A. General Considerations in Microbial Control
Chapter Outline
11.1. Controlling Microorganisms
A. General considerations in microbial control
1. Sterilization
2. Disinfection
3. Antisepsis
4. Decontamination
B. Relative resistance of microbial forms
1. Highest: prions, endospores
2. Moderate: Protozoan cysts, sexual fungal spores, naked viruses, Mycobacterium
3. Least: Most bacterial vegetative cells, ordinary fungal spores, enveloped viruses, yeasts, and protozoan trophozoites
C. Methods of microbial control
1. Sterilization
a. Sterile
b. Sterilants
c. Disinfection
d. Sepsis
e. Asepsis
f. Antiseptics
g. A note about prions
2. The agents versus the process
a. –cide
i. Bactericide
ii. Germicide
iii. Fungicide
iv. Virucide
v. Microbicide
b. –static
i. Bacteristatic
ii. Microbistatic
3. Decontamination
a. Sanitization
b. Degermation
4. Practical concerns in microbial control
D. What is microbial death?
1. Factors that affect death rate
a. Numbers
b. Nature of microorganism
c. Temperature and pH
d. Concentration of agent
e. Mode of action of agent
f. Presence of solvents, organic matter, and inhibitors
E. How antimicrobial agents work: Their modes of action
1. The effects of agents on the cell wall
a. Cells deprived of functional cell wall lyse easily
2. How agents affect the cell membrane
a. Surfactants lower surface tension of cell membranes
3. Agents that affect protein and nucleic acid synthesis
a. Ribosome function
b. DNA and RNA function
4. Agents that alter protein function
a. Denaturation of proteins
11.2. Methods of Physical Control
A. Heat as an agent of microbial control
1. Mode of action and relative effectiveness of heat
a. Moist heat
b. Dry heat
2. Heat resistance and thermal death of spores and vegetative cells
3. Susceptibility of microbes to heat: Thermal death measurements
a. Thermal death time
b. Thermal death point
c. Common methods of moist heat control
d. Dry heat: Hot air and incineration
i. Incineration: most rigorous heat treatment
ii. Hot-air oven
iii. Dry oven
B. The effects of cold and desiccation
1. Cold retards activities of microbes
2. Dessication
3. Lyophilization
C. Radiation as a microbial control agent
1. Modes of action of ionizing versus non-ionizing radiation
2. Ionizing radiation: Gamma rays, X rays, and cathode rays
a. Cold sterilization
b. Applications of ionizing radiation
3. Non-ionizing radiation: Ultraviolet rays
a. Pyrimidine dimers
b. Applications of ultraviolet radiation
D. Decontamination by filtration: Techniques for removing microbes
1. Pore diameters
2. Applications of filtration
E. Osmotic pressure
11.3. Chemical Agents in Microbial Control
A. Selecting a microbicidal chemical
1. Levels of chemical decontamination
a. High
b. Intermediate
c. Low
B. Factors affecting the germicidal activity of chemicals
1. Nature of microorganism
2. Nature of material being treated
3. Degree of contamination
4. Time of exposure
5. Strength and chemical action of germicide
C. Germicidal categories according to chemical group
1. The halogen antimicrobial chemicals
a. Chlorine and its compounds
b. Chlorine compounds in disinfection and antisepsis
i. Hypochlorites
ii. Chloramines
c. Iodine and its compounds
i. Free iodine
ii. Iodophors (alcohol and iodine complex)
d. Applications of iodine solutions
i. Iodine tincture
1. Surgical antiseptic
ii. Iodophore
1. Betadine
a. Surgical handscrub
2. Phenol and its derivatives
a. Phenolics
b. Aromatic carbon ring with added functional groups
3. Applications of phenolics
a. Triclosan
4. Chlorhexidine
a. Surgical hand scrub
b. Obstetric antiseptic
5. Alcohols as antimicrobial agents
a. Applications of alcohols
b. Ethanol, isopropyl alcohol
6. Hydrogen peroxide and related germicides
a. Applications of hydrogen peroxide
i. 3% hydrogen peroxide as an antiseptic
ii. Vaporized hydrogen peroxide
iii. Ozone used to disinfect air and water
7. Chemicals with surface action: Detergents
a. Surfactants
i. Anionic detergents (negatively charged): limited microbicidal power
ii. Cationic detergents (positively charged): much more effective
b. Applications of detergents and soaps
i. Quaternary ammonium compounds:
1. Benzalkonium chloride
2. Zephiran
8. Heavy metal compounds
a. Oligodynamic action
b. Drawbacks:
i. Toxic to humans
ii. Allergic reactions
iii. Neutralized by biological fluids and wastes
iv. Resistance can develop in treated microbes
c. Applications of heavy metals
i. Merthiolate
ii. Metaphen
iii. Silver nitrate
9. Aldehydes as germicides
a. Glutaraldehyde; formaldehyde
b. Applications of the aldehydes
10. Gaseous sterilants and disinfectants
a. Ethylene oxide
b. Chlorine dioxide
c. Applications of gases and aerosols
11. Dyes as antimicrobial agents
a. Crystal violet and malachite green
b. Acridine dyes
12. Acids and alkalis