Cultivation and Assay of Viruses

Viruses replicate only within living cells. Some viruses are restricted in the kinds of cells in which they replicate, and a few have not yet been cultivated at all under laboratory conditions. However, most viruses are grown in cultured cells, embryonated hen's eggs, or laboratory animals. In veterinary virology, the natural host animal is used for the cultivation of viruses; indeed the earliest viral assay has been carried out with foot-and-mouth disease virus in cattle. The natural host is still useful for the studies of pathogenesis and immunology, experiments in chemotherapy, and occasionally for diaglostic purposes. However, the in vitro cultivation of viruses in cell cultures is essential for the study of their mode of replication and for diagnostic virology. Cells may be grown in vitro as explants of tissue, such as respiratory or intestinal epithelium, or as cell cultures. Explant cultures are occasionally used for research purposes or for the cultivation of certain viruses, but almost all diagnostic and research work involving viral cultivation is carried out in cell cultures—usually in monolayers, occasionally as suspension cultures. To produce cell monolayers, tissue is cut into small pieces and placed in a medium containing a proteolytic enzyme such as trypsin. After the cells have dispersed into a single-cell suspension, they are washed, counted, and diluted in a growth medium and permitted to settle on the flat surface of a glass or plastic container. Most types of cells adhere quickly and under optimal conditions, they divide about once a day until the surface is covered with a confluent monolayer.