ACID FERMENTATION and BUTANEDIOL FERMENTATION). The system includes a formate dehydrogenase (which eliminates CO2 from formic acid) and a HYDROGENASE; electrons derived from the first reaction appear to be transferred to hydrogenase via one or more electron carriers. Anaerobically, Escherichia coli (and certain other bacteria) can oxidize formate using nitrate or fumarate as terminal electron acceptor; a different formate dehydrogenase is involved, and hydrogen is not evolved. N-formimidoylthienamycin See THIENAMYCIN. formol–ether method See FORMALIN–ETHER METHOD. Formvar Polyvinyl formal: a substance, soluble in e.g. chloroform, used in the preparation of support films for ELECTRON MICROSCOPY. Forssman antigen (1) Any HETEROPHIL ANTIGEN. (2) Any heterophil antigen which can elicit antibodies capable of combining with determinant(s) on sheep RBCs. (The first-described Forssman antigen is an antigen found e.g. on sheep RBCs and on certain guinea-pig cells.) smaller than that in the gamont test. Variations may occur in this generalized life cycle: e.g., in Glabratella sulcata the gametes are exchanged directly between two gamonts in contact, rather than being released into the sea. Most foraminifera are marine benthic organisms, although some (e.g. GLOBIGERINA, GLOBIGERINOIDES) are pelagic (planktonic), and a few (e.g. Nonion germanicum) occur in brackish waters. The organisms are typically holozoic and omnivorous, feeding on bacteria, small protozoa, microalgae, etc. [Invasive activity of Allogromia pseudopodial networks: JP (1985) 32 9–12.] Photosynthetic endosymbionts occur in many foraminifera, including most or all of the large forms (e.g. Amphistegina) which occur – often abundantly – in shallow tropical and subtropical seas; depending on species, these endosymbionts may be dinoflagellates (e.g. Amphidinium, ‘Symbiodinium’), green algae (e.g. Chlamydomonas, Chlorella), frustule-less diatoms (which can reconstitute their frustules in culture), or a unicellular red alga (Porphyridium?). The endosymbionts seem able to satisfy at least a significant proportion of the carbon and energy requirements of the protozoon, but ingestion of prey is apparently necessary to supply adequate levels of e.g. nitrogen and phosphorus [e.g. in Globigerinoides sacculifer : Limn. Ocean. (1985) 30 1253–1267]. In Nonion germanicum chloroplasts from ingested algae appear to survive digestion for long enough to supply the protozoon with some photosynthate. [Symbiosis: Book ref. 129, pp. 37–68; potential symbionts in bathyal species: Science (2003) 299 861.] Fossil foraminiferan tests occur abundantly in certain oceanic deposits and sedimentary rocks, sometimes being a major component of thick deposits of chalk (such as the white cliffs of Dover and Normandy). (See also COCCOLITH.) Numerous genera and species of fossil foraminifera have been documented, the earliest dating from the late Precambrian to early Cambrian (primitive globular tests with a single aperture). Fossil tests are commonly ca. 0.2–2.0 mm in diam., but some ‘fusulinids’ (Carboniferous to Permian) and ‘nummilitids’ (Eocene to Oligocene) are much larger: e.g. the discoid multiloculate tests of Nummilites gizehensis may be >120 mm. A rapid succession of distinctive species of fossil foraminifera has occurred through the geological ages, making them ideal markers in the biostratigraphy of sedimentary rocks e.g. for oil prospecting. [Stratigraphy of planktonic foraminifera: Book ref. 136, pp. 11–328.] foreign body giant cell See GIANT CELL. forespore See ENDOSPORE (sense 1 (a)). forest yaws Syn. PIAN BOIS. form genus (mycol.) A non-phylogenetic category, equivalent to ‘genus’, which is distinguished on the basis of one or more (typically) morphological features. Form species within a form genus are referred to by Latin binomials (see NOMENCLATURE). (The prefix ‘form’ – although taxonomically correct – is frequently omitted.) In the DEUTEROMYCOTINA, form genera are used to classify e.g. those fungi which have no known sexual stage; such form genera are based primarily on the characteristics (including mode of development) of the conidia. If the sexual stage of a form species is discovered, the name of the form species – based on the anamorph – can continue to be used, but the taxonomically valid name is that of the teleomorph. Form genera are also used for certain members of the Aphyllophorales: see e.g. FOMES, POLYPORUS and PORIA. form species See FORM GENUS. forma specialis (special form; f. sp., sometimes written f.; plural : formae speciales, ff. sp.) An intraspecific taxonomic 320 Francisella Fort Bragg fever (pretibial fever) Human LEPTOSPIROSIS, caused by Leptospira interrogans serotype autumnalis, which is characterized by a pretibial rash, malaise, coryza and fever; it occurs in SE Asia, Japan, and the USA. Fort Morgan virus See ALPHAVIRUS. fortimicin An AMINOGLYCOSIDE ANTIBIOTIC which contains neither streptidine nor deoxystreptamine. forward mutation A MUTATION which results in a mutant phenotype, i.e., a phenotype which differs from that of the WILD-TYPE organism; a forward mutation typically involves the inactivation of e.g. a structural gene. (cf. BACK MUTATION.) forward primer In the context of a given gene whose nucleotides are numbered: a primer whose sequence of nucleotides corresponds to a sequence within the gene which reads in a numerically ascending order in the 5
-to-3
direction; thus, a forward primer may have a sequence which corresponds to e.g. nucleotides 10 → 25 (5
-to-3
) of a given gene. The nucleotides of a reverse primer read in a numerically descending order in the 5
-to-3
direction – e.g. nucleotides 350 → 335 in the other strand. fos See FBJ OSTEOSARCOMA VIRUS. Foscarnet sodium See PHOSPHONOFORMIC ACID. fosfomycin (phosphonomycin; cis-1,2-epoxypropyl-1-phosphonic acid) A broad-spectrum ANTIBIOTIC (obtained from Streptomyces strains) which blocks PEPTIDOGLYCAN biosynthesis by binding (irreversibly) and inhibiting phosphoenolpyruvate: UDPGlcNAc enolpyruvyl transferase – thus preventing the synthesis of N-acetylmuramic acid; other reactions involving PEP are apparently not inhibited. Fosfomycin is taken up by the glycerolphosphate transport system in susceptible cells, and resistance to the drug develops readily by alteration or loss of this transport system; plasmid-borne resistance involving intracellular modification of the drug also occurs [JGM (1985) 131 1649–1655]. Fosfonet sodium See PHOSPHONOACETIC ACID. Foshay test A SKIN TEST in which the antigen is a suspension of (or a protein extract from) killed cells of Francisella tularensis. Foshay’s vaccine An anti-TULARAEMIA vaccine prepared from killed cells of Francisella tularensis. fossil microorganisms See e.g. COCCOLITH; CYANOBACTERIA; DASYCLADALEAN ALGAE; FORAMINIFERIDA; GYROGONITES; RADIOLARIA; STROMATOLITES. (See also BIOSTRATIGRAPHY.) [Microfossils of the Proterozoic/Phanerozoic periods in China: Nature (1985) 315 655–658.] foulbrood See AMERICAN FOULBROOD and EUROPEAN FOULBROOD. Fourier transform See NUCLEAR MAGNETIC RESONANCE. foveate Pitted with small holes or depressions. foveolate Minutely pitted. fowl adenovirus See AVIADENOVIRUS. fowl cholera (avian pasteurellosis) A POULTRY DISEASE (affecting e.g. chickens, turkeys, ducks, game birds) caused by Pasteurella multocida. Symptoms of the acute form of the disease may include anorexia, bloody diarrhoea, discharge from nostrils, eyes and mouth, and haemorrhages of internal organs and mucous membranes; death may occur within hours or days. In the chronic form the disease mainly affects the respiratory system. fowl paralysis Syn. MAREK’S DISEASE. fowl pest (1) Syn. FOWL PLAGUE. (2) Syn. NEWCASTLE DISEASE. fowl plague A POULTRY DISEASE caused by strains of influenzavirus A. Symptoms resemble those of NEWCASTLE DISEASE; mortality rates may be high. Wild birds may be an important source of infection. fowl pox A POULTRY DISEASE caused by an AVIPOXVIRUS. Infection occurs via wounds (particularly in unfeathered regions such as the comb, wattles etc) or mucous membranes. Greyish-white or yellow eruptions form and may develop into large wartlike growths, and/or yellowish, caseating vesicles develop in the throat, forming a characteristic diphtheritic membrane. Live attenuated vaccines are available. fowl typhoid An acute infectious POULTRY DISEASE caused by Salmonella gallinarum; it affects mainly adult chickens and turkeys. Symptoms: e.g. lethargy, anorexia, yellow or greenish diarrhoea; the liver is enlarged and has a typical bronzed appearance post-mortem. Death may occur within days, or the disease may become chronic with severe, progressive emaciation and anaemia. Survivors become chronic carriers. Transmission occurs via the faecal–oral route or transovarially. Wild birds (e.g. rooks, pigeons) may provide a reservoir of infection. Lab. diagnosis and treatment: as for PULLORUM DISEASE; antisera which are positive for S. pullorum cross-react with S. gallinarum. fowlpox subgroup See AVIPOXVIRUS. foxfire BIOLUMINESCENCE in decaying wood. FP-cellulose ‘Filter-paper cellulose’; filter paper may be used as a substrate for determining the ability of an organism or enzyme to degrade crystalline CELLULOSE. FP2 plasmid See INCOMPATIBILITY. fps See FES. fractalkine See CHEMOKINES. Fraction 1 antigen See YERSINIA (Y. pestis). fraction I protein See CHLOROPLAST. fractional sterilization Syn. TYNDALLIZATION. fractionation See CELL FRACTIONATION. Fragilaria See DIATOMS. fragilis yeast See SINGLE-CELL PROTEIN. fragmin See ACTIN. framboesia (frambesia) Syn. YAWS. frameshift mutation (phase-shift mutation) A type of MUTATION in which a number of nucleotides not divisible by three is inserted into or deleted from a coding sequence, causing an alteration in the reading frame of the entire sequence downstream of the mutation. For example, if a wild-type gene containing a sequence UUG-GAG-UGU-AGU (encoding Leu-Glu-Cys-Ser: see GENETIC CODE) suffers an insertion mutation in the UUG codon which generates the sequence UAUG (a +1 frameshift), the reading frame will be shifted backwards by one nucleotide, and the sequence will read UAU-GGA-GUG-UAG-U (encoding Tyr-Gly-Val-stop, UAG being a nonsense codon). Similarly, deletion of one nucleotide (a −1 frameshift) will shift the reading frame forwards: e.g., if the second U is deleted, the above sequence becomes UGG-AGU-GUA-GU, encoding Trp–Ser–Val. A −1 frameshift may suppress the effects of a +1 frameshift: see SUPPRESSOR MUTATION. Frameshift mutations may occur spontaneously (see SPONTANEOUS MUTATION) and are induced by certain types of mutagen (e.g. certain INTERCALATING AGENTS). framycetin An AMINOGLYCOSIDE ANTIBIOTIC produced by a strain of Streptomyces lavendulae; it may be identical to one form of neomycin. Francisella A genus (incertae sedis) of aerobic, chemoorganotrophic, Gram-negative bacteria which occur e.g. as parasites and pathogens in man and other animals. Cells (when actively growing): non-motile cocci, coccobacilli or rods, according to species and growth conditions; subsequently filamentous and pleomorphic. The cells stain poorly and often bipolarly; a bipolarly stained rod may give the false impression of two cocci. Metabolism: respiratory (oxidative). One species (F. tularensis) 321 frange in a cryoprotectant (e.g. 20% skim milk) to a cell density of ca. 108 cells/ml. An aliquot of the suspension (ca. 0.1 ml) is put into a sterile VIAL. (This account assumes the use of a vial 10 cm long, 6 mm internal diameter.) The COTTON WOOL plug is replaced and pressed into the vial to a position ca. 2–3 cm above the suspension. With precautions to avoid heating the suspension, a region of the vial 2–3 cm from its open end is heated and drawn out into a capillary neck. The suspension is then frozen (see FREEZING) at −60 to −80° C, e.g. by placing the closed end of the vial in a mixture of solid CO2 (‘dry ice’) and ethanol. The vial is transferred to a freeze-drying machine (where the low temperature is maintained) and subjected to a high vacuum for a period of 18 hours to several days, depending on temperature. The vial is removed, fitted to a vacuum line, and sealed by melting the capillary neck. Vials are best stored at 5° C or below. (For many species, viability is retained for 10–30 years or longer.) When a culture is required, a vial is opened and the contents reconstituted with broth or distilled water. A sealed vial can be opened (preferably in a SAFETY CABINET) by ‘nicking’ the glass over the centre of the cotton wool plug and momentarily applying a red-hot glass rod to the nick; the vial (usually) cracks around its circumference, permitting the slow ingress of air – which is filtered through the cotton wool. In the double-vial method, the material is freeze-dried in a vial which is sealed only with a cotton wool plug; this vial is placed in an outer vial, containing a desiccant, which is evacuated and sealed by fusion of the glass. freeze-etching A technique used for examining the topography of surface(s) exposed by the fracture (cleavage) of e.g. a deepfrozen cell. The specimen is frozen rapidly (see FREEZING). While at ca. −100 to −196° C (depending on apparatus), the specimen is fractured: it may be split with a knife-blade in a microtome-based apparatus, or, in one form of the ‘snapped tube’ apparatus, a capillary tube containing a homogenate is deepfrozen and snapped at a pre-scored position. During fracture, part of the cleavage generally follows the boundaries of internal structures (e.g. organelles), and membranes may be split to expose their hydrophobic faces. Surfaces exposed by cleavage are then ‘etched’ by sublimation of volatiles (mainly water) from the surfaces at a temperature of ca. −100° C under high vacuum; organelles and non-volatiles are thus thrown into relief. A thin replica of the etched surface is then prepared (see ELECTRON MICROSCOPY (a)) and examined under the TEM. [Book ref. 4, pp. 279–341. For nomenclature of freeze-etched surfaces see Science (1975) 190 54–56. For potential artefacts see JUR (1983) 82 123–133.] freeze-fracturing The process of FREEZE-ETCHING omitting the etching step. freeze pressing The operation of a HUGHES PRESS. freeze-sectioning (cryosectioning) The preparation of sections, for MICROSCOPY or ELECTRON MICROSCOPY, by cutting a FROZENHYDRATED SPECIMEN with a cryotome (a modified MICROTOME which maintains the specimen in a frozen state). freeze-substitution A procedure for preparing a specimen e.g. for X-RAY SPECTROCHEMICAL ANALYSIS. Essentially, the specimen is rapidly frozen and then immersed for e.g. 14 days in anhydrous ethanol or acetone at e.g. −70° C; during this time the ice is gradually replaced by ethanol or acetone. This method helps to maintain the positions of e.g. water-soluble ions within the specimen – thus facilitating subsequent determination of the in vivo locations of these ions. freezing Sub-zero (° C) freezing is used for the preservation (cryopreservation) of certain microorganisms, foods etc, and is nutritionally fastidious (requiring e.g. cysteine for growth), while the other (F. novicida) can grow e.g. on nutrient gelatin. Carbohydrates are metabolized slowly and anaerogenically. Optimum growth temperature: ca. 37° C. Oxidase −ve. Catalase test: weakly positive. GC%: ca. 33–36. Type species: F. tularensis. F. novicida. Cells: non-capsulated, ca. 0.2–0.3 × 0.3 µm in tissues, 0.5 × 0.5–0.9 µm on solid media, 0.7 × 1.7 µm in liquid media. Unlike F. tularensis, F. novicida does not require cysteine for growth. F. novicida is apparently not pathogenic for man but can produce tularaemia-like lesions, experimentally, in e.g. guinea pigs and white mice. F. tularensis (formerly Pasteurella tularensis). The causal agent of TULARAEMIA. Cells: ca. 0.2 × 0.2–0.7 µm; virulent strains have a non-immunogenic capsule, ca. 0.02–0.04 µm in thickness. F. tularensis can be grown e.g. on glucose–cysteine–blood agar; colonies (2–4 days, 37° C) are 1–4 mm diam., typically smooth, grey, easily emulsifiable, each being surrounded by a zone of greenish discoloration. F. tularensis is generally sensitive to e.g. kanamycin, streptomycin and tetracyclines. [Book ref. 22, pp. 394–399.] frange A band of perioral ciliature characteristic of some members of the HYPOSTOMATIA. Frankia A genus of bacteria (order ACTINOMYCETALES, wall type III) which form ACTINORRHIZAE but which can also occur as freeliving soil saprotrophs that appear not to fix nitrogen. (At least some of the nif genes have been found to be located on a large plasmid in one strain of Frankia [MGG (1986) 204 492–495].) The organisms form branching, septate hyphae, and give rise to non-motile spores in sporangia which develop on the hyphae. GC%: ca. 68–72. Type species: F. alni. [Media and methods: Book ref. 46, pp. 1991–2003.] Frateuria A genus of Gram-negative, obligately aerobic, chemoorganotrophic bacteria (family PSEUDOMONADACEAE) which occur e.g. on the fruit of the raspberry (Rubus parvifolius). Cells: polarly flagellate or non-motile rods, 0.5–0.7 × 0.7–3.5 µm. Round, entire, dark brown colonies are formed on glucose–yeast extract–CaCO3 agar; optimum growth temperature: 25–30° C. Growth can occur at pH 3.6. Acid is formed from various carbon sources, including ethanol. Catalase +ve. Oxidase −ve. GC%: 62–64. Type species: F. aurantia. [Book ref. 22, pp. 210–213.] Frazier’s medium An agar medium containing 0.4% GELATIN; it is used to detect gelatin hydrolysis. Following culture of the test organism on the medium, the plate is flooded with a solution of mercuric chloride in dilute HCl; unhydrolysed gelatin rapidly becomes opaque – clear zones indicating gelatin hydrolysis. frd genes See FUMARATE RESPIRATION. free cell formation See ASCUS. free electrophoresis See ELECTROPHORESIS. free induction decay (FID) See NUCLEAR MAGNETIC RESONANCE. free-living Living without being directly dependent on other organisms. (cf. SYMBIOSIS.) free residuals See WATER SUPPLIES. freeze-cleaving Syn. FREEZE-FRACTURING. freeze-drying (lyophilization) A process used e.g. for the longterm preservation of certain microorganisms, certain foodstuffs etc; in this process volatile substances (mainly water) are removed from the deep-frozen specimen or material by sublimation under high vacuum (cf. CRITICAL POINT DRYING). Freeze-drying of bacterial cultures. A fresh broth culture of e.g. 108 cells/ml is gently centrifuged and the cells resuspended 322 Friend virus e.g. in FREEZE-DRYING and FREEZE-ETCHING vials, are then transferred to the ‘freezing chamber’ of a liquid nitrogen apparatus for 20–30 min, and subsequently placed in liquid nitrogen. Thawing should be rapid to preserve viability [Nature (1980) 286 511–514]. Vials in liquid nitrogen may be equilibrated in solid CO2 (−78° C) before being transferred to a 37° C waterbath. When working with liquid nitrogen, thawing vials etc, it is usual to wear a protective face-shield and gloves. (b) See FOOD PRESERVATION (b). Frei test A SKIN TEST, analogous to the TUBERCULIN TEST, used in the diagnosis of LYMPHOGRANULOMA VENEREUM; it involves the intradermal injection of antigen derived from a chick embryo culture of Chlamydia trachomatis. Fremyella See RIVULARIACEAE. French moult See BUDGERIGAR FLEDGLING DISEASE. French press (French pressure cell) An apparatus for CELL DISRUPTION. A suspension or paste of cells is put into a cooled steel cylinder, and a piston is forced into the cylinder by hydraulic pressure (ca. 10 tons/inch2 , i.e., ca. 160 MPa). Cells emerge from a small hole in the cylinder and are broken by the high shear forces during their exit; they tend to explode owing to the sudden drop in pressure. The process readily breaks most Gramnegative bacteria but not Gram-positive cocci or endospores. Frenching disease See COTTON WILT. Frenkelia A genus of coccidian protozoa (suborder EIMERIORINA) parasitic in rodents (intermediate hosts) and birds (final hosts); the organisms closely resemble species of SARCOCYSTIS. Freon (trade name) Any of a range of halogenated hydrocarbons. For example, Freon 22 (= halocarbon 22; CHClF2 ) melts at −160° C and boils at −41° C; it is used e.g. as a CRYOGEN. Freon 113 (= halocarbon 113; CCl2 F.CClF2 ) melts at −35° C and boils at +48° C; it is used e.g. as an intermediate fluid in CRITICAL POINT DRYING. FRET See PCR. Freund’s adjuvant Freund’s incomplete ADJUVANT is a waterin-oil emulsion made with a mineral oil. Freund’s complete adjuvant consists of the incomplete adjuvant together with dead mycobacteria dispersed in the oil phase; in a modified form, MDP (q.v.) is used instead of the mycobacteria. In both types of adjuvant the antigen is dispersed in the water phase. (cf. ADJUVANT 65.) fried egg colony See MYCOPLASMA. Friedländer’s bacillus A species of Klebsiella isolated by Friedländer (in 1883) from the lungs of postmortem cases of PNEUMONIA. Friedländer’s pneumonia See PNEUMONIA (d). Friedmannia A genus of sarcinoid, soil-inhabiting green algae related to TREBOUXIA. Friend virus (FV) A virus complex consisting of a mixture of a replication-competent MURINE LEUKAEMIA VIRUS (F-MuLV or Fr-MuLV) and a replication-defective component designated ‘spleen focus-forming virus’ (SFFV) because of its ability to induce macroscopic foci of erythroid cell proliferation in the spleen of an infected mouse. (FV does not transform cells in vitro.) The F-MuLV component alone can induce erythroleukaemia only in newborn mice of certain strains, or in adult mice after a long latent period; the F-MuLV-SFFV complex can cause disease in both adult and neonatal mice of most inbred strains, causing anaemia (variant FV-A, containing SFFVA ) or polycythaemia (variant FV-P, containing SFFVP ) after a short latent period. Both SFFV variants are believed to have arisen by recombination between F-MuLV and an endogenous xenotropic techniques (see also CRYOFIXATION). (a) Cryopreservation of living cells. Cryopreservation may be used to preserve viability in many species of bacteria, fungi and protozoa, many viruses, and tissue cultures; this method of preservation (or freeze-drying) is generally preferable, when practicable, to preservation by repeated subculture (see STABILATE). However, unless precautions are taken, cells may be killed or (reversibly) damaged when they are frozen. Damage or death may be due to one (or both) of two main causes. First, the formation of intracellular and extracellular ice crystals may result in e.g. mechanical or osmotic damage. (Even after water freezes, ice crystals continue to grow at temperatures above a certain critical temperature; the temperature range between the freezing point and the critical temperature is called the critical temperature zone.) Secondly, as the temperature continues to fall below 0° C, the ongoing formation of crystals of pure ice results in the development of concentrated intracellular solutions of salts as the eutectic temperature is approached; these concentrated solutions are believed to damage certain cell components (e.g. enzymes). The continued presence of these solutions at temperatures found in certain types of commercial freezer (e.g. −10 to −20° C) precludes the use of such equipment for effective microbial preservation. Cell damage can be minimized if, prior to freezing, the cells are equilibrated with a cryoprotectant: an antifreeze which, by depressing the freezing point, reduces the time that cells are subjected to temperatures in the critical temperature zone. Cryoprotectants may be used in both rapid and slow freezing (see below); they include e.g. glycerol (widely used e.g. for bacteria and yeasts) and DIMETHYLSULPHOXIDE (DMSO) (used e.g. for protozoa and tissue cultures). These agents are used e.g. at 10% v/v in growth medium; they can penetrate cell membranes and permeate the cytoplasm. Other cryoprotectants (e.g. skim milk, dextran, polyvinylpyrrolidone) appear to function extracellularly. Rapid freezing (e.g. ca. 100 to 1000 centigrade degrees per second) tends to prevent ice crystal formation by encouraging vitrification, i.e. the non-crystalline (amorphous) solidification of water; it is commonly used to preserve viability in eukaryotes (e.g. Saccharomyces cerevisiae). The use of a cryoprotectant encourages vitrification by compressing the critical temperature zone. The specimen (e.g. one drop of suspension) may be initially quench-frozen in e.g. FREON 22 at ca. −150° C and then transferred to liquid nitrogen (−196° C). Liquid nitrogen may be used as the sole cryogen (despite the LEIDENFROST PHENOMENON) if the specimen is frozen in subcooled nitrogen (at ca. −200 to −210° C); heat transferred from the specimen raises the temperature of the nitrogen – but not as far as its boiling point. [Instruments which facilitate rapid freezing: JM (1982) 126 221–229; relative efficiency of cryogens used for plunge-cooling biological specimens: JM (1987) 145 89–96.] Slow freezing is often used e.g. for bacteria; it may permit extracellular ice formation and intracellular dehydration by osmosis. A cell suspension is equilibrated with a cryoprotectant, and aliquots are placed in vials; vials containing non-pathogens are usually sealed. Freezing may be carried out at a rate of ca. one degree/min down to ca. −30° C, then at 30 degrees/min down to −150° C; vials are then stored in liquid nitrogen or in the vapour phase above liquid nitrogen (ca. −150° C). Tissue culture cells may be initially equilibrated for 1 hour at 4° C in a mixture of growth medium, serum and ca. 10% v/v DMSO to permit penetration of DMSO. The cells, in sealed 323 Frings Acetator fruit spoilage See e.g. CANNING; GREY MOULD; PECTIC ENZYMES; PICKLING; SOFT ROT (2). fruiting body (fructification; fruit body; sporocarp) Any specialized structure (plectenchymatous in fungi) which bears or contains sexually- or asexually-derived spores. Examples include e.g. APOTHECIUM, PYCNIDIUM, pycnium and the resting structures of the MYXOBACTERALES. Some fruiting bodies (e.g. those formed by species of Heterobasidion and Poria) are perennial. frustule See DIATOMS. fruticose Shrub-like. Of a LICHEN: having a thallus which is thread-like (terete) or strap-like (more or less flattened) and either erect and shrubby or pendulous; the thallus may be attached to the substratum by a holdfast or may be unattached. The mechanical strength of the thallus may derive from a dense axial strand (in USNEA) or from the cortex; thalli of the latter type may or may not be hollow. (cf. CRUSTOSE; FOLIOSE.) FTA-ABS test (fluorescent treponemal antibody-absorption test) A TREPONEMAL TEST used in the diagnosis of SYPHILIS. Initially, the patient’s serum is absorbed with an extract of Reiter treponemes to remove non-specific anti-treponemal antibodies, and the absorbed serum is applied to a slide on which there is a film of killed, fixed cells of Treponema pallidum; the slide is then briefly incubated to allow any specific antibodies in the serum to combine with the surface antigens of T. pallidum. The slide is rinsed free of serum and briefly re-incubated with fluorescent conjugate (anti-human globulin conjugated with a fluorescent dye); uncombined conjugate is rinsed off and the slide is examined by fluorescence MICROSCOPY. Antibodies which combine with T. pallidum combine also with the conjugate – their presence being indicated by fluorescent treponemes. FTO agar A selective medium used for the culture of Micrococcus spp. It consists of trypticase–soy agar containing 0.1% yeast extract, 0.5% Tween 80, and small amounts of furoxone and Oil Red O. [Preparation and use: Book ref. 46, p. 1542.] ftsA gene (Caulobacter) See CAULOBACTER. FtsH An ATP- and zinc-dependent protease within the cytoplasmic membrane of Escherichia coli and many other bacteria; it is encoded by gene ftsH (also called hflB and tolZ ). In E. coli, FtsH is involved e.g. in the degradation of certain membrane and regulatory proteins (including the HEAT-SHOCK PROTEIN s32 ), and it may also act as a chaperone. [FtsH (review): FEMS Reviews (1999) 23 1–11.] ftsI gene See PENICILLIN-BINDING PROTEINS. ftsQ gene (Caulobacter) See CAULOBACTER. FtsY protein See PROTEIN SECRETION (type II systems). ftsZ gene See SOS SYSTEM. FtsZ protein See CELL CYCLE (b) and SOS SYSTEM. 5-FU 5-Fluorouracil (see e.g. PHENOTYPIC SUPPRESSION). fuberidazole (2-(2
-furyl)-benzimidazole) An agricultural antifungal agent (see BENZIMIDAZOLES); it is particularly useful as a seed treatment for the control of Fusarium infections (e.g. F. nivale in cereals). Fucales See FUCUS and PHAEOPHYTA. fucan A polymer of FUCOSE. (See also FUCOIDIN.) fuchsin (fuchsine) Basic fuchsin is a red-purple DYE consisting mainly of pararosanilin and rosanilin (see TRIPHENYLMETHANE DYES). (See also CARBOLFUCHSIN and BRUCELLA.) Acid fuchsin, derived from basic fuchsin by sulphonation, is used e.g. in ANDRADE’S INDICATOR. fucidin See FUSIDIC ACID. fucoidin (fucoidan; fucan) A group of complex, branched, sulphated heteropolysaccharides present in phaeophycean algae; fucoidins contain fucose (usually sulphated), xylose, glucuronic virus sequence; the glycoprotein product of the recombinant env gene has been implicated in the enhanced oncogenicity of FV as compared with that of F-MuLV alone. (cf. MCF VIRUSES; RAUSCHER VIRUS.) Frings Acetator See VINEGAR. Frings trickling generator See VINEGAR. Fritschiella See CHAETOPHORA. frog virus 3 (FV3) A virus (genus RANAVIRUS) originally isolated from a leopard frog (Rana pipiens); it is apparently nonpathogenic in adult leopard frogs, but is lethal for tadpoles and for Fowler toads. FV3 can replicate in a wide range of cells in culture, including amphibian, avian, mammalian and piscine cells. Growth temperature: ca. 12–32° C. The virion (ca. 160–200 nm diam.) apparently contains ca. 29 structural proteins and at least six enzymic activities. An envelope may or may not be present; enveloped virions, which are more infective than naked virions, enter the host cell by endocytosis (see ENVELOPE), while the naked virions usually enter by fusion between the virion shell and the host cell plasmalemma [JGV (1985) 66 283–293]. Initial rounds of viral DNA replication occur in the host cell nucleus, resulting in DNA molecules up to twice the length of the viral genome. The DNA is then transferred to the cytoplasm where subsequent rounds of replication occur; concatemers >10 times the genome length are formed – apparently by recombination (as in e.g. BACTERIOPHAGE T4). The virion DNA and cytoplasmic viral DNA – but not the viral DNA synthesized in the nucleus – is highly methylated, >20% of the cytosine residues being methylated at the C-5 position; methylation is carried out in the cytoplasm by a viral early-gene product (a methyltransferase), and may protect the DNA from virus-encoded endonucleases. Virus assembly occurs in morphologically distinct regions of the cytoplasm delimited by aggregated host-cell INTERMEDIATE FILAMENTS [JGV (1986) 67 915–922]. [Review: AVR (1985) 30 1–19.] frond twist disease See ALGAL DISEASES. front focal plane Of a convex lens: the focal plane nearest the light source. Frontonia See HYMENOSTOMATIDA. frost damage (in plants) See e.g. ICE NUCLEATION BACTERIA. frosty pod (of cacao) See POD ROT. frozen-hydrated specimen A specimen which has been frozen hard without prior dehydration – as e.g. in FREEZE-SECTIONING. fructan (fructosan) A polysaccharide composed mainly or solely of fructosyl residues (see e.g. INULIN and LEVAN). Fructans are synthesized by the addition of fructose residues to SUCROSE, and hence each fructan chain ends in a (non-reducing) sucrose residue. Fructans can be hydrolysed by a wide range of microorganisms. fructification (1) Syn. FRUITING BODY. (2) The formation or development of a fruiting body. fructosan Syn. FRUCTAN. fructose (‘fruit sugar’) A ketohexose found e.g. in plants (e.g. in sweet fruits), in honey (with glucose and sucrose), and as a component of many oligo- and polysaccharides (e.g. MELEZITOSE, RAFFINOSE, SUCROSE; see also FRUCTAN). D-Fructose (‘laevulose’) can be metabolized by a wide range of microorganisms. Fructose is sweeter than sucrose and is being used on an increasing scale as a sweetener in foods (see e.g. HIGH FRUCTOSE CORN SYRUP). (See also INULINASE.) fructose bisphosphatase See GLUCONEOGENESIS. b-fructosidase See INVERTASE and INULINASE. fruit (verb) To produce a FRUITING BODY. 324 fungi under aerobic conditions and repressed under anaerobic conditions. Fumarate reductase is bound to the cytoplasmic face of the cytoplasmic membrane [JGM (1984) 130 2851–2855]; fumarate respiration thus requires a fumarate transport system. Fumarate reductase in E. coli consists of an FAD-containing protein subunit (frdA gene product), an iron–sulphur protein subunit (frdB gene product), and two small, hydrophobic ‘anchor proteins’ (frdC and frdD gene products) which anchor the enzyme in the membrane; these four cistrons occur in an operon with a promoter–operator region and a transcriptional terminator. The energy yield of fumarate respiration in E. coli is uncertain, but has been calculated to be ca. 0.66 ATP/fumarate [MR (1984) 48 222–271]. In Wolinella succinogenes, fumarate is used as an electron acceptor for the oxidation of formate (HCOO− + H+ + fumarate → succinate + CO2 ) or H2 , yielding, by oxidative phosphorylation, 1–2 ATP per fumarate reduced. fumigant See FUMIGATION. fumigation The exposure of an enclosed space (e.g. a horticultural glasshouse, a hospital room) to a gaseous or vapour-phase DISINFECTANT or STERILANT. Control of humidity is commonly important for effective fumigation. Agents used for fumigation (fumigants) include e.g. DAZOMET, ETHYLENE OXIDE, FORMALDEHYDE, SULPHUR DIOXIDE. fumitremorgin A tremorgenic MYCOTOXIN produced e.g. by Aspergillus fumigatus and Penicillium piscarium. functional immunity Syn. IMMUNITY (3). fungaemia (fungemia) The presence of fungi in the bloodstream. fungemia See FUNGAEMIA. fungi (sing. fungus) Unicellular, multicellular or coenocytic, heterotrophic, eukaryotic MICROORGANISMS which do not contain chlorophyll (cf. ALGAE) and which characteristically form a rigid CELL WALL containing CHITIN and/or CELLULOSE (cf. PROTOZOA). In some taxonomic schemes [see e.g. Book ref. 64] the fungi are divided into two divisions: EUMYCOTA (‘true fungi’) and MYXOMYCOTA; since members of the Myxomycota are commonly regarded as protozoa, the following account refers specifically to the ‘true fungi’. In most fungi the vegetative form (thallus) consists of hyphae (see HYPHA), while in e.g. many YEASTS the thallus is predominantly or exclusively unicellular (cf. DIMORPHIC FUNGI). (See also GROWTH (fungal).) The majority of fungi are nonmotile; however, flagellated reproductive/disseminative forms are produced by many of the LOWER FUNGI. (Phylogenetically, motility is regarded as a ‘primitive’ feature among the fungi.) Most fungi can reproduce asexually – often by the formation of conidia (see CONIDIUM). In sexual reproduction, interaction between individuals may involve e.g. anisogamy, GAMETANGIAL CONTACT, isogamy, oogamy (see MONOBLEPHARIDALES), or SOMATOGAMY. (See also PARASEXUAL PROCESSES.) Specialized asexual and/or sexual SPORE-bearing structures (see e.g. FRUITING BODY) are formed by most fungi; many are macroscopic, and some (e.g. the mushrooms and puffballs) may be several centimetres or more in size. Fungi are osmotrophic chemoheterotrophs which, collectively, utilize substrates ranging from simple sugars to e.g. cellulose (see CELLULASES), HYDROCARBONS, LIGNIN, pectins (see PECTIC ENZYMES) and xylans (see XYLANASES). Energy-yielding metabolism may involve RESPIRATION and/or FERMENTATION. Although fungi are typically aerobic organisms, some are facultative or obligate anaerobes (see e.g. RUMEN). The fungi are widespread in nature; most species are terrestrial, but some (see e.g. NIA, OOMYCETES, SPATHULOSPORALES) are acid, and – in some species – galactose and/or mannose, and they may be linked to protein. Fucoidins occur in the algal CELL WALLS and intercellular matrix and in the mucilage exuded from the surfaces of the fronds. They appear to play a role in preventing desiccation of the alga when it is exposed by tides: algae which are permanently submerged contain much less fucoidin than those growing in the intertidal zone. fucosans See PHAEOPHYTA. L-fucose 6-Deoxy-L-galactose. fucosterol See PHAEOPHYTA. fucoxanthin See CAROTENOIDS. Fucus A genus of algae (division PHAEOPHYTA) which occur primarily in marine and intertidal zones. The vegetative organism consists typically of dichotomously branched, flat leathery blades, each having a pronounced central midrib; according to species, the blade may have entire or serrated edges. The stipe is attached to rocks etc by a holdfast. Some species, e.g. F. vesiculosus (bladder wrack), have PNEUMATOCYSTS. As in other members of the Fucales, the haploid phase of the life cycle is represented only by the gametes of the sexual organs in diploid individuals. fuel biodeterioration See PETROLEUM. fuel cell See BIOFUEL CELL. fuel ethanol See INDUSTRIAL ALCOHOL. Fujinami sarcoma virus See AVIAN SARCOMA VIRUSES. Fuligo A genus of slime moulds (class MYXOMYCETES) which form aethalia containing both capillitium and pseudocapillitium; the capillitial threads bear nodes of lime. F. septica – a common species on rotting wood, humus, manure, etc. – forms aethalia measuring up to 70 cm across (the largest sporocarp of any myxomycete); the aethalia (sometimes known as ‘flowers of tan’) are initially brownish-yellow, and have a thick calcareous covering which eventually breaks open to reveal the black spores. Fulvia A genus of fungi of the HYPHOMYCETES. F. fulva, formerly Cladosporium fulvum, is the causal agent of leaf mould of the tomato plant. fumagillin An ANTIBIOTIC produced by Aspergillus fumigatus; it has a complex structure which includes two epoxide rings and is an ester of decatetraenedioic acid. Fumagillin apparently has no activity against bacteria or fungi but it is a potent AMOEBICIDE; it also inhibits the development of certain bacteriophages, and may show some antiviral activity in tissue cultures. fumarase See Appendix II(a). fumarate reductase See e.g. FUMARATE RESPIRATION and PROPIONIC ACID FERMENTATION [Appendix III(h)]. fumarate respiration A type of ANAEROBIC RESPIRATION, carried out by a range of bacteria and by certain eukaryotic microorganisms, in which exogenous fumarate acts as the terminal electron acceptor for re-oxidation of reduced pyridine nucleotides, for oxidative phosphorylation, etc; fumarate is reduced to succinate, the E
0 for the succinate/fumarate redox couple being +30 mV. (cf. e.g. PROPIONIC ACID FERMENTATION.) In Escherichia coli, fumarate respiration occurs during anaerobic growth in the presence of a non-fermentable carbon source (e.g. glycerol) and fumarate; the respiratory chain involved comprises fumarate reductase (EC 1.3.99.1), menaquinone, dehydrogenases (e.g. formate, NADH and lactate dehydrogenases), and possibly a b-type cytochrome. Fumarate reductase is distinct from succinate dehydrogenase (which catalyses the same reaction): fumarate reductase is repressed in the presence of O2 and NO3 − but is expressed under anaerobic conditions in the presence of fumarate, whereas succinate dehydrogenase is induced 325 Fungi Imperfecti funicular cord See CYATHUS. funiculus See CYATHUS. funoran A mucilaginous sulphated galactan produced by Gloiopeltis furcata; it consists mainly of D-galactose and 3,6anhydro-L-galactose residues, with some L-galactose residues. In Japan, funoran is used as an adhesive and sizing agent. Fur protein A multifunctional regulatory protein encoded by the fur gene. In many bacteria, Fur is involved e.g. in the control of ferric iron uptake (hence ferric uptake regulator); for example, after uptake of adequate ferric iron in Escherichia coli, accumulated intracellular ferrous ions cause the Fur protein to repress transcription from various genes (e.g. fep genes) which encode cell envelope proteins involved in ferric iron uptake. Repression involves the binding of Fe2+ –Fur to an operator sequence (the ‘iron box’) upstream of the coding region in the affected genes. (See also OPERATOR.) Fur has many functions other than iron regulation – e.g. in Salmonella typhimurium it is involved in the acid tolerance response [JB (1996) 178 5683–5691]. furacin Nitrofurazone: see NITROFURANS. furadantin Nitrofurantoin: see NITROFURANS. furalaxyl See PHENYLAMIDE ANTIFUNGAL AGENTS. furaltadone See NITROFURANS. furamide See DILOXANIDE. furazlocillin A b-LACTAM ANTIBIOTIC which binds exclusively to PBP-3 (see PENICILLIN-BINDING PROTEINS). furazolidone See NITROFURANS. furcellaran (Danish agar) A sulphated galactan, similar to kCARRAGEENAN, isolated from Furcellaria fastigiata; it has been used as a gelling agent in the food industry. Furcellaria See RHODOPHYTA. furfuraceous Scurfy; covered with small scales. furocoumarin Syn. PSORALEN. furoviruses See SOIL-BORNE WHEAT MOSAIC VIRUS. furoxone Furazolidone: see NITROFURANS. furuncle Syn. BOIL. furunculosis (1) A FISH DISEASE affecting mainly freshwater fish, particularly salmonids. Pathogen: Aeromonas salmonicida. The typical disease occurs in two forms. In the acute form external symptoms are usually absent; internal organs become inflamed and haemorrhagic, and death is usually rapid. In the subacute form furuncle-like swellings (containing necrotic tissue, blood and bacteria) appear on the body. A chronic, subclinical carrier state is also known. Infection may occur via gills or wounds; high temperature (12–19° C) is a predisposing factor. Atypical forms of the disease (‘ulcerative furunculosis’), caused by atypical strains of A. salmonicida, occur in salmonids and nonsalmonids (e.g. goldfish) and involve progressive ulceration of skin and underlying muscle. (cf. ULCER DISEASE.) A. salmonicida produces several extracellular toxins, including a leucocytolytic factor, proteases (‘caseinase’ and ‘gelatinase’), and two haemolysins: H-lysin and T-lysin; H-lysin is a broad-spectrum haemolysin [purification and properties: JGM (1985) 131 1603–1609], while T-lysin appears to be active specifically against trout RBCs. These products may, together, play a role in the pathogenicity of A. salmonicida. (2) (med.) The (simultaneous or sequential) formation of a number of BOILS. furylfuramide (AF-2) See NITROFURANS. fusarenon-X See TRICHOTHECENES. fusaric acid (2-carboxyl-5-n-butylpyridine) A TOXIN produced – sometimes together with dehydrofusaric acid and 10-hydroxyfusaric acid – by several species of Fusarium. It apparently primarily or exclusively aquatic. [Review of marine fungi: Biol. Rev. (1983) 58 423–459.] The terrestrial species include freeliving saprotrophs which occur e.g. in soil and dung; parasites and pathogens of man and other animals – see e.g. INSECT DISEASES, MYCOSIS, NEMATOPHAGOUS FUNGI (see also MYCOTOXINS); plant-pathogenic species – see e.g. DOWNY MILDEWS, POWDERY MILDEWS, RUSTS (sense 2) and SMUTS (sense 2); MYCOPARASITES; and fungi involved in other associations – see e.g. AMBROSIA FUNGI, FUNGUS GARDENS, LICHENS, MYCETOCYTE, MYCORRHIZA, TERMITE–MICROBE ASSOCIATIONS. Some of the aquatic fungi are pathogenic – see e.g. BLACK MAT SYNDROME, BURNED SPOT DISEASE, FISH DISEASES and OSTRACOBLABE IMPLEXA. (See also ANTIFUNGAL AGENTS.) Certain fungi are used commercially e.g. in the production of antibiotics (e.g. some PENICILLINS), enzymes (see e.g. PROTEASES), FERMENTED FOODS, INDUSTRIAL ALCOHOL and SINGLECELL PROTEIN. Fungi may also be important agents in the BIODETERIORATION of certain materials. Fungi Imperfecti See DEUTEROMYCOTINA. fungicidal Able to kill at least some types of fungus. (cf. FUNGISTATIC.) fungicide See ANTIFUNGAL AGENTS. fungicidin Syn. NYSTATIN. fungicole Any organism which grows on or in fungi; hence adj. fungicolous. fungimycin Syn. PERIMYCIN. fungistatic Able to inhibit the growth and reproduction of at least some types of fungus. (cf. FUNGICIDAL.) fungitoxic Toxic to fungi. fungus See FUNGI. fungus ball Syn. MYCETOMA (sense 2). fungus combs See TERMITE–MICROBE ASSOCIATIONS. fungus gardens Subterranean ‘gardens’ of fungi cultivated by parasol ants (Formicidae, subfamily Myrmicinae, tribe Attini). The ants cut leaf fragments or collect other organic particles and carry them to the nest, where the fragments are pulped, treated with faecal material, inserted into the garden, and inoculated with fungal mycelium acquired from established parts of the garden or – if a new garden is being started – from a small pellet of the fungus carried in an infrabuccal pocket at the back of the mouth. Hyphae growing in the garden commonly develop terminal swellings (variously called bromatia, gongylidia or staphyla), and these are eaten by adult ants and fed to the larvae; the fungi are the sole source of food for the larvae, but the adults may also feed on plant juices. The fungi appear to occur only in fungus gardens; they generally appear to be basidiomycetes of the genera Auricularia, Agaricus or Lepiota. The fungal hyphae contain proteases (as well as cellulases, pectic enzymes etc), but proteases are not secreted to any significant extent, and the fungi grow slowly if supplied with polypeptides as sole source of nitrogen. When the fungus is ingested by the ants, the enzymes pass through the ants’ gut and are still active in the faecal material. Thus, treatment of plant fragments with faecal material increases availability of amino acids by proteolysis, augmenting growth of the fresh inoculum of fungi, and probably also facilitates fungal penetration of the plant material by pectinolysis. [Book ref. 77, pp. 163–166.] The fungus is maintained in almost pure culture by the activities of the ants, although it is not clear how this is achieved. (See also INSECT–MICROBE ASSOCIATIONS.) fungus gnats Small flies (family Mycetophilidae) whose larvae feed on fungal mycelium and fruiting bodies; fungus gnats (particularly Sciara spp) can cause severe damage in mushroom beds. 326 fusion protein eye infections and skin lesions in man and animals; such infections are usually or always secondary to trauma or debilitation in the host. In immunocompromised patients, invasive infection by Fusarium may mimic aspergillosis; a PCR-based examination may be useful for detecting this organism in clinical specimens [JCM (1999) 37 2434–2438]. Fusarium spp have been used commercially as sources of ENNIATINS, GIBBERELLINS for horticultural use, ZEARALENONE derivatives for medical and veterinary use, and in the production of single-cell protein. (See also PAPER SPOILAGE and TIMBER STAINING.) [Applied aspects: Book ref. 131.] fusarium wilt A VASCULAR WILT caused by a Fusarium sp – usually F. oxysporum (‘oxysporum wilt’). Strains of F. oxysporum may have a narrow, intermediate or wide host range: e.g. F. oxysporum f. sp. cubense infects banana plants (‘Panama disease’); f. sp. lini infects flax; f. sp. lycopersici infects tomatoes (cf. LYCOMARASMIN); f. sp. conglutinans infects cruciferous plants; f. sp. vasinfectum infects e.g. coffee, cotton, cowpea, rubber (Hevea), soybean, and many other plants. Typical symptoms of fusarium wilts include brown or black vascular discoloration and wilting, sometimes preceded by e.g. epinasty, vein-clearing and leaf chlorosis; affected plants generally die. fusarubin See NAPHTHAZARINS. Fuscidea See LECIDEA. fusel oil A mixture of higher alcohols (mainly amyl, isoamyl, isobutyl, and propyl alcohols) formed in small quantities as byproducts of ALCOHOLIC FERMENTATION; the alcohols are formed by deamination, decarboxylation and reduction of amino acids, and from keto acid precursors of amino acids. fusicoccin A toxin, produced by Fusicoccum amygdali, which can affect a range of plants. Fusicoccin can increase the rate at which a plant loses water, e.g. by promoting the opening of stomata. It can also stimulate H+ efflux across the plasmalemma, thus facilitating uptake of e.g. K+ and certain energy-yielding substrates; this may at least partly account for the observed growth-promoting activity of fusicoccin. Fusicoccum A genus of fungi of the COELOMYCETES. fusidic acid An ANTIBIOTIC synthesized e.g. by Cylindrocarpon spp (= Fusidium coccineum) and Acremonium (Cephalosporium) spp; it is a steroid structurally related to cephalosporin P1 – with which it exhibits cross-resistance. Sodium fusidate (fucidin) is active (bacteriostatic) against Gram-positive bacteria, particularly staphylococci; it inhibits PROTEIN SYNTHESIS by binding to EF-G, preventing the dissociation of EF-G and GDP from the ribosome following translocation. The EF-Gs (= EF-2s) of certain archaeans (e.g. methanogens, halobacteria) are sensitive to fusidic acid, whereas those from e.g. Desulfurococcus mobilis, Sulfolobus solfataricus, Thermococcus celer and Thermoplasma acidophilum are insensitive [JB (1986) 167 265–271]. Fusidium See CYLINDROCARPON. fusiform Spindle-shaped: tapered at both ends. Fusiformis An obsolete bacteria genus. (See BACTEROIDES and FUSOBACTERIUM.) fusigen A SIDEROPHORE produced by e.g. Aspergillus spp. fusion protein (1) In e.g. viruses of the genus PARAMYXOVIRUS (e.g. Sendai virus): a protein which promotes fusion between host cells, as well as between virus envelope and cell membrane. (2) (biotechnol.) A protein containing amino acid sequences from each of two distinct proteins; it is formed by the expression of a recombinant genetic construct prepared by joining together damages plant cell membranes and may play a role in the pathogenesis of FUSARIUM WILT; the toxin can also inhibit plant polyphenol oxidases, thus possibly interfering with the host defence mechanisms. Some plants can convert fusaric acid to the less toxic N-methyl derivative. fusaritoxicosis A MYCOTOXICOSIS due to toxins produced by FUSARIUM spp. Fusarium A genus of economically important fungi (class HYPHOMYCETES) which form a septate mycelium and produce macroconidia and, in some species, microconidia and/or chlamydospores. The macroconidia are produced from phialides which are generally borne on sporodochia; they are hyaline, usually crescent-shaped and multiseptate, and each has a characteristically shaped foot cell (forming one pole of the macroconidium) in which a ‘heel’ is present. Microconidia are also formed from phialides and are usually single-celled and spherical or ovoid. Conidia intermediate in form between macro- and microconidia may be produced. Fusaria are notoriously variable in culture, often making identification difficult. Many species have teleomorphs in the Hypocreales (e.g. GIBBERELLA, NECTRIA), but some – e.g. F. culmorum, F. oxysporum – have no known teleomorph. Fusarium spp have a world-wide distribution and occur e.g. in soil, decaying organic materials, etc. Some strains may be wholly saprotrophic, but the majority can also be weakly to highly pathogenic – mainly in plants and invertebrates; no strain appears to be obligately parasitic. Examples of plant-pathogenic species: F. avenacearum (Gibberella avenaceae) and F. culmorum cause seedling blight, foot and root rot, and head blight in cereals; F. coeruleum causes DRY ROT (sense 2) in potatoes; F. decemcellulare causes gall formation on cacao pods; F. graminearum (Gibberella zeae) causes seedling blight and foot and root rots in small-grain cereals (cf. WHEAT SCAB), and stalk and ear rot of maize; F. lateritium (Gibberella baccata) causes wilt, dieback and canker in woody plants, bud rot in apples, etc; F. moniliforme (Gibberella fujikuroi ) causes ear and stalk rot of maize, BAKANAE DISEASE of rice, PITCH CANKER of pines, etc; F. nivale causes pre-emergence blight, root rot and head blight of cereals (see also SNOW MOULD); F. oxysporum causes e.g. FUSARIUM WILT, a form of CELERY YELLOWS, and basal rots of bulbs and corms (e.g. Narcissus, tulips, gladioli); F. poae causes SILVER TOP in grasses; F. solani (Nectria haematococca) causes root rots in a wide range of plants; F. sulphureum (Gibberella cyanogena) can cause dry rot of stored potatoes. Many plant-pathogenic fusaria produce non-host-specific TOXINS (see e.g. FUSARIC ACID, LYCOMARASMIN, NAPHTHAZARINS), although the significance of these in pathogenesis is uncertain (cf. GIBBERELLINS). In chlamydosporeforming species the chlamydospores are believed to be the main agents of infection [Book ref. 131, pp. 71–93]. Fusarium spp can be important pathogens in e.g. crustacea (see e.g. BLACK GILL DISEASE and BURNED SPOT DISEASE) and insects – e.g. strains of F. solani can be pathogenic in larvae and adults of the Southern pine beetle Dendroctonus frontalis, and in the larvae of the elm beetle Scolytus scolytus, while F. larvarum and F. lateritium have been reported to be pathogenic in scale insects. Toxins such as the naphthazarin pigments may account for at least some of the insecticidal activity of these fungi. (See also AMBROSIA FUNGI.) In man and other mammals, Fusarium spp are rarely pathogenic, although they may indirectly cause disease by producing mycotoxins in foodstuffs (see e.g. TRICHOTHECENES and ZEARALENONE). Certain strains have been associated with e.g. 327 Fusobacterium useful experimental approach, certain reporter genes have been found to influence the activity of some promoters [JB (1994) 176 2128–2132]. (‘Fusion protein’ is also used to refer to the products of certain viral ONCOGENES – e.g. MYC.) Fusobacterium A genus of Gram-negative bacteria (family BACTEROIDACEAE) which occur e.g. in the human mouth and intestine, and in the RUMEN, and which include human and animal pathogens (see NECROBACILLOSIS and FOOT-ROT; see also SWINE DYSENTERY). Cells: typically fusiform or non-fusiform rods or filaments; all species are non-motile. Fermentation of peptone or carbohydrates yields butyric acid as a major product; isobutyric and isovaleric acids are not formed, but small amounts of acetic, formic, lactic or propionic acids may be formed. Optimum growth temperature: ca. 37° C. GC%: ca. 26–34. Type species: F. nucleatum. F. necrophorum (formerly Fusiformis necrophorus, Sphaerophorus necrophorus). Fusiform or round-ended cells. Propionate is formed from both threonine and lactate; aesculin is not hydrolysed; indole +ve; copious gas is usually formed from glucose. F. nucleatum (formerly Fusiformis fusiformis, Fusobacterium polymorphum, Sphaerophorus fusiformis). Cells: typically fusiform rods or filaments. Propionate is formed from threonine but not from lactate; aesculin is not hydrolysed; indole +ve; typically, little gas is formed from glucose. Other species: F. gonidiaformans, F. mortiferum, F. naviforme, F. necrogenes, F. perfoetens, F. prausnitzii, F. russii and F. varium. Fusulina See FORAMINIFERIDA. fusulinids See FORAMINIFERIDA. Fusulinina See FORAMINIFERIDA. futile cycle The cyclic interconversion of two compounds by irreversible reactions catalysed by two or more enzymes which are active at the same time within a given cell, the result being an apparently useless dissipation of energy. For example, in e.g. Escherichia coli phosphofructokinase catalyses ATP-dependent phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate [see Appendix I(a)], and fructose bisphosphatase hydrolyses fructose 1,6-bisphosphate to fructose 6phosphate and inorganic phosphate (Pi) (see GLUCONEOGENESIS); if both enzymes were to be active simultaneously, the net result would be the hydrolysis of ATP to ADP + Pi. Presumably, futile cycles are normally prevented by control of the presence/activity of enzymes with opposing functions; however, they may have a role under certain conditions: e.g., it has been suggested that a futile cycle that results in ATP hydrolysis could provide Pi from endogenous ATP when the exogenous supply of Pi is interrupted [ARM (1984) 38 459–486 (471–473)]. FV3 FROG VIRUS 3. two coding sequences with their reading frames in phase. (See also GENE FUSION.) Fusion proteins are constructed for various purposes. A fusion protein may be constructed e.g. in order to facilitate identification/detection/assay of a given gene product which (for any reason) may be difficult to identify, detect or assay on its own. In such cases, the gene in question may be fused with a (partner or carrier ) gene such as the lacZ gene of Escherichia coli ; this results in a fusion protein with the properties of bgalactosidase – which can be readily detected/assayed e.g. on media containing XGAL. Another type of partner gene is that encoding glutathione S-transferase (GST); the resulting fusion protein can be isolated e.g. by affinity chromatography (using glutathione in the stationary matrix). In this latter example GST is the affinity tail on the protein of interest. If a given gene encodes an intracellular protein, the product may be made secretable by fusing the gene to a partner gene whose product is secreted; this may facilitate isolation of the required gene product. Fusion may also be used to determine the intracellular location of a given gene product. For example, the location of the FtsZ protein during cell division in Escherichia coli has been studied by fusing ftsZ with the gene encoding GREEN FLUORESCENT PROTEIN – the (fluorescent) fusion protein being detected by fluorescence microscopy [PNAS (1996) 93 12998–13003]. Again, the gene for GFP has been fused with smc in a study on chromosome partitioning [GD (1998) 12 1254–1259]. [Other GFP fusions: TIM (1998) 6 234–238.] Another use of fusion is to obtain the product of a gene which is normally not highly expressed – the gene of interest being fused downstream of a highly expressed partner gene in an expression vector. A well-chosen partner gene may also confer the additional benefit of greater solubility and/or stability. If a fusion protein has been isolated it may be cleaved to yield separate products for each of the two genes. One method is to use a site-specific protease. An alternative approach is to use a chemical agent such as cyanogen bromide (which cleaves at methionine residues) or hydroxylamine (which cleaves between asparagine and glycine residues); potential problems with chemical agents include (i) possible cleavage at sites within the required protein, and (ii) unwanted chemical modification of the required protein. Fusion is also useful for studying the regulation of gene expression. If the expression of a gene is difficult to monitor (owing, e.g. to difficulty in assaying the gene product) the activity of the gene’s promoter may be studied separately by fusing it upstream of a promoter-less reporter gene – i.e. a gene whose product can be readily monitored and whose expression reflects the activity of the promoter. Reporter genes include those encoding e.g. galactokinase, chloramphenicol acetyltransferase (CAT) and green fluorescent protein. While this is generally a 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 328 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 G G (1) Guanine (or the corresponding nucleoside or nucleotide) in a nucleic acid. (2) Glycine (see AMINO ACIDS). G-actin See ACTIN. (G + C)% value See GC%. G loop If the DNA of BACTERIOPHAGE MU is extracted from virions obtained by induction of a lysogenic population of bacteria and is then denatured and re-annealed, a proportion of the resulting dsDNA molecules show a ‘bubble’ or loop of unpaired strands in the G region of the DNA; this loop – the G loop – reflects localized non-homology generated by inversion of the G segment during lysogeny. (G loops are rarely observed in phage DNA derived from lytic infections.) G phases (of cell cycle) See CELL CYCLE. G proteins (GTP-binding proteins) In eukaryotic cells: GTPbinding proteins involved mainly in intracellular signalling. There are two main types of G protein: (i) the heterotrimeric type (each G protein consisting of a, b and g subunits), and (ii) the (smaller) proteins of the Ras superfamily (= p21 family). Heterotrimeric G proteins are characteristically associated with the cytoplasmic domain of certain transmembrane receptors (e.g. the receptors for CHEMOKINES). (See also ADENYLATE CYCLASE.) In the inactive state, a GDP residue binds to the a subunit of the protein. During activation of the G protein (through receptor–ligand binding), the a subunit separates from the other two subunits (which stay together), and GDP on the a subunit is exchanged for GTP; when activated in this way (i.e. when binding GTP), the a subunit can behave as an effector molecule in the signalling pathway – as can the b + g entity. During de-activation of the G protein, GTPase activity (inherent in the a subunit) cleaves the terminal phosphate from GTP – so that the a subunit is then bound to GDP; the a subunit (with bound GDP) then re-associates with b + g to form the inactive G protein. G proteins of the Ras superfamily interact with several factors during their cycle of activation and de-activation. During activation, a guanosine nucleotide exchange factor promotes the exchange of (bound) GDP for GTP; the activated G protein (i.e. with bound GTP) can then act as an effector molecule in the signalling pathway. During de-activation, a GTPase-activating factor stimulates the latent GTPase activity of the G protein, thus giving rise to the inactive (GDP-bound) state. Another factor – guanosine nucleotide dissociation inhibitor – inhibits the exchange of GDP for GTP. G1 -event model See CELL CYCLE. Gaeumannomyces A genus of fungi of the order DIAPORTHALES. G. graminis (formerly Ophiobolus graminis), the causal agent of TAKE-ALL, forms dark, thick-walled perithecia which occur immersed in leaf sheaths and stem bases with their beaks (i.e., necks) protruding; the ascospores are septate, filiform or fusiform. (See also MYCOVIRUS.) Gaeumannomyces graminis virus See MYCOVIRUS. gaffkaemia (gaffkemia) A fatal septicaemic disease of lobsters (Homarus spp) caused by Aerococcus viridans (formerly Gaffkya homari ); infection occurs via breaks in the integument. Gaffkya An obsolete bacterial genus [nom. rejic. IJSB (1971) 21 104–105]. (See also AEROCOCCUS and PEPTOSTREPTOCOCCUS.) gag gene See RETROVIRIDAE. gal operon See OPERON and OPERATOR. galactan A polymer of GALACTOSE: see e.g. AGAR; AMYLOVORIN; CARRAGEENAN; FUNORAN; FURCELLARAN; GALACTOCAROLOSE; PECTIC POLYSACCHARIDES. galactitol (dulcitol) The POLYOL corresponding to galactose. It occurs in certain HONEYDEWS, in two red algae (Iridaea laminarioides and Bostrychia scorpioides), in certain yeasts and other fungi, and in certain higher plants. Galactitol can be metabolized e.g. by a number of fungi. galactocarolose An extracellular polysaccharide from Penicillium charlesii ; it consists of (1 → 5)-linked D-galactofuranosyl residues. galactose An aldohexose; both D- and L-forms occur naturally. Galactose is found e.g. in GALACTANS, LACTOSE, MELIBIOSE, RAFFINOSE and STACHYOSE. Many bacteria can metabolize galactose; uptake may involve a PEP-PTS or permease transport system, and subsequent metabolism may involve the tagatose 6-phosphate or Leloir pathways, respectively – see Appendix III(a). galactose 1-phosphate pathway Syn. Leloir pathway: see LACTOSE and Appendix III(a). b-galactosidase (lactase; b-D-galactoside galactohydrolase; EC 3.2.1.23) An ENZYME which hydrolyses LACTOSE to glucose and galactose. (cf. ALLOLACTOSE.) b-Galactosidases are produced by a range of microorganisms and are obtained commercially mainly from yeasts (e.g. Kluyveromyces marxianus) or moulds (e.g. Aspergillus spp). The commercial enzyme is used mainly to hydrolyse the lactose in milk and milk products (to make them acceptable to lactose-intolerant consumers, to increase their sweetness, and to avoid crystallization of lactose in frozen or condensed milk products) and in WHEY (to make it more amenable as a microbial substrate). (See also ONPG TEST and XGAL.) galacturonan A polysaccharide composed of galacturonic acid residues – see PECTINS. galacturonic acid The URONIC ACID corresponding to galactose. (See also PECTINS.) Galerina See AGARICALES (Cortinariaceae). gall See GALLS. Gallego’s solution See BROWN–HOPPS MODIFICATION. gallid herpesviruses 1 and 2 See GAMMAHERPESVIRINAE. gallid herpesvirus 3 See HERPESVIRIDAE. Gallionella A genus of Gram-negative, microaerophilic IRON BACTERIA. The cells are stalked and are coccoid to kidney shaped; they divide by binary fission, the non-stalked daughter cell being flagellated. The stalk consists of a bundle of (apparently inanimate) fibrils. Metabolism is apparently chemolithoautotrophic; energy appears to be obtained by the oxidation of Fe2+ (thus accounting for the deposition of ferric hydroxide during growth). (See also EXTRACYTOPLASMIC OXIDATION.) Type species: G. ferruginea. galls (cecidia) (plant pathol.) Abnormal plant structures formed e.g. in response to parasitic attack by certain insects or microorganisms. Galls may develop either by localized cell proliferation or increase in cell size. In at least some cases gall-formation involves a PHYTOHORMONE imbalance. Gall-inducing organisms often exhibit considerable host specificity; the host plant is rarely killed. 329 galvanotaxis Bacterium-induced galls include e.g. CROWN GALL, FASCIATION (some instances), HAIRY ROOT, and OLIVE KNOT; galls can also be induced by Erwinia herbicola. (See also ROOT NODULES.) Fungus-induced galls (mycocecidia) include e.g. CLUBROOT, WART DISEASE and WITCHES’ BROOM; galls are also induced by e.g. Albugo candida on certain crucifers, by some smut fungi (see SMUTS sense 2), and by members of the EXOBASIDIALES. (See also ‘cedar apples’ under GYMNOSPORANGIUM.) Virus-induced galls include those caused by the RICE GALL DWARF VIRUS and the WOUND TUMOUR VIRUS. (See also ENATIONS and FIJI DISEASE.) galvanotaxis A TAXIS in which the stimulus is an electrical potential gradient. An organism may move (with its anterior end pointing in the direction of motion as in normal motility) towards the cathode (negative galvanotaxis) or anode (positive galvanotaxis). Paramecium is said to be negatively galvanotactic, but with a higher potential difference it may move towards the anode with its (normally) posterior end leading. Some protozoa are positively galvanotactic. Gambierdiscus See CIGUATERA. gametangial contact (gametangy) A process in which one or more male nuclei pass from a male gametangium to a female gametangium either (according to species) via a pore which develops at the point of contact of the two gametangia, or via a TRICHOGYNE which grows between the gametangia. gametangial copulation The occurrence of plasmogamy and karyogamy in a process which may involve either the complete fusion of two gametangia (‘gametangial fusion’) or the transfer of the contents of one gametangium into the other. gametangial fusion See GAMETANGIAL COPULATION. gametangium A structure which gives rise to gametes or which, in its entirety, functions as a gamete. gametangy Syn. GAMETANGIAL CONTACT. gamete A haploid cell or nucleus involved in sexual reproduction – during which two gametes fuse to form a zygote. (See also ANISOGAMY; ISOGAMY; OOGAMY.) gametic meiosis MEIOSIS, in a meiocyte other than a zygote, preceding gamete formation in a life cycle in which DIPLOPHASE predominates. (cf. ZYGOTIC MEIOSIS.) gametocytaemia The presence in the bloodstream of gametocyte stages of a parasite. gametocyte Any cell which gives rise to gamete(s); a gametocyte may be haploid (see ALTERNATION OF GENERATIONS) or diploid (see GAMETIC MEIOSIS). gametogony (gamogony) (1) The formation of gametocytes and gametes. (2) The formation of gametes from gametocytes. gametophyte See ALTERNATION OF GENERATIONS. gametothallus See ALTERNATION OF GENERATIONS. gamma chain (immunol.) See HEAVY CHAIN. gamma-delta (gd) See entry Tn3. gamma globulins A subclass of serum globulins (which includes the IMMUNOGLOBULINS) characterized by low electrophoretic mobility towards the anode – or, under certain conditions, movement towards the cathode (see ELECTROENDOSMOSIS). gamma interferon See INTERFERONS. gamma particle (1) See PSEUDOCAEDIBACTER. (2) (mycol.) In the zoospores of some members of the Chytridiomycetes (e.g. Blastocladiella emersonii ): an ovoid particle, one or more of which appear to play a role in cyst wall formation; in B. emersonii the gamma particles (each ca. 500 nm diam.) exhibit chitin synthase activity. [Book ref. 175, pp. 544–545.] gamma-rays (g-rays) See IONIZING RADIATION. Gammacell A laboratory apparatus used as a source of gammaradiation. (See also IONIZING RADIATION.) gd See entry Tn3. Gammerferon See INTERFERONS. Gammaherpesvirinae (‘lymphotropic or lymphoproliferative group’) A subfamily of viruses of the HERPESVIRIDAE (q.v.). The prototype virus of the subfamily is the EPSTEIN–BARR VIRUS (human (gamma) herpesvirus 4; human herpesvirus 4, HHV4; type species of the genus Lymphocryptovirus [Intervirol. (1986) 25 141–143]). Other herpesviruses which have been either accepted or proposed as members of the subfamily include Herpesvirus saimiri (main natural host: squirrel monkeys, Saimiri sciureus), equine herpesvirus 2 (EHV2); Herpesvirus ateles (main natural host: spider monkeys, Ateles spp), gallid herpesvirus 1 (MAREK’S DISEASE virus), gallid herpesvirus 2 (turkey herpesvirus) and leporid herpesvirus 1 (rabbit herpesvirus). These gammaherpesviruses vary in the duration of their replication cycles and in the type of CPEs which they produce. They are generally able to replicate in lymphoblastoid cells, and some can also lyse epithelioid and fibroblastoid cells. In either B or T lymphocytes (according to virus), infection often does not result in the production of progeny virions; the viral genome may persist in the cells with minimal viral gene expression and no cell lysis, or cells may lyse without formation of complete virions. Latent infection in vivo appears to occur commonly in lymphoid cells. Gammaherpesviruses are associated with the formation of tumours either in their natural hosts (see e.g. EPSTEIN–BARR VIRUS and MAREK’S DISEASE) or in experimental animals; for example, neither Herpesvirus saimiri nor Herpesvirus ateles are pathogenic in their main natural hosts but are highly oncogenic in marmosets (Saguinus spp) and in other New World primates, causing malignant tumours of the lymphatic system [review: Book ref. 139, pp 253–332]. The most recent addition to the subfamily is human (gamma) herpesvirus 8 (human herpesvirus 8, HHV8). The genome is reported to be ∼170 kb but to vary in size in different isolates (owing to duplication events); it includes sequences which resemble those of various human regulatory genes (e.g. those encoding a CYCLIN and a CYTOKINE). HHV8 has a tropism for B lymphocytes (and also infects endothelial cells) and it has been associated with B-cell lymphomas and with KAPOSI’S SARCOMA. [HHV8 (review): Lancet (1997) 349 558–563. PCR-based study on specimens of biopsy-proven Kaposi’s sarcoma: Am. J. Path. (1997) 150 147–153.] Activation of latent HHV8 in vitro has been achieved by demethylation of the promoter region of a transactivator [PNAS (2001) 98 4119–4124]. gamogony Syn. GAMETOGONY. gamone A name used for various types of sex PHEROMONE. For example, the two mating types of Blepharisma japonicum (types I and II) form, respectively, gamone 1 (= blepharmone; a glycoprotein) and gamone 2 (= blepharismone or blepharismin: calcium 3-(2 -formylamino-5 -hydroxybenzoyl)-lactate) [Book ref. 28, pp. 339–340]. gamont (1) Syn. GAMETOCYTE. (2) See ALTERNATION OF GENERATIONS. ganciclovir A nucleoside analogue used as an ANTIVIRAL AGENT; it is activated by phosphorylation, intracellularly, and inhibits viral DNA synthesis. Ganciclovir is used mainly for the treatment of cytomegalovirus infections; it has a number of sideeffects and is contraindicated in pregnancy. gangliosides See SPHINGOLIPID. gangrene (1) (med., vet.) Death of tissues or organs in the body owing to a deficiency in or loss of local blood supply. Dry 330 gasteroid gangrene results from a gradual reduction in local blood supply, with darkening, drying and shrivelling of the tissues. Moist gangrene is sudden in onset, resulting e.g. from injury followed by bacterial infection and putrefaction; it may spread very rapidly. (cf. GAS GANGRENE; see also e.g. ERGOTISM.) (2) (plant pathol.) A disease of stored potato tubers, caused by Phoma exigua (e.g. var. foveata in the UK), in which the tubers become decayed and hollow. Surface contamination of the tubers occurs during harvesting, an important source of the pathogen being pycnidia from senescent potato stems; infection occurs via wounds made during or after harvesting. The disease may be controlled with e.g. benomyl or thiabendazole. (See also POTATO DISEASES.) gangrenous coryza Syn. MALIGNANT CATARRHAL FEVER. Ganoderma See APHYLLOPHORALES (Ganodermataceae). Ganodermataceae See APHYLLOPHORALES. gap (in dsDNA) See NICK. GAR-936 See TETRACYCLINES. Gardner–Arnstein feline sarcoma virus See FES. Gardnerella A genus (incertae sedis) of catalase-negative, oxidase-negative, chemoorganotrophic, apparently Gram typenegative bacteria which occur in the human genital and urinary tracts and which are commonly associated with BACTERIAL VAGINOSIS. Cells: pleomorphic rods, ca. 0.5 × 1.5–2.5 µm. The genus includes both obligately anaerobic and facultatively anaerobic strains. The organisms are nutritionally fastidious: little or no growth occurs on nutrient agar. Neither X FACTOR nor V FACTOR is required. Growth occurs e.g. on certain bloodcontaining media and on peptone–starch– D-glucose medium; most strains can grow aerobically on blood agar at 35° C. Clear (‘b’) haemolysis occurs on human (but not sheep) blood plates. Most strains ferment maltose (and some ferment lactose) forming acid but not gas. Nitrate is not reduced. Optimum growth temperature: 35–37° C. GC%: ca. 42–44. Type species: G. vaginalis (formerly Haemophilus vaginalis). [Book ref. 22, pp. 587–591.] Gardneriella See RHODOPHYTA. gari A West African food made by the fermentation of cassava pulp. The essential organisms appear to be Leuconostoc and Candida spp. gas bladder (algol.) Syn. PNEUMATOCYST. gas gangrene A type of acute, rapidly spreading GANGRENE (sense 1) which typically results from the infection of a wound with certain anaerobic bacteria, particularly Clostridium spp (e.g. C. perfringens type A, C. septicum, C. novyi ); these organisms produce a variety of extracellular enzymes (see CLOSTRIDIUM) which promote tissue destruction. The a-toxin of Clostridium perfringens (which hydrolyses lecithin and also has sphingomyelinase activity) appears to be the major lethal toxin [Mol. Microbiol (1995) 15 191–202]. Gas gangrene may occur in e.g. subcutaneous tissues (clostridial CELLULITIS), muscle (clostridial myonecrosis), or internal organs. Infected tissues become necrotic, discoloured, and swollen with serosanguinous fluid and pockets of gas (hydrogen, carbon dioxide) produced by the clostridia; the gas exerts pressure on adjacent tissues, further disrupting the blood supply to these tissues and promoting spread of the gangrene. Treatment: e.g. surgery; antibiotic therapy; treatment with hyperbaric oxygen. (Other organisms occasionally implicated in gas gangrene include anaerobic streptococci and Escherichia coli ; mixed infections also occur.) gas–liquid chromatography See CHROMATOGRAPHY. gas vacuole A subcellular organelle, found only in prokaryotes, which consists of clusters of hollow, cylindrical, gas-filled vesicles (gas vesicles). Gas vacuoles appear bright and refractile under the light microscope. They occur in planktonic cyanobacteria, particularly bloom-forming species (e.g. Anabaena flosaquae, Trichodesmium), and in certain archaeans and bacteria (e.g. strains of Halobacterium, Methanosarcina, Pelodictyon and Rhodopseudomonas). In cyanobacteria, gas vacuole formation is usually constitutive, but it may be inducible in hormogonia (see HORMOGONIUM). The gas vesicles of cyanobacteria are hollow cylinders with conical ends; they are ca. 60–250 nm in diameter and of variable length (e.g. 300 nm); they have a ribbed construction. Vesicles from other prokaryotes (including Methanosarcina) are generally similar in both morphology and construction (although they may differ in size), but those of Halobacterium are usually spindle-shaped. Vesicles are composed of a rigid monolayer of a single type of protein (gas vesicle protein – GVP); they are permeable to gases but not to water. GVPs from e.g. cyanobacteria and halobacteria show some homology [JGM (1984) 130 2709–2715]. The primary function of a gas vacuole is to give buoyancy to the (free-floating) organism. Gas vesicles collapse when subjected to pressure (ca. 5 atmospheres for many freshwater cyanobacteria, ca. 50 atmospheres for Trichodesmium). The vesicles in some freshwater species can be collapsed by high turgor pressures within the cells, thus providing a mechanism whereby these cells can regulate their buoyancy and, hence, their position in the water: an increase in light intensity causes an increase in turgor pressure in the cell (due to lightstimulated accumulation of potassium ions and the accumulation of photosynthate), resulting in the collapse of part of the gas vacuole and hence loss of buoyancy. New gas vesicles form when the light intensity is low, thus increasing buoyancy. (There is some evidence that the GVP of collapsed vesicles disaggregates and may be re-used in the assembly of new vesicles [JGM (1984) 130 1591–1596].) Another mechanism for the regulation of buoyancy occurs e.g. in Oscillatoria agardhii and in a strain of Microcystis [JGM (1985) 131 799–809]: when the intensity of light is high, gas vesicles do not collapse – but existing vesicles are ‘diluted out’ by cell growth and division. gas vesicle (1) See GAS VACUOLE. (2) Syn. PNEUMATOCYST. gasohol A mixture of gasoline and (up to ca 20% v/v) ethanol which is used as a fuel. (See also INDUSTRIAL ALCOHOL.) GasPak An ANAEROBIC JAR which consists of a strong, cylindrical, transparent plastic (polycarbonate) chamber with a flat, gas-tight lid which is secured by means of a screw clamp. The jar is loaded (with e.g. plates), and an indicator of anaerobiosis (e.g. a pad soaked with a METHYLENE BLUE solution) is placed inside the jar. Water is then added to a small foil sachet containing (i) sodium borohydride and (ii) a mixture of powdered sodium bicarbonate and citric acid – which (with water) yield H2 and CO2 , respectively; the sachet is quickly placed in the jar in an upright position and the lid is clamped in place. Attached to the inside of the lid is a ‘cold’ catalyst (see MCINTOSH AND FILDES’ ANAEROBIC JAR) which promotes the elimination of O2 . Anaerobiosis within the jar develops within a few hours. Since this type of jar does not require evacuation, plates can be incubated upside down (i.e. lid-side down); this avoids the problem (which can arise with the McIntosh and Fildes’ jar) in which water of condensation drops from the inside of the Petri dish lid onto the agar surface. gasteroid Refers to the type of fruiting body which is characteristic of fungi of the GASTEROMYCETES: angiocarpic, forming STATISMOSPORES. (cf. AGARICOID.) 331 gasteroid basidiospore gasteroid basidiospore A term proposed for a BASIDIOSPORE which is not forcibly discharged from the basidium [BBMS (1983) 17 82–94 (86)]. (cf. STATISMOSPORE.) Gasteromycetes A class of fungi (subdivision BASIDIOMYCOTINA) characterized by the formation of epigean or hypogean, sessile or stipitate basidiocarps which undergo ANGIOCARPIC DEVELOPMENT and give rise to STATISMOSPORES. The typical basidiocarp consists of e.g. fleshy or cartilaginous fertile tissue (the GLEBA) enclosed by a limiting wall (PERIDIUM); in some species the basidiocarp contains a definite hymenium, while in others the fertile cells are scattered, singly or in groups, throughout the spore-bearing tissue. (See also CAPILLITIUM.) The dispersal of mature basidiospores depends e.g. on the disintegration or perforation of the peridium. Gasteromycetes occur typically as saprotrophs on soil, dead wood and dung; some species form mycorrhizal associations. The orders [Book ref. 64, p. 160] are: Gautieriales. Hymenium-forming, hypogean saprotrophs or mycorrhizal fungi. Hymenogastrales. Hymenium-forming, typically hypogean saprotrophs, mycorrhizal fungi and parasites. Some authors include gasteroid members of the Russulales in this order. Genera include e.g. Hymenogaster and Rhizopogon (which forms mycorrhizal associations with conifers). LYCOPERDALES (q.v.). Melanogastrales. Hypogean or marine; mature gleba mucilaginous, lacking a true hymenium. Genera include Melanogaster and NIA. NIDULARIALES (q.v.). PHALLALES (q.v.). Podaxales. An order which includes some of the SECOTIOID FUNGI. SCLERODERMATALES (q.v.). Tulostomatales. An order which includes the stipitate puffballs (e.g. Battarrea, Calostoma). gastric flu A term sometimes used to refer to the syndrome caused by infection with Influenzavirus type B – in which some studies have recorded involvement of the gastrointestinal tract. gastriole Syn. FOOD VACUOLE. gastroenteritis Inflammation of the lining of the stomach and intestine (usually the small intestine), usually resulting in DIARRHOEA and vomiting. (cf. ENTERITIS.) It may be due e.g. to allergic reactions, chemical poisoning (e.g. by excessive alcohol consumption), or to microbial infection or toxins (see e.g. FOOD POISONING). gastrointestinal tract flora The mammalian gastrointestinal tract is normally sterile at birth, but thereafter becomes rapidly colonized by a range of bacteria. The gut flora of the human infant initially includes Lactobacillus and Bifidobacterium spp, coliforms and clostridia, but the flora depends to some extent on diet: the gut of breast-fed infants contains a higher proportion of bifidobacteria and lactobacilli – and fewer coliforms – than that of infants bottle-fed on cows’ milk [Book ref. 107, pp 1–26]. Acid production by the lactic acid bacteria is believed to discourage the growth of enterobacteria, including enteric pathogens. On weaning, the gut flora becomes more complex and soon resembles that of the adult. In adult humans, many of the bacteria ingested with food (and oral bacteria swallowed with saliva) are killed by the acidity in the stomach. However, despite the normal acidity of the stomach, it appears that this region of the gut is colonized by Helicobacter pylori (see HELICOBACTER) in a high proportion of the human population. (A greater number and variety of organisms may be present in the stomach in cases of achlorhydria.) Bacterial numbers (in colony-forming units/gram) increase from about 103 in the duodenum to about 1011 in the colon. Anaerobes such as Bacteroides and Fusobacterium generally predominate but many other organisms are usually present–e.g. species of Clostridium and Peptostreptococcus, enterobacteria, various yeasts (e.g. Blastocystis hominis, Candida spp), protozoa (e.g. Endolimax nana, Entamoeba spp, Trichomonas hominis) and viruses such as enteroviruses. The numbers and nature of the gut microorganisms may be affected by e.g. the nature of the diet, malnutrition, local immune responses, antibiotic therapy etc. Under certain conditions – e.g. abnormal peristalsis, trauma, diminished gut secretions – abnormally high numbers of bacteria may develop in the small intestine; symptoms of such ‘bacterial overgrowth’ include steatorrhoea, malabsorption of carbohydrates, and symptoms of iron and vitamin B12 deficiencies. TRANSLOCATION (sense 2) of gut bacteria may occur normally but may sometimes lead to extraintestinal disease; however, the importance of translocation in pathogenesis has been questioned [Critical Care Medicine (2003) 31 598–607]. The microflora has various roles: (i) production of shortchain fatty acids – e.g. butyric acid (a nutrient for epithelial cells); (ii) recycling of BILE ACIDS. (iii) production of vitamin K (see QUINONES); (iv) stimulation of the gut immune system – helping to maintain a normal immune response; (v) inhibition of pathogens by competing for nutrients and space, and by producing antimicrobial agents (e.g. BACTERIOCINS) [Lancet (2003) 360 512–519]. (The gut epithelium also secretes antimicrobial peptides (e.g. defensins) [Nature (2002) 415 389–395, (2003) 422 478–479].) However, a possible role for gut bacteria in the aetiology of carcinoma of the colon has been postulated: certain substances (particularly bile acids) in the gut may be metabolized by gut bacteria to potential carcinogens or co-carcinogens [Book ref. 107, pp 347–363]; the bacteria implicated include the socalled ‘nuclear dehydrogenating clostridia’ (NDC) which can desaturate the A-ring in various bile acid derivatives to form 4-ene-3-one structures [JMM (1985) 20 233–238]. (See also LIGNANS.) Certain gut parasites (e.g. Entamoeba histolytica) appear to require the presence of gut bacteria (as food) in order to establish infection. GATC Dam methylation site: see DAM GENE. gatifloxacin A QUINOLONE ANTIBIOTIC (q.v.). gating The switching on or off of a function or facility as a consequence of the value of a given parameter reaching a critical level. Thus, e.g., certain transport systems and membrane PROTON ATPASE activity are inhibited when proton motive force falls below a certain level. Gäumannomyces Syn. GAEUMANNOMYCES. Gautieriales See GASTEROMYCETES. Gazdar murine sarcoma virus (Gz-MSV) A MURINE SARCOMA VIRUS originally isolated from a spontaneous tumour in a (NZB × NZW)F1 mouse. Gz-MSV resembles MOLONEY MURINE SARCOMA VIRUS (Mo-MSV) in containing V-mos, but the mos sequence is inserted at a different location in the genome. The pathologies of Gz-MSV and Mo-MSV infections are similar. Gb3 See SHIGA TOXIN and STARFISH. GC% (GC value; %GC value; %G + C value; mol% G + C; etc) Of (usually chromosomal) DNA: the ratio (G + C)/(A + T + G + C) in which G, C, A and T represent the relative molar amounts of guanine, cytosine, adenine and thymine, respectively, in a given organism; the ratio is expressed as a percentage. (cf. BASE RATIO.) GC% values vary according to genus and species, 332 geminiviruses and provide a useful criterion in microbial TAXONOMY; however, a similarity in GC% values, by itself, does not necessarily indicate a close taxonomic relationship. The possession of a PLASMID (particularly one with a high copy number) may affect the observed GC% value of a cell unless the method used for extracting chromosomal DNA discriminates against extrachromosomal nucleic acids. The GC% of a DNA sample may be obtained, indirectly, by determining its ‘melting point’ (Tm : see THERMAL MELTING PROFILE), since Tm is linearly related to GC% (i.e., a high Tm correlates with a high GC%). Alternatively, an indirect determination of GC% may be achieved by buoyant density (Bd) measurement (e.g. in a CsCl density gradient – see CENTRIFUGATION) since Bd is also linearly related to GC%. GC% values in bacteria fall within the approximate range 25–75; as a guideline, intra-species variation should be not more than 4–5% and intra-genus variation not more than 10% [Book ref. 59, p. 14]. [Methods for determining DNA base composition: Book ref. 138, pp. 1–31.] GC type See BASE RATIO. GDP Guanosine 5 -diphosphate. Geastrum See LYCOPERDALES. gel diffusion In serology: gel diffusion (= immunodiffusion) refers to any procedure in which antibodies (usually in an antiserum) and/or antigens diffuse through a gel medium (e.g. AGAR or agarose) – forming a precipitate in the gel where homologous (or cross-reacting) antigens and antibodies meet in OPTIMAL PROPORTIONS. (Agar/agarose at concentrations of ca. 0.5% to 1.5% allows the diffusion of antibody-sized molecules but hinders random mechanical and thermal movements in the medium.) Lines, bands or haloes of whitish precipitate are obtained (within hours or a day or two) in number(s) and position(s) which depend on the number and nature of the homologous (or cross-reacting) pairs of antigens and antibodies within the diffusing system. The precipitate may or may not be visible to the naked eye; visibility may be enhanced by washing the gel free of uncombined antigen and antibody, and staining the precipitate in situ with a protein stain (e.g. COOMASSIE BRILLIANT BLUE). Gel diffusion may involve SINGLE DIFFUSION or DOUBLE DIFFUSION of the single- or double-DIMENSION type. (See also IMMUNOELECTROPHORESIS and ELEK PLATE.) gel electrophoresis See ELECTROPHORESIS. gel filtration (molecular sieving) A method for separating molecules from a mixture by passing the mixture through a column of e.g. Sephadex, polyacrylamide, or agarose gel; molecules are separated primarily according to their size and shape. gelatin The product obtained by boiling COLLAGEN. Gelatin is soluble in water at temperatures above ca. 40° C, and a solution of concentration >ca. 1–2% (w/v) forms a gel on cooling below ca. 28–35° C; gels of ca. 4–12% are used e.g. to test the ability of certain microorganisms to liquefy (i.e., hydrolyse) gelatin (see GELATINASE). (See also FRAZIER’S MEDIUM.) gelatinase A GELATIN-hydrolysing enzyme. Gelatin can be hydrolysed (to soluble oligopeptides) by clostridial COLLAGENASES and by any of a range of extracellular proteolytic enzymes produced e.g. by various bacteria and fungi. The activity of many gelatinases requires, or is stimulated by, calcium ions. Gelbstoff Collectively, the various dissolved and colloidal organic substances which occur e.g. in lakes and coastal waters and which may absorb significant amounts of light – particularly at the blue end of the spectrum. Geleia See KARYORELICTID GYMNOSTOMES. Gelidium See RHODOPHYTA. gellan gum An extracellular polysaccharide produced by ‘Pseudomonas elodea’. Gellan gum is a linear polymer containing residues of glucose, rhamnose and glucuronic acid which may be mainly (1 → 4)-b-linked; the polymer may be O-acetylated. In water containing monovalent or divalent cations, gellan gums form gels on heating and cooling; the nature of the gel depends on the degree of acetylation – the more de-acetylated the polymer the firmer and more brittle the gel. A clarified, de-acetylated form of gellan gum (Gelrite; Keloc, San Diego, USA) can be used as a substitute for AGAR in bacteriological media; the setting temperature varies from 35° C to >50° C, depending on the concentrations of Gelrite and cations. The gels can be re-melted under standard autoclaving conditions. Gelrite gels containing 0.1% calcium chloride are not prone to SYNERESIS at high temperatures (unlike agar gels) and may thus be suitable for use in media for thermophiles. Gelrite may also be used e.g. as a substitute for agarose in gel electrophoresis (Gelrite gels are clearer than agar gels). Gelrite has been investigated for use as a gelling agent, in place of agar, in transport media used for PCR-based studies. This investigation was prompted by the finding that agar inhibits PCR in a concentration-dependent manner [JCM (1998) 36 275–276], an observation suggesting that detection of pathogens by PCR can be inhibited by the use of an agar-containing transport medium (such as STUART’S TRANSPORT MEDIUM). These studies yielded superior results from gellan gum, inhibition of PCR occurring with agar [JMM (2001) 50 108–109]. In industry, the polymer may be useful e.g. as a substitute for pectins or gelatin in jams, jellies etc. or as a matrix for the IMMOBILIZATION of cells or enzymes. [Book ref. 62, pp 231–253.] Gelman filter See FILTRATION. Gelrite See GELLAN GUM. gelsolin See ACTIN. Gemella A genus of Gram-type-positive bacteria of the family Streptococcaceae; the organisms occur e.g. in the mammalian respiratory tract. Cells: cocci (diam. <1 µm), which occur singly or in pairs. Colonies on rabbit- or horse-blood agar are each surrounded by a zone of clear haemolysis. GC%: ca. 33. Type species: G. haemolysans. gemifloxacin A QUINOLONE ANTIBIOTIC (q.v.). geminiviruses A group of small, ssDNA-containing PLANT VIRUSES in which the virions have a unique morphology: each consists of a pair of isometric particles (incomplete icosahedra, each ca. 18 × 20 nm), composed of a single type of protein (MWt ca. 2.7–3.4 × 104 ). The group includes: MAIZE STREAK VIRUS (MSV; type member), BEAN GOLDEN MOSAIC VIRUS (BGMV), cassava latent virus (CLV), Chloris striate mosaic virus (CSMV), tomato golden mosaic virus (TGMV; possibly = tomato yellow mosaic virus). Probable members include: beet curly top virus (BCTV), Euphorbia mosaic virus, mungbean yellow mosaic virus, Paspalum striate mosaic virus, tobacco leafcurl (= tomato yellow dwarf) virus, tobacco yellow dwarf virus (TYDV; = bean summer death virus), tomato yellow leafcurl virus, wheat dwarf virus. Each virus has a narrow host range; some cause economically important diseases in cultivated plants, symptoms including e.g. curling and distortion of the leaves (e.g. BCTV) or chlorosis (e.g. BGMV). Depending on virus, transmission occurs via leafhoppers or whitefly – in which the viruses are persistent (circulative). Each geminivirus virion contains one molecule of circular, positive-sense ssDNA, MWt ca. 7–8 × 105 (<2.6 kb). In 333 gemma gene conversion See RECOMBINATION. gene dosage The number of copies of a given gene per cell or nucleus. Increased gene dosage may cause higher levels of gene product if the gene is not autogenously regulated. gene-for-gene concept (plant. pathol.) See AVIRULENCE GENE. gene fusion In genetic engineering: the fusion of two DNA sequences from different genes to form a ‘hybrid’ gene (see FUSION PROTEIN, sense 2). The term is sometimes used synonymously with OPERON FUSION. (See also MUDLAC SYSTEM.) [Gene fusions (commentary): JB (2000) 182 5935–5938.] gene gun A device for inoculating a DNA VACCINE directly into the epidermis; it bombards the surface with gold particles (∼ 1 µm) that are coated with the target DNA. [Example of use: Inf. Immun. (2005) 73 2974–2985.] gene library See LIBRARY. gene therapy Genetic modification of certain cells specifically to correct an inherited disorder or to treat some infectious diseases; it has been successful e.g. with adenosine deaminase deficiency but has apparently caused leukaemia-like illness in one patient [Nature (2002) 420 116–118]. In the ex vivo mode, cells from the patient are modified in vitro and then re-introduced. For example, in ADENOSINE DEAMINASE DEFICIENCY the patient’s stem cells are supplemented with the normal, wild-type gene before re-introduction; lymphocytes – derived from the stem cells – can then develop normally. (In some cases stem cells from a normal (histocompatible) donor are introduced into the patient.) For selecting stem cells the CD34 marker can be used in immunomagnetic enrichment techniques (see DYNABEADS). In the in vivo mode, modification takes place within the patient. Viruses used as vectors (for carrying genetic material into cells) include adenoviruses (see ADENOVIRIDAE), adenoassociated viruses (see DEPENDOVIRUS) and certain retroviruses (RETROVIRIDAE), e.g. lentiviruses. Viral vectors are genetically modified to be replication-deficient and non-pathogenic; the gene (etc.) to be introduced into cells is inserted into the viral genome and enters cells during infection. Sequences/transgenes can be inserted into various types of dividing or non-dividing cell e.g. by AAVs (small sequences), adenoviruses (larger sequences) and lentiviruses. With some viruses the broad range of host cells is problematic when specific type(s) of cell are to be targeted. In such cases a virus can be modified by PSEUDOTYPING – e.g. adenovirus type 5 was modified to produce a more specifically targeted vector [JV (2001) 75 2972–2981]. In CYSTIC FIBROSIS, respiratory epithelial cells have been targeted (via an inhaler) by LIPOSOMES containing the normal gene. [Bioch. J. (2005) 387 1–15.] some (whitefly-transmitted) members (e.g., CLV, BGMV, TGMV) the genome appears to be bipartite, i.e., two ssDNA molecules – similar in size but differing in nucleotide sequence – are present in a given population of virions [nucleotide sequence of CLV genome: Nature (1983) 301 260–262, and of BGMV: PNAS (1985) 82 3572–3576]; however, the (leafhopper-transmitted) members CSMV and (possibly) TYDV seem to have only one type of ssDNA [Book ref. 80, pp. 59–62]. Virus replication occurs in the plant cell nucleus, where large aggregates of virus particles accumulate. There is evidence (at least in the case of BGMV and TGMV) that replication may involve a supercoiled ccc dsDNA intermediate (which may also act as a template for transcription), and that a rolling circle mechanism may be involved [Book ref. 80, pp. 49–58]. [Model for gene regulation in geminiviruses: PNAS (1985) 82 3572–3576.] [Review: ARPpath. (1985) 23 55–82; AVR (1985) 30 139– 177.] gemma A thick-walled, irregularly-shaped or spheroidal, asexually derived spore formed by certain fungi (e.g. Saprolegnia spp) from a portion of a somatic hypha; gemmae may be formed singly or in chains. Gemmata obscuriglobus A species of freshwater BUDDING BACTERIA; the life cycle includes a multitrichous swarmer stage. GC%: ca. 64. [AvL (1984) 50 261–268.] Gemmiger A genus of Gram-variable, anaerobic, budding bacteria which can ferment carbohydrates to butyric acid. Cells: cocci, occurring in pairs or chains. GC%: ca. 59. [G. formicilis in the avian caecum: JAB (1983) 54 7–22.] gen. nov. See GENUS NOVUM. gene A sequence of nucleotides in a genetic nucleic acid (chromosome, plasmid etc) which encodes a functional polypeptide chain or RNA molecule (rRNA, tRNA, etc). In certain cases a gene may share a sequence with another gene (see OVERLAPPING GENES); some genes contain non-coding sequences (see e.g. MRNA and SPLIT GENE). (See also ALLELE; CISTRON; GENETIC CODE; PSEUDOGENE.) gene amplification The generation of multiple copies of one or more genes in a cell or nucleus – e.g. as an arrangement of tandemly repeating genes within a chromosome, or in the form of extrachromosomal elements (see e.g. RRNA). (See also GENE DOSAGE.) Amplification occurs in both eukaryotic and prokaryotic cells. For example, in some mammalian cells in culture (and in bacteria) resistance to certain drugs involves the amplification of genes specifying enzymes which either detoxify or bypass the effects of the drug. [Gene amplification in eukaryotes: ARB (1984) 53 447–491.] In bacteria, amplification occurs spontaneously at frequencies which depend e.g. on the location of the amplified sequences, and is generally readily reversible. Amplification of particular genes may help the organism to adapt to particular environmental conditions: e.g., a substrate normally used inefficiently by a given strain may be used more effectively following amplification of genes specifying the appropriate catabolic enzymes. Amplified regions are generally found to be flanked by directly repeated sequences, and amplification commonly appears to involve RecA-dependent homologous RECOMBINATION (e.g. between daughter molecules). [Possible mechanism: CSHSQB (1984) 49 443–451.] gene bank Syn. LIBRARY. gene cloning See CLONING. gene cluster A cluster of functionally related genes, each of which encodes a separate protein. (cf. CLUSTER GENE.) Gene therapy against infectious diseases For this type of disease a wider range of modalities may be employed. For example, RIBOZYMES can be targeted to specific pathogen-associated RNA; useful activity was reported for the anti-pol ribozyme in infection by simian immunodeficiency virus (SIV) [JGV (2004) 85 1489–1496]. Intrabodies (= single-chain antibodies) are engineered antibodies that remain intracellular [see NAR (2003) 31 e23]. DNA VACCINES are being developed against e.g. Mycobacterium tuberculosis [Inf. Immun. (2005) 73 5666–5674] and Leishmania donovani [Inf. Immun. (2005) 73 812–819]. RNA INTERFERENCE (RNAi) has shown promise in vitro but pathogens may escape RNAi by mutation [e.g. JV (2004) 78 2601–2605; Blood (2005) 106 818–826]. 334 genetic drift GENETIC CODE: the ‘universal’ code Transdominant negative proteins (TNPs) – encoded e.g. in the genome of a viral vector – can be expressed in infected or preinfected cells; TNPs may be engineered anti-function versions of particular pathogen-associated proteins which are necessary e.g. for the completion of a given phase of replication. A TNP may be inhibitory e.g. by forming an inactive complex with its wild-type counterpart or by competing with it for specific binding site(s). One anti-AIDS concept is to modify CD34+ bone marrow stem cells or CD4+ T cells so that they inhibit HIV replication. In one in vitro study the TNP consisted of a modified form of the Rev protein (see HIV) whose functions include facilitating export of viral transcripts from the nucleus; HIV-1 replication was strongly inhibited [JV (2001) 75 3590–3599]. (See also TRIPLEX DNA.) gene transfer agent See CAPSDUCTION. GeneAmp In situ PCR System 1000 See IN SITU PCR. genera Plural of genus (see TAXONOMY). general export pathway See PROTEIN SECRETION. general recombination See RECOMBINATION. general secretory pathway See PROTEIN SECRETION. generalized transduction See TRANSDUCTION. generation time (of bacteria) Syn. doubling time – see GROWTH (b). generative hyphae See HYPHA. generic Of a genus: e.g. generic name (see BINOMIAL). genetic code The ‘code’ in which information for the synthesis of proteins is contained in the nucleotide sequence of a DNA molecule (or, in certain viruses, of an RNA molecule). During PROTEIN SYNTHESIS, the coded information in DNA is initially transmitted to mRNA (q.v.), i.e., the deoxyribonucleotide sequence is transcribed into a complementary sequence of ribonucleotides. (In RNA viruses the viral RNA may itself function as mRNA, or may need to be transcribed into complementary RNA: see VIRUS.) The mRNA is then translated into one or more polypeptides, each amino acid of a polypeptide being encoded by a particular sequence of three nucleotides (called a codon) in the mRNA; an mRNA molecule contains a series of non-overlapping nucleotide triplets (codons) which, together, specify the amino acid sequence of a polypeptide. Polypeptide synthesis begins at a specific initiator codon in the mRNA (see PROTEIN SYNTHESIS), and the translating machinery reads the triplets sequentially from this point; thus, although (theoretically) triplets in a sequence of nucleotides could be read in three different ways (reading frames) – e.g. AUC-AUC-AUC-AUC or A-UCAUCA-UCA-UC or AU-CAU-CAU-CAU-C – in practice the correct reading frame is determined by the fixed starting point. (cf. FRAMESHIFT MUTATION.) (N.B. The term codon is also applied to the corresponding triplet in the coding or non-coding strand of DNA.) A codon does not interact directly with its corresponding amino acid; the amino acid must be linked to an adaptor molecule, tRNA (q.v.), which contains a triplet of nucleotides (the anticodon) complementary to the codon for that amino acid. For example, a codon ACG is recognized by a tRNA containing the anticodon CGU. (Codons and anticodons, like other nucleotide sequences, are conventionally written in the 5 -to-3 direction: hence, the first base in a codon pairs with the third base in the anticodon.) The four bases (A, C, G and U) in mRNA can generate 64 possible triplet combinations; 61 of these encode 20 amino acids: i.e., in most cases an amino acid is encoded by two or more (synonymous) codons, a phenomenon called degeneracy (see table). The remaining three codons – UAA (ochre codon), UAG (amber codon) and UGA (opal or umber codon) – are nonsense First base (5 end) Second base Third base (3 end) U C A G U Phe Phe Leu Leu Ser Ser Ser Ser Tyr Tyr ochre amber Cys Cys opal Trp U C A G C Leu Leu Leu Leu Pro Pro Pro Pro His His Gln Gln Arg Arg Arg Arg U C A G A Ile Ile Ile Met Thr Thr Thr Thr Asn Asn Lys Lys Ser Ser Arg Arg U C A G G Val Val Val Val Ala Ala Ala Ala Asp Asp Glu Glu Gly Gly Gly Gly U C A G codons (‘stop codons’, termination codons) which specify the termination of polypeptide chain synthesis. (cf READTHROUGH.) The three bases of a codon do not contribute equally to the specificity of the codon: the 2nd base contributes maximally, the 3rd minimally; thus, certain codons which specify the same amino acid differ from each other only in the 3rd base (see table). (See also WOBBLE HYPOTHESIS.) There is some relationship between codons which specify related amino acids: e.g. all codons in which U is the 2nd base specify hydrophobic amino acids. [Hypothesis on the physicochemical rationale of the genetic code: FEBS (1985) 189 159–162.] The frequency with which synonymous codons occur in mRNAs is believed to parallel the population sizes of the corresponding tRNAs in the cell, i.e., the most commonly used codons correspond to the most abundant of the isoaccepting tRNA molecules. The codons in a given gene may thus affect the rate of translation of that gene; e.g., it has been suggested that the presence of rare codons in sequences encoding signal peptides (see SIGNAL HYPOTHESIS) may serve – or may have served at some evolutionary stage – to delay translation, allowing time for the ribosome–polypeptide complex to become associated with the cell membrane [FEBS (1985) 189 318–324]. (See also CODON BIAS.) The genetic code as shown in the table is often said to be ‘universal’, i.e., applicable to all living systems. However, there are exceptions. For example in at least some plant, animal and fungal mitochondria – and apparently also in Mycoplasma capricolum [PNAS (1985) 82 2306–2309] – UGA functions not as a stop codon but as a sense codon specifying tryptophan; in mammalian mitochondria AUA specifies methionine, while AGA and AGG function as stop codons. In the ciliate MACRONUCLEUS, UAA and UAG specify glutamine, UGA apparently acting as the sole stop codon. It is believed that these distinct codes must have evolved from a common, more primitive code rather than one from another. [Evolutionary aspects: MR (1992) 56 229–264.] genetic drift Changes in the genotype of a species or strain which are due not to selection pressures but to the persistence of neutral mutations, i.e., mutations which appear to be neither 335 genetic engineering advantageous nor disadvantageous to the organism. (cf. bacterial species – references for some of them can be found in the entry BACTERIA. Sequences have also been published e.g. for the genome of the plant Arabidopsis thaliana [Nature (2000) 408 791–826] and the human genome [Nature (2001) 409 813–958]. genomic library See LIBRARY. genomic masking See PHENOTYPIC MIXING. genophore A nucleic acid which carries genetic information; the term is usually applied to a prokaryotic chromosome, viral genome, ctDNA or mtDNA. genospecies Any given group of strains among which genetic exchange can occur. genotoxin Any DNA-damaging agent (e.g. a MUTAGEN). genotype The genetic constitution of an organism: i.e., the organism’s content of genetic information, either in total or with respect to one or more particular named alleles, regardless of whether or not that information is being – or can be – expressed under a given set of conditions. (cf. GENOME; PHENOTYPE.) gentamicin Any of several (related) AMINOGLYCOSIDE ANTIBIOTICS (gentamicins A, B, C1 , C1a , C2 etc) produced by Micromonospora spp. The drug ‘gentamicin’ used clinically contains gentamicins C1 , C1a , and C2 ; it is active against e.g. some strains of Pseudomonas aeruginosa. gentian violet A mixture of dyes which includes CRYSTAL VIOLET and METHYL VIOLET. gentiobiose A reducing disaccharide: b-D-glucopyranosyl-(1 → 6)-D-glucose. It occurs naturally in several plant glycosides (e.g. AMYGDALIN) and as a degradation product of glucans containing (1 → 6)-b-D-glucosyl linkages. genus A taxonomic rank (see TAXONOMY and NOMENCLATURE). genus novum (gen. nov.; gen. n.; n. gen.) A designation used to indicate a newly proposed genus at the time of its initial publication. Geodermatophilus A genus of aerobic, catalase-positive bacteria (order ACTINOMYCETALES, wall type III) which occur in soil. The organisms form a simple substrate mycelium of thick strands, each a mass of cuboid cells formed by repeated division in both transverse and longitudinal modes; the mycelium gives rise to zoospores and to non-motile cuboid or coccoid cells. Budding occurs in zoospores and in young mycelia. GC%: ca. 73–75. Type species: G. obscurus. (cf. DERMATOPHILUS.) Geoglossum See HELOTIALES. geophilic Living or growing in or on the earth. (cf. DERMATOPHYTES.) geosmin (trans-1,10-dimethyl-trans-9-decalol) A substance which is produced by certain aquatic actinomycetes and cyanobacteria (e.g. Anabaena, Lyngbya and Oscillatoria spp) and which imparts an ‘earthy’ or ‘musty’ taste and odour to the water and to fish living in the water. (cf. METHYLISOBORNEOL.) geotaxis A TAXIS in which the stimulus is gravity; a negatively geotactic organism moves upwards. Geotoga See BACTERIA (taxonomy). Geotrichum A genus of fungi of the class HYPHOMYCETES; teleomorphs occur in the genus Dipodascus. [DNA relatedness between Geotrichum and Dipodascus: CJB (1985) 63 961–966.] The mycelium is septate, each septum having a single, central pore; pores as small as 50 nm (micropores) have been reported e.g. in G. candidum. Conidiogenesis is thallicarthric (see CONIDIUM). In e.g. G. candidum the septa which separate adjacent conidia are multiperforate. (See also CHEESE SPOILAGE, PAPER SPOILAGE, SEWAGE FUNGUS, VILIA.) geotropism See TROPISM (sense 1). GEP See PROTEIN SECRETION. GERL Part of the GOLGI APPARATUS from which LYSOSOMES are derived. ANTI- GENIC DRIFT.) genetic engineering The in vitro manipulation of nucleic acid molecules e.g. to generate new combinations of genes or sequences (see e.g. FUSION PROTEIN), to place a given gene or genes under the control of a different regulatory system (see e.g. OPERON FUSION), to introduce specific mutations into a molecule (see SITE-SPECIFIC MUTAGENESIS), etc. The ‘engineered’ DNA may be produced in quantity by any of various CLONING techniques. (See also BIOTECHNOLOGY.) genetic footprinting A procedure which may be used to determine the need for expression of each of a range of specific genes during growth under particular, chosen conditions. (cf. FOOTPRINTING.) The procedure involves three stages. Initially, a population of cells is subjected to transposon mutagenesis, transposons inserting randomly and extensively in the genome; from this mutagenized population, a sample (labelled T0, for time zero) is taken and set aside for subsequent analysis. The remainder of the mutagenized cells are then grown for many population doublings under conditions that repress further transposition. Finally, genomic DNA is isolated from (i) cells of the T0 population and (ii) cells of the cultured mutagenized population, and PCR is used to amplify sequences from specific genes from each of these two groups of cells. If a given gene is necessary for viability under particular growth conditions, then transposon insertions may be detected in that gene from the T0 population, but fewer (or none) will be found in that gene from cultured populations of the cells. [Genetic footprinting in bacteria (Escherichia coli ): JB (2001) 183 1694–1706.] (See also IVET.) genetic immunization The use of a DNA VACCINE. (See also FILOVIRIDAE.) genetic reactivation See REACTIVATION. genetic vaccination The use of a DNA VACCINE. (See ALSO FILOVIRIDAE.) genital herpes See HERPES SIMPLEX. genital wart See PAPILLOMA. genitourinary tract flora The distal part of the human urethra (in either sex) may harbour any of a range of organisms which include various Gram-positive cocci (e.g. Staphylococcus spp), Gram-negative bacteria (e.g. species of Acinetobacter, Escherichia), and yeasts (e.g. Candida albicans); in the female, the urethral microflora is generally influenced by the resident vaginolabial flora (see VAGINA MICROFLORA). The microflora of the external genitalia in both sexes may include those organisms mentioned above together with e.g. various coliform bacteria and Mycobacterium smegmatis. (See also UREAPLASMA.) Other regions of the genitourinary tract (e.g. urinary bladder, ureters) do not normally have a resident microflora. (See also BODY MICROFLORA and URINARY TRACT INFECTION.) genome A term which is used with various meanings, including the following: (1) The genetic material of an organism. For example: (i) the chromosome in a bacterial cell (or one of each type of chromosome if more than one type is present – e.g. both the small and large chromosomes in Vibrio cholerae [Nature (2000) 406 477–483]); (ii) the DNA or RNA in a virion; (iii) the chromosome(s) together with any associated plasmids – for example, the chromosome and the two small plasmids in the bacterium Buchnera [Nature (2000) 407 81–86]. (2) All the (different) genes in a cell or virion. (3) The haploid set of chromosomes or genes in a cell. The ‘complete genome sequence’ (i.e. the complete sequence of nucleotides in the genome) has been published for a number of 336 gibberellins disc’ or ‘sucker’ by means of which the parasite is believed to adhere to the intestinal wall of its host (see also GIARDIASIS). The cell contains a pair of AXOSTYLES, and one or two deeply staining, rod- or comma-shaped bodies (median or ‘parabasal’ bodies) which occur in the centre of the cell and are orientated more or less perpendicularly to the long axis of the cell. The trophozoites reproduce by binary fission; they stain well with e.g. iron–haematoxylin. [Culture of G. lamblia in serum-free medium: J. Parasitol. (1983) 69 1181–1182. Lipid metabolism in Giardia: TIP (2001) 17 316–319.] G. lamblia forms ovoid cysts (ca. 11–15 × 7–10 µm) in the gut lumen, and these are expelled with the faeces. Mature cysts generally contain 4 nuclei which are commonly clustered at one pole. The nuclei and portions of the median bodies and flagella can be seen in cysts stained with e.g. iodine. The classification of Giardia strains into species remains controversial. Strains are often named according to the host in which they are found – e.g. G. canis (from dogs) – but such speciation may not be justified. It has been proposed that only three species be recognized on the basis of morphology and host range: G. agilis (from frogs and tadpoles), G. muris (from rodents and birds), and G. duodenalis (from mammals); strains currently known as ‘G. lamblia’ would thus be included in the species G. duodenalis. giardiasis (lambliasis) A human disease caused by Giardia lamblia. Infection occurs by ingestion of cysts; transmission may occur by person-to-person contact of via contaminated food or drinking water (cysts can survive levels of chlorination used for treating WATER SUPPLIES). Incubation period: ∼4–7 days. Infection may be asymptomatic, or there may be acute or chronic diarrhoea, often with nausea, weight loss, fatty stools and flatulence. Symptoms are believed to be due to mechanical obstruction of the intestinal mucosa; tissue invasion does not occur. Although adhesion of G. lamblia to the mucosa of the small intestine is generally believed to depend on the organism’s ‘sucker’ (see GIARDIA), adhesion may involve a specific giardial LECTIN which is activated by gut proteases [Science (1986) 232 71–73]. Diagnosis. Trophozoites may be detected by microscopy in (fresh) saline-mounted specimens of diarrhoea; cysts, rather than trophozoites, may be found in specimens of formed stool. An ELISA-based method, using polyclonal antibodies, may be used to detect Giardia antigens in faeces. Chemotherapy: e.g. METRONIDAZOLE, QUINACRINE. [Giardia in drinking water supplies (cyst levels, parasite viability, health impact): AEM (1996) 62 47–54.] Gibberella A genus of fungi (order HYPOCREALES) which include plant-pathogenic species (see e.g. BAKANAE DISEASE); anamorphs occur in the genus FUSARIUM. Gibberella spp form blue- or violet-coloured superficial perithecia, and ovoid to fusiform ascospores. (See also GIBBERELLINS and ZEARALENONE.) gibberellic acid GIBBERELLIN A3 . gibberellins A class of PHYTOHORMONES which are synthesized e.g. in the leaves of plants and which regulate stem elongation, seed germination, etc; gibberellins were originally discovered as products of the fungus Gibberella fujikuroi (Fusarium moniliforme). Many gibberellins have now been identified and designated gibberellins A1 , A2 etc (or GA1 , GA2 etc); all share a common tetracyclic structure, but differ in the position and nature of their substituents. The production of gibberellins by G. fujikuroi is believed to play an important role in the pathogenesis of BAKANAE DISEASE; another example of a plant pathogen which produces germ sporangium (mycol.) A SPORANGIUM produced at the distal end of a germ tube formed by a germinating zygospore. germ tube (mycol.) A short, hypha-like structure which develops from certain types of spore on germination, or – in e.g. Candida albicans – from yeast cells undergoing yeast-to-mycelium transition. A germ tube usually develops into a hypha. germ warfare See BIOLOGICAL WARFARE. Germall 115 A water-soluble imidazolidinyl urea compound with antibacterial and antifungal properties; it is used as a PRESERVATIVE in e.g. cutting oil emulsions. German measles Syn. RUBELLA. germicide (1) A general term for any antimicrobial chemical agent used for disinfection, antisepsis or sterilization – regardless of whether its action is microbicidal or microbistatic. (2) Any microbicidal disinfectant, antiseptic or sterilant. germinal centres See ANTIBODY FORMATION. germinant See ENDOSPORE (sense 1(a)). germination (1) The process in which a spore gives rise to a vegetative cell or hypha. (2) (of bacterial endospores) See ENDOSPORE (sense 1(a)). germination by repetition (mycol.) On the germination of a spore: the formation of a secondary spore (rather than a germ tube). Gerstmann–Sträussler–Scheinker syndrome A human, inherited, autosomal-dominant TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHY (one of a number of inherited prion diseases). Characteristically, PrP–amyloid plaques in the cerebellar cortex are associated with pyramidal tract degeneration. Onset occurs in the third–seventh decade, and the mean duration of the illness is 5 years; clinical features (including the presence/absence of dementia) appear to depend on genotype. Getah virus An ALPHAVIRUS which is closely related to ROSS RIVER VIRUS; it is apparently non-pathogenic in man, but in horses it can cause a febrile disease with e.g. oedema of the limbs. GF stain GRIDLEY FUNGUS STAIN. GFP GREEN FLUORESCENT PROTEIN. ghost (1) An empty phage capsid (i.e., a capsid devoid of its genome). (2) An empty vesicle, composed of cytoplasmic membrane, obtained e.g. from a bacterial cell by removal of its peptidoglycan followed by cell lysis. Gi protein See ADENYLATE CYCLASE. giant cells Large multinucleate cells found in the tissues in certain diseases – e.g. CHICKENPOX, MEASLES, TUBERCULOSIS (cf. GRANULOMA); they are formed by the fusion of MACROPHAGES. Langhans giant cells contain relatively few nuclei which are situated at the periphery of the cytoplasm; foreign body giant cells contain many nuclei distributed throughout the cytoplasm. giant condyloma acuminatum Syn. BUSCHKE–LÖWENSTEIN TUMOUR. Giardia A genus of parasitic flagellate protozoa of the DIPLOMONADIDA. Strains occur in the intestinal tract in a wide range of vertebrates; the organisms are transmitted as cysts. G. lamblia (= e.g. G. intestinalis, Lamblia intestinalis) occurs in the small intestine of man (causing GIARDIASIS), other primates, and pigs. Morphologically, the trophozoite resembles one half of a longitudinally-bisected pear, the flat side being ventral; it is commonly ca. 10–20 × 5–10 µm. The two nuclei occur symmetrically in the broad anterior region. The 8 flagella arise ventrally between, and slightly anterior to, the nuclei; three pairs emerge laterally, and the distal ends of the fourth pair become free at the extreme posterior end of the cell. The anterior ventral surface of the cell body bears a large ‘sucking 337 gibbon ape leukaemia virus on various tissues within the mouth; systemic symptoms may include anorexia, fever, and the involvement of the submandibular and cervical lymph nodes. The causal agent is the type 1 HERPES SIMPLEX virus. Acute necrotizing ulcerative gingivitis occurs principally in young adults and is apparently stress-linked. It involves necrosis of the gingiva and the development of ulcers which usually become secondarily infected by spirochaetes. The primary aetiological agents are believed to include Bacteroides intermedius. In chronic (non-specific) gingivitis the gingiva may show signs of oedema, hyperplasia or atrophy. The severity of gingivitis often appears to correlate with stress and/or with hormonal influences; thus, e.g. the severity of the disease may increase during puberty or pregnancy. Giovannolaia A subgenus of PLASMODIUM. girdle See e.g. DINOFLAGELLATES. girdle bands See DIATOMS. Glabratella See FORAMINIFERIDA. glabrous Hairless; smooth. glanders An acute or chronic, often fatal disease which affects mainly equines (horse, mule, donkey), although some other animals – including man – are susceptible; the causal agent, Pseudomonas mallei, appears to enter the body mainly via the mouth or, less commonly, via the nose or skin lesions. Among equines, the acute form of the disease (more common in mules and donkeys) typically involves fever, coughing, a highly infectious nasal discharge, ulceration of the nasal mucosa, and nodular skin lesions on the limbs or abdomen; death from septicaemia may occur within days. Chronic glanders (more common in horses) may be largely pulmonary, with coughing, dyspnoea and epistaxis; alternatively, the prominent features may include ulcerating nasal lesions and/or subcutaneous ulcerating nodules (often in the hock region) which discharge a dark-honey-coloured pus. In farcy, a cutaneous manifestation of glanders, ulcerating lesions occur in cutaneous and subcutaneous tissues, and the regional lymph nodes and ducts become swollen and hard (the so-called farcy buds and farcy pipes, respectively). Lab. diagnosis: e.g. a CFT and/or the MALLEIN TEST. Treatment: e.g. sulphonamides together with a formalinized vaccine prepared from P. mallei. (cf. EPIZOOTIC LYMPHANGITIS.) In man, the disease (which can be fatal) affects mainly the respiratory tract and/or cutaneous tissues, but the viscera, muscles or bones may also be affected. glandular fever Syn. INFECTIOUS MONONUCLEOSIS. Glanzmann’s thrombasthenia See INTEGRINS. glass (biodeterioration of) In warm, humid conditions certain fungi (e.g. Aspergillus glaucus, A. fumigatus, Cladosporium spp, Penicillium spp) can grow on glass surfaces (e.g. microscope lenses), deriving nutrients either from adjacent substrates (e.g. lens sealing compounds) or from the film of material which tends to condense and collect on the glass; the surface of the glass may be etched by the extracellular products of such fungi. Certain lichens can grow on and etch stained glass windows etc. Damage to glass may be avoided by frequent cleaning, storage under dry conditions (where possible), use of fungicides, etc. Glasser’s disease (infectious polyarthritis and polyserositis) An acute disease of young pigs, often fatal within days. Onset is sudden, with fever, anorexia, dyspnoea, and lameness with swollen joints; the causal agent is a species of Haemophilus – often H. parasuis – although Mycoplasma spp can also be isolated in some cases. Treatment: e.g. tetracyclines, given early. glaucescent Becoming GLAUCOUS. Glaucocystis See GLAUCOPHYTA. a gibberellin (A4 ) is Sphaceloma manihoticola, causal agent of ‘superelongation disease’ in cassava [BBRC (1979) 91 35–40]. Stunting of plants infected with certain viruses may be reversed by the application of gibberellins; it has been suggested that such viruses may affect the functioning of endogenous plant gibberellins. Gibberellins obtained commercially from G. fujikuroi have been used e.g. for controlling premature fruit-drop, improving fruit setting, improving the quality and yield of grapes, and inducing rapid and uniform germination in malting barley (e.g. for BREWING). gibbon ape leukaemia virus (GALV) A C-type retrovirus (subfamily ONCOVIRINAE) first isolated from a female gibbon with disseminated lymphosarcoma; GALV can be transmitted horizontally and can cause a substantial incidence of leukaemia in gibbon colonies. GALV is infectious for a wide range of host cells, including e.g. human, monkey, cat, mink, rabbit, rat (but not mouse), and various avian (but not chicken) cells. [Book ref. 114, pp. 130–137.] (cf. SIMIAN SARCOMA VIRUS.) Giemsa’s stain See ROMANOWSKY STAINS. Gigartacon See ACANTHAREA. Gigartina A genus of marine algae (division RHODOPHYTA). The organisms, which attach to rocks etc by means of a holdfast, vary widely in form according to species – often being flat (ribbonlike) and more or less branched; the surface frequently bears numerous papillate outgrowths. Gigartina spp are used e.g. as a source of CARRAGEENAN. Gigaspora See ENDOGONALES. Gilbertella See MUCORALES. Gilchrist’s disease Syn. BLASTOMYCOSIS. gill (mycol.) Syn. LAMELLA. gill disease (in fish) See BACTERIAL GILL DISEASE. gill fungi Fungi which form lamellae: see LAMELLA. gill rot A FISH DISEASE caused by Branchiomyces sanguinis or B. demigrans. The fungus grows on and into the gills, obstructing capillary circulation and leading to necrosis; the fish soon dies from suffocation. Gill rot is encouraged by high temperatures and the presence of decaying organic matter. Gilpinia hercyniae NPV See BACULOVIRIDAE. Giménez stain A procedure for staining rickettsiae. Add 4% aqueous phenol (250 ml) and distilled water (650 ml) to 10% basic fuchsin in 95% ethanol (100 ml); allow to stand for 2 days at 37° C, and then add 2 volumes to 5 volumes of phosphate buffer (0.2 M, pH 7.45), mix, and filter immediately before use. A heat-fixed smear is stained for 1–2 min and washed with tap water; the smear is then stained for 5–10 sec with 0.8% w/v malachite green oxalate, washed in tap water, stained again for 5–10 sec with malachite green oxalate, washed thoroughly in tap water, and blotted dry. Rickettsiae stain red, background green. gin See SPIRITS. gin gene See BACTERIOPHAGE MU. ginger blotch A MUSHROOM DISEASE caused by a distinct strain of the Pseudomonas fluorescens complex; superficial pale-brown to ginger-coloured lesions develop on the caps [JAB (1982) 52 43–48]. (cf. BROWN BLOTCH.) gingivitis An acute or chronic inflammation of the gingiva (i.e., the gums) affecting areas of gum tissue (the ‘attached gingiva’) other than those immediately adjacent to the tooth–gum junction (which are referred to as the ‘free gingiva’); unlike PERIODONTITIS, gingivitis does not bring about a permanent loss of periodontal structures. Acute herpetic gingivostomatitis occurs most commonly in children and generally lasts about two weeks. It involves reddening of the gingiva and the formation of vesicular lesions 338 Gloeocapsa in a number of bacteria and cyanobacteria. In bacteria of the Cytophaga–Flexibacter group, sulphonolipids in the cell envelope appear to be necessary for gliding motility [Nature (1986) 324 367–369]. In some gliding bacteria certain stages in the life cycle (e.g. the myxospore in myxobacteria) are non-motile. In Flexibacter elegans (cells ca. 1–50 µm) gliding occurs only in those cells which are longer than ca. 5 µm. In some cyanobacteria, motility has been associated with the junctional pore complex (JPC): a channel in the cell envelope through which mucilage (carbohydrate) is secreted; each organism has a number of JPCs, and secretion via these channels is believed to provide the thrust that is necessary for locomotion [JB (2000) 182 1191–1199]. Results of studies on Myxococcus xanthus (involving shockfreezing and freeze-drying) have indicated that the motility apparatus in this organism resembles a helical ‘continuum’ or band (∼170–380 nm wide) that is wrapped around the protoplast (i.e. within the periplasmic space) to form a closed loop; ‘nodes’, associated with the continuum, may travel along the trichomes in a wave-like fashion (but with variable internode distance), each node possibly corresponding to a region of close contact, or adhesion, between the trichome and substratum [Microbiology (2001) 147 939–947]. Gliocladium See HYPHOMYCETES and GLIOTOXIN. gliotoxin An epidithiapiperazinedione MYCOTOXIN (see EPIPOLYTHIAPIPERAZINEDIONES) obtained from Trichoderma viride and from species of Aspergillus, Gliocladium and Penicillium; it inhibits replication of polioviruses, echoviruses and measles virus, and also has antibacterial, antifungal and antitumour activity. In eukaryotic cells an early effect of gliotoxin is apparently disruption of microfilaments [JCS (1986) 85 33–46]. glmM gene In Helicobacter pylori : a gene (formerly ureC ) which encodes phosphoglucosamine mutase. gln genes See e.g. NTR GENES. global regulatory system Syn. REGULON. global warming See GREENHOUSE EFFECT. Globigerina A genus of pelagic foraminifera (order FORAMINIFERIDA). The test is calcareous and multiloculate, the rounded locules being spirally arranged. G. bulloides may reach ca. 800 µm diam., and numerous fine calcareous spines radiate from the test; G. bulloides and e.g. G. inflata, G. pachyderma and G. quinqueloba occur in subpolar and cold temperate seas. Globigerinoides A genus of pelagic foraminifera (order FORAMINIFERIDA). G. sacculifer is often abundant in tropical and subtropical seas, occuring in the upper euphotic zone; it is ca. 0.5–1.0 mm diam., bears radial spines 1–2 mm long, and contains numerous zooxanthellae. globomycin See e.g. BRAUN LIPOPROTEIN. globose Spherical. globoside (parvovirus receptor) See ERYTHROVIRUS. Gloeobacter A genus of unicellular CYANOBACTERIA (section I) in which the rod-shaped cells divide by equal binary fission in one plane and occur in colonial aggregates surrounded by a sheath; the cells are characterized by their lack of THYLAKOIDS – photosynthesis and respiration apparently being associated with the cytoplasmic membrane. Phycobiliproteins occur as an electron-dense layer of characteristically shaped phycobilisomes (apparently composed of bundles of rods) beneath the cytoplasmic membrane [Arch. Micro. (1981) 129 181–189]. GC% (one strain): 64. Gloeocapsa A genus of unicellular CYANOBACTERIA (section I) in which the cells are coccoid and divide by equal binary Glaucoma A genus of ciliates (order HYMENOSTOMATIDA). G. scintillans is ovoid, ca. 40–80 µm, with uniform somatic ciliature; it is common in freshwater habitats containing decomposing organic matter (see also SAPROBITY SYSTEM). G. dragescui also is ovoid, but G. frontata is elongate with a pointed posterior end. Glaucophyta A phylum of ‘algae’ which includes those species that contain CYANELLES instead of CHLOROPLASTS. The phylum contains wall-less organisms (e.g. the flagellated, cryptomonadlike Cyanophora) and species which have a cell wall (e.g. Glaucocystis). In Cyanophora the (cyanobacterial) endosymbiont is a species of Cyanocyta; synthesis of phycocyanin by the endosymbiont is apparently partially dependent on the host’s ribosomes. glaucous Bluish-green, or covered with a bluish-green powdery or waxy bloom. GLC Gas–liquid CHROMATOGRAPHY. GlcNAc N-ACETYL-D-GLUCOSAMINE. gleba In the fruiting bodies of e.g. fungi of the GASTEROMYCETES: the spore-bearing tissue which is (at least initially) enclosed by the PERIDIUM (or by the wall of the peridiole in members of the NIDULARIALES). Glenodinium See DINOFLAGELLATES. gliding bacteria A non-taxonomic group of bacteria and cyanobacteria which exhibit GLIDING MOTILITY during at least some stage in their life cycles; gliding bacteria typically occur in soil and in freshwater and marine habitats. The group comprises the genera listed under CYTOPHAGALES, the MYXOBACTERALES, the CHLOROFLEXACEAE, members of the cyanobacteria – both unicellular (e.g. Synechococcus) and filamentous (e.g. Anabaena, Oscillatoria, Phormidium, Spirulina) – and e.g. Capnocytophaga and Desulfonema. Although not formally included among the gliding bacteria, some species of Mycoplasma exhibit a form of surfaceassociated locomotion referred to as gliding. gliding motility A continuous-type (i.e. smooth, non-jerky) MOTILITY which occurs in some prokaryotes (GLIDING BACTERIA), algae (e.g. some diatoms and desmids) and protozoa (e.g. Toxoplasma gondii ) which have no obvious locomotory organelles; gliding motility appears to occur only on a solid surface. In the diatom Amphora coffeaeformis, some studies have suggested a possible mechanism that involves actin- and tubulinbased structures; this is compatible with the theory that gliding results from the translocation of membrane proteins along the raphe canal – the outermost part of such proteins being attached to the immobile slime secretion which has adhered to the substratum [Cell Mot. (1985) 5 103–122]. In the gregarine Gregarina garnhami (which apparently does not contain actin) it has been suggested that gliding may involve two sets of filaments which run longitudinally in the pellicle [JUR (1984) 88 66–76]. Gliding motility has been extensively investigated in prokaryotes [early review: ARM (1981) 35 497–529]. In some species, individual cells glide quite slowly (a few micrometres per minute) while in other species the cells may achieve speeds of up to ca. 150 micrometres per minute; some gliding bacteria can also perform other types of movement (e.g. flexing, twitching – see e.g. SPIRULINA). The path of a gliding cell is marked by a slime trail, and on an agar surface the path may be ‘etched’ into the agar; at least some gliding bacteria can penetrate agar gels. Gliding is also exhibited by trichomes and by aggregates of cells (‘swarms’). Typically, gliding bacteria glide when nutrient levels are low; high levels can suppress gliding. Gliding appears to require the presence of calcium ions and is inhibited by EGTA 339 Gloeochloris fission in two or three planes, giving rise to clusters of cells enclosed in a sheath. GC%: 40–46. The organisms grow – often in masses – on wet rocks e.g. in the supralittoral zones of rocky coasts. (cf. GLOEOTHECE.) Gloeochloris A genus of algae of the XANTHOPHYCEAE. The vegetative cells occur in a spherical palmelloid colony; asexual reproduction occurs by the formation and release of biflagellate zoospores. Gloeodinium A genus of DINOFLAGELLATES which form palmelloid colonies and reproduce by zoospore formation. One species (G. montanum), found in freshwater peaty marshes. Gloeothece A genus of unicellular CYANOBACTERIA (section I); the cells are rod-shaped (ca. 5–6 µm wide), possess thylakoids (cf. GLOEOBACTER), divide by equal binary fission in one plane (cf. GLOEOCAPSA), and occur in colonial aggregates surrounded by a well-defined sheath (cf. SYNECHOCOCCUS). The function of the sheath seems not to include the protection of nitrogenase from oxygen [oxygen relationships in nitrogen fixation: MR (1992) 56 346–347]. Strains of Gloeothece can carry out NITROGEN FIXATION aerobically, despite the absence of heterocysts, and were formerly regarded as nitrogen-fixing strains of GLOEOCAPSA. GC%: 40–43. Gloeotrichia A phycological genus of the RIVULARIACEAE (now included in the genus CALOTHRIX). ‘G. echinulata’ is a gas-vacuolate, freshwater planktonic species which may form blooms; in nature, it forms characteristic spherical urchin-like colonies in which the terminal heterocysts occur in the centre of the colony and the trichomes radiate outwards. Hair cells may be formed e.g. under phosphorus-deficient conditions. Gloiopeltis See RHODOPHYTA. glomerulonephritis Inflammation of the kidney glomeruli. Acute glomerulonephritis is characterized by blood and red cell casts in the urine, and a reduced glomerular filtration rate leading to oliguria, oedema and hypertension. It may be due to deposition in the glomeruli of antigen-antibody complexes formed during certain infectious diseases, e.g., SYPHILIS (see also POST-STREPTOCOCCAL GLOMERULONEPHRITIS); in mice it may be induced e.g. by the LCM VIRUS. (See also PYELONEPHRITIS.) Glomospira See FORAMINIFERIDA. Glomus See ENDOGONALES. glorin See POLYSPHONDYLIUM. Glossina (tsetse fly) A genus (22 species) of haematophagous (blood-sucking) flies (order Diptera) which include vectors of salivarian trypanosomes; they occur in sub-Saharan Africa in a belt approximately 15° N–15° S, and in SE Africa, including Mozambique. Glossina spp are distinguished from other dipterans e.g. by the chopper-shaped ‘hatchet cell’ in the wing venation. Both the male and female flies suck blood. glove box A cabinet, the interior of which is accessible via gloves attached to openings in the front panel. (See also SAFETY CABINET.) glove juice test A test used (in the USA) to determine the efficacy of a given antimicrobial agent used as a surgical hand scrub [Book ref. 65, pp. 958–961]. GlpF See MIP CHANNEL. glucan A homoglycan (see POLYSACCHARIDE) consisting of glucose residues – see e.g. CELLULOSE, GLYCOGEN, LUTEOSE, NIGERAN, PARAMYLON, STARCH. (See also CELL WALL (fungal).) glucitol Syn. SORBITOL. glucoamylases Syn. g-AMYLASES. glucobrassicin See CLUBROOT. glucocorticoids Steroid hormones, produced in the adrenal cortex, whose activity includes regulation of carbohydrate and protein metabolism. Glucocorticoids also have anti-inflammatory activity; thus, e.g. they inhibit the ability of IFN-g and lipopolysaccharides to induce the synthesis of CYTOKINES in macrophages. glucogenesis Syn. GLUCONEOGENESIS. glucomannans See MANNANS. gluconate See GLUCONIC ACID. gluconeogenesis (glucogenesis) The synthesis of glucose (and hence other hexoses) from non-carbohydrate substrates such as acetate, glycerol, pyruvate, succinate etc. Essentially, this may be achieved by conversion (where necessary) of the substrate to an intermediate of the EMBDEN–MEYERHOF–PARNAS PATHWAY [Appendix I(a)], followed by reversal of reactions of the EMP pathway – the three irreversible reactions in this pathway being bypassed by other enzymes. Thus, e.g., in Escherichia coli phosphoenolpyruvate can be synthesized from pyruvate or from the TCA CYCLE intermediates oxaloacetate and malate [see Appendix II(b)]; phosphoenolpyruvate is then converted to fructose 1,6-bisphosphate by reversal of reactions of the EMP pathway. Fructose 1,6-bisphosphate is hydrolysed by fructose bisphosphatase to fructose 6-phosphate which can in turn be converted to glucose 6-phosphate by phosphohexose isomerase. gluconic acid Gluconic acid and its salts are metabolized by various microorganisms (see e.g. ENTNER–DOUDOROFF PATHWAY), and are synthesized (by the oxidation of glucose) e.g. by Aspergillus niger and other hyphomycetes and by Gluconobacter oxydans. A. niger and G. oxydans are used as commercial sources of these compounds which have a range of uses in the food and pharmaceutical industries; for example, gluconic acid (a chelating agent) is used for sequestering metal ions; sodium gluconate is used for cleaning glassware and for removing rust from metals; calcium and iron gluconates are used therapeutically for the treatment of calcium and iron deficiencies. Gluconobacter A genus of Gram-type-negative bacteria of the ACETOBACTERACEAE; the organisms occur e.g. on certain fruits and flowers, and are used e.g. for the production of MONOBACTAMS and in the SORBOSE FERMENTATION. Cells: typically ovoid to rod-shaped, 0.5–0.8 × 0.9–4.2 µm, non-motile or lophotrichously flagellate. Catalase-positive. Typically, ethanol is oxidized to acetic acid on e.g. CARR MEDIUM; neither acetate nor lactate is oxidized to CO2 . Ketogenic growth occurs on polyalcohol substrates, and all strains form 2-ketogluconic acid (and most form 5-ketogluconic acid) from D-glucose. Substrates utilized include e.g. D-mannitol, sorbitol, glycerol (see DIHYDROXYACETONE FERMENTATION), D-fructose and D-glucose; all strains require growth factors (which may include e.g. pantothenate, niacin, thiamine, PABA) when D-mannitol is used as the sole carbon source. The organisms lack a complete TCA cycle; the breakdown of sugars and polyols to CO2 occurs mainly via the HEXOSE MONOPHOSPHATE PATHWAY. Optimum growth temperature: 25–30° C. GC%: ca. 56–64. Type species: G. oxydans – which incorporates strains previously referred to as Acetobacter suboxydans. glucose dehydrogenase See QUINOPROTEIN and EXTRACYTOPLASMIC OXIDATION. glucose effect (glucose repression) (1) Syn. CATABOLITE REPRESSION. (2) Syn. CRABTREE EFFECT. glucose isomerase (xylose isomerase; D-xylose ketolisomerase; EC 5.3.1.5) An intracellular enzyme which converts D-glucose to D-fructose, although its main function in vivo is probably the conversion of D-xylose to D-xylulose; the enzyme is typically induced in the presence of xylose. Glucose isomerase, obtained e.g. from Bacillus coagulans (see IMMOBILIZATION (1)) or Streptomyces spp, is used commercially on a large scale for the 340 glycocalyx conversion of D-glucose to D-fructose e.g. in the manufacture of HIGH-FRUCTOSE CORN SYRUP. (See also ENZYMES.) glucose oxidase (b-D-glucose:oxygen 1-oxidoreductase, EC 1.1.3.4) An enzyme (produced e.g. by Aspergillus and Penicillium spp) which oxidizes glucopyranose in the presence of oxygen to Dglucono-d-lactone and H2 O2 – the former rapidly hydrolysing to D-gluconic acid. Glucose oxidase from e.g. Aspergillus niger (a glycoprotein containing 2FAD/molecule) is widely used in clinical tests for the detection of glucose in urine, blood etc; in general, such tests rely on a colour change resulting from the oxidation of a chromogen by H2 O2 released by glucose oxidase in the presence of glucose and oxygen. Glucose oxidase is also used in the food industry e.g. for removing glucose from foods (e.g. eggs preparatory to drying) to prevent browning, and for removing oxygen from e.g. beer, wine, mayonnaise etc. (See also ENZYMES.) b-glucosidase See CELLULASES. b-glucoside permease (in Escherichia coli ) See CATABOLITE REPRESSION. glucosinolase See CLUBROOT. Glugea A genus of protozoa (class MICROSPOREA) which are parasitic in fish; tumour-like cysts (XENOMAS, ‘glugea cysts’) ca. 2–4 mm diam. develop in the gut mucosa and liver of the infected fish. glume blotch A CEREAL DISEASE, caused by Leptosphaeria nodorum (Septoria nodorum), which can affect e.g. wheat and barley. Leaf lesions are initially yellow, later becoming golden-brown and sometimes coalescing. Lesions on glumes (usually spreading from the tip) are a dark purplish-brown, and they later exhibit dark or black pycnidia. Control: e.g. CARBENDAZIM or PROCHLORAZ with a DITHIOCARBAMATE. L-glutamate See GLUTAMIC ACID. glutamate dehydrogenase See AMMONIA ASSIMILATION. glutamate synthase See AMMONIA ASSIMILATION. L-glutamic acid An amino acid synthesized by the reductive amination of 2-oxoglutarate – see Appendix IV(a). It is produced commercially on a large scale for use in the food industry – mainly as a flavour enhancer (monosodium glutamate) – and is obtained from strains of bacteria (e.g. Corynebacterium glutamicum, ‘glutamic acid bacteria’) grown aerobically on e.g. molasses or starch hydrolysates. (Some strains can be grown e.g. on acetic acid.) The strains used require biotin for growth and have little or no 2-oxoglutarate dehydrogenase activity (so that metabolism of 2-oxoglutarate via the TCA cycle is limited or cannot occur). Production of glutamic acid is maximal under suboptimal growth conditions (achieved e.g. by limiting biotin). The glutamic acid must be secreted by the cells to avoid feedback inhibition of its synthesis; permeability of the cells may be increased e.g. by biotin deficiency, by treatment with fatty acid derivatives or antibiotics (e.g. penicillin), or by using strains auxotrophic for glycerol or oleic acid grown on low levels of glycerol and oleic acid, respectively. Some L-glutamine is also formed during glutamic acid production; the proportion of glutamine formed can be increased by adjusting conditions of pH, nutrient availability etc. L-glutamine An amino acid synthesized from GLUTAMIC ACID: see Appendix IV(a). It is an important intermediate in various metabolic pathways, being involved e.g. in some types of AMMONIA ASSIMILATION and in various transamination reactions – e.g. in the biosynthesis of tryptophan and of purine nucleotides [see Appendixes IV(f) and V(a)]. glutamine synthetase See AMMONIA ASSIMILATION. glutaraldehyde (CHO(CH2 )3 CHO) A dialdehyde whose two aldehyde groups can substitute amino and other groups in proteins and nucleic acids and which can thus form cross-links in and between molecules (cf. FORMALDEHYDE); it is used e.g. for pre-FIXATION (e.g. 1–10% in phosphate or cacodylate buffer) and as a DISINFECTANT or STERILANT (e.g. for medical and other equipment). Glutaraldehyde (e.g. 2% aqueous) is rapidly lethal for a wide range of vegetative microorganisms and viruses, but activity may be slow against e.g. Mycobacterium tuberculosis and bacterial ENDOSPORES: the dried spores of Bacillus subtilis, for example, have been reported to need up to 10 hours in 2% glutaraldehyde for complete inactivation (cf. STERILANT). The antimicrobial activity of glutaraldehyde is much greater (but stability is reduced) under alkaline or neutral conditions than under acidic conditions. [For antimicrobial activity and uses of glutaraldehyde see JAB (1980) 48 161–190.] glutarimide antibiotics A group of ANTIBIOTICS characterized by two structural components: a b-(2-hydroxyethyl)-glutarimide and a cyclic or non-cyclic ketone. They include e.g. CYCLOHEXIMIDE and inactone. glutathione peroxidase (glutathione:hydrogen peroxide oxidoreductase; EC 1.11.1.9) An enzyme which catalyses the oxidation of reduced glutathione by hydrogen peroxide. (See also CATALASE.) glycan Syn. POLYSACCHARIDE. glyceollin Any of four isoflavonoid PHYTOALEXINS (glyceollins I–IV) formed by the soybean plant. glycerate pathway See TCA CYCLE. glycerokinase (ATP:glycerol 3-phosphotransferase; EC 2.7.1.30) An enzyme which catalyses the ATP-dependent phosphorylation of glycerol. It is obtained commercially from e.g. ‘Candida mycoderma’ and Escherichia coli, but a more stable enzyme is obtained from Bacillus stearothermophilus. It is used in diagnostic kits for the determination of serum triglyceride levels [PTRSLB (1983) 300 399–410]. (See also ENZYMES.) glycerol (CH2 OH.CHOH.CH2 OH) Glycerol is produced by various organisms as a (usually minor) product of e.g. ALCOHOLIC FERMENTATION and other fermentations; it is formed by the reduction of dihydroxyacetone phosphate (DHAP, an intermediate of the EMBDEN–MEYERHOF–PARNAS PATHWAY) to glycerol 3-phosphate, followed by hydrolysis to yield free glycerol. (See also NEUBERG’S FERMENTATIONS.) Some bacteria can use glycerol as a growth substrate: e.g. Escherichia coli can use glycerol (taken up by FACILITATED DIFFUSION) as sole source of carbon and energy in the presence of an external electron acceptor such as O2 , NO3 − , or fumarate. Glycerol is phosphorylated by an ATP-dependent kinase, the resulting glycerol 3-phosphate being oxidized by an NAD-dependent dehydrogenase to yield DHAP which can be further metabolized via the EMP pathway. (See also DIHYDROXYACETONE FERMENTATION.) Klebsiella pneumoniae and certain lactobacilli can ferment glycerol: some glycerol is converted to DHAP as above, and the NADH thus formed is reoxidized by a parallel pathway in which glycerol is converted to b-hydroxypropionaldehyde (by a coenzyme B12 -dependent glycerol dehydrase) which is in turn reduced to 1,3-propanediol by an NADH-dependent reductase. (See also ALCOHOLS (as antimicrobial agents) and COMPATIBLE SOLUTE.) glycerol facilitator See MIP CHANNEL. glycerol fermentation See NEUBERG’S FERMENTATIONS and GLYCEROL. glycine betaine See OSMOREGULATION. glycine biosynthesis See e.g. Appendix IV(c). glycobiarsol See ARSENIC. glycocalyx In eukaryotic cells: a layer of polysaccharide and/or glycoprotein that surrounds many types of cell. In bacterial 341 glycocholic acids glyoxysome A type of MICROBODY which occurs in plant cells, and in some eukaryotic microorganisms, and which contains enzymes of the glyoxylate cycle. glypiated protein A protein which is anchored to a component of the eukaryotic cell membrane (glycosylphosphatidylinositol) via a carbohydrate chain (to which the protein is attached). Gm allotype See ALLOTYPE. GM-CSF See COLONY-STIMULATING FACTORS. GM gangliosides See SPHINGOLIPID. GMP Guanosine 5 -monophosphate: see Appendix V(a). GMS Gomori METHENAMINE–SILVER STAIN. GN broth ‘Gram-negative enrichment broth’ (used e.g. for the enrichment of shigellae from food or faeces); it contains enzymic digest of casein and animal tissue, D-glucose, citrate, deoxycholate, NaCl, phosphate buffer (pH 7.0). [Recipe: Book ref. 47, p. 652.] Gnomonia See DIAPORTHALES. gnotobiotic Refers to a microbiologically monitored environment or animal, i.e., one in which the identities of all microorganisms present are known. (cf. SPECIFIC PATHOGEN FREE.) goat pox A goat disease, clinically similar to SHEEP POX, caused by a Capripoxvirus which is antigenically distinguishable from the sheep pox virus. GOGAT See AMMONIA ASSIMILATION. gold leaching See LEACHING. gold standard (lab. microbiol.) Any well-established procedure (e.g. a diagnostic test) which is generally regarded as the superior and definitive method, and which is used as a reference against which other, proposed or alternative methods may be compared (often in terms of sensitivity and/or specificity). In many cases culture is regarded as the gold standard for diagnostic purposes. However, with the advent of nucleic-acidbased methods, some authors refer to an expanded or combined gold standard; using such a standard, a true-positive result is one that is positive by the gold standard method (usually culture) and by one nucleic-acid-amplification test [example of use of the term expanded /combined gold standard: JCM (1997) 35 957–959]. For diagnostic tests on Chlamydia trachomatis, some authors have used an aggregate gold standard in which a sample is deemed to give a true-positive result if it is either culture-positive or positive in two non-culture assays. Goldberg–Hogness box See PROMOTER. golden algae CHRYSOPHYTES. Golgi apparatus An intracellular organelle consisting typically of a number of stacks of flattened membranous vesicles (cisternae); at least one Golgi apparatus occurs in most types of eukaryotic cell (cf. e.g. DIPLOMONADIDA). Within the cell the Golgi apparatus is characteristically orientated such that one end of the stacked cisternae (the proximal or ‘forming’ end) is nearer the nucleus; the opposite end is called the distal or ‘maturing’ end. Functions of the Golgi apparatus include the formation of LYSOSOMES and the formation of secretory vesicles within which e.g. macromolecules or cell envelope substructures can be transported to the cell surface (see e.g. PALADE PATHWAY and CELL WALL (a)). Secretory molecules reach the Golgi apparatus primarily from the ENDOPLASMIC RETICULUM; thus, e.g., a portion of smooth ER forms a vesicle which encloses molecules from the lumen of the ER, and the vesicle passes through the cytoplasm and fuses with a similar vesicle or with a cisterna at the ‘forming’ end of the Golgi apparatus. Presumably as a result of repeated budding and fusion, the contents of a vesicle eventually reach the ‘maturing’ end of the Golgi apparatus and are finally enclosed within a secretory vesicle which cells: a CAPSULE – of either a polysaccharide or S LAYER type [ARM (1981) 35 299–324] – or, specifically, a polysaccharide capsule. glycocholic acids See BILE ACIDS. glycogen A highly branched D-glucan formed as a storage polysaccharide in animal cells, in many bacteria, fungi (excluding oomycetes) and protozoa, and in certain plants. Glycogen resembles amylopectin (see STARCH) but the average chain length is generally shorter (average 10–12 glucose residues) and the molecule is more extensively branched. It forms reddish-brown complexes with iodine and may be stained (non-specifically) e.g. by the PAS REACTION. Biosynthesis of glycogen involves the successive transfer of glucose residues from UDP-glucose (in eukaryotes) or ADPglucose (in bacteria) to a (1 → 4)-a-D-glucan acceptor in the presence of glycogen synthase; the branches are then introduced by a branching enzyme: an enzyme that transfers a segment of a (1 → 4)-a-glucan chain from a 4-OH to a 6-OH position. (ADP/UDP-glucose is formed from ATP/UTP and glucose 1phosphate, the latter being formed from glucose 6-phosphate by phosphoglucomutase.) In bacteria, at least, initiation of glycogen synthesis may involve the addition of glucosyl residues to a protein acceptor [BBA (1978) 540 190–196]; different enzymes may be necessary for the synthesis of the glucoprotein primer and the glycogen molecule. [Book ref. 62, pp. 171–178; ARM (1984) 38 419–458.] Degradation of glycogen typically involves the transfer of glucose residues from the non-reducing end of a chain to inorganic phosphate, a reaction catalysed by glycogen phosphorylase. (See also amylases and DEBRANCHING ENZYMES.) glycolate metabolism (glycollate metabolism) See e.g. entry TCA CYCLE. glycolate pathway See PHOTORESPIRATION. glycols (as antimicrobial agents) See ALCOHOLS. glycolysis Syn. EMBDEN–MEYERHOF–PARNAS PATHWAY. glycomacropeptide See CASEIN. glycosaminoglycan Syn. MUCOPOLYSACCHARIDE. glycosaminopeptide Syn. MUCOPEPTIDE. glycosome A MICROBODY which contains glycolytic enzymes and which occurs in members of the KINETOPLASTIDA; in e.g. Trypanosoma brucei most of the enzymes involved in the glycolytic pathway from glucose to 3-phosphoglycerate occur exclusively in glycosomes. [BBA (1986) 866 179–203 (190–193).] glycosylase (DNA glycosylase) See ADAPTIVE RESPONSE and URACIL-DNA GLYCOSYLASE. glyodin (2-heptadecyl-2-imidazoline acetate) An ANTIFUNGAL AGENT used e.g. for the control of apple scab and various fungal diseases of cherries and ornamental plants. glyoxalase See METHYLGLYOXAL BYPASS. glyoxylate cycle A pathway which occurs in a variety of organisms, including some bacteria and fungi: see entry TCA CYCLE and Appendix II(b); a key enzyme in the cycle is isocitrate lyase. The glyoxylate cycle permits the use of 2-carbon compounds and e.g. fatty acids as sources of carbon. (See also PHOTORESPIRATION.) Following phagocytosis, the persistence of Mycobacterium tuberculosis within macrophages is reported to require the activity of the (bacterial) isocitrate lyase [Nature (2000) 406 735–738]. The glyoxylate cycle is reported to be required for the expression of virulence in certain fungi [Nature (2001) 412 83–86]. glyoxylate metabolism See TCA CYCLE. glyoxylate shunt See TCA CYCLE. 342 gradostat fuses with the cytoplasmic membrane. During transit through the Golgi apparatus a molecule characteristically increases its degree of glycosylation – fucose, galactose and/or neuraminic acid residues typically being added. (See also DICTYOSOME.) Gomori stain Syn. METHENAMINE–SILVER STAIN. Gomphonema A genus of pennate DIATOMS. In e.g. G. olivaceum the cell adheres to the substratum by a ‘stalk’ of secreted mucilage. gonal nucleus See CONJUGATION (1)(c). Gonapodya See MONOBLEPHARIDALES. Gonatobotrys See HYPHOMYCETES and CONIDIUM. Gonatobotryum See HYPHOMYCETES. gongylidia (sing. gongylidius) Nodular fungal structures which develop in the FUNGUS GARDENS of parasol ants or in the fungus combs of termites (see TERMITE–MICROBE ASSOCIATIONS). gonidium A cell which is involved in asexual reproduction: see e.g. LEUCOTHRIX and VOLVOX. (The term was once applied to a photobiont cell in a lichen in the mistaken belief that such cells served a reproductive function.) Gonimochaete See ENTOMOPHTHORALES. gonococcal ophthalmia See GONORRHOEA. gonococcus Neisseria gonorrhoeae. Gonometa virus See PICORNAVIRIDAE. gonorrhoea (gonorrhea) An acute or chronic VENEREAL DISEASE which in nature affects only man; it is caused by Neisseria gonorrhoeae (the ‘gonococcus’ – see NEISSERIA). Incubation period: 2–5 days or longer. Gonococci adhere to the urethral epithelium by means of fimbriae; they penetrate the mucosa, causing an acute inflammatory response, and the infection spreads readily to adjacent tissues. In males, infection may be asymptomatic or may involve URETHRITIS, with painful urination and a yellowish mucopurulent discharge; prostatitis and epididymitis may occur. In females, infection may be asymptomatic or there may be a purulent vaginal discharge; infection may spread via the endometrium, fallopian tubes and ovaries to the pelvic peritoneum (pelvic inflammatory disease, PID). (See also CURTIS–FITZ-HUGH SYNDROME.) In either sex, sterility may result. Some strains of gonococci may become disseminated: bacteraemia, associated with e.g. fever, may be followed by skin lesions, arthritis, and (sometimes fatal) endocarditis and/or meningitis. Gonococcal contamination of the eyes may lead to conjunctivitis (gonococcal ophthalmia) which can cause blindness if untreated; infants may be infected during birth (ophthalmia neonatorum), and neonates may therefore be prophylactically treated with eye-drops containing e.g. 1% w/v silver nitrate (Credé procedure) or penicillin. Lab. diagnosis: smears of the discharge are stained (e.g. by the Gram stain or fluorescent antibody techniques) to reveal large numbers of diplococci in PMNs. Culture (e.g. on THAYER–MARTIN AGAR) may be necessary, especially for diagnosing females. A PROBEbased test (the PACE 2C test) has been used for detecting N. gonorrhoeae and Chlamydia trachomatis in a single assay (particularly useful because co-infection with these two pathogens is quite common) [evaluation of the PACE 2C test with endocervical specimens: JCM (1995) 33 2587–2591]. A test for detecting N. gonorrhoeae based on the ligase chain reaction (see LCR) reported results superior to those obtained by culture [JCM (1997) 35 239–242]. Chemotherapy depends on (changing) patterns of antibiotic resistance in the pathogen; penicillins (with probenecid), tetracycline, erythromycin and spectinomycin have been used. Gonyaulax See DINOFLAGELLATES; BIOLUMINESCENCE; RED TIDE. gonyautoxins Toxins produced by Gonyaulax spp and other RED TIDE dinoflagellates. The toxins are sulphonated derivatives of SAXITOXIN and neosaxitoxin; they have a low toxicity, but can be converted to saxitoxin/neosaxitoxin by low pH (e.g. in the stomach). Gonyostomum See CHLOROMONADS. Good buffers Zwitterionic pH buffers, e.g. BICINE and HEPES. goose parvovirus See PARVOVIRUS. Gordona An obsolete genus of bacteria currently included in the genus RHODOCOCCUS. Gorgonzola cheese See CHEESE-MAKING. Gouda cheese See CHEESE-MAKING. Gower coefficient See entry SG . gp (1) Gene product. For example, gp6 = the polypeptide product of gene 6. (2) In retroviruses, gp designates a glycoprotein gene product (see RETROVIRIDAE). Gracilaria See RHODOPHYTA. (See also HOLMSELLA.) Gracilicutes A proposed division of the PROCARYOTAE comprising bacteria in which the CELL WALL is typically of the Gramnegative type; classes: ANOXYPHOTOBACTERIA; OXYPHOTOBACTERIA; SCOTOBACTERIA. gradient differential centrifugation See CENTRIFUGATION. gradient elution Elution during which the eluent, A, is progressively diluted with another solvent or solution, B, until it is finally pure B. gradient plate An agar plate within which a given antibiotic is distributed such that its concentration varies, in gradient fashion, from one side of the plate to the other; the plate is used to isolate antibiotic-resistant mutants. To prepare a gradient plate, a volume of antibiotic-free agar is allowed to set in a Petri dish – one side of which is raised ca. 5 mm above the horizontal during setting; with the Petri dish level, an equal volume of antibiotic-agar is then added and allowed to set. Plates are left for ca. 1 day before inoculation; this permits equilibration of antibiotic concentrations in a vertical (but not lateral) direction. A two-dimensional gradient plate may be prepared in a square dish; two wedge-shaped layers are prepared as described above, and a further two layers are added perpendicular to the first two. A given layer may incorporate a specific electrolyte, acid etc [JGM (1985) 131 2865–2869]. Gradocol filter A type of membrane filter. (See also FILTRATION.) gradostat An apparatus used for a specialized form of CONTINUOUS CULTURE in which two different solutions flow, simultaneously but in opposite directions, through each of a number of serially-linked chemostats; under steady-state conditions the concentration of each of the given solutes exhibits a stepped gradient along the line of chemostats. In one type of gradostat five chemostats are arranged at progressively higher levels so that one of the solutions can flow through the system under gravity – i.e., overflowing a weir in each chemostat and passing down to the chemostat below it; the other solution is pumped into the lowest chemostat, and pumped from each chemostat to the one above it. In this type of gradostat organisms as well as solutes are transferred between chemostats. By contrast, in the diffusion-coupled gradostat (Herbert model) several chemostats (each with its own independent inflow and outflow of medium) are connected in a horizontal row such that the medium in one chemostat is separated from that in the next by a membrane filter; thus, solutes can diffuse between chemostats but the organisms in a given chemostat remain in that chemostat. Such a system can be used e.g. to investigate metabolic interactions between different species [FEMS Ecol. (1985) 31 239–247]. [Gradostats and other novel growth systems: MS (1985) 2 53–60.] 343 Graff–Reinet disease to the lower permeability of their cell walls: the PEPTIDOGLYCAN of Gram-positive cell walls is more extensively cross-linked than that in Gram-negative species. (N.B. Mammalian cells are uniformly Gram-negative.) The cells of some bacteria (e.g. Bacillus spp) are strongly Gram-positive when young, but tend to become Gram-negative in ageing cultures; this may reflect degenerative changes in the cell wall. Some bacteria give a Gram-variable reaction, i.e., they are sometimes Gram-positive, sometimes Gram-negative; this could reflect e.g. minor variations in staining technique. To avoid the taxonomic problems of Gram-variable bacteria it has been suggested that classification be based on GRAM TYPE rather than on GRAM REACTION. Gram type A taxonomically useful designation (either Gram type positive or Gram type negative) which is based on biochemical and anatomical properties of a bacterial cell (see e.g. CELL WALL) rather than on GRAM REACTION; designation on this basis avoids the taxonomic problems caused by a Gram-variable reaction in the GRAM STAIN. [JGM (1982) 128 2261–2270.] Gram-variable See GRAM STAIN. gramicidins A group of linear peptide ANTIBIOTICS, each of which has a formyl group at the N-terminal and an ethanolamine residue at the C-terminal. (‘Gramicidin J’, also called ‘gramicidin S’, is misnamed: it is a TYROCIDIN.) Gramicidins act as channel-forming IONOPHORES, forming transient ion-conducting dimers in lipid bilayers; such channels permit the passage of a range of hydrated monovalent cations as well as protons. Against Gram-positive bacteria gramicidins may be bacteriostatic or (at higher concentrations) bactericidal; most Gram-negative bacteria tend to be resistant. grana See CHLOROPLAST. granulin See BACULOVIRIDAE. granulocyte See PMN. granuloma A type of lesion resulting from a local inflammatory response to a persistent antigen or toxin at a given site in the body; the lesion consists of a hard mass composed mainly of macrophages and strands of fibrous connective tissue – although other types of cell (e.g. lymphocytes, plasma cells, polymorphs etc) may also be present, and the lesion may also contain GIANT CELLS. Granuloma formation involves a T cell-dependent immune response. (See also DELAYED HYPERSENSITIVITY.) granuloma inguinale (donovanosis) A chronic human disease, occuring mainly in the tropics and subtropics, in which granulomatous ulcers develop chiefly on the genitals; the causal agent, Calymmatobacterium granulomatis, is transmitted by intimate contact. In smears (stained e.g. with Wright’s stain) the pathogen appears as clusters of blue or black ‘Donovan bodies’ in the cytoplasm of large mononuclear cells. granuloplasm See e.g. SARCODINA. Granuloreticulosea A class of protozoa (superclass RHIZOPODA) which form delicate, finely granular or hyaline reticulopodia (or, rarely, finely pointed, granular, non-anastomosing pseudopodia). Orders: Athalamida (amoebae naked: e.g., Arachnula, Biomyxa); Monothalamida (amoebae enclosed in a singlechambered organic or calcareous test, not exhibiting an alternation of generations: e.g., Lieberkuehnia); FORAMINIFERIDA. granulose (1) (noun) A (1 → 4)-a-glucan which accumulates in the cells of Clostridium spp prior to sporulation; it is rapidly degraded during sporulation, and may thus function as an endogenous reserve of carbon and energy for sporulation. (2) (adj.) Having a granular appearance. granulosis An INSECT DISEASE caused by a GRANULOSIS VIRUS. granulosis viruses (GVs) Viruses belonging to subgroup B of the BACULOVIRIDAE (q.v.). The viruses replicate mainly in the Graff–Reinet disease Syn. MAEDI. Grahamella A genus of Gram-negative bacteria (family BARTONELLACEAE); the organisms occur worldwide and are parasitic (though rarely, if ever, pathogenic) in various small mammals (e.g. rodents) but not in man. Grahamella spp (G. talpae, the type species; G. peromysci ) are non-flagellated bacilli or coccobacilli, commonly ca. 1 µm long; the organisms are transmitted e.g. by fleas and occur in the erythrocytes of the infected vertebrate host. grain bloat See BLOAT. Grain convention A system for numbering the microtubular triplets in a kinetosome. Looking at a cross-section of a ciliary kinetosome from the direction of the interior of the cell, the number 1 triplet is that which is aligned with the axis of the KINETY and which is near the posterior side of the kinetosome (relative to the cell’s anterior–posterior axis); triplets 2–9 follow in a clockwise sequence from number 1. (cf. PITELKA CONVENTION.) Gram-negative See GRAM STAIN. Gram-negative enrichment broth See GN BROTH. Gram-positive See GRAM STAIN. Gram reaction The result (positive, negative, or variable) of the GRAM STAIN; the Gram reaction does not necessarily indicate the chemical nature of the cell envelope (cf. GRAM TYPE). In a nonstaining method for determining Gram reaction, bacterial growth is emulsified with one drop of aqueous KOH (3%); Gramnegative bacterial growth becomes viscid, or gels, in 5–60 sec [AEM (1982) 44 992–993]. Gram stain An important bacteriological STAINING procedure discovered empirically in 1884 by the Danish scientist Christian Gram. When bacteria are stained with certain basic dyes the cells of some species (Gram-negative species) can be easily decolorized with organic solvents such as ethanol or acetone; cells of Gram-positive species resist decolorization. The ability of bacteria to either retain or lose the stain generally reflects fundamental structural differences in the CELL WALL, and is an important taxonomic feature; the Gram stain is therefore used as an initial step in the identification of bacteria. Procedure (one of many variations). A heat-fixed smear of bacteria on a slide is stained for ca. 1 min with one of certain basic TRIPHENYLMETHANE DYES (commonly crystal violet); most bacteria take up the dye. The smear is then rinsed briefly under running water, treated for 1 min with LUGOL’S IODINE, and again rinsed with water. Decolorization is then attempted by treating the smear with ethanol, acetone, or iodine–acetone. This is the critical step: solvent is allowed to run over the tilted slide until dye no longer runs freely from the smear (ca. 1–3 sec), and the smear is immediately rinsed under running water; at this stage any Gram-negative bacteria in the smear will be colourless, while any Gram-positive cells will still be violet. The smear is then counterstained for ca. 30 sec (e.g. with dilute carbolfuchsin) to stain any Gram-negative bacteria present in the smear. Finally, the slide is rinsed briefly under the tap, blotted dry, and examined under the microscope. (In Hucker’s method the dye solution is made by adding 20 ml crystal violet (10% w/v in 95% ethanol) to 80 ml of 1% w/v aqueous ammonium oxalate, and filtering after 24 h. The decolorizing agent is 95% ethanol. The counterstain is safranin O.) (See also BROWN–HOPPS MODIFICATION and KOPELOFF MODIFICATION.) The mechanism of Gram-staining is still not understood. The integrity of the CELL WALL is necessary for Gram-positivity: disrupted Gram-positive cells do not retain the dye. It may be that Gram-positive cells retain the dye–iodine complex owing 344 gregarine movement green spot disease See ALGAL DISEASES. green sulphur See SULPHUR. green sulphur bacteria See GREEN BACTERIA. Greenberg–Smith sampler An ALL-GLASS IMPINGER. greenhouse effect The warming of the planet due to rising levels of CO2 , methane and certain other gases (which effectively ‘trap’ some of the radiant energy reaching the Earth’s surface). In the CARBON CYCLE, large amounts of CO2 are produced by animals, plants and microbes during respiration or fermentation, but large amounts are used (‘fixed’) during PHOTOSYNTHESIS; globally, the important balance between biological production and uptake of CO2 is being upset e.g. by the burning of vast amounts of fossil fuel (gas, petroleum etc.). (See also METHANOTROPHY.) It might be thought that raised levels of CO2 would stimulate growth in plants, and, thereby – due to greater sequestration of CO2 by photosynthesis – tend to offset the rise in CO2 . Some agricultural, crop-type plants do respond to increased CO2 by improved growth. However, experimental tropical ecosystems under increased CO2 have shown, in addition to increased root growth, an increased efflux of CO2 from soil to atmosphere; this efflux was apparently due to stimulated metabolic activity of microorganisms in the RHIZOSPHERE. Thus, an increased uptake of CO2 by plants may itself be offset by increased efflux of CO2 from the soil [Science (1992) 257 1672–1675]. More recently it was suggested that certain pasture grasses in the South American savannas may offer a useful sink for carbon in their particularly massive and deep root systems [Nature (1994) 371 236–238]. It has been pointed out that global warming may itself result in increased levels of greenhouse gases – i.e. a potential positive feedback effect [BC (1996) 5 1069–1083]. It has been assumed that the avoidance of deforestation is of major importance in the context of global warming because such a policy preserves a valuable means for controlling levels of atmospheric carbon dioxide [Nature (2001) 410 429]. However, recent experiments have indicated that the ability of forest systems to sequester carbon dioxide may be limited by shortages of nutrients and water and/or by the relatively rapid turnover of organic carbon in the litter layer [Nature (2001) 411 469–472; Nature (2001) 411 466–469; commentary: Nature (2001) 411 431–433]. greening (1) See HAEMOLYSIS. (2) See MEAT SPOILAGE and DFD MEAT. gregarin The trophozoite form of a gregarine. Gregarina A genus of the GREGARINASINA. Gregarinasina (Gregarinidia) A subclass of protozoa (class SPOROZOASIDA) parasitic in invertebrates (e.g. annelids, insects). In members of this subclass the mature gametocytes occur extracellularly in the host (cf. COCCIDIASINA); the mature organism generally has a MUCRON or an EPIMERITE. In the cephaline gregarines (e.g. Gregarina spp) the mature organism is septate – the nucleus usually being in the DEUTOMERITE; in acephaline gregarines (e.g. MONOCYSTIS) the mature organism is aseptate. Species whose life cycles do not include schizogony (the eugregarines) are commonly non-pathogenic; species which undergo schizogony as well as sexual reproduction (the schizogregarines) are commonly pathogenic. Some authors divide the schizogregarines into the archigregarines (parasitic in annelids) and the neogregarines (parasitic in insects). Genera include Actinocephalus, Stenophora, Urospora. gregarine A member of the GREGARINASINA. gregarine movement See MOTILITY. nucleus, but some replication can also occur in the cytoplasm. The virion contains a single nucleocapsid within the envelope. Type species: Trichoplusia ni (cabbage looper) GV (TnGV); other members: Plodia interpunctella (Indian meal moth) GV, Cydia pomonella (codling moth) GV, and viruses isolated from a wide range of other lepidopteran insects. granum (pl. grana) See CHLOROPLAST. Granville wilt (Southern bacterial wilt) A soil-borne TOBACCO DISEASE, involving wilting and yellowing of leaves, caused by Pseudomonas solanacearum. (See also SCOPOLETIN.) granzymes See APOPTOSIS. Graphidales An order of fungi of the ASCOMYCOTINA; members include crustose LICHENS (photobiont generally trentepohlioid). Ascocarp: APOTHECIOID, or immersed and ostiolate, with paraphyses. Asci: unitunicate and cylindrical with a thickened apex. Genera: e.g. Diploschistes, GRAPHIS, Phaeographis, Thelotrema. Graphiolaceae See USTILAGINALES. Graphis A genus of crustose LICHENS (order GRAPHIDALES); photobiont: a green alga. Thallus: thin, smooth or wrinkled. Apothecia: black LIRELLAE; ascospores: colourless, multiseptate. Species occur mainly on bark. graphitization An early term for the anaerobic corrosion of iron by sulphate-reducing bacteria. Graphium A synnema-forming genus of the HYPHOMYCETES. (See also SAP-STAIN.) GRAS ‘Generally recognized as safe’ – a category of food additives (including some food PRESERVATIVES) recognized in the USA. grass diseases Diseases which can affect non-cereal grasses include e.g. CHOKE, DOLLAR SPOT, SILVER TOP, SNOW MOULD, and ophiobolus patch disease (see TAKE-ALL). (See also FIJIVIRUS, CEREAL DISEASES and SUGARCANE DISEASES.) Grasseella See EIMERIORINA. graticule See MICROMETER. gratuitous inducer See OPERON. gravity slide sampler Syn. DURHAM SHELTER. gray (Gy) A unit of absorbed radiation equal to 1 joule of energy absorbed by 1 kg of material. (1 Gy = 100 RAD; 103 Gy = 1 kGy.) gray mold See GREY MOULD. greasy pig disease (exudative dermatitis) An acute, often fatal disease of piglets characterized by a non-pruritic dermatitis, the presence of a greasy exudate on the skin, dehydration and emaciation; aetiology: uncertain, but a Gram-positive coccus (Staphylococcus hyicus) and/or a virus may be involved. green algae Algae of the CHLOROPHYTA. green bacteria (green sulphur bacteria) (1) Bacteria of the suborder CHLOROBIINEAE. (2) Bacteria of the family CHLOROBIACEAE. green ear disease See PHYLLODY. green fluorescent protein (GFP) A fluorescent protein, found in jellyfish of the genus Aequorea, which emits green light (l = 508 nm) when irradiated with blue light (l = 395 nm). The gene for GPF has been widely used as a partner in gene fusion experiments (see FUSION PROTEIN sense 2). [Green fluorescent protein (review): ARB (1998) 67 509– 544.] green laver See LAVER and ULVA. green manure See Azolla in NITROGEN FIXATION. green monkey fever Syn. MARBURG FEVER. green oak Oak wood which has been stained green as a result of infection by Chlorociboria aeruginascens (= Chlorosplenium aeruginascens, often called Chlorosplenium aeruginosum). Green oak is valued as a medium for inlay work etc. (cf. BROWN OAK.) 345 Gregarinia Gregarinia (1) A subclass of protozoa (class Telosporea) later classified as the subclass GREGARINASINA. (2) A subclass of protozoa (class Sporozoea [JP (1980) 27 37–58]) equivalent to the GREGARINASINA. Gregarinidia Syn. GREGARINASINA. grepafloxacin See QUINOLONE ANTIBIOTICS. grex See DICTYOSTELIUM. grey mould (‘botrytis’) A disease, caused by Botrytis cinerea, which can affect a wide range of plants as well as stored fruits and vegetables (e.g. beans, peas, raspberries and strawberries); the disease is encouraged by cool, damp conditions, and is characterized by a grey, fluffy surface mould overlying a soft, commonly brown, rot. (See also DAMPING OFF.) grid See ELECTRON MICROSCOPY (a). Gridley fungus stain A staining procedure that depends on the oxidation (by chromic acid) of adjacent hydroxyl groups in fungal cell wall polysaccharides, followed by the reaction of the resulting aldehyde groups with e.g. SCHIFF’S REAGENT. [Method: Book ref. 53, p. 2207.] Griffith’s tube An apparatus used for grinding tissues. It consists of a stout, large-diameter glass tube which is closed at the bottom; the lower, inner part of the tube has a roughened surface – against which the tissue is ground by means of a closefitting pestle-like glass rod. Griffith’s typing (of group A streptococci) A serological (slide agglutination) TYPING method based on strain differences in the M and/or T antigens. Griffithsia See RHODOPHYTA. Grifola See APHYLLOPHORALES (Polyporaceae). grippe Syn. INFLUENZA. griseofulvin (7-chloro-4,6-dimethoxycoumaran-3-one-2-spiro-1 (2 -methoxy-6 -methylcyclohex-2 -en-4 -one)) An antifungal ANTIBIOTIC produced by Penicillium griseofulvum. It is inhibitory for actively growing dermatophytes and certain other filamentous fungi but is inactive against e.g. yeasts and bacteria. Griseofulvin acts primarily as a MICROTUBULE inhibitor, preventing the assembly of tubulin dimers into microtubules and thereby inhibiting e.g. mitosis. Additionally, the antibiotic interferes with the pattern of glucan and glycoprotein deposition in the fungal cell wall; as a result the tips of the hyphae become characteristically curled (hence the early name for the antibiotic: ‘curling factor’). Griseofulvin is widely used in the treatment of dermatophyte infections of the skin, hair and nails. It is administered orally (it is not effective topically) and accumulates in keratinized tissues; it apparently forms a complex with keratin which may decrease the susceptibility of the keratin to fungal keratinases. Griseofulvin-resistant dermatophytes occur [Arch. Derm. (1981) 117 16–19]. Griseofulvin is active against certain plant-pathogenic fungi (e.g. Botrytis spp, Alternaria solani ). It can be taken up by plant roots and has low phytotoxicity; it has limited use as a systemic ANTIFUNGAL AGENT in plants. griseoviridin An antibiotic structurally related to streptogramin A (see STREPTOGRAMINS). gRNAs See RNA EDITING. Grocott–Gomori stain Syn. METHENAMINE–SILVER STAIN. groEL gene See HEAT-SHOCK PROTEINS. groES gene See HEAT-SHOCK PROTEINS. Gromia See FILOSEA. Gromiida See FILOSEA. groove (in DNA) See DNA. Grossglockneria See CILIOPHORA. ground squirrel hepatitis virus (GSHV; ground squirrel hepatitis B virus, GSHBV) A virus of the HEPADNAVIRIDAE which infects Beechey ground squirrels (Spermophilus beecheyi ); infected animals show little or no evidence of hepatitis and appear not to develop hepatocellular carcinoma. (cf. WOODCHUCK HEPATITIS VIRUS.) groundnut crinkle virus See CARLAVIRUSES. groundnut eyespot virus See POTYVIRUSES. groundwater See WATER SUPPLIES. group I/II introns See SPLIT GENE (b) and (c). group-phase antigens See PHASE VARIATION. group stage See TOXOPLASMOSIS. group translocation (1) See TRANSPORT SYSTEMS. (2) Syn. VECTORIAL GROUP TRANSLOCATION. group treponemal antigens Syn. REITER ANTIGENS. grouping antiserum Any antiserum which can agglutinate all the members of a particular group of serotypes within a given genus, i.e., serotypes which have in common one or more antigens which are not found in serotypes outside the group (see e.g. KAUFFMANN–WHITE CLASSIFICATION). growth In a single living cell: a coordinated increase in the mass of essential cell components leading, typically, to progress through the CELL CYCLE; in a population of cells or a mycelium: a coordinated increase in biomass, or an increase in the number of cells in the population. (An increase in mass does not necessarily signify growth since it may be due e.g. to the accumulation of an intracellular storage compound.) Growth is said to be balanced when the increase in mass of all essential cell components occurs exponentially and is coincident with the rate of increase in total biomass or in cell numbers. In unbalanced growth there is a change in the rate of synthesis of some cell component(s) relative to that of other components; under these conditions (which occur e.g. during the adaptive changes in the lag phase of BATCH CULTURE) changes in biomass are not directly proportional to changes in cell numbers. (See also CONTINUOUS CULTURE.) Growth may be measured e.g. by monitoring cell numbers (see COUNTING METHODS), by measuring the increase in dry weight of biomass formed in a given time interval, by monitoring the uptake and metabolism (or release) of particular substances, or by measuring the amount of a radioactive metabolite incorporated in biomass in a given time. Growth may be expressed in the form of a growth curve: a graph in which the number of cells, or the biomass, is plotted against time. (a) Bacterial growth. Growth in individual bacterial cells commonly leads to (asexual) reproduction (i.e., cell division) – usually by BINARY FISSION, but sometimes by BUDDING or by MULTIPLE FISSION; the events which occur during growth in a single cell are described under CELL CYCLE. In some species, growth may be followed by differentiation, e.g. the formation of a dormant cell (see e.g. ENDOSPORE). The growth of a bacterial population. When cells divide by binary fission, each individual cell gives rise to 2, 4, 8, 16, 32. . . . cells after 1, 2, 3, 4, 5. . . . generations (i.e. rounds of division). Hence, the number of cells (N) derived from a single cell after n generations is given by: N = 2n Similarly, after n generations, the number of cells derived from an initial cell population of N0 is given by: N = 2n N0 For bacteria dividing by binary fission, a growth curve can be prepared by plotting cell numbers on a log2 scale; this permits the number of generations per unit time to be read off directly 346 GSHV (since each unit on the log2 scale is equal to one generation), and the graph also indicates clearly any changes in the growth rate. (The log2 and log10 of any number can be interconverted by use of the formula: log10 N = 0.301 log2 N.) The rate of growth may be expressed in terms of the SPECIFIC GROWTH RATE, m. When m is constant (e.g. during exponential growth in batch culture), the relationship between m and the doubling time (i.e. the time taken for the biomass – or population – to double) is given by: td = bisexualis) that the localized decrease in pH in the hyphal tip may affect vesicle–cytoskeleton interaction, thereby regulating the translocation and localization of vesicles. Interestingly, the polarity of growth of various filamentous fungi can be manipulated experimentally by applied electrical fields [JGM (1986) 132 2515–2525]. The development of a hyphal branch is apparently preceded by the commencement of charge influx at the given location. Branching can be enhanced by CYTOCHALASINS A and E (which may affect microfilament systems involved in vesicle transport) and by IONOPHORES such as A23187 and CCCP (thus implicating cytoplasmic ions in the control of branch initiation) [JGM (1986) 132 213–219]. Hyphal growth (both linear and branching) requires the coordinated action of autolytic and synthetic enzymes in the region of growth – together with an intracellular turgor pressure to act as driving force for surface expansion. Antibiotics which inhibit cell wall formation (e.g. POLYOXINS) cause the development of bulbous hyphae and distorted germ tubes in isotonic media; hyphal tips (and budding yeasts) burst in hypotonic media. [Cell wall synthesis in apical hyphal growth: Int. Rev. Cytol. (1986) 104 37–79.] Growth in unicellular fungi. Growth may involve BUDDING or BINARY FISSION. BATCH CULTURES of unicellular fungi exhibit lag, exponential, stationary and death phases of growth. Growth in dimorphic fungi. Whether growth occurs in the unicellular or mycelial phase may be determined by temperature (e.g. in Paracoccidioides brasiliensis), by the availability of particular nutrients (e.g. in Candida albicans, Mucor rouxii ), or by a combination of factors which include temperature and redox potential (e.g. in Histoplasma capsulatum). In a given organism the composition of the CELL WALL (q.v.) may differ in the unicellular and mycelial phases. In C. albicans the distribution of ACTIN in a growing bud differs from that in a growing hypha [JGM (1986) 132 2035–2047]. (See also SECONDARY METABOLISM.) growth curve See GROWTH and BATCH CULTURE. growth cycle (1) Syn. CELL CYCLE. (2) A term used by some authors to refer to the (non-cyclical) sequence of phases in BATCH CULTURE (i.e. lag phase to death phase). growth precursor cell (shut-down cell) A cell which differs from its parent cell in that it proceeds through the CELL CYCLE only under (presumably) favourable environmental conditions. Cells conforming to this description include e.g. the swarmer cells of Caulobacter crescentus, Hyphomicrobium sp, and Rhodomicrobium vannielii – cells which are characterized by low levels of endogenous metabolism, no synthesis of DNA or rRNA, and the apparent ability to respond to external growthtriggering stimuli. It has been suggested that a growth precursor cell may function as a survival cell and/or as a stage from which one of various cell types may develop according to particular environmental stimuli. [Review: Book ref. 28, pp. 187–247.] growth rate constant (1) Syn. SPECIFIC GROWTH RATE. (2) The exponential growth rate constant – see GROWTH (a). growth-specific yield coefficient See YIELD COEFFICIENT. growth yield coefficient Syn. YIELD COEFFICIENT. Grunstein–Hogness procedure Syn. COLONY HYBRIDIZATION. Gruyère cheese See CHEESE-MAKING. Gs protein See ADENYLATE CYCLASE. GS/GOGAT See AMMONIA ASSIMILATION. GSHBV See GROUND SQUIRREL HEPATITIS VIRUS. GSHV GROUND SQUIRREL HEPATITIS VIRUS. 0.693 m in which td is the doubling time. For some cells (including those of e.g. Escherichia coli ) the minimum doubling time (= minimum generation time) is of the order of minutes, while for others (e.g. those of certain Mycobacterium spp) it may be many hours. (Variation may occur among the doubling times of individual cells in a single-species population – see also SYNCHRONOUS CULTURE.) Provided that m is constant, the number of generations (= number of doublings of biomass, or of cell numbers) per unit time is given by k: k= (log2 Nt − log2 N0 ) t – k being the exponential growth rate constant (sometimes, confusingly, symbolized by µ instead of k), and Nt and N0 being the populations at time t and time zero, respectively. The quantity 1/k is the mean generation time (= mean doubling time). (b) Fungal growth. According to species and conditions, growth may involve or lead to (i) an increase in cell size or (in mycelial fungi) an increase in hyphal length and/or hyphal branching; (ii) vegetative differentiation – e.g. a phase change in DIMORPHIC FUNGI; (iii) asexual reproduction; (iv) sexual reproduction. Hyphal growth. Linear hyphal growth occurs by the incorporation of new materials at the extreme distal end of the hypha (i.e., ‘tip growth’); precursors of the CELL WALL and CYTOPLASMIC MEMBRANE are assembled in the GOLGI APPARATUS and then transported, in vesicles, to the hyphal apex where fusion occurs between the vesicles and the cytoplasmic membrane. The unidirectional flow of vesicles to the hyphal apex appears to be related to a phenomenon observed in growing (but not in nongrowing) hyphae: the continual circulation of positive charges through the terminal region of the hypha; thus, positive charges from the environment pass into the hypha at, and close to, the hyphal apex, and pass out into the environment a short distance proximal to the apical region. Such transhyphal currents of positive charge have been observed in several fungi [JGM (1984) 130 3313–3318] and may be common to all growing fungal hyphae. (Axially aligned currents of positive charge also occur e.g. in zygotes of the brown algae Fucus and Pelvetia and in growing plant roots and pollen tubes.) In the water mould Achlya bisexualis the current consists of protons; the proton current is apparently due to the preferential distribution of proton–amino acid symporters in the apical region of the hypha, and PROTON ATPASES in the adjacent region. (In e.g. Pelvetia the circulating current appears to consist primarily of calcium ions.) The way in which these circulating currents contribute to the unidirectional flow of intracellular cell-building vesicles is not clear; it has been suggested e.g. that currents of charge may promote the ‘electrophoretic’ translocation of vesicles, or (in Achlya 347 GSP GV GRANULOSIS VIRUS. Gy See GRAY. Gyalecta See GYALECTALES. Gyalectales An order of mainly lichenized fungi of the ASCOMYCOTINA. Ascocarp: APOTHECIOID; hamathecium: paraphyses. Asci: clavate or cylindrical. Genera: e.g. Dimerella, Gyalecta, Petractis. GYC agar See ACETOBACTER. Gymnamoebia A subclass of naked (non-testate) amoebae (AMOEBIDA, PELOBIONTIDA and SCHIZOPYRENIDA). (cf. TESTACEALOBOSIA.) Gymnoascales An order of mainly keratinophilic fungi of the subdivision ASCOMYCOTINA; many species have anamorphs among the DERMATOPHYTES. Ascocarp: cleistothecioid, with or without appendages, sometimes insubstantial (consisting e.g. of a loose hyphal network); sessile or stipitate. Asci: unitunicate. Ascospores: unicellular, spherical, discoid or fusiform, sometimes forming a mazaedial mass at maturity. Genera: e.g. AJELLOMYCES, Aphanoascus, ARTHRODERMA, Gymnoascus, Myxotrichum, NANNIZZIA and Onygena (see also MAZAEDIUM); of these, Aphanoascus and Onygena belong in the family Onygenaceae, and the others in the family Gymnoascaceae. Gymnoascus See GYMNOASCALES. gymnocarpic development (also: gymnocarpous development) (mycol.) The mode of development of a fruiting body in which the spores are exposed to the environment throughout their differentiation and maturation. (See e.g. APHYLLOPHORALES; cf. ANGIOCARPIC DEVELOPMENT.) Gymnodinioides See HYPOSTOMATIA. Gymnodinium See DINOFLAGELLATES; RED TIDE; ZOOXANTHELLAE. Gymnomyxa A protistan phylum (equivalent to the mycological division MYXOMYCOTA) comprising the subphyla MYCETOZOA, Plasmodiophorina (equivalent to PLASMODIOPHOROMYCETES), and Labyrinthulina (equivalent to LABYRINTHULOMYCETES). [Book ref. 147, pp. 4–5.] Gymnosphaera See CENTROHELIDA. Gymnosporangium A genus of typically heteroxenous rust fungi (class UREDINIOMYCETES) whose primary and secondary hosts are commonly members of the families Juniperaceae and Rosaceae, respectively. Most species are DEMICYCLIC RUSTS. The aecial and telial stages often form on GALLS which develop on the host plant as a response to infection by the rust; thus, e.g. G. juniperi-virginianae, which parasitizes the juniper (Juniperus) and apple (Malus), produces telia which develop in (and later project from) spherical galls (‘cedar apples’) which form on the juniper. (The cedar (Cedrus) is not susceptible to this rust.) Gymnostomatia A subclass of ciliates (class KINETOFRAGMINOPHOREA) in which the cytostome is superficial (i.e., it does not occur at the base of an organized buccal cavity or vestibulum), and the cytopharyngeal apparatus is of the RHABDOS type. Somatic ciliature is commonly uniform. The organisms are typically large and carnivorous. Genera include e.g. Acaryophrya, Acineria, ACTINOBOLINA, COLEPS, Didesmis, DIDINIUM, DILEPTUS, Holophrya, Lacrymaria, KENTROPHOROS, LOXODES, Loxophyllum, Mesodinium, Metacystis, Nanophrya, PRORODON, STEPHANOPOGON, TRACHELOPHYLLUM and Urotricha. (See also KARYORELICTID GYMNOSTOMES; PRIMOCILIATID GYMNOSTOMES.) gymnostome A ciliate of the GYMNOSTOMATIA. gyrA, gyrB genes See GYRASE. gyrase (DNA gyrase) A type II TOPOISOMERASE which catalyses the negative supercoiling of relaxed or positively supercoiled GSP See PROTEIN SECRETION. GSR Generalized SHWARTZMAN REACTION. GT . . . AG rule See SPLIT GENE (a). GT1b See FOOD POISONING (Vibrio). GTA Gene transfer agent: see CAPSDUCTION. GTP Guanosine 5 -triphosphate. (See also ATP and NUCLEOTIDE.) GTP-binding proteins Syn. G PROTEINS. GU . . . AG rule See SPLIT GENE (a). guanidine ((H2 N)2 C=NH) Guanidine salts (e.g. guanidine hydrochloride: (H2 N)2 C=NH2 + Cl− ) act as ANTIVIRAL AGENTS against a number of picornaviruses (e.g. polioviruses) by inhibiting viral RNA synthesis. Although cytotoxicity is low, guanidine is not used clinically since resistant mutants arise rapidly; guanidinedependent mutants have also been isolated. guanidine antifungal agents See e.g. DODINE and GUAZATINE. guanosine See NUCLEOSIDE and Appendix V(a). guanosine tetraphosphate (guanosine 5 -diphosphate 3 -diphosphate) See STRINGENT CONTROL (sense 1). guanylate cyclase See ETEC (ST-I). guanylin See EAST1 and ETEC (ST-I). guanylyl transferase See MRNA (b). Guarnieri bodies (B-type inclusion bodies) Eosinophilic INCLUSION BODIES, each surrounded by a clear non-staining ‘halo’, characteristically formed in the cytoplasm of cells infected with vaccinia, variola or cowpox viruses (see POXVIRIDAE). guazatine A guanidine ANTIFUNGAL AGENT used e.g. as a seed treatment for the control of bunt of wheat, and – mixed with e.g. imazalil – as a spray against cereal smuts, net blotch, etc. Guepiniopsis See DACRYMYCETALES. guide RNAs (gRNAs) See RNA EDITING. guide sequence See SPLIT GENE. Guillain–Barré syndrome (GBS) An immune-mediated disorder of the peripheral nerves involving e.g. a progressive motor weakness in at least two limbs. In the majority of patients GBS is preceded by an infectious disease – e.g. a (very) small proportion of patients with a Campylobacter infection may develop GBS. Guilliermondella A genus of fungi (family SACCHAROMYCETACEAE) which form budding yeast cells as well as a true septate mycelium. One species: G. selenospora. [Book ref. 100, pp. 151–153.] guinea-pig agent Legionella micdadei. guinea-pig cytomegalovirus See BETAHERPESVIRINAE. gullet (1) The BUCCAL CAVITY in certain ciliates. (2) See EUGLENOID FLAGELLATES. guluronic acid See ALGINATE. gum xanthan Syn. XANTHAN GUM. Gumboro disease Syn. infectious bursal disease (see INFECTIOUS BURSAL DISEASE VIRUS). gumma See e.g. SYPHILIS. gummosis Any plant disease in which the lesions exude a sticky liquid; examples: gummosis of cucumber (caused by Cladosporium cucumerinum), of Axonopus spp (Gramineae) (caused by Xanthomonas axonopodis), and of sweet orange (Citrus sinensis) (caused by Phytophthora parasitica). Gunflintia See STROMATOLITES. Gunnera See NOSTOC. gut flora (human) See GASTROINTESTINAL TRACT FLORA. guttulate Containing one or more GUTTULES. guttule (mycol.) A globule or droplet (of oil?) within a spore. Guttulina See ACRASIOMYCETES. Guttulinopsis See ACRASIOMYCETES. 348 Gyrosigma ds cccDNA (see DNA) in an ATP-dependent process. (In the absence of ATP, the gyrase from Escherichia coli can relax negative, but not positive, supercoils.) Gyrase functions as a tetramer consisting of two of each of two types of subunit, A and B (i.e. A2 B2 ); subunits A and B are the products of genes gyrA (= nalA) and gyrB (= cou), respectively. The enzyme binds to dsDNA at specific (but apparently non-homologous) sites. The DNA becomes wrapped around the surface of the enzyme in a positive supercoil; this necessarily introduces a compensating negative supercoil elsewhere in the molecule. The remaining steps effectively remove the positive supercoil. The B subunits bind ATP, a step which alters the functional (conformational?) state of the enzyme, and the A subunits bring about a staggered break in the dsDNA, the resulting 5 ends each projecting beyond the 3 ends by four bases. [Determinants of site-specific DNA breakage by gyrase: EMBO (1986) 5 1411–1418.] Each 5 end is covalently bound to one of the A subunits via a phosphotyrosine bond; this allows conservation of the energy of the phosphodiester bond for later use in resealing. A length of DNA from elsewhere in the molecule (possibly from the region wrapped around the enzyme) is passed through the (double-stranded) break while both ends of the break are held so that they cannot rotate; the break is then resealed. This decreases the linking number of the molecule by two. (If the translocated length of DNA belongs to a second molecule, rather than to another part of the same molecule, the result is a CATENANE.) Finally, the ATP bound to the B subunits is hydrolysed and the original conformation of the enzyme is restored; the whole cycle can then begin again. (Note: ATP binding allows a single round of supercoiling, but ATP hydrolysis is necessary for subsequent rounds.) The ability of gyrase to introduce negative supercoiling appears to decrease with increasing levels of negative supercoiling. In E. coli, the appropriate level of supercoiling in vivo is apparently achieved/maintained by the opposing activities of gyrase, on the one hand, and topoisomerases I and IV (see TOPOISOMERASE) on the other hand. Gyrase is apparently essential in vivo for e.g. DNA REPLICATION, recombination, transposition (see TRANSPOSABLE ELEMENT), and some cases of TRANSCRIPTION; this requirement seems to be due, at least in part, to the need for supercoiled DNA in these processes, and implies that gyrase has an important role in generating and maintaining the correct level of supercoiling of DNA in the cell. (cf. ILLEGITIMATE RECOMBINATION.) There is also evidence for a more direct role for gyrase at the replication forks during DNA replication. (cf. REVERSE GYRASE; see also COUMERMYCINS; NALIDIXIC ACID; NOVOBIOCIN; OXOLINIC ACID.) [QUINOLONE ANTIBIOTICS and other gyrase-targeted antibiotics: TIM (1997) 5 102–109.] Some candidate antibiotics affect gyrase at sites distinct from those targeted by quinolone antibiotics (and do not exhibit crossresistance with the quinolones) [TIM (1998) 6 269–275]. Gyrodinium See DINOFLAGELLATES. Gyrodontium See APHYLLOPHORALES (Coniophoraceae). gyrogonites Fossilized charophyte oogonia. Gyromitra See PEZIZALES. gyrose Sinuous; curving; marked with undulating lines. In lichenology: refers to a type of apothecium (e.g. in some Umbilicaria spp) in which there is a spiral or maze-like pattern of fertile ridges and sterile furrows. Gyrosigma See DIATOMS. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 349 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 H H Histidine (see AMINO ACIDS). H antigens Bacterial flagellar antigens. H antigens are detected in the serological characterization of certain enterobacteria (see e.g. KAUFFMANN–WHITE CLASSIFICATION). Agglutination of cells with homologous H antiserum occurs rapidly, is characteristically floccular, and the agglutinated cells can be readily dispersed by shaking (since flagella are easily broken). H antigens are heatlabile and are destroyed by alcohol but not by dilute formalin. (See also IMMOBILIZATION TEST.) In a few Salmonella serotypes (e.g. S. typhi ) and other enterobacteria the flagella exhibit only one type of antigenic pattern, i.e. the H antigens are monophasic. However, most salmonellae can produce either of two different patterns of H antigens, i.e. in these organisms the H antigens are diphasic (see PHASE VARIATION). H+ -ATPase See PROTON ATPASE. H+ /2e− ratio During the operation of an ELECTRON TRANSPORT CHAIN: the number of protons translocated across the membrane during the passage of two electrons along the chain. H-lysin See FURUNCULOSIS (sense 1). h mutant HOST-RANGE MUTANT. H+ /O ratio During the operation of an ELECTRON TRANSPORT CHAIN with oxygen as terminal electron acceptor: the number of protons translocated across the membrane per oxygen atom reduced. (cf. P/O RATIO.) H+ /P ratio The number of protons passing through a PROTON ATPASE per molecule of ATP released at the catalytic site. H+ -PPase PROTON PPASE. H strand ‘Heavy strand’: that strand of a dsDNA molecule which exhibits the higher buoyant density (owing to a higher content of thymine and guanine) when the dsDNA is denatured and subjected to CsCl density gradient CENTRIFUGATION; the other (‘light’) strand is designated the L strand. The designation ‘heavy’ or ‘H’ is also used to refer to a strand labelled with a heavy isotope such as 15 N, the unlabelled strand being designated ‘light’ or ‘L’. H1 protein Syn. H-NS PROTEIN. H-1 virus See PARVOVIRUS. HII phase See CYTOPLASMIC MEMBRANE. HAA See HEPATITIS B VIRUS. HAART Highly active antiretroviral therapy: a form of chemotherapy, used for HIV-positive and AIDS patients, involving a combination of different kinds of ANTIRETROVIRAL AGENT; such treatment should, theoretically, inhibit completely the replication of HIV, and in some studies HIV is reported to be undetectable in plasma. One example of HAART is the combination of (i) efavirenz (a non-nucleoside reverse transcriptase inhibitor), (ii) nelfinavir (a protease inhibitor) and (iii) a nucleoside reverse transcriptase inhibitor [NEJM (1999) 341 1874–1881]. Even with effective HAART, low-level replication can occur. This may involve (i) long-lived HIV-infected cells and (ii) selection of drug-resistant mutants [JV (2005) 79 9625–9634]. Haber–Weiss reaction See HYDROXYL RADICAL. HACCP Hazard analysis critical control point system: detailed appraisal of a production process and control of known/potential hazards at certain critical points in the process. (‘Hazard’ refers to any influence or factor which would, unless avoided, affect adversely the integrity of the process and/or the quality of the final product.) Initially, an objective assessment of known and potential hazards is carried out using a flow-chart which shows all aspects of the process – details that may include e.g. source(s) of raw materials, a step-wise coverage of all stages of processing, route(s) of distribution of the product, and the movement of product to the point of sale (if applicable). Central to the HACCP system is the identification of a number of critical control points (CCPs) – specific stages in the process at which effective action can be taken to prevent or counteract the effects of particular hazard(s). Monitoring is carried out at CCPs to ensure that processing continues within pre-determined tolerance limits (in terms of quality, contamination etc.). Microbiological monitoring at CCPs may aim to detect specific organism(s) or it may assess the level of general microbial contamination. Monitoring tests should give rapid results so that corrective action, if necessary, can be taken swiftly; the traditional approach to monitoring, based on colony counting, may be too slow to permit timely intervention in a modern production process. In the food industry, many of the microbiological tests are designed to detect contamination by non-specific organisms; these tests monitor factors such as carbon dioxide (produced by contaminating organisms) and/or the presence of ATP (released, in vitro, from contaminating organisms). An example of such tests is the screening of UHT milk for microbial contamination by the Lumac Autobiocounter M 4000 (manufactured by Perstorp Analytical LUMAC, Landgraaf, The Netherlands); in this system, samples of UHT milk are examined for their content of (intracellular) ATP – the presence of which indicates contamination with metabolizing (living) organisms. The Autobiocounter M 4000 is used e.g. for testing samples derived from a continuous production process that operates on a batch system (see below). Before testing, each sample is pre-incubated (at e.g. 30° C for 48 hours) to allow the growth of any contaminants that may be present. After incubation, an aliquot (e.g. 50 µl) of sample is transferred to an ATP-free cuvette (container) and is treated first with ATPase to hydrolyse (destroy) any background ATP in the sample. Subsequently, the aliquot is treated with a reagent that degrades (permeabilizes) the cytoplasmic membrane of any contaminating cells – allowing leakage of ATP from such cells. ATP, if present, is then detected by the luciferin–luciferase system (see BIOLUMINESCENCE) which produces light in the presence of even minute quantities of ATP; emitted light is measured by a photomultiplier (light meter), the result being shown digitally. Measurement of ATP is a fully automated procedure that is completed in about 15–20 minutes. Each batch of product is held back until the test results on samples from that batch become available – being released for distribution when tested samples are found to be satisfactory; despite the delay for sample preparation (incubation), the manufacturing process itself can be continuous, each batch of product being held only for the period required to examine samples. The LIMULUS AMOEBOCYTE LYSATE TEST, which detects and/or quantifies LIPOPOLYSACCHARIDE (LPS), can be used to monitor contamination by Gram-negative bacteria; any LPS-containing cell, and even isolated LPS, can give a positive test, so that this approach can detect both living and dead cells in the sample. 350 haemolysin Spontaneous elution (e.g. of influenza viruses) can be inhibited by carrying out haemagglutination tests at 4° C. (c) Passive haemagglutination: see PASSIVE AGGLUTINATION. (d) The action of certain LECTINS. (e) Combination between RBCs and bacterial fimbrial haemagglutinins (see FIMBRIAE). haemagglutination-inhibition test (HAI or HI test) Any test in which specific antibody or antigen is detected or quantified by its ability to inhibit HAEMAGGLUTINATION. HI tests are used e.g. as clinical diagnostic tests to detect serum antibodies to haemagglutinating viruses (e.g. influenza and rubella viruses); essentially, such tests determine the ability of a patient’s serum to neutralize the haemagglutinating properties of a suspension of the given virus – neutralization (a positive test) being indicated by the failure of the serum–virus mixture to agglutinate added RBCs. In many cases the serum used in such tests must be pre-treated to (i) abolish its ability to inhibit haemagglutination non-specifically, and/or (ii) to abolish its ability to promote haemagglutination. For example, serum may be pre-absorbed with kaolin (to sequester certain non-specific inhibitors of haemagglutination) and/or pre-treated with RBCs (of the type used in the test) in order to absorb HETEROPHIL ANTIBODIES; serum used in HI tests for e.g. influenza viruses is pre-treated with e.g. the enzyme RDE, and is then heated (prior to the test) in order to destroy RDE activity. [Book ref. 53, pp. 2034–2048.] HI tests for soluble antigens may involve inhibition of passive haemagglutination (see PASSIVE AGGLUTINATION) – antigen being preabsorbed to RBCs. Thus, the amount of antigen in a sample can be measured by the amount of homologous antibody it can complex (sequester) when mixed with a fixed volume of antibody; the amount of free (uncombined) antibody remaining in the mixture (an indirect measure of antigen) is determined by the addition of antigen-coated RBCs (see BOYDEN PROCEDURE.) To ensure antigen-specificity in the test, the antiserum should be preabsorbed with (non-coated) RBCs of the type used in the test. haemagglutinin Any agent or substance which can bring about HAEMAGGLUTINATION. Haemamoeba A subgenus of PLASMODIUM. haematein A red amphoteric DYE prepared by atmospheric oxidation of an aqueous solution of the colourless compound haematoxylin; haematein is usually referred to as ‘haematoxylin’. haematin Protoporphyrin IX–Fe(III) hydroxide. (cf. HAEMIN.) haematogenous (hematogenous) Produced by or derived from the blood, or disseminated via the bloodstream. haematophagous Feeding on blood. haematotropic Syn. HAEMOTROPIC. haematoxylin See HAEMATEIN. haematuria The presence of blood in the urine. haemin (1) Any protoporphyin IX–ferric iron complex: cf. HAEM sense 2. (2) Specifically: protoporphyrin IX–Fe(III) chloride. (cf. HAEMATIN.) Haemobartonella A genus of Gram-negative bacteria of the family ANAPLASMATACEAE. Cells: cocci, coccobacilli or rods (range 0.1–1.5 µm) which occur tightly bound to, or within, erythrocytes in the infected host. Haemobartonella spp (H. canis, H. felis and H. muris) have a wide host range and occur worldwide; they are transmitted e.g. by biting insects. haemocytometer See COUNTING CHAMBER. haemoglobinuria The presence of free haemoglobin in the urine. Haemogregarina See ADELEORINA. haemogregarines An imprecise name for those protozoa of the suborders ADELEORINA and EIMERIORINA which infect red and/or white blood cells in the vertebrate host. haemolysin (1) Any antibody homologous to the surface antigens of red blood cells (RBCs). (See also IMMUNE HAEMOLYSIS.) A particular type of microbial contaminant (e.g. a specific pathogen) may be detectable by using a PROBE-based approach. Moreover, PCR may provide a useful test system given organismspecific primers. HACEK Acronym for a group of bacteria which occur in the mouth and upper respiratory tract and which are sometimes responsible for lesions on heart valves: Haemophilus spp, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens and Kingella spp. hadacidin (CHO.NOH.CH2 .CO2 H) An ANTIBIOTIC and antitumour agent. It is an analogue of L-aspartic acid and blocks adenylosuccinate synthetase (and hence adenine nucleotide synthesis: see Appendix V(a)) but not other aspartic acid-requiring reactions. (cf. AZASERINE.) HaeII A RESTRICTION ENDONUCLEASE from Haemophilus aegyptius; PuGCGC/Py. HaeIII A RESTRICTION ENDONUCLEASE from Haemophilus aegyptius; GG/CC. Haeckel’s theory See ONTOGENY. haem (heme) (1) A generic term for any compound in which iron is coordinately bound to a PORPHYRIN or related compound (e.g. a CHLORIN); the four pyrrole nitrogen atoms each contribute an electron pair to form coordinate bonds with the iron atom – which may be in the ferrous or ferric state. The 5th and 6th coordination positions of the iron may be filled by electron pairs donated e.g. from water, chloride, or – in a haemoprotein – basic amino acid residues in the protein. Iron–protoporphyrin IX (= protohaem IX, ‘protohaem’) is the prosthetic group in haemoproteins such as CATALASE and certain CYTOCHROMES. Biosynthesis of protohaem IX involves the insertion of an iron atom into the porphyrin, catalysed by ferrochelatase. (See also X FACTOR.) (2) The term ‘haem’ is often used to refer specifically to protoporphyrin IX–ferrous iron – cf. HAEMIN. haemadsorption The adherence of RBCs to cells which are infected with certain types of virus (e.g. paramyxoviruses); thus, if a tissue culture is infected by such viruses infection may be detected by the cells’ ability to adsorb the RBCs of certain species (e.g. guinea pig) under appropriate conditions. haemadsorption-inhibition test Any test used to detect serum antibodies homologous to HAEMADSORPTION-promoting viruses. Essentially, the patient’s INACTIVATED SERUM (in various dilutions) is incubated (e.g. for 1 hour) with a suspension of such a virus – the whole then being used to inoculate a tissue culture; after ca. 5 days’ incubation the (washed) tissue culture is tested for haemadsorption by the addition of RBCs. In a positive test at least some dilutions of the patient’s serum will have neutralized the virus – thus preventing viral infection of the cells and (hence) preventing subsequent haemadsorption. haemagglutination The AGGLUTINATION of red blood cells (RBCs). Haemagglutination may result from any of the following. (a) Combination between RBCs and antibodies homologous to their surface antigens. (b) Combination between certain types of virus (e.g. orthomyxoviruses, paramyxoviruses) and specific cell membrane receptor sites on the RBCs of certain species – the viruses acting as links between the RBCs. After a period of time (which depends e.g. on temperature) some viruses may subsequently elute, spontaneously, owing to their NEURAMINIDASE activity which destroys the neuraminic acidcontaining receptor sites. RBCs from which such viruses have eluted cannot again be agglutinated by viruses of the same strain, and are said to be ‘stabilized’. (Sometimes RBCs stabilized by one strain of virus can be agglutinated by a different strain.) 351 haemolysis (2) (Syn. haemolytic toxin) Any of various microbial products which cause HAEMOLYSIS by damaging the cytoplasmic membrane of RBCs. (a) Aeromonas salmonicida haemolysins. See e.g. FURUNCULOSIS (H-lysin and T-lysin). (b) Bacillus haemolysins. See e.g. SURFACTIN. (c) Clostridium haemolysins. See CLOSTRIDIUM. (d) Escherichia coli haemolysins. a-Haemolysin is an extracellular haemolysin (MWt ca. 107000) specified by either chromosomal or plasmid-borne genes (hly genes); chromosomal and plasmid-borne hly genes are closely homologous, and identical hly genes can occur on plasmids of different incompatibility groups. In cultures of E. coli, production of a-haemolysin occurs under aerobic and anerobic conditions, but is enhanced by aerobiosis and inhibited by >100 µM iron. a-Haemolysin can lyse the RBCs of e.g. sheep, rabbits and man – causing clear haemolysis on blood agar plates; it is cytotoxic for human leucocytes and is lethal when injected into mice. a-Haemolysin appears to insert into lipid bilayers, generating hydrophilic transmembrane pores of effective diameter ca. 3 nm [Inf. Immun. (1986) 52 63–69]. [Review: MR (1984) 48 326–343; role of a-haemolysin in the pathogenesis of experimental E. coli infection in mice: JGM (1985) 131 395–403.] b-Haemolysin is a cell-bound haemolysin active against e.g. rabbit and human RBCs; it causes clear haemolysis on blood agar plates. g-Haemolysin has been reported in nalidixic acid-resistant E. coli mutants, and its production is stimulated by nalidixic acid; it is not lytic for human or rabbit RBCs. (e) Staphylococcus haemolysins are produced by both coagulase-positive and -negative strains; the best known are the protein EXOTOXINS a-, b-, g- and d-haemolysins, although others also occur. No particular pattern of haemolysin production has been associated with virulence. a-Haemolysin (a-toxin) is a hydrophobic, surface-active protein with (apparently) no enzymic activity. It shows the Arrhenius effect: it is inactivated on heating to 60° C but is partly reactivated on further heating to ca. 100° C. a-Haemolysin is >100 times more effective against rabbit RBCs than human RBCs. It appears to bind to the cell membrane, causing release of K+ – possibly due to the formation of transmembrane pores by amphiphilic hexamers of the haemolysin. a-Haemolysin can be lethal for man and animals; depending on animal species, it can be leucocidal (= the Neisser–Wechsberg LEUCOCIDIN), can aggregate and lyse blood platelets, and has a range of toxic effects e.g. on the nervous and vascular systems (e.g. it increases vascular permeability). When injected subcutaneously it can have dermonecrotic effects which may be due to direct necrotizing activity or to prolonged vasospasm. a-Haemolysin has been shown to play a major role in the pathogenesis of gangrenous mastitis in sheep and cattle. b-Haemolysin (b-toxin) is a SPHINGOMYELINASE C specific for sphingomyelin (or lysolecithin); it is inactivated at 60° C, requires Mg2+ for activity, and is inhibited e.g. by Zn2+ and chelating agents. It can hydrolyse the sphingomyelin in sphingomyelin-rich RBCs (e.g. those from sheep or ox) without causing haemolysis; however, lysis can subsequently be induced by chilling (see HOT–COLD LYSIS) or by the addition of the CAMP factor (see CAMP TEST). b-Haemolysin is most frequently produced by staphylococci isolated from animals other than man. Reports that it can be lethal are controversial. It is not dermonecrotic (although it may cause necrosis in lactating mammary gland), but it can show toxic effects on the cardiovascular system and can lyse platelets and guinea-pig macrophages (but not human PMNs). Its role in pathogenesis, if any, is unknown. g-Haemolysin (g-toxin) consists of two protein components which appear to act synergistically in bringing about haemolysis. Rabbit RBCs are more susceptible than human or sheep RBCs; horse RBCs are insensitive. g-Haemolysin can also increase membrane permeability in cultured fibroblasts, and may have some leucocidal activity. It is inactivated at 60° C and by certain acidic polymers – including agar, heparin, and dextran sulphate (but not e.g. agarose, hyaluronic acid or chondroitin sulphate), by various lipids (e.g. cholesterol, fatty acids), and by EDTA and citrate. d-Haemolysin (d-toxin) is a heat-stable, hydrophobic, strongly surface active peptide containing 26 amino acid residues. It can lyse RBCs from many (possibly all) species; it is also active on a wide range of natural and synthetic membranes, lysing e.g. leucocytes, all kinds of mammalian cells in culture, certain bacteria, mitochondria, protoplasts, liposomes, etc. At relatively high concentrations, it probably exerts its effect by non-specific solubilization of membrane lipids and/or proteins; at low concentrations it may act by forming transmembrane pores and/or by activating endogenous phospholipase A2 . It is inhibited by various phospholipids and serum lipoproteins, and is poorly antigenic. Different d-haemolysins are produced by different biotypes of S. aureus. [Reviews: Book ref. 44, pp. 619–744.] (f) Streptococcus haemolysins. See e.g. STREPTOLYSIN O and STREPTOLYSIN S. (g) Vibrio parahaemolyticus haemolysin. See KANAGAWA PHENOMENON. haemolysis The lysis of erythrocytes (red blood cells). Haemolysis may occur in any of the following ways. (a) By the action of a COMPLEMENT-dependent or -independent HAEMOLYSIN. (b) By a complement-fixing union between an antibody and an erythrocyte-bound particulate antigen (passive haemolysis). (c) By REACTIVE LYSIS. (d) By osmotic lysis in a hypotonic medium. (e) By viral activity: certain viruses (e.g. the mumps virus) can lyse the erythrocytes of certain species. The ability of certain microorganisms to cause haemolysis can be detected e.g. by growing the organisms on a suitable BLOOD AGAR; an extracellular haemolysin causes the development of a differentiated zone around each colony. Some types of haemolysin bring about a glass-clear, colourless zone. Such clear haemolysis is often called ‘b-haemolysis’; however, ‘b-haemolysis’ can be a misleading term since clear haemolysis can be caused e.g. by the a-haemolysin of staphylococci or by the a- or b-haemolysins of Escherichia coli. A zone in which the blood is greenish, but still opaque, is often called ‘a-haemolysis’; such greening can be caused by various types of haemolysin. The absence of haemolysis is sometimes called (confusingly) ‘g-haemolysis’. haemolytic immune body (HIB; amboceptor) Antibody to the surface antigens of erythrocytes. (See also HAEMOLYTIC SYSTEM.) haemolytic system In a COMPLEMENT-FIXATION TEST: a suspension of sensitized erythrocytes, i.e. erythrocytes (commonly sheep erythrocytes) which have been exposed to a subagglutinating dose of antibodies homologous to their surface antigens. The system exhibits no HAEMOLYSIS in the absence of COMPLEMENT, and almost complete haemolysis (ca. 90% of the erythrocytes lysed) when exposed to the maximum concentration of complement used in a CFT; with amounts of complement between these extremes, the proportion of erythrocytes which lyse is dependent on the amount of free (i.e. unfixed) complement. Thus, the 352 Haemophilus haemolytic system indicates the amount of complement (if any) remaining unfixed in each dilution at the end of the first stage in a CFT. (See also EAC.) haemolytic uraemic syndrome (HUS) A potentially fatal disease (of children and adults) involving renal impairment/acute renal failure (see also TNF). An early stage of (bloody) diarrhoea is common, though not invariably present. Most cases with prodromal diarrhoea appear to be caused by O157 strains of Escherichia coli (see EHEC) or by Shigella dysenteriae. The major symptoms of HUS are toxin-dependent; attempts are being made to sequester toxins in the gut (see SYNSORB Pk ). Complications may include e.g. CNS involvement. Antibiotics have not been found useful, and may be actually harmful. Non-diarrhoeal HUS is sometimes associated with EHEC but may be induced by certain drugs (e.g. mitomycin C, quinine), and there are several other distinct causes. haemophagocytic syndrome (HS) A syndrome which is characterized by fever and e.g. (commonly) cytopenia, coagulopathy, hepatosplenomegaly and haemophagocytosis – i.e. phagocytosis of haemopoietic cells by HISTIOCYTES. Primary HS is associated with inherited conditions. Secondary HS may be associated with certain infections, malignancy or non-malignant conditions (e.g. autoimmune disease), or may be drug-associated (phenytoin). Infection-associated HS (IAHS) includes virus-associated HS (VAHS) and bacterium-associated HS (BAHS). The pathogenesis of HS is not understood but it appears to involve unregulated T lymphocytes, monocytes, macrophages and cytokines; elevated levels of e.g. IFN-g (interferon-g) and IL-18 (interleukin-18) have been found in serum, and an imbalance in Th1/Th2 cells has also been reported. Some 60% of cases have been reported to occur in patients with preexisting immunodeficiency, suggesting that immune status may be important as a predisposing factor. Diagnosis includes demonstrating phagocytosis of erythrocytes (red blood cells), leukocytes and platelets by examination of bone marrow. Microorganisms associated with HS include: Epstein–Barr virus (apparently responsible for most cases of VAHS) and other herpesviruses (e.g. cytomegalovirus, HHV6 and HHV8); species of Borrelia, Brucella, Mycobacterium, Rickettsia, Staphylococcus and Streptococcus; Cryptococcus; and Leishmania donovani. [Haemophagocytic syndrome associated with infections: BCH (2000) 13 163–178.] Haemophilus A genus of Gram-negative bacteria of the PASTEURELLACEAE. Cells: pleomorphic coccobacilli, rods (often ca. 0.4 × 1–2 µm) or – especially under suboptimal growth conditions – filaments. Some strains have capsules. Fimbriae occur in haemagglutinating strains (H. parasuis, H. paragallinarum, strains of H. aegyptius). Optimum growth temperature: 35–37° C. Growth may require the X FACTOR and/or the V FACTOR (see PORPHYRIN TEST and SATELLITE PHENOMENON). Media for primary isolation include e.g. CHOCOLATE AGAR, FILDES’ ENRICHMENT AGAR, or LEVINTHAL’S MEDIUM; some species (e.g. H. ducreyi, H. parasuis) require serum. Growth in some species (e.g. H. aphrophilus, H. paragallinarum) requires, or is enhanced by, incubation with 5–10% CO2 . Colonies on rich media are usually non-pigmented or slightly yellowish, 0.5–2.0 mm diameter (48 hours/37° C); those of capsulated strains are more mucoid and may appear iridescent in obliquely transmitted light. Most species produce acetic, lactic and succinic acids (usually without gas) from glucose. Most strains do not ferment lactose. NO3 − is reduced to NO2 − ; NO2 − may also be reduced. GC%: 37–44. Type species: H. influenzae. Haemophilus spp are parasitic on the mucous membranes (particularly of the respiratory tract) in man and other animals. Some species are pathogenic; others can be opportunist pathogens, causing e.g. dental abscesses, ENDOCARDITIS etc. Some strains form IgA1 proteases (q.v.). Plasmid-mediated antibiotic resistance has become common in some species (e.g. H. influenzae). H. actinomycetemcomitans. See ACTINOBACILLUS. H. aegyptius. Causes e.g. PINK-EYE, mainly in hot climates. Requires X FACTOR (q.v.) and V factor. Indole −ve; urease +ve; non-haemolytic; usually forms acid (no gas) from glucose but not from xylose; catalase +ve. H. ducreyi. Causes CHANCROID. Requires X factor but not V factor. Indole −ve; urease −ve; non-capsulated; some strains weakly haemolytic; sugars are generally not metabolized (acid from glucose may be delayed +ve); peptidases are produced; catalase −ve. H. influenzae. Parasitic and pathogenic in man. Requires V and X factors; non-haemolytic; acid (no gas) from glucose and xylose; catalase +ve. Eight biotypes (I–VIII) are distinguished e.g. on indole, urease and ODC reactions. Some strains have Haemophilus influenzae: differentiation of biotypes Biotype I II III IV V VI VII VIII a Indole Urease ODCa + + − − + − + − + + + + − − − − + − − + + + − − Ornithine decarboxylase test (see DECARBOXYLASE TESTS). capsules, and six serotypes are distinguished on the basis of capsular antigens (polysaccharides, at least some of which contain ribosyl groups). Strains of serotype b (usually biotype I) may cause acute diseases such as CELLULITIS, EPIGLOTITIS, MENINGITIS (in children), and PNEUMONIA (caused also by other serotypes). Antisera to serotype b cross-react with antigens from a wide range of bacteria – e.g. staphylococci, streptococci, Bacillus spp. Non-capsulated strains (usually biotypes II or III) occur as commensals (e.g. in the nasopharynx) but can occasionally cause chronic conditions such as BRONCHITIS, CONJUNCTIVITIS, OTITIS MEDIA and SINUSITIS. [Conjugate vaccine against H. influenzae type b (for protection against e.g. meningitis and epiglottitis); RMM (1996) 7 231–241.] H. paragallinarum (including strains formerly called H. gallinarum). Causes coryza in fowl. Requires V factor but not X factor. Indole −ve; urease −ve; non-haemolytic; acid (no gas) from glucose and (some strains) xylose; catalase −ve. H. parainfluenzae. Commensal in man, occasionally implicated in e.g. endocarditis. Requires V factor but not X factor. Indole −ve; urease −ve (biotype I) or +ve (biotypes II and III); non-haemolytic; acid (±gas; biotype III: no gas) from glucose but not from xylose; catalase +ve/−ve. H. parasuis (including most strains formerly called H. suis or H. influenzae-suis). Causes GLASSER’S DISEASE in pigs. Requires 353 haemoproteins V factor but not X factor. Indole −ve; urease −ve; nonhaemolytic; acid (no gas) from glucose but not from xylose; catalase +ve. H. piscium. A fish pathogen (see ULCER DISEASE) now regarded as an atypical strain of Aeromonas salmonicida. H. vaginalis: see GARDNERELLA. Other species: H. aphrophilus, H. avium, H. haemoglobinophilus, H. haemolyticus, H. paracuniculus, H. parahaemolyticus, H. paraphrohaemolyticus, H. paraphrophilus, H. pleuropneumoniae, H. segnis. Species incertae sedis: ‘H. somnus’, ‘H. agni’ and ‘H. equigenitalis’, pathogenic in cattle, sheep and horses, respectively. [Book ref. 22, pp. 558–569.] haemoproteins See HAEM. Haemoproteus A genus of protozoa of the HAEMOSPORORINA. Species are parasitic in e.g. birds; the vectors are hippoboscid flies. haemorrhagic colitis COLITIS associated with bloody diarrhoea; when caused by EHEC it usually occurs without fever. haemorrhagic enteritis of turkeys An acute disease of turkeys (particularly turkey poults 6–12 weeks of age) caused by an AVIADENOVIRUS. Symptoms: depression, loss of appetite, bloody faeces; haemorrhages also occur in the muscles and internal organs. The disease may be fatal. [Book ref. 116, pp. 537–539.] (cf. MARBLE SPLEEN DISEASE.) haemorrhagic fever with renal syndrome See HANTAVIRUS. haemorrhagic fevers See VIRAL HAEMORRHAGIC FEVERS. haemorrhagic nephrosonephritis Syn. KOREAN HAEMORRHAGIC FEVER. haemorrhagic septicaemia (1) (in fish) See INFECTIOUS DROPSY and EGTVED DISEASE. (2) (barbone) An acute, often fatal, septicaemic CATTLE DISEASE which appears to be a primary PASTEURELLOSIS caused by strains of Pasteurella multocida. Onset is sudden, with fever, profuse salivation, and petechial haemorrhages in submucosal tissues; death may occur within ca. 1 day. A carrier state occurs. A vaccine is available. haemorrhagic syndrome See TRICHOTHECENES. Haemosporina See HAEMOSPORORINA. Haemospororina A suborder of protozoa (order EUCOCCIDIORIDA) previously classified as the suborder Haemosporina (sic) of the order Eucoccida [JP (1964) 11 7–20] or Eucoccidiida [JP (1980) 27 37–58]. In this suborder the organisms lack a conoid, syzygy does not occur, and motile zygotes are formed (cf. ADELEORINA, EIMERIORINA); some species form pigment from host cell haemoglobin (cf. PIROPLASMASINA). The organisms are heteroxenous, the asexual phase taking place in a vertebrate host and the sexual phase occurring in a blood-sucking insect. Genera: e.g. HAEMOPROTEUS, LEUCOCYTOZOON, PLASMODIUM. haemotropic (haematotropic; hemotropic; hematotropic) Having an affinity for the blood. (Used e.g. of blood parasites such as Plasmodium.) haemozoin (malarial pigment) A brown or black pigment which occurs in the intraerythrocytic schizont of Plasmodium spp (see PLASMODIUM and MALARIA). Haemozoin derives from the erythrocyte’s haemoglobin; the parasite ingests haemoglobin, uses the protein part, and (normally) detoxifies the remaining ferriprotoporphyrin IX (FP) by polymerizing it to haemozoin. (Unless detoxified by polymerization, FP can lyse the parasite’s membranes.) Haemozoin remains behind in the parasitized erythrocyte following release of merozoites. Quinoline-type antimalarial agents (see AMINOQUINOLINES, appear to act by blocking the parasite’s ‘haem polymerase’ activity, thus killing the parasite by inhibiting detoxification of FP [Acta Tropica (1994) 56 157–171]. In order to facilitate studies on (i) the mechanism by which Plasmodium detoxifies FP, and (ii) the mode of action of quinoline-type antimalarials, the crystal structure has been determined for a synthetic compound, b-haematin, which is chemically identical to haemozoin [Nature (2000) 404 307–310]. Hafnia A genus of (usually non-capsulated) Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Most strains are motile at 25–30° C, but many are non-motile at 35° C. MR and VP reactions are variable at 35–37° C but MR −ve, VP +ve at 22–25° C. Lactose −ve (but Lac plasmids may occur); lysine and ornithine decarboxylases +ve; arginine dihydrolase −ve. Growth occurs e.g. on DCA and in KCN media. One species: H. alvei (formerly e.g. Enterobacter alvei or E. hafniae); many biotypes. H. alvei occurs in man, animals and birds, and in soil, sewage and water. H. alvei has been associated with diarrhoeal disease [JCM (1994) 32 2335–2337]; strains of H. alvei can produce A/E lesions (see entry EPEC). H. alvei has also been associated with liver abscesses, septicaemia, peritonitis and pneumonia [characterization of H. alvei isolates from human clinical extra-intestinal specimens: JMM (2001) 50 208–214]. (cf. KOSERELLA and OBESUMBACTERIUM.) hag gene In enterobacteria: the structural gene for FLAGELLIN. HAI test HAEMAGGLUTINATION-INHIBITION TEST. haidai See LAMINARIA. hair cell A narrow, elongated, apparently non-pigmented cell present at the ends of trichomes in certain filamentous CYANOBACTERIA. In ‘rivularian’ species (e.g. GLOEOTRICHIA), at least, hair cells seem to be formed under conditions of phosphorus or fixed nitrogen limitation; their function, if any, is unknown. hairpin In a single strand of DNA or RNA: a double-stranded region formed by base-pairing between sequences in the strand which are complementary and opposite in polarity (e.g., 5
. . . GGCATG. . . . CATGCC . . . 3
). (cf. CRUCIFORM; see also STEMAND-LOOP STRUCTURE.) hairy-cell leukaemia (1) A T cell LEUKAEMIA (see HTLV-II in HTLV). (2) A B cell leukaemia. Both diseases are rare, and in each the cells have fine, hair-like projections. hairy leukoplakia (oral hairy leukoplakia) In e.g. HIV-infected individuals: whitish areas, typically found on the sides of the tongue, associated with replication of the EPSTEIN–BARR VIRUS; it may resemble oral candidiasis but, unlike candidiasis, it cannot be removed. A high proportion of patients with hairy leukoplakia subsequently develop AIDS. Hairy leukoplakia has been treated with e.g. high-dose ACYCLOVIR but it tends to recur on cessation of treatment. [Opportunistic oral infections in patients infected with HIV-1: RMM (1996) 7 151–163 (hairy leukoplakia: 157–158).] hairy root A disease which can affect various dicotyledonous plants; it is characterized by the proliferation of adventitious roots from the site of a wound infected with Agrobacterium rhizogenes (see AGROBACTERIUM). Pathogenesis appears to be analogous to that in CROWN GALL, involving the transfer, integration, and expression in plant host cells of a segment of an Ri (root-inducing) plasmid. (Ri and Ti plasmids belong to different incompatibility groups.) Conjugal transfer of Ri plasmids appears to occur constitutively at rather high frequencies, and induction of conjugal transfer by opines or other compounds has not been observed. Two types of Ri plasmid are known: the agropine type and the mannopine type; some of the catabolic QUININE) 354 Halomonas some species lyse (owing to weakening of the cell envelope rather than to an osmotic effect per se). The organisms are chemoorganotrophs which use amino acids and/or carbohydrates as principal sources of carbon, and which typically grow aerobically; metabolism is respiratory (oxidative). The organisms form menaquinones rather than ubiquinones. Under microaerobic or anaerobic conditions some strains can use light energy (see PURPLE MEMBRANE; see also PHOTOTAXIS). Two genera: HALOBACTERIUM (the type genus) and HALOCOCCUS. Halobacterium A genus of catalase-positive, oxidase-positive archaeans of the HALOBACTERIACEAE. The cells are lophotrichously flagellated (or non-motile) frequently pleomorphic rods or filaments, or discs; they often contain gas vacuoles (which are plasmid-encoded in at least some strains), and they lyse in dilute solutions (e.g. NaCl ∼ca. 0.8–1.5 M). Binary fission occurs by constriction. Most strains can be cultured in e.g. well-aerated tryptone – yeast extract – salt media. Optimum growth temperature: 40–50° C. Some strains form a PURPLE MEMBRANE under microaerobic or anaerobic conditions. The DNA in Halobacterium spp is often composed of a major component (ca. 60–90% of the total DNA) with a GC% of ca. 66–68, and a minor component (‘satellite DNA’ or plasmid DNA) with a GC% of ca. 57–60. Type species: H. salinarium. H. halobium. See H. salinarium. H. pharaonis. Motile, alkalophilic (optimum pH: 8.5) rods which do not use sugars; substrates include e.g. formate, fumarate, pyruvate. Growth occurs optimally in 3.5 M NaCl. H. saccharovorum. Motile rods which form acid (mainly acetic acid) from sugars. Growth occurs optimally in 3.5–4.5 M NaCl. H. salinarium (= H. halobium). Motile, typically rod-shaped cells which grow aerobically or (strains with a purple membrane) anaerobically in the light. Amino acids are used for carbon and energy, although growth may be stimulated by carbohydrates (without acid formation). Growth occurs optimally in 4–5 M NaCl. H. vallismortis. Motile, very pleomorphic rods which use sugars, often with the formation of acid. Facultatively anaerobic. Growth occurs optimally in 4.3 M NaCl. H. volcanii. Mainly discoid or cup-shaped cells which are (apparently) non-flagellated but which are sometimes capable of a rotatory movement. Sugars are used for carbon and energy. Growth occurs optimally in 1.5–2.5 M NaCl. halocarbon See FREON. halocins See BACTERIOCINS. Halococcus A genus of catalase-positive and oxidase-positive archaeans of the HALOBACTERIACEAE. The cells are non-motile, orange- or red-pigmented cocci (diameter ca. 0.8–1.5 µm) which occur in pairs or in regular or irregular groups. At least 2.5 M NaCl is needed for growth; growth occurs optimally in 3.5–4.5 M NaCl. The organisms are much more resistant than are Halobacterium spp to hypotonic solutions. Amino acids are used for carbon and energy. Optimum growth temperature: 30–37° C. GC%: ca. 61–66. Type species: H. morrhuae [Book ref. 22, pp. 266–267]. halofantrine See MALARIA. halogens (as antimicrobial compounds) See entries CHLORINE, FLUORIDES, IODINE. Halomonas A genus (incertae sedis) of aerobic, facultatively anaerobic, chemoorganotrophic, halotolerant, Gram-negative functions of the agropine strains are carried not by the Ri plasmid but by another plasmid which can cointegrate with it [MGG (1983) 190 204–214]. [Book ref. 55, pp. 271–286.] hairy shaker disease Syn. BORDER DISEASE. halazone See CHLORINE. half-a-Gram stain A non-specific staining procedure used e.g. for detecting cells of Legionella in smears of sputum or aspirates etc. The slide is flooded for 0.5–1 min with a solution of crystal violet, drained, and flooded with Gram’s iodine solution for 1–2 min. The slide is then rinsed well in tap-water and dried in air. (cf. GRAM STAIN.) half-chiasma See RECOMBINATION. half-reduction potential See REDOX POTENTIAL. Halicystis See DERBESIA. Halidrys See PHAEOPHYTA. Halimeda A genus of siphonaceous, mainly tropical or subtropical green seaweeds (division CHLOROPHYTA). The thallus is segmented and calcified, and contains both chloroplasts and amyloplasts. The cell wall contains a xylan (see XYLANS) as the main structural component, and is more or less heavily calcified with aragonite. When mature, the entire thallus is converted to gametangia, after which it dies. Related holocarpic, usually calcified algae include species of CAULERPA, PENICILLUS and UDOTEA. Haliscomenobacter A genus of Gram-negative, rod-shaped bacteria which occur e.g. in activated sludge. Individual cells are apparently non-motile; they commonly occur in sheathed trichomes. The organisms can utilize a range of carbon and nitrogen sources; PHB is not formed. H. hydrossis has a GC% of ca. 49. [Book ref. 45, pp. 425–440 (436–439).] hallucinogenic mushrooms Certain agarics whose fruiting bodies contain hallucinogenic compounds; they include certain species of Conocybe, Psilocybe (e.g. P. mexicana, P. semilanceata) and Stropharia. At least some of these fungi contain psilocybin and/or psilocin, hallucinogenic tryptamine derivatives related to serotonin. Some species of Lycoperdon also contain hallucinogenic substance(s). halo blight A typically seed-borne disease of beans (Phaseolus) caused by Pseudomonas phaseolicola. Small, translucent lesions on leaves later darken, each becoming surrounded by a yellowish ‘halo’; leaves later exhibit interveinal yellowing, and brownish spots may develop on pods. halo spot A CEREAL DISEASE, caused by Selenophoma donacis (Septoria oxyspora) which can affect e.g. barley, wheat and rye. Small, pale lesions, each with a purplish-brown margin, occur mainly on the leaves, and rows of dark-coloured pycnidia develop in line with the veins. (Halo spot is called ‘eyespot’ in some parts of the world: cf. EYESPOT sense 2.) halobacteria (1) Strains or species of Halobacterium. (2) Members of the family Halobacteriaceae. Halobacteriaceae A family of extremely halophilic archaeans which occur e.g. in salt lakes, evaporated brines, and salted fish (in which they can cause spoilage). Cells: rods, cocci or discs, containing orange, red or mauve carotenoid pigments (predominantly bacterioruberins). Division occurs by binary fission. The Gram reaction is negative. (See also CELL WALL, GAS VACUOLE and S LAYER.) Growth requires a minimum concentration of 1.5 M NaCl, good growth typically needing 3–4 M NaCl; cells accumulate electrolyte (mainly KCl) to an intracellular concentration at least equivalent to that in the environment. High concentrations of electrolyte are required to maintain the structural integrity of e.g. the cytoplasmic membrane and the ribosomes; in dilute solutions 355 halonitrosoureas bacteria which occur e.g. in salterns and (presumably) in salt lakes etc. Cells: non-pigmented or yellow-pigmented, polarly or laterally flagellated rods, 0.6–0.8 × 1.6–1.9 µm, sometimes pleomorphic or filamentous. Metabolism may be respiratory (oxidative), with O2 or nitrate (anaerobic respiration) as terminal electron acceptor, or fermentative; anaerobic growth, without nitrate, can occur with glucose but not with other carbohydrates. Carbon sources include a wide range of alcohols, amino acids, organic acids and sugars. Salt (NaCl) tolerance: 0.1–32.5% w/v. Catalase +ve. Oxidase +ve. GC%: ca. 60. Type species: H. elongata. [Book ref. 22, pp. 340–343.] halonitrosoureas See N-NITROSO COMPOUNDS. halo-opsin See OPSIN. halophile An organism which grows optimally only in the presence of electrolyte (commonly NaCl) at concentrations above ca. 0.2 M, and which typically grows poorly, or not at all, in low concentrations of electrolyte; examples include e.g. species of ACTINOPOLYSPORA, DACTYLOCOCCOPSIS, HALOBACTERIUM, HALOCOCCUS, PARACOCCUS and VIBRIO. (cf. HALOTOLERANT.) A number of authors have accepted the following categories: slightly halophilic (optimum growth in the presence of 0.2–0.5 M electrolyte); moderately halophilic (optimum growth in 0.5–2.5 M electrolyte); extremely halophilic (optimum growth in >2.5 M electrolyte). [Halophilic and halotolerant organisms: Book ref. 157, pp. 171–214, and Book ref. 191, pp. 55–81.] Halopteris See PHAEOPHYTA. halorhodopsin A RETINAL-containing protein pigment (MWt ca. 25000) which occurs in the PURPLE MEMBRANE of Halobacterium salinarium and which is involved in transmembrane ion translocation. Maximum absorption appears to occur at ca. 580 nm, the absorption characteristics being stabilized by anions (primarily Cl− ). On illumination, halorhodopsin undergoes cyclical changes in its absorption characteristics with a periodicity of ca. 5–10 msec; several photointermediates appear to be involved. Halorhodopsin undergoes bleaching when illuminated in the presence of hydroxylamine. Some strains of Halobacterium salinarium (e.g. JW-5, W-296), which cannot synthesize retinal, produce bacterio-opsin but are repressed in the synthesis of haloopsin. Halothrix See PHAEOPHYTA. halotolerant Refers to the ability of a non-HALOPHILE to grow in the presence of high concentrations of electrolyte (e.g. up to ca. 2.5 M); examples of halotolerant organisms include e.g. HALOMONAS and strains of Staphylococcus. Halteria A genus of freshwater ciliates (order OLIGOTRICHIDA). Cells: roughly spherical, with very conspicuous oral ciliature and a number of groups of ‘cirri’ or ‘bristles’ forming an equatorial band. H. grandinella is ca. 25–50 µm in diam., has a ‘bouncing’ type of movement, and occurs e.g. in ponds and organically polluted waters (see SAPROBITY SYSTEM) where it feeds on bacteria; the organism is fairly tolerant of low O2 levels, but is sensitive to NH3 and H2 S. ham See CURING (1); MEAT SPOILAGE; SOFT CORE HAM. hamanatto (black beans) An oriental food made by fermenting whole soybeans with strains of Aspergillus oryzae. Beans are steamed, coated with wheat flour, inoculated with A. oryzae, and incubated 1–2 days. Brine and spices are added and incubation is continued 6–12 months; the beans are then dried. hamathecium A term which refers, collectively, to the various types of structure which may occur between and around the asci in an ascocarp: see e.g. PARAPHYSIS, PARAPHYSOID, PSEUDOPARAPHYSIS and PERIPHYSES. Hamigera See TALAROMYCES. Hammondia hammondi Syn. Isospora datusi. hamycin A heptaene POLYENE ANTIBIOTIC produced by Streptomyces pimprina; it is effective against only a limited range of fungi, but is particularly effective against Candida albicans. hand, foot and mouth disease An acute, highly infectious, self-limiting disease (mainly of children) characterized by the development of vesicular-ulcerative lesions in the mouth with similar lesions scattered on the soles of the feet and on the palms of the hands. The causal agent is usually coxsackievirus A16, but A4, A5, A9, A10, B2 and B5 have also been implicated. (cf. FOOT AND MOUTH DISEASE.) hanging drop A direct, visual procedure used for determining the MOTILITY of microorganisms in a fluid medium. A COVERGLASS is placed flat on the bench, and a drop of fluid (e.g. from a broth CULTURE) is transferred (e.g. by a LOOP) from the culture to the centre of the cover-glass. A plasticine ring (diam. ca. 1.5 cm, ca. 4 mm thick) is gently pressed onto one face of a SLIDE; the ring (attached to the slide) is then gently pressed onto the cover-glass so that the ring becomes sandwiched between the (parallel) faces of slide and cover-glass. The whole is then inverted, so that the drop hangs freely beneath the cover-glass, and can be examined under the microscope. This method avoids microcurrents which could give a false impression of motility. Hanks’ BSS Hanks’ BALANCED SALT SOLUTION; it contains (g/l): NaCl (8.0), KCl (0.4), CaCl2 .2H2 O (0.185), MgSO4 .7H2 O (0.1), MgCl2 .6H2 O (0.1), Na2 HPO4 .H2 O (0.06), KH2 PO4 (0.06), NaHCO3 (0.35), glucose (1.0), and phenol red (0.01); pH: 7.0–7.2. Hanseniaspora A genus of fungi (family SACCHAROMYCETACEAE). The genus contains the teleomorphs of KLOECKERA spp. Ascospores may be bowler-hat-shaped, 1–4 per ascus, and released at maturity, or they may be spherical, with or without an equatorial ridge, smooth or warty, and not released at maturity. [Book ref. 100, pp. 154–164.] Hansen’s bacillus Mycobacterium leprae. Hansen’s disease Syn. LEPROSY. Hansenula A genus of yeasts (family SACCHAROMYCETACEAE) in which the cells are generally spheroidal, ellipsoidal or elongate; vegetative reproduction occurs by multilateral budding. Pseudomycelium or true mycelium may be formed. Asci are generally dehiscent, occasionally persistent. Ascospores: hemispheroidal, Saturn-shaped, or bowler-hat-shaped. Species may be homothallic or heterothallic. Most species can ferment glucose, some can also ferment other sugars, and some (e.g. H. canadensis) are non-fermentative. NO3 − is assimilated. Thirty species are recognized, including e.g. H. anomala (anamorph: Candida pelliculosa), H. canadensis (conspecific with H. wingei ), H. capsulata, H. jadinii (anamorph: Candida utilis), H. polymorpha (see also METHYLOTROPHY), H. wickerhamii. Species have been isolated from tree exudates, soil, insect larvae and frass, etc. [Book ref. 100, pp. 165–213.] Hantaan virus See HANTAVIRUS. Hantavirus A genus of viruses of the family BUNYAVIRIDAE [taxonomic proposal: Virol. (1983) 131 482–491]. The enveloped, spherical virions each contain a negative-sense ssRNA genome consisting of three fragments. Approximately half of the >20 known species are associated with human disease – either (i) the hantavirus pulmonary syndrome (HPS), or (ii) haemorrhagic fever with renal syndrome (HFRS). Unlike other members of the Bunyaviridae (which are maintained in arthropods) hantaviruses are found in populations of 356 HBB (persistently infected) rodents – including rats (Seoul virus), mice (Sin Nombre virus, Dobrava-Belgrade virus, Hantaan virus) and voles (Puumala virus); human infection appears to occur mainly by inhalation of aerosols of the infected urine, faeces etc. of rodents. Species of Hantavirus include: Hantaan virus (HFRS; see also KOREAN HAEMORRHAGIC FEVER), Seoul virus (HFRS), Puumala virus (HFRS), Dobrava-Belgrade virus (HFRS), Sin Nombre virus (HPS), Andes virus (HPS), Black Creek Canal Virus (HPS) and Laguna Negra virus (HPS). [Hantaviruses (persistence in rodent reservoirs): TIM (2000) 8 61–67. Hantaviruses (review): JMM (2000) 49 587–599.] Haploangium See MYXOBACTERALES. haploid Having a PLOIDY of one. (cf. DIPLOID, POLYPLOID.) haploidization (mycol.) See PARASEXUAL PROCESSES. haplokinety (1) The infraciliary base (only) of a stichodyad PARORAL MEMBRANE. (2) A stichodyad paroral membrane. haplomycosis Syn. ADIASPIROMYCOSIS. haplont (1) (noun) An organism in whose life cycle only the zygote is diploid. (cf. DIPLONT.) (2) The haploid form of an organism, e.g. a gametophyte (see ALTERNATION OF GENERATIONS). (3) (haplontic) (adj.) Refers to either (1) or (2) above. haplophase In organisms which reproduce sexually: the haploid phase of the life cycle. In many higher fungi haplophase and DIPLOPHASE are separated by DIKARYOPHASE. Haplospora See PHAEOPHYTA. Haplosporangium See ADIASPIROMYCOSIS. Haplosporidium A genus of protozoa (class STELLATOSPOREA) which are parasitic in various aquatic invertebrates (including annelids and molluscs). The spores are operculate and characteristically bear filamentous extensions which arise from the outer spore coat and are typically coiled around the spore. [Spore ultrastructure in H. lusitanicum: J. Parasitol. (1984) 70 358–371.] The vegetative stage is plasmodial. haplosporosomes See STELLATOSPOREA. hapten A substance which can elicit an immune response only when combined with another molecule or particle (carrier ); if administered on its own it fails e.g. to stimulate antibody production (but cf. autocoupling hapten, below). The free hapten can, however, combine with antibodies raised against the hapten–carrier complex; such combination may or may not result in the precipitation of the hapten–antibody complex. A substance which acts as a hapten in one species may act as a complete antigen in another – e.g. the pneumococcal capsular polysaccharides are haptenic in the rabbit but antigenic in man. A hapten may be artificially bound to a protein in order to increase the immunological specificity of that protein. Autocoupling haptens bind spontaneously to tissue carriers in vivo, thus becoming antigenic. (See also ALLERGEN.) hapteron See CYATHUS. haptocyst An EXTRUSOME in the tentacles of suctorian ciliates. haptonema A thread-like appendage present on unicellular algae of the PRYMNESIOPHYCEAE. Structurally, a haptonema bears some resemblance to a eukaryotic FLAGELLUM but contains fewer microtubules (which are arranged differently from those in a flagellum) and has a sheath composed of several concentric membranes, the innermost enclosing the microtubules. Haptonemata range from relatively short (e.g. in Prymnesium parvum) to very long (>80 µm in Chrysochromulina spp); most haptonemata (but not that of Prymnesium) can coil and uncoil: e.g., in Chrysochromulina spp the haptonema remains coiled while the organism is swimming and becomes extended when the cell comes to rest. The haptonema is believed to function as an attachment organelle. Haptophyceae Syn. PRYMNESIOPHYCEAE. hard cider (American) Syn. CIDER. hard gill See WATERY STIPE. hard-pad A form of CANINE DISTEMPER which begins with tenderness and keratinization of the feet, usually followed by CNS involvement and death. hard swell See SWELL. hare fibroma virus See LEPORIPOXVIRUS. Harpellales See TRICHOMYCETES. harpin (plant. pathol.) Any of certain types of glycine-rich protein, encoded by plant-pathogenic bacteria, which (at least in culture) can be secreted via a type III PROTEIN SECRETION system and which are able to elicit a heat-stable HYPERSENSITIVITY reaction in the leaves of tobacco and certain other plants [see e.g. JB (1997) 179 5655–5662]. (See also AVIRULENCE GENE.) Harpochytriales See CHYTRIDIOMYCETES. Harposporium See HYPHOMYCETES and NEMATOPHAGOUS FUNGI. Hartig net See MYCORRHIZA. Hartmannella A genus of amoebae which are similar to ACANTHAMOEBA spp except that they do not produce acanthopodia from the (usually well-defined) hyaline cap of the pseudopodium. Hartmannula See HYPOSTOMATIA. hartrot (bronze leaf wilt; Coronie wilt) A disease of the coconut palm (Cocos nucifera) caused by a species of PHYTOMONAS which invades the phloem. Symptoms: yellowing and falling of leaves and blackening of inflorescences; unripe, internally blackened coconuts fall prematurely. Trees may die within three months of the initial signs. hart’s truffle See TRUFFLES. Harvey murine sarcoma virus (Ha-MSV) A replication-defective, v-onc+ , fibroblast-transforming MURINE SARCOMA VIRUS which was isolated from BALB/c mice inoculated neonatally with rat-passaged MOLONEY MURINE LEUKAEMIA VIRUS (MoMuLV). Ha-MSV carries the oncogene v-ras (v-Ha-ras) and apparently arose by recombination between Mo-MuLV and a rat cellular ras sequence (Ha-ras: see RAS). Infection of newborn rodents with Ha-MSV results in anaemia, splenomegaly (resulting from proliferation of erythroblasts) and erythroleukaemia, as well as sarcomas. (cf. KIRSTEN MURINE SARCOMA VIRUS.) HAT medium See HYBRIDOMA. haustorium A specialized hyphal structure formed e.g. by certain plant-parasitic fungi (e.g. Erysiphe, Peronospora) in order to obtain nutrients from the host plant; it develops within a host cell after a small hyphal branch has penetrated the host cell wall, but it remains external to the host cell’s cytoplasmic membrane. A haustorium may be spherical, club-shaped or elongated (branched or unbranched) according to the species of fungus which forms it. (Haustoria are also formed by the mycobionts of certain lichens.) Haverhill fever See RAT-BITE FEVER. Hawii virus See SMALL ROUND STRUCTURED VIRUSES. hay fever See TYPE I REACTION. Hayflick medium A medium used for the culture of Mycoplasma. spp. It consists of heart infusion broth supplemented with membrane-filtered horse serum (which provides sterols), fresh yeast extract solution, calf thymus DNA solution, thallium acetate and benzylpenicillin; the pH is adjusted to 6.8–7.8 according to strain. The solid medium is made by using a purified agar (e.g. Noble agar) as gelling agent. HBB (2-(a-hydroxybenzyl)-benzimidazole) An ANTIVIRAL AGENT which selectively inhibits the replication of a number of picornaviruses in cell cultures; it inhibits the initiation of viral 357 HBcAg GroES, GroEL and DnaK seem to prevent misfolding or aggregation of unfolded proteins, while the Lon protease degrades abnormal proteins. The heat-shock genes constitute a REGULON composed of several unlinked operons (for example: promoter–dnaK –dnaJ and promoter–groES –groEL). The heat-shock regulon is under the control of a positive transcriptional activator encoded by gene rpoH (previously called htpR). RpoH (MWt ca. 32000) is a SIGMA FACTOR (s32 ) which interacts with RNA POLYMERASE, producing a holoenzyme (Es32 ) that promotes transcription from heat-shock promoters more efficiently than does Es70 . (Transcription of the rpoH gene itself requires Es70 [JB (1986) 166 380–384].) Following heat shock, the increased levels of RpoH are due mainly to increased translation of the protein – rather to increased transcription of rpoH. The heat-induced translation from rpoH mRNA involves an effect of temperature on a secondary structure formed in the mRNA by base-pairing between a site immediately downstream of the start codon (a downstream box ) and another region in the coding sequence. This secondary structure normally inhibits translation, but on temperature upshift it is destabilized so that translation can occur; thus, the mRNA acts as a thermosensor (an RNA thermometer ) for expression of s32 [GD (1999) 13 633–636]. Although synthesized under normal conditions, s32 has a short half-life (about 1 min); it appears that, within the cell, s32 forms complexes with DnaK, DnaJ and GrpE and is subsequently degraded by the FtsH protease. During heat shock DnaK binds preferentially to denatured proteins – leaving RpoH free to function as a sigma factor [EMBO (1996) 15 607–617]. The heat-shock response in other bacteria. In some bacteria the mechanism for regulating the heat-shock response differs from that in E. coli. In some cases there is a regulatory sequence upstream of heat-shock genes; this inverted repeat sequence, termed CIRCE (controlling inverted repeat of chaperone expression), occurs e.g. in strains of Bacillus and Clostridium and is also found in some Gram-negative bacteria. In Bradyrhizobium japonicum the control mechanism includes at least two distinct regulatory systems involving CIRCE and a s32 -like sigma factor [JB (1996) 178 5337–5346]. More recently it has been reported that control of the heatshock response involves negative regulation by repressor proteins (in conjunction with cis-acting DNA sequences) which inhibit transcription of the HSP genes under normal physiological conditions [Mol. Microbiol. (1999) 31 1–8]. (See also SOS SYSTEM.) Strains of E. coli mutant in the rpoH gene have been useful in the OVERPRODUCTION of recombinant proteins. heated lid cycler See THERMOCYCLER. heavy chain A class-specific polypeptide IMMUNOGLOBULIN component (MWt ca. 50000–70000, depending on Ig class). The various types of heavy chain are designated alpha, gamma, delta, epsilon and mu – corresponding to Ig classes IgA, IgG, IgD, IgE and IgM, respectively. In IgM, different C-terminal sequences occur in the heavy chains of the membrane-associated and pentameric forms. Certain Ig classes can be divided into subclasses on the basis of minor differences in their heavy chains. heavy metals (and their compounds) (as antimicrobial agents) Although trace amounts of certain heavy metal ions are essential for the growth of microorganisms, higher concentrations may exhibit antimicrobial activity. Heavy metal ions bind to certain groups (particularly thiol groups) and appear to exert antimicrobial activity largely by inactivating proteins, nucleic acids and RNA synthesis. HBB-resistant mutants emerge rapidly; HBBdependent mutants have also been isolated. HBcAg See HEPATITIS B VIRUS and HEPATITIS B. HBeAg See HEPATITIS B VIRUS and HEPATITIS B. HBsAg See HEPATITIS B VIRUS and HEPATITIS B. HBV HEPATITIS B VIRUS. HCAM Syn. CD44. HCC HEPATOCELLULAR CARCINOMA. HCMV Human cytomegalovirus: see BETAHERPESVIRINAE. HCV HEPATITIS C VIRUS. HCVs (human caliciviruses) See SMALL ROUND STRUCTURED VIRUSES. HD protein Helix-destabilizing protein (see SINGLE-STRAND BINDING PROTEIN). HD50 (CH50 ; C
H50 ) Haemolytic dose (50%): the quantity of COMPLEMENT needed to lyse 50% of a standardized suspension of sensitized erythrocytes. In a COMPLEMENT-FIXATION TEST the HD50 may be used as the unit in place of MHD (q.v.); this gives a more accurate end-point (since the graph ‘%lysis of cells versus quantity of complement’ is sigmoidal). HDAg See DELTA VIRUS. HDCV Human diploid cell vaccine (see RABIES). hDNA Hybrid DNA. (See e.g. CLONING.) H-DNA See TRIPLEX DNA. HE-cellulose (HEC) Hydroxyethylcellulose: a neutral cellulose derivative sometimes used in preference to CM-CELLULOSE in CELLULASE assays. HE markers See TRANSFORMATION (1). headful mechanism A mechanism of genome packaging during virion assembly: packaging is initiated by insertion into a prohead (or provirion) of one end of a concatemeric form of the genome (which, at or before insertion, may be cut at a particular sequence); incorporation of the genome then continues until the head is full. Thus, termination of packaging is determined only by the length of the nucleic acid packaged, and not by a specific sequence in the nucleic acid. (See e.g. BACTERIOPHAGE MU and BACTERIOPHAGE P22; cf. e.g. BACTERIOPHAGE l.) Heaf test A form of TUBERCULIN TEST. PPD is spread over a small area of skin and is then pressed into the skin with a ‘Heaf gun’: an instrument which punctures the skin with several small needles. heart rot See TREE DISEASES. heartwater See COWDRIA. heat-fixed smear See FIXATION. heat-shock proteins (HSPs) Proteins which are synthesized at greatly (but transiently) increased levels in an organism in response to a sudden rise in temperature or to certain other types of stress (e.g. ultraviolet radiation, virus infection, ethanol, or the intracellular accumulation of abnormal proteins); HSPs may be necessary for survival of the organism at higher temperatures, and they are produced at the expense of ‘normal’ proteins. Such a heat-shock response has been observed in a wide range of species – from bacteria and yeasts to plants and animals; at least some HSPs have been highly conserved through evolution, but the response is regulated differently in different organisms. In Escherichia coli there are at least 17 HSPs which include the products of genes dnaJ, dnaK, groEL, groES, rpoD, lysU and lon. Synthesis of these proteins reaches a peak ca. 5–10 min after a rise in temperature (for example, 30 → 42° C), and subsequently declines to a new steady state which is greater than that at the lower temperature. Some HSPs apparently cope with stress-induced damage – for example, the molecular chaperones 358 Helicobacter Helicases I and III both act processively and move in the 5
-to3
