Growth pattern of Rickettsia tsutsugamushi in irradiated L cells.

Irradiated L cells infected with Rickettsia tsutsugamushi were studied under the electron microscope to define the morphological growth pattern of the organism. For 2 days after inoculation, no rickettsiae were found either extra- or intracellularly; after 2 days multiple rickettsiae appeared within the host cells without morphological evidence of their entry. These observations showed that the rickettsiae within the cell were assembled in situ by segregation of portions of the granular cytoplasm and subsequent internal differentiation and surface membrane assembly of the segregated bodies. The protoplasmic (P) bodies, which seemed to be formed by shedding infected-cell granular cytoplasm, consistently appeared on the surface and within the phagosomes of the host cells. Rickettsiae were occasionally seen entering host cells in the later phase of infection; these were apparently the ones assembled within the P bodies. This suggested that the P bodies, and not the rickettsiae, were the major infectious particles that transmitted the rickettsial genetic substance among the host cells. On the basis of the present morphological observations, viral-type multiplication for R. tsutsugamushi is proposed.