1. Chapter 9 Controlling Microorganisms

2. Controlling Microbial Growth
Sterilization
destroy all microbial life
Disinfection
reduces number of pathogens
pose no danger of disease
Decontamination
renders contaminated surface safe to handle
Antisepsis
kills microorganisms on living tissue
Microbiocidal
killing of microorganisms
Microbiostatic
inhibiting the growth of microorganisms

3. Microbial Death Rate Decimal reduction time Heat treatments D-value
time required to kill 90% of cells
Heat treatments
temperature
type of microorganism
physiological state
other substances
Figure 9.1

4. Sterilization Thermal death point Thermal death time
lowest temperature
kill all microorganisms
10 minutes
Thermal death time
minimal time
liquid suspension
at given temperature

5. Physical Controls Heat Moist Dry boiling autoclave Pasteurization
kills most vegetative cells, not sterile
autoclave
steam under pressure: 15 lbs./in2 at 121oC for 15 min.
kills endospores
Pasteurization
kills certain microorganisms
Dry
long periods
160oC for 2 hours
171oC for 1 hour

6. Physical controls Cold Radiation does not kill many die slowly
useful in slowing growth
Radiation
UV light
sterilizes by damaging DNA
Ionizing: X-rays and gamma rays
sterilizes by stripping electrons

7. Physical controls Filtration Drying Osmotic strength
physically removes microorganisms
heat-sensitive liquids
media, vitamins, antibiotics, hormones
air filters (HEPA)
Drying
not sterilization
evaporation
lyophilization
Osmotic strength
salts and sugars
damage cells by plasmolysis
preserving foods

8. Chemical controls Antimicrobial agents 1200 germicides Selection
chemotherapeutic agents
used to treat disease
germicides and germistats
disinfectants and antiseptics
1200 germicides
sold by 330 manufacturers in U.S.
Selection
damage the tissue or object
control the target microorganism(s)
purpose of treatment

9. Testing germicides Paper disc method germicide placed on paper disc
disc placed on agar seeded with microorganism
after incubation, observe zone of inhibition
Figure 9.2

10. Classes of germicides Phenols and phenolics denature proteins
act on lipids
disrupt cytoplasmic membrane
Examples:
Paracresol
Lysol
Hexachlorophene
prescription use
Figure 9.3

11. Classes of germicides Alcohols disrupt lipids denature proteins
attack cytoplasmic membrane
denature proteins
50-70% solutions more effective
increase plasmolysis after membrane is damaged
Figure 9.1

12. Classes of germicides Oxidizing agents Halogens
inactivate proteins
oxidizes functional groups
Halogens
iodine: antiseptic
betadine, tincture of iodine
chlorine: disinfectant
Hydrogen peroxide (H2O2)
3% solution-weak antiseptic
broken down to water and oxygen
bubbling when applied to skin

13. Classes of germicides Surfactants hydrophobic and hydrophilic parts
penetrate cytoplasmic membrane
quaternary ammonium salts
charged nitrogen with four hydrophobic groups
Cepacol
Zephiran
Figure 9.5

14. Classes of germicides Alkylating agents inactive proteins Formalin
attach short chains of carbons (alkyl groups)
Formalin
37% solution of formaldehyde
Ethylene oxide
gas sterilization
Figure 9.1

15. Food preservation Temperature pH Water Chemicals refrigeration canning
Pasteurization
63oC for 30 minutes
72oC for 15 seconds pH
vinegar
lactic acid fermentation
Water
drying
evaporation
freeze-drying
salting
high osmotic strength
Chemicals
preservatives