Uniport of monoanionic L-malate in membrane vesicles from Leuconostoc oenos

Macromolecular Crowding, Phase Separation, and Homeostasis in the Orchestration of Bacterial Cellular Functions

Macromolecular crowding affects the activity of proteins and functional macromolecular complexes in all cells, including bacteria. Crowding, together with physicochemical parameters such as pH, ionic strength, and the energy status, influences the structure of the cytoplasm and thereby indirectly macromolecular function. Notably, crowding also promotes the formation of biomolecular condensates by phase separation, initially identified in eukaryotic cells but more recently discovered to play key functions in bacteria. Bacterial cells require a variety of mechanisms to maintain physicochemical homeostasis, in particular in environments with fluctuating conditions, and the formation of biomolecular condensates is emerging as one such mechanism. In this work, we connect physicochemical homeostasis and macromolecular crowding with the formation and function of biomolecular condensates in the bacterial cell and compare the supramolecular structures found in bacteria with those of eukaryotic cells. We focus on the effects of crowding and phase separation on the control of bacterial chromosome replication, segregation, and cell division, and we discuss the contribution of biomolecular condensates to bacterial cell fitness and adaptation to environmental stress.

The Role of Yeasts and Lactic Acid Bacteria on the Metabolism of Organic Acids during Winemaking

The main role of acidity and pH is to confer microbial stability to wines. No less relevant, they also preserve the color and sensory properties of wines. Tartaric and malic acids are generally the most prominent acids in wines, while others such as succinic, citric, lactic, and pyruvic can exist in minor concentrations. Multiple reactions occur during winemaking and processing, resulting in changes in the concentration of these acids in wines. Two major groups of microorganisms are involved in such modifications: the wine yeasts, particularly strains of Saccharomyces cerevisiae, which carry out alcoholic fermentation; and lactic acid bacteria, which commonly conduct malolactic fermentation. This review examines various such modifications that occur in the pre-existing acids of grape berries and in others that result from this microbial activity as a means to elucidate the link between microbial diversity and wine composition.

l-Malate (-2) Protonation State is Required for Efficient Decarboxylation to l-Lactate by the Malolactic Enzyme of Oenococcus oeni

Malolactic fermentation (MLF) is responsible for the decarboxylation of l-malic into lactic acid in most red wines and some white wines. It reduces the acidity of wine, improves flavor complexity and microbiological stability. Despite its industrial interest, the MLF mechanism is not fully understood. The objective of this study was to provide new insights into the role of pH on the binding of malic acid to the malolactic enzyme (MLE) of Oenococcus oeni. To this end, sequence similarity networks and phylogenetic analysis were used to generate an MLE homology model, which was further refined by molecular dynamics simulations. The resulting model, together with quantum polarized ligand docking (QPLD), was used to describe the MLE binding pocket and pose of l-malic acid (MAL) and its l-malate (−1) and (−2) protonation states (MAL⁻ and MAL²⁻, respectively). MAL²⁻ has the lowest ∆G_binding, followed by MAL⁻ and MAL, with values of −23.8, −19.6, and −14.6 kJ/mol, respectively, consistent with those obtained by isothermal calorimetry thermodynamic (ITC) assays. Furthermore, molecular dynamics and MM/GBSA results suggest that only MAL²⁻ displays an extended open conformation at the binding pocket, satisfying the geometrical requirements for Mn²⁺ coordination, a critical component of MLE activity. These results are consistent with the intracellular pH conditions of O. oeni cells—ranging from pH 5.8 to 6.1—where the enzymatic decarboxylation of malate occurs.

Cell Fuelling and Metabolic Energy Conservation in Synthetic Cells

We are aiming for a blue print for synthesizing (moderately complex) subcellular systems from molecular components and ultimately for constructing life. However, without comprehensive instructions and design principles, we rely on simple reaction routes to operate the essential functions of life. The first forms of synthetic life will not make every building block for polymers de novo according to complex pathways, rather they will be fed with amino acids, fatty acids and nucleotides. Controlled energy supply is crucial for any synthetic cell, no matter how complex. Herein, we describe the simplest pathways for the efficient generation of ATP and electrochemical ion gradients. We have estimated the demand for ATP by polymer synthesis and maintenance processes in small cell-like systems, and we describe circuits to control the need for ATP. We also present fluorescence-based sensors for pH, ionic strength, excluded volume, ATP/ADP, and viscosity, which allow the major physicochemical conditions inside cells to be monitored and tuned.

Global Transcriptomic Analysis and Function Identification of Malolactic Enzyme Pathway of Lactobacillus paracasei L9 in Response to Bile Stress

Tolerance to bile stress is crucial for Lactobacillus paracasei to survive in the intestinal tract and exert beneficial actions. In this work, global transcriptomic analysis revealed that 104 genes were significantly changed (log₂FoldChange > 1.5, P < 0.05) in detected transcripts of L. paracasei L9 when exposed to 0.13% Ox-bile. The different expressed genes involved in various biological processes, including carbon source utilization, amino acids and peptide metabolism processes, transmembrane transport, transcription factors, and membrane proteins. It is noteworthy that gene mleS encoding malolactic enzyme (MLE) was 2.60-fold up-regulated. Meanwhile, L-malic acid was proved to enhance bile tolerance, which could be attributed to the intracellular alkalinization caused by MLE pathway. In addition, membrane vesicles were observed under bile stress, suggesting a disturbance in membrane charge without L-malic acid. Then, genetic and physiological experiments revealed that MLE pathway enhanced the bile tolerance by maintaining a membrane balance in L. paracasei L9, which will provide new insight into the molecular basis of MLE pathway involved in bile stress response in Lactic acid bacteria.

