Low Protonmotive Force Research Articles

Bioenergetic suppression by redox-active metabolites promotes antibiotic tolerance in Pseudomonas aeruginosa

The proton-motive force (PMF), consisting of a pH gradient and a membrane potential (ΔΨ) underpins many processes essential to bacterial growth and/or survival. Yet bacteria often enter a bioenergetically diminished state characterized by a low PMF. Consequently, they have increased tolerance for diverse stressors, including clinical antibiotics. Despite the ubiquity of low metabolic rates in the environment, the extent to which bacteria have agency over entry into such a low-bioenergetic state has received relatively little attention. Here, we tested the hypothesis that production of redox-active metabolites (RAMs) could drive such a physiological transition. Pseudomonas aeruginosa is an opportunistic pathogen that produces phenazines, model RAMs that are highly toxic in the presence of molecular oxygen (O2). Under oxic conditions, the phenazines pyocyanin and phenazine-1-carboximide, as well as toxoflavin-a RAM produced by Burkholderia species-suppress the ΔΨ in distinct ways across distributions of single cells, reduce the efficiency of proton pumping, and lower cellular adenosine-triphosphate (ATP) levels. In planktonic culture, the degree and rate by which each RAM lowers the ΔΨ correlates with the protection it confers against antibiotics that strongly impact cellular energy flux. This bioenergetic suppression requires the RAM's presence and corresponds to its cellular reduction rate and abiotic oxidation rate by O2; it can be reversed by increasing the ΔΨ with nigericin. RAMs similarly impact the bioenergetic state of cells in (hyp)oxic biofilