Structural characterization of pellicle polysaccharides of Acetobacter tropicalis SKU1100 wild type and mutant strains

Mutants of Acetobacter tropicalis SKU1100 (R) strain, ΔpolE and ΔgalE, defective in pellicle formation, excrete exopolysaccharide (EPS) instead of capsular polysaccharide (CPS) that is produced by the wild-type. We carried out structural analysis of wild type CPS and mutant EPSs using monosaccharide composition analysis, methylation analysis, and 1H and 13C NMR spectroscopy. Wild-type CPS and ΔpolE mutant EPS had a branched hexasaccharide repeating unit composed of 2 moles of 2,3-α-l-rhamnopyranosyl, and 1 mole each of 6-β-d-galactopyranosyl and 2-α-d-glucopyranosyl residues, of which the rhamnosyl residues were branched by terminal-β-d-galactofuranosyl and terminal-α-d-glucopyranosyl residues. The EPS of ΔgalE mutant showed a branched tetrasaccharide repeating unit composed of 2,3-α-l-rhamnopyranosyl, 2-α-d-glucopyranosyl, and 3-α-l-rhamnopyranosyl residues and terminal-α-d-glucopyranosyl branch residue. By comparing the three structures, it was suggested that PolE may control the switching of EPS to CPS by adding some residue, e.g. β-d-galactopyranosyl residue, to 2,3-α-l-rhamnopyranosyl residue to make 2,3,4-α-l-rhamnosyl residue which leads to CPS formation.