Murine cytomegalovirus particle types in relation to sources of virus and pathogenicity

Murine cytomegalovirus (MCMV) preparations from mouse embryo fibroblasts and from infected salivary glands were purified on potassium tartrate density gradients and examined by electron microscopy. The cell culture virus contained multicapsid enveloped virions which are ruptured easily and account for the excess number of free capsids in these preparations. Salivary gland virus consisted of single-capsid enveloped virus and equal numbers of free capsids. Particle types were imperfectly separated on density gradients, but successful separation was achieved by filtration through 220 and 450 nm Millipore membrane filters. Naked capsids were not infectious and could not be rendered infectious by centrifugal adsorption, showing that centrifugal enhancement of MCMV infectivity is not mediated through this mechanism. Although present in excess number in (avirulent) cell culture virus preparations, naked capsids did not interfere with the action of (virulent) salivary gland virus in newborn mice.