We can be certain that all forms of microbial life are completely destroyed only when sterilizing techniques are used. The term sterilization is an absolute one; it means total, irreversible destruction of living cells. A number of physical environmental agents—such as ultraviolet or ionizing radiation, ultrasonic waves, or total dryness—exert stress on microorganisms and may kill them, but they cannot destroy large concentrations of microorganisms in a laboratory culture or a clinical specimen. Even small numbers of microorganisms may not be totally destroyed when exposed to ultraviolet rays or drying if they are distributed throughout and protected by the fabrics contained in a clean surgical pack, for example.

Ultraviolet light does not penetrate most substances, including fabrics, and therefore is used primarily to inactivate microorganisms located on surfaces. In microbiology laboratories, ultraviolet lamps are used inside of biological safety cabinets to decontaminate their surfaces, usually at the end of the day.

Of all the physical agents that exert antimicrobial effects, heat is the most effective. It is an excellent sterilizing agent when applied at high enough temperatures for an adequate period of time, because it effectively stops cellular activities. Depending on whether it is moist or dry, heat can coagulate cellular proteins (think of a boiled egg) or oxidize cell components (think of a burned finger or a flaming piece of paper). Heat is also nonselective in its effects on microorganisms (or other living cells), but we must bear in mind that this advantage is offset by its capacity to destroy all materials, whether living or not.

In Exercises Moist and Dry Heat and The Autoclave we shall see some examples of sterilization by use of moist and dry heat.