1. Control of Microorganisms by Physical and Chemical Agents
Chapter 8
Control of Microorganisms by Physical and Chemical Agents

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)

3. 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

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

5. Antisepsis Antisepsis Antiseptics
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

6. Figure 8.1

7. Antimicrobial agents –cide: to kill -static: to stand still (inhibit)
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

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

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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10. More conditions… duration of exposure to agents temperature
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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11. The Use of Physical Methods in Control
heat
low temperatures
filtration
radiation
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12. Heat moist heat dry heat sterilization
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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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. exponential plot of survivors versus minutes of exposure to heating at 121 degrees; D121 value is 1 minute; data from table 7.1
Figure 8.3

15. Moist heat….

16. Figure 8.4

17. Moist heat… Pasteurization flash 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

18. 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

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

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

21. Figure 9.9 Filtration equipment-overview

22. Filtering air surgical masks cotton plugs on culture vessels
high-efficiency particulate air (HEPA) filters
used in laminar flow biological safety cabinets
laminar flow biological safety cabinet
Figure 7.6a

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

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

27. Alcohols bactericidal, fungicidal, but not sporicidal
inactivate some viruses
denature proteins and possibly dissolve membrane lipids

28. 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

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

30. Heavy Metals Ions of mercury, silver, arsenic, zinc, and copper
effective but usually toxic
combine with and inactivate proteins; may also precipitate proteins

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

32. Aldehydes highly reactive molecules
sporicidal and can be used as chemical sterilants
combine with and inactivate nucleic acids and proteins

33. Sterilizing Gases used to sterilize heat-sensitive materials
microbicidal and sporicidal
combine with and inactivate proteins

34. Phenol coefficient test