Emergence of diauxie as an optimal growth strategy under resource allocation constraints in cellular metabolism

Abstract

The sequential rather than simultaneous consumption of carbohydrates in bacteria such as E. coli, a phenomenon termed diauxie, has been hypothesized to be an evolutionary strategy which allows the organism to maximize its instantaneous specific growth, thus giving the bacterium a competitive advantage. Currently, computational techniques used in industrial biotechnology fall short of explaining the intracellular dynamics underlying diauxic behavior, in particular at a proteome level. Some hypotheses postulate that diauxie is due to limitations in the catalytic capacity of bacterial cells. We developed a robust iterative dynamic method based on expression- and thermodynamically enabled flux models (dETFL) to simulate the temporal evolution of carbohydrate consumption and cellular growth. The dETFL method couples gene expression and metabolic networks at the genome scale, and successfully predicts the preferential uptake of glucose over lactose in E. coli cultures grown on a mixture of carbohydrates. The observed diauxic behavior in the simulated cellular states suggests that the observed diauxic behavior is supported by a switch in the content of the proteome in response to fluctuations in the availability of extracellular carbon sources. We are able to model both the proteome allocation and the proteomic switch latency induced by different types of cultures. Our models suggest that the diauxic behavior of the cell is the result of the evolutionary objective of maximization of the specific growth of the cell. We propose that genetic regulatory networks, such as the lac operon in E. coli, are the biological implementation of a robust control system to ensure optimal growth.