1. The Control of Microbial Growth
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 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
Hot-air
Autoclave
Equivalent treatments
170˚C, 2 hr
121˚C, 15 min

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
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. 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
Soap
Degerming
Acid-anionic detergents
Sanitizing
Quarternary ammonium compounds Cationic detergents
Bactericidal, Denature proteins, disrupt plasma membrane

23. 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

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

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
Chemical agent
Effectiveness against
Endospores
Mycobacteria
Phenolics
Poor
Good
Quats
None
Chlorines
Fair
Alcohols
Glutaraldehyde