Study of Viruses
In 1892, Dmitri Ivanovsky, a Russian scientist working in St. Petersburg, demonstrated that the sap of leaves infected with tobacco mosaic disease retains its infectious properties even after filtration through Chamberland filter candles. This was an important observation, because it provided an opera-tional definition of viruses and also an experimental technique by which an agent could be considered as a virus.
Beijerinck, a Dutch soil microbiologist, showed that the fil-tered sap could be diluted and then regain its strength after replication in living and growing tissue of the plant. The agent could reproduce itself (which meant that it was not a toxin) but only in living tissues, not in the cell-free sap of the plant. This explained the failure to culture the pathogen outside its host. All these observations contributed immensely to the dis-covery of an organism smaller than bacteria (a filterable agent) that is not observable in the light microscope and is able to reproduce itself only in living cells or tissues. Beijerinck called this agent a contagiumvivumfluidum, or a contagious living liquid.
The concept of contagiumvivumfluidum or a contagious living liquid began a 25-year debate about the nature of viruses; whether they were liquids or particles? This conflict was laid to rest when d’Herelle developed the plaque assay in 1917 and subsequent development of electron microscopy by Ruska (1934), when the first electron micrographs of tobacco mosaic virus (TMV) were taken in 1939. The viruses were accepted as particles.
Loeffler and Frosch (1898) described and isolated the first filterable agent from animals, the foot-and-mouth disease virus of cattle. Walter Reed and his team in Cuba (1902) recognized the first human filterable virus, yellow fever virus. Landsteiner and Popper (1909) demonstrated that poliomyelitis was caused by a filterable virus and also successfully transmitted the infec-tion to monkeys. Goodpasture (1930) used chick embryos for cultivation of viruses.
The term virus (taken from the Latin for slimy liquid or poison) was at that time used interchangeably for any infectious agent and so was applied to TMV and then further to all agents of the class.
Twort and d’Herelle (1915) independently observed a lytic phenomenon in bacterial cultures, which they attributed to viruses. d’Herelle named these viruses as bacteriophages. He developed the use of limiting dilutions with the plaque assay to titer the virus preparation. He suggested that the appearance of plaques in the plaque assay show the viruses to be particulate, or corpuscular.
d’Herelle also demonstrated that the attachment (adsorp-tion) of the virus to the host cell is the first step in the pathogen-esis of a virus infection. The attachment of a virus occurred only when bacteria sensitive to the virus were mixed with it, demon-strating the host range specificity of a virus at the adsorption step. He described the process of cell lysis and subsequently the release of infectious virus particles. He developed many other techniques that are still used in virology. d’Herelle was in many ways one of the founders of the principles of modern virology.