Wholly Rickettsia! Reconstructed Metabolic Profile of the Quintessential Bacterial Parasite of Eukaryotic Cells

Reductive genome evolution has purged many metabolic pathways from obligate intracellular Rickettsia (Alphaproteobacteria; Rickettsiaceae). While some aspects of host-dependent rickettsial metabolism have been characterized, the array of host-acquired metabolites and their cognate transporters remains unknown. This dearth of information has thwarted efforts to obtain an axenic Rickettsia culture, a major impediment to conventional genetic approaches. Using phylogenomics and computational pathway analysis, we reconstructed the Rickettsia metabolic and transport network, identifying 51 host-acquired metabolites (only 21 previously characterized) needed to compensate for degraded biosynthesis pathways. In the absence of glycolysis and the pentose phosphate pathway, cell envelope glycoconjugates are synthesized from three imported host sugars, with a range of additional host-acquired metabolites fueling the tricarboxylic acid cycle. Fatty acid and glycerophospholipid pathways also initiate from host precursors, and import of both isoprenes and terpenoids is required for the synthesis of ubiquinone and the lipid carrier of lipid I and O-antigen. Unlike metabolite-provisioning bacterial symbionts of arthropods, rickettsiae cannot synthesize B vitamins or most other cofactors, accentuating their parasitic nature. Six biosynthesis pathways contain holes (missing enzymes); similar patterns in taxonomically diverse bacteria suggest alternative enzymes that await discovery. A paucity of characterized and predicted transporters emphasizes the knowledge gap concerning how rickettsiae import host metabolites, some of which are large and not known to be transported by bacteria. Collectively, our reconstructed metabolic network offers clues to how rickettsiae hijack host metabolic pathways. This blueprint for growth determinants is an important step toward the design of axenic media to rescue rickettsiae from the eukaryotic cell.IMPORTANCE A hallmark of obligate intracellular bacteria is the tradeoff of metabolic genes for the ability to acquire host metabolites. For species of Rickettsia, arthropod-borne parasites with the potential to cause serious human disease, the range of pilfered host metabolites is unknown. This information is critical for dissociating rickettsiae from eukaryotic cells to facilitate rickettsial genetic manipulation. In this study, we reconstructed the Rickettsia metabolic network and identified 51 host metabolites required to compensate patchwork Rickettsia biosynthesis pathways. Remarkably, some metabolites are not known to be transported by any bacteria, and overall, few cognate transporters were identified. Several pathways contain missing enzymes, yet similar pathways in unrelated bacteria indicate convergence and possible novel enzymes awaiting characterization. Our work illuminates the parasitic nature by which rickettsiae hijack host metabolism to counterbalance numerous disintegrated biosynthesis pathways that have arisen through evolution within the eukaryotic cell. This metabolic blueprint reveals what a Rickettsia axenic medium might entail.

Physiological Role of Two-Component Signal Transduction Systems in Food-Associated Lactic Acid Bacteria

Two-component systems (TCSs) are widespread signal transduction pathways mainly found in bacteria where they play a major role in adaptation to changing environmental conditions. TCSs generally consist of sensor histidine kinases that autophosphorylate in response to a specific stimulus and subsequently transfer the phosphate group to their cognate response regulators thus modulating their activity, usually as transcriptional regulators. In this review we present the current knowledge on the physiological role of TCSs in species of the families Lactobacillaceae and Leuconostocaceae of the group of lactic acid bacteria (LAB). LAB are microorganisms of great relevance for health and food production as the group spans from starter organisms to pathogens. Whereas the role of TCSs in pathogenic LAB (most of them belonging to the family Streptococcaceae) has focused the attention, the roles of TCSs in commensal LAB, such as most species of Lactobacillaceae and Leuconostocaceae, have been somewhat neglected. However, evidence available indicates that TCSs are key players in the regulation of the physiology of these bacteria. The first studies in food-associated LAB showed the involvement of some TCSs in quorum sensing and production of bacteriocins, but subsequent studies have shown that TCSs participate in other physiological processes, such as stress response, regulation of nitrogen metabolism, regulation of malate metabolism, and resistance to antimicrobial peptides, among others.

Application of flow cytometry to wine microorganisms

Flow cytometry (FCM) is a powerful technique allowing detection and enumeration of microbial populations in food and during food process. Thanks to the fluorescent dyes used and specific probes, FCM provides information about cell physiological state and allows enumeration of a microorganism in a mixed culture. Thus, this technique is increasingly used to quantify pathogen, spoilage microorganisms and microorganisms of interest. Since one decade, FCM applications to the wine field increase greatly to determine population and physiological state of microorganisms performing alcoholic and malolactic fermentations. Wine spoilage microorganisms were also studied. In this review we briefly describe FCM principles. Next, a deep revision concerning enumeration of wine microorganisms by FCM is presented including the fluorescent dyes used and techniques allowing a yeast and bacteria species specific enumeration. Then, the last chapter is dedicated to fluorescent dyes which are used to date in fluorescent microscopy but applicable in FCM. This chapter also describes other interesting "future" techniques which could be applied to study the wine microorganisms. Thus, this review seeks to highlight the main advantages of the flow cytometry applied to wine microbiology.

