Definitions Bank

The smallest, cell-wall-free prokaryotes that can live freely in nature and on artificial medium. They are pleomorphic and can pass through bacterial filters (0.2 µm), are 0.1–0.3 µm, and have a triple-layered membrane containing ~36% sterols which prevents osmotic lysis. Being wall-less they are resistant to all β-lactams. ⭐ The only term set in both modern papers.

Bacille Calmette-Guérin: a tuberculosis vaccine consisting of a freeze-dried preparation of a LIVE, ATTENUATED BOVINE organism, produced by Calmette and Guérin from 1908. It is used in national immunisation programmes, and converts the tuberculin test to positive — which is why a positive test in a vaccinated person does not prove infection.

A surface protein of Staphylococcus aureus, covalently bound to the peptidoglycan, which binds non-specifically to the Fc fragment of IgG. As a virulence factor it is anti-phagocytic (antibody is bound the wrong way round, leaving no Fc handle for the phagocyte), damages platelets, activates B cells and induces hypersensitivity and inflammation. Laboratory use: the coagglutination test.

Round or oval, EOSINOPHILIC, INTRACYTOPLASMIC inclusion bodies, observed under the light microscope within the cytoplasm of nerve cells infected with RABIES virus — classically the pyramidal cells of the hippocampus and Purkinje cells of the cerebellum, on HE stain. It is a form of inclusion body and part of the cytopathic effect, and is the classic histological diagnosis of rabies.

The form of gene transfer and recombination in bacteria in which a piece of free (“naked”) DNA is taken up by a competent bacterial cell and integrated into the recipient genome. Demonstrated by Griffith's 1928 pneumococcal experiment, in which heat-killed smooth strains transformed live rough strains into virulent ones.

A wall-less bacterium that is still able to grow and divide. Wall loss follows lytic enzymes such as lysozyme, or antibiotics blocking peptidoglycan synthesis. L-forms are difficult to culture (needing solidified medium of proper osmotic strength) and can revert to the original bacterial shape, causing chronic infection. Compare spheroplast (outer membrane present) and protoplast (absent), and Mycoplasma, which never had a wall and never reverts.

Long, slender, helical, soft and actively motile prokaryotes, with an outer membrane (sheath), axial filaments (endoflagella) and a protoplasmic cylinder. They divide by binary fission and are penicillin-sensitive. Three medically important genera: Treponema (syphilis), Borrelia (Lyme disease, relapsing fever) and Leptospira (leptospirosis).

A virus that is genetically deficient — lacking genes necessary for a complete infectious cycle — and is therefore incapable of producing infectious progeny virions. It may be rescued by a helper virus supplying the missing function (AAV with adenovirus; HDV with HBV), or act as a DIP — defective interfering particle — occupying host-cell materials and interfering with normal replication.

A form of persistent infection in which, after the acute infection, the viruses are not eliminated; there is intermittent multiplication and shedding, the virus is continuously detectable (often at low level), and mild or no symptoms are present — e.g. hepatitis B. Distinguish from latent infection, in which no infectious virus is produced until reactivation.

The form of gene transfer and recombination between bacteria in which bacterial DNA is carried from a donor cell to a recipient cell by a bacteriophage. Two types: generalised transduction, where any fragment of host DNA is mis-packaged into a phage head during lytic assembly, so any gene may transfer; and specialised transduction, where a prophage excises imprecisely and carries only the host genes adjacent to its integration site.

An agent (a protein) produced by a bacterium that inhibits or kills closely related species, or even different strains of the same species. Unlike an antibiotic — which is a microbial product inhibiting or killing microorganisms generally — a bacteriocin is narrow-spectrum, and is used in typing (colicin typing of E. coli, pyocin typing of Pseudomonas).

A neutralisation test between the toxin streptolysin O (SLO) and its specific antitoxin anti-streptolysin O (ASO), helping to diagnose rheumatic fever. It works because SLO is immunogenic (SLS is only weakly so). In active rheumatic fever the titre is particularly high; positive above 400 units.

The smallest, cell-wall-free prokaryotes that can live freely in nature and on artificial medium. They are pleomorphic and can pass through bacterial filters (0.2 µm), are 0.1–0.3 µm, and have a triple-layered membrane containing ~36% sterols which prevents osmotic lysis. Being wall-less they are resistant to all β-lactams. ⭐ The only term set in both modern papers.

