Abstract
Shigella flexneri is the major pathogen causing bacillary dysentery in developing countries. S. flexneri is divided into at least 16 serotypes based on the combination of antigenic determinants present in the O-antigen. All the serotypes (except for serotype 6) share a basic O-unit containing one N-acetyl-d-glucosamine and three l-rhamnose residues, whereas differences between the serotypes are conferred by phage-encoded glucosylation and/or O-acetylation. Serotype Xv is a newly emerged and the most prevalent serotype in China, which can agglutinate with both MASF IV-1 and 7,8 monoclonal antibodies. The factor responsible for the presence of MASF IV-1 (E1037) epitope has not yet been identified. In this study, we analyzed the LPS structure of serotype Xv strains and found that the MASF IV-1 positive phenotype depends on an O-antigen modification with a phosphoethanolamine (PEtN) group attached at position 3 of one of the rhamnose residues. A plasmid carried gene, lpt-O (LPS phosphoethanolamine transferase for O–antigen), mediates the addition of PEtN for serotype Xv and other MASF IV-1 positive strains. These findings reveal a novel serotype conversion mechanism in S. flexneri and show the necessity of further extension of the serotype classification scheme recognizing the MASF IV-1 positive strains as distinctive subtypes.
Highlights
Shigella flexneri is the major pathogen causing bacillary dysentery in developing countries
All the Oantigen modification genes known to date are encoded by seven temperate bacteriophages or prophages (SfI, SfIC, SfII, Sf6, SfIV, SfV and SfX), which are integrated into the conserved sites of the host S. flexneri genome [3,6,8,9,10,11,13,14]
The gene named as lpt-O, encoding an LPS phosphoethanolamine transferase for O-antigen was identified to be responsible for carrying out the PEtN modification in serotype Xv and other MASF IV-1 positive strains and was found to be carried on a plasmid
Summary
Shigella flexneri is the major pathogen causing bacillary dysentery in developing countries. GtrA and gtrB are highly conserved and interchangeable, while gtr (type) is unique and encodes the glucosyltransferase responsible for the attaching of a glucosyl group to a specific sugar in the tetrasaccharide repeat unit of the O-antigen [3,12]. Compared to typical serotype X, serotype Xv strains carry an additional new Oantigenic epitope MASF IV-1 ( called E1037, an antigenic determinant specific for MASF IV-1 antibody) [2]. The gene named as lpt-O, encoding an LPS phosphoethanolamine transferase for O-antigen was identified to be responsible for carrying out the PEtN modification in serotype Xv and other MASF IV-1 positive strains and was found to be carried on a plasmid
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