Structures of two cell wall-associated polysaccharides of a Streptococcus mitis biovar 1 strain - A unique teichoic acid-like polysaccharide and the group O antigen which is a C-polysaccharide in common with pneumococci

The cell wall of Streptococcus mitis biovar 1 strain SK137 contains the C-p olysaccharide known as the common antigen of a closely related species Stre ptococcus pneumoniae, and a teichoic acid-like polysaccharide with a unique structure. The two polysaccharides are different entities and could be par tially separated by gel chromatography. The structures of the two polysacch arides were determined by chemical methods and by NMR spectroscopy. The tei choic acid-like polymer has a heptasaccharide phosphate repeating unit with the following structure: 6)-beta -D-Galp-(1 -->6)-beta -D-Galp-(1 -->6)-be ta -D-Galf-(1 -->3)-beta -D-Glcp(1 -->6)-beta -D-Galf-(1 -->6)-alpha -D-Gal p-(1 -->6)-alpha -D-Glcp-1-P-(O --> The structure neither contains ribitol nor glycerol phosphate as classical teichoic acids do, thus we have used th e expression teichoic acid-like for this polysaccharide. The following stru cture of the C-polysaccharide repeating unit was established: [GRAPHICS] O)-P-Cho 6 O)-P-Cho 6 6)-beta -D-Glcp-(1 -->3)-alpha -AATp-(1 -->4)-alpha - D-GalpNAc-(1 -->3)-beta -D -GalpNAc-(1 -->1)-ribitol-5-P-(O --> where AAT i s 2-acetamido-4-amino-2,4,6-trideoxy-D-galactose. It has a carbohydrate bac kbone identical, to that of one of the two structures of C-polysaccharide p reviously identified in S. pneumoniae. C-polysaccharide of S. mitis is char acterized by the presence, in each repeating unit, of two residues of phosp hocholine and both galactosamine residues in the N-acetylated form. Immunoc hemical analysis showed that C-polysaccharide constitutes the Lancefield gr oup O antigen. Studies using mAbs directed against the backbone and against the phosphocholine moiety of the C-polysaccharide revealed several differe nt patterns of these epitopes among 95 S. mitis and Streptococcus oralis st rains tested and the exclusive presence of the group O antigen in the major ity of S. mitis biovar 1 strains.