Structural and genetic characterization of the closely related O-antigens of Escherichia coli O85 and Salmonella enterica O17.

O-Antigen is a part of the lipopolysaccharide present in the outer membrane of Gram-negative bacteria, which confers major antigenic variability to the cell surface. In this study, we report on a previously undefined pair of Escherichia coli and Salmonella enterica with closely related O-antigens. The O-polysaccharides were isolated from the lipopolysaccharides of E. coli O85 and S. enterica O17 by mild acid degradation and studied by sugar analysis and NMR spectroscopy. The following structure was established for the O-unit of the E. coli O85-polysaccharide: The S. enterica O17-polysaccharide has the same carbohydrate backbone and, in addition, contains an O-acetyl group at position 2 of ~80% β-Galf residues. The O-antigen gene cluster of E. coli O85 was found to be closely related to that of S. enterica O17. Screening of type strains of all E. coli and S. enterica O-serogroups revealed two genes specific to the E. coli O85 O-antigen gene cluster, which can be used for development of PCR-based assays for identification and detection of E. coli O85 strains.