Pharmaceutical Microbiology: Glossary, Technical Words, Terms

O Antigen. A polysaccharide antigen extending from the outer membrane of some gram-nega-tive bacterial cell walls ; it is part of the lipopolysaccharide.

Obligate Anaerobes. Microorganisms that cannot tolerate the presence of oxygen and die when exposed to it.

One-step Growth Experiment. An experiment used to study the reproduction of lytic phages in which one round of phage reproduction occurs and ends with the lysis of the host bacterial population.

Open Reading Frame (ORF). A reading frame sequence not interrupted by a stop codon ; it is usually determined by nucleic acid sequencing studies.

Opportunistic Microorganism or Pathogen. A microorganism that is usually free-living or a part of the host’s normal microbiota, but which may become pathogenic under certain circum-stances, such as when the immune system is compromised.

Opsonization. The action of opsonins in making bacteria and other cells more readily phagocytosed. Antibodies, complement (especially C₃b) and fibronectin are potent opsonins.

Optical Tweezer. The use of a focused laser beam to drag and isolate a specific microorganism from a complex microbial mixture.

Organotrophs. Organisms that use reduced organic compounds as their electron source.

Osmophilic Microorganisms. Microorgnisms that grow best in or on media of high solute con-centration.

Osmotolerant. Organisms that grow over a fairly wide range of water activity or solute concen-tration.

Outer Membrane. A special membrane located outside the peptidoglycan layer in the cell walls of Gram-negative bacteria.

Oxidative Burst. The generation of reactive oxygen species, primarily superoxide anion ( O₂^– ) and hydrogen peroxide (H₂O₂) by a plant or an animal, in response to challenge by a potential bacterial, fungal, or viral pathogen.

Oxygenic Photosynthesis. Photosynthesis that oxidizes water to form oxygen ; the form of pho-tosynthesis characteristic of algae and cyanobacteria.

Parasite. An organism that lives on or within another organism (the host) and benefits from the association while harming its host. Often the parasite obtains nutrients from the host.

Parasitism. A type of symbiosis in which one organism benefits from the other and the host is usually harmed.

Parfocal. A microscope that retains proper focus when the objectives are changed.

Pasteur Effect. The decrease in the rate of sugar catobolism and change to aerobic respiration that occurs when microorganisms are switched from anaerobic to aerobic conditions.

Pasteurization. The process of heating milk and other liquids to destroy microorgnisms that can cause spoilage or disease.

Pathogen. Any virus, bacterium, or other agent that causes disease.

Pathogen-Associated Molecular Pattern (PAMP). Conserved molecular structures that occur in patterns on microbial surfaces. The structures and their patterns are unique to particular micro-organisms and invariant among members of a given microbial group.

Pathogenicity. The condition or quality of being pathogenic, or the ability to cause disease.

Pathogenicity Island. A large segment of DNA in some pathogens that contains the genes re-sponsible for virulence ; often it codes for the type III secretion system that allows the pathogen to secrete virulence proteins and damage host cells. A pathogen may have more than one pathogenicity island.

Pathogenic Potential. The degree that a pathogen causes morbid signs and symptoms.

Ped. A natural soil aggregate, formed partly through bacterial and fungal growth in the soil.

Pencillins. A group of antibiotics containing a β-lactam ring, which are active against gram-positive bacteria.

Peptic Ulcer Disease. A gastritis caused by Helicobacter pylori.

Peptidoglycan. A large polymer composed of long chain of alternating N-acetyl-glucosamine and N-acetylmuramic acid residues. The polysaccharide chains are linked to each other through connections between tetrapeptide chains attached to the N-acetylmuramic acids. It provides much of the strength and rigidity possessed by bacterial cell walls.

Peptones. Water-soluble digests or hydrolysates of proteins that are used in the preparation of culture media.

Period of Infectivity. Refers to the time during which the source of an infectious disease is infectious or is disseminating the pathogen.

