  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.
