Controlling Microbial Growth Notes

Terminology Related to Microbial Growth Control

  • Decontamination: Removal of contaminants.
  • Sterilization: The process of making something free from bacteria or other living microorganisms.
  • Disinfection: The process of cleaning something, especially with a chemical, in order to destroy bacteria.
  • Disinfectant: A chemical liquid that destroys bacteria.
  • Antiseptic: A substance that prevents the growth of disease-causing microorganisms.
  • Bacteriostatic: Preventing the growth of bacteria.
  • Bacteriocidal: Capable of killing bacteria.

Controlling Microbial Growth: Physical Methods - Moist Heat

  • Boiling water kills vegetative bacterial cells but does not kill endospores.
  • Sterilization can be achieved by steam under pressure.
  • An autoclave is standard equipment in microbiology labs and hospital settings.
  • Conditions for sterilization in an autoclave: 121°C + 15 lbs/in² pressure for 15–60 minutes.
  • Reusable lab equipment, growth media & solutions, and old cultures are sterilized by autoclave.

Controlling Microbial Growth: Physical Methods - Moist Heat in Food Processing

  • Food processing uses pressure cooking or pasteurization to remove microbes and spores.
  • Pressure cooking is required for canning non-acidic foods to prevent the growth of botulism spores and production of a deadly exotoxin.
  • Pasteurization is used to reduce microbial levels or sterilize milk, eggs, or other beverages.
  • Pasteurization involves very short, high-heat treatments.

Controlling Microbial Growth: Physical Methods - Dry Heat

  • Dry heat can be used to sterilize equipment that cannot be exposed to moisture.
  • Dry heat sterilization is not as efficient as moist heat, requiring 170°C for 2+ hours.

Controlling Microbial Growth: Physical Methods - Radiation

  • Non-ionizing (UV) radiation
    • Kills bacteria by damaging DNA.
    • Does not penetrate solids – used for surface sterilization.
    • Used in operating rooms (ORs), labs, and the food industry to sterilize rooms.
    • Used in water treatment facilities.
  • Ionizing (gamma rays or X-rays) radiation
    • Penetrates solid objects.
    • Used to sterilize food for non-refrigerated aseptic packaging, such as MREs (Meals, Ready-to-Eat).
    • Used for non-autoclavable medical equipment.
    • Ionizing radiation does NOT make the food radioactive.

Controlling Microbial Growth: Physical Methods - Filtration

  • Filters can be used to catch microbes or particles as liquid or air is passed through.
  • Pore size of the filter determines what categories of microbes can be removed.
  • Liquids:
    • Drinking water (large-scale building/municipal, and personal-scale water treatment).
    • Non-autoclavable medical solutions such as vaccines and antibiotics.
  • Air:
    • Building-level air filters: Hospitals, labs, manufacturing clean rooms.
    • Masks: N95 or better for hospital use.

Controlling Microbial Growth: Chemical Methods - Germicides

  • Germicides are antimicrobial agents.
  • Disinfectants are chemicals used to reduce the number of microbes on inanimate objects.
  • Antiseptics are chemicals used to reduce the number of microbes on skin or mucous membranes.

Controlling Microbial Growth: Chemical Methods - Germicide Rating Criteria

  • Low-level germicides: Destroy vegetative bacteria except acid-fast bacteria, fungi, and some viruses.
  • Medium-level germicides: Destroy all vegetative bacteria, fungi, and viruses.
  • High-level germicides: Destroy all microbial life, including endospores if not highly concentrated, but action against prions is not determined.

Controlling Microbial Growth: Chemical Methods - Germicides for Reducing or Eliminating Microbes

  • Low-Level Germicides
    • Detergents
      • Mode of Action: Target lipid membranes.
      • Pros: Cheap, low toxicity, pleasant scent, usable as disinfectant and antiseptic.
      • Cons: Activity is decreased in hard water, easily contaminated by Pseudomonas bacteria.
  • Intermediate-Level Germicides
    • Alcohols (Isopropanol, Ethanol)
      • Mode of Action: Target proteins and lipid membranes.
      • Pros: Cheap, easily applied, usable as disinfectant and antiseptic.
      • Cons: Flammable, can react with plastics.
    • Phenols
      • Mode of Action: Target proteins and lipid membranes.
      • Pros: Easy to apply, effective in hard water, usable as disinfectant and antiseptic.
      • Cons: Leave residue, irritants, harsh on surfaces, medicinal scent, Sensitive to water hardness
  • High-Level Germicides
    • Aldehydes (Formaldehyde, Glutaraldehyde)
      • Mode of Action: Target proteins, nucleic acids.
      • Pros: Achieve sterility at certain concentrations
      • Cons: Toxic, irritants, and leave a residue
    • Halogens (Chlorine, Iodine)
      • Mode of Action: Oxidizing agents, mainly target proteins, nucleic acids.
      • Pros: Sterilants at higher concentrations, cheap, usable as disinfectant and antiseptic.
      • Cons: Rapidly inactivated by organic material, corrosive, discolor fabrics.
    • Peroxygens (Hydrogen peroxide, Peracetic acid)
      • Mode of Action: Oxidizing agents that mainly target nucleic acids and proteins.
      • Pros: Effective sterilization at high concentrations, usable as disinfectant and antiseptic, peracetic acid is effective in spite of organic material present and has no residue.
      • Cons: Most are readily inactivated by organic matter, corrosive, irritants.
    • Ethylene oxide
      • Mode of Action: Target proteins, nucleic acids.
      • Pros: Can treat items that can't withstand heat or moisture, gentle on equipment.
      • Cons: Toxic and flammable.
        *Denotes the highest possible germicide level of the agent. Any germicide that is greatly diluted or improperly applied will have a low effect.
        Higher concentrations usually provide a higher disinfection potential (or even Sterilize).

Controlling Microbial Growth: Equipment Criteria

  • Critical:
    • Contact with sterile body sites and/or vascular system.
    • Must be sterilized.
  • Semicritical:
    • Contact with mucous membranes or non-intact skin.
    • Not necessary to remove small numbers of endospores.
  • Noncritical:
    • Only contact intact skin.

Controlling Microbial Growth: Chemical Methods - Special Considerations by Microbe Type

  • Mycobacterium
    • Waxy walls resist all but the strongest disinfectants.
    • Prevention of airborne transmission via filtration.
  • Endospores
    • Autoclave, high-heat hydrogen peroxide treatment, or chemicals marked as sporicidal are necessary for destruction.
  • Protozoans
    • Main concerns are waterborne transmission.
    • Filtration or boiling of untreated water.
    • Filtration, ozone, CO_2 treatments of municipal water.
    • Some species resist chlorine treatment, which is a common water treatment method.

Controlling Microbial Growth: Chemical Methods - Special Considerations by Microbe Type (Continued)

  • Viruses
    • Because viruses are dormant outside host cells, they MAY be resistant to some chemicals.
    • Naked viruses are more difficult to eliminate than enveloped viruses.
  • Prions
    • Non-living infectious protein particles.
    • Resistant to standard autoclave procedures; requires increased temperature and pressure combined with chemical treatments.
    • Resistant to cooking.