The Control of Microbial Growth

1. 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 Clostridium 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

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

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

9. Steam Sterilization • Steam must contact item’s surface. Figure 7.3

10. Physical Methods of Microbial Control • Pasteurization reduces spoilage organisms and pathogens • Equivalent treatments • 63°C (145.4°F) for 30 min • High temperature, short time: 72°C for 15 sec • Ultra high temperature: 140°C (284°F) 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 • Low temperature inhibits microbial growth • Refrigeration • Deep freezing • Lyophilization • Desiccation prevents metabolism

13. Physical Methods of Microbial Control • Radiation damages DNA • Ionizing radiation (X rays, gamma rays, electron beams) – damages many molecules • Nonionizing radiation (UV) – damages DNA • (Microwaves kill by heat; not especially antimicrobial)

14. Figure 7.5

15. Chemical Methods of Microbial Control • Evaluating a disinfectant • Use-dilution test • Metal rings dipped in test bacteria are dried. • Dried cultures are placed in disinfectant for 10 min at 20°C. • Rings are 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: Hexacholorphene, Triclosan • Disrupt plasma membranes Figure 7.7

18. Types of Disinfectants • Halogens: Iodine, chlorine • Oxidizing agents • Bleach is hypochlorous acid (HOCl)

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

20. Types of Disinfectants • Heavy metals: Ag, Hg, and Cu • Oligodynamic action • Denature proteins

21. Types of Disinfectants • Surface-active agents or surfactants

22. Figure 7.10

23. Types of Disinfectants • Chemical food preservatives • Organic acids • Inhibit metabolism • Sorbic acid, benzoic acid, and calcium propionate • Control molds and bacteria in foods and cosmetics • 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 and formaldehyde

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

26. Microbial Characteristics and Microbial Control Figure 7.11

27. Microbial Characteristics and Microbial Control Table 7.7

28. Table 7.8 (1 of 4)

29. Table 7.8 (2 of 4)

30. Table 7.8 (3 of 4)