Chapter 8
Control of Microorganisms
by Physical and Chemical
Agents
1
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)
2
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
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
4
Antisepsis
• 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
Figure 8.1
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
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
8
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
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
9
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
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
10
The Use of Physical Methods in Control
•
•
•
•
heat
low temperatures
filtration
radiation
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
11
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
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
12
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
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
13
Figure 8.3
14
Moist heat….
15
Figure 8.4
16
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
– 72°C for 15 seconds then rapid cooling
• ultrahigh-temperature (UHT) sterilization
– 140 to 150°C for 1 to 3 seconds
17
Dry Heat (Oven)
• Usually an electric oven
• Coils radiate heat within an enclosed
compartment
• Exposure to 150°C to 180°C for 2 to 4 hours
• Used for heat-resistant items that do not
sterilize well with moist heat
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
19
Filtration
• Reduces microbial population or sterilizes solutions of heatsensitive 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
20
Figure 9.9 Filtration equipment-overview
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
22
Figure 7.6b
Copyright © The McGraw-Hill
Companies, Inc. Permission
required for reproduction or
23
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
24
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
26
Alcohols
• bactericidal, fungicidal, but not sporicidal
• inactivate some viruses
• denature proteins and possibly dissolve
membrane lipids
27
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
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
29
Heavy Metals
• Ions of mercury, silver, arsenic, zinc, and
copper
• effective but usually toxic
• combine with and inactivate proteins; may
also precipitate proteins
30
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
31
Aldehydes
• highly reactive molecules
• sporicidal and can be used as chemical
sterilants
• combine with and inactivate nucleic acids
and proteins
32
Sterilizing Gases
• used to sterilize heat-sensitive materials
• microbicidal and sporicidal
• combine with and inactivate proteins
33
Phenol coefficient test
34