Important Microbiology Terms, Glossary, and Definitions

Microbiology Terms

Note- We are updating the microbiology terms on this page in regular intervals.

Microbiology Terms from A

AB toxins

  • Bacterial exotoxins consisting of two polypeptides. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • Exotoxins composed of two parts (A and B). The B portion is responsible for toxin binding to a cell but does not directly harm it; the A portion enters the cell and disrupts its function. (Prescott’s Microbiology)

ABC (ATP-binding cassette) transporter

  • A membrane transport system consisting of three proteins, one of which hydrolyzes ATP, one of which binds the substrate, and one of which functions as the transport channel through the membrane. (Brock Biology of Microorganisms)


  • The belief in spontaneous generation as a source of life. (Foundation in Microbiology by Talaro and Chess)


  • Nonliving factors such as soil, water, temperature, and light that are studied when looking at an ecosystem. (Foundation in Microbiology by Talaro and Chess)

ABO blood group system

  • Developed by Karl Landsteiner in 1904; the identification of different blood groups based on differing isoantigen markers characteristic of each blood type. (Foundation in Microbiology by Talaro and Chess)
  • The classification of red blood cells based on the presence or absence of A and B carbohydrate antigens. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • A circumscribed pus-filled lesion characteristic of staphylococcal skin disease; also called a boil. (Alcamo’s Fundamentals of Microbiology)
  • A localized infection characterized by the production of pus. (Brock Biology of Microorganisms)
  • An inflamed, fibrous mass enclosing a core of pus. (Foundation in Microbiology by Talaro and Chess)
  • Localized accumulation of pus. (Microbiology: An Introduction by Tortora, Funke, and Case)

Abyssal zone

  • The environment at the bottom of the oceanic trenches. (Alcamo’s Fundamentals of Microbiology)
  • The deepest region of the ocean; a sunless, high-pressure, cold, anaerobic habitat. (Foundation in Microbiology by Talaro and Chess)


  • Abzyme is defined as a monoclonal antibody that has catalytic activity. (Kuby Immunology)

Acetyl CoA (Acetyl-Coenzyme A)

  • One of the starting compounds for the Krebs cycle. (Alcamo’s Fundamentals of Microbiology)
  • A molecule made 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. (Prescott’s Microbiology)


  • A substance that releases hydrogen ions (H+) in solution. (Alcamo’s Fundamentals of Microbiology)
  • A substance that dissociates into one or more hydrogen ions (H+) and one or more negative ions. (Microbiology: An Introduction by Tortora, Funke, and Case)

Acid-fast stain

  • A staining process in which mycobacteria resist decolorization with acid alcohol. (Alcamo’s Fundamentals of Microbiology)
  • A property of Mycobacterium species; cells stained with basic fuchsin dye resist decolorization with acidic alcohol. (Brock Biology of Microorganisms)
  • A term referring to the property of mycobacteria to retain carbol fuchsin even in the presence of acid alcohol. The staining procedure is used to diagnose tuberculosis. (Foundation in Microbiology by Talaro and Chess)
  • A differential stain used to identify bacteria that are not decolorized by acid-alcohol. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A staining procedure that differentiates between bacteria based on their ability to retain a dye when washed with an acid alcohol solution. (Prescott’s Microbiology)


  • A solution with a pH value below 7 on the pH scale. (Foundation in Microbiology by Talaro and Chess)

Acidic dye

  • A negatively charged colored substance in a solution that is used to stain an area around cells. (Alcamo’s Fundamentals of Microbiology)
  • A salt in which the color is in the negative ion; used for negative staining. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • A microorganism that grows at acidic pHs below 4. (Alcamo’s Fundamentals of Microbiology)
  • An organism that grows best at acidic pH values. (Brock Biology of Microorganisms)
  • A bacterium that grows below pH 4. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A microorganism that has its growth optimum between about pH 0 and 5.5. (Prescott’s Microbiology)

Acquired immune response

  • The acquired immune response is the immunity mediated by lymphocytes and characterized by antigen-specificity and memory. (Roitt’s Immunology)

Acquired Immunodeficiency Syndrome (AIDS)

