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Control of Microbial Growth

Control of Microbial Growth

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Control of Microbial Growth

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  1. Control of Microbial Growth • Chapter 5

  2. 5.1 Approaches to Control • Physical methods • Heat • Irradiation • Filtration • Mechanical (e.g., washing) • Chemical methods • Disinfectants • Antibiotics

  3. Principles of Control • Sterilization • Disinfectants • Germicides • Antiseptics (topical) • Pasteurization • Decontamination • Sanitation • Preservation

  4. Situational Considerations • Daily life • Hospitals • Microbiology laboratories • Food production facilities • Water treatment facilities • Others

  5. 5.2 Selection of an Antimicrobial Procedure • Type of microorganism • Bacterial endospores and Mycobacterium species are heat and chemical resistant • Vegetative cells are susceptible to most disinfectants • Number of microbes initially present • “99.9% effective” - still a lot of live bacteria! • Environmental conditions (dirt, grease, etc.)

  6. Potential risk of infection • Critical instruments - sharps • Semicritical instruments - mucous membranes • Noncritical instruments - intact skin • Composition of the item • Metals are damaged by some disinfectants • Plastics are damaged by irradiation

  7. 5.3 Using Heat to Destroy Microorganisms and Viruses • Moist heat • Boiling kills most bacteria and viruses • Pasteurization is effective for many food-borne microbes • Heat to 72° C for 15 sec for most liquids • Other objects can withstand higher temps and durations • Autoclave • High pressure increases boiling temp of water • Typically reach 120° C at 15 psi

  8. Moist heat (cont.) • Commercial canning process • Most serious threat is Clostridium botulinum endospores • Canning facilities use retorts, which are large autoclaves • Prevents spores from germinating inside the can, thus prevents botulism toxin production • Dry heat • Heating items to 200° C or more

  9. 5.4 Using Other Physical Methods to Remove or Destroy Microbes • Filtration of fluids • Nylon membranes of defined pour sizes • 0.45 µm • 0.20 µm • Filters are sterilized by irradiation • Filter out bacteria • Will not filter viruses or prions

  10. Filtration of air • High-efficiency particulate air filters (HEPA) • Used in a variety of settings • Room ventilation • Laminar flow hoods • Powered Air-Purifying Respirator (PAPR)

  11. Radiation • Ionizing • Ultraviolet • Microwaves • High pressure (up to 130,000 psi)

  12. 5.5 Using Chemicals to Destroy Microorganisms and Viruses • Potency of germicidal chemical formulations • Sterilants - everything • High-level disinfectants - viruses and all vegetative bacteria • Intermediate-level disinfectants - all vegetative bacteria and most viruses • Low-level disinfectants - all vegetative bacteria and enveloped (membrane-bound) viruses

  13. Selecting the appropriate germicidal chemical • Toxicity • Activity in the presence of organic matter • Residue • Cost and availability • Storage and stability • Environmental risk

  14. Classes of germicidal chemicals • Alcohols - denature proteins, solubilize membranes • Aldehydes - chemically modify proteins and nucleic acids • Glutaraldehyde • Formaldehyde • Orthophthaladehyde

  15. Classes of germicidal chemicals (cont.) • Biguanides adhere to skin and mucous membranes • Chlorhexidines • Surgical scrubbing • Mouthwash • Skin abrasions • Ethylene oxide • Highly oxidative • Used for surgical garbs

  16. Classes of germicidal chemicals (cont.) • Halogens • React with organic molecules to form toxic substances • Chlorine • Iodine • Ozone • Oxidizing agent • Drinking water and wastewater

  17. Classes of germicidal chemicals (cont.) • Peroxygens • Oxidizing agents • Hydrogen peroxide (H2O2) • Peracetic acid (CH3C[=O]OOH) • Phenolic compounds • Dissolve membranes • Denature proteins • Quaternary ammonia compounds • Cationic (+ charge) detergents • Bind to negatively-charged membrane lipids

  18. 5.6 Preservation of Perishable Products • Chemical preservatives • Benzoic and sorbicacids - breads, cheeses, juices • Nitrates - inhibit spore germination • Low temperature - reduces biochemical activities of microbes • Reduce availability of water - salting/drying