direction along the strand to which they are bound. Helicase I is encoded by the traI gene of the F plasmid [PNAS (1983) 80 4659–4663]; it seems to be responsible for nicking at oriT in the F plasmid [JBC (1991) 266 16232–16237], and may be involved in unwinding the plasmid DNA for transfer during CONJUGATION (sense 1b). Helicase III cannot replace either the Rep protein or helicase II. Some bacteriophages encode their own helicases; for example, the dda gene product of phage T4 is a helicase, while the gp4 of phage T7 is a protein with both helicase and PRIMASE activities. The DnaB helicase, which is active e.g. in DNA replication, is a ring-shaped hexamer of DnaB proteins [see e.g. Cell (1996) 86 177–180]. Helicobacter A genus of asporogenous Gram-negative bacteria classified within the e (epsilon) subdivision (= rRNA superfamily VI) of the taxon Proteobacteria (which also includes the family CAMPYLOBACTERACEAE). The cells are typically curved/helical rods with one or more sheathed flagella (unsheathed e.g. in H. pullorum). Currently there are ∼20 named species, many of which have been isolated from the intestines (and often liver) of animals (e.g. dogs, sheep, pigs, rodents, chickens). However, attention has been focused primarily on the species H. pylori owing to its association with e.g. gastritis and peptic-ulcer disease in humans. H. pylori was formerly classified within the genus Campylobacter, but was re-classified [IJSB (1989) 39 397–405]. Cells: curved/helical, ca. 0.5–0.9 µm wide and up to ∼3 µm in length; motile (several flagella). Genome: circular, approx. 1700 kb; GC%: average 39, but significant differences occur in specific regions – one such region including the cag PATHOGENICITY ISLAND [H. pylori strain 26695 (complete genome): Nature (1997) 388 539–547]. Chemoorganoheterotrophic. Microaerophilic. Culture on enriched media (e.g. blood, chocolate agar) needs high humidity (freshly poured, undried plates) and a gaseous phase of e.g. oxygen/carbon dioxide/nitrogen (5%/10%/85%); grey, translucent colonies (<2 mm in diam.) are obtained after 3–7 days at 35–37° C. The medium may be made selective for H. pylori by adding e.g. amphotericin, nalidixic acid and vancomycin (to which the organism is resistant). Oxidase +ve, catalase +ve, urease +ve. The pathogenesis of H. pylori -associated gastric disease is not understood. One suggestion was based on the presence of lipopolysaccharide antigens identical to the Lewis x and Lewis y antigens found e.g. in human gastric mucosa; it was postulated that these antigens might provoke an antibody response and give rise to autoimmune inflammation [molecular mimicry of H. pylori : TIM (1997) 5 70–73]. However, this idea was questioned because the predominant anti-LPS antibodies found in a number of infected patients were unrelated to the given LPS antigens [Inf. Immun. (1998) 66 3006–3011]. Pathogenesis seems likely to involve the cag pathogenicity island and an unlinked gene, vacA, which is often jointly expressed. The joint expression of cag and vacA (in ‘type I’ clinical strains) appears to correlate with severe gastrointestinal disease, while type II strains (which lack cag, and do not express vacA) are generally not associated with severe disease. The reason why infection with H. pylori gives rise to gastric cancer in only some individuals may depend on host genetic factors such as polymorphisms which affect levels of the proinflammatory cytokine interleukin-1b (IL-1b) [Nature (2000) 404 398–402]. other microbial constituents; their activity is often microbistatic, and they typically have little or no activity against bacterial endospores. The antimicrobial activity of e.g. mercury ions may be reversed by the presence of substances such as cysteine, glutathione etc which presumably complex free ions. In some cases the antimicrobial activity of one metal can be antagonized by another; thus, e.g., the inhibition of Lactobacillus spp by zinc can be reversed by the addition of manganese. Some ions appear to have antimicrobial activity only when they are in a particular redox state (see e.g. ANTIMONY and ARSENIC). (See also OLIGODYNAMIC EFFECT.) Among bacteria, resistance to heavy metals and their compounds can be PLASMID-borne. Thus, e.g., some bacteria can synthesize an inducible, plasmid-encoded, NADPH-linked mercuric ion reductase which can reduce Hg2+ to (relatively nontoxic) metallic mercury which is readily lost from the cell; such cells may also synthesize an organomercurial lyase which cleaves the carbon–mercury bond in organomercurials such as phenylmercuric acetate. [Microbial resistance to mercury and organomercurials: MR (1984) 48 95–124.] In Staphylococcus aureus, plasmid-encoded resistance to cadmium may involve a pmf-dependent efflux of cadmium ions (via a cadmium–proton antiporter) which prevents intracellular accumulation of Cd2+ . [Reviews on bacterial resistance to metal ions: Book ref. 160, pp. 345–367; FEMS Reviews (1985) 32 39–54.] (See also METALLOTHIONEIN; TN501.) Heavy metals and their compounds have been used e.g. as agricultural antifungal agents, as antiseptics, as chemotherapeutic agents for the treatment of diseases of bacterial, fungal and protozoal aetiology, as preservatives for e.g. textiles and wood, and in the disinfection of water. (See also ANTIMONY; ARSENIC; COPPER; MERCURY; SILVER; TIN; ZINC; and TELLURITE MEDIA.) heavy strand (mol. biol.) See H STRAND. HEC See HE-CELLULOSE. hedgehog fungus Hydnum repandum (see APHYLLOPHORALES). Hektoen medium A medium selective for certain enterobacteria; it contains lactose, sucrose, salicin, bromthymol blue, Andrade’s indicator, ferric citrate, and deoxycholate. Strains (e.g. most salmonellae) which do not ferment sucrose, lactose or salicin form blue-green colonies; those which can ferment one or more of these substrates form salmon-pink colonies. H2 S-producing colonies have black centres. HeLa An ESTABLISHED CELL LINE derived, in 1952, from a human cervical carcinoma; the cells are heteroploid and epithelioid. HeLa cultures support the replication of a wide range of viruses. Helber chamber See COUNTING CHAMBER. helicases (DNA unwinding proteins) Proteins with ssDNA-dependent ATPase activity which can unwind and separate the strands of dsDNA given a single-stranded ‘leader sequence’ or gap. Escherichia coli has several types of helicase: the Rep protein (encoded by the rep gene), the DnaB protein (see DNAB GENE), helicases II and III, and the F PLASMID-specified helicase I. The Rep protein binds to a region of ssDNA in a dsDNA molecule, moving along (3
-to-5
) into the double-stranded region and unwinding it processively; the separated strands may be stabilized by SINGLE-STRAND BINDING PROTEIN. Two molecules of ATP are hydrolysed for each base pair separated. The Rep protein is required for RF replication in certain phages (see SSDNA PHAGES), but its role in the host cell is unknown. Helicase II (uvrD gene product) is involved in EXCISION REPAIR. This enzyme may act processively in the 3
-to-5
direction [see e.g. JBC (1987) 262 2066–2076]. 359 Helicobasidium the period between successive initiations of DNA replication; it is equal to the doubling time. As each I period ends, another I period begins. The principle of the Helmstetter–Cooper model is illustrated in the figure. Some of the factors that may influence or determine the length of the I period are indicated in the entry CELL CYCLE. Helotiales An order of fungi (subdivision ASCOMYCOTINA) which are typically saprotrophs or plant parasites. Ascocarp: APOTHECIOID with paraphyses, sessile or stipitate, sometimes setose. Asci: unitunicate, typically cylindrical; inoperculate, but with an apical pore. Ascospores: typically not elongated. Genera: e.g. Ascocoryne, Botryotinia, Bulgaria, Chlorociboria (see also GREEN OAK), Chlorosplenium, Ciboria, Dasyscyphus, DIPLOCARPON, Geoglossum, Korfia, Microglossum, Mitrula (see also MYCOPARASITE), Monilinia, Pezizella (see also MYCORRHIZA), Phacidium (see also SNOW BLIGHT), SCLEROTINIA, Spathularia, Trichoglossum. helotism (mycol.) Literally: serfdom. The term has been used to refer to the (supposedly) subservient role of the photobiont in a LICHEN. helper component (plant virol.) See NON-CIRCULATIVE TRANSMISSION. helper T cell See T LYMPHOCYTE. helper virus Any virus which can provide functions necessary for the replication of a DEFECTIVE VIRUS or e.g. of a SATELLITE RNA. Helvella A genus of fungi (order PEZIZALES) in which the ascocarp is a stipitate APOTHECIUM which is typically saddle-shaped (‘saddle fungi’); the smooth to warty, guttulate ascospores are commonly colourless or pale brown. helveticin J See BACTERIOCINS. heme See HAEM. Hemiascomycetes A class of ascomycetes characterized by the formation of naked asci (i.e., asci not formed on or in an organized ascocarp) which do not develop from a system of ascogenous hyphae; the hemiascomycetes include most of the yeasts (e.g. Saccharomyces spp) and e.g. Taphrina spp. The class is not recognized in most modern taxonomic schemes; most of the hemiascomycetes are now placed in the ENDOMYCETALES. Hemiaulus See DIATOMS. Hemibasidiomycetes See TELIOMYCETES. hemicelluloses The non-cellulosic polysaccharide fraction obtained from isolated plant cell walls by extraction with alkali after the PECTIC POLYSACCHARIDES have been extracted. Hemicelluloses are closely (but non-covalently) associated with CELLULOSE and occur in the matrix of the plant cell wall; they include e.g. galactomannans, glucomannans (see MANNANS), mixed bglucans, XYLANS, xyloglucans etc. hemidesmosome See TRYPANOSOMA. Hemidiscus A genus of DIATOMS which are generally regarded as centric, although the valves are often markedly asymmetric. Blooms of H. hardmannianus in estuarine and neritic waters in India have been associated with high mortalities of fish and invertebrates [Limn. Ocean. (1985) 30 910–911]. Hemileia See UREDINIOMYCETES and COFFEE RUST. hemimethylated DNA dsDNA in which only one of the strands is methylated (see DNA METHYLATION). Hemitrichia A genus of slime moulds (class MYXOMYCETES). H. serpula forms conspicuous, yellowish, sessile, reticulate plasmodiocarps (up to several centimetres across) on humus or on rotting wood. Hemming filter An apparatus used for FILTRATION. Two bijou bottles are clamped together, mouth-to-mouth, with a filter Species of Helicobacter have also been detected in liver tissue from patients with primary sclerosing cholangitis and primary biliary cirrhosis [JCM (2000) 38 1072–1076]. [H. pylori infection (diagnosis, treatment): Lancet (1997) 349 265–269. Vaccines against H. pylori : Drugs (1996) 52 799–804; BCID (1997) 4 413–433. E test (susceptibility to antibiotics): JCM (1997) 35 1842–1846. Electroporation of H. pylori : AEM (1997) 63 4866–4871. H. pylori epidemiology, metabolism, genetics, pathogenesis, diagnosis, therapy etc.: BCID (1997) 4 239–484. H. pylori (detection by PCR): JCM (1999) 37 772–774.] Helicobasidium See AURICULARIALES. helicosporae See SACCARDOAN SYSTEM. helicotrichous Refers to flagella inserted in a helical zone on the cell surface of Centipeda periodontii [JGM (1984) 130 185–191]. Heliobacterium A genus of photosynthetic bacteria; H. chlorum contains a novel bacteriochlorophyll (Bchl g: see CHLOROPHYLLS) which, when in situ, shows a long-wavelength absorption peak at 788 nm. From 16S rRNA sequence analysis, H. chlorum is apparently related more closely to the Gram-positive bacteria than to other photosynthetic bacteria [Science (1985) 229 762–765]. Heliozoea A class of protozoa (superclass ACTINOPODA) in which numerous separate axopodia or filopodia radiate from the (more or less spherical) cell body, giving rise to the popular name of ‘sun animalcules’. The cells lack a central capsule (cf. RADIOLARIA); some species have a ‘skeleton’ of siliceous scales or spines. The cytoplasm is more or less clearly differentiated into a highly vacuolated (‘frothy’) ectoplasm and a less vacuolated endoplasm which contains e.g. one or more nuclei, contractile vacuole(s), food vacuoles, etc. The ectoplasmic vacuoles appear to aid flotation. In general, the axopodia are not effective locomotory organelles, serving mainly to capture prey; however, in some members a type of rolling motility can be achieved by the sequential retraction of the axopodia. (See also TAXOPODIDA.) Reproduction occurs by binary fission or (in some uninucleate species) by budding. Sexual processes have been recorded in some species. Most heliozoa are free-floating freshwater organisms; however, some occur attached to a substratum by a stalk, and some are marine. They feed on other protozoa, microalgae, etc. Orders: ACTINOPHRYIDA, CENTROHELIDA, DESMOTHORACIDA, TAXOPODIDA. helix–coil transition In proteins or nucleic acids: the transition from an ordered, helical conformation to a disordered ‘random coil’ conformation. When used of double-helical DNA the term usually means simply strand separation (‘melting’). helix destabilizing protein Syn. SINGLE-STRAND BINDING PROTEIN. Helminthosporium A genus of fungi (class HYPHOMYCETES) which form septate mycelium and pigmented, septate conidia; conidiogenesis is tretic. The organisms occur e.g. on woody plants. (See also SILVER SCURF.) Some species have been transferred from Helminthosporium to the genus DRECHSLERA. Helminthosporium maydis virus See MYCOVIRUS. Helmstetter–Cooper model (I + C + D model) A model which relates chromosome replication (in bacteria) to the (prokaryotic) CELL CYCLE. Essentially, the cell cycle is divided into a linear sequence of three main periods designated I, C and D. Chromosome replication starts at the beginning of the C period and finishes at the end of the C period. Septum formation begins, and is completed, during the D period – at the end of which the cell divides. The I period is the inter-initiation period, that is, 360 (a) Slow growth Rapid growth (b) I C D I C D I C D I I C D I C D I C D I HELMSTETTER–COOPER MODEL (see entry). (a) Chromosome replication during growth at different rates (cells and chromosomes are represented diagrammatically). Replication begins at a specific location in the chromosome, the origin, which is shown here as a small circle. During slow growth (upper sequence) each new daughter cell has exactly one chromosome because, following duplication of the parental chromosome, a new round of replication does not begin before the parent cell has divided fully. During faster growth (lower sequence), a new round of replication has begun before the previous round has been completed – well before cell division; as a result, each new daughter cell has fractionally more than one chromosome. (b) The cell division cycle may be represented as a linear sequence of three periods: I, C and D. C is the period during which chromosome replication occurs. D is the period in which the septum forms – cell division occurring at the end of the D period. I is the period between each successive initiation of chromosomal replication. Each I period starts when the previous one ends. (Continued on page 362.) 361 hemp retting it consists of a circular (but not covalently closed), partially double-stranded DNA molecule in which one strand (the L, ‘long’, or (−) strand) is of fixed length (ca. 3000–3300 nucleotides long) and has unique 3
and 5
ends and a protein (the ‘DNA-linked protein’) covalently attached to the 5
end. The other strand (the S, ‘short’, or (+) strand) has a fixed 5
end but a variable 3
end, and varies in length from 50 to 100% of that of the L(−) strand. (The 3
end of the S(+) strand can be extended by the endogenous viral polymerase, the L(−) strand acting as template.) The circularity of the molecule is maintained by basepairing between the 5
ends of the two strands (the ‘cohesive overlap’); in all hepadnaviruses there is an 11-bp direct repeat sequence on either side of the cohesive overlap. In mammalian hepadnaviruses, the L(−) strand has four open reading frames, orientated in the same direction, designated S, C, P and X regions. The S region comprises the S gene, the preS1 region and the pre-S2 region; it encodes the ‘major protein’ (S gene), ‘middle protein’ (pre-S2 + S gene) and ‘large protein’ (pre-S1 + pre-S2 + S gene) of the viral envelope. The C region comprises the C gene (which encodes the major core protein) and the pre-C region. The product of the P region (which overlaps the whole of the S region and parts of the C and X regions) is a large histidine-rich protein (MWt ca. 90000) which is a DNA polymerase with REVERSE TRANSCRIPTASE activity. The X region includes the cohesive overlap sequence and encodes a polypeptide of unknown function. (DHBV lacks an X region.) The viral genes are expressed via at least two unspliced primary transcripts. No known viral protein is encoded by the S(+) strand. Hepadnavirus DNA replication occurs by a characteristic mechanism involving reverse transcription of an RNA intermediate (see RETROID VIRUSES). When a virion infects a host cell, the viral DNA enters the nucleus and is converted to a supercoiled ccc dsDNA molecule. The L(−) strand is transcribed to form multiple copies of a 3.5-kb RNA called the pre-genome; this RNA is then encapsidated with the viral DNA polymerase and ‘DNA-linked protein’, and an L(−) strand of DNA is synthesized by reverse transcription of the pre-genome. The pre-genome is then apparently degraded by an RNase H activity, and an S(+) strand is synthesized on the L(−) strand template. The ‘DNA-linked protein’ may act as primer for L(−) strand DNA synthesis, while a small fragment of RNA may be the primer for S(+) strand synthesis. (cf. RETROVIRIDAE.) The assembly of the virions and their release from the cell can occur before S(+) strand synthesis is complete, accounting for the partially double-stranded nature of the genome in virions liberated into the bloodstream. [Review (mainly of HBV): Nature (1985) 317 489–495; Book ref. 110, pp. 23–41.] heparin A highly sulphated proteoglycan present e.g. in MAST CELLS and other cells. Heparin varies in MWt according e.g. between them; one bijou contains the sample, the other acts as the receiving vessel. On centrifugation, liquid passes through the filter into the receiver. hemp retting See RETTING. Henneguya A large genus of protozoa (class MYXOSPOREA) which are important histozoic parasites in various fish. Species are commonly specific for a given host and for a particular tissue within that host: e.g. the Channel catfish (Ictalurus punctatus) may be parasitized by H. adiposa (adipose fin), H. diversis (liver and kidney), or H. exilis (gills), while the Pacific salmon (Oncorhynchus spp) can be infected by H. salminicola (muscles). Typically, conspicuous spore-filled cysts (up to 15 mm across in e.g. salmon infected with H. salminicola) are formed. Heavy infestations may kill the fish (particularly when the gills are affected). HEP High egg passage: refers to the number of times that a given strain of virus has undergone SERIAL PASSAGE in eggs. (See e.g. FLURY VIRUS; cf. LEP.) HEp-2 (H.Ep2, Hep-2, hep-2 etc) An ESTABLISHED CELL LINE derived from a human laryngeal carcinoma; the cells are heteroploid and epithelioid. HEp-2 cultures support the replication of a wide range of viruses. HEPA filter High-efficiency particulate air filter, used e.g. in a SAFETY CABINET. Hepadnaviridae A family of enveloped DNA-containing animal viruses which can cause hepatitis in man, animals or birds; the family includes HEPATITIS B VIRUS (HBV) of man, GROUND SQUIRREL HEPATITIS VIRUS (GSHV), DUCK HEPATITIS B VIRUS (DHBV), and WOODCHUCK HEPATITIS VIRUS (WHV) [Intervirol. (1986) 25 14–29], and probably tree squirrel (Sciurus carolinensis pennsylvanicus) hepatitis B virus (THBV) [PNAS (1986) 83 2994–2997]. These viruses closely resemble one another in structure and genetic organization, but differ e.g. in their interactions with their hosts. The host’s immune response to hepadnavirus infection has been implicated in the pathology of the virus-induced liver disease. Although all of the hepadnaviruses are highly hepatotropic, HBV DNA has also been detected in e.g. pancreatic, spleen and kidney cells, and DHBV can apparently infect duck pancreatic and kidney cells. The infectious hepadnavirus virion (ca. 42 nm diam.) consists of a nucleocapsid (an internal core containing the viral genome) surrounded by a lipoprotein envelope. The core contains a major protein (designated P22C in HBV) and is associated with a DNA polymerase and a protein kinase which can phosphorylate the major core protein. In HBV, at least, the envelope contains three proteins: a ‘major protein’ (glycosylated form: GP27S , nonglycosylated form: P24S ), the ‘middle protein’ (two glycosylated forms: GP33S and GP36S ), and the ‘large protein’ (glycosylated form: GP42S ; non-glycosylated form: P39S ). The genome, as present in hepadnavirus virions isolated from the blood of an infected host, has a characteristic structure: HELMSTETTER–COOPER MODEL (continued) In a given set of I + C + D sequences, the relationship between a particular sequence and the one immediately above it is that of a daughter cell to its parent cell. The upper set of I + C + D sequences represents successive cell cycles during slow, steady growth. Notice that, when I = C + D, a new round of chromosome replication does not start until after the D period, i.e. after completion of cell division. The lower set of I + C + D sequences represents successive cell cycles during faster steady growth. Here, with I shorter than C, chromosome replication is initiated before the previous round has been completed. Hence, at the end of D the new round of replication is already well advanced. Thus, in cells growing rapidly, the chromosome in a new daughter cell will have reached a stage of replication which is determined by the timing of initiation (i.e. start of the C period) in the parent cell. In Escherichia coli, C and D increase linearly in generation times >60–70 minutes [Microbiology (2003) 149 1001–1010]. Reproduced from Bacteria, 5th edition, Figure 3.2, pages 44–45, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 362 hepatitis B virus Lab. diagnosis involves detection of HBV antigens in the serum. HBsAg (see HEPATITIS B VIRUS) appears in the serum during the incubation period and persists through the symptomatic phase of the disease. It usually begins to disappear ca. 2–3 months after the onset of symptoms; if it persists for >6 months, the patient has become a carrier and may develop chronic liver disease. Anti-HBsAg antibodies appear only late in convalescence; the high levels of HBsAg present in the blood may induce immune tolerance to this antigen. HBeAg appears in the serum just before symptoms begin; its appearance is associated with high infectivity and the presence of whole (infectious) virions (‘Dane particles’) in the blood. HBcAg does not appear in the serum, but anti-HBcAg antibodies can be detected as symptoms begin. Anti-HBcAg IgM antibodies disappear late in the convalescence phase, but anti-HBcAg IgG antibodies may persist for years (possibly for life), and detection of the latter is a useful indicator of previous infection. [Screening blood samples for HBV by PCR: Lancet (1999) 353 359–363.] Treatment. There is no specific treatment for the clinical disease. Antiserum may be used for pre- or post-exposure prophylaxis. Vaccines containing HBsAg are available [PM (1984) 75 199–211]. hepatitis B virus (HBV) A virus of the HEPADNAVIRIDAE (q.v.) which causes HEPATITIS B in man, and which is also apparently a causal agent of human hepatocellular carcinoma (HCC). (HBV can also infect chimpanzees, but does not cause chronic hepatitis or HCC in these animals.) (cf. WOODCHUCK HEPATITIS VIRUS.) On infection of a new host (see HEPATITIS B), the HBV virion attaches to the surface of a hepatocyte and penetrates the cell by endocytosis or by fusion of the viral envelope with the host cell plasma membrane. HBV surface (envelope) antigen (HBsAg) and core antigen (HBcAg) become associated with the plasma membrane of the infected cell, and subsequently large amounts of HBsAg are released into the circulation in the form of non-infectious (DNA-free), spherical or tubular lipoprotein particles (ca. 22 nm diam.). HBsAg (formerly known e.g. as ‘Australia antigen’, Au antigen, or hepatitis-associated antigen, HAA) is a dimer of two molecules, linked by disulphide bonds, of the envelope ‘major protein’ (see HEPADNAVIRIDAE). HBcAg is the major capsid polypeptide (P22C ). The soluble e antigen (HBeAg) is a product of proteolytic cleavage of HBcAg. The appearance in the serum of infected individuals of antibodies to these antigens provides a useful means of monitoring disease development (see HEPATITIS B). HBV strains can be serotyped on the basis of their HBsAg. HBsAg contains a group-specific determinant (designated a), common to all subtypes, together with subtype-specific determinants (designated d, y, w and r ); strains typically contain either d or y with either w or r. The four principal serotypes, which differ in geographical distribution, are adw, adr, ayw and ayr. The subtype determinants are useful epidemiological markers. In ca. 5–10% of individuals infected with HBV, HBsAg persists in the circulation for >6 months, and a carrier state is established which may persist for life. The carrier state may be asymptomatic (latent infection) or symptomatic (with e.g. chronic active hepatitis, cirrhosis, and/or HCC), and is most likely to develop in e.g. perinatally-infected or immunodeficient individuals. In HBV carriers, viral DNA may integrate at one or more (apparently random) sites in the host cell genome; the integrated sequences may be complete genomes or subgenomic fragments, and in HCC cells they are often extensively rearranged, with e.g. multiple deletions, inversions, duplications, etc. Rearrangements also occur in the flanking cellular sequences. to the methods used for its isolation; that used as an ANTICO(heparin inhibits thrombin activity) has a MWt of ca. 25000. In addition to anticoagulant activity, heparin can e.g. modify (limit) delayed hypersensitivity reactions in mice and guinea pigs (mechanism unknown), stimulate phagocytosis, inhibit the alternative pathway of complement fixation (by enhancing the activity of Factor H), and inhibit eosinophilmediated ADCC. Heparin is cleaved by heparinases, and its activity can be neutralized by e.g. protamines. hepatitis Inflammation of the liver. Hepatitis is often caused by viruses (e.g. HEPATITIS A, HEPATITIS B, HEPATITIS C); hepatitis D and hepatitis E viruses are less-common causal agents. The significance of infection with hepatitis G virus is currently uncertain [transfusion-associated hepatitis G virus infection: NEJM (1997) 336 747–754; RMM (1998) 9 207–215]. Hepatitis may also occur as a symptom of non-viral infectious diseases (see e.g. amoebic DYSENTERY). [Diagnostic perspective of viral hepatitis: RMM (1997) 8 197–207.] hepatitis A (infectious hepatitis; epidemic hepatitis; short-incubation hepatitis) A viral HEPATITIS caused by the hepatitis A virus (HAV, = ENTEROVIRUS type 72); it tends to occur in epidemics among children and young adults, particularly in closed institutions (e.g. mental hospitals). Man is probably the only natural host, but certain other primates (e.g. chimpanzees) can be infected. A carrier state is not known. Infection occurs mainly by consumption of water, milk or food contaminated with human faeces – e.g. shellfish harvested from sewage-polluted waters. HAV replicates in hepatocytes and is presumed to reach the intestine (and hence faeces) via the bile duct. The peak excretion of virus occurs during the latter half of the incubation period (2–6 weeks). Symptoms: fatigue, anorexia, low-grade fever, some abdominal pain, and sometimes jaundice. The disease is usually self-limiting within a few weeks; rarely, a severe, fulminant, often fatal hepatitis develops. Lab. diagnosis: demonstration of HAV in faeces; serological tests for anti-HAV IgM antibodies in serum. hepatitis B (serum hepatitis; homologous serum jaundice; longincubation hepatitis) A viral HEPATITIS caused by the HEPATITIS B VIRUS (HBV). Incubation period: 2–6 months (but occasionally as short as 2 weeks). Infection occurs by direct inoculation or by contamination of mucous membranes. The virus is present in the blood and body fluids (saliva, semen, mucus, wound exudates etc) of an infected person. Transmission may thus occur by the transfusion of blood or blood products from infected donors; by tattooing, ear-piercing, vaccination, renal dialysis, etc, using contaminated needles; by sexual contact; or (in infants) during birth. (Intrauterine infection is uncommon since HBV does not normally cross the placenta.) HBV reaches the liver via the blood and replicates in the hepatocytes. The disease may be mild or subclinical, and a carrier state is recognized (see HEPATITIS B VIRUS). In clinical cases, onset is gradual with jaundice and e.g. nausea, malaise, pains in muscles and joints, with or without fever; occasionally acute hepatic failure occurs. (See also DELTA VIRUS.) Pathogenesis appears to involve the host’s immune response to viral antigens in the plasma membranes of infected hepatocytes. The condition may be self-limiting, or chronic liver disease may develop; chronic infection is associated with an increased risk of hepatocellular carcinoma (HCC: see HEPATITIS B VIRUS) and possibly other tumours, including KAPOSI’S SARCOMA [JAMA (1984) 251 1007–1008]. HCC is a major cause of death in regions where HBV is endemic and neonatal infection is common (e.g. in regions of China and tropical Africa). AGULANT 363 hepatitis C Six major genotypes and >60 subtypes of HCV have been distinguished. Treatment of hepatitis C is apparently influenced by the specific genotype involved and the level of viraemia; disease caused by genotype 2 or 3, in conjunction with low-level viraemia, is reportedly more likely to respond in a sustained fashion to treatment with IFN-a, while disease caused by genotype 1 or 4 is apparently more refractory. [Quantification of HCV in plasma samples: JCM (1997) 35 187–192.] In many cases HCV causes persistent infection. Persistence may be facilitated by the relatively poor immunogenicity of the virus and the low-level viraemia. Changes in viral antigens, due e.g. to mutations in HVR-1, may also contribute to persistence; however, genetic drift in HCV may be independent of the host’s (weak) immune pressure [JV (2000) 74 2541–2549]. (See also CD81.) hepatitis D, E See HEPATITIS. hepatitis D virus Syn. DELTA VIRUS. hepatitis delta virus Syn. DELTA VIRUS. hepatitis G virus See HEPATITIS. hepatitis X Hepatitis in dogs caused by ingestion of food contaminated with AFLATOXINS; the liver becomes necrotic and congested, with haemorrhagic oedema of the gall bladder mucosa. hepatocellular carcinoma (HCC) A CARCINOMA of liver cells. Primary HCC can be caused e.g. by HEPATITIS B VIRUS and WOODCHUCK HEPATITIS VIRUS. (See also HEPATITIS C.) hepatotoxin A TOXIN which acts on the liver. hepatotropism See TROPISM (sense 2). HEPES A zwitterionic pH buffer: N-2-hydroxyethylpiperazineN
-2-ethanesulphonic acid. HEPES is widely used e.g. in TISSUE CULTURES, and has also been used in bacterial cultures; it is readily soluble in water, and does not bind divalent cations such as Ca2+ , Mg2+ or Cu2+ . At 37° C the pKa of HEPES is ca. 7.3; at 25° C it is ca. 7.5. (cf. BICINE.) Herbert’s pits See TRACHOMA. herbicolin A An acyl peptide antibiotic produced by Erwinia herbicola. It can inhibit the growth of yeasts and filamentous fungi, and can kill sterol-requiring mollicutes [JAC (1985) 16 449–455]; it is inactive against other bacteria. herbivorous Refers to protozoa which feed on bacteria and/or algae. (cf. CARNIVOROUS.) herd immunity See EPIDEMIOLOGY. herd structure See EPIDEMIOLOGY. Heribaudiella See PHAEOPHYTA. Hericium A genus of fungi of the APHYLLOPHORALES. hermaphroditism The phenomenon in which an individual has both male and female sexual organs; such an individual may exhibit HOMOTHALLISM or HETEROTHALLISM. herpangina An acute (usually self-limiting) infectious PHARYNGITIS, occurring mainly in children and young adults, caused by coxsackieviruses A2–A6, A8 and A10. Small, vesicular, ulcerating lesions develop in the pharyngeal region, and there may be varying degrees of fever and/or prostration. herpes (1) Any disease caused by a herpesvirus (especially HERPES SIMPLEX but also e.g. HERPES ZOSTER) and characterized by the formation of small vesicular lesions on skin and mucous membranes. (2) Used increasingly as a synonym of genital herpes (see HERPES SIMPLEX). herpes genitalis See HERPES SIMPLEX. herpes gladiatorum See HERPES SIMPLEX. herpes labialis See HERPES SIMPLEX. herpes simplex An infectious disease, caused by herpes simplex virus (HSV) type 1 or 2 (see ALPHAHERPESVIRINAE), which [Duplications of flanking cellular sequences and model for HBV DNA integration: PNAS (1985) 82 4458–4462.] Integrated viral DNA may continue to be expressed, resulting in the continued synthesis of e.g. HBsAg; however, in HCC cells virus replication and gene expression appear generally to be absent. Integrated HBV DNA has been found in the majority of HCC cells examined, and integration is generally assumed to precede carcinogenesis; however, the relationship between viral DNA integration and the induction of HCC is unknown. HCC generally does not develop for many years after the establishment of the carrier state; in at least some cases additional factors – such as genetic predisposition, immunological factors, or exposure to chemical co-carcinogens (e.g. AFLATOXINS) – may be involved. [Biology and epidemiology of HBV: Book ref. 110, pp. 43– 65; molecular biology of HBV: Nature (1985) 317 489–495.] hepatitis C A viral HEPATITIS caused by the HEPATITIS C VIRUS (HCV); most cases of hepatitis formerly referred to as NANB (non-A-non-B) hepatitis are now believed to have been hepatitis C. In nature, hepatitis C is limited to humans (chimpanzees can be infected experimentally). The route of transmission of HCV is commonly parenteral (including blood transfusion); sexual and perinatal transmission also occur. The incubation period (4–12 weeks) may be followed by nonspecific symptoms that resolve within weeks or months. Infection is often asymptomatic; jaundice is uncommon. HCV RNA is detectable in serum (e.g. by rtPCR) before seroconversion. In a high proportion of patients infection becomes chronic (indicated e.g. by the continuing presence of HCV RNA in serum); usually asymptomatic, the chronic disease may nevertheless give rise to e.g. hepatocellular carcinoma or cirrhosis in a proportion of cases after 20–30 years. (Preliminary data have suggested that there is a cerebral effect (altered cerebral metabolism) in patients chronically infected with HCV [Lancet (2001) 358 38–39].) Lab. diagnosis: e.g. screening (with a third-generation ELISA) for specific antibodies; antigens used in the assay consist of synthetic peptides/recombinant proteins representing products which are encoded by particular regions of the HCV genome. Any specimen positive in this assay is re-tested by a supplemental assay such as the recombinant immunoblot assay (RIBA). In a RIBA test, a nitrocellulose strip is coated with peptides/recombinant proteins similar to those used in the ELISA, each type of peptide and protein forming a separate band on the strip. The strip is exposed to a sample of serum (permitting antibody–antigen binding) and then to an anti-human IgG–enzyme conjugate. After washing, bound antibodies are detected by adding the enzyme’s substrate and examining the strip for the (coloured) product of hydrolysis; the presence of particular antibodies in the serum is deduced from the development of colour at particular band(s) on the strip. Nucleic-acid-based tests are useful for the early detection of HCV – and also for following the efficacy of antiviral therapy. [Quantification of hepatitis C virus by bDNA assay: JCM (1997) 35 187–192; rtPCR-based screening for HCV in a blood bank: Lancet (1999) 353 359–363.] hepatitis C virus (HCV) A small, enveloped, positive-sense ssRNA virus which causes HEPATITIS C in man. A population or sample of HCV from a given source exists as a QUASISPECIES. HCV has not been cultured. The 5
non-coding end of the HCV genome is followed (5
→ 3
) by sequences which encode: core protein, envelope proteins, membrane-binding function, helicase and protease, membranebinding function, RNA polymerase and a 3
non-coding region. One of the envelope protein genes includes a hypervariable region designated HVR-1. 364 herpes zoster chickenpox, such lesions may appear before development of the characteristic skin lesions.) [Virology of the mouth (including HSV infections): RMM (1994) 5 209–216.] Lab. diagnosis. For infections other than encephalitis: scrapings from the edges of lesions, or biopsies from skin or liver, are examined microscopically for multinucleate giant cells with eosinophilic intranuclear inclusion bodies; HSV can be demonstrated e.g. by immunofluorescence techniques, ELISA etc. For encephalitis, laboratory diagnosis may be made e.g. by PCR-based examination of cerebrospinal fluid (CSF), thus avoiding the risks of brain biopsy [see e.g. JCM (1997) 35 691–696]. The PCR-based detection of HSV (types 1 and 2) may be facilitated by a commercial system involving the use of MOLECULAR BEACON PROBES that hybridize to amplicons of both HSV type 1 and HSV type 2 (HSVision ; Stratagene). (HSV type 2 is not a common cause of viral encephalitis; it may be a causal agent e.g. in immunocompromised individuals.) Chemotherapy. A number of antiviral agents (e.g. ACYCLOVIR, IDOXURIDINE, PENCICLOVIR, trifluorothymidine, VIDARABINE) are active against HSV and are useful in some cases; for example, vidarabine has been used in cases of HSV encephalitis, while acyclovir, or penciclovir, is used in creams for the topical treatment of lesions on skin and mucous membranes. In general, such agents are not effective in preventing recurrence or transmission. herpes simplex virus group See ALPHAHERPESVIRINAE. herpes zoster A human disease resulting from the reactivation of a latent human (alpha) herpesvirus 3, present in a dorsal nerve ganglion, subsequent to an attack of CHICKENPOX that may have occurred (many) years earlier. The virus replicates in the ganglion, and in adjacent nervous tissue, and is transported, within the axon, to nerve endings in the corresponding region of skin; viral replication is believed to account for pain experienced during the acute phase of the disease – the pain being accompanied by an eruption of characteristic clusters of vesicular lesions. (Lesions may develop in areas not served by the affected nerves; this is thought to be due to viral dissemination in the bloodstream.) (See also RAMSAY HUNT SYNDROME.) Reactivation of the virus may occur spontaneously or may be provoked e.g. by immunosuppression, certain drugs, or other diseases. Post-herpetic neuralgia (PHN) is prolonged pain which appears to be due to damage sustained by the peripheral nervous system as a result of viral replication. Some regard PHN as pain which persists following the disappearance of the rash, while others have defined PHN as e.g. pain which persists for at least 30 days from the onset of the rash. In an attempt to provide a usable definition (for clinical trials of antiviral agents), the concept of zoster-associated pain (ZAP) has been introduced; ZAP includes the continuum of acute and chronic pain. Chemotherapy. The use of antiviral agents in the acute phase of herpes zoster is generally beneficial, and is particularly important in immunocompromised patients in whom herpes zoster may become a life-threatening condition; in the latter patients, intravenous ACYCLOVIR is the first-choice therapy (which may decrease the risk of dissemination). For immunocompetent patients with moderate to severe pain during the acute phase, systemic antiviral therapy should reduce the duration of the rash and pain if given immediately after the onset of the rash (within a maximum of 72 hours after onset); useful drugs: ACYCLOVIR, FAMCICLOVIR, VALACICLOVIR (each of which can be administered orally). Acyclovir should be given intravenously if severe complication (e.g. encephalitis) develop. [Treatment of zoster: RMM (1995) 6 165–174.] (typically) involves the formation of thin-walled vesicles that ulcerate, crust and heal; vesicles occur, often in clusters, on skin and/or mucous membranes. Transmission occurs mainly by person-to-person contact – e.g. sexual contact, kissing, contact sports such as wrestling (‘herpes gladiatorum’). Infection may occur at any of a range of body sites; in general, HSV type 2 is associated with genital (and hence neonatal) infections; HSV type 1 accounts for most of the other forms. Incubation period: 2–12 (average 6) days. The disease varies from subclinical to severe, and is occasionally fatal. HSV can remain latent in nerve cells near the site of infection; reactivation may occur spontaneously or in response to other infections, stress, immunosuppression etc. In general, recurrent episodes become progressively less severe and less frequent. In neonates and immunodeficient individuals HSV may become disseminated and may involve e.g. the liver, adrenal glands, brain etc. Cutaneous herpes. Skin lesions may occur on any part of the body, often in clusters which may coalesce. Other symptoms of primary infection may include pain, fever, oedema etc. HSV type 1 tends to be associated with skin lesions above the waist, type 2 with those below the waist. (See also ECZEMA and PARONYCHIA.) Encephalitis. HSV type 1 is a common cause of non-epidemic viral ENCEPHALITIS. Onset is acute, with headache, nausea, vomiting, fever, myalgia, mental deterioration, convulsions, coma etc.; the fatality rate is typically >50%. Neurological sequelae are common in survivors. (See also aseptic MENINGITIS.) Genital herpes (herpes genitalis) is a SEXUALLY TRANSMITTED DISEASE which usually involves HSV type 2. It may be mild and self-limiting. Lesions occur in the genital region and may spread to adjacent areas of skin (e.g. buttocks, thighs). Other symptoms may include e.g. fever, dysuria, pain, malaise. In women the cervix may be the main site of genital infection (herpetic cervicitis); this may be asymptomatic, or there may be ulceration with hyperplasia. (HSV type 2 infection in women is associated with an increased risk of abortion and of cervical cancer.) Genital infections may give rise to subclinical shedding of virus, with associated risk of transmission. Labial herpes (herpes labialis) is typically a recurrent lip lesion (cf. COLD SORE) which usually crusts and heals in 3–10 days. Labial herpes may be associated with lip cancer. Neonatal herpes is usually acquired (during birth) from a mother with genital herpes – hence HSV type 2 is commonly responsible. Herpetic vesicles or ulcers, or keratoconjunctivitis, may or may not be present. Fatality rates may be 50% or more in untreated cases; surviving infants commonly exhibit neurological and/or ocular sequelae. (See also TORCH DISEASES.) Ocular herpes. HSV infection of the eye is a recurrent, usually unilateral, keratitis, sometimes with conjunctivitis; corneal scarring can lead to blindness. The characteristic lesion is a dendritic (branching) corneal ulcer with can be readily stained with either FLUORESCEIN or ROSE BENGAL [photograph of stained lesion: Book ref. 224, pp 67–68]. Oral herpes (herpetic stomatitis) occurs as a manifestation of initial HSV infection in adults and children. Primary herpetic gingivostomatitis involves shallow vesicular and ulcerative lesions on the labial and buccal mucosa, and tongue, with GINGIVITIS, fever and regional lymphadenopathy. The lesions usually heal in 7–10 days. Reactivation rarely involves the oral mucosa (except in immunocompromised individuals, in whom the oral lesions may be atypical in appearance). (Oral lesions may also be due to infection with the varicella zoster virus (VZV); in 365 Herpesviridae variety of criteria – such as the disease caused by the virus (e.g. Marek’s disease virus, herpes simplex virus); CPEs induced by the virus (e.g. cytomegalovirus); host species (e.g. Herpesvirus saimiri ); names of pioneer workers (e.g. Epstein–Barr virus, Lucké virus). It has been recommended [Book ref. 139, pp 1–23] that the name of a herpesvirus should usually be based on the family or subfamily to which its primary host belongs (subfamily for bovine and primate viruses, family in other cases) – viruses isolated from humans being prefixed by ‘human’; the name may, where applicable, indicate the subfamily of herpesviruses to which the virus belongs (alpha, beta or gamma), and is followed by a number – numbers being allocated sequentially as new viruses are discovered. Thus, e.g. Marek’s disease virus becomes gallid herpesvirus 1; herpes simplex type 1 virus becomes human (alpha) herpesvirus 1; Epstein–Barr virus becomes human (gamma) herpesvirus 4. Many herpesviruses, including some important pathogens of animals, were not listed in an earlier (1982) classification scheme [Book ref. 23, pp 47–51]. These viruses include e.g. bovine herpesvirus 1 (causal agent of various cattle diseases, such as INFECTIOUS BOVINE RHINOTRACHEITIS, encephalitis in newborn calves, infectious pustular vulvovaginitis and abortion in cows etc.); anatid herpesvirus 1 (= anserid herpesvirus1, causal agent of DUCK VIRUS ENTERITIS); gallid herpesvirus 3 (causal agent of AVIAN INFECTIOUS LARYNGOTRACHEITIS); herpesviruses of amphibians (e.g. Lucké virus, causal agent of renal adenocarcinoma in frogs, Rana pipiens) [review: Book ref. 140, pp 367–384]; and herpesviruses of reptiles and fish (see e.g. CARP POX and CHANNEL CATFISH VIRUS DISEASE) [review: Book ref. 140, pp 319–366]. Herpesvirus ateles See GAMMAHERPESVIRINAE. herpesvirus B Syn. B VIRUS. Herpesvirus saimiri See GAMMAHERPESVIRINAE. Herpesvirus simiae Syn. B VIRUS. herpetic Pertaining to or caused by a herpesvirus – see e.g. HERPES SIMPLEX. herpetic cervicitis See HERPES SIMPLEX. herpetic stomatitis See HERPES SIMPLEX. Herpetomonas A genus of homoxenous parasitic protozoa (family TRYPANOSOMATIDAE) which occur (typically in the gut) in flies, bugs and hymenopterous insects. Promastigote, paramastigote and opisthomastigote forms occur during the life cycle; biflagellate organisms may result from rapid division of promastigotes. Herpetosiphon A genus of GLIDING BACTERIA (see CYTOPHAGALES) which occur in aquatic (freshwater to marine) habitats. The organisms are gliding filaments (up to ca. 2 µm wide) which may be sheathed; they typically contain carotenoid glycoside pigments ranging from yellow or orange. Metabolism appears to be chemoorganotrophic. GC%: ca. 44–53. Herpetosoma A subgenus of TRYPANOSOMA within the STERCORARIA; species include parasites of rodents and man. The trypomastigote form typically has a subterminal kinetoplast, a pointed posterior end, and a free flagellum. T. (H.) lewisi, a parasite in rats, is transmitted cyclically and contaminatively by fleas, e.g. Xenopsylla cheopis; it is cultivable e.g. in NNN medium at 25–27° C. In the vertebrate host, the reproductive forms (epimastigotes) undergo multiple fission or unequal binary fission, resulting eventually in the production of non-dividing trypomastigotes. T. (H.) rangeli is parasitic in man (and in e.g. cats, dogs etc) in Central and South America, and is transmitted cyclically – mainly by bugs of the genus Rhodnius. Although classified in the Stercoraria, this species is transmitted by bite: the Herpesviridae A large family of enveloped VIRUSes containing a linear dsDNA genome. All herpesviruses are structurally similar: the virion (ca. 120–200 nm diam.) contains a core, i.e. a DNA–protein complex, within an icosahedral capsid (ca. 100–110 nm diam.; 12 pentameric and 150 hexameric capsomers) [capsid protein (structure): EMBO (2003) 22 757–765]; the nucleocapsid is enclosed by a lipoprotein envelope (bearing glycoprotein projections at the surface) – the (apparently) amorphous region (tegument) between capsid and envelope containing virus-encoded proteins. Virions are readily inactivated by lipid solvents, lipases or heat. Herpesviruses have been isolated from a wide range of animals, including mammals, birds, reptiles, amphibians and fish; some of the viruses have a narrow host range. Many herpesviruses can cause disease in their primary host(s). Many (possibly all) can remain latent in the host’s tissues, often for the lifetime of the host. The transmission of herpesviruses commonly occurs by direct contact between mucosal surfaces – but some can be transmitted via body fluids, such as milk, or via the placenta etc. (See also DNA TUMOUR VIRUSES.) The herpesvirus genome ranges from ∼125 to ∼240 kb in length and it characteristically contains repeated terminal and/or internal sequences which can take part in recombinational events (and which may therefore allow the development of variant forms of the genome). In the large genome (e.g. ∼80 genes in EPSTEIN–BARR VIRUS), most of the genes encode structural or regulatory functions; relatively few genes are expressed during viral latency. Interestingly, in some human herpesviruses the genome contains sequences that are partially homologous to certain CYTOKINES and/or cytokine receptors. Herpesviruses may be typed (fingerprinted) by techniques such as RFLP for epidemiological and other purposes [e.g. molecular epidemiology of herpes simplex virus type 1: RMM (1998) 9 217–224]. Replication cycle. Initially, the virion attaches to a host cell and the viral envelope then fuses with the cell membrane (see ENVELOPE). The nucleocapsid is transported across the cytoplasm to the nuclear membrane where the DNA is released into the nucleus via a nuclear pore. Within the nucleus, the viral immediate–early genes are transcribed, and the transcripts enter the cytoplasm for synthesis of viral polypeptides. Products of the immediate–early genes include proteins that serve regulatory functions and which are necessary for the expression of early genes (encoding e.g. a DNA polymerase) and late genes (encoding e.g. components of the capsid). Viral DNA is replicated, and the nucleocapsids are assembled, in the nucleus, with various structural proteins that enter the nucleus from the cytoplasm. Nucleocapsids can leave the nucleus in two ways. They can bud through the inner lamella of the nuclear membrane, thus acquiring an envelope; in this case, the mature virions are transported to the cell surface within membrane-bounded vacuoles and are then released to the exterior. Alternatively, nucleocapsids may enter the cytoplasm in an immature form which, only later, acquires an envelope by budding into a cytoplasmic vacuole. Classification and nomenclature. Some of the (many) known herpesviruses have been allocated to one of three subfamilies: ALPHAHERPESVIRINAE, BETAHERPESVIRINAE and GAMMAHERPESVIRINAE [Book ref. 23, pp 47–51] – mainly on the basis of biological characteristics. The naming of individual herpesviruses has been complicated by the widespread use of common names which are based on a 366 heterokont helps to maintain the anaerobic state. [Protection of nitrogenase from oxygen in heterocystous cyanobacteria: MR (1992) 56 352–362.] In Anabaena strain PCC 7120, which forms heterocysts in both terminal and intercalary locations, differentiation of heterocysts depends on a functional hetR gene. In genetically engineered cells of this strain, controlled expression of hetR can induce differentiation of heterocysts even under conditions that would normally repress their formation. However, in cells with a mutant patA gene – which form heterocysts almost exclusively at terminal locations – the expression of hetR did not induce the formation of intercalary heterocysts; this was interpreted to mean that, while raised levels of HetR are necessary to promote the formation of heterocysts, PatA may play a role in regulating the expression of hetR [PNAS (2001) 98 2729–2734]. During nitrogen fixation, the heterocysts and vegetative cells are metabolically interdependent. The vegetative cells supply heterocysts with e.g. fixed carbon, reduced sulphur, and glutamate; in the heterocyst, fixed nitrogen is transferred to glutamate by glutamine synthetase (see AMMONIA ASSIMILATION), and the resulting glutamine is passed back to vegetative cells to supply their nitrogen requirements. Communication between a heterocyst and adjacent vegetative cell(s) occurs via fine pores (microplasmodesmata) in their juxtaposed cytoplasmic membranes; juxtaposition of these membranes occurs at the vegetative-cell end of a channel (the pore channel ) which passes through the thick heterocyst envelope. (Microplasmodesmata are also found between vegetative cells.) [Heterocyst differentiation and function: Book ref. 75, pp. 219–242, 265–280.] heterocytotropic antibodies CYTOPHILIC ANTIBODIES (particularly REAGINIC ANTIBODIES) which can attach to the cells of two or more different host species. (cf. HOMOCYTOTROPIC ANTIBODIES.) heteroduplex Any double-stranded nucleic acid in which some, or many, of the bases in one strand are not complementary to bases in the corresponding positions in the other strand; such mismatching of base pairs is reflected in the thermal stability of the duplex. (See also DNA HOMOLOGY and HOMODUPLEX.) The term ‘heteroduplex’ is also used for any duplex in which each strand originates from a different parent duplex (see e.g. RECOMBINATION), even though such strands may in some cases be strictly complementary. heteroecious Syn. HETEROXENOUS. heterofermentation (1) Any fermentation in which there is more than one major end-product. (2) Syn. HETEROLACTIC FERMENTATION. heterogeneous nuclear RNA See HNRNA. heterogenote See MEROZYGOTE. heteroglycan See POLYSACCHARIDE. heteroimmune phages See SUPERINFECTION IMMUNITY. heteroimmunization The stimulation of an immune response to antigens derived from another species. (cf. ISOIMMUNIZATION.) heterokaryon (heterocaryon) (mycol.) A hyphal cell, mycelium, organism, or spore which contains genetically different nuclei. (cf. HOMOKARYON.) heterokaryosis (mycol.) The condition in which genetically different nuclei exist in the same cell, mycelium or spore; it may arise e.g. by the entry of a genetically different nucleus into a HOMOKARYON (e.g. by hyphal fusion) or by mutation in a binucleate or multinucleate homokaryotic cell or mycelium. Heterokaryosis is important e.g. in PARASEXUAL PROCESSES. heterokont Refers to a pair of flagella (on a biflagellate cell) in which one flagellum differs from the other in length and often also in type: see e.g. XANTHOPHYCEAE. (cf. ISOKONT.) organism develops both in the gut and (as epimastigotes and trypomastigotes) in the salivary glands of the vector, but forms which develop in the gut are not infective for mammals. In man, bloodstream forms are trypomastigotes, ca. 25–35 µm in length. T. (H.) rangeli appears to be non-pathogenic. It can be cultivated e.g. in NNN medium. Other species include T. microti, T. nabiasi and T. zapi. Herpomyces See LABOULBENIALES. Herxheimer reaction Syn. JARISCH–HERXHEIMER REACTION. Hessea See MICROSPOREA. Hesseltinella See MUCORALES. heteroantibody Any antibody formed in one species against an antigen derived from another species. (cf. ISOANTIBODY.) heteroauxin See AUXINS. Heterobasidion A genus of fungi of the APHYLLOPHORALES (family Polyporaceae). H. annosum (formerly Fomes annosus) forms perennial, bracket-shaped or resupinate, typically corky or woody fruiting bodies in which the context is dimitic and lacks clamp connections; the bracket-shaped fruiting body typically has a rust-coloured upper surface and a pale, porous, lower surface. Fruiting bodies of several years’ standing consist of vertically stacked (‘stratified’) layers of vertically orientated tubules. Basidiospores: spheroidal, colourless, ca. 5 × 4 µm. H. annosum grows on felled timbers, and is parasitic on conifers and e.g. red oak (Quercus borealis); in e.g. parts of the United Kingdom, the stumps of freshly-felled trees are inoculated with Peniophora gigantea which colonizes the cut surface, inhibiting infection by H. annosum. (See also TIMBER SPOILAGE.) heterocaryon Syn. HETEROKARYON. Heterochlorida See PHYTOMASTIGOPHOREA. Heterochloris See XANTHOPHYCEAE. heterochromatin See CHROMATIN. heteroclicity The phenomenon in which a cross-reactive antigen binds an antibody with an AFFINITY greater than that shown by the homologous antigen; the cross-reactive antigen, and the antibody, are described as heteroclitic. heteroclitic See HETEROCLICITY. heterococcolith See COCCOLITH. heterocyst A type of cell, morphologically and functionally distinct from a vegetative cell, which occurs under certain conditions in some filamentous CYANOBACTERIA (members of sections IV and V); heterocysts function as specialized anaerobic compartments within which NITROGEN FIXATION can occur in aerobic environments. (cf. AKINETE.) Heterocysts differentiate from vegetative cells (intercalary or terminal, depending on species) in response to limiting levels of combined nitrogen; differentiation is inhibited in the presence of e.g. nitrate and ammonia. Mature heterocysts apparently cannot divide. Differentiation from a vegetative cell involves e.g. (i) the formation of a thick, multilayered envelope – whose thickness (in Anabaena flos-aquae) has been found to increase with increased partial pressure of oxygen [JGM 1992) 138 2673–2678]; (ii) rearrangement of thylakoid membranes (heterocysts are filled with contorted membranes); (iii) inactivation of photosystem II, resulting in the cessation of oxygen evolution – cyclic photophosphorylation (which involves photosystem I) apparently continues to occur; (iv) degradation of phycocyanin (see PHYCOBILIPROTEINS); (v) cessation of carbon dioxide fixation (e.g. by inactivation of RuBP carboxylase); (vi) synthesis of NITROGENASE (see NIF GENES). [Role of transcription in differentiation: JGM (1984) 130 789–796]. Respiratory activity apparently 367 Heterokontae heteroresistance (1) The resistance (of an organism) to two or more related antibiotics – e.g. two or more b-lactam antibiotics. (2) See MRSA. heterothallism The phenomenon in which sexual reproduction requires the involvement of two different thalli – an individual thallus being self-sterile even when (as is common) it is hermaphroditic; gametes etc which can unite sexually are said to be compatible. (cf. HOMOTHALLISM; see also COMPATIBILITY sense 2.) In morphological heterothallism (= dioecism) compatibility is determined solely on a sexual basis: there are separate male and female thalli, and sexual reproduction can occur between any male thallus and any female thallus of the same species. Dioecism is uncommon among fungi; it occurs e.g. in some species of Achlya and of Laboulbenia. In physiological heterothallism male and female organs generally occur on the same thallus, and in these cases compatibility is determined not only on a sexual (male–female) basis: sexual union can occur between male and female gametes etc only if each has appropriate mating type allele(s) (see MATING TYPE); gametes etc which have the same mating type allele(s) cannot unite sexually and are said to be incompatible. In e.g. many zygomycetes, male and female organs are not distinguishable; in such cases sexually compatible thalli are often designated ‘plus’ and ‘minus’. In bipolar physiological heterothallism compatibility is determined at one mating type locus which may contain either of 2 alleles; if these alleles be designated A and a, then mating can occur between one individual (mating type) which has the A allele and another which has the a allele. Bipolar heterothallism is also called one-locus two-allele heterothallism, unifactorial heterothallism (since only one locus is involved), or DIMIXIS (since two alleles are involved). In tetrapolar physiological heterothallism compatibility is determined at two mating type loci, each locus containing one of two alleles. If these loci be designated A, B (with alleles a, b, respectively), four mating types can be distinguished: AB, Ab, aB and ab. Mating can occur only between individuals which contain complementary alleles at both loci; thus, e.g. mating can occur between AB and ab individuals, and between Ab and aB individuals. Tetrapolar heterothallism is also called bifactorial heterothallism. In some cases of one- and two-locus heterothallism, a locus may be occupied by any allele of an allelomorphic series; in such cases there can be many mating types. (See also DIAPHOROMIXIS.) Heterothallism provides a constraint on inbreeding, and consequently increases the potential for genetic reassortment within a species. Heterotrichida An order of free-living and parasitic ciliate protozoa (class POLYHYMENOPHOREA) in which the organisms are typically large, often contractile (see MYONEME), and sometimes pigmented, and in which somatic ciliature is commonly abundant and oral ciliature is well developed. Genera include e.g. Balantidioides, BLEPHARISMA, Brachonella, Bursaria, Condylostoma, Metopus, SPIROSTOMUM and STENTOR. heterotrichous (1) Having different types of cilia or flagella. (2) Of certain filamentous algae: having a vegetative thallus consisting of both prostrate and erect systems of filaments (as e.g. in CHAETOPHORA). heterotroph An organism which uses organic compounds for most or all of its carbon requirements. (cf. AUTOTROPH; ORGANOTROPH.) The term ‘heterotroph’ is often used to refer specifically to CHEMOORGANOHETEROTROPHS – although chemolithotrophs and phototrophs may also be heterotrophic. Heterokontae Former taxon for the xanthophytes. heterolactic fermentation A type of LACTIC ACID FERMENTATION in which sugars (e.g. lactose, glucose) are fermented to a range of products. There are two distinct pathways, phosphoketolase being a key enzyme in each. The ‘classical’ (6-phosphogluconate) pathway occurs in certain LACTIC ACID BACTERIA (e.g. Leuconostoc spp, betabacteria – e.g. Lactobacillus brevis): glucose is fermented to lactic acid, CO2 , and acetic acid and/or ethanol (the ratio of acetic acid to ethanol depending e.g. on the redox potential of the system); PENTOSES are fermented to lactic and acetic acids [Appendix III(b)]. In the Bifidobacterium (‘bifidus’) pathway the products of glucose fermentation are lactic and acetic acids in the molar ratio 2:3 [Appendix III(b)]. heterologous (1) Derived from or associated with a species different from that being referred to (cf. HOMOLOGOUS (2)). (2) See ALTERNATION OF GENERATIONS. (3) (immunol.) Of an antibody (or antigen): one which is not HOMOLOGOUS (sense 4) to a given antigen (or antibody). A heterologous antibody, for example, may be a CROSS-REACTING ANTIBODY or may not combine at all with the given antigen. A heterologous vaccine is a vaccine prepared against one organism but capable of giving protective immunity against another. heterologous interference See INTERFERENCE (1). Heteromastix See MICROMONADOPHYCEAE. heteromerous (1) (lichenol.) Refers to a lichen thallus in which the photobiont is confined to a distinct layer within the thallus (see LICHEN). (2) (mycol.) In an agaric: refers to a pileus and stipe composed of two distinct types of cell: SPHAEROCYSTS and hyphae (see RUSSULALES). (3) (ciliate protozool.) See MACRONUCLEUS. (cf. HOMOIOMEROUS.) heteromixis Collectively: DIMIXIS, DIAPHOROMIXIS and secondary HOMOTHALLISM. heteromorphic Morphologically dissimilar. heteromorphic alternation of generations See ALTERNATION OF GENERATIONS. Heteronema A genus of EUGLENOID FLAGELLATES, closely related to PERANEMA, in which the second emergent flagellum is not attached to the pellicle. H. acus occurs e.g. in activated sludge. heterophil antibodies (heterophile antibodies) Antibodies which can combine with specific HETEROPHIL ANTIGENS. Such antibodies may occur in a host which has not had immunological contact with the corresponding antigen; e.g., patients with INFECTIOUS MONONUCLEOSIS usually develop heterophil antibodies which agglutinate the red blood cells of the sheep, horse and ox – species whose RBCs share a common heterophil antigen (see PAUL–BUNNELL TEST). heterophil antigen (heterophile antigen) Any antigen which occurs (in an identical or closely related form) in several or many widely differing species (e.g. sheep, horse). If introduced into a species in which it is normally absent, a heterophil antigen elicits the synthesis of HETEROPHIL ANTIBODIES. (See also FORSSMAN ANTIGEN.) heterophilic binding (of cell adhesion molecules) See IMMUNOGLOBULIN SUPERFAMILY. Heterophrys See CENTROHELIDA. heteroplasmon See CYTODUCTION. heteroploid Having a complement of chromosomes differing from that characteristic of the species. (cf. EUPLOID sense 2.) A heteroploid cell may be EUPLOID (sense 1) – e.g. a polyploid cell arising in a haploid or diploid population – or it may be ANEUPLOID. heteropolysaccharide See POLYSACCHARIDE. 368 high-dry objective reactions of the EMBDEN–MEYERHOF–PARNAS PATHWAY; similarly, glyceraldehyde 3-phosphate may be converted to pyruvate via the latter part of the EMP pathway. In organisms with a functional TCA CYCLE, pyruvate can be oxidized to yield energy via the TCA cycle and a respiratory chain. In organisms which lack a complete TCA cycle, pyruvate may be converted to acetyl-CoA and thence to acetic acid (as in some acetic acid bacteria). Alternatively, under certain conditions, glyceraldehyde 3-phosphate can be converted to glucose 6-phosphate (by reactions of GLUCONEOGENESIS) which can then re-enter the HMP pathway; in this case, for every six molecules of glucose entering the pathway, one molecule is effectively completely oxidized. If reducing equivalents from NADPH can be transferred to NAD+ (see TRANSHYDROGENASE) and thence to an electron acceptor via a respiratory chain, the pathway can be used to generate energy even in the absence of a TCA cycle. Other functions of the HMP pathway include the metabolism of those pentoses which can be converted to intermediates of the pathway. (See also RMP PATHWAY.) hexulose phosphate pathway Syn. RMP PATHWAY. hexuronic acids URONIC ACIDS corresponding to hexoses. hexylresorcinol See PHENOLS. hfl genes (in E. coli ) See BACTERIOPHAGE l. hflB gene See FTSH. Hfr donor High frequency recombination donor: a conjugal donor (see CONJUGATION sense 1b) created by the insertion of a CONJUGATIVE PLASMID (sense 1) into a bacterial chromosome. The chromosome of an Hfr donor can be mobilized by the plasmid so that chromosomal genes can be transmitted to a recipient during conjugation; thus, in a mixed population of recipients and Hfr donors, recombination between recipient and (transferred) donor DNA will occur at high frequency in the recipient population. The (relatively few) conjugative plasmids which can give rise to Hfr donors include the F PLASMID. A conjugative plasmid which cannot normally integrate into the bacterial chromosome may be able to do so if it first integrates e.g. with the genome of an A+ B − strain of BACTERIOPHAGE MU, integration of the plasmid–Mu complex into the chromosome then being mediated by Mu; a recipient mated with such an Hfr donor must be lysogenic for a c+ strain of Mu to avoid ZYGOTIC INDUCTION when the Mu prophage is transferred to the recipient. (See also PRIME PLASMID and INTERRUPTED MATING.) HFRS Haemorrhagic fever with renal syndrome (see HANTAVIRUS). HFT (of plasmids) See EPIDEMIC SPREAD. HFT lysate See TRANSDUCTION. HGV Hepatitis G virus (see HEPATITIS). HHV1 Human (alpha) herpesvirus 1 (see ALPHAHERPESVIRINAE). HHV2 Human (alpha) herpesvirus 2 (see ALPHAHERPESVIRINAE). HHV3 Human (alpha) herpesvirus 3 (see ALPHAHERPESVIRINAE). HHV4 Human (gamma) herpesvirus 4 (see GAMMAHERPESVIRINAE). HHV5 Human (beta) herpesvirus 5 (see BETAHERPESVIRINAE). HHV6 Human (beta) herpesvirus 6 (see BETAHERPESVIRINAE). HHV7 Human (beta) herpesvirus 7 (see BETAHERPESVIRINAE). HHV8 Human (gamma) herpesvirus 8 (see GAMMAHERPESVIRINAE). HI test HAEMAGGLUTINATION-INHIBITION TEST. HIB HAEMOLYTIC IMMUNE BODY. Hibitane See CHLORHEXIDINE. high-dry objective Any objective lens which has a numerical aperture between 0.65 and 0.95, and which is not used as an immersion lens. heteroxenous (heteroecious) Refers to a parasite which carries out part of its life cycle in each of two or more different host species. (cf. HOMOXENOUS.) heterozygote A HETEROZYGOUS cell or organism. heterozygous Refers to a cell or organism (e.g. a diploid eukaryote or a bacterial MEROZYGOTE) in which one or more specified genes, or all of the genes, are present in the form of different ALLELES. (cf. HOMOZYGOUS.) hex genes See TRANSFORMATION (1). Hexacapsula See MYXOSPOREA. hexachlorophane (hexachlorophene) See BISPHENOLS. hexagonal II phase See CYTOPLASMIC MEMBRANE. hexamer See ICOSAHEDRAL SYMMETRY. hexamethylenetetramine See HEXAMINE. hexamine (hexamethylenetetramine; methenamine) A condensation product of ammonia and formaldehyde used as a urinary antiseptic. It has no intrinsic antimicrobial activity, but under acid conditions it slowly releases FORMALDEHYDE. It is not effective against urea-splitting bacteria since these produce ammonia which raises the pH of the urine. Hexamine is usually used with MANDELIC ACID (hexamine mandelate, Mandelamine) or with hippuric acid (Hiprex). Hexamita A genus of protozoa of the DIPLOMONADIDA. H. meleagridis is an important pathogen of turkeys (see HEXAMITIASIS); cells are pear-shaped, ca. 10 µm in length, with two anteriorly located nuclei, two clearly separate, parallel, longitudinal axostyles, and 8 flagella – two pairs orientated anteriorly, one pair laterally, and the fourth pair projecting posteriorly. H. intestinalis is a cyst-forming species which occurs e.g. in the cloaca of the frog. hexamitiasis An acute infectious disease of e.g. turkeys and pheasants (but not affecting chickens, ducks or geese); it is caused by Hexamita meleagridis. Infection occurs by ingestion of food, water etc contaminated with faeces from infected or carrier birds. Symptoms: listlessness; anorexia; passage of watery, frothy, foul-smelling stools; coma. Mortality rates are often very high in young birds. Survivors commonly become carriers. Treatment: e.g. furazolidone. (See also POULTRY DISEASES.) hexokinase An enzyme which phosphorylates various hexoses in the 6-position (see e.g. EMBDEN–MEYERHOF–PARNAS PATHWAY). hexon See ADENOVIRIDAE. hexose bisphosphate pathway Syn. EMBDEN–MEYERHOF–PARNAS PATHWAY. hexose monophosphate pathway (HMP pathway; HMP shunt; oxidative pentose phosphate pathway; pentose phosphate pathway/cycle; phosphogluconate pathway; Warburg–Dickens pathway) A metabolic pathway present in a wide range of prokaryotic and eukaryotic microorganisms as well as in plants and animals; it involves the oxidative decarboxylation of glucose 6-phosphate, via 6-phosphogluconate, to ribulose 5phosphate, followed by a series of reversible, non-oxidative interconversions whereby hexose and triose phosphates are formed from pentose phosphates. The generally accepted scheme for the HMP pathway is shown in Appendix I(b). The HMP pathway can serve various functions, the major ones probably being to provide NADPH (2 molecules per molecule of glucose converted to ribulose 5-phosphate) necessary for biosyntheses (e.g. of fatty acids), and to provide precursors for various biosynthetic pathways (e.g. pentoses for histidine and nucleotide biosynthesis [Appendices IV(g) and V], erythrose 4-phosphate for aromatic amino acid biosynthesis [Appendix IV(f)]). Fructose 6-phosphate may be converted to glucose 6-phosphate and re-enter the pathway, or may be converted to pyruvate via 369 high-fructose corn syrup reagent.) In a quicker method, a solution of sodium hippurate is heavily inoculated with the test organism; after incubation (37° C/2hours), ninhydrin reagent is added. A deep purple colour indicates the presence of glycine (positive test). [Methods: Book ref. 2, p. 416.] Hiprex See HEXAMINE. hircinol A dihydroxyphenanthrene PHYTOALEXIN produced by the orchid Loroglossum hircinum in response to infection with e.g. Rhizoctonia spp. (cf. ORCHINOL.) Hirst spore trap (automatic volumetric spore trap) An instrument used e.g. for sampling the airborne microflora. (See also AIR.) Air, drawn in through a vertical slit in an upright cylindrical housing, strikes a slowly-moving greased microscope slide or, in the modern version (Burkard trap), a slowly-rotating drum coated with transparent adhesive tape; the slit is kept facing into the wind by a vane attached to the housing. Hirsutella A genus of fungi of the HYPHOMYCETES. Most species are entomopathogenic; H. thompsonii infects mites. his operon (histidine operon) An OPERON (located at ca. 44 min on the chromosome of Escherichia coli ) which contains 9 structural genes (hisGDCBHAFIE ) encoding enzymes necessary for the biosynthesis of histidine [Appendix IV(g)]. The transcription of the his structural genes is inversely related to the level of histidyl-tRNA in the cell. Regulation is apparently achieved solely by attenuation (see OPERON, cf. TRP OPERON); the sequence encoding the leader peptide contains seven successive histidine codons. HisJ, HisM, HisP, HisQ See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. hispid Bearing hairs, bristles or spines. Hispid flagellum: see FLAGELLUM (b). Hiss serum water A medium made by mixing 1 volume of serum (horse, ox or sheep) with 3 volumes of water, and adding phenol red and a sugar (final concentrations ca. 0.0025% and 1.0%, respectively). histamine An amine formed by the decarboxylation of histidine; it occurs e.g. in MAST CELLS and is involved in ANAPHYLACTIC SHOCK and INFLAMMATION. (See also ANAPHYLATOXINS.) Histamine causes contraction of e.g. tracheobronchial and intestinal smooth muscle, and increases vascular permeability in the skin and other regions by acting as a vasodilator of terminal arterioles and as a vasoconstrictor of postcapillary venules. [Agents that release histamine from mast cells: ARPT (1983) 23 331–351.] The microbial production of toxic amounts of histamine in certain foods has been implicated as a cause of food poisoning: for example, Lactobacillus buchneri can produce histamine in Swiss cheese [AEM (1985) 50 1094–1096] (see also FISH SPOILAGE). histamine-sensitizing factor Syn. PERTUSSIS TOXIN. histidase See HISTIDINE DEGRADATION. L-histidine biosynthesis See Appendix IV(g); see also HIS OPERON. histidine degradation Histidine can be used as a substrate by various bacteria; in e.g. Pseudomonas aeruginosa histidine is initially deaminated by histidase to urocanate which is eventually degraded to glutamate and formate. Some bacteria can decarboxylate histidine to histamine: see e.g. FISH SPOILAGE. (See also BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM.) histidine kinase A KINASE whose active site contains a histidine residue. (See also CHEMOTAXIS and TWO-COMPONENT REGULATORY SYSTEM.) [Histidine kinases and signal transduction (review): TIG (1994) 10 133–138.] histidine operon Syn. HIS OPERON. high-fructose corn syrup A sweetening agent produced industrially from glucose by GLUCOSE ISOMERASE (see also IMMOBILIZATION). high-mobility group proteins See NON-HISTONE PROTEINS. high mutability gene Syn. MUTATOR GENE. high-performance liquid chromatography See CHROMATOGRAPHY. high-potential iron–sulphur protein See HIPIP. high-pressure liquid chromatography See CHROMATOGRAPHY. high zone tolerance See IMMUNOLOGICAL TOLERANCE. higher fungi Fungi of the subdivisions ASCOMYCOTINA, BASIDIOMYCOTINA and DEUTEROMYCOTINA. (cf. LOWER FUNGI.) Highlands J virus See ALPHAVIRUS. highly active antiretroviral therapy See HAART. HIGM See RNA EDITING. Hikojima variant See VIBRIO (V. cholerae). hilar appendix A small protuberance on a basidiospore near its region of attachment to the sterigma. Hildenbrandia See RHODOPHYTA. Hill reaction In PHOTOSYNTHESIS: light-dependent evolution of oxygen by isolated CHLOROPLASTS in the absence of CO2 but in the presence of e.g. ferric oxalate which acts as an electron acceptor for PS-II. hilum The mark, scar or small projection on a spore corresponding to the region at which it was formerly attached to the spore-bearing structure. him genes (in E. coli ) See BACTERIOPHAGE l. Himanthalia See PHAEOPHYTA. Himmelweit pipette A PASTEUR PIPETTE with a hole in the side of the capillary end; it is used e.g. in the harvesting of allantoic fluid from an EMBRYONATED EGG. hin gene See PHASE VARIATION. HindII A RESTRICTION ENDONUCLEASE from Haemophilus influenzae; GTPy/PuAC. HindIII A RESTRICTION ENDONUCLEASE from Haemophilus influenzae; A/AGCTT. HinfI See RESTRICTION ENDONUCLEASE (table). hinge region In the heavy chain of an IMMUNOGLOBULIN: the amino acid sequence at the junction of the Fd portion and the Fc portion; it appears to confer conformational flexibility on the molecule. Structurally, the hinge region is the most variable part of the ‘constant’ region of the heavy chain. In alpha and gamma chains (but not in mu chains) the hinge region is rich in proline. Hinozan (edifenphos; O-ethyl-S,S-diphenyl phosphorodithioate) An antifungal ORGANOPHOSPHORUS COMPOUND used as a protectant against rice blast disease; it can act e.g. as a powerful cutinase inhibitor (see CUTIN). hip gene (in E. coli ) See BACTERIOPHAGE l. HiPIP High-potential iron–sulphur protein: a type of IRON–SULPHUR PROTEIN which has a single [4Fe–4S] centre and a highly positive Em ; the HiPIP in Chromatium vinosum has an Em of +350 mV. hippurate hydrolysis Certain bacteria (e.g. Streptococcus spp, Campylobacter spp) produce a hippuricase which hydrolyses hippurate (C6 H5 .CO.NH.CH2 .CO2 − ) to benzoate and glycine. Hippuricase activity may be detected by adding a calculated amount of acid FeCl3 to a 4-day culture of the organism in a medium containing 1.0% sodium hippurate; the FeCl3 forms a persistent precipitate with benzoate (positive test). (A precipitate is also formed with hippurate, but this is more soluble with excess FeCl3 ; the amount of FeCl3 used in the test is that which just dissolves the hippurate precipitate initially formed in an uninoculated control tube on dropwise addition of the 370 HIV lung function or to secondary bacterial infection. Infrequently, histoplasmosis becomes disseminated (usually in young children or in the elderly, debilitated or immunocompromised); lesions may occur in lymph nodes, liver, spleen, intestine, skin, heart, kidneys, CNS etc. Disseminated histoplasmosis is often rapidly fatal. Lab. diagnosis: serological tests (e.g. a CFT); microscopic and cultural examination of clinical specimens. Chemotherapy: e.g. amphotericin B, ketoconazole. histotope A site on an MHC class I or II antigen recognized by a T lymphocyte. histozoic Living (parasitic) within tissues. (cf. COELOZOIC.) Histriculus See HYPOTRICHIDA. hit-and-run oncogenesis Oncogenesis which is initiated by a particular agent (e.g. a virus) but, once initiated, can proceed in the absence of the initiating agent. HIV Human immunodeficiency virus, infection with which can give rise to AIDS (q.v.). HIV is a retrovirus (family RETROVIRIDAE) classified within the subfamily LENTIVIRINAE. HIV was previously referred to as AIDS-associated retrovirus (ARV), ‘HTLVIII’ (cf. HTLV), immunodeficiency-associated virus (IDAV) and lymphadenopathy-associated virus (LAV). Evidence for the involvement of HIV in AIDS was obtained in 1983 [Science (1983) 220 868–871]. Subsequently a distinct, but related, retrovirus was detected in AIDS patients in West Africa [Nature (1987) 326 662–669]. These two viruses are now designated HIV-1 and HIV-2 respectively. HIV-1 and HIV-2 are similar in both structure and genome. The origin of HIV is unknown. One hypothesis supposes that the major (M) strain of HIV arose through the use of an antipoliomyelitis vaccine that had been prepared by culturing polio virus in non-human primate cells; it is proposed that a simian strain of immunodeficiency virus (initially present in the culture cells) contaminated the vaccine and (thus) entered the vaccinated human population (located mainly in the Congo) in the late 1950s. Subsequent studies on viruses from the Congo area do not support this hypothesis [Nature (2001) 410 1047–1048]. The HIV virion is ∼100 µm in diameter. Its innermost region consists of a cone-shaped core that includes (i) two copies of the (positive-sense) ssRNA genome, (ii) the enzymes REVERSE TRANSCRIPTASE, integrase and protease, (iii) some minor proteins, and (iv) the major core protein. The core is surrounded by a protein matrix, and the whole is enclosed within an envelope (a lipid bilayer); the envelope is penetrated by a number of ‘spikes’ (gp41), each spike bearing a distal (outermost) trimeric glycoprotein (gp120). [Fine structure of HIV: Virol. (1987) 156 171–176.] The genome (∼9.2 kb) includes sequences corresponding to the gag, pol and env regions of other exogenous retroviruses (see RETROVIRIDAE). The gag region encodes structural proteins, including the major core protein (p24) and the matrix protein (p17). The pol region encodes regulatory proteins: REVERSE TRANSCRIPTASE, protease (involved in the maturation of virions), RNASE H and integrase. (The enzymes reverse transcriptase and protease are major targets for antiretroviral drugs.) The env region encodes the envelope-associated proteins, gp120 (required for initial binding of virus to target cell) and gp41 (involved in fusion of the viral envelope with the membrane of the target cell). Other HIV genes include: tat and rev (whose products are needed for viral replication); vif (‘viral infectivity factor’); and vpu (whose product promotes efficient budding of the (HIV-1) virion during productive infection of a cell). (See also Rev in GENE THERAPY.) histidine permease See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. histiocyte A MACROPHAGE which occurs in extravascular tissues. histocompatibility restriction (MHC restriction) The restriction on (physical) cell–cell interaction, involving cells of the immune system, imposed by the need for such cells to have, or to recognize, particular MHC-specified cell-surface antigens as a prerequisite for interaction. histolyticolysin See THIOL-ACTIVATED CYTOLYSINS. Histomonas A genus of protozoa of the TRICHOMONADIDA. H. meleagridis is the causal agent of BLACKHEAD in turkeys. Intracellular cells of H. meleagridis are spherical to ovoid, nonflagellate (though they contain kinetosomes), ca. 10–15 µm; they exhibit amoeboid motility. Within the caecal lumen of the turkey, cells of H. meleagridis (ca. 10–25 µm) may exhibit both amoeboid and flagellar motility, each cell having 1–4 flagella (commonly one). histomoniasis Syn. BLACKHEAD. histone-like proteins Bacterial DNA-binding proteins believed to resemble the HISTONES associated with eukaryotic DNA. (Compare HU PROTEIN.) histones Basic proteins, rich in arginine and/or lysine, which are major components of CHROMATIN in most eukaryotes. (cf. DINOFLAGELLATES.) The main classes of histones are designated H1 (‘lysine-rich’), H2A and H2B (‘slightly lysine-rich’), and H3 and H4 (‘arginine-rich’). Histones interact with DNA primarily via salt bridges between the positively-charged arginine/lysine residues and the negatively-charged phosphate groups of the DNA. Constitutive histone variants can effect changes in chromatin structure and dynamics that affect e.g. DNA replication, repair and transcription [GD (2005) 19 295–316]. Histoplasma A genus of fungi of the HYPHOMYCETES; teleomorph: AJELLOMYCES. H. capsulatum, the causal agent of HISTOPLASMOSIS, is a dimorphic fungus which occurs e.g. in soil that is contaminated with the droppings of birds, bats and other animals. At room temperature or e.g. 25° C, H. capsulatum forms white to golden, cottony, septate mycelium, but at 37° C it forms ovoid, budding, yeast-like cells 2–4 µm (var. capsulatum) or 8–15 µm (var. duboisii ); in vivo, grape-like clusters of cells may occur in macrophages or in regions of caseation. H. capsulatum gives rise to microconidia (2–5 µm) and macroconidia (ca. 8–15 µm), the latter (also called ‘chlamydospores’) typically bearing short, finger-like projections. H. farciminosum. See EPIZOOTIC LYMPHANGITIS. (See also ENDOCARDITIS and PAS REACTION.) histoplasmin test A SKIN TEST, analogous to the TUBERCULIN TEST, used in the diagnosis of HISTOPLASMOSIS; it involves the intradermal injection of histoplasmin (a filtrate of a mycelial culture of Histoplasma capsulatum). The skin usually becomes reactive to histoplasmin within one to two months of the onset of disease, and reactivity may persist for many years; crossreactions with other fungal antigens (e.g. coccidioidin) occur. histoplasmosis (Darling’s disease) A disease of man and animals, found worldwide, caused by Histoplasma capsulatum (cf. EPIZOOTIC LYMPHANGITIS). Infection occurs by inhalation of spores (e.g. from disturbed soil). The disease may be subclinical, acute or chronic. The acute condition may resemble a common cold or influenza. In the chronic condition, granulomatous, inflamed lesions occur in the lungs; the primary lesion may become inactive and calcify on healing. The primary infection is usually self-limiting, but occasionally progressive lung disease occurs and may eventually be fatal due e.g. to loss of 371 HIV-positive The LTR also includes a regulatory region for polyadenylation of transcripts. Synthesis of viral mRNA, and of genomic RNA, is carried out by the host’s RNA polymerase. Viral envelope proteins are translocated to, and insert into, the cell membrane (with gp120 located at the outer surface). The assembled core acquires an envelope by budding through the cell membrane. Culture. Culture may be useful e.g. for diagnosis of HIV infection in neonates; in these young patients the presence of anti-HIV antibody may simply reflect acquisition from an HIV-positive mother. Cultures of HIV characteristically exhibit CPE (cytopathic effects), including the formation of giant multinucleated cells and cell lysis. Culture can be carried out in CD4+ T cells; the virus can also infect other types of cell such as monocytes and promyelocytes [Virol. (1985) 147 441–448]. HIV may give rise to a persistent, non-cytopathic and productive (virion-producing) infection of human CD4+ T cells in culture [Science (1985) 229 1400–1402]. Inactivation of HIV by disinfection. Hypochlorite (effective conc. 10000 parts per million available chlorine) is a useful disinfectant. Glutaraldehyde (dangerous chemical!!!) has been used for disinfecting surfaces in a wellventilated environment; safety regulations must be observed. Phenolics and ethanol are unsuitable. In serum, inactivation of HIV may require more than 30 minutes at 56° C [e.g. Book ref. 219, p 814]. Genetic variability of HIV isolates. Isolates of HIV from different patients tend to be genetically heterogeneous, particularly in the env sequence [minireview: Cell (1986) 46 1–4], and variation may also be observed in isolates obtained sequentially from the same patient over a period of time [Science (1986) 232 1548–1553]. As env encodes the (antigenic) viral surface protein gp120, such ANTIGENIC VARIATION presents a problem for the development of an effective anti-HIV vaccine; in this context, studies have been carried out to determine the effect of genetic variation on the immunogenetic potential of gp120 [Arch. Virol. (2000) 145 2087–2103]. HIV-positive Refers to an individual who is infected with the human immunodeficiency virus (see HIV). (See also AIDS.) hives Syn. URTICARIA. Hjärre’s disease (coli-granuloma) A POULTRY DISEASE in which granulomatous lesions are formed in the wall of the intestine (particularly in the caecum); the causal agent is Escherichia coli. HLA-A, HLA-B etc. See MAJOR HISTOCOMPATIBILITY COMPLEX. Hly plasmid See ESCHERICHIA. HlyA (of Escherichia coli ) See RTX TOXINS. HMA Hydroxymethionine analogue: DL-a-hydroxy-g-methiolbutyrate; HMA is used in animal feedstuffs as a sulphur amino acid supplement [ARB (1983) 52 215–216]. (See also SINGLE-CELL PROTEIN.) HMG box That region, in certain types of protein, which can e.g. interact with sharply angular or kinked DNA (in eukaryotic cells); it consists of a sequence of about 80 amino acid residues (including a high proportion of aromatic amino acids) with a net positive charge. HMG boxes are found in certain NON-HISTONE PROTEINS. (See also HU PROTEIN.) HMG proteins See NON-HISTONE PROTEINS. HMP pathway HEXOSE MONOPHOSPHATE PATHWAY. HMS-1 b-lactamase See b-LACTAMASES. HMT toxin A long-chain polyketide host-specific toxin produced by Drechslera (Helminthosporium) maydis race T; it has been suggested that HMT toxin acts as an ionophore. Attachment of the virion to a target cell. The principal high-affinity binding site for viral gp120 glycoprotein is the CD4 antigen (see CD4) (formerly referred to as OKT4); target cells for HIV therefore include the CD4+ (‘T helper’) subset of T LYMPHOCYTES, monocytes, macrophages and dendritic cells. CD4+ cells also occur in the central nervous system (microglial cells), and infection of these cells by HIV may account for neurological symptoms in HIV-positive patients; multinucleate (syncytial) cells have been observed in brain tissue from such patients. For infection of a cell by HIV, the cell must express not only CD4 antigen but also receptors for particular CHEMOKINES – e.g. those for certain CXC-type and CC-type chemokines (the CXCR4 and CCR5 receptors) [JLB (1999) 65 552–565]. Some (T-cell-tropic) strains of HIV require the co-receptor CXCR4 while other (M-tropic) strains require CCR5; M-tropic strains can infect macrophages and monocytes as well as T cells. Yet other (dual-tropic) strains of HIV can use both co-receptors. On the basis of their use of particular co-receptors, strains of HIV have been categorized as X4, R5 and R5X4 respectively. Some CD34+ haemopoietic stem cells co-express the CD4 antigen (and co-receptors) but these cells are apparently not susceptible to infection by pathogenic strains of HIV; however, such cells are susceptible to infection by genetically engineered viral ‘constructs’ (based on HIV) which may be of use in GENE THERAPY. Although stem cells may not be susceptible to natural strains of HIV, inhibition of haemopoiesis may nevertheless occur in HIV+ patients owing to perturbation of the cytokine milieu in bone marrow. [Haematological aspects of HIV infection: BCH (2000) 13 215–230.] Internalization and integration of HIV. Fusion between viral envelope and cell membrane is achieved in a gp41-dependent manner; the viral core then enters the target cell’s cytoplasm. On (intracellular) release of viral RNA and enzymes, a dsDNA form of the genome is synthesized using viral reverse transcriptase and RNase H (see: RETROVIRIDAE). [HIV entry inhibitors: PNAS (2002) 99 16249–16254.] The 3
and 5
LTRs of the dsDNA are involved in circularization of the molecule (forming a plasmid-like structure); this structure enters the nucleus and is integrated into the host’s genome in a process involving the viral integrase. In some cases the integrated (provirus) form of HIV remains latent (inactive), or weakly active, for some time (e.g. ∼10 years), the patient remaining essentially asymptomatic; in other cases disease progresses rapidly. Replication of HIV. Replication is apparently an inefficient process in which a high proportion of progeny virions are defective. Transcription of the integrated viral genome appears to depend on various (viral and host) regulatory proteins which have binding sites on the LTR. For example, the viral tat gene product, with a binding site designated Tat-responsive region (TAR), has an important transactivation role; the activity of this protein is essential in the upregulation of replication. The host’s nuclear transcription factor, NF-kB (see CYTOKINES), is also important as a (positive) regulator of viral gene expression and it, too, has a binding site on the LTR; it is therefore possible that expression of viral genes may be promoted by superinfection with certain opportunist pathogens that activate NF-kB by inducing synthesis of appropriate cytokines. (This suggests a possible mechanism for the exacerbation of HIV-mediated disease that may be seen during opportunist infections.) 372 homogenote hnRNA Heterogeneous nuclear RNA: the high-MWt RNA synthesized by RNA polymerase II in the nucleus of a eukaryotic cell; hnRNA occurs in the form of ribonucleoprotein (hnRNP) and includes pre-mRNA (i.e., MRNA before the generation of 3
ends, polyadenylation and splicing). H-NS protein (syn. H1 protein) In bacteria (e.g. Escherichia coli ), a small protein (136 amino acids, 15.6 kDa) encoded by the hns gene. H-NS protein is able to (i) regulate the expression of various unrelated genes (e.g. bgl, proU ), (ii) constrain negative supercoiling, and (iii) contribute to the physical organization of the bacterial nucleoid. In uropathogenic strains of E. coli (UPEC) the H-NS protein has been implicated in the temperature-dependent expression of P FIMBRIAE: at the non-permissive temperature of 25° C, the H-NS protein is reported to act as a methylation-blocking factor, repressing transcription [Mol. Microbiol. (1998) 28 1121–1137]. In Proteus mirabilis it is reported to act as a repressor of the transcriptional activator gene, ureR, in the absence of urea induction [JB (2000) 182 2649–2653]. H-NS has also been implicated in the regulation of DNA repair in Shigella [JB (1998) 180 5260–5262]. H-NS protein is able to condense supercoiled plasmid DNA, and appears to play a major part in condensing DNA in the nucleoid. H-NS-mediated condensation of DNA has been investigated by studying the structure of H-NS–DNA complexes by atomic force microscopy [NAR (2000) 28 3504–3510]. Hoarella A genus of protozoa (suborder EIMERIORINA) which form dizoic oocysts each containing 16 sporocysts. Hoechst 33258 A bis-benzimidazole derivative: a FLUOROCHROME used e.g. for staining chromosomes. hog cholera Syn. SWINE FEVER. hog diseases See PIG DISEASES. Hogness box See PROMOTER. holdfast An organ or organelle by means of which an organism or cell attaches to the substratum or to another organism. Examples include the root-like region of LAMINARIA, and the specialized regions in SPORICHTHYA and members of the ASTOMATIDA. holding–flush jar procedure In the culture of anaerobes: a procedure which minimizes the exposure of plates to O2 ; essentially, uninoculated plates, or inoculated plates awaiting incubation, are held in an anaerobic jar (with unclamped lid) while a stream of O2 -free gas (e.g. N2 or CO2 ) is continually directed through a tube to the bottom of the jar. holin See LYSIS PROTEIN. Hollandina See SPIROCHAETALES. Holliday junction See RECOMBINATION. Holliday model See RECOMBINATION (Figure 1). Holliday structure See RECOMBINATION. Holmsella A genus of algae (division RHODOPHYTA). H. pachyderma is a colourless organism which is parasitic on the red alga Gracilaria; PIT CONNECTIONS have been observed between H. pachyderma and Gracilaria. Holobasidiomycetidae A subclass of fungi (class HYMENOMYCETES) characterized by the formation of holobasidia (see BASIDIUM). Orders [Book ref. 64, p. 189]: AGARICALES, APHYLLOPHORALES, BOLETALES, BRACHYBASIDIALES, CANTHARELLALES, DACRYMYCETALES, EXOBASIDIALES, RUSSULALES and TULASNELLALES. holobasidium See BASIDIUM. holoblastic development See CONIDIUM. holocarpic Refers to those organisms in which the entire thallus takes on a reproductive function. (cf. EUCARPIC.) holocellulose A complex of cellulose and hemicelluloses (formed e.g by the removal of lignin from lignocellulosic plant material). holocentric (holokinetic) Refers to a chromosome in which the CENTROMERE is diffuse, being distributed along the length of the chromosome; the kinetochores are correspondingly long, and microtubules are attached along the whole length of the chromosome. holococcolith See COCCOLITH. holoenzyme A ‘whole’ enzyme – i.e., an enzyme in its complete, active form – consisting either of an apoenzyme plus PROSTHETIC GROUP or of two or more distinct protein subunits (see e.g. DNA POLYMERASE). holokinetic Syn. HOLOCENTRIC. holomictic lake A lake in which the entire water mass undergoes a complete turnover. (cf. MEROMICTIC LAKE.) holomorph (mycol.) Any fungus considered in its entirety, i.e., including all latent or expressed (sexual and/or asexual) forms and potentialities. (cf. ANAMORPH; ANA-HOLOMORPH; TELEOMORPH.) Holophrya See GYMNOSTOMATIA. holophytic nutrition Plant-type nutrition, i.e., the endogenous formation of nutrients by photosynthesis. (cf. HOLOZOIC NUTRITION.) Holospora A genus of Gram-negative bacteria which are obligate endosymbionts in the micronucleus (H. elegans, H. undulata) or the macronucleus (H. caryophila, H. obtusa) of Paramecium spp; Holospora spp do not confer a killer characteristic (cf. CAEDIBACTER). Cells: non-motile rods or filaments. Type species: H. undulata. [Book ref. 22, pp. 802–803; H. obtusa in P. caudatum (infection and maintenance): JCS (1985) 76 179–187.] Holosticha See HYPOTRICHIDA. holothallic conidium See CONIDIUM. holotoxin The form in which an exotoxin is secreted by a pathogen. After uptake by a eukaryotic cell, the holotoxin may be processed (e.g. by proteolytic cleavage) to release the active component (see e.g. DIPHTHERIA TOXIN and SHIGA TOXIN). holotrich Any of the relatively primitive ciliate protozoa previously classified in the subclass Holotrichia [JP (1964) 11 7–20] but now classified in the classes KINETOFRAGMINOPHOREA and OLIGOHYMENOPHOREA. holotype The actual specimen which an author has designated as the NOMENCLATURAL TYPE of a new taxon. holozoic nutrition Animal-type nutrition, i.e., nutrition involving the ingestion of other organisms or components of other organisms. (cf. HOLOPHYTIC NUTRITION.) homing (intron, bacterial) See INTRON HOMING. homing-associated cell adhesion molecule Syn. CD44. homoacetate fermentation See ACETOGENESIS. homocaryon Syn. HOMOKARYON. homocysteine See e.g. Appendix IV(d). homocytotropic antibodies (1) Syn. REAGINIC ANTIBODIES. (2) Antibodies which bind only to cells of the species in which they were formed. (cf. HETEROCYTOTROPIC ANTIBODIES.) homodiaphoromixis See HOMOTHALLISM. homodimixis See HOMOTHALLISM. homoduplex Any double-stranded nucleic acid in which each strand is completely complementary to the other. (cf. HETERODUPLEX.) homoeomerous Syn. HOMOIOMEROUS. homofermentation (1) Any fermentation in which there is only one major end-product. (2) Syn. HOMOLACTIC FERMENTATION. homogeneous immersion See IMMERSION OIL. homogenization (of milk) See MILK. homogenote See MEROZYGOTE. 373 homoglycan homoglycan See POLYSACCHARIDE. homoheteromixis See HOMOTHALLISM. homoimmune phages See SUPERINFECTION IMMUNITY. homoiomerous (homoeomerous) (1) (lichenol.) Refers to a lichen thallus in which the photobiont is more or less evenly distributed throughout the thallus (see LICHEN). (2) (mycol.) In an agaric: refers to a pileus or stipe composed of hyphae only. (3) (ciliate protozool.) See MACRONUCLEUS. (cf. HETEROMEROUS.) homoisocitrate pathway See icl− SERINE PATHWAY. homokaryon (homocaryon) (mycol.) A hyphal cell, mycelium, organism or spore in which all the nuclei are genetically identical. (cf. HETEROKARYON.) homokaryotic (1) (mycol.) Refers to a HOMOKARYON. (2) (ciliate protozool.) Having only one type of nucleus: see PRIMOCILIATID GYMNOSTOMES. homolactic fermentation A type of LACTIC ACID FERMENTATION in which e.g. glucose is converted entirely, or almost entirely, to lactic acid. Many LACTIC ACID BACTERIA (e.g. Lactococcus lactis, thermobacteria and streptobacteria) carry out a homolactic fermentation of glucose (or lactose) under conditions of glucose (or lactose) excess (cf. LACTIC ACID STARTERS); glucose is metabolized by the EMBDEN–MEYERHOF–PARNAS PATHWAY, and most or all of the pyruvate thus formed is converted to lactic acid. (See also LACTOSE.) However, under glucose or lactose limitation, or with different substrates (e.g. PENTOSES), the same organisms may form other products, sometimes with little or no lactic acid; for example, many strains of L. lactis produce mainly formic and acetic acids and ethanol when grown in glucose- or lactose-limited continuous culture (see Appendix III(c)). (See also DIACETYL.) homologous (1) Similar in form or structure, but not necessarily in function; homology suggests evolutionary relatedness. (2) Derived from or associated with the same species as that being referred to. (cf. HETEROLOGOUS (1).) (3) See ALTERNATION OF GENERATIONS. (4) (immunol.) The antibody elicited by a given antigen is said to be homologous to that antigen; similarly, the antigen is said to be homologous to the antibody. (cf. HETEROLOGOUS (3).) (5) (genetics) Of chromosomes or chromatids: containing the same sequence of genes – but not necessarily identical alleles. (6) (genetics) Of nucleic acids: see e.g. DNA HOMOLOGY. homologous interference See INTERFERENCE (1). homologous recombination See RECOMBINATION. homologue (of chromosomes) In a diploid eukaryotic nucleus: each of a pair of CHROMOSOMES which encode the same types of characteristics and which are usually morphologically similar; in general, homologous chromosomes carry the same sequences of genes, though not necessarily the same alleles. homology (of DNA) See DNA HOMOLOGY. homomixis See HOMOTHALLISM. homonym Any name used to refer to each of two (or more) different microbial taxa; a name of a higher animal or plant which is identical to that of a microorganism is not considered to be a homonym. The name published first is the senior homonym, while any name published later is a junior homonym; junior homonyms are usually suppressed. homophilic binding (of cell adhesion molecules) See IMMUNOGLOBULIN SUPERFAMILY. homoplasmid segregant See INCOMPATIBILITY. homopolymer tailing See TAILING. homopolysaccharide See POLYSACCHARIDE. homoserine See e.g. Appendix IV(d). homothallism (homomixis) Self-fertility: the phenomenon in which sexual reproduction can involve the fusion of e.g. gametes derived from the same individual (as well as the fusion of those derived from different individuals); no ‘mating types’ are involved (cf. HETEROTHALLISM). Secondary homothallism (= homoheteromixis) occurs in some fungi which are basically heterothallic; it can arise e.g. when nuclei of compatible mating types are incorporated in the same spore – the thallus developing from such a spore being selffertile. (This occurs regularly in e.g. Neurospora tetrasperma.) If secondary homothallism involves nuclei whose compatibility is determined on a one-locus two-allele basis the phenomenon is called homodimixis; if it is determined on a one- or two-locus multi-allele basis the phenomenon is called homodiaphoromixis. homothetogenic Refers to the type of cell division typical of ciliate protozoa: transverse (PERKINETAL) BINARY FISSION in which the daughter cells (the PROTER and OPISTHE) are not mirror images of one another – although they may be similar or identical. For example, when Tetrahymena divides, the anterior end of the opisthe is near the plane of division. (cf. SYMMETROGENIC; see also INTERKINETAL.) homoxenous (monoxenous, autoecious) Refers to a parasite which completes its life cycle in a single host species. (cf. HETEROXENOUS.) homozygote A HOMOZYGOUS cell or organism. homozygous Refers to a cell or organism (e.g. a diploid eukaryote or a bacterial MEROZYGOTE) in which one or more specified genes, or all of the genes, are present in the form of identical ALLELES. (cf. HETEROZYGOUS.) honey-bee diseases See BEE DISEASES. honey fungus See ARMILLARIA. honeydew A sugary secretion produced e.g. by sap-sucking insects such as aphids. Sap-sucking insects feed on phloem sap which is rich in sugars but low in nitrogen; thus, large quantities of sap must be ingested to satisfy the insect’s nitrogen requirements, the excess sugars being excreted as honeydew. Insect honeydews vary in composition (depending e.g. on insect); some contain e.g. melezitose (rare in nature), and some are rich in polyols (galactitol, ribitol, etc). Certain fungi – particularly SOOTY MOULDS – may grow abundantly on insect honeydews, and it has been suggested that honeydews falling on soil below insect-infested plants may enhance nitrogen-fixation by freeliving bacteria in the soil [SBB (1984) 16 203–206]. (See also CLAVICEPS.) Hong Kong flu See INFLUENZAVIRUS. Hoogsteen hydrogen bonding See TRIPLEX DNA. hook (flagellar) See FLAGELLUM (a). hop latent virus See CARLAVIRUSES. hop mosaic virus A member of the CARLAVIRUSES which can cause severe to lethal disease in certain (Golding-type) cultivars of hops (Humulus lupulus); symptoms include chlorotic veinbanding, curling of leaf-margins, stunting etc. Transmission occurs via aphids. Weeds (e.g. Urticaria urens, Chenopodium album, Plantago major ) may be important reservoirs of the virus. Sensitive hop cultivars have been replaced by tolerant cultivars in endemic areas. Hop protein (plant pathol.) Hrp-dependent outer protein: a proposed designation for proteins secreted via the Hrp system (see AVIRULENCE GENE) – some of which may lack an avirulence function (and are therefore inappropriately referred to as avr proteins) [JB (1997) 179 5655–5662 (5659–5660)]. hop stunt viroid See VIROID. hopanes See bacterial CYTOPLASMIC MEMBRANE. HOQNO HYDROXYQUINOLINE-N-OXIDE. hordeiviruses (barley stripe mosaic virus group) A group of multicomponent PLANT VIRUSES in which the virions are elongated, 374 hot–cold lysis rigid particles (ca. 100–150 × 20 nm) composed of ssRNA and helically-arranged subunits of a single type of coat protein (MWt ca. 21000). The genome appears to consist of 2–4 molecules (depending on strain) of linear positive-sense ssRNA, two or three of which are necessary for infection. The 3
end of the RNA has an internal poly(A) sequence and a 3
-terminal tRNAlike structure which can be amino-acylated with tyrosine in vitro [Virol. (1982) 119 51–58]. Viruses accumulate mainly in the cytoplasm but also in the nuclei of infected cells. Type member: barley stripe mosaic virus (BSMV), an important pathogen of barley in many regions of the world; some strains of BSMV can also infect wheat, oats etc. Transmission is mechanical and seed-borne. Seeds from BSMV-infected plants may show increased frequencies of triploidy and aneuploidy, and some BSMV strains induce a heritable genetic anomaly (‘aberrant ratio phenomenon’ [ARPpath (1984) 22 77–94]) characterized by abnormal segregation ratios for one or more genetic markers. hordeolum Syn. STYE. horizontal resistance (plant pathol.) In a given cultivar: the existence of similar levels of resistance to each of the races of a given pathogen. Unlike VERTICAL RESISTANCE, horizontal resistance tends to be a permanent (though not necessarily complete) form of resistance which does not readily break down when genetic changes occur in the pathogen; it is commonly inherited polygenically. horizontal transmission (lateral transmission) Transmission of a disease or parasite from one individual to another contemporary individual. (cf. VERTICAL TRANSMISSION.) Hormidium Syn. KLEBSORMIDIUM. Hormiscium dermatitidis See WANGIELLA. Hormoconis A genus of fungi of the HYPHOMYCETES. H. resinae (formerly Cladosporium resinae, the ‘creosote fungus’ or ‘kerosene fungus’) can utilize e.g. creosote and various hydrocarbons as sources of carbon (see also PETROLEUM). [Mechanism of dodecane uptake by H. resinae: JGM (1986) 132 751–756.] The teleomorph of H. resinae is the ascomycete Amorphotheca resinae. [Discussion and references on re-naming of C. resinae: MS (1986) 3 169.] hormocyst In certain cyanobacteria: short filaments – composed of granular cells surrounded by a common, condensed sheath – formed in intercalary or terminal positions in the vegetative trichome; hormocysts become detached from the parent trichome and appear to function as propagules (cf. AKINETE and HORMOGONIUM). (See also LEMPHOLEMMA.) Hormodendrum dermatitidis See WANGIELLA. Hormodendrum resinae Syn. Hormoconis resinae. hormogonium A short filament of undifferentiated cells (i.e., lacking heterocysts or akinetes) formed by filamentous CYANOBACTERIA (members of sections III, IV and V) and by e.g. Beggiatoa; hormogonia may be formed by the fragmentation of whole filaments (e.g. in Oscillatoria and Cylindrospermum) or may develop from the tips of trichomes (e.g. in Scytonema). (See also NECRIDIUM.) Hormogonia may differ from the parent trichome e.g. in having smaller cells; they are typically motile (by gliding) and usually move some distance before developing into mature filaments, thus functioning as propagules. In some cyanobacteria (e.g. Nostoc muscorum [JGM (1983) 129 263–270], Calothrix spp), GAS VACUOLES are formed only in the hormogonia, the increased buoyancy they confer presumably aiding in the dispersal of the hormogonia. hormone A substance which is released from specialized cells in specific part(s) of a multicellular organism and which, in small quantities, can bring about one or more specific responses when translocated to other part(s) of the same individual. hormone A See PHEROMONE. hormone B See PHEROMONE. horse diseases Horses may be affected by a wide range of diseases, some of which are specific to equines. (a) Bacterial diseases: see e.g. CONTAGIOUS EQUINE METRITIS; CONTAGIOUS PUSTULAR DERMATITIS (sense 2); GLANDERS; PARATYPHOID FEVER; POTOMAC HORSE FEVER; SLEEPY FOAL DISEASE; STRANGLES; TULARAEMIA; TYZZER’S DISEASE; ULCERATIVE LYMPHANGITIS. (See also RHODOCOCCUS (R. equi ).) (b) Fungal diseases: see e.g. ASPERGILLOSIS; EPIZOOTIC LYMPHANGITIS; EQUINE PHYCOMYCOSIS; SPOROTRICHOSIS. (See also PASPALUM STAGGERS and PENITREMS.) (c) Protozoal diseases: see e.g. DOURINE; EQUINE PROTOZOAL MYELOENCEPHALITIS; SURRA. (d) Viral diseases: see e.g. AFRICAN HORSE SICKNESS; BORNA DISEASE; EASTERN EQUINE ENCEPHALOMYELITIS; EQUINE COITAL EXANTHEMA; SWAMP FEVER (equine infectious anaemia); VENEZUELAN EQUINE ENCEPHALOMYELITIS; VESICULAR STOMATITIS; WESTERN EQUINE ENCEPHALOMYELITIS. horse mushroom See AGARICUS. horse sickness fever See AFRICAN HORSE SICKNESS. horseradish peroxidase A haemin-containing PEROXIDASE, used e.g. in the IMMUNOPEROXIDASE METHODS. host-controlled modification The modification of bacteriophage DNA by its host’s modification enzymes (see RESTRICTION – MODIFICATION SYSTEM). If a given phage replicates in a particular bacterial strain, the DNA of the progeny phages will carry the modification pattern characteristic of that strain; such phages will be able to infect other cells of the same strain with high efficiency. If these phages infect another strain of bacteria in which there is a different R-M system, a low efficiency of plating is generally observed since, in most of the cells, infection is followed by restriction of the phage DNA; however, in a few cells restriction may fail to occur, and the phage DNA then acquires the modification pattern characteristic of the new strain. The modified DNA can replicate, the daughter strands being modified by the host enzymes; the progeny phages can efficiently infect cultures of the new strain, but will infect cultures of the original strain with low efficiency. host-dependent mutant A CONDITIONAL LETHAL MUTANT bacteriophage which contains e.g. an amber mutation in an essential gene and can thus replicate only in a (permissive) host which contains an intergenic amber SUPPRESSOR MUTATION. host-range mutant (h mutant; hr mutant) A mutant virus in which the mutation alters the host range of the virus (i.e. the type(s) of cell which it can infect). hot-air oven An apparatus used for the dry-heat STERILIZATION of e.g. clean glassware and those materials (such as mineral oils) which cannot be sterilized in an AUTOCLAVE owing to their impermeability to steam. It consists of an electrically heated, thermostatically controlled, heat-insulated cabinet which, ideally, incorporates an internal fan to prevent temperature stratification. Typical sterilizing conditions are 160–170° C/1 hour. hot–cold lysis A phenomenon in which incubation of susceptible red blood cells (RBCs) with e.g. staphylococcal b-HAEMOLYSIN at 37° C causes little or no haemolysis, but subsequent chilling to 0–4° C causes rapid and extensive haemolysis. The sensitivity of RBCs from different species depends on the SPHINGOMYELIN content of their membranes, those from sheep, goat and ox being the most sensitive. Hot–cold lysis is still not fully understood; it may reflect the combined effects of degradation of sphingomyelin in the outer leaflet of the cell membrane (with consequent condensation of the resulting ceramide into droplets in the outer layer) and the weakening of hydrophobic forces 375 hot-spot but is now referred to as ‘parvovirus B19’ or ‘B19 parvovirus’ (see on cooling [Book ref. 44, pp. 721–725]. Hot–cold lysis with staphylococcal b-toxin is most easily demonstrated in the testtube; blood agar plates may give less consistent results. Hot–cold lysis can also occur with the a-toxin of Clostridium perfringens. hot-spot (mol. biol.) A region of DNA which is particularly prone to e.g. transposition (see TRANSPOSABLE ELEMENT) or mutation (see SPONTANEOUS MUTATION). hot-start PCR A form of PCR in which an essential component of the reaction mixture is withheld, or blocked, until the temperature of the mixture has, for the first time, risen above the primer-binding temperature; the object of this procedure is to avoid mis-priming (i.e. binding of primers to inappropriate sequences) – which, in the standard form of PCR, tends to occur primarily in the initial pre-cycling stage, i.e. when all components are present but the mixture is still at room temperature. By avoidance (or minimization) of mis-priming, the hot-start technique promotes the specificity of a PCR assay; it can also promote the sensitivity of an assay by concentrating the full potential of the system on amplification of the required target sequence. Avoidance or minimization of non-specific products also serves to reduce the background against which the legitimate target is to be detected. In one commercial procedure, a chemically modified form of the polymerase (AmpliTaq Gold DNA polymerase; PerkinElmer) is initially inactive (when added to the reaction mixture) but is thermally activated at 95° C when the temperature first rises to the denaturation level. In another commercial system, the reaction mixture is prepared with all components except the polymerase; a layer of molten wax (AmpliWax ; Perkin-Elmer) is allowed to set on the surface of the mixture, and the polymerase is then added above the wax barrier. Once cycling begins, the wax melts when the temperature first exceeds ∼75–80° C; the reactants then mix, and PCR proceeds normally. [See also Nature (1996) 381 445–446.] housefly virus (HFV) A virus pathogenic in the housefly (Musca domestica); it resembles viruses of the REOVIRIDAE in having a dsRNA genome (10 genes), but is apparently not serologically related to reoviruses. [Book ref. 83, p 5.] Howell–Jolly body In some erythrocytes in a stained blood smear: a small, dark, rounded body generally considered to be a fragment of the precursor cell’s disintegrating nucleus. Hoyer’s medium (Frateur’s modification) A medium in which ethanol is the sole source of carbon, and (NH4 )2 SO4 the sole source of nitrogen; it includes K2 HPO4 , KH2 PO4 , MgSO4 , FeCl3 , biotin, calcium pantothenate, thiamine, folic acid, PABA, vitamin B12 , pyridoxal-HCl, niacin and riboflavin. (See e.g. ACETOBACTER.) HpaI A RESTRICTION ENDONUCLEASE from Haemophilus parainfluenzae; GTT/AAC. HpaII A RESTRICTION ENDONUCLEASE from Haemophilus parainfluenzae; C/CGG. HPI layer See DEINOCOCCUS. HPLC High-pressure liquid CHROMATOGRAPHY. HPr See PTS. HPr kinase See CATABOLITE REPRESSION. hprK gene (in Staphylococcus xylosus) See CATABOLITE REPRESSION. HPUra (Hpura) 6-(p-Hydroxyphenylazo)uracil: an inhibitor of DNA polymerase III in e.g. Gram-positive bacteria. HPV Human papillomavirus – see PAPILLOMA and PAPILLOMAVIRUS. (Note. ‘Human parvovirus’ was formerly referred to as HPV ERYTHROVIRUS).) HQNO HYDROXYQUINOLINE-N-OXIDE. hr mutant HOST-RANGE MUTANT. HR756 Syn. cefotaxime (see CEPHALOSPORINS). Hrp secretion pathway (plant pathol.) See AVIRULENCE GENE. HRP-2 protein See MALARIA. HS HAEMOPHAGOCYTIC SYNDROME. HSA (serol.) Human serum albumin. hsp genes Genes encoding HEAT-SHOCK PROTEINS. HSV HERPES SIMPLEX virus. HT-2 toxin See TRICHOTHECENES. HTF Mastigocladus See CHLOROGLOEOPSIS. HTLV Human T-lymphotropic virus (= human T-cell leukaemia virus, human T-cell leukaemia/lymphoma virus): a generic term for exogenous, replication-competent human retroviruses (see RETROVIRIDAE) which (at least HTLV-I and HTLV-II) can infect and transform CD4+ T LYMPHOCYTES; other types of cell (including macrophages) can also be infected. HTLV-I (formerly ‘HTLV’) was first identified in the neoplastic T cells of patients with ADULT T CELL LEUKAEMIA (ATL) and is now regarded as a causal agent of that disease; this virus has also been associated with some non-malignant diseases which include spastic paraparesis and uveitis. Infection with HTLV-I is permanent (i.e. life-long). A carrier state is recognized. Transmission of the virus appears to occur mainly via breast-feeding, sexual contact and blood transfusion. Endemic areas include Japan, the Caribbean, parts of Africa and Latin America. The virion of HTLV-I is ∼75 µm in diameter. The genome of HTLV-I integrates at random sites in the host’s DNA; transformed T cells from ATL patients contain only one, or a few, copies of the viral genome per cell. The genome of HTLV-I (∼9–10 kb) includes the genes gag, pol and env (analogous to those of other retroviruses). A further gene, pro, encodes a protease. Near the 3
end of the genome (the pX region) are several genes that include rex and tax, both of which are involved in viral replication. The genome lacks a viral oncogene. In addition to its role in viral replication, the Tax protein induces the expression of a number of host genes – including that encoding INTERLEUKIN-2 and the ONCOGENE c-fos. Expression of such host proteins may promote leukaemogenesis. (In vitro, HTLV-I is reported to transform normal human peripheral blood T cells, the transformed cells continuing to proliferate in the absence of exogenous interleukin-2.) HTLV-I is closely related to SIMIAN T-CELL LEUKAEMIA VIRUS (strains of which are >85% homologous with HTLV-I in terms of nucleotide sequence); it is also related to bovine leukosis virus (BLV). HTLV-II has been rarely isolated – initially from a T cell line derived from a case of (T cell) HAIRY CELL LEUKAEMIA [Science (1982) 218 571–573]. The nucleotide sequence of HTLV-II is 70% homologous with that of HTLV-I. The pathogenic potential of HTLV-II is uncertain, although one report described a patient with disease closely resembling HTLV-I-associated myelopathy in a patient infected with HTLV-II. ‘HTLV-III’ was a former name for the human immunodeficiency virus (HIV). Another human T-cell lymphotropic retrovirus – related to but distinct from other HTLVs – was detected in some cases of MULTIPLE SCLEROSIS (MS), but an aetiological role (if any) for such a virus in MS has yet to be determined [Nature (1985) 318 154–160]. 376 hybridization Hungate technique See ROLL-TUBE TECHNIQUE. Hungate tube A gas-tight, tube-shaped glass vessel with a rubber stopper which is held in place by a screw cap; it is used in the ROLL-TUBE TECHNIQUE. hupA, hupB genes In Escherichia coli, the genes which encode the two subunits of the HU PROTEIN. Huroniospora See STROMATOLITES. HUS See HAEMOLYTIC URAEMIC SYNDROME. hut genes Genes involved in histidine utilization (see HISTIDINE DEGRADATION). Hutchinson’s teeth See SYPHILIS. HUVEC Human umbilical vascular endothelial cell. HVR-1 (in hepatitis C virus) See HEPATITIS C VIRUS. hyaline Transparent, translucent, or colourless. hyaline cap See PSEUDOPODIUM. hyalodendrins See EPIPOLYTHIAPIPERAZINEDIONES. hyalodidymae See SACCARDOAN SYSTEM. hyaloplasm (1) Syn. CYTOSOL. (2) See SARCODINA. Hyalospora See UREDINIOMYCETES and AMPHISPORE. Hyalotheca A genus of filamentous placoderm DESMIDS. The filaments are composed of chains of cylindrical, flat-ended cells; each cell has only a slight median constriction. hyaluronan Syn. HYALURONIC ACID. hyaluronate lyase (‘spreading factor’; EC 4.2.2.1) A LYASE which cleaves HYALURONIC ACID at the (1 → 4)-b-linkages; the end product is a disaccharide containing a 4,5-unsaturated uronic acid residue. (cf. HYALURONIDASE). It is produced e.g. by most coagulase +ve staphylococci, Streptococcus pneumoniae, S. pyogenes and Clostridium perfringens (µ toxin); it may promote invasiveness. It can be assayed by its ability to reduce viscosity in a solution of hyalauronic acid. [Action of enzyme from S. pneumoniae: EMBO (2000) 19 1228–1240.] hyaluronic acid A linear polysaccharide in which repeating disaccharide units of b-D-glucuronopyranosyl-(1 → 3)-N-acetyl-Dglucosamine are connected by (1 → 4)-b-linkages. Hyaluronic acid occurs e.g. as an intercellular constituent of various animal tissues, in synovial fluid, and in the capsules of certain group A streptococci. Hyaluronic acid forms highly viscous solutions in water and readily forms gels. hyaluronidase Any enzyme which cleaves HYALURONIC ACID; they include HYDROLASES and LYASES. The hydrolases include hyaluronate 4-glucanhydrolase (EC 3.2.1.35) – present e.g. in testicular extracts and produced e.g. by certain streptomycetes. The name ‘hyaluronidase’ is often used (erroneously) as a synonym for the enzyme HYALURONATE LYASE. hybrid antibody See CONJUGATION (2). hybrid-arrested translation A method for identifying a protein encoded by a given cDNA. mRNAs from the cell are translated in the presence of cloned, single-stranded copies of the cDNA. mRNAs which do not bind cDNA yield proteins incorporating a labelled amino acid. Translation is repeated without cDNAs, allowing translation of the mRNA previously blocked by hybridization to cDNA. The protein of interest is the extra protein formed in the second translation. hybrid plasmid (1) Any recombinant PLASMID (see e.g. CLONING). (2) A recombinant plasmid containing sequences derived from organisms which can normally exchange genetic information. (cf. CHIMERIC PLASMID.) hybridization (1) The formation of a double-stranded nucleic acid by base-pairing between single-stranded nucleic acids derived (usually) from different sources; some authors use the term specifically for the association of ssRNA with ssDNA. (See also DNA HOMOLOGY and SOUTHERN HYBRIDIZATION.) [Early review (HTLV-I -II -III): Nature (1985) 317 395–403. Human T-cell leukaemia virus: BCH (1995) 8 131–148. Detection of HTLV-I and HTLV-II by multiplex PCR: PNAS (1999) 96 6394–6399. Human T-lymphotropic virus type I infection: BCH (2000) 13 231–243.] htpR gene See HEAT-SHOCK PROTEINS. HTST See APPERTIZATION and PASTEURIZATION. HU HYDROXYUREA. HU protein A small, basic protein, abundant in Escherichia coli, which binds strongly to angular/kinked dsDNA and which can inhibit cruciform extrusion from PALINDROMIC SEQUENCES in vitro. It functions e.g. in replication from oriC in E. coli, in some SITE-SPECIFIC RECOMBINATION systems, and in Tn10 transposition. It resembles mammalian HMG1 protein (whose DNA-binding is more structure- than sequence-dependent) [Mol. Microbiol. (1993) 7 343–350]. The HU protein is reported to be involved in regulating the expression of gene rpoS (encoding SIGMA FACTOR sS ) [Mol. Microbiol. (2001) 39 1069–1079]. Hucker’s method See GRAM STAIN. Huffia A subgenus of PLASMODIUM. Hugh and Leifson’s test OXIDATION–FERMENTATION TEST. Hughes press An apparatus for CELL DISRUPTION. A frozen suspension or paste of cells in a cylinder is forced by hydraulic pressure (up to ca. 30 tons/inch2 , i.e., ca. 480 MPa) through a small hole in the containing vessel; as the cells emerge they are broken by the solid shear forces due to the presence of intracellular ice crystals. The frozen sample may incorporate an abrasive (e.g. finely powdered glass) for more efficient cell breakage. Huilia See LECIDEA. hülle cell See EMERICELLA. human (alpha) herpesviruses See ALPHAHERPESVIRINAE. human (beta) herpesviruses See BETAHERPESVIRINAE. human caliciviruses See SMALL ROUND STRUCTURED VIRUSES. human coronavirus See CORONAVIRIDAE. human cytomegalovirus See BETAHERPESVIRINAE. human diploid cell vaccine (HDCV) See RABIES. human enteric coronavirus See CORONAVIRIDAE. human foamy viruses See SPUMAVIRINAE. human (gamma) herpesviruses See GAMMAHERPESVIRINAE. human genome (sequence) See GENOME. human herpesvirus (HHV) Any virus of the family HERPESVIRIDAE which can infect humans; HHVs occur in each of the three subfamilies ALPHAHERPESVIRINAE, BETAHERPESVIRINAE and GAMMAHERPESVIRINAE. (See also HHV.) human immunodeficiency virus See HIV. human papilloma virus See PAPILLOMAVIRUS. human parvovirus See PARVOVIRUS. human T-cell leukaemia/lymphoma virus See HTLV. human T-lymphotropic virus See HTLV. Humicola See HYPHOMYCETES; see also COMPOSTING and SOFT ROT (sense 1). humicolous Growing in or on soil or humus. humoral antibodies Antibodies present (free) in plasma and in other body fluids. humoral immunity (1) ANTIBODY-dependent IMMUNITY; humoral immunity can be transferred to a non-immune individual by the transfer of cell-free plasma or serum. (cf. CELL-MEDIATED IMMUNITY; see also IMMUNIZATION (sense 1); ADCC; JONES–MOTE SENSITIVITY.) (2) Immunity derived from any factor(s) in the body fluids – e.g. antibodies, LYSOZYME, COMPLEMENT. humus tank See SEWAGE TREATMENT. 377 hybridization protection assay hydrocarbons Compounds which contain only carbon and hydrogen; they occur e.g. in PETROLEUM. Many types of hydrocarbon can be used as substrates for growth by various microorganisms. Such organisms are ecologically important e.g. in the degradation of petroleum pollutants, and some may be commercially useful e.g. in the production of SINGLE-CELL PROTEIN from hydrocarbons; however, certain hydrocarbon-utilizing organisms can cause spoilage of hydrocarbon products such as fuels (see PETROLEUM). Hydrocarbon metabolism is strictly aerobic and appears always to involve the introduction of oxygen into the molecule in a process requiring a monooxygenase (= hydroxylase) or dioxygenase (see OXYGENASE). Aliphatic hydrocarbons. Straight-chain paraffins (n-alkanes) can be utilized by bacteria (e.g. strains of Acinetobacter, Corynebacterium, Mycobacterium, Nocardia, Pseudomonas), by yeasts (e.g. species of Candida), and by mycelial fungi (e.g. species of Aspergillus, Botrytis, Fusarium, Helminthosporium, Hormoconis, Penicillium). Some organisms can use only shortchain alkanes, some can use only long-chain alkanes. In most cases n-alkane metabolism appears to occur by !-oxidation, i.e., a terminal methyl group of the alkane is oxidized by a monooxygenase to form a primary alcohol which is in turn (apparently) oxidized, via the aldehyde, to the corresponding fatty acid by alcohol dehydrogenase and aldehyde dehydrogenase activities; the fatty acid can then be degraded by conventional b-oxidation. In e.g. Pseudomonas strains the alkane-oxidizing enzyme system is complex, involving an !-monooxygenase, a rubredoxin (see IRON–SULPHUR PROTEINS), and an NADH-rubredoxin oxidoreductase; in eukaryotes, and possibly in certain bacteria (e.g. Acinetobacter strains [FEMS (1981) 11 309–312]), the alkane monooxygenase is linked to the cytochrome P-450 electroncarrier system (see CYTOCHROMES). (See also METHANOTROPHY.) In some organisms an alkane may be oxidized at both ends, resulting in the formation of a dicarboxylic acid. Subterminal oxidation may also occur. Branched-chain alkanes (alkylalkanes) and unsaturated hydrocarbons (alkenes, olefins) are generally somewhat less susceptible to microbial degradation than are n-alkanes; they may be oxidized in the same way as n-alkanes, but alkenes may also be oxidized at the double bond, resulting in the formation of a diol. Organisms growing on alkanes have certain characteristic structural features. For example, yeasts growing on hydrocarbons generally contain numerous PEROXISOMES which are apparently involved in fatty acid metabolism rather than in alkane oxidation per se. Alkane-utilizing bacteria generally contain intracytoplasmic membranes together with ‘hydrocarbon inclusions’: electron-translucent, spherical bodies (ca. 0.2 µm diam.) limited by a non-unit-type (monolayer) membrane; the inclusions occur at the cell periphery or in close association with the intracytoplasmic membranes, and apparently contain unmodified alkane, protein, phospholipid and neutral lipid. Since alkane metabolism occurs intracellularly, the hydrocarbon must be taken into the cell. Uptake may occur by different mechanisms in different organisms, and in at least some cases may require prior emulsification of the hydrocarbon by an extracellular BIOSURFACTANT or bioemulsifier. Alicyclic hydrocarbons (cycloparaffins, cycloalkanes) are cyclic, non-aromatic hydrocarbons; they are generally less susceptible to microbial attack than either aliphatic or aromatic compounds. A suggested pathway for the degradation of e.g. cyclohexane by strains of Nocardia or Pseudomonas involves oxidation of the cyclohexane to cyclohexanol by a cyclohexane monooxygenase; cyclohexanol is oxidized to cyclohexanone, (2) The formation of e.g. a HYBRID PLASMID. (3) The formation of a hybrid cell (see e.g. HYBRIDOMA), or of a hybrid organism (e.g. by a cross between genetically dissimilar organisms). hybridization protection assay See TMA. hybridoma The product and/or progeny of cell fusion (‘somatic cell hybridization’) between a tumour cell and a non-tumour cell; the in vitro production of hybridomas is carried out to provide continuously replicating (hybrid) cells which exhibit some or all of the characteristics of the non-tumour cell. Hybridomas have been formed, for example, between normal B LYMPHOCYTES and MYELOMA cells; such hybridomas are used e.g. as sources of MONOCLONAL ANTIBODIES. In one method of B cell hybridoma formation, spleen (or lymph node) cells are mixed with similar numbers of myeloma cells and centrifuged. The cell pellet is briefly exposed to polyethylene glycol (PEG) which promotes cell fusion. (Lysolecithin or inactivated Sendai virus has been used in place of PEG.) (cf. ELECTROFUSION.) The PEG is diluted out, and the fusion mixture is then centrifuged; the pellet is re-suspended in a growth medium, divided into aliquots, and incubated. Small cluster(s) of hybridoma cells appear in the aliquots after ca. 1 week. (The number of hybridomas depends e.g. on the initial cell density: the fusion rate may be e.g. 1 in 104 .) In order to prevent the culture from being overgrown by the myeloma cells, use is made of (pre-selected) mutant myeloma cells which are blocked in one of their two (normal) pathways for nucleotide synthesis; such mutants lack either thymidine kinase (TK− mutants) or hypoxanthine-guaninephosphoribosyltransferase (HGPRT− mutants) so that they cannot use the ancillary (‘salvage’) pathway for nucleotide synthesis and (hence) cannot use exogenous thymidine or hypoxanthine (or guanine). Accordingly, to suppress the growth of non-fused myeloma cells, the fusion mixture is incubated in a medium (HAT medium) which contains aminopterin (a folic acid analogue which blocks the main pathway of nucleotide synthesis) together with hypoxanthine and thymidine; thus, the main pathway for nucleotide synthesis is blocked in both myeloma and non-myeloma cells, while fused (hybridoma-forming) myeloma cells can continue to grow by using TK or HGPRT supplied by their non-mutant partner cells. In order to prevent the formation of hybrid Ig (molecules which contain heavy and/or light chains from each of both partner cells), use is made of mutant myeloma cells which do not secrete Ig components. Activated or proliferating B cells fuse to myeloma cells more readily than do quiescent B cells; hence, prior to hybridoma formation, B cells of the required antibody specificity should be activated e.g. by antigenic challenge. (The consequent expansion of particular B cell clone(s) will also increase the incidence with which the cells of such clones participate in hybridoma formation.) T cell hybridomas have been used e.g. in the study of cytokines, and for characterization of the different T LYMPHOCYTE subsets. Hydnaceae See APHYLLOPHORALES. Hydnum See APHYLLOPHORALES (Hydnaceae). Hydra viridis See ZOOCHLORELLAE. Hydramoeba A genus of amoebae (order AMOEBIDA) which occur both free-living and as parasites in the gastrodermis of Hydra sp. hydrocarbon inclusions See HYDROCARBONS. 378 hydrogen swell and then an oxygen atom is introduced into the ring (forming a lactone) by a cyclohexanone monooxygenase. The lactone can then be hydrolysed to form a (non-cyclic) dicarboxylic acid. Aromatic hydrocarbons (benzene, naphthalene, anthracene etc) are present in e.g. petroleum and are formed by the incomplete combustion of almost any organic material; they are therefore common pollutants, and many are recognized carcinogens. Bacteria (e.g. Pseudomonas spp) metabolize aromatic hydrocarbons by initially incorporating two atoms of oxygen into the substrate to form a cis-dihydrodiol; the reaction is catalysed by a multicomponent enzyme system comprising a dioxygenase, a flavoprotein, and IRON–SULPHUR PROTEINS. The cis-dihydrodiol is oxidized to a catechol which is in turn a substrate for another dioxygenase system which breaks open the aromatic ring. In contrast, fungi oxidize aromatic hydrocarbons using a cytochrome P-450-dependent monooxygenase to form a reactive arene oxide; this can either undergo isomerization to form a monohydric phenol, or enzymic hydrolysis to form a trans-dihydrodiol. [Reviews on microbial hydrocarbon metabolism: Book ref. 155.] Hydroclathrus See PHAEOPHYTA. Hydrodictyon A genus of freshwater green algae (division CHLOROPHYTA) which form free-floating net-like colonies (‘water nets’) composed of large, coenocytic cells. hydrogen (as a metabolite) Gaseous (molecular) hydrogen is produced e.g. in various types of FERMENTATION (sense 1), and is formed during NITROGENASE activity. Gaseous hydrogen may be consumed e.g. during METHANOGENESIS and may be used for lithotrophic metabolism by e.g. HYDROGEN-OXIDIZING BACTERIA and some SULPHATE-REDUCING BACTERIA. [Production and uptake of molecular hydrogen by unicellular cyanobacteria: JGM (1985) 131 1561–1569.] (See also FORMATE HYDROGEN LYASE and INTERSPECIES HYDROGEN TRANSFER.) hydrogen bacteria Syn. HYDROGEN-OXIDIZING BACTERIA. hydrogen dehydrogenase Syn. HYDROGENASE. hydrogen hypothesis See EUKARYOTE. hydrogen-oxidizing bacteria (the ‘hydrogen bacteria’; knallgas bacteria) A phrase sometimes used to refer specifically to a (non-taxonomic) category of aerobic bacteria which can grow chemolithoautotrophically by obtaining energy from the oxidation of gaseous hydrogen by oxygen via an electron transport chain (see KNALLGAS REACTION); most species can also grow chemoorganoheterotrophically and/or mixotrophically. [Details of the obligately chemolithoautotrophic hydrogenoxidizing species Hydrogenobacter thermophilus (gen. nov., sp. nov.): IJSB (1984) 34 5–10.] The hydrogen-oxidizing bacteria, sensu stricto, are distinct from those bacteria which can oxidize H2 without autotrophic CO2 fixation and/or which can carry out anaerobic oxidation of H2 coupled to the reduction of e.g. SO4 2− , CO2 or fumarate. The hydrogen-oxidizing bacteria (which include most or all CARBOXYDOBACTERIA) include e.g. those strains of Alcaligenes denitrificans formerly called A. ruhlandii ; Aquaspirillum autotrophicum; Bacillus schlegelii [JGM (1979) 115 333–341]; Paracoccus denitrificans; Pseudomonas facilis and P. saccharophila; and bacteria referred to as Alcaligenes eutrophus, Nocardia autotrophica, and N. opaca. Most hydrogen-oxidizing bacteria (including e.g. Paracoccus denitrificans) have only a membrane-bound, NAD-independent HYDROGENASE (see also EXTRACYTOPLASMIC OXIDATION); some strains have only a soluble (cytoplasmic), NAD-reducing hydrogenase (‘hydrogen dehydrogenase’), while others (e.g. strains of ‘Alcaligenes eutrophus’) have both forms of the enzyme. [Habitats, culture, and descriptions of individual hydrogenoxidizing bacteria: Book ref. 45, pp. 865–893.] hydrogen peroxide (a) (as an antimicrobial agent) Hydrogen peroxide (H2 O2 ) is an OXIDIZING AGENT which can be an effective disinfectant at low concentrations (e.g. 0.1% or less) and can act as a sterilant (for inanimate objects) at high concentrations (e.g. 10–25%); sporicidal activity at high concentrations is enhanced by ultrasonic energy and by certain metal ions, particularly Cu2+ . Dilute aqueous solutions of H2 O2 are used for the treatment of wounds; here, antimicrobial activity may be largely mechanical – tissue CATALASE causing a rapid evolution of oxygen which can dislodge contaminating foreign matter, thus facilitating its removal. However, the strongly oxygenated environment thus created in the wound can inhibit the development of anaerobic pathogens. The antimicrobial action of H2 O2 may be due to its reduction to the highly reactive HYDROXYL RADICAL which can react e.g. with membrane lipids and nucleic acids. [Book ref. 65, pp. 240–244.] Escherichia coli and Salmonella typhimurium, when exposed to sublethal concentrations of H2 O2 , acquire resistance to higher doses of H2 O2 and other oxidants; in S. typhimurium this ‘adaptation’ to oxidative stress involves the induction of various proteins, some of which are apparently encoded by a regulon under the positive control of a locus designated oxyR [Cell (1985) 41 753–762]. [Toxicity, mutagenesis and stress responses induced in E. coli by H2 O2 : JCS (1987) Supplement 6 289–301.] Peracids (acids containing the peroxy group, −O−O−) – which can be regarded as derivatives of H2 O2 – are oxidizing agents whose activity is greater than that of H2 O2 . Peracetic acid (CH3 .COO.OH) is the most active antimicrobial agent of the organic peracids, and can act as a sterilant at quite low concentrations (e.g. 1% or less). An aqueous solution of sodium perborate (NaBO3 ) acts as a mixture of borate and hydrogen peroxide; a paste of sodium perborate in water or glycerol has been used for the treatment of oral infections involving anaerobes (e.g. Vincent’s angina). [H2 O2 and peracetic acid as antimicrobial agents: JAB (1983) 54 417–423.] (b) (as a metabolite) H2 O2 is produced e.g. by the spontaneous or enzymatic dismutation of SUPEROXIDE (see also SUPEROXIDE DISMUTASE). Endogenously formed H2 O2 may be inactivated, intracellularly, by e.g. CATALASE or GLUTATHIONE PEROXIDASE, thus preventing the toxic effects of H2 O2 (H2 O2 can inactivate some types of superoxide dismutase, and can give rise to HYDROXYL RADICAL and SINGLET OXYGEN). However, extracellular H2 O2 , produced by the activities of certain organisms, may be important e.g. in LIGNIN degradation and in BROWN ROT (sense 1). (See also DIANISIDINE and LACTOPEROXIDASE–THIOCYANATE–HYDROGEN PEROXIDE SYSTEM.) hydrogen sulphide (H2 S) Certain bacteria can produce H2 S e.g. by the reductive degradation of sulphur-containing amino acids (e.g. cysteine) or by DISSIMILATORY SULPHATE REDUCTION; H2 S production can be plasmid mediated – see e.g. Hys plasmid in ESCHERICHIA. (See also KLIGLER’S IRON AGAR and TSI AGAR.) H2 S can be formed from sulphite by Saccharomyces cerevisiae [JGM (1985) 131 1417–1424]. H2 S can be used by some bacteria as an electron donor in lithotrophic metabolism; these bacteria include e.g. species of Beggiatoa and Thiobacillus, members of the Rhodospirillaceae and other photosynthetic bacteria, and the cyanobacterium Oscillatoria. (See also SULPHUR CYCLE.) hydrogen swell A SWELL caused by hydrogen production due to internal corrosion of the can (see also TIN). Hydrogen swells, 379 hydrogenase thus, e.g. prokaryotes occur in the gill cells of the giant clam (Calyptogena magnifica) and in the TROPHOSOME tissue of the giant tube worm, Riftia pachyptila. A living community at a given vent appears to last for several years to several decades. [Biology and microbiology of hydrothermal vents: Science (1985) 229 713–725; evidence for chemosynthetic primary production in hydrothermal vents: Book ref. 202, pp. 319–360.] hydroxamates (in iron uptake) See SIDEROPHORES. hydroxyapatite (hydroxylapatite; Ca10 (PO4 )6 (OH)2 ) A mineral which is used as a stationary phase in adsorption CHROMATOGRAPHY e.g. for separating ssDNA from dsDNA or RNA. Under certain ionic conditions the mineral adsorbs dsDNA; the dsDNA is desorbed at higher ionic concentrations. hydroxycobalamin See VITAMIN B12 . hydroxyethylcellulose See HE-CELLULOSE. hydroxyl radical (OH· or ·OH) A hyper-reactive radical formed e.g. during a reaction involving SUPEROXIDE and HYDROGEN PEROXIDE (the ‘Haber–Weiss reaction’); this reaction can be catalysed by iron complexes (e.g. Fe–EDTA, Fe–transferrin) and may be represented in two phases: (i) the reduction of ferric (with the formation of ferrous complexes and complexes by O− 2̇ SINGLET OXYGEN), and (ii) the oxidation of ferrous complexes by H2 O2 (with the formation of ferric complexes, OH− and OH·) [FEBS (1978) 86 139–142]. (Hydroxyl radical is also reported to be formed e.g. in a reaction involving superoxide and hypochlorite, and by the action of ultraviolet radiation on hydrogen peroxide.) During PHAGOCYTOSIS, LACTOFERRIN from activated NEUTROPHILS may catalyse the formation of OH·, thus enhancing antimicrobial activity. Scavengers of OH· used in experimental systems include e.g. benzoate, dimethyl sulphoxide, mannitol and salicylate. hydroxylamine (as a MUTAGEN) Hydroxylamine (NH2 OH) reacts mainly with cytosine residues (and, much more slowly, with adenine residues) in DNA (particularly ssDNA), replacing the amino group with a hydroxyamino group (−NHOH); this favours tautomerization of the base such that, during subsequent DNA replication, mispairing occurs (resulting predominantly in G·C to A·T transitions). Although hydroxylamine is an effective mutagen when used to treat certain viruses or e.g. transforming DNA, any mutagenic effect it may have in living cells is generally masked by its toxic or lethal effects. The related compound methoxyamine (NH2 OCH3 ) acts in a similar way. hydroxylapatite Syn. HYDROXYAPATITE. hydroxylase Syn. MONOOXYGENASE. hydroxymethionine analogue See HMA. 6-(p-hydroxyphenylazo)uracil See HPURA. hydroxypyrimidine antifungal agents A group of agricultural systemic ANTIFUNGAL AGENTS which have a highly selective action against powdery mildews; resistance to these fungicides tends to develop rapidly. Hydroxypyrimidines include e.g. DIMETHIRIMOL and ETHIRIMOL. 8-hydroxyquinoline (oxine; 8-quinolinol) (as antimicrobial agent) A chelating agent which, in the presence of Cu2+ or Fe2+ , has antifungal and antibacterial activity; Gram-positive bacteria are much more susceptible than Gram-negative bacteria. The potentiating effects of Cu2+ and Fe2+ are antagonized by trace amounts of Co2+ , while Ni2+ acts as a competitive inhibitor. COPPER –oxine has been used in agricultural antifungal sprays and as a rot-proofing agent for e.g. textiles. Some halogenated derivatives of oxine are used e.g. for the treatment or prophylaxis of amoebic dysentery. They include characterized by a high proportion of hydrogen in the headspace, may take months to develop; they are commonly associated with certain acidic fruits and with foods containing curing salts. hydrogenase An OXIDOREDUCTASE (EC 1.12.–. – ) which catalyses the reaction: H2 ↔ 2H+ + 2e− . Hydrogenases occur in various algae (see e.g. PHOTOREDUCTION) and e.g. in HYDROGENOXIDIZING BACTERIA, some SULPHATE-REDUCING BACTERIA, some nitrogen-fixing bacteria (see NITROGENASE), Chloroflexus aurantiacus [FEMS (1985) 28 231–235], and Escherichia coli (e.g. as part of the FORMATE HYDROGEN LYASE system). Hydrogenases are IRON–SULPHUR PROTEINS, and in many species of bacteria they have been shown to contain nickel. Hydrogen-oxidizing bacteria may contain a membrane-bound NAD-independent hydrogenase (see also EXTRACYTOPLASMIC OXIDATION) and/or a soluble NAD+ reducing hydrogenase (‘hydrogen dehydrogenase’). [Hydrogenases – their structure and applications in hydrogen production: Book ref. 132, pp. 75–102.] (See also KNALLGAS REACTION.) Hydrogenobacter See HYDROGEN-OXIDIZING BACTERIA. hydrogenogen See ACETOGEN. Hydrogenomonas An obsolete bacterial genus which included certain species of HYDROGEN-OXIDIZING BACTERIA (e.g. H. facilis, now Pseudomonas facilis). hydrogenosome A membrane-limited intracellular organelle present in certain eukaryotic microorganisms – e.g. trichomonads (but not e.g. Entamoeba histolytica); in trichomonads it contains various IRON–SULPHUR PROTEINS and flavoproteins as components of an electron transport chain that is used in the oxidation of pyruvate to acetate, CO2 and H2 (protons being used as electron acceptors). In isolated hydrogenosomes of Trichomonas vaginalis, METRONIDAZOLE is reduced, anaerobically, to an active (cytotoxic) derivative [JGM (1985) 131 2141–2144]; in vivo such active product(s) may damage the hydrogenosome itself and/or may leave the hydrogenosome and affect other target(s). (cf. MICROBODY.) hydrolases ENZYMES (EC class 3) which catalyse the hydrolytic cleavage of bonds, including C−O, C−N and C−C bonds. The names of such enzymes are commonly formed by adding ‘-ase’ to the name of the substrate (e.g. esterase, glycosidase, peptidase); the systematic name has the form substrate hydrolase. hydrophobia Syn. RABIES. hydrops fetalis (non-immune) A condition in which the fetus develops e.g. anaemia and oedema; death may result from severe anaemia. In a proportion of cases hydrops fetalis is due to infection by parvovirus B19 (see ERYTHROVIRUS). hydroresorufin See RESAZURIN TEST. hydrothermal vent In certain geographical locations: a region of the ocean floor where seawater, which has permeated the earth’s crust for several kilometres, emerges as a warm (5–25° C) or hot (270–380° C) hydrothermal fluid which is particularly rich in sulphide; vents which discharge hot fluids ≤300° C are called ‘white smokers’, while those discharging fluids at ca. 350° C are called ‘black smokers’ – an allusion to the clouds of black metal sulphides formed when these fluids meet the cold (2° C) ambient seawater. One of the best-studied vents is situated at the Galápagos Rift (near the equator at 86° W). The sulphide (and e.g. hydrogen, methane and ferrous iron) in hydrothermal fluids supports the chemolithotrophic growth of various bacteria (e.g. species of THIOBACILLUS and THIOMICROSPIRA) – thus permitting chemosynthetic PRIMARY PRODUCTION; these bacteria form the basis of a food chain which supports communities of e.g. giant clams, limpets, mussels and tube worms in and around the vents. Some of the lithotrophic prokaryotes form symbiotic associations with some of the invertebrates; 380 hypha hypericin See STENTORIN. hyper-IgM syndrome See RNA EDITING. hyperimmune Refers to the condition of an individual whose plasma contains a high titre of a particular antibody following repeated exposure of the individual to the homologous antigen. Hyperimmune serum is serum derived from such an individual. Hypermastigida An order of protozoa (class ZOOMASTIGOPHOREA) which have numerous flagella, the kinetosomes being arranged in complete or incomplete circles; the organisms are associated with insects. Genera: e.g. Barbulanympha, Lophomonas, Microjoenia, Spirotrichonympha, TRICHONYMPHA. hyperparasite A parasite of a parasite. hyperplasia The enlargement of an organ or tissue owing to an increase in the number of cells. (Hence adj. hyperplastic.) (cf. HYPERTROPHY and NEOPLASIA.) hypersensitivity (1) (immunol.) The state of a PRIMED individual who, on further exposure to the relevant antigen, gives an exaggerated immune response that causes varying degrees of harm – ranging from a mild local inflammatory reaction to death. (See IMMEDIATE HYPERSENSITIVITY and DELAYED HYPERSENSITIVITY; see also BRUCELLOSIS.) (2) (plant pathol.) The expression of extreme reactivity by a plant in response to a potential parasite or pathogen, the plant’s response commonly serving to limit or prevent parasitization/disease. Hypersensitivity typically involves rapid cell death at the (localized) site of infection and the concomitant induction of e.g. PHYTOALEXIN(s) by microbial elicitors; the induction of phytoalexins by microbial elicitors involves the expression of new host genes and the formation of new species of mRNA and new enzymes. (In addition to phytoalexin production, microbial elicitors can also apparently e.g. stimulate the formation of ETHYLENE and bring about the accumulation of LIGNIN.) (See also PATHOGENESIS-RELATED PROTEINS.) In many cases the combination of cell death and accumulation of phytoalexin(s) at the site of infection appears to provide the level of resistance needed to prevent disease development. When the challenging microorganism is an obligate parasite (i.e., a BIOTROPH), the death of host cells may, in itself, be sufficient to prevent the spread of the parasite to uninfected tissues. hypersensitivity pneumonitis Syn. EXTRINSIC ALLERGIC ALVEOLITIS. hypertrophy Enlargement of an organ or tissue owing to an increase in the size of pre-existing cells. (cf. HYPERPLASIA.) hypervariable region (immunol.) Any of several amino acid sequences, located in the VARIABLE REGIONS of the heavy chain and light chain of IMMUNOGLOBULINS, which exhibits a degree of variability (in composition) greater than that found in other parts of the variable region. (See also COMPLEMENTARITY-DETERMINING REGIONS.) hypha (pl. hyphae) (1) (bacteriol., mycol.) In many (mycelial) fungi and in some bacteria (see ACTINOMYCETALES): a branched or unbranched filament, many of which together constitute the vegetative form of the organism and (in some species) form the sterile portion of a fruiting body; a mass of vegetative hyphae is referred to as a MYCELIUM. Fungal hyphae. Each hypha has a tubular CELL WALL which, in many species, is divided into compartments or cells by crosswalls (see SEPTUM (b)); in both septate and aseptate species, the cell wall is external to the CYTOPLASMIC MEMBRANE which encloses the cytoplasm, NUCLEUS or nuclei, and other components typical of eukaryotic organization. (See also VACUOLE.) The diameter of a hypha ranges from about 1 µm to macroscopic dimensions, depending e.g. on species. Hyphae grow mainly by apical extension – see GROWTH (fungal). 7-iodo-8-hydroxyquinoline-5-sulphonate (chiniofon); 5,7-diiodo8-hydroxyquinoline (diiodohydroxyquin; Diodoquin); 5-chloro7-iodo-8-hydroxyquinoline (iodochlorohydroxyquin; Enterovioform; Vioform). (Some of these derivatives also have useful antibacterial and antifungal properties.) hydroxyquinoline-N-oxide (HOQNO; HQNO) 2-Alkyl-4-hydroxyquinoline-N-oxides (e.g. 2-n-heptyl-4-HOQNO) act as RESPIRATORY INHIBITORS in mitochondria and in certain bacteria – some bacteria are apparently impermeable to HOQNOs; HOQNOs appear to resemble ANTIMYCIN A (q.v.) in their mode of action. HOQNOs also inhibit the Na+ pump in Vibrio alginolyticus (see SODIUM MOTIVE FORCE). 2-hydroxystilbamidine An aromatic DIAMIDINE which has trypanocidal and antifungal activity; it has been used e.g. in the treatment of blastomycosis and African trypanosomiasis. 5-hydroxytryptamine Syn. SEROTONIN. hydroxyurea (HU; HONH.CO.NH2 ) A reagent which prevents the synthesis of deoxyriboNUCLEOTIDES by specifically inhibiting ribonucleoside diphosphate reductase. Hyella See PLEUROCAPSA GROUP. Hygrocybe See AGARICALES (Hygrophoraceae). hygromycin B An AMINOGLYCOSIDE ANTIBIOTIC which, like STREPTOMYCIN, appears to act at a single ribosomal site. hygrophilic (hygrophilous) Requiring a moist habitat for optimum growth. Hygrophoraceae See AGARICALES. Hygrophorus See AGARICALES (Hygrophoraceae). hymenium (mycol.) A layer of ascospore- or basidiosporeforming tissue – e.g. that lining an APOTHECIUM or that forming the surface of a LAMELLA; a hymenium may also contain sterile structures – see e.g. PARAPHYSIS and PSEUDOPARAPHYSIS. Hymenochaetaceae See APHYLLOPHORALES. Hymenochaete See APHYLLOPHORALES (Hymenochaetaceae). Hymenogaster See GASTEROMYCETES (Hymenogastrales). Hymenogastrales See GASTEROMYCETES. Hymenomonas See COCCOLITHOPHORIDS. Hymenomycetes A class of fungi (subdivision BASIDIOMYCOTINA) in which the typical fruiting body is a well-developed, macroscopic, gymnocarpous or semiangiocarpous structure containing BALLISTOSPORE-bearing basidia in a HYMENIUM. Subclasses: HOLOBASIDIOMYCETIDAE and PHRAGMOBASIDIOMYCETIDAE. hymenophore That part of a fruiting body which bears a HYMENIUM or SUBHYMENIUM, or the entire (hymenium-bearing) fruiting body. (cf. SPOROPHORE.) Hymenostomatia A subclass of (mostly) freshwater ciliates (class OLIGOHYMENOPHOREA) in which somatic ciliature is often uniform, the buccal cavity (when present) is ventral, and kinetodesmata are usually conspicuous; sedentary and colonial types are not common. Orders: ASTOMATIDA, HYMENOSTOMATIDA, SCUTICOCILIATIDA. Hymenostomatida An order of protozoa (subclass HYMENOSTOMATIA) in which the buccal cavity is well defined and contains characteristic membranelles. A SCUTICA does not appear during stomatogenesis. Genera (which may be distinguished e.g. by the number of post-oral kineties) include e.g. COLPIDIUM, Frontonia, GLAUCOMA, ICHTHYOPHTHIRIUS, LAMBORNELLA, Ophryoglena, PARAMECIUM and TETRAHYMENA. hyperauxinic Containing or producing abnormally high levels of AUXINS. hyperchromic shift A change in the amount of ultraviolet radiation absorbed by a nucleic acid during changes from the double-stranded to the single-stranded condition, or vice versa (see e.g. THERMAL MELTING PROFILE). 381 hyphal body In basidiomycetes the primary hyphae are uninucleate haploid hyphae which develop when basidiospores germinate; the secondary (dikaryotic) hyphae develop following DIKARYOTIZATION. A basidiocarp may, according to species, consist of one, two or three distinct types of hypha – corresponding, respectively, to a monomitic, dimitic or trimitic sporocarp. A monomitic sporocarp contains only generative hyphae: typically thin-walled, branched, septate hyphae which give rise to basidia; generative hyphae may contain CLAMP CONNECTIONS if the organism normally forms clamps on its vegetative mycelium. A dimitic sporocarp contains generative hyphae together with either skeletal hyphae or binding hyphae (both of which originate from generative hyphae). Skeletal hyphae are typically thick-walled and unbranched; they are sterile (i.e., do not form basidia) and do not form clamps. Binding (= ligative) hyphae are sterile, highly branched, and do not form clamps; in trimitic sporocarps they bind together the generative and skeletal hyphae. (2) (bacteriol.) See PROSTHECA. hyphal body In some species of the ENTOMOPHTHORALES: a piece of fragmented mycelium which can germinate to form a sporebearing structure. hyphenated dyad symmetry See e.g. PALINDROMIC SEQUENCE. Hyphochytriaceae See HYPHOCHYTRIOMYCETES. Hyphochytriales See HYPHOCHYTRIOMYCETES. Hyphochytridiomycetes See HYPHOCHYTRIOMYCETES. Hyphochytriomycetes A class of fungi (subdivision MASTIGOMYCOTINA) which form zoospores having one anteriorly-directed tinsel flagellum. The organisms include terrestrial species as well as aquatic saprotrophs and parasites of other fungi and of freshwater and marine algae; they closely resemble members of the CHYTRIDIALES in morphology and life cycle, but they apparently do not carry out sexual processes. One order: Hyphochytriales; three families: Anisolpidiaceae, Hyphochytriaceae and Rhizidiomycetaceae (e.g. RHIZIDIOMYCES). This class has been called ‘Hyphochytridiomycetes’ by many authors. [Validation of the name ‘Hyphochytriomycetes’: Book ref. 174, p. 285.] Hypholoma See AGARICALES (Strophariaceae). Hyphomicrobium A genus of PROSTHECATE BACTERIA found in soils and in freshwater, estuarine and marine habitats. Morphologically, Hyphomicrobium resembles RHODOMICROBIUM, and its cell cycle is very similar to the ‘simplified’ cell cycle of R. vannielii ; it differs from R. vannielii e.g. in the absence of pigments and in that its swarm cells bear a single subpolar flagellum. (Phages specific for developing daughter cells have been isolated [JGV (1979) 43 29–38].) The organisms are chemoorganotrophs which can use one-carbon compounds (e.g. methanol, methylamine) as sole sources of carbon and energy (see METHYLOTROPHY); one-carbon compounds are metabolized via the icl− serine pathway. Growth occurs either aerobically or anaerobically with nitrate (which is reduced to nitrogen via nitrite); optimum growth temperature: 25–30° C. GC%: ca. 59–67. [Review: ARM (1981) 35 567–594.] Hyphomonas A genus of Gram-negative, aerobic, asporogenous, non-photosynthetic, heterotrophic, PROSTHECATE BACTERIA which reproduce by budding; daughter cells are motile (swarm cells), having one or more flagella according to species. The cell body may vary in size and shape during the cell cycle, and usually has a single polar prostheca (‘hypha’). Iron and manganese salts are not deposited on the cell surface (cf. PEDOMICROBIUM). Amino acids are the preferred substrates for growth; onecarbon compounds are not used (cf. HYPHOMICROBIUM). Species: H. neptunium (isolated from seawater) and H. polymorpha (type species, isolated from nasal mucus from a patient with sinusitis) [IJSB (1984) 34 71–73]. Proposed species: H. hirschiana, H. jannaschiana and H. oceanitis [IJSB (1985) 35 237–243]. Hyphomycetales See HYPHOMYCETES. Hyphomycetes A class of fungi of the subdivision DEUTEROMYCOTINA; most of the conidium-forming species do not form conidiomata (order Hyphomycetales), while the others form sporodochia (order Tuberculariales) or synnemata (order Stilbellales) – and some species do not form conidia at all (order AGONOMYCETALES). (cf. COELOMYCETES.) The vegetative form of the organisms is commonly a well-developed mycelium, but the class includes some fungi which can grow as unicellular organisms and which are sometimes classified in the class BLASTOMYCETES. The class Hyphomycetes (ca. 1000 genera) includes e.g. ACICULOCONIDIUM, ACREMONIUM, ALTERNARIA, ARTHROBOTRYS, ASPERGILLUS, AUREOBASIDIUM, BEAUVERIA, BLASTOMYCES, BOTRYTIS, BRETTANOMYCES, CALCARISPORIUM, CANDIDA, CERCOSPORA, Chalara, CHRYSOSPORIUM, Cladobotryum, CLADOSPORIUM, COCCIDIOIDES, CRYPTOCOCCUS, CRYPTOSTROMA, CULICINOMYCES, Curvularia, CYLINDROCARPON, DACTYLARIA, DACTYLELLA, DRECHSLERA, Exophiala, Fonsecaea, FULVIA, FUSARIUM, GEOTRICHUM, Gliocladium, Gonatobotrys, Gonatobotryum, GRAPHIUM, HARPOSPORIUM, HELMINTHOSPORIUM, HIRSUTELLA, HISTOPLASMA, HORMOCONIS, HUMICOLA, HYPHOZYMA, KLOECKERA, MADURELLA, MALASSEZIA, MERIA, METARHIZIUM, MONACROSPORIUM, Monodictys, Mycocentrospora, MYCOGONE, MYROTHECIUM, NOMURAEA, Oidiopsis, Oidium, OOSPORIDIUM, Ovulariopsis, PAECILOMYCES, PARACOCCIDIOIDES, PENICILLIUM, Periconia, Pesotum, PHAFFIA, PHIALOPHORA, PITHOMYCES, PSEUDOCERCOSPORELLA, PYRICULARIA, Ramularia, RHINOCLADIELLA, RHODOTORULA, RHYNCHOSPORIUM, Scedosporium, SCHIZOBLASTOSPORION, Scopulariopsis, Sphacelia, SPOROTHRIX, SPOROTRICHUM, STACHYBOTRYS, STENELLA, STERIGMATOMYCES, THERMOMYCES, THIELAVIOPSIS, Tolypocladium, TORULA, TRICHODERMA, TRICHOSPORON, TRICHOTHECIUM, TRIGONOPSIS, Triposporina, Tritirachium, VERTICILLIUM, WANGIELLA, XYLOHYPHA, ZALERION. hyphomycosis destruens Syn. EQUINE PHYCOMYCOSIS. hyphopodium In the mycelium of the BLACK MILDEWS: a short, one- or two-celled branch on a somatic hypha. In a capitate hyphopodium the distal cell is enlarged; this type of hyphopodium may act as an appressorium and give rise to a haustorium. Hyphozyma A genus of yeast-like fungi of the HYPHOMYCETES. [Key to species: AvL (1986) 52 39–44.] hypnospore A thick-walled resting cell. hypnozoite A dormant form of a parasite in a living host. (See e.g. PLASMODIUM.) hypobiosis See DORMANCY. Hypocenomyce See LECIDEA. hypochlorites (as antimicrobial agents) Hypochlorites (and hypochlorous acid) are strong OXIDIZING AGENTS which are microbicidal e.g. for many bacteria (including endospores) and viruses; their antimicrobial activity is not inhibited by most anionic and non-ionic detergents, but they are readily inactivated by organic matter and they have a tendency to decompose, forming e.g. chlorate (ClO3 − ) and HCl. (Decomposition is enhanced by e.g. heavy metal ions, acidity, light, and heat.) In solution, hypochlorites occur in equilibrium with hypochlorous acid, HOCl. The highly microbicidal undissociated form of HOCl is favoured by low pH; however, low pH reduces the stability of hypochlorites, so that commercial 382 Hyuga fever sodium hypochlorite solutions are often stabilized with sodium hydroxide. Sodium hypochlorite is used in a range of DISINFECTANTS and bleaches (e.g. Chloros, Domestos, Milton). Aerosols of hypochlorite solutions have been used for the disinfection of air. (N.B. Hypochlorite solutions react with formaldehyde to form the carcinogen bis-chloromethylether.) (See also DICHLOROISOCYANURATE.) hypocone See DINOFLAGELLATES. Hypocrea See HYPOCREALES. Hypocreales An order of mainly saprotrophic, plant-parasitic or fungicolous fungi of the subdivision ASCOMYCOTINA; anamorphs occur e.g. in the genera ACREMONIUM, CYLINDROCARPON and FUSARIUM. Ascocarp: perithecioid or closed, frequently on or within a stroma and often brightly coloured. Asci: cylindrical to clavate. Ascospores: usually colourless, aseptate to multiseptate or muriform. Some genera in the order have traditionally been distinguished on the basis of the number of septa in their ascospores; spore septation has not been recognized as a valid taxonomic criterion [Mycol. Pap. (1983) Nr 150 ]. Genera: e.g. Calonectria, GIBBERELLA, Hypocrea, NECTRIA. Hypocrella See CLAVICIPITALES. Hypoderma See RHYTISMATALES. hypogean Occuring underground (cf. EPIGEAN). Hypogymnia A genus of LICHENS (order LECANORALES). The thallus is foliose, with often hollow, inflated lobes, grey to greyishbrown above, black beneath; rhizines are absent. H. physodes is very common e.g. in the UK; it is tolerant of air pollution and is often the only macrolichen to be found in and around towns and cities. hypolimnion In a lake: the layer of water between the METALIMNION and the lake bottom; characteristically, it is anaerobic and rich in sulphide. Hypomyces A genus of mainly fungicolous fungi of the order CLAVICIPITALES; anamorph: Cladobotryum. (See also COBWEB DISEASE.) hyponeuston See NEUSTON. hypophloeodal Syn. ENDOPHLOEODAL. Hypopylaria A category (‘section’) of LEISHMANIA species which develop in the midgut and hindgut of the arthropod vector; it includes ‘L. agamae’ and ‘L. ceramodactyli ’. (cf. PERIPYLARIA, SUPRAPYLARIA.) hyposensitization (immunol.) Syn. DESENSITIZATION. Hyposoter exiguae virus See POLYDNAVIRIDAE. hypostatic allele See EPISTASIS. Hypostomatia A subclass of ciliates (class KINETOFRAGMINOPHOREA) in which the cytostome (when present) generally occurs on the ventral surface, and the cytopharyngeal apparatus is of the CYRTOS type; the body is typically cylindrical or dorsoventrally flattened, and the somatic ciliature is often reduced. The orders are: Apostomatida. Cytostome inconspicuous or absent; rosette (secretory organelle?) commonly present in the oral area; somatic ciliature spirally arranged in mature organisms; cysts common (see also TOMITE); often associated with marine crustacea. Genera: e.g. Chromidina, Foettingeria, Gymnodinioides, Polyspira. Chonotrichida. Typically vase-shaped and sedentary with a non-contractile stalk, attached to crustacea; reproduction by budding, producing motile ciliated forms (see also BROOD POUCH); ciliature in adult cell restricted to atrial region. Genera: e.g. Actinichona, CHILODOCHONA, Spirochona, Stylochona. Cyrtophorida. Body flattened, FRANGE absent or vestigial. Genera: e.g. CHILODONELLA, Chlamydodon, Dysteria, Hartmannula. Nassulida. Frange typically limited and restricted to left side of ventral surface. Genera: e.g. Microthorax, NASSULA, Pseudomicrothorax. Rhynchodida. Cells with a single, TOXICYST-bearing anterior tentacle, typically associated with the gills of marine bivalve molluscs. Genera: e.g. Ancistrocoma. Synhymeniida. Body commonly cylindrical and completely ciliated; frange extensive. Genera: e.g. Orthodonella. hypothallus (1) (prothallus) (lichenol.) A pale or black layer of non-lichenized (i.e. photobiont-free) hyphae which extends from the periphery of the thallus in certain crustose (or squamulose) lichens; a hypothallus may also be evident between the areolae of certain areolate species (see e.g. RHIZOCARPON). (2) (of myxomycetes) A thin, often transparent, sometimes calcified deposit which is secreted by the plasmodium during fruiting, and which forms a base for the fruiting bodies, remaining on the substratum following fruiting body formation. In some cases the hypothallus may be simply a remnant of the plasmodial slime sheath. hypotheca (1) See DINOFLAGELLATES. (2) See DIATOMS. hypothecium See APOTHECIUM. Hypotrichida An order of typically free-living ciliate protozoa (class POLYHYMENOPHOREA). Cells: typically ovoid to elongate and dorsoventrally flattened, with a conspicuous oral ciliature and a number of groups of cirri on the ventral side; the dorsal side often carries pairs of short cilia (‘sensory bristles’). Genera include e.g. ASPIDISCA, EUPLOTES, Histriculus, Holosticha, Isosticha, Klonostricha, Oxytricha, Psammomitra, Strongylidium, Stylonychia, Trachelostyla, Uronychia, Urosoma, Urostyla. hypovalve See DIATOMS. hypovirulence A reduced level of virulence in a strain of pathogen due e.g. to genetic changes in the pathogen, or to the effects on the pathogen of an infectious agent (contagious or transmissible hypovirulence: see e.g. CHESTNUT BLIGHT and DUTCH ELM DISEASE). hypoxanthine The base in the nucleoside inosine [see Appendix V(a) for structure]. (See also NITROUS ACID and WOBBLE HYPOTHESIS.) hypoxanthine-DNA glycosylase See NITROUS ACID. Hypoxylon See SPHAERIALES. Hys plasmid See ESCHERICHIA. hystrichosphaerids (hystrichospheres; hystricospores) Fossil resting spores of DINOFLAGELLATES; they date from the late Triassic onwards. [Book ref. 136, pp. 847–964.] Hyuga fever See EHRLICHIA. 383 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 I I Isoleucine (see AMINO ACIDS). (I + C + D) model Syn. HELMSTETTER–COOPER MODEL. I-like pili See PILI. I period See HELMSTETTER–COOPER MODEL. Ia antigens Class II antigens of the murine MAJOR HISTOCOMPATIBILITY COMPLEX. (See also IR GENES.) Ia protein The Escherichia coli OmpF PORIN. IAA Indole 3-acetic acid (see AUXINS). IAEA International Atomic Energy Agency. IAHS Infection-associated haemophagocytic syndrome: see HAEMOPHAGOCYTIC SYNDROME. IAM Institute of Applied Microbiology, University of Tokyo, Yayoi, Bunko-Ku, Tokyo, Japan. IAP Intracisternal A-TYPE PARTICLE. iatrogenic Refers to any disease or infection which is caused or exacerbated, unintentionally, as a result of medical intervention – e.g. examination or treatment. IAVI International AIDS Vaccine Initiative. Ib protein The Escherichia coli OmpC PORIN. ibotenic acid See AMATOXINS. IBV Infectious bronchitis virus (CORONAVIRIDAE). ICAD See APOPTOSIS. ICAM-1 Syn. CD54. ICAM-2 See CD102. ICAM-3 See IMMUNOGLOBULIN SUPERFAMILY. ICDH Isocitrate dehydrogenase: see TCA CYCLE. ICE (syn. caspase-1) See APOPTOSIS. ice algae See DIATOMS. ice nucleation bacteria Those bacteria which, at temperatures just below 0° C, promote water-to-ice transition by acting as nuclei around which ice crystals can form. Such bacteria (e.g. strains of Erwinia herbicola, Pseudomonas fluorescens, P. syringae and Xanthomonas campestris) – which include some common epiphytes – have been implicated as contributory factors in frost damage in various agricultural crops. A DNA fragment from P. fluorescens, when cloned in Escherichia coli, can confer the ice-nucleation phenotype; the fragment may encode an OUTER MEMBRANE protein of MWt ca. 180000 [EMBO (1986) 5 231–236]. When grown at 15° C, most ice-nucleation-positive strains of Erwinia herbicola shed into the medium vesicles composed of outer membrane; these vesicles (‘cell-free ice nuclei’) can behave as ice-nucleation centres which are active at −2° C to −10° C [JB (1986) 167 496–502]. Some plant tissues survive, without damage, when ‘supercooled’ to several degrees below 0° C – but may suffer frost damage if ice-nucleation bacteria are present. Above (approx.) −5° C, the incidence of frost injury can be decreased by reducing the size of the populations of plant-contaminating ice-nucleation bacteria. This can be done e.g. by treatment of plant surfaces with antibiotics such as streptomycin or oxytetracycline; an alternative method is to treat plant surfaces with non-ice-nucleating bacteria (a form of BIOLOGICAL CONTROL). A combination of antibiotic treatment and biological control has been found to act additively in the control of frost injury in pear trees [Phytopathol. (1996) 86 841–848]. Iceland moss See CETRARIA. ich See ICHTHYOPHTHIRIASIS. ichthyo- Prefix denoting fish – e.g. ichthyopathology: the study of FISH DISEASES; ichthyotoxic: toxic to fish. ichthyophonosis A systemic, granulomatous FISH DISEASE caused by Ichthyophonus hoferi (see ICHTHYOPHONUS); external symptoms vary in nature and severity (cf. TAUMELKRANKHEIT). I. hoferi occurs in various host tissues, mainly as spherical, thick-walled, multinucleate cells (‘cysts’, ‘resting spores’) up to 200 µm or more in diameter; immediately after the death of the host the ‘cysts’ germinate to form branching hyphae. The disease can affect e.g. mackerel and salmonids, and has caused mass fatalities in Atlantic herring; infected fish are, or rapidly become, unsuitable for human consumption as the muscles rapidly degenerate after the death of the fish. [Review: Book ref. 1, pp. 243–269.] Ichthyophonus A genus of fungi (or protozoa? – taxonomic affinity unknown) parasitic in fish. Species: I. hoferi (see ICHTHYOPHONOSIS) and I. gasterophilum. (cf. ICHTHYOSPORIDIUM.) ichthyophthiriasis (white spot, ich, ick) A freshwater FISH DISEASE caused by the ciliate Ichthyophthirius multifiliis. White spots 1 mm or so across, each containing one or more ciliates, appear on skin and fins; affected fish are weakened and may die. Secondary SAPROLEGNIASIS is common. I. multifiliis needs well-oxygenated waters and may be controlled e.g. by raising the temperature to diminish dissolved oxygen levels. Ichthyophthirius A genus of ciliates (order HYMENOSTOMATIDA); I. multifiliis is the causal agent of ICHTHYOPHTHIRIASIS in freshwater teleost fish. (cf. CRYPTOCARYON.) The life cycle of I. multifiliis involves three stages. The stage infective for fish is an ovoid or fusiform cell (called a tomite or theront), ca. 15 × 40 µm, which is covered with cilia, and which has a filamentous projection at the anterior end. Within the fish’s epidermis the tomite becomes spherical and develops an oral apparatus – becoming a trophozoite (= trophont) which grows to ca. 100–1000 µm diam. When free of the host, the trophozoite forms a gelatinous exocyst, loses its oral apparatus, and undergoes schizogony. Cysts, each several hundred micrometres in size, may contain up to ca. 1000 tomites which are released to complete the life cycle. [SEM of stages in the life cycle of I. multifiliis: J. Parasitol. (1985) 71 218–226; development of I. multifiliis in gill epithelium: JP (1986) 33 369–374.] Ichthyosporidium A genus used by some to include members of the genus ICHTHYOPHONUS, and by others to include microsporidean protozoa parasitic in fish. ichthyotoxin A TOXIN which is active against fish. ick See ICHTHYOPHTHIRIASIS. icl See SERINE PATHWAY. ICL Isocitrate lyase: see TCA CYCLE and SERINE PATHWAY. icm genes (of Legionella pneumophila) See end of section (a) in PHAGOCYTOSIS. ICMSF International Commission on Microbiological Specifications for Foods, an organization of the International Union of Microbiological Societies. ICNV International Committee on Nomenclature of Viruses, superseded by the ICTV. icosahedral symmetry (5-3-2 symmetry) The symmetry exhibited by an ICOSAHEDRON and by the capsid of certain types of virus: 5-fold rotational symmetry through each of the 12 apexes; 3-fold rotational symmetry about an axis through the centre of each of the 20 triangular faces; and 2-fold rotational symmetry about an axis through the centre of each of the 30 edges. 384 IgA1 proteases IDAV See HIV. idiogram See KARYOTYPE. idiolite A product of SECONDARY METABOLISM. idiopathic (of a disease) Of unknown cause. idiophase In BATCH CULTURE, that phase in which SECONDARY METABOLISM is dominant (cf. TROPHOPHASE); it corresponds to the latter part of the log phase together with the stationary phase. idiosome (protozool.) See XENOSOME (2). idiotope See IDIOTYPE. idiotype An Ig molecule defined by the sum total of antigenic determinants (idiotopes) on its variable (VH and/or VL ) domains. idli A type of steamed bread from India. It is made from a batter of milled rice and black gram (Phaseolus mungo) which is left to ferment overnight. The fermentation is carried out mainly by Leuconostoc mesenteroides which produces acid and gas; Enterococcus faecalis, and sometimes Pediococcus cerevisiae, contribute acidity, and yeasts may also be involved. The leavened batter is then steamed. idling During DNA replication: repeated cycles of addition and excision of nuleotides at a 3
terminal with no net synthesis or degradation of DNA. [Idling as a mechanism in lagging strand synthesis: GD (2004) 18 2764–2773.] idoxuridine (IDU; 5-iodo-2
-deoxyuridine; IUdR) An ANTIVIRAL AGENT which acts as an analogue of thymidine (5-methyl-2
deoxyuridine). Selective toxicity is poor, and the drug is too toxic for systemic use; it may be used topically for the treatment of e.g. herpes simplex keratitis, but is being superseded by more recent antiviral drugs. IDU IDOXURIDINE. IEF ISOELECTRIC FOCUSING. IEM IMMUNOELECTRON MICROSCOPY. IEP (1) ISOELECTRIC POINT. (2) IMMUNOELECTROPHORESIS. IF INTERMEDIATE FILAMENT. IF-1, IF-2, IF-3 See PROTEIN SYNTHESIS. IF1 protein See PROTON ATPASE. IFA test Syn. IFAT. IFAT See indirect IMMUNOFLUORESCENCE. IFN INTERFERON. IFO Institute for Fermentation, 17–85 Juso-Honmachi, 2-Chome, Yodogawa-Ku, Osaka, Japan. IFT Syn. IFAT. Ig IMMUNOGLOBULIN. IgA Immunoglobulin A, the predominant IMMUNOGLOBULIN in saliva, tears, colostrum and other body fluids, but comprising only ca. 10% of the total plasma Igs. The IgA monomer (MWt ca. 160000; S20 W 7) contains alpha HEAVY CHAINS, kappa or lambda LIGHT CHAINS, and ca. 8% carbohydrate. Most IgA at mucosal surfaces (secretory IgA, sIgA, SIgA) is in dimeric form: two monomers are linked via a J CHAIN, and the whole is linked to the SECRETORY COMPONENT. Polymers larger than dimers are also formed. IgA can fix complement via the alternative pathway. There are two subclasses: IgA1 and IgA2. (See also entry IgA1 proteases.) IgA1 proteases Various extracellular bacterial enzymes which specifically cleave the human IgA1 antibody at the heavychain hinge region – but which do not affect IgA2; some are sensitive to EDTA. These enzymes are synthesized by e.g. Haemophilus influenzae, Neisseria gonorrhoeae, N. meningitidis and Streptococcus pneumoniae – bacteria which are typically pathogenic at or via the mucosal surfaces; presumably, IgA1 proteases counteract the host’s IgA1 defences, but the role of these enzymes in pathogenesis has not been established. At least some IgA1 proteases are secreted by a type IV system (see PROTEIN SECRETION). A viral capsid having strict icosahedral symmetry would be constructed from exactly 60 identical building units arranged in positions of exact equivalence on the surface of a hypothetical sphere, the units being held together via identical (strictly equivalent) interunit contacts throughout. In most viruses, however, the genome is too large to fit into a capsid composed of 60 reasonably sized units, and in many ‘icosahedral’ viruses the number of units which form the capsid is a certain multiple of 60 (n60); in some viruses (e.g. polioviruses) the n60 units include units of more than one type, while in other viruses all the units are identical. The construction of a symmetrical capsid with n60 units requires that the units be arranged in 60-unit sets, the members of each set being distributed throughout the capsid. In such an arrangement of (n60) units, the interunit contacts are not precisely identical throughout the capsid; however, since all interunit bonding involves the same general type of contact, the interunit bonds in such a capsid are described as ‘quasi-equivalent’. In the smaller icosahedral capsids consisting of n60 units, the units are (of necessity) arranged into 5- and 6-membered rings, each ring forming a tight cluster – referred to as a PENTAMER or hexamer, respectively – that may be distinguishable in electron micrographs; the clustering of units in this way maximizes contact between the units. In such capsids a pentamer occupies each of the 12 apexes. (The existence of 5- or 6-membered clusters does not necessarily mean that members of a given cluster are bound more strongly to each other than they are to adjacent units; thus, e.g. many viruses which have 180-unit capsids yield dimers (2-unit particles) when gently disrupted.) In some cases a pentamer or hexamer may appear, under the electron microscope, as a single entity – which may be referred to as a morphological unit or capsomer (= capsomere). In those icosahedral viruses whose capsids comprise up to 240 building units, the value of n (in n60) can be 1, 3 or 4 (depending on virus); in these viruses, n (i.e., the number of 60-unit sets in the capsid) corresponds to the triangulation number (T number, or T). The large icosahedral viruses (e.g. adenoviruses) deviate from the geometrical and structural criteria obeyed by the smaller icosahedral viruses. [Principles of virus structure: Book ref. 148, pp. 27–44.] icosahedron A solid figure contained by 20 plane faces, all the faces being equilateral triangles of the same size. Many viruses have ICOSAHEDRAL SYMMETRY. ICP (insecticidal crystal protein) See BIOLOGICAL CONTROL. ICPB International Collection of Phytopathogenic Bacteria, Department of Bacteriology, University of California, Davis, California 95616, USA. ICR compounds See ACRIDINES. IcsA protein See DYSENTERY and PROTEIN SECRETION (type IV systems). icteroanaemia (porcine) See EPERYTHROZOONOSIS. icterus Jaundice. ICTV International Committee on Taxonomy of Viruses. (cf. ICNV.) id reaction The formation of skin lesions at sites remote from a focus of infection; such lesions are commonly sterile, and they may represent a local HYPERSENSITIVITY response to the infection. If the infecting organism is e.g. Trichophyton the reaction is termed trichophytid. ID50 Infectious dose (50%): that dose of a given infectious agent which, when given to each of a number of experimental test systems or animals, brings about the infection of 50% of the systems/animals under given conditions. 385 IgD IgD Immunoglobulin D, an IMMUNOGLOBULIN present in very low concentrations in plasma; it occurs mainly as a surface receptor for antigen on the membranes of B LYMPHOCYTES. The IgD molecule (MWt ca. 175000; S20 W 7) contains delta HEAVY CHAINS, kappa or lambda LIGHT CHAINS, and ca. 12–13% carbohydrate. IgE Immunoglobulin E, an IMMUNOGLOBULIN found in minute quantities in plasma. The IgE molecule (MWt ca. 190000; S20 W 8) contains epsilon HEAVY CHAINS, kappa or lambda LIGHT CHAINS, and ca. 12% carbohydrate. IgE binds to basophils and mast cells via its Fc portion, causing degranulation in the presence of specific allergen and thus effecting a TYPE I REACTION; cytophilic activity is abolished e.g. by heating (56° C/3–4 hours). IgG Immunoglobulin G, the predominant IMMUNOGLOBULIN in plasma (ca. 9–16 mg/ml, 75% of total plasma Ig). The IgG molecule (MWt ca. 150000; S20 W 7) contains gamma HEAVY CHAINS, kappa or lambda LIGHT CHAINS, and ca. 3% carbohydrate. There are four subclasses – IgG1, IgG2, IgG3 and IgG4 – distinguished by differences in amino acid sequences and by serology; human IgG subclasses are numbered in order of their concentrations in plasma: IgG1 comprises ca. 65% of plasma IgG. IgG1 and IgG3 are bivalent COMPLEMENT-FIXING ANTIBODIES (half-life ca. 21 and 7 days, respectively) which are important e.g. as opsonins and antitoxins in extravascular regions as well as in the bloodstream; in humans, they can cross the placenta and are important in the protection of the fetus and neonate. (Efficient binding of complement component C1 appears to require an IgG doublet, i.e. two molecules side by side; a single IgG molecule binds C1 only weakly.) IgG antibodies generally appear to predominate in the secondary (anamnestic) humoral reponse to antigen. (See also ALLOTYPE.) IgG protease See e.g. ELASTASE. IgM (macroglobulin) Immunoglobulin M, a minor plasma IMMUNOGLOBULIN (comprising ca. 5–10% of total plasma Ig); phylogenetically, IgM is apparently the most primitive class of Igs. The IgM monomer (MWt ca. 180000) contains mu HEAVY CHAINS, kappa or lambda LIGHT CHAINS, and ca. 12% carbohydrate; monomeric IgM occurs as a surface receptor for antigen on the B LYMPHOCYTE membrane. The typical form of IgM is a pentamer (S20 W 19) in which the five monomers are arranged radially (Fc portions directed towards the centre) and connected via disulphide bonds, with a J CHAIN at the centre of the polymer; in the free (uncombined) form, an IgM antibody is therefore stellate, but pentameric IgM is sufficiently flexible to assume a spider-like conformation in order to combine with multiple-repeating antigens on a surface. (IgM antibodies are thus particularly good agglutinators of those antigens, e.g. bacteria, which display a pattern of repeated antigenic determinants.) IgM antibodies are COMPLEMENT-FIXING ANTIBODIES with a theoretical valency of 10. Maximum valency is generally exerted only with small haptens; with larger antigens the effective valency is often only 5. (It appears that an IgM antibody must bind antigen at more than one of its combining sites in order to be able to fix complement [Mol.Immunol. (1981) 18 863–868, cited in Book ref. 42, p. 647].) In man, IgM does not pass through blood vessel walls into the tissues, and does not cross the placenta; its half-life is ca. 5 days. IgM antibodies are often the first to be formed in the humoral response to antigen; the humoral response to THYMUS-INDEPENDENT ANTIGENS (such as purified pneumococcal polysaccharides) generally involves, almost exclusively, the formation of IgM antibodies. The Wassermann antibody (see WASSERMANN REACTION) and the rheumatoid factor (see RHEUMATOID ARTHRITIS) are largely or exclusively IgM antibodies. IGS (mol. biol.) Internal guide sequence: see SPLIT GENE (b). IHF INTEGRATION HOST FACTOR. IkBa See CYTOKINES. IL Abbreviation for interleukin. See separate entries for INTERLEUKIN-1 (IL-1), INTERLEUKIN-2 (IL-2) etc. IL-1 INTERLEUKIN-1. IL-1g Syn. INTERLEUKIN-18. IL-2 INTERLEUKIN-2. ilarviruses (‘isometric labile ringspot viruses’; tobacco streak virus group) A group of tripartite ssRNA-containing PLANT VIRUSES which have a wide host range; transmission occurs via seeds and pollen and can readily occur mechanically. Type member: tobacco streak virus; other members include e.g. apple mosaic virus, Citrus leaf rugose virus, Citrus variegation virus, Prunus necrotic ringspot virus, spinach latent virus, Tulare apple mosaic virus. Virions: quasi-isometric, occasionally bacilliform; several types occur, differing in size (ca. 26–35 nm diam.) and in S20 W. Genome: three molecules of linear positive-sense ssRNA: RNA1 (MWt ca. 1.1 × 106 ), RNA2 (MWt ca. 0.9 × 106 ) and RNA3 (MWt ca. 0.7 × 106 ); the coat protein mRNA (‘RNA4’) is also encapsidated. RNAs 1, 2 and 3, together with coat protein or RNA4, are required for infectivity (cf. ALFALFA MOSAIC VIRUS). ileocaecal syndrome Syn. NEUTROPENIC ENTEROCOLITIS. Ilheus virus See FLAVIVIRIDAE. illegitimate name In NOMENCLATURE: a validly published name which contravenes any rule(s) of the relevant nomenclatural code. illegitimate recombination Fortuitous RECOMBINATION which occurs between DNA sequences which are non-homologous or which have very short regions of homology. In Escherichia coli illegitimate recombination is recA-independent and may be mediated by GYRASE: strands cleaved during gyrase action at different sites may undergo crossing over by the exchange of gyrase subunits covalently bound to the cut ends [model: PNAS (1983) 80 2452–2456]. imazalil An agricultural systemic ANTIFUNGAL AGENT, an imidazolyl derivative which acts as an inhibitor of sterol biosynthesis, thereby disrupting the cytoplasmic membrane in susceptible fungi. Imazalil is used – mixed with e.g. guazatine or thiophanate-methyl – as a seed treatment against many seedborne pathogens of cereals (e.g. Ustilago and Pyrenophora spp). It is also effective against Penicillium diseases of citrus fruits. imbricated Overlapping, like tiles on a roof. ImD unit See MICROCOMPLEMENT FIXATION. IMF INTERMEDIATE FILAMENT. imf phosphorylation See ELECTRON TRANSPORT PHOSPHORYLATION. Imhoff tank A large tank formerly widely used for SEWAGE TREATMENT. Essentially, crude sewage flows into a sludge settlement chamber located within a larger tank; sludge falls from a slot in the settlement chamber into the main digester compartment where it undergoes ANAEROBIC DIGESTION. The tank is vented for the escape of gases. IMI Imperial Mycological Institute, the former name of the Commonwealth Mycological Institute, Kew, UK. imidazole antifungal agents See AZOLE ANTIFUNGAL AGENTS. imipenem See THIENAMYCIN. immediate hypersensitivity (immunol.) HYPERSENSITIVITY mediated by humoral antibodies: see TYPE I REACTION, TYPE II REACTION, TYPE III REACTION, TYPE V REACTION (cf. DELAYED HYPERSENSITIVITY). Immediate hypersensitivity reactions typically occur within minutes or hours of contact with specific 386 immune globulin and labour of preparing purified immobilized enzymes. Cells can be immobilized by the methods used for enzymes (see above), but entrapment is the most commonly used method; gels used include e.g. agar, alginate, k-carrageenan, polyacrylamide, and polyurethane. Dead cells may be used for simple one-step or two-step reactions (provided they retain specific enzymic activity), but living cells are necessary for multistep transformations in which several enzymes act sequentially and/or in which there is a need for the regeneration of cofactors. Examples of the industrial use of dead immobilized cells include e.g. Escherichia coli in a k-carrageenan gel for the production of L-aspartic acid from ammonium fumarate, and ‘Brevibacterium flavum’ in a k-carrageenan gel for the production of L-malic acid from fumaric acid. High cell densities can be achieved with immobilized living cells. Thus, e.g., in one process used for the production of ethanol, Saccharomyces cerevisiae (‘S. carlsbergensis’) is immobilized in carrageenan, and ‘beads’ of the gel (containing 3.5 × 106 cells/ml) are incubated in a shaken nutrient medium for 60 hours: cell density rises to over 5 × 109 cells/ml – ca. 10 times higher than that possible in a conventional broth culture – with the yeast cells densely packed within the surface layer of each bead. When such beads are packed into a column and fed with a glucose-containing nutrient medium they produce ethanol efficiently for long periods of time. Other uses of immobilized living cells include the production of hydrogen from cells of Anabaena cylindrica bound to glass beads, and the production of an extracellular a-amylase by Bacillus subtilis immobilized in a polyacrylamide gel. [Book ref. 3, pp. 203–222, and PTRSLB (1983) 300 369–389 (cells); Book ref. 31, pp. 331–367 (enzymes); Science (1983) 219 722–727.] (2) The inhibition of MOTILITY in a microorganism – e.g. by a specific antiserum (see IMMOBILIZATION TEST). immobilization test Any test which involves the inhibition of MOTILITY in a microorganism. Immobilization tests can be used e.g. to detect specific antibodies (see e.g. TPI TEST) or organisms (e.g. a given strain of Vibrio cholerae may be identified by determining which types of specific antibody inhibit its motility). Antibodies may immobilize a motile organism e.g. by mediating the adhesion of flagella, or by promoting IMMUNE CYTOLYSIS. immune (1) (adj.) Refers to the state of IMMUNITY (sense 2 or 3). (2) (noun) See EPIDEMIOLOGY. immune adherence A phenomenon in which antigen–antibody complexes that have triggered COMPLEMENT FIXATION adhere firmly to receptors on phagocytes; the enhanced adhesion (which adds to that due to direct antigen–antibody–phagocyte interaction) is due mainly to component C3b. The phrase ‘immune adherence’ is sometimes used also to refer to the totality of pathogen–phagocyte adherence, i.e. including both antibody-mediated and complement-mediated adhesion. immune complex An antigen–antibody complex. Small, soluble (i.e. non-precipitating) immune complexes are formed e.g. under ANTIGEN EXCESS conditions. (See also TYPE III REACTION.) immune cytolysis The lysis of a cell which has been sensitized (SENSITIZATION sense 3) and which has triggered COMPLEMENT FIXATION; lysis may result directly or indirectly from the formation of a pore, in the membrane, created by the membrane attack complex. (cf. REACTIVE LYSIS.) immune deficiency-associated virus See AIDS. immune electron microscopy IMMUNOELECTRON MICROSCOPY. immune globulin Antiserum (or its immunoglobulin fraction) containing antibodies to a particular antigen or antigens. antigen; they are also referred to as immediate-type hypersensitivity reactions to include those which require hours or days to develop. immediate-type hypersensitivity See IMMEDIATE HYPERSENSITIVITY. Immedium filter See SEWAGE TREATMENT. immersion lens See RESOLVING POWER. immersion oil Cedarwood or synthetic oil (refractive index ca. 1.5) used with oil-immersion objectives (see RESOLVING POWER). In homogeneous immersion the immersion oil, objective lens, and cover-glass all have the same refractive index. Oil should be removed from the objective with a lens tissue immediately after use; unless otherwise recommended, a suitable solvent is 1,1,1trichloroethane, but benzene or xylene can be used. Prolonged contact with solvents may damage lens mountings. immobilization (1) (biotechnol.) Microbial, plant or animal cells, or macromolecules, may be ‘immobilized’ by attachment to solid structures, incorporation in gels etc for use in e.g. BIOCONVERSIONS (sense 1) – in some cases on an industrial scale. Various immobilized macromolecules are used e.g. in procedures involving affinity CHROMATOGRAPHY. Enzymes are immobilized because soluble enzymes are difficult to separate from substrates or products (and, hence, cannot be re-used in industrial processes), and most are unstable under conditions of use. The main methods of enzyme immobilization are: (a) Binding to a solid carrier or support. Supports used for covalent binding include e.g. cellulose, ceramic, glass, steel and synthetic polymers; usually, the support is ‘activated’ (see e.g. CYANOGEN BROMIDE), and the enzyme is then allowed to bind (commonly via its amino or carboxyl groups) to the activated support. The active site of the enzyme can be protected by allowing binding to occur in the presence of the enzyme’s substrate. Supports used for ionic binding include ion exchangers such as DEAE-cellulose. (b) Cross-linking with bifunctional reagents to form insoluble aggregates. Reagents used include GLUTARALDEHYDE (which binds enzymes via their amino groups), or diamines (e.g. hexamethylenediamine) which bind enzymes via their carboxyl groups after these groups have been ‘activated’ with carbodiimides; e.g., immobilized glucose isomerase is manufactured by treating pellets of homogenized cells of Bacillus coagulans with glutaraldehyde, yielding waterinsoluble aggregates containing glucose isomerase and other proteins. (c) Encapsulation. Enzymes are enclosed within LIPOSOMES or hollow fibres which are permeable to low-MWt substrates and products. (d) Entrapment within polymeric gels such as calcium ALGINATE, k-CARRAGEENAN and polyacrylamide. Enzymes are added to a solution of the polymer which is then gelled e.g. by the addition of a gelling agent. Leakage of enzymes from the gel may be counteracted by cross-linking them e.g. with glutaraldehyde. Entrapment is suitable primarily for bioconversion of low-MWt substrates which can diffuse through the gel. Immobilized enzymes are used for simple one-step or twostep bioconversions in which there is no need for regeneration of coenzymes. [Potential for immobilization of coenzymedependent enzymes: PTRSLB (1983) 300 335–367.] In some cases immobilization can improve the stability of enzymes which may otherwise be inadequate under operational conditions; e.g. immobilization of proteases inhibits their mutual degradation, and multipoint binding of an enzyme to its support helps to retain the conformational integrity of the enzyme – protecting it from inactivation by various denaturing agents (e.g. heat). Immobilized cells can be used for bioconversions which are not possible with isolated enzymes; the use of cells saves the cost 387 immune haemolysis immunodeficiency The inability to respond with a normal immune response to antigenic stimulation. (See also e.g. AIDS.) immunodepression Syn. IMMUNOSUPPRESSION. immunodiffusion See GEL DIFFUSION. immunodominant See DETERMINANT and ANTIGENIC COMPETITION. immunoelectron microscopy A procedure used e.g. for locating particular antigens in cells or tissues. Essentially, the given antigen is isolated and an antiserum is raised against it. The homologous antibodies are labelled with e.g. FERRITIN and then allowed to combine with the antigen in the cells or tissue; on examination by transmission ELECTRON MICROSCOPY the location of the given antigen is indicated by the ferritin label. (The use of ferritin in this way is called the immunoferritin technique.) (cf. IMMUNOSORBENT ELECTRON MICROSCOPY; see also IMMUNOGOLD TECHNIQUE.) immunoelectrophoresis (IEP) Any procedure in which antibodies (or other serum proteins) and/or antigens are subjected to ELECTROPHORESIS prior to their detection, characterization, or quantification (e.g. by means of specific antigen–antibody precipitation or staining). In one form of IEP the serum (or other) sample is first subjected to electrophoresis in a strip of agarose (or other) gel. This separates components into discrete zones – each zone containing one or more proteins characterized by a specific electrophoretic mobility. (Some proteins migrate in an unexpected direction – see ELECTROENDOSMOSIS.) A rectangular slot (‘trough’), cut into the gel strip parallel to the line of electrophoretic migration, is filled with antiserum containing antibodies to some or all of the proteins in the sample; the proteins and their homologous (and/or cross-reacting) antibodies diffuse through the agar and form lines or arcs of precipitate where they meet in OPTIMAL PROPORTIONS (see GEL DIFFUSION). (See also CHARGE-SHIFT IMMUNOELECTROPHORESIS; COUNTERCURRENT IMMUNOELECTROPHORESIS; ROCKET IMMUNOELECTROPHORESIS; TWO-DIMENSIONAL ELECTROPHORESIS.) immunoferritin technique See IMMUNOELECTRON MICROSCOPY. immunofluorescence The use of a fluorescent antibody (i.e. antibody conjugated with a FLUOROCHROME such as FITC, FLUORESCEIN, or RHODAMINE isothiocyanate) for the detection and/or quantification of a specific antigen (or antibody). In the simplest (direct) immunofluorescence techniques, used to detect antigen, the specimen (tissue section, smear etc) is exposed to the conjugate (i.e. dye-linked antibody) for an appropriate time, washed free of conjugate, and examined by fluorescence MICROSCOPY; antigens homologous to the fluorescent antibodies are readily identified by regions of fluorescence in the specimen. In the indirect immunofluorescence technique (e.g. the indirect fluorescent antibody test, IFAT) the presence of a given antigen can be determined by first exposing the specimen to unstained antibodies (e.g. a serum containing antibodies homologous to the given antigen); the specimen is then washed free of uncombined antibodies, exposed to fluorescent anti-Ig antibodies, washed, and examined by fluorescence microscopy. Any antigen–antibody combination on the specimen can thus be detected by the presence of fluorescent antibodies (which bind to the unstained antibodies); if particular antigens are known to be present in the specimen, the test can be used to detect homologous antibodies in the serum. Another type of indirect immunofluorescence technique is referred to as the SANDWICH TECHNIQUE. In all immunofluorescence techniques adequate controls are necessary to preclude an interpretation based on the non-specific localization of conjugate, and to take into account any primary FLUORESCENCE. immune haemolysis IMMUNE CYTOLYSIS of erythrocytes. immune lysis Syn. IMMUNE CYTOLYSIS. immune response The response(s) of a person or animal to immunological contact with an antigen; such responses may involve e.g. priming (see PRIMED), ANTIBODY FORMATION, an ANAMNESTIC RESPONSE, a HYPERSENSITIVITY reaction, or IMMUNOLOGICAL TOLERANCE. (See also ANTIGENIC COMPETITION.) immune serum Syn. ANTISERUM. immune surveillance Syn. IMMUNOSURVEILLANCE. immune tolerance See IMMUNOLOGICAL TOLERANCE. immunity (1) A state characterized by the tendency of the body to reject, eliminate or otherwise counteract foreign (‘non-self’) or seemingly foreign materials, or organisms, on or within its tissues; in this (broadest) sense immunity includes AUTOIMMUNITY, HYPERSENSITIVITY and IMMUNOLOGICAL TOLERANCE as well as other forms of SPECIFIC IMMUNITY, and NON-SPECIFIC IMMUNITY. (2) The state of an individual who, having had prior contact with a given antigen, can react to further contact with that antigen more vigorously than can a non-immune individual; in this sense immunity includes all the (beneficial and harmful) manifestations of HUMORAL IMMUNITY and CELL-MEDIATED IMMUNITY, including HYPERSENSITIVITY. (3) (protective immunity; functional immunity) Relative insusceptibility to specific harmful agents (e.g. pathogens) involving only the beneficial effects of humoral immunity and/or cell-mediated immunity. immunity breakdown See COLICINS. immunity groups See e.g. COLICIN E. immunity protein See e.g. COLICIN PLASMID; COLICINS; KILLER FACTOR. immunization (1) Any procedure in which specific microorganisms and/or antigenic materials (see VACCINE), or pre-formed antibodies, are introduced into the body in order to bring about specific protective IMMUNITY. (cf. INOCULATION (2) and VACCINATION.) Stimulation of the body’s own immune response (by administration of a vaccine) is referred to as active immunization; the (parenteral) administration of pre-formed antibodies is called passive immunization (see also PASSIVE IMMUNITY). (2) The administration of antigen to elicit any form of SPECIFIC IMMUNITY. (See also ISOIMMUNIZATION; HETEROIMMUNIZATION; NON-SPECIFIC IMMUNIZATION.) immunoadjuvant See ADJUVANT (1). immunoassay Any procedure in which the specificity of the antigen–antibody reaction is used for detecting and quantifying antigens, antibodies, or substance(s). (See e.g. ELISA; ERYTHROIMMUNOASSAY; ENZYME IMMUNOASSAY; IMMUNORADIOMETRIC ASSAY; PREGNANCY TEST; RADIOIMMUNOASSAY.) immunoblotting See BLOTTING. immunocompromised Unable to exhibit a normal immune response. immunoconglutinins Autoantibodies homologous to certain bound components of COMPLEMENT, particularly C3b; titres of serum immunoconglutinins are raised in long-term autoallergic conditions and in some diseases of microbial aetiology. Immunoconglutinins possibly play a role in the agglutination and phagocytosis of small, C3b-containing complexes. (cf. CONGLUTININ.) immunocyte Any functional cell of the immune system. immunocyte adherence technique (rosette technique) A method used e.g. for detecting cells which form specific antibodies. When such cells are incubated with homologous particulate antigen (or with erythrocytes coated with homologous soluble antigens) the antigenic particles adhere to specific receptor sites on the surfaces of the antibody-forming cells, forming rosettes. (See also PROTEIN A.) 388 immunoradiometric assay immunoliposome See LIPOSOME. immunological drift Syn. ANTIGENIC DRIFT. immunological paralysis IMMUNOLOGICAL TOLERANCE – particularly that induced by pneumococcal polysaccharides. immunological surveillance Syn. IMMUNOSURVEILLANCE. immunological tolerance Inability to respond normally to a given antigen (by humoral and/or by cell-mediated mechanisms) as a consequence of prior exposure to that antigen; unrelated antigens may elicit normal responses in the same individual. (cf. SELF TOLERANCE.) Tolerance may be total (i.e. no detectable response to antigen) or may involve e.g. failure to produce certain class(es) of antibody or some type(s) of immune response. Failure to react normally to some antigens may be beneficial in that harmful HYPERSENSITIVITY reactions may be avoided. The extent, duration, and type of acquired tolerance is influenced e.g. by the size of dose of the inducing antigen (the toleragen), the nature of the antigen, the route of administration, and the state of immunological maturity of the individual when tolerance is induced; the induction of tolerance is inhibited e.g. by adjuvants and is favoured e.g. by ANTIGENIC COMPETITION and by non-specific immunosuppression. Typically, thymus-dependent antigens (e.g. proteins) can induce tolerance when administered in doses higher than the normal immunizing dose (high-zone tolerance) or lower than the normal immunizing dose (low-zone tolerance); in high-zone tolerance both specific B cell and T cell clones are unresponsive (clonal deletion), while in low-zone tolerance only the helper T cell clone is unresponsive. Thymus-independent antigens usually induce only high-zone tolerance. Direct access of antigens to the gastrointestinal tract favours the induction of tolerance (see also SULZBERGER–CHASE PHENOMENON). In general, tolerance to a given antigen is more readily induced in the fetus or neonate than in the adult, and in a naive subject than in a primed one. The duration of tolerance may be extended by repeated administration of antigen. Diverse mechanisms of tolerance induction have been suggested (including e.g. enhancement of suppressor T cell activity), and different types of tolerance may involve different mechanisms. Exposure of immature B lymphocytes to specific antigen or to anti-Ig makes them subsequently unresponsive (i.e., unable to secrete antibody) when exposed to specific antigen (clonal abortion); this suggests a reason why tolerance can be induced more easily in the fetus or neonate than in adults. immunological unresponsiveness (1) Syn. IMMUNOLOGICAL TOLERANCE. (2) Inability, or reduced ability, to respond to antigens in general, due e.g. to IMMUNOSUPPRESSION. immunomagnetic separation See DYNABEADS. immunomodulation Specific or generalized alteration of the immune response (see IMMUNOSTIMULATION; IMMUNOSUPPRESSION; IMMUNOLOGICAL TOLERANCE). immunopathogenesis The development of pathological effects as a direct result of the immune response. immunoperoxidase method Any method in which antibodyconjugated, or otherwise complexed, peroxidases (e.g. horseradish peroxidase) are used to locate specific antigens on cells or tissues (see e.g. ABC IMMUNOPEROXIDASE METHOD and PAP TECHNIQUE). immunopotentiation (1) Syn. IMMUNOSTIMULATION. (2) Nonspecific immunostimulation. immunoprecipitable Capable of being precipitated by homologous antibodies. immunoradiometric assay A highly sensitive IMMUNOASSAY by which specific antigens are quantified using radioactive antibodies – which may be prepared as follows. Antigen is adsorbed (See also FLOW MICROFLUOROMETRY.) immunogen (1) Syn. ANTIGEN. (2) An antigen which can stimulate protective immunity. immunogenicity The capacity to function as an IMMUNOGEN. (Hence adj. immunogenic.) immunoglobulin superfamily A category of CELL ADHESION MOLECULES characterized by a molecular structure homologous to that of immunoglobulins – i.e. a number of immunoglobulin domains, each containing an intra-domain disulphide bond. These molecules are involved e.g. in calcium-independent cell–cell adhesion and antigen-specific interaction. The first member of the family to be isolated was the neural cell adhesion molecule (NCAM) which appears early in ontogeny and is important e.g. in the development of the central nervous system; binding is homophilic, i.e. NCAM binds to NCAM – in contrast to most members of the superfamily which bind to dissimilar molecules (i.e. heterophilic binding). Other members are important in the immune system – e.g. the T CELL RECEPTOR and the CD4 and CD8 molecules. Interactions between T cells and other cells also involve the leukocyte function-associated antigens LFA-2 (= CD2) and LFA-3 (= CD58); CD2 occurs on T cells, while CD58 occurs on many types of cell. CD2 and CD58 bind to one another. (N.B. LFA-1 is a member of the integrin family: see CD11a/CD18.) The intercellular adhesion molecules (ICAMs) have important roles in cell–cell adhesion. ICAM-1 (= CD54), ICAM-2 (CD102) and ICAM-3 (CD50) – widely distributed (e.g. on endothelium, monocytes, lymphocytes) – bind to integrin ligands (see CD11a/CD18). In at least some circumstances, cell-surface expression of ICAMs can be upregulated or down-regulated. (See also CD31 and CD106.) immunoglobulins (Igs) A class of proteins present e.g. in plasma, colostrum, tears and other body fluids; all antibodies are immunoglobulins (see ANTIBODY and ANTIBODY FORMATION). The (monomeric) form of an immunoglobulin consists of four polypeptide chains: two identical HEAVY CHAINS and two identical LIGHT CHAINS which are linked by interchain disulphide bonds to form a Y-shaped macromolecule. The two heavy chains are adjacent for part of their length, their N-terminal ends diverging (at the HINGE REGION) to form the two limbs of the Y; two or more disulphide bonds connect the heavy chains at the hinge region. One light chain runs alongside each of the two limbs of the Y, and is attached to it by a disulphide bond. The distal (N-terminal) end of each limb of the Y is, compositionally, a highly variable region – VH and VL referring to the so-called variable (distal) DOMAINS (V regions) of the heavy and light chains, respectively. The composition of the rest of the molecule is relatively constant, and is therefore termed the constant region (C region). The C region of each heavy chain comprises several DOMAINS which are designated CH 1, CH 2 etc (numbering from the V region end), while the constant region of the light chain consists of a single domain designated CL . Certain enzymes, e.g. PAPAIN, cleave the Ig monomer at the hinge region, producing two identical FAB PORTIONS and an FC
PORTION. Pepsin cleaves monomeric Ig to form an F(ab )2 portion (q.v.). Five Ig classes are distinguished: see entries for IgA, IgD, IgE, IgG and IgM. [Book ref. 42, pp. 131–219.] immunogold technique A form of IMMUNOELECTRON MICROSCOPY in which the specific antibodies (or PROTEIN A molecules) are complexed with gold prior to use – the gold serving as an electron-dense marker. 389 immunorecessive to an inert carrier (e.g. cellulose) and exposed to purified immunoglobulins obtained from an antiserum containing the homologous antibody; uncombined antibodies are removed by washing, and the combined antibodies are radiolabelled (e.g. with 125 I). The radioactive antibodies are then eluted for use in the assay. In one form of assay, excess radioactive antibody is added to the sample containing antigen at an unknown concentration; the reaction mixture is then exposed to fresh, cellulosebound antigen which adsorbs the uncombined antibodies and permits their separation from the reaction mixture. The amount of combined antibody can then be measured by determining the level of radioactivity remaining in the reaction mixture; this allows the concentration of antigen in the sample to be determined from a previously prepared standard curve (radioactivity versus antigen concentration). (cf. RADIOIMMUNOASSAY.) immunorecessive See DETERMINANT. immunosilent See DETERMINANT. immunosorbent assay Any IMMUNOASSAY in which antigen or antibody is immobilized by adsorption to a solid surface – see e.g. ELISA. immunosorbent electron microscopy (ISEM) ELECTRON MICROSCOPY in which particulate antigens (e.g. specific viruses) are detected or examined by first complexing them with homologous antibodies; prior to antigen–antibody interaction the antibodies may be e.g. fixed to an electron microscope grid (Derrick’s method, antibody-coated grid technique, A-CGT), adsorbed to a PROTEIN A-coated grid (Shukla’s method, protein A-coated grid technique, PA-CGT), or adsorbed directly to the protein A of cells of Staphylococcus aureus (protein A-coated bacteria technique, PA-CBT). [Review: AVR (1984) 29 169–194.] (cf. IMMUNOELECTRON MICROSCOPY.) immunostaining The staining of a specific antigen or structure by any method in which the stain (or stain-generating system) is complexed with specific antibody (e.g. ABC IMMUNOPEROXIDASE METHOD). immunostimulation Specific or non-specific potentiation of the immune response – e.g. by the administration of a VACCINE or by the use of an ADJUVANT. (cf. IMMUNOPOTENTIATION.) immunosuppression (immune suppression) Complete or partial suppression of normal IMMUNE RESPONSES – either in respect of specific antigen(s) (see also IMMUNOLOGICAL TOLERANCE) or in respect of all antigens (generalized suppression). Immunosuppression can be induced e.g. by ionizing radiation; specific antimetabolites (e.g. the folic acid antagonist methotrexate); a variety of antimitotic/antitumour agents (e.g. 6-mercaptopurine, prednisone, CYCLOSPORIN A, cyclophosphamide, azathioprine); antilymphocyte serum. Immunosuppression induced by radiation or by drugs tends to be generalized. immunosurveillance The continual monitoring of the tissues for abnormal antigens (e.g. on tumour cells) by cells of the immune system. immunotoxin An antibody–toxin conjugate intended to destroy specific target cells (e.g. tumour cells) which bear antigens homologous to the antibody. [Anti-CD22 immunotoxin BL22, which uses a fragment of EXOTOXIN A, in chemotherapy-resistant HAIRY-CELL LEUKAEMIA: NEJM (2001) 345 241–247.] Imotest A form of TUBERCULIN TEST in which a disposable plastic unit is used. The tuberculin is contained in a sealed plastic tube which is first broken and then squeezed to inoculate the nine points used to penetrate the skin. IMP Inosine 5
-monophosphate: see Appendix V(a). impactor See AIR. impedimetry A technique in which microbial growth and activity are measured in terms of changes in the electrical impedence of the growth medium due to the effects of metabolism. [JAB (1982) 53 423–426.] imperfect stage (mycol.) See ANAMORPH. impetigo A superficial, highly infectious skin disease, common in children, caused by Streptococcus pyogenes and/or Staphylococcus aureus. In the streptococcal form, spreading, inflamed pustules develop, rupture, and form thick brownish-yellow crusts; lesions may be secondarily infected by staphylococci. In the staphylococcal form (bullous impetigo) the lesions contain watery fluid rather than pus, and a thin crust forms over the centre of the lesion. Bullous impetigo occurs most commonly in very young infants; superinfection with streptococci is rare. Chemotherapy: e.g. oral penicillin (for streptococcal impetigo) or erythromycin. [Book ref. 17, pp. 341–346.] impinger See AIR. IMS See DYNABEADS. IMVEC tests IMVIC TESTS plus the EIJKMAN TEST. IMViC tests Tests used for the identification of bacteria e.g. of the family Enterobacteriaceae: INDOLE TEST, METHYL RED TEST, VOGES–PROSKAUER TEST, CITRATE TEST. (cf. EIJKMAN TEST.) in-block staining See ELECTRON MICROSCOPY (a). in situ gene amplification See IN SITU PCR. in situ hybridization See PROBE. in situ PCR PCR-based amplification of a target inside intact cells. The cells may be initially attached to glass slides (e.g. with AAS in acetone) and are fixed (e.g. with a formalin-based solution). The cells are then treated (permeabilized ) so as to allow entry of PCR reagents without encouraging the exit of PCR products. The reaction mixture is added to the cell layer (or tissue section), and is held in place with a cover slip. Temperature cycling may be carried out in a specialized form of THERMOCYCLER in which each slide is processed in a close-fitting slot that ensures good thermal contact (e.g. the GeneAmp In situ PCR System 1000; PerkinElmer). After amplification, another phase of fixation promotes intracellular retention of the products. Detection of products may be direct or indirect. Direct detection involves the use of labelled nucleotides or primers in the reaction mixture, and detection of the label histochemically. One disadvantage of this approach is that all amplified products will be labelled – even those resulting from mis-priming; as a consequence, the specificity of the assay is reduced. Improved specificity is obtained by the indirect mode of detection which involves the use of a labelled probe that is complementary to an internal sequence in the amplicon; the use of such a probe is equivalent to adding a stage of in situ hybridization (ISH), and procedures which involve indirect detection of products in this way have been referred to as PCR-ISH (‘in situ PCR’ being reserved for those procedures involving direct detection of products). Collectively, in situ PCR and PCR-ISH have been referred to as in situ gene amplification (IS-GA) [RMM (1997) 8 157–169]. This approach has been widely used for studying virusinfected cells, and may be particularly useful e.g. for those viruses which remain latent in tissues in small numbers. One early study detected intracellular lentiviral DNA [PNAS (1990) 87 4971–4975]. PCR-ISH has been used e.g. in studies on g-herpesvirus type 8 [AJP (1997) 150 147–153]. in vitro complementation assay An assay based on the COMPLEMENTATION of a defective (mutant) protein by an active (wildtype) form of that protein; such assays are used e.g. to determine which of several proteins is/are involved in a particular process. For example, to identify a protein involved in DNA synthesis, a cell-free DNA-synthesizing system is initially prepared 390 incompatibility incompatibility (in plasmids) The inability of PLASMIDS to coexist, stably, within the same cell when they have similar or identical systems for replication (see PLASMID) and/or PARTITION. Two incompatible plasmids which occupy the same cell will (in the absence of a selective pressure for both plasmids) tend to segregate to different cells during cell division. The stable intracellular co-existence of one plasmid with another requires that each plasmid be able to control, independently of the other, its own replication/partition such that it can establish and maintain a stable COPY NUMBER; the inability of a given plasmid to maintain a stable copy number in the presence of another plasmid is the characteristic feature of incompatibility. In many cases, incompatibility arises from the synthesis of plasmid-encoded trans-acting elements involved in the negative control of plasmid replication. Since plasmids with identical replication systems form identical trans-acting elements, the elements synthesized by one plasmid will affect the replication of other plasmids (in the same cell) which have an identical replication system. In this case, plasmid replication initially occurs randomly – e.g., two (incompatible) plasmids will each have an equal chance of being replicated. Once replicated, however, a plasmid of one type will be present at a higher copy number and will then be more likely to be involved in subsequent rounds of replication; this effect tends to be cumulative and to give rise to progeny cells that contain only one type of plasmid (‘homoplasmid segregants’). In plasmids which have similar but not identical replication systems, incompatibility may result in the preferential exclusion of one particular plasmid; for example, in a cell containing certain derivatives of the COLE1 PLASMID and of plasmid pMB1, the ColE1 derivative is rapidly excluded – possibly owing to differential sensitivity of the two plasmids to RNA I. (See also DISLODGEMENT and ONE-WAY INCOMPATIBILITY.) In a number of plasmids the negative control elements which govern incompatibility are small RNA molecules: see e.g. RNA I under COLE1 PLASMID, and CopA under R1 PLASMID. In the pT181 family of Staphylococcus aureus plasmids, too, replication is regulated by negative control elements (countertranscripts – see ANTISENSE RNA); however, in the pT181 family of plasmids incompatibility is not governed by such elements because the negative control elements of different plasmids in this group are not interchangeable. Among pT181 plasmids incompatibility is due to the existence of a common (interchangeable) Rep protein (an initiator protein which promotes replication by binding to the origin) [MGG (1986) 204 341–348]. Incompatibility and the classification of plasmids. The expression of incompatibility or compatibility between plasmids provides a useful criterion for classifying them. Thus, plasmids are classified into so-called incompatibility groups (Inc groups) in such a way that all the plasmids in a given Inc group express mutual incompatibility. Inc groups exist e.g. for plasmids which occur in enterobacteria, and separate Inc groups exist for plasmids which occur in other bacteria; some (‘promiscuous’) plasmids can occur e.g. in both enterobacteria and Pseudomonas spp, and some of these plasmids have been allocated to an Inc group in both the enterobacterial and pseudomonad grouping schemes. Enterobacterial Inc groups are designated by the prefix ‘Inc’ followed by a letter and, sometimes, by a Roman numeral or a Greek letter – e.g. IncFII, IncIa. The letter (e.g. F, I) indicates the type of conjugation system specified by the TRANSFER OPERON; thus, e.g. an IncF plasmid specifies a conjugative system like that of the F PLASMID or of an F-like plasmid, and encodes either F pili or F-like pili (see PILI). The Roman from cells which have a temperature-sensitive mutation in a gene essential for DNA synthesis; such a system can synthesize DNA at permissive temperatures but not at restrictive temperatures. Proteins purified from a wild-type strain of the same species can then be added, separately, to the system to determine which of them can restore DNA-synthesizing activity at restrictive temperatures. in vivo expression technology See IVET. Inaba variant See VIBRIO (V. cholerae). inactivated serum Serum which has been heated to 56° C for 30 min; heating inactivates COMPLEMENT in the serum. inactivated vaccine (killed vaccine) Any VACCINE consisting of microorganisms which have been treated (e.g. with formalin) so that they are no longer capable of multiplying, although their PROTECTIVE ANTIGENS remain effective. (See e.g. SALK VACCINE; SEMPLE VACCINE.) inactivation (of a microorganism) (1) The killing of a microorganism. (2) The temporary (reversible) inhibition of growth (or other activity) of a microorganism. inactone See GLUTARIMIDE ANTIBIOTICS. inaequihymeniiferous See LAMELLA. Inc group See INCOMPATIBILITY. incB locus See F PLASMID. IncC group See INCOMPATIBILITY. incC locus See F PLASMID. incertae sedis Of uncertain taxonomic position. IncF groups See INCOMPATIBILITY. IncI groups See INCOMPATIBILITY. incident light microscopy See MICROSCOPY (e) and (g). inclusion blennorrhoea See OPHTHALMIA NEONATORUM. inclusion bodies (‘inclusions’) Discrete structures of various types present (normally or abnormally) within cells. In virology, the term generally refers to those structures which develop in certain virus-infected cells; such structures consist of virions and/or viral components and/or cellular material, and may be characteristic in form and location for a given type of virus. (See also CYTOPATHIC EFFECT; GUARNIERI BODIES; NEGRI BODIES; POLYHEDRA; VIROPLASM.) In bacteriology, the term may refer to structures within a bacterial cell (e.g. granules of GLYCOGEN, POLY-b-HYDROXYBUTYRATE or POLYSULPHATE; ‘hydrocarbon inclusions’ (see HYDROCARBONS); CARBOXYSOMES; GAS VACUOLES; R BODY) or to bacterial cells within a (parasitized) eukaryotic host cell (see e.g. ANAPLASMA and CHLAMYDIA). (See also OVERPRODUCTION.) inclusion body hepatitis A POULTRY DISEASE apparently caused by certain strains of AVIADENOVIRUS (e.g. the ‘Tipton’ strain). The liver of an infected bird is enlarged, yellowish, with haemorrhagic patches; haemorrhages may also occur in the muscles. The disease may be associated with INFECTIOUS BURSAL DISEASE VIRUS infection. inclusion body rhinitis A common, generally mild disease of young pigs, caused by suid herpesvirus 2 (see BETAHERPESVIRINAE). Infection occurs by inhalation; symptoms: e.g. sneezing, nasal discharge etc. The disease may become generalized, resulting in anaemia and sudden death. inclusion conjunctivitis In adults: CONJUNCTIVITIS caused by Chlamydia trachomatis, commonly serotype D, E, F, G, H, I, J or K; if untreated, the disease (which is also called paratrachoma) typically resolves spontaneously after some months. (cf. TRACHOMA.) In newborn infants: see OPHTHALMIA NEONATORUM. IncN group See INCOMPATIBILITY. 391 incompatible numeral or Greek letter indicates a particular type of plasmid replication system; thus e.g. IncFI and IncFII plasmids have different replication systems (i.e., they are compatible) though they specify similar conjugation systems. Some examples of enterobacterial Inc groups are given below. IncFI includes the F PLASMID, the COLICIN PLASMID ColV-K94, and the R PLASMID R386. IncFII includes the R1 PLASMID, R6 and R100. IncIa (sometimes written IncI1 or IncI1 ) includes ColIb-P9, the delta plasmid (see DELTA), and R64. IncIg includes R621a. IncN includes N3, R46, and R269N-1. IncX includes the R6K PLASMID. Pseudomonas Inc groups are designated IncP-1 to IncP-13, the numbers referring to each of the 13 different types of replication system. The conjugation systems of Pseudomonas plasmids are less well known than those of enterobacterial plasmids, but the pili encoded by Pseudomonas plasmids have been characterized. [Pili encoded by Pseudomonas plasmids: JGM (1983) 129 2545–2556; Pseudomonas R plasmids: Book ref. 198, pp. 265–293.] Some of the Pseudomonas Inc groups are given below. IncP-2 includes the CAM PLASMID, the OCT PLASMID, and the R plasmid pMG1. IncP-6 includes the R plasmid Rms149. IncP-7 includes Rms148. IncP-8 includes plasmid FP2 (which encodes resistance to mercury). IncP-9 includes R2, the SAL PLASMID, and the TOL PLASMID. IncP-10 includes R91. IncP-11 includes RP8 and R151. IncP-12 includes R716. IncP-13 includes pMG25. Shared enterobacterial and pseudomonad Inc groups. Some of the shared groups are given below, the enterobacterial Inc group being given first. IncC (≡ IncP-3) includes R55. IncP (≡ IncP-1) includes R68, R751, RK2, RP1 and RP4. IncQ (≡ IncP-4) includes RSF1010. (A plasmid apparently related to this group, pFM739, has been isolated from Neisseria sicca [JGM (1986) 132 2491–2496].) incompatible (non-compatible) (plant pathol.) Refers to an interaction between a plant and a microorganism (e.g. an avirulent strain of a pathogen) which does not result in the development of disease in the plant. (cf. COMPATIBLE.) incomplete antibodies (1) Syn. BLOCKING ANTIBODIES (sense 1). (2) FAB PORTIONS. incomplete Freund’s adjuvant See FREUND’S ADJUVANT. IncP groups See INCOMPATIBILITY. IncQ group See INCOMPATIBILITY. incubation The maintenance of e.g. inoculated media or other types of material at a particular ambient temperature over a period of hours, days, weeks, or longer; the purpose of incubation is to provide conditions suitable for growth etc. Incubation is commonly carried out within closed, thermally insulated, thermostatically controlled chambers (incubators) or within vessels suspended in a WATER BATH; refrigerated incubators maintain the lower range of temperatures. Specialized incubation permits control of humidity, light, radiation etc as well as temperature. (cf. PHYTOTRON.) Cultures or specimens are sometimes left outside the incubator, i.e., they are exposed to the (often variable) conditions within the laboratory – a procedure known as ‘incubation at room temperature’. incubation period (med., vet.) The time interval between INFECTION (sense 1), or e.g. exposure to a TOXIN, and the appearance of the first symptoms of disease. (cf. PREPATENT PERIOD.) incubator See INCUBATION. IncX group See INCOMPATIBILITY. independent assortment (genetics) During e.g. MEIOSIS: the chance distribution of unlinked genes (see LINKAGE) among progeny cells. Thus, e.g., if the (diploid) parent cell contained the unlinked allelic pairs A/a and Z/z, any one of the four possible allele combinations (A and Z, A and z, a and Z, a and z ) may occur, with equal probability, in a given (haploid) progeny cell. indicator (pH) See PH INDICATOR. indicator organism Any organism whose presence and/or numbers serve to indicate the condition or quality of a material or environment. For example, in studies on faecal pollution in rivers, counts of the faecal bacteria Escherichia coli and Enterococcus faecalis are often used to indicate the degree of sewage pollution at given sites. The same organisms are also used for testing the efficiency of disinfection in treated drinking water, their presence in treated water supplies indicating a failure of the treatment process or contamination by sewage effluent subsequent to treatment. These organisms are used as indicators because they are common intestinal bacteria: their presence in water signals the potential presence of enteric pathogens. Water samples are not routinely tested for each of the (many) different types of enteric pathogen because, even in sewage-contaminated water, a given pathogen may occur only intermittently. Moreover, in sewage-contaminated water the indicator bacteria greatly outnumber pathogens so that they permit detection of very low levels of faecal pollution. Endospores of Clostridium perfringens, which remain viable for long periods of time, have been used to indicate previous contamination. Bifidobacterium spp may specifically indicate human faecal pollution [JAB (1983) 55 349–357], while faecal pollution from human and animal sources may be differentiated by differences in the antibiotic resistance patterns of ‘faecal streptococci’ [AEM (1996) 62 3997–4002]. Traditional tests for faecal indicator bacteria include the MULTIPLE-TUBE METHOD (for E. coli ) and membrane FILTRATION for FAECAL STREPTOCOCCI. Indicator organisms are also monitored in the assessment of food-handling conditions/hygiene [indicator organisms in meat: J. Food Protect. (1984) 47 672–677]. (See also SAPROBITY SYSTEM and lichens and pollution in the entry LICHEN.) indigoidine See ERWINIA (E. chrysanthemi ). indinavir See ANTIRETROVIRAL AGENTS. indirect agglutination Syn. PASSIVE AGGLUTINATION. indirect fluorescent antibody test (IFAT) See indirect IMMUNOFLUORESCENCE. indirect immunofluorescence See IMMUNOFLUORESCENCE. individual quick blanch (IQB) In the preparation of certain foods for CANNING: blanching by exposure of individual items of food to steam for a short period of time. indole 3-acetic acid See AUXINS. indole test An IMVIC TEST used to determine the ability of an organism to produce indole from tryptophan [see Appendix IV(f)]. The organism is grown in PEPTONE WATER or TRYPTONE WATER at 37° C. (N.B. The presence of carbohydrate (which may repress TRYPTOPHANASE) or nitrite in the medium can give false-negative results.) Test procedures vary. (a) KOVÁCS’ INDOLE REAGENT (0.5 ml) is added to ca. 5 ml of e.g. a 48-hour culture with gentle shaking; the reagent forms a floating layer which, 392 infectious bronchitis in a positive test (i.e. indole produced), becomes pink or red. (b) Xylene (ca. 1 ml) is added to ca. 5 ml of e.g. a 48-hour culture with vigorous shaking (to extract the indole). EHRLICH’S REAGENT (ca. 0.5 ml) is then allowed to run down the side of the slanted tube and forms a layer beneath the (floating) xylene; in a positive test a red colour develops in the reagent layer. (c) An oxalic acid-impregnated paper strip is inserted between the tube and stopper before incubation; during incubation (up to 7 days) indole (volatile at 37° C) reacts with the test strip to give a pink or red colour. indophenol oxidase test See OXIDASE TEST. indoxyl acetate (3-acetoxyindole) A reagent used in identification tests for e.g. species of Arcobacter and Campylobacter. Growth (from a non-selective medium) is suspended in distilled water, and a paper disc, impregnated with indoxyl acetate, is added; on incubation at room temperature for 20 minutes, a blue coloration indicates hydrolysis of indoxyl acetate. inducer exclusion (catabolite inhibition) The inhibition by glucose (or certain other readily metabolizable, ‘preferred’ substrates) of the uptake of certain substrates by bacteria; for example, if the bacteria are growing in a medium containing two substrates, one of the substrates is preferentially taken up and metabolized, and this substrate prevents the uptake of the other. There are several mechanisms for inducer exclusion; for exclusion of e.g. lactose in enterobacteria see CATABOLITE REPRESSION. Uptake of other (structurally unrelated) non-PTS sugars – e.g. maltose, melibiose – is inhibited in a similar way. In other cases, inducer exclusion may be due to competition between structurally related substrates for the same transport system or for shared components of the PTS; for example, in enterobacteria glucose can directly inhibit the uptake of galactose via a transport system common to both sugars. In certain Gram-positive bacteria (e.g. Streptococcus spp, Lactobacillus casei), accumulated sugar phosphates are actually expelled from the cell when e.g. glucose is added (inducer expulsion); this involves a two-step mechanism in which intracellular dephosphorylation of the sugar is followed by efflux of the free sugar. In these organisms regulation of sugar accumulation has been associated with the CcpA system (see CATABOLITE REPRESSION); phosphorylation of HPr at the Ser-46 site reduces its ability to be phosphorylated at the (routine) His-15 site by enzyme I, and this should reduce phosphate transfer to the PTS permeases and (hence) inhibit the uptake of sugars. inducer expulsion See INDUCER EXCLUSION. inducer protein Syn. activator: see OPERON. induction (1) (of enzymes, gene expression) See e.g. OPERON. (2) (of bacteriophage) See LYSOGENY. industrial alcohol (microbiological aspects) Large amounts of ethanol are produced commercially by the microbial fermentation of carbohydrates; ethanol is used primarily as a fuel (e.g. in GASOHOL), as a solvent, and as a feedstock in the chemical industry. ‘Fermentation ethanol’ (= ‘fuel ethanol’) may be produced by the ALCOHOLIC FERMENTATION of substrates such as MOLASSES and STARCH hydrolysates by strains of the yeast Saccharomyces; laboratory- and pilot-scale evaluations have been made of the economics of ethanol production from materials as diverse as cassava, Jerusalem artichoke, LIGNOCELLULOSE, sweet sorghum and WHEY – using various yeasts, certain bacteria, and mixtures of organisms. Since many organisms can utilize only mono- or disaccharides, polymers such as cellulose and starch must be subjected to acid or enzyme-catalysed hydrolysis before they can be fermented; strains of e.g. Clostridium thermocellum (which form extracellular CELLULASES) can use lignocellulose without prior hydrolysis, but the potential of this organism for ethanol production is limited by its poor tolerance of ethanol. On hydrolysis, most usable polysaccharides yield mainly hexoses – from which ethanol can be formed as a major product by various yeasts (including e.g. species of Candida, Kluyveromyces, Saccharomyces and Schizosaccharomyces) and by some bacteria (e.g. Zymomonas mobilis). The hydrolysis of HEMICELLULOSES yields significant quantities of PENTOSES, e.g. xylose; some organisms (e.g. Fusarium oxysporum) can convert xylose to xylulose, and can ferment the latter to ethanol via reactions of the HEXOSE MONOPHOSPHATE PATHWAY and the EMBDEN–MEYERHOF–PARNAS PATHWAY. Certain other organisms (e.g. Candida tropicalis, Pachysolen tannophilus) can also form ethanol from xylose, while a number of organisms (e.g. Saccharomyces, Schizosaccharomyces spp) can form ethanol from xylulose. A strain of Paecilomyces can give good yields of ethanol from a range of hexoses and pentoses [Nature (1986) 321 887–888]. industrial microbiology For examples of microorganisms and/or microbial products used on an industrial or commercial scale see e.g. AGAR, ALCOHOLIC BEVERAGES, ALGINATE, ASCORBIC ACID, BIOLOGICAL CONTROL, BIOMIMETIC TECHNOLOGY, BIOPOL, CARRAGEENAN, CITRIC ACID, COCOA, COFFEE, DAIRY PRODUCTS, DEXTRANS, DIATOMACEOUS EARTH, ENZYMES, ERGOT ALKALOIDS, FERMENTED FOODS, FUNORAN, FURCELLARAN, GELLAN GUM, GLUCONIC ACID, GLUTAMIC ACID, INDUSTRIAL ALCOHOL, INTERESTERIFICATION, KELP, LEACHING, MUSHROOM (senses 3 and 4), OVERPRODUCTION, RETTING, RIBOFLAVIN, SILAGE, SINGLE-CELL PROTEIN, STEROID BIOCONVERSIONS, VINEGAR, VITAMIN B12 , XANTHAN GUM and ZEARALENONE. infant botulism See BOTULISM (a). infantile gastroenteritis See EPEC. infantile paralysis Syn. POLIOMYELITIS. infection (1) The initial entry of a pathogen into a host. (2) The condition in which a pathogen has become established in or on the cells or tissues of a host; such a condition does not necessarily constitute or lead to disease. (3) Synonymous with disease of microbial aetiology. (See also CATHETER-ASSOCIATED INFECTION and TRANSFUSION-TRANSMITTED INFECTION.) (4) The establishment of a microbial symbiont in a host organism (see e.g. ROOT NODULES). infection-associated haemophagocytic syndrome (IAHS) See HAEMOPHAGOCYTIC SYNDROME. infection court (plant pathol.) The site at which a pathogen or parasite initiates infection of a plant (e.g. a wound). infection immunity Syn. PREMUNITION. infection peg See APPRESSORIUM. infection thread See ROOT NODULES. infectious abortion (vet.) See e.g. BRUCELLOSIS, CAMPYLOBACTERIOSIS, ENZOOTIC ABORTION. infectious ascites (of fish) See INFECTIOUS DROPSY. infectious bovine keratoconjunctivitis See INFECTIOUS KERATITIS. infectious bovine rhinotracheitis (red nose) A highly infectious CATTLE DISEASE caused by strains of bovine herpesvirus 1 (see HERPESVIRIDAE). (Rarely, pigs may be affected.) Infection occurs mainly by droplet inhalation. Incubation period: 3–7 days or longer. Onset is sudden, with anorexia, fever, necrotic lesions on the mucous membranes of the nasal septum, serous discharge from the eyes and nose, increased salivation etc; sudden death may result from obstructive bronchiolitis. Abortion is a common sequel. [Book ref. 33, pp. 798–804.] infectious bronchitis See AVIAN INFECTIOUS BRONCHITIS. 393 infectious bulbar paralysis mononucleosis and lymphocytosis; (c) the presence of atypical lymphocytes (enlarged, with foamy, vacuolated cytoplasm). (The atypical lymphocytes are not virus-infected B cells; they are T cells expressing the CD3 molecule.) [Haematology and cell-mediated immunity in infectious mononucleosis: BCH (1995) 8 165–199 (170–175).] (See also XLP SYNDROME; cf. CYTOMEGALIC INCLUSION DISEASE and EHRLICHIA.) infectious necrotic hepatitis (vet.) Syn. BLACK DISEASE. infectious nucleic acid (virol.) A nucleic acid which, when extracted from virions, can infect a host cell and give rise to progeny virions. Such a nucleic acid must be capable of being expressed and replicated in the cell in the absence of virion enzymes. Examples include DNA genomes which can gain access to the cell nucleus and which can be recognized and transcribed by host enzymes, and positive-sense ssRNA genomes which can be recognized and translated by host proteinsynthesizing machinery. Negative-sense ssRNA genomes are not infectious since an RNA-dependent RNA polymerase (present in the virion) is necessary for the synthesis of positive-sense mRNA. An infectious nucleic acid (e.g. that from polioviruses) may be able to infect cells not normally susceptible to the intact virion since the nucleic acid does not depend on the presence of a specific cell-surface receptor to initiate the infection process. (See also TRANSFECTION.) infectious pancreatic necrosis (IPN) A FISH DISEASE affecting hatchery-reared salmonid fry during the onset and early stages of feeding. Pancreatic tissue degenerates and becomes necrotic; mortality is often high. The causal agent is the IPN VIRUS. infectious polyarthritis (of pigs) Syn. GLASSER’S DISEASE. infectious spread (of plasmids) See EPIDEMIC SPREAD. infectious stunting syndrome (of chickens) See STUNTING SYNDROME. infective endocarditis See ENDOCARDITIS. infectivity (1) (of pathogenic microorganisms) The ability of a pathogen to become established on or within the tissues of a host. (See also VIRULENCE.) (2) (of a disease) The ease with which an infectious disease may be contracted under given conditions. (3) (viral) See END-POINT DILUTION ASSAY. inflammation In man and other animals: an acute or chronic response to tissue damage or to the presence of an allergen, or certain types of microorganism, within the tissues; inflammation may be initiated and sustained by microbial components and/or products. Inflammation is a protective response which commonly enables the body to overcome infection and return to normal function. Acute inflammation is characterized by redness, swelling, warmth (in the affected region), pain and loss of function. This manifestation of the body’s innate response to infection involves a diverse range of intercellular and intracellular signalling events – illustrated in the following generalized account of inflammation triggered by a Gram-negative bacterial pathogen which has breached the epithelial barrier. For convenience only, events mediated by complement and those mediated by cytokines are considered below under separate headings; the events described are representative of a process which is much more complex and which is still not understood fully. Complement-mediated events in inflammation. Activation of COMPLEMENT by LIPOPOLYSACCHARIDES (LPS) gives rise to (among other products) the fragments C3a and C5a; these fragments are anaphylatoxins which elicit HISTAMINE from e.g. MAST CELLS; C5a also acts as a chemotactic factor which attracts MONOCYTES and NEUTROPHILS to the infected site. infectious bulbar paralysis Syn. AUJESZKY’S DISEASE. infectious bursal disease virus (IBDV) A virus which causes severe inflammation of the BURSA OF FABRICIUS in chickens. IBDV closely resembles IPN VIRUS in morphology, genome etc. infectious centre See ONE-STEP GROWTH EXPERIMENT. infectious disease (communicable disease) Any disease which, under natural conditions, can be transmitted from one individual to another by a causal agent which passes either directly (by physical contact) or indirectly (e.g. by DROPLET INFECTION or via FOMITE or VECTOR) from the infected to the noninfected individual (i.e. HORIZONTAL TRANSMISSION); diseases passed on by VERTICAL TRANSMISSION include some which are normally ‘infectious’ (e.g. AIDS) as well as others (e.g. certain TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES) which are hereditary, i.e. not considered ‘infectious’. A transmissible disease may be transmitted from the affected individual to another individual, or to a fetus, by any means, including those processes that are involved in horizontal and vertical transmission as well as those involved in experimental infection (the latter form of transmission may or may not occur naturally in any given instance). TETANUS is an example of a non-infectious disease. (cf. CONTAGIOUS DISEASE; see also IATROGENIC.) [The transmission of infection (review): RMM (1995) 6 217–227.] infectious dropsy (of carp) (syn. rubella; infectious ascites; also: haemorrhagic septicaemia) An ‘umbrella term’ for at least two distinct FISH DISEASES affecting carp (Cyprinus spp): see SPRING VIRAEMIA OF CARP and CARP ERYTHRODERMATITIS; abdominal distension with generalized oedema are common to both diseases. infectious endocarditis See ENDOCARDITIS. infectious enterohepatitis Syn. BLACKHEAD. infectious equine arteritis virus Syn. equine arteritis virus: see ARTERIVIRUS. infectious foot-rot See FOOT-ROT. infectious haematopoietic necrosis (IHN) A FISH DISEASE characterized by massive destruction of the blood-forming tissues; the causal agent is a virus (see RHABDOVIRIDAE). infectious hepatitis Syn. HEPATITIS A. infectious jaundice See LEPTOSPIROSIS. infectious keratitis (infectious bovine keratoconjunctivitis; pinkeye) In cattle: KERATITIS or KERATOCONJUNCTIVITIS which occurs world-wide, particularly in young animals, usually in summer and autumn. The common causal agent is apparently Moraxella bovis. [Microbial flora of the bovine eye: VR (1986) 118 204–206.] (See also CATTLE DISEASES.) infectious mononucleosis (syn. glandular fever) An acute, selflimiting infectious disease of the lymphatic system which primarily affects children and young adults; it is caused by the EPSTEIN–BARR VIRUS (EBV). Transmission occurs by direct oral contact (‘kissing disease’) and may also occur by droplet infection. The incubation period is 1–7 weeks. In the typical ‘glandular fever’ syndrome (the anginose form), common in young adults, there is fever, pharyngitis (often with exudative tonsilitis) and swollen tender lymph nodes (with or without rash); jaundice and/or hepatosplenomegaly occur less commonly. Infrequently, the disease develops with fulminant hepatitis and VAHS (see HAEMOPHAGOCYTIC SYNDROME). Therapeutic approaches to the severe form of the disease may involve the use of agents such as ACYCLOVIR, CYCLOSPORIN A and corticosteroids. Lab. diagnosis may include demonstration of (a) increased levels of heterophil antibodies (see PAUL–BUNNELL TEST); (b) 394 influenza virus C Infection generally occurs by inhalation of virus-containing aerosols, virions being deposited onto alveolar membrane or (larger droplets) onto mucous membrane lining the respiratory tract; in the latter case, virions may bind to sialic acid residues in mucoproteins (rather than to receptors on respiratory epithelium) – but such (abortive) binding can be broken by the viral neuraminidase. Incubation period: typically 1–3 days. Infection may be asymptomatic, or there may be a sudden onset with chills, fever, headache, muscular aches, anorexia, malaise etc.; fever subsides after 1–5 days, and respiratory symptoms (coryza, cough etc.) then become prominent. Recovery is usually rapid, although cough and malaise may persist for several weeks. Viral infection damages ciliated epithelium in the trachea and bronchi; loss of cilia predisposes to secondary bacterial infection and development of e.g. bronchial pneumonia involving organisms such as Streptococcus pneumoniae and/or Haemophilus influenzae. Secondary infections are particularly problematic e.g. in the elderly, debilitated and immunocompromised; apart from pneumonia, secondary infections may lead to complications such as tracheo-bronchitis, sinusitis, otitis media etc. (One early report indicated that, in mice, the severity of disease can be reduced by ozone [AEM (1982) 44 723–731].) Chemotherapy. Drugs such as AMANTADINE and rimantadine may afford some protection against influenza virus type A if administered before or soon after infection. (See also ZANAMIVIR.) Vaccines. The original, formalin-inactivated, whole-virus vaccines produce local and systemic side-effects. Split-product (split-virus) vaccines, containing H and N components of the virus (as opposed to entire virions), are tolerated better. Various alternative approaches to vaccines have been tried. For example, attempts have been made to prepare attenuated ‘live’ vaccines by using REASSORTANT VIRUSes containing e.g. surface glycoproteins from a currently epidemic strain and internal components from an attenuated strain. In some cases the attenuated strain was a temperature-sensitive (cold-adapted) virus, while in other cases it was a strain from another species with low virulence for man [e.g. avian strains: Book ref. 86, pp 395–405]. Vaccines have been prepared from synthetic oligopeptides that mimic glycoprotein epitopes [oligopeptide vaccines: MS (1984) 1 55–58] and with virosomes [Lancet (1994) 344 160–163]. Owing to the effects of antigenic variation, the components of influenza vaccines must be regularly reviewed to ensure that, when used, a given vaccine will generate protection relevant to the existing strain(s) of virus. [Efficacy of influenza vaccines: RMM (1996) 7 23–30.] influenza virus A See INFLUENZAVIRUS. influenza virus B See INFLUENZAVIRUS. influenza virus C A virus of the ORTHOMYXOVIRIDAE. It is physicochemically and morphologically similar to members of the genus INFLUENZAVIRUS, but differs in containing only 7 RNA segments (total MWt ca. 4–5 × 106 ) and in having only one type of surface glycoprotein (HA) which has both haemagglutinating and receptor-destroying activities, but apparently no neuraminidase activity. It has been shown that, contrary to previous assumptions, sialic acid may nevertheless be an essential component of the host cell-surface receptor [Virol. (1985) 141 144–147]. Influenza virus C primarily infects man, although it has been isolated from pigs in China; it generally causes only mild or subclinical disease of the upper respiratory tract. Antigenic drifting occurs slowly, but antigenic shift has not been observed (cf. INFLUENZAVIRUS). Histamine increases the permeability of small blood vessels, allowing efflux of plasma (hence the swelling) and exposing the pathogen to (i) increased amounts of antimicrobial agents (e.g. complement and antibodies) and (ii) cells of the immune system attracted by C5a. Histamine is also reported to stimulate the early expression of SELECTIN molecules (specifically P-selectins) on local vascular endothelium (apparently within minutes of stimulation). Cytokine-mediated events in inflammation. CYTOKINES (e.g. IL1, IL-8, TNF-a) are produced by local cells on stimulation by the pathogen. IL-1 and TNF-a promote the expression of E-selectins on local vascular endothelium. Leukocytes in local blood vessels bind weakly to endothelial selectins; binding involves certain carbohydrate ligands in the leukocyte membrane – e.g. sialyl-Lewis x (= CD15). This (labile) selectin–CD15 binding causes leukocytes to slow down and roll along the vascular endothelium in the direction of blood flow, bonds being continually made and broken; this is referred to as tethering. As well as CD15, the leukocyte cell membrane also contains adhesion molecules of the INTEGRIN family – although, in the absence of infection, these molecules are normally inactive; an example is the leukocyte functionassociated antigen 1 (LFA-1; CD11a/CD18). Chemokines, e.g. IL-8 (see above), promote the activation of LFA-1 (via phosphorylation), so that LFA-1 can then bind (strongly) to counterreceptors (e.g. ICAM-1, ICAM-2) on vascular endothelium. [Leukocyte rolling and firm adhesion (biophysical view): PNAS (2000) 97 11262–11267.] The captured leukocytes flatten, and then ‘crawl’ over the endothelium to a cell–cell junction – subsequently migrating between endothelial cells (a process called diapedesis or extravasation) into the affected area (apparently guided by the chemokine gradient). The mechanism of extravasation is not known, but platelet–endothelial cell adhesion molecule-1 (PECAM-1) appears to be required. As indicated, cytokines are important for the recruitment of immune cells to the local site of infection. (Neutrophils are commonly the dominant type of cell in the early stages of acute inflammation; macrophages often become prominent after ∼8 hours, and their ability to secrete FIBRONECTIN may be an important factor in tissue repair [wound healing (review): Science (1997) 276 75–81].) The importance of leukocyte recruitment to infected tissues is emphasized by those genetic disorders in which this process is inhibited (e.g. LEUKOCYTE ADHESION DEFICIENCY). Cytokines are also involved in the production of ACUTE-PHASE PROTEINS (q.v.). Antigen-stimulated T lymphocytes release g-interferon (IFNg) which activates MACROPHAGES. The pain associated with inflammation may be due e.g. to pressure on nerves and/or to PROSTACYCLIN. Chronic inflammation, which occurs e.g. in some persistent intracellular infections, is characterized by the formation of new connective tissue and by the presence of concentrations of macrophages; it often leads to the formation of a GRANULOMA. infliximab See TNF. influenza (flu; grippe) An acute, highly infectious human disease caused by influenza virus A (often widespread epidemics, occasionally pandemics) or influenza viruses B or C (typically sporadic outbreaks, often among children and young adults). (See INFLUENZAVIRUS for details of strains, including those in the 1997 bird flu in Hong Kong, the 1999 outbreak in Guangdong (China), and the 2004–2006 problem with strain H5N1; see also INFLUENZA VIRUS C, GASTRIC FLU and COMMON COLD.) 395 Influenzavirus are apparently resistant to drying at room temperature, and survive at low temperatures in water. The virions can be preserved for long periods at −70° C. Infection of host cells; replication. The virion binds, via the HA protein, to specific cell surface receptors containing sialic acid residues; it is then internalized in a vesicle (endocytosis). Low pH within the vesicle promotes cleavage of the HA protein (by a host protease), and this leads to fusion of the viral envelope with the vesicle’s membrane; as a result, viral ribonucleoprotein and transcriptase are released into the cytoplasm of the host cell. (See also ENVELOPE.) The infectivity of a virion is abolished if antibodies have bound to the globular HA1 portion of the HA glycoprotein. Transcription of the (negative-sense) viral RNA occurs in the nucleus; it involves viral transcriptase but depends on the priming activity of the host’s (a-AMANITIN-sensitive) RNA polymerase II. (See also RIBAVIRIN.) Transcription gives rise to ten molecules of mRNA (two from each of genome segments 7 and 8 – each of which have two reading frames). Viral mRNAs are polyadenylated in the nucleus. Replication of the viral genome also occurs in the nucleus and involves viral transcriptase and the host’s RNA polymerase II. The progeny ribonucleoprotein structures are assembled in the nucleus, but details of the mode of assembly are not available. HA and NA proteins are synthesized on rough endoplasmic reticulum and subsequently migrate to localized sites in the plasma membrane. The type B influenzaviruses (but apparently not type A viruses) encode another glycoprotein, NB, whose reading frame overlaps that of NA; NB does not occur in the virion. Other non-structural (NS) proteins are produced in cells infected with A- or B-type viruses; their role(s) are unknown. Maturation of the virion occurs by budding through the cell membrane at sites containing HA and NA. Classification/nomenclature of influenzaviruses. Influenzaviruses are classified as subtypes on the basis of their HA antigens (15 different subtypes) and NA antigens (9 different subtypes). A given strain of virus is designated by a formula which indicates the following data: (i) type (i.e. A, B or C); (ii) the animal from which the strain was first isolated (omitted if the host was human); (iii) the place where the strain was first isolated; (iv) the strain number; (v) the year of isolation; and (vi) the particular HA (= H) and NA (= N) antigens. Example: Influenzavirus A genus of viruses (family ORTHOMYXOVIRIDAE) which includes influenza virus type A and influenza virus type B; both viruses cause sporadic or epidemic INFLUENZA in man, but type A strains also cause epizootics in pigs, horses or birds (see e.g. FOWL PLAGUE and SWINE INFLUENZA) and (occasionally) pandemics in humans. (See also INFLUENZA VIRUS C.) Influenzaviruses are transmitted via aerosols or water, or by direct contact. Virions. The enveloped virions are pleomorphic, ca. 80–120 nm across; ‘filamentous’ forms of up to several micrometres in length are common. The genome consists of eight molecules of linear, negative-sense ssRNA (total MWt ca. 5 × 106 ); these RNA segments form a helical ribonucleoprotein complex with the (virus-encoded) NP protein. Associated with the RNA–NP complex are the viral proteins P1, P2 and P3 (also called PA, PB1 and PB2) which, together, constitute a transcriptase (i.e. an RNA-dependent RNA polymerase) which is required for synthesis of viral mRNA and for replication of the RNA genome. [Function of transcriptase (model): Book ref. 86, pp 73–84.] The ribonucleoprotein is surrounded by a layer of (virusencoded) matrix protein M1; this layer of M1 protein is wholly enclosed by the viral envelope. The viral M2 protein forms part of the envelope. It is encoded by genome segment 7 (as is M1); M2 (MWt ca. 13 × 103 ) is translated from a spliced transcript. The viral envelope (containing lipids derived from host cell membrane) forms the outermost layer of the virion. Two types of structure arise from the surface of the envelope. These structures are usually referred to as spikes and mushroomshaped projections (names which reflect their appearance under the electron microscope). Spikes. Each spike (∼10–15 nm long) consists of three identical glycoproteins. Because these glycoproteins can bind to red blood cells (and cause HAEMAGGLUTINATION) they are referred to as haemagglutinins (HA). However, the glycoproteins can bind to any type of cell which has a suitable sialic acid (= Nacetylneuraminic acid, NANA) receptor – e.g. cells of the respiratory epithelium. Each molecule of HA glycoprotein consists of two dissimilar polypeptide subunits which are linked by a disulphide bond; these subunits are termed HA1 (the larger subunit) and HA2. (The HA protein is encoded by segment 4 of the viral genome. It is synthesized as a single polypeptide chain which undergoes post-translational cleavage: an N-terminal signal sequence is removed, and the molecule is commonly cleaved to form HA1 (328 amino acid residues) and HA2 (221 amino acid residues) – which remain linked by a disulphide bond.) In each spike, the proximal HA molecule is attached to the envelope via a site in the HA2 subunit. Mushroom-shaped projections. Each individual mushroomshaped projection is a tetramer of another glycoprotein, NEURAMINIDASE (NA) (q.v.); each protein in the tetramer has a functional enzymic site. Propagation of influenzaviruses in vitro. The influenzaviruses can be propagated in embryonated hens’ eggs and e.g. in chick embryo cell cultures. Some authors prefer monkey kidney or human embryo kidney cells for primary isolation. (Eggs are used for vaccine virus production.) Inactivation of virions. Virions are rapidly inactivated by low pH (<5), by mild heat (e.g. 60° C/30 min), by lipid solvents, detergents and soaps, by formaldehyde and ultraviolet radiation, and by phenolic disinfectants and other agents; they A/duck/Ukraine/1/63(H3N8) The pandemic of 1957 – so-called Asian flu – was caused by a strain formerly referred to as ‘strain A2’ but now designated: A/Singapore/1/57(H2N2) The 1968 pandemic – so-called Hong Kong flu – was due to: A/Hong Kong/1/68(H3N2) It appears that these two strains of influenzavirus were circulating human strains which had acquired novel antigens through genetic reassortment with avian strains (see REASSORTANT VIRUS). The sudden acquisition of new antigens (‘antigenic shift’ – see below) meant that radically new strains of virus had appeared in a human population which, immunologically, was completely naive. It was previously assumed that if a ‘new’ subtype appears then the ‘old’ strain ceases to circulate. However, subtype H1N1, which was active early in the 20th century, re-appeared in 1977 and was involved in the pandemic of so-called Russian flu. 396 Inonotus The surface glycoproteins of influenzaviruses undergo ANTIby this means, the virus may avoid the host’s immune response and give rise to new outbreaks of influenza. New strains of virus are generally not inactivated by antibodies induced by a previous strain – i.e. antibody-based protection tends to be strain-specific. Variation in surface antigens may involve ANTIGENIC DRIFT or ANTIGENIC SHIFT. Antigenic drift involves relatively small changes in the surface antigens, and this is associated with the regular occurrence of epidemic influenza. Antigenic shift involves major (and abrupt) changes in the surface antigens, and it happens infrequently. It has been recorded only in type A influenzaviruses; this apparently reflects the fact that type A strains can infect animals as well as man, and that, in mixed infections (involving viruses from different species), segments of the genome may undergo reassortment – resulting in the formation of novel hybrid subtypes containing genome segments from different co-infecting parent virions. In recent times, human strains typically displayed H1, H2 or H3, and N1, N2 or N3 antigens. Many other types of H and N antigens occur in animal strains, suggesting scope for the formation of new pandemic strains by reassortment. Fortunately, influenzaviruses are generally highly species-specific – so that transfer across the species barrier is assumed to occur only rarely. Transfer occurred when American harbour seals were infected by an avian H7N7 virus in 1980 [Virology (1981) 113 712–724] and pigs were infected by avian strains [JV (1996) 70 8041–8046]. Transfer across the species barrier may occur in a two-step process; for example, transmission of the human H3N2 subtype to avian hosts can be achieved after reassortment in pigs. The 1997 outbreak of bird flu in Hong Kong, which caused six deaths, resulted from avian-to-human transmission of a fowl plague strain (H5N1). Subsequently, no deaths from this strain were reported following large-scale slaughter of poultry. In 1999, two children in Hong Kong were infected with influenza A subtype H9N2 (strains A/Hong Kong/1073/99; A/Hong Kong/1074/99); these strains closely resemble a strain isolated from quail in 1997 (A/quail/Hong Kong/G1/97). Also in 1999, H9N2 viruses were isolated from patients with influenzalike illness in Guangdong province (China). The H5N1 and (Hong Kong) H9N2 human isolates are similar in the six genes that encode internal components of the virion, suggesting that H9N2 arose from reassortment of H5N1. Avian strains with these six genes might have a tendency to infect humans and may therefore have the potential to give rise to a novel human pathogen. [Relationship between H9N2 and H5N1 human isolates: PNAS (2000) 97 9654–9658]. H5N1 caused over 50 human deaths in East Asia in 2004–2005. Isolates from humans and those from birds consisted of two distinct clades. In all isolates the genes were of avian influenzaviruses, indicating that they were not derived through reassortment with human strains. Human-to-human transmission was described. [Evolution of H5N1 avian influenza viruses in Asia: EID (2005) 11(10). Influenza (special issue): EID (2006) 12(1) (January).] informational suppressor See SUPPRESSOR MUTATION. informosome See MRNA (b). infraciliature (ciliate protozool.) The subpellicular system which includes the kinetosomes and all the various associated microfibrillar and microtubular structures (see e.g. KINETODESMA; POSTCILIARY MICROTUBULES; TRANSVERSE MICROTUBULES). infradian rhythms In certain parameters of a eukaryotic cell or organism: innate rhythmical changes which occur with a periodicity >24 hours. (cf. CIRCADIAN RHYTHMS.) infrared radiation See dry-heat STERILIZATION. infrasubspecific rank Any rank below subspecies (see e.g. VARIETY). infundibulum (1) In some ciliates: the lower or inner (often tubular) region of the BUCCAL CAVITY; in peritrichs it corresponds to that part of the buccal cavity which excludes the PERISTOME (sense 1). (2) Any funnel-shaped structure. infusion agar Infusion broth (see NUTRIENT BROTH) gelled with 1.5–2.0% w/v AGAR. infusion broth See NUTRIENT BROTH. infusoria An archaic term for the various microscopic organisms – particularly ciliates – found in aqueous infusions of vegetable matter. INH Isonicotinic acid hydrazide: ISONIAZID. inhibitory medium See MEDIUM. initial(s) (microbiol.) Any part of a cell or organism which subsequently develops and/or differentiates to become a distinct and characteristic structure; the earliest or early stage(s) of a given structure. initial body See CHLAMYDIALES. initiation complex See PROMOTER. initiation mass See CELL CYCLE. initiator (of an operon) See OPERON. ink-cap fungi Species of COPRINUS whose autodigesting lamellae form a black, basidiospore-containing, ink-like liquid. Inkoo virus See BUNYAVIRUS. inlA gene See LISTERIOSIS. innate (mycol.) Refers to any fungal structure which is embedded in a substratum, or in the fungal thallus, such that its outer surface is more or less level with the surface of the substratum or thallus. inner primers See NESTED PCR. inner veil (mycol.) Syn. PARTIAL VEIL. inoculation (1) The placing of material (the INOCULUM) on or into a MEDIUM, TISSUE CULTURE, animal etc using an ASEPTIC TECHNIQUE. If the purpose of inoculation is to initiate a CULTURE, the inoculated medium must be incubated (see INCUBATION). (2) (immunol.) Syn. VACCINATION (1) or (2). inoculative infection (parasitol.) Infection in which a pathogen is introduced into the vertebrate host from the ANTERIOR STATION. (cf. CONTAMINATIVE INFECTION.) inoculum (1) Material used for the INOCULATION of a MEDIUM, TISSUE CULTURE, animal etc; it typically comprises or contains viable microorganisms. (2) (plant pathol.) Pathogenic microorganisms which have become established within a plant and which may give rise to disease. inoculum effect In the in vitro testing of bacteria for resistance to antibiotic(s): the phenomenon in which the MIC increases when the size of the test inoculum (cells/ml) is increased; for a given strain of bacteria the inoculum effect may differ greatly with different antibiotics. Inocybe See AGARICALES (Cortinariaceae). Inonotus A genus of fungi of the APHYLLOPHORALES (family Hymenochaetaceae) which form basidiocarps in which the context is reddish-brown and fibrous. I. dryadeus (formerly Polyporus dryadeus) forms bracket-type basidiocarps (context: monomitic) with a pale grey (later darkening) upper surface and a porous underside; basidiospores: whitish, spheroidal, ca. 7 µm diameter. It is parasitic on various types of oak (Quercus). (See also BUTT ROT and POCKET ROT.) GENIC VARIATION; 397 inoperculate RFI v strand at a specific site in the IG region; the free 3
