Shigella flexneri is an important human pathogen causing shigellosis. Until recently, strains of this species have been classified into 15 O-serotypes. Except for serotype 6, their O‑antigens share a linear →2)-α-l-RhapIII-(1→2)-α-l-RhapII-(1→3)-α-l-RhapI-(1→3)-β-d‑GlcpNAc-(1→ basal structure, which defines group O‑factor 3,4 of serotype Y. Type O‑factors I-VII and group O‑factors 6 and 7,8 of the other ‘classical’ serotypes (1a,b-5a,b, 7a,b, and X) are conferred by phage-encoded glucosylation or/and O‑acetylation. Another S. flexneri serotype conversion mechanism discovered recently involves a plasmid-borne gene (lpt-O)-mediated modification of the O‑antigens with phosphoethanolamine at RhaII or/and RhaIII giving rise to ‘variant’ serotypes 4av, Yv, and Xv [1, 2]. Monoclonal antibody MASF IV-I binds specifically a phosphoethanolamine-associated epitope.
We demonstrated that all ‘classical’ S. flexneri serotypes can be transformed with a lpt-O carrying plasmid to ‘variant’ serotypes. However, some Glc-associated O-factors were lost upon transformation owing to interference between phosphorylation and glucosylation. For instance, the O-antigen of S. flexneri serotype 2a, which is most frequently isolated from patients, converted into that of serotype Yv as phosphorylation at RhaII blocked glucosylation at RhaI (type O-factor II). This finding may account for the absence of ‘variant’ serotype 2av in nature. We also established the O-antigen structure of serotype 1d characterized by a so far unknown combination of type O‑factor I and group O‑factor 7,8. Adding this novel ‘classical’ serotype and three ‘variant’ serotypes extends the typing scheme of S. flexneri to 19 serotypes.
1. Q. Sun, Y. A. Knirel, R. Lan, J. Wang, S. N. Senchenkova, D. Jin, A. S. Shashkov, S. Xia, A. V. Perepelov, Q. Chen, Y. Wang, H. Wang, J. Xu. PLoS ONE 2012, 7 (9): e46095.
2. Y. A. Knirel, R. Lan, S. N. Senchenkova, J. Wang, A. S. Shashkov, Y. Wang, A. V. Perepelov, Y. Xiong, J. Xu, Q. Sun. Glycobiology 2013, 23: 475-485.
