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Chapter 7

Chapter 7. The Control of Microbial Growth. The Control of Microbial Growth. Sepsis refers to microbial contamination. Asepsis is the absence of significant contamination. Aseptic surgery techniques prevent microbial contamination of wounds. Terminology.

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Chapter 7

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  1. Chapter 7 The Control of Microbial Growth

  2. The Control of Microbial Growth • Sepsis refers to microbial contamination. • Asepsis is the absence of significant contamination. • Aseptic surgery techniques prevent microbial contamination of wounds.

  3. Terminology • Sterilization: Removal of all microbial life • Commercial Sterilization: Killing C. botulinum endospores • Disinfection: Removal of pathogens • Antisepsis: Removal of pathogens from living tissue • Degerming: Removal of microbes from a limited area • Sanitization: Lower microbial counts on eating utensils • Biocide/Germicide: Kills microbes • Bacteriostasis: Inhibiting, not killing, microbes • Bacteriocidal: Kills bacteria

  4. Bacterial populations die at a constant logarithmic rate. Figure 7.1a

  5. Effectiveness of antimicrobial treatment depends on: • Number of microbes • Environment (organic matter, temperature, biofilms) • Time of exposure • Microbial characteristics • Condition of microbes - lag, log or sport Figure 7.1b

  6. Actions of Microbial Control Agents Mechanisms of action for Antimicrobial control agents • Alternation of membrane permeability • Damage to proteins • Damage to nucleic acids • Interfere with metabolic pathways • Methods may be Physical or Chemical • Physical like heat • Chemical like disinfectants

  7. Physical Methods of Microbial Control Heat is very commonly used, heat may be moist or dry. • Heat • Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min. • Thermal death time (TDT): Time to kill all cells in a culture • Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature

  8. Moist Heat • Moist heat denatures proteins • Autoclave: Steam under pressure Figure 7.2

  9. Physical Methods of Microbial Control • Pasteurization reduces spoilage organisms and pathogens • Equivalent treatments • 63°C for 30 min • High-temperature short-time 72°C for 15 sec • Ultra-high-temperature: 140°C for <1 sec • Thermoduric organisms survive

  10. Physical Methods of Microbial Control • Dry Heat Sterilization kills by oxidation • Flaming • Incineration • Hot-air sterilization

  11. Physical Methods of Microbial Control • Filtration removes microbes • Low temperature inhibits microbial growth • Refrigeration • Deep freezing • Lyophilization • High pressure denatures proteins • Desiccation prevents metabolism • Osmotic pressure causes plasmolysis

  12. Physical Methods of Microbial Control Electromagnetic spectrum, shorter wavelength means great energy and can penetrate further. • Radiation damages DNA • Two types used: • Ionizing radiation (X rays, gamma rays, electron beams) • Non-ionizing radiation (UV) • (Microwaves kill by heat; not especially antimicrobial)

  13. Electromagnetic Spectrum Figure 7.5

  14. Chemical Methods of Microbial Control • Principles of effective disinfection • Concentration of disinfectant • Organic matter • pH • Time • Often chemical methods are compared to Phenol. If more effective given a phenol coefficient of >1. If less effective give a phenol coefficient of <1.

  15. Chemical Methods of Microbial Control • Evaluating a disinfectant • Use-dilution test 1. Metal rings dipped in test bacteria are dried 2. Dried cultures placed in disinfectant for 10 min at 20°C 3. Rings transferred to culture media to determine whether bacteria survived treatment

  16. Chemical Methods of Microbial Control • Evaluating a disinfectant • Disk-diffusion method Figure 7.6

  17. Types of Disinfectants • Phenol • Phenolics. Lysol • Bisphenols. Hexachlorophene, Triclosan • Disrupt plasma membranes Figure 7.7

  18. Types of Disinfectants • Biguanides. Chlorhexidine • Disrupt plasma membranes

  19. Types of Disinfectants • Halogens. Iodine, Chlorine • Oxidizing agents • Bleach is hypochlorous acid (HOCl)

  20. Types of Disinfectants • Alcohols. Ethanol, isopropanol • Denature proteins, dissolve lipids Table 7.6

  21. Types of Disinfectants • Heavy Metals. Ag, Hg, Cu • Oligodynamic action • Denature proteins

  22. Types of Disinfectants • Surface-Active Agents or Surfactants

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

  24. Types of Disinfectants • Aldehydes • Inactivate proteins by cross-linking with functional groups (–NH2, –OH, –COOH, —SH) • Glutaraldehyde, formaldehyde

  25. Types of Disinfectants • Gaseous Sterilants • Denature proteins • Ethylene oxide - not Ethyl oxide

  26. Types of Disinfectants • Peroxygens • Oxidizing agents • O3, H2O2, peracetic acid

  27. Microbial Characteristics and Microbial Control Figure 7.11

  28. Microbial Characteristics and Microbial Control

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