The secrets of the helical shape of H. pylori

The helical shape of Helicobacter pylori (H. pylori) bacteria is essential for its pathogenicity. The Salama lab (Human Biology Division), whose research focuses on these bacteria, demonstrated in the past that mutant H. pylori that lost their helical shape were less efficient at infiltrating the stomach mucosa and thus less pathogenic. However, how this helical shape is controlled is barely known. H. pylori cell wall is mostly composed of peptidoglycan (PG), a polymer of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), and its structure dictates the cell shape. Some proteins such as CcmA and MreB have been shown to impact cell shape, but the exact mechanism is poorly understood. Whereas CcmA is thought to play a role of scaffold for this wall, MreB has been shown to modulate PG synthesis. The Salama lab sought to investigate in detail the regulation of PG synthesis in wild type helix-shaped or straight rod mutant H. pylori strains. Their work was recently published in Elife.

In order to objectively assess the shape of bacteria, the authors performed superresolution imaging and 3D image analysis and defined the curvature of different region of the cell wall. Indentations in the cell shape were identified as negative Gaussian curvatures and buldges as positive Gaussian curvatures. A flat cell wall would have a null Gaussian curvature.

3D superresolution images and computed analysis of H. pylori curvatures. On the righ, blue and red lines are the minor and major helical axes respectively. Illustration from publication

To study the incorporation of new PG in the cell wall, the researchers confirmed that bacteria were able to use modified MurNAc to build the cell wall when cultured. Then, they incubated bacteria with the modified MurNAc, used click chemistry to add a fluorescent tag to the modified MurNAc and localized the fluorescent signal by microscopy. Dr. Jennifer Taylor, first author of the study, and colleagues demonstrated that PG metabolites were mostly incorporated at negative gaussian curvature and along the major helical axis (see figure). No PG appeared to be synthetized at the poles of the bacteria. The researchers reasoned that differential synthesis or incorporation of monomers in the cell wall depending on the curvature would physically regulate cell shape. They wondered if this could be controlled by CcmA and MreB.

Foreground: Rendering of 3D H. pylori cell surface reconstruction showing fluorescence signal of interest (right), Gaussian curvature (middle), and Gaussian curvature with triangular surface meshwork displayed (left). Background: structured illumination micrograph of stained H. pylori cell walls (blue) and CcmA-FLAG (yellow). Illustration provided by Dr. Salama

Interestingly, CcmA localization correlated mostly with positive curvatures and cell wall synthesis (along the major helical axis) whereas MreB was enriched at the negative curvatures. Furthermore, genetic ablation of ccmA or mreB perturbed PG synthesis and led to ill-shaped bacteria. When purified in vitro, CcmA protein spontaneously formed filaments, bundles and lattices. Altogether, the results of Taylor et al. suggest that localized expression of CcmA and MreB regulate PG synthesis and scaffolding, thus tightly controlling cell shape.

This work may ultimately lead to therapeutic strategies targeting the key determinant of H. Pylori helical shape and therefore limiting its impact on related diseases.

This work was supported by the National Institutes of Health and the National Science Foundation.

Fred Hutch/UW Cancer Consortium member Dr Salama contributed to this research.

Taylor JA, Bratton BP, SIchel SR, Blair KM et al. 2020. Distinct cytoskeletal proteins define zones of enhanced cell wall synthesis in Helicobacter pylori. Elife 9:e52482. DOI:https://doi.org/10.7554/eLife.52482.