end is elongated as in other ssDNA phages (q.v.). Genome lengths of displaced v ssDNA are cleaved by gp2, and each is circularized to form v ss cccDNA. At first, new v strands are converted to RFs. However, a phage-coded SSB protein (gp5) gradually accumulates, eventually coating the newly formed v strands and blocking their conversion to RFs (stage III). Phage coat proteins (at least gp8 and gp3) are synthesized as soluble cytoplasmic precursors with a ‘signal peptide’ at the amino end (see SIGNAL HYPOTHESIS); these proteins become inserted asymmetrically into the cytoplasmic membrane, the signal peptides are removed, and the proteins are ‘anchored’ within the membrane [PNAS (1982) 79 5200–5204]. Phage assembly – which requires THIOREDOXIN [PNAS (1985) 82 29–33] – occurs (in f1) at zones of adhesion between the cytoplasmic and outer membranes, f1 gp1 possibly being involved in the formation of these adhesion zones [JB (1985) 163 1270–1274] (cf. ADHESION SITE); gp8 displaces gp5, and the mature virion is finally extruded through the host cell envelope. insect diseases Bacterial pathogens of insects include e.g. Bacillus spp (see e.g. AMERICAN FOULBROOD, DELTA-ENDOTOXIN, MILKY DISEASE), Melissococcus pluton (see EUROPEAN FOULBROOD), and species of RICKETTSIELLA, SERRATIA and SPIROPLASMA. (See also CECROPINS and SARCOTOXINS.) Fungal entomopathogens include species of e.g. Ascosphaera (see CHALK-BROOD); ASCHERSONIA; Aspergillus (see e.g. STONEBROOD); BEAUVERIA; Coelomomyces (see BLASTOCLADIALES); CORDYCEPS; CULICINOMYCES; ENTOMOPHTHORA, ERYNIA and other fungi of the ENTOMOPHTHORALES; FUSARIUM; HIRSUTELLA; METARHIZIUM; NOMURAEA; SEPTOBASIDIALES; TERMITARIA; VERTICILLIUM. Protozoal entomopathogens include e.g. species of MALPIGHAMOEBA, NOSEMA and THELOHANIA. (See also CRITHIDIA, HERPETOMONAS and LEPTOMONAS.) For viral entomopathogens see e.g. BACULOVIRIDAE; CYTOPLASMIC POLYHEDROSIS VIRUS GROUP; DENSOVIRUS; DROSOPHILA X VIRUS; ENTOMOPOXVIRINAE; FLACHERIE; HOUSEFLY VIRUS; NODAVIRIDAE; NUDAURELIA b VIRUS GROUP; PICORNAVIRIDAE; POLYDNAVIRIDAE; SIGMA VIRUS. (See also INVERTEBRATE DISEASES and BIOLOGICAL CONTROL.) insect iridescent viruses See CHLORIRIDOVIRUS and IRIDOVIRUS. insect–microbe associations See e.g. AMBROSIA FUNGI; DUTCH ELM DISEASE; FUNGUS GARDENS; FUNGUS GNATS; INSECT DISEASES; MYCETOCYTE; TERMITE–MICROBE ASSOCIATIONS; WOODWASP FUNGI. insecticidal crystal protein (ICP) See BIOLOGICAL CONTROL. insertion mutation (addition mutation) A type of MUTATION in which one or more nucleotides are inserted into the genome; if the number of nucleotides inserted is not divisible by 3, the mutation will be a FRAMESHIFT MUTATION. Insertion mutations commonly result from the insertion of a TRANSPOSABLE ELEMENT or prophage (see also BACTERIOPHAGE CONVERSION). (cf. DELETION MUTATION.) insertion sequence (IS element) A TRANSPOSABLE ELEMENT which contains no genetic information other than that necessary for its transposition. (cf. TRANSPOSON.) An IS element may transpose independently, when it may be detectable by the effect(s) of its insertion at a new site, or it may occur as a terminal part of a composite (class I) TRANSPOSON. An IS element is defined by its ends: an INVERTED REPEAT sequence of ca. 10–40 bp has been found to occur at both ends in every IS element so far analysed; these inverted repeats are apparently necessary for transposition and are probably the recognition sites for the inoperculate Lacking an OPERCULUM. inorganic pyrophosphate See PYROPHOSPHATE. iNOS See NITRIC OXIDE. inosine See NUCLEOSIDE and Appendix V(a). inositol Hexahydroxycyclohexane. Theoretically there are nine stereoisomers; of these, meso-inositol (= myo-inositol), scyllitol, d-inositol and l-inositol occur naturally, and only meso-inositol is common. Meso-inositol occurs widely e.g. in the phospholipids of microorganisms; it is required, in small amounts, for the growth of a number of microorganisms – including many yeasts (see e.g. BREWING). Inositol is used as a substrate in biochemical characterization tests for bacteria; it is attacked e.g. by Klebsiella pneumoniae. (See also CYCLITOL ANTIBIOTICS.) Inoue–Melnick virus A virus which has been isolated from patients with chronic CNS disease (including MULTIPLE SCLEROSIS) [JCM (1985) 21 698–701]. Inoviridae A family of SSDNA PHAGES in which the virions are rod-shaped or filamentous, the capsid subunits being arranged helically around one piece of ss cccDNA; the virions are chloroform-sensitive. On entry into a host cell the ssDNA is converted into a double-stranded replicative form from which progeny ssDNA can be synthesized. Mature phages are extruded through the host cell envelope. The host remains viable but grows more slowly than an uninfected cell. There are two genera: INOVIRUS and PLECTROVIRUS. [Book ref. 23, pp. 78–79.] Inovirus (filamentous phages) A genus of non-lytic SSDNA PHAGES of the INOVIRIDAE. Hosts: enterobacteria (for e.g. phages fd, f1, M13, If1, If2, IKe, ZJ/2), Pseudomonas (phages Pf1, Pf2), Xanthomonas (phages Xf, Xf2, Cf), and Vibrio (phage v6); most or all inoviruses can infect only ‘male’ cells (i.e. those containing CONJUGATIVE PLASMIDS). Proposed type member: bacteriophage fd; f1 and M13 are very similar to fd. The inovirus virion is a flexible filament (760–1950 × 6 nm, according to phage) comprising ss cccDNA (MWt ca. 1.9–2.7 × 106 ), one major coat protein (gp8, B-protein), and minor coat proteins A (gp3), C (gp9) and D (gp6) at 5, 10 and 5 copies/virion, respectively. Proteins A and D occur at one end of the virion; C seems to occur at the opposite end. The A-protein (gp3) is necessary for adsorption; the roles of C and D are unknown. The DNA loop fits inside the capsid (internal diam. ca. 2 nm) such that the two adjacent (antiparallel) strands of the loop are wound around each other in a type of double helix (pitch ca. 2.7 nm) despite the lack of homology between them; the helix seems to be stabilized by the coat protein [JMB (1982) 157 321–330]. The following refers to fd, f1 and M13; other inoviruses seem to be generally similar. Phages fd, f1 and M13 adsorb specifically to the tips of F and F-like pili (cf. BACTERIOPHAGE IKE, BACTERIOPHAGE IF1, LEVIVIRIDAE). During penetration of the host (which may involve pilus retraction) the capsid proteins (at least gp3 and gp8) become localized asymmetrically in the cell membrane. At it penetrates the cell, the phage DNA becomes coated with host SSB protein, initiating stage I (see SSDNA PHAGES). A unique 508-nucleotide intergenic (IG) region between genes 2 and 4 may remain uncoated; this region binds host RNA POLYMERASE holoenzyme which may recognize the secondary or tertiary structure of DNA in the IG region (which can form several hairpin loops). The RNA polymerase synthesizes an RNA primer at a specific site (ori ) in the IG region. The complementary (c) strand, and hence RF, is completed as in other ssDNA phages (q.v.). RF is supercoiled by GYRASE to become RFI (see RF). The c strand of RFI functions as template for transcription of phage genes. An early product is an endonuclease (gp2) which initiates stage II by cleavage of the 398 intein integrative suppression The phenomenon in which the integration of a plasmid into a bacterial chromosome suppresses the effect of a chromosomal mutation which has inhibited the initiation of chromosome replication; replication of the joint plasmid–chromosome replicon is apparently initiated at a site within the plasmid. Thus, e.g., as a result of integrative suppression, a temperature-sensitive dnaA mutant of Escherichia coli can carry out limited chromosome replication at a non-permissive temperature. integrins A family of CELL ADHESION MOLECULES found on various types of cell and involved in many aspects of cell physiology; the molecule is a heterodimer of proteins consisting of an a subunit non-covalently linked to a b subunit – the particular combination of a subunit (many types) and b subunit (few types) determining the biological activity of each integrin. Integrins of the b1 subfamily are found e.g. on various types of epithelial cell – either on the basolateral surface or (e.g. a6 b1 ) on the basal surface (i.e. adjacent to the basement membrane) [integrins (review): HJ (1993) 25 469–477]. These integrins have a major role in cell–cell and cell–matrix interaction – binding to ligands that include collagen, fibronectin and laminin; however, such interaction involves more than cell position and maintenance of tissue structure: integrin–ligand binding can also trigger signals that modulate a wide range of cell functions, including aspects of the cell cycle. The b1 integrins are also implicated as receptors for the adhesins of invasive pathogenic bacteria; for example, adhesins of Yersinia enterocolitica have been found to bind (in vitro) to the a5 b1 integrin of epithelial cells. The b2 integrins are found primarily on leukocytes; their expression on a given cell is apparently dependent on activation – e.g. cytokine-stimulated cells may exhibit the leukocyte function-associated antigen-1 (LFA-1; = CD11a/CD18; = aL b2 ) and the Mac-1 integrin (= CD11b/CD18; = aM b2 ); both of these integrins can bind to ICAM ligands (see IMMUNOGLOBULIN SUPERFAMILY and INFLAMMATION). The b2 integrins can also serve as receptors for bacterial adhesins. Thus, many integrins bind (via their a subunit) to ligands that contain the amino acid sequence Arg-GlyAsp (RGD), and this sequence is mimicked by the adhesin filamentous haemagglutinin (FHA) of Bordetella pertussis; the binding between FHA (RGD sequence) and Mac-1 (aM b2 ) of an activated macrophage assists bacterial invasion of the phagocyte. The b3 integrin aIIb b3 (gpIIbIIIa; = CD41b/CD61) is found on platelets and megakaryocytes; its ligands include fibrinogen, VON WILLEBRAND FACTOR and fibronectin. Failure to synthesize adequate amounts of this integrin results in deficient platelet aggregation (Glanzmann’s thrombasthenia). (See also LEUKOCYTE ADHESION DEFICIENCY.) [Defects in human integrins: Lancet (1999) 353 341–343.] integron A region of DNA which includes (i) a gene encoding a site-specific recombinase (see SITE-SPECIFIC RECOMBINATION) and (ii) a corresponding site for recombination into which one or more genes may be inserted [see e.g. JAC (1999) 43 1–4]. Integrons may have a role as a general gene-capture mechanism in bacterial evolution [PNAS (2001) 98 652–657]. intein In certain types of protein: an internal sequence of amino acids apparently capable of catalysing a self-splicing reaction in which the intein is excised (forming a separate protein) and the terminal parts of the original polypeptide (the two exteins) are joined to form a functional protein. [Review: TIBS (1995) 20 351–356. Intein sequences (review): NAR (1997) 25 1087–1093.] corresponding transposition enzymes. Most IS elements also have the potential for encoding 1–3 polypeptides, presumed to include TRANSPOSASE component(s). In many IS elements a small open reading frame occurs within the larger open reading frame but in the opposite orientation; IS50 has overlapping genes (see Tn5 ). IS elements (designated IS1, IS2 etc) may be classified according to their terminal inverted repeats and the number of base pairs duplicated at the target site on insertion. Different IS elements transpose with different frequencies depending e.g. on the structure of the donor and target sequences, on the physiological state of the host cell, etc; frequencies of transposition range from ca. 10−9 to 10−5 per IS element per cell division. (See also e.g. IS1, IS5, IS101.) [Review: Book ref. 20, pp. 159–221.] insertion vector A CLONING vector which has a single site at which a sequence of exogenous DNA can be inserted (cf. REPLACEMENT VECTOR). inside-to-outside model A model which describes the mode of incorporation of PEPTIDOGLYCAN into the CELL WALL of a growing Gram-positive bacillus [FEMS Reviews (1986) 32 247–254; J. Theor. Biol. (1985) 117 137–157]. Essentially, layers of newly formed peptidoglycan are laid down just external to the cytoplasmic membrane – a region in which they are subject to minimal hydrostatic stress; as growth continues, the layers move outwards towards the cell surface (being replaced by new layers) and accordingly bear an increasing proportion of the hydrostatic pressure. Finally, near the region of maximum stress, the peptidoglycan becomes increasingly susceptible to autolysins, the (older) layers of peptidoglycan commonly being shed in fragments. inspissation The process of thickening. In microbiology: a process in which certain heat-coagulable substances (e.g. serum, homogenized hens’ eggs) – or media containing such substances – are solidified by heating to ca. 85° C for ca. 1 hour. (See e.g. LÖWENSTEIN–JENSEN MEDIUM.) instructive theories of antibody-formation Theories (now untenable) which hold that the ability to produce specific antibodies develops in antibody-producing cells only after their initial exposure to the antigen, i.e., it is assumed that pre-existing specificity does not occur. In such theories antigen is supposed to act as a template for the production of specific antibodies or for the formation of an antibody-specifying nucleic acid. (cf. CLONAL SELECTION THEORY.) int (1) See e.g. BACTERIOPHAGE LAMBDA and BACTERIOPHAGE P22. (2) See MOUSE MAMMARY TUMOUR VIRUS. integral membrane protein See CYTOPLASMIC MEMBRANE. integrase See e.g. BACTERIOPHAGE l. integrase-type recombinase See SITE-SPECIFIC RECOMBINATION. integration host factor (IHF) In e.g. Escherichia coli: a heterodimeric protein consisting of polypeptide chains of apparent MWt 11000 and 9500 [JBC (1981) 256 9246–9253]; IHF, which has the ability to induce bends in DNA, has been associated with various roles that include e.g. stimulating the transcription of nitrogen-fixing operons [Cell (1990) 63 11–22], promoting expression of the tra operon in the F plasmid [JB (1990) 172 4603–4609], stimulating helicase-catalysed nicking of the F plasmid at oriT [JBC (1995) 270 28374–28380] and promoting gene specificity for a given sigma factor [EMBO (2000) 19 3028–3037]. IHF is also one of the factors needed for the on–off switching of type I fimbriae in Escherichia coli. (See also SIDD.) 399 interallelic complementation interallelic complementation See COMPLEMENTATION TEST. intercalary Not apical or terminal: e.g. formed or situated between apex and base of a given structure or between two cells in a chain of cells. intercalating agent Any dye possessing a planar chromophore which can insert (intercalate) between adjacent base pairs in dsDNA or in base-paired regions in ssDNA (e.g. HAIRPIN loops); bifunctional (bis) intercalating agents have two such chromophores per molecule (see e.g. QUINOXALINE ANTIBIOTICS). Intercalation forces the base pairs apart and causes (a) local unwinding of the helix, the degree of unwinding (unwinding angle, f) depending on the nature of the dye, and (b) an increase in the length of the DNA. In linear dsDNA these effects lead to an increase in viscosity and a decrease in sedimentation coefficient (s) of the DNA in proportion to the amount of dye intercalated. Intercalative binding is limited to a maximum of one dye molecule per 2–3 bp; the binding of a dye molecule at one site precludes binding of others at the adjacent sites (the neighbour exclusion principle). In circular dsDNA intercalation causes a local increase in pitch and thus a decrease in local (and hence average overall) twist. It follows from W r = Lk − T w (see DNA) that, as increasing amounts of dye intercalate in a negatively supercoiled molecule, the amount of writhe progressively decreases to zero (molecule behaves as relaxed), and then increases again as the molecule becomes positively supercoiled. The changes in writhe are accompanied by changes in viscosity of the DNA solution (increase to a maximum, then decrease) and in sedimentation coefficient of the DNA (decrease, then increase). Since intercalation relieves the strain of negative supercoiling, a negatively supercoiled cccDNA molecule has a higher affinity for intercalating agents than does the equivalent nicked or linear DNA, while a relaxed or positively supercoiled cccDNA has a lower affinity than the equivalent nicked or linear DNA. However, a cccDNA molecule has a limited capacity to unwind, and can therefore bind fewer molecules of intercalating agent than can equivalent linear or nicked circular DNA molecules (see e.g. ETHIDIUM BROMIDE). Many intercalating agents have antimicrobial and antitumour activity, inhibiting e.g. transcription, DNA replication etc, and inducing FRAMESHIFT MUTATIONS. In vivo, intercalation is often followed by DNA strand breakage, possibly due to the action of nucleases which recognize distortion of the helix at the intercalation sites. At lower concentrations some intercalators (e.g. aminoACRIDINES, ETHIDIUM BROMIDE) can cause the selective loss of small cccDNAs such as plasmids (see CURING (2)), mtDNA (see PETITE MUTANT), ctDNA, and kinetoplast DNA. (See also ACTINOMYCIN D; ANTHRACYCLINE ANTIBIOTICS; QUINOXALINE ANTIBIOTICS; TILORONE. cf. MITOMYCIN C and PSORALENS.) [Review of DNA-binding drugs: Bioch. J. (1987) 243 1–13.] intercistronic complementation See COMPLEMENTATION TEST. intercrines Syn. CHEMOKINES; thus, a-intercrines = a-chemokines, b-intercrines = b-chemokines. interesterification An industrial process in which fatty acyl residues are interchanged among the various triglycerides in a mixture of lipids; the process, which is catalysed e.g. by sodium methoxide, is carried out in order to modify the composition (and hence properties) of oils and fats. Interesterification can be carried out using certain microbial LIPASES for the production of useful mixtures of triglycerides which cannot be made by conventional chemical processes. [Enzyme-catalysed modification of oils and fats: PTRSLB (1985) 310 227–233.] interference (1) (virol.) The phenomenon in which the replication of one virus (the challenge virus) is partially or completely inhibited by the presence in the host cells of another (interfering) virus. In homologous interference the challenge and interfering viruses are of similar or identical types, while in heterologous interference they are unrelated (see e.g. RUBIVIRUS). Interference may result e.g. from competition between the viruses for components of the replication apparatus, etc. (See also DEFECTIVE INTERFERING PARTICLE.) (2) (genetics) The phenomenon in which the occurrence of one recombinational event affects the chances of another occurring between adjacent regions of the same molecules of nucleic acid. In chiasma interference (= positive interference) the occurrence of a cross-over tends to prevent the occurrence of a second cross-over between nearby regions in the same duplexes; this effect decreases with increasing distance from the cross-over. In markers which are closely linked, a localized negative interference is sometimes observed: the occurrence of one recombinational event promotes recombination in an adjacent region; this can be due to the correction of adjacent sequences in hybrid DNA during gene conversion (co-conversion – see RECOMBINATION). interference-contrast microscopy See MICROSCOPY (d). interfering virus See INTERFERENCE (1). interferons (IFNs) A category of CYTOKINES which are able to inhibit the replication of many types of virus in (i) the IFNproducing cell, and (ii) cells exposed to exogenous IFNs; IFNs also have other important roles in host defence mechanisms (see later). Human interferons are divided into two main groups that are designated types I and II. The type I interferons (stable at pH 2) include IFN-a (∼17–26 kDa), IFN-b (∼21 kDa) and IFN-!; these interferons appear to be related – their genes (all of which are intron-less) may have developed from a common ancestral gene. The IFN-a gene exists in many allelic forms (subtypes). The type II (acid-labile) interferon IFN-g (∼25 kDa) exhibits little homology with type I interferons in terms of amino acid sequence; even so, the three-dimensional structure of the molecule has features in common with IFN-b. The gene encoding IFN-g contains three introns. Distinct types of interferon are found in other animals; for example, IFN-t (structurally related to IFN-!) occurs in cows and sheep. These animals also form e.g. interferons related to IFN-b. (A given type of interferon from one species may or may not be active in another species.) Human interferons of both types have antiviral activity, although this function appears to be of major importance primarily in type I interferons. The type II interferon (IFN-g) plays a major role in various immune defence mechanisms. Alpha interferon (IFN-a). IFN-a is produced by leukocytes. At least 14 subtypes of IFN-a occur in humans; these are designated IFN-a1, IFN-a2 etc. Beta interferon (IFN-b). IFN-b is produced by fibroblasts. Compared with IFN-a, the amino acid sequence of IFN-b exhibits only 30% homology; the two IFNs are antigenically distinct. Both IFN-a and IFN-b can induce intracellular antiviral effects that include degradation of viral RNA and inhibition of protein synthesis (described later). These IFNs also upregulate MHC class I molecules at the surface of virus-infected cells; this is of value in that class I molecules act as binding sites for CD8+ cytotoxic T cells (which kill virus-infected cells). Gamma interferon (IFN-g). IFN-g is produced by antigenically and/or mitogenically stimulated T lymphocytes. (See also 400 interleukin-1 an enzyme, RNase L (also called RNase F), which can cleave viral and cellular RNA; however, 2–5A is unstable, being enzymically degraded by 2
,5
-phosphodiesterase. The antiviral activity of interferons has been assayed in vitro by a plaque-reduction method in which cultured cells are treated with IFN and subsequently challenged with virus; the titre of IFN may be taken as the reciprocal of the highest dilution of IFN that produces a 50% decrease in the number of plaques. Therapeutic uses of interferons. Both natural and recombinant forms of interferon (types I and II) have been evaluated for clinical use in a range of diseases, including various types of malignancy, Kaposi’s sarcoma (IFN-a), multiple sclerosis (IFN-b), diseases of viral aetiology, and disease caused by Mycobacterium tuberculosis (IFN-g). Commercial products include Alferon N (multicomponent IFN-a; Interferon Sciences, USA); Betaferon and Betaseron (recombinant IFN-b; Schering AG); Gammaferon (recombinant IFN-g; Rentschler, Germany). [Clinical uses of interferons: Book ref. 226, pp 237–271.] The enhanced capacity of lymphocytes to secrete IFN-g when thalidomide is used for adjuvant therapy may offer an explanation for the ability of thalidomide to bring about clinical improvement in patients infected with e.g. Mycobacterium tuberculosis and HIV [AAC (2000) 44 2286–2290]. (See also ELISPOT ASSAY (for TB).) intergenic complementation See COMPLEMENTATION TEST. intergenic repeat unit Syn. ERIC SEQUENCE. intergenic spacer region See RIBOTYPING. intergenic suppression See SUPPRESSOR MUTATION. intergenote A partial zygote formed by heterologous TRANSFORMATION (i.e., transformation in which the donor DNA is derived from a species different from that of the recipient). intergranal frets See CHLOROPLAST. interkinesis See MEIOSIS. interkinetal Refers e.g. to the plane of cell division typical of members of the OPALINATA: longitudinal, between the rows of kineties. (cf. HOMOTHETOGENIC.) interleukin-1 (IL-1) A cytokine (see INTERLEUKINS) secreted by various types of cell, including monocytes, fibroblasts, dendritic cells and endothelial cells. There are several forms of IL-1: IL1a (previously called lymphocyte activating factor, LAF) and IL-1b (both pro-inflammatory cytokines with similar or identical functions) and IL-1ra; IL-1ra binds to the cognate receptor of IL-1a and IL-1b but, having no agonist activity, it functions as an antagonist of the a and b forms of IL-1. (There are two types of receptor for IL-1 but only one is functional; the other (‘decoy’) receptor does not initiate intracellular signals when IL-1a or IL-1b binds to it.) For IL-1g see INTERLEUKIN-18. The mature forms of IL-1a and IL-1b are produced by intracellular cleavage of precursor pro-proteins. IL-1b is cleaved (activated) by a cysteine protease referred to as IL-1b-converting enzyme (ICE). (ICE also has a role in APOPTOSIS.) Cells stimulated to produce IL-1 generally form both the a and b types; however, in at least some cases (e.g. a monocyte responding to LPS), the amount of IL-b produced is much greater than that of IL-1a. The known or presumed functions of IL-1a and IL-1b in vivo include: Activation, and induction of cytokine synthesis in macrophages. Induction of ACUTE-PHASE PROTEINS in the liver. Stimulation of activated B cells to proliferate and form antibody (and possibly stimulation of T cells). INTERLEUKIN-18.) The active form of IFN-g is a non-covalentlybound dimer. Like type I interferons, IFN-g can upregulate class I MHC molecules. It can also upregulate class II MHC molecules on certain types of cell (including fibroblasts and endothelial cells); class II molecules promote the ability of cells to present antigen to CD4+ T cells (for which MHC class II molecules are binding sites). IFN-g is categorized as a pro-inflammatory cytokine (see CYTOKINES). In addition to its ability to upregulate MHC molecules, IFN-g can also e.g. (i) activate macrophages, (ii) stimulate the synthesis of NITRIC OXIDE, and (iii) promote class switching in B cells – upregulating e.g. IgG1 (and murine IgG2a) antibodies. Induction of interferons. Agents which induce the synthesis of interferons include many types of virus, certain bacteria and protozoa, mitogens, lipopolysaccharides, and double-stranded nucleic acids (e.g. poly(I:C) – a polymer consisting of one strand each of polyinosinic acid and polycytidylic acid). (See also TILORONE.) Many of the studies on induction of interferons have investigated transcriptional regulation of the gene encoding IFNb – apparently because this gene lacks the many subtypes of IFN-a and because, unlike IFN-g, it lacks introns. Induction of the gene encoding IFN-b during viral infection appears to involve activation of the transcription factor NF-kB; this seems to depend on phosphorylation of IkB by a dsRNA-dependent kinase designated PKR [PNAS (1994) 91 6288–6292]. (IkB normally inhibits NF-kB by binding to it; phosphorylation (and hence degradation) of IkB therefore results in the activation of NF-kB.) Viral infection also appears to promote synthesis of ‘interferon regulatory factor-1’ (IRF-1) which, together with activated NF-kB, may promote transcription from the regulatory region of the IFN-b gene. Antiviral activity of interferons. Exogenous interferons initially bind to specific cell-surface receptors. There are various kinds of receptor for type I interferons; some (not all) are common to IFN-a, IFN-b and IFN-!. The receptor for IFN-g is distinct from those that bind type I interferons. In all cases, however, the receptor for an interferon consists of a heterodimeric structure associated with tyrosine kinases on the cytoplasmic side; the kinases are involved in initiating the intracellular signal that leads to transcriptional upregulation of certain genes. A generalized account of antiviral activity is as follows. The binding of interferon to a specific receptor activates a JAK–STAT signalling pathway (see JAK and STATs in CYTOKINES); this promotes transcription of certain genes encoding antiviral proteins. The type of interferon-inducible protein produced in any given case will depend on the type of cell and the type of interferon. Proteins induced by interferons include e.g. receptors for TNF; class I and II MHC antigens; nitric oxide synthetase; CD54 (inducible only by IFN-g); Mx protein (an inhibitor of influenzavirus replication inducible only by type I interferons); PKR (a protein kinase); and 2–5A synthetase. The two latter proteins are involved in generalized antiviral activity, as follows. When activated by dsRNA, the PKR kinase phosphorylates (and therefore inactivates) the a subunit of the protein synthesis initiation factor 2; this inhibits formation of initiation complexes and, hence, inhibits the synthesis of both viral and cellular proteins. The 2–5A synthetase (activated by dsRNA) catalyses the synthesis of 2–5A (= 2,5-oligoadenylate). The 2–5A activates 401 interleukin-2 but, unlike e.g. TNF, it does not induce shock when administered experimentally. interleukin-10 (IL-10) A cytokine (see INTERLEUKINS) produced e.g. by the Th2 subset of T cells and by monocytes stimulated by LPS. IL-10 has anti-inflammatory activity; for example, it inhibits proliferation and release of cytokines in antigenstimulated Th1 cells, it inhibits release of pro-inflammatory cytokines from monocytes and macrophages, and it promotes proliferation and synthesis of antibodies in activated B cells. interleukin-12 (IL-12; natural killer stimulatory factor) A cytokine (see INTERLEUKINS) produced by B cells and also by macrophages acting as antigen-presenting cells. IL-12 appears to be essential for development of the Th1 subset of T cells from naive T cells; it also stimulates cytotoxicity in NK cells and enhances the release of cytokines from NK cells, macrophages and T cells. IL-12 also has anti-angiogenic activity (it inhibits development of new blood vessels). The ability of macrophages to synthesize IL-12 is inhibited by the binding of measles virus to the complement receptor CD46. interleukin-18 (syn. interleukin-1g) An INTERLEUKIN produced e.g. by macrophages, keratinocytes and (murine) Kupffer cells. IL-18 is a pro-inflammatory cytokine which induces IFN-g in T cells (and murine spleen cells), enhances cytotoxicity in NK cells, and promotes the Th1 (rather than Th2) immune response. IL-18 also triggers activation of NF-kB and synthesis of e.g. IL-1b, TNF-a and Fas ligand. IL-18 is synthesized in a biologically inactive form; it is activated by IL-1b converting enzyme (ICE; also referred to as caspase-1). IL-18-mediated induction of IFN-g can be inhibited by the binding of poxvirus-encoded proteins to IL-18 [PNAS (1999) 96 11537–11542 (correction: PNAS (2000) 97 11673)]. interleukin-21 An INTERLEUKIN which, in vitro, influences the proliferation/maturation of NK cells from bone marrow, the proliferation of B cells co-stimulated with anti-CD40, and the proliferation of T cells co-stimulated with anti-CD3. IL-21 is synthesized by activated CD4+ T cells. [Nature (2000) 408 57–63.] interleukins Certain CYTOKINES which are secreted by, and effective on, leukocytes (white blood cells) – but which, in some cases, can also be secreted by other types of cell (e.g. endothelial cells). It should be noted that the nomenclature is inconsistent in that some cytokines which conform to this description (e.g. tumour necrosis factor) are not formally referred to as interleukins; moreover, some interleukins are also classified in other named categories (e.g. interleukin-8 is a chemokine). Each interleukin (IL) is designated by a number – e.g. IL-4, IL-6, IL-21. (See also separate entries for individual interleukins.) intermediate filaments (IFs; IMFs) In most types of eukaryotic cell: protein filaments, ca. 7–11 nm thick, which form part of the CYTOSKELETON. IMFs appear to be much more stable than either microfilaments or microtubules, and their roles are believed to include the strengthening of the cytoskeleton. IMFs differ e.g. according to cell type and species, and the monomers of different IMFs vary widely in size; the monomer vimentin (MWt ca. 55000) occurs in many types of cell. intermediate host In heteroxenous coccidia: the host in which the asexual phase occurs (see EIMERIORINA). internal guide sequence See SPLIT GENE (b). internalin A See LISTERIOSIS. interphase (1) See MITOSIS. (2) See DICTYOSTELIOMYCETES. interrupted gene Syn. SPLIT GENE. Induction of synthesis of endothelial adhesion molecules during INFLAMMATION; induction of e.g. IL-1 and colony-stimulating factors in endothelial cells. Induction of fever (apparently by stimulation of prostaglandins in the temperature regulatory region of the brain). The biological activities of IL-1 overlap with those of tumour necrosis factor (TNF); IL-1b and TNF-a have been found to be synergistic when jointly administered to animals. Moreover, IL1b and TNF-a stimulate the production of each other (as well as their own synthesis). However, despite the overlap in activities, it appears that, unlike TNF-a, IL-1 cannot promote cytotoxic activity via apoptosis. interleukin-2 (IL-2; killer helper factor, KHF; T cell growth factor, TCGF) A cytokine (see INTERLEUKINS) produced by T lymphocytes. In vitro, IL-2 can stimulate proliferation of antigenically or mitogenically activated T cells; in vivo, it appears to promote the growth/differentiation of T cells and to enhance the cytotoxicity of CD8+ T cells and NK cells. (See also CD28 and ANTIBODY FORMATION.) IL-2 promotes development of the Th1 subset of T lymphocytes, and is one of the cytokines secreted by these proinflammatory cells. The gene encoding IL-2 (like that encoding IFN-g) contains three introns. (See also SEVERE COMBINED IMMUNODEFICIENCY.) interleukin-4 (IL-4) A cytokine (see INTERLEUKINS) produced by lymphocytes of the Th2 subset. IL-4 promotes differentiation of Th0 to Th2 cells, and in antigenically stimulated B cells it seems to e.g. induce class switching to IgG4 and IgE (IgG1 and IgE in mice). IL-4 is also involved in the formation of eosinophils – probably through its ability to promote the Th2 subset and (consequent) secretion of interleukin-5 (a cytokine required for the normal development of eosinophils). The ability of naive CD4+ T cells to secrete IL-4 is inhibited when ICAM-1 is co-expressed with antigen on the antigenpresenting cell [PNAS (1999) 96 3023–3028]. interleukin-5 (IL-5) A cytokine (see INTERLEUKINS) produced by the Th2 subset of T lymphocytes (see also INTERLEUKIN-4). IL5 appears to be the main stimulus for eosinophilia in certain parasitic infections. [Eosinophilia and helminthic infections: BCH (2000) 13 301–317.] interleukin-6 (IL-6) A cytokine (see INTERLEUKINS) produced by a variety of cells, including monocytes/macrophages, fibroblasts, endothelial cells and the Th2 subset of T lymphocytes. Synthesis of IL-6 in monocytes/macrophages can be induced by LPS (lipopolysaccharides) or by cytokines (TNF-a or IL-1); the P FIMBRIAE of uropathogenic Escherichia coli are reported to be potent inducers of IL-6. IL-6 appears to have many roles in vivo. These roles include stimulation of growth, differentiation and antibody production in B cells, and induction of ACUTE-PHASE PROTEINS; in at least some cases IL-6 may contribute to the development of fever. interleukin-8 (IL-8) A cytokine (see INTERLEUKINS) produced by many types of cell, including monocytes/macrophages, neutrophils, fibroblasts, epithelial and endothelial cells, keratinocytes, NK cells and T cells. Agents that stimulate production of IL-8 include LPS and the cytokines IL-1 and TNF; production of IL-8 is inhibited by certain cytokines (e.g. IFN-g and IL-4) and by glucocorticoids. IL-8 acts as a chemoattractant and activator primarily for neutrophils (see CHEMOKINES), and its main role in vivo is believed to be the recruitment of leukocytes to sites of infection/INFLAMMATION. IL-8 is found in many inflammatory diseases 402 invertase intravital staining See VITAL STAINING. intrinsic resistance (to antibiotics) See ANTIBIOTIC. intron See SPLIT GENE. intron homing (in bacteria) The process in which introns (of either group I or group II) spread, in replicative fashion, to allelic intron-less genes. The mechanism of intron homing differs in the two types of intron. Intron homing in group I introns is initiated when a DNA endonuclease (homing endonuclease), encoded within the intron sequence, acts at the potential insertion site in an intron-less allele, typically making a double-stranded cut. The lesion is repaired by enzymes which use the intron-containing gene as a template; in this way, a copy of the intron is synthesized in the insertion site of the intron-less allele. In group II introns, the process begins with an intron-encoded protein which forms a complex with an RNA copy of the intron. This RNA–protein (RNP) complex is able to recognize the insertion site in an intron-less allele. The RNP complex can then (i) cut both strands at the target site, covalently inserting the RNA into one strand, and (ii) form a cDNA copy of the RNA (by virtue of the protein’s reverse transcriptase function); the RNA is then replaced by DNA. The involvement of a reversetranscribed copy of the intron (cf. group I introns) is reflected in the term retrohoming for the process in bacterial group II introns. Intron homing (in both group I and II bacterial introns) is outlined in a recent minireview [JB (2000) 182 5281–5289 (5281–5282)]. inulin A linear FRUCTAN consisting of (2 → 1)-b-linked fructofuranose residues. Inulins occur e.g. as storage polysaccharides in certain plants (e.g. in dahlia tubers). inulinase An enzyme (b-fructosidase, 2,1-b-D-fructan-fructanohydrolase, EC 3.2.1.7) which hydrolyses INULIN to FRUCTOSE. Many microbial inulinases also show INVERTASE activity, and some can hydrolyse bacterial LEVAN. Microbial inulinases may prove useful in the commercial production of fructose from plant inulins [AAM (1983) 29 139–176]. inv gene (Yersinia) See FOOD POISONING (Yersinia). inv–spa genes See PATHOGENICITY ISLAND and FOOD POISONING (Salmonella). invasin In certain pathogenic bacteria: a type of cell-surface molecule which promotes uptake of the pathogen by (eukaryotic) host cells. Examples of invasins include ‘invasin’ (see FOOD POISONING (Yersinia)) and ‘internalin A’ (see LISTERIOSIS). In strains of UPEC (q.v.), the FimH adhesin appears to function as an invasin. invasion (of phagocytes by pathogens) See end of section (a) in PHAGOCYTOSIS. invasiveness (of a pathogenic microorganism) The ability of a pathogen to spread through a host’s tissues; the degree of invasiveness reflects the relative insusceptibility of the pathogen to the host’s defense mechanisms. (See also AGGRESSIN.) invasomes See FOOD POISONING (Salmonella). inversion (mol. biol.) See e.g. CHROMOSOME ABERRATION and RECOMBINATIONAL REGULATION. invert sugar See INVERTASE. invertase (1) (saccharase; sucrase; b-fructosidase; b-D-fructofuranoside fructohydrolase; EC 3.2.1.26) An enzyme which hydrolyses SUCROSE to glucose and fructose; it can also hydrolyse raffinose to fructose and melibiose. Invertase occurs in many yeasts and other fungi and in higher plants; it is obtained commercially mainly from BAKERS’ YEAST in which it occurs in the cell wall as a mannoprotein containing ca. 50% mannose. Invertase is used chiefly in the conversion of sucrose to ‘invert interrupted mating A technique used for studying gene transfer during bacterial CONJUGATION. For example, a population of cells of an Hfr strain of Escherichia coli is mixed with an F− (recipient) population (time zero). At regular intervals of time, a sample is taken from the mating mixture, agitated to separate mating cells, diluted to reduce the probability of further mating contacts, and plated on a medium which will support the growth of particular recombinant(s) but not of the parent Hfr or recipient cells. Samples taken within ca. 5 min of time zero generally do not contain recombinants; samples taken after increasing periods of time will contain recombinants with an increasing number of donor genes. Since donor genes are transferred sequentially, a given donor gene will appear in a recombinant after a characteristic time interval which depends on the distance of the gene from the origin of transfer and on the efficiency with which the gene recombines with the recipient’s chromosome. Thus, the time of transfer of a gene gives an indication of the position of that gene in the donor chromosome, and interrupted mating can be a useful means of genetic mapping. (In practice, the further a gene is from the origin of transfer, the less frequently will it be transferred owing to the increased chances of strand breakage during the longer conjugation times; thus, the yield of recombinants expressing a given donor gene is also indicative of the position of that gene.) In E. coli the time taken to transfer the entire chromosome is typically ca. 100 min at 37° C (estimated using several Hfr strains differing in the position of the origin of transfer). The position of any gene can be given in terms of the time (in minutes) of transfer of that gene relative to a given origin of transfer – arbitrarily standardized at the thr (threonine synthesis) locus (zero minutes). interspecies hydrogen transfer The mutually beneficial, unidirectional transfer of H2 from H2 -producing to H2 -utilizing organisms in a given ecosystem (see e.g. ANAEROBIC DIGESTION). interstitial pneumonia See PNEUMONIA. intervening sequence See SPLIT GENE. intestinal tract flora See GASTROINTESTINAL TRACT FLORA. intimin See EPEC and PATHOGENICITY ISLAND. intine See CYST (a). Intoshellina See ASTOMATIDA. intoxication Poisoning. See e.g. FOOD POISONING and MYCOTOXICOSIS. intra-epithelial lymphocytes (IELs) See T LYMPHOCYTE. intra vitam staining See VITAL STAINING. intracisternal A-type particles See A-TYPE PARTICLES. intracistronic complementation See COMPLEMENTATION TEST. intracytoplasmic membranes (also: cytomembranes) (bacteriol.) Membranous systems or structures present within the cytoplasm in certain bacteria; according to type, the membranes may or may not be continuous with the CYTOPLASMIC MEMBRANE. Intracytoplasmic membranes occur e.g. in members of the METHYLOCOCCACEAE and NITROBACTERACEAE, in HYDROCARBONutilizing bacteria, in photosynthetic bacteria (see CHLOROSOMES, CHROMATOPHORE (sense 2), THYLAKOIDS), etc. (See also MESOSOME and POLAR MEMBRANE.) intragenic complementation See COMPLEMENTATION TEST. intragenic suppression See SUPPRESSOR MUTATION. Intrapes See UREDINIOMYCETES. Intrasporangium A genus of aerobic bacteria (order ACTINOMYCETALES, wall type I). The organisms form branching substrate mycelium (hyphae: 0.4–1.2 µm in diam.) but no aerial hyphae; intercalary or subterminal swellings (‘sporangia’) occur in old cultures. Type species: I. calvum. 403 invertebrate diseases molecular form (I2 ); however, an iodine solution was reported to inactivate poliovirus (at 25° C) more efficiently at pH 10 than at pH 6 (at pH 10 iodine occurs mainly as hypoiodous acid, HIO) [AEM (1982) 44 1064–1071]. Both I2 and HIO are strongly microbicidal; the ratio I2 :HIO in a solution of iodine is affected not only by pH but also by the total concentration of dissolved iodine and by the presence of organic matter [Book ref. 65, pp. 183–196]. The staining, corrosive and irritant properties of iodine can be overcome by complexing the element with certain high-MWt surface-active compounds – e.g. polyvinyl pyrrolidone, certain quaternary ammonium compounds; such complexes are referred to as iodophores (or iodophors). Iodophores have the antimicrobial properties of iodine together with the detergent properties of the surfactant, but only ca. 80% of the bound iodine is released; their response to pH is similar to that of iodine. Iodophores have been used e.g. for the disinfection of swimming-pool water and dairy equipment; polyvinyl pyrrolidone–iodine (povidone–iodine, PVP–iodine) has been used for treating skin and mucous membrane infections involving species of Streptococcus, Staphylococcus and Candida. Betadine and Wescodyne are iodophores used as antiseptics/disinfectants. (cf. IODOFORM; IODONIUM COMPOUNDS; see also CHLORINE.) (b) (as a stain) See e.g. LUGOL’S IODINE. iodinin A red, water-soluble pigment: 1,6-phenazinediol-5,10dioxide; crystals of iodinin are formed in the culture medium e.g. by Microbispora parva. iodochlorohydroxyquin See 8-HYDROXYQUINOLINE. 5-iodo-2
-deoxyuridine Syn. IDOXURIDINE. iodoform (CHI3 ) A compound once widely used as an antiseptic; its antimicrobial activity is poor compared with that of IODINE. iodonium compounds Compounds of the form R2 IX, in which R is an organic group and X is an inorganic ion; such compounds (e.g. diphenyliodonium chloride) contain trivalent iodine, are strongly basic, and possess antimicrobial properties, but their activity against spores is doubtful. iodophores (iodophors) See IODINE (a). ion-exchange chromatography See CHROMATOGRAPHY. ion-motive force phosphorylation See ELECTRON TRANSPORT PHOSPHORYLATION. ion transport Ion fluxes across ENERGY-TRANSDUCING MEMBRANES are involved e.g. in certain types of energy conversion (see e.g. PROTON ATPASE, PROTON PPASE, END-PRODUCT EFFLUX), in the ion-linked uptake of certain substrates, in the regulation of intracellular osmotic pressure and pH, in the generation of electrochemical gradients of specific ions, and (apparently) in morphogenetic processes (see GROWTH (fungal)). Since ions cannot pass freely across such membranes (cf. OUTER MEMBRANE) their transmembrane translocation requires a driving force and more or less specific TRANSPORT SYSTEMS which may involve UNIPORT, ANTIPORT or SYMPORT processes. The energy needed for transmembrane ion translocation may be provided, directly, by (a) a concentration gradient of the ion itself; (b) a gradient of another ion – e.g. H+ (see pmf in CHEMIOSMOSIS) or Na+ (see SODIUM MOTIVE FORCE); (c) ATP hydrolysis (see e.g. PROTON ATPASE); (d) the direct coupling of metabolism to ion transport (see ELECTRON TRANSPORT CHAIN and SODIUM MOTIVE FORCE); or (e) light (see PHOTOSYNTHESIS and PURPLE MEMBRANE). Ion electrochemical gradients as driving forces in ion transport. A difference in the concentrations of a given ion on either side of a membrane constitutes an electrochemical gradient which may be able to provide a driving force for the transmembrane transfer of the ion itself or of other types of ion. However, the presence of such a gradient, and the existence of transmembrane route(s) for a given ion, does not necessarily mean that sugar’ (a mixture of glucose and fructose) used as a sweetening agent. (See also ENZYMES.) (2) (mol. biol.) A recombinase responsible for the inversion of a segment of DNA: see RECOMBINATIONAL REGULATION. invertebrate diseases See e.g. CRUSTACEAN DISEASES; INSECT DISEASES; NEMATOPHAGOUS FUNGI; OYSTER DISEASES. inverted repeat (IR) In a double-stranded nucleic acid molecule: either of two regions in which the sequences of base pairs are similar or identical and have the same polarity but are opposite in orientation, e.g.: 5
. . . GGCT . . . AGCC . . . 3
3
. . . CCGA . . . TCGG . . . 5
(The repeated sequences may or may not be contiguous.) In a single-stranded nucleic acid, IRs are sequences which are complementary and opposite in polarity, i.e., equivalent to one of the strands of a double-stranded IR. (cf. DIRECT REPEAT; see also HAIRPIN and PALINDROMIC SEQUENCE.) invertible DNA See RECOMBINATIONAL REGULATION. invJ gene (Salmonella typhimurium) See NEEDLE COMPLEX. involution forms (of bacteria) Morphologically or otherwise aberrant cells often seen in old cultures or among organisms cultured under harsh or hostile conditions (e.g. in the presence of sub-lethal concentrations of antibiotics). Involution forms are generally regarded as degenerate cells, degeneracy commonly being ascribed to the action of autolysins, failure of cell wall synthesis, or failure of cell division without cessation of macromolecule synthesis. (cf. L-FORM.) Iodamoeba A genus of protozoa of the AMOEBIDA. I. bütschlii occurs in the human intestine; it is non-pathogenic. Trophozoites (ca. 9–20 µm diam.) contain numerous vacuoles and form blunt pseudopodia slowly; the single nucleus lacks peripheral chromatin and contains a large karyosome which is variable in shape and location and which is surrounded by clusters of granules. Cysts are irregular in shape, ca. 5–16 µm in diam., containing (usually) a single nucleus and a large persistent glycogen granule which stains a characteristic dark-brown with iodine; chromatoid bodies are usually absent. iodinated density-gradient media Media used e.g. for the separation of macromolecules and sub-cellular fractions by isopycnic ultraCENTRIFUGATION. High-density, non-ionic media in this category include e.g. a tri-iodinated benzamido derivative of glucose (Metrizamide), and a tri-iodinated derivative of benzoic acid (substituted with three highly hydrophilic aliphatic side chains) having a MWt of 821 and a density of 2.1 g/ml (Nycodenz – trade mark of Nyegaard & Co., Oslo, Norway). iodine (a) (as an antimicrobial agent) Iodine has microbicidal activity against a wide range of bacteria (including endospores), fungi, and viruses; in many cases its antimicrobial activity appears to be due to the combination of molecular iodine with proteins (e.g. tyrosine is irreversibly iodinated). Iodine is readily soluble in organic solvents; tincture of iodine (used as an ANTISEPTIC) contains ca. 2.5% iodine in an ethanol–water solution of potassium iodide. The (low) solubility of iodine in water is increased by the presence of iodide, when the tri-iodide (I3 − ) is formed; I3 − itself is not antimicrobial, but it readily decomposes to release free iodine. For skin antisepsis aqueous solutions are generally preferred since they are less irritant than alcoholic solutions; the latter have been used e.g. for the disinfection of thermometers. The antimicrobial activity of aqueous iodine solutions is sometimes said to be maximal under acid conditions (pH 6 or below) – when dissolved iodine is present largely in the 404 ionizing radiation the ion will be translocated: ion transport is subject to various intracellular and extracellular regulatory factors; moreover, the transmembrane route taken by a given ion may vary according e.g. to the extracellular concentration of that ion. The potential energy of a transmembrane gradient of ions can be considered in terms of the change in Gibbs free energy which occurs when a charged species is transferred down an electrochemical gradient. Thus, e.g., if one mole of an ion, Xm+ , is transferred down an electrical potential gradient of 1y millivolts, from concentration [Xm+ ]
to [Xm+ ]

, the net change in Gibbs free energy (1G) can be obtained from the general electrochemical equation: 1G = −mF1y + 2.3RT log10 [Xm+ ]

[Xm+ ]
Ca2+ uptake are modified by the operation of Ca2+ efflux systems, and a steady-state level of intramitochondrial Ca2+ is maintained by the continual cycling of Ca2+ across the membrane. Modes of ion transport include: (a) Uniport driven by pmf. Examples include pmf-mediated ATP synthesis (see PROTON ATPASE) and the uptake of Ca2+ by mitochondria [Book ref. 126, pp. 251–255]. (b) Uniport coupled to ATP hydrolysis. One example is the so-called Kdp transport system for K+ uptake in Escherichia coli. In this system, ATP hydrolysis (rather than pmf) at a membrane-bound K+ -ATPase (‘K+ -dependent ATPase’, ‘K+ stimulated ATPase’, ‘K+ -pump’) brings about the transmembrane translocation of K+ into the cell against a concentration gradient; the Kdp system becomes derepressed under conditions of K+ starvation. [K+ pathway in E. coli : Book ref. 126, pp. 653–666.] Enterococcus faecalis has been reported to use an Na+ -ATPase for Na+ extrusion [JB (1984) 158 844–848], and appears to use a Ca2+ -ATPase for Ca2+ extrusion. (c) Antiport driven by pmf or smf. A Ca2+ /H+ antiporter has been reported to operate in membrane vesicles of Bacillus subtilis for Ca2+ extrusion [JB (1985) 164 1294–1300]. Similar systems appear to function in E. coli and Azotobacter vinelandii for Ca2+ extrusion, while a Ca2+ /Na+ antiporter may occur in halophilic organisms (e.g. Halobacterium spp). Under some conditions the Na+ /H+ antiporter in E. coli can be driven by pmf, extruding Na+ and thus generating a SODIUM MOTIVE FORCE which can be used e.g. for melibiose uptake (Na+ /melibiose symport) and/or for the regulation of intracellular pH. However, in media of high pH, containing high concentrations of Na+ , the Na+ /H+ antiporter can generate pmf by proton extrusion [JB (1985) 163 423–429]. The K+ /H+ antiporter in E. coli is believed to function e.g. for the prevention of over-alkalinization of the cytoplasm in respiring cells. (d) Antiport coupled to ATP hydrolysis. An Na+ /K+ -ATPase (‘sodium pump’) occurs in eukaryotic cells but not in prokaryotic cells or in mitochondria; energy derived from ATP hydrolysis at this ATPase is used to pump Na+ out and K+ in – the Na+ /K+ exchange ratio probably being ca. 3:2. The sodium pump is used e.g. to maintain intracellular osmotic stability, and to generate the smf needed for the uptake of certain substrates; it is inhibited by ouabain. (e) Symport systems. In E. coli, LACTOSE is generally taken up by an H+ /lactose symport, while melibiose uptake involves an Na+ /melibiose symport. Bacillus subtilis, grown aerobically on citrate-containing media, can accumulate Ca2+ by a Ca2+ /citrate symport. An electrogenic H+ /lactate symport is used by some fermentative bacteria for energy conversion (see END-PRODUCT EFFLUX). In mitochondria, phosphate (Pi) uptake occurs by electroneutral symport with H+ . (f) Other ion transport systems. The so-called Trk system is responsible for much of the uptake of K+ in E. coli, and its activity is influenced by extracellular osmotic pressure. Trk is apparently not an ATPase, but it may be regulated by ATP or by a related compound [JGM (1985) 131 77–85]. ionizing radiation (in sterilization) Ionizing radiations (e.g. gamma-rays, beta-rays (electrons), X-rays) are used e.g. for the STERILIZATION of pre-packed medical and biological equipment such as surgical sutures, syringes, plastic Petri dishes, etc; currently they are not used to any great extent for the treatment of foodstuffs because they can cause toxicity and a deterioration in organoleptic properties in foods (but see FOOD PRESERVATION (g)). (See also SEWAGE TREATMENT.) (1) in which R is the gas constant, T is the absolute temperature, F is the Faraday constant, and m is the charge on the ion. This expression can be readily converted to the ion electrochemical potential gradient, 1m̃Xm+ : 1m̃Xm+ = m1y − Z log10 [Xm+ ]

[Xm+ ]
(2) where Z is the constant 2.3RT/F. When applied to the gradient of an ion which crosses the membrane by electrogenic or electrophoretic uniport, equation 2 gives a measure of the driving force (due to the gradient itself) tending to cause movement of that ion across the membrane. (When an ion is translocated across a membrane by antiport or symport mechanisms equation 2 must be modified so as to accommodate the additional forces acting on the ion.) If a proton gradient is maintained (imposed) across an energytransducing membrane (e.g. by the use of metabolic energy) it will influence the electrochemical equilibria of each of the various types of ion distributed across the membrane. A given cation which crosses the membrane by electrical uniport will tend to redistribute across the membrane so as to reach an electrochemical equilibrium with the membrane potential of the pmf. For example, in the case of the bacterial cytoplasmic membrane (interior negative) a given cation which, initially, is largely extracellular will tend to be accumulated within the cytoplasm; if the cation reaches electrochemical equilibrium, the Gibbs free energy associated with its gradient becomes zero (i.e., 1m̃Xm+ = 0), and the relationship of the imposed membrane potential to the intracellular and extracellular concentrations of the given cation is then given by the Nernst equation: 1y = [Xm+ ]int Z log10 m+ m [X ]ext (3) Equation 3 is obtained by re-arranging equation 2 in the special case when 1m̃Xm+ = 0. In living cells, transmembrane ion translocation may affect either or both components of pmf. When ions are translocated by antiport or symport processes, the effect of such translocation on pmf depends e.g. on the stoichiometry of the process. For example, under some conditions the Na+ /H+ antiporter in Escherichia coli consumes pmf, H+ influx exceeding Na+ efflux. (Since the H+ /Na+ ratio is >1, operation of the antiporter in this mode consumes 1y as well as 1pH.) However, with high extracellular pH and Na+ the antiporter operates in such a way that H+ efflux is greater than Na+ influx – with consequent augmentation of pmf. Ion uptake or extrusion commonly leads to compensatory ion fluxes. Thus, e.g. in mitochondria the effects of pmf-dependent 405 b-ionone ring genome of two linear pieces of dsRNA (MWts ca. 2.3 × 106 and 2.5 × 106 ). Each strand of both RNA molecules is covalently linked by its 5
end to a protein – the first known case of genome-linked protein in a dsRNA virus. A minor protein component (VP105) is reputed to have dsRNA-dependent RNA transcriptase activity. [Review: CJM (1983) 29 377–384.] Virus transmission occurs vertically and horizontally. IPNV is representative of a family of bisegmented dsRNA animal viruses, Birnaviridae [approved: Intervirol. (1986) 25 141–143], which includes the INFECTIOUS BURSAL DISEASE VIRUS of chickens, DROSOPHILA X VIRUS, and various ‘IPNV-like’ viruses isolated from freshwater and marine fish (e.g. Eel Virus European, EVE, isolated from Japanese eels: Anguilla japonica; see also SPINNING DISEASE) and from marine bivalve molluscs (e.g. oysters – Ostrea and Crassostrea spp – and Tellina tenuis). ipomeamarone The (+)-enantiomer of 2-methyl-2-(4-methyl-2oxopentyl)-5-(3-furyl) tetrahydrofuran: a PHYTOALEXIN produced by the sweet potato (Ipomoea batatas) in response to infection by certain fungal pathogens (e.g. the ‘black rot’ fungus Ceratocystis fimbriata). Ipomeamarone and its (−)-enantiomer ngaione (a normal constituent of certain trees and shrubs) are both hepatotoxic and can cause liver disease in livestock. (See also 4-IPOMEANOL.) 4-ipomeanol A substance, (1-(3-furyl)-4-hydroxy-1-pentanone), produced by the sweet potato (Ipomoea batatas) in response to infection by certain fungal pathogens. It is toxic to animals, causing pulmonary oedema and massive pleural effusion. Together with e.g. ipomeanine (a dihydro derivative of 4-ipomeanol) it may be the ‘lung oedema factor’ responsible for the fatal respiratory disease in livestock (especially cattle) which follows ingestion of mould-damaged sweet potatoes. (cf. IPOMEAMARONE.) iprodione See DICARBOXIMIDES. IPTG Isopropyl-b-D-thiogalactoside: a gratuitous inducer of the LAC OPERON. IQB INDIVIDUAL QUICK BLANCH. IR INVERTED REPEAT. Ir genes Immune response genes: genes, associated with the MHC, which determine the ability of a mouse to make humoral and cell-mediated immune responses to a wide range of antigens; certain of these genes encode the IA ANTIGENS. Irgasan DP 33 Syn. TRICLOSAN. Iridaea See RHODOPHYTA. iridescence Coloured light formed by interference when white light is reflected from both surfaces of a thin film. iridescent insect viruses See CHLORIRIDOVIRUS and IRIDOVIRUS. Iridia See FORAMINIFERIDA. Iridoviridae A family of icosahedral dsDNA-containing VIRUSes which infect invertebrates (mainly insects) and poikilothermic vertebrates. The family contains four genera: CHLORIRIDOVIRUS, IRIDOVIRUS, LYMPHOCYSTIVIRUS and RANAVIRUS. (cf. AFRICAN SWINE FEVER.) Virion: icosahedral (ca. 120–300 nm diam.) and complex in structure, consisting of a nucleoprotein core surrounded by a layer composed of a lipoprotein membrane beneath an icosahedral protein lattice. Some virions may have an additional lipoprotein ENVELOPE derived from the host cell plasmalemma or endoplasmic reticulum; the envelope is apparently not essential for infectivity. Virions are generally stable at pH 3–10 and at 4° C; they can be inactivated e.g. at 55° C for 15–30 min. Members of the genera Ranavirus and Lymphocystivirus are sensitive to ether and non-ionic detergents. Genome: one molecule (possibly two in some cases) of linear dsDNA (MWt ca. 100–250 × 106 ); in at least some iridoviruses Ionizing radiations sterilize by supplying energy which permits a great variety of lethal chemical reactions to occur in contaminating organisms. In general, resistance to ionizing radiation increases in the order: multicellular organisms; Gram-negative bacteria; Gram-positive bacteria and moulds; bacterial endospores, viruses and viroids. Exceptions include e.g. Deinococcus radiodurans, which is more resistant than most or all bacterial endospores. Low (sublethal) doses of radiation can be mutagenic: e.g., in Escherichia coli the SOS SYSTEM – and hence error-prone (mutagenic) repair – may be induced. (In some cases, susceptibility to ionizing radiation is enhanced by the presence of water and/or oxygen.) Typically, toxins and other microbial products are more resistant than cells to ionizing radiations. Ionizing radiation may affect the materials being sterilized – e.g., a sterilizing dose of radiation can damage certain plastics. Large-scale industrial sterilization is generally carried out with either a source of gamma-rays or a source of high-energy electrons. In a gamma-radiation plant the source of radiation is 60 Co (cobalt-60): a radioactive isotope (half-life ca. 5.3 years) which provides both gamma-rays and (relatively) low-energy electrons. High-energy electrons (ca. 5–10 MeV) are produced in electron accelerators. The high-energy electron radiation used for sterilization carries more energy than does the gamma-radiation, although it has poorer powers of penetration; sterilization processes using high-energy electrons require exposures of the order of seconds, while those using gamma-rays require minutes or hours. The actual sterilizing dose used should, ideally, take into account e.g. the numbers and types of contaminants. In many countries the recommended minimum sterilizing dose is ca. 2.5 Mrad (25 kGy, see GRAY); a higher minimum is recommended in Scandinavia. X-rays (a form of electromagnetic radiation) have been used e.g. for sterilizing items of high density which are impermeable to a beam of high-energy electrons. [Quality assurance in radiation sterilization (theoretical treatment): JAB (1985) 58 303–313.] (See also ULTRAVIOLET RADIATION.) b-ionone ring See CAROTENOIDS. ionophore Any substance which actually or effectively increases the permeability of biological membranes (or of artificial lipid bilayers) to one or more types of ion. Channel-forming ionophores (e.g. GRAMICIDINS) form pores in the membrane; such ionophores typically exhibit little discrimination between different ions, and may allow the passage of up to ca. 107 ions per pore per second. Mobile carrier ionophores (e.g. PROTON TRANSLOCATORS) diffuse within the membrane; they can exhibit a high degree of specificity for ions. Mobile carriers may effect ion UNIPORT (see e.g. VALINOMYCIN) or ion ANTIPORT (see e.g. A23187 and NIGERICIN). (See also ENNIATINS, LASALOCID, MACROTETRALIDES, SALINOMYCIN and ION TRANSPORT.) Iotech process A process, involving explosive depressurization, in which the lignin–hemicellulose–cellulose complex from plant material is disrupted to release CELLULOSE for use as a feedstock in certain biotechnological processes (e.g. SINGLE-CELL PROTEIN or INDUSTRIAL ALCOHOL production). IP10 (chemokine) See CHEMOKINES. IpaA protein See DYSENTERY. IpaB protein See APOPTOSIS, DYSENTERY and PROTEIN SECRETION (type III systems). IPN virus (IPNV) The causal agent of INFECTIOUS PANCREATIC NECROSIS disease of salmonid fish. The virion has an unenveloped, icosahedral capsid (ca. 59 nm diam.) containing a 406 iron–sulphur proteins Listeria monocytogenes uses siderophores from other bacteria and various iron-chelating agents in the environment and mammalian hosts; a cell-surface ferric reductase may recognize all of these agents. A pathogen’s absolute need for iron has prompted the search for new vaccines that stimulate protective antibodies against the cell-surface receptors for iron-chelating agents. Pseudomonas aeruginosa can reduce PYOCYANIN to leucopyocyanin which, in turn, can reduce the (ferric) iron in iron– transferrin complexes [Inf. Immun. (1986) 52 263–270]; iron thus released from transferrin may be available to iron-limited bacteria. Virulence in the fish pathogen Vibrio anguillarum (see VIBRIO) depends partly on a plasmid-encoded siderophore, anguibactin. [Plasmid-mediated iron transport and bacterial virulence: ARM (1984) 38 69–89.] In plant-pathogenic Erwinia chrysanthemi, mutants defective in iron transport were non-pathogenic [JB (1985) 163 221–227]. [Iron metabolism in pathogenic bacteria: ARM (2000) 54 881–941.] Applied aspects. Siderophore-producing strains of Pseudomonas putida used for the BACTERIZATION of seed potatoes apparently increase the yield in short-rotation crops [NJPP (1986) 92 249–256]; thus, siderophores may modify the RHIZOSPHERE microflora in favour of the potato plant. Interestingly, some microbial ferrisiderophores are taken up by plants [see for example FEBS (1986) 209 147–151]. iron bacteria Bacteria whose growth is associated with the extracellular deposition of oxides or hydroxides of iron and/or manganese; such deposits usually cover or impregnate the bacterial capsule, sheath or stalk. Iron bacteria occur e.g. in various ironand manganese-containing soils and waters, and in water distribution systems (see also TUBERCLE (sense 2)). Evidence that the deposition of metal oxides/hydroxides is a direct result of microbial metabolism has not been obtained for the majority of the iron bacteria (cf. GALLIONELLA) – many of which have not even been grown in pure culture; the opportunity for biological oxidation of Fe2+ is relatively poor under aerobic conditions (in which Fe2+ undergoes rapid autoxidation) but becomes significant under microaerobic conditions, particularly at pH 5–6, in which autoxidation is slow. (Autoxidation of Mn2+ is slow below pH 9.) The iron bacteria include e.g. CLONOTHRIX, CRENOTHRIX, GALLIONELLA, LEPTOTHRIX, METALLOGENIUM, OCHROBIUM and SIDEROCAPSA; some authors include the acidophilic iron oxidizers, e.g. Thiobacillus ferrooxidans. [Review: ARM (1984) 38 515–550.] iron box See OPERATOR. iron corrosion See CATHODIC DEPOLARIZATION THEORY. iron–sulphur centre See IRON–SULPHUR PROTEINS. iron–sulphur proteins (Fe−S proteins) A category of iron- and sulphur-containing proteins which are widely distributed in cells of all types and which participate e.g. in electron transfer processes (see e.g. ELECTRON TRANSPORT CHAIN and NITROGENASE); except in rubredoxins (see later) each iron–sulphur protein contains two or more iron atoms linked to one another by an equal number of (acid-labile) sulphur atoms, the entire complement of iron and acid-labile sulphur forming one or more discrete iron–sulphur centres (= iron–sulphur clusters) linked to the protein typically by thiolate side-chains of cysteine residues. (See also RHODANESE.) Rubredoxins are traditionally included with the Fe−S proteins, although they contain only one iron atom and no acidlabile sulphur; the iron is linked to the protein via four thiolate (Chilo iridescent virus, frog virus 3, lymphocystis virus) the DNA is circularly permuted and terminally redundant – features unique among animal viruses (but quite common among bacteriophages). The molecular biology of iridoviruses has been studied mainly in FROG VIRUS 3. Iridovirus (small iridescent insect virus group) A genus of viruses (family IRIDOVIRIDAE) which infect insects; infected larvae – and purified virus pellets – exhibit a blue iridescence. Virions (ca. 120 nm diam.) are ether-resistant. Type species: Chilo iridescent virus (insect iridescent virus type 6) [Intervirol. (1986) 25 141–143]. Other members include e.g. Tipula iridescent virus, which infects e.g. craneflies (Tipula spp) and many other insects, and insect iridescent viruses 1, 2, 9, 10 and 16–29. Probable member: Chironomus plumosus iridescent virus (C. plumosus is a type of non-biting midge, with haemoglobin-containing larvae known as ‘blood-worms’). Possible member: Octopus vulgaris disease virus. Irish moss See CHONDRUS. iron Most microorganisms need iron to form e.g. CATALASE, various oxidases, IRON–SULPHUR PROTEINS, ribonucleotide reductase, certain SUPEROXIDE DISMUTASES and/or CYTOCHROMES. Rarely – as e.g. in the type strain of Lactobacillus plantarum [FEMS (1983) 19 29–32] – iron is not an absolute requirement for growth. In many cases microorganisms synthesize and export SIDEROPHORES (q.v.) specifically for the sequestration and uptake of iron, particularly when iron is scarce. Theoretically, siderophores could behave simply as shuttles – remaining extracellular and delivering iron from the environment to the cell surface for internalization by a membrane TRANSPORT SYSTEM. However, in Escherichia coli, the siderophore enterochelin (= enterobactin) carries (ferric) iron into the cytoplasm in an energy-dependent uptake process – binding initially at the cell-surface FEPA PROTEIN; within the cytoplasm, cleavage of the siderophore (by an esterase) releases the iron. Fe3+ is reduced to Fe2+ . Some strains of E. coli produce aerobactin, a siderophore whose outer membrane receptor is the Iut protein. Interestingly, E. coli can also use iron–coprogen and iron–ferrichrome complexes (receptors: FhuE and FhuA (= TonA) proteins, respectively) – even though E. coli produces neither coprogen nor ferrichrome. E. coli can also take up iron–citrate complexes. Iron uptake: regulation. In e.g. E. coli, accumulation of intracellular Fe2+ results in binding of ferrous ions to the regulator FUR PROTEIN which then represses the expression of a number of genes (including fep). Iron in pathogenicity. Pathogens in animals must compete successfully with the host for available iron. [Iron in pathogenicity: RMM (1998) 9 171–178.] Thus, many pathogenic strains of E. coli secrete aerobactin; this promotes bacterial growth in extracellular concentrations of iron ca. 500-fold lower than those needed by the enterochelin system [Inf. Immun. (1986) 51 942–947]. Aerobactin seems able to compete successfully with the SIDEROPHILIN transferrin [Mol. Microbiol. (1994) 14 843–850]. For extracellular growth, Mycobacterium tuberculosis uses EXOCHELINs (which may obtain iron from siderophilins) and cellsurface ferric iron chelators: MYCOBACTINs (which internalize iron transferred from the exochelin). Some pathogens obtain iron directly from siderophilins; thus, e.g. Neisseria gonorrhoeae and N. meningitidis have receptors for human TRANSFERRIN. 407 ironophores ligands. A rubredoxin with an Em of ca. −60 mV occurs in Clostridium pasteurianum. (See also HYDROCARBONS.) Other iron–sulphur proteins contain, per molecule, one or more of the following types of centre: [2Fe−2S] (see also RIESKE PROTEIN), [3Fe−3S] (found e.g. in Azotobacter vinelandii ), and/or [4Fe−4S]; 2S, 3S and 4S refer specifically to acid-labile sulphur. Iron–sulphur proteins can be classified as either simple (e.g. rubredoxins, FERREDOXINS) or complex (= conjugated), the latter having additional prosthetic groups such as a flavin or haem or a non-iron metal atom (e.g. molybdenum). Complex iron–sulphur proteins include e.g. the sulphite reductase of Escherichia coli (EC 1.8.7.1), which includes four [4Fe−4S] centres together with molecules of FAD, FMN and sirohaem, and a chloroplast nitrate reductase (EC 1.7.7.1) containing one [4Fe−4S] centre and sirohaem. [Review: Book ref. 146, pp. 79–120.] ironophores Syn. SIDEROPHORES. Irpex See APHYLLOPHORALES (Polyporaceae). IRS Internal resolution site: see Tn3. IS element See INSERTION SEQUENCE. IS1 A 768-bp INSERTION SEQUENCE present e.g. in R plasmids (see also Tn9 ), the genome of BACTERIOPHAGE P1, the chromosome of Escherichia coli K12 (in multiple copies), etc. IS1 seems to transpose preferentially to target sites which are rich in AT, frequently cleaving at a GC pair within such a region. Transposition can apparently generate either 9- or 8-bp duplications of target DNA [PNAS (1985) 82 839–843]. IS1 encodes two genes, insA and insB, which are required for IS1-mediated transposition and cointegration. IS5 A 1195-bp INSERTION SEQUENCE present in the chromosome of Escherichia coli K12 strains; it has a high target specificity (see TRANSPOSABLE ELEMENT). IS10 An INSERTION SEQUENCE that has been found only in association with Tn10 (q.v.). IS50 See Tn5. IS101 A 209-bp INSERTION SEQUENCE which is defective and carries no genes; its transposition depends entirely on gdencoded transposase and resolvase functions (see Tn3 ). (cf. Tn951.) Its inverted repeat sequences are closely related to, but not identical with, those of gd. IS900 An INSERTION SEQUENCE reported to be specific to Mycobacterium paratuberculosis. IS900 has been used e.g. as a target for PCR-based detection of M. paratuberculosis in milk [AEM (1998) 64 3153–3158]. IS1000 Syn. gd (see Tn3 ). IS6110 An INSERTION SEQUENCE regarded as specific to members of the Mycobacterium tuberculosis complex (i.e. M. tuberculosis, M. bovis, BCG, M. africanum and M. microti ). The number of copies of IS6110 per genome varies with strain; a few strains of M. tuberculosis apparently lack the sequence, while most strains of M. bovis appear to contain only a single copy. Some 16 copies of IS6110 occur in the genome of the H37rv reference strain of M. tuberculosis [Nature (1998) 393 537–544]. Some authors have questioned the specificity of IS6110, indicating that a central region of the sequence is similar to a sequence in non-tuberculous mycobacteria [JCM (1997) 35 799–800]; others have affirmed the specificity of this sequence for members of the M. tuberculosis complex [JCM (1997) 35 800–801]. IS6110 has been widely used for TYPING isolates of M. tuberculosis. However, using SPOLIGOTYPING, some authors have found that, in a particular subset of multidrug-resistant strains, the rate of transposition of IS6110 within the genome may be too high for reliable RFLP-based typing even over a period of a few years [JCM (1999) 37 788–791]. By contrast, others have reported that IS6110 -based typing gives insufficient discrimination [JCM (1999) 37 3022–3024]. isatin-b-thiosemicarbazone (IBT) An ANTIVIRAL AGENT. IBT and some of its derivatives (e.g. methisazone: N-methyl-IBT) inhibit the replication of poxviruses, apparently acting at a late stage in the replication cycle. Methisazone has been used prophylactically in cases of exposure to smallpox, and in the treatment of complications of smallpox vaccination. (See also THIOSEMICARBAZONES.) ISEM IMMUNOSORBENT ELECTRON MICROSCOPY. IS-GA See IN SITU PCR. ISH See PROBE. isidiate (lichenol.) Bearing or consisting of isidia (see ISIDIUM). isidium (lichenol.) A small protuberance (ca. 0.01–0.3 × 0.5–3.0 mm) which arises from the surface of the thallus in certain lichens; it consists of photobiont cells and medullary tissue, typically covered by a cortex, and may be e.g. spherical, flattened, cylindrical, branched (‘coralloid’), according to species. In many isidiate lichens the isidia are easily broken off and probably serve as propagules; in other cases they may serve to increase the surface area – and hence the assimilative capacity – of the thallus. islandic acid A water-soluble, acidic, extracellular, (1 → 6)-blinked glucan containing malonic acid residues; it is produced by Penicillium islandicum. islet-activating protein Syn. PERTUSSIS TOXIN. isoaccepting tRNA See TRNA. isoamylase See DEBRANCHING ENZYMES. isoantibody Any antibody formed against an antigen derived from a genetically different individual of the same species. isocitrate dehydrogenase See TCA CYCLE. isocitrate dehydrogenase kinase–phosphatase See TCA CYCLE. isocitrate lyase See TCA CYCLE and SERINE PATHWAY. isoconazole See AZOLE ANTIFUNGAL AGENTS. isodiametric Rounded or polyhedral: having roughly the same diameter in all directions. isoelectric focusing (IEF) A type of ELECTROPHORESIS in which amphoteric molecules (e.g. proteins) are separated from one another on the basis of their differing ISOELECTRIC POINTS (IEPs). In principle, a column of gel (e.g. polyacrylamide) containing a mixture of AMPHOLYTES (each with a different IEP) is subjected to an electrical potential difference until the ampholytes are distributed (according to their IEPs) to form a stable pH gradient. The sample (e.g. mixture of proteins) is then added and subjected to an electrical potential difference so that each protein migrates to the region corresponding to its IEP; proteins may then e.g. be stained in situ or removed from the gel. isoelectric point (IEP; pI) In substances (e.g. proteins) whose charge depends on the pH of the medium: the pH at which the molecule has no net charge and hence zero electrophoretic mobility. isoelectrofocusing Syn. ISOELECTRIC FOCUSING. isoenzyme (isozyme) One of a number of ENZYMES which catalyse the same reaction(s) but differ from each other e.g. in primary structure and/or electrophoretic mobility. (See also ZYMODEME.) isofloridoside 1-O-Glycerol-a-D-galactopyranoside: a storage and osmoregulatory glycoside in e.g. Poterioochromonas malhamensis and also (often with FLORIDOSIDE) in members of the Rhodophyta. 408 itraconazole isopsoralen See PCR. isopycnic centrifugation See CENTRIFUGATION. isorenieratene See LIGHT-HARVESTING COMPLEX. isoschizomers RESTRICTION ENDONUCLEASES which recognize the same sequence in DNA but which are derived from different species. [Restriction endonucleases and their isoschizomers: NAR (1991) 19 2077–2109.] Isosphaera See BACTERIA (taxonomy). Isospora A genus of parasitic protozoa (suborder EIMERIORINA) which form disporic, tetrazoic oocysts (cf. e.g. EIMERIA); the genus, as originally defined, includes only homoxenous species, but it has been expanded by some authors [see AP (1982) 20 403–406] to include heteroxenous species which form disporic, tetrazoic oocysts (typically shed unsporulated) and which may form tissue cysts in the intermediate host. The expanded genus thus includes e.g. organisms otherwise placed in the genera BESNOITIA and TOXOPLASMA, as well as the obligately heteroxenous Isospora datusi (= Hammondia hammondi). For generalized homoxenous and heteroxenous life cycles see EIMERIORINA; see also COCCIDIOSIS (q.v. for refs). Isosticha See HYPOTRICHIDA. isosulfazecin See MONOBACTAMS. isotopic labelling (probe) See PROBE. Isotricha A genus of ciliates related to DASYTRICHA. isotype (class) Each of the five main variant forms (classes) of IMMUNOGLOBULIN (IgA, IgD, IgE, IgG and IgM); isotypes differ in the amino acid sequence in the constant region of their heavy chains. Immunoglobulins of all five isotypes occur in every normal individual (cf. ALLOTYPE). A given isotype may be divided into subclasses (distinguishable serologically). For example, there are several subclasses of IgG (designated IgG1, IgG2 etc.). isotype switching See ANTIBODY FORMATION. isovelleral See LACTARIUS. IsoVitaleX A commercial preparation – containing e.g. vitamin B12 , amino acids, purines, D-glucose, thiamine and TPP, NAD+ , Fe(NO3 )3 , PABA – used for the enrichment of media for isolating and/or cultivating fastidious bacteria (e.g. Haemophilus spp). isozyme Syn. ISOENZYME. Issatchenkia A genus of yeasts (family SACCHAROMYCETACEAE) which reproduce by multilateral budding; pseudomycelium is formed. Asci are persistent. Ascospores: spheroidal, roughsurfaced. Cells contain ubiquinone-7 (Q-7). Sugars may be fermented; NO3 − is not assimilated. A pellicle develops on liquid cultures. Species – I. occidentalis (anamorph: Candida sorbosa), I. orientalis (anamorph: Candida krusei ), I. scutulata, I. terricola – have been isolated from soil, fruit, etc. [Book ref. 100, pp. 214–223.] isthmus (biol.) Any narrow region connecting two structures in an organism or cell: see e.g. placoderm DESMIDS. iteron One of a number of repeated (‘reiterated’) nucleotide sequences which occur in and/or near the replication origin(s) in certain plasmids – e.g. the F PLASMID and the R6K PLASMID. Itersonilia A genus of fungi incertae sedis; the organisms form mycelium containing CLAMP CONNECTIONS, and it has been suggested that I. perplexans forms a one-spored basidium [TBMS (1983) 80 365–368]. itraconazole A clinically useful AZOLE ANTIFUNGAL AGENT (a triazole); it has a high MWt and is highly lipophilic. Itraconazole has a broad spectrum of activity, inhibiting yeasts, dimorphic fungi and filamentous fungi; it appears to be less toxic to mammalian tissues than e.g. miconazole or ketoconazole owing isogamy The union of gametes which are alike in form and physiology. (cf. ANISOGAMY.) isogenic Refers to two or more organisms or cells which have identical genotypes. isoimmunization The stimulation of an immune response to antigens derived from another individual of the same species. (cf. HETEROIMMUNIZATION.) isokont Refers to a pair of flagella (on a biflagellate cell) which are similar in length and type. (cf. HETEROKONT.) isolation (1) (microbiol.) Any procedure in which a given species of organism, present in a particular sample or environment, is obtained in pure CULTURE. According to organism, isolation may involve e.g. culture on a selective MEDIUM, ENRICHMENT, and/or the use of techniques such as FILTRATION or MICROMANIPULATION. (2) (med., vet.) The separation of patient(s) or animals(s) from others in order e.g. to prevent the spread of disease. (See also QUARANTINE.) L-isoleucine biosynthesis Many microorganisms synthesize isoleucine by the pathway shown in Appendix IV(d). Certain bacteria generate a-oxobutyrate by alternative pathways: e.g., from L-glutamate in Escherichia coli, from L-methionine in obligately anaerobic bacteria, from pyruvate in Leptospira sp, and from propionate in Clostridium sporogenes [JGM (1984) 130 309–318, q.v. for refs]. isolichenin A linear a-D-glucan containing (1 → 3) and (1 → 4) linkages; it occurs in certain lichens (e.g. Cetraria islandica). (cf. LICHENIN.) isomaltose A reducing disaccharide: a-D-glucopyranosyl-(1 → 6)-D-glucopyranose. (cf. GENTIOBIOSE.) isomarticin See NAPHTHAZARINS. isomerases ENZYMES (EC class 5) which catalyse isomerizations (i.e., intramolecular rearrangements). They include e.g. epimerases (e.g. that catalysing the interconversion of Dribulose 5-phosphate and D-xylulose 5-phosphate), racemases (e.g. those catalysing L- and D-amino acid interconversions), etc. isometric labile ringspot viruses Syn. ILARVIRUSES. isometric ssDNA phages Syn. MICROVIRIDAE. isometric virus A virus in which the virion is more or less spherical, usually exhibiting ICOSAHEDRAL SYMMETRY. isomorphic Morphologically similar or identical. isomorphic alternation of generations See ALTERNATION OF GENERATIONS. isoniazid A synthetic ANTIBIOTIC, isonicotinic acid hydrazide (INH), bactericidal for actively growing cells of Mycobacterium tuberculosis, M. bovis and, to a lesser extent, other Mycobacterium species. INH is an important anti-TUBERCULOSIS drug whose function depends on prior activation by a catalase (KatG; katG gene product) within the bacterium; the activated drug apparently inhibits enzymic step(s) in the synthesis of essential cell-wall MYCOLIC ACID (Microbiology (2000) 146 289–296]). Resistance to isoniazid can follow mutation in any of several genes, including katG and ahpC ; isoniazid is used in combination therapy for the treatment of tuberculosis. (See also RISE-RESISTANT TUBERCULOSIS.) In M. tuberculosis, susceptibility to isoniazid, oxidative stress response and iron regulation are related features, but precise details of relationships remain to be elucidated [see TIM (1998) 6 354–358]. isopanose a-Maltosyl-(1 → 6)-D-glucose. isopenicillin N See PENICILLIN N. isopropanol See ALCOHOLS. isopropyl-b-D-thiogalactoside A gratuitous inducer of the LAC OPERON. 409 IUB IVET (in vivo expression technology) A technique used for detecting those genes (of a pathogen) which are transcribed only during infection of the host; such genes may be virulence genes, and this can be studied further e.g. by animal tests involving strains of the pathogen which are mutant for the given gene. IVET is outlined in the figure. [Some examples of IVET: TIM (1997) 5 509–513. Behaviour of bacteria in the host, and the methodology for studying it: TIM (1998) 6 239–243.] IVS (mol. biol.) Intervening sequence (= intron): see SPLIT GENE. iwatake The edible lichen Umbilicaria esculenta. to its much higher selectivity for the fungal rather than the mammalian cytochrome P-450. [Review: Arch. Derm. (1986) 122 399–402.] [Pharmocology of itraconazole (review): Drugs (2001) 61 (supplement 1) 27–37.] IUB International Union of Biochemistry. iucA–iucD genes See SIDEROPHORES. IUdR See IDOXURIDINE. IUPAC International Union of Pure and Applied Chemistry. iutA gene See SIDEROPHORES. ivanolysin See THIOL-ACTIVATED CYTOLYSINS. Population of small cccDNA molecules, constructed in vitro , each containing (in order of transcription): Population of cells of the pathogen (1) a random fragment of the pathogen's chromosome (2) a promoter −less gene encoding chloramphenicol acetyltransferase (3) a promoter −less gene encoding β-galactosidase Population of recombinant cells of the pathogen used to infect a test animal whose food has been supplemented with chloramphenicol Recover the bacteria from test animal and plate on a medium containing Xgal; select white colonies IVET (in vivo expression technology): the principle (diagrammatic). The figure shows one of several forms of IVET. IVET detects those genes of a pathogen whose expression is induced during (and only during) infection of the host animal. Vector molecules (see top, right) are inserted, by transformation, into a population of cells of the pathogen. In each transformed cell, the (random) fragment of chromosome in the vector inserts into the corresponding part of the pathogen’s chromosome (by an ‘insertion–duplication’ mechanism); because the vector molecules contain different fragments of the chromosome they will insert into different chromosomal sites in different cells – forming a heterogeneous population of recombinant cells. In some recombinant cells, the vector’s two promoter-less genes will have been inserted ‘in frame’ with an upstream promoter. In such cells, both of these genes will be transcribed if the promoter is active. The recombinant cells of the pathogen are used to infect a test animal whose food contains the antibiotic chloramphenicol (to which the pathogen is normally susceptible). Under these conditions, a recombinant cell can grow only if it produces chloramphenicol acetyltransferase (CAT), i.e. only if the CAT gene (in the vector molecule) is controlled by an active promoter; thus, the fact that a given recombinant cell grows within the animal indicates that its CAT gene is controlled by a promoter which is active within the test animal. (Because synthesis of CAT points to an active promoter, the CAT gene is sometimes called a ‘reporter’ gene.) Cells producing CAT can form large populations which greatly outnumber cells that do not form CAT. We need to know whether the promoter controlling an expressed CAT gene is active only when the pathogen is in the test animal – or whether it is also active when the pathogen is cultured (e.g. on agar media). If active only in the test animal, this indicates that the gene normally controlled by that promoter is induced during infection; such a gene is of interest because of its possible association with virulence. To study promoter activity further, the recovered bacteria are plated on a medium which lacks chloramphenicol but which contains the agent XGAL (q.v.); all the cells will grow. If a given promoter is active in culture then b-galactosidase will be formed and will give rise to a blue-green colony; such a colony tells us that activation of the given promoter does not occur only within the test animal. A white (lac− ) colony indicates that b-galactosidase (and CAT) can be formed only within the test animal, i.e. the relevant promoter is active only when the pathogen is actually infecting the animal. The gene which is normally controlled by this promoter can be isolated, cloned and sequenced and examined for its role in virulence. Reproduced from Bacteria, 5th edition, Figure 11.3, pages 304–305, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 410 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 J J chain A cystine-rich polypeptide (MWt ca. 15000) involved in the formation of dimeric and polymeric forms of both IgA and IgM (q.v.). In an IgA dimer the J chain forms a link between cystine residues situated near the C-terminal ends of the heavy chains in the two monomers. jaagsiekte (pulmonary adenomatosis) A SHEEP DISEASE characterized by chronic progressive pneumonia in which the alveolar spaces become occluded by adenomatous ingrowths of the epithelium; the causal agent(s) appear to be viral (a herpesvirus and/or a retrovirus), but the causal agent of MAEDI is not involved. Incubation period: 1–3 years. Symptoms: dyspnoea, emaciation, and a profuse watery nasal discharge which may contain hyperplastic adenomatous epithelial cells; fever and inflammation are absent. Death occurs within ca. 4 months. [Book ref. 33, pp. 807–808.] Jaccard coefficient See entry SJ . jack bean lectin CONCANAVALIN A. jack-in-the-box ascus Syn. BITUNICATE ASCUS. Jacquard coefficient See entry SJ . JAK See CYTOKINES. janiemycin A polypeptide ANTIBIOTIC which inhibits transglycosylation in PEPTIDOGLYCAN synthesis. Janthinobacterium A genus of Gram-negative, strictly aerobic, catalase-positive, chemoorganotrophic bacteria which occur e.g. in soil and water; the organisms are motile (flagellated) rods, 0.8–1.2 × 2.5–6.0 µm. Metabolism is respiratory (oxidative); acid (no gas) is formed from e.g. glucose. Growth occurs in/on simple media (e.g. peptone water); violet, often gelatinous, colonies are formed on nutrient agar. Optimum growth temperature: 25° C, maximum 32° C. MR −ve; VP −ve; usually oxidase +ve. GC%: ca. 61–67. Type species: J. lividum (formerly Chromobacterium lividum). [Book ref. 22, pp. 376–377.] Janus green A blue basic DYE which has both azine and azo chromophores; it is used e.g. in VITAL STAINING. Janus kinase See CYTOKINES. Japanese B encephalitis (Japanese encephalitis; Russian autumn encephalitis) A viral ENCEPHALITIS which affects man, pigs and horses; it occurs in epidemics in e.g. Japan, China, Korea and parts of India. The disease is usually acute, but may run a protracted course with acute exacerbations. The causal agent is a flavivirus (see FLAVIVIRIDAE) which occurs e.g. in wild birds and is transmitted by mosquitoes (usually Culex sp). An inactivated vaccine has been used in Japan. [Review: ARM (1986) 40 395–414.] Japanese river fever Syn. SCRUB TYPHUS. Japonochytrium See THRAUSTOCHYTRIDS. Jarisch–Herxheimer reaction (Herxheimer reaction) A potentially fatal reaction which may follow the first effective chemotherapy against e.g. brucellosis or syphilis – or, in general, diseases caused by certain bacteria (particularly spirochaetes) and protozoa. Symptoms (e.g. an initial rise in temperature) are associated with a cascade of CYTOKINES which seem to be responsible for at least some of the pathophysiology. [Review: BCID (1994) 1 65–74.] The reaction has been prevented by treatment with antibodies against tumour necrosis factor (TNF) [NEJM (1996) 335 311–315]. jarrah dieback A disease of the jarrah (Eucalyptus marginata) – an important timber tree e.g. in Western Australia; the causal agent is generally believed to be Phytophthora cinnamomi, but waterlogging of the soil may be an important contributory factor [New Phyt. (1985) 101 743–753]. (See also TREE DISEASES.) javanicin See NAPHTHAZARINS. JC virus See POLYOMAVIRUS. JCM Japanese Collection of Microorganisms, Riken, Wako-shi, Saitama 351, Japan. jelly fungi Those fungi of the TREMELLALES (e.g. Exidia glandulosa, Tremella mesenterica) which form gelatinous fruiting bodies – or all fungi (including e.g. Auricularia spp) which do so. jelly lichens Gelatinous lichens e.g. of the genera COLLEMA, LEMPHOLEMMA, LEPTOGIUM. Jembrana disease A CATTLE DISEASE, involving e.g. fever, generalized lymphadenopathy, nasal discharge, haemorrhage and splenomegaly; it was formerly assumed to be a tick-borne rickettsial disease but is now known to be caused by the bovine immunodeficiency virus. jet loop fermenter A LOOP FERMENTER in which culture is continually withdrawn from the column and pumped back (together with air/gas) via a nozzle at the base of the DRAFT TUBE; this promotes liquid circulation and dispersion of the injected gas. In the plunging jet design of the Vogelbusch IZ fermenter, re-cycled culture (and air) is introduced under pressure at the top of the column (which lacks a draft tube); this design achieves a high level of dissolved oxygen. Jew’s ear fungus See AURICULARIA. Jeyes fluid A household and horticultural DISINFECTANT consisting of coal tar acids solubilized with a soap prepared from pine resin and alkali. Jirovecella See ASTOMATIDA. JK coryneforms Coryneform bacteria which are resistant to (usually many) antibiotics; they are being increasingly reported as causes of serious nosocomial infections in immunosuppressed patients and patients with implants (pacemakers, prosthetic heart valves, etc). [Biochemical and cultural characteristics: JCP (1986) 39 654–660.] Johne’s bacillus Mycobacterium paratuberculosis. Johne’s disease (chronic bacillary diarrhoea; paratuberculosis) A chronic intestinal CATTLE DISEASE which may also affect other ruminants (e.g. sheep, goats, deer); it occurs worldwide and is caused by Mycobacterium paratuberculosis. Infection occurs on ingestion of contaminated water, grass etc. In cattle, the incubation period is long (usually more than 12 months); symptoms are uncommon in animals under 2 years old. The pathogen multiplies in the intestinal wall which becomes thickened and poorly absorptive; scouring is at first intermittent but becomes persistent and severe. The animal becomes emaciated and anaemic. There is no effective treatment. In goats, the mesenteric lymph nodes become oedematous and there is emaciation, but scouring is not a feature [VR (1983) 113 464–466]. In sheep, the disease involves emaciation but not scouring. joint-breaker fever Syn. O’NYONG-NYONG FEVER. joint-ill (vet.) In e.g. calves, foals and lambs: lameness involving inflammation of the articular surfaces of the joints, caused by any of a variety of organisms – e.g. Actinobacillus equuli, Actinomyces pyogenes, Erysipelothrix rhusiopathiae (particularly in piglets), salmonellae, Staphylococcus aureus. joint molecule (mol. biol.) A term sometimes used to refer to a structure composed of two dsDNA molecules held together only 411 Jones–Mote sensitivity by hydrogen bonding – as e.g. in early intermediates formed during homologous RECOMBINATION. Jones–Mote sensitivity (cutaneous basophil hypersensitivity) A form of hypersensitivity which may develop e.g. when a human subject is primed with a soluble protein antigen, or a guinea pig is primed with a protein in incomplete Freund’s adjuvant; subsequent challenge with the relevant allergen (within a few days of priming) leads to a weak, non-indurated, erythematous skin reaction in which the lesion contains a high proportion of BASOPHILS. (These reponses do not occur in mice.) Jones–Mote sensitivity can be transferred to a non-primed animal by serum (cf. DELAYED HYPERSENSITIVITY). Jonesia A genus which contains the former species Listeria denitrificans [IJSB (1987) 37 266–270]. Jopling reactions See LEPROSY. JPC See JUNCTIONAL PORE COMPLEX. Jud. Comm. JUDICIAL COMMISSION. Judicial Commission A taxonomic body concerned with the interpretation and/or amendment of the rules of microbial nomenclature. juglone (5-hydroxy-1,4-naphthoquinone) A compound produced in the leaves and roots of walnut trees (Juglans spp); it has antifungal and antibacterial activity, and its presence in the tree may play a role in resistance to disease. [Juglone as a transcription-blocking agent: NAR (2001) 29 767–773.] jumping gene Syn. TRANSPOSABLE ELEMENT. juncopox virus See AVIPOXVIRUS. junctional pore complex (JPC) A carbohydrate-export apparatus, reported to occur in certain motile cyanobacteria, which (in at least some cases) includes a tubular structure (minimum diameter 13 nm) that is capable of spanning the cell envelope. It has been suggested that JPCs form the mechanistic basis of GLIDING MOTILITY in cyanobacteria [JB (2000) 182 1191–1199]. jungle yellow fever See YELLOW FEVER. Junı́n virus See ARENAVIRIDAE and ARGENTINIAN HAEMORRHAGIC FEVER. junior synonym See SYNONYM. jute retting See RETTING. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 412 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 K and antitumour activity. In the presence of divalent cations, kanchanomycin binds to DNA and inhibits DNA and RNA synthesis. Kaposi’s sarcoma (KS) A rare multifocal neoplastic disease which occurs in two forms: (i) slow and indolent (limited mainly to the skin), and (ii) rapid and fulminant (involving skin and gastrointestinal tract). The milder form occurs in certain ethnic groups (e.g. Ashkenazi Jews). The aggressive form occurs in children in tropical Africa and is also a common feature in HIVinfected patients. (Note that Kaposi’s sarcoma occurs also in some immunosuppressed transplant patients.) Kaposi’s sarcoma appears to be associated with human (gamma) herpesvirus 8 (HHV8) [see e.g. Lancet (1997) 349 558–563] in conjunction with the immunosuppressive effects of HIV. [PCR-based investigation of HHV8 in KS biopsies: Am. J. Path. (1997) 150 147–153.] Activation of latent HHV8 in vitro has been achieved by demethylation of the promoter of a transactivator region by means of the reagent tetradecanoylphorbol acetate (TPA), and studies on the level of methylation of the transactivator region in biopsies have suggested a relationship between methylation status and the development of HHV8associated disease [PNAS (2001) 98 4119–4124]. Kaposi’s varicelliform eruption May refer either to eczema herpeticum or eczema vaccinatum (see ECZEMA). kappa chain See LIGHT CHAIN. kappa particles See CAEDIBACTER. Karatomorpha See PROTEROMONADIDA. Kärber method See END-POINT DILUTION ASSAY. Karelian fever See SINDBIS VIRUS. Karnal bunt (partial bunt; new bunt) A wheat disease caused by Neovossia indica (formerly e.g. Tilletia indica); originally a minor disease confined to NW India, it has recently spread through northern India and has become established in Afghanistan, Iraq, Pakistan, and Mexico, apparently transmitted on and in wheat seed. Usually only some of the grains in an ear are attacked; infected parts of the grain are initially grey but gradually turn black and emit a foul odour (trimethylamine). [Bot. Rev. (1983) 49 309–330.] (See also COMMON BUNT.) karyogamy The coalescence of nuclei (cf. PLASMOGAMY). karyogram See KARYOTYPE. karyokinesis Syn. MITOSIS. karyological relict (ciliate protozool.) Any present-day ciliate whose characteristics (particularly nuclear constitution) resemble those of (presumably) phylogenetically ancient ciliates. (See also KARYORELICTID GYMNOSTOMES and PRIMOCILIATID GYMNOSTOMES.) karyolysis (histopathol.) The dissolution of a cell’s nucleus with consequent loss of affinity for basic dyes. (cf. KARYORRHEXIS.) karyomastigont A nucleus together with its associated flagellum (or flagella) and, when present, axostyle. (See also DIPLOMONADIDA.) karyomere See MACRONUCLEUS. karyonide (caryonide) (ciliate protozool.) A clone of cells in which all the macronuclei have been derived from the same macronucleus. karyoplast A nucleus which has been isolated from a (eukaryotic) cell and which is enclosed within a sac of cytoplasmic membrane containing a small amount of cytoplasm. (cf. CYTOPLAST.) K Lysine (see AMINO ACIDS). K antigens Capsular antigens – usually capsular polysaccharides (see CAPSULE). Examples include the capsular antigens of Streptococcus pneumoniae, COLOMINIC ACID, and VI ANTIGENS. In Gram-negative bacteria, K antigens can mask O ANTIGENS; in some bacteria the K antigens can be removed by heating (see e.g. VI ANTIGEN), but in others (e.g. Klebsiella spp) they are heat-stable. In Escherichia coli, several surface antigens originally designated K – e.g. K88, K99 – are actually (proteinaceous) fimbrial antigens, and it has been proposed that they be renamed F antigens (K88 = F4, K99 = F5) [Book ref. 68, pp. 61–64]; polysaccharide K antigens may occur together with fimbrial antigens in certain strains of E. coli. (See also ETEC and TEICHOIC ACIDS.) K+ -ATPase See ION TRANSPORT. K cells Killer cells: lymphoid cells which have cytotoxic/cytolytic activity against target cells; syn. NK CELLS. K+ pump See ION TRANSPORT. K+ transport See ION TRANSPORT. K virus See POLYOMAVIRUS. K vitamins See QUINONES. K1 killer strain (of Saccharomyces cerevisiae) See KILLER FACTOR. K1 RNA, K2 RNA See RNASE P. K88, K99 In Escherichia coli : fimbrial antigens of certain strains pathogenic in animals – see ETEC, FIMBRIAE and K ANTIGENS. Kabackosome A type of vesicle formed by the hypotonic lysis of a SPHAEROPLAST. A Kabackosome (which contains little or no cytoplasm) is composed of CYTOPLASMIC MEMBRANE; the inner and outer faces of the membrane correspond to those in the original cell. (cf. ETP.) KAF Syn. FACTOR I. Kaffir pox See SMALLPOX. Kagami fever See EHRLICHIA. Kahn test A STANDARD TEST FOR SYPHILIS. kala-azar See VISCERAL LEISHMANIASIS. Kanagawa phenomenon The phenomenon in which those strains of Vibrio parahaemolyticus isolated from human patients exhibit clear (b) haemolysis when grown on WAGATSUMA AGAR containing human RBCs but not on that containing horse RBCs (a Kanagawa +ve reaction), while almost all strains isolated from other sources, including food suspected of causing V. parahaemolyticus food poisoning, do not (i.e. are Kanagawa −ve). (Discoloration (a-haemolysis) and clear haemolysis on both human and horse RBC-containing media are both regarded as Kanagawa −ve results.) The Kanagawa haemolysin is heatstable, extracellular, cytotoxic and cardiotoxic, and is haemolytic for human, dog and rat RBCs, weakly so for rabbit and sheep RBCs, and inactive against horse RBCs. In feeding experiments (in man), only Kanagawa +ve strains were capable of causing gastroenteritis, but the role of the haemolysin in pathogenesis is unknown; Kanagawa +ve strains appear to be better able to multiply in the intestine than are Kanagawa −ve strains. (See also FOOD POISONING (h).) kanamycin Any of several related AMINOGLYCOSIDE ANTIBIOTICS (kanamycins A, B, C) produced by Streptomyces kanamyceticus; the drug used clinically is composed mainly of kanamycin A. kanchanomycin (albofungin) A complex polycyclic ANTIBIOTIC, produced by Streptomyces sp, which has both antibacterial 413 karyorelictid gymnostomes Kawasaki disease An acute, occasionally fatal, febrile disease of unknown cause which affects infants and young children. Kawasaki disease may be a manifestation of an immune-complex-mediated systemic vasculitis [ADC (1984) 59 405–409]; more recently it has been suggested that at least some of the symptoms may be due to a SUPERANTIGEN. kb (kilobase) Of a DNA or RNA strand: a unit of length equal to 103 bases; the unit may also be used for 103 base pairs (bp) in dsDNA or dsRNA – although kbp (kilobase-pairs) is also used in this context. kbp See previous entry. Kcat mechanism See PHASEOLOTOXIN. KCN broth (cyanide broth; Møller’s [= Moeller’s] cyanide medium; potassium cyanide broth) A MEDIUM used in the CYANIDE TEST; it consists of an aqueous solution of peptone (1.0%), NaCl (0.5%), KH2 PO4 (0.0225%), Na2 HPO4 (0.56%), and KCN (0.0075%). The medium may be stored at 4° C for up to ca. 2 weeks. KCN test See CYANIDE TEST. kDa Kilodalton: 103 DALTONS. kDNA KINETOPLAST DNA. KDO See LIPOPOLYSACCHARIDE. Kdp system See ION TRANSPORT. kefir A fermented milk beverage made in parts of Russia, Bulgaria and the former Yugoslavia. Milk is fermented by a mixed and varied population of organisms which usually include lactobacilli [SAAM (1983) 4 286–294; JAB (1984) 56 503–505], Lactococcus lactis, and yeasts (for example, Saccharomyces spp); lactic acid, ethanol and carbon dioxide (which causes foaming) are the main products. The organisms become embedded in an extracellular polysaccharide (kefiran) to form whitish, gelatinous granules [scanning electron microscopy: JAB (1980) 48 265–268] which are carried to the surface by bubbles of carbon dioxide. The granules are collected and used as an inoculum for subsequent fermentations; they can be stored for several days in cold milk or water. (See also DAIRY PRODUCTS.) kefiran See KEFIR. Kellogg classification (of Neisseria gonorrhoeae) An early classification based on e.g.: (i) appearance of colony; (ii) autoagglutinability; and (iii) virulence. Types T1 and T2 correspond to fimbriate, virulent strains, while types T3 and T4 correspond to afimbriate, non-virulent strains. keloidal blastomycosis Syn. LOBOMYCOSIS. kelp (1) Any of various seaweeds that are used to obtain kelp (sense 2). (2) The ashes obtained by burning various large brown seaweeds such as Ascophyllum, Fucus, Laminaria, Macrocystis; such ashes have been used as an agricultural fertilizer and as a source of iodine, potash and soda (sodium carbonate). Kelsey–Sykes test A CAPACITY TEST used to determine the efficacy of each of several dilutions of a given disinfectant under simulated practical conditions. (cf. USE-DILUTION TEST.) To 3 ml of a given dilution of disinfectant is added a 1-ml aliquot of bacterial suspension at times 0, 10 and 20 min; at 8, 18 and 28 min the disinfectant is subcultured to each of five tubes of nutrient broth which are then incubated to detect the presence or absence of viable bacteria. A given dilution passes the test if no growth is obtained in at least two of the five tubes inoculated from it at 8 and 18 min. The test organism used is Pseudomonas aeruginosa NCTC 6749, Proteus vulgaris NCTC 4635, Escherichia coli NCTC 8196 or Staphylococcus aureus NCTC 4163; in a given test the organism used is that which is the most resistant to the test disinfectant. To examine the efficacy of the disinfectant under ‘dirty’ conditions a yeast karyorelictid gymnostomes Presumptively primitive ciliates (subclass GYMNOSTOMATIA, order Karyorelictida) which contain diploid, non-dividing macronuclei; genera include e.g. Geleia, KENTROPHOROS, LOXODES, Remanella and Tracheloraphis. karyorrhexis (histopathol.) The fragmentation of a cell’s nucleus (cf. KARYOLYSIS; PYKNOSIS). karyoskeleton Syn. NUCLEOSKELETON. karyosome A NUCLEOLUS or nucleolus-like body. karyotic (of cells) Nucleated. karyotype The chromosomal constitution of a (eukaryotic) cell in terms of the number, size and morphology of the chromosomes at metaphase. A systematized diagrammatic representation of a karyotype is called an idiogram; a systematized photographic representation may be referred to as an idiogram or as a karyogram. kasugamycin An atypical AMINOGLYCOSIDE ANTIBIOTIC whose molecule contains neither streptidine nor deoxystreptamine. Kasugamycin is bacteriostatic and is also active against certain fungi (e.g. Pyricularia oryzae and Rhizoctonia solani ). kat See KATAL. Kata virus See PESTE DES PETITS RUMINANTS. Katadyn silver Metallic SILVER – containing traces of impurities (gold, palladium etc) – deposited on sand (or other filtering medium) used for the filtration and disinfection of water; the impurities facilitate ionization of the silver. katal (abbrv. kat) A unit of enzyme activity: that which increases the rate of conversion of a given chemical reaction by one mole per second under defined conditions. katG gene See ISONIAZID. Katodinium See DINOFLAGELLATES. Kauffmann–White classification A scheme for the classification and identification of the numerous serotypes of SALMONELLA (e.g. for epidemiological purposes). Each serotype is defined by its O ANTIGENS and, where applicable, its H ANTIGENS and VI ANTIGENS, and is given a specific ANTIGENIC FORMULA which indicates the nature of these antigens in the order O, Vi (if present) : H phase 1 : H phase 2. For example, S. typhi is designated by the formula 9, 12, [Vi]:d:–. This means that the organism has O antigens 9 and 12, may have Vi antigen (variable presence indicated by [ ]), and has phase 1 flagellar antigen d and no phase 2 flagellar antigen (indicated by a dash) – i.e. PHASE VARIATION does not occur in this serotype. The antigenic formula for S. panama is 1, 9, 12:1, v:1, 5 where 1, 9 and 12 are O antigens, and either phase 1 flagellar antigens 1 and v or phase 2 flagellar antigens 1 and 5 may be present; this serotype has no Vi antigen. Underlining (as in O antigen 1 in S. panama) indicates that the antigen is present as a result of BACTERIOPHAGE CONVERSION. (Phage conversion may lead to a change in serotype: thus, e.g., S. anatum, 3, 10:e, h:1, 6, may be converted to S. newington, 3, 15:e, h:1, 6. Similar effects can occur in bacteria of other genera, including e.g. Shigella [serotpe-converting bacteriophages and O-antigen modification in Shigella flexneri : TIM (2000) 8 17–23].) For further examples of antigenic formulae see SALMONELLA. (See also SMOOTH–ROUGH VARIATION.) Serotypes thus identified are placed into groups (O groups), the serotypes in each group having in common at least one major O antigen (the group antigen) which is not found in members of any other group. For example, the group antigen of group A is O antigen 2; that of group B is O antigen 4; group C, O antigen 6; group D, O antigen 9; group E, O antigen 3. Thus, both S. typhi and S. panama are group D strains. (Minor O antigens may occur in more than one group: e.g., O antigen 12 occurs in strains from groups A, B and D.) 414 Kineosporia kidney stones (in humans) See UROLITHIASIS kieselguhr Syn. DIATOMACEOUS EARTH. kievitone An isoflavonoid PHYTOALEXIN produced by the French bean (Phaseolus vulgaris). Cell-free extracts of Rhizoctonia solani elicit high levels of kievitone from excised bean hypocotyls, but those of Fusarium solani elicit only trace amounts. Kievitone can be converted to the less fungitoxic kievitone hydrate by an extracellular enzyme produced by Fusarium solani f.sp solani. Kilham rat virus See PARVOVIRUS. killed vaccine Syn. INACTIVATED VACCINE. killer cells (immunol.) Commonly refers to NK CELLS (q.v.) but may also refer e.g. to cytotoxic T cells. killer factor Any of several protein toxins secreted by ‘killer’ strains of Saccharomyces cerevisiae and encoded by one of two cytoplasmically co-inherited dsRNA elements; the K1 toxin (formed by K1 or type 1 killer strains) binds initially to a cell wall D-glucan of a sensitive cell and then transfers to the cytoplasmic membrane where it disrupts the membrane potential. One of the dsRNA elements (designated M) encodes both the toxin and an ‘immunity protein’ which confers on the producing cell immunity to the toxin; the toxin contains two subunits, a and b, both derived by proteolytic processing of a precursor polypeptide during toxin secretion. The precursor polypeptide apparently functions as the immunity protein – possibly by competing with the mature toxin for binding sites on the membrane [Cell (1986) 46 105–113]. The other dsRNA element (designated L) encodes a protein required for the (separate) encapsidation of M and L. The encapsidated particles (termed VLPs: virus-like particles) are variously regarded as plasmids or as viruses (Saccharomyces cerevisiae viruses, ScVs – see MYCOVIRUS). [Review: MR (1984) 48 125–156.] (cf. KILLER PLASMIDS; see also BACTERIOCIN.) killer helper factor Syn. INTERLEUKIN-2. killer paramecia Strains of Paramecium which are able to kill other (‘sensitive’) paramecia. See: CAEDIBACTER, LYTICUM, MATE KILLER, PSEUDOCAEDIBACTER, R BODY, SPIN KILLING. (cf. TECTIBACTER.) killer plasmids (in yeasts) In some strains of Kluyveromyces marxianus var. lactis the cells contain multiple copies of each of two linear, cytoplasmically inherited dsDNA plasmids, pGl1 and pGl2; cells containing these plasmids secrete a glycoprotein toxin capable of killing other (sensitive) strains of e.g. Candida, Kluyveromyces and Saccharomyces by inhibiting adenylate cyclase activity. These plasmids can be transferred to Saccharomyces cerevisiae (e.g. by protoplast fusion) on which they confer the killer phenotype. [ARM (1983) 37 253–276.] (cf. KILLER FACTOR.) killer yeasts See KILLER FACTOR and KILLER PLASMIDS. kilobase See entry kb. kimchi A Chinese and Korean food made by the lactic acid fermentation of vegetables (usually cabbage or radishes). Preparation resembles that of SAUERKRAUT. KinA, KinB See ENDOSPORE (sense 1). kinase An ENZYME which catalyses the transfer of a phosphate group from one substrate (commonly ATP) to another; examples include hexokinase and pyruvate kinase (both involved in the Embden–Meyerhof–Parnas pathway: see Appendix I(a)), ADENYLATE KINASE, and PPi:phosphofructose dikinase (see PYROPHOSPHATE). Kineosporia A genus of bacteria (order ACTINOMYCETALES, wall type I). The organisms form a substrate mycelium but no aerial hyphae; sporangia, each containing one zoospore, develop on suspension is incorporated in each dilution of the disinfectant. The test conditions are standardized – e.g. the bacteria, yeast and disinfectant are each diluted in standard hard water. [PJ (1974) 213 528–530.] Kemerovo subgroup See ORBIVIRUS. kennel cough See MASTADENOVIRUS. Kentrophoros A genus of marine ciliates (subclass GYMNOSTOMATIA) related to LOXODES. Cells: very elongated and flattened (ribbon-like), ciliated only on one side, and associated with ectosymbiotic bacteria; there is no cytostome. keratin A highly insoluble protein found e.g. in hair, wool, horn, skin, feathers. Keratin is degraded by relatively few organisms – e.g. Candida albicans, DERMATOPHYTES. (See also PILIMELIA and THERMOMONOSPORA.) keratitis Inflammation of the cornea. (cf. KERATOCONJUNCTIVITIS; see also INFECTIOUS KERATITIS.) keratoconjunctivitis Inflammation of the cornea and conjunctiva. It may be caused e.g. by Chlamydia trachomatis (see e.g. TRACHOMA), Staphylococcus aureus, HERPES SIMPLEX virus, or adenoviruses (see e.g. EPIDEMIC KERATOCONJUNCTIVITIS). keratomycosis KERATITIS due to a fungus; it is usually due to an opportunist pathogen (e.g. Aspergillus spp, Candida spp). kerion (exudative ringworm; tinea kerion) A severe form of RINGWORM (usually tinea capitis) in which the dry, scaling skin lesions become inflamed and suppurative. kerogen Solvent-insoluble, condensed (i.e., covalently crosslinked and/or aromatized) organic matter which forms the major part of OIL SHALE; it is formed by the gradual transformation of sedimented biomolecules in the absence of high temperatures and pressures. (Protokerogen is a less mature form of kerogen from more recent sediments.) Kerogen is believed to be the precursor material of PETROLEUM and natural gas; on heating it yields HYDROCARBONS. (See also MICROBIAL MAT.) kerosene See PETROLEUM. kerosene fungus See HORMOCONIS. kethoxal (2-keto-3-ethoxyl-n-butyraldehyde) A reagent which reacts with and modifies only unpaired guanine residues in DNA or RNA. ketoconazole (1,3-dioxolanylmethylimidazole) An AZOLE ANTIFUNGAL AGENT which is effective against various mucocutaneous and cutaneous mycoses (including those caused by fungi resistant to GRISEOFULVIN) as well as systemic mycoses such as aspergillosis, blastomycosis, coccidioidomycosis, histoplasmosis, etc; unlike most of the imidazole antifungal agents, ketoconazole can be administered orally. ketogenic fermentations Commercial fermentation processes in which polyhydric alcohols are converted to ketoses: see e.g. DIHYDROXYACETONE FERMENTATION and SORBOSE FERMENTATION. a-ketoglutarate dehydrogenase See TCA CYCLE. ketopentose See PENTOSES. kGy See GRAY. Khawkinea See EUGLENOID FLAGELLATES. KIA KLIGLER’S IRON AGAR. Kickxellales An order of fungi (class ZYGOMYCETES) which are typically saprotrophic in soil and dung; all species form one-spored sporangiola – numbers of which are borne on a short hyphal branch (sporocladium). Genera: e.g. Coemansia, Linderina, Martensella (species can be parasitic on other fungi), and Spiromyces. kidney disease (Dee disease) A systemic, usually chronic FISH DISEASE affecting salmonids. The kidneys become pale and swollen with characteristic greyish necrotic lesions; mortality may be high. Pathogen: Renibacterium salmoninarum. 415 kinesis the hyphae. Growth occurs at/below 30° C, but not at 37° C. Type species: K. aurantiaca. kinesis (1) A behavioural response in which the speed (but not the direction) of locomotion of a motile organism depends on the intensity of an external stimulus; e.g., when light intensity increases, an organism may swim faster (positive photokinesis) or slower (negative photokinesis). (See also PHOTOTAXIS.) Kinesis, unlike TAXIS, is a continuous response and is not cancelled by ADAPTATION. (2) A behavioural response in which the rate of activity of an organism depends on the intensity of an external stimulus. When the activity concerned is speed of swimming, the phenomenon is known as orthokinesis (syn. kinesis sense 1). When the activity concerned is frequency of directional change, the phenomenon is termed klinokinesis. [Photochem. Photobiol. (1977) 26 559–560.] kinete A motile zygote, or a motile form derived from a zygote. kinetic response Syn. KINESIS. kinetid (ciliary corpuscle; kinetosomal territory) In ciliates: a unit of the KINETY; it typically includes a CILIUM with its associated kinetosome, KINETODESMA, cytoplasmic membrane and alveolus (see PELLICLE), and may also include e.g. a MUCOCYST, PARASOMAL SAC and TRICHOCYST together with various microfibrils and/or microtubules (e.g. nematodesmata). kinetin 6-Furfurylaminopurine, a compound formed during the hydrolysis of DNA; it has CYTOKININ activity but it apparently does not occur in plants. kinetochore An electron-dense MICROTUBULE-ORGANIZING CENTRE, one of which develops on each side of the CENTROMERE during MITOSIS. kinetochore fibre See MITOSIS. kinetocyst See AXOPODIUM. kinetodesma (kinetodesmos) In some ciliates: one of a series of overlapping endoplasmic fibrils which, by light microscopy, may appear as a single fibre running parallel with, and to the right of, a row of basal bodies in a somatic KINETY; each kinetodesma arises from a kinetosome (near triplets 5–8, GRAIN CONVENTION), passes a little to the right, and then runs anteriorly to terminate in a position where it may overlap the kinetodesma(ta) of the more anterior kinetosome(s). Kinetodesmata have crossstriations of periodicity ca. 30 nm. The role of the kinetodesmata is unknown; they are commonly believed not to be involved in ciliary coordination. (See also RULE OF DESMODEXY; SILVER LINE SYSTEM.) kinetofragments In many members of the KINETOFRAGMINOPHOREA: patches or short rows of kinetids (sometimes with non-ciliferous kinetosomes) in the vicinity of the oral area; their evolutionary origin is presumed to have been the anterior ends of somatic kineties. Kinetofragminophorea A class of protozoa (phylum CILIOPHORA) which characteristically have an apical or subapical cytostome and a CYTOPHARYNGEAL APPARATUS which is usually conspicuous; COMPOUND CILIATURE is typically absent, and ciliature in the oral region is not obviously differentiated from that in other region(s) of the body. STOMATOGENESIS is typically telokinetal (apparently apokinetal in the ENTODINIOMORPHIDA). This class includes many of the ciliates previously referred to as the ‘lower holotrichs’ (e.g. the apostomes, chonotrichs, gymnostomes and trichostomes) together with e.g. the suctorians. Subclasses: GYMNOSTOMATIA, HYPOSTOMATIA, SUCTORIA, VESTIBULIFERIA. kinetoplast In protozoa of the KINETOPLASTIDA: a unique structure which forms a distinct region of the mitochondrion; it comprises a complex network of numerous catenated circular DNA molecules (kDNA), including several thousand small circles (minicircles) and fewer (ca. 20–50) identical larger circles (maxicircles). The maxicircles are the functional counterpart of the mitochondrial DNA of other eukaryotes, encoding e.g. rRNA and proteins. Minicircles encode most of the gRNAs for RNA EDITING (their only known function). [kDNA: Eukaryotic Cell (2002) 1 495–502]. During cell division, the kinetoplast (and mitochondrion) divides before nuclear division, Kinetoplast division is inhibited by certain drugs, including e.g. ACRIDINES, DIAMIDINES and PHENANTHRIDINE derivatives. (See also DYSKINETOPLASTY.) The kinetoplast stains red with ROMANOWSKY STAINS. Kinetoplastida An order of protozoa (class ZOOMASTIGOPHOREA); each organism has a single nucleus, one or two flagella (which arise from flagellar pockets and which often contain a PARAXIAL ROD) and a single, simple or branched, typically elongated mitochondrion which usually contains a KINETOPLAST located close to the flagellar basal body. (See also GLYCOSOME.) Two suborders: BODONINA and TRYPANOSOMATINA. kinetosomal territory Syn. KINETID. kinetosome (protozool.) Syn. BASAL BODY (b). kinety In ciliates: typically, a longitudinal (‘meridianal’) row of somatic KINETIDS; a kinety corresponds to the ‘primary meridian’ in the SILVER LINE SYSTEM. kingdom A major taxonomic category (see TAXONOMY) ranking above phylum. Organisms are grouped into kingdoms in various ways according to different schemes; e.g., according to the ‘fivekingdoms’ classification scheme the kingdoms are Animalia, FUNGI, MONERA, Plantae, and PROTISTA (sense 2). Kingella A genus of catalase-negative, oxidase-positive bacteria (family NEISSERIACEAE) which occur e.g. in the upper respiratory tract. (See also HACEK.) Cells: rods, 1 × 2–3 µm, in pairs or chains. Growth occurs aerobically, and has been reported to occur anaerobically on blood agar with a gaseous phase of 95% hydrogen and 5% carbon dioxide. The organisms are nutritionally fastidious. Optimal growth temperature: 33–37° C. GC%: ∼47–55. Type species: K. kingae. kinins See CYTOKININS. Kinyoun stain An ACID-FAST STAIN which resembles ZIEHL– NEELSEN’S STAIN but differs in that the stain contains higher concentrations of phenol and basic fuchsin and is used cold; methylene blue may be used as counterstain. [Methods: Book ref. 53, pp. 1381, 1384–1385.] Kirby–Bauer technique See DISC DIFFUSION TEST. kirromycin See POLYENE ANTIBIOTICS (b). kirrothricin See POLYENE ANTIBIOTICS (b). Kirsten murine sarcoma virus (Ki-MSV) A replicationdefective MURINE SARCOMA VIRUS isolated from rats inoculated neonatally with a cell-free extract from thymic lymphomas of C3H mice. Ki-MSV resembles HARVEY MURINE SARCOMA VIRUS in carrying the oncogene v-ras (v-Ki-ras: see RAS) and in causing erythroleukaemia as well as sarcomas in newborn mice. kissing disease See INFECTIOUS MONONUCLEOSIS. Kitasatoa A genus of bacteria which resemble Streptomyces spp in e.g. wall type and phage sensitivity; species have been regarded as members of the genus Streptomyces [JGM (1983) 129 1743–1813] and as species incertae sedis [Book ref. 73, pp. 97–98]. kitazin (O,O-diethyl-S-benzyl phosphorothioate) An ORGANOPHOSPHORUS COMPOUND used as an antifungal agent against rice blast disease; kitazin and its isopropyl analogue (kitazin P) is readily absorbed by plant roots and is rapidly translocated in the 416 Kluyveromyces the thread-like molecules of DNA thus become more electrondense than the background. In a modification of this technique, benzalkonium chloride is used instead of cytochrome c; since, in this case, the DNA is not thickened with adherent protein, its relationship to other macromolecules (e.g. polymerases) can be determined more easily. Klenow fragment (Klenow enzyme) The larger of two fragments obtained by proteolytic cleavage (using e.g. subtilisin) of the DNA POLYMERASE I of Escherichia coli. The Klenow fragment retains 5 -to-3 polymerase and 3 -to-5 exonuclease activities but lacks 5 -to-3 exonuclease activity; it has various uses in genetic engineering techniques and in DNA sequencing methods. Klett–Summerson colorimeter See TURBIDIMETRY. Klett unit A unit of turbidity as measured with the Klett–Summerson colorimeter using monochromatic light of a specified colour. Kligler’s iron agar (KIA) A double-sugar–iron agar (cf. TSI AGAR) used e.g. for distinguishing between members of the Enterobacteriaceae. It is prepared as a slope with a deep butt, and consists of nutrient agar (pH 7.4) supplemented with glucose (ca. 0.1%), lactose (1%), sodium thiosulphate (0.05%), ferric ammonium citrate (0.05%), and phenol red (ca. 0.0025%). In general, bacterial reactions with KIA are similar to those with TSI agar; however, the absence of sucrose in KIA means that the lactose-negative reaction given by some organisms is not masked by their ability to attack sucrose. klinokinesis See KINESIS (sense 2). Kloeckera A genus of yeasts (class HYPHOMYCETES) which have teleomorphs in the genus HANSENIASPORA. Vegetative cells are more or less lemon-shaped (apiculate) and reproduce by bipolar budding in basipetal succession; pseudomycelium may be formed. All species ferment glucose; some ferment sucrose, none ferments maltose (although some species can assimilate maltose). NO3 − is not assimilated. All species require inositol and pantothenate for growth. Species (K. africana, K. apiculata, K. apis, K. corticis, K. japonica, K. javanica) have been isolated from fruit, soil, bark etc. [Book ref. 100, pp. 873–881.] Klonostricha See HYPOTRICHIDA. Klossiella See ADELEORINA. Kluyver effect The phenomenon in which certain yeasts can utilize particular disaccharides aerobically, but not anaerobically. The mechanism underlying the effect is unknown [JGM (1982) 128 2303–2312]. Kluyvera A genus of motile bacteria of the ENTEROBACTERIACEAE. MR +ve; VP −ve; usually indole +ve and citrate +ve. 2-Oxoglutarate is formed from glucose. The organisms occur in food, soil and sewage, and may be opportunist pathogens in man. [Book ref. 22, pp. 511–513.] Kluyveromyces A genus of yeasts (family SACCHAROMYCETACEAE) in which the cells are spheroidal, ovoid, ellipsoidal, or cylindrical to elongate; vegetative reproduction occurs by multilateral budding. Pseudomycelium may be formed. Asci are evanescent; they contain one to many spores which may be crescent-shaped, reniform, spheroidal etc, and which tend to agglutinate after liberation. Cells contain ubiquinone-6 (Q6). All species can ferment glucose; K. marxianus var. lactis and some strains of K. marxianus var. marxianus can ferment LACTOSE. NO3 − is not assimilated. Eleven species have been recognized primarily on the basis of hybridization data; on this basis some former species (e.g. K. bulgaricus, K. fragilis, K. lactis) have been relegated to varieties of K. marxianus – e.g., ‘K. fragilis’ is K. marxianus var. marxianus (anamorph: Candida kefyr, = ‘C. pseudotropicalis’), and transpiration stream. It appears to function by inhibiting CHITIN synthesis. klebicin (klebecin, klebocin) See BACTERIOCIN. Klebs–Löffler bacillus Corynebacterium diphtheriae. Klebsiella (1) A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE (q.v.). Cells: straight rods, ca. 0.3–1.0 × 0.6– 6.0 µm, occurring singly, in pairs, or in short chains; non-motile. The cells are capsulated and usually form convex, glistening, viscid (mucoid) colonies on carbohydrate-rich media. Serotyping [Book ref. 68, pp. 143–164] is based on capsular (K) rather than on O antigens. Some strains may possess mannose-sensitive (type 1) and/or mannose-resistant (type 3) FIMBRIAE. Typical reactions: acid (sometimes with gas) from glucose; H2 S −ve; phenylalanine deaminase −ve; ornithine decarboxylase −ve; myo-inositol +ve; growth in KCN media. Many strains are lactose +ve. GC%: 53–58. Type species: K. pneumoniae. K. oxytoca. Indole +ve; MR −ve; VP +ve; no gas from lactose at 44.5° C; growth occurs at 10° C; melezitose sometimes +ve; pectate +ve. Occurs in the intestines of man and animals and in plant and aquatic environments. K. planticola (= K. trevisanii [IJSB (1986) 36 486–488]). Indole and MR reactions variable; VP +ve; no gas from lactose at 44.5° C; growth occurs at 10° C; melezitose −ve; pectate −ve. Occurs mainly in plant, soil and aquatic environments. K. pneumoniae (including strains sometimes called K. aerogenes and K. edwardsii – cf. AEROBACTER). Indole −ve; no growth at 10° C; melezitose −ve; pectate −ve. Some strains carry out NITROGEN FIXATION. Strains occur in the intestines and respiratory tract of man and animals and may be pathogenic, causing e.g. bovine MASTITIS, equine metritis, OZAENA, PNEUMONIA, and nosocomial URINARY TRACT INFECTION. Subspecies pneumoniae: gas from lactose at 44.5° C; MR −ve, VP +ve (but see METHYL RED TEST); urease +ve. Subspecies ozaenae (formerly K. ozaenae): lactose fermentation variable; MR (usually) +ve; VP −ve; urease variable. Subspecies rhinoscleromatis (formerly K. rhinoscleromatis): lactose −ve; MR +ve; VP −ve; urease −ve. K. terrigena. Indole −ve; no gas from lactose at 44.5° C; growth occurs at 10° C; melezitose +ve; pectate −ve. Occurs mainly in soil and aquatic environments. K. trevisanii. See K. planticola. [Book ref. 22, pp. 461–465.] (2) A genus of EUGLENOID FLAGELLATES found in marine or brackish waters; each cell is partly enclosed in a yellow to brown LORICA. Klebsormidium (Hormidium) A genus of filamentous, unbranched, freshwater or terrestrial green algae (division CHLOROPHYTA). The filaments lack a holdfast and are composed of uninucleate cells each containing a single plate-like chloroplast. Zoospores (rarely formed) are asymmetric, biflagellate and naked, and are released via a pore in the mother cell wall. Sexual reproduction is isogamous; gametes are biflagellate. (cf. STICHOCOCCUS.) Kleinschmidt monolayer technique A method for preparing a nucleic acid for examination by ELECTRON MICROSCOPY. Essentially, a solution of e.g. DNA is mixed with a solution of the globular protein cytochrome c (which adheres to DNA), and one drop of this mixture is allowed to run down a sloping surface onto an air–liquid interface. This gives rise to a monolayer of denatured cytochrome c containing extended molecules of DNA which are thickened by a coating of cytochrome c; dsDNA appears thicker than ssDNA. A small area of the monolayer is transferred to a prepared grid, and the preparation is either shadowed on a rotating platform or stained e.g. with uranyl acetate; 417 k-MTs field diaphragm. (v) Focus on the specimen with a low-power (e.g. ×10) objective, and adjust the substage condenser slightly so that the edge of the field diaphragm is in sharp focus in the plane of the specimen. (vi) Open the field diaphragm until the small disc of light in the centre of the field of view has expanded to cover the entire field. Under these conditions an image of the lamp’s filament is formed in the lower focal plane of the substage condenser, and divergent rays from each point of the filament’s image pass through the condenser and emerge as parallel rays which illuminate the specimen. koji A preparation consisting of mould (usually Aspergillus oryzae) growing on cooked cereal and/or soybeans; the mould produces enzymes (including a range of proteases, amylases, pectinases, glutaminase, etc) and is used in the production of e.g. SOY SAUCE and MISO. The inoculum for making koji is a koji starter or tane koji : a powder consisting of mould spores. Tane koji is typically made by inoculating steamed polished rice with spores of selected fungal strains; the rice is spread in shallow trays, incubated at 30° C for ca. 5 days, and the spores are then harvested and dried. kojibiose A reducing disaccharide: a-D-glucopyranosyl-(1 → 2)D-glucopyranose. It can occur e.g. as a product of enzymic degradation of certain plant carbohydrates, and is present in glycerol TEICHOIC ACIDS of group D streptococci (in which it is linked to the C-2 position of glycerol and may be esterified with D-alanine). kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone) A secondary metabolite produced e.g. by Aspergillus spp (particularly the flavus–oryzae group) when grown on glucose, xylose or certain other sugars. It is a chelating agent which gives a strong blood-red colour with Fe3+ ; it also has weak antibiotic activity (enhanced by certain metal ions), being effective mainly against Gram-negative bacteria. Kolmer CFT See STANDARD TESTS FOR SYPHILIS. Kolpoda Syn. COLPODA. kombu See LAMINARIA. Konservomat See BATCH RETORT. Kopeloff modification A modification of the GRAM STAIN used for staining anaerobic bacteria. The technique involves the addition of ca. 5 drops of NaHCO3 solution (5%) to the smear after the latter has been flooded with crystal violet (1%); the iodine solution (iodine 2%, KI 0.1%) includes NaOH (0.4%), decolorization is carried out with acetone–alcohol, and safranin (2%) is used for counterstaining. Koplik’s spots Small, white, necrotic lesions formed in the mouth in the early stage of MEASLES. Kordyana See EXOBASIDIALES. Korean haemorrhagic fever (epidemic haemorrhagic fever) A VIRAL HAEMORRHAGIC FEVER involving renal dysfunction, proteinuria and oliguria; the mortality rate may be up to 30%. The causal agent is the Hantaan virus (see HANTAVIRUS); reservoirs of infection are found in rodent populations. Infection typically occurs by inhalation of aerosols of urine, faeces etc. from infected rodents. The disease occurs in Asia and Europe. Korfia See HELOTIALES. Kornberg enzyme See DNA POLYMERASES. Koserella A genus of Gram-negative bacteria of the ENTEROBACTERIACEAE. K. trabulsii (formerly known as ‘enteric group 45’ and originally identified as an atypical strain of Hafnia alvei ) has been isolated from clinical specimens; it is MR +ve; VP −ve; indole −ve; citrate +ve; H2 S − ve; urease −ve; LDC ‘K. lactis’ is K. marxianus var. lactis (anamorph: Candida sphaerica). Other species include e.g. K. aestuarii, K. africanus, K. blattae, K. polysporus, K. thermotolerans (anamorph: Candida dattila). [Book ref. 100, pp. 224–251.] Kluyveromyces spp occur in a wide range of habitats: e.g. sea-water (K. aestuarii ), soil, insects, fruit and other plant material, foods and beverages, etc. (See also CHEESE-MAKING (c), SINGLE-CELL PROTEIN, YEAST EXTRACT, and KILLER PLASMIDS.) k-MTs MICROTUBULES attached to a KINETOCHORE. knallgas bacteria Syn. HYDROGEN-OXIDIZING BACTERIA. knallgas reaction (oxyhydrogen reaction) The oxidation of gaseous hydrogen by oxygen. In the HYDROGEN-OXIDIZING BACTERIA this reaction, which provides energy for growth, involves one or more types of HYDROGENASE and a respiratory chain which may contain e.g. a-, b-, c- and o-type cytochromes, iron–sulphur proteins and ubiquinones. [Ubiquinones in hydrogen-oxidizing bacteria: SAAM (1983) 4 181–183.] In e.g. chemolithoautotrophically grown Paracoccus denitrificans, electrons from hydrogen appear to be transferred to oxygen via hydrogenase, UQ-10 and cytochromes b562 and o. In e.g. ‘Alcaligenes eutrophus’ (which has both NAD-independent and NAD-reducing hydrogenases) it has been suggested that electrons from hydrogen may be donated to the respiratory chain and/or to NAD – the NADH being oxidized via the respiratory chain. knockout mice Mice in which specific gene(s) have been disrupted (by genetic manipulation at the embryo stage) such that the corresponding gene product(s) are not synthesized in active form. knopvelsiekte The severe form of LUMPY SKIN DISEASE. knot (in DNA) An entanglement within a single, circularly closed DNA molecule, resolution of which (to form a knotfree cccDNA molecule) requires the activity of a TOPOISOMERASE (q.v.). koala (chlamydiosis in) See CHLAMYDIA. Koch–Weeks bacillus Haemophilus aegyptius. Koch’s blue bodies See EAST COAST FEVER. Koch’s phenomenon The phenomenon in which different responses are given by healthy and tuberculous guinea pigs to a subcutaneous injection of virulent tubercle bacilli; reactions in the tuberculous animals are manifestations of DELAYED HYPERSENSITIVITY (q.v.). Koch’s postulates According to Robert Koch (1843–1910): a set of conditions which should be fulfilled in order to establish that a given organism is the causal agent of a particular disease. (i) The organism must be present in every case of the disease. (ii) It should be isolable in pure culture. (iii) Inoculation of susceptible animals with the isolated organism must produce the disease. (iv) The organism must be observable in, and/or isolable from, the (experimental) diseased animals. Koehler illumination Syn. KÖHLER ILLUMINATION. Kofoidinium A genus of DINOFLAGELLATES. Köhler illumination (Koehler illumination) In MICROSCOPY: illumination in which the entire field is lit with uniform intensity, regardless of any non-uniformity in the light source (cf. CRITICAL ILLUMINATION). To obtain Köhler illumination with an external lamp having a condensing lens with an iris diaphragm (field diaphragm) outside it: (i) Close the substage CONDENSER diaphragm (aperture diaphragm). (ii) Open the field diaphragm. (iii) Using the microscope’s plane mirror, image the lamp’s filament on the underside of the aperture diaphragm; thereafter, keep lamp and microscope in the same relative positions. (iv) Open the aperture diaphragm and almost close the 418 Kyzylagach virus +ve; ODC +ve; and it produces acid from cellobiose and melibiose but not from glycerol, lactose or sucrose. [JCM (1985) 21 39–42.] Koser’s citrate medium A medium, used for the CITRATE TEST, containing (per 100 ml of water): NaCl (0.5 g), MgSO4 ·7H2 O (0.02 g), (NH4 )H2 PO4 (0.1 g), K2 HPO4 (0.1 g), citric acid (0.2 g); pH 6.8. Koster’s stain A stain for detecting Brucella spp in mammalian tissues. The smear or section is stained for 1 min in a solution made by mixing saturated aqueous safranin and normal KOH (2:5 v/v); the slide is then washed in tap water and differentiated in 0.1% sulphuric acid (two changes for a total period of 10–20 sec). The slide is washed in tap water and counterstained with 1% carbol methylene blue. Brucella stains orange, tissues blue. Kotonkan virus See LYSSAVIRUS. koumiss (kumiss) An acidic, mildly alcoholic, fermented milk beverage made in the former USSR. Traditionally made from mares’ milk, it is now also made from cows’ milk. Organisms involved in the fermentation include Lactobacillus bulgaricus (the main acid-producer) and lactose-fermenting yeasts; the main products of fermentation are lactic acid, ethanol and CO2 . (See also DAIRY PRODUCTS.) Kovács’ indole reagent An INDOLE TEST reagent: p-dimethylaminobenzaldehyde (5 g) is dissolved in 75 ml amyl (or isoamyl) alcohol, and conc. HCl (25 ml) is slowly added; it is stored at 4° C in the dark. Kovács’ oxidase reagent A 1% aqueous solution of tetramethylp-phenylenediamine dihydrochloride. (See also OXIDASE TEST.) kPa 103 Pa (see PASCAL). KpnI See RESTRICTION ENDONUCLEASE (table). krad 103 rad (see RAD). kraft pulps See PAPER SPOILAGE. Krebs cycle Syn. TCA CYCLE. Krebspest Syn. CRAYFISH PLAGUE. Kruse’s bacillus Archaic name for Shigella sonnei (Kruse’s ‘Bacillus pseudodysenteriae type E’). Kudoa See MYXOSPOREA. kumiss Syn. KOUMISS. Kunin antigen Syn. ENTEROBACTERIAL COMMON ANTIGEN. Kunjin virus See FLAVIVIRIDAE. Kupffer cells Actively phagocytic cells which line the liver sinusoids. Kurthia A genus of Gram-positive, asporogenous, catalasepositive, obligately aerobic bacteria which occur e.g. in meat and meat products. In the exponential growth phase the organisms occur in long chains of rods (or of short filaments) but in the stationary phase coccoid forms or short rods predominate; individual rods are peritrichously flagellated. Kurthia spp are chemoorganotrophs which can utilize certain alcohols (e.g. ethanol, ethanediol), amino acids (e.g. L-alanine) and fatty acids (e.g. butyric acid). VP −ve. Indole-negative. The type species, K. zopfii, grows in the temperature range 5–35° C and is killed by heating to 55° C/20 minutes. K. gibsonii grows in the range 5–45° C and survives heating to 55° C/20 minutes [SAAM (1983) 4 253–276]. GC%: ca. 36–38. kuru A human TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHY [early description: NEJM (1957) 257 974–978]. The incubation period may range from a few years to some 30 years or more; characteristically there is ataxia, tremor, dysarthria and sometimes late dementia, and death occurs within a year of the onset of symptoms. Kuru is found in certain regions of Papua New Guinea and is believed to result from the ingestion of prion-containing flesh during ritual cannibalism; as this practice has declined, the incidence of kuru has decreased. Kuru can be transmitted experimentally to chimpanzees and to other primate and non-primate animals; goats (but not sheep) inoculated with tissue from kuru victims develop a disease that closely resembles SCRAPIE. [Review of kuru: Book ref. 159, pp. 483–544 (497–510).] Kusnezovia A genus of poorly-characterized, manganesedepositing bacteria found in mud; the organisms have not been obtained in pure culture, and the validity of the genus (and of an apparently similar genus, Caulococcus) has been questioned [Book ref. 45, pp. 529–530]. Kyasanur Forest disease An acute, tick-borne human disease (vector: mainly Haemaphysalis sp) caused by a flavivirus (see FLAVIVIRIDAE); it occurs in Mysore State, India. Onset is sudden, with e.g. fever, headache, severe myalgia, haemorrhages, etc. Mortality rates are generally low (<5%). Monkeys may act as a reservoir of infection. Kyzylagach virus See ALPHAVIRUS. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 419 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 L The vegetative stage of the organism consists of spindleshaped cells which produce a branching, anastomosing network of membrane-enclosed ‘slime-ways’ (variously called the ectoplasmic net, filoplasmodium, net plasmodium, rhizoplasmodium, etc); the cells move (by gliding) within these slime-ways. The vegetative cells contain a characteristic organelle, the sagenogenetosome or sagenogen (formerly ‘bothrosome’), which is apparently involved in the production of the ectoplasmic net and possibly also in the movement of the cells through the net. The cells feed osmotrophically; enzymes are secreted from the ectoplasmic net, and the soluble products of digestion diffuse back to the cells within the net. In at least some labyrinthulas biflagellate zoospores are formed, each bearing one anteriorly directed tinsel flagellum and one posteriorly directed whiplash flagellum; an orange eyespot occurs at the base of the flagellar apparatus. The zoospores are haploid, their formation involving meiosis. They apparently develop directly into spindle-shaped cells; copulation is presumed to occur between two of these cells. labyrinthulids See LABYRINTHULAS. Labyrinthulina See GYMNOMYXA. Labyrinthuloides See THRAUSTOCHYTRIDS. Labyrinthulomycetes A class of organisms (division MYXOMYCOTA) which includes the LABYRINTHULAS and the THRAUSTOCHYTRIDS; these organisms are apparently unrelated to other members of the Myxomycota. lac operon (lactose operon) An OPERON containing genes which encode proteins involved in the uptake and utilization of bgalactosides such as LACTOSE. The lac operon occurs e.g. in Escherichia coli – in which it is located at ca. 8 minutes on the chromosome map. A simplified scheme for the lac operon is shown in the figure. The lacZ gene encodes b-galactosidase, lacY encodes a transport protein, ‘b-galactoside permease’, and lacA encodes thiogalactoside transacetylase. (LacA catalyses the transfer of an acetyl group from acetyl-CoA to a b-galactoside. Its function in vivo is unknown; it has been suggested that it may be involved in the detoxification of non-metabolizable sugars by bringing about their acetylation and, hence, excretion.) According to an earlier view, control of the lac operon (illustrated in the figure) is as follows. In the absence of an inducer, the regulator protein (repressor), which is active in tetramer form, binds tightly to the OPERATOR. The operator (O) is a region of ca. 16 bp which occurs immediately downstream of the promoter (P); the operator is an imperfect PALINDROMIC SEQUENCE centred at about nucleotide +10 in relation to the transcription start site (see PROMOTER). The binding of repressor to the operator reduces transcription to a minimal level (only a few molecules of product being produced per cell). If an inducer (ALLOLACTOSE or e.g. IPTG) is present, it binds to the operator-bound repressor and causes the repressor to undergo a conformational change that reduces its affinity for the operator; the repressor then detaches from the operator, and the level of lacZYA transcription increases rapidly; up to ca. 10000 molecules of b-galactosidase may appear in the cell within minutes of induction. (The presence of inducer does not necessarily lead to expression of the lac operon: see CATABOLITE REPRESSION.) While the above description gives the essential mode of control of the lac operon, the actual process is rather more complex [Mol. Microbiol. (1992) 6 2419–2422]. Thus, in L Leucine (see AMINO ACIDS). L-flagella Lateral flagella – see VIBRIO. L-form (L-phase variant) A defective bacterial cell of spherical or irregular shape, formed either spontaneously (e.g. by Streptobacillus moniliformis) or as a result of various stimuli (e.g. temperature shock, osmotic shock, or antibiotics which inhibit cell wall biosynthesis); in an L-form the cell wall is either partly or totally absent. (cf. PROTOPLAST and SPHAEROPLAST.) On removal of the stimulus, an L-form may resume cell wall synthesis and revert to the condition of the original strain, or it may continue to reproduce as an L-form (stable L-form). Lforms have been observed in various bacteria – including species of Bacillus, Proteus, Streptococcus and Vibrio. L-form colonies often resemble those of Mycoplasma spp. (L-forms were named after the Lister Institute of Preventive Medicine, London.) (See also CHLORAZOL BLACK E.) L-phase variant Syn. L-FORM. L protein See LEVIVIRIDAE. L ring See FLAGELLUM (a). L-selectin See SELECTINS. l-strand (in adenoviruses) See ADENOVIRIDAE. L strand See H STRAND. L-type starter See LACTIC ACID STARTERS. L1 acholeplasmavirus group Syn. PLECTROVIRUS. L1–L34 proteins See RIBOSOME. L2 acholeplasmavirus group Syn. PLASMAVIRIDAE. L3 acholeplasmavirus group Syn. MV-L3 PHAGE GROUP. L18-MDP(A) See ADJUVANT. L-573,655 See ENDOTOXIC SHOCK. L-749,345 ERTAPENEM. La Crosse virus A virus of the genus BUNYAVIRUS; this virus is an occasional causal agent of human meningoencephalitis. La France disease See WATERY STIPE. labial herpes See HERPES SIMPLEX. label (probe) See PROBE. Laboulbenia See LABOULBENIALES. Laboulbeniales An order of fungi (subdivision ASCOMYCOTINA) which include species that are obligately parasitic on insects. The minute, non-mycelial but multicellular thallus attaches to the host via a specialized organelle (see FOOT). Some species are monoecious, others dioecious (see also HETEROTHALLISM). The ascocarp is a perithecium. Conidial stages are apparently unknown. Genera: e.g. Ceratomyces, Herpomyces, Laboulbenia, Rickia. (cf. SPATHULOSPORALES.) Labyrinthomorpha A phylum of the PROTOZOA which includes the LABYRINTHULAS and the THRAUSTOCHYTRIDS. Labyrinthula See LABYRINTHULAS. labyrinthulas (labyrinthulids; ‘net slime moulds’) A group of eukaryotic organisms of uncertain taxonomic position; they are classified together with the THRAUSTOCHYTRIDS, being variously regarded as fungi e.g. of the LABYRINTHULOMYCETES, as protozoa of a distinct phylum, Labyrinthomorpha, or as protists of the subphylum Labyrinthulina, phylum GYMNOMYXA. There is probably only one genus, Labyrinthula, species of which generally occur in marine habitats on plants and algae; they appear to be at least partially parasitic, and are believed to play a role in the ecologically important ‘wasting disease’ of eelgrass (Zostera marina). One Labyrinthula species occurs in fresh water and is apparently parasitic on Vaucheria. 420 lac operon (a) DNA lac I P lac Z O lac Y lac A RNA polymerase mRNA protein regulator protein (b) DNA lac I P O lac Z lac Y lac A mRNA RNA polymerase protein inactive regulator− inducer (allolactose) inducer complex b-galactosidase b-galactoside permease thiogalactoside transacetylase THE lac OPERON in Escherichia coli (a simplified scheme). The lac operon consists of (i) three regulated genes: lacZ, lacY and lacA (see entry); (ii) a promoter (P); (iii) an operator (O); and (iv) the regulator gene, lacI, encoding a repressor. lacI is transcribed constitutively, at a low rate, from its own independent promoter. (a) In the absence of an inducer, the regulator protein (LacI) binds to the operator, O, and minimizes transcription of genes lacZ, lacY and lacA by the RNA polymerase (seen here at the promoter); there is some ‘leakage’: a few molecules of product are formed under these conditions in each cell. (b) In the presence of lactose (taken up by proton–lactose symport), b-galactosidase converts some of the lactose to allolactose, the inducer of the lac operon. Allolactose binds to, and inactivates, the regulator protein – which dissociates from the operator, thus permitting transcription and translation of the regulated genes. (The single mRNA transcript contains an initiator codon and a stop codon for each of the three genes.) When all the lactose has been metabolized, transcription of the lac operon is no longer needed; the inducer (allolactose) is no longer formed, and the (now active) regulator protein represses the operon. The lac operon is subject to CATABOLITE REPRESSION (q.v.). See entry for further details. Reproduced from Bacteria, 5th edition, Figure 7.11, page 138, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. As well as the main CRP-binding site (centred ∼62 bp upstream from the start site), there is a secondary CRP-binding site which coincides with the (primary) operator sequence. CRP has less affinity for this site, and its role is unknown. Isopropyl-b-D-thiogalactoside (IPTG) is a gratuitous inducer of the lac operon, i.e. it acts as an inducer of the operon but is not metabolized by the cell. IPTG is useful e.g. in recombinant DNA technology. Mutants defective in regulation of the operon. lacI − mutants either fail to produce a repressor or produce an inactive one; transcription of lacZYA in these mutants occurs constitutively, but it can be repressed by supplying a functional repressor, i.e. in this case the lac− phenotype is recessive. However, in socalled lac−d mutants the mutant phenotype is dominant over the addition to the primary operator (O), two other specific sites have been found to bind repressor (albeit with lower affinity): O-2 (located within the lacZ gene) and O-3 (found upstream of the promoter); a tetramer of the repressor protein appears to bind to the primary operator (O) and, simultaneously, to either O-2 or O-3, thus forming a DNA loop that may stabilize repressor–operator binding. Moreover, in addition to the primary promoter (P1), there are at least five other overlapping promoter-like sequences – two upstream and three downstream of P1 – but their role(s) are unknown; some of these promoter-like sequences can be activated by a single base-pair substitution to give a CRPindependent Lac+ phenotype (but the significance of this is unknown). 421 Lac plasmid wild-type; in these cells, a defective (mutant) repressor subunit can interact with wild-type subunits to form a defective tetramer (an example of negative complementation). Mutants designated lacI s produce a ‘super-repressor’ which retains its affinity for the operator but has negligible affinity for the inducer – the repressor remains tightly bound to the operator even in the presence of inducer (i.e. the operon is non-inducible), and the defect cannot be overcome by the presence of normal repressor (i.e. the mutant phenotype is dominant). A mutation in the operator may reduce the affinity of the operator for the repressor; such mutants (designated lacO c or oc ) express the lac genes constitutively, and the oc allele exhibits CIS-DOMINANCE. Lac plasmid Syn. LACTOSE PLASMID. lacA gene See LAC OPERON. laccase A monophenol oxidase (EC 1.14.18.1). Many basidiomycetes (and some ascomycetes and deuteromycetes) produce extracellular laccase which appears to be involved in the degradation of LIGNIN and e.g. in the induction of basidiocarp development [JGM (1982) 128 2763–2770]. Most or all fungal laccases are copper-containing ‘blue proteins’ which can oxidize o- and p-phenols and aromatic amines. Lachnospira A genus of bacteria (family BACTEROIDACEAE) which occur e.g. in the bovine RUMEN. Cells: curved rods, 0.4–0.6 × 2.0–4.0 µm, with one lateral or subpolar flagellum per cell; the cells from young cultures may give a weak Grampositive reaction, while those from older cultures may stain Gram-negatively. Glucose fermentation yields acetate, ethanol, formate, lactate, CO2 and H2 ; these products, together with methanol, are also formed from pectin fermentation. Butyrate, propionate and succinate are not formed. Growth is flocculent in glucose–rumen fluid media. Type species: L. multiparus. [Book ref. 22, pp. 661–662.] lachrymal canaliculitis Inflammation of the lachrymal ducts and adjacent tissues, together with the formation of yellowish concretions within the ducts; the disease is non-invasive, usually mild, and the causal agent is commonly Arachnia propionica or Actinomyces israelii. laciniate Lobed or notched. lacrimoid (lacrioid) Shaped like a tear-drop. Lacrymaria See GYMNOSTOMATIA. lactacin A See BACTERIOCINS. lactacin B See BACTERIOCINS. lactacin F See BACTERIOCINS. b-lactam antibiotics A family of ANTIBIOTICS, each of which contains the b-lactam ring (see figure). CEPHALOSPORINS and PENICILLINS are produced by fungi, while the more recently discovered members (see CARBAPENEMS, CLAVAMS, MONOBACTAMS, NOCARDICINS) are produced by bacteria [carbapenems and clavams: TIM (1998) 6 203–208]; synthetic and semi-synthetic members have been manufactured. Most b-lactam antibiotics are antibacterial, but some of the newer ones are antifungal. As well as possessing weak or moderate antibacterial activity, CLAVULANIC ACID and many CARBAPENEMS also inhibit b-LACTAMASES (see also b-LACTAMASE INHIBITOR). The antibacterial action of penicillins and cephalosporins has been widely studied in Escherichia coli. One of the earliest proposals was that penicillin acted as an analogue of D-alanine– D-alanine, thus inhibiting the cross-linking of PEPTIDOGLYCAN with resultant lysis of the structurally weakened cells. The b-lactam antibiotics are now known to have multiple killing sites, and various PENICILLIN-BINDING PROTEINS (PBPs) have been identified as lethal targets. In many cases a given blactam antibiotic has a roughly similar degree of affinity for more than one essential PBP – e.g., amoxycillin (PBPs 1A and 2), ampicillin (2, 3, 1A), benzylpenicillin (2, 3), carbenicillin (1A, 3), cefotaxime (1A, 3), cefuroxime (3, 1A), and cloxacillin (1A, 3). However, some b-lactam antibiotics bind preferentially or exclusively to one particular PBP – e.g., cefsulodin, cephalexin, cephaloridine and cephalothin bind preferentially to PBP-1A, mezlocillin binds preferentially to PBP-3, furazlocillin binds exclusively to PBP-3, and mecillinam binds exclusively to PBP2 (cf. CEFOXITIN, CLAVULANIC ACID and THIENAMYCIN). Resistance to b-lactam antibiotics may be intrinsic or acquired. In Pseudomonas aeruginosa resistance to many blactam antibiotics is due to poor penetration of these antibiotics through the outer membrane. In Escherichia coli resistance can be due e.g. to a mutation resulting in the loss of the ompF PORIN. (In Gram-positive species the CELL WALL offers little hindrance to penetration by molecules the size of b-lactam antibiotics.) Another form of resistance involves mutation resulting in PBPs with lower affinities for b-lactam antibiotics; such PBPs have been detected in e.g. methicillin-resistant strains of Staphylococcus aureus, and in benzylpenicillin-resistant strains of Streptococcus pneumoniae [JGM (1983) 129 1247–1260]. Resistance to b-lactam antibiotics may also be due to the action of chromosomal or plasmid-specified b-LACTAMASES. An important source of resistance to b-lactam antibiotics is the mecA gene which encodes a novel PBP that has low affinity for b-lactams; this gene can confer high-level resistance to methicillin (and other b-lactams) e.g. in Staphylococcus aureus – see MRSA. [Evidence for in vivo transfer of mecA between staphylococci: Lancet (2001) 357 1674–1675.] Cephalosporins and penicillins are decomposed by ions of the transition metals (e.g. copper, zinc). [Nomenclature: JAC (1982) 10 365–372; data review of clinically useful b-lactam antibiotics and those under evaluation: JAC (1986) 17 5–36. Clinical pharmacology and therapeutic uses of penicillins (review): Drugs (1993) 45 866–894.] b-lactamase inhibitor A compound which inhibits the inactivation of b-LACTAM ANTIBIOTICS by b-LACTAMASES; b-lactamase inhibitors include certain b-lactam compounds (e.g. CLAVULANIC ACID) which themselves have little or no antimicrobial activity but which can be combined with b-lactamase-sensitive antibiotics to form clinically useful drugs: see e.g. AUGMENTIN and TIMENTIN. b-lactamases A class of enzymes (EC 3.5.2.6) which hydrolyse the b-lactam (C–N) bond in susceptible b-LACTAM ANTIBIOTICS, such hydrolysis inactivating the antibiotics. (Certain molecular features of a given b-lactam antibiotic may confer resistance to specific b-lactamases; such features include the 7-a-methoxy group in CEPHAMYCINS and the cysteamine side-chain in THIENAMYCIN.) Bacteria generally exhibit a substantial level of resistance to a given b-lactam antibiotic if they encode a b-lactamase which is active against that antibiotic (but see b-LACTAMASE INHIBITOR). (See also Microbial resistance to antibiotics in the entry ANTIBIOTIC.) b-Lactamases are produced by many species of bacteria, both Gram-positive and Gram-negative. In some cases the enzymes are inducible and liberated into the medium, while in other cases they are synthesized constitutively and retained within the cell envelope. (Some b-lactam antibiotics – including e.g. cefoxitin and imipenem – are particularly good inducers of blactamases.) Studies on the transmembrane signalling pathway in staphylococci indicate that the induction of b-lactamases may depend on a novel mechanism that involves sequential 422 b-lactamases dihydrothiazine ring b-lactam ring thiazolidine ring b-lactam ring H H R N CH3 S penicillin amidase 5 6 7 N 1 4 R′ N S 1 2 CH3 8 3 N 3 5 4 O COOH O 2 6 7 CH2R″ b-lactamase b-lactamase (a) COOH (b) H R R″ R′ N OH R′′′ N H N N O O R′′′′ C COOH H (c) (d) oxazolidine ring R′ R″ O N O OH 6 5 7 N 4 O COOH (e) 1 2 3 COOH (f) b-LACTAM ANTIBIOTICS. (a) Penicillin nucleus. In 6-aminopenicillanic acid (6-APA): R = H. (b) Cephalosporin nucleus. In 7aminocephalosporanic acid (7-ACA): R = H, R = O.CO.CH3 ; in cephalosporin C: R = CO.(CH2 )3 .CHNH2 .COOH, R = O.CO.CH3 . (c) Nocardicin nucleus. (d) Monobactam nucleus. (e) Carbapenem nucleus. (f) Clavulanic acid (a clavam). active primarily against penicillins (i.e. penicillinases); type III enzymes included plasmid-encoded b-lactamases active against both cephalosporins and penicillins, but not against oxacillin or carbenicillin; type IV b-lactamases included the chromosomeencoded enzymes of Klebsiella spp that are active primarily against penicillins; type V b-lactamases included plasmidencoded enzymes active against oxacillin and/or carbenicillin. Subsequently, classification of plasmid-encoded b-lactamases of Gram-negative bacteria was based on ISOELECTRIC FOCUSING, and more than 11 types were distinguished [BMB (1984) 40 18–27]. This classification included b-lactamases designated e.g. TEM-1, TEM-2, SHV-1 and HMS-1; these particular enzymes are active against many of (the early) cephalosporins and penicillins, although poorly active against carbenicillin, cloxacillin, methicillin and oxacillin. The OXA-1, -2 and -3 enzymes are active against oxacillin (and e.g. ampicillin and cloxacillin) but poorly active against carbenicillin. The PSE-1, -2, -3 and -4 enzymes of Pseudomonas spp are active against carbenicillin (and e.g. ampicillin), while PSE-2 is also active against e.g. oxacillin, methicillin and moxalactam. Subsequent cleavage of regulatory proteins [Science (2001) 291 1962–1965; commentary: Science (2001) 291 1915–1916]. In this scheme, the regulation of b-lactamase genes involves initial binding of a b-lactam antibiotic to a cell-surface sensor–transducer protein, BlaR1; the resulting cleavage of BlaR1 gives rise to a (hypothetical) product BlaR2 which, directly or indirectly, brings about cleavage of the transcriptional repressor of b-lactamase genes, allowing expression of these genes. b-Lactamases may be encoded by chromosomal genes and/or by genes in a PLASMID or transposon (see e.g. entry for Tn3 ). Genes which encode b-lactamases are designated bla genes. The detection of b-lactamases in vitro can be facilitated e.g. by the use of NITROCEFIN. Classification of b-lactamases. An early scheme classified the b-lactamases according e.g. to their substrate specificities and the location of genes encoding them. The type I blactamases included those chromosome-encoded enzymes of enterobacteria and pseudomonads that are active mainly against cephalosporins (i.e. cephalosporinases); type II b-lactamases included chromosome-encoded enzymes of Proteus spp that are 423 Lactarius The LDV genome is a single molecule of positive-sense ssRNA, and the virus has been placed in the arterivirus group (see ARTERIVIRUS). LDV replicates only in a particular subset of MACROPHAGES, the macrophages being destroyed in the process. [Determinants of macrophage susceptibility: JGV (1985) 66 1469–1477.] Infection results in a persistent viraemia (maintained by the ongoing production of new susceptible cells) and gives rise to elevated levels of certain plasma enzymes – particularly certain (positively charged) lactate dehydrogenase isoenzymes. Elevated levels of these enzymes result from their reduced rates of clearance from the plasma; such enzymes may be cleared in the normal mouse by the particular subset of macrophages destroyed by LDV. Infected mice generally show little sign of disease, but they exhibit slight splenomegaly and disturbances in the immune system (altering the growth of tumours, response to infection etc.). In the acute phase of infection there is transient polyclonal activation of antibody-producing cells – resulting in increased immunoglobulin levels – and transient depression of cell-mediated immunity; in the chronic phase, the humoral response is depressed, while cell-mediated immunity returns to normal. Antibodies to LDV are produced, and virus–antibody complexes circulate in the blood; the immune complexes cause little tissue reaction – possibly because they do not bind complement. LDV can apparently cause age-dependent polioencephalomyelitis (resulting in paralysis) in certain strains of mouse. LDV is apparently transmitted by blood-sucking parasites and by fighting; transplacental transmission can occur if the mother becomes infected during pregnancy or lactation, but generally does not occur in chronically infected mothers. [LDV (review): JGV (1985) 66 2297–2312.] lactic acid (CH3 .CHOH.COOH) A compound present e.g. in many fermented DAIRY PRODUCTS, and also widely used as a food additive. Only the L(+)-isomer can be readily assimilated by man and animals. L(+)-Lactic acid is obtained commercially by the LACTIC ACID FERMENTATION of sucrose by LACTIC ACID BACTERIA; it is used primarily in the food industry, being added e.g. to confectionary, pickles, beverages, etc [AvL (1983) 49 86]. lactic acid bacteria A (non-taxonomic) group of Gram-positive, non-sporing bacteria which carry out a LACTIC ACID FERMENTATION of sugars; it includes species of Lactobacillus, Lactococcus, Leuconostoc and Pediococcus. (Bifidobacterium is sometimes included.) Lactic acid bacteria are used extensively in the food industry (e.g. BREAD-MAKING (sourdough), DAIRY PRODUCTS, SALAMI); however, they can also cause food spoilage (see e.g. MEAT SPOILAGE; cf MALOLACTIC FERMENTATION). (See also SILAGE.) Lactic acid bacteria are nutritionally fastidious and weakly proteolytic [for proteolytic systems see AvL (1983) 49 225–245]. Many activities of the lactic acid bacteria (e.g. LACTOSE metabolism) appear to be plasmid-linked [AvL (1983) 49 259–274]. Although lactic acid bacteria are primarily fermentative, many strains can grow in the presence of air and can carry out certain oxidation reactions using flavin-containing oxidases, peroxidases, or NAD-independent dehydrogenases – oxidizing at least some of the pyruvic and lactic acids (formed by fermentative pathways) to acetic acid and carbon dioxide [AvL (1983) 49 209–224]. (See also LEUCONOSTOC.) The hydrogen peroxide formed during these oxidations has some preservative action in fermented foods (see FOOD PRESERVATION (e)). to the designation of these (early) b-lactamases, many new b-lactam antibiotics have been introduced, but b-lactamases have tended to evolve in parallel – see later under Evolution of b-lactamases. The b-lactamases have also been classified into four categories (A–D) on the basis of sequence homology (apparent evolutionary relationships). For example, class A b-lactamases (which include e.g. TEM-1, encoded by gene blaTEM−1 ) contain a serine residue at the active site of the enzyme; they are found in both Gram-positive and Gram-negative bacteria and are mainly plasmid-encoded enzymes. The class B b-lactamases include zinc-requiring cephalosporinases from Bacillus cereus, while class C enzymes include chromosome-encoded cephalosporinases from Gram-negative bacteria. Evolution of b-lactamases; extended-spectrum b-lactamases (ESBLs). The use of successive ‘generations’ of b-lactam antibiotics, sometimes with b-lactamase inhibitors, created a selective pressure for the development of new b-lactamases that are able to protect bacteria against some of the most recently introduced drugs. Initially, a new generation of so-called ‘extendedspectrum’ b-lactam antibiotics were designed to be effective in the presence of the (then) existing b-lactamases, but, following the use of these newer drugs, new variant (mutant) forms of b-lactamase have evolved; these can hydrolyse at least some of the newer antibiotics and are becoming widely disseminated. These so-called extended-spectrum b-lactamases (ESBLs) are able to inactivate oxyimino-b-lactam antibiotics (such as the third-generation cephalosporin cefotaxime) and the monobactam AZTREONAM. The ESBLs appear to have evolved mainly from TEM and SHV b-lactamases (both class A enzymes) [b-lactamases (evolution): TIM (1998) 6 323–327]. (Interestingly, evolutionary relationships may exist between blactamases and PENICILLIN-BINDING PROTEINS [ARB (1983) 52 853–856].) ESBLs include e.g. TEM-50 (which incorporates resistance to inhibitors) [AAC (1997) 41 1322–1325] and SHV18 (a plasmid-encoded enzyme from Klebsiella pneumoniae K6) [AAC (2000) 44 2382–2388]. Resistance to the fourthgeneration cephalosporin cefepime has been associated with hyperproduction of SHV-5 [JCM (1998) 36 266–268]. ESBLs are frequently encoded by plasmids that also carry other antibiotic-resistance genes, so that the choice of drugs for chemotherapy against ESBL-producing strains may be limited. It has been suggested that the choice of b-lactam drugs suitable for treating ESBL-positive infections may (at present) be limited to CARBAPENEMS or CEPHAMYCINS [treatment options for extendedspectrum b-lactamase producers: FEMS (2000) 190 181–184]. Infections with strains of Acinetobacter and Pseudomonas aeruginosa that encode ESBLs of the PER-1 type have been reported to be associated with an increased level of fatalities [JMM (2001) 50 642–645]. Lactarius A genus of fungi (family RUSSULACEAE) which form mushroom-shaped, lamellate fruiting bodies that exude a milklike fluid (‘latex’) when cut; Lactarius spp are sometimes called ‘milk caps’. The lactiferous (latex-containing) hyphae appear to store precursors of the pungent dialdehydes velleral and isovelleral; these dialdehydes may protect the fungus from mycovores [Mycol. (1984) 76 355–358]. lactase Syn. b-GALACTOSIDASE. lactate dehydrogenase virus (LDV; lactic dehydrogenase virus; lactate dehydrogenase-elevating virus; Riley virus) An enveloped, roughly spherical RNA virus which causes a persistent, life-long infection in mice of the species Mus musculus and M. caroli ; apparently, other animals, including other rodents, are not susceptible. 424 Lactobacillus and prevent its translation. [Improvement and optimization of two engineered phage-resistance mechanisms in Lactococcus lactis: AEM (2001) 67 608–616.] lactic acidosis (vet.) See ACIDOSIS (1). lactic dehydrogenase virus Syn. LACTATE DEHYDROGENASE VIRUS. lacticin 3147 A pore-forming LANTIBIOTIC produced by Lactobacillus lactis subsp lactis. It consists of two peptide components, both of which are needed for activity; each component requires modification by a separate enzyme [Microbiology (2000) 146 2147–2154]. In sensitive cells, the lesions formed by lacticin 3147 are ion-specific pores. A lacticin 3147-producing starter culture has been tested for its ability to inhibit Listeria monocytogenes in the manufacture of cottage cheese [JAM (1999) 86 251–256]. lactiferous hyphae See LACTARIUS. Lactobacillaceae A family of bacteria (sole genus: LACTOBACILLUS). [Book ref. 21, p. 576.] Lactobacillus A genus of Gram-positive, asporogenous bacteria which are anaerobic, microaerophilic, or facultatively aerobic. Cells are rods or coccobacilli, occurring singly or in chains. Most strains are non-motile. Catalase −ve, but may be catalase +ve in haem-containing media. Metabolism is predominantly fermentative, lactic acid being formed from glucose either homofermentatively or heterofermentatively. (See LACTIC ACID BACTERIA.) Species are commonly aciduric or acidophilic. Nutritional requirements are complex, generally including carbohydrates, peptones, vitamins etc. Lactobacilli may be grown e.g. on APT agar; growth is stimulated by Tween 80. The genus (GC%: ca. 32–53) is divided into three subgenera: Betabacterium, Streptobacterium and Thermobacterium. Betabacteria carry out HETEROLACTIC FERMENTATION of glucose (and hence require thiamine and produce CO2 from glucose); fructose is fermented and can be reduced to mannitol (see e.g. SILAGE), and PENTOSES may be fermented. Streptobacteria and thermobacteria carry out HOMOLACTIC FERMENTATION of glucose (CO2 not produced, thiamine not required); fructose is usually fermented but it cannot be reduced to mannitol. Streptobacteria can grow at 15° C (and may or may not grow at 45° C), can ferment pentoses, and produce CO2 from gluconate. Thermobacteria can generally grow at or above 45° C but not at 15° C, and cannot ferment pentoses or produce CO2 from gluconate. Species are distinguished by biochemical tests. The nature of the lactic acid produced can be a useful criterion: betabacteria produce DL-lactic acid, streptobacteria and thermobacteria produce (mainly) L(+)-, D(−)- or DL-lactic acid, according to species. Betabacteria include e.g. L. brevis, L. buchneri (possibly a subspecies of L. brevis), L. cellobiosus, L. confusus, L. desidiosus, L. fermentum, L. hilgardii, L. viridescens; streptobacteria include e.g. L. alimentarius, L. casei, L. coryniformis, L. curvatus, L. farciminis, L. plantarum, L. xylosus (cf. LACTOCOCCUS); thermobacteria include e.g. L. acidophilus, L. bulgaricus, L. delbrueckii (type species of the genus), L. helveticus, L. lactis, L. leichmannii, L. ruminis, L. salivarius. [Book ref. 46, pp. 1653–1679.] (L. bifidus: see BIFIDOBACTERIUM.) [Classification of L. bulgaricus, L. lactis and L. leichmannii as subspecies of L. delbrueckii : SAAM (1983) 4 552–557.] Lactobacilli are found e.g. in man and animals (see e.g. DENTAL CARIES, GASTROINTESTINAL TRACT FLORA, VAGINA MICROFLORA), in MILK, DAIRY PRODUCTS and other fermented foods and beverages (see e.g. sourdough BREAD-MAKING, PULQUE, SOY SAUCE), and – usually in very small numbers – on plant material (see e.g. SILAGE, PICKLING). (See also BEER SPOILAGE, MALOLACTIC FERMENTATION, MEAT SPOILAGE, MILK SPOILAGE, WINE [Genetics, metabolism and applications of lactic acid bacteria (symposium); FEMS Reviews (1993) 12 1–272.] lactic acid fermentation A type of FERMENTATION (1), carried out e.g. by LACTIC ACID BACTERIA, in which sugars (e.g. LACTOSE, glucose, PENTOSES) are converted either entirely (or almost entirely) to lactic acid (HOMOLACTIC FERMENTATION) or to a mixture of lactic acid and other products (HETEROLACTIC FERMENTATION) [AvL (1983) 49 209–224]. Lactic acid bacteria produce either L(+)or D(−)-lactic acid, or both – the nature of the isomer(s) formed being an important taxonomic characteristic (see e.g. LACTOBACILLUS). Which isomer is produced depends on the specificity of the NAD-dependent lactate dehydrogenase (LDH) present; species which produce racemic mixtures usually possess both D- and L-specific LDHs, but in certain lactobacilli the mixture results from the combined action of L-specific LDH and lactic acid racemase. (See also LACTIC ACID and DAIRY PRODUCTS.) lactic acid starters STARTER cultures used to initiate a LACTIC ACID FERMENTATION in the commercial production of e.g. DAIRY PRODUCTS; they may be supplied in freeze-dried or frozen concentrated form. The starters used for dairy products usually contain ‘homolactic’ lactobacilli or lactococci as the main lactic acid producers, with leuconostocs and/or strains of Lactococcus lactis as ‘aroma bacteria’ (see DIACETYL). The lactic acid-producing strains carry out HOMOLACTIC FERMENTATION of the lactose in milk – conditions of lactose excess for these bacteria; cows’ milk contains ca. 4–7% w/v lactose, much of which remains in the product after fermentation. Only strains which use the PTS/P-b-gal system of LACTOSE (q.v.) metabolism achieve sufficiently rapid growth and produce enough lactic acid to be useful commercially. Mesophilic starters (used e.g. for making BUTTER) usually contain strains of Lactococcus lactis as the main acidproducers; starters which contain only acid-producers (no aroma bacteria) are called N-type (= O-type) starters. B-type (= Ltype) starters contain, in addition to N-type organisms, a Leuconostoc aroma species – usually Leuconostoc cremoris (= ‘Betacoccus cremoris’, hence ‘B’ starter). D-type starters contain N-type organisms and aroma strains of Lactococcus lactis. BD-type (= BL-type) starters contain both B- and D-type organisms. Thermophilic starters (used e.g. for YOGHURT and some CHEESE-MAKING processes) include e.g. Lactobacillus bulgaricus, L. helveticus and/or Streptococcus thermophilus. A genetically engineered starter culture has been used to control the (food-borne) pathogen Listeria monocytogenes in cheddar cheese [AEM (1998) 64 4842–4845]. For cottage cheese, use has been made of a starter containing organisms that produce lacticin 3147 [JAM (1999) 86 251–256]. Starter strains are susceptible to infection by certain phages, and this can lead to problems such as slow (or absent) fermentation associated with economic losses. Efforts have therefore been made to produce ‘engineered’ starter strains that incorporate resistance to specific phages. For example, some engineered strains exhibit the Per (phage-encoded resistance) phenotype which is conferred by the presence of a high-copy-number plasmid that contains a sequence equivalent to the origin of replication of a particular infecting phage; it appears that the plasmidborne phage origin (in multiple copies) inhibits phage replication by competing with replication function(s) of the infecting phage. In a different approach, transcription of a plasmid-borne sequence (within the engineered bacterium) gives rise to an antisense RNA molecule that can bind to a specific phage mRNA 425 lactocin 27 bacteriostatic against Gram-positive, catalase-negative bacteria (e.g. streptococci and lactobacilli). The LP system has a potential application in the preservation of (refrigerated or uncooled) milk; for this purpose, levels of H2 O2 in the milk could be boosted e.g. by the addition of a glucose/glucose oxidase H2 O2 -generating system and (if necessary) SCN− . Components which activate the LP system have been incorporated into an ‘anti-plaque’ toothpaste to inhibit oral streptococci (see DENTAL CARIES). [Review: J. Food Protect. (1984) 47 724–732.] lactophenol cotton blue A general mycological MOUNTANT and DYE. Phenol (20 g), water (20 ml), lactic acid (20 ml), and glycerol (30 ml) are warmed together, and cotton blue (0.05 g) is added. lactose A disaccharide (b-D-galactopyranosyl-(1 → 4)-D-glucopyranose) present in MILK. Lactose is the major source of carbon and energy for LACTIC ACID BACTERIA growing in milk (see also LACTIC ACID FERMENTATION). The ability to metabolize lactose is an important characteristic in the identification of members of the family Enterobacteriaceae. In Escherichia coli and other enterobacteria, and in most lactobacilli (though not Lactobacillus casei ), lactose is generally taken up by a lactose-specific proton symport TRANSPORT SYSTEM; the lactose is then split into glucose and galactose by b-galactosidase (see also LAC OPERON), the glucose and galactose being metabolized via the EMP and Leloir pathways, respectively: see Appendix III(a). In e.g. Lactobacillus casei, Lactococcus lactis and Staphylococcus aureus, lactose is taken up by a PEP-dependent phosphotransferase system (see PTS) – entering the cell as lactose phosphate (apparently as lactose bisphosphate in L. lactis [Book ref. 11, pp 196–203]) which is then hydrolysed by phosphob-D-galactosidase (P-b-gal); the glucose (glucose 6-phosphate in L. lactis) and galactose 6-phosphate moieties are metabolized via the EMP and D-tagatose 6-phosphate pathways, respectively – see Appendix III(a). In at least some strains of L. lactis, all the enzymes for lactose metabolism – from transport to the formation of triose phosphates via the D-tagatose 6phosphate pathway – appear to be plasmid-encoded [JB (1983) 153 76–83]; strains lacking the lactose plasmid can metabolize lactose via the permease/b-galactosidase mechanism and Leloir pathway, but their rates of lactose metabolism and growth are much reduced (cf. LACTIC ACID STARTERS). In yeasts, Kluyveromyces marxianus var. lactis and var. marxianus (some strains) are among the few capable of fermenting lactose (see SINGLE-CELL PROTEIN); however, other yeasts may be able to metabolize lactose oxidatively (cf. KLUYVER EFFECT). lactose killing Lactose-mediated SUBSTRATE-ACCELERATED DEATH. lactose operon Syn. LAC OPERON. lactose plasmid (Lac plasmid) A PLASMID which specifies the uptake and metabolism of LACTOSE. Conjugative lactose plasmids occur e.g. in some lactococci. lacunose Covered with pits or indentations. lacY gene See LAC OPERON. lacZ gene See LAC OPERON. LAD LEUKOCYTE ADHESION DEFICIENCY. Laemmli electrophoresis Syn. SDS-PAGE. laeoplectic metachrony See METACHRONAL WAVES. Laetiporus See APHYLLOPHORALES (Polyporaceae). laevulose D-Fructose. LAF See INTERLEUKIN-1. lag phase (of growth) See BATCH CULTURE. Lagarde retort An air–steam BATCH RETORT. SPOILAGE.) Lactobacilli are generally non-pathogenic, although some strains (particularly of L. casei subsp. rhamnosus) have been associated with endocarditis, septicaemia and abscesses, and another species (proposed name: L. piscicola [IJSB (1984) 34 393–400]) is apparently an opportunist pathogen in salmonid fish. [Lactobacillus casei strain Shirota has been reported to have potent PROBIOTIC activity when administered intraurethrally in an Escherichia coli urinary tract infection in mice: AAC (2001) 45 1751–1760.] lactocin 27 See BACTERIOCINS. lactococcin G See BACTERIOCINS. Lactococcus A genus of LACTIC ACID BACTERIA which occur e.g. in DAIRY PRODUCTS; the organisms were previously classified in the genus Streptococcus (most strains react positively with streptococcal Lancefield group N antiserum). Cells: non-motile cocci, or coccoid forms, which occur singly or in pairs or chains. Metabolism: fermentative, L(+)-lactic acid being the main product of glucose fermentation. Growth can occur at 10° C but not at 45° C. GC% 34–43. Type species: L. lactis. L. lactis includes organisms previously known as Streptococcus lactis, including the subspecies lactis and diacetylactis (= diacetilactis), Streptococcus cremoris (including S. lactis subspecies cremoris), and Lactobacillus xylosus. It was suggested that Lactococcus should also include the species L. plantarum (= Streptococcus plantarum) and L. raffinolactis (= Streptococcus raffinolactis). [Validation of the names Lactococcus, L. lactis, L. lactis subspecies cremoris and lactis, and L. plantarum: IJSB (1986) 36 354–356.] lactoferrin A SIDEROPHILIN (MWt ca. 90000) which has two ironbinding sites per molecule; it occurs e.g. in milk and tears, and in granules within NEUTROPHILS. Lactoferrin in milk may serve an antimicrobial function by sequestering iron (IRON being an essential requirement for most microorganisms). The lactoferrin in neutrophils may serve an antimicrobial function (during PHAGOCYTOSIS and INFLAMMATION) by catalysing the formation of HYDROXYL RADICAL; levels of lactoferrin in plasma are known to rise during inflammatory processes. Lactoferrin released by neutrophils may enhance the adhesion of neutrophils to tissue cells, and to each other, and may promote chemotaxis in other neutrophils. lactoflavin Syn. RIBOFLAVIN. lactoperoxidase–thiocyanate–hydrogen peroxide system (LP system; LPS) An antimicrobial system, present in various body fluids, in which lactoperoxidase (an enzyme present e.g. in saliva, milk etc) catalyses the oxidation, by hydrogen peroxide (H2 O2 ), of thiocyanate (SCN− ). SCN− is a product of the metabolism of e.g. sulphur-containing amino acids and certain glycosides; it is present at relatively high concentrations e.g. in milk from cattle fed on clover or cruciferous plants such as kale or rape. The H2 O2 may be derived from indigenous microorganisms (as a metabolic by-product – see HYDROGEN PEROXIDE (b)) or from PMNs. Although the final products of the oxidation (SO4 2− , CO2 and NH3 ) are much less toxic than H2 O2 , various short-lived antibacterial intermediates – including e.g. hypothiocyanite, OSCN− – are generated during the reaction. The LP system inhibits various bacterial enzymes (including e.g. the EMP enzyme glyceraldehyde 3-phosphate dehydrogenase) and damages the bacterial cytoplasmic membrane, increasing the permeability of the membrane to ions and small molecules. The system can be bactericidal for Gram-negative, catalase-positive bacteria (e.g. enterobacteria, pseudomonads), but is usually 426 Lampropedia e.g. in Coprinus spp; they are rectangular in transverse crosssection (opposite faces of the lamella being parallel), and the basidia at the lower (free) end reach maturity before those higher up the lamella. (See also BASIDIOSPORES.) lamellate (mycol.) Having lamellae (see LAMELLA). lamina (algol.) Syn. BLADE. laminaran Syn. LAMINARIN. Laminaria A genus of large marine algae (division PHAEOPHYTA) which exhibit a heteromorphic ALTERNATION OF GENERATIONS. The sporophyte is a large blade with a stipe and holdfast; growth occurs at the region between stipe and blade, and at the MERISTODERM. The gametophyte is a microscopic filamentous structure. L. japonica is used as a food in China (where it is called ‘haidai’) and e.g. in Japan (‘kombu’) [Book ref. 130, pp. 688–690]. (See also ALGINATE.) laminaribiose A reducing disaccharide: b-D-glucopyranosyl(1 → 3)-D-glucopyranose. laminarin (laminaran) The storage polysaccharide characteristic of members of the Phaeophyta. Laminarins are essentially linear (1 → 3)-b-D-glucans in which the chains may terminate with D-mannitol (M-chains, non-reducing) or with D-glucose (G-chains, reducing); low levels of branching via (1 → 6)b-glucosidic linkages may occur. Laminarins from different sources vary in their mannitol content (usually ca. 2–4%), degree of branching, and average chain length. [Book ref. 37, pp. 481–486.] Laminarins may be attacked e.g. by certain fungi and by Nocardia spp. (cf. CHRYSOLAMINARIN.) lamivudine (−)2 -deoxy-3 -thiacytidine: an ANTIVIRAL AGENT with activity against the hepatitis B virus; this nucleoside analogue inhibits the viral DNA polymerase, thus inhibiting viral replication. As a candidate therapeutic agent, lamivudine was able significantly to reduce serum levels of viral DNA in phase III clinical trials. However, resistance to the drug may appear during long-term treatment programmes; such resistance has been found to be associated with mutant forms of the polymerase. [PCR-based detection of mutant strains of HBV with reduced susceptibility to lamivudine: JCM (1999) 37 3338–3347.] Lamivudine is also an ANTIRETROVIRAL AGENT which acts as a NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITOR; it is one of the drugs used against AIDS. Lampit See CHAGAS’ DISEASE. Lamprene CLOFAZIMINE. Lamprocystis A genus of photosynthetic bacteria (family CHROMATIACEAE). Cells of the sole species, L. roseopersicina, are motile cocci, 3–3.5 µm, which contain Bchl a, various carotenoids (cell suspensions are typically purple), and gas vacuoles. L. roseopersicina is a strict anaerobe which requires sulphide and vitamin B12 . Lamproderma See MYXOMYCETES. Lampropedia A genus (incertae sedis) of aerobic, oxidasepositive, catalase-positive, chemoorganotrophic, Gram-negative bacteria which have been isolated e.g. from stagnant and polluted waters. Cells: non-flagellated, non-pigmented, rounded to cuboid, ca. 1.0–1.5 × 1.0–2.5 µm, which occur in thin, hydrophobic sheets (on liquid and solid media) – each sheet consisting of a number of roughly square ‘tablets’ of 16–64 cells arranged, with sides adjacent, in the same plane; cell division occurs synchronously. Metabolism: respiratory (oxidative). Carbon and energy sources include TCA cycle intermediates; carbohydrates are not used. Optimum growth temperature: 30° C. GC%: ca. 61. Type species: L. hyalina. [Book ref. 22, pp. 402–406.] Lagenidiales An order of aquatic fungi (class OOMYCETES) which are parasitic on e.g. other fungi, oyster larvae and crabs’ eggs. The thallus is either a single cell or a short filament, according to organism. Genera include Lagenidium, Olpidiopsis, Petersenia (see also ALGAL DISEASES) and Sirolpidium. Lagenidium See LAGENIDIALES. Lagenophrys See PERITRICHIA. lager-making See BREWING. lagging-strand See DNA REPLICATION. Lagos bat virus See LYSSAVIRUS. Laguna Negra virus See HANTAVIRUS. laking (of erythrocytes) Lysis, with release of haemoglobin. LAL test LIMULUS AMOEBOCYTE LYSATE TEST. lamb dysentery An acute, fatal disease of young lambs (<10 days old) involving diarrhoea, dysentery and toxaemia; the causal agent is Clostridium perfringens type B. Overcrowding is a predisposing factor. LamB protein (‘maltoporin’) In Escherichia coli : an OUTER MEMBRANE protein encoded by the lamB gene; it acts as a receptor for bacteriophage lambda, and as a channel-forming protein involved in the uptake of maltose and maltodextrins – and certain other solutes (e.g. some unrelated sugars, and amino acids). (cf. PORIN.) LamB-deficient mutants exhibit impaired active transport of maltose when the external maltose concentration is <10 µM. The channel-forming role of LamB in the uptake of maltodextrins (and possibly also maltose) may depend on its interaction with the periplasmic maltose-binding protein (see BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM). Synthesis of the LamB protein is inducible by maltose and is subject to CATABOLITE REPRESSION. lambda chain See LIGHT CHAIN. lambda particles See LYTICUM. lambdoid phages A group of temperate phages whose genomes can undergo recombination with each other; the group includes e.g. phages l, 21, 82, 434, f80, and P22 (see BACTERIOPHAGE l and BACTERIOPHAGE P22). The genomes of these phages have a more or less common gene sequence, but many of the gene products are genome-specific: e.g., the gpN antiterminators of phages l, 21 and P22 show little homology and interact only with their own genomes. Phages l, f80, 21 and 434 each has a unique immunity region (genes cI, cro, oL and oR ) and hence a lysogen containing one of these phages does not show SUPERINFECTION IMMUNITY to the others. Lamblia intestinalis See GIARDIA. lambliasis Syn. GIARDIASIS. Lambornella A genus of ciliates of the HYMENOSTOMATIDA; L. clarki is endoparasitic in certain mosquitoes. lamella (gill) (mycol.) Blade-like sheets of tissue which hang vertically from the underside of the pileus in fruiting bodies of fungi of the AGARICALES and of e.g. PLEUROTUS spp; each lamella consists of a TRAMA (see also SPHAEROCYST) bearing a HYMENIUM which gives rise to basidiospores. In some species (e.g. Agaricus) the lamellae are arranged radially around a central stipe; in others (e.g. Pleurotus) they radiate fan-wise from a lateral stipe. Lamellae may branch and/or anastomose; their colour at maturity usually reflects the colour of the basidiospores. (See also SPORE PRINT.) The shape of lamellae can be taxonomically important. In most agarics (including Agaricus) the lamellae are aequihymeniiferous, i.e., they are wedge-shaped in transverse crosssection (tapering from the attached to the free end), and they form basidia which mature at approximately the same time at all regions in the hymenium. Inaequihymeniiferous lamellae occur 427 LAMPs (convexities in the upper surface corresponding to hollows in the lower surface); asci contain one or two large spores (those of Umbilicaria contain 8 smaller spores). (L. pustulata: formerly U. pustulata.) lasalocid A MACROTETRALIDE antibiotic which is used as an anticoccidial agent in poultry, and which has been used as a FEED ADDITIVE for ruminants (its mode of action apparently being similar to that of MONENSIN). Lassa fever A VIRAL HAEMORRHAGIC FEVER caused by the Lassa virus (see ARENAVIRIDAE); it occurs in Africa, especially Nigeria, Liberia and Sierra Leone. Early symptoms resemble those of other VHFs; petechiae may be present, and there is marked pharyngitis with white patchy exudate adhering to the soft palate and pharyngeal region. In fatal cases death usually occurs during the second week of infection; mortality rates may be 30–66% among hospital cases, but subclinical infections may occur. The natural reservoir of the virus is the multimammate rat Mastomys natalensis. Human infection may occur e.g. by contamination of wounds with urine from infected rats; personto-person transmission also occurs. Lassa virus See ARENAVIRIDAE and LASSA FEVER. latamoxef Syn. MOXALACTAM. late blight (of potato) A POTATO DISEASE caused by Phytophthora infestans (see PHYTOPHTHORA). Dark patches appear on the upper surfaces of leaves (and sometimes on stems), and patches of white ‘mildew’ (sporangium-bearing sporangiophores) develop on corresponding regions on the undersides of leaves. Disease may spread rapidly from plant to plant. Tubers develop dark surface patches and succumb to a dry brown rot – and sometimes to a secondary soft rot. Protectants used against late blight include e.g. MANEB and ZINEB. Late blight was responsible for the Irish potato famine in the 1840s. [Review: Book ref. 58, pp. 3–17.] late genes See VIRUS. latent infection (1) (animal virol.) An asymptomatic infection in which infectious virions are not produced. (See PERSISTENCE.) (2) (plant virol.) Any viral infection in which symptoms do not occur – whether or not virus replication occurs. latent period See ONE-STEP GROWTH EXPERIMENT. latent virus A virus which can establish a LATENT INFECTION (sense 1 or 2). lateral body (virol.) See POXVIRIDAE. lateral transmission See HORIZONTAL TRANSMISSION. laterosporolysin See THIOL-ACTIVATED CYTOLYSINS. latex (1) A product of the rubber tree, Hevea brasiliensis. Natural latex is a colloidal suspension of polyisoprenoid particles in an aqueous phase containing e.g. carbohydrates and proteins; it is readily attacked by a variety of microorganisms. (See also RUBBER SPOILAGE.) (2) See LATEX PARTICLE TEST. (3) See LACTARIUS. latex particle test (latex test) (serol.) A PASSIVE AGGLUTINATION test in which antibody or soluble antigen is adsorbed to particles of a synthetic latex: e.g. a suspension of particles of polystyrene (commonly ca. 0.81 µm diam.), a substance to which proteins tend to adsorb. For example, to detect antibodies to a given antigen, antigencoated particles of latex are exposed to the patient’s serum; the presence of homologous antibodies in the serum is indicated by agglutination of the latex particles (which are bound together by antigen–antibody bridges). Uncoated particles of latex, and known negative serum, are used for controls. Latex particle tests have a wide range of uses, being able not only to detect serum antibodies and soluble antigens but also to LAMPs See PHAGOCYTOSIS (a). lamziekte A form of BOTULISM in cattle. It occurs in S. Africa and is contracted by the ingestion of carrion contaminated with C. botulinum (usually) type D. Lancefield’s streptococcal grouping test A procedure for identifying and classifying streptococci; it involves the extraction and identification, by serology, of cell components known as C SUBSTANCES. On the basis of their particular C substances, most strains of STREPTOCOCCUS can be placed into one or other of a number of Lancefield groups which are designated A, B, C. . . . etc; all the strains in a particular Lancefield group possess a C substance which is serologically distinct from those C substances which characterize the other Lancefield groups. (Some streptococci have more than one type of C substance; thus, e.g., some strains of enterococci contain the C substances of Lancefield groups D and Q.) The test is often carried out as a RING TEST in a capillary tube: an extract of the strain under test is layered onto an antiserum to a given C substance, and the test is repeated with antisera to different C substances until a positive result (a precipitate at the extract–antiserum interface) is obtained. Langerhans’ cells Non-phagocytic cells, derived from bone marrow, found in the skin. Morphologically they resemble DENDRITIC CELLS but, in addition to class II MHC antigens, they bear cell-surface receptors for complement components and for IgG Fc portions; they also contain racket-shaped cytoplasmic organelles (Birbeck granules). Langerhans’ cells are involved in CONTACT SENSITIVITY. Langhans giant cell See GIANT CELL. lankamycin See MACROLIDE ANTIBIOTICS. Lankesterella A genus of coccidian protozoa (suborder EIMERIORINA) parasitic in amphibia and transmitted by invertebrate vectors. Asexual and sexual development occur in the vertebrate host, sporozoites occurring in red and/or white blood cells. lantibiotics Polycyclic peptide BACTERIOCINS whose molecules include some uncommon residues – e.g. the sulphur-containing amino acid lanthionine (hence lantibiotic) and D-amino acids. Lantibiotics are produced by strains of e.g. Bacillus and Lactococcus, and are typically active against Gram-positive bacteria; they form pores in the cytoplasmic membrane of a target cell, thereby causing ionic leakage and loss of pmf. Examples include NISIN and SUBTILIN. The presence of lanthionine (and of e.g. D-amino acids) in these peptides results from post-translational modification [TIBS (1992) 17 481–485]. [Biosynthesis and biological activities of lantibiotics: ARM (1998) 52 41–79.] [Lantibiotics: structure, biosynthesis and mode of action: FEMS Reviews (2001) 25 285–308.] LAO binding protein See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. LAP See ENDOTOXIN. lapinized vaccine A VACCINE containing live organisms whose virulence for a given host species has been attenuated by SERIAL PASSAGE through rabbits. Laredo-type strains See ENTAMOEBA (E. histolytica). large granular lymphocytes See NK CELLS. large iridescent insect viruses See CHLORIRIDOVIRUS. large T antigen See POLYOMAVIRUS. lariat (lariat RNA) See SPLIT GENE (a) and (c). las genes See QUORUM SENSING. Lasallia A genus of LICHENS (order LECANORALES). The thallus resembles that of UMBILICARIA spp but is pustulate in appearance 428 LCR LCR Ligase chain reaction: a PROBE-based method for copying (‘amplifying’) a given sequence of nucleotides in DNA; like PCR (but unlike NASBA or SDA), LCR involves thermal cycling. Essentially, sample DNA is heat-denatured in order to expose the target sequence (amplicon) in one strand – and its complementary sequence in the other strand; on lowering the temperature, two oligonucleotide probes bind (anneal ) in adjacent positions to cover the entire amplicon – while another two probes bind to the amplicon’s complementary sequence. If a given pair of probes binds correctly to the target, the 5 terminus of one probe will abut the 3 terminus of the other; when this occurs, a covalent link can be made between the 5 and 3 termini by a heat-stable DNA LIGASE. A pair of ligated probes will thus form a copy of the target sequence and can be used as a target (by other probes) in the subsequent rounds of amplification. Ligation and amplification of the sequence-specific probes argues for the presence of the given target sequence in the sample DNA. Note that, for ligation to occur, both juxtaposed bases in a given pair of probes must base-pair correctly with the corresponding nucleotides in the template strand (even if mismatches occur elsewhere in the probe–target duplex); moreover, the 5 end of one probe must be phosphorylated in order for ligation to occur. Commercial LCR-based assays (LCx assays; Abbott Diagnostics) are used to detect specific sequences of DNA from certain pathogens in clinical specimens; such assays include tests for Chlamydia trachomatis, Neisseria gonorrhoeae and Mycobacterium tuberculosis. These assays differ from the (basic) format described above. Thus, the two probes in a bound pair are separated by a gap of one to several nucleotides. Consequently, part of each cycle includes a period of extension, from the 3 end of one probe, by a DNA polymerase present in the reaction mixture; closure of the gap is followed by ligation. Cycling thus involves three phases of temperature; in the Abbott LCx assay for Chlamydia trachomatis, these phases are: (i) 93° C for denaturing the dsDNA sample, (ii) 59° C for annealing, and (iii) 62° C for extension/ligation. Corresponding values in the LCx assay for M. tuberculosis are (i) 94° C, (ii) 64° C and (iii) 69° C. An amplification of at least 10 million-fold is commonly achieved in an assay of 30–40 cycles. Products of the LCx assays are detected by a system called microparticle enzyme immunoassay (MEIA). Each of a pair of probes carries – at its non-ligating end – a molecule (hapten) that is able to bind a specific antibody; the two probes carry different haptens – i.e. they bind different types of antibody. Hence, after cycling, each ligated probe pair consists of a copy of the target sequence flanked by two different haptens (haptens 1 and 2). Microparticles (<1 µm diam.), coated with hapten 1-specific antibodies, are added to the system; the microparticles bind hapten 1 on both ligated and nonligated probes. The microparticles are then immobilized on a matrix. The non-ligated probes bearing hapten 2 are removed by washing. A conjugate, consisting of alkaline phosphatase covalently linked to a hapten 2-specific antibody, is then added to the bound microparticles; the conjugate binds to hapten 2 (via the antibody), thus labelling ligated probe pairs (on the microparticle) with alkaline phosphatase. A substrate for the enzyme, 4-methylumbelliferyl phosphate, is then added and is cleaved to the fluorescent product 4-methylumbelliferone; under ultraviolet radiation, the rate at which fluorescence is generated is proportional to the number of (enzyme-labelled) ligated probe pairs. Readings above a certain cut-off value indicate a positive result, i.e. evidence of a given sequence in the sample; readings distinguish between different strains of a given pathogen [e.g. rapid detection of mecA-positive and mecA-negative strains of coagulase-negative staphylococci by an anti-PBP 2a slide latex agglutination test: JCM (2000) 38 2051–2054.] When specific antibodies are bound to the latex particles, the test is often described as a reverse (or reversed ) passive latex agglutination test. Thus, e.g. staphylococcal enterotoxins can be tested for (in e.g. food samples) by using a reverse passive latex agglutination test in which the particles are coated with antibodies to the various enterotoxins; agglutination of such particles may occur in the presence of very low concentrations of enterotoxins in the samples. [Reversed passive latex agglutination test for detecting ‘free’ verocytotoxin produced by strains of Escherichia coli O157 (EHEC): LAM (2001) 32 370–374.] Latin binomial See BINOMIAL. Latino virus See ARENAVIRIDAE. lattice hypothesis (serol.) A hypothesis advanced by Marrack in 1934 to account for the observation that antibody and antigen can combine in varying proportions; it postulates that each molecule of antibody has a valency of at least two, and that antigens are multivalent. Thus, when antigens and antibodies combine in suitable proportions (see OPTIMAL PROPORTIONS) large insoluble aggregates (lattices) are formed; under ANTIGEN EXCESS conditions small, soluble complexes are formed. The precipitability of a complex may depend e.g. on its hydrophobicity as well as on its size. Laurell’s rocket immunoelectrophoresis Syn. ROCKET IMMUNOELECTROPHORESIS. lauryl tryptose lactose broth A medium, containing sodium lauryl sulphate (sodium dodecyl sulphate), used e.g. in the EIJKMAN TEST. Lauth’s violet Syn. THIONIN. LAV (1) See HIV. (2) LEAFHOPPER A VIRUS. laver (laver bread) Edible algae, particularly certain species of Porphyra (known as ‘zicai’ in China, ‘nori’ in Japan, and ‘purple laver’ in some other countries). (See also ULVA.) Laverania A subgenus of PLASMODIUM. law of recapitulation See ONTOGENY. lawn plate A PLATE whose entire surface bears confluent growth of microorganisms of one species or strain; it may be prepared e.g. by incubating a FLOOD PLATE. Lb LEGHAEMOGLOBIN. LB broth See LURIA–BERTANI BROTH. LBP See CD14. Lc protein A PORIN of Escherichia coli. LCM virus (lymphocytic choriomeningitis virus, LCMV) The type species of the genus Arenavirus (see ARENAVIRIDAE). The main natural host is the house mouse (Mus musculus), but the syrian or golden hamster (Mesocricetus auratus) can also be an epidemiologically important host. LCM virus can replicate in a wide range of cultured cells in which infection is usually persistent and without CPE. Embryo or neonatal (immunologically immature) mice inoculated with LCMV survive the infection to become persistent carriers, shedding virus in their urine, faeces, saliva etc throughout their lives (see PERSISTENCE). However, adult mice inoculated intracerebrally with LCMV rapidly succumb to the infection by a mechanism which seems to involve interferon: a ‘docile’ strain of LCMV which does not induce interferon does not cause a lethal infection [JGV (1983) 64 1827–1830]. (See also GLOMERULONEPHRITIS.) In man, LCMV is a rare cause of disease – which may be an influenza-like illness, an acute, benign aseptic MENINGITIS, or a severe meningoencephalomyelitis. 429 LD50 be extracted by cyanidation in the usual way. The ore is crushed and subjected to bacterial action in highly aerated bio-oxidation tanks; arsenopyrite (FeAsS) is solubilized by oxidation (to FeAsO4 and sulphuric acid), thus exposing the gold for cyanidation. After cyanidation, gold is separated from the gold–cyanide complex e.g. by adsorption to charcoal, and the spent, cyanide-containing liquor is detoxified by bacteria; detoxification involves the oxidation of cyanide to carbon dioxide and urea, and the oxidation of urea to nitrate. In addition to their use in leaching, bacteria have also been used as ‘bioadsorbents’ for the recovery of low levels of uranium in certain liquid wastes. (See also THIOBACILLUS.) [Microbiology of metal leaching (symposium report): FEMS Reviews (1993) 11 1–267. Mining with microbes (review): Biotechnology (1995) 13 773–778. Predominant types of bacteria used in commercial bioleaching processes: Microbiology (1999) 145 5–13.] leader (leader sequence) A region of an RNA molecule (e.g. certain mRNA molecules, pre-rRNA) between the 5 (promoter) end and the sequence corresponding to the first (or only) structural gene. The leader sequence is commonly non-coding, but may encode a small regulatory peptide (a leader peptide): see OPERON (attenuator control) and TNA OPERON. leader peptidase See SIGNAL HYPOTHESIS. leader peptide (1) See LEADER. (2) See SIGNAL HYPOTHESIS. leading strand See DNA REPLICATION. leaf blotch (scald) A CEREAL DISEASE, caused by Rhynchosporium secalis, which can affect e.g. barley and rye; large, pale brown lesions with dark brown or purple margins appear mainly on leaves, and crop losses may be severe. (See also FLUTRIAFOL and PROPICONAZOLE.) leaf curl (of peach) See TAPHRINA. leaf nodules See PHYLLOBACTERIUM. leafhopper A virus (LAV) An unclassified reovirus-like virus which was isolated from the leafhopper Cicadulina bimaculata during the search for the causal agent of MAIZE WALLABY EAR DISEASE; attempts to infect maize plants with LAV have been unsuccessful, and any connection between LAV and MWED has yet to be proved. Although LAV resembles viruses of the genus FIJIVIRUS, it cannot be included in the genus since it is not known to infect any plant species. leaky mutation A MUTATION which results in only partial inactivation of the sequence in which it occurs. (cf. NULL MUTATION.) leather spoilage Leather has a relatively low pH (usually ca. 3–4) and even at high humidities is fairly resistant to bacterial attack; however, at relative humidities of >80% leather is highly susceptible to attack by moulds which may cause significant loss of tensile strength. Aspergillus spp are commonly implicated, but other moulds (e.g. species of Mucor, Paecilomyces and Rhizopus) may also be involved. Heavy fungal contamination may raise the pH sufficiently to allow some bacterial growth; e.g. Bacillus sphaericus has been isolated from mouldy leather. Preservatives which have been used for treating leather include phenylmercuric salts of fatty acids, p-nitrophenol, and bnaphthol. (For the bating of hides by microbial enzymes see PROTEASES.) leben A type of concentrated YOGHURT consumed in e.g. Egypt. Milk is fermented with organisms which include Lactococcus lactis, Streptococcus thermophilus, Lactobacillus bulgaricus and certain lactose-fermenting yeasts. The curd is concentrated by draining off the whey and may then be dried. (See also DAIRY PRODUCTS.) in an ‘equivocal zone’ (defined in terms of the ratio: sample reading/cut-off value) indicate repeat testing. Examples of assays with LCx systems include studies on Chlamydia trachomatis [JCM (1998) 36 94–99], Neisseria gonorrhoeae [JCM (1997) 35 239–242], and Mycobacterium tuberculosis [JCM (2002) 40 2305–2307]. Improved reproducibility with LCx assays for Chlamydia trachomatis and Neisseria gonorrhoeae has been reported following modification of the recommended equivocal zones: from 0.8–0.99 to 0.8–1.5 for C. trachomatis, and from 0.8–1.2 to 0.8–2.0 for N. gonorrhoeae [JCM (2000) 38 2416–2418]. [LCR in clinical microbiology: Book ref. 221, pp 152–167.] LD50 Lethal dose (50%): that dose of a given lethal agent which, when given to each of a number of test animals, kills 50% of those animals under given conditions. LDC Lysine decarboxylase: see DECARBOXYLASE TESTS. LE markers See TRANSFORMATION (1). leaching (of ores) Bacteria can promote the leaching (solubilization) of certain metals from their ores. Bacterial leaching (biomining) is carried out e.g. for the commercial recovery of copper from low-grade ores containing chalcopyrite (CuFeS2 ) and for the recovery of gold from recalcitrant arsenopyrite ores. In one process for recovering copper, a liquid containing sulphuric acid and chemolithotrophic bacteria is continually recycled through a mound of crushed ore; in dump leaching the mound consists of rough, ungraded ores and waste, while in heap leaching it consists of more concentrated, finer ore. Ferrous ions, which leach spontaneously, are oxidized by the bacteria to ferric ions – resulting in ongoing solubilization of iron; the solubilization of iron releases sulphur, which is oxidized to sulphate. These processes may be carried out by e.g. ‘Leptospirillum ferrooxidans’ (an iron oxidizer), Thiobacillus ferrooxidans (an iron and sulphur oxidizer) and Thiobacillus thiooxidans (a sulphur oxidizer). The bacteria obtain nutrients from the percolating liquor and from the ore itself – for example, carbon dioxide may be derived from acidification of carbonates within the ore. Copper in the leachate (up to 5 grams per litre) is periodically removed (e.g. by electrophoresis); the liquor is recycled for months, being supplemented with water to replace losses due to evaporation. Heat generated by bacterial metabolism maintains a temperature of about 40–50° C in the leachate, and the convective upflow of air helps to provide an aerobic environment. The development of an efficient leaching flora may arise via a succession of species, each being particularly suited e.g. to a given pH. Very low pH is advantageous for leaching copper ores because it inhibits the precipitation of ferric ions; leaching under these conditions has been achieved with strains identified as Thiobacillus thiooxidans and ‘Leptospirillum ferrooxidans’ – the organisms being identified by characterization of their rRNA genes in DNA extracted from the leachate [AEM (1997) 63 332–334]. Studies on the sulphur chemistry of bacterial leaching suggest that the leaching of pyrite may involve a cyclical degradative process in which thiosulphate and polythionates are among the intermediate products; ferric ions, produced by the ongoing bacterial oxidation of ferrous ions, may have an active role in the oxidation of both pyrite and thiosulphate [AEM (1996) 62 3424–3431]. Bioleaching is also used in e.g. South Africa, Australia and Brazil as a preliminary process in the extraction of gold from certain recalcitrant (arsenopyrite) ores. In these ores, the gold is embedded in a matrix of arsenopyrite so that it cannot 430 Legionellaceae phosphate group of phosphatidic acid is esterified with choline: (CH3 )3 N+ (CH2 )2 OH. lecithinase A PHOSPHOLIPASE (q.v.) which cleaves LECITHIN. Extracellular lecithinases are formed e.g. by Bacillus cereus, Clostridium bifermentans, C. novyi (g-toxin), C. perfringens (atoxin), and strains of Staphylococcus aureus; the lecithinase of C. perfringens apparently requires Zn2+ for activity. Those organisms (e.g. C. novyi, C. perfringens) which form lecithinases that are C-type phospholipases produce zones of opacity around colonies when grown on e.g. EGG-YOLK AGAR due to the formation of insoluble, phosphate-free diacylglycerides (‘diglycerides’). (See also MILK SPOILAGE.) lecithovitellin agar Syn. EGG-YOLK AGAR. lectin pathway See COMPLEMENT FIXATION (c). lectins Various non-enzymic proteins (usually glycoproteins) which are not products of the immune system (thus excluding antibodies) and which can each bind to specific carbohydrate group(s) and can agglutinate (or precipitate) cells or other entities having these particular (and accessible) group(s). The term ‘lectin’ originally referred to certain plant products which bind to and agglutinate particular types of erythrocyte (cf. PHYTOHAEMAGGLUTININ sense 2); it is now used for materials of plant, animal or microbial origin which have specific carbohydratebinding ability – though not necessarily in relation to erythrocytes. Lectins have been obtained e.g. from plants [Book ref. 37, pp. 537–547; ARPphys. (1985) 36 209–234], lichens [Lichenol. (1983) 15 303–305], the snail (Helix pomatia), the horseshoe crab (Limulus polyphemus), slime moulds (e.g. discoidins from Dictyostelium discoideum, pallidins from Polysphondylium pallidum), fungi and bacteria (e.g. mannose-sensitive FIMBRIAE). Lectins have been used e.g. for haemagglutination in blood grouping studies. Many lectins behave as MITOGENS and are used e.g. for lymphocyte activation in immunological studies. In nature, lectins may be involved in plant–microbe symbioses [Book ref. 38, pp. 658–677]; they may play a role in the pathogenesis of some plant diseases [ARPpath. (1983) 21 300–302] and possibly in certain animal diseases (e.g. lectins in raw beans may promote disease in pigs by enhancing attachment of ETEC to the ileal epithelium [CRM (1981) 8 303–338 (324)]). (See also GIARDIASIS.) [Interaction of bacteria and fungi with lectins: ARM (1981) 35 85–112.] (See also CONCANAVALIN A; LIMULIN; PHYTOHAEMAGGLUTININ; POKEWEED MITOGEN; TRIFOLIIN A; WHEAT GERM AGGLUTININ.) lectotype A specimen, chosen to serve as NOMENCLATURAL TYPE, derived from the original material studied by an author who did not specify a HOLOTYPE; it has nomenclatural precedence over a NEOTYPE. LEE See PATHOGENICITY ISLAND. leek yellow stripe virus See POTYVIRUSES. left-handed DNA See e.g. Z-DNA. left splice site See SPLIT GENE. leghaemoglobin (leghemoglobin) A red haem-containing pigment present (in multiple forms) in the host cell cytoplasm in the ROOT NODULES of leguminous plants. The protein and haem moieties are specified by plant genes and bacteroid genes, respectively. Leghaemoglobins are structurally and functionally similar to animal myoglobins and haemoglobins but have a much higher affinity for oxygen; they release bound oxygen only at low levels of oxygen and thus permit aerobic respiration in the bacteroids while protecting NITROGENASE from oxygen inactivation. [Book ref. 55, pp. 73–79; ARPphys. (1984) 35 443–478.] Legionella See LEGIONELLACEAE. Legionellaceae A family of Gram-negative, asporogenous bacteria. Cells: rod-shaped or filamentous, 2.0 to >20.0 × LECAM-1 See SELECTINS. Lecanactis See OPEGRAPHALES. Lecanidiales An order of fungi of the ASCOMYCOTINA; members include crustose LICHENS (photobiont commonly green), and saprotrophic and lichenicolous fungi. Ascocarp: APOTHECIOID, sometimes lirelliform. Asci: bitunicate. Genera: e.g. RHIZOCARPON. Lecanora A large genus of LICHENS (order LECANORALES); photobiont: a green alga. Thallus: crustose; apothecia: lecanorine, sessile; spores: aseptate. The genus includes several species which are highly tolerant of air pollution. L. conizaeoides is common (on trees, non-basic rocks, brick etc) in industrial and densely populated regions of Europe, but is rare elsewhere; it forms a grey-green, granular-sorediate, sometimes areolate thallus, and apothecia with yellowish- or greenish-brown discs are usually present. L. dispersa may occur on concrete, asbestos etc; the thallus may be superficial and whitish, or may grow endolithically, appearing only as a dark stain on rocks, walls etc. The apothecia have yellowish- to dark-brown discs which are often pruinose when young. L. muralis has become increasingly common on pavements, walls etc in towns; it forms a more or less circular thallus with radiating lobes and apothecia crowded in the centre. lecanoralean ascus See BITUNICATE ASCUS. Lecanorales An order of fungi of the ASCOMYCOTINA; members include many LICHENS (photobiont usually green), and saprotrophic and lichenicolous fungi. Ascocarp: APOTHECIOID, typically discoid (sometimes stipitate), but podetia (see PODETIUM) are formed by Cladonia spp; paraphyses are present. Asci: bitunicate (see BITUNICATE ASCUS), commonly elongated with a thickened apex. Genera: e.g. ACAROSPORA, ALECTORIA, ANAPTYCHIA, BAEOMYCES, BRYORIA, BUELLIA, CANDELARIELLA, CETRARIA, CLADONIA, COLLEMA, DIPLOICIA, EVERNIA, HYPOGYMNIA, LASALLIA, LECANORA, LECIDEA, LEMPHOLEMMA, LEPTOGIUM, LICHINA, PANNARIA, PARMELIA, PHYSCIA, PILOPHORUS, RAMALINA, STEREOCAULON, UMBILICARIA, USNEA. lecanorine apothecium (lichenol.) An APOTHECIUM which has a THALLINE MARGIN; such apothecia occur e.g. in lichens of the genera Lecanora and Xanthoria. (cf. LECIDEINE APOTHECIUM.) Lecidea A genus of crustose LICHENS (order LECANORALES); photobiont: a green alga (e.g. Trebouxia). Apothecia: lecideine. Many species of the (originally very large) genus have been transferred to other genera, including e.g. Fuscidea, Huilia, Hypocenomyce, Lecidella, Psilolechia, Psora, Trapelia. Thus, e.g., L. limitata (thallus: crustose, greenish- to yellowish-grey, sometimes with a dark hypothallus; apothecia: grey or black) is now Lecidella elaeochroma; L. lucida (thallus: leprose, yellow to yellow-green; apothecia: yellow to pale-brown) is now Psilolechia lucida; L. macrocarpa (thallus: crustose, grey, with a black hypothallus; apothecia: black) is now Huilia macrocarpa; L. scalaris (thallus: squamulose, with imbricated squamules) is now Hypocenomyce scalaris. lecideine apothecium (lichenol.) An APOTHECIUM which lacks a THALLINE MARGIN, i.e., it has only a PROPER MARGIN. Such apothecia occur e.g. in lichens of the genera Buellia, Lecidea and Rhizocarpon. (cf. LECANORINE APOTHECIUM.) Lecidella See LECIDEA. lecithin Phosphatidylcholine, a type of phospholipid found e.g. in egg-yolk and in the CYTOPLASMIC MEMBRANES of mammalian cells and of some Gram-negative bacteria. Lecithins are derivatives of phosphatidic acid (itself a derivative of glycerol 3phosphate in which the hydroxyl groups at positions 1 and 2 are each esterified with a long-chain fatty acid); in a lecithin, the 431 legionellosis 0.3–0.9 µm; motile, with one, two or more polar or lateral flagella. Cellular fatty acids are predominantly branched. Aerobic; oxidase −ve or weakly +ve. Chemoorganotrophic, using amino acids (non-fermentatively) as carbon and energy sources; carbohydrates are generally not metabolized. Growth requires L-cysteine and iron (Fe3+ ) salts, and can occur e.g. on blood agar supplemented with L-cysteine and iron, on Mueller–Hinton agar supplemented with haemoglobin and IsoVitaleX, or on buffered charcoal–yeast extract agar (BCYE), but not e.g. on unsupplemented blood agar or nutrient agar. Optimum growth temperature: 35–37° C. Gelatinase +ve; catalase +ve; urease −ve; NO3 − not reduced. Several species exhibit a bluish-white autofluorescence under WOOD’S LAMP, and most species produce a diffusible brown pigment on tyrosinecontaining media. Species occur in various aquatic habitats: e.g. surface waters, moist soils, thermally polluted streams, domestic water systems, etc; most or all species can be pathogenic for man (see LEGIONELLOSIS). GC%: ca. 39–43; type genus: Legionella. According to some authors [Book ref. 22, pp. 279–288], Legionella (type species L. pneumophila) is the sole genus of the Legionellaceae, but other authors have divided the family into three genera: Fluoribacter, Legionella and Tatlockia [IJSB (1980) 30 609–614; IJSB (1981) 31 111–115]. Species include e.g. L. bozemanii (Fluoribacter bozemanae; ‘WIGA bacterium’), L. dumoffii (F. dumoffii ; ‘Tex-KL bacterium’), L. feeleii [AIM (1984) 100 333–338], L. gormanii (F. gormanii ), L. jordanis, L. longbeachae, L. micdadei (Tatlockia micdadei; ‘TATLOCK agent’; ‘guinea-pig agent’; Pittsburg pneumonia agent), L. oakridgensis, L. pneumophila, L. sainthelensi (AEM (1984) 47 369–373], L. wadsworthii ). [Taxonomy and typing of legionellae: RMM (1994) 5 79–90.] Sequencing of the mip gene has been reported to provide a basis for unambiguous identification of 39 species of the genus Legionella [Mol. Microbiol. (1997) 25 1149–1158; JCM (1998) 36 1560–1567.] legionellosis Any (human) disease caused by a species of Legionella (see LEGIONELLACEAE). Most species appear to be capable of causing PNEUMONIA in man. Legionnaires’ disease is a pneumonia caused by L. pneumophila. Infection is believed to occur by inhalation of contaminated aerosols (common sources of infection are air-conditioning cooling towers, showers, nebulizers, etc). [Survival of L. pneumophila in a model hot water distribution system: JGM (1984) 130 1751–1756; distribution and persistence of Legionella in water systems: MS (1985) 2 40–43.] Incubation period: 2–10 days. Symptoms: malaise, myalgia, headache, followed by fever, chills, and a consolidating pneumonia which primarily involves the alveoli and terminal bronchioles; an intra-alveolar exudate is characteristic of the disease. Other common symptoms include chest and abdominal pains, vomiting, diarrhoea, and mental confusion. Mortality rates may be high, particularly in compromised individuals. [Review: Chest (1984) 85 114–120.] An important feature of pathogenesis is the ability of L. pneumophila to invade alveolar macrophages, to inhibit phagosome–lysosome fusion, and to multiply within these cells; such activity may be facilitated by expression of the pathogen’s icm (= dot) genes (see PHAGOCYTOSIS (a)) because mutations in these genes can give rise to modified virulence [TIM (1998) 6 253–255]. Moreover, the results of in vitro studies have suggested that the pathogen’s zinc metalloprotease (a 38 kDa secreted protein; = tissue-destructive protease) may promote pathogenesis by inhibiting both the oxidative burst and chemotaxis in phagocytic cells [JMM (2001) 50 517–525]. Pontiac fever is an acute, febrile, non-pneumonic, selflimiting illness caused by L. pneumophila (and by L. feeleii [AIM (1984) 100 333–338]). Incubation period: ca. 36 hours. Symptoms: fever, chills, headache, myalgia, arthralgia, diarrhoea, and sometimes neurological symptoms. Pittsburg pneumonia is a pneumonia caused by L. micdadei. Lab. diagnosis: direct or indirect immunofluorescence tests on e.g. sputum samples; microscopic examination of specimens stained with e.g. DIETERLE SILVER STAIN; analysis of the fatty acids of the isolated organism by GLC. [Treatment: JAC (1999) 43 747–752.] Legionnaires’ disease See LEGIONELLOSIS. legume bloat See BLOAT. legume yellows virus See LUTEOVIRUSES. leguminous root nodules See ROOT NODULES. Leidenfrost phenomenon The formation of a heat-insulating layer of gaseous nitrogen around a specimen when it is plunged into liquid nitrogen. (See also FREEZING.) Leifson’s flagella stain (for bacterial flagella) Mix equal volumes of 1.5% w/v aqueous NaCl and 3% w/v aqueous tannic acid; to this mixture add half its volume of a solution of pararosanilin acetate (0.9% w/v) and pararosanilin hydrochloride (0.3% w/v) in 95% ethanol. Prepare a medium-free, slightly cloudy suspension of bacteria in distilled water. Using a wax pencil, draw a ring on a chemically clean glass slide enclosing ca. 2 cm2 . Spread one loopful of the bacterial suspension within the ringed area; dry at room temperature. Do not fix. Add 1 ml stain; allow to act for ca. 10 min and rinse off gently under the tap. Flagella appear to be coated with a colloidal tannic acid–dye complex, rendering them visible by light microscopy. Leighton tube A test-tube in which part of the wall (near the closed end) forms a flat, rectangular region which can accommodate a cover-glass on which a cell monolayer can be formed. Leiotrocha See PERITRICHIA. Leishman–Donovan bodies Amastigote forms of Leishmania in the vertebrate host. Leishmania A genus of parasitic protozoa (family TRYPANOSOMATIDAE) which resemble TRYPANOSOMA species in their ultrastructure. The organisms are the causal agents of LEISHMANIASIS in mammals, and are transmitted by sandflies of several genera (e.g. Lutzomyia, Phlebotomus, Sergentomyia); species of Leishmania which are normally dermatotropic sometimes give rise to visceral leishmaniasis, while typically visceralizing strains sometimes cause cutaneous infections. Most leishmaniases are zoonotic. In the vertebrate host Leishmania grows in the intracellular, amastigote form (ca. 2–10 µm in diam.) – primarily in mononuclear phagocytes (e.g. macrophages). (Leishmania species can grow within phagolysosomes: see PHAGOCYTOSIS; amastigote forms of L. donovani have been found to contain CATALASE and glutathione peroxidase in amounts greater than those found in the promastigote stage. The ability of Leishmania spp to undergo ANTIGENIC VARIATION may contribute to their pathogenicity.) In the sandfly the pathogen occurs in the paramastigote and promastigote forms, the latter being the form infective for man. In cell-free media (e.g. in NNN MEDIUM), and in tissue culture, the organisms grow in the promastigote form. Leishmania spp include L. aethiopica, L. braziliensis, L. donovani, L. major, L. mexicana and L. tropica. Some species are divided into subspecies; e.g., L. braziliensis includes e.g. L. braziliensis braziliensis (= L. b. braziliensis), L. b. guyanensis and L. b. peruviana. Organisms which cause visceral leishmaniasis have been given separate names in particular geographical 432 lepromin test regions: e.g. L. infantum (Mediterranean), L. chagasi (S. America), and L. archibaldi (Sudan); some authors regard all of these as one species, L. donovani [Book ref. 72, p. 215]. Leishmania spp have been divided into groups (‘sections’) on the basis of their location within the arthropod vector: see HYPOPYLARIA, PERIPYLARIA, SUPRAPYLARIA; at least some of the reptile-infecting species in the Hypopylaria and Peripylaria appear to be trypanosomes (‘L. tarentolae’ has been renamed Trypanosoma platydactyli ). leishmanial form Obsolete Syn. AMASTIGOTE. leishmaniasis Any disease caused by a species of LEISHMANIA; see CUTANEOUS LEISHMANIASIS, MUCOCUTANEOUS LEISHMANIASIS, POST-KALA-AZAR DERMAL LEISHMANIASIS and VISCERAL LEISHMANIASIS. [Book ref. 72, pp. 183–249.] leishmaniasis recidivans An intractable form of CUTANEOUS LEISHMANIASIS which may develop after an apparent cure of a previous episode of cutaneous leishmaniasis; lesions, typically non-ulcerating, form around the rim of scar tissue. leishmanin test Syn. MONTENEGRO TEST. Leishman’s stain See ROMANOWSKY STAINS. Leloir pathway See LACTOSE and Appendix III(a). Lelystad virus See BLUE-EARED PIG DISEASE. Lemanea See RHODOPHYTA. Leminorella A new genus of non-motile Gram-negative bacteria of the ENTEROBACTERIACEAE; two species have been described: L. grimontii (the proposed type species) and L. richardii [JCM (1985) 21 234–239]. Reactions: e.g. VP −ve; indole −ve; H2 S +ve; LDC −ve; ODC −ve; ADH −ve; no growth with KCN. Species are relatively inactive biochemically; acid is produced from L-arabinose and D-xylose, but not from a wide range of other sugars (including lactose). Strains have been isolated from human faeces and urine. Lempholemma A genus of gelatinous LICHENS (order LECANORALES); photobiont: Nostoc. Thallus: homoiomerous, lacking a cortex. HORMOCYSTS are formed in specialized swollen regions of the thallus (termed hormocystangia), and hyphae of the mycobiont are – or may be – associated with the gelatinous sheath of the hormocyst. lens-shaped A phrase sometimes used to describe the shape of a structure or colony etc; as a descriptive phrase it is of doubtful value since a lens may be double-convex, plano-convex, meniscus etc. (cf. LENTICULAR.) lentic Pertaining to aquatic habitats with standing (still) water (e.g. ponds, swamps). (cf. LOTIC.) lenticular (lentiform) Shaped like a lentil or a double-convex lens. lentiform Syn. LENTICULAR. Lentinula A genus of fungi of the family Tricholomataceae (order AGARICALES). L. edodes (formerly Lentinus edodes) forms edible fruiting bodies: see SHII-TAKE. [Chemically defined medium for the fruiting of L. edodes: Mycol. (1983) 75 905–908; note on the reclassification of Lentinus edodes to Lentinula edodes: MS (1986) 3 171; effect of light and aeration on fruiting of L. edodes: TBMS (1987) 88 9–20.] (See also MYCOVIRUS.) Lentinus A genus of lignicolous fungi of the APHYLLOPHORALES (family Polyporaceae) which form tough but pliant basidiocarps in which the hymenium is borne on lamellae (see LAMELLA). L. lepideus forms a mushroom-shaped basidiocarp in which the fawn-coloured pileus bears dark scales and is supported by a central or eccentric stipe; the lamellae are whitish and decurrent, and the basidiospores are ellipsoidal, hyaline, and ca. 10 × 5 µm. L. lepideus causes TIMBER SPOILAGE (see also STAG’S HORN FUNGUS); it can tolerate quite high concentrations of CREOSOTE. (For L. edodes see LENTINULA.) Lentivirinae A subfamily of viruses of the RETROVIRIDAE which includes HIV and the FELINE IMMUNODEFICIENCY VIRUS; other viruses which have been classified in this subfamily (on the basis of e.g. nucleotide sequences) include a number of viruses isolated from diseased sheep and goats – e.g. CAPRINE ARTHRITIS–ENCEPHALITIS virus, VISNA virus and zwoegerziekte virus – and the causal agent of SWAMP FEVER (equine infectious anaemia virus). At least some lentiviruses (including HIV and visna virus) can be cultured; the high rate of transcription of the visna virus in infected cells has been attributed to the presence of both cis-acting enhancer sequences and trans-acting transcriptional activating factors [Science (1985) 229 482–485]. As well as replicating within (and killing) host cells, at least some lentiviruses (HIV and feline immunodeficiency virus) have been associated with instances of neoplastic disease (e.g. lymphomas); however, in these cases the mechanism of pathogenesis is unclear, and it may be that lentivirus infection permits neoplastic development by (e.g.) undermining immune surveillance. Lenzites See APHYLLOPHORALES (Polyporaceae). LEP Low egg passage: refers to the number of times that a given strain of virus has undergone SERIAL PASSAGE in eggs. (See e.g. FLURY VIRUS; cf. HEP.) Lepiota A genus of fungi (AGARICALES, Agaricaceae) which typically form colourless spores. L. procera (the edible ‘parasol mushroom’) forms a flattened, umbonate, brownish pileus whose upper surface bears loose, darker brown scales. (See also FUNGUS GARDENS.) Lepista A genus of fungi (AGARICALES, Tricholomataceae) which form pink basidiospores. Some species are edible: see BLEWIT. leporid herpesvirus 1 See GAMMAHERPESVIRINAE. Leporipoxvirus (myxoma subgroup) A genus of viruses of the CHORDOPOXVIRINAE which infect leporids (hares and rabbits) and squirrels. The viruses commonly cause tumours (fibromas, myxomas) in their hosts, and are commonly transmitted (mechanically) by arthropod vectors. Haemagglutinin is not formed; infectivity of the virion is destroyed e.g. by ether. Type species: myxoma virus (see MYXOMATOSIS); other members: hare fibroma virus, rabbit (Shope) fibroma virus, and squirrel fibroma virus. (See also NON-GENETIC REACTIVATION.) lepra cells See LEPROSY. lepra reaction See LEPROSY. Lepraria A genus of LICHENS (of the AGONOMYCETALES) in which the (leprose) thallus forms a soft, powdery crust consisting of loosely associated fungal hyphae and green algal cells; fruiting bodies are unknown. In e.g. L. incana the thallus is thick, whitish to greenish grey, and parts may consist only of the (white) mycobiont; L. incana is fairly tolerant of air pollution and occurs e.g. on walls, bark etc in damp situations. leproma See LEPROSY. lepromin A preparation obtained by grinding, autoclaving and filtering lepromatous tissue (see LEPROSY); it thus contains heat-stable fractions and includes human antigens unrelated to leprosy. lepromin test A SKIN TEST involving the intra-dermal administration of LEPROMIN. There are three types of reaction: an immediate (Medina) reaction; the Fernandez reaction, an inflammatory response 24–48 hours after the injection; and the Mitsuda reaction which occurs after ca. 3–4 weeks. In a positive Mitsuda reaction a hard granulomatous nodule occurs at the site of injection. Since normal adults can give a 433 leprose Laboratory diagnosis. Biopsies from skin lesions, or e.g. nasal secretions, are stained by acid-fast stains; in lepromatous leprosy the foamy Virchow cells, packed with acid-fast bacilli, are diagnostic. Tuberculoid leprosy may be difficult to differentiate from sarcoidosis or non-leprosy mycobacterial granulomas. (See also LEPROMIN TEST.) The carbohydrate moiety of PGLI (an M. leprae cell-surface glycolipid) may be useful in a serodiagnostic test. Chemotherapy. Currently, antimicrobial treatment involves a combination of drugs: DAPSONE and rifampicin with or without CLOFAZIMINE. Other drugs (e.g. ofloxacin, minocycline) are also effective for treating leprosy. Thalidomide is useful in some cases of ENL. [Various aspects: TRSTMH (1993) 87 499–517; epidemiology: FEMS (1996) 136 221–230; review: RMM (1998) 9 39–48.] lepto- Prefix meaning fine, narrow, thin. Leptogium A genus of gelatinous LICHENS (order LECANORALES); photobiont: Nostoc. The thallus has a cortex, but otherwise resembles that of COLLEMA spp (although generally less gelatinous and often felt-like or tomentose). [Studies on some species: Lichenol. (1983) 15 109–125.] Leptolegnia See SAPROLEGNIALES. Leptomitales An order of aquatic fungi (class OOMYCETES) which are typically saprotrophic – though one species, Leptomitus lacteus, may be an opportunist invader of epidermal lesions in fish. The thallus consists of hyphae which bear characteristic constrictions along their lengths, the constrictions often being plugged with CELLULIN GRANULES; in some genera (Apodachlya, Leptomitus) there is evidence that the cell wall contains both CHITIN and CELLULOSE [Book ref. 174, pp. 62–63]. Other genera include Aqualinderella (an anaerobe found in polluted waters) and Rhipidium. Leptomitus See LEPTOMITALES. leptomonad form Obsolete syn. PROMASTIGOTE. Leptomonas A genus of homoxenous parasitic protozoa (family TRYPANOSOMATIDAE) which typically occur in the gut (sometimes in the haemocoele or in the salivary glands) of insects. The organisms occur in amastigote and promastigote forms, the latter usually being 10–40 µm in length (excluding flagellum), or about twice as long including the flagellum; in some species the cell may be helical, up to 200 µm in length. leptomycin B A product of SECONDARY METABOLISM (in Streptomyces spp) which blocks progression through the cell cycle in eukaryotic cells. Leptomyxa See ACARPOMYXEA. Leptonema A genus of bacteria of the order SPIROCHAETALES. [Motility in L. illini : see e.g. JB (1996) 178 6539–6545.] Leptosphaeria A genus of fungi (order DOTHIDEALES) which include saprotrophs and plant parasites (see e.g. BLACKLEG (sense 2), CANE BLIGHT and GLUME BLOTCH); anamorphs occur in e.g. Camarosporium and Septoria. Leptospira A genus of obligately aerobic bacteria (family LEPTOSPIRACEAE) which occur as free-living organisms in soil and aquatic (freshwater and marine) habitats, and as parasites or pathogens in man and other animals (see LEPTOSPIROSIS). The cells are motile, helical (≥18 coils/cell), 0.1 µm in diameter and from 6 to over 12 µm in length; a single periplasmic flagellum arises at each end of the cell. In liquid media, one or both ends of the cell are typically bent or hooked. Growth occurs on e.g. serum-containing media; optimum growth temperature: 28–30° C. Metabolism is respiratory (oxidative); fatty acids are metabolized by b-oxidation to acetate and CO2 . Catalase and/or positive Mitsuda reaction it is not diagnostic for leprosy, but a negative Mitsuda reaction is considered to indicate susceptibility to leprosy. The lepromin test can help to classify leprosy: the Mitsuda reaction is negative in lepromatous cases, positive in tuberculoid cases. leprose (lichenol.) Refers to a thallus which lacks a cortex and is covered with or consists entirely of soredia (see e.g. LEPRARIA). leprosins Ultrasonicates of tissue-free cells of Mycobacterium leprae (derived from lesions). leprosy (Hansen’s disease) A chronic human disease, caused by Mycobacterium leprae, which is currently found mainly in the tropics and subtropics. It is not readily transmissible. Infection may occur by direct contact, by droplet infection, and/or by contact with fomites. Incubation period: ca. 1–10 years. M. leprae has a predilection for the peripheral nervous system; the CNS is apparently not affected. The disease characteristically involves destruction of nerves – leading to loss of sensitivity (allowing damage to fingers etc. by trauma) as well as weakness/paralysis of muscles. Leprosy occurs in different forms; the specific type of T cell response to M. leprae in a given host correlates with the form of disease. In lepromatous leprosy the CD4+ :CD8+ T cell ratio in lesions is ca. 1:2; major T-cell-derived cytokines include interleukins 4 and 10. Bacilli are abundant (e.g. 1010 bacilli/gram tissue). The multiple skin lesions enlarge/coalesce to form nodules (lepromas) – commonly on ear lobes, nose, brows and forehead (but possible elsewhere). Affected skin is typically dry and hairless. Lepromas develop through infiltration of the dermis by lepra cells (= Virchow cells): histiocytes (macrophages) containing large numbers of bacilli; the infiltrate also contains compact masses of (intra- and extracellular) bacilli associated with cellular debris – ‘globi’ or ‘brown bodies’ – which may reach 200 µm in diameter. Nerve damage is associated with an inflammatory process. Various organs may become involved – e.g. eyes (sometimes leading to blindness), oesophagus, lymphatic system, liver. In a diffuse form of lepromatous leprosy the infiltrate occurs as a continuous thin sheet in the dermis; an associated necrotizing vasculitis leads to ulceration (the Lucio phenomenon). In tuberculoid leprosy the CD4+ :CD8+ T cell ratio in lesions is ca. 2:1; major T-cell-derived cytokines include INFg and interleukin 2. Skin lesions (epithelioid-cell granulomas infiltrated with lymphocytes) are few, dry, anaesthetic, hypoor hyperpigmented (or erythematous); Virchow cells are absent, and bacilli are rare/absent. The granulomas, which apparently result from delayed-type hypersensitivity, often damage nerve trunks. Borderline (= dimorphous) leprosy is characterized by histopathological features of both lepromatous and tuberculoid forms; the features of one form may predominate. Most cases of leprosy are borderline. Some patients treated for leprosy may develop one of two types of reaction state (acute, immunologically-based episodes). The lepra reaction (= Jopling type 1 reaction; reversal reaction) occurs in some borderline cases and may involve e.g. necrosis/ulceration of bacilli-containing tissues and the formation of abscesses in nerve trunks. Erythema nodosum leprosum (ENL; Jopling type 2 reaction) occurs in some lepromatous and borderline cases (not in tuberculoid cases); it may result from vasculitis caused by antibody–antigen complexes, and inflammation may affect e.g. eyes, joints, nose and peripheral nerves. ENL may recur repeatedly. 434 Leuconostoc peroxidase +ve; oxidase +ve. GC%: 35–41, but one strain 53. Type species: L. interrogans. Two species are recognized [Book ref. 22, pp. 62–67]: L. biflexa (free living, capable of growth at 13° C, usually resistant to 225 µg/ml 8-azaguanine) and L. interrogans (parasitic/pathogenic, usually sensitive to 225 µg/ml 8-azaguanine, and growth inhibited at 13° C). Each species consists of a large number of named serotypes (= serovars) which are classified into named serogroups; thus, e.g. the Icterohaemorrhagiae serogroup of L. interrogans contains nearly twenty serotypes, including e.g. dakota and icterohaemorrhagiae. Leptospiraceae A family of aerobic bacteria of the order SPIROCHAETALES; the cells typically have hooked ends, their cell wall PEPTIDOGLYCAN contains the diamino acid diaminopimelic acid, and their carbon and energy sources are long-chain fatty acids or long-chain fatty alcohols (cf. SPIROCHAETACEAE). One genus: LEPTOSPIRA. Leptospirillum ferrooxidans See LEACHING. leptospirosis A disease of man and animals, caused by any of various serotypes of Leptospira interrogans. A wide range of animals can act as carriers, the leptospires normally occurring in the kidney tubules; e.g., serotypes icterohaemorrhagiae and australis A are carried by rats and other rodents, canicola by dogs and pigs, pomona by cattle and pigs, hardjo and grippotyphosa by cattle. In domestic animals the disease ranges from mild to fatal, depending on the animal and serotype involved; symptoms commonly include fever, jaundice, haematuria (cf. RED WATER), and abortion. (Rodents appear not to be significantly affected by leptospire infection.) Animals may continue to excrete leptospires in their urine for several months after recovery. Man acquires infection either by direct contact with animals or animal carcasses (cf. ZOONOSIS) or via water contaminated with urine from infected animals. Infection occurs via wounds or mucous membranes. Incubation period averages ca. 10 days. In the mild form of leptospirosis, onset is abrupt with shivering, fever, prostration, headache, anorexia and myalgia; nausea and vomiting may occur. Renal function is typically impaired, with proteinuria, haematuria etc. Uveitis is a late complication. A severe form of leptospirosis (Weil’s disease, infectious jaundice) is caused by serotype icterohaemorrhagiae; symptoms are more intense, with e.g. marked prostration, severe nausea, vomiting and diarrhoea, enlargement of the liver, jaundice, renal dysfunction, haemorrhages, MENINGITIS. Mortality rates: ca. 15–40%. Lab. diagnosis: demonstration of rising titres of antibodies (which appear 7–10 days after onset); identification/culture of the pathogen from blood during the first week, later from urine. [ELISA for detection of specific IgM and IgG in human leptospirosis: JGM (1985) 131 377–385.] leptotene stage See MEIOSIS. Leptotheca See MYXOSPOREA. Leptothrix A genus of Gram-negative IRON BACTERIA. Cells: polarly flagellated single rods, or chains of rods within a sheath. Type species: L. ochracea. Leptotrichia A genus of Gram type-negative bacteria (family BACTEROIDACEAE) which occur e.g. in the human mouth. Cells: non-motile, straight or slightly curved rods, 0.8–1.5 × 5.0–15.0 µm, often occurring in pairs, chains or septate filaments; the cells of young cultures may stain Gram-positively, but the cell wall resembles those of Gram-negative bacteria. Acid (no gas) is formed from carbohydrates; glucose fermentation yields lactic acid with small amounts of acetic and succinic acids. GC%: ca. 25. Type species: L. buccalis. lesion A localized region of tissue which has been damaged physically or altered by any pathological process. letA gene See F PLASMID. letD gene See F PLASMID. lethal mutation A mutation which kills the organism in which it occurs (cf. CONDITIONAL LETHAL MUTANT). lethal yellowing disease (of coconut) See COCONUT LETHAL YELLOWING. lethal zygosis During CONJUGATION (sense 1b) between HFR DONORS and F− recipients: the phenomenon in which recipients are killed if the ratio of donor cells to recipient cells is high (e.g. ca. 10:1) and the culture is aerated during mating. The effect is not exhibited in F+ × F− crosses. lettuce big vein agent A virus-like agent which causes big vein disease in lettuces (Lactuca sativa) and is pathogenic in other plants of the Compositae. In lettuces, symptoms include veinclearing and vein-banding, and significant economic losses may result – particularly in hydroponically cultured plants. The agent is transmitted by Olpidium brassicae. lettuce downy mildew See BREMIA. lettuce mosaic virus See POTYVIRUSES. lettuce necrotic yellows virus See RHABDOVIRIDAE. L-leucine biosynthesis See Appendix IV(b). leucine-responsive regulator protein See LRP. leuco compound A colourless derivative of a dye. (See e.g. leuco METHYLENE BLUE.) leucocidin Any microbial toxin which can damage or kill one or more types of LEUCOCYTE. Leucocidal activity may be shown by e.g. certain staphylococcal HAEMOLYSINS and STREPTOLYSIN S; the PANTON–VALENTINE LEUCOCIDIN has specific leucocidal activity. leucocyte (leukocyte) Any white cell of the blood and lymphoid system, including granulocytes (= PMNS), MACROPHAGES, LYMPHOCYTES, and the precursor cells of each of these types. (The term ‘leucocyte’ is often used to refer solely to the PMNs.) (See also LEUCOCIDIN.) leucocytosis An increase in the number of leucocytes in the blood. leucocytosis-promoting factor Syn. PERTUSSIS TOXIN. Leucocytozoon A genus of protozoa of the HAEMOSPORORINA. Species are parasitic in birds; the vectors are commonly bloodsucking flies of the Simuliidae. leucolysin A toxin which lyses leucocytes. leucomycin See MACROLIDE ANTIBIOTICS. leuconocin S See BACTERIOCINS. Leuconostoc A genus of Gram-positive bacteria of the family Streptococcaceae; the organisms occur e.g. in various dairy products and in fermented beverages and fermenting vegetables (see e.g. BUTTERMILK, CHEESE-MAKING, IDLI, LACTIC ACID STARTERS, PULQUE, ROPINESS, SAUERKRAUT and WINE SPOILAGE). Cells: nonmotile, spherical or lenticular (maximum dimension ca. 1 µm), typically in pairs or chains. Growth requirements include a suitable carbohydrate and a range of amino acids and vitamins, e.g. BIOTIN, NICOTINIC ACID and THIAMINE. Glucose is metabolized anaerobically by a HETEROLACTIC FERMENTATION in which D(−)-lactic acid, CO2 and ethanol are the main products; in the presence of O2 , glucose is used more efficiently than it is under anaerobic conditions, and the end-products include acetic acid rather than ethanol – NADH being reoxidized by NAD(P)H oxidase activity [JGM (1986) 132 1789–1796]. (See also LACTIC ACID BACTERIA.) Optimum growth temperature: 20–30° C. GC%: ca. 38–44. Type species: L. mesenteroides. L. cremoris (formerly L. citrovorum) can ferment very few carbohydrates (e.g. glucose, galactose and lactose). In the presence of a suitable carbohydrate (as energy source), L. cremoris can break down citrate in a non-energy-yielding manner with 435 Leucopaxillus as a result of immunological or non-immunological stimuli; leukotrienes can e.g. cause contraction of smooth muscle, stimulate vascular permeability, and act as attractants for leucocytes. [Review: ARB (1983) 52 355–377.] leukoviruses Former name for retroviruses of the ONCOVIRINAE, particularly the TYPE C ONCOVIRUS GROUP. levamisole A drug used to stimulate the immune response; inhibit tumour formation; stimulate the phagocytic activity of macrophages; inhibit alkaline phosphatases; treat intestinal disease caused by certain helminths – e.g. that caused by Ascaris lumbricoides. levan A linear FRUCTAN consisting mainly of (2 → 6)-b-linked fructofuranose residues. Levans are widespread as storage polysaccharides in higher plants (especially grasses), and are produced as extracellular slime by certain bacteria (e.g. species of Bacillus and Streptococcus). (See also ROPINESS and DENTAL PLAQUE.) Leveillula See ERYSIPHALES. Levinea See CITROBACTER. Levinthal’s medium A medium used for the isolation of Haemophilus spp; it contains both V FACTOR and X FACTOR. Levinthal stock is made by adding defibrinated horse blood (10%) to a boiling brain–heart infusion broth and filtering through e.g. glass wool and then through a membrane filter. Autoclaved peptone broth is then added, and the whole may be mixed with sterile molten agar and dispensed into Petri dishes. [Recipe: Book ref. 46, pp. 1375–1376.] Leviviridae A family of BACTERIOPHAGES [Book ref. 23, p. 136]; the genome is linear, positive-sense ssRNA (MWt ca. 1.0–1.3 × 106 ), and the virion is icosahedral (ca. 22–26 nm diam.). One genus: Levivirus. Leviviruses occur e.g. in enterobacteria (e.g. phages a15, b, f2, fr, M12, MS2, µ2, Qb, R17 and R23), in Pseudomonas (e.g. PP7, 7S) and in Caulobacter. Enterobacterial leviviruses (common in sewage) have been classified into four groups (I–IV) on the basis of serological and physiological properties. Phages of group I (e.g. f2, fr, MS2, R17) and group III (e.g. Qb) have been the most intensively studied. (MS2, f2 and R17 apparently differ only in a number of point mutations.) The group I virion consists of (i) 180 copies of a single type of coat protein, (ii) one molecule of A protein (= ‘maturation protein’ – necessary for infectivity and phage maturation), (iii) the RNA genome, and (iv) at least in MS2, spermidine (presumably to neutralize the negative charge on RNA). (The Qb virion is similar but contains one molecule of an A2 protein (equivalent to A protein in group I phages) and about three molecules of an A1 protein (= IIb protein); the A1 protein is formed by READTHROUGH of the coat protein gene.) [Capsomeric structure of Qb: Nature (1982) 298 819–824.] The group I phage genome encodes: (i) the A protein; (ii) the coat protein; (iii) a replicase subunit; and (iv) a ‘lysis protein’ (the gene for which overlaps the coat and replicase genes in the +1 reading frame [Nature (1982) 295 35–41]). The lysis protein (L protein) of MS2 forms specific adhesion sites (junctions of the cytoplasmic and outer membranes); these sites may facilitate lysis of the cell envelope [JB (1989) 171 3331–3336]. In Qb and other leviviruses the replicase subunit combines with three host proteins – the S1 protein of the 30S ribosomal subunit, and the elongation factors EF-Tu and EF-Ts (see PROTEIN SYNTHESIS) – to form the active replicase (i.e. RNAdependent RNA polymerase) necessary for phage genome replication. Expression of viral genes is controlled in both quantity the formation of acetoin, DIACETYL, acetate and CO2 . Dextrans are not formed. L. dextranicum. Many carbohydrates (although not e.g. arabinose) are fermented. DEXTRANS are formed. L. lactis. Many carbohydrates (though not e.g. dextrin) are fermented. Dextrans are not formed. L. mesenteroides. Many carbohydrates (including e.g. arabinose) are fermented. DEXTRANS are formed. L. oenos. Occurs in wine (see also MALOLACTIC FERMENTATION). Fermentable carbohydrates include e.g. fructose and trehalose. Dextrans are not formed. Growth can occur in the presence of 10% ethanol, and can occur at pH 3.7. L. paramesenteroides. Many carbohydrates (including dextrin) are fermented. Dextrans are not formed. It has been proposed that L. cremoris and L. dextranicum be re-classified as subspecies of L. mesenteroides [IJSB (1983) 33 118–119]. [Characterization of Leuconostoc spp: Book ref. 143, pp. 147–178.] Leucopaxillus A genus of fungi of the Tricholomataceae (AGARICALES). leucopenia The presence of reduced numbers of leucocytes in the blood. leucoplast Any colourless PLASTID. leucosin Syn. CHRYSOLAMINARIN. leucosis Syn. LEUKOSIS. Leucosporidium See SPORIDIALES and CANDIDA. Leucothrix A genus of GLIDING BACTERIA (see CYTOPHAGALES) which occur e.g. as epiphytes on marine algae. The organisms are non-sheathed, sessile (attached), colourless filaments whose constituent cells (including non-terminal cells) can round up and become individual gliding gonidia; sulphur is not deposited intracellularly. Metabolism appears to be chemoorganotrophic. GC%: ca. 46–51. Leucotrichaceae See CYTOPHAGALES. leucovorin See FOLIC ACID. leukaemia Any of various progressive, malignant diseases in which increased numbers of (usually abnormal or immature) leucocytes occur in the blood; numbers of circulating erythrocytes and platelets are reduced. Symptoms characteristic of leukaemia include anaemia, a tendency to haemorrhage, and increased susceptibility to infection. Leukaemias are classified on the basis of the time course of the disease (acute or chronic) and the types of cell involved (e.g., granular PMNs and their precursors, B or T lymphocytes or lymphoblasts, or monocytes). Certain retroviruses can cause leukaemias in man, animals or birds: see e.g. ADULT T-CELL LEUKAEMIA; AVIAN ACUTE LEUKAEMIA VIRUSES; BOVINE VIRAL LEUKOSIS; FELINE LEUKAEMIA VIRUS; MURINE LEUKAEMIA VIRUSES. leukocyte See LEUCOCYTE. leukocyte adhesion deficiency (LAD) A genetic disorder (autosomal, recessive) characterized by the failure of leukocytes adequately to express b2 integrin adhesion molecules (see INTEGRINS); such cells are unable to give a normal response during INFLAMMATION, and individuals with LAD have correspondingly lower resistance to infection. [See also Lancet (1999) 353 341–343.] leukocyte function-associated antigen See INTEGRINS. leukosis (leucosis) The (abnormal) proliferation of leucocyteforming tissue. (See e.g. BOVINE VIRAL LEUKOSIS; LYMPHOID LEUKOSIS.) leukotrienes Derivatives of ARACHIDONIC ACID, and of certain other fatty acids, formed e.g. by leucocytes and macrophages 436 lichen and time; thus, e.g. coat protein is synthesized greatly in excess of A protein, and it acts as a repressor which, later in infection, terminates translation of the replicase subunit gene. Regulation of gene expression may involve changes in the secondary structure of the RNA which alter the availability of ribosome binding sites [e.g. Nature (1982) 295 35–41]. Virion assembly apparently occurs largely or exclusively by spontaneous aggregation. Most or all enterobacterial leviviruses are ‘male-specific’ (ANDROPHAGES); f2, MS2, R17, fr, M12 and Qb all adsorb specifically to the sides of F pili (cf. INOVIRUS). The Pseudomonas phages PP7 and 7S adsorb to the chromosomally specified polar (PSA) fimbriae. Caulobacter phages also adsorb to polar fimbriae; only the pre-divisional and swarmer stages are susceptible to infection. Infection of the host cell may, at least in some cases, involve pilus (or fimbria) retraction. The genome enters the host cell together with – and probably mediated by – the A/A2 protein (which is cleaved during penetration). Levivirus See LEVIVIRIDAE. levofloxacin A QUINOLONE ANTIBIOTIC (q.v.). levulose D-Fructose. lexA gene See SOS SYSTEM. LFA-1 See INTEGRINS. LFA-2 See IMMUNOGLOBULIN SUPERFAMILY. LFA-3 See IMMUNOGLOBULIN SUPERFAMILY. LFT (of plasmids) See EPIDEMIC SPREAD. LFT lysate See TRANSDUCTION. LGLs Large granular lymphocytes: see NK CELLS. LHC LIGHT-HARVESTING COMPLEX. LHC complex See CELLULOSE. library (gene bank; gene library) (mol. biol.) A collection of recombinant DNA molecules which, together, commonly represents the entire genome of an organism (a genomic library) (see figure). Another type of library consists of a collection of CDNA molecules derived from a mixture of polyadenylated mRNAs isolated from a population of cells of a given strain; this kind of library does not represent the entire genome of the strain. Licea See MYXOMYCETES. Liceales See MYXOMYCETES. lichen A ‘composite organism’ consisting of a fungus (the mycobiont) and an alga or cyanobacterium (the photobiont – cf. PHYCOBIONT) in symbiotic association; a lichen thallus is morphologically and physiologically quite unlike that of either symbiont growing separately. Lichens are widely distributed, being found from tropical to polar regions in a very wide range of habitats, both terrestrial (e.g. on trees, rocks and soil) and aquatic (freshwater and marine). Taxonomically, lichens are generally regarded as ‘lichenized fungi’, classification being based mainly on the nature and origin of their fruiting structures. Most mycobionts appear not to occur in free-living form (but cf. XANTHORIA). The majority are ascomycetes (‘ascolichens’) belonging mainly to the orders ARTHONIALES, CALICIALES, DOTHIDEALES, GRAPHIDALES, GYALECTALES, LECANIDIALES, LECANORALES, OPEGRAPHALES, PELTIGERALES, PERTUSARIALES, PYRENULALES, TELOSCHISTALES, and VERRUCARIALES. Some lichens belong to the Deuteromycotina, and a few to the Hymenomycetes (‘basidiolichens’). The photobiont is usually either a green alga (e.g. Coccomyxa, Myrmecia, Phycopeltis, Pseudotrebouxia, Trebouxia or Trentepohlia) or a cyanobacterium (usually Nostoc or Calothrix ). Some lichens may have more than one photobiont: see e.g. SOLORINA and CEPHALODIUM. Free-living counterparts of lichen photobionts occur in nature, but in the lichen the cells DNA ligase LIBRARY: making a (bacterial) genomic library (diagrammatic). Initially, chromosomes are isolated from a population of bacteria of a given strain. The chromosomes are exposed to a RESTRICTION ENDONUCLEASE which cuts at specific sites in each chromosome (top, left), forming many fragments of different sizes (centre, left). As in CLONING (q.v.), copies of a given plasmid (top, right) provide vector molecules. The plasmid is one which can be cut by the same enzyme as that used to cut the chromosomes; note, however, that the plasmid has only one cutting site for the given enzyme – so that cutting simply linearizes the plasmid molecule (centre, right). Chromosomal fragments and linearized plasmids are then mixed in the presence of ATP and DNA ligase, an enzyme that catalyses the formation of phosphodiester bonds. Fragments and plasmids join together, randomly, via their sticky ends to form a large number of recombinant molecules, many consisting of a plasmid circularized with chromosomal DNA. (Plasmid–fragment binding may be promoted by the phosphatase approach described in CLONING.) Because the fragments are of different sizes, the recombinant plasmids will also be of different sizes (bottom of diagram). As many millions of fragments and plasmids are used, the recombinant plasmids will, collectively, carry all the DNA of the chromosome, and this collection of recombinant molecules is therefore called a genomic library. By using TRANSFORMATION or ELECTROPORATION, the library can be inserted into a population of bacteria – different cells receiving different fragments of the chromosome; these cells can then be allowed to form individual colonies, and colonies containing a particular fragment of the chromosome can be detected by various screening processes (see e.g. CLONING). Reproduced from Bacteria 5th edition, Figure 8.7, page 184, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 437 lichen Vitamins found to be necessary for the growth of the mycobiont in culture are assumed to be supplied by the photobiont in nature. Lichens accumulate high concentrations of certain metal ions (including heavy metals), but lichen mineral metabolism is not well understood. Water (in liquid or vapour form) is readily absorbed by lichen thalli; the absorbed water occurs intercellularly and is readily lost by evaporation. The ability to absorb and retain water may be influenced by thallus morphology – e.g., surface area and hence absorptive capacity will be greater in a thallus which is extensively lobed or perforated than in one which is not. (See also RHIZINE.) ‘Lichen substances’ (‘lichen acids’). Lichen mycobionts produce a range of water-insoluble secondary metabolites (e.g. higher aliphatic acids and esters, depsides, depsidones, dibenzofurans, anthraquinones) which may be deposited on the medullary hyphae, on the surface of the cortex (often giving a pruinose appearance), in the hymenial discs (see e.g. RHODOCLADONIC ACID), etc. Many of these substances are unique to lichens; they are generally not formed by the isolated mycobionts. Their functions are largely unknown, but may include e.g. protection of the photobiont from high light intensities (see e.g. PARIETIN); inhibition of potentially competing plants (particularly bryophytes) and/or soil microflora in the vicinity of the thallus (e.g. EVERNIC ACID and USNIC ACIDS have antibiotic activity); prevention of grazing of the thallus by invertebrates; etc. The content of lichen substances in a given lichen is a useful feature in taxonomy and identification. The compounds may be identified e.g. by chromatography, mass spectrometry, microcrystallography etc. Some lichen substances give characteristic colour reactions with certain reagents, and this forms the basis of simple colour tests widely used for identification of lichens in the field. The reagents used are: saturated aqueous calcium hypochlorite (‘C’), 10–25% aqueous KOH (‘K’), and 1–5% ethanolic p-phenylenediamine (‘P’ or ‘PD’). For example, parietin and other anthraquinones give a purple colour with K. Growth of a lichen thallus generally appears to occur at or near the periphery (or apex in fruticose thalli). [Growth in circular thalli: Lichenol. (1981) 13 265–287.] Lichen growth is characteristically slow; the radius of a circular thallus may increase by (e.g.) 0.01–2.50 mm per year, depending on species, rainfall, light intensity, temperature, etc. (See also LICHENOMETRY.) In certain crustose and squamulose lichens (e.g. Pannaria, Pertusaria, Rhizocarpon) a photobiont-free margin (the hypothallus) may be formed around the thallus as the hyphae of the mycobiont grow outwards; the photobiont reproduces by cell division or e.g. by autospore formation, and the daughter cells spread into the region of new fungal growth. Reproduction. Asexual (vegetative) reproduction in lichens may be achieved e.g. by SOREDIUM, ISIDIUM or HORMOCYST formation, or by fragmentation of the thallus. Some lichens may be able to generate new thalli from conidia (which may or may not be produced in pycnidia, according to species). Sexual reproduction commonly involves ascocarp formation. Lichen ascocarps tend to be longer-lived than those of nonlichenized ascomycetes, and may continue to produce ascospores for a period of years. Since ascospores do not contain the photobiont, the germinating ascospores must acquire the photobiont from the environment. In a few cases (e.g. Endocarpon sp, Pertusaria pertusa) cells of the photobiont may adhere to the ascospores as they are released, thereby allowing their dissemination with the ascospores. In most cases, however, the fungus may be modified (e.g., they may be smaller, may lack motile stages, and filamentous forms may become unicellular). The bionts of a lichen can be separated (e.g. by micromanipulation) and can be maintained in culture, but it is notoriously difficult to induce them to re-synthesize a lichen thallus; re-synthesis has been achieved e.g. for Endocarpon pusillum. Culture of the photobiont may be necessary for its identification. Morphology and anatomy. The lichen thallus may be e.g. CRUSTOSE, FOLIOSE, FRUTICOSE, LEPROSE or SQUAMULOSE, or it may have a composite form, with e.g. a squamulose primary thallus from which arises a fruticose secondary thallus (see PSEUDOPODETIUM; cf. PODETIUM). The simplest type of thallus consists of a loose association of fungal hyphae and photobiont cells, with little or no differentiation (see e.g. LEPRARIA). In gelatinous lichens such as COLLEMA and LEMPHOLEMMA, both bionts are more or less evenly distributed throughout the thallus (i.e. the thallus is homoiomerous), with little differentiation into tissues (cf. LEPTOGIUM). In most crustose, foliose and fruticose lichens the thallus is heteromerous (stratified): the mycobiont is differentiated to form a surface layer, the cortex (a layer of compressed, conglutinated cells or hyphae), which covers the medulla (a fibrous tissue of loosely interwoven hyphae), and the photobiont occurs in a layer between the upper cortex and the medulla. One or more differentiated somatic structures may also be present, depending on species: see e.g. CEPHALODIUM; CILIUM (sense 2); CYPHELLA; ISIDIUM; PSEUDOCYPHELLA; RHIZINE; TOMENTUM. (See also PORED EPICORTEX.) Within the thallus, the hyphae of the mycobiont may be closely appressed to the cells of the photobiont, the walls of both bionts being thinner at the point of contact, or the mycobiont may penetrate the photobiont cells by means of haustoria; the haustoria do not penetrate the plasma membrane in living cells, but may do so in dead cells. [Cytology of mycobiont–photobiont interaction: Lichenol. (1984) 16 111–127.] Physiology. Although traditionally regarded as a mutualistic association, a lichen is now thought to be more a case of ‘controlled parasitization’ of the photobiont by the mycobiont. It has long been assumed that all the carbon requirements of the mycobiont are fulfilled by photosynthate released by the photobiont; cyanobionts release glucose, while phycobionts release a sugar alcohol (Trebouxia, Myrmecia and Coccomyxa release ribitol, Trentepohlia releases erythritol, Stichococcus releases sorbitol), and these compounds are converted by the mycobiont to e.g. mannitol and/or arabitol. However, the carbon requirement for lichen growth is small, and could – at least theoretically – be fulfilled e.g. by absorption of organic carbon from the environment; it has also been shown that, in the dark, the mycobiont in Peltigera sp can fix substantial amounts of CO2 mainly via PEP carboxylase activity [Physiol. Plant. (1983) 57 285–290]. The photosynthate received by the mycobiont from the photobiont may thus be necessary not so much for growth as for the maintenance of high levels of soluble carbohydrate (particularly polyols) which appear to function as a reserve of respirable carbon (necessary e.g. when the thallus is hydrated, and hence metabolically active, during periods of darkness) and to protect cell components during periods of desiccation. Nitrogen requirements may be fulfilled by absorption of e.g. nitrate from the environment. Some lichens contain nitrogenfixing cyanobacteria – either as the primary photobiont or as an additional biont in specialized cephalodia (see CEPHALODIUM); nitrogen fixed by the cyanobacteria is transferred to the mycobiont. 