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10/10/2016
1
Control of microbial growth
Chapter 7
BIO 220
Biocides (germicides)
• Kill microbes
Examples include
• Fungicide – kills fungi
• Virucide – deactivates or destroys virus
Bacteriostatic agents
• Inhibit growth and reproduction of microbes,
but do not kill them
• Once the bacteriostatic agent is removed,
growth and reproduction could resume
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More vocabulary
• Sepsis – indicates bacterial contamination
• Asepsis – absence of significant contamination
Rate of microbial death
• When bacterial populations are heated or
treated with antimicrobial chemicals, they
usually die at a constant rate.
• For example . . .
Fig. 7.1
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Effectiveness of antimicrobial treatments is
depend on . . .
• Number of microbes
Fig. 7.1b
Effectiveness of antimicrobial treatments is
depend on . . .
• Environmental influences – i.e. temperature
• Presence of organic matter
• Microbes incorporated in a biofilm?
• Medium the microbes are suspended in
• Time of exposure
• Microbial characteristics
Actions of microbial agents
• Alteration of membrane permeability
• Damage to proteins
• Damage to nucleic acids
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Heat
• Thermal death point (TDP) – lowest
temperature at which all of the microbes in a
liquid suspension will be killed in 10 minutes
• Thermal death time (TDT) – minimal length of
time for all bacteria in a liquid culture to be
killed at a given temperature
• Decimal reduction time (DRT) – time required
to kill 90% of the population at a particular
temperature
Autoclave
Fig. 7.2
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Fig. 7.3
Pasteurization
• Use of heat to kill pathogenic microbes and
reduce the number of spoilage microbes in
foods and beverages
• i.e. for milk, heat to 62.8 °C for 30 minutes or
71.7 °C for 15 seconds
• Phosphatase test for pasteurization
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Filtration
• Used to sterilize heat-
sensitive materials (i.e.
antibiotic solutions,
enzymes, vaccines)
• HEPA filters remove
microbes larger than 0.3
micrometers in diameter Fig. 7.4
Radiant energy spectrum
Fig. 7.5
Chemical antimicrobials
• Things we should keep in mind
– Most disinfectants do not work equally well on all
microbes
– Concentration
– Material to be disinfected
– Will disinfectant be able to contact microbes?
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Evaluating a disinfectant
• Use – dilution tests
– Stainless steel “carriers” are dipped in bacteria grown in liquid media
– Carriers are dried
– Each carrier is transferred to a test tube containing disinfectant (manufacturer’s dilution)
– Carriers then transferred to tubes containing liquid growth medium then incubated
– Number of carriers showing growth assessed
– More qualitative than quantitative
Evaluating a disinfectant
• Disk – diffusion method
Fig. 7.6
Phenolics and bisphenols
Fig. 7.7
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Action of heavy metals
Fig. 7.8
The ability of very
small amounts of
heavy metals (i.e.
silver and copper) to
exert antimicrobial
activity is called
oligodynamic action.
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Quats
• Bactericidal against gram (+) bacteria, less so
against gram (-) bacteria
• Do not kill endospores or mycobacteria
• Pseudomonas can grow in quats
Fig. 7.9Fig. 7.10
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