Microbial Control pt. 2

• Physical Methods Used to Control Microbial Growth

  • Heat
  • Filtration
  • Cold
  • High Pressure
  • Desiccation
  • Osmotic Pressure
  • Radiation
• Moist heat


  • Boiling or flowing steam
  • Autoclaving
• Moist heat


  • Kills vegetative bacterial and fungal pathogens and many viruses within 10 min; less effective on endospores
  • Very effective method of sterilization; at about 15 psi of pressure (121°C), all vegetative cells and their endospores are killed in about 15 min
• Pasteurization


  Heat treatment for milk (72°C for about 15 sec) that kills all pathogens and most nonpathogens
• Dry heat


  • Direct flaming
  • Incineration
  • Hot-air sterilization
• Dry heat


  • Very effective method of sterilization
  • Used for inoculating loops
  • Very effective method of sterilization
  • Used for disposal of contaminated dressings, animal carcasses, and paper
  • Very effective method of sterilization but requires temperature of 170°C for about 2 hr, used for empty glassware
• Filtration


  Separation of bacteria from suspending liquid
• Filtration


  • Removes microbes by passage of a liquid or gas through a screenlike material; most filters in use consist of cellulose acetate or nitrocellulose
  • Useful for sterilizing liquids (e.g., enzymes, vaccines) that are destroyed by heat
• Refrigeration


  Decreased chemical reactions and possible changes in proteins
• Refrigeration


  • Has a bacteriostatic effect
• Deep-freezing


  Decreased chemical reactions and possible changes in proteins
• Deep-freezing


  • An effective method for preserving microbial cultures, food, and drugs
• Lyophilization


  Decreased chemical reactions and possible changes in proteins
• Lyophilization


  • Most effective method for long-term preservation of microbial cultures, food, and drugs
• High Pressure


  Alteration of molecular structure of proteins and carbohydrates
• High Pressure


  • Preserves colors, flavors, nutrient values of fruit juices
• Desiccation


  Disruption of metabolism
• Desiccation


  • Involves removing water from microbes; primarily bacteriostatic
• Osmotic Pressure


  • Results in loss of water from microbial cells
• Ionizing Radiation


  Destruction of DNA
• Ionizing Radiation


  • Used for sterilizing pharmaceuticals and medical and dental supplies
• Nonionizing Radiation


  Damage to DNA
• Nonionizing Radiation


  • Radiation is not very penetrating
• Chemical agents are used to control the growth of microbes on both living tissue and inanimate objects
• Few chemical agents achieve sterility; most of them merely reduce microbial populations to safe levels or remove vegetative forms of pathogens from objects
• The concentration of a disinfectant affects its action, so it should always be diluted exactly as specified by the manufacturer
• The nature of the material being disinfected can interfere with the action of the disinfectant
• The pH of the medium often has a great effect on a disinfectant's activity
• Disinfection is a gradual process, so a disinfectant might need to be left on a surface for several hours to be effective
• Use-Dilution Tests


  Standard method to evaluate the effectiveness of disinfectants and antiseptics
• Use-Dilution Tests


  • Metal or glass cylinders are dipped into standardized cultures of test bacteria, dried, placed in disinfectant solution for 10 minutes, then transferred to growth medium to see if any bacteria survive
• Disk-Diffusion Method


  Used in teaching labs to evaluate the efficacy of a chemical agent
• Disk-Diffusion Method


  • A disk soaked in chemical is placed on an agar plate inoculated with test organism, and a clear zone around the disk indicates inhibition of growth
• Phenol


  Carbolic acid, first used by Lister to control surgical infections
• Phenol


  • Rarely used as antiseptic or disinfectant due to skin irritation and disagreeable odor, but can have antimicrobial effect at concentrations above 1%
• Phenolics


  Derivatives of phenol that contain a molecule of phenol that has been chemically altered
• Phenolics


  • Exert antimicrobial activity by injuring lipid-containing plasma membranes, remain active in presence of organic compounds, are stable, and persist for long periods
• Phenolics


  • O-phenylphenol (main ingredient in Lysol)
  • Cresols (derived from coal tar)
• Bisphenols


  Derivatives of phenol that contain two phenolic groups connected by a bridge
• Bisphenols


  • Hexachlorophene (ingredient in pHisoHex lotion)
  • Triclosan (ingredient in antibacterial products)