Acidophile An organism that grows best at acidic pH values.
Aerobe An organism that grows in the presence of O2; may be facultative, obligate, or microaerophilic.
Aerotolerant anaerobe An anaerobic microorganism whose growth is not inhibited by O2.
Algae Phototrophic eukaryotic micro- and macro organisms.
Alkaliphile An organism that grows best at high pH.
Anabolic reactions (anabolism) The biochemical processes involved in the synthesis of cell constituents from simpler molecules, usually requiring energy.
Anaerobe An organism that grows in the absence of O2; some may even be killed by O2 (obligate or strict anaerobes).
Anaerobic respiration Use of an electron acceptor other than O2 in an electron transport–based oxidation leading
Anammox Anoxic ammonia oxidation.
Anoxic Oxygen-free. Usually used in reference to a microbial habitat.
Anoxygenic photosynthesis The use of light energy to synthesize ATP by cyclic photophosphorylation without O2 production.
Antibiotic A chemical substance produced by a microorganism that kills or inhibits the growth of another microorganism.
Antibiotic resistance The acquired ability of a microorganism to grow in the presence of an antibiotic to which the microorganism is usually sensitive.
Antibody A soluble protein produced by B lymphocytes and plasma cells that interacts specifically with antigen; also called immunoglobulin.
Antigen A molecule capable of interacting with specific components of the immune system.
Antimicrobial Harmful to microorganisms by either killing or inhibiting growth.
Antimicrobial agent A chemical that kills or inhibits the growth of microorganisms.
Antimicrobial drug resistance The acquired ability of a microorganism to grow in the presence of an antimicrobial drug to which the microorganism is usually susceptible.
Antiseptic (germicide) A chemical agent that kills or inhibits growth of microorganisms and is sufficiently nontoxic to be applied to living tissues.
Antiserum A serum containing antibodies.
Archaea Phylogenetically related prokaryotes distinct from Bacteria.
Aseptic technique The manipulation of sterile instruments or culture media in such a way as to maintain sterility.
ATP Adenosine triphosphate, the principal energy carrier of the cell.
Autoclave A sealed sterilizing device that destroys microorganisms with temperature and steam under pressure.
Autotroph An organism able to grow on CO2 as sole source of carbon.
Auxotroph An organism that has developed a nutritional requirement through mutation. Contrast with prototroph.
Bacteria Phylogenetically related prokaryotes distinct from Archaea.
Bacteriochlorophyll A pigment of phototrophic organisms consisting of light-sensitive magnesium tetrapyrroles.
Bacteriocidal agent An agent that kills bacteria.
Bacteriocins Agents produced by certain bacteria that inhibit or kill closely related species.
Bacteriophage A virus that infects prokaryotic cells.
Bacteriorhodopsin A protein containing retinal that is found in the membranes of certain extremely halophilic Archaea and that is involved in light-mediated ATP synthesis.
Bacteriostatic agent An agent that inhibits bacterial growth.
Basal body The “motor” portion of the bacterial flagellum, embedded in the cytoplasmic membrane and wall.
Base composition In reference to nucleic acids, the proportion of the total bases consisting of guanine plus cytosine or thymine plus adenine base pairs. Usually expressed as a guanine plus cytosine (GC) value, for example, 60% GC.
Batch culture A closed-system microbial culture of fixed volume.
Beta-Lactam antibiotic An antibiotic such as penicillin that contains the four-membered heterocyclic β-lactam ring.
Binary fission Cell division whereby a cell grows by intercalary growth to twice its minimum size and then divides to form two cells.
Biocatalysis The use of microorganisms to synthesize a product or carry out a specific chemical transformation.
Biofilm Microbial colonies encased in an adhesive, usually polysaccharide material and attached to a surface.
Biofuel A fuel made by microorganisms from the fermentation of carbon-rich feedstocks.
Brewing The manufacture of alcoholic beverages such as beer and ales from the fermentation of malted grains.
Broad-spectrum antibiotic An antibiotic that acts on both gram-positive and gram-negative Bacteria.
Calvin cycle The series of biosynthetic reactions by which most photosynthetic organisms convert CO2 to organic compounds.
Capsid The protein shell that surrounds the genome of a virus.
Capsule A dense, well-defined polysaccharide or protein layer closely surrounding a cell.
Carboxysome Polyhedral cellular inclusions of crystalline ribulose bisphosphate carboxylase (RubisCO), the key enzyme of the Calvin cycle.
