Studies on transformation of hamster cells by polyoma virus in vitro

The proportion of hamster cells transformed in vitro was determined after exposure to various doses of polyoma virus. Preliminary experiments were made to characterize the transformed cells; the plating efficiency was determined with different types and numbers of irradiated feeder cells, and in the presence of excess normal hamster cells. Virus was adsorbed to hamster cells in suspension, and known numbers of cells were plated. Subsequently, colonies of transformed and normal cells were counted. After exposure to 96 PFU per cell the proportion of transformed colonies was 10−4 of original cells plated. This indicated transformation of between 10−2 and 10−3.7 of the original cells, depending on the plating efficiency immediately after virus adsorption. In various experiments involving exposure to 0.13 to 96 PFU per cell, the number of transformed colonies was directly related to virus dose. The size of some transformed colonies after 7 days suggested that there was little delay between virus infection and alteration of the hereditary characteristics of the cell.