The sRNA Regulated Protein DdbA Is Involved in Development and Maintenance of the Chlamydia trachomatis EB Cell Form

The chlamydial transcriptional regulator Euo is a key switch in cell form developmental progression but is not involved in the committed step to the formation of the infectious form

Bacteria in the genus Chlamydia are a significant health burden worldwide. They infect a wide range of vertebrate animals, including humans and domesticated animals. In humans, C. psittaci can cause zoonotic pneumonia, while C. pneumoniae causes a variety of respiratory infections. Infections with C. trachomatis cause ocular or genital infections. All chlamydial species are obligate intracellular bacteria that replicate exclusively inside of eukaryotic host cells. Chlamydial infections are dependent on a complex infection cycle that depends on transitions between specific cell forms. This cycle consists of cell forms specialized for host cell invasion, the elementary body (EB), and a form specialized for intracellular replication, the reticulate body (RB). In addition to the EB and RB, there is a transitionary cell form that mediates the transformation between the RB and the EB, the intermediate body (IB). In this study, we ectopically expressed the regulatory protein Euo and showed that high levels of expression resulted in reversible arrest of the development cycle. The arrested chlamydial cells were trapped phenotypically at an early IB stage of the cycle. These cells had exited the cell cycle but had not shifted gene expression from RB like to IB/EB like. This arrested state was dependent on continued expression of Euo. When ectopic expression was reversed, Euo levels dropped in the arrested cells which led to the repression of native Euo expression and the resumption of the developmental cycle. Our data are consistent with a model where Euo expression levels impact IB maturation to the infectious EB but not the production of the IB form.

IMPORTANCE

Bacterial species in the Chlamydiales order infect a variety of vertebrate animals and are a global health concern. They cause various diseases in humans, including genital and respiratory infections. The bacteria are obligate intracellular parasites that rely on a complex infectious cycle involving multiple cell forms. All species share the same life cycle, transitioning through different states to form the infectious elementary body (EB) to spread infections to new hosts. The Euo gene, encoding a DNA-binding protein, is involved in regulating this cycle. This study showed that ectopic expression of Euo halted the cycle at an early stage. This arrest depended on continued Euo expression. When Euo expression was reversed, the developmental cycle resumed. Additionally, this study suggests that high levels of Euo expression affect the formation of the infectious EB but not the production of the cell form committed to EB formation.

Recent advances in genetic systems in obligate intracellular human-pathogenic bacteria

The ability to genetically manipulate a pathogen is fundamental to discovering factors governing host-pathogen interactions at the molecular level and is critical for devising treatment and prevention strategies. While the genetic "toolbox" for many important bacterial pathogens is extensive, approaches for modifying obligate intracellular bacterial pathogens were classically limited due in part to the uniqueness of their obligatory lifestyles. Many researchers have confronted these challenges over the past two and a half decades leading to the development of multiple approaches to construct plasmid-bearing recombinant strains and chromosomal gene inactivation and deletion mutants, along with gene-silencing methods enabling the study of essential genes. This review will highlight seminal genetic achievements and recent developments (past 5 years) for Anaplasma spp., Rickettsia spp., Chlamydia spp., and Coxiella burnetii including progress being made for the still intractable Orientia tsutsugamushi. Alongside commentary of the strengths and weaknesses of the various approaches, future research directions will be discussed to include methods for C. burnetii that should have utility in the other obligate intracellular bacteria. Collectively, the future appears bright for unraveling the molecular pathogenic mechanisms of these significant pathogens.

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 multicell form developmental cycle. Infection of the host is initiated by the elementary body (EB). Once in the host, the EB cell differentiates into the noninfectious, 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+), intermediate body (IB; hctAprom+), and EB (hctBprom+). The kinetic data from the three dual-promoter constructs were 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. 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 noninfectious 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 levels. 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.