Periplasmic Space or Periplasm. The space between the plasma membrane and the outer membrane in Gram-negative bacteria, and between the plasma membrane and the cell wall in Gram-positive bacteria.

Pertussis. An acute, highly contagious infection of the respiratory tract, most frequently affect-ing young children, usually caused by Bordetella pertussis or B. parapertussis. Consists of pecu-liar paroxysms of coughing, ending in a prolonged crowing or whooping respiration ; hence the name whooping cough.

Petri Dish. A shallow dish consisting of two round, overlapping halves that is used to grow microorganisms on solid culture medium ; the top is larger than the bottom of the dish to prevent contamination of the culture.

Phase-contrast Microscope. A microscope that converts slight differences in refractive index and cell density into easily observed differences in light intensity.

Phenetic System. A classification system that groups organisms together based on the similarity of their observable characteristics.

Phenol Coefficient Test. A test to measure the effectiveness of disinfectants by comparing their activity against test bacteria with that of phenol.

Photolithotrophic Autorophs. Organisms that use light energy, an inorganic electron source (e.g., H₂O, H₂, H₂S) and CO₂ as a carbon source.

Photoorganotrophic Heterotrophs. Microorganisms that use light energy and organic electron donors, and also employ simple organic molecules rather than CO₂ as their carbon source.

Phototrophs. Organisms that use light as their energy source.

Phycobiliproteins. Photosynthetic pigments that are composed of proteins with attached tetrapyrroles ; they are often found in cyanobacteria and red algae.

Phycobilisomes. Special particles on the membranes of cyanobacteria that contain photosyn-thetic pigments and electron transport chains.

Phylogenetic Tree. A graph made of nodes and branches, much like a tree in shape, that shows phylogenetic relationships between groups of organisms and sometimes also indicates the evolu-tionary development of groups.

Phytoplankton. A community of floating photosynthetic organisms, largely composed of algae and cyanobacteria.

Phytoremediation. The use of plants and their associated microorganisms to remove, contain, or degrade environmental contaminants.

Plankton. Free-floating, mostly microscopic microorganisms that can be found in almost all waters ; a collective name.

Plaque. 1. A clear area in a lawn of bacteria or a localized area of cell destruction in a layer of animal cells that results from the lysis of the bacteria by bacteriophages or the destruction of the animal cells by animal viruses, 2. The term also refers to dental plaque, a film of food debris, polysaccharides, and dead cells that cover the teeth.

Plasmid Fingerprinting. A technique used to identify microbial isolates as belonging to the same strain because they contain the same number of plasmids with the identical molecular weights and similar phenotypes.

Plasmodial (acellular) Slime Mold. A member of the devision Myxomycota that exists as a thin, streaming, multinucleate mass of protoplasm which creeps along in an amoeboid fashion.

Plasmodium (pl. plasmodia). A stage in the life cycle of myxomycetes (plasmodial slime molds) ; a multinucleate mass of protoplasm surrounded by a membrane. Also, a parasite of the genus Plasmodium.

Plastid. A cytoplasmic orgenelle of algae and higher plants that contains pigments such as chlo-rophyll, stores food reserves, and often carries out processes such as photosynthesis.

Pleomorphic. Refers to bacteria that are variable in shape and lack a single, characteristic form.

Poly-β-hydroxybutyrate (PHB). A linear polymer of β-hydroxybutyrate used as a reserve of carbon and energy by many bacteria.

Polymerase Chain Reaction (PCR). An in vitro technique used to synthesize large quantities of specific nucleotide sequences from small amounts of DNA. It employs oligonucleotide primers complementary to specific sequences in the target gene and special heat-stable DNA polymerases.

Porin Proteins. Proteins that form channels across the outer membrane of Gram-negative bacte-rial cell walls. Small molecules are transported through these channels.

Pour Plate. A petri dish of solid culture medium with isolated microbial colonies growing both on its surface and within the medium, which has been prepared by mixing microorganisms with cooled, still liquid medium and then allowing the medium to harden.

