A classic problem in microbiology is that bacteria display two types of growth behavior when cultured on a mixture of two carbon sources: the two sources are sequentially consumed one after another (diauxie) or they are simultaneously consumed (co-utilization). The search for the molecular mechanism of diauxie led to the discovery of the lac operon. However, questions remain as why microbes would bother to have different strategies of taking up nutrients. Here we show that diauxie versus co-utilization can be understood from the topological features of the metabolic network. A model of optimal allocation of protein resources quantitatively explains why and how the cell makes the choice. In case of co-utilization, the model predicts the percentage of each carbon source in supplying the amino acid pools, which is quantitatively verified by experiments. Our work solves a long-standing puzzle and provides a quantitative framework for the carbon source utilization of microbes.

微生物学中的一个经典问题是，当细菌在两种碳源的混合物上培养时，细菌会表现出两种类型的生长行为：这两种源被依次一个接一个地消耗（diauxie）或同时被消耗（共同利用）。对diauxie分子机理的探索导致了lac的发现操纵子。然而，仍然存在疑问，为什么微生物会费心采取不同的养分吸收策略。在这里，我们显示可以从代谢网络的拓扑特征中了解双生与协同利用。蛋白质资源的最佳分配模型定量地解释了为什么以及如何做出选择。在共同利用的情况下，该模型可以预测每种碳源在提供氨基酸库中所占的百分比，这一点已通过实验进行了定量验证。我们的工作解决了一个长期的难题，并为微生物碳源的利用提供了定量框架。