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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

  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 Figure 7.1b

  6. Actions of Microbial Control Agents • Alternation of membrane permeability • Damage to proteins • Damage to nucleic acids

  7. Physical Methods of Microbial Control • 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. Heat • Moist heat denatures proteins • boiling • Autoclave e: 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 • High-efficiencyparticuiateair(HEPA) filters remove • almost all microorganisms larger than about 0.3 Micron in diameter. • 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 • Radiation damages DNA • Ionizing radiation (X rays, gamma rays, electron beams) • Nonionizing radiation (UV) • (Microwaves kill by heat; not especially antimicrobial)

  13. Figure 7.5

  14. Chemical Methods of Microbial Control • Principles of effective disinfection • Concentration of disinfectant • Organic matter • pH • Time

  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. One of the most freque ntly used phenolics is derived fromcoal ta r, a group of chemicals called cresols. • Bisphenols. • Hexachlorophene:used for surgical and hospital microbial control procedures. • Triclosan • An ingredient in antibacterial soaps and at least one toothpaste. • Triclosan has even been incorporated in to kitchen cutting boards and the handles of knives an d other plastic kitchenware. • Disrupt plasma membranes Figure 7.7

  18. Types of Disinfectants • Biguanides. Chlorhexidine • Disrupt plasma membranes • They are effective against gram-positive and gram-negative bacteria. • Biguanides are not sporicidal but have some activity against enveloped viruses. • chlorhexidine is much used for surgical hand scrubs and preoperative skin preparation in patients.

  19. Types of Disinfectants • Halogens. Iodine, Chlorine • Iodineactive against all kinds of bacteria, many endospores , various fungi, and some viruses. • Iodine impairs protein synthesis and alters cell membranes, apparently by forming complexes with amino acids and unsaturated fatty acids. • Iodine is available as a tincture- that is solution in aqueous alcohol- and as an iodophor. An iodophor is a combination of iodine and an organic molecule, from which the iodine is released slowly.

  20. Chlorine (CI2), Oxidizing agents • Bleach is hypochlorous acid (HOCl) • used extensively for disinfecting municipal drinking water, water in swimming pools, and sewage. • chloramines, combinations of chlorine and ammonia. Most municipal water-treatment systems mix ammonia with chlorine to form chloramines.

  21. Types of Disinfectants • Alcohols. Ethanol, isopropanol • effectively kill bacteria and fungi but not endospores and nonenveloped viruses. • Denature proteins, dissolve lipids. • The recommended optimum concentration of ethanol is • 70%, but concentrations between 60% and 95% seem to kill as well. • Ethanol and isopropanol are often used to enhance the effectiveness of other chemical agents. Table 7.6

  22. Types of Disinfectants • Heavy Metals. Ag, Zn, Cu • Oligodynamic action • When the metal ions combine with the sulfhydryl groups on cellular proteins, Denature proteins. • Silver is used as an antiseptic in a 1% silver nitrate solution. • copper compounds arc effective in concentrations of • one part per million of water. • To prevent fungi, copper compounds such as copper 8-hydroxyquinoline are sometimes included in paint. • Zinc chloride is a common ingredient in mouthwashes. • Inorganic mercurycompounds, such as mercuric chloride have a long history of use as disinfectants.

  23. Types of Disinfectants • Surface-Active Agents or Surfactants can decrease surface tension among molecules of a liquid.

  24. 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 • Antibiotics. Nisin and natamycin prevent spoilage of cheese

  25. Types of Disinfectants • Aldehydes • Inactivate proteins by cross-linking with functional groups (–NH2, –OH, –COOH, —SH) • Glutaraldehyde, formaldehyde • Formaldehyde gasis an excellent disinfectant. However, it is more commonly available as formalin, a 37% aqueous solution of formaldehyde gas. • Glutaraldehyde is a chemical relative of formaldehyde that is less irritating and more effective than formaldehyde. • Glutaraldehyde is used to disinfect hospital instruments,

  26. Types of Disinfectants • Gaseous Sterilants • Denature proteins • Ethylene oxide kills all microbes and endospores but requires a lengthy exposure period of several hours. • It is toxic and explosive in its pure form, so it is usually mixed with a nonflammable gas, such as carbon dioxide.

  27. Types of Disinfectants • Peroxygens • are a group of oxidizing agents that includes hydrogen peroxide and peracetic acid . • O3, H2O2, peracetic acid • Hydrogen peroxide is an antiseptic found in many household medicine cabinets and in hospital supply rooms. • Heated hydrogen peroxide can be used as a gaseous sterilant.

  28. Microbial Characteristics and Microbial Control • Gram-negative bacteria are generally more resistant than gram-positive bacteria to disinfectants and antiseptics. • 2. Mycobacteria, endospores, and protozoan cysts and oocysts are • very resistant to disinfectants and antiseptics. • 3. Nonenveloped viruses are generally more resistant than enveloped viruses to disinfectants and antiseptics. • 4. Prions are resistant to disinfection and autodaving. Figure 7.11

  29. Microbial Characteristics and Microbial Control

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