Carcinogen A substance that causes the initiation of tumor formation. Frequently a mutagen.
Catalysis An increase in the rate of a chemical reaction.
Catalyst A substance that promotes a chemical reaction without itself being changed in the end.
Cell The fundamental unit of living matter.
Cell wall A rigid layer present outside the cytoplasmic membrane that confers structural strength on the cell and protection from osmotic lysis.
Chemolithotroph An organism that obtains its energy from the oxidation of inorganic compounds.
Chemoorganotroph An organism that obtains its energy from the oxidation of organic compounds.
Chemostat A continuous culture device controlled by the concentration of limiting nutrient and dilution rate.
Chemotaxis Movement toward or away from a chemical.
Chemotherapeutic agent An antimicrobial agent that can be used internally.
Chemotherapy Treatment of infectious disease with chemicals or antibiotics.
Chitin A polymer of N-acetyl glucosamine commonly found in the cell walls of fungi.
Chlorination A highly effective disinfectant procedure for drinking water using chlorine gas or other chlorine-containing compounds as disinfectant.
Chlorine A chemical used in its gaseous state to disinfect water. A residual level is maintained throughout the distribution system.
Chlorophyll A pigment of phototrophic organisms consisting of light sensitive magnesium tetrapyrroles.
Chloroplast The chlorophyll-containing organelle of phototrophic eukaryotes.
Chlorosome A cigar-shaped structure enclosed by a nonunit membrane and containing the light-harvesting bacteriochlorophyll (c, d, or e) in green sulfur bacteria and in Chloroflexus.
Chromogenic Producing color
Chromosome A genetic element carrying genes essential to cellular function. Prokaryotes typically have a single chromosome consisting of a circular DNA molecule. Eukaryotes typically have several chromosomes, each containing a linear DNA molecule.
-cidal Suffix indicating killing; for example, a bacteriocidal agent kills bacteria. Compare with -static.
Citric acid cycle A cyclical series of reactions resulting in the conversion of acetate to CO2 and NADH. Also called the tricarboxylic acid cycle or Krebs cycle.
Coccus A spherical bacterium.
Coenzyme A low-molecular-weight molecule that participates in an enzymatic reaction by accepting and donating electrons or functional groups. Examples: NAD+, FAD.
Coliform Gram-negative, nonsporulating, facultatively aerobic rod that ferments lactose with gas formation within 48 hours at 35°C.
Colonization The multiplication of a microorganism after it has attached to host tissues or other surfaces.
Colony A macroscopically visible population of cells growing on solid medium, arising from a single cell.
Complementary Nucleic acid sequences that can base-pair with each other.
Conidia Asexual spores of fungi.
Conjugation The transfer of genes from one prokaryotic cell to another by a mechanism involving cell-to-cell contact.
Culture A particular strain or kind of organism growing in a laboratory medium.
Culture medium An aqueous solution of various nutrients suitable for the growth of microorganisms.
Cyanobacteria Prokaryotic oxygenic phototrophs containing chlorophyll a and phycobilins.
Cyst A resting stage formed by some bacteria and protists in which the whole cell is surrounded by a thick-walled chemically and physically resistant coating; not the same as a spore or endospore.
Cytochrome An iron-containing porphyrin complexed with proteins, which functions as an electron carrier in the electron transport system.
Cytoplasm The fluid portion of a cell, bounded by the cell membrane.
Cytoplasmic membrane A semipermeable barrier that separates the cell interior (cytoplasm) from the environment.
Defined medium Any culture medium whose exact chemical composition is known. Compare with complex medium.
Deoxyribonucleic acid (DNA) A polymer of nucleotides connected via a phosphate–deoxyribose sugar backbone; the genetic material of cells and some viruses.
Differential media A growth medium that allows identification of microorganisms based on phenotypic properties.
Dipicolinic acid A substance unique to endospores that confers heat resistance on these structures.
Diploid In eukaryotes, an organism or cell with two chromosome complements, one derived from each haploid gamete.
Disease An injury to a host organism, caused by a pathogen or other factor that affects the host organism’s function.
Disinfectant An antimicrobial agent used only on inanimate objects.
Disinfection The elimination of pathogens from inanimate objects or surfaces.
Doubling time The time needed for a population to double. See also generation time.
Ecology Study of the interrelationships between organisms and their environments.
Ecosystem A dynamic complex of organisms and their physical environment interacting as a functional unit.
