Sterilization
Sterilization is a term that refers to the complete killing or elimination of living organisms in the sample being treated. Sterilization is absolute. After the treatment the sample is either devoid of life, or the possibility of life (as from the subsequent germination and growth of bacterial spores), or it is not.
There are four widely used means of sterilization. Standard sterilization processes utilize heat, radiation, chemicals, or the direct removal of the microorganisms.
The most widely practiced method of sterilization is the use of heat. There are a number of different means by which heat can be applied to a sample. The choice of which method of delivery depends on a number of factors including the type of sample. As an example, when bacterial spores are present the heating conditions must be sufficient to kill even these dormant forms of the bacteria.
A common type of heat sterilization that is used many types each day in a microbiology laboratory is known as incineration. Microorganisms are burned by exposing them to an open flame of propane. "Flaming" of inoculating needles and the tops of laboratory glassware before and after sampling are examples of incineration.
Another form of heat sterilization is boiling. Drinking water can be sterilized with respect to potentially harmful microorganisms such as Escherichia coli by heating the water to a temperature of 212°F (100°C) for five minutes. However, the dormant cyst form of the protozoan Giardia lamblia that can be present in drinking water, can survive this period of boiling. To ensure complete sterility, the 212°F (100°C) temperature must be maintained for 30 minutes. Even then, some bacterial spores, such as those of Bacillus or Clostridium can survive. To guarantee sterilization, fluids must be boiled for an extended time or intermittent boiling can be done, wherein at least three—and up to 30—periods of boiling are interspersed with time to allow the fluid to cool.
Steam heat (moist heat) sterilization is performed on a daily basis in the microbiology laboratory. The pressure cooker called an autoclave is the typical means of steam heat sterilization. Autoclaving for 15 minutes at 15 pounds of pressure produces a temperature of 250°F (121°C), sufficient to kill bacterial spores. Indeed, part of a quality control regiment for a laboratory should include a regular inclusion of commercially available bacterial spores with the load being sterilized. The spores can then be added to a liquid growth medium and growth should not occur.
Pasteurization is employed to sterilize fluids such as milk without compromising the nutritional or flavor qualities of the fluid.
The final form of heat sterilization is known as dry heat sterilization. Essentially this involves the use of an oven to heat dry objects and materials to a temperature of 320–338°F (160–170°C) for two hours. Glassware is often sterilized in this way.
Some samples cannot be sterilized by the use of heat. Devices that contain rubber gaskets and plastic surfaces are often troublesome. Heat sterilization can deform these materials or make them brittle. Fortunately, other means of sterilization exist.
Chemicals or gas can sterilize objects. Ethylene oxide gas is toxic to many microorganisms. Its use requires a special gas chamber, because the vapors are also noxious to humans. Chemicals that can be used to kill microorganisms include formaldehyde and glutaraldehyde. Ethanol is an effective sterilant of laboratory work surfaces. However, the exposure of the surface to ethanol must be long enough to kill the adherent microorganisms, otherwise survivors may develop resistance to the sterilant.
Another means of sterilization utilizes radiation. Irradiation of foods is becoming a more acceptable means of sterilizing the surface of foods (e.g., poultry). Ultraviolet radiation acts by breaking up the genetic material of microorganisms. The damage is usually too severe to be repaired. The sole known exception is the radiation-resistant bacteria of the genus Deinococcus.
The final method of sterilization involves the physical removal of microorganisms from a fluid. This is done by the use of filters that have extremely small holes in them. Fluid is pumped through the filter, and all but water molecules are excluded from passage. Filters—now in routine use in the treatment of drinking water—can be designed to filter out very small microorganisms, including many viruses.
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