Overview
This lecture covers methods for controlling microbial growth, focusing on physical techniques, resistance levels among microbes, and key concepts such as sterilization, disinfection, and antisepsis.
Key Concepts in Microbial Control
- Sepsis means microbial contamination; asepsis means absence of contamination.
- Aseptic technique in labs and surgery prevents contamination.
- Controlling microbes also reduces food spoilage and foodborne illness.
- Sterilization destroys all microbial life, including endospores.
- Disinfection reduces/inhibits microbes on non-living surfaces; antisepsis does so on living tissue.
- Degerming is mechanical removal of microbes from a limited area (e.g., handwashing).
- Sanitation lowers microbial counts on eating utensils via washing and chemicals.
Methods and Effectiveness of Microbial Control
- Microbial kill rate is proportional to the population size and occurs at a constant rate.
- More microbes present = longer time to eliminate them.
- Effectiveness influenced by initial number, agent concentration, temperature, pH, organic matter, evaporation, exposure time, and microbial species/age.
- Few agents achieve true sterility; most reduce counts.
Microbial Resistance Hierarchy
- Prions (infectious proteins) and endospores are extremely resistant to control methods.
- Mycobacteria resist due to waxy cell walls; protozoan cysts are protected by chitin walls.
- Gram-negative bacteria are more resistant than Gram-positive due to outer membrane porins.
- Fungi and spores are moderately resistant; enveloped viruses and Gram-positive bacteria are least resistant.
Mechanisms of Antimicrobial Agents
- Disrupt cell membranes, cause protein denaturation, damage nucleic acids, or create free radicals.
Physical Methods of Microbial Control
- Moist heat (e.g., autoclaving) denatures proteins; standard autoclave: 121°C, 15 min, 15 psi.
- Pasteurization (e.g., milk at 72°C for 15 sec) reduces pathogens/spoilage organisms but does not sterilize.
- Dry heat (e.g., flaming, hot air) kills by oxidation but requires longer exposure.
- Low temperatures (e.g., refrigeration) are bacteriostatic (inhibit growth, not kill).
- High pressure denatures proteins; effective for vegetative cells, not endospores.
- Desiccation (drying) prevents metabolism; is bacteriostatic.
- High salt/sugar concentrations cause plasmolysis; used to preserve foods, is bacteriostatic.
- Filtration removes microbes from heat-sensitive fluids using membrane filters or HEPA filters.
- Ionizing radiation (X-rays, gamma rays) creates DNA-damaging free radicals and sterilizes items.
- Non-ionizing radiation (UV) damages DNA but only disinfects surfaces; not deeply penetrating.
- Microwaves may kill by heat but are unreliable for sterilization.
Key Terms & Definitions
- Sterilization — Removal/destruction of all forms of microbial life, including endospores.
- Disinfection — Destruction of vegetative pathogens on inanimate objects.
- Antisepsis — Destruction of vegetative pathogens on living tissue.
- Degerming — Mechanical removal of microbes from a limited area.
- Sanitation — Lowering microbial counts to safe levels.
- Bactericidal — Agent that kills bacteria (suffix -cide).
- Bacteriostatic — Agent that inhibits bacterial growth (suffix -stat).
- HEPA filter — High-efficiency particulate air filter removing particles >0.3 μm.
Action Items / Next Steps
- Review physical control methods for exam preparation.
- Refer to the study guide for required detail level.
- No need to study chemical methods of microbial control for this class.