Sterilization, Disinfection and Antimicrobial Agents
Domain 5 tests the distinction between sterilization, disinfection and antisepsis, the physical and chemical methods used, and the mechanisms of antimicrobial agents. Questions often ask which method is appropriate for a heat-labile solution or why boiling is not sterilization.
Core rules and concepts
- Sterilization destroys all microbial life, including bacterial spores. Disinfection reduces pathogens on inanimate objects but may not kill spores. Antisepsis applies antimicrobial chemicals to living tissue. Sanitization lowers microbial numbers to safe levels.
- Moist heat under pressure: autoclave at 121°C, 15 psi, 15 minutes for typical loads. Dry heat: 160°C for 2 hours or 170°C for 1 hour. Pasteurization (63°C for 30 min or 72°C for 15 s) reduces pathogens but is not sterilization.
- Filtration removes microbes from heat-labile liquids and air. UV light is useful for surface and air disinfection but has poor penetration. Gamma irradiation is used for heat-sensitive disposable plastics.
- Antibiotics act on cell walls (penicillin), 70S ribosomes (streptomycin, tetracycline), nucleic acid synthesis (rifampicin) or folate synthesis (sulfonamides). Bactericidal agents kill; bacteriostatic agents inhibit growth.
Worked example
A heat-labile vitamin solution must be sterilized. Reasoning: autoclaving would destroy the vitamin, and boiling would not reliably kill spores. Passing the solution through a 0.22 µm membrane filter removes bacteria and spores without heat. The filter and receiving vessel must be sterile, and the process performed aseptically.
Mark-losing traps
- Boiling at 100°C does not kill all spores and is not sterilization.
- 70% ethanol is more effective than 100% because water aids protein denaturation and penetration.
- UV does not penetrate glass, liquids or dust well; it is not a substitute for autoclaving.
- Antibiotics do not treat viral infections, and misuse promotes resistance.