Chapter 7: Controlling Microbial Growth

Introduction

• Assumption: Often aim to kill every microbe on a surface or tissue.
• Sterilization: Killing all living microbes, including endospores.
  • Not always necessary or practical, especially on living tissue or food items.
  • Example: Commercial sterilization targets Clostridium botulinum spores.

Methods of Microbial Control

• Sterilization: Complete destruction of all microbial life.
• Disinfection and Antisepsis:
  • Disinfection: Destroying vegetative pathogens on inanimate objects.
  • Antisepsis: Destroying microbes on living tissue.
• De-germing:
  • Reducing microbial numbers, e.g., using alcohol swabs before injections.
• Sanitization:
  • Removing or reducing microbes on non-living surfaces like utensils.

Key Concepts

• Microbicidal vs. Microbial Static:
  • Microbicidal: Kills microbes.
  • Microbial Static: Inhibits microbial growth.
• Microbial death rate is constant over time.
• Factors Affecting Microbial Control:
  • Population size/density affects treatment time.
  • Different microbes react differently to the same treatment.

Physical Methods of Microbial Control

Temperature

• Heat:
  • Denatures proteins and macromolecules by disrupting hydrogen bonds.
  • Moist Heat: More effective due to better heat transfer.
• Autoclaving:
  • Uses high pressure and high temperature steam to sterilize.
  • Considerations: material sensitivity to moisture, proper wrapping, and indicator tape for sterilization verification.
• Dry Heat: Removes water, takes longer, used when moisture is not suitable.
• Pasteurization: Reduces microbes in food to prevent spoilage.

Low Temperature

• Slows down microbial metabolism, may be bactericidal or static.
• Freezing forms ice crystals that may rupture cell membranes.

Desiccation

• Removes water, leading to microbial dormancy rather than death.

Filtration

• Sterilizes liquids by filtering out microbes.
• Uses filter paper with pores smaller than microbes.

Radiation

• Ionizing Radiation (X-rays, Gamma rays):
  • Oxidizes molecules, removing electrons and causing damage.
• Non-ionizing Radiation (UV):
  • Causes DNA damage, particularly thymine dimers, leading to cell death.
  • Example application: UV treatment plants for reducing microbes in water.

Conclusion

• Different methods balance effectiveness with preservation of the material being treated.
• Choice of method depends on the context and nature of the material or surface.