Tamil Nadu HSE Second Year Microbiology key terms

The outdated belief that living organisms can arise spontaneously from non-living matter, such as maggots from meat. Controlled experiments by Pasteur and others disproved it and supported biogenesis instead.

The principle that living organisms arise only from other living organisms, never from non-living material. It replaced spontaneous generation and underpins sterile technique in the laboratory.

The concept that specific microorganisms cause specific infectious diseases. Established by Koch and Pasteur, it transformed medicine and led to hygiene, vaccination and pure-culture methods.

Four criteria linking a microbe to a disease: the organism must be found in all cases, be isolated in pure culture, reproduce the disease when inoculated, and be re-isolated from the experimental host.

The two-part naming system devised by Linnaeus, giving each organism a capitalised genus and lowercase species name, both italicised or underlined, for example Escherichia coli.

The science of classifying, naming and identifying organisms. It uses a hierarchical order from domain down to species, based on structural, biochemical and genetic similarities.

A population of microorganisms descended from a single isolation and differing in some identifiable character from other members of the same species, such as a vaccine or industrial strain.

A cell lacking a membrane-bound nucleus and organelles, with DNA in a nucleoid. Bacteria and archaea are prokaryotes, typically 1 to 5 micrometres in size.

A cell with a true nucleus enclosed by a membrane and with membrane-bound organelles such as mitochondria and chloroplasts. Fungi, protozoa and algae are eukaryotic microorganisms.

The mesh-like polymer of alternating N-acetylglucosamine and N-acetylmuramic acid cross-linked by short peptides, forming the bacterial cell wall and the target of penicillin.

A highly resistant dormant structure formed inside certain Gram-positive bacteria such as Bacillus and Clostridium, surviving heat, drying and chemicals, and requiring moist heat for destruction.

A gelatinous layer outside the cell wall, usually polysaccharide, that protects against phagocytosis and helps adhesion. It contributes to virulence and is demonstrated by negative staining.

A long filamentous appendage used for motility, built from flagellin protein and anchored by a basal body. Arrangement may be monotrichous, lophotrichous, amphitrichous or peritrichous.

Short hair-like surface structures of bacteria. Fimbriae mediate adhesion to host cells, while the sex pilus forms a conjugation tube for transfer of plasmid DNA.

A differential staining method using crystal violet, iodine, alcohol decolouriser and safranin. Thick peptidoglycan retains the purple complex; thin walls stain pink with the counterstain.

A staining technique that divides bacteria into groups by colour, revealing structural or chemical differences. Examples include Gram stain and acid-fast stain.

A staining method using hot carbol fuchsin that detects mycolic acid-rich cell walls. Mycobacteria retain the red dye despite acid-alcohol decolourisation; other bacteria appear blue.

A technique in which the background, not the cell, is stained with an acidic dye such as nigrosin, leaving the organism as a clear halo. It demonstrates capsules and avoids heat fixing.

A single basic dye applied to a heat-fixed smear to show cell shape, size and arrangement. It does not differentiate organisms by chemical property.

An organism that builds its organic carbon from carbon dioxide using light or inorganic chemical energy. Photoautotrophs and chemoautotrophs are the two main types.

An organism requiring preformed organic compounds as its carbon source. Most bacteria of medical importance, and all fungi and protozoa studied here, are heterotrophic.

A culture medium containing substances that inhibit unwanted microbes while permitting the growth of the target group, such as salts suppressing Gram-positive organisms.

A medium containing indicators or specific substrates that produce distinguishable colonies, allowing different species growing together to be told apart, as on MacConkey agar.

A medium supplemented with nutrients such as blood or serum to support fastidious organisms that fail to grow on ordinary media.

A laboratory culture containing only one species or strain, obtained by aseptic transfer or single-colony isolation. It is essential for identification and for Koch's postulates.

The complete destruction or removal of all microorganisms, including endospores and viruses, from an object or medium, usually by autoclaving, dry heat or filtration.

