Quiz Me!	Control of Microbial Growth: Physical & Chemical Factors
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Last revised: Tuesday, February 15, 2000
Reading: Ch. 7 in Prescott et al, Microbiology, 4th Ed.
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 2000. Thomas M. Terry

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1. Overall Effectiveness (from least to most specific)

1. Sterilizing Agents-- kill everything (e.g. heat, radiation)
2. Disinfectants-- kill most things. Too strong for living tissues (e.g. lysol, NH₃)
3. Antiseptics-- milder in action. Can be used topically, but not ingested. (e.g. alcohol, iodine)
4. Chemotherapeutics-- can be ingested (e.g. penicillin, sulfa drugs)

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2. Sterilizing Agents

A. Heat

1. Boiling. OK for most food, but not sterilizing. Endospore formers, hepatitis virus can resist.
2. Autoclaving. Most common sterilizing procedure. 15 min @ 121 deg. Celsius. Adequate for l liter volumes. Longer times for larger volume.
3. Dry Heat. Used for dry products. Typically 170-200 deg. C. overnite.
4. Pasteurization. Not a sterilizing treatment, but kills pathogens in milk. 63-67 deg. C. for 30', Now 71 deg. C. for 15 sec.

B. Membrane Filters.

1. 0.45 um filters retard bacteria. Good for heat-labile materials. Rapid. But expensive, and filters will clog.
2. View examples of membrane filters

C. Chemicals

• Ethylene Oxide = alkylating agent

  

• chemically adds on to proteins, nucleic acids, etc.
• Widely used (in special cabinets) to sterilize heat-sensitive items, e.g. syringes, disposable plastics, rubber items, surgical supplies, etc.
• usually carried out for 1-10 hours at 60 deg. C. Very flammable, must use special precautions. Products retain residues of gases. (e.g. petri dishes have enough to cause mutations in bacteria).

D. Radiation

1. UV light. Reacts with DNA, causes DNA damage -- death. Thymine dimers!. But much damage can be repaired, esp. if light available (PHOTOREACTIVATION). NOTE: cannot penetrate glass.
2. Ionizing radiation. Gamma rays produce free radicals, destroy all kinds of chemicals. E.g. OH^-

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3. Disinfectants & Antiseptics (Not mutually exclusive; depends on concentration)

1. heavy metals (Mercury, Silver, Arsenic)- cause protein denaturation
2. halogens (Chlorine, Iodine, Hypochlorite)- oxidizing agents. Not usually used as antiseptics, but good for swimming pools, hot tubs, water supplies. Household bleach = 5% soln of hypochlorite- good for all-purpose disinfectant. But don't use with ammonium compounds or acids, can produce explosive gases (nitrogen tricholoride or chlorine gas).
3. phenols & cresols- dissolve membranes, denature proteins
4. alcohols- denature proteins, dissolve membranes.
5. detergents- dissolve membranes

Note: disinfectants are classified into 3 groups:

1. High level: effective against all life, incl. endospores. E.g. ethylene oxide, 2% glutaraldehyde. May require 10 hours to kill all pop of endospore-forming bacteria
2. Intermediate level: defined as tuberculocidal (kill Mycobacterium tuberculosis ), as well as more resistant viruses (hepatitis, rhinovirus). Not effective against endospores.
3. Low level: not effective against tuberculosis or endospores, or viruses without membranes. But do kill vegetative bacteria and fungi, used extensively. Economical, not overly toxic to humans. E.g. Lysol, detergents, mercurials.

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