Equivalence between Vaccinia Particles Counted by Electron Microscopy and Infectious Units of the Virus

Abstract

Studies of the relation between the number of infectious or pock-forming units of vaccinia virus and the number of virus particles indicate a ratio of 1.51 under conditions described. A modified centrifugation cell is described with which virus suspensions can be concentrated and the particles then counted by electron microscopy. Virus particles are sedimented directly onto coated electron microscope grids which when removed from the centrifugation cell are ready for shadow-casting and examination. The precision of this method is comparable to that obtained by other techniques.