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SUMMARY: M-protein and the opacity factor (OF) were both extracted by Lancefield's method from certain types of group A streptococci. The two proteins could not be separated by fractional precipitation with ammonium sulphate, column chromatography or polyacrylamide gel electrophoresis. The OF appeared to be closely associated with the high molecular weight fraction of the M-antigen, and there is some evidence that the two may form part of the same molecule.M-protein and the OF were sought in various other extracts and preparations of M-positive and M-negative variants of serotypes which gave a positive serum opacity reaction (SOR). In all preparations from M-positive variants, the two proteins were always present. In M-negative variants, the OF present in the wall-membrane fraction could be extracted only by ‘phage-associated.’ lysin. All other extracts from M-negative variants were SOR-negative and M-negative. In the SOR-positive extract obtained by phage lysin treatment of M-negative variants, no M-antigen could be detected by precipitin tests, though the extracts contained a protein of similar electrophoretic mobility to the precipitin-positive protein in the extracts of the M-positive variants of the same serotype.

The Relationship between M-antigen and Opacity Factor in Group A Streptococci