438 light chain regions which were subjected to fall-out from atmospheric nuclear weapons tests in the 1950s/1960s; lichens in this region form the basis of the food chain: lichen → reindeer (caribou) → man, and correspondingly high levels of e.g. 137 Cs were recorded in Lapps who consumed reindeer meat at that time (although the levels were not considered to be dangerous). [Lists of references on lichens and air pollution are published regularly in The Lichenologist.] Uses of lichens. Some lichens are used as food by people in certain countries: e.g. Cetraria islandica (‘Iceland moss’) is consumed e.g. in Iceland, Umbilicaria esculenta (‘iwatake’) in Japan. Species of Cladonia and Cetraria form a major part of the winter diet of reindeer (caribou) and other deer in arctic regions. Various lichens have been used since ancient times as sources of dyes: e.g. Ochrolechia tartarea yields the purple dye cudbear, Parmelia spp a brown dye, and Roccella spp LITMUS and the purple dye orchil. Certain lichens – particularly Evernia prunastri (‘oakmoss’) – are used in the manufacture of perfumes, both for their own scent and for their ability to stabilize (fix) other scents. Lichens were formerly widely used in folk medicine; currently, USNIC ACIDS have some medical uses. In Scandinavian countries, Cladonia stellaris is harvested on a commercial scale for ornamental uses in wreaths, floral decorations etc [Lichenol. (1979) 11 85–89]. lichen starch Syn. LICHENIN. lichenan Syn. LICHENIN. lichenicolous Growing on lichens. lichenin (lichenan; lichen starch; ‘moss starch’) A linear b-Dglucan consisting mainly of (1 → 3)-b-linked cellotriose units (i.e. units of three (1 → 4)-b-linked glucose residues); it occurs in certain lichens (e.g. Cetraria islandica). (cf. ISOLICHENIN.) lichenometry A method for dating glacial deposits, stone monuments etc, based on a knowledge of the growth rates of particular (usually crustose) lichens. The assumption is made that the largest crustose thalli present on a given exposed surface started to grow when the surface first became exposed; the thalli are measured, and their ages estimated from a standard growth curve for the species – thus giving an approximate age for the substratum. Certain thalli of Rhizocarpon geographicum have been estimated to be ca. 1000–4500 years old. [Lichenometry with R. geographicum: Lichenol. (1983) 15 249–261.] Lichina A genus of small, fruticose LICHENS (order LECANORALES); photobiont: a cyanobacterium. The thallus is more or less gelatinous, brown to black, erect, branching, terete (L. confinis) or flattened (L. pygmaea), forming tufts or mats on rocks on the seashore at or just above the high-tide level. Apothecia occur immersed in the swollen ends of the branches. Lieberkuehnia See GRANULORETICULOSEA. Lieberkühn’s organelle (watchglass organelle) A refractive, lenticular body which occurs, subpellicularly, in or near the buccal cavity in certain hymenostomes. lif gene See LYSOSTAPHIN. lig gene Gene for DNA LIGASE. ligase chain reaction See LCR. ligases (synthetases) ENZYMES (EC class 6) which catalyse covalent bond formation using energy obtained from the cleavage of a pyrophosphate bond (e.g. in ATP); C−O, C−S, C−N and C−C bonds are formed by different subclasses of ligases. (See e.g. DNA LIGASE; cf. SYNTHASE.) ligative hyphae See HYPHA. light chain (immunol.) The smaller (MWt ca. 23000) of the two types of polypeptide chain in an IMMUNOGLOBULIN monomer. About half the (ca. 215) amino acid residues in a light chain must incorporate free-living cells, or may acquire the appropriate photobiont from propagules (e.g., soredia: probably an important source for TREBOUXIA-containing lichens) present on the substratum. In some lichen species the ascospores may germinate only in the presence of cells of the photobiont; in others, germination gives rise to a mat of hyphae which subsequently incorporates algal or cyanobacterial cells in its immediate vicinity. The mechanism by which the ‘correct’ photobiont is selected is unknown; some evidence implies a role for LECTINS in the initial recognition step [Lichenol. (1984) 16 103–110]. However, in at least some cases the initial incorporation of algal or cyanobacterial cells seems to be non-selective, all cells except those of the prospective photobiont being killed by the fungus [Nature (1981) 289 169–172]. Habitat. Lichens occur in a wide range of habitats – usually attached to substrata such as rocks, soil, bark, leaves, mosses etc, but sometimes unattached. Saxicolous species may be ENDOLITHIC (q.v.) or EPILITHIC, and are often specific for particular types of rock (e.g. acidic or basic); they may facilitate the weathering of rocks both by physical mechanisms (e.g. rhizine penetration, thallus expansion and contraction on wetting and drying) and by chemical mechanisms (e.g. metal chelation by lichen substances, corrosion by CO2 and/or oxalic acid). Some species can grow on and etch the surface of glass, marble etc, and have caused significant deterioration of stone monuments, stained glass windows etc. Many lichens can grow in situations where they are exposed to extremes of temperature and desiccation which can be tolerated by few other forms of life – e.g. in hot arid deserts, in polar regions, and at high altitudes. In some harsh environments lichens are important primary producers, and terricolous species may play an important role in stabilization of the soil in such environments. A few lichens (e.g. Dermatocarpon weberi, Verrucaria aquatilis) are aquatic, occurring e.g. on rocks submerged in streams. Some lichens occur on rocks by the sea – at or above the intertidal zone, depending on species (see e.g. LICHINA, VERRUCARIA). Lichens have been found in some unusual locations: e.g. on the carapaces of giant tortoises, and on the backs of (living) flightless weevils of the genus Gymnopholus. Lichens and pollution. Many lichens are highly sensitive to air pollution, probably as a result of their ability to accumulate high concentrations of pollutants. They are killed by low concentrations of e.g. SO2 , fluoride, fertilizer dust etc; the pollutant primarily responsible for lichen death in industrial regions is generally SO2 , one effect of which is to degrade chlorophyll to phaeophytin. The various sensitivities of different lichen species to pollution makes them useful biological indicators of pollution levels in a given area (see also INDICATOR ORGANISM); however, sensitivities may be influenced by moisture, temperature and substratum (e.g. corticolous lichens are more sensitive than saxicolous ones). Some lichens are inherently resistant to pollution (see e.g. LECANORA). Lichen thalli can accumulate high concentrations of heavy metal ions [Lichenol. (1984) 16 173–188] – apparently with little adverse effect on themselves; lichens have thus been used to monitor environmental levels of heavy metals. (See also ACAROSPORA.) Since lichens are used as food (by man and animals) in some regions, their ability to accumulate toxic substances can be a potential health hazard. For example, lichens accumulated radioactive isotopes (e.g. strontium-90, caesium-137) in arctic 439 light green units connected by C−C and C−O−C links; it is formed by the peroxidase-initiated dehydrogenative polymerization of substituted p-hydroxycinnamyl alcohols (p-coumaryl, coniferyl and sinapyl alcohols). Lignin is a major component in the vascular tissues of terrestrial plants, and constitutes ca. 20–35% of the weight of wood; it occurs with hemicelluloses in the matrix of the cell walls, ‘waterproofing’ the xylem vessels and providing mechanical strength and protection from enzymic attack. [Lignification in disease resistance in plants: ARPpath. (1980) 18 259–288.] Lignin is tightly bonded to CELLULOSE and HEMICELLULOSES – mainly by hydrogen bonds but also by some covalent bonds. Lignins are extremely resistant to chemical and enzymic degradation; biological degradation is achieved mainly by fungi – most efficiently by white-rot basidiomycetes (see WHITE ROT) – but also by certain actinomycetes. In the white-rot fungi Phanerochaete chrysosporium and Coriolus versicolor, lignin degradation is a secondary metabolic process, occurring under low levels of nutrient nitrogen and requiring the presence of a carbon source such as glucose or cellulose [Book ref. 39, pp. 67–90]; it appears not to yield significant carbon or energy, but allows access to the polysaccharide components of the cell walls (see CELLULASES). P. chrysosporium produces an extracellular haemoprotein ‘ligninase’ which, in the presence of H2 O2 , can degrade lignin by bringing about oxidative cleavage of the backbone between Ca and Cb, oxidation and hydroxylation of benzylic methylene groups, oxidation of phenols and benzyl alcohols, etc. The haemoprotein functions as a peroxidase. Reaction of the enzyme with H2 O2 generates a highredox-potential porphyrin cation radical (oxy-ferryl complex) which can extract a single electron from an aromatic ring in the lignin substrate to generate an aromatic cationic radical; this is followed by a variety of spontaneous degradative reactions via radical and cation intermediates [FEBS (1985) 183 7–12, 13–16]. P. chrysosporium produces multiple forms of the haemoprotein lignin peroxidase [homology among multiple extracellular peroxidases from P. chrysosporium: JBC (1987) 262 419–424]. Other enzymes implicated in aerobic lignin degradation include phenol oxidases (e.g. LACCASE) and alcohol oxidase. Quinones, which can be toxic to organisms, are common products of phenol oxidase action; several white-rot fungi produce a cellobiose:quinone oxidoreductase which couples the oxidation of cellobiose with the reduction of quinones [Book ref. 26, pp. 25–26], thus linking cellulose and lignin degradation. [Anaerobic lignin degradation: AEM (1984) 47 998–1004.] ligninolytic Capable of degrading LIGNIN. (cf. LIGNOLYTIC.) lignite biodegradation See COAL BIODEGRADATION. lignocellulose A complex containing LIGNIN, CELLULOSE, and (usually) HEMICELLULOSES. lignolytic Capable of degrading wood. (cf. LIGNINOLYTIC.) Limacella See AGARICALES (Amanitaceae). limax amoebae ‘Slug-like’ amoebae, i.e., cylindrical (not flattened) MONOPODIAL AMOEBAE, or cylindrical amoebae which ‘flow’ along without forming obvious pseudopodia. limberneck A form of BOTULISM in birds, especially poultry, caused by C. botulinum type A. (cf. WESTERN DUCK DISEASE.) Limburger cheese See CHEESE-MAKING. lime sulphur See SULPHUR. limit dextrin See AMYLASES. limit dextrinase See DEBRANCHING ENZYMES. limit of resolution See RESOLVING POWER. limited enrichment method See AUXOTROPH. limulin A LECTIN from the horseshoe crab, Limulus polyphemus; it is presumed to bind at sites in LIPOPOLYSACCHARIDES and constitute the variable, N-terminal region (VL ), while the remainder form the constant, C-terminal region (CL ). There are two structurally distinct types of light chain: kappa and lambda; both types occur in all classes of immunoglobulin, but a given Ig molecule contains only kappa or only lambda chains. In human Igs, the ratio kappa:lambda is ca. 70:30, while in mouse Igs it is ca. 95:5. light green A green acid TRIPHENYLMETHANE DYE. light-harvesting complex (LHC) In or on membranes involved in PHOTOSYNTHESIS: a pigment-containing complex which receives radiant energy and transfers energy to REACTION CENTRES; an antenna is an array of LHCs. Protein–pigment complexes in LHCs are sometimes indicated by the letter ‘B’ followed by the absorption maximum or maxima – e.g. B800 indicates absorption at 800 nm. LHCs in oxygenic organisms. In e.g. green algae (CHLOROPHYTA), DIATOMS and DINOFLAGELLATES, LHCs occur within the photosynthetic membranes; in addition to CHLOROPHYLL a, LHCs in these organisms include Chl b (green algae) or Chls c2 and/or c1 (diatoms, dinoflagellates). In CRYPTOPHYTES, RHODOPHYTA and CYANOBACTERIA the LHCs include PHYCOBILIPROTEINS which, in cyanobacteria and Rhodophyta, occur in PHYCOBILISOMES attached to thylakoid membranes. Factors such as light quality and the availability of nitrogen and sulphur can influence the composition of LHCs. [Environmental effects on the LHC of cyanobacteria: JB (1993) 175 575–582.] LHCs in anoxygenic organisms. In bacteria of the RHODOSPIRILLALES the LHCs include bacteriochlorophylls and CAROTENOIDS. In the ‘purple’ photosynthetic bacteria (RHODOSPIRILLINEAE) LHCs occur in ‘intracytoplasmic membranes’ (CHROMATOPHORE sense 2) and include bacteriochlorophyll a or b as well as various carotenoids. In the ‘green’ photosynthetic bacteria (CHLOROBIINEAE) the LHCs characteristically occur in CHLOROSOMES; in green-coloured species of the Chlorobiaceae, the LHCs include Bchls c or d and the carotenoid chlorobactene, while those of brown-coloured species include Bchl e and isorenieratene. In members of the Chloroflexaceae the LHCs include Bchl c and b-carotene. light reaction See PHOTOSYNTHESIS. light strand (mol. biol.) See H STRAND. light trap (1) A zone of focused light in a liquid medium. Organisms capable of behavioural responses to light may accumulate in, or be excluded from, such a zone of light. Accumulation may occur e.g. when the light–dark boundary offers an intensity gradient sufficient to trigger a PHOBIC RESPONSE such that any attempt by the organisms to leave the light zone incites a change in direction. (2) The reaction centre in a photosynthetic pigment system. LightCycler See PCR. LightCycler Red 640 See PCR. LightCycler Red 705 See PCR. lignans A class of dibenzylbutane derivatives which occur in higher plants and in fluids (bile, serum, urine etc) in man and other animals. Mammalian lignans include enterodiol (2,3bis(3-hydroxybenzyl)butane-1,4-diol), and enterolactone (trans2,3-bis(3-hydroxybenzyl)-g-butyrolactone); these compounds, which have a potential anti-cancer role, can be synthesized in vitro by human faecal flora [JAB (1985) 58 37–43]. lignicolous Living or growing in or on wood. Ligniera See PLASMODIOPHOROMYCETES. lignin A complex, variable, hydrophobic, cross-linked, threedimensional aromatic polymer of p-hydroxyphenylpropanoid 440 lipopolysaccharide glycerol moiety. Lipases function well at the interface between their hydrophobic substrates and the aqueous phase, and can act on triglycerides in micelles or emulsions. Microbial lipases have some commercial applications. They are used e.g. in digestive aids, in the manufacture of certain cheeses (to improve or accelerate the development of flavour and aroma) and in INTERESTERIFICATION. Tests for bacterial lipase production: (a) growth on EGG-YOLK AGAR; (b) TWEEN HYDROLYSIS; (c) inoculation onto a nutrient agar medium containing a fat (or e.g. corn oil) and NILE BLUE (lipaseproducing colonies have a blue halo). A given lipase may not react positively in all tests. (See also FILM AND SPOTS.) lipid A See LIPOPOLYSACCHARIDE. lipid A-associated protein See ENDOTOXIN. lipid cyst See METHYLOCOCCACEAE. lipid stain See e.g. BURDON’S STAIN. lipid X See LIPOPOLYSACCHARIDE. lipoamino acids See bacterial CYTOPLASMIC MEMBRANE. lipofection Lipid-aided insertion of nucleic acid into mammalian (and other) cells; uptake is promoted when (e.g.) DNA forms LIPOSOMES with mixed cationic and neutral lipids. lipoic acid (a-lipoic acid; thioctic acid) A coenzyme which is involved in the oxidative decarboxylation of keto-acids: e.g. pyruvate and 2-oxoglutarate are decarboxylated to acetyl-CoA and succinyl-CoA, respectively [see Appendix II(a)]. (See also THIAMINE.) The oxidized form of the coenzyme contains a disulphide bond (−S−S−) which can be reduced to form two −SH groups, the reduced coenzyme (dihydrolipoic acid) being 6,8-dithiooctanoic acid: CH2 SH.CH2 .CHSH.(CH2 )4 .COOH. Lipoic acid is required as a growth factor e.g. by Tetrahymena spp; it can replace acetate as a growth factor for e.g. Lactobacillus casei. (See also ARSENIC.) Lipomyces A genus of yeasts (family SACCHAROMYCETACEAE). Cells are spherical or ovoid; vegetative reproduction occurs by multilateral budding or by bud-fission. In most species the cells are each surrounded by a thick mucoid capsule which often contains a starch-like polysaccharide. Cells accumulate lipid, and those from older cultures usually contain one or more lipid globules. Asci typically develop from ‘active buds’; an active bud resembles a vegetative bud but develops into an ascus – either following conjugation between the active bud and a protuberance on another cell (or the same cell), or in the absence of conjugation. The mother cell of the active bud may also become an ascus. Ascospores are ellipsoidal or oblong-ellipsoidal, light amber to brown, smooth, or e.g. rough with longitudinal ridges, 2–20 or more per ascus (depending on species). [Ascospore morphology and ultrastructure: IJSB (1984) 34 80–86.] The organisms are non-fermentative and do not assimilate NO3 − . Species (L. anomalus, L. kononenkoae, L. lipofer, L. starkeyi, L. tetrasporus) have been isolated from soil. [Book ref. 100, pp. 252–262.] lipo-oligosaccharide (LOS) In certain Gram-negative bacteria (e.g. Neisseria meningitidis): a (normal) component of the OUTER MEMBRANE which is anatomically equivalent to LIPOPOLYSACCHARIDE (q.v.) but in which the O-specific chains are shorter than those (for example) in the enterobacteria [CRM (1998) 24 281–334]. lipopolysaccharide (LPS) Strictly: any lipid-containing polysaccharide; however, the term ‘lipopolysaccharide’ is commonly used to refer specifically to the endotoxic component of the OUTER MEMBRANE in Gram-negative bacteria. Outer membrane LPS is a complex molecule consisting of three covalently linked can agglutinate Escherichia coli and many other Gram-negative bacteria. Limulus amoebocyte lysate test (LAL test) A test used for the detection or quantification of LIPOPOLYSACCHARIDE. The test depends on the ability of LPS to cause gelation of a lysate of the blood cells (amoebocytes) of the horseshoe crab, Limulus polyphemus. Quantities of LPS of the order of 10−9 g/ml can be detected; different types of LPS differ in their ability to cause gelation. Substances other than LPS (e.g. lipoteichoic acids, polynucleotides) can also cause gelation [FEMS (1983) 20 343–346]. lincomycin See LINCOSAMIDES. lincosamides A group of MLS ANTIBIOTICS which contain lincosamine (6-amino-6,8-dideoxyoctose); the group includes e.g. lincomycin (consisting of a substituted pyrrolidine linked to methyl-a-thiolincosamine, produced by Streptomyces lincolnensis), clindamycin (7-chloro-7-deoxylincomycin, a semisynthetic derivative of lincomycin), and celesticetin. Lincomycin is active mainly against Gram-positive bacteria (e.g. staphylococci, streptococci, clostridia, Corynebacterium diphtheriae) and may be useful e.g. against penicillin-resistant strains. Clindamycin is more active than lincomycin against Gram-negative bacteria, and is effective e.g. against most Gram-positive and Gram-negative anaerobic pathogens (including Bacteroides fragilis); it is also active against Plasmodium spp, and has been used (with QUININE) in the treatment of malaria. Lincosamides inhibit PROTEIN SYNTHESIS; they bind to the 50S subunit of 70S ribosomes and appear to block the peptidyltransferase reaction. Antiplasmodial action is presumably due to inhibition of mitochondrial protein synthesis. lincosamine See LINCOSAMIDES. Linderina See KICKXELLALES. line probe assay See PROBE. linezolid See OXAZOLIDINONES. linkage (genetics) The degree to which the alleles of two or more given genes are inherited together. Alleles of unlinked genes exhibit independent assortment and occur e.g. on different chromosomes. Linked genes occur on the same chromosome (or other genetic molecule) and are said to constitute a linkage group; the closer together two genes are on a chromosome, the more tightly linked they are – i.e., the lower is their RECOMBINATION FREQUENCY. (Genes far apart on a chromosome may behave as unlinked genes; in bacteriology, genes on the chromosome are sometimes said to be unlinked if they are merely non-contiguous, regardless of the distance between them.) linked recognition (immunol.) Syn. COGNATE HELP. linker DNA (1) See CHROMATIN. (2) A synthetic oligodeoxyribonucleotide which contains the cleavage site of a particular restriction endonuclease, and which can be ligated to a vector or exogenous DNA fragment prior to CLONING; a linker may serve e.g. to facilitate retrieval of the cloned fragment. (See also POLYLINKER.) linker polypeptide See PHYCOBILISOMES. linking number (a) See DNA. linseed rust Syn. FLAX RUST. lipase (triacylglycerol acylhydrolase; EC 3.1.1.3) A type of enzyme (see ENZYMES) which hydrolyses glycerides to free fatty acids and (ultimately) glycerol. Non-specific lipases release fatty acids from all three positions of the glycerol moiety of a triglyceride – 1,2(2,3)-diglycerides, 1,3-diglycerides and monoglycerides being formed as intermediates. Other lipases release fatty acids only from the outer (1 and 3) positions of the 441 lipopolysaccharide-binding protein (KDO-PEtN) GlcNAc O-specific chain Glc Gal (Gal) (Hep) Glc Hep (PPEtN) (KDO) Hep KDO Lipid A P LIPOPOLYSACCHARIDE. General structure of the core oligosaccharide in Salmonella typhimurium; the structure is somewhat variable, and possible sites of variation are indicated by parenthesis. Glc = D-glucose; Gal = D-galactose; Hep = ‘heptose’ (L-glycero-D-mannoheptose); KDO = 3-deoxy-D-manno-octulosonate; GlcNAc = N-acetyl-D-glucosamine; PEtN = phosphorylethanolamine; PPEtN = pyrophosphorylethanolamine; P = phosphate. [Data from e.g. Book ref. 138, pp. 157–207.] little experimental support.) A proportion of the LPS can be released from enterobacteria by treatment of cells with EDTA. LPS can be extracted from cells with a phenol–water mixture (ca. 45% w/v phenol in water); an LPS–protein complex (the Boivin antigen) can be extracted by treatment of cells with trichloroacetic acid (TCA). (See also LIMULUS AMOEBOCYTE LYSATE TEST.) Biosynthesis of LPS. (Most of the studies on LPS biosynthesis have been carried out with S. typhimurium.) LPS is assembled at the cytoplasmic membrane, and completed LPS molecules are subsequently transferred to the outer membrane – apparently at a limited number of sites which may correspond to ADHESION SITES. Lipid A is built up from glucose 1-phosphate via monosaccharide intermediates such as 2,3-diacylglucosamine 1-phosphate (‘lipid X’) [JBC (1985) 260 15536–15541] and variously substituted glucosamine disaccharide derivatives. [For lipid A biosynthesis see e.g. TIM (1998) 6 154–159.] In S. typhimurium, the core oligosaccharide is synthesized on the lipid A precursor by the sequential addition of KDO (from a CMP precursor) and other sugars (mostly from UDP precursors, although the nature of the heptose precursor is unknown). The oligosaccharide subunits of the O-specific chains are synthesized on BACTOPRENOL carrier molecules by the sequential addition of sugars from nucleotide precursors (presumably) at the inner face of the cytoplasmic membrane; the bactoprenol–oligosaccharide complex is presumed to be translocated across the cytoplasmic membrane, where the subunits are polymerized on the core oligosaccharide of an incomplete LPS molecule. (Bactoprenol pyrophosphate is released and is dephosphorylated by a specific phosphatase, the monophosphate derivative returning to the inner face of the cytoplasmic membrane for another round of synthesis.) Branches on the O-specific chains may be added before or after polymerization: e.g., in S. typhimurium the abequose residue is added to the bactoprenol-linked oligosaccharide, whereas in S. minneapolis and other serogroup E3 (O3, 15, 34) salmonellae various residues in the O-specific chains are glucosylated after polymerization. Inhibition of lipid A biosynthesis is a property of the compound L-573,655; this agent inhibits the enzyme deacetylase (product of the lpxC gene) that plays an essential role in the biosynthesis of lipid A [see e.g. Science (1996) 274 980–982]. The antibiotic BACITRACIN inhibits polymerization of the Ospecific chain. [Reviews: CTMT (1982) 17 79–151; Book ref. 92; LPS nomenclature: JB (1986) 166 699–705.] lipopolysaccharide-binding protein See CD14. lipoprotein signal peptidase See SIGNAL HYPOTHESIS. liposan A bioemulsifier (see BIOSURFACTANTS) produced by Candida lipolytica when growing e.g. on hexadecane [AEM (1985) 50 965–970]. regions: lipid A–core oligosaccharide–O-specific chain. (cf. SMOOTH–ROUGH VARIATION.) Lipid A is a glycolipid which is largely responsible for the endotoxic activity of LPS (see ENDOTOXIN sense 1) and for the ability of LPS to act as a POLYCLONAL ACTIVATOR. In enterobacteria, it consists of a substituted b-linked glucosamine disaccharide – usually (1 → 6)-linked, but apparently (1 → 4)linked e.g. in strains of Escherichia coli ; the disaccharide is fully substituted with a number of long-chain (e.g. C12 – 16 ) saturated fatty acids (including 3-hydroxyacids, usually 3hydroxymyristic acid), phosphate, and the core oligosaccharide (linked to the C-6 position of the non-reducing glucosamine residue). In many enterobacteria the hydroxy fatty acids are 3-O-acylated to form characteristic 3-myristoxymyristic acid residues. Variations in the structure of lipid A occur in other bacteria: e.g., in certain photosynthetic bacteria the glucosamine disaccharide is replaced by a diaminohexose monomer (e.g. 2,3-diamino-2,3-dideoxyglucose in Rhodopseudomonas viridis) [LPS in photosynthetic bacteria: ARM (1979) 33 215–239]; 3-hydroxymyristic acid is absent e.g. in Brucella strains; 2hydroxy fatty acids are present e.g. in Pseudomonas aeruginosa strains. Core oligosaccharide is a complex oligosaccharide usually linked to lipid A via ‘2-keto-3-deoxyoctonate’, KDO (= 3-deoxy-D-manno-octulosonate). In enterobacteria (e.g. S. typhimurium – see figure) cores appear to contain a KDO dimer. E. coli K12 strains appear to have cores similar to that of S. typhimurium, while B strains have cores containing only glucose, heptose, and KDO. In P. aeruginosa the core contains glucose, heptose, KDO, rhamnose, galactosamine and alanine. Certain bacteria (e.g. strains of Bacteroides) appear to lack both KDO and heptose. O-specific chain (O side chain) is the immunodominant part of the LPS molecule in the intact bacterial cell. It consists of a variable-length chain of identical (linear or branched) oligosaccharide subunits; the number of subunits may vary even in the same cell. In Salmonella serotypes each oligosaccharide subunit typically contains 3, 4 or 5 monosaccharide residues (including hexoses, pentoses, 6-deoxyhexoses, and 2,6-dideoxyhexoses). The O-specific chains determine the specificity of the O ANTIGENS of a given serotype, antigenic specificity being determined by the presence of a particular sugar (see e.g. ABEQUOSE, COLITOSE, PARATOSE, or TYVELOSE), by the positions of glycosidic bonds (e.g. 1 → 4 or 1 → 6 linkages), by the anomeric configuration (a or b) of the glycosidic linkages, by substituents (e.g. acetyl groups) on sugar residues, etc. LPS appears to be bound in the outer membrane by hydrophobic bonds and by divalent cations. (Early reports of pyrophosphate bridges between lipid A moieties appear to have received 442 listeriosis liposarcoma See SARCOMA. liposome An artificial phospholipid vesicle. Liposomes have many uses in biology. For example, various components of biological membranes (e.g. PORINS, viral ENVELOPE proteins) can be incorporated into liposomes, allowing investigation of their properties under controlled conditions. Enzymes can be immobilized (see IMMOBILIZATION) by encapsulation within liposomes. Certain drugs may be encapsulated within liposomes e.g. to protect them from metabolic degradation and/or to prevent them from exerting general toxicity when delivered into the bloodstream; the liposomes fuse with or are endocytosed by cells of the reticuloendothelial system, where they release their contents. [Potential of liposome-mediated antiviral therapy: Antiviral Res. (1985) 5 179–190.] Drug-loaded liposomes can be ‘targeted’ to specific cells by the incorporation of antibodies to antigens in the target cell surface; thus, e.g., ‘immunoliposomes’ incorporating a monoclonal antibody to herpes simplex virus glycoprotein D were found to bind specifically to HSV-infected rabbit corneal cells in vitro [J. Imm. (1986) 136 681–685]. liposome swelling assay A method for determining the rate of diffusion of ions or molecules through PORIN channels; purified porins are incorporated into LIPOSOMES, and the rate of influx of specified ions or molecules is estimated from the effects of the (osmotically-induced) influx of water. [Method: JB (1983) 153 241–252.] lipoteichoic acid See TEICHOIC ACIDS. Lipschütz body An intranuclear inclusion body formed (late in infection) in cells infected with e.g. herpes simplex virus type 1 or 2. lirella (lirelliform apothecium) (lichenol.) A slit-like (long, narrow) apothecium; lirellae may appear as black branched or unbranched lines with (in Graphis spp) or without (in Opegrapha spp) raised margins. lirelliform apothecium Syn. LIRELLA. lissamine rhodamine (acid rhodamine B; also: sulphorhodamine B) A red FLUOROCHROME (fluorescence: orange-red) used for labelling proteins; it is initially treated with phosphorus pentachloride and acetone to form the highly reactive sulphonyl chloride derivative, and protein–dye conjugation is then effected by a sulphamido condensation. Listeria A genus of Gram-positive, asporogenous, aerobic/ facultatively anaerobic bacteria. Cells: rods or coccobacilli (ca. 0.5 × 0.5–2 µm), occurring singly or in small groups (‘Chinese letter’ arrangements), palisades or filaments. Optimum temperature range: 25–37° C; growth occurs slowly at 4° C. Most strains catalase +ve. Oxidase −ve. Indole −ve. Aesculin is hydrolysed. Sugars are fermented to acid (no gas). Generally motile. Colonies of L. monocytogenes: grey, translucent, about 1 mm in diameter after 24–48 hours; a characteristic bluish colour in obliquely transmitted light. L. monocytogenes may give clear haemolysis on e.g. horse- and/or sheep-blood agar. Listeria spp occur (apparently as saprotrophs) e.g. in soil, decaying plant matter, silage, etc., and have been isolated from e.g. faeces of healthy humans and animals; some haemolytic strains of Listeria can be pathogenic in man and also in animals (see LISTERIOSIS and MENINGITIS). [Wild birds and silage as reservoirs of Listeria in the agricultural environment: JAB (1985) 59 537–543.] [The pathogenicity of L. monocytogenes (a public health perspective): RMM (1997) 8 1–14.] Some (human) infections with L. monocytogenes have been linked to the ingestion of certain types of cheese. One approach to the control of L. monocytogenes in cheese (during manufacture) involves the use of specific types of starter culture [see e.g. AEM (1998) 64 4842–4845; JAM (1999) 86 251–256]. Studies of e.g. DNA relatedness among strains of L. monocytogenes sensu lato have established five distinct ‘genomic groups’, each of which has been given a species designation. Group 1 contains the type strain of L. monocytogenes and related strains, and corresponds to L. monocytogenes sensu stricto. Group 2 (L. monocytogenes serovar 5) has been named L. ivanovii [IJSB (1984) 34 336–337]. Group 3 (now L. innocua) includes non-haemolytic, non-pathogenic strains. Groups 4 and 5 (now L. welshimeri and L. seeligeri, respectively [IJSB (1983) 33 866–869]) also include strains which are (apparently) non-pathogenic. L. grayi and L. murrayi have been retained in the genus [IJSB (1987) 37 298–300]; L. denitrificans is now in Jonesia [IJSB (1987) 37 266–270]. [Taxonomy of Listeria: IJSB (1991) 41 59–64, (1993) 43 26–31.] [Typing of L. monocytogenes by repetitive element sequencebased PCR: JCM (1999) 37 103–109.] [Culture etc: Book ref. 46, pp. 1680–1687.] listeriolysin See THIOL-ACTIVATED CYTOLYSINS. listeriosis Any disease of man or animals caused by Listeria monocytogenes. In man, the groups commonly affected are pregnant women (particularly during the third trimester), infants, immunosuppressed adults and the elderly. In pregnant women, the disease may involve e.g. fever, chills, backache and flu-like symptoms; while the mother herself is usually not severely affected, intrauterine infection may lead to premature birth, stillbirth or abortion. (Note that L. monocytogenes is one of the few bacterial pathogens which can cross the blood–brain barrier and the placenta.) Early-onset neonatal disease (i.e. within ∼2 days) may be largely ‘septicaemic’, with disseminated infection, cardiopulmonary and CNS problems; mortality may be ∼50–60%. Late-onset neonatal disease (starting after ∼5–6 days) typically involves meningitis; compared with early-onset disease, mortality is somewhat lower. In adults, the common manifestations are CNS involvement, septicaemia and endocarditis. L. monocytogenes can invade intestinal epithelium, invasion being promoted by the bacterial cell-surface INVASIN internalin A (encoded by gene inlA); the eukaryotic cell-surface receptor for internalin A is a CADHERIN (E-cadherin) which occurs on basolateral surfaces of epithelial cells. Interaction between internalin A and E-cadherin apparently initiates uptake of the bacterium by the host cell. (Interestingly, even latex beads coated with internalin A are internalized by some types of mammalian cell [Inf. Immun. (1997) 65 5309–5319].) Invasion (uptake of the pathogen) resembles the ‘zipper’ mechanism involved in the uptake of Yersinia enterocolitica (see Y. enterocolitica under FOOD POISONING). L. monocytogenes escapes from the phagosome by secreting a pore-forming toxin, listeriolysin O (a THIOL-ACTIVATED CYTOLYSIN). Once free in the host cell’s cytoplasm, the pathogen grows and spreads to adjacent cells by actin-based motility (as described for Shigella dysenteriae in DYSENTERY); in L. monocytogenes, the ActA protein plays the same role as IcsA in Shigella. When propelled into an adjacent cell, the pathogen becomes enclosed by a double membrane (one membrane from each cell); escape (into the cytoplasm of the adjacent cell) involves secretion of the enzyme lecithinase (encoded by gene plcB ) – which hydrolyses the lecithin content of the double membrane. [Invasion and actin-based motility: EMBO (1998) 17 3797–3806. Internalin–E-cadherin complex (structure): Cell (2002) 111 825–836. Pathogenicity of L. monocytogenes (review): RMM (1997) 8 1–14.] Infection appears to occur primarily via food; foods which have been implicated include e.g. Mexican-style cheese, certain 443 Listonella littoral A term used with different meanings by different authors; thus, the littoral region of the marine benthic zone is variously regarded as e.g. (a) the ‘intertidal region’, i.e., the zone which is uncovered at low tide and under water at high tide; (b) a zone below the average low-tide mark which is uncovered only occasionally; (c) that region between the high-tide mark and a depth of 200 metres. In freshwater environments the littoral region may refer to those parts of the benthic zone which occur in shallower waters of unspecified depth. LIV-I binding protein See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. live vaccine Any VACCINE containing viable, usually attenuated, microorganisms (see e.g. BCG and STERNE STRAIN). liver of sulphur See SULPHUR. liver-stage-antigen 1 See MALARIA. liverwort symbioses See e.g. NOSTOC and MYCORRHIZA. lividomycin Either of two AMINOGLYCOSIDE ANTIBIOTICS (lividomycins A, B), both of which may be regarded as derivatives of NEOMYCIN B. Lk Linking number: see DNA. LktA (of Pasteurella haemolytica) See RTX TOXINS. LMD Culture Collection, Laboratory of Microbiology, Julianalaan 67A, 2628 BC Delft, The Netherlands. lmp genes (EBV) See EPSTEIN–BARR VIRUS. LmrA (Lactococcus lactis) See ABC TRANSPORTER. lobar pneumonia See PNEUMONIA. Lobaria A genus of foliose LICHENS (order PELTIGERALES). Photobiont: Myrmecia (in e.g. L. pulmonaria) or Nostoc (in e.g. L. scrobiculata); Myrmecia-containing species may have Nostoc-containing cephalodia – cephalodia being fruticose in L. amplissima (see CEPHALODIUM). Thallus: large (e.g. up to ca. 20 cm across), lobed, both surfaces corticate; the underside is usually more or less tomentose, often with glabrous patches, and in L. linita and L. pulmonaria the upper surface is coarsely reticulate. Soredia may be present in e.g. L. pulmonaria and L. scrobiculata. Apothecia: lecanorine, usually with reddish or brown discs, abundant (in e.g. L. laetevirens) or scarce. Found on rocks, bark etc, and generally very sensitive to air pollution. Loboa A genus of fungi (? ENTOMOPHTHORALES). L. loboi causes LOBOMYCOSIS and has not been cultured in vitro. In tissues, it occurs as spherical, thick-walled, hyaline, yeast-like cells (ca. 5–12 µm diam.) which reproduce by budding; sequential budding may lead to the formation of chains of cells. lobomycosis (keloidal blastomycosis) A MYCOSIS, which apparently affects only man and dolphins, caused by Loboa loboi ; it occurs in Central and South America. Infection is presumed to occur via wounds; chronic, nodular, keloidal (elevated and scarlike) skin lesions develop chiefly (in man) on exposed areas of the body. lobopodium See PSEUDOPODIUM. lobose pseudopodium See PSEUDOPODIUM. Lobosea A class of naked and testate amoebae (superclass RHIZOPODA) which form either lobopodia or filiform subpseudopodia from a broader hyaline lobe. The cells are typically uninucleate; the few multinucleate members do not form plasmodia. Fruiting structures do not occur, but many members can form resistant cysts. Orders: AMOEBIDA, ARCELLINIDA, PELOBIONTIDA, SCHIZOPYRENIDA, TRICHOSIDA. lobster diseases See e.g. BURNED SPOT DISEASE and GAFFKAEMIA. local lesion (plant virol.) A discrete lesion (e.g. a patch of chlorosis or necrosis, or a RINGSPOT) which develops as a result of localized viral infection in a leaf, stem etc. A plant which readily develops local lesions when infected with a given virus other cheeses, coleslaw and milk. However, risk assessment is problematic as isolates of L. monocytogenes vary widely in their pathogenic potential, and there is no simple test for pathogenicity. A DNA-based method has been used to type 64 isolates of L. monocytogenes from human, animal and food sources; the strains clustered primarily according to origin, and, on the basis of the results, it was suggested that only a small proportion of food-borne strains may be pathogenic for humans [JCM (1999) 37 103–109]. In animals, outbreaks may be associated with the consumption of incompletely fermented SILAGE (pH > 5.5); L. monocytogenes cannot multiply in properly made silage (pH 4–4.5). The disease may affect e.g. cattle, sheep and goats, and may lead to manifestations such as abortion. The disease is more common in temperate regions. Listonella A genus of Gram-negative bacteria that includes the species L. anguillarum, L. damsela and L. pelagia (formerly Vibrio anguillarum, V. damsela and V. pelagia, respectively) [SAAM (1985) 6 171–182; validation of names: IJSB (1986) 36 354–356]. (See also PHOTOBACTERIUM.) L. anguillarum is an important pathogen of freshwater and marine fish (causing e.g. disease in salmonids, and RED PEST in eels); it is also pathogenic for oyster larvae. The organism is facultatively halophilic, capable of growing on nutrient agar with 0.5–6.0% NaCl, but not with 8% NaCl. Oxidase +ve. Sucrose (usually) +ve. [Agglutination typing of Vibrio (Listonella) anguillarum: AEM (1984) 47 1261–1265.] lithobiont A SAXICOLOUS organism. lithocholic acid See BILE ACIDS. Lithophyllum See RHODOPHYTA. lithophyte Refers to a SAXICOLOUS plant. lithotroph An organism which uses an inorganic substance as substrate in energy metabolism. (cf. ORGANOTROPH; AUTOTROPH.) Among chemolithotrophs (see CHEMOTROPH) energy may be obtained by the oxidation of e.g. NH3 (see NITRIFICATION), H2 (see e.g. HYDROGEN-OXIDIZING BACTERIA, DISSIMILATORY SULPHATE REDUCTION), sulphur, sulphide and/or thiosulphate (see e.g. THIOBACILLUS), or Fe2+ (e.g. in Thiobacillus ferrooxidans). Photolithotrophs use inorganic substrates (e.g. water, sulphur, sulphide, H2 , or thiosulphate) as electron donors in PHOTOSYNTHESIS. litmus A PH INDICATOR: pH 4.5–8.3 (red to blue); it is obtained from lichens (Roccella spp) and is a complex mixture of compounds. litmus milk A bacteriological medium which consists of skim milk (whole MILK without cream) containing sufficient litmus solution to impart a blue-purple colour. Prior to use, the medium is either steamed (20/30 min on each of 3 successive days) or autoclaved (115° C/10 min), and the pH is adjusted to 7. A given bacterial strain growing in litmus milk may give one of the following reactions. (a) No visible change in the medium. (b) Acid production from lactose (indicated by the LITMUS). (c) Alkali production – commonly due to hydrolysis of casein. (d) Reduction (decolorization) of the litmus. (e) The production of an acid clot; the clot, which is soluble in alkali, forms at about pH5 (at or near the IEPs of the various types of CASEIN). The production of acid and gas may give rise to a STORMY CLOT. (f) The formation of a clot at or near pH7. Such clots are formed by certain bacteria (e.g. Bacillus spp) which produce rennin-like proteases; the litmus may be reduced before or during clot formation, and peptonization (digestion of the clot) may subsequently occur. (See also MILK SPOILAGE.) little t antigen (small t antigen) See POLYOMAVIRUS. 444 low-temperature steam disinfection formed into a closed loop at the free end. Before and after each use the loop and adjacent wire are sterilized by FLAMING and allowed to cool. In use, the sterile loop is brought into contact with e.g. a colony so that a small quantity of material adheres to it; this can then be used as an INOCULUM. If a sterile loop is dipped into e.g. a broth CULTURE and withdrawn, the loop retains a circular film of liquid (containing microorganisms) which can be used as an inoculum; the volume of liquid may be varied by using loops of different sizes. In use, no part of the loop handle should enter the culture vessel; loops with longer wires may be used e.g. for inoculating media in bottles. (See also STRAIGHT WIRE; STREAKING.) loop fermenter A FERMENTER in which the culture is continually and efficiently circulated from the bottom of the column to the top – ‘loop’ referring to the circulatory nature of the flow. Most loop fermenters consist of a vertical cylindrical column containing a coaxial DRAFT TUBE (see e.g. AIRLIFT FERMENTER; JET LOOP FERMENTER; PROPELLER FERMENTER). loop model (of proton translocation) A model, based on the concept of VECTORIAL GROUP TRANSLOCATION, which proposes that electrons travel in a loop within an energy-transducing membrane, starting and finishing at the same surface of the membrane, while protons pass to the opposite side of the membrane. In this model, the binding and release of protons at certain sites in an ELECTRON TRANSPORT CHAIN is seen as a direct consequence of the variation in pK of those sites due to their reduction or oxidation during electron flow. looping (DNA) (transcriptional control) See OPERATOR. loose smut See SMUTS (sense 2). LOPAT Levan formation, oxidase reaction, potato rotting capacity, arginine dihydrolase production, and tobacco sensitivity: criteria which have been used for classifying plant-pathogenic fluorescent pseudomonads. lophine See CHEMILUMINESCENCE. Lophomonas See HYPERMASTIGIDA. lophotrichous Of e.g. flagella: arranged as a tuft at one or both poles of a cell. Lordsdale virus A SMALL ROUND STRUCTURED VIRUS. lorica In certain eukaryotic microorganisms – including certain ciliates (e.g. TINTINNINA), choanoflagellates (see CHOANOFLAGELLIDA), CHRYSOPHYTES, and EUGLENOID FLAGELLATES: a vase-like or bottle-like, commonly mineralized, loose-fitting shell which partially or completely surrounds the cell. (cf. TEST.) Loricate organisms may be free-swimming (e.g. TRACHELOMONAS) or sedentary (e.g. DINOBRYON). loricate Possessing a LORICA. LOS LIPO-OLIGOSACCHARIDE. loss mutant (loss variant) A variety of an organism which has lost one or more phenotypic traits as a result of mutation(s). lotic Pertaining to aquatic habitats with running water (e.g. streams, rivers). (cf. LENTIC.) louping-ill (ovine encephalomyelitis) An acute, sometimes fatal, usually tick-borne encephalomyelitis which affects mainly sheep; the causal agent is a flavivirus (see FLAVIVIRIDAE), and the tick vector is probably always Ixodes ricinus. Incubation period: ca. 1–2 weeks; viraemia and fever are followed by e.g. leaping (‘louping’) movements and paralysis. Death may occur in ca. 1 week. The louping-ill virus can also cause a fatal infection in Red grouse (Lagopus lagopus scoticus). Infection of cattle may result in a disease similar to that in sheep. In man, the virus can cause a mild, febrile, influenza-like illness. low temperature disease See COLD WATER DISEASE. low-temperature steam disinfection A process in which e.g. blankets, endoscopes and other types of hospital equipment can may be used for assaying that virus, and is termed a local lesion host (= assay host ); the number of infective virus particles present in a preparation can be assessed from the number of lesions produced when the leaves of a suitable local lesion host are inoculated with a known volume of the preparation. (See also PLANT VIRUSES.) Locke’s solution A solution of inorganic salts used e.g. as a liquid overlay for certain solid media. [Recipe: Book ref. 53, p. 2175.] lockjaw Syn. TETANUS. locomotor fringe Syn. TROCHAL BAND. locule (loculus) A small chamber or cavity: see e.g. ASCOSTROMA and FORAMINIFERIDA. Loculoascomycetes A class of ascomycetes characterized by the formation of bitunicate asci in ascostromata; the genera include e.g. Elsinoë, Mycosphaerella, Myriangium and Venturia. The class is not recognized in most modern taxonomic schemes. locus (genetics) The position of a given gene, operon, mutation etc on a chromosome, plasmid, or other genetic molecule. locus of enterocyte effacement See PATHOGENICITY ISLAND. Lodderomyces A genus of yeasts (family SACCHAROMYCETACEAE). Cells are spheroidal, ellipsoidal, or cylindrical, and reproduce by multilateral budding; pseudomycelium may be formed. Asci are persistent. Ascospores: smooth, oblong to fusiform, 1 or 2 per ascus. One species, L. elongisporus, isolated e.g. from fruit juice and soil. [Book ref. 100, pp. 263–265.] lodging (plant pathol.) The collapse of cereal plants due to bending or breaking of the stems; lodging may occur in certain diseases. Loeffler’s methylene blue See METHYLENE BLUE. Loeffler’s serum A solid medium, usually prepared as a slope, used e.g. for the culture of Corynebacterium diphtheriae. The medium contains sterile serum and nutrient broth (3:1 by volume) and glucose (ca. 0.25–0.5% w/v); this mixture may be coagulated by TYNDALLIZATION or, if precautions are taken to prevent foaming, by autoclaving. log dilutions See SERIAL DILUTIONS. log phase (of growth) See BATCH CULTURE. logarithmic phase (of growth) See BATCH CULTURE. Lolium enation virus See FIJIVIRUS. lomasomes Membranous tubular, vesicular or sac-like structures which occur, generally between the cell wall and cytoplasmic membrane, in many fungi; they may be formed by invagination of the cytoplasmic membrane, and often contain particulate matter. The function of lomasomes is unknown; they have been implicated e.g. in cell wall synthesis. lon gene In Escherichia coli, a gene which encodes an ATPdependent protease that e.g. inactivates the SulA (= SfiA) protein (see SOS SYTEM); the Lon protein is a HEAT-SHOCK PROTEIN. long-incubation hepatitis Syn. HEPATITIS B. longitudinal binary fission See BINARY FISSION. longus A large plasmid-encoded fimbria∗ (up to 20 µm in length) formed by strains of ETEC. The appendage consists of a repeated 22 kDa subunit, the N-terminal sequence of which shows some homology with that of the subunits of various type IV FIMBRIAE (including the TCP of Vibrio cholerae and ‘bundle-forming pili’ of EPEC). [Mol. Microbiol. (1994) 12 71–82. ∗ In this paper the appendage is referred to as a ‘pilus’.] lonomycin See MACROTETRALIDES. loop An instrument widely used in bacteriology and mycology for the manipulation of small quantities of solid or liquid material – e.g. during INOCULATION; the rod-shaped metal handle holds a piece of platinum or nickel–steel wire (3–10 cm long) 445 low-temperature steam–formaldehyde sterilization be disinfected by exposure to saturated steam below 100° C at subatmospheric pressure in an apparatus resembling a vacuum AUTOCLAVE. Essentially, the chamber – containing the load – is first evacuated to an absolute pressure of ca. 2 kPa or less; steam is then admitted to the chamber and held at a temperature of ca. 73–80° C for 10–15 min – the absolute pressure in the chamber being ca. 39–47 kPa; saturated steam at these temperatures disinfects more efficiently than does water at corresponding temperatures. The load is then dried under vacuum. low-temperature steam–formaldehyde sterilization (LTSFS) A process which resembles the LOW-TEMPERATURE STEAM DISINFECTION process but which incorporates the use of FORMALDEHYDE. One of several possible schedules is as follows. The chamber is evacuated and formaldehyde is admitted; this step is carried out three times. Subsequently, formaldehyde, and then steam, are admitted to the chamber, and a temperature of ca. 73° C is maintained for two hours. The chamber is then evacuated, steam is admitted, and the chamber is again evacuated; this last evacuation removes traces of formaldehyde and steam and gives rise to a dry, odourless load. [Biological indicators for LTSFS: JAB (1985) 58 207–214.] low zone tolerance See IMMUNOLOGICAL TOLERANCE. Löwenstein–Jensen medium A complex solid medium used for isolating Mycobacterium spp from e.g. sputum. The medium is prepared by the INSPISSATION of a mixture of homogenized whole hens’ eggs and a solution containing asparagine, KH2 PO4 , MgSO4 , citrate, glycerol, potato starch, and malachite green; the pH is ca. 7.0. The medium is prepared (as slopes) in screw-cap bijou or universal bottles. [Recipe: Book ref. 53, p. 1415.] lower fungi Fungi of the MASTIGOMYCOTINA and the ZYGOMYCOTINA (collectively: the phycomycetes). For some authors ‘lower fungi’ includes the MYXOMYCOTA as well as the phycomycetes. lox site See BACTERIOPHAGE P1. Loxocephalus See SCUTICOCILIATIDA and PARATENY. Loxodes A genus of freshwater ciliates (subclass GYMNOSTOMATIA) related to KENTROPHOROS. Cells: somewhat flattened, elliptical in outline, with a slit-like cytostome located in a subapical concavity (giving the anterior end of the cell a hook-like appearance). The cytoplasm contains MÜLLER’S VESICLES. [Photobehaviour of Loxodes: JP (1986) 33 139–145.] Loxophyllum See GYMNOSTOMATIA. LP system LACTOPEROXIDASE–THIOCYANATE–HYDROGEN PEROXIDE SYSTEM. lpp gene In Escherichia coli : the gene which encodes the BRAUN LIPOPROTEIN. (See also MRNA (a).) LPP group A category of filamentous CYANOBACTERIA (section III) which includes the poorly defined genera LYNGBYA, PHORMIDIUM and PLECTONEMA, as well as MICROCOLEUS and Schizothrix. In LPP group A the trichomes are straight, sheathed and non-motile, and are composed of disc-shaped cells; motile hormogonia are produced. In LPP group B the trichomes are straight, sheathed or non-sheathed, motile or non-motile, and are composed of isodiametric or cylindrical cells. GC% for the group: 42–67. Species occur in freshwater and marine habitats. (See also STROMATOLITE and EMULCYAN.) LPS (1) LIPOPOLYSACCHARIDE. (2) LACTOPEROXIDASE–THIOCYANATE–HYDROGEN PEROXIDE SYSTEM. lpxC gene See ZINC and LIPOPOLYSACCHARIDE. Lrp In Escherichia coli : leucine-responsive regulator protein, a homodimeric protein involved in the regulation of a range of genes/operons – apparently by acting as an activator or (in some cases) repressor of transcription; the activity of Lrp is inhibited by exogenous leucine. Lrp is reported to be one of several factors which regulate expression of Es70 (see SIGMA FACTOR). [Lrp: JBC (2002) 277 40309–40323.] LS binding protein See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. LSD See ERGOT ALKALOIDS. LT See ETEC. LTA LipoTEICHOIC ACID. LTH See PASTEURIZATION. LTLT See PASTEURIZATION. LTR Long terminal repeat: see RETROVIRIDAE. LuIII virus See PARVOVIRUS. lubimin A sesquiterpenoid PHYTOALEXIN produced by the potato plant (Solanum tuberosum). lucerne enation virus See RHABDOVIRIDAE. lucerne transient streak virus (LTSV) See VELVET TOBACCO MOTTLE VIRUS and VIRUSOID. Lucibacterium An obsolete bacterial genus; L. harveyi is now Vibrio harveyi (see VIBRIO). luciferase See BIOLUMINESCENCE. luciferin See BIOLUMINESCENCE. lucigenin See CHEMILUMINESCENCE. Lucio phenomenon See LEPROSY. Lucké virus See HERPESVIRIDAE. Ludwig’s angina An inflammatory, purulent condition of tissues around the submaxillary gland due to infection by oral bacteria – usually streptococci. luer fitting A type of joint between tubular parts of two items of apparatus: one part is tapered and fits coaxially into the other (correspondingly flared) part, forming a rigid, airtight joint. Lugol’s iodine Iodine (5% w/v) and potassium iodide (10% w/v) in distilled water. Diluted Lugol’s iodine is used e.g. in the GRAM STAIN, and as a stain for starch granules, cysts of Entamoeba histolytica, etc. lumber spoilage See TIMBER SPOILAGE. lumicolchicine See COLCHICINE. luminescence See CHEMILUMINESCENCE and BIOLUMINESCENCE. luminol The compound 5-amino-2,3-dihydro-1,4-phthalazinedione which emits light when exposed to unstable oxygen species (e.g. O2̇ − , H2 O2 , OH·); it is used e.g. as a probe (‘chemiluminogenic probe’) in studies on CHEMILUMINESCENCE – luminol either acting as the sole light emitter or supplementing natural light emitter(s). Lumi-Phos 530 A dioxetane substrate (Lumigen, Detroit, USA) which yields chemiluminescence when cleaved by alkaline phosphatase. lumper See TAXONOMY. lumpy jaw See ACTINOMYCOSIS (1). lumpy skin disease Either of two infectious diseases of cattle characterized by the sudden development of flattened, firm, intradermal nodules (up to ca. 4 cm in diameter), regional or generalized lymphadenitis, oedema and dysgalactia; lesions may also occur on the nasal and buccal mucosae. The more severe of the two diseases (knopvelsiekte, found in various parts of Africa) is caused by the ‘Neethling virus’ – a Capripoxvirus [genome: JV (2001) 75 7122–7130], while the milder disease (pseudolumpyskin disease) is caused by a member of the Alphaherpesvirinae apparently identical to bovine mammilitis virus; these diseases may be transmitted by insect vectors. (cf. ALLERTON DISEASE.) lumpy wool (‘mycotic dermatitis’) A SHEEP DISEASE, caused by Dermatophilus congolensis, characterized by skin lesions and matting of the wool. Infection appears to occur via minor 446 Lyme disease LV agar See EGG-YOLK AGAR. lyases ENZYMES (EC class 4) which catalyse either the nonhydrolytic removal of a group from a substrate with the resulting (sometimes transient) formation of a double bond, or the reverse reaction; when the reverse reaction is the more significant (physiologically) the enzyme is often called a SYNTHASE. Subclasses are recognized on the basis of the nature of the bond split: e.g., C−C, C−O or C−N. Lyases include e.g. decarboxylases, aldolases and dehydratases. (See e.g. HYALURONATE LYASE and PECTIC ENZYMES.) Lyb antigens In mice: antigens which occur on certain mature B lymphocytes. Lyb 5+ cells can respond to polyclonal B cell activators (e.g. LPS) and also to soluble polysaccharide antigens (e.g. certain bacterial capsular antigens); Lyb 5− cells can respond to polyclonal B cell activators but not to soluble polysaccharides. Lycogala A genus of slime moulds (class MYXOMYCETES) in which the fruiting bodies are aethalia. L. epidendrum – a common species on rotting logs – forms clusters of hemispherical, puff-ball-like, pink to yellowish-brown aethalia, each ca. 0.3–1.5 cm diam. lycomarasmin A TOXIN produced by certain strains of Fusarium oxysporum (e.g. some strains of F. oxysporum f. sp. lycopersici, the causal agent of FUSARIUM WILT in tomatoes). It chelates metal ions, and at high concentrations can cause wilting in plants; however, its role in pathogenesis – if any – is unknown. lycopene See CAROTENOIDS. Lycoperdales An order of fungi (class GASTEROMYCETES) in which the basidiocarp consists of highly convoluted, hymeniumbearing fertile tissue enclosed by a peridium composed of two or more layers; the organisms are characteristically humicolous or lignicolous saprotrophs. The so-called ‘earth stars’ (e.g. Geastrum) form globose or onion-shaped basidiocarps ca. 2–10 cm in diameter. When the basidiocarp is mature, the exoperidium (the outer layer(s) of the peridium) ruptures and peels back to form a stellate fringe; in Geastrum, an apical pore develops in the remaining peridium (endoperidium) to permit spore release. (In Myriostoma coliformis many pores develop in the endoperidium.) The (non-stipitate) ‘puffballs’ include species of Bovista, Calvatia and Lycoperdon. In Calvatia spore release depends on peridial rupture and disintegration; C. gigantea (the giant puffball) may reach ca. 1 metre across. In Lycoperdon the spores are released via a pore which develops in the peridium. Lycoperdon See LYCOPERDALES. lycoperdon nut A hypogean fruiting body of a member of the Elaphomycetales. Lyell’s syndrome Syn. SCALDED SKIN SYNDROME. Lyme disease (Lyme borreliosis) A disease of humans (and e.g. canines [VR (1995) 136 244–247]) caused by Borrelia burgdorferi (see BORRELIA) and transmitted by the bite of (mainly ixodid) ticks. Symptoms (in humans): typically, an initial rash (erythema migrans, EM) spreading from the tick bite, followed (after days/weeks) by neurological/musculoskeletal/cardiac signs and, later, joint/neurological/skin symptoms; symptoms vary in both nature and timing. Diagnosis: mainly serological (ELISA) (seroconversion after ∼4 weeks); detection of spirochaetes e.g. by culture or PCR examination of blood. Chemotherapy: e.g. ampicillin (during EM), ceftriaxone (for arthritis, cardiac infection). Failure of chemotherapy may be due to inaccessibility to antibiotics of intracellular spirochaetes [see e.g. AAC (1996) 40 1552–1554]. abrasions or other lesions – particularly when the skin is wet or damp. (cf. DERMATOPHILOSIS.) lunar caustic See SILVER. lunate Crescent-shaped. Lundegårdh-type respiration A type of respiration in which electrons, derived from the substrate, are translocated across a membrane [Arkiv för Botanik (Stockholm) (1945) 32A (No. 12) 1–139 (23–49)]. (cf. EXTRACYTOPLASMIC OXIDATION.) lung oedema factor (vet.) See 4-IPOMEANOL. lupinosis (vet.) A MYCOTOXICOSIS, involving liver damage with jaundice, caused by ingestion of lupin plants (Lupinus spp) infected with (or supporting the saprotrophic growth of) Phomopsis leptostromiformis. The disease occurs mainly in sheep; other animals (e.g. cattle) may be affected but are less commonly fed lupins. Lupinosis may follow consumption of living plants or lupin hay, and either sweet or bitter lupin cultivars may be involved; the condition is distinct from ‘lupin poisoning’ caused by alkaloids present in bitter cultivars. In lupins (especially L. luteus), P. leptostromiformis causes ‘stem blight’ (‘stem spot’), a disease characterized by brownish, sunken, linear lesions on the stem, circular lesions on the pods, and brownish discolorations on the seeds. The fungus can grow saprotrophically on dead lupin plant material and on mature living plants. Luria–Bertani broth A medium containing (per litre): 10 g tryptone, 5 g yeast extract and 10 g (or 5 g) of sodium chloride (NaCl); the pH is adjusted to 7.5 (or 7) with sodium hydroxide (NaOH). (The alternative values indicate variation in different sources of information.) Luria–Delbrück fluctuation test See FLUCTUATION TEST. luteic acid A water-soluble, acidic, extracellular, (1 → 6)-blinked glucan containing malonic acid residues; it is produced by ‘Penicillium luteum’. lutein See CAROTENOIDS. luteose A b-(1 → 6)-linked glucan produced by ‘Penicillium luteum’. luteoskyrin See YELLOW RICE. luteoviruses (barley yellow dwarf virus group) A group of PLANT VIRUSES in which the virions are icosahedral (ca. 25–30 nm diam.), each containing a single molecule of linear, positive-sense ssRNA (MWt. ca. 2 × 106 ). Luteoviruses can infect various monocotyledonous and/or dicotyledonous plants, and many can cause important economic losses in crop plants; transmission occurs persistently (circulatively) via aphids, and weeds may provide important reservoirs of infection. Virus replication is confined to the phloem tissues of the host plant. Symptoms of infection typically include severe stunting with yellowing or reddening of the leaves (which may also become brittle); the severity of the symptoms depends on the virus, species and cultivar of host plant, and environmental conditions. Type member: barley yellow dwarf virus (BYDV), MAV isolate. Other members include e.g. beet mild yellowing virus, beet western yellows virus (BWYV), carrot red leaf virus, legume yellows virus, pea leaf roll virus, potato leaf roll virus, soybean dwarf virus, subterranean clover red leaf virus, tobacco necrotic dwarf virus, turnip yellows virus. Probable members: milk-vetch dwarf virus, subterranean clover stunt virus, and tomato yellow top virus. Possible members include e.g. banana bunchy top virus, beet yellow net virus, celery yellow spot virus, cotton anthocyanosis virus, millet red leaf virus, raspberry leaf curl virus, tobacco vein distorting virus, tobacco yellow net virus, tomato yellow net virus. lux operon See BIOLUMINESCENCE. luxS gene See QUORUM SENSING. 447 lymphadenitis basis of the predominant cell type involved and on the degree of differentiation of the neoplastic tissue. (See also BURKITT’S LYMPHOMA, BOVINE VIRAL LEUKOSIS, MAREK’S DISEASE.) lymphomatosis A condition in which multiple lymphomas are formed. lymphon An individual’s entire immune system, including e.g. lymphoid tissues, LYMPHOCYTES, MACROPHAGES, and the COMPLEMENT system. lymphopoiesis The development of LYMPHOCYTES. lymphoproliferative virus group See GAMMAHERPESVIRINAE. lymphosarcoma See SARCOMA. lymphotactin (chemokine) See CHEMOKINES. lymphotoxin a Syn. Tumour necrosis factor-b (TNF-b). lymphotoxin b A membrane-anchored glycoprotein related to, but distinct from, TNF-a and TNF-b. lymphotropic virus group See GAMMAHERPESVIRINAE. lymphotropism See TROPISM (sense 2). Lyngbya A genus of freshwater and marine cyanobacteria of the LPP GROUP. L. majuscula is responsible for causing dermatitis in swimmers (swimmers’ itch) e.g. in waters off the coasts of Hawaii and Japan; this species can produce several toxins, including the brominated phenolic aplysiatoxin, various derivatives of aplysiatoxin (e.g. debromo-, bromo and dibromoaplysiatoxin [structures: PAC (1986) 58 339–350 (345)]), the indole alkaloid lyngbyatoxin A, etc. (See also GEOSMIN and PHORMIDIUM.) lyngbyatoxin See LYNGBYA. lyophilization Syn. FREEZE-DRYING. lysate The product of cell lysis. lysergic acid diethylamide See ERGOT ALKALOIDS. lysigenous (lysigenic) Syn. LYSOGENOUS. lysin An antibody or other entity which, under appropriate conditions, can cause the lysis (rupture) of cells. See e.g. HAEMOLYSIN. lysine biosynthesis See AMINOADIPIC ACID PATHWAY and DIAMINOPIMELIC ACID PATHWAY. lysine decarboxylase test See DECARBOXYLASE TESTS. lysis from without Lysis of a host cell when large numbers of virions adsorb to the host cell surface. (See e.g. T-EVEN PHAGES.) lysis inhibition In certain bacteriophages (e.g. T-even phages, RNA phages): a delay in the lysis of an infected host cell on superinfection of the cell with another phage of the same strain. Mechanism: unknown. Lysis inhibition results in the formation of plaques which each have a turbid halo. (cf. RAPID LYSIS MUTANT.) lysis protein (1) (syn. holin) A bacteriophage-encoded protein that forms a pore in the cytoplasmic membrane of an infected bacterium, thus enabling an ENDOLYSIN to gain access to the peptidoglycan. In bacteriophage l, for example, a holin is encoded by gene S ; this protein oligomerizes in the cytoplasmic membrane, forming lesions which apparently allow the R gene product to reach peptidoglycan [JB (1990) 172 912–921]. The requirement for the S gene product in bacteriophage l apparently reflects the absence of a signal sequence (see SIGNAL HYPOTHESIS) on the peptidoglycan-degrading R gene product, i.e. the latter, by itself, could not cross the cytoplasmic membrane and therefore could not effect lysis. In phage T4 the product of the t gene appears to have the same role. In BACTERIOPHAGE f29 (which infects Gram-positive bacteria), an analogous product is encoded by gene 14 [JB (1993) 175 1038–1042]. In a phage-infected cell, a holin must be active only after normal phage assembly has been completed; if lysis occurred too early then fewer (or no) mature phages would be released [Review: RMM (1998) 9 99–107; quantitative assay of B. burgdorferi in tissues: JCM (1999) 37 1958–1963; treatment of early Lyme disease (review): Drugs (1999) 57 157–173.] lymphadenitis Inflammation of lymph node(s). lymphadenopathy A pathological condition of lymph node(s). lymphadenopathy-associated virus See AIDS. lymphadenopathy syndrome See AIDS. lymphangitis Inflammation of lymph vessel(s). lymphatic leukosis viruses See AVIAN LEUKOSIS VIRUSES. Lymphocryptovirus See GAMMAHERPESVIRINAE. lymphocystis A FISH DISEASE affecting higher freshwater and marine teleosts. Small, benign, whitish, tumour-like nodules or clusters of nodules develop on skin and fins; the nodules contain greatly hypertrophied cells (to 1 mm diam.) rich in DNA. The fish seems to be little harmed. The causal agent is a virus of the genus Lymphocystivirus (family IRIDOVIRIDAE). Lymphocystivirus A genus of viruses (family IRIDOVIRIDAE) which infect fish; the genus includes the causal agent of LYMPHOCYSTIS. lymphocyte A type of LEUCOCYTE found e.g. in blood, lymph and lymph nodes; lymphocytes are essential for humoral immunity (see ANTIBODY FORMATION) and for CELL-MEDIATED IMMUNITY. (See B LYMPHOCYTE and T LYMPHOCYTE.) Lymphocytes are roughly spherical (ca. 8–15 µm in diameter) with non-granular cytoplasm and a large, non-lobed, rounded nucleus which occupies much or most of the cell. Morphologically, B and T lymphocytes are identical; they can be distinguished e.g. by their cell-surface antigens (e.g. using immunofluorescence techniques). B and T lymphocytes can be separated e.g. by passing a mixed suspension through a column of glass beads coated with antiglobulin; the B cells are retained. B cells are less numerous than T cells in the bloodstream. (See also NK CELLS and NULL CELLS.) lymphocyte activating factor See INTERLEUKIN-1. lymphocyte homing See CD44 and CD102. lymphocytic choriomeningitis virus See LCM VIRUS. lymphocytosis The condition in which increased numbers of lymphocytes are present in the blood. lymphogranuloma venereum A VENEREAL DISEASE caused by strains of Chlamydia trachomatis; symptoms typically include a lesion on the genitals, followed by inflammation and swelling of lymph nodes in the groin. Chemotherapy: e.g. tetracyclines or sulphonamides. (See also FREI TEST.) lymphoid leukosis A POULTRY DISEASE caused by an avian type C oncovirus (see AVIAN LEUKOSIS VIRUSES); B cell lymphomas originate in the bursa of Fabricius and metastasize to other organs, including the liver (‘big liver disease’), spleen, kidney, etc. Transmission occurs chiefly via the egg, although horizontal transmission can occur. Only a small proportion of the birds infected actually develop the disease. (cf. MAREK’S DISEASE.) lymphokines The former name for a wide range of biologically active glyco(proteins), produced and secreted by lymphocytes and certain other cells, which are now called CYTOKINES. Among the agents regarded as lymphokines were: (i) B cell growth factors (BCGFs), a term which may refer e.g. to the cytokine now known as interleukin-14; (ii) B cell differentiation factors (BCDFs), which may have included e.g. interleukin-6; (iii) INTERFERONS; (iv) chemotactic factors (see also CHEMOKINES and MIF); (v) various factors that regulate haemopoiesis. [Early account of B cell growth and differentiation factors: Imm. Rev. (1984) 78 .] lymphoma Any tumour of lymphoid tissues, or one derived from lymphoid cells. Lymphomas may be classified e.g. on the 448 Lyssavirus optimum of ca. 5.0. (The acidity within a lysosome is maintained by a PROTON ATPASE in the lysosome membrane.) [The lysosome membrane: TIBS (1986) 11 365–368.] Lysosomes develop from the distal end of the GOLGI APPARATUS; they are involved primarily in intracellular digestion: see AUTOLYSIN, FOOD VACUOLE, PHAGOCYTOSIS. A primary lysosome is one which has not yet coalesced with an autophagic or food vacuole or a phagosome; following such coalescence the whole is termed a secondary lysosome. When digestion is complete the entity remaining is termed a residual body. Certain viruses (e.g. Reovirus) depend on lysosomal enzymes for uncoating. Lysosomes may be damaged or disrupted e.g. by streptolysin O or phalloidin. lysostaphin A 22-kDa BACTERIOCIN produced by Staphylococcus simulans (bv staphylolyticus) which is active against strains of Staphylococcus aureus. Lysostaphin is an endopeptidase which cleaves the glycyl–glycyl peptide bridges in staphylococcal PEPTIDOGLYCAN, causing lysis. Immunity to lysostaphin in the secreting organism is apparently due to modification of its own peptidoglycan by an enzyme encoded by the lif gene. Lysostaphin has been investigated for use in reducing numbers of staphylococci in the nose and throat of carriers and for the treatment of staphylococcal infections resistant to b-lactam antibiotics. lysozyme (N-acetylmuramidase; endolysin; endo-b-N-acetylmuramide glycanhydrolase; EC 3.2.1.17) An enzyme which hydrolyses the N-acetylmuramyl-(1 → 4)-b-linkages in PEPTIDOGLYCAN; it can thus cause BACTERIOLYSIS in species whose peptidoglycan is accessible to enzyme action. (Micrococcus luteus – formerly M. lysodeikticus – is particularly susceptible and is used in lysozyme assays.) Lysozyme is widely distributed in nature. Animal lysozyme (MWt ca. 14500; IEP ca. 11) occurs e.g. in egg-white, milk, saliva, tears, mucosal secretions, macrophages etc (cf. NON-SPECIFIC IMMUNITY). Structurally distinct enzymes are produced by some bacteria and are coded for by certain bacteriophages (e.g. T2, T4, l). An extracellular lysozyme-like enzyme (endo-b-N-acetylglucosaminidase, EC 3.4.99.17, which cleaves the glucosaminidyl-(1 → 4)-b-linkages in peptidoglycan) is produced by most coagulase-positive and some coagulase-negative staphylococci [Book ref. 44, pp. 746–748]. lyssa Syn. RABIES. Lyssavirus (rabies virus group) A genus of viruses within the RHABDOVIRIDAE; type species: RABIES virus. The rabies virus is neurotropic; the amino acid sequence of the virion glycoprotein has been shown to resemble that of certain snake venom curaremimetic neurotoxins which bind to acetylcholine receptors on nerve cells [Science (1984) 226 847–848]. The rabies virus can multiply in nearly all types of mammalian and avian cells in culture; it apparently replicates in the host cell cytoplasm, budding predominantly from intracytoplasmic membranes. Unlike VSV (see VESICULOVIRUS), it does not inhibit cellular macromolecule synthesis. (See also FIXED VIRUS; FLURY VIRUS; STREET VIRUS.) The genus currently contains seven genotypes – all of which, except Lagos bat virus, are able to cause death in humans and/or animals in nature; the genotypes are: when the cell burst (i.e. the BURST SIZE would be affected). Such early lysis was reported with a strain of phage l containing a mutant S gene [Mol. Microbiol. (1994) 13 495–504]. In some holin-encoding genes there are two start codons (separated by one or two codons), i.e. the gene encodes two polypeptides of slightly different length. The shorter product is the holin, while the longer polypeptide seems to be an inhibitor of holin activity; this arrangement may be a mechanism for controlling the timing of cell lysis [Mol. Microbiol. (1996) 21 675–682]. (2) The product of the ‘lysis gene’ in a colicin plasmid (see COLICINS). (3) (syn. holin) A protein, product of the tcdE gene in Clostridium difficile, which appears to promote the release (across the cytoplasmic membrane) of toxin molecules (encoded by genes tcdA and tcdB ) that lack a signal sequence [JMM (2001) 50 613–619]. Lysobacter A genus of unicellular GLIDING BACTERIA; GC%: 65–68. [IJSB (1978) 28 367–393.] lysochrome Any non-ionic coloured substance which dissolves in and thus imparts colour to the lipids of cells or tissues. (See e.g. SUDAN BLACK B and NILE BLUE A; cf. DYE.) lysogen A lysogenic cell, or a population of lysogenic cells: see LYSOGENY. lysogenic (1) See LYSOGENY. (2) Syn. LYSOGENOUS. lysogenic conversion See BACTERIOPHAGE CONVERSION. lysogenic immunity See SUPERINFECTION IMMUNITY. lysogenous (lysogenic; lysigenous; lysigenic) Refers to a structure, cavity or aperture (e.g. in a fungal fruiting body) which is formed as a result of the breakdown of cells. lysogeny The phenomenon in which the genome of a (temperate) BACTERIOPHAGE establishes a stable, non-lytic presence in its living host without producing progeny virions; the host cell (which is said to be lysogenic) can continue to grow and divide, and replication of the phage genome (the prophage) is coordinated with that of the host chromosome such that, at each cell division, the prophage is inherited by both daughter cells. The prophage is maintained either by integration into the host chromosome (as in the majority of temperate phages: see e.g. BACTERIOPHAGE l, BACTERIOPHAGE MU and BACTERIOPHAGE f105) or as an extrachromosomal plasmid (see e.g. BACTERIOPHAGE P1 and BACTERIOPHAGE F116). (cf. PROVIRUS.) The host cell may or may not exhibit an altered phenotype (see BACTERIOPHAGE CONVERSION). A prophage normally retains the potential to initiate a lytic cycle in which progeny virions are produced and released by cell lysis; maintenance of the lysogenic state necessitates the repression of lytic functions in the prophage by means of phagedirected synthesis of a repressor protein (see e.g. BACTERIOPHAGE l). (See also SUPERINFECTION IMMUNITY.) Derepression – e.g. by inactivation of the repressor protein – results in induction of the lytic cycle; induction may occur spontaneously in a small proportion of cells in a lysogenic population. (See also ZYGOTIC INDUCTION.) (See also PSEUDOLYSOGENY and TRANSDUCTION.) Lysol See PHENOLS. lysopeptide Syn. signal peptide: see SIGNAL HYPOTHESIS. lysophagosome See PHAGOCYTOSIS. lysopine See CROWN GALL. lysosome In eukaryotic cells (only): a closed, intracellular membranous sac (often ca. 0.5 µm) containing a wide range of enzymes which, collectively, can degrade various classes of biological macromolecules; the enzymes typically have a pH 1. 2. 3. 4. 5. 449 Rabies virus Lagos bat virus Mokola virus Duvenhage virus European bat lyssavirus type 1 lysU gene Lyt antigens Cell-surface antigens of murine T cells. lytic bacteriophage Syn. VIRULENT BACTERIOPHAGE. lytic cycle (virol.) The sequence of events which begins with adsorption of a virus to a susceptible cell and terminates with cell lysis and release of progeny virions. lytic infection (virol.) The infection and subsequent lysis of susceptible cells by a VIRUS. (cf. e.g. LYSOGENY.) Lyticum A genus of Gram-negative bacteria which occur as endosymbionts in the cytoplasm of certain strains of Paramecium aurelia. Cells: peritrichously flagellated rods, ca. 0.6–0.9 × 2.0–10 µm, which stain well with toluidine blue; in vitro cultivation has been reported. Lyticum spp do not contain R bodies (cf. CAEDIBACTER) but they produce labile toxin(s) which lyse sensitive paramecia. Type species: L. flagellatum. L. flagellatum (earlier name: lambda particles) includes typically straight rods, while L. sinuosum (earlier name: sigma particles) includes curved and spiral rods. [Book ref. 22, pp. 808–811.] lyxose An aldopentose. (cf. STREPTOSE.) 6. European bat lyssavirus type 2 7. Australian bat lyssavirus The first four genotypes correspond to the four earlier serotypes. Lagos bat virus was isolated from a fruit bat. Duvenhage virus can cause a rabies-like disease in man. Mokola virus (isolated e.g. from shrews and cats) can cause CNS disease in man. Australian bat lyssavirus (reported in 1997) is infective for humans. Kotonkan virus, isolated from Culicoides sp, may cause ephemeral febrile disease in animals and man. This virus, and the Obodhiang virus, isolated from Mansonia uniformis, have been associated with the genus Lyssavirus but have not been assigned to a genotype. Phylogenetic analysis of the genus Lyssavirus has been able to distinguish two phylogroups: phylogroup I (rabies virus, European bat lyssavirus types 1 and 2, Duvenhage virus and the Australian bat lyssavirus), and phylogroup II (Lagos bat virus and Mokola virus); it is believed that this classification reflects more closely the biological characteristics of lyssaviruses than do the serotypes and genotypes [JV (2001) 75 3268–3276]. lysU gene See HEAT-SHOCK PROTEINS. 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 450 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 M m Maintenance coefficient: see YIELD COEFFICIENT. M Methionine (see AMINO ACIDS). M antigen (1) Streptococcal M PROTEIN. (2) A protein–LPS surface antigen in Brucella spp. (3) The COLANIC ACID capsular antigen in certain enterobacteria. M-associated protein See MAP. M bands (of Stentor) See MYONEME. M cells See DYSENTERY. M fibres (of Stentor) See MYONEME. M fimbriae FIMBRIAE which occur on some pyelonephritic strains of Escherichia coli ; they bind to the amino-terminal part of glycophorin AM . M phase (of cell cycle) See CELL CYCLE. M phenotype In strains of Streptococcus pyogenes and S. pneumoniae: an efflux-based resistance to MACROLIDE ANTIBIOTICS (q.v.). M plasmid See MICROCINS. M protein (1) (M antigen) (of streptococci) A heat-stable, trypsin-sensitive protein antigen associated with virulence in certain (MC ) strains of group A streptococci. The M protein occurs as a fibrillar ‘fuzz’ covering the entire cell surface: the protein molecules apparently occur in extended form as double-stranded, a-helical coiled coils (ca. 50 nm long) anchored in the cell wall by their C-terminal ends [PNAS (1981) 78 4689–4693]. The M protein protects the cell from phagocytosis, and may play a role in adhesion. MC strains often form matt or mucoid colonies on solid media, and may under certain cultural conditions produce an extracellular protease which degrades the M protein. M protein occurs in different antigenic forms, providing a basis for the serological typing of MC strains. (See also MAP; R PROTEIN; T PROTEIN.) (2) (in e.g. Escherichia coli ) The product of the lacY gene of the LAC OPERON. M ring See FLAGELLUM (a). M1 RNA See RNASE P. mAb Monoclonal antibody. MAC (1) (immunol.) Membrane attack complex: see COMPLEMENT FIXATION. (2) (bacteriol.) (syn. MAIS complex) Mycobacterium avium complex: a category of slow-growing mycobacteria which include M. avium, M. intracellulare and M. scrofulaceum. These organisms are causal agents in three main types of infection: (i) cervical adenitis in children, (ii) pulmonary disease in adults, and (iii) disseminated disease in patients with advanced AIDS; treatment for cervical adenitis involves resection of affected node(s), while treatment for pulmonary and disseminated infections involves extended chemotherapy with a multiple drug regimen that includes clarithromycin or azithromycin. [Infections due to MAC and their treatment: BCID (1997) 4 25–61. PCR-based detection of MAC bacteraemia in AIDS patients: JCM (1999) 37 90–94. Cross-reactivity between M. avium (and M. kansasii ) and M. tuberculosis in AMTDT: JCM (1999) 37 175–178.] Mac-1 See INTEGRINS. MacConkey’s agar An agar-based medium used e.g. for the primary isolation of Salmonella and Shigella from faeces, for the differentiation of lactose-fermenting enterobacteria from NLFs, etc. The medium contains (w/v): peptone (2%); lactose (1%); sodium taurocholate or glycocholate (0.5%); NaCl (0.5%); NEUTRAL RED (0.003%); agar (ca. 1.5%). Final pH: ca. 7.4. Colonies of lactose fermenters (e.g. Escherichia coli ) are pink or red, those of NLFs (e.g. most strains of Proteus, Salmonella and Shigella) are colourless. Most non-enteric bacteria are inhibited by the bile salts. Some commercial preparations also contain crystal violet (1 mg/l) to inhibit Gram-positive bacteria. MacConkey’s broth A broth with the same formulation as MACCONKEY’S AGAR but without the agar. Bromcresol purple may be used instead of neutral red as pH indicator. Machupo virus See ARENAVIRIDAE and BOLIVIAN HAEMORRHAGIC FEVER. macrocapsule See CAPSULE. macroconidium The larger of two types of CONIDIUM formed by certain fungi (see e.g. FUSARIUM). macroconjugant See CONJUGATION (1) (c). macrocyclic rusts (eu-form rusts) (1) Those rust fungi (e.g. Puccinia graminis) which form five clearly distinct types of spore: pycniospores, aeciospores, uredospores, teliospores and basidiospores (see UREDINIOMYCETES). (2) Those rust fungi which form more than one type of binucleate spore – i.e., aeciospores and/or uredospores in addition to teliospores. (cf. DEMICYCLIC RUSTS.) macrocyst A type of resting stage formed under certain conditions by (at least) some dictyostelid slime moulds (e.g. strains of Dictyostelium discoideum, D. mucoroides, Polysphondylium violaceum). It is formed when myxamoebae aggregate, and the aggregate becomes enclosed in a fibrillar sheath; one of the cells (the ‘phagocytic’ or ‘cytophagic’ cell) enlarges and begins to engulf neighbouring amoebae. When all the peripheral amoebae have been ingested, the remaining giant cell develops a cellulose wall, its single large nucleus divides to form numerous smaller nuclei, and the resulting multinucleate macrocyst enters a period of dormancy. On germination, the macrocyst protoplast divides into large, uninucleate ‘proamoebae’ which in turn divide to form normal-sized vegetative myxamoebae; these are released when the macrocyst wall breaks down. Macrocysts apparently represent a sexual stage in the dictyostelid life cycle. The large nucleus of the cytophagic cell is believed to be that of a zygote; meiosis is presumed to occur when this nucleus divides to form the multinucleate cyst. [Review: Book ref. 144, pp. 178–221.] Macrocystis See PHAEOPHYTA. macrodilution broth test See DILUTION TEST. macrofibre In a culture of Bacillus subtilis grown under certain conditions: a bundle of aligned chains of cells which can exhibit a right-handed or left-handed helical conformation; the cell wall in the cells of a helically twisted macrofibre is believed to be under torsional stress. The helical conformation appears to depend on the presence of intact glycan backbone chains (rather than intact cross-links) in the cell wall PEPTIDOGLYCAN [JGM (1986) 132 2377–2385]. macrogamete See ANISOGAMY. macroglobulin (1) Syn. IgM (q.v.). (2) a2 -Macroglobulin is a fibrinolysin inhibitor secreted by macrophages. macrolide antibiotics A large group of MLS ANTIBIOTICS which includes e.g. angolamycin, carbomycin, chalcomycin, cirramycin, erythromycin, lankamycin, leucomycin (D turimycin), 451 macronucleus pathway from haemopoietic stem cells in the bone marrow, early stages including the monoblast and MONOCYTE. [Macrophages: JCS (1986) supplement 4, 267–286.] Macrophages have important roles in host resistance to pathogenic microorganisms (see PHAGOCYTOSIS, INFLAMMATION, ANTIBODY FORMATION and DELAYED HYPERSENSITIVITY). (See also GIANT CELLS, HISTIOCYTE, KUPFFER CELLS and PMN.) Macrophages can express a wide range of cell-surface receptors, some of which are expressed constitutively while others can be induced by appropriate stimuli. They include: (i) receptors for the FC PORTION of IgG antibodies; (ii) receptors for various components of COMPLEMENT (e.g. CD46 and CD11c/CD18); (iii) binding sites for lipopolysaccharides (see CD14); (iv) binding sites for type I FIMBRIAE (see CD48); and (v) receptors for various CYTOKINES. Macrophages also display MHC class II molecules and function as ANTIGEN-PRESENTING CELLS. Under appropriate conditions (and with suitable stimulation), macrophages secrete a wide range of products, including: (i) certain components of COMPLEMENT (e.g. FACTOR B, PROPERDIN, C1–C5); (ii) FIBRONECTIN; (iii) certain PROSTAGLANDINS; and (iv) various CYTOKINES (e.g. INTERLEUKIN-1, INTERLEUKIN-6, INTERLEUKIN-8, INTERLEUKIN-12, INTERLEUKIN-18, TNF). Activation of macrophages. On activation, a macrophage e.g. (i) becomes more aggressive towards pathogens which have been engulfed by phagocytosis, and (ii) secretes various cytokines. Thus, an activated macrophage is associated with a strong respiratory burst (see NEUTROPHIL) in which it produces significant amounts of e.g. SUPEROXIDE, hydrogen peroxide and NITRIC OXIDE; in the activated state a macrophage is able to kill certain ingested pathogens which may not be killed in nonactivated macrophages. Activation may occur e.g. when a macrophage presents antigen to an antigen-specific T LYMPHOCYTE of the Th1 subset (this is an MHC class II-restricted interaction – see HISTOCOMPATIBILITY RESTRICTION). Following such interaction with the macrophage, the T cell secretes an important macrophageactivating factor: interferon-gamma (IFN-g) (see INTERFERONS). IFN-g causes the macrophage to mount a respiratory burst (see above) and to secrete cytokines such as IL-1, IL-6, IL-12, IL18 (see individual entries) and TNF-a (see TNF). Apparently, IFN-g and lipopolysaccharides (LPS) can act synergistically to enhance the respiratory burst [JBC (1990) 265 20241–20246] and to stimulate the synthesis of cytokines by macrophages. (The ability of IFN-g and LPS to stimulate cytokine production in macrophages may be inhibited by glucocorticoids.) Cytokines secreted by a T cell may provide BYSTANDER HELP (q.v.). Following uptake, some pathogenic bacteria can kill macrophages [see e.g. EMBO (1996) 15 3853–3860; TIM (1998) 6 253–255]; Yersinia down-regulates production of the proinflammatory cytokine TNF-a in macrophages [Mol. Microbiol. (1998) 27 953–965]. [Interactions between Mycobacterium tuberculosis and phagocytes: TIM (1998) 6 328–335.] Macrophages may be maintained for some time in culture. Viable macrophages adhere to glass and plastics and accumulate dyes such as TRYPAN BLUE. macrophage-activating factor See MAF. macropinocytosis See DYSENTERY. macroscopic Visible to the unaided eye. macrostome See TETRAHYMENA. macrotetralides A group of ANTIBIOTICS which function as IONOPHORES; they inhibit Gram-positive bacteria. The cyclic megalomycin, methymycin, narbomycin, niddamycin, oleandomycin, relomycin, roxithromycin, spiramycins, troleandomycin and tylosin; they are produced e.g. by species of Streptomyces. The molecule of a macrolide antibiotic contains a large lactone ring (12–14 carbon atoms) substituted with various functional groups and linked glycosidically to one or more sugars (including an aminosugar and, frequently, an unusual deoxysugar); thus, e.g. erythromycin (see also POLYKETIDE) consists of a 14-member ring linked to the aminosugar desosamine and the deoxysugar cladinose. (Macrolides which contain a conjugated polyene system form a separate group: see POLYENE ANTIBIOTICS (a).) The macrolides are typically bacteriostatic and are active mainly against Gram-positive bacteria (including Streptococcus spp and Clostridium spp), although certain Gram-negative bacteria (e.g. Haemophilus spp, Neisseria spp, some species of Bacteroides) may be susceptible. Macrolides inhibit PROTEIN SYNTHESIS by binding to the 50S subunit of 70S ribosomes, apparently inhibiting the peptidyltransferase reaction and/or affecting translocation. The macrolide AZITHROMYCIN inhibits the ability of Pseudomonas aeruginosa strain PAO1 to produce virulence factors by inhibiting the pathogen’s QUORUM SENSING circuitry [AAC (2001) 45 1930–1933]. In e.g. Escherichia coli, resistance to macrolides may be due to a mutation which results in the modification of a protein (L4, or possibly L22) in the 50S ribosomal subunit. In staphylococci, streptococci and streptomycetes resistance to macrolides (and other MLS antibiotics) may be due to methylation of the ribosomal 23S rRNA (see STREPTOGRAMINS). In Streptococcus pyogenes and S. pneumoniae, strains exhibiting the ‘M phenotype’ contain an efflux-based mechanism which confers resistance specifically to macrolides – i.e. the mechanism does not affect susceptibility to lincosamides or B streptogramins. The efflux mechanism is encoded by a sequence (mefA) within a 7244-bp chromosomal element designated Tn1207.1 [AAC (2000) 44 2585–2587]. macronucleus (ciliate protozool.) The larger of the two types of nucleus characteristically present in ciliates. (cf. MICRONUCLEUS.) Many ciliates contain a single macronucleus, but some contain more than one; according to species, the macronucleus may be e.g. ovoid, elongated, moniliform, C-shaped etc. Macronuclei contain nucleoli and are typically polyploid (they are typically diploid in the karyorelictid gymnostomes – e.g. Kentrophoros, Loxodes); macronuclear DNA, which is actively transcribed, directs the ‘somatic’ activities of the cell. During CONJUGATION the macronucleus usually disintegrates – a new one subsequently developing from a micronucleus. In Oxytricha nova macronuclear DNA is in the form of small, gene-sized molecules; during its development (from micronuclear DNA) the DNA undergoes a complex series of changes which include polytenization (with the loss of certain sequences), fragmentation, modification (the addition of inverted terminal repeats), and encapsulation within vesicles followed by release and replication [PNAS (1982) 79 3255–3259]. A heteromerous macronucleus is one which is subdivided into two parts (karyomeres) – the orthomere and paramere – which differ in their content of DNA and RNA; such nuclei occur in many hypostomes. (See also ENDOSOME (2).) A homoiomerous (D ‘homomerous’) macronucleus is one which is not subdivided; such nuclei occur in the majority of ciliates. macrophage A large (10–20 µm), relatively long-lived, amoeboid, phagocytic and pinocytotic LEUCOCYTE present in blood, lymph and other tissues; macrophages develop via the myeloid 452 maize chlorotic dwarf virus maedi-visna See MAEDI. MAF Macrophage-activating factor: a former name for the CYTOKINE interferon-g (IFN-g). magenta Syn. basic FUCHSIN. Magnaporthe See POLYSTIGMATALES and AVIRULENCE GENE. magnetic separation See DYNABEADS. magnetosome See MAGNETOTACTIC BACTERIA. magnetotactic bacteria A non-taxonomic grouping of aquatic, microaerophilic, flagellate, Gram-negative bacteria, each of which forms intracellular, enveloped magnetic bodies (magnetosomes); the presence of magnetosomes causes the cells to align with lines of magnetic force so that cell motility is given direction (but not assistance) by a magnetic field (magnetotaxis). These bacteria are widespread geographically, occurring in sediments in freshwater and marine habitats. Aquaspirillum magnetotacticum has been obtained in pure culture; each cell contains a chain of ca. 20 cuboidal or polyhedral magnetosomes (of side ca. 40 nm) which contain magnetite (Fe3 O4 ). [Precipitation of magnetite in magnetotactic bacteria: PTRSLB (1984) 304 529–536. Magnetosomes (review): JGM (1993) 139 1663–1670.] magnetotaxis See MAGNETOTACTIC BACTERIA. magnification The extent to which the image of an object is larger than the object itself. With a compound (light) microscope the total magnification is the magnification of the objective lens multiplied by that of the eyepiece lens; it is determined by the focal lengths of the objective and eyepiece lenses. RESOLVING POWER involves other factors and is distinct from magnification. Different objective lenses having the same magnification may have different resolving powers; resolving power (but not magnification) may be affected by sub-optimal illumination of the specimen. Any increase in magnification which is not accompanied by an increase in resolution is said to be empty magnification. The maximum useful magnification obtainable with a given objective lens is approximately 1000 times its numerical aperture. Good-quality light microscopes can give a maximum magnification of about 1200ð. Higher magnifications are obtainable by ultraviolet microscopy (see MICROSCOPY (f)). In ELECTRON MICROSCOPY magnification can be e.g. 1000000ð. Maillard reaction A reaction between a reducing sugar (e.g. lactose) and the free amino groups in proteins, the product being a brown-coloured compound; the extent of the reaction depends on the concentrations of the sugar and protein and on temperature. (See also MILK (treatment processes).) maintenance coefficient (m) See YIELD COEFFICIENT. maintenance energy The energy used by a cell for purposes other than growth; maintenance energy is used e.g. for motility, for the maintenance of osmotic pressure in the cell, for maintaining the energized state of the cytoplasmic membrane etc. At high rates of growth the maintenance energy requirement is generally relatively minimal, but at low rates of growth maintenance energy may account for an appreciable proportion of the overall energy requirement. (See also YIELD COEFFICIENT.) maintenance medium See MEDIUM and TISSUE CULTURE. MAIS complex Mycobacterium avium–intracellulare–scrofulaceum complex: a non-taxonomic grouping of species which is convenient for laboratory diagnostic purposes. (See MAC.) maize chlorotic dwarf virus (MCDV; Ohio corn stunt agent) A PLANT VIRUS in which the virions are polyhedral (ca. 30 nm diam.) and contain one molecule of positive-sense ssRNA (MWt ca. 3.2 ð 106 ). Host range is limited to members of the Gramineae; symptoms (in maize) include stunting and chlorotic striping of tertiary leaf veins (cf. CORN STUNT DISEASE). Transmission occurs via leaf-hoppers (e.g. Graminella nigrifrons) macrotetralides (called ‘actins’ or ‘nactins’) each contain four similar tetrahydrofuran rings linked via ester bonds; nonactin is substituted with four methyl groups, monactin with three methyl and one ethyl, dinactin with two methyl and two ethyl, and trinactin with one methyl and three ethyl groups. Openchain macrotetralides (polyether antibiotics) include alborixin, dianemycin, LASALOCID, lonomycin, MONENSIN, NIGERICIN and tetronomycin. [Structure and biosynthesis of polyether antibiotics: J. Nat. Prod. (1986) 49 35–47.] macule (macula) On e.g. skin: an unelevated spot of discoloration. (cf. PAPULE.) maculopapular Refers to a rash in which both MACULES and PAPULES occur. mad cow disease (BSE) See BOVINE SPONGIFORM ENCEPHALOPATHY. mad itch Syn. AUJESZKY’S DISEASE. Madin–Darby canine kidney cells See MDCK CELLS. Madura foot Syn. MADUROMYCOSIS. Madurella See HYPHOMYCETES and MADUROMYCOSIS. maduromycetes Certain sporoactinomycetes with have a type III cell wall (see ACTINOMYCETALES) and which typically contain madurose; the group includes Actinomadura, Excellospora, Microbispora, Microtetraspora, Planobispora, Planomonospora, Spirillospora and Streptosporangium, but excludes the (morphologically distinguishable) genera Dermatophilus and Frankia (both wall type III, and both containing madurose) [Book ref. 73, pp. 105–115]. maduromycosis (Madura foot) A non-infectious human disease characterized by the formation – usually on feet or hands – of granulomatous, suppurative tumours (mycetomas) of the subcutaneous tissues; eventually the bone may be affected. The causal agent may be any of various fungi (e.g. Acremonium spp, Aspergillus nidulans, Madurella spp, Pseudallescheria boydii ) or actinomycetes (e.g. Actinomadura spp, Nocardia spp, Streptomyces somaliensis) – the tumours being termed, respectively, eumycetomas (D eumycotic mycetomas) and actinomycetomas (D actinomycotic mycetomas). Infection occurs via wounds. Within the tissues, the pathogen forms characteristic aggregates (called granules or grains); depending on pathogen, the granules may be white, red, yellow, brown or black, soft or hard, and range in size from <15 µm (Nocardia spp) to >4 mm (e.g. Actinomadura madurae). (cf. ACTINOMYCOSIS.) Treatment of eumycetomas is usually surgical; some actinomycetomas can be controlled with antibiotics (e.g. sulphamethoxazole-trimethoprim C streptomycin). madurose A sugar (3-O-methyl-D-galactose) which occurs e.g. in MADUROMYCETES. MAECT Miniature anion exchange centrifugation technique: a method for detecting the presence of e.g. trypanosomes in the blood of patients suspected of suffering from SLEEPING SICKNESS. Blood, diluted with buffer, is passed through a DEAE (ionexchange) column; erythrocytes are retained in the column, but any trypanosomes which may be present pass through the column and can be concentrated by centrifugation and examined by dark-field MICROSCOPY. maedi (maedi-visna; ovine progressive interstitial pneumonia; Graff–Reinet disease) A chronic, progressive, fatal pneumonia which occurs mainly in sheep (also in goats); the causal agent, a virus of the LENTIVIRINAE, is apparently identical to the VISNA virus, but maedi and visna rarely occur together in one animal. Incubation period: at least two years. Symptoms: progressive emaciation and dyspnoea, with or without coughing and nasal discharge, typically lasting up to six months. (cf. JAAGSIEKTE.) [Book ref. 33, pp. 805–807.] 453 maize diseases mal del pinto Syn. PINTA. mal du coit Syn. DOURINE. malachite green A green basic TRIPHENYLMETHANE DYE used e.g. as an ENDOSPORE STAIN. malaria An acute or chronic, sometimes recurrent, infectious disease caused by species of PLASMODIUM and responsible, each year, for some 1–2 million deaths (including many children); the disease is prevalent in tropical and subtropical regions. Malaria may be caused by Plasmodium falciparum, P. malariae, P. ovale or P. vivax. (Mixed plasmodial infections can also occur; moreover, P. knowlesi and P. simium, normally parasites of non-human primates, can infect man under natural conditions, causing ‘simian malaria’.) Natural transmission of the malaria parasite – from an infected to an uninfected individual – occurs via an insect vector: a female mosquito of the genus Anopheles. [Systematics of malaria vectors (in particular, the Anopheles punctulatus group): IJP (2000) 30 1–17.] If, during a blood meal, the mosquito ingests viable plasmodial gametocytes, male and female gametes can develop and fertilization can take place in the insect’s gut; eventually, sporozoites – forms of the parasite infective for a human host – appear in the salivary glands of the mosquito, and these can be passed to an uninfected individual during the insect’s next blood meal (see figure). [The malaria parasite in the mosquito: PT (2000) 16 196–201.] Transmission may also occur e.g. by transfusion of infected blood [NEJM (2001) 344 1973–1978]. Incubation period: ca. 7–28 days, or longer. The typical illness is characterized by a series of paroxysms, each involving an attack of rigors followed by high fever and then profuse sweating. These paroxysms occur every ¾36–48 hours in falciparum malaria (‘malignant tertian malaria’), ¾42–48 hours in vivax malaria (‘benign tertian malaria’), ¾50 hours in ovale malaria (‘tertian malaria’), and ¾72 hours in malariae malaria (‘quartan malaria’). Other symptoms include e.g. headache, malaise, nausea and vomiting, anaemia, and pains in the muscles, joints and abdomen; enlargement of the liver and/or spleen may occur. Falciparum malaria exhibits the most severe symptoms, and causes the majority of deaths from malaria. In certain blood vessels (the post-capillary venules), parasitized erythrocytes adhere to endothelial cells – although the significance of this in pathogenesis is not fully understood. Such adherence may lead to mechanical obstruction of the vessels; it may, perhaps additionally, activate or damage endothelial cells and/or e.g. stimulate the release of cytokines. Some authors indicate that disease manifestations result almost entirely from cytoadherence (adherence of red blood cells to the endothelium) together with the induction of certain cytokines (TNF-a, IL-1 and IL-6) [BCID (1995) 2 227–247]. In some cases, severe disease has been correlated with a variant form of the TNF-a gene promoter [Nature (1994) 371 508–510]. Another factor in falciparum malaria is ‘rosetting’: the phenomenon in which uninfected erythrocytes bind to infected red cells; again, the significance of this in pathogenesis is not clear, although some studies suggest an association between rosetting and cerebral malaria. Cytoadherence, rosetting and impaired erythrocyte deformability have been discussed in the context of abnormal microcirculatory blood flow [PT (2000) 16 228–232]. Complications that may be associated with severe malaria include anaemia, DISSEMINATED INTRAVASCULAR COAGULATION, hypoglycaemia and renal failure. (See also BLACKWATER FEVER.) [Pathogenesis of severe falciparum malaria: BCID (1995) 2 249–270.] and is semipersistent. The virus overwinters in ‘Johnson grass’ (Sorghum halepense) and other weed grasses. (cf. RICE TUNGRO VIRUS.) maize diseases See CEREAL DISEASES. maize dwarf mosaic virus See POTYVIRUSES. maize mosaic virus See RHABDOVIRIDAE. maize rayado fino virus group A group of isometric ssRNAcontaining PLANT VIRUSES which are obligately transmitted by leafhoppers. maize rough dwarf virus See FIJIVIRUS. maize streak virus (MSV) The type member of the GEMINIVIRUSES. MSV is an important cause of disease in maize (Zea mays) in many parts of Africa; leaves of infected plants show e.g. broken, narrow streaks of chlorosis along the veins. MSV is transmitted by leafhoppers of the genus Cicadulina [epidemiology: ARE (1978) 23 259–282]. maize stripe virus See RICE STRIPE VIRUS GROUP. maize wallaby ear disease (MWED) A disease of maize (Zea mays) first described in Australia; symptoms include small swellings on secondary veins on the lower surfaces of leaves, stunting etc. MWED is associated with infestation by leafhoppers (Cicadulina, Nesoclutha); the causal agent was thought to be a virus (cf. LEAFHOPPER A VIRUS), but there is evidence to suggest that the disease is due to an insect toxin [Ann. Appl. Biol. (1983) 103 185–189]. major facilitator superfamily (MFS) See MFS. major groove (in DNA) See DNA. major histocompatibility complex (MHC) In all mammalian species studied: linked genetic loci that dominate the immune response; these genes, and their products, are designated by the prefixes HLA (human leukocyte antigens) in man, H-2 in mice, RT1 in rats, ChLA in chimpanzees etc.; the human (HLA) genes occur on chromosome 6. MHC gene products occur on cell surfaces and serve as markers which e.g. distinguish ‘self’ (or syngeneic) tissue from ‘non-self’ (foreign) tissue; thus, MHC gene products determine the outcome of tissue transplants (see also MLR), and they are also involved e.g. in ANTIBODY FORMATION. Genes in the MHC complex occur in three main groups (classes I, II and III). Most of the genes belong to classes I and II, and the products of these genes are designated class I and class II antigens, respectively. (Class III genes encode the COMPLEMENT components C2, C4 and factor B.) Class I and class II antigens are distinguishable on a structural and functional basis. Class I antigens (encoded by HLA-A, HLA-B and HLAC genes) each consist of a single transmembrane polypeptide. These antigens are expressed on most nucleated cells in the body, and they are involved e.g. in the recognition of target cells by cytotoxic CD8C T cells. Class II antigens (encoded by the HLA-DP, HLA-DQ and HLA-DR genes) each consist of a two-polypeptide transmembrane structure. These antigens have a limited distribution, being found e.g. on B cells and other ANTIGEN-PRESENTING CELLS; they are involved e.g. in the recognition of APCs by CD4C T cells. For some cells the expression of MHC antigens may be induced, or upregulated, by certain CYTOKINES. For example (at least in vitro), IFN-g (see INTERFERONS) can induce expression/upregulation of class II antigens on e.g. endothelial cells and macrophages (and can also stimulate expression of class I antigens). mal de caderas In South America: (1) Syn. SURRA. (2) A fatal paralytic disease of cattle caused by the rabies virus and transmitted by blood-sucking vampire bats. 454 455 MALARIA. The life cycle of the malaria parasite (Plasmodium) (diagrammatic) showing the stages which are targets for antimalarial drugs (see table). Sporozoites, injected into the bloodstream via mosquito bite, penetrate liver cells. A sporozoite grows and develops into a tissue schizont which undergoes asexual reproduction (schizogony) to produce a number of cells called merozoites. On rupture of the liver cell, merozoites enter the bloodstream and infect erythrocytes (red blood cells). Within an erythrocyte, a merozoite develops as a trophozoite (growing stage); the young trophozoite is known as the ring stage owing to its appearance in stained preparations under the microscope. The mature trophozoite (blood schizont) undergoes schizogony, and many merozoites are released [in a two-step process: PNAS (2001) 98 271–276] when the erythrocyte ruptures. These merozoites infect fresh erythrocytes. Some merozoites become gametocytes (i.e. cells with the potential to develop into male and female gametes). If ingested by a mosquito, gametocytes give rise to gametes – and the sexual phase of the life cycle follows. In two species of Plasmodium the sporozoite can form a dormant stage (hypnozoite) in liver cells; hypnozoites may remain dormant for months/years. For this form of malaria a ‘radical cure’ involves treatment with an agent that kills hypnozoites (e.g. primaquine) as well as other drug(s) to deal with the intra-erythrocytic parasites. Reproduced from Antimicrobial Drug Action, Figure 7.1, pages 120–121, R. A. D. Williams, P. A. Lambert & P. Singleton (1996) copyright Bios Scientific Publishers Ltd, UK (ISBN 1-872748-81-3) with permission from the publisher. malaria Chemotherapy. Most antimalarials kill intraerythrocytic stages of the parasite. The choice of drug(s) depends e.g. on the local prevalence of parasites resistant to given antimalarials. (See QUININE for the formulae of some antimalarials.) Fansidar (D PYRIMETHAMINE C sulphadoxine) is an oral treatment for uncomplicated falciparum malaria. Quinoline-type antimalarials include QUININE, quinidine (used e.g. in the USA), chloroquine and mefloquine (see AMINOQUINOLINES). Parenteral quinine is considered to be the drug of choice for severe falciparum malaria because few strains of the parasite are resistant to this form of treatment. Chloroquine is used orally for uncomplicated falciparum malaria and for most cases of vivax and ovale malaria. Mefloquine is used orally for uncomplicated malaria and, via nasogastric tube, for severe malaria; neuro-psychiatric sideeffects of this drug may be important [BMJ (1995) 310 709–714]. Halofantrine is used orally for uncomplicated malaria. This drug (9-phenanthrene methanol), unlike e.g. quinine, seems to be active against the ring-stage trophozoite. The mechanism of action is unknown. Halofantrine is not used for prophylaxis owing e.g. to variable absorption. High levels of the drug have been (rarely) linked with cardiac arrhythmias [Lancet (1993) 341 1054–1056]. Atovaquone (hydroxynaphthoquinone) is a candidate oral antimalarial. It appears to inhibit electron transport in the parasite. Primaquine (an 8-aminoquinoline) is active against hypnozoites (but not against blood forms) and is used to prevent relapse in vivax and ovale malaria; for a ‘radical cure’, primaquine is used in conjunction with e.g. chloroquine. WR238605 (a primaquine analogue) is being developed by the US army (Walter Reed Army Institute of Research). It has shown success in animal systems. Natural resistance to falciparum malaria occurs in people heterozygous for the sickle-cell anaemia gene. Resistance to falciparum malaria may also be associated with an absence of the Sla antigen, or low levels of CR1 (CD35), on erythrocytes, as Sla is involved as a binding site in the rosetting phenomenon [Nature (1997) 388 292–295]. Vivax, ovale and malariae malarias may be self-limiting, even without treatment; however, in subsequent years, species which form hypnozoites can give rise to relapses – at increasing intervals of time – as merozoites periodically emerge from the liver. The receptor for P. vivax merozoites on the red blood cell is the Duffy glycoprotein (a marker in the Duffy blood group system), and this glycoprotein is essential for infection [see e.g. Blood (1997) 89 3077–3091]. Many black Africans lack this receptor [a mutation inhibits expression of the Duffy gene: Nature Genetics (1995) 10 224–228] and are accordingly resistant to vivax malaria. The immune response to plasmodial infection. Interactions between plasmodia and the immune system are complex. Immunity develops slowly and tends to be transient; the majority of antigens do not induce immunological memory. Nevertheless, there is an antibody response to sporozoites, asexual blood-stage parasites and gametocytes, and a cell-mediated response which involves e.g. cytotoxic T cells and NK (natural killer) cells. [Clinical immunology of malaria: BCID (1995) 2 227–247.] Vaccines against malaria. Recognition of the need to stimulate immunity by using an ‘antigenic cocktail’ lead to the development of the Colombian synthetic subunit vaccine (SPf66). The future development of vaccines is aimed at improving the protective immune response. [Strategies towards the design of a synthetic subunit malaria vaccine: BCID (1995) 2 389–403.] The plasmodial antigen PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) is displayed at the surface of those red blood cells (RBCs; erythrocytes) which are parasitized by P. falciparum. PfEMP1 promotes cytoadherence, i.e. the adherence of parasitized RBCs to sites such as the CD36 receptor on endothelial cells; PfEMP1 is therefore a potential target for candidate antimalarial vaccines. In the context of vaccines, however, one problem is that PfEMP1 occurs in variant forms in different strains of the pathogen, and an immune response to a given variant is predominantly variant-specific; nevertheless, studies using monoclonal antibodies have detected cross-reacting epitopes on a number of strains of P. falciparum, suggesting the possibility of developing an effective vaccine directed against PfEMP1 [PNAS (2001) 98 2664–2669]. The case has been argued for developing a vaccine against the liver (pre-erythrocytic) stage of the pathogen, thus attacking the disease in the initial stage of infection; a candidate antigen for such a vaccine is liver-stage-antigen-1 (LSA-1), a 200 kDa protein that is expressed specifically by the parasite within liver cells and that appears to have a generally conserved sequence among different isolates. [Pre-erythrocytic immunity: the case for an LSA-1 vaccine: TIP (2001) 17 219–223.] Lab. diagnosis. (i) Detection of parasitaemia by staining blood films (Giemsa). (ii) Detection of P. falciparum protein-2 (HRP2) in haemolysed blood by means of anti-HRP-2 MONOCLONAL ANTIBODIES [PT (1994) 10 494–495]. This principle is used e.g. in the ParaSight-F test (Becton Dickinson, Maryland, USA). (iii) PCR-based testing with species-specific primers [TRSTMH (1993) 87 649–653]. (iv) A genus-specific and (multiplex) species-specific approach by PCR [JCM (1999) 37 1269–1273]. [Rapid diagnostic techniques: TIP (2001) 17 307–309.] ANTIMALARIAL DRUGSa Drug Chemical description Active againstb Artemisinin Sesquiterpene lactone endoperoxide Hydroxynaphthoquinone 4-Aminoquinoline 9-Phenanthrene methanol 4-Aminoquinoline 8-Aminoquinoline Diguanide Diaminopyrimidine Quinoline-type (alkaloid) Quinoline-type (alkaloid) Sulphonamide 8-Aminoquinoline BS BS BS RS? BS TS, HYP TS, BS BS BS BS BS HYP, GAM Atovaquonec Chloroquine Halofantrine Mefloquine Primaquine Proguanil Pyrimethamine Quinidine Quinine Sulphadoxine WR238605c a b See entry QUININE for the formulae of some antimalarials. See the figure for stages in the plasmodial life cycle. BS D blood schizont; TS D tissue schizont; HYP D hypnozoite; GAM D gametocyte; RS D ring stage. c Candidate antimalarial. Reproduced from Antimicrobial Drug Action, Table 7.1, page 121, R. A. D. Williams, P. A. Lambert & P. Singleton (1996) copyright Bios Scientific Publishers Ltd, UK (ISBN 1-87274881-3) with permission from the publisher. 456 Mamiella indicated, after 48 hours, by local oedema and a severe purulent conjunctivitis. Mallomonas A genus of unicellular CHRYSOPHYTES which resemble Chromulina spp but have a coating of silicified scales; in some species the scales bear long siliceous spines. malolactic fermentation A type of FERMENTATION (sense 2), carried out e.g. by species of Lactobacillus, Leuconostoc and Pediococcus, in which L-malic acid is converted to L-lactic acid and CO2 . The reaction is apparently catalysed by a single enzyme (the malolactic enzyme) which has no L-lactate dehydrogenase activity; the malolactic enzyme from e.g. Lactobacillus plantarum has an absolute requirement for Mn(II) and NADC , but the NADC is not reduced. There is no evidence that the reaction yields ATP. In certain fermented products (e.g. wines, pickled cucumbers, soy sauce, silage) the malolactic fermentation has the effect of reducing the acidity of the product (lactic acid being a weaker acid than malic acid); this may or may not be desirable, depending on the degree and nature of acidity required in the product (see e.g. WINE-MAKING). Malolactic fermentation in wine tends to be slow and may be unpredictable; where required, it may be speeded up by the addition of a pure culture of suitable bacteria (commonly Leuconostoc oenos). The gene for malolactic fermentation has been transferred from Lactobacillus delbrueckii to a wine yeast in an attempt to produce a yeast capable of simultaneous malolactic and alcoholic fermentations [AEM (1984) 47 288–293]; however, the resulting yeast exhibited only low levels of malolactic fermentation. Malpighamoeba A genus of amoebae (order AMOEBIDA). M. mellificae is parasitic in bees (see BEE DISEASES), M. locustae is parasitic in locusts [Parasitol. (1976) 72 127–135]. malt agar (malt extract agar) A medium made by diluting MALT EXTRACT to a specific gravity of 10° Balling and solidifying it with 2% agar. malt extract An extract of malt, used e.g. as a medium for the culture of a wide range of yeasts and moulds. Malt (1 kg) is mixed with water (2.6 litres). The mixture is held at 45° C for 3 h (with stirring), and then at 63° C for 1 h; it is then filtered and autoclaved. The specific gravity of the filtrate is adjusted to 15° Balling using a Balling saccharometer. Malt extract is available commercially in dehydrated form. (See also BREWING.) Malta fever See BRUCELLOSIS. Maltaner antigen A cardiolipin–lecithin antigen used in certain STANDARD TESTS FOR SYPHILIS. maltase An a-glucosidase (EC 3.2.1.20) which hydrolysis (1 ! 4)-a-D-glucosidic bonds; thus, e.g. maltose is converted to free glucose. maltobiose Syn. MALTOSE. maltoporin See LAMB PROTEIN. maltose (maltobiose) A reducing disaccharide: a-D-glucopyranosyl-(1 ! 4)-D-glucopyranose. Maltose is an intermediate in the metabolic mobilization of STARCH in plants (e.g. during the germination of cereal grains – see e.g. BREWING); it also occurs e.g. as a major product of photosynthesis in Chlorella symbionts of Hydra viridis and of Paramecium bursaria (although it is not produced by the non-symbiotic alga). Maltose can be metabolized by a wide range of microorganisms. (For maltose transport in e.g. Escherichia coli see BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM.) (cf. ISOMALTOSE; see also AMYLASES and MALTASE.) maltotriose A trisaccharide: a-D-glucopyranosyl-(1 ! 4)-a-Dglucopyranosyl-(1 ! 4)-D-glucopyranose. (See also AMYLASES.) Mamiella See MICROMONADOPHYCEAE. Proguanil (D Paludrine, CHLORGUANIDE) is a FOLIC ACID ANTAwith e.g. chloroquine it is used for prophylaxis. Of alternative prophylactic agents, mefloquine is absolutely contraindicated in early pregnancy. See also QINGHAOSU. In general, while antimalarials are used prophylactically by travellers, such use is considered undesirable for residents in endemic areas because it lowers the level of acquired resistance, resulting in high mortality when the drug is withdrawn. [Chemotherapy: Book ref. 213, pp 119–128.] [Malaria feature (vaccines, drugs, mosquito engineering etc.): Science (2000) 290 428–441.] malarial pigment Syn. HAEMOZOIN. Malassezia (syn. Pityrosporum) A genus of non-fermentative imperfect yeasts (class HYPHOMYCETES) which occur on the skin in man and animals. The cells are spherical, oblong-ellipsoidal, or cylindrical, and reproduce by monopolar bud-fission. True hyphae are formed infrequently in culture, but are common in skin scales. Growth is commonly stimulated by certain oils (e.g. olive oil); optimum growth temperature: 35–37° C. The organisms can grow on glucose-peptone-yeast extract agar (overlaid with olive oil for M. furfur ); they do not grow in the liquid media used in standard physiological tests for yeasts. Two species are recognized [Book ref. 100, pp. 882–885]: M. furfur (including ‘Pityrosporum ovale’ and ‘P. orbiculare’, which are currently considered to be variant cultural forms of M. furfur ), and M. pachydermatis (D P. pachydermatis, P. canis). M. furfur occurs e.g. on human skin (see also PITYRIASIS VERSICOLOR); M. pachydermatis has been isolated from the ears of dogs (in which it may be associated with otitis externa) and from human skin. malate synthase See TCA CYCLE. male-specific phage Syn. ANDROPHAGE. malic enzyme (1) See TCA CYCLE. (2) Syn. malolactic enzyme (see MALOLACTIC FERMENTATION). malignant (med.) (1) (of disease) Becoming progressively worse (when untreated), usually resulting in death. (2) (of tumours) Refers to a neoplasm which invades adjacent tissues and which may metastasize (see METASTASIS). (cf. CANCER.) malignant catarrhal fever (gangrenous coryza) An acute CATTLE DISEASE caused by a cytomegalovirus-like herpesvirus (see BETAHERPESVIRINAE). The disease is characterized by high fever with severe inflammatory and degenerative lesions in the mucous membranes of the upper respiratory tract and intestinal tract; the eyes may be affected, and blindness may result. Mortality rates are generally high (usually >95%). The disease may also affect certain other ungulates (e.g. sheep, wildebeest, deer). [Review of the disease: VR (1984) 114 581–583.] malignant oedema (vet.) Tissue oedema and necrosis, often involving frank GAS GANGRENE, due to wound infection by one or more species of Clostridium – e.g. C. chauvoei, C. novyi, C. septicum, C. sordellii. Symptoms: fever, with a soft, red, local swelling which becomes dark; in some cases gas production gives rise to a frothy exudate from the wound. Death usually occurs within 1–2 days. Treatment: parenteral administration of antibiotics, antitoxin therapy (appropriate only in early stages of the disease), irrigation of the wound with hydrogen peroxide. (See also SWELLED HEAD.) malignant pustule Cutaneous ANTHRAX. malignant tertian malaria See MALARIA. mallein test (vet.) A test for GLANDERS (sense 1) involving the intradermal injection of mallein (containing antigens of the pathogen) e.g. into the lower eyelid; a positive reaction is GONIST; 457 Mancini test maple bark strippers’ disease An EXTRINSIC ALLERGIC ALVEOLITIS associated with inhalation of the spores of Cryptostroma corticale. Maranil Dodecylbenzolsulphonate: an agent used e.g. in BTB AGAR (0.005%) to inhibit swarming by Proteus spp. Marasmius See AGARICALES (Tricholomataceae); see also MYCORRHIZA. marble bone Syn. OSTEOPETROSIS. marble spleen disease A disease of pheasants caused by an adenovirus very similar to the causal agent of HAEMORRHAGIC ENTERITIS OF TURKEYS. The disease is acute and rapidly fatal, and involves severe pulmonary oedema with enlargement and mottling of the spleen. [Book ref. 116, pp. 537–539.] Marburg fever (green monkey fever) A VIRAL HAEMORRHAGIC FEVER, caused by the Marburg virus (see FILOVIRIDAE), first reported in 1967 when an outbreak occurred in Marburg (W. Germany) among laboratory personnel working with infected tissues of the African green monkey (Cercopithecus aethiops). Incubation period: ca. 6–10 days. Onset is sudden, with e.g. fever, mental confusion, diarrhoea, haemorrhages from mucous membranes, and often a maculopapular rash. The disease is often fatal. (Ebola virus causes a similar disease.) Marburg virus See FILOVIRIDAE. Marek’s disease (fowl paralysis) A POULTRY DISEASE caused by gallid herpesvirus 1 (see GAMMAHERPESVIRINAE); chickens, turkeys, pheasants, ducks, pigeons and other birds are susceptible, but the principal natural hosts appear to be domestic chickens and their close relatives (Gallus spp). Infection seems to occur primarily by inhalation of ‘feather dust’ released from the feather follicles of infected birds. The classical (chronic) form of the disease involves the development of T-cell lymphomas in the peripheral nerves, causing progressive paralysis. A visceral (acute) form also occurs in which lymphomas develop in internal organs; mortality rates for the acute disease are commonly high. Infected birds remain carriers – probably for life. Live vaccines are available: e.g., vaccination with gallid herpesvirus 2 (‘turkey herpesvirus’) protects chickens and turkeys against Marek’s disease. [Review of Marek’s disease and its virus: Book ref. 105, pp. 155–183.] Marek’s disease virus can also cause chronic atherosclerosis in SPF chickens [AJP (1986) 122 62–70]; MDV infection apparently prevents activation of cytoplasmic cholesteryl esterase in arterial smooth muscle cells [JBC (1986) 261 7611–7614]. These observations lend support to the hypothesis that herpesvirus infection may play a role in the pathogenesis of atherosclerosis (‘hardening of the arteries’) in man [review: MS (1986) 3 50–52]. Marfanil (sulphamylon; p-aminomethylbenzenesulphonamide: NH2 CH2 .C6 H4 .SO2 .NH2 ) A drug which has been used e.g. in the topical treatment of wounds, burns, etc. It differs from other SULPHONAMIDES in that it is not a sulphanilamide derivative, the p-aminomethyl group replacing the p-amino group of sulphanilamide. marginal zone B cells (MZ B cells) A subset of B LYMPHOCYTES found within the marginal zone of the spleen (i.e. the junction of red and white pulp) – together with DENDRITIC CELLS and macrophages; cells in the marginal zone are considered to represent an early form of defence against blood-borne pathogens. MZ B cells are important in responses to T-independent antigens, and they are particularly sensitive to lipopolysaccharides. [See e.g. Science (2000) 290 89–92.] Marinomonas See ALTEROMONAS. Mancini test See SINGLE DIFFUSION. mancozeb An agricultural antifungal agent consisting of a complex of zinc and MANEB; it is used, for example, against late blight of potato. Mandelamine See HEXAMINE. mandelic acid Hydroxyphenylacetic acid: C6 H5 .CHOH.COOH. Mandelic acid has bacteriostatic properties and is used as a urinary antiseptic – being excreted unchanged if the urine is sufficiently acidic; it may be used in combination with HEXAMINE. Mandler filter See FILTRATION. maneb (dithane M-22) Manganese ethylene-BISDITHIOCARBAMATE, an agricultural antifungal agent with a spectrum of activity similar to that of ZINEB; it is used e.g. to control diseases caused by Alternaria solani and Phytophthora infestans. manganese (in microbiology) See e.g. IRON BACTERIA, PSEUDOCATALASE and SUPEROXIDE DISMUTASE. mango black spot See BLACK SPOT (3). mannans Polysaccharides composed of manosyl residues. Linear (1 ! 4)-b-linked mannans occur in some algae of the Chlorophyta (e.g. Acetabularia, Codium) – in which they may replace cellulose as the main structural component of the CELL WALL – and in certain plants (e.g. in many palm seeds); in many yeasts the CELL WALL has a high mannan content. Heteroglycans consisting of (1 ! 4)-b-linked D-glucosyl and D-mannosyl residues (glucomannans) are major HEMICELLULOSES in gymnosperms. mannitol The POLYOL corresponding to mannose; it occurs e.g. in many plants, algae, fungi and lichens, and is formed by the reduction of fructose by certain bacteria (see e.g. SILAGE). Mannitol is a major carbohydrate in members of the Phaeophyta and in chrysophytes (in which it is the primary photosynthetic carbohydrate) and in ascomycetous and basidiomycetous fungi; earlier reports that it occurs in the Rhodophyta appear to be incorrect [Book ref. 37, pp. 162–165]. Mannitol can have an osmoregulatory role in certain algae and fungi exposed to fluctuating salinities. mannopine An opine of the agropine type: see e.g. HAIRY ROOT. mannose An aldohexose found e.g. in MANNANS and in animal mucolipids and mucoproteins. (See also NEURAMINIC ACIDS; cf. MANNITOL.) mannose-binding lectin See COMPLEMENT FIXATION (c). mannose-resistant haemagglutination See FIMBRIAE. mannose-sensitive haemagglutination See FIMBRIAE. mantle (mycol.) See MYCORRHIZA. Mantonella See EIMERIORINA. Mantoniella See MICROMONADOPHYCEAE. Mantoux test See TUBERCULIN TEST. manure gas poisoning Poisoning of man or animals by gases (e.g. H2 S, NH3 , CO2 ) produced by anaerobic bacteria in manure stored in large slurry pits; exposure to high concentrations of these gases – e.g. during mixing of slurries prior to emptying pits – can be rapidly fatal. MAP (1) (M-associated protein) A protein often associated with the streptococcal M PROTEIN; it also occurs in streptococci of groups C and G that have M-like antigens. (2) MICROTUBULE-ASSOCIATED PROTEIN. (3) METHIONINE AMINOPEPTIDASE. MAP kinase See ANTHRAX TOXIN. map lichens Rhizocarpon spp, particularly R. geographicum. map unit (map distance) (1) Syn. CENTIMORGAN. (2) Any unit used for indicating the position of markers in a genome: e.g. the chromosome of Escherichia coli is divided into 100 minutes (see INTERRUPTED MATING). 458 Mastadenovirus host sequences – often undergoes amplification. Transformation requires the integration of at least the E1a region, but the mechanism of transformation is unknown. [Adenoviral genes and transformation: Book ref. 110, pp. 125–172.] Human adenoviruses have been classified into subgenera (subgroups) on the basis of e.g. DNA characteristics, oncogenicity, subgenus-specific antigens, morphology, haemagglutinating properties, etc: Subgenus A (Ad12, 18 and 31): GC% of the DNA: 47–49; length of virion fibres: 28–31 nm. Viruses in this subgenus are highly oncogenic when injected into newborn hamsters or rats, causing tumours after ca. 2–4 months; in man, they are associated with upper respiratory tract infections and diarrhoea, or infection may be subclinical. Subgenus B (Ad3, 7, 11, 14, 16, 21, 34 and 35): GC% of the DNA: 49–52; length of the virion fibres: 9–11 nm. Viruses in this subgenus are weakly oncogenic when injected into newborn hamsters, causing tumours at low incidence after ca. one year; in man, they cause e.g. ARD (Ad3, 7, 14, 21), acute haemorrhagic CYSTITIS (Ad11), PHARYNGOCONJUNCTIVAL FEVER, pneumonia, etc. Subgenus C (Ad1, 2, 5 and 6): GC% of the DNA: 57–59; length of the virion fibres: 23–31 nm. These viruses are nononcogenic in newborn rodents, but can cause transformation of rodent cells in vitro; they can cause mild to severe upper respiratory tract disease in young children. Subgenus D (Ad8–10, 13, 15, 17, 19, 20, 22–30, 32, 33, 36, 37–39): GC% of the DNA: 57–60; length of the virion fibres: 12–13 nm. The viruses are non-oncogenic in newborn hamsters, but may cause mammary adenomas in newborn female rats and can transform rodent cells in vitro. Ad8 and Ad19 are causal agents of EPIDEMIC KERATOCONJUNCTIVITIS. Subgenus E (Ad4): GC% of the DNA: 57; length of virion fibres: 17 nm. Non-oncogenic in newborn hamsters. Ad4 can cause EPIDEMIC KERATOCONJUNCTIVITIS, acute respiratory disease (see ARD), and PHARYNGOCONJUNCTIVAL FEVER. Two further subgenera, F (Ad40) and G (Ad41), have been recognized but are not yet well characterized; these include the ‘enteric’ or ‘fastidious’ adenoviruses [review: Arch. Virol. (1986) 88 1–17] which cause diarrhoea in infants and young children, and which cannot be propagated in conventional cell cultures. They can be grown in an Ad5-transformed human embryonic kidney cell line. Most human adenoviruses will not replicate readily in simian cell lines, but may do so in the presence of simian virus 40 (SV40); SV40 can interact extensively with human adenoviruses, e.g. forming SV40–adenovirus hybrids [Book ref. 116, pp. 399–449]. Animal adenovirus nomenclature follows at least two different conventions; a species may be designated by a three-letter prefix derived either from the name of the animal plus ‘AV’ (e.g. BAV for bovine adenovirus, SAV for simian adenovirus [Book ref. 116, p. 500]) or from the genus name of the host animal (e.g. bos for cattle, sus for pigs, equ for horses, mus for mice [Book ref. 23, p. 60]), followed in either case by the serotype (species) number – e.g. BAV-3 or bos-3, MAV-1 or mus-1, etc. Infection of animals by adenoviruses may result in (usually mild) respiratory or diarrhoeal illnesses, conjunctivitis, etc. Canine adenovirus type 1 (CAV-1, can-1) can cause encephalitis in foxes and e.g. hepatitis in dogs; CAV-2 is thought to be a causal agent of laryngotracheitis (‘kennel cough’) in dogs. Several animal adenoviruses (11 simian, 2 bovine and 2 canine species) can induce tumours when injected into newborn hamsters. [Animal adenoviruses: Book ref. 116, pp. 497–562.] marker (genetics) A genetic locus which is associated with a particular (usually readily detectable) phenotypic characteristic: e.g., an antibiotic resistance gene. marker rescue Conferment on a gene, or genes, of the ability to replicate as a result of the integration of the gene(s) with a replicon. (See also REACTIVATION sense 1.) Marseilles fever Syn. BOUTONNEUSE. marsh gas See ANAEROBIC DIGESTION. Marssonina See MELANCONIALES. marsupium Syn. BROOD POUCH. Marteilia See STELLATOSPOREA. Martensella See KICKXELLALES. marticin See NAPHTHAZARINS. Mason–Pfizer monkey virus (MPMV) An exogenous, xenotropic retrovirus of the TYPE D RETROVIRUS GROUP. MPMV was isolated from a carcinoma of a rhesus monkey; it has not been shown to be oncogenic either in its natural host or in other primates. (cf. SIMIAN AIDS.) MASP MBL-associated serine protease: see COMPLEMENT FIXATION (c). mast cell A cell which can respond to certain stimuli by rapidly secreting e.g. vasoactive products; it is important e.g. in immediate-type hypersensitivity reactions and in acute INFLAMMATION. Unlike the BASOPHIL, the mast cell is primarily a noncirculating cell, occurring e.g. in blood vessels and in the lymphatic system. The (basophilic) cytoplasmic granules in the mast cell contain a variety of biologically active products – e.g. HISTAMINE, HEPARIN, SEROTONIN (significant amounts in some species), and proteases – which are released, within 15–20 sec, on activation and degranulation of the cell. Mast cells can be activated and degranulated e.g. by certain CYTOKINES, ANAPHYLATOXINS, IgE (and certain IgG) antibodies together with their homologous antigens, IgE aggregates (not monomeric IgE), bivalent anti-IgE receptor antibodies, and some LECTINS (e.g. PHA). The mast cell surface contains many high-affinity receptor sites for the Fc portion of IgE, and the cross-linking of surfacebound IgE antibodies by antigen (allergen) appears to be a trigger for degranulation (see also TYPE I REACTION). (Degranulated mast cells gradually replace their histamine.) Activated mast cells also produce e.g. SUPEROXIDE, H2 O2 , and factors which are chemotactic for NEUTROPHILS and EOSINOPHILS. Mastadenovirus (mammalian adenoviruses) A genus of adenoviruses (family ADENOVIRIDAE) which infect mammals (including man); type species: human adenovirus type 2 (D h2, D Ad2). Almost all mammalian adenoviruses share a common (groupspecific) antigen (a hexon surface antigen); subgenus-specific and species-specific antigens can also be detected. Human adenovirus species are designated by the prefix Ad (or h) followed by the species (serotype) number: Ad1, Ad2, etc (or h1, h2, etc). Human adenoviruses can cause a wide range of diseases (as well as subclinical infections) in humans. Several can induce tumours (mainly sarcomas) when injected into newborn rodents, but none is known to cause tumours in man under natural conditions; however, most human adenoviruses can cause morphological transformation in certain types of cell cultures, causing the formation of characteristic foci of small, often polygonal cells containing scanty cytoplasm and capable of dividing indefinitely. Transformation of cells by adenoviruses involves the integration of viral DNA sequences into host chromosomal DNA. Depending on the host cell, the whole viral genome or (more commonly) a fragment of it may be integrated; integration apparently occurs at random sites in the host DNA, and the viral DNA – together with flanking 459 Mastigocladus maternal inheritance A form of CYTOPLASMIC INHERITANCE in which certain characteristics are transmitted to sexually derived progeny only from the female parent cell. Various hypotheses have been proposed to account for maternal inheritance, and different mechanisms may be applicable in different cases. Thus, in certain cases where maternally inherited genes occur in mitochondria, mitochondria from the male parent are presumed to be excluded from the zygote (see e.g. POKY MUTANT). To account for the maternal inheritance of genes in chloroplast DNA (a phenomenon recorded in some isogamous green algae) it has been proposed that chloroplast DNA from the male parent is preferentially digested [MS (1985) 2 267–270]. mating Syn. CONJUGATION (sense 1). mating type A strain of an organism which can interact sexually only with other, genetically distinct, strains of the same species. (See also HETEROTHALLISM and SYNGEN.) In Saccharomyces cerevisiae mating type is determined at a single genetic locus, designated MAT, on chromosome III. In haploid cells, MAT is occupied by either the MAT a allele or the MAT a allele – which occur in a and a mating types respectively. According to the widely-accepted ‘a1-a2 hypothesis’, MAT a and MAT a regulate the same groups of structural genes, but their different modes of regulation result in the development of phenotypically different mating types; thus, groups of genes known as ‘a-specific genes’ and ‘a-specific genes’ (which specify the a and a phenotypes, respectively) occur in the cells of both a and a mating types, and the way in which these genes are controlled by the MAT locus determines the cell’s mating type. MAT a is transcribed in two parts from a central promoter region, the two RNA transcripts being copied from different DNA strands; the parts are called MAT a1 and MAT a2. MAT a1 expression is necessary for the transcription of the a-specific genes – whose products include the a-f actor (a PHEROMONE). MAT a2 is a negative regulator of a-specific genes. The (single) MATa product has no effect in the a-type haploid vegetative cell; by default, a-type genes (including that encoding the a-type pheromone a-factor) are expressed in cells of this mating type. In most natural isolates of S. cerevisiae, spontaneous conversion of the MAT a allele to the MAT a allele, and vice versa, occurs quite frequently – with concomitant switching of mating type; a given population will therefore consist of a mixture of compatible mating types (a and a) and will give the appearance of being homothallic. (Stability at the MAT locus involves another locus, designated HO for homothallism, located on chromosome IV.) Switching involves a cassette mechanism, unexpressed copies of MATa and MATa being stored on chromosome III at sites designated, respectively, HMR and HML; these genes are silenced by the transcription-resistant form of their chromatin. Switching requires a double-stranded cut at the MAT locus, made by the HO (‘YZ’) endonuclease, followed by excision of the resident MAT allele and its replacement with a copy of the allele of the other mating-type (gene conversion). When a-type and a-type cells are mixed, the pheromone from a given mating type causes cells of the other type to stop in the G1 phase of the CELL CYCLE, and to exhibit a mating typespecific glycoprotein component of the cell wall which promotes fusion between cells of opposite mating types. The zygote, which lacks mating potential and mating type characteristics, is designated a/a. In the a/a zygote, the a2 and a1 proteins jointly switch off various genes, including a-specific and a-specific genes and the ‘haploid-specific’ genes active in both mating types. Mastigocladus See FISCHERELLA and CHLOROGLOEOPSIS. Mastigomycota In some taxonomic schemes: a division equivalent to the subdivision MASTIGOMYCOTINA. Mastigomycotina A subdivision of unicellular and (more commonly) mycelial fungi (division EUMYCOTA); most species (though not e.g. Peronospora spp) form flagellated spores or gametes. Classes: CHYTRIDIOMYCETES, HYPHOCHYTRIOMYCETES, OOMYCETES. mastigoneme See FLAGELLUM (b). Mastigophora A subphylum of protozoa (phylum SARCOMASTIGOPHORA) which have one or more flagella; cell division is typically symmetrogenic. Classes: PHYTOMASTIGOPHOREA and ZOOMASTIGOPHOREA. mastigosome Syn. BASAL BODY (b). mastitis Inflammation of the mammary gland; it may or may not be of microbial causation. (a) (med.) Puerperal mastitis may occur in lactating women, commonly ca. 1 month after childbirth, and may be caused by staphylococci or streptococci. (b) (vet.) Mastitis is quite common among farm animals, but is uncommon in mares. In cows, common causal agents include e.g. Actinomyces pyogenes (‘summer mastitis’ in dry cows and precalving heifers), Escherichia coli (‘coliform mastitis’ is caused by E. coli or other coliforms), Staphylococcus aureus (see also BLACK POX), and Streptococcus agalactiae, but any of a wide range of organisms may be involved, including various other bacteria (e.g. species of Pasteurella, Pseudomonas), fungi (e.g. Aspergillus, Candida) and algae (Prototheca spp). In ewes, mastitis is commonly caused by E. coli, S. aureus or S. agalactiae but may be due e.g. to Mycoplasma agalactiae, while in mares the causal agent may be e.g. Streptococcus equi. In cows, predisposing factors include age and the stage of lactation; thus, e.g. cows which have had ca. four or more lactations, and those within the first two months of lactation, are more susceptible than others. Symptoms may range from mild to an acute inflammatory condition with fever and other systemic effects; the milk contains an increased number of white blood cells, and may contain a significant number of pathogenic microorganisms. (See also CALIFORNIA MASTITIS TEST and CAMP TEST.) Treatment: e.g. parenteral administration or intramammary infusion of antibiotics [Book ref. 33, pp. 451–500]. [Chemoprophylaxis in bovine mastitis: Book ref. 121, pp. 193–204.] [DNA fingerprinting and ribotyping in epidemiological investigations of outbreaks of Pseudomonas aeruginosa mastitis among Irish dairy herds: AEM (1999) 65 2723–2729.] mat (microbial) See MICROBIAL MAT. MAT locus See MATING TYPE. matching (in taxometrics) See entry SSM . MATE Multidrug and toxic compound extrusion: an EFFLUX MECHANISM which e.g. mediates resistance to antibiotic(s) in certain bacteria (including e.g. Escherichia coli and Vibrio parahaemolyticus). [Mol. Microbiol. (1999) 31 393–395.] mate killer The term for certain (endosymbiont-containing) strains of ciliate protozoa which, on conjugation (mating), cause the death of the conjugal partner; mate killers include strains of Paramecium aurelia (endosymbiont: Caedibacter paraconjugatus or Pseudocaedibacter conjugatus) and of Euplotes crassus and E. patella (endosymbionts unnamed). In each case mating is essential for killing; thus, e.g. the ingestion of C. paraconjugatus by sensitive paramecia does not cause harmful effects. maternal immunity PASSIVE IMMUNITY acquired by a fetus or neonate from its mother either via the placenta or from COLOSTRUM. 460 measles between an ecotropic MuLV and an endogenous xenotropic retrovirus. Since many MCF viruses have a leukaemogenic potential greater than that of the parent MuLV, it has been suggested that an MCF component may be responsible for ‘MuLV-induced’ leukaemogenesis. (See also AKV; cf. FRIEND VIRUS.) McFadyean’s test A confirmatory test for ANTHRAX carried out, post-mortem, on an infected guinea pig. Blood (from the heart), or a spleen imprint, is stained with polychrome methylene blue; in a positive test, the stained smear reveals large, blue, squareended bacilli surrounded by a reddish granular capsule. McIntosh and Fildes’ anaerobic jar (modified) An ANAEROBIC JAR which consists of a strong cylindrical metal chamber with a flat, circular, gas-tight lid; such jars are referred to by various trade names (e.g. Torbal jar). Media etc are placed in the jar, and the lid is secured by means of a screw clamp. The jar is then evacuated, by a suction pump, via one of two screw-controlled needle valves in the lid; this valve is then closed. (To prevent the vacuum sucking the agar from Petri dishes, plates are incubated lid-side up – cf. GASPAK.) The jar is filled, via the other valve, with gas(es) – e.g. H2 , or CO2 and H2 – from a compressed-gas cylinder or from a gas-filled rubber bladder; this valve is closed, and the cycle of evacuation and re-filling is carried out several times. Attached to the inside of the lid is a gauze envelope containing a catalyst (e.g. palladium-coated pellets of alumina) which promotes chemical combination between hydrogen and the last traces of oxygen in the jar; such catalysts work at room temperatures and are called ‘cold’ catalysts. The jar also has a side-arm to which is attached a small, thick-walled, closed glass vessel which communicates with the interior of the jar and which contains an indicator of anaerobiosis (e.g. METHYLENE BLUE solution). MCP (1) Methyl-accepting chemotaxis protein (see CHEMOTAXIS). (2) Membrane co-factor protein (see COMPLEMENT FIXATION (b).) MCS (multiple cloning site) See POLYLINKER. M-CSF See COLONY-STIMULATING FACTORS. MDa Megadalton: 106 DALTONS. MDCK cells Madin–Darby canine kidney cells: a heteroploid cell line derived from the kidney of an apparently normal dog. MDCK cell cultures have been used e.g. for the primary isolation and passage of influenzavirus type A [PNAS (2000) 97 9654–9658]. MDP Muramyl dipeptide: N-acetylmuramyl-L-alanyl-D-isoglutamine; a water-soluble component of a modified form of complete FREUND’S ADJUVANT. mean doubling time See GROWTH (a). mean generation time See GROWTH (a). measles (morbilli; cf. RUBEOLA) An acute and highly infectious human disease which affects mainly children. (Other primates can be affected.) The causal agent is a MORBILLIVIRUS. Transmission occurs by droplet infection or via fomites; the conjunctivae may be important sites of infection. Incubation period: 8–12 days. In the prodromal stage (2–4 days) there is coryza, cough, fever, and the appearance of KOPLIK’S SPOTS. Subsequently, a maculopapular rash develops on the head, then on the limbs and trunk. The spots are initially bright pink but become deeper red and then brownish – a phenomenon known as ‘staining’. Viruses are present in the urine, nasopharyngeal secretions etc during the prodromal period and for several days after the appearance of the rash. Complications may include ENCEPHALITIS [NEJM (1984) 310 137–141]; secondary bacterial infection may result in e.g. matrix protein (bacteriol.) A former term for PORIN. maturase See SPLIT GENE (b) and (c). Maurer’s clefts (Maurer’s dots) Large dots, streaks, or ‘commas’ sometimes observed, on suitable staining, in erythrocytes infected with certain stages of Plasmodium falciparum. Maus Elberfeld virus See CARDIOVIRUS. maxicell A bacterial ‘cell’ obtained by UV irradiation of recA, uvrA mutant strains of e.g. Escherichia coli. Irradiation of such mutants results in damage to and degradation of chromosomal DNA (cf. RECKLESS DNA DEGRADATION); however, if small, multicopy plasmids are present in such cells, some of the plasmids may be undamaged by the UV radiation and can subsequently replicate. These cells therefore synthesize plasmidencoded proteins almost exclusively; the proteins can be labelled by adding e.g. [35 S]methionine to the medium. [JB (1979) 137 692–693.] (cf. MINICELL.) maxicircle DNA See KINETOPLAST. maximum specific growth rate (µmax ) See SPECIFIC GROWTH RATE. Mayaro fever An epidemic febrile human disease caused by an ALPHAVIRUS and transmitted by mosquitoes (e.g. Haemagogus janthinomys); it occurs in the Caribbean and parts of S. America. Symptoms: e.g. headache, myalgia, arthralgia, and a maculopapular rash. Monkeys are probably the main natural hosts. Mayorella A large genus of amoebae (order AMOEBIDA) which characteristically form several bluntly conical, hyaline pseudopodia of similar lengths. (See also UROID.) Some species have a surface covering of complex scales (not discernible by light microscopy). Species occur in freshwater habitats and e.g. sewage treatment plants. M. viridans contains ZOOCHLORELLAE. maytansine See MICROTUBULES. mazaedium A type of ascocarp in which the asci disintegrate to release a powdery mass of ascospores (with sterile elements); mazaedia are formed in members of the Caliciales and in Onygena. (The term ‘mazaedium’ may also refer to the spore mass per se.) MBC fungicides See BENZIMIDAZOLES (a). MBL Mannose-binding lectin: see COMPLEMENT FIXATION (c). Mbl protein See CELL CYCLE (b) (determination of shape). MBP Mannose-binding protein, referred to more commonly as MBL (mannose-binding lectin) – see COMPLEMENT FIXATION (c). MBSA Methylated bovine serum albumin. MC29 See AVIAN ACUTE LEUKAEMIA VIRUSES. Mcc plasmid See MICROCINS. McCartney bottle A bottle similar to a UNIVERSAL BOTTLE but which has a narrower neck. McClung Toabe egg-yolk agar (modified) A medium used for the isolation and identification of clostridia and other anaerobes; it permits detection of lecithinase, lipase and proteolytic activity. The medium contains tryptone, yeast extract, glucose, egg yolk, Na2 HPO4 , NaCl, MgSO4 and agar. McDonough feline sarcoma virus See FMS. MCF viruses Mink cell focus-forming viruses: variant MURINE LEUKAEMIA VIRUSES (MuLVs) which can be detected and assayed by their ability to induce foci of transformed cells in monolayer cultures of a mink lung cell line. MCF viruses were originally isolated from mouse strains (e.g. AKR) which show a very high incidence of spontaneous leukaemia. The env gene of an MCF virus resembles a mixture of those of murine ecotropic and xenotropic viruses, and confers on it a DUALTROPIC host range. MCF viruses are believed to have arisen by recombination 461 meat spoilage OTITIS MEDIA or PNEUMONIA. Measles is usually self-limiting, but may be fatal in young infants or in malnourished children; death is usually due to respiratory complications. Life-long immunity usually follows recovery (cf. SUBACUTE SCLEROSING PANENCEPHALITIS). Lab. diagnosis: microscopic examination of smears of the nasal mucosa for epithelial GIANT CELLS; detection of the measles virus in blood, urine or nasal secretions. Chemotherapy: none. Measles immune globulin may be used prophylactically in high-risk patients exposed to measles. Live attenuated vaccines (cf. MR and MMR) may be given to infants and young children. meat spoilage The occurrence, nature and extent of meat spoilage depends on e.g. the initial numbers and types of contaminating microorganisms, the conditions of storage (particularly temperature and oxygen tension), the length of storage, and the nature of the meat itself (e.g. its WATER ACTIVITY, glucose content [see e.g. AEM (1982) 44 521–524], pH). Spoilage may involve slime formation, discoloration, souring, and off-odours (due e.g. to the volatile products of microbial amino acid metabolism). (a) Carcasses. Carcass meat is normally surface-contaminated with bacteria and fungi derived from the animal’s skin, intestine, faeces etc., from soil, and e.g. from the hands and tools of workers. Even under good hygienic conditions, carcasses are likely to carry 102 –105 bacteria/cm2 , but spoilage commonly becomes apparent only after bacterial numbers reach ca. 107 –109 per cm2 . After slaughter, the pH of meat gradually falls as the muscle glycogen is converted to lactic acid – the final pH depending on the amount of glycogen present at the time of death. The final pH of red meats is commonly 5.4–5.8; chicken leg meat normally has a pH of ca. 6.5, although the breast meat usually has a pH of ca. 5.7. Meat from stressed animals has a higher pH and is more susceptible to spoilage – see DFD MEAT. For short-term storage, carcasses may be chilled to e.g. 0–10° C. Below ca. 10° C psychrotrophic spoilage organisms increase greatly in numbers; by contrast, pathogenic organisms (cf. FOOD POISONING) grow very slowly, and their presence is usually not obvious. In most meats, including uncured pig meat and poultry, the commonest spoilage bacteria under aerobic conditions are strains of Pseudomonas [JAB (1982) 52 219–228 (taxonomic study)], often occurring together with smaller numbers of bacteria such as Acinetobacter, Alteromonas, Brochothrix and Lactobacillus spp. Of the spoilage organisms, Pseudomonas spp have the highest growth rate at low temperatures, and this advantage tends to become more marked with decreasing temperature. (In poultry, Acinetobacter replaces Pseudomonas as the commonest spoilage organism at temperatures above ca. 10° C.) However, Brochothrix thermosphacta and Lactobacillus spp, for example, are more tolerant than pseudomonads of low aw , and these may become dominant on drying carcasses. On cured meats (see CURING), e.g. bacon, the commonest spoilage bacteria under aerobic conditions are halophilic, psychrophilic vibrios (e.g. Vibrio costicola) and species of Acinetobacter and Micrococcus; the micrococci are lipolytic and proteolytic, and some strains can reduce nitrite. Fungi found on chilled carcasses include Cladosporium, Penicillium, Mucor, Rhizopus and Thamnidium, and the yeasts Candida, Cryptococcus and Rhodotorula; these may cause e.g. surface discolorations. For long-term storage, temperatures of ca.
12° C to
18° C are often used; the process of freezing kills or damages a proportion of contaminants, although lower temperatures (e.g.
30° C) are less lethal (see FREEZING). At temperatures between ca.
5° C and
10° C fungal contaminants may become important spoilage agents (see e.g. BLACK SPOT sense 1). (b) Packaged meats. As well as the factors cited above, the nature of spoilage of packaged meats depends on the type of packaging – particularly on the permeability of the packing material to air. Spoilage of chilled red meats typically occurs more rapidly in the presence of oxygen than under vacuum. Under aerobic conditions (e.g. with oxygen-permeable packing film) Pseudomonas spp are the commonest spoilage organisms; Acinetobacter may be present in significant numbers if the pH is above ca. 6.0. Under anaerobic conditions (e.g. in oxygen-impermeable vacuum packs) these organisms are inhibited, permitting the growth of e.g. Lactobacillus spp and Brochothrix thermosphacta; CARBON DIOXIDE per se (ca. 10–20% v/v) inhibits the growth of e.g. Pseudomonas spp, Alteromonas putrefaciens, Acinetobacter sp, and Yersinia enterocolitica, but not of e.g. B. thermosphacta or lactobacilli. (cf. DFD MEAT.) At low temperatures (e.g. 5° C) vacuum-packed cured meats (e.g. bacon) are susceptible to spoilage by e.g. Lactobacillus spp (which may cause e.g. souring) [AvL (1983) 49 327–336]; lactobacilli are resistant to nitrite, smoke (see SMOKING), and low concentrations of salt. At higher salt concentrations, Micrococcus spp may become dominant. Cooked hams and cooked sausage (in which CATALASE has been destroyed) may undergo a type of greening caused by e.g. lactobacilli (especially L. viridescens); these organisms produce H2 O2 which oxidizes the porphyrin ring in meat haem pigments to form a green compound (cf. DFD MEAT). (Higher concentrations of H2 O2 can produce yellow or colourless compounds.) [Book ref. 30.] (See also CANNING; FOOD SPOILAGE; SALAMI; SOFT CORE HAM.) mecA gene See MRSA. MECAM A (synthetic) functional analogue of enterobactin used e.g. in studies on IRON uptake [see e.g. JB (1986) 167 666–680]. mechanical transmission Transmission of a parasite by passive transfer from one host to another, e.g. via a mechanical VECTOR, via contaminated tools or instruments etc. mechanical vector See VECTOR (1). mechanosensitive channel (stretch-activated channel) A channel through the CYTOPLASMIC MEMBRANE which responds to increases in the cell’s turgor pressure by increasing in pore size, thus facilitating efflux of water and certain solutes [JBC (1997) 272 32150–32157]; one example is the MscL channel in Escherichia coli [mechanosensitive channels in E. coli : ARP (1997) 59 633–657]. Such channels occur in many types of bacteria and appear to have an important role in OSMOREGULATION; they open when the cell’s turgor pressure is just below the level at which fatal disruption would occur [EMBO (1999) 18 1730–1737]. [The gating mechanism of the large mechanosensitive channel in Mycobacterium tuberculosis: Nature (2001) 409 720–724.] mecI, mecR1 genes See MRSA. mecillinam (Coactin; amdinocillin) A derivative of 6-b-amidinopenicillanic acid (cf. PENICILLINS) which, in e.g. Escherichia coli, binds exclusively to PBP-2 (see PENICILLIN-BINDING PROTEINS). medallion clamp See CLAMP CONNECTION. median lethal dose Syn. LD50 (q.v.). medical flat A flat-sided glass screw-cap bottle obtainable in various sizes. medicarpin An isoflavonoid PHYTOALEXIN produced e.g. by the broad bean (Vicia faba). (cf. WYERONE.) Medina reaction See LEPROMIN TEST. 462 meiosis medium In microbiology: any liquid or solid preparation made specifically for the growth, storage or transport of microorganisms or other types of cell. Growth media for microorganisms may be used e.g. for the initiation of a CULTURE (or a SUBCULTURE), for ENRICHMENT, or for diagnostic (identification) tests – i.e. tests in which the identity of a given organism may be deduced from the characteristics of its growth in or on particular media. (See also TRANSPORT MEDIUM.) Specialized media are used for the growth/maintenance of mammalian (and other) cells in TISSUE CULTURE (q.v.). Before use, a medium is normally STERILE (sense 1). Many chemolithautotrophic bacteria can be cultured in simple aqueous media containing mainly, or only, mixtures of inorganic salts. Nutritionally undemanding heterotrophs can be grown on/in a range of culture media (see e.g. NUTRIENT BROTH and PETONE WATER). Nutritionally ‘fastidious’ heterotrophs may be grown on an ENRICHED MEDIUM. Strict anaerobes are often cultured on ‘prereduced’ media (see ANAEROBE). Solid media usually consist of liquid media which have been solidified (‘gelled’) with an agent such as AGAR or GELATIN; other gelling agents include e.g. ALGINATE, Gelrite (see GELLAN GUM), and PLURONIC POLYOL F127. (See also SILICA GEL.) Solid media (such as e.g. NUTRIENT AGAR) may be used in order to obtain colonies (see COLONY) of a given organism; colonies may be required for identification purposes or e.g. for the determination of viable cell counts (see COUNTING METHODS). Media containing an unusually high concentration of solidifying agent (e.g. stiff AGAR) are used e.g. to inhibit SWARMING; those containing less than normal concentrations of solidifying agent (e.g. semi-solid or ‘sloppy’ agar) may be used e.g. for CRAIGIE’S TUBE METHOD. Basal medium. One which, without supplement, can support the growth of various nutritionally undemanding species; see e.g. NUTRIENT BROTH. Defined medium. One in which all the constituents (including trace substances) are quantitatively known. Differential medium. A solid medium on/in which different types of organism may be distinguished by their different forms of growth; see e.g. MACCONKEY’S AGAR; SS AGAR. Diphasic medium. See DIPHASIC MEDIUM. Enriched medium. See ENRICHED MEDIUM. Enrichment medium. See ENRICHMENT MEDIUM. Inhibitory medium. One containing certain constituent(s) which suppress the growth of certain type(s) of microorganism. Maintenance medium. One used for the initial growth and subsequent storage (under conditions of minimal growth) of microorganisms or other cells. For microorganisms, a maintenance medium is used to prepare a CULTURE of the given organism which is then stored either at room temperature or under refrigeration; subculture is required at intervals ranging from 1 to 12 months. Ideally, the constituents of a maintenance medium are the minimum consistent with the need to maintain viability of the organisms. Examples: COOKED MEAT MEDIUM and DORSET’S EGG. (See also TISSUE CULTURE.) Pre-reduced medium. See ANAEROBE. Selective medium. One which allows or encourages the growth of some type(s) of organism in preference to others. The term usually refers to media such as DCA, MacConkey’s agar (which generally supports the growth of enteric – but not nonenteric – bacteria), and all the various types of enrichment media; however, all media are selective to some extent. Test medium. One used for diagnostic (identification) tests. Test media are often made by incorporating additional substance(s) into standard or modified liquid or solid media; often a PH INDICATOR is included to monitor certain aspects of metabolic activity. Examples: KCN BROTH, LITMUS MILK, TSI AGAR. Transport medium. See TRANSPORT MEDIUM. MEE MULTILOCUS ENZYME ELECTROPHORESIS. mefA gene See MACROLIDE ANTIBIOTICS. mefloquine See AMINOQUINOLINES and MALARIA. megacins See BACTERIOCIN. megacolon See CHAGAS’ DISEASE. megalomycin See MACROLIDE ANTIBIOTICS. Megasphaera A genus of Gram-negative bacteria (family VEILLONELLACEAE) which occur e.g. in the RUMEN and in the human intestine. Cells: cocci, ca. 2.0–2.5 µm diam., commonly occurring in pairs. The fermentation of glucose yields mainly caproate, while that of lactate yields acetate, propionate, valerate, C4 fatty acids, much CO2 and a little H2 – but little or no caproate. GC%: ca. 54. Type species: M. elsdenii (formerly Peptostreptococcus elsdenii ). Megatrypanum A subgenus of TRYPANOSOMA within the STERCORARIA; species are parasitic in e.g. cattle and sheep. The organisms are large, sometimes greater than 100 µm in length; the kinetoplast is situated approximately mid-way between the nucleus and the posterior end of the cell, and there is a free flagellum. T . (M.) theileri occurs in cattle, and is probably transmitted by contaminative infection of mucous membranes etc with metacyclic forms via tabanid vectors; it is generally non-pathogenic. T . (M.) melophagium occurs in sheep, and T . (M.) theodori occurs in goats. megrims Syn. STAGGERS. MEIA (microparticle enzyme immunoassay) See LCR. meiocyte A cell whose nucleus undergoes MEIOSIS. meiosis (reduction division) The process in which a eukaryotic nucleus divides into nuclei whose ploidy is lower than that of the parent nucleus (typically, haploid nuclei being formed from diploid nuclei) and in which RECOMBINATION usually occurs. (cf. MITOSIS.) Meiosis occurs e.g. during the formation of gametes from diploid cells, and at the inception of haplophase in those organisms which exhibit an ALTERNATION OF GENERATIONS. (See also GAMETIC MEIOSIS; SPORIC MEIOSIS; ZYGOTIC MEIOSIS.) The best-known form of meiosis is that which occurs in higher animals; other forms of meiosis (which differ in one or more aspects) occur in various microorganisms. Meiosis in higher animals involves two distinct nuclear divisions: the first meiotic division (meiosis I) and the second meiotic division (meiosis II) – each division exhibiting a prophase, metaphase, anaphase and telophase. (cf. MITOSIS.) Prophase I. Individual chromosomes, each having previously replicated to form two chromatids, become visible as long, single threads (leptotene stage). In the zygotene stage each chromosome pairs, lengthwise, with its HOMOLOGUE, the pair of closely adjacent chromosomes being called a bivalent ; the pairing of chromosomes (synapsis) involves the SYNAPTONEMAL COMPLEX. In this stage and/or the next (pachytene) stage CROSSING OVER occurs between pairs of non-sister chromatids. In the diplotene stage each chromosome begins to separate from its homologue – but the chromosomes of each homologous pair remain in contact at sites of crossing over (see CHIASMA). In the final stage of prophase I (diakinesis) the chromosomes condense, and each homologous pair is seen to consist of four chromatids (i.e., a tetrad ). By the end of diakinesis the nucleolus and nuclear membrane have disintegrated. Metaphase I. Homologous pairs of chromosomes form a layer at the equator of a microtubular spindle (as in MITOSIS). Anaphase I. The two chromosomes in each homologous pair move to opposite poles of the spindle. 463 meiosporangium L-arabinose. The disease occurs primarily in tropical regions, particularly in S. E. Asia. Infection usually occurs via wounds or (infrequently) by inhalation of contaminated dust or by ingestion. The causal organism occurs as a free-living bacterium in both soil and water; infection by B. (P.) pseudomallei (through cuts, abrasions) has been reported to occur more often during the wet season. Person-to-person transmission of the disease has been reported [Lancet (1991) 337 1290–1291]. In man, the disease is very variable, ranging from a local suppurative lesion at the site of infection to an acute, fulminating septicaemia. The disease may flare up after long periods of time (as found e.g. in American veterans of the Vietnam war). In a recent study of melioidosis in Taiwan, all the isolates of B. (P.) pseudomallei were reported to be susceptible to the following antibiotics: amoxycillin-clavulanate, piperacillintazobactam (tazobactam is a b-lactamase inhibitor), imipenem and meropenem [EID (2001) 7 428–433]. Meliola See BLACK MILDEWS. Melissococcus See EUROPEAN FOULBROOD. Melittangium See MYXOBACTERALES. Melksham virus See SMALL ROUND STRUCTURED VIRUSES. Melolontha melolontha EPV See ENTOMOPOXVIRINAE. Melosira A genus of freshwater and marine centric DIATOMS in which the cylindrical cells (diam. ca. 5–100 µm) are typically joined valve-to-valve to form long chains. In many species the valves are highly pleomorphic; in e.g. M. islandica valve morphology apparently depends on nutrient (phosphorus and silicon) levels [Limn. Ocean. (1985) 30 414–418]. Sexual reproduction is oogamous. melting (of DNA) See HELIX–COIL TRANSITION. Meltzer’s reagent A mycological stain used e.g. for studying spore septation and ascus structure; it consists of iodine (0.5 g), potassium iodide (1–1.5 g), and chloral hydrate (20 g) in distilled water (20 ml). membrane anchor sequence See SIGNAL HYPOTHESIS. membrane attack complex See COMPLEMENT FIXATION (a). membrane co-factor protein See COMPLEMENT FIXATION (b). membrane filter See FILTRATION. membrane fusion protein (MFP) In certain transport systems of Gram-negative bacteria: a protein which appears to act as a link between inner and outer membranes (see e.g. ABC EXPORTER and RND). membrane-inlet mass spectrometry A method used for the direct and continuous monitoring of gas(es) dissolved in a liquid medium, e.g. a bacterial culture. Essentially, a hydrophobic membrane (e.g. PTFE – see FILTRATION) separates the liquid from a vacuum; dissolved gas(es) pass through the membrane into the vacuum and thence into the mass spectrometer. [MS (1984) 1 200–203; Book ref. 132, pp. 239–262; some applications: Book ref. 132, pp. 271–294.] membrane potential See CHEMIOSMOSIS. membrane trigger hypothesis See SIGNAL HYPOTHESIS. membranelle (protozool.) A discrete tuft or band of closely packed, apparently coherent cilia which behave as a unified organelle. (cf. SYNCILIUM.) The term is sometimes used to refer specifically to any one of the serially arranged membranelles of the AZM (q.v.). (See also COMPOUND CILIATURE.) membranous croup Syn. DIPHTHERIA. memory cells (immunol.) Lymphocytes which have had initial contact with specific antigen and which, on subsequent challenge with antigen, can give a rapid and heightened response (compared with the response that followed initial challenge). Telophase I. Each of the two groups of chromosomes becomes enclosed by a nuclear membrane. The interval between the first and second meiotic divisions is called interkinesis or interphase II. The second meiotic division is analogous to mitosis, and consists of prophase II, metaphase II, anaphase II and telophase II. [Control of meiosis: SEBS (1984) 38.] meiosporangium A SPORANGIUM within which MEIOSIS occurs (see e.g. ALLOMYCES). meiospore A haploid spore formed by meiotic cell division. (See also ALTERNATION OF GENERATIONS.) meiotic Of or pertaining to MEIOSIS. Mel B Syn. melarsoprol (see ARSENIC). Mel T See ARSENIC (a). Melampsora A genus of homoxenous and heteroxenous rust fungi (class UREDINIOMYCETES) which typically form caeomatoid aecia, peridiate uredia, and sessile teliospores. (See also FLAX RUST.) Melanconiales An order of fungi (class COELOMYCETES) in which the vegetative mycelium occurs within the substrate or host, and the conidiomata are formed in the superficial layers of the host, becoming erumpent at maturity; mature conidiomata are either acervuli or sporodochia. Some species are important plant pathogens (see e.g. ANTHRACNOSE). Genera include COLLETOTRICHUM, Cylindrosporium, Entomosporium, Marssonina and Pestalotia. (See also DIPLOCARPON.) melanin A high-MWt, amorphous polymer of indole quinone; melanins are pigments found in plants, animals (including the skin and hair in man), insects etc, and also in certain microorganisms (e.g. in actinomycetes, in hyphal walls of dematiaceous fungi, in the spore walls of e.g. Mucor spp). Melanins are relatively non-specific enzyme inhibitors; they may e.g. protect fungal cell walls from enzymic attack [Book ref. 38, pp. 373–374] and prevent cell lysis in old cultures. Melanins in plants may play a role in resistance to fungal infection. The first step in melanin biosynthesis involves the ohydroxylation of L-tyrosine (by tyrosinase) to form L-3,4dihydroxyphenylalanine (L-dopa); L-dopa is used e.g. in the treatment of Parkinson’s disease, and attempts have been made to adapt melanin-producing microorganisms for the commercial production of L-dopa [Book ref. 31, p. 567]. Melanogaster See GASTEROMYCETES (Melanogastrales). Melanogastrales See GASTEROMYCETES. Melanoplus sanguinipes EPV See ENTOMOPOXVIRINAE. Melanospora See SORDARIALES. Melanotaenium A genus of smut fungi (order USTILAGINALES) which are parasitic on the pteridophyte Selaginella. melarsoprol See ARSENIC. Melasmia A genus of fungi of the COELOMYCETES. (See also RHYTISMA.) melezitose A trisaccharide – a-D-glucopyranosyl-(1 ! 3)-b-Dfructofuranosyl-(2 $ 1)-a-D-glucopyranoside – found in nature e.g. in certain HONEYDEWS. It can be metabolized e.g. by certain Lactobacillus spp. Hydrolysis of melezitose by a-glucosidase yields glucose and turanose (a-D-glucopyranosyl-(1 ! 3)-Dfructose). melibiose A reducing disaccharide: a-D-galactopyranosyl-(1 ! 6)-D-glucopyranose, formed from RAFFINOSE by the action of b-fructosidase (invertase). In Escherichia coli melibiose uptake occurs by NaC /melibiose symport (see ION TRANSPORT). melioidosis A disease of man and animals (particularly rodents) caused by Burkholderia (Pseudomonas) pseudomallei – commonly by strains of the biotype that are unable to assimilate 464 mercury Unlike other lymphocytes in the circulation, memory cells persist for long periods of time; they are also distinguishable by certain cell-surface antigens, e.g. CD44. (See also ANTIBODY FORMATION.) menadione See QUINONES. menaquinones See QUINONES. Mendosicutes A proposed division of the PROCARYOTAE comprising the single class ARCHAEOBACTERIA. mengovirus See CARDIOVIRUS. meningitis Inflammation of the meninges (membranes covering the brain and spinal cord). Meningitis may be of non-microbial causation, or it may result from primary or secondary infection by any of a range of microorganisms which gain access to the meninges via the blood or lymph systems, head wounds, or paranasal sinuses. (a) Bacterial meningitis. Symptoms typically include a severe, throbbing headache, stiff neck, fever, delirium and coma; bacterial meningitis may be rapidly fatal. Lab. diagnosis: the pathogen can often be detected by microscopic examination of a Gram-stained smear of CSF sediment. Meningococcal meningitis (‘epidemic meningitis’) – caused by Neisseria meningitidis (the meningococcus) – occurs sporadically or in epidemics. Transmission occurs by direct contact or by droplet infection; asymptomatic carriers harbour meningococci in the nasopharynx. Incubation period: ca. 2–10 days. Onset is usually abrupt, with rapid progression to confusion and coma; meningitic symptoms are often accompanied by petechial or purpuric skin lesions which may become gangrenous. Mortality rates: ca. 25–75% in untreated cases, ca. 7–10% in treated cases. A severe, fulminating form involving cyanosis, coma, and skin and adrenal haemorrhages (Waterhouse–Friderichsen syndrome) may be fatal within a few hours. Chemotherapy: benzylpenicillin, chloramphenicol. Rifampicin can eliminate meningococci from carriers. [A meningococcal C vaccine in teenagers: Lancet (2003) 361 675–676.] Haemophilus influenzae type b is a common cause of bacterial meningitis in infants and young children, often secondary to otitis media, pneumonia or viral respiratory disease. Mortality rates (treated cases): ca. 3–10%. Chemotherapy: e.g. chloramphenicol. Pneumococcal meningitis (caused by Streptococcus pneumoniae) occurs mostly in infants and the elderly or debilitated. It is commonly associated with pneumococcal pneumonia, but may follow otitis media, head wounds etc. Mortality rates: ca. 20–70%. Chemotherapy: e.g. benzylpenicillin, chloramphenicol. Various other bacteria can cause meningitis in adults – e.g. other streptococci, staphylococci, Acinetobacter calcoaceticus, Listeria monocytogenes, enterobacteria. Neonates are particularly vulnerable to meningitis caused by Escherichia coli and other enterobacteria, group B streptococci, Staphylococcus aureus, L. monocytogenes, Flavobacterium meningosepticum etc; mortality rates may be high, and motor or intellectual impairment often persists in survivors. (b) Aseptic meningitis is generally a benign, self-limiting condition characterized by a lymphocytic infiltration of the meninges and raised protein levels in the CSF. It is usually caused by a virus – commonly mumps virus or enteroviruses (e.g. echoviruses or coxsackieviruses), also e.g. ENCEPHALITIS viruses, herpes simplex virus and LCM VIRUS. A lymphocytic meningitis may also occur in e.g. CRYPTOCOCCOSIS, LEPTOSPIROSIS, SYPHILIS and TUBERCULOSIS; following administration of nerve-tissue rabies vaccines (e.g. Semple vaccine); and in certain non-microbial diseases (e.g. cerebral tumours). (See also MENINGOENCEPHALITIS.) meningococcus Neisseria meningitidis. meningoencephalitis Inflammation of the brain and its meninges (cf. ENCEPHALITIS and MENINGITIS). Primary amoebic meningoencephalitis is a human disease which affects mainly children and young adults and is usually fatal. It is usually caused by Naegleria fowleri. Infection occurs via the nasal mucosa and is usually associated with swimming in e.g. freshwater lakes rich in organic matter. Symptoms may include headache, fever, vomiting, disturbances to taste, smell and vision, and coma; death may occur within ca. 72 h. Amoebae of the Acanthamoeba-Hartmannella group may cause a subacute or chronic granulomatous meningoencephalitis. Lab. diagnosis: identification of amoebae in the CSF. Chemotherapy: e.g. amphotericin B (see also MICONAZOLE). meningoencephalomyelitis See ENCEPHALITIS. meningomyelitis Inflammation of the spinal cord and its meninges (cf. MENINGITIS). Meniscus A genus of Gram-negative, anaerobic (but aerotolerant) bacteria which occur e.g. in anaerobic digester sludge. The cells are capsulated, straight or curved, non-motile rods, 0.7–1.0 ð 2.0–3.0 µm, which contain gas vacuoles. The organisms are chemoorganotrophs with a strictly anaerogenic fermentative metabolism; growth requirements include vitamin B12 , thiamine, and a raised partial pressure of CO2 . Optimum growth temperature: ca. 30° C. GC%: ca. 45. Type species: M. glaucopsis. [Book ref. 22, pp. 135–137.] Menoidium See EUGLENOID FLAGELLATES. menthol See PHENOLS. mepacrine Syn. QUINACRINE. merbromin See MERCURY. 2-mercaptoethanesulphonic acid Coenzyme M: see METHANOGENESIS. mercaptoethanol (HSCH2 CH2 OH) A reagent used e.g. to reduce disulphide bonds in proteins (e.g. immunoglobulins – see FAB PORTION). Mercurochrome See MERCURY. mercury (as an antimicrobial agent) Mercury is a HEAVY METAL whose compounds include many effective antimicrobial agents; such agents are typically reversibly microbistatic, and they have little or no effect on the viability of bacterial endospores – though they can delay germination. The functional part of an antimicrobial mercurial is the mercury cation or the mercury-containing moiety; antimicrobial activity may involve the binding of these entities to certain groups (particularly thiols – but also e.g. amides and amines, purines and pyrimidines) in e.g. enzymes, cell walls, cytoplasmic membranes and nucleic acids. Microbistatic activity can be reversed with e.g. thiols – which probably compete with the mercury-binding sites of the organisms. In bacteria, resistance to mercurials can be plasmid-mediated. Inorganic mercurials are toxic and irritant to tissues; their activity is readily inhibited or reversed by organic matter (e.g. blood or serum). Mercuric chloride (corrosive sublimate) has been used for the disinfection of inanimate objects when other methods are unsuitable, and as a preservative for e.g. paper and timber. Mercurous chloride (calomel) has restricted use as a fungicide in horticulture – e.g. as a control against clubroot. Yellow mercuric oxide has been used in ointments for the treatment of conjunctivitis and blepharitis. Ammoniated mercury (aminomercuric chloride; white precipitate; HgNH2 Cl) has been used in ophthalmic preparations and for the treatment of superficial mycoses. 465 meriOrganic mercurials are generally less toxic (to man), are less irritant, and are typically more effective antimicrobial agents. Mercurochrome (merbromin; disodium 20 ,70 -dibromo-40 hydroxymercurifluorescein) was one of the first organic mercurial antiseptics, but it is among the least effective and is no longer widely used. Nitromersol (Metaphen; 4-nitro-3-hydroxymercurio-cresol anhydride) is used e.g. in antimicrobial solutions and ointments for the treatment of e.g. conjunctivitis and superficial mycoses. (See also THIOMERSAL.) Phenylmercuric borate, nitrate and acetate (the most soluble) all have similar antimicrobial properties; they have been used as antiseptics, as preservatives for pharmaceutical preparations, paper, wood etc, and as seed dressings. Microbial resistance to mercury and organomercurials: see HEAVY METALS. meri- Combining form signifying ‘part’. Meria See HYPHOMYCETES and NEMATOPHAGOUS FUNGI. Merismopedia A phycological genus of unicellular ‘blue-green algae’ (Chroococcales) in which the cells occur (in nature) in rectangular ‘plates’. See SYNECHOCYSTIS. meristem A region of an organism at which new, permanent tissue is formed, i.e., a ‘growth region’ of a plant or multicellular microorganism. meristematic nodules See ROOT NODULES. meristoderm In certain algae (e.g. Laminaria): a surface layer of cells which, on division, gives rise to an increase in the thickness of the thallus. meristogenous development (mycol.) The development of certain fungal structures (e.g. pycnidia, stromata) by the proliferation and differentiation of a small number of adjacent cells in a single hypha. (cf. SYMPHOGENOUS DEVELOPMENT.) mermaid’s wine-glass See ACETABULARIA. mero- Combining form signifying ‘part’. Merocystis See EIMERIORINA. merodiploid See DIPLOID. merogony Syn. SCHIZOGONY. meromictic lake A permanently stratified lake in which only the upper layers undergo turnover, the lower layer (hypolimnion) being typically stagnant, anaerobic, and rich in sulphide. (cf. HOLOMICTIC LAKE.) meropenem A broad-spectrum 1-b-methylCARBAPENEM antibiotic. (See also ERTAPENEM.) merosporangium An elongated (‘cylindrical’) SPORANGIUM containing either a single spore or a small number of spores in a row; merosporangia are formed e.g. by Piptocephalis and Syncephalastrum. merozoite See SCHIZOGONY. merozygote A bacterium which is part DIPLOID, part haploid; it may be formed in certain processes in which chromosomal genes pass from one bacterium (the donor) to another (the recipient) – see CONJUGATION, TRANSDUCTION and TRANSFORMATION. The genetic complement of a merozygote comprises the endogenote (the recipient’s own chromosome) and the fragment of genetic material (the exogenote) received from the donor. If corresponding ALLELES in the endogenote and exogenote are identical the merozygote is said to be a homogenote; if not, it is said to be a heterogenote. merthiolate Syn. THIOMERSAL. Merulius See APHYLLOPHORALES (Corticiaceae). (cf. SERPULA.) MES A pH buffer based on 2-(N-morpholino)-ethanesulphonic acid. Meselson–Radding model See RECOMBINATION (Figure 2). mesocyst See CYST (b). Mesodinium See GYMNOSTOMATIA. meso-inositol See INOSITOL. mesokaryotic See DINOFLAGELLATES. mesophile An organism whose optimum growth temperature lies within a range generally accepted as ca. 20–45° C; bearing in mind the definition of PSYCHROPHILE, it would be reasonable to extend this range to 15–45° C. mesoplankton See PLANKTON. mesoporphyrins See PORPHYRINS. mesosaprobic zone See SAPROBITY SYSTEM. mesosomes (chondrioids) Intracellular membranous structures, apparently continuous with the CYTOPLASMIC MEMBRANE, which have been observed in electron micrographs of many bacteria. In Gram-positive species they may appear e.g. as parallel or concentric lamellae or as vesicles (perhaps tubules) which may be interconnected; in Gram-negative bacteria mesosomes appear to be smaller and less intricate, and are often seen as simple invaginations of the cytoplasmic membrane. Mesosomes are commonly associated with the developing septum in a dividing cell, but their function – if any – is unknown; they are widely believed to be artefacts derived from the cytoplasmic membrane during preparation of specimens for electron microscopy [see e.g. AvL (1984) 50 433–460 (439–443)]. (cf. INTRACYTOPLASMIC MEMBRANES.) Mesostigma See MICROMONADOPHYCEAE. Mesotaenium A genus of saccoderm DESMIDS. messenger RNA See MRNA. messenger RNA-interfering complementary RNA See ANTISENSE RNA. metabasidium A developing BASIDIUM at the stage at which MEIOSIS occurs, or that part of a developing basidium in which meiosis occurs. According to species, the metabasidium may be a later stage of (but morphologically identical to) the PROBASIDIUM, or it may arise – as a separate structure – from the probasidium. According to species, a metabasidium gives rise to sterigmata (each STERIGMA bearing one terminal basidiospore) or to basidiospores borne directly on its surface (i.e., without sterigmata). A metabasidium may be septate (as e.g. in Tremella) or aseptate (as e.g. in Agaricus). In e.g. rust fungi the metabasidium is a septate structure (D ‘promycelium’) which grows out from the teliospore. metabiosis The phenomenon in which one organism alters environmental conditions in such a way as to allow the growth of another or others; e.g., utilization of oxygen by aerobic organisms may create anaerobic microenvironments in which strict anaerobes can grow. metabisulphites See SULPHUR DIOXIDE. metabolic inhibition test Any test in which an agent is detected or quantified by its ability to inhibit metabolic activity. For example, a toxin, virus, or other cytocidal agent can be assayed by determining its ability to inhibit the normal metabolic processes in a tissue culture; normally, the medium in a tissue culture becomes acidic with time, and the titre of the cytocidal agent can be estimated by determining the highest dilution of the agent which delays or inhibits acidification of the medium. metaboly See EUGLENA. metachromasy The phenomenon in which certain substances (termed chromotropes) develop a colour different to that of the dye used to stain them. See e.g. METACHROMATIC GRANULES. metachromatic dye Any DYE which gives rise to METACHROMASY when used to stain chromotropic substances. metachromatic granules Intracellular POLYPHOSPHATE-containing granules which, on staining with certain basic dyes (e.g. polychrome methylene blue or toluidine blue – see ALBERT’S STAIN), 466 Methanobacteriaceae produces depsipeptide entomotoxins (destruxins) which apparently play an important role in pathogenesis. M. anisopliae has been used for the BIOLOGICAL CONTROL of e.g. froghoppers (Mahanarva posticata) in Brazilian sugar-cane plantations. metastasis (med.) The dissemination of disease from a localized site in the body, with the formation of new foci at distant sites. (Usually used in the context of CANCER, but also of certain infectious diseases – e.g. TUBERCULOSIS.) (Hence verb metastasize, adj. metastatic.) metatrypanosomes Syn. METACYCLIC FORMS. Metchnikovella See RUDIMICROSPOREA. methacycline See TETRACYCLINES. metham sodium (vapam) Sodium N-methyl-DITHIOCARBAMATE (CH3 .NH.CS.SNa), a soil fumigant which, in soil, breaks down to form the volatile methyl-isothiocyanate. (cf. DAZOMET, QUINTOZENE.) methanal Syn. FORMALDEHYDE. methane monooxygenase (MMO) A MONOOXYGENASE which occurs in methanotrophs (see METHYLOCOCCACEAE) and which catalyses the oxidation of methane to methanol by molecular oxygen; MMO can also catalyse the oxidation of substrates such as n-alkanes and n-alkenes: e.g., propene (propylene) is oxidized to 1,2-epoxypropane. (See also CO-METABOLISM.) MMO may be ‘particulate’ (i.e., membrane bound) or soluble, depending on growth conditions. Thus, e.g., in cells of Methylococcus capsulatus and ‘Methylosinus trichosporium’ grown on methane, the MMO is soluble or particulate according to whether the culture medium contains low (e.g. 1 µM) or high (e.g. 5 µM) levels of Cu2C respectively. However, in M. capsulatus only particulate MMO is formed when growth occurs on methanol – in this case an increase in the level of Cu2C enhancing MMO activity [JGM (1985) 131 155–163]. The soluble MMO of M. capsulatus is a three-component enzyme comprising proteins A (believed to be the oxygenase [JBC (1984) 259 53–59]), B and C, and that of ‘Methylosinus trichosporium’ is apparently similar. The substrate specificities of particulate and soluble MMOs may differ appreciably. Thus, e.g. the soluble MMO of ‘M. trichosporium’ can oxidize n-alkanes, n-alkenes, aromatic and alicyclic compounds, while the particulate MMO of the same organism cannot oxidize aromatic or alicyclic substrates [JGM (1984) 130 3327–3333]. In various types of methanotroph the reducing power requirement for MMO activity in vitro (i.e. in cell extracts) appears to be satisfied by NADH for both the soluble and particulate forms of the enzyme. However, it has been proposed that the reducing power needed for in vivo particulate MMO activity in ‘M. trichosporium’ may be derived from a reduced c-type cytochrome. It has been suggested that reducing power for in vivo particulate MMO activity in M. capsulatus may be supplied by an electron transfer protein (e.g. an Fe–S protein) the reduction of which depends on reversed electron transport [JGM (1983) 129 3487–3497]. methane production See METHANOGENESIS. methane utilization See METHANOTROPHY. Methanobacillus omelianskii A syntrophic association of two species of prokaryote: Methanobacterium bryantii and an obligate proton reducer (the ‘S’ organism) which oxidizes ethanol to acetate and hydrogen gas. Methanobacteriaceae A family of METHANOGENS (order METHANOBACTERIALES); two genera: METHANOBACTERIUM and METHANOBREVIBACTER. (This family formerly included all the methane-producing species.) exhibit METACHROMASY. [Composition of metachromatic granules: JB (1984) 158 441–446.] metachronal waves (protozool.) Waves which may be seen on the surface of a ciliate due to the beating of its cilia – an effect similar to that produced by an intermittent breeze on a field of barley or tall grass. While the cilia of a given ciliate beat with similar or identical frequencies, they do not, in general, beat synchronously – i.e., all the cilia do not pass simultaneously through the same phase of the beat cycle (see CILIUM). Instead, ciliary beating is usually metachronous: adjacent cilia pass through slightly different phases of the beat cycle at any given instant – the phase difference between a given cilium and other cilia in a KINETY increasing and decreasing cyclically with distance from the given cilium. (This results in the smooth motion characteristic of ciliates.) In one type of metachrony (symplectic metachrony) a sequence of cilia which, at a given instant, are all passing through (different stages of) the effective stroke of the beat cycle, tend to bunch together to form the crest of a metachronal wave; the crests of such waves move in the direction of the effective stroke. In antiplectic metachrony, cilia passing through the effective stroke do not bunch together, and the metachronal crests move in a direction opposite to that of the effective stroke. In both symplectic and antiplectic metachrony the effective and recovery strokes occur in the plane of the kinety; these types of metachrony are termed orthoplectic metachrony. In diaplectic metachrony the effective and recovery strokes occur in a plane perpendicular to the kinety; the effective stroke may be to the right (dexioplectic metachrony) or to the left (laeoplectic metachrony) when looking in the direction of the metachronal wave. Symplectic metachrony is quite rare in ciliates, but antiplectic and diaplectic (dexioplectic) types of metachrony are quite common. metacyclic forms (metatrypanosomes) Trypanosomes, infective for the vertebrate host, which develop at the end of the period of cyclical development in the invertebrate vector; they are trypomastigote in form, with or without a short free flagellum. In Trypanosoma vivax a ‘surface coat’ (see VSG) has been reported to occur on metacyclic forms prior to contact with the mammalian host [Parasitol. (1986) 92 75–82]. Metacystis See GYMNOSTOMATIA. Metadinium See ENTODINIOMORPHIDA. metal leaching See LEACHING. metalaxyl See PHENYLAMIDE ANTIFUNGAL AGENTS. metalimnion (thermocline) In a lake: the layer of water between the EPILIMNION and the HYPOLIMNION; this layer is characterized by the maximum rate of decrease in temperature with depth. Metallogenium A genus of IRON BACTERIA. The organisms have been reported to be capable of passing through a filter of pore size 0.2 µm and of inhibiting various prokaryotic and eukaryotic microorganisms when cultured with them [Curr. Micro. (1984) 11 349–356]. metalloporphyrins See PORPHYRINS. metallothionein A cysteine-rich, low-MWt, inducible protein which binds (and may detoxify) HEAVY METAL ions; metallothioneins from a range of organisms (e.g. Synechococcus, Neurospora, Saccharomyces, crab, horse, man) are structurally similar. metaphase See MITOSIS and MEIOSIS. metaphase plate See MITOSIS. Metaphen See MERCURY. Metarhizium A genus of entomopathogenic fungi (class HYPHOMYCETES). M. anisopliae can infect a wide variety of insects; it 467 Methanobacteriales M. voltae. Cells ca. 0.5–3.0 µm diam. Mesophilic. Growth is optimal in 0.4 M NaCl. Methane is produced from carbon dioxide and hydrogen and from formate. methanofuran See METHANOGENESIS. methanogen Any member of the domain ARCHAEA which can carry out METHANOGENESIS; all known methanogens are obligate anaerobes which occur e.g. in muds and in the RUMEN (some forming close physical associations with protozoa [MS (1986) 3 100–105]). (See also ANAEROBIC DIGESTION and WETWOOD.) Methanogens have been classified in three orders (METHANOBACTERIALES, METHANOCOCCALES, METHANOMICROBIALES) on the basis of 16S rRNA sequence analysis. Many methanogens can grow chemolithoautotrophically in mineral salts solution with carbon dioxide and hydrogen (although some need e.g. acetate and amino acids). Methanogens are not necessarily confined to simple substrates such as acetate, methanol and methylamines; thus, pure cultures of e.g. Methanospirillum can be grown on 2-propanol and certain other alcohols [AEM (1986) 51 1056–1062]. The methanogens have been divided into two groups: group I (carbon dioxide and hydrogen, or formate, typically used), and group II (carbon dioxide and hydrogen, or formate, typically not used); group II includes the family METHANOSARCINACEAE [AvL (1984) 50 557–567]. Autotrophic carbon dioxide fixation apparently occurs via a pathway resembling that used for the synthesis of acetylCoA from carbon dioxide in ACETOGENESIS, although formyl tetrahydrofolate synthase does not seem to be involved. AcetylCoA appears to be reductively carboxylated to pyruvate – which is converted to phosphoenolpyruvate (PEP); carboxylation of PEP gives rise to oxaloacetate. Oxaloacetate appears to be metabolized to 2-oxoglutarate as in the TCA cycle [see Appendix II(a)] – via citrate in Methanosarcina barkeri, via malate in Methanobacterium thermoautotrophicum. Some species (e.g. Methanococcus thermolithotrophicus, Methanosarcina barkeri ) can carry out NITROGEN FIXATION [Nature (1984) 312 284–288; FEMS Ecol. (1985) 31 47–55]. methanogenesis The biosynthesis of methane (CH4 ): an energyyielding process carried out by certain members of the domain Archaea (METHANOGENS) in anaerobic environments that are characterized by an Eh below about
330 mV (see e.g. ANAEROBIC DIGESTION, RUMEN and WETWOOD). Methane can be formed from several substrates: carbon dioxide and hydrogen (or carbon dioxide and formate), acetate, methanol, or methylamines. Methanogenesis involves a number of novel enzymes and coenzymes/cofactors [FEMS Reviews (1999) 23 13–38]. Methane from carbon dioxide and hydrogen. Essentially, the carbon atom of carbon dioxide is bound, sequentially, to each of several C1 -carrier molecules and undergoes progressive reduction (via formyl and methyl stages) to methane (see figure). The final stage (reduction of CoM-S-CH3 ) is thermodynamically 0 favourable (1G0 greater than
100 kJ). Methane from acetate. Growth on acetate is slower than that on carbon dioxide and hydrogen; the free energy of the reaction Methanobacteriales An order of METHANOGENS comprising those species whose cell wall contains PSEUDOMUREIN. Two families: METHANOBACTERIACEAE, METHANOTHERMACEAE. Methanobacterium A genus of facultatively autotrophic archaeans (family METHANOBACTERIACEAE). Cells: non-motile rods. Methane may be produced from carbon dioxide and hydrogen and/or from formate. GC% ca. 33–50. M. bryantii. Mesophilic. Cells: morphologically indistinguishable from M. formicicum (see below), but the organisms cannot use formate. Growth is stimulated by acetate and B vitamins. (See also METHANOBACILLUS OMELIANSKII.) M. formicicum. Cells: round-ended rods, ca. 0.6 ð 2–15 µm, often in chains. Formate can be used as an electron donor, and acetate stimulates growth. M. thermoautotrophicum. Thermophilic (opt. temperature: 65–70° C). Formate is not metabolized. Methanobrevibacter A genus of archaeans of the family METHANOBACTERIACEAE. Cells: cocci or short rods, often in pairs or chains. Mesophilic. Methane may be produced from carbon dioxide and hydrogen and/or from formate. GC% ca. 27–32. M. arboriphilus. A facultative autotroph which occurs e.g. in soil (see also WETWOOD). Formate is not used. M. ruminantium. Cells: rods, ca. 0.7 ð 2.0 µm, often in chains. Habitat: the RUMEN. Methane can be formed from carbon dioxide and hydrogen or from formate. Strain M-1 has been used for the BIOASSAY of coenzyme M. M. smithii. Morphologically similar to M. ruminantium. Coenzyme M is not required for growth. methanochondrion A term proposed for the apparent organelle formed (in a number of methane-producing archaeans) from invaginated cytoplasmic membrane and assumed to be the site of methanogenesis. (Such a structure is not invariably present in methanogenic species [CJM (1984) 30 594–604].) Methanococcaceae See METHANOCOCCALES. Methanococcales An order of METHANOGENS comprising one family (Methanococcaceae) and one genus, METHANOCOCCUS. Methanococcoides A genus of METHANOGENS (see METHANOSARCINACEAE). The cells of M. methylutens are non-flagellated cocci (ca. 1 µm diam.); the cell wall consists of an S LAYER only. Methanol and methylamine are methanogenic substrates. GC%: ca. 42. Methanococcus A genus of mesophilic and thermophilic archaeans (order METHANOCOCCALES) which occur e.g. in saltmarsh sediments and marine muds. Cells: motile cocci in which the cell wall consists solely of an S LAYER; the GRAM REACTION is negative. Methane is typically produced from carbon dioxide and hydrogen and/or from formate. Most species (but not M. voltae) can grow autotrophically. (The original type species, M. mazei, differs markedly from the other species, and has been transferred to METHANOSARCINA (Methanosarcina mazei ), Methanococcus vannielii being designated the new type species of the (conserved) genus Methanococcus [IJSB (1986) 36 491].) GC%: ca. 30–33. M. jannaschii. Thermophilic (optimum: 86° C). Methane is produced from carbon dioxide and hydrogen, but not from formate. The growth rate is the highest of all known methanogens (doubling time: 30 min). Isolated from a HYDROTHERMAL VENT. M. maripaludis. Cells 1.2–1.6 µm diam. Mesophilic. M. mazei. See METHANOSARCINA. M. thermolithotrophicus. Cells ca. 1.5 µm diam. Thermophilic (optimum: 65° C). M. vannielii. Cells ca. 3–4 µm diam. Mesophilic. Methane is produced from carbon dioxide and hydrogen and from formate. CH3 CO2
C H2 O

! CH4 C HCO3
is about
37 kJ. Note that this pathway differs from the carbon dioxide/hydrogen pathway e.g. in the requirement for an initial stage of ATP-dependent activation. Methanogenesis from acetate is inhibited by cyanide (which inhibits CO dehydrogenase). Methane from methanol. In this pathway, the methyl group from one molecule of methanol is oxidized to provide electrons 468 METHANOGENESIS. Two major pathways in the biosynthesis of methane: (i) synthesis from carbon dioxide, and (ii) synthesis from acetate. (Methanol and methylamines are alternative substrates.) The final stage of biosynthesis, i.e. reduction of methyl-coenzyme M (see later), is common to all the methane-forming pathways. (i) Methane from carbon dioxide. In this pathway, the reduction of carbon dioxide can be linked to the oxidation of either hydrogen or formate; the reactions shown occur in both hydrogen- and formate-oxidizing pathways. Initially, carbon dioxide is reduced to a formyl residue on the coenzyme methanofuran (MFR; earlier name: carbon dioxide reduction factor, CDR factor). Methanofuran is a long-chain molecule containing an aromatic ring and a terminal furan ring. This initial reduction is catalysed by formylmethanofuran dehydrogenase. The formyl group is then transferred to tetrahydromethanopterin (THMP). Part of the THMP molecule resembles FOLIC ACID, but the molecule is much larger than that of folic acid and it includes a sugar residue. The conversion of 5-formyl-THMP to 5,10-methenyl-THMPC is catalysed by the enzyme N5 , N10 -methenyltetrahydromethanopterin cyclohydrolase. Subsequent reductions in some methanogens (e.g. strains of Methanosarcina barkeri) involve an enzyme acting in association with coenzyme F420 (factor 420), a 5-deazaflavin derivative that serves as an electron carrier. Strains of e.g. Methanococcus thermolithotrophicus use an F420 -independent dehydrogenase to convert 5,10-methenyl-THMPC to 5,10-methylene-THMP. Finally, the methyl group is transferred from THMP to coenzyme M (CoM-SH; 2-mercaptoethanesulphonic acid; HS(CH2 )2 SO3
) in preparation for its reduction to methane by the enzyme methyl-coenzyme M reductase; in all cases, the electron donor for this reduction is coenzyme B (CoB; 7-mercaptoheptanoylthreonine phosphate). In the reductases from Methanobacterium thermoautotrophicum the active site of the enzyme contains two molecules of coenzyme F430 (factor 430), a nickel-containing porphinoid. Reduction of CoM-S-S-CoB regenerates the two coenzymes. (ii) Methane from acetate. The sequence of reactions shown are those for Methanosarcina thermophila; the pathway differs slightly in some methanogens. (Continued on page 470.) 469 Methanogenium for the reduction of methyl groups from (three) other molecules of methanol, the products being methane, carbon dioxide and water. For methane synthesis, methyl groups must be bound to coenzyme M in preparation for methanogenic reduction involving CoB-SH (see figure). Contributing to this pathway in Methanosarcina barkeri is an enzyme system that includes methanol:corrinoid methyltransferase which catalyses the initial transfer of methyl groups to the corrinoid moiety; further enzymic activity transfers the methyl groups to CoM-SH. In nature, methanogens are believed to produce most of the methane from acetate; in vitro studies on the degradation of rice straw in anoxic paddy soil have indeed shown that a high proportion of methane is synthesized from acetate under those conditions [FEMS Ecol. (2000) 31 153–161]. Earlier work had suggested that acetate may be the preferred substrate at 10° C, with hydrogen being used at higher temperatures [FEMS Ecol. (1997) 22 145–153]. However, despite the overall picture, in certain locations the contribution of hydrogen to methanogenesis may be ¾70–100% [FEMS Ecol. (1999) 28 193–202]. The production of methane in anoxic soils can be suppressed e.g. by nitrate; such inhibition appears to be due to the effects of intermediates of the denitrification process [FEMS Ecol. (1999) 28 49–61]. Methanogenium A genus of archaeans of the family METHANOMICROBIACEAE. Cells: cocci in which the cell wall consists of an S LAYER only; the GRAM REACTION is negative. Species include M. cariaci and M. marisnigri, both peritrichously flagellated organisms with an optimum growth temperature of ca. 20° C. methanol utilization See METHYLOTROPHY. Methanolobus A genus of METHANOGENS (see METHANOSARCINACEAE). The cells of M. tindarius are monoflagellated cocci (ca. 1 µm diam.); the cell wall consists of an S LAYER only. Methanol and methylamine are methanogenic substrates. GC%: ca. 46. Methanomicrobiaceae A family of METHANOGENS (order METHANOMICROBIALES); genera: METHANOGENIUM, METHANOMICROBIUM, METHANOSPIRILLUM. Methanomicrobiales An order of METHANOGENS comprising the families METHANOMICROBIACEAE, METHANOPLANACEAE and METHANOSARCINACEAE. Methanomicrobium A genus of archaeans of the family METHANOMICROBIACEAE. Cells: typically short rods in which the cell wall consists of an S LAYER only; the GRAM REACTION is negative. M. mobile. Polarly flagellated rods which need e.g. acetate, THIAMINE, PYRIDOXINE and PABA for growth. Habitat: RUMEN. M. paynteri. Non-motile coccobacilli which occur in marine sediments. Methanoplanaceae A family of METHANOGENS (order METHANOMICROBIALES) [validation: IJSB (1984) 34 270]. One genus: METHANOPLANUS. Methanoplanus A genus of archaeans of the family METHThe cells of M. limicola, the sole species, are plate-like and are bounded by an S LAYER; this organism has strong affinities with Methanogenium spp [Book ref. 157, p 17]. Methanosaeta A genus of obligately acetotrophic METHANOGENS (methanogens which synthesize methane only from acetate). The organisms, which include M. concilii and M. thermoacetophila, are found e.g. in rice fields. Methanosarcina A genus of archaeans of the family METHANOSARCINACEAE. Cells: non-motile, coccoid to irregularly shaped forms, ca. 1–3 µm; the heteropolysaccharide-containing cell wall is a thick (up to ca. 200 nm) layer which may be laminated at the periphery. GAS VACUOLES are found in some strains. The original type species, M. methanica, has been rejected and replaced by M. barkeri [IJSB (1986) 36 492]. M. acetivorans. An acetotrophic marine species [AEM (1984) 47 971–978]. M. barkeri. Cells grow as aggregates. Most strains can produce methane from carbon dioxide and hydrogen, acetate, methanol or methylamine. Strain FR-19 lyses spontaneously in substrate-depleted media [JGM (1985) 131 1481–1486]. M. mazei (cf. METHANOCOCCUS) is similar to M. barkeri but differs e.g. in that M. mazei metabolizes acetate and methanol readily but carbon dioxide and hydrogen poorly or not at all. Methanosarcinaceae A family of METHANOGENS (order METHANOMICROBIALES). Depending on species or strain, some or all of the following substrates are used for METHANOGENESIS: acetate, CO2 C H2 , methanol, methylamines. Genera: METHANOSARCINA, METHANOTHRIX; it has been proposed that the genera METHANOCOCCOIDES and METHANOLOBUS be incorporated in the family [IJSB (1984) 34 444–450]. Methanospirillum A genus of archaeans of the family METHANOMICROBIACEAE. M. hungatei is a facultative autotroph which occurs in sewage sludge; the cells are curved, sheathed rods or filaments that are lophotrichously flagellated. Methanothermaceae A family of archaeans of the order METHANOBACTERIALES. One genus: METHANOTHERMUS. Methanothermus A genus of archaeans (family METHANOTHERMACEAE) in which the cell wall consists of two layers: a PSEUDOMUREIN-containing layer and an (outermost) S LAYER. Thermophilic (optimum: 83° C). M. fervidus. Cells: non-motile rods, ca. 0.4 ð 1–3 µm; the GRAM REACTION is positive. Methane is formed from carbon dioxide and hydrogen but not from formate; yeast extract is needed for growth. GC%: ca. 33. Methanothrix A genus of archaeans of the family METHANOSARCINACEAE. M. soehngenii is common in anaerobic digestors; cells: rods, ca. 0.8 ð 2 µm, or long, sheathed filaments – each filament often consisting of several hundred rods. The GRAM REACTION is negative. Methane is formed from acetate but not from carbon dioxide and hydrogen or from formate, methanol or methylamine. ANOPLANACEAE. METHANOGENESIS (continued) Initially, acetate undergoes ATP-dependent activation to acetyl-CoA – catalysed by the two enzymes acetate kinase and phosphotransacetylase. (In Methanosaeta, this section of the pathway differs in that the formation of acetyl-CoA is catalysed by acetate thiokinase.) The methyl group is then transferred to tetrahydrosarcinapterin (THSP; analogous to THMP, above) by an enzyme complex that includes CO dehydrogenase and CoA synthase; the activity of these enzymes results in the cleavage of carbon–carbon and carbon–sulphur bonds – and also oxidation of the carbonyl group (from acetyl-CoA) to carbon dioxide. Electrons derived from oxidation of the carbonyl group are involved in the final methanogenic reduction of the methyl group after the latter has been transferred to coenzyme M. In Methanosarcina thermophila the cam gene encodes a CARBONIC ANHYDRASE which apparently includes a signal peptide (see SIGNAL HYPOTHESIS); it has been suggested that, by hydrating carbon dioxide in the periplasm, this enzyme may facilitate removal of carbon dioxide from the cytoplasm – and may thus promote conditions thermodynamically more favourable for methanogenesis. 470 methylene blue MAP belongs to a diverse family of physiologically important aminopeptidases [bacterial aminopeptidases: FEMS Reviews (1996) 18 319–344]. L-methionine biosynthesis See Appendix IV(d). (See also MICROCINS.) methionine deformylase In bacteria: an enzyme which cleaves the formyl group from methionine at the N-terminal of a newly synthesized protein. L-methionine-DL-sulphoximine See MSX. methisazone See ISATIN-b-THIOSEMICARBAZONE. Methocel See METHYLCELLULOSE. methotrexate See FOLIC ACID ANTAGONIST. methoxatin See QUINOPROTEIN. methoxsalen 8-MethylPSORALEN. methoxyamine (as a mutagen) See HYDROXYLAMINE. 6-methoxymellein A PHYTOALEXIN elicited from the carrot plant e.g. by cupric or mercuric chloride. methyl-accepting chemotaxis protein See CHEMOTAXIS. methyl blue (cotton blue) A blue acid TRIPHENYLMETHANE DYE; cf. LACTOPHENOL COTTON BLUE. methyl bromide (CH3 Br) Methyl bromide vapour is fungicidal, insecticidal and herbicidal; it has good powers of penetration and is used as a fumigant. methyl green A green basic TRIPHENYLMETHANE DYE. (See also METHYL GREEN–PYRONIN STAIN.) methyl green–pyronin stain (Unna–Pappenheim stain) A stain which can distinguish DNA (which stains green) from RNA (which stains red); it can be used e.g. to distinguish lymphocytes from (ribosome-rich) plasma cells. methyl orange A PH INDICATOR: pH 3.0–4.4 (red to orangeyellow); pKa 3.6. methyl red A PH INDICATOR: pH 4.4–6.2 (red to yellow); pKa 5.1. methyl red test (MR test) An IMVIC TEST which determines the ability of an organism to acidify a phosphate-buffered glucose–peptone medium to pH 4.4 or below. The medium is inoculated and incubated for 48 h at 37° C or 5 days at 30° C. Two drops of METHYL RED solution (0.02% w/v in ca. 50% ethanol) are added to the culture; a red coloration indicates a positive reaction. (N.B. Too short an incubation time may give a false-positive reaction: some organisms (e.g. Klebsiella pneumoniae subsp. pneumoniae) initially form acidic metabolic products which are later metabolized further.) The VOGES–PROSKAUER TEST can be performed after the MR test using the same culture. methyl violet A composite TRIPHENYLMETHANE DYE which includes crystal violet and 4,40 -bis(dimethylamino)-400 -(methylamino) triphenylmethyl chloride. methyl viologen (Paraquat) 1,10 -Dimethyl-4,40 -dipyridinium, a widely used herbicide. (See also PHOTOSYNTHESIS.) The reduced form of methyl viologen is used e.g. as a reducing agent in laboratory studies. 3-methyladenine-DNA glycosylase I See EXCISION REPAIR. 3-methyladenine-DNA glycosylase II See ADAPTIVE RESPONSE. methylation (of DNA) See DNA METHYLATION. methylcellulose (Methocel) An unbranched polymer (MWt ca. 143000) used e.g. to increase the viscosity of a medium in order to slow down rapidly motile organisms – see COUNTING METHODS. (cf. FICOLL.) methylcobalamin See VITAMIN B12 . methylene blue A blue basic thiazine DYE used e.g. in STAINING and VITAL STAINING and as a redox indicator (E0 D C11 mV at pH 7). (See also e.g. METHYLENE BLUE TEST; TOXOPLASMA DYE TEST.) Leuco methylene blue is the reduced (colourless) form of methylene blue. Loeffler’s methylene blue is M. concilii is an acetotrophic species isolated from sewage sludge [CJM (1984) 30 1383–1396]. [Acetate and carbon dioxide assimilation by M. concilii : JB (1985) 162 905–908.] methanotroph Any organism capable of METHANOTROPHY. methanotrophy The use of methane as the sole source of carbon and energy: a mode of metabolism which occurs under aerobic and microaerobic conditions in certain bacteria (see e.g. METHYLOCOCCACEAE). (cf. METHYLOTROPHY.) Most methanotrophic bacteria are obligately methanotrophic; these organisms include species of Methylococcus and Methylomonas. The methanotrophic bacteria occur e.g. in soil, water and sediments. The oxidation of methane by methanotrophs significantly affects the global methane budget in that much less methane reaches the atmosphere; this reduces the contribution of methane to global warming. In members of the Methylococcaceae, methane is initially oxidized to methanol by METHANE MONOOXYGENASE (q.v.); the subsequent oxidations are: methanol ! formaldehyde ! formate ! carbon dioxide. When methane is used as the sole source of carbon and energy, some formaldehyde is assimilated into cell biomass while the remainder is oxidized to yield energy. [Methane oxidation in natural waters: AEM (1982) 44 435– 446. Methane oxidation by microorganisms: Book ref. 132, pp 173–200. Methanotrophic bacteria (review): MR (1996) 60 439–471. Ecology of methanotrophic species studied by molecular methods: FEMS Ecol. (1998) 27 103–114. Detection and classification of atmospheric methane-oxidizing bacteria in soil: Nature (2000) 405 175–178.] methazotrophic Refers to any organism capable of using methylamines as the sole source of nitrogen (but not as sole source of carbon and energy); methazotrophs include various yeasts, e.g. Candida utilis. [Oxidation of methylamines by yeasts: Book ref. 169, pp. 155–164.] methenamine See HEXAMINE. methenamine–silver stain (Grocott–Gomori stain; Gomori stain) A stain used e.g. for detecting actinomycetes or fungi in tissue sections. The section is oxidized in 5% chromic acid for 1 h, washed in tap water, washed for 1 min in 1% sodium bisulphite to eliminate chromic acid, and washed for 10 min in tap water. After 3–4 changes of distilled water the section is immersed in a solution made by adding 0.4% aqueous borax (108 ml) to a mixture of 10% silver nitrate (7 ml) and 3% methenamine (100 ml); the temperature is kept at 58–60° C until the section becomes yellow-brown (30–60 min). The slide is rinsed repeatedly in distilled water, washed for 2–5 min in 2% sodium thiosulphate to remove unreduced silver, washed in tap water, and counterstained for ca. 1 min in a solution of light green (0.04% w/v) containing 0.04% v/v glacial acetic acid. Mycelium stains brown-black, tissues green. methicillin See PENICILLINS. methicillin-resistant Staphylococcus aureus See MRSA. methicillinase See MRSA. methionine aminopeptidase (MAP) In bacteria: an enzyme required for excision of the N-terminal methionine of a newly synthesized protein. In e.g. Escherichia coli, the N-terminal methionine is cleaved from about 50% of proteins. Whether or not such cleavage occurs in a given protein is influenced by the identity of the penultimate N-terminal amino acid; cleavage, or otherwise, of the N-terminal methionine thus determines the identity of the N-terminal amino acid in the mature protein, and, in this way, influences the protein’s half-life (according to the N-END RULE). 471 methylene blue test prepared by adding 0.01% KOH (100 ml) to 1% methylene blue in 95% ethanol (30 ml). If allowed to ‘ripen’ (i.e. oxidize slowly) over several months this stain becomes polychrome methylene blue: a mixture of methylene blue and certain products of its spontaneous oxidation (e.g. the metachromatic dyes azure A and azure B). Methylocaldum A genus of methylotrophic bacteria classified within the (gamma) PROTEOBACTERIA. Methylococcaceae A family of Gram-negative, obligately methylotrophic bacteria characterized by the ability to use methane as the sole source of carbon and energy under aerobic or microaerobic conditions (see METHANOTROPHY); bacteria which can use other C1 compounds, but not methane, are excluded. Two genera: METHYLOCOCCUS and METHYLOMONAS. [Book ref. 22, pp. 256–261.] (cf. METHYLOMONADACEAE.) Members of the Methylococcaceae occur e.g. in soil and in water overlaying anaerobic mud. The cells range from cocci to rods, and all contain a complex arrangement of intracytoplasmic membranes when grown on methane; some strains are motile (polarly flagellated). Pigments, some water-soluble, are formed by some strains and may be e.g. blue, brown, red or yellow. Certain strains form desiccation-resistant resting stages; these stages may be cysts (similar to those of Azotobacter spp), ‘lipid cysts’ (which contain PHB inclusions, lack intracellular membranes, and have a thick pericellular coating), or ‘exospores’ (which develop by polar budding, and which are resistant to 85° C/15 min). Metabolism is respiratory (oxidative) with oxygen as the terminal electron acceptor; cytochromes of the a, b, c and o types occur in those strains examined. According to strain, the TCA CYCLE may be complete or e.g. may lack 2-oxoglutarate dehydrogenase (in which case the pathway is primarily biosynthetic). All strains can use methanol as a sole source of carbon and energy, and many or all can use formaldehyde. Methane oxidizers can use other carbon compounds, simultaneously with methane, as co-oxidizable substrates to provide carbon and energy; such supplementary carbon and energy sources include e.g. acetate, alkanes, formate, primary alcohols, and various alicyclic, aromatic and heterocyclic compounds. At least one species, Methylococcus capsulatus, can fix CO2 via the Calvin cycle, and some methanotrophs can carry out NITROGEN FIXATION [JGM (1983) 129 3481–3486]. All strains which have been examined are catalase Cve and oxidase Cve. Aerobic methane-oxidizing bacteria were divided into two major physiological groups; these groups included species which were not formally included in the Methylococcaceae. Type I organisms (e.g. Methylococcus capsulatus, Methylomonas methanica) form intracytoplasmic membranes comprising bundles of vesicular discs, may form cysts, have an incomplete TCA cycle, and assimilate carbon by the RMP PATHWAY. Type II organisms (e.g. METHYLOBACTERIUM, METHYLOSINUS TRICHOSPORIUM) form paired intracytoplasmic membranes aligned with the cell periphery, may form ‘lipid cysts’ or ‘exospores’, have a complete TCA cycle, and assimilate carbon by the SERINE PATHWAY. [Ecology, isolation and culture: Book ref. 45, pp. 894–902.] Methylococcus A genus of Gram-negative bacteria (family METHYLOCOCCACEAE) which occur e.g. in mud, soil and water. Cells: non-pigmented, non-motile cocci, ca. 1 µm in diameter. The organisms may be cultured on e.g. nitrate–mineral salts media with a gaseous phase consisting of a 30:70 mixture of methane and air; growth occurs between 30 and 50° C (optimally at 37° C), enrichment and primary isolation being carried out at 45° C. GC%: ca. 62.5. Type species: M. capsulatus. Methylomonadaceae An obsolete bacterial family which consisted of methylotrophic organisms – the use of methane not being an obligatory requirement. (cf. METHYLOCOCCACEAE.) Methylomonas A genus of Gram-negative bacteria within the family METHYLOCOCCACEAE. Cells: straight or curved, sometimes branched, motile (polarly flagellated) rods, 0.5–1.0 ð 1.0–4.0 µm. Growth occurs optimally at 30° C, no growth METHYLENE BLUE As a redox indicator (see e.g. GASPAK and REDOX POTENTIAL), methylene blue is often used in a TRIS- or phosphate-buffered solution at pH 9; at pH 9 the dye becomes colourless at a lower Eh than it does at pH 7. methylene blue test (for milk) A test used to assess the numbers of (certain types of) microorganisms in untreated or pasteurized MILK. To a standard volume of milk is added a fixed volume of a solution of METHYLENE BLUE (or methylene blue thiocyanate); the whole is incubated at 37° C and examined periodically. If the milk contains large numbers of organisms that are metabolically active at 37° C, the dye will be quickly reduced to the colourless leuco form; decolorization is delayed if the milk contains relatively few organisms. The time taken for dye decolorization is thus a guide to the microbial load. (cf. RESAZURIN TEST.) methylglyoxal bypass A sequence of reactions which can bypass reactions of the EMBDEN–MEYERHOF–PARNAS PATHWAY between triose phosphates and pyruvate. Methylglyoxal synthase converts dihydroxyacetone phosphate to methylglyoxal (2-oxopropanal: CH3 .CO.CHO) and inorganic phosphate; methylglyoxal can then be converted to D-lactate (by two glutathione-dependent enzymes, glyoxalase I and glyoxalase II), or to pyruvate (by methylglyoxal dehydrogenase). Although these enzymes have been identified in various bacteria and yeasts, their significance in vivo remains unclear; they could allow the formation of pyruvate (and hence acetyl-CoA) under conditions of phosphate deficiency which would otherwise limit the formation of 1,3bisphosphoglycerate from glyceraldehyde 3-phosphate. Methylglyoxal is cytotoxic, apparently reacting with 7-methylguanosine in rRNA and thereby inhibiting ribosomal function; the glyoxalase system probably plays an important role in detoxifying methylglyoxal formed from dihydroxyacetone phosphate or during other metabolic processes (e.g. the catabolism of certain amino acids). [Methylglyoxal metabolism: ARM (1984) 38 49–68.] 2-methylisoborneol A substance which is produced by certain aquatic actinomycetes and cyanobacteria (e.g. Lyngbya); in concentrated form, 2-methylisoborneol has a camphor-like smell, but when diluted it has a ‘musty’ or ‘earthy’ odour which can taint WATER SUPPLIES and fish living in the water. (cf. GEOSMIN.) methylmethane sulphonate See ALKYLATING AGENTS. N-methyl-N
-nitro-N-nitrosoguanidine See MNNG. N-methyl-N-nitrosourea See ALKYLATING AGENTS. Methylobacterium A genus of facultatively methylotrophic and methanotrophic, rod-shaped bacteria which can alternatively use glucose and other complex substrates as sole sources of carbon and energy. Type species: M. organophilum. (See also METHYLOCOCCACEAE and PROTOMONAS.) 472 metronidazole occurring at 37° C; enrichment and primary isolation are otherwise as described under METHYLOCOCCUS. A pink pellicle is formed at 30° C; colonies are pink or yellow, consistently pink on subculture. A blue diffusible pigment may be formed in iron-deficient media. GC%: ca. 52. Type species: M. methanica. (‘M. methanitrificans’ and ‘M. methanooxidans’ may be synonyms of Methylosinus trichosporium.) Methylophaga A proposed genus of marine methylotrophic bacteria. The organisms are Gram-negative, strictly aerobic rods which grow on methanol, methylamine(s) or fructose (but not methane) and which require vitamin B12 ; those strains tested use the RMP pathway for carbon assimilation. Two species: M. marina, M. thalassia. [IJSB (1985) 35 131–139.] Methylophilus methylotrophus A species of obligately methylotrophic, non-methanotrophic bacteria (see METHYLOTROPHY) which assimilate carbon via the RMP pathway and which have been used in the production of SINGLE-CELL PROTEIN. Methylosinus trichosporium A species of methanotrophic, obligately methylotrophic, typically rod-shaped or vibrioid bacteria (see METHANOTROPHY, METHANE MONOOXYGENASE, METHYLOCOCCACEAE). [Effect of growth conditions on intracytoplasmic membranes and MMO activity: JGM (1981) 125 63–84.] methylotroph Any organism capable of METHYLOTROPHY. methylotrophy Oxidative metabolism characterized by (i) obligate or facultative use of C1 COMPOUNDS as the sole source of carbon and energy, and (ii) assimilation of formaldehyde (produced during methylotrophic metabolism) as a major source of carbon. Methylotrophy includes METHANOTROPHY. Methylotrophic organisms include certain bacteria (e.g. Hyphomicrobium, Methylobacterium spp, members of the family METHYLOCOCCACEAE, Methylophaga spp, Methylophilus methylotrophus, Microcyclus and Paracoccus denitrificans) and some fungi (e.g. species of Candida, Hansenula and Pichia). (cf. CARBOXYDOBACTERIA.) (a) Methylotrophy in bacteria. Most of the C1 compounds can be oxidized via a sequence of reactions which end in HCHO ! HCOOH ! CO2 . Formaldehyde may be assimilated by either the RMP PATHWAY or the SERINE PATHWAY, according to species. (The serine pathway also assimilates some carbon dioxide.) Energy may be derived from direct (non-cyclical) oxidative pathways (e.g. methanol ! formaldehyde ! formate ! carbon dioxide) or from the (cyclical) RMP PATHWAY operating in a dissimilatory mode. The TCA cycle appears to play little or no part in the generation of energy from C1 substrates. Methanol is used by most methylotrophs. It is oxidized to formaldehyde by a QUINOPROTEIN: methanol dehydrogenase (MDH; EC 1.1.99.8), whose electron acceptor is a soluble form of cytochrome cL . [Roles for c cytochromes in methylotrophs: JGM (1984) 130 3319–3325.] Methane is oxidized to formaldehyde via methanol (see METHANOTROPHY). Methylamines are oxidized directly to formaldehyde in reactions that usually involve dehydrogenases or monooxygenases; electrons from these substrates may be transferred to cyt b (via a flavoprotein), or may be passed to cyt c via the ‘blue protein’ amicyanin. The oxidation of formaldehyde to formate may be catalysed by various enzymes – e.g. (i) MDH; (ii) NAD- and reduced-glutathione-dependent formaldehyde dehydrogenase (EC 1.2.1.1); or (iii) NAD(P)-independent non-specific aldehyde dehydrogenase. Formate is oxidized by NAD-dependent formate dehydrogenase (EC 1.2.1.2). (Some methylotrophs can use exogenous formate as a source of energy and carbon.) [Formaldehyde metabolism: Book ref. 169, pp 315–323. Physiology and biochemistry of methylotrophic bacteria: AvL (1987) 53 23–28. Energetics of bacterial metabolism of C1 compounds: AvL (1987) 53 37–45.] (b) Methylotrophy in fungi. Of C1 compounds, only methanol has been studied extensively (as a substrate for certain yeasts); however, other C1 compounds have been reported to be used as substrates by fungi. The assimilatory and energy-yielding pathways of methanol metabolism in yeasts differ from those in bacteria. Methanol can be oxidized via the sequence: methanol ! formaldehyde ! formate ! carbon dioxide. The oxidation of methanol to formaldehyde takes place within PEROXISOMES and is catalysed by ‘methanol oxidase’ (EC 1.1.3.13), a non-specific alcohol oxidase which can oxidize e.g. several alkanols and alkenols, and which consists of eight FAD-containing subunits. The oxidation of methanol yields hydrogen peroxide as well as formaldehyde; CATALASE, within the peroxisome, eliminates the peroxide (as water and oxygen). Formaldehyde may be assimilated, via the XMP PATHWAY, or may be oxidized to yield energy. The oxidation of formaldehyde occurs within the cytoplasm. In Candida boidinii this reaction appears to occur in three stages, the first stage involving NADC - and GSH-linked (reduced-glutathione-linked) formaldehyde dehydrogenase (EC 1.2.1.1): GSH C HCHO C NADC

! HCO-SG C NADH C HC HCO-SG (S-formylglutathione) is hydrolysed by S-formylglutathione hydrolase (EC 3.1.2.12): HCO-SG C H2 O

! GSH C HCOOH Finally, HCOOH may be oxidized to carbon dioxide by NADlinked formate dehydrogenase – although the low substrate affinity of this enzyme in cell-free systems has raised doubts as to its function in vivo. Methylotrophy has also been recorded in the yeast Hansenula polymorpha. NADH generated in methanol-grown yeasts is oxidized in a (ROTENONE-insensitive but cyanide-sensitive) reaction involving an NADH dehydrogenase located at the outer surface of the inner mitochondrial membrane – the maximum energy yield being 2 ATP/NADPH. (In e.g. glucose-grown cells, most NADH is generated intra-mitochondrially and is oxidized by an NADH dehydrogenase located at the inner surface of the inner mitochondrial membrane, the maximum yield of energy being 3 ATP/NADH.) [Methylotrophic yeasts: AvL (1987) 53 29–36.] methymycin See MACROLIDE ANTIBIOTICS. metiram An agricultural antifungal agent used e.g. for the control of late blight of potato; it is a complex of ZINEB and polyethylene THIURAM DISULPHIDE. Metopus See HETEROTRICHIDA. metritis Inflammation of the uterus. (cf. ENDOMETRITIS; see also CONTAGIOUS EQUINE METRITIS.) Metrizamide See IODINATED DENSITY-GRADIENT MEDIA. metrocyte See SARCOCYSTIS. metronidazole (Flagyl) 1-(2-Hydroxyethyl)-2-methyl-5-nitroimidazole: a NITROIMIDAZOLE antimicrobial agent used to treat e.g. amoebic and balantidial DYSENTERY, GIARDIASIS, genitourinary TRICHOMONIASIS and infections caused by anaerobic bacteria, e.g. PSEUDOMEMBRANOUS COLITIS and VINCENT’S ANGINA. The bactericidal effect of metronidazole on Helicobacter pylori appears to depend on the reduced (active) 473 Metschnikowia Mickle shaker An apparatus used e.g. for the ballistic disintegration of tissue or cells; in principle it resembles the BRAUN MSK TISSUE DISINTEGRATOR. (See also BALLOTINI, CELL DISRUPTION and ULTRASONICATION.) miconazole (1-[2,4-dichloro-b-(2,4-dichlorobenzyloxyl)phenethylimidazole]) An AZOLE ANTIFUNGAL AGENT with a broad spectrum of antifungal activity; it also has activity against certain bacteria (particularly Gram-positive species) and e.g. against Naegleria fowleri. Miconazole is used mainly in the treatment of superficial mycoses; it is usually administered topically, but can be given intravenously (in cases of invasive or systemic mycosis) or orally (e.g. to reduce intestinal populations of Candida spp). Micrasterias A genus of placoderm DESMIDS. microaerobic Refers to an environment in which oxygen is present (cf. ANAEROBIC) but at a partial pressure which is (usually significantly) lower than that in air. microaerophilic Refers to any organism which grows optimally in a MICROAEROBIC environment. (Frequently the term is also used as a synonym of microaerobic.) microalgae Microscopic algae – particularly unicellular algae such as CHLAMYDOMONAS, CHLORELLA, DIATOMS, etc. microarray (DNA) See DNA CHIP. Microascales An order of fungi of the ASCOMYCOTINA; the organisms are primarily saprotrophs in soil and dung, but some can be pathogenic in man and other animals. Ascocarp: perithecioid or cleistothecioid, dark and solitary, sometimes setose; hamathecium: absent. Asci: rounded, thin-walled, evanescent. Ascospores: aseptate, pigmented. Genera: e.g. Microascus, PSEUDALLESCHERIA. Microascus See MICROASCALES. Microbacterium A genus of catalase-positive, asporogenous bacteria (order ACTINOMYCETALES, wall type VI – see also PEPTIDOGLYCAN) which frequently occur in dairy products (e.g. spraydried milk, some cheeses) – presumably as a result of improper cleansing of dairy equipment. In culture the organisms are slender, pleomorphic, non-motile rods. Growth occurs optimally at ca. 30° C on e.g. media containing yeast extract, peptone and milk; L-(C)-lactic acid is formed from glucose. Some strains produce a yellowish pigment. All strains grow aerobically. The organisms are typically thermoduric, surviving e.g. 63° C/30 min and 72° C/15 min. GC%: ca. 69–70. Type species: M. lacticum. (For M. thermosphactum see BROCHOTHRIX.) microbe Syn. MICROORGANISM. microbial insecticides See BIOLOGICAL CONTROL. microbial leaching (of ores) See LEACHING. microbial mat A complex, benthic microbial community which forms a cohesive layer and which is typically dominated by CYANOBACTERIA or other photosynthetic prokaryotes – and occasionally e.g. by microalgae; microbial mats occur in those regions where environmental stress tends to exclude, or reduce the numbers of, metazoans – e.g. in intertidal and hypersaline regions, in hot springs, and around HYDROTHERMAL VENTS. Microorganisms which occur in microbial mats include e.g. species of Lyngbya and Microcoleus, various purple photosynthetic bacteria, Chloroflexus, and certain aerobic and anaerobic chemotrophs, including SULPHATE-REDUCING BACTERIA. Microbial mats are believed to have given rise to STROMATOLITES, and, under certain conditions, may have been precursor material for KEROGEN and hydrocarbon deposits; in the latter context it has been suggested that oil is more likely to have been derived from marine organic deposits than from terrestrial plant material which, under appropriate conditions, gives rise to coal. [Book ref. 154.] form of the drug rather than on toxic oxygen radicals formed during its reduction [JAC (1998) 41 67–75]. Metschnikowia A genus of unicellular and pseudomycelial fungi (family METSCHNIKOWIACEAE) in which multilateral budding occurs. Ascospores: acicular, 1–2 per ascus. Six species are recognized; they occur in aquatic and terrestrial habitats and as parasites in invertebrates. M. pulcherrima and M. reukaufii are teleomorphs of Candida pulcherrima and Candida reukaufii, respectively. [Book ref. 100, pp. 266–278.] Metschnikowiaceae (Nematosporaceae; Spermophthoraceae) A family of fungi (order ENDOMYCETALES) which includes unicellular, pseudomycelial and mycelial forms. Ascospores: fusiform to acicular (cf. SACCHAROMYCETACEAE). Genera include e.g. Ashbya, COCCIDIASCUS, Eremothecium, METSCHNIKOWIA, NEMATOSPORA, Spermophthora. Members are generally parasitic in plants and/or invertebrates (see e.g. STIGMATOMYCOSIS). Species of Ashbya and Eremothecium are important commercial sources of riboflavin. metula In the conidiophores of certain fungi: a cell, or a short extension or branch (or part of a branch), which bears one or more phialides (see CONIDIUM) at its apex – see e.g. ASPERGILLUS and PENICILLIUM. Mexico virus A SMALL ROUND STRUCTURED VIRUS. mezlocillin See PENICILLINS. MF0 F1 H+ -ATPase See PROTON ATPASE. MFP See MEMBRANE FUSION PROTEIN. MFS Major facilitator superfamily: a category of TRANSPORT SYSTEMS which occur in prokaryotes and eukaryotes; they include pmf-dependent efflux pumps which mediate extrusion of antibiotics – and which therefore confer resistance to particular antibiotic(s). Examples of MFS systems are the various efflux pumps in Gram-negative bacteria which confer resistance to tetracyclines. Each pump encoded by a tetA gene is an integral inner membrane protein containing multiple transmembrane helices; tetA genes have been found on transposons and plasmids. MG fungus See APHANOMYCES. MGIT 960 A fully automated BACTEC system (Becton Dickinson) (MGIT D mycobacteria growth indicator tube) used e.g. for detecting Mycobacterium tuberculosis [evaluation: JCM (1999) 37 748–752]. MHA (immunol.) Major histocompatibility antigen. MHA-TP test Microhaemagglutination assay for Treponema pallidum antibodies, a miniaturized form of the TPHA TEST. MHC (immunol.) MAJOR HISTOCOMPATIBILITY COMPLEX. MHC restriction Syn. HISTOCOMPATIBILITY RESTRICTION. MHD (1) (serol.) Minimum haemolytic dose. (a) The smallest quantity of COMPLEMENT needed to lyse completely a fixed quantity of a standardized suspension of sensitized erythrocytes. (See also HAEMOLYTIC SYSTEM.) (b) The smallest quantity of a complement-dependent haemolysin needed to lyse completely a fixed quantity of erythrocytes in the presence of excess complement. (c) The smallest quantity of streptolysin O needed to lyse completely a fixed quantity of erythrocytes in the ANTISTREPTOLYSIN O TEST. (The MHD may be referred to as ‘1 unit’.) (See also HD50 .) (2) (virol.) Minimum haemagglutinating dose: the smallest quantity of a haemagglutinating virus capable of bringing about maximum agglutination of the erythrocytes in a given volume of a standardized suspension. mi-1 gene See POKY MUTANT. MIC (of an antibiotic) Minimum inhibitory concentration: the lowest concentration of a given antibiotic which inhibits a given type of microorganism under given conditions. (See also E TEST.) 474 microcomplement fixation microbicidal Able to kill at least some types of microorganism. (cf. MICROBISTATIC.) microbiocoenosis A natural community of microorganisms (including algae, bacteria and protozoa) characterized by phases of short-lived stability. microbiological safety cabinet Syn. SAFETY CABINET. microbiology The study of MICROORGANISMS and their interactions with other organisms and the environment (cf. CELLULAR MICROBIOLOGY). There is considerable overlap between microbiology and certain other disciplines: e.g. biochemistry, immunology, molecular biology and PARASITOLOGY. Microbiology is also directly relevant to certain aspects of medicine, veterinary science, agriculture, the food industry (see FOOD MICROBIOLOGY) and various commercial processes (see INDUSTRIAL MICROBIOLOGY) as well as ecology and pollution studies (see e.g. CARBON CYCLE, NITROGEN CYCLE, SULPHUR CYCLE, SEWAGE TREATMENT and WATER SUPPLIES). microbiota Microscopic organisms, particularly those in soil. Microbispora A genus of bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes) which occur e.g. in soil. The organisms form branched aerial and substrate mycelium, the former (commonly pink) giving rise to pairs of elongated spores; when grown on certain solid media some species (e.g. M. parva) deposit crystals of IODININ in the medium. The GC% of the type species (M. rosea) has been reported to be ca. 74. [Ecology, isolation and cultivation: Book ref. 46, pp. 2103–2117.] microbistatic Able to inhibit the growth and reproduction of at least some types of microorganism. (cf. MICROBICIDAL.) microbody A particle, typically less than 1 µm diam., consisting of a collection of functionally related enzymes enclosed in a membranous sac; microbodies occur in eukaryotic cells: see e.g. GLYCOSOME, GLYOXYSOME, PEROXISOME. [Microbodies in urateutilizing yeasts: AvL (1985) 51 33–43. ‘How proteins get into microbodies’ (review): BBA (1986) 866 179–203.] (cf. HYDROGENOSOME; LYSOSOME.) microcapsule See CAPSULE. microcarrier cell culture A system for culturing eukaryotic cells in which the cells grow to form a confluent layer on the surface of small solid particles suspended in a slowly agitated medium. [Review: AAM (1986) 31 139–179.] (See also TISSUE CULTURE.) microcins A category of low-MWt BACTERIOCINS produced by members of the Enterobacteriaceae and effective e.g. against bacteria of the same family. The microcins differ from COLICINS in their size, and also differ in that their synthesis is not induced by conditions which trigger the SOS SYSTEM. Typically, microcins are synthesized in the stationary phase, and synthesis tends to be repressed in rich media. The typical microcin ranges in size from a single modified amino acid (microcin A15 appears to be a derivative of methionine) to short or medium-sized peptides (up to ¾50 amino acid residues in length). Microcins are characteristically thermostable (e.g. 100° C/30 min), soluble in methanol–water (5:1), and resistant to extremes of pH and to certain proteases. The microcins can be grouped according to their modes of action: type A microcins inhibit certain metabolic pathways, type B microcins inhibit DNA replication etc. For example, the bacteriostatic microcin A15 inhibits homoserine succinyltransferase in the methionine biosynthetic pathway (see Appendix IV(d)). The bactericidal microcin B17 inhibits DNA gyrase and induces the SOS SYSTEM in Escherichia coli K12 [JGM (1986) 132 393–402]. Microcin C7 is a heptapeptide which blocks protein synthesis. (See also COLICIN V.) Plasmids which specify microcins (M plasmids or, preferably, Mcc plasmids) appear not to encode resistance to conventional antibiotics. Some of these plasmids are conjugative – e.g. that encoding B17 (pMccB17, an IncFII plasmid) and that encoding C7 (pMccC7, an IncX plasmid). Micrococcaceae A family of asporogenous, Gram-positive cocci which includes the genera MICROCOCCUS (the type genus), STAPHYLOCOCCUS and STOMATOCOCCUS. Micrococcus A genus of Gram-positive, aerobic, chemoorganotrophic, asporogenous, catalase-positive, generally non-motile bacteria which have a GC value (see GC%) of ca. 65–75 and which are typically resistant to lysostaphin; the micrococcal cell wall typically contains little or no teichoic acid (cf. TEICHURONIC ACIDS), and the ratio of glycine to lysine in the PEPTIDOGLYCAN is characteristically <2. (cf. STAPHYLOCOCCUS.) Micrococcus spp typically do not utilize glucose anaerobically, and typically do not form acid, aerobically, in a glycerol–erythromycin medium. Metabolism is respiratory (see RESPIRATION). The organisms occur e.g. on mammalian skin and in raw MILK. They can be isolated e.g. on FTO AGAR. Cells: cocci (ca. 1–2 µm diam.) which occur in tetrads and clusters. Most species form a waterinsoluble pigment. Type species: M. luteus. M. agilis. A red-pigmented, flagellated, psychrophilic species which does not form acid from glucose or mannose, is resistant to lysozyme, and is VP
ve. M. denitrificans. See PARACOCCUS. M. kristinae. An orange-pigmented species which forms acid from glucose and mannose, is resistant to lysozyme, and is VP Cve. M. luteus (formerly M. lysodeikticus, Sarcina pelagia). A yellow-pigmented species. Acid is generally not formed from glucose or mannose. Most strains are sensitive to LYSOZYME. VP
ve. M. radiodurans. See DEINOCOCCUS. M. roseus (formerly M. tetragenus, M. roseofulvus). A redor pink-pigmented species which often forms acid from glucose (but not from mannose), which may be weakly or strongly resistant to lysozyme, and which can hydrolyse gelatin. Other species: M. halobius, M. lylae, M. nishinomiyaensis, M. sedentarius, M. varians. [Book ref. 46, pp. 1539–1547.] Microcoleus A phycological genus of filamentous ‘blue-green algae’ (Nostocales) which has been included in the LPP GROUP [JGM (1979) 111 1–61 (p. 23)]. The organisms may form extensive mats on sedimented silt in shallow water [JGM (1984) 130 983–990], in salt-marshes, on mudflats etc, and may also grow on damp soil. microcolony (1) A small clone of bacteria in which the cells may be e.g. in direct physical contact or embedded in a common polysaccharide matrix; such colonies occur e.g. in aquatic habitats. (cf. COENOBIUM, CONSORTIUM.) [Microcolony formation and consortia: Book ref. 108, pp. 373–393.] (2) See CHLAMYDIA. (3) Any very small colony. microcomplement fixation A method for comparing the amino acid sequence in a given reference protein (e.g. an enzyme) with that in the same protein derived from a different strain or organism; the method is used e.g. in microbial TAXONOMY. Essentially, the protein under test is examined for its ability to fix COMPLEMENT when mixed with an antiserum raised against the purified reference protein; the (quantitative) difference in the complement-fixing ability of the test and reference proteins is taken as a measure of the difference between the amino acid sequences of the two proteins. The results are usually given in ImD units; 5 ImD units represent ca. 1% difference between the amino acid sequences. 475 microconidium microconidium (1) A SPERMATIUM. (2) A small CONIDIUM (cf. MACROCONIDIUM). microconjugant See CONJUGATION (1)(c). microcycle sporulation Sporulation immediately following outgrowth of a spore. [Microcycle sporulation in Bacillus subtilis: Book ref. 164, pp. 163–167.] microcyclic rusts Those rust fungi (see UREDINIOMYCETES) which form only one type of binucleate spore, the teliospore – i.e., aeciospores and uredospores are not formed; they include e.g. some species of Puccinia. Microcyclus (1) A genus of fungi of the Dothideales. (2) A genus of Gram-negative, obligately aerobic bacteria which occur e.g. in soil and freshwater habitats. The cells are capsulated curved rods, 0.3–1.0 ð 1.0–3.0 µm; one strain has a single polar flagellum, but most strains are non-motile. The cells of some strains contain GAS VACUOLES. The organisms grow chemoorganotrophically, but chemolithoautotrophic growth, with H2 as electron donor, has been reported for some strains; all strains tested are facultative methylotrophs (see METHYLOTROPHY) which can utilize e.g. formate and methanol. Catalase and oxidase Cve. GC%: 66–69. Type species: M. aquaticus. (Since the fungal genus Microcyclus (Saccardo 1904) takes precedence, a new name has been proposed for the bacterial genus: Ancylobacter [IJSB (1983) 33 397–398].) [Book ref. 22, pp. 133–135.] microcyst (1) (bacteriol.) A type of resting cell – which may or may not be resistant to heat, desiccation etc – formed by certain bacteria: e.g. Sporocytophaga and certain Nocardia spp. (2) (bacteriol.) In the MYXOBACTERALES: syn. myxospore. (3) (bacteriol.) Syn. coccoid body (see COCCOID BODIES). (4) In some SLIME MOULDS: a resting structure formed by the rounding up and encystment of an individual myxamoeba (cf. MACROCYST). microcystin See MICROCYSTIS. Microcystis A phycological genus of unicellular ‘blue-green algae’ (CYANOBACTERIA) which may be referrable to SYNECHOCYSTIS. The cells are spherical, occur in mucilaginous colonies, and contain GAS VACUOLES. Species are traditionally distinguished on the basis of cell size and colony form: M. aeruginosa and M. flos-aquae form colonies in which numerous cells are embedded in a diffluent mucilage, while colonies of M. marginata are surrounded by a distinct envelope. (Colony form may be lost in culture.) The organisms do not fix nitrogen. Microcystis spp are planktonic in fresh water, often forming BLOOMS e.g. in reservoirs. Some strains of M. aeruginosa produce a cyclic peptide toxin, microcystin, which has hepatotoxic effects in animals drinking the contaminated water; the toxin appears to associate with cytoskeletal actin in a manner similar to phalloidin (see PHALLOTOXINS). Another (peptide?) toxin (‘fast death factor’, microcystin-c) is a potent neurotoxin responsible for convulsions, respiratory failure, and rapid death in animals. microdilution broth test See DILUTION TEST. Microellobosporia See STREPTOMYCES. microfauna See MICROFLORA (2). microfilament Any very thin filament – particularly one of polymerized ACTIN. microflora (1) In a microbiological context: the totality of microorganisms normally associated with a given environment or location (see e.g. BODY MICROFLORA); the term ‘flora’ is often used as an abbreviation in this context. (2) In a broader biological context: the microscopic plants and ‘plant-like’ organisms (e.g. bacteria, fungi, algae etc) normally present in a given environment or location, the term microfauna being used to refer to the microscopic animals and ‘animal-like’ organisms (e.g. protozoa, nematodes) of that environment/location. microforge See MICROMANIPULATION. microfungi Those fungi in which the individual fructifications are too small to be seen in detail with the unaided eye. microgamete See ANISOGAMY. microgamont A cell which gives rise to microgametes (i.e., male gametes). Microglossum See HELOTIALES. Microjoenia See HYPERMASTIGIDA. micromanipulation (micrurgy) Any procedure involving the physical manipulation of individual cells (or spores etc) or their components. Examples include: the isolation and culture of single cells for genetic studies; dissection of the ciliate infraciliature in cytophysiological studies; intercellular transplantation of nuclei, mitochondria etc. Dissection and transplantation procedures are sometimes referred to as microsurgery. Micromanipulation may be carried out with any conventional microscope that is fitted with a good-quality mechanical stage and a high-power dry (i.e. non-immersion) objective lens; phase contrast microscopy is preferable for certain types of operation. A vibration-free workplace is essential. The material under study may be arranged e.g. on the surface of an agar block, or as a hanging drop preparation, and is commonly contained within a partially enclosed moist chamber on the microscope stage. (The use of a heated objective lens mounting prevents condensation of water vapour on the lower lens.) The actual manipulation is carried out with glass microtools which are prepared in a microforge: essentially, an apparatus used to control the movement of a thin glass tube or rod relative to an electrically heated filament – the operation being observed under the low power of a microscope. Needles are fine glass filaments which, in the region of the point, may be ca. 0.2 µm in diameter. Loops are made in various sizes. Pipettes, which may be <1 µm diameter, are made by drawing out glass tubing which has been heated to plasticity; they have been used e.g. for holding cells during microsurgery: the (heat-polished) tip of the pipette is held against a cell and a gentle suction is applied. Each microtool is fixed in a metal holder which is clamped to a micropositioner: essentially, a means whereby the coarse movements of the hand are scaled down to give the finer movements necessary for the control of the microtools. By such means, microtools can make controlled movements, with speed and precision, along all three axes in space. In a simpler method (used for uncomplicated operations) the microtool holder is clamped to the nosepiece of a second microscope; rotation of the fine focusing control of this microscope moves the microtool in a vertical plane, while movements of the microtool relative to the specimen in a horizontal plane can be made by moving the specimen with the mechanical stage of the first microscope. micrometer In light MICROSCOPY: a device used for the measurement of a specimen or parts of a specimen. The simplest micrometer is the ocular micrometer (D eyepiece micrometer, graticule, reticle, reticule): a glass or plastic disc, marked with a series of regular (though arbitrary) graduations, which is placed in the front focal plane of the eyepiece lens, i.e., resting on the FIELD DIAPHRAGM (sense 2); when the micrometer is in place its graduations can be seen superimposed on the image of the specimen. Calibration of the micrometer may be carried out by focusing the microscope on a slide micrometer: a glass slide etched with graduations precisely 5 or 10 µm apart; if, say, 10 graduations of the ocular micrometer correspond to 5 µm on the slide, then 1 graduation will measure 0.5 µm in a specimen. The filar micrometer consists of a thin moveable wire in the front focal plane of the eyepiece, the wire being moveable by 476 micropore (ca. 53–56) than that characteristic of the genus (71–73). Many species are cellulolytic. Some species produce useful antibiotics (see e.g. GENTAMICIN). Type species: M. chalcea. [Book ref. 73, pp. 67–69.] micron (µ) 10
6 m (D MICROMETRE). micronemes Small, convoluted, thread-like structures, oval or circular in cross-section, which occur in members of the APICOMPLEXA; function unknown. micronucleus (ciliate protozool.) The smaller (commonly <5 µm) of the two types of nucleus characteristically present in ciliates (cf. MACRONUCLEUS); according to species a cell may contain one, several or many micronuclei, although micronuclei are absent in some viable strains of species which are normally micronucleate. Micronuclei lack nucleoli, are characteristically diploid, and their DNA (which occurs in chromosome form) is transcriptionally inactive; they are involved in genetic recombination (see AUTOGAMY and CONJUGATION) and in the regeneration of macronuclei. microorganisms According to common usage: microscopic organisms (and taxonomically related macroscopic organisms) within the categories ALGAE, ARCHAEA, BACTERIA, FUNGI (including LICHENS), PROTOZOA, VIRUSES and SUBVIRAL AGENTS. Some authors do not regard viruses and subviral agents as microorganisms – reserving the term ‘organism’ for an entity which consists of one or more cells [Book ref. 159, pp. 11–13]; other authors include PLASMIDS (as well as viruses) within the category ‘organisms’ [Book ref. 161, pp. 3–16]. micropalaeontology The study of microscopic fossils. (See also FOSSIL MICROORGANISMS.) microparticle enzyme immunoassay See LCR. microperoxidase Part of a cytochrome c molecule (the haem moiety together with part of the protein) which has peroxidase activity and which is used e.g. in CHEMILUMINESCENCE studies. microphotography Photography in which very small photographs are made of macroscopic or large objects. (cf. PHOTOMICROGRAPHY.) microplasmodesmata Minute pores which occur in the septa in certain filamentous prokaryotes: e.g. in hyphae of certain actinomycetes (see ACTINOMYCETALES) and in cyanobacterial TRICHOMES (see also HETEROCYST). Micropolyspora A (proposed) genus of bacteria (order ACTINOMYCETALES, wall type IV) which are found e.g. in mouldy hay, compost and soil. The organisms form substrate and aerial mycelium, both of which give rise to chains of spores; they do not form mycolic acids. M. brevicatena, the former type species of Micropolyspora, forms mycolic acids and has been transferred to the genus Nocardia [JGM (1982) 128 503–527], leaving Micropolyspora a proposed genus nomen conservandum [Book ref. 73, pp. 123–124]. [Proposal to conserve the name Micropolyspora (with M. faeni as type species) and new genus description: IJSB (1984) 34 505–507.] M. faeni, a common causal agent of FARMERS’ LUNG, forms substrate mycelium of some shade of yellow, orange or brown, and a white aerial mycelium. (M. faeni and M. rectivirgula appear to be synonyms [JGM (1979) 115 343–354].) M. viridinigra and M. rubrobrunea have been transferred to the genus Excellospora. Some authors use the name Faenia rectivirgula as a synonym of M. faeni. micropore (1) (formerly: micropyle, ultracytostome) In members of the APICOMPLEXA: a small invagination of the pellicle (usually) coincident with a discontinuity of the inner layer of the pellicle. One or more micropores may be present in a given cell; they may function as sites for the ingestion of food. (2) See GEOTRICHUM. means of a knob that controls a fine-threaded screw; calibration is carried out e.g. with a slide micrometer, the distance (in µm) travelled by the wire being read off on a scale around the control knob. micromethods (miniaturized methods) In clinical microbiology: various procedures used for routine testing of specimens or microbial isolates in a manner economical of time and/or space and material. Micromethods are commonly used e.g. to carry out, simultaneously, a range of biochemical identification tests on a given (usually bacterial) isolate; these procedures generally make use of commercially available kits, some of which are briefly described below. The API system consists of a plastic strip holding a number of microtubes, each containing a different dehydrated medium; each microtube is inoculated with a suspension of the test organism, mineral oil is added to certain microtubes (to exclude air), and the strip is incubated. Subsequently, reagents are added to detect particular metabolic products, and the results are read and analysed. The Enterotube II system consists of a tube divided into a sequence of 12 compartments, each compartment containing a different agar-based medium; the media are inoculated by passing an inoculum-bearing needle axially through the tube. The Minitek system consists of a plastic plate containing a number of wells, each well containing a paper disc impregnated with a dehydrated medium; each disc is inoculated with a suspension of the test organism, some are then overlaid with mineral oil, and the plate is incubated. Reagents are then added (to certain wells) and the results are read and analysed. The PathoTec system consists of a number of test strips, each impregnated with the dehydrated medium appropriate to a given test; each strip is incubated in a suspension of the test organism (or inoculated directly from a colony) and the test is read after a specified time. Some systems are designed for anaerobic incubation. The AN-IDENT system permits the testing of anaerobes in a strip incubated aerobically. [Descriptions of some identification kits: Book ref. 120, pp. 52–65.] (See also BACTEC.) micrometre (µm) 10
