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13: Control of Microbial Growth

  • Page ID
    5192
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    How clean is clean? People wash their cars and vacuum the carpets, but most would not want to eat from these surfaces. Similarly, we might eat with silverware cleaned in a dishwasher, but we could not use the same dishwasher to clean surgical instruments. As these examples illustrate, “clean” is a relative term. Car washing, vacuuming, and dishwashing all reduce the microbial load on the items treated, thus making them “cleaner.” But whether they are “clean enough” depends on their intended use. Because people do not normally eat from cars or carpets, these items do not require the same level of cleanliness that silverware does. Likewise, because silverware is not used for invasive surgery, these utensils do not require the same level of cleanliness as surgical equipment, which requires sterilization to prevent infection.

    Why not play it safe and sterilize everything? Sterilizing everything we come in contact with is impractical, as well as potentially dangerous. As this chapter will demonstrate, sterilization protocols often require time- and labor-intensive treatments that may degrade the quality of the item being treated or have toxic effects on users. Therefore, the user must consider the item’s intended application when choosing a cleaning method to ensure that it is “clean enough.”

    A photo of the inside of a car with a table identifying the average CFUs per 6.5 x 6.5 cm area.  Door latch – 256. Door lock – 14. Door lock control – 182. Door handle – 29. Window control – 4. Cruise control button – 69. Steering wheel – 239. Interior steering wheel – 390. Radio volume know – 99. Gear shifter – 115. Center console – 506.
    Figure \(\PageIndex{1}\): Most environments, including cars, are not sterile. A study1 analyzed 11 locations within 18 different cars to determine the number of microbial colony-forming units (CFUs) present. The center console harbored by far the most microbes (506 CFUs), possibly because that is where drinks are placed (and often spilled). Frequently touched sites also had high concentrations. (credit “photo”: modification of work by Jeff Wilcox)

    • 13.1: Controlling Microbial Growth
      Inanimate items, such as doorknobs, toys, or towels, which may harbor microbes and aid in disease transmission, are called fomites. Two factors heavily influence the level of cleanliness required for a particular fomite and, hence, the protocol chosen to achieve this level. The first factor is the application for which the item will be used and the second factor is the level of resistance to antimicrobial treatment by potential pathogens.
    • 13.2: Using Physical Methods to Control Microorganisms
      For thousands of years, humans have used various physical methods of microbial control for food preservation. Common control methods include the application of high temperatures, radiation, filtration, and desiccation (drying), among others. Many of these methods nonspecifically kill cells by disrupting membranes, changing membrane permeability, or damaging proteins and nucleic acids by denaturation, degradation, or chemical modification.
    • 13.3: Using Chemicals to Control Microorganisms
      In addition to physical methods of microbial control, chemicals are also used to control microbial growth. A wide variety of chemicals can be used as disinfectants or antiseptics. This section describes the variety of chemicals used as disinfectants and antiseptics, including their mechanisms of action and common uses.
    • 13.4: Testing the Effectiveness of Antiseptics and Disinfectants
      Several environmental conditions influence the potency of an antimicrobial agent and its effectiveness. For example, length of exposure is particularly important, with longer exposure increasing efficacy. Similarly, the concentration of the chemical agent is also important, with higher concentrations being more effective than lower ones. Temperature, pH, and other factors can also affect the potency of a disinfecting agent.
    • 13.E: Control of Microbial Growth (Exercises)

    Footnotes

    1. 1 R.E. Stephenson et al. “Elucidation of Bacteria Found in Car Interiors and Strategies to Reduce the Presence of Potential Pathogens.” Biofouling 30 no. 3 (2014):337–346.

    Thumbnail: Scanning electron microscope image of Vibrio cholerae bacteria, which infect the digestive system. (Public Domain; T.J. Kirn, M.J. Lafferty, C.M.P Sandoe and R.K. Taylor).


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