[Plaque cloning of the velogenic viscerotropic Newcastle disease virus]

A plaque clone was successfully produced and the plaque characteristic was studied of a velogenic viscerotropic Newcastle disease virus (strain II). The isolated 3 plaque mutants were found to form bright plaques in chick embryo fibroblast cultures, having a different size: those of clone I had a 4 mm dia, those of clone II - 2.5 mm dia, and those of clone III - 1.5 mm dia. The clones were differing in their virulence to day-old and six-week-old susceptible birds both from one another and from the parent virus. With regard to chick embryos, however, the difference in their virulence was negligible so far as the average time of perishing was concerned. Two methods of obtaining clones of the virus were comparatively tested: (1) infecting the cells as a monolayer (method of Dulbecco) and (2) infecting the cells set as a suspension. More advantageous proved the suspension method.