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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University

Control of Microorganisms by Physical and Chemical Agents

Study Outline

  1. Definition of Frequently Used Terms
    1. Sterilization-destruction or removal of all viable organisms from an object or from a particular environment
    2. Disinfection-killing, inhibition, or removal of pathogenic microorganisms (usually on inanimate objects)
    3. Sanitization-reduction of the microbial population to a safe level as determined by public health standards
    4. Antisepsis-prevention of infection of living tissue by microorganisms
    5. Antimicrobial agents fall into one of two broad categories denoted by suffixes indicating effect
      1. -cide-a suffix indicating that the agent will kill the kind of organism in question (e.g., viricide, fungicide)
      2. -static-a suffix indicating that the agent will prevent the growth of the type of organism in question (e.g., bacteriostatic, fungistatic)
  2. The Pattern of Microbial Death
    1. Microorganisms are not killed instantly when exposed to a lethal agent; rather, the population decreases by a constant fraction at constant intervals (exponential killing)
    2. A microorganism is usually considered dead when it is unable to grow in conditions that would normally support its growth and reproduction
  3. Conditions Influencing the Effectiveness of Antimicrobial Agent Activity
    1. Population size-larger populations take longer to kill than smaller populations
    2. Populations consisting of different species or of cells at different developmental stages (e.g., endospores versus vegetative cells or young cells versus old cells) differ markedly in their sensitivity to various agents
    3. Concentration or intensity of the antimicrobial agent-higher concentrations or intensities are generally more efficient, but the relationship is not linear
    4. Duration of exposure-the longer the exposure, the greater the number of organisms killed
    5. Temperature-a higher temperature will usually (but not always) increase the effectiveness of killing
    6. Local environment-environmental factors, such as pH, viscosity, and concentration of organic matter can profoundly influence the effectiveness of a particular antimicrobial agent
  4. The Use of Physical Methods in Control
    1. Heat
      1. Measuring heat killing efficiency
        1. The thermal death time (TDT) is the shortest time necessary to kill all microorganisms in a suspension at a specific temperature and under defined conditions
        2. The decimal reduction time (D, or D value) is the time required to kill 90% of the microorganisms or spores in a sample at a specific temperature
        3. The Z value is the increase in temperature required to reduce D to 1/10 of its previous value
        4. The F value is the time in minutes at a specific temperature (usually 250ºF or 121.1ºC) necessary to kill a population of cells or spores
      2. Killing with moist heat
        1. Boiling water is effective against vegetative cells and eucaryotic spores
        2. Autoclaving (steam under pressure) is effective against vegetative cells and most bacterial endospores
        3. Pasteurization, a process involving brief exposure to temperatures below the boiling point of water, reduces the total microbial population and thereby increases the shelf life of the treated material; it is often used for heat-sensitive materials that cannot withstand prolonged exposure to high temperatures
          1. Low-temperature long-term (LTLT) pasteurization-63ºC for 30 min
          2. High-temperature short-term (HTST) flash pasteurization-72ºC for 15 sec
          3. Ultrahigh temperature (UHT) pasteurization-140º to 150ºC for 1 to 3 sec
      3. Dry heat can be used to sterilize moisture-sensitive materials such as powders, oils, and similar items; it is less efficient than moist heat because it usually requires higher temperatures (160º to 170ºC) and longer exposure times (2 to 3 hours)
    2. Low temperatures-these slow (refrigeration) or prevent (freezing) microbial growth and reproduction, but do not necessarily kill microorganisms; refrigeration and freezing are particularly important in food microbiology
    3. Filtration-sterilizes heat-sensitive liquids and gases by removing microorganisms rather than destroying them 1. Depth filters are thick fibrous or granular filters that remove microorganisms by physical screening, entrapment, and/or adsorption 2. Membrane filters are thin filters with defined pore sizes that remove microorganisms, primarily by physical screening 3. High-efficiency particulate air (HEPA) filters are used in laminar flow biological safety cabinets to sterilize the air circulating in the enclosure
    4. Radiation
      1. Ultraviolet (UV) radiation is effective, but its use is limited to surface sterilization because UV radiation does not penetrate glass, dirt films, water, and other substances
      2. Ionizing radiation (X rays, gamma rays, etc.) is effective and penetrates the material; the Food and Drug Administration and the World Health Organization have approved food irradiation and declared it safe; however, it is not widely used because of cost and concerns about the effects of the radiation on food
  5. The Use of Chemical Agents in Control
    1. Phenolics-laboratory and hospital disinfectants; act by denaturing proteins and disrupting cell membranes
    2. Alcohols-widely used disinfectants and antiseptics; will not kill endospores; act by denaturing proteins and possibly by dissolving membrane lipids
    3. Halogens-widely used antiseptics and disinfectants; iodine acts by oxidizing cell constituents and iodinating cell proteins; chlorine acts primarily by oxidizing cell constituents
    4. Heavy metals-effective but usually toxic; act by combining with proteins and inactivating them
    5. Quaternary ammonium compounds-cationic detergents used as disinfectants for food utensils and small instruments, and because of low toxicity, as antiseptics for skin; act by disrupting biological membranes and possibly by denaturing proteins
    6. Steri
    7. Aldehydes-reactive molecules that can be used as chemical sterilants; may irritate the skin; act by combining with nucleic acids and proteins and inactivating them lizing gases (e.g., ethylene oxide, betapropiolactone)-can be used to sterilize heat-sensitive materials such as plastic petri dishes and disposable syringes; act by combining with proteins and inactivating them; recently, vapor-phase hydrogen peroxide has been used to decontaminate biological safety cabinets
  6. Evaluation of Antimicrobial Agent Effectiveness
    1. The phenol coefficient test is a useful initial screening test in which the potency of a disinfectant is compared to that of phenol
    2. A more realistic test is the use of a dilution test; in this test, stainless steel cylinders are contaminated with specific bacterial species under carefully controlled conditions and then exposed to the disinfectant