Primary (frank) Pathogen. Any organism that causes a disease in the host by direct interaction with or infection of the host.

Primary Metabolites. Microbial metabolites produced during the growth phase of an organism.

Primary Producer. Photoautotrophic and chemoautotrophic organisms that incorporate carbon dioxide into organic carbon and thus provide new biomass for the ecosystem.

Primary Production. The incorporation of carbon dioxide into organic matter by photosyn-thetic organisms and chemoautotrophic bacteria.

Probiotic. A living organism that may provide health benefits beyond its nutritional value when it is ingested.

Procaryotic Cells. Cells that lack a true, membrane-enclosed nucleus ; bacteria are procaryotic and have their genetic material located in a nucleoid.

Procaryotic Species. A collection of strains that share many stable properties and differ signifi-cantly from other groups of strains.

Propagated Epidemic. An epidemic that is characterized by a relatively slow and prolonged rise and then a gradual decline in the number of individuals infected. It usually results from the introduction of an infected individual into a susceptible population, and the pathogen is transmit-ted from person to person.

Prostheca. An extension of a bacterial cell, including the plasma membrane and cell wall, that is narrower than the mature cell.

Protein Engineering. The rational design of proteins by constructing specific amino acid se-quences through molecular techniques, with the objective of modifying protein characteristics.

Proteobacteria. A large group of bacteria, primarily Gram-negative, that 16S rRNA sequence comparisons show to be phylogenetically related ; proteobacteria contain the purple photosyn-thetic bacteria and their relatives and are composed of the α, β, γ, δ and ε subgroups.

Proteome. The complete collection of proteins that an organism produces.

Protists. Eucaryotes with unicellular organization, either in the form of solitary cells or colonies of cells lacking true tissues.

Protoplast. A bacterial or fungal cell with its cell wall completely removed. It is spherical in shape and osmotically sensitive.

Protoplast Fusion. The joining of cells that have had their walls weakened or completely removed.

Prototroph. A microorganism that requires the same nutrients as the majority of naturally occurring members of its species.

Protozoan or Protozoon (pl. Protozoa). A microorganism belonging to the Protozoa subkingdom. A unicellular or acellular eucaryotic protist whose organelles have the functional role of or-gans and tissues in more complex forms. Protozoa vary greatly in size, morphology, nutrition, and life cycle.

Protozoology. The study of protozoa.

Pseudopodium or Pseudopod. A nonpermanent cytoplasmic extension of the cell body by which amoebae and amoeboid organisms move and feed.

Psittacosis (ornithosis). A disease due to a strain of Chlamydia psittaci, first seen in parrots and later found in other birds and domestic fowl (in which it is called ornithosis). It is transmissible to humans.

Psychrophile. A microorganisms that grows well at 0°C and has an optimum growth tempera-ture of 15°C or lower and a temperature maximum around 20°C.

Psychrotroph. A microorganism that grows at 0°C, but has a growth optimum between 20 and 30°C, and a maximum of about 35°C.

Puerperal Fever. An acute, febrile condition following childbirth ; it is characterized by infection of the uterus and/or adjacent regions and is caused by streptococci.

Pulmonary Anthrax. A form of anthrax involving the lungs. Also known as woolsorter’s disease.

Pure Culture. A population of cells that are identical because they arise from a single cell.

Putrefaction. The microbial decomposition of organic matter, especially the anaerobic break-down of proteins, with the production of foul-smelling compounds such as hydrogen sulfide and amines.

Quellung Reaction. The increase in visibility or the swelling of the capsule of a microorganism in the presence of anitbodies against capsular antigens.

Quorum Sensing. The process in which bacteria monitor their own population density by sens-ing the levels of signal molecules that are released by the microorganisms. When these signal molecules reach a threshold concentration, quorum-dependent genes are expressed.

