Poxvirus virions: their surface ultrastructure and interaction with the surface membrane of host cells

Virions of vaccinia and orf viruses were examined by ultrahigh-resolution scanning electron microscopy using a non-coating method. Intracellular mature particles of vaccinia virus appeared to be covered with a net and ultrastructurally their surface consists of many fine ridges and globules, while the surfaces of orf virus mature particles recovered from infected cells consist of spirally running protrusions. The ridge-like structures of vaccinia virus were presumed to correspond to surface tubules shown by negative staining of this virus, while the spiral protrusions of orf virus were presumed to correspond to spiral threads having a criss-cross appearance by the same staining. Using scanning electron microscopy in which the samples were prepared by the conventional method, we observed: (i) many virions, i.e. one or two hundreds, or occasionally more reaching about one thousand particles, of the IHD strain of vaccinia virus, (ii) many or a moderate number of virions, i.e. about one hundred or fewer particles, of the 58 strain of cowpox virus and (iii) rather few virions, i.e. several tens or fewer particles, of the Iwate strain of orf virus on the free surface of each cell infected with these viruses. It must be noted that the number of virions detected considerably differed in respective cells examined. Virus budding was frequently observed at the cell surface of monolayer cells infected with vaccinia virus but it was never detected with cowpox or orf virus, indicating a difference in the mechanism of virus release between vaccinia and the other two viruses. When whole cells infected with vaccinia virus were examined by a combination of high-voltage and scanning electron microscopies, virions on the cell surface and those inside the cells were clearly differentiated. All virions on the cell surface had an envelope, and some of the envelopes had a slack and/or one or more bulges.