The type III antigen of Lancefield's group B streptococci is known to be responsible for the virulence of the organism. Some of the chemical, physical and immunologic properties of the antigen were studied. The antigen was extracted from the bacteria by hot HCl extractions. It was further purified by treatment with pepsin, trypsin, and by fractionation on a DEAE-cellulose, ion-exchange, chromatographic column.

The type III antigen was characterized by hydrolysis and chromatography, nitrogen, phosphorus, amino acid and carbohydrate content. It is a polysaccharide composed of 38.9% galactose, 22.8% glucosamine and 17.8% glucose. The latter sugars were present at a ratio of 2:1:1 µmols/mg, respectively. The antigen also contains 3.1% uronic acid, 0.8% muramic acid and less than 1% each of alanine, aspartic acid, glutamic acid, serine and threonine. The antigen has a sedimentation coefficient of 1.67S and a m.w. of 15,561. The partial specific volume was 0.62 g/ml. Immunoelectrophoresis of the antigen resulted in migration toward the cathode. The cathodal migration explains the separation which was obtained on the DEAE-cellulose column.

Glucuronic acid appears to be one of the immunodominant components of the antigen. It offers 34% inhibition of the quantitative precipitin reaction between the type III antigen and type III antisera.

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