1. Methods for Control of Microbial Growth

2. Controlling Microorganisms
Physical, chemical, and mechanical methods can be used to destroy or reduce undesirable microbes in a given area
Primary targets are microorganisms capable of causing infection or spoilage:
vegetative bacterial cells and endospores
fungal hyphae and spores, yeasts
protozoan trophozoites and cysts
worms
viruses

3. Hierarchy of Resistance
Most resistant = bacterial endospores
Moderately resistant = cysts, fungal zygospores, naked viruses
Least resistant = vegetative bacterial cells

4. Types of Control
Sterilization – a process that destroys all viable microbes, including viruses and endospores; microbiocidal
Disinfection – a process to destroy vegetative pathogens, not endospores; for inanimate objects
Antisepsis – disinfectants applied directly to exposed body surfaces
Sanitization – any cleansing technique that mechanically removes microbes
Degermation – reduces the number of microbes

5. A Number of Factors Influence the Effectiveness of Control Agents
Number of microbes
Nature of microbes in the population
Temperature and pH of environment
Concentration or dosage of agent
Mode of action of the agent
Presence of solvents, organic matter, or inhibitors

6. Microbiocidals Cause Microbial Death

7. Stasis Agents Slow or Retard Growth, but Do Not Kill Microbes

8. Microbial Death
The Permanent Loss of Reproductive Capability Under Optimal Growth Conditions

9. Cellular Targets of Control
Cell wall
Cell membrane
Nucleic Acids (DNA, RNA)
Proteins

10. Physical Control Practices

11. Heat-Based Microbial Control Processes
Incineration/ baking achieve sterility
Autoclaving: sterilization with live steam and pressure, very widely used
Pasteurization: flash heat treatment (63°C - 66°C for 30 minutes) that reduces the bio-burden of food materials (kills Salmonella and Listeria)
Boiling disinfection – does not achieve sterility

12. Radiation Sterilization
Ionizing radiation (X-rays, gamma particles) penetrates and damages DNA and other vital cell components
Non-ionizing radiation (UV light) causes adjacent T-T pairs in DNA to fuse
UV has limited penetration; use for surfaces and films of liquids

15. Ultra-filtration Mechanically excludes organisms from a liquid
Membranes have a specific pore size; any particle larger cannot pass through
Achieves sterilization

16. Filtration

17. Targets of Chemical Agents
Cell Membrane - detergents
Key Proteins – denaturing and cross-linking agents
Nucleic Acids – alkylating and cross-linking agents

18. Halogen Antimicrobials - Denature Proteins
Chlorine compounds (gaseous Cl, bleach, chloramine); can be sporicidal
Iodine (tincture, Betadine)
Halogens can react with any organic matter – a surface should be clean before applying them!

19. Phenolics- Disrupt Cell Membranes & Precipitate Proteins
Lysol
PhisoHex (not any more)
Benzalkonium chloride
Triclosan

20. Alcohols: Dissolve Membranes and Coagulate Proteins
Ethanol (70%)
Isopropanol
Act as surfactants dissolving membrane lipids and coagulating proteins of vegetative bacterial cells and fungi
Cannot destroy spores at room temperature

21. Hydrogen Peroxide – Attacks DNA and Proteins
3% solution is effective as wound antiseptic, but is potentially damaging to tissues

22. Heavy Metals
Mercury, Silver Salts Kill Vegetative Cells Present in low Numbers by Inactivating Proteins

23. Aldehydes – Cross-link DNA and Proteins
Formaldehyde – formalin
Glutaraldehyde - Cidex
A soak of dental or surgical instruments in glutaraldehyde Does Not guarantee sterility!

24. Gases and Aerosols Ethylene oxide, propylene oxide
Strong alkylating agents
High level
Sterilize and disinfect plastics and prepackaged devices, foods