Section II. MICROORGANISMS OF IMPORTANCE
Microorganisms are extremely important in our everyday lives. In some
instances, as the production of certain cheeses, the presence of certain microorganisms
is greatly desired. In other cases, as spoiling food or bacterial infections,
microorganisms are not desirable. In this section, the major types of microorganisms,
their characteristics, and the diseases they produce will be discussed.
a. Characteristics. Viruses are the smallest microorganisms. A virus can only
be seen with the aid of an electron microscope. The diameter of the smallest viruses
can be as little as 10 millimicrons (or 39/1,000,000,000 of an inch). Because of their
size, most viruses can easily pass through filters that would capture bacteria.
Fortunately, our bodies generally develop long-lasting immunity against many viruses.
Viruses are composed of an outer coat of protein and an inner coat of either DNA
(deoxyribonucleic acid) or RNA (ribonucleic acid). Viruses do not cause a disease like
most bacteria. Instead, they change the metabolic and reproductive activity of the host
cell. This causes necrosis or death of the host cell.
b. Reproductive Characteristics. Viruses are obligate intracellular parasites.
This means they must be inside a living cell in order to live. Viruses use the substances
of that host cell to reproduce since they do not have the internal structures required for
life. Basically, the virus "injects" its internal components (that is, DNA or RNA) into the
host cell. Then, the metabolism of the host cell is controlled by the virus. After other
viruses are produced inside the host cell, the host cell ruptures, and the viruses are
released into the environment.
c. Viruses and Diseases. Viruses cause many types of diseases. The disease
range from the common cold to polio, rabies, and acquired immune deficiency
syndrome (AIDS). Influenza (flu) is actually a viral infection. The virus herpes simplex
causes cold sores in humans; in rabbits, a herpes simplex infection is fatal. Two strains
of herpes simplex exist: one strain produces cold sores and the other strain produces a
type of venereal disease that cannot be cured with existing medications. Another
species of the same genus, herpes zoster, causes a condition of the skin called
shingles (large flakes of skin come off in scales). Some childhood diseases are caused
by viruses. Examples include chickenpox (varicella), German measles (rubella), and
"red" measles (rubeola). Also, more severe types of diseases, such as smallpox (which
has almost been eradicated from the world), poliomyelitis (an inflammation of the gray
matter of the spinal cord and brain), rabies, and yellow fever, are caused by viruses.
You should remember that German measles (rubella) is especially dangerous for
pregnant women since the virus can pass through the placental barrier and cause birth