Prokaryote viruses studied by electron microscopy.

This review summarizes the electron microscopical descriptions of prokaryote viruses. Since 1959, nearly 6300 prokaryote viruses have been described morphologically, including 6196 bacterial and 88 archaeal viruses. As in previous counts, the vast majority (96.3 %) are tailed, and only 230 (3.7 %) are polyhedral, filamentous, or pleomorphic. The family Siphoviridae, whose members are characterized by long, noncontractile tails, is by far the largest family (over 3600 descriptions, or 57.3 %). Prokaryote viruses are found in members of 12 bacterial and archaeal phyla. Archaeal viruses belong to 15 families or groups of family level and infect members of 16 archaeal genera, nearly exclusively hyperthermophiles or extreme halophiles. Tailed archaeal viruses are found in the Euryarchaeota only, whereas most filamentous and pleomorphic archaeal viruses occur in the Crenarchaeota. Bacterial viruses belong to 10 families and infect members of 179 bacterial genera, mostly members of the Firmicutes and γ-proteobacteria.