Rabies. An acute infectious disease of the central nervous system, which affects all warmblooded animals (including humans). It is caused by an ssRNA virus belonging to the genus Lyssavirus in the family Rhabdoviridae.

Radappertization. The use of gamma rays from a cobalt source for control of microorganisms in foods.

Radioimmunoassay (RIA). A very sensitive assay technique that uses a purified radioisotope-labeled antigen or antibody to compete for antibody or antigen with unlabeled standard and samples to determine the concentration of a substance in the samples.

Recombinant DNA Technology. The techniques used in carrying out genetic engineering ; they involve the identification and isolation of a specific gene, the insertion of the gene into a vector such as a plasmid to form a recombinant molecule, and the production of large quantities of the gene and its products.

Recombinant-vector Vaccine. The type of vaccine that is produced by the introduction of one or more of a pathogen’s genes into attenuated viruses or bacteria. The attenuated virus or bacte-rium serves as a vector, replicating within the vertebrate host and expressing the gene(s) of the pathogen. The pathogen’s antigens induce an immune response.

Recombination. The process in which a new recombinant chromosome is formed by combin-ing genetic material from two organisms.

Red Tides. Red tides occur frequently in coastal areas and often are associated with population blooms of dinoflagellates. Dinoflagellate pigments are responsible for the red colour of the water. Under these conditions, the dinoflagellates often produce saxitoxin, which can lead to paralytic shellfish poisoning.

Reductive Dehalogenation. The cleavage of carbon-halogen bonds by anaerobic bacteria that creates a strong electron-donating environment.

Regulatory Mutants. Mutant organisms that have lost the ability to limit synthesis of a product, which normally occurs by regulation of activity of an earlier step in the biosynthetic pathway.

Reservoir. A site, alternate host, or carrier that normally harbors pathogenic organisms and serves as a source from which other individuals can be infected.

Reservoir Host. An organism other than a human that is infected with a pathogen that can also infect humans.

Residuesphere. The region surrounding organic matter such as a seed or plant part in which microbial growth is stimulated by increased organic matter availability.

Resolution. The ability of a microscope to separate or distinguish between small objects that are close together.

Restricted Transduction. A transduction process in which only a specific set of bacterial genes are carried to another bacterium by a temperate phage ; the bacterial genes are acquired because of a mistake in the excision of a prophage during the lysogenic life cycle.

Retroviruses. A group of viruses with RNA genomes that carry the enzyme reverse transcriptase and form a DNA copy of their genome during their reproductive cycle.

Ribotyping. Ribotyping is the use of E.coli rRNA to probe chromosomal DNA in Southern blots for typing bacterial strains. This method is based on the fact that rRNA genes are scattered throughout the chromosome of most bacteria and therefore polymorphic restriction endonuclease patterns result when chromosomes are digested and probed with rRNA.

Rocky Mountain Spotted Fever. A disease caused by Rickettsia rickettsii.

Root Nodule. Gall-like structures on roots that contain endosymbiotic nitrogen-fixing bacteria (e.g., Rhizobium or Bradyrhizobium is present in legume nodules).

Run. The straight line movement of a bacterium.

Salmonellosis. An infection with certain species of the genus Salmonella, usually caused by ingestion of food containing salmonellae or their products. Also known as Salmonella gastroenteritis or Salmonella food poisoning.

Sanitization. Reduction of the microbial population on an inanimate object to levels judged safe by public health standards ; usually, the object is cleaned.

Saprophyte. An organism that takes up nonliving organic nutrients in dissolved form and usu-ally grows on decomposing organic matter.

Saprozoic Nutrition. Having the type of nutrition in which organic nutrients are taken up in dissolved form ; normally refers to animals or animal-like organisms.

Scanning Electron Microscope (SEM). An electron microscope that scans a beam of electrons over the surface of a specimen and forms an image of the surface from the electrons that are emitted by it.

Scanning Probe Microscope. A microscope used to study surface features by moving a sharp probe over the object’s surface (e.g., the Scanning Tunneling Microscope).

