Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 77

The terms “in vitro” and “in vivo” are common in the virology literature. In vitro means “in glass” and refers to experiments carried out in an artificial environment, such as a glass or plastic test tube. Unfortunately, the phrase “experiments performed in vitro” is used to designate not only work done in the cell-free environment of a test tube but also work done within cultured cells. The use of the phrase in vitro to describe living cultured cells leads to confusion and is inappropriate. In vivo means “in a living organism” but may be used to refer to either cells or animals. Those who work on plants avoid this confusion by using the term “in planta.”

In this textbook, we use in vitro to designate experiments carried out in the absence of cells, e.g., in vitro translation. Work done in cells in culture is done ex vivo, while research done in animals is carried out in vivo.

The time required for the development of cytopathology varies considerably among animal viruses. For example, depending on the size of the inoculum, enteroviruses and herpes simplex virus can cause cytopathic effects in 1 to 2 days and destroy the cell monolayer in 3. In contrast, cytomegalovirus, rubella virus, and some adenoviruses may not produce such effects for several weeks.

Figure 2.5 Development of cytopathic effect. (A) Cell rounding and lysis during poliovirus infection. Shown are uninfected cells (upper left) and cells 5.5 h after infection (upper right), 8 h after infection (lower left), and 24 h after infection (lower right). (B) Syncytium formation induced by murine leukemia virus. The field shows a mixture of individual refractile small cells and flattened syncytia (arrow), which are large, multinucleated cells. Courtesy of R. Compans, Emory University School of Medicine. (C) Schematic illustration of syncytium formation. Viral glycoproteins on the surface of an infected cell bind receptors on a neighboring cell, causing fusion.

The development of characteristic cytopathic effects in infected cell cultures is frequently monitored in diagnostic virology after isolation of viruses from specimens obtained from infected patients or animals. In the research laboratory, observation of cytopathic effect can be used to monitor the progress of an infection, and is often one of the phenotypic traits that characterize mutant viruses.

Some viruses multiply in cells without causing obvious cytopathic effects. For example, many members of the families Arenaviridae, Paramyxoviridae, and Retroviridae do not cause obvious damage to cultured cells. Infection by such viruses must therefore be assessed using alternative methods, as described in “Assay of Viruses” below.