Abstract
Cell size regulation in bacteria is a function of two basic cellular processes: the expansion of the cell envelope and its constriction at spatially defined points at what will eventually become the division plane. In most bacterial species, both cell wall expansion and restriction are dependent on peptidoglycan (PG), a structural polymer comprised of sugars and amino acids that imparts strength and rigidity to bacterial membranes. Pathogenic Chlamydia species are unique in that their cell walls contain very little PG, which is restricted almost entirely to the apparent division plane of the microbe’s replicative forms. Very little is known about the degree to which PG affects the size and shape of C. trachomatis during its division process, and recent studies suggest the process is initiated via a polarized mechanism. We conducted an imaging study to ascertain the dimensions, orientation, and relative density of chlamydial PG throughout the organism’s developmental cycle. Our analysis indicates that PG in replicating C. trachomatis can be associated with four, broad structural forms; polar/septal disks, small/thick rings, large rings, and small/thin rings. We found that PG density appeared to be highest in septal disks and small/thick rings, indicating that these structures likely have high PG synthesis to degradation ratios. We also discovered that as C. trachomatis progresses through its developmental cycle PG structures, on average, decrease in total volume, indicating that the average cell volume of chlamydial RBs likely decreases over time. When cells infected with C. trachomatis are treated with inhibitors of critical components of the microbe’s two distinct PG synthases, we observed drastic differences in the ratio of PG synthesis to degradation, as well as the volume and shape of PG-containing structures. Overall, our results suggest that C. trachomatis PG synthases differentially regulate the expansion and contraction of the PG ring during both the expansion and constriction of the microbe’s cell membrane during cell growth and division, respectively.
Highlights
All Chlamydia species share a biphasic developmental cycle (Figure 1a)
Cells were subsequently immunolabeled with a monoclonal antibody specific for the pathogen’s Major Outer Membrane Protein (MOMP)
Cell size regulation is critical for bacterial species, enabling microbes to respond to favorable and detrimental changes in their respective environments
Summary
All Chlamydia species share a biphasic developmental cycle (Figure 1a) These obligate intracellular pathogens alternate between a small (~0.3 μm) extracellular, infectious form (Elementary Body or EB) and a larger (~1 μm), intracellular, replicative form (Reticulate Body or RB; Elwell et al, 2016). Sixteen to twenty-four hours after entering a host cell these RBs begin to asynchronously decrease in size (Lee et al, 2018), transition into intermediate forms (IBs), express cysteine-rich outer membrane proteins, and transition into EBs. EBs exit the cell by one of two routes: (i) the inclusion expands enough to lyse the cell, expelling the newly formed EBs, or (ii) portions of the inclusion begin to “bleb off ” into the extracellular environment in a process termed extrusion (Hybiske and Stephens, 2007). The disruption of either of these transition events (EB to RB or RB to EB) effectively halts the developmental cycle and prevents the bacterium from entering new host cells (Beatty et al, 1994)
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