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Two antigens present in group O streptococci have been described and are designated as type antigens. No other antigens are present in extracts of group O cells in significant quantity. Antigen I is a cell wall protein, and II is a nucleoprotein of undetermined cell location. The immunological specificity of the latter resides only in the protein. Both antigens have been extracted from the cell with water or 0.2 n HCl (pH 2) at 100 C, and antigen I can be removed by trypsin. After purification, pH 2 was found to destroy the serological activity of each antigen. Antgens I and II and also the O polysaccharide antigen can be distinguished on a single agar-gel diffusion plate with group O whole-cell antiserum. N- and C-terminal amino acid analyses and total amino acid composition prove that I and II are separate molecular species. Each possesses properties which separate them from the streptococcal M, R, and T proteins. Antigen I separated into three fractions, each possessing serological specificity, on a diethylaminoethyl-Sephadex column at pH 8.6 during an increase in buffer molarity. The protein is considered to possess a multiple unit structure. The results indicate that the O streptococci are limited to two serological types. Group O whole-cell antisera, after adsorption with a strain not containing the O polysaccharide, can now be prepared for the positive identification of the types of group O streptococci.

Immunological and Chemical Studies of the Streptococcal Group O Protein Type Antigens