7.1 The Terminology of Microbial Control

• Sterilization is the removal or destruction of all living microorganisms.

• A sterilizing agent is called a sterilant.

• Control directed at destroying harmful microorganisms is called disinfection.

• When this treatment is directed at living tissue, it is called antisepsis, and the chemical is then called an antiseptic.

• For example, when someone is about to receive an injection, the skin is swabbed with alcohol—the process of degerming (or degermation), which mostly results in the mechanical removal, rather than the killing, of most of the microbes in a limited area.

• Restaurant glassware, china, and tableware are subjected to sanitization, which is intended to lower microbial counts to safe public health levels and minimize the chances of disease transmission from one user to another.

• A biocide, or germicide, kills microorganisms (usually with certain exceptions, such as endospores); a fungicide kills fungi; a virucide inactivates viruses; and so on.

• Other treatments only inhibit the growth and multiplication of bacteria; their names have the suffix -stat or -stasis, meaning to stop or to steady, as in bacteriostasis.

• Sepsis, from the Greek for decay or putrid, indicates bacterial contamination, as in septic tanks for sewage treatment.

• Asepsis is the absence of significant contamination.

  • Aseptic techniques are important in surgery to minimize contamination from the instruments, operating personnel, and the patient.

7.2 The Rate of Microbial Death

• When bacterial populations are heated or treated with antimicrobial chemicals, they usually die at a constant rate.

• Several factors influence the effectiveness of antimicrobial treatments:

  • The number of microbes. The more microbes there are to begin with, the longer it takes to eliminate the entire population.

  • Environmental influences. Most disinfectants work somewhat better in warm solutions.

• Heat is also measurably more effective under acidic conditions.

  • Time of exposure. Chemical antimicrobials often require extended exposure to affect more-resistant microbes or endospores.

  • Microbial characteristics. The concluding section of this chapter discusses how microbial characteristics affect the choice of chemical and physical control methods.

7.3 Actions of Microbial Control Agents

• Damage to the lipids or proteins of the plasma membrane by antimicrobial agents causes cellular contents to leak into the surrounding medium and interferes with the growth of the cell.

• The nucleic acids DNA and RNA are the carriers of the cell’s genetic information.

  • Damage to these nucleic acids by heat, radiation, or chemicals is frequently lethal to the cell; the cell can no longer replicate, nor can it carry out normal metabolic functions such as the synthesis of enzymes.

7.4 Physical Methods of Microbial Control

• Thermal death point (TDP) is the lowest temperature at which all the microorganisms in a particular liquid suspension will be killed in 10 minutes.

• Thermal death time (TDT), the minimal length of time for all bacteria in a particular liquid culture to be killed at a given temperature.

• Decimal reduction time (DRT), or D value, is a third concept related to bacterial heat resistance.

• These high temperatures are most commonly achieved by steam under pressure in an autoclave.

  • Autoclaving is the preferred method of sterilization in health care environments, unless the material to be sterilized can be damaged by heat or moisture.

  • The higher the pressure in the autoclave, the higher the temperature.

• The intent of pasteurization of milk was to eliminate pathogenic microbes.

• Many relatively heat-resistant (thermoduric) bacteria survive pasteurization, but these are unlikely to cause disease or cause refrigerated milk to spoil.

• This treatment, known as high-temperature short-time (HTST) pasteurization, is applied as the milk flows continuously past a heat exchanger.

  • In addition to killing pathogens, HTST pasteurization lowers total bacterial counts, so the milk keeps well under refrigeration.

• Milk can also be sterilized—something quite different from pasteurization—by ultra-high-temperature (UHT) treatments.

  • It can then be stored for several months without refrigeration.

• Equivalent treatments are as the temperature is increased, much less time is needed to kill the same number of microbes.

• One of the simplest methods of dry heat sterilization is direct flaming.

  • Another form of dry heat sterilization is hot-air sterilization.

• High-efficiency particulate air (HEPA) filters remove almost all microorganisms larger than about 0.3 mm in diameter.

• Membrane filters are composed of such sub- stances as cellulose esters or plastic polymers, which have become popular for industrial and laboratory use.

• In the absence of water, known as desiccation, microorganisms cannot grow or reproduce but can remain viable for years.

• Ionizing radiation are gamma rays, X rays, and high-energy electron beams that has a wavelength shorter than that of non-ionizing radiation, less than about 1 nm.

• Non-ionizing radiation has a wavelength longer than that of ionizing radiation, usually greater than about 1 nm.

• Microwaves do not have much direct effect on microorganisms, and bacteria can readily be isolated from the interior of recently operated microwave ovens.

7.5 Chemical Methods of Microbial Control

• The disk-diffusion method is used in teaching laboratories to evaluate the efficacy of a chemical agent.

  • A disk of filter paper is soaked with a chemical and placed on an agar plate that has been previously inoculated and incubated with the test organism.

  • After incubation, if the chemical is effective, a clear zone representing inhibition of growth can be seen around the disk.

• Lister was the first to use phenol (carbolic acid) to control surgical infections in the operating room.

• The halogens, particularly iodine and chlorine, are effective antimicrobial agents, both alone and as constituents of inorganic or organic compounds.

• Iodine is available as a degerming—that is, in solution in aqueous alcohol—and as an iodophor.

• An iodophor is a combination of iodine and an organic molecule, from which the iodine is released slowly.

7.6 Microbial Characteristics and Microbial Control

• It is important to remember that microbial control methods, especially biocides, are not uniformly effective against all microbes.