Surface morphology of Rickettsia tsutsugamushi-infected mouse fibroblasts

Sequential morphological changes of the L-cell surface after infection with Rickettsia tsutsugamushi (Gilliam strain) have been examined by scanning electron microscopy and ruthenium red staining technique. Adherence of inoculated rickettsiae to the host-cell surface and their engulfment by the cell were seen at 30 min and have still proceeded at 24 hr post infection (p.i.). Progeny rickettsiae which were lifting up the host cell membrane by budding were observed on the cell surface at 48 hr p.i. The budding of rickettsiae increased gradually in time and, at 96 hr after infection, covered almost the all host-cell surface except of the cell margin. Numerous microvilli observed on the surface of uninfected L-cells decreased gradually p.i.; they had almost disappeared when progeny rickettsiae occurred. Ruthenium red staining specimens clearly showed that the budding rickettsiae were surrounded with the host cell membrane. The following layers were distinguished from outside on: (1) ruthenium red positive fuzzy coat (25 nm thick); (2) a triple-layered cell membrane (5-6 nm); (3) outer and inner leaflets of the rickettsial cell-wall (7-8 nm and 2-2.5 nm, respectively); (4) periplasmic space (15-20 nm); (5) a triple-layered rickettsial cytoplasmic membrane (5-6 nm).