Pharmaceutical Microbiology: Glossary, Technical Words, Terms

Eclipse Period. The initial part of the latent period in which infected host bacteria do not contain any complete virions.

Effacing Lesion. The type of lesion caused by enteropathogenic strains of E. coli (EPEC) when the bacteria destroy the brush border of intestinal epithelial cells. The term AE (attaching-effac-ing) E. coli is now used to designate true EPEC strains that are an important cause of diarrhea in children from developing countries and in traveler’s diarrhoea.

Ehrlichiosis. A tick-borne (Dermacentor andersoni, Amblyomma americanum) rickettsial disease caused by Ehrlichia chaffeensis. Once inside leukocytes, a nonspecific illness develops that resembles Rocky Mountain spotted fever.

Endogenous Infection. An infection by a member of an individual’s own normal body microbiota.

Endosymbiont. An organism that lives within the body of another organism in a symbiotic asso-ciation.

Endosymbiosis. A type of symbiosis in which one organism is found within another organism.

Endosymbiotic Theory or Hypothesis. The theory that eucaryotic organelles such as mitochondria and chloroplasts arose when bacteria established an endosymbiotic relationship with the eucaryotic ancestor and then evolved into organelles.

Enteric Bacteria (enterobacteria). Members of the family Enterobacteriaceae (Gram-nega-tive, peritrichous or nonmotile, facultatively anaerobic, straight rods with simple nutritional re-quirements); also used for bacteria that live in the intestinal tract.

Enterohemorrhagic E. coli (EHEC). EHEC strains of E. coli (O157:H7) produce several cytotoxins that provoke fluids secretion in traveller’s diarrhea; however, their mode of action is unknown.

Enterionvasive E. coli (EIEC). EIEC strains of E. coli cause traveller’s diarrhoea by penetrat-ing and binding to the intestinal epithelial cells, EIEC may also produce a cytotoxin and enterotoxin.

Enteropathogenic E. coli (EPEC). EPEC strains of E. coli attach to the brush border of intes-tinal epithelial cells and cause a specific type of cell damage called effacing lesions that lead to traveller’s diarrhea.

Enterotoxigenic E. coli (ETEC). ETEC strains of E. coli produce two plasmid-encoded enterotoxins (which are responsible for traveller’s diarrhea) and the distinguished by their heat stability : heat-stable enterotoxin (ST) and heat-labile enterotoxin (LT).

Epidemic (louse-borne) Typhus. A disease caused by Rickettsia prowazekii that is transmitted from person to person by the body louse.

Epsilon-proteobacteria. One of the five subgroups of proteobacteria, each with distinctive 16S rRNA sequences. Slender Gram-negative rods, some of which are medically important (Campylobacter and Helicobacter).

Ergot. The dried sclerotium of Claviceps purpurea. Also, an ascomycete that parasitizes rye and other higher plants causing the disease called ergotism.

Ergotism. The disease or toxic condition caused by eating grain infected with ergot ; it is often accompanied by gangrene, psychotic delusions, nervous spasms, abortion, and convulsions in hu-mans and in animals.

Eucarya. The domain that contains organisms composed of eucaryotic cells with primarily glycerol fatty acyl diesters in their membranes and eucaryotic rRNA.

Excystation. The escape of one or more cells or organisms from a cyst.

Exergonic reaction. A reaction that spontaneously goes to completion as written ; the standard free energy change is negative, and the equilibrium constant is greater than one.

Exogenote. The piece of donor DNA that enters a bacterial cell during gene exchange and recom-bination.

Exotoxin. A heat-labile, toxic protein produced by a bacterium as a result of its normal me-tabolism or because of the acquisition of a plasmid or prophage. It is usually released into the bacterium’s surroundings.

Exponential Phase. The phase of the growth curve during which the microbial population is growing at a constant and maximum rate, dividing and doubling at regular intervals.

Extracutaneous Sporotrichosis. An infection by the fungus Sporothrix schenckii that spreads throughout the body.

Extreme Barophilic Bacteria. Bacteria that require a high-pressure environment to function.

Extreme Environment. An environment in which physical factors such as temperature, pH, salin-ity, and pressure are outside of the normal range for growth of most microorganisms ; these condi-tions allow unique organisms to survive and function.

