Diversity and Genetic Basis of Polysaccharide Biosynthesis in Vibrio cholerae

Abstract

Vibrio cholerae elaborates three types of polysaccharide structures: lipopolysaccharide (LPS), a component of which is the O-polysaccharide or O-antigen, capsular polysaccharide (CPS) or K-antigen, and “rugose” polysaccharide also known as exopolysaccharide (EPS) or Vibrio polysaccharide (VPS). The major protective antigen for V. cholerae is the O-antigen. A strain typing scheme based on the somatic O-antigen has been in use for a number of years. There are 206 serogroups identified so far and of these only O1 and O139 are known to cause epidemic/pandemic cholera, although a handful of non-O1/non-O139 strains are known to possess the major virulence factors. The O-antigen diversity is due to the number and composition of monosaccharide components, linkages, addition of non-sugar moieties, modal length of the polysaccharide chain, and biosynthesis mechanisms. The genetic basis of this diversity is just beginning to be understood with the sequencing of a number of gene clusters that encode O-polysaccharide (OPS)/capsule structures. In this review, we summarize our current knowledge on the biochemical composition and structure of some of the O-polysaccharides, genes involved in their biosynthesis, and touch upon the role of horizontal gene transfer in creating this diversity and possible mechanisms that may be operative in this process. We highlight the fact that the distinction between OPS and CPS seems to be less evident in V. cholerae than in other species since the genes encoding these structures are shared and map in the same region of the genome. We also describe our current understanding of the genetics and regulation of EPS/VPS synthesis and its role in biofilm formation and environmental survival of V. cholerae.