Electron acceptor A substance that accepts electrons during an oxidation–reduction reaction.
Electron donor A compound that donates electrons in an oxidation–reduction reaction.
Electron transport phosphorylation Synthesis of ATP involving a membrane-associated electron transport chain and the creation of a proton motive force. Also called oxidative phosphorylation.
Endospore A differentiated cell formed within the cells of certain gram-positive bacteria that is extremely resistant to heat as well as to other harmful agents.
Endosymbiosis The engulfment of one cell type by another cell type and the subsequent and stable association of the two cells.
Enriched media Media that allow metabolically fastidious organisms to grow because of the addition of specific growth factors.
Enrichment culture Use of selective culture media and incubation conditions to isolate specific microorganisms from natural samples.
Entropy A measure of the degree of disorder in a system; entropy always increases in a closed system.
Enveloped In reference to a virus, having a lipoprotein membrane surrounding the virion.
Enzyme A catalyst, usually composed of protein, that promotes specific reactions or groups of reactions
Eukarya The phylogenetic domain containing all eukaryotic organisms
Eukaryote A cell or organism having a unit membrane–enclosed nucleus and usually other organelles; a member of the Eukarya.
Evolution Descent with modification; DNA sequence variation and the inheritance of that variation.
Exponential growth Growth of a microbial population in which the cell number doubles within a fixed time period.
Exponential phase A period during the growth cycle of a population in which growth increases at an exponential rate.
Expression The ability of a gene to function within a cell in such a way that the gene product is formed.
Extreme halophile An organism whose growth is dependent on large concentrations (generally .9%) of NaCl.
Extreme piezophile A piezophilic organism unable to grow at a pressure of 1 atm and typically requiring several hundred atmospheres of pressure for growth.
Extremophile An organism that grows optimally under one or more chemical or physical extremes, such as high or low temperature or pH.
Facultative Indicates that an organism is able to grow in either the presence or absence of an environmental factor (for example, “facultative aerobe”).
Fermentation Anaerobic catabolism of an organic compound in which the compound serves as both an electron donor and an electron acceptor and in which ATP is usually produced by substrate-level phosphorylation.
Fermentation (industrial) A large-scale microbial process.
Fermenter An organism that carries out the process of fermentation.
Fermentor A growth vessel, usually quite large, used to culture microorganisms for the production of some commercially valuable product.
Ferredoxin An electron carrier of very negative reduction potential; small protein containing iron–sulfur clusters.
Filamentous In the form of very long rods, many times longer than wide.
Filtration The removal of suspended particles from water by passing it through one or more permeable membranes or media (e.g., sand, anthracite, or diatomaceous earth).
Fimbria (plural, fimbriae) Short, filamentous structure on a bacterial cell; although flagella-like in structure, it is generally present in many copies and not involved in motility. Plays a role in adherence to surfaces and in the formation of pellicles. See also pilus.
Flagellum (plural, flagella) A thin, filamentous organ of motility in prokaryotes that functions by rotating. In motile eukaryotes, the flagellum, if present, moves by a whip like motion.
Flavoprotein A protein containing a derivative of riboflavin, which functions as an electron carrier in the electron transport system.
Fluorescent Having the ability to emit light of a certain wavelength when activated by light of another wavelength.
Free energy (G) Energy available to do work; G0 is free energy under standard conditions.
Fruiting body A macroscopic reproductive structure produced by some fungi (for example, mushrooms) and some Bacteria (for example, myxobacteria), each distinct in size, shape, and coloration.
Fungi Nonphototrophic eukaryotic microorganisms that contain rigid cell walls.
Fungicidal agent An agent that kills fungi.
Fungistatic agent An agent that inhibits fungal growth.
Gene A unit of heredity; a segment of DNA (or RNA in some viruses) specifying a particular protein or polypeptide chain, or a tRNA or an rRNA.
Gene expression Transcription of a gene followed by translation of the resulting mRNA into protein(s).
General-purpose medium A growth medium that supports the growth of most aerobic and facultatively aerobic organisms.
Generation time The time required for a cell population to double. See also doubling time.
Genetics Heredity and variation of organisms.
Genome The total complement of genetic information of a cell or a virus.
Genomics The discipline that maps, sequences, analyzes, and compares genomes.
Genus A taxonomic group of related species.
Germicide (antiseptic) A chemical agent that kills or inhibits growth of microorganisms and is sufficiently nontoxic to be applied to living tissues.
