Demonstration of inapparent heterogeneity in a population of an animal virus by single-burst analyses

Single-burst analyses and standard plaque assays were performed on populations of plaque types of vesicular exanthema of swine virus (VESV). The plaque assay method yielded 1% of a minute type in one of the experimental populations. When the same virus population was analyzed by single-burst experiments, up to 23% of the bursts yielded only the minute type. When one plaque type was grown en masse, other types appeared as contaminants. Two new plaque types not previously discovered in standard plaque assays were also recovered as homogeneous populations from single bursts. Experiments conclusively excluded the hypothesis of host-induced modifications as an explanation for the discrepancies between the results of the two methods of analyzing virus populations. It was concluded that some of the plaque morphology mutants of VESV have a low plating efficiency under the standard conditions of plaque assay.