between the mycoplasmas and typical bacterial cells is that mycoplasmas completely
lack a cell wall; therefore, they assume a coccoid shape. Cell reproduction is more
complex than typical bacteria. They vary in their growth requirements, but all can be
grown on special artificial media. Mycoplasmas cause a number of infections
Section III. VIRUSES
Viruses are submicroscopic, obligate, filterable intracellular parasites. In other
words, they are organisms that are too small to be seen with an ordinary light
microscope and which are totally dependent upon a host cell for survival. They range in
size from about 15 m to 300 m in diameter. Viruses are true parasites, in that they
have no means of metabolism of their own. They cannot be grown on artificial media,
as they need living cells to exist. They are considered by some authorities as falling
within the gray area between the lowest forms of life and complex organic chemical
2-13. MORPHOLOGY AND PHYSIOLOGY OF VIRUSES
a. Morphology. Because of its minute size, a virus must be studied with the
electron microscope. The virus is much simpler than the bacterial cell, consisting of a
core of nucleic acid--either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)--
enclosed in a protective membrane of protein (called the capsid). Some viruses have a
lipid membrane called an envelope around the outside of the capsid. The shape of
viruses varies widely, but in general, they tend to conform to the following:
(1) Viruses of humans, animals, and plants are spherical rod-shaped, or
many sided. (Figure 2-7).
(2) Viruses of bacteria (bacteriophages) are shaped like a lunarlander
spacecraft. (Figure 2-7).
b. Physiology. Being an obligate parasite, the virus requires a living host cell
for existence. When a virus comes into contact with a susceptible host cell, it becomes
attached to the surface. The virus or its nucleic acid then enters the host cell, where it
takes control and utilizes the protoplasm of the host cell to produce new virus particles.
The new virus particles then invade additional host cells, and multiplication continues.
There is no cell division in the growth and reproduction of viruses. A new virus is
formed by chemical synthesis of the viral nucleic acid and capsid proteins (replication).
Host cells that have been attacked by a virus may be completely destroyed, or they may
suffer little or no harm.