Control of microbial growth

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Control of microbial growth Antimicrobial
agents physical chemical How much is
enough? efficacy toxicity specificity
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General categories of antimicrobials Physical he
at, irradiation, filtration, scrubbing Chemical
(antimicrobial mechanisms are considered separate
ly)
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Considerations Sterility- completely free of
endospores, viruses and all other microbes
(except prions) Living or nonliving
surfaces Killing bacteria (bactericidal) or
preventing their grwoth (bacteriostatic)
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p. 110
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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
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Some microbes are harder to kill than
others Endospores- extremely heat-resistant Myco
bacterium- chemical resistant Pseudomonas- can
metabolize many chemicals Naked viruses
(protein, genome only) more resistant than
enveloped viruses
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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
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How many cycles needed for 1000 organisms?
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Antimicrobial treatment can be enhanced
by Scrubbing (removing contaminants as well
as microbes) Heat High or low pH
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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
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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-temperat
ure up to 150oC, then aseptic packaging no
refrigeration needed
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Autoclaves high temperature, high
pressure 121oC, 15 psi Time varies depending on
content and volume Really high temperatures
(132oC) may destroy prions
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What materials can be autoclaved? Commercial
canning Parameters designed to kill spore
formers (such as C. botulinum) 12 D process
(would kill 1012 organisms)
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Dry heat can be used to sterilize certain
items rapidly and/or that cant be
autoclaved Takes much longer (e.g., 200oC for
1.5 hours vs 121oC for 15 minutes)
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Chemicals- disrupt tissue
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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
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p. 119
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Filtration membrane- sterilizes
heat-sensitive liquids air filtration- HEPA
(high-efficiency particular air) filtlers remove
particles bigger than 0.3?m Laminar flow hoods
protect workers and materials from contamination
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Radiation Ionizing radiation (gamma) is highly
penetrating used to sterilize (or pastuerize)
heat- sensitive materials, including food UV
irradiation can sterilize surfaces Microwaves
kill some microbes with generated heat
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Preservatives nontoxic for food acids lower pH
(benzoic, sorbic) nitrates inhibit spore
formers Cold storage Drying- desiccation or
lyophilization Packing in salt or sugar
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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
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Can we overdo the microbial control? Is
toxicity of substances a bigger risk than
the microbes? allergens mutagens pollutants D
o we kill good microbes along with the
bad? Are we producing resistant microbes?