Streptococcus suis serotype 3 and serotype 18 capsular polysaccharides contain di-N-acetyl-bacillosamine

Streptococcus suis serotype 3 is counted among the S. suis serotypes causing clinical disease in pigs. Yet, limited information is available on this serotype. Here we determined for the first time the chemical composition and structure of serotype 3 capsular polysaccharide (CPS), a major bacterial virulence factor and the antigen at the origin of S. suis classification into serotypes. Chemical and spectroscopic data gave the repeating unit sequence for serotype 3: [4)D-GlcA (β1-3)d-QuiNAc4NAc(β1-]n. To the best of our knowledge, this is the first report of di-N-acetyl-d-bacillosamine (QuiNAc4NAc) containing polysaccharides in Streptococci and the second time this rare diamino sugar has been observed in a Gram-positive bacterial species since its initial report. This led to the identification of homologues of UDP-QuiNAc4NAc synthesis genes in S. suis serotype 18. Thus, the repeating unit sequence for serotype 18 is: [3)d-GalNAc(α1-3)[d-Glc (β1-2)]d-GalA4OAc(β1-3)d-GalNAc(α1-3)d-QuiNAc4NAc(α1-]n. A correlation between S. suis serotypes 3 and 18 CPS sequences and genes of these serotypes' cps loci encoding putative glycosyltransferases and polymerase responsible for the biosynthesis of the repeating unit was tentatively established. Knowledge of CPS structure and composition will contribute to better dissect the role of this bacterial component in the pathogenesis of S. suis serotypes 3 and 18.