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.