Flux analysis: a basic tool of microbial physiology.

Flux analysis (FA) is a means of organizing data to show flux through the central metabolic pathways (CMPs). It quantifies flux from uptake of carbon to the outputs of the CMPs, which are the precursors used for biosynthesis, acetate excretion and CO2. Fluxes to precursors reflect the commands of the genome and acetate excretion balances fluxes to precursor supply when uptake exceeds the capacity of the CMPs to allocate carbon in exactly the correct amount to each precursor. No other products have been detected in 11 phenotypes of Escherichia coli ML308. FA of each of these 11 phenotypes (with some additional variations in culture conditions, some selected mutations and one genetic construct) are shown as flux (mol (kg dry weight biomass)-1 h-1) and are the starting point for further exploration of the physiology of E. coli: FAs suggest the possibility of four strategies to reduce acetate excretion and these have been tested in two of the phenotypes (glucose and pyruvate). All are successful to some degree but results are not always what were expected. FA of such interventions suggest that some 'global' control mechanisms operate in E. coli ML308 independent of carbon source. There is a division in the CMPs between those pathways that use phosphorylated intermediates and those that do not and these, in turn, are divided into the Krebs cycle and the C2 and C3 monocarboxylic acids. Altogether, there are four 'compartments' and each contains intermediates that are also precursors.