Microbial Growth Control Notes

Terminology Related to Microbial Growth Control

  • Decontamination
  • Sterilization
  • Disinfection
  • Disinfectant
  • Antiseptic
  • Bacteriostatic
  • Bacteriocidal

Controlling Microbial Growth - Physical Methods

  • Moist Heat
    • Boiling water kills vegetative bacterial cells but not endospores.
    • Sterilization via steam under pressure in an autoclave (121^oC + 15 lbs/in² pressure for 15 – 60 minutes).
    • Used for reusable lab equipment, growth media, solutions, and decontamination of old cultures.
    • Food processing: pressure cooking or pasteurization.
      • Pressure cooking for canning non-acidic foods to prevent botulism spores.
      • Pasteurization reduces microbial levels in milk, eggs, and beverages using short, high-heat treatments.
  • Dry Heat
    • Used for equipment that cannot be exposed to moisture; less efficient than moist heat (170^oC for 2+ hours).
  • Radiation
    • Non-ionizing (UV) radiation:
      • Damages DNA, used for surface sterilization in ORs, labs, food industry, and water treatment.
    • Ionizing (gamma rays or X-rays) radiation:
      • Penetrates solid objects, used to sterilize food for non-refrigerated aseptic packaging and non-autoclavable medical equipment.
  • Filtration
    • Filters catch microbes/particles from liquid or air.
    • Pore size determines removed microbes.
    • Liquids: drinking water, vaccines, and antibiotics.
    • Air: building-level air filters (hospitals, labs), masks (N95 or better).

Controlling Microbial Growth - Chemical Methods

  • Germicides
    • Disinfectants: reduce microbes on inanimate objects.
    • Antiseptics: reduce microbes on skin/mucous membranes.
  • Germicide Rating Criteria
    • Low: destroy vegetative bacteria (except acid-fast), fungi, some viruses.
    • Medium: destroy all vegetative bacteria, fungi, and viruses.
    • High: destroy all microbial life, including endospores (if not highly concentrated); action against prions undetermined.

Germicides for Reducing or Eliminating Microbes

  • Low Level
    • Germicide: Detergents
    • Mode of Action: Target lipid membranes
  • Intermediate Level
    • Germicide: Alcohols (Isopropanol, Ethanol), Phenols
    • Mode of Action: Target proteins and lipid membranes
  • High Level
    • Germicide: Aldehydes (Formaldehyde, Glutaraldehyde), Halogens (Chlorine, Iodine), Peroxygens (Hydrogen peroxide, Peracetic acid), Ethylene oxide
    • Mode of Action: Target proteins, nucleic acids; Oxidizing agents

Equipment Criteria

  • Critical: contact with sterile body sites/vascular system - must be sterilized.
  • Semicritical: contact with mucous membranes/non-intact skin - endospore removal not always necessary.
  • Noncritical: contact with intact skin.

Special Considerations by Microbe Type

  • Mycobacterium: waxy walls resist disinfectants; prevent airborne transmission via filtration.
  • Endospores: require autoclave, high heat, hydrogen peroxide, or sporicidal chemicals.
  • Protozoans: waterborne transmission; use filtration or boiling of untreated water, ozone, CO2 treatments; some resist chlorine.
  • Viruses
    • May be resistant to chemicals due to dormancy outside host cells.
    • Naked viruses are harder to eliminate than enveloped viruses.
  • Prions
    • Resistant to standard autoclave procedures; increase temp/pressure with chemical treatments.
    • Resistant to cooking.