Involvement of lipid rafts in the budding-like exit of Orientia tsutsugamushi

Orientia tsutsugamushi is an intracellular parasite that causes scrub typhus. After entering the cytoplasm by induced phagocytosis, O. tsutsugamushi escapes from the primary phagosome into the host cytosol, where it replicates slowly. Subsequently, it is released from the host cells by a process resembling viral budding with a remaining bacterial aggregate near the nucleus. Lipid rafts have been implicated in the life cycle of a wide variety of pathogenic microorganisms. We have observed that proteins of O. tsutsugamushi were co-fractionated with the lipid rafts over a sucrose density gradient, suggesting the possible involvement of lipid rafts during the intracellular life cycle of O. tsutsugamushi. The entry of O. tsutsugamushi into the host cells was shown to be independent on lipid rafts as judged by the inability of lipid raft-disrupting agents to inhibit bacterial entry and no co-localization of bacterial proteins with caveolin. To our interest, a 47-kDa protein (HtrA) was observed to be co-localized with caveolin at the cell membrane at 72 h after infection, when bacterial particles move to the cell membrane and initiate the exit into the extracellular environment. Our results suggest that O. tsutsugamushi involves lipid rafts of the host cells in the budding-like process to exit from host cells.