Computational Modeling of the Chlamydial Developmental Cycle Reveals a Potential Role for Asymmetric Division

Chlamydia trachomatis is an obligate intracellular bacterium that progresses through an essential multi cell form developmental cycle. Infection of the host is initiated by the elementary body (EB). Once in the host, the EB cell differentiates into the non-infectious, but replication competent, reticulate body, or RB. After multiple rounds of replication, RBs undergo secondary differentiation eventually producing newly infectious EBs. Here we generated paired cell type promoter reporter constructs and determined the kinetics of the activities of the euo, hctA and hctB promoters. The paired constructs revealed that the developmental cycle produces at least three phenotypically distinct cell types; the RB (euoprom+), IB (intermediate body, hctAprom+) and EB (hctBprom+). Additionally, results from this study showed that RBs amplify in number early in infection before reaching a plateau mid cycle. This data, along with the kinetic data from the three dual promoter constructs, was used to generate two computational agent-based models to reproduce the chlamydial developmental cycle. Both models simulated EB germination, RB amplification, IB formation and EB production but differed in the mechanism that generated the IB. The Direct Conversion and the Asymmetric Production models predicted different behaviors for the RB population which were experimentally testable. In agreement with the Asymmetric Production model, RBs acted as stem cells after the initial amplification stage, producing one IB and self-renewing after every division. Additionally, the data indicated that cell division was required for IB and EB production which supports the Asymmetric Production model over the Direct Conversion model. We also demonstrated that IBs are a transient cell population, maturing directly into EBs after formation without the need for cell division. The culmination of these results suggests that the developmental cycle can be described by a four stage model, EB germination, RB amplification/maturation, IB production, and EB formation. Importance Chlamydia trachomatis is an obligate intracellular bacterial pathogen responsible for both ocular and sexually transmitted infections. All Chlamydiae are reliant on a complex developmental cycle, consisting of both infectious and non-infectious cell forms. The EB cell form initiates infection, whereas the RB cell replicates. The infectious cycle requires both cell types as RB replication increases the cell population while EB formation disseminates the infection to new hosts. The mechanisms of RB to EB development are largely unknown. Here, we developed unique dual promoter reporters and used live cell imaging and confocal microscopy to visualize the cycle at the single cell and kinetic level. These data were used to develop and test two agent-based models, simulating either direct conversion of RBs to EBs or production of EBs via asymmetric RB division. Our results suggest that RBs mature into a stem cell-like population producing intermediate cell forms through asymmetric division, followed by maturation of the intermediate cell type into the infectious EB. Ultimately a more complete mechanistic understanding of the developmental cycle will lead to novel therapeutics targeting cell type development to eliminate chlamydial dissemination.