Cell surface polysaccharides in the gut microbiota: occurrence, structure and role

The gastrointestinal (GI) tract is colonized by trillions of microorganisms living in a symbiotic relationship with the host. Commensal bacteria in the gut engage in cross-talks with epithelial and immune cells through effector molecules secreted or attached to the cell surface. Although cell surface polysaccharides have mainly been studied in the context of pathogen-host interactions, these are increasingly being recognized as important factors of the symbiotic interaction between the gut microbiota and the host conferring biological activities and physiological functions. In this review, we focus on the structure and role of polysaccharides surrounding the bacterial cell wall, namely capsular polysaccharide (CPS) and cell wall polysaccharides (CWPS), both tightly linked to the cell surface, and exopolysaccharides (EPS) which are loosely attached to the extracellular surface or secreted into the environment. We will focus on structurally characterized CPS, CWPS and EPS from both gut commensal bacteria and food-derived bacteria found in the gut. These polysaccharides show high structural diversity and are important for the bacteria to adapt to the GI environment and/or influence host immune response. The combined diversity of microbes in the gut provides a vast array of glycans that could be harnessed to benefit human health.