Mutants having enhanced antibiotic-producing capacity are obtained by subjecting an antibiotic-producing strain of a species of the genus Streptomyces to the mutagenic action of a compound selected from 1-(5-nitrofurfurylidene amino)-2-imidazolino thione, 5-nitro-2-furaldehyde semi carbazone, N-(5-nitro-2-furfurylidene)-1-amino-hydantoin and 5-nitro-2-furaldehyde, and selecting in a conventional manner from the various colonies produced a strain of improved antibiotic activity. Mutation is effected in a nutrient medium containing from 1.0 to 20.0 g /ml of the said compound and also 0.01 to 0.5% by volume of an acid amide e.g. dimethylformamide. The species may be Streptomyces aureofaciens, S. rimosus, S. antibioticus, S. erythreus, S. fradiae, S. griseus, S. niveus, S. noursei and S. hydroscopicus.ALSO:Mutants having enhanced antibiotic-producing capacity are obtained by subjecting an antibiotic-producing strain of a species of the genus Streptomyces to the mutagenic action of a compound selected from 1-(5-nitrofurfurylidene amino) - 2 - imidazolino thione, 5 - nitro-2 - furaldehyde semicarbazone, N - (5 - nitro-2 - furfurylidene) - 1 - amino - hydantoin and 5 - nitro - 2 - furaldehyde, and selecting in a conventional manner from the various colonies produced a strain of improved antibiotic activity. Mutation is effected in a nutrient medium containing from 1.0-20.0 g /ml. of the said compound and also 0.01 to 0.5% by volume of an acid amide such as dimethylformamide. The species may be Streptomyces aureofaciens, S. rimosus, S. antibioticus, S. erythreus, S. fradiae, S. griseus, S. niveus, S. noursei and S. hydroscopicus. Examples relate to the preparation of Streptomyces aureofaciens A-802.934 (deposited at the State Hygienic Institute Budapest, Hungary under No. 640.304.26) and to the cultivation of this mutant for the preparation of tetracycline in a medium containing chloride ions.
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