  • Acquired Immunodeficiency Syndrome (AIDS) is defined as a disease caused by the human immunodeficiency virus (HIV) that is marked by significant depletion of CD4+ T cells and that results in increased susceptibility to a variety of opportunistic infections and cancers. (Kuby Immunology)

Activation-induced cytidine deaminase (AID)

  • Activation-induced cytidine deaminase (AID) is defined as an enzyme that removes an amino group from deoxycytidine, forming deoxyuridine. This is the first step in the processes of both somatic hypermutation and class switch recombination. (Kuby Immunology)

Active immunity

  • Active immunity is defined as an adaptive immunity that is induced by natural exposure to a pathogen or by vaccination. (Kuby Immunology)

Acute lymphocytic leukemia (ALL)

  • Acute lymphocytic leukemia (ALL) is defined as a form of cancer in which there is an uncontrolled proliferation of a cell of the lymphoid lineage. The proliferating cells usually are present in the blood. (Kuby Immunology)


  • Addressin is defined as the cell adhesion molecule present on the luminal surface of blood and lymph vessel endothelium and recognized by homing molecules that direct leukocytes to tissues with the appropriate “address.” (Roitt’s Immunology)


  • The antibody is defined as the proteins consisting of two identical heavy chains and two identical light chains, that recognize a particular epitope on an antigen and facilitates clearance of that antigen. The membrane-bound antibody is expressed by B cells that have not encountered antigen; secreted antibody is produced by plasma cells. Some antibodies are multiples of the basic four-chain structure. (Kuby Immunology)


  • The antigen is defined as any molecule capable of being recognized by an antibody or T-cell receptor. (Roit Immunology)
  • The antigen is defined as any substance (usually foreign) that binds specifically to an antibody or a T-cell receptor. (Kuby Immunology)

Microbiology Terms from W


  • An epidermal tumor caused by papillomaviruses. (Foundation in Microbiology by Talaro and Chess)


  • The liquid derived from domestic sewage or industrial sources, which cannot be discarded in untreated form into lakes or streams. (Brock Biology of Microorganisms)

Wastewater treatment

  • The use of physical and biological processes to remove particulate and dissolved material from sewage and to control pathogens. (Prescott’s Microbiology)

Water activity (aw)

  • An expression of the relative availability of water in a substance. Pure water has an aw of 1.000. (Brock Biology of Microorganisms)
  • A quantitative measure of water availability in the habitat; the water activity of a solution is one-hundredth of its relative humidity. (Prescott’s Microbiology)

Water molds

  • A collective name for protists. Formerly thought to be fungi. (Prescott’s Microbiology)

West Nile fever

  • A neurological disease caused by the West Nile virus, a virus transmitted by mosquitoes from birds to humans. (Brock Biology of Microorganisms)

Western blot

  • The detection of specific proteins by separating them via electrophoresis, transferring them to a membrane, and adding specific antibodies. (Brock Biology of Microorganisms)
  • A procedure for separating and identifying antigen or antibody mixtures by two-dimensional electrophoresis in polyacrylamide gel, followed by immune labeling. (Foundation in Microbiology by Talaro and Chess)
  • A technique that uses antibodies to detect the presence of specific proteins separated by electrophoresis. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • An enlarged, hive-like zone of puffiness on the skin, often due to an allergic reaction. (Alcamo’s Fundamentals of Microbiology)
  • A welt; marked, slightly red, usually itchy bumps on the skin that change in size and shape as they spread. It is surrounded by a red patch (the flare). The reaction is triggered by cutaneous contact or intradermal injection of allergens in sensitive individuals. (Foundation in Microbiology by Talaro and Chess)


  • The residual fluid from milk coagulation that separates from the solidified curd. (Foundation in Microbiology by Talaro and Chess)
  • The fluid portion of milk that separates from curd. (Microbiology: An Introduction by Tortora, Funke, and Case)

White blood cell

  • Blood cells having innate or acquired immune functions. These cells are named for the white or buffy layer in which they are found when blood is centrifuged. (Prescott’s Microbiology)

White piedra

  • A fungus disease of hair, especially of the scalp, face, and genitals, caused by Trichosporon beigelii. The infection is associated with soft, mucilaginous, white-to-light-brown nodules that form within and on the hair shafts. (Foundation in Microbiology by Talaro and Chess)


