The drop-plate method of “simultaneous antagonism” used by others for the detection of bacteriocinogeny in group-A streptococci is unsuitable to differentiate the inhibitory action of true bacteriocines from that of peroxide produced by the test strains. Among 36 group-A streptococcal strains examined, 78 % displayed inhibitory activity on appropriate indicator strains but failed to do so when tested under anaerobic conditions or in the presence of catalase. Most of these strains were shown to give a positive peroxide test on heated blood agar, all of them released hydrogen peroxide during growth in aerated liquid cultures as measured quantitatively by polarography, and none of them lost its inhibitory effect in the presence of trypsin. No strains were encountered that, apart from bacteriophages, produced any catalase-stable, trypsin-sensitive inhibitory substances. For inhibitory effects to be considered to reflect the action of bacteriocines, it is therefore necessary to show that inhibition cannot be ascribed to the toxic effect of liberated peroxide.


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