Mechanisms of host cell exit by the intracellular bacterium Chlamydia

The mechanisms that mediate the release of intracellular bacteria from cells are poorly understood, particularly for those that live within a cellular vacuole. The release pathway of the obligate intracellular bacterium Chlamydia from cells is unknown. Using a GFP-based approach to visualize chlamydial inclusions within cells by live fluorescence videomicroscopy, we identified that Chlamydia release occurred by two mutually exclusive pathways. The first, lysis, consisted of an ordered sequence of membrane permeabilizations: inclusion, nucleus and plasma membrane rupture. Treatment with protease inhibitors abolished inclusion lysis. Intracellular calcium signaling was shown to be important for plasma membrane breakdown. The second release pathway was a packaged release mechanism, called extrusion. This slow process resulted in a pinching of the inclusion, protrusion out of the cell within a cell membrane compartment, and ultimately detachment from the cell. Treatment of Chlamydia-infected cells with specific pharmacological inhibitors of cellular factors demonstrated that extrusion required actin polymerization, neuronal Wiskott-Aldrich syndrome protein, myosin II and Rho GTPase. The participation of Rho was unique in that it functioned late in extrusion. The dual nature of release characterized for Chlamydia has not been observed as a strategy for intracellular bacteria.