Secondary Metabolites. Products of metabolism that are synthesized after growth has been completed.

Secondary Treatment. The biological degradation of dissolved organic matter in the process of sewage treatment ; the organic material is either mineralized or changed to settleable solids.

Selective Media. Culture media that favor the growth of specific microorganisms ; this may be accomplished by inhibiting the growth of undesired microorganisms.

Selective Toxicity. The ability of a chemotherapeutic agent to kill or inhibit a microbial patho-gen while damaging the host as little as possible.

Sepsis. Systemic response to infection. The systemic response is manifested by two or more of the following conditions as a result of infection : temperature > 38 or < 36 °C ; heart rate > 90 beats per min ; respiratory rate > 20 breaths per min, or pCO₂ < 32 mm Hg ; leukocyte count > 12,000 cells per ml³ or > 10% immature (band) forms. Sepsis also has been defined as the pres-ence of pathogens or their toxins in blood and other tissues.

Septicemia. A disease associated with the presence in the blood of pathogens or bacterial toxins.

Septic Shock. Sepsis associated with severe hypotension despite adequate fluid resuscitation, along with the presence of perfusion abnormalities that may include, but are not limited to, lactic acidosis, oliguria, or an acute alternation in mental status. Gram-positive bacteria, fungi, and endotoxin-containing Gram-negative bacteria can initiate the pathogenic cascade of sepsis leading to septic shock.

Septum. A partition or crosswall that occurs between two cells in a bacterial (e.g. actinomycete or fungal filament, or which partitions off fungal structures such as spores. Septa also divide parent cells into two daughter cells during bacterial binary fission.

Serotyping. A technique or serological procedure that is used to differentiate between strains (serovars or serotypes) of microorganisms that have differences in the antigenic composition of a structure or product.

Serum (pl. Serums or Sera). The clear, fluid portion of blood lacking both blood cells and fibrinogen. It is the fluid remaining after coagulation of plasma, the noncellular liquid faction of blood.

Serum Resistance. The type of resistance that occurs with bacteria such as Neisseria gonorrhoeae because the pathogen interferes with membrane attack complex formation during the comple-ment cascade.

Settling Basin. A basin used during water purification to chemically precipitate out fine particles, microorganisms, and organic material by coagulation or flocculation.

Sex Pilus. A thin protein appendage required for bacterial mating or conjugation. The cell with sex pili donates DNA to recipient cells.

Sheath. A hollow tubelike structure surroundings a chain of cells and present in several genera of bacteria.

Shigellosis. The diarrheal disease that arises from an infection with Shigella spp. Often called bacillary dysentery.

Shine-Dalgarno Sequence. A segment in the leader of procaryotic mRNA that binds to a special sequence on the 16S rRNA of the small ribosomal subunit. This helps properly orient the mRNA on the ribosome.

Shingles (Herpes Zoster). A reactivated form of chickenpox caused by the varicella-zoster virus.

Signal Peptide. The special amino-terminal sequence on a peptide destined for transport that delays protein folding and is recognized in bacteria by the Sec-dependent pathway machinery.

Silent Mutation. A mutation that does not result in a change in the organism’s proteins or pheno-type even though the DNA base sequence has been changed.

Simple Matching Coefficient (S_SM). An association coefficient used in numerical taxonomy ; the proportion of characters that match regardless of whether or not the attribute is present.

Site-specific Recombination. Recombination of nonhomologous genetic material with a chromosome at a specific site.

S-layer. A regularly structure layer composed of protein or glycoprotein that lies on the surface of many bacteria. It may protect the bacterium and help give it shape and rigidity.

Slime. The viscous extracellular glycoproteins or glycolipids produced by staphylococci and Pseudomonas aeruginosa bacteria that allows them to adhere to smooth surfaces such as pros-thetic medical devices and catheters. More generally, ther term often refers to an easily removed, diffuse, unorganized layer of extracellular material that surrounds a bacterial cell.

