Overview
This lecture covers methods to control microbial growth, distinctions between sterilization and disinfection, factors affecting antimicrobial effectiveness, microbial resistance hierarchies, and physical methods of microbial control.
Concepts of Microbial Control
- Sepsis means microbial contamination; asepsis is the absence of significant contamination.
- Aseptic technique prevents contamination in labs and surgeries.
- Microbial control prevents food spoilage and illness.
Levels and Terms of Control
- Sterilization destroys all microbial life, including endospores, typically by heat (e.g., autoclave).
- Disinfection reduces microbes on non-living surfaces; antisepsis reduces microbes on living tissues.
- Disinfectants and antiseptics do not destroy endospores.
- Degerming removes (not kills) microbes from a limited area (e.g., handwashing).
- Sanitation lowers microbial counts on eating utensils, usually by high-temp washing.
- "-cide" suffix indicates outright killing; "-stat/stasis" indicates inhibition, not killing.
Factors Influencing Microbial Death
- Microbes die at a constant rate (logarithmic decline) when treated.
- Higher initial population takes longer to eliminate.
- Effectiveness depends on number of microbes, agent concentration/age, temperature, pH, presence of organic matter, evaporation, diffusibility, and exposure time.
- Few chemicals achieve true sterility.
Microbial Resistance Hierarchy
- Prions (infectious proteins) are most resistant; destroyed by incineration and proteases.
- Endospores are highly resistant due to tough protein shells.
- Mycobacteria resist due to waxy cell walls (mycolic acid).
- Protozoan cysts resist via thick chitin walls; vegetative protozoans via pellicle.
- Gram-negative bacteria resist via outer membrane with porins.
- Fungi (especially spores) are moderately resistant.
- Viruses with lipid envelopes are least resistant; non-enveloped viruses are more resistant.
- Gram-positive bacteria and enveloped viruses are less resistant.
Mechanisms of Antimicrobial Agents
- Disrupt cell membranes, causing leakage.
- Denature proteins and inhibit enzymes.
- Damage nucleic acids, preventing replication/metabolism.
- Produce free radicals (oxidizing agents).
Physical Methods of Microbial Control
- Moist heat denatures proteins (boiling, autoclaving at 121°C, 15 min, 15 psi).
- Pasteurization (e.g., 72°C for 15 sec) reduces spoilage organisms, does not sterilize.
- Dry heat sterilizes by oxidation (flaming, incineration, hot air at 170°C for 2 hours).
- Low temperature (refrigeration, freezing) is bacteriostatic, not bactericidal.
- High pressure denatures proteins but does not affect endospores.
- Desiccation (drying) is bacteriostatic; inhibits metabolism.
- Osmotic pressure (high salt/sugar) causes plasmolysis; mainly inhibits, does not kill.
- Filtration removes microbes from heat-sensitive liquids with various pore sizes.
- Ionizing radiation (X-rays, gamma rays) creates free radicals; used for sterilization.
- Non-ionizing radiation (UV light) damages DNA, used mainly on surfaces; not penetrating.
- Microwaves mostly kill by heat but are not always reliable for sterilization.
Key Terms & Definitions
- Sterilization — Destruction of all microbial life, including endospores.
- Disinfection — Reducing or inhibiting microbes on inanimate surfaces.
- Antisepsis — Reducing or inhibiting microbes on living tissues.
- Degerming — Mechanical removal of microbes from a limited area.
- Sanitation — Lowering microbial counts on food-contact surfaces.
- Bactericidal — Killing bacteria.
- Bacteriostatic — Inhibiting bacterial growth without killing.
- Prion — Infectious, highly resistant protein particle.
- Endospore — Tough, dormant bacterial structure resistant to control measures.
- Autoclave — Device using pressurized steam for sterilization.
Action Items / Next Steps
- Review physical (not chemical) microbial control methods for exams.
- Use study guide for required details.
- Remember: Pasteurization does not achieve sterilization.