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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