Generation of transformed cell foci

The same titration principles can be used to measure other biological effects of virus infection. Under certain conditions, some DNA viruses can transform cells so that normal growth control

Fig. 10.5 Visualization of virus plaques. Under appropriate conditions, virus infection can be localized to the vicinity of the originally infected cells. If a limited number of infectious units of virus (PFUs) are incubated in a culture dish or on tissue in which virus can cause a cytopathic effect, virus plaques can be visualized.

  • a) A continuous line of monkey cells (Vero cells) was grown in the six-well culture dish. When the cells reached confluence, they were infected in duplicate with a series of 10-fold dilutions of a HSV-1 stock. After 48 hours, the cells were partially dehydrated (fixed) with ethanol and stained. Areas of cell death show as white plaques, each representing a single infectious event with the input virus solution.
  • b) A portion of the surface of a Petri dish containing agar with bacterial nutrient medium. A "lawn" of E. coli was grown on the plate's surface, and this layer of cells was infected with a solution containing a genetically "engineered" version of bacteriophage X that can be used to clone inserted genes. (See Chapter 22, Part V for some general details.) Bacteriophages that contain an inserted gene form clear plaques due to inactivation of an indicator gene (|-galactosidase) and viruses without the insert form dark-colored plaques. (c) Assay of tobacco mosaic virus (TMV). Leaves of a resistant (left) and a susceptible (right) plant that have been infected with small amounts of virus. (d) A higher magnification of plaque development. (Photographs in
  • c) courtesy of J. Langland.)
  • a) 10-6 10-7 10-8
  • a) 10-6 10-7 10-8
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(b) (c)

is altered. As outlined earlier in this chapter, transformed cells have a different morphology and tend to overgrow normal cells to form clumps of proliferating cells. Each infectious event, even if it is abortive and does not produce new virus, will result in the formation of transformed cell clumps, called a focus of transformation. An example of a focus of transformed cells is shown in Fig. 10.6. The changes in cell morphology that form a focus (a type of cyto-pathic effect of transformation) are clearly evident. The number of focus-forming units can be counted just as with PFUs, but here one is counting the spread of transformed cells, not the spread of virus.

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