Experimental design and characterization. a) Schematic diagram of the lac operon illustrating the decoupling of regulation and metabolism using IPTG that induces but is not metabolized, and Pgal that is metabolized but does not induce. b) Measured growth rates g(I, P) for a number of combinations of IPTG and Pgal in a minimal M9 medium plus casamino acids (see Materials and Methods). At low Pgal concentrations, inducing the lac operon products results in an expression cost that is larger than the benefit, resulting in a decrease of the growth rate. For higher Pgal concentrations the benefit will dominate the cost. Each data point is obtained from three separate measurements. c) The wild-type induction profile in the absence of Pgal, fitted with a Hill curve (Additional file 1).