Infection cycle of Rickettsia rickettsii in chicken embryo and L-929 cells in culture

Abstract

The infection cycle of Rickettsia rickettsii, studied in slide chamber cultures of chicken embryo and L-929 cells, was found to be complex and did not conform to a one-step growth cycle. Initial uptake kinetics resembled those established for Rickettsia prowazekii, but subsequent events showed very marked differences. Intracytoplasmic growth commenced exponentially without measurable lag. However, very soon after infection, intracytoplasmic rickettsiae began to escape from the host cell into the medium in large numbers, resulting in (i) failure of large numbers of rickettsiae to accumulate in the cytoplasm, (ii) sustained rapid division of the organisms in the cytoplasm, (iii) substantial accumulation of extracellular rickettsiae, and (iv) rapidly spreading infection in the culture, with most cells infected in 48 to 72 h. In the occasional cell, rickettsiae were found in the nucleus, where they multiplied to form compact masses. Thus, analysis of the growth characteristics of R. rickettsii must consider the entire culture as a unit in which the rickettsiae are distributed among three compartments in which they behave in different ways: (i) intranuclear, (ii) intracytoplasmic, and (iii) extracellular. The rickettsial traffic is bidirectional across the host cell plasma membrane and dominantly monodirectional across the nuclear membranes. The implications of this behavior with respect to location and range of receptors and substrates involved in membrane penetration are discussed. In older cultures, unique intracytoplasmic ring or doughnut colonies were common, indicating a change in the intracytoplasmic environment. The possible significance of the growth characteristics in cell culture to the characteristics of infection in humans and animals is discussed.

Penetration of Cultured Mouse Fibroblasts (L Cells) by Rickettsia prowazeki

“…At this ratio approximately 14 rickettsiae were associated with each L cell after 30 min. This was similar to previous reports that the association of R. tsutsugamushi (5), R. rickettsii (24), and R. prowazeki (27) varied directly with MOL.…”

Heparin protects human endothelial cells infected by Rickettsia rickettsii

“…Because the endothelial cell is the putative target cell in human infections, our studies over the past several years have focused on the interaction of R. rickettsii with endothelial cells. The kinetics of infection of endothelial cells in culture (10) parallel that shown by Wisseman et al (20) for chicken embryo fibroblasts and mouse L cells. Typically, following internalization by uptake and subsequent escape from the phagosome into the cytoplasm (17,18), the microorganisms replicate by binary fission and rapidly spread to adjacent host cells in the culture.…”

“…Typically, following internalization by uptake and subsequent escape from the phagosome into the cytoplasm (17,18), the microorganisms replicate by binary fission and rapidly spread to adjacent host cells in the culture. In a small percentage of infected host cells, the bacteria also enter the nucleus, a characteristic common to members of the spotted fever group rickettsiae (20).…”

Penetration of Rickettsia tsutsugamushi into Cultured Mouse Fibroblasts (L Cells): An Electron Microscopic Observation

“…Cohn et al (I) first showed that the entrance of Rickettsia tsutsugamushi into cultured mouse MB III lymphoblasts requires active participation of both the rickettsiae and the host, and a similar result was reported by Walker and Winkler (10) in the R. protoazekiimouse fibroblast (L cell) system. Wisseman et al (13) indicated rapid, bidirectional traffic of R. rickettsii directly across the host cell membrane. However Wisse-251 man and Waddel (14) suggested another mode of entry of R. protoazekii into cultured chicken embryo cells; that is, by phagocytosis into the phagosome and escape from the phagosome into the cell cytoplasm.…”