Bacteria are very resistant to all environments. A
protective coating on the surface helps the bacteria
evade the defense mechanisms of the body and
generally makes it more durable.
Not all bacteria will take on the form of a spores
shell-like coating to withstand unfavorable conditions.
Bacteria in a spore state remain alive but passive, and
they are resistant to the effects of heat, drying, and
most bactericidal chemicals. They will remain capable
of becoming virulent (strongly pathogenic) again
under favorable conditions.
unfavorable conditions, they will either die or remain
dormant in a spore state until another opportunity for
growth presents itself.
Viruses are micro-organisms that are much
smaller than bacteria. Viruses vary in size, from being
the size of a single protein molecule to the size of a
more complicated bacterial cell. They can be so small
that they can be seen only through an electron
Viruses cannot live long or reproduce outside of a
living body (host). They must be able to enter and live
in specific cells. For descriptive purposes, they are
customarily divided into three subgroups, based on
1. Bacterial viruses
2. Animal viruses (including those that attack
3. Plant viruses
Some of the most common diseases caused by
viruses are colds, smallpox, measles, rubella, herpes
simplex, AIDS, infectious hepatitis, and serum
Viruses are usually not affected by
therapeutic treatment with antibiotics. Generally,
therapeutic treatment is not used to combat a viral
infection, but used to treat a secondary bacterial
infection that may develop.
Most viruses are susceptible to immersion in
boiling water for at least 20 minutes. There are two
major exceptions to this rule, infectious hepatitis and
serum hepatitis. Because of these exceptions to heat
resistance, autoclaving for a minimum of 20 minutes at
270°F, or dry heat sterilization for 90 minutes at 320°F
are the only safe procedures for control of these two
Protozoa are single-celled animals that do not have
a rigid cell wall.
Some protozoa cause parasitic
diseases but not all are pathogens. Most species are
harmless, living on dead organic matter or bacteria.
Protozoa that are pathogenic survive freely in nature
and must be spread by a carrier.
Most protozoa pass through a life-cycle, meaning
that they have definite stages of development. These
stages vary for each species and are usually very
complicated. Malaria is an example of a disease that is
caused by protozoa.
Fungi, like bacteria, are plants that lack
chlorophyll. They are free-living organisms that are
smaller than protozoa. Mold and yeast forms of fungi
have firm cell walls and resemble plants more than
Molds usually form cells in long chains or threads
that grow into tangled masses. Some threads of the
mass bear clusters of seedlike spores that, when dry,
are easily blown into the air like dust. Each
microscopic seed is capable of growing new mold
upon settling in a suitable place. Mold spores are
easily destroyed by heat. The most common infections
in humans because of mold are athletes foot and
ringworm. The mold penicillium is very common in
nature and contributes to the spoilage of food. The drug
penicillium is derived from this mold.
INFECTION CONTROL TERMS AND
The following terms and their definitions will
help you understand the material that is in this chapter
and in chapter 10, Sterilization and Disinfection,
AsepsisThe prevention of contact with micro-
Automated washer processorWasher,
sterilizer, dishwasher, or other mechanical washing
Barrier techniqueThe use of rubber, plastic,
foil, or other fluid resistant materials to cover surfaces
and protect them from contamination.
Bioburden The number of micro-organisms
contaminating an object. Also known as bioload or