Elementary mode analysis to study the preculturing effect on the metabolic state of Lactobacillus rhamnosus during growth on mixed substrates.

Quantification of metabolism through elementary modes offers insights into the working of a metabolic network. We have determined the fluxes of elementary modes through linear optimization using the stoichiometry of the elementary modes as a constraint. We apply this methodology to obtain insights into the effect of preculturing on growth of Lactobacillus rhamnosus on medium containing mixed substrates. L. rhamnosus, a microaerophilic organism, produces flavor compounds such as diacetyl and acetoin during growth on glucose and citrate. The uptake of citrate has been shown to be sensitive to preculturing states of the cells. Elementary modes demonstrated that citrate was utilized by the organism as a sole carbon source. Further, both glucose and citrate was catabolized by this organism through aerobic and anaerobic routes. The flux analysis indicated that only 21 elementary modes were operational during growth of L. rhamnosus on glucose and citrate. Glucose specifically accounted for 6 elementary modes, while the remaining 15 involved citrate as substrate. The modes associated with glucose were mainly operational when cells were precultured on glucose. It was observed that all the 21 modes contributed to the fluxes when the cells were precultured on citrate. The NADH recycling through lactate formation and oxygen uptake were dependent on the preculturing state. The analysis also demonstrated that preculturing on citrate yielded better productivity of diacetyl and acetoin.