In Leuconostoc mesenteroides subsp. mesenteroides 19D, citrate is transported by a secondary citrate carrier (CitP). Previous studies of the kinetics and mechanism of CitP performed in membrane vesicles of L. mesenteroides showed that CitP catalyzes divalent citrate Hcit In bacteria, metabolic energy present in the form of ATP and ion gradients of H ϩ and Na ϩ are used to drive various endergonic reactions associated with cellular growth. The two forms of metabolic energy can be interconverted by the action of membrane-bound F 0 F 1 -ATPases that couple the translocation of an H ϩ (or Na ϩ ) to the hydrolysis/synthesis of ATP. In fermentative organisms, ATP is usually formed by substratelevel phosphorylation (glycolysis; arginine deaminase pathway), which is subsequently used to generate an electrochemical gradient of protons across the cytoplasmic membrane (proton motive force [pmf]) (16). Recently, a different mechanism of pmf generation was discovered that is of particular importance in the energetics of certain anaerobes and allows the F 0 F 1 -ATPase to function in the synthesis mode, i.e., the pmf drives the synthesis of ATP. The mechanism involves the action of secondary transporters and is therefore termed secondary metabolic energy generation (7). The pmf is formed indirectly during the metabolic breakdown of weak acids. The anionic forms of the acids are transported into the cell by an electrogenic secondary carrier that translocates net negative charge into the cell, generating a membrane potential. The internal degradation of the substrate involves a decarboxylation step that consumes a scalar proton, which results in the formation of a pH gradient (11). The anions are taken up in exchange with a metabolic end product of the pathway (precursor/product exchange) or by a uniport mechanism, in which case the end product leaves the cell by passive diffusion.Examples of pathways using the exchange type of uptake are oxalate fermentation in Oxalobacter formigenes (1), malolactic fermentation in Lactococcus lactis (17), and histidine decarboxylation in Lactobacillus buchneri (15), and an example of a pathway using the uniporter mechanism is malate and citrate fermentation in the acidophilic bacterium Leuconostoc oenos (18,20).Cometabolism of glucose and citrate by the lactic acid bacterium Leuconostoc mesenteroides results in a growth advantage relative to growth on glucose alone. The increased growth rate is usually attributed to a metabolic shift in the heterofermentative pathway for glucose breakdown, yielding additional ATP (2, 4, 9, 21). In the absence of citrate, acetyl-P formed from glucose is reduced to ethanol, which balances the redox equivalents produced in the other steps of the phosphoketolase pathway (see also Fig. 1). In the presence of citrate, the redox equivalents are shuttled to pyruvate produced from citrate, yielding D-lactate, and acetyl-P is converted into acetate via the acetate kinase pathway, which results in the production of ATP. In a previous study in this laboratory, the cataly...
The growth of Lactobacillus delbrueckii subsp. bulgaricus (L. delbrueckii subsp. bulgaricus) on lactose was altered upon aerating the cultures by agitation. Aeration caused the bacteria to enter early into stationary phase, thus reducing markedly the biomass production but without modifying the maximum growth rate. The early entry into stationary phase of aerated cultures was probably related to the accumulation of hydrogen peroxide in the medium. Indeed, the concentration of hydrogen peroxide in aerated cultures was two to three times higher than in unaerated ones. Also, a similar shift from exponential to stationary phase could be induced in unaerated cultures by adding increasing concentrations of hydrogen peroxide. A significant fraction of the hydrogen peroxide produced by L. delbrueckii subsp. bulgaricus originated from the reduction of molecular oxygen by NADH catalyzed by an NADH:H 2 O 2 oxidase. The specific activity of this NADH oxidase was the same in aerated and unaerated cultures, suggesting that the amount of this enzyme was not directly regulated by oxygen. Aeration did not change the homolactic character of lactose fermentation by L. delbrueckii subsp. bulgaricus and most of the NADH was reoxidized by lactate dehydrogenase with pyruvate. This indicated that NADH oxidase had no (or a very small) energetic role and could be involved in eliminating oxygen.Lactobacillus delbrueckii subsp. bulgaricus (L. delbrueckii subsp. bulgaricus) is an important species of lactic acid bacteria currently used in the industrial production of fermented milk products. L. delbrueckii subsp. bulgaricus is an aerotolerant anaerobe that obtains most of its energy from homolactic fermentation (12). It does not require strict anaerobic growth conditions and tolerates the concentration of O 2 in air. Even though L. delbrueckii subsp. bulgaricus does not use O 2 in its energetic metabolism, it is likely that the presence of oxygen in its environment can influence its physiology. Indeed, some lactic acid bacteria possess oxidases that utilize molecular oxygen to oxidize substrates such as pyruvate (22) or NADH (2,5,8,16,(23)(24)(25). As these oxidation reactions cannot occur under anaerobic conditions, metabolism in the presence of oxygen cannot be identical to that in the absence of oxygen. Also, the activities of these oxidases can produce partially reduced oxygen species such as the superoxide radical (O 2 .Ϫ ), hydrogen peroxide (H 2 O 2 ), the hydroxyl radical (HO . ), and other peroxyl radicals or peroxides that will cause an oxidative stress in the cell. It is therefore expected that the presence of oxygen will induce a specific cellular response to such oxidative stress. In this work, a difference in the growth of L. delbrueckii subsp. bulgaricus in the presence and absence of oxygen was observed. It was also found that L. delbrueckii subsp. bulgaricus could reduce oxygen into hydrogen peroxide with an NADH oxidase, probably to eliminate the oxygen present. However, this detoxification of oxygen led to an overproducti...
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