SMALL COLONY VARIANTS OFESCHERICHIA COLI

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Abstract
In the presence of copper ions, populations of Escherichia coli undergo a series of predictable population shifts. Slow growth of the wild type inoculum is followed by the establishment in the population of small colony variants. The variant cells are able to establish themselves temporarily because of their greatly increased resistance to copper, but are completely displaced again by phenotypically normal cells of a type somewhat more resistant to copper than the original population. This replacement is due not to a physiological conversion of variants into normal cells but to the presence, in the original population, of small numbers of wild type cells with increased copper resistance. A correlation between copper resistance on one hand, and colony size and ability to ferment lactose on the other, was observed. The great variability observed in the proportion of variants establishing themselves in the population can be ascribed as (i) competitive interplay between phenotypically normal and variant cells, followed by gradual elimination of the variants from the population; (2) presence of wild types cells with increased resistance to copper; and (3) variations in the oxidation-reduction potential of the cultures. Cysteine, chelating copper in the monovalent state, and versene, chelating it in the divalent state, abolish both the growth inhibition and the induction of small colony variants by copper. The same is true of anaerobic conditions; the presence of air is necessary for the variants to occur. Free radical trapping agents had no effect. These and other experiments indicate that copper initially must be present as the free ion and is active in the formation of variants only if bound to a cell site in the divalent state.