Competitive growth of lactose-positive and lactose-negative strains of Bact. coli mutabile

Naturally occurring strains of Bact. coli mutabile are known to show long initial lags in the utilization of lactose ($\text{lac}^{-}$ $\text{strains}$). By growth in lactose media $\text{lac}^{+}$ strains are obtained. $\text{Lac}^{+}$ strains (though showing a rapid but limited increase in lactose lag during intervening cultures without lactose) do not revert to the $\text{lac}^{-}$ state for a very long time. Thus when artificial mixtures of $\text{lac}^{-}$ $\text{and}$ $\text{lac}^{+}$ strains are grown in a glucose medium competition for predominance occurs as between independent types of organism. One or other type may gain predominance according to circumstances, and the determining factor is found to be not the initial relative proportions of the strains, as the theory of 'periodic selection' suggests, but the degree of general adaptation (in respects having nothing to do with the $\text{lac}^{+}$ $\text{or}$ $\text{lac}^{-}$ character) of the one or other strain to the medium in which the competitive growth occurs.