M ■ uring the late nineteenth century, many bacteria, M M fungi, and protozoa were identified as infectious A organisms. Most of these organisms could be readily seen with a microscope, and they generally could be grown in the laboratory. In the 1890s, D. M. Iwanowsky and Martinus Beijerinck found that a disease of tobacco plants, called mosaic disease, was caused by an agent different from anything that was known. About 10 years later, F. W. Twort in England and F. d'Herelle in France showed that infectious agents existed that could destroy bacteria and had the same unusual properties. They were so small that they could not be seen with the light microscope, and they passed through filters that retained almost all known bacteria. These same agents could only be grown in media if it contained living cells. The agents were called filterable viruses by Beijerinck. Virus means "poison," a term that once had been applied to all infectious agents. With time, the adjective filterable was dropped and only the word virus was retained.
Viruses had many features that were more characteristic of complex chemicals than of cells. For example, tobacco mosaic virus (TMV) could be precipitated from a suspension with ethyl alcohol and would still remain infective. A similar treatment destroyed the infectivity of bacteria. Further, in 1935, Wendell Stanley of the University of California, Berkeley, crystallized tobacco mosaic virus. Its physical and chemical properties obviously differed from those of cells, which cannot form crystals. Surprisingly, the crystallized tobacco mosaic virus could still cause the disease described by Iwanowsky and Beijerinck 40 years before.
—A Glimpse of History
VIRUSES POSED A MYSTERY TO SCIENTISTS AS recently as 50 years ago. They were clearly smaller than known bacteria and possessed properties different from those of cells. The nature of these curious agents, some of which infected animals and others plants, became clearer through the study of viruses that infect bacteria. Because bacterial cells can be grown easily and multiply rapidly, viruses that infect them, called bacteriophage or phage (phago means "to eat''), can be cultivated much more readily than the viruses that infect other organisms. Bacteriophages have been studied extensively, and the knowledge gained from these studies has contributed enormously to an understanding both of the viruses and of the molecular biology of all organisms.
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