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13.1 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.

13.2 Using Physical Methods to Control Microorganisms

  • Heat is a widely used and highly effective method for controlling microbial growth.
  • Dry-heat sterilization protocols are used commonly in aseptic techniques in the laboratory. However, moist-heat sterilization is typically the more effective protocol because it penetrates cells better than dry heat does.
  • Pasteurization is used to kill pathogens and reduce the number of microbes that cause food spoilage. High-temperature, short-time pasteurization is commonly used to pasteurize milk that will be refrigerated; ultra-high temperature pasteurization can be used to pasteurize milk for long-term storage without refrigeration.
  • Refrigeration slows microbial growth; freezing stops growth, killing some organisms. Laboratory and medical specimens may be frozen on dry ice or at ultra-low temperatures for storage and transport.
  • High-pressure processing can be used to kill microbes in food. Hyperbaric oxygen therapy to increase oxygen saturation has also been used to treat certain infections.
  • Desiccation has long been used to preserve foods and is accelerated through the addition of salt or sugar, which decrease water activity in foods.
  • Lyophilization combines cold exposure and desiccation for the long-term storage of foods and laboratory materials, but microbes remain and can be rehydrated.
  • Ionizing radiation, including gamma irradiation, is an effective way to sterilize heat-sensitive and packaged materials. Nonionizing radiation, like ultraviolet light, is unable to penetrate surfaces but is useful for surface sterilization.
  • HEPA filtration is commonly used in hospital ventilation systems and biological safety cabinets in laboratories to prevent transmission of airborne microbes. Membrane filtration is commonly used to remove bacteria from heat-sensitive solutions.

13.3 Using Chemicals to Control Microorganisms

  • Heavy metals, including mercury, silver, copper, and zinc, have long been used for disinfection and preservation, although some have toxicity and environmental risks associated with them.
  • Halogens, including chlorine, fluorine, and iodine, are also commonly used for disinfection. Chlorine compounds, including sodium hypochlorite, chloramines, and chlorine dioxide, are commonly used for water disinfection. Iodine, in both tincture and iodophor forms, is an effective antiseptic.
  • Alcohols, including ethyl alcohol and isopropyl alcohol, are commonly used antiseptics that act by denaturing proteins and disrupting membranes.
  • Phenolics are stable, long-acting disinfectants that denature proteins and disrupt membranes. They are commonly found in household cleaners, mouthwashes, and hospital disinfectants, and are also used to preserve harvested crops.
  • The phenolic compound triclosan, found in antibacterial soaps, plastics, and textiles is technically an antibiotic because of its specific mode of action of inhibiting bacterial fatty-acid synthesis..
  • Surfactants, including soaps and detergents, lower the surface tension of water to create emulsions that mechanically carry away microbes. Soaps are long-chain fatty acids, whereas detergents are synthetic surfactants.
  • Quaternary ammonium compounds (quats) are cationic detergents that disrupt membranes. They are used in household cleaners, skin disinfectants, oral rinses, and mouthwashes.
  • Bisbiguanides disrupt cell membranes, causing cell contents to gel. Chlorhexidine and alexidine are commonly used for surgical scrubs, for handwashing in clinical settings, and in prescription oral rinses.
  • Alkylating agents effectively sterilize materials at low temperatures but are carcinogenic and may also irritate tissue. Glutaraldehyde and o-phthalaldehyde are used as hospital disinfectants but not as antiseptics. Formaldehyde is used for the storage of tissue specimens, as an embalming fluid, and in vaccine preparation to inactivate infectious agents. Ethylene oxide is a gas sterilant that can permeate heat-sensitive packaged materials, but it is also explosive and carcinogenic.
  • Peroxygens, including hydrogen peroxide, peracetic acid, benzoyl peroxide, and ozone gas, are strong oxidizing agents that produce free radicals in cells, damaging their macromolecules. They are environmentally safe and are highly effective disinfectants and antiseptics.
  • Pressurized carbon dioxide in the form of a supercritical fluid easily permeates packaged materials and cells, forming carbonic acid and lowering intracellular pH. Supercritical carbon dioxide is nonreactive, nontoxic, nonflammable, and effective at low temperatures for sterilization of medical devices, implants, and transplanted tissues.
  • Chemical preservatives are added to a variety of foods. Sorbic acid, benzoic acid, propionic acid, and their more soluble salts inhibit enzymes or reduce intracellular pH.
  • Sulfites are used in winemaking and food processing to prevent browning of foods.
  • Nitrites are used to preserve meats and maintain color, but cooking nitrite-preserved meats may produce carcinogenic nitrosamines.
  • Nisin and natamycin are naturally produced preservatives used in cheeses and meats. Nisin is effective against gram-positive bacteria and natamycin against fungi.

13.4 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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