A mathematical model of the link between growth and L-malic acid consumption for five strains of Oenococcus oeni

In winemaking, after the alcoholic fermentation of red wines and some white wines, L-malic acid must be converted into L-lactic acid to reduce the acidity. This malolactic fermentation (MLF) is usually carried out by the lactic acid bacteria Oenococcus oeni. Depending on the level of process control, selected O. oeni is inoculated or the natural microbiota of the cellar is used. This study considers the link between growth and MLF for five strains of O. oeni species. The kinetics of growth and L-malic acid consumption were followed in modified MRS medium (20 °C, pH 3.5, and 10 % ethanol) in anaerobic conditions. A large variability was found among the strains for both their growth and their consumption of L-malic acid. There was no direct link between biomass productivities and consumption of L-malic acid among strains but there was a link of proportionality between the specific growth of a strain and its specific consumption of L-malic acid. Experiments with and without malic acid clearly demonstrated that malic acid consumption improved the growth of strains. This link was quantified by a mathematical model comparing the intrinsic malic acid consumption capacity of the strains.

Channel-mediated lactic acid transport: a novel function for aquaglyceroporins in bacteria

MIPs (major intrinsic proteins), also known as aquaporins, are membrane proteins that channel water and/or uncharged solutes across membranes in all kingdoms of life. Considering the enormous number of different bacteria on earth, functional information on bacterial MIPs is scarce. In the present study, six MIPs [glpF1 (glycerol facilitator 1)–glpF6] were identified in the genome of the Gram-positive lactic acid bacterium Lactobacillus plantarum. Heterologous expression in Xenopus laevis oocytes revealed that GlpF2, GlpF3 and GlpF4 each facilitated the transmembrane diffusion of water, dihydroxyacetone and glycerol. As several lactic acid bacteria have GlpFs in their lactate racemization operon (GlpF1/F4 phylogenetic group), their ability to transport this organic acid was tested. Both GlpF1 and GlpF4 facilitated the diffusion of D/L-lactic acid. Deletion of glpF1 and/or glpF4 in Lb. plantarum showed that both genes were involved in the racemization of lactic acid and, in addition, the double glpF1 glpF4 mutant showed a growth delay under conditions of mild lactic acid stress. This provides further evidence that GlpFs contribute to lactic acid metabolism in this species. This lactic acid transport capacity was shown to be conserved in the GlpF1/F4 group of Lactobacillales. In conclusion, we have functionally analysed the largest set of bacterial MIPs and demonstrated that the lactic acid membrane permeability of bacteria can be regulated by aquaglyceroporins.

Role of secondary transporters and phosphotransferase systems in glucose transport by Oenococcus oeni

Glucose uptake by the heterofermentative lactic acid bacterium Oenococcus oeni B1 was studied at the physiological and gene expression levels. Glucose- or fructose-grown bacteria catalyzed uptake of [(14)C]glucose over a pH range from pH 4 to 9, with maxima at pHs 5.5 and 7. Uptake occurred in two-step kinetics in a high- and low-affinity reaction. The high-affinity uptake followed Michaelis-Menten kinetics and required energization. It accumulated the radioactivity of glucose by a factor of 55 within the bacteria. A large portion (about 80%) of the uptake of glucose was inhibited by protonophores and ionophores. Uptake of the glucose at neutral pH was not sensitive to degradation of the proton potential, Δp. Expression of the genes OEOE_0819 and OEOE_1574 (here referred to as 0819 and 1574), coding for secondary transporters, was induced by glucose as identified by quantitative real-time (RT)-PCR. The genes 1574 and 0819 were able to complement growth of a Bacillus subtilis hexose transport-deficient mutant on glucose but not on fructose. The genes 1574 and 0819 therefore encode secondary transporters for glucose, and the transports are presumably Δp dependent. O. oeni codes, in addition, for a phosphotransferase transport system (PTS) (gene OEOE_0464 [0464] for the permease) with similarity to the fructose- and mannose-specific PTS of lactic acid bacteria. Quantitative RT-PCR showed induction of the gene 0464 by glucose and by fructose. The data suggest that the PTS is responsible for Δp-independent hexose transport at neutral pH and for the residual Δp-independent transport of hexoses at acidic pH.

Activity of ethanol-stressed Oenococcus oeni cells: a flow cytometric approach

AIMS

To study the effect of ethanol on Oenococcus oeni activity at the single cell level.

