SUMMARY: In addition to R outgrowths, strains of growing on a carbohydrate-free medium also formed smooth secondary colonies situated on the primary colonies. These secondary colonies arose after about 6 days of incubation and were of two types: when centrally situated they formed papillae; when near the margin of the mother colony they often spilt over and formed fan-shaped outgrowths adjacent to the parent colony. Cultures of several different species of were examined; secondary colony formation was confined almost exclusively to strains of The fan-shaped outgrowths were pure cultures of organisms possessing a shorter generation time than those comprising the mother colony. An inverse relationship existed on crowded and sparsely inoculated plates between numbers of papillae and fan-shaped outgrowths/colony observed. The distribution of colonies bearing different numbers of secondary colonies did not follow a Poisson distribution; the random origin of fast-growing variants could not be established. On subculture the fast-growing variants produced colonies with a slightly different morphology from that of the wild-types. This difference was stable and persisted through numerous subcultures. In the presence of small quantities of fermentable carbohydrates secondary colony formation was not observed and this possibly explains how the wild type competes successfully with fast-growing variants in natural environments.


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