Streptomyces producing ionophore antibiotics were searched for in various natural substrata, such as soil, sea water, and coastline sea mud, in many geographical regions. A total of 393 Streptomyces strains were isolated, 308 of which proved to be capable of producing antibiotics; 135 of these contained water-insoluble antibiotically active substances in the mycelium. Studies by radioindication, spectrophotometry, and potentiometry demonstrated that these partially purified hydrophobic antibiotics possessed ionophore properties. Selectivity to monovalent potassium and ammonium cations seems to be a characteristic feature of hydrophobic antibiotics derived from Streptomyces differing them from other bacteria.
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