Pyruvate is the substrate for diacetyl and acetoin synthesis by lactobacilli. Exogenous pyruvate stimulates acetoin production when glucose is present as an energy source. In Lactobacillus plantarum ATCC 8014, the energy derived from glucose via glycolysis generated a constant proton motive force of about-120 mV. At a low external pH, energized cells rapidly transported and accumulated pyruvate but did not do so when they were deenergized by nigericin. When large amounts of pyruvate were transported and subsequently accumulated internally, the cotransported protons rapidly lowered the internal pH. The conversion of pyruvate to acetoin instead of acidic end products contributed to the maintenance of pH homeostasis. This is the first report showing that the conversion of pyruvate to acetoin serves as a mechanism of pH homeostasis.

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Proton motive force (PMF), intracellular end product concentrations, and ATP levels were determined when a steady-state Lactobacillus plantarum 8014 anaerobic chemostat culture was shifted to an aerobic condition or was shifted from pH 5.5 to 7.5. The PMF and intracellular ATP levels increased immediately after the culture was shifted from anaerobic to aerobic conditions. The concentrations of intracellular lactate and acetate, which exported protons that contributed to the proton gradient, changed in the same fashion. The H+/lactate stoichiometry, n, varied from 0.8 to 1.2, and the H+/acetate n value changed from 0.8 to 1.6 at 2 h after the shift to aerobic conditions. The n value for acetate excretion remained elevated at aerobic steady state. When the anaerobic culture was shifted from pH 5.5 to 7.5, intracellular ATP increased 20% immediately even though the PMF decreased 50% as a result of the depletion of the transmembrane proton gradient. The H+/ lactate n value changed from 0.7 to 1.8, and n for H+/acetate increased from 0.9 to 1.9 at pH 7.5 steady state. In addition, the H+/acetate stoichiometry was always higher than the n value for H+/lactate; both were higher in alkaline than aerobic conditions, demonstrating that L. plantarum 8014 coexcreted more protons with end products to maintain intracellular pH homeostasis and generate proton gradients under aerobic and alkaline conditions. During the transient to pH 7.5, the n value for H+/acetate approached 3, which would spare one ATP.The circulation of ions across biological membranes, especially protons and sodium ions, is one of the fundamental processes in cellular energetics. Transport systems pump ions across membranes at the expense of some energy source, establishing an ion gradient whose electrochemical potential represents stored energy. Return of that ion across the membrane is mediated by a second transport system which links the downhill flux of the ion to the performance of some useful work (4).Mitchell's chemiosmotic hypothesis (13,14) proposed that electron transport and phosphorylation are not chemically linked, but are coupled by a transmembrane current of protons or proton motive force (PMF). The electron transport chain is a metabolic pathway arranged within and across the membrane to translocate protons across it. Since the membrane has a low conductance for protons and ions, a gradient of pH (ApH) and of membrane potential (A/@) will develop across the membrane to form the total PMF (AP) as given by the equation erichia coli. In this energy-recycling model, metabolic end products and protons are coexcreted via specific symport proteins in the cytoplasmic membrane. As result of this proton translocation, a PMF is generated which can contribute to the overall production of metabolic energy (23,25). Using Lactococcus cremoris, a homofermentative lactateproducing organism, as a model system, they observed that proton excretion coupled to lactate export supplied a significant quantity of metabolic energy to cells (17,18,24,26). Lactobacillu...