Characterization of the linkage between the type III capsular polysaccharide and the bacterial cell wall of group B Streptococcus

The capsular polysaccharide of group B Streptococcus is a key virulence fac tor and an important target for protective immune responses. Until now, the nature of the attachment between the capsular polysaccharide and the bacte rial cell has been poorly defined. We isolated insoluble cell wall fragment s from lysates of type III group B Streptococcus and showed that the comple xes contained both capsular polysaccharide and group B carbohydrate covalen tly bound to peptidoglycan. Treatment with the endo-N-acetylmuramidase muta nolysin released soluble complexes of capsular polysaccharide linked to gro up B carbohydrate by peptidoglycan fragments. Capsular polysaccharide could be enzymatically cleaved from group B carbohydrate by treatment of the sol uble complexes with beta-N-acetylglucosaminidase, which catalyzes hydrolysi s of the beta-n-GlcNAc(1-->4)beta-D-MurNAc subunit produced by mutanolysin digestion of peptidoglycan. Evidence from gas chromatography/mass spectrome try and (31)p NMR analysis of the separated polysaccharides supports a mode l of the group B Streptococcus cell surface in which the group B carbohydra te and the capsular polysaccharide are independently linked to the glycan b ackbone of cell wall peptidoglycan; group B carbohydrate is linked to N-ace tylmuramic acid, and capsular polysaccharide is linked via a phosphodiester bond and an oligosaccharide linker to N-acetylglucosamine.