Viruses Part 1
About
a century ago at the time of Louis Pasteur (1822-1895) and Robert Koch
(1843-1910), the word “virus” was generally referred to as a poison
associated with disease and death. The present notion of virus is
entirely different. Now viruses are recognized as particles of nucleic
acid often with a protein coat. They replicate in living cells and cause
many diseases such as influenza, hepatitis, small pox and AIDS. In this
section the focus is on the properties of viruses and life cycle of
bacterial viruses, also known as bacteriophages. Some diseases caused by
viruses shall also be discussed in this section. The branch which deals
with the study of viruses is known as virology.
The word virus is derived from Latin word venome
meaning poisonous fluid. It can be defined as non cellular infectious
entities which contain either RNA or DNA, normally encased in
proteinaceous coat, and reproduce only in living cells. Viruses utilize
the biosynthetic machinery of the host for its own synthesis and then
transfer themselves efficiently to other Cell.
Some
viral diseases have been known for centuries. In fact, the first
infectious disease against which effective method of prevention was
developed was a viral disease. In 1796, Edward Jenner first vaccinated
an 8 years old boy with material removed from cowpox lesion on the hand
of milkmaid. After six weeks the boy was inoculated with pus from a
small pox victim, but he did not develop the disease. Later Jenner used
material for vaccination from cowpox lesions and successfully vaccinated
23 persons. As the material he used was obtained from cow (latin vacca), latter the term vaccination was used by Louis Pasteur for inoculation against disease.
In
1884, one of Pasteur’s associates, Charles Chamberland, found that
bacteria can not pass through porcelain filters, while agent responsible
for rabies (a disease which is transferred to human by bites of rabid
dogs, foxes, cats, bats and other animals) can pass through these
filters. As in those days the word virus was loosely used to describe
any toxic substance that caused disease, those unseen filterable agents
of disease were described as filterable viruses. In 1892, Ivanowski
discovered that the agent which caused tobacco mosaic disease was
filterable. He obtained bacteria free filtrate from ground up infected
plants and placed it on healthy leaves of tobacco. He observed that
filtrate produced the disease in healthy plants. After that, presence of
similar filter-passing, ultramicroscopic agents was seen in the victims
of many diseses, including foot and mouth disease (1898) and yellow
fever (1901).
Bacteriophages,
viruses that infect bacteria were discovered independently by Twort in
1915 and D’Herelle in 1917. Twort observed that bacterial colonies
sometimes undergo Lysis (dissolved and disappeared) and that this Lysis
can be transferred from one colony to other. Even highly diluted
material from Lysed colony can transfer the Lytic effect. However,
heating the filtrate destroyed its Lytic property. From these
observations he concluded that Lytic agent might be a virus. D’Herelle
rediscovered this phenomena in 1917 and used the word bacteriophages
meaning “bacteria eater”.
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The
filterable agents were first purified in 1935, when Stanley was
successful in crystallizing the tobacco mosaic virus. Chemical analysis
of these particles showed that they contained only nucleic acid and
protein. This suggested that, unlike other forms, viruses are of simple
chemical composition.
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