METHODS AND RESULTS

The active extrusion of the fluorescent probe carboxy fluorescein (cF) was used to assess the metabolic activity of ethanol-stressed O. oeni cells. Subsequent flow cytometric analysis revealed that O. oeni cells extrude the accumulated cF upon energizing with l-malic acid. However, O. oeni cells exposed to 12% (v/v) ethanol for 1 h showed a decreased capacity for active extrusion of cF. Moreover, two subpopulations could be distinguished, one of which being able to extrude cF and the other one remaining cF fluorescent. Growing cells in the presence of 8% (v/v) ethanol resulted in robust cells that maintained the capacity to actively extrude cF after being exposed to 12% (v/v) ethanol, which in turn correlated with the high levels of ATP observed in these ethanol stressed, malolactic fermentation (MLF) performing cells.

CONCLUSION

From our results, it becomes evident that active extrusion of cF can be used to assess malolactic activity in O. oeni.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study provides information for the development of a rapid method to assess the malolactic activity of individual O. oeni cells performing MLF during wine production.

Dual effect of organic acids as a function of external pH in Oenococcus oeni

In this study we analyzed under various pH conditions including low pH, the effects of L-malic acid and citric acid, combined or not, on the growth, the proton motive force components and the transcription level of selected genes of the heterolactic bacterium Oenococcus oeni. It is shown here that L-malate enhanced the growth yield at pH equal or below 4.5 while the presence of citrate in media led to a complete and unexpected inhibition of the growth at pH 3.2. Nevertheless, whatever the growth conditions, both L-malate and citrate participated in the enhancement of the transmembrane pH gradient, whereas the membrane potential decreased with the pH. These results suggested that it was not citrate that was directly responsible for the inhibition observed in cultures done at low pH, but probably its end products. This was confirmed since, in media containing L-malate, the addition of acetate substantially impaired the growth rate of the bacterium and slightly the membrane potential and pH gradient. Finally, study of the expression of genes involved in the metabolism of organic acids showed that at pH 4.5 and 3.2 the presence of L-malate led to an increased amount of mRNA of mleP encoding a malate transporter.

Microbial transport: Adaptations to natural environments

The cytoplasmic membrane of bacteria is the matrix for metabolic energy transducing processes such as proton motive force generation and solute transport. Passive permeation of protons across the cytoplasmic membrane is a crucial determinant in the proton motive generating capacity of the organisms. Adaptations of the membrane composition are needed to restrict the proton permeation rates especially at higher temperatures. Thermophilic bacteria cannot sufficiently restrict this proton permeation at their growth temperature and have to rely on the much lower permeation of Na + to generate a sodium motive force for driving metabolic energy-dependent membrane processes. Specific transport systems mediate passage across the membrane at physiological rates of all compounds needed for growth and metabolism and of all end products of metabolism. Some of transport systems, the secondary transporters, transduce one form of electrochemical energy into another form. These transporters can play crucial roles in the generation of metabolic energy. This is especially so in anaerobes such as Lactic Acid Bacteria which live under energy-limited conditions. Several transport systems are specifically aimed at the generation of metabolic energy during periods of energy-limitation. In their natural environment bacteria are also often exposed to cytotoxic compounds, including antibiotics. Many bacteria can respond to this live-threatening condition by overexpressing powerful drug-extruding multidrug resistance systems.

The 2-hydroxycarboxylate transporter family: physiology, structure, and mechanism

The 2-hydroxycarboxylate transporter family is a family of secondary transporters found exclusively in the bacterial kingdom. They function in the metabolism of the di- and tricarboxylates malate and citrate, mostly in fermentative pathways involving decarboxylation of malate or oxaloacetate. These pathways are found in the class Bacillales of the low-CG gram-positive bacteria and in the gamma subdivision of the Proteobacteria. The pathways have evolved into a remarkable diversity in terms of the combinations of enzymes and transporters that built the pathways and of energy conservation mechanisms. The transporter family includes H+ and Na+ symporters and precursor/product exchangers. The proteins consist of a bundle of 11 transmembrane helices formed from two homologous domains containing five transmembrane segments each, plus one additional segment at the N terminus. The two domains have opposite orientations in the membrane and contain a pore-loop or reentrant loop structure between the fourth and fifth transmembrane segments. The two pore-loops enter the membrane from opposite sides and are believed to be part of the translocation site. The binding site is located asymmetrically in the membrane, close to the interface of membrane and cytoplasm. The binding site in the translocation pore is believed to be alternatively exposed to the internal and external media. The proposed structure of the 2HCT transporters is different from any known structure of a membrane protein and represents a new structural class of secondary transporters.

Escherichia coli glutamate- and arginine-dependent acid resistance systems increase internal pH and reverse transmembrane potential

Due to the acidic nature of the stomach, enteric organisms must withstand extreme acid stress for colonization and pathogenesis. Escherichia coli contains several acid resistance systems that protect cells to pH 2. One acid resistance system, acid resistance system 2 (AR2), requires extracellular glutamate, while another (AR3) requires extracellular arginine. Little is known about how these systems protect cells from acid stress. AR2 and AR3 are thought to consume intracellular protons through amino acid decarboxylation. Antiport mechanisms then exchange decarboxylation products for new amino acid substrates. This form of proton consumption could maintain an internal pH (pHi) conducive to cell survival. The model was tested by estimating the pHi and transmembrane potential (DeltaPsi) of cells acid stressed at pH 2.5. During acid challenge, glutamate- and arginine-dependent systems elevated pHi from 3.6 to 4.2 and 4.7, respectively. However, when pHi was manipulated to 4.0 in the presence or absence of glutamate, only cultures challenged in the presence of glutamate survived, indicating that a physiological parameter aside from pHi was also important. Measurements of DeltaPsi indicated that amino acid-dependent acid resistance systems help convert membrane potential from an inside negative to inside positive charge, an established acidophile strategy used to survive extreme acidic environments. Thus, reversing DeltaPsi may be a more important acid resistance strategy than maintaining a specific pHi value.