6 m, i.e., 1/1000th mm. Micromonadophyceae A proposed class of unicellular, scaly or naked, flagellated green algae (division CHLOROPHYTA) in which the interzonal mitotic spindle persists during cytokinesis [Book ref. 123, pp. 29–72]. Examples of constituent genera are Cymbomonas, Dolichomastix, Mamiella (scaly), Mantoniella (scaly), Mesostigma, Micromonas (naked), Monomastix, Nephroselmis (D Heteromastix) (scaly), Pedinomonas (naked), Pterosperma, Pyramimonas (scaly), Scourfieldia (naked), Trichloris; many of these genera were formerly included in the PRASINOPHYCEAE. (cf. TETRASELMIS.) Micromonas See MICROMONADOPHYCEAE. Micromonospora A genus of bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in soil, decaying vegetation and aquatic habitats. The organisms form a non-fragmenting, branching substrate mycelium, but usually do not form an aerial mycelium; most species form a pigment (e.g. yellow, blue, purple). Non-motile, darkly-pigmented spores are borne singly at the tips of sporophores which arise from the mycelium; the spores have some resistance to heat (e.g. 70° C/30 min) and to desiccation. Most species are aerobic; some obligately anaerobic species are currently included in the genus, but the GC% of one (‘M. ruminantium’) has been reported to be much lower 477 micropositioner micropositioner See MICROMANIPULATION. micropyle (1) In some species of coccidia (e.g. Eimeria stiedae): a thin, polar region in the oocyst wall. (2) See MICROPORE (1). microRNA (miRNA) See ANTISENSE RNA. Microscilla See BACTERIA (taxonomy). microscope slide Syn. SLIDE. microscopy The use of an instrument (microscope) for examining objects or details which are too small to be seen (or seen clearly) by the unaided eye; the microscope forms an enlarged image of the specimen. In light microscopy images are formed by light or ultraviolet radiation (cf. ELECTRON MICROSCOPY). (a) Bright-field microscopy is the commonest form of light microscopy: the form usually meant by the term ‘microscopy’. Essentially, light from a lamp is concentrated by the CONDENSER and directed onto the specimen (Fig. 1). The objective lens (‘objective’) forms a magnified image of the specimen, and this MICROSCOPY. Figure 1. A research microscope (Leitz Orthoplan) set up for Köhler illumination. Reproduced by permission of Leitz (Instruments) Ltd, Luton, United Kingdom. MICROSCOPY. Figure 2. Image formation in bright-field microscopy, using a diffraction grating as a specimen. Light entering one slit of the grating gives rise to zero-order rays (single arrowhead), first-order rays (double arrowhead) and second-order rays (terminally dashed lines). An image is formed as a result of interference between diffracted and non-diffracted rays. 478 microscopy MICROSCOPY. Figure 3. Amplitude and phase relationships in phase-contrast microscopy. (See text for explanation.) MICROSCOPY. Figure 4. Image formation in phase-contrast microscopy: simplified diagrammatic scheme showing spatial relationships between zero-order rays (solid lines from specimen to image plane) and first-order diffracted rays (dashed lines). (See text for explanation.) image is further magnified by the eyepiece lens (see MAGNIFICAThe specimen is usually examined on a SLIDE which rests on the stage. (See also HANGING DROP; MICROTOME; SMEAR; WET MOUNT.) For an image to be formed, light which has passed through the specimen must differ from the background light in amplitude (intensity) and/or in composition (e.g. colour); thus, an image of the specimen is usually seen against a lighter (bright-field) background. Some specimens are translucent and are therefore poorly visible; such specimens can be made light-absorbing by suitable STAINING. Image formation is often depicted by ray diagrams. However, for objects below a certain size, image formation can be described satisfactorily only in terms of wave optics; this is because such objects cause significant diffraction of light, and the formation of an image depends on interference between diffracted and nondiffracted light waves. Image formation is shown in Fig. 2; the object shown is a diffraction grating, but biological specimens form images in a similar way. Fig. 2 shows that, for image formation, the objective must collect at least the undeviated (zeroorder) light waves and the diffracted (first-order) light waves. For a perfect image all the diffracted light should be collected, but this is impracticable. To collect the maximum amount of diffracted light an objective should have a high numerical aperture: see RESOLVING POWER. The specimen should be uniformly illuminated, and the (divergent) beam of light from the specimen should fill the entire aperture of the objective (see CRITICAL ILLUMINATION; KÖHLER ILLUMINATION). When using a HIGH-DRY OBJECTIVE, good image formation also requires the use of a COVER-GLASS of the correct thickness. (See also oil immersion objective under RESOLVING POWER; cf. CONFOCAL SCANNING LIGHT MICROSCOPY.) (b) Dark-field microscopy (dark-ground microscopy) is used e.g. for examining objects too small to be seen by bright-field microscopy. For low- or medium-power work, the underside of TION). 479 microsome the substage condenser is fitted with a central opaque disc (the stop or patch stop) which allows only the peripheral rays to pass through the condenser – thus forming a hollow cone of light whose apex is focused in the plane of the specimen; in the absence of a specimen these rays diverge at such an angle that none enters the objective so that the field of view appears dark or black. (The NA of the condenser must be greater than that of the objective.) A specimen scatters the rays, some of which enter the objective and form a bright image of the specimen against a dark background. For high-power work use is made of a special oil-immersion condenser (e.g. a cardioid condenser) within which the incident rays are twice reflected so that the emergent rays form a hollow cone of large apical angle. (See also RHEINBERG ILLUMINATION.) (c) Phase-contrast microscopy. A colourless specimen which absorbs little light (e.g. a non-pigmented living cell) is not clearly visible by bright-field microscopy but may be seen by phase-contrast microscopy. When such a specimen diffracts light, the first-order waves are retarded by approximately 1/4wavelength (1/4l) and are of amplitude smaller than that of the zero-order waves (Fig. 3(i)). If these zero-order and firstorder waves are allowed to interfere they will produce resultant waves of amplitude (intensity) similar to that of the background waves but differing from them in phase (Fig. 3(ii)); the phase difference cannot be detected by the eye so that no image is seen. (Background waves are waves which do not pass through the specimen before entering the objective lens of the microscope.) If, however, the zero-order and background waves are first retarded by 1/4l (to eliminate the phase difference between the first-order waves and the zero-order and background waves), interference between zero-order and first-order waves will produce a resultant wave (the image-forming wave) of amplitude greater than that of the background wave (Fig. 3(iii)), i.e., a visible image will be formed. In a phase-contrast microscope the condenser has a ringshaped aperture in its front focal plane so that a hollow cone of light can be focused (as a small bright ring) onto a phase plate located in the back focal plane of the objective (Fig. 4). The phase plate is a glass disc on which has been deposited a ring of material (e.g. magnesium fluoride) of such thickness that it retards by 1/4l the light it transmits. In the absence of a specimen all the incident light is focused on and transmitted via the ring. When a specimen is examined, the zero-order and background light is transmitted via the ring, but the firstand higher-order beams pass through the phase plate via regions not located in the ring (Fig. 4). In the image plane, interference occurs between zero-order and diffracted light; the resultant waves form a visible image because their amplitude is greater than that of the background waves (Fig. 3(iii)). To increase image contrast further the phase plate ring has deposited on it a thin layer of light-absorbing material which decreases the amplitude of background and zero-order waves; thus the amplitude of the image-forming wave becomes greater relative to that of the background wave (Fig. 3(iv)). (The clarity of the image can be improved by using a green filter in the microscope condenser.) In the above scheme (negative or ‘bright’ phase-contrast microscopy) the zero-order beam is retarded by 1/4l, and the specimen appears lighter than the background. In positive or ‘dark’ phase-contrast microscopy the zero-order beam is advanced 1/4l; the specimen appears darker than the background because destructive interference occurs between the zeroorder and first-order beams. [Some improvements in phase-contrast microscopy: JM (1983) 129 49–62.] (d) Interference-contrast microscopy (differential interference-contrast microscopy). This form of microscopy avoids the ‘haloes’ which surround images formed by phase-contrast microscopy, and gives clearer images of fine detail. It can detect minute differences in specimen thickness and/or refractivity – such differences being indicated by colour differences in the image. In microscopes based on the shear system (e.g. the Nomarski microscope) light from the source is initially split into two parallel beams. The object beam passes through the specimen, while the reference beam passes through a clear or relatively clear area of the slide; subsequently the beams interfere to form an image. In the double-focus system both beams are co-axial – the object beam being focused in the plane of the specimen, and the reference beam being focused either above or below this plane; subsequently, the beams interfere to form an image. (e) Fluorescence microscopy. In fluorescence microscopy a (usually) FLUOROCHROME-treated specimen is irradiated with ultraviolet radiation (or, in some cases, blue light) and the light emitted (see FLUORESCENCE) forms the image of the specimen in a manner similar in principle to that in bright-field microscopy. (Some types of specimen exhibit autoFLUORESCENCE.) In a fluorescence microscope, the beam from a mercury-arc lamp, or a tungsten lamp, is initially passed through an exciter filter which transmits only the exciting radiation. The exciting radiation is then focused onto the specimen either from below (transmittedlight fluorescence microscopy) or from above (incident-light fluorescence microscopy, or epifluorescence microscopy). In epifluorescence microscopy exciting radiation enters the microscope tube from the side and is reflected downwards, by a dichroic mirror, through the objective onto the specimen; the imageforming light emitted from the specimen passes through the objective, through the dichroic mirror (which does not reflect light of visible wavelength), and through the eyepiece. In both transmitted-light and epifluorescence microscopes, a barrier filter (D stopping filter) is placed below the eyepiece lens; this transmits emitted light of longer wavelength but stops the short wavelength exciting radiation – thus protecting the eyes and, in photomicrography, protecting the film against fogging. One advantage of epifluorescence microscopy, compared with transmitted-light fluorescence microscopy, is that the emitted light is not partially absorbed within the thickness of the specimen; thus a brighter image can be obtained. (See also CITIFLUOR.) (N.B. In all types of fluorescence microscopy it is necessary to use non-fluorescent lenses, slides, cover-glasses, immersion oil, etc.) (See also IMMUNOFLUORESCENCE; ACRIDINE ORANGE; AURAMINE–RHODAMINE STAIN; CALCOFLUOR WHITE.) (f) Ultraviolet microscopy is a form of microscopy in which ultraviolet radiation is used both for illumination of the specimen and for image formation; the image is recorded on a fluorescent screen or on a photographic plate. Ultraviolet microscopy can resolve finer detail than can bright-field microscopy because the wavelength of the image-forming radiation is shorter than that of visible light (see RESOLVING POWER). (g) Epimicroscopy is any form of microscopy in which the illuminating beam is focused on the specimen by the objective; in this type of illumination (D vertical or incident-light illumination) the objective acts simultaneously as the condenser and objective lens. An example is epifluorescence microscopy: see (e) above. microsome One of many small heterogeneous membranous vesicles obtained e.g. when eukaryotic cells are homogenized and centrifuged; microsomes include e.g. vesicles formed from the rough and smooth ENDOPLASMIC RETICULUM and from the CYTOPLASMIC MEMBRANE. 480 microtubules microsource See BIOLUMINESCENCE. Microsphaera See ERYSIPHALES. Microspora A genus of filamentous, unbranched green algae (division CHLOROPHYTA). Microspora A phylum of PROTOZOA [JP (1980) 27 37–58] which are obligate intracellular parasites in a wide range of animals, including vertebrates and invertebrates. Mitochondria are absent. The organisms form spores which are small (commonly <10 µm diam.), unicellular in origin, and have walls which lack pores and are not composed of valves (cf. MYXOZOA); each spore contains a single uninucleate or binucleate sporoplasm (an amoeboid infective cell which is released on germination of the spore), and a single tubular, coiled, extrusile polar filament which is not enclosed within a polar capsule. A polar cap covers the attached end of the tubular filament. In many species there is a membranous (vacuolar) region beneath the polar cap (the polaroplast or polar sac). Classes: MICROSPOREA; RUDIMICROSPOREA. (See also ENTEROCYTOZOON; cf. ASCETOSPORA.) Microsporea A class of protozoa (phylum MICROSPORA) which are parasitic in insects and other invertebrates, in poikilothermic vertebrates, and (rarely) in mammals. The microsporean spore has (typically) a three-layered, dense, refractile wall, and contains a single sporoplasm and a complex extrusion apparatus comprising the polar filament (which extends back from the polar cap and coils around the inside of the spore wall) and (typically) a polaroplast. When a spore is ingested by a host animal, the polar filament is discharged explosively, everting to form a long tube; the explosive force is such that the filament penetrates any host cell in its path, and the infective sporoplasm can then pass through the filament and into the host cell. [Role of pH-triggered Ca2C influx in the discharge mechanism: JCB (1985) 100 1834–1838.] The class contains two orders: Minisporida (e.g. Burkea, Chytridiopsis, Hessea) and Microsporida (e.g. Amblyospora, ENCEPHALITOZOON, GLUGEA, NOSEMA, Pleistophora, THELOHANIA). Microsporida See MICROSPOREA. microsporidean (Pertaining to) a protozoon of the MICROSPORA (formerly of the class Microsporidea, subphylum Cnidospora). microsporidiosis Any disease caused by protozoa of the phylum MICROSPORA. In man, infections occur in both immunocompetent and immunocompromised individuals, although most isolations appear to come from the latter group (particularly AIDS patients). Infections may occur in any of various sites. For example, travellers’ diarrhoea in immunocompetent hosts can be due to intestinal infection by e.g. Encephalitozoon intestinalis or Enterocytozoon bieneusi, while Encephalitozoon hellem, Nosema ocularum and Vittaforma corneae have been isolated from the eye. Species of Pleistophora infect skeletal muscle, while Trachipleistophora hominis infects skeletal muscle, eye, kidney and nasopharynx. [Lab identification: JCM (2002) 40 1892–1901.] In immunocompromised individuals, infection by microsporidia is reported to be more common in patients with CD4C counts below 100/µl. [Microsporidiosis: JMM (2000) 49 947–967 (948–952).] Microsporum See DERMATOPHYTES. microstome See TETRAHYMENA. microstrainer See SEWAGE TREATMENT. microsurgery See MICROMANIPULATION. Microtetraspora A genus of bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes) which occur e.g. in soil. The organisms form a substrate mycelium (usually colourless, grey, or yellowish) and aerial hyphae which give rise to spores (typically) in chains of four. Type species: M. glauca. [Ecology, isolation, cultivation: Book ref. 46, pp. 2103–2117.] Microthamnion A genus of filamentous green algae related to TREBOUXIA; the filaments are branched and possess holdfasts. Microthorax See HYPOSTOMATIA. Microthrix parvicella A species of filamentous bacteria isolated from activated sludge [see e.g. JGM (1984) 130 2035–2042]. microtome An instrument for cutting thin sheets (sections) of tissue (or of individual cells) for examination by MICROSCOPY. Essentially, the instrument consists of a fixed knife and a mechanism which moves the specimen relative to the knife; after each section is cut the specimen is moved towards the knife by a distance equal to the thickness of the section. Sections for ELECTRON MICROSCOPY are cut with an ultramicrotome: a more sophisticated instrument having a glass or diamond knife. To ensure rigidity during cutting, the fixed, dehydrated specimen is embedded in paraffin wax (for light microscopy) or e.g. in an epoxy resin (for electron microscopy). Sections for light microscopy are usually about 5–10 µm thick. Sections for electron microscopy are 100 nm or less; these are cut and selected with the aid of a dissecting microscope. microtubule-associated proteins (MAPs) Proteins associated stoichiometrically with the surfaces of MICROTUBULES; certain MAPs are present in only some types of cell. MAPs include e.g. Mg2C -activated GTPase; the ATPase dynein; the various tau proteins (apparently involved in the assembly and/or stability of MTs); the ‘high-MWt MAPs’ designated MAP1 and MAP2 (which occur mainly in the cells of higher animals); and the cold stability factor (CSF) which occurs e.g. in mammalian brain cells and which increases the resistance of MTs to cold. (CSF–MT binding is Ca2C -sensitive in brain cells.) [Review: ARCB (1986) 2 421–457.] microtubule-organizing centre (MTOC) In eukaryotic cells: any intracellular body or region involved in the initiation and/or assembly of MICROTUBULES: see e.g. BASAL BODY (b), centrosome in CENTRIOLE, DEUTEROSOME, KINETOCHORE and SPINDLE POLE BODY. microtubules (MTs) Non-contractile protein tubules, each having a mean external diameter of ca. 24 nm, which occur singly, in pairs or triplets, or in bundles, in most types of eukaryotic cell – including the cells of microorganisms, higher plants and animals. (MTs do not occur in disc-shaped mammalian erythrocytes, although they are present in the elliptical erythrocytes of the Camelidae, and even in blood platelets.) MTs are involved e.g. in the maintenance of cell shape and structure (see e.g. CYTOSKELETON), in some types of MOTILITY (see e.g. CILIUM and FLAGELLUM (b)), in spindle formation during MITOSIS, and (apparently) in the intracellular translocation of vesicles, LYSOSOMES and other organelles. The wall of each MT consists of several (typically 13) longitudinally adjacent rows of TUBULIN molecules, each row being called a protofilament; MTs can change their length (and, apparently, their intracellular location) by the assembly/disassembly of tubulin units at one or both ends of the tubule. The outer surface of the MT wall bears a number of MICROTUBULE-ASSOCIATED PROTEINS (MAPs). Even when MTs occur in bundles there is seldom direct contact between one MT and another; they are usually separated from one another by an ‘exclusion zone’ of ca. 10 nm width – a zone into which the MAPs project. The assembly of tubulin into MTs is apparently associated with the binding of guanine nucleotides to the tubulin molecule: GTP binds at the ‘E’ site and (more firmly) at the ‘N’ site; at some stage (during or after incorporation of the tubulin molecule) GTP at the E site is hydrolysed by an Mg2C -activated GTPase (an MAP) and inorganic phosphate is released (GDP 481 Microviridae micrurgy Syn. MICROMANIPULATION. Middelburg virus See ALPHAVIRUS. middle T antigen See POLYOMAVIRUS. Middlebrook 7H-9 broth A growth medium used for Mycobacterium spp; it contains mineral salts, pyridoxine hydrochloride, biotin, sodium glutamate, sodium citrate, and either Tween 80 or glycerol. [Recipe: Book ref. 53, p. 1415.] midland cattle disease A form of BOTULISM in cattle; the disease is due to ingestion of forage or carrion contaminated with C. botulinum type Cb (cf. FORAGE POISONING). mid-point potential See REDOX POTENTIAL. Miescher’s tubes Sarcocysts in tissues (see SARCOCYSTIS). MIF Migration inhibition factor: a CYTOKINE originally obtained from specifically reactive T lymphocytes exposed to the relevant antigen; in vitro, this factor is characterized by it ability to inhibit the movement of macrophages away from the source of MIF. (Other in vitro activities include the activation of NO synthase (see NITRIC OXIDE) in macrophages.) MIF is associated with DELAYED HYPERSENSITIVITY reactions and may account for the accumulation and activity of macrophages in DH lesions. The human MIF is a 12 kDa protein; its sources include the pituitary gland. The precise in vivo role of MIF has not been established; it may e.g. exert pro-inflammatory activity as a counter-balance to raised levels of (anti-inflammatory) glucocorticoids resulting from stimulation of the hypothalamic–pituitary–adrenal axis. migration inhibition factor See MIF. mikamycins See STREPTOGRAMINS. mildews (plant pathol.) See e.g. DOWNY MILDEWS and POWDERY MILDEWS. Miles and Misra’s method (drop method; drop plate method) A COUNTING METHOD for determining the viable cell count of a bacterial suspension. Serial dilutions (often log dilutions) of the suspension are prepared. One drop (of known volume) from each dilution is placed at a separate, recorded position on the surface of a pre-dried agar PLATE; the drops are allowed to dry. The plate is then incubated until visible colonies develop in the small, circular areas corresponding to the drops. If a sufficient number of dilutions has been prepared, a drop from one of the dilutions will give rise to a countable number of discrete colonies. Assuming that each viable cell in the drop gave rise to a separate colony (and knowing the volume of the drop) the viable count can be calculated. In practice, a count is calculated from each of several drops on the plate, and the average is taken. milfuram See PHENYLAMIDE ANTIFUNGAL AGENTS. Milgo See ETHIRIMOL. miliary Resembling millet seed. Miliary TUBERCULOSIS: a form in which numerous small tubercles develop. Miliolina See FORAMINIFERIDA. milk (microbiological aspects) Milk consists primarily of water containing proteins, soluble carbohydrates, electrolytes, lipids and vitamins. The major protein constituents of milk are CASEINS (which occur in the form of complex micelles); there are also the socalled WHEY PROTEINS such as lactalbumin and lactoglobulin. IMMUNOGLOBULINS occur in high concentrations in COLOSTRUM and in relatively lower concentrations in milk. Various enzymes, e.g., alkaline phosphatase (see PHOSPHATASE TEST), are normally present in raw (i.e., untreated) milk. (See also LACTOFERRIN and LACTOPEROXIDASE–THIOCYANATE–HYDROGEN PEROXIDE SYSTEM.) The major carbohydrate, LACTOSE (ca. 50 g/l in cow’s milk), contributes ca. 50% of the osmotic pressure of milk (which is isotonic with blood plasma); small amounts of e.g. glucose and galactose are also present. remaining bound to the tubulin). However, the in vitro assembly of MTs can occur (albeit more slowly) with GDP–tubulin (or with non-hydrolysable GTP analogues) – suggesting that energy is not required for MT assembly. In contrast, MT disassembly seems to need GTP and to be energy-dependent. Although assembly and disassembly can occur at both ends of an MT, there is evidence that, at least under certain in vitro conditions, ‘treadmilling’ can occur; treadmilling means the incorporation of tubulin at one end of a microtubule with concomitant disassembly at the other end. The initiation of MT assembly in vivo occurs usually, but apparently not always, at a MICROTUBULE-ORGANIZING CENTRE. Various factors and agents can promote or inhibit MT assembly or disassembly. Thus, e.g. the in vitro assembly of tubulin into MTs occurs only above a certain minimal temperature, and is promoted by GTP, Mg2C , and a low concentration of Ca2C . Disassembly is promoted e.g. by cold, by high hydrostatic pressures, and by dilution (MTs disassemble below a certain critical concentration); in some cases MT disassembly can be brought about by Ca2C (above a certain concentration), while in other cases Ca2C is effective only in combination with CALMODULIN. Other agents which can inhibit the assembly of MTs include e.g. BENZIMIDAZOLES, COLCHICINE, GRISEOFULVIN, the macrolide maytansine, PODOPHYLLOTOXIN, steganacin (a polycyclic lactone with a trimethoxy-substituted aromatic ring), and certain VINCA ALKALOIDS. MT assembly is promoted e.g. by TAXOL; high concentrations of glycerol stabilize MTs by lowering the rates of assembly and disassembly. [Reviews: Book ref. 165; MT structure studied by cryoelectron microscopy: JM (1986) 141 361–373.] Microviridae (fX phage group; isometric ssDNA phages) A large, diverse family of icosahedral, lytic, ssDNA-containing BACTERIOPHAGES; hosts: enterobacteria. Members are abundant e.g. in sewage. One genus: Microvirus; type species: fX174. Microviruses fX174, S13, G4, G6, G13, G14, a3, fA, fB, fC, and fR infect C strains of Escherichia coli ; St-1, fK, fXtB, and U3 infect E. coli K12. Growth temperature range, antiserum cross-reactions, host receptor site, and host dna function requirements suggest close relationships between fX174, S13, G6, fA and fB, probably between St-1 and fK, and possibly between G4 and fC; G14 and U3 differ from one another and from other microviruses. The microvirus has an icosahedral capsid (28–32 nm diam.) whose vertices each bear a spike; spikes seem to function in adsorption of phage to host LIPOPOLYSACCHARIDE. The ss cccDNA genome (MWt ca. 1.7 ð 106 ) comprises 11 genes (A–H , J , K and AŁ ), four of which are OVERLAPPING GENES: gene B occurs within A, E within D, K overlaps A and C; B, E and K are translated in different reading frames from A, D and C. gpAŁ is the C-terminal portion of gpA. Gene products from different microviruses differ slightly in composition. gpF (60 molecules per virion) is the major component of the capsid; each spike comprises gpG (5 molecules) and gpH (1 molecule at the tip). Several copies of gpJ and possibly one of gpAŁ are associated with the capsid. gpA functions in genome replication; gpB, gpC, gpD, and possibly gpJ function in phage morphogenesis. gpE is necessary for host cell lysis; it may interfere with the cell membrane and/or may activate host autolytic enzymes. (For microvirus replicative cycles see SSDNA PHAGE, BACTERIOPHAGE fX174 and BACTERIOPHAGE G4.) Microvirus See MICROVIRIDAE. microwave radiation (in sterilization) Microwaves (2.45 gHz) have been used e.g. for the STERILIZATION of plastic tissue-culture vessels [AEM (1982) 44 960–964]. 482 Milton microorganisms may contaminate (and survive within) milk following collection. (See also DAIRY PRODUCTS and MILK SPOILAGE.) milk caps See LACTARIUS. milk ring test (abortus Bang reaction/ringprobe; ABR) A test for detecting BRUCELLOSIS in e.g. cattle. Milk from cows infected with Brucella abortus contains antibodies to B. abortus adsorbed to the fat globules. In the test, a small sample of milk is mixed with a suspension of killed, stained cells of B. abortus, shaken, and incubated for up to one hour. In a positive test (antibodies present) the stained cells react with the antibodies and are carried to the surface with the fat globules to form a coloured layer (‘ring’) of cream. In a negative test the cells remain dispersed in the milk. milk spoilage Souring (acidification) of milk (which often results in clotting – see CASEIN) can be caused e.g. by the growth of Lactococcus lactis or other LACTIC ACID BACTERIA. ROPINESS can be caused by slime-forming strains of Lactobacillus spp (e.g. L. brevis, L. casei ). ‘Broken cream’ (‘bitty cream’), usually due to Bacillus cereus or B. mycoides, is characterized by the development (in the cream layer) of small particles which fail to emulsify on shaking; it is thought to involve the activity of microbial LECITHINASES on the milk fat globule membrane (see MILK) with consequent coalescence of globules. Usually, the milk beneath a layer of broken cream subsequently exhibits a non-acid clot (‘sweet curdling’) due to the effects of extracellular rennin-like microbial enzymes on the milk CASEIN. Malty off-flavours can be due e.g. to certain strains of Lactococcus lactis which convert amino acids to aldehydes (particularly leucine to 3-methylbutanal). (See also LITMUS MILK and FOOD SPOILAGE.) milk-vetch dwarf virus See LUTEOVIRUSES. milkers’ nodule (milkers’ node) In man: non-ulcerating, red, nodular (papular) lesions on the hands of milkers caused by localized infection (via abrasions, cuts etc) with the PSEUDOCOWPOX virus. milky disease An INSECT DISEASE affecting larvae of beetles of the family Scarabaeidae. Originally, two forms of the disease were recognized: type A, caused by Bacillus popilliae, and type B, caused by B. lentimorbus (D B. popilliae var. lentimorbus); other species associated with milky disease – e.g. B. fribourgensis and B. euloomarahae – may be varieties of B. popilliae. Beetle larvae become infected when they ingest spores of the pathogen; the spores germinate in the gut, and the vegetative cells proliferate, subsequently invading the haemolymph. Sporulation then occurs, resulting in ca. 5 ð 109 spores/ml and giving the haemolymph the characteristic milky appearance. B. popilliae, but not B. lentimorbus, forms a parasporal crystal similar to that of B. thuringiensis (see DELTAENDOTOXIN), but this apparently plays no role in milky disease. The milky disease pathogens have been used successfully in the USA for the BIOLOGICAL CONTROL of the soil-dwelling larvae of the Japanese beetle (Popillia japonica); the spores are produced commercially by the artificial inoculation of living insect larvae. The pathogen may persist in suitable environments, precluding the need for repeated application. B. popilliae cannot grow at 37° C and is therefore unlikely to infect mammals. millet red leaf virus See LUTEOVIRUSES. millimicron (mµ) 10
3 µ (D NANOMETRE). Millipore filter See FILTRATION. Milstem See ETHIRIMOL. Milton See HYPOCHLORITES. Electrolytes include Ca2C , Mg2C , NaC and KC together with phosphate, chloride and citrate ions. Milk lipids are mainly triacylglycerides (‘triglycerides’) containing the residues of various saturated and unsaturated, branched and unbranched fatty acids; there are also some free fatty acids and phospholipids. Lipids occur as globules (up to ca. 6 µm diam. in freshly secreted milk), each globule being surrounded by a phospholipid-rich ‘milk fat globule membrane’ derived from the cytoplasmic membrane of the secretory cell. (Homogenization, which involves pumping milk under high pressure, reduces the size of the globules (which then do not rise to the surface as ‘cream’); the resulting partial loss of phospholipid membrane, and the increased total surface area of the globules, renders the lipid more susceptible to enzymic lipolysis.) The microflora of raw (untreated ) milk. Milk drawn aseptically from a healthy animal usually contains few, if any, microorganisms. However, milk may contain large numbers of bacteria if e.g. it is drawn from an animal suffering from asymptomatic or clinical MASTITIS. Freshly drawn milk may be subsequently contaminated e.g. by microorganisms on the exterior of the udder, on milking equipment, or in manure or SILAGE; the temporary storage of raw milk in refrigerated vessels provides an opportunity for the growth of PSYCHROTROPHIC contaminants (e.g. some species of Pseudomonas). Organisms frequently found in raw milk include certain species of e.g. ESCHERICHIA (and other enterobacteria), LACTOBACILLUS, LEUCONOSTOC, MICROBACTERIUM, MICROCOCCUS, PSEUDOMONAS, STAPHYLOCOCCUS and Streptococcus (see also LACTOCOCCUS), as well as certain yeasts and moulds; various actinomycetes, other organisms, and endospores of e.g. Bacillus spp may also be present. The nature and extent of the microbial contamination of raw milk can affect the keeping qualities of the milk following processing; thus, e.g., certain contaminating organisms form heat-stable extracellular enzymes (including lipases) that remain active after the cells that produced them have been killed during processing. Microbiological examination of milk. An assessment of microbial contamination may be made by a total bacterial count (D standard plate count); this can be carried out e.g. by plating an aliquot of milk on yeast extract–milk agar (or similar medium) and incubating aerobically at ca. 30–32° C for several days; lowtemperature incubation (for a longer period) may be used to detect psychrotrophs. Colony counts may range from <1000 to >106 colonies ml
1 milk according to the sample’s history. (See also DEFT, METHYLENE BLUE TEST and SPIRAL PLATE COUNT METHOD.) [Assessing the bacterial content of milk (review): JAB (1983) 55 187–201.] Milk treatment processes. See e.g. PASTEURIZATION and APPERTIZATION (for UHT treatment). (Certain processes involving heat treatment are carefully designed to avoid or minimize the occurrence of the MAILLARD REACTION.) When milk is derived from animals that have been treated with penicillin, the milk may contain sufficient penicillin to inhibit STARTER strains; in e.g. some CHEESE-MAKING plants such milk is routinely treated with PENICILLINASE. Pathogenic microorganisms in milk can be derived from a site of infection within the milk-producing animal itself; the consumption of such contaminated milk can give rise to disease such as e.g. BRUCELLOSIS, Q FEVER (caused by the ENDOSPOREforming bacterium Coxiella burnetii ) and TUBERCULOSIS. (See also LISTERIOSIS and ZOONOSIS.) Any of a variety of pathogenic 483 min genes A mixture of mini-Mu and helper Mu phages can function more effectively in generalized TRANSDUCTION to rec C recipients than can Mu alone (which transduces only at very low frequencies); this is probably due to the higher proportion of phage particles containing significant amounts of bacterial DNA which can undergo rec-dependent recombination with homologous regions of the recipient’s chromosome. However, if the mini-Mu DNA has an intact A gene, recA
recipients can be used; in this type of transduction (termed mini-muduction) the transduced DNA is integrated at random locations in the recipient’s chromosome, the inserted DNA being flanked by two mini-Mu genomes in the same orientation. (See also MUDLAC SYSTEM.) mini-muduction See MINI-MU. minimum haemagglutinating dose See MHD. minimum haemolytic dose See MHD. minimum inhibitory concentration (of an antibiotic) See MIC. minimum lethal dose (MLD) (1) The minimum dose of a given lethal agent sufficient to cause 100% mortality in a population of test animals under given conditions. (2) The minimum dose of a given lethal agent sufficient to kill an individual of a given species of specified body weight under given conditions. Minisporida See MICROSPOREA. Minitek system See MICROMETHODS. mink Aleutian disease See ALEUTIAN DISEASE OF MINK. mink cell focus-forming viruses See MCF VIRUSES. mink enteritis virus See PARVOVIRUS. mink leukaemia virus (MiLV) An endogenous type C retrovirus (subfamily ONCOVIRINAE) isolated from the Mv-1-Lu mink lung cell line. It is highly infectious for dog and donkey cells, weakly infectious for mink cells, and non-infectious for murine, goat, human and monkey cells. [Book ref. 114, pp. 118–120.] minocycline See TETRACYCLINES. minor groove (in DNA) See DNA. minus-progamone See PHEROMONE. minus strand (of a gene) (mol. biol.) See CODING STRAND. minus strand (virol.) See VIRUS. minus 10 sequence See PROMOTER. minus 35 sequence See PROMOTER. minute virus of canines See CANINE PARVOVIRUS. minute virus of mice See PARVOVIRUS. minutes (in relation to genetic loci) See INTERRUPTED MATING and MAP UNIT. MIP channel A molecular pathway, consisting of a single protein in the cytoplasmic membrane (CM), which permits transmembrane movement of water and/or certain other uncharged molecules. Such a channel (designated Aqp1) was first described in the erythrocyte (red blood cell) membrane; it was later shown to be analogous to the major intrinsic protein (MIP) in mammalian lens fibre, to the tonoplast intrinsic proteins in plants, and to the GlpF protein in Escherichia coli. MIP proteins (found in all classes of living organisms) share highly conserved regions. A MIP channel consists of a single polypeptide which apparently passes through the CM a number of times in a series of loops; this structure is believed to form a pathway for either water (channel D aquaporin) or glycerol (and/or other uncharged molecules) (channel D glycerol facilitator ). (In Lactococcus lactis the MIP channel transports both water and glycerol.) The aquaporin in E. coli (AqpZ) is encoded by gene aqpZ, and the glycerol facilitator (GlpF) is encoded by gene glpF. Mutants in aqpZ appear to grow poorly under low osmotic min genes See CELL CYCLE (b). MinCDE See CELL CYCLE (b). Minchinia A genus of protozoa (class STELLATOSPOREA) which are parasitic in aquatic invertebrates, including oysters; M. costalis and M. nelsoni are economically important pathogens of the American oyster Crassostrea virginica, sometimes causing massive mortalities in oyster-beds. Minchinia spp have a plasmodial vegetative phase. mineralization In nature: the process in which organic materials are broken down into inorganic materials; mineralization is effected mainly by saprotrophic bacteria and fungi. (cf. BIOMINERALIZATION.) Mineralization in organically polluted rivers and lakes may lead to SELF PURIFICATION (cf. EUTROPHICATION). Mineralization plays an essential part in the cycles of matter: see e.g. CARBON CYCLE, NITROGEN CYCLE, SULPHUR CYCLE. miniature anion exchange centrifugation technique See MAECT. miniaturized methods See MICROMETHODS. minicell In bacteria: an abnormal, small product of cell division which lacks a chromosome and which is formed from a mutant parent cell by septation at a polar (rather than mid-cell) location; although lacking genomic DNA, a minicell may contain one or more plasmids derived from the parent cell. Minicells do not grow, but they contain the components necessary for transcription and translation; they can be useful in genetic studies in that, following infection or transfection with e.g. phage DNA, they will synthesize only phage-encoded proteins. Minicells can be formed in e.g. Escherichia coli and Bacillus subtilis by mutation in the minCD genes (see prokaryotic CELL CYCLE); MinCD are septation-inhibitory proteins, and their absence permits septation at polar sites as well as at the midcell site. Minicells can also be formed from cells in which the FtsZ protein (see CELL CYCLE) is overproduced, i.e. raised levels of FtsZ can override the septation-inhibitory activity of the min proteins at polar sites. (See also MAXICELL.) minichromosome (1) A form in which the genome of certain DNA viruses occurs in the host (animal or plant) cell; the circular viral DNA complexes with histones to form a structure similar or identical to that of the host’s CHROMATIN. Minichromosomes are formed e.g. by CAULIFLOWER MOSAIC VIRUS, EPSTEIN–BARR VIRUS, and viruses of the PAPOVAVIRIDAE. Those of SV40 are widely used in studies on chromatin structure; a mature SV40 minichromosome contains ca. 27 nucleosomes, or ca. 25 nucleosomes with a ‘nucleosome-free gap’. [Structure of replicating SV40 minichromosomes: JMB (1986) 189 189–204.] (2) A plasmid which contains a chromosomal replication origin (usually the oriC of Escherichia coli ) as sole origin of replication. minicircle DNA See KINETOPLAST. mini-F plasmid Any small, self-replicating plasmid constructed from a fragment of the F PLASMID – see e.g. PML31. minimal medium (MM) A type of culture medium which lacks certain growth factors – i.e., it does not support the growth of some or all auxotrophic strains of a given organism, but permits the growth of prototrophic strains. (cf. COMPLETE MEDIUM.) mini-Mu A defective form of BACTERIOPHAGE MU in which the genome lacks certain sequences but has intact ends; owing to the ‘headful’ mechanism by which Mu DNA is packaged into virions, the shortened genome of a mini-Mu phage is associated with a long sequence of bacterial DNA at the S end. Although mini-Mu phages are defective, they can be replicated and matured in the presence of helper Mu. 484 mitochondrion humans, some types of cancer have been linked to mutations in genes homologous to mutS and mutL.) Recent work suggests that asymmetry of ATPases on the MutS dimer is crucial for mismatch repair and controls timing of the repair cascade [EMBO (2003) 22 746–756]. miso (soy paste; bean paste) A condiment made by fermenting soybeans, usually mixed with cereal. A KOJI is prepared by inoculating steamed rice with Aspergillus oryzae. Salt and steamed soybeans are added, and the mixture is inoculated with a starter culture of yeasts and bacteria (e.g. Zygosaccharomyces rouxii, Pediococcus halophilus, Enterococcus faecalis), blended, packed tightly into vats, and fermented at 25–30° C for ca. 2–3 months. After ageing for ca. 2 weeks, the product is blended, pasteurized and packaged, or freeze-dried and sold as a powder. [Book ref. 5, pp. 39–86.] mis-sense mutation A MUTATION in which a codon specifying one amino acid is altered so as to specify a different amino acid. The effects of such a mutation range from none (a SILENT MUTATION) to total inactivation of the gene product, depending e.g. on the location of the amino acid in the product and on the nature of the substituted amino acid relative to that of the original. (cf. NONSENSE MUTATION; see also POINT MUTATION.) MISTRESS Milk into steam treatment research equipment small scale: equipment used for research into high-temperature (e.g. UHT) methods of preservation in the food industry. Essentially, milk is sprayed into a steam-containing vessel and, at the end of the holding time, it is drawn through a hole into a vacuum chamber where it cools by expansion. mithramycin See CHROMOMYCIN. mitochondrion A semi-autonomous intracellular organelle, one or more (usually many) of which occur in most types of eukaryotic cell (being absent e.g. in protozoa of the orders Diplomonadida and Pelobiontida); RESPIRATION and the reactions of the TCA CYCLE occur within the mitochondrion. Mitochondria differ in size (<1 µm to >10 µm), shape, and number, according to the type of cell in which they occur and to the physiological state of that cell; a mitochondrion may be spherical, ovoid, elongated, calyciform or irregularly shaped, and some are branched. Structure and composition. A mitochondrion consists of two closed membranous sacs, one fitting closely within the other, and an amorphous matrix (enclosed by the inner membrane) which contains the mitochondrial DNA and enzymes of the TCA cycle. The mitochondrial inner membrane is extensively corrugated and forms plate-like, tubular, or finger-like structures (cristae) which project into the matrix – often more or less perpendicular to the longitudinal axis of the mitochondrion; the inner membrane contains components of the ELECTRON TRANSPORT CHAIN, PROTON ATPASES and TRANSPORT SYSTEMS for e.g. nucleotides, inorganic phosphate, and Ca2C . (See also ION TRANSPORT.) The mitochondrial outer membrane contains PORINS (see also VDAC). Mitochondrial DNA (mtDNA) is typically in the form of covalently closed circular, double-stranded molecules (differing from the nuclear DNA e.g. in base composition and buoyant density); mtDNA is linear in e.g. certain ciliates (including Paramecium), Physarum polycephalum, and Hansenula mrakii. Mitochondrial genes employ a GENETIC CODE (q.v.) which differs in some respects from the ‘universal’ code, and some fungal mitochondrial genes contain introns (see SPLIT GENE). Genetic recombination can occur between mtDNAs. (See also PETITE MUTANT.) pressure – conditions which, in wild-type strains, appear to stimulate expression of aqpZ ; this suggests that AqpZ has an important physiological role. MIP channels occur in some members of the Archaea (e.g. Archaeoglobus fulgidus, Methanobacterium thermoautotrophicum) but apparently not in others (e.g. Methanococcus jannaschii ). They also seem to be absent in e.g. Chlamydia trachomatis, Helicobacter pylori, Mycobacterium tuberculosis and Treponema pallidum. [The importance of aquaporin water channel protein structures: EMBO (2000) 19 800–806. Microbial MIP channels: TIM (2000) 8 33–38.] Mirabilis mosaic virus See CAULIMOVIRUSES. mirror yeasts (shadow yeasts) BALLISTOSPORE-forming yeasts (see SPOROBOLOMYCETACEAE); when these yeasts are grown in a Petri dish, the pattern of discharged ballistospores adhering to the lid is a mirror image (or shadow) of the pattern of growth beneath. Mischococcus A genus of freshwater epiphytic algae (class XANTHOPHYCEAE) in which the coccoid, non-motile vegetative cells occur in colonies composed of dichotomously branching gelatinous tubes. mismatch repair A DNA REPAIR system which can correct mismatched (i.e. non-Watson–Crick) base pairs – e.g. those formed during DNA replication when misincorporated nucleotides have escaped proof-reading. Mismatch repair effectively enhances the accuracy of replication. (cf. MUTATOR GENE.) In Escherichia coli, mismatch repair (Dam-directed mismatch repair; DDMR) in the newly replicated daughter strand occurs soon after the failure of proof-reading and before Dam methylation (see DAM GENE) has occurred, i.e. while the new daughter strand is still (transiently) undermethylated. The mismatched base-pair is detected by MutS (mutS gene product), and this activates an endonuclease, MutH. [Mutational analysis of the MutH protein from Escherichia coli : JBC (2001) 276 12113–12119.] (MutH exhibits sequence homology with a restriction endonuclease, Sau3AI [EMBO (1998) 17 1526–1534].) MutH cleaves the new strand at a site 50 to an unmethylated GATC (Dam) sequence – which may be up to ¾1000 nucleotides from the actual site of mismatch; co-ordination between the mismatch location and the GATC cleavage site may be mediated by the MutL protein. When nicked by MutH, the sequence of daughter strand between the nick site and a site on the other side of the mismatch is removed. It has been assumed that removal of this sequence depends on helicase II (D MutU; D UvrD) and a DNA exonuclease; however, mutants lacking the 30 -to-50 exonuclease ExoI and the 50 -to-30 exonucleases ExoVII and RecJ appear to be capable of normal mismatch repair [JB (1998) 180 989–993]. On removal of the sequence, the single-stranded gap is filled by a DNA polymerase and sealed by a ligase. The mismatch repair system in E. coli is apparently not equally effective against all possible mismatched base pairs [PNAS (1985) 82 503–505]. Mismatched nucleotides can arise not only by misincorporation of bases during replication but also by the formation of heteroduplex DNA during RECOMBINATION. In E. coli, if one strand of a heteroduplex is unmethylated, DDMR can apparently correct that strand; if both strands are fully methylated, either strand can apparently be corrected by a mismatch repair system involving MutS and UvrD [JMB (1986) 188 147–157]. (See also TRANSFORMATION.) Strains of E. coli lacking mut products have defective DNA repair systems, and they exhibit a mutator phenotype. (In 485 mitogen an intermediate and may bind DNA non-covalently and intercalatively (see INTERCALATING AGENT). Preferred sites for alkylation are the O-6 groups of guanine residues. (Reduced mitomycin will also react with RNA and, to a lesser extent, protein.) Alkylation of B-DNA by mitomycin can cause conformational changes that may involve the transition to Z-DNA. Mitomycin is a potent inducer of lysogenic bacteriophages; viral DNA synthesis is relatively resistant to mitomycin. mitoplast The inner membrane of a MITOCHONDRION together with the mitochondrial matrix. mitosis (karyokinesis) A sequence of events which culminates in the division of a eukaryotic nucleus into two genetically similar or identical nuclei whose ploidy is the same as that of the parent nucleus; mitosis occurs during asexual (vegetative) cell division. (cf. MEIOSIS.) Different types of mitosis occur among the various types of eukaryotic cell. The best-known (‘typical’) form of mitosis is that which occurs in the cells of higher animals and e.g. in some protozoa; it involves the following stages (in chronological order). Prophase. Individual CHROMOSOMES, each having previously divided into two chromatids, condense and become visible by light microscopy. The NUCLEOLUS becomes indistinct and then invisible. MICROTUBULES of the CYTOSKELETON disassemble, forming a pool of tubulin subunits. From each of two pairs of CENTRIOLES (lying close to each other in the cytoplasm) arises a radial array of microtubules (referred to as an aster ). Those microtubules which extend from each centriole pair towards the other elongate – the centriole pairs concomitantly moving apart until each eventually forms one pole of a bipolar microtubular mitotic spindle; microtubules from each of the centriole pairs (D polar microtubules or polar fibres) overlap in the mid-region of the spindle. Prometaphase. The nuclear membrane breaks into pieces. KINETOCHORES develop on the chromosome centromeres, and from each kinetochore kinetochore microtubules (D k-MTs; kinetochore fibres) grow out more or less perpendicularly to the long axis of the chromosome. The k-MTs apparently begin to interact with the spindle (in some as yet unknown way) causing the chromosomes to execute rapid movements. Metaphase. The chromosomes become arranged in a layer (the metaphase plate) midway between the poles of the spindle and perpendicular to its axis. The metaphase plate may be maintained for some time, each chromosome possibly being held in dynamic equilibrium by equal and opposite forces generated within the spindle. Anaphase. The two chromatids of each chromosome appear to move along stationary k-MTs to opposite poles of the spindle; de-polymerization of the k-MTs apparently occurs at the kinetochores – the kinetochores possibly being involved in generating the force needed to propel the chromatids towards their respective poles [JCB (1987) 104 9–18]. As the k-MTs shorten, the polar microtubules elongate and the spindle poles move further apart. Telophase. The chromatids become less clearly visible, and a nuclear membrane assembles around each group of chromatids to form two new nuclei – in each of which a new nucleolus appears. [Cell-free system for studying reassembly of the nuclear envelope: Cell (1986) 44 639–652.] Telophase is followed by CYTOKINESIS. The non-dividing nucleus is called the interphase nucleus; DNA replication occurs during interphase. In many microorganisms mitosis differs significantly from that described above; thus, e.g. there are at least four distinct types of Origin and semi-autonomy of mitochondria. It is generally believed that mitochondria are formed by the division or fragmentation of pre-existing mitochondria – or (see later) by the development of promitochondria – and that these organelles incorporate new material (i.e., grow) during interdivision periods. The components of mitochondria are encoded partly by the cell’s nuclear DNA and partly by the mtDNA. [Nuclear genes encoding mitochondrial proteins in yeast: TIBS (1985) 10 192–194.] Thus, in Saccharomyces cerevisiae, mtDNA encodes e.g. tRNAs, two types of rRNA, one ribosomal protein, the apoprotein of cytochrome b in Complex III, and a protein designated ‘var1’. Certain mitochondrial components are synthesized in the mitochondrion itself. However, while a mitochondrion can synthesize DNA and RNA, and can carry out protein synthesis, many of the proteins needed for these processes are encoded by nuclear genes, synthesized on cytoplasmic ribosomes, and incorporated into the mitochondrion; control of the synthesis of these proteins may be largely at the transcriptional level. Mitochondrial protein synthesis differs from cytoplasmic protein synthesis e.g. in that it is sensitive to those agents (e.g. CHLORAMPHENICOL, ERYTHROMYCIN) which inhibit bacterial PROTEIN SYNTHESIS; it is not sensitive to CYCLOHEXIMIDE; and it is characterized by the incorporation of N-formylmethionine as the first amino acid in a polypeptide chain. These features have lent support to a popular hypothesis which supposes that mitochondria have their evolutionary origins in endosymbiotic prokaryotes. [Mitochondrial origins: PNAS (1985) 82 4443–4447.] (An alternative hypothesis supposes that mitochondria evolved from plasmids.) In at least some facultatively fermentative organisms (including e.g. Saccharomyces cerevisiae) cells growing under anaerobic conditions do not contain functional mitochondria; such cells contain promitochondria: organelles which resemble mitochondria but which lack components of the electron transport chain. On exposure to aerobic conditions promitochondria apparently develop into functional mitochondria; reversion to promitochondria occurs if anaerobic growth is resumed. In e.g. certain fungi mitochondrial defects can alter the nature of the cell’s metabolism (see e.g. PETITE MUTANT and POKY MUTANT), and can affect the expression of nuclear genes which are involved e.g. in determining the nature of cell-surface antigens [TIBS (1982) 7 147–151]; inhibition of mitochondrial protein synthesis can affect both meiotic and apomictic sporulation in Saccharomyces cerevisiae [Yeast (1985) 1 39–47]. mitogen Any agent which, under appropriate conditions, can promote MITOSIS, non-specifically, and can bring about BLAST TRANSFORMATION. (cf. POLYCLONAL ACTIVATORS.) Mitogens include e.g. certain LECTINS (such as PHYTOHAEMAGGLUTININ). (Substances which can promote early events in the activation process – including DNA synthesis – but which cannot promote mitosis are not, strictly speaking, mitogens.) mitogen-activated protein kinase See ANTHRAX TOXIN. mitomycin C (‘mitomycin’) An ANTIBIOTIC (from Streptomyces spp) with general cytotoxic activity; it contains a quinone group and an aziridine (ethyleneimine) ring. It binds to DNA and causes inhibition of DNA replication followed by degradation of DNA; synthesis of proteins and RNA may continue for a time. Mitomycin activity depends on its reduction in vivo (by NADPH-dependent enzymes) to the highly reactive hydroquinone derivative which behaves as a bifunctional ALKYLATING AGENT and forms e.g. covalent cross-links between complementary strands of dsDNA; the semiquinone seems to be formed as 486 MM virus been reported e.g. for an iron-oxidizing bacterium [JGM (1984) 130 1337–1349] and for ‘Alcaligenes eutrophus’ [JGM (1984) 130 1987–1994]. (See also CHEMOLITHOHETEROTROPH.) Miyagawanella See CHLAMYDIA. MK Menaquinone: see QUINONES. MK broth MÜLLER–KAUFFMANN BROTH. MK-0787 See THIENAMYCIN. MLD MINIMUM LETHAL DOSE. MLEE MULTILOCUS ENZYME ELECTROPHORESIS. MLOs Mycoplasma-like organisms: wall-less, non-helical, prokaryotic organisms which are associated with invertebrates and plants and which are the causal agents of a number of plant YELLOWS diseases. MLOs have not been cultured in vitro and have not been formally classified, although they are usually associated, taxonomically, with the MOLLICUTES. Various MLOs have been shown to lack a Spiroplasma-specific antigen [JGM (1983) 129 1959–1964]. [Book ref. 22, pp. 792–793; wall-less prokaryotes and plants: ARPpath. (1984) 22 361–396.] From the mid-1990s, MLOs were referred to by the trivial name phytoplasma. Individual MLOs are now being called by names such as Candidatus Phytoplasma australiense. [Phytoplasma: ARM (2000) 54 221–255.] MLR (immunol.) Mixed lymphocyte reaction (D mixed leucocyte reaction or mixed leukocyte reaction): mutual interaction, with blast transformation and proliferation of T cells, which occurs when leukocytes from two allogeneic individuals are mixed and cultured; the greater the disparity between antigens (e.g. MHC class II antigens) on donor and recipient cells the greater the degree of proliferation. (The degree of proliferation may be assessed e.g. by measuring the amount of radioactive thymidine (a DNA precursor) taken up by the cells.) MLR (which takes several days to perform) is used e.g. to assess the compatibility of transplant donors and recipients. A negative reaction (no proliferation of T cells) is given when cells are derived from identical twins. MLS antibiotics Macrolide, lincosamide and streptogramin Btype antibiotics: a group of structurally distinct antibiotics all of which act on the 50S subunit of 70S ribosomes (see MACROLIDE ANTIBIOTICS, LINCOSAMIDES and STREPTOGRAMINS). [Symposium on MLS antibiotics: JAC (1985) 16 (supplement A).] Co-resistance to MLS antibiotics in staphylococci, streptococci and streptomycetes can be due to N 6 -methylation of adenine residues in 23S rRNA (see STREPTOGRAMINS). Strains of Streptococcus pyogenes and S. pneumoniae that express the ‘M phenotype’ contain a macrolide-efflux mechanism which confers resistance to macrolides concomitant with sensitivity to lincosamides and B streptogramins (see MACROLIDE ANTIBIOTICS). MLSB=c strains See STREPTOGRAMINS. MLST (multilocus sequence typing) A method for TYPING pathogenic bacteria which permits long-term, global-scale tracking of virulent and antibiotic-resistant strains, with information made available on the Internet. In MLST, strains are characterized and classified on the basis of nucleotide sequences in specific alleles, this approach giving rise to categorization which is apparently stable over long periods of time. [MLST (review): TIM (1999) 12 482–487. MLST of methicillin-resistant and methicillin-sensitive clones of Staphylococcus aureus: JCM (2000) 38 1008–1015.] MluI See RESTRICTION ENDONUCLEASE (table). MLV MURINE LEUKAEMIA VIRUS. MM MINIMAL MEDIUM. MM virus See CARDIOVIRUS. mitosis–cytokinesis even within the green algae [Book ref. 130, pp. 95–99]. In many algae (e.g. Dunaliella, DINOFLAGELLATES) and fungi (e.g. Albugo spp, Mucor hiemalis, Saprolegnia ferax, many yeasts) the nuclear envelope persists throughout mitosis (‘closed mitosis’) – although in some of these organisms the nuclear envelope develops a number of holes (fenestrae). In some organisms the spindle may be wholly intranuclear; this occurs e.g. in many yeasts, including Saccharomyces, in which the spindle microtubules arise, at each pole, from a SPINDLE POLE BODY. In EUGLENOID FLAGELLATES the nuclear envelope remains intact, the nucleolus is persistent, and microtubules develop within the nucleus; however, the microtubules apparently do not function as a conventional spindle. In some algae part of the spindle is used as a section of the newly forming cell wall (see PHRAGMOPLAST). (cf. AMITOSIS; see also CELL CYCLE and PARASEXUAL PROCESSES.) [Mitosis in fungi: Book ref. 168, pp. 12–26, 85–112; sites of microtubule assembly and disassembly in the spindle: Cell (1986) 45 515–527.] mitosporangium A SPORANGIUM within which MITOSIS occurs (see e.g. ALLOMYCES). mitotic crossing over See PARASEXUAL PROCESSES. mitotic spindle See MITOSIS. Mitrula See HELOTIALES. Mitsuda reaction See LEPROMIN TEST. mixed acid fermentation A FERMENTATION (sense 1) carried out e.g. by certain enterobacteria, including Escherichia coli and species of Proteus, Salmonella and Shigella. In E. coli the products of glucose fermentation may include e.g. acetic and lactic acids, smaller amounts of formic acid (or CO2 C H2 ) and succinic acid, and ethanol [see Appendix III(e)]; products are formed in relative proportions which depend on organism and growth conditions. Those organisms (e.g. E. coli ) which have a FORMATE HYDROGEN LYASE system split formic acid (under acid conditions) to CO2 and H2 ; some of the CO2 may be assimilated during the formation of succinate. In E. coli formate accumulates under alkaline conditions. Species of e.g. Shigella, which lack the formate hydrogen lyase system, carry out the fermentation anaerogenically. Enterobacteria which carry out the mixed acid fermentation give a positive METHYL RED TEST. (See also PHOSPHOROCLASTIC SPLIT.) mixed bed exchangers Ion-exchange material containing both cation and anion exchangers. mixed culture See CULTURE. mixed-function oxidase Syn. MONOOXYGENASE. mixed infection The concurrent infection of a given individual with more than one pathogen. mixed leucocyte reaction See MLR. mixed lymphocyte reaction See MLR. mixed vaccine A single VACCINE designed to give protection against more than one pathogen; such a vaccine contains PROTECTIVE ANTIGENS and/or toxoids from each of the pathogens. (See e.g. TRIPLE VACCINE; cf. POLYVALENT VACCINE.) mixis The fusion of gametes (fertilization). mixoploid Refers to a population of cells in which the chromosome complement per cell varies among the cells of the population. mixotrophy A mode of metabolism in which energy is obtained by the oxidation of an inorganic substrate, and carbon is obtained from an organic substrate (and sometimes also from the fixation of CO2 via an autotrophic pathway). In some cases of mixotrophy, energy appears to be derived from the metabolism of an organic substrate as well as an inorganic substrate; this has 487 MMLV reverse transcriptase widely used as a relatively cheap substrate for the commercial production of various microorganisms and microbial products (see e.g. BAKERS’ YEAST; CITRIC ACID; INDUSTRIAL ALCOHOL; SINGLE-CELL PROTEIN). mold See MOULD. molecular beacon probes Oligonucleotide probes used for (i) monitoring progress in a nucleic-acid-amplification procedure, and (ii) (in PCR), estimating the initial (pre-amplification) number of target sequences in the sample. The general structure and mode of action of a molecular beacon probe are described in the figure. Like TAQMAN PROBES (q.v.), molecular beacon probes are added (in large numbers) to the reaction mixture prior to amplification. In NASBA, molecular beacon probes can be used for real-time monitoring of amplification [NAR (1998) 26 2150–2155]. In PCR, the probes are useful for estimating the initial number of target sequences in a sample (see TAQMAN PROBES for rationale). Because these probes can be labelled with different kinds of fluorophore (having different emission characteristics), it is possible to carry out a multiplex PCR in which the amplification of each type of target can be followed separately by monitoring the distinct emission from target-specific probes. [Multiplex detection of four pathogenic retroviruses using molecular beacons: PNAS (1999) 96 6394–6399.] Compared with linear probes (such as TaqMan), molecular beacon probes exhibit greater specificity in binding; for example, a beacon probe of equivalent length can more readily distinguish a one-nucleotide difference between two sequences. [Thermodynamic basis of the enhanced specificity of structured DNA probes: PNAS (1999) 96 6171–6176.] molecular chaperone See FIMBRIAE. molecular cloning See CLONING. molecular sieving Syn. GEL FILTRATION. Møller’s cyanide medium See KCN BROTH. Møller’s decarboxylase broth A medium used for DECARBOXYLASE TESTS. It contains (g/l): peptic digest of animal tissue (5.0), beef extract (5.0), glucose (0.5), BROMCRESOL PURPLE (0.1), cresol red (0.005), pyridoxal (0.005) and L-arginine or L-lysine or L-ornithine (1% final concentration). The pH of the broth is adjusted to 6.0. mollicute A proposed trivial name for any member of the class MOLLICUTES; use of this name could permit ‘mycoplasmas’ to be used specifically for members of the genus Mycoplasma. Mollicutes A class of small, cell-wall-less bacteria which are typically parasitic (sometimes pathogenic) in animals or plants. (cf. TENERICUTES; see also MOLLICUTE.) The class comprises a single order, MYCOPLASMATALES, and two genera incertae sedis (ANAEROPLASMA and THERMOPLASMA) [Book ref. 22, pp. 740–793]. (See also MLO.) molluscum contagiosum A human skin disease caused by a virus of the POXVIRIDAE; the virus is apparently unrelated to other poxviruses, and has not been grown in cell cultures or in the CAM. Molluscum contagiosum is characterized by the formation of multiple firm, rounded, whitish, translucent nodules which become crater-like, revealing a white core; the lesions may persist for months or even years. The virus is spread by body contact (e.g. sexual contact) or via fomites. Moloney murine leukaemia virus (Mo-MuLV) A replicationcompetent v-onc
MURINE LEUKAEMIA VIRUS originally isolated following passage of a cell-free extract of a transplantable tumour (‘Sarcoma 37’) in neonatal mice. Mo-MuLV can induce MMLV reverse transcriptase See REVERSE TRANSCRIPTASE. MMO METHANE MONOOXYGENASE. MMR Measles–mumps–rubella vaccine. MMS Methylmethane sulphonate: see ALKYLATING AGENTS. MMTV MOUSE MAMMARY TUMOUR VIRUS. MNNG (N-methyl-N 0 -nitro-N-nitrosoguanidine; also: ‘nitrosoguanidine’, NTG) A mutagenic N-NITROSO COMPOUND, the mutagenic activity of which appears to depend primarily on methylation of the O-6 position of guanine (see ALKYLATING AGENTS). MNNG acts preferentially at the replication fork in replicating DNA, producing clusters of closely linked mutations (co-mutagenesis); it also methylates proteins. MNPV See NUCLEAR POLYHEDROSIS VIRUSES. MNU N-Methyl-N-nitrosourea: see ALKYLATING AGENTS. mobile genetic element See TRANSPOSABLE ELEMENT. Mobilina See PERITRICHIA. mobilization (mol. biol.) See CONJUGATION (1b) (i). Mobiluncus A genus of anaerobic, asporogenous bacteria which occur in the human vagina and which are associated with BACTERIAL VAGINOSIS. Cells: Gram-negative and Gram-variable curved rods (ca. 1.5–3.0 µm in length) which are motile by multiple subpolar flagella; the CELL WALL is of the Gram-positive type [IJSB (1986) 36 288–296]. Metabolic products include acetate and succinate. GC%: 49–52 (Tm ). Type species: M. curtisii ; other species: M. mulieris. [IJSB (1984) 34 177–184.] mocimycin See POLYENE ANTIBIOTICS (b). modification (of DNA) See DNA MODIFICATION. modulator protein See Tn21. modulin Any virulence factor which causes damage by inducing inappropriate activity of CYTOKINES. The modulins include e.g. endotoxins, exotoxins and SUPERANTIGENS. [MR (1996) 60 316–341.] Moellerella A genus of bacteria of the ENTEROBACTERIACEAE. M. wisconensis (formerly ‘enteric group 46’) has been isolated from human faeces and is apparently associated with diarrhoea in humans. It is non-motile; MR Cve; citrate Cve (Simmons); VP
ve; indole
ve; LDC, ODC, ADH and phenylalanine deaminase
ve; anaerogenic. On MacConkey’s agar colonies are bright red and surrounded by precipitated bile. [JCM (1984) 19 460–463.] Moeller’s cyanide medium See KCN BROTH. Moeller’s decarboxylase broth See MØLLER’S DECARBOXYLASE BROTH. moenomycin (Flavomycin) A phosphorus-containing glycolipid ANTIBIOTIC in which the lipid moiety (moenocinol) is a C-25 compound structurally analogous to undecaprenylphosphate; it inhibits transglycosylation in PEPTIDOGLYCAN synthesis in Grampositive bacteria and, at higher concentrations, in Gram-negative bacteria. MoFe protein See NITROGENASE. moi MULTIPLICITY OF INFECTION. moiré image In microscopy: a misleading image due to overlap among multiple structures. moist heat Refers to the use of steam for STERILIZATION. (See also AUTOCLAVE.) Mokola virus See LYSSAVIRUS. mol% G + C See GC%. molar growth yield See YIELD COEFFICIENT. molasses A by-product of the sugar industry: the syrupy liquid which remains after the crystallization of sucrose from sugarcane or sugar-beet syrup. Molasses contains ca. 50–60% sucrose (which cannot be removed by crystallization), 16–20% water, and various non-sugar and inorganic (‘ash’) components; it is 488 Monilia MOLECULAR BEACON PROBES: structure and mode of action (diagrammatic). A molecular beacon probe is an oligonucleotide probe sequence flanked by two, short mutually complementary oligonucleotides (side-arms), the molecule being capable of adopting a stem-and-loop conformation (top); the terminals of the beacon probe are labelled with a fluorophore ( ) and a quencher (
) such that fluorescence is quenched (by juxtaposition of fluorophore and quencher) when the molecule adopts the stem-and-loop conformation. When a probe (the loop region) binds to the target sequence in an amplicon, the probe–target duplex is both longer and more stable than the stem duplex; hence, on probe binding, the two side-arms of the stem dissociate – thus separating the fluorophore from the quencher and giving rise to a functionally fluorescent probe. Like TAQMAN PROBES, molecular beacon probes are added to the reaction mixture prior to amplification; they are added in large numbers so that a probe will be available for each amplicon at the end of amplification. In NASBA, an isothermal process, fluorescence from the bound probes is monitored continually. In PCR, fluorescence is monitored during the primer annealing stage (because only at this stage are fluorescent probe–amplicon duplexes formed). Reproduced from Figure 4.13, page 102, in DNA Methods in Clinical Microbiology (ISBN 07923-6307-8), Paul Singleton (2000), with kind permission from Kluwer Academic Publishers, Dordrecht, The Netherlands. ž thymic leukaemia, disseminated lymphosarcoma, or lymphatic leukaemia in mice after a latent period of ca. 2–3 months. (See also MOLONEY MURINE SARCOMA VIRUS.) Moloney murine leukaemia virus reverse transcriptase See REVERSE TRANSCRIPTASE. Moloney murine sarcoma virus (Mo-MSV) A replicationdefective v-oncC MURINE SARCOMA VIRUS isolated from a BALB/c mouse infected with MOLONEY MURINE LEUKAEMIA VIRUS (Mo-MuLV) and suffering from a rhabdosarcoma; several variants are known, each containing Mo-MuLV-derived sequences and the oncogene v-mos (see MOS). Infection of newborn mice with Mo-MSV can result in the appearance of sarcomas within a few days; sarcomas induced in older mice commonly undergo spontaneous regression and recurrence. (cf. GAZDAR MURINE SARCOMA VIRUS.) molybdoenzymes The molybdenum-containing enzymes; they include e.g. aldehyde oxidase, CO OXIDASE, dissimilatory nitrate reductase, NITROGENASE, sulphite oxidase and xanthine oxidase. molybdoferredoxin See NITROGENASE. mom gene See BACTERIOPHAGE MU. momilactone Either of two diterpene PHYTOALEXINS (momilactones A and B) formed by the rice plant (Oryza sativa). (See also WL 28325.) Monacrosporium See HYPHOMYCETES and NEMATOPHAGOUS FUNGI. monactin See MACROTETRALIDES. -monas A suffix signifying ‘a unit’. monascoflavin See ANG-KAK. monascorubrin See ANG-KAK. Monascus See PEZIZALES. monensin (monensin sodium; syn. rumensin) A MACROTETRALIDE antibiotic synthesized by Streptomyces cinnamonensis; it is used e.g. as an anti-coccidial agent in poultry, and as a FEED ADDITIVE for ruminants (e.g. cattle, sheep). (See also FOG FEVER.) As a feed additive, monensin appears to act by inhibiting those RUMEN microorganisms which produce acetic acid and hydrogen, thereby favouring those which produce propionic acid; thus, it appears that energetically wasteful methanogenesis is suppressed (by as much as 30–40%) as a result of the lack of hydrogen (rather than by direct inhibition of methanogens), and that it is the corresponding increase in propionic acid which leads to increased feed conversion efficiency. In pure cultures of methanogens both monensin and LASALOCID directly affect membrane function and proton motive force, and in fermenting organic wastes monensin inhibits methanogens and causes an accumulation of acetate. The different susceptibilities of rumen and sewage methanogens may be due to the high NaC concentration in rumen fluid; the inhibition of methanogenesis by monensin is known to be prevented by high concentrations of NaC [Book ref. 157, pp. 530–531]. Monensin can inhibit the replication of certain viruses whose maturation involves the PALADE PATHWAY. Monera The prokaryotic protists – see PROTISTA. Monilia Syn. CANDIDA. 489 Moniliales monocins See BACTERIOCIN. monocistronic mRNA See MRNA. monoclonal antibodies A population of identical antibodies (see ANTIBODY), all of which recognize the same specific DETERMINANT on a simple or complex antigen or hapten. (cf. POLYCLONAL ANTISERUM.) A given population of monoclonal antibodies may be able to react with each of two apparently unrelated antigens if, by chance, those antigens happen to share a common determinant; thus, e.g., monoclonal antibodies which combine with a particular mouse myeloma protein can combine with human C-reactive protein. (cf. CROSS-REACTING ANTIBODY.) Monoclonal antibodies are obtained from B-cell HYBRIDOMAS. Monoclonal antibodies have an enormous range of actual or potential uses. These include e.g. characterization and detection of viruses [e.g. plant viruses: MS (1984) 1 73–78] and other microorganisms; tissue typing; the study of changes in cellsurface antigens on lymphoid cells during maturation; and the preparation of IMMUNOTOXINS. monocystid gregarine Syn. acephaline gregarine. Monocystis A genus of protozoa (subclass GREGARINASINA) many species of which are parasitic in the earthworm (Lumbricus spp). Sporozoites enter the seminal vesicles. The developing parasites (trophozoites) occur intracellularly in the sperm morulae; the fully-grown, extracellular acephaline trophozoites (gametocytes) associate in pairs (syzygy) and each pair forms a gametocyst within which many gametes develop. The gametes fuse in pairs, and each zygote forms a cyst (sporocyst, pseudonavicella) within which 8 sporozoites develop. monocyte A large (diam. 10–20 µm) phagocytic LEUCOCYTE which contains a single, large, spherical or indented nucleus, and azurophilic peroxidase-containing granules; ca. 200–800 monocytes/ml occur in normal adult human blood. Monocytes differentiate to become MACROPHAGES. Suspensions of monocytes may be prepared e.g. by density flotation [JIM (1984) 69 71–77] or by the use of Sephadex [JIM (1984) 71 25–36]. monocytosis Syn. MONONUCLEOSIS. Monod equation See SPECIFIC GROWTH RATE. Monodictys See HYPHOMYCETES; see also SOFT ROT (sense 1). monoecious Refers to an organism in which the male and female reproductive structures occur in the same individual. (cf. DIOECIOUS.) monogeneric Refers to a taxon which has only one genus. monogenic mRNA See MRNA. monokaryotic Having one nucleus per cell. monokines Certain soluble, biologically active products of MACROPHAGES – e.g. INTERLEUKIN-1. monolayer culture See TISSUE CULTURE. Monomastix See MICROMONADOPHYCEAE. monomitic See HYPHA. monomorphic Refers to an organism which occurs in only one form or MORPHOTYPE. (cf. POLYMORPHISM.) Mononegavirales A taxon containing viruses which have nonsegmented, negative-sense ssRNA genomes, including members of the FILOVIRIDAE, PARAMYXOVIRIDAE and RHABDOVIRIDAE – and (probably) the BORNA DISEASE VIRUS. mononuclear leucocyte (mononuclear phagocyte) (1) Syn. MACROPHAGE. (2) Syn. MONOCYTE. (3) A macrophage or monocyte. mononuclear phagocyte See MONONUCLEAR LEUCOCYTE. mononucleosis (monocytosis) The condition in which increased numbers of monocytes are present in the blood. (cf. INFECTIOUS MONONUCLEOSIS.) monooxygenase (hydroxylase; mixed-function oxidase; mixedfunction oxygenase) An OXYGENASE (EC 1.13.12) which catalyses the incorporation of one atom of molecular oxygen into Moniliales An order of fungi; depending on taxonomic scheme, the Moniliales includes either most or all of the organisms which constitute the class HYPHOMYCETES. moniliasis Syn. CANDIDIASIS. monilicolin A See PHASEOLLIN. moniliform Resembling a string of beads. Monilinia See HELOTIALES. Moniliophthora A genus of fungi of the APHYLLOPHORALES; M. roreri (anamorph: ‘Monilia roreri ’) causes POD ROT of cacao. monkey-bite encephalomyelitis See B VIRUS. monkeypox A rare disease of man and other primates; it is caused by the monkeypox virus (see ORTHOMYXOVIRUS). In man, the disease involves generalized exanthema and closely resembles SMALLPOX; it is not readily transmissible from person to person. Monkeypox generally affects people who have not been vaccinated against smallpox and who live in villages in the tropical rain forests of West and Central Africa; the natural reservoir of the virus is unknown. Monkeypox also occurs e.g. in captive monkeys; the symptoms are similar to those in man. The monkeypox virus is highly lethal in mice and typically forms small, opaque, haemorrhagic pocks on CAM; only B-type inclusion bodies are seen in infected cells. Genome MWt: ca. 128 ð 106 . monoazide ethidium See ANISOTROPIC INHIBITOR. monobactams A class of monocyclic b-LACTAM ANTIBIOTICS (q.v. for formula). Natural monobactams include e.g. sulfazecin (D ‘monobactam I’, R0 D D-glu-D-ala, R00 D OCH3 , R000 D H, R0000 D SO3 H), produced e.g. by Gluconobacter and Acetobacter spp, and isosulfazecin, produced by Agrobacterium radiobacter and Pseudomonas sp. Synthetic derivatives include e.g. AZTREONAM and the monophosphams (2-oxoazetidine-1-phosphonates, where R0000 D PO3 X), monocarbams (R0000 D CONHSO2 X), and monosulfactams (R0000 D OSO3
). [Review: AAM (1986) 31 181–205.] Monoblepharella See MONOBLEPHARIDALES. Monoblepharidales An order of typically aquatic, saprotrophic fungi (class CHYTRIDIOMYCETES) in which sexual reproduction is oogamous, involving a small, motile male gamete and a large non-motile female gamete (a type of sexual reproduction not found in other fungi) (cf. OOMYCETES). The thallus is a welldeveloped branching mycelium in which the cytoplasm is highly vacuolated (‘foamy’); in at least some species the cell wall contains CHITIN. Asexual reproduction involves the formation of elongated, typically cylindrical sporangia containing large numbers of zoospores; each zoospore can give rise to a new thallus. In sexual reproduction, distinct male and female gametangia (antheridia and oogonia, respectively) are formed; the male gametes (‘antherozoids’) swim to the non-motile oospheres. (In some species the oogonium forms more than one oosphere, and oospheres may be fertilized within or outside the oogonium, according e.g. to species.) After fertilization, the oosphere develops into a thick-walled resting oospore which later germinates to form a new thallus. Genera include Gonapodya, Monoblepharella and Monoblepharis. Monoblepharis See MONOBLEPHARIDALES. monocarbams See MONOBACTAMS. monocentric (1) Refers to a thallus in which only one region has a reproductive function. (cf. POLYCENTRIC.) (2) Refers to a chromosome which has a single, discrete CENTROMERE. (cf. HOLOCENTRIC.) Monocercomonas See TRICHOMONADIDA. Monocercomonoides See OXYMONADIDA. monochloramine See CHLORINE. 490 Morbillivirus an appropriate liquid medium and incubated. The Moore-swab technique appears to be more effective in small sewers [AEM (1986) 51 425–426]. Moorella See CLOSTRIDIUM. Mopeia virus See ARENAVIRIDAE. Morax–Axenfeld bacillus See MORAXELLA. Moraxella A genus of oxidase Cve, typically catalase Cve Gram type-negative bacteria of the family MORAXELLACEAE which occur on mucous membranes (e.g. respiratory tract) in man and other mammals; strains of Moraxella can act as opportunist pathogens. Cells: short rods/cocci which may occur singly or in pairs; non-motile, but may exhibit TWITCHING MOTILITY. Nonpigmented. Some moraxellae are nutritionally fastidious; all strains can grow on blood agar (some better on chocolate agar); many strains are able to grow on rich, non-blood-containing media or even nutrient agar. Acid is not produced from carbohydrates. Nitrate is reduced by some strains. Optimum growth temperature: 33–36° C. Unlike species of Neisseria, no strain produces CARBONIC ANHYDRASE. GC%: ¾40–47.5. Type species: M. lacunata. Species include: M. atlantae. Non-haemolytic. Serum is needed for growth; coagulated serum is not liquefied. Growth is stimulated by bile salts. Nitrate is not reduced. M. bovis. A causal agent of e.g. INFECTIOUS KERATITIS. Some strains are catalase
ve. Usually haemolytic (on human blood). Coagulated serum is usually liquefied. Nitrate is usually not reduced. Acetate, butyrate, ethanol and lactate can be metabolized in complex media. M. lacunata (the Morax–Axenfeld bacillus). A causal agent of CONJUNCTIVITIS. Non-haemolytic on human blood. Serum is required for growth; coagulated serum is liquefied. Nitrate is reduced. Phenylalanine deaminase and urease are not formed. Acetate, butyrate and lactate can be metabolized in complex media. M. catarrhalis (formerly Branhamella catarrhalis). A causal agent of e.g. respiratory tract infections, acute OTITIS MEDIA and SINUSITIS. Non-haemolytic. Many strains can grow on nutrient agar. On blood agar, the colonies are whitish, convex, and 1–2 mm in diam. after 24 hours. Most strains reduce nitrate and nitrite, and most or all strains are reported to produce a DNase. [Review: Drugs (1986) 31 (supplement 3) 1–142.] M. caviae, M. cuniculi and M. ovis have been regarded as members of a subgenus, Branhamella, within the genus Moraxella or as species incertae sedis within the genus Neisseria (so-called ‘false neisseriae’) [Book ref. 22, pp 290–296]. Moraxellaceae A family of Gram type-negative bacteria comprising the genera ACINETOBACTER, MORAXELLA and PSYCHROBACTER [IJSB (1991) 41 310–319]. morbidity (1) The state or condition of being diseased. (2) (morbidity rate) The number of cases of a given disease in relation to the overall population (quoted e.g. as cases per 100000 or per million per annum). (cf. MORTALITY RATE.) morbilli Syn. MEASLES. morbilliform Resembling measles. Morbillivirus (measles–rinderpest–distemper (MRD) virus group) A genus of viruses of the PARAMYXOVIRIDAE in which the virions have haemagglutinin but no neuraminidase activity. Virions contain proteins N (nucleocapsid), P (nucleocapsid-associated phosphorylated), H (haemagglutinin), F (fusion) and M (membrane or matrix). (cf. PARAMYXOVIRUS.) All members share a a substrate molecule, the other oxygen atom being reduced to water; the reducing power needed for monooxygenase activity may be supplied e.g. by NADH. (See also METHANE MONOOXYGENASE.) monophasic serotypes (in Salmonella) See H ANTIGEN. monophosphams See MONOBACTAMS. monophyletic Refers to taxa which have arisen from a common evolutionary line. (cf. POLYPHYLETIC.) monoplanetism In certain OOMYCETES: the phenomenon in which only one type of zoospore is formed and only one swarm period occurs. (cf. DIPLANETISM.) monopodial (adj.) Refers to a ‘tree-like’ form or mode of development of a branching structure – i.e., one with a main, central stem from which lateral branches arise. (cf. SYMPODIAL.) monopodial amoebae Amoebae which normally produce only one lobose PSEUDOPODIUM at a time (although a second, lateral pseudopodium may be produced when the organism changes direction). (See also LIMAX AMOEBAE; cf. POLYPODIAL AMOEBAE.) Monosiga See CHOANOFLAGELLIDA. monosodium glutamate See GLUTAMIC ACID. monosome See PROTEIN SYNTHESIS. monosomic See CHROMOSOME ABERRATION. monospecific Refers to a genus which has only one species. monospecific antiserum An antiserum (e.g. a GROUPING ANTISERUM) which is capable of reacting specifically with one particular antigen or antigenic determinant – rather than with several (or many) of the antigens present on cells etc in a given suspension. (cf. POLYVALENT ANTISERUM.) monostichomonad membrane See PARORAL MEMBRANE. Monostroma A genus of green seaweeds (division CHLOROPHYTA) which show a heteromorphic alternation of generations; the gametophyte is a monostromatic sheet or blade (cf. ULVA), the sporophyte is unicellular. monostromatic Refers to a structure which is only one cell thick (see e.g. MONOSTROMA). (cf. DISTROMATIC.) monosulfactams See MONOBACTAMS. Monothalamida See GRANULORETICULOSEA. monothetic classification Any form of classification based on a small number of carefully selected criteria – which are usually interpreted unequivocally (e.g. ‘present’ or ‘absent’); such classification is necessarily biased by the choice of criteria. (cf. POLYTHETIC CLASSIFICATION.) monotrichous Refers to the presence of only one flagellum (or e.g. pilus) per cell. monotype (1) The sole species of a newly proposed genus. (2) Any taxon based on a single TYPE (sense 1). monoxenous Syn. HOMOXENOUS. Monsur medium (Monsur’s tellurite–bile salt medium) A medium for the primary isolation of Vibrio cholerae. It contains e.g. peptone, NaCl (1%), Na2 CO3 , sodium taurocholate, and (added after autoclaving) potassium tellurite; pH 9.0–9.2. The liquid medium may be used as a TRANSPORT MEDIUM for specimens containing V. cholerae, or may be solidified with agar. [Recipe: Book ref. 46, pp. 1276–1277.] (See also TELLURITE MEDIA.) Montenegro test A SKIN TEST sometimes used in the diagnosis of cutaneous leishmaniasis; the antigen is a suspension of dead Leishmania promastigotes. Moore swab (sewer swab) A SWAB used for detecting certain types of pathogen (e.g. species of Salmonella and Vibrio) in a sewer; it consists of a pad of cotton gauze, or of a length of such gauze wrapped around a wire. The swab, attached by a cord, is suspended in flowing sewage for several days; when retrieved, the swab (or liquid expressed from it) is transferred to 491 Morchella Mortierellaceae See MUCORALES. mos An ONCOGENE first identified as the transforming determinant of MOLONEY MURINE SARCOMA VIRUS (Mo-MSV). The c-mos sequences are hypermethylated, apparently transcriptionally silent (or expressed at very low levels) in normal and transformed murine cells, and lack introns. Wild-type Mo-MSV expresses v-mos only at very low levels in cell culture, but a temperature-sensitive mutant (ts110) produces higher levels of an unstable gag-mos fusion protein (MWt ca. 85000) which has serine or threonine protein kinase activity [review: JGV (1985) 66 1845–1853]. [Properties of mouse and human c-mos loci: Book ref. 113, pp. 281–289.] mosaic (plant pathol.) A form of leaf VARIEGATION in which two or more colours, or shades of colour, develop – the differently coloured areas being somewhat angular with relatively sharp, clear-cut boundaries between them. (cf. MOTTLE.) Mosaics occur in plants infected systemically with certain PLANT VIRUSES; however, similar leaf patterns may be an inherited characteristic or may result from the effects of toxins produced by leaf-feeding arthropods. mosquito iridescent virus See CHLORIRIDOVIRUS. moss starch Syn. LICHENIN. most probable number method See MULTIPLE-TUBE METHOD. motB gene See FLAGELLUM (a). motile (microbiol.) Capable of MOTILITY. motile colonies (1) (bacteriol.) The colonies of certain bacteria, e.g. Bacillus circulans, can migrate across the surface of a culture plate; the track of such movement is often marked by lines of bacterial growth which arise from cells left behind by the migrating colony. (2) (algol.) See e.g. SYNURA and VOLVOX. motility (microbiol.) Locomotion, i.e. an active process in which a cell or organism moves from place to place by expending energy to exert force(s) against the surrounding medium or the contiguous substratum; passive movements due to changes in buoyancy (see e.g. GAS VACUOLE) are not regarded as true motility (see also TWITCHING MOTILITY). Not all microorganisms are motile, i.e. capable of exhibiting motility; however, those which are motile occur among the algae, archaeans, bacteria, fungi and protozoa. Some nonmotile organisms have motile stages in their life cycles – e.g. propagules, gametes. Motility is an advantage because it enables an organism to respond e.g. to certain environmental stimuli. (See also CHEMOTAXIS.) Locomotion may be achieved in various ways, depending on organism. (a) Amoeboid movement is characteristic e.g. of some sarcodines (see PSEUDOPODIUM). (b) CILIUM-mediated locomotion occurs in members of the CILIOPHORA (see also METACHRONAL WAVES). (c) Euglenoid movement: see EUGLENA. (d) FLAGELLAR MOTILITY involves the activity of one or more flagella (or periplasmic flagella in the SPIROCHAETALES) and is found in many bacteria, protozoa (see MASTIGOPHORA) and algae, and in some fungi (limited to certain lower fungi – e.g. the zoospores of chytrids). (e) GLIDING MOTILITY occurs in some prokaryotes (e.g. Beggiatoa, Oscillatoria), and e.g. in some diatoms and desmids. [The motility apparatus in shock-frozen gliding cells of Myxococcus xanthus (electron micrographs): Microbiology (2001) 147 939–947.] (f) Gregarine movement, which occurs in some gregarines, is apparently similar or identical to GLIDING MOTILITY (q.v.). common antigen. Type species: MEASLES virus (MV); other members: CANINE DISTEMPER virus (CDV), PESTE DES PETITS RUMINANTS virus (D Kata virus), and RINDERPEST virus (RV). [Antigenic relationship between MV, CDV and RV: JGV (1986) 67 1381–1392.] In addition to causing acute disease in their respective hosts, MV and CDV are also associated with persistent infection which may lead to progressive, degenerative neurological disease (see CANINE DISTEMPER and SUBACUTE SCLEROSING PANENCEPHALITIS). It has also been suggested that MV may play a role in the pathogenesis of MULTIPLE SCLEROSIS – possibly by triggering the formation of autoantibodies to neural antigens in certain genetically predisposed individuals [Book ref. 88, pp. 155–165]. Morchella A genus of fungi of the order PEZIZALES. The fruiting body, a yellowish to brown APOTHECIUM, consists of a hollow, ovoid or elongated, waxy to brittle sponge-like pileus borne on a stout, hollow stipe – the whole being ca. 5–12 cm in height and ca. 5 cm at the broadest part; the outer surface of the pileus is conspicuously ridged and pitted, the hymeniumlined pits being separated by sterile ridges. Asci: cylindrical, operculate. M. esculenta and M. vulgaris form edible fruiting bodies (‘morels’). mordant See STAINING. Moredun medium See WHOOPING COUGH. morel See MORCHELLA. Morganella A genus of Gram-negative bacteria of the tribe PROTEEAE. Cells: ca. 0.6–0.7 ð 1.0–1.7 µm. Swarming does not occur. Reactions: urease Cve; lipase
ve; gelatin not liquefied at 22° C; H2 S
ve; citrate
ve; acid from glucose and mannose. Both nicotinic and pantothenic acids are required for growth. Species: M. morganii (formerly Proteus morganii ), found (rarely) in the intestines of animals; in man, it can be an opportunist pathogen and may cause diarrhoea. GC%: 50. [Book ref. 22, pp. 497–498.] morganocin Any BACTERIOCIN from Morganella. Moro test See TUBERCULIN TEST. moromi See SOY SAUCE. morph (mycol.) Any (sexual or asexual) stage or state of a given fungus. (See e.g. ANAMORPH; TELEOMORPH; and DIMORPHIC FUNGI (sense 1).) morphogene See CELL CYCLE. morpholine antifungal agents A group of agricultural systemic ANTIFUNGAL AGENTS which have eradicant properties. They function by inhibiting ergosterol biosynthesis, thereby disrupting the CYTOPLASMIC MEMBRANE in susceptible fungi; the mode of action differs from that of e.g. PROPICONAZOLE. Morpholines include e.g. FENPROPIMORPH and TRIDEMORPH. morphological heterothallism See HETEROTHALLISM. morphological unit (virol.) See ICOSAHEDRAL SYMMETRY. morphology agar A complex agar-based medium used for determining morphological characteristics of yeasts; it contains amino acids, D-glucose, vitamins, salts and trace elements [composition: Book ref. 100, pp. 100–101]. It is available commercially in dehydrated form. morphotype Any distinct, more or less stable morphological form of a given organism. (See also VARIETY and POLYMORPHISM sense 1.) morphovar See VARIETY. mortality rate Of a given disease: the number of deaths expressed as a proportion of the number of individuals who have contracted the disease in a particular outbreak, or over a specified period of time. (cf. MORBIDITY.) Mortierella See MUCORALES. 492 mRNA predisposition, hormonal status, number of pregnancies, etc. A critical factor in tumorigenesis is believed to involve the insertion of the provirus at particular sites in the mouse genome; two such sites have been implicated (int-1 and int-2), neither of which shows homology with any known cellular or viral ONCOGENE. Integration of an MMTV provirus near one of these loci appears to activate the expression of a host gene which is normally silent, possibly by exerting an enhancer function. [Reviews of oncogenesis by MMTV: Book ref. 110, pp. 175–195; JGV (1985) 66 931–943.] mouse poliomyelitis See THEILER’S MURINE ENCEPHALOMYELITIS VIRUS. mousepox See ECTROMELIA VIRUS. mouth fungus (in fish) See COLUMNARIS DISEASE. mouth microflora The microflora of the human mouth may include e.g. species of ACTINOMYCES, BACTEROIDES, CAMPYLOBACTER, CAPNOCYTOPHAGA, EIKENELLA, FUSOBACTERIUM, LEPTOTRICHIA, ROTHIA, SELENOMONAS, STAPHYLOCOCCUS, STREPTOCOCCUS, TREPONEMA, WOLINELLA, VEILLONELLA, and various yeasts (e.g. Candida albicans) and protozoa (e.g. Entamoeba gingivalis and Trichomonas tenax ). Some of these organisms occur primarily in microaerobic or anaerobic microenvironments such as the gingival sulcus. (See also BODY MICROFLORA; DENTAL PLAQUE; GASTROINTESTINAL TRACT FLORA; RESPIRATORY TRACT MICROFLORA; STOMATITIS.) moving-boundary electrophoresis See ELECTROPHORESIS. moxalactam (latamoxef) An OXACEPHEM antibiotic which is active against a wide range of Gram-negative bacteria. moxifloxacin A QUINOLONE ANTIBIOTIC (q.v.). Mozambique virus See ARENAVIRIDAE. mozzarella cheese See CHEESE-MAKING. MPa 106 PASCAL. MPEDn In a heat STERILIZATION process: the most probable effective dose of thermal energy required to achieve an n-log10 reduction in the numbers of a given microbial contaminant. mpl gene See PEPTIDOGLYCAN. MPN method See MULTIPLE-TUBE METHOD. MppA See PEPTIDOGLYCAN. MPT64 See ESAT-6. Mr RELATIVE MOLECULAR MASS. MR test See METHYL RED TEST. MR vaccine Measles–rubella vaccine. Mrad 106 rad (see RAD). mrcA gene See PENICILLIN-BINDING PROTEINS. mrcB gene See PENICILLIN-BINDING PROTEINS. MRD virus group See MORBILLIVIRUS. MreB protein See CELL CYCLE (b) (determination of shape). MRHA Mannose-resistant haemagglutination (see FIMBRIAE). mRNA Messenger RNA: any RNA molecule which functions as a template for the assembly of amino acids during PROTEIN SYNTHESIS (see GENETIC CODE). In all cells, and in DNA viruses, the mRNA is transcribed from DNA (see TRANSCRIPTION); in RNA viruses, the genome may or may not be able to function directly as mRNA (see VIRUS). (a) Prokaryotic mRNAs. Many bacterial mRNAs are functional as primary transcripts, i.e. they do not require posttranscriptional maturation. An mRNA molecule may contain the transcript of a single gene (monocistronic or monogenic mRNA) or, more commonly, transcripts of two or more genes (polycistronic or polygenic mRNA). (See also OPERON.) A polycistronic mRNA contains several distinct regions: a 50 LEADER; coding regions (structural gene transcripts), each beginning with a translation initiation codon and consisting of a (g) Motility (mechanism unknown) reported in certain strains SYNECHOCOCCUS (q.v.) [see also JB (1997) 179 2524–2528]. (h) In Serratia marcescens: flagellum-independent spreading (which may involve surface tension) [JB (1995) 177 987–991]. (i) Actin-based motility: motility exhibited by certain pathogenic bacteria within eukaryotic cells – see DYSENTERY (1a) and LISTERIOSIS. Direct (microscopical) determination of motility can usually be achieved easily. However, in some organisms – particularly archaeans and bacteria – BROWNIAN MOVEMENT may be mistaken for motility. (See also HANGING DROP and COUNTING METHODS.) Motility in certain organisms may be determined macroscopically: see e.g. SWARMING. Motile and non-motile bacteria may be separated e.g. by CRAIGIE’S TUBE METHOD. MOTT Mycobacteria other than tuberculosis: refers to certain species of Mycobacterium (excluding M. leprae) which may be associated with human disease but which are not members of the Mycobacterium tuberculosis complex [Book ref. 219, p 329]. The phrase non-tuberculous mycobacteria (NTM) is synonymous, and is preferred by some authors [e.g. RMM (1997) 8 125–135]. mottle (plant pathol.) A form of leaf VARIEGATION in which two or more colours, or shades of colour, develop – the differently coloured areas being somewhat ill-defined with indistinct boundaries between them. (cf. MOSAIC.) Mottling occurs in plants infected systemically with certain PLANT VIRUSES. Mougeotia A genus of filamentous green algae related to SPIROGYRA. Each cell contains a single plate-like chloroplast in the centre of the cell; the chloroplast can rotate within the cell in response to changes in light intensity. mould (mold) A general, imprecise term for any fungus which forms a visible layer of mycelium and/or spores on the surface of foods, walls etc, but which does not form macroscopic fruiting bodies. (See e.g. BREAD SPOILAGE.) mountant (mounting medium) Any substance used to immerse or impregnate a specimen or smear for microscopical examination. For temporary preparations (see e.g. WET MOUNT) the mountant may be e.g. water, saline, or glycerol. Mountants for permanent preparations include e.g. DPX and CANADA BALSAM; before being mounted in these substances, specimens must be fixed (see FIXATION), dehydrated, and cleared (see CLEARING). For fungal material LACTOPHENOL COTTON BLUE is a useful mountant (and stain) for temporary or permanent preparations. mouse Elberfeld virus See CARDIOVIRUS. mouse mammary tumour virus (MMTV; Bittner virus) A type B retrovirus (see TYPE B ONCOVIRUS GROUP) which induces carcinomas in the mammary glands of several inbred strains of mice. The virus replicates in the alveolar cells of the mammary gland in lactating mice, and is transmitted to the suckling offspring via the milk. MMTV also occurs as an ENDOGENOUS RETROVIRUS in most inbred mouse strains, the provirus (usually present in multiple copies) being transmitted genetically (see RETROVIRIDAE). MMTV is replication-competent, v-onc
, does not transform cells in culture, and induces carcinomas after a long latent period. The MMTV genome contains – in addition to gag, pol and env sequences – a potential coding sequence, designated orf (for open reading frame), near the 30 end distal to and partly overlapping env ; the function of this sequence, if any, in carcinogenesis is unknown. Tumours induced by MMTV are clonal (arising mainly from a single infected cell); several factors may influence the development of tumours: e.g. genetic of 493 mRNA linear sequence of codons; and sometimes a 30 -terminal trailer. The coding regions may or may not be separated by non-coding intercistronic regions (see also OVERLAPPING GENES). The synthesis, translation and degradation of bacterial mRNA are closely linked phenomena; typically, bacterial mRNAs have a very short half-life, and in some cases degradation may begin even before transcription has been completed. mRNA is particularly prone to degradation in the absence of translation: see POLAR MUTATION. (cf. RRNA.) (See also S LAYER.) The secondary structure of bacterial mRNA may be important in determining its stability. Stable mRNAs typically contain multiple secondary structures, particularly in the 30 -terminal region, and these may serve to protect the molecule from 30 to-50 processive exonucleases present in the cell. (One function of the GC-rich stem-and-loop structure in a rho-independent terminator – see TRANSCRIPTION – may be to protect the 30 end of the transcript from degradation by exonucleases.) (See also RETROREGULATION.) Some bacterial mRNAs – e.g. that transcribed from the Escherichia coli gene lpp (which encodes an outer membrane lipoprotein) – have atypically long half-lives. Moreover, the rate of degradation of mRNAs can be affected by growth conditions. For example, in E. coli the half-life of at least some mRNAs is greatly extended during slow anaerobic growth (doubling time: 700 minutes); in these conditions the rate of synthesis of mRNA is much lower, so that extended half-life may be a mechanism for maintaining the expression of such genes during anaerobiosis [Mol. Microbiol. (1993) 9 375–381]. In Escherichia coli, no known exonuclease can degrade RNA in the 50 -to-30 direction (i.e. in the direction of transcription and translation). mRNA degradation appears to involve the activity of endonuclease(s) and (30 -to-50 ) exonuclease(s) (e.g. RNASE II); small pieces of RNA (¾10 nucleotides in length) are apparently degraded to single nucleotides by an enzyme designated RNaseŁ [JB (1991) 173 4653–4659]. Polyadenylation. Many prokaryotic mRNAs have a short (e.g. 10–50 nt) polyadenylate ‘tail’ at the 30 end. This poly(A) (AA-A. . .) tail, whose synthesis involves a poly(A)polymerase, may influence the half-life (stability) of those mRNAs whose 30 terminus contains a stem–loop structure encoded by a rhoindependent transcription terminator; such 30 stem–loop structures tend to resist enzymic degradation, and polyadenylation may facilitate degradation by providing a suitable binding site for enzymes such as 30 -exonuclease polynucleotide phosphorylase. Interestingly, mutations in gene pcnB – which encodes the Escherichia coli poly(A)polymerase I (PAP I) – can reduce the copy number of plasmid ColE1. Apparently, the plasmidencoded antisense molecule RNA I (which inhibits RNA II) is normally polyadenylated, polyadenylation assisting degradation of the molecule; the absence of polyadenylation in pcnB mutants tends to stabilize RNA I, i.e. to enhance its inhibitory action on RNA II – causing inhibition of plasmid replication and, hence, a reduction in copy number. A poly(A) tail of 10 nucleotides was reported earlier in the archaean Methanococcus vannielii [JB (1985) 162 909–917.] The poly(A) tail facilitates isolation of mRNAs by affinity chromatography; thus, mRNAs can be isolated by exploiting base-pairing specificity between the A-A-A. . . of mRNAs and the T-T-T. . . of oligo(dT)-cellulose in the matrix. Isolation of mRNAs can also be achieved e.g. by the use of DYNABEADS coated with oligo-dT. [Polyadenylation of mRNA in bacteria (review): Microbiology (1996) 142 3125–3133. Characterization of Escherichia coli poly(A)polymerase: NAR (2000) 28 1139–1144. Polyadenylation in mycobacteria: Microbiology (2000) 146 633–638.] Introns. As in typical eukaryotic genes, some bacterial, archaean and phage genes include introns (see SPLIT GENE). In the transcripts of such genes any part corresponding to an intron must be removed in order to produce a ‘mature’ mRNA that correctly encodes the gene product. Bacterial introns are autocatalytic, i.e. enzymic activity that develops within a correctly folded mRNA molecule automatically ‘splices out’ the relevant sequence of nucleotides. In at least some cases, splicing may occur even while the nascent transcript is bound to the ribosome, the ribosome pausing during excision of the intron sequence. [Splicing and translation in bacteria: GD (1998) 12 1243–1247.] (b) Eukaryotic mRNAs. In eukaryotes mRNA is synthesized (by RNA POLYMERASE II) in the nucleus and must be exported to the cytoplasm for translation; typically, mRNA is synthesized as a large precursor molecule (pre-mRNA) which undergoes an extensive maturation process involving capping at the 50 end (see below), polyadenylation of the 30 end, and (commonly) splicing out of introns (see SPLIT GENE). The mature mRNA molecule is typically functionally monocistronic. (cf. POLYPROTEIN; see also SUBGENOMIC MRNA.) Some eukaryotic genes are atypical in that they do not contain introns – thus obviating the need for splicing of mRNA; such genes include most or all of those encoding histones as well as e.g. the gene encoding interferon-b. In the cytoplasm, mRNAs are relatively stable, and they occur complexed with proteins to form messenger ribonucleoproteins, mRNPs (D ‘informosomes’). [Review: TIBS (1985) 10 162–165.] Polyadenylation. Most eukaryotic mRNAs have a poly(A) (polyadenylate) sequence – about 200 adenylate residues – at the 30 end. This poly(A) tail is not added to the complete transcript: at the end of transcription an endonuclease cuts the transcript at a specific site ¾10–30 nucleotides downstream of an AAUAAA (or similar) sequence – thus creating an appropriate 30 terminus from which another enzyme, poly(A)polymerase, can synthesize the polyadenylate tail. The function of the poly(A) tail is unknown. Some eukaryotic mRNAs do not have a poly(A) tail; for example most or all mRNAs that encode histones are apparently poly(A)
. Capping. Almost all eukaryotic mRNAs are modified (capped ) at the 50 end. Capping occurs shortly after the beginning of transcription: an enzyme (guanylyl transferase) adds to the newly formed 50 end of the molecule an ‘inverted’ guanosine residue, i.e. a guanosine residue linked 50 -to-50 to the 50 -triphosphate end of the growing pre-mRNA molecule. A methyl group is then transferred (by a specific 7-methyltransferase) to the N7 position of the G cap to form the characteristic cap structure: 0 0 0 0 0 m7 G5 ppp5 N3 p5 N3 p . . . A cap methylated only at this position is said to be of type 0. 0 Type 1 caps have an additional methyl group at the O2 -position of the next (2nd) nucleotide, and sometimes also at the N6 position of a penultimate adenine residue; type 2 caps contain, in addition to the type 1 methyl group(s), a methyl group at 0 the O2 -position of the 3rd nucleotide. Apparently, unicellular eukaryotes contain only type 0 caps; type 1 caps predominate in other eukaryotes. The 50 cap enhances translation by promoting the formation of the translation initiation complex; the recognition of the cap by 494 muciferous body culture media typically contain 4% NaCl and they are incubated at e.g. 30–35° C. MRSA grows in 24–48 hours when inoculated (104 cfu) onto Mueller–Hinton agar containing NaCl (4%) and oxacillin (6 µg/ml) or methicillin (10 µg/ml) [Book ref. 120, p. 151]. Heteroresistance refers to the expression of resistance in only a minority of cells in a genetically resistant population; most strains of MRSA are heteroresistant. [Mechanisms of heteroresistance: AAC (1994) 38 724–728.] Nasal MRSA in carriers may be eliminated by MUPIROCIN; hair and skin may be disinfected by povidone–iodine (see IODINE) or by 4% CHLORHEXIDINE. Some strains of MRSA exhibit plasmidencoded resistance to mupirocin and/or to chlorhexidine. MRSA is a serious problem in many areas. Levels of MRSA are low e.g. in Holland – where control includes strict isolation of infected/colonized patients [e.g. RMM (1998) 9 109–116]. [Rapid discrimination between methicillin-sensitive and methicillin-resistant S. aureus by intact cell mass spectroscopy: JMM (2000) 49 295–300.] [Multilocus sequence typing of methicillin-resistant and methicillin-sensitive S. aureus: JCM (2000) 38 1008–1015.] [Rapid (¾1 hour) detection of MRSA without prior culture: JCM (2004) 42 1875–1884.] [Rapid detection of mecA-positive and mecA-negative coagulase-negative staphylococci by an anti-PBP 2a slide latex agglutination test: JCM (2000) 38 2051–2054.] [Genotyping of European isolates of MRSA by fluorescent amplified fragment length polymorphism analysis and PFGE: JMM (2001) 50 588–593.] MS ring See FLAGELLUM (a). MscL channel See MECHANOSENSITIVE CHANNEL. msDNA See RETRON. MseI A RESTRICTION ENDONUCLEASE; T/TAA. MSHA Mannose-sensitive haemagglutination (see FIMBRIAE). MspI See RESTRICTION ENDONUCLEASE (table). MSU Mid-stream urine (see URINARY TRACT INFECTION). MSV MAIZE STREAK VIRUS. MSX L-Methionine-DL-sulphoximine: an inhibitor of glutamine synthetase – and hence of AMMONIA ASSIMILATION via the GS/ GOGAT pathway. MT Microtubule (see MICROTUBULES). mtDNA Mitochondrial DNA (see MITOCHONDRION). MTOC MICROTUBULE-ORGANIZING CENTRE. µ (mu) (1) An abbreviation for ‘micro-’ (i.e. one-millionth). (2) MICRON. (3) SPECIFIC GROWTH RATE. (4) One of the symbols used to designate the exponential growth rate constant – see GROWTH (a). (cf. BACTERIOPHAGE MU.) Mu See BACTERIOPHAGE MU. mu chain (immunol.) See HEAVY CHAIN. 2µ circle Syn. TWO-MICRON DNA PLASMID. mu particle The trivial name for certain bacteria (see CAEDIBACTER and PSEUDOCAEDIBACTER) which are endosymbionts in strains of Paramecium and which confer the MATE KILLER characteristic. 2µ plasmid See TWO-MICRON DNA PLASMID. 3µ plasmid See THREE-MICRON DNA PLASMID. mucA gene (1) See SOS SYSTEM. (2) See ALGINATE. Mucambo virus See ALPHAVIRUS. mucate A salt of MUCIC ACID. mucB gene See SOS SYSTEM. mucic acid HOOC(CHOH)4 COOH. It is obtained e.g. by the oxidation of galactose. muciferous body See MUCOCYST. specific cap-binding protein(s) (CBP) is an important factor in the regulation of gene expression. (The cap may also be involved e.g. in pre-mRNA processing.) [Cap-binding proteins: Cell (1985) 40 223–224. Cap structure and interaction with CBP: Prog. Mol. Subcell. Biol. (1985) 9 104–155.] (See also SNRNA.) Among plant and animal viruses, viral RNA may be capped but not polyadenylated (e.g. TOBACCO MOSAIC VIRUS), polyadenylated but not capped (e.g. poliovirus), or polyadenylated and capped (e.g. ADENOVIRIDAE, POXVIRIDAE). Polioviruses apparently bring about the switch from translation of host (capped) mRNA to translation of viral (uncapped) mRNA at least partly by inactivating the host CBP. (See also e.g. REOVIRUS.) mRNP See MRNA (b). MRSA Methicillin-resistant Staphylococcus aureus. Resistance to methicillin is generally accompanied by resistance to other b-lactam antibiotics, and is often accompanied by resistance to e.g. aminoglycosides and macrolides. MRSA has been commonly susceptible to VANCOMYCIN and/or TEICOPLANIN (see also QUINUPRISTIN/DALFOPRISTIN) [but see e.g. Lancet (1999) 353 1587–1588; JCM (2001) 39 591–595]. High-level resistance seems to have been acquired from Enterococcus faecalis via transposon Tn1546 [Science (2003) 302 1569–1571]. One report has suggested that strains of MRSA may arise, in vivo, through transfer of the mecA gene (see below) between strains of staphylococci [Lancet (2001) 357 1674–1675]. High-level (‘intrinsic’) resistance to methicillin is (chromosomally) encoded by the mecA gene whose product is a novel 78 kDa PENICILLIN-BINDING PROTEIN, PBP 2a (D 20 ), with a low affinity for b-LACTAM ANTIBIOTICS. Expression of mecA is controlled by various regulatory genes. For example, some strains of MRSA contain a mecI gene which encodes a strong repressor; thus, resistance to methicillin may arise e.g. through mutation which inactivates the MecI product or which affects transcription from mecI. MecR1 is a sensor–transducer protein in the cell envelope. The binding of a b-lactam antibiotic to MecR1 brings about, indirectly, transcription of the mecA gene; transcription involves prior cleavage (i.e. inactivation) of MecI, the repressor of the mecA gene. Such induction of mecA-based resistance may parallel the induction of b-LACTAMASES (q.v.) [see e.g. Science (2001) 291 1962–1965; commentary: Science 291 1915–1916]. [PCR-based detection of mecA, mecR1 and mecI genes in clinical isolates: JAC (2001) 47 297–304.] (See also ‘Induction of resistance to antibiotics’ within the entry ANTIBIOTIC.) mecA-mediated resistance can be lost e.g. by mutations in the fem genes (fem D factor essential for methicillin resistance). The femAB operon encodes factors needed for synthesis of normal PEPTIDOGLYCAN cross-links, and in some femAB mutants the (abnormal) cross-links are poor substrates for the (transpeptidase) activity of PBP 2a; as the ‘routine’ (methicillin-sensitive) PBPs are enzymically more effective than PBP 2a on the abnormal cross-links, the (mutant) cell is susceptible to methicillin. Non-mecA-based methicillin resistance can be due to overproduction of b-lactamases (in which case it may be overcome by inhibitors such as CLAVULANIC ACID), to mutant PBPs which have lower affinity for b-lactam antibiotics, or to a reported methicillinase. [Genetics of MRSA (review): RMM (1998) 9 153–162.] The phenotypic expression of methicillin resistance is governed by factors that include temperature and salt concentration; 495 mucigel mucigel A mucilaginous layer covering the root tips and root hairs of plants; it is composed mainly of pectin-like polysaccharides and is produced by the epidermal cells of the root tips. The mucigel may protect against desiccation, aid in the uptake of plant nutrients, and help to bind soil particles; it often contains large numbers of bacteria. [Book ref. 38, pp. 565–568.] mucigenic body See MUCOCYST. mucocutaneous leishmaniasis LEISHMANIASIS in man and other animals in which the pathogen gives rise (in man) to invasive granulomatous lesions primarily in the oral/nasal mucous membranes; in man the disease may follow an earlier, apparently cured, episode of CUTANEOUS LEISHMANIASIS with a latent period of up to many years. The disease occurs mainly in South America where it is called espundia, the causal agent being Leishmania braziliensis; a mucocutaneous leishmaniasis caused by L. donovani occurs in parts of Africa. Diagnosis may involve e.g. the MONTENEGRO TEST; pentavalent ANTIMONY compounds and/or antibiotics (e.g. amphotericin B) may be used for treatment. mucocyst (muciferous body; mucigenic body) In many ciliates and some flagellates: a small, sac-like organelle which, when appropriately stimulated, discharges a mucoid material through a pore at the cell surface. In ciliates the mucocyst is an invagination of the outermost membrane, the closed, inner part of the sac being located in the cytoplasm; mucocysts often occur in longitudinal rows parallel to the rows of kinetosomes. In some species mucocysts may be involved e.g. in cyst formation. mucoid Viscous; slimy; mucus-like. The term is often applied to the colonies of certain capsulated bacteria (see CAPSULE; see also CYSTIC FIBROSIS). mucolytic agent A chemical agent used e.g. on a sample of sputum prior to use of the sample as an inoculum on a culture plate; the purpose of the agent is to liquefy the sputum so that (i) the sputum can be manipulated, and (ii) any target cells within the sputum will be exposed to reagents. Mucolytic agents include dithiothreitol [example of use: JCM (1997) 35 193–196] and N-acetyl-L-cysteine [examples of use: JCM (1999) 37 175–178; JCM (1999) 37 137–140]. mucopeptide (glycosaminopeptide) (1) Syn. PEPTIDOGLYCAN. (2) Any peptide-containing mucopolysaccharide. mucopolysaccharide (glycosaminoglycan) A polysaccharide that contains a high proportion of amino sugars and (commonly) uronic acids: e.g. HYALURONIC ACID. mucoprotein A protein covalently linked to amino sugars. mucopurulent Containing mucus and pus. Mucor A genus of fungi (order MUCORALES) which typically occur as saprotrophs on soil, vegetable matter and dung. The vegetative thallus is usually a branched, aseptate mycelium, though some species are dimorphic. The organisms form ZYGOSPORES (see also SPOROPOLLENIN); many species exhibit HETEROTHALLISM (see also COMPATIBILITY sense 2 and PHEROMONE). (For M. miehei and M. pusillus see RHIZOMUCOR.) Mucoraceae See MUCORALES. Mucorales An order of fungi (class ZYGOMYCETES) which occur as saprotrophs on decaying vegetable matter, soil or dung, or as parasites or pathogens of plants or animals (see e.g. ZYGOMYCOSIS). The vegetative thallus is typically a well-developed aseptate mycelium; some species (e.g. Mucor rouxii, other species of Mucor, Mycotypha spp) are DIMORPHIC FUNGI (sense 1). The organisms commonly form asexually derived spores in sporangia and/or sporangiola. Most species form ZYGOSPORES (q.v.) – cf. Saksenaeaceae (below). Nine families are recognized [Book ref. 64, pp. 247–248] on the basis of reproductive structures: Choanephoraceae. Sporangia and sporangiola are borne on different types of sporangiophore. Genera: Blakeslea (see also PHEROMONE), Choanephora. [The family has been redefined to contain Blakeslea (sporangiola dehiscent), Choanephora (sporangiola non-dehiscent), and Poitrasia (sporangiola not formed; sporangia distinctive): Mycol. Pap. (1984) Nr 152.] Cunninghamellaceae. Sporangiola: one-spored; sporangia absent. Genera: e.g. Cunninghamella (see also ZYGOMYCOSIS), Thamnocephalis. Mortierellaceae. Columellae vestigial or absent. Genera: e.g. Mortierella (see also COMPOSTING and ZYGOMYCOSIS). Mucoraceae. Sporangia are columellate. Genera: e.g. Absidia (see also ZYGOMYCOSIS), Actinomucor, Gilbertella, MUCOR, Phycomyces (see also THIAMINE), RHIZOMUCOR, RHIZOPUS and Zygorhynchus. Pilobolaceae. Sporangia are columellate; in some species the sporangia are forcibly discharged. Genera: e.g. PILAIRA, PILOBOLUS. Radiomycetaceae. Sporangia are absent; sporangiola are often formed on stoloniferous sporangiophores. Genera: e.g. Hesseltinella. Saksenaeaceae. Zygospores are apparently not formed. Sporangia are columellate or acolumellate. Genera: Echinosporangium, Saksenaea (see also ZYGOMYCOSIS). Syncephalastraceae. Merosporangia are formed. Genera: Syncephalastrum. Thamnidiaceae. Sporangiola (each containing one to several spores) are formed – sometimes with columellate sporangia on the same or on separate (morphologically similar) sporangiophores. Genera: e.g. Chaetocladium, Mycotypha, Thamnidium. mucoran A water-soluble, alkali-soluble heteroglucan (containing fucose, mannose and glucuronic acid) present e.g. in the cell walls of Mucor rouxii ; it appears to be maintained in an insoluble form in the cell wall as a result of binding to insoluble glucosamine-containing polymers. mucormycosis See ZYGOMYCOSIS. mucosal disease (MD) A severe CATTLE DISEASE caused by the BVD-MD virus (see PESTIVIRUS). It apparently results from persistent BVD-MD virus infection acquired congenitally (with the consequent development of specific immune tolerance to the virus). MD is characterized by fever, anorexia, nasal discharge, profuse watery diarrhoea, buccal ulceration, etc; dehydration and emaciation are usually followed by death. [VR (1985) 117 240–245.] (cf. BOVINE VIRUS DIARRHOEA.) mucron In acephaline gregarines: a differentiated region of the anterior end of the cell by means of which the developing parasite remains attached to the host cell (cf. EPIMERITE). Mudlac system A system for bringing about GENE FUSIONS or OPERON FUSIONS using a defective form of BACTERIOPHAGE MU (‘Mud phage’) which carries the lacZ gene of Escherichia coli (and e.g. an antibiotic resistance marker); when the genome of such a phage transposes in the chromosome of a host cell, lacZ is inserted at more or less random sites and hence can be fused to almost any gene or operon in the chromosome. Two basic types of Mud phage can be used. Mud I phages contain an intact lacZ gene which can be expressed only if it can be transcribed from the promoter of the gene into which it has inserted; lacZ expression can be detected e.g. by plating the cells on Xgal plates (see XGAL). Mud II phages contain lacZ minus its translation initiation signals; in this case lacZ expression depends on both the promoter and the translation initiation signals of the gene into which it has inserted, the product being a FUSION PROTEIN. (See also MINI-MU.) 496 multiplex PCR loss during an epiphytotic is less likely than it would be for a genetically uniform susceptible cultivar. [Review: ARPpath. (1985) 23 251–273.] multiloculate (multilocular) Consisting of or having many LOCULES: see e.g. FORAMINIFERIDA. multilocus enzyme electrophoresis (MEE) A method in which the electrophoretic mobilities of a range of enzymes from one organism are compared with those of equivalent enzymes from one or more closely related organisms; MEE is used e.g. for TYPING microorganisms. In this method, enzymes are isolated and tested under conditions in which enzymic activity is retained; enzymes can be identified in gels by specific colour-generating substrates. Variation in the electrophoretic mobility of a given enzyme can arise e.g. when a variant form of the enzyme is specified by a different allele of the enzyme-encoding gene; such an enzyme – which may exhibit a different mobility when only a single amino acid differs – is called an alloenzyme. multilocus sequence typing See MLST. multipartite virus Syn. MULTICOMPONENT VIRUS. multiple cloning site Syn. POLYLINKER. multiple drug resistance See ANTIBIOTIC. multiple fission A type of fission in which one cell gives rise to many cells. Multiple fission may involve repeated BINARY FISSION within a common envelope (as e.g. in CYANOBACTERIA of section II), or it may involve a series of nuclear divisions followed by the incorporation of each nucleus in a separate daughter cell (see e.g. SCHIZOGONY). multiple sclerosis A chronic demyelinating disease of the CNS in humans. It is generally believed to be an autoimmune disease, possibly triggered by viral infection; it has been suggested that any of various enveloped viruses may be able to trigger the autoimmune response – which could be mounted against host lipids incorporated (with viral proteins) in the viral envelope [Nature (1986) 321 386]. (See also HTLV; INOUE–MELNICK VIRUS; MORBILLIVIRUS; cf. THEILER’S MURINE ENCEPHALOMYELITIS VIRUS.) multiple-tube method (most probable number (MPN) method) A method for estimating the number of viable organisms (usually bacteria) suspended in a liquid. The method is used e.g. for the bacteriological examination of natural and treated waters; for this purpose, one of several possible procedures is as follows. Five tubes, each containing a suitable liquid growth medium, are each inoculated with a fixed volume of the sample water (or a dilution of it). Another five tubes, containing the same medium, are each inoculated with a smaller fixed volume of the sample, and further sets of tubes are similarly inoculated with progressively smaller volumes. Following incubation, each tube is examined for the presence or absence of growth. (It is assumed that growth will occur in any tube which receives at least one viable organism.) Growth usually occurs in those tubes inoculated with the larger volumes of the sample since these tubes are more likely to have received at least one viable organism in the inoculum. The number of cells in the original sample can be calculated from the pattern of positive (growth) and negative (no growth) tubes by the use of statistical (probability) tables; if a diluted sample were used, the count indicated by the tables is multiplied by the dilution factor. By using a selective medium in the tubes it is possible to estimate the numbers of a particular type of organism in the sample. The multiple-tube method is characterized by a large sampling error. (See also COUNTING METHODS.) [Improved method of computing MPNs: AEM (1983) 45 1646–1650.] multiplex PCR A form of PCR in which twoŁ or more different targets are amplified, simultaneously, in a single assay. The reaction mixture usually contains two types of primer for each target; muduction See MINI-MU. Mueller–Hinton medium A general-purpose medium containing beef infusion, casamino acids (or peptone), and starch. It can be used as a broth or gelled with agar, and may be used e.g. as a base for blood-containing media. [Recipe: Book ref. 53, p. 1403.] MUG test A test which helps to detect Escherichia coli in a mixed microflora. An agar-based medium which includes the reagent 4-methylumbelliferyl-b-D-glucuronide (MUG) is inoculated with the sample; after incubation, colonies are examined under ultraviolet radiation (wavelength 366 nm). Most strains of E. coli form b-glucuronidase – an enzyme which cleaves MUG to a compound that fluoresces greenish-blue under radiation of 366 nm; the presence of fluorescent colonies allows a presumptive identification of E. coli. (N.B. Other sources of bglucuronidase – found e.g. in shellfish samples – can give rise to a false-positive indication.) In an alternative approach, a colony is smeared on filter paper which has been impregnated with MUG; the filter paper is then examined under ultraviolet radiation. mukB gene See CELL CYCLE (b). Müller–Kauffmann broth A tetrathionate-containing ENRICHMENT MEDIUM used e.g. in the isolation of salmonellae from preenrichment cultures of meat products in buffered peptone media. Müller’s vesicle In certain gymnostome ciliates (e.g. Loxodes): a composite organelle consisting of a membrane-enclosed mineral body (the statolith), a vacuole, and structures derived from the overlying kinety; it appears to act as a gravity sensor. [JP (1986) 33 69–76.] multi-L-arginyl-poly(L-aspartic acid) Syn. CYANOPHYCIN. multicellular Consisting of more than one (usually many) cells. multicistronic mRNA Syn. polycistronic mRNA (see MRNA). multicomponent virus (multipartite virus; multiparticle virus; covirus) A virus which exists as two or more separate particles, each containing only part of the genome; all the particles must be present in a given host cell for the complete replication cycle to occur. (cf. SATELLITE VIRUS.) Many PLANT VIRUSES are multicomponent viruses. multicopy inhibition The phenomenon in which transposition of a single-copy Tn10 element, resident in a bacterial chromosome, is inhibited 3- to 10-fold in the presence of a multicopy plasmid carrying IS10-R (see Tn10 ). The phenomenon is due to inhibition of translation (not transcription) of the transposase gene – apparently as a result of pairing between the start region of the transposase gene transcript and a short complementary RNA molecule (see ANTISENSE RNA) transcribed from the opposite IS10 strand. [Book ref. 20, pp. 275–277.] (cf. TRANSPOSITION IMMUNITY.) multicopy plasmid A PLASMID whose COPY NUMBER (in a given cell) is high, e.g., 10–30. Multicopy plasmids include e.g. the COLE1 PLASMID and plasmids pBR322, pMB1 and R6K. In a cell containing a multicopy plasmid, the replication of individual plasmids occurs randomly (as indicated by Meselson–Stahl density-shift experiments), and there is no active mechanism for PARTITION. multicopy single-stranded DNA See RETRON. multidrug and toxic compound extrusion (MATE) See MATE. multifidene See PHEROMONE. multiline cultivar (plant pathol.) A plant cultivar which is genetically non-uniform, comprising a mixture of plant lines which exhibit different degrees of resistance to a given pathogen or to particular race(s) of a given pathogen. The spread of a particular pathogen through such a cultivar is inhibited, and total crop 497 multiplicity of infection muramic acid 3-O-Lactylglucosamine: see PEPTIDOGLYCAN. muramidase An enzyme which cleaves glycosidic bonds in PEPTIDOGLYCAN: see e.g. LYSOZYME. A muramidase obtained from the bacteriolytic hyphomycete Chalara (D ‘Chalaropsis’) sp is commonly used in studies on peptidoglycan; this enzyme hydrolyses the same glycosidic bond as lysozyme but differs e.g. in substrate specificity [JBC (1967) 242 5586–5590]. muramyl dipeptide See MDP. murein Syn. PEPTIDOGLYCAN. murein lipoprotein Syn. BRAUN LIPOPROTEIN. mureinoplast Any Gram-negative bacterial cell from which the outer membrane, but not the peptidoglycan, has been removed. murid (beta) herpesvirus 1 See BETAHERPESVIRINAE. murid herpesvirus 2 See BETAHERPESVIRINAE. muriform Refers to a spore which has both transverse and longitudinal septa. murine Of or pertaining to rats or mice (Muridae). murine cytomegalovirus group See BETAHERPESVIRINAE. murine encephalomyocarditis virus See CARDIOVIRUS. murine hepatitis virus See CORONAVIRIDAE. murine leukaemia viruses (MLVs; MuLVs) A group of type C retroviruses (subfamily ONCOVIRINAE), nearly all of which are replication-competent and v-onc
(but cf. ABELSON MURINE LEUKAEMIA VIRUS), and some of which can induce e.g. lymphatic leukaemia in mice after a long latent period (see e.g. MOLONEY MURINE LEUKAEMIA VIRUS). (All replication-competent murine type C retroviruses are termed ‘murine leukaemia viruses’, although most are usually non-leukaemogenic [Book ref. 114, p. 70].) MuLV-induced leukaemogenesis requires viraemia (i.e., it requires expression and replication of the virus) and an active cellular immune response to the MuLV env gene product; it has been suggested that chronic immune stimulation may be a factor in leukaemogenesis. Although the env gene product is thought to play a direct role in leukaemogenesis, it has been shown that the primary oncogenic determinant in one MuLV lies outside the env region – possibly in the LTR [JV (1983) 47 317–328]. (See also MCF VIRUSES.) MuLVs have given rise to a range of defective, often acutely oncogenic variants: see e.g. FRIEND VIRUS, HARVEY MURINE in some cases, however, one primer can be shared by two targets – e.g. 16S rRNA sequences from two species, Bacteroides forsythus and Prevotella intermedia, were simultaneously amplified with one forward primer (a BROAD-RANGE PRIMER common to both species) and two species-specific reverse primers [JCM (1999) 37 1621–1624]. Owing to the use of multiple primers, it is particularly important to ensure the absence of complementarity between the 30 ends of the primers in order to avoid the formation of PRIMER-DIMERS. The possibility of competition between different targets in a multiplex PCR may be investigated by adding a few copies of another target which is amplifiable but distinct from the other targets. Failure of this target to amplify (when other(s) are positive) would suggest that competition/inhibition has affected the efficiency of amplification; in these circumstances, the failure of amplification of other target(s) in the same assay would not be considered meaningful [JCM (1998) 36 191–197]. Examples of the use of multiplex PCR include: detection of chlamydiae [JCM (1999) 37 575–580]; detection of vancomycin-resistance genes in enterococci [JCM (1999) 37 2090–2092]; detection of mecA and coa genes in Staphylococcus aureus [JMM (1997) 46 773–778]; detection of different types of virus in cerebrospinal fluid (CSF) [JCM (1997) 35 691–696]; and detection of HIV-1, HIV-2, HTLV-I and HTLV-II [PNAS (1999) 96 6394–6399]. Ł The amplification of two targets is also referred to as duplex PCR. multiplicity of infection (moi) The ratio of virions to susceptible cells in a given system; e.g., a high moi means a large number of virions per cell. multiplicity reactivation See REACTIVATION. multiseriate Arranged in or forming several rows; more than one cell wide. multisite mutation A MUTATION which involves the alteration of two or more contiguous nucleotides. (cf. POINT MUTATION.) multivalent vaccine Syn. POLYVALENT VACCINE. Multivalvulida See MYXOSPOREA. MuLV MURINE LEUKAEMIA VIRUS. µm MICROMETRE. 2µm circle (2µm plasmid) Syn. TWO-MICRON DNA PLASMID. mumps (contagious, infectious or epidemic parotitis) An acute infectious human disease which affects mainly children; it is caused by a PARAMYXOVIRUS. Transmission occurs by droplet infection. Incubation period: 2–3 weeks. The disease ranges from subclinical to severe. Typical symptoms include fever with pain and swelling of one or more salivary glands (usually the parotid). Common complications include MENINGITIS, often with mild encephalitis, and orchitis; oophoritis, mastitis, pancreatitis etc may also occur. Infection seems to confer life-long immunity. Lab. diagnosis: serological tests (CFT, HAI test) for mumps virus antibodies; isolation of mumps virus e.g. from pharynx or urine. mundticin A 43-amino-acid class IIa BACTERIOCIN produced by Enterococcus mundtii. mungbean yellow mosaic virus See GEMINIVIRUSES. mupirocin (pseudomonic acid A) An antibiotic produced by strains of Pseudomonas fluorescens. It is active against staphylococci and streptococci associated with skin infections, and the topically-applied antibiotic has been shown to be effective against experimental infection of surgical wounds by Staphylococcus aureus [JAC (1985) 16 519–526]. Mupirocin appears to act by inhibiting isoleucyl-tRNA synthetase. [Review: JAC (1987) 19 1–5.] SARCOMA VIRUS, MOLONEY MURINE SARCOMA VIRUS, RAUSCHER VIRUS. murine poliovirus Syn. THEILER’S MURINE ENCEPHALOMYELITIS VIRUS. murine sarcoma viruses (MSV) A group of highly oncogenic, replication-defective, C-type retroviruses (subfamily ONCOVIRINAE): see e.g. FBS OSTEOSARCOMA VIRUS, GAZDAR MURINE SARCOMA VIRUS, HARVEY MURINE SARCOMA VIRUS, KIRSTEN MURINE SARCOMA VIRUS, and MOLONEY MURINE SARCOMA VIRUS. MSV replication requires the presence of e.g. a MURINE LEUKAEMIA VIRUS as helper. murine typhus See TYPHUS FEVERS. MurNAc N-Acetylmuramic acid: see PEPTIDOGLYCAN. muropeptides Products of muramidase (e.g. LYSOZYME) digestion of PEPTIDOGLYCAN. In Escherichia coli they are designated C1–C8, according to their migration rates during chromatography; e.g., C3 is a dimeric subunit consisting of two GlcNAc–MurNAc disaccharides bound by a cross-link; C4 consists of a tetrasaccharide – (GlcNAc–MurNAc)2 – in which there is a cross-link between the two MurNAc residues; C5 and C6 are monomeric subunits (GlcNAc–MurNAc) linked to short peptides. [Book ref. 122, pp. 79–81.] murrain Syn. REDWATER FEVER. 498 mutasynthesis Murray Valley fever (Australia X disease) An acute human disease caused by a flavivirus (see FLAVIVIRIDAE) and transmitted by mosquitoes (e.g. Culex annulirostris); it occurs in parts of Australasia. The disease is usually a mild, febrile condition, but encephalitis may occur and may be fatal in young children. Birds may act as a reservoir of infection. Murraya See LISTERIA. muscarine (muscarin) A quaternary ammonium derivative of tetrahydrofuran produced e.g. by Amanita muscaria (the fly agaric) and Inocybe spp; it is toxic to flies, and infrequently causes a fatal intoxication in humans. (See also MYCETISM.) muscicolous Growing on mosses. mushroom (1) Loosely: any umbrella-shaped fungal fruiting body; such a fruiting body consists of a horizontally orientated, circular PILEUS supported, at its centre, by a vertical, columnar STIPE. ‘Mushroom’ is often reserved for those fruiting bodies in which the pileus has radially arranged lamellae (see LAMELLA) or lamella-like structures on its underside. (2) Any AGARIC. (3) The (edible) fruiting body of Agaricus brunnescens, the common cultivated mushroom (see AGARICUS and MUSHROOM CULTIVATION). (See also MUSHROOM DISEASES.) (4) Any edible mushroom – including e.g. ENOKITAKE, OYSTER FUNGUS and SHII-TAKE. (cf. TOADSTOOL.) mushroom cultivation The mycelium of the edible mushroom Agaricus brunnescens (D A. bisporus) is cultivated on composted material – traditionally a mixture of straw and horse manure (see COMPOSTING; [SEM of flora: JAB (1983) 55 293–304]). After an initial composting period, the compost undergoes ‘peak-heating’ (‘pasteurization’) to ca. 50–60° C; during this stage thermophilic actinomycetes and fungi continue to grow, but the number of mesophiles – including many mushroom parasites and pathogens – is reduced. (See also OLIVEGREEN MOULD.) The compost is then allowed to cool to ca. 25° C and inoculated with SPAWN of a selected strain of A. brunnescens. After ca. 14 days, during which the mycelium of A. brunnescens grows through the compost, a ‘casing layer’ of soil or a mixture of peat and chalk or limestone is spread over the compost. The temperature is maintained at ca. 25° C (RH >90%) for ca. 1 week. When the mycelium reaches the surface of the casing layer, the temperature and RH are allowed to fall (e.g. to ca. 16–18° C or less, RH 80–90%) to induce initiation of fruiting. Mature fruiting bodies begin to appear ca. 3 weeks after casing, and then appear in flushes at intervals of ca. 7–10 days. During the growth of A. brunnescens the previous (thermophilic) flora declines, and progressive colonization by mesophilic fungi (e.g. species of Aspergillus and Trichoderma) occurs [TBMS (1980) 74 465–470]. The composting and pasteurization processes appear to be necessary to provide low C:N ratios with high levels of organic nitrogen (ammonia being inhibitory to A. brunnescens). The microbial population established during these stages may act as a direct source of nutrients for A. brunnescens, which produces a variety of extracellular enzymes (including a bN-acetylmuramidase) that can lyse and degrade bacteria [JGM (1984) 130 761–769] and fungal mycelium [JGM (1985) 131 1735–1744]. The casing is necessary to switch growth of A. brunnescens from vegetative to reproductive. This switch appears to be associated with the activities of bacteria (especially pseudomonads) in the casing layer, possibly involving the removal by the bacteria of certain ‘self-inhibitory’ fungal metabolites; the casing may also serve to trap some volatile product(s) of A. brunnescens or other fungi in the compost [Book ref. 39, p. 291]. Certain other edible agarics are cultivated in some countries; some (e.g. PADI-STRAW MUSHROOM) are grown on waste plant materials, others (e.g. ENOKITAKE, OYSTER FUNGUS, SHII-TAKE) are grown on wood, sawdust, woodchips etc. (See also MUSHROOM DISEASES.) mushroom diseases Diseases of the cultivated mushroom Agaricus brunnescens (D A. bisporus) can be caused by bacteria (see e.g. BROWN BLOTCH, GINGER BLOTCH), fungi (see e.g. BUBBLE DISEASES, COBWEB DISEASE, OLIVE-GREEN MOULD) or viruses (see e.g. WATERY STIPE). (See also FUNGUS GNATS.) mushroom virus 3 See MYCOVIRUS. mussel poisoning See SHELLFISH POISONING. must See WINE-MAKING. mustard gas See SULPHUR MUSTARDS. mut genes See MUTATOR GENE. mutagen An agent (chemical or physical) which promotes MUTAGENESIS. Most mutagens interact directly with (and damage) DNA, but mutagenesis commonly requires the active participation of the cell itself; e.g., a mutagen may alter a base in DNA such that, although not itself a mutation, it may pair with the wrong base during a subsequent round of DNA replication, resulting in a mutation. The damaging effects of some potential mutagens can be counteracted, to varying extents, by particular DNA REPAIR systems, one of which (error-prone repair: see SOS SYSTEM) plays an important role in mutagenesis induced by certain mutagens. A given mutagen may be mutagenic in one organism but not in another, or may act differently in different organisms. Physical agents which promote mutagenesis in at least some organisms include e.g. IONIZING RADIATION, ULTRAVIOLET RADIATION and heat (heat apparently increases the rates of spontaneous reactions in DNA, thus increasing the SPONTANEOUS MUTATION rate). Chemical mutagens include e.g. ALKYLATING AGENTS, BASE ANALOGUES, BISULPHITE, HYDROXYLAMINE, certain INTERCALATING AGENTS, and NITROUS ACID; many of these are more reactive with ssDNA than with dsDNA and thus probably act preferentially in singlestranded regions generated e.g. during replication. [Chemical mutagenesis: ARB (1982) 52 655–693.] mutagenesis The generation of one or more MUTATIONS. Mutagenesis may involve SPONTANEOUS MUTATION (see also MUTATOR GENE) or it can be induced in vitro by the use of a MUTAGEN or by recombinant nucleic acid technology (see SITE-SPECIFIC MUTAGENESIS, TRANSPOSON MUTAGENESIS and SIGNATURE-TAGGED MUTAGENESIS). mutagenic Capable of causing MUTAGENESIS. mutagenic repair See SOS SYSTEM. mutagenicity test A test used to detect the mutagenic potential of a particular (usually chemical) agent. Mutagenicity tests are widely used for screening chemicals for potential carcinogenic activity since most direct CARCINOGENS are mutagenic (although not all mutagens are carcinogenic). See e.g. AMES TEST and SOS CHROMOTEST. mutan A water-insoluble glucan, containing mostly (1 ! 3)-a(with some (1 ! 6)-a-) glycosidic bonds, produced by certain strains of Streptococcus mutans; mutan is a component of DENTAL PLAQUE. [Mutan synthesis and cariogenicity in S. mutans: JGM (1981) 127 407–415.] mutanolysin Endo-N-acetylmuramidase. mutant (adj. or noun) (Refers to) an organism, population, gene, chromosome etc which differs from the corresponding WILD TYPE by one or more MUTATIONS. mutasynthesis The synthesis of novel end-products by a mutant organism which is blocked in the given biosynthetic pathway 499 Mutatest accuracy in the polymerizing (nucleotide selection) activity and/or in the proof-reading activity of the enzyme. Other mutator genes in E. coli include genes involved in the MISMATCH REPAIR system. Another mutator, designated mutT, specifically increases the level of AÐT ! CÐG transversions. [Escherichia coli mutator genes: TIM (1999) 7 29–36.] Some DNA polymerase mutants of E. coli exhibit mutation rates which are lower than those in wild-type cells; the corresponding gene is then called an antimutator gene. An antimutator gene product presumably permits fewer errors than occur normally during replication – possibly by increasing the efficiency of the proof-reading/editing function of the enzyme. (See also BACTERIOPHAGE MU.) mutator phage Any phage which, on infection of a host bacterium, causes an increase in the rate of mutation in the host cell (see BACTERIOPHAGE MU and BACTERIOPHAGE D108). mutD gene See DNA POLYMERASE. mutH gene See MISMATCH REPAIR. Mutinus See PHALLALES. mutL gene See MISMATCH REPAIR. mutS gene See MISMATCH REPAIR. mutualism SYMBIOSIS (sense 1) in which both symbionts derive benefit from the association. (cf. COMMENSALISM.) Some authors reserve the term for an obligatory association, i.e., an association of organisms which, in nature, cannot live independently. Optional (‘facultative’) mutualism is sometimes called PROTOCOOPERATION. MV-L3 phage group (L3 acholeplasmavirus group) A group of MYCOPLASMAVIRUSES probably belonging to the PODOVIRIDAE. Type member: bacteriophage MV-L3 (D MVL3; D L3 strain L3). The MV-L3 virion has a polyhedral head (diam. ca. 60 nm) with a short tail (ca. 20 ð 9 nm) attached at one vertex via a collar; it includes five or more proteins and ca. 2% by weight of fucose. Genome: linear dsDNA (MWt 2.6 ð 107 ). Host: Acholeplasma laidlawii ; plaques clear, minute. Infected cells are killed but not lysed; progeny virions seem to be released in membrane vesicles which subsequently rupture. Mx protein See INTERFERONS. mxi-spa genes See DYSENTERY. myalgia Muscle pain. Myambutol Syn. ETHAMBUTOL. myb An ONCOGENE originally identified as the transforming determinant in avian myeloblastosis virus (AMV: see AVIAN ACUTE LEUKAEMIA VIRUSES); v-myb is an altered form of a cellular sequence amv, differing from amv in gene structure, transcript structure, gene product structure, and in the intracellular location (nucleus) of its product [Book ref. 113, pp. 143–151]. v-mybC AMV can transform chicken haematopoietic cells in culture, but differs from other acutely transforming retroviruses in that it does not transform fibroblasts in culture; it causes a rapidly fatal leukaemia only in chickens. myc An ONCOGENE originally identified as the transforming determinant of avian myelocytomatosis virus (MC29: see AVIAN ACUTE LEUKAEMIA VIRUSES). The MC29 v-myc product is a gagmyc fusion protein (P110gag
myc ) which has no protein kinase activity; it binds to dsDNA and occurs – possibly as a chromatin component – in the nucleus. In humans, c-myc is located on chromosome 8 and is involved in the pathogenesis of BURKITT’S LYMPHOMA. In chickens, c-myc activation by AVIAN LEUKOSIS VIRUSES appears to result in the development of lymphoid leukosis. mycangium Syn. MYCETANGIUM. and which is provided with unnatural substrates with which to complete that pathway. Mutasynthesis has been used e.g. to synthesize new antibiotics. Mutatest Syn. AMES TEST. mutation A stable, heritable change in the nucleotide sequence of a genetic nucleic acid (DNA or – in RNA viruses, viroids etc – RNA) which typically results in the generation of a new allele and a new phenotype (cf. SILENT MUTATION and CONDITIONAL LETHAL MUTANT). (In a diploid cell, a mutant allele may be recessive to the corresponding WILD TYPE allele present in the same cell, in which case it will not affect the phenotype of the cell.) The generation of a mutation (mutagenesis) may occur spontaneously (see SPONTANEOUS MUTATION) or may result from the activity of a MUTAGEN. See also e.g. BACK MUTATION; DELETION MUTATION; FORWARD MUTATION; FRAMESHIFT MUTATION; INSERTION MUTATION; LEAKY MUTATION, MIS-SENSE MUTATION; MULTISITE MUTATION; NONSENSE MUTATION; NULL MUTATION; PLEIOTROPIC mutation; POINT MUTATION; POLAR MUTATION; SAME-SENSE MUTATION; SUPPRESSOR MUTATION. mutation rate (bacteriol.) The probability that any given cell in a growing population will undergo, spontaneously, a specific MUTATION during its division cycle. The concept of mutation rate can be expressed: m aD d where a D mutation rate, m D the number of specific mutations which have occurred within the population, and d D the number of individual division cycles within the population. In a SYNCHRONOUS CULTURE, d may be regarded as (n
n0 ) where n is the final number of cells, and n0 is the initial number; thus (n
n0 ) indicates the number of newly formed chromosomes (assuming one per cell) – i.e., the number of genomes in which a mutation could occur. However, in a normal, non-synchronous culture (n
n0 ) will not indicate the number of newly formed chromosomes because chromosome replication will have occurred in some cells which have not divided at the time of counting; (n
n0 ) therefore gives an underestimate of the new genomes, and a statistical correction is applied to the expression for mutation rate, which becomes: aD m ð 0.69 n
n0 Examples of some measured mutation rates: resistance to phage T1 in Escherichia coli 3 ð 10
8 ; loss of ability to ferment galactose (E. coli ) 10
10 . (A value such as 5 ð 10
8 means that, on average, one mutant is formed for every 2 ð 107 division cycles within the population.) For two independent mutations having rates of, say, 10
6 and 10
8 , the mutation rate for a double mutant (i.e., a cell having both mutations) would be 10
14 . The actual (observed) mutation rate in respect of a given phenotypic characteristic may be influenced e.g. by the presence of a TRANSPOSABLE ELEMENT. (See also MUTATOR GENE.) mutator gene (mutator) Any gene (designated mut) within which certain mutations cause an increase in the rate of SPONTANEOUS MUTATION in other genes. For example, in Escherichia coli, mutations in the gene encoding the e subunit of DNA POLYMERASE III (dnaQ and mutD alleles) can result in extremely high levels of spontaneous mutation; mutant alleles may differ from the wildtype allele e.g. by only one or two single base substitutions, leading to one or two amino acid changes in the e subunit [MGG (1986) 205 9–13]. The mutation(s) may lead to reduced 500 Mycobacterium Mycelia Sterilia See AGONOMYCETALES. mycelium A group or mass of discrete hyphae (see HYPHA): the form of the vegetative thallus in many types of fungi and in certain bacteria (see ACTINOMYCETALES). (See also AERIAL MYCELIUM, SPROUT MYCELIUM; SUBSTRATE MYCELIUM; cf. PLECTENCHYMA.) Mycena See AGARICALES (Tricholomataceae) and BIOLUMINESCENCE. mycetangium (mycangium) In certain insects: a specialized region within which symbiotic fungi are carried; see e.g. AMBROSIA FUNGI and WOODWASP FUNGI. mycetism (mycetismus) Poisoning due to the ingestion of certain mushrooms (MUSHROOM sense 1) – e.g. the poisonous species of Amanita or Cortinarius. (cf. MYCOSIS, MYCOTOXICOSIS; see also e.g. AMATOXINS, MUSCARINE and PHALLOTOXINS.) Mycetism can occur in e.g. sheep and cattle as well as in humans; thus, e.g. the toxins in Cortinarius speciosissimus are known to cause renal failure in both humans and sheep. (See also ORELLANIN POISONING.) mycetismus Syn. MYCETISM. mycetocyte In certain invertebrates, particularly insects: a specialized cell which contains intracellular bacterial or fungal symbionts; if the endosymbiont is a bacterium the term BACTERIOCYTE may be used – although ‘mycetocyte’ is often used regardless of the nature of the endosymbiont. Mycetocytes may be irregularly distributed in certain tissues (e.g. the gut lining) or they may be aggregated into specialized organelles (mycetomes) which are usually associated with the gut. In at least some cases the microflora of the mycetome supplies essential nutrients to the insect host. (See also MYCETANGIUM and TROPHOSOME.) [Molecular biology of symbiotic bacteria in aphid mycetocytes: MS (1986) 3 117–120.] mycetoma (1) Syn. MADUROMYCOSIS. (2) (fungus ball) A tumourlike mycelial mass formed in the tissues in certain mycoses (see e.g. ASPERGILLOSIS and COCCIDIOIDOMYCOSIS). mycetome See MYCETOCYTE. mycetophagous Syn. MYCOPHAGOUS. Mycetozoa A subphylum (phylum GYMNOMYXA) comprising two classes: Eumycetozoa (see EUMYCETOZOEA) and Acrasea (see ACRASIOMYCETES). Mycoacia See APHYLLOPHORALES (Corticiaceae). mycobacteriophage Any BACTERIOPHAGE which can infect one or more Mycobacterium spp. Most mycobacteriophages have a hexagonal head and a non-contractile tail (contractile in I3), and many are readily inactivated by organic solvents. Mycobacteriophages include both temperate and virulent types; in certain cases phage progeny may be released from the living host cell. [Book ref. 54, pp. 326–342.] Mycobacterium A genus of Gram-positive, aerobic to microaerophilic, non-motile, asporogenous bacteria (order ACTINOMYCETALES, wall type IV) that are acid-fast during at least some stage of growth. Cells: typically straight or curved rods, ca. 0.2–0.8 ð 1–10 µm, but may occur as coccoid forms, branched rods or fragile filaments; some strains are capsulated (see also MYCOSIDE C). Individual cells may stain uniformly or may exhibit banding or beading. The CELL WALL contains MYCOLIC ACIDS (see also CORD FACTOR, WAX D and PEPTIDOGLYCAN). Some strains form CAROTENOID pigments: see PHOTOCHROMOGEN and SCOTOCHROMOGEN. Species of Mycobacterium occur in soil as free-living saprotrophs, in water [review: JAB (1984) 57 193–211], on plants, and as parasites and pathogens of man and other animals (including fish) (see e.g. BURULI ULCER, JOHNE’S DISEASE, LEPROSY, SCROFULA and TUBERCULOSIS). Metabolism is respiratory and, typically, chemoorganotrophic – although chemolithotrophic strains of e.g. M. marinum and M. smegmatis have been reported. Nutritionally, mycobacteria are generally not fastidious; sources of carbon and nitrogen include e.g. sugars, hydrocarbons and amino acids. In a number of species glycerol and asparagine are preferred sources of C and N, respectively. Growth may be stimulated e.g. by serum or egg-yolk (see also LÖWENSTEIN–JENSEN MEDIUM) or by an increase in the partial pressure of carbon dioxide; in members of the Mycobacterium tuberculosis complex growth is enhanced by pyruvate or (in some species) by glycerol. [Nutrition/metabolism in mycobacteria: Book ref. 54, pp 185–271.] In ‘slow-growing’ (SG) strains, visible growth on solid media may not appear for ¾4–6 weeks (up to ¾12 weeks in M. malmoense). In ‘rapidly growing’ (RG) strains, visible growth on solid media appears within 1 week, often 4–6 days. Note that in BACTEC (liquid) media growth can be detected more rapidly than it can on solid media. Tests used in the identification of mycobacteria include e.g. the ARYLSULPHATASE TEST, catalase test (that is, persistence of CATALASE activity after incubation at 68° C/20 minutes in neutral phosphate buffer [Book ref. 53, p 1707]), NIACIN TEST, NITRATE REDUCTION TEST, T2H TEST and TWEEN HYDROLYSIS. There are also various molecular methods that are used for detecting, identifying, characterizing and typing certain species: see e.g. PROBE, SPOLIGOTYPING and TMA. The genus comprises ¾50 species. Medically important species are found in the following groups: (i) the M. tuberculosis complex (M. tuberculosis, M. bovis, BCG, M. africanum and M. microti ) – all species associated with tuberculosis; (ii) the M. avium complex (M. avium, M. intracellulare and M. scrofulaceum) – see MAC; (iii) M. leprae; (iv) ‘nontuberculous mycobacteria’, or ‘mycobacteria other than tuberculosis’: see MOTT. There is a growing number of infrequently isolated mycobacteria – some with actual or potential ability to cause human disease [RMM (1997) 8 125–135]. GC%: ca. 62–70. Type species: M. tuberculosis. The genus Mycobacterium includes the following species: M. africanum. SG; microaerophilic. Similar to M. bovis, but results are variable in the niacin and nitrate reduction tests. Sensitive to PYRAZINAMIDE. Can cause human tuberculosis; found primarily in Africa. M. avium. SG; non-pigmented. Typical tests: arylsulphatase
ve; catalase (68° C) variable; growth at 25–42° C Cve; niacin
ve; nitrate reduction
ve; T2H test Cve; Tween hydrolysis
ve. Pathogenic for birds. M. avium, M. intracellulare and M. scrofulaceum form the ‘M. avium complex’; these species are associated with various human disease syndromes: see MAC. M. bovis (‘M. tuberculosis var. bovis’). SG; microaerophilic. Non-pigmented. Typical reactions: catalase (68° C)
ve; growth at 42° C
ve; growth enhancement by pyruvate Cve; niacin
ve; reduction of nitrate
ve; T2H test
ve; Tween hydrolysis variable. Resistant to pyrazinamide. Most strains of M. bovis are reported to contain only a single copy of the insertion sequence IS6110. M. bovis is a causal agent of TUBERCULOSIS in both animals and man. Differentiation between M. bovis and M. tuberculosis may be facilitated by SPOLIGOTYPING. M. bovis BCG is a strain of M. bovis which differs e.g. in that growth occurs aerobically. (See also BCG.) 501 Mycobacterium M. branderi. SG; non-pigmented/scotochromogenic. Typical tests: arylsulphatase Cve; niacin
ve; reduction of nitrate weak; Tween hydrolysis
ve; growth 25–45° C Cve. [Original description of species: IJSB (1995) 45 549–553.] M. branderi has been isolated from the respiratory tract on a number of occasions and may be a causal agent of disease. M. celatum. SG; non-pigmented. Typical tests: catalase (68° C) Cve; arylsulphatase variable; niacin
ve; reduction of nitrate
ve; Tween hydrolysis
ve; T2H test Cve. [Original description of species: IJSB (1993) 43 539–548.] Strains of M. celatum have been classified into three types (I–III) on the basis of RFLP analysis. Type I strains may give false-positive results with a commercial probe for M. tuberculosis [JCM (1994) 32 536–538]. M. celatum has been isolated from a range of infections, some in patients with AIDS [e.g. Lancet (1994) 344 332; Lancet (1994) 344 1020–1021]. M. chelonei (D M. chelonae). RG; non-pigmented. Similar to M. fortuitum but e.g. does not grow at 42° C (optimum growth temperature ¾25° C) and does not reduce nitrate. M. chelonei has been isolated e.g. from infected wounds. M. farcinogenes. RG. Resembles M. fortuitum (on the basis of DNA homology studies). A causal agent of bovine FARCY. M. flavum. See XANTHOBACTER. M. fortuitum. RG; non-pigmented. Typical tests: arylsulphatase Cve; growth at 42° C Cve; Tween hydrolysis variable; reduction of nitrate Cve. An opportunist pathogen. [DNA relatedness study of the M. fortuitum–M. chelonae complex: IJSB (1986) 36 458–460.] M. gordonae (previously called M. aquae). SG; scotochromogen. The organism occurs in water, and is an occasional cause of pulmonary disease; it is not susceptible to isoniazid or pyrazinamide but is sensitive to some other antimycobacterial drugs. A DNA probe (for identifying isolates) is available commercially. M. haemophilum. SG; non-pigmented. Requires haemin for growth. Growth occurs at 30° C, not at 37° C. Causes disease in the immunosuppressed and in healthy children. [Clinical and laboratory aspects of M. haemophilum infections: RMM (1998) 9 49–54.] M. interjectum. SG; scotochromogen. Typical tests: catalase (68° C) Cve; arylsulphatase variable; niacin
ve; reduction of nitrate
ve; Tween hydrolysis variable; growth at 31–37° C Cve; T2H test Cve. [Original description of species: JCM (1993) 31 3083–3089.] M. interjectum was first isolated from a child with chronic submandibular lymphadenitis (and a positive TUBERCULIN TEST). Partial resection of the lymph node and treatment with antituberculosis drugs failed to resolve the infection; subsequent total resection of the node and treatment with isoniazid, clarithromycin and protionamide was successful. M. interjectum has been associated with cervical lymphadenitis in a child [JCM (2001) 39 725–727]. M. intracellulare. SG; non-pigmented. Similar to M. avium but typically arylsulphatase Cve, catalase (68° C) Cve. Tween hydrolysis
ve. Growth occurs at 25–40° C (some strains grow at 40–45° C). (See also MAC.) M. kansasii. SG; usually photochromogenic. Typical tests: arylsulphatase Cve; catalase (68° C) Cve; growth at 25–40° C Cve (some strains grow at 42° C); niacin
ve; reduction of nitrate Cve; T2H test Cve; Tween hydrolysis Cve. M. kansasii can cause human disease similar to pulmonary tuberculosis; moreover, M. kansasii can cross-react with M. tuberculosis in the AMTDT [JCM (1999) 37 175–178]. However, chemotherapy against M. kansasii differs from that used against M. tuberculosis (e.g. M. kansasii is not susceptible to pyrazinamide); consequently, early identification of the pathogen is essential to ensure correct treatment. [Identification of M. kansasii isolates with a DNA probe: JCM (1999) 37 964–970.] M. leprae. SG. The causal agent of LEPROSY. This species can be cultured in the footpads of mice but has not been cultured in cell-free laboratory media. [Review: Book ref. 54, pp 273–307. Various aspects: Ann. Mic. (1982) 133B 5–171. Carbon metabolism in M. leprae: JGM (1983) 129 1481–1495.] An examination of the genome sequence of M. leprae has revealed massive gene decay, with less than 50% of the genome containing protein-encoding genes [Nature (2001) 409 1007–1011]. M. lepraemurium. SG; non-pigmented. The causal agent of murine leprosy. Limited growth has been reported to occur on egg-yolk media when very large inocula are used. M. malmoense. SG; growth on egg media at 37° C may take up to 12 weeks. This organism can cause disease similar to that associated with members of the M. avium complex (see MAC). To facilitate detection/identification of M. malmoense a PCR-based method has been devised and tested successfully [JCM (1999) 37 1454–1458]. M. marinum. SG; photochromogenic. Typical tests: arylsulphatase Cve; catalase (68° C) Cve; growth at 30° C Cve; growth at 37° C
ve on primary isolation but Cve after serial subculture; niacin
ve; reduction of nitrate
ve; Tween hydrolysis Cve. This species causes disease in fish and skin granulomas/ulcers in humans [Arch. Derm. (1986) 122 698–703]; it is more common in temperate regions than in tropical regions (cf. M. ulcerans). M. microti. SG; aerobic. Non-pigmented. Similar to M. tuberculosis but typically gives a variable nitrate reduction test and a negative T2H test. Causal agent of tuberculosis in voles. M. paratuberculosis (‘M. johnei ’). SG; non-pigmented. Typically (results from few strains): catalase (68° C) Cve; niacin
ve; reduction of nitrate
ve; T2H test Cve; Tween hydrolysis variable. (See also IS900 and MYCOBACTINS.) M. paratuberculosis is the causal agent of JOHNE’S DISEASE. (See also CROHN’S DISEASE.) (See also PULSED ELECTRIC FIELD.) M. phlei. RG; scotochromogenic. Typical tests: arylsulphatase
ve (at 3 days) or variable (at 1 week); growth at 52° C Cve; Tween hydrolysis Cve. M. phlei is found in soil and on vegetation; it has also been isolated from (i) synovial fluid in a case of Reiter’s syndrome, and (ii) a foot wound, and is regarded as an opportunist pathogen. M. scrofulaceum. SG; scotochromogenic. Typical test results: arylsulphatase
ve; catalase (68° C) Cve; growth at 25–42° C Cve; niacin
ve; reduction of nitrate
ve; T2H test Cve; Tween hydrolysis
ve. A causal agent of SCROFULA. (See also MAC.) M. senegalense. RG; scotochromogen. A causal agent of bovine FARCY. M. simiae. SG; some strains photochromogenic, others nonchromogenic. Generally similar to M. scrofulaceum but the niacin test is Cve in some strains. M. smegmatis. RG; non-pigmented. Typical test results: arylsulphatase Cve (at 1 week); growth at 45° C Cve; Tween hydrolysis Cve. Found in smegma; believed to be non-pathogenic. M. terrae. SG; non-chromogenic. With M. nonchromogenicum and M. triviale, forms the ‘Terrae group’ or M. terrae complex – species which are pathogenic but apparently rare. M. thermoresistibile. RG; scotochromogen. Typical test results: arylsulphatase
ve; niacin
ve; reduction of nitrate 502 Mycoplasma
ve; growth at 52° C Cve; Tween hydrolysis Cve; T2H test Cve. Found e.g. in soil. Reported to be pathogenic in lung (several cases); skin and lymph nodes; and breast tissue [JCM (1992) 30 1036–1038]. M. triplex. SG; non-pigmented. Isolated from various clinical specimens. [Description of species: JCM (1996) 34 2963–2967.] [M. triplex associated with infection in a liver transplant patient: JCM (2001) 39 2033–2034.] M. tuberculosis. SG; aerobic. Non-pigmented. Typical test results: catalase (68° C)
ve; growth at 42° C
ve; growth at 37° C Cve; growth enhanced by glycerol and by pyruvate; niacin Cve; reduction of nitrate Cve; Tween hydrolysis variable; sensitive to pyrazinamide. Usually forms rough, raised, whitish/pale buff colonies (‘eugonic’ growth). A casual agent of TUBERCULOSIS. [Genome sequence of M. tuberculosis, strain H37rv: Nature (1998) 393 537–544.] The insertion sequence IS6110 (q.v.) is frequently used for TYPING isolates of M. tuberculosis. M. ulcerans. SG; pigmentation variable. Typical test results: catalase (68° C) Cve; growth at 30° C Cve; growth at 37° C
ve; niacin variable; reduction of nitrate
ve; Tween hydrolysis
ve. This species is found in tropical regions e.g. on vegetation; it causes BURULI ULCER. M. xenopi. SG; pigmentation variable. Found in water, and able to cause lesions in the (human) lung. Does not grow at 25° C, grows poorly at 37° C, and has an optimum growth temperature of ca. 42–45° C. mycobactins A family of complex, lipophilic compounds which occur in the cell envelope in most species of Mycobacterium (not in M. paratuberculosis or in some strains of M. avium); they chelate trivalent metal ions, particularly solubilized ferric ions, and are believed to function in iron transport – iron being released after enzymic reduction to the ferrous form. For in vitro growth M. paratuberculosis needs mycobactin or e.g. ferric ammonium citrate. [Structure of mycobactins: Book ref. 54, pp. 242–245.] (See also EXOCHELINS and SIDEROPHORES.) Related compounds occur in Nocardia. mycobiont A fungal symbiont – e.g. in a LICHEN or MYCORRHIZA. Mycobionta Syn. EUMYCOTA. Mycocalia See NIDULARIALES. Mycocaliciaceae See CALICIALES. mycocecidia GALLS induced by fungi. Mycocentrospora See HYPHOMYCETES; see also CROWN ROT. mycochrome See PHOTOINDUCTION and PHOTOINHIBITION. mycodextran Syn. NIGERAN. Mycogone See HYPHOMYCETES; see also BUBBLE DISEASES. mycoherbicide See BIOLOGICAL CONTROL. mycolic acids a-Substituted, b-hydroxylated fatty acids (having the general formula R0 CHOH.CHR00 .COOH), esters of which are found in the cell walls of e.g. species of Corynebacterium, Mycobacterium, Nocardia, and Rhodococcus; in Mycobacterium spp the mycolic acids fall within the approximate range C60 –C90 , in Nocardia C40 –C60 , and in Corynebacterium C20 –C40 . In mycobacterial mycolic acids R0 is usually a C50 –C60 chain which often includes double bonds, cyclopropane rings etc, while R00 is a C22 –C24 chain. [Structure and biosynthesis: Book ref. 54, pp. 113–128. Mycolic acid patterns in various strains of Mycobacterium: JGM (1983) 129 889–891; (1984) 130 363–367, 2733–2736.] (See also WAX D.) [Mycolic acids in Corynebacterium spp: JGM (1984) 130 513–519.] mycology The study of FUNGI. mycoparasite A fungus which is parasitic on other fungi. Mycoparasites include e.g. Christiansenia pallida [life history: Mycol. (1984) 76 9–22]; PIPTOCEPHALIS; and Rozella spp: endobiotic and holocarpic organisms which parasitize e.g. Polyphagus euglenae (itself a parasite of Euglena spp) [Mycol. (1984) 76 1039–1048] and other fungi and algae. Certain mycoparasites can apparently exert some control on pathogens of higher plants – e.g., in cases of CLOVER ROT, Trichoderma viride (or e.g. Mitrula sclerotiorum) can bring about a reduction of the numbers of sclerotia in the soil [Bot. Rev. (1984) 50 491–504]. [Susceptibility of e.g. Pythium spp to the mycoparasite Pythium oligandrum: SBB (1986) 18 91–96.] (See also CONTACT BIOTROPHIC MYCOPARASITE.) mycophagous (mycetophagous) Fungus-eating. mycophenolic acid An ANTIBIOTIC produced e.g. by Penicillium brevicompactum in aerial hyphae formed on solid media [AEM (1981) 41 729–736]. It has antimicrobial and antitumour activity, blocking GMP synthesis by inhibiting the formation of XMP from IMP (see Appendix V(a)). Mycoplana A genus of Gram-negative, aerobic bacteria of uncertain taxonomic affinity; species occur e.g. in soil. The organisms form branching filaments which fragment into irregular, flagellated rods. Some strains can fix nitrogen under microaerobic conditions [JGM (1982) 128 2073–2080]. GC%: ca. 64–69. [Book ref. 46, pp. 2118–2119.] mycoplasma (1) A member of the class MOLLICUTES. (cf. MOLLICUTE.) (2) A member of the genus MYCOPLASMA. Mycoplasma A genus of cell wall-less, sterol-requiring, catalasenegative bacteria (family MYCOPLASMATACEAE) which occur as parasites and pathogens e.g. in the respiratory and urogenital tracts in man and other animals; diseases caused by, or associated with, Mycoplasma spp include e.g. AIR SACCULITIS, BRONCHITIS, CONTAGIOUS BOVINE PLEUROPNEUMONIA, GLASSER’S DISEASE, NONGONOCOCCAL URETHRITIS, ovine MASTITIS, and PRIMARY ATYPICAL PNEUMONIA (sense 2). (Mycoplasma spp are also common contaminants in TISSUE CULTURES.) Cells: typically non-motile (but see below) and pleomorphic, ranging from spherical, ovoid or pear-shaped (ca. 0.3–0.8 µm diam.) to branched filamentous forms of near-uniform diameter, several µm to ca. 150 µm in length; filaments, the typical forms in young cultures under optimum conditions, subsequently transform into chains of coccoid cells which later break up into individual cells that are capable of passing through membrane filters of pore size 0.22 µm or 0.45 µm. The cells of some species have a ‘tip’ structure (possibly part of a microfibrillar ‘cytoskeleton’) which has been thought to be involved in attachment to host cells, and which (in motile species) appears to have a role in GLIDING MOTILITY – the tip always pointing in the direction of motion. The attachment organelle and cytoadherence proteins of M. pneumoniae have been detected by immunofluorescence microscopy [JB (2001) 183 1621–1630]. The trilaminar cytoplasmic membrane contains sterols (in addition to e.g. phospholipids and proteins) – thus rendering the cells susceptible to POLYENE ANTIBIOTICS and to lysis by e.g. digitonin (which complexes sterols). Some species bear a capsule or slime layer – that in M. mycoides subsp mycoides being a galactan. Replication of the genome may precede cytoplasmic division; hence, ‘multinucleate’ filaments may exist for a time before individual cells are delimited by constriction. Budding can also occur. Most Mycoplasma spp are facultatively anaerobic, some apparently being obligately anaerobic on primary isolation. All species are chemoorganotrophic. ‘Fermentative’ species can use 503 mycoplasma virus type 1 phages Mycoplasmatales An order of cell wall-less bacteria of the class MOLLICUTES; it comprises three families: MYCOPLASMATACEAE (non-helical cells which require sterols for growth), ACHOLEPLASMATACEAE (non-helical cells which do not require sterols), and SPIROPLASMATACEAE (cells often helical; sterols required for growth). [Book ref. 22, pp. 741–787.] mycoplasmaviruses BACTERIOPHAGES which infect members of the MYCOPLASMATALES: see MV-L3 PHAGE GROUP, PLASMAVIRIDAE, PLECTROVIRUS, SPIROPLASMAVIRUSES. [Review: Intervirol. (1982) 18 177–188.] mycoplasmosis Any disease caused by a species of MYCOPLASMA (q.v.) mycorrhiza A stable, usually mutualistic association between a fungus and the root (or rhizoid) of a plant. Mycorrhizas occur in the majority of plants, including vascular and some nonvascular species (e.g. liverworts). The fungi involved (e.g. basidiomycetes, ascomycetes, deuteromycetes) are always associated with the primary cortex of the root, and many appear never to occur as free-living saprotrophs. The formation of mycorrhizas leads to improved uptake of nutrients by the host plant; nutrients are apparently absorbed by hyphae (which may extend some distance from the root) and are transported back to the root to be released into the host tissue. Mycorrhiza formation and efficacy is greatest in nutrient-poor soils, and may be reduced or eliminated by application of soil fertilizers. Three major types of mycorrhiza are recognized. Ectomycorrhizas (‘ectotrophic mycorrhizas’) occur mainly in temperate forest trees; the fungi involved include basidiomycetes (e.g. agarics, boletes), ascomycetes (e.g. Tuber spp) and zygomycetes (Endogone). A given tree may associate with more than one species of fungus. In an ectomycorrhiza the fungal hyphae occur on the root surface and may penetrate between the cortical cells of the root, but the cortical cells themselves are not penetrated. Typically, the host root becomes completely enclosed by a sheath of pseudoparenchymal fungal tissue (the mantle); hyphae from the mantle may penetrate the soil surrounding the root and also penetrate between the cortical cells of the root to enmesh individual cortical cells in a network of hyphae (the Hartig net). The root is morphologically distinct from an uninfected root: e.g., it lacks root hairs and a root cap; it is thicker than an uninfected root and may be a different colour; it may branch extensively and characteristically – e.g. pinnately (in Fagus spp) or dichotomously (in Pinus spp) – or not at all (e.g. in Quercus spp). In certain cases an ectomycorrhiza may develop in the form of nodules (D tubercles), each consisting of a rounded, dense mass of mycorrhizal roots. Ectomycorrhizal fungi appear to have only limited (or no) ability to use complex carbohydrates (e.g. cellulose); they obtain simple sugars (e.g. glucose, fructose, sucrose) from the plant and store them (e.g. as mannitol, trehalose or glycogen) in the mantle. Benefits to the plant, in addition to improved uptake of nutrients (particularly phosphate), include increased protection from certain pathogenus – e.g., the fungus may produce antibiotic(s), or the mantle may function as a mechanical barrier to infection. Many ectomycorrhizal fungi produce PHYTOHORMONES, but the significance of this to the plant is unknown. In certain cases a mycorrhizal association appears to be essential for the normal development of the plant (e.g. in certain Pinus spp). Endomycorrhizas (‘endotrophic mycorrhizas’) involve the development of the fungus within the cells of the root cortex; there is usually little or no change in root morphology, and an external fungal sheath is usually not formed. Typically, the fungal hyphae penetrate the cortical cells of the root and develop sugars such as glucose (metabolized to e.g. lactic acid via the EMP pathway), while ‘non-fermentative’ species can use e.g. arginine. All species need cholesterol or related sterols (e.g. cholestanol or stigmasterol). The organisms have a flavinterminated electron transport chain which lacks both quinones and cytochromes. NADH oxidase occurs in the cytoplasm (cf. ACHOLEPLASMA). Growth occurs on complex media (e.g. HAYFLICK MEDIUM); fastidious mycoplasmas may be grown on diphasic SP-4 medium [recipe: Book ref. 22, p. 746]. Colonies (usually <1 mm diam.) are typically of the ‘fried egg’ type: an opaque, granular central region, embedded in the agar, surrounded by non-granular surface growth. Optimum growth temperature of mammalian strains: 36–37° C. Many species produce weak or clear haemolysis; haemolysis appears to be due to the secretion of H2 O2 (a product which is believed to account for some aspects of pathogenicity). Mycoplasmas are commonly sensitive to chloramphenicol and to tetracyclines; most species can tolerate 1:2000/4000 thallous acetate. Broth cultures of Mycoplasma spp (supplemented with DMSO or glycerol) can be stored at
70° C; alternatively, broth cultures may be lyophilized. GC%: ca. 23–40. Type species: M. mycoides. The genus currently contains over 60 species which are differentiated on the basis of certain tests: e.g., utilization of glucose and mannose, arginine hydrolysis, phosphatase production, the FILM AND SPOTS reaction, and haemadsorption. M. glycophilum. An avian species [JGM (1984) 130 597–603]. M. laidlawii. Re-classified as Acholeplasma laidlawii. M. mycoides. Non-motile cells which often form repeatedly branching filaments. Under certain conditions a culture may contain cells called rho-forms; a rho-form contains an intracellular organelle (function unknown) which consists essentially of an axial fibre (ca. 40–120 nm diam.) extending the length of the cell and occupying a major part of the cell’s volume. M. mycoides subsp mycoides causes contagious bovine pleuropneumonia. M. pneumoniae (Eaton’s agent). A slowly-growing species which causes a primary atypical pneumonia in man. On primary isolation, the colonies (after 5–10 days’ incubation) are ca. 50–100 µm in diameter and are entirely granular, i.e., they are not typical ‘fried egg’ colonies; fried egg colonies generally develop on subculture. The organisms are generally highly sensitive to erythromycin. M. pneumoniae respiratory tract infection is commonly investigated serologically by: (i) a complement-fixation test (CFT) that measures titres of anti-Mycoplasma antibodies, or (ii) a more rapid (and more sensitive) immunofluorescence assay for specific IgM antibodies. Following comparison of serology, culture and a PCR-based assay in the diagnosis of M. pneumoniae respiratory tract infection in children, it was considered appropriate to use PCR for rapid diagnosis and to confirm/reject any PCR-negative specimens on the basis of CFT examination of paired sera [JCM (1999) 37 14–17]. T-strain mycoplasmas. See UREAPLASMA. [Book ref. 22, pp. 742–770. Mycoplasma characterization: Book ref. 98. Mycoplasma evolutionary tree from 5S rRNA sequencing data: PNAS (1985) 82 1160–1164.] (See also MYCOPLASMAVIRUSES.) mycoplasma virus type 1 phages Syn. PLECTROVIRUS. mycoplasma virus type 2 phages Syn. PLASMAVIRIDAE. Mycoplasmataceae A family of non-helical, sterol-requiring, cell wall-less bacteria of the order MYCOPLASMATALES. Two genera: MYCOPLASMA (urease-negative) and UREAPLASMA (ureasepositive). 504 mycovirus HISTOPLASMOSIS; LOBOMYCOSIS; MADUROMYCOSIS; ONYCHOMYCOSIS; PARACOCCIDIOIDOMYCOSIS; PHAEOHYPHOMYCOSIS; PITYRIASIS NIGRA; PITYRIASIS VERSICOLOR; RHINOSPORIDIOSIS; RINGWORM; SPOROTRICHOSIS; WHITE PIEDRA; ZYGOMYCOSIS. intracellularly; subsequently, the hyphae are digested by the root cell, leaving a knot of undigested hyphal wall material in the cell. As the root grows, the fungus invades new cells behind the root meristem; thus, a balance is set up between fungal invasion of the plant and plant digestion of the fungus. There are three main types of endomycorrhiza. (a) The vesicular-arbuscular (VA) type, found in a very wide range of plants, in which the (aseptate) fungal hyphae spread through the primary cortex of unsuberized roots and penetrate the cortical cells. A characteristic, much-divided haustorium (the arbuscule) is formed, and both intracellular and extracellular hyphae usually develop spherical, lipid-rich, intercalary or terminal swellings (vesicles). VA mycorrhizal fungi belong to the genera Acaulospora, Gigaspora, Glomus and Sclerocystis (all formerly Endogone spp); they have not yet been grown in pure culture. VA mycorrhizas improve uptake of nutrients (particularly phosphate) by the host plant. (b) The ericoid type, found in members of the Ericaceae, in which the fungi colonize the fine terminal roots of the host plant and form coils or loops within the host cells. The fungi involved all belong to, or are closely related to, the species Pezizella ericae. (c) The orchid type, found in embryos and roots in members of the Orchidaceae, in which the fungi penetrate the host cells and form intracellular hyphal coils. All known orchid mycorrhizal fungi are also normal soil saprotrophs or parasites of other plants; they are usually basidiomycetes (e.g. Armillaria, Ceratobasidium, Marasmius, Thanatephorus, Tulasnella). Mycorrhizal associations appear to be essential to the orchid – at least for germination and seedling growth; orchid seeds are very small and have little or no food reserve, so that (under natural conditions) nutrients must be supplied by an invading fungus for successful germination to occur. Saprotrophic orchids may depend on mycorrhizal fungi throughout their lives, associating with fungi (e.g. Armillaria mellea) which can degrade substrates such as cellulose and pectin to simple compounds which the orchid can assimilate. Green orchids generally associate with Rhizoctonia (Thanatephorus) spp, but may lose their mycorrhizal fungi when mature. Ectendomycorrhizas (‘ectendotrophic mycorrhizas’) are intermediate in form between ecto- and endomycorrhizas; an organized fungal sheath is formed, and inter- and intracellular penetration of the root cortex occurs. This type of mycorrhiza is formed by only a limited number of plants, including certain members of the Ericaceae – e.g. Monotropa (a genus of achlorophyllous herbaceous plants) and Arbutus – and the seedlings of certain conifers. The fungi involved are apparently basidiomycetes; some fungi which form ectendomycorrhizas in plants of the Arbutae and Monotropaceae may form ectomycorrhizas in other plants. [Host–fungus interactions: Book ref. 55, pp. 225–253. General: Book ref. 56. A check list of British mycorrhizas: New Phyt. (1987) 105 (Supplement, 1–102).] mycosamine An aminosugar (3-amino-3,6-dideoxy-D-mannose) found e.g. in NYSTATIN and AMPHOTERICIN B. mycoside C A peptidoglycolipid, distinct from WAX D, formed by some species of Mycobacterium (e.g. M. avium); in M. lepraemurium mycoside C occurs as a fibrillar or crystalline capsule. [Book ref. 54, pp. 28–30.] mycosin Syn. CHITOSAN. mycosis Any human or animal disease resulting from infection with a fungus (cf. MYCOTOXICOSIS and MYCETISM). See e.g. ADIASPIROMYCOSIS; ASPERGILLOSIS, BLACK PIEDRA; BLASTOMYCOSIS, CANDIDIASIS; CHROMOBLASTOMYCOSIS; COCCIDIOIDOMYCOSIS; mycosis fungoides A chronic human cutaneous T-cell lymphoma. HTLV-I provirus has been detected in a very small proportion of cases, but HTLV-I is not thought to be an aetiological agent. (cf. ADULT T-CELL LEUKAEMIA.) Mycosphaerella A genus of fungi (order DOTHIDEALES) which include saprotrophs and plant parasites (see e.g. BANANA LEAF SPOT). (See also CLADOSPORIUM.) mycotête See TERMITE–MICROBE ASSOCIATIONS. mycotic dermatitis Syn. LUMPY WOOL. mycotoxicosis Any disease of man or animals resulting from the ingestion of MYCOTOXINS (cf. MYCETISM; MYCOSIS). See e.g. ALIMENTARY TOXIC ALEUKIA; DENDRODOCHIOTOXICOSIS; ERGOTISM; LUPINOSIS, PASPALUM STAGGERS; STACHYBOTRYOTOXICOSIS; YELLOW RICE. mycotoxin A TOXIN produced by a fungus. The term is usually reserved for fungal metabolites that are toxic to man and/or animals and are produced by moulds growing on foodstuffs (cf. MYCETISM). See e.g. AFLATOXINS; ASTELTOXIN; AUROVERTINS; CHLOROPEPTIDE; CITREOVIRIDIN; CITRININ; CYCLOPIAZONIC ACID; ERGOCHROMES; ERGOT ALKALOIDS; OCHRATOXINS; PATULIN; PENITREMS; RUBRATOXINS; SLAFRAMINE; SPORIDESMINS; STERIGMATOCYSTIN; TRICHOTHECENES; XANTHOMEGNIN; ZEARALENONE. (cf. IPOMEAMARONE and 4-IPOMEANOL.) mycotrophein See CONTACT BIOTROPHIC MYCOPARASITE. mycotrophic (1) (of plants) Refers to the plant partner in a MYCORRHIZA. (2) (of animals) Refers to an animal (e.g. an invertebrate) that feeds on fungi. Mycotypha See MUCORALES. mycovirus Any VIRUS which can infect one or more fungi. Mycoviruses have been observed in a wide range of fungi; the majority are isometric (diam. ca. 25–50 nm) dsRNA viruses either with monopartite genomes or with segmented genomes, each segment being encapsidated in a separate particle. Many of the dsRNA virions have been shown to contain an RNADEPENDENT RNA POLYMERASE. Six groups of isometric dsRNA mycoviruses have been defined [Intervirol. (1984) 22 17–23]. Group A: genome monopartite (4.8–6.1 kbp), virion diam. ca. 35–43 nm; members: Saccharomyces cerevisiae viruses ScV-L1 (D ScV-LA) and ScV-L2; probable members: Ustilago maydis viruses UmV-P1-H1, UmV-P4-H1 and UmV-P6-H1. Group B : genome monopartite (8.3 kbp), virion diam. ca. 48 nm; member: Drechslera (formerly Helminthosporium) maydis virus (HmV). Group C : genome bipartite (1.6 and 1.4 kbp), virion diam. ca. 30–35 nm; member: Penicillium stoloniferum virus S (PsV-S); probable member: Diplocarpon rosae virus (DrV). Group D (Gaeumannomyces graminis virus group 1): genome bipartite (1.8–1.9 and 1.7–1.8 kbp), virion diam. ca. 35 nm; members: G. graminis viruses GgV-019/6-A, GgV-OgA-B and GgVF6-C. Group E (Gaeumannomyces graminis virus group 2): genome bipartite (2.2–2.3 and 2.0–2.1 kbp), virion diam. ca. 35 nm; members: G. graminis viruses GgV-F6-B, GgV-T1-A and GgV-OgA-A. Group F : genome tripartite (ca. 3.2, 3.0 and 2.9 kbp), virion diam. ca. 35–40 nm; members: Penicillium chrysogenum virus (PcV), P. brevicompactum virus (PbV) and ‘P. cyaneofulvum’ (D P. chrysogenum) virus (Pc-fV). (Pc-fV actually has four dsRNA segments, the 4th probably being a variant, satellite or defective dsRNA.) Two families, Totiviridae and Partitiviridae, have been created to accommodate isometric dsRNA viruses with, respectively, monopartite and bipartite genomes [Intervirol. (1986) 25 141–143]. 505 mycovore Most dsRNA mycoviruses appear to have a single-layered capsid composed of a single polypeptide species. However, a dsRNA virus from Lentinula edodes appears to have a doubleshelled capsid [Virol. (1982) 123 93–101] and to resemble a PHYTOREOVIRUS. Other mycoviruses include e.g. ‘Boletus virus’ (a possible member of the POTEXVIRUSES) and mushroom virus 3 (virions bacilliform, ca. 50 ð 19 nm); DNA-containing lytic viruses have been observed in certain lower fungi (e.g. Rhizidiomyces [Virol. (1983) 130 10–20, 21–28]). Mycoviruses are apparently transmissible only by intracellular routes (e.g. during mating, via spores, or by hyphal anastomosis). In many cases the host fungus shows little evidence of infection (cf. MUSHROOM DISEASES); in certain cases virus infection may affect the virulence of a pathogenic fungus: e.g. an isometric dsRNA virus appears to be associated with virulence in strains of Rhizoctonia solani [JGV (1985) 66 1221–1232] – cf. CHESTNUT BLIGHT and DUTCH ELM DISEASE. (See also KILLER FACTOR.) mycovore A fungus-eating animal. myelitis Inflammation of the spinal cord. myeloblastosis-associated viruses See AVIAN LEUKOSIS VIRUSES. myeloma A tumour consisting of a single clone of B LYMPHOCYTES; myelomas can occur spontaneously or they can be induced. A given myeloma secretes Ig (‘paraprotein’) molecules, all of which are structurally and electrophoretically homogeneous (monoclonal); some myeloma Igs react with environmental antigens. Myeloma cells are used e.g. for HYBRIDOMA formation. (See also BALB/C MICE.) myeloperoxidase (MPO) A haemoprotein (MWt ca. 150000) found e.g. in MACROPHAGES and in the azurophilic granules of NEUTROPHILS; MPO catalyses the oxidation of halide ions to hypohalite. [Role of MPO in oxygen-dependent microbicidal activity of human neutrophils: Blood (1983) 61 483–492.] myo-inositol See INOSITOL. myonecrosis (clostridial) See GAS GANGRENE. myoneme (ciliate protozool.) A bundle, ribbon or sheet of intracellular protein microfibrils, numbers of which occur in certain ciliates; myonemes are apparently contractile and may thus account for the observed contractility in those organisms which contain them. Myonemes occur below the infraciliature in e.g. Stentor and Spirostomum; in Stentor they are also known as M bands or M fibres. (See also SPASMONEME.) Myoneme contraction appears to be related to environmental and/or intracellular levels of calcium ions. myosin A rod-shaped protein which occurs in most types of eukaryotic cell; the molecule consists of six polypeptides: two ‘heavy chains’ and four ‘light chains’, the light chains occurring at one end of the molecule and contributing to its bulbous ‘head’. Myosin molecules can aggregate into bundles, each bundle consisting of a number of longitudinally adjacent molecules arranged so that head regions occur at each end. When contact occurs between a myosin head and an ACTIN microfilament, the head apparently undergoes a conformational change which causes the myosin molecule (or bundle) to move relative to the microfilament; moreover, when interposed between suitably aligned microfilaments, a myosin bundle can apparently cause one microfilament to move relative to the other. Continuing movement of a microfilament relative to a myosin head seems to involve a continual sequence of reversible changes in the conformation of the myosin head – the head repeatedly making and breaking contact with the microfilament, and repeatedly binding and hydrolysing ATP. Myoviridae A family of BACTERIOPHAGES which are characterized by the possession of a long, complex, contractile tail consisting of a central tube surrounded by a coaxial contractile sheath; the phage head may be isometric or elongated. Type species: T2. Other members include e.g. the T-EVEN PHAGES, BACTERIOPHAGE P1, BACTERIOPHAGE P2, and many other enterobacterial phages; Bacillus phages PBS1 (q.v.), SP3, SP8 and SP50; Clostridium phage HM3; Pseudomonas phage fW-14 (q.v.); etc. Myriangium See DOTHIDEALES. Myrica root nodule See ACTINORRHIZA. Myriodesma See PHAEOPHYTA. Myriostoma See LYCOPERDALES. Myrmecia A genus of unicellular green algae (division CHLOROPHYTA) which may occur as photobionts in certain lichens (see e.g. BAEOMYCES and LOBARIA) or as free-living organisms. The cells are spherical, and each contains a single cup-shaped chloroplast with no pyrenoid; autospores and zoospores may be formed. myrotheciotoxicosis Syn. DENDRODOCHIOTOXICOSIS. Myrothecium See HYPHOMYCETES; see also DENDRODOCHIOTOXICOSIS and TRICHOTHECENES. myxamoeba A non-flagellated amoeboid cell which occurs in the life cycles of the CELLULAR SLIME MOULDS and MYXOMYCETES. Myxidium A genus of protozoa (class MYXOSPOREA) which are coelozoic parasites of e.g. sticklebacks (M. gasterostei ), salamanders (M. serotinum) etc. The mature spore contains two polar capsules, one at each end of the spore. myxin An antibiotic produced by Lysobacter sp. myxo- A prefix derived from the Greek word for mucus or slime. myxobacter Any member of the MYXOBACTERALES. Myxobacterales An order of GLIDING BACTERIA which form fruiting bodies (see below) and which have GC values of ca. 68–72%; all species are Gram-negative. (cf. CYTOPHAGALES.) Myxobacteria occur e.g. in soil, on decaying vegetation, and on dung. Vegetative cells, generally 0.4–1.4 ð 2–15 µm according to species and growth conditions, may be blunt-ended rods (Chondromyces, Nannocystis, Polyangium) or tapered rods (Archangium, Cystobacter, Melittangium, Myxococcus, Stigmatella); they bear extracellular slime. The cell wall resembles that of other Gram-negative bacteria (see CELL WALL), although in at least one species (Myxococcus xanthus) the peptidoglycan occurs in patches rather than in a continuous layer; the myxobacteria are generally much more sensitive than enteric bacteria to hydrophobic antibiotics such as actinomycin D and novobiocin – suggesting structural differences in the outer membrane. The cells generally contain non-carotenoid pigments and membrane-bound, photoinducible CAROTENOID glycosides (e.g. myxobacton: 10 ,20 -dihydro-10 -glucosyl-4-oxotorulene, and myxobactin: 10 ,20 -dihydro-10 -glucosyl-3,4-dehydrotorulene). Studies on the motility apparatus of Myxococcus xanthus have been carried out with shock-frozen gliding cells [Microbiology (2001) 147 939–947]. Myxobacteria are obligate aerobic chemoorganotrophs. Most species release enzymes which lyse bacteria, yeasts and moulds – organisms which form the primary food source; Polyangium cellulosum is atypical in being strongly cellulolytic. Myxobacteria generally grow on solid media containing live or dead microorganisms; the colony or aggregate of cells (‘swarm’ or ‘pseudoplasmodium’) is typically thin, flat and wrinkled. Reproduction occurs by transverse binary fission. Fruiting in myxobacteria may be induced by starvation (or e.g. by the addition of cAMP); the vegetative cells aggregate, and about 80% of them lyse – possibly to provide nutrients for the remainder which become the resting cells (myxospores) from which the fruiting body is formed. [Development in Myxococcus xanthus: QRB (1984) 59 119–138.] In some genera (e.g. Chondromyces, Nannocystis, Polyangium) the myxospores resemble 506 Myxomycetes PLASMODIUM which typically exhibits shuttle streaming (rhythmic reversible flow) of its protoplasm. Multispored, usually macroscopic fruiting bodies develop from the plasmodium. Sexual reproduction occurs in most (if not all) species, and the life cycle typically exhibits alternate haploid and diploid phases. Myxomycetes occur in various habitats, e.g., soil, humus, rotting wood, treebark, dung, decaying vegetation, etc. [Ecologically significant features of myxomycetes: TBMS (1984) 83 1–19.] Life cycle. A haploid spore germinates to release one or more uninucleate myxamoebae or flagellated amoeboid cells (which are typically biflagellated and heterokont); the two types of cell are interconvertible – e.g. myxamoebae may become flagellated in the presence of water. The cells feed phagocytically and osmotrophically, and divide mitotically (flagellated cells first withdrawing their flagella); under unfavourable conditions they may form MICROCYSTS. The amoeboid/flagellated cells function as gametes: cells of compatible mating type fuse, and the zygote develops into the main vegetative stage of the organism, the plasmodium. Three main types of plasmodium are distinguished. A protoplasmodium is microscopic, resembling a multinucleate amoeba, and lacks reticulations or ‘veins’. Protoplasmodia occur e.g. in ECHINOSTELIUM and many Licea spp; each gives rise to a single fruiting body. An aphanoplasmodium (formed e.g. by Stemonitis spp) is thin, delicate, and inconspicuous, with a reticulum of ‘veins’ leading into a broad, fan-shaped, advancing front; aphanoplasmodia lack a slime sheath, and their protoplasm is relatively non-granular. A phaneroplasmodium (formed e.g. in PHYSARUM) resembles an aphanoplasmodium but is much thicker and more robust, and is often conspicuous; its protoplasm is very granular, and it is surrounded by a gelatinous slime sheath – traces of which are left behind on the substratum over which the plasmodium migrates. Shuttle streaming (a rapid, rhythmic, back-and-forth flow) of the protoplasm occurs in aphanoplasmodia and phaneroplasmodia; in protoplasmodia streaming is very slow and irregular. Plasmodia are commonly yellow or white, but they may be e.g. orange, brown, red, violet or black; the larger plasmodia may measure several centimetres to a metre or more across. As it migrates over a substratum, the plasmodium feeds by ingesting bacteria, microalgae, protozoa, yeasts, particles of organic detritus etc, as well as myxamoebae and flagellated cells of its own species (‘cannibalism’); as it grows, its nuclei divide synchronously. Coalescence of two or more plasmodia may occur. Eventually, the plasmodium gives rise to fruiting bodies (or, in some species, to a SCLEROTIUM). Prior to fruiting, the plasmodium generally flows out onto an exposed surface; in some species light is apparently necessary to initiate fruiting. Three major types of fruiting body are formed, in each of which the spores develop in masses within a persistent or evanescent peridium. A sporangium is a relatively small, sessile or stalked structure which is more or less uniform in size and shape for a given species; many sporangia usually develop from a single plasmodium. (See e.g. ARCYRIA and STEMONITIS.) An aethalium is typically sessile, relatively large, hemispherical or pulvinate, derived from an entire plasmodium or from a major part of one; aethalia may vary in size and shape even within a single species. (See e.g. LYCOGALA.) (Pseudoaethalia, consisting of numerous compacted sporangia, occur in some species.) A plasmodiocarp is sessile, irregularly shaped and often branched or reticulate; it is derived from the major veins of an aphanoplasmodium or phaneroplasmodium. (See e.g. HEMITRICHIA.) Depending on species, fruiting bodies may contain e.g. a CAPILLITIUM, COLUMELLA, PSEUDOCAPILLITIUM and/or PSEUDOCOLUMELLA. In some species certain parts of the vegetative cells, while in others (e.g. Myxococcus, Stigmatella) they are shorter, or spherical, and refractile; in at least some genera (e.g. Myxococcus) the myxospores are more resistant than vegetative cells to e.g. heat and desiccation. (In e.g. Myxococcus xanthus myxospore formation – without fruiting – can be induced in broth cultures e.g. by high concentrations of glycerol.) Myxospores can, under suitable conditions, germinate to form vegetative cells – thus completing the life cycle. The myxobacterial fruiting bodies vary in size (from microscopic to ca. 1 mm), shape and composition, according to species; they are often brightly coloured. In some myxobacteria (e.g. Archangium, Myxococcus xanthus) the fruiting body is merely a small, sessile (non-stalked), rounded or irregularly shaped mound of myxospores set in slime; in M. stipitatus it consists of a spherical mass of myxospores raised on a stalk of hardened slime. In many genera the myxospores are enclosed within one or more hard, sac-like structures termed cysts or sporangia. In Nannocystis individual sporangia are embedded in the substrate, while in Cystobacter and Polyangium the fruiting body consists of a single sessile sporangium or a group of sessile sporangia; in Chondromyces, Melittangium and Stigmatella one or more sporangia occur on simple or branched stalks. Other myxobacterial genera include e.g. Angiococcus, Corallococcus, Haploangium and Sorangium. myxobactin See MYXOBACTERALES. myxobacton See MYXOBACTERALES. Myxobionta Syn. MYXOMYCOTA. Myxobolus A genus of protozoa (class MYXOSPOREA) which are histozoic parasites in fish. M. pfeifferi causes ‘boil disease’ in cyprinid fish, in which the sporonts develop in large (up to 7 cm) tumour-like cysts. M. cyprini is a common parasite of the common carp (Cyprinus carpio); it develops primarily in the muscle fibres of skeletal muscle, the mature spores subsequently being liberated into the bloodstream where they are retained in the capillaries [Parasitol. (1985) 90 549–555]. Myxococcus See MYXOBACTERALES. myxoflagellate A flagellated amoeboid cell formed by members of the MYXOMYCETES. Myxogastria See EUMYCETOZOEA. myxoma subgroup See LEPORIPOXVIRUS. myxoma virus See LEPORIPOXVIRUS and MYXOMATOSIS. myxomatosis An acute infectious disease which primarily affects the European rabbit Oryctolagus cuniculus; it is caused by the myxoma virus (see LEPORIPOXVIRUS). In O. cuniculus the disease, which is usually rapidly fatal, is characterized by the development of gelatinous tumours (myxomas) in the skin – particularly at mucocutaneous junctions (around the nose, mouth, ears and genitals); the tumours result from accumulation of mucinous material together with cellular proliferation. Transmission occurs (mechanically) via arthropod vectors (rabbit fleas, mosquitoes etc), and by direct contact etc. In its natural host, Sylvilagus brasiliensis (a forest rabbit native to Brazil and Uruguay), the myxoma virus usually causes only a localized lesion. Hares and jackrabbits (Lepus spp) are generally resistant. The myxoma virus has been used for the BIOLOGICAL CONTROL of rabbit populations – first in Australia in the early 1950s, later in France and the UK; initially, mortality rates in O. cuniculus were very high (up to 99%) but have subsequently declined owing to attenuation of the virus and natural selection of resistant rabbits. Myxomycetes (acellular slime moulds, or ‘true’ slime moulds) A class of SLIME MOULDS of the MYXOMYCOTA (cf. EUMYCETOZOEA). The major vegetative stage is a multinucleate, diploid, migratory 507 Myxomycota particular tissue (in histozoic species: e.g. species of Henneguya, Kudoa, Myxobolus, Myxosoma) or body fluid (in coelozoic species: e.g. Chloromyxum spp, Leptotheca spp, Myxidium spp, Zschokkella floridanae). The sporoplasm grows and its nucleus divides many times, resulting in the formation of a multinucleate plasmodium – the main vegetative stage of the organism. Eventually, many of the nuclei in the plasmodium each become segregated into cells (sporonts) destined to become spores; depending on species, a sporont may develop into a single spore (monosporous sporont) or into two or more spores within a common envelope (the whole being termed a pansporoblast). A sequence of nuclear divisions then ensues, each division apparently giving rise to a ‘somatic’ nucleus and a ‘generative’ nucleus. Cells containing somatic nuclei are involved in the formation of the spore wall, polar filament(s) and polar capsule(s), while generative nuclei are incorporated in the sporoplasm(s) (commonly two nuclei per sporoplasm). The mature spores may be released into the bloodstream or may remain in situ until the host is eaten e.g. by another fish or by a bird; in the latter case, the spores may be passed unharmed with the bird’s faeces, thus possibly being transported to fresh aquatic habitats. (cf. WHIRLING DISEASE.) myxosporidean (Pertaining to) a protozoon of the MYXOZOA (formerly of the class Myxosporidea, subphylum Cnidospora). Myxotheca See FORAMINIFERIDA. Myxotrichum See GYMNOASCALES. myxoviruses A (non-taxonomic) group of viruses: the ORTHOMYXOVIRIDAE and PARAMYXOVIRIDAE. myxoxanthophyll See CAROTENOIDS. Myxozoa A phylum of PROTOZOA [JP (1980) 27 37–58] which are mainly intracellular parasites of invertebrates and poikilothermic vertebrates. The organisms form spores of multicellular origin; a spore contains one or more extrusile polar filaments, each coiled within a separate sac (the polar capsule), and one or more sporoplasms (amoeboid cells which are the infective forms of the organism). The spore wall consists of one, two or three (rarely more) valves. Two classes are recognized: ACTINOSPOREA and MYXOSPOREA; however, since two apparently distinct organisms – one from each class – have been found to be different stages in the life cycle of the same organism (see WHIRLING DISEASE), a reappraisal of myxozoan taxonomy may be necessary [discussion: JP (1985) 32 589–591]. (See also MICROSPORA and ASCETOSPORA.) MZ B cells MARGINAL ZONE B CELLS (q.v.). fruiting body (e.g. peridium, capillitium) bear deposits of lime. Meiosis is believed to occur in the maturing spores; three of the four resulting nuclei apparently degenerate. Several hundred species of myxomycetes are recognized [descriptions and illustrations: Book ref. 145]. Orders: Echinosteliales (e.g. ECHINOSTELIUM); Echinosteliopsidales (Bursulla, Echinosteliopsis); Liceales (e.g. Cribraria, Enteridium, LYCOGALA, Licea, Tubifera); Physarales (e.g. Badhamia, Didymium, FULIGO, PHYSARUM); Stemonitales (e.g. Comatricha, Lamproderma, STEMONITIS); Trichiales (e.g. ARCYRIA, HEMITRICHIA, Perichaena, Trichia). Myxomycota (Myxobionta) In some mycological classification schemes [e.g. Book ref. 64]: a division which includes the SLIME MOULDS and organisms which resemble slime moulds in certain respects. Classes: ACRASIOMYCETES, Ceratiomyxomycetes (see CERATIOMYXA), DICTYOSTELIOMYCETES, LABYRINTHULOMYCETES, MYXOMYCETES, PLASMODIOPHOROMYCETES, PROTOSTELIOMYCETES. (cf. EUMYCOTA.) myxophage Any bacteriophage whose host is a member of the MYXOBACTERALES. Currently, only DNA myxophages are known; these are designated Mx1, Mx4 etc. Myxophyceae Obsolete class for the CYANOBACTERIA. Myxosarcina A genus of unicellular CYANOBACTERIA) (section II) in which growth and binary fission result in cubical aggregates of cells; baeocytes initially lack a fibrous outer cell wall layer and are motile (by gliding). GC%: 43–44. Myxosoma A genus of protozoa (class MYXOSPOREA) which are histozoic parasites in fish (see WHIRLING DISEASE). myxospore See MYXOBACTERALES. Myxosporea A class of protozoa (phylum MYXOZOA) which are parasitic (coelozoic or histozoic) in poikilothermic (‘coldblooded’) vertebrates, particularly fish; many members are economically important fish pathogens. The myxosporean spore contains between one and six (typically two) polar capsules (each containing a coiled filament), and one or two sporoplasms; the spore wall consists of two valves (order Bivalvulida: e.g. Ceratomyxa, Chloromyxum, HENNEGUYA, MYXOBOLUS, MYXOSOMA, MYXIDIUM, Sinuolinea, Sphaeromyxa, Sphaerospora, Thelohanellus) or of three or more valves (order Multivalvulida: e.g. Hexacapsula, Kudoa, Trilospora). Generalized life cycle. When ingested by a host animal, the spore explosively discharges its coiled filament(s), the discharged filament apparently serving to attach the organism to the gut epithelium. The amoeboid sporoplasm(s) emerges and passes through the gut wall, subsequently localizing in a 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 508 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 N N Asparagine (see AMINO ACIDS). N-end rule The observed relationship between the N-terminal amino acid of a protein and the half-life of that protein in vivo. In Escherichia coli, for example, proteins whose N-terminal amino acid is either arginine or lysine are characterized by a short halflife; early degradation of such proteins may be avoided in strains mutant in the AAT GENE. n. gen. See GENUS NOVUM. N-Serve See NITRAPYRIN. n. sp. See SPECIES NOVA. N-type starter See LACTIC ACID STARTERS. NA Numerical aperture: see RESOLVING POWER. Na+ -ATPase See ION TRANSPORT and SODIUM MOTIVE FORCE. Na+ -motive force See SODIUM MOTIVE FORCE. Na+ pump See ION TRANSPORT. nabam (dithane D-14) Disodium ethyleneBISDITHIOCARBAMATE, an agricultural antifungal agent sometimes used in conjunction with zinc sulphate and lime – when ZINEB is formed. nactins See MACROTETRALIDES. NAD (coenzyme I; cozymase) Nicotinamide adenine dinucleotide (formerly known as diphosphopyridine nucleotide, DPN). NAD is an important coenzyme, functioning as a hydrogen carrier in a wide range of redox reactions; the H is carried on the nicotinamide residue (see figure). The oxidized form of the coenzyme is written NAD+ , the reduced form as NADH (or NADH + H+ ); hence: the HEXOSE MONOPHOSPHATE PATHWAY, by the isocitrate dehydrogenase reaction of the TCA CYCLE, and by PHOTOSYNTHESIS in photosynthetic eukaryotes. (See also TRANSHYDROGENASE.) [NADPH production and consumption in yeasts: JGM (1983) 129 953–964.] In addition to its role as a hydrogen carrier, NAD+ also functions as an ADP-ribosyl donor in ADP-RIBOSYLATION reactions. Biosynthesis of NAD and NADP. NAD may be synthesized de novo from aspartate and dihydroxyacetone phosphate or from tryptophan (see NICOTINIC ACID; cf. V FACTOR). Nicotinamide released during the degradation of NAD+ (by NAD+ nucleosidase) may be re-cycled (i.e., incorporated into new NAD molecules) via the ‘pyridine nucleotide salvage cycle’; such a cycle may also be operative in certain organisms which cannot synthesize nicotinic acid/nicotinamide de novo (e.g. Bordetella pertussis [AvL (1984) 50 33–37]). NADP is synthesized by the ATP-dependent phosphorylation of NAD by NAD+ kinase. NAD+ kinase See NAD. NADH–UQ oxidoreductase Complex I of the mitochondrial ELECTRON TRANSPORT CHAIN. NADP See NAD. Nadsonia A genus of yeasts (family SACCHAROMYCETACEAE) in which the cells are usually lemon-shaped; vegetative reproduction occurs by bipolar bud-fission. Pseudomycelium is not formed. Ascus formation follows conjugation between a bud and its mother cell; ascospores contain a prominent lipid globule and are spherical, brownish, warty or spiny, 1 or 2 per ascus. Glucose is fermented by N. elongata and N. fulvescens, but not by N. commutata. Nitrate is not assimilated. Maximum growth temperature: ca. 24–26° C. Species have been isolated from soil and from the slime flux of trees. [Book ref. 100, pp. 279–284.] Naegleria A genus of amoeboflagellates (order SCHIZOPYRENIDA). Some species (e.g. N. gruberi ) are entirely free-living. N. fowleri (= N. aerobia, N. invades) is an opportunist pathogen (see MENINGOENCEPHALITIS); it is ca. 10 µm in diameter when rounded, and its single nucleus contains a large central nucleolus. (See also AMOEBOSTOME.) In N. fowleri interconversion can occur between the amoebic form and a transient, non-dividing biflagellate form [JB (1981) 147 217–226]. Amoebae can encyst. [ARM (1982) 36 101–123.] XH2 + NAD+ −−→ X + NADH + H+ The E0 of the NAD+ /NADH couple is −320 mV. NAD phosphate (NADP: formerly triphosphopyridine nucleotide, TPN, or coenzyme II) functions in the same way as NAD; the E0 of the NADP+ /NADPH couple is −317 mV. Many oxidoreductases are specific for either NAD or NADP, although some can function with either. (When either coenzyme can be used in a given reaction – or e.g. when different coenzymes serve the same function in different organisms – the designation NAD(P) is generally used.) As a broad generalization, NADP is more commonly associated with biosynthetic reactions, NAD with catabolic and energy-yielding reactions. Reduced NADP necessary for biosynthetic reactions may be generated e.g. by NH2 N N N CONH2 N O O 1′ H H 2′ OH 5′ CH2 H + N O CONH2 +2H −2H +H N O O 4′ P O O− P O− H 3′ OH adenosine mononucleotide O CH2 H R H H OH OH H nicotinamide mononucleotide NAD+ NADH NAD (nicotinamide adenine dinucleotide). NADP = NAD 2 -phosphate. 509 + nafcillin nafcillin See PENICILLINS. naftifine See ALLYLAMINES. NAG vibrios Non-agglutinable vibrios – see NON-CHOLERA VIBRIOS. nagana Any of a range of trypanosomiases which affect domestic animals (e.g. cattle) in various parts of Africa and in which the vector is a biting fly, often a species of Glossina; the pathogens include Trypanosoma brucei, T. congolense, T. suis and T. vivax. Symptoms typically include anaemia, cachexia, and damage to e.g. heart, kidneys and liver; affected cattle typically exhibit a generalized immunosuppression. Abortion and infertility are common in certain areas. Nagler’s reaction A test used to detect strains of Clostridium perfringens which produce a diffusible LECITHINASE (the atoxin). (An antigenically similar phospholipase C, which also cleaves lecithin, is produced by C. bifermentans.) When grown on EGG-YOLK AGAR such strains give rise to colonies each of which is surrounded by a zone of opacity. In the Nagler reaction, one half of a plate of egg-yolk agar is spread with anti-atoxin, and the plate is inoculated with the unknown strain – the inoculum forming a line which extends into both halves of the plate; following incubation, strains which produce a-toxin produce zones of opacity only in that half of the plate which does not contain antitoxin – although growth occurs in both halves of the plate. Nairobi sheep disease A disease of sheep and goats caused by a virus of the genus NAIROVIRUS. It occurs mainly in East Africa. Symptoms include haemorrhagic gastroenteritis, splenomegaly, nephritis, and myocardial degeneration; mortality rates are high. The virus is transmitted by the tick Rhipicephalus appendiculatus. Nairovirus A genus of viruses of the BUNYAVIRIDAE. Host range: various vertebrates; vectors: primarily ticks. MWts of L, M and S RNAs: ca. 4.1–4.9, 1.5–1.9, and 0.6–0.7 × 106 , respectively. MWts of proteins L, G1, G2 and N: ca. 145–200, 72–84, 30–40 and 48–54 × 103 , respectively. The genus comprises ca. 6 serogroups, including e.g. the Crimean–Congo haemorrhagic fever group, NAIROBI SHEEP DISEASE group, and the Qalyub group. naive (immunol.) Unprimed (cf. PRIMED). naked (1) (of viruses) Not enveloped. (2) (of nucleic acids, VIROIDS etc) Not associated with protein. nalA gene See GYRASE. nalidixic acid A QUINOLONE ANTIBIOTIC. Nalidixic acid inhibits bacterial DNA replication (and, to a lesser extent, RNA and protein synthesis) and causes degradation of DNA in the cell. It acts as a specific inhibitor of GYRASE, interfering with the function of the A subunits (cf. NOVOBIOCIN) and preventing the breakage-and-reunion activity – and hence the supercoiling and relaxing activities – of the enzyme. The drug apparently causes the formation of a very stable gyrase–DNA complex. In vitro, treatment of the complex with e.g. SDS triggers the cleavage of DNA strands with consequent covalent binding of the 5 ends to the A subunits; in vivo, the complex may block the movement of replication forks [ARB (1981) 50 901–902]. A related quinolone, norfloxacin, has been shown to inhibit gyrase by binding not to the enzyme itself but to regions of ssDNA, suggesting that gyrase is actually inhibited by a DNA–quinolone complex [PNAS (1985) 82 307–311]. [Gyrase-targeted antibiotics: TIM (1998) 6 269–275.] name bearer Syn. NOMENCLATURAL TYPE. naming of microorganisms See NOMENCLATURE. NANA N-AcetylNEURAMINIC ACID. NANB hepatitis NON-A–NON-B HEPATITIS. Nannizzia A genus of heterothallic fungi (order GYMNOASCALES) which occur e.g. in soil; anamorphs occur in the genus Microsporum (see DERMATOPHYTES). Nannoceratopsis A fossil DINOFLAGELLATE (lower Jurassic). Nannochloris A genus of green algae. N. bacillaris can tolerate a wide range of salinities [stepwise adaptation to salinity: CJB (1985) 63 327–332]. Nannocystis See MYXOBACTERALES. Nannomonas A subgenus of TRYPANOSOMA within the SALIVARIA; species occur, in parts of Africa, as parasites and pathogens in a wide range of wild and domestic animals including cattle, equines and suids (see also NAGANA). In the vertebrate host, bloodstream forms are trypomastigotes, <20 µm in length, in which the kinetoplast is subterminal (and typically close to the pellicle) and in which there is no free flagellum. In the vector (Glossina spp), trypomastigotes and epimastigotes – which also lack a free flagellum – may reach ca. 40 µm in length. Species include T . (N.) congolense and T . (N.) simiae (an important pathogen of the pig). Nannophrya See GYMNOSTOMATIA. nannoplankton See PLANKTON. nanobacteria Minute bacteria, detected e.g. in humans and bovines, which have been classified in the a-2 subgroup of Proteobacteria; the cells are generally about 0.2–0.5 µm, but smaller cells may be seen under the electron microscope (accounting for the ability of these organisms to pass through membrane filters of pore size 0.1 µm). The cells can form a carbonate apatite deposit on their envelope at near-neutral pH, and it has been suggested that they may contribute e.g. to the development of kidney stones [PNAS (1998) 95 8274–8279] (see also UROLITHIASIS). Nanochlorum A genus of very small, unicellular, possibly primitive, eukaryotic green algae (division CHLOROPHYTA). nanometre (nm) 109 m, or 10−3 µm. nanoplankton See PLANKTON. NANP See PLASMODIUM. naphthazarins (naphthazarines) A class of red pigments (5,8dihydroxynaphthoquinones substituted at position C-2 and sometimes at C-3) produced by FUSARIUM spp; they readily chelate multivalent metal ions, and are toxic to plants and insects. Naphthazarins include e.g. fusarubin, isomarticin, javanicin and marticin. naphthoquinones See QUINONES. naramycin B See CYCLOHEXIMIDE. narasin An ionophore antibiotic, produced by Streptomyces aureofaciens, which is used as an anti-coccidial agent in poultry and as a FEED ADDITIVE for ruminants. narbomycin See MACROLIDE ANTIBIOTICS. NarI See RESTRICTION ENDONUCLEASE (table). narrow groove Syn. minor groove (see DNA). NARTC Nalidixic acid-resistant thermophilic campylobacters: CAMPYLOBACTER strains which are related to but distinct from C. jejuni and C. coli. NASBA (nucleic acid sequence-based amplification) A method used for the isothermal amplification of RNA target sequences; NASBA, which is based on the replication strategy of retroviruses, involves several enzymes (including REVERSE TRANSCRIPTASE) and is typically run at ∼41° C for e.g. ∼90 min. The products include ssRNA amplicons (which are complementary to the target RNA) and double-stranded cDNA. An outline of NASBA is given in the figure. (Transcription-mediated amplification is a very similar process – see TMA.) 510 amplicon 3′ 1 5′ 3′ 2 5′ 3 5′ 3′ 4 5′ 5 3′ 5′ 6 5′ 3′ 7 5′ cyclic phase 3′ 8 5′ 3′ 9 5′ NASBA (nucleic acid sequence-based amplification): isothermal amplification of an RNA target; amplification is carried out at ∼41° C. The dashed lines are strands of RNA, the solid lines strands of DNA. For each strand the polarity is indicated by a 3 and/or 5 label. With clinical specimens, amplification of targets is preceded by treatment of the sample to ensure that target sequences, if present, are made accessible. Such pre-treatment necessarily includes release of nucleic acid from organisms and heating to denature target RNA; although RNA is generally single-stranded, intra-strand base-pairing can occur in mRNA and occurs extensively in rRNA. Pre-treatment may also involve inactivation of inhibitory substances. The following stages are involved in amplification: 1. A strand of RNA showing the target sequence (amplicon) delimited by two, short vertical bars. A primer (primer 1) has bound to the 3 end of the amplicon. The 5 end of this primer is tagged with a short sequence (
) containing the promoter of an RNA polymerase. 2. The enzyme reverse transcriptase has extended the primer to form a strand of cDNA, thus giving rise to a hybrid RNA:DNA duplex intermediate. 3. The enzyme RNase H has degraded (removed) the RNA strand. Another primer, primer 2, has bound to the 3 end of the cDNA. 4. Reverse transcriptase (which can also synthesize DNA on a DNA template) has extended primer 2 to form double-stranded cDNA; notice that the promoter sequence in primer 1 has also become double stranded as primer 2 has been fully extended on the template to form the complementary strand () of the promoter. Now double-stranded, the promoter is functional; an RNA polymerase () has bound to the promoter and will synthesize an RNA strand in the direction of the arrow, i.e. in a 5 -to-3 direction. 5. The newly synthesized strand of RNA. Note the strand’s polarity: it is complementary to the amplicon in the sample RNA (compare 5 with 1). 6. The RNA strand has bound primer 2. 7. Reverse transcriptase has synthesized cDNA on the RNA strand. 8. RNA has been removed by RNase H. Primer 1 has bound to the cDNA. 9. Reverse transcriptase has synthesized a complementary strand of cDNA; note that the 3 end of the template cDNA has also been extended to form a functional (double-stranded) promoter for the RNA polymerase. RNA polymerase has bound to the promoter and will (repeatedly) synthesize RNA strands identical to the one shown at stage 5. These strands can participate in the cyclic phase, leading to high-level amplification of the target. (Continued on page 512.) 511 nasopharyngeal carcinoma As in other nucleic-acid-amplification methods, products can be detected by gel electrophoresis and staining. Alternatively, the amplicons can be monitored by means of MOLECULAR BEACON PROBES [see e.g. NAR (1998) 26 2150–2155]. In clinical microbiology, NASBA is useful e.g. for monitoring the development of transcriptional activity in latent DNA viruses. For example, by monitoring transcripts of the late genes of cytomegalovirus (CMV) it is possible to identify patients at risk for CMV-related disease [JCM (1998) 36 1341–1346]; a commercial assay kit for this purpose has been marketed. [NASBA in diagnostic microbiology, research and other noncommercial applications: RMM (1999) 10 185–196.] [Isothermal nucleic-acid-amplification methods (NASBA, TMA, SDA): Book ref. 221, pp 126–151.] nasopharyngeal carcinoma (NPC) A carcinoma arising from the epithelial lining of the post-nasal space. Undifferentiated NPC is associated with EPSTEIN–BARR VIRUS (EBV) infection; the epithelial tumour cells contain multiple copies of EBV DNA, but chromosomal abnormalities have not been detected (cf. BURKITT’S LYMPHOMA). NPC is generally rare, but occurs with high incidence among certain ethnic groups, particularly Cantonese Chinese and Eskimos. Since EBV is ubiquitous, genetic and/or environmental factors are believed to play a role in pathogenesis; nitrosamines (present in salted and dried fish eaten by Cantonese people) and certain components (e.g. phorbol esters) of Chinese herbal medicines have been implicated as potentiating factors. nasopharynx microflora See RESPIRATORY TRACT MICROFLORA. nasse (protozool.) Syn. CYRTOS. Nassellarida See RADIOLARIA. Nassula A genus of freshwater protozoa (subclass HYPOSTOMATIA). Cells: ovoid to elongate, ca. 150–200 µm in length, completely covered with cilia. Food: e.g. cyanobacteria. Nassulida See HYPOSTOMATIA. natamycin Syn. PIMARICIN. Natronobacterium A genus of halophilic, alkalophilic archaeans [FEMS Reviews (1986) 39 9–15]. Natronococcus A genus of halophilic and alkalophilic archaeans [FEMS Reviews (1986) 39 9–15]. natto A Japanese food made by fermenting soybeans. Soybeans are soaked, steam-cooked, wrapped in rice straw, and incubated at e.g. ca. 40° C for 1–2 days. ‘Bacillus natto’ (a strain of B. subtilis), supplied by the straw or added as a pure-culture starter, develops and coats the beans with a viscous, sticky polymer; the product develops a strong persistent flavour. [Book ref. 8, pp. 520–521.] natural antibodies Immunoglobulins, present in plasma, which act as antibodies towards antigens with which the body has apparently not been in immunological contact; they may derive e.g. from previous or ongoing subclinical infections or from immunological contact with CROSS-REACTING ANTIGENS. natural antigen Syn. ANTIGEN (= IMMUNOGEN sense 1). natural immunological tolerance Syn. SELF TOLERANCE. natural killer cells See NK CELLS. Naumanniella A genus of bacteria found e.g. in soil and wellwaters [see e.g. Book ref. 45, pp. 1049–1059]. Navicula A genus of pennate DIATOMS in which the cells are characteristically spindle- or boat-shaped and show gliding motility. Species occur on mud or other substrates in freshwater, marine and terrestrial habitats. NBT Nitroblue TETRAZOLIUM. NCAM See IMMUNOGLOBULIN SUPERFAMILY. NCCLS National Committee for Clinical Laboratory Standards (USA). Ncd cells See PROTOPLAST FUSION. NCDO National Collection of Dairy Organisms, Food Research Institute (Reading), Shinfield, Reading, Berkshire RG2 9AT, UK. NCIB National Collection of Industrial Bacteria, Torry Research Station, P.O. Box 31, 135 Abbey Road, Aberdeen AB9 8DG, Scotland, UK. NCMB National Collection of Marine Bacteria, Torry Research Station, P.O. Box 31, 135 Abbey Road, Aberdeen AB9 8DG, UK. NcoI See RESTRICTION ENDONUCLEASE (table). NCPPB National Collection of Plant Pathogenic Bacteria, Plant Pathology Laboratory, Hatching Green, Harpenden, UK. NCS NEOCARZINOSTATIN (q.v.). NCTC National Collection of Type Cultures, Central Public Health Laboratory, Colindale Avenue, London NW9 5HT, UK. NCV NON-CHOLERA VIBRIO. NCVP Non-O1 Vibrio cholerae pilus (non-cholera-vibrio pilus): a filamentous cell-surface appendage found in non-O1 strains of Vibrio cholerae, including many strains of the O139 Bengal serogroup. (See also CHOLERA.) NDC Nuclear dehydrogenating clostridia: see GASTROINTESTINAL TRACT FLORA. 2
-NDG 2 -Nor-2 -deoxyguanosine: see DHPG. Ndumu virus See ALPHAVIRUS. NDV NEWCASTLE DISEASE virus. neamine (neomycin A) An AMINOGLYCOSIDE ANTIBIOTIC whose molecule contains an aminosugar linked to DEOXYSTREPTAMINE. Nebraska calf diarrhoea See ROTAVIRUS. necridium (sacrificial cell) In certain filamentous bacteria (e.g. certain cyanobacteria, Beggiatoa): a specialized intercalary cell which, by death and lysis, allows fragmentation of the trichome and liberation of hormogonia. necrobacillosis Any of various diseases of man or animals caused by Fusobacterium necrophorum. In humans, F. necrophorum can cause e.g. an acute systemic disease characterized by sore throat, submandibular lymphadenopathy, fever with rigors, and subsequently formation of lung abscesses with consequent pleuritic pain and haemoptysis. In animals, F. necrophorum may invade the skin or mucous membranes, usually via damaged tissues, causing necrosis and sometimes abscess formation. In cattle and lambs the pathogen may infect via the navel or rumen and then localize e.g. in the liver. Infections localized to the mouth and larynx (‘calf diphtheria’) or to the mouth only (‘necrotic stomatitis’) may also occur. (See also FOOT-ROT.) NASBA (continued) The double-stranded cDNA templates in stage 9 are permanent products and are continually transcribed by the RNA polymerase. Operation of the cyclic phase produces many copies of the amplicon in the form of (i) complementary RNA (the major product), and (ii) cDNA. In NASBA, amplification involves three enzymes: reverse transcriptase, RNA polymerase and RNase H; in a similar process, TMA, the role of RNase H is carried out by the reverse transcriptase, so that only two enzymes are used. Reproduced from Bacteria, 5th edition, Figure 8.25, pages 226–227, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 512 Neisseria of the needle, and that the length of the needle is regulated by the invJ gene product [PNAS (2000) 97 10225–10230]. [Structure and composition of the Shigella flexneri needle complex: Mol. Microbiol. (2001) 39 652–663.] Studies on Yersinia enterocolitica suggest that the needle is formed by polymerization of a 6 kDa protein, and that polymerization of this protein provides the force necessary for puncturing the cytoplasmic membrane of a eukaryotic cell [PNAS (2001) 98 4669–4674]. Neethling virus See LUMPY SKIN DISEASE. negative complementation See COMPLEMENTATION TEST. negative contrast See ELECTRON MICROSCOPY (a). negative control (of an operon) See OPERON. negative interference (genetics) See INTERFERENCE (2). negative Pasteur effect Syn. CUSTERS EFFECT. negative phase (immunol.) The phase in which the in vivo titre of a given antibody is reduced following administration of the homologous antigen – due to combination of the antigen with circulating antibody. negative-sense genome See VIRUS. negative staining See STAINING and ELECTRON MICROSCOPY (a). negative-strand virus A VIRUS (q.v.) with a negative-sense genome. Negishi virus See FLAVIVIRIDAE. Negri bodies Acidophilic, intracytoplasmic INCLUSION BODIES which develop in cells of the CNS in most cases of human or animal RABIES. They may occur only in the late stage of the disease, and are not formed in FIXED VIRUS infections. negro coffee mosaic virus See POTEXVIRUSES. neighbour exclusion principle See INTERCALATING AGENT. Neisser–Wechsberg leucocidin See staphylococcal a-HAEMOLYSIN. Neisseria A genus of aerobic, oxidase-positive, Gram typenegative bacteria (family NEISSERIACEAE) which occur as parasites or as primary or opportunist pathogens on the mucous membranes of mammals. Cells: typically cocci, 0.6–1.0 µm in diameter (which often occur singly, or in pairs with adjacent sides flattened or concave), but in one species (N. elongata) the cells are rod-shaped, ca. 0.5 µm in width, and often occur as diplobacilli; the cells tend to resist decolorization in the Gram stain. Cells may exhibit TWITCHING MOTILITY, and in some species the FIMBRIAE are associated with pathogenicity. (See also ANTIGENIC VARIATION and IGA1 PROTEASES.) N. gonorrhoeae and N. meningitidis are nutritionally fastidious (isolation and general culture media including e.g. THAYER–MARTIN AGAR, MUELLER–HINTON MEDIUM and CHOCOLATE AGAR); the other species of Neisseria are able to grow on unenriched nutrient agar. The growth of all species is improved by a high relative humidity (ca. 50%). Optimum growth temperature: ca. 35–37° C. All strains form CARBONIC ANHYDRASE (cf. MORAXELLA). Most strains form catalase – some strains of N. elongata are catalase-negative. (See also SUPEROXOL TEST.) Neisseria spp are highly susceptible to desiccation and to exposure to direct sunlight. GC%: ca. 46.5–53.5. Type species: N. gonorrhoeae. N. canis. Some strains are haemolytic on rabbit blood; no acid from glucose or other sugars; nitrate is usually reduced; a yellow pigment is formed. Strains have been isolated e.g. from the throats of cats. N. caviae. See FALSE NEISSERIAE. N. cinerea. Non-haemolytic; no acid from glucose or other sugars. N. cuniculi. See FALSE NEISSERIAE. necrosis Localized death and degeneration of tissues in a living organism – due e.g. to infection or injury. (See also CASEATION.) necrotic stomatitis (vet.) See NECROBACILLOSIS. necrotizing enteritis Syn. ENTERITIS NECROTICANS. necrotizing enterocolitis (NEC) A severe disease of newborn infants; it is characterized by vomiting, abdominal distension, bloody stools, pneumatosis intestinalis, ischaemic necrosis of the intestinal wall and sloughing of the mucosa. The aetiology of NEC is unknown; microorganisms – particularly Clostridium perfringens – are believed to play a role in at least some cases. [Book ref. 107, pp. 15–26.] necrotizing fasciitis A condition characterized by soft-tissue oedema and necrosis, with histology-based evidence of necrosis of the superficial fascia, fascial oedema, and an infiltrate of PMNs. A range of bacteria have been isolated from such tissues – including Staphylococcus aureus, enterobacteria, and group A streptococci; streptococci have been causally connected with the disease in about 10% of cases. Identification of the causal agent in a given case facilitates the choice of appropriate chemotherapy; moreover, necrotizing fasciitis may be associated with a risk of familial transmission – a possible indication for chemoprophylaxis of close contacts. Tissue samples/blood may be culture-negative owing to prior chemotherapy. A small PCR-based study, designed to detect streptococci, found that all 10 cases of confirmed streptococcal aetiology could be identified by testing for the speB gene (which encodes exotoxin B in almost all group A streptococci) [JCM (1998) 36 1769–1771]. necrotizing toxin A TOXIN which can cause NECROSIS. necrotroph (1) An organism which kills part or all of another organism before deriving nutrients from it; the term is usually applied to an association between a plant and a microorganism. (2) An organism which derives nutrients from dead plant or animal tissues, whether or not it is responsible for the death of those tissues. (cf. BIOTROPH; PERTHOTROPH.) nectin Protein which forms the ‘stalk’ region of mitochondrial PROTON ATPASE. Nectria A genus of fungi (order HYPOCREALES) which include plant-pathogenic species (see e.g. APPLE CANKER and BEECH BARK DISEASE; see also PISATIN); anamorphs occur in the genera ACREMONIUM, CYLINDROCARPON and FUSARIUM. N. cinnabarina occurs mainly on the dead branches of woody plants; however, it can cause dieback (particularly in e.g. Acer spp), gaining access e.g. via wounds or via dead tissues. Unicellular conidia are formed on simple or sparsely branched conidiophores which develop on orange-pink sporodochia (ca. 1–5 mm diam.) – hence the popular name ‘coral spot’. Dark red perithecia (the overwintering stage) subsequently develop on the same sporodochium; each ascus contains eight two-celled ascospores. N. galligena differs from N. cinnabarina e.g. in that the conidia are multiseptate and the perithecia are formed singly, without the development of a stroma. needle (bacteriol.) Syn. STRAIGHT WIRE. needle complex Part of a type III PROTEIN SECRETION system: a multi-protein structure consisting of a base, which spans the cell envelope, and a needle-like extension from the surface of the bacterium; in overall structure, this complex resembles the hook–basal body portion of a bacterial FLAGELLUM. Studies on the type III protein secretion system of Salmonella typhimurium have found that the prgI gene product is the main component 513 Neisseriaceae N. subflava. Non-haemolytic; acid is usually formed from glucose and maltose, and sometimes from fructose and sucrose. Some strains are saline-agglutinable. Neisseriaceae A family of aerobic, chemoorganotrophic, nonmotile, asporogenous Gram type-negative bacteria which are typically parasitic and/or pathogenic in warm-blooded hosts. Most or all strains are oxidase +ve. The family Neisseriaceae formerly included the genera ACINETOBACTER, KINGELLA, MORAXELLA and NEISSERIA [Book ref. 22, pp 288–309]. The genera Acinetobacter and Moraxella have been transferred to the family MORAXELLACEAE. nekton Collectively, the actively motile organisms in a body of water (e.g. lake, sea). (cf. PLANKTON; see also NEUSTON.) nelfinavir See ANTIRETROVIRAL AGENTS. Nelsonian illumination Syn. CRITICAL ILLUMINATION. nematode-trapping fungi See NEMATOPHAGOUS FUNGI. nematodesma One of a number of bundles of microtubules which reinforce the walls of the ciliate CYTOPHARYNX. (cf. TRICHITE.) nematophagous fungi A heterogeneous group of fungi which derive nutrients from nematode worms. Many are predatory, trapping and killing their prey before invading its tissues, while others infect and apparently parasitize nematodes; most can also grow saprotrophically. Nematophagous fungi occur e.g. in soil and decaying organic matter. (a) Nematode-trapping fungi. Most fungi in this category produce various types of specialized ‘trap’, generally in response to the presence of nematodes or of compounds such as certain peptides or amino acids (collectively termed ‘nemin’). These traps may be simple adhesive knobs (e.g. in Acaulopage pectospora [CJB (1985) 63 1386–1390], Dactylaria candida [CJB (1977) 55 2956–2970] and Monacrosporium drechsleri ), relatively undifferentiated adhesive lateral hyphal branches (e.g. in Triposporina aphanopaga) or adhesive hyphal branches which branch and anastomose to form two- or three-dimensional networks (e.g. in Arthrobotrys oligospora, Dactylella cionopaga [CJB (1984) 62 674–679], and some Monacrosporium spp). [A. oligospora: AvL (1985) 51 385–398 (EM study), 399–407 (microbodies in trap cells); FEMS Ecol. (1985) 31 17–21 (trap formation in liquid culture).] Non-adhesive hyphal rings are formed by some species, and these rings may be constricting (e.g. in Arthrobotrys dactyloides, Dactylaria brochopaga [CJB (1977) 55 2945–2955], Dactylella spp) or non-constricting (e.g. Dactylaria candida – which also forms adhesive knobs). When a nematode enters a ring of the constricting type, the ring constricts abruptly, trapping and killing the animal; the mechanism of constriction is unknown, but apparently involves the rapid swelling of cells of the ring in response to contact between the inner surface of the ring and the nematode. In the case of a non-constricting ring, a nematode of appropriate size may be trapped when it begins to move through the ring but is too wide to pass right through; in D. candida the rings are apparently not effective traps [CJB (1977) 55 2956–2970]. Some fungi (e.g. Stylopage) can trap nematodes without the formation of specialized structures; when a nematode contacts a hypha, the hypha secretes at the point of contact an adhesive which traps the animal. (b) Fungi endoparasitic in nematodes. Some species (e.g. Meria coniospora) produce adhesive conidia which adhere to a nematode and germinate, the hyphae then invading the animal; after the death of the host, conidiophores emerge from the body. Harposporium spp produce non-adhesive conidia, and a nematode becomes infected by ingestion of these conidia. Certain N. denitrificans. Non-haemolytic; acid is usually formed from glucose, fructose, mannose and sucrose, but not from maltose. N. elongata. Cells rod-shaped; non-haemolytic; can grow in unenriched peptone media; some strains (subsp. glycolytica) cause weak acidification of glucose media and are catalasepositive, while others (subsp. elongata) do not acidify glucose media and are catalase-negative. N. flavescens. Non-haemolytic; no acid is produced from glucose or other sugars; yellow pigment is formed on Loeffler’s serum medium. N. gonorrhoeae (the gonococcus). The causal agent of GONORRHOEA (and a common cause of ophthalmia neonatorum) in man, but apparently not pathogenic in other animals. Culture is carried out under 3–10% CO2 . Colonies (48-hour): typically round, smooth, convex, greyish-white, opaque, 0.6 mm to >1 mm in diameter, usually becoming mucoid on further incubation. Nonhaemolytic. Acid (no gas) is usually formed from glucose but not from other sugars. (See also FIMBRIAE; PEPTIDOGLYCAN; TRANSFORMATION.) Certain Opa+ strains of N. gonorrhoeae (see OPA PROTEINS) are capable of TRANSCYTOSIS [see e.g. TIM (1998) 6 489–495]. N. gonorrhoeae can be detected in clinical specimens by culture and e.g. by a PROBE-based test [PACE 2C test: JCM (1995) 33 2587–2591] and by the ligase chain reaction [Abbott LCx assay (target: opa gene): JCM (1998) 36 1630–1633]. [N. gonorrhoeae (various molecular TYPING methods): RMM (1998) 9 1–8.] N. lactamica. Some strains are haemolytic (horse blood); acid is usually formed from glucose and maltose, and (unlike other neisseriae) from lactose. N. meningitidis (the meningococcus). A causal agent of MENINGITIS. Culture is carried out under 5–10% CO2 (which enhances growth). Colonies (24-hour) on Mueller–Hinton agar are typically round, smooth, translucent and butyrous, 1 mm or more in diameter, becoming viscid on further incubation. Non-haemolytic. Acid (no gas) is usually formed from glucose and maltose but not from fructose, lactose, mannose or sucrose. Nitrate is not reduced. Serogroups A, B, C, D, H–L, X, Y, Z, Z (= 29E) and W-135 have been distinguished on the basis of polysaccharide capsular antigens. Strains of serogroups A and C (sometimes B) are commonly associated with epidemics. Strains of serogroups H–L are apparently not pathogenic. Strains of serogroups Z and Z are reported to be killed by normal human serum, and to give rise to disease only in those with underlying health problems. PCR-based typing of N. meningitidis (culture-independent typing, i.e. typing direct from the clinical specimen) may be achieved with specimens that have become culture-negative owing to preadmission chemotherapy [JCM (1997) 35 1809–1812]. For multilocus sequence typing (see MLST) of N. meningitidis, a database of allelic profiles may be found at the website: http://mlst.zoo.ox.ac.uk [MLST: TIM (1999) 12 482–487.] N. mucosa. Non-haemolytic; acid is usually formed from glucose, maltose, fructose and sucrose; colonies are typically mucoid; unlike most other neisseriae N. mucosa reduces nitrate. N. ovis. See FALSE NEISSERIAE. N. sicca. Haemolysis may be exhibited on horse, human, rabbit and/or sheep blood agar; acid is usually formed from glucose, fructose, maltose and sucrose; colonies are dry and wrinkled, and the growth agglutinates spontaneously in saline. 514 Nephroma chytridiomycetes (e.g. Catenaria), oomycetes (e.g. Nematophthora gynophila) and zygomycetes (e.g. Gonimochaete pyriforme [EM study: CJB (1985) 63 2326–2331]) can infect and parasitize nematodes; N. gynophila can apparently play a role in the control of the cereal cyst nematode, Heterodera avenae [Nematol. (1980) 26 57–68]. Nematophthora See NEMATOPHAGOUS FUNGI. Nematospora A genus of fungi (family METSCHNIKOWIACEAE) in which multilateral budding occurs; a well-developed pseudomycelium is usually formed, and true mycelium may be formed. Ascospores: fusiform with a whip-like appendage at one end, usually 8 per ascus. Species occur mainly as parasites and pathogens of plants (see e.g. STIGMATOMYCOSIS). [Book ref. 100, pp. 285–288.] Nematosporaceae Syn. METSCHNIKOWIACEAE. Nematostelium See PROTOSTELIOMYCETES. nemin See NEMATOPHAGOUS FUNGI. neoantigen An antigen formed in vivo by the combination of a protein with an exogenous substance. (See also CONTACT SENSITIVITY.) Neocallimastix A genus of anaerobic fungi found in the RUMEN; species include N. patriciarum [TBMS (1986) 86 178–181, 103–109] and N. frontalis. N. frontalis forms multiflagellate zoospores [ultrastructure: CJB (1983) 61 295–307] and produces an extracellular cellulase which is more effective than any other known cellulases in solubilizing the extensively hydrogenbonded cellulose in cotton fibres [FEMS (1986) 34 37–40]. It has been proposed that Neocallimastix be placed in an amended class CHYTRIDIOMYCETES. [Mitosis in and phylogeny of the genus Neocallimastix: CJB (1985) 63 1595–1604.] neocarzinostatin (NCS) An antitumour agent, produced by Streptomyces carzinostaticus, which consists of an acidic 109amino-acid protein (containing two disulphide bonds) linked non-covalently to a complex, highly labile, non-protein chromophore which is responsible for the activity of the drug. In vitro, NCS interacts with DNA (its chromophore behaving as an INTERCALATING AGENT), causing single-stranded breaks in which the 3 and 5 ends generally bear 3 -phosphate and 5 -aldehyde groups, respectively; strand breakage occurs mainly at thymidine and deoxyadenosine residues and requires the presence of a thiol and O2 as cofactors. NCS-induced strand breakage in intact cells appears to occur by a reaction similar to that in vitro [Biochem. (1987) 26 384–390]. NCS can also cause e.g. release of free bases (particularly thymine and adenine) from the DNA. Neodiprion sertifer NPV See BACULOVIRIDAE. neogregarines See GREGARINASINA. neomycin Either of two related AMINOGLYCOSIDE ANTIBIOTICS (neomycins B, C) produced by Streptomyces fradiae; the drug used clinically consists mainly of neomycin B with some neomycin C. (cf. NEAMINE.) neonatal calf diarrhoea coronavirus See CORONAVIRIDAE. neonatal herpes See HERPES SIMPLEX. neoplasia Progressive, uncontrolled cell division which, if the progeny cells remain localized (at least initially), results in the formation of an abnormal growth (tumour, neoplasm). (cf. HYPERPLASIA.) In man or animals, a neoplasm may be BENIGN or MALIGNANT (sense 2). (See also CANCER; cf. LEUKAEMIA.) neoplasm See NEOPLASIA. Neorickettsia A genus of Gram-negative bacteria of the tribe EHRLICHIEAE. Cells: coccoid, often pleomorphic, non-motile, maximum dimension ca. 0.5 µm. Growth occurs intracytoplasmically in lymphoid cells of canine animals. Growth does not occur in cell-free media. Type species: N. helminthoeca. N. helminthoeca, the sole species, is the causal agent of ‘salmon poisoning’ in dogs: a disease (usually fatal in untreated cases) which is transmitted by trematodes (flukes) and in which the pathogen occurs singly or in colonies in the cytoplasm of reticuloendothelial cells in lymphoid tissue; the pathogen has not been detected microscopically in circulating lymphocytes, but the blood of a diseased animal is infectious. The fluke-borne disease of dogs known as Elokomin fluke fever appears to be a different disease since immunity to this disease does not confer immunity to salmon poisoning. [Book ref. 22, pp. 710–711.] neosaxitoxin See SAXITOXIN. Neo-Sensitabs Antibiotic-containing tablets which are used (in place of antibiotic-impregnated paper discs) in antibioticsensitivity tests; they are used e.g. in Denmark, Holland, Belgium, Norway, Finland and other countries. Neo-Sensitabs (produced by A/S ROSCO, Taastrup, Denmark) are colour-coded for identification, and most are stable at room temperature for a minimum of 4 years. neosolaniol See TRICHOTHECENES. neostibosan See ANTIMONY. neoteny The persistence, in a mature organism, of features which are characteristic of the immature organism. neotype Any strain which has been described and published in accordance with the relevant nomenclatural code in order to serve as a NOMENCLATURAL TYPE in place of e.g. a nomenclatural type which is no longer extant. Neovossia See USTILAGINALES and KARNAL BUNT. neoxanthin See CAROTENOIDS. nephelometry Any procedure in which the concentration of cells (or particles) in a suspension is estimated by passing a beam of (monochromatic) light through the suspension and measuring the intensity of the scattered light – commonly at 90° to the incident light (cf. TURBIDIMETRY). Light scattering is due to intracellular refraction, diffraction, and reflection from the cell surface; the amount of light scattered becomes greater with increases in cell concentration, cell size, and length of light path through the suspension, and with decreases in the wavelength of the incident light. The intensity of the scattered light varies with its angular displacement from the axis of the incident light – exhibiting several peaks of intensity; these peaks are due to interference phenomena. The relationship between the intensities of the incident and scattered light and the concentration of cells is given by: log10 (I0 /Iscat ) ∝ 1/(cell concentration) where I0 is the intensity of incident light, and Iscat is the intensity of scattered light. The sensitivity of a nephelometer is lower when cell concentrations are high owing to the effects of secondary scattering, i.e., redirection of a deflected ray by other cell(s). (See also COUNTING METHODS.) nephridial network See CONTRACTILE VACUOLE. nephritic (1) Pertaining to, or affected with, nephritis. (2) Of or pertaining to the kidneys. nephritis Inflammation of the kidney. (See PYELONEPHRITIS.) nephritogenic Causing nephritis. Nephroma A genus of foliose LICHENS (order PELTIGERALES); photobiont: Coccomyxa or Nostoc, according to species. Thallus: lobed, corticate on both surfaces, grey, green or brown; apothecia (rare e.g. in N. expallidum, common e.g. in N. laevigatum) are formed on the lower surface of the thallus near the margins; fertile lobes are usually held erect. Other structures which may be present include Nostoc-containing cephalodia (in Coccomyxa-containing species – e.g. N. arcticum), tubercles on 515 Nephroselmis of amplification (20 cycles) the annealing temperature was lowered to 45° C – permitting binding of the inner primers and leading to the formation of second-phase amplicons. Detection of the second-phase amplicons involved pre-labelling of inner primers. One type of primer was 5 -biotinylated while the other was 5 labelled with DIGOXIGENIN – so that the double-stranded amplicon was labelled with both compounds. By means of the biotin tag, amplicons were captured on an AVIDIN-coated microtitre plate; an antidigoxigenin–alkaline phosphatase conjugate was used to label the amplicon (by digoxigenin–antidigoxigenin binding), and the label (enzyme) was detected by addition of a reagent (p-nitrophenylphosphate) and monitoring colorimetrically at 405 nm for the enzymic cleavage products. Other examples of nPCR include detection of Mycobacterium malmoense [JCM (1999) 37 1454–1458], and nested duplex PCR for the detection of Bordetella pertussis and B. parapertussis [JCM (1999) 37 606–610]. net blotch A CEREAL DISEASE, caused by Pyrenophora teres, which affects mainly winter barley. Pale, yellowish blotches, later turning brown, appear on leaves in the autumn; the brown lesions, which have a net-like appearance, may coalesce to form longitudinal stripes. (See also FLUTRIAFOL, GUAZATINE, PROCHLORAZ, PROPICONAZOLE.) net plasmodium See LABYRINTHULAS. net slime moulds See LABYRINTHULAS. netilmicin An AMINOGLYCOSIDE ANTIBIOTIC. netropsin A basic oligopeptide ANTIBIOTIC (isolated from ‘Streptomyces netropsis’) containing two N-methylpyrrole rings; the functionally similar distamycin A contains three such rings. Both netropsin and distamycin bind specifically and avidly to BDNA, with a strong preference for AT-rich regions [DNA–drug specificity: PNAS (1985) 82 1376–1380]; the drug appears to bind (non-intercalatively) in the minor groove (see DNA). Both drugs show antimicrobial and antitumour activity (apparently by inhibiting synthesis of RNA and DNA) and inhibit the development of DNA viruses, but they are too toxic for clinical use. Low concentrations of netropsin have been reported to inhibit sporulation without affecting growth in Bacillus subtilis [CJM (1980) 26 420–426]. Neuberg’s fermentations Neuberg’s ‘1st form of fermentation’ is the ALCOHOLIC FERMENTATION as normally carried out by yeasts such as Saccharomyces. Neuberg’s ‘2nd form of fermentation’ (= ‘glycerol fermentation’, ‘sulphite fermentation’) refers to the alcoholic fermentation perturbed by the presence of sodium bisulphite (NaHSO3 ). Bisulphite forms an addition compound with acetaldehyde, preventing its reduction (by NADH) to ethanol; NADH (from the EMP pathway) instead reduces dihydroxyacetone phosphate to glycerol-3-phosphate – which is dephosphorylated to GLYCEROL. Neuberg’s ‘3rd form of fermentation’ refers to the alcoholic fermentation perturbed by imposed alkalinity; the end products are glycerol, ethanol and acetic acid. Neufchatel cheese See CHEESE-MAKING. Neufeld quellung phenomenon See QUELLUNG PHENOMENON. neuraminic acid A C9 -compound (5-amino-3,5-dideoxy-D-glycero-D-galactononulosonic acid) derived from mannosamine and pyruvate; N-acetyl and O-acetyl derivatives (e.g. Nacetylneuraminic acid, NANA) are called sialic acids. Sialic acids occur widely in animal tissues and fluids – e.g. in glycolipids, mucopolysaccharides and mucoproteins in mucous secretions, serum, brain glycolipids (gangliosides), milk (Nacetylneuraminyllactose), etc. Terminal sialic acid residues in glycoprotein or glycolipid cell membrane components serve as the lower surface (N. resupinatum), soredia on the upper surface (N. parile) and tomentum on the lower surface (many species). Nephroma spp occur mainly on mossy trees and rocks e.g. in boreal regions. Nephroselmis See MICROMONADOPHYCEAE. nephrotoxic Toxic to the kidneys. nepoviruses (‘nematode-transmitted polyhedral viruses’; tobacco ringspot virus group) A group of ssRNA-containing PLANT VIRUSES which have a wide host range. Infected plants may be symptomless, or the leaves may initially develop ringspots, spots, mottling, curling etc; leaves produced subsequently may be symptomless. Vesicular inclusion bodies occur in the cytoplasm of infected cells, usually near the nucleus. Transmission occurs via seeds, via soil nematodes (e.g. Longidorus spp), and (experimentally) mechanically; replication does not occur in the nematode vector. Type member: tobacco ringspot virus (TobRV); other members include e.g. cherry leaf roll virus, chicory yellow mottle virus, cocoa necrosis virus, peach rosette mosaic virus, potato black ringspot virus, raspberry ringspot virus, tomato black ring virus, tomato ringspot virus. Possible members: cherry rasp leaf virus, satsuma dwarf virus, strawberry latent ringspot virus, tomato top necrosis virus. Virion: icosahedral, ca. 28 nm diam.; genome: two linear, positive-sense ssRNA species, RNA1 (MWt ca. 2.8 × 106 ) and RNA2 (MWt ca. 1.3–2.4 × 106 ). Virus particles may contain either RNA1 (B particles) or RNA2 (M particles), M and B particles being distinguished by their sedimentation coefficients; T particles contain no RNA, while some (B-like) particles may contain two molecules of RNA2. Both M and B particles are necessary for infectivity. Particles of some nepovirus isolates may also contain a SATELLITE RNA. RNA1 and RNA2 show little sequence homology; they are polyadenylated at the 3 end and each is covalently linked to a small polypeptide (MWt ca. 3000–6000) which is necessary for infectivity. RNA1 encodes the genome-linked polypeptide and (possibly) the virus-specific replicase; RNA2 encodes the coat protein. neritic Of or pertaining to coastal waters. Nernst equation See e.g. ION TRANSPORT. nested PCR (nPCR) A variant form of PCR carried out in two phases. In the initial phase, a standard form of PCR is run for e.g. ∼25 cycles; in the second phase, an aliquot of product from the first phase is used as template for another phase of amplification using a (different) pair of primers that are complementary to subterminal (internal) sequences within the first-phase amplicon. Primers used in the first and second phases have been referred to as ‘outer’ and ‘inner’ primers, respectively. The object of nPCR is to improve sensitivity and specificity in a PCR assay; such improvements may reflect e.g. the extra stage of amplification and the competitive advantage of a shorter amplicon during the second phase. nPCR has been particularly useful for amplifying samples of DNA which are in poor condition or which are of limited quantity. nPCR assays may suffer from susceptibility to contamination during preparation of the second stage of amplification. However, the problem of contamination can be minimized by using so-called ‘one-tube’ protocols. One-tube nPCR has been used e.g. for detecting Trichomonas vaginalis in vaginal discharge [JCM (1997) 35 132–138]. Both sets of primers were included in a single reaction mixture (which used the hot-start approach). In the first phase of amplification (30 cycles) the annealing temperature was 62° C; at this temperature only the outer primers were able to bind. In the second phase 516 Newcombe experiment neutropenic enterocolitis (ileocaecal syndrome) (med.) A syndrome characterized by neutropenia (a diminished number of circulating neutrophils) and ulceration, haemorrhage and necrosis of the distal ileum or caecum; Clostridium septicum may play a major causative role in the symptoms. neutrophil An amoeboid, phagocytic PMN (q.v.) responsive to chemotactic stimuli – often the major type of cell involved early in acute INFLAMMATION. Neutrophils contain many enzymes (e.g. LYSOZYME, MYELOPEROXIDASE) (see also BPI PROTEIN, LACTOFERRIN). Stimulation (e.g. by opsonized bacteria or concanavalin A) leads to a ‘respiratory burst’ (metabolic burst) of increased oxygen consumption (over several minutes) starting within seconds of stimulation; much of the oxygen may be used to form SUPEROXIDE radicals (see PHAGOCYTOSIS). (cf. MACROPHAGE.) (See also CALPROTECTIN.) neutrophile An organism which has an optimum growth pH within the pH range 6–8. (cf. ACIDOPHILE and ALKALOPHILE.) nevirapine See ANTIRETROVIRAL AGENTS and AIDS. Nevskia A poorly characterized genus of aquatic bacteria [see e.g. Book ref. 45, pp. 520–523]. new combination In NOMENCLATURE: syn. COMB. NOV. (q.v.). new variant CJD See CREUTZFELDT–JAKOB DISEASE. New World arenaviruses See ARENAVIRIDAE. Newcastle disease (avian pneumoencephalitis; Ranikhet disease) A highly infectious POULTRY DISEASE caused by a PARAMYXOVIRUS. Chickens, turkeys and pheasants may be affected, and mild infections have been reported in ducks, geese and pigeons. Transmission may occur by droplet inhalation or by ingestion of contaminated water or food (e.g. food contaminated with infected pigeon droppings [VR (1984) 115 601–602]). Incubation period: ca. 4–12 days. Symptoms: variable, but commonly include e.g. loss of appetite, diarrhoea, laboured breathing, discharge from eyes, nostrils and mouth, often followed by neurological signs (drowsiness, weakness, paralysis). Mortality rates vary from 0 to ca. 100%. Lab. diagnosis: identification of the virus; an HAI test. Control: slaughter of infected flocks; vaccination using either live or inactivated vaccines. The Newcastle disease virus has been reported to cause conjunctivitis in man. Newcombe experiment An experiment by which Newcombe (in 1949) demonstrated that the apparent adaptation of a bacterial population to an altered environment involves the selection of pre-existing mutants. Nutrient agar plates are each spread with the same quantity of inoculum from a single culture of phage-sensitive bacteria. The plates are incubated for a few hours, during which time each viable cell gives rise to a clone. The plates are then divided into two groups. In group A, the bacterial growth on each plate is redistributed (with a sterile glass spreader) so that the cells of each clone are dispersed over the agar surface. Group B plates are left undisturbed. All plates are then sprayed with a suspension of virulent phage and re-incubated. All the sensitive bacteria lyse; thus, colonies which subsequently develop on any plate must have arisen from cells which were, or had become, resistant to phage. If phage-resistant bacteria developed adaptively, resistant cells would be formed only after exposure of phage; the redistribution of growth in group A plates would therefore not influence the subsequent development of resistant cells, and similar numbers of resistant cells – and hence colonies – should appear on all the plates. If, however, phage-resistant cells arose spontaneously before exposure to phage, each resistant (mutant) cell would form a discrete clone of resistant cells; subsequent redistribution receptor sites for the adsorption of certain viruses (see e.g. INFLUENZAVIRUS). (See also COLOMINIC ACID; NEURAMINIDASE; COMPLEMENT FIXATION (b).) neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18; sialidase) An enzyme which hydrolyses a-ketosidic linkages between terminal N-acetylNEURAMINIC ACIDS and sugar residues in polysaccharides, glycoproteins, etc. Extracellular neuraminidases are produced by certain bacteria (e.g. Streptococcus pneumoniae, Vibrio cholerae) and occur in certain viruses – e.g. members of the Orthomyxoviridae (see INFLUENZAVIRUS) and Paramyxoviridae (genus PARAMYXOVIRUS). The viral neuraminidase (a RECEPTOR-DESTROYING ENZYME, RDE) lowers the viscosity of mucous secretions, exposing the cell surface receptor sites; it also assists the release of progeny virions by eluting them from their receptors. (See also HAEMAGGLUTINATION.) Neuraminidases from different sources may have different specificities, but most cannot hydrolyse COLOMINIC ACID (q.v. for exceptions); they are generally inhibited by EDTA. neuroexocytosis See TETANOSPASMIN. Neurospora A genus of fungi (order SORDARIALES) which occur e.g. on burnt or decomposing vegetation; the organisms form longitudinally-ribbed ascospores in asci borne in perithecia. N. crassa and N. sitophila are both hermaphroditic and heterothallic; other species (e.g. N. tetrasperma) are homothallic or secondarily homothallic (see HOMOTHALLISM and HETEROTHALLISM). N. sitophila forms a TRICHOGYNE-bearing ascogonium but no antheridium, plasmogamy occurring by spermatization or conidiation; eight-spored asci are formed in dark, pyriform perithecia. Conidial stages occur in N. crassa and N. sitophila. (See also BREAD SPOILAGE, ONCOM, POKY MUTANT, RAGI, TEMPEH.) [Control of growth and nuclear division cycle in N. crassa: MR (1981) 45 99–122; Neurospora plasmids: Book ref. 179, pp. 245–258.] neurotoxin A TOXIN which acts on nerve cells or tissues, or at neuromuscular junctions. See e.g. BOTULINUM TOXIN, SAXITOXIN, TETANOSPASMIN. neurotransmitter See BOTULINUM TOXIN and TETANOSPASMIN. neurotropism See TROPISM (sense 2). neuston The community of organisms which live in the surface film of a body of water (e.g. lake, sea); some members of the community (the epineuston) are associated with the upper surface and project into the air, while others (the hyponeuston) are associated with the underside of the film and project into the water below. (cf. NEKTON, PLANKTON.) neutral dye See DYE. neutral petite See PETITE MUTANT. neutral red A red, basic azine DYE used e.g. in VITAL STAINING and as a PH INDICATOR: pH 6.8–8.0 (red to yellow). (See also BIOFUEL CELL.) neutralism (1) The coexistence of two different organisms in an association in which neither is affected by the presence of the other. (2) SYMBIOSIS (sense 1) in which one symbiont is (permanently or temporarily) dependent on the association, and the other derives neither benefit nor harm from the association. (cf. COMMENSALISM.) neutralization test Any test in which antibody prevents or inhibits the expression of a biological function by a microorganism or toxin. Thus, e.g., a microorganism may be identified by its failure to exhibit certain features (e.g. growth, ability to infect cells or organisms) in the presence of known, specific antiserum; a toxin may be identified or quantified by its failure to produce toxic effects (in vitro or in vivo) following admixture with specific antitoxin. (See also DANYSZ’ PHENOMENON.) 517 nexin nicotinic acid (niacin) Pyridine 3-carboxylic acid; the corresponding amide, nicotinamide, is a component of the important coenzymes NAD (q.v.) and NADP. Niacin may be biosynthesized (via quinolinic acid) from aspartate and dihydroxyacetone phosphate (e.g. in many bacteria, and in Saccharomyces cerevisiae growing under anaerobic conditions), or from tryptophan (e.g. in Xanthomonas spp and in most fungi – including S. cerevisiae growing under aerobic conditions). Organisms auxotrophic for niacin or nicotinamide include e.g. Lactobacillus spp (used in the BIOASSAY of niacin), Morganella and Proteus spp (but not Providencia), Bordetella pertussis, some yeasts and fungi (e.g. Mucor rouxii requires niacin under anaerobic conditions but not under aerobic conditions), and certain protozoa (e.g. Tetrahymena pyriformis, coccidia). (See also NIACIN TEST; V FACTOR; VITAMIN.) niddamycin See MACROLIDE ANTIBIOTICS. Nidula See NIDULARIALES. Nidulariales An order of humicolous, lignicolous or coprophilous fungi (class GASTEROMYCETES) characterized by a type of basidiocarp in which the GLEBA occurs in several parts, each part contained within a closed chamber (peridiole) which is physically separate from the other chambers; in some species each peridiole is attached to the inside of the (typically cup-shaped or funnel-shaped) peridium by a complex, extensible structure. The appearance of peridioles at the bottom of the peridium (which is open at maturity) has earned these fungi the name ‘bird’s nest fungi’. Genera include Crucibulum, CYATHUS, Mycocalia and Nidula. nif genes Genes whose expression is necessary for NITROGEN FIXATION. In Klebsiella pneumoniae there are at least 17 nif genes clustered (close to the his operon) in 8 contiguous transcriptional units (nifJ, nifHDK, nifY, nifENX, nifUSVM, nifF, nifLA, and nifBQ) spanning ca. 23 kb on the chromosome. Genes nifHDK are structural genes for NITROGENASE: nifD and nifK specify the two subunits of the MoFe protein (‘Kp1’), nifH specifies the subunit of the Fe protein (‘Kp2’). The nifJ product is the pyruvate:flavodoxin oxidoreductase which transfers electrons from pyruvate to the flavodoxin (nifF product) that reduces the Fe protein of nitrogenase. The nifB, nifE, nifN and nifV genes all appear to be involved in the synthesis of the FeMo cofactor; nifQ appears to be involved in molybdate uptake; nifM, nifS and nifU may be involved in the processing of nitrogenase components; nifL and nifA are concerned with nif gene expression. Expression of nif genes in K. pneumoniae is regulated at the transcriptional level and is repressed e.g. by alternative sources of nitrogen, by high oxygen levels, etc. The nifA gene specifies an activator of transcription which is necessary for the expression of most nif genes. The nifL gene specifies a negative regulator which represses transcription in the presence of e.g. amino acids, low levels of NH4 + , and oxygen; it appears not to interact directly with DNA but may complex with and inactivate the NifA protein. Transcription of nif genes is regulated by the availability of combined nitrogen and is subject to control by the NTR GENES. The nif genes of other bacteria are generally less well characterized: Azotobacter appears to have additional copies of nifH; in Bradyrhizobium strains nifH is separated from nifDK; Rhodopseudomonas capsulata has multiple copies of nifHDK, while its other nif genes are apparently more widely dispersed on the chromosome than are those of K. pneumoniae. In Rhizobium spp nif genes are located on the Sym plasmid (see entry ROOT NODULES). In most nitrogen-fixing CYANOBACTERIA, nif genes are expressed only in HETEROCYSTS. In vegetative cells of e.g. of growth on the group A plates would disperse the resistant cells over the agar surface, so that each resistant cell on a group A plate would subsequently form a single colony. On the group B plates, however, each undisturbed clone of resistant cells would form only one colony. Hence, far more colonies would be formed on the group A plates than on the group B plates – as is found in practice. (cf. FLUCTUATION TEST.) nexin See FLAGELLUM (b). NF-kB See CYTOKINES. ngaione See IPOMEAMARONE. NGU NON-GONOCOCCAL URETHRITIS. NHI protein NON-HAEM IRON PROTEIN. Ni protein See ADENYLATE CYCLASE. Nia A genus of marine fungi (GASTEROMYCETES, Melanogastrales). N. vibrissa occurs on submerged wood; it forms globose basidiocarps (2–4 mm diam.) and five-limbed basidiospores. niacin NICOTINIC ACID. niacin test A test used to determine whether or not a detectable amount of niacin is liberated into a solid medium when a given strain of Mycobacterium is grown on that medium. Water or saline (ca. 1 ml) is allowed to stand in contact with the medium for 15 min to extract any niacin present. To ca. 0.5 ml extract is added 0.5 ml aniline (4% in ethanol) followed by 0.5 ml cyanogen bromide (CNBr: 10% aqueous); an immediate yellow coloration indicates the presence of niacin (a positive test). (To avoid the formation of free HCN from CNBr, used materials should be treated with NaOH prior to disposal.) In an alternative method the extract is tested with commercial niacin-test strips instead of with aniline and CNBr. [Book ref. 53, p. 1708.] niacinamide Nicotinamide. NIAID National Institute of Allergy and Infectious Diseases (USA). nic See CONJUGATION (b)(i). NICED National Institute of Cholera and Enteric Diseases, Beliaghata, Calcutta 700010, India. Nichols’ treponeme (1) A virulent strain of Treponema pallidum subsp. pallidum. The organism has been reported to grow and divide on the surface of a tissue culture of cotton-tail rabbit epithelium cells (Sf1EP) under 1.5% oxygen [Inf. Immun. (1981) 32 908–915]. (2) Strains of Treponema refringens. nick A break in one strand of a dsDNA molecule due to the loss (or absence) of a phosphodiester linkage in an otherwise intact strand, as opposed to the loss (or absence) of one or more nucleotides from the strand, which is termed a gap. A nick can be repaired by a DNA LIGASE, a gap cannot. nick-closing enzyme A type I TOPOISOMERASE. nick translation technique A technique used e.g. for preparing a radioactively labelled DNA strand. A dsDNA molecule is nicked, using an appropriate endonuclease, and treated with DNA POLYMERASE I; this enzyme, using the nicks as starting points, can degrade the nicked strand using its 5 → 3 exonucleolytic activity, replacing it with a new strand using its polymerase activity. The presence of plenty of radiolabelled dNTPs in the medium ensures that the new strand will be extensively labelled. nickase An ENDONUCLEASE which makes a single-strand nick in dsDNA (e.g., gpA of BACTERIOPHAGE fX174). nicking enzyme An enzyme that nicks one strand of dsDNA: see NICKASE and RESTRICTION ENDONUCLEASE. Nicollia See BABESIA. nicotinamide See NICOTINIC ACID. nicotinamide adenine dinucleotide See NAD. nicotinamide nucleotide transhydrogenase See TRANSHYDROGENASE. 