Tailor-made polysaccharides containing uniformly distributed repeating units based on the xanthan gum skeleton

Xanthan gum, whose structure determines its physicochemical properties, is an important microbial polysaccharide. Currently, marketed xanthan products are produced by wild-type strains followed by post-fermentation separation or chemical modification, which are complicated and labor-intensive. In the present study, we designed eight polysaccharides containing uniformly distributed repeating units and different rheological properties based on natural xanthan skeleton and according to the relationship between property and structure. The customized polysaccharides were produced in Xanthomonas campestris CGMCC 15155 using marker-less gene knockout and gene overexpression methods. The results showed that their different homogeneous primary structures determined their specific secondary structures and rheological properties, especially the terminal mannose, the pyruvyl group, and the acetyl group attached to the internal mannose of the side chain. Polysaccharides lacking a terminal mannose, such as xanthan XdM-0 and XdM-A, had reduced zero-shear viscosity and modulus values. The internal acetyl group of the side chain stabilized the helix structure (e.g., in XG-A0), while the pyruvate group had the opposite effect (e.g., in XG-AP and XG-0P). The eight xanthan variants provide a promising theoretical foundation to further study the structure-activity relationship of xanthan and will help to construct xanthan-containing block copolymers.