  The heterogeneity of reaction fluxes present in a metabolic network within a
single flux state can be exploited to construct the so-called backbone as a
reduced version of metabolism. The backbone maintains all significant fluxes
producing or consuming metabolites while displaying a substantially decreased
number of interconnections and, hence, it becomes a useful tool to extract
primary metabolic routes. Here, we disclose the metabolic backbone of
Escherichia coli using the computationally predicted fluxes which maximize the
growth rate in glucose minimal medium, and we compare it with the backbone of
Mycoplasma pneumoniae, a much simpler organism. We find that the central core
in both backbones is mainly composed of reactions in ancient pathways, still
playing at present a key role in energy metabolism. In E. coli, the analysis of
the backbone reveals that the synthesis of nucleotides and the metabolism of
lipids form smaller cores which rely critically on energy metabolism; but not
conversely. At the same time, an analysis of the dependence of this backbone on
media composition leads to the identification of pathways sensitive to
environmental changes. The metabolic backbone of an organism is thus useful to
trace simultaneously both its evolution and adaptation fingerprints.