Extremophiles. Microorganisms that grow under harsh or extreme environmental conditions such as very high temperatures or low pHs.

Extrinsic Factor. An environmental factor such as temperature that influences microbial growth in food.

Facultative Anaerobes. Microorganisms that do not require oxygen for growth, but do grow better in its presence.

Fecal Coliform. Coliforms whose normal habitat is the intestinal tract and that can grow at 44.5°C.

Fecal Enterococci. Enterococci found in the intestine of humans and other warm-blooded ani-mals. They are used as indicators of the fecal pollution of water.

Fimbria (fimbriae). A fine, hairlike protein appendage on some gram-negative bacteria that helps attach them to surfaces.

Flagellin. The protein used to construct the filament of a bacterial flagellum.

Flagellum (flagella). A thin, threadlike appendage on many prokaryotic and eukaryotic cells that is responsible for their motility.

Fluorescence Microscope. A microscope that exposes a specimen to light of a specific wave-length and then forms an image from the fluorescent light produced. Usually the specimen is stained with a fluorescent dye or fluorochrome.

Fomite (fomites). An object that is not in itself harmful but is able to harbor and transmit patho-genic organisms. Also called fomes.

Food-borne Infection. Gastrointestinal illness caused by ingestion of microorganisms, fol-lowed by their growth within the host. Symptoms arise from tissue invasion and/or toxin produc-tion.

Food Web. A network of many interlinked food chains, encompassing primary producers, con-sumers, decomposers, and detritivores.

Gamma-proteobacteria. One of the five sub-groups of proteobacteria, each with distinctive 16S rRNA sequences. This is the largest subgroup and is very diverse physiologically ; many important genera are facultatively anaerobic chemoorganotrophs.

Gas Gangrene. A type of gangrene that arises from dirty, lacerated wounds infected by anaerobic bacteria, especially species of Clostridium. As the bacteria grow, they release toxins and fer-ment carbohydrates to produce carbon dioxide and hydrogen gas.

Gastroenteritis. An acute inflammation of the lining of the stomach and intestines, characterized by anorexia, nausea, diarrhea, abdominal pain, and weakness. It has various causes including food poisoning due to such organisms as E. coli, S. aureus, Campylobacter (campy-lobacteriosis), and Salmonella species ; consumption of irritating food or drink ; or psychological factors such as anger, stress, and fear. Also called enterogastritis.

Gas Vacuole. A gas-filled vacuole found in cyanobacteria and some other aquatic bacteria that provides flotation. It is composed of gas vesicles, which are made of protein.

Generalized Transduction. The transfer of any part of a bacterial genome when the DNA frag-ment is packaged within a phage capsid by mistake.

General Recombination. Recombination involving a reciprocal exchange of a pair of homolo-gous DNA sequences ; it can occur any place on the chromosome.

Generation Time. The time required for a microbial population to double in number.

Genetic Engineering. The deliberate modification of an organism’s genetic information by directly changing its nucleic acid genome.

Genital Herpes. A sexually transmitted disease caused by the herpes simplex virus type 2.

Germicide. An agent that kills pathogens and many nonpathogens but not necessarily bacterial endospores.

Giardiasis. A common intestinal disease caused by the parasitic protozoan Giardia lamblia.

Glycocalyx. A network of polysaccharides extending from the surface of bacteria and other cells.

Gnotobiotic. Animals that are germfree (microorganisms free) or live in association with one or more known microorganisms.

Gonococci. Bacteria of the species Neisseria gonorrhoeae—the organism causing gonorrhea.

Gonorrhea. An acute infectious sexually transmitted disease of the mucous membranes of the genitourinary tract, eye, rectum, and throat. It is caused by Neisseria gonorrhoeae.

Gram Stain. A differential staining procedure that divides bacteria into Gram-positive and Gram-negative groups based on their ability to retain crystal violet when decolorized with an organic solvent such as ethanol.

Greenhouse Gases. Gases released from the Earth’s surface through chemical and biological processes that interact with the chemicals in the stratosphere to decrease the release of radiation from the Earth. It is believed that this leads to global warming.

