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Controlling Microbial Growthchapter 8
Destroy pathogens and prevent
their transmission
Reduce or eliminate microorganismsfrom water, food and othersubstances
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Control Terms
Sterilization
Disinfection
Sanitization reduction of microbial population to safelevel
Antisepsis reduction of microbial population on livingtissue
Chemotherapy use of chemical agents to kill orinhibit growth of microorganisms within host tissue
Cidal
Static
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Patterns of Microbial Death
microorganisms are not killed instantly
population death usually occursexponentially
microorganisms were previouslyconsidered to be dead when they did notreproduce in conditions that normallysupported their reproduction
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Tab. 8.1
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Fig. 8.2
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Conditions influencing how effective
antimicrobials agents will be.
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Catagories of Control
Physical methods
Chemical methods
Mechanical methods Biological methods
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1) PHYSICAL Control
Physical agents
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Moist Heat Sterilization
must be carried out above 100oCwhich requires saturated steamunder pressure
carried out using an autoclave
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Ex: The Autoclave orSteam Sterilizer
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Pasteurization
controlled heating at temperatureswell below boiling
used for milk, beer and otherbeverages
process does not sterilize but doeskill pathogens present
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Dry Heat Sterilization
less effective than moist heatsterilization, requiring highertemperatures and longer exposuretimes items subjected to 160-170oC for 2 to 3
hours
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Dry Heat Incineration
bench topincinerators areused to sterilize
inoculating loopsused inmicrobiologylaboratories
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Measuring Heat-Killing Efficiency
thermal death time (TDT)
shortest time needed to kill all microorganismsin a suspension at a specific temperature and
under defined conditions
decimal reduction time (D or D value)
time required to kill 90% of microorganisms orspores in a sample at a specific temperature
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Example:
Zvalue
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Low Temperatures
freezing stops microbial reproduction due to
lack of liquid water
some microorganisms killed by icecrystal
refrigeration
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Filtration
depth filtersthickfibrousorgranularfiltersthatremove
microorganismsbyphysical
membranefiltersporousmembraneswith definedporesizes
thatremovemicroorganismsprimarilyby
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Filtering air
surgical masks
cotton plugs onculture vessels
high-efficiencyparticulate air(HEPA) filters
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Radiation
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Ultraviolet (UV) Radiation
- limited to surfacesterilization
because it does notpenetrate glass, dirtfilms, water, andother substances
-
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Ionizing Radiation
penetrates deep into objects
destroys bacterial endospores;
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2) Chemical Control
Chemical agents
Phenolics
Alcohols Halogens
Heavy metals
Quaternary ammonium compounds
Aldehydes Gases
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Phenolics
Antiseptic and disinfectant
Denature proteins and disrupt membranes
Effective in the presence of organic material
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Alcohols
Antisepticanddisinfectant
Most widelyused
Denatureproteins / dissolvemembranes
Bactericidal / fungicidal
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Halogens
One of fivecompounds:
Fluorine, chlorine,bromine, iodineand astitine
Disinfectants (F,Cl, Br)
Antiseptic (I)
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Heavy metals
Germicides: mercury, silver,arsenic, zinc, copper
Combine with proteins anddeactivate them
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Quartenary Ammonium compounds
QAC detergents
Amphipathic
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Aldehydes
Formaldehyde, glutaraldehyde
Sporicidal and can be used as chemicalsterilants
Highly reactive molecules
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Effects of Glutaraldehyde
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Sterilizing gases
Ethylene oxide gases
Very strong alkylating agent
sterilization
Microbicidal, sporicidal
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Chemotherapeutic Agents
chemicals that can be used internally tokill or inhibit the growth of microbeswithin host cells
their selective toxicity allows them totarget the microbe without harming thehost
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3) MECHANICAL Control
Filtration
High pressure washing
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4) Biological Control
Natural control processes
Bacteriophage
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Determining the effectiveness of
antimicrobial agents
complex process regulated byfederal agencies
EnvironmentalProtection
Agency (E
PA)
Food and Drug Administration (FDA)
Health Canada Canada
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Phenol coefficient test