Virus Particle Aggregation and the Plaque-Forming Unit

Abstract

Summary Dilute suspensions of vaccinia virus, prepared in various ways, have been sedimented upon a smooth surface and examined by electron microscopy. The observed aggregation, probably similar to that which exists in plaque titration experiments, is found to involve usually over 60% and often over 90% of the particles. Sonic waves are found to be more effective than repeated freezing and thawing, enzymes or pipetting for dispersal of aggregation among the virus particles. This comparison is done by counting and expressing as a percentage of the total count, the number of groups or potentially active units seen in the pictures. The rate of reaggregation of dispersed suspensions was similarly measured. Plaque titration shows the effects of dispersive agents which increase the percentage of active units (AU) and of plaques as well, whereas the total virus particle count remains unchanged. The observed increase in plaques (84% in one experiment) is less, however, than the corresponding increase in AU. This is attributed to the fact that only a small fraction of the virus particles are capable of producing plaques.