Guillain-Barré Syndrome. A relatively rare disease affecting the peripheral nervous system, especially the spinal nerves, but also the cranial nerves. The cause is unknown, but it most often occurs after an influenza infection or flu vaccination. Also called French Polio.

Halophile. A microorganism that requires high levels of sodium chloride for growth.

Harborage Transmission. The mode of transmission in which an infectious organism does not undergo morphological or physiological changes within the vector.

Healthy Carrier. An individual who harbors a pathogen, but is not ill.

Hemolysis. The disruption of red blood cells and release of their hemoglobin. There are several types of hemolysis when bacteria such as streptococci and staphylococci, grow on blood agar. In α-hemolysis, a narrow greenish zone of incomplete hemolysis forms around the colony. A clear zone of complete hemolysis without any obvious colour change is formed during β-hemolysis.

Hemolytic Uremic Syndrome. A kidney disease characterized by blood in the urine and often by kidney failure. It is caused by enterohemorrhagic strains of Escherichia coli O157 : H7 that produce a Shiga-like toxin, which attacks the kidneys.

Hepatitis A. (formerly infectious hepatitis). A type of hepatitis that is transmitted by fecal - oral contamination ; it primarily affects children and young adults, especially in environments where there is poor sanitation and overcrowding. It is caused by the hepatitis A virus, a single-stranded RNA virus.

Hepatitis B. (formely serum hepatitis). This form of hepatitis is caused by a double-stranded DNA virus (HBV) formerly called the “DNA particle”. The virus is transmitted by body fluids.

Hepatitis C. About 90% of all cases of viral hepatitis can be traced to either HAV or HBV. The remaining 10% is believed to be caused by one and possibly several other types of viruses. At least one of these is hepatitis C (formerly non-A, non-B).

Hepatitis D (formerly delta hepatitis). The liver diseases caused by the hepatitis D virus in those individuals already infected with the hepatitis B virus.

Hepatitis E (formerly enteric-transmitted NANB hepatitis). The liver disease caused by the hepatitis E virus. Usually, a subclinical, acute infection results, however, there is a high mortality in women in their last trimester of pregnancy.

Heterolactic Fermenters. Microorganisms that ferment sugars to form lactate, and also other products such as ethanol and CO₂.

Heterotroph. An organism that uses reduced, preformed organic molecules as its principal car-bon source.

Heterotrophic Nitrification. Nitrification carried out by chemoheterotrophic microorganisms.

Hfr strain. A bacterial strain that denotes its genes with high frequency to a recipient cell during conjugation because the F factor is integrated into the bacterial chromosome.

High Oxygen Diffusion Environment. A microbial environment in close contact with air and through which oxygen can move at a rapid rate (in comparison wtih the slow diffusion rate of oxygen through water).

Holdfast. A structure produced by some bacteria and algae that attaches them to a solid object.

Holozoic Nutrition. In this type of nutrition, nutrients (such as bacteria) are acquired by phagocytosis and the subsequent formation of a food vacuole or phagosome.

Homolactic Fermenters. Organisms that ferment sugars almost completely to lactic acid.

Host. The body of an organism that harbors another organism. It can be viewed as a microenviron-ment that shelters and supports the growth and multiplication of a parasitic organism.

Host Restriction. The degradation of foreign genetic material by nucleases after the genetic material enters a host cell.

Human Immunodeficiency Virus (HIV). A lentivirus of the family, Retroviridae that is asso-ciated with the onset of AIDS.

Hypermutation. A rapid production of multiple mutations in a gene or genes through the activa-tion of special mutator genes. The process may be deliberately used to maximize the possibility of creating desirable mutants.

Hyperthermophile. A bacterium that has its growth optimum between 80°C and about 113°C. Hyperthermophiles usually do not grow well below 55°C.

Hypha (hyphae). The unit of structure of most fungi and some bacteria ; a tubular filament.

Identification. The process of determining that a particular, isolate or organism belongs to a recognized taxon.

Immobilization. The incorporation of a simple, soluble substance into the body of an organism, making it unavailable for use by other organisms.

Inclusion Bodies. Granules of organic or inorganic material lying in the cytoplasmic matrix of bacteria.

