Development of secondary inclusions in cells infected by Chlamydia trachomatis

The chlamydiae are obligate intracellular bacteria that occupy a non-acidified vacuole (the inclusion) during their entire developmental cycle. These bacteria produce a set of proteins (Inc proteins) that localize to the surface of the inclusion within infected cells. Chlamydia trachomatis IncA is also commonly found in long fibers that extend away from the inclusion. We used standard and confocal immunofluorescence microscopy to demonstrate that these fibers extend to newly developed inclusions, termed secondary inclusions, within infected cells. Secondary inclusions observed at early time points postinfection were devoid of chlamydial reticulate bodies. Later in the developmental cycle, secondary inclusions containing variable numbers of reticulate bodies were common. Reticulate bodies were also observed within the IncA-laden fibers connecting primary and secondary inclusions. Quantitative differences in secondary inclusion formation were found among clinical isolates, and these differences were associated with serovar. Isolates of serovar G consistently produced secondary inclusions at the highest frequency (P < 0.0001). Similar quantitative studies demonstrated that secondary inclusion formation was associated with segregation of inclusions to daughter cells following cytokinesis. We conclude that the production of secondary inclusions via IncA-laden fibers allows chlamydiae to generate an expanded intracellular niche in which they can grow and may provide a means for continuous infection within progeny cells following cell division.