The plaque assay also permits the isolation of pure virus strains. is because if a plaque has arisen from a single virion,all the viruses in this plaque should be genetically identical. of the virions from this plaque can be picked and inoculated into a fresh bacteria culture to establish a pure virus line. development of the plaque assay technique was as important for the advancement of virology as Kroch's development of solid media (section 1.8) was for pure culture microbiology.