Inclusion Conjunctivitis. An infectitious disease that occurs worldwide. It is caused by Chlamydia trachomatis that infects the eye and causes inflammation and the occurrence of large inclusion bodies.

Incubation Period. The period after pathogen entry into a host and before signs and symptoms appear.

Incubatory Carrier. An individual who is incubating a pathogen but is not yet ill.

Indicator Organism. An organism whose presence indicates the condition of a substance or envi-ronment, for example, the potential presence of pathogens. Coliforms are used as indicators of fecal pollution.

Infection. The invasion of a host by a microorganism with subsequent establishment and multipli-cation of the agent. An infection may or may not lead to overt disease.

Infection Thread. A tubular structure formed during the infection of a root by nitrogen-fixing bacteria. The bacteria enter the root by way of the infection thread and stimulate the formation of the root nodule.

Infectious Disease Cycle (Chain of Infection). The chain or cycle of events that describes how an infectious organism grows, reproduces, and is disseminated.

Infectious Dose 50 (ID₅₀). Refers to the dose or number of organisms that will infect 50% of an experimental group of hosts within a specified time period.

Infectivity. Infectiousness ; the state or quality of being infectious or communicable.

Integration. The incorporation of one DNA segment into a second DNA molecule to form a new hybrid DNA. Integration occurs during such processes as genetic recombination, episome incorpo-ration into host DNA, and prophage insertion into the bacterial chromosome.

Integrins. A large family of α/β heterodimers. Integrins are cellular adhesion receptors that mediate cell-cell and cell-substratum interactions. Integrins usually recognize linear amino acid sequences on protein ligands.

Integron. A genetic element with an attachment site for site-specific recombination and an integrase gene. It can capture genes and gene cassettes.

Intercalating Agents. Molecules that can be inserted between the stacked bases of a DNA double helix, thereby distorting the DNA and including insertion and deletion mutations.

Interferon (IFN). A glycoprotein that has nonspecific antiviral activity by stimulating cells to produce antiviral proteins, which inhibit the synthesis of viral RNA and proteins. Interferons also regulate the growth, differentiation, and/or function of a variety of immune system cells. Their production may be stimulated by virus infections, intracellular pathogens (chlamydiae and rickettsias), protozoan parasites, endotoxins, and other agents.

Interleukin. A glycoprotein produced by macrophages and T cells that regulates growth and dif-ferentiation, particularly of lymphocytes. Interleukins promote cellular and humoral immune responses.

Intermediate Filaments. Small protein filaments about 8 to 10 nm in diameter, in the cytoplas-mic matrix of eucaryotic cells that are important in cell structure.

Interspecies Hydrogen Transfer. The linkage of hydrogen production from organic matter by anaerobic heterotrophic microorganisms to the use of hydrogen by other anaerobes in the re-duction of carbon dioxide to methane. This avoids possible hydrogen toxicity.

Intertriginous Candidiasis. A skin infection caused by Candida species. Involves those areas of the body, usually opposed skin surfaces, that are warm and moist (axillae, groin, skin folds).

Intoxication. A disease that results from the entrance of a specific toxin into the body of a host. The toxin can induce the disease in the absence of the toxin producing organisms.

Intrinsic Factors. Food-related factors such as moisture, pH, and available nutrients that influ-ence microbial growth.

Invasiveness. The ability of a microorganism to enter a host, grow and reproduce within the host, and spread throughout its body.

Kirby-Bauer Method. A disk diffusion test to determine the susceptibility of a microorganism to chemotherapeutic agents.

Koch’s Postulates. A set of rules for proving that microorganism causes a particular disease.

Lactic Acid Fermentation. A fermentation that produces lactic acid as the sole or primary product.

Lager. Pertaining to the process of aging beers to allow flavor development.

Lag Phase. A period following the introduction of microorganisms into fresh culture medium when there is no increase in cell numbers or mass during batch culture.

Latent Period. The initial phase in the one-step growth experiment in which no phages are released.

Lectin Complement Pathway. The lectin pathway for complement activation is triggered by the binding of a serum lectin (mannan-binding lectin ; MBL) to mannose-containing proteins or to carbohydrates on viruses or bacteria.

Leishmanias. Zooflagellates, members of the genus Leishmania, that cause the disease leishmaniasis.

