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Michael, J. Gabriel (House of the Good Samaritan, Children's Hospital Medical Center, Boston, Mass.),Benedict F. Massell, and Robert E. Perkins . Effect of sublethal concentrations of penicillin on the virulence and antigenic composition of group A streptococci. J. Bacteriol.85: 1280–1287. 1963.—Virulent strains of group A streptococci were grown in sublethal concentrations of penicillin and tested for possible changes in colonial morphology, virulence, and antigenic composition. After overnight growth in broth containing penicillin, there was a marked reduction in precipitable group A and M protein content of the bacteria. Upon transfer to broth without antibiotic, the streptococci regained their ability to produce these substances. After multiple transfers in broth containing non-bactericidal levels of penicillin, variants developed which lacked group and type substances and which were also relatively resistant to penicillin. These variants were avirulent for mice and susceptible to the bactericidal action of normal human blood. Two of three strains tested, when grown on blood agar, failed to produce beta hemolysis. The mutants produced by multiple transfers in broth containing small amounts of penicillin, when passed through mice, regained all of their original properties.

EFFECT OF SUBLETHAL CONCENTRATIONS OF PENICILLIN ON THE VIRULENCE AND ANTIGENIC COMPOSITION OF GROUP A STREPTOCOCCI