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Chapter 7 The Control of Microbial Growth

Chapter 7 The Control of Microbial Growth. TERMINOLOGY Sepsis refers to microbial contamination. Asepsis is the absence of significant contamination. Sterilization: Removal of all microbial life Commercial Sterilization:. Disinfection: Killing of vegetative microbes on nonliving objects

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Chapter 7 The Control of Microbial Growth

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  1. Chapter 7The Control of Microbial Growth TERMINOLOGY • Sepsis refers to microbial contamination. • Asepsis is the absence of significant contamination. • Sterilization: Removal of all microbial life • Commercial Sterilization:

  2. Disinfection: Killing of vegetative microbes on nonliving objects • Antisepsis: Killing of vegetative microbes in/on living tissue • Degerming: Removal of microbes from a limited area • Biocide/Germicide: Kills microbes • Bacteriostasis: Inhibits, does not kill microbes

  3. Microbe Death by Antimicrobial Agents • Factors influencing death rate are: • Number of microbes • Environmental influences (ie. Organic substances, biofilms, temperature, pH) • Biofilms: • Time of exposure • Microbial characteristics • [antimicrobial agent]

  4. Actions of Microbial Control Agents • Alteration of membrane permeability • Lipids and proteins are altered • Intracellular substances leak out of cell • Damage to proteins • Denaturation of enzymes, etc. • Loose their structure and function • Damage to nucleic acids • Damaged DNA and RNA prevents production of vital proteins

  5. Moist Heat • Heat denatures proteins • Boiling water • (May not kill spores) • Autoclave: generates very high pressure and temperature via steam • Steam must contact surface to kill microbes Figure 7.2

  6. Pasteurization • Pasteurization reduces spoilage organisms and pathogens via heating • Thermoduric (i.e. thermophiles) organisms survive • Pasteurization treatments • High-temperature short-time (HTST): 72°C for 15s • Ultra-high-temperature (UHT): 140°C for 3s

  7. Dry Heat • Dry Heat Sterilization: • Direct flaming • Incineration • Hot-air sterilization

  8. Other Physical Methods of Microbial Control • Filtration removes microbes • Low temperature inhibits microbial growth: • Refrigeration • Deep freezing • Desiccation prevents metabolism • Hypertonicity causes plasmolysis

  9. Other Physical Methods of Microbial Control • Radiation damages DNA and other organic molecules • Ionizing radiation (X rays, gamma rays, electron beams) • Nonionizing radiation (UV) • Microwaves kill by heat; not especially antimicrobial

  10. Chemical Methods of Microbial Control • Evaluating a disinfectant • Use-dilution test • 1. Stainless steel cylinders are inoculated with Staphylococcus aureus, Salmonella choleraesuis, Pseudomonas aeruginosa • 2. Cylinders are dried in an incubator • 3. Cylinders are placed in test disinfectant • 4. Cylinders are incubated in broth media

  11. Disk-diffusion method

  12. Types of Disinfectants/Antiseptics • Phenol • Phenolics: Modified phenol • reduced irritation, enhanced antibacterial action • ie. Lysol • Bisphenols: • Hexachlorophene • Triclosan (2nd link) • Found in many household items Figure 7.7

  13. Types of Disinfectants/Antiseptics • Chlorhexidine • surgical hand scrub • preoperative patient scrub • Halogens: • Iodine: skin disinfectant; water treatment • Chlorine (ie. Bleach): water, sewage treatment; swimming pools; restaurants, etc.

  14. Types of Disinfectants/Antiseptics • Alcohols: Ethanol, isopropanol • May not be appropriate for wound cleaning • May “seal in” microbes • Alcohol-based handrubs Table 7.6

  15. Types of Disinfectants/Antiseptics • Heavy Metals: ie. Ag, Cu • Silver compounds: impregnated dressings; burn creams; catheters • Copper sulfate: kills green algae in reservoirs, pools, ponds

  16. Types of Disinfectants • Chemical Food Preservatives • Organic Acids - Inhibit metabolism • Sorbic acid, benzoic acid, calcium propionate • Control molds and bacteria in foods and cosmetics • Nitrite prevents endospore germination in meats • Antibiotics. Nisin and natamycin prevent spoilage of cheese

  17. Types of Disinfectants • Aldehydes: • Glutaraldehyde: • sterilizes* • hospital instrument disinfection • used for embalming

  18. Types of Disinfectants • Gaseous Sterilants • Ethylene oxide: • Peroxygens • Ozone = O3: ie. water treament • Hydrogen peroxide = H2O2: ie. Food packaging; NOT good for open wounds

  19. Types of Disinfectants/Antiseptics • Surface-Active Agents or Surfactants

  20. Relative Resistances to Antimicrobials Figure 7.11

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