9.1 Controlling Microbial Growth

  • Inanimate items that may harbor microbes and aid in their transmission are called fomites. The level of cleanliness required for a fomite depends both on the item’s use and the infectious agent with which the item may be contaminated.
  • The CDC and the NIH have established four biological safety levels (BSLs) for laboratories performing research on infectious agents. Each level is designed to protect laboratory personnel and the community. These BSLs are determined by the agent’s infectivity, ease of transmission, and potential disease severity, as well as the type of work being performed with the agent.
  • Disinfection removes potential pathogens from a fomite, whereas antisepsis uses antimicrobial chemicals safe enough for tissues; in both cases, microbial load is reduced, but microbes may remain unless the chemical used is strong enough to be a sterilant.
  • The amount of cleanliness (sterilization versus high-level disinfection versus general cleanliness) required for items used clinically depends on whether the item will come into contact with sterile tissues (critical item), mucous membranes (semicritical item), or intact skin (noncritical item).
  • Medical procedures with a risk for contamination should be carried out in a sterile field maintained by proper aseptic technique to prevent sepsis.
  • Sterilization is necessary for some medical applications as well as in the food industry, where endospores of Clostridium botulinum are killed through commercial sterilization protocols.
  • Physical or chemical methods to control microbial growth that result in death of the microbe are indicated by the suffixes -cide or -cidal (e.g., as with bactericides, viricides, and fungicides), whereas those that inhibit microbial growth are indicated by the suffixes -stat or-static (e.g., bacteriostatic, fungistatic).
  • Microbial death curves display the logarithmic decline of living microbes exposed to a method of microbial control. The time it takes for a protocol to yield a 1-log (90%) reduction in the microbial population is the decimal reduction time, or D-value.
  • When choosing a microbial control protocol, factors to consider include the length of exposure time, the type of microbe targeted, its susceptibility to the protocol, the intensity of the treatment, the presence of organics that may interfere with the protocol, and the environmental conditions that may alter the effectiveness of the protocol.

9.2 Testing the Effectiveness of Antiseptics and Disinfectants

  • Chemical disinfectants are grouped by the types of microbes and infectious agents they are effective against. High-level germicides kill vegetative cells, fungi, viruses, and endospores, and can ultimately lead to sterilization. Intermediate-level germicides cannot kill all viruses and are less effective against endospores. Low-level germicides kill vegetative cells and some enveloped viruses, but are ineffective against endospores.
  • The effectiveness of a disinfectant is influenced by several factors, including length of exposure, concentration of disinfectant, temperature, and pH.
  • Historically, the effectiveness of a chemical disinfectant was compared with that of phenol at killing Staphylococcus aureus and Salmonella enterica serovar Typhi, and a phenol coefficient was calculated.
  • The disk-diffusion method is used to test the effectiveness of a chemical disinfectant against a particular microbe.
  • The use-dilution test determines the effectiveness of a disinfectant on a surface. In-use tests can determine whether disinfectant solutions are being used correctly in clinical settings.


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Allied Health Microbiology by Open Stax and Linda Bruslind is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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