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

10. 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

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

12. 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

13. 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)

14. Figure 7.5

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

16. 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

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

18. 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

19. 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.

20. 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.

21. 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.

22. 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

24. 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.

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

26. 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

27. 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,

28. 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.

29. 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.

30. 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

31. Microbial Characteristics and Microbial Control