518 nitrate respiration [Synergistic action of nisin and CO2 against Listeria monocytogenes: AEM (2000) 66 769–774. Bioassay for nisin in food: AEM (2003) 69 4214–4218.] Nitella See CHAROPHYTES. nitrapyrin (N-Serve) 2-Chloro-6-(trichloromethyl)pyridine, a NITRIFICATION INHIBITOR. nitratase Syn. nitrate reductase. nitrate broth Nutrient broth (or peptone water) containing KNO3 (0.1–0.2% w/v). nitrate reductase See ASSIMILATORY NITRATE REDUCTION; DENITRIFICATION; NITRATE RESPIRATION. nitrate reduction See ASSIMILATORY NITRATE REDUCTION and DISSIMILATORY NITRATE REDUCTION. nitrate reduction test (nitrate test) A test used to indicate whether or not a given bacterial strain can reduce nitrate; the medium and test conditions used depend on the particular organism being examined. For many bacteria (including enterobacteria and pseudomonads, but excluding e.g. Mycobacterium spp) the organism is cultured for 2–5 days in NITRATE BROTH. The culture is then tested for the presence of nitrite by adding 0.5 ml ‘reagent A’ (0.8% w/v sulphanilic acid in 5 N acetic acid) followed by 0.5 ml ‘reagent B’ (0.6% w/v N,N-dimethyl-a-naphthylamine in 5 N acetic acid); if present, nitrite reacts with sulphanilic acid to form a diazonium salt which, with the naphthylamine, forms a soluble red azo dye. The absence of coloration could mean that either (a) nitrate has not been reduced, or (b) the nitrite has itself been reduced. To resolve this question a trace of zinc dust is added; if nitrate is present it is reduced (by the zinc dust) to nitrite (which gives a red coloration) – the absence of coloration indicating possibility (b). For Mycobacterium spp, a small loopful of growth is dispersed in 0.02 M phosphate buffer (pH 7.0) containing 0.01 M NaNO3 , and the suspension is incubated (37° C/2hours). To the suspension is then added 1 drop of HCl (concentrated HCl:water mixed 1:1 v/v) followed by 2 drops of sulphanilamide solution (0.2% aqueous) and 2 drops of N-naphthylethylenediamine dihydrochloride (0.1% aqueous); pink or red coloration indicates reduction of nitrate to nitrite. If no colour develops, the addition of zinc dust causes a red coloration if the medium still contains nitrate – and fails to do so if the nitrate had been reduced beyond nitrite. nitrate respiration ANAEROBIC RESPIRATION in which, typically, nitrate (as terminal electron acceptor) is reduced to nitrite in a reaction catalysed by an enzyme (dissimilatory nitrate reductase) located at the end of an ELECTRON TRANSPORT CHAIN; this reaction, which is equivalent to the first stage of DENITRIFICATION, generates proton motive force (pmf: see CHEMIOSMOSIS) which can be used e.g. for ATP synthesis. (cf. DISSIMILATORY NITRATE REDUCTION.) In some organisms the nitrite is excreted, but in others it can be reduced to ammonia. In nitrate respiration the energy yield is less than that obtainable with oxygen as electron acceptor. In some bacteria (including e.g. Escherichia coli ) nitrate can be used for the anaerobic oxidation of e.g. NADH, succinate and/or formate – pmf being generated in each case; the resulting nitrite inhibits aconitase and fumarase, thus inhibiting the TCA cycle [Appendix II(a)]. In e.g. Escherichia coli a soluble (‘assimilatory’) nitrite reductase can catalyse the anaerobic reduction of nitrite to ammonia in a reaction coupled to the oxidation of NADH. This reaction does not, in itself, yield energy; however, by regenerating NAD+ from NADH (produced during the metabolism of e.g. glucose) it tends to prevent the (energetically wasteful) synthesis of ethanol – see Anabaena, the nifH and nifD genes are contiguous, but nifK is separated from nifD by ca. 11 kb. It has been shown that, late in heterocyst differentiation, the DNA undergoes certain rearrangements, including the excision and circularization of the 11-kb segment between nifD and nifK; this results in the fusion of nifK with nifHD and allows coordination of transcription of the nifHDK genes. [nif and associated genes in Acetobacter diazotrophicus: JB (2000) 182 7088–7091.] nifuratel See NITROFURANS. nifuroxime See NITROFURANS. nifurprazine See NITROFURANS. nigeran (mycodextran) A linear a-D-glucan in which glucopyranose residues are linked by alternating (1 → 3)-a- and (1 → 4)-a-glycosidic linkages; it is soluble in hot water. Nigeran occurs in the hyphal walls of Aspergillus spp (but not in A. nidulans, which contains PSEUDONIGERAN) and Penicillium spp. [Nigeran as a phylogenetic marker: Mycol. (1978) 70 1201–1211.] nigericin A linear MACROTETRALIDE antibiotic which can act as a mobile carrier IONOPHORE for the transport of monovalent cations – K+ or Rb+ (or, to a lesser extent, e.g. Na+ ). In order to transport an ion the nigericin molecule loses a proton and binds the ion, and the (electrically neutral) nigericin–ion complex diffuses across the membrane; since nigericin can also cross the membrane in a protonated state, it can bring about the exchange of H+ for K+ (see ELECTRONEUTRAL TRANSPORT). Nigericin is used e.g. to alter a membrane pH gradient (see CHEMIOSMOSIS). nigerose a-D-Glucopyranosyl-(1 → 3)-D-glucopyranose. nigrosin A black composite indulin-based DYE used e.g. for negative STAINING. nikA gene See CONJUGATION (1b)(i). nikkomycins A group of antibiotics which closely resemble POLYOXINS both in structure and in mode of action; nikkomycins have a wider spectrum of activity than polyoxins, being more active e.g. against the yeast forms of Mucor rouxii and Candida albicans. Nikolsky sign Wrinkling of the skin (evident on gentle stroking) due to separation of the outer epidermis from the basal layer. (See also EPIDERMOLYTIC TOXIN.) Nile blue A (Nile blue sulphate) A blue basic oxazine DYE used e.g. in VITAL STAINING and as a lipid stain. In aqueous solution some of the dye is spontaneously oxidized to the phenoxazone LYSOCHROME Nile red (soluble in aqueous Nile blue A); this allochromatic mixture stains fatty acids and phospholipids blue, and neutral lipids red. Nile blue A can be used as a fluorescent stain for poly-b-hydroxybutyrate [AEM (1982) 44 238–241]; Nile red is useful as a vital stain for intracellular lipid droplets, being strongly fluorescent only in a hydrophobic environment [JBC (1985) 100 965–973]. Nile red See NILE BLUE A. nisin A low-MWt LANTIBIOTIC produced by strains of Lactococcus; genes for nisin synthesis appear to be plasmid-borne. [Nisin (review): AAM (1981) 27 85–123.] Nisin is bactericidal against Gram-positive bacteria, and has some sporicidal activity; it is both heat-stable and acid-stable, but stability and solubility decrease with increasing pH, and nisin is inactivated by certain proteases (e.g. chymotrypsin). Nisin is not used clinically; it is employed as a food PRESERVATIVE, generally in conjunction with heat treatment, in e.g. milk and DAIRY PRODUCTS (in which it occurs naturally) and certain canned vegetables. Canned, lowacid foods in which nisin is used must still receive adequate heat treatment because Clostridium botulinum is relatively resistant to nisin. (cf. DIPLOCOCCIN and SUBTILIN.) 519 nitrate test aquatic environments, and in the upper (aerobic) region of some sediments. (See also SEWAGE TREATMENT.) Nitrification is carried out by autotrophic bacteria of the family NITROBACTERACEAE and by many heterotrophic organisms, including some fungi. It has been generally supposed that only the autotrophic nitrifying bacteria carry out nitrification to an ecologically significant extent; however, while this appears to be the case e.g. in the sediments of a Scottish estuary [JGM (1984) 130 2301–2308], the potential for heterotrophic nitrification was found to be greater than that for autotrophic nitrification in a Californian forest soil [AEM (1984) 48 802–806]. Autotrophic nitrification occurs in two stages, each stage being carried out by one of the two categories of nitrifying bacteria. (a) Ammonia to nitrite (‘nitrosofication’). Ammonia is initially oxidized to hydroxylamine (NH2 OH) in a MONOOXYGENASEdependent, non-energy-yielding reaction; NH2 OH is then oxidized to nitrite by an enzyme complex, electrons being transferred to molecular oxygen with concomitant oxidative phosphorylation. (b) Nitrite to nitrate. The oxidation of nitrite to nitrate provides energy by oxidative phosphorylation: in at least some nitrite-oxidizing bacteria NO2 − is oxidized in the reaction: NO2 − + H2 O → NO3 − + 2H+ + 2e− ; the electrons are believed to be transferred to cytochrome (cyt) a1 which in turn transfers them via cyt aa3 to molecular oxygen, 1ATP being formed for each pair of electrons transferred. In the nitrite-oxidizing bacteria (and probably also in NH4 + oxidizers) pyridine nucleotide coenzymes are reduced by reversed electron transport. In at least some nitrite oxidizers, electrons appear to pass from cyt a1 , via cyts c and b, to a flavoprotein and thence to the pyridine nucleotides; this process is highly energy-dependent, so that nitrifying bacteria need large quantities of NO2 − to sustain their relatively slow rates of growth. Agriculturally, nitrification can be disadvantageous under certain conditions: NO3 − is readily leached from the topsoil by rain – thereby becoming unavailable as a source of crop nitrogen. (By contrast, owing to its charge, NH4 + is more strongly adsorbed to soil particles and is relatively immobile in top-soils.) Nevertheless, when e.g. the crop’s nitrogen demand is high and the rainfall (and hence risk of leaching) is likely to be low, rapid nitrification may actually be desirable since (a) optimal growth in many or most plants appears to occur only when both NH4 + and NO3 − are available, and (b) NH4 + may be assimilated (in preference to NO3 − ) by soil microorganisms so that, in the absence of nitrification, much of the available soil nitrogen may be lost to crops as a consequence of being immobilized in microbial biomass. However, when e.g. the risk of leaching is high, the use of NITRIFICATION INHIBITORS can be advantageous; these agents have a temporary effect (they are gradually lost by evaporation and/or decomposition), and they not only benefit soil fertility but also reduce the pollution of groundwater by nitrate. (See also NITROGEN CYCLE.) nitrification inhibitors Agents which retard or block NITRIFICATION; nitrification inhibitors appear to be typically bacteriostatic under field conditions. The agriculturally important agents inhibit microbial oxidation of ammonia and may also inhibit the oxidation of nitrite – though nitrite oxidation is generally much less sensitive to these agents; agents which block only nitrite oxidation cannot be used since they could cause accumulation of (phytotoxic) nitrite ions. Commercial nitrification inhibitors – some of which also have useful antifungal properties – include e.g. DICYANDIAMIDE, ETRIDIAZOLE and NITRAPYRIN. (See also BARBAN and CARBON DISULPHIDE.) [Review: Book ref. 121, pp. 33–43.] Appendix III(e) – thereby increasing the yield of acetic acid and, hence, conserving energy via acetyl phosphate. Since the oxidation of NADH by nitrite (i) depends on an exogenous electron acceptor, (ii) results in an overall energy yield greater than that obtainable by the mixed acid fermentation, and (iii) results in a net oxidation of the substrate, it conforms to the definition of RESPIRATION and may be described as ‘nitrite respiration’. In Desulfovibrio gigas the coupling of nitrite reduction to the oxidation of hydrogen appears to generate pmf; accordingly, this process, too, may be referred to as nitrite respiration. Some anaerobic bacteria (e.g. Clostridium perfringens) can carry out dissimilatory nitrate reduction [JGM (1975) 87 120–128]. This process, in which nitrate is reduced to ammonia, apparently does not generate pmf; however, it is coupled to the regeneration of NAD+ from NADH, and brings about an increase in the proportion of those intermediate metabolites which can participate in substrate-level phosphorylation. This process, which is sometimes referred to as ‘fermentative nitrate reduction’, may be regarded as another example of nitrate respiration. So-called dissimilatory reduction of nitrate to ammonia (DRNA) is a respiratory process reported to take place e.g. in marine sediments, particularly where nitrate concentrations are low [see FEMS Ecol. (1996) 19 27–38]. nitrate test See NITRATE REDUCTION TEST. nitric oxide (NO) On suitable activation, various types of cell (including macrophages) synthesize an inducible form of the enzyme nitric oxide synthase (iNOS); synthesis of iNOS is positively regulated at the level of transcription by certain cytokines (e.g. TNF-a, IFN-g). iNOS catalyses the NADPHdependent oxidative deamination of L-arginine to L-citrulline and nitric oxide. Nitric oxide gives rise to a range of reactive species by interaction with oxygen, water and/or other reactive intermediates; they include peroxynitrite (ONOO− ) (formed by the reaction between superoxide and nitric oxide), nitrogen dioxide and nitrogen trioxide. Peroxynitrite forms an adduct with carbon dioxide that is potentially toxic. In e.g. a target bacterium, nitric oxide and the various reactive species inhibit oxygen-binding haemcontaining enzymes (such as terminal oxidases) and certain other iron-containing enzymes (e.g. the TCA cycle enzyme aconitase). Damage to an infecting bacterium may include failure of DNA replication and/or the loss of important metabolic processes, leading to death of the cell. The production of large amounts of nitric oxide by iNOS therefore serves a protective role. However, ongoing production of high levels of nitric oxide in a persistent, unresolved infection can suppress the host’s immune system and bring about various forms of disease. Inhibitors of iNOS inhibit the killing of Mycobacterium tuberculosis by IFN-g-activated murine macrophages; moreover, studies in mice indicate that deletion of IFN-g genes increases the susceptibility of the mice to M. tuberculosis – this being associated with a deficiency of iNOS. Bacterial reaction to nitric oxide includes e.g. increased activity of enzymes involved in the antioxidant response to oxidative stress. [Effects of nitric oxide on pathogens and host: RMM (1998) 9 179–189. Niric oxide in sepsis and endotoxaemia: JAC (1998) 41 (supplement A) 31–39. Bacterial responses to nitric oxide and nitrosative stress: Mol. Microbiol. (2000) 36 775–783.] nitric oxide synthase See NITRIC OXIDE. nitrification The oxidation of ammonia to nitrite, and nitrite to nitrate: a process which occurs e.g. in aerobic soils, in 520 nitrogen fixation In laboratory studies, acetylene has been used to block ammonia oxidation, and chlorate to block nitrite oxidation, in autotrophic nitrifiers [AEM (1984) 48 802–806]. nitrifying bacteria See NITROBACTERACEAE. nitriloacetic acid See NTA. nitrite (a) (as a food additive and preservative) See e.g. CURING (sense 1). (b) (in the environment) See NITROGEN CYCLE. nitrite reductase See ASSIMILATORY NITRATE REDUCTION; DENITRIFICATION; NITRATE RESPIRATION. nitrite respiration See NITRATE RESPIRATION. Nitrobacter A genus of Gram-negative bacteria of the NITROBACTERACEAE; strains occur in soils and in freshwater and marine habitats. Cells of the type species, N. winogradskyi, are rods ca. 0.6–0.8 × 1.0–2.0 µm; they are commonly non-motile but may have a single polar flagellum. Peripherally arranged cytomembranes form a cap at one pole of the cell. The cells reproduce by budding. Some strains are obligate chemolithoautotrophs which obtain energy by oxidizing nitrite to nitrate; other strains are facultatively chemoorganoheterotrophic. Storage compounds include PHB. Growth occurs optimally at ca. 25–30° C and at a pH of ca. 7.5–8.0. GC%: ca. 61. (See also CARBOXYSOMES.) Nitrobacteraceae (nitrifying bacteria) A family of Gramnegative, asporogenous, obligately aerobic bacteria which occur e.g. in soil and in freshwater and marine habitats; the organisms obtain energy by the oxidation of ammonia or nitrite (see NITRIFICATION), and all except some strains of Nitrobacter winogradskyi are obligate chemolithoautotrophs. The family includes coccoid, rod-shaped, spiral and lobate bacteria. Cells may be non-motile or may have polar, subpolar or peritrichous flagella. The cells of many species contain characteristic cytomembranes. Most species divide by binary fission; budding occurs in Nitrobacter winogradskyi. Species which oxidize ammonia to nitrite are placed in the following genera: NITROSOCOCCUS, NITROSOLOBUS, NITROSOMONAS, NITROSOSPIRA and NITROSOVIBRIO; nitrite-oxidizing species are placed in the genera NITROBACTER, NITROCOCCUS and NITROSPINA. nitroblue tetrazolium See TETRAZOLIUM. nitrocefin A chromogenic cephalosporin used to test for bLACTAMASES; when cleaved by a b-lactamase it changes from yellow to red. (See also PADAC.) Nitrococcus A genus of Gram-negative marine bacteria of the NITROBACTERACEAE. Cells of the type species, N. mobilis, are coccoid, ca. 1.5–1.8 µm diameter, each having one or two subpolar flagella; the cells contain tubular cytomembranes arranged randomly in the cytoplasm. The cells reproduce by binary fission. GC%: ca. 61. nitrofurans A family of synthetic ANTIBIOTICS which are derivatives of 2-substituted 5-nitrofurans; they are active against many Gram-positive and Gram-negative bacteria, certain fungi, and some protozoa. Some nitrofurans are used as chemotherapeutic agents, but many are known to be mutagenic/oncogenic. The antibacterial activity of a nitrofuran depends on the (stepwise) intracellular reduction of the nitro group, the intermediate(s) formed being able to cause strand breakage in DNA. (cf. NITROIMIDAZOLES.) In e.g. Escherichia coli, the reduction of a nitrofuran involves NADH-linked ‘nitrofuran reductase 1’ (NR1) and NADPH-linked ‘nitrofuran reductase 2’ (NR2); mutants lacking NR1 exhibit high-level resistance to nitrofurans. (Little is known about plasmid-encoded resistance to nitrofurans; if it exists, the incidence appears to be negligible.) [Resistance to nitrofurans: Book ref. 160, pp. 317–344.] Examples of nitrofurans: Furaltadone is used e.g. against colibacillosis and salmonellosis in pigs and cattle. (See also PULLORUM DISEASE.) Furazolidone (furoxone) has been used e.g. against bacillary dysentery and Salmonella infections in man, and is used e.g. in the treatment of human GIARDIASIS and certain diseases of animals (e.g. BLACKHEAD). Furylfuramide (AF-2 ) is an oncogenic nitrofuran once used as a food preservative in Japan. Nifuratel has been used e.g. against vaginal trichomoniasis. Nifuroxime has been used, topically, as an antifungal agent, being active against e.g. Candida albicans; it has also been used (in conjunction with furazolidone) against vaginal trichomoniasis. Nifurprazine is used as a topical, broad-spectrum antibacterial agent. Nitrofurantoin (furadantin) is the most widely-used nitrofuran. It is used as a broad-spectrum antibacterial agent against infections of the (human) urinary tract; many strains of Pseudomonas aeruginosa are resistant, and resistant strains of e.g. Proteus also occur. Nitrofurazone (furacin) is used, rarely, for the topical treatment of e.g. infected wounds. nitrofurantoin See NITROFURANS. nitrofurazone See NITROFURANS. nitrogen cycle In nature: the cyclical interconversion of nitrogen and its compounds by living organisms and non-biological processes. The major microbial conversions are shown on page 522. (See also AMMONIA ASSIMILATION, AMMONIFICATION, ASSIMILATORY NITRATE REDUCTION, DENITRIFICATION, DISSIMILATORY NITRATE REDUCTION, DISSIMILATORY REDUCTION OF NITRATE TO AMMONIA, NITRATE RESPIRATION, NITRIFICATION, NITROGEN FIXATION.) (cf. CARBON CYCLE and SULPHUR CYCLE.) nitrogen fixation (dinitrogen fixation) The reduction of gaseous nitrogen (dinitrogen, N2 ) to ammonia. (See also NITROGEN CYCLE.) Biological nitrogen fixation is carried out only by certain prokaryotes; these nitrogen-fixing organisms are called diazotrophs. The diazotrophs include various cyanobacteria (e.g. Anabaena, Nostoc, Plectonema, Trichodesmium); certain species of Bacillus and Clostridium; Azospirillum; Klebsiella pneumoniae (some strains); members of the Azotobacteriaceae, Methylococcaceae, Rhizobiaceae and Rhodospirillales; and certain species of the Archaea. That plants are unable to fix nitrogen is interesting in that, during evolution, plants are believed to have incorporated prokaryotes as organelles (e.g. mitochondria). However, during that period of evolution, prokaryotes may not have yet developed nitrogen-fixing ability and, moreover, plants may not have been subjected to sufficient selection pressure [PTRSLB (1992) 338 409–416]. The free-living diazotrophs occur e.g. in soil and water, while a number of diazotrophs form symbiotic associations with other organisms (which benefit from a supply of fixed nitrogen). AZOSPIRILLUM fixes nitrogen in the RHIZOSPHERE of various plants, including some important crop plants. Certain species of AZOTOBACTER fix nitrogen in association with the roots of a grass, Paspalum notatum, and the surface of the green seaweed Codium. Clostridium spp are found in waterlogged (anaerobic) soils, and may also fix nitrogen in the intestines of man and other animals. Frankia spp form symbiotic associations with certain non-leguminous plants (see ACTINORRHIZA), while Rhizobium/Bradyrhizobium spp are found in leguminous ROOT NODULES and STEM NODULES. 521 nitrogen fixation NO3− nitra te r e sp ira ti o ct io n n n du tr ni o a ti ifi c Nitrobacter Nitrococcus a ss im il a to ry te tra ni Clostridium spp Escherichia coli re Bacillus licheniformis Pseudomonas stutzeri Thiobacillus denitrificans − NO2 denit r i fi cat ion − N2 on n it r ifi c at io n on p ir a ti es )r it e re du ct io n io n Nitrosococcus Nitrosomonas n il a t tra te nitrogen fixati ni sim ry as to organic matter Some cyanobacteria Azotobacteraceae Rhizobium ia il a ica tio m on si am mo n if as NO2 NH3 am m n t itra e (n it r + NH4 NITROGEN CYCLE – microbial aspects, with examples of organisms. (For cross-references see entry NITROGEN CYCLE.) Not shown is the activity of ANAMMOX BACTERIA (q.v.). Reproduced from Bacteria 5th edition, Figure 10.2, page 258, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. Nitrogen fixation is catalysed by the enzyme complex NITROThe reaction requires a strong reducing agent (source of electrons) and consumes ∼12–16 molecules of ATP per molecule of nitrogen fixed; electrons from a source such as hydrogen or NADPH are transferred e.g. to a FERREDOXIN or FLAVODOXIN and thence to nitrogenase. (For in vitro work, the artificial reductant dithionite can be used.) In some diazotrophs (e.g. Bacillus polymyxa, Clostridium pasteurianum, Klebsiella pneumoniae, some cyanobacteria) reducing power and ATP can both be obtained from pyruvate. Reducing power is provided by the oxidative decarboxylation of pyruvate to acetyl-CoA (catalysed by the enzyme pyruvate:flavodoxin oxidoreductase); the acetyl-CoA is converted to acetyl phosphate – from which phosphate can be transferred to ADP in a substrate-level phosphorylation. The ammonia produced by nitrogen fixation may be assimilated e.g. via the GOGAT pathway (see AMMONIA ASSIMILATION). Nitrogenase is highly sensitive to oxygen, and many diazotrophs (e.g. clostridia) fix nitrogen anaerobically or microaerobically. In some cyanobacteria the process is carried out within specialized cells (see HETEROCYST). Certain diazotrophs have specific mechanisms for protecting nitrogenase during aerobic nitrogen fixation; for example, rhizobium bacteroids in ROOT Nitrogen-fixing cyanobacteria are an important source of nitrogen e.g. in rice paddies; for example, Anabaena azollae forms a symbiotic association with a small, floating fresh-water fern, Azolla, that is used as ‘green manure’ in rice-growing regions of the Far East. (See also NOSTOC.) Cyanobacteria can also form associations with certain LICHENS (see e.g. CEPHALODIUM). Both free-living and symbiotic nitrogen-fixers occur in the open ocean, and such organisms may contribute significantly to the nitrogen budget of these waters. [Nitrogen-fixing microorganisms in tropical and subtropical oceans: TIM (2000) 8 68–73.] The benefits enjoyed by those plants that have nitrogenfixing symbiotic partners have suggested that the growth of certain crop plants, such as cereals, may be improved by infecting them experimentally with nitrogen-fixing organisms. Experiments have been carried out to determine whether wheat can be infected with the nitrogen-fixing bacterium Azorhizobium caulinodans (which nodulates a tropical legume, Sesbania rostrata); A. caulinodans became established endophytically (in the xylem and root meristem), and the wheat showed significantly increased dry weight and nitrogen content compared with uninoculated control plants [Proc. RSLB (1997) 264 341–346]. GENASE. 522 Nitrosomonas are protected by LEGHAEMOGLOBIN. (See also TRIStrains of Azotobacter and possibly Gloeothece [JGM (1984) 130 495–503] exhibit oxygen-scavenging ‘respiratory protection’: they show high rates of respiration in the presence of high levels of oxygen, their branched respiratory chains being poorly coupled to ATP synthesis; respiration thus serves primarily to consume oxygen. In those organisms which have an ‘uptake hydrogenase’ (see NITROGENASE), the transfer of electrons from hydrogen – via an electron transport chain to oxygen (as terminal electron acceptor) – may also help to protect nitrogenase from oxygen. In unicellular, aerobic, nitrogen-fixing cyanobacteria of the genus Cyanothece, nitrogen fixation was found to occur only in the dark period during alternating 12-hour light/dark cycles, but was continuous during 24-hour illumination; nitrogen fixation during the dark period would avoid exposure to the oxygen evolved by photosynthesis, but continuous fixation during continual illumination requires additional explanation – and may involve the ‘inclusion granules’ that develop under nitrogen-fixing conditions [JB (1993) 175 1284–1292]. [Cyanobacterial nitrogen fixation (oxygen relationships): MR (1992) 56 340–373. Nitrogen fixation by non-heterocystous cyanobacteria: FEMS Reviews (1997) 19 139–185.] nitrogen mustards Highly reactive bifunctional ALKYLATING AGENTS, most of which correspond to the general formula Cl(CH2 )2 .NR.(CH2 )2 Cl. They are toxic (e.g. causing blisters on contact with the skin), mutagenic and carcinogenic, and promote IMMUNOSUPPRESSION. Nitrogen mustards (and the SULPHUR MUSTARDS) react with DNA e.g. at the N-7 position of guanine residues, forming mono-adducts as well as inter- and intra-strand cross-links. nitrogen-regulated genes See NTR GENES. nitrogenase (dinitrogenase) An enzyme complex (EC 1.18.2.1) which catalyses biological NITROGEN FIXATION: it combines with N2 and reduces it to NH3 . Nitrogenase can also reduce other substrates – e.g., acetylene, azide, cyanide, nitrous oxide; the reduction of acetylene to ethylene (detectable by gas chromatography) is the basis of an assay for nitrogenase. The nitrogenase complex typically comprises two components: the MoFe (or FeMo) protein (also called e.g. component I, molybdoferredoxin, azofermo, or dinitrogenase), and the Fe protein (= e.g. component II, azoferredoxin, azofer, dinitrogenase reductase). The MoFe protein (MWt ca. 220000–250000) is an a2 b2 tetramer containing ca. 50 iron atoms, 2 molybdenum atoms, and ca. 30 acid-labile sulphide groups; the molybdenum occurs in an FeMo cofactor (FeMoco) containing at least 1Mo per 6Fe [ARB (1984) 53 231–257]. (An alternative nitrogenase has been found in a strain of Azotobacter chroococcum in which the MoFe protein is replaced by a vanadium-containing protein [Nature (1986) 322 388–390].) The Fe protein (MWt ca. 55000–60000) contains two identical protein subunits, 4 iron atoms and 4 acid-labile sulphide groups. The substrate (e.g. N2 ) is bound by the MoFe protein and, in the presence of Mg-ATP, electrons are transferred from reduced ferredoxin or flavodoxin to the Fe protein which, in turn, reduces the MoFe protein with concomitant ATP hydrolysis. (At least 2ATP are hydrolysed for each electron transferred.) In an apparently inevitable side-reaction of nitrogenase, electrons are transferred to protons, resulting in the ATP-dependent evolution of H2 ; H2 evolution thus competes with nitrogen fixation for energy and reducing power. Some organisms (e.g. some Rhizobium bacteroids [Book ref. 55, pp. 179–203], Azotobacter spp, and some cyanobacteria) possess an oxygen-dependent enzyme system termed ‘uptake hydrogenase’ which oxidizes the H2 to yield ATP; this enzyme system also serves to remove H2 (a competitive inhibitor of nitrogenase) from the active centre of nitrogenase. Nitrogenase action is slow, and large amounts of the enzyme are necessary for effective nitrogen fixation; the enzyme complex may comprise up to 10% of the cellular protein in an actively diazotrophic cell. Control of nitrogenase may occur at the transcriptional level (see NIF GENES) or at the level of the enzyme itself. Nitrogenase is inhibited e.g. by H2 , NH3 (indirectly), and NO3 − (probably via nitrite); addition of NH3 to cultures of nitrogen-fixing photosynthetic bacteria or Azospirillum results in inactivation of the Fe protein by covalent modification. Nitrogenase is highly oxygen-labile (see NITROGEN FIXATION). nitroimidazoles A group of synthetic antimicrobial agents which are maximally effective against anaerobes. Their activity appears to depend on at least partial reduction of the nitro group by metabolic pathways linked to low-redox-potential electron transport components (such as ferredoxin) in the susceptible organism; reduction results in the formation of short-lived cytotoxic intermediate(s) – see HYDROGENOSOME. (cf. NITROFURANS.) Nitroimidazoles include e.g. azomycin (2-nitroimidazole), carnidazole [1-(2-ethylcarbamothioic acid O-methyl ester)2-methyl-5-nitroimidazole], DIMETRIDAZOLE, flunidazole [1(2-hydroxyethyl)-2-(p-fluorophenyl)-5-nitroimidazole], METRONIDAZOLE, ornidazole [1-(3-chloro-2-hydroxypropyl)-2-methyl-5nitroimidazole], panidazole [1-(2-(4-pyridyl)-ethyl)-2-methyl-5nitroimidazole], secnidazole [1-(2-hydroxypropyl)-2-methyl-5nitroimidazole], and tinidazole [1-(2-(ethylsulphonyl)-ethyl)2-methyl-5-nitroimidazole]. [Comparative evaluation of the 2-methyl-5-nitroimidazoles against Bacteroides fragilis: AAC (1985) 28 561–564.] nitromersol See MERCURY. nitrophenols (as disinfectants) See PHENOLS. N-nitroso compounds Compounds which contain the group −N−N=O. Many nitroso compounds are effective MUTAGENS: e.g., dialkylnitrosamines (R2 N.N=O), N-alkylnitrosoureas (NH2 .CO.NR.N=O), and N-alkyl-N  -nitro-N-nitrosoguanidines (NO2 .NH.C=NH.NR.N=O; see e.g. MNNG) act as ALKYLATING AGENTS (after metabolic activation in the case of dialkylnitrosamines). Halonitrosoureas – e.g. 1,3-bis-(2-chloroethyl)1-nitrosourea (BCNU), bis-(2-fluoroethyl)-nitrosourea (BFNU), and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) – apparently transfer haloethyl, hydroxyethyl and aminoethyl groups to various positions in bases in DNA. Nitrosococcus A genus of Gram-negative bacteria of the NITROBACTERACEAE; the organisms occur in soil and in freshwater and marine habitats. Cells: cocci, diameter 1.5 µm or greater, which divide by binary fission. N. mobilis and N. oceanus are motile with one or more flagella; they differ in the arrangement of their cytomembranes. (See also CARBOXYSOMES.) N. nitrosus is non-motile. GC% (N. oceanus): ca. 51. nitrosofication See NITRIFICATION. nitrosoguanidine See MNNG. Nitrosolobus A genus of Gram-negative bacteria of the NITROBACTERACEAE; the organisms occur in soil. Cells: irregularly rounded (‘lobular’), ca. 1.0–1.5 µm in diameter, with 1–20 flagella; the cells divide by binary fission. Cytomembranes compartmentalize the cell into several central regions and a larger number of peripheral regions. GC%: ca. 53–55. Nitrosomonas A genus of Gram-negative bacteria of the NITROBACTERACEAE; strains occur e.g. in soils and in freshwater and marine habitats. Cells of the type species, N. europaea, are typically rod-shaped, ca. 0.8–1.0 × 1.0–2.0 µm, and either NODULES CHODESMIUM.) 523 Nitrosospira cytotoxic T cells – which are MHC class I-restricted. (NK cells are not MHC class I-restricted; in fact, class I molecules inhibit the killing activity of NK cells.) This role of NK cells appears to be particularly valuable in that many viruses are able to inhibit the display of MHC class I-linked viral antigens at the cell surface. The killing activity of NK cells is stimulated by certain cytokines, including INTERLEUKIN 12. NLF A non-lactose-fermenting bacterium. nm (nanometre) 10−9 m, or 10−3 µm. NMR NUCLEAR MAGNETIC RESONANCE. NNN medium (Novy, Nicolle and MacNeal’s medium) A DIPHASIC MEDIUM used e.g. for the culture of certain Leishmania spp and stercorarian trypanosomes. It is a blood agar SLOPE (containing defibrinated mammalian blood) to which has been added a small volume of e.g. Locke’s solution containing antibiotics to inhibit bacterial growth. The inoculated slope is incubated (e.g. 25–28° C) for days or weeks; a loopful of fluid can be removed at intervals for examination. In culture, pathogenic flagellates may form only certain morphological types; Leishmania donovani grows in the promastigote form, although Trypanosoma cruzi cultures contain e.g. epimastigotes and metacyclic forms. NNRTI NON-NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITOR. Noble agar A purified (washed) form of AGAR. noble rot (pourriture noble; Edelfäule) A rot of grapes caused by Botrytis cinerea. On ripe or over-ripe white grapes the mould penetrates the grape skins and leads to loss of water; must from such grapes is thus more concentrated than normal and is used for making high-quality sweet wines (see WINE-MAKING). (The same mould on unripe or black grapes renders them useless for wine-making.) nocardamine A colourless, trihydroxamate SIDEROPHORE produced e.g. by certain actinomycetes and by Pseudomonas stutzeri under iron-deficient conditions. [JGM (1980) 118 125–129.] Nocardia A genus of Gram-positive, aerobic, chemoorganotrophic, non-motile bacteria (order ACTINOMYCETALES, wall type IV) which occur e.g. in soil and as pathogens of man and other animals (see e.g. MADUROMYCOSIS, NOCARDIOSIS; see also NOCARDICINS); the degree of acid-fastness appears to be variable even within a given isolate. The organisms typically form a branched (substrate and aerial) mycelium (hyphae 1 µm or less in diam.) which, according to species, tends to fragment more or less readily into rods and coccoid forms, some of the latter being heat- and/or desiccation-resistant (see MICROCYST (1)). Cell walls contain MYCOLIC ACIDS. Growth occurs readily at 30° C and 37° C on enriched media; colonies are often wrinkled or ridged, and may be any of a variety of colours. Glucose is metabolized oxidatively. GC%: 64–72. Type species: N. asteroides. The genus includes e.g. N. amarae (isolated e.g. from the foam on an activated-sludge sewage treatment plant) and also N. asteroides, N. brasiliensis and N. farcinica (Kyoto-1 group), N. otitidis-caviarum, and N. vaccinii (a species reported to produce galls in the blueberry plant). ‘N. autotrophica’, ‘N. mediterranea’ and ‘N. rugosa’ each have type IV actinomycete walls but they do not contain mycolic acids and have been removed from the genus [Book ref. 73, pp. 89–91, 129]. (See also e.g. HYDROCARBONS and RUBBER SPOILAGE.) nocardicins A class of monocyclic b-LACTAM ANTIBIOTICS (q.v. for structure) produced by a species of Nocardia. nocardioform actinomycetes A category within the order ACTINOMYCETALES which includes Caseobacter, Corynebacterium, non-motile or motile with 1–2 subpolar flagella. The cells contain lamellar cytomembranes in a mainly peripheral arrangement. Marine strains (and some other strains) have an S LAYER. (See also CARBOXYSOMES.) GC%: ca. 47–51. Nitrosospira A genus of Gram-negative bacteria of the NITROBACTERACEAE; the organisms occur in soil. Under the light microscope the cells appear as rods, ca. 0.3–0.4 × 0.8–1.0 µm, but they appear as tightly coiled spirals under the electron microscope; the cells may be non-motile, or motile with 1–6 flagella, and they divide by binary fission. Cytomembranes are absent. Type species: N. briensis. GC%: ca. 54. nitrosoureas See N-NITROSO COMPOUNDS. Nitrosovibrio A genus of Gram-negative bacteria of the NITROBACTERACEAE. The type species, N. tenuis, is represented by a single isolate from soil in the Hawaiian Islands; cells: straight to curved rods, ca. 0.3–0.4 × 1.1–3.0 µm (coccoid in stationary-phase cultures), with 1–4 lateral or subpolar flagella. Extensive cytomembranes are lacking. Cells divide by binary fission. Nitrospina A genus of Gram-negative marine bacteria of the NITROBACTERACEAE. Cells of the type species, N. gracilis, are non-motile rods, ca. 0.3–0.4 × 2.6–6.5 µm, which divide by binary fission; the cells may contain numerous deposits of glycogen. GC%: ca. 58. (See also CARBOXYSOMES.) nitrous acid (as a MUTAGEN) Nitrous acid (HNO2 ) can cause the oxidative deamination of guanine (G), cytosine (C) and adenine (A) to form xanthine (X), uracil (U) and hypoxanthine (HX), respectively. When DNA treated with nitrous acid undergoes replication, X pairs with C which, on subsequent replication, pairs with G: i.e., no mutation results from G deamination. U pairs with A, and HX pairs with C: thus deamination of C and A leads to G·C-to-A·T and A·T-to-G·C transitions, respectively. (HNO2 can also cause e.g. cross-linking between bases.) In e.g. Escherichia coli the abnormal bases HX and U can be removed from DNA by specific hypoxanthine- and URACIL-DNA GLYCOSYLASES. Nitschkea See SORDARIALES. Nitzschia See DIATOMS. nivalenol See TRICHOTHECENES. NK cells Natural killer cells: lymphoid cells, derived from bone marrow stem cells, which are related to, but antigenically distinct from, T LYMPHOCYTES; NK cells do not express the T cell receptor (TCR), and do not express CD3, CD4 or CD8 antigens but they do express the CD16 antigen. NK cells are classified as large granular lymphocytes (LGLs). NK cells have little phagocytic activity, but they can exert cytotoxic/cytolytic activity against various pathogens, against host cells infected with certain bacteria (e.g. Listeria monocytogenes) or viruses, and against certain tumours; moreover, NK cells can be stimulated to release CYTOKINES, including IFN-g (see INTERFERONS). NK cells may bind to target cells (e.g. virus-infected cells) via the CD16 receptor – i.e. CD16 binding to the Fc portion of antibodies that have bound to the target cell (see ADCC). Alternatively, binding to target cells may occur in an antibodyindependent way – receptors on NK cells binding directly to ligands on the target cell. The killing activity of NK cells appears to involve the release of perforins and granzymes which induce APOPTOSIS in target cells. NK cells can attack virus-infected host cells which do not express viral antigen, linked to MHC class I molecules, at the cell surface; such cells would not normally be targeted by CD8+ 524 nomenclature Mycobacterium, Nocardia and Rhodococcus – organisms which form either non-mycelial growth or a fragmenting mycelium and which have a cell wall of chemotype IV. (cf. SPOROACTINOMYCETES.) Nocardioides A genus of aerobic soil bacteria (order ACTINOMYCETALES, wall type I) which form branching substrate and aerial mycelia that fragment into coccoid and rod-shaped pieces. GC%: ca. 66–67. Type species: N. albus. [Book ref. 73, pp. 100–101.] Nocardiopsis A genus of bacteria (order ACTINOMYCETALES, wall type III) which occur e.g. in soil. The organisms form branching, fragmenting substrate mycelium, and an aerial mycelium which gives rise to long chains of spores – each spore tending to form at an angle to its neighbours, thus giving the chain an undulating appearance. GC%: ca. 70–76. Type species: N. dassonvillei (formerly Actinomadura dassonvillei ). [Book ref. 73, pp. 117–118.] nocardiosis An acute or chronic disease of man and animals, caused by a species of Nocardia (commonly N. asteriodes). The disease may be systemic: suppurative lesions are formed mainly in the lungs, but the infection may disseminate to other organs (brain, kidney etc). Nodular, often suppurating, subcutaneous lesions may result from disseminated infection or from primary infection via a wound. (See also MADUROMYCOSIS.) nocardomycolic acids MYCOLIC ACIDS of Nocardia spp. Noctiluca A genus of marine, bioluminescent, heterotrophic (holozoic) DINOFLAGELLATES. The vegetative cells are large (e.g. 200–2000 µm diam.), roughly spherical (with no girdle), and have highly vacuolated cytoplasm; the flagella are very short, the transverse flagellum being reduced to a small projection. There is a deep sulcus from the region of which arises a retractable tentacle; the tentacle is apparently sticky and aids in the capture of prey. A cytostome is present near the base of the tentacle. Some strains of Noctiluca contain endosymbiotic unicellular green flagellates in vacuoles within the cell. (See also BIOLUMINESCENCE.) Nodamura virus See NODAVIRIDAE. Nodaviridae (Nodamura virus group) A family of ssRNA viruses which infect insects of the Diptera, Coleoptera and Lepidoptera. The virions are roughly spherical, naked, 29 nm diam., and each contains one major polypeptide species and two molecules of non-polyadenylated ssRNA (MWts ca. 1.15 and 0.46 × 106 ), both of which are necessary for infection. The type species, Nodamura virus, was isolated from mosquitoes (Culex tritaeniorhynchus) in Japan; it can replicate in various insects, can cause a lethal infection in suckling mice, and can replicate in various vertebrate and invertebrate cell cultures (including e.g. BHK cells). Other member: Black beetle virus, isolated from the New Zealand Black beetle, Heteronychus arator. Possible members include e.g. Arkansas bee virus. Nodosaria See FORAMINIFERIDA. Nodularia A genus of filamentous freshwater and marine (littoral) CYANOBACTERIA (section IV) in which the trichomes are made up of disc-shaped vegetative cells. Heterocysts may be intercalary or terminal. GC%: ca. 41. N. spumigena is a gasvacuolate, primarily freshwater, bloom-forming species which may form toxin(s) responsible for severe vomiting and bloody diarrhoea, or chronic liver and kidney damage, in animals which drink the water. nodules (plant microbiol.) See ACTINORRHIZAE; PHYLLOBACTERIUM; ROOT NODULES; STEM NODULES. nodulins ROOT NODULE-specific proteins specified by host plant genes; they appear to be involved in symbiotic nitrogen fixation and nitrogen metabolism. C-nodulins (‘common nodulins’) occur in all (leguminous) root nodules; S-nodulins are specific to the nodules of certain plant species. [Book ref. 55, pp. 79–83; review of nodulins and nodulin genes in Glycine max (soybean): PMB (1986) 7 51–61.] nogalamycin See ANTHRACYCLINE ANTIBIOTICS. nom. cons. Nomen conservandum: a CONSERVED NAME. nom. dub. NOMEN DUBIUM (q.v.). nom. nud. NOMEN NUDUM (q.v.). nom. rejic. NOMEN REJICIENDUM (q.v.). nom. rev. NOMEN REVICTUM. Nomarski interference-contrast microscopy See MICROSCOPY (d). nomen conservandum (nom. cons.) A CONSERVED NAME. nomen dubium (nom. dub.) A name which cannot, with certainty, be applied to a specific organism; the uncertainty may be due e.g. to an incomplete description of an organism in the original publication. nomen nudum (nom. nud.) In NOMENCLATURE: a name which, when proposed, was not supported by an adequate description of the relevant taxon. nomen rejiciendum (nom. rej.; nom. rejic.) Any name which has been rejected by a recognized taxonomic authority. nomen revictum (nom. rev.) A revived name. nomenclatural type (name bearer, nominifer, or ‘type’) Commonly, a viable specimen of a given organism to which a name has been attached and which is regarded as the permanent representative, or reference specimen, of a given taxon; otherwise, e.g., a preserved specimen or a photographic (or other) representation – sometimes simply a description – of an organism to which is permanently attached the name of a given taxon. (See also TYPE SPECIES and TYPE STRAIN.) [Bacterial types: Book ref. 22, p. 21.] nomenclature (microbiol.) The naming of organisms: one of the functions of TAXONOMY. Nomenclature is formally governed by an array of rules and priorities promulgated by various ad hoc bodies – each concerned with a particular type of organism; thus, e.g. bacterial names are regulated by the International Committee on Systematic Bacteriology which promulgates the International Code of Nomenclature of Bacteria [Book ref. 71 and subsequent revisions]. (Nevertheless, such rules and regulations are not always universally adhered to – due e.g. to a lack of awareness or to disagreement with published opinions.) With some exceptions (see later) each species of microorganism is referred to by a unique BINOMIAL. (The names of species and genera are usually printed in italics; in typewritten and handwritten manuscripts such names are usually underlined.) Where ambiguity cannot arise the generic name may be abbreviated; thus, e.g. Staphylococcus aureus may be written Staph. aureus or S. aureus. The name of a species is sometimes followed by an authority citation; this gives the name(s) of the author(s) who first proposed the name in a ‘valid publication’ and may also give the date of that publication – e.g. Bacillus cereus Frankland and Frankland 1887. (A valid publication is one which contains an adequate description of a newly proposed taxon and a correctly formulated Latin or Latinized name; it must be in printed form and must be available to the public or to the scientific community. A publication which lacks an adequate description but which fulfils the other requirements is called an effective publication.) Sometimes a double citation is used – e.g. Escherichia coli (Migula 1895) Castellani and Chalmers 1919; here, Migula is the author who described the organism under the name Bacillus coli, and Castellani and Chalmers are the authors who first described 525 nomenspecies NOMENCLATURE: some suffixes used in microbial nomenclature Algae Division (phylum) Subdivision (subphylum) Superclass Class Subclass Superorder Order Suborder Superfamily Family Subfamily Tribe Subtribe Genus Bacteria Fungi Protozoa -phyta -phytina -mycota -mycotina -phyceae -phycidae -mycetes -mycetidae -a -a -a -ea -ia -idea -ida -ina -oidea -idae -inae -ini -ales -ineae -ales -ineae -ales -ineae -aceae -oideae -eae -inae -aceae -oideae -eae -inae -aceae -oideae -eae -inae Viruses -viridae -virinae -virus the organism as Escherichia coli. (Sometimes an organism is known by one or more trivial names, as well as by its scientific name; these may be descriptive – e.g. ‘orange-peel fungus’ – or may have historical or other interest.) Taxa below the rank of species may be referred to by the binomial of the given species followed by a further designation(s): e.g., Campylobacter sputorum subsp bubulus, Fusarium solani f. sp. pisi, Pseudomonas syringae pv. savastanoi, Puccinia graminis var. phlei-pratensis Race 1. (See also TERNARY COMBINATION.) A taxon of the rank genus or above is referred to by a unique uninomial (one-name designation); the rank of a particular named taxon is often indicated by a standardized suffix (see table above). While an organism may bear only one valid taxonomic name, certain fungi may be referred to by a second binomial (see e.g. FORM GENUS); in deuteromycetes the name of the TELEOMORPH (where known) is the valid taxonomic name – that of the anamorph sometimes being written, in parenthesis, afterwards: e.g. Ajellomyces dermatitidis (Blastomyces dermatitidis). nomenspecies (1) One or more strains designated by a given binomial, i.e., a strain or group of strains treated as a species but not necessarily recognized as such taxonomically. (2) A type species. nominifer Syn. NOMENCLATURAL TYPE. Nomuraea A genus of fungi of the HYPHOMYCETES. N. rileyi has been evaluated for the BIOLOGICAL CONTROL of caterpillars of noctuid moths; it can be grown on artificial media [inexpensive media and methods: J. Inv. Path. (1986) 48 246–248], and its conidia can remain infective when sprayed onto crops. Conidia germinate on the surface of a caterpillar, and hyphae penetrate the insect, subsequently growing in the haemocoel; the caterpillar usually dies within 1–2 days and becomes covered with a dense mycelial mat from which conidia are eventually produced. non-A–non-B hepatitis (NANB hepatitis) See HEPATITIS C. nonactin See MACROTETRALIDES. non-agglutinable vibrios Syn. NON-CHOLERA VIBRIOS. non-agglutinating antibodies BLOCKING ANTIBODIES (sense 1). non-brewed condiment See VINEGAR. non-cellular Syn. ACELLULAR. non-cholera vibrios (NCVs; non-agglutinable vibrios) Those strains of Vibrio cholerae which are not agglutinated by O1 (or O139) antiserum; strains of V. cholerae which cause CHOLERA (q.v.) are characteristically agglutinated by O1 (or O139) antiserum. NCVs are classified serologically into serogroups O2 to O138; they are generally regarded as less pathogenic than O1 (or O139) strains, although they may cause mild to severe, choleralike disease. (See also NCVP.) non-circulative transmission (of viruses) (plant pathol.) (1) A mode of transmission in which viruses are temporarily associated with surface region(s) of an insect VECTOR; non-circulatively transmitted viruses can be acquired from an infected plant and transmitted to a healthy plant within seconds or minutes, i.e., there is no latent period (cf. CIRCULATIVE TRANSMISSION). A distinction is sometimes made between those cases in which virus is retained by the vector for minutes or hours (nonpersistent transmission) and those cases in which virus is retained for up to several days (semi-persistent transmission). In non-circulative transmission the insect’s ability to transmit a previously acquired virus is lost during moulting. When an insect moults it replaces its external body covering, parts of the pharynx, and the anterior and posterior regions of the gut lining; hence, any virus carried superficially is shed, and the loss of infectivity during moulting is regarded as a diagnostic feature of non-circulative transmission. In many cases, non-circulative transmission involves more than the simple mechanical contamination of the surface of an insect vector. For example, aphid-borne transmission of purified POTYVIRUSES occurs only if the aphid vector acquires a ‘helper component’ (HC) before or simultaneously with – but not subsequent to – acquiring the virus; HC is a virus-encoded protein (serologically distinct from either virus structural protein or ‘pinwheel’ protein) which occurs in potyvirus-infected leaves, and which can be obtained in the laboratory by cellfree translation of viral RNA. The uptake of virus by the aphid vector is apparently not influenced by the presence or absence of HC [MS (1984) 1 191–193]. Interestingly, the HC of a given potyvirus can also promote the aphid-borne transmission of certain other potyviruses – and even some unrelated viruses (e.g. potato aucuba mosaic virus) – which are not aphidtransmissible in the absence of HC. (HC is not necessary for mechanical, i.e., manual, infection of plants by potyviruses.) In certain CAULIMOVIRUSES, aphid-borne transmission is – as in potyviruses – dependent on helper factors (termed ‘transmission 526 non-sulphur purple bacteria factors’ in this virus group), and the aphid-borne transmission of certain caulimoviruses which are not normally aphidtransmissible can be promoted by transmission factor from other caulimoviruses [JGV (1985) 66 921–923]. (2) The term ‘non-circulative transmission’ (or ‘non-persistent transmission’) is sometimes used to refer to the transmission of viruses by vectors other than insects; examples include transmission of SOIL-BORNE WHEAT MOSAIC VIRUS and TOBACCO NECROSIS VIRUS by species of Polymyxa and Olpidium, respectively, and the transmission of NEPOVIRUSES by nematodes of the genera Longidorus and Xiphinema. non-coding strand (of dsDNA) See CODING STRAND. non-cognate interaction (immunol.) Interaction between a B cell and a T cell in which each cell recognizes, and binds to, a separate (physically distinct) molecule (cf. COGNATE HELP); the T cell may influence the B cell via soluble factors (i.e. CYTOKINES). non-complementing diploid See PROTOPLAST FUSION. non-conjugative plasmid (1) As commonly used: a PLASMID which does not encode one or more of the functions needed for its own intercellular transmission by CONJUGATION (sense 1b). (cf. CONJUGATIVE PLASMID.) (2) According to some authors [ARG (1979) 13 99–125 (105–106)]: a plasmid which does not encode one or more of the functions (e.g. pilus formation) needed for specific cell–cell contact, and which may or may not be capable of being mobilized. non-disjunction Atypical distribution of chromosomes or chromatids (or parts thereof) to the poles of the spindle during MEIOSIS or MITOSIS. (See e.g. PARASEXUAL PROCESSES.) non-genetic reactivation (virol.) A phenomenon in which an inactivated virus (which cannot complete its replication cycle when infecting cells on its own) can produce viable progeny virions in the presence of a second, closely related virus as a result of COMPLEMENTATION between viral proteins. An example is the ‘Berry–Dedrick phenomenon’ in which rabbits infected with a mixture of heat-inactivated myxoma virus and infectious (non-damaged) rabbit fibroma virus may develop myxomatosis. (cf. REACTIVATION.) non-gonococcal urethritis (NGU; non-specific urethritis, NSU) Sexually transmitted URETHRITIS caused by any organism other than Neisseria gonorrhoeae (cf. GONORRHOEA and URINARY TRACT INFECTION). The causal agent is commonly Chlamydia trachomatis which infects the epithelium of the urinary tract. Symptoms of chlamydial infection may be mild, or there may be severe urethritis with a profuse purulent discharge (usually in males); the infection may spread to the genital tract epithelium, causing e.g. epididymitis in males, salpingitis in females [BMB (1983) 39 133–150]. Other causal agents of NGU include e.g. Candida albicans, Trichomonas vaginalis, Mycoplasma hominis, and (possibly) Ureaplasma urealyticum. Chemotherapy: tetracyclines. non-haem iron protein A category of iron-containing proteins in which the iron is not present as part of a HAEM (sense 1) moiety; they were once believed to include all IRON–SULPHUR PROTEINS but some of the latter are now known to contain e.g. SIROHAEM. non-histone proteins A range of different proteins, other than HISTONES, associated with CHROMATIN; many occur in relatively small amounts. Examples include those proteins involved in transcription (e.g. RNA polymerases) and so-called HMG proteins (high-mobility group proteins) – the latter being characterized by their DNA-binding domains (see HMG BOX). The HMG1 protein is a small (25 kDa), highly charged protein which contains two similar HMG boxes (designated A and B). It occur in large numbers (e.g. 106 copies per nucleus) and binds to dsDNA in an apparently non-sequence-specific manner – although it binds strongly to specific DNA structures such as angular or kinked DNA or four-way junctions. HMG1 can also interact with certain other DNA-binding proteins and, in this way, can cause sharp bends to develop in dsDNA; this ability can enhance the activity of certain transcription factors, and HMG1 apparently affects the expression of particular genes. HMG1 also has extra-nuclear activity. In vitro, it can be secreted by macrophages on stimulation by CYTOKINES or ENDOTOXIN, and it may have a role e.g. in inflammation. More recently it has been reported that HMG1 is released by certain damaged or necrotic cells, including endothelial cells, and that it can act as a chemoattractant, induce cell migration and bring about reorganization in the cytoskeleton in certain cells [JCB (2001) 152 1197–1206]. Nonidet P-40 A non-ionic detergent of the polyoxyethylene p-toctyl phenol series. Nonion See FORAMINIFERIDA. non-Mendelian inheritance Syn. CYTOPLASMIC INHERITANCE. non-nucleoside reverse transcriptase inhibitors (NNRTIs) A category of ANTIRETROVIRAL AGENTS (q.v.) that includes delavirdine, efavirenz and nevirapine. The NNRTIs are structurally diverse compounds; they bind to reverse transcriptase and inhibit enzyme activity. The clinically used NNRTIs are reported to be effective against the reverse transcriptase of HIV-1 but not against that of HIV-2. (See also NU-1320.) [NNRTIs and other antiretroviral agents in the treatment of AIDS: BMJ (2001) 322 1410–1412.] non-O1 Vibrio cholerae pilus See NCVP. non-permissive conditions See CONDITIONAL LETHAL MUTANT. non-persistent transmission See NON-CIRCULATIVE TRANSMISSION. non-photochromogen A term used (very loosely) to refer e.g. to any strain of Mycobacterium which (i) forms little or no pigment, (ii) does not form pigment during early growth, and/or (iii) does not need to grow in the light in order to produce pigment (cf. SCOTOCHROMOGEN). non-propagative viruses See CIRCULATIVE TRANSMISSION. non-reciprocal recombination See RECOMBINATION. nonsense codon See GENETIC CODE. nonsense mutation A MUTATION in which a codon specifying an amino acid is altered to a nonsense (chain-terminating) codon: e.g. an amber codon (amber mutation) or an ochre codon (ochre mutation). (cf. MIS-SENSE MUTATION.) nonsense suppressor See SUPPRESSOR MUTATION. non-specific immunity The constitutive (or, in certain cases, acquired) resistance of the body to foreign materials in general – including those (living and non-living) materials with which it has had no previous contact. Non-specific immunity involves e.g. PHAGOCYTOSIS and the activities of INTERFERONS, LYSOZYME and PROPERDIN. (cf. SPECIFIC IMMUNITY.) non-specific immunization The administration of certain immunostimulants (e.g. BCG) in order to elicit a heightened immune response to antigen(s) unrelated to the immunostimulant. non-specific immunotherapy The therapeutic use of NONSPECIFIC IMMUNIZATION. non-specific urethritis Syn. NON-GONOCOCCAL URETHRITIS. non-specific vaginitis Syn. BACTERIAL VAGINOSIS. non-sterilizing immunity Syn. PREMUNITION. non-steroidal anti-inflammatory drugs (NSAIDs) See PROSTAGLANDINS. non-sulphur purple bacteria See RHODOSPIRILLACEAE. 527 non-treponemal tests in discrete cephalodia (see CEPHALODIUM). In liverworts (e.g. Blasia, Cavicularia) and hornworts (Anthoceros), Nostoc occurs in cavities in the underside of the thallus. Among angiosperms, Gunnera (a genus of rhubarb-like marsh plants) contains ‘Nostoc punctiforme’ in mucilage-filled glands in the plant stem at the bases of petioles; the cyanobiont penetrates between the cortical cells and enters meristematic cells, thus becoming an intracellular symbiont. [Development of Nostoc in Gunnera mucilage glands: Bot. Gaz. (1985) 146 56–62.] Cycads are the only gymnosperms known to be capable – via symbionts – of nitrogen fixation; they develop coralloid root nodules which contain symbiotic nitrogen-fixing cyanobacteria (Nostoc) in mucilage-filled spaces in the root cortex [ultrastructural studies of Nostoc–Zamia association: New Phyt. (1985) 101 707–716]. Nostocales See CYANOBACTERIA. notatin A flavoprotein GLUCOSE OXIDASE produced intracellularly e.g. by ‘Penicillium notatum’ (= P. chrysogenum). It can show antimicrobial activity due to the production of HYDROGEN PEROXIDE. NotI See RESTRICTION ENDONUCLEASE (table). notifiable diseases Those diseases (of man, animals or plants) the occurrence (or suspected occurrence) of which must, by law, be notified to the appropriate authorities. novel antigen Any weakly immunogenic or non-immunogenic molecule which can be made (more) immunogenic by CONJUGATION (sense 2) with an immunogenic molecule. (cf. HAPTEN; NEOANTIGEN.) novobiocin (albamycin) An ANTIBIOTIC (produced e.g. by Streptomyces spheroides and S. niveus): 4,7-dihydroxy-3-amino-8methylcoumarin linked via the 3-amino group to a substituted p-hydroxybenzoic acid and via the 7-hydroxy group to a substituted hexose (noviose). Novobiocin is more active against Gram-positive than Gram-negative bacteria; resistance develops readily. It primarily inhibits synthesis of DNA and, to a lesser extent, RNA. It selectively and competitively inhibits the binding of ATP to GYRASE B subunits; thus DNA supercoiling by gyrase is inhibited, but the (ATP-independent) DNA-relaxing gyrase activity is not. (See also CURING.) Secondary effects of novobiocin include inhibition of protein and cell wall synthesis. (See also COUMERMYCINS; cf. NALIDIXIC ACID.) Novyella A subgenus of PLASMODIUM. Nowakowskiella A genus of eucarpic, polycentric fungi (order CHYTRIDIALES) which are typically aquatic saprotrophs. The vegetative thallus includes a RHIZOMYCELIUM whose branching rhizoids penetrate the substratum. Operculate zoosporangia are formed, and each liberated zoospore germinates to form a rhizomycelium. Thick-walled resting spores are formed on pseudoparenchymatous structures, but whether or not these are sexual structures is unknown. noxythiolin (N-methyl-N  -hydroxymethylthiourea) A broadspectrum antimicrobial agent which has been used e.g. for the treatment of bacterial peritonitis; its activity appears to depend on the release of FORMALDEHYDE. Noxythiolin can kill blastospores of Candida albicans in vitro [JAB (1986) 60 319–325]. nPCR See NESTED PCR. NPV NUCLEAR POLYHEDROSIS VIRUS. NR1 plasmid See R100 PLASMID. nrdA gene (dnaF gene) A gene which codes for ribonucleotide reductase. nrdB gene See SPLIT GENE (e). nrdD gene See SPLIT GENE (e). non-treponemal tests Syn. STANDARD TESTS FOR SYPHILIS. non-vibrio cholera Any CHOLERA-like disease caused by organisms other than Vibrio cholerae – e.g. enterotoxigenic strains of Escherichia coli (see ETEC). nopaline An opine: N 2 -(1,3-dicarboxypropyl)-L-arginine – see CROWN GALL. nopalinic acid See CROWN GALL. 2
-nor-2
-deoxyguanosine See DHPG. norfloxacin See QUINOLONE ANTIBIOTICS. nori See LAVER. Normandina A genus (incertae sedis) of LICHENS; photobiont: Trebouxia. The thallus is squamulose, the squamules being shellshaped, 1–2 mm across, bluish-green, with raised, often sorediate margins. Fruiting structures are unknown; however, ascocarps of a (probably parasitic) fungus (Sphaerulina chlorococca) may be visible as dark dots on the surface of the thallus. The sole species, N. pulchella, occurs as an epiphyte on bryophytes. North American blastomycosis Syn. BLASTOMYCOSIS. Northern blot See BLOTTING. Norwalk virus (Norwalk agent) A small (27 nm) non-enveloped virus categorized as a SMALL ROUND STRUCTURED VIRUS (q.v.); it was first described in Norwalk, Ohio, in 1972. Norwalk virus can cause outbreaks of diarrhoeal disease/gastroenteritis, and is transmitted by the faecal–oral route. (cf. SNOW MOUNTAIN AGENT; see also WATER SUPPLIES and FOOD POISONING (i).) nose microflora See RESPIRATORY TRACT MICROFLORA. Nosema A genus of protozoa (class MICROSPOREA) which are parasitic mainly in insects. N. apis is parasitic in epithelial cells of the midgut in the honeybee (Apis mellifera); spores liberated by disruption of the epithelial cells are voided in the faeces. Infection occurs by ingestion; transovarial transmission apparently does not occur. (See also BEE DISEASES.) N. bombycis causes pébrine – a commonly fatal disease of silkworms (larvae of the silk-moth, Bombyx mori ). Infection may occur by ingestion of mulberry leaves contaminated with sporecontaining faeces from an infected silkworm. The spores germinate in the gut; reproduction occurs first in gut epithelium–but later in various tissues. Infected silkworms have characteristic dark spots. Transmission can occur transovarially. N. cuniculi : see ENCEPHALITOZOON. Other species may parasitize mosquitoes, moths, fish, etc; N. algerae and N. locustae have been tested for use in the BIOLOGICAL CONTROL of mosquitoes and grasshoppers, respectively. nosocomial disease A disease acquired in hospital. Infection rates of about 30% have been reported for patients in adult intensive-care units [review: Lancet (2003) 361 2068–2077]. Nostoc A genus of filamentous CYANOBACTERIA (section IV) in which the vegetative cells are spherical, ovoid or cylindrical; young trichomes formed from hormogonia have terminal heterocysts, but in longer trichomes heterocysts may also occur in intercalary positions. Akinetes, if formed, often occur in chains and develop half-way between two heterocysts. In N. muscorum, at least, gas vacuoles occur only in hormogonia (see HORMOGONIUM). GC%: 39–46. In nature, free-living Nostoc spp occur in various habitats (e.g. river-beds, wet rocks, damp soil), typically forming green to blackish gelatinous colonies up to several centimetres across; the colony form is rarely retained in culture. Nostoc spp form symbiotic associations with a wide range of eukaryotes, the latter deriving benefit from the ability of the cyanobiont to fix nitrogen. In LICHENS, Nostoc may be the main photobiont (e.g. in Collema, some Peltigera spp) or may occur 528 nuclear magnetic resonance NRRL Culture collection of the Northern Regional Research Laboratories, US Dept. Agriculture, 1815 North University St, Peoria, Illinois 61604, USA. NRTIs NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS. NruI See RESTRICTION ENDONUCLEASE (table). NS protein See ADENYLATE CYCLASE. NSAIDs Non-steroidal anti-inflammatory drugs. (See also PROSTAGLANDINS.) NSU See NON-GONOCOCCAL URETHRITIS. nt Abbreviation for nucleotide(s). NTA Nitriloacetic acid, N(CH2 COOH)3 (= triglycollamic acid), a chelating agent which complexes a range of cations (e.g. Ca2+ , Zn2+ ) – though less efficiently than does EDTA. It may be carcinogenic. NTG Nitrosoguanidine: see MNNG. NTM Non-tuberculous mycobacteria (see MOTT). NTP NUCLEOSIDE-5 -triphosphate. ntr genes In enterobacteria, rpoN (= ntrA, glnF ), ntrB (= glnL) and ntrC (= glnG): genes whose products control the expression of nitrogen-regulated genes – the latter including e.g. glnA (encoding glutamine synthetase, GS), NIF GENES, and genes concerned with transport and degradation of amino acids; transcription of nitrogen-regulated genes is typically repressed in the presence of high concentrations of ammonia and induced when ammonia becomes limiting. (See also AMMONIA ASSIMILATION.) rpoN (ntrA) encodes an enhancer-dependent SIGMA FACTOR, s54 , involved in the control of initiation of transcription of nitrogen-regulated genes. [s54 (minireview): JB (2000) 182 4129–4136.] NtrC is an activator protein that permits transcription from Es54 (see SIGMA FACTOR). NtrB appears to be involved in the activation of NtrC. The glnA operon in Salmonella typhimurium contains at least two promoters. P2, nearest the structural genes, is the major nitrogen-regulated promoter; transcription from P2 requires the rpoN and ntrC gene products. (Es70 apparently cannot initiate transcription from P2.) P1 is a secondary promoter, upstream from P2, which is activated by the cAMP–CRP complex (see NifL CATABOLITE REPRESSION); it is not dependent on the rpoN and ntrC gene products. In Klebsiella pneumoniae, transcription of all the NIF GENES is dependent on the sigma factor s54 (rpoN gene product). Transcription of the first operon, nifLA, requires the NtrC activator; after transcription of this operation, the NifA product functions as an activator for s54 in all subsequent nif genes/operons (see figure). However, while NtrC and NifA are functionally related, NtrC – but not NifA – is activated by phosphorylation. nu particle See PSEUDOCAEDIBACTER. NU-1320 (syn. NU1320) A candidate anti-AIDS drug within the category NON-NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS. It is reported to be several million times more potent than AZT but to have a very low level of toxicity. The drug was discovered by Tronchet’s team in Geneva and is being developed by the French company Mayoly Spindler. nuclear area (nuclear body) (bacteriol.) See CHROMOSOME. nuclear dehydrogenating clostridia See GASTROINTESTINAL TRACT FLORA. nuclear factor-kB See CYTOKINES. nuclear localization signal See CROWN GALL. nuclear magnetic resonance (NMR) The absorption and consequent emission of electromagnetic radiation by the nuclei of certain types of atom in the presence of a strong magnetic field. This phenomenon is the basis of a non-destructive, non-invasive method used (e.g. in biochemistry, medicine and microbiology) for elucidating the structures, conformations and interactions of molecules in both in vitro and in vivo systems; NMR is used primarily for investigating small molecules (MWt <2000) in solution. Principle. The types of nuclei particularly suitable for NMR studies occur in those atoms which have an odd mass number. In such atoms (e.g. 1 H, 13 C, 31 P) the nucleus has a magnetic moment, and in the presence of a strong external magnetic field the nuclear magnetic axis ‘wobbles’ about the field’s axis with a characteristic precession frequency; exposure to a pulse of radiation at the precession frequency will increase the amplitude of the wobble (= nuclear magnetic resonance). When the resonance decays it generates electromagnetic radiation which NifA nifLA σ 54 ntrC ntrB σ 54 σ 54 rpoN ntr GENES: control of the nif genes by the ntr genes in Klebsiella pneumoniae (diagrammatic) (see entry). Transcription of all the nif genes/operons requires the enhancer-dependent sigma factor s54 (rpoN gene product); additionally, transcription initially requires an activated (phosphorylated) NtrC protein – which is present when combined nitrogen is limiting. NtrB is apparently involved in regulating the activity of NtrC. The first nif genes to be transcribed are those in the nifLA operon. NifA, which is functionally related to NtrC, takes over the role of activator of E s54 for all subsequent nif genes/operons. NifL can inhibit NifA in the presence of combined nitrogen; moreover, NifA is inactivated by oxygen. Nitrogen fixation is blocked completely by null mutations in rpoN, ntrC or nifA. 529 nuclear petite can be detected by its ability to induce a signal (a radiofrequency alternating voltage) in a coil tuned to the particular resonance frequency. Usually, a single excitatory pulse produces only a weak resonance signal, so that several hundred or several thousand intermittent pulses are commonly used, the resulting resonance signals being added together in a computer. The resonance frequency of a given nucleus is influenced by the precise location of the atom within a molecule: if atoms of a given type (e.g. 1 H) are located in different molecular environments (e.g. CH2 , CH3 , OH) their nuclei will have slightly different resonance frequencies. When, due to a chemical reaction, an atom moves to a different molecular environment, the resonance frequency of its nucleus will change to that specified by the new location. A given type of nucleus within a particular molecular environment may be identified by its chemical shift (d): the relative amount by which its resonance frequency is shifted from that of a given reference standard – which, for 1 H, is the agent tetramethylsilane (TMS), (CH3 )4 Si; chemical shift is expressed in the dimensionless unit ‘parts per million’ (ppm). If, for a given type of nucleus (e.g. 1 H), the nuclear resonance frequency is determined (under standard conditions) for each type of molecular environment, it is possible e.g. to follow the progress of a chemical reaction by monitoring changes in signal strength at the various resonance frequencies; this is possible because the signal strength at a given resonance frequency is proportional to the number of nuclei occupying a given type of molecular environment. An NMR signal is a composite radio-frequency trace which incorporates simultaneous signals at various resonance frequencies; it is called a free induction decay (FID) since signal intensity decreases to zero as all the nuclei relax into their former (pre-excited) state of thermal equilibrium. The various resonance signals are separated out from the FID by mathematical manipulation involving the Fourier transform: a function which relates a complex waveform to its component spectrum of frequencies. The apparatus used for NMR includes an electromagnet or a superconducting magnet in order to obtain a strong magnetic field. Field strength directly influences the absolute resonance frequencies of nuclei; thus, e.g., resonance frequencies of 1 H nuclei are centred around ca. 42.6 megacycles (= megahertz, MHz) when the magnetic field is 1 tesla (= 10000 gauss), while at 4.7 tesla the 1 H nucleus resonates at ca. 200 MHz. Other resonance frequencies include e.g. (MHz per tesla): 10.7(13 C), 4.31(15 N) and 17.24 (31 P). [Book ref. 151; NMR and its clinical applications: BMB (1984) 40 113–201; NMR in biochemistry: Book ref. 152, pp. 1–67; NMR and drug design: Book ref. 153, pp. 267–281; microbiological applications of NMR: MS (1985) 2 203–211, 340–345.] nuclear petite See PETITE MUTANT. nuclear polyhedrosis viruses (NPVs) Viruses belonging to subgroup A of the BACULOVIRIDAE (q.v.). Virus replication and occlusion body formation occur exclusively in the host cell nucleus (cf. CYTOPLASMIC POLYHEDROSIS VIRUS GROUP). The NPV virion may contain a single nucleocapsid (S NPV) or many nucleocapsids (M NPV). Type species: Autographa californica NPV (AcM NPV); other members include e.g. Bombyx mori (silkworm) NPV (BmS NPV), Orygia pseudotsugata (Douglas Fir tussock moth) NPVs (OpS NPV and OpM NPV), and viruses from many other insects and from certain crustacea. Nuclearia A genus of amoebae (class FILOSEA). N. delicatula feeds by ingesting filamentous cyanobacteria [feeding behaviour and structure: JP (1986) 33 369–374]. nuclease Any enzyme which can cleave the sugar–phosphate backbone of a NUCLEIC ACID. (See also EXONUCLEASE and ENDONUCLEASE.) nuclease S1 Syn. ENDONUCLEASE S1 . nucleation site See CELL CYCLE (b). nucleic acid A polymer of NUCLEOTIDES in which the 3 position of one nucleotide sugar is linked to the 5 position of the next by a phosphodiester bridge. In a linear nucleic acid strand, one end typically has a free 5 -phosphate group, the other a free 3 -hydroxyl group. (See DNA and RNA.) nucleic acid sequence-based amplification See NASBA. nucleocapsid A structure containing nucleic acid and protein which forms part (or all) of a virion. Depending on virus, ‘nucleocapsid’ may refer e.g. to the CORE plus the CAPSID (see e.g. HERPESVIRIDAE, TOGAVIRIDAE) or to a helical nucleoprotein structure (see e.g. ARENAVIRIDAE, PARAMYXOVIRIDAE, TOBACCO MOSAIC VIRUS); it corresponds to the ‘core’ in the REOVIRIDAE. nucleoid (1) (bacteriol.) The bacterial CHROMOSOME [review: MR (1994) 58 211–232]. During cell division, the mechanism by which nucleoids are partitioned to daughter cells is unknown [partitioning in Escherichia coli: JB (1992) 174 7883–7889; in Bacillus: JB (1998) 180 547–555]. (2) (virol.) The ribonucleoprotein structure within the core, or the core itself, in the virion of a retrovirus (see RETROVIRIDAE). (3) Mitochondrial or chloroplast DNA. (4) The electron-dense central core in a PEROXISOME. nucleolar organizer See NUCLEOLUS. nucleolus Within most types of eukaryotic NUCLEUS: a distinct region, not delimited by a membrane, in which at least some species of rRNA are synthesized and assembled into ribonucleoprotein subunits of RIBOSOMES. In the nucleolus rRNA is transcribed from a nucleolar organizer, i.e., a group of tandemly repeated chromosomal genes which encode rRNA and which are transcribed by RNA polymerase I. (See also RRNA.) [Nucleolar structure and ribosome biogenesis: TIBS (1986) 11 438–442.] In most cases nucleoli do not persist during MITOSIS (cf. ENDOSOME sense 3). Nucleoli do not occur e.g. in the ciliate MICRONUCLEUS. nucleoside A compound consisting of a purine or pyrimidine base covalently linked to a pentose – usually ribose (in ribonucleosides) or 2-deoxyribose (in deoxyribonucleosides); the figure shows some of the bases commonly found in nucleosides. Ribonucleosides containing the bases adenine, guanine, cytosine, uracil, thymine and hypoxanthine are called (respectively) adenosine, guanosine, cytidine, uridine, thymidine and inosine. The corresponding deoxyribonucleosides are called deoxyadenosine, deoxyguanosine etc. (Note that ‘thymidine’ usually refers to deoxythymidine as thymine is associated primarily with DNA.) (cf. NUCLEOTIDE; see also Appendix V.) nucleoside anitbiotic A NUCLEOSIDE-containing ANTIBIOTIC: see e.g. POLYOXINS, PSICOFURANINE, PUROMYCIN, SHOWDOMYCIN. nucleoside reverse transcriptase inhibitors (NRTIs) A category of deoxyribonucleoside analogues which are used e.g. as ANTIRETROVIRAL AGENTS (q.v.); all NRTIs have a similar mode of action – see AZT for details. These agents are involved in chemotherapy against AIDS [BMJ (2001) 322 1410–1412]. Examples of NRTIs: AZT (zidovudine); didanosine (2 ,3 dideoxyinosine); LAMIVUDINE; stavudine (2 ,3 -dideoxythymidine); and zalcitabine (2 ,3 -dideoxycytidine). NRTIs can be mutually antagonistic: see e.g. AZT. The anti-HIV activity of an NRTI may be raised by increasing the ratio of NRTIs to the cell’s own dNTPs; this can be done e.g. by using hydroxyurea to inhibit the enzyme ribonucleotide 530 Nudaurelia b virus group NH2 O C 6 N1 5 C C N HN 1 7 8 HC 2 3 4C N H2N O C 2 O C C 1 CH 5 6 CH C 4 C N 7 C2 3 CH 9 N H N guanine NH2 4 5 8 9 N H adenine N3 6 CH HN 3 O C2 4 1 O C 5 6 C CH CH3 4 HN 3 O C2 1 5 CH 6 N H N H N H cytosine thymine uracil CH NUCLEOSIDE: some of the bases commonly found in nucleosides. Reproduced from Bacteria 5th edition, Figure 7.3, page 112, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. nucleotropism See TROPISM (sense 2). Nuclepore filter See FILTRATION. nucleus Within a eukaryotic cell: a membrane-limited body which contains e.g. CHROMOSOMES and one or more nucleoli (see NUCLEOLUS). (See also NUCLEOSKELETON.) The number of chromosomes per nucleus depends e.g. on species. (See also PLOIDY.) The nuclear membrane consists of a double unittype membrane which is perforated by a number of pores; the outermost membrane is continuous with the ENDOPLASMIC RETICULUM, and the space between the two membranes is called the perinuclear space. A cell may contain more than one nucleus (see e.g. COENOCYTE and SYNCYTIUM), and ciliate protozoa typically have two types of nuclei (see MACRONUCLEUS and MICRONUCLEUS). The fungal nucleus is characteristically much smaller than the nuclei of most other eukaryotes, and in many cases fungal chromosomes cannot be seen by light microscopy [the fungal nucleus: Book ref. 168]. Nudaurelia b virus group A family of non-enveloped, icosahedral (ca. 35 nm diam.), ssRNA-containing viruses which infect lepidopteran insects; the genome is a single molecule of positivesense ssRNA (MWt ca. 1.8–1.9 × 106 ). reductase (this causing a decrease in endogenous dNTPs). This approach was found to give clinical benefit when hydroxyurea was combined with didanosine in the Swiss HIV Cohort Study (1998). At least some NRTIs have been used against DNA viruses, i.e. they also inhibit DNA-dependent DNA polymerases (see e.g. LAMIVUDINE). nucleoskeleton In a eukaryotic cell: an intranuclear framework, analogous to the CYTOSKELETON, which is believed to include e.g. points of anchorage for complexes involved in the replication and transcription of DNA; the concept of a nucleoskeleton is supported e.g. by demonstrations of intranuclear actin and myosin, and by the demonstration that DNA polymerases are retained within nuclei, under isotonic conditions, following treatment of the encapsulated nuclei with restriction endonucleases [EMBO (1986) 5 1403–1410]. There is also evidence that the cytoskeleton and nucleoskeleton are connected at the nuclear periphery. Transport between nucleus and cytoplasm may involve an ATPand actomyosin-dependent control of the nuclear pore complex [JCB (1986) 102 859–862]. [Organization beyond the gene: TIBS (1986) 11 249–252.] nucleosome See CHROMATIN. nucleotide A NUCLEOSIDE in which the sugar carries one or more phosphate groups (see figure); nucleotides are the subunits of NUCLEIC ACIDS. (See also ATP.) Pathways for the de novo biosynthesis of the main nucleotides are shown in Appendix V. Deoxyribonucleotides are synthesized by the reduction of ribonucleotides catalysed by the enzyme ribonucleotide reductase. In Escherichia coli, deoxyribonucleotides are synthesized mainly by a THIOREDOXIN-dependent reduction of ribonucleoside 5 -di phosphates, while in e.g. Lactobacillus leichmannii ribonucleoside 5 -tri phosphates are reduced in a VITAMIN B12-dependent reaction. In addition to de novo synthesis, nucleotides can be synthesized using purine and pyrimidine bases derived from pre-formed nucleotides in a so-called ‘salvage cycle’. (See also NAD and THYMIDINE KINASE.) Antibiotics which affect nucleotide biosynthesis include AZASERINE; DON; HADACIDIN; MYCOPHENOLIC ACID and PSICOFURANINE. nucleotide excision repair See EXCISION REPAIR. Base OH 5′ HO P O CH2 O O H H 3′ H 1′ H 2′ OH OH NUCLEOTIDE. The figure shows a ribonucleotide; ribonucleotides are the subunits in RNA. In a deoxyribonucleotide the 2 position of ribose carries −H rather than −OH; deoxyribonucleotides are the subunits in DNA. The ribonucleotide in the figure is a ribonucleoside monophosphate; a ribonucleoside triphosphate (such as ATP) carries a chain of three phosphate groups – a, b and (terminal) g – at the 5 position of the ribose. In a dideoxyribonucleotide (used e.g. in DNA SEQUENCING), the 2 and 3 positions in the ribose carry −H (instead of −OH). 531 null cells formed at the 100% level of similarity. Although strains within a given cluster are mutually 100% similar, no information is given here on the degree of similarity which exists between the OTUs of one cluster and those of another. The remaining OTUs are progressively added at successive, lower levels of similarity (e.g. 97%, 95%, 80%, 75%, 50%) to one or other of the initially formed clusters. Additionally, two or more clusters may unite at a given level of similarity; thus, e.g. in the above example the clusters 43–47 – and 29–31–33 – may form a single cluster at, say, 75% – meaning that one or more OTUs in a given cluster has a mutual similarity of at least 75% with one or more OTUs in the other cluster. The mode in which OTUs are added to a cluster, and in which clusters unite, may be varied according to the purpose of the taxonomist: see e.g. SINGLE LINKAGE and UPGMA. At the end of the study the result of clustering may be shown e.g. in a PHENOGRAM or in a SHADED MATRIX. The processed data can then be examined to determine whether or not a given phenon can be regarded as a TAXON. Nummilites See FORAMINIFERIDA. nummilitids See FORAMINIFERIDA. nungham The freshwater red alga Lemanea mamillosa (division RHODOPHYTA) used as a food in India. nu/nu mice A strain of congenitally athymic mice. nusA gene See TRANSCRIPTION and BACTERIOPHAGE LAMBDA. nutrient agar NUTRIENT BROTH gelled with 1.5–2.0% AGAR: a general-purpose solid basal MEDIUM. nutrient broth A liquid basal MEDIUM; the name may refer to either extract broth or infusion broth. Extract broth (the type most commonly used) may be prepared as an aqueous solution of beef extract and peptone (each ca. 0.5–1.0% w/v) and sodium chloride (ca. 0.5% w/v); it is sterilized by autoclaving, and the final pH is ca. 7.2–7.4. Powdered (dehydrated) extract broth is available commercially; it is reconstituted with distilled (or tap) water, dispensed into tubes, and autoclaved. Infusion broth is prepared by infusing overnight, at 4° C, 50 g of minced lean beef in 100 ml distilled water; peptone (ca. 1% w/v) and sodium chloride (ca. 0.5% w/v) are added, and the mixture is boiled, filtered, and sterilized by autoclaving. Final pH: ca. 7.3–7.5. nutritionally variant streptococci (NVS) Strains of Streptococcus, present e.g. in the normal human throat and urogenital tract, which are characterized by a requirement for cysteine, or for pyridoxal hydrochloride or pyridoxamine hydrochloride, when grown on complex media; the organisms were originally detected as satellite colonies (see SATELLITE PHENOMENON) of viridans streptococci associated with colonies of Staphylococcus. Two species are distinguished: S. adjacens and S. defectivus, the latter species differing from S. adjacens e.g. in fermenting trehalose and forming b-galactosidase. The organisms may form a red pigment. NVS may be responsible for ∼5% of those cases of endocarditis which involve oral streptococci. Nuttallia See BABESIA. nvCJD New variant CREUTZFELDT–JAKOB DISEASE. NVDP See PLASMODIUM. NVL No visible lesion. NVS NUTRITIONALLY VARIANT STREPTOCOCCI. Nycodenz See IODINATED DENSITY-GRADIENT MEDIA. nystatin (fungicidin) An antifungal POLYENE ANTIBIOTIC, produced by Streptomyces noursei and named after the New York State Health Department (where it was discovered). It contains MYCOSAMINE and is usually classed as a tetraene, although it contains six double bonds (four conjugated double bonds separated from two others by two methylene groups). Nystatin is used primarily for the topical treatment of superficial mycoses (e.g. mucosal Candida infections); it is not absorbed when taken orally. Type species: Nudaurelia b virus (NbV), isolated from the pine emperor moth – Nudaurelia cytherea capensis (= Imbrasia cytherea) – whose larvae cause extensive defoliation of pine trees in South Africa. Other members of the family have been isolated from various lepidopteran hosts – e.g. Darna trima (a pest of coconut and oil palms in SE Asia), Thosea asigna (a pest of oil palms in Malaysia), Trichoplusia ni (cabbage looper). The viruses have not been propagated in cell culture, and must be cultivated by inoculation into suitable host larvae; virus replication occurs in the cytoplasm of gut cells. Infected larvae become flaccid. [Review in JGV (1985) 66 647–659.] null cells (immunol.) Lymphoid cells which lack characteristic B cell and T cell antigens; some are non-cytolytic, others are cytolytic – the latter including NK CELLS. null mutation A MUTATION which results in complete loss of function of the sequence in which it occurs. (cf. LEAKY MUTATION.) numerical aperture See RESOLVING POWER. numerical taxonomy (taxometrics) A computer-assisted form of TAXONOMY in which the organisms in a given study are sorted into groups of mutually similar organisms on the basis of a large number (ideally >50) of observable properties (characters); typically, every character is regarded as having the same degree of importance, i.e. equal ‘weighting’, in the computation of similarities (cf. ADANSONIAN TAXONOMY; see also POLYTHETIC CLASSIFICATION). (The use of a computer is not strictly essential, but the calculations involved would otherwise take an inordinate length of time.) Numerical taxonomy is primarily a form of PHENETIC CLASSIFICATION in which the aim is to define TAXOSPECIES (see also PHENON); however, the availability of certain fundamental data (derived e.g. from DNA homology studies) has permitted the method to be modified for use in PHYLOGENETIC CLASSIFICATION. In numerical taxonomy each strain – or OTU (q.v.) – among those to be classified is initially examined or tested in respect of each of the biochemical, morphological or other characters used as criteria in the study. The nature or value of a given test result is then expressed in a form such as + or − (or e.g. as 0 or 1); for example, ‘growth at 45° C’ could be written as ‘+’, while ‘−’ could be used for ‘no growth at 45° C’. Each OTU in the study may thus be defined by a series of simple symbols; this is the form in which the data are fed into the computer. In the computer the first step in classification is the comparison of each OTU with every other OTU – likenesses being expressed as percentage similarities; for example, two given OTUs which match in any 60 out of 80 characters would have a percentage similarity of 75%. (There are actually several different ways in which a given pair of OTUs can be compared: see entry SSM and compare it with SG , SJ and SP ; cf. entry DP .) The result of these initial comparisons is a list of percentage similarities – each having been derived from the comparison of one given OTU with another given OTU; these data are usually printed out as an S MATRIX. The computer may be programmed to search the percentage similarities for pairs or groups of ‘identical organisms’, i.e., pairs or groups which exhibit a mutual similarity of 100%; such pairs or groups are called ‘100% clusters’. On the computer printout such clusters, if present, may appear thus: 100% 5–10–11–19– 43–47– 29–31–33– in which each number (5, 10 etc) is the designation of one of the OTUs in the study; in this example three clusters are 532 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 O O antigens (Boivin antigens) The heat-stable, alcohol-resistant LIPOPOLYSACCHARIDE –protein somatic antigens of Gram-negative bacteria (particularly members of the Enterobacteriaceae); O antigens are important in the serological characterization of enterobacteria (see e.g. KAUFFMANN–WHITE CLASSIFICATION). Antigenic specificity is determined by the polysaccharide O-specific chains (see LIPOPOLYSACCHARIDE). O antigens may be modified e.g. by BACTERIOPHAGE CONVERSION or by mutation (e.g. in salmonellae a mutation may give rise to T1 side-chains, consisting of ribose and galactose residues, instead of normal O-specific chains – see also SMOOTH–ROUGH VARIATION). (See also VI ANTIGENS.) O side chain See LIPOPOLYSACCHARIDE. O-specific chain See LIPOPOLYSACCHARIDE. O-type starter See LACTIC ACID STARTERS. O1 strains (of Vibrio cholerae) See VIBRIO. O/129 2,4-Diamino-6,7-diisopropylpteridine: a water-soluble agent bacteriostatic for e.g. many members of the Vibrionaceae (e.g. most Vibrio strains). Sensitivity to O/129 may be determined using O/129-impregnated discs on nutrient agar containing 0.5% w/v NaCl. Some strains of Vibrio cholerae produce a trimethoprimresistant dihydrofolate reductase (DHFR) and are resistant to both trimethoprim and O/129; the gene encoding the DHFR may be borne on a plasmid or on a transposon [Ann. Mic. (1985) 136B 265–273]. O139 Bengal (Vibrio cholerae) See CHOLERA. O157:H7 E. coli See EHEC. oak butt rot See BUTT ROT. oak-moss Evernia prunastri. oak wilt A disease of oak trees (Quercus spp) caused by Ceratocystis fagacearum. Infection occurs via wounds (made e.g. by man or woodpeckers). The fungus forms pressure cushions (sclerotia) between the infected xylem and phloem, bursting open the overlying bark and allowing entry of insects, particularly sap-feeding beetles of the Nitidulidae, which act as vectors of the disease. Oakley–Fulthorpe test See DOUBLE DIFFUSION. oar weed Any species of Laminaria which has an oar-shaped thallus, e.g. L. digitata and L. hyperborea. oat diseases See CEREAL DISEASES. oat mosaic virus See POTYVIRUSES. oat necrotic mottle virus See POTYVIRUSES. oat sterile dwarf virus See FIJIVIRUS. oat striate virus See RHABDOVIRIDAE. Obesumbacterium A genus of non-motile, slow-growing bacteria of the ENTEROBACTERIACEAE (apparently related to Hafnia alvei ), found as contaminants in fermenting beer wort. Cells: pleomorphic rods, 0.8–2.0 × 1.5–100 µm (short, fat rods predominate in wort). [Book ref. 22, pp. 506–509.] objective (objective lens) See MICROSCOPY (a). obligate Refers to an essential attribute of a given organism. For example, an obligate aerobe is an organism which grows only under aerobic conditions, and an obligate parasite is an organism which, in nature, can grow only as a parasite. (cf. FACULTATIVE.) obligate proton reducers See ANAEROBIC DIGESTION. Obodhiang virus See LYSSAVIRUS. occlusion bodies See BACULOVIRIDAE. Occlusosporida See STELLATOSPOREA. Oceanospirillum A genus of marine (coastal), Gram-negative, asporogenous bacteria formerly included in the genus SPIRILLUM. Cells: rigid, helical, 0.3–1.4 × ca. 5–20 µm. POLAR MEMBRANES and intracellular PHB granules are present. COCCOID BODIES predominate in old cultures in all species except O. japonicum. Motile, with bipolar tufts of flagella (or, in O. pusillum, a single flagellum at each pole). Aerobic; metabolism is strictly respiratory. Nitrate respiration does not occur, but some species can reduce NO3 − . Oxidase +ve; catalase and phosphatase variable; indole −ve; arylsulphatase −ve. Optimum growth temperature: 25–32° C. Seawater is required for growth. Carbon sources: amino acids or salts of organic acids, but not carbohydrates. GC%: 42–51. Type species: O. linum. Nine species are recognized [Book ref. 22, pp. 104–110]. ocellus See EYESPOT (sense 1). Ochoterenaia See TRICHOSTOMATIDA. ochratoxins A group of MYCOTOXINS produced by species of Penicillium and Aspergillus. Ochratoxin A is a dihydrocoumarin derivative linked to L-phenylalanine via an amide bond; it inhibits phosphoenolpyruvate carboxykinase. Ingestion of ochratoxin A in contaminated food causes nephropathy in e.g. cattle, pigs and man. ochre codon See GENETIC CODE. ochre mutation See NONSENSE MUTATION. ochre suppressor See SUPPRESSOR MUTATION. Ochrobium A genus of IRON BACTERIA. Cells: ellipsoidal to rod-shaped. Type species: O. tectum. [Population ecology of Ochrobium: JGM (1981) 125 85–93.] Ochrolechia A genus of crustose LICHENS (order PERTUSARIALES). Photobiont: a green alga. Thallus: often thick, warty, irregular in shape. Apothecia: lecanorine, with characteristically thick thalline margins and yellowish or pinkish discs. Species generally occur on rocks, sometimes also on bark; O. frigida occurs on the ground on mosses etc. O. tartarea has been harvested on a commercial scale for the production of CUDBEAR. Ochromonas A genus of unicellular CHRYSOPHYTES. The cell is usually ovoid, contains (typically) two parietal chloroplasts, and has two flagella of unequal length. (See also FLAGELLAR MOTILITY (b).) The organisms are both photosynthetic and phagotrophic, apparently requiring organic nutrients (e.g. VITAMIN B12 ) or prey for survival. Some forms have a delicate, cup-shaped LORICA composed of microfibrils of CHITIN; typically, the lorica adheres via a stalk to the substratum or to other cells. (Loricate forms of O. malhamensis are sometimes known as Poterioochromonas stipitata.) Ockelbo disease See SINDBIS VIRUS. OCT plasmid An IncP-2 Pseudomonas PLASMID (ca. 500 kb) which encodes the capacity to metabolize octane and decane. (cf. TOL PLASMID.) [Characterization of the OCT plasmid: JB (1986) 165 650–653.] octapeptins A group of ANTIBIOTICS closely related to the POLYMYXINS; they differ e.g. in having a shorter side-chain and a longer b-hydroxy fatty acid residue. A mixture of octapeptins A and B is up to ten times more effective than polymyxin B against certain Gram-positive bacteria. Octomyxa See PLASMODIOPHOROMYCETES. octopine N 2 -(D-1-carboxyethyl)-L-arginine – see CROWN GALL. Other, related opines include histopine and lysopine. octopinic acid See CROWN GALL. 533 Octopus vulgaris disease virus Oidium See HYPHOMYCETES and ERYSIPHALES. Oikomonas A genus of colourless, unicellular CHRYSOPHYTES which resemble Chromulina spp; the organisms occur in organically polluted waters, activated sludge, etc. oil biodeterioration See PETROLEUM. oil-immersion condenser See RESOLVING POWER and MICROSCOPY (b). oil-immersion objective See RESOLVING POWER. oil shale A grey or black ancient sedimentary rock which consists primarily of KEROGEN with a small proportion of solvent-soluble organic matter. OJ locus See FINOP SYSTEM. Okazaki fragments See DNA REPLICATION. OKT antigens The former name for surface antigens on human T LYMPHOCYTES. The designation ‘OKT’ was replaced by ‘CD’ – for example, OKT4 is now CD4. old dog encephalitis See CANINE DISTEMPER. old tuberculin See TUBERCULIN. oleandomycin A MACROLIDE ANTIBIOTIC (from Streptomyces antibioticus) closely related to erythromycin. (cf. SIGMAMYCIN.) olefin metabolism See HYDROCARBONS. oligo(dT)-cellulose See MRNA (a). oligodynamic effect The antibacterial (or antimicrobial) action exerted by low concentrations of certain metals in elemental form (see HEAVY METALS); the effect may be observed by placing small metal discs on a flood plate and measuring the zone of no growth surrounding each disc following appropriate incubation. Oligohymenophorea A class of protozoa (phylum CILIOPHORA) in which, typically, the cytostome occurs at the base of a BUCCAL CAVITY, and the oral ciliature is obviously differentiated from the somatic ciliature; the PARORAL MEMBRANE is characteristically of the stichodyad type; and STOMATOGENESIS is buccokinetal or parakinetal. This class includes ciliates previously referred to as ‘higher holotrichs’ (e.g. the hymenostomes) together with e.g. the astomes. Subclasses: HYMENOSTOMATIA, PERITRICHIA. oligomycin An antibiotic, produced by Streptomyces spp, which binds non-covalently to mitochondrial (F0 F1 )-type PROTON ATPASES, inhibiting the transfer of protons through the F0 site and, hence, inhibiting both ATP hydrolysis and OXIDATIVE PHOSPHORYLATION. Oligomycin does not collapse pmf (cf. UNCOUPLING AGENTS). oligonucleotide cataloguing See RRNA OLIGONUCLEOTIDE CATALOGUING. oligonucleotide-directed mutagenesis See SITE-SPECIFIC MUTAGENESIS. oligonucleotide fingerprinting Any FINGERPRINTING technique in which the fingerprint consists of a strain-specific pattern of oligonucleotides. For example, to differentiate strains of an RNA virus, the genome is digested with a sequence-specific enzyme (such as RNASE T1) and the resulting oligonucleotide fragments are subjected to gel ELECTROPHORESIS; the pattern (bands) of fragments in the gel constitute the characteristic fingerprint of the given strain of virus. oligosaccharide A carbohydrate consisting of 2 to ca. 10 monosaccharides joined by glycosidic linkages. oligosaprobic zone See SAPROBITY SYSTEM. Oligotrichida An order of ciliate protozoa (class POLYHYMENOPHOREA) which are typically free-living, marine, pelagic organisms, but which include some freshwater and edaphic species. Cells: rounded to elongate with a thickened pellicle and (in many species) a perilemma; somatic ciliature is typically reduced, and in some species the oral ciliature is divided into two parts: one part within the buccal cavity, the other encircling the anterior end of the cell. Some species are loricate. Octopus vulgaris disease virus See IRIDOVIRUS. Octosporella See EIMERIORINA. ocular Syn. eyepiece (see MICROSCOPY). ocular herpes See HERPES SIMPLEX. ocular micrometer See MICROMETER. OD Optical density (see TURBIDIMETRY). ODC Ornithine decarboxylase: see DECARBOXYLASE TESTS. Odontostomatida An order of typically freshwater, ‘polysaprobic’ ciliate protozoa (class POLYHYMENOPHOREA) in which the cells are laterally compressed, have reduced somatic ciliature, and often have posterior spines or cirri. Genera include e.g. Epalxella (formerly Epalxis), Pelodinium, SAPRODINIUM. oedema (edema; ‘dropsy’) The abnormal accumulation of fluid in body tissues. (cf. ASCITES.) oedema disease (bowel oedema) A PIG DISEASE which typically attacks young animals subjected to stress (e.g. change of diet at weaning). It is apparently caused by toxigenic strains of Escherichia coli. Infection may result in sudden death, or there may be oedema of certain tissues (especially the stomach wall and mesentery) with signs of nervous disturbance (incoordination, loss of balance) usually followed by coma and death. oedematolysin See THIOL-ACTIVATED CYTOLYSINS. Oedogonium A genus of freshwater (commonly epiphytic), unbranched filamentous green algae (division CHLOROPHYTA). Each filament is composed of uninucleate cells and is usually attached to the substratum by a differentiated basal cell. The individual cells each contain a single parietal reticulate chloroplast with several pyrenoids. In some cells of the filament the cell wall has a characteristic series of ridges (the ‘cap’) at one end; these ridges result from an unusual mode of cell division: shortly after mitosis, the cell wall ruptures and the cell increases rapidly in size before the cross-wall is formed. Asexual reproduction occurs by the formation of multiflagellate, naked zoospores; sexual reproduction is oogamous. (See also BULBOCHAETE.) Oerskovia A genus of asporogenous bacteria (order ACTINOMYCETALES, wall type VI) which occur e.g. in soil. In culture the organisms form a primary mycelium which fragments into small, monotrichously flagellated rods (ca. 0.4 × 1.1 µm) that develop into larger, peritrichously flagellated rods, the latter giving rise to a branched mycelium in which the hyphal diameter is ca. 0.5 µm. Growth occurs aerobically (organisms catalasepositive) or anaerobically (organisms catalase-negative) on rich media (e.g. trypticase–soy agar) with the formation of yellowish colonies; glucose can be metabolized either oxidatively or fermentatively. GC%: ca. 70–75. Type species: O. turbata. [Book ref. 73, pp. 52–53.] O–F test OXIDATION–FERMENTATION TEST. OFAGE Orthogonal-field-alternation gel electrophoresis: a form of PFGE (q.v.) used e.g. for separating chromosomes by size. OFAGE has been used e.g. to obtain a karyotype of the yeast Saccharomyces cerevisiae [PNAS (1985) 82 3756–3760]. ofloxacin See QUINOLONE ANTIBIOTICS. Ogawa variant See VIBRIO (V. cholerae). ogi A Nigerian food: a type of sour porridge made by the natural fermentation of ground maize, sorghum or millet. A range of organisms is present, including e.g. Lactobacillus plantarum and various yeasts. O’Hara’s disease See TULARAEMIA. Ohio corn stunt agent Syn. MAIZE CHLOROTIC DWARF VIRUS. Oidiopsis See HYPHOMYCETES and ERYSIPHALES. oidium (mycol.) A thin-walled THALLOSPORE; oidia may function as disseminative spores or as male gametes. 534 oncogene Members of the suborder Oligotrichina are typically small and rounded organisms whose somatic ciliature may consist of only a few short rows of ‘bristles’, but which may have groups of ‘cirri’ or ‘bristles’ forming an equatorially encircling band; genera include e.g. HALTERIA, Strombidium, Strombilidium, and Tontonia. The other suborder, TINTINNINA (q.v.), comprises a distinctive group of organisms. Oligotrichina See OLIGOTRICHIDA. oligotroph Any organism which can grow in relatively nutrientpoor environments (cf. COPIOTROPH). oligotrophic (1) Of or pertaining to an OLIGOTROPH. (2) Of lakes, rivers etc: containing low levels of nutrients, particularly of those nutrients which support the growth of aerobic photosynthetic organisms. (cf. EUTROPHIC.) olivanic acids An ill-defined group of CARBAPENEM antibiotics derived from Streptomyces olivaceus and other Streptomyces spp. olive-green mould Chaetomium olivaceum: a mould which frequently grows on mushroom compost after pasteurization (see MUSHROOM CULTIVATION) and which inhibits mycelial development in Agaricus brunnescens by competition for nutrients. [Biological control by a thermophilic Bacillus sp: AEM (1983) 45 511–515.] olive knot A disease of the olive, oleander and privet, in which small galls develop on young twigs and leaves. The causal agent is Pseudomonas syringae pv. savastanoi ; the pathogen produces IAA and cytokinin which may be responsible for the formation of the galls. In at least some strains IAA production appears to be plasmid-mediated. Infection occurs via wounds. Transmission may occur via the olive fly (Dacus oleae). olives (pickled) See PICKLING. olivomycin See CHROMOMYCIN. Olpidiopsis See LAGENIDIALES. Olpidium A genus of unicellular, endobiotic, holocarpic fungi (order CHYTRIDIALES) which are parasitic on algae and other aquatic organisms and on some terrestrial plants; Olpidium spp are also involved in the transmission of the LETTUCE BIG VEIN AGENT and the TOBACCO NECROSIS VIRUS. In the asexual cycle of O. viciae a zoospore settles on a susceptible host (Vicia), encysts, and the cyst’s contents enter a host cell; after intracellular growth and a number of nuclear divisions, the fungus eventually forms an inoperculate zoosporangium – the numerous zoospores subsequently escaping via one or more pores. In the sexual cycle, two morphologically indistinguishable zoospores fuse and then encyst on a susceptible host; the cyst’s contents enter a host cell and develop into thick-walled resting sporangia. Karyogamy (and probably meiosis) occur before the germination of resting sporangia – each of which produces a number of zoospores. OM OUTER MEMBRANE. omasum See RUMEN. O’Meara’s method See VOGES–PROSKAUER TEST. !-oxidation See HYDROCARBONS. ! (omega) protein See TOPOISOMERASE. omicron An endosymbiotic, Gram-negative, rod-shaped bacterium which occurs in the cytoplasm of Euplotes spp; it does not confer a killer characteristic, and it appears to be essential for the viability of its protozoan host. [Book ref. 22, p. 798.] omnivorous Refers to protozoa which feed on microscopic ‘animals’ (e.g. other protozoa) and ‘plants’ (e.g. algae, bacteria). (cf. CARNIVOROUS; HERBIVOROUS.) Omp The designation of certain major OUTER MEMBRANE PROTEINS, each type of protein being distinguished as OmpA, OmpC etc. Some of these proteins are PORINS – for example OmpC and OmpF (see OSMOREGULATION). In Escherichia coli, OmpA (also called e.g. 3A, d and II∗ ) is a product of the ompA gene (also called e.g. con, tolG and tut); it apparently spans the thickness of the outer membrane but does not seem to function as a porin. OmpA-deficient mutants have an unstable outer membrane and are deficient in conjugation and in sensitivity to colicin L and phages K3 and TuII. In E. coli, OmpT is a protease involved in type IV (autotransporter) protein secretion. OMP Orotidine 5 -monophosphate [see Appendix V(b)]. OmpA See OMP. Omphalotus See AGARICALES (Tricholomataceae). OmpR See OSMOREGULATION. OmpT protein See PROTEIN SECRETION (type IV systems). Omsk haemorrhagic fever See VIRAL HAEMORRHAGIC FEVERS and FLAVIVIRIDAE. onc See ONCOGENE. Oncobasidium See TULASNELLALES and VASCULAR-STREAK DIEBACK. oncogen See ONCOGENIC. oncogene A gene which can (potentially) induce neoplastic transformation in the cell in which it occurs or into which it is introduced. Oncogenes occur e.g. in certain viruses, particularly members of the RETROVIRIDAE. Viral oncogenes (v-onc), originally identified as the transforming determinants of acutely oncogenic retroviruses, were subsequently found to have closely related cellular homologues (designated c-onc or proto-onc) in the chromosomes of eukaryotes; c-onc sequences have been highly conserved through evolution, and homologous genes have been found in all vertebrates examined, as well as in lower eukaryotes such as Drosophila and Saccharomyces. In the normal cell, at least some oncogenes are known to have one or more major roles in cell growth/development. Retroviral v-onc sequences may have arisen from c-onc genes by recombination (see RETROVIRIDAE) and may differ from them e.g. in lacking introns; they are often expressed with adjacent viral sequences as fusion proteins. Retroviral and cellular oncogenes are usually referred to by three-letter designations based on the retrovirus in which they were first identified: e.g. myc from myelocytomatosis virus, fes from feline sarcoma virus, etc; viral genes are prefixed by v- (e.g. v-myc), their cellular homologues by c- or proto- (e.g. c-myc or proto-myc). A v-onc gene introduced into a cell by an infecting retrovirus may induce the neoplastic transformation of that cell in various ways. The products of many retroviral oncogenes (e.g. v-abl, v-fes, v-fps, v-ros, v-src, v-yes) have tyrosine-specific protein kinase activity, transferring g-phosphate from ATP (or GTP) to tyrosine residues in proteins. (See ABL, FES and SRC; cf. ERB and MOS.) In normal (non-neoplastic) cells, tyrosine kinase activity is associated with the receptors for certain growth-promoting peptides: e.g. epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and insulin. Oncogenic tyrosine kinase activity has been targeted by selective inhibitors of proteins encoded by e.g. bcr-abl and v-abl ; some inhibitors can be effective against a range of oncogenic kinases [e.g. Pharmacology and Experimental Therapeutics (2000) 295 139–145], and this has encouraged the screening of existing clinically useful drugs for activity against the emergent drug-resistant mutant kinases [PNAS (2005) 102 11011–11016]. v-myc and v-myb products lack tyrosine kinase activity (see MYC and MYB). v-sis encodes a protein almost identical to the B chain of human PDGF. 535 oncogenesis of giving rise to a single plaque – regardless of the number of virions it may contain. (The latent period lasts ca. 20–25 min in phage T2/Escherichia coli B systems, but may be of the order of hours in virus-infected animal cells.) Aliquots taken sequentially after the latent period give progressively increasing numbers of plaques; during this period the infected cells begin to lyse, releasing large numbers of infectious virions each capable of giving rise to a plaque. The number of plaques per aliquot reaches a plateau when all the cells have lysed. (See also BURST SIZE.) If the virions are labile the plateau is followed by a decay period characterized by a fall in plaque titres. If, during the latent period, the cells in each aliquot are lysed artificially – e.g. by the addition of SDS (for animal cells) or chloroform (for Gram-negative bacteria) – a so-called eclipse period can be distinguished. In this period, which begins at the same time as the latent period, infectious virions cannot usually be detected owing to the ‘uncoating’ of the virus following infection; the eclipse period ends when the first progeny virions are assembled. (The latent period normally continues until the host cell lyses naturally, i.e., after the assembly of many progeny virions.) one-tube nPCR See NESTED PCR. one-way incompatibility INCOMPATIBILITY exhibited by two given plasmids only when one particular plasmid is introduced into a cell containing the other; if the second plasmid is introduced into a cell containing the first, incompatibility is not exhibited. [One-way incompatibility between IncH1 plasmids and the F plasmid: JGM (1985) 131 1523–1530.] onion rot White rot (which can also affect e.g. leeks) is caused by Sclerotium cepivorum; leaves become yellow and die, and the rotting roots and scale bases become covered with white, fluffy mycelium. Black sclerotia develop in or on necrotic tissue. Neck rot, caused by Botrytis aclada (= B. allii), is more common in stored onions. A soft rot can be caused by Burkholderia cepacia (formerly Pseudomonas cepacia). (See also SMUDGE). onion yellow dwarf virus See POTYVIRUSES. onium compounds Syn. QUATERNARY AMMONIUM COMPOUNDS. ONPG test A test used to detect b-D-galactosidase activity in bacteria. A loopful of growth of the test organism (grown on e.g. TSI agar) is introduced into phosphate-buffered peptone water (pH 7.0) containing ONPG (o-nitrophenyl-b-Dgalactopyranoside); the medium is then incubated at 37° C. b-GALACTOSIDASE hydrolyses ONPG to galactose and o-nitrophenol – the latter (being yellow) acting as a test indicator. Organisms which give a positive ONPG test do not necessarily metabolize lactose since they may not synthesize a lactose permease. (ONPG can enter cells without a specific permease.) ontjom Syn. ONCOM. ontogeny The range of stages which occur during the development of an individual organism to the mature or adult stage. The supposition that ontogeny recapitulates PHYLOGENY (the ‘law of recapitulation’ or the ‘recapitulation theory’) was made by the 19th-Century biologist Haeckel. onychomycosis MYCOSIS of nails (cf. PARONYCHIA). [Review: Clinical Microbiology Reviews (1998) 11 415–429.] Onygena See GYMNOASCALES. o’nyong-nyong fever (joint-breaker fever) A human disease caused by an ALPHAVIRUS and transmitted by anopheline mosquitoes; it occurs e.g. in Africa. Symptoms: fever, headache, myalgia, maculopapular rash, and severe arthralgia. Wild primates may act as a reservoir of the virus. oocyst (1) (mycol.) Syn. OOGONIUM. (2) (protozool.) A structure in which sporozoites are formed. (3) An encysted fertilized female gamete. Modification or activation of a c-onc sequence is apparently involved in certain cases of oncogenesis, and may even prove to be a common mechanism of oncogenesis in general. Oncogenesis could result from alterations in the properties of a c-onc gene product (e.g. as a result of mutation – see RAS), or from abnormal regulation of a c-onc gene; for example, a c-onc gene which is normally silent or expressed at very low levels in a particular cell may be activated by the insertion of a retroviral provirus adjacent to and upstream of the c-onc sequence, the sequence thus coming under the control of the viral transcriptional regulation signals (see e.g. AVIAN LEUKOSIS VIRUSES). The regulation of a c-onc gene may also be disturbed as a result of its translocation to a novel context by chromosomal rearrangements (see e.g. ABL and BURKITT’S LYMPHOMA). Viral genes involved in oncogenesis induced by certain DNA viruses are sometimes referred to as oncogenes, although they differ in several respects from the oncogenes of retroviruses; they are viral genes which apparently have no cellular homologues and which encode proteins essential for virus replication. (See e.g. MASTADENOVIRUS and POLYOMAVIRUS; cf. EPSTEIN–BARR VIRUS.) oncogenesis The development of a (benign or malignant) tumour (or other neoplastic condition: e.g. LEUKAEMIA). A tumour may develop e.g. as a result of the effect on an ONCOGENE of a rearrangement of chromosomes or of mutation (see e.g. ABL, BURKETT’S LYMPHOMA, RAS). oncogenic Strictly: refers to any agent which can induce the formation of a tumour; more generally, the term is often applied to any agent which can bring about the neoplastic transformation of cells – either in vivo (resulting in the formation of a tumour or e.g. LEUKAEMIA) or in TISSUE CULTURE. Such agents may be termed oncogens (cf. CARCINOGEN). oncom (ontjom) An Indonesian food made by the fermentation of (usually) peanut press-cake from which at least some of the oil has been extracted. The presscake is broken into fragments, soaked, steamed and cooled, placed in shallow trays, inoculated with a starter (RAGI) of Neurospora intermedia or Rhizopus oligosporus, and incubated at ca. 30° C; the mould binds the fragments into a cake which becomes coated with pink-orange, powdery conidia (Neurospora) or black sporangia (Rhizopus). [Book ref. 8, pp. 513–515.] (cf. TEMPEH.) oncornaviruses Former name for viruses of the ONCOVIRINAE. Oncovirinae A subfamily of viruses of the RETROVIRIDAE; the subfamily includes all of the oncogenic retroviruses as well as many closely related but non-oncogenic retroviruses. On the basis of e.g. morphology and mode of development, the subfamily has been divided into the type B viruses (genus TYPE B ONCOVIRUS GROUP), type C viruses (genus TYPE C ONCOVIRUS GROUP), and type D viruses (proposed genus TYPE D RETROVIRUS GROUP) [Book ref. 23, p. 125]. (See also HTLV.) one-carbon compounds See C1 COMPOUNDS. one-step growth experiment A procedure devised by Ellis and Delbrück [JGP (1939) 22 365–384] for the quantitative study of lytic bacteriophage multiplication; it has since been modified for the study of other lytic viruses. A suspension of susceptible cells is inoculated with enough viruses to infect every cell. After allowing time for virus adsorption, unadsorbed viruses are removed. The cell suspension is then incubated at a suitable temperature, and aliquots are periodically withdrawn and subjected to e.g. PLAQUE ASSAY. During the initial period (latent period ) the number of plaques per aliquot typically remains fairly constant; in this period each virus-infected cell behaves as a single ‘infectious centre’ capable 536 operator [Role of Opa proteins in interactions with host (eukaryotic) cells: TIM (1998) 6 489–495.] opacity-associated proteins (of Neisseria) See OPA PROTEINS. opal codon See GENETIC CODE. Opalina See OPALINATA. Opalinata A subphylum of protozoa (phylum SARCOMASTIGOPHORA); the organisms are typically parasitic in amphibians. Individual cells are often oval or elongated, ca. 60–600 µm in length (according to species), and are generally flattened and leaf-like; each contains two or more (typically many) identical nuclei, and none has a cytostome. Each cell bears many short flagella (which are described as ‘cilia’ by some authors). Asexual reproduction typically occurs by longitudinal (INTERKINETAL) binary fission. In Opalina spp (rectal parasites of the frog) a phase of rapid asexual multiplication and encystment coincides with the breeding season of its amphibian host; it is believed that this phase is triggered by the host’s hormonal activity. Cysts, the infective stages, are voided by the frog and ingested by tadpoles – in which excystment is followed by repeated longitudinal fission and the production of flagellated gametes. After syngamy, the encysted zygote is voided and ingested by another tadpole in which the zygote may undergo fission and gamete formation, or develop to form the mature, multinucleate trophic stage. (See also ENTAMOEBA (E. paulista).) Other genera include Cepedea and Zelleriella. Opegrapha A genus of crustose LICHENS (order OPEGRAPHALES). Photobiont: Trentepohlia. Fruiting structures may be lirelliform (see LIRELLA) or rounded. Species occur on rocks, bark etc. Opegraphales An order of fungi of the ASCOMYCOTINA; members include crustose to fruticose LICHENS (photobiont commonly trentepohlioid) and lichenicolous fungi. Ascocarp: APOTHECIOID, with a typically thick, carbonaceous excipulum. Asci: bitunicate, elongated to clavate. Genera: e.g. Lecanactis, OPEGRAPHA, ROCCELLA. open complex See TRANSCRIPTION. open culture Syn. CONTINUOUS CULTURE. open reading frame (ORF) A sequence of codons (see GENETIC CODE), starting with an initiator codon and ending with a stop codon, which encodes a known or (as yet) unidentified polypeptide–or (see later) part of a polypeptide; the existence of an ORF is revealed by sequencing studies (see DNA SEQUENCING). A reading frame is said to be ‘blocked’ if a stop codon is located close to the initiator codon. In some cases, two adjacent ORFs jointly encode a single polypeptide; translation of such a polypeptide requires some kind of mechanism (see e.g. READTHROUGH) that permits such coupling. In one case, the mechanism of translational bypassing of a 50-nucleotide gap between two ORFs appears to involve (i) a stem–loop structure between the first ORF and the coding gap, and (ii) a sequence of amino acid residues within the nascent peptide translated from the first ORF [EMBO (2000) 19 2671–2680]. open-system culture Syn. CONTINUOUS CULTURE. operational taxonomic unit See OTU. operator (mol. biol.) Any specific nucleotide sequence to which a regulatory element can bind in order to control the expression (transcription) of a given gene or operon. An operator commonly consists of, or contains, a PALINDROMIC SEQUENCE, and it may lie e.g. between the PROMOTER and the coding sequence of a gene, or between the promoter and first structural gene in an operon – or (as in the TRP OPERON) it may occur entirely within the promoter region. Oocystis A genus of unicellular, non-motile, ellipsoidal or lemon-shaped, freshwater green algae (division CHLOROPHYTA); asexual reproduction occurs by autospore formation. Oodinium See DINOFLAGELLATES and VELVET DISEASE. oogamy Fertilization involving morphologically distinguishable male and female elements – typically a large non-motile female gamete and a small motile male gamete (see e.g. MONOBLEPHARIDALES; cf. OOMYCETES). (See also ANISOGAMY.) oogonium A female structure in which one or more gametes (oospheres) develop; a fertilized oosphere is called an oospore. ookinete A motile zygote. Oomycetes A class of aquatic and terrestrial fungi (subdivision MASTIGOMYCOTINA) which typically give rise to zoospores having one anteriorly-directed tinsel flagellum and one posteriorlydirected whiplash flagellum (cf. PERONOSPORA). The organisms include saprotrophs and parasites – the latter including some important pathogens of cultivated crops. According to species, the thallus ranges from a simple, unicellular, holocarpic structure to a well-developed mycelium; in at least some species the thallus is diploid. The CELL WALL is generally believed to lack chitin and to contain CELLULOSE; however, there is evidence that CHITIN and cellulose both occur in the wall in e.g. certain members of the LEPTOMITALES and Saprolegniales. Some oomycetes exhibit DIPLANETISM, others MONOPLANETISM. Sexual reproduction is oogamous, involving the fusion of non-motile male and female elements within gametangia (cf. MONOBLEPHARIDALES). Orders: LAGENIDIALES, LEPTOMITALES, PERONOSPORALES, SAPROLEGNIALES. Some authors include this class of fungi in the subdivision Diplomastigomycotina of the division Mastigomycota. oophoritis Inflammation of the ovaries. oosphere See OOGONIUM. oospore See OOGONIUM. Oosporidium A genus of non-fermentative imperfect yeasts (class HYPHOMYCETES). The cells are variously shaped, reproduce by multilateral bud-fission, and usually remain attached to each other to form long chains. Asexual endospores are formed, arthrospores are not. Pink or orange-yellow non-carotenoid pigments are synthesized. One species: O. margaritiferum, isolated from the slime flux of trees. [Book ref. 100, pp. 886–889.] Opa proteins (of Neisseria) In pathogenic strains of Neisseria: OUTER MEMBRANE adhesins, encoded by opa genes, whose presence affects the opacity (hence Opa) and colour of the colonies (owing to increased bacterial aggregation); the Opa proteins mediate adhesion to (and uptake by) epithelial cells or opsonin-independent phagocytosis by PMNs. (Non-pathogenic strains of Neisseria contain genes with sequences homologous to those of the opa genes; it is possible that the products of these genes may mediate mucosal colonization by commensal strains.) (See also CD66.) Some Opa+ strains exhibit TRANSCYTOSIS. Within a given cell, the chromosome contains several allelic (variant) forms of opa gene, and the organisms exhibit phase variation, i.e. a given cell may change (e.g.) from expressing a single type of opa gene to expressing more than one type (or no type) of opa gene. Phase variation is regulated by certain DNA re-arrangements in the 5 leader sequence of the opa genes. The 5 region of these genes contains multiple copies of a pentameric sequence (CTCTT), and the number of copies of this ‘coding repeat’ can vary; thus, while all the opa genes are transcribed constitutively, translation of a transcript will give rise to a functional Opa protein only if the number of (pentameric) coding repeats is such that the coding sequence of the gene is in the correct reading frame. 537 Opercularia In operons under negative promoter control, the binding of repressor to operator sequence(s) inhibits initiation of transcription. The gal operon (encoding utilization of galactose, and biosynthesis of UDP-galactose) contains two operators (located on either side of the promoter region); it appears that, in the repressed state, these two sequences may be physically linked by an oligomer of the repressor protein (GalR) to form a DNA loop that (by sequestering the promoter region) inhibits initiation of transcription. (See also LAC OPERON.) Such DNA looping is also used for transcriptional control in the araBAD operon (see ARA OPERON). This operon contains four operator sequences, and the way in which the regulatory protein (AraC) binds to particular sequences (influenced by the presence/absence of arabinose) controls the activity of the araBAD operon. When arabinose is absent, AraC binds to the I1 and O2 operators, forming a loop that represses transcription. In the presence of arabinose, the arabinose–AraC complex binds to the I1 and I2 operators (which are close together and also referred to, jointly, as the initiator, araI ); this promotes transcription of araBAD. In Pseudomonas aeruginosa, synthesis of the siderophore PYOVERDIN is apparently controlled by a transcriptional regulator, the FUR PROTEIN, via an operator designated the ‘Fur box’ or ‘iron box’; with adequate iron, Fur binds to the operator, blocking transcription of pvdS – a gene encoding a SIGMA FACTOR necessary for the synthesis of pyoverdin [see JB (1996) 178 2299–2313]. Iron-regulated transcriptional control occurs at the operator of the tox gene encoding diphtheria toxin. Repression of tox by transitional metal ions apparently involves a helix-to-coil change in the repressor protein (DtxR); in this (iron-modified) state, dimers of DtxR inhibit transcription by binding on either side of the operator region [Nature (1998) 394 502–506]. Opercularia A genus of sedentary ciliate protozoa (subclass PERITRICHIA) which occur in various freshwater habitats; some species (e.g. O. coarctata) are common in SEWAGE TREATMENT plants. Opercularia resembles CARCHESIUM, but the zooid lacks a flared rim, and the stalks are non-contractile. operculate ascus An ASCUS whose wall has a large, annular, apical thickening that encloses a circular region of thinner wall tissue; the circular region of tissue ruptures prior to ascospore discharge. operculum (mycol.) A hinged lid or an easily detachable portion of the wall in a sac-like spore-bearing structure (see e.g. OPERCULATE ASCUS). operon In prokaryotes: two or more genes whose expression can be co-ordinated from (i) a common PROMOTER, the contiguous genes being transcribed as a single polycistronic mRNA (see e.g. LAC OPERON) or (ii) a common regulatory region – mRNAs being formed by DIVERGENT TRANSCRIPTION from different promoters. The genes in an operon are often – though not necessarily – functionally related: e.g. they may encode enzymes of a particular metabolic pathway. The organization of genes into an operon may allow their expression to be co-ordinately ‘turned on’ (induced ) or ‘turned off’ (repressed ) according to the cell’s needs. For example, enzymes of a catabolic pathway may be induced in the presence of the substrate of that pathway, and repressed again when the substrate has been used up; enzymes of an anabolic pathway must be induced when the end-product of the pathway becomes limiting and repressed when adequate levels of the product have been synthesized. Regulation of structural gene expression may occur at the level of transcription initiation (promoter control ), transcription termination (attenuator control ), or a combination of both; exceptionally, regulation occurs at the level of translation. Promoter control. An operon under promoter control is commonly regulated by the product of a specific regulator gene which may or may not be contiguous with the structural genes. (Opinions differ as to whether or not the regulator gene should be regarded as part of the operon.) The regulator gene generally has its own promoter and may be expressed constitutively or may be subject e.g. to AUTOGENOUS REGULATION. The operon is said to be under negative control if the structural genes are expressed until ‘turned off’ by the regulator protein, and under positive control if they are not expressed until ‘turned on’ by the regulator protein. (The regulator protein may require a low-MWt effector molecule for activity.) In a negatively controlled operon the regulator gene encodes a repressor protein which binds to a specific region of the DNA (the OPERATOR) upstream of the structural genes, thereby preventing the initiation of transcription. If the structural genes encode enzymes of a catabolic pathway, the presence of the substrate brings about the induction of gene expression. The molecule effecting induction is called an inducer; it is not necessarily the substrate of the pathway itself, but may be e.g. a derivative of it. (Certain analogues of the natural inducer may also act as inducers; an analogue which can act as an inducer but which cannot be metabolized by the cell is called a gratuitous inducer.) The inducer inactivates the repressor, causing it to release the operator and allow transcription to proceed. When the substrate is removed (e.g. when all of the substrate has been metabolized) the repressor again binds to the operator and transcription ceases; since mRNA is generally short-lived in bacteria (see MRNA), enzyme synthesis also rapidly ceases. (See e.g. LAC OPERON.) Structural genes encoding enzymes involved in an anabolic pathway may also be subject to negative control. In this case, the regulator gene encodes an aporepressor which cannot itself bind to the operator of the operon, but which can do so when it interacts with the end-product of the pathway (or a derivative or analogue of it), the corepressor. In a positively-controlled operon the regulator protein may be an activator (= inducer protein) which binds to a region of the operon (the initiator ), thereby turning on (activating) transcription initiation. In the case of a catabolic operon the regulator gene may encode an apo-activator which must interact with the substrate (or a derivative of it), the co-activator, to form a functional activator. (See e.g. ARA OPERON.) Attenuator control. Attenuation is a regulatory mechanism in which gene expression is prevented by the termination of transcription at a rho-independent terminator (see TRANSCRIPTION), the attenuator, in the LEADER region of the mRNA; thus, transcription is terminated before the RNA polymerase reaches the first structural gene of the operon. Whether or not transcription terminates at the attenuator depends on the secondary structure adopted by the leader region of the mRNA: in one conformation the stem-and-loop structure of the rho-independent terminator is formed and transcription is terminated, while in the alternative conformation the terminator structure is not formed and transcription continues into the structural genes. Attenuation is involved e.g. in the regulation of various operons concerned with amino acid biosynthesis. In these operons the leader encodes a small peptide which contains a relatively high proportion of the amino acid whose biosynthesis is encoded by the operon. If the amino acid is present at levels 538 oral hairy leukoplakia adequate to meet the cell’s requirements, the leader peptide is synthesized and the mRNA adopts the conformation which terminates transcription at the attenuator. However, when levels of the amino acid fall, the corresponding aa-tRNA becomes limiting and the ribosome translating the leader peptide stalls when it reaches the codon(s) for that amino acid; the presence of a stalled ribosome in this position causes the mRNA to adopt the alternative conformation (sometimes called the ‘antiterminator’), and transcription continues through the attenuator and into the structural genes of the operon. Operons subject to attenuator control include e.g. the HIS OPERON and the TRP OPERON. Attenuation is also involved in the regulation of e.g. genes involved in pyrimidine nucleotide biosynthesis [Appendix V(b)]; pyrBI (encoding subunits of aspartate carbamoyltransferase) and pyrE (encoding orotate phosphoribosyl transferase) are regulated in response to the level of cellular UTP [review: TIBS (1986) 11 362–365]. (cf. TNA OPERON.) Translational control. Translational control is exemplified by the IF3-L35-L20 operon (containing genes infC-rpmI-rplT respectively) in Escherichia coli ; this operon (transcribed as a polycistronic mRNA) encodes two ribosomal proteins (L35, L20) and a protein factor involved in protein synthesis (IF3: translation initiation factor 3). Translation of L35 and L20 is repressed by L20, i.e. the concentration of L20 in the cell regulates gene expression – high levels of L20 repressing translation. Repression by L20 appears to involve the binding of L20 to mRNA at a site upstream of rpmI. The mechanism of repression by L20 is unknown – L20 could simply block mRNA–ribosome binding, or it could bind the ribosome to form an inactive complex. However, in vitro studies suggest the possibility that regulation may involve an RNA ‘pseudoknot’ formed by base-pairing between mRNA sites upstream of rpmI ; it has been suggested that L20 may stabilize the pseudoknot (which includes the Shine–Dalgarno sequence and initiator codon of rpmI ) so as to inhibit ribosome binding and, hence, prevent translation of both rpmI and rplT [EMBO (1996) 15 4402–4413]. (See also RIBOSOME.) operon fusion In genetic engineering: the fusion of two operons (which are transcribed in the same direction) e.g. by the deletion of a region of DNA between them; the structural genes of the second operon thus come under the control of the regulatory sequences (promoter, operator etc) of the first. (cf. GENE FUSION; see also MUDLAC SYSTEM.) Ophiobolus See DOTHIDEALES. (For O. graminis see GAEUMANNOMYCES.) ophiobolus patch disease See TAKE-ALL. Ophiocytium A genus of unicellular algae (class XANTHOPHYCEAE) in which the vegetative cell is rod-shaped to filamentous and is either free-floating or attached to the substratum by a stalk; the cell is usually multinucleate and reproduces by zoospore or aplanospore formation. Ophiostoma See OPHIOSTOMATALES. Ophiostomatales An order of saprotrophic and plant-parasitic fungi of the subdivision ASCOMYCOTINA. Ascocarp: a black, longnecked, superficial or partly immersed perithecium; hamathecium: absent. Asci: unitunicate and thin-walled, rounded to ellipsoidal, evanescent. Genera: e.g. CERATOCYSTIS. (The genus Ophiostoma is distinguished by some authors.) Ophryoglena See HYMENOSTOMATIDA. ophthalmia Inflammation of any part of the eye, especially the conjunctiva. (See also CONJUNCTIVITIS.) ophthalmia neonatorum Ophthalmia affecting newborn babies, usually contracted from the birth canal; it is commonly caused by Neisseria gonorrhoeae (see GONORRHOEA) or Chlamydia trachomatis. (The disease caused by C. trachomatis is also called inclusion blennorrhoea or inclusion conjunctivitis.) [Chlamydial ophthalmia neonatorum (aetiology, epidemiology, diagnosis, prophylaxis and treatment): Book ref. 193, pp. 297–304.] opine See CROWN GALL. opisthe (ciliate protozool.) The posterior of the two cells formed during HOMOTHETOGENIC cell division. (cf. PROTER.) opisthomastigote See HERPETOMONAS and TRYPANOSOMATIDAE. oppBCDF genes See PEPTIDOGLYCAN. opportunist pathogens Organisms which are normally freeliving, or which occur as part of the normal body flora, but which may adopt a pathogenic role under certain conditions – e.g. when the normal antimicrobial defence mechanisms of the host have become impaired; thus, e.g. Bacteroides spp (common inhabitants of the human gut) may give rise to peritonitis following bowel surgery. (See also ENDOGENOUS INFECTION.) opsin The apoprotein of e.g. BACTERIORHODOPSIN (bacterioopsin), HALORHODOPSIN (halo-opsin) or SLOW-CYCLING RHODOPSIN (slow-opsin). opsonic index The ratio of phagocytic activity of a patient’s blood to that of a normal (non-immune) subject in respect of a specific opsonin. To determine the opsonic index: both (citrated) blood samples are incubated for 15 min with a standard number of the relevant antigenic particles (usually microbial cells); the average number of particles engulfed by the phagocytes is then determined by microscopy of smears from each sample – the ratio of the two averages being the opsonic index. opsonin Any molecule which, when combined with a particulate antigen, increases the susceptibility of that antigen to PHAGOCYTOSIS. Opsonins include: (i) certain types of antibody (e.g. most types of IgG) – receptors for the FC PORTION of an antibody occurring e.g. on the macrophage surface; (ii) the C3b and C4b components of COMPLEMENT; (iii) certain ACUTE-PHASE PROTEINS. opsonization The process in which a particulate antigen (e.g. a microbial cell) becomes more susceptible to PHAGOCYTOSIS as a result of combination with an OPSONIN. The term may refer to either a complement-dependent or complement-independent increase in susceptibility. optical bleach Syn. FLUORESCENT BRIGHTENER. optical brightener Syn. FLUORESCENT BRIGHTENER. optical density See TURBIDIMETRY. optical staining Syn. RHEINBERG ILLUMINATION. optimal proportions (optimal ratio; equivalence) (serol.) In a PRECIPITIN TEST: the proportions in which antigen and antibody must be mixed in order to obtain the maximum amount of precipitate; for a given antigen–antibody system different optimal proportions are observed in the DEAN AND WEBB TITRATION and the RAMON TITRATION. (See also ANTIGEN EXCESS and LATTICE HYPOTHESIS.) optochin Ethylhydrocupreine hydrochloride (EHC): a reagent which strongly inhibits the growth of Streptococcus pneumoniae; other streptococci which form colonies similar to those of S. pneumoniae are commonly resistant to ten or more times the concentration of optochin that inhibits S. pneumoniae. Optochin is used to detect the presence of S. pneumoniae in a mixed flora; the sample is streaked onto the surface of a solid medium, and an optochin-impregnated paper disc is placed on the medium prior to incubation. A significant decrease in the number of colonies in the vicinity of the disc may indicate the presence of S. pneumoniae. oral hairy leukoplakia See HAIRY LEUKOPLAKIA. 539 oral herpes although changes in pH may also affect e.g. transport systems and/or surface properties in the target cells. In membrane vesicles of Escherichia coli, parabens and sorbic acid were found to eliminate 1pH – leaving 1y much less disturbed [JGM (1985) 131 73–76]. organism See MICROORGANISM. organoleptic Affecting the organs of sense – particularly those of taste and smell. organophosphorus compounds Phosphorus-containing organic compounds are widely used as insecticides and acaricides, and some are also effective agricultural ANTIFUNGAL AGENTS; the latter include e.g. ditalimfos (used against barley powdery mildew), HINOZAN, KITAZIN, and PYRAZOPHOS. [Accumulation, metabolism and effects of organophosphorus insecticides on microorganisms: AAM (1982) 28 149–200.] organotin See TIN. organotroph An organism which obtains energy by the metabolism of an organic substrate (i.e., a chemoorganotroph) or which uses an organic substrate as an electron donor in phototrophic metabolism (i.e. a photoorganotroph). (cf. LITHOTROPH; HETEROTROPH.) Chemoorganotrophs may obtain energy from organic substrates e.g. by FERMENTATION (sense 1) and/or by RESPIRATION; substrates may include any of a wide range of compounds, e.g. (depending on species) various sugars, amino acids, fatty acids, cyclic compounds, etc. Photoorganotrophs include e.g. certain photosynthetic bacteria which use relatively simple substrates (e.g. acetate, caproate, lactate) as electron donors in PHOTOSYNTHESIS. orgmet See BIOMIMETIC TECHNOLOGY. ori A designation which usually indicates an origin of DNA REPLICATION (see e.g. oriC ); oriT is an origin of transfer (see CONJUGATION sense 1b). ori1 See F PLASMID. ori2 See F PLASMID. oriC An origin of chromosome replication. In Escherichia coli the oriC region is ca. 245 bp long and contains sequences for recognition by and interaction with initiation factors; these sequences are precisely separated by ‘spacer’ sequences [Book ref. 69, pp. 257–273; interaction of oriC with proteins: pp. 289–301]. (See also DNA REPLICATION.) oriental sore See CUTANEOUS LEISHMANIASIS. origin (mol. biol.) See DNA REPLICATION. oriS See F PLASMID. orisome See DNA REPLICATION. oriT See CONJUGATION (1b) (i). oriV The/an origin of vegetative replication in a PLASMID. (cf. oriT in CONJUGATION (1b) (i).) Orleans process See VINEGAR. ornidazole See NITROIMIDAZOLES. L-ornithine See e.g. Appendix IV(a). ornithine carbamoyltransferase See ARGININE DIHYDROLASE and Appendix IV(a). ornithine decarboxylase test See DECARBOXYLASE TESTS. Ornithocercus A genus of tropical marine DINOFLAGELLATES in which the cells have elaborate thecal extensions (‘wings’). ornithocoprophilous Preferring a habitat rich in bird faeces. ornithosis See PSITTACOSIS. orotate Uracil 4-carboxylate: see Appendix V(b). orotidine monophosphate See Appendix V(b). Oroya fever See BARTONELLOSIS. orphan virus A virus which is not known to cause any disease. orseille Syn. ORCHIL. Orthocerina See FORAMINIFERIDA. oral herpes See HERPES SIMPLEX. oral microbiology Microbiology of the mouth: see e.g. MOUTH MICROFLORA. (See also DENTAL CARIES, DENTAL PLAQUE and PERIODONTITIS.) oral rehydration solution A solution of salts and glucose recommended by the WHO (World Health Organization) for use as an oral treatment in cases of severe diarrhoeal disease and dysentery. One litre contains: sodium chloride (3.5 g), sodium citrate (2.9 g), potassium chloride (1.5 g) and anhydrous glucose (20.0 g). orange peel fungus See ALEURIA. Orbivirus A genus of viruses of the REOVIRIDAE. The capsid contains at least seven polypeptide species, of which four are major structural components. Genome: 10 molecules of dsRNA. Type species: bluetongue virus. Orbiviruses infect a wide range of arthropods (mainly insects) and vertebrates, and some can cause disease in man or animals (see e.g. AFRICAN HORSE SICKNESS, BLUETONGUE; cf. COLORADO TICK FEVER); transmission is by vectors such as midges, mosquitoes, sandflies and ticks. The genus comprises many named viruses which are classified in the following serological subgroups (main vertebrate and invertebrate hosts in parentheses): African horse sickness (equines; Culicoides); bluetongue (ruminants; Culicoides); Changuinola (rodents; Phlebotomus); Colorado tick fever (man and rodents; ticks) – but see COLORADO TICK FEVER; Corriparta (birds, mosquitoes); epizootic haemorrhagic disease (deer; ?Culicoides); equine encephalosis (equines; unknown); Eubenangee (kangaroos, wallabies; mosquitoes); Kemerovo (various; ticks); Palyam (unknown; Culicoides, mosquitoes); Wallal (kangaroos, etc; Culicoides); Warrego (cattle, kangaroos, wallabies; Culicoides). [Review: Book ref. 83, pp. 287–357; biochemistry: Arch. Virol. (1984) 82 1–18.] orchid fleck virus See RHABDOVIRIDAE. orchid mycorrhiza See MYCORRHIZA. orchil (orseille) A purple dye obtained from species of the lichen ROCCELLA. orchinol 2,4-Dimethoxy-6-hydroxy-9,10-dihydrophenanthrene: a wide-spectrum antifungal PHYTOALEXIN produced by the tubers (and, to a lesser degree, by the roots and stems) of certain orchids (e.g. Orchis militaris) in response to infection with parasitic or mycorrhizal fungi (e.g. Rhizoctonia repens) or with certain soil bacteria. orchitis Inflammation of the testes. orcinol 3,5-Dihydroxytoluene – a compound obtained from various lichen species. order A taxonomic rank (see TAXONOMY and NOMENCLATURE). orellanin poisoning MYCETISM, involving renal dysfunction, due to the ingestion of Cortinarius orellanus. ores (microbial leaching) See LEACHING. orf Syn. CONTAGIOUS PUSTULAR DERMATITIS (1). ORF OPEN READING FRAME. orf subgroup See PARAPOXVIRUS. orf18 gene See CONJUGATIVE TRANSPOSITION. organelle Any structure which forms part of a cell and which performs a specialized function: e.g. a FLAGELLUM, a MITOCHONDRION, or the PARORAL MEMBRANES of a ciliate. organic acids (as antimicrobial agents) Certain organic acids and their salts or esters are used as PRESERVATIVES and e.g. as antifungal agents for the treatment of skin diseases (see e.g. BENZOIC ACID, PARABENZOATES, SALICYLIC ACID, SORBIC ACID and UNDECYLENIC ACID). Typically, their antimicrobial activity is maximum in the lower pH range; this is commonly taken as evidence that activity depends on the unionized molecule, 540 osmoregulation osmic acid See OSMIUM TETROXIDE. osmiophilic Having an affinity for OSMIUM TETROXIDE. osmium tetroxide (‘osmic acid’; OsO4 ) A volatile compound used e.g. for post-FIXATION (1–2% w/v in phosphate or cacodylate buffer); the specimen is immersed in (or exposed to the vapour of) the solution. OsO4 may cross-link and stabilize e.g. lipids by reacting with their double bonds. Longer exposure to OsO4 may be used for positive staining in ELECTRON MICROSCOPY. osmolyte Syn. OSMOTICUM. osmophilic Refers to an organism which grows optimally in or on media of high osmotic pressure. osmoregulation A form of ADAPTATION in which a cell increases or decreases its intracellular osmotic pressure in response to an increase or decrease, respectively, in extracellular osmotic pressure. Maintenance of internal osmotic pressure, within certain limits, is necessary for growth/viability; for example, osmotic withdrawal of water from a cell – without adaptive response – would tend to raise the concentrations of some solutes to levels that may be inhibitory or lethal. In microorganisms osmoregulation involves the regulation of intracellular concentrations of certain types of solute and/or (in some eukaryotes) the action of CONTRACTILE VACUOLES. Solutes used in osmoregulation include certain ions (e.g. K+ ) and a number of low-MWt organic compounds such as glutamate and glycine betaine ((CH3 )3 .N+ .CH2 COO− ), a substance common in nature. (See also COMPATIBLE SOLUTE.) Solutes (osmotica) may be taken up from the environment and/or synthesized in situ. Uptake or pumping out of ions through the cytoplasmic membrane involves e.g. the activity of particular types of ATPase; this form of osmoregulation is common in bacteria: see e.g. the Escherichia coli Kdp-based system in TWO-COMPONENT REGULATORY SYSTEMS. Another form of osmoregulation in E. coli is controlled by a different twocomponent system. In this system, increased osmolarity in the environment stimulates the kinase activity of a cell-envelope sensor, EnvZ, which transfers phosphate to the regulator protein, OmpR; activated OmpR (i) enhances transcription from ompC (the gene encoding OmpC PORIN) and (ii) inhibits transcription from the OmpF porin gene. Hence, increased osmolarity results in a modified outer membrane containing a higher proportion of OmpC (which forms a pore smaller than that of OmpF). Following osmotic up-shift, uptake of glycine betaine is common in both Gram-positive and Gram-negative bacteria. In E. coli this compatible solute is taken up very efficiently by a binding-protein-dependent transport system (a so-called ABC importer) designated ProU [FEMS Reviews (1994) 14 3–20]. The bacterium Sinorhizobium meliloti actually metabolizes most of the known osmoprotectants, including glycine betaine; in S. meliloti, osmotic up-shift triggers the accumulation of endogenous solutes (e.g. glutamate and N-acetylglutaminylglutamine amide) – accumulation being stimulated e.g. by exogenous sucrose [JB (1998) 180 5044–5051]. [Regulation of compatible solute accumulation in bacteria: Mol. Microbiol. (1998) 29 397–407.] Osmotic down-shift triggers efflux systems. For example, K+ ions and glutamate are released rapidly by E. coli (efflux apparently being energy-independent); much of the K+ released is believed to pass through mechanosensitive channels in the CYTOPLASMIC MEMBRANE: stretch-activated channels which respond to increases in the cell’s turgor pressure by increasing their pore size [JBC (1997) 272 32150–32157]. In some bacteria (including E. coli ) the membrane contains structures called aquaporins that may contribute to osmoregulation. orthochromatic dye A dye which does not give metachromatic effects (see METACHROMASY). Orthodonella See HYPOSTOMATIA. orthokinesis See KINESIS (sense 2). orthomere See MACRONUCLEUS. Orthomyxoviridae A family of pleomorphic, enveloped VIRUSes which have a segmented, negative-sense ssRNA genome (cf. PARAMYXOVIRIDAE). Virus replication in host cells is inhibited by actinomycin D and by a-amanitin. Orthomyxoviruses are mainly pathogens of the upper respiratory tract in man, animals and birds; they include influenza viruses A, B and C, distinguished on the basis of their ribonucleoprotein antigens. Influenza viruses A and B constitute one genus: INFLUENZAVIRUS; INFLUENZA VIRUS C probably constitutes a separate genus. Two other viruses (DHORI VIRUS and THOGOTO VIRUS), previously considered to have affinities with the Bunyaviridae, have been shown to resemble orthomyxoviruses in structure; both viruses have a segmented genome of seven ssRNA molecules. [Virol. (1983) 127 205–219.] orthoplectic metachrony See METACHRONAL WAVES. Orthopoxvirus (vaccinia subgroup) A genus of viruses of the CHORDOPOXVIRINAE. The orthopoxviruses are morphologically indistinguishable from each other and are very closely related serologically; they are distinguished chiefly on the basis of their biological properties. Orthopoxviruses grow on the CAM, producing ‘pocks’ characteristic for each virus; they can also replicate in certain cells in culture (e.g. chick embryo fibroblasts, Vero cells, HeLa cells, etc). All members produce a lipoprotein haemagglutinin in infected cells; this haemagglutinin apparently becomes incorporated in the host cell membrane, and may be present on the surface of (extracellular) virions. Recombination between different orthopoxviruses occurs readily: e.g. a recombinant has been derived from vaccinia and ectromelia viruses which exhibits specific pathogenicity markers of both parents [JGV (1985) 66 621–626]. (See POXVIRIDAE for replication cycle etc.) Type species: VACCINIA VIRUS; other members: buffalopox virus, camelpox virus, COWPOX VIRUS, ECTROMELIA VIRUS, MONKEYPOX virus, rabbitpox virus, VARIOLA VIRUS. Orthoreovirus Syn. REOVIRUS. Oryctes rhinoceros virus See BACULOVIRIDAE. Orygia pseudotsugata NPV See NUCLEAR POLYHEDROSIS VIRUSES. Oscillatoria A genus of filamentous CYANOBACTERIA (section III) in which the trichomes are straight, motile (see GLIDING MOTILITY), and composed of disc-shaped cells with little or no constriction between them; sheath formation is never pronounced. GAS VACUOLES may be present, and hormogonia are formed by fragmentation of whole trichomes. GC%: 40–50. Species include e.g. O. agardhii, O. limnetica, O. princeps, and O. rubescens; O. redekei strains may belong to the genus Pseudanabaena. (See also TRICHODESMIUM.) The organisms occur in a wide range of habitats, including aquatic (freshwater, brackish and marine) and damp terrestrial environments, often forming dense conspicuous mats. (See also GEOSMIN and PAINT SPOILAGE.) O. limnetica has a high sulphide tolerance and characteristically occurs, together with purple and green photosynthetic bacteria, in sulphide-rich anaerobic habitats, where it can carry out facultative anoxygenic phototrophy using H2 S as an electron donor for CO2 assimilation (see CYANOBACTERIA). Oscillochloris See CHLOROFLEXACEAE. OSCP Oligomycin-sensitivity-conferring protein: see PROTON ATPASE. Osler’s nodes See ENDOCARDITIS. 541 osmotic shock osteosarcoma See SARCOMA. ostiole (mycol.) A tubular or pore-like opening to the exterior in e.g. a perithecium, pseudoperithecium or pycnidium. Ostracoblabe implexa A coenocytic, mycelial fungus (taxonomic position uncertain) which occurs on the shells of marine molluscs, obtaining nutrients from the organic matrix of the shell. In the European flat oyster (Ostrea edulis) the fungus causes shell disease: it penetrates between the shell and mantle of the oyster, causing severe irritation; the oyster responds by producing a highly proteinaceous shell which further encourages growth of the fungus. Shell disease occurs mainly in shallow waters where temperatures commonly exceed 18° C. [Book ref. 1, pp. 216–218.] (See also OYSTER DISEASES.) Ostreobium A genus of siphonaceous green algae (division CHLOROPHYTA) which grow endolithically in shells and corals (and beneath the layer of zooxanthellae in corals) in tropical and temperate oceans; the cells contain siphonein and siphonoxanthin, and their photosynthetic pigments have absorption maxima in the far-red. ostreogrycins See STREPTOGRAMINS. ostropalean ascus An ASCUS whose wall contains a substantial, non-refractive apical thickening (apical cap); at maturity, the ascospores are discharged via a narrow channel in the cap. Such asci characteristically contain filiform ascospores. Ostropales An order of saprotrophic, plant-parasitic and lichenized fungi of the ASCOMYCOTINA. Ascocarp: apothecioid or perithecioid. Asci: unitunicate, inoperculate, cylindrical. Ascospores: commonly filiform. OT Old TUBERCULIN. Oth-25/89/J virus A SMALL ROUND STRUCTURED VIRUS. otitis media Inflammation of the middle ear. The condition is commonly suppurative: pyogenic bacteria from the throat infect the middle ear via the eustachian tube – often following an upper respiratory tract infection or tonsillitis. The eustachian tube becomes blocked, and pus formation causes bulging and sometimes rupture of the eardrum. Causal agents include e.g. Haemophilus influenzae, Moraxella (Branhamella) catarrhalis, Streptococcus pneumoniae and S. pyogenes. otomycosis A MYCOSIS affecting the ears (see e.g. ASPERGILLOSIS). Ottens’ antigens Polysaccharide antigens which occur in many strains of Streptococcus milleri but which seldom occur in other streptococci. OTU Operational taxonomic unit: a designation applied to each individual strain, species, genus etc used in classification carried out by NUMERICAL TAXONOMY; in bacteriology OTU commonly refers to a given strain. ouabain (g-strophanthin) A polycyclic, L-rhamnose-containing glycoside which inhibits Na+ /K+ -ATPase (see ION TRANSPORT). Ouchterlony test A GEL DIFFUSION test of the DOUBLE DIFFUSION, double DIMENSION type. A pattern of holes (‘wells’) is cut into a sheet of agar (or similar material) contained (e.g.) in a Petri dish. The number and arrangement of wells depend on the number and nature of the samples to be tested, and on the purpose of the test. Each well is filled with an antigen, antibody (usually antiserum), or control preparation; the whole is covered (to prevent drying) and incubated (usually at room temperature) for hours or (more usually) days. The diagram shows the arrangement of wells, and some possible results, for the serological comparison of two samples of antiserum. Oudemansiella See AGARICALES (Tricholomataceae). Oudin test See SINGLE DIFFUSION. out genes See PROTEIN SECRETION (type II systems). outer envelope See SPIROCHAETALES. In the alga DUNALIELLA, glycerol appears to play a major role in osmoregulation. In this organism, the GLYCEROL cycle (glycerol → dihydroxyacetone (DHA) → DHA-phosphate (DHAP) → glycerol phosphate → glycerol) can be used to accumulate glycerol when external osmotic pressure is high – DHAP being derived from the Calvin cycle or from starch. In Poterioochromonas malhamensis, ISOFLORIDOSIDE is used as an osmoregulatory agent. osmotic shock (‘cold shock’) In cells or subcellular organelles etc.: sudden change(s) in osmotic pressure sufficient to cause physical disruption. Osmotic shock has been used e.g. to release periplasmic enzymes from Gram-negative bacteria. The cells are pre-incubated in 20% sucrose containing EDTA and TRIS buffer, and then rapidly diluted in cold (e.g. 4° C), dilute MgCl2 solution (e.g. 0.005 M); on centrifugation the liberated enzymes remain in the supernatant. During pre-incubation sucrose enters the periplasmic space, causing plasmolysis; on dilution, water enters the periplasmic space at a rate much greater than that of sucrose efflux via the porin channels – resulting in the rupture of the OUTER MEMBRANE. osmotic shock-sensitive permease See BINDING PROTEIN-DEPENDENT TRANSPORT SYSTEM. osmotica See OSMOTICUM. osmoticum (pl. osmotica) A substance which contributes significantly to intracellular osmotic pressure. (See also OSMOREGULATION.) osmotrophy A mode of nutrition in which soluble nutrients are absorbed through the cell surface – as occurs e.g. in bacteria and fungi. (cf. PHAGOTROPHY.) osteomyelitis Inflammation of bone. Acute osteomyelitis is caused by bacterial infection via wounds or via the blood from another site of infection (e.g. skin, throat); it may occur as a complication of septicaemia. The commonest causal agent is Staphylococcus aureus. Acute osteomyelitis occurs most commonly in children and affects mainly the long bones of the arms and legs. Symptoms: e.g. fever and bone pain, with inflammation of the overlying skin; subsequent bone growth may be prevented. Chronic osteomyelitis may be due e.g. to tubercle bacilli, the vertebrae being most commonly involved (Pott’s disease); spinal deformity may result. Chronic osteomyelitis may also occur as a complication of Salmonella infection (see e.g. FOOD POISONING (e)) – especially in patients with sickle-cell disease, bone defects, or prostheses; other causal agents include e.g. Pseudomonas aeruginosa (especially in drug addicts and open-heart surgery patients) and Escherichia coli. osteopetrosis (‘marble bone’) A disease characterized by the formation of abnormally dense bone. In chickens, osteopetrosis (‘big bone disease’) may result from infection with an AVIAN LEUKOSIS VIRUS (ALV), and involves the abnormal thickening of bones – particularly the long bones of the legs – as a result of abnormal growth and differentiation of ALV-infected osteoblasts. Most exogenous ALVs appear to have the potential for inducing osteopetrosis, although strains differ in the time of onset of disease after infection, and in the severity of the disease induced. The time of onset and severity of disease seem to correlate with the amount of viral DNA in osteoblasts; affected cells each contain several to many copies of unintegrated viral DNA as well as integrated (proviral) DNA [Virol. (1985) 141 130–143]. It has been reported that sequences near the 5 -LTR of the ALV genome are responsible for osteopetrosis induction [JV (1986) 59 45–49], while 3 -env-LTR sequences influence the time of onset and severity of the disease [Virol. (1985) 145 94–104]. 542 outer membrane Ag Ag Aga X Agb Z Aga Agc Y Ab Ab AbaAbc (a) (b) (c) OUCHTERLONY TEST. (a) Reaction of identity. With wells located symmetrically, the lines of precipitate are symmetrical and do not overlap. Since the antigens (Ag) in the top two wells react identically with the antibody (Ab) they are taken to be serologically identical. (b) Reaction of partial identity. X is a line of precipitate formed where antibody and its homologous antigen (Aga ) meet in optimal proportions. At Y, antibody and a cross-reactive antigen (Agb ) meet in optimal proportions. In this instance, Agb has less affinity than Aga for the antibody, and antibody which has diffused beyond Y forms a precipitate with Aga at Z; Z, which is called a spur, points towards the cross-reactive antigen. (c) Reaction of non-identity. Two independent precipitating systems are present: each antibody reacts only with its homologous antigen. the heptose residue(s) have significantly lower amounts of OM protein. Porins (and e.g. OmpA protein) are bound to the underlying layer of peptidoglycan by ionic links; OmpA is readily released by treatment with SDS at temperatures below 56° C, but porins remain bound even after treatment with 2% SDS at 60° C – their removal requiring drastic treatment with SDS or e.g. with 0.5 M NaCl. At a number of locations the OM is linked covalently to peptidoglycan via the Braun lipoprotein (see CELL WALL), the lipid portion of the Braun lipoprotein apparently being anchored in the hydrophobic interior of the OM. At various localized regions there are apparent fusions between the OM and the cytoplasmic membrane (see ADHESION SITE); these sites are believed to be involved e.g. in the export of LPS and porin proteins from the cytoplasmic membrane to the OM. (Although widely accepted, a few authors question the existence of adhesion sites.) Permeability of the OM. In wild-type (smooth) enteric bacteria the OM is permeable to small ions and small hydrophilic molecules; these entities can thus be taken up across the outer membrane in an energy-independent manner. The OM is generally much less permeable (or impermeable) to hydrophobic molecules – and to amphipathic molecules such as bile salts and SDS; such molecules do not pass easily through the porin channels, and their inability to penetrate the OM reflects the low permeability of the asymmetric lipid bilayer. In many cases, therefore, uptake across the outer membrane is energy-dependent (see e.g. TONB PROTEIN). In deep rough mutants (e.g. the Rd and Re mutants of Salmonella typhimurium) the permeability of the OM to e.g. crystal violet, nafcillin, phenol and other hydrophobic molecules – and to anionic and cationic detergents – is much higher. This increased permeability may be due e.g. to the loss of some LPS molecules and their replacement by phospholipid molecules, resulting in the formation of phospholipid bilayers which are presumed to be more permeable to lipophilic substances. Alternatively, or perhaps additionally, rough mutants may be defective in LPS–LPS cross-linking; thus, the absence of phosphate group(s) in the core oligosaccharide may permit hydrophobic molecules to pass through gaps between the LPS molecules. [Review: CRM (1986) 13 1–62.] Experimental disruption of the OM. Some of the LPS molecules (up to ca. 50% of the total) can be removed from the OM by the action of metal ion chelators, such as EDTA (q.v.), in the presence of TRIS buffer – or by high concentrations of TRIS outer membrane (OM) The outer layer of the CELL WALL in Gram-negative bacteria: a composite layer external to the PEPTIDOGLYCAN sacculus. The OM is an important permeability barrier (see later) which helps to control the cell’s internal environment and give protection against certain antibiotics and other harmful agents. In pathogenic Gram-negative bacteria the hydrophilicity of the OM may help the cells to evade phagocytosis. Composition of the OM. Most studies on the OM have been carried out on Escherichia coli and Salmonella typhimurium, and the following refers primarily to these and related enterobacteria. In electronmicrographs, the OM is seen as a trilaminar structure. Much of it consists of an asymmetrical lipid bilayer: a monolayer of phospholipid molecules forming the inner layer, and LIPOPOLYSACCHARIDE (LPS) molecules forming the outer (external) layer; the LPS molecules are so arranged that lipid A contributes to the hydrophobic interior of the membrane, and the (hydrophilic) O-specific chains are directed outwards. Almost half the mass of the OM consists of proteins, and the various species of protein are distributed throughout the membrane – either embedded or partly embedded in the lipid bilayer. The quantitatively dominant ‘major proteins’ include the specialized channel-forming (‘pore-forming’) transmembrane proteins (see PORIN), the BRAUN LIPOPROTEIN, and (in E. coli ) the OmpA protein (see OMP). Other proteins include those involved in the uptake of specific substance(s) (see e.g. BTUB PROTEIN, FEPA PROTEIN, LAMB PROTEIN, TONA PROTEIN, TSX PROTEIN), and enzymes such as proteases (see e.g. OmpT protein in type IV PROTEIN SECRETION) and phospholipases. In members of the Enterobacteriaceae the outer surface of the OM carries a significant amount of ENTEROBACTERIAL COMMON ANTIGEN. A stable and functional OM depends on ionic and other interactions between the various components. The core oligosaccharides of adjacent LPS molecules appear to be linked by divalent cations, particularly magnesium and calcium ions; in this context the phosphate group(s) of the heptose residue(s) may be important because their loss has been correlated with increased leakiness of the OM to hydrophobic molecules. Each LPS molecule is also hydrophobically bound (via lipid A) to the fatty acid chains of phospholipids. Some of the proteins appear to be linked to heptose residue(s) – or to the adjacent glucose residue – of the core oligosaccharide; thus, rough mutants which lack distal residues of the core oligosaccharide retain normal amounts of OM protein, while deep rough mutants which lack 543 outer membrane protein (e.g. 0.1 M, pH 7.2) without EDTA; by sequestering divalent cations, chelators increase the electrostatic repulsion between LPS molecules (and between LPS and acidic protein molecules), while TRIS may further destabilize the membrane by replacing LPS-bound cations. Cells thus treated become permeable to various hydrophobic molecules (e.g. actinomycin D, novobiocin). (See also OSMOTIC SHOCK.) An EDTA–TRIS– LYSOZYME mixture can be used for the gentle lysis of Gram-negative bacteria, while the same procedure carried out in hypertonic sucrose solution can be used for SPHAEROPLAST formation. POLYMYXINS, polylysines and polyornithines are examples of other agents which disrupt the OM and which increase its permeability. [Agents that increase the permeability of the OM: MR (1992) 56 395–411.] (See also BPI PROTEIN.) Low concentrations of magnesium ions (e.g. 0.1–5.0 mM) can help to stabilize the OM in deep rough mutants, but high concentrations of divalent cations (particularly calcium, but also magnesium) can destabilize/disrupt the wild-type OM; thus, e.g. 100 mM magnesium ions in the cold (0–4° C) causes the release of periplasmic b-lactamases in Salmonella typhimurium. High concentrations of cations may cause these effects by raising the melting point of the membrane lipids: at low temperatures the fluidity of the membrane may be decreased to such an extent that it readily ruptures; this effect is used to promote TRANSFORMATION and TRANSFECTION in enterobacteria. outer membrane protein In Gram-negative bacteria: any of a range of proteins in the OUTER MEMBRANE – see e.g. OMP, FEPA PROTEIN, LAMB PROTEIN, TONA PROTEIN and PORIN. (cf. TONB PROTEIN.) outer primers See NESTED PCR. outer sheath See SPIROCHAETALES. outer veil (mycol.) Syn. UNIVERSAL VEIL. outgrowth (of bacterial endospores) See ENDOSPORE (sense 1(a)). overgrowth syndrome See GASTROINTESTINAL TRACT FLORA. overlapping genes Two (or more) GENES in which part or all of one gene is coextensive with part of another. The genes may be translated in different reading frames (see GENETIC CODE) or in the same reading frame with different start and/or stop points (see e.g. LEVIVIRIDAE and MICROVIRIDAE) or different splicing patterns. The phenomenon of overlapping genes maximizes the coding capacity of a genome, and can also provide a means for the regulation of expression of the genes. overoxidation See ACETOBACTER. overpressure retort See BATCH RETORT. overproduction Synthesis of high concentrations of a given protein, usually a heterologous (i.e. ‘foreign’) protein, in a genetically modified organism. This procedure has been used e.g. for the synthesis of STREPTOKINASE and other therapeutic agents in Escherichia coli. E. coli is commonly used for overproduction, but it cannot express certain genes – for example, eukaryotic genes whose products require specific post-translational modification in a eukaryotic cell. Moreover, genes containing introns would not be expressed (although this problem can be circumvented by using cDNA). Efficient, high-level expression of heterologous genes in E. coli requires careful attention to specific aspects of the procedure; some of the relevant factors are considered below. Transcription of the heterologous gene should be optimized by using a strong, tightly regulated promoter in the expression vector. The use of a tightly regulated promoter is necessary because gene expression should not occur until the population of E. coli has reached a high density; this avoids an early depression of growth rate and maximizes the yield of the required protein. Such tight regulation is clearly essential if the gene product is frankly toxic for E. coli. Moreover, the choice of transcription regulator should be appropriate; thus, e.g. a toxic inducer such as IPTG (isopropyl-b-D-thiogalactoside) would not be optimal for the overproduction of therapeutic proteins. The efficiency of translation depends on factors such as the precise Shine–Dalgarno (SD) sequence and the number of nucleotides between the SD sequence and the start codon; these aspects of translational efficiency must be considered when constructing the expression vector. Additionally, some genes contain a DOWNSTREAM BOX whose precise sequence may be an important factor in efficient translation in E. coli. A further consideration is CODON BIAS. One poorly understood problem is the appearance of ‘inclusion bodies’ – aggregates of unfolded or incorrectly folded proteins which form insoluble masses in the cytoplasm; this may result in little or no biologically active product. Approaches to this problem include (i) the use of a lower growth temperature, (ii) co-expression of so-called ‘chaperone’ proteins to promote correct folding, and (iii) use of gene fusion techniques; the latter method may involve fusion of the heterologous gene with another gene whose product promotes solubility of the fusion protein. (If, however, the inclusion body proteins can be correctly folded in vitro, the development of such bodies can be advantageous in that they may facilitate purification of the protein product and help to prevent degradation of the product by intracellular proteases.) A further problem to consider is the risk of proteolysis by E. coli proteases. This risk may be avoided or minimized by targeting the protein product to the bacterial periplasm (in which there are fewer proteases). Another approach involves elimination of particular proteolytic site(s) in the product by re-coding the gene in a way which does not alter the product’s biological activity. A further possibility is to fuse the heterologous gene with another gene whose product is secreted. Re-coding a gene may also help to increase the stability of certain proteins which are (inherently) unstable owing to the presence of particular amino acids (e.g. arginine, leucine, lysine, tryptophan) at the N-terminal. (In re-coding, it should be noted that the penultimate N-terminal amino acid in a bacterial protein is also important in that it may potentiate removal of the terminal N-formylmethionine (by methionine aminopeptidase) and thus itself become the terminal amino acid.) A different approach to proteolysis involves the use of cells mutant in the rpoH gene (see HEAT-SHOCK PROTEINS); because RpoH promotes expression of the Lon protease these mutants are deficient in Lon, and such mutants have been found to give greatly increased yields of heterologous proteins. [Achieving high-level expression of genes in E. coli : MR (1996) 60 512–538.] overwound DNA See DNA. ovine diseases See SHEEP DISEASES. ovine encephalomyelitis Syn. LOUPING-ILL. ovine foot-rot See FOOT-ROT. ovine progressive interstitial pneumonia See MAEDI. Ovivora See EIMERIORINA. Ovulariopsis See HYPHOMYCETES and ERYSIPHALES. OXA-type b-lactamase See b-LACTAMASES. oxacephems Compounds (e.g. MOXALACTAM) structurally resembling cephalosporins but having oxygen instead of sulphur at position 1 (see b-LACTAM ANTIBIOTICS, Figure (b)). 544 Oxymonadida of the medium). Acidification in only the uncovered medium indicates oxidative attack on the carbohydrate, while acidification in the covered tube indicates that the carbohydrate can be attacked fermentatively; no reaction in either tube usually indicates that the particular carbohydrate is not utilized by the test organism. [Modified medium for marine bacteria: AEM (1985) 49 1541–1543.] oxidation–reduction potential REDOX POTENTIAL. oxidative metabolism See RESPIRATION. oxidative pentose phosphate pathway Syn. HEXOSE MONOPHOSPHATE PATHWAY. oxidative phosphorylation As commonly used: the phosphorylation of ADP to ATP, or the formation of PYROPHOSPHATE from inorganic phosphate, in a reaction which is driven by energy derived from proton motive force (see CHEMIOSMOSIS) generated by means of a respiratory chain; electron transport may be driven by aerobic RESPIRATION or by ANAEROBIC RESPIRATION. Oxidative phosphorylation may be catalysed by a PROTON ATPASE or a PROTON PPASE. (cf. ELECTRON TRANSPORT PHOSPHORYLATION and SUBSTRATE-LEVEL PHOSPHORYLATION.) oxidative rancidity RANCIDITY due to AUTOXIDATION of a food’s unsaturated fatty acids; such oxidation is encouraged e.g. by metal ions and by light. The occurrence of oxidative rancidity can be inhibited or delayed e.g. by refrigeration, by removing oxygen (e.g. by vacuum packing), or by the use of ANTIOXIDANTS. oxidizing agents (as antimicrobial agents) Strong or moderately strong oxidizing agents can behave as effective antimicrobial agents when they are present in adequate concentrations under suitable conditions. However, their use as DISINFECTANTS is often limited by their high reactivity – which renders them liable to inactivation in the presence of excess organic matter. Inactivation by inert organic matter can be compensated for by using a concentration of the oxidizing agent sufficient to oxidize the inert matter and to leave a residual concentration which can act against viable microorganisms – see e.g. chlorine demand in WATER SUPPLIES. (For ‘adaptation’ of bacteria to oxidizing agents see HYDROGEN PEROXIDE.) Antimicrobial oxidizing agents include e.g. CHLORINE, HYDROGEN PEROXIDE, HYPOCHLORITES, OZONE and POTASSIUM PERMANGANATE. oxidoreductases ENZYMES (EC class 1) which catalyse oxidation–reduction reactions. (See also DEHYDROGENASE; OXIDASE; OXYGENASE; PEROXIDASE.) oxine Syn. 8-HYDROXYQUINOLINE. Oxobacter See CLOSTRIDIUM. 2-oxoglutarate dehydrogenase See TCA CYCLE. oxolinic acid An analogue of NALIDIXIC ACID; it resembles nalidixic acid in its mode of action but is much more potent. 6-oxo-PGF1a See PROSTACYCLIN. oxycarboxin See CARBOXIN. Oxyfume Sterilants A range of STERILANTS containing ETHYLENE OXIDE. oxygenase An OXIDOREDUCTASE which catalyses the incorporation of one or both atoms of a molecule of oxygen into a molecule of substrate. A MONOOXYGENASE catalyses the incorporation of one atom of oxygen, the other being reduced to water; a dioxygenase (EC 1.13.11) catalyses the incorporation of both atoms of oxygen. Oxygenases are frequently involved e.g. in the first step of the bacterial degradation of HYDROCARBONS. oxygenic photosynthesis See PHOTOSYNTHESIS. oxyhydrogen reaction KNALLGAS REACTION. Oxymonadida An order of protozoa (class ZOOMASTIGOPHOREA) which are parasitic in insects (e.g. termites). Cells have one oxacillin See PENICILLINS. oxadixyl See PHENYLAMIDE ANTIFUNGAL AGENTS. Oxalobacter formigenes See UROLITHIASIS. Oxalophagus A proposed genus of bacteria containing organisms which have been included in the genus Clostridium [IJSB (1994) 44 812–826]. oxathiin antifungal agents See e.g. CARBOXIN. oxazolidinones Synthetic antibiotics which inhibit PROTEIN SYNTHESIS in (mainly) Gram-positive bacteria; they include linezolid and eperezolid. Oxazolidinones bind to the 50S ribosomal subunit in the central region of domain V of 23S rRNA – a site similar to that targeted by chloramphenicol and lincomycin; this site is within the peptidyltransferase region of the ribosome, and it is thought that these drugs may affect function(s) associated with this location of the ribosome [JB (2000) 182 5325–5331]. Linezolid (administered orally or intravenously) is a bacteriostatic agent which is effective (MIC 1–4 mg/l) against e.g. S. pneumoniae, staphylococci and enterococci; it can be useful against MRSA and other multidrug-resistant pathogens. [Review: TIM (1997) 5 196–200.] Linezolid has been licenced for use in the USA. [Linezolid (when to use?): JAC (2000) 46 347–350.] [Linezolid: review of use in the management of serious Gram-positive infections: Drugs (2001) 61 525–551.] Clinical resistance to linezolid has been reported in vancomycin-resistant enterococci (VRE) [Lancet (2001) 357 1179]. Oxford strain A strain of Staphylococcus aureus (NCTC strain 6571) which is sensitive to a wide range of antibiotics; it may be used as a control in antibiotic sensitivity tests. oxic Refers to an environment in which oxygen is present. oxiconazole See AZOLE ANTIFUNGAL AGENTS. oxidase An OXIDOREDUCTASE which catalyses a reaction in which electrons removed from a substrate are donated directly to O2 . oxidase reagent See OXIDASE TEST. oxidase test A test used to detect the presence, in bacteria, of cytochrome c and its associated oxidase. In one form of the test, filter paper is moistened with a few drops of KOVÁCS’ OXIDASE REAGENT, and a small amount of bacterial growth is smeared onto the moist filter paper with a glass spatula or a platinum loop. (Nichrome or iron-containing loops may give a false-positive reaction.) In a positive test (cyt c and cyt c oxidase present) a purple coloration (due to oxidation of the reagent) develops immediately or within 10 sec; electrons are transferred from the reagent to cyt c and thence, via cyt c oxidase, to molecular oxygen. In the so-called cytochrome oxidase test (= indophenol oxidase test) the reagent used is a mixture of equal volumes of 1% a-naphthol in 95% ethanol and 1% aqueous p-aminodimethylaniline oxalate; when the reagent is poured over bacterial growth, a positive result is indicated by a strong blue coloration (due to indophenol blue formation) within ca. 30 sec. (cf. BENZIDINE TEST.) oxidation–fermentation test (Hugh and Leifson’s test; O–F test) A test used to indicate whether a given carbohydrate (usually glucose) is attacked oxidatively or fermentatively (or not at all) by a given strain of bacterium. The peptone–agar medium includes NaCl, K2 HPO4 , the carbohydrate, and a pH indicator (usually BROMTHYMOL BLUE); the unused medium is green (if bromthymol blue is used) and has a pH of 7.1. In the test, each of two tubes of the (oxygen-free) medium is stab-inoculated with the test organism, and the medium in one of the tubes is immediately covered with a layer of sterile liquid paraffin ca. 1 cm in depth. Both tubes are then incubated and subsequently examined for evidence of carbohydrate utilization (acidification: yellowing 545 Oxymonas (cf. SHII-TAKE); oyster fungi have also been grown on a number of waste materials such as sawdust, cereal straws, cotton waste, waste paper etc. [Mycol. (1983) 75 351–360.] ozaena (ozena) Rhinitis, caused by Klebsiella pneumoniae subsp ozaenae, characterized by a profuse, mucopurulent, fetid nasal discharge with crusting of the nasal mucosa. ozena Syn. OZAENA. ozone (as an antimicrobial agent) Ozone (O3 ) is an OXIDIZING AGENT which reacts readily with most types of organic matter and which is active against a wide range of microorganisms; it is more efficient at lower temperatures, and requires a minimum relative humidity of ca. 60% for the disinfection of surfaces. Ozone has been used for the preservation of certain foods, but it is active against only surface contaminants, and it is not suitable for use with fats, meat etc which are likely to be oxidized. Ozone is sometimes used for the disinfection of potable WATER SUPPLIES; for this purpose it is superior to chlorine in that it is a stronger oxidizing agent, it is less affected by pH, and it does not impart a taste or odour. The treatment of wastewater (processed sewage) with ozone, rather than with chlorine, is advantageous e.g. in that it avoids the production of possibly toxic chlorinated organic compounds. The disinfection of water by ozone has been studied by monitoring the consumption of ozone-generated hydroxyl free radicals [Wat. Res. (1984) 18 473–478]. (See also INFLUENZA.) to many axostyles (which, in some genera, are contractile) and one or more karyomastigonts, each containing four flagella arranged typically in two pairs; the kinetosomes of each flagellar pair are connected by a ‘preaxostyle’: a paracrystalline structure into which are inserted the anterior ends of the axostylar microtubules. Mitochondria and Golgi apparatus are absent. Genera: e.g. Monocercomonoides, Oxymonas, Pyrsonympha. Oxymonas See OXYMONADIDA. Oxyphotobacteria A proposed class of oxygenic, photosynthetic prokaryotes (division GRACILICUTES) comprising the CYANOBACTERIA and the PROCHLOROPHYTES. oxyR locus See HYDROGEN PEROXIDE. Oxyrrhis See DINOFLAGELLATES. oxysome Syn. FERNÁNDEZ–MORÁN PARTICLE. oxysporum wilt See FUSARIUM WILT. oxytetracycline See TETRACYCLINES. Oxytricha See HYPOTRICHIDA. oyster cap Syn. OYSTER FUNGUS. oyster diseases For pathogens of oysters (e.g., Ostrea and Crassostrea spp) see e.g. LAGENIDIALES; MINCHINIA; OSTRACOBLABE IMPLEXA; PERKINSUS; VAHLKAMPFIA; VIBRIO (e.g. V. anguillarum). (See also INVERTEBRATE DISEASES.) oyster fungus (oyster mushroom or oyster cap) Any species of Pleurotus, but especially P. ostreatus. A number of Pleurotus spp are edible and can easily be cultivated by inoculating logs 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 546 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 P p (1) Prefix denoting the polypeptide product of a gene (see e.g. (2) Designation for a precursor molecule (as e.g. in pTP in ADENOVIRIDAE). (3) Prefix denoting a PLASMID, usually a recombinant plasmid. P Proline (see AMINO ACIDS). P agar An agar medium containing peptone (1%), yeast extract (0.5%), NaCl (0.5%), glucose (0.1%) and agar (1.5%). P antigen (parvovirus receptor) See ERYTHROVIRUS. P fimbriae Chromosome-encoded FIMBRIAE found e.g. on many strains of Escherichia coli associated with human PYELONEPHRITIS or CYSTITIS. P fimbriae mediate mannoseresistant adhesion; they bind to a-D-galactopyranosyl-(1 ! 4)b-D-galactopyranoside receptors on ceramide-linked glycosphingolipids in human uroepithelium. Adhesion of E. coli to urinary tract epithelium elicits the release of various CYTOKINES, including IL-6 and the chemokine IL-8. P fimbriae are reported to be more potent than other bacterial adhesins as an elicitor of IL-6; this cytokine can e.g. stimulate the synthesis of immunoglobulins and promote the release of ACUTE-PHASE PROTEINS. The expression of P fimbriae is regulated by temperature, and is believed to involve the H–NS PROTEIN (q.v.). P fimbriae are encoded by genes of the pap operon. These genes include papA (encoding the major fimbrial subunit), papCD (encoding, respectively, ‘usher’ and ‘chaperone’ proteins – see FIMBRIAE), and papB (encoding a regulatory protein). [P fimbriae: structure, genetics, assembly: FEMS Reviews (2000) 24 21–44 (27–31).] Interestingly, the PapB protein is involved in regulatory crosstalk between the fim and pap adhesin operons in E. coli – PapB being able to inhibit expression of type I fimbriae (see FIMBRIAE) [EMBO (2000) 19 1450–1457]. P fimbriae are also called ‘Pap pili’ (Pap D pili associated with pyelonephritis). 987P fimbriae See F6 FIMBRIAE. P ring See FLAGELLUM (a). P segment See RECOMBINATIONAL REGULATION. P-selectin See SELECTINS. P site (of a ribosome) See PROTEIN SYNTHESIS. P1 plasmid See BACTERIOPHAGE P1. p15A See COLE1 PLASMID. p21 family (Ras superfamily) See G PROTEINS. P48 See PROTEIN SECRETION. p55 (CD 120a) See TNF. p75 (CD 120b) See TNF. P-450 See CYTOCHROMES. P582 A sirohaem-containing sulphite reductase which occurs in Desulfotomaculum nigrificans. (cf. DESULFOVIRIDIN.) Pa See PASCAL. PABA (PAB) p-AMINOBENZOIC ACID. pac site A packaging initiation site in the DNA of certain bacteriophages: see e.g. BACTERIOPHAGE P22 and BACTERIOPHAGE P1. (See also TRANSDUCTION.) PA-CBT See IMMUNOSORBENT ELECTRON MICROSCOPY. PACE 2C test See PROBE. PA-CGT See IMMUNOSORBENT ELECTRON MICROSCOPY. pachyman A water- and alkali-insoluble (1 ! 3)-b-glucan comprising ca. 250–700 glucose residues with several (1 ! 6) -b-linked branch points per molecule; it occurs in the sclerotia of Poria cocos. Pachysolen A genus of yeasts (family SACCHAROMYCETACEAE). Cells are spheroidal to ellipsoidal and reproduce by multilateral budding; pseudomycelium is poorly developed or absent. Ascus formation is characteristic: an ascus develops at the tip of a tube which grows from a vegetative cell; the walls of the cell and tube become thickened and refractile except at the ascospore-bearing end – where the wall is thin and splits to release bowler-hatshaped ascospores. Glucose is fermented; NO3
is assimilated. One species: P. tannophilus, isolated from concentrated tanning liquors. [Book ref. 100, pp. 289–291.] pachytene stage See MEIOSIS. Pachytichospora A genus of yeasts (family SACCHAROMYCETACEAE). [Book ref. 100, pp. 292–294.] packet (of cells) A more or less regular arrangement of cells formed as a result of successive cell divisions along three mutually perpendicular axes, the progeny cells failing to separate. Packets are formed e.g. by Sarcina and Sarcinosporon. (cf. SARCINIFORM.) pactamycin An antibiotic which specifically inhibits the initiation of translation during PROTEIN SYNTHESIS – apparently by inhibiting binding of the initiator tRNA to the initiation complex. Pactamycin has been used for genome mapping e.g. in picornaviruses which synthesize a POLYPROTEIN from a single initiation site (see PICORNAVIRIDAE). The antibiotic is added, together with radioactively-labelled amino acids, to virus-infected cells. Label is incorporated only into those polypeptides which were initiated before addition of pactamycin. A polyprotein molecule subsequently completed will contain an amount of label which depends on the stage of elongation reached when the drug and label were added; different polyprotein molecules will therefore contain different amounts of label – a minority bearing label near the N-terminus and a majority bearing label near the Cterminus (corresponding to the 50 and 30 ends of the mRNA, respectively). Thus, the amount of label present in the polypeptides derived from the polyprotein indicates the relative positions of their structural genes in the genome. PADAC A violet-coloured pyridine-2-azo-p-dimethylanilide cephalosporin derivative which turns yellow on hydrolysis (e.g. by b-LACTAMASES); it is used in assays for b-lactamase levels. (See also PENICILLINASE.) paddy-straw mushroom Syn. PADI-STRAW MUSHROOM. PADGEM See SELECTINS. padi-straw mushroom (paddy-straw, rice-straw or straw mushroom) Either of the tropical edible mushrooms Volvariella volvacea and V. diplasia, widely cultivated in S. E. Asia and the Far East. The mushrooms are traditionally grown on soaked (uncomposted) rice-straw, but the modern method – using e.g. composted rice-straw and cotton waste – resembles that for cultivating Agaricus brunnescens (see MUSHROOM CULTIVATION). Paecilomyces A genus of fungi of the class HYPHOMYCETES; teleomorphs occur in e.g. BYSSOCHLAMYS. (See also COAL BIODEGRADATION; INDUSTRIAL ALCOHOL; LEATHER SPOILAGE; PAPER SPOILAGE; SINGLE-CELL PROTEIN.) PAGE POLYACRYLAMIDE GEL ELECTROPHORESIS. PAI See PATHOGENICITY ISLAND. paint spoilage The susceptibility of liquid paints to microbial attack depends mainly on the nature of the solid phase and RETROVIRIDAE). 547 paired organelles its solvent. Liquid emulsion paints may contain a variety of organic substances (e.g. as stabilizers, thickeners), inorganic constituents, and a high content of water, and may be susceptible to attack by bacteria (e.g. Pseudomonas spp) and fungi (e.g. Fusarium spp); biocides incorporated in such paints (to extend shelf-life) include e.g. organo-tins (such as tributyl tin oxide), certain QUATERNARY AMMONIUM COMPOUNDS, and mercurials such as phenylmercuric acetate. Oil-based liquid paints are less susceptible to spoilage, but the hardened films of both oil-based and emulsion paints are susceptible to attack – particularly under humid conditions and when surfaces are soiled; spoilage organisms may obtain nutrients from the film itself, from surface contamination, or from the substrate (wood etc) beneath the film. Spoilage organisms include species of the fungi Aspergillus, Aureobasidium, Cladosporium and Penicillium; cyanobacteria (e.g. Nostoc, Oscillatoria) and algae (e.g. Trentepohlia) may grow given adequate moisture and light. Biocides are incorporated in paints to help paint films resist microbial attack; these include e.g. organo-mercury compounds, SALICYLANILIDES, halogenated phenolics, and a variety of copper and zinc compounds. Antifouling paints are used e.g. on ships’ hulls to inhibit both spoilage and colonization by microorganisms and other organisms. In soluble-matrix paints the binder constituent is (slowly) soluble in seawater, low levels of biocide continually becoming available at the surface. In other paints, biocide diffuses slowly to the surface of an insoluble matrix. Biocides used include e.g. copper and zinc compounds, phenols and dithiocarbamates; some (e.g. tributyl tin oxide) inhibit algae as well as fungi. paired organelles Syn. RHOPTRIES. paired sera Two samples of serum from a given patient – the second sample taken e.g. days or weeks after the first. Paired sera are used e.g. to monitor changes in antibody titres. Palade pathway In a eukaryotic cell: the path followed by many types of protein from their site of synthesis to their final cellular or extracellular location. Thus, e.g., a protein destined for secretion is synthesized on a ribosome bound to the cytoplasmic face of the rough ENDOPLASMIC RETICULUM (RER); it is translocated to the luminal face of the RER (see SIGNAL HYPOTHESIS) and is then enclosed within a vesicle (cisterna) which buds from the smooth endoplasmic reticulum and subsequently coalesces with a vesicle in the GOLGI APPARATUS. Finally, a secretory vesicle (containing the protein) buds from the Golgi apparatus and subsequently fuses with the cytoplasmic membrane to release the protein at the cell surface. Other types of protein – e.g. those destined for the cytoplasmic membrane itself, or for the lumen of a LYSOSOME – follow an analogous pathway; additionally, in cells infected with certain enveloped viruses the viral ENVELOPE proteins are transported to their appropriate membrane via this pathway. The production of secretory vesicles at the Golgi apparatus, and the subsequent fusion of these vesicles with the cytoplasmic membrane, can be inhibited e.g. by MONENSIN – which can therefore inhibit the replication of certain viruses. Characteristically, proteins become progressively more glycosylated during their passage along the Palade pathway, and their intracellular location may even by determined by their degree of glycosylation. The glycosylation of newly synthesized (cell or viral) proteins may be inhibited e.g. by TUNICAMYCIN. Proteins that are not transported via the Palade pathway include e.g. the small subunit of ribulose-1,5-bisphosphate carboxylase–oxygenase. This protein is synthesized (with an Nterminal signal peptide) on a cytoplasmic (i.e., not membranebound) ribosome and is taken up, post-translationally, by chloroplasts – being proteolytically processed during or after uptake. PalI See RESTRICTION ENDONUCLEASE (table). palindromic sequence A region of a nucleic acid which contains a pair of INVERTED REPEAT sequences. In a double-stranded molecule, such a region shows two-fold rotational (dyad) symmetry – or hyphenated dyad symmetry if the two IR sequences are separated by another sequence. A double-stranded palindromic sequence can adopt either of two possible conformations: a linear structure with interstrand hydrogen-bonding – e.g. 50 . . . TCCACATGTGGA . . . 30 30 . . . AGGTGTACACCT . . . 50 – or a cruciform structure in which the two strands each form HAIRPINS by intrastrand hydrogen-bonding. For steric reasons, non-paired bases occur at the proximal and distal ends of the hairpins in a cruciform, and the linear form is normally the more stable form. However, the formation of a cruciform tends to relieve the torsional strain of supercoiling in negatively supercoiled DNA, and purified negatively supercoiled DNA containing palindromic sequences tends to adopt a cruciform structure in vitro [JBC (1982) 257 6292–6295]; nevertheless, cruciforms appear not to be formed in vivo [Book ref. 69, pp. 19–28]. Palindromic sequences occur e.g. in many operators, DNA replication origins, transcription terminators, etc, and are apparently important e.g. in protein–DNA recognition. (See also e.g. DEPENDOVIRUS.) [Properties of palindromic sequences: JB (1985) 161 1103– 1111.] palindromic unit Syn. REP SEQUENCE. palisade A number of elongated cells (or other structures) arranged side by side in a row, adacent cells being in contact. palisade plectenchyma See PLECTENCHYMA. pallidins See LECTINS. Palmaria A genus of algae of the RHODOPHYTA. P. palmata (formerly Rhodymenia palmata) is used as food (‘dulse’) in various countries. Palmella A genus of freshwater green algae (division CHLOROPHYTA). (See also PALMELLOID PHASE.) palmella phase Syn. PALMELLOID PHASE. palmelloid phase (palmella phase) A vegetative phase formed under certain environmental conditions (e.g. on damp soil, agar media etc) by some unicellular green algae which are usually flagellated and motile (e.g. CHLAMYDOMONAS); the flagellated cells lose or withdraw their flagella and form a type of colony in which an indefinite number of cells is embedded in a gelatinous or mucilaginous matrix. The cells continue to divide within the matrix, and under suitable conditions (e.g. when the colony is flooded with water) they can develop flagella and escape from the colony. A ‘palmelloid’ type of colony is the usual vegetative form in some Chlamydomonas-like algae (e.g. Palmella, TETRASPORA); in these algae motile cells are generally released from the colonies only as gametes or zoospores. Paludrine See CHLORGUANIDE. Palyam subgroup See ORBIVIRUS. palynology The study of (fossil and living) pollen and spores. pAMb1 See CONJUGATION (1b) (ii). Panacide Dichlorophane (see BISPHENOLS). Panaeolus See AGARICALES (Strophariaceae). Panama disease See FUSARIUM WILT. pandemic An EPIDEMIC affecting (simultaneously) large numbers of people in a major geographical area, or worldwide. 548 papilloma PAP technique (peroxidase–antiperoxidase technique) An IMMUNOPEROXIDASE METHOD which involves three antibody-mediated stages. Initially, separate antisera are raised (e.g. in rats) against (i) the specific antigen and (ii) a peroxidase; if these antisera are rat-derived, anti-rat Ig antiserum is raised e.g. in a rabbit. To locate specific antigen in a section, the section is first treated with rat antiserum homologous to the antigen (‘primary’ antiserum) and then washed free of uncombined antibody. The section is then treated with excess rabbit anti-rat Ig antiserum; under such antiserum-excess conditions, rabbit anti-rat Ig antibodies tend to bind to rat antibodies with only one of their combining sites. The section is washed and then treated with rat anti-peroxidase antiserum – these antibodies being bound by the free combining sites of the rabbit anti-rat Ig antibodies. (The rabbit anti-rat Ig antibodies are called ‘bridging’ or ‘linking’ antibodies.) Peroxidase is then added and combines with rat anti-peroxidase. Peroxidase (and hence antigen) is detected by incubating the section with reagents as in the ABC IMMUNOPEROXIDASE METHOD. papain A heat-stable, basic thiol PROTEASE (EC 3.4.22.2) obtained from the latex and unripe fruit of the pawpaw (Carica papaya). (See also IMMUNOGLOBULINS.) Papaya mosaic virus See POTEXVIRUSES. Papaya ringspot virus See POTYVIRUSES. PapB See P FIMBRIAE. paper partition chromatography See CHROMATOGRAPHY. paper spoilage Pulpwood (timber selected for pulping) may be attacked e.g. by cellulolytic fungi such as Poria and Stereum (see also TIMBER SPOILAGE). Wood pulp (produced from pulpwood by mechanical or chemical methods) is a watery suspension containing e.g. cellulose, hemicelluloses, sugars and proteins; it is susceptible to attack by a range of cellulolytic fungi (e.g. species of Aspergillus, Cladosporium, Fusarium, Phoma and Stereum) and fungi which cause darkening or staining (e.g. species of Aureobasidium, Paecilomyces and Penicillium). Chemically prepared pulps – e.g. those containing acidified Ca(HSO3 )2 (acid sulphite pulps) or NaOH and Na2 SO3 (kraft pulps) – are generally less susceptible to spoilage than are mechanically prepared pulps. Cellulolytic attack on either pulpwood or pulp shortens the cellulose fibres, resulting in inferior (weaker) paper. Slime formation in pulps is due e.g. to species of Alcaligenes, coliforms and other bacteria (Serratia forms red slime), and to fungi such as Fusarium and Geotrichum; slime may cause weaknesses, holes and/or discoloration in the paper. PRESERVATIVES used in pulps include e.g. dichlorophane, pentachlorophenol, and salicylanilides; slime formation has been controlled e.g. by chlorine and mercurials. Finished paper and board products have a low moisture content and are subject to fungal spoilage (particularly under high humidity) by e.g. Chaetomium globosum, Stachybotrys atra and other cellulolytic species; Aspergillus and Penicillium spp are common causes of discoloration. Paper and board preservatives include e.g. copper and zinc compounds, dichlorophane, and dithiocarbamates. papilla A microcolony, within and visibly distinct from, a larger colony. For example, a colony of Lac
mutant cells of Escherichia coli may contain a papilla that consists of a subpopulation of LacC revertants; on MacConkey’s agar, such a papilla would be pink/red in a colourless colony. papilloma (pl. papillomas or papillomata) A small tumour of epithelial tissue (skin, mucous membranes, or glandular ducts), caused by a PAPILLOMAVIRUS; papillomas affecting the skin are commonly known as warts or verrucas. Papillomaviruses are largely host-specific, and different types of papillomavirus may cause morphologically distinct types of papilloma in a given Pandorina A genus of freshwater flagellate green algae (division which form motile, ball-like coenobia consisting of clusters of 4–32 cells with no central space. (cf. VOLVOX.) panelling Refers to the condition of a can (see CANNING) in which, due to atmospheric pressure, part of the wall of the can has collapsed inwards. Panelling occurs e.g. in large highvacuum cans, in cans cooled under excessive pressure, or in cans whose walls are made of thin tinplate. pangola stunt virus See FIJIVIRUS. panidazole See NITROIMIDAZOLES. panleukopenia A significant reduction in the numbers of both lymphocytes and neutrophils – a condition brought about e.g. by the FELINE PANLEUKOPENIA VIRUS. panmictic Of, or pertaining to, PANMIXIS. panmixis (panmixia) Unrestricted interbreeding. Pannaria A genus of LICHENS (order LECANORALES). Photobiont: Nostoc. Thallus: mainly squamulose, bluish-grey, with a dark hypothallus. Apothecia: lecanorine with reddish-brown discs. Species occur among mosses on bark, rocks etc. panose a-Glucosyl-(1 ! 6)-D-maltose. Pansoil See ETRIDIAZOLE. pansporoblast See MYXOSPOREA. pantetheine See PANTOTHENIC ACID. pantoic acid See PANTOTHENIC ACID. Panton–Valentine leucocidin (PVL) A staphylococcal LEUCOCIDIN which specifically lyses human or rabbit PMNs and macrophages; no other type of cell appears to be affected. PVL consists of two synergistically acting components, designated ‘fast’ (F) and ‘slow’ (S) (indicating their migration rates on CMC columns). According to a recent model, the S component binds to GM1 receptors on sensitive cells, causing activation of an endogenous (membrane-bound) phospholipase A2 and stimulating uptake of Ca2C ; F then binds to products of phospholipase action and rapidly induces the formation of KC -specific ion channels in the membrane. The resulting secondary effects lead to lysis. [Book ref. 44, pp. 732–737.] Insensitive cells may bind S but do not bind F – possibly because activation of phospholipase does not occur. The role of PVL in pathogenesis is unclear; in e.g. staphylococcal abscesses it may kill phagocytes and thus facilitate the survival and spread of the pathogen. pantotheine See PANTOTHENIC ACID. pantothenic acid A water-soluble VITAMIN which consists of pantoic acid (2,4-dihydroxy-3,3-dimethylbutyric acid) linked by an amide bond to b-alanine. Pantothenic acid is linked to 2-mercaptoethylamine (D cysteamine) to form pantetheine (D pantotheine); pantetheine 40 -phosphate is a component of COENZYME A (q.v. for formula) and is also the prosthetic group of acyl-carrier proteins (ACPs) involved e.g. in fatty acid biosynthesis. Pantothenic acid is required as a growth factor e.g. by many yeasts (e.g. Candida spp, Saccharomyces spp), the filamentous fungus Polyporus texanus, certain bacteria (e.g. Acetobacter spp, Lactobacillus spp, Morganella spp), and certain protozoa (e.g. Crithidia spp, Leishmania spp, Tetrahymena spp). The requirement for pantothenic acid may be satisfied by its precursor(s) – e.g., b-alanine, pantoic acid. pantropic (virol). Refers to a virus which can infect many types of cell. (See also TROPISM sense 2.) PAP (1) 30 -Phosphoadenosine-50 -phosphate (‘adenosine-30 ,50 -diphosphate’): see ASSIMILATORY SULPHATE REDUCTION. (2) Poly(A)polymerase: see MRNA. pap operon See FIMBRIAE. Pap pili See P FIMBRIAE. CHLOROPHYTA) 549 Papillomavirus host. In man, papillomas are caused by human papilloma viruses (HPVs): e.g. plantar warts, caused by HPV-1, occur on the soles of the feet and are flat, being pressed into the dermis by the weight of the body; genital warts (venereal warts, papillomata acuminata, condylomata acuminata), caused by HPV-6, occur in the anogenital region and are transmitted by sexual contact (cf. BUSCHKE–LÖWENSTEIN TUMOUR and MOLLUSCUM CONTAGIOSUM). Papillomas are usually benign, but those caused by certain viruses can become malignant in the presence of certain ‘cofactors’ – e.g., certain animal papillomas can become malignant on exposure to sunlight. (cf. EPIDERMODYSPLASIA VERRUCIFORMIS.) In cattle, papillomas (caused by bovine papilloma virus 4) in the oesophagus and forestomachs can become malignant if the cattle eat bracken (Pteridium aquilinum). In humans, HPVs associated with genital warts are believed to play a role in the development of cancer of the genital tract (particularly of the cervix) [BMJ (1984) 288 735–737; Lancet (1986) i 573–575]. Papillomavirus A genus of viruses of the PAPOVAVIRIDAE. Type species: cottontail rabbit (Shope) papilloma virus (CRPV) [genomic structure: PNAS (1985) 82 1580–1584]; other members include human papilloma viruses (many types, designated HPV-1, HPV-2 etc), bovine papilloma viruses (BPV-1, BPV-2 etc), and papilloma viruses infecting deer, dogs, goats, horses, rats and sheep. Papilloma viruses can induce benign squamous epithelial tumours of the cutaneous and mucosal epithelium of their hosts (see PAPILLOMA). The virus can infect – but not replicate in – the basal cells of the epidermis or mucosa, establishing a persistent infection in which multiple copies of the viral DNA occur in the nucleus in the form of circular extrachromosomal molecules; however, production of virions can occur only in the non-dividing, differentiated squamous epithelial cells of the stratum spinosum and stratum corneum. (Since these cells cannot be grown in culture, papilloma viruses cannot be propagated in vitro, and molecular biological studies have depended largely on the use of cloned viral DNA.) Although the papillomas induced by papilloma viruses are usually benign, certain human and animal papilloma viruses may be involved in the development of malignant tumours; in most of these systems infection with the papilloma virus is by itself insufficient for malignant transformation, and additional physical, chemical and/or biological factors are necessary for carcinogenesis (see PAPILLOMA and EPIDERMODYSPLASIA VERRUCIFORMIS). In experimental systems, several papilloma viruses can induce fibroblastic tumours in hamsters and can transform rodent cells in vitro (although such cells do not produce virions). The mechanism of transformation is unknown. In BPV-1, the presence of only a portion (69%) of the viral genome is necessary for transformation, and the expression of viral genes is apparently necessary; BPV-1 appears to carry multiple transforming genes [PNAS (1985) 82 1030–1034]. There is evidence that viral DNA may integrate into the host chromosome in at least some cases of malignancy – e.g. integrated HPV-16 DNA has been detected in malignant tumours of the human genital tract [JGV (1985) 66 1515–1522; see also Gynecologic Oncology (1992) 47 263–266]. Papovaviridae (papovavirus group) A family of non-enveloped, small, icosahedral DNA viruses, most of which infect mammals (cf. BUDGERIGAR FLEDGLING DISEASE VIRUS); most or all members can induce tumours in certain host animals. Viral DNA replication and assembly occur in the host cell nucleus. The family contains two genera: PAPILLOMAVIRUS and POLYOMAVIRUS, distinguished on the basis of size, genome characteristics, a genusspecific antigen (detected in disrupted virions), and biological properties. The papovavirus virion consists of an icosahedral capsid, diam. ca 45 nm (Polyomavirus) or 55 nm (Papillomavirus), which comprises 72 capsomers and which contains a supercoiled, covalently closed circular dsDNA genome; the DNA – MWt ca. 3 ð 106 (ca. 5 kb) in Polyomavirus, ca. 5 ð 106 (ca. 8 kb) in Papillomavirus – is associated with hostderived histones. (In the host cell, the viral DNA occurs in the nucleus – usually as a MINICHROMOSOME.) Papovavirus virions do not contain lipid or carbohydrates and are resistant to lipid solvents, low pH, heat treatment, etc. papovaviruses Viruses of the PAPOVAVIRIDAE. PAPS 30 -Phosphoadenosine-50 -phosphosulphate (also called adenosine-30 phosphate-50 -sulphatophosphate or 30 -phosphoadenylylsulphate). See ASSIMILATORY SULPHATE REDUCTION. (cf. APS.) papulacandin B An antibiotic which is effective against yeasts but apparently not against other fungi, bacteria or protozoa; it inhibits the formation of CELL WALL b-D-glucan in whole cells and sphaeroplasts of e.g. Saccharomyces cerevisiae and Candida albicans. (cf. ACULEACIN A.) papule On skin: a small, discrete, raised, solid lesion. (cf. MACULE, VESICLE.) par locus See PARTITION. para-aminobenzoic acid See p-AMINOBENZOIC ACID. parabactin See SIDEROPHORES. Parabasalidea A superorder of protozoa (class ZOOMASTIGOPHOREA) which consists of the two orders HYPERMASTIGIDA and TRICHOMONADIDA. parabens PARABENZOATES. parabenzoates (parabens) Antifungal and antibacterial esters of p-hydroxybenzoic acid used e.g. as PRESERVATIVES in various foods, cosmetics and pharmaceuticals; they are more effective against fungi and Gram-positive bacteria than Gram-negative bacteria. Antimicrobial activity and insolubility in water both increase with increases in the chain length of the substituent group. Parabens are active at neutral as well as at acidic values of pH. (cf. BENZOIC ACID.) paracentric (of a chromosome) To one side of the CENTROMERE; e.g., a paracentric inversion is a CHROMOSOME ABERRATION in which the inverted segment does not contain the centromere. (cf. PERICENTRIC.) paracoagulation Clumping of cells (rather than clotting of plasma) which occurs e.g. when plasma (containing an anticoagulant) is mixed with a suspension of cells whose surface components include either staphylococcal clumping factor (see COAGULASE) or protamine sulphate. Paracoccidioides A genus of fungi of the class HYPHOMYCETES. P. brasiliensis, the causal agent of PARACOCCIDIOIDOMYCOSIS, is a dimorphic fungus which occurs within tissues in a yeast-like form; the spherical, yeast-like cells (ca. 5–60 µm) can form several large buds (each bud attached to the parent cell by a narrow neck) or many small buds (forming the so-called ship’s wheel form). When cultured at 37° C on blood-agar, P. brasiliensis grows in the yeast-like form; when cultured at room temperature or at 25° C on Sabouraud’s dextrose agar (see SABOURAUD’S MEDIUM) it gives rise to a colourless, septate mycelium and to ‘chlamydospores’ and cells which resemble conidia. (See also CELL WALL (c) and PSEUDONIGERAN.) paracoccidioidomycosis (South American blastomycosis) A chronic granulomatous human disease caused by Paracoccidioides brasiliensis; it occurs mainly in Central and South America. Infection may result in a benign or progressive lung disease, in a mucocutaneous disease (with formation of granulomatous ulcers chiefly on the mucous membranes of the mouth, nose and 550 Paramyxovirus eyes, accompanied by regional lymphadenopathy), or in a systemic disease involving various internal organs. Lab. diagnosis: microscopy of infected tissues; serological tests (e.g. an EIA [SAB (1984) 22 73–78]). (cf. BLASTOMYCOSIS.) Paracoccus A genus (incertae sedis) of aerobic, oxidasepositive, catalase-positive, Gram-negative bacteria which occur e.g. in soil and in meat-curing brines. Cells: non-motile cocci or rods which typically contain granules of PHB. Metabolism: respiratory (oxidative), the terminal electron acceptor being O2 or (in anaerobic respiration) nitrate, nitrite or nitrous oxide. One species (P. halodenitrificans) is chemoorganotrophic and halophilic, the other (P. denitrificans) is facultatively chemolithotrophic and methylotrophic (see METHYLOTROPHY) and is non-halophilic. Carbon sources include a wide range of organic compounds. Optimum growth temperature: 25–30° C. GC%: ca. 64–67. Type species: P. denitrificans. P. denitrificans (formerly Micrococcus denitrificans). Cells (on e.g. nutrient agar): cocci, ca. 0.5–0.9 µm diam., or rods, ca. 0.9–1.2 µm in length. (See also PORINS.) P. halodenitrificans. Cells (on e.g. nutrient agar containing 1.0 M (5.8%) NaCl): cocci, ca. 0.5–0.9 µm diam.; cells grown in lower concentrations of electrolyte (e.g. 0.6 M NaCl) are usually distorted, and no growth occurs below ca. 0.5 M NaCl. Isolated from meat-curing brines. [Book ref. 22, pp. 399–402.] paracolon An outdated term for those enterobacteria which do not metabolize lactose within 24/48 hours at 37° C. paracytosis Invasive behaviour by certain pathogenic bacteria in which the pathogen crosses a layer of (eukaryotic) cells by passing between the cells; thus, e.g. Haemophilus influenzae can pass between the cells of lung epithelium [paracytosis by H. influenzae in tissue cultures of lung epithelial cells: Inf. Immun. (1995) 63 4729–4737]. (See also TRANSCYTOSIS.) paraffin metabolism See HYDROCARBONS. paraflagellar rod Syn. PARAXIAL ROD. paraformaldehyde (HO(CH2 O)n H) A solid, white, polymeric form of FORMALDEHYDE which, when heated, yields gaseous formaldehyde. parainfluenza viruses See PARAMYXOVIRUS. parakinetal stomatogenesis See STOMATOGENESIS. paralytic shellfish poisoning See SHELLFISH POISONING. paramastigote A form assumed by the cells of species of Herpetomonas and Leishmania during certain stages in their life cycles: see TRYPANOSOMATIDAE. Paramecium (‘slipper animalcule’) A genus of (typically) freshwater protozoa (order HYMENOSTOMATIDA). Cells: ovoid or elongate with uniform somatic ciliature; length varies with species: P. aurelia is ca. 120–200 µm, P. bursaria ca. 90–150 µm, P. caudatum ca. 180–300 µm, P. multimicronucleatum ca. 200–300 µm, and P. trichum ca. 50–120 µm. The buccal cavity is ventral and contains a single PARORAL MEMBRANE (D endoral membrane), two peniculi (see PENICULUS) and a QUADRULUS. STOMATOGENESIS is buccokinetal. Many species have one micronucleus and one macronucleus; P. aurelia has an additional micronucleus, and P. multimicronucleatum has three or four micronuclei. The presence of two CONTRACTILE VACUOLES is typical, and most species have large numbers of TRICHOCYSTS. Cysts are not formed. Paramecia are said to be negatively galvanotactic. Many strains of Paramecium contain endosymbionts: see CAEDIBACTER, HOLOSPORA, LYTICUM, PSEUDOCAEDIBACTER, TECTIBACTER; see also KILLER PARAMECIA, MU PARTICLE, ZOOCHLORELLAE. [Endonucleobiosis in ciliates: Int. Rev. Cytol. (1986) 102 169–213.] Asexual reproduction occurs by binary fission. AUTOGAMY and occur in at least some species; mating types are divided into SYNGENS. Paramecia consume bacteria and e.g. small flagellates. Certain species of Paramecium are a major food source for DIDINIUM. paramere See MACRONUCLEUS. Paramoeba A genus of marine amoebae (order AMOEBIDA). Species are generally free-living. However, P. perniciosa is apparently an obligate parasite of the blue crab (Callinectes sapidus) in which it can cause high mortality rates during moulting; another Paramoeba sp has been implicated as the cause of mass mortalities among sea urchins in Nova Scotia [J. Parasitol. (1985) 71 559–565]. paramylon (syn. paramylum) A storage polysaccharide, a (1 ! 3)-b-D-glucan, characteristic of the EUGLENOID FLAGELLATES. Paramylon occurs in refractile granules of various forms – discs, rings, rods etc – depending on species. Paramylon granules give no colour reaction with iodine. In Euglena gracilis paramylon is synthesized on a protein primer [Bioch. J. (1978) 174 283–290]. paramylum Syn. PARAMYLON. Paramyxa See PARAMYXEA. Paramyxea A class of protozoa (phylum ASCETOSPORA) in which the spores are bicellular, consisting of a parietal cell and a sporoplasm. The spore wall has no orifice. One order: Paramyxida; genera include Paramyxa. Paramyxoviridae A family of enveloped VIRUSes which have a non-segmented, negative-sense ssRNA genome. The family includes some important pathogens of humans, animals and birds – e.g. the causal agents of CANINE DISTEMPER, MEASLES, MUMPS, NEWCASTLE DISEASE, RINDERPEST, and various respiratory diseases. The viruses are transmitted mainly via the air (aerosols etc); vectors are unknown. Virion: pleomorphic (usually roughly spherical), ca. 150–200 nm or more in diameter, containing 5–7 polypeptide species, one molecule of negative-sense linear ssRNA (MWt ca. 5–7 ð 106 ) (some particles may contain positive-sense RNA strands), and 20–25% by weight of lipid. The envelope, which encloses a helical nucleocapsid, is derived from the host cell plasma membrane and contains virus glycoproteins (which form surface projections ca. 8 nm long) and a non-glycosylated virus protein (M). The virions contain various enzymes, including e.g. transcriptase, polyadenylate transferase, mRNA methyltransferase. The virions are inactivated by lipid solvents, non-ionic detergents, formaldehyde, oxidizing agents, etc. Members of the Paramyxoviridae can be propagated in various types of cell culture. Infection occurs by fusion of the virus envelope with the plasma membrane of the host cell. Transcription and replication occur in the cytoplasm, the nucleocapsid functioning as template. Virions mature by budding through the host cell plasma membrane at sites containing the virus envelope proteins. Infected host cells commonly lyse, but temperate and persistent infections also occur, commonly resulting in cell fusion and syncytium formation, inclusions, haemadsorption, etc. The family is divided into three genera – MORBILLIVIRUS, PARAMYXOVIRUS and PNEUMOVIRUS – on the basis of e.g. neuraminidase activity, virion structure, etc. Paramyxovirus A genus of viruses of the PARAMYXOVIRIDAE in which the virions have both haemagglutinin and NEURAMINIDASE activities. The genus includes e.g. NEWCASTLE DISEASE virus (NDV) (type species), MUMPS virus, and parainfluenza viruses type 1 human, type 1 murine (D Sendai virus), type 2 (human), type 3 (human, bovine and ovine), type 4 (human), and type 5 CONJUGATION 551 paramyxoviruses Parapoxvirus virions are ovoid, ca. 250 ð 160 nm (for orf virus) to 300 ð 190 nm (for pseudocowpox virus); the outer surface of the virion exhibits a characteristic criss-cross pattern in negatively stained preparations. DNA MWt: ca. 85 ð 106 . Haemagglutinin is not formed. Infected cells contain only B-type inclusion bodies. Infectivity of virions can be destroyed e.g. by heating at 58–60° C for 30 min. Parapoxviruses do not produce pocks in CAM. They can bring about NON-GENETIC REACTIVATION of inactivated orthopoxviruses. paraprotein Immunoglobulin produced by a neoplastic clone of B cells. (See also MYELOMA.) parapyle See RADIOLARIA. Paraquat Syn. METHYL VIOLOGEN. pararosanilin A reddish TRIPHENYLMETHANE DYE. pararotaviruses (group B rotaviruses) Viruses which are morphologically indistinguishable from ROTAVIRUSES but which lack the group-specific antigen common to all other known (avian and mammalian) rotaviruses; the 11 dsRNA segments also have a unique and characteristic electrophoretic migration pattern. Pararotaviruses have been isolated from human stool specimens, but their clinical significance – if any – is unknown. parasexual processes (mycol.) Non-sexual processes which can result in the formation of recombinant nuclei. The process first reported in Aspergillus nidulans (and later found in many ascomycetes and in some deuteromycetes and basidiomycetes) has been called the standard parasexual process. This process starts with the formation of a heterokaryotic cell or mycelium (see HETEROKARYOSIS) and involves the fusion of two genetically dissimilar haploid nuclei (in the heterokaryon) to form a single diploid nucleus. Recombination can then occur through mitotic crossing over – i.e., the (infrequent) reciprocal exchange of chromatid segments between homologous chromosomes during mitotic propagation of the diploid nucleus – or (e.g. in Saccharomyces cerevisiae) predominantly by gene conversion [MR (1985) 49 33–58 (43–47)]. Haploidization (the formation of haploid progeny nuclei) involves NON-DISJUNCTION in which there is a stepwise loss of one member of each pair of homologous chromosomes during successive mitoses. For the imperfect fungi parasexuality brings the advantage of genetic recombination. The importance of parasexual processes in nature is uncertain. [Review: Book ref. 168, pp. 191–214.] ParaSight-F test See MALARIA. parasitaemia (parasitemia) The presence of parasites in the bloodstream. parasite See PARASITISM. A parasite may be obligatorily or facultatively parasitic; it may be host- or strain-specific or able to parasitize a range of hosts. Parasites which remain external to the host’s cells or tissues are called ectoparasites, while those that live intracellularly or within tissues are called endoparasites. If the host suffers significant harm, or is killed, through the activity of a microbial parasite, the parasite is referred to as a PATHOGEN. Compared to its free-living (non-parasitic) relatives, a parasite often exhibits certain aspects of degeneracy – e.g. a reduction in, or loss of, structures, organelles, or metabolic mechanisms involving the ingestion and digestion of food, respiration, osmoregulation or excretion. Adaptation to the parasitic mode of life may be regarded as a form of specialization; some parasites are so well integrated into the economy of their hosts that they can be cultivated in vitro only with difficulty, or not at all. parasitism SYMBIOSIS (sense 1) in which one organism (the PARASITE) benefits at the expense of the other (the host); typically, the parasite obtains its nutrients from the host, and the association is detrimental to the host. (cf. COMMENSALISM.) (avian, canine and simian). (See also CALF PNEUMONIA, COMMON (g).) The paramyxovirus virion consists of a helical nucleocapsid, composed of the genomic ssRNA and proteins NP, P and L, surrounded by an envelope containing a non-glycosylated protein (M) in the inner layer, and two glycoproteins which extend across the width of the envelope and beyond the outer surface to form spikes. Proteins P and/or L may have transcriptase activity. Protein M plays an important role in virus assembly. The larger of the envelope glycoproteins (designated HN) has cellbinding, haemagglutinating and neuraminidase activities, while the smaller (F) has haemolytic activity and promotes fusion between the virus envelope and the host plasma membrane. The F protein can also promote cell–cell fusion, a property exploited e.g. in the use of UV-inactivated Sendai virus in the production of hybrid cells (see e.g. HYBRIDOMA). In the Sendai and mumps viruses the F protein is synthesized as an inactive precursor, F0 , which is activated by proteolytic cleavage. [Sequence of the F protein gene in Sendai virus: JGV (1985) 66 317–331.] Host cell infection occurs by adsorption, via HN, to the cell surface, followed by F protein-mediated fusion between the virus envelope and the host plasma membrane; antibodies to HN and F proteins appear to be important in limiting or preventing infection in vivo. Within the host cell, virus RNA is transcribed into six (mostly monocistronic) mRNAs. In addition to the six structural proteins, two non-structural proteins (C and C0 ) are synthesized from a reading frame overlapping that of the P gene. HN and F proteins are synthesized on membranebound polysomes, glycosylated, and inserted into the host plasma membrane; during maturation, the virions bud through the region of the membrane containing these proteins. paramyxoviruses (1) Viruses of the PARAMYXOVIRIDAE. (2) Viruses of the genus PARAMYXOVIRUS. Parana virus See ARENAVIRIDAE. Paranaplasma A genus of bacteria of the family Anaplasmataceae [Book ref. 21, pp. 908–909]; P. caudatum is now Anaplasma caudatum [Book ref. 22, p. 722]. parapertussis See WHOOPING COUGH. paraphysis (1) A type of sterile, basally-attached, branched or unbranched filament (of unknown function), many of which occur among the hymenial asci in the ascocarps of certain ascomycetes; paraphyses may or may not project above the level of the asci. (cf. EPITHECIUM and HAMATHECIUM.) Apical paraphyses are sterile filaments which originate in the upper wall of a pseudothecial locule and grow downwards into the centrum, their tips remaining unattached. (cf. PSEUDOPARAPHYSIS.) (2) Syn. BASIDIOLE (sense 3). paraphysoid (1) In some (particularly lichenized) ascomycetes: a structure which resembles a PSEUDOPARAPHYSIS but which generally has few septa, and which may be formed before or after the asci have developed. (2) In basidiomycetes: a sterile hymenial structure, e.g. a BASIDIOLE (sense 3). Paraphysomonas A (probably heterogeneous) genus of freshwater and marine, non-pigmented CHRYSOPHYTES (q.v.) which bear various types (according to species) of silicified scales. paraplectenchyma See PLECTENCHYMA. Parapoxvirus (orf subgroup) A genus of viruses of the CHORDOPOXVIRINAE which infect ungulates and may infect man. Type species: orf virus (see CONTAGIOUS PUSTULAR DERMATITIS sense 1); other members: bovine pustular stomatitis virus, chamois contagious ecthyma virus, PSEUDOCOWPOX virus (D MILKERS’ NODULE virus, paravaccinia virus). COLD, PNEUMONIA 552 parthenogenesis parasitology The study of certain PARASITES: their life cycles, ecology, transmission, etc; those parasites studied in parasitology include some which are classified as microorganisms (e.g. amoebae, trypanosomes, malarial parasites) as well as various arthropods, helminths, etc. Pathogenic bacteria, fungi and viruses are traditionally not included within the scope of parasitology – even though they can live parasitically. parasol ants See FUNGUS GARDENS. parasol mushroom See LEPIOTA. parasomal sac In many ciliates (e.g. Paramecium): a small flasklike invagination of the surface membrane near the ciliary bases, possibly involved in pinocytosis. Parasponia See BRADYRHIZOBIUM. parasporal crystal See DELTA-ENDOTOXIN and MILKY DISEASE. paratenic Refers to an alternative host (of a parasite) which is incidental to the normal life cycle of the parasite: i.e. the paratenic host is neither essential for nor interferes with the completion of the parasite’s life cycle. parateny (ciliate protozool.) The condition in which part(s) of a cell have lines of kinetids arranged perpendicularly to the longitudinal axis of the cell; parateny is characteristic of some scuticociliates (e.g. Loxocephalus). paratope (1) The COMBINING SITE of an antibody. (2) The site at which a cell-surface receptor combines with an exogenous molecule. paratose (3,6-dideoxy-D-glucose) A sugar, first isolated from Salmonella paratyphi, which occurs in the O-specific chains of the LIPOPOLYSACCHARIDE in certain Salmonella serotypes and which contributes to the specificity of O antigen 2 in group A salmonellae (see KAUFFMANN–WHITE CLASSIFICATION). paratrachoma See INCLUSION CONJUNCTIVITIS. paratuberculosis Syn. JOHNE’S DISEASE. paratyphoid fever (paratyphoid) (1) (med.) An acute infectious disease caused by Salmonella paratyphi A, B or C. It resembles TYPHOID FEVER in modes of transmission and infection, but symptoms are milder and are often accompanied by gastroenteritis. (2) (vet.) Any disease (paratyphoid, salmonellosis) caused by a serotype of Salmonella and characterized by septicaemia (particularly in young animals) and/or enteritis. In cattle, paratyphoid is often caused by S. dublin, and sometimes e.g. by S. typhimurium; in the acute enteric form symptoms include fever and diarrhoea, while chronic enteritis may cause general unthriftiness. In pigs, the causal agents include S. choleraesuis and S. typhimurium; the acute enteric form, involving fever and diarrhoea, is often complicated by pulmonary involvement, while in the (usually fatal) septicaemic form there may be nervous involvement, red-purple skin discoloration (e.g. on the ears and abdomen), and subcutaneous petechial haemorrhages. In horses, S. typhimurium can cause e.g. an acute febrile condition with diarrhoea and dehydration, while in very young foals it can cause a fatal septicaemia. An asymptomatic carrier state can occur in cattle, pigs and (to a limited extent) horses. Parauronema See SCUTICOCILIATIDA. paravaccinia virus See PARAPOXVIRUS. paraxial rod (paraflagellar rod) In the FLAGELLUM of many members of the KINETOPLASTIDA, DINOFLAGELLATES and EUGLENOID FLAGELLATES: a filament or rod which runs, within the flagellar membrane, parallel to the axoneme or (in the helical transverse flagellum of a dinoflagellate) parallel to the axis of the axoneme. [Paraxial rod in trypanosomatids: JP (1986) 33 552–557.] paracentric objectives In a given microscope: a set of objective lenses each of which gives a field of view whose centre is identical to that of the others. (cf. PARFOCAL OBJECTIVES.) parC gene See CELL CYCLE (b). parE gene See QUINOLONE ANTIBIOTICS. parenteral Administered by any route except the alimentary canal. parenthesome See DOLIPORE SEPTUM. parfocal objectives In MICROSCOPY: a set of objective lenses which, in a given microscope, can be interchanged (by rotating the nosepiece) without the need to re-adjust the focusing controls. (cf. PARCENTRIC OBJECTIVES.) parietal Lying near to, or attached to, the inner face of the wall of a structure; may refer e.g. to algal chloroplasts which lie close to the cell wall. parietin An orange anthraquinone pigment found in lichens of the Teloschistaceae, in certain free-living fungi (e.g. Aspergillus and Penicillium spp), and in a few higher plants; it gives a purple colour with KOH. Parietin is present at higher concentrations in Xanthoria parietina thalli growing in bright light than in those growing in shade, and probably serves to protect the photobiont from high-intensity light. pariobasidium In certain basidiomycetes: the distal, functional part of a mature METABASIDIUM whose proximal region consists of remnants of the probasidium [TBMS (1973) 61 497–512]. Park nucleotide UDP-N-acetylmuramyl pentapeptide: an intermediate in the biosynthesis of PEPTIDOGLYCAN. Parmelia A large genus of LICHENS (order LECANORALES); photobiont: a green alga. The thallus is foliose, usually with rhizines on the underside; according to species, the upper surface may be yellowish, grey, greyish-green, brown etc, and soredia, isidia, or pseudocyphellae may be formed. Apothecia are lecanorine. Species occur on rocks, bark, wood etc. Parmesan cheese See CHEESE-MAKING. PARNA See CUCUMOVIRUSES. paromomycin Either of two related AMINOGLYCOSIDE ANTIBIOTICS, produced by Streptomyces rimosus, which may be regarded as derivatives of neomycins B and C. paronychia Inflammation of the folds of tissue surrounding a fingernail or toenail. It may be caused e.g. by Candida spp (see CANDIDIASIS), by certain bacteria, or by HERPES SIMPLEX virus (‘herpetic paronychia’). (cf. ONYCHOMYCOSIS; WHITLOW.) paroral membrane In many ciliates: a membrane, composed of multiple cilia, which occurs on the right-hand side or border of the buccal cavity; such membranes are often undulatory and are sometimes called ‘undulating membranes’. In ciliates of the OLIGOHYMENOPHOREA (e.g. hymenostomes) the paroral membrane is characteristically of the stichodyad type: a double row of kinetosomes arranged in a zigzag pattern, only the outermost kinetosomes commonly bearing cilia (see also HAPLOKINETY); in Paramecium the paroral membrane is also called the endoral membrane. In many or most ciliates of the POLYHYMENOPHOREA the paroral membrane is of the stichomonad (D monostichomonad ) type in which the infraciliature consists of a single row of kinetosomes; other polyhymenophoreans have diplostichomonad membranes (the kinetosomes forming two parallel rows) or – in some hypotrichs – polystichomonad membranes (the kinetosomes forming more than two parallel rows). In some ciliates of the SCUTICOCILIATIDA the paroral membrane is clearly divided into two or three parts. (cf. AZM.) parotitis Inflammation of the parotid gland. (cf. MUMPS.) parrot fever Syn. PSITTACOSIS. parsnip mosaic virus See POTYVIRUSES. parthenogenesis The development of progeny from a female reproductive structure without the involvement of a male gamete. The term is sometimes used to include cases of AMIXIS and APOMIXIS. 553 partial veil Genera: DENSOVIRUS, DEPENDOVIRUS, ERYTHROVIRUS, PARVO[Cell infection: JV (2004) 78 6709–6714.] Parvovirus (parvovirus group) A genus of autonomous, helperindependent viruses (family PARVOVIRIDAE) which infect vertebrates, causing important diseases in domestic and other animals; parvoviruses responsible for human diseases are classified in the genus ERYTHROVIRUS. (So-called ‘parvovirus-like agents’ – associated with human food poisoning – are unrelated (ssRNA) viruses currently referred to as SMALL ROUND STRUCTURED VIRUSES.) The type species is r-1 (D Kilham rat virus or ‘rat virus’). Other members include e.g. Aleutian mink disease virus (see ALEUTIAN DISEASE OF MINK), bovine parvovirus, feline parvovirus (including the subspecies FELINE PANLEUKOPENIA VIRUS, CANINE PARVOVIRUS, mink enteritis virus), goose parvovirus (causal agent of a fatal hepatitis in goslings aged 8–30 days), H-1 virus, LuIII virus, porcine parvovirus (an important cause of infertility, abortion and stillbirth), and minute virus of mice (MVM). Several parvoviruses (e.g. H-1, LuIII) were originally isolated from cell cultures. Most parvoviruses can agglutinate the erythrocytes (red blood cells) of at least one species of animal. [Differences in haemagglutinating activity among strains of canine parvovirus: JGV (1988) 69 349–354.] The autonomous parvoviruses are characterized by their need to replicate in dividing cells, and in at least some cases (see ERYTHROVIRUS) a virus may be highly tropic for cells in a particular phase of the cell cycle. (The frequent association of parvoviruses with tumours may be due to their preference for dividing cells – rather than to an oncogenic potential; in fact, parvoviruses are reported to have anti-neoplastic potential [Book ref. 97, pp 344–345].) In some parvoviruses the virions contain only negative-strand DNA; in others, the virions in a given population may contain either negative-strand or positive-strand DNA. In autonomous parvoviruses, replication of the DNA is believed to follow a ‘rolling hairpin’ model [modified rolling hairpin model: JV (1985) 54 171–177], which is outlined below. Replication of the single-stranded DNA genome is facilitated by the presence of a PALINDROMIC SEQUENCE at both ends of the strand; in B19 parvovirus (see ERYTHROVIRUS) the two terminal palindromes are identical. Replication begins at the 30 end, where the palindromic sequence folds back on itself to form a T-shaped double-stranded structure; strand synthesis from the 30 terminus gives rise to a double-stranded replicative form which extends to the 50 terminus of the genome. (During strand synthesis, the T-shaped structure at the 50 end is apparently ‘opened up’ by strand displacement.) The original 50 end of the genome thus becomes paired with a newly synthesized strand that includes a (complementary) copy of the palindrome. The palindrome in each strand spontaneously forms a double-stranded T-shaped structure – so that the 30 end is now positioned to use the newly synthesized strand as a template for ongoing synthesis. Strand synthesis along this (newly synthesized) template continues with synthesis along the original genome strand, resulting in a double-stranded genomic dimer. Nicking at specific sites leads to regeneration and recycling of intermediates and the formation of progeny genomes (see reference for further details). parvoviruses (1) Viruses of the PARVOVIRIDAE. (2) Viruses of the genus PARVOVIRUS. PAS (p-aminosalicylic acid) A chemotherapeutic agent which is bacteriostatic for some Mycobacterium spp, e.g., M. tuberculosis; it is used, together with e.g. ISONIAZID, in the treatment partial veil (inner veil) (mycol.) In the immature fruiting bodies of certain agarics (e.g. Agaricus spp): a membranous tissue which extends from the margin of the pileus to the stipe; when intact, the partial veil seals off the gill cavity. As the fruiting body grows, the partial veil tears – leaving a ring of tissue (the annulus) around the stipe, and (often) a ragged ring of tissue (the cortina) attached to the periphery of the pileus. (cf. UNIVERSAL VEIL.) partition (of plasmids) Segregation to daughter cells during cell division. In high-COPY-NUMBER plasmids (e.g. COLE1 PLASMID) partition appears to depend on random segregation to daughter cells: there appear to be no special mechanisms for active partition. In low-copy-number plasmids (e.g. the F plasmid, IncFII plasmids) active mechanisms are essential for accurate partition; such mechanisms prevent the segregation of plasmid-free daughter cells at cell division and help to maintain a stable copy number. These plasmids contain specific loci (designated e.g. par, sop or sta) associated with partition. The ParM protein of plasmid R1 polymerizes at a parC -ParR complex, the resulting filaments apparently forcing the plasmids to segregate [EMBO (2002) 21 3119–3127]. Plasmids with similar/identical partition sites would, if present in the same cell, exhibit instability in partition and (hence) copy number, i.e. INCOMPATIBILITY. Some plasmids which form multimers have efficient mechanisms for monomerization; thus, e.g. BACTERIOPHAGE P1 encodes a SITE-SPECIFIC RECOMBINATION system. Plasmid R46 encodes a recombinase at the per locus. Partition in R1 appears to involve a mitosis-like process mediated by actin-like filaments. The par (partitioning) locus includes: parC, a centromere-like region, and the genes parM and parR. Partitioning seems to occur as follows. A pair of plasmids (formed at the cell’s mid-cell replication machinery) become linked by the binding of ParR to the parC region of each plasmid; parC -ParR-parC is the partitioning complex that acts as a nucleation site on which ATP-ParM molecules polymerize to form actin-like filaments. Continued polymerization lengthens the filaments, forcing the plasmids to opposite poles [EMBO (2002) 21 3119–3127]. partition chromatography See CHROMATOGRAPHY. Partitiviridae See MYCOVIRUS. partner gene See FUSION PROTEIN. partridge wood Wood which has undergone a pocket rot caused e.g. by Stereum frustulatum. parvaquone A naphthoquinone drug used e.g. in the treatment of EAST COAST FEVER. parvo-like viruses See SMALL ROUND STRUCTURED VIRUSES. parvobacteria A term sometimes used to refer to some or all of the small Gram-negative bacilli, e.g. species of Brucella and Pasteurella. Parvoviridae A family of non-enveloped, isometric ssDNA viruses which infect invertebrates or vertebrates; virus replication occurs in the host cell nucleus. Virion: ¾15–28 nm diameter, typically with icosahedral symmetry and usually containing three or four types of polypeptide; genome: one molecule of linear ssDNA. In some members of the family each virion contains negative-sense DNA; in other members, some of the virions in a population contain negative-sense DNA while others contain positive-sense DNA. The virions are typically highly resistant to adverse conditions, surviving e.g. 56° C/>60 min, pH 3–9, lipid solvents, and many types of protease; they can generally be inactivated by formaldehyde, hydroxylamine, b-propiolactone, oxidizing agents or ultraviolet radiation. VIRUS. 554 Pasteurella of TUBERCULOSIS. PAS is incorporated in FOLIC ACID (in place of PABA), giving rise to an analogue with impaired activity. PAS reaction (periodic acid–Schiff reaction) A procedure for demonstrating the presence of certain carbohydrates – e.g. CELLULOSE, GLYCOGEN, STARCH. Adjacent hydroxyl groups in the carbohydrate are oxidized to aldehyde groups by periodic acid (HIO4 ); the aldehyde groups react with SCHIFF’S REAGENT to give a purple coloration. (Tetraacetic acid or chromic acid may be used in place of periodic acid.) A positive PAS reaction is also given by sugars containing an unsubstituted amino group adjacent to a hydroxyl group (as in CHITOSAN). The PAS reaction has been used e.g. to detect certain pathogenic fungi (e.g. Blastomyces dermatitidis, Histoplasma capsulatum) in tissue sections. pascal (Pa) The SI unit of pressure; 1 pascal is equal to 1 newton/metre2 . 1 mmHg D 1 torr D 133.3 Pa D 0.133 kPa. (1 atmosphere D 14.7 lb/inch2 D 101.325 kPa.) (cf. BAR.) paspalinines See PASPALUM STAGGERS. paspalum staggers A MYCOTOXICOSIS which affects mainly cattle but also sheep and horses; it is caused by the ingestion of tremorgenic toxins (paspalinines) when grazing Paspalum grasses ergotized by Claviceps paspali. Symptoms: hypersensitivity to noise and movement, followed by muscle tremors and incoordination. Recovery is usually rapid following removal from affected pastures. Paspalum striate mosaic virus See GEMINIVIRUSES. passage See SERIAL PASSAGE. passive agglutination (serol.) Any form of AGGLUTINATION in which the combination of antibodies with antigens is made readily detectable by the prior adsorption of the antigen or antibody to an unrelated particulate entity – e.g. a cell or a colloidal particle. Thus, e.g., a soluble antibody or antigen may be adsorbed to bentonite or latex (see LATEX PARTICLE TEST) or to erythrocytes (see BOYDEN PROCEDURE); antibodies may be adsorbed e.g. to staphylococci (see SENSITIZATION (sense 5) and PROTEIN A). (If erythrocytes are used, the procedure is referred to as passive haemagglutination.) passive agglutination test Any test, involving PASSIVE AGGLUTINATION, used for detecting or quantifying specific antibodies or antigens – e.g., antibodies homologous to soluble antigens, or particulate antigens homologous to particle-bound antibodies; in such tests the combination of particle-bound antigen (or antibody) with homologous antibody (or antigen) causes AGGLUTINATION of the particles. Passive agglutination tests are appreciably more sensitive than PRECIPITIN TESTS. passive haemagglutination See PASSIVE AGGLUTINATION. passive haemolysis See HAEMOLYSIS. passive immunity (adoptive immunity) Temporary immunity brought about by the transfer of pre-formed antibody, or specifically sensitized lymphocytes, from an immune individual to a non-immune individual – the latter thus becoming immune without necessarily having had contact with the corresponding antigen(s). Passive immunity can be acquired e.g. by transfer of antibody via the placenta, in COLOSTRUM, or by passive IMMUNIZATION. passive immunization See IMMUNIZATION. Pasteur effect In organisms capable of both fermentative and respiratory modes of metabolism (e.g. Saccharomyces cerevisiae): the inhibition of glycolysis in the presence of O2 , manifested by the inhibition of ethanol formation and a decrease in the amount of glucose used. This effect reflects the higher energy yield obtained from glucose by RESPIRATION than by FERMENTATION. The Pasteur effect can apparently be explained, at least in part, by the allosteric modulation of phosphofructokinase (see EMB(cf. CUSTERS EFFECT.) The Pasteur effect is not observed e.g. in yeast cells growing in the presence of high concentrations of glucose: cf. CRABTREE EFFECT. Pasteur pipette An open-ended glass tube (internal diam. ca. 5 mm) with one end drawn out to an internal diam. ca. 1 mm; the wide end is fitted with a rubber bulb. It is used e.g. for nonquantitative transfer of liquids, but may be roughly calibrated: e.g. if 50 drops are delivered per ml, each drop is 0.02 ml. For aseptic work Pasteur pipettes are plugged (see PLUGGING) and sterilized before use. Pasteurella A genus of Gram-negative bacteria of the PASTEURELLACEAE. (The following account is based on data from Book ref. 22, pp. 552–558, but cf. final paragraphs of entry for proposals for a revised classification.) Cells: usually coccoid, rod-shaped or pleomorphic, ca. 0.3–1.0 ð 1–2 µm, occurring singly or sometimes in pairs or short chains. Bipolar staining is common. On primary isolation virulent strains usually have a carbohydrate (e.g. hyaluronic acid) capsule which may be lost on subculture. Optimum growth temperature: 37° C. Only P. aerogenes and P. haemolytica can grow on MacConkey’s agar. Most strains are non-haemolytic (cf. P. haemolytica). Glucose is usually fermented anaerogenically – gas being formed only by P. aerogenes and Pasteurella ‘gas’ (a urease Cve strain which may be a biotype of P. pneumotropica). MR
ve; VP
ve; oxidase Cve (some strains of P. multocida are
ve); gelatinase
ve; LDC
ve; ADH
ve; usually lactose
ve. GC%: 40–45. Type species: P. multocida. P. aerogenes. Gas from glucose; indole
ve; urease Cve. Commensal in the intestines of pigs. P. gallinarum. Indole
ve; urease
ve. Occurs e.g. in the upper respiratory tract of poultry and may be associated with chronic respiratory disease in poultry. P. haemolytica. Haemolytic (e.g. on bovine blood agar); indole
ve; urease
ve. Can cause primary or secondary pneumonia in e.g. cattle and sheep, and is a major cause of ‘shipping fever’ (see BOVINE RESPIRATORY DISEASE); it can also cause e.g. mastitis in ewes and septicaemia in lambs. (See also RTX TOXINS.) P. multocida (formerly P. septica). Indole Cve; urease
ve. Pathogenic in a range of animals, causing e.g. HAEMORRHAGIC SEPTICAEMIA (in cattle), FOWL CHOLERA, ‘shipping fever’, and secondary infections (see e.g. SWINE INFLUENZA). Can be an opportunist pathogen in man e.g. following animal bites (see e.g. CAT-BITE FEVER). [Genomic sequence of a common avian strain of P. multocida: PNAS (2001) 98 3460–3465.] P. pestis: see YERSINIA. P. pneumotropica. Indole Cve; urease Cve. Occurs in the nasopharynx in rodents and other animals, and can cause e.g. respiratory tract infection in laboratory rodents. P. pseudotuberculosis: see YERSINIA. P. tularensis: see FRANCISELLA. P. ureae. Indole
ve; urease Cve. Occurs occasionally in the upper respiratory tract in humans. In a proposal for a revised classification [IJSB (1985) 35 309–322] it was suggested that the genus should include the following species: P. anatis sp. nov.; P. avium; P. canis sp. nov. (includes e.g. the former P. multocida biotype 6); P. dagmatis sp. nov. (includes organisms previously called P. ‘gas’ or P. pneumotropica type Henriksen); P. gallinarum; P. langaa sp. nov.; P. multocida (includes three subspecies: gallicida, multocida and septica); P. stomatis sp. nov.; P. volantium sp. activity in response to the ADENYLATE ENERGY CHARGE DEN–MEYERHOF–PARNAS PATHWAY). 555 Pasteurellaceae causes disease in the absence of the microbe itself would not be regarded as a pathogen. Thus, a toxinogenic strain of Gonyaulax may – indirectly – cause paralytic SHELLFISH POISONING in man, but it is not regarded as a human pathogen since it has never been known to cause disease by infecting man. Similarly, while Claviceps purpurea can be regarded as a pathogen of certain cereals, it cannot be called a human pathogen – even though the ergot alkaloids it produces can cause ERGOTISM in man. By analogy, Clostridium botulinum (often described as a pathogen in medical circles) is not acting as a pathogen when its toxin, in the absence of the bacterium, causes BOTULISM in an adult human, although it does act as a pathogen sensu stricto when it causes infant botulism, wound botulism, or certain (apparently atypical) C. botulinum infections in adults. (See also OPPORTUNIST PATHOGEN.) By custom, disease-causing higher organisms, such as trematode worms, are referred to as parasites rather than pathogens. (See also PARASITOLOGY.) pathogenesis The process or mechanism of disease development. pathogenesis-related proteins (plant pathol.) A group of lowMWt, low-IEP proteins, resistant to proteases, formed de novo in plant tissues in response to infection with a pathogen; of unknown function, they are thought to have a role in resistance to disease. They were first found in tobacco cultivars exhibiting HYPERSENSITIVITY to tobacco mosaic virus infection, but occur in other plants infected with viruses, viroids, fungi or bacteria. (cf. PHYTOALEXINS.) pathogenic Able to behave as a PATHOGEN. pathogenicity island (PAI) In a bacterial pathogen: a cluster or ‘cassette’ of genes encoding a set of virulence factors; such sequences usually occur in the chromosome but their GC% is typically unlike that of the chromosome – suggesting acquisition by horizontal transfer. Many PAIs (e.g. cag in Helicobacter pylori ) are flanked by direct repeats. (cf. AVIRULENCE GENE.) A PAI is often located at a site adjacent to a tRNA gene. The LEE (locus of enterocyte effacement) PAI is found in all strains of EPEC and in many strains of EHEC. LEE is a 35.5 kb chromosomal sequence which is not flanked by direct repeats; in EPEC it is located 16 bp downstream of the selC gene (which encodes the tRNA for selenocysteine) [PNAS (1995) 92 1664–1668]. The LEE PAI encodes components of a type III secretory system (see PROTEIN SECRETION) as well as various effector molecules. [Type III secretory systems and pathogenicity islands (review): JMM (2001) 50 116–126.] Specific genes (and products) include: eae – an adhesin (intimin). espA – a cell–cell linking structure (?) espB – an activator of epithelial cell enzymes (?) escN – an ATPase for the secretory system (?) tir – (espE in EHEC) Tir protein (see EPEC). (The names of genes encoding components of the type III system in EPEC differ from those in the earlier literature; these genes are now designated esc – e.g. escN was formerly sepB.) At least some genes in LEE (including eae, and those of secreted proteins, such as espB ) are under transcriptional control from a regulatory region (per ) in a plasmid (EPEC adherence factor plasmid; EAF plasmid) found in all strains of EPEC. This plasmid also encodes the so-called bundle-forming pili (BFP) which may be involved in the formation of microcolonies on the intestinal epithelium (see EPEC). [Model of EPEC pathogenesis: TIM (1998) 6 169–172.] Other PAIs include cag in Helicobacter pylori [PNAS (1996) 93 14648–14653; Nature (1997) 388 539–547], and the island nov. (V-factor-requiring avian and human strains). Organisms referred to as P. haemolytica, P. pneumotropica biotypes Jawetz and Heyl, P. testudinis and P. ureae were excluded from the genus, and were said to be more closely related to the Actinobacillus group; P. aerogenes and P. multocida biotype 1 were also excluded. It was later proposed that P. ureae be transferred to the genus Actinobacillus (as A. ureae) [IJSB (1986) 36 343–344]. Pasteurellaceae A family of Gram-negative, asporogenous, facultatively anaerobic bacteria. Cells: coccoid to rodshaped, sometimes pleomorphic or filamentous; non-motile. Chemoorganotrophic (respiratory and fermentative); demethylmenaquinones (see QUINONES) are involved in aerobic and anaerobic electron transport. Nutritionally fastidious, requiring organic nitrogen and various growth factors. Chocolate or blood agars can be used for primary isolation of most strains. NO3
is reduced to NO2
