Chapter 7- Control of Microbial Growth
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I. The Terminology of Microbial Growth
A. While the last chapter dealt with the growth of microbes and the needs for that action. This chapter deals with the control of such growth, immensely applicable to the hospital setting.
B. Sterilization removal or destruction of all forms of microbial life.
C. Sterilant an agent that can cause sterilization
D. Commercial Sterilization a limited heat treatment that is long enough to kill the endospores of Clostridium botulinum, some thermophilic endospores survive this treatment.
E. Disinfection the destruction of vegetative (non-endospore-forming) pathogens.
F. Degerming the mechanical removal of microbes from a surface (i.e. alcohol swabbing of a surface).
G. Sanitization processes designed to lower the microbial counts to a safe level for human consumption, minimizing the risk of pathogen transmittal.
H. Sepsis indicates bacterial contamination
I. Asepsis indicates the absence of significant contamination
II. The Rate of Microbial Death
A. When microbes are treated with antimicrobial agents or actions they tend to die at a constant rate, resulting in a logarithmic death curve.
B. The effectiveness of antimicrobial treatments is governed by several factors.
1. The number of microbes the more you have to start, the more have to die in the end.
2. Environmental Factors
a. The presence of organic matter can shield microbes
b. Acidic solutions are usually more effective
c. The presence of fats or proteins can shield microbes
3. Time of Exposure
4. Microbial Characteristics, not all microbes are affected at the same rate by any one microbial agent.
III. Actions of Microbial Control Agents
A. Alterations of membrane permeability
B. Damage to the protein and/or nucleic acid structures
IV. Physical Methods of Microbial Control
A. Early Methodologies
1. Heating direct sterilization by exposure to flame
2. Desiccation Drying material to lower the available water content
3. Salting Osmotic prevention of microbial growth
B. Heat Sterilization
1. Thermal Death Point (TDP) The lowest temperature at which all microbes in a particular suspension are killed in 10 minutes.
2. Thermal Death Time (TDT) The minimum amount of time required to kill microbes in particular suspension
3. Decimal Reduction Time (DRT) The amount of time, in minutes, required to cause the death of 90% of the organisms in a suspension.
4. Moist Heat
a. Steam temperatures approach boiling and can kill most bacteria and fungi within ten (10) minutes.
b. Some viruses and endospores can take much longer to kill
c. Reliable sterilization requires temperatures above the normal boiling temperature of water, which can be accomplished in a higher pressure atmosphere. This is accomplished through a device called an autoclave.
d. Successful sterilization requires the steam to contact the material to be sterilized or a liquid material in contact. Therefore materials can not be wrapped in steam-proof materials (i.e. aluminum foil).
e. Sterilization is guaranteed by placing a sample of very thermophilic endospores in the material and attempting to culture these endospores after the process. Typically Bacillus sterothermophilus is used.
5. Pasteurization
a. Full sterilization can result in adverse flavors and a cooked taste to foodstuffs, making it unsuitable for the preservation of food.
b. Pasteurization the quick heating of material to a high temperature, followed by rapid cooling. The purpose is to remove pathogenic microbes and preserve the quality of the foodstuffs.
6. Dry Heat
a. Like the caveman, we still sterilize some things by directly heating them in flames (i.e. the loops we use in lab)
b. Hot-Air Sterilization dry heat is much less effective at killing organisms and can requires hours to effectively sterilize items.
C. Low Temperatures
D. High Pressure
E. Desiccation
F. Osmotic Pressure
G. Radiation
1. Ionizing Radiation the use of high energy radiation to destroy microbes by directly damaging their DNA.
2. UV useful, but has very little penetration (Shadow Effect)
3. Microwaves Little use in sterilization
V. Chemical Methods of Microbial Control
A. Evaluation methods for disinfectants
1. Use-Dilution Tests
2. Diffusion Disk Method
3. A variety of microorganisms are used in the tests so that the effectiveness across the entire spectrum of the microbe world can be seen.
B. Types of Disinfectants
1. Phenolics Derivatives of phenol (C6H6O, the standard against which all disinfectants are judged) method of action is disruption of the plasma membrane and possibly denaturation of proteins.
2. Biguanides Disruption of the plasma membrane.
3. Halogens Strong oxidizing agents
4. Alcohols lipid dissolution and protein denaturation
5. Heavy Metals denaturation of enzymes
6. Soaps and Detergents mechanical removal
7. Acid-anionic Sanitizers unknown method of action
8. Quaternary Ammonium Compounds Enzyme inhibition, protein denaturation
9. Organic Acids metabolic inhibition
10. Nitrates selective enzyme inhibition
11. Aldehydes protein denaturation
12. Gaseous Chemosterilizers protein denaturation
13. Peroxygens Strong oxidizers
VI. Microbial Characteristics and Microbial Control
A. The type of microorganism determines the effectiveness of a particular antimicrobial strategy.
B. Types of Microorganisms listed from those most sensitive to antimicrobial agents to those least sensitive.
1. Lipid Enveloped Viruses
2. Gram Positive Bacteria
3. Unenveloped Viruses
4. Fungi
5. Gram Negative Bacteria
6. Vegetative Protozoa
7. Cysts of Protozoa
8. Mycobacteria
9. Endospores
10. Prions