Virology
Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy.
The identification of the causative agent of tobacco mosaic disease as a novel pathogen by Martinus Beijerinck is now acknowledged as being the official beginning of the field of virology as a discipline distinct from bacteriology. He realized the source was neither a bacterial nor a fungal infection, but something completely different. Beijerinck used the word "virus" to describe the mysterious agent in his 'contagium vivum fluidum'. Rosalind Franklin proposed the full structure of the tobacco mosaic virus in 1955.
One main motivation for the study of viruses is because they cause many infectious diseases of plants and animals. The study of the manner in which viruses cause disease is viral pathogenesis. The degree to which a virus causes disease is its virulence. These fields of study are called plant virology, animal virology and human or medical virology.
Virology began when there were no methods for propagating or visualizing viruses or specific laboratory tests for viral infections. The methods for separating viral nucleic acids and proteins, which are now the mainstay of virology, did not exist. Now there are many methods for observing the structure and functions of viruses and their component parts. Thousands of different viruses are now known about and virologists often specialize in either the viruses that infect plants, or bacteria and other microorganisms, or animals. Viruses that infect humans are now studied by medical virologists. Virology is a broad subject covering biology, health, animal welfare, agriculture and ecology.
History
was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected by microscopes. In 1884, the French microbiologist Charles Chamberland invented the Chamberland filter with pores small enough to remove all bacteria from a solution passed through it. In 1892, the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus: crushed leaf extracts from infected tobacco plants remained infectious even after filtration to remove bacteria. Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but he did not pursue the idea. At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium—this was part of the germ theory of disease.In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that the filtered solution contained a new form of infectious agent. He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum and reintroduced the word virus. Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanley, who proved they were particulate. In the same year, Friedrich Loeffler and Paul Frosch passed the first animal virus, aphthovirus, through a similar filter.
In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages, in 1915. The French-Canadian microbiologist Félix d'Herelle announced his independent discovery of bacteriophages in 1917. D'Herelle described viruses that, when added to bacteria on an agar plate, would produce areas of dead bacteria. He accurately diluted a suspension of these viruses and discovered that the highest dilutions, rather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension. Phages were heralded as a potential treatment for diseases such as typhoid and cholera, but their promise was forgotten with the development of penicillin. The development of bacterial resistance to antibiotics has renewed interest in the therapeutic use of bacteriophages.
By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to pass filters, and their requirement for living hosts. Viruses had been grown only in plants and animals. In 1906 Ross Granville Harrison invented a method for growing tissue in lymph, and in 1913 E. Steinhardt, C. Israeli, and R.A. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue. In 1928, H. B. Maitland and M. C. Maitland grew vaccinia virus in suspensions of minced hens' kidneys. Their method was not widely adopted until the 1950s when poliovirus was grown on a large scale for vaccine production.
Another breakthrough came in 1931 when the American pathologist Ernest William Goodpasture and Alice Miles Woodruff grew influenza and several other viruses in fertilised chicken eggs. In 1949, John Franklin Enders, Thomas Weller, and Frederick Robbins grew poliovirus in cultured cells from aborted human embryonic tissue, the first virus to be grown without using solid animal tissue or eggs. This work enabled Hilary Koprowski, and then Jonas Salk, to make an effective polio vaccine.
The first images of viruses were obtained upon the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll. In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it was mostly made of protein. A short time later, this virus was separated into protein and RNA parts.
The tobacco mosaic virus was the first to be crystallised and its structure could, therefore, be elucidated in detail. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in 1941. Based on her X-ray crystallographic pictures, Rosalind Franklin discovered the full structure of the virus in 1955. In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its protein coat can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells.
The second half of the 20th century was the golden age of virus discovery, and most of the documented species of animal, plant, and bacterial viruses were discovered during these years. In 1946, bovine viral diarrhoea was first described. In 1957 equine arterivirus was discovered. In 1963 the hepatitis B virus was discovered by Baruch Blumberg. In 1965 Howard Temin described the first retrovirus. Reverse transcriptase, the enzyme that retroviruses use to make DNA copies of their RNA, was first described in 1970 by Temin and David Baltimore independently. In 1983 Luc Montagnier's team at the Pasteur Institute in France, first isolated the retrovirus now called HIV. In 1989 Michael Houghton's team at Chiron Corporation discovered hepatitis C.
Detecting viruses
There are several approaches to detecting viruses and these include the detection of virus particles or their antigens or nucleic acids and infectivity assays.Electron microscopy
Viruses were seen for the first time in the 1930s when electron microscopes were invented. These microscopes use beams of electrons instead of light, which have a much shorter wavelength and can detect objects that cannot be seen using light microscopes. The highest magnification obtainable by electron microscopes is up to 10,000,000 times whereas for light microscopes it is around 1,500 times.Virologists often use negative staining to help visualise viruses. In this procedure, the viruses are suspended in a solution of metal salts such as uranium acetate. The atoms of metal are opaque to electrons and the viruses are seen as suspended in a dark background of metal atoms. This technique has been in use since the 1950s. Many viruses were discovered using this technique and negative staining electron microscopy is still a valuable weapon in a virologist's arsenal.
Traditional electron microscopy has disadvantages in that viruses are damaged by drying in the high vacuum inside the electron microscope and the electron beam itself is destructive. In cryogenic electron microscopy the structure of viruses is preserved by embedding them in an environment of vitreous water. This allows the determination of biomolecular structures at near-atomic resolution, and has attracted wide attention to the approach as an alternative to X-ray crystallography or NMR spectroscopy for the determination of the structure of viruses.
Growth in cultures
Viruses are obligate intracellular parasites and because they only reproduce inside the living cells of a host these cells are needed to grow them in the laboratory. For viruses that infect animals cells grown in laboratory cell cultures are used. In the past, fertile hens' eggs were used and the viruses were grown on the membranes surrounding the embryo. This method is still used in the manufacture of some vaccines. For the viruses that infect bacteria, the bacteriophages, the bacteria growing in test tubes can be used directly. For plant viruses, the natural host plants can be used or, particularly when the infection is not obvious, so-called indicator plants, which show signs of infection more clearly.Viruses that have grown in cell cultures can be indirectly detected by the detrimental effect they have on the host cell. These cytopathic effects are often characteristic of the type of virus. For instance, herpes simplex viruses produce a characteristic "ballooning" of the cells, typically human fibroblasts. Some viruses, such as mumps virus cause red blood cells from chickens to firmly attach to the infected cells. This is called "haemadsorption" or "hemadsorption". Some viruses produce localised "lesions" in cell layers called plaques, which are useful in quantitation assays and in identifying the species of virus by plaque reduction assays.
Viruses growing in cell cultures are used to measure their susceptibility to validated and novel antiviral drugs.