Glycocalyx Polysaccharide components outside of the bacterial cell wall; usually a loose network of polymer fibers extending outward from the cell.
Glycolysis Reactions of the Embden–Meyerhof–Parnas pathway in which glucose is converted to pyruvate.
Glycosidic bond A type of covalent bond that links sugar units together in a polysaccharide.
Glyoxylate cycle A series of reactions including some citric acid cycle reactions that are used for aerobic growth on C2 or C3 organic acids.
Gram-negative cells A major phylogenetic lineage of prokaryotic cells with a cell wall containing relatively little peptidoglycan, and an outer membrane composed of lipopolysaccharide, lipoprotein, and other complex macromolecules; stain pink in the Gram stain.
Gram-positive cells A major phylogenetic lineage of prokaryotic cells containing mainly peptidoglycan in their cell wall; stain purple in the Gram stain.
Gram stain A differential staining technique in which cells stain either pink (gram-negative) or purple (gram-positive), depending upon their structural makeup.
Green fluorescent protein (GFP) A protein that fluoresces green and is widely used in genetic analysis.
Green sulfur bacteria Anoxygenic phototrophs containing chlorosomes and bacteriochlorophyll c, d, or e as light-harvesting chlorophyll.
Growth In microbiology, an increase in cell number.
Growth rate The rate at which growth occurs, usually expressed as the generation time.
Habitat An environment within an ecosystem where a microbial community could reside.
Halophile An organism requiring salt (NaCl) for growth.
Halotolerant Capable of growing in the presence of NaCl, but not requiring it.
Haploid An organism or cell containing only one set of chromosomes.
Heliobacteria Anoxygenic phototrophs containing bacteriochlorophyll g.
HEPA filter A high-efficiency particulate air filter used in laboratories and industry to remove particles, including microorganisms, from intake or exhaust air flow.
Heterocyst A differentiated cyanobacterial cell that carries out nitrogen fixation.
Heterotroph An organism that requires organic carbon as its carbon source; also a chemoorganotroph.
Hfr cell A cell with the F plasmid integrated into the chromosome.
Homofermentative In reference to lactic acid bacteria, producing only lactic acid as a fermentation product.
Homologous Describes genes related in sequence to an extent that implies common genetic ancestry; includes both orthologs and paralogs.
Horizontal gene transfer The transfer of genetic information between organisms as opposed to its vertical inheritance from parental organism(s).
Host (or host cell) An organism or cell type capable of supporting the growth of a virus or other parasite.
Humus Dead organic matter.
Hydrolysis Breakdown of a polymer into smaller units, usually monomers, by addition of water; digestion.
Hyperthermophile A prokaryote having a growth temperature optimum of 80°C or higher.
Industrial microbiology The large-scale use of microorganisms to make products of commercial value.
Inhibition In reference to growth, the reduction of microbial growth because of a decrease in the number of organisms present or alterations in the microbial environment.
Irradiation In food microbiology, the exposure of food to ionizing radiation to inhibit microorganisms and insect pests or to retard growth or ripening.
Koch’s postulates A set of criteria for proving that a given microorganism causes a given disease.
Lag phase The period after inoculation of a culture before growth begins.
Lichen A fungus and an alga (or a cyanobacterium) living in symbiotic association.
Lipopolysaccharide (LPS) Complex lipid structure containing unusual sugars and fatty acids found in most gram-negative Bacteria and constituting the chemical structure of the outer membrane.
Lophotrichous Having a tuft of polar flagella.
Lyophilization (freeze-drying) The process of removing all water from frozen food under vacuum.
Lysis Loss of cellular integrity with release of cytoplasmic contents.
Lysogen A prokaryote containing a prophage. See also temperate virus.
Lysogenic pathway After virus infection, a series of steps that leads to a state (lysogeny) in which the viral genome is replicated as a provirus along with that of the host.
Lysogeny A state following virus infection in which the viral genome is replicated as a provirus along with the genome of the host.
Lytic pathway A series of steps after virus infection that lead to virus replication and the destruction (lysis) of the host cell.
Magnetotaxis The directed movement of bacterial cells by a magnetic field.
Medium (plural, media) In microbiology, the nutrient solution(s) used to grow microorganisms.
Mesophile An organism living in the temperature range near that of warm-blooded animals and usually showing a growth temperature optimum between 25 and 40°C.
Metabolism All biochemical reactions in a cell, both anabolic and catabolic.