Slime Layer. A layer of diffuse, unorganized, easily removed material lying outside the bacte-rial cell wall.

Slow Sand Filter. A bed of sand through which water slowly flows ; the gelatinous microbial layer on the sand grain surface removes waterborne microorganisms, particularly Giardia, by adhesion to the gel. This type of filter is used in some water purification plants.

Sorocarp. The fruiting structure of the Acrasiomycetes.

Sorus. A type of fruiting structure composed of a mass of spores or sporangia.

Source. The location or object from which a pathogen is immediately transmitted to the host, either directly or through an intermediate agent.

Species. Species of higher organisms are groups of interbreeding or potentially interbreeding natural populations that are reproductively isolated. Bacterial species are collections of strains that have many stable properties in common and differ significantly from other groups of strains.

Spheroplast. A relatively spherical cell formed by the weakening or partial removal of the rigid cell wall component (e.g., by pencillin treatment of Gram-negative bacteria). Spheroplasts are usually osmotically sensitive.

Spirillum. A rigid, spiral-shaped bacterium.

Spirochete. A flexible, spiral-shaped bacterium with periplasmic flagella.

Spore. A differentiated, specialized form that can be used for dissemination, for survival of adverse conditions because of its heat and dessication resistance, and/or for reproduction. Spores are usually unicellular and may develop into vegetative organisms or gametes. They may be produced asexually or sexually and are of many types.

Sporulation. The process of spore formation.

Spread Plate. A petri dish of solid culture medium with isolated microbial colonies growing on its surface, which has been prepared by spreading a dilute microbial suspension evenly over the agar surface.

Stalk. A nonliving bacterial appendage produced by the cell and extending from it.

Staphylococcal Food Poisoning. A type of food poisoning caused by ingestion of improperly stored or cooked food in which Staphylococcus aureus has grown. The bacteria produce exotoxins that accumulate in the food.

Staphylococcal Scalded Skin Syndrome (SSSS). A disease caused by staphylococci that pro-duce an exfoliative toxin. The skin becomes red (erythema) and sheets of epidermic may sepa-rate from the underlying tissue.

Starter Culture. An inoculum, consisting of a mixture of carefull selected microorganisms, used to start a commercial fermentation.

Stationary Phase. The phase of microbial growth in a batch culture when population growth ceases and the growth curve levels off.

Stem-nodulating Rhizobia. Rhizobia (members of the genera Rhizobium, Bradyrhizobium, and Azorhizobium) that produce nitrogen-fixing structures above the soil surface on plant stems. These most often are observed in tropical plants and produced by Azorhizobium.

Sterilization. The process by which all living cells, viable spores, viruses, and viroids are either destroyed or removed from an objector habitat.

Strain. A population of organisms that descends from a single organism or pure culture iso-late.

Streak Plate. A petri dish of solid culture medium with isolated microbial colonies growing on its surface, which has been prepared by spreading a microbial mixture over the agar surface, using an inoculating loop.

Streptococcal Pneumonia. A endogenous infection of the lungs caused by Streptococcus pneumoniae that occurs in predisposed individuals.

Streptococcal Sore Throat. One of the most common bacterial infections of humans. It is com-monly referred to as “strep throat”. The disease is spread by droplets of saliva or nasal secre-tions and is caused by Streptococcus spp. (particularly group A streptococci).

Streptolysin-O (SLO). A specific hemolysin produced by Streptococcus pyogenes that is inacti-vated by oxygen (hence the “O” in its name). SLO casuses beta-hemolysis of blood cells on agar plates incubated anaerobically.

Streptolysin-S (SLS). A product of Streptococcus pyogenes that is bound to the bacterial cell but may sometimes be released. SLS causes beta hemolysis on aerobically incubated blood-agar plates and can act as a leukocidin by killing leukocytes that phogocytose the bacterial cell to which it is bound.