Leishmaniasis. The disease caused by the protozoa called leishmanias.

Lepromatous (progressive) Leprosy. A relentless, progressive form of leprosy in which large numbers of Mycobacterium lepae develop in skin cells, killing the skin cells and resulting in the loss of features. Disfiguring nodules from all over the body.

Leprosy or Hansen’s Disease. A severe disfiguring skin disease caused by Mycobacterium leprae.

Lethal Dose 50 (LD₅₀). Refers to the dose or number of organisms that will kill 50% of an experi-mental group of hosts within a specified time period.

Leukemia. A progressive, malignant disease of blood-forming organs, marked by distorted proliferation and development of leukocytes and their precursors in the blood and bone marrow. Certain leukemias are caused by viruses (HTLV-1, HTLV-2).

Leukocidin. A microbial toxin that can damage or kill leukocytes.

Lichen. An organism composed of a fungus and either green algae or cyanobacteria in a symbiotic association.

Liebig’s Law of the Minimum. Living organisms and populations will grow until lack of a resource begins to limit further growth.

Lipopolysaccharide (LPSs). A molecule containing both lipid and polysaccharide, which is important in the outer membrane of the Gram-negative cell wall.

Listeriosis. A sporadic disease of animals and humans, particularly those who are immunocompromised or pregnant, caused by the bacterium Listeria monocytogenes.

Lithotroph. An organism that uses reduced inorganic compounds as its electron source.

Low Oxygen Diffusion Environment. An aquatic environment in which microorganisms are surrounded by deep water layers that limit oxygen-diffusion to the cell surface. In contrast, microorganisms in thin water films have good oxygen transfer from air to the cell surface.

LPS-Binding Protein. A special plasma protein that binds bacterial lipopolysaccharides and then attaches to receptors on monocytes, macrophages, and other cells. This triggers the release of IL-1 and other cytokines that stimulate the development of fever and additional endotoxin effects.

Lymphogranuloma Venereum (LGV). A sexually transmitted disease caused by Chalmydia trachomatis serotypes L₁ – L₃, which affect the lymph organs in the genital area.

Lysogens. Bacteria that are carrying a viral prophage and can produce bacteriophages under the proper conditions.

Lysogeny. The state in which a phage genome remains within the bacterial cell after infection and reproduces along with it rather than taking control of the host and destroying it.

Lysosome. A spherical membranous eucaryotic organelle that contains hydrolytic enzymes and is responsible for the intracellular digestion of substances.

Macrolide Antibiotic. An antibiotic containing a macrolide ring, a large lactone ring with multiple keto and hydroxyl groups, linked to one or more sugars.

Macromolecule Vaccine. A vaccine made of specific, purified macromolecules derived from pathogenic microorganisms.

Macronucleus. The larger of the two nuclei in ciliate protozoa. It is normally popyploid and directs the routine activities of the cell.

Macrophage. The name for a large mononuclear phagocytic cell, present in blood, lymph, and other tissues. Macrophages are derived from monocytes. They phagocytose and destroy pathogens ; some macrophages also activate B cells and T cells.

Maduromycosis. A subcutaneous fungal infection caused by Madurella mycetoma ; also termed an eumycotic mycetoma.

Madurose. The sugar derivative 3-O-methyl-D-galactose, which is characteristic of several actinomycete genera that are collectively called maduromycetes.

Magnetosomes. Magnetite particles in magnetotactic bacteria that are tiny magnets and allow the bacteria to orient themselves in magnetic fields.

Malaria. A serious infectious illness caused by the parasitic protozoan Plasmodium. Malaria is characterized by bouts of high chills and fever that occur at regular intervals.

Mash. The soluble materials released from germinated grains and prepared as a microbial growth medium.

Mean Growth Rate Constant (k). The rate of microbial population growth expressed in terms of the number of generations per unit time.

Meiosis. The sexual process in which a diploid cell divides and forms two haploid cells.

Melting Temperature (T_m). The temperature at which double-standard DNA separates into individual strands ; it is dependent on the G + C content of the DNA and is used to compare genetic material in microbial taxonomy.

Membrane Filter Technique. The use of a thin porous filter made from cellulose acetate or some other polymer to collect microorganisms from water, air and food.

