Identification of the common antigenic determinant shared by Streptococcus pneumoniae serotypes 33A, 35A, and 20...

In order to better understand cross-reactions of serogroup 33 polysaccharides and the typing sera, the structure of pneumococcal capsular polysaccharide serotype 33A was elucidated. Serotype 33A has been shown to have an identical polysaccharide backbone as that of serotype 33F, with two additional sites of O-acetylation at C5, and C6 of the 3-β-Galf residue in serotype 33A. This finding is consistent with the presence of an additional functional acetyltransferase gene (wcjE) in the cps biosynthetic locus of serotype 33A compared to 33F. The identical polysaccharide backbone with at least one common O-acetylation site (C2 of 5-β-Galf) shared by serotype 33A and 33F polysaccharides is proposed to be the epitope recognized by typing serum 33b. In addition, a 5,6-di-O-acetylated →3)-β-d-Galf5,6Ac-(1→3)-β-d-Glcp-(1→ disaccharide unit, a common structural motif present in serotypes 33A, 20, and 35A polysaccharides, is proposed to be the antigenic determinant recognized by typing serum 20b.