Stromatolite. Dome-like microbial mat communities consisting of filamentous photosynthetic bacteria and occluded sediments (often calcareous or siliceous). They usually have a laminar structure. Many are fossilized, but some modern forms occur.

Superinfection. A new bacterial or fungal infection of a patient that is resistant to the drug(s) being used for treatment.

Swab. A wad of absorbent material usually wound around one end of a small stick and used for applying medication or for removing material from an area ; also, a dacron-tipped polystyrene applicator.

Symbiosis. The living together or close association of two dissimilar organisms, each of these organisms being known as a symbiont.

Syntrophism. The association in which the growth of one organism either depends on, or is improved by, the provision of one or more growth factors or nutrients by a neighboring organism. Sometimes both organisms benefit.

Systematic Epidemiology. The field of epidemiology that focuses on the ecological and social factors that influence the development of emerging and reemerging infectious disease.

Systematics. The scientific study of organisms with the ultimate objective being to characterize and arrange them in an orderly manner ; often considered synonymous with taxonomy.

Taxon. A group into which related organisms are classified.

Taxonomy. The science of biological classification; it consists of three parts : classification, nomenclature and identification.

T Cell or T Lymphocyte. A type of lymphocyte derived from bone marrow stem cells that matures into an immunologically competent cell under the influence of the thymus. T cells are involved in a variety of cell-mediated immune reactions.

T-Cell Antigen Receptor (TCR). The receptor on the T cell surface consisting of two antigen-binding peptide chains ; it is associated with a large number of other glycoproteins. Binding of antigen to the TCR, usually in association with MHC, activates the T cell.

Teichoic Acids. Polymers of glycerol or ribitol joined by phosphates ; they are found in the cell walls of Gram-positive bacteria.

Temperate Phages. Bacteriophages that can infect bacteria and establish a lysogenic relation-ship rather than immediately lysing their hosts.

Tetanolysin. A hemolysin that aids in tissue destruction and is produced by Clostridium tetani.

Tetrapartite Associations. A symbiotic association of the same plant with three different types of microorganisms.

Theory. A set of principles and concepts that have survived rigorous testing and that provide a systematic account of some aspects of nature.

Thermal Death Time (TDT). The shortest period of time needed to kill all the organisms in a microbial population at a specified temperature and under defined conditions.

Thermoacidophiles. A group of bacteria that grow best at acid pHs and high temperatures ; they are members of the Archaea.

Thermophile. A microorganism that can grow at temperatures of 55°C or higher ; the mini-mum is usually around 45°C.

Thrush. Infection of the oral mucous membrane by the fungus Candila albicans ; also known as oral candidiasis.

Toxigenicity. The capacity of an organism to produce a toxin.

Toxin. A microbial product or component that injures another cell or organism. Often the term refers to a poisonous protein, but toxins may be lipids and other substances.

Transformation. A mode of gene transfer in bacteria in which a piece of free DNA is taken up by a bacterial cell and integrated into the recipient genome.

Transgenic Animal or Plant. An animal or plant that has gained new genetic information from the insertion of foreign DNA. It may be produced by such techniques as injecting DNA into animal eggs, electroporation of mammalian cells and plant cell protoplasts, or shooting DNA into plants cells with a gene gun.

Transmission Electron Microscope (TEM). A microscope in which an image is formed by passing an electron beam through a specimen and focusing the scattered electrons with magnetic lenses.

Transovarian Passage. The passage of a microorganisms such as a rickettsia from generation to generation of hosts through tick eggs. (No humans or other mammals are needed as reservoirs for continued propagation.)

Traveller’s Diarrhoea. A type of diarrhoea resulting from ingestion of viruses, bacteria, or protozoa normally absent from the traveller’s environment. A major pathogen is enterotoxigenic Escherichia coli.

Trichomoniasis. A sexually transmitted disease caused by the parasitic protozoan Trichomonas vaginalis.

