Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 39

A defining characteristic of viruses is their absolute dependence on a living host for reproduction: they are obligate parasites. Transmission of plant viruses such as tobacco mosaic virus can be achieved readily, for example, by applying extracts of an infected plant to a scratch made on the leaf of a healthy plant. Furthermore, as a single infectious particle of many plant viruses is sufficient to induce a characteristic lesion (Fig. 1.11), the concentration of the infectious agent could be measured. Plant viruses were therefore the first to be studied in detail. Some viruses of humans and other species could also be propagated in laboratory animals, and methods were developed to quantify them by determining the lethal dose. The transmission of yellow fever virus to mice by Max Theiler in 1930 was an achievement that led to the isolation of an attenuated strain, still considered one of the safest and most effective ever produced for the vaccination of humans.

After specific viruses and appropriate host organisms were identified, it became possible to produce sufficient quantities of virus particles for study of their physical and chemical properties and the consequences of infection for the host. Features such as the incubation period, symptoms of infection, and effects on specific tissues and organs were investigated. Laboratory animals remain an essential tool in investigations of the pathogenesis of viruses that cause disease. However, real progress toward understanding the mechanisms of virus reproduction was made only with the development of cell culture systems. The first and the simplest, but crucial to both virology and molecular biology, were cultures of bacterial cells.