Cell Type Development inChlamydia trachomatisFollows a Program Intrinsic to the Reticulate Body

AbstractThe obligate intracellular bacterial pathogenChlamydia trachomatis(Ctr) is reliant on an unusual developmental cycle consisting of two cell forms termed the elementary body (EB) and the reticulate body (RB). The EB is infectious and utilizes a type III secretion system and preformed effector proteins during invasion, but does not replicate. The RB replicates in the host cell but is non-infectious. This developmental cycle is central to chlamydial pathogenesis. In this study we developed mathematical models of the chlamydial developmental cycle that account for potential factors influencing the timing of RB to EB cell type switching during infection. Our models predicted that two broad categories of regulatory signals for RB to EB development could be differentiated experimentally; an “intrinsic” cell autonomous program inherent to each RB or an “extrinsic” environmental signal to which RBs respond. To experimentally differentiate between these hypotheses, we tracked the expression ofCtrdevelopmental specific promoters using fluorescent reporters and live cell imaging. These experiments indicated that EB production was not influenced by increased MOI or by superinfection, suggesting the cycle follows an intrinsic program that is not influenced by environmental factors. Additionally, live cell imaging of these promoter constructs revealed that EB development is a multistep process linked to RB growth rate and cell division. The formation of EBs followed a cell type gene expression progression with the promoters foreuoandihtAactive in RBs, while the promoter forhctAwas active in early EBs/intermediate cells and finally the promoters for the true late genes,hctB, scc2, andtarpactive in the maturing EB.ImportanceChlamydia trachomatisis an obligate intracellular bacteria that can cause trachoma, cervicitis, urethritis, salpingitis, and pelvic inflammatory disease. To establish infection in host cellsChlamydiamust complete a multi cell type developmental cycle. The developmental cycle consists of two specialized cells; the EB which mediates infection of new cells and the RB which replicates and eventually produces more EB cells to mediate the next round of infection. By developing and testing mathematical models to discriminate between two competing hypotheses for the nature of the signal controlling RB to EB cell type switching. We demonstrate that RB to EB development follows a cell autonomous program that does not respond to environmental cues. Additionally, we show that RB to EB development is a function of cell growth and cell division. This study serves to further our understanding of the chlamydial developmental cycle that is central to the bacterium’s pathogenesis.