Immunochemical Characterization of Native Polysaccharides from Group B Streptococcus: The Relationship of the Type III and Group B Determinants

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Abstract
Three polysaccharides were purified from a type III strain of Group B Streptococcus grown under conditions varying in exposure to acid by washing the cells with an EDTA buffer or extracting with cold 10% TCA. Extraction of cells grown in a medium buffered with a pH titrator resulted in a polysaccharide antigen of 1.2 × 106 daltons having only type III serologic specificity. The antigen extracted with EDTA from cells grown in a medium where acid accumulated had a molecular size of approximately 600,000, and demonstrated both type III and group B serologic activity. The antigen extracted with TCA had a molecular size of approximately 500,000 and also reacted with type III and group B sera. Chemical studies revealed that all three polysaccharides contained varying concentrations of galactose, glucose, glucosamine, and sialic acid. Only the TCA antigen contained rhamnose. Alcohol fractionation of the TCA antigen allowed separation of type III from group B serologic reactivity. However, this method failed to separate the two determinants of the EDTA-non-pH titrated antigen. Other physical, chemical, and immunologic methods similarly failed to separate these two determinants. Mild hydrolysis of the EDTA-pH-titrated antigen exposed a determinant immunologically identical to the formamide-extracted group B antigen. These studies as well as electron microscopy and affinity chromatography demonstrated that two determinants exist on the native polysaccharide antigen of type III, Group B Streptococcus. One is the superficial capsular type specific determinant, which in the most native state masks the second determinant, a common group B determinant. Quantitative precipitation inhibition studies confirmed that l-rhamnose was the immunodominant sugar for the formamide-extracted B antigen. No monosaccharide inhibited the cross-reactive determinant on the EDTA-non pH titrated antigen. Despite this observation, both the formamide-extracted B antigen and the EDTA-non pH titrated antigen were demonstrated to be precipitating antibodies of identical specificity.