  • A deep inflammation of the finger or toe, especially near the tip or around the nail. Whitlow is a painful herpes simplex virus infection that can last several weeks and is most common among health care personnel who come in contact with the virus in patients. (Foundation in Microbiology by Talaro and Chess)

Whole blood

  • A liquid connective tissue consisting of blood cells suspended in plasma. (Foundation in Microbiology by Talaro and Chess)

Whole-cell vaccine

  • A vaccine made from complete pathogens, which can be either killed microorganisms or live, attenuated microbes. (Prescott’s Microbiology)

Whole-genome shotgun sequencing

  • Genome sequencing in which random fragments of a complete genome are individually sequenced. The nucleotide sequences of the fragments are placed in the proper order based on overlapping identical sequences. (Prescott’s Microbiology)

Whooping cough

  • Infection by Bordetella pertussis. A highly communicable disease that causes the acute respiratory syndrome. Pertussis can be life-threatening in infants, but vaccination on the recommended schedule can prevent infection. (Foundation in Microbiology by Talaro and Chess)

Widal test

  • An agglutination test for diagnosing typhoid fever and determining the antibody titer. (Foundation in Microbiology by Talaro and Chess)

Wild type

  • The form of an organism or gene isolated from nature. (Alcamo’s Fundamentals of Microbiology)
  • A strain of microorganism isolated from nature or one used as a parent in a genetics investigation. The usual or native form of a gene or organism. (Brock Biology of Microorganisms)
  • The natural, nonmutated form of a genetic trait. (Foundation in Microbiology by Talaro and Chess)
  • A prevalent form of a gene or phenotype. (Prescott’s Microbiology)

Winogradsky column

  • A glass column packed with mud and overlaid with water to mimic an aquatic environment in which various bacteria develop over a period of months. (Brock Biology of Microorganisms)
  • A glass column with an anoxic lower zone and anoxic upper zone, which allows the growth of microorganisms under conditions similar to those found in a nutrient-rich lake. (Prescott’s Microbiology)


  • In reference to protein synthesis, a less rigid form of base pairing allowed only in the codon-anticodon pairing. (Brock Biology of Microorganisms)
  • The loose base pairing between an anticodon and a codon at the third position of the codon. (Prescott’s Microbiology)


  • A sugary liquid produced from crushed malted grain and water to which is added yeast and hops for the brewing of beer. (Alcamo’s Fundamentals of Microbiology)
  • The clear fluid derived from a soaked mash that is fermented for beer. (Foundation in Microbiology by Talaro and Chess)
  • The filtrate of malted grains used as the substrate for the production of beer and ale by fermentation. (Prescott’s Microbiology)

Microbiology Terms from X


  • Substances from the heme fraction of blood hemoglobin. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • The ionizing radiation that can be used to sterilize objects. (Alcamo’s Fundamentals of Microbiology)


  • A synthetic compound not produced by organisms in nature. (Brock Biology of Microorganisms)
  • Synthetic chemicals that are not readily degraded by microorganisms. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • A method of diagnosis based on exposing a parasite-free normal host to the parasite and then examining the host for parasites. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • A tissue graft between members of different species, such as between a pig and a human. (Alcamo’s Fundamentals of Microbiology)
  • The transfer of a tissue or an organ from an animal of one species to a recipient of another species. (Foundation in Microbiology by Talaro and Chess)
  • A tissue graft from another species. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A tissue graft between animals of different species. (Prescott’s Microbiology)


  • An organism adapted to growth at very low water potentials. (Brock Biology of Microorganisms)
  • Microorganisms that grow best under low water activity (aw) conditions, and may not be able to grow at high aw values. (Prescott’s Microbiology)

Microbiology Terms from Y


  • A tropical disease caused by Treponema pertenue that produces granulomatous ulcers on the extremities and occasionally on the bone but does not produce a central nervous system or cardiovascular complications. (Foundation in Microbiology by Talaro and Chess)


