An Experimental Model: Bacterial Specialists and Generalists Competing in Chemostats

An experimental model was used to test the ecological axiom that a generalist is less efficient than a specialist. The generalist in this model were strains of Escherichia coli which can use both maltose and lactose. The specialists were strains of E. coli which can use only maltose because of a deletion of the lactose genes. These generalists and specialists competing for maltose in energy—limited chemostats containing both sugars achieved coexistence under certain conditions. The equilibrium frequency depended upon the pair of strains used and the percentage of the sugar which is lactose. A genetic change was important in the outcome of these experiments: a lac constitutive mutation replaced the lac / of the generalist. (Cells with a lactose constitutive mutation produce lactose enzyme whether or not lactose is present.) This allowed the generalist to coexist on lower amounts of lactose that it would have been able to otherwise. We propose that the lactose consititutive is selected for over the wild—type in a lactose—limited chemostat because the concentration of lactose required for growth, if the operon is constitutively depressed, is less than that required for continued depression of the lactose operon. The equilibrium frequency predicted by chemostate competition theory gave a much higher frequency of the generalist than seen in the experiments. When only the generalist was added to a chemostat containing both maltose and lactose, an equilibrium of both lac / and lac constitutive cells was established. This implies that there exists "resource interference." Resource interference happens when an organism is not as efficient at using a resource when it is using other resources as when it was using that resource alone. When a constant for resource interference was incorporated into the equations, the theoretical predictions agreed with the experimental results. We hypothesize that this resource interferences is due to competition for permease sites.