An agglutination test performed to detect rickettsial antibody that can agglutinate certain strains of Proteus vulgaris (OX-2, OX-19, OX-K), helping to diagnose rickettsial disease. It works because rickettsiae share a common polysaccharide antigen with those Proteus strains. OX-19 for epidemic and murine typhus; OX-K for scrub typhus; OX-19 and OX-2 for Rocky Mountain spotted fever. It is non-specific and insensitive.

Transmission of a virus from one generation to the next — from parent to offspring — via the placenta, the birth canal, or breastfeeding; contrasted with horizontal, host-to-host transmission. Rubella, CMV and HSV cause congenital malformation; HBV and HIV cause congenital infection without malformation. ⚠️ Not to be confused with vertical gene transfer in bacterial genetics.

A minor antigenic change in the haemagglutinin or neuraminidase of influenza virus, occurring gradually through the accumulation of point mutations — an adaptation to host antibody. Each drifted strain prevails 2–5 years, causes epidemics, and forces annual reformulation of the vaccine. Contrast antigenic shift: a major change by reassortment of genome segments with an animal virus, in type A only, causing pandemics.

Acquired immunodeficiency syndrome — the final stage of HIV infection, in which destruction of CD4⁺ T cells leaves the immune system badly damaged. The CD4⁺ count falls below 200 per ml, the viral titre rises and the remaining immune response collapses, giving opportunistic infections (tuberculosis, candidiasis), tumours (Kaposi's sarcoma, from HHV-8) and AIDS dementia complex. Death in about 2 years without intervention.

The smallest, cell-wall-free prokaryotes that can live freely in nature and on artificial medium. They are pleomorphic and can pass through bacterial filters (0.2 µm), are 0.1–0.3 µm, and have a triple-layered membrane containing ~36% sterols which prevents osmotic lysis. Being wall-less they are resistant to all β-lactams. ⭐ The only term set in both modern papers.

An extrachromosomal genetic element capable of autonomous replication. The F (fertility) plasmid encodes the sex pilus of conjugation; R plasmids carry several antibiotic-resistance genes together.

The process that reduces the number of pathogenic microorganisms — but NOT necessarily bacterial spores and other resistant microorganisms — on objects or in materials, to a level which is not harmful to health. Contrast sterilisation, which kills or removes ALL microorganisms including endospores; the criterion of sterilisation is killing spores.

A quantitative agglutination test for enteric fever, detecting a patient's antibodies to the specific O antigen of S. typhi and the H antigens of S. typhi, S. paratyphi A, B and C.

A microorganism capable of causing disease. Distinguish pathogenicity (the disease-causing ability — a yes/no) from virulence (the quantitative ability to cause disease, involving adherence, persistence, invasion and toxigenicity).

The smallest infectious and ACELLULAR microbe, consisting of only ONE kind of nucleic acid (DNA or RNA), which replicates in host cells. It passes 0.2 µm filters, is an obligate intracellular parasite, and self-replicates. A virion is the complete mature (intact infectious) particle.

A form of persistent infection with a long incubation lasting months or years, during which no clinical symptoms appear although infectious virus may multiply; then chronic and progressive disease appears and eventually becomes fatal. Examples: HIV → AIDS; prions → kuru and CJD.

A MAJOR antigenic change in influenza HA or NA, producing a new subtype, caused by genetic REASSORTMENT of genome segments between human and animal viruses. It occurs in type A only — because type B has no animal reservoir — and causes PANDEMICS; the 1957 and 1968 pandemics were both reassortments with avian influenza A.

Viruses from various families which are transmitted via ARTHROPODS from one vertebrate to another. They cause encephalitis, febrile disease and haemorrhagic fever — e.g. encephalitis B virus and dengue virus (mosquito), forest encephalitis and Xinjiang haemorrhagic fever virus (tick).

The process of killing or removing ALL living microorganisms, including endospores. The criterion of sterilisation is to kill spores, because the endospore is the most resistant form in microbiology.

See Unit 1 for the full three-way classification into prokaryotic, non-cellular and eukaryotic microorganisms.

A particular (non-essential) bacterial structure which shields the bacteria from immune and phagocytic responses — the second of the six virulence factors. It is the most important virulence factor of S. pneumoniae, and is poly-D-glutamate in B. anthracis.