The obligate intracellular bacterial pathogen Chlamydia trachomatis (Ctr) is reliant on a 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 developmental cycle that account for potential factors influencing the timing of RB to EB cell type switching during infection. Our models predicted that two 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 of Ctr developmental 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 revealed that EB development is a multistep process linked to RB growth rate and cell division. The formation of EBs followed a gene expression progression with the promoters for euo and ihtA active in RBs, while the promoter for hctA was active in early EBs/IBs and finally the promoters for the true late genes, hctB, scc2, and tarp active in the maturing EB.

Cell Type Development in Chlamydia trachomatis Follows a Program Intrinsic to the Reticulate Body.