AB toxins The structure and activity of many exotoxins are based on the AB model. In this model, the B portion of the toxin is responsible for toxin binding to a cell but does not directly harm it. The A portion enters the cell and disrupts its function. accessory pigments Photosynthetic pigments such as carotenoids and phycobiliproteins that aid chlorophyll in trapping light energy. acetyl coenzyme A (acetyl-CoA) A combination of acetic acid and coenzyme A that is energy rich; it is produced by many catabolic pathways and is the substrate for the tricarboxylic acid cycle, fatty acid biosynthesis, and other pathways. acid dyes Dyes that are anionic or have negatively charged groups such as carboxyls. acid fast Refers to bacteria like the mycobacteria that cannot be easily decolorized with acid alcohol after being stained with dyes such as basic fuchsin. acid-fast staining A staining procedure that differentiates between bacteria based on their ability to retain a dye when washed with an acid alcohol solution. acidophile A microorganism that has its growth optimum between about pH 0 and 5.5. acquired enamel pellicle A membranous layer on the tooth enamel surface formed by selectively adsorbing glycoproteins (mucins) from saliva. This pellicle confers a net negative charge to the tooth surface. acquired immune deficiency syndrome (AIDS) An infectious disease syndrome caused by the human immunodeficiency virus and is characterized by the loss of a normal immune response, followed by increased susceptibility to opportunistic infections and an increased risk of some cancers. acquired immune tolerance The ability to produce antibodies against nonself antigens while "tolerating" (not producing antibodies against) self-antigens. acquired immunity Refers to the type of specific immunity that develops after exposure to a suitable antigen or is produced after antibodies are transferred from one individual to another. actinobacteria A group of gram-positive bacteria containing the actinomycetes and their high G 1 C relatives. actinomycete An aerobic, gram-positive bacterium that forms branching filaments (hyphae) and asexual spores. actinorhizae Associations between actinomycetes and plant roots. activated sludge (sluj) Solid matter or sediment composed of actively growing microorganisms that participate in the aerobic portion of a biological sewage treatment process. The microbes readily use dissolved organic substrates and transform them into additional microbial cells and carbon dioxide. activation energy The energy required to bring reacting molecules together to reach the transition state in a chemical reaction. active carrier An individual who has an overt clinical case of a disease and who can transmit the infection to others. active immunization The induction of active immunity by natural exposure to a pathogen or by vaccination. active site The part of an enzyme that binds the substrate to form an enzyme-substrate complex and catalyze the reaction. Also called the catalytic site. active transport The transport of solute molecules across a membrane against an electrochemical gradient; it requires a carrier protein and the input of energy. acute carrier See casual carrier. acute infections Virus infections with a fairly rapid onset that last for a relatively short time. acute viral gastroenteritis An inflammation of the stomach and intestines, normally caused by Norwalk and Norwalklike viruses, other caliciviruses, rotaviruses, and astroviruses. acyclovir A synthetic purine nucleoside derivative with antiviral activity against herpes simplex virus. adenine A purine derivative, 6-aminopurine, found in nucleosides, nucleotides, coenzymes, and nucleic acids. adenosine diphosphate The nucleoside diphosphate usually formed upon the breakdown of ATP when it provides energy for work. adenosine 5_-triphosphate (ATP) The triphosphate of the nucleoside adenosine, which is a high energy molecule or has high phosphate group transfer potential and serves as the cell's major form of energy currency. adhesin (ad-he_zin) A molecular component on the surface of a microorganism that is involved in adhesion to a substratum or cell. Adhesion to a specific host tissue usually is a preliminary stage in pathogenesis, and adhesins are important virulence factors. adjuvant Material added to an antigen to increase its immunogenicity. Common examples are alum, killed Bordetella pertussis, and an oil emulsion of the antigen, either alone (Freund's incomplete adjuvant) or with killed mycobacteria (Freund's complete adjuvant) adult T-cell leukemia A type of white blood cell cancer caused by the HTLV-1 virus. aerobe An organism that grows in the presence of atmospheric oxygen. aerobic anoxygenic photosynthesis Photosynthetic process in which electron donors such as organic matter or sulfide, which do not result in oxygen evolution, are used under aerobic conditions. aerobic respiration A metabolic process in which molecules, often organic, are oxidized with oxygen as the final electron acceptor. 