Meningitis. A condition that refers to inflammation of the brain or spinal cord meninges (mem-branes). The disease can be divided into bacterial (septic) meningitis and aseptic meningitis syn-drome (caused by nonbacterial sources).

Mesophile. A microorganism with a growth optimum around 20 to 45°C, a minimum of 15 to 20°C, and a maximum about 45°C or lower.

Metachromatic Granules. Granules of polyphosphate in the cytoplasm of some bacteria that appear a different colour when stained with a blue basic dye. They are storage reservoirs for phosphate. Sometimes called volutin granules.

Methanogens. Strictly anaerobic archaeons that derive energy by converting CO₂, H₂, formate, acetate, and other compounds to either methane or methane and CO₂.

Methylotroph. A bacterium that uses reduced one-carbon compounds such as methane and metha-nol as its sole source of carbon and energy.

Microaerophile. A microorganism that requires low levels of oxygen for growth, around 2 to 10%, but is damaged by normal atmospheric oxygen levels.

Microbial Ecology. The study of microorganisms in their natural environments, with a major emphasis on physical conditions, processes, and interactions that occur on the scale of individual microbial cells.

Microbial Loop. The mineralization of organic matter synthesized by photosynthetic phytoplankton through the activity of microorganisms such as bacteria and protozoa. This process “loops” minerals and carbon dioxide back for reuse by the primary producers and makes the organic matter unavailable to higher consumers.

Microbial Mat. A firm structure of layered microorganisms with complementary physiological activities that can develop on surfaces in aquatic environments.

Microbiology. The study of organisms that are usually too small to be seen with the naked eye. Special techniques are required to isolate and grow them.

Microbivory. The use of microorganisms as a food source by organisms that can ingest or phagocytose them.

Microenvironment. The immediate environment surrounding a microbial cell or other struc-ture, such as a root.

Microorganism. An organism that is too small to be seen clearly with the naked eye.

Miliary Tuberculosis. An acute form of tuberculosis in which small tubercles are formed in a number of organs of the body because of disemination of M. tuberculosis throughout the body by the bloodstream. Also known as reactivation tuberculosis.

Mineralization. The release of inorganic nutrients from organic matter during microbial growth and metabolism.

Minimal Inhibitory Concentration (MIC). The lowest concentration of a drug that will pre-vent the growth of a particular microorganism.

Minimal Lethal Concentration (MLC). The lowest concentration of a drug that will kill a particular microorganism.

Mitochondrion. The eucaryotic organelle that is the site of electron transport, oxidative phosphorylation, and pathways such as the Krebs cycle ; it provides most of a nonphotosynthetic cell’s energy under aerobic conditions. It is constructed of an outer membrane and an inner mem-brane, which contains the electron transport chain.

Mitosis. A process that takes place in the nucleus of a eucaryotic cell and results in the formation of two new nuclei, each with the same number of chromosomes as the parent.

Mixed Acid Fermentation. A type of fermentation carried out by members of the family Enterobacteriaceae in which ethanol and a complex mixture of organic acids are produced.

Mixotrophic. Refers to microorganisms that combine autotrophic and heterotrophic meta-bolic processes (they use inorganic electron sources and organic carbon sources).

Modified Atmosphere Packaging (MAP). Addition of gases such as nitrogen and carbon diox-ide to packaged foods in order to inhibit the growth of spoilage organisms.

Mold. Any of a large group of fungi that cause mold or moldiness and that exist as multicellular filamentous colonies ; also the deposit or growth caused by such fungi. Molds typically do not produce macroscopic fruiting bodies.

Most Probable Number (MPN). The statistical estimation of the probable population in a liquid by diluting and determining end points for microbial growth.

Mucociliary Blanket. The layer of cilia and mucus that lines certain portions of the respiratory system ; it traps microorganisms up to 10 μm in diameter and then transports them by ciliary action away from the lungs.

Mucociliary Escalator. The mechanism by which respiratory ciliated cells move material and microorganisms, trapped in mucus, out of the pharynx, where it is spit out or swallowed.

Multi-drug-resistant Strains of Tuberculosis (MDR-TB). A multi-drug-resistant strain is de-fined as Mycobacterium tuberculosis resistant to isoniazid and rifampin, with or without resist-ance to other drugs.