Absence of malolactic activity is a characteristic of H+-ATPase-deficient mutants of the lactic acid bacterium Oenococcus oeni

The lack of malolactic activity in H(+)-ATPase-deficient mutants of Oenococcus oeni selected previously was analyzed at the molecular level. Western blot experiments revealed a spot at 60 kDa corresponding to the malolactic enzyme only in the parental strain. Moreover, the mleA transcript encoding the malolactic enzyme was not detected by reverse transcription (RT)-PCR analysis of mutants. These results suggest that the malolactic operon was not transcribed in ATPase-deficient mutants. The mleR gene encoding a LysR-type regulatory protein which should be involved in expression of the malolactic genes was described previously for O. oeni. Results obtained in this study show that the mleR transcript was not detected in the mutants by RT-PCR. No mutation in the nucleotide sequences of the mleR gene and the malolactic operon was found. The effect of a reduction in H(+)-ATPase activity on L-malate metabolism was then investigated by using other malolactic bacteria. Spontaneous H(+)-ATPase-deficient mutant strains of Lactococcus lactis and Leuconostoc mesenteroides were isolated by using neomycin resistance. Two mutants were selected. These mutants exhibited ATPase activities that were reduced to 54 and 70% of the activities obtained for the L. lactis and L. mesenteroides parental strains, respectively. These mutants were also acid sensitive. However, in contrast to the ATPase-deficient mutants of O. oeni, activation of L-malate metabolism was observed with the L. lactis and L. mesenteroides mutants under optimal or acidic growth conditions. These data support the suggestion that expression of the genes encoding malolactic enzymes in O. oeni is regulated by the mleR product, as it is in L. lactis. Nevertheless, our results strongly suggest that there is a difference between the regulation of expression of the malolactic locus in O. oeni and the regulation of expression of this locus in less acidophilic lactic acid bacteria.

Flow cytometric assessment of membrane integrity of ethanol-stressed Oenococcus oeni cells

The practical application of commercial malolactic starter cultures of Oenococcus oeni surviving direct inoculation in wine requires insight into the mechanisms involved in ethanol toxicity and tolerance in this organism. Exposure to ethanol resulted in an increase in the permeability of the cytoplasmic membrane, enhancing passive proton influx and concomitant loss of intracellular material (absorbing at 260 nm). Cells grown in the presence of 8% (vol/vol) ethanol revealed adaptation to ethanol stress, since these cells showed higher retention of compounds absorbing at 260 nm. Moreover, for concentrations higher than 10% (vol/vol), lower rates of passive proton influx were observed in these ethanol-adapted cells, especially at pH 3.5. The effect of ethanol on O. oeni cells was studied as the ability to efficiently retain carboxyfluorescein (cF) as an indicator of membrane integrity and enzyme activity and the uptake of propidium iodide (PI) to assess membrane damage. Flow cytometric analysis of both ethanol-adapted and nonadapted cells with a mixture of the two fluorescent dyes, cF and PI, revealed three main subpopulations of cells: cF-stained intact cells; cF- and PI-stained permeable cells, and PI-stained damaged cells. The subpopulation of O. oeni cells that maintained their membrane integrity, i.e., cells stained only with cF, was three times larger in the population grown in the presence of ethanol, reflecting the protective effect of ethanol adaptation. This information is of major importance in studies of microbial fermentations in order to assign bulk activities measured by classical methods to the very active cells that are effectively responsible for the observations.

Characterization of Schizosaccharomyces pombe malate permease by expression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, L-malic acid transport is not carrier mediated and is limited to slow, simple diffusion of the undissociated acid. Expression in S. cerevisiae of the MAE1 gene, encoding Schizosaccharomyces pombe malate permease, markedly increased L-malic acid uptake in this yeast. In this strain, at pH 3.5 (encountered in industrial processes), L-malic acid uptake involves Mae1p-mediated transport of the monoanionic form of the acid (apparent kinetic parameters: Vmax = 8.7 nmol/mg/min; Km = 1.6 mM) and some simple diffusion of the undissociated L-malic acid (Kd = 0.057 min(-1)). As total L-malic acid transport involved only low levels of diffusion, the Mae1p permease was further characterized in the recombinant strain. L-Malic acid transport was reversible and accumulative and depended on both the transmembrane gradient of the monoanionic acid form and the DeltapH component of the proton motive force. Dicarboxylic acids with stearic occupation closely related to L-malic acid, such as maleic, oxaloacetic, malonic, succinic and fumaric acids, inhibited L-malic acid uptake, suggesting that these compounds use the same carrier. We found that increasing external pH directly inhibited malate uptake, resulting in a lower initial rate of uptake and a lower level of substrate accumulation. In S. pombe, proton movements, as shown by internal acidification, accompanied malate uptake, consistent with the proton/dicarboxylate mechanism previously proposed. Surprisingly, no proton fluxes were observed during Mae1p-mediated L-malic acid import in S. cerevisiae, and intracellular pH remained constant. This suggests that, in S. cerevisiae, either there is a proton counterflow or the Mae1p permease functions differently from a proton/dicarboxylate symport.

