Serological identification and prevalence of a novel O-antigen epitope linked to 3- and 4-O-acetylated rhamnose III of lipopolysaccharide in Shigella flexneri.

Shigella flexneri is the major cause of shigellosis in developing countries. All serotypes except for serotype 6 share an O-antigen backbone composed of a → 2)-α-L-Rhap(III)-(1 → 2)-α-l-Rhap(II)-(1 → 3)-α-l-Rhap(I)-(1 → 3)-β-D-GlcpNAc-(1 → tetrasaccharide repeat. It can be modified by the addition of a glucosyl group to one or more sugar residues and/or an O-acetyl group to Rha(I) and/or a phosphoethanolamine to Rha(II) and/or Rha(III). These modifications give rise to type I-, IC-, II-, IV-, and V- and to group 6-, 7,8-, and MASF IV-1-specific antigenic determinants, which comprise the current serotyping scheme of S. flexneri. Recently, another O-antigen modification created by adding an O-acetyl group to Rha(III) at position 3 or 4 (3/4-O-acetylation) has been found in S. flexneri serotypes 1a, 1b, 2a, 5a, Y, and 6. A new O-acyltransferase gene named oacB has been shown to mediate the 3/4-O-acetylation in serotypes 1a, 1b, 2a, 5a, and Y but not in 6. In this work, we studied the distribution of the 3/4-O-acetylation in S. flexneri and the antigenicity that resulted from this modification. PCR screening of the oacB gene in clinical isolates of S. flexneri demonstrated that the oacB-mediated 3/4-O-acetylation is widespread in serotypes 1a, 1b, 2a, 5a, and Y. Serological analysis indicated that this modification confers the host with a novel antigenic determinant that is provisionally named group O factor 9. These findings enhance our understanding of the varieties of O-antigenic determinants related to O-antigen modification in S. flexneri and will assist epidemiological studies and vaccine development.