[Antibiotic production by inactive cultures of actinomycetes after treatment with ethidium bromide].

Inactive strains of actinomycetes isolated from natural sources were treated with ethidium bromide, an intercalating agent. After the treatment the cultures formed active variants at a frequency of more than 0.1 per cent which was rather high. 41 out of 93 cultures formed variants active against gram-positive bacteria. The majority of the active variants synthesized antibiotics only on agarized media. Daunorubicin was also used as an intercalating agent and shown to induce at a high frequency the formation of active variants with the same antibacterial spectrum as the variants induced by ethidium bromide. Induction of antibiotic production by the intercalating agents in the inactive strains of actinomycetes was likely due to the activation of the silent genes responsible for the biosynthesis of secondary metabolites.