Viruses: Historical Account and Origin | Microbiology

In this article we will discuss about:- 1. Historical Account of Viruses 2. General Characteristics of Viruses 3. Origin 4. Chemical Composition.

Historical Account of Viruses:

Some of the plant diseases, although not attributed to viruses at that time, were observed quite early. Breaking of Tulip was recorded as early as 1576 while potato curl disease and peach yellows were recorded in 1765 and 1791, respectively.

Around 1868, Variegation in Abutilon gained importance in Europe. It was sometime in 1870 that tobacco crop was affected by a severe disease in Holland. Adolf Mayer, Director of Agriculture experiment station, Wageningen began his studies on this disease in 1880 and published his results in 1886.

Mayer named the disease as Mosaik krankhet which means mosaic. He was able to demonstrate the infectious nature of the sap of diseased leaves which could pass through double filters. It is, therefore, considered that the science of viruses or Virology was born in 1886.

However, a Russian scientist named Iwanowsky in 1892 demonstrated that Tobacco Mosaic could be transmitted to healthy plants through the sap from diseased plants even though the sap had passed through double filters, fine enough to remove all bacteria.

In addition, the sap, when inoculated on culture medium, could not produce any living thing capable of producing the tobacco mosaic disease. Beijerinck (1898) from Holland also found that the infectious juice remained infective even after passing through bacterial filters and remained sterile.

He recognised the existence of a new type of infectious agent and he called it as “Contagium vivum fluidum” (infectious living fluid). Subsequently he referred this infective agent of tobacco mosaic disease as Virus.

By 1892 viral diseases of vertebrates were well known since the noted scientist Louis Pasteur had been studying canine rabies for quite some time. The first viral disease of human beings—the yellow fever—was discovered by Walter Reed and his associates in 1900. At that time the term virus included toxins, bacteria and viruses.

However, more important events in the development of plant virology have been listed below in chronological order:

General Characteristics of Viruses:

Viruses are non-cellular, ultra-microscopic particles (or structures) of protein and nucleic acid which grow and multiply only in living cells. They cause many highly infectious diseases of animals including man, angiosperms, bacteria, fungi and algae including the blue-greens.

All known viruses are pathogens. No saprophytic virus has been discovered so far. The viruses can exist outside the host free in air or water for long periods but remain quite inert in the free state. They become activated only when they enter a living cell of a host organism.

They can neither multiply nor metabolize even in complex mixtures of nutrients including the extracts of cells which they normally parasitize. It is possible to cultivate them only on intact living cells.

The viruses thus are obligate parasites and are the smallest of all infective agents, much smaller than the smallest bacteria. Some are of the dimensions of large protein molecules. They are not visible with the light microscope.

Even the largest ones are seen only as tiny dots at the highest possible magnification. The viruses can however, be observed and photographed with the electron microscope. In electron micrographs some appear rod-shaped.

Others have spherical cubical or tadpole forms. Most of them have been crystallized. The virus crystals resemble simple chemical compounds and can be treated like them.

Within the host cells, the viruses possess some of the properties of life but not all. They grow, reproduce and undergo mutation but no virus has any respiration of its own. They depend on the enzymes of the host cells to do their work.

The viruses cannot be placed in either the plant or animal kingdom. The branch of science which deals with them is called virology. One who specializes in this branch of science is called a virologist.

Origin of Viruses:

The supporters of the living hypothesis are divided into three camps about the origin of viruses. Some consider viruses as simple or primitive forms of living matter which represent a transition stage between the living and the non-living world.

They hold them to be living forms with certain characteristics of the non-living materials. According to this view, the viruses suggest the way how life might have arisen from lifeless matter.

It is generally held that life originated from simple, inorganic compounds by slow chemical reactions spread over millions and millions of years. In this way the pre-cellular first inhabitants of the earth known as the protoviruses may have originated. The present-day viruses are survivors of these inhabitants of the earth.

The supporters of the second hypothesis look the other way. They consider viruses as degenerate microorganism degraded from primitive, free living microorganisms.

The supporters of this regressive theory hold that the viruses have evolved from intracellular bacterial and rickettsial forms by losing most of the functions and structures characteristic of free living organisms.

Mathew (1970) derives viruses from mycoplasma-like organisms which are the simplest of the cellular microorganisms by loss of plasma membrane and change of genetic material from DNA to RNA.

The members of the third camp have put forth the cell component theory. They derive viruses from normal cell constituents such as mitochondrium or chloroplast.

They hold that virus is nothing but a part of the cell, the nucleic acids or genetic elements, which have developed the ability to leave the cell and be transferred to other cells. Such material is independent enough to pass from cell to cell and impose its patterns on the infected host cells.

Chemical Composition of Viruses:

Chemically viruses are nucleoproteins which are made up of two components, protein and nucleic acid. The nucleic acid forms the core of the virus particle or virion. It is covered by a tightly fitting protein coat shell or sheath. Each virion consists of one nucleic acid molecule but the protein coat or shell is formed of many similar protein molecules.

The nucleic acid is the infective agent. The protein coat is protective in function. It is inert and non-genetic. The proportion of nucleic acid varies from about 1-40%. The nucleic acid in virus particle is of a particular type, it is either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) but not both.

Plant viruses generally contain RNA. The only exceptions are Cauliflower Mosaic Virus and Dahlia Mosaic Virus which contain double-stranded DNA. The RNA in plant viruses is usually single-stranded.

Double-stranded RNA is, however, known to occur in two namely, Wound Tumor Virus and Rice Dwarf Virus. The DNA almost always occurs as a double stand. The phages contain DNA. About half of the animal viruses contain DNA and half RNA.

Lipids and carbohydrates so common in cellular organisms are usually absent in the viruses. The top cowpox virus, however, contains lipid, carbohydrates, copper and one or two vitamin-like substances.

Even such large and complex viruses exhibit no independent metabolic activity outside the host. On entering the host cell, the virus particle sheds its protein coat. The nucleic acid is released. It passes into the healthy cell and brings about multiplication of the virus.

The viruses, thus represent the simplest forms of life known at present. They are below the level of cellular organisation and do not appear to have anything resembling cytoplasm, nucleus or chromosomes.

The only thing they have in common with other living forms is the presence of nucleic acid, RNA or DNA but never both. Thus they have definite and stable, hereditary mechanisms. The only physiological activity of viruses is self-duplication.

They exhibit no other physiological function and no metabolism. They are strict parasites alive at least while in the living cells of their hosts. They do not possess energy yielding or synthetic enzymes systems which are essential for an independent existence.

Owing to the absence of the enzyme systems they cannot themselves convert energy. Energy is essential for duplication. Entering the host the virus can make use of the enzyme systems present in the cells of the host and shunt their energy to themselves. It is used for rapid multiplication at the expense of the raw materials of the host cell.