Electron-microscopic Studies on Rickettsia sennetsu by Ultrathin Sectioning

Abstract

For the purpose of bringing out the fine structure and reproductive process of Rickettsia sennetsu (Misao and Kobayashi, 1956), identified as a causative agent of infectious mononucleosis in western Japan, the agent was propagated in tissue culture L cells and the infected preparations were processed for examination by electron microscopy with ultrathin sectioning.
At first, the rickettsia particles have been absorbed at the surface of the host cell and succeedingly taken into the vacuoles of the host cell by the mechanism of phagocytosis of the cell. However, soon after, the rickettsia particles appear in the peripheral region of the host-cell cytoplasm, and with the progress of the infection, immigrate into the central region of the host-cell cytoplasm, where the rickettsia particles multiply by binary fission.
When the agents are released from the host cell in the last stage of the infection, they are enveloped by the host-cell membrane. Therefore, extracellular rickettsia particles are surrounded by double membranes, one is their specific membrane, another is from the host cell.
Rickettsia particles are almost spherical or ellipsoidal and 400 to 700 mμ in diameter, but rarely rodshaped, measuring up to 1200 mμ in longitudinal diameter. The internal structure is homogenously composed of small granules and fine filaments, but sometimes unhomogenously as a result of electronic dense concentration distributed in the viroplasm. The membrane of the rickettsia particles in the hostcell cytoplasm is a tri-layered structure (two dense layers separated by a pale layer) and about 270 Å in breadth.
The host cells degenerate with the progress of the infection, specially the mitochondria are hardly recognizable and its cristae decrease and disappear. At a glance, degenerated mitochondria closely resemble the rickettsia particle, however they are distinguished one from the other by their internal structure and membrane.