We can be certain that all forms of microbial life are completely destroyed only
techniques are used. The term sterilization
is an absolute one; it
means total, irreversible destruction of living cells. A number of physical
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.