Overview
This lecture covers various physical, chemical, and mechanical methods for controlling microbial growth, explaining key definitions and techniques for sterilization, disinfection, and antisepsis.
Types of Microbial Control Methods
- Three primary methods: physical (heat, radiation), chemical (gases, liquids), and mechanical (filtration).
- Microbial control targets a spectrum from bacteria to prions.
Key Terms in Microbial Control
- Sterilization removes all forms of microbial life, including endospores and viruses.
- Disinfection reduces pathogen numbers on non-living objects but does not remove endospores.
- Antisepsis is disinfection for living tissues.
- Sanitization reduces microbial numbers on surfaces to safe levels.
- Degermination is mechanical removal of microbes, e.g., handwashing.
Physical Methods
- Heat: moist heat (autoclave, boiling, pasteurization) and dry heat (incineration, dry oven).
- Autoclave: 121°C, 15 min, 15 psi for sterilization.
- Pasteurization: 72°C, 15 sec (disinfection only).
- Incineration: oxidizes cells, sterilization.
- Cold & Desiccation: slow microbial growth (microbiostatic), do not kill microbes.
- Radiation:
- Ionizing (X-rays, gamma rays): deep penetration, sterilization.
- Non-ionizing (UV): surface disinfection, causes DNA mutations.
- Filtration: removes microbes from air or liquids; HEPA filters (air), membrane filtration (liquids).
Chemical Methods
- High-level germicides kill spores, used on heat-sensitive critical items.
- Intermediate-level kill fungal spores, tubercle bacteria, viruses.
- Low-level eliminate vegetative cells and some viruses.
- Effectiveness depends on nature of treated material, contamination level, exposure time, and concentration.
Major Chemical Agents
- Halogens (chlorine, iodine): denature proteins by disrupting disulfide bonds.
- Phenols: disrupt membranes, denature proteins, not sporicidal.
- Chlorhexidine: pre-surgical/hand washes, disrupts membranes and proteins.
- Alcohols: dissolve lipids, denature proteins, not effective against spores.
- Hydrogen peroxide: forms toxic radicals, effective against anaerobes, sporicidal at high concentrations.
- Aldehydes (glutaraldehyde, formaldehyde): denature proteins, high-level disinfectants.
- Gaseous agents (ethylene oxide): sterilize plastics and packaged foods, sporicidal.
- Detergents & soaps: surfactants remove microbes mechanically; quats disrupt membranes.
- Heavy metals (silver, mercury): inactivate proteins, low-level agents.
- Dyes, acids, alkalis: inhibit microbial and fungal growth, very low-level activity.
Factors Influencing Effectiveness
- Population size, type of microbe, temperature, pH, agent concentration, duration, and presence of organic matter.
- Consider item sensitivity, reuse possibility, and cost when choosing control methods.
Key Terms & Definitions
- Sterilization — elimination of all microbial life forms.
- Disinfection — removal of pathogens, not endospores, from non-living surfaces.
- Antisepsis — reduction of microbes on living tissue.
- Sanitization — lowering microbe count to safe levels.
- Degermination — mechanical removal of microbes from limited area.
- Cidal — suffix meaning to kill (e.g., bactericidal).
- Static — suffix meaning to inhibit growth (e.g., microbiostatic).
Action Items / Next Steps
- Review textbook tables 11-1, 11-2, 11-4, and 11-13 for charts of resistance, definitions, and agents.
- Prepare to discuss application scenarios for different decontamination methods.
- Bring questions to office hours or send by email.