Overview
This lecture covered the physical and chemical methods for controlling microorganisms, focusing on the definitions and distinctions among sterilization, disinfection, antisepsis, sanitation, and the methods and agents used to achieve microbial control.
Historical and Basic Microbial Control Methods
- Ancient methods of food preservation include salting, smoking, pickling, drying, and sunlight exposure.
- Sunlight (UV) and drying kill microbes; heavy metals in containers inhibit microbial growth.
- Burning and incineration historically controlled the spread of disease.
Key Definitions in Microbial Control
- Sterilization: Destroys all viable microorganisms including endospores; gold standard for critical items.
- Disinfection: Eliminates most pathogens except some endospores; used mostly for surfaces.
- Antisepsis: Application of antimicrobials to living tissue to reduce risk of infection.
- Sanitation/Decontamination: Mechanical removal of most microbes to safe levels; lowest standard.
Levels and Resistance of Microorganisms
- Microbes vary in resistance: prions and bacterial endospores are hardest to kill; enveloped viruses are easiest.
- Destruction of endospores is the main goal of sterilization.
- Boiling water disinfects but does not sterilize (endospores survive).
Physical Control Methods
- Moist heat (autoclave): Uses steam under pressure (121°C, 15 psi, 15–20 mins) to sterilize, including endospores.
- Dry heat: Requires higher temperatures and longer times than moist heat to sterilize.
- Cold: Slows microbial growth but does not kill microorganisms.
- Desiccation: Removes water, inhibiting microbial growth.
- Radiation:
- Ionizing (gamma/X-rays): Sterilizes by causing DNA breaks.
- Nonionizing (UV): Causes DNA damage (thymine dimers) but not deeply penetrating.
- Filtration: Physically removes microbes from air or liquids; only ultra-fine filters can sterilize.
Chemical Methods and Agents
- Ideal chemicals: rapid action, broad spectrum, stable, non-corrosive, affordable.
- High-level germicides: Kill endospores (e.g., glutaraldehyde, bleach at high concentration).
- Intermediate-level: Kill fungal spores, not all bacterial spores; used on semi-critical items.
- Low-level: Kill vegetative cells and enveloped viruses; used on non-critical surfaces.
Types of Chemical Agents
- Halogens (chlorine, iodine): Disrupt proteins/enzymes; some can sterilize at high concentration.
- Phenolics: Disrupt membranes and proteins but may be toxic (e.g., triclosan in soaps).
- Alcohols: Denature proteins and disrupt membranes; 60–70% is effective.
- Oxidizing agents: (Hydrogen peroxide, ozone) damage proteins/DNA; high concentrations can sterilize.
- Detergents/Surfactants: Disrupt cell membranes; effective mainly as sanitizers.
- Heavy metals: Antimicrobial by disrupting microbial enzymes and proteins (e.g., silver, copper).
- Aldehydes: (Glutaraldehyde, formaldehyde) cross-link proteins/nucleic acids; effective sterilants.
- Gaseous sterilants: (Ethylene oxide) used for heat-sensitive materials; cold sterilization.
Factors Affecting Microbial Death Rate
- Microbial load, nature of microbes, environment (pH, temp), concentration of chemical, exposure time, and presence of organic matter all influence effectiveness.
Key Terms & Definitions
- Sterilization — complete destruction of all microbial life, including spores.
- Disinfection — removal of most pathogens from inanimate objects.
- Antisepsis — chemical treatment to reduce microbes on living tissue.
- Sanitation/Decontamination — reduction of microbes to safe public health levels.
- Cidal (e.g., bactericidal) — kills a specific group of microbes.
- Static (e.g., bacteriostatic) — inhibits growth, does not kill.
- Endospore — tough, dormant bacterial structure highly resistant to killing.
- Autoclave — device using pressurized steam to sterilize.
- Halogens — antimicrobial elements (chlorine, iodine).
- Aldehyde — chemical sterilant (e.g., glutaraldehyde).
Action Items / Next Steps
- Review key definitions and differences among sterilization, disinfection, antisepsis, and sanitation.
- Memorize which methods/agents are used for different levels of microbial control.
- Prepare for lab by understanding proper use of disinfectants and exposure times.