154, aerotolerant anaerobes Microorganisms that grow equally well whether or not oxygen is present. aflatoxin A polyketide secondary fungal metabolite that can cause cancer. agar (ahg_ar) A complex sulfated polysaccharide, usually extracted from red algae, that is used as a solidifying agent in the preparation of culture media. agglutinates The visible aggregates or clumps formed by an agglutination reaction. agglutination reaction The formation of an insoluble immune complex by the cross-linking of cells or particles. agglutinin (ah-gloo²tùõ-nin) The antibody responsible for an agglutination reaction. AIDS See acquired immune deficiency syndrome. AIDS-related complex (ARC) A collection of symptoms such as lymphadenopathy (swollen lymph glands), fever, malaise, fatigue, loss of appetite, and weight loss. It results from an HIV infection and may progress to frank AIDS. airborne transmission The type of infectious organism transmission in which the pathogen is truly suspended in the air and travels over a meter or more from the source to the host. akinetes Specialized, nonmotile, dormant, thick-walled resting cells formed by some cyanobacteria. alcoholic fermentation A fermentation process that produces ethanol and CO2 from sugars. alga (al_gah) A common term for a series of unrelated groups of photosynthetic eucaryotic microorganisms lacking multicellular sex organs (except for the charophytes) and conducting vessels. algicide (al_jùõ-søõd) An agent that kills algae. algology (al-gol_o-je) The scientific study of algae. alkalophile A microorganism that grows best at pHs from about 8.5 to 11.5. allergen (al_er-jen) A substance capable of inducing allergy or specific susceptibility. allergic contact dermatitis An allergic reaction caused by haptens that combine with proteins in the skin to form the allergen that produces the immune response. allergy (al_er-je) See hypersensitivity. allograft (al_o-graft) A transplant between genetically different individuals of the same species. allosteric enzyme (al_o-ster_ik) An enzyme whose activity is altered by the binding of a small effector or modulator molecule at a regulatory site separate from the catalytic site; effector binding causes a conformational change in the enzyme and its catalytic site, which leads to enzyme activation or inhibition. allotype Allelic variants of antigenic determinant(s) found on antibody chains of some, but not all, members of a species, which are inherited as simple Mendelian traits. alpha hemolysis A greenish zone of partial clearing around a bacterial colony growing on blood agar. alpha-proteobacteria One of the five subgroups of proteobacteria, each with distinctive 16S rRNA sequences. This group contains most of the oligotrophic proteobacteria; some have unusual metabolic modes such as methylotrophy, chemolithotrophy, and nitrogen fixing ability. Many have distinctive morphological features. alternative complement pathway An antibody-independent pathway of complement activation that includes the C3-C9 components of the classical pathway and several other serum protein factors (e.g., factor B and properdin). alveolar macrophage A vigorously phagocytic macrophage located on the epithelial surface of the lung alveoli where it ingests inhaled particulate matter and microorganisms. amantadine (ah-man_tah-den) An antiviral compound used to prevent type A influenza infections. amebiasis (amebic dysentery) (am²e-bi_ah-sis) An infection with amoebae, often resulting in dysentery; usually it refers to an infection by Entamoeba histolytica. amensalism A relationship in which the product of one organism has a negative effect on another organism American trypanosomiasis (Chagas' disease) See trypanosomiasis. Ames test A test that uses a special Salmonella strain to test chemicals for mutagenicity and potential carcinogenicity. amino acid activation The initial stage of protein synthesis in which amino acids are attached to transfer RNA molecules. aminoacyl or acceptor site (A site) The site on the ribosome that contains an aminoacyl-tRNA at the beginning of the elongation cycle during protein synthesis; the growing peptide chain is transferred to the aminoacyl-tRNA and lengthens by an amino acid. aminoglycoside antibiotics A group of antibiotics synthesized by Streptomyces and Micromonospora, which contain a cyclohexane ring and amino sugars; all aminoglycoside antibiotics bind to the small ribosomal subunit and inhibit protein synthesis. amnesic shellfish poisoning The disease arising in humans and animals