The Plaque Assay of Animal Viruses

This chapter discusses many factors that contribute to the accuracy and convenience of the plaque assay. Plaque assays are introduced for animal viruses as a direct analogy with the plaque assay of bacteriophage, with the object of improving the methods of handling a wide range of animal viruses, so as to obtain more rigorous information. The aim is to count individual infective particles in the same way that individual bacteria or bacteriophages can be counted, to assay virus infectivity more accurately, to express virus concentrations in terms of these particles rather than as arbitrary units, to achieve the simplification that tissue culture brings compared with animals or fertile eggs, and to isolate clones of virus from single infective particles, thus facilitating genetic studies. The plaque assay consists of the inoculation of a statistically adequate yet easily countable number of infective virus particles onto an immobilized layer of cells. Each particle is allowed to replicate under conditions where the resulting lesion remains local, and the lesions are finally counted. Each lesion is caused by a single particle, and this enables the calculation of the infective particle content of the original inoculum.