Ultrastructure of a Japanese Rickettsial strain genetically identified as Rickettsia helvetica which was originally found in Europe

A rickettsial strain IO-1 has been isolated from a tick, Ixodes ovatus, in Japan and genetically identified as Rickettsia helvetica, a member of the spotted fever group rickettsiae. Ultrastructural observations were made on the microorganism. The ultrastructure of R. helvetica IO-1 appeared to be generally the same as that previously shown for other rickettsiae of the spotted fever and typhus groups. The rickettsiae were primarily found free in the cytoplasm of L929 cultured cells. Occasionally, the rickettsiae may also invade the host cell nucleus; however, the frequency of the nuclear localization was very low.