Metagenome The total genetic complement of all the cells present in a particular environment.
Microaerophile An organism requiring O2 but at a level lower than that in air.
Microbial leaching The extraction of valuable metals such as copper from sulfide ores by microbial activities.
Microorganism A microscopic organism consisting of a single cell or cell cluster, also including the viruses, which are not cellular.
Mixotroph An organism that uses organic compounds as carbon sources but uses inorganic compounds as electron donors for energy metabolism.
Molds Filamentous fungi.
Morphology The shape of an organism.
Motility The property of movement of a cell under its own power.
Mushroom The aboveground fruiting body, or basidiocarp, of basidiomycete fungi.
Mutagen An agent that induces mutation, such as radiation or certain chemicals.
Mutant An organism whose genome carries a mutation.
Mutation An inheritable change in the base sequence of the genome of an organism.
Neutrophile An organism that grows best around pH 7.
Nucleocapsid The complete complex of nucleic acid and protein packaged in a virus particle.
Nucleoid The aggregated mass of DNA that makes up the chromosome of prokaryotic cells.
Nucleoside A nucleotide minus phosphate.
Nucleotide A monomeric unit of nucleic acid, consisting of a sugar, a phosphate, and a nitrogenous base.
Nucleus A membrane-enclosed structure in eukaryotes containing the genetic material (DNA) organized in chromosomes.
Nutrient A substance taken by a cell from its environment and used in catabolic or anabolic reactions.
Obligate Indicates an environmental condition always required for growth (for example, “obligate anaerobe”).
Oligotrophic Describes a habitat in which nutrients are in low supply.
Organelle A bilayer membrane–enclosed structure such as the mitochondrion found in eukaryotic cells.
Outer membrane A phospholipid- and polysaccharide-containing unit membrane that lies external to the peptidoglycan layer in cells of gram-negative Bacteria.
Oxic Containing oxygen; aerobic. Usually used in reference to a microbial habitat.
Oxidation A process by which a compound gives up electrons (or H atoms) and becomes oxidized.
Oxidation–reduction (redox) reaction A pair of reactions in which one compound becomes oxidized while another becomes reduced and takes up the electrons released in the oxidation reaction.
Oxidative (electron transport) phosphorylation The non-phototrophic production of ATP at the expense of a proton motive force formed by electron transport.
Oxygenic photosynthesis The use of light energy to synthesize ATP and NADPH by noncyclic photophosphorylation with the production of O2 from water
Pasteurization The use of controlled heat to reduce the microbial load, including disease-producing microorganisms and spoilage microorganisms, in heat-sensitive liquids.
Pathogen A disease-causing microorganism.
Pathogenicity The ability of a pathogen to cause disease
Penicillin A class of antibiotics that inhibit bacterial cell wall synthesis; characterized by a β-lactam ring.
Pentose phosphate pathway A major metabolic pathway for the production and catabolism of pentoses (C5 sugars).
Peptide bond A type of covalent bond joining amino acids in a polypeptide.
Peptidoglycan The rigid layer of the cell walls of Bacteria, a thin sheet composed of N-acetylglucosamine, N-acetylmuramic acid, and a few amino acids.
Periplasm The area between the cytoplasmic membrane and the outer membrane in gram-negative Bacteria.
Peritrichous flagellation In flagellar arrangements, having flagella attached to many places on the cell surface.
pH The negative logarithm of the hydrogen ion (H+) concentration of a solution.
Phage bacteriophage.
Phospholipid A lipid containing a substituted phosphate group and two fatty acid chains on a glycerol backbone.
Photoautotroph An organism able to use light as its sole source of energy and CO2 as its sole carbon source.
Photoheterotroph An organism using light as a source of energy and organic compounds as a carbon source.
Photophosphorylation The synthesis of energy-rich phosphate bonds in ATP using light energy.
Photosynthesis The series of reactions in which ATP is synthesized by light-driven reactions and CO2 is fixed into cell material. See also anoxygenic photosynthesis and oxygenic photosynthesis.
Phototaxis Movement of a cell toward light.
Phototroph An organism that obtains energy from light.
Phycobilin The light-capturing open chain tetrapyrrole component of phycobiliproteins.
Phycobiliprotein The accessory pigment complex in cyanobacteria that contains phycocyanin or phycoerythrin coupled to proteins.
Phylogeny The evolutionary (natural) history of organisms.