Analysis of the role of the Listeria monocytogenes F0F1 -AtPase operon in the acid tolerance response

As little is known about the genes involved in the induction of an acid tolerance response in Listeria monocytogenes, the role of the F0F1-ATPase was analyzed as a consequence of its role in the acid tolerance of a number of other bacteria and its conserved nature. It was found that acid adapted cells treated with N,N'-dicyclohexylcarbodiimide (DCCD) exhibited greatly enhanced sensitivity to low pH stress. Degenerate primers were designed to amplify and sequence a portion of the atpD gene. Subsequently, a PCR product from atpA to atpD was identified. While we were unable to create a deletion in the atpA gene, the plasmid pORI19 was inserted in a region between atpA and atpG to reduce, rather than eliminate, expression of the downstream genes. As expected this mutant displayed enhanced resistance to neomycin and exhibited slower growth than the wild type strain. This mutant could still induce an acid tolerance response and remained susceptible to DCCD treatment, but its relative acid sensitivity was difficult to assess as a consequence of its slow growth.

Acid sensitivity of neomycin-resistant mutants of Oenococcus oeni: a relationship between reduction of ATPase activity and lack of malolactic activity

Mutants of Oenococcus oeni were isolated as spontaneous neomycin-resistant mutants. Three of these mutants harbored a significantly reduced ATPase activity that represented 50% of that of the wild-type strain. Their growth rates were also impaired at pH 5.3 (46-86% of the wild-type level). However, the profiles of sugar consumption appeared identical to those of the parental strain. At pH 3.2, all the mutant strains failed to grow and a drastic decrease in viability was observed after an acid shock. Surprisingly, all the isolated mutants were devoid of malolactic activity. These results suggest that the ATPase and malolactic activities of O. oeni are linked to each other and play a crucial role in the mechanism of resistance to an acid stress.

Mechanism of citrate metabolism in Lactococcus lactis: resistance against lactate toxicity at low pH

Measurement of the flux through the citrate fermentation pathway in resting cells of Lactococcus lactis CRL264 grown in a pH-controlled fermentor at different pH values showed that the pathway was constitutively expressed, but its activity was significantly enhanced at low pH. The flux through the citrate-degrading pathway correlated with the magnitude of the membrane potential and pH gradient that were generated when citrate was added to the cells. The citrate degradation rate and proton motive force were significantly higher when glucose was metabolized at the same time, a phenomenon that could be mimicked by the addition of lactate, the end product of glucose metabolism. The results clearly demonstrate that citrate metabolism in L. lactis is a secondary proton motive force-generating pathway. Although the proton motive force generated by citrate in cells grown at low pH was of the same magnitude as that generated by glucose fermentation, citrate metabolism did not affect the growth rate of L. lactis in rich media. However, inhibition of growth by lactate was relieved when citrate also was present in the growth medium. Citrate did not relieve the inhibition by other weak acids, suggesting a specific role of the citrate transporter CitP in the relief of inhibition. The mechanism of citrate metabolism presented here provides an explanation for the resistance to lactate toxicity. It is suggested that the citrate metabolic pathway is induced under the acidic conditions of the late exponential growth phase to make the cells (more) resistant to the inhibitory effects of the fermentation product, lactate, that accumulates under these conditions.

Bacterial solute uptake and efflux systems

The recent discovery of binding protein dependent secondary transporters and the ever-growing family of membrane potential generating secondary transporters emphasize the diversity of transport systems in both the mechanistical and physiological sense. The vast amount of data on the lactose permease is now beginning to crystallize in a model that relates functional events to structural changes of the protein. Evidence has been presented that multidrug transporters pick up their substrates from the membrane, and the binding of a number of substrates to the binding-protein components of ATP-driven transporters is now understood in detail.

Membrane potential-generating malate (MleP) and citrate (CitP) transporters of lactic acid bacteria are homologous proteins. Substrate specificity of the 2-hydroxycarboxylate transporter family

Membrane potential generation via malate/lactate exchange catalyzed by the malate carrier (MleP) of Lactococcus lactis, together with the generation of a pH gradient via decarboxylation of malate to lactate in the cytoplasm, is a typical example of a secondary proton motive force-generating system. The mleP gene was cloned, sequenced, and expressed in a malolactic fermentation-deficient L. lactis strain. Functional analysis revealed the same properties as observed in membrane vesicles of a malolactic fermentation-positive strain. MleP belongs to a family of secondary transporters in which the citrate carriers from Leuconostoc mesenteroides (CitP) and Klebsiella pneumoniae (CitS) are found also. CitP, but not CitS, is also involved in membrane potential generation via electrogenic citrate/lactate exchange. MleP, CitP, and CitS were analyzed for their substrate specificity. The 2-hydroxycarboxylate motif R1R2COHCOOH, common to the physiological substrates, was found to be essential for transport although some 2-oxocarboxylates could be transported to a lesser extent. Clear differences in substrate specificity among the transporters were observed because of different tolerances toward the R substituents at the C2 atom. Both MleP and CitP transport a broad range of 2-hydroxycarboxylates with R substituents ranging in size from two hydrogen atoms (glycolate) to acetyl and methyl groups (citromalate) for MleP and two acetyl groups (citrate) for CitP. CitS was much less tolerant and transported only citrate and at a low rate citromalate. The substrate specificities are discussed in the context of the physiological function of the transporters.

