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Control of microbial growth Antimicrobial agents physical chemical How much is enough? efficacy toxicity specificity. General categories of antimicrobials Physical heat, irradiation, filtration, scrubbing Chemical (antimicrobial mechanisms are considered separately).

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Presentation Transcript
slide1

Control of microbial growth

Antimicrobial agents

physical

chemical

How much is enough?

efficacy

toxicity

specificity

slide2

General categories of antimicrobials

Physical

heat, irradiation, filtration, scrubbing

Chemical

(antimicrobial mechanisms are considered

separately)

slide3

Considerations

Sterility- completely free of endospores, viruses

and all other microbes (except prions)

Living or nonliving surfaces

Killing bacteria (bactericidal) or preventing their

grwoth (bacteriostatic)

slide5

What is the context?

Home

Hospital

Laboratory (research, production, diagnostic)

Food production and processing

There is no perfect, universal antimicrobial

What is the best for the situation

slide6

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

slide7

How long does it take to kill microbes?

Depends on how many are present

D values can be calculated: the amount of time

required to kill 90% of organisms

The more organisms are present, the more

processes are required

Assumption: death occurs at a constant rate

Death rate can be accelerated by antimicrobial

treatments

slide9

Antimicrobial treatment can be enhanced by:

Scrubbing (removing contaminants as well as

microbes)

Heat

High or low pH

slide10

In hospitals:

Critical items come into contact with body tissue

must be sterilized

Semicritical items contact mucous membranes

but nothing deeper

kill vegetative cells and viruses

Noncritical come into contact with unbroken skin

slide11

Heat treatment

Boiling- kills most microbes, not spores

Pasteurization- reduces numbers of microbes

to tolerable levels

Classic: 62oC for 30 minutes

High-temperature short-time 72oC (more or less)

for 15 seconds

Ultra-high-temperature up to 150oC, then

aseptic packaging

no refrigeration needed

slide12

Autoclaves: high temperature, high pressure

121oC, 15 psi

Time varies depending on content and volume

Really high temperatures (132oC) may destroy

prions

slide14

What materials can be autoclaved?

Commercial canning

Parameters designed to kill spore formers

(such as C. botulinum)

12 D process (would kill 1012 organisms)

slide16

Dry heat can be used to sterilize certain items

rapidly and/or that can’t be autoclaved

Takes much longer (e.g., 200oC for 1.5 hours

vs 121oC for 15 minutes)

slide18

Disinfectants

chemicals kill microbes; some more than

others

antiseptics can be used on skin

Criteria for use:

toxicity

interactions with other chemicals or with

the material treated (residue)

cost

storage

environmental risk

slide21

Filtration

membrane- sterilizes heat-sensitive

liquids

air filtration- HEPA (high-efficiency

particular air) filtlers remove particles

bigger than 0.3m

Laminar flow hoods protect workers and

materials from contamination

slide22

Radiation

Ionizing radiation (gamma)

is highly penetrating

used to sterilize (or pastuerize) heat-

sensitive materials, including food

UV irradiation can sterilize surfaces

Microwaves kill some microbes with generated

heat

slide24

Preservatives

nontoxic for food

acids lower pH (benzoic, sorbic)

nitrates inhibit spore formers

Cold storage

Drying- desiccation or lyophilization

Packing in salt or sugar

slide25

These mechanisms reduce but do not eliminate

microbes

Cold- psychrophiles can grow

also refrigerator temperatures must be

about 4oC or under 40oF

Chemical preservatives may have side effects

Some organisms (such as S. aureus) can grow

in high-salt conditions

None of these mechanisms kills everything

slide26

Can we overdo the microbial control?

Is toxicity of substances a bigger risk than the

microbes?

allergens

mutagens

pollutants

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

Are we producing resistant microbes?