Sympatric metabolic diversification of experimentally evolved Escherichia coli in a complex environment

Sympatric diversification in bacteria has been found to contravene initial evolutionary theories affirming the selection of the fittest type by competition for the same resource. Studies in unstructured (well-mixed) environments have discovered divergence of an ancestor strain into genomically and phenotypically divergent types growing both on single and mixed energy sources. This study addresses the metabolic diversification in an Escherichia coli population that evolved over ~1,000 generations under aerobic conditions in the nutritional complexity offered by Luria-Bertani (LB) broth. The medium lacked glucose but contained a variety of other resources. Two distinct metabolically-diverged types, coinciding with colony morphologies, were found to dominate the populations. One type was an avid carbohydrate consumer, which could quickly utilize the available (alternative) substrates feeding into glycolysis. The second type was a slow grower, which was able to specifically consume acetate. The capacity to utilize acetate might be providing an advantage to this second type, suggesting an increased capability to deal with adverse conditions that occur in the later stages of growth. The diverged metabolic preferences of the two forms suggested differential and interactive ecological roles within the population. We postulate that these types used different alternative metabolic strategies occupying different niches in a sympatric manner as an outcome of adaptation to the complex environment.