Summary: A new M-type of group A streptococcus, provisionally designated type 65, is described. The vaccine and other initially isolated strains of this type attracted attention because of the T-agglutination reactions 2/25, not previously encountered among pyoderma streptococci. The investigations characterizing the strains as members of a new type were done with streptococci isolated from patients with pyoderma. However, type 65 was subsequently found to cause both pyoderma and acute pharyngitis.

The T-2 agglutination reactions encountered with original members of this type, plus the cross-reactions later seen with type 65 antiserum and M-type 2 streptococci, prompted a comparison of this new type with M-type 2 streptococci, including those with the T-2 agglutination and others with the 8-25-Imp.19 complex. The two M-antigens were clearly distinguished from one another in reciprocal bactericidal and precipitin tests with absorbed antisera. They were further distinguished in that all type 65 strains were opacity-factor (OF) negative, whereas type 2 streptococci were uniformly OF-positive. Most M-type 65 strains subsequently found in surveillance studies were shown to be members of the 8-25-1mp.19 T-complex. Type 65 is thus a newly described type which shares with M-types 55 and 57 a common T-agglutination pattern and, like members of these types, fails to produce opacity factor. In our collection of strains, from both pyoderma and pharyngitis, shown to be members of the 8-25-1mp.19 complex, and OF-negative, only type 65 has been identified to date. In contrast to types 55 and 57, the new type 65 does not appear to be of major importance in causing acute glomerulonephritis.


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