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Control of Microbial Growth

Control of Microbial Growth. Chapter 5. 5.1 Approaches to Control. Physical methods Heat Irradiation Filtration Mechanical (e.g., washing) Chemical methods Disinfectants Antibiotics. Principles of Control Sterilization Disinfectants Germicides Antiseptics (topical) Pasteurization

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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

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