Microbiology: Controlling Microbial Growth

Key Concepts

• Microbial Growth: Refers to an increase in the number of microbes, not the size.
• Control vs. Kill: Sometimes we need to control or stop growth rather than completely kill the microbes.

Considerations for Controlling Microbial Growth

• Purpose: What you're trying to achieve influences the method.
• Resistance: Some microbes, like endospores, are more resistant, affecting method choice.
• Disease Prevention: Main goal is to prevent disease spread.

Biological Safety Levels (BSL)

• BSL 1: Minimal risk, non-pathogenic strains.
• BSL 2: Can cause disease in specific environments.
• BSL 3: More severe diseases that can cause illness.
• BSL 4: Lethal diseases, often aerosol-transmitted; require HEPA filters.

Terminology

• Sterilization: Complete removal/killing of all life forms, usually in labs/medical facilities.
• Disinfection: Inactivates microbes on surfaces using chemicals/heat.
• Antiseptics: Disinfects living skin (e.g., alcohol, hydrogen peroxide).
• Degerming: Physical removal of microbes (e.g., hand scrubbing).
• Sanitation: Reducing microbes to safe levels for public health.
• -cide and -static:
  • -cide: Kills microbes (e.g., bacteriocide).
  • -static: Inhibits growth (e.g., bacteriostatic).

Methods of Microbial Control

Physical Methods

1. Heat
   • Boiling: Kills some bacteria/viruses but not endospores.
   • Incineration: Direct flame, sterilization method.
   • Autoclave: High temp (121°C) and pressure, sterilizes including endospores.
   • Pasteurization: Reduces pathogens, not sterilization.
2. Cold: Slows down microbial growth, does not sterilize.
3. Pressure: Can kill some microbes, extends shelf life.
4. Drying/Osmotic Pressure: Removes water (e.g., beef jerky, jellies).
5. Radiation
   • Ionizing: Sterilizes, penetrates cells (e.g., x-ray, gamma).
   • Non-ionizing: UV light, does not penetrate.
6. Filtration
   • HEPA filters: Air filtration.
   • Membrane filters: Liquid filtration, necessary if heat cannot be used.

Chemical Methods

1. Phenolics: Phenol derivatives, disinfectants.
2. Heavy Metals: Mercury, silver, copper, zinc slow microbial growth.
3. Halogens: Iodine and chlorine (e.g., bleach).
4. Alcohol: Most effective at 70%, hand sanitizers.
5. Bispoquinides: Used in surgical scrubs.
6. Alkylating Agents (e.g., Ethylene Oxide): Chemical sterilant.

Study Tips

• Focus on methods that can sterilize vs. those that cannot.
• Understand appropriate techniques for different scenarios.