Symmetric Inheritance of the Cytoplasmic Bacterium Orientia tsutsugamushi During Mitosis

For long-term stable residence within host cells, endosymbionts must evolve mechanisms to ensure their segregation into both daughter cells during cell division, and to coordinate their division with the host cell cycle. This regulation was also a prerequisite for the evolution of mitochondria, enabling their transition from free-living bacteria to an organelle. Here, we examine Orientia tsutsugamushi and Rickettsia rhipicephali , two obligate intracellular Rickettsiales that replicate directly in the host cytoplasm, during host mitosis. Using cell cycle synchronisation and immunofluorescence microscopy, we demonstrate that O. tsutsugamushi frequently localise near the spindle poles during mitosis, and are distributed between daughter cells in a manner dependent on its surface protein ScaC, that drives dynein-dependent motility of the bacterium along microtubules. In contrast, R. rhipicephali , which does not interact with microtubules, largely inhibits host mitosis and is randomly distributed in the few cells that divide. Overall, our findings reveal distinct strategies for mitotic adaptation among these two Rickettsiales and suggest that associations with the host’s cytoskeletal elements may be critical for near-equal distribution into daughter cells.