1 Chapter 8 Control of Microorganisms by Physical and Chemical
Agents
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2 Definition of Frequently Used Terms Sterilization Removes all
viable microorganisms including viruses Material is said to be
sterile Usually for inanimate objects Mostly performed with heat
Autoclave use saturated steam under pressure to reach temperatures
(above boiling)
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Definitions. Disinfection disinfectants agents, usually
chemical, used for disinfection 5% bleach solution The use of a
physical process or chemical agent (disinfectant) to destroy
vegetative pathogens Does not destroy bacterial endospores Usually
used only on inanimate objects Also removes toxins
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4 Decontamination & Sanitization Any cleansing technique
that mechanically removes microorganisms to reduce contamination to
safe levels (reduction of microbial population to levels deemed
safe (based on public health standards) Sanitizer: compound such as
soap or detergent that sanitizes Sanitary: may not be free from
microbes but are safe for normal use
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Antisepsis prevention of infection of living tissue by
microorganisms Antiseptics chemical agents that kill or inhibit
growth of microorganisms when applied directly to exposed body
surfaces to destroy or inhibit vegetative pathogens Sepsis: the
growth of microorganisms in the blood and other tissues Asepsis:
any practice that prevents the entry of infectious agents into
sterile tissues
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Figure 8.1
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Antimicrobial agents cide: to kill Bactericide: chemical that
destroys bacteria (not endospores) Fungicide: a chemical that can
kill fungal spores, hyphae, and yeasts Virucide: a chemical that
inactivates viruses Sporicide: can destroy bacterial endospores
Germicide and microbicide: chemical agents that kill microorganisms
-static: to stand still (inhibit) Bacteristatic: prevent the growth
of bacteria Fungistatic: inhibit fungal growth Microbistatic:
materials used to control microorganisms in the body, for
example
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8 The Pattern of Microbial Death microorganisms are not killed
instantly population death usually occurs exponentially
microorganisms are considered to be dead when they are unable to
reproduce in conditions that normally support their
reproduction
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 9 Conditions Influencing the
Effectiveness of Antimicrobial Agent Activity population size
larger populations take longer to kill than smaller populations
population composition microorganisms differ markedly in their
sensitivity to antimicrobial agents Concentration of an
antimicrobial agent usually higher concentrations or intensities
kill more rapidly relationship is not linear
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 10 More conditions duration of
exposure to agents longer exposure more organisms killed
temperature higher temperatures usually increase amount of killing
local environment Many other factors such as pH, viscosity and
concentration of organic matter can profoundly impact
effectiveness
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 11 The Use of Physical Methods in
Control heat low temperatures filtration radiation
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 12 Heat moist heat effective against
all types of microorganisms degrades nucleic acids, denatures
proteins, and disrupts membranes dry heat sterilization less
effective, requiring higher temperatures and longer exposure times
oxidizes cell constituents and denatures proteins
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 13 Measuring heat-killing efficiency
thermal death time (TDT) shortest time needed to kill all
microorganisms in a suspension at a specific temperature and under
defined conditions decimal reduction time (D or D value) time
required to kill 90% of microorganisms or spores in a sample at a
specific temperature
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14 Figure 8.3
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15 Moist heat.
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16 Figure 8.4
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17 Moist heat Pasteurization controlled heating at temperatures
well below boiling reduces total microbial population and thereby
increases shelf life of treated material flash pasteurization high
temperature short-term HTST 72C for 15 seconds then rapid cooling
ultrahigh-temperature (UHT) sterilization 140 to 150C for 1 to 3
seconds
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Dry Heat (Oven) Usually an electric oven Coils radiate heat
within an enclosed compartment Exposure to 150C to 180C for 2 to 4
hours Used for heat-resistant items that do not sterilize well with
moist heat
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19 Low Temperatures To slow growth of cultures and microbes in
food during processing and storage Cold does not kill most
microbes; freezing can actually preserve cultures freezing stops
microbial reproduction due to lack of liquid water some
microorganisms killed by ice crystal disruption of cell membranes
refrigeration slows microbial growth and reproduction
Lyophilization : a combination of freezing and drying; used to
preserve microorganisms and other cells in a viable state for many
years
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20 Filtration Reduces microbial population or sterilizes
solutions of heat- sensitive materials by removing microorganisms
also used to reduce microbial populations in air depth filters
thick fibrous or granular filters that remove microorganisms by
physical screening, entrapment, and/or adsorption membrane filters
porous membranes with defined pore sizes that remove microorganisms
primarily by physical screening Filters are usually thin membranes
of cellulose acetate, polycarbonate, and a variety of plastic
materials
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Figure 9.9 Filtration equipment-overview
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22 Filtering air surgical masks cotton plugs on culture vessels
high-efficiency particulate air (HEPA) filters used in laminar flow
biological safety cabinets Figure 7.6a
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Copyright The McGraw-Hill Companies, Inc. Permission required
for reproduction or display. 23 Figure 7.6b
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24 Radiation ultraviolet (UV) radiation (non ionizing) limited
to surface sterilization because UV radiation does not penetrate
glass, dirt films, water, and other substances Germicidal lamp: 254
nm not as penetrating as ionizing radiation ionizing radiation
(Gamma Rays, X Rays) penetrates deep into objects destroys
bacterial endospores; not always effective against viruses used for
sterilization and pasteurization of medical products and food
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26 Phenolics commonly used as laboratory and hospital
disinfectants act by denaturing proteins and disrupting cell
membranes tuberculocidal, effective in presence of organic
material, and long lasting disagreeable odor and can cause skin
irritation
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27 Alcohols bactericidal, fungicidal, but not sporicidal
inactivate some viruses denature proteins and possibly dissolve
membrane lipids
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Halogens Iodine skin antiseptic Microbicidal and sporicidal
with longer exposure oxidizes cell constituents and iodinates
proteins at high concentrations may kill spores skin damage,
staining, and allergies can be a problem iodophore iodine complexed
with organic carrier
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29 Halogens Chlorine oxidizes cell constituents important in
disinfection of water supplies and swimming pools, used in dairy
and food industries, effective household disinfectant destroys
vegetative bacteria and fungi, but not spores can react with
organic matter to form carcinogenic compounds
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30 Heavy Metals Ions of mercury, silver, arsenic, zinc, and
copper effective but usually toxic combine with and inactivate
proteins; may also precipitate proteins
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31 Quaternary Ammonium Compounds Detergents organic molecules
with hydrophilic and hydrophobic ends act as wetting agents and
emulsifiers cationic detergents are effective disinfectants kill
most bacteria, but not Mycobacterium tuberculosis or endospores
safe and easy to use, but inactivated by hard water and soap
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32 Aldehydes highly reactive molecules sporicidal and can be
used as chemical sterilants combine with and inactivate nucleic
acids and proteins
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33 Sterilizing Gases used to sterilize heat-sensitive materials
microbicidal and sporicidal combine with and inactivate
proteins