Exchange of glutamate and gamma-aminobutyrate in a Lactobacillus strain

Lactobacillus sp. strain E1 catalyzed the decarboxylation of glutamate (Glu), resulting in a nearly stoichiometric release of the products gamma-aminobutyrate (GABA) and CO2. This decarboxylation was associated with the net synthesis of ATP. ATP synthesis was inhibited almost completely by nigericin and about 70% by N,N'-dicyclohexylcarbodiimide (DCCD), without inhibition of the decarboxylation. These findings are consistent with the possibility that a proton motive force arises from the cytoplasmic proton consumption that accompanies glutamate decarboxylation and the electrogenic Glu/GABA antiporter and the possibility that this proton motive force is coupled with ATP synthesis by DCCD-sensitive ATPase.

In vitro reassembly of the malolactic fermentation pathway of Leuconostoc oenos (Oenococcus oeni)

The mechanism of metabolic energy generation by malolactic fermentation was studied with artificial membrane vesicles of Leuconostoc oenos (Oenococcus oeni). (Note that although L. oenos was recently reclassified as O. oeni [L. M. T. Dicks, F. Dellaglio, and M. D. Collins, Int. J. Syst. Bacteriol. 45:395-397, 1995], the old designation was kept in the present work.) Purified malolactic enzyme was entrapped in artificial membrane vesicles prepared from L. oenos cells able to transport L-malate. We show that the in vitro reconstituted system, including an electrogenic L-malate carrier and the decarboxylating malolactic enzyme, generated a proton motive force that was able to drive intravesicular accumulation of leucine.

The proton motive force generated in Leuconostoc oenos by L-malate fermentation

In cells of Leuconostoc oenos, the fermentation of L-malic acid generates both a transmembrane pH gradient, inside alkaline, and an electrical potential gradient, inside negative. In resting cells, the proton motive force ranged from -170 mV to -88 mV between pH 3.1 and 5.6 in the presence Of L-malate. Membrane potentials were calculated by using a model for probe binding that accounted for the different binding constants at the different pH values at the two faces of the membrane. The delta psi generated by the transport of monovalent malate, H-malate-, controlled the rate of fermentation. The fermentation rate significantly increased under conditions of decreased delta psi, i.e., upon addition of the ionophore valinomycin in the presence of KCl, whereas in a buffer depleted of potassium, the addition of valinomycin resulted in a hyperpolarization of the cell membrane and a reduction of the rate of fermentation. At the steady state, the chemical gradient for H-malate- was of the same magnitude as delta psi. Synthesis of ATP was observed in cells performing malolactic fermentation.

Proton motive force generation by citrolactic fermentation in Leuconostoc mesenteroides

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 HCit2-/H+ symport, indicative of metabolic energy generation by citrate metabolism via a secondary mechanism (C. Marty-Teysset, J. S. Lolkema, P. Schmitt, C. Divies, and W. N. Konings, J. Biol. Chem. 270:25370-25376, 1995). This study also revealed an efficient exchange of citrate and D-lactate, a product of citrate/carbohydrate cometabolism, suggesting that under physiological conditions, CitP may function as a precursor/product exchanger rather than a symporter. In this paper, the energetic consequences of citrate metabolism were investigated in resting cells of L. mesenteroides. The generation of metabolic energy in the form of a pH gradient (delta pH) and a membrane potential (delta psi) by citrate metabolism was found to be largely dependent on cometabolism with glucose. Furthermore, in the presence of glucose, the rates of citrate utilization and of pyruvate and lactate production were strongly increased, indicating an enhancement of citrate metabolism by glucose metabolism. The rate of citrate metabolism under these conditions was slowed down by the presence of a membrane potential across the cytoplasmic membrane. The production of D-lactate inside the cell during cometabolism was shown to be responsible for the enhancement of the electrogenic uptake of citrate. Cells loaded with D-lactate generated a delta psi upon dilution in buffer containing citrate, and cells incubated with citrate built up a pH gradient upon addition of D-lactate. The results are consistent with an electrogenic citrate/D-lactate exchange generating in vivo metabolic energy in the form of a proton electrochemical gradient across the membrane. The generation of metabolic energy from citrate metabolism in L. mesenteroides may contribute significantly to the growth advantage observed during cometabolism of citrate and glucose.

