Antigens of Streptococcus mutans: cellular localization of the serotype-specific polysaccharide of strain AHT and release during exponential growth.

The serotype-specific polysaccharide of Streptococcus mutans AHT (serotype a) was shown to be loosely associated with the cell surface of this organism. The antigen was extracted from whole cells by boiling in sodium acetate buffer, pH 4.0, for 10 min. The purified product was found to be a diheteroglycan of galactose and glucose (3.6:1, molar ratio). The antigen possessed serological characteristics similar to the a antigen previously extracted from purified cell walls with hot formamide. Its physicochemical structure was identical to the previously studied wall antigen. Electron micrographs, developed after immunocytological labeling of this antigen on whole cells, revealed it to compose a dense microcapsule surrounding the microbe. Analyses of spent culture fluids indicated that the antigen was released during exponential growth at a rate directly proportional to the increase in culture biomass. It is concluded that the serotype-specific antigen may be a prime immunogen due to its surface localization at both capsule and wall sites.