  • (1) A type of unicellular, nonfilamentous fungus that resembles bacterial colonies when grown in culture. (2) A term sometimes used to denote the unicellular form of pathogenic fungi. (Alcamo’s Fundamentals of Microbiology)
  • The single-celled growth form of various fungi. (Brock Biology of Microorganisms)
  • Single-celled, budding fungi. (Foundation in Microbiology by Talaro and Chess)
  • Nonfilamentous, unicellular fungi. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A unicellular, uninuclear fungus that reproduces either asexually by budding or fission, or sexually through spore formation. (Prescott’s Microbiology)

Yeast Artificial Chromosome (YAC)

  • A genetically engineered chromosome with yeast origin of replication and centromere sequence. (Brock Biology of Microorganisms)
  • Engineered DNA that contains all the elements required to propagate a chromosome in yeast and is used to clone foreign DNA fragments in yeast cells. (Prescott’s Microbiology)

Yeast infection

  • Disease caused by the growth of certain yeasts in a susceptible host. (Microbiology: An Introduction by Tortora, Funke, and Case)

Yellow fever

  • Best-known arboviral disease. Yellow fever is transmitted by mosquitoes. Its symptoms include fever, headache, and muscle pain that can proceed to oral hemorrhage, nosebleeds, vomiting, jaundice, and liver and kidney damage. (Foundation in Microbiology by Talaro and Chess)

Microbiology Terms from Z

Zone of inhibition

  • The area of no bacterial growth around an antimicrobial agent in the disk-diffusion method. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • An animal disease that may be transmitted to humans. (Alcamo’s Fundamentals of Microbiology)
  • Any disease primarily of animals that is occasionally transmitted to humans. (Brock Biology of Microorganisms)
  • An infectious disease indigenous to animals that humans can acquire through direct or indirect contact with infected animals. (Foundation in Microbiology by Talaro and Chess)
  • A disease that occurs primarily in wild and domestic animals but can be transmitted to humans. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A disease that can be transmitted from animals to humans. (Prescott’s Microbiology)


  • The collection of nonphotosynthetic microorganisms (protozoa, tiny animals) that float in the upper regions of aquatic habitat and together with phytoplankton comprise the plankton. (Foundation in Microbiology by Talaro and Chess)


  • An asexual algal spore; has two flagella. (Microbiology: An Introduction by Tortora, Funke, and Case)


  • A dinoflagellate found living symbiotically within cnidarians (corals) and other invertebrates. (Prescott’s Microbiology)


  • A group of fungi that usually has aseptate hyphae. Sexual reproduction normally involves the formation of zygospores. (Prescott’s Microbiology)


  • A phylum of fungi whose members have coenocytic hyphae and form zygospores, among other notable characteristics. (Alcamo’s Fundamentals of Microbiology)


  • A sexually produced spore formed by members of the Zygomycota. (Alcamo’s Fundamentals of Microbiology)
  • A thick-walled sexual spore produced by the zygomycetes fungi. It develops from the union of two hyphae, each bearing nuclei of opposite mating types. (Foundation in Microbiology by Talaro and Chess)
  • A sexual fungal spore characteristic of the zygomycetes. (Microbiology: An Introduction by Tortora, Funke, and Case)
  • A thick-walled, sexual, resting spore characteristic of the zygomycetes fungi. (Prescott’s Microbiology)


  • In eukaryotes, the diploid cell resulting from the union of two haploid gametes. (Brock Biology of Microorganisms)
  • A diploid cell produced by the fusion of two haploid gametes. (Microbiology: An Introduction by Tortora, Funke, and Case)

References and Sources

  1. Alcamo’s Fundamentals of Microbiology
  2. Brock Biology of Microorganisms
  3. Foundation in Microbiology by Talaro and Chess
  4. Microbiology: An Introduction by Tortora, Funke, and Case
  5. Prescott’s Microbiology

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He attended St. Xavier’s College, Maitighar, Kathmandu, Nepal, to complete his Master of Science in Microbiology. He worked as a Lecturer at St. Xavier’s College, Maitighar, Kathmandu, Nepal, from Feb 2015 to June 2019. After teaching microbiology for more than four years, he joined the Central Department of Microbiology, Tribhuvan University, to pursue his Ph.D. in collaboration with Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. He is interested in research on actinobacteria, myxobacteria, and natural products. He has published more than 15 research articles and book chapters in international journals and well-renowned publishers.

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