Structure and function of a novel osmoregulated periplasmic fiber-forming high-molecular-weight carbohydrate of Myxococcus xanthus

Osmoregulation is of central importance for living cells. In Gram-negative bacteria, strategies for osmoregulation and turgor maintenance in hypotonic environments include the synthesis, accumulation, and modification of periplasmic oligosaccharides. These osmoregulated periplasmic glucans (OPGs, formerly known as membrane-derived oligosaccharides or MDOs) promote water uptake and retention, keeping the cells in an optimal state of hydration. While our understanding of OPG-dependent osmoregulation in a number of model organisms like Escherichia coli is quite detailed, less is known about these processes in bacteria that live in environments characterized by strongly fluctuating osmolarity, such as soil. Here we describe that the soil bacterium Myxococcus xanthus lacks a canonical low-molecular-weight OPG, but instead possesses a novel high-molecular-weight, fiber-forming polysaccharide. Chemical analysis reveals that this polysaccharide is several thousand kilodaltons in size, composed of a highly branched decasaccharide repeat unit containing mannose, glucose, N-acetylglucosamine, and rhamnose. Physiological experiments indicate that the polysaccharide is osmoregulated thereby functionally replacing the canonical OPG. Moreover, experiments indicate that this high-molecular-weight periplasmic polysaccharide forms a fibrillar meshwork that stabilizes the cell envelope during glycerol spore formation, a process during which the entire peptidoglycan of the cell is degraded and the rod-shaped vegetative cells convert into spherical spores. SignificanceOsmoprotection is a necessity for every living cell, particularly in an environment with fluctuating osmolarity. In Gram-negative bacteria, low-molecular-weight osmoregulated periplasmic glucans (OPGs) are an important component of the osmotic stress response in hypotonic environments. Here, we describe that the soil bacterium Myxococcus xanthus does not possess such an OPG but instead accumulates a novel high-molecular-weight fiber-forming polysaccharide in the periplasm in response to hypotonic conditions. This polymer is important for osmoprotection of the cells and plays a key role in the stabilization of the cell envelope during the conversion of rod-shaped vegetative cells into spherical spores. These results indicate that bacteria may use non-OPG carbohydrates for osmoprotection and cell wall stabilization during processes like cellular differentiation.