Actinomycete Variants with Impaired Differentiation: Production in High Yield, Recognition and Phenotypic Characterization in Seven Species

S ummary . When plated on mineral synthetic media with D‐fructose as a sole carbon and energy source 7 strains of actinomycetes belonging to different streptomycete species regularly produced secondary variant colonies in high yield. Beside the ability of strepto‐mycetes to utilize fructose as a sole carbon source, the following factors were shown to be of importance in the control of variational events: the state of the parent culture used for platings; the composition of fructose‐containing media used for the production of variants. Indole, anthranilate and phenol inhibited formation of secondary colonies by most strains. Phenotypically all the variants obtained shared in common the loss of ability to form secondary (aerial) mycelium and spores as well as a tendency of the substrate mycelium to fragment. These traits were shown to be: hereditarily stable on all media tested for variants derived from Streptcmyces ruber, Str. roseolus, Str. lateritius and Str. roseoflavus var. roseofungini; less stable and nutritionally affected for variants derived from Str. albocyaneus, Str. roseoflavus and Str. anthocyaneus; unstable on all except fructose‐containing mineral medium for the variants of Str. flavofungini. Vegetative growth of some of the variants obtained was not inferior to that of parent cultures; some variants produced increased amounts of intracellular antibiotics. Some implications of the reported findings are discussed.