Type III Capsular Polysaccharide of Group B Streptococci: Role in Virulence and the Molecular Basis of Capsule Expression

Group B streptococci (GBS) are weakly beta-hemolytic, facultatively anaerobic, gram-positive cocci which have emerged over the past 50 years as the most common cause of neonatal pneumonia, sepsis, and meningitis in the United States, western Europe, and some developing countries. The type III capsular polysaccharide is a high-molecular-weight polymer composed of repeating pentasaccharide subunits containing a trisaccharide backbone of galactose, glucose, and N-acetylglucosamine with a side chain of galactose and a terminal sialic acid moiety. In epidemiological studies, a correlation was observed between newborns with antibody to the sialylated capsule and protection from infection, which is in contrast to the case for infants who lacked antibody or who had antibody to the desialylated polysaccharide. Subcutaneous injection of the acapsular or asialo mutants into neonatal rats resulted in similar 50% lethal doses that were at least 2 orders of magnitude higher than those observed for the parent strain. The genetic characterization of complex polysaccharide production by gramnegative bacteria has proceeded more quickly because of advances in the molecular technology available for these organisms. Molecular analysis of the genes involved in GBS capsule biosynthesis should provide an understanding of complex polysaccharide biosynthesis and transport in gram-positive organisms. Similar studies investigating capsule synthesis by Streptococcus pneumoniae and Streptococcus pyogenes are being pursued.