Pathways of genetic adaptation: multistep origin of mutants under selection without induced mutagenesis in Salmonella enterica.

In several bacterial systems, mutant cell populations plated on growth-restricting medium give rise to revertant colonies that accumulate over several days. One model suggests that nongrowing parent cells mutagenize their own genome and thereby create beneficial mutations (stress-induced mutagenesis). By this model, the first-order induction of new mutations in a nongrowing parent cell population leads to the delayed accumulation of visible colonies. In an alternative model (selection only), selective conditions allow preexisting small-effect mutants to initiate clones that grow and give rise to faster-growing mutants. By the selection-only model, the delay in appearance of revertant colonies reflects (1) the time required for initial clones to reach a size sufficient to allow the second mutation plus (2) the time required for growth of the improved subclone. We previously characterized a system in which revertant colonies accumulate slowly and contain cells with two mutations, one formed before plating and one after. This left open the question of whether mutation rates increase under selection. Here we measure the unselected formation rate and the growth contribution of each mutant type. When these parameters are used in a graphic model of revertant colony development, they demonstrate that no increase in mutation rate is required to explain the number and delayed appearance of two of the revertant types.