Developmental differentiation in a cytoplasm-dwelling obligate intracellular bacterium

Developmental differentiation has been described for several vacuole-dwelling obligate intracellular bacteria but never for an obligate intracellular bacterium that resides in the cytoplasm. Here, we show that the cytoplasm-dwelling obligate intracellular bacterium Orientia tsutsugamushi (Ot) exists in five distinct subpopulations. We show that Ot differentiates into a distinct, metabolically inactive, extracellular state upon budding from the surface of host cells and that this stage is preceded by a surface-associated maturation stage. We identify proteins that are differentially expressed in intracellular, replicative bacteria and extracellular, metabolically inactive bacteria. Metabolic activity resumes rapidly upon entry into the cytoplasm and is triggered by the host cell reducing environment. This example of developmental differentiation in a species of Rickettsiaceae provides a new model system for studying synchronized differentiation in a bacterium that has a minimal genome and where the interactions between bacterium and host cell are more direct than they are for bacteria separated from the eukaryote cell host by a vacuolar membrane.