Phylum A major lineage of cells in one of the three domains of life.
Phytopathogen A microorganism that causes plant disease.
Piezophile An organism that lives optimally at high hydrostatic pressure.
Pilus (plural, pili) A fimbria-like structure that is present on fertile cells, both Hfr and F+, and is involved in DNA transfer during conjugation. Sometimes called a sex pilus. See also fimbria.
Plaque A zone of lysis or cell inhibition caused by virus infection on a lawn of cells.
Plasmid An extrachromosomal genetic element that is not essential for growth and has no extracellular form.
Plate count A viable counting method in which the number of colonies on a plate is used as a measure of cell number.
Point mutation A mutation that involves a single base pair.
Polar flagellation In flagellar arrangements, having flagella attached at one end or both ends of the cell.
Poly-beta-hydroxybutyrate (PHB) A common storage material of prokaryotic cells consisting of a polymer of β-hydroxybutyrate (PHB) or other β-alkanoic acids (PHA).
Polymerase chain reaction (PCR) Artificial amplification of a DNA sequence by repeated cycles of strand separation and replication.
Polynucleotide A polymer of nucleotides bonded to one another by phosphodiester bonds.
Polypeptide Several amino acids linked together by peptide bonds.
Polysaccharide A long chain of monosaccharides (sugars) linked by glycosidic bonds.
Porins Protein channels in the outer membrane of gram-negative Bacteria through which small to medium-sized molecules can flow.
Primary metabolite A metabolite excreted during the exponential growth phase.
Prion An infectious protein whose extracellular form contains no nucleic acid
Probiotic A live microorganism that, when administered to a host, may confer a health benefit.
Prokaryote A cell or organism lacking a nucleus and other membrane enclosed organelles and usually having its DNA in a single circular molecule. Members of the Bacteria and the Archaea.
Prophage The state of the genome of a temperate virus when it is replicating in synchrony with that of the host, typically integrated into the host genome.
Protist A unicellular eukaryotic microorganism; may be flagellated or aflagellate, phototrophic or nonphototrophic, and most lack cell walls; includes algae and protozoa.
Proton motive force A source of energy resulting from the separation of protons from hydroxyl ions across the cytoplasmic membrane,generating a membrane potential.
Protoplasm The complete cellular contents, cytoplasmic membrane,cytoplasm, and nucleus/nucleoid of a cell.
Protoplast A cell from which the wall has been removed.
Prototroph The parent from which an auxotrophic mutant has been derived. Contrast with auxotroph.
Protozoa Unicellular eukaryotic microorganisms that lack cell walls.
Psychrophile An organism able to grow at low temperatures and showing a growth temperature optimum of less than 15°C.
Psychrotolerant Able to grow at low temperature but having a growth temperature optimum of >20°C.
Pure culture A culture containing a single kind of microorganism.
Resolution In microbiology, the ability to distinguish two objects as distinct and separate under the microscope.
Respiration Catabolic reactions producing ATP in which either organic or inorganic compounds are primary electron donors and organic or inorganic compounds are ultimate electron acceptors.
Rhizosphere The region immediately adjacent to plant roots.
Ribonucleic acid (RNA) A polymer of nucleotides connected via a phosphate–ribose backbone; involved in protein synthesis or as genetic material of some viruses.
Ribosome A structure composed of RNAs and proteins upon which new proteins are made.
Ribotyping A means of identifying microorganisms from analysis of DNA fragments generated from restriction enzyme digestion of genes encoding their 16S rRNA.
Ribulose monophosphate pathway A reaction series in certain methylotrophs in which formaldehyde is assimilated into cell material using ribulose monophosphate as the C1 acceptor molecule.
Rickettsias Obligate intracellular bacteria that cause disease, including typhus, spotted fever rickettsiosis, and ehrlichiosis.
RNA life A hypothetical ancient life form lacking DNA and protein,in which RNA had both a genetic coding and a catalytic function.
RNA polymerase An enzyme that synthesizes RNA in the 5’ → 3’ direction using an antiparallel 3’ → 5’ DNA strand as a template.
RubisCO The acronym for ribulose bisphosphate carboxylase, a key enzyme of the Calvin cycle.
S-layer A paracrystalline outer wall layer composed of protein or glycoprotein and found in many prokaryotes.
Selective medium A growth medium that enhances the growth of certain organisms while inhibiting the growth of others due to an added media component
Slime molds Nonphototrophic eukaryotic microorganisms lacking cell walls, which aggregate to form fruiting structures (cellular slime molds) or simply masses of protoplasm (acellular slime molds).
