Long-term evolution of the natural isolate of Escherichia coli 536 in the mouse gut colonized after maternal transmission reveals convergence in the constitutive expression of the lactose operon.

In vitro experimental evolution has taught us many lessons on the molecular bases of adaptation. To move towards more natural settings, evolution in the mice gut has been successfully performed. Yet, these experiments suffered from the use of laboratory strains as well as the use of axenic or streptomycin treated mice to maintain the inoculated strains. To circumvent these limitations, we conducted a one-year experimental evolution in vivo using a natural isolate of E. coli, strain 536, in conditions mimicking as much as possible natural environment with mother to offspring microbiota transmission. Mice were then distributed in 24 independent cages and separated in two different diets: a regular one (Chow diet, CD) and high-fat high-sugar one (Western diet, WD). Genome sequences revealed an early and rapid selection during the breast-feeding period that selected the constitutive expression of the well-characterized lactose operon. E. coli was lost significantly more in CD than WD, howev...