(3) Sedimentation. Sedimentation is the process by which bacteria and
other particles suspended in a liquid coagulate and settle to the bottom. This process
finds its practical application in water purification and sewage treatment.
b. Heat. Heat is the most widely used and most effective means of sterilization.
All species of microorganisms are killed by heat. The conditions under which the heat is
applied, the temperature achieved, the length of time the temperature is maintained,
and the species of microorganism involved are all factors affecting sterilization by heat.
The temperature that will kill a species of microorganism in 10 minutes is known as the
thermal death point. The time required to kill all microorganisms at a given temperature
is known as the thermal death time.
(1) Dry heat. Dry heat is used primarily for sterilizing objects made of glass,
metal, and substances that are damaged by moisture. For complete sterilization, a
temperature of 160-170C (320-338F) must be maintained for at least 1 to 3 hours.
Most fabrics are damaged by this much dry heat.
(2) Moist heat. Moist heat provides complete sterilization at lower
temperatures and in a shorter time than does dry heat. Whereas dry heat kills by
oxidation, a relatively slow process, moist heat kills by coagulation of the protein in the
microorganism--a relatively fast process. Moist heat may be applied in three ways.
(a) Boiling. Boiling is the most common means of sterilization. Boiling
will kill vegetative forms of pathogenic microorganisms in 5 minutes or less. Spores are
much more heat resistant, but they can be destroyed by boiling for 15 minutes. Boiling
is not considered a completely effective sterilizing agent, since the spores of certain
thermophiles (heat-loving organisms) can survive prolonged boiling.
(b) Steam. Free flowing steam provides about the same degree of
effectiveness as boiling, if applied in sufficient volume to maintain a temperature of
(c) Steam under pressure. The most effective means of
sterilization is by means of the autoclave. The autoclave is an airtight cylinder in which
steam may be introduced under pressure. Where as free-flowing steam has a
temperature of 100C, steam under 15 pounds of pressure per square inch has a
temperature of 121.5C (251F). Steam under 15-20 pounds of pressure per square
inch will kill all organisms and spores in 15-45 minutes.
(d) Flaming. Some instruments or objects, such as platinum wire loops
used in inoculating laboratory cultures, may be sterilized by holding the object in a gas
or alcohol flame until it glows.
(e) Incineration. Incineration is the approved method for sterilizing and
disposing of contaminated materials that have no further value. Care must be exercised