. Acid (usually without gas) is usually formed from glucose but may be difficult to detect by conventional methods. Typically oxidase Cve; catalase Cve; alkaline phosphatase Cve. Species are parasitic in man, animals and birds; some are pathogenic. Type genus: PASTEURELLA; other genera: ACTINOBACILLUS and HAEMOPHILUS. [Book ref. 22, pp. 550–575.] pasteurellosis (vet.) Any of a range of (primarily) septicaemic or pneumonic diseases caused by species of Pasteurella. (See e.g. shipping fever in BOVINE RESPIRATORY DISEASE; HAEMORRHAGIC SEPTICAEMIA (2).) Pasteuria A genus of BUDDING BACTERIA which typically occur attached to particulate matter in aquatic environments; in these organisms the (proteinaceous) cell wall lacks peptidoglycan. P. ramosa is a parasite of water-fleas (Daphnia spp). [Book ref. 45, pp. 490–492.] A strain formerly designated P. ramosa was reclassified in a new genus, Pirella [IJSB (1984) 34 492– 495]. pasteurization A form of heat treatment which kills certain pathogens and/or spoilage organisms in e.g. milk and certain foods; temperatures below 100° C are used. (cf. APPERTIZATION.) For MILK, an early form of pasteurization involved holding the milk at a minimum of 63° C for at least 30 min (low temperature holding, LTH; low temperature long time, LTLT). In a more recent method milk is held at a minimum of 72° C for at least 15 sec (flash pasteurization; high temperature short time, HTST). Pasteurization is lethal for the causal agents of a number of milktransmissible diseases (e.g. salmonellosis, tuberculosis) as well as for a proportion of the milk microflora. It also inactivates certain bacterial enzymes (e.g. lipases) which would otherwise cause milk spoilage. (Both the LTLT and the HTST methods may be inadequate for the inactivation of Coxiella burnetii (see Q FEVER).) The term ‘pasteurization’ has also been used for the mild heat disinfection (80° C/5 min) of medical instruments. pasture bloat See BLOAT. patch stop See MICROSCOPY (b). patch test Any form of SKIN TEST in which the antigen is applied to the surface of the skin – see e.g. VOLLMER PATCH TEST. patching See CAPPING (sense 3). patent period (med., vet.) The period during which parasitic organisms can be demonstrated in blood, tissues, faeces etc of the host. (cf. PREPATENT PERIOD.) pathogen Any MICROORGANISM which, by direct interaction with (infection of) another organism (by convention, a multicellular organism), causes DISEASE in that organism. A pathogen is not, therefore, any microorganism which is causally connected with disease: for example, a microbe which produces a TOXIN that 556 PCR PCBs Polychlorinated biphenyls (see BIOREMEDIATION). PCC Pasteur Culture Collection. pCF-10 See SURFACE EXCLUSION. PCNB See QUINTOZENE. pcnB gene See MRNA (a) and PLASMID (replication). PCP See BIOREMEDIATION. PCR (polymerase chain reaction) A method for copying (‘amplifying’) specific sequences of nucleotides (see AMPLICON senses 1 and 2) in DNA; a modified form of the process (REVERSE TRANSCRIPTASE PCR) is used for copying sequences in RNA. PCR depends on the ability of a (thermostable) DNA polymerase to extend a primer on a template containing the amplicon (see figure). (Patents covering PCR are owned by HoffmannLa Roche.) Other methods for amplifying nucleic acids include LCR, NASBA and SDA; unlike PCR, the latter two methods are conducted isothermally. Repeated rounds of replication (i.e. ongoing cycling) in PCR can yield millions of amplicons within hours; usually, the sequence being copied is less than 2000 base-pairs in length, but longer sequences have been copied. In the basic form of PCR the reaction mixture includes: (i) the sample (dsDNA); (ii) the primers: oligonucleotides (¾20–30 base-pairs in length) of two types – one type complementary to the 30 end of the amplicon in one strand, the other type complementary to the 30 end of the amplicon in the other strand; (iii) deoxyribonucleoside triphosphates of all four types; (iv) a thermostable DNA polymerase – e.g. the Taq DNA polymerase (see DNA POLYMERASE for examples of polymerases used in PCR). Temperature cycling (see figure) can be controlled manually; however, it is usually achieved by carrying out the reaction in an instrument (a thermal cycler or thermocycler ) in which timing is controlled automatically (see THERMOCYCLER). In most cases, each cycle of PCR involves a sequence of three levels of temperature: (i) e.g. 94/95° C for strand separation (denaturation of the sample duplex DNA); (ii) a temperature suitable for the binding of primers (annealing); and (iii) a temperature suitable for extension, i.e. ongoing synthesis of a DNA strand, from the primer, on the template strand. In some cases a two-temperature protocol has been used – e.g. a denaturing temperature of 94° C and combined annealing/extension at 68° C; this is possible e.g. with the GeneAmp 9700 system (Perkin-Elmer) amplifying a 500-nucleotide amplicon of phage l DNA. While denaturation and extension temperatures are generally ¾94/95° C and 72° C, respectively, the temperature used for annealing varies widely according to the particular assay. One factor which influences annealing temperature is the GC% of the primers – GC%-rich primers requiring a higher annealing temperature to avoid the formation of spurious products through ‘mispriming’ (binding of primers to inappropriate sequences in the template). (Examples of some primer-binding temperatures: GC% 33: 45° C [JCM (1999) 37 772–774]; GC% 62: 63° C [NAR (1998) 26 3614–3615].) One way of finding an optimal annealing temperature is TOUCHDOWN PCR. For some purposes (e.g. ARBITRARILY PRIMED PCR) a primer is required to bind to a target sequence to which it is not fully complementary. In such cases, PCR can be run under so-called low-stringency conditions, i.e. particular values of temperature, pH and electrolyte concentration which optimize primer–target binding by helping to overcome, or minimize, the effects of mismatches between primer and target. Highstringency conditions are those which allow primer–target binding to occur only when the primer and target sequences are exactly complementary, or very nearly so. The greater the encoding so-called ‘toxin co-regulated pili’ (TCP) in Vibrio cholerae. Salmonella spp contain at least five distinct PAIs. For example, SPI-1 includes the inv -spa genes that encode a type III secretory system used in the invasion of epithelial cells. SPI3 encodes products needed for survival in macrophages; like LEE, this PAI is located downstream of selC. SPI-5 apparently contributes specifically to enteropathogenicity [Mol. Microbiol. (1998) 29 883–891]. In Staphylococcus aureus, the gene for toxic shock toxin is carried by a mobile pathogenicity island [Mol. Microbiol. (1998) 29 527–543]. pathognomonic Of symptoms: characteristic of, or specific to, a given disease. PathoTec system See MICROMETHODS. pathotype, pathovar See VARIETY. patricins See STREPTOGRAMINS. patronym (patronymic) In NOMENCLATURE: any name derived from that of a person. Pattern coefficient See entry Sp . Pattern difference See entry Dp . patulin (clavacin, clavatin, claviformin) An antibacterial, antifungal, antimitotic (and possibly carcinogenic) MYCOTOXIN formed by species of Penicillium, Aspergillus and Byssochlamys. It is a heterocyclic compound which is soluble e.g. in water and alcohol and which reacts with sulphydryl groups. It occurs e.g. in animal feeds and has been implicated in fatalities in cattle. It is formed by P. expansum in apples and has been detected in commercial apple juice; it is destroyed by SO2 and by alcoholic fermentation. [Mode of action on Saccharomyces cerevisiae: AEM (1983) 45 110–115.] Paul–Bunnell test A haemagglutination test used to detect and quantify a HETEROPHIL ANTIBODY (the ‘Paul–Bunnell antibody’) which usually appears in cases of INFECTIOUS MONONUCLEOSIS. (Heterophil antibodies may not be detectable in very young children (e.g. <4 years of age), so that the test may be unreliable in this group.) The antibody appears 1–3 weeks after the onset of symptoms and may persist for months; it agglutinates the red blood cells of the sheep, horse and OX. The test determines the titre of serum which agglutinates sheep RBCs. In a modified form of the test, one sample of serum is pre-absorbed with OX RBCs (which bind the Paul–Bunnell antibody); another sample is absorbed with guinea-pig tissue (which binds e.g. Forssman antibodies but not the Paul–Bunnell antibody); as Forssman antibodies can agglutinate sheep RBCs, pre-absorption makes the test more specific for the Paul–Bunnell antibody. Paulinella A genus of testate amoebae similar to EUGLYPHA spp. P. chromatophora, a small (20–30 µm long), ovoid, freshwater species, contains two CYANELLES (Synechococcus); on division, each daughter receives one cyanelle, which subsequently divides. Paxillus See BOLETALES. PBP PENICILLIN-BINDING PROTEIN. PBP 2a (PBP 20 ) See PENICILLIN-BINDING PROTEINS and MRSA. pbpA gene See PENICILLIN-BINDING PROTEINS. pbpB gene See PENICILLIN-BINDING PROTEINS. pBR322 A small, multicopy, non-conjugative PLASMID (4362 bp) which carries genes for resistance to ampicillin and tetracycline; it resembles the COLE1 PLASMID in its mode of replication. pBR322 is widely used as a CLONING vector [review: Gene (1986) 50 3–40]; insertion of DNA at restriction sites in resistance gene(s) is detected by loss of antibiotic resistance. PBS (1) Phosphate-buffered saline. (2) Primer binding sites: see RETROVIRIDAE. 557 PCR amplicon (a) 5′ 3′ 3′ 5′ 5′ 3′ (b) 3′ 5′ (c) PCR (polymerase chain reaction): the basic method (principle, diagrammatic). (a) Part of a double-stranded molecule of DNA: two antiparallel strands held together by base-pairing. The sequence to be amplified (the amplicon) is delimited by short vertical bars. (b) Initial heating to e.g. 95° C has separated the two strands of DNA. Transient cooling to 45–65° C has permitted the primers – shown as short, solid lines – to bind (D anneal) to their respective (complementary) sites on the template strands; notice the locations of the bound primers in relation to the site of the amplicon. The temperature is then increased to e.g. 72° C, and DNA synthesis (dashed line) occurs from the 30 end of each primer. Note that each of the newly synthesized strands in this first round of replication is longer than the amplicon. (c) The second round of replication begins with initial heating (to ¾95° C) so that the new daughter strands (synthesized in the first round of replication) are separated from their templates. Each new daughter strand (as well as the two parental strands) can then act as a template. In the diagram, a primer has bound to each new daughter strand and has been extended (dashed line). Note that the strand synthesized on a new daughter strand is the same length as the amplicon; such strands rapidly become numerically dominant in subsequent rounds of replication. Reproduced from Figure 4.1, page 57, in DNA Methods in Clinical Microbiology (ISBN 07923-6307-8), Paul Singleton (2000), with kind permission of Kluwer Academic Publishers, Dordrecht, The Netherlands. stringency (e.g. the higher the temperature) the greater will be the degree of matching needed between primer and target for successful binding to occur. Difficulties in designing a primer – owing e.g. to insufficient data on the target sequence – may be approached by using a ‘universal nucleotide’ in place of one or more of the unknown nucleotides [universal nucleotides: Nature (1994) 369 492–493]. Successful PCR requires close attention to detail and also exclusion of contamination by DNA from the environment and/or from the apparatus. Contamination by extraneous DNA. Extraneous DNA may contain (i) sequences that are able to bind the primers being used and/or (ii) sequences able to prime irrelevant sites in the sample DNA; either way, unwanted sequences may be amplified – forming a troublesome mixture of products and reducing the efficiency of amplification of the required sequence. There are two main approaches to the control of contamination. First, amplicon containment; this involves division of the working space into a number of dedicated areas – each area being used for only certain stage(s) of the procedure. In one common scheme there are five designated areas, two of which are reserved for (i) cycling and (ii) analysis of products. The second approach, amplicon inactivation, can be achieved by either of two methods. In one method, isopsoralen (see PSORALENS) is added to the reaction mixture before cycling. After cycling, the reaction mixture is exposed to ultraviolet radiation – e.g. 365 nm/15 minutes/4° C; this results in covalently cross-linked double-stranded amplicons which – as they cannot be denatured into single-stranded templates – cannot jeopardize later assays in the same apparatus/laboratory. (Activation of isopsoralen at low temperature is more efficient than at room temperature [JCM (1999) 37 261–262].) In the second form of amplicon inactivation, all PCR assays are carried out routinely with deoxyuridine triphosphate (dUTP) instead of the usual deoxythymidine triphosphate (dTTP); as a result, dUMP is incorporated into all amplified products during cycling. (Such products can be detected and analysed 558 PCR was satisfactory when targets were present at the limit of the test’s sensitivity [JCM (1998) 36 191–197]. Even when known inhibitors have been taken into account it is mandatory to include suitable controls to detect the effects of unknown/unexpected inhibitors (which may occur in only certain batches of a given type of specimen). Amplification facilitators are agents which can enhance the efficiency of PCR in the presence of inhibitory substance(s); such agents may also e.g. improve the specificity of PCR and increase the fidelity of DNA synthesis. Amplification facilitators include e.g. bovine serum albumin (BSA) and betaine; some facilitators are effective against a range of inhibitors while others have more selective activity. [Effects of amplification facilitators on diagnostic PCR in the presence of blood, faeces and meat: JCM (2000) 38 4463–4470.] Detection of PCR products. In the ‘standard’ form of PCR the products consist of millions of amplicons whose precise length (exact number of base pairs) is determined by the targetbinding sites of the primer pair (see figure). Such products, all the same size, can be readily detected/identified by gel electrophoresis as a single band at a precise location in the gel. Typically, electrophoresis is carried out in an agarose gel, and the band of products is detected by staining with ethidium bromide and examining the gel under ultraviolet radiation. To facilitate identification, another lane in the same gel is used for electrophoresis of a set of fragments of known size (e.g. 100, 200, 300. . . base pairs); this gives a scale (‘ladder’) with which the experimental band(s) can be compared. Gel electrophoresis is also used for detecting the products in MULTIPLEX PCR (in which products of more than one size are formed) and in ARBITRARILY PRIMED PCR (in which there are products of very many sizes – the multiple bands of products forming a fingerprint of the sample DNA). Band(s) of products within a gel strip may be further examined by Southern blotting followed by SOUTHERN HYBRIDIZATION. The identity of amplicons from a given reaction can also be checked by subjecting them to a particular RESTRICTION ENDONUCLEASE and checking (by electrophoresis) to see whether fragments of expected sizes have been formed. Amplicons can be rapidly detected by a plate-based hybridization assay. First, single-stranded amplicons are prepared by denaturing the products. These amplicons are then exposed to detector probes which are complementary to a given sequence in the amplicon; each PROBE is labelled with the enzyme alkaline phosphatase. The whole is then transferred to a microtitre plate whose wells have been coated with capture probes – probes complementary to a different sequence in the amplicon. Incubation at 37° C permits amplicons (if present) to bind to the (surfacebound) capture probes. After washing (to remove unbound detector probes), a substrate for alkaline phosphatase is added to detect any bound, enzyme-labelled amplicons; if present, the enzyme will cleave the substrate, and this can be detected by monitoring the optical density of the mixture. This method has been used for detecting Bordetella pertussis in clinical specimens (by detecting a sequence of nucleotides specific to the pathogen) [JCM (1997) 35 117–120]. Products can also be detected rapidly by PCR DIG-ELISA. In this method, the PCR reaction mixture includes DIGOXIGENINtagged deoxyuridine triphosphate (dUTP) so that dUMP is incorporated into the products. Moreover, the primers are 50 biotinylated so that, after cycling, products can be captured on the STREPTAVIDIN-coated surface of a well in a microtitre plate. Any products captured on the well’s surface can then in the usual way.) In each assay, the reaction mixture also includes the enzyme URACIL-DNA GLYCOSYLASE (D uracil-Nglycosylase, UNG); consequently, if amplicons from previous assay(s) contaminate a given assay, then those contaminating amplicons will be degraded by the dual effects of (i) enzymic removal of uracil by UNG, and then (ii) denaturation and strand breakage caused by the high initial temperature of the first cycle in the new assay. (This high temperature also inactivates UNG; this is necessary in order to avoid degradation of products in the new assay.) Inhibitors and facilitators of PCR. Some (e.g. clinical) specimens may contain substance(s) which inhibit PCR – so that a false-negative result may be obtained even when a given specimen contains the particular amplicon of interest. Inhibitors are found in a wide range of clinical specimens, e.g. bone marrow aspirates, cerebrospinal fluid, faeces and urine. PCR-inhibitory components of blood cells include haemoglobin (in erythrocytes) and lactoferrin (in leukocytes). [Purification and characterization of PCR-inhibitory components in blood cells: JCM (2001) 39 485–493.] Inhibitors of PCR also include the blood anticoagulants SPS (sodium polyanetholesulphonate) [JCM (1998) 36 2810–2816] and HEPARIN; haemin; and agar (see GELLAN GUM for an alternative to the agar component in transport media). The problem of inhibitors may be approached in several ways. For example, specimens may be diluted in order to reduce the concentration of the inhibitory substance(s); however, this also lowers the number of copies of the target sequence (if any) in the specimen and may lower the sensitivity of the assay. Inhibitors in faeces appear to include complex polysaccharides of dietary origin; these inhibitors can be removed (at least partially) by means of a modified form of a commercial process used for preparing purified DNA from tissues and viruses [JCM (1997) 35 995–998]. The removal of some polysaccharide inhibitors of PCR may be achieved by means of agarose–DNA preparations [NAR (1998) 26 3309–3310]. In another form of nucleic-acid amplification, TMA (q.v.), certain inhibitors can be inactivated by treating specimens with the detergent sodium dodecyl sulphate (SDS) [JCM (1997) 35 307–310]. SDS has been used for a similar purpose in tests based on the ligase chain reaction (LCR) [JCM (1998) 36 1324–1329]. A distinct approach to the problem of inhibitors is offered by the use of magnetic separation techniques (see e.g. DYNABEADS); following capture on Dynabeads, specific molecules may be washed free of inhibitory substance(s). The presence of inhibitors may be detected in several ways. For example, an aliquot of the specimen may be ‘spiked’ with the given target DNA prior to assay; in general, this is not a good idea because it involves extra handling of the specific target sequence and, hence, increases the risk of contamination. An alternative approach uses an internal control (IC) consisting of molecules of nucleic acid that (i) contain binding sites for the given primers, and (ii) are distinguishable from the actual target by the presence of a unique PROBE-binding region. On efficient amplification of a specimen containing specific target and IC (but lacking inhibitors), two types of product are likely to be obtained: (i) amplicons from the actual target, and (ii) copies of the IC (identifiable by a specific probe). Failure to obtain copies of both target and IC may indicate the presence of inhibitors; amplification of the IC in the absence of target amplification suggests lack of target sequences in the specimen. In one experiment, 20 copies of the IC were added to each test sample, and a positive IC signal was taken to indicate that amplification 559 PCR DIG-ELISA In microbiology, PCR is used e.g. for detecting and identifying specific microorganisms in medical or environmental specimens; this use depends on the amplification and examination of sequences of nucleotides which are specific to the organisms of interest. Such an approach is particularly useful for those organisms which cannot be cultured – or which grow very slowly (e.g. Mycobacterium tuberculosis). For diagnostic purposes, primers must be specific to sequences found only in a given pathogen (for example, a sequence in a gene encoding a unique toxin). Examples of the use of PCR for detecting pathogens include: Bacteroides forsythus [JCM (1999) 37 1621–1624], Bordetella pertussis [JCM (1999) 37 606–610], chlamydiae [JCM (1999) 37 575–580], Escherichia coli O157 [JCM (1998) 36 1801–1804], Fusarium spp [JCM (1999) 37 2434–2438], group A streptococci [JCM (1998) 36 1769–1771], HIV-1, HIV-2, HTLV-I, HTLV-II [PNAS (1999) 96 6394–6399], rhinoviruses [JCM (1999) 37 524–530] and Trichomonas vaginalis [JCM (1997) 35 132–138]. For PCR in TYPING see: AFLP FINGERPRINTING, ARBITRARILY PRIMED PCR, PCR-RIBOTYPING, REP-PCR, RFLP, SPOLIGOTYPING and SSCP. PCR is also used e.g. for (i) detecting specific virulence factors – such as toxins (e.g. diphtheria toxin) [JCM (1997) 35 1651–1655]; (ii) detecting genes that confer resistance to antibiotics (e.g. the mecA gene in staphylococci [JCM (1999) 37 3029–3030]); (iii) surveying the diversity of species in environmental samples by means of BROAD-RANGE PRIMERS; (iv) DIFFERENTIAL DISPLAY. (See also ARBITRARILY PRIMED PCR; ASYMMETRIC PCR; HOTSTART PCR; MULTIPLEX PCR; NESTED PCR; REP-PCR; REVERSE TRANSCRIPTASE PCR.) [PCR in clinical microbiology: Book ref. 221, pp 56–125.] PCR DIG-ELISA See PCR. PCR-ISH See IN SITU PCR. PCR-RFLP See RFLP. PCR-ribotyping An alternative to RIBOTYPING (q.v.) in which, initially, PCR is used to amplify a particular sequence in the rrn operon (usually the intergenic spacer region between the 16S and 23S genes); in many bacteria the chromosome contains multiple copies of this sequence and, moreover, the length of the sequence may vary even within the same chromosome. The amplicons are examined by gel electrophoresis, giving a fingerprint of the strain under test. Strains are classified into ‘PCR ribotypes’ on the basis of differences in their fingerprints. PCR-ribotyping has been used e.g. for typing strains of Burkholderia [JCM (1992) 30 2084–2087] and Clostridium difficile [RMM (1997) 8 (supplement 1) S55–S56], and detecting species of Rhizobium [LAM (1999) 28 137–141]. PCR-SSCP See SSCP. PDGF Platelet-derived growth factor. pea early-browning virus See TOBRAVIRUSES. pea enation mosaic virus (PEMV) An ssRNA-containing PLANT VIRUS which has a narrow host range; transmission occurs (circulatively) via aphids (e.g. Acyrthrosiphon pisi ). Virion: icosahedral, ca. 28 nm diam.; two types occur, differing in S20 w. Genome: two molecules of linear positive-sense ssRNA (MWts 6 ca. 1.7 and 1.3 ð 10 ), sometimes with a third molecule of MWt ca. 0.3 ð 106 . PEMV replicates in the nucleus; vesicular cytopathological structures arise from the nuclear membrane. pea leaf roll virus See LUTEOVIRUSES. pea mosaic virus See POTYVIRUSES. pea streak virus See CARLAVIRUSES. peach leaf curl See TAPHRINA. be detected by exposing the well to a conjugate consisting of antidigoxigenin (the antibody to digoxigenin) covalently bound to alkaline phosphatase; any bound, digoxigenin-tagged products will bind the conjugate (via the antibody) and, hence, become labelled with alkaline phosphatase. As in the previous method, the enzymic label is detected by adding an appropriate substrate and monitoring the optical density. This method has been used for screening salmonellae [JCM (1997) 35 714–718]. Note that, unlike the previous method, this method does not confirm the identity of the products by specific hybridization(s). Real-time detection of products is possible e.g. with TAQMAN PROBES (q.v.) and MOLECULAR BEACON PROBES. The LightCycler (Roche) is a thermocycler in which detection (and/or quantification) of products can be achieved by at least two distinct methods. One method uses SYBR Green I (Molecular Probes Inc., Eugene, OR, USA) – a dye which fluoresces strongly (530 nm) when it binds to the minor groove of dsDNA. In PCR, this dye can be used to monitor the cycle-bycycle build-up of products by including it in the reaction mixture and measuring its fluorescence at the end of the extension stage in each cycle – that is, when all amplicons are double-stranded, the quantity of dsDNA is at a maximum in that cycle, and the maximum amount of dye has been bound. With ongoing cycling, there is a progressive increase in the amount of dsDNA at the end of extension and, hence, in fluorescence from the bound dye. Detection of products by the LightCycler can also be achieved by a probe-based format. Two types of probe, included (in large numbers) in the reaction mixture, can bind to adjacent sites on an internal (non-terminal) sequence in the amplicon. When bound, the 30 end of probe 1 (labelled with fluorescein) is juxtaposed to the 50 end of probe 2 (labelled with the dye LightCycler Red 640). When probes are bound in this way, fluorescence from probe 1 provides excitation for the dye on probe 2 (by a process called fluorescence resonance energy transfer, FRET); when excited in this way, the dye on probe 2 emits radiation of wavelength 640 nm. During cycling, probes 1 and 2 bind to amplicons at the annealing stage, and the signal (at 640 nm) is read at this stage. Subsequent primer extension displaces the probes – so that (because the dyes are then no longer juxtaposed) fluorescence from probe 2 ceases. During ongoing cycling, probes bind to increasing numbers of amplicons at the annealing stage in each cycle and give rise to a progressively increasing signal at that stage. Optionally, the LightCycler can work in duplex mode, i.e. detecting two targets (in the same sample) by using two pairs of primers and two (different) pairs of probes; one probe in one pair is labelled with LightCycler Red 640, and one probe in the other pair is labelled with LightCycler Red 705 – so that two independent signals are obtained from the one sample. [Duplex LightCycler assay for detecting (i) the mecA gene and (ii) a Staphylococcus aureus-specific marker: JCM (2000) 38 2429–2433.] Quantification in PCR. The number of target sequences in a sample before amplification can be estimated by several commercial systems. These include TaqMan probes (PE Applied Biosystems) and the probe-based and SYBR Green I formats of the LightCycler (Roche); in all cases, quantification in a given reaction requires identification of the threshold cycle – see TAQMAN PROBES for rationale. (See also MOLECULAR BEACON PROBES.) Uses of PCR. PCR is an extremely flexible technique which has found use in fields as diverse as palaeobiology and forensic science. 560 Pediococcus boiling water (with or without EDTA) or dilute acid (cf. HEMICELLULOSES); they generally include PECTINS, pectic acid and its salts (pectates), and certain neutral polysaccharides (e.g. arabinogalactans) which occur in association with pectins. pectic substances Syn. PECTIC POLYSACCHARIDES. pectin See PECTINS. pectin methylesterase See PECTIC ENZYMES. pectinases See PECTIC ENZYMES. pectinate (1) (adj.) Arranged like the teeth of a comb. (2) (noun) A salt of ‘PECTINIC ACID’. Pectinatus A genus of Gram-negative bacteria (family BACTEROIDACEAE) which have been isolated from spoilt beers. Cells: round-ended, slightly curved or helical rods or filaments, 0.7–0.8 ð 2.0–32.0 µm; lateral flagella arise in a line along the concave side of the cell. Acid is formed from the fermentation of e.g. arabinose, fructose, glucose, glycerol, lactate, maltose and mannitol; lactate fermentation yields acetic, propionic and succinic acids, and CO2 , as major products. GC%: ca. 40. Type species: P. cerevisiiphilus. pectinella One of the circumferential bands of cilia which encircle some ciliates (e.g. Didinium). (cf. TROCHAL BAND.) pectinesterases See PECTIC ENZYMES. pectinic acid An outmoded term for colloidal galacturonans which have a methyl ester content intermediate between that of PECTINS and pectic acids. pectinolytic (pectolytic) Able to degrade PECTINS – see PECTIC ENZYMES. pectins The major component of the PECTIC POLYSACCHARIDES. Pectins consist essentially of (1 ! 4)-a-D-linked galacturonic acid residues (D pectic acid); a variable number of the C-6 carboxyl groups are methyl-esterified, and, commonly, a-Lrhamnose residues occur at intervals within the chain (forming a rhamnogalacturonan). Other neutral sugars (e.g. L-arabinose, D-galactose) may occur in (usually short) side-chains. Pectins occur in the primary cell walls and intercellular regions of plants (many fruits have a high pectin content); they also occur in the cell walls of certain algae. Pectins are powerful gelling agents and are widely used in the food industry (e.g. as a setting agent in jams) and in pharmaceuticals and cosmetics. Pectin-degrading enzymes (PECTIC ENZYMES) are produced by a wide range of microorganisms but not by higher animals or by man. (See also RUMEN.) Pectobacterium See ERWINIA (Carotovora group). pectolytic Syn. PECTINOLYTIC. Pediastrum A genus of freshwater colonial green algae (division CHLOROPHYTA). Each colony is flat and disc-like (up to ca. 100 µm diam.) and is typically made up of a central cell surrounded by concentric rings of cells; the outermost cells each have two lobes projecting radially outwards, so that the colony resembles a miniature gear-wheel. pedicel A small stalk. In the fruiting bodies of some myxobacteria: a branch of the common (main) stalk bearing a single sporangium. Pedinomonas See MICROMONADOPHYCEAE. pediocin PA-1 A 44-amino-acid, pore-forming BACTERIOCIN produced by species of Pediococcus; it is active against Listeria spp. Pediococcus A genus of Gram-positive bacteria of the family Streptococcaceae. Cells: non-motile cocci (ca. 1 µm diam.), commonly in pairs and tetrads. Certain vitamins and amino acids are essential for growth. Growth occurs optimally under microaerobic conditions; glucose is fermented homofermentatively and anaerogenically to DL-lactic acid, L(C)-lactic acid generally predominating. (See also LACTIC ACID FERMENTATION.) peach rosette mosaic virus See NEPOVIRUSES. peanut (toxicity associated with fungal contamination) See AFLATOXINS. peanut clamp virus See TOBRAVIRUSES. peanut clump virus See SOIL-BORNE WHEAT MOSAIC VIRUS and TOBAMOVIRUSES. peanut mottle virus See POTYVIRUSES. peanut stunt virus See CUCUMOVIRUSES. peanut yellow mottle virus See TYMOVIRUSES. pear decline disease A YELLOWS disease of pear trees caused by an MLO; oxytetracycline hydrochloride is used for control. peat scours See SCOURS. pébrine See NOSEMA. pectase See PECTIC ENZYMES. pectates Salts of pectic acid – see PECTINS. pectic acid See PECTINS. pectic enzymes (‘pectinases’) Enzymes which degrade PECTINS; they are widely distributed among microorganisms. There are two broad categories of pectic enzyme: those which de-esterify pectins (pectinesterases) and those which degrade the galacturonan backbone chain (depolymerases). Pectinesterase (pectin methylesterase, pectase, pectin pectylhydrolase, EC 3.1.1.11) de-esterifies pectins, producing methanol and pectic acid; pectinesterases occur in higher plants, numerous fungi (including some yeasts) and some bacteria. Pectic depolymerases include both hydrolases, which hydrolyse glycosidic linkages in the galacturonan, and lyases, which cleave a glycosidic bond with the formation of a double bond between C-4 and C-5 in the residue at the newly formed non-reducing end (b-elimination). These enzymes vary in their specificities; most function mainly or only on non-esterified galacturonans (pectic acids). Depolymerases which act as endoenzymes include e.g. endopolygalacturonase (poly-(1 ! 4)-a-D galacturonide glycanhydrolase, EC 3.2.1.15), produced by many plant-pathogenic and saprotrophic fungi and bacteria and by some yeasts; endopectate lyase (polygalacturonic acid trans-eliminase, endopolygalacturonate lyase, EC 4.2.2.2), produced by various bacteria – especially SOFT ROT (2) agents – and some plant-pathogenic fungi; and endopectin lyase (pectin lyase, polymethoxylgalacturonide lyase, EC 4.2.2.10 – the only depolymerase specific for highly esterified pectins), produced mainly by fungi. Exoenzymes include exopolygalacturonase (poly (1 ! 4)-a-D-galacturonide galacturonohydrolase, EC 3.2.1.67), produced e.g. by plants, fungi and some bacteria; and exopectate lyase (EC 4.2.2.9 – an enzyme which liberates unsaturated dimers or, less commonly, trimers [e.g. JGM (1982) 128 2661–2665] from the reducing end of a pectic acid), produced e.g. by certain bacteria. Pectinolytic organisms can be important plant pathogens and causal agents of spoilage of fresh and preserved fruit and vegetables (see e.g. SOFT ROT (2); PICKLING; CANNING). Many plants have cell wall-associated proteins which act as inhibitors of endopolygalacturonases in vitro; these proteins are thought to serve a defensive role against invasion by pectinolytic pathogens, although it has been reported that they may not be effective in vivo [PP (1985) 34 54–60]. (See also TANNINS.) Pectinolytic organisms also fulfil useful roles: see e.g. COFFEE; RETTING; RUMEN. Preparations of pectic enzymes, obtained mainly from Aspergillus niger, are used e.g. to facilitate pressing in the extraction of fruit juices, to clarify (reduce turbidity and viscosity of) fruit juices, etc. (See also ENZYMES.) pectic polysaccharides (syn. pectic substances) Polysaccharides obtained from isolated plant cell walls by extraction with 561 pedology and is separated from it by an electron-translucent layer ca. 15–20 nm thick. In the apicomplexan Gregarina garnhami the three-membrane-thick pellicle forms a series of longitudinal folds, the crests of which are associated with two sets of longitudinal filaments which have been postulated to have a role in gliding motility [JUR (1984) 88 66–76]. The typical ciliate pellicle consists of three layers of unit-type membrane, the outermost of which (the plasmalemma) covers the entire organism, including the cilia. The two inner layers are sometimes called ‘alveolar membranes’; the space between these membranes is partitioned, forming a number of vesicles or alveoli which lie beneath the plasmalemma. Adjacent alveoli may or may not be in communication; in some species (see e.g. COLEPS) the alveoli contain inorganic deposits and constitute a semi-rigid endoskeleton. The alveolar pellicle does not cover the entire organism, and in some ciliates it forms only a relatively small part of the cell envelope. Even when the alveolar pellicle forms the major part of the cell envelope it is absent in the region of the CYTOSTOME, and it is characteristically penetrated by the MUCOCYSTS and by the kinetosomes of the cilia – which penetrate below the level of the innermost membrane. The inner surface of the innermost membrane may be in contact with a dense fibrillar layer of peripheral cytoplasm (the epiplasm); the epiplasm may be the site of connection and/or anchorage of subpellicular structures and/or pellicular structures. Some authors consider the epiplasm to be part of the pellicle. (See also CORTEX; KINETY; SILVER LINE SYSTEM.) pellis (cutis; cuticle) (mycol.) Non-velar cortical layers of a basidiocarp. pellucid Transparent or translucent. Pelobacter A genus of Gram-negative, rod-shaped anaerobic bacteria. Metabolism: fermentative; sugars are not utilized. Species include P. acidigallici ; P. venetianus [name validation: IJSB (1984) 34 91–92]; P. carbinolicus and P. propionicus [name validation: IJSB (1984) 34 355–357]. Pelobiontida An order of naked, typically multinucleate amoebae (class LOBOSEA) which, when in motion, are typically cylindrical or elongate-ovoid, and which lack mitochondria but contain endosymbiotic bacteria. Flagellate stages are unknown. The organisms are free-living in freshwater (typically microaerobic) habitats, feeding mostly on non-motile or slow-moving algae (e.g. diatoms). The order includes the genus Pelomyxa. (See also PSEUDOPODIUM and UROID.) Pelochromatium roseo-viride See CHLOROCHROMATIUM AGGREGATUM. Pelodictyon A genus of photosynthetic bacteria (family CHLOROBIACEAE). The cells are rod-like or coccoid and contain GAS VACUOLES. Cell division occurs predominantly by binary fission; cells of P. clathratiforme form chains which, owing to the occurrence of ternary fission in some cells in the chains, develop into colonial structures consisting of three-dimensional networks. Pelodinium See ODONTOSTOMATIDA. Pelomyxa See PELOBIONTIDA. Pelosigma A proposed genus of filamentous bacteria which occur, in S-shaped bundles, on mud and in fresh and brackish waters. [Book ref. 22, p. 138.] pelotons Coils or balls of fungal hyphae within root cortical cells in certain endomycorrhizas. Peltier system (thermocyclers) See THERMOCYCLER. Peltigera A genus of foliose LICHENS (order PELTIGERALES). Photobiont: Coccomyxa (e.g. in P. aphthosa) or Nostoc (e.g. in P. canina); Coccomyxa-containing species commonly have Nostoc-containing cephalodia (see CEPHALODIUM). Thallus: typically large and more or less deeply lobed, lobes ca. 1–10 cm or The nomenclature of the species of Pediococcus is confused [for discussion see Book ref. 143, pp. 179–211]. Some authors have recognized the following species as validly published: P. acidilactici, P. damnosus (formerly P. cerevisiae), P. dextrinicus, P. halophilus, P. parvulus and P. pentosaceus. (See also BEER SPOILAGE; IDLI; LACTIC ACID BACTERIA; MALOLACTIC FERMENTATION; MISO; PICKLING; SOY SAUCE.) pedology Soil science. Pedomicrobium A genus of budding PROSTHECATE BACTERIA found e.g. in soils; the life cycle is similar to that of HYPHOMICROBIUM but differs e.g. in that the mother cell may have many prosthecae and typically bears a coating of iron and/or manganese compounds. peduncle disease See COLD WATER DISEASE. PEF PULSED ELECTRIC FIELD. pefloxacin See QUINOLONE ANTIBIOTICS. PEG POLYETHYLENE GLYCOL. pegylated Refers to the form of a given molecule, prepared in vitro, which consists of a construct containing a polyethylene glycol moiety. peitschengeissel flagellum See FLAGELLUM (b). Pekilo process See SINGLE-CELL PROTEIN. Pekin duck hepatitis B virus Syn. DUCK HEPATITIS B VIRUS. pelagic May refer to that region of an aquatic habitat (e.g. lake, sea) comprising the entire body of water but excluding the BENTHIC zone, or may refer to organisms living in that region. (See also NEKTON; NEUSTON; PLANKTON.) Pelagophycus See PHAEOPHYTA. Pelargonium blackleg See BLACKLEG (2). pellet (1) The packed sedimented material formed by CENTRIFUGATION. (2) A spherical colony formed by the growth of a mycelial fungus within a liquid medium. pellicle (1) (bacteriol., mycol.) A continuous or fragmentary film, or a mat of organisms, formed at the surface of a liquid culture by certain bacteria or fungi. A bacterial pellicle may comprise bacterial cells, extracellular products, or both. Strains of Acetobacter xylinum (see ACETOBACTER) form a tough pellicle of CELLULOSE; each cell produces a single cellulose ribbon composed of a number of microfibrils which apparently emanate from distinct sites along the outer membrane [Book ref. 38, pp. 526–528]. The cellulose ribbons from many cells aggregate to form a network which floats to the surface of the medium, carrying the cells with it – presumably a mechanism for maintaining the organisms in the upper, oxygenated regions of the medium. Among fungi, pellicles are formed on liquid cultures e.g. by various yeasts (see e.g. ‘flor’ yeasts in WINE-MAKING) and by some mycelial fungi (e.g. Penicillium spp). (2) (mycol.) Any superficial membranous structure which can easily be removed from the tissues beneath it. (3) (protozool.) A composite membranous structure which forms the limiting envelope in many protozoa. In some flagellates (e.g. Trypanosoma) the pellicle consists essentially of the cytoplasmic membrane (D plasmalemma) supported from beneath by a system of microtubules; some authors do not regard such a simple type of cell envelope as a pellicle. (cf. EUGLENOID FLAGELLATES.) In coccidia (phylum APICOMPLEXA) the pellicle of the sporozoite and merozoite stages consists of three unit-type membranes, the outermost membrane (plasmalemma) enclosing the entire cell; the middle and inner membranes are closely apposed and together form the inner pellicular complex – a structure which underlies the plasmalemma (except at the MICROPORES and at regions delimited by the POLAR RINGS) 562 penicillins more in length; only the upper surface has a cortex, the lower surface often being tomentose and marked with ‘veins’. Conspicuous rhizines are present on the lower surface in many species. Apothecia are formed on the upper surface – often borne on narrow, erect, stalk-like extensions of the thallus margin. Species occur e.g. on soil, rotting logs etc; many are widely distributed. Peltigerales An order of fungi of the ASCOMYCOTINA; members include foliose LICHENS (photobiont green or cyanobacterial) which often have cephalodia. Ascocarp: APOTHECIOID, with filiform paraphyses not forming a distinct epithecium. Asci: bitunicate, cylindrical. Ascospores are often elongated and multiseptate. Genera: e.g. LOBARIA, NEPHROMA, PELTIGERA, PSEUDOCYPHELLARIA, SOLORINA, STICTA. peltigeroside b-D-Galactofuranosyl-(1 ! 3)-D-mannitol, found in Peltigera spp. Pelvetia See PHAEOPHYTA. pemphigus neonatorum See SCALDED SKIN SYNDROME. penams Compounds which contain the penicillin nucleus (see b-LACTAM ANTIBIOTICS). (cf. PENEMS.) penciclovir A guanine nucleoside analogue ANTIVIRAL AGENT; it is effective against alphaherpesviruses. Penciclovir is used e.g. as a topical agent (1% in creams) for the treatment of labial and genital herpes simplex infections. (See also FAMCICLOVIR.) penems A class of antibiotics; a penem contains a penicillin-like nucleus in which a double bond occurs between C-2 and C-3 (see b-LACTAM ANTIBIOTICS, Figure (a)). penicillic acid A secondary metabolite of certain Penicillium and Aspergillus spp; it is carcinogenic in rats when administered by subcutaneous injection. penicillin Any of the PENICILLINS, or (specifically) penicillin G. penicillin acylase Syn. PENICILLIN AMIDASE. penicillin amidase (penicillin acylase; penicillin amidohydrolase) An enzyme (EC 3.5.1.11) produced by many Gram-positive and Gram-negative bacteria; it cleaves the side-chain of PENICILLINS to form 6-aminopenicillanic acid. penicillin amidohydrolase Syn. PENICILLIN AMIDASE. penicillin-binding proteins (PBPs) Proteins which occur in the cell envelope of Gram-positive and Gram-negative bacteria and which bind covalently to PENICILLINS and to related b-LACTAM ANTIBIOTICS; according to species, there may be up to eight distinct types of PBP in a given cell. Different PBPs vary in their ability to bind particular b-lactam antibiotics. The PBPs of a given species are numbered in order of decreasing MWt (PBP-1, PBP-2 etc.); similar patterns of PBPs occur in taxonomically related species. PBPs have enzymic roles in the biosynthesis of PEPTIDOGLYCAN; they are thus involved e.g. in cell elongation and septum formation. (See also CELL CYCLE and THREE-FOR-ONE MODEL.) Some PBPs are essential for cell viability, and these constitute the killing sites of the b-lactam antibiotics; a b-lactam antibiotic acylates (thus inactivating) a catalytic site on a PBP. Low-MWt (¾40000–50000) PBPs are typically efficient carboxypeptidases (see PROTEASES) and/or endopeptidases. In vivo they appear to play a subordinate role, and are apparently non-essential for cell viability. High-MWt (>60000) PBPs are less abundant in the cell but at least one type appears to be essential for cell viability; they are generally more sensitive than the low-MWt PBPs to penicillins. The enzymic roles of these PBPs include mediation in transglycosylation and transpeptidation, i.e. the assembly of glycan subunits and the formation of peptide cross-links, respectively. In Escherichia coli the high-MWt PBPs are PBP-1A, PBP-1B, PBP-2 and PBP-3 (see below). PBP-1A and PBP-1B (encoded by genes mrcA and mrcB, respectively) are involved in cell elongation; inactivation of both of these PBPs leads to rapid lysis of growing cells. PBP-2 (encoded by gene pbpA) is essential for maintaining the rod shape of the cell during growth; a null mutation in pbpA gives rise to spherical cells that eventually lyse. PBP-2 may be capable of substituting for PBP-1B in mutant cells which lack a functional PBP-1B. (See also MECILLINAM.) PBP-3 (encoded by gene ftsI; also called pbpB ) is required for peptidoglycan synthesis in the SEPTUM. Inactivation of PBP-3 leads to formation of filaments and eventual lysis. The low-MWt PBPs (PBP-4, PBP-5, PBP-6) are encoded by genes dacB, dacA and dacC, respectively. PBPs 4 and 5 have carboxypeptidase activity, PBP-5 being responsible e.g. for cleavage of the terminal D-ala from the pentapeptide side chain. [Location of PBPs in E. coli : JB (1986) 165 269–275.] In Staphylococcus aureus, PBP 2a (D PBP20 ) is associated with high-level resistance to methicillin: see MRSA. penicillin G Syn. BENZYLPENICILLIN. penicillin N (syn. cephalosporin N; synnematin B) An antibiotic (6-(D-a-aminoadipyl)aminopenicillanic acid) produced by Acremonium kiliense (D Cephalosporium acremonium) (cf. PENICILLINS). Isopenicillin N has an L-a-aminoadipyl side-chain. penicillin V Syn. PHENOXYMETHYLPENICILLIN. penicillinase A b-LACTAMASE active mainly or solely against PENICILLINS, forming PENICILLOIC ACIDS. Penicillinase production (e.g. by staphylococci) can be demonstrated e.g. with N-phenyl-1-naphthylamine-azo-o-carboxybenzene, a substance which is yellow and soluble in alkaline form, purple and insoluble in acid form. Agar plates bearing growth of the test organism are flooded with a solution of the alkaline (yellow) form, dried, and then flooded with a solution of penicillin; penicillinase-producing colonies develop a purple colour. Other methods are based on the ability of penicilloic acid to react with iodine and to decolorize a starch–iodine complex; for example, the bacteria are grown on an agar medium containing 1% w/v soluble starch, and the plate is flooded with a solution of benzylpenicillin and 0.1 N iodine in KI; penicillinaseproducing colonies are each surrounded by a white halo against a blue-black background. (See also PADAC.) [Staphylococcal penicillinase: Book ref. 44, pp. 768–775.] penicillins A class of antibiotics, each characterized by a b-lactam ring fused to a thiazolidine ring (see b-LACTAM ANTIBIOTICS for structures and mode of action). The original penicillins (BENZYLPENICILLIN and PHENOXYMETHYLPENICILLIN) are not effective against b-LACTAMASE-producing Grampositive bacteria, and they are generally rather ineffective against Gram-negative species owing to poor penetration of the outer membrane; they are produced by species of Penicillium (cf. PENICILLIN N). Cleavage of benzylpenicillin by PENICILLIN AMIDASE yields 6-aminopenicillanic acid (6APA), a precursor from which can be prepared many semisynthetic penicillins with improved antimicrobial properties. Semi-synthetic penicillins which are resistant to many blactamases (particularly those of staphylococci) include e.g. methicillin (6-(2,6-dimethoxybenzamido)penicillanic acid), nafcillin (6-(2-ethoxy-1-naphthamido)penicillanic acid), cloxacillin, dicloxacillin, flucloxacillin and oxacillin; typically, these penicillins are poorly effective against Gram-negative bacteria. Improved activity against Gram-negative species (coupled with resistance to some b-lactamases) is given by ampicillin (6(a-aminobenzylamido)penicillanic acid) and its derivatives (e.g. 563 penicilliosis AMOXYCILLIN, APALCILLIN, azlocillin, becampicillin, mezlocillin, pivampicillin, and talampicillin), and by carbenicillin (6-(acarboxybenzamido)penicillanic acid) and its derivatives (e.g. carfecillin and ticarcillin); typically, these penicillins are less effective than benzylpenicillin against Gram-positive species. Penicillins for oral administration (which must be acidstable) include ampicillin, phenoxymethylpenicillin and phenoxyethylpenicillin; benzylpenicillin, carbenicillin and methicillin are administered parenterally. Infrequently, penicillins can act as ALLERGENS, the various penicillins commonly exhibiting cross-allergenicity (cf. CEPHALOSPORINS). (See also MECILLINAM.) [Clinical pharmacology and therapeutic uses of penicillins (review): Drugs (1993) 45 866–894.] penicilliosis A disease of man or animals caused by a species of Penicillium. For example, P. marneffei can cause a progressive, disseminated, fatal human disease characterized by proliferation of yeast-like fungal cells within histiocytes, followed by the development of foci of necrosis and eventually abscess formation [AJCP (1985) 84 323–327]. Penicillium A genus of fungi of the class HYPHOMYCETES; teleomorphs occur e.g. in the genus TALAROMYCES. The organisms are widespread in nature and are characteristically saprotrophic; however, P. marneffei can cause PENICILLIOSIS (see also SUBEROSIS), and many species form toxins (see e.g. CHLOROPEPTIDE, CITREOVIRIDIN, CYCLOPIAZONIC ACID, ERGOCHROMES, FUMITREMORGIN, GLIOTOXIN, OCHRATOXINS, PATULIN, PENITREMS, RUBRATOXINS, VERRUCOLOGEN). Some species can cause spoilage of various types of material (see e.g. BREAD SPOILAGE, CHEESE SPOILAGE, PAPER SPOILAGE), some are used in manufacturing processes (see e.g. CHEESE MAKING), and some produce useful antibiotics (see e.g. GRISEOFULVIN, MYCOPHENOLIC ACID, PENICILLINS). Penicillium spp form a well-developed septate mycelium; in most species the aerial mycelium is colourless, but mycelium within the substratum may exhibit distinctive colours – the underside (‘reverse’) of a colony often being some shade of yellow, orange, red or purple; diffusible pigments are formed by some species. The conidiophores each consist essentially of an erect or prostrate hypha (stipe), often ca. 100–250 µm in length, which may be unbranched or branched. The tip of an unbranched conidiophore may terminate in a cluster of phialides (see CONIDIUM) (as in e.g. P. frequentans) or in a cluster of metulae (see METULA) (as in e.g. P. variabile). In branched conidiophores, each branch (ramus) terminates in one or more metulae. (The term ‘ramus’ is generally reserved for any cell or hypha which occurs between a metula and its stipe; however, confusingly, the terms ‘ramus’ and ‘metula’ are sometimes used as though they were synonymous.) The terminal region of a conidiophore (including phialides, metulae and/or rami) is termed the penicillus. In most species the conidiophores occur as discrete structures, but in e.g. P. claviforme and P. concentricum they form synnemata, and in e.g. P. expansum they form coremia. The conidia are spherical to ellipsoidal, aseptate, up to ca. 5 µm; in masses, they may be e.g. blue-green, grey-green or yellow, according to species. Colonies may appear e.g. velvety or woolly, and in some species they typically bear surface drops of exudate. Sclerotia are formed e.g. by P. raistrickii and P. thomii. Species include e.g. P. brevicompactum, P. charlesii, P. chrysogenum, P. citreoviride, P. digitatum, P. frequentans, P. griseofulvum, P. islandicum, P. italicum, P. janthinellum, P. marneffei, P. oxalicum, P. roqueforti, and P. rubrum. [Identification manual and atlas of Penicillium spp: Book ref. 185.] Penicillium brevicompactum virus See MYCOVIRUS. Penicillium chrysogenum virus See MYCOVIRUS. Penicillium cyaneofulvum virus See MYCOVIRUS. Penicillium stoloniferum virus S See MYCOVIRUS. penicilloic acids Compounds formed when the b-lactam (C–N) bond in penicillins (see b-LACTAM ANTIBIOTICS) is hydrolysed by a b-LACTAMASE. penicillopepsin An acid aspartyl PROTEASE from Penicillium janthinellum; it has two catalytically important aspartyl residues in its active centre. penicillus See PENICILLIUM. Penicillus A genus of siphonaceous, calcified green seaweeds related to HALIMEDA; when mature, the thallus bears an apical tuft of branches and somewhat resembles a shaving brush. peniculus A ribbon-like membranelle consisting of a number of rows of cilia; peniculi occur in the buccal cavity of Paramecium and related ciliates. Peniophora See APHYLLOPHORALES (Corticiaceae) and HETEROBASIDION. penitrems Steroid MYCOTOXINS produced by Penicillium spp – e.g. P. verrucosum (P. cyclopium); ingestion by e.g. sheep, cattle or horses of feed contaminated with penitrems can lead to weakness, tremors, convulsions and death. These toxins (produced by soil penicillia) may play a role in the pathogenesis of ryegrass staggers: a condition associated with grazing perennial ryegrass (Lolium perenne) pastures. [Book ref. 33, pp. 1161–1162, 1167–1168.] Penium A genus of placoderm DESMIDS. The cells are cylindrical, round-ended, and have a median suture line but no sinus (cf. CLOSTERIUM). pennate diatoms See DIATOMS. Pennoxide A STERILANT containing ETHYLENE OXIDE (12%) and dichlorodifluoromethane (88%). penny bun See BOLETALES. pentachlorophenol See PHENOLS and BIOREMEDIATION. pentamer See ICOSAHEDRAL SYMMETRY. pentamidine isethionate An aromatic DIAMIDINE used e.g. in the treatment of African trypanosomiasis and Pneumocystis carinii pneumonia. Pentatrichomonas See TRICHOMONAS. Penthesta process See AIV PROCESS. pentitols Five-carbon POLYOLS (e.g. ARABITOL, RIBITOL, xylitol). Pentitols can be metabolized e.g. by many yeasts and by certain bacteria; e.g., strains of Lactobacillus casei metabolize xylitol and ribitol as shown in Appendix III(d), the end-products including acetate, ethanol (cf. PENTOSES), and a mixture of D-and L-lactate (even though L-lactate is the main product of glucose fermentation in this species). penton See ADENOVIRIDAE. pentosans Polysaccharides consisting of PENTOSE residues: see e.g. XYLANS. pentose phosphate cycle Syn. HEXOSE MONOPHOSPHATE PATHWAY. (cf. CALVIN CYCLE.) pentose phosphate pathway Syn. HEXOSE MONOPHOSPHATE PATHWAY. pentoses Five-carbon sugars. Aldopentoses (general formula CH2 OH(CHOH)3 CHO) include e.g. ARABINOSE, RIBOSE and XYLOSE. The corresponding ketopentoses (general formula CH2 OH.CO.(CHOH)2 CH2 OH) are called pentuloses (e.g. RIBULOSE, XYLULOSE). (cf. PENTITOLS and PENTOSANS.) Pentoses can be used as substrates by many bacteria and fungi, being metabolized via different pathways in different species. Heterofermentative LACTIC ACID BACTERIA and ‘homofermentative’ streptococci, pediococci and streptobacteria ferment pentoses to equimolar quantities of lactic and acetic 564 peptidoglycan In many other Gram-negative bacteria the PG is more or less similar to that of E. coli, but many variations in PG structure occur among the Gram-positive bacteria; for example, backbone chains in Mycobacterium spp contain Nglycollylmuramic acid, and in Staphylococcus aureus 30–50% of the muramic acid residues are not acetylated. Even in strains of Neisseria gonorrhoeae, though, the PG is extensively Oacetylated, and it contains 1,6-anhydromuramyl residues [JGM (1985) 131 3397–3400]. The stem peptides may contain e.g. L-lysine in place of meso-DAP (in S. aureus), and isoglutamine instead of D-glutamic acid (in S. aureus and mycobacteria). Cross-links between stem peptides may occur via a short ‘interpeptide bridge’ (e.g. a penta- or hexaglycine bridge in S. aureus), and may occur in different positions in different species. The degree of cross-linking also varies with species, being generally more extensive in Gram-positive species. [PG structure in staphylococci and mycobacteria: Book refs 44 and 54, respectively.] Classification based on PG structure. The structure of PG in a given strain tends to be constant and independent of environmental conditions (although, exceptionally, the amino acid content of the interpeptide bridge in staphylococcal PG can be affected to some extent by the availability of amino acids in the growth medium). PG structure may therefore be a useful feature in taxonomy. Depending on the mode of cross-linking, two basic ‘PG types’ can be recognized. In group A PGs, crosslinks occur between the distal amino group of the diamino acid in position 3 of one stem peptide and the D-alanine in position 4 of another stem peptide; the cross-link may be direct (as in E. coli ) or may involve an interpeptide bridge (as in S. aureus). In group B PGs an interpeptide bridge occurs between the carboxyl group of D-glutamic acid in position 2 of one stem peptide and the carboxyl group of D-alanine in position 4 of another. Group B PGs are much less common than group A PGs; they are found in certain coryneforms (e.g. strains of Agromyces, Arthrobacter, Brevibacterium, Curtobacterium and Microbacterium), in certain anaerobes (e.g. Acetobacterium woodii ) and in Erysipelothrix rhusiopathiae. PG is also classified e.g. on the basis of the nature of the peptide bridge and of the amino acid at position 3. (See also ACTINOMYCETALES.) [Analysis of PG composition and structure: Book ref. 138, pp 123–156.] Peptidoglycan in relation to the cell envelope. In Grampositive bacteria, PG is the major component of the cell wall and is covalently linked to e.g. TEICHOIC ACIDS. (See also e.g. WAX D.) The PG layer is significantly thicker than that found in most Gram-negative bacteria. Interestingly, the thickness of the PG layer in many cyanobacteria is similar to that typical of Grampositive species, and in at least some strains of cyanobacteria the degree of cross-linking resembles that found in Gram-positive bacteria [JB (2000) 182 1191–1199]. In Gram-negative bacteria PG occurs as a layer between the cytoplasmic and outer membranes (see CELL WALL), and is linked covalently to the outer membrane by the BRAUN LIPOPROTEIN. (See also ADHESION SITE.) The content of PG may depend on the phase of growth: e.g. in E. coli the PG is reported to be 0.7–0.8% or 1.4–1.9% of the dry weight of log-phase or stationary-phase cells, respectively [JB (1985) 163 208–212]. It has been suggested that, in E. coli, the PG occurs as a gel which fills the entire space between the inner and outer membranes (‘periplasmic gel’) [JB (1984) 160 143–152]; it was proposed that this gel may be more extensively cross-linked near the outer membrane, becoming progressively less crosslinked towards the cytoplasmic membrane, and that periplasmic acids via xylulose 5-phosphate and the phosphoketolase reaction (Appendix III (d and b), [AvL (1983) 49 209–224]); the pentoses are taken up either by specific permeases or by PEPdependent phosphotransferase systems (see PTS). (Thermobacteria are unable to ferment pentoses.) In e.g. Bacillus spp, some enterobacteria and certain yeasts pentoses/pentuloses are converted [as in Appendix III(d)] to xylulose 5-phosphate, ribulose 5-phosphate or ribose 5-phosphate which can enter the HEXOSE MONOPHOSPHATE PATHWAY [Appendix I(b)]. (See also ARA OPERON and INDUSTRIAL ALCOHOL.) pentostatin See VIDARABINE. pentraxin proteins See ACUTE-PHASE PROTEINS. pentulose See PENTOSES. PEP Phosphoenolpyruvate. PEP-dependent phosphotransferase system See PTS. PEP potential The sum of the intracellular concentrations of phosphoenolpyruvate (PEP) and 2-phosphoglycerate [JB (1977) 130 583–595]. In cells of e.g. streptococci, high intracellular levels of these compounds develop during starvation, ensuring that when sugars become available they can be taken up rapidly by the (PEP-dependent) PTS. (See also FLUORIDES.) peplomer See ENVELOPE (sense 1). peplos Syn. ENVELOPE (sense 1). pepsin A PROTEASE (EC 3.4.23.1) which is present in the stomach in most vertebrates and which shows maximum activity at low pH (ca. 1–2); it is denatured at pH >6. pepstatin A peptide inhibitor of aspartyl proteases such as PENICILLOPEPSIN. peptidase Syn. PROTEASE. peptide antibiotics See e.g. BACITRACIN, CECROPINS, DEFENSINS, EDEINES, GRAMICIDINS, POLYMYXINS, TYROCIDINS. peptide nucleic acid (PNA) See PNA. peptidoglycan (murein; mucopeptide; glycosaminopeptide) A heteropolymer which is found in the CELL WALL of most bacteria (cf. e.g. PASTEURIA and PLANCTOMYCES), and which is also present in the cortex of bacterial ENDOSPORES, but which is not found in members of the ARCHAEA (cf. PSEUDOMUREIN). Peptidoglycan (PG) is the component which is primarily responsible for the mechanical strength of the cell wall (protecting the cell e.g. against osmotic lysis) and for maintaining the shape of the cell. Structure of PG. PG consists essentially of linear, heteropolysaccharide backbone chains that are cross-linked, via short peptides, to form a net-like molecule (the ‘murein sacculus’ – see (a) in the figure); in almost all cases the sacculus envelops the bacterial protoplast (incomplete, or ‘patchy’, peptidoglycan occurs e.g. in Myxococcus xanthus: see MYXOBACTERALES). In Escherichia coli, the backbone chains consist of alternating (1 ! 4)-b-linked residues of N-acetyl-D-glucosamine (GlcNAc) and its 3-O-D-lactyl ether derivative, N-acetylmuramic acid (MurNAc). A MurNAc residue may be linked, via its carboxyl group, to a short peptide (the stem peptide) – commonly the tetrapeptide L-alanine– D-glutamic acid–meso-diaminopimelic acid– D-alanine. Cross-links between the backbone chains are formed directly between the D-alanine of one stem peptide and the meso-diaminopimelic acid (meso-DAP) of another on an adjacent chain (see (b) in figure). The backbone chains run perpendicular to the long axis of the cell. Each peptide bridge shown in the figure links a given backbone chain to one of the other backbone chains. However, as described in the legend, some peptide bridges link together more than two backbone chains, and such links are essential features in the THREE-FOR-ONE MODEL of peptidoglycan growth. 565 M M G M G G M G free peptide G M G G M M M M G M G G M G M G M G M G G M M G backbone chains M M G cross-link M M G M G G N -acetylmuramic acid residue G N -acetylglucosamine residue G G M M G G M M G (a) N -acetylmuramic acid residue 6 4 O H CH2OH 5 H N -acetylglucosamine residue O H H 3 H CH2OH H O 1 O H OH H 2 H O H H NH H NH lysozyme acts here O CH.CH3 CO CO.CH3 CO.CH3 CH2OH H NH CO L-ala H CH.COOH O (CH2)2 CO D-glu H (CH2)3 CO meso-DAP O H H CO.CH3 NH CO.CH3 NH NH CH.CH3 CH.CH3 CO NH2CH.COOH CO free ε-amino group available for trimer formation H H NH CH.CH3 NH CH.CO H OH O H H NH O H H O CH.CH3 CH2OH O NH CH.COOH D-ala (CH2)2 cr os CO s- lin k NH CH.CO (CH2)3 NH.CH.COOH NH CH.CH3 COOH (b) PEPTIDOGLYCAN. The structures shown are typical of those in Escherichia coli; similar types of peptidoglycan are found in many other Gram-negative bacteria. (Continued on page 567.) 566 peptidoglycan proteins may diffuse freely in the gel. However, this would imply multilayered PG, and measurements of the turnover of PG in E. coli have suggested that the lateral wall of the cell may contain a monolayer of PG, with multilayered PG probably at the poles of the cell [JB (1993) 175 7–11]. Biosynthesis of peptidoglycan in E. coli. The initial step in biosynthesis may be regarded as the formation of the UDP (uridine 50 -diphosphate) derivative of N-acetylglucosamine (here designated UDP-GlcNAc); this occurs in the cytoplasm. Some of the UDP-GlcNAc is converted to UDP-Nacetylmuramic acid (UDP-MurNAc). This conversion begins with a reaction between UDP-GlcNAc and phosphoenolpyruvate catalysed by the enzyme phosphoenolpyruvate:UDP-GlcNAc enolpyruvyl transferase; the UDP-GlcNAc-3-enolpyruvyl ether intermediate is reduced to UDP-MurNAc in an NADPHdependent reaction. (Conversion of UDP-GlcNAc to UDPMurNAc is blocked by FOSFOMYCIN.) Still within the cytoplasm, amino acids are added sequentially to UDP-MurNAc – by enzymes that require ATP and magnesium (or manganese) ions – to form UDP-N-acetylmuramylpentapeptide (the ‘Park nucleotide’), which includes a terminal D-alanyl-Dalanine (added as a dipeptide). (See also CYCLOSERINE.) The Park nucleotide is then transferred, with release of UMP, to BACTOPRENOL monophosphate in the cytoplasmic membrane. UDP-GlcNAc is subsequently incorporated, with release of UDP, to complete the bactoprenol–disaccharide–pentapeptide subunit (a step which is inhibited by NISIN). Disaccharide–pentapeptide subunits are transferred from the cytoplasmic membrane to the periplasmic region. Such transfer (blocked by VANCOMYCIN) involves release of the bactoprenol–pyrophosphate (bactoprenol–PP) – which thus remains in the cytoplasmic membrane. Bactoprenol–PP must be dephosphorylated (by a membrane-bound pyrophosphatase) to the monophosphate form before it can again act as an acceptor of the P–MurNAc–pentapeptide; dephosphorylation is inhibited by BACITRACIN. In the periplasm, the disaccharide–pentapeptide subunits are inserted into the growing peptidoglycan sacculus. According to one scheme [Microbiology (1996) 142 1911–1918], the disaccharides are polymerized (forming new backbone chains) by transglycosylation reactions that involve the enzymic activities of PENICILLIN-BINDING PROTEINS (see also MOENOMYCIN). (Although transglycosylation reactions seem to be catalysed primarily by penicillin-binding proteins, a penicillin-insensitive glycan polymerase, distinct from PBPs, has been reported in E. coli [FEBS (1984) 168 155–160].) These new chains are inserted into pre-existing peptidoglycan by the formation of new peptide cross-links (transpeptidation – again, involving penicillin-binding proteins) in a manner described by the THREEFOR-ONE MODEL (q.v.). During transpeptidation reactions, the terminal D-alanine of a pentapeptide is cleaved, and the transpeptidase binds to the remaining D-alanine residue to form an acyl-enzyme complex; this conserves the energy of the peptide bond and allows the formation of a new peptide bond – in the absence of ATP – between the acyl group and an amino group of a peptide on another glycan chain. The liberated D-alanine reenters the cytoplasm via a specific transport system (see also CYCLOSERINE). (See also b-LACTAM ANTIBIOTICS; LYSOSTAPHIN; LYSOZYME; MUROPEPTIDES.) One report has linked peptidoglycan synthesis with the synthesis of membrane phospholipids [Microbiology (1996) 142 2871–2877]. PG turnover during growth in E. coli. During growth in E. coli the PG is continually degraded (by periplasmic enzymes) and partly re-cycled. In one model, the PG is degraded to disaccharide–tripeptide subunits which complex with a periplasmic binding protein designated MppA; this complex is then taken up across the cytoplasmic membrane by a specific transport system (encoded by genes oppBCDF). Within the cytoplasm, the tripeptide (i.e. L-alanyl– D-glutamyl–meso-DAP) is cleaved from the subunit by an amidase and linked to UDP-MurNAc by a ligase [mpl gene product: JB (1996) 178 5347–5352] – thus reentering the pathway of peptidoglycan biosynthesis [JB (1998) 180 1215–1223]. Biosynthesis of PG in Gram-positive bacteria. The general scenario is similar to that in Gram-negative species – differences including e.g. the use of other types of derivative in the glycan chains (such as N-glycollylmuramic acid in mycobacteria). When the PG contains an interpeptide bridge, this is added during the intra-membrane stage; in Staphylococcus aureus, for example, the pentaglycine bridge is incorporated from a specific glycyl-tRNA donor. The addition of new PG to the (thick) Grampositive cell wall has been described by the INSIDE-TO-OUTSIDE MODEL. Transport through the peptidoglycan barrier. The cell envelope must, on occasion, be permeable to certain large molecules (e.g. a DNA–protein complex during at least some types of conjugation); moreover, discontinuity in the PG is necessary for the assembly of various TRANSPORT SYSTEMS. It follows that the PG PEPTIDOGLYCAN (continued) (a) The three-dimensional net-like peptidoglycan molecule: backbone chains of alternating residues of Nacetylglucosamine (G) and N-acetylmuramic acid (M) are held together by short peptides which link N-acetylmuramic acid residues. Each of the peptide bridges shown in the diagram links a given backbone chain to one other backbone chain. However, there are also peptide bridges which link together more than two backbone chains; the different types of peptide bridge in E. coli are briefly described in (b), below. (b) Part of two adjacent backbone chains showing the mode of linkage between them. Each N-acetylmuramic acid residue bears a short stem peptide – in this example, the tetrapeptide L-alanine– D-glutamic acid–meso-diaminopimelic acid(meso-DAP)– D-alanine (shown in dotted boxes); in this kind of peptide bridge there is a direct, covalent link between the D-alanine of one stem peptide and the e-amino group of meso-DAP in the other stem peptide. As this particular type of peptide bridge involves two stem peptides, each consisting of four amino acid residues, it has been referred to as a tetra-tetra dimer (or a tetra-tetra). Another type of peptide bridge, the tetra-tri-dimer (tetra-tri), is formed from one tetrapeptide and one tripeptide. A trimer peptide bridge is one which links together three of the glycan backbone chains; this occurs when a stem peptide on a third backbone chain forms a covalent linkage with the free e-amino group (see diagram) of a dimer bridge. An appreciation of these different types of peptide bridge is essential for understanding the 3-for-1 model (THREE-FOR-ONE MODEL) of replication of the peptidoglycan sacculus. The enzyme lysozyme hydrolyses the N-acetylmuramyl-(1!4) linkages in the backbone chain (see diagram); such activity weakens the cell envelope, and may lead to osmotic lysis. Reproduced from Bacteria, 5th edition, Figure 2.7, pages 20–21, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. 567 peptidyl transferase anaerobia be included within the genus Peptostreptococcus as P. tetradius [IJSB (1983) 33 683–698]. (For P. elsdenii see MEGASPHAERA.) (cf. PEPTOCOCCUS.) per locus See PARTITION. Per phenotype Phage-encoded resistance phenotype (see LACTIC barrier must be able to be breached without sacrificing the cell’s structural integrity. The assembly of transport-specific or other structures which cross the cell envelope is therefore likely to require local modification of the PG, and the assembly of some of these structures has indeed been associated with peptidoglycan metabolism [JB (1996) 178 5555–5562]. peptidyl transferase See PROTEIN SYNTHESIS. Peptococcus A genus of Gram-positive, asporogenous, anaerobic bacteria which can ferment amino acids or peptones and which may or may not ferment carbohydrates. Cells: cocci (ca. 1 µm diam.) occurring in pairs, tetrads, clumps and short chains. The organisms have been isolated e.g. from lesions (abscesses etc) in man, often as part of a mixed microflora. (See also VAGINA MICROFLORA.) It has been proposed (mainly on the basis of GC values) that the species P. asaccharolyticus, P. indolicus, P. magnus and P. prevotii be transferred to the genus PEPTOSTREPTOCOCCUS [IJSB (1983) 33 683–698]; the GC% of the type species, Peptococcus niger, is ca. 51. (For P. saccharolyticus see STAPHYLOCOCCUS (S. saccharolyticus).) peptone A soluble product of protein hydrolysis; peptones are not coagulated by heat, are not precipitated by saturated (NH4 )2 SO4 , but are precipitated by phosphotungstic acid. peptone water A MEDIUM containing 1–2% PEPTONE and 0.5% NaCl in water. peptone-water sugars (‘sugars’) A range of liquid media each of which consists of PEPTONE WATER to which has been added a particular carbohydrate (final concentration often ca. 1% w/v) and a pH indicator (e.g. ANDRADE’S INDICATOR); such media are used to determine which (if any) of a series of carbohydrates are metabolized by a given strain of microorganism. The carbohydrates used include e.g. arabinose, galactose, glucose, lactose, maltose, mannitol, mannose, rhamnose, sucrose, and xylose. Bacterial growth in a given peptone-water sugar may give rise to acid, or acid and gas, as breakdown product(s) of the carbohydrate; acid is detected by the pH indicator, and gas production is detected by a DURHAM TUBE. ‘Broth sugars’ are used in the same way as peptone-water sugars; in broth sugars the carbohydrate is dissolved in ‘broth’ (e.g. peptone and meat extract) rather than in peptone water alone. For nutritionally fastidious organisms, peptone-water sugars or broth sugars are supplemented with e.g. serum (‘serum sugars’ or ‘serum water sugars’), or with particular growth requirement(s), in order to permit growth to occur. Some bacteria (e.g. certain Bacillus spp) produce an alkaline reaction in conventional sugars owing to the formation of excess alkaline breakdown products from the peptone; since this would mask acid production from carbohydrates, such organisms are tested in e.g. inorganic salts–carbohydrate media such as AMMONIUM SALT SUGARS. peptone–yeast extract–glucose medium See PYG MEDIUM. peptonization The formation of PEPTONE from protein. Peptostreptococcus A genus of Gram-positive, asporogenous, anaerobic bacteria which can ferment amino acids or peptones and which may or may not ferment carbohydrates. Cells: cocci (ca. 1 µm diam.) which typically occur in chains. The organisms have been isolated e.g. from lesions in man, often as part of a mixed microflora. (See also GASTROINTESTINAL TRACT FLORA.) Species: P. anaerobius (the type species; GC%: ca. 33–34), P. micros (GC%: ca. 28), P. parvulus (GC%: ca. 44–45) and P. productus (GC%: ca. 44); it has been suggested that the saccharolytic species (P. parvulus and P. productus) be omitted from the genus [FEMS (1983) 17 197–200]. It has been proposed that organisms previously referred to as Gaffkya ACID STARTERS). PER-1 ESBLs See b-LACTAMASES. peracetic acid See HYDROGEN PEROXIDE (a). peracids See HYDROGEN PEROXIDE (a). Peranema A genus of colourless, heterotrophic (holozoic) EUGLENOID FLAGELLATES which ingest prey (unicellular algae and protozoa) by means of a specialized organelle (the pharyngeal rod organ) consisting of two parallel tapering rods (pharyngeal rods, trichites); essentially, ingestion of prey involves protrusion of the pharyngeal rods which adhere to the prey and then retract, the prey eventually being engulfed in a food vacuole at a cytostome situated subapically. (The rods can also be used to puncture or cut larger prey, allowing access to the cell contents.) Peranema has a characteristic smooth, gliding type of motility. One of the emergent flagella is extended forwards and held straight, movement being limited to a coiling or flickering of the tip, while a second is folded back along the cell and adheres to the pellicle. Genera closely related to Peranema include Entosiphon and HETERONEMA. perborate See HYDROGEN PEROXIDE (a). perchlorate degradation See BIOREMEDIATION. percolating filter See SEWAGE TREATMENT. percurrent proliferation In conidiogenesis (see CONIDIUM): the development, by the conidiogenous cell, of a new conidiumforming apex which grows through the previous apex; percurrent proliferation occurs e.g. in annellidic development (see CONIDIUM) and in the formation of new conidiogenous ampullae in Gonatobotryum. perfect stage (mycol.) See TELEOMORPH. perforins See APOPTOSIS. perfringocins See BACTERIOCIN. perfringolysin See THIOL-ACTIVATED CYTOLYSINS. peribacteroid membrane See ROOT NODULES. pericentric (of a chromosome) On both sides of, or including, the CENTROMERE; e.g., a pericentric inversion is a CHROMOSOME ABERRATION in which the inverted segment contains the centromere. (cf. PARACENTRIC.) pericentriolar structure See CENTRIOLE. Perichaena See MYXOMYCETES. periclinal Parallel to a given surface. (cf. ANTICLINAL.) Periconia See HYPHOMYCETES. peridermioid Refers to a peridiate, typically tongue-shaped (flattened) aecium of the type formed e.g. by species of Cronartium and Pucciniastrum. Peridermium See UREDINIOMYCETES. peridiate Having a PERIDIUM. peridinin See CAROTENOIDS. Peridinium See DINOFLAGELLATES. peridiole See NIDULARIALES and PISOLITHUS. peridium The outer enveloping wall which encloses the spores, or the spore-bearing structures, in the fruiting bodies of various fungi (e.g. GASTEROMYCETES) and acellular slime moulds (MYXOMYCETES). (See also e.g. CLEISTOTHECIUM and PERITHECIUM.) Périgord truffle See TRUFFLES. perihepatitis Inflammation of the peritoneal coat of the liver; it can result from localized infection, trauma, or systemic inflammatory disease. [Review of chlamydial perihepatitis: BMB (1983) 39 159–162.] (See also CURTIS–FITZ-HUGH SYNDROME.) 568 Peritrichia perilemma In many oligotrich ciliates: a membrane external to the pellicle. perimycin (syn. fungimycin) A PEROSAMINE-containing heptaene POLYENE ANTIBIOTIC produced by a strain of Streptomyces coelicolor. perinuclear space See NUCLEUS. periodic acid–Schiff reaction See PAS REACTION. periodontitis An acute or chronic inflammation of toothsupporting tissues. The periodontium includes the periodontal ligament: a cup-like mass of fibrous connective tissue which binds the root of the tooth to the tooth socket in the jaw bone; the term ‘periodontium’ may also include the cementum (a bone-like substance which invests the root of the tooth), the free gingiva (see GINGIVITIS), and the alveolar bone (i.e. that part of the jaw bone which forms the tooth socket that binds the periodontal ligament. Periodontitis is promoted by the accumulation of DENTAL PLAQUE in or near the gingival sulcus: the shallow groove (1–2 mm deep) at the tooth–gum junction; under these conditions the gum may shrink away from the tooth (the gingival sulcus enlarging to form a periodontal pocket) – thus exposing the cementum and the periodontal ligament to microbial attack. In periodontal disease there is an increased flow of CREVICULAR FLUID. Various bacterial species have been causally connected with periodontitis, but the aetiology of the disease is not yet fully understood; the human oral microflora has not yet been fully characterized, and it is possible that currently uncultured organisms may contribute to periodontitis. In one study, samples of healthy and periodontic material were examined with the object of investigating (from PCR-determined sequence data) whether any specific types of organism correlate with disease; one organism, whose data suggested a close relationship to asaccharolytic Eubacterium spp, was found only in deep pockets in periodontitis patients, possibly indicating an association with advanced periodontitis [JCM (1999) 37 1469–1473]. Bacteria which have been aetiologically linked to periodontitis include species of BACTEROIDES (e.g. B. gingivalis) and FUSOBACTERIUM; Actinobacillus actinomycetemcomitans and Capnocytophaga sp have been implicated in ‘juvenile periodontitis’. (See also CENTIPEDA.) Multiplex PCR has been used for the simultaneous detection of Bacteroides forsythus and Prevotella intermedia in 152 clinical samples, including 38 specimens of plaque [JCM (1999) 37 1621–1624]. peripheral membrane cylinder See ASCUS. peripheral membrane protein See CYTOPLASMIC MEMBRANE. periphyses (sing. periphysis) Short, sterile filaments which form a lining in the ostiolar canal in a perithecium or pseudoperithecium. periphyton community Syn. AUFWUCHS. periplasm Syn. PERIPLASMIC REGION. periplasmic binding protein Syn. BINDING PROTEIN (sense 1). periplasmic fibrils See SPIROCHAETALES. periplasmic flagella See SPIROCHAETALES. periplasmic gel See PEPTIDOGLYCAN. periplasmic region (periplasmic space; periplasm) In Grampositive bacteria and archaeans: the region between the CYTOPLASMIC MEMBRANE and the CELL WALL; in Gram-negative bacteria: the region between the cytoplasmic membrane and the OUTER MEMBRANE – or, according to some authors, between the cytoplasmic membrane and the layer of PEPTIDOGLYCAN. Certain enzymes and other proteins are located partially or entirely in the periplasmic region and may be released from the cell e.g. by OSMOTIC SHOCK. DEPENDENT TRANSPORT SYSTEM (See also BINDING PROTEINand EXTRACYTOPLASMIC OXIDA- TION.) periplasmic space Syn. PERIPLASMIC REGION. periplast The cell envelope, or particular layer(s) of the cell envelope, in certain algae and protozoa; see e.g. PELLICLE (sense 3) and CHOANOFLAGELLIDA. Peripylaria A category (‘section’) of LEISHMANIA species which multiply in the anterior part of the hindgut of the arthropod vector, and then migrate to the pharynx, cibarium and proboscis; it includes e.g. L. braziliensis and ‘L. tarentolae’. (cf. HYPOPYLARIA, SUPRAPYLARIA.) periseptal annulus In Gram-negative bacteria: a continuous circumferential ‘fusion’ between the OUTER MEMBRANE and the cytoplasmic membrane, one annulus occurring on each side of the site of cell division – thus delimiting a small periplasmic compartment at the site of future septum formation. [PNAS (1983) 80 1372–1376.] (cf. ADHESION SITE.) peristome (1) The buccal cavity, or the extreme distal part of the buccal cavity, together with the somatic region adjacent to it; the term is used primarily for the appropriate parts in peritrichs and in certain spirotrichs, but it is sometimes used for the convoluted distal region of some chonotrichs. (cf. INFUNDIBULUM.) (2) Syn. BUCCAL CAVITY. perithecioid Having the general characteristics of a PERITHECIUM. perithecioid pseudothecium See ASCOSTROMA. perithecium A hollow, spheroidal or flask-shaped ASCOCARP (ca. 100–300 µm) which opens to the exterior via an OSTIOLE; asci develop inside the perithecium, commonly in the form of a hymenial layer or as a basal tuft. According to species, perithecia may develop on the surface of a substratum or fungal STROMA, or embedded within a fungal stroma, and the wall (peridium) of a perithecium may be membranous or brittle, brightly coloured or dark. Some perithecia contain (in addition to asci) sterile structures such as paraphyses. (For ascospore discharge see ASCOSPORE.) Perithecia are formed e.g. by members of the SORDARIALES and SPHAERIALES. (cf. ASCOSTROMA (pseudoperithecium).) peritonitis Localized or generalized inflammation of the peritoneum (the membrane which lines the abdominal cavity and covers the viscera). peritrich A member of the PERITRICHIA. Peritrichia A subclass of freshwater, estuarine and marine ciliate protozoa (class OLIGOHYMENOPHOREA); one order: Peritrichida. In mature peritrichs, the somatic ciliature is limited or absent, but the oral ciliature is well developed and includes rows of cilia which – looking into the buccal cavity – spiral downwards towards the cytostome in an anticlockwise (counterclockwise) direction. According to species, individual cells may be shaped like an egg, bell or bottle etc; in some species the cell (zooid ) is attached to the substratum either directly or by means of a contractile or non-contractile stalk. Some species are loricate, and some form colonies. STOMATOGENESIS is buccokinetal. Asexual reproduction in sedentary species may occur e.g. by budding with the formation of a motile, ciliated ‘larva’ (telotroch). Predominantly sedentary peritrichs are placed in the suborder Sessilina (genera: e.g. Apiosoma, Campanella, CARCHESIUM, Ellobiophrya, Epistylis, Lagenophrys, OPERCULARIA, Rhabdostyla, Scyphidia, Vaginicola, VORTICELLA, Zoothamnium). The mobile peritrichs (all ecto- or endoparasitic, occasionally pathogenic, in freshwater or marine vertebrates and invertebrates) are placed in the suborder Mobilina (genera: e.g. Leiotrocha, Trichodina, Vauchomia). 569 Peritrichida peroxidase–antiperoxidase technique See PAP TECHNIQUE. peroxin See PEX MUTATION. peroxynitrite See NITRIC OXIDE. peroxisome A type of MICROBODY found in many kinds of eukaryotic cell, including certain fungi, protozoa and algae. Peroxisomes contain a range of enzymes – including e.g. peroxidase, CATALASE, D-amino acid oxidase, dehydrogenase and acyl transferase. Each peroxisome is typically rounded, with a granular matrix and commonly an electron-dense core (D nucleoid); it is bounded by a single trilaminar membrane ¾4.5–8 nm in thickness. Peroxisomes apparently develop by incorporating their component proteins (synthesized on free polysomes) and eventually dividing into daughter peroxisomes; this contrasts with the earlier view of biogenesis which supposed that peroxisomes budded from the endoplasmic reticulum. In animal-type cells, peroxisomes carry out a range of functions which (depending on type of cell) include respiration, oxidation of alcohol and fatty acids, synthesis of plasmalogens, and purine metabolism; in plant/algal cells, functions include GLUCONEOGENESIS and PHOTORESPIRATION. In methylotrophic yeasts (see METHYLOTROPHY) the peroxisomes, which contain methanol oxidase, occur in greater number and size when the yeasts are grown on methanol (rather than e.g. glucose). [Peroxisomes in methylotrophic yeasts: AMP (1983) 24 1–82.] (See also HYDROCARBONS.) [Recent developments in peroxisome biology: Endeavour (1996) (new series) 20 68–73.] [Peroxisome membrane proteins (role in biogenesis and function of the peroxisome): FEMS Reviews (2000) 24 291–301.] persistence (virol.) The phenomenon in which a virus remains within a cell, organism or population [Book ref. 90] for a prolonged period of time, during which it may or may not replicate to produce infectious progeny virions; if the virus persists in cryptic form, without production of infectious virions and without causing symptoms in the host, it is said to be latent. (Some authors reserve the term ‘persistence’ for those cases in which infectious virions are continually produced [Book ref. 148, p. 161].) In a host organism, persistence (sensu lato) may be established directly or may follow an acute phase of disease. Various categories of persistent infection can be distinguished. (a) Infectious virions are not produced and infection is asymptomatic (‘latent infection’); in some cases the virus may, under certain conditions, be ‘reactivated’ to produce infectious virions and cause sporadic recurrences of illness (see e.g. HERPES ZOSTER). (b) Infectious virions are not produced, but disease may nevertheless eventually occur (see e.g. SUBACUTE SCLEROSING PANENCEPHALITIS). (c) Infectious virions are continually produced but no symptoms are apparent – resulting in a CARRIER (sense 1). (See also LCM VIRUS.) (d) Infectious virions are continually produced, and ongoing chronic disease results. Viral strategies for persistence in cells/hosts include e.g. integration of viral nucleic acid into host chromosomal DNA (as in the RETROVIRIDAE), or maintenance of viral nucleic acid in a stable extrachromosomal form (see e.g. EPSTEIN–BARR VIRUS); in some cases impaired immunological responses or immunological tolerance may be important factors. In cell cultures, persistence may result from the formation of viral mutants or of DEFECTIVE INTERFERING PARTICLES. (See also CARRIER CULTURE.) persistent generalized lymphadenopathy See AIDS. persistent virus (1) See CIRCULATIVE TRANSMISSION. (2) A virus which can establish a persistent infection (see PERSISTENCE). pertactin A cell-surface adhesin found in species of Bordetella. Peritrichida See PERITRICHIA. peritrichous Refers to flagella (or e.g. fimbriae) which are distributed more or less uniformly over the cell surface. perkinetal (ciliate protozool.) Refers to the plane of cell division: perpendicular to the kineties. (cf. HOMOTHETOGENIC.) Perkinsasida A class of parasitic, homoxenous protozoa (phylum APICOMPLEXA) in which the form of the conoid is atypical; reproduction is asexual. One species: Perkinsus marinus (see PERKINSUS). Perkinsea A class of protozoa [JP (1980) 27 37–58] equivalent to the PERKINSASIDA. Perkinsus A genus of protozoa of the PERKINSASIDA [J. Parasitol. (1978) 64 549] formerly regarded as fungi of the genus Dermocystidium. P. marinum is a pathogen of oysters. permeaplast A cyanobacterial cell which has been treated with lysozyme and EDTA; permeaplasts have been used for the TRANSFORMATION of certain cyanobacteria. permease (porter; transporter) A TRANSPORT SYSTEM or the (nonenzymic) component of a multicomponent transport system that effects transmembrane translocation of the substrate. (See also ION TRANSPORT.) permissive cell (virol.) A cell which permits the replication of a given virus, with the consequent production of infectious progeny virions. (cf. ABORTIVE INFECTION.) permissive conditions See CONDITIONAL LETHAL MUTANT. Peronophythora See PERONOSPORALES. Peronosclerospora See PERONOSPORALES. Peronospora A genus of obligately plant parasitic fungi of the order PERONOSPORALES. Within the host plant, the (intercellular) mycelium gives rise to short club-shaped or branching filamentous haustoria. Conidia develop singly on curved, tapering and branching conidiophores which emerge from the stomata. On germination, the conidia form germ tubes – as do the (sexually-produced) oospores. P. destructor and P. parasitica cause DOWNY MILDEWS on onions and crucifers, respectively. P. tabacina causes the TOBACCO DISEASE blue mould – a disease which occurs worldwide; blue mould is encouraged by wet weather, and has been controlled e.g. by dithiocarbamates or metalaxyl. Peronosporales An order of aquatic and terrestrial fungi (class OOMYCETES) which include both saprotrophs and parasites – some of the latter being major plant pathogens (see e.g. DOWNY MILDEWS). (See also PHENYLAMIDE ANTIFUNGAL AGENTS.) The thallus is characteristically a well-developed, coenocytic, branching mycelium which, in parasitic species, may give rise to haustoria. Asexually-derived zoospores are formed by most species; they are typically produced in rounded sporangia which are borne either on undifferentiated hyphae or on distinct sporangiophores, according to species. In Bremia and Peronospora the asexually-derived spores are conidia (see CONIDIUM) rather than sporangiospores. Sexual reproduction is oogamous (see OOMYCETES). According to species, oogonia and antheridia are borne on the same or on different hyphae, and each may be uninucleate or multinucleate. Fertilization involves gametangial contact, the oosphere subsequently developing into a thick-walled oospore. Genera include ALBUGO, BASIDIOPHORA, BREMIA, Bremiella, Peronophythora, Peronosclerospora, PERONOSPORA, PHYTOPHTHORA, PLASMOPARA, Pseudoperonospora, PYTHIUM, Sclerophthora, SCLEROSPORA, and ZOOPHAGUS. perosamine An aminosugar (4-amino-4,6-dideoxy-D-mannose) found e.g. in PERIMYCIN. peroxidase An OXIDOREDUCTASE which catalyses a reaction in which the oxidation of a substrate is coupled with the reduction of hydrogen peroxide. See e.g. LIGNIN; cf. CATALASE. 570 petroleum perthophyte Syn. PERTHOTROPH. perthotroph (perthophyte) (1) A parasite which kills tissues within a living host, subsequently deriving nutrients from the dead tissues. (2) An organism which derives nutrients from dead tissues within a living host – whether or not it kills those tissues itself. (The term is usually applied to plant/microorganism interactions.) (cf. BIOTROPH; NECROTROPH.) Pertusaria A genus of crustose LICHENS (order PERTUSARIALES); photobiont: a green alga. Apothecia frequently occur several together in warts on the thallus; they are initially peritheciumlike with a pore-like opening, but in some species they subsequently open out to form flat or concave discs. In P. pertusa, the periphery of the thallus is surrounded by a white hypothallus; the apothecia do not open out as they mature. In P. amara, the thallus bears scattered white soralia (0.5–1.5 mm diam.) which have a bitter taste due to the presence of picrolichenic acid; apothecia are rare, but when present they are solitary and the mature discs are open and sorediate. Species occur e.g. on bark. Pertusariales An order of fungi of the ASCOMYCOTINA; members include crustose LICHENS (photobiont commonly green), and saprotrophic and lichenicolus fungi. Ascocarp: APOTHECIOID, sometimes lirelliform. Asci: bitunicate. Genera: e.g. OCHROLECHIA, PERTUSARIA. pertussigen Syn. PERTUSSIS TOXIN. pertussis Syn. WHOOPING COUGH. pertussis toxin (pertussigen; islet-activating protein; histaminesensitizing factor; leucocytosis-promoting factor) An exotoxin, produced by Bordetella pertussis, which is one of the virulence factors associated with WHOOPING COUGH (see also CYCLOLYSIN, DERMONECROTIC TOXIN, TRACHEAL CYTOTOXIN). The structure of pertussis toxin (PT) resembles that of e.g. CHOLERA TOXIN and SHIGA TOXIN: all of these toxins have an AB5 arrangement of subunits in which five B subunits form a pentameric ring, and the A subunit occupies a central position to one side of the plane of the ring. The B subunits of PT are not identical; some of them have binding sites for specific receptor molecules on eukaryotic target cells. The A subunit is a 26 kDa protein with ADP-ribosyl transferase activity. Within the target cell it ADP-ribosylates a particular heterotrimeric G protein, Gi (D Ni ), which normally inhibits the activity of ADENYLATE CYCLASE; ADP-ribosylation inhibits Gi , leading to an increase in the intracellular concentration of cyclic AMP (cAMP). (cf. CHOLERA TOXIN.) B. parapertussis apparently contains a silent (i.e. nonexpressed) gene for pertussis toxin. Pesotum See CERATOCYSTIS and HYPHOMYCETES. Pestalotia See MELANCONIALES. peste des petits ruminants (syn. pseudorinderpest) A disease of sheep and goats which occurs in W. Africa. Symptoms include fever, nasal and ocular discharge, and necrotic stomatitis, followed by severe enteritis and pneumonia; mortality rates may be >90%. The causal agent is a virus (the ‘Kata virus’) of the genus MORBILLIVIRUS. pesticin I A BACTERIOCIN produced by strains of Yersinia pestis; it is active against Y. pseudotuberculosis and strains of Escherichia coli. Production of pesticin I correlates with the production of a fibrinolytic factor and a coagulase. pestis minor A mild form of PLAGUE in which buboes are formed without systemic symptoms. Pestivirus A genus of viruses (family TOGAVIRIDAE) which infect pigs (SWINE FEVER virus) and ruminants (BOVINE VIRUS DIARRHOEA/MUCOSAL DISEASE (BVD-MD) virus, and the closely related BORDER DISEASE virus). The viruses do not replicate in invertebrates and are not vector-borne. Pestiviruses can be transmitted transplacentally, and may cause abortion or malformation of the fetus. petal breaking Syn. COLOUR BREAKING. petechia See PURPURA. Petersenia See LAGENIDIALES and ALGAL DISEASES. petite mutant In certain yeasts (including e.g. Saccharomyces cerevisiae): a mutant strain characterized by the formation of abnormally small colonies; petite mutants are respirationdeficient owing to functionally defective mitochondria (see MITOCHONDRION), and they exhibit a low growth rate since they are restricted to the less efficient fermentative mode of metabolism. In nuclear petite (D segregational petite) mutants the mutant characteristic is due to mutation(s) in the nuclear (chromosomal) DNA which affect mitochondrial function; such mutations arise with frequencies similar to those of other chromosomal mutations, and they can be induced by mutagenic agents such as ultraviolet radiation. Crosses between nuclear petites and wildtype strains yield petite and wild-type progeny in the ratio 1:1. In cytoplasmic petite mutants the mutant characteristic is due to mutation(s) in the mitochondrial DNA. Neutral petite (D rho0 petite) mutants are completely devoid of mtDNA; they are the main or sole types of petite mutant which arise in the presence of high concentrations of INTERCALATING AGENTS. In crosses between neutral petites and wild-type strains the rho0 genotype is recessive. In suppressive petite (D rho
petite) mutants the mtDNA has suffered deletion(s) such that, in general, much less than 50% of the wild-type mtDNA remains. In crosses between rho
and wild-type strains, some of the progeny have only the petite genotype, the wild-type genotype having been lost – an effect called suppression; in highly suppressive petites, loss of wild-type mtDNA is apparently due to the preferential replication of petite mtDNA – possibly owing to higher concentrations of origin-like sequences in the petite mtDNA. (cf. POKY MUTANT.) Petractis See GYALECTALES. Petri dish A (usually) round, shallow, flat-bottomed, glass or plastic dish (often ca. 10 cm diam.) with a vertical side, together with a similar, slightly larger structure which forms a looselyfitting lid. Petri dishes are used in microbiology e.g. for the preparation of PLATES. (See also VENT.) Petriellidium See PSEUDALLESCHERIA. Petroderma See PHAEOPHYTA. petroleum Crude oil, i.e., oil in the form obtained from oil deposits within the earth. (See also KEROGEN.) Petroleum is an extremely complex mixture of components, including aliphatic, alicyclic and aromatic hydrocarbons together with e.g. various heterocyclic compounds. It can be converted into various ‘refined’ products – used e.g. as fuels for internal combustion engines – which are still mostly complex mixtures (e.g. kerosene may contain 5000–10000 different compounds). Many of the components of petroleum and its products can be degraded by microorganisms (see e.g. HYDROCARBONS). Contamination of petroleum-derived fuels and oils by certain microorganisms can be a serious economic problem, resulting in e.g. corrosion of fuel tanks, blockage of fuel pipes and filters, alteration of chemical and functional properties of lubrication and cutting oils, etc. Microbial growth requires the presence of at least some water (which may result e.g. from condensation), 571 Petunia vein-clearing virus restriction endonucleases (see DNA FINGERPRINTING). According to some authors, PFGE is the ‘gold standard’ among molecular typing methods. [Guidelines for typing bacterial strains with PFGE: JCM (1995) 33 2233–2239.] PFGE has been used e.g. for subtyping strains of the Penner HS11 serotype of Campylobacter jejuni [JMM (1998) 47 353–357], isolates of Shigella dysenteriae type 1 [JMM (1999) 48 781–784], and strains of Serratia marcescens [JCM (1997) 35 325–327] and Vibrio parahaemolyticus [JCM (1999) 37 2473–2478]. CHEF has been used e.g. for typing strains of Neisseria gonorrhoea [JMM (1993) 38 366–370]. [Comparison of PFGE with five other molecular methods for typing methicillin-resistant strains of Staphylococcus aureus: JMM (1998) 47 341–351.] pFM739 See INCOMPATIBILITY. pfmdr1 See AMINOQUINOLINES. pfu PLAQUE-FORMING UNIT. Pfu DNA polymerase See DNA POLYMERASE. PG (1) PROSTAGLANDIN. (2) PEPTIDOGLYCAN. PGD2 See PROSTAGLANDINS. PGE2 See PROSTAGLANDINS. PGF2a See PROSTAGLANDINS. Pgh1 See AMINOQUINOLINES. PGI2 See PROSTACYCLIN. pGKl plasmids See KILLER PLASMIDS. PGL (persistent generalized lymphadenopathy) See AIDS. pH indicator A substance which changes colour over a particular pH range(s): see e.g. ANDRADE’S INDICATOR; BROMCRESOL GREEN; BROMCRESOL PURPLE; BROMPHENOL BLUE; BROMTHYMOL BLUE; CHLORPHENOL RED; CONGO RED; CRESOL RED; LITMUS; METHYL ORANGE; METHYL RED; NEUTRAL RED; PHENOL RED; PHENOLPHTHALEIN; THYMOL BLUE; THYMOLPHTHALEIN. (See also e.g. LITMUS MILK; MACCONKEY’S AGAR; METHYL RED TEST.) pH-stat See CONTINUOUS CULTURE. PHA (1) PHYTOHAEMAGGLUTININ. (2) Passive haemagglutination (see PASSIVE AGGLUTINATION). (3) Poly-b-hydroxyalkanoate: see POLY-b-HYDROXYBUTYRATE. Phacidium See HELOTIALES. Phacus A genus of EUGLENOID FLAGELLATES in which the cell has a rigid pellicle and is usually flattened with a posterior point, often appearing somewhat leaf-shaped. Phaeodactylum A genus of pennate DIATOMS. P. tricornutum is polymorphic, occurring in three main morphotypes: oval, fusiform, and triradiate (cells with three corners – often extended into arms); the latter two forms may occur in chains, and the fusiform type is the most stable of the three. Daughter cells are (initially) of the same morphotype as the parent cell. The cell wall is almost entirely organic, the only siliceous structures being the gridle bands (up to 10); one valve is partially silicified in the oval morphotype. Phaeodarea See RADIOLARIA. phaeodium See RADIOLARIA. Phaeographis See GRAPHIDALES. phaeohyphomycosis Any of a range of human and animal diseases caused by any of several black moulds (e.g. Dactylaria gallopava, Drechslera spp, Exophiala spp, Wangiella dermatitidis, Xylohypha bantiana (formerly Cladosporium bantianum)) which form dark-walled septate mycelium in the tissues (cf. CHROMOBLASTOMYCOSIS). Such diseases generally affect only debilitated or immunosuppressed individuals; they usually involve the formation of chronic, progressive, subcutaneous abscesses or warty lesions in various parts of the body. The disease may be systemic, particularly when caused by X. bantiana (which tends growth occurring in the water or at the oil–water or fuel–water interface. Organisms commonly involved include hydrocarbonutilizing microbes such as Hormoconis resinae (formerly Cladosporium resinae) (particularly in aviation kerosene), strains of Aspergillus and Pseudomonas, etc. However, not all spoilage organisms are necessarily hydrocarbon utilizers: some may obtain nutrients from solutes in the aqueous phase, from impurities in the fuel, etc. (See also BIOBOR and EGME.) [Petroleum microbiology: Book ref. 155.] Petunia vein-clearing virus See CAULIMOVIRUSES. pex mutation Any mutation which results in a deficiency in PEROXISOME biogenesis; pex genes encode various peroxins, i.e. protein constituents of the peroxisome. Most peroxins so far identified are components of the peroxisome membrane. pexicyst An EXTRUSOME which occurs e.g. in Didinium; on discharge it adheres to the prey. Peziza A genus of fungi (order PEZIZALES) which form lightbrown, sessile, discoid or cup-shaped apothecia, often several centimetres across; the ascospores are typically biguttulate. Pezizales An order of fungi (subdivision ASCOMYCOTINA) which occur e.g. on wood, soil and dung. Ascocarp: typically APOTHECIOID (but closed and hypogean in some species), with paraphyses. Asci: unitunicate, cylindrical to globose, with a nonthickened apex which typically opens, at maturity, by a slit or operculum (see OPERCULATE ASCUS). Genera: e.g. ALEURIA, Ascobolus, Carbomyces, Gyromitra, HELVELLA, Monascus, MORCHELLA, PEZIZA, PYRONEMA, Sarcoscypha, SCUTELLINIA, TUBER, VERPA. Pezizella See HELOTIALES. PF4 (chemokine) See CHEMOKINES. Pf155 See PLASMODIUM. PFA PHOSPHONOFORMIC ACID. Pfeiffer phenomenon The rapid lysis of Vibrio cholerae by specific antibody and COMPLEMENT. Pfeifferinella See EIMERIORINA. Pfeiffer’s bacillus Haemophilus influenzae. Pfeiffer’s disease INFECTIOUS MONONUCLEOSIS. PFGE Pulsed-field gel electrophoresis: any of several forms of gel ELECTROPHORESIS used for separating large molecules of nucleic acid (up to ¾106 base pairs in length). Essentially, electric fields are applied to the gel alternately from different angles so that the net migration of a given molecule depends on the speed with which it can change its direction of movement; this separates the nucleic acid molecules according to size. The range of sizes of molecule which can be separated is determined by the pulse length: short pulses are used for separating the (relatively) smaller molecules, while longer pulses are used for separating the larger molecules. (It may also be possible to choose a pulse length which includes plasmids in the assay; such a pulse length may exclude the largest chromosomal fragments.) Following PFGE, bands of fragments may be detected in the gel e.g. by staining with ethidium bromide. Various forms of PFGE differ e.g. in the number, type and location of electrodes; in some forms, electrodes can periodically switch polarity. In contour-clamped homogeneous electric field electrophoresis (CHEF), electrophoresis is carried out in a hexagonal chamber in which electrodes are located on sides 2, 3, 5 and 6; this procedure is reported to be reproducible and highly discriminatory. (See also OFAGE.) PFGE is used e.g. in TYPING. For example, this approach is used for separating large fragments of DNA produced when bacterial chromosomes are cleaved with so-called ‘rare-cutting’ 572 phagocytosis of the grid is inoculated with one drop of a phage suspension (at ROUTINE TEST DILUTION) – each square being inoculated with a different phage; the plate is then incubated. Each of the phages used for typing is lytic for one strain, or a limited number of strains, of the species under test; a phage which lyses the particular strain being tested will form a clear macroscopic area amid the opaque layer of bacterial growth. The test strain can then be defined in terms of the phages to which it is sensitive. (The results of phage typing may be affected if the bacterium under test carries a PLASMID encoding a RESTRICTION ENDONUCLEASE which destroys phage nucleic acid.) Strains of Salmonella typhi which bear the VI ANTIGEN may be typed by a range of phages each of which is specific for one or more of the Vi strains of S. typhi ; this procedure is often termed Vi phage typing. phagemid A CLONING vector which includes (i) a plasmid’s origin of replication; (ii) a multiple cloning site (MCS), i.e. a POLYLINKER; (iii) a marker gene (e.g. a gene encoding resistance to kanamycin); and (iv) the origin of replication of a filamentous phage (e.g. that of the ssDNA phage f1). The DNA sample (to be cloned) is inserted via the MCS, and the phagemid is then inserted into a bacterium e.g. by transformation. A phagemid can be used e.g. for making double-stranded copies of the given fragment (i.e. used as a conventional cloning vector). It can also be used for making single-stranded copies if the phagemid-carrying bacteria are infected with a ‘helper phage’, e.g. phage M13, which promotes replication from the phage origin in the phagemid; the single-stranded copies are packaged into phage coat proteins (encoded by the helper phage) and exported to the medium. When freed from phage coat proteins, the single-stranded copies of the insert can be used e.g. for sequence analysis (see DNA SEQUENCING). A phagemid may contain a promoter on each strand, either side of the MCS, so that, if required, the cloned fragment can be transcribed. phagocyte Any cell which can carry out PHAGOCYTOSIS – particularly a neutrophil or macrophage. phagocytosis The process in which particulate matter is ingested, and usually digested, by certain types of cell or microorganism (cf. PINOCYTOSIS). In mammals, phagocytosis by certain leucocytes is an important aspect of the immune defence system, while in some microorganisms phagocytosis is a mode of feeding. (a) In mammals, phagocytosis is carried out e.g. by NEUTROPHILS and MACROPHAGES. Essentially, the particle (e.g. an opsonized bacterium) initially binds to the surface of the phagocyte; the phagocyte’s cytoplasmic membrane then invaginates to form a pocket which contains the particle. (Different intracellular signals may be generated in the phagocyte according to whether engulfment was triggered from antibody- or complement-binding sites.) Subsequently, the invaginated membrane forms a closed, intracellular sac or vacuole, the phagosome. (A phagosome which contains a parasite is called a parasitophorous vacuole.) The pH within the phagosome decreases, and the phagosome coalesces with one or more LYSOSOMES to form a phagolysosome (D lysophagosome) within which the bacterium etc. may be killed and degraded. The maturation of a phagosome into a phagolysosome can be monitored from changes in (i) its internal pH, and (ii) the associated proteins; for example, a phagolysosome is characterized by so-called lysosome-associated membrance proteins (LAMPs). Antimicrobial activity within phagocytes is promoted by (i) the wide range of lysosomal enzymes, and (ii) the intracellular formation of certain toxic derivatives of oxygen – see to localize in the brain, but which may also cause pulmonary infections) or by W. dermatitidis. Phaeophyceae The sole class within the PHAEOPHYTA. Phaeophyta (brown algae) A division of predominantly marine ALGAE (freshwater members including species of e.g. Bodanella and Heribaudiella). The thallus ranges from an adherent disc or crust to hollow bulbous or whip-like forms, branched filaments and giant complex forms – the latter reaching 50 metres or more in length and containing sieve tubes within which photosynthate can be translocated; there are no unicellular species. The CELL WALL contains cellulose and e.g. FUCOIDIN and ALGINATES. Within the CHLOROPLASTS the THYLAKOIDS characteristically occur in groups of three. Brown algae contain CHLOROPHYLLS a, c1 andc2 ; certain CAROTENOIDS (e.g. fucoxanthin); carbohydrates such as MANNITOL and LAMINARIN; sterols (e.g. fucosterol and cholesterol); and sometimes large amounts of TANNINS (‘fucosan’) stored in ‘fucosan vesicles’ (D physodes) in the cytoplasm. Sexual processes include isogamy and oogamy, and many species exhibit a heteromorphic or isomorphic ALTERNATION OF GENERATIONS; the biflagellate, pyriform gametes each have one anteriorly-directed tinsel FLAGELLUM, and one posteriorlydirected whiplash flagellum. Some species form PHEROMONES. Life cycles are often complex. Orders include [Book ref. 130]: Ascoseirales (e.g. Ascoseira); Chordariales (e.g. Chordaria, Cladosiphon, Halothrix ); Cutleriales (e.g. Cutleria); Desmarestiales (e.g. Desmarestia); Dictyosiphonales (e.g. Dictyosiphon, Striaria); Dictyotales (e.g. Dictyota, Zonaria); Durvillaeales (Durvillaea); Ectocarpales (e.g. Ectocarpus, Petroderma, Ralfsia); Fucales (e.g. Ascophyllum, Cystoseira, FUCUS, Halidrys, Himanthalia, Myriodesma, Pelvetia, Sargassum, Turbinaria); Laminariales (e.g. Alaria, Chorda, Costaria, LAMINARIA, Macrocystis, Pelagophycus, Thalassiophyllum, UNDARIA); Scytosiphonales (e.g. Colpomenia, Hydroclathrus, Scytosiphon); Sphacelariales (e.g. Cladostephus, Halopteris, Sphacelaria); Sporochnales (e.g. Sporochnus); Tilopteridaceae (e.g. Haplospora, Tilopteris). [Evolution of the Phaeophyta (particularly Fucales): Book ref. 167, pp. 11–46.] phaeophytin A derivative of CHLOROPHYLL in which Mg2C is replaced by two protons. (cf. BACTERIOPHAEOPHYTIN.) phaeosporae See SACCARDOAN SYSTEM. Phaffia A genus of imperfect yeasts (class HYPHOMYCETES). The cells are ellipsoidal, occurring singly, in pairs, or occasionally in short chains; they reproduce by budding. Spheroidal chlamydospores containing refractile granules are formed e.g. after several days in malt extract. Colonies on e.g. malt agar are orange-red or salmon-pink due to the synthesis of carotenoid pigments, predominantly astaxanthin (3,30 -dihydroxy-4,40 -diketo-bcarotene, a red pigment common among animals – particularly crustaceans, echinoderms and tunicates – but rare in plants). Glucose and certain other sugars are fermented. Neither inositol nor NO3
is assimilated. One species: P. rhodozyma, isolated from tree exudates. [Book ref. 100, pp. 890–892.] phage Abbreviation for BACTERIOPHAGE. (For ‘phage G4’, ‘phage l’ etc see entries BACTERIOPHAGE G4, BACTERIOPHAGE l etc.) phage conversion Syn. BACTERIOPHAGE CONVERSION. phage typing A form of TYPING in which strains of bacteria are distinguished on the basis of differences in their susceptibilities to a range of BACTERIOPHAGES. Initially, a FLOOD PLATE is prepared from a culture of the strain under test; the surface of the plate is then dried (by leaving the plate in a 37° C incubator, with the lid partly off, for 10–20 minutes) and a grid is drawn on the base of the plate. The agar surface corresponding to each square 573 phagolysosome and (species formed during the ‘oxidative burst’ (D ‘respiratory burst’) characteristic of some types of phagocyte); such derivatives are formed e.g. by neutrophils and macrophages. In NEUTROPHILS (q.v.), the oxygen derivatives are generated within 30–60 seconds of cell-surface stimulation. Ferric ions bound to lysosomal LACTOFERRIN may serve to promote the formation of hydroxyl radical from hydrogen peroxide and superoxide. During the oxidative burst, the consumption of oxygen by the phagocyte increases sharply. (See also CHEMILUMINESCENCE.) Reactive oxygen derivatives are also released to the extracellular environment. Some bacteria can resist ingestion by phagocytes as a consequence of their capsular or cell-wall components. For example, the capsule of Bacillus anthracis is anti-phagocytic (cf. STERNE STRAIN). Again, the PROTEIN A cell-wall component of Staphylococcus aureus may serve to inhibit opsonization by binding the Fc portion of antibodies. Some pathogens can kill phagocytes: see e.g. APOPTOSIS and LEUCOCIDINS. Certain pathogens (e.g. Coxiella, Leishmania spp, Listeria monocytogenes and Mycobacterium lepraemurium) can survive or grow within phagosomes, or phagolysosomes, avoiding destruction by various mechanisms. For example, in the case of Chlamydia spp, Legionella pneumophila and Mycobacterium tuberculosis the phagosome does not acidify and does not fuse with the lysosome. Under certain circumstances, some pathogens (e.g. Mycobacterium leprae, Rickettsia mooseri, Trypanosoma cruzi ) can escape from the phagosome and grow within the cytoplasm of the phagocyte. (See also REOVIRUS.) Phagosomes containing live Toxoplasma gondii resist fusion with lysosomes, and T. gondii may also fail to trigger the phagocyte’s oxidative mechanism and may inbibit development of the normal acidification of the phagosome. M. leprae apparently fails to stimulate the generation of superoxide anion by the phagocyte. In some cases a pathogen is not taken up and killed because it has actually invaded the phagocyte. Invasion may involve uptake via adhesins and receptors which differ from those involved in routine phagocytosis. The initial contact between pathogen and phagocyte may be important in determining the pathogen’s fate; for example, of the various modes of binding, some may favour survival by failing to trigger an oxidative burst within the phagocyte. In the case of Legionella pneumophila, the phagosome in a macrophage maintains near-neutral pH, and the pathogen is able to multiply, killing the phagocyte. In this organism the icm (intracellular multiplication) genes – also called dot (defect in organelle trafficking) genes – are believed to encode a secretory system (and associated effector molecules) involved e.g. in blocking phagosome–lysosome fusion; mutations in these genes have been found to affect the pathogen’s ability to multiply within, and kill, macrophages [see e.g. TIM (1998) 6 253–255]. It has been reported that the zinc metalloprotease secreted by L. pneumophila inhibits (i) the oxidative burst, and (ii) chemotaxis in phagocytes [JMM (2001) 50 517–525]. In the case of Mycobacterium tuberculosis, invasion/survival may be influenced by (i) the cell-surface chemistry of the given strain; (ii) the site of infection (e.g. lung); and (iii) the timing of pathogen–phagocyte contact (e.g. at initial infection, or later). The opsonization of M. tuberculosis may occur via any of four pathways of COMPLEMENT FIXATION (q.v.); however, nonopsonic (complement- and antibody-independent) binding of the pathogen to the phagocyte may be important for initial infection e.g. SUPEROXIDE, HYDROGEN PEROXIDE, SINGLET OXYGEN of the normal lung. [M. tuberculosis–phagocyte interactions: TIM (1998) 6 328–335.] [Bacterial invasion of host cells: Book ref. 218, pp. 223–272.] (b) Among microorganisms, particulate food is ingested e.g. by many sarcodines (see also PSEUDOPODIUM), ciliates and flagellates; in (at least) most of the ciliates, the essential features of phagocytosis appear to resemble those described in (a) above: particle(s) are enclosed within an invagination of the cytoplasmic membrane which gives rise to a closed intracellular vesicle (FOOD VACUOLE). In many ciliates there is a single, specialized site for phagocytosis (see CYTOSTOME), while in suctorians ingestion may occur at the tip of each of the tentacles. phagolysosome See PHAGOCYTOSIS. Phagomyxa A genus of endoparasitic, phagocytic, plasmodial organisms. P. algarum occurs in the cells of marine algae; it appears to resemble members of the PLASMODIOPHOROMYCETES but apparently does not form cysts. phagosome See PHAGOCYTOSIS. phagotrophy A mode of nutrition in which nutrients are ingested in particulate form, as in some protozoa. (cf. OSMOTROPHY.) phagotype See VARIETY. phagovar See VARIETY. Phalacrocleptes A genus of protozoa (placed tentatively in the subclass SUCTORIA [Book ref. 135, p. 239]) which have short tentacles but which lack both cilia and infraciliature at all stages of the life cycle. The organisms are parasitic on polychaete annelids. phallacidin See PHALLOTOXINS. Phallales An order of humicolous and lignicolous fungi (class GASTEROMYCETES) which include both epigean and hypogean species. The immature basidiocarp (‘egg’) is rounded, ca. one to several centimetres in size, and consists essentially of a gleba enveloped in gelatinous material and enclosed within a membranous peridium. In some species the peridium is indehiscent, the basidiospores within being dispersed, when mature, on rupture of the peridium e.g. by burrowing animals. In other species (the ‘stinkhorns’) the ‘egg’ contains a mass of spongy tissue (the receptacle) which, during maturation of the basidiocarp, expands (rupturing the peridium) to form a typically hollow, columnar structure – also called the receptacle (or pseudostem) – which carries the hymenium-bearing gleba at its apex; autolysis of the glebal material gives rise to an often evil-smelling liquid which attracts certain insects, the latter dispersing the basidiospores. The stinkhorns include e.g. species of Clathrus, Mutinus and Phallus. phallin B See PHALLOTOXINS. phallisin See PHALLOTOXINS. phalloidin See PHALLOTOXINS. phalloin See PHALLOTOXINS. phallotoxins A group of cyclic heptapeptide toxins present in Amanita phalloides and e.g. in A. verna. Phallotoxins include e.g. phallacidin, phallin B, phallisin, phalloidin and phalloin; they generally contain unusual amino acids: e.g. phalloidin, a bicyclic heptapeptide, contains D-threonine, 4(cis)-hydroxyL-proline and g, d-dihydroxy-L-leucine, and is resistant to all proteases which have been tested. Phallotoxins produce clinical symptoms within a few hours of ingestion and are generally less toxic than AMATOXINS; symptoms of phallotoxin poisoning include weakness, severe vomiting and diarrhoea, and sometimes death. Degenerative changes occur in the cells of the liver. Phalloidin binds specifically and strongly to F-actin [binding site: EMBO (1985) 4 2815–2818], effectively accelerating Gto F-actin polymerization (see ACTIN) and increasing the stability HYDROXYL RADICAL 574 phase variation of actin filaments; consequently, cellular functions associated with the microfilament system (e.g. cytoplasmic streaming, cell motility) are irreversibly blocked. (See also MYCETISM.) Phallus See PHALLALES. Phanerochaete See APHYLLOPHORALES (Corticiaceae). phaneroplasmodium See MYXOMYCETES. pharyngeal basket Syn. CYRTOS. pharyngeal rod organ See PERANEMA. pharyngitis Inflammation of the pharynx. Acute pharyngitis is often caused by Streptococcus pyogenes; symptoms include e.g. sore throat, fever, pharyngeal exudate. Other causes include any of a range of viruses – e.g. adenovirus (see PHARYNGOCONJUNCTIVAL FEVER), COMMON COLD viruses, coxsackieviruses (see also HERPANGINA), EB virus (see INFECTIOUS MONONUCLEOSIS). pharyngoconjunctival fever A (usually mild) disease, affecting mainly children and young adults, caused by certain adenoviruses (see MASTADENOVIRUS). Symptoms: fever, inflammation of conjunctivae and/or pharynx, and enlargement of cervical lymph nodes. phase-contrast microscopy See MICROSCOPY (c). phase degradation Phase variation in BORDETELLA. phase-shift mutation Syn. FRAMESHIFT MUTATION. phase variation Spontaneous switching from the synthesis of a given cell-surface component to the synthesis of a different form of that component – or the starting, or stopping, of synthesis of such a component; each form of a given component depends on the expression of a specific allele, so that the number of different forms of that component depends on the allelic complement of the organism (cf. ANTIGENIC VARIATION). The term ‘phase variation’ was initially applied to the alternation of flagellar components in Salmonella, but is now applied more generally to a range of phenomena involving variation in cell surface components. Many of these phenomena are known to involve re-arrangements of DNA. Some well-studied examples of phase variation include the prototypical flagellar phase variation in Salmonella (see below); the on/off switching of type I FIMBRIAE (q.v.) in Escherichia coli; and the expression of OPA PROTEINS in Neisseria. Variation in the Vi antigen of Citrobacter freundii may be a further example. In most strains of Salmonella the filament of the FLAGELLUM contains either of two distinct types of FLAGELLIN encoded by genes H1 (D fliC) and H2 (D fljB); normally, only one of these genes is expressed at any given time, so that the flagellar filament contains either the H1 gene product or the H2 gene product. (Note that there are many different alleles of gene H1; the product of a given H1 allele – i.e. a particular phase 1 antigen (formerly called a ‘specific-phase antigen’) – is found only in certain serotypes of Salmonella. Gene H2 also occurs in a number of allelic forms. However, in many cases, a given product of gene H2 – i.e. a particular phase 2 antigen (formerly called a ‘group-phase antigen’) – occurs in a wide range of serotypes of Salmonella; for example, the phase 2 antigen ‘1’ occurs in many, if not most, of the serotypes. Some salmonellae, including S. typhi, do not have phase 2 antigens. (See also H ANTIGEN, KAUFFMANN–WHITE CLASSIFICATION and CRAIGIE’S TUBE METHOD.) One of the consequences of phase variation in Salmonella is the formation of flagella which may differ e.g. antigenically and/or in susceptibility to FLAGELLOTROPIC PHAGES. The switch from H1 to H2 gene expression is reversible and occurs at a frequency of about 10
5 to 10
3 per cell per generation. Switching involves the inversion of a specific, 995-bp sequence of nucleotides which includes the promoter of the H2 gene. When this sequence is in one orientation, the promoter can be used for transcription of the H2 gene – so that, in this case, the H2 gene product is incorporated into the flagellar filament. The H2 gene actually forms part of an operon that includes a gene (rh1 or rH1 ) whose product represses the H1 gene; thus, with the sequence orientated this way, the H2 product is formed and the H1 product is repressed. Inversion of the control sequence prevents transcription of H2 – and also prevents formation of the H1 repressor; hence, mRNA H2 H1 repressor H2 H1 repressor H1 mRNA H1 PHASE VARIATION: alternation of flagellar proteins in Salmonella (principle, diagrammatic; see entry). In most strains of Salmonella the flagellar filament contains either of two distinct types of protein, encoded by genes H1 and H2. Normally, only one of these genes is expressed at any given time, so that the flagellar filament contains either the H1 gene product or the H2 gene product. The promoter ( ) of the H2 operon is flanked by two short inverted repeats ( ) which are recognized by a site-specific recombinase; that is, the sequence containing the promoter can be inverted by site-specific recombination. Top. The position of the H2 promoter is such that the H2 operon can be transcribed. The H2 gene product is incorporated in the flagellar filament, and the H1 repressor protein blocks transcription of the H1 gene. Below. Site-specific recombination has occurred between the two inverted repeats, so that the sequence containing the H2 promoter has been inverted. In this new orientation the H2 promoter is no longer functional. Because the H2 operon is not transcribed, the H1 repressor is not formed, and this permits transcription of the H1 gene – so that the flagellar filament now contains the H1 gene product. Reproduced from Bacteria, 5th edition, Figure 8.3, page 168, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. ž 575 phaseollidin EXCLUSION.) When cultured in a fresh medium, F
phenocopies revert to the donor state. Studies on F
phenocopies reveal e.g. a low level of nicking at oriT in the late stationary phase – although the F pilus transfer apparatus appears to be maintained in the cell envelope even after cessation of transcription of the main transfer region [Microbiology (1998) 144 2579–2587]. phenogram Any dendrogram which expresses phenetic relationships. (cf. CLADOGRAM.) phenol (as a disinfectant) See PHENOLS. phenol coefficient A number which expresses the antimicrobial activity of a given phenolic DISINFECTANT relative to that of phenol under standard conditions; it can be determined by e.g. the RIDEAL–WALKER TEST or the CHICK–MARTIN TEST. phenol red A PH INDICATOR: pH 6.8–8.4 (yellow to red); pKa 7.9. phenolates (in iron uptake) See SIDEROPHORES. phenolic disinfectants See PHENOLS. phenolphthalein A PH INDICATOR: pH 8.3–10.0 (colourless to red); pKa 9.6. phenols (as ANTISEPTICS and/or DISINFECTANTS) Phenol and its derivatives (‘phenolics’ – cresols, xylenols etc) are microbistatic or microbicidal, depending on e.g. concentration and temperature; they may act e.g. by affecting bacterial membrane potentials or membrane permeability, or by coagulating the cytoplasm. The antimicrobial activity of phenolics increases with boiling point, but so too does their insolubility in water and (in general) their susceptibility to inactivation by organic matter; high-boilingpoint phenolics tend to be less toxic to tissues. Phenolic disinfectants typically have a high DILUTION COEFFICIENT, and they can be inactivated by non-ionic surfactants; those which are poorly soluble in water may be solubilized by alkali or prepared as suspensions with soaps. Phenol (carbolic acid, C6 H5 OH) is soluble in water and (under appropriate conditions) may be bactericidal, fungicidal and virucidal; its activity against bacterial endospores and acidfast bacteria tends to be very slow. It is toxic to tissues. Phenol (now produced mainly by chemical synthesis) is used e.g. as a preservative in animal glues. In general, alkyl- and halogen-substituted phenolics exhibit enhanced antimicrobial activity, tend to be less toxic to tissues, and are typically less water-soluble than phenol. In 4alkylphenols, activity increases with alkyl chain length up to 6 carbon atoms. Among halogenated phenols, 4-halophenols are the most active; in phenolics containing both alkyl and halogen substituents, activity is maximum when these are in the 2 and 4 positions, respectively. Nitrophenols tend to be more toxic than other phenolics to tissues. Polyhydric phenols generally have weak antimicrobial activity. (See also BISPHENOLS.) Lysol consists of 2-, 3- and 4-cresols (methylphenols) solubilized by soap. Aqueous Lysol (0.5%) kills many non-sporing pathogens in 15 min, but endospores may survive in 2% Lysol for several days. Amyl-3-cresol is used e.g. in antiseptic lozenges and mouthwashes. Sudol consists of xylenols (dimethylphenols) and ethylphenols; it is more active and less corrosive than Lysol and is active against e.g. staphylococci and tubercle bacilli. Aqueous Sudol (2%) can inactivate Clostridium perfringens endospores in about 4 hours, but inactivation of Bacillus subtilis endospores has been found to require 6 hours in a 66% solution. Hexylresorcinol (4-n-hexyl-1,3-dihydroxybenzene) has been used as a skin wound cleanser, a urinary antiseptic, and a constituent of throat lozenges. Menthol (3-methyl-6isopropylphenol) and carvacrol (2-methyl-5-isopropylphenol) with the sequence in this orientation the product of the H1 gene is incorporated into the flagellar filament. Inversion of the control sequence – in either direction – involves SITE-SPECIFIC RECOMBINATION between two short INVERTED REPEATS, one of which is located at each end of the control sequence. The recombinase which mediates inversion is encoded by gene hin (D vH2 or vh2 ) which, like the H2 promoter, occurs on the control sequence. The scheme for flagellar phase variation in Salmonella is shown diagrammatically in the figure. (See also RECOMBINATIONAL REGULATION.) phaseollidin An isoflavonoid PHYTOALEXIN produced by the French bean (Phaseolus vulgaris). phaseollin An isoflavonoid PHYTOALEXIN (7-hydroxy-30 ,40 -dimethylchromanocoumarin) produced by the French bean (Phaseolus vulgaris). It can be elicited e.g. by the polypeptide elicitor monilicolin A which can be obtained from culture filtrates of Monilinia fructicola. phaseolotoxin A chlorosis-inducing TOXIN produced by Pseudomonas syringae pv. phaseolicola; it irreversibly inactivates ornithine carbamoyltransferase, an enzyme which catalyses the formation of citrulline from ornithine and carbamoyl phosphate. The toxin–enzyme interaction involves the so-called ‘Kcat’ mechanism; in this mechanism the target molecule (enzyme in this case) activates the toxin, and the activated toxin then brings about specific alkylation of the enzyme. P. syringae pv. phaseolicola can produce a form of ornithine carbamoyltransferase which is insensitive to the toxin. Phaseolotoxin may also inhibit HYPERSENSITIVITY (sense 2) in the host plant. phasins See e.g. POLY-b-HYDROXYBUTYRATE. phasmid (1) A construct formed by recombination between e.g. a phage l genome and a plasmid containing the l att site and cos site. Integration of the plasmid into the phage genome is mediated (within a host bacterial cell) by the phage integrase site-specific recombination system (see BACTERIOPHAGE LAMBDA). The resulting genome–plasmid hybrid can be encapsidated during the assembly of phage virions. Phasmids are useful tools in genetic engineering since genes carried by the original plasmid can readily be introduced into other host cells by infection of those cells by phasmid-containing phage virions. (cf. COSMID.) (2) (biol.) A caudal chemoreceptor in nematodes of the class Phasmidia. PHB POLY-b-HYDROXYBUTYRATE. Phellinus See APHYLLOPHORALES (Hymenochaetaceae). phenanthridine An INTERCALATING AGENT which has trypanocidal and antitumour activity. Phenanthridine derivatives include e.g. ETHIDIUM BROMIDE. phenethicillin Syn. PHENOXYETHYLPENICILLIN. phenetic classification Any form of classification which is based on the observable (phenotypic and genotypic) characteristics of organisms – regardless of their ancestral lineage. (cf. PHYLOGENETIC CLASSIFICATION.) The relationship between two given organisms may be expressed as the proportion or percentage of similarities or differences on the basis of a certain minimum number of specified criteria (see e.g. NUMERICAL TAXONOMY). phenocopy A cell or organism whose phenotype has become modified in a way which mimics a phenotypic modification that is due to genetic change. For example, FC donors of Escherichia coli (see F PLASMID) may lose their donor characteristics under certain conditions – e.g. when grown to maximum density in aerated broth; such transfer-deficient cells mimic F
cells and are therefore called F
phenocopies. (See also SURFACE 576 pheromone are both used in antiseptic lozenges and mouthwashes. Thymol (5-methyl-2-isopropylphenol) is used as a disinfectant, as an antiseptic (e.g. in mouthwashes), and as a preservative (e.g. in urine samples pending biochemical tests). 2-Phenylphenol (2-hydroxydiphenyl) is primarily an antifungal agent but also has antibacterial activity; it has been used e.g. as a fungicide in the paper industry, as a preservative in waxed papers, and as a constituent of PINE DISINFECTANTS. Sodium-o-phenylphenate is used e.g. for the initial antifungal disinfection of stored fruits. 2-Benzyl-4-chlorophenol is active against Gram-positive and Gram-negative bacteria, protozoa and fungi; it is used e.g. in antiseptic soaps. Chlorocresol (4-chloro-3-methylphenol) is used e.g. as a preservative for pharmaceutical products and e.g. for glues and paints. It is also used as a biocide in a lowtemperature (98–100° C) STERILIZATION process. Chloroxylenols (e.g. 4-chloro-3,5-dimethylphenol) are used e.g. as topical antiseptics; they are active ingredients in Dettol. Pentachlorophenol is used e.g. as a fungicide in wood pulp and as a preservative for leather, paper and textiles. Picric acid (2,4,6-trinitrophenol), which has antibacterial and antifungal properties, is also used as a chemical FIXATIVE. 4-Nitrophenol is used as a preservative in the leather industry. (See also DINITROPHENOLS.) (See also BLACK FLUIDS; CREOSOTE; SALICYLIC ACID; WHITE FLUIDS.) phenon Any group established on the basis of PHENETIC CLASSIFICATION; in some cases a phenon may be considered to be more or less equivalent to a TAXON. phenosafranine A green redox dye – see REDOX POTENTIAL. phenotype The observable characteristics of an organism, either in total or with respect to one or more particular named characteristics. The phenotype of an organism is the manifestation of gene expression in that organism. (cf. GENOTYPE.) phenotypic adaptation See ADAPTATION. phenotypic lag A delay in the phenotypic expression of a new allele – acquired e.g. by mutation or by gene transfer (e.g. transduction) – due e.g. to SEGREGATION LAG. phenotypic mixing In a cell simultaneously infected by genetically different viruses: the formation of progeny virions which either incorporate the genome of one virus within the capsid of another (transcapsidation or genomic masking) or contain structural components derived from different ‘parent’ viruses. (See also PSEUDOTYPE.) phenotypic suppression The suppression of a mutant phenotype by non-genetic (environmental) factors. For example, during mRNA synthesis, 5-fluorouracil (5-FU) can be incorporated instead of uracil and can be misread as cytosine during subsequent translation; thus, e.g., the nonsense codon UAG may occasionally be misread as CAG (specifying glutamine), thus simulating a nonsense SUPPRESSOR MUTATION. The accuracy of translation may also be affected by alteration of ribosomes e.g. in the presence of STREPTOMYCIN; the resulting mis-reading of certain codons may lead to the suppression of certain mutations. phenoxetol See ALCOHOLS. phenoxyethanol See ALCOHOLS. b-phenoxyethyldimethyldodecylammonium bromide Domiphen bromide (see QUATERNARY AMMONIUM COMPOUNDS). phenoxyethylpenicillin (phenethicillin) A semisynthetic penicillin; R D C6 H5 .O.CH(CH3 ).CO (see b-LACTAM ANTIBIOTICS). Its activity is similar to that of benzylpenicillin, but it can be administered orally. phenoxymethylpenicillin (penicillin V) One of the first natural PENICILLINS to be produced (from P. chrysogenum); in phenoxymethylpenicillin R D C6 H5 .OCH2 .CO (see b-LACTAM ANTIBIOTICS). Its activity is similar to that of BENZYLPENICILLIN, but it can be administered orally. phenylalanine agar A medium (usually prepared as a slope) containing (g/100 ml water): yeast extract (0.3), L-phenylalanine (0.1), Na2 HPO4 (0.1), NaCl (0.5), agar (1.2). L-phenylalanine biosynthesis See AROMATIC AMINO ACID BIOSYNTHESIS. phenylalanine deaminase test A test used to determine the ability of an organism to deaminate phenylalanine to phenylpyruvic acid. In one form of the test, the organism is grown overnight on PHENYLALANINE AGAR, and 0.2 ml of 10% aqueous FeCl3 is added to the growth; phenylpyruvic acid, if present, gives a green coloration with the FeCl3 . Bacteria which usually give a positive test include e.g. Proteus vulgaris and Moraxella phenylpyruvica. phenylamide antifungal agents A group of ANTIFUNGAL AGENTS which includes the acylalanines benalaxyl, furalaxyl and metalaxyl, the butyrolactones milfuram and cyprofuram, and the oxazolidinone ring-containing oxadixyl. The phenylamides show in vitro activity against members of the Peronosporales and certain other fungi [Mycol. (1985) 77 424–432]. Metalaxyl (trade name: e.g. Ridomil) is widely used as an agricultural antifungal agent against a range of peronosporalean plant pathogens; it is a systemic fungicide which is most effective in young, actively growing foliage, and is often mixed with a contact fungicide such as copper, captan or mancozeb for maximum efficacy. phenylethanol See ALCOHOLS. phenylmercuric salts See MERCURY. phenylmethanol See ALCOHOLS. phenylpenicillin A semi-synthetic penicillin; R D C6 H5 .CO
(see b-LACTAM ANTIBIOTICS). It is less active than benzylpenicillin. 2-phenylphenol See PHENOLS. pheromone A type of molecule, released by an organism, which can cause one or more (non-harmful, non-nutritional) effects in another individual – typically an individual of the same species. (cf. ANTIBIOTIC; HORMONE). For example, a ‘sex pheromone’ can encourage sexual interaction between individuals e.g. by acting as a chemoattractant (see CHEMOTAXIS), by causing a TROPISM, and/or by promoting cell–cell adhesion (see below). Bacterial pheromones. Pheromones (small linear peptides) are secreted e.g. by those cells of Enterococcus faecalis that are potential recipients for certain plasmids; a given pheromone can act on other (potential donor) cells of E. faecalis that are carrying a particular CONJUGATIVE PLASMID – causing them to form a cell-surface adhesin called ‘aggregation substance’ which promotes donor–recipient adhesion. (Because culture filtrates of recipients also cause donor–donor aggregation, these pheromones have been called ‘clumping-inducing agents’.) On receipt of a plasmid (by CONJUGATION), a recipient stops secreting the corresponding pheromone (but may continue to secrete the pheromones corresponding to other plasmids). Each type of E. faecalis pheromone is designated according to the particular plasmid to which its function was originally linked; thus, e.g. pheromones cAD1 and cAM373 act on (donor) cells carrying the plasmids pAD1 and pAM373 respectively. [Review: CRM (1986) 13 309–334.] In Bacillus subtilis, pheromones are involved in the development of competence for transformation and in the initiation of endospore formation (see QUORUM SENSING). Fungal pheromones. Many fungi form sex pheromones. For example, in Allomyces spp, female gametangia and gametes release the pheromone sirenin (a sesquiterpene diol); male gametes take up and inactivate sirenin, and move chemotactically towards its source. 577 f In dioecious species of Achlya, the vegetative female thallus forms a steroid pheromone (antheridiol, ‘hormone A’) which stimulates the male thallus to form antheridial hyphae; the male thallus takes up, and inactivates, the pheromone and forms another steroid pheromone (‘hormone B’) which stimulates the female thallus to form oogonia. In some bipolarly heterothallic zygomycetes (e.g. strains of Blakeslea trispora and Mucor mucedo), development of sexual structures in the ‘plus’ and ‘minus’ mating types (see HETEROTHALLISM) is governed by trisporic acid derivatives (formerly referred to as GAMONE) – terpene carboxylic acids synthesized from b-carotene. The plus strains synthesize and secrete the pheromone prohormone P C (‘plus-progamone’), a methyldihydrotrisporate; minus strains synthesize and secrete a trisporol pheromone called prohormone P
(formerly ‘minusprogamone’). It appears that when each pheromone is taken up by strains of the opposite mating type it is converted to trisporic acid in situ. The trisporic acid promotes sexual development and increases prohormone synthesis. Pheromones produced by Saccharomyces cerevisiae include the a-factor and the a-factor (see MATING TYPE). The a-factor is a tridecapeptide (13 amino acid residues) synthesized as a highMWt precursor (which, interestingly, resembles the hormone precursors in higher eukaryotes); the precursor is sequentially cleaved by three proteinases: yscF, yscIV (formerly dipeptidylaminopeptidase A, or X-prolyldipeptidylaminopeptidase), and (apparently) ysca. The a-factor, an undecapeptide (11 residues), is apparently also synthesized in precursor form. [MS (1986) 3 107–114.] Algal pheromones. Pheromones are produced by various algae. In some cases a given pheromone can act on algae of different genera; for example, among brown algae, the pheromone ectocarpene (a cyclic hydrocarbon) can act on Ectocarpus and Sphacelaria, while multifidene is effective in Chorda and Cutleria. The pheromone desmarestene (structurally similar to ectocarpene) acts as an attractant to male gametes in the brown alga Cladostephus [Naturwissenschaften (1986) 73 99–100]. f Unwinding angle: see INTERCALATING AGENT. f6 phage group Syn. CYSTOVIRIDAE. phialide (phialid) See CONIDIUM. phialidic development See CONIDIUM. Phialoascus See ENDOMYCETALES. Phialophora A genus of fungi (class HYPHOMYCETES) which include plant-pathogenic species – e.g. P. cinerescens (causal agent of carnation wilt). (See also CHROMOBLASTOMYCOSIS.) [Species parasitic on bdelloid rotifers: Mycol. (1984) 76 1107–1110.] The mycelium is septate, and the conidia (produced from phialides) are non-septate, colourless or pigmented. (For P. dermatitidis see WANGIELLA.) phialopore See VOLVOX. Phialospora See DOTHIDEALES. Philadelphia chromosome An abnormal human chromosome 22 associated with chronic myelogenous leukaemia: see ABL. Philaster See SCUTICOCILIATIDA. Philomiragia bacterium A non-motile bacterium pathogenic in muskrats. It was formerly called Yersinia philomiragia, but has been excluded from Yersinia on the basis of DNA homology studies [Book ref. 22, p. 506]. pHisoHex See BISPHENOLS. fX phage group Syn. MICROVIRIDAE. Phlebia See APHYLLOPHORALES (Corticiaceae). Phlebotomus A genus of blood-sucking sandflies which includes the vectors of certain diseases. Phlebovirus (sandfly fever group) A genus of viruses of the BUNYAVIRIDAE. Host range: various vertebrates; vectors: primarily phlebotomine sandflies. The genus appears to include only one serological group with >30 viruses (including e.g. Punta Toro virus, RIFT VALLEY FEVER virus, Rio Grande virus, Sicilian sandfly fever virus). MWts of L, M and S RNAs: ca. 2.6–2.8, 1.8–2.2, and 0.7–0.8 ð 106 , respectively; MWts of proteins L, G1, G2 and N: ca. 145–200, 55–70, 50–60, and 20–30 ð 103 , respectively. The phlebovirus M RNA appears to resemble that of the bunyaviruses (see BUNYAVIRIDAE) in its genetic organization; however, the S RNA of Punta Toro virus appears to be an AMBISENSE RNA, encoding the NSs protein on a viral-sense subgenomic mRNA and the N protein on a viral-complementary subgenomic mRNA. Phleogena See AURICULARIALES. phlobaphenes See TANNINS. phloem necrosis (of coffee) A disease of the coffee plant (e.g. Coffea liberica, C. arabica) caused by Phytomonas sp. Symptoms: leaves become yellow and fall prematurely, and CALLOSE is deposited in the sieve tubes. Only plants older than ca. two years are affected. Vectors: unknown, but heteropterans are suspected. phloeophagous ‘Bark-eating’ (see e.g. AMBROSIA FUNGI). Phlogiotis See TREMELLALES. phlogistic Able to cause inflammation. phloretin See PHLORIDZIN. phlorhizin Syn. PHLORIDZIN. phloridzin (phlorhizin) A phenolic glycoside formed in certain tissues in apple trees (Malus spp) and in other members of the Rosaceae. The aglycone part of phloridzin, phloretin (b(4-hydroxyphenyl)-propionophlorophenone), can be liberated by b-glycosidases and oxidized by polyphenol oxidase to yield polymers that are toxic to e.g. Venturia inaequalis. PHLS Public Health Laboratory Service, 61 Colindale Avenue, London NW9, United Kingdom. pho regulon (phosphate regulon) In Escherichia coli: a REGULON comprising the pho genes: phosphate-controlled genes whose activity depends on the level of available Pi (inorganic orthophosphate). The cell’s response to Pi starvation is complex and not fully understood, but the following model has been suggested for regulation of the pho genes. Regulation is exerted partly through the products of genes phoR and phoB. PhoR and PhoB are associated in a TWOCOMPONENT REGULATORY SYSTEM in which PhoR is a sensor protein in the cytoplasmic membrane, and PhoB acts as a regulator protein. With low levels of phosphate, PhoR undergoes ATP-dependent autophosphorylation and transfers phosphate to PhoB. PhoB¾P behaves as a transcriptional activator by binding to the so-called phosphate box: a nucleotide sequence associated with the promoters of e.g. the phoA, phoE and phoS genes; PhoA is a periplasmic alkaline PHOSPHATASE (which releases Pi from organic phosphate esters), PhoE is a PORIN (for phosphate uptake), and PhoS is a Pi-binding protein. If phosphate levels rise, PhoB¾P is de-phosphorylated by the joint activity of PhoR and the product of gene phoU; this switches off the pho regulon. Regulation of pho genes appears also to involve element(s) of the pst (phosphate-specific transport) operon because phosphatase (PhoA) is produced constitutively in pst mutants. phoA gene See PHO REGULON. phoB gene See PHO REGULON. phobic response (shock movement; shock reaction; avoiding reaction) A transient behavioural response of a motile organism to a noxious stimulus, where the stimulus is an abrupt change in an external parameter (e.g., a light/dark boundary). The nature of 578 phosphorescence is normally present in raw (i.e., untreated) milk, is inactivated by PASTEURIZATION – so that the test determines the efficiency of the pasteurization process. Milk containing alkaline-buffered disodium p-nitrophenyl phosphate is incubated at 37° C for 2 hours and then examined (e.g. by colorimetry) for the presence or intensity of yellow coloration due to p-nitrophenol. A falsepositive result can be obtained if, subsequent to pasteurization, the milk has been contaminated with phosphatase-producing organisms. Enzymic activity may reappear (‘reactivation’) in e.g. cream which has been treated by HTST pasteurization; reactivation can be promoted by the addition of MgCl2 . phosphate box See PHO REGULON. phosphate-controlled genes See PHO REGULON. phosphate potential See CHEMIOSMOSIS. phosphate regulon See PHO REGULON. phosphatidases Syn. PHOSPHOLIPASES. phosphatidic acid See LECITHIN. phosphatidylcholine See LECITHIN. phosphatidylethanolamine A product of esterification of ethanolamine (NH2 (CH2 )2 OH) with phosphatidic acid (see LECITHIN); it occurs e.g. in the CYTOPLASMIC MEMBRANE in many bacteria. phosphoadenosine phosphosulphate See PAPS. phosphoenolpyruvate carboxykinase See Appendix II(b). phosphoenolpyruvate carboxylase See Appendix II(b). phosphoenolpyruvate carboxytransphosphorylase See Appendix II(b). phosphoenolpyruvate-dependent phosphotransferase system See PTS. phosphoenolpyruvate synthase See Appendix II(b). phosphofructokinase See EMBDEN–MEYERHOF–PARNAS PATHWAY. 6-phosphogluconate pathway May refer to either HETEROLACTIC FERMENTATION or the HEXOSE MONOPHOSPHATE PATHWAY. phosphoketolase See HETEROLACTIC FERMENTATION. phospholipases (phosphatidases) Enzymes that hydrolyse phospholipids, releasing fatty acids or other groups (cf. LIPASES). Phospholipases A1 and A2 release fatty acids from, respectively, positions 1 and 2 of a phospholipid; phospholipases C and D release the base – in phosphorylated and unphosphorylated form, respectively – from e.g. phosphatidylcholines (lecithins), sphingomyelins etc. Phospholipases may act specifically (or preferentially) on particular phospholipids – see e.g. LECITHINASE and SPHINGOMYELINASE. phosphonoacetic acid (PAA) An ANTIVIRAL AGENT (used as disodium phosphonoacetate, D Fosfonet sodium) which is similar to, but more toxic than, PHOSPHONOFORMIC ACID. phosphonoformic acid (PFA) An ANTIVIRAL AGENT (used as trisodium phosphonoformate, NaO.CO.PO.(ONa)2 , D Foscarnet sodium) which acts by selectively inhibiting the DNA polymerases of a number of viruses (and the reverse transcriptase of retroviruses) – apparently by acting as an analogue of pyrophosphate. It is effective e.g. in the topical treatment of herpesvirus infections. phosphonomycin Syn. FOSFOMYCIN. phosphorelay See ENDOSPORE (sense 1). phosphorescence (1) The emission of light which occurs when certain substances absorb radiation; emission occurs as electrons excited to the triplet state (T1 ) are returning to the (unexcited) ground state (S0 ). The light emitted is of wavelength longer than that of the exciting radiation; emission typically lasts for 10
6 sec to 1 sec or longer. (cf. FLUORESCENCE.) (2) The term is sometimes applied, incorrectly, to BIOLUMINESCENCE. the response depends on the organism, but typically the organism stops and then either reverses or reorientates and proceeds in a new, randomly selected direction. If conditions in the new direction are not significantly better, the phobic response may be repeated until the organism eventually moves into a more favourable region – when smooth swimming tends to supervene; thus, repeated phobic responses may result in a net progress in the more beneficial direction (‘phobotaxis’). phobotaxis See PHOBIC RESPONSE. phoE gene See PHO REGULON and PORIN. Pholiota See AGARICALES (Strophariaceae). Phoma A genus of fungi (order SPHAEROPSIDALES) which include many plant pathogens (see e.g. BLACKLEG sense 2, and GANGRENE sense 2). Colourless, ovoid conidia develop singly on conidiophores borne in brown, spherical or spheroidal, thin-walled, papillate, uniloculate, ostiolate pycnidia immersed in the substratum. Phomopsis See SPHAEROPSIDALES. phoR gene See PHO REGULON. phorbol ester 12-O-tetradecanoylphorbol-13-acetate. (See also ZEBRA.) -phore A suffix which signifies ‘carrier of’ or ‘bearer of’ – e.g. SPOROPHORE, IONOPHORE. Phormidium A genus of cyanobacteria of the LPP GROUP. Species occur e.g. with other filamentous cyanobacteria (e.g. Lyngbya, Oscillatoria) in mats on sediments in marine lagoons, saltmarshes etc. (See also STROMATOLITE and EMULCYAN.) phoront See TOMITE. phorophyte The host plant of an epiphyte. phoS gene See PHO REGULON. phosphatase (orthophosphoric monoester phosphohydrolase) Any enzyme which hydrolyses esters of phosphoric acid; phosphatases are classified (according to their pH optima) as acid phosphatases (EC 3.1.3.2) or alkaline phosphatases (EC 3.1.3.1). (See also ENZYMES.) They are widely distributed among living organisms, including many bacteria. Staphylococcal phosphatases. An acid phosphatase (pH optimum 5.2) is produced by most or all coagulase-positive staphylococci and (generally in smaller amounts) by some coagulase-negative strains. The enzyme may be secreted into the medium or may be loosely or tightly cell-bound – the proportions of each depending on strain and medium composition. An alkaline phosphatase (pH optimum 10.8) is apparently cell-bound. Phosphatase production may be demonstrated by growing the test organism on a solid medium containing the sodium salt of phenolphthalein diphosphate. After incubation, the plate is exposed to gaseous ammonia; colonies of phosphatase-producing strains become deep pink owing to the presence of free PHENOLPHTHALEIN. (Coagulase-negative strains which initially appear phosphatase-negative may produce phosphatase on prolonged incubation.) Phosphatase may be assayed e.g. using the artificial substrate p-nitrophenyl phosphate; pnitrophenol (yellow at alkaline pH) is released by acid or alkaline phosphatases and can be assayed spectrophotometrically (at 400 nm). (See also XP.) Staphylococcal phosphatase production appears to have little or no significance in pathogenicity. [Staphylococcal phosphatases: Book ref. 44, pp. 775–780.] In e.g. Escherichia coli a periplasmic alkaline phosphatase is produced in response to inorganic phosphate starvation: see PHO REGULON. phosphatase test (for milk) A test used to detect the presence of alkaline PHOSPHATASE in pasteurized MILK; the enzyme, which 579 phosphoribulokinase under aerobic conditions; this effect appears to be due largely to the formation of SINGLET OXYGEN [JAB (1985) 58 391–400]. ETHIDIUM BROMIDE can cause a light-induced nicking of DNA. photoheterotroph A phototrophic HETEROTROPH. photoinduction and photoinhibition (in fungi) In many fungi, light is needed for, or encourages, the development of fruiting bodies or spores, or the formation of particular compounds (photoinduction); in other fungi light inhibits growth and/or sporulation (photoinhibition). Commonly, light in the blue-violet region of the spectrum is effective in photoinduction/photoinhibition. In some cases light may be needed only for the completion of a particular phase of development, e.g., the initiation of fruiting in Pyronema omphalodes, or the development of the pileus in Lentinus lepideus (cf. STAG’S HORN FUNGUS). Conidium formation is photoinducible in e.g. Trichoderma viride, and in Choanephora cucurbitarum it is stimulated by darkness preceded by a period of exposure to light. In some species of Coprinus fruiting bodies develop more rapidly in the light, i.e., light encourages fruiting but is not essential for it. In Phycomyces blakesleeanus carotenoid synthesis is stimulated by light, while some species of Fusarium and Verticillium which normally form negligible amounts of carotenoids form significant amounts of these compounds in the presence of light. Photoinhibition of growth and/or sporulation occurs e.g. in at least some species of Alternaria and Penicillium. In e.g. Alternaria cichorii, A. tomato and Botrytis cinerea, conidium formation is controlled by a photoreceptor system referred to as mycochrome. On exposure to blue light mycochrome adopts the MNUV form, in the presence of which conidia are not formed and conidiophores may revert to vegetative hyphae; on exposure to near-ultraviolet mycochrome adopts the MB form, in the presence of which conidia can be formed. In A. cichorii the blue-light inhibition of conidium formation can be reversed by a period of darkness [CJB (1986) 64 1016–1017]. photoinhibition See PHOTOINDUCTION AND PHOTOINHIBITION. photokinesis A KINESIS (sense 1 or 2) in which the stimulus is light intensity. photolithotroph See LITHOTROPH. photolyase In some bacteria (including Escherichia coli ): an enzyme which can repair thymine dimers in DNA damaged by ULTRAVIOLET RADIATION; the energy needed for repair is derived from (visible) light. In vitro studies on the repair of thymine dimers may be facilitated by using the reagent potassium permanganate (KMnO4 ) [NAR (1998) 26 3940–3943]. photomicrograph A photograph obtained by PHOTOMICROGRAPHY. photomicrography Photography of microscopic objects through a microscope; it may involve the attachment of a camera to a microscope, or the use of a photomicroscope: a microscope which incorporates a camera. (cf. MICROPHOTOGRAPHY.) photomicroscope See PHOTOMICROGRAPHY. photomorphogenesis The formation of new cells or tissues under the stimulus of light. photoorganotroph See ORGANOTROPH. photophobic response A light-induced PHOBIC RESPONSE. photophosphorylation See PHOTOSYNTHESIS and PURPLE MEMBRANE. photoreactivation (photorestoration) A mechanism of DNA REPAIR in which pyrimidine dimers (formed as a result of exposure to ULTRAVIOLET RADIATION) are cleaved by exposure of the cells to light of wavelength ca. 300–600 nm; in Escherichia coli the reaction is mediated by the product of the phr gene (DNA photolyase) which, when bound to a pyrimidine dimer, phosphoribulokinase See CALVIN CYCLE. phosphoroclastic split A reaction, analogous to hydrolysis, in which a molecule is cleaved with the addition of the components of phosphoric acid. The term is usually applied to the ‘pyruvic phosphoroclasm’ – the reaction pyruvate C H3 PO4 ! acetyl phosphate C formate (or CO2 and H2 ), a reaction which occurs e.g. in the MIXED ACID FERMENTATION and in the BUTYRIC ACID FERMENTATION, and which is also carried out by many SULPHATEREDUCING BACTERIA (including species of Desulfobacter, Desulfotomaculum and Desulfovibrio). However, the initial product of pyruvate cleavage is actually acetyl-CoA, acetyl phosphate being formed from acetyl-CoA by the action of phosphotransacetylase; acetyl phosphate is subsequently dephosphorylated in the presence of ADP to yield acetate and ATP (enzyme: acetokinase). phosphorylation potential See CHEMIOSMOSIS. phosphotransferase transport system See PTS. photic zone That zone of an aquatic habitat which is penetrable by sunlight and in which PHOTOSYNTHESIS can occur. In the upper part of the zone (euphotic zone) the amount of oxygen produced by photosynthesis exceeds that consumed by respiration (over a 24-hour period); the opposite situation occurs in the lower part of the zone (dysphotic zone), while between these zones (at the compensation level ) photosynthetic oxygen production and respiratory oxygen consumption are balanced. (cf. APHOTIC ZONE.) photoautotroph A phototrophic AUTOTROPH. Photobacterium A genus of Gram-negative bacteria of the family VIBRIONACEAE. Cells: straight rods, 0.8–1.3 ð 1.8–2.4 µm. Motile, with one to three unsheathed polar flagella. All species grow at 20° C but not at 40° C; P. phosphoreum and some strains of P. angustum can grow at 4° C. Growth requires NaC (opt. ca. 3% NaCl). Many strains are oxidase
ve. PHB accumulates in cells under certain conditions, but exogenous b-hydroxybutyrate is not utilized. Strains are sensitive to O/129. P. phosphoreum and P. leiognathi (formerly P. mandapamensis) exhibit bluegreen BIOLUMINESCENCE and occur in seawater, on and in marine animals (see also FISH SPOILAGE), and in specialized luminous organs in certain marine fish. (See also SUPEROXIDE DISMUTASE.) GC%: 40–44. Type species: P. phosphoreum. It has been suggested that the genus should include P. leiognathi and P. phosphoreum, and that Vibrio fischeri and V. logei should be transferred from the genus VIBRIO (q.v.) as P. fischeri and P. logei, respectively; it was also suggested that P. angustum is a biovar of P. leiognathi [SAAM (1985) 6 171–182]. photobiont A photosynthetic symbiont. (See also PHYCOBIONT.) photochromogen Any strain of MYCOBACTERIUM in which a period of exposure to light is necessary for the development of pigmentation. (cf. SCOTOCHROMOGEN; NON-PHOTOCHROMOGEN.) Photocyta (photocytes) A (suggested) URKINGDOM (sense 2) comprising the halobacteria (e.g. Halobacterium spp) and the EUBACTERIA, the proposed relatedness of these organisms being based on three-dimensional RIBOSOME structure [PNAS (1985) 82 3716–3720]; it has been proposed that the photocytes, together with the methanogens, gave rise to the Eocyta (see ARCHAEBACTERIA) and the eukaryotes [Nature (1986) 319 626]. photocytes See PHOTOCYTA. photodissociation Light-induced chemical dissociation (see e.g. CARBON MONOXIDE (b)). photodynamic effect Light-induced damage or death which occurs when some microorganisms (particularly bacteria) are stained with certain fluorescent dyes (e.g. ACRIDINE ORANGE, EOSIN, ROSE BENGAL) and subsequently exposed to strong light 580 photosynthesis photorestoration Syn. PHOTOREACTIVATION. photosynthate Any carbohydrate product of photosynthesis. photosynthesis In certain (photosynthetic) organisms: the process in which radiant energy is absorbed by specialized CHLOROPHYLL-containing pigment system(s) and is converted to forms of energy (including chemical energy) which can be used for metabolic and other purposes (cf. PURPLE MEMBRANE); radiation of wavelengths in the visible spectrum is used by plants, ALGAE and photosynthetic bacteria (including CYANOBACTERIA and members of the RHODOSPIRILLALES), while some photosynthetic bacteria can also use infrared radiation. Light reaction refers, collectively, to those photochemical events involved in the conversion of radiant energy; dark reaction (D light-independent reaction) generally refers, collectively, to those reactions in which photosynthetically derived chemical energy is used for the synthesis of carbohydrate(s). In all cases, the conversion of radiant energy occurs in a specialized ENERGY-TRANSDUCING MEMBRANE containing CHLOROPHYLLS or bacteriochlorophylls, accessory pigments, and components of ELECTRON TRANSPORT CHAIN(S). Within such a photosynthetic membrane there are well defined chlorophyllcontaining REACTION CENTRES towards which radiant energy is channelled via LIGHT-HARVESTING COMPLEXES. On excitation of a reaction centre with radiant energy, the mid-point REDOX POTENTIAL of the reaction centre chlorophyll changes from a high positive value to a high negative value, and electrons are consequently ejected (‘charge separation’) from the chlorophyll (the ‘primary electron donor’). The ejected electrons, by virtue of the high negative redox potential of their source, are associated with a certain amount of energy; these electrons can therefore pass down an electron transport chain in the direction of less negative (or more positive) potentials and, in doing so, can provide the energy for (a) the transmembrane pumping of protons (thereby contributing to a proton motive force – see CHEMIOSMOSIS) or (b) the reduction of pyridine nucleotides. The precise nature of photosynthetic energy conversion differs among the different groups of photosynthetic organisms. (a) Photosynthesis in bacteria of the Rhodospirillales (anoxygenic photosynthesis) occurs only under anaerobic conditions, and only one type of reaction centre appears to occur in a given species. In the ‘purple’ photosynthetic bacteria (RHODOSPIRILLINEAE), electrons which have been ejected from the bacteriochlorophyll follow a cyclic path: e.g. from the bacteriochlorophyll to BACTERIOPHAEOPHYTIN and thence – probably via QUINONES, b-and c-type cytochrome(s) – back to the reaction centre; this cyclic electron flow causes the transmembrane pumping of protons, i.e., it generates pmf. The pmf can be used, by means of membrane-bound PROTON ATPASES, for the synthesis of ATP (photophosphorylation). At least some of these organisms can obtain reducing power by REVERSE ELECTRON TRANSPORT; in this process, photosynthetically generated pmf is used to reduce NADC at a membrane-bound NADH dehydrogenase – electrons being obtained from an external electron donor (such as hydrogen or sulphide). NADPH is obtained via a TRANSHYDROGENASE. In the ‘green’ photosynthetic bacteria (CHLOROBIINEAE), the light-energized reaction centre reaches a much lower (more negative) potential (ca.
550 mV) than is the case in the purple bacteria, and – consequently – the ejected electrons are able to effect the reduction of NADC (the Em,7 of the NADC /NADH redox couple being
320 mV); the reduction of NADC involves a non-cyclic electron transport path which is believed to include iron–sulphur proteins. Electrons which are removed from the absorbs light energy and cleaves the bonds between the bases (‘monomerization’). [Action mechanism of E. coli DNA photolyase: JBC (1987) 262 478–485, 486–491, 492–498.] The phr gene product may also play a role in UvrABC-dependent repair of UV damage in the dark [JB (1985) 161 602–608]. Photoreactivation does not occur e.g. in Bacillus subtilis. photoreceptor An organelle or region specialized for receiving light stimuli: see e.g. EYESPOT (sense 1), STENTORIN and STENTOR. photoreduction In photosynthetic organisms (see PHOTOSYNTHESIS): any light-dependent reduction – e.g., the reduction of NADC , or of CO2 via the Calvin cycle; electron donors used in photoreductions include e.g. water and H2 S. In certain algae (e.g. Chlorella, Chlamydomonas, Scenedesmus) hydrogen can act as electron donor for the reduction of CO2 ; HYDROGENASE is involved, and the process can occur only after the algae have been incubated anaerobically with hydrogen, followed by illumination with low levels of light. photorespiration (C2 carbon oxidation cycle) In photosynthetic organisms which fix CO2 via the CALVIN CYCLE: a lightdependent process in which oxygen is consumed and CO2 is evolved, but which is distinct from true (energy-yielding) RESPIRATION; photorespiration occurs under conditions of relatively high O2 and low CO2 concentrations and high light intensities, and results from the ability of RIBULOSE 1,5BISPHOSPHATE CARBOXYLASE–OXYGENASE (RuBisCO) to act as an oxygenase under these conditions. RuBisCO catalyses the oxygenation and cleavage of ribulose 1,5-bisphosphate (RuBP) to 3-phosphoglycerate and 2-phosphoglycolate. (The dependence of photorespiration on light is thus explained by the need for photosynthesis to drive the CALVIN CYCLE and hence supply RuBP.) The 2-phosphoglycolate is dephosphorylated to glycolate which may be excreted – e.g. by certain microalgae and cyanobacteria (often resulting in relatively high levels of glycolate in aquatic environments) – and/or may be metabolized further: e.g. glycolate ! glyoxylate ! glycine ! serine ! hydroxypyruvate ! glycerate ! 3-phosphoglycerate (the ‘glycolate pathway’). The step glycolate ! glyoxylate is also an O2 -consuming step (catalysed by glycolate oxidase); H2 O2 generated during the reaction is rapidly degraded by catalase. (The glycolate ! glyoxylate conversion occurs in PEROXISOMES in photosynthetic eukaryotes.) The 3-phosphoglycerate generated by the glycolate pathway and by the original cleavage of RuBP by RuBisCO may then enter the Calvin cycle. An alternative pathway for glycolate metabolism in some organisms involves the oxidation of glycolate to glyoxylate which is then converted to glycerate via tartronic semialdehyde (TSA). In either pathway, CO2 is lost: in the glycolate pathway during the glycine ! serine reaction (during which two glycine molecules are converted to one of serine with loss of both CO2 and NH3 ), and in the TSA pathway during the formation of TSA from glyoxylate (2CHO.COOH ! CHO.CHOH.COOH C CO2 ). Photorespiration thus results in a net loss of photosynthetic productivity. Its physiological significance is still not understood. In green algae, photorespiration is suppressed by CO2 -concentrating mechanisms which become operative at low CO2 levels. [Review of photorespiration: ARPphys (1984) 35 415–442.] (cf. WARBURG EFFECT.) A process very similar to photorespiration (although independent of light) occurs in some chemolithoautotrophic bacteria (e.g. ‘Alcaligenes eutrophus’, Thiobacillus neapolitanus) which fix CO2 via the Calvin cycle; by analogy with photorespiration, this process has been called ‘chemorespiration’. [Book ref. 115, pp. 129–173.] 581 photosynthesis photosystem II photosystem I (PS-II) (PS-I) P*700 −1.0 iron-sulphur proteins P*680 E m(volts) −0.5 ferredoxin phaeophytin quinones 0 NADP + cytochrome b6 / f complex light plastocyanin P700 0.5 H2O 2e− 1.0 1 2 light P680 O2 + 2H+ PHOTOSYNTHESIS. Simplified ‘Z scheme’ for non-cyclic, oxygenic photosynthetic electron transport (scale approximate). Each photosystem (PS-I and PS-II) consists of a reaction centre together with its associated accessory pigments. P680 and P700 represent the chlorophylls in the reaction centres of PS-II and PS-I, respectively; PŁ680 and PŁ700 represent the excited state in P680 and P700 , respectively. The cytochrome b6 /f complex includes cyts b6 and f, quinone, and a Rieske iron–sulphur protein. reaction centre during non-cyclic electron flow must be replaced if photosynthesis is to continue; these electrons are obtained (via a pathway containing cytochrome c555 ) from certain substrates (e.g. sulphide). Water is never used as an electron donor, so that this type of photosynthesis is always anoxygenic. The green photosynthetic bacteria appear to be also capable of cyclic elctron flow – electrons returning to the reaction centre via a menaquinone and cytochrome c555 . In many photosynthetic bacteria the dark reaction involves the fixation of carbon dioxide via the CALVIN CYCLE; however, at least some of the green photosynthetic bacteria use the REDUCTIVE TRICARBOXYLIC ACID CYCLE for this purpose. Apart from photochemical reactions, most of the reactions which occur in photosynthetic bacteria are known to occur in nonphotosynthetic species; thus, e.g. the Calvin cycle occurs in chemolithotrophic bacteria. (b) Photosynthesis in algae and cyanobacteria. Photosynthesis in these organisms differs from that described in (a) in that: (i) it occurs aerobically; (ii) it involves a composite photosynthetic system (as shown in the figure); (iii) it involves non-cyclic electron flow in which the terminal electron donor, water, is oxidized to oxygen – such photosynthesis thus being oxygenic. [How does PSII split water? (review) TIBS (1996) 21 44–49.] (Some species of cyanobacteria, e.g. Oscillatoria limnetica, can also carry out DCMU-insensitive, anaerobic, anoxygenic photosynthesis in which H2 S is used as external electron donor.) (See also PHOTOREDUCTION.) Oxygenic photosynthesis in algae and cyanobacteria (and Prochloron) closely resembles that in higher green plants. The characteristic (oxygenic, non-cyclic) mode of photosynthesis in algae and cyanobacteria is generally depicted by the socalled Z scheme (see figure). In this scheme, the midpoint redox potential of chlorophyll in the reaction centre of photosystem II (PS-II) changes from ca. C1.0 volt to between ca.
550 and
600 mV when the reaction centre receives a photon of light; electrons which are ejected from PS-II (and which are replaced by the oxidation of water) flow ‘downhill’ (i.e., towards less negative or more positive potentials) via QUINONES (generally believed to include plastoquinones), the CYTOCHROME b6 /f complex, and PLASTOCYANIN, to the reaction centre of photosystem I (PS-I). This electron flow is associated with the development of a proton gradient across the thylakoid membrane. In chloroplasts the inner surface of the thylakoid membrane becomes positive with respect to the outer (stromal) surface; the associated pmf can be utilized by membrane-bound PROTON ATPASES for ATP synthesis (photophosphorylation) – the CF1 part of the ATPase, and (hence) ATP synthesis, occurring at the outer surface of the thyalkoid membrane. On excitation of PS-I by light, the midpoint redox potential of the reaction centre chlorophyll changes to a value which exceeds ca.
1.0 volt; electrons ejected from the reaction centre of PS-I can therefore flow downhill to the ferredoxin-NADPC reductase complex and can be used for the reduction of NADPC to NADPH. A revised eukaryotic Z scheme includes the reduction of NADP by PSII [TIBS (1996) 21 121–122]. 582 phycobilisomes Phragmobasidiomycetidae A subclass of fungi (class HYMENOMYCETES) characterized by the formation of phragmobasidia (see BASIDIUM). Orders [Book ref. 64, p. 189]: AURICULARIALES, SEPTOBASIDIALES, TREMELLALES. phragmobasidium See BASIDIUM. phragmoplast In e.g. some green algae (e.g. Chara): a plate which develops in the cleavage plane between two newly forming cells during the telophase stage of MITOSIS; it consists of microtubules of the persistent mid-region of the mitotic spindle together with vesicles (derived from dictyosomes) which collect between the microtubules. (cf. PHYCOPLAST.) phragmosporae See SACCARDOAN SYSTEM. phthalimide antifungal agents A group of agricultural nonsystemic ANTIFUNGAL AGENTS which includes CAPTAFOL, CAPTAN, FOLPET etc. phthalylsulphathiazole See SULPHONAMIDES. phthiocerols Complex waxes found in mycobacteria [JGM (1983) 129 859–863]. phthisis Syn. TUBERCULOSIS. phycobilin See PHYCOBILIPROTEINS. phycobiliproteins (biliproteins) Water-soluble pigments which occur in CYANOBACTERIA, in algae of the RHODOPHYTA, and in CRYPTOPHYTES. In cryptophytes the pigments occur between the thylakoids and are antigenically unrelated to the cyanobacterial and rhodophytan pigments (which occur bound to thylakoids – see PHYCOBILISOMES). A phycobiliprotein consists of a protein ‘monomer’ comprising two distinct polypeptide chains (a and b), each of which is linked covalently (via a thioether bond) to an open-chain tetrapyrrole chromophore (phycobilin or bilin). The monomers tend to form trimers or hexamers which are the structural units of phycobilisomes. Phycobiliproteins function as light-harvesting pigments for photosystem II (see PHOTOSYNTHESIS); there are three main classes: allophycocyanins (lmax 650–671 nm), phycocyanins (lmax 617–620 nm), and phycoerythrins (including phycoerythrocyanin: lmax 545–568 nm). Allophycocyanins and phycocyanins apparently occur in all cyanobacteria and red algae; phycoerythrins occur in most red algae and in many cyanobacteria, phycoerythrocyanin occurs in some cyanobacteria (e.g. Anabaena variabilis, ‘Mastigocladus laminosus’ ). Cryptophytes contain either phycocyanin or phycoerythrin, according to species. In cyanobacteria, the content of phycobiliproteins can be affected by many environmental factors, including temperature, CO2 concentration, nitrogen starvation, wavelength and intensity of light [Book ref. 76, pp. 182–188]; in addition to its function as a photopigment, phycocyanin serves as a nitrogen reserve in cyanobacteria, being degraded by a specific protease induced during nitrogen starvation both in vegetative cells and in differentiating HETEROCYSTS. [Structure and function of cyanobacterial phycobiliproteins: Book ref. 75, pp. 23–42; the phycocyanin operon and light regulation of its expression in Anabaena: EMBO (1987) 6 871–884.] phycobilisomes Structures, each ca. 20–70 nm across, which are attached in regular arrays to the surface of thylakoid membranes in CYANOBACTERIA (cf. GLOEOBACTER) and red algae (RHODOPHYTA). Phycobilisomes are composed mainly of PHYCOBILIPROTEINS together with ‘linker’ polypeptides; they vary in composition and morphology (being typically hemidiscoid or hemispherical) according e.g. to species. The (mainly nonpigmented) linker polypeptides seem to have many functions – e.g. to stabilize the specific structure of phycobilisomes, to modify the phycobiliproteins so as to promote unidirectional [Excited-state redox potentials in the Z scheme: TIBS (1985) 10 382–383.] In addition to non-cyclic electron flow, a pmf-generating cyclic flow of electrons can occur via PS-I and cytochromes b6 and f. There is also evidence for a cyclic flow of electrons around PS-II, possibly via cyt b559 . [Photosynthetic electron transfer: Book ref. 85, pp. 95–148.] In algae and cyanobacteria the dark reaction characteristically involves the CALVIN CYCLE. Inhibitors of the photosynthetic electron transport chain include e.g. dibromomethyl-isopropyl p-benzoquinone (DBMIB) and DCMU, both of which inhibit electron flow between PS-II and PS-I, and hydroxylamine, which blocks electron flow from water to the reaction centre of PS-II. Experimental electron donors to PS-II include e.g. benzidine and catechol, while electron acceptors for PS-II include e.g. ferricyanide; electron donors for PS-I include e.g. reduced phenylenediamines, while electron acceptors for PS-I include e.g. ferricyanide and METHYL VIOLOGEN. [Molecular biology of the photosynthetic apparatus: Book ref. 163.] See also CAROTENOID BAND SHIFT; ENHANCEMENT EFFECT; HILL REACTION; PHOTORESPIRATION; RED DROP; WARBURG EFFECT.) photosynthetic bacteria A term which usually refers to bacteria of the RHODOSPIRILLALES (cf. CYANOBACTERIA). photosystems I and II See PHOTOSYNTHESIS. phototaxis A TAXIS in which an organism moves, directly or indirectly, from a region of low light intensity to one of higher light intensity (positive phototaxis) or vice versa (negative phototaxis). (A phototactic organism may be positively phototactic at low light intensities but negatively phototactic at high light intensities.) In phototactic bacteria, changes in light intensity may affect the frequency of tumbling (in peritrichously flagellated organisms) and/or speed of swimming (cf. KINESIS sense 1). In purple photosynthetic bacteria, at least, phototaxis appears to be mediated by changes in proton motive force (cf. AEROTAXIS and CHEMOTAXIS) resulting from changes in the rate of energy conversion in the photosynthetic reaction centre. [Role of pmf in sensory transduction: ARM (1983) 37 551–573.] Halobacterium salinarium responds differently to light of different wavelengths. The cells of this species are bipolarly flagellated; they swim in the direction of the long axis of the cell, with spontaneous reversals in direction at intervals of ca. 10–50 sec. A sudden decrease in yellow-green light or a sudden increase in blue/UV light elicits an extra reversal in direction, while an increase in yellow-green light or a decrease in blue/UV light leads to a suppression of the spontaneous reversals. (See also SLOW-CYCLING RHODOPSIN.) Thus cells tend to acumulate in regions illuminated by yellowgreen or white light (areas optimum for photophosphorylation by the purple membrane) but avoid areas exposed to (possibly damaging) UV radiation [SEBS (1983) 36 207–222]. See also STENTOR. phototopotaxis A light-induced TOPOTAXIS. phototroph An organism which can use light (and, in some species, infrared radiation) as a primary source of energy for metabolism and growth. (See also PHOTOSYNTHESIS and PURPLE MEMBRANE.) Some phototrophs are facultative CHEMOTROPHS.) (See also LITHOTROPH and ORGANOTROPH.) phototropism See TROPISM (sense 1). phoU gene See PHO REGULON. phr gene See PHOTOREACTIVATION. Phragmidium See UREDINIOMYCETES. 583 phycobiont energy flow within the phycobilisomes and to the reaction centres, and to link phycobilisomes to the photosynthetic membranes. In general, each phycobilisome comprises two major structural domains: (i) a ‘core’ (consisting of two or three cylindrical elements), and (ii) five or six ‘rods’ (composed of stacked discs) that radiate from the core to form the hemidiscoid or hemispherical structure. The core is composed of allophycocyanin (APC) and is anchored to the thylakoid membrane by a linker polypeptide. Where the rods are adjacent to the core, they consists of discs of phycocyanin (PC), but discs of phycoerythrin (PE) and/or phycoerythrocyanin (PEC), when present, occur at the periphery. Absorbed light energy is transferred from the periphery to the core, and thence to chlorophyll a – thus: PE/PEC ! PC ! APC ! Chl a; this transfer is almost 100% efficient. [Structure of a ‘simple’ phycobilisome (in Synechococcus): Ann. Mic. (1983) 134 B 159–180. Phycobilisome structure: JB (1993) 175 575–576.] phycobiont An algal symbiont. In lichenology, ‘phycobiont’ commonly refers to the main algal or cyanobacterial partner in a lichen, although in the latter case the term ‘cyanobiont’ is more appropriate. The term photobiont is a useful non-specific term for the principal photosynthetic partner in a LICHEN. (See also PHYCOZOAN.) phycocyanins See PHYCOBILIPROTEINS. phycoerythrins See PHYCOBILIPROTEINS. phycoerythrocyanin See PHYCOBILIPROTEINS. phycology The study of ALGAE. Phycomyces See MUCORALES. phycomycetes See LOWER FUNGI. phycomycosis See ZYGOMYCOSIS and EQUINE PHYCOMYCOSIS. Phycopeltis A genus of algae closely related to TRENTEPOHLIA. The thallus is typically a circular monostromatic disc (ca. 1–3 mm diam.) of compacted, branched filaments. Species occur mainly as epiphytes, growing on – not beneath – the leaf cuticle (cf. CEPHALEUROS), and also as photobionts in certain (mainly tropical) lichens. phycoplast In many green algae: a plate which develops in the cleavage plane between two newly forming cells in species in which the mitotic spindle is non-persistent; it contains microtubules orientated perpendicular to the axis of the former spindle. (cf. PHRAGMOPLAST.) phycosymbiodeme See CEPHALODIUM. phycotoxin Any TOXIN produced by an alga (see e.g. BREVETOXINS, CIGUATERA, GONYAUTOXINS, PRYMNESIUM, SAXITOXIN, ULVA) or – traditionally – by a cyanobacterium (‘blue-green alga’: see e.g. ANABAENA, APHANIZOMENON, LYNGBYA, MICROCYSTIS, NODULARIA). phycovirus A VIRUS which infects one or more ALGAE. (cf. CYANOPHAGE.) Several polyhedral dsDNA viruses have been observed in green algae: e.g. a tailed polyhedral virus, resembling (at least superficially) a tailed bacteriophage, has been found in Uronema gigas [Virol. (1980) 100 156–165, 166–174], and various polyhedral dsDNA viruses have been observed in Chlorella-like algae, including the zoochlorellae of Paramecium bursaria and Hydra viridis [PNAS (1982) 79 3867–3871]. phycozoan A term proposed to refer to a ‘compound organism’ in which cells (or chloroplasts) of an alga (the phycobiont) live within the tissues or cells of an animal (the zoobiont: an invertebrate) [Book ref. 129, pp. 5–17]. (See e.g. ZOOCHLORELLAE and ZOOXANTHELLAE; cf. LICHEN.) phyletic classification Syn. PHYLOGENETIC CLASSIFICATION. Phyllactinia See ERYSIPHALES. Phyllobacterium A genus of Gram-negative bacteria of the RHIZOBIACEAE. Cells (in vitro) are straight rods which, in liquid media, form characteristic star-shaped clusters. Acid is formed from a wide range of carbohydrates. GC%: 59.6–61.3. Type species: P. myrsinacearum. P. myrsinacearum and P. rubiacearum occur as pleomorphic cells (rod-shaped, ellipsoidal or branched) within leaf nodules which they induce in certain tropical plants of the Rubiaceae (e.g. madder), Myrsinaceae, Myrtaceae and Dioscoreaceae (e.g. yam). They have not been shown to be capable of nitrogen fixation. [Book ref. 22, pp. 254–256.] phyllocladia See STEREOCAULON. phyllody (plant pathol.) A malformation in which normal floral components are replaced by leaf-like structures; it is a symptom of certain plant diseases: e.g. green ear disease of millet (Pennisetum typhoides) and other grasses caused by Sclerospora graminicola. Phyllopertha horticola EPV See ENTOMOPOXVIRINAE. phylloplane The surface(s) of a leaf. (cf. PHYLLOSPHERE.) [Book ref. 158.] Phylloporus See BOLETALES. phylloquinone See QUINONES. phyllosphere (1) Syn. PHYLLOPLANE. (2) The region immediately surrounding, and influenced by, a leaf. [Quantification of bacterial sugar consumption in the phyllosphere: PNAS (2001) 98 3446–3453.] (See also RHIZOSPHERE.) Phyllosticta See SPHAEROPSIDALES. phylogenetic classification (phyletic classification) Any form of classification in which the aim is to group organisms according to their ancestral lineage (i.e. evolutionary relationships). (cf. PHENETIC CLASSIFICATION.) phylogeny The range of developmental stages in the evolution of an organism. (cf. ONTOGENY.) phylotype A taxon which is defined solely by nucleic acid sequence data (no representative species of the taxon having been cultured). [Example of use: JCM (1999) 37 1469–1473.] phylum A taxonomic rank (see TAXONOMY and NOMENCLATURE). Physarales See MYXOMYCETES. Physarum A large genus of slime moulds (class MYXOMYCETES) which usually form sporangia (some species form plasmodiocarps). P. polycephalum is widely used in laboratory studies. It generally forms a robust, conspicuous yellow phaneroplasmodium in which a network of major and subsidiary ‘veins’ leads to the advancing front edge. The sporangia are stalked, greyish, and typically multilobed. Physcia A genus of small foliose LICHENS (order LECANORALES); photobiont: a green alga. The thallus generally consists of grey or greyish-brown, often pruinose, narrow branching lobes which are often radially orientated to form a rosette. Several species have cilia; some have soredia. Apothecia: lecanorine, with grey or blue-black, often pruinose discs. Ascospores: brown, oneseptate. Species are generally nitrophilous, occurring e.g. on bark and/or on rocks. physiological heterothallism See HETEROTHALLISM. physiological race (mycol.) Organisms which are morphologically similar to, but which differ in physiological (and/or other) characteristics from, other members of the same FORMA SPECIALIS. A given race may be designated by a number, letter etc (see e.g. NOMENCLATURE). physode See PHAEOPHYTA. phytoalexins Low-MWt antimicrobial compounds which are synthesized by, and accumulate in, higher plants exposed to 584 Phytophthora certain (pathogenic and non-pathogenic) microorganisms – or to heavy metals (e.g. copper, mercury), detergents or certain other chemicals, ultraviolet radiation or physical (e.g. freeze–thaw) damage. Phytoalexins are formed by many angiosperms (particularly by members of the Leguminosae and Solanaceae) and by some gymnosperms; they have not been detected in lower plants. Under pre-stimulus conditions, phytoalexins may be either absent from plant tissues or present in extremely low concentrations. The production of phytoalexins appears to be a local effect in the plant – their translocation has yet to be conclusively demonstrated; if phytoalexins are involved in a plant’s natural resistance to disease, their localized accumulation may be an important factor in such resistance. In some cases the ability to produce phytoalexin(s) appears to enable a plant to resist the development of disease following infection by a pathogen; thus, e.g. resistance of the French bean to Colletotrichum lindemuthianum seems to correlate with the accumulation of a phytoalexin, phaseollin. In other cases the accumulation of phytoalexins appears not to be the sole factor which determines the plant’s resistance to disease. Pathogens which are able successfully to invade a plant may (i) fail to induce the production of phytoalexins; (ii) be relatively insensitive to the phytoalexin(s) produced; or (iii) be able to convert a phytoalexin to an inactive form (see e.g. KIEVITONE and PISATIN). (See also HYPERSENSITIVITY sense 2.) Elicitors of phytoalexins, i.e. specific chemical entities which promote the synthesis of phytoalexins, include the polypeptide monilicolin A (which elicits PHASEOLLIN) and various polysaccharides (such as b-1,3- and b-1,6-glucans and CHITOSAN) present e.g. in fungal cell walls or culture filtrates. Physical damage to plant tissues may cause release of the plant’s own elicitors (constitutive or endogenous elicitors). [Phytoalexins and their elicitors: ARPphys. (1984) 35 243–275.] Phytoalexins are structurally/chemically diverse compounds; they include e.g. CAMALEXIN; CAPSIDIOL; DEOXYHEMIGOSSYPOL; FALCARINDIOL; GLYCEOLLIN; HIRCINOL; IPOMEAMARONE; KIEVITONE; LUBIMIN; MOMILACTONES (see also WL 28325); ORCHINOL; PHASEOLLIN; PISATIN; PTEROSTILBENE; RISHITIN; VIGNAFURAN; VINIFERINS; and WYERONE. (cf. AVENACIN; CAFFEIC ACID; CHLOROGENIC ACID; PATHOGENESISRELATED PROTEINS.) [Phytoalexins (structure, role etc.): ARPpath. (1999) 37 285–306.] phytobiont A plant symbiont – e.g. the plant partner in a MYCORRHIZA. phytoflagellates See PHYTOMASTIGOPHOREA. phytoflavin See FLAVODOXINS. phytoglycogen A type of GLYCOGEN-like polysaccharide found e.g. in certain algae. (cf. CYANOPHYCEAN STARCH.) phytohaemagglutinin (PHA) (1) A mitogenic LECTIN from the red kidney bean, Phaseolus vulgaris; it can agglutinate erythrocytes, and it binds to both B and T cells but is mitogenic mainly in T cells. PHA activity requires Ca2C and Mg2C and is inhibited e.g. by N-acetyl-D-galactosamine. (2) Any plant lectin which can agglutinate erythrocytes. phytohormones (plant hormones) Compounds which are produced by plants and which regulate the plants’ own metabolism, cell division, seed germination etc; they include ABSCISIC ACID, AUXINS, CYTOKININS, ETHYLENE and GIBBERELLINS. phytokinins See CYTOKININS. Phytomastigophorea (the phytoflagellates) A class of flagellated protozoa (subphylum MASTIGOPHORA) which either have chloroplasts or bear an obvious relationship to other members which have chloroplasts. Most phytoflagellates can also be regarded as algae and are alternatively classified in algal taxonomic schemes. Orders: Chloromonadida (the CHLOROMONADS); Chrysomonadida (the CHRYSOPHYTES); Cryptomonadida (the CRYPTOPHYTES); Dinoflagellida (the DINOFLAGELLATES); Euglenida (the EUGLENOID FLAGELLATES); Heterochlorida (unicellular, flagellate and/or amoeboid members of the XANTHOPHYCEAE); Prasinomonadida (cf. PRASINOPHYCEAE); Prymnesiida (D PRYMNESIOPHYCEAE); Silicoflagellida (the SILICOFLAGELLATES); Volvocida (including genera such as CHLAMYDOMONAS and VOLVOX). Phytomonas (1) A genus of protozoa (family TRYPANOSOMATIDAE) parasitic in certain plants – particularly lactiferous plants (e.g. Euphorbiaceae) but also e.g. coffee plants (see e.g. PHLOEM NECROSIS) and palms; they are transmitted by sap-sucking bugs. The organisms occur in the promastigote form and are usually ca. 20 µm in length; some species have been grown in vitro. [Plant diseases caused by Phytomonas: ARPpath. (1984) 22 115–132; cultivation of P. françai, a species associated with a root rot disease of cassava (Manihot esculenta): JP (1986) 33 511–513.] (2) Obsolete name for Xanthomonas. phytoncide (plant pathol.) A broad term for any substance which is produced (constitutively or inducibly) by a plant and which confers on that plant resistance to disease or infestation by killing or inhibiting the growth of a potential pathogen or parasite. phytone A proteolytic digest (by PAPAIN) of plant material (e.g. soybean meal), used in certain media (see e.g. TRYPTICASE–SOY AGAR). phytopathology Plant pathology. Phytophthora A genus of fungi (order PERONOSPORALES) which include some important phytopathogenic species. In many geographical regions, P. infestans (causal agent of LATE BLIGHT of potato) appears to overwinter as mycelium in infected tubers. In the spring, when the tubers produce shoots, the fungus grows into the developing shoots and subsequently gives rise to distinctive sporangiophores – bearing lemon-shaped sporangia – which emerge from the stomata of the leaves; the sporangia are dispersed e.g. by wind. A sporangium germinates to form either zoospores or a germ tube, depending on conditions. A zoospore subsequently encysts and later germinates, forming a germ tube; a germ tube may enter a host plant by means of an APPRESSORIUM or, directly, via a stoma. Within the new host a branching mycelium develops intercellularly, and elongated haustoria are formed; later, sporangiophores emerge from the stomata. Sexual reproduction occurs between strains of appropriate MATING TYPE. During gametangial development the oogonial initial penetrates and grows through the antheridium such that, when fully developed, the expanded, globose oogonium carries the antheridium as an encircling collar around its base; a fertilization tube develops between the two gametangia. The sexually-derived thick-walled oospore typically germinates to form a germ tube that terminates in a zoosporangium. Oospore formation is very rare in most geographical regions – though not in Mexico. Other pathogenic species include e.g. P. cactorum (see CROWN ROT); P. cinnamomi, a worldwide pathogen of various trees and other plants (see e.g. JARRAH DIEBACK); P. citricola (see BLEEDING CANKER); P. fragariae (see RED CORE); P. megasperma var. sojae (causal agent of root and stem rot of soybean) [review: Book ref. 58, pp. 19–30]; P. palmivora (see BLACK POD DISEASE); and P. parasitica (see GUMMOSIS). P. syringae causes e.g. storage rot of apples and pears. 585 phytoplankton olives and cucumbers – cf. SAUERKRAUT). [Lactic acid bacteria in vegetable fermentations: Food Mic. (1984) 1 303–313.] Fermentative yeasts may also be present; these may ferment any sugars remaining after the lactic acid bacteria have been inhibited by acidity. The combination of salt, anaerobiosis and low pH serves to discourage the growth of undesirable organisms. Microbial spoilage of pickled vegetables may be due e.g. to oxidative pectinolytic moulds and yeasts which cause softening of the vegetable tissue; these organisms form a surface film unless precautions are taken to exclude air. Oxidative yeasts may also metabolize lactic acid and hence reduce acidity, allowing the subsequent growth of other spoilage organisms. Spoilage bacteria (e.g. coliforms, clostridia) tend to grow when the pH is too high and/or the salt concentration too low; they can cause softening, off-flavours and odours, and/or the formation of gas pockets within the vegetable (called ‘bloater damage’ in cucumbers, ‘fisheye spoilage’ in olives). picodnaviruses Rejected name for the PARVOVIRIDAE. a-picolinic acid Pyridine 2-carboxylic acid, a toxin produced by Pyricularia oryzae. (See also BLAST DISEASE and PIRICULARIN.) Picornaviridae A family of small, ssRNA-containing VIRUSes in which the icosahedral virion (ca. 22–30 nm diam.) is naked and ether-resistant. Most picornaviruses have a narrow host range, and some are important pathogens of man and animals; transmission generally occurs mechanically. The family includes four genera: APHTHOVIRUS, CARDIOVIRUS, ENTEROVIRUS and RHINOVIRUS; within the genera, species are distinguished by their susceptibility to specific neutralizing antibodies. Some picornaviruses are currently not classified into genera; these include equine rhinovirus types 1 and 2, and insect picornaviruses such as cricket paralysis virus, Drosophila C virus and Gonometa virus. Possible picornaviruses include ‘unclassified small RNA viruses of invertebrates’: e.g., bee acute paralysis virus, bee slow paralysis virus, bee virus X, black queen cell virus, Drosophila P and A viruses, FLACHERIE virus and SACBROOD virus. [Review of small RNA viruses of insects: JGV (1985) 66 647–659.] Virion structure. The capsid comprises 60 subunits, a subunit consisting of one molecule each of the 4 major capsid polypeptides (VP1–VP4); portions of VP1, VP2 and VP3 are exposed at the virion surface, while VP4 is probably internal and may be associated with the RNA. [Virion structure in polioviruses: Science (1985) 229 1358–1365, and in rhinoviruses: Nature (1985) 317 145–153.] The RNA genome is a single molecule of linear positive-sense ssRNA (MWt ca. 2.5 ð 106 ) which is polyadenylated at its 30 end and covalently linked at its 50 end to a virus-encoded protein, VPg. In cardioviruses and aphthoviruses (but not in enteroviruses or rhinoviruses) the RNA contains a poly(C) tract near the 50 end, the length of which varies with strain. [Poly(C) secondary structure in aphthoviruses: JGV (1985) 66 1919–1929.] Replication cycle. Virus replication occurs in the host cell cytoplasm. Infection begins when a virion attaches to receptors in the host cell plasma membrane; in e.g. poliovirus type 1 and mouse Elberfeld virus, entry into the host cell apparently occurs by endocytosis, and uncoating apparently occurs in endosomes and/or lysosomes [JGV (1985) 66 483–492]. In polioviruses, translation of the viral RNA is initiated at one site near the 50 end of the RNA, resulting in the formation of a polyprotein; the polyprotein is initially cleaved, during translation, into three precursor proteins (P1, P2 and P3), each of which undergoes further cleavage. P1 gives rise to 1A, 1B, 1C and 1D (capsid polypeptides); P2 gives rise to 2A, 2B (a host-range determinant), and 2C (involved in RNA synthesis); P3 undergoes phytoplankton See PLANKTON. phytoplasma See MLOS. Phytoreovirus (plant reovirus subgroup 1) A genus of PLANT VIRUSES of the REOVIRIDAE. Genome: 12 linear dsRNA molecules. Replication occurs in cytoplasmic viroplasms; mRNA transcripts are capped. Transmission occurs propagatively via cicadellid leafhoppers; transovarial transmission occurs in the insect vector. The genus includes WOUND TUMOUR VIRUS (type species), RICE DWARF VIRUS, and RICE GALL DWARF VIRUS. [Review: Book ref. 83, pp. 505–563.] phytotoxin (1) A TOXIN produced by a microorganism and active against a plant or against plant cells/tissues. (2) A toxin produced by a plant. phytotron An apparatus consisting of a chamber within which plants etc can be grown under a variety of accurately controlled conditions of temperature, light intensity, humidity, etc. pI See ISOELECTRIC POINT. Pi Inorganic orthophosphate (PO4 3
) pi particle See PSEUDOCAEDIBACTER. p protein (Pi protein) See R6K PLASMID. pian Syn. YAWS. pian bois (forest yaws) A form of LEISHMANIASIS in which the pathogen spreads, typically via the lymphatic route, producing a chain of ulcers in the peripheral lymphatic system; pian bois occurs in South America. Pichia A large, apparently polyphyletic genus of yeasts (family SACCHAROMYCETACEAE). Cells are spheroidal, ellipsoidal or elongate; vegetative reproduction occurs by multilateral budding. Pseudomycelium and, to a limited extent, true mycelium may be formed. Arthrospores may be formed by some species. Species are homothallic or heterothallic. Asci are usually dehiscent. Ascospores: bowler-hat-shaped, Saturn-shaped, hemispheroidal, or spheroidal, smooth-surfaced, usually 1–4 per ascus. Sugars may or may not be fermented; NO3
is not assimilated. More than 56 species are recognized; type species: P. membranaefaciens. Several species have anamorphs in the form-genus Candida: e.g. the anamorph of P. burtonii is C. variabilis (and Trichosporon variabile), that of P. guilliermondii is C. guilliermondii, that of P. membranaefaciens is C. valida, etc. Pichia species are found in a wide range of habitats: e.g. tunnels and frass of wood-boring beetles, tree exudates, pickling brines, tanning liquors, grain and flour (e.g. P. farinosa), wine (e.g. P. carsonii [formerly P. vini ], P. membranaefaciens), naturally fermented apple juice (e.g. P. delftensis), cacti (e.g. P. cactophila, P. opuntiae), human skin, faeces, etc. [Book ref. 100, pp. 295–378.] (See also ALCOHOL OXIDASE, ALCOHOLIC FERMENTATION, BEER SPOILAGE, CIDER, METHYLOTROPHY, WINE SPOILAGE.) Pichinde virus See ARENAVIRIDAE. pickling A traditional method of FOOD PRESERVATION in which the pH is lowered either by direct addition of acid (e.g. VINEGAR, LACTIC ACID) or by a LACTIC ACID FERMENTATION of the food; cabbage (see SAUERKRAUT), cucumbers and olives are often preserved by fermentation [Book ref. 5, pp. 227–258]. Fresh vegetable material has a large and varied microflora, of which lactic acid bacteria form only a very small component; fermentation conditions must therefore specifically encourage the growth of the lactic acid bacteria. The vegetables are graded and washed (Spanish-style olives may be treated with NaOH to remove the bitter phenolic glucoside oleuropein), and salt is added; the salt extracts the sugar-containing juices, encouraging the growth of the lactic acid bacteria (mainly Lactobacillus brevis, L. plantarum and Pediococcus pentosaceus in the case of 586 pili intramolecular self-cleavage to form 3A, 3B (VPg), 3C (a protease which carries out most of the proteolytic cleavages), and 3D (an RNA polymerase which can elongate nascent RNA chains on an RNA template). [Nomenclature of picornavirus proteins: JV (1984) 50 957–959.] In contrast to polioviruses, aphthoviruses have two distinct translation initiation sites [JGV (1985) 66 2615–2626]. Replication of the RNA genome is associated with the smooth endoplasmic reticulum; (
)-strand synthesis may involve a primer formed by hairpin folding of the (C)-strand template, resulting in a product up to twice the length of the genome (at least in vitro) [JV (1986) 58 790–796]. Virus assembly is preceded by cleavage of P1 to form an immature 5S protomer containing VP0, VP1 and VP3. The immature protomers become associated with an RNA genome to form a provirion. During maturation most or all of the VP0 molecules are cleaved to form VP2 and VP4. The mature virions are eventually released by host cells lysis. (Some picornaviruses – e.g. hepatitis A virus – can cause non-lytic infections.) Most picornaviruses can be propagated in cell cultures. Infected cells undergo drastic changes in their macromolecular metabolism: the rate of RNA synthesis declines shortly after infection, and cellular protein synthesis is inhibited soon after. [Review of inhibition of protein synthesis: Book ref. 150, pp. 177–221.] Characteristic CPEs generally develop in the infected cells: e.g., accumulation of chromatin on the inside of the nuclear envelope; appearance of membranous vesicles in the cytoplasm; formation of crystalline arrays of virions in the cytoplasm; alterations in membrane permeability leading to shrivelling of the cell; etc. picornaviruses Viruses of the PICORNAVIRIDAE. picric acid (2,4,6-trinitrophenol) An antimicrobial agent (see PHENOLS), a fixative (see e.g. BOUIN’S FLUID), and a DYE. piedra See BLACK PIEDRA and WHITE PIEDRA. Piedraia A genus of fungi of the DOTHIDEALES (anamorph: Trichosporon). (See BLACK PIEDRA.) piericidin An antibiotic, produced by Streptoverticillium mobaraense, which acts as a RESPIRATORY INHIBITOR in mitochondrial and bacterial ELECTRON TRANSPORT CHAINS (although intact cells may be impermeable and hence insensitive to piericidin); the (non-covalent) binding site(s) and mechanism of action of piericidin are apparently similar to those of ROTENONE. pif A locus in the F PLASMID associated with the inability of certain phages (e.g. T7) to replicate in cells containing the F plasmid. (pif D phage-inhibitory function.) (See also FEMALESPECIFIC PHAGE.) pig bel Syn. ENTERITIS NECROTICANS. pig diseases (a) Bacterial diseases: see e.g. EPERYTHROZOONOSIS, FOOT-ROT, GLASSER’S DISEASE, GREASY PIG DISEASE, OEDEMA DISEASE, PARATYPHOID FEVER (sense 2), SWINE DYSENTERY, SWINE ERYSIPELAS, TULARAEMIA. (b) Viral diseases: see e.g. AFRICAN SWINE FEVER, AUJESZKY’S DISEASE, FOOT AND MOUTH DISEASE, INCLUSION BODY RHINITIS, PARVOVIRUS, SWINE FEVER, SWINE INFLUENZA, SWINE POX, SWINE VESICULAR DISEASE, TALFAN DISEASE, TESCHEN DISEASE, TRANSMISSIBLE GASTROENTERITIS, VESICULAR EXANTHEMA, VESICULAR STOMATITIS, VOMITING AND WASTING DISEASE. pigeonpox virus See AVIPOXVIRUS. pil genes The designation of genes which encode type IV FIMBRIAE in various Gram-negative bacteria – e.g. Neisseria gonorrhoeae and Pseudomonas aeruginosa; for example, in N. gonorrhoeae, pilC encodes the terminal adhesin, pilE encodes the major subunit (see also ANTIGENIC VARIATION), and pilQ is a which is stabilized by an outer membrane lipoprotein encoded by pilP. Pilaira A genus of fungi (order MUCORALES) which occur on the dung of herbivores. The long, positively phototropic sporangiophore bears a single non-projectile sporangium which, on maturity, becomes adhesive. (cf. PILOBOLUS.) pileate (mycol.) Having a PILEUS. pileus The structure on which the spore-bearing tissue is carried in the sexually-derived fruiting bodies of certain basidiomycetes and ascomycetes. Examples include the entire lamella-bearing structure (‘cap’) carried by the stipe in mushroom-shaped basidiocarps; the corky or leathery, non-stipitate fruiting bodies formed by certain wood-rotting fungi; and the ridged and pitted stipe-borne cylindrical structures of the morels (Morchella spp). pili (singular : pilus) (1) (conjugative pili; sex pili) Elongated or filamentous, proteinaceous, plasmid-encoded structures which extend from the surface of those (Gram-negative) bacteria which contain an (expressed) CONJUGATIVE PLASMID. Some types of pili have been shown to play an essential role in CONJUGATION (sense 1b), and it is generally believed (though not proven) that all pili have essential roles in conjugation. (cf. FIMBRIAE; SPINA; FLAGELLUM.) Commonly, only a few pili (sometimes only one) occur on a given donor cell (cf. FIMBRIAE). At least some types of pili appear to be retractable, but the mechanism of retraction is unknown. In general, thin, flexible pili mediate so-called ‘universal’ conjugation systems, i.e. systems in which conjugation can occur equally well within a body of liquid (e.g. broth) or on a moist (but not submerged) solid surface (e.g. an agar plate or membrane filter). Rigid pili often mediate ‘surface-obligatory’ mating, i.e. conjugation which occurs at a negligible frequency in liquid media but which may occur at high frequency on a moist, non-submerged solid surface. Thick, flexible pili may mediate in either ‘surface-preferred’ mating (in which conjugal transfer occurs more readily on solid surfaces than within liquids) or universal mating. [Surface mating systems in Escherichia coli: JB (1980) 143 1466–1470; surface mating systems in Pseudomonas spp: JGM (1983) 129 2545–2556.] F pili (encoded by the F PLASMID) are thin, flexible filaments that are one to several micrometres in length, ca. 8 nm in diameter; each F-pilus has an axial channel, ca. 2 nm diameter, with a hydrophilic lumen. It has been assumed that the base of a pilus is associated with an ADHESION SITE or similar structure. F pili are composed mainly or solely of a single type of subunit, pilin; the 70-amino acid pilin molecule is a single polypeptide containing one residue of D-glucose and two phosphate residues. The way in which the pilin subunits are assembled to form the pilus is not known. In one model, rings of five pilin subunits are stacked along the axis of the pilus, each ring being rotated 29° relative to the previous ring. In a second model, the pilin molecules are arranged helically. [Structure of F-pilin and a model for the organization of pilin subunits in the F pilus: Mol. Microbiol. (1997) 23 423–429.] Bacteriophages which can bind to F pili include f1, fd and M13 (see INOVIRUS), and MS2 and Qb (see LEVIVIRIDAE). (See also ANDROPHAGE.) (Note that the term pilin is also used to refer to the subunits of other types of pili – and also to the subunits of fimbriae.) F-like pili are serologically similar or identical to F pili, and they bind similar types of phage. They are encoded e.g. by the colicin plasmids ColV and ColI-K94. SECRETIN 587 Pilimelia bacteria, but were poorly active against Gram-positive species. Modern pine fluids (with RW coefficients of ca. 3–5) typically contain phenolics (e.g., 2-phenylphenol, 4-chloro-3,5-xylenol) together with e.g. terpineol (an antimicrobial constituent of pine oil) and a solubilizing agent, e.g., potassium ricinoleate; these fluids are active against both Gram-negative and Gram-positive bacteria. The activity of at least one pine fluid against Pseudomonas aeruginosa is greatly increased at temperatures above 30° C [Book ref. 13, pp. 85–90]. The recommended maximum dilution of a pine fluid is 20 times its RW coefficient. pink-eye (pinkeye) (1) Syn. CONJUNCTIVITIS. (2) An acute contagious conjunctivitis caused by Haemophilus aegyptius. (3) Syn. INFECTIOUS KERATITIS (of cattle). Pinnularia See DIATOMS. pinocytosis The ingestion, by various types of eukaryotic cell (e.g. amoebae, macrophages), of minute droplets of fluid; in pinocytosis (as in PHAGOCYTOSIS) a small region of the CYTOPLASMIC MEMBRANE invaginates and is subsequently pinched off to form a closed, intracellular membranous sac (pinocytotic vesicle, pinosome) containing the ingested fluid. In one form of pinocytosis (fluid-phase pinocytosis) pinocytotic vesicles are formed continually at non-specific sites in the cytoplasmic membrane – samples of extracellular fluid being taken randomly; such vesicles often fuse with LYSOSOMES. Receptor-mediated pinocytosis (D absorptive pinocytosis) is a selective process in which specific types of macromolecule (or e.g. virus particle) are taken up. The macromolecules bind to specific receptors located at the outward-facing surface of the cytoplasmic membrane, groups of such receptors occurring in discrete regions of the cytoplasmic membrane known as coated pits; a coated pit bears on its inner (i.e., cytoplasmic) surface a characteristic array of molecules of the fibrous protein clathrin (MWt ca. 215000). Clathrin occurs in propeller-shaped (‘three-legged’) trimeric units (triskelions) which assemble into pentagons and hexagons, forming a polygonal network which comprises the cytoplasmic face of the coated pit. (Often, bundles of ACTIN microfilaments – stress fibres – occur in the cytoplasm beneath a coated pit.) Following receptor-mediated binding of macromolecules to a coated pit, the coated pit region invaginates and eventually pinches off to form a pinocytotic vesicle – the outer (cytoplasm-facing) surface of the vesicle being covered by a lattice-like network of clathrin; the vesicle, enclosed within its clathrin ‘cage’, is called a coated vesicle. Before fusing with e.g. a lysosome, a coated vesicle must have its clathrin removed in an ATP-dependent process. [Enzymatic recycling of clathrin from coated vesicles: Cell (1986) 46 5–9.] pinosome See PINOCYTOSIS. pinosylvin 3,5-Dihydroxy-trans-stilbene: an antifungal compound formed by many species of pine (Pinus); it appears to be at least partly responsible for the relative resistance of pine heartwood to fungal attack. (See also TIMBER PRESERVATION.) pinta (mal del pinto; syn. spotted sickness) A chronic infectious human disease caused by Treponema carateum; it occurs in Central and South America. Symptoms: spots or patches of abnormally pigmented or depigmented skin; other tissues are rarely affected. Chemotherapy: e.g. penicillins. pInv plasmid See EIEC. pionnotes In Fusarium spp: a flat spore mass which has a fat-like or greasy appearance. pipemidic acid See QUINOLONE ANTIBIOTICS. Piptocephalis A genus of fungi (order ZOOPAGALES) which are parasitic on other fungi, often other zygomycetes. The vegetative hyphae develop extracellularly on the host – from which I-like pili resemble F pili morphologically but they differ serologically and they bind different phages – e.g. If1 and If2 (see INOVIRUS). They are encoded e.g. by the ColIb-P9 plasmid. Short, rigid, nail- or thorn-like pili are encoded e.g. by IncN enterobacterial plasmids and Pseudomonas IncP-7 plasmids. The precise role of the pilus is as yet unknown – see CONJUGATION. The currently favoured model (at least for the F pilus and related pili) is one in which the pilus makes initial contact with the recipient and then retracts to draw donor and recipient into wall-to-wall contact. Evidence supportive of this model includes the demonstration that a mutant donor with a temperature-dependent block in DNA transfer can achieve wall-to-wall contact with a recipient independently of DNA transfer; DNA transfer can occur subsequently at the permissive temperature, thus excluding the need for an extended pilus as a conducting tube for DNA [JB (1985) 162 584–590]. The role of the short, rigid pili encoded e.g. by IncN plasmids remains unknown. (2) Collectively, pili (sense 1, above) and FIMBRIAE. Use of the term in this way should be discouraged owing to the confusion it causes (see FIMBRIAE for rationale). Pilimelia A genus of keratinophilic bacteria (order ACTINOMYCETALES, wall type II) which occur e.g. in soil. The organisms resemble Actinoplanes spp but differ e.g. in that they form parallel chains of rod-shaped zoospores in spherical or cylindrical sporangia. Type species: P. terevasa. [Book ref. 73, pp. 65–66.] pilin See FIMBRIAE and PILI. Pillotina See SPIROCHAETALES. Pilobolaceae See MUCORALES. Pilobolus A genus of fungi (order MUCORALES) which occur on the dung of herbivores. In Pilobolus spp a single sporangium is carried on a subsporangial vesicle at the distal end of each long, positively phototropic sporangiophore; the (non-dehiscent) sporangium is propelled from the sporangiophore by a jet of liquid which escapes, explosively, from the subsporangial vesicle. (cf. PILAIRA.) Pilophorus A genus of LICHENS (order LECANORALES); photobiont: a green alga. Erect pseudopodetia (see PSEUDOPODETIUM) – which may bear terminal, convex, black apothecia – arise from a granular primary thallus which may be evanescent; cephalodia are generally present. Species grow e.g. on rocks, particularly in mountainous regions. pilot protein A protein which is believed to aid the transfer of DNA from a donor cell to a recipient during bacterial CONJUGATION (sense 1b) or from a bacteriophage virion to a host cell during certain types of phage infection. pilus See PILI. pimaricin (natamycin; tennecetin) A MYCOSAMINE-containing tetraene POLYENE ANTIBIOTIC produced by Streptomyces natalensis. It is used e.g. in the treatment of mycotic keratitis and as an antifungal food preservative e.g. in sausages, fruit juices, cheese etc; it can be inactivated by enzymes produced by certain moulds (e.g. Aspergillus flavus). pin gene See RECOMBINATIONAL REGULATION. pine diseases Diseases of pine trees (Pinus spp) include BLISTER RUST and PITCH CANKER. Pine wilt may be caused by a species of the fungus Ceratocystis (vector: bark beetles) or by the nematode Bursaphelenchus xylophilus (vector: the pine sawyer). (See also TREE DISEASES.) pine disinfectants (pine fluids) DISINFECTANTS which include certain volatile oils obtained synthetically or by steam-distillation of pine wood. The early pine fluids were made by solubilizing pine oil with soap; they were active mainly against Gram-negative 588 plague vesicle) appears on each side of the plug; the cytoplasmic membrane thus remains continuous from one cell to another. (cf. PLASMODESMA.) A secondary pit connection may develop between two cells which have become juxtaposed. (See also CHOREOCOLAX.) pitch canker A disease of pine trees (Pinus spp) caused by Fusarium moniliforme var. subglutinans; symptoms include dieback of terminal and lateral branches, and the formation of CANKERS characterized by a copious flow of resin. Pitelka convention A system for numbering the microtubular triplets in a kinetosome. Looking at a cross-section of a ciliary kinetosome from the direction of the interior of the cell, the triplet associated with the ribbon of POST-CILIARY MICROTUBULES is designated number 5; the numbering sequence is clockwise – as in the GRAIN CONVENTION. Pithomyces See HYPHOMYCETES; see also SPORIDESMINS. pitted keratolysis A human skin disease characterized by focal erosion of the stratum corneum, usually on the soles and heels of the feet; it appears to be caused by an organism resembling Dermatophilus congolensis. Pittsburg pneumonia agent Legionella micdadei. pityriasis nigra (tinea nigra) A benign superficial human dermatomycosis in which brownish or black macules are formed (usually on the palms of the hands). Causal agents: Exophiala werneckii (D Cladosporium werneckii ) or Stenella araguata (D Cladosporium castellanii ). pityriasis versicolor (tinea versicolor) A mild, chronic, superficial human dermatomycosis caused by Malassezia furfur (‘Pityrosporum orbiculare’); flat or slightly raised, scaly, brownish or fawn spots (which gradually enlarge and coalesce) develop mainly on the chest, back, arms and neck. Lesions may fluoresce greyish-yellow under Wood’s lamp. Pityrosporum Syn. MALASSEZIA. pivampicillin See PENICILLINS. Pixuna virus See ALPHAVIRUS. P–K test See PRAUSNITZ–KÜSTNER TEST. PKA See CYCLIC AMP. PKC kinase See CHEMOKINES. PKD PROLIFERATIVE KIDNEY DISEASE. PKDL POST-KALA-AZAR DERMAL LEISHMANIASIS. pKM101 See SOS SYSTEM and AMES TEST. PKR kinase See INTERFERONS. Placidiopsis See VERRUCARIALES. placoderm desmid See DESMIDS. placodioid (placoid) (lichenol.) Refers to a crustose thallus which is generally circular in outline with radial lobes at the periphery. placoid Syn. PLACODIOID. Plagiacantha See RADIOLARIA. Plagonium See RADIOLARIA. plague An acute infectious disease which primarily affects rodents but which can be transmitted to man; the causal agent is Yersinia pestis (see also VIRULON). Before the availability of antibiotic therapy, epidemics and pandemics of plague caused enormous loss of life; for example, an outbreak in Europe during the 14th century (the ‘Black Death’) involved ca. 25 million deaths. The World Health Organization has recently recognized plague as a ‘re-emerging disease’. An epidemic of human plague is commonly preceded by an epizootic among urban rats; Rattus rattus populations are rapidly decimated in such epizootics, but wild rodents in rural or wooded areas are less susceptible and provide a reservoir of infection. nutrients are abstracted by means of haustoria. Asexually derived spores are formed in merosporangia carried on (typically) deciduous ‘head cells’ at the distal ends of dichotomously branched sporangiophores; each merosporangium contains one or a few spores. Most species are apparently homothallic. Piptoporus A genus of lignicolous fungi of the APHYLLOPHORALES (family Polyporaceae) which form typically annual, non-stipitate basidiocarps having a whitish or pale brown, corky, dimitic, non-xanthochroic context; the bracket-type basidiocarp has a porous hymenophore on the underside. P. betulinus (‘razor-strop fungus’) is saprotrophic and parasitic on birch (Betula); the upper surface of the basidiocarp has a thin, pale brown, separable outer layer, and the lower porous surface is whitish. Basidiospores: colourless, cylindrical, ca. 5 ð 2 µm. (See also BROWN ROT and TIMBER SPOILAGE; cf. POLYPORUS.) pir gene See R6K PLASMID. Pirella See PASTEURIA. piricularin (pyricularin) A toxin produced by Pyricularia oryzae (see BLAST DISEASE). Piricularin is toxic to the fungus itself, but P. oryzae produces a copper-containing ‘piricularin-binding protein’ which complexes the toxin and renders it non-toxic for the fungus without affecting its toxicity for the rice plant. CHLOROGENIC ACID can complex piricularin and inactivate it. Piromonas A genus of fungi found in the RUMEN. piroplasm (1) A member of the PIROPLASMASINA. (2) The intraerythrocytic stage of Theileria spp. Piroplasmasina A subclass of protozoa (class SPOROZOASIDA) in which the APICAL COMPLEX is less well developed than in other members of the Sporozoasida, and in which the conoid is absent; reproduction occurs asexually (by binary fission or schizogony) and sexual reproduction occurs in at least some species. Motility involves e.g. cell flexion. Species are parasitic in the erythrocytes (and other cells) of vertebrates but do not form pigment from host cell haemoglobin (cf. HAEMOSPORORINA). Piroplasms are heteroxenous and are transmitted by ticks. Genera: BABESIA, Dactylosoma, THEILERIA. Piroplasmea A class of parasitic protozoa classified within the subphylum Sarcomastigophora [JP (1964) 11 7–20] and subsequently, as the subclass PIROPLASMASINA, in the class SPOROZOASIDA. Piroplasmia A subclass of protozoa [JP (1980) 27 37–58] equivalent to the PIROPLASMASINA. Piry virus A virus (genus VESICULOVIRUS) antigenically related to VSV. pisatin An isoflavonoid PHYTOALEXIN (3-hydroxy-7-methoxy40 ,50 -methylenedioxychromanocoumaran) produced by the pea plant (Pisum sativum). Some isolates of Nectria haematococca which are pathogenic for P. sativum can tolerate pisatin and can ‘detoxify’ the phytoalexin by demethylating it. Pisolithus A genus of saprotrophic or mycorrhizal fungi (order SCLERODERMATALES). P. arhizus (D P. tinctorius) forms large, club-shaped, partly hypogean, ochre to brownish basidiocarps (up to ca. 25 cm high) which contain closed pockets or chambers (peridioles) that eventually desiccate and crumble to release their content of basidiospores; this organism forms mycorrhizal associations with various trees and can apparently enhance the growth of e.g. pine trees under conditions which are normally suboptimal for their growth. pit connection In many multicellular red algae: a structure located between two adjacent cells. A primary pit connection forms between two daughter cells during cell division: a central hole in the intercellular septum becomes blocked by a plug of electron-dense material, and a plug cap (formed from a flattened 589 plague PLAGUE: areas where endemic plague is known to have persisted near the end of the 20th century. Areas in black are regions in which there are populations of wild rodents that carry the plague bacillus. Reproduced from Medical Microbiology 14th edition, Figure 36.1, page 406, David Greenwood, Richard Slack & John Peutherer (eds) (1992) copyright Churchill Livingstone (Harcourt Publishers Ltd, Edinburgh) (ISBN 0-443-04256-X) by courtesy of the author, Dr J. D. Coghlan, and with permission from the publisher. antigen (see YERSINIA). PCR-based diagnosis (amplification of a sequence of the Y. pestis caf1 gene) under field conditions in Madagascar was found to be less sensitive than culture or serology and was not recommended [JCM (2000) 38 260–263]. [Rapid mAb/F1-based test for bubonic/pneumonic plague: Lancet (2003) 361 211–216.] Chemotherapy: e.g. tetracyclines, streptomycin or chloramphenicol. Vaccines containing either inactivated virulent cells or live avirulent cells may offer short-term protection. [Molecular and cell biology aspects of plague: PNAS (2000) 97 8778–8783.] Endemic plague persists in various regions of the world (see figure) and, significantly, transferable (plasmid-mediated) multidrug resistance has been reported in Yersinia pestis [NEJM (1997) 337 677–680]. [Dynamics of plague in the rat population, and transmission of the disease to humans: Nature (2000) 407 903–906. Bubonic plague (a metapopulation model of a zoonosis): Proc. RSLB (2000) 267 2219–2230.] planachromat See ACHROMAT. planapochromat See APOCHROMAT. Planctomyces A genus of planktonic freshwater bacteria. Cells: cocci, or coccoid to pear-shaped forms, ca. 0.3–2.0 µm, which develop non-prosthecate, bristle-like appendages when mature; the cell wall contains no peptidoglycan. Reproduction occurs by budding. Types species: P. bekefii. Other species include e.g. P. guttaeformis and P. stranskae [IJSB (1984) 34 470–477]; Transmission from rat to rat, and from rat to man, occurs via the bite of the rat flea (especially Xenopsylla cheopsis); the gut of an infected flea becomes blocked by plague bacilli which are regurgitated into the next bite. Person-to-person transmission may occur via the human flea, Pulex irritans (a less effective vector), or, in pneumonic plague, by droplet infection. Plague bacilli have been reported to occur in the throats of symptomless human carriers. In man, plague occurs in three clinical forms. In bubonic plague, the commonest form, the incubation period is 2–6 days; onset is sudden, with fever, prostration, haemorrhages and the development of buboes: swollen, inflamed, necrotic lymph nodes. Septicaemia may result in e.g. meningitis or secondary pneumonia. (cf. PESTIS MINOR.) In primary septicaemic plague (incubation period: 2–6 days) there is a sudden onset with high fever, haemorrhages, vomiting and bloody diarrhoea, but no buboes. Primary pneumonic plague results from infection by inhalation (incubation period: 2–3 days). There is high fever, prostration, severe pneumonia and frequently delirium and coma; the sputum contains large numbers of plague bacilli. Mortality rates in untreated cases: e.g. bubonic plague 25–50%; pneumonic and septicaemic plague ¾100%; early antibiotic therapy is associated with significantly lower mortality. Lab. diagnosis. Demonstration of the pathogen in fluid from buboes, blood, CSF or sputum (by microscopy and/or culture) and/or serology for antibodies against the F1 (‘fraction 1’) 590 plant viruses the latter bearing elongated sporangia, singly or in groups, each sporangium containing a serially-arranged pair of spores; liberated spores become motile (flagellated). Type species: P. longispora. [Book ref. 73, pp. 111–112.] Planococcus A genus of Gram-positive, asporogenous, catalasepositive, chemoorganotrophic, aerobic bacteria which occur as marine saprotrophs. Cells: cocci (each having 1–3 flagella) which occur in pairs and tetrads; the cell wall contains no teichoic acids. A yellow-brown pigment is formed. Carbohydrates are not utilized. Growth occurs e.g. on nutrient agar containing 12% NaCl. GC%: ca. 39–51. Species: P. citreus (type species) and P. halophilus. planofluorite See FLUORITE OBJECTIVE. planogamete A motile GAMETE. Planomonospora A genus of bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes) which occur in soil. The organisms form branching substrate and aerial mycelium, the latter bearing rows of parallel, elongated sporangia, each sporangium being attached at one end to the hypha; each sporangium contains one spore which becomes motile (flagellated) after release. Type species: P. parontospora. [Book ref. 73, 112–113.] planospore A motile SPORE. plant viruses VIRUSes which can infect and replicate in plants. (cf. VIROID; VIRUSOID.) Commonly, plant viruses show a low degree of host specificity, and some can also replicate in insects. Infection of a plant by a given virus may be systemic – spreading rapidly via the phloem (or occasionally xylem) or slowly from cell to cell via plasmodesmata – or may remain localized at the site of infection. (Spread is apparently a virus-specific function involving virus-encoded or virus-induced proteins [review: AVR (1984) 29 313–364].) Infection may be symptomless (silent, ‘latent’), or may involve the development of symptoms ranging from mild to severe or lethal; symptoms, when present, often depend on the species and variety of the host plant, on the particular virus strain, and on environmental conditions, and may be affected by the presence of another virus or of a ‘P. gracilis’ has been excluded from the genus [IJSB (1984) 34 465–469]. plankter See PLANKTON. plankton Collectively, the microorganisms (chiefly microalgae and protozoa), small invertebrates (e.g. copepods and other small crustacea) and fish larvae etc which drift more or less passively in water currents within the pelagic zone of lakes, seas and other bodies of water; although many planktonic organisms are actively motile, even these organisms depend on water currents for significant amounts of displacement. The plant and plantlike components of plankton (including e.g. the photosynthetic flagellates) are referred to as phytoplankton, while the animal or animal-like components (including e.g. protozoa) are called zooplankton. An individual planktonic organism is called a plankter. Plankton, particularly phytoplankton, is important as the basis of the aquatic food chain; it is consumed by a range of invertebrates and by certain fish and whales. Microbial marine plankton includes e.g. DIATOMS, DINOFLAGELLATES, RADIOLARIA, members of the PRYMNESIOPHYCEAE and TINTINNINA, and certain foraminifera (e.g. GLOBIGERINA, GLOBIGERINOIDES). Microbial freshwater plankton includes e.g. CYANOBACTERIA, DESMIDS, DIATOMS, and members of the HELIOZOEA. Various schemes have been proposed for the classification of planktonic organisms according to size; thus, e.g. nannoplankton (D nanoplankton ) may include (depending on author) organisms which are less than 20 µm, less than 50 µm, 5–50 µm, 5–60 µm, or greater than 75 µm. Similarly, mesoplankton may include organisms of 0.5–1.0 mm, 0.2–2.0 mm, greater than 1.0 mm or 1–5 mm. (cf. NEKTON, NEUSTON; see also BLOOM.) Planktothrix A genus of gas-vacuolate CYANOBACTERIA. [Variation in gas vesicle genes: Microbiology (2000) 146 2009–2018.] Planobispora A genus of bacteria (order ACTINOMYCETALES, wall type III; group: maduromycetes) which occur in soil. The organisms form branching substrate and aerial mycelium, PLANT VIRUSES: basic characteristics Genome Envelope Nucleocapsid morphology Virus group (or unclassified virus)a dsDNA ssDNA dsRNA


isometric paired isometric particles isometric ssRNA ssRNA monopartite C š rod-shaped caulimoviruses geminiviruses plant reoviruses (Reoviridae), rice ragged stunt virus, some cryptic viruses plant rhabdoviruses (Rhabdoviridae), tomato spotted wilt virus
isometric monopartite
rod-shaped or filamentous bipartite
isometric bipartite tripartite tripartite multipartite



rod-shaped or filamentous š isometric š rod-shaped filamentous a luteoviruses, maize chlorotic dwarf virus, sobemoviruses, tobacco necrosis virus, tombusviruses, tymoviruses carlaviruses, closteroviruses, potexviruses, potyviruses, tobamoviruses comoviruses, dianthoviruses, nepoviruses, pea enation mosaic virus, velvet tobacco mottle virus furoviruses?, hordeiviruses (some strains), tobraviruses bromoviruses, cucumoviruses, ilarviruses alfalfa mosaic virus, hordeiviruses (some strains) rice stripe virus group See separate entries for details. 591 Plantago mottle virus e.g. REOVIRIDAE VIRUS.) SATELLITE VIRUS or SATELLITE RNA. Virus-infected plants may exhibit one or more of the following symptoms: CHLOROSIS, COLOUR-BREAKING, curling or distortion of leaves, ENATIONS, GALL-formation, MOSAIC, MOTTLE, NECROSIS, RINGSPOT, ROSETTEformation, STREAK, stunting of growth, TOP NECROSIS, VEIN BANDING, VEIN CLEARING, VIRESCENCE, yellowing (see YELLOWS), etc. (Plant structures which are fully matured at the time of infection may fail to develop symptoms.) Transmission of many plant viruses can occur mechanically – e.g. by contact between plants or by contamination of a plant with sap from an infected plant (present e.g. on pruning implements, hands etc); in the laboratory mechanical transmission can often be achieved by rubbing the leaves of a plant with a virus preparation – often mixed with a mild abrasive. Some viruses can be transmitted via soil, via seeds or tubers, or by grafting (e.g. from an infected stock to an uninfected scion). Many plant viruses can be transmitted only by specific VECTORS such as sap-sucking or biting insects, mites, nematodes, fungi, or parasitic plants such as the dodder (Cuscuta spp). (See CIRCULATIVE TRANSMISSION and NON-CIRCULATIVE TRANSMISSION.) Identification of a plant virus is seldom possible from symptoms exhibited by infected plants in the field, but may be achieved e.g. by light or electron microscopy of infected tissues (many viruses induce characteristic inclusion bodies in infected cells), by inoculation of the virus into a range of suitable DIFFERENTIAL HOSTS, and/or by serological tests; plant viruses are generally effective immunogens, and specific antibodies against a known virus can be raised in animals and used for the identification of an unknown virus (e.g. by ELISA, IMMUNOELECTRON MICROSCOPY, IMMUNOSORBENT ELECTRON MICROSCOPY, precipitin tests, etc). A plant virus may be assayed e.g. using a LOCAL LESION host. Cultivation in vitro of many plant viruses can be achieved in plant protoplasts derived e.g. from cell suspension cultures or mesophyll tissue of leaves [AVR (1984) 29 215–262] or from shoot cultures [e.g. of potatoes: JGV (1985) 66 1341–1346]. Control of virus diseases of plants may involve any of various preventive measures: e.g. use of virus-free seed (obtained e.g. by meristem culture), control of vector populations, ROGUING, removal of volunteer plants or weeds in which the virus may overwinter, selection of resistant or tolerant plant cultivars, etc. Once a plant is infected with a virus there is no effective chemical treatment; however, certain viruses can be inactivated by subjecting the infected plant (or seeds in the case of seedborne viruses) to air temperatures of 30–35° C (for days or weeks), to water immersion at ca. 40 or 50° C for shorter periods, etc (‘thermotherapy’). [Heat treatment of lettuce seeds carrying lettuce mosaic virus: Ann. Appl. Biol. (1985) 107 137–145.] Nomenclature and classification. Individual plant viruses are generally given names based on the common, first-recognized, or most important host plant, and on the main symptom(s) produced by the virus in that plant under natural conditions. (Examples: apple chlorotic leafspot virus; barley yellow dwarf virus.) In the majority of plant viruses the virus genome consists of RNA which is often segmented and sometimes distributed between two or more separate virus components (see MULTICOMPONENT VIRUSES). On the basis of the nature of the genome, shape of the virus particle(s), presence or absence of an envelope, etc, plant viruses are classified into ‘groups’ (see table); the name of a group is commonly a sigla derived e.g. from the name of the type virus (e.g. caulimoviruses from cauli flower mosaic virus). A few plant viruses are related to certain animal viruses and are included in animal virus classification schemes: see and RHABDOVIRIDAE. (cf. TOMATO SPOTTED WILT Plantago mottle virus See TYMOVIRUSES. Plantago severe mottle virus See POTEXVIRUSES. Plantago virus See CAULIMOVIRUSES. Plantago virus X See POTEXVIRUSES. plantar wart See PAPILLOMA. plantaricin S See BACTERIOCINS. plaque (1) A discrete, macroscopic or microscopic, usually circular region in a cell monolayer (see TISSUE CULTURE) or in a film or layer of bacterial growth (see LAWN PLATE) in which some or all of the cells have been lysed, or their growth has been strongly inhibited, as a result of the activity of a virus or other agent (e.g. BDELLOVIBRIO). In virology, it is generally assumed that each plaque develops following the replication of a single virion; thus, e.g., when the first-infected cell lyses, the progeny virions diffuse outwards, progressively infecting and lysing other cells until a visibly depopulated area (the plaque) is produced. Plaque characteristics – e.g. size, shape, nature of the margin – can be useful e.g. in identifying the infecting virus, or in determining whether or not a cell population is homogeneous in its susceptibility to the virus; thus, e.g., clear plaques (total cell lysis) on a lawn of susceptible bacteria are typically produced by virulent bacteriophages, while turbid plaques are characteristic of temperate phages – which lyse only some of the cells in the plaque area – and of non-lytic phages (see e.g. INOVIRIDAE). (2) (verb) To inoculate a lawn plate or cell monolayer with a virus suspension in order to obtain plaques (sense 1). (3) See DENTAL PLAQUE. (4) (med., vet.) A patch or flat lesion e.g. on skin or mucous membranes (see e.g. DOURINE). plaque assay A procedure for determining the PLAQUE TITRE of a given viral suspension. Bacteriophages. Log dilutions of the phage suspension are initially prepared, and a measured volume of a given dilution is added to a known volume of molten (ca. 45–46° C) semisolid agar containing an excess of sensitive bacteria; the whole is well mixed and poured onto a plate of solid nutrient agar, forming an upper layer 1–2 mm thick. An identical procedure is carried with each dilution; the plates are incubated at the growth temperature of the bacterium and subsequently examined for PLAQUES. Since each plaque is assumed to have been caused by a single phage, the plaque titre of the original phage suspension can be calculated from the volumes and dilutions used. Other viruses. Plaque assay can be carried out for other viruses if suitably sensitive monolayer cultures (see TISSUE CULTURE) are available. In one method, each of a number of monolayer cultures is drained of growth medium and inoculated with a known volume of one of a range of log dilutions of the viral suspension; the cultures are rocked to enable viruses to adsorb throughout the cell sheet. The monolayers are then overlaid with buffered growth medium in semi-solid purified agar, incubated, and subsequently examined for plaques. (The agar overlay inhibits the spread of viruses from the plaque areas.) plaque-forming unit (pfu) An entity – usually a virion, but also e.g. an ‘infectious centre’ (see ONE-STEP GROWTH EXPERIMENT) – which can give rise to a single PLAQUE under appropriate conditions. (See also EOP and PLAQUE TITRE.) plaque mutant Syn. PLAQUE-TYPE MUTANT. plaque-reduction assay See INTERFERONS. plaque titre The number of PLAQUE-FORMING UNITS per unit volume. plaque-type mutant (plaque mutant; plaque morphology mutant) A mutant virus which forms PLAQUES (sense 1) that differ from 592 plasmid environment does not select for the genetic diversity needed by species in more challenging environments [FEMS Reviews (1998) 22 255–275]; however, some intracellular bacteria (e.g. Chlamydia spp) are known to contain plasmids [see e.g. Microbiology (1997) 143 1847–1854]. Many plasmids encode product(s) and/or functions(s) which modify the phenotype of the host cell. Almost any feature may be encoded by plasmids, including e.g. resistance to particular ANTIBIOTIC(s) and/or HEAVY METAL(s) (see R PLASMID), and the synthesis of AGROCINS, BACTERIOCINS, GAS VACUOLES (e.g. in certain strains of Halobacterium), restriction endonucleases (see also PHAGE TYPING), SIDEROPHORES, toxins (see e.g. ANTHRAX TOXIN, ETEC) or virulence factors (see e.g. CROWN GALL and VW ANTIGENS). Some plasmid-encoded products alter the antigenic characteristics of a cell (see e.g. ANTIGEN GAIN), and some confer the ability to utilize particular substrate(s) (see e.g. LACTOSE PLASMID, OCT PLASMID, TOL PLASMID). One plasmid may counteract the effects of another; for example, the pSa plasmid can abolish the specific adherence properties of Agrobacterium strains which are due to the presence of the Ti plasmid [JGM (1983) 129 3657–3660]. (See also CRYPTIC PLASMID.) The presence of a plasmid within a cell may often be inferred from the cell’s phenotype: certain characteristics are commonly plasmid-specified. Whether or not a plasmid is present may be determined e.g. by gel ELECTROPHORESIS of a CLEARED LYSATE, or by treating the cells with certain agents that eliminate plasmids (see CURING sense 2) and observing for the loss of particular phenotypic properties. [Plasmid technology: Book ref. 177.] Some plasmids can promote their own intercellular transfer by CONJUGATION (sense 1b): see CONJUGATIVE PLASMID; some conjugative plasmids contain a TRANSFER OPERON which is DEREPRESSED for conjugal transfer. (See also EPIDEMIC SPREAD.) In addition to their own intercellular transfer, some conjugative plasmids can mobilize the host cell’s chromosome (or certain types of non-conjugative plasmid) for intercellular transfer (see CONJUGATION sense 1b). More than one type of plasmid can occur in a given cell; plasmids which have different modes of replication (see below) and of PARTITION are said to exhibit compatibility, i.e., they can co-exist stably in the same cell. Plasmids which have similar or identical replication/partition systems are incompatible, i.e., they cannot co-exist stably in the same cell; the phenomenon of incompatibility enables plasmids to be classified into a number of ‘incompatibility groups’ (Inc groups): see INCOMPATIBILITY. Plasmid replication. A plasmid controls its own replication, using the cell’s biosynthetic machinery to make any plasmidencoded proteins etc. that are needed; plasmid DNA REPLICATION requires various host proteins – commonly e.g. the product of the DNAB GENE, the DNAC GENE, the dnaE gene (D polC gene: see DNA POLYMERASE) and the dnaG gene (see PRIMASE) in Escherichia coli. (See also DNAA GENE.) Replication is not coupled to any specific stage or event in the CELL CYCLE, and in some plasmids it occurs throughout the cell cycle. Replication and PARTITION appear to be independent processes, although both can determine INCOMPATIBILITY. The rate-limiting step in replication is initiation, and the rate of initiation is controlled in ways that differ according to plasmid. In some plasmids the control of initiation involves constitutive synthesis of small, trans-acting ANTISENSE RNA molecules. Thus, e.g. in plasmid ColE1 control involves the synthesis of two, free (i.e. non-duplexed) strands of plasmid-encoded RNA, one those formed by the wild-type virus in a population of sensitive cells. plasma (1) (in vivo) The fluid (non-particulate) part of blood. (2) (in vitro) Fluid obtained (e.g. by centrifugation) from blood which has been pre-treated with an ANTICOAGULANT. (cf. SERUM.) plasma cell (immunol.) An antibody-secreting cell that develops from a B LYMPHOCYTE on appropriate antigenic stimulation (see ANTIBODY FORMATION); plasma cells occur e.g. in the spleen and (particularly) in lymph nodes which are draining an infected site. In the earliest stage of the immune response the secreted antibodies are of the IgM class; however (for TD antigens), antibodies of other classes are secreted subsequently following class switching (see ANTIBODY FORMATION). Typical plasma cells are ovoid, ca. 10 ð 15 µm, with an offcentre nucleus which, when stained, may have a ‘clock-face’ or ‘cartwheel’ appearance due to the arrangement of chromatin. The (non-granular) cytoplasm contains a well-developed, ribosomerich endoplasmic reticulum (see also METHYL GREEN–PYRONIN STAIN). plasma clotting See FIBRIN; COAGULASE; ANTICOAGULANT. plasma membrane Syn. CYTOPLASMIC MEMBRANE. plasma volume extenders (for blood transfusion) See DEXTRANS. plasmagel See e.g. SARCODINA. plasmalemma Syn. CYTOPLASMIC MEMBRANE. plasmalogen A glycerophospholipid in which the glycerol bears a 1-alkenyl ether group. Plasmalogens occur e.g. in the cytoplasmic membrane in anaerobic bacteria. plasmasol See e.g. SARCODINA. Plasmaviridae (mycoplasma virus type 2 phages; L2 acholeplasmavirus group) A family of enveloped, pleomorphic MYCOPLASMAVIRUSES containing dsDNA. One genus: Plasmavirus; type species: BACTERIOPHAGE MV-L2. Other members: bacteriophage 1307 and possibly v1, v2, v4, v5 and v7. Plasmavirus See PLASMAVIRIDAE. plasmid In many types of prokaryotic and eukaryotic cell: a linear or covalently closed circular (ccc) molecule of DNA (q.v.) – distinct from chromosomal DNA, mtDNA, ctDNA or kDNA – which can replicate autonomously (i.e., independently of other replicons) and which is commonly dispensable to the cell; the term ‘plasmid’ is sometimes used to refer also to the dsRNA elements which occur in certain yeasts: see KILLER FACTOR. (cf. EPISOME.) (The stable, extrachromosomal (i.e., non-integrated) prophages of certain bacteriophages (e.g. BACTERIOPHAGE P1) conform to the definition of plasmid and are commonly referred to as plasmids.) Some plasmids appear to have arisen from chromosomal or mitochondrial DNA: see e.g. THREE-MICRON DNA PLASMID. The following refers primarily to prokaryotic plasmids. Plasmids occur in both Gram-positive and Gram-negative bacteria; in size they range from ca. 1.5 kb (MWt ca. 106 ) to ca. 300 kb (MWt ca. 2 ð 108 ). Most bacterial plasmids are ccc dsDNA molecules, and these are normally supercoiled (see DNA). However, some plasmids can occur as ccc ssDNA molecules [PNAS (1986) 83 2541–2545], while linear plasmids occur e.g. in Streptomyces and in Borrelia [Mol. Microbiol. (1993) 10 917–922]. Many bacterial plasmids carry one or more TRANSPOSABLE ELEMENTS. (cf. KILLER PLASMIDS; TWO-MICRON DNA PLASMIDS.) Certain obligate pathogens in the alpha subclass of Proteobacteria normally lack plasmids, although plasmids occur in other members of the same subclass. It has been suggested that species of Anaplasma, Bartonella, Brucella and Rickettsia evolved without plasmids because their relatively constant (intracellular) 593 plasmid pInv growth conditions. Such plasmids (e.g. ColE1) are said to be under ‘relaxed control’; a typical ‘relaxed’ plasmid is a small, non-conjugative MULTICOPY PLASMID whose copy number under normal conditions is ca. 10–30. (A ‘relaxed’ plasmid can exceed its normal copy number only under abnormal conditions. See also CLONING.) Plasmids which fail to replicate on inhibition of protein synthesis are said to be under ‘stringent control’; a typical ‘stringent’ plasmid is a large, low-copy-number CONJUGATIVE PLASMID. Whether a plasmid is ‘relaxed’ or ‘stringent’ depends on its system of replication control. Thus, e.g. in the ‘relaxed’ COLE1 PLASMID (q.v.), not only is ongoing protein synthesis not required for replication, but the inhibition of protein synthesis eliminates a protein (Rom/Rop) which is inhibitory to plasmid replication. Conversely, in ‘stringent’ plasmids replication involves e.g. the synthesis of proteins that are essential for the initiation of replication. [Control of plasmid replication: Book ref. 161, pp. 189–214.] Some plasmids have mechanisms for ensuring their persistence within a bacterial strain: see e.g. ccd mechanism in entry F PLASMID. Nomenclature. Plasmids are designated in various ways. COLICIN PLASMIDS are designated according to the colicins they encode (e.g. ColE1-K30 encodes colicin E1, ColV,I-K94 encodes colicins V and Ia). Many antibiotic-resistance plasmids are designated by the letter ‘R’ followed by a number, e.g. R1, R46 etc; others include e.g. R6K, RK2, RP1 etc. Recombinant plasmids are often indicated by the prefix ‘p’, e.g. pML31. A number of plasmids are given trivial names (e.g., 1), while the names of some indicate particular functions (see e.g. Ti plasmid under CROWN GALL, and TOL PLASMID). (See also entries for individual plasmids: e.g. COLE1 PLASMID; DELTA; F PLASMID; PBR322; PMB1; PML31; PSC101; R1 PLASMID; R46 PLASMID; R100 PLASMID; R6K PLASMID; RP1 PLASMID.) plasmid pInv See EIEC. plasmid pYV See FOOD POISONING (Yersinia). plasmin See FIBRINOLYSIN. plasminogen (profibrinolysin) An inactive precursor of FIBRINOLYSIN normally present in blood; it is a large (¾90 kDa) glycoprotein. Plasminogen has been found to bind disease-associated PRIONS, but not the normal protein (PrPC ); this property may be useful in diagnostic tests for TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES [Nature (2000) 408 479–483]. plasmodesma (pl. plasmodesmata) A fine channel in a plant cell wall or a fungal SEPTUM through which cytoplasm can pass; plasmodesmata allow continuity between the cytoplasm of adjacent cells. (See also DESMOTUBULE, PIT CONNECTION and PLANT VIRUSES.) plasmodiocarp See MYXOMYCETES. Plasmodiophora See PLASMODIOPHOROMYCETES and CLUBROOT. Plasmodiophorea See RHIZOPODA. Plasmodiophorina See GYMNOMYXA. Plasmodiophoromycetes (‘endoparasitic slime moulds’) A class of eukaryotic organisms (division MYXOMYCOTA) which are obligate intracellular parasites in plants, algae and fungi. The vegetative stage of the organisms is a plasmodium (a naked, multinucleate protoplast); hyphae and fruiting bodies are apparently never formed. Although traditionally regarded as fungi, the true taxonomic position of these organisms remains uncertain; they are apparently unrelated to other members of the Myxomycota. rRNA data studies support the idea that they may be at the base of an evolutionary line leading to the of which, RNA II, can bind at the plasmid’s origin and act as a primer, thus permitting replication. The other strand, RNA I, can interfere by base-pairing with RNA II – such that the outcome of RNA I–RNA II interaction determines the occurrence, or otherwise, of replication (for further details see COLE1 PLASMID). (It appears that RNA I is normally polyadenylated, such polyadenylation facilitating its degradation; in Escherichia coli, mutations in the pcnB gene, which encodes a poly(A)polymerase, result in RNA I molecules that are stabilized through lack of polyadenylation – the effect of which is to inhibit replication of ColE1, i.e. to reduce its copy number.) Once initiated, replication in ColE1 proceeds as in the E. coli chromosome, although in this plasmid replication occurs unidirectionally from the origin. In the R1 PLASMID (q.v.), antisense RNA molecules bind to, and inhibit translation of, the mRNA of the RepA protein – which, in turn, is involved in regulating the frequency of plasmid replication. Many of the small circular plasmids in Gram-positive bacteria (and some in Gram-negative bacteria) replicate by a ROLLING CIRCLE MECHANISM. Replication is initiated by a plasmid-encoded Rep protein which, by nicking a specific strand at the origin, generates a 30 end for synthesis. One round of replication produces a complete dsDNA plasmid and a circular ssDNA copy, the latter then being replicated to the dsDNA state from an RNA primer. In some of these plasmids initiation is controlled by regulation of the synthesis/activity of the Rep protein (as the quantity of Rep protein affects the frequency of replication); in some cases small, plasmid-encoded RNA molecules bind to the rep mRNA and bring about either premature termination of transcription or inhibition of translation. Alternatively, the Rep protein may be inactivated after only one use by binding to a small plasmid-encoded oligonucleotide. [Replication control in rolling circle plasmids: TIM (1997) 5 440–446.] In many plasmids of Gram-negative bacteria, the Rep initiator protein binds to ‘direct repeats’ of nucleotides (iterons) in the region of the plasmid’s origin of replication, thus contributing to a nucleoprotein complex that initiates replication. For example, in the F PLASMID, RepE (the E protein) carries out this function; replication proceeds by a bidirectional Cairns-type mechanism (as in the E. coli chromosome). (See also R6K PLASMID.) For each plasmid, the iterons have a characteristic number, spacing and composition. In some plasmids, Rep can also repress transcription of its own gene by binding to inverted repeat sequences overlapping the promoter; in some cases (including the F plasmid) it has been shown that, while monomers of Rep bind to iterons (promoting replication), dimers bind to the promoters of their own genes (inhibiting replication). RepA monomers (as opposed to dimers) of the Pseudomonas plasmid pPS10 may be structurally suitable for RepA–iteron binding [EMBO (1998) 17 4511–4526]. In general, plasmid replication and PARTITIONing seem likely to depend on some kind of association between the plasmid and the cytoplasmic membrane [Mol. Microbiol. (1997) 23 1–10]. Under normal growth conditions, replication is initiated at a frequency which, for a given plasmid in a given type of cell under given conditions, leads to the establishment of a characteristic COPY NUMBER. During steady-state conditions each plasmid replicates, on average, once per cell division – although the inter-replication period may vary widely from one plasmid to another. If protein synthesis is inhibited (e.g. by CHLORAMPHENICOL), some plasmids can continue to replicate, and can achieve copy numbers much higher than those achieved under normal 594 Plasmodium Chytridiomycetes, Ascomycotina and Basidiomycotina, but do not exclude the possibility of a protozoal ancestry [TBMS (1983) 80 107–112]. (cf. RHIZOPODA and GYMNOMYXA.) Genera include Ligniera, Octomyxa, Plasmodiophora, Polymyxa, Sorodiscus, Sorosphaera, Spongospora, Tetramyxa, Woronina. (cf. ENDEMOSARCA; PHAGOMYXA.) Species are parasitic in e.g. higher plants (e.g. Plasmodiophora brassicae in crucifers – see CLUBROOT; Spongospora subterranea in potatoes – see POWDERY SCAB; Ligniera betae in beet; L. junci in various monocots and dicots); in algae (e.g. Woronina in Vaucheria, Sorodiscus in Chara); and in certain aquatic fungi (e.g. Woronina in Achlya, Pythium or Saprolegnia). Infected tissues often exhibit hypertrophy and sometimes hyperplasia. In e.g. Plasmodiophora brassicae and Polymyxa betae, infection of a new host is initiated when a (‘primary’) zoospore attaches to the wall of a root hair, withdraws its flagella, and encysts. Within the encysted cell there develops a tubular structure, the Rohr, containing an osmiophilic spine-like structure, the Stachel. The Rohr evaginates rapidly, and the Stachel punctures the host cell wall; the protoplast of the encysted zoospore then enters the host cell and develops into a vegetative primary (D sporangiogenous) plasmodium. When mature, this plasmodium gives rise to uninucleate ‘secondary’ zoospores, each with two smooth flagella of different lengths. [Secondary zoospore development in P. brassicae: TBMS (1984) 82 339–342.] The zoospores are released either by disintegration of the host cell or via an ‘exit tube’ arising from the zoosporangium. The role of these zoospores in the life cycle is unclear; fusion between two zoospores has been reported, but karyogamy – when it occurs – is apparently delayed. A zoospore (or fused pair of zoospores?) infects another host cell, in which it develops to form a (diploid?) secondary (D cystogenous) plasmodium. When mature, this plasmodium eventually cleaves to form a number of uninucleate, thick-walled, resistant cysts (resting spores); meiosis has been observed at this stage in several species. [Ultrastructure of meiosis in Woronina pythii : Mycol. (1984) 76 1075–1088.] In some species the cysts are arranged in characteristic clusters (cystosori). [Fine structure of the cystosorus in Spongospora subterranea: CJB (1985) 63 2278–2282.] On disintegration of the host cell the cysts are liberated into the soil where they may remain viable for long periods (e.g. years). On germination, each cyst gives rise to a biflagellate (heterokont) zoospore which can infect another host, thus initiating a new cycle. Some members of the Plasmodiophoromycetes are important as vectors of certain plant viruses: see e.g. BEET NECROTIC YELLOW VEIN VIRUS, SOIL-BORNE WHEAT MOSAIC VIRUS, POTYVIRUSES, TOBAMOVIRUSES. plasmodium A multinucleated, usually motile mass of protoplasm which is usually naked (i.e., bounded only by a plasma membrane) and which is generally variable in size and form; plasmodia are the main vegetative forms in e.g. members of the ACARPOMYXEA, MYXOMYCETES (q.v. for types), MYXOSPOREA and PLASMODIOPHOROMYCETES. (cf. PSEUDOPLASMODIUM.) Plasmodium A genus of protozoa of the suborder HAEMOSPORORINA. The genus includes the common causal agents of MALARIA in man: P. falciparum [genome: Nature (2002) 419 498–511], P. malariae (formerly P. rodhaini), P. ovale and P. vivax. P. cynomolgi and P. knowlesi are parasitic in non-human primates; they occasionally cause malaria in man, and have been widely used as models in research on human malaria. P. berghei and P. yoelii (also spelt P. yoelli ) cause rodent malaria; they have been used for testing antimalarial drugs. The life cycle is basically similar in all species. In Plasmodium spp, sexual reproduction occurs in a mosquito and leads to the formation of large numbers of fusiform or needle-shaped uninucleate cells (sporozoites), ca. 6–18 µm in length, that are infective for the vertebrate host. (Below ca. 15° C P. falciparum will not develop within the mosquito, and below ca. 18° C its development is retarded; P. vivax can develop in mosquitoes at somewhat lower temperatures – which probably accounts for its presence in temperate as well as tropical and subtropical regions.) Transmission of the parasite occurs when the (female) mosquito takes a blood meal by inserting her proboscis into the tissues of a vertebrate host: sporozoites, present in the insect’s saliva, quickly reach the bloodstream and initiate the cycles of asexual reproduction. In man, the sporozoites rapidly enter parenchymatous cells in the liver; there, they typically grow and eventually undergo SCHIZOGONY, rupturing the liver cells and releasing into the bloodstream large numbers of minute cells (merozoites), each ca. 1 µm in diameter. This stage is called pre-erythrocytic schizogony or exoerythrocytic schizogony; its duration ranges from ca. 5 1/2 days (in P. falciparum) to ca. 15 days (in P. malariae), and is ca. 8 and 9 days respectively, in P. vivax and P. ovale. Instead of growth and schizogony, some or all of the sporozoites of some species – e.g. P. cynomolgi, P. ovale, P. vivax – form specialized cells (hypnozoites) which can remain dormant within the liver for e.g. 8–10 months; on resuming development, the hypnozoites follow the normal course of growth and schizogony and give rise to the (periodic) relapses which are characteristic of malaria caused by these species. Merozoites bind to, and enter, erythrocytes – except e.g. in P. berghei and P. yoelii, in which the merozoites parasitize reticulocytes (erythrocyte precursor cells). Within an erythrocyte a merozoite forms a central vacuole and develops into the characteristic ‘ring’ or ‘signet-ring’ stage; in stained preparations this commonly appears as a ring of cytoplasm containing a bulging nucleus. The growing cell (now called a trophozoite) obtains nutrients from e.g. the protein part of the host cell’s haemoglobin; P. falciparum apparently obtains its iron from outside the parasitized erythrocyte (see TRANSFERRIN; cf. HAEMOZOIN). At this stage the erythrocyte and/or parasite may exhibit certain inclusions: see HAEMOZOIN, MAURER’S CLEFTS, SCHÜFFNER’S DOTS and ZIEMANN’S DOTS. A protein designated exp-1 was detected in a cytoplasmic compartment in P. falciparum-infected erythrocytes; it was suggested that parasite-encoded proteins pass through this compartment on their way to the erythrocyte’s plasmalemma [EMBO (1987) 6 485–491]. The fully grown parasite (now called a schizont), which no longer appears ring-shaped, subsequently undergoes schizogony to form a number of merozoites (each ca. 1 µm diam.) which are released from the ruptured erythrocyte; these merozoites can infect fresh erythrocytes. Typically, merozoites are released more or less synchronously from large numbers of parasitized erythrocytes, thus accounting for the periodic symptoms of malaria; the duration of the erythrocytic cycle (i.e., from infection to rupture) typically ranges from ca. 2 days (P. falciparum) to ca. 3 days (P. malariae), but is only 1 day in P. knowlesi. After one or more erythrocytic cycles, some of the intraerythrocytic merozoites develop into gametocytes (rather than schizonts); within the bloodstream, gametocytes may remain viable for one month or more, but they do not develop further unless ingested by an anopheline mosquito. On ingestion by a mosquito 595 plasmogamy plasmogamy Coalescence of the cytoplasm of two or more cells; in fertilization, plasmogamy is followed by KARYOGAMY. Plasmopara A genus of fungi of the PERONOSPORALES; species include P. viticola and P. halstedi, the causal agents of DOWNY MILDEWS on grape and sunflower, respectively. plasmotomy In certain multinucleate protozoa (e.g. Actinosphaera, Pelomyxa): an asexual reproductive process in which fission results in the formation of two or more multinucleate daughter cells, the nuclei of the parent cell being more or less evenly distributed between them; nuclear and cytoplasmic divisions appear to occur independently. plastics (from bacteria) See BIOPOL. plastid Any of various types of membrane-limited, typically DNA-containing organelle which occur within the cells of plants and algae but not in those of animals or prokaryotes. Examples: AMYLOPLAST, CHLOROPLAST, CHROMOPLAST, ELAIOPLAST, ETIOPLAST and LEUKOPLAST. (See also PLASTOME.) plastocyanin A copper-containing BLUE PROTEIN which acts as an electron donor to photosystem I (see PHOTOSYNTHESIS). In certain algae (e.g. Scenedesmus) plastocyanin can apparently be replaced by a c-type cytochrome in the absence of copper. [Structure and function of plastocyanin: Book ref. 146, pp. 19–31.] plastoglobuli Osmiophilic lipid globules in the stroma of a CHLOROPLAST. plastome The genetic complement of a PLASTID. plastoquinone See QUINONES and PHOTOSYNTHESIS. plate (1) A solid MEDIUM in a PETRI DISH prepared (using an ASEPTIC TECHNIQUE) by pouring a sterile molten (AGAR- or GELATIN-based) medium into a Petri dish to a depth of 3–5 mm and allowing it to set. Freshly-poured plates to be used e.g. for STREAKING should be left for 10–20 min in a 37° C hot-air incubator with the lid partly off so that the surface moisture can evaporate; such ‘drying’ before inoculation prevents unwanted spreading of the inoculum in the surface film of moisture. (See also SPREAD PLATE.) (2) A plate CULTURE. (3) (verb) To inoculate a plate: see PLATING. plate dilution test See DILUTION TEST. platelet factor 4 (PF4) See CHEMOKINES. platelets (thrombocytes) Flat, disc-like, membrane-bounded bodies (ca. 2.5 µm diam.) which occur in mammalian blood (ca. 250000/mm3 in human blood); they are formed by fragmentation of megakaryocytes in red bone marrow. Platelets adhere to rough or damaged surfaces and assist in blood clotting (though clotting can occur in their absence). Platelets contain e.g. HISTAMINE and SEROTONIN. plating (1) The act of distributing an INOCULUM on the surface of a PLATE – see e.g. SPREAD PLATE and STREAKING. (2) (serol.) The use of specifically coated plastic (or other) surfaces for the separation of specific cells (or other entities) from heterogeneous populations. The procedure is based on antigen–antibody specificity; thus, e.g., antibody, immobilized (by its Fc portion) on an inert support, will bind, and thus remove from a mixture, those cells which bear homologous cell-surface antigens. Platyamoeba See AMOEBIDA. Platymonas See TETRASELMIS. plcB gene See LISTERIOSIS. plectenchyma (mycol.) A tissue composed of hyphae or cells; such tissues occur e.g. in LICHEN thalli, in fungal fruiting bodies, etc. There are two principal types of plectenchyma: prosenchyma (prosoplectenchyma), in which the hyphae remain distinguishable, are rather loosely woven, and are more or less parallel to each other; and pseudoparenchyma (paraplectenchyma), in the gametocytes develop into male and female gametes. In this process the (male) microgametocyte undergoes exflagellation: several nuclear divisions occur, up to eight thread-like appendages are formed, and the microgametocyte eventually breaks up to form uninucleate, uniflagellate male gametes (microgametes). A microgamete fuses with a (female) macrogamete to form a zygote which subsequently becomes motile (and is then called an ookinete or vermicule). The ookinete passes through the mosquito’s gut wall, rounds off, and forms an oocyst in the outer layer of the gut wall (adjacent to the haemocoele). Sporozoites develop within the oocyst; they are subsequently released into the haemocoele, and many find their way to the insect’s salivary glands, thus completing the cycle. Plasmodial antigens and antimalarial vacines. Certain plasmodial antigens have been used in the preparation of antimalarial vaccines. One such antigen is the circumsporozoite protein (CS protein), a protein (MWt ca. 60000) which forms a thick surface coat on the (mature) sporozoite. [CS proteins: Cell (1985) 42 401–403.] Monoclonal antibodies to CS proteins inhibit infectivity of sporozoites both in vitro and in vivo. (cf. CSP REACTION.) The CS proteins from different species of Plasmodium all contain a block of tandemly repeated peptides (though these vary in nature according to species); in e.g. P. falciparum there are multiple copies of the tetrapeptide asparagine–alanine–asparagine–proline (designated NANP), and these sequences are involved in the binding of anti-CS protein monoclonal antibodies. One experimental antimalarial vaccine consisted of a suspension of polypeptides each containing multiple repeats of NANP (and of another tetrapeptide, asparagine–valine–aspartic acid–proline (NVDP), also found in P. falciparum). Another experimental vaccine was prepared from an NANP trimer conjugated to tetanus toxoid. The ring-infected erythrocyte surface antigen (RESA) is a parasite-encoded protein found in the plasmalemma in erthrocytes infected with the ring stage of P. falciparum; it is a protein of MWt ca. 155000, and is sometimes referred to as Pf155. Antibodies to a repeated amino acid sequence in RESA inhibit the invasion of erythrocytes by merozoites in vitro. The possibility of a transmission-blocking vaccine arose from the observation that anti-gamete antibodies raised in an animal host can neutralize the gametes (of Plasmodium) in a mosquito which has taken a blood meal from that host; transmission of the parasite to a new host is therefore inhibited. The problems of developing a satisfactory antimalarial vaccine are exacerbated by ANTIGENIC VARIATION in Plasmodium. For more recent data on anti-malaria vaccines see the entry MALARIA. Subgenera of Plasmodium. Subgenera are defined on the basis of (a) the vertebrate host(s), and (b) the morphology of the erythrocytic schizont and the gametocyte. Subgenera include: Haemamoeba. Hosts: birds. Vector: the mosquito Mansonia crassipes (experimental vectors include Aedes aegypti, Anopheles spp, Culex spp). Schizonts are large, gametocytes spherical. Laverania. Hosts: primates. Vector: anopheline mosquitoes. Schizonts are large, gametocytes elongated and crescentic. Species include Plasmodium (Laverania) falciparum. Plasmodium. Hosts: primates. Vector: anopheline mosquitoes. Schizonts are large, gametocytes spherical. Species include Plasmodium (Plasmodium) cynomolgi, P. (P.) knowlesi, P. (P.) malariae, P. (P.) ovale, and P. (P.) vivax. Vinckeria. Hosts: non-primate mammals. Vector: anopheline mosquitoes. Schizonts are small, gametocytes spherical. Other (avian) subgenera: Giovannolaia, Novyella, Huffia. 596 Pluronic polyol F127 which hyphae are usually not distinguishable as such, the tissue consisting of closely packed, isodiametric or oval cells (resembling the parenchyma of higher plants). ‘Palisade plectenchyma’ occurs in the cortex of many lichen thalli, and consists of short hyphae orientated more or less perpendicular to the thallus surface. Plectonema A genus of cyanobacteria of the LPP GROUP. Although non-heterocystous, P. boryanum can carry out NITROGEN FIXATION under microaerobic (but not under aerobic) conditions. Plectrovirus (mycoplasma virus type 1 phages; L1 acholeplasmavirus group) A genus of BACTERIOPHAGES of the INOVIRIDAE. Host: Acholeplasma laidlawii ; plaques turbid (sometimes with a clear centre), 0.5–6.0 mm across. Other strains of Acholeplasma and Mycoplasma may be infected. The virions are short, naked, bullet-shaped rods (70–90 ð 14–16 nm) comprising ss cccDNA (MWt 1.5 ð 106 ) and four proteins. Proposed type member: MV-L51 (D L1 strain L51). Other member: MV-L1 (D L1 strain L1). Possible member: SV-C1 (see SPIROPLASMAVIRUSES). See also MYCOPLASMAVIRUSES. pleiotropic Refers to e.g. a gene or a mutation which has multiple effects: e.g., expression of a pleiotropic gene affects more than one phenotypic characteristic. Pleistophora See MICROSPOREA. pleomorphic Refers to e.g. an organism which exhibits PLEOMORPHISM. pleomorphism (1) In general: an inherent variability in size and shape e.g. among the cells in a pure culture of a given organism. Cells of a pleomorphic organism (e.g. Corynebacterium spp, Propionibacterium spp) may occur in a wide range of indeterminate forms which are often variations of a single basic shape: e.g. the cells may be basically rod-shaped but may be swollen at one or both ends, curved or straight, branched or unbranched, etc. (cf. POLYMORPHISM sense 1.) (2) Syn. POLYMORPHISM sense 1. (3) (mycol.) Degenerative changes – e.g. loss of the ability to form conidia and/or to synthesize pigment – which occur in some fungi (e.g. certain dermatophytes) when propagated in laboratory cultures. Pleospora See DOTHIDEALES. Plesiomonas (‘C27 organisms’) A genus of Gram-negative bacteria of the VIBRIONACEAE. Cells: straight, round-ended rods, 0.8–1.0 ð 1–3 µm. Usually motile with 2–5 lophotrichous unsheathed flagella; lateral flagella may be formed in young cultures on solid media. Optimum growth temperature: ca. 35–38° C. Growth does not require NaCl and cannot occur with 7.5% NaCl. Growth media: e.g. SS, XLD or Hektoen agars [media, identification etc: Book ref. 46, pp. 1285–1288]. Oxidase Cve; acid but not gas is formed from glucose and e.g. inositol; extracellular DNase, gelatinase and lipase are not formed; most strains are sensitive to O/129. Strains may possess the enterobacterial common antigen, and a few share an O antigen with Shigella sonnei. GC%: 51. Type species: P. shigelloides – found in association with fish and other aquatic animals and in mammals; strains can cause e.g. gastroenteritis in man. [Book ref. 22, pp. 548–550.] According to 5S rRNA sequence analysis, P. shigelloides is more closely related to Proteus mirabilis than is Proteus vulgaris and should therefore be included in the genus PROTEUS as Proteus shigelloides [SAAM (1985) 6 171–182]. (See also ENTEROBACTERIAL COMMON ANTIGEN.) Pleurasiga See CHOANOFLAGELLIDA. Pleurastrophyceae See TETRASELMIS and TREBOUXIA. Pleurastrum A genus of filamentous green algae related to the filaments are branched and lack holdfasts. P. terrestre may also grow as unicells under certain conditions. Pleuroascus See EUROTIALES. Pleurocapsa See PLEUROCAPSA GROUP. Pleurocapsa group A large and diverse group of unicellular CYANOBACTERIA (section II) in which growth and binary fission lead to the formation of irregular, sometimes pseudofilamentous cell aggregates; baeocytes initially lack a fibrous outer cell wall layer and are motile (by gliding). In some members (subgroup I) growth of the baeocyte is symmetrical, resulting in a spherical vegetative cell, while in others (subgroup II) growth is asymmetrical and vegetative cells are elongated prior to the onset of binary fission. The group includes organisms previously included in many phycological genera (e.g. Hyella, Pleurocapsa) which were based on characters such as endolithic habit, arrangement of vegetative cells in aggregates, etc. GC%: 39–47. pleurocapsalean (1) Refers to BAEOCYTE-forming CYANOBACTERIA (section II). (2) Refers to any member of the phycological order Pleurocapsales. Pleurocapsales See CYANOBACTERIA. Pleurococcus A genus of unicellular green algae (division CHLOROPHYTA) which are extremely common on wood, bark etc; the cells generally occur in characteristic packets. The organisms have been referred to by various names (Pleurococcus viridis, P. vulgaris, Protococcus viridis, etc), but apparently should be called Desmococcus olivaceus [Taxon (1985) 34 671–672]. pleurodynia See BORNHOLM DISEASE. Pleuronema See SCUTICOCILIATIDA. Pleurosigma See DIATOMS. Pleurotus A genus of fungi of the APHYLLOPHORALES (family Polyporaceae) in which the basidiocarp is commonly pileate and stipitate (stipe often eccentric or lateral); the hymenium is borne on decurrent lamellae (see LAMELLA). P. ostreatus is an edible species (see OYSTER FUNGUS) which grows on wood and which can be pathogenic for certain trees, particularly beech (Fagus). plicate Folded. Plodia interpunctella GV See GRANULOSIS VIRUSES. ploidy The number of (complete) sets of chromosomes (or AUTOSOMES) in a cell. (See also DIPLOID, HAPLOID, POLYPLOID, ANEUPLOID.) plugging (of pipettes) The insertion of a small plug of COTTON WOOL into the wide end of a pipette or PASTEUR PIPETTE before sterilization; liquids being pipetted are thus protected from contamination from the rubber bulb, pi-pump etc. plum pox (sharka disease) A disease affecting many species of Prunus (e.g. plum, apricot, peach, blackthorn); it is caused by a virus of the POTYVIRUSES and is transmitted by aphids and by infected rootstocks. Symptoms commonly include diffuse mottling or ring-spots on the leaves, premature fruit-drop, and (depending on fruit type) dark bands or rings, or grooving and pitting, on the fruits. plumbagin 5-Hydroxy-2-methyl-1,4-naphthoquinone. plunging jet See JET LOOP FERMENTER. pluracidomycins CARBAPENEM antibiotics. plurivorous Able to use a range of hosts or substrates. Pluronic polyol F127 A copolymer of ethylene oxide and polypropylene oxide used e.g. as a solidifying agent for culture media. (See also MEDIUM.) A polyol-solidified medium can be liquefied by cooling, the gel–liquid transition temperature depending on the concentration of polyol used. The clear, paste-like medium is apparently unsuitable for streaking, but is TREBOUXIA; 597 plus-progamone Diagnosis of Pneumocystis pneumonia typically involves microscopy of induced sputum or of fluid from broncho-alveolar lavage. Asci can be seen by staining with either TOLUIDINE BLUE or Gomori’s silver stain. Individual cells are detected with Giemsa or Wright’s stain [AJCP (1984) 81 511–514]. Immunofluorescence stains are also available for P. carinii. [Pneumocystis carinii (epidemiology, pneumonia, laboratory diagnosis, therapy etc.): BCID (1995) 2 409–576. Immune response to P. carinii infection: TIM (1998) 6 71–75. P. carinii pneumonia in pigs (immunohistochemical study): VR (2000) 147 544–549.] pneumolysin See THIOL-ACTIVATED CYTOLYSINS. pneumonia Inflammation of the lungs. It may involve the alveoli (cf. ALVEOLITIS) and/or the interstitial tissues (interstitial pneumonia); inflammation may affect most of the parenchyma in one or more lobes (lobar pneumonia) or may be diffuse. Pneumonia is commonly due to bacterial infection, often secondary to e.g. viral URTI (such as INFLUENZA) or other infection (e.g. MEASLES); other causal agents include e.g. viruses (see also PNEUMOCYSTIS). The pathogen may gain access to the lungs by inhalation or via the blood or lymph systems (see e.g. bubonic PLAGUE). Conditions which predispose to pneumonia include chronic lung disease, debilitating illness, immunosuppression, etc. Symptoms of pneumonia generally include fever, chills, rigors, malaise, dyspnoea and chest pain; cough may be initially dry, later productive of purulent, sometimes bloody sputum. Lab. diagnosis: examination of sputum, pleural fluid etc by microscopy, culture, and/or serological techniques (e.g. CIE). Sputum may be obtained by percutaneous aspiration from the trachea, rather than by expectoration, to reduce contamination by members of the normal respiratory tract flora. (a) Pneumococcal pneumonia (‘classical lobar pneumonia’), caused by certain strains of Streptococcus pneumoniae (the ‘pneumococcus’), is the commonest form of bacterial pneumonia. Infection may be primary or secondary. The pathogen, present e.g. in the upper respiratory tract of carriers, gains access to the lungs by inhalation. Incubation period: 1–3 days. Affected lobes typically become consolidated, i.e., solidifed with inflammatory material (leucocytes, fibrin, pneumococci etc). Sputum is initially purulent, becoming ‘rusty’ or blood-stained as the consolidation breaks down. Complications include e.g. pericarditis, meningitis etc. Chemotherapy: e.g penicillins, erythromycin. Polyvalent vaccines containing polysaccharide antigens of various common pneumococcal serotypes are available for certain high-risk patients [NEJM (1984) 310 651–653; AIM (1986) 104 106–109, 110–112, 118–120]. (b) Haemophilus influenzae (usually type b) may cause primary pneumonia (mainly in children) or secondary pneumonia in patients with e.g. viral respiratory tract infection. The pneumonia may be diffuse or lobar, and abscesses may develop. Chemotherapy: e.g. ampicillin; tetracycline; chloramphenicol. (c) Staphylococcus aureus can cause lobar or diffuse interstitial pneumonia, usually following e.g. influenza or staphylococcal septicaemia. Abscesses may develop in lungs and/or pleura. Sputum is yellow, purulent, and sometimes bloody. Proteases from some strains of S. aureus may activate influenza virus virions (by cleaving their haemagglutinins: see INFLUENZAVIRUS), thus promoting combined viral–bacterial pneumonia [Nature (1987) 325 536–537]. Chemotherapy: e.g. b-lactamaseresistant b-lactam antibiotics. (d) Friedländer’s pneumonia (caused by Klebsiella pneumoniae) is a severe form which usually occurs as a complication of e.g. chronic lung disease. Typically, one or more lobes become reported to be suitable for use e.g. in the pour plate technique for heat-sensitive organisms, and in gradient gel systems. [JGM (1984) 130 731–733.] plus-progamone See PHEROMONE. plus strand (of a gene) (mol. biol.) See CODING STRAND. plus strand (virol.) See VIRUS. Pluteaceae See AGARICALES. PM2 phage group Syn. CORTICOVIRIDAE. pMB1 A small, multicopy PLASMID which carries genes for colicin E1, the colicin E1 immunity protein, and the EcoRI restriction and modification enzymes. pMB1 belongs to the same incompatibility group as the COLE1 PLASMID, and its ori region shares extensive homology with that of the ColE1 plasmid. pmf Proton motive force: see CHEMIOSMOSIS. pMG25 See INCOMPATIBILITY. PML PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY. pML31 An autonomous 9-kb plasmid obtained by digestion of the F PLASMID with EcoRI restriction endonuclease and the ligation of a part of the plasmid (containing an origin of replication) with an antibiotic-resistance determinant. (See also MINI-F PLASMID.) PMN Polymorphonuclear leucocyte (‘polymorph’; granulocyte): a type of LEUCOCYTE which has a lobed nucleus and granular cytoplasm (hence ‘granulocyte’); diam. ca. 10–15 µm. PMNs include NEUTROPHILS, BASOPHILS and EOSINOPHILS – so named because of the affinity of their cytoplasmic granules for particular dyes; they are short-lived cells which do not replicate and which are continually replaced by cells derived from bone marrow. ‘Polymorph’ may refer to PMNs in general, but is sometimes used to refer solely to neutrophils. PNA (peptide nucleic acid) An analogue of DNA, the backbone chains of which are modified polypeptides (rather than sugar–phosphate units). Two strands of PNA can hybridize to form a DNA-like helical duplex [Nature (1994) 368 561–563]; the existence of such a molecule has prompted the suggestion that pre-biotic nucleic acids may not necessarily have had a sugar–phosphate backbone. A strand of PNA can hybridize with a strand of DNA; this may be of use in DNA CHIP technology. PNA probes have been used for identifying and enumerating specific microorganisms by an in situ hybridization technique [JAM (2001) 90 180–189]. pncA gene See PYRAZINAMIDE. pneumatocyst (air bladder/vesicle; bladder; gas bladder) A gasfilled structure normally present in the thallus of certain algae (e.g. species of Fucus, Pelagophycus and Sargassum); a given thallus may contain many pneumatocysts. Pneumatocysts, which are generally observable as distinct swellings, contribute buoyancy to the thallus. (cf. GAS VESICLE.) pneumococcal pneumonia See PNEUMONIA (a). pneumococcus Streptococcus pneumoniae. pneumococcus capsule swelling reaction Syn. QUELLUNG PHENOMENON. Pneumocystis A genus of fungi (formerly regarded as protozoa) now classified as members of the subdivision Ascomycotina. P. carinii is an important causal agent of pneumonia in HIVinfected patients; however, it can also give rise to infection in e.g. children and elderly individuals in the absence of HIV infection [BCID (1995) 2 461–470]. P. carinii is also a causal agent of disease in animals (e.g. pigs). The cells of P. carinii are ¾1–5 µm; asci (10–12 µm) each contain up to eight ascospores. No satisfactory culture technique has been developed. 598 poky mutant subsequently darken. [Phytopathol. Paper number 24 (March, 1981).] Podaxales See GASTEROMYCETES. podetium (lichenol.) A vertical, stem-like, lichenized structure of generative tissue (sometimes called a ‘secondary thallus’) which develops from a crustose, squamulose or foliose basal (‘primary’) thallus and which supports one or more hymenial discs (‘apothecia’); the entire podetium – together with its hymenial disc(s) – should be regarded as a fruticose APOTHECIUM [Lichenol. (1982) 14 105 – 113]. (cf. PSEUDOPODETIUM.) Podetia are formed e.g. by most species of CLADONIA; they are usually hollow and may be branched or unbranched or – in some Cladonia spp – may terminate in a cup- or funnel-shaped structure (the ‘scyphus’). (According to the above definition, structures resembling podetia but lacking algae are not true podetia; such structures are formed e.g. by Baeomyces spp and Cladonia caespiticia.) Podophrya See SUCTORIA. podophyllotoxin A substance of plant origin (obtained from Podophyllum peltatum) which has antitumour activity. Podophyllotoxin, which contains a trimethoxy-substituted aromatic ring, binds to TUBULIN at a site close to (or identical with) that at which COLCHICINE binds – inhibiting the assembly of MICROTUBULES. Some epipodophyllotoxins (which have been used as antitumour agents) appear to affect DNA synthesis (rather than microtubule assembly) – apparently by inhibiting topoisomerase activity. Podoscypha See APHYLLOPHORALES (Stereaceae). Podosphaera See ERYSIPHALES. Podospora A genus of fungi (order SORDARIALES) which occur e.g. on soil and dung; the organisms form dark ascospores, each bearing gelatinous appendages, in persistent asci formed in perithecia. In P. anserina the four binucleate ascospores per ascus each contain nuclei of compatible mating types (cf. secondary HOMOTHALLISM). (See also AMIXIS.) Podoviridae A family of DNA-containing BACTERIOPHAGES which have isometric or elongated heads and short (ca. 20 nm) non-contractile tails. Members include BACTERIOPHAGE N4, P22, f29, T3, T7 (type species), and TBILISI PHAGE (see separate entries). (See also MV-L3 PHAGE GROUP and SPIROPLASMAVIRUSES (SV-C3).) Pogosta disease See SINDBIS VIRUS. Poikilovirus See ALPHAHERPESVIRINAE. point mutation (1) A type of MUTATION in which a single nucleotide is replaced by another: see TRANSITION MUTATION and TRANSVERSION MUTATION. A point mutation in a structural gene may result in a MIS-SENSE MUTATION, a NONSENSE MUTATION, or a SAME-SENSE MUTATION. Mutagens which can induce point mutations include e.g. 5-BROMOURACIL, HYDROXYLAMINE and NITROUS ACID. (2) Any mutation involving a single nucleotide, including the gain or loss of a nucleotide (resulting in a FRAMESHIFT MUTATION) as well as transition and transversion mutations. (cf. MULTISITE MUTATION.) poising system See REDOX POTENTIAL. Poitrasia See MUCORALES. pokeweed mitogen (PWM) Any of five mitogenic LECTINS (designated Pa-1 to Pa-5) obtained from the roots, leaves and berries of the pokeweed, Phytolacca americana. All can stimulate T cells; Pa-1, in the presence of T cells and macrophages, can stimulate Ig secretion by B cells. PWM binds to oligomers of b-D-acetylglucosamine. poky mutant A mutant strain of Neurospora which has a subnormal rate of growth owing to a deficiency in its respiratory densely infiltrated, and thin-walled abscesses occur. Sputum is often viscid, gelatinous and blood-stained. Chemotherapy: e.g. gentamicin; amikacin; cephalosporins. (e) Streptococcus pyogenes may cause primary or secondary pneumonia which is usually diffuse and often leads to empyema. Chemotherapy: e.g. penicillins. (f) Various other Gram-negative bacteria may cause primary or secondary pneumonia – particularly in the very young or elderly, debilitated, or immunosuppressed, often following antibiotic therapy. Nosocomial pneumonia may be due e.g. to species of Enterobacter, Klebsiella, Proteus, Serratia, Escherichia coli or Pseudomonas aeruginosa. Anaerobes (e.g. Bacteroides spp) may infect the lungs, often with abscess formation; infection originates from the mouth or – via the lymph or blood systems – from e.g. intra-abdominal abscesses. (See also LEGIONELLOSIS.) (g) Certain viruses (e.g. adenoviruses, coxsackieviruses, parainfluenza viruses, respiratory syncytial virus) can cause pneumonia, particularly in small children. (See also (c), above.) Cf. PNEUMONITIS and PRIMARY ATYPICAL PNEUMONIA. See also e.g. ASPERGILLOSIS; COCCIDIOIDOMYCOSIS; HISTOPLASMOSIS; PLAGUE; PNEUMOCYSTIS. For pneumonia in animals see e.g. ATYPICAL INTERSTITIAL PNEUMONIA; BOVINE RESPIRATORY DISEASE; CALF PNEUMONIA; CONTAGIOUS BOVINE PLEUROPNEUMONIA: JAAGSIEKTE; MAEDI. pneumonia of mice virus See PNEUMOVIRUS. pneumonic pasteurellosis See BOVINE RESPIRATORY DISEASE. pneumonitis (1) Syn. PNEUMONIA. (2) Inflammation of the lungs as part of a more generalized syndrome, or resulting from chemical or physical injury or allergy (as opposed to infection). (cf. ALVEOLITIS.) pneumotropic Having an affinity for the lungs. Pneumovirus A genus of viruses of the PARAMYXOVIRIDAE in which the virions have neither haemagglutinin nor neuraminidase activity. Type species: (human) respiratory syncytial virus (RSV), an important cause of respiratory disease in humans (particularly infants and young children) – see e.g. BRONCHIOLITIS, COMMON COLD, CROUP, PNEUMONIA (g). [Review of RSV: Arch. Virol. (1985) 84 1–52. Experimental models for RSV infections: RMM (1996) 7 115–122.] Other members of the genus: bovine respiratory syncytial virus (see BOVINE RESPIRATORY DISEASE) and pneumonia of mice virus (common in laboratory mice, in which infection is latent or mild). PNPase POLYNUCLEOTIDE PHOSPHORYLASE. P/O ratio (P:O ratio) A measure of the efficiency of OXIDATIVE PHOSPHORYLATION: the number of molecules of ATP formed per atom of oxygen used. (cf. HC /O RATIO.) POC Particulate organic carbon. (cf. DOC.) Pocheina See ACRASIOMYCETES. pock (1) (med.) A cutaneous pustule, as formed e.g. in SMALLPOX. (2) (virol.) A lesion produced on the CAM (see EMBRYONATED EGG) by certain viruses; the nature and number of pocks formed can be useful in the identification and assay of the virus (see e.g. COWPOX VIRUS, VACCINIA VIRUS and VARIOLA VIRUS). pocket rot (of timber) A form of WHITE ROT in which decay is limited to small, discrete regions (‘pockets’) and LIGNIN is attacked preferentially. Pocket rot fungi include Phellinus pini (in conifers) and Inonotus sp (in oaks). [Mycol. (1983) 75 552–556.] pod rot (frosty pod; Quevedo disease) A CACAO DISEASE caused by Moniliophthora roreri and characterized by a whitish (‘frosted’) powdery covering of spores on the pods; the spores 599 pol genes polaroplast See MICROSPORA. polB gene See DNA POLYMERASE. polC gene See DNA POLYMERASE. poliomyelitis (polio; infantile paralysis) An acute infectious disease of humans, particularly children, caused by any of three serotypes of human poliovirus (see ENTEROVIRUS). (A few nonhuman primates, e.g. chimpanzees, are also susceptible.) Transmission occurs by the faecal–oral route (e.g. by person-toperson contact or via sewage-contaminated water). Incubation period: 3–35 (usually 7–14) days. Infection may be asymptomatic; clinical forms are usually mild and self-limiting, with fever, headache, nausea and vomiting, and pharyngitis (abortive poliomyelitis). In a minority of cases the virus may invade the CNS, causing inflammation and destruction of the lower motor neurones in the brain stem and spinal cord. In non-paralytic poliomyelitis symptoms include those of MENINGITIS (e.g. stiffness of the neck and back). In paralytic poliomyelitis there is weakness and eventually flaccid paralysis of muscles (commonly of the legs); the paralysis may or may not be permanent. Polio is not usually fatal, but may be if respiratory muscles become involved. Lab. diagnosis: serological tests; culture of the virus from pharyngeal secretions and faeces. (See also SALK VACCINE and SABIN VACCINE.) polioviruses See ENTEROVIRUS. Polish infectious dropsy (of carp) See CARP ERYTHRODERMATITIS. pollen mould Bettsia alvei (q.v.). pollution (environmental) See ENVIRONMENTAL POLLUTION. poly(A) tail See MRNA. polyacetylenes (polyynes) Linear compounds, containing both double and triple bonds, which are produced by certain higher fungi (particularly basidiomycetes) and which, in some cases, exhibit antifungal and antibacterial activity; the polyacetylene molecule usually has an oxygen-containing terminal group (e.g. a carboxyl or hydroxyl group). For example, mycorrhizal Leucopaxillus cerealis produces diatretyne nitrile (HOOC
