New approach to pathogens’ bacterial polysaccharide export

There are currently only two known mechanisms for polysaccharide export in Gram-negative bacteria, which include some of the most dangerous human pathogens. A third, completely undiscovered mechanism for the export of polysaccharides has now been discovered by a study team. These discoveries open the door to a thorough comprehension of the processes governing the defence, motility, and interaction of several bacterial pathogens.

In addition to using sugar as a source of carbon and energy, bacteria also generate and release a wide range of so-called polysaccharides. Strings of sugar called polysaccharides are the most prevalent biopolymers on the planet. The lengthy sugar chains are crucial to the survival of pathogenic, commensal, and free-living bacteria. They are also essential for protecting bacteria from environmental challenges such desiccation, immunological effectors, and predators by covering the cells.

Their structural and adhesive properties also aid in the colonisation of surfaces and the development of biofilms. They are crucial for the effective use of anti-bacterial vaccinations. They therefore possess the knowledge necessary to comprehend and manage both advantageous and harmful interactions between humans, animals, and plants and microbes. Last but not least, polysaccharides are employed in the food, drug, and healthcare sectors. Due to the huge size and chemical diversity of the molecules, polysaccharide export is a significant issue.

There are only two known methods for the export of polysaccharides in Gram-negative bacteria, which include some of the most dangerous human pathogens: an outer membrane OPX protein (in the so-called Wzx/Wzy- and ABC transporter-dependent pathways) and an outer membrane b-barrel protein (in the so-called synthase-dependent pathways). However, there are examples of paths that do not appear to adhere to these straightforward plans: For example, in Vibrio cholerae and Myxococcus xanthus, outer membrane b-barrel proteins were recognised to be critical for polysaccharide export, but the precise mechanism remained unknown.

Additionally, shorter OPX proteins without the component needed to integrate with the outer membrane are described in other investigations. It is unknown how these proteins may facilitate polysaccharide export in this situation.

These issues were clarified by a study team at the Max-Planck-Institute for Terrestrial Microbiology under the direction of Lotte Sogaard-Andersen. The researchers present evidence for a completely unique mechanism for how bacteria can export polysaccharides over the outer membrane using tests and computational structural biology. The study’s co-authors, Dr. Maria Perez-Burgos and graduate student Johannes Schwabe, state that they “began by closely examining the Wzx/Wzy-dependent pathway for the synthesis of a secreted polysaccharide termed EPS in M. xanthus.” EPS would, in accordance with current knowledge, be secreted over the outer membrane by an integrated OPX protein.

The team did discover that EpsX, an outer membrane b-barrel protein, is crucial for EPS export. Then, unexpectedly, we found a comparable periplasmic short OPX protein called EpsY that is deficient in the component needed to bridge the outer membrane. We also discovered, in collaboration with Dr. Timo Glatter, that EpsX and EpsY interact directly.

The researchers suggest that EpsX and EpsY represent a novel type of translocon for polysaccharide export across the outer membrane, where a b-barrel protein explicitly serves as the outer membrane-spanning part in a bipartite complex with an entirely periplasmic OPX protein. This hypothesis is based on their observations and computational structural biology.

According to Lotte Sogaard-Andersen, this in-depth understanding might lead to the development of brand-new strategies for limiting dangerous germs. She clarifies: “Using computational genomics, Marco Herfurth, a graduate student in my research group, discovered that comparable composite structures are prevalent in Gram-negative bacteria.

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For instance, the VPS polysaccharide, which is crucial for the development of biofilms and pathogenicity, is secreted by V. cholerae according to this new system. Therefore, our findings have deep implications for our understanding of polysaccharide export generally in Gram-negative bacteria as well as major implications for our understanding of polysaccharide export in M. xanthus specifically.”
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