Control of Bacterial Growth
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Definitions
– Sterilization - Processes that kills living
organisms including spores
– Disinfection - Destruction of vegetative
pathogens (not spores) on inanimate surfaces
– Antisepsis –A chemical method of disinfection
from skin and mucus membranes
– Degerming - Removal of microbes from a
limited area (alcohol swab prior to injection)
– Sanitization – The removal of microbes from
eating utensils and food preparation areas
Control of Bacterial Growth
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Factors that influence effectiveness of
antimicrobial treatment
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Number
Environmental Influences
Time of exposure
Inherent characteristics
Control of Bacterial Growth
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Physical Methods of Control
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Temperature Effects (Hot & Cold)
Filtration
Desiccation
Osmotic Pressure & High Pressure
Radiation
See chart page 197
Control of Bacterial Growth
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Effects of Temperature
– Moist Heat
– Dry Heat
– Pasteurization
– Refrigeration
Control of Bacterial Growth
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Thermal Death Point (TDP) lowest
temperature at which all microbes in liquid
suspension will be killed with 10 min of
exposure
Thermal Death Time (TDT) minimal length
of time for all bacteria in liquid culture to be
killed at a given temperature
Decimal Reduction Time (DRT) Time in
which 90% of a population at a given
temperature will be killed
Control of Bacterial Growth
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Temperature - Moist Heat
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Autoclave: 120-130°C (15-20 lbs/in2)
Effect on instruments
Foil or cloth wrap solid materials
Must use on liquids
Control of Bacterial Growth
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Temperature - Dry Heat
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Glassware and instruments
160-170°C for at least 90 min
Usually foil wrap
Incineration (loops & needles)
Control of Bacterial Growth
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Temperature - Pasteurization
– Milk: 63°C for 30 min (old)
– Milk: 72°C for 15 sec (new)
– Juices
Control of Bacterial Growth
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Temperature - Refrigeration
– Does not usually kill
– Slows metabolism although psychrophillic
organisms can still grow
– Best between 0° and 7° C
– Freeze thaw cycles can kill some organisms
– Used for culture preservation
– Lyophilization or freeze-drying long term
storage – effective in preserving foods
Control of Bacterial Growth
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Filtration
– Used for heat sensitive liquids like serum
or urea containing media
– 0.45µ or 0.22µ pores in membrane
– Often used commercially with beer, wine
and fruit juices
Control of Bacterial Growth
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Desiccation
– Inhibits growth more than kills
– Dried meats and vegetables
– Freeze dry processes in foods
Control of Bacterial Growth
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Osmotic Pressure
– High concentrations of salts or sugars
inhibit growth of bacteria; examples
includes jams and jellies, sugar and salt
cured meats
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High Pressure
– High pressure is transferred evenly
throughout culture suspension can
denature proteins & glycoproteins
Radiation & Control of
Microorganisms
Control of Bacterial Growth
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Radiation
– Short wave (X-rays, gamma rays) high
penetration power; breaks DNA
– Non-ionizing (UV) longer wave; no
penetrating power; forms thymine dimers
– Organisms contain multiple repair systems