A tyrothricin concentration of 0.01 mg. per ml. causes rapid lysis of Staphylococcus aureus; this lyric action is not enhanced by phage. A concentration of 0.001 mg. per ml. of tyrothricin is likewise capable of lysing staphylococci, and the addition of phage does not appreciably alter the rate of lysis. A concentration of 0.0001 mg. per ml. of tyrothricin is unable to effect lysis though it enhances the lytic activity of phage. The tyrocidine fraction of tyrothricin appears to be responsible for the latter's lytic action.

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