Current understanding on biosynthesis of microbial polysaccharides.

The surfaces of almost all microbes are decorated with remarkable variations of polysaccharides such as O-antigen, capsular polysaccharides (CPS), and exopolysaccharides (EPS) in bacteria, lipoarabinomannans (LAM) in mycobacteria and lipophosphoglycan (LPG) in Leishmania. These polysaccharides play important roles in many biological processes, and they can function as the virulence determinants in the pathogens. The basic structures of these polysaccharides are known, but they show species-specificity or stage-specificity. For example, there are 186 O-serotypes and 80 capsular serotypes in E. coli. Despite the variation, the range of strategies used for the biosynthesis and assembly of these microbial polysaccharides is limited. Depending on the assembly and translocation mechanisms, O-antigen biosynthesis is subdivided into three pathways, of which the Wzy-dependent pathway is widely used not only in O-antigen, but also in CPS and EPS.