Prescott’s Microbiology, 9th Edition
Chapter 8 –Control of Microorganisms in the Environment
GUIDELINES FOR ANSWERING THE MICRO INQUIRY QUESTIONS
Figure 8.1Which types of agents can be used for sterilization? Which can be used for antisepsis? What is the difference?
Agents in all 4 groups (physical, chemical, mechanical, and biological)can be used for sterilization purposes (observe the graphic and note which techniques have a final endpoint of sterilization as indicated by the black box). Only those techniques that are not harmful or toxic to human tissues can be used for antisepsis (as indicated by the grey boxes). There are two primary differences, the first is that sterilization is an absolute criterion; there is no partial sterilization. Any surviving microbial life is failure to sterilize. Disinfection and antisepsis comes in levels, low, medium and high. These are reductions in microbial life. The other difference is that antisepsis occurs on living tissues, such as a patient’s skin. Therefore, the antiseptic agent must also be non-harmful to the patient as well as controlling microbial growth. Thus, while ethanol is an acceptable skin antiseptic, boiling water is not.
Figure 8.3Examine graph (a). How long would it take to kill one-half of the original population of microorganisms? Keep in mind that the Y axis is exponential.
The original number of organisms pre-treatment (zero minutes of exposure) is 1 x 106, or one million. Half of this is 500,000, or 5 x 105. That value on the Y axis matches 0.5 minutes on the X axis. This makes sense, because one D value (90% reduction in viability) takes one minute under these conditions.
Figure 8.4How might one verify that filtration removed all microorganisms?
This is relatively easy to do by streaking some of the filtered liquid on some non-selective media such as nutrient agar or blood agar, or adding the liquid to some broth. Any growth indicates lack of sterility.
Figure 8.10 Why is it important that all of these compounds are relatively hydrophobic?
Disinfectants and antiseptics typically possess hydrophobic regions so that they can penetrate the cell membrane, with the assistance of a transporter. Some disinfectants, such as hydrogen peroxide, are mostly hydrophilic.
Figure 8.11 Why are cross-linking agents such as glutaraldehyde often called “fixatives” or are said to “fix the cells”?
They are considered fixatives because of the extensive crosslinkage in cellular proteins that occurs, resulting in loss of protein function. Thus the proteins cannot participate in any cellular reactions-the cell surface is no longer dynamic. This simultaneously kills the cells, and also ‘fixes’ their structure to be static, often performed before observation via microscopy.
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