Regulation of carbon and energy metabolism during the linear growth phase in batch fermentations of the acetogenic methylotroph Eubacterium limosum on methanol/CO 2

Abstract

Growth of the acetogenic anaerobe, Eubacterium limosum, on methanol/CO 2 in a fully defined medium was characterized by a prolonged linear growth phase following a short exponential growth period. Decelerating growth resulted from a progressively diminishing capacity of this bacterium to metabolize methanol and led to a shift in organic acid end products and degradation of the energy status of the cells. A butyrate efflux transporter was induced late in the fermentation and further contributed to the diminished energy status of the cells due to a protonophore effect resulting from a butyrate gradient other than that arising from cross-membrane pH considerations. The mechanism employed by this excretion phenomenon was postulated to be an anion uniport since steady-state butyrate gradients were equilibrated with the transmembrane electrical gradient. The shift from exponential to linear growth was associated with pyruvate accumulation together with a fall in the pool concentrations of all phosphorylated gluconeogenic intermediates. It appears that growth limitations result in carbon flux towards anabolic pathways being regulated at the level of PEP synthetase by ATP availability, thus balancing carbon and energy availability for biomass synthesis.