Identification of a gene cluster for the biosynthesis of a long galactose-rich exopolysaccharide in Lactobacillus rhamnosus GG and functional analysis of the priming glycosyltransferase.

Cell surface polysaccharides have an established role as virulence factors in human bacterial pathogens. Less documented are the biosynthesis and biological functions of surface polysaccharides in beneficial bacteria. We identified a gene cluster that encodes the enzymes, regulatory and transporter proteins for the different steps in the biosynthesis of extracellular polysaccharides (EPS) of the well-documented probiotic strain Lactobacillus rhamnosus GG. Subsequent mutation of the welE gene encoding the priming glycosyltransferase within this cluster, and comparative phenotypic analyses of wild-type versus mutant confirmed the specific function of this gene cluster in the biosynthesis of high molecular weight galactose-rich heteropolymeric EPS molecules. The phenotypic analyses included monomer composition determination, polymer length estimation of the isolated EPS molecules and single-molecule force spectroscopy of the surface polysaccharides. Further characterization of the welE mutant also showed that deprivation of these long galactose-rich EPS molecules results in an increased adherence and biofilm formation capacity of L. rhamnosus GG, possibly by less shielding of adhesins such as fimbriae-like structures.