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Variation of Actinomyces chromogenes var. trienicus 141-18 MSU, an organism producing trienin was studied under laboratory conditions. Nine stable spontaneous variants were isolated from the population of the initial culture when grown on Gause medium No. 1. The variants varied in differentiation and biosynthetic capacity, including such characteristics as size and form of the colonies, ability for formation of the aerial mycelium and its colour, capacity for sporulation, form of the spore chains and antibiotic production property. In the secondary structures the spores formed only in 6 variants out of 9 isolates. The spore form and spore membrane surface were close in all sporogenic variants, while there were significant differences in the structure of the sporophores. The variants forming the aerial mycelium of the same colour as that of the initial culture did not differ from it also by the nature of the spore chains (spirals with 3--8 turns). The variants with lighter aerial mycelium than that of the initial population formed straight sporophores or spirals with a small number of the turns (1--3). The comparative study of the antimicrobial spectrum of the variants and the component composition of the synthesized antibiotic complex showed that the asporogenic variants and dwarf variant signifcantly differed with respect to their phenotypes from the other cultures and had no antagonistic action. One of the assporogenic variants had only insignificant activity. All the spore forming variants did not differ from the initial culture in the complex of the antibiotics synthesized.

The necessity of taking into account the specific characteristics of the colony differentiation for isolation of spontaneous variants of the initial culture with the method of monospore plating was shown on the examples of the geliomycin-producing culture Actinomyces chromogenes var. trienicus 141-18. It was found that the population density of the colonies had a significant effect on the differentiation rate during the subsequent stages and completeness of the culture development. The less was the population density, the longer was the period of the colony complete development. Definite levels of the colony organization (fractionation of morphogenesis) corresponded to different dilutions of the monospore suspension providing definite population densities of the colonies on the plates. On the basis of the data on the colony differentiation compared with the data on the population density effect upon differentiation it was possible to determine without mistake stable variants on the plates at any dilution of the monospore culture, i.e. with any population density of the colony.