During the long-term study of sore throat caused by group-A streptococci in a village, M-positive members of type 12 disappeared and were replaced by streptococci that appeared to be M-negative variants of the original strain.

The new strain resembled the original M-type-12 streptococcus in possessing the T-antigen of type 12, but further investigation showed that it was not M-negative; it had a hitherto undescribed M-antigen. It also differed from the M-type-12 strain in having the ability to produce opacity in horse serum and in not being lysogenic for a phage active on the indicator strain K56.

A minority-population of the new strain was found in 7 of 11 cultures of the original M-type-12 streptococcus. This is taken as evidence that the new strain has arisen as a variant of the M-type-12 streptococcus.


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