Control of microbial growth

Antimicrobial agents

physical

chemical

How much is enough?

efficacytoxicityspecificity

General categories of antimicrobials

Physicalheat, irradiation, filtration, scrubbing

Chemical(antimicrobial mechanisms are consideredseparately)

Considerations

Sterility- completely free of endospores, virusesand all other microbes (except prions)

Living or nonliving surfaces

Killing bacteria (bactericidal) or preventing theirgrowth (bacteriostatic)

What is the context?

HomeHospitalLaboratory (research, production, diagnostic)Food production and processing

There is no perfect, universal antimicrobial

What is the best for the situation

Some microbes are harder to kill than others

Endospores- extremely heat-resistant

Mycobacterium- chemical resistant

Pseudomonas- can metabolize many chemicals

Naked viruses (protein, genome only)more resistant than enveloped viruses

How long does it take to kill microbes?

Depends on how many are present

D values can be calculated: the amount of timerequired to kill 90% of organisms

The more organisms are present, the more processes are required

Assumption: death occurs at a constant rateDeath rate can be accelerated by antimicrobial

treatments

How many cycles needed for 1000 organisms?

Antimicrobial treatment can be enhanced by:

Scrubbing (removing contaminants as well asmicrobes)

HeatHigh or low pHSummarized in Table 5.1

In hospitals:

Critical items come into contact with body tissuemust be sterilized

Semicritical items contact mucous membranesbut nothing deeperkill vegetative cells and viruses

Noncritical come into contact with unbroken skin

Heat treatment

Boiling- kills most microbes, not spores

Pasteurization- reduces numbers of microbesto tolerable levels

Classic: 62oC for 30 minutesHigh-temperature short-time 72oC (more or less)

for 15 secondsUltra-high-temperature up to 150oC, then

aseptic packagingno refrigeration needed

Autoclaves: high temperature, high pressure

121oC, 15 psiTime varies depending on content and volume

Really high temperatures (132oC) may destroyprions

What materials can be autoclaved?

Commercial canning

Parameters designed to kill spore formers(such as C. botulinum)

12 D process (would kill 1012 organisms)

Filtrationmembrane- sterilizes heat-sensitiveliquids

air filtration- HEPA (high-efficiencyparticular air) filters remove particlesbigger than 0.3m

Laminar flow hoods protect workers and materials from contamination

Electromagnetic spectrum• Ionizing radiation

– Highly penetrating– Sterilize heat-

sensitive materials• UV irradiation

– Sterilizes surfaces• Microwaves

– Generates heat; can kill some microbes

Chemicals- disrupt tissue

Disinfectantschemicals kill microbes; some more thanothers

antiseptics can be used on skin

Criteria for use:toxicityinteractions with other chemicals or with

the material treated (residue)coststorageenvironmental risk

Preservativesnontoxic for food

acids lower pH (benzoic, sorbic)

nitrates inhibit spore formers

Cold storage

Drying- desiccation or lyophilization

Packing in salt or sugar

These mechanisms reduce but do not eliminatemicrobes

Cold- psychrophiles can growalso refrigerator temperatures must beabout 4oC or under 40oF

Chemical preservatives may have side effects

Some organisms (such as S. aureus) can growin high-salt conditions

None of these mechanisms kills everything

Can we overdo the microbial control?

Is toxicity of substances a bigger risk than themicrobes?allergensmutagenspollutants

Do we kill “good” microbes along with the bad?

Are we producing resistant microbes?