Exchange of aspartate and alanine. Mechanism for development of a proton-motive force in bacteria

We examined the idea that aspartate metabolism by Lactobacillus subsp. M3 is organized as a proton-motive metabolic cycle by using reconstitution to monitor the activity of the carrier, termed AspT, expected to carry out the electrogenic exchange of precursor (aspartate) and product (alanine). Membranes of Lactobacillus subsp. M3 were extracted with 1.25% octyl glucoside in the presence of 0. 4% Escherichia coli phospholipid and 20% glycerol. The extracts were then used to prepare proteoliposomes loaded with either aspartate or alanine. Aspartate-loaded proteoliposomes accumulated external [3H]aspartate by exchange with internal substrate; this homologous self-exchange (Kt = 0.4 mm) was insensitive to potassium or proton ionophores and was unaffected by the presence or absence of Na+, K+, or Mg2+. Alanine-loaded proteoliposomes also took up [3H]aspartate in a heterologous antiport reaction that was stimulated or inhibited by an inside-positive or inside-negative membrane potential, respectively. Several lines of evidence suggest that these homologous and heterologous exchange reactions were catalyzed by the same functional unit. Thus, [3H]aspartate taken up by AspT during self-exchange was released by a delayed addition of alanine. In addition, the spontaneous loss of AspT activity that occurs when a detergent extract is held at 37 degrees C prior to reconstitution was prevented by the presence of either aspartate (KD(aspartate) = 0.3 mm) or alanine (KD(alanine) > or = 10 mm), indicating that both substrates interact directly with AspT. These findings are consistent with operation of a proton-motive metabolic cycle during aspartate metabolism by Lactobacillus subsp. M3.

Membrane potential-generating transport of citrate and malate catalyzed by CitP of Leuconostoc mesenteroides

Citrate uptake in Leuconostoc mesenteroides subsp. mesenteroides 19D is catalyzed by a secondary citrate carrier (CitP). The kinetics and mechanism of CitP were investigated in membrane vesicles of L. mesenteroides. The transporter is induced by the presence of citrate in the medium and transports both citrate and malate. In spite of sequence homology to the Na(+)-dependent citrate carrier of Klebsiella pneumoniae, CitP is not Na(+)-dependent, nor is CitP Mg(2+)-dependent. The pH gradient (delta pH) is a driving force for citrate and malate uptake into the membrane vesicles, whereas the membrane potential (delta psi) counteracts transport. An inverted membrane potential (inside positive) generated by thiocyanide diffusion can drive citrate and malate uptake in membrane vesicles. Analysis of the forces involved showed that a single unit of negative charge is translocated during transport. Kinetic analysis of citrate counterflow at different pH values indicated that CitP transports the dianionic form of citrate (Hcit2-) with an affinity constant of approximately 20 microns. It is concluded that CitP catalyzes Hcit2-/H+ symport. Translocation of negative charge into the cell during citrate metabolism results in the generation of a membrane potential that contributes to the protonmotive force across the cytoplasmic membrane, i.e. citrate metabolism in L. mesenteroides generates metabolic energy. Efficient exchange of citrate and D-lactate, a product of citrate/carbohydrate co-metabolism, is observed, suggesting that under physiological conditions, CitP may function as an electrogenic precursor/product exchanger rather than a symporter. The mechanism and energetic consequences of citrate uptake are similar to malate uptake in lactic acid bacteria.

Role of scalar protons in metabolic energy generation in lactic acid bacteria

Lactic acid bacteria are able to generate a protonmotive force across the cytoplasmic membrane by various metabolic conversions without involvement of substrate level phosphorylation or proton pump activity. Weak acids like malate and citrate are taken up in an electrogenic process in which net negative charge is translocated into the cell thereby generating a membrane potential. The uptake is either an exchange process with a metabolic end-product (precursor/ product exchange) or a uniporter mechanism. Subsequent metabolism of the internalized substrate drives uptake and results in the generation of a pH gradient due to the consumption of scalar protons. The generation of the membrane potential and the pH gradient involve separate steps in the pathway. Here it is shown that they are nevertheless coupled. Analysis of the pH gradient that is formed during malolactic fermentation and citrate fermentation shows that a pH gradient, inside alkaline, is formed only when the uptake system forms a membrane potential, inside negative. These secondary metabolic energy generating systems form a pmf that consists of both a membrane potential and a pH gradient, just like primary proton pumps do. It is concluded that the generation of a pH gradient inside alkaline, upon the addition of a weak acid to cells is diagnostic for an electrogenic uptake mechanism translocating negative charge with the weak acid.

Solute transport and energy transduction in bacteria

In bacteria two forms of metabolic energy are usually present, i.e. ATP and transmembrane ion-gradients, that can be used to drive the various endergonic reactions associated with cellular growth. ATP can be formed directly in substrate level phosphorylation reactions whereas primary transport processes can generate the ion-gradients across the cytoplasmic membrane. The two forms of metabolic energy can be interconverted by the action of ion-translocating ATPases. For fermentative organisms it has long been thought that ion-gradients could only be generated at the expense of ATP hydrolysis by the F0F1-ATPase. In the present article, an overview is given of the various secondary transport processes that form ion-gradients at the expense of precursor (substrate) and/or end-product concentration gradients. The metabolic energy formed by these chemiosmotic circuits contributes to the 'energy status' of the bacterial cell which is particularly important for anaerobic/fermentative organisms.