M protein was detected in the concentrated broth-culture supernates of strains of group-A streptococci, belonging to a wide variety of M types, by double-diffusion in gel, and its presence was confirmed in several strains of M-type 12 by complement fixation with absorbed anti-M serum and by the neutralisation of human “bactericidal” antibody. Extracellular M proteins retained their serological activity after heating at 95°C for 10 min. at neutral or acid , but were destroyed by proteolytic enzymes. Both the acid-extracted and extracellular M proteins of a type-12 strain had isoelectric points between 5·0 and 5·1.

The ratios of extracellular to acid-extractable M protein were significantly higher in M types associated with pyoderma than in those associated with infection of the upper respiratory tract. This was related to a striking tendency for “skin” strains to have less extractable M protein than many of the “throat” strains, and to the fact that only 50% of the “throat” strains had detectable extracellular M protein. On the other hand the majority of “skin” strains (18 of 21) produced extracellular M antigen. The molecular-weight distribution in extracellular and acid-extracted M proteins was inferred from chromatography on Sephadex G100 of culture supernates and corresponding acid extracts. In the “respiratory” M-types 5,12 and 30 the range of molecular weights was wide (5000 to ≧ 150 000) and very similar in the two preparations. In the “skin” serotypes 49, 55, 57 and 60, however, extracellular M protein fell into the range 55 000 to ≧ 150 000, a distribution strikingly different from that in the acid extracts of the corresponding strains (5000 to ≧ 150 000). This suggests that acid extraction may modify the M proteins of pyoderma serotypes more than those of respiratory serotypes.


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