CHDCH
CC
CC
CN) which is active e.g. against Phytophthora cinnamomi. polyacrylamide gel electrophoresis ELECTROPHORESIS using a polyacrylamide gel – see e.g. SDS-PAGE. polyadenylated mRNA See MRNA. polyamines Polycationic compounds containing two or more amino groups; they appear to occur in all animal, plant and microbial cells and in certain viruses. Putrescine [NH2 (CH2 )4 NH2 ], spermidine [NH2 (CH2 )3 NH (CH2 )4 NH2 ] and spermine [NH2 (CH2 )3 NH(CH2 )4 NH(CH2 )3 NH2 ] appear to be almost universally present in prokaryotic and eukaryotic microorganisms. However, their cellular location(s) and biological function(s) are still not known, although numerous effects on e.g. nucleic acid structure and enzyme activity have been observed in vitro; polyamines may have a role e.g. in protein synthesis in bacteria, and are necessary for the replication of at least some bacteriophages, including the T-even phages (which contain putrescine and spermidine) and bacteriophage l. (See also BACTERIOPHAGE fW-14.) Polyamines are synthesized primarily from amino acids – e.g. putrescine may be formed by the decarboxylation of ornithine or by the combined action of arginine decarboxylase and agmatine ureohydrolase. (See also DECARBOXYLASE TESTS.) In bacteria the polyamine content is significantly affected by the growth conditions. A number of novel polyamines have been described from extreme thermophiles: e.g. ‘Thermus thermophilus’ contains polyamines such as thermine chain. The mutant gene (D mi-1 or maternal inheritance-1 gene) appears to occur in the mitochondrial DNA, and is transmitted to sexually-derived progeny only from a female (ascogonial) parent; such MATERNAL INHERITANCE is presumed to be due to the absence of mitochondria in the (male) microconidium. (cf. PETITE MUTANT.) pol genes (1) See DNA POLYMERASE. (2) See RETROVIRIDAE. polA gene See DNA POLYMERASE. polar body An intracellular granule at one or each pole of a cell – e.g. the deposits of PHB in Beijerinckia. polar cap See MICROSPORA. polar capsule See MYXOZOA. polar fibre See MITOSIS. polar filament See MICROSPORA and MYXOZOA. polar membrane A multilaminar intracellular membrane, attached to the inside of the cytoplasmic membrane by barlike links, present at the poles (sites of flagellar attachment) of the cells in species of helical and vibrioid bacteria (e.g. Aquaspirillum, Campylobacter, Ectothiorhodospira, Oceanospirillum, Rhodospirillum, Spirillum). polar microtubule See MITOSIS. polar mutation A mutation which, in addition to affecting the gene in which it occurs, reduces the expression of any gene(s) in the same (polycistronic) operon on the promoter-distal side of the mutation – a phenomenon known as polarity; genes between the promoter and the mutation are not affected. Thus, e.g., in the LAC OPERON a nonsense mutation in the y gene may eliminate permease synthesis, does not affect b-galactosidase synthesis, and reduces the amount of transacetylase synthesis. The mutation is said to be strongly polar when the expression of promoterdistal genes is strongly inhibited; weakly polar mutations cause only slight inhibition. The strength of polarity depends on the distance between the mutation and the end of the gene in which it occurs (i.e., between the mutation and the next translation initiation site), being greatest when this distance is greatest. According to one model for polarity, cessation of translation (due e.g. to the presence of a nonsense mutation) may expose in the mRNA a r-dependent TRANSCRIPTION termination site which is normally protected from the r factor by the presence of ribosomes; interaction of r with this site results in premature termination of transcription and hence loss of expression of promoter-distal genes. Polar mutations can be suppressed by mutations in the rho gene, the mutant r factor apparently failing to interact with the premature termination signal; polarity suppressors thus restore expression of genes downstream of the mutant gene but not of the mutant gene itself. (cf. SUPPRESSOR MUTATION.) The polar effects of certain TRANSPOSABLE ELEMENTS (e.g. IS2 ) have been attributed to the presence of a r-dependent terminator within the TE sequence. In some cases a polar mutation does not appear to affect transcription; in such cases ribosomes, having dissociated from the mRNA at a nonsense mutation, may be unable to re-initiate translation at the next initiation site – possibly because of the formation of a secondary structure in the untranslated mRNA. polar plaque Syn. SPINDLE POLE BODY. polar ring An osmiophilic annular structure, one or more of which occur at the anterior end in members of the APICOMPLEXA. Polar ring(s) may act as support(s) for the opening in the inner layer of the pellicle, and they may be involved in the function of the conoid. polarilocular Refers to a lichen spore consisting of two cells which are separated by a thick septum that has a central canal. (See also TELOSCHISTALES.) 600 poly-b-hydroxybutyrate and small molecules; the polyene molecule may form a pore in the membrane through which such leakage may occur. These antibiotics are microbistatic at low concentrations, microbicidal at higher concentrations. Macrolide polyenes are effective against yeasts and other fungi and also against protozoa, but not against most bacteria (whose membranes lack sterols – cf. MYCOPLASMA). The selective toxicity of the clinically useful macrolide polyenes is apparently due, at least in part, to their greater affinity for ergosterol (a principal sterol in fungal cytoplasmic membranes) compared with cholesterol (the main sterol in mammalian cell membranes) (cf. FILIPIN). Macrolide polyenes include e.g. AMPHOTERICIN B; AUREOFUNGIN; CANDICIDIN B; ETRUSCOMYCIN; FILIPIN; HAMYCIN; NYSTATIN; PERIMYCIN; PIMARICIN; and TRICHOMYCIN. (b) A group of structurally novel polyenes (produced by actinomycetes [FEMS (1984) 25 121–124]) which are active against prokaryotes, inhibiting PROTEIN SYNTHESIS. In bacteria, aurodox, azdimycin, dihydromocimycin, efrotomycin, factumycin, kirromycin (D mocimycin) and kirrothricin all act by binding to EF-Tu and preventing its dissociation from the ribosome; the structurally related antibiotic pulvomycin binds EFTu-GTP and prevents the formation of the aa-tRNA-GTP-EF-Tu complex, thus preventing the binding of aa-tRNA to the ribosomal A site. In archaeans, elongation factors are generally insensitive to kirromycin, and the elongation factors of only some archaeans are sensitive to pulvomycin [JB (1986) 167 265–271]. polyether antibiotics See MACROTETRALIDES. polyethylene glycol (PEG; Carbowax) A polymer, H(OCH2 . CH2 )n OH, available in a range of grades of mean MWt between ca. 200 and ca. 20000; above MWt ca. 1000 it is a solid, below it is a liquid. PEG is used e.g. in cell fusion (see e.g. HYBRIDOMA), in PROTOPLAST FUSION, in artificial TRANSFORMATION systems, and as a precipitating agent in clinical chemistry. PEG is also used for concentrating, by osmosis, serum and other aqueous solutions and suspensions; serum etc is placed in a U-shaped tube of semipermeable material (e.g. Visking tubing) and the tubing partly submerged in a concentrated aqueous solution of PEG. poly(ethylene oxide) A high-MWt polymer (
CH2 CH2 O
)n used e.g. to increase the viscosity of a medium in order to slow down rapidly motile organisms – see COUNTING METHODS. (cf. FICOLL.) polygenic mRNA See MRNA. polyhedra (virol.) Large inclusion bodies formed in the cells of insects infected with certain viruses (see CYTOPLASMIC POLYHEDROSIS VIRUS GROUP and BACULOVIRIDAE); a polyhedron is composed of a matrix of crystalline, virus-specific protein (polyhedrin) in which mature virions are embedded (‘occluded’). Polyhedra are stable structures which appear to give some protection to the virions they contain; they serve as vehicles for virus transmission: ingestion of polyhedra by a new host is followed by degradation of polyhedrin in the gut and the release of infectious virions. Polyhedra can be isolated from infected cells e.g. by differential centrifugation. polyhedral bodies See CARBOXYSOMES. polyhedrin See POLYHEDRA and BACULOVIRIDAE. polyhedrosis An INSECT DISEASE caused by a NUCLEAR POLYHEDROSIS VIRUS or by a virus of the CYTOPLASMIC POLYHEDROSIS VIRUS GROUP. poly-b-hydroxyalkanoate See POLY-b-HYDROXYBUTYRATE. poly-b-hydroxybutyrate (PHB) A linear polymer of b-hydroxybutyrate (D(
)-3-hydroxybutyrate): H-[O.CH(CH3 ).CH2 .CO]n OH. It occurs as refractile granules in the cells of various [D sym-norspermine: NH2 (CH2 )3 NH(CH2 )3 NH(CH2 )3 NH2 ], sym-homospermidine [NH2 (CH2 )4 NH(CH2 )4 NH2 ], thermospermine [NH2 (CH2 )3 NH(CH2 )3 NH(CH2 )4 NH2 ] and also caldopentamine [NH2 (CH2 )3 NH(CH2 )3 NH(CH2 )3 NH(CH2 )3 NH2 ]. These substances have been implicated in the maintenance of thermostability in cell components, enzymes, etc; however, symhomospermidine has also been found in mesophilic bacteria [FEMS (1983) 20 159–161]. [Reviews: MR (1985) 49 81–99; ARB (1984) 53 749–790.] Polyangium See MYXOBACTERALES. poly(A)polymerase See MRNA. polybetahydroxybutyrate See POLY-b-HYDROXYBUTYRATE. polycentric Refers to a thallus in which a number of regions, in different parts of the thallus, each have a reproductive function. (cf. MONOCENTRIC.) polychlorinated biphenyls (PCBs) See BIOREMEDIATION. polychrome methylene blue See METHYLENE BLUE. polycistronic mRNA See MRNA. polyclonal activator (immunol.) Any non-specific activator of lymphocytes, i.e. any agent which can bring about differentiation/proliferation in each of a number of different clones. Polyclonal activators for B LYMPHOCYTES include lipopolysaccharides and various mitogens; such agents are THYMUSINDEPENDENT ANTIGENS. SUPERANTIGENS are polyclonal activators for T LYMPHOCYTES. polyclonal antiserum Any antiserum prepared by injecting an animal with a given antigen; it contains a heterogeneous population of antibodies – each antibody being specific to one of the (usually many) determinants present on the antigen. (cf. MONOCLONAL ANTIBODIES.) polycycline See TETRACYCLINES. Polycystinea See RADIOLARIA. Polydnaviridae A family of viruses [Intervirol. (1986) 25 141–143], previously classified as subgroup D of the BACULOVIRIDAE, characterized by their polydisperse ccc dsDNA genomes. Virus replication occurs in host cell nuclei; virions are not occluded. The viruses infect certain species of parasitic hymenopteran insects. Two distinct groups (genera) are recognized. In one group (genus unnamed) the virion has a cylindrical nucleocapsid of variable length, and virions are surrounded – either individually or in groups – by a unit membrane envelope; these viruses infect braconid wasps. In the other group (genus Polydnavirus) the virions have fusiform nucleocapsids surrounded by two envelopes; these viruses infect ichneumonid wasps (e.g. Hyposoter spp). Viruses of both groups can apparently be transmitted vertically [Virol. (1986) 155 120–131]. Type species: polydnavirus type 1 (Hyposoter exiguae virus). polyene antibiotics ANTIBIOTICS which contain region(s) of conjugated double bonds. There are two structurally and functionally distinct types. (a) A group of MACROLIDE ANTIBIOTICS (produced by streptomycetes) characterized by a large lactone ring containing a rigid, lipophilic region of (generally) unsubstituted, all-trans conjugated double bonds and a flexible, hydrophilic, hydroxylated region; the lactone may be substituted with aminosugar, carboxyl, aliphatic or aromatic groups. These polyenes are classified according to the number of conjugated double bonds they contain (e.g. tetraenes, heptaenes). They are poorly soluble in water but are soluble in organic solvents; they are unstable in solution in the presence of light owing to photo-oxidation of the double bonds. The macrolide polyenes interact with sterols in the cytoplasmic membrane of sensitive organisms, causing leakage of ions 601 Polyhymenophorea which is synthesized by a multifunctional enzyme, polyketide synthase. The possibility of developing new drugs by genetic engineering of the polyketide synthetic pathway has been hindered by the difficulty of carrying out such work with actinomycetes; to address this problem, synthetic potential (including polyketide synthase activity) has been incorporated into a strain of Escherichia coli – which can now synthesize the nucleus of the erythromycin molecule [Science (2001) 291 1790–1792; commentary: Science (2001) 291 1683]. polylinker A region of DNA containing close/overlapping sites for different RESTRICTION ENDONUCLEASES. Synonym: multiple cloning site (MCS). polymerase chain reaction See PCR. polymetaphosphate See POLYPHOSPHATE. polymorph (polymorphonuclear leucocyte) (1) Syn. PMN. (2) Syn. NEUTROPHIL. polymorphic Refers to e.g. an organism which exhibits POLYMORPHISM. polymorphism (1) The ability of an organism to occur in two or more morphologically distinct forms (MORPHOTYPES), according e.g. to conditions (as in PHAEODACTYLUM) or to the stage in the life cycle (e.g. TRYPANOSOMATIDAE). (cf. PLEOMORPHISM sense 1). (2) Syn. PLEOMORPHISM (sense 1). (3) (in nucleic acids) A variant region of nucleotides in a related nucleic acid molecule – e.g. a variant sequence found in a given allele in one or more strains of an organism but not in all strains of that organism. Some authors use the term even when the variation in sequence consists of only a point mutation. Polymyxa See PLASMODIOPHOROMYCETES. polymyxins A group of peptide ANTIBIOTICS produced by various species of Bacillus (e.g. B. polymyxa); they include e.g. antibiotics formerly classified as circulins and colistins. Polymyxins are active against many Gram-negative bacteria – exceptions including e.g. many strains of Neisseria and Proteus, and the eltor biotype of Vibrio cholerae (the cholerae biotype is susceptible); most Gram-positive bacteria and most fungi are resistant, but some strains of e.g. Candida are susceptible. Against bacteria, polymyxins are bactericidal to both growing and nongrowing cells. Polymyxins have MWts of ca. 1000–1200, and they contain a high proportion of a,g-diaminobutyric acid (DAB). The polymyxin molecule is a cyclic heptapeptide with a peptide side-chain that terminates in a fatty acid residue; the fatty acid is 6-methyloctanoic acid in e.g. polymyxins A, B1, D1 and E1 (D colistin A), and is 6-methylheptanoic acid in e.g. polymyxins B2, D2 and E2. Variations also occur in the constituents of the cyclic and linear peptides. [Structure of polymyxins: Book ref. 14, pp. 189–194.] In Gram-negative bacteria, polymyxins act by increasing the permeability of the CYTOPLASMIC MEMBRANE to small molecules and by damaging the OUTER MEMBRANE. Polymyxin B nonapeptide, a derivative of polymyxin B which lacks the fatty acid residue, lacks the bactericidal activity of polymyxin B but is able to damage the outer membrane [CJM (1986) 32 66–69]. (See also OCTAPEPTINS.) polynucleotide kinase An enzyme used e.g. for the 50 -end labelling of a PROBE (q.v.). polynucleotide phosphorylase (PNPase) An enzyme, present e.g. in Escherichia coli and other prokaryotes, which in vitro can catalyse e.g. the phosphorolysis of poly- or oligoribonucleotides in the presence of phosphate, and the polymerization of ribonucleoside diphosphates. polyol (polyhydric alcohol) Strictly: any compound containing more than one alcohol group. As commonly used, the term refers PROKARYOTES, including cyanobacteria and members of the Chromatiaceae and Rhodospirillaceae. PHB synthase (and possibly also de-polymerizing enzymes) occur at the surface of the granules. Each mature granule is bounded by a layer apparently <4 nm in thickness – believed to be a monolayer consisting mainly of amphiphilic proteins (phasins) and phospholipids, all the molecules orientated with their hydrophilic regions facing the cytoplasm [JBM (1997) 37 45–52]. PHB granules can be stained in situ (i.e. in the cell) by dyes such as NILE BLUE A. In many bacteria PHB accumulates when nutrients other than carbon sources become limiting, being degraded when these conditions are reversed; PHB thus acts as a reservoir of carbon and/or energy. Azotobacter beijerinckii accumulates very large quantities (up to 80% of the cell mass) of PHB under conditions of oxygen limitation, PHB serving as a ‘sink’ for excess reducing power under these conditions. Typically, PHB is synthesized via the pathway: acetyl-CoA ! acetoacetyl-CoA ! b-hydroxybutyryl-CoA ! PHB (enzymes: b-ketothiolase, b-hydroxybutyryl-CoA dehydrogenase, PHB synthase). PHB degradation: PHB ! b-hydroxybutyrate ! acetoacetate ! acetoacetyl-CoA ! acetyl-CoA; the first enzyme in this pathway, PHB depolymerase, is reported to be granule-bound in some bacteria, soluble in others. Some bacteria, e.g. certain pseudomonads, can degrade exogenous PHB. Other poly-b-hydroxyalkanoates (PHAs): e.g. Bacillus megaterium may contain small amounts of b-hydroxyheptanoate together with PHB, and Pseudomonas oleovorans forms polyb-hydroxyoctanoate from n-octane. [PHAs, including PHB and Biopol: FEMS Reviews (1992) 103 91–376.] (See also BIOPOL.) Polyhymenophorea A class of protozoa (phylum CILIOPHORA) in which the oral ciliature is typically well developed and conspicuous (often extending out of the buccal cavity); the PARORAL MEMBRANE is of the stichomonad, diplostichomonad or polystichomonad type; the somatic ciliature is often reduced or replaced by cirri; kinetodesmata are rare; STOMATOGENESIS is apokinetal or parakinetal; and neither toxicysts nor trichocysts are formed. Some species are sedentary, and some are loricate. Most species are free-living. All members are placed in the single sublcass Spirotrichia which is divided into four orders: HETEROTRICHIDA, HYPOTRICHIDA, ODONTOSTOMATIDA, OLIGOTRICHIDA. poly(I:C) See INTERFERONS. polykaryocyte A multinucleate cell (e.g. a GIANT CELL). polyketide A compound (often a secondary metabolite) which is, or may be regarded as, a condensation product of subunits such as acetate, propionate and/or malonate; polyketides, which may be classified according to the number of C2 units they contain (e.g. a tetraketide is a C8 compound), may undergo cyclization, glycosylation, oxidation etc. Metabolites derived from polyketides include some antibiotics (e.g. erythromycin, tetracyclines) and mycotoxins (e.g. PATULIN – a tetraketide, AFLATOXINS, CYTOCHALASINS). Such compounds are synthesized from precursors such as acetyl-CoA and malonyl-CoA; for example, in aflatoxin biosynthesis, malonylCoA subunits condense with an acetyl-CoA ‘starter’, with concomitant decarboxylation, in a process analogous to fatty acid biosynthesis but minus dehydration and reduction [AAM (1983) 29 53–92]. The molecule of erythromycin, a MACROLIDE ANTIBIOTIC obtained from actinomycetes (e.g. Streptomyces erythreus and Saccharopolyspora erythraea), has a cyclic polyketide nucleus 602 Polyporus to a sugar alcohol, i.e., the compound obtained when the aldo or keto group of a sugar is reduced to a hydroxy group. For example, reduction of galactose gives galactitol (dulcitol), ribose gives ribitol, etc. (cf. PLURONIC POLYOL F127.) [Polyol metabolism in fungi: AMP (1984) 25 149–193.] (See also e.g. PENTITOLS.) polyoma virus (polyomavirus) (1) Mouse (murine) polyoma virus: see POLYOMAVIRUS. (2) Any virus of the genus Polyomavirus (q.v.). Polyomavirus A genus of viruses of the PAPOVAVIRIDAE. Type species: mouse (murine) polyoma virus (‘polyoma virus’); other members include e.g. BK virus and JC virus (human viruses), K virus (a mouse virus), SIMIAN VIRUS 40 (SV40), and stump-tailed macaque virus (STMV). (See also BUDGERIGAR FLEDGLING DISEASE VIRUS.) Polyoma viruses can be propagated readily in suitable cell cultures (see e.g. SIMIAN VIRUS 40; cf. PAPILLOMAVIRUS). The viruses generally appear to be common in their natural hosts, in which infection is usually asymptomatic (although JC virus – which seems to be common in humans – can cause PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY in certain circumstances); however, many polyoma viruses can be oncogenic in other hosts: e.g. SV40, JC and BK can induce tumours in newborn rodents and can transform several types of cell in culture. JC virus can also induce brain tumours in adult monkeys. Cells transformed by SV40 or murine polyoma virus contain viral DNA integrated into the host chromosomal DNA; integration occurs at non-specific sites and may involve the entire viral genome or only a portion of it. However, integration appears not to be essential for transformation by BK virus, and its significance in SV40 and polyoma virus transformation is unknown. Expression of early viral genes appears to be necessary for transformation in all cases. [Polyomaviruses and disease of the central nervous system: RMM (1998) 9 79–85.] In murine polyoma virus, at least three distinct genes appear to act in a coordinated manner to induce transformation in primary rat embryo fibroblasts; the product of each gene induces particular alterations in cell structure and growth control [Book ref. 113, pp. 109–116]. The three products are ‘large T protein’ or ‘large T antigen’ (MWt ca. 105000), ‘middle T protein’ or ‘middle T antigen’ (MWt ca. 56000), and ‘small T (or t) protein’ or ‘small T antigen’ (MWt ca. 22000). The large T protein is a DNA-binding protein which localizes in the host cell nucleus in association with the chromatin. The middle T protein localizes in the host cell plasma membrane; it is tightly associated with a tyrosine-specific protein kinase – identified as the c-src product pp60c-src (see SRC [EMBO (1984) 3 585–591]. The small T protein is found in the soluble cytoplasmic fraction of the cell; its activities are unknown, but it may disrupt actin cables in the cell. Viruses SV40, BK and JC appear to encode proteins corresponding to large T and small T proteins but do not encode a middle T protein. In SV40, the large T antigen (‘T antigen’) is a multifunctional phosphoprotein MWt ca. 92000) which can alone induce neoplastic transformation in cells from a variety of species; it also acts as a key regulatory protein in the lytic cycle of SV40 in permissive monkey cells. The protein binds to SV40 DNA at multiple sites near the replication origin, has ATPase activity, and binds to a host protein of MWt 53000. The SV40 small T protein may also have a role in transformation; it has been detected in the host cell cytoplasm and in the nucleus [Book ref. 113, pp. 369–375]. polyoxins A group of antifungal nucleoside–oligopeptide antibiotics produced by Streptomyces cacaoi. Polyoxins are competitive inhibitors of chitin synthase, being structural analogues of UDP-N-acetylglucosamine; other enzymes using this substrate seem not to be affected. Susceptible fungi take up polyoxins via peptide transport systems; resistant strains, which emerge readily, appear to have altered peptide permeases. Polyoxins have been used for the control of certain plant pathogens: e.g. Alternaria kikuchiana (black spot of pear); they have no clinical application. (See also GROWTH (fungal).) polypectate Syn. pectate (see PECTINS). Polyphagus See CHYTRIDIALES and MYCOPARASITE. polyphasic taxonomy See TAXONOMY. polyphenoloxidases See e.g. TANNINS. polyphosphate (volutin) A linear phosphate polymer (polymetaphosphate: [PO3 2
]–O–[PO3
]n –[PO3 2
]) which occurs in bacteria, fungi, algae, protozoa, and certain higher eukaryotes. In eukaryotic microorganisms (e.g. yeasts) polyphosphate appears to occur chiefly in the periplasmic region and/or in the cell vacuole; in most bacteria (including cyanobacteria) polyphosphate occurs mainly in the cytoplasm. (See METACHROMATIC GRANULES.) Polyphosphate is believed to function as a reserve of phosphate and, in at least some organisms, of energy; it may also play a role in the storage or sequestration of cations. Polyphosphate is synthesized in some bacteria by the transfer of g-phosphate from ATP to nascent polyphosphate, catalysed by an Mg2C -dependent polyphosphate kinase (ATP-polyphosphate phosphotransferase). Other bacteria, and e.g. Neurospora crassa, synthesize polyphosphate by the transfer of phosphate from the 1-position of 1,3-bisphosphoglycerate. Some organisms (e.g. Escherichia coli ) possess both enzyme systems. Degradation of polyphosphate can be achieved by several enzymes, including e.g. polyphosphate kinase, and an Mg2C -dependent polyphosphate glucokinase – present in certain bacteria – which can catalyse the transfer of phosphate from polyphosphate to glucose to form glucose 6-phosphate. [Polyphosphate metabolism: AMP (1983) 24 83–171.] (See also PYROPHOSPHATE.) polyphyletic Refers to taxa which have not arisen from a common evolutionary line but which are classified together because they share certain feature(s) – due e.g. to convergent evolution. (cf. MONOPHYLETIC.) polyplanetism The phenomenon in which secondary zoospores (see DIPLANETISM) undergo repeated cycles of encystment and excystment, successive populations of secondary zoospores being formed. polyploid Having a PLOIDY greater than two. polypodial amoebae Amoebae which usually have several pseudopodia at a given time. (cf. MONOPODIAL AMOEBAE.) Polyporaceae See APHYLLOPHORALES. Polyporales Syn. APHYLLOPHORALES. polypore Any species of the APHYLLOPHORALES (particularly bracket fungi of the Polyporaceae) in which the basidiocarp has a porous hymenophore. Polyporus (1) A genus of typically lignicolous fungi of the APHYLLOPHORALES (family Polyporaceae) which form brackettype to stipitate (mushroom-shaped) fruiting bodies in which the stipe may be central, eccentric, lateral, or virtually nonexistent; the hymenophore is porous. P. squamosus is the causal agent of heart rot in a range of deciduous trees, particularly elm (Ulmus); the fruiting body (context: dimitic) has a light brown, scaly upper surface and a pale underside, and the basidiospores are colourless, cylindrical, ca. 12 ð 5 µm. For P. dryadeus see INONOTUS. (See also BLACKFELLOWS’ BREAD.) (2) Polyporus is sometimes used as a form genus to include those polypores which form annual, fleshy to leathery fruiting 603 polyprotein polyvalent antiserum Any antiserum which contains antibodies to a number of different antigens. polyvalent vaccine A single VACCINE containing the PROTECTIVE ANTIGENS and/or toxoids from each of several different strains of one species of pathogen. (cf. MIXED VACCINE.) polyvinyl alcohol fixative A FIXATIVE: SCHAUDINN’S FLUID with five times the usual concentration of glacial acetic acid, supplemented with polyvinyl alcohol (5% w/v) and glycerol (1.5 ml per 100 ml fluid). Specimens can remain in PVA fixative, without damage, for long periods of time. polyvinyl pyrrolidone–iodine See IODINE (a). polyynes Syn. POLYACETYLENES. pomona fever LEPTOSPIROSIS caused by L. interrogans serotype pomona. Pontiac fever See LEGIONELLOSIS. porcine circovirus (PCV) A small isometric virus (diam. 17 nm) containing ccc ssDNA (MWt 0.58 ð 106 ). PCV was isolated from a porcine cell line, and is presumed to infect pigs since specific antibodies to the virus were present in randomly collected pig sera but not in the sera of other animals. PCV apparently represents a new family of ANIMAL VIRUSES. [Nature (1982) 295 64–66.] porcine haemagglutinating encephalitis virus See CORONAVIRUS and VOMITING AND WASTING DISEASE. porcine icteroanaemia See EPERYTHROZOONOSIS. porcine parvovirus See PARVOVIRUS. porcine reproductive and respiratory syndrome See BLUEEARED PIG DISEASE. porcine transmissible gastroenteritis virus See CORONAVIRIDAE and TRANSMISSIBLE GASTROENTERITIS. pored epicortex (lichenol.) A polysaccharide layer (ca. 0.6 µm thick) which overlies the upper cortex in certain foliose lichen thalli and which is perforated by numerous more or less regular perforations ca. 15–25 µm or more in diameter. The underlying cortex is continuous but is generally thin (e.g. 2–3 cell layers thick), the cells being relatively loosely packed; these cortical features, together with the pored epicortex, apparently allow sufficient gas exchange to obviate the need for pseudocyphellae or cyphellae [Lichenol. (1981) 13 1–10]. porfiromycin The N-methyl derivative of MITOMYCIN C. Poria (1) A genus of lignicolous fungi of the APHYLLOPHORALES (family Polyporaceae) which form resupinate fruiting bodies (ranging from millimetres to centimetres in thickness) in which the hymenophore is porous; the whitish context is monomitic and lacks clamp connections. In perennial fruiting bodies the tubules are stratified (cf. HETEROBASIDION). (See also BROWN ROT, COAL BIODEGRADATION, DRY ROT, PACHYMAN and PAPER SPOILAGE.) (2) Poria is sometimes used as a form genus to include those polypores which form annual or perennial resupinate fruiting bodies. porin (formerly: matrix protein) In the OUTER MEMBRANE of Gram-negative bacteria: a type of protein molecule which – usually in trimeric form (but sometimes dimeric or monomeric) – forms a water-filled transmembrane channel (‘pore’) that permits the passage of certain ions and small molecules; porins are also found e.g. in the mitochondrial outer membrane. (See also LAMB PROTEIN and TSX PROTEIN.) Note that the term ‘porin’ is also used to refer to the complete pore structure – whether it be composed of one, two or three porin proteins. Bacterial porins. Porins have been demonstrated in many types of Gram-negative bacteria, but most studies have been carried out on E. coli and Salmonella typhimurium. Although porins are typically trimeric, dimeric porins have been found bodies; thus, e.g. Piptoporus betulinus is equivalent to the form species Polyporus betulinus. (cf. FOMES sense 2.) polyprotein (virol.) A protein which is produced by the uninterrupted translation of two or more adjacent genes on a polycistronic mRNA; the products of individual genes are separated by post- or co-translational cleavage of the polyprotein. (See e.g. COWPEA MOSAIC VIRUS, PICORNAVIRIDAE and RETROVIRIDAE.) (Certain polypeptides of eukaryotes are also synthesized as polyproteins.) polyribosome See PROTEIN SYNTHESIS. polysaccharide (glycan) Any macromolecule that consists of monosaccharide (glycose) residues joined by glycosidic linkages. Polysaccharides containing only one kind of monosaccharide are called homopolysaccharides (D homoglycans ), those containing two or more different kinds are called heteropolysaccharides (D heteroglycans ). [Nomenclature: JBC (1982) 257 3352–3354.] Several polysaccharides obtained from microorganisms have important commercial uses: see e.g. AGAR, ALGINATE, CARRAGEENAN, CURDLAN, DEXTRAN, FUNORAN, FURCELLARAN, GELLAN GUM and XANTHAN GUM. polysaprobic zone See SAPROBITY SYSTEM. Polysiphonia A large genus of filamentous marine algae (division RHODOPHYTA). (See also CHOREOCOLAX.) polysome See PROTEIN SYNTHESIS. Polysphondylium A genus of cellular slime moulds (class DICTYOSTELIOMYCETES) in which the sorocarp stalk tube resembles that of DICTYOSTELIUM in containing empty cells, but differs in bearing whorls of branches along its length; globose sori are borne on the tips of the stalks and at the ends of the branches. The spores are purple in P. violaceum, white to hyaline in other species (e.g. P. pallidum). In P. violaceum the ACRASIN is not cAMP but an unusual dipeptide, ‘glorin’ (N-propionylg-L-glutamyl-L-ornithine-d-lactam ethyl ester [PNAS (1982) 79 7376–7379]). In this species a single cell, the founder cell, serves as the focus for aggregation. Six species are recognized [descriptions and key: Book ref. 144, pp. 368–392]. Polyspira See HYPOSTOMATIA. polystichomonad membrane See PARORAL MEMBRANE. Polystigma See POLYSTIGMATALES. Polystigmatales An order of primarily plant-pathogenic fungi of the subdivision ASCOMYCOTINA. Ascocarp: perithecioid, immersed. Asci: unitunicate, typically clavate. Ascospores: aseptate to multiseptate. Genera: e.g. Magnaporthe (see also PYRICULARIA), Polystigma, Sphaerodothis. polystromatic Refers to a structure which is more than two cells thick. (cf. MONOSTROMATIC; DISTROMATIC.) polysulphide See SULPHUR. polytenization The repeated replication of a chromosome without separation of the daughter chromosomes; a polytene chromosome is thus a ribbon-like arrangement of multiple chromosomes lying parallel to one another. polythetic classification Any form of classification based on a large number of criteria. (cf. MONOTHETIC CLASSIFICATION.) Polythrix See RIVULARIACEAE. polytropic Syn. DUALTROPIC. polyurethanase Any enzyme which can degrade polyurethane. Such enzymes are produced by certain types of microorganism. Bacterial polyurethanase activity may be detected by a simple plate assay in which organisms are tested for their ability to degrade a colloidal form of polyurethane in the presence of rhodamine B, hydrolysis of the substrate being indicated by the development of orange fluorescence in the medium [LAM (2001) 32 211–214]. 604 porphyrins glucose derivatives; a similar preference is shown by porin channels in Pseudomonas aeruginosa, but those in e.g. Neisseria gonorrhoeae (and those in the mitochondrial outer membrane) are preferentially permeable to anions. Negative charge may make little difference (or enhance) the passage of a molecule through the PhoE channel in E. coli. [Role of porins in outer membrane permeability (review): JB (1987) 169 929–933.] Porin channels in LIPOSOMES can be opened and closed by adjusting the value of the transmembrane potential; in this way they resemble the voltage-controlled mitochondrial porin channels (referred to as ‘voltage-dependent anion channels’ – VDACs). However, while the outer membrane can exhibit a transmembrane potential under certain conditions, the opening and closing of porin channels in intact cells has not been shown to be regulated in this way. (See also LIPOSOME SWELLING ASSAY.) poroconidia See CONIDIUM. porogenous development See CONIDIUM. porospore See CONIDIUM. porphin A cyclic TETRAPYRROLE in which the four pyrrole groups are linked by their a-carbon atoms via methene (
CHD) bridges; porphin is the parent compound of the PORPHYRINS. porphobilinogen A substituted monopyrrole: an intermediate in the biosynthesis of PORPHYRINS. Porphyra A genus of marine algae (division RHODOPHYTA). The nature of the thallus varies according to the stage in the life cycle; the thallus can be sheet-like (one or two cells thick) or filamentous – the filamentous form being called the ‘Conchocelis stage’ since it was previously thought to be a different organism (designated Conchocelis). The organisms, which range from reddish-purple to olive green, attach to rocks etc by means of a holdfast. (See also LAVER and PORPHYRAN.) porphyran A group of complex, mucilaginous, sulphated and methylated galactans (containing D- and L-galactose and 3,6anhydro-L-galactose) produced by Porphyra spp. Porphyridium A genus of unicellular, spherical algae (division RHODOPHYTA) which occur e.g. in freshwater habitats and on soil. Each cell contains a stellate chloroplast with a central pyrenoid; there is no cell wall, each cell being surrounded by a gelatinous polysaccharide capsule. When viewed in masses, the cells may be blood-red (e.g. P. purpureum – formerly P. cruentum) to blue-green (e.g. P. aerugineum), depending on the relative proportions of phycoerythrin and phycocyanin which they contain. porphyrin test A test for determining the X FACTOR requirement of a Haemophilus strain by determining the ability of the strain to convert d-aminolaevulinic acid (ALA) to porphyrins or porphobilinogen. A medium (0.5-ml aliquot) containing ALA (2 mM) and MgSO4 (0.8 mM) in phosphate buffer (0.1 M, pH 6.9) is inoculated with a loopful of bacteria from an agar plate culture and incubated for ca. 4 hours at 37° C. A red fluorescence from the culture (or culture fluid) under WOOD’S LAMP (360 nm) indicates the formation of porphyrins (a positive test, i.e., the strain does not require the X factor); alternatively, 0.5 ml Kovács’ indole reagent may be added, shaken vigorously, and the phases allowed to separate – a red coloration in the lower (aqueous) phase indicating the formation of porphobilinogen (a positive test). [Book ref. 22, pp. 561–562.] porphyrins PORPHIN derivatives in which the pyrrole bcarbons are variously substituted: e.g., aetioporphyrins (D etioporphyrins) have 4 methyl and 4 ethyl substituents, mesoporphyrins have 4 methyl, 2 ethyl and 2 propionic acid substituents, and protoporphyrins have 4 methyl, 2 vinyl and e.g. in Paracoccus denitrificans [JB (1985) 162 430–433]; monomeric porins occur in the cyanobacteria [JB (2000) 182 1191–1199]. The number of types of porin in a cell may vary according to conditions. For example, in E. coli the PhoE porin is induced when phosphate levels are low (see PHO REGULON). Moreover, extra porins may be acquired through LYSOGENY: the protein 2 (Lc protein) porin of E. coli K12 is present only when the cells are lysogenized by coliphage PA2. The porins of E. coli typically include the products of genes ompC, ompF and (if phosphate is limiting) phoE; these products are designated OmpC (D Ib), OmpF (Ia) and PhoE, respectively. (See also OMP.) The OmpC and OmpF porins play a part in OSMOREGULATION (q.v.). In addition to their role as channel components, porins may serve e.g. as receptor sites for phages; for example, in Escherichia coli the OmpF porin acts as a receptor for phages T2 and TuIa, while OmpC is a receptor for T4 and TuIb. During infection by Gram-negative bacteria, porins are reported to induce the release of CYTOKINES from monocytes and lymphocytes. In E. coli, each transmembrane pore is formed by a group of three porin proteins. Structurally, the OmpC and OmpF pores are similar: a single channel from the inner (periplasmic) side of the pore branches into three channels which exit from the outer (exterior) face of the pore; the OmpF channel is somewhat wider than the OmpC channel. When extracted with SDS, these porins can be isolated as undenatured trimers. The PhoE pore differs in that it contains three channels which remain separate across the width of the pore; a similar structure is found e.g. in the protein F porin of Pseudomonas aeruginosa. Cyanobacterial porin proteins are larger than those of other Gram-negative bacteria (50–70 kDa versus 30–40 kDa), and they occur as monomers [JB (2000) 182 1191–1199 (1193)]. Salmonella typhimurium typically contains the OmpC, OmpD and OmpF porins (previously referred to as 36K, 34K and 35K proteins, respectively). In Pseudomonas aeruginosa, the porin channels have been reported to be larger [MR (1985) 49 1–32] and relatively smaller [FEMS (1984) 21 119–123] than those of E. coli. The rate of diffusion of molecules through a porin channel depends strongly on the size, hydrophobicity and charge of the molecules. E. coli porin channels permit the passage of hydrophilic molecules of MWt up to ca. 650–850 (i.e. the ‘exclusion limit’ is ca. 650–850), but they are poorly permeable or impermeable to hydrophobic molecules (including many b-lactam antibiotics). By contrast, the pores in Haemophilus influenzae type b appear to be larger, and they offer little hindrance to the influx of a range of penicillins and cephalosporins [JB (1985) 162 918–924]. In E. coli the main porin channels are not equally permeable: glucose (for example) diffuses through the OmpC channel at about half the rate at which it diffuses through the OmpF channel, and the differential is even greater with larger, hydrophobic or charged molecules. Mutants which lack OmpF channels exhibit increased resistance to e.g. chloramphenicol, tetracyclines and b-lactam antibiotics. [In Enterobacter aerogenes, resistance to e.g. cefepime and other cephalosporin antibiotics has been associated with altered porins: Microbiology (1998) 144 3003–3009.] The main porin channels in E. coli are preferentially permeable to cations (as opposed to anions), and they permit the entry of glucose at rates much higher than those of negatively charged 605 port 2 propionic acid substituents. Isomers of a given porphyrin are usually denoted by Roman numerals: e.g. aetioporphyrin I is 1,3,5,7-tetramethyl-2,4,6,8-tetraethylporphin, and aetioporphyrin III is 1,3,5,8-tetramethyl-2,4,6,7-tetraethylporphin. Protoporphyrin IX (D protoporphyrin III) is 1,3,5,8-tetramethyl2,4-divinylporphin-6,7-dipropionic acid. (The numbering system used in the Dictionary – illustrated in the figure for CHLOROPHYLLS – is the widely used Fischer system; alternative systems have been proposed in which, e.g., all carbon atoms in the tetrapyrrole ring are numbered sequentially, 1–20, starting with the a-carbon of ring I next to the d-methene position [PAC (1979) 51 2251–2304].) Porphyrins can readily chelate various metals, the metalloporphyrins being components of several important biological pigments: see e.g. CHLOROPHYLLS, CYTOCHROMES, HAEM. (See also CHLORIN; cf. PHYCOBILIPROTEINS.) port See WINE-MAKING. Port Salut cheese See CHEESE-MAKING. porter See PERMEASE. positive control (of an operon) See OPERON. positive interference See INTERFERENCE (2). positive-sense genome See VIRUS. positive staining See ELECTRON MICROSCOPY (a). positive-strand virus A VIRUS (q.v.) with a positive-sense genome. post-antibiotic effect (PAE) The suppression of bacterial growth (typically for hours) which follows a short exposure to some types of antimicrobial agent – e.g. aminoglycosides, 4quinolones, b-lactams, streptogramins and macrolides. PAE is a factor when considering the dose/dosage frequency of these antibiotics; it can be advantageous when coincident with low concentrations of an intermittently administered drug. PAE may involve different mechanisms in different organism–antibiotic combinations [AAC (1995) 39 1314–1319]. The post-antibiotic sub-MIC effect may be more relevant, clinically, than the PAE [JAC (1999) 43 71–77]. post-ciliary microtubules (ciliate protozool.) A ribbon of MICROTUBULES which arises from the right-hand posterior side of a kinetosome (close to triplet 9, GRAIN CONVENTION), extends upwards (towards the pellicle) and to the right, and then (sometimes) runs posteriorly between the kineties, the wider face of the ribbon being perpendicular to the body surface. (See also PITELKA CONVENTION.) posterior station (parasitol.) The hindgut and/or rectum of an arthropod vector; infective forms of parasites in the posterior station can be transmitted to the vertebrate host via the faeces of the vector (see e.g. CHAGAS’ DISEASE). (cf. CONTAMINATIVE INFECTION.) post-fixation See FIXATION. post-herpetic neuralgia See HERPES ZOSTER. post-kala-azar dermal leishmaniasis A cutaneous leishmaniasis which may occur in a patient up to two years after that patient has been cured of VISCERAL LEISHMANIASIS. [PCR-based diagnosis of post-kala-azar dermal leishmaniasis: JCM (1998) 36 1621–1624.] post-meiotic segregation The segregation of alleles during the first nuclear division after MEIOSIS; it results from the separation of strands of uncorrected heteroduplex DNA (in a chromatid) formed previously during recombinational events in meiosis (see RECOMBINATION). postoral suture See SYSTÈME SÉCANT. post-replication repair (1) A DNA REPAIR process which operates after DNA replication: e.g. daughter-strand gap repair (see RECOMBINATION REPAIR) and some cases of MISMATCH REPAIR. (2) Syn. daughter-strand gap repair. post-streptococcal glomerulonephritis (PSGN) GLOMERULONEPHRITIS which occurs as a late complication of Streptococcus pyogenes infection (usually IMPETIGO – cf. RHEUMATIC FEVER); PSGN is caused only by certain (‘nephritogenic’) strains of streptococci, and occurs mainly in young children. There may be a latent period of ca. 10–21 days between the original infection and the onset of PSGN. post-transfusion hepatitis HEPATITIS transmitted by blood transfusion. post-zygotic exclusion See PRE-ZYGOTIC EXCLUSION. potassium cyanide broth See KCN BROTH. potassium permanganate (as an antimicrobial agent) Potassium permanganate (KMnO4 ) is an OXIDIZING AGENT which is active against a wide range of microorganisms at concentrations ranging from 0.001% to 1% w/v; it has been used for the treatment of certain superficial mycoses, but it can act as an irritant at high concentrations. As an algicide, potassium permanganate has been reported to be superior to copper sulphate. At concentrations >ca. 0.002% w/v, potassium permanganate imparts a pink coloration to water; this, together with the toxicity of its manganese component, makes potassium permanganate unsuitable for the disinfection of potable water. potassium pump See ION TRANSPORT. potassium transport See ION TRANSPORT. potato aucuba mosaic virus See POTEXVIRUSES. potato black ringspot virus See NEPOVIRUSES. potato blight See EARLY BLIGHT and LATE BLIGHT. potato diseases See e.g. BLACKLEG (2); EARLY BLIGHT; GANGRENE (2); LATE BLIGHT; POTATO SCAB; POTATO VIRUS Y; POWDERY SCAB; SILVER SCURF; SPRAING; WART DISEASE. potato leaf roll virus See LUTEOVIRUSES. potato mop top virus See SOIL-BORNE WHEAT MOSAIC VIRUS, SPRAING and TOBAMOVIRUSES. potato paracrinkle virus See CARLAVIRUSES. potato scab (common scab) A POTATO DISEASE caused by Streptomyces scabies. The disease occurs particularly in light, sandy, alkaline soils; corky scabs of various sizes typically develop on the surface of tubers, deeper lesions sometimes occurring. In heavy, wet soils POWDERY SCAB may develop. potato slope A wedge-shaped piece of potato autoclaved (in a test-tube or bottle) and used as a MEDIUM. potato spindle tuber viroid See VIROID. potato virus A See POTYVIRUSES. potato virus M See CARLAVIRUSES. potato virus S See CARLAVIRUSES. potato virus T A filamentous (about 640 ð 12 nm), ssRNAcontaining virus (possible member of the CLOSTEROVIRUSES) which has a rather limited host range; in potato plants, infection may be symptomless (latent) or may result in a mild leaf mottle. Transmission occurs via seeds in some solanaceous plants. potato virus X See POTEXVIRUSES. potato virus Y (PVY) The type member of the POTYVIRUSES. Virion: ca. 730 ð 11 nm. PVY infects mainly plants of the Solanaceae, causing important diseases in several commercial crop plants (e.g. potato, capsicum, tobacco, tomato). In the potato, symptoms depend on virus strain, host cultivar, and environmental conditions. PVY YN strains may cause necrotic rings or spots, with mild mottling appearing later in the growing season; symptoms appearing in the second and subsequent years may include mild to severe mottling. PVY YO strains may cause necrosis, mottling or yellowing of leaflets, leaf-drop, and 606 Powassan encephalitis mosaic virus, carnation vein mottle virus, carrot thin leaf virus, celery mosaic virus, cocksfoot streak virus, cowpea aphid-borne mosaic virus (of which Azuki bean mosaic virus is a strain), leek yellow stripe virus, lettuce mosaic virus, onion yellow dwarf virus, Papaya ringspot virus, parsnip mosaic virus, peanut mottle virus, PLUM POX virus, potato virus A, soybean mosaic virus, sugarcane mosaic virus (of which maize dwarf mosaic virus is a strain), tobacco etch virus, tulip breaking virus, turnip mosaic virus. Many other viruses have been tentatively included as ‘possible members’ of the potyviruses. These include (a) certain aphidborne viruses: e.g. carrot mosaic virus, celery yellow mosaic virus, groundnut eyespot virus, tobacco vein mottling virus, tomato (Peru) mosaic virus, wheat spindle streak virus, wheat streak virus; (b) a group of viruses which are transmitted by eriophyid mites: e.g., Agropyron mosaic virus, oat necrotic mottle virus, ryegrass mosaic virus, Spartina mottle virus, wheat streak mosaic virus; (c) the whitefly-transmitted virus sweet potato mild mottle virus; and (d) a group of viruses transmitted by Polymyxa graminis: barley yellow mosaic virus, oat mosaic virus, rice necrosis mosaic virus, wheat spindle streak mosaic virus, wheat yellow mosaic virus. Some or all of these ‘possible members’ may actually belong to or constitute separate groups. pouch (food processing) See RETORT POUCH. poultry diseases Poultry (chickens, ducks, turkeys, etc) can be affected by a wide range of diseases – some specific for one species of bird, others affecting a range of birds. (a) Bacterial diseases: see e.g. AIR SACCULITIS; ARIZONOSIS; BUMBLEFOOT; COLISEPTICAEMIA; DIPHTHEROID STOMATITIS; erysipelas (see SWINE ERYSIPELAS); FOWL CHOLERA; FOWL TYPHOID; HJÄRRE’S DISEASE; PSITTACOSIS; PULLORUM DISEASE. (b) Fungal diseases: see e.g. ASPERGILLOSIS; CANDIDIASIS; FAVUS. (c) Protozoal diseases: see e.g. BLACKHEAD; COCCIDIOSIS; HEXAMITIASIS. (d) Viral diseases: see e.g. AVIAN ACUTE LEUKAEMIA VIRUSES; AVIAN ENCEPHALOMYELITIS; AVIAN INFECTIOUS BRONCHITIS; AVIAN INFECTIOUS LARYNGOTRACHEITIS; AVIAN RETICULOENDOTHELIOSIS VIRUSES; AVIAN SARCOMA VIRUSES. DUCK VIRUS ENTERITIS; DUCK VIRUS HEPATITIS; FOWL PLAGUE; FOWL POX; HAEMORRHAGIC ENTERITIS OF TURKEYS; INCLUSION BODY HEPATITIS; INFECTIOUS BURSAL DISEASE VIRUS; LYMPHOID LEUKOSIS; MAREK’S DISEASE; NEWCASTLE DISEASE; OSTEOPETROSIS. (See also EGG-DROP SYNDROME, PARVOVIRUS and STUNTING SYNDROME.) (e) Diseases due to ingestion of microbial toxins include e.g. LIMBERNECK; TURKEY X DISEASE; WESTERN DUCK DISEASE. poultry spoilage See MEAT SPOILAGE. pour plate (1) A procedure (used e.g. as a COUNTING METHOD) in which a measured volume of liquid INOCULUM is placed in a PETRI DISH, a molten medium (e.g. an agar medium at 45–48° C) is added, and the whole swirled to disperse the inoculum throughout the medium; when set, the plate is incubated. (The inoculum may instead be mixed with the medium before the latter is poured into the Petri dish.) The medium may incorporate a reducing agent (e.g. sodium thioglycollate) so that even under aerobic incubation some anaerobes can grow within the medium. (2) A CULTURE prepared as in (1). pourriture noble (French) syn. NOBLE ROT. povidone–iodine See IODINE (a). POW virus See POWASSAN ENCEPHALITIS. Powassan encephalitis An acute, occasionally fatal, human ENCEPHALITIS caused by a flavivirus (see FLAVIVIRIDAE); it occurs e.g. in forested areas of Canada and Northern USA. The Powassan virus (POW virus) occurs in small mammals and is transmitted by ticks (usually Ixodes spp). [ARE (1981) 26 84–85.] premature death (see also RUGOSE MOSAIC DISEASE). PVY YC strains may cause necrosis, mottling, crinkling, and necrotic streaks on leaves, petioles and stem; necrosis may occur in the tubers. Symptoms of PVY infection in capsicum, tobacco and tomato typically include mild mottling, but YN strains can cause a severe disease in tobacco (‘tobacco veinal necrosis disease’) which may result in total loss of the crop. potato yellow dwarf virus See RHABDOVIRIDAE. Poterioochromonas See OCHROMONAS. potexviruses (potato virus X group) A group of PLANT VIRUSES in which the virions are flexuous filaments (ca. 470–580 ð 13 nm) comprising one type of coat protein (MWt ca. 18000–23000) and one molecule of linear positive-sense ssRNA (MWt ca. 2.1 ð 106 ). The RNA is capped at the 50 end 0 (sequence: m7 G5 pppGpA . . .) but is not polyadenylated at the 30 end; the adenine content of the RNA is ca. 30%. The host range for the group is wide, including monocots and dicots, but individual members infect only a narrow host range. Plants infected with potexviruses typically develop mosaic and ringspot symptoms. Virus particles form fibrous, sometimes banded, often large aggregates in the cytoplasm of infected cells; some members also induce nuclear inclusions. Transmission occurs mechanically. Type member: potato virus X (PVX). Other members include e.g. cactus virus X, cassava common mosaic virus, clover yellow mosaic virus, Cymbidium mosaic virus, Papaya mosaic virus, Plantago severe mottle virus, Plantago virus X, white clover mosaic virus. Possible members include e.g. artichoke curly dwarf virus, bamboo mosaic virus, barley B-1 virus, Boletus virus, negro coffee mosaic virus, potato aucuba mosaic virus. Potomac horse fever (equine monocytic ehrlichiosis) A HORSE DISEASE caused by Ehrlichia risticii, a species antigenically related to E. canis and E. sennetsu (and which can be transmitted, experimentally, to cattle [VR (2001) 148 86–87]). Symptoms are variable, and may include e.g. fever, depression, anorexia, colic, mild to severe diarrhoea, oedema, etc; mortality rates may exceed 30%. Potter–Elvehjem homogenizer A type of blender used e.g. for preparing subcellular fractions of eukaryotic cells. Pott’s disease Tuberculosis of the spine – see OSTEOMYELITIS. potyviruses (potato virus Y group) A large group of PLANT VIRUSES in which the virions are flexuous filaments (ca. 680–900 ð 11 nm) comprising one type of coat protein (MWt ca. 32000–36000) and one molecule of linear positive-sense ssRNA (MWt ca. 3.0–3.5 ð 106 ). As a group, the viruses infect a wide range of host plants, including monocots and dicots, but individual members typically have a narrow host range; some members can cause important economic losses in crop plants. Symptoms depend on plant cultivar, virus strain, and environmental conditions, but typically include mosaic or mottle in the leaves, colour-breaking in flowers, mottling and/or distortion in the fruits; characteristic proteinaceous inclusion bodies (serologically unrelated to virus coat protein) are formed in the cytoplasm of infected plant cells, appearing as ‘pinwheels’ in transverse section or as ‘bundles’ in longitudinal section. A few members also form crystalline nuclear inclusions. Potyviruses are transmitted (non-persistently) by aphids (see NON-CIRCULATIVE TRANSMISSION sense 1) and can be transmitted mechanically; some viruses tentatively included in the group are transmitted by whitefly, mites or fungi. Type member: POTATO VIRUS Y. Other members include e.g. bean common mosaic virus, bean yellow mosaic virus (of which pea mosaic virus is a strain), beet mosaic virus, blackeye cowpea 607 powdery mildews VIRUS). Infection begins when the virion adsorbs to a host cell; vaccinia virus appears to bind to the host cell-surface epidermal growth factor (EGF) receptor [Nature (1985) 318 663–665]. The virus apparently enters the cell by fusion between viral and cell membranes and/or by viropexis. An early consequence of infection is the inhibition of host cell DNA, RNA and protein synthesis [review: Book ref. 150, pp. 391–429]. The viral core is uncoated in the cytoplasm, and enzymes of the core begin transcription of the viral DNA; early mRNA is synthesized by an a-amanitin-resistant viral RNA polymerase, and viral enzymes then process the RNA to form the functional polyadenylated, capped and methylated mRNA. (Early mRNAs, at least, are apparently not spliced.) DNA replication then begins; this may involve nicking of the DNA to provide free 30 -ends which can act as primers. The onset of DNA replication allows initiation of the synthesis of late mRNA which codes for most of the structural proteins of the virion. Proteolytic processing of the structural proteins appears to be coupled to virus assembly. In the case of vaccinia virus, the mature virions each become enclosed in a double membrane derived from the Golgi apparatus and are transported to the cell periphery; it was suggested that the outer viral membrane may fuse with the cell membrane, resulting in the release of virions enclosed by a single layer of Golgi membrane [JGV (1985) 66 643–646]. The discrete cytoplasmic sites of virus replication are termed ‘virus factories’, ‘factory areas’, or ‘viroplasms’; they are visible in suitably stained preparations as B-type inclusion bodies (D GUARNIERI BODIES). Some poxviruses (e.g. COWPOX VIRUS) form a second type of inclusion body, the A-type inclusion body: acidophilic structures which may or may not contain virions. poxvirus group See POXVIRIDAE. pp Prefix denoting (1) phosphoprotein (see e.g. RETROVIRIDAE), or (2) diphospho- or pyrophospho- (as in ppGpp). PPAR Peroxisome proliferator-activated receptor: see PROSTAGLANDINS. PPD See TUBERCULIN. PPFM Pink-pigmented facultatively methylotrophic bacterium. ppGpp See (i) CELL CYCLE; (ii) STRINGENT CONTROL (sense 1); and (iii) SPOT GENE. ppGpp has been classified as an ALARMONE. PPi Inorganic PYROPHOSPHATE (P2 O7 4
). PPLOs Pleuropneumonia-like organisms: an early name for Mycoplasma spp. PQQ See QUINOPROTEIN. Pr Prefix denoting a polyprotein product of transcription (see e.g. RETROVIRIDAE). PR proteins PATHOGENESIS-RELATED PROTEINS. Pr65 gag precursor protein (of FeLV) See FELINE LEUKAEMIA VIRUS. PRAS media Pre-reduced anaerobically-sterilized media. Prasinocladus See TETRASELMIS. Prasinomonadida See PHYTOMASTIGOPHOREA. Prasinophyceae A class of unicellular, flagellated green algae which have scaly flagella arising from an anterior flagellar pit. The class, which is no longer recognized [Book ref. 123, pp. 65–66], included e.g. TETRASELMIS and many of the genera in the MICROMONADOPHYCEAE. Prasiola A genus of marine and freshwater green algae (division CHLOROPHYTA). The thallus is foliose or filamentous. Asexual reproduction occurs by the formation of diploid aplanospores. In at least some species sexual reproduction is preceded by meiosis in the cells of the upper portion of the blade followed by several mitotic divisions; this results in the formation of powdery mildews Plant diseases caused by members of the ERYSIPHALES and characterized by the formation of at least partially superficial hyphal growth carrying a dense layer of conidia in which individual conidium-bearing structures are difficult to distinguish under low magnification (cf. DOWNY MILDEWS); dark-coloured cleistothecia may develop among the hyphae during the summer – being the overwintering stage in some species. Powdery mildews of cereals (including barley and wheat) are caused by Erysiphe graminis. White to grey (later brownish) mycelium develops on small chlorotic spots on leaves, and may also occur on the glumes (particularly in wheat); lodging may occur. Control: antifungals such as CARBENDAZIM and PROCHLORAZ. powdery scab A POTATO DISEASE caused by Spongospora subterranea (PLASMODIOPHOROMYCETES). Small wart-like swellings develop on the stolons, roots and tubers of infected plants; a dry brownish powder (consisting of masses of cystosori) forms within the tuber lesions. The cysts can persist in the soil for many years. The disease is favoured by wet soil conditions. The pathogen can also infect tomato roots. powdery yeasts See FLOCCULATION (sense 2). pOX1 plasmid See ANTHRAX TOXIN. pOX2 plasmid See ANTHRAX TOXIN. Poxviridae (the poxvirus group) A family of dsDNA-containing VIRUSes which infect mammals (including man), birds or insects; some members are important pathogens (e.g. the causal agents of SMALLPOX, MONKEYPOX, CONTAGIOUS PUSTULAR DERMATITIS (sense 1), etc). The family includes two subfamilies, CHORDOPOXVIRINAE (vertebrate poxviruses) and ENTOMOPOXVIRINAE (insect poxviruses), together with several unclassified viruses (e.g. MOLLUSCUM CONTAGIOSUM virus, TANAPOX VIRUS, YABA MONKEY TUMOUR POXVIRUS) [Book ref. 23, pp. 42–46]. The poxvirus virion is the largest and structurally the most complex of all animal viruses. Depending on virus, it may be ovoid or brick-shaped, ca. 200–450 ð 170–260 nm. It contains a central core (incorporating the genome) which is commonly bilaterally concave; on either side of the core – coincident with each concavity – is an ellipsoidal structure called the lateral body. (cf. ENTOMOPOXVIRINAE.) The core and its two associated lateral bodies are enclosed within a lipoprotein outer membrane which bears a characteristic pattern of ridges or tubules on its surface. (In some poxviruses the extracellular virions have an extra lipoprotein layer, the envelope, acquired from the host cell; these are believed to be the infective forms of the virus.) The virion contains a variety of enzymes, including enzymes involved in e.g. DNA replication, transcription, and RNA processing. In general, poxvirus virions are stable at room temperature and are resistant to desiccation; most are etherresistant but may be inactivated e.g. by chloroform and by heating (e.g. 60° C/20 min). The genome is a single molecule of dsDNA (MWt in the range 85–250 ð 106 ) in which – in at least some poxviruses – the two strands are covalently joined at each end of the molecule (i.e., the molecule is actually a single uninterrupted circular polynucleotide chain). The genome contains highly conserved sequences in the central region, and terminal hypervariable regions. [Genomes: orf virus, bovine papular stomatitis virus [JV (2004) 78 168–177]; goatpox, sheeppox [JV (2002) 76 6054–6061]; deerpox [JV (2005) 79 966–977.] Replication cycle. Poxvirus replication occurs in the cytoplasm of the host cell; studies on replication processes have been carried out mainly with orthopoxviruses (particularly VACCINIA 608 Pribnow box prephenate See AROMATIC AMINO ACID BIOSYNTHESIS. preprimosome See PRIMOSOME. pre-protein See SIGNAL HYPOTHESIS. pre-reduced medium See ANAEROBE. preservation (1) (of materials, products) The use of physical and/or chemical means to kill or prevent the growth of those microorganisms which, by their growth and/or activities, may cause BIODETERIORATION of a given material or product. Physical methods (e.g. heat STERILIZATION, freezing – see e.g. FOOD PRESERVATION) either kill contaminating organisms or render the materials or products unsuitable for the (rapid) growth of likely contaminants. Chemical agents (PRESERVATIVES) may be microbistatic or microbicidal. (2) (of microorganisms) Viable populations of microorganisms may be preserved (i.e. maintained for reference purposes etc) by e.g. FREEZING, FREEZE-DRYING or DESICCATION. Some bacterial cultures can be preserved by sealing with a layer of sterile mineral oil and storing at ca. 4° C. A culture may be preserved by periodic SUBCULTURE (see also STABILATE). preservative Any chemical used for the PRESERVATION of materials or products; in some industries such chemicals are referred to as BIOCIDES. For examples of preservatives and their usage see e.g. FOOD PRESERVATION, PAINT SPOILAGE, PAPER SPOILAGE, SAUSAGE, TEXTILE SPOILAGE, TIMBER PRESERVATION; see also ALCOHOLS, BISPHENOLS, CHLORBUTANOL, EGME, FORMALDEHYDE, GERMALL 115, PHENOLS, QUATERNARY AMMONIUM COMPOUNDS, SALICYLANILIDES, THIOMERSAL. A preservative may be inactivated e.g. by colloids (such as magnesium trisilicate or bentonite – cf. BENZOIC ACID; CHLORBUTANOL) or by certain container materials (polyethylene can inactivate e.g. dichlorophenol and other phenolics, while polypropylene can inactivate e.g. dichlorophenol and sorbic acid). prespore See ENDOSPORE (sense 1(a)). pressure-cycle fermenter See AIRLIFT FERMENTER. presumptive coliform count See COLIFORM TEST. pretibial fever Syn. FORT BRAGG FEVER. pretrichocyst See TRICHOCYST. pretyrosine See AROMATIC AMINO ACID BIOSYNTHESIS. Prevotella A genus of Gram-negative, asporogenous, anaerobic, rod-shaped bacteria found e.g. in the human oral cavity; some species form black or brown pigments when grown on bloodcontaining media. Most species ferment sucrose and lactose. Using quantitative culture on a non-selective blood-agar medium, studies on the anaerobic oral microflora in the first year of life found that organisms in the Prevotella melaninogenica group were among the early colonizers (18% at 2 months, rising to 57% at 6 months and 77% at 12 months); members of the P. intermedia group were apparently late colonizers, whereas isolation of non-pigmented species of Prevotella increased from ‘occasional’ at 2 months to 75% at 12 months [RMM (1997) 8 (supplement 1) S19–S20]. Species of Prevotella have been associated with e.g. abscesses and PERIODONTITIS. [Simultaneous detection of Prevotella intermedia and Bacteroides forsythus by multiplex PCR in clinical samples: JCM (1999) 37 1621–1624.] pre-zygotic exclusion During processes such as CONJUGATION (sense 1b) and TRANSDUCTION: the absence of expression of a given donor gene in the recipient as a result of the failure of that gene to enter the recipient. If the gene entered the recipient but failed to recombine with the recipient’s genome, the absence of expression of that gene is referred to as post-zygotic exclusion. prgI gene (Salmonella typhimurium) See NEEDLE COMPLEX. Pribnow box See PROMOTER. polystromatic micro- and macrogametangial regions which give rise to biflagellate microgametes and non-motile macrogametes, respectively. Prausnitz–Küstner antibodies Syn. REAGINIC ANTIBODIES. Prausnitz–Küstner test (P–K test) A test used for demonstrating the presence of IgE antibodies homologous to a given allergen. Serum from a hypersensitive subject is injected intradermally into a normal subject; subsequent injection of the normal subject with the given allergen causes a WHEAL-AND-FLARE response to develop within minutes (a positive test). The P–K reaction can be inhibited by the prior injection of a fragment of e-chain [Nature (1985) 315 577–578]. PRD See CATABOLITE REPRESSION. PRD1 phage group Syn. TECTIVIRIDAE. preaxostyle See OXYMONADIDA. pre-B cell See B LYMPHOCYTE. prebiotic (1) A non-digestible part of a food (often an oligosaccharide) which, by selectively stimulating certain type(s) of colon bacteria, benefits the host. (2) (adj.) The time before life appeared on Earth. prebuccal area In some ciliates (e.g. Paramecium): a depression in the body surface leading to the BUCCAL CAVITY; its ciliature is essentially somatic (cf. VESTIBULUM). precipitation (serol.) The formation of a precipitate when antibodies and soluble antigens react in suitable proportions (see e.g. OPTIMAL PROPORTIONS; cf. ANTIGEN EXCESS and AGGLUTINATION). (See also GEL DIFFUSION.) precipitin An ANTIBODY which forms a precipitate with its homologous soluble antigen. (cf. AGGLUTININ.) precipitin test Any serological test in which the interaction of antibodies with soluble antigens is detected by the formation of a precipitate. precipitinogen (1) An antigen which elicits a PRECIPITIN. (2) Any agent (e.g. PROTEIN A) which can combine with antibody to form a precipitate. precyst See CYST (a). prednisone See IMMUNOSUPPRESSION. pre-erythrocytic schizogony (in Plasmodium) See PLASMODIUM. pre-filtration See FILTRATION. pre-fixation See FIXATION. pre-genome See HEPADNAVIRIDAE. pregnancy test An IMMUNOASSAY in which urine is examined for human chorionic gonadotrophin (hCG), a hormone that appears during pregnancy. Urine is added to serum containing antihCG antibodies, which combine with hCG. Then, the absence of uncombined antibodies (a positive test) is shown by nonagglutination when latex-bound hCG is added. pre-initiation complex See PROMOTER. Preisz–Nocard bacillus See CORYNEBACTERIUM. pre-lesion terminus In damaged DNA: the nucleotide immediately 50 of the lesion site. premunition (concomitant immunity; non-sterilizing immunity) (immunol.) Protective immunity, in respect of a given pathogen, due to the persistence of small numbers of that pathogen in the host; the host can resist superinfection (i.e., infection by the same or closely related pathogens) but cannot achieve STERILIZING IMMUNITY. In an apparently analogous phenomenon, cells of a given tumour type fail to develop when injected into an animal suffering from the same type of tumour. preoral suture See SYSTÈME SÉCANT. prepatent period In a parasitic infection: the time interval between the initial infection and the appearance of parasites, or their cysts, in blood, tissues or faeces. 609 Pril primate T-cell leukaemia virus See SIMIAN T-CELL LEUKAEMIA VIRUS. prime plasmid A PLASMID which has undergone aberrant excision from a bacterial chromosome. In some types of prime plasmid (‘type I’), part of the plasmid has been left behind in the chromosome, and the plasmid carries a sequence of bacterial DNA; a ‘type II’ prime plasmid consists of the entire plasmid carrying a sequence of bacterial DNA. The particular sequence of bacterial DNA carried by a prime plasmid depends on the site within the chromosome occupied by the plasmid before excision; thus, e.g. it is possible to isolate various F0 plasmids (derived from the F PLASMID) because this plasmid can integrate at various sites in the bacterial chromosome. (See also specialized TRANSDUCTION.) primed (immunol.) (1) (of persons, animals) Refers to those individuals who have had an initial immunological contact with a given antigen; such individuals may respond e.g. with ANTIBODY FORMATION. Subsequent exposure of a primed individual to the given antigen may bring about a heightened response (see ANAMNESTIC RESPONSE and HYPERSENSITIVITY (sense 1)). (See also IMMUNE RESPONSE.) (2) (of cells) Refers to those cells whose responsiveness to a given antigen has been heightened as a consequence of their previous contact with that antigen (or of previous contact between their precursor cells and that antigen). (cf. PRIMING.) primer See (i) DNA REPLICATION; (ii) PCR; (iii) NASBA; (iv) SDA; and (v) FORWARD PRIMER. primer-dimer In a PCR reaction mixture: an artefact resulting from the use of a pair of primers whose 30 terminal sequences are mutually complementary; during extension, each primer uses the other as a template – forming products that consist essentially of two double-stranded primers in tandem. priming (1) (mol. biol.) The initiation of synthesis of a DNA strand e.g. by the synthesis of an RNA primer (see DNA REPLICATION). (For ‘specific’ and ‘general’ priming see DNAB GENE.) (2) (immunol.) The process in which an individual becomes PRIMED. primite The anterior organism of a pair of gregarines in SYZYGY. (cf. SATELLITE.) primociliatid gymnostomes Presumptively primitive ciliates (subclass GYMNOSTOMATIA, order Primociliatida) which are homokaryotic; species of STEPHANOPOGON are the only known examples. primordium (microbiol.) Syn. INITIAL(s). primosome A multiprotein complex involved in certain cases of primer synthesis (see DNA REPLICATION): e.g. in BACTERIOPHAGE fX174 DNA synthesis, and in Okazaki fragment synthesis during Escherichia coli chromosome replication and e.g. COLE1 PLASMID replication. Primosome formation on ssDNA coated with SINGLESTRAND BINDING PROTEIN is initiated by protein n0 (D factor Y) which recognizes and binds to specific short base sequences in the DNA. In the presence of ATP, a complex of DnaB (see DNAB GENE) and DnaC proteins is formed which, together with proteins i (D factor X), n, and n00 combine with n0 to form a prepriming intermediate (preprimosome) at or near the n0 recognition site. The preprimosome is recognized and bound by the DnaG protein (PRIMASE) to complete the primosome. The primosome migrates along the ssDNA template in a 50 -to-30 (anti-elongation) direction (i.e., in the direction of replication fork movement in dsDNA replication), driven by ssDNAdependent ATP hydrolysis by n0 , and primase synthesizes short RNA primers at many different sites on the template DNA. The Pril A quaternary ammonium detergent used e.g. in certain media to prevent swarming by Proteus spp. prill A solid in granular form. primaquine See MALARIA (chemotherapy). primary amoebic meningoencephalitis See MENINGOENCEPHALITIS. primary atypical pneumonia (1) Any of a group of pneumonias characterized by symptoms distinct from those of ‘classical’ lobar PNEUMONIA and by a causal agent which cannot readily be isolated by routine laboratory methods; the group includes LEGIONELLOSIS; Mycoplasma pneumoniae pneumonia (see (2) below); PSITTACOSIS; Q FEVER. (2) PNEUMONIA caused by Mycoplasma pneumoniae; it affects mainly children. Infection occurs by droplet inhalation. Incubation period: 2–3 weeks. Onset is insidious, with fever and malaise followed by symptoms of an upper respiratory tract infection; a persistent, non-productive cough typically develops, and ear infection is common. The disease is usually mild and self-limiting, with few chest signs; in only a minority of cases does frank pneumonia develop. Chemotherapy: e.g. erythromycin. primary constriction Syn. CENTROMERE. primary culture (1) A CULTURE prepared by inoculating a medium directly from a natural source of microorganisms (cf. SUBCULTURE). For example, a bacteriological medium may be inoculated directly from a faecal specimen. In TISSUE CULTURE a primary culture is one prepared from cells or tissues taken directly from an animal or plant. (2) The process of preparing a culture as in (1). primary fixation Syn. pre-FIXATION. primary fluorescence See FLUORESCENCE. primary host (of rust fungi) See UREDINIOMYCETES. primary hyphae See HYPHA. primary metabolism Metabolism which is essential for and geared towards growth – i.e., energy metabolism, synthesis of cell components, etc. (cf. SECONDARY METABOLISM.) primary mycelium (in actinomycetes) Syn. SUBSTRATE MYCELIUM. primary production The production of new biomass by photosynthetic organisms as a result of their use of light (solar) energy. The phrase ‘chemosynthetic primary production’ has been used to refer to the production of biomass by those nonphotosynthetic organisms which are believed to derive energy from the oxidation of e.g. sulphide and/or methane in the vicinity of HYDROTHERMAL VENTS in the ocean floor; these organisms (which include species of Thiobacillus and Thiomicrospira) appear to be at the start of a food chain which is unique in that it is not based on photosynthetic organisms. primary septum See SEPTUM (b). primary zoospore See e.g. DIPLANETISM and PLASMODIOPHOROMYCETES. primase (DNA primase) An enzyme which, in vitro, can polymerize molecules of ribonucleoside triphosphate (rNTPs) and/or dNTPs in the 50 -to-30 direction to form short oligonucleotides on an ssDNA template. In vivo, Escherichia coli primase (dnaG gene product) synthesizes short RNA primers for the formation of Okazaki fragments in DNA REPLICATION, and also synthesizes primers for the initiation of DNA replication of isometric ssDNA phages. Primase either recognizes a specific region of DNA directly (as in BACTERIOPHAGE G4) or, more commonly, it requires the co-operation of additional proteins (see PRIMOSOME). Certain phages (e.g. T4, T7) encode their own primases. Primase action is resistant to rifampicin (cf. RNA POLYMERASE). 610 probe others use superscripts which indicate the specific disease (e.g. ‘GSS’ for Gerstmann–Sträussler–Scheinker syndrome). [Protein misfolding in prion disease: Cell (1997) 89 499–510; human prion diseases: Book ref. 215, pp. 39–77.] (2) Strains of the yeast Saccharomyces cerevisiae contain a protein release factor (see PROTEIN SYNTHESIS) designated eRF3 (D Sup35, product of gene SUP35 ) which, through variation in its effective concentration in the cytoplasm, can influence the fidelity with which translation is terminated on the ribosome; thus, a lowered concentration of the release factor promotes readthrough at certain stop codons. A non-genetic mechanism which can control the concentration of eRF3 may therefore permit heritable variation in the organism without changes in the genome [see e.g. Nature (2000) 407 477–483]. Sup35 can exist in an aggregated, prion form (the [PSIC ] state) or in a soluble form (the [psi
] state). In [PSIC ] cells, aggregation of Sup35 protein reduces the amount available for terminating translation on the ribosomes, and this allows readthrough at certain stop codons. Propagation of [PSIC ] involves protein–protein interaction following the mixing of cytoplasm from different cells (e.g. during mating); in this process the soluble form of Sup35 is converted to the aggregated (prion) form. (Interestingly, if a [PSIC ] strain of S. cerevisiae is grown in the presence of 2 M glycerol (or certain other agents) the aggregates of Sup35 break down to the soluble form of Sup35, i.e. a [psi
] strain of the yeast is formed.) [Elimination of [PSIC ] by guanidine hydrochloride: Mol. Microbiol. (2001) 40 1357–1369.] [eRF3 concentration in the termination of translation: Microbiology (2001) 147 255–269 (262–263).] In vitro studies involving fusion between the prion domain of Sup35 proteins from S. cerevisiae and Candida albicans may help to explain factor(s) which determine the ability of a prion to cross a species barrier [Nature (2001) 410 223–227]. pristane An isoprenoid alkane: 2,6,10,14-tetramethylpentadecane. pristinamycins See STREPTOGRAMINS. pRNA See BACTERIOPHAGE f29. PRNP gene See PRION. probasidium (1) A developing BASIDIUM at the stage at which karyogamy occurs; in many basidiomycetes the terms probasidium and METABASIDIUM refer to the same structure at different developmental stages. (2) That part of a developing basidium in which karyogamy occurs – e.g. the teliospore (or its contents) of the rust and smut fungi, or, in Septobasidium, the typically thick-walled structure from which arises, by outgrowth, the septate, basidiosporebearing metabasidium. probe A short strand of DNA or RNA (often 10–25 nucleotides in length) which is complementary to a given target sequence of nucleotides; because a probe can hybridize specifically with its target sequence, it can be used e.g. to detect, identify or locate the target. To make the probe detectable it is ‘labelled’ either before or after it hybridizes to the target sequence (see below). (In addition to DNA or RNA, probes may also be prepared from PNA [JAM (2001) 90 180–189].) Probe–target hybridization is affected e.g. by temperature, pH and concentration of electrolyte. Conditions can be chosen such that the probe will hybridize only to the exact target sequence, i.e. conditions under which mis-matches are not tolerated; such high-stringency conditions are used to promote specificity in probe–target binding. Under low-stringency conditions the effect of mis-matched base(s) can be minimized so direction of primase action is opposite to that of primosome migration; primer synthesis is assumed to occur within the domain of the primosome. [JBC (1981) 256 5273–5286.] primulin A polycyclic fluorescent dye. (See also SCAR.) prion (1) In man/animals: an aberrant form of a normal, chromosome-encoded glycoprotein; prions are now believed to be the aetiological agents of TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES. [Mapping the parameters of prion-induced neuropathology: PNAS (2000) 97 10573–10577.] The normal protein, which is designated PrPc , is encoded (in humans) by the PRNP gene (on chromosome 20). PrPc is a component of the cytoplasmic membrane in certain types of cell – e.g. neurones, astrocytes, dendrocytes, lymphocytes and macrophages. Synthesis of PrPc involves intranuclear transcription of PRNP and translation on the endoplasmic reticulum; post-translational modification includes glycosylation (although some molecules remain unglycosylated) and attachment to a glycolipid phosphatidylinositol (GPI) ‘anchor’. The PrPc molecule is apparently anchored at the cell surface for a short period of time and then internalized and degraded in a lysosome. The function of PrPc has not been established. When a prion infects a cell, it appears that normal breakdown of (at least some) PrPc does not occur, and that the secondary structure of these PrPc molecules is changed to that of the prion. This is thought to occur either by interaction between individual PrPc and prion molecules (heterodimer model), or by interaction between individual PrPc molecules and organized aggregates of prion molecules (the ‘seed’ or ‘nucleus’ model). Accumulation of prions in the form of crystalloid structures eventually leads to the death of the cell. (Interestingly, specific parts of PrPc (particularly the central region: residues 90–120) undergo spontaneous re-arrangement to a conformation resembling that of the infectious form when immobilized on a surface with recombinant antibodies [EMBO (2001) 20 1547–1554].) In the prion model of pathogenesis, prions may be present through infection – either via ingestion (as in KURU) or through their introduction by medical or surgical procedures. The inherited prion diseases, in which the PRNP gene is almost invariably heterozygous (wild-type and mutant alleles), are believed to result from the effect of mutant (prion) proteins on wild-type proteins. In the so-called ‘sporadic’ prion diseases, conversion of normal protein to prion protein is considered to result e.g. from random, spontaneous events. The presence of normal (PrPc ) protein appears to be necessary for the development of prion diseases: knockout mice which are unable to form PrPc do not develop disease on challenge with prion-containing tissue; this suggests that a prion cannot direct the production of further prions without a supply of the normal protein. Prions do not differ, biochemically, from the normal protein, i.e. the amino acid sequence is identical. However, the conformation is different: about ¾40% of normal protein is in the soluble, protease-sensitive a-helical state – with little or no bsheet formation; in contrast, the prion form (isoform) is insoluble and protease-resistant, and is composed of ¾50% b-sheet and ¾20% a-helix forms. (See also PLASMINOGEN.) Prions are resistant to e.g. heat, ionizing and ultraviolet radiation, formalin, b-propiolactone and bleach (as well as to proteases). The prion form of PrPc is often designated PrPres (indicating resistance). Some authors use PrPsc (which, strictly, indicates the SCRAPIE prion) as a generalized designation for any prion; 611 probenecid that hybridization may occur even when the probe–target duplex contains mis-matched base(s). The detection of RNA targets may be achieved with greater rapidity and specificity by means of 20 -O-methyl oligoribonucleotide probes (compared with 20 -deoxy oligoribonucleotide probes) [NAR (1998) 26 2224–2229]. Labelling of probes. Probes may be labelled directly and covalently with radioactive isotopes (e.g. 32 P); such isotopic labels can be detected by autoradiography. Both DNA and RNA probes can be labelled at the 50 end with e.g. [g32 P]ATP (i.e. ATP with 32 P in the g position); this reaction is mediated by polynucleotide kinase. Radioactive labels are robust, and are associated with good sensitivity (but limited resolution). Non-radioactive (non-isotopic) labelling with e.g. enzymes or fluorescent molecules is common. Labelling may be either direct or indirect. Thus, for example, existing probes may be labelled with a reactive fluorophore which binds covalently to the probe; such labelling systems are available commercially (e.g. FluoReporter , marketed by Molecular Probes Inc., Eugene, OR, USA). In the second form of direct fluorescent labelling, probes are synthesized in a reaction mixture containing fluorophore-conjugated nucleotides (which are thus incorporated into the probe). Such fluorescent nucleotides are available commercially (e.g. ChromaTide ; Molecular Probes Inc.). Fluorescent labels are detected by examining the preparation under ultraviolet light. CHEMILUMINESCENCE is used in probe detection in certain commercial diagnostic tests (see e.g. hybridization protection assay in the entry TMA). Note that, in direct labelling, the probe is already labelled prior to hybridization. For indirect labelling (e.g. with an enzyme or fluorophore), a small molecule (e.g. a biotin or DIGOXIGENIN ‘tag’) is incorporated into the probe before use. After hybridization, the probe is detected by using a conjugate to link the tag with a fluorophore or enzyme – see e.g. DOT-BLOT. (It has been reported that short (¾10-nucleotide) 50 -biotinylated probes give satisfactory results when labelled with a STREPTAVIDIN –alkaline phosphatase conjugate prior to the hybridization step [NAR (1999) 27 703–705].) Uses of probes. Probes can be used e.g. to detect, identify or locate particular pathogens in clinical specimens (by demonstrating the presence of pathogen-specific sequences); specimens are commonly subjected to pre-test procedures in which any target nucleic acids that may be present are exposed (in singlestranded form) to specific probes. One example is the PACE 2C test (Gen-Probe, San Diego, USA) which, in a single assay, can detect Chlamydia trachomatis and/or Neisseria gonorrhoea in urogenital specimens [see e.g. JCM (1995) 33 2587–2591]. In situ hybridization (ISH) involves the use of strain- or species-specific probes e.g. for detecting pathogens in situ – i.e. at the actual site of infection within tissues or cells; bacterial, viral, fungal and protozoal pathogens may be detected. ISH is particularly useful for demonstrating nucleic acids which are focally distributed – e.g. virus-infected tissue. For example, ISH was found to be as sensitive as PCR for the detection of human papillomavirus in cervical scrapings [HJ (1995) 27 54–59]. Much of the early work in ISH was conducted with radioactive probes; currently, fluorescent probes are common (fluorescence in situ hybridization: FISH). FISH is both rapid and flexible. For example, the target area can be interrogated simultaneously with several different types of probe (each labelled with a different fluorophore); thus, when different probes bind to closely adjacent targets they can be distinguished by their emission characteristics. Probes are also used in a line probe assay for detecting certain point mutations which confer resistance to antibiotics in Mycobacterium tuberculosis. Essentially, PCR is used to amplify a sequence in the chromosome associated with common resistance-conferring mutations, and the amplicons are denatured to the single-stranded state; these products are then tested for hybridization against a range of probes which include nonmutant and mutant copies of the given chromosomal sequence. The presence/absence of hybridization between the amplicons and each of the probes provides an indication of the probable resistance or susceptibility of the test strain to given antibiotic(s). [Evaluation of a line probe assay: JCM (1997) 35 1281–1283.] In nucleic acid amplification processes, specialized probes can provide a real-time indication of progress in the reaction, and can also be used to assess the concentration of target molecules present in the sample prior to amplification (see TAQMAN PROBES). (See also MOLECULAR BEACON PROBES.) [Probe-based methods in clinical microbiology: Book ref. 221, pp 44–55.] probenecid A drug which e.g. delays the excretion of – and (thus) helps to maintain blood levels of – certain penicillins and cephalosporins. Probenecid, which inhibits glucuronidation, may affect the metabolism of the antiretroviral drug AZT. probiotic Any potentially beneficial preparation consisting typically of a culture of bacteria (or bacterial spores) of a type normally found in the healthy gut microflora. Oral administration of probiotics has been reported to restore normal gut microecology and e.g. to enhance intestinal IgA responses; perinatal and postnatal administration of the probiotic Lactobacillus rhamnosus (D Lactobacillus GG; ATCC 53103) has been associated with a significant reduction in the incidence of atopic disease (eczema) in at-risk infants [Lancet (2001) 357 1076–1079; see also commentary: Lancet (2001) 357 1057–1059]. The Shirota strain of Lactobacillus casei is reported to be useful therapeutically against an Escherichia coli urinary tract infection when administered intraurethrally [AAC (2001) 45 1751–1760]. Probiotic activity has been reported for various strains of Bacillus. For example, the product Enterogermina (an aqueous suspension of spores) is used in Italy with the object of preventing or treating bacterial diarrhoea. [Molecular characterization of bacteria (Bacillus clausii ) from samples of Enterogermina: AEM (2001) 67 834–839.] (See also YAKULT and VSL#3; cf PREBIOTIC.) procapsid A capsid precursor formed during the assembly of a virus. Procaryotae An obsolete kingdom which comprised the PROKARYOTES; proposed divisions of the Procaryotae: FIRMICUTES; GRACILICUTES; MENDOSICUTES; TENERICUTES. procaryote Syn. PROKARYOTE. processive enzyme An enzyme which binds to a (macromolecular) substrate and then moves along it, functioning as it goes. (See e.g. HELICASES; DNA POLYMERASES.) Prochlorales See PROCHLOROPHYTES. prochloraz (trade name: e.g. Sportak) An agricultural systemic ANTIFUNGAL AGENT which inhibits sterol biosynthesis, thereby disrupting the cytoplasmic membrane in susceptible fungi. It is used e.g. against rhynchosporium, net blotch and powdery mildew of cereals, and – mixed with e.g. mancozeb – for the control of yellow rust (Puccinia striiformis) on wheat. Mixed with CARBENDAZIM (‘Sportak Alpha’) it is useful e.g. against eyespot, sharp eyespot, septoria and other CEREAL DISEASES. 612 promoter progressive pneumonia virus See LENTIVIRINAE. proguanil See CHLORGUANIDE. prohead A DNA-free phage head precursor formed during bacteriophage assembly. (See e.g. BACTERIOPHAGE T4; see also SCAFFOLDING PROTEIN.) prohormone See PHEROMONE. prokaryote (procaryote) A type of CELLULAR (sense 2 or 3) MICROORGANISM in which the CHROMOSOME(s) are not separated from the cytoplasm by a specialized membrane; the CYTOPLASMIC MEMBRANE is typically devoid of sterols; chloroplasts and mitochondria are absent (cf. INTRACYTOPLASMIC MEMBRANES); RIBOSOMES have a sedimentation coefficient of ca. 70S; a CELL WALL is typically present and may contain structural components such as PEPTIDOGLYCAN or PSEUDOMUREIN, or may consist e.g. of an S LAYER; storage compounds may include e.g. POLYb-HYDROXYBUTYRATE; flagella (when present) are structurally relatively simple (see FLAGELLUM (a) and (c)). CHEMOLITHOAUTOTROPHS, and the ability to carry out NITROGEN FIXATION, occur only among the prokaryotes. The prokaryotes comprise two DOMAINS: the domain ARCHAEA (formerly referred to as the kingdom Archaebacteria) and the domain BACTERIA [JB (1994) 176 1–6]. Members of the two domains differ in their 16S rRNA and e.g. in the composition, structure and/or mode of assembly of the cytoplasmic membrane, cell wall and flagella; differences also occur e.g. in the composition of the ribosomal proteins. According to one hypothesis, EUKARYOTES arose from an energy-based symbiotic relationship between cells from the two prokaryotic domains [Nature (1998) 392 37–41]. prolamellar body In an ETIOPLAST: a paracrystalline array of tubules bearing a small lamellar structure (prothylakoid ) which contains certain thylakoid components (e.g. the CF1 unit of PROTON ATPASE); on illumination, prolamellar bodies disperse, and prothylakoids eventually develop into THYLAKOIDS. prolate Of a phage head: having a shape in which the distance from one pole to the other is greater than the diameter at the mid-point of the head; an example of a prolate head occurs in BACTERIOPHAGE f29. proliferative kidney disease (PKD) A FISH DISEASE which can be economically important in young salmonid fish. Symptoms include exophthalmia, anaemia, abdominal swelling and kidney hypertrophy; mortality rate: 10–95%. PKD is caused by an unclassified protozoon which apparently has affinities with members of the MYXOZOA [JP (1985) 32 254–260]. Proliferobasidium See BRACHYBASIDIALES. L-proline biosynthesis See Appendix IV(a). proloculus See FORAMINIFERIDA. promastigote A form assumed by the cells of many species of the TRYPANOSOMATIDAE (q.v.) during at least certain stages of their life cycles. prometaphase See MITOSIS. Promicromonospora A genus of asporogenous bacteria (order ACTINOMYCETALES, wall type VI) which occur e.g. in soil. The organisms form a yellow fragmenting mycelium. GC%: ca. 73. Type species: P. citrea. [Book ref. 73, pp. 53–54.] promiscuous plasmids Those CONJUGATIVE PLASMIDS (sense 1) – e.g. IncP1 plasmids – which are capable of self-transmission between bacteria of a wide range of different species and genera. promitochondrion See MITOCHONDRION. promoter In a DNA strand: a nucleotide sequence which is recognized (directly or indirectly) and bound by a DNAdependent RNA POLYMERASE (RPase) during the initiation of TRANSCRIPTION. Transcription is initiated at a position (the start Prochlorococcus marinus See PROCHLOROPHYTES. Prochloron See PROCHLOROPHYTES. Prochlorophyta See PROCHLOROPHYTES. prochlorophytes A category of unicellular prokaryotes (order Prochlorales, division Prochlorophyta) which contain eukaryotictype chlorophylls a and b (but lack phycobiliproteins) and carry out oxygenic photosynthesis. The first member identified was Prochloron didemni (formerly Synechocystis didemni ). The cells are more or less spherical, 6–25 µm in diameter, and the photosynthetic pigments are attached to THYLAKOIDS. The organisms occur as ectosymbionts on marine didemnid ascidians (sea-squirts) in tropical and subtropical coastal waters. Species identified later include Prochlorothrix hollandica and Prochlorococcus marinus. Taxonomically (in terms of genomic sequence data), the prochlorophytes have been grouped with the cyanobacteria and chloroplasts. [Photosynthetic machinery in prochlorophytes: FEMS Reviews (1994) 13 393–414.] Prochlorothrix hollandica See PROCHLOROPHYTES. prodigiosin A red, water-insoluble, ethanol-soluble tripyrrole derivative. Prodigiosin and similar pigments are produced by some species and biotypes of SERRATIA and by other bacteria (e.g. Streptomyces spp). prodromal Refers to e.g. symptom(s) which precede the main symptoms of an infectious disease (e.g. Koplik’s spots in MEASLES). prodrug The precursor form of a drug – see e.g. FAMCICLOVIR. proenzyme Syn. ZYMOGEN. professional phagocytes PHAGOCYTES (including NEUTROPHILS and MACROPHAGES) which can kill and digest microorganisms which they have ingested. profibrinolysin See FIBRINOLYSIN. profilin See ACTIN. proflavine (proflavin) See ACRIDINES. progametangium In zygomycetes: an enlarged ZYGOPHORE which differentiates into a distal gametangium and a proximal SUSPENSOR. (See also ZYGOSPORE.) progamone See PHEROMONE. progenote A (hypothetical) primitive organism; progenotes are presumed to be the phylogenetic progenitors of both prokaryotes and eukaryotes. progressive multifocal leukoencephalopathy (PML) A human demyelinating disease that occurs primarily in those with an underlying immunological dysfunction (e.g. Hodgkin’s disease, chronic lymphatic leukaemia, AIDS); multiple small areas of demyelination develop in the white matter of the brain, resulting in progressive paralysis, mental deterioration etc. – death usually occurring within 6–12 months of the first symptoms. Characteristic of PML are enlarged nuclei in the oligodendrocytes (glial cells associated with myelin formation). It is widely belived that the causal agent of PML is a POLYOMAVIRUS: the JC virus (JC D the initials of a patient who died from PML); it is also believed that some form of immunosuppression is a prerequisite for the development of the disease. Other factor(s) may be involved; for example, one in vitro study found that the Tat protein, encoded by the AIDS virus (HIV-1), could activate the late promoter of JC virus in glial cells [PNAS (1990) 87 3479–3483]. [Murine model for PML: Cell (1986) 46 13–18. Polyomaviruses and disease of the central nervous system: RMM (1998) 9 79–85.] 613 promoter control point or start site) which is commonly within the promoter sequence. The first nucleotide to be transcribed is designated C1; nucleotides downstream of this position (i.e. those in the direction of RNA elongation: 50 -to-30 with respect to the RNA) are numbered C2, C3, C4 etc., and nucleotides in the opposite (upstream) direction are numbered
1,
2,
3 etc. Bacteria have several RPase holoenzymes (differing in their SIGMA FACTORS), each one recognizing a distinct class of promoter. In Escherichia coli most transcription is carried out by Es70 ; the enzyme binds to a region of DNA extending from ca. 50 bp upstream to ca. 20 bp downstream of the start point. Comparison of the sequences of many promoters recognized by Es70 has revealed that certain short sequences are more or less conserved. Thus, a CONSENSUS SEQUENCE, TATAAT (the Pribnow box or
10 sequence), is centred ca. 10 bp upstream from the start point, and another consensus sequence, TTGACA (the
35 sequence) is centred ca. 35 bp upstream of the start point. The start point itself is usually a purine, often an adenine residue in the sequence CAT. Promoters recognized by Es70 in E. coli differ to varying extents from the theoretical ‘consensus promoter’, and may also differ from one another in functional efficiency (efficient promoters being described as ‘strong’, inefficient ones as ‘weak’). Efficiency may be affected e.g. by changes in the consensus sequences or in the transcribed region downstream of the start point. Changes at one site may be compensated for by changes at another: thus, promoters with the same efficiencies may have different base sequencies; furthermore, a (synthetic) promoter containing the theoretical consensus sequences has been found to be relatively inefficient. Bacterial promoters of other classes (recognized by different RPase holoenzymes) also typically have two short, distinctive conserved sequences upstream from the start point; however, the actual sequences vary from one class of promoter to another. For example, in Bacillus subtilis the main holoenzyme, Es43 , recognizes the same type of promoter as does the Es70 of E. coli, but Es37 (see SIGMA FACTOR) recognizes a consensus sequence AGGATTTNA (N D any nucleotide) in the
35 region and GGAATTNTTT in the
10 region; the corresponding sequences for Es29 are TTNAAA and CATATT. (The phage T4 Esgp55 recognizes a promoter which apparently lacks a
35 sequence; in promoters recognized by the EsgpntrA of e.g. Klebsiella a
35 sequence is apparently absent, but there is a consensus sequence in the
20 region.) In eukaryotes, promoters are not recognized directly by the RNA polymerases; transcription initiation factors (TIFs) first bind to a promoter to form a pre-initiation complex, and only then does an RPase bind to form an initiation complex. Promoters for RPase II generally contain a short consensus sequence, TATA(A/T)A(A/T) (the TATA box, Goldberg–Hogness box, or Hogness box ), ca. 25–30 bp upstream from the transcription start point; this appears to be the only well-conserved sequence in RPase II promoters [NAR (1986) 14 10009–10026], although other sequences upstream of the TATA box may be important sites for the action of promoter-specific TIFs [review: Nature (1985) 316 774–778]. (See also ENHANCER.) The TATA box, found in many RPase II promoters, also occurs in a minority of type III promoters. The ‘classical’ RPase III promoters occur within the associated gene, i.e. intragenically, downstream of the start point; binding of the RPase at the promoter leads to initiation of transcription at a particular distance upstream of the binding site. One factor, the TATA box-binding protein (TBP), is necessary for the formation of an initiation complex by each of the three types of RPase (I, II and III) – regardless of whether or not their promoters include a TATA box; TBP was first identified as a DNA-binding element in complexes assembled on type II, TATA-containing promoters, but in TATA-lacking promoters TBP appears to bind not to DNA but (directly or indirectly) to a DNA-binding protein (as well as to the polymerase) in the initiation complex. [TBP (minireview): Cell (1993) 72 7–10.] promoter control See OPERON. promycelium See METABASIDIUM. Pronase (proprietary name) A mixture of endopeptidases and exopeptidases (including carboxypeptidase(s) and aminopeptidases) which has non-specific protease activity. It is obtained from Streptomyces griseus. Prontosil An early sulphonamide, first used clinically in the 1930s; its activity was due to its breakdown in vivo to sulphanilamide. It has been superceded by the more effective, less toxic modern SULPHONAMIDES. pronucleus (1) The haploid nucleus of a gamete. (2) (protozool.) See AUTOGAMY and CONJUGATION. proof-reading (mol. biol.) A system in which the accurary of a process is increased by the removal of ‘errors’ immediately after they have occurred: see e.g. DNA REPLICATION and PROTEIN SYNTHESIS. propagative viruses See CIRCULATIVE TRANSMISSION. propagule Any disseminative unit of an organism, e.g. a spore, a mycelial fragment. propamidine An aromatic DIAMIDINE used as an antiseptic; it is active mainly against asporogenous Gram-positive bacteria and certain fungi. propeller loop fermenter A LOOP FERMENTER in which a motordriven propeller promotes circulation in the column by impelling the culture vertically up (or down) the DRAFT TUBE. proper margin (excipulum proprium) (lichenol.) Non-lichenized (i.e., wholly fungal) tissue forming the excipular margin (rim) of a lichen APOTHECIUM. A proper margin is often the same colour as the hymenial disc. (cf. THALLINE MARGIN; see also LECIDEINE APOTHECIUM.) properdin A g-globulin (MWt ca. 220000) which occurs in normal serum (ca. 25 µg/ml). Properdin promotes the alternative pathway of COMPLEMENT FIXATION by stabilizing the C3 convertase (C3bBb) in the presence of specific activators of this pathway. prophage See LYSOGENY. prophage immunity See SUPERINFECTION IMMUNITY. prophase See MITOSIS and MEIOSIS. prophylaxis Measure(s) taken to prevent the occurrence of disease – e.g. DISINFECTION, IMMUNIZATION. (See also PROTECTANT.) propiconazole (1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan2,2-yl-methyl]-1H-1,2,4-triazole; trade names: e.g. Radar, Tilt) An agricultural AZOLE ANTIFUNGAL AGENT which has both contact and systemic action against a wide range of plant pathogenic fungi; it has both eradicant and protectant properties against e.g. cereal diseases such as eyespot, net blotch, powdery mildew, rhynchosporium, septoria, yellow and brown rusts, etc. It may be mixed with e.g. CARBENDAZIM to give a preparation with an even broader antifungal spectrum. propidium A phenanthridine trypanocidal agent and INTERCALATING AGENT (apparent unwinding angle: 26° ). b-propiolactone (BPL) A water-miscible and non-inflammable cyclic ether (b.p. 155° C) used e.g. (in vapour form) as a surface STERILANT; it has low powers of penetration, needs a high relative humidity (ca. 70%) for maximum activity, and may be poorly effective against e.g. dried spores (cf. ETHYLENE 614 prostaglandins OXIDE). BPL acts as an ALKYLATING AGENT, substituting proteins etc with propionic acid residues. Above 25° C it is readily hydrolysed to b-hydroxypropionic acid (hydracrylic acid); under acid conditions it forms an open-chain ester-linked polymer. BPL is an irritant and may be carcinogenic. Propionibacterium A genus of Gram-positive, asporogenous, chemoorganotrophic, anaerobic bacteria which occur e.g. in dairy products (see e.g. CHEESE-MAKING) and on the human skin, and which form propionic acid as a main product in the PROPIONIC ACID FERMENTATION of hexoses or lactate. Cells: nonmotile, pleomorphic, branched or unbranched rods or coccoid forms which may exhibit a ‘Chinese letter’ arrangement in stained preparations; some form pigments. The organisms can be cultured e.g. on yeast extract-lactate-peptone media. GC%: ca. 57–67. Type species: P. freudenreichii. P. acnes (formerly Corynebacterium parvum). The organisms ferment glucose (but not maltose or sucrose), do not hydrolyse aesculin, and are indole-positive; they digest casein and liquefy gelatin. [Extracellular proteolysis: JAB (1983) 54 263–271.] Colonies older than ca. 4 days may become reddish. (See also ACNE and SKIN MICROFLORA.) [Pathogenicity: RMM (1994) 5 163–173.] [Possible association between P. acnes and sciatica: Lancet (2001) 357 2024–2025.] Other species: P. acidi-propionici; P. freudenreichii (which incorporates P. shermanii ) (see also PYROPHOSPHATE); P. jensenii (which incorporates P. peterssonii and P. raffinosaceum); P. thoenii. [Book ref. 46, pp. 1894–1902.] propionic acid fermentation A FERMENTATION (sense 1), carried out e.g. by Propionibacterium spp, Clostridium propionicum and Megasphaera elsdenii, in which e.g. glucose and/or lactate yield propionic acid and acetic acid as the main end products. Propionibacterium spp ferment glucose or lactate via succinate [Appendix III(h)]; when lactate is the substrate the reduction of (endogenous) fumarate generates proton motive force (pmf), permiting ATP synthesis by electron transport phosphorylation. (cf. FUMARATE RESPIRATION.) C. propionicum and M. elsdenii ferment lactate via a different pathway (the acrylate pathway): lactate ! lactyl-CoA ! acrylyl-CoA ! propionyl-CoA ! propionic acid. Propionigenium A genus of anaerobic, asporogenous bacteria. P. modestum converts succinate to propionate by reactions which include the decarboxylation of methylmalonyl-CoA; this decarboxylation is coupled to the outward pumping of NaC – generating a SODIUM MOTIVE FORCE which can be used for ATP synthesis by a membrane-bound ATPase [EMBO (1984) 3 1665–1670]. Propionispira A genus of Gram-negative, obligately anaerobic, asporogenous, nitrogen-fixing bacteria which occur in trees affected with WETWOOD. The cells are curved rods or long spiral filaments; peritrichously flagellate. Various compounds are fermented: e.g. galacturonate, lactate, mannitol, and a number of sugars (e.g. glucose, lactose, sucrose); propionate, CO2 and acetate are major metabolic products. GC%: ca. 37. Type species: P. arboris. [JGM (1982) 128 2771–2779.] proplastid See e.g. CHLOROPLAST. propylene glycol See ALCOHOLS. propylene oxide A water-miscible cyclic ether (b.p. 34° C) used (in vapour form) e.g. as a disinfectant for treating certain foods; on hydrolysis it yields propylene glycol. It is also used in the preparation of specimens for ELECTRON MICROSCOPY. Prorocentrum A genus of ‘primitive’ DINOFLAGELLATES in which the cell is flattened, lacks a girdle and sulcus, and is divided longitudinally into two halves, each half containing a relatively thick, convex thecal plate. The two flagella are inserted apically. Prorodon A genus of freshwater and marine carnivorous ciliates (subclass GYMNOSTOMATIA). Cells: ovoid, up to ca. 200 µm in length, with uniform somatic ciliature, an apical cytostome, and a cytopharynx with associated nematodesmata. Encystment is promoted e.g. by desiccation. prosenchyma See PLECTENCHYMA. prosome A stable ribonucleoprotein particle (ca. 19S) present in the nucleus and cytoplasm of various types of eukaryotic cell. Prosomes are believed to be involved in the post-transcriptional regulation of gene expression. [Prosomes – ubiquity and interspecies variation: JMB (1986) 187 479–493.] prosoplectenchyma See PLECTENCHYMA. prostacyclin (PGI2 ) A potent vasodilator, derived from PROSTAGLANDIN PGH2 , which degrades spontaneously (in minutes) to 6-oxo-PGF1a ; PGI2 promotes vascular permeability and induces pain. 6-Oxo-PGF1a levels are raised in exudates from chronic granulomas. prostaglandins (PGs) Compounds formed from ARACHIDONIC ACID via the activity of cyclo-oxygenase; this enzyme, which exists in two isoforms (COX-1 and COX-2), catalyses a twostep reaction in which the central region of the linear arachidonic acid molecule undergoes oxidative cyclization to form PGH2 , a precursor of PGF2a , PGD2 , PGE2 , PGI2 and thromboxane. Prostaglandins are included within the category eicosanoids. Cyclo-oxygenases are inhibited e.g. by compounds designated non-steroidal anti-inflammatory drugs (NSAIDs). Many of these drugs can block the activity of both COX-1 and COX-2 by binding to the enzyme in the region of an arginine residue at position 120. (The common drug aspirin (acetylsalicylic acid) – made synthetically, but also found naturally in the willow (Salix spp) – inhibits cyclo-oxygenase by acetylating a serine residue in the enzyme: Ser-530 in the COX-1 isoform, Ser-516 in COX-2.) Some NSAIDs (e.g. the tri-cyclic coxibs celecoxib and rofecoxib) specifically inhibit COX-2, such inhibition apparently involving the region near a valine residue at position 523 (isoleucine occupies 523 in COX-1); such specific inhibitors of COX-2 may be useful e.g. against certain types of cancer. Cyclo-oxygenase is apparently activated in vitro by NITRIC OXIDE [PNAS (1993) 90 7240–7244]. COX-2 is inducible (e.g. by ENDOTOXIN, immune complexes, certain CYTOKINES – e.g. INTERLEUKIN-1 and TNF-a (although expression is inhibited by IL-4 and IL-10) – and phorbol esters). COX-1 is typically produced constitutively and seems to have a ‘housekeeping’ function. (It is generally thought that NSAIDs work by inhibiting COX-2 at sites of inflammation, and that their adverse gastrointestinal effects result from inhibition of COX-1.) Particular PGs tend to be synthesized by particular types of cell. PGD2 is formed e.g. by mast cells, PGE2 and PGF2a by macrophages and monocytes, and PGI2 by endothelial cells. Following synthesis, prostaglandins may exit the cell and then act on cell-surface receptors in an autocrine or paracrine fashion; for example, PGE2 may promote an increase in the concentration of cAMP (cyclic AMP) by binding to the EP1 or EP4 receptor. Receptors for prostaglandins also occur in the cell nucleus – the so-called peroxisome proliferator-activated receptor (PPAR). Prostaglandins have many physiological roles/effects – including vasodilation (e.g. PGE2 ), vasoconstriction (e.g. PGF2a ) and the febrile response. Prostaglandins, which occur e.g. in inflammatory exudates (see also INFLAMMATION), are associated with both inflammatory and anti-inflammatory activities. Prostaglandins have other roles e.g. in kidney function and certain aspects of reproduction. 615 prostheca the addition of microbial proteases to domestic detergents for the digestion of proteinaceous stains in fabrics. Since most commercial detergents contain tripolyphosphate as a sequestering agent, Ca2C -requiring proteases (e.g. THERMOLYSIN) are not suitable for this purpose, and alkaline serine proteases (see SUBTILISINS) from Bacillus spp are usually used. Enzymes from Rhizomucor pusillus, R. miehei and Endothia parasitica have some use as rennin (rennet) substitutes in CHEESE-MAKING (see also CASEIN); the proteolytic-to-coagulating activity ratio of these enzymes is higher than that of rennin, and their use requires some modification of the cheese-making process in order to avoid the excessive formation of bitter peptides. Microbial proteases have some application in the manufacture of leather, although chemical methods are still cheaper; alkaline proteases from alkalophilic Bacillus spp may be used with lime for de-hairing hides, and proteases from e.g. Aspergillus oryzae or Bacillus spp may be used for bating hides (a process which makes the leather softer and more elastic). Other uses include e.g. treatment of flour to adjust its gluten composition (and hence baking qualities); as a digestive aid; and in the débridement of wounds (experimental) and dissolution of blood clots (see e.g. BRINASE). (See also PRONASE.) Proteases produced by certain pathogenic microorganisms may act as virulence factors (see e.g. ELASTASE). proteasome A hollow, intracellular proteolytic structure formed by the self-assembly (autocompartmentalization) of proteases and ATPases; proteolytic sites occur in the interior of the proteasome so that protein degradation can be carried out intracellularly without risk to the cell’s integrity. A narrow entrance to the proteasome’s interior restricts access to unfolded proteins. In eukaryotes, proteasomes are used for the ATP-dependent degradation of various UBIQUITIN-bound proteins. For example, the ubiquitin–proteasome pathway is reported to be essential during changes in morphology (e.g. trypomastigote ! amastigote) in Trypanosoma cruzi [Biochem. (2001) 40 1053– 1062]. Proteasomes also occur in prokaryotes – e.g. the archaeans Methanosarcina thermophila and Thermoplasma acidophilum, and the bacteria Mycobacterium tuberculosis and Rhodococcus erythropolis. [Proteasomes in prokaryotes: TIM (1999) 7 88–92.] (See also DEGRADOSOME.) protectant (prophylactic) (plant pathol.) Any chemical agent which prevents the occurrence of particular disease(s) among plants treated with that agent. (cf. ERADICANT.) protective antigens Those antigens of a pathogen which can elicit an immune response that gives protective immunity against the pathogen. protective immunity Syn. IMMUNITY (3). Proteeae A tribe of bacteria (family ENTEROBACTERIACEAE) divided (e.g. on the basis of DNA relatedness) into three genera: MORGANELLA, PROTEUS and PROVIDENCIA. All members can oxidatively deaminate a range of amino acids. Most strains are motile (but may be non-motile at temperatures >30° C), urease Cve (cf. PROVIDENCIA), lactose
ve (although some strains may contain a Lac plasmid), indole Cve (cf. PROTEUS), VP
ve, MR Cve. Acid (š gas) is produced from glucose, and a reddish-brown pigment is produced on nutrient agar containing 5% tryptophan. Species are generally resistant to bacitracin, polymyxins and erythromycin. [Book ref. 46, pp. 1204–1224.] protein 2 A PORIN of Escherichia coli. protein A (SpA) A cell wall protein (MWt ca. 42000) found in most strains of Staphylococcus aureus; it is covalently bound (See also PROSTACYCLIN.) prostheca In certain bacteria (PROSTHECATE BACTERIA): a narrow extension of the cell, bounded by the cell wall and cytoplasmic membrane; a cell may have one or more prosthecae – which may be long and narrow (e.g. in Ancalomicrobium) or short and conical (in Prosthecomicrobium), unbranched or branched. (A long, narrow prostheca is sometimes called a hypha.) In some genera prosthecae are involved in reproduction (see e.g. RHODOMICROBIUM); in Caulobacter the prostheca functions in ADHESION. (cf. STALK.) prosthecate bacteria Those bacteria which form one or more prosthecae during at least some stage of their life cycle. Genera in which the prostheca appears to have an essential reproductive role include HYPHOMICROBIUM, RHODOMICROBIUM, and some species of RHODOPSEUDOMONAS. See also ANCALOMICROBIUM, ASTICCACAULIS, CAULOBACTER, HYPHOMONAS, PEDOMICROBIUM, PROSTHECOMICROBIUM and STELLA. Prosthecochloris See CHLOROBIACEAE and CHLOROBIINEAE. Prosthecomicrobium A genus of chemoorganotrophic, strictly aerobic PROSTHECATE BACTERIA found in natural waters. The cells are coccoid, ca. 1 µm, each bearing ca. 10–30 conicallyshaped prosthecae. P. pneumaticum (type species) is nonmotile and contains GAS VACUOLES; P. enhydrum is monotrichously flagellated and lacks gas vacuoles. P. hirschii is motile (monotrichously flagellated), lacks gas vacuoles, and forms short and/or long (Ancalomicrobium-like) prosthecae [IJSB (1984) 34 304–308]. Cell division apparently occurs by budding [ibid.]. prosthetic group In an enzyme: any low-MWt, non-protein component which is firmly bound to the APOENZYME and which is necessary for the activity of the enzyme. (cf. COENZYME sense 1.) protamines Low-MWt, strongly basic proteins found e.g. in association with DNA in the sperm of fish and birds. (See also PARACOAGULATION.) Protargin See SILVER. Protargol A silver proteinate (see SILVER) used e.g. as an antiseptic and as a reagent for staining protozoan flagella. protease IV See SIGNAL HYPOTHESIS. protease inhibitors (med.) A category of ANTIRETROVIRAL AGENTS that includes the drugs amprenavir, indinavir, nelfinavir, ritonavir and saquinavir. Protease inhibitors bind to, and inhibit, the viral protease. The target enzyme, encoded by the pol region of HIV, is necessary for production of mature virions as it functions in posttranslational processing of precursor proteins. Protease inhibitors have been associated with side-effects such as lipodystrophy. proteases (peptidases) ENZYMES which hydrolyse peptide bonds in proteins and peptides. Endopeptidases cleave bonds within the peptide chain with varying degrees of specificity for particular amino acyl residues (e.g. the pancreatic enzyme trypsin hydrolyses arginyl and lysyl residues). Endopeptidases include e.g. serine proteases (characterized by a catalytically active serine residue in the active centre) and thiol proteases (in which free –SH groups occur). Serine proteases include e.g. trypsin, chymotrypsin and SUBTILISINS; they are inactivated by organic phosphate esters (e.g. diisopropylfluorophosphate, DFP) which acylate the active serine residue. Thiol proteases include e.g. BROMELAIN and PAPAIN; they are inhibited by SH-reagents such as heavy metals. Exopeptidases remove amino acids sequentially from one end of a peptide chain and include N-terminal exopeptidases (aminopeptidases) and C-terminal exopeptidases (carboxypeptidases). Proteases are termed acid, alkaline or neutral if they are active at low, high or netural pH, respectively. Microbial proteases have a number of commercial applications [Book ref. 31, pp. 49–114]. A major commercial use is 616 protein secretion (and export) the alternative name for type II systems: the ‘sec-dependent pathway’); the products of certain sec genes form a secretory channel through the CM. Proteins secreted by the GSP are synthesized with an Nterminal signal sequence (see SIGNAL HYPOTHESIS). In one form of type II secretion, the initial phase (transport across the CM: general export pathway, GEP) begins in the cytoplasm when the secretory protein (i.e. the one being transported) binds to a chaperone protein, SecB; binding may occur during or after translation, but before folding (folding of sec-exported proteins occurs in the periplasm). SecB is targeted to a CM-linked ATPase, SecA. SecA is associated with a SecYEG protein complex (the translocon) [JB (1997) 179 5699–5704; Nature (2002) 418 662–665] which forms a secretory channel through the CM. SecB is released (to chaperone another protein), and translocation of the protein through the CM is associated with ATP hydrolysis at SecA. During, or directly after, translocation, the signal sequence is cleaved by a specific enzyme (a signal peptidase). E. coli has two distinct signal peptidases; leader peptidase (D leader peptidase I ) is functional on a wide range of proteins, while lipoprotein signal peptidase (D signal peptidase II ) acts only on certain modified proteins (e.g. BRAUN LIPOPROTEIN). (Signal peptidases may constitute a class of serine proteases mechanistically related to b-lactamases [TIBS (1992) 17 474–478].) Excised signal peptides are degraded by a signal peptide peptidase; in E. coli at least one enzyme (‘protease IV’) carries out this function. In a variant type II pathway, transport to the CM necessarily begins at an early stage in translation. One scheme for this pathway (in E. coli ) is as follows. Initially, the nascent polypeptide (with its ribosome) binds, via the N-terminal sequence, to a signal recognition particle (SRP) that consists of (i) a 4.5S RNA molecule, and (ii) a 48 kDa multifunctional protein (with GTPase activity) designated P48 (or Ffh). It is not known whether protein–SRP binding inhibits further translation on the ribosome (as occurs in eukaryotic systems); it has been suggested that, in prokaryotes, translation arrest may not be necessary, and that the SRP pathway may simply accelerate association of nascent polypeptide chains with the translocon. A free cytosolic FtsY protein binds to the complex, and the FtsY–SRP–ribosome–polypeptide entity binds transiently, via FtsY, to the CM; hydrolysis of GTP energizes dissociation of the components, and the nascent polypeptide (with its ribosome) binds at a SecYEG (or similar) translocon. [Model for the SRP pathway in E. coli: EMBO (1998) 17 2504–2512.] (Interestingly, in Escherichia coli, reduced levels of SRP lead to the induction of a heat-shock response (see HEAT-SHOCK PROTEINS); this may help to maintain viability by increasing the cell’s capacity to degrade mislocalized CM proteins [JB (2001) 183 2187–2197].) For any given nascent polypeptide, the choice of pathway – i.e. SecB–SecA or SRP – is believed to depend on the hydrophobicity of the N-terminal sequence. Among secreted proteins, the SRP pathway may be used preferentially by those (e.g. b-lactamases) which have relatively hydrophobic Nterminal sequences. However, the SRP pathway may be used mainly for targeting and elaboration of CM proteins – whose N-terminal sequences are characteristically very hydrophobic; for these proteins, this pathway may be advantageous in that completion of translation at a membrane-associated site could avoid the risk of aggregation of hydrophobic domains which may occur in the cytoplasm. In vitro studies involving replacement or modification of the signal peptide sequence indicate that to the wall peptidoglycan and can be released (intact) e.g. by LYSOSTAPHIN. SpA binds to the FC PORTION of IgG1 , IgG2 and IgG4 ; one SpA molecule can bind two IgG molecules. (cf. PROTEIN G.) SpA can also bind, apparently via the Fab region, to IgE, IgA, IgM, and some IgG3 . SpA can elicit Arthus- and anaphylaxis-type reactions, histamine release from granulocytes, and complement fixation by the classical pathway. Applications. (a) SpA immobilized on e.g. sepharose can be used in affinity CHROMATOGRAPHY for IgG purification; IgGs are eluted at pH ca. 4. (IgM and IgA elute at a lower pH.) (b) SpAcontaining staphylococci can bind to specific (IgG) antibodies (via their Fc portions) and will then agglutinate on exposure to the homologous antigen (CO-AGGLUTINATION); such antibodycoated staphylococci can be used e.g. in slide co-agglutination tests for detecting and/or identifying bacteria, toxins, etc. (c) SpA may be bound to red blood cells (e.g. with glutaraldehyde) and used e.g. to detect IgG on the surface of lymphocytes (by rosette formation). (d) SpA conjugated with an enzyme (e.g. b-lactamase, horseradish peroxidase) can be used e.g. for the assay of IgG in serum. Antigen is immobilized, incubated with the serum, and any homologous antibody binding to the antigen is detected with the SpA–enzyme conjugate; the amount of SpA bound, and hence the amount of homologous IgG in the serum, is determined by an assay of the enzymic activity (e.g. PADAC hydrolysis for b-lactamase conjugates) [PTRSLB (1983) 300 399–410]. [Properties and applications of protein A: Book ref. 44, pp. 429–480.] (See also SORTASE.) protein G A cell wall protein from group G streptococci; it resembles staphylococcal PROTEIN A in binding specifically to the Fc region of an IgG molecule, but it binds a broader range of IgG subclasses. [Structure of the IgG-binding regions of protein G: EMBO (1986) 5 1567–1575.] protein i See PRIMOSOME. protein kinase An enzyme which phosphorylates particular amino acid residues in a protein. The functions of certain proteins are regulated by phosphorylation/dephosphorylation both in eukaryotic cells (see e.g. ONCOGENE) and in Escherichia coli (see e.g. TCA CYCLE (isocitrate dehydrogenase)). (See also CYCLIC AMP.) protein kinase A See CYCLIC AMP. protein n (also n
, n

) See PRIMOSOME. protein phosphatase An enzyme which removes covalently bound phosphate from a protein; protein phosphatases have important roles e.g. in the regulation of the eukaryotic CELL CYCLE – in which they complement the activity of PROTEIN KINASES. Some protein phosphatases can cleave phosphate from more than one type of amino acid residue. (See also CYCLIN.) protein priming See DNA REPLICATION (linear genomes). protein secretion (and export) (in Gram-negative bacteria) At least five major modes of protein secretion occur in Gramnegative bacteria; they have been designated types I–V (but see note on types IV and V at the end of this entry). Type I systems. Secretion by type I systems is energized by ATP hydrolysis and involves one-step translocation from cytoplasm to exterior without a free (periplasmic) intermediate; it is exemplified by secretion of a-haemolysin by the ABC EXPORTER of Escherichia coli. Type II systems. Secretion by type II systems (the general secretory pathway, GSP) requires energy from both ATP and pmf – and, in some cases, from GTP. The first stage, translocation across the cytoplasmic membrane (CM), depends on the products of genes analogous to the sec genes of E. coli (hence, 617 protein synthesis the EspB and EspE proteins of enteropathogenic E. coli (EPEC); and the various Yops of Yersinia [the Yersinia Yop virulon: Mol. Microbiol. (1997) 23 861–867]. [Type III secretion systems (review): TIM (1997) 5 148–156.] Type IV systems are mechanistically distinct and not fully understood; the following is an outline of a current model. In these systems the secreted protein is synthesized as part of a larger protein that consists of (i) an N-terminal signal peptide, (ii) an a domain, and (iii) a C-terminal b domain. Initially, the whole protein is translocated across the CM in a sec-dependent manner, with cleavage of the N-terminal signal peptide. To pass through the outer membrane, the b domain is believed to form a ‘b-barrel’ pore in the membrane through which the a (‘passenger’) domain is translocated. At the bacterial surface, the a domain may (i) remain attached (exposed to the environment); (ii) undergo cleavage, but remain (non-covalently) attached; (iii) undergo cleavage and release to the environment. Cleavage may be autocatalytic, or it may involve another outer membrane protein such as the SopA protease of Shigella or the OmpT protease of Escherichia coli. In a type IV system, the translated protein (i.e. N-terminal C a domain C b domain) has been termed an autotransporter. (This term has also been used by some authors to refer specifically to the b domain; other authors have used the term to refer specifically to the a domain.) Proteins transported by type IV systems include the (cellsurface-retained) IcsA protein, associated with Shigella invasiveness in dysentery; the IGA1 PROTEASES of Haemophilus influenzae, Neisseria gonorrhoeae and N. meningitidis; and the AIDA-I system of Escherichia coli. The latter system has been used for AUTODISPLAY. Type V systems. These include the mechanism by which TDNA is transferred from Agrobacterium tumefaciens to the host cell in CROWN GALL, and the secretion of pertussis toxin by Bordetella pertussis. Note on the designation of type IV and type V systems. The name ‘type IV’ secretion system has been widely used for autotransporters by a number of authors [see e.g. TIM (1998) 6 370–378; Book ref. 218 (1999), p 42; Book ref. 223 (1999), p 94]. Some authors [see Inf. Immun. (2001) 69 1231–1243] have suggested that (i) autotransporters be called type V systems, and that (ii) secretory mechanisms referred to above as type V systems be called type IV systems. In this dictionary the designation ‘type IV’ has been retained for autotransporters. protein synthesis A protein consists essentially of one or more polypeptides – a polypeptide being a chain of amino acids linked by peptide bonds (
CO.NH
). Polypeptides are folded into a three-dimensional structure that is stabilized mainly by hydrogen and/or disulphide bonds formed between amino acids in different parts of the chain; the specific three-dimensional structure of a given protein is associated with its biological role. The biosynthesis of a protein is a complex process in which the composition of the protein (i.e. the number, nature and sequence of its amino acids) is decoded from information contained in specific sequence(s) of nucleotides. In the majority of organisms (including all cells), proteins are encoded by DNA; in certain viruses, proteins are encoded by RNA. The information in nucleic acids is expressed via a GENETIC CODE (q.v.). Unlike the polymerization of nucleotides (see e.g. DNA SYNTHESIS), polypeptide chains are not synthesized by the polymerization of amino acids on a template. In fact, protein synthesis occurs in several stages. In all cases (in both prokaryotic and eukaryotic cells) the first stage is the synthesis of an RNA copy of the particular DNA the targeting pathway of Escherichia coli pre-secretory and integral membrane proteins is specified by the hydrophobicity of the targeting signal [PNAS (2001) 98 3471–3476]. The second stage of the GSP, translocation across the OUTER MEMBRANE, depends on a number of specific secretion factors. Although the GEP occurs in E. coli, this organism apparently does not secrete proteins via the GSP, i.e. it does not carry out the second stage of the GSP. However, other species of the Enterobacteriaceae do secrete proteins via this pathway. For example, Klebsiella oxytoca secretes a lipoprotein enzyme, pullulanase (see DEBRANCHING ENZYME), whose translocation across the outer membrane requires the products of 14 pul genes. (Interestingly, if all the pul genes from K. oxytoca are expressed in E. coli, this organism can secrete pullulanase.) Again, the GSP is used by Erwinia chrysanthemi for secreting the enzymes pectinase and cellulase, secretion needing expression of the out genes. (E. coli encoding a gene for pectinase can secrete this enzyme if supplied with out genes.) The secretory systems in K. oxytoca and E. chrysanthemi are similar though not identical: K. oxytoca can synthesize – but not secrete – a pectinase encoded by a gene from E. chrysanthemi. (See also SECRETON.) Pseudomonas aeruginosa secretes an exotoxin, and the enzymes elastase and alkaline phosphatase, via a secretory system involving the xcp genes. Interestingly, XcpA is identical to the PilD protein (involved in the formation of fimbriae) and is also homologous to PulO, a protein involved in the secretion of pullulanase, and it has been suggested that, in at least some cases, the second phase of the GSP may involve a protein complex similar to the fimbrial assembly apparatus [TIG (1992) 8 317–322; see also EMBO (2000) 19 2221–2228]. Folded proteins may cross the CM via the pmf-dependent Tat system [see Microbiology (2003) 149 547–556]. Type III systems. Secretion by type III systems occurs as a onestep process via a channel/pore which crosses the cell envelope from cytoplasm to cell surface; unlike type I systems, the type III pathway may involve 20 or more distinct proteins. (See also NEEDLE COMPLEX.) Type III systems occur in certain pathogens (including pathogens of plants [JB (1997) 179 5655–5662]). (See also VIRULON.) [Type III secretory systems and pathogenicity islands (review): JMM (2001) 50 116–126.] Components of type III systems in various species exhibit homology. Moreover, some type III components exhibit homology with components of other secretory systems – e.g. components of the basal body of the (bacterial) FLAGELLUM. Proteins secreted via type III systems include an N-terminal sequence which may be targeted to the pore or channel – but which it is not cleaved. The energy requirements of these systems are unknown, but a pore-associated protein (YscN) in a type III system of Yersinia includes binding sites typical of ATPases. In some type III systems, protein secretion seems to be activated by contact with a eukaryotic cell; at least some proteins are believed to be secreted directly into the eukaryotic cytoplasm because e.g. (i) proteins of the kind secreted by type III systems typically have little or no effect on eukaryotic cells in the absence of the secreting bacterium; (ii) some secreted proteins have been detected within eukaryotic cells; and (iii) some proteins (Yops) secreted by Yersinia have been associated with specific effects within eukaryotic cells (see VIRULON). In EHEC and EPEC strains of Escherichia coli, type III systems are reported to be regulated by QUORUM SENSING (q.v.). Proteins secreted by type III systems include e.g. the IpaB protein of Shigella (which promotes APOPTOSIS in macrophages); 618 protein synthesis sequence that encodes the given protein (see TRANSCRIPTION); as this RNA transcript carries the genetic ‘message’ from DNA it is called messenger RNA (or, more usually, mRNA) – see MRNA. In some cases (commonly in eukaryotes, less commonly in prokaryotes) the initial transcript must be processed to remove non-coding part(s) (called introns) from the nucleotide sequence – see SPLIT GENE; the resulting (mature) mRNA is then used to guide assembly of the polypeptide chain – the stage of protein synthesis called translation. mRNAs which lack introns can be used directly for translation. (See also RNA EDITING.) Along the length of the mRNA molecule, groups of three consecutive bases each encode a particular amino acid. Each of these three-base groups is called a codon; thus, e.g. the codon UCA (uracil-cytosine-adenine) encodes the amino acid serine (see table in GENETIC CODE). Hence, the sequence of codons in mRNA encodes the sequence of amino acids in a polypeptide. For the synthesis of a polypeptide, each amino acid must first bind to a small ‘adaptor’ molecule of RNA which is specific for that amino acid and which also has a binding site (a threebase anticodon) for the appropriate codon. These RNA adaptor molecules are called transfer RNA (tRNA) – see TRNA. The binding of a given amino acid to its tRNA (i.e. the ‘charging’ of a tRNA molecule) occurs in a two-step reaction. First, the amino acid reacts with ATP in the presence of a specific aminoacyltRNA synthetase (forming an enzyme-bound aminoacyl-AMP complex); the aminoacyl group is then transferred to the 20 or 30 position of the terminal adenosine residue in the tRNA (with release of AMP and synthetase). (See also MUPIROCIN.) The synthesis of a polypeptide on mRNA (i.e. the translation process) takes place on a RIBOSOME. In prokaryotes, correct positioning of mRNA on the ribosome is facilitated by a particular sequence of nucleotides in the mRNA (the Shine–Dalgarno sequence) located ‘upstream’ of the coding region; this sequence base-pairs with part of a 16S rRNA molecule in the 30S subunit of the ribosome. Essentially, following mRNA–ribosome binding, the ribosome moves along the mRNA molecule in such a way that each codon, in turn, binds the relevant tRNA, and each successive amino acid is linked, covalently, to the previous one. The following outlines a generally accepted scenario for protein synthesis in Escherichia coli; some details of the process are still unknown. The process appears to be essentially similar in most or all other bacteria. Translation is initiated when mRNA binds to the ribosome. Each ribosome has two (adjacent) binding sites for charged tRNA molecules: the A site (D acceptor or entry site) and the P site (D peptidyl or donor site). At the start of translation, the first codon of mRNA (the initiator codon) coincides with the P site. The initiator codon is usually AUG, but is sometimes e.g. GUG, UUG or AUU. The initiator codon is recognized by a distinct initiator tRNA (tRNAi ) which is charged with methionine (Met). In E. coli there are two tRNAs specific for methionine: tRNAm and tRNAf ; only tRNAf can function as tRNAi (tRNAm being used for incorporation of Met in subsequent location(s) in the polypeptide chain). In the charged tRNAf the a-amino group of the methionyl residue is formylated (via N 10 -formyltetrahydrofolate), giving fMet-tRNAf ; hence, the first (N-terminal) amino acid to be incorporated in the polypeptide chain is N-formylmethionine. (The incorporation of N-formylmethionine as N-terminal amino acid is typical of prokaryotes – and of mitochondria and chloroplasts; in the eukaryotic cytoplasm, and in at least some archaeans, the methionine residue is not formylated, so that the N-terminal amino acid in these cases is methionine.) Before discussing subsequent stages in protein synthesis, further consideration can be given to the initial binding of mRNA to the ribosome. In E. coli this preliminary step requires three protein initiation factors: IF-1, IF-2 and IF-3. The timing and mode of action of these factors is not yet fully understood, and there are several models for this phase of protein synthesis. According to one model: (i) mRNA binds to the (separate) 30S subunit of the ribosome; (ii) fMet-tRNAf binds to AUG at the P site on the 30S subunit before addition of the 50S subunit; (iii) IF-2 and GTP are needed for the binding of fMet-tRNAf to AUG; (iv) all three initiation factors will have been released by the time the 50S subunit has been added. During assembly of the complete 70S ribosome, the association of 30S and 50S subunits is linked to the hydrolysis of GTP; IF-2 acts as a GTPase. [Bacterial initiation factors in protein synthesis (review): Mol. Microbiol. (1998) 29 409–417.] Initiation can apparently also occur with a 70S monosome. This may happen e.g. when initiation occurs at an internal initiator codon in a polycistronic mRNA, the ribosome terminating synthesis of one polypeptide and initiating synthesis of the next without dissociating from the mRNA. Once tRNAi is bound to AUG at the P site on a 70S ribosome, the elongation process proceeds by sequential addition of amino acids according to the codons that follow AUG. Elongation beings when an aminoacyl-tRNA binds to the vacant A site – which is adjacent to the P site and coincident with the second codon; this decoding step requires GTP and the elongation factor T (EF-T; ‘transfer factor’). EF-T comprises two components: EF-Tu (D EF-1A; see also TUFA, TUFB GENES) and EF-Ts (D EF-1B). A complex consisting of these two components interacts with GTP, EF-Ts being displaced and the EF-Tu–GTP complex being involved in binding of the incoming tRNA to the A site; during this process GTP is hydrolysed, EFTu–GDP and phosphate being released. EF-Ts displaces GDP, re-forming the original complex – which can then react with GTP to repeat the cycle. The binding of an incoming tRNA to the A site is followed by transpeptidation, i.e. the formation of a peptide bond between the carboxyl group of fMeT-tRNAf and the a-amino group of the second amino acid (at the A site); transpeptidation is catalysed by the enzyme peptidyltransferase. (Peptidyltransferase is part of the 50S ribosomal subunit.) At this stage a dipeptidyl-tRNA occupies the A site. In the next stage, translocation, the ribosome moves along the mRNA by a distance equal to one codon (in the 50 -to30 direction), the tRNA at the P site being displaced in the process; the result of this movement is that the dipeptidyltRNA entity now occupies the P site – i.e. the A site is left vacant (and therefore ready to receive the third aminoacyltRNA). Translocation, which involves elongation factor EF-G, is accompanied by GTP hydrolysis (catalysed by EF-G); GTP hydrolysis may be necessary to effect detachment of EF-G from the ribosome [JMB (1986) 189 653–662]. (Release of EF-G from the ribosome is essential for continuation of elongation as EF-G and EF-Tu apparently bind to similar or identical sites.) The third aminoacyl-tRNA now enters the A site (coincident with the third codon of mRNA); transpeptidation follows, the carboxyl of the dipeptide being covalently joined to the a-amino group of the third amino acid. The cycle of events outlined above – decoding, transpeptidation, translocation – is repeated for each incoming aminoacyltRNA during elongation. The scheme described above is the two-site model for translocation. In the three-site model [MGG (1986) 204 221–228, q.v. 619 protein synthesis for references], described for E. coli and e.g. ‘Halobacterium halobium’, tRNAs remain bound to the ribosome both before and after translocation, i.e. deacylated tRNA is not released from the P site during translocation; instead, it is transferred to a third ribosomal site (the E site) concomitantly with translocation. Release of deacylated tRNA from the E site is triggered by the entry into the A site of the next (incoming) aminoacyl-tRNA. Base-pairing between mRNA codons and tRNA anticodons is not sufficiently stable, in itself, to guarantee the degree of accuracy and efficiency needed in translation. It has been suggested that proof-reading may occur e.g. at the level of amino acid selection by aminoacyl-tRNA synthetases; when synthetase specificity is not high enough to prevent binding of the ‘wrong’ amino acid, the latter may be recognized and removed either at the aminoacyl-AMP stage (pre-transfer proof-reading) or at the aminoacyl-tRNA stage (post-transfer proof-reading) [see e.g. NAR (1986) 14 7529–7539]. Proof-reading may also occur during elongation; aminoglycosides such as STREPTOMYCIN may exert their effect on translational accuracy by interfering with this proof-reading system. The ribosome itself may provide help, and this appears to be given by the rRNA [see e.g. Nature (1994) 370 597–598]. Termination of translation (i.e. cessation of polypeptide synthesis) occurs when a specific termination codon (UAA, UAG or UGA in E. coli – see GENETIC CODE) appears at the A site. These codons are recognized by protein release factors (RFs); RF-1 recognizes UAA and UAG, while RF-2 recognizes UAA and UGA. The recognition of a termination codon by an RF is followed by hydrolysis of the ester bond between the tRNA (at the P site) and the polypeptide chain it carries – thus releasing the completed polypeptide; hydrolysis is catalysed by peptidyltransferase. A third factor, RF-3 (D ‘S protein’), may stimulate the activities of RF-1 and RF-2. Although the ‘stop’ signal is traditionally regarded as a triplet of bases (e.g. UAA), there is evidence that the base following a stop codon influences the efficiency of termination, and it may be that the actual stop signal is a four-base (or even longer) sequence [Mol. Microbiol. (1996) 21 213–219]. In E. coli the efficiency of termination at a stop codon is also influenced by the 50 (upstream) nucleotide context of the codon, the mechanism involving the C-terminal amino acid residues in the nascent polypeptide. [Translation termination and stop codon recognition (review): Microbiology (2001) 147 255–269.] As one ribosome translocates along the mRNA another can fill the vacated initiation site and start translation of another molecule of the polypeptide; thus, a given mRNA molecule may carry a number of ribosomes along its length – forming a polyribosome or polysome. (A single ribosome is sometimes called a monosome.) At some time after the start of elongation, the formyl group of formylmethionine (see earlier) – or formylmethionine itself – is excised from the polypeptide. In E. coli, formylmethionine is cleaved from about 50% of proteins; whether or not cleavage occurs in a given protein appears to depend on the identity of the penultimate N-terminal amino acid residue – the longer the side-chain on this residue the smaller the probability of cleavage of formylmethionine. When such cleavage does occur it is effected by METHIONINE AMINOPEPTIDASE (q.v.). When the formyl group (only) is cleaved, the reaction is catalysed by methionine deformylase (D peptide deformylase, PDF). (PDF is inhibited by certain derivatives of hydroxamic acid, but these compounds are unlikely to be useful as broad-spectrum antibacterial agents because (i) they were bacteriostatic in the organisms tested, and (ii) resistance develops readily [AAC (2001) 45 1058–1064].) Signal sequences. Commonly, a protein which will form part of the cytoplasmic membrane, or pass through it, is synthesized with a special N-terminal sequence of amino acids (called a signal sequence) which facilitates passage into, or through, the membrane (see SIGNAL HYPOTHESIS). Protein folding. As mentioned earlier, polypeptide chains must be folded correctly in order to form functional, biologically active proteins. Folding of periplasmic, membrane and secreted proteins, frequently involves the formation of disulphide bonds between specific amino acid residues in the polypeptide chain. (In E. coli the cytoplasmic proteins generally do not contain disulphide bonds as structural features; in the cytoplasm, disulphide bonds are usually reduced by enzymes such as the NADPH-dependent thioredoxin system [but see EMBO (1998) 17 5543–5550].) In E. coli, disulphide bonds are catalysed in the periplasmic region by the Dsb proteins (encoded by genes dsbA and dsbB ). Mutations which affect dsb genes are pleiotropic – i.e. they have multiple effects; the reason for this is that disulphide bonds stabilize a range of different proteins (with widely differing functions) – for example, flagellar proteins, membrane proteins and various secreted proteins. In pathogens, mutations in the dsb genes may affect secreted protein virulence factors (and thus possibly reduce the virulence of the organism). Proteins containing proline residues may need peptidyl–prolyl isomerases for normal folding; several such isomerases (e.g. FkpA [function: Mol. Microbiol. (2001) 39 199–210], PpiA) are found in the periplasmic region in E. coli. At least some protein-folding enzymes appear to be synthesized under the regulation of a TWO-COMPONENT REGULATORY SYSTEM [see e.g. GD (1997) 11 1169–1182]. Transcription of the genes encoding certain protein-folding enzymes appears to involve sigma factor sE [GD (1997) 11 1183–1193]. Protein folding is facilitated by so-called molecular chaperones (D ‘chaperones’): molecules which bind to, and stabilize, nascent proteins – promoting correct folding/inhibiting incorrect folding; one or more chaperones may be involved in a given stage of folding or translocation. Chaperones may bind to proteins co-translationally [JBC (1997) 272 32715–32718] or post-translationally. (See also HEAT-SHOCK PROTEINS.) Although chaperones are typically proteins, the membrane lipid phosphatidylethanolamine acts as a chaperone for the E. coli protein LacY [EMBO (1998) 17 5255–5264]. [Disulphide bond formation (review): Mol. Microbiol. (1994) 14 199–205. Protein folding and misfolding (some concepts and themes): EMBO (1998) 17 5251–5254.] Some proteins are folded in the cytoplasm and may be exported via the Tat system [Microbiology (2003) 149 547–556]. [Protein folding (review): TIBS (2000) 25 611–618.] Peptide synthesis without ribosomes. Some very short polypeptides are synthesized by a multi-enzyme complex (rather than by ribosomes and RNA molecules). Such a process is used e.g. for the synthesis of certain antibiotics (including GRAMICIDINS and TYROCIDINS). [Non-ribosomal biosynthesis of peptide antibiotics (review): Eur. J. Bioch. (1990) 192 1–15.] Protein synthesis in eukaryotes. Some differences between prokaryotic and eukaryotic protein synthesis are listed below. Aminoacyl-tRNA synthetases occur in high-MWt multienzyme complexes [review: Bioch. J. (1986) 239 249–255]. There is apparently no equivalent to the Shine–Dalgarno sequence of prokaryotes. In the cytoplasmic proteins of eukaryotes (and at least some archaeans) the N-terminal methionine is not formylated. 620 proton ATPase P. morganii. See MORGANELLA. P. myxofaciens. Indole
ve; maltose Cve; ornithine decarboxylase
ve; abundant slime produced e.g. in trypticase–soy broth. P. rettgeri. See PROVIDENCIA. P. shigelloides. See PLESIOMONAS. P. vulgaris. Indole Cve; maltose Cve; ornithine decarboxylase
ve. P. mirabilis and P. vulgaris occur e.g. in soil, polluted water, intestines of (healthy) man and animals, etc. P. mirabilis, in particular, is an important opportunist pathogen, causing nosocomial URINARY TRACT INFECTION, pneumonia, septicaemia, etc. P. myxofaciens was isolated from gypsy moth larvae (Porthetria dispar ). [Book ref. 22, pp. 491–494.] prothallus (lichenol.) (1) Syn. HYPOTHALLUS (sense 1). (2) A lichen thallus in the initial stages of development. prothylakoid See PROLAMELLAR BODY. proticins BACTERIOCINS from Proteus spp. protist A member of the PROTISTA. Protista (1) A taxon (kingdom) which includes the ALGAE, FUNGI and PROTOZOA (collectively, the eukaryotic protists), and the PROKARYOTES. (2) A KINGDOM comprising the eukaryotic protists – see (1) above. (cf. MONERA.) protoaecidium See UREDINIOMYCETES stage I. protoaecium See UREDINIOMYCETES stage I. Protococcidiorida An order of protozoa of the subclass COCCIDIASINA. Protococcus See PLEUROCOCCUS. protocooperation In a mixed population of microorganisms: an interaction between two (or more) different microorganisms in which each organism benefits from the activities of the other (e.g. each organism may produce a substance which stimulates the growth of the other), but in which the interaction is not obligatory for either organism. (See e.g. YOGHURT; cf. MUTUALISM.) protofilament See MICROTUBULES. Protogonyaulax tamarensis Syn. Gonyaulax tamarensis. protohaem See HAEM. protokaryote Archaic term for PROKARYOTE. protokerogen See KEROGEN. protomerite In a cephaline gregarine: the anterior of the two main regions of the (septate) cell (cf. DEUTOMERITE). In the late trophozoite stage the anterior part of the protomerite may bear an EPIMERITE. protometer In a cell: a hypothetical sensor which can respond to changes in proton motive force. protomite See TOMITE. Protomonas A genus of methylotrophic bacteria; the organisms have been re-classified in the genus Methylobacterium [IJSB (1985) 35 209]. proton ATPase (proton-translocating ATPase; HC -ATPase) Any ATPASE which can couple ATP hydrolysis to the energydependent translocation of protons across an ENERGYTRANSDUCING MEMBRANE (see CHEMIOSMOSIS) and/or which can catalyse the pmf-dependent phosphorylation of ADP to ATP. (cf. PROTON PPASE.) The most-studied HC -ATPases are the (F0 F1 )-type HC ATPases (also referred to as (F1 F0 )-type HC -ATPases) which occur e.g. in the bacterial cytoplasmic membrane, in the mitochondrial (inner) membrane, and in the thylakoid membranes of chloroplasts; these enzymes have a MWt of ca. 500000. Bacterial (F0 F1 )-type HC -ATPases consist of two distinct parts: (a) the Initiation of synthesis of these proteins usually occurs at the AUG codon nearest the 50 end of the mRNA, and generally requires a capped mRNA (see MRNA) for maximum efficiency. At least nine initiation factors appear to be required. The eukaryotic eEF-1, equivalent to EF-G in E. coli, differs from the bacterial factor in being susceptible to ADPribosylation by DIPHTHERIA TOXIN. (In at least some members of the Archaea the corresponding factor is susceptible to diphtheria toxin [Book ref. 157, pp 379–410].) The sole eukaryotic release factor, designated eRF, requires GTP for activity and can recognize any termination codon. Antibiotics affecting protein synthesis: AMINOGLYCOSIDE ANTIBIOTICS, ANISOMYCIN, AURINTRICARBOXYLIC ACID, CHLORAMPHENICOL, CYCLOHEXIMIDE, FUSIDIC ACID, LINCOSAMIDES, MACROLIDE ANTIBIOTICS, OXAZOLIDINONES, PACTAMYCIN, POLYENE ANTIBIOTICS (b), PUROMYCIN, SPARSOMYCIN, STREPTOGRAMINS (including e.g. QUINUPRISTIN/DALFOPRISTIN), TETRACYCLINES, TIAMULIN, THIOSTREPTON, VIOMYCIN. proteinase (1) Syn. protease (see PROTEASES). (2) Syn. endopeptidase (see PROTEASES). proteinase K A non-specific PROTEASE obtained from Tritirachium album. (cf. PRONASE.) Proteobacteria A taxonomic category (D PURPLE BACTERIA, sense 2) within the domain BACTERIA defined on the basis of 16S rRNA sequences. Some constituent genera are listed below. Alpha subdivision. Anaplasma, Agrobacterium, Azospirillum, Bartonella, Beijerinckia, Brucella, Ehrlichia, Hyphomicrobium, Methylobacterium, Rickettsia, Rochalimaea, Rhodopseudomonas, Rhodospirillum, Wolbachia. Beta subdivision. Alcaligenes, Bordetella, Chromobacterium, Neisseria, Nitrosomonas, Rhodocyclus, Spirillum. Gamma subdivision. Acinetobacter, Coxiella, Erwinia, Escherichia, Haemophilus, Legionella, Methylobacter, Methylocaldum, Methylococcus, Methylomicrobium, Methylomonas, Nitrosococcus, Pseudomonas, Proteus, Vibrio. Delta and epsilon subdivisions. Bdellovibrio, Campylobacter, Desulfuromonas, Helicobacter, Myxococcus, Wolinella. proteomics (1) The study of a cell’s proteome (collectively, all the proteins that can be expressed) as an integrated whole. (2) As in (1) with genetic and other aspects. [Microbiology and Molecular biology Reviews (2002) 66 39–63.] proteose A soluble product of protein hydrolysis; proteoses are not coagulated by heat but are precipitated in saturated (NH4 )2 SO4 . proter (ciliate protozool.) The anterior of the two cells formed during HOMOTHETOGENIC binary fission. (cf. OPISTHE.) Proteromonadida An order of parasitic protozoa (class ZOOMASTIGOPHOREA). The cells have one or two pairs of flagella, without paraxial rods, and a single mitochondrion which lacks a kinetoplast; cysts are formed. Genera: e.g. Karatomorpha, Proteromonas. Proteromonas See PROTEROMONADIDA. Proteus A genus of Gram-negative bacteria of the tribe PROTEEAE. Cells: ca. 0.4–0.8 ð 1–3 µm. Most strains swarm at 37° C, typically forming characteristic concentric zones of growth on the moist surface of an agar or gelatin medium (see SWARMING and DIENES PHENOMENON). (Some strains may form a spreading, uniform film of growth.) Typical reactions: H2 S (in TSI) Cve (delayed in P. myxofaciens); gelatin is liquefied at 22° C; lipase Cve; urease Cve; mannose and sugar alcohols are not attacked. Nicotinic acid (but not pantothenic acid) is required for growth. GC%: 38–41. Type species: P. vulgaris. P. inconstans. See PROVIDENCIA. P. mirabilis. Indole
ve; maltose
ve, ODC Cve. 621 proton circuit F0 moiety, a hydrophobic protein (typically containing three kinds of subunit – designated a, b and c in Escherichia coli ) embedded in the membrane bilayer, and (b) the F1 moiety, a hydrophilic protein (typically consisting of five kinds of subunit) which has ATPase activity, and which projects – like a knob on a short ‘stalk’ – from the cytoplasmic face of the membrane. (Some authors regard the projecting stalked knob as an artefact [JUR (1985) 93 138–143].) The ‘knob’ of F1 consists of two types of subunit (designated a and b), and the ‘stalk’ consists of three types of subunit (g, d and e). Mitochondrial (F0 F1 )-type HC -ATPases are essentially similar in structure but are somewhat more complex; the stalk region of the enzyme complex includes an OLIGOMYCIN-sensitivity-conferring protein (OSCP), which does not occur in bacterial or chloroplast ATPases, and a protein (called ‘inhibitor protein’, ‘I’, or ‘IF1 ’) which inhibits the hydrolysis of newly-made ATP. The mitochondrial, bacterial and chloroplast ATPases all contain, in the F0 moiety, a DCCD-binding protein which confers sensitivity to DCCD. Nomenclature of HC -ATPases. HC -ATP-ases from different sources are designated as follows: CF0 F1 (from chloroplasts); EF0 F1 (from Escherichia coli ); MF0 F1 (from mitochondria); TF0 F1 (from certain thermophilic bacteria, e.g. ‘PS3’). Individual components may be designated e.g. CF0 , EF1 , MF0 etc. The three types of subunit in TF0 may be referred to as bands 4, 6 and 8 (from the SDS–gel electrophoresis pattern). Other types of HC -ATPase include the (E1 E2 )-type enzymes which occur e.g. in the cytoplasmic membrane in certain fungi; these enzymes differ from (F0 F1 )-type ATPases e.g. in that their activity involves the formation of a phosphorylated intermediate. (See also ATPASE for a general note on the nomenclature of ATPases.) ATP synthesis and hydrolysis. HC -ATPase-catalysed ATP synthesis requires a minimum pmf of ca. 100–200 mV. During ATP synthesis protons flow through the enzyme system, passing down the proton concentration gradient; the actual route of proton flow appears to include the side-chains of particular amino acids – e.g. aspartic acid, glutamine, lysine and tyrosine. (Interestingly, it has been proposed that the proton-conducting pathway through another energy-transducing system, the purple membrane (see BACTERIORHODOPSIN), involves a similar range of amino acids.) Much or most of the energy derived from proton translocation may be used to effect the release of ATP from the tightly cohesive ATP–HC -ATPase complex – rather than to carry out the phosphorylation step itself. In one model it is proposed that protons passing through the F1 moiety cause conformational changes in the enzyme which decrease the affinity of ATP for the catalytic site. This type of model is believed to be compatible with variable HC /P ratios – the number of protons needed to cause a given conformational change (with concomitant release of ATP) being postulated to vary with the prevailing pmf. Under some conditions HC -ATPases may catalyse either the synthesis or hydrolysis of ATP in order that GATP and pmf be more or less balanced (see CHEMIOSMOSIS). However, when pmf is below a certain level, HC -ATPases are rapidly (and reversibly) inactivated; such inactivation prevents depletion of ATP e.g. under those conditions in which, for any reason, the membrane is temporarily de-energized. In chloroplasts and bacteria inactivation of HC -ATPases occurs as a result of the tight binding between ADP and the catalytic site, while in mitochondria the enzyme is inactivated by the inhibitor protein. [Regulation of HC -ATPase activity: TIBS (1986) 11 32–35.] Inhibitors of (F0 F1 )-type HC -ATPases include e.g. AUROVERTINS, CITREOVIRIDIN, DCCD, EFRAPEPTIN, OLIGOMYCIN, QUERCETIN and tributyltin chloride; TF1 (in contrast to the F1 moieties of mesophilic bacteria) is resistant to e.g. aurovertins, efrapeptin and quercetin. [HC -ATPases, structure and function: ARB (1983) 52 801–824; HC -ATPase as an energy-converting enzyme: Book ref. 127, pp. 149–176.] proton circuit See CHEMIOSMOSIS. proton motive force See CHEMIOSMOSIS. proton PPase (HC -PPase) Proton pyrophosphatase: a membranebound enzyme or enzyme complex which couples the hydrolysis or synthesis of PYROPHOSPHATE to the energy-linked transmembrane translocation of protons; HC -PPases occur widely e.g. in mammalian and yeast mitochondria, in chloroplasts, and in certain photosynthetic bacteria. HC -PPases function in a way analogous to that of PROTON ATPASES. Thus, e.g. PPi hydrolysis by an HC -PPase can generate or augment a proton motive force (see CHEMIOSMOSIS) which can be used for energy-requiring processes such as ION TRANSPORT, reverse electron transport, and ATP synthesis (via an HC ATPase). Conversely, an HC -PPase can use pmf, directly, for the synthesis of PPi (see also OXIDATIVE PHOSPHORYLATION); such synthesis has been reported to occur e.g. in mitochondria, in the chloroplasts of the alga Acetabularia mediterranea and of the pea plant, and in the bacterium Rhodospirillum rubrum. (See also ELECTRON TRANSPORT CHAIN (figure).) proton pump Any biochemical process which promotes the energy-requiring translocation of protons across an ENERGYTRANSDUCING MEMBRANE: see e.g. ELECTRON TRANSPORT CHAIN and PROTON ATPASE. proton reducers See ANAEROBIC DIGESTION. proton-reducing acetogen See ACETOGEN. proton translocators (protonophores) IONOPHORES which effectively increase the permeability, to protons, of the lipid bilayer in biological and/or artificial membranes; such agents act as mobile carriers of (specifically) protons, and can dissipate proton motive force (see CHEMIOSMOSIS) by effectively ‘short circuiting’ the membrane. (See also UNCOUPLING AGENTS.) Proton translocators include a number of lipophilic weak organic acids which retain lipophilicity even in the deprotonated (dissociated) state (owing to delocalization of charge within the molecule); in the protonated state proton translocators deliver protons to one side of the membrane, while in the deprotonated state they accept protons from the other side. Examples include e.g. CCCP (carbonylcyanide-m-chlorophenylhydrazone), 2,4dinitrophenol, and FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone). protonmotive force Proton motive force (see CHEMIOSMOSIS). protonmotive Q-cycle See Q-CYCLE. protonophore Syn. PROTON TRANSLOCATOR. proto-onc See ONCOGENE. protoplasmic cylinder See SPIROCHAETALES. protoplasmodium See MYXOMYCETES. protoplast (1) The spherical or near-spherical, osmotically sensitive structure formed by the complete removal of the cell wall (e.g. by enzymic action) from a cell suspended in an isotonic (or hypertonic) medium; a protoplast thus consists of the cytoplasmic membrane and cell contents. Protoplasts can continue to metabolize – and may be able to revert to normal cells – under appropriate cultural conditions; however, protoplasts cannot divide. Bacterial protoplasts are prepared more easily from Gram-positive cells (e.g. by using LYSOZYME) than from Gram-negative cells; they are resistant to phage infection, 622 Providencia cell. Species occur e.g. in soil and water, and some can behave as opportunist pathogens (see PROTOTHECOSIS). protothecosis A disease, which affects man and animals, caused by a species of Prototheca (commonly P. zopfii ); typically, it involves the formation of localized skin lesions – which may be nodular, ulcerating and papular, or (rarely) granulomatous. (See also MASTITIS.) A severe, often fatal, systemic form of the disease also occurs, particularly in animals. prototroph A strain of microorganism whose nutritional requirements do not exceed those of the corresponding wild-type strain. (cf. AUXOTROPH.) prototunicate ascus An ASCUS whose wall consists of a thin, delicate membrane; the ascospores are released when the ascus wall ruptures or deliquesces. Such asci are formed e.g. by ascogenous yeasts. protoxin A precursor form of a TOXIN: see e.g. DELTA-ENDOTOXIN. protozoa A diverse group of eukaryotic, typically unicellular microorganisms; they are classified as a subkingdom (Protozoa) within the kingdom Animalia or the kingdom PROTISTA. (Some of these organisms – e.g. the SLIME MOULDS and members of the PHYTOMASTIGOPHOREA – are also classified in botanical taxonomic schemes.) The earliest known protozoa lived in the late Precambrian (see e.g. FORAMINIFERIDA). The protozoa exhibit a great variety of forms, structures and life styles, and are divided into seven phyla: APICOMPLEXA, ASCETOSPORA, CILIOPHORA, LABYRINTHOMORPHA, MICROSPORA, MYXOZOA and SARCOMASTIGOPHORA [JP (1980) 27 37–58]. Many protozoa are free-living organisms which occur in freshwater, brackish or marine habitats. Some protozoa occur e.g. in soil, some are common in SEWAGE TREATMENT plants, and some inhabit the gut in vertebrates or invertebrates (see e.g. RUMEN and TERMITE–MICROBE ASSOCIATIONS). Parasitic species include e.g. members of the Apicomplexa, Ascetospora, Microspora and Myxozoa; some protozoa are pathogenic in vertebrates (including man) or invertebrates (see e.g. BABESIA, BALANTIDIUM, EIMERIORINA, ENTAMOEBA, GIARDIA, HISTOMONAS, NAEGLERIA, NOSEMA, PLASMODIUM, TOXOPLASMA, TRYPANOSOMA). PHYTOMONAS is parasitic in certain plants. Protozoa range in size from ca. 1 µm to several millimetres; some have internal or external ‘skeletons’ of e.g. calcareous, siliceous and/or organic material. Many exhibit MOTILITY but some (see e.g. PERITRICHIA) are sedentary. Probably most protozoa are aerobic organisms, but some can grow microaerobically or anaerobically (see e.g. RUMEN and SAPROBITY SYSTEM). Feeding may occur saprotrophically and/or by pinocytosis – and/or it may occur holozoically (e.g. in many amoebae and ciliates), these organisms being classified as CARNIVOROUS, HERBIVOROUS or OMNIVOROUS. Some protozoa (see PHYTOMASTIGOPHOREA) can carry out PHOTOSYNTHESIS. Asexual reproduction may involve binary fission, multiple fission and/or budding. Sexual processes occur in some species (see e.g. AUTOGAMY and CONJUGATION). According to species, protozoa may be haploid, diploid or polyploid; some (the foraminifera) exhibit an ALTERNATION OF GENERATIONS. Populations of viable protozoa may be preserved by FREEZING (suitable e.g. for Plasmodium, Tetrahymena, Trypanosoma) or by repeated subculturing. CYST-forming species (e.g. Didinium, certain amoebae) may be encouraged to encyst (and thus survive for protracted periods) e.g. by slow desiccation or by starvation. protrichocyst Syn. EJECTOSOME. Providencia A genus of Gram-negative bacteria of the tribe PROTEEAE. Cells: ca. 0.6–0.8 ð 1.5–2.5 µm. Swarming does not occur. Species do not produce H2 S or lipase, do not but those from previously infected cells can support phage replication. (See also SPHAEROPLAST; AUTOPLAST; L-FORM; PROTOPLAST FUSION.) (2) In an intact cell: the cytoplasmic membrane and all structures internal to it. protoplast fusion A technique for in vitro genetic transfer in which a hybrid PROTOPLAST (and ultimately a hybrid cell) is formed by fusing protoplasts derived from different strains, species or genera. Protoplasts are initially prepared e.g. by treating cells with wall-degrading enzymes (e.g. LYSOZYME for some Gram-positive bacteria); populations of two types of protoplast are then mixed. Treatment of the mixture with e.g. POLYETHYLENE GLYCOL (PEG) increases the (low) spontaneous rate of fusion. (cf. ELECTROFUSION.) The protoplasts are then transferred to an appropriate (hypertonic) solid medium for cell wall regeneration. The resulting cells can be screened for those with genetic markers from both parents; e.g., if both parents were auxotrophic (with different growth requirements) the cells can be screened for prototrophs. Hybrid bacterial protoplasts, containing a genome from each parent, are called biparentals. The two genomes may undergo recombination, or the diploids may be unstable, the two genomes segregating when the cell divides. In e.g. Bacillus subtilis, many of the progeny cells are unstable complementary diploids (i.e., genes on both genomes are expressed and can exhibit COMPLEMENTATION); however, some progeny cells are unstable non-complementing diploids (Ncd cells) in which one of the two parental genomes is expressed while the other is inactive until segregated on cell division. (cf. CONJUGATION sense 1b (ii).) (See also TRANSFORMATION.) protoplast membrane Syn. CYTOPLASMIC MEMBRANE. protoporphyrins See PORPHYRINS. Protosporangium See PROTOSTELIOMYCETES. Protostelia See EUMYCETOZOEA. protostelids See PROTOSTELIOMYCETES. Protosteliia See EUMYCETOZOEA. Protosteliomycetes (protostelids) A class of primitive slime moulds (division MYXOMYCOTA – cf. EUMYCETOZOEA) in which the vegetative phase consists of a small reticulate plasmodium or of simple amoebae which form filose – and sometimes also lobose – pseudopodia; in some species the amoebae bear one or more smooth flagella. ‘Shuttle streaming’ (i.e., rhythmic reversible flow of protoplasm) does not occur. An individual amoeba, or a segment of a plasmodium, gives rise to a fruiting body consisting of one to several spores borne on a slender tubular stalk, the whole being bounded by a sheath. Ballistospores are formed e.g. by Protostelium nocturnum [Mycol. (1984) 76 443–447], P. expulsum [TBMS (1981) 76 303–309], and Schizoplasmodium cavostelioides [sporocarp ultrastructure and development: Mycol. (1985) 77 848–860]. [Ultrastructural study of trophozoite and cyst stages of P. pyriformis: JP (1986) 33 405–411.] Protostelids are found in various habitats, including bark, rotting wood, etc. Other genera include e.g. Cavostelium, Ceratiomyxella, Endostelium [Mycol. (1984) 76 884–891], Nematostelium and Protosporangium. (cf. CERATIOMYXA.) Protostelium See PROTOSTELIOMYCETES. Prototheca A genus of unicellular eukaryotic microorganisms which are generally regarded as achlorophyllous strains of CHLORELLA. The cells are hyaline and roughly spherical or ovoid (ca. 1.3–13.4 ð 1.3–16.0 µm, depending on species, growth conditions etc); when mature, a cell divides to form 2–20 or more ‘endospores’ which are released by rupture of the mother 623 provirus liquefy gelatin at 22° C, and can metabolize mannose and one or more sugar alcohols; colonies on DCA typically develop yellow-orange centres due to the precipitation of Fe(OH)3 by alkaline metabolic products. Nicotinic and pantothenic acids are not required for growth. GC%: 39–42. Type species: P. alcalifaciens. P. alcalifaciens (formerly included in Proteus inconstans). Urease
ve (weak or delayed Cve in a few strains); trehalose
ve; inositol.
ve; adonitol Cve. P. rettgeri (including most of the strains formerly regarded as Proteus rettgeri ). Urease Cve; trehalose
ve; inositol Cve; adonitol Cve. P. stuartii (incorporating strains formerly included in Proteus inconstans and Proteus rettgeri ). Urease
ve (weak or delayed Cve in a few strains); trehalose Cve; inositol Cve; adonitol
ve. Providencia spp are rare in the normal human intestine, and their natural habitat is unknown. Some strains can be opportunist pathogens – P. rettgeri and P. stuartii causing e.g. nosocomial UTIs, P. alcalifaciens being more often associated with diarrhoeal illnesses. [Book ref. 22, pp. 494–496.] provirus Viral DNA which has become integrated into the chromosomal DNA of the host cell; in viruses of the RETROVIRIDAE the DNA is a transcript of the RNA genome, while in some DNA viruses the genome itself, or a portion of the genome, may integrate (see e.g. DEPENDOVIRUS, HEPATITIS B VIRUS and MASTADENOVIRUS). prozone (serol.) (1) In a titration involving antibodies and particulate antigens: absence of visible AGGLUTINATION in tubes containing the higher concentration(s) of antibody. A prozone may be due e.g. to (i) relatively low titres of BLOCKING ANTIBODIES (sense 1) which are diluted out in tubes containing agglutinating titres of normal homologous antibodies; (ii) gross ANTIBODY EXCESS; or (iii) the presence in the antiserum of substances which inhibit agglutination non-specifically. (2) In a titration involving antibodies and soluble antigens: the absence of visible precipitate in those tubes in which there is an ANTIGEN EXCESS. prozygosporangium See ZYGOSPORE. PrP 27–30 (PrP) See SCRAPIE. PrPc , PrPsc See PRION. PRRS Porcine reproductive and respiratory syndrome: see BLUEEARED PIG DISEASE. pruinose (of surfaces) Powdery in appearance. Prunus necrotic ringspot virus See ILARVIRUSES. pruritis Itching. Pruteen See SINGLE-CELL PROTEIN. Prymnesiida See PHYTOMASTIGOPHOREA. Prymnesiophyceae (Haptophyceae) A class of unicellular, uninucleate, commonly biflagellated algae (sometimes regarded as protozoa – see PHYTOMASTIGOPHOREA). Typically, the motile cell has two smooth flagella of more or less equal length, together with a HAPTONEMA; the golden-brown chloroplasts (usually two per cell) contain chlorophylls a, c1 and c2 , and fucoxanthin. Storage compounds include CHRYSOLAMINARIN. Many members can ingest food phagocytically. Most prymnesiophytes bear a covering of delicate, typically oval scales embedded in a mucilaginous matrix external to the plasmalemma; these scales are produced in the Golgi apparatus (or in vesicles probably derived therefrom) before being passed to the cell exterior (cf. CHRYSOPHYTES). The scales may be entirely organic (e.g. in Chrysochromulina and Prymnesium) or may be calcified (in certain stages of the COCCOLITHOPHORIDS). Some of the scales may bear long or short spines, and – according to species – different types of scales may be present on the same cell and/or on different cells in different stages of the life cycle. The nature of the scales (determined by electron microscopy) is an important taxonomic characteristic. Prymnesiophytes are mostly marine organisms, forming an important component of the nannoplankton. Some species (e.g. PRYMNESIUM) occur in brackish waters, others in fresh water. Prymnesium A genus of unicellular algae (class PRYMNESIOPHYCEAE) in which the HAPTONEMA is short and immobile. Species occur in brackish water. P. parvum produces a potent, cation-activated, pH-dependent, haemolytic PHYCOTOXIN (comprising at least 6 components) which can cause mass mortalities in fish and other gill-bearing vertebrates (as well as in invertebrates) in e.g. brackish-water culture ponds. PS3 A strain of thermophilic bacteria of uncertain taxonomic affiliation. PS-5 A CARBAPENEM antibiotic. PSA fimbriae (‘PSA pili’) See FIMBRIAE. Psalliota See AGARICUS. Psammetta See XENOPHYOPHOREA. Psammina See XENOPHYOPHOREA. Psammomitra See HYPOTRICHIDA. psammophilic Preferring a sandy habitat. Psathyrella See AGARICALES (Coprinaceae). pSC101 A 9.4-kb, low-copy-number PLASMID which encodes tetracycline resistance and which has a single EcoRI cleavage site in a non-essential region, making it a useful CLONING vector. Replication of pSC101 occurs unidirectionally, does not require DNA polymerase I (cf. e.g. COLE1 PLASMID) or the host cell’s DnaA protein (see DNAA GENE) but requires a pSC101-encoded initiator protein. [Nucleotide sequence of pSC101: NAR (1984) 12 9427–9440.] PSE-type b-lactamases See b-LACTAMASES. Pseudallescheria (syn. Petriellidium) A genus of fungi of the order MICROASCALES. P. boydii (‘Allescheria boydii ’) (anamorph: Scedosporium apiospermum) occurs e.g. in soil and in faecally contaminated cattle feed. It can cause mycotic abortion e.g. in cattle; in man it is a causal agent of MADUROMYCOSIS and may be implicated in infections of the respiratory tract, sinuses etc, particularly in immunosuppressed patients. [Clinical significance of P. boydii : Mayo Clin. Proc. (1985) 60 531–537.] Pseudanabaena A genus of filamentous CYANOBACTERIA (section III) in which the trichomes are composed of cylindrical cells, with constrictions between adjacent cells; trichomes are motile and non-sheathed, and the cells contain polar GAS VACUOLES. GC%: 44–52. The genus probably includes at least some strains of ‘Oscillatoria redekei ’. pseudoaethalium See MYXOMYCETES. pseudoappendicitis See e.g. FOOD POISONING (j); see also PSEUDOTUBERCULOSIS. Pseudocaedibacter A genus of Gram-negative bacteria which occur as endosymbionts in strains of Paramecium. Cells: nonmotile rods, ca. 0.25–0.7 ð 0.5–4.0 µm, which do not contain R bodies (cf. CAEDIBACTER) and which may or may not (according to species) confer a killer characteristic on their host paramecia. GC%: ca. 35–39. Type species: P. conjugatus. P. conjugatus (see also MU PARTICLE) confers the MATE KILLER characteristic; in vitro cultivation has been reported. P. falsus (nu and pi particles) does not confer a killer characteristic. P. minutus (gamma particles) confers the killer characteristic. [Book ref. 22, pp. 807–808.] 624 Pseudomonas pseudocapillitium In the aethalia or pseudoaethalia of certain MYXOMYCETES: a system of threads (generally irregular in width) and/or of membranous or perforated plate-like structures present among the spores; it has been suggested that these structures may be remnants of sporangial walls. pseudocatalase A non-haemoprotein substance which catalyses the breakdown of H2 O2 into water and O2 ; pseudocatalases occur e.g. in various lactic acid bacteria, including strains of Lactobacillus, Enterococcus faecalis, and in some species of Veillonella. Unlike CATALASE, pseudocatalases are not inhibited by low concentrations of azide or cyanide. A manganesecontaining protein pseudocatalase (a ‘manganicatalase’) has been isolated from a strain of Lactobacillus plantarum [JBC (1983) 258 6015–6019]. Pseudocercosporella See HYPHOMYCETES; see also EYESPOT (sense 2). pseudocilia See TETRASPORA. pseudocoagulase In certain strains of Staphylococcus: any protease that mimics the effects of staphyloCOAGULASE by proteolytic activation of prothrombin, thus causing a false-positive reaction in a COAGULASE TEST. ‘Pseudocoagulase’ is believed to be a metalloprotease(s) and can be inhibited by adding EDTA to the plasma. [Book ref. 44, p. 530.] pseudocolumella In the fruiting bodies of certain MYXOMYCETES: a COLUMELLA-like, spherical or rod-shaped, calcareous structure which occurs in the centre of the sporangium; it is not attached to the peridium, but may be attached to capillitial threads. pseudocowpox A mild, usually benign CATTLE DISEASE characterized by the formation of papular, scab-forming lesions on the teats which may predispose to MASTITIS; the causal agent is a PARAPOXVIRUS. (See also MILKERS’ NODULE; cf. COWPOX.) pseudocyphella (lichenol.) A small pore (ca. 0.1–2.0 mm diam.) in the (upper and/or lower) cortex of the thallus in various lichens (e.g. species of Cetraria, Parmelia, Pseudocyphellaria). Pseudocyphellae have no definite margin (cf. CYPHELLA); they appear to be formed by the disintegration of the pseudoparenchymatous cortex, and are believed to facilitate gas exchange [Lichenol. (1981) 13 1–10]. (cf. PORED EPICORTEX.) Pseudocyphellaria A genus of foliose LICHENS (order PELTIGERALES) which closely resemble STICTA spp but differ in that the tomentose underside is perforated with PSEUDOCYPHELLAE. The upper surface may bear soredia which are yellow in P. crocata, bluish-grey in P. intricata. Species occur on soil, trees, rocks, etc. pseudocyst (protozool.) See e.g. CHAGAS’ DISEASE and TOXOPLASMOSIS. pseudoepithecium In some types of APOTHECIUM: a layer, at the surface of the hymenium, which consists of the tips of paraphyses immersed in an amorphous matrix. (cf. EPITHECIUM.) pseudo-Fiji disease See FIJI DISEASE. pseudogene In a eukaryotic chromosome: a DNA segment which resembles a known functional gene but differs in carrying deleterious mutations and in having no apparent function. Many pseudogenes resemble cDNA copies of cellular mRNAs: they have a short A–T tract at the 30 end (corresponding to the 30 poly(A) of mRNA), lack introns, and are often flanked by direct repeats; such pseudogenes are believed to have arisen by reverse transcription of cellular mRNAs. Pseudogenes have also been found in prokaryotes. For example, in Mycobacterium leprae about 27% of the genome is reported to consist of pseudogenes [Nature (2001) 409 1007–1011]. pseudoglanders Syn. EPIZOOTIC LYMPHANGITIS. pseudohaemoptysis (pseudohemoptysis) The apparent expectoration of blood due to the coloration of sputum by red-pigmented bacteria (e.g. Serratia sp). Pseudohydnum See TREMELLALES. pseudohyphae Syn. SPROUT MYCELIUM. Pseudoklossia See EIMERIORINA. pseudolumpy skin disease See LUMPY SKIN DISEASE. pseudolysogeny A phenomenon in which an association between a BACTERIOPHAGE and its host mimics LYSOGENY in that host cell lysis is delayed or does not occur, but true lysogeny is not established. A bacterial population exhibiting pseudolysogeny can be freed of phage by prolonged incubation with neutralizing anti-phage antiserum or by serial subculture from isolated colonies. Different types of pseudolysogeny occur in different phage–bacterium systems. For example, when a population of susceptible bacteria (e.g. Shigella dysenteriae) is exposed to a virulent bacteriophage (e.g. T7) at a low multiplicity of infection, a few cells become infected. On lysis, these cells may release an enzyme which destroys the phage receptors on other cells in the population; phage virions therefore fail to adsorb to these cells – which, as a result, become (temporarily) resistant to infection (phenotypic resistance). These cells regain their susceptibility to infection after subculture in the absence of phage. In other cases the phage genome may be carried, but not expressed or replicated, within a cell in a susceptible population of bacteria, and the genome is passed to only one of the daughter cells at each cell division; a culture containing a proportion of such ‘carrier cells’ is known as a CARRIER CULTURE. After several successive transits from cell to daughter cell, the phage genome enters the lytic cycle and progeny phages are released by cell lysis. These may infect other cells which may in turn become carrier cells. This type of pseudolysogeny may occur e.g. when most of the host cells are repressed for one or more functions needed for phage replication. In Azotobacter strain O certain phages can apparently establish a pseudolysogenic state accompanied by the phenotypic conversion of the host cells (see BACTERIOPHAGE CONVERSION); the cells lose their polysaccharide capsule, become flagellated and motile, and acquire a yellow pigmented appearance [Virol. (1980) 102 267–277]. pseudomembranous colitis A severe, acute form of COLITIS. There is a profuse, watery diarrhoea, cramps, fever, and the formation of pseudomembranous patches of inflammatory exudate on the colonic mucosa (sometimes also in the small intestine). It may follow chemotherapy (e.g. with some b-LACTAM ANTIBIOTICS, clindamycin) and is linked to CLOSTRIDIUM difficile; toxins A and B kill gut epithelial cells e.g. by disrupting tight junction proteins. Inadequate IgG response may promote disease/relapse. [Review: BCG (2003) 17 475–493.] pseudomethylotroph An organism which oxidizes C1 compounds to CO2 and which assimilates C1 carbon only via the Calvin cycle. Pseudomicrothorax See HYPOSTOMATIA. Pseudomonadaceae A family of Gram-negative, aerobic, chemoorganotrophic or facultatively chemolithotrophic, typically motile (usually polarly flagellate), rod-shaped bacteria. GC%: 58–71. Genera: FRATEURIA, PSEUDOMONAS, XANTHOMONAS, ZOOGLOEA. [Book ref. 22, pp. 140–219.] Pseudomonadales A poorly defined, obsolete order of bacteria. Pseudomonas A genus of Gram-negative, aerobic, chemoorganotrophic or facultatively chemolithoautotrophic bacteria of the family PSEUDOMONADACEAE; species occur as free-living organisms in soil and aquatic habitats, and as pathogens in man, 625 Pseudomonas groups I
V. Group I included fluorescent species (e.g. P. aeruginosa, P. aureofaciens, P. chlororaphis, P. putida, P. syringae) and non-fluorescent species (e.g. P. alcaligenes, P. pseudoalcaligenes, P. stutzeri ), all of which (apart from some strains of P. pseudoalcaligenes) do not accumulate polyb-hydroxybutyrate (PHB). Species of groups II and III do accumulate PHB; group II included pathogens (e.g. P. cepacia, P. mallei, P. solanacearum), while group III consisted of nonpathogens (e.g. P. acidovorans, P. testosteroni ) and included some autotrophs which can grow chemolithotrophically (e.g. P. facilis, P. flava, P. paleronii and P. saccharophila). Group IV consisted of P. diminuta and P. vesicularis, while group V contained only P. maltophila; these three species require growth factors, and the last two are unable to assimilate nitrate. For group II see BURKHOLDERIA. For group V see P. maltophila, below. Many species have not been assigned to rRNA groups, and their relationship to rRNA-grouped species is uncertain; such species include e.g. P. agarici, P. amygdali, P. andropogonis, P. aurantiaca, P. avenae, P. cattleyae, P. fragi, P. fulva, P. lemoignei, P. marina (see DELEYA), P. nautica, P. spinosa and P. woodii. P. aeruginosa (formerly P. pyocyanea). Occurs in soil, river water etc, and is an important opportunist pathogen in man and other animals – being isolated from infected burns, urinary tract infections, etc. (See also BLUE PUS; EXOENZYME S; EXOTOXIN A.) It can occasionally be pathogenic in (stressed) plants. Motile strains usually have one polar flagellum per cell; rarely nonmotile. Polarly fimbriate. Colonies on nutrient agar are often roundish, flat, dull and butyrous (R-type), but can be e.g. smooth and umbonate (S-type). Mucoid (M-type) colonies are formed by ALGINATE-producing strains isolated e.g. from CYSTIC FIBROSIS patients; only some mucoid strains can produce alginate on minimal agar. Most strains produce PYOCYANIN, and may also produce e.g. PYOVERDIN, while a few strains can form a brown pigment (pyomelanin) or a red pigment (pyorubin). (Pyomelanin production is enhanced in mineral salts media containing 1% Ltyrosine; 1% DL-glutamate enhances pyorubin production.) All strains are catalase Cve and oxidase Cve; many are urease Cve. Optimum growth temperature: 37° C; all strains can grow at 42° C. GC%: ca. 67. The production of virulence factors by P. aeruginosa may be inhibited through inhibition of the organism’s QUORUM SENSING mechanisms by the macrolide antibiotic azithromycin [AAC (2001) 45 1930–1933]. P. alcaligenes. A non-fluorescent species which cannot utilize e.g. glucose, fructose, maltose or gluconate, but which can use e.g. L-arginine, malate, and propionate. Monotrichously flagellate. Growth can occur at 41° C; optimum temperature ca. 35° C. GC%: 64–68. P. andropogonis (D P. stizolobii ). A non-fluorescent, typically oxidase
ve species pathogenic e.g. for sorghum. The organisms form sheathed flagella at ca. 28° C but are nonflagellate when grown at 34° C. P. carboxydohydrogena. See CARBOXYDOBACTERIA. P. carboxydovorans. See CARBOXYDOBACTERIA. P. cepacia. A former species (see BURKHOLDERIA) found e.g. in soil, water. Oxidase Cve. Lophotrichously flagellate. Nonfluorescent pigments often formed. Opt. growth: 30–35° C. Pathogenic in man (see CYSTIC FIBROSIS) and a causal agent of soft rot in onions. P. cichorii. A fluorescent, PYOVERDIN-forming species pathogenic for chicory: Lophotrichously flagellate. Weakly oxidase Cve. Optimum growth temperature: ca. 30° C. GC%: 59. other animals, and plants – see e.g. BLUE PUS, BROWN BLOTCH, GINGER BLOTCH, OLIVE KNOT and TABTOXIN. [Mechanisms of plant pathogenesis: CJM (1985) 31 403–410.] (See also BUTTER and MEAT SPOILAGE.) The cells are straight or slightly curved rods, 0.5–1.0 ð 1.5–5.0 µm. In most species the majority of strains are motile with one or several unsheathed polar flagella per cell; however, sheathed flagella are formed e.g. by P. andropogonis, and some species produce both polar and lateral flagella – particularly when grown on solid media. [Biochemical and serological properties of flagella of some Pseudomonas spp: JGM (1985) 131 873–883.] Polarly fimbriate strains may exhibit TWITCHING MOTILITY. Cell walls and cytoplasmic membranes resemble those of other Gram-negative bacteria. (See also PORIN, OUTER MEMBRANE and S LAYER.) Catalase Cve; many strains are oxidase Cve. GC%: 58–70. Type species: P. aeruginosa. Nutrition, metabolism, growth. Typically, the organisms are nutritionally and metabolically highly versatile: most species will grow on inorganic salts with an organic carbon source, while some can grow chemolithoautotrophically; however, certain species (e.g. P. diminuta, P. vesicularis) require factors such as biotin, cyanocobalamin and pantothenate. Metabolism is respiratory (oxidative); many species can carry out NITRATE RESPIRATION. The TCA CYCLE operates in all species investigated. In e.g. P. aeruginosa, P. fluorescens and P. putida, glucose can be oxidized by EXTRACYTOPLASMIC OXIDATION to gluconate which may then be metabolized via the ENTNER–DOUDOROFF PATHWAY. Carbohydrates are metabolized anaerogenically. [Alternative pathways of carbohydrate metabolism in Pseudomonas: ARM (1984) 38 359–387.] HYDROCARBONS can be metabolized by some strains. Some metabolic activities are plasmid-dependent: e.g., the ability to degrade camphor, salicylate, toluate and xylene is associated with plasmids CAM, SAL, TOL and XYL, respectively. Under nitrogen-limiting conditions some species accumulate POLY-b-HYDROXYBUTYRATE. Under iron-deficient conditions, siderophores (some fluorescent) may be secreted (see NOCARDAMINE, PYOVERDIN). (See also PYOCYANIN.) Some species produce bacteriocins (see e.g. PYOCIN). In many species growth occurs optimally at ca. 28–30° C and is generally inhibited at or below ca. pH 4.5. Sensitivity to antimicrobial agents. In general, pseudomonads are resistant to a range of common antimicrobial agents; resistance may or may not be plasmid-mediated. P. aeruginosa may be sensitive to e.g. certain AMINOGLYCOSIDE ANTIBIOTICS (e.g. amikacin, gentamicin); it is resistant to many common DISINFECTANTS (cf. DETTOL CHELATE) but is usually lysed rapidly by EDTA. Susceptibility to phages. Pseudomonas spp are susceptible to a variety of phages: see e.g. BACTERIOPHAGE f6; BACTERIOPHAGE fW-14; INOVIRUS; TECTIVIRIDAE. The transducing phages F116 and G101 were used for mapping the chromosome of P. aeruginosa. Lysogeny is very common in P. aeruginosa, but is uncommon in other species. Genetic aspects. Genetic studies have often been carried out with P. aeruginosa strain PAO. [Chromosomal maps of P. aeruginosa PAO and P. putida PPN compared: JGM (1985) 131 885–896.] Gene transfer can occur by transduction and conjugation; most conjugative plasmids transfer better on solid media than in liquids [JGM (1983) 129 2545–2556]. Transformation in P. aeruginosa has been achieved with CaCl2 -treated cells. Taxonomy. A number of pseudomonads were classified by rRNA oligonucleotide cataloguing (q.v.) into rRNA 626 pseudopodetium pisi (peas); P. syringae pv. savastanoi (olive trees – see OLIVE P. syringae pv. syringae (lilac) (see also SYRINGOMYCIN); P. syringae pv. tabaci (tobacco – see TABTOXIN). P. thermocarboxydovorans. See CARBOXYDOBACTERIA. P. tolaasii. The causal agent of BROWN BLOTCH in mushrooms. [Book ref. 22, pp. 140–199; ecology, isolation, culture: Book ref. 45, pp. 655–741; the biology of Pseudomonas: Book ref. 198.] pseudomonic acid A Syn. MUPIROCIN. pseudomurein A CELL WALL polymer present in some members of the ARCHAEA (METHANOBACTERIALES); it consists of (1 ! 3)b-linked backbone chains – containing N-acetyl-D-glucosamine (and/or N-acetyl-D-galactosamine, according to species) and Nacetyl-L-talosaminuronic acid – cross-linked by peptides which contain only L-amino acids (lys, glu, ala or thr). (cf. PEPTIDOGLYCAN.) Pseudomurein is resistant to LYSOZYME and to antibiotics such as penicillins and vancomycin. [Book ref. 157. pp. 416–423.] pseudomycelium Syn. SPROUT MYCELIUM. pseudonavicella See MONOCYSTIS. pseudonigeran A water-insoluble, alkali-soluble (1 ! 3)-a-Dglucan present in the CELL WALL in certain fungi: e.g. Aspergillus nidulans, Paracoccidioides brasiliensis (yeast form), Schizophyllum commune. Pseudonigeran can act as a reserve carbohydrate for cleistothecium formation in the teleomorph of A. nidulans (Emericella nidulans). (cf. NIGERAN.) Pseudonocardia A genus of bacteria (order ACTINOMYCETALES, wall type IV) which occur e.g. in manure and soil. The organisms form non-fragmenting substrate mycelium, and unbranched aerial hyphae which give rise to chains of cylindrical spores. Mycolic acids are not formed. GC%: ca. 79. Type species: P. thermophila. [Book ref. 73, pp. 125–126; ecology, isolation, cultivation: Book ref. 46, pp. 2103–2117.] pseudoparaphysis A type of sterile, typically branched and anastomosing, usually septate filament (of unknown function) which originates in the upper wall of a pseudothecial locule and grows downwards between the asci, subsequently fusing with the lower wall. (cf. PARAPHYSIS; see also HAMATHECIUM.) pseudoparenchyma See PLECTENCHYMA. pseudoperithecium See ASCOSTROMA. Pseudoperonospora See PERONOSPORALES. pseudopilins Certain components of the SECRETON which exhibit a region of homology with the pilins of type IV fimbriae; homology is limited to the 30 (hydrophobic) amino acids found at the N-terminal of the protein, i.e. those residues which, in the pilins, interact during fimbrial assembly. Studies on those bacteria which encode both the secreton and type IV fimbriae suggested that a functional relationship may exist between the operation of the secreton and the mechanism of fimbriation. Subsequently, when genes encoding the pullulanase secreton of Klebsiella oxytoca were expressed in Escherichia coli K-12, one of the pseudopilins, PulG, was found to assemble into a pilus-like structure [EMBO (2000) 19 2221–2228]. pseudoplasmodium (1) A multicellular, commonly motile structure formed by the aggregation of amoeboid cells (prior to fruiting) in CELLULAR SLIME MOULDS. (2) The ectoplasmic net of members of the LABYRINTHULOMYCETES. (3) In the MYXOBACTERALES: a number of individual cells embedded in a slime matrix. pseudopodetium (lichenol.) A vertical, fruticose, branched or unbranched, usually solid secondary thallus which develops by growth from the primary thallus and is thus a purely thalline (vegetative, somatic) structure (cf. PODETIUM); ascocarp initials ‘P. cocovenenans’ (incertae sedis). See BONGKREKIC ACID. P. facilis (formerly Hydrogenomonas facilis). A nonfluorescent, non-pigmented species which occurs e.g. in soil. Monotrichously flagellate. Oxidase Cve. Glucose, fructose and malonate can be utilized; extracellular PHB is hydrolysed. The organisms can grow chemolithoautotrophically using hydrogen and oxygen (see KNALLGAS REACTION). Optimum growth temperature: ca. 28° C. GC%: 62–64. P. fluorescens. A fluorescent, PYOVERDIN-forming species which occurs e.g. in soil and water. Lophotrichously flagellate. Oxidase Cve. Gelatinase Cve. Growth can occur at 4° C but not at 41° C; optimum growth temperature: 25–30° C. GC%: ca. 59–61. (See also MUPIROCIN.) P. fragi. A psychrophilic, peritrichously fimbriate species which can cause spoilage of refrigerated foods (e.g. BUTTER). P. mallei. A non-motile, non-pigmented species which causes GLANDERS. Oxidase Cve, gelatinase Cve. D-xylose is utilized, D-ribose is not. (See BURKHOLDERIA.) P. maltophila. A former species found in e.g. soil, water and milk. Lophotrichously flagellate. Most strains need cystine or methionine; PHB is not accumulated. Strongly lipophilic. No growth at 4° C or 41° C. Re-classified initially as Xanthomonas maltophila [IJSB (1983) 33 409–413] and then as Stenotrophomonas maltophila (see CYSTIC FIBROSIS). P. marina. See DELEYA. P. nigrifaciens. See ALTEROMONAS. P. phaseolicola. See P. syringae. P. pseudoalcaligenes. A non-fluorescent species, some strains of which are pathogenic for watermelon. Monotrichously flagellate. Fructose can be utilized (cf. P. alcaligenes). GC%: 62–64. P. pseudomallei. Occurs e.g. in soil and is the causal agent of MELIOIDOSIS. Motile, lophotrichously flagellate. Oxidase Cve. Gelatinase Cve. D-Ribose can be utilized, D-xylose cannot. Yellow or orange pigments may be formed. (See BURKHOLDERIA.) P. putida. A fluorescent species which occurs e.g. in soil and water. Lophotrichously flagellate. Oxidase Cve. Optimum growth temperature: 25–30° C. Two biovars: A and B; strains of biovar B can utilize L-tryptophan and testosterone. P. putrefaciens. See ALTEROMONAS. P. pyocyanea. See P. aeruginosa. P. saccharophila. The sole strain of P. saccharophila was isolated from mud. Monotrichously flagellate. Growth can occur chemolithoautotrophically with hydrogen as electron donor. PHB is accumulated. Oxidase Cve. Optimum growth temperature: ca. 30° C. GC%: ca. 69. P. solanacearum. A plant-pathogenic species which causes wilts in e.g. members of the Solanaceae. Lophotrichously flagellate. Oxidase Cve. Some strains form brown pigments. GC%: ca. 66–68. (See BURKHOLDERIA.) P. stizolobii. See P. andropogonis. P. stutzeri. Occurs e.g. in soil and water. Non-fluorescent. PHB is not accumulated. Monotrichously flagellate, but lateral flagella are formed under certain conditions. Oxidase Cve. Starch is hydrolysed. GC%: 60–66. (See also BIOMIMETIC TECHNOLOGY.) P. syringae. A fluorescent, plant-pathogenic species. [Genetic determinants of pathogenicity in pathovar syringae: PNAS (1985) 82 406–410.] Lophotrichously flagellate. Oxidase
ve. Optimum growth temperature: ca. 25–30° C. GC%: 59–61. The species is divided into a large number of pathovars; these include (with their hosts) e.g. P. syringae pv. coronafaciens (e.g. oats); P. syringae pv. glycinea (soybean); P. syringae pv. morsprunorum (Prunus spp – see CANKER); P. syringae pv. phaseolicola (formerly P. phaseolicola) (e.g. beans); P. syringae pv. KNOT); 627 pseudopodium develop on the pseudopodetia. Pseudopodetia occur e.g. in species of PILOPHORUS and STEREOCAULON. pseudopodium An extension of an amoeboid cell formed by extrusion of the cytoplasm (bounded by the plasmalemma). Pseudopodia are characteristically formed by protozoa of the SARCODINA; according to species, pseudopodia may function in locomotion and/or in feeding, and one or more may be formed at a time by a given cell. Various types of pseudopodia are formed by sarcodines. A lobopodium (lobose pseudopodium) is blunt-ended and generally broad, and often has a clear ectoplasmic area (the hyaline cap); lobopodia may be formed eruptively, i.e., by a burst of cytoplasmic movement, or more slowly by a steady cytoplasmic flow. A filopodium (filose or filiform pseudopodium) is slender, filamentous, sometimes tapered and sometimes branched, but lacks rigid axial elements (cf. AXOPODIUM) – although it may contain microtubules. Fine, filamentous pseudopodia which branch and anastomose to form a network are called reticulopodia or rhizopodia. In some sarcodines more or less fine ‘subpseudopodia’ may arise from a broader pseudopodial lobe (see e.g. ACANTHAMOEBA). In e.g. Amoeba proteus (see AMOEBA) the lobose pseudopodia function in both feeding and locomotion. When a prey organism comes into the vicinity of a pseudopodium, the pseudopodium flows around it, initially forming a ‘cup’ (food cup) containing the prey; the plasmalemma eventually closes around the prey to form a FOOD VACUOLE. (cf. AXOPODIUM.) Locomotion in A. proteus involves the extension of pseudopodia which apparently adhere to the substratum; cytoplasm then continues to flow into one of the pseudopodia, which expands to accommodate it, and further pseudopodia are extended, etc. This type of locomotion is often regarded as typical ‘amoeboid movement’, although other forms of motility occur in other sarcodines. For example, in Pelomyxa the cell moves along by steady cytoplasmic flow without the formation of obvious pseudopodia. (See also HELIOZOEA and TAXOPODIDA.) Apart from functioning in active motility, pseudopodia in some amoebae (e.g. Vannella spp) can apparently function as flotation devices; such amoebae can, under certain conditions, produce distinct ‘flotation forms’ characterized by fine, usually hyaline pseudopodia which radiate from the central cell mass. The mechanism of pseudopodial movement is not understood. It appears to involve microfilaments of ACTIN which lie just beneath – and which interact with – the plasmalemma. pseudorabies Syn. AUJESZKY’S DISEASE. pseudoraphe See DIATOMS. pseudorinderpest Syn. PESTE DES PETITS RUMINANTS. pseudorubella Syn. EXANTHEM SUBITUM. pseudoseptum See SEPTUM (b). pseudospores See USTILAGINALES. pseudostem See PHALLALES. pseudotemperate bacteriophage A BACTERIOPHAGE which can establish PSEUDOLYSOGENY. pseudothecium See ASCOSTROMA. Pseudotrebouxia A genus of sarcinoid unicellular green algae (division CHLOROPHYTA) which occur as photobionts in many LICHENS. The cells are more or less spherical (up to ca. 30 µm in diam.). Pseudotrebouxia spp closely resemble TREBOUXIA spp except in that asexual reproduction occurs not only by the formation of zoospores and aplanospores but also by ‘vegetative cell division’, i.e., basically, the division of one cell into two non-motile vegetative cells, etc – the resulting cells remaining in contact (at least initially) to form sarciniform groupings. [Pseudotrebouxia species in lichens, and discussion of vegetative cell division: Lichenol. (1981) 13 65–86.] pseudotuberculosis A self-limiting or fatal (septicaemic) disease of animals (especially rodents) due to Yersinia pseudotuberculosis; symptoms e.g.: mesenteric adenitis, chronic diarrhoea, internal caseous lesions. (Infected humans may have mesenteric adenitis or, if debilitated, septicaemia.) Wild animals form a reservoir of the pathogen. pseudotype (virol.) A virus particle which, as a result of PHENOTYPIC MIXING, contains the genome of one virus and polypeptide(s) encoded by another virus; e.g., the genome of a replication-defective virus may be encapsidated by proteins encoded by a replication-competent helper virus. pseudotyping (genetic engineering) A technique in which e.g. the envelope proteins of a retrovirus are replaced by the G glycoprotein of vesicular stomatitis virus; the pseudotyping of retroviruses enables them to infect a wider range of types of cell. [Example of use (in gene therapy): PNAS (1999) 96 10379–10384.] Some viruses used for gene delivery (in GENE THERAPY) have the disadvantage that they infect too many types of cell. Thus, if a gene is to be delivered to a specific type of cell, then viruses with a wide spectrum of target cells may have to be administered in high doses in order to allow for losses during interaction with other types of cell; such high doses of virus may expose the patient to the risk of adverse reactions. One such virus with a wide tropism is adenovirus type 5; in vitro, plasmid-mediated modification of the virion’s attachment site (pseudotyping) has been able to de-target the virus from its (widely expressed) host receptor, allowing the development of an engineered virion which is more specifically targeted [JV (2001) 75 2972–2981]. pseudovirion A ‘virion’ which contains only host-cell nucleic acid. (See e.g. TRANSDUCTION.) pseudovitellus The mycetome of an aphid. pseudo-wild phenotype See e.g. SUPPRESSOR MUTATION. y Pseudouridine (see TRNA). [PSI+ ] See PRION (sense 2). psicofuranine (9-b-D-psicofuranosyladenine) An antitumour, antimicrobial NUCLEOSIDE ANTIBIOTIC from Streptomyces hygroscopicus. It (and its close relative decoyinine) inhibits GMP synthesis by mimicking the allosteric inhibition by adenosine of XMP aminase [see Appendix V(a)]. psilocin See HALLUCINOGENIC MUSHROOMS. Psilocybe See AGARICALES (Strophariaceae) and HALLUCINOGENIC MUSHROOMS. psilocybin See HALLUCINOGENIC MUSHROOMS. Psilolechia See LECIDEA. psittacine beak and feather disease A disease of psittacines due to an ssDNA circovirus; feather, beak and immune cells are killed and the birds may die of other infections. psittacosis (parrot fever; ornithosisŁ ) An acute infectious disease which primarily affects birds – both psittacine (parrots, parakeets etc) and non-psittacine (e.g. pigeons, sparrows, canaries, domestic fowl) – but which is also transmissible to man; it is caused by Chlamydia psittaci. In birds, C. psittaci usually causes a mild or asymptomatic infection but can cause a fatal disease. The spleen and liver may be enlarged, with areas of focal necrosis; symptoms may include e.g. ocular and nasal discharge, weakness and diarrhoea. C. psittaci is present in the faeces, nasal secretions, feathers etc of infected birds. Man usually becomes infected by inhalation of dust from contaminated birds; personto-person transmission is also known. Incubation period: 4–15 days. The disease may be mild and self-limiting, resembling influenza, but it can be fatal; symptoms may include e.g. fever, cough, headache, photophobia, myalgia, and lobular pneumonitis 628 PTS PTS (phosphoenolpyruvate-dependent phosphotransferase system) A TRANSPORT SYSTEM, found in both Gram-negative and Grampositive bacteria, in which the substrate (e.g. a hexose, disaccharide or hexitol) is phosphorylated in a reaction which is an essential part of the transport process. The source of energy (and of phosphate) is normally phosphoenolpyruvate (PEP); the formula of PEP is given in Appendix I(a). (Certain enzymes can generate PEP from ATP or GTP.) In members of the Enterobacteriaceae, sugars whose uptake is PTS-dependent (‘PTS sugars’) include glucose, fructose, mannose, mannitol, sorbitol, glucosamine, N-acetylglucosamine and N-acetylmannosamine. (cf. LACTOSE.) (Lactose is a PTS sugar e.g. in Staphylococcus aureus.) Because sugars are often metabolized via their phosphate derivatives – e.g. glucose 6-phosphate in Appendix I(a) – phosphorylation during uptake is a positive aspect of PTS transport. (Note, however, that a non-metabolizable substrate may be phosphorylated and transported by a PTS – e.g. the sugar analogue methyl a-glucoside can be transported by the glucosePTS of Escherichia coli and of Clostridium pasteurianum.) (See also STREPTOZOTOCIN.) A PTS consists of both cytoplasmic (soluble) and membranebound protein components; the early nomenclature for PTS components (enzymes I, II and III) has been somewhat modified. In the simplest case (mannitol uptake in E. coli: see diagram), PEP feeds energy and phosphate into the PTS by sequential phosphorylation of two soluble energy-coupling proteins: enzyme I (here designated I) and a histidine-containing protein, HPr (here designated H). (Genes encoding I and H occur in the same (pts) operon.) The phosphate and energy are transferred from H to a sugar-specific permease (the so-called II complex) in the cytoplasmic membrane; the permease includes a transmembrane component (IIC), involved in the binding and translocation of substrate, and two phosphorylation sites (IIA, IIB) – phosphorylation and concomitant transport (uptake) of mannitol apparently occurring only at the IIB site. Uptake of glucose by PTS (see diagram) differs in that the IIA component is soluble – adding a further cytoplasmic step to the chain of phosphorylation. Phosphorylated forms of components I and H have high free energies of hydrolysis (approximately that of PEP). The I and H components are not substrate-specific, i.e. they are required for the phosphorylation and transport of all PTS substrates; thus, mutation causing loss of one or both of these components will be pleiotropic, i.e. the mutant will be unable to transport any PTS-dependent substrate. In contrast, inactivation of a given II complex will affect the transport of only the relevant permeasespecific substrate(s). [PTS (review): Mol. Microbiol. (1994) 13 755–764.] In enterobacteria, some complex II components apparently contain oxidizable sulphhydryl groups, and it seems that only reduced forms are active [Biochem. (1985) 24 47–51]; this may indicate an energy-dependent regulation of the PTS, and may account for the observed pmf-related regulation of PTS activity. In some II complexes the components are grouped in other ways. For example, the fructose permease of enteric bacteria consists of a membrane-bound protein, containing IIC and IIB, and a separate protein containing the functions of both IIA and HPr. In Rhodobacter capsulatus, the fructose permease includes a single protein which carries out the functions of IIA, HPr and enzyme I. with an alveolar and interstitial exudate (cf. PRIMARY ATYPICAL Lab. diagnosis: serological tests. Chemotherapy: e.g. tetracyclines. [Review: BMB (1983) 39 163–167.] Ł Although the terms ‘psittacosis’ and ‘ornithosis’ are used by some authors as synonyms for the disease caused by C. psittaci in both psittacine and non-psittacine birds and in man, others reserve the term ‘ornithosis’ for the disease in (i) psittacine and non-psittacine birds (i.e., excluding man); (ii) non-psittacine birds and humans infected from non-psittacine birds; or (iii) nonpsittacine birds only. Psora See LECIDEA. psoralens (furocoumarins) Three-ringed heterocyclic compounds found e.g. in certain fungi and tropical fruits. A psoralen binds intercalatively (see INTERCALATING AGENT) and non-covalently to DNA or RNA in the dark. On irradiation of this complex with light (365 nm) the psoralen becomes covalently linked to a pyrimidine residue in one strand; it may then react photochemically with a second pyrimidine to form a cross-link. Owing to the small size of a psoralen molecule, two pyrimidines can be cross-linked only when they are very close (e.g. on complementary strands in a double helix or a hairpin loop). Psoralens are useful in studies on the secondary structures of DNA and RNA molecules [ARBB (1981) 10 69–86]. psoroptic mange A disease of animals caused by mite bites (see e.g. SHEEP SCAB). pst operon See PHO REGULON. PstI A RESTRICTION ENDONUCLEASE from Providencia stuartii ; CTGCA/G. PSTV Potato spindle tuber VIROID. Psychrobacter A genus of Gram type-negative bacteria formerly proposed as a member of the family Neisseriaceae [IJSB (1986) 36 388–391] but later classified within the family Moraxellaceae [IJSB (1991) 41 310–319]. Strains of P. immobilis are oxidase Cve and catalase Cve coccobacilli found e.g. in fish and processed meat products; most are psychrotrophic (optimum growth temperature: 20–25° C). GC% 44–46. psychrophile An organism which grows optimally at or below 15° C, which has an upper limit for growth of ca. 20° C, and which has a lower limit for growth of 0° C or below [Bact. Rev. (1975) 39 144–167]. (This is currently the most widely accepted definition; however, some authors still use the term, very loosely, to include PSYCHROTROPHS.) Psychrophilic organisms include certain algae and fungi, a number of Gram-negative bacteria (e.g. some species of Pseudomonas and Vibrio), and a few Grampositive bacteria (e.g. some Clostridium spp). [Ecology and physiology of psychrophilic bacteria: Book ref. 191, pp. 1–23.] (cf. MESOPHILE and THERMOPHILE.) psychrotroph An organism which can grow at low temperatures (e.g. 0–5° C) but which has an optimum growth temperature >15° C and an upper limit for growth >20° C. (cf. PSYCHROPHILE.) Psychrotrophs include e.g. certain algae and fungi, and various Gram-negative and Gram-positive bacteria. pT181 plasmids See INCOMPATIBILITY. Pterosperma See MICROMONADOPHYCEAE. pterostilbene A stilbene PHYTOALEXIN produced by the grapevine (Vitis vinifera). pteroylglutamic acid Syn. FOLIC ACID. PTFE Polytetrafluoroethylene (Teflon). PTLV Primate T-cell leukaemia virus: see SIMIAN T-CELL LEUKAEMIA VIRUS. PNEUMONIA). 629 PTS regulation domain mannitol glucose IIC IIA CM IIC P IIB IIB mannitol 1-phosphate P P H H I P IIA glucose 6-phosphate P P P PEP PTS (phosphoenolpyruvate-dependent phosphotransferase system). Transport of mannitol and glucose across the cytoplasmic membrane (CM) by the PTS in Escherichia coli (simplified, diagrammatic); transport is from the periplasm (above) to the cytoplasm (below). Note that mannitol and glucose are taken up by different membrane-associated systems (permeases). P The source of energy for transport is phosphoenolpyruvate (PEP). Energy is fed into the system by the sequential transfer of phosphate  from PEP to enzyme I (shown here as I) and then to the HPr protein (shown here as H). In mannitol transport, phosphate is transferred from H direct to a membrane-bound permease (enzyme II complex, here designated II). The permease for mannitol consists of three domains: IIA, IIB and IIC. IIC is a hydrophobic, transmembrane domain which binds the specific substrate and appears to contain, or contribute to, a channel through which the substrate is translocated. The IIA and IIB domains each contain a phosphorylation site, but phosphorylation of the substrate seems directly to involve only the IIB domain. In the glucose permease, IIA is a separate (cytoplasmic, soluble) protein; however, the sequence of phosphorylation is the same as that in the mannitol permease, i.e. IIA ! IIB ! substrate. In all cases, phosphorylation of the substrate is an integral part of the transport mechanism. Reproduced from Bacteria, 5th edition, Figure 5.12, page 91, Paul Singleton (1999) copyright John Wiley & Sons Ltd, UK (ISBN 0471-98880-4) with permission from the publisher. In the proposed nomenclature of PTS permeases [JB (1992) 174 1433–1438], the mannitol permease of E. coli is designated IICBAMtl,Eco , and the glucose permease is IICBGlc,Eco C IIAGlc,Eco (see diagram). In some cases a cell responds chemotactically to substrates taken up by a PTS (see CHEMOTAXIS). Moreover, a PTS can control some aspects of carbon utilization; thus, PTS phosphate carriers (see diagram) can regulate the activity of certain operons involved in the catabolism of carbon sources – sometimes via regulator sites designated ‘PTS regulation domains’ (PRD) [Mol. Microbiol. (1998) 28 865–874] (see CATABOLITE REPRESSION). PTS regulation domain (PRD) See CATABOLITE REPRESSION. pts operon See PTS. Ptychodiscus See RED TIDE and BREVETOXINS. Pu Purine (nucleotide). PU Palindromic unit: see REP SEQUENCE. pubescent Downy. Puccinia A genus of typically macrocylic and heteroxenous rust fungi (class UREDINIOMYCETES) which characteristically form cupulate aecia, non-peridiate uredia, and pedicellate, two-celled teliospores. Puccinia species are parasitic on a wide range of plant hosts, including monocotyledonous and dicotyledonous species. See UREDINIOMYCETES for the life cycle of P. graminis tritici. (See also BLACK STEM RUST, BROWN RUST, CHRYSANTHEMUM WHITE RUST, CROWN RUST and YELLOW RUST.) Pucciniastrum See UREDINIOMYCETES and PERIDERMIOID. puerperal fever (childbed fever; puerperal sepsis) An acute, febrile condition, following childbirth, due to infection of the uterus and/or adjacent regions – usually by streptococci but sometimes by e.g. Clostridium spp; complications (e.g. septicaemia) may occur, and mortality rates are high in untreated cases. puerperal mastitis See MASTITIS. puerperal sepsis Syn. PUERPERAL FEVER. puffball See LYCOPERDALES and GASTEROMYCETES (Tulostomatales). puffing (mycol.) See ASCOSPORE. pul genes See PROTEIN SECRETION (type II systems). pullorum disease (bacillary white diarrhoea, BWD) An acute infectious POULTRY DISEASE caused by Salmonella pullorum; it affects mainly chicks, but also occurs e.g. in turkey poults and pheasants. Symptoms: e.g. lethargy, anorexia, white diarrhoea, weakness and neurological symptoms; lungs or joints may be affected. Mortality rates may be high. Survivors become chronic carriers: the pathogen localizes in the ovaries and infects a proportion of the eggs; chicks hatching from these eggs develop the disease, and infection can then spread to other chicks – either by ingestion of food contaminated with faeces from infected (or carrier) birds, or by inhalation of fluff from infected chicks. Lab. diagnosis: identification of S. pullorum from e.g. liver, spleen or ovaries. Carriers are detected by an agglutination test (cf. FOWL TYPHOID). Treatment: e.g. furazolidone, furaltadone. pullulan An extracellular, water-soluble, linear D-glucan synthesized by Aureobasidium (Pullularia) pullulans; it consists predominantly of MALTOTRIOSE units linked by (1 ! 6)a-glucosidic bonds. pullulanase See DEBRANCHING ENZYMES. Pullularia Syn. AUREOBASIDIUM. pullulation Asexual reproduction by BUDDING. pulmonary adenomatosis (vet.) Syn. JAAGSIEKTE. 630 putrefaction pulpwood spoilage See PAPER SPOILAGE. pulpy kidney An acute toxaemic disease of ruminants (especially lambs) caused by growth and toxigenesis by Clostridium perfringens type D in the intestines. Symptoms: diarrhoea, convulsions, paralysis, sudden death. pulque A white, viscous, acidic, alcoholic Mexican beverage made by fermenting the juices of Agave spp. The alcohol is produced by Saccharomyces cerevisiae (‘S. carbajali ’) and Zymomonas mobilis. Acidity and viscosity are contributed, respectively, primarily by Lactobacillus plantarum and dextranproducing strains of Leuconostoc. Pulque is rich in B vitamins. Tequila is made by the distillation of pulque. (See also WINEMAKING; SPIRITS.) pulse–chase technique A technique in which e.g. living cells are exposed to a pulse of labelled substrate for a fixed period of time – followed immediately by a high concentration of the unlabelled substrate (the ‘chase’ phase); the chase effectively stops further uptake/use of the labelled substrate, thus defining the duration of the pulse. The technique has been used e.g. to follow the intracellular movement of a secreted protein from the time of its synthesis to the time of secretion; for this experiment, the period of the pulse is increased in a stepwise manner. pulsed electric field (PEF) A technique involving electric fields of typically >20 kV/cm applied in pulses of up to several µs (microseconds) for the inactivation of susceptible microorganisms in a liquid medium; exposure to PEF treatment appears to bring about irreversible structural damage to the cells. In one experiment, the use of 2500 pulses of a 30 kV/cm field at 50° C reduced the level of viable cells of Mycobacterium paratuberculosis in milk by ca. 5.9 log10 cfu/ml [AEM (2001) 67 2833–2836]. This technique may have applications e.g. in the food industry. pulsed-field gel electrophoresis See PFGE. pulverulent Powdery. pulvinate Cushion-like; forming a swelling. pulvomycin See POLYENE ANTIBIOTICS (b). punctate Having surface dot(s), pore(s) etc. punctiform Dot-like. Punta Toro virus See PHLEBOVIRUS. PUO Pyrexia (fever) of unknown origin. pure culture See CULTURE. purified protein derivative See TUBERCULIN. purine nucleotide biosynthesis See NUCLEOTIDE and Appendix V(a). purity plate A plate inoculated from a given culture and subsequently incubated to determine whether the culture was pure or contaminated – the presence of two or more types of colony on the purity plate indicating a contaminated culture. puromycin (6-dimethyl-30 -deoxy-30 -p-methoxyphenylalanylamino adenosine) A NUCLEOSIDE ANTIBIOTIC obtained from Streptomyces alboniger or synthesized chemically; it is active against prokaryotes and eukaryotes. It acts primarily as an inhibitor of PROTEIN SYNTHESIS, acting as an analogue of the aminoacyl-adenyl portion of an aminoacyl-tRNA. When a peptidyl-tRNA occupies a ribosomal P site, puromycin can enter the A site and a peptide bond can be formed between the amino group of puromycin and the C-terminus of the peptide. However, puromycin does not have a hydrolysable ester bond like that linking the aminoacyl group to a tRNA, and puromycin binds only weakly to the ribosome; thus, nascent polypeptide chains (in which the C-terminus is blocked by puromycin) are released. purple bacteria (1) Bacteria of the RHODOSPIRILLINEAE. (2) A category (D PROTEOBACTERIA – q.v.) within the domain BACTERIA originally distinguished on the basis of 16S rRNA sequence analysis [SAAM (1985) 6 143–151]. Organisms in this category are believed to have evolved from photosynthetic bacteria similar to members of the Rhodospirillineae; they are divided into the alpha, beta, gamma, delta and epsilon subdivisions. purple laver See LAVER. purple membrane In some strains of Halobacterium salinarium: functionally and structurally differentiated, purple-pigmented regions of the cytoplasmic membrane which develop under microaerobic or anaerobic conditions in the light; purple membrane may occupy up to 50% of the total membrane area – the remaining cytoplasmic membrane (which contains red carotenoid pigments) often being called the ‘red membrane’. The purple membrane utilizes light energy to effect transmembrane translocation of protons, thus generating or augmenting a proton motive force (pmf) (see CHEMIOSMOSIS); cells can therefore grow as phototrophs under anaerobic conditions (see HALOBACTERIUM). The protein component of the purple membrane consists almost entirely of BACTERIORHODOPSIN (q.v.) but includes another retinal-containing purple pigment, HALORHODOPSIN, and (in at least some cases) SLOW-CYCLING RHODOPSIN. (Strains of H. salinarium which cannot synthesize retinal may form an apoprotein-containing ‘white membrane’.) The lipids of the purple membrane are similar to those in the remainder of the cytoplasmic membrane, but they include a glycosulpholipid which is apparently unique to the purple membrane. While photocycling in bacteriorhodopsin causes an outward efflux of protons, photocycling in halorhodopsin appears to cause an inward pumping of chloride ions (rather than an outward pumping of sodium ions as was previously thought). The combined activities of light-energized bacteriorhodopsin and halorhodopsin may account for the various ion fluxes across the membrane and for the complex pattern of changes in transmembrane pH differential. When whole cells are illuminated, the steady-state or ‘resting’ membrane potential (typically ca. 100–110 mV) is increased by ca. 10–40 mV, and the ‘resting’ transmembrane pH differential (ca. 2 units at pH 5, ca. 0 units at pH 8) changes by 0–1 unit; the increased membrane potential serves to energize e.g. photophosphorylation (phosphorylation of ADP), uptake of potassium ions and efflux of sodium ions. purple non-sulphur bacteria See RHODOSPIRILLACEAE. purple sulphur bacteria See CHROMATIACEAE. purpura (med.) A condition characterized by reddish or purple patches due to haemorrhage into skin or mucous membrane and underlying tissues. A pin-point-sized haemorrhage is called a petechia, a larger one is known as an ecchymosis. purse (mycol.) See CYATHUS. purulent Containing, consisting of, or forming PUS. pus A thick, usually yellowish, fluid product of inflammation formed at the site of an infection. Pus contains proteins, leucocytes, cell fragments, and e.g. living and/or dead bacteria. (cf. BLUE PUS.) pustulan A (1 ! 6)-b-D-glucan produced by Lasallia (Umbilicaria) pustulata. pustule A small, raised, PUS-filled skin lesion. (cf. VESICLE.) pusule In many DINOFLAGELLATES: an intracellular vesicular structure which opens to the exterior via a canal and a pore; it is believed to function in osmoregulation but is apparently non-contractile (cf. CONTRACTILE VACUOLE). putidaredoxin See FERREDOXINS. putrefaction The microbial degradation of proteinaceous materials with the formation of evil-smelling products such as amines 631 putrescine cysteine-HCl, NaCl, resazurin, and glucose (added after autoclaving), and may be used as a broth (PYGB) or as an agar medium (PYGA). [Recipe: Book ref. 53, p. 1428.] Py-GLC PYROLYSIS GAS–LIQUID CHROMATOGRAPHY. pyknosis (pycnosis) (histopathol.) The shrinkage of a cell’s nucleus, resulting in the formation of a smaller, densely staining body. (cf. KARYOLYSIS; KARYORRHEXIS.) pyo- Prefix meaning pus. pyocin (pyocine; aeruginocin) Any BACTERIOCIN produced by Pseudomonas aeruginosa; pyocins are bactericidal – log-phase target cells apparently being the most sensitive. R-type pyocins (MWt ca. 107 ) resemble phage tails and are relatively insensitive to proteases; S-type pyocins (MWt ca. 105 ) are amorphous and are sensitive to proteases. Pyocin typing: see BACTERIOCIN TYPING. pyocyanin (pyocyanine) A blue-green, water-soluble and nonfluorescent phenazine pigment synthesized by Pseudomonas aeruginosa from chorismate (an intermediate in the biosynthesis of aromatic amino acids: see Appendix IV(f)); it becomes red-purple when acidified. Pyocyanin has oxygen-dependent antimicrobial activity which appears to involve the (enzymeindependent) oxidation of NADH with production of SUPEROXIDE and H2 O2 [JB (1980) 141 156–163]; it also inhibits the beating of human respiratory cilia in vitro [JCI (1987) 79 221–229]. (cf. CYANOMYCIN; see also IRON.) pyoderma Any purulent skin disease – e.g. IMPETIGO. pyofluorescein Syn. PYOVERDIN. pyogenic Able to cause formation of pus. pyomelanin See PSEUDOMONAS (P. aeruginosa). pyorubin See PSEUDOMONAS (P. aeruginosa). pyoverdin (pyoverdine; pyofluorescein; ‘fluorescein’) Any of a range of yellowish, fluorescent, water-soluble pigments produced by certain species of Pseudomonas under iron-deficient conditions; the pyoverdins act as SIDEROPHORES. A model for the iron-regulated induction of pyoverdin in P. aeruginosa proposes that, under iron-deficient conditions, the (regulatory) Fur protein dissociates from a transcriptional control site upstream of the pvdS gene, allowing synthesis of PvdS; PvdS is a sigma factor which promotes transcription of genes involved in the synthesis of pyoverdin [JB (1996) 178 2299–2313]. The pyoverdin of P. fluorescens contains a quinoline chromophore linked to a cyclic peptide; this compound represses synthesis of another siderophore, quinolobactin [AEM (2000) 66 487–492]. [Functional analysis of PvdS: JB (2000) 182 1481–1491.] PYR A substrate (N,N-dimethylaminocinnamaldehyde) which is apparently hydrolysed by all strains of Streptococcus pyogenes but by no other streptococci. In an identification test for S. pyogenes, the organisms are grown aerobically on agar plates overnight at 35° C; when PYR is added to the surface growth, colonies of S. pyogenes become red, while those of other streptococci become yellow or do not change colour. PYR is also hydrolysed by strains of ENTEROCOCCUS. Pyr(6–4)Pyo See ULTRAVIOLET RADIATION. pyracarbolid (2H-3,4-dihydro-6-methylpyran-5-carboxanilide) An agricultural systemic ANTIFUNGAL AGENT which is active e.g. against various rusts, smuts, and Rhizoctonia spp. Pyramimonas See MICROMONADOPHYCEAE. pyrazinamide 1,4-Diazine carboxamide: a drug active against Mycobacterium tuberculosis in vivo (including cells within phagosomes) and (at e.g. pH 5.6) in vitro. M. bovis is not susceptible. (e.g. cadaverine and putrescine formed by the decarboxylation of lysine and ornithine, respectively) and HYDROGEN SULPHIDE (from sulphur-containing amino acids). Putrefaction is typically an anaerobic process and may be carried out e.g. by proteolytic clostridia; however, certain pseudomonads can bring about the aerobic putrefaction of e.g. meat. putrescine See POLYAMINES, DECARBOXYLASE TESTS and PUTREFACTION. Puumala virus See HANTAVIRUS. pv. Pathovar: see VARIETY. PVA See POLYVINYL ALCOHOL FIXATIVE. pvdS gene See PYOVERDIN. PVP–iodine See IODINE (a). PvuI See RESTRICTION ENDONUCLEASE (table). PWM POKEWEED MITOGEN. Py Pyrimidine (nucleotide). pyaemia (pyemia) A particular form of SEPTICAEMIA in which pyogenic bacteria are disseminated via the bloodstream. pycnidiospore A CONIDIUM formed in a PYCNIDIUM. pycnidium A hollow, typically spherical, flattened, or flaskshaped fungal fruiting body (generally ca. 100–500 µm in size) within which conidia (D pycnidiospores) are produced, and which, at maturity, is – or becomes – open to the environment e.g. via one or more pores. Pycnidia, which may superficially resemble perithecia or cleistothecia, are formed by many members of the SPHAEROPSIDALES. According to species, a pycnidium may be thick-walled or thin-walled, unilocular or multilocular, black, brown or brightly coloured (e.g. orange or yellow), glabrous or setose; it may develop superficially or may be immersed – or partly immersed – within the substratum or within a fungal stroma. [Symphogenous development of pycnidia in Chaetomella acutiseta: CJB (1980) 58 1129–1137.] pycniospore A mononucleate, haploid SPERMATIUM formed in a pycnium (see UREDINIOMYCETES). pycnium See UREDINIOMYCETES stage 0. Pycnomonas A subgenus of TRYPANOSOMA within the SALIVARIA. T. (P.) suis occurs in wild and domestic pigs in parts of Africa (e.g. Zaire) and is transmitted by Glossina spp. The organisms are stout trypomastigotes, ca. 10–20 µm, in which there is a subterminal kinetoplast and a free flagellum. pycnosis Syn. PYKNOSIS. pycnospore An obsolete term for a pycnidiospore or a pycniospore. Pycnothyriales An order of fungi (class COELOMYCETES) in which the vegetative mycelium and conidiomata may be superficial or immersed in the substratum; conidiomata may be e.g. multiloculate, and in some species each is elevated on a central columnar structure. pyelitis See PYELONEPHRITIS. pyelonephritis Inflammation of the kidney (nephritis) and the renal pelvis (pyelitis). (The renal pelvis is the region at the kidney–ureter junction.) Pyelitis and/or nephritis may be caused by infection with e.g. enterobacteria (particularly strains of Escherichia coli ), staphylococci or streptococci; infection may occur either via the bloodstream or via the urethra, bladder and ureter. Symptoms usually include fever and pain in the loins, and there may be bacteraemia. (See also CYSTITIS, GLOMERULONEPHRITIS, UPEC and URINARY TRACT INFECTION.) pyemia See PYAEMIA. PYG medium Peptone–yeast extract–glucose medium, used e.g. for culture of anaerobes. It contains peptone, yeast extract, 632 Pyrrhophyta Pyrazinamide is a prodrug which is converted, in vivo, to the active compound pyrazinoic acid; conversion involves the enzyme pyrazinamidase, which is encoded by gene pncA in Mycobacterium tuberculosis. Most pyrazinamide-resistant clinical isolates of M. tuberculosis have been found to have mutations in the pncA gene. Expression of M. smegmatis pyrazinamidase in mutant (pyrazinamide-resistant) M. tuberculosis confers hypersensitivity to pyrazinamide and related amides [JB (2000) 182 5479–5485]. pyrazinoic acid See PYRAZINAMIDE. pyrazophos An antifungal ORGANOPHOSPHORUS COMPOUND which is active against various powdery mildews. It is readily absorbed by leaves and shoots, and is translocated within the plant; it is poorly absorbed by roots. pyrBI, pyrE genes See OPERON. pyrenoid A dense proteinaceous body which occurs within a CHLOROPLAST; in some cases a pyrenoid may be ‘stalked’ – protruding from the chloroplast but always contained by the chloroplast envelope. The pyrenoid appears to be the site of synthesis of storage polysaccharide; the enzyme RuBisCO has been isolated as the main protein component of the pyrenoids in some species. Pyrenomycetes A class of ascomycetes characterized by the formation of unitunicate asci in a perithecium or a cleistothecium. The class is not recognized in most modern taxonomic schemes. Pyrenophora See DOTHIDEALES. Pyrenula See PYRENULALES. Pyrenulales An order of saprotrophic and lichenized fungi of the ASCOMYCOTINA. Ascocarp: ascolocular and perithecioid (see ASCOCARP). Asci: bitunicate. Ascospores: multiseptate. Genera: e.g. Acrocordia, Pyrenula. Pyricularia A genus of fungi of the HYPHOMYCETES; teleomorph: Magnaporthe. P. oryzae, the causal agent of rice BLAST DISEASE, forms a septate mycelium and gives rise to typically pyriform, commonly biseptate conidia. pyricularin Syn. PIRICULARIN. pyridine haemochrome See CYTOCHROMES. pyridine nucleotide coenzymes NAD (q.v.) and NADP. pyridine nucleotide salvage cycle See NAD. pyridoxal phosphate See PYRIDOXINE. pyridoxamine See PYRIDOXINE. pyridoxine (pyridoxin; vitamin B6 ; pyridoxol) A water-soluble and photolabile VITAMIN: 2-methyl-3-hydroxy-4,5-di-(hydroxymethyl)pyrimidine; related compounds such as pyridoxal and pyridoxamine (bearing, respectively,
CHO and
CH2 NH2 at the 4-position) may also be referred to as ‘vitamin B6 ’. The coenzyme form of the vitamin is pyridoxal 5-phosphate (‘codecarboxylase’); pyridoxamine 5-phosphate is involved in some reactions. The coenzyme is important in amino acid metabolism – e.g., in reactions involving transamination, decarboxylation, racemization (interconversion of D-and L-amino acids), serine–glycine interconversion, etc. Certain microorganisms (e.g. Clostridium spp, Lactobacillus spp, some yeasts, Crithidia fasciculata, Tetrahymena spp) require an exogenous supply of pyridoxine; the various derivatives are not necessarily equally effective in this respect. pyridoxol Syn. PYRIDOXINE. pyriform Pear-shaped. pyrimethamine (2,4-diamino-5-(p-chlorophenyl)-6-ethylpyrimidine) A FOLIC ACID ANTAGONIST used – usually in combination with e.g. a sulphonamide – in the treatment of MALARIA and of toxoplasmosis and other coccidioses; it is active against the trophozoite, but resistance tends to develop readily. pyrimidine dimer See ULTRAVIOLET RADIATION. pyrimidine nucleotide biosynthesis See NUCLEOTIDE and Appendix V(b). pyrimine See SERRATIA. Pyrocystis See BIOLUMINESCENCE. Pyrodictium A genus of chemolithoautotrophic prokaryotes (order THERMOPROTEALES) isolated from a submarine volcanic region. Growth occurs as a network of filaments associated with ‘discs’, each ca. 0.3–2.5 µm in diameter. Optimum growth temperature: 105° C. Growth occurs in up to 12% NaCl. Pyrodinium See BIOLUMINESCENCE. pyrogen (1) A fever-inducing agent. (2) LIPOPOLYSACCHARIDE. pyrogenic exotoxin C See TOXIC SHOCK SYNDROME. pyrogram See PYROLYSIS GAS–LIQUID CHROMATOGRAPHY. pyrolysate See PYROLYSIS. pyrolysis A technique, used e.g. in microbial taxonomy, in which a small sample of a microbial culture is thermally degraded, in a vacuum or in an inert atmosphere (e.g. nitrogen), to form a range of volatile low-MWt compounds characteristic of the organism(s); the products (pyrolysate) may be analysed by e.g. Py-GLC or Py-MS (see following entries). In filament pyrolysers the sample may be coated onto a platinum filament which is then heated by the transmission of an electric current. (See also CURIE POINT PYROLYSIS.) Laser pyrolysers have also been used. pyrolysis gas–liquid chromatography (Py-GLC) An analytical technique, intended e.g. for the rapid characterization of microorganisms, in which microbial cells are subjected to PYROLYSIS and the pyrolysate components are separated by gas–liquid CHROMATOGRAPHY and recorded as a complex trace (pyrogram). pyrolysis–mass spectrometry (Py-MS) An analytical technique, intended e.g. for the rapid characterization of microorganisms, in which microbial cells are subjected to PYROLYSIS and the pyrolysate is analysed by mass spectrometry. Pyronema A genus of fungi (order PEZIZALES) which occur e.g. on burnt soils. The apothecium (commonly 1–2 mm diam.), which develops on a subiculum, may be white, pale orange or pinkish. Ascospores: uninucleate, colourless, eguttulate. pyronin A red basic xanthene DYE. pyrophosphate (inorganic pyrophosphate; PPi) Pyrophosphate (P2 O7 4
) is involved in various metabolic reactions in both prokaryotic and eukaryotic organisms. In many organisms PPi is released from nucleoside triphosphates e.g. during the biosynthesis of nucleic acids and proteins; in some organisms it is synthesized by photophosphorylation (e.g. in Rhodospirillum rubrum) or OXIDATIVE PHOSPHORYLATION (e.g. in animal and plant mitochondria) involving a PROTON PPASE. In e.g. Propionibacterium freudenreichii (formerly P. shermanii ) and Entamoeba histolytica, PPi is used for the (reversible) phosphorylation of fructose 6-phosphate catalysed by PPi:phosphofructose dikinase – a reaction which appears to be important in the glycolytic pathway in these organisms. In DESULFOTOMACULUM (q.v.) PPi is used for the SUBSTRATELEVEL PHOSPHORYLATION of ADP; sulphate-reducing bacteria which lack the enzyme acetate:PPi phosphotransferase (e.g. Desulfovibrio) cannot use PPi in this way. pyrophosphotransferase See STRINGENT CONTROL (1). Pyrosequencing A rapid method for sequencing short DNA templates based on detection of the pyrophosphate released when a series of (known) nucleotides sequentially extend a primer [principle, details: Science (1998) 281 363–365]. Pyrrhophyta Syn. PYRROPHYTA. 633 pyrrolo-(1,4)-benzodiazepine antibiotics tube which penetrates the host plant – giving rise to both intercellular and intracellular mycelium; haustoria are not formed. Later, spherical or ovoid, non-deciduous sporangia develop, in terminal or intercalary positions, on undifferentiated sporangiophores. A sporangium germinates by extruding a thin tube which terminates in a thin-walled, bubble-like vesicle; the multinucleate contents of the sporangium flow into the vesicle where they differentiate into zoospores. In sexual reproduction, a spherical oogonium and a smaller, club-shaped or elongated antheridium develop on the hyphae in a terminal or intercalary position. (Most species of Pythium appear to be homothallic, but at least one species, P. sylvaticum, is heterothallic.) Meiosis is generally thought to occur in the gametangia. A fertilization tube develops between antheridium and oogonium, and a male nucleus passes into the oogonium where fertilization occurs. The resulting thick-walled oospore germinates to form a germ tube which may terminate in a zoosporangium. Pythium red rot See ALGAL DISEASES. Pythonella See EIMERIORINA. pyuria The presence of pus in the urine. ‘Sterile pyuria’ (D ‘abacterial pyuria’) is a condition in which pus is repeatedly found in samples of urine from which no bacteria can be cultured by routine methods; common causes are probably chlamydiae or mycoplasmas. pYV plasmid See VIRULON. pyrrolo-(1,4)-benzodiazepine antibiotics See ANTHRAMYCIN. pyrrolo-quinoline quinone See QUINOPROTEIN. Pyrrophyta (Pyrrhophyta) A division of the algae which includes the DINOFLAGELLATES and sometimes also the CRYPTOPHYTES. Pyrsonympha See OXYMONADIDA. pyruvate An intermediate in a wide range of (aerobic and anaerobic) metabolic pathways; for examples of formation and fates of pyruvate see Appendices I, II, III and IV(b, e). pyruvate carboxylase A BIOTIN ENZYME which is an important anaplerotic enzyme (see Appendix II (b)). [Structure: FEMS Reviews (1993) 104 330–332.] pyruvate dehydrogenase complex See e.g. Appendix II(a). pyruvate synthase See TCA CYCLE and REDUCTIVE TRICARBOXYLIC ACID CYCLE. pyruvic phosphoroclasm See PHOSPHOROCLASTIC SPLIT. Pythium A genus of aquatic and terrestrial fungi (order PERONOSPORALES) which include saprotrophs, parasites, and pathogens of higher plants (see e.g. DAMPING OFF), certain algae (see ALGAL DISEASES), and mammals (see EQUINE PHYCOMYCOSIS). (See also MYCOPARASITE.) The thallus is a well-developed, branching mycelium, and sporangia are borne on sporangiophores which typically resemble somatic hyphae. The generalized life cycle of a representative species, P. debaryanum, is as follows. A reniform zoospore (flagella arising from the concave surface) encysts and later forms a germ 1. Words in SMALL CAPITALS are cross-references to separate entries. 2. Keys to journal title abbreviations and Book ref. numbers are given at the end of the Dictionary. 3. The Greek alphabet is given in Appendix VI. 4. For further information see ‘Notes for the User’ at the front of the Dictionary. 634 Dictionary of Microbiology and Molecular Biology, Third Edition Paul Singleton and Diana Sainsbury © 2006 John Wiley & Sons Ltd. ISBN: 0-470-03545-5 Q Q (1) See TEMPERATURE COEFFICIENT. (2) QUEUOSINE. (3) Glutamine (see AMINO ACIDS). Q-band (in ESR) See ELECTRON SPIN RESONANCE. Q bases Bases formed by the modification of guanine in tRNA; they contain a pentenyl ring attached (via NH) to the methyl group of N-methylguanine. An example is queuine, the Q base of QUEUOSINE. (See also Y BASES.) Q-cycle (protonmotive Q-cycle) A hypothetical pathway originally proposed (as part of the classical respiratory LOOP MODEL) to account e.g. for proton extrusion at Complex III of the mitochondrial ELECTRON TRANSPORT CHAIN; this pathway requires the presence of oxidized and reduced forms of ubiquinone (UQ and UQH2 , respectively) and the free radical, semiquinone (UQH). In one form of the Q-cycle, an electron which enters the matrix side of Complex III is taken up, together with one proton from the matrix side, by UQH – forming UQH2 . On passing to the cytoplasmic side, UQH2 undergoes oxidation to UQH – one proton being extruded, and one electron passing back to the matrix side via b-type cytochromes; this electron, together with a proton from the matrix side, reduces UQ, thus regenerating UQH at the matrix side. At the cytoplasmic side, the UQH formed from UQH2 undergoes oxidation to regenerate UQ, one proton being extruded, and one electron passing to cyt c1 . [Thermodynamic and kinetic considerations of Q-cycle mechanisms: JBB (1985) 17 51–64.] Q enzyme A BRANCHING ENZYME which can convert amylose into an amylopectin-type polysaccharide; it cannot introduce branches into amylopectin. It occurs in certain algae and in higher plants. Q fever In man, an acute disease caused by Coxiella burnetii (see COXIELLA). Infection may occur e.g. by the inhalation of contaminated dust or the ingestion of contaminated milk. After an incubation period of 2–3 weeks there is a sudden onset with headache, malaise, fever, muscular pain, and (often) respiratory symptoms (pneumonitis); there is no rash. Complications (e.g. endocarditis) may occur but the disease is rarely fatal. Diagnosis may include e.g. serological tests and/or attempts to culture C. burnetii from blood or sputum samples. Tetracyclines and chloramphenicol have been used therapeutically. Reservoirs of infection occur e.g. in sheep and cattle and in argosid and ixodid ticks. [Minireview: JMM (1996) 44 77–78.] [Goat-associated Q fever in Newfoundland: EID (2001) 7 413–419.] QAC See QUATERNARY AMMONIUM COMPOUNDS. Qalyub group See NAIROVIRUS. Qiagen plasmid kit Any of several commercial kits (marketed by Qiagen GmbH, Hilden, Germany) used for isolating plasmids from bacteria (e.g. Escherichia coli ); up to 20 µg of plasmid DNA can be isolated with the ‘mini’ kit, and up to 10 mg can be isolated with larger kits. For E. coli (and other Gram-negative bacteria) the outer membrane is initially disrupted by a Tris–EDTA buffer. Then, controlled lysis of the cytoplasmic membrane with SDS (sodium dodecyl sulphate) permits release of plasmids and soluble proteins etc. (but not chromosomal DNA); contaminating RNA is digested with RNase A. SDS and proteins (but not plasmids) are precipitated in buffer, and a cleared lysate (containing the plasmids) is obtained by centrifugation. The spun lysate is passed through Qiagen anion-exchange resin; the resin is washed to remove contaminants (and to remove any DNA-binding proteins), and the plasmids are eluted with buffer of appropriate pH and ionic strength. Plasmid DNA is precipitated with isopropanol; after centrifugation, the supernatant is discarded and the DNA is washed with 70% ethanol, air-dried, and suspended in buffer. qinghaosu (artemisinine, artemisinin, or arteannuin) An orally administered ANTIMALARIAL AGENT obtained from qinghao (Artemisia annua): a Chinese medicinal herb which has been used in China for at least 2000 years for the treatment of malaria [Chinese Medical Journal (1979) 92 811–816, cited in BMB (1982) 38 197]. Qinghaosu, a sesquiterpene lactone endoperoxide, is structurally distinct from all other known antimalarials. [Structure and chemical synthesis: PAC (1986) 58 817–824.] Qinghaosu is a rapidly acting blood schizonticide. It appears to be taken up selectively by infected erythrocytes [TRSTMH (1984) 78 265–270], and is apparently effective against Plasmodium falciparum (including strains resistant to chloroquine) and P. vivax. In culture, the drug rapidly inhibits protein synthesis and, subsequently, nucleic acid synthesis in P. falciparum. It seems that iron, within the parasite, activates the drug and gives rise to potent free radicals [TRSTMH (1994) 88 (supplement 1) 31–32]. Parenteral derivatives of qinghaosu, artemether and artesunate, have been used successfully for treating severe malaria e.g. in Indochina [BCID (1995) 2 309–330]. QPCR Quantitative PCR. qscR gene See QUORUM SENSING. quadrulus A four-rowed ribbon of cilia which runs, in a spiral, down the buccal cavity in Paramecium. quail disease Ulcerative enteritis in chickens, quail and pheasants; the causal agent is Clostridium colinum [IJSB (1985) 35 155–159]. quail pea mosaic virus See COMOVIRUSES. quailpox virus See AVIPOXVIRUS. Quantiplex See BDNA ASSAY. quarantine (1) The isolation of an individual (usually an animal), e.g. prior to entering a country, in order to determine whether or not that individual is suffering from a particular infectious disease; the period of isolation should be equal to or longer than the longest incubation period of the suspected disease. (From Italian quarantina = 40 days.) (2) The isolation of an individual suffering from an infectious disease in order to prevent the transmission of the disease to others. quarg (quark) A soft, unripened German cheese made from e.g. skim milk in much the same way as is cottage cheese (see CHEESE-MAKING (e)); a B-type LACTIC ACID STARTER and rennin are used to coagulate the milk protein. [AvL (1983) 49 83–84.] quartan malaria See MALARIA. quasi-equivalence See ICOSAHEDRAL SYMMETRY. quasispecies Of a given RNA virus: a population or sample which, as a consequence of the normal rate of replication error, is genetically heterogeneous; the range of genomes present will depend e.g. on the effects of selection. [See JINF (1997) 34 201–203.] quaternary ammonium compounds (QACs; quats; syn. onium compounds) A group of cationic surfactants used as ANTISEPTICS, DISINFECTANTS and PRESERVATIVES; they appear to act by 635 quats Quevedo disease See POD ROT. quinacrine A weakly basic, yellow fluorescent ACRIDINE dye (chromophore: 2-methoxy-6-chloro-9-aminoacridine) which has been used in the chemotherapy of e.g. malaria and giardiasis, and which is used as a chromosome stain. quinapyramine An anti-trypanosomal agent used e.g. for the treatment of dourine. quinghaosu See QINGHAOSU. quinic acid See CHLOROGENIC ACID. quinidine See QUININE. quinine An alkaloid obtained from the bark of the Cinchona tree; the sulphate and hydrochloride are ANTIMALARIAL AGENTS effective against the intraerythrocytic stage (blood schizont) of the parasite (Plasmodium). The diastereoisomer of quinine, quinidine, is used in place of quinine e.g. in the USA; compared to quinine it is less proteinbound but more cardiotoxic. Quinine appears to interfere with the parasite’s detoxification of ferriprotoporphyrin IX (FP). FP, formed when the parasite digests haemoglobin, is able to lyse the parasite unless it is detoxified by polymerization to insoluble granules of pigment (see HAEMOZOIN); such polymerization seems to involve a ‘haem polymerase’ [Nature (1992) 355 108–109], and the quinoline-type drugs (including e.g. quinine) possibly inhibit this enzymic activity. In a photoaffinity labelling experiment, quinine competitively inhibited the binding of a chloroquine analogue to specific proteins (perhaps ‘haem polymerase’?) within parasitized erythrocytes – suggesting the presence of targets common to the quinoline-type antimalarial agents [JBC (1994) 269 6955–6961]. 8-quinolinol Syn. 8-HYDROXYQUINOLINE. quinolobactin A SIDEROPHORE, from Pseudomonas fluorescens, whose synthesis is repressible by pyoverdin [AEM (2000) 66 487–492]. quinolone antibiotics A group of synthetic ANTIBIOTICS whose targets are GYRASE [gyrase-targeted antibiotics: TIM (1997) 5 102–109] and topoisomerase IV; each antibiotic contains a substituted 4-quinolone ring. The original antibiotics in this group – cinoxacin, NALIDIXIC ACID, OXOLINIC ACID and pipemidic acid – are active mainly against Gram-negative bacteria (but not against Pseudomonas aeruginosa); being rapidly excreted in urine, they have been used for treating UTIs caused by members of the Enterobacteriaceae. However, these drugs are not well absorbed when given orally and/or are readily inactivated in the body; they have a limited antibacterial spectrum, and resistance develops readily. Antibiotics subsequently added to the group were fluorinated, piperazinyl-substituted derivatives (so-called fluoroquinolones); they include ciprofloxacin, enoxacin, norfloxacin, ofloxacin and pefloxacin. Compared to the earlier quinolones, these drugs have a wider spectrum of activity (being active against e.g. Pseudomonas aeruginosa and certain Gram-positive cocci, including MRSA), are effective at much lower in vivo concentrations and are more stable in the body; moreover, resistance to these drugs develops less readily. [Comparison of activities of various quinolones: JAC (1985) 16 475–484, 485–490. Laboratory and clinical evaluation of pefloxacin: JAC (1986) 17 (supplement B) 1–118.] The fluoroquinolone danofloxacin has been used e.g. against Mycoplasma bovis in the treatment of CALF PNEUMONIA; little resistance to danofloxacin has developed in M. bovis, although significant levels of resistance have developed against other antiM. bovis antibiotics, including oxytetracycline, spectinomycin and the macrolide tilmicosin [VR (2000) 146 745–747]. disrupting the cytoplasmic membrane and (at high concentrations) by denaturing proteins. A QAC may be regarded as an ammonium halide with four substituents: a long-chain alkyl group (C8 to C18 for high antimicrobial activity) and short-chain alkyl and/or benzyl or heterocyclic groups. QACs are bacteriostatic at low concentrations, bactericidal at high concentrations. They are typically more active against Gram-positive than Gramnegative bacteria, are reported to be fungistatic, trypanocidal, and active against certain viruses, but have little or no activity against Mycobacterium tuberculosis, Pseudomonas aeruginosa and bacterial endospores. They have a DILUTION COEFFICIENT of 1, and are inhibited e.g. by low pH, organic matter (serum, faeces etc), anionic detergents, soaps, phospholipids, and certain cations (e.g. Ca2+ , Mg2+ ) which may compete for sites on the (negatively charged) cell surface. Various substances, (e.g. agar, glass) strongly adsorb QACs. QACs are used e.g. for pre-operative skin cleansing and surgical dressings, and for the disinfection of equipment used in the food and dairy industries. They include e.g. benzalkonium chloride (= Zephiran, a mixture of alkyldimethylbenzylammonium chlorides) – used e.g. as a preservative in ophthalmic solutions; benzethonium chloride (a substituted benzalkonium chloride); Cetrimide (= Cetavlon, a mixture of hexadecyl- (cetyl-), tetradecyl- and dodecyl-trimethylammonium bromides); cetylpyridinium chloride (1-hexadecylpyridinium chloride); CTAB (cetyltrimethylammonium bromide); Domiphen bromide (dodecyldimethyl-2-phenoxyethylammonium bromide). quats QUATERNARY AMMONIUM COMPOUNDS. Quayle cycle Syn. RMP PATHWAY. Queensland tick