He could pass a solution through it and completely remove bacteria from the solution.
He showed that crushed leaf extracts from infected tobacco plants were still infectious after passing through the filter (remember, viruses are 100 times smaller than bacteria). He suggested that the infection might be caused by a toxin produced by bacteria but did not pursue the idea.
At that time it was thought that all infectious agents
* could be retained by filters (too large for viruses)
* grown on a nutrient medium (viruses need host cells to grow)
In 1898 Martinus Beijerinck repeated the experiments and became convinced this was a new type of infectious agent. He observed that the agent multiplied only in dividing cell and used the word virus but thought it was liquid in nature.
In 1899, Friedrich Loeffler and Frosch passed the agent of foot and mouth disease (aphthovirus) through a similar filter and ruled out the possibility of a toxin because of the high dilution; they concluded that the agent could replicate.
In the early 20th century, the English bacteriologist Frederick Twort discovered the viruses that infect bacteria, which are now called bacteriophages.
the French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria on agar, 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 suspension.
By the end of the nineteenth century, viruses were defined in terms of their infectivity, filterability, and their requirement for living hosts. Viruses had only been grown in plants and animals.
In 1906, 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.[12] 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 grew influenza and several other viruses in fertilised chickens' eggs.[14] In 1949 John F. Enders, Thomas Weller and Frederick Robbins grew polio virus in cultured human embryo cells, the first virus to be grown without using solid animal tissue or eggs. This work enabled Jonas Salk to make an effective polio vaccine.[15]
With the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll came the first images of viruses.[16] In 1935 American biochemist and virologist Wendell Stanley examined the Tobacco mosaic virus and found it to be mostly made from protein.[17]
A short time later, this virus was separated into protein and RNA parts.
Tobacco mosaic virus was the first one to be crystallised and whose 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 pictures, Rosalind Franklin discovered the full structure of the virus in 1955.[19] In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified Tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably how viruses assembled within their host cells.[20]
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