Metabolic pathways and ΔpH regulation in Escherichia coli during the fermentation of glucose and glycerol in the presence of formate at pH 6.5: the role of FhlA transcriptional activator

Role of the FhlA transcriptional activator in metabolic changes in Escherichia coli during fermentation of mixed carbon sources at acidic pH

Escherichia coli formate hydrogen lyase (FHL) complexes are involved in the acid resistance mechanisms during fermentation, neutralizing intracellular formate to molecular hydrogen (H2). The current study elucidates the role of the FhlA transcriptional activator and formate metabolism in metabolic regulation during the fermentation of glucose, glycerol and formate at pH 5.5. It was shown that FhlA has a role in bacterial growth properties. The extracellular formate concentration was higher by ∼10 mM (at 20 h) and ∼8 mM (at 72 h) in cells lacking FhlA. Moreover, bacterial cells regulate ΔpH and maintain viability by the formate-lactate exchange during glycerol utilization. The formate to ethanol pathway is suggested for the regulation of pHin. It can be concluded that fermenting cells efficiently regulate metabolic pathways to adapt and maintain viability under acidic conditions.

Growth characteristics, redox potential changes and proton motive force generation in Thermus scotoductus K1 during growth on various carbon sources

The extremophile microorganism Thermus scotoductus primarily exhibits aerobic metabolism, though some strains are capable of anaerobic growth, utilizing diverse electron acceptors. We focused on the T. scotoductus K1 strain, exploring its aerobic growth and metabolism, responses to various carbon sources, and characterization of its bioenergetic and physiological properties. The strain grew on different carbon sources, depending on their concentration and the medium's pH, demonstrating adaptability to acidic environments (pH 6.0). It was shown that 4 g L-1 glucose inhibited the specific growth rate by approximately 4.8-fold and 5.6-fold compared to 1 g L-1 glucose at pH 8.5 and pH 6.0, respectively. However, this inhibition was not observed in the presence of fructose, galactose, lactose, and starch. Extracellular and intracellular pH variations were mainly alkalifying during growth. At pH 6.0, the membrane potential (ΔΨ) was lower for all carbon sources compared to pH 8.5. The proton motive force (Δp) was lower only during growth on lactose due to the difference in the transmembrane proton gradient (ΔpH). Moreover, at pH 6.0 during growth on lactose, a positive Δp was detected, indicating the cells' ability to employ a unique energy-conserving strategy. Taken together, these findings concluded that Thermus scotoductus K1 exhibits different growth and bioenergetic properties depending on the carbon source, which can be useful for biotechnological applications. These findings offer valuable insights into how bacterial cells function under high-temperature conditions, which is essential for applying bioenergetics knowledge in future biotechnological advancements.

Proton conductance and regulation of proton/potassium fluxes in Escherichia coli FhlA-lacking cells during fermentation of mixed carbon sources

Escherichia coli uptake potassium ions with the coupling of proton efflux and energy utilization via proton FOF1-ATPase. In this study contribution of formate hydrogen lyase (FHL) complexes in the proton/potassium fluxes and the formation of proton conductance (CMH+) were investigated using fhlA mutant strain. The proton flux rate (JH+) decreased in fhlA by ∼ 25 % and ∼70 % during the utilization of glucose and glycerol, respectively, at 20 h suggesting H+ transport via or through FHL complexes. The decrease in JK+ in fhlA by ∼40 % proposed the interaction between FHL and Trk secondary transport system during mixed carbon fermentation. Moreover, the usage of N,N'-dicyclohexylcarbodiimide (DCCD) demonstrated the mediation of FOF1-ATPase in this interaction. CMH+ was 13.4 nmol min-1 mV-1 in WT at 20 h, which decreased by 20 % in fhlA. Taken together, FHL complexes have a significant contribution to the modulation of H+/K+ fluxes and the CMH + for efficient energy transduction and regulation of the proton motive force during mixed carbon sources fermentation.