Sterilisation, Disinfection and Antimicrobial Agents
Domain 5 distinguishes sterilisation from disinfection and matches methods to materials. Sterilisation kills or removes all microorganisms including endospores. Disinfection reduces pathogens on inanimate objects; antisepsis applies to living tissue; sanitisation lowers counts to safe levels. Bactericidal agents kill; bacteriostatic agents inhibit growth.
Physical methods
Moist heat in an autoclave at 121°C and 15 psi for 15–20 minutes is sporicidal. Pasteurisation (63°C for 30 minutes, 72°C for 15 seconds, or ultra-high temperature at 135–150°C for 1–2 seconds) reduces pathogens but does not sterilise. Dry heat uses a hot-air oven at 160–170°C for 2 hours; flaming and incineration also sterilise. Filtration through 0.22 µm membranes treats heat-labile liquids; HEPA filters clean air. Ultraviolet radiation around 260 nm is used for surfaces and air but has poor penetration; gamma rays sterilise disposables. Tyndallisation uses repeated 100°C heating.
Chemical agents
Alcohols (70% ethanol is more effective than 100%), aldehydes such as glutaraldehyde, phenols, halogens such as hypochlorite and iodine, quaternary ammonium compounds and ethylene oxide gas. Antibiotics act on the cell wall (penicillin), protein synthesis (streptomycin, tetracycline), nucleic acid synthesis (rifampicin) or folate synthesis (sulphonamides). Resistance arises by enzymatic inactivation, efflux, target alteration or reduced uptake.
Worked example
Why use 121°C in an autoclave rather than boiling at 100°C? Reasoning: pressurised steam raises the boiling point; at 15 psi saturated steam reaches about 121°C, which kills endospores in 15–20 minutes. Boiling at 100°C does not reliably kill spores.
Traps
Pasteurisation is not sterilisation. Ultraviolet light does not penetrate glass, liquids or dust. Autoclave tape shows heat exposure, not sterility. Absolute alcohol coagulates proteins and is less effective than 70%. Organic matter can inactivate disinfectants.