Mutation. A permanent, heritable change in the genetic material.

Mutualist. An organism associated with another in an obligatory relationship that is beneficial to both.

Mycelium. A mass of branching hyphae found in fungi and some bacteria.

Mycolic Acids. Complex 60 to 90 carbon fatty acids with a hydroxyl on the β-carbon and an aliphatic chain on the α-carbon, found in the cell walls of mycobacteria.

Mycoplasma. Bacteria that are members of the class Mollicutes and order Mycoplasmatales ; they lack cell walls and cannot synthesize peptidoglycan precursors ; most require sterols for growth ; they are the smallest organisms capable of independent reproduction.

Mycoplasmal Pneumonia. A type of pneumonia caused by Mycoplasma pneumoniae. Spread involves airborne droplets and close contact.

Mycorrhizosphere. The region around ectomycorrhizal mantles and hyphae in which nutrients released from the fungus increase the microbial population and its activities.

Mycotoxicology. The study of fungal toxins and their effects on various organisms.

Myxobacteria. A group of Gram-negative, aerobic soil bacteria characterized by gliding motility, a complex life cycle with the production of fruiting bodies, and the formation of myxospores.

Myxospores. Special dormant spores formed by the myxobacteria.

Narrow-spectrum Drugs. Chemotherapeutic agents that are effective only against a limited variety of microorganisms.

Natural Classification. A classification system that arranges organisms into groups whose members share many characteristics and reflect as much as possible the biological nature of organisms.

Necrotizing Fasciitis. A disease that results from a severe invasive group. A streptococcus infection. Necrotizing fasciitis is an infection of the subcutanious soft tissues, particularly of fibrous tissue, and is most common on the extremities. It begins with skin reddening, swelling, pain and cellulitis and proceeds to skin breakdown and gangrene after 3 to 5 days.

Negative Staining. A staining procedure in which a dye is used to make the background dark while the specimen is unstained.

Neurotoxin. A toxin that is poisonous to or destroys nerve tissue ; especially the toxins secreted by C. tetani, Corynebacterium diphtheriae, and Shigella dysrenteriae.

Neustonic. The microorganisms that live at the atmospheric interface of a water body.

Neutrophile. Microorgansims that grow best at a neutral pH range between pH 5.5 and 8.0.

Niche. The function of an organism in a complex system, including place of the organism, the resources used in a given location, and the time of use.

Nitrifying Bacteria. Chemolithotrophic, Gram-negative bacteria that are members of the family Nitrobacteriaceae and convert ammonia to nitrate and nitrite to nitrate.

Nitrogen Fixation. The metabolic process in which atmospheric molecular nitrogen is reduced to ammonia ; carried out by cyanobacteria, Rhizobium, and other nitrogen-fixing procaryotes.

Nitrogen Oxygen Demand (NOD). The demand for oxygen is sewage treatment, caused by nitrifying microorganisms.

Nocardioforms. Bacteria that resemble members of the genus Nocardia ; they develop a substrate mycelium that readily breaks up into rods and coccoid elements (a quality sometimes called fugacity).

Nomenclature. The branch of taxonomy concerned with the assignment of names to taxonomic groups in agreement with published rules.

Nondiscrete Microorganism. A microorganism, best exemplified by a filamentous fungus, that does not have a defined and predictable cell structure or distinct edges and boundaries. The organism can be defined in terms of the cell structure and its cytoplasmic contents.

Normal Microbiota (also indigenous microbial population, microflora, microbial flora). The microorganisms normally associated with a particular tissue or structure.

Nucleoid. An irregularly shaped region in the procaryotic cell that contains its genetic material.

Nucleolus. The organelle, located within the eucaryotic nucleus and not bounded by a mem-brane, that is the location of ribosomal RNA synthesis and the assembly of ribosomal subunits.

Numerical Aperture. The property of a microscope lens that determines how much light can enter and how great a resolution the lens can provide.

Nutrient. A substance that supports growth and reproduction.

Nystatin. A polyene antibiotic from Streptomyces noursei that is used in the treatment of Candida infections of the skin, vagina, and alimentary tract.