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Control of microbial growth

Chapter 7

BIO 220

Biocides (germicides)

• Kill microbes (with some exceptions like

endospores)

Examples include

• Fungicide – kills fungi

• Virucide – inactivates 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

dependent on . . .

• Number of microbes

Fig. 7.1b

Effectiveness of antimicrobial treatments is

dependent on . . .

• Environmental influences – i.e. temperature

• Presence of organic matter

• Microbes incorporated in a biofilm? (EPS)

• Medium the microbes are suspended in

– i.e. fats and proteins

• Time of exposure

• Microbial characteristics

– i.e. gram-positive vs. gram-negative bacteria

Actions of antimicrobial agents

• Alteration of membrane permeability

– Target is cytoplasmic membrane

• Damage to proteins

• Damage to nucleic acids

– Often lethal

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Heat

• Microbial resistance to heat is variable

• 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

Moist heat kills primarily by coagulating proteins, requires temperatures above that

of boiling water – i.e. achieved by steam under pressure

Autoclave

Fig. 7.2

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Autoclave

Fig. 7.2 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, conditions sufficient to kill Coxiella

burnetii – Q fever

– 62.8 °C for 30 minutes (batch processing)

• 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

• In the lab, filters usually

have pore size 0.22 µm or

0.45 µm, but can be smallerFig. 7.4

Radiant energy spectrum

Fig. 7.5

Chemical antimicrobials

• Things we should keep in mind

– Few achieve sterility

– Most disinfectants do not work equally well on all

microbes

– Concentration

– Material to be disinfected (organic material

present?)

– 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

Phenol, phenolics and bisphenols

Fig. 7.7Phenolics are effective against mycobacteria and are active in the presence of organic material.

pHisoHex - nurseriesNot Pseudomonas aeruginosa

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Biguanides are effective against most gram-negative bacteria, but not Pseudomonas,

some activity against enveloped viruses. i.e. Alexidine

Iodine inhibits protein synthesis and alters membranes. Available as tincture or iodophore.

Hypochlorite is an oxidizer that interferes with enzymatic function.

Clostridium difficile and non-enveloped viruses tend to be resistant to alcohol-based

hand sanitizers. Metals like silver, mercury, and copper can be biocidal or antiseptic.

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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 anti-

microbial activity is

called oligodynamic

action. Heavy metals

denature proteins.

Quats

• Bactericidal against gram (+) bacteria, less so

against gram (-) bacteria

• Fungicidial, amebicidal, and virucidal (envel

viruses)

• Do not kill endospores or mycobacteria

• Pseudomonas can grow in quats

Fig. 7.9Fig. 7.10

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