Tamil Nadu HSE First Year Microbiology key terms

The branch of biology that studies organisms too small to be seen clearly with the naked eye, including bacteria, fungi, algae, protozoa and viruses, and their structure, growth, genetics and effects on humans and the environment.

The old belief that living organisms can arise directly from non-living matter, such as maggots from rotting meat. Controlled experiments with sealed flasks finally disproved it and supported biogenesis instead.

The principle that living organisms arise only from pre-existing living organisms, never from non-living material. It replaced spontaneous generation and underpins aseptic technique and modern sterilisation practice.

A set of four criteria used to prove that a particular microorganism causes a particular disease: the organism must be found in all cases, be isolated in pure culture, reproduce the disease when inoculated, and be recovered again from the infected host.

Single-celled prokaryotic microorganisms that lack a membrane-bound nucleus and organelles, usually reproduce by binary fission, and are classified by shape, cell wall type and staining reactions.

An obligate intracellular parasite consisting of nucleic acid enclosed in a protein coat, with no independent metabolism. It can replicate only inside a living host cell and is unaffected by antibiotics.

The protein coat that surrounds and protects the nucleic acid genome of a virus. It is built from repeating protein subunits called capsomeres and helps the virus attach to and enter host cells.

A virus that infects bacteria, attaching to the cell surface and injecting its nucleic acid inside. Lytic phages destroy the host cell, while temperate phages can integrate into the bacterial chromosome as a prophage.

The mesh-like polymer of sugar chains cross-linked by short peptides that forms the main structural component of bacterial cell walls. Its thickness differs between Gram-positive and Gram-negative bacteria and it is the target of penicillin.

A differential staining method that divides bacteria into Gram-positive (purple) and Gram-negative (pink) groups. The result depends on cell wall structure: thick peptidoglycan retains the crystal violet-iodine complex, thin peptidoglycan does not.

A highly resistant dormant structure formed inside certain Gram-positive bacteria such as Bacillus and Clostridium. It survives heat, drying and chemicals, resists ordinary staining, and germinates into a vegetative cell when conditions improve.

The asexual mode of reproduction in bacteria in which one cell grows and divides into two identical daughter cells after chromosome replication and septum formation. It produces exponential growth in batch culture.

The time taken for a microbial population to double in number during exponential growth. It varies with species, nutrients and temperature and is used to calculate growth rates and predict culture yield.

The graph of cell number or biomass against time in a closed batch culture, showing lag, exponential (log), stationary and death phases. Each phase reflects changes in nutrient supply and waste accumulation.

A prepared mixture of nutrients that supports the growth of microorganisms in the laboratory. It supplies carbon, nitrogen, minerals and water and may be solid, semi-solid or liquid, defined or complex.

A culture medium containing substances that inhibit unwanted organisms while allowing the target microorganism to grow, such as salts, dyes or antibiotics. It is used to isolate a particular species from a mixed sample.

A laboratory culture containing only one species or strain of microorganism, obtained by techniques such as streak plating or serial dilution. Pure cultures are essential for identification, biochemical testing and industrial production.

The complete destruction or removal of all living microorganisms, including bacterial endospores and viruses, from an object or medium. Methods include moist heat, dry heat, filtration and radiation.

The destruction or inhibition of most pathogenic microorganisms on inanimate surfaces or skin by chemical or physical agents, without necessarily killing all spores. It reduces infection risk but does not achieve sterility.

A chemical agent applied to living tissues such as skin or wounds to kill or inhibit microorganisms. It is usually less toxic than a disinfectant because it must not damage host cells at its working concentration.

An instrument that sterilises materials using saturated steam under pressure, typically at 121 degrees Celsius and 15 psi for 15 to 20 minutes. The pressure raises the steam temperature so that heat-resistant endospores are destroyed.

A substance produced by or derived from a microorganism that kills or inhibits other microorganisms at low concentrations. Antibiotics act on specific targets such as cell wall synthesis, protein synthesis or nucleic acid replication.

The ability of a microorganism to survive and grow in the presence of an antibiotic that would normally inhibit or kill it. It may arise by mutation or by acquiring resistance genes through transformation, transduction or conjugation.

A small, circular, extrachromosomal DNA molecule that replicates independently in bacteria and often carries genes for antibiotic resistance or metabolic functions. Plasmids are widely used as vectors in genetic engineering.

The uptake of free DNA from the environment by a competent bacterial cell and its incorporation into the cell's genome or plasmids. It was first demonstrated in Streptococcus pneumoniae and is a natural route of gene transfer.

The transfer of DNA from one bacterium to another through direct cell-to-cell contact, usually via a conjugative pilus. It commonly spreads plasmids carrying antibiotic resistance genes between bacteria.

The transfer of bacterial DNA from one host cell to another by a bacteriophage that accidentally packages host DNA instead of, or along with, viral DNA. It is a form of horizontal gene transfer.

A viral life cycle in which a temperate phage inserts its DNA into the bacterial chromosome and is replicated along with it as a prophage, without immediately killing the host. The prophage may later enter the lytic cycle.

The body's ability to resist infection and neutralise foreign material through coordinated defence mechanisms. It includes innate, non-specific responses and adaptive, specific responses involving lymphocytes and antibodies.

Any substance, usually a protein or polysaccharide, that is recognised by the immune system and can trigger a specific immune response. Antibodies and T-cell receptors bind to specific regions of the antigen called epitopes.

A Y-shaped protein produced by plasma cells that binds specifically to an antigen and helps neutralise or eliminate it. Antibodies are immunoglobulins and form the basis of serological tests.

A preparation of weakened, inactivated or subunit antigens that stimulates protective adaptive immunity without causing serious disease. It creates immunological memory so that the body responds rapidly on later exposure to the pathogen.

A microorganism or virus that is capable of causing disease in a host. Pathogenicity depends on factors such as adhesion, invasiveness, toxin production and the host's immune status.

The resident community of microorganisms that normally lives on and inside a healthy body, especially on the skin and in the gut and mouth. It usually causes no harm and can protect against pathogens.

The conversion of atmospheric nitrogen gas into ammonia by microorganisms such as Rhizobium, Azotobacter and cyanobacteria. It makes nitrogen available for plant growth and is a key process in the nitrogen cycle.

The natural circulation of elements such as carbon, nitrogen, sulphur and phosphorus between living organisms and the physical environment. Microorganisms drive many of these transformations, including decomposition and nitrification.

The breakdown of dead organic matter into simpler compounds by microorganisms and detritivores, releasing nutrients back into the environment. Aerobic decomposition yields carbon dioxide and water, while anaerobic decomposition can produce methane.

An anaerobic metabolic process in which microorganisms convert sugars and other organic compounds into products such as ethanol, lactic acid or acetic acid, generating energy without an external electron acceptor. It is widely used in food and industrial microbiology.

A heat treatment that kills most pathogenic and spoilage microorganisms in milk and other liquids without sterilising them, usually by rapid heating to 63 to 72 degrees Celsius followed by rapid cooling. It preserves nutritional quality.

The deterioration of food quality caused by microbial growth, chemical reactions or physical damage, producing off-flavours, odours, discolouration or slime. Spoilage organisms are not always pathogenic but make food unacceptable.