1.1 DISCOVERY OF VIRUSES

it is instructive and interesting to consider how this knowledge came about. It was only just over 100 years ago at the end of the nineteenth century that the germ theory of disease was formulated, and pathologists were then confident that a causative microorganism would be found for each infectious disease. Further they believed that these agents of disease could be seen with the aid of a microscope, could be cultivated on a nutrient medium, and could be retained by filters. There were, admittedly, a few organisms which were so fastidious that they could not be cultivated in vitro (literally, in glass, meaning in the test tube), but the other two criteria were satisfied. However, a few years later, in 1892, Dmitri Iwanowski was able to show that the causal agent of a mosaic disease of tobacco plants, manifesting as a discoloration of the leaf, passed through a bacteria-proof filter, and could not be seen or cultivated. Iwanowski was unimpressed by his discovery, but Beijerinck repeated the experiments in 1898, and becameconvinced this represented a new form of infectious agent which he termed contagium vivum fluidum, what we now know as a virus. In the same year Loeffler and Frosch came to the same conclusion regarding the cause of foot-and-mouth disease. Furthermore, because foot-and-mouth disease could be passed from animal to animal, with great dilution at each passage, the causative agent had to be reproducing and thus could not be a bacterial toxin. Viruses of other animals were soon discovered. Ellerman and Bang reported the cell-free transmission of chicken leukemia in 1908, and in 1911 Rous discovered that solid tumors of chickens could be transmitted by cell-free filtrates. These were the first indications that some viruses can cause cancer. Finally bacterial viruses were discovered. In 1915, Twort published an account of a glassy transformation of micrococci. He had been trying to culture the smallpox agent on agar plates but the only growth obtained was that of some contaminating micrococci. Upon prolonged incubation, some of the colonies took on a glassy appearance and, once this occurred, no bacteria could be subcultured from the affected colonies. If some of the glassy material was added to normal colonies, they too took on a similar appearance, even if the glassy material was first passed through very fine filters. Among the suggestions that Twort put forward to explain the phenomenon was the existence of a bacterial virus or the secretion by the bacteria of an enzyme which could lyse the producing cells. This idea of self-destruction by secreted enzymes was to prove a controversial topic over the next decade. In 1917 d’Hérelle observed a similar phenomenon in dysentery bacilli. He observed clear spots on lawns of such cells, and resolved to find an explanation for them. Upon noting the lysis of broth cultures of pure dysentery bacilli by filtered emulsions of feces, he immediately realized he was dealing with a bacterial virus. Since this virus was incapable of multiplying except at the expense of


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