Identification of cpsD, a gene essential for type III capsule expression in group B streptococci

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Abstract
Summary: We showed previously that a mutant strain of group B Streptococcus (GBS) defective in capsule production was avirulent. This study describes the derivation of an unencapsulated mutant from a highly encapsulated wild‐type strain of type III GBS, COH1, by transposon mutagenesis with Tn916ΔE. The mutant, COH1‐13, was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat sepsis model compared with the wild‐type strain. No capsular polysaccharide was evident in the cytoplasm or on the cell surface of the mutant strain. The Tn916ΔE insertion site in COH1‐13 was mapped to the same chromosomal location as the Tn916 insertion site in the unencapsulated type III mutant COH31‐15 reported previously. Nucleotide sequencing of DNA flanking the insertion site in COH1‐13 revealed an open reading frame, designated cpsD, with significant homology to the rfbP gene of Salmonella typhimurium. RfbP encodes a galactosyl transferase enzyme that catalyses the transfer of galactose to undecaprenol phosphate, the initial step in O‐polysaccharide synthesis. A particulate fraction of a lysate of wild‐type strain GBS COH1 mediated the transfer of galactose from UDP‐galactose to an endogenous acceptor. The galactose–acceptor complex partitioned into organic solvents, suggesting it is lipid in nature or membrane‐associated. Galactosyl transferase activity was significantly reduced in the unencapsulated mutant strain COH1‐13. These results, together with the similarity in deduced amino acid sequence between cpsD and rfbP suggest that cpsD encodes a galactosyl transferase essential for assembly of the GBS type III capsular polysaccharide.