Control of microbial growth Antimicrobial agents physical chemical How much is enough? efficacy toxicity specifici

1. Control of microbial growth Antimicrobial agents physical chemical How much is enough? efficacy toxicity specificity

2. General categories of antimicrobials Physical heat, irradiation, filtration, scrubbing Chemical (antimicrobial mechanisms are considered separately)

3. Considerations Sterility- completely free of endospores, viruses and all other microbes (except prions) Living or nonliving surfaces Killing bacteria (bactericidal) or preventing their growth (bacteriostatic)

5. What is the context? Home Hospital Laboratory (research, production, diagnostic) Food production and processing There is no perfect, universal antimicrobial What is the best for the situation

6. Some microbes are harder to kill than others Endospores- extremely heat-resistant Mycobacterium- chemical resistant Pseudomonas- can metabolize many chemicals Naked viruses (protein, genome only) more resistant than enveloped viruses

7. How long does it take to kill microbes? Depends on how many are present D values can be calculated: the amount of time required to kill 90% of organisms The more organisms are present, the more processes are required Assumption: death occurs at a constant rate Death rate can be accelerated by antimicrobial treatments

8. How many cycles needed for 1000 organisms?

9. Antimicrobial treatment can be enhanced by: Scrubbing (removing contaminants as well as microbes) Heat High or low pH Summarized in Table 5.1

11. In hospitals: Critical items come into contact with body tissue must be sterilized Semicritical items contact mucous membranes but nothing deeper kill vegetative cells and viruses Noncritical come into contact with unbroken skin

12. Heat treatment Boiling- kills most microbes, not spores Pasteurization- reduces numbers of microbes to tolerable levels Classic: 62oC for 30 minutes High-temperature short-time 72oC (more or less) for 15 seconds Ultra-high-temperature up to 150oC, then aseptic packaging no refrigeration needed

13. Autoclaves: high temperature, high pressure 121oC, 15 psi Time varies depending on content and volume Really high temperatures (132oC) may destroy prions

15. What materials can be autoclaved? Commercial canning Parameters designed to kill spore formers (such as C. botulinum) 12 D process (would kill 1012 organisms)

17. Filtration membrane- sterilizes heat-sensitive liquids air filtration- HEPA (high-efficiency particular air) filters remove particles bigger than 0.3m Laminar flow hoods protect workers and materials from contamination

18. Electromagnetic spectrum • Ionizing radiation • Highly penetrating • Sterilize heat-sensitive materials • UV irradiation • Sterilizes surfaces • Microwaves • Generates heat; can kill some microbes

19. Chemicals- disrupt tissue

20. Disinfectants chemicals kill microbes; some more than others antiseptics can be used on skin Criteria for use: toxicity interactions with other chemicals or with the material treated (residue) cost storage environmental risk

22. Preservatives nontoxic for food acids lower pH (benzoic, sorbic) nitrates inhibit spore formers Cold storage Drying- desiccation or lyophilization Packing in salt or sugar

23. These mechanisms reduce but do not eliminate microbes Cold- psychrophiles can grow also refrigerator temperatures must be about 4oC or under 40oF Chemical preservatives may have side effects Some organisms (such as S. aureus) can grow in high-salt conditions None of these mechanisms kills everything

24. Can we overdo the microbial control? Is toxicity of substances a bigger risk than the microbes? allergens mutagens pollutants Do we kill “good” microbes along with the bad? Are we producing resistant microbes?