Tripartite Associations. A symbiotic association of the same plant with two types of microor-ganisms.

Trophozoite. The active, motile feeding stage of a protozoan organism ; in the malarial parasite, the stage of schizogony between the ring stage and the schizont.

Tropism. The movement of living organisms toward or away from a focus of heat, light, or other stimulus.

Tubercle. A small, rounded nodular lesion produced by Mycobacterium tuberculosis.

Tuberculoid (neural) Leprosy. A mild, nonprogressive form of leprosy that is associated with delayed-type hypersensitivity to antigens on the surface of Mycobacterium leprae. It is charac-terized by early nerve damage and regions of the skin that have lost sensation and are surrounded by a border of nodules.

Tuberculosis (TB). An infectious disease of humans and other animals resulting from an infec-tion by a species of Mycobacterium and characterized by the formation of tubercles and tissue necrosis, primarily as a result of host hypersensitivity and inflammation. Infection is usually by inhalation, and the disease commonly affects the lungs (pulmonary tuberculosis), although it may occur in any part of the body.

Tularemia. A plaguelike disease of animals caused by the bacterium Francisella tularensis subsp. tularensis (Jellison type A), which may be transmitted to humans.

Tumble. Random turning or tumbling movements made by bacteria when they stop moving in a striaght line.

Turbidostat. A continuous culture system equipped with a photocell that adjusts the flow of medium through the culture vessel so as to maintain a constant cell density or turbidity.

Ultramicrobacteria. Bacteria that can exist normally in a miniaturized form or which are capa-ble of miniaturization under low-nutrient conditions. They may be 0.2 μm or smaller in diameter.

Ultraviolet (UV) Radiation. Radiation of fairly short wavelength, about 10 to 400 nm, and high energy.

Vector-borne Transmission. The transmission of an infectious pathogen between hosts by means of a vector.

Vehicle. An inanimate substance or medium that transmits a pathogen.

Vibrio. A rod-shaped bacterial cell that is curved to form a comma or an incomplete spiral.

Virology. The branch of microbiology that is concerned with viruses and viral diseases.

Virulence. The degree or intensity of pathogenicity of an organism as indicated by case fatality rates and/or ability to invade host tissues and cause disease.

Virulence Factor. A bacterial product, usually a protein or carbohydrate, that contributes to virulence or pathogenicity.

Virus. An infectious agent having a simple acellular oganization with a protein coat and a single type of nucleic acid, lacking independent metabolism and reproducing only within living host cells.

Vitamin. An organic compound required by organisms in minute quantities for growth and re-production because it cannot be synthesized by the organism ; vitamins often serve as enzyme cofactors or parts of cofactors.

Whole-genome Shotgun Sequencing. An approach to genome sequencing in which the com-plete genome is broken into random fragments, which are then individually sequenced. Finallly the fragments are placed in the proper order using sophisticated computer programs.

Whole-organism Vaccine. A vaccine made from complete pathogens, which can be of four types : inactivated viruses ; attenuated viruses ; killed microorganisms ; and live, attenuated microbes.

Widal Test. A test involving agglutination of typhoid bacilli when they are mixed with serum containing typhoid antibodies from an individual having typhoid fever ; used to detect the presence of Salmonella typhi and S. paratyphi.

Winogradsky Column. A glass column with an anaerobic lower zone and an aerobic upper zone, which allows growth of microorganisms under conditions similar to those found in a nutri-ent-rich lake.

Xenograft. A tissue graft between animals of different species.

Xerophilic Microorganisms. Microorganisms that grow best under low a_w conditions, and may not be able to grow at high a_w values.

Yellow Fever. An acute infectious disease caused by a flavivirus, which is transmitted to hu-mans by mosquitoes. The liver is affected and the skin turns yellow in this disease.

YM Shift. The change in shape by dimorphic fungi when they shift from the yeast (Y) form in the animal body to the mold or mycelial form (M) in the environment.