On the relation of colony variants to the time dependency of colony formation during adaptive mutation of Escherichia coli FC40.

When Escherichia coli FC40 formed adaptive Lac+ revertants on a selective agar medium containing lactose as the carbon source, the colonies which accumulated over several days were of two readily distinguishable types. Colonies of both types appeared both early and late on the plates. Cells of colonies that appeared early and late on the plates, irrespective of the type, when grown in liquid medium and replated, all formed colonies on the selective medium within 48 h. Cells of each colony type gave rise to colonies of both types and attempts to isolate cells of each type in pure culture were unsuccessful. It was concluded that the presence of two colony types in the cultures plated did not contribute to the observed time dependency of colony formation during adaptive mutation. The proportions of the two colony types arising from cultures of the Lac+ revertants varied from culture to culture.