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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. [ABSTRACT FROM AUTHOR]

O-antigen is part of the lipopolysaccharide present in the outer membrane of Gram-negative bacteria, and contributes the major antigenic variability to the cell surface. Screening for the Escherichia coli O-serogroup is the conventional method for identifying E. coli clones. In this study, we investigated the structural characteristics of the E. coli O99 O-antigen and the organization of the genes involved in its synthesis. On the basis of sugar and methylation analysis and nuclear magnetic resonance spectroscopy data, we established the structure of the branched hexasaccharide repeat unit of the O-polysaccharide. This unit consists of fourd-rhamnose (d-Rha) moieties in the backbone and twod-glucose (d-Glc) moieties in the side chain, as shown below: The O-antigen gene cluster of E. coli O99, which was located between galF and gnd, was found to contain putative genes for the synthesis ofd-Rha, genes encoding sugar transferases, and ATP-binding cassette (ABC) transporter genes ( wzm and wzt). Our findings indicate that in E. coli O99, the synthesis and translocation of the O-antigen occurs by an ABC transporter-dependent process. [ABSTRACT FROM AUTHOR]

The occurrence of unilateral flagellar phase variation was previously demonstrated in Escherichia coli strains carrying the non-fliC flagellin-specifying locus flk. In this study, we investigated the mechanism involved in this process. By using sequencing and sequence analysis, the flk region between the chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains, including the H35 strain identified in this study (the other strains used are H3, H36, H47, and H53 strains), and this region was found to contain a putative integrase gene and flanking direct repeats in addition to the flk flagellin-specifying gene flkA and a fliC repressor gene, flkB, indicating that there is a typical genomic islet (GI), which was designated the flk GI. The horizontal transfer potential of the flk GI was indicated by detection of the excised extrachromosomal circular form of the flk GI. By generating fliC-expressing variants of H3 and H47 strains, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and a complementation test. The potential integration sites of the flk GI were identified. A general model for flagellar phase variation in flk-positive E. coli strains can be expressed as fliCoff + flkAon fliCon + flkAnone. This is the first time that a molecular mechanism for flagellar phase variation has been reported for E. coli. [ABSTRACT FROM AUTHOR]