Spheroplast A spherical, osmotically sensitive cell derived from a bacterium by loss of some but not all of the rigid wall layer. If all of the rigid wall layer has been completely lost, the structure is called a protoplast.
Spirilla (singular, spirillum) Spiral-shaped cells.
Spontaneous generation The hypothesis that living organisms can originate from nonliving matter.
Spontaneous mutation A mutation that occurs “naturally” without the help of mutagenic chemicals or radiation.
Spore A general term for resistant resting structures formed by many prokaryotes and fungi.
Sporozoa Nonmotile parasitic protozoa.
Stalk An elongate structure, either cellular or excreted, that anchors a cell to a surface.
-static Suffix indicating inhibition of growth. For example, a bacteriostatic agent inhibits bacterial growth. Compare with -cidal.
Stationary phase The period during the growth cycle of a microbial population in which growth ceases.
Sterilant (sterilizer, sporicide) A chemical agent that destroys all forms of microbial life.
Sterile Free of all living organisms and viruses.
Sterilization The killing or removal of all living organisms and viruses from a growth medium.
Strain A population of cells of a single species all descended from a single cell; a clone.
Substrate The molecule that undergoes a specific reaction with an enzyme.
Substrate-level phosphorylation The synthesis of energy-rich phosphate bonds through reaction of inorganic phosphate with an activated organic substrate.
Symbiosis An intimate relationship between two organisms, often developed through prolonged association and coevolution.
Systematics The study of the diversity of organisms and their relationships; includes taxonomy and phylogeny.
Teichoic acid A phosphorylated polyalcohol found in the cell wall of some gram-positive Bacteria.
Temperate virus A virus whose genome is able to replicate along with that of its host without causing cell death in a state called lysogeny.
Thermophile An organism with a growth temperature optimum between 45 and 80°C.
Toxin A microbial substance able to induce host damage.
Toxoid A toxin modified so that it is no longer toxic but is still able to induce antibody formation.
Transcription The synthesis of an RNA molecule complementary to one of the two strands of a double-stranded DNA molecule.
Transcriptome The complement of all RNAs produced in an organism under a specific set of conditions.
Transduction The transfer of host genes from one cell to another by a virus.
Transfection The transformation of a prokaryotic cell by DNA or RNA from a virus. Used also to describe the process of genetic transformation in eukaryotic cells.
Transfer RNA (tRNA) A small RNA molecule used in translation that possesses an anticodon at one end and has the corresponding amino acid attached to its other end.
Transformation The transfer of genetic information via free DNA.
Transition A mutation in which a pyrimidine base is replaced by another pyrimidine or a purine is replaced by another purine.
Translation The synthesis of protein using the genetic information in a messenger RNA as a template.
Transporters Membrane proteins that function to transport substances into and out of the cell.
Transversion A mutation in which a pyrimidine base is replaced by a purine or vice versa.
Turbidity A measurement of suspended solids in water.
Vaccination (immunization) The inoculation of a host with inactive or weakened pathogens or pathogen products to stimulate protective immunity.
Vaccine An inactivated or weakened pathogen, or an innocuous pathogen product used to stimulate protective immunity.
Vector (1) A self-replicating DNA molecule that carries DNA segments between organisms and can be used as a cloning vector to carry cloned genes or other DNA segments for genetic engineering. (2) A living agent, usually an insect or other animal, able to carry pathogens from one host to another.
Viable Alive; able to reproduce.
Viable count A measurement of the concentration of live cells in a microbial population.
Viricidal agent An agent that stops viral replication and activity.
Virion A virus particle; the virus nucleic acid surrounded by a protein coat and in some cases other material.
Viroid Small, circular, single-stranded RNA that causes certain plant diseases.
Virulence The relative ability of a pathogen to cause disease.
Virulent virus A virus that lyses or kills the host cell after infection; a nontemperate virus.
Virus A genetic element that contains either RNA or DNA and that replicates in cells; has an extracellular form.
Wild type A strain of microorganism isolated from nature. The usual or native form of a gene or organism.
Xenobiotic A completely synthetic chemical compound not naturally occurring on Earth.
Xerophile An organism adapted to growth at very low water potentials.
Yeast The single-celled growth form of various fungi.
Zygote In eukaryotes, the single diploid cell resulting from the union of two haploid gametes.