The destruction or inhibition of pathogenic microorganisms on inanimate surfaces by chemical agents. It reduces infection risk but does not necessarily kill all spores.

An instrument that sterilises by saturated steam under pressure, normally at 121 degrees Celsius and 15 psi for 15 minutes, which reliably kills bacterial endospores.

A chemical agent applied to living tissue, such as skin, to inhibit or destroy microorganisms. It is usually less toxic than a disinfectant used on inanimate objects.

A substance produced by one microorganism that inhibits or kills another, or a semisynthetic derivative of such a substance. Examples include penicillin and streptomycin.

The ability of a microorganism to survive a drug that would normally inhibit it, gained by mutation, enzyme inactivation, efflux or target change, and spread by plasmids.

A small circular extrachromosomal DNA molecule that replicates independently of the bacterial chromosome and often carries genes for resistance or transfer. It is a common cloning vector.

Transfer of genetic material between bacteria through direct cell-to-cell contact via a sex pilus. Typically a plasmid, sometimes with chromosomal genes, passes to the recipient.

The uptake and incorporation of free DNA from the environment by a competent bacterial cell, which may acquire new traits such as capsule formation or drug resistance.

Transfer of bacterial DNA from one cell to another by a bacteriophage, either accidentally carrying host genes or through lysogenic conversion.

A heritable change in the DNA base sequence, produced by replication errors or mutagens. It may be silent, missense, nonsense or a frameshift affecting many codons.

An enzyme that cuts DNA at a specific recognition sequence, protecting the cell from foreign DNA. It is used to create recombinant molecules in genetic engineering.

Any substance, usually a protein or polysaccharide, that is recognised by the immune system and can provoke a specific immune response, including antibody production.

A Y-shaped immunoglobulin protein produced by plasma cells that binds specifically to an antigen, marking it for neutralisation, phagocytosis or complement lysis.

The state of resistance to a particular pathogen, gained actively through infection or vaccination, or passively through transferred antibodies, and mediated by humoral and cell-mediated responses.

A preparation of weakened, killed or subunit antigen that stimulates protective immunity without causing serious disease, often requiring several doses and cold storage.

The degree of pathogenicity of a microorganism, measured by its ability to invade, colonise and damage the host. It depends on factors such as capsules, toxins and enzymes.

A soluble protein toxin released by living bacteria, typically heat-labile and highly specific, often inactivated to form a toxoid used for immunisation.

A heat-stable lipopolysaccharide component of the Gram-negative outer membrane, released on cell lysis, causing fever and shock. It is not converted to a toxoid.

An infection acquired in a hospital or other healthcare setting and not present or incubating on admission, often caused by resistant organisms and prevented by hand hygiene.

Conversion of atmospheric nitrogen gas into ammonia by free-living or symbiotic bacteria such as Rhizobium, making nitrogen available to plants and to the soil.

The cycling of elements such as carbon, nitrogen and sulphur between living organisms and the physical environment, largely driven by microbial transformation reactions.

Use of microorganisms or their enzymes to remove or detoxify pollutants such as oil spills, pesticides and heavy metals from soil and water, converting them to harmless products.

An anaerobic metabolic process in which microorganisms convert sugars into products such as alcohol, lactic acid or acetic acid, exploited in food and industrial production.

A heat treatment that kills pathogens and reduces spoilage organisms in milk and other liquids without sterilising them, preserving flavour better than boiling.

Laboratory practices that prevent contamination of cultures, media and the worker, including flaming loops, working near a flame and never opening plates unnecessarily.

An indicator tape that changes colour after exposure to steam at the required temperature, giving a visible check that an autoclave load has been processed.

A procedure, usually in semi-solid agar or a hanging-drop preparation, that shows whether bacteria possess flagella by spreading growth or by directed swimming.

A wet-mount technique in which a drop of suspension hangs from a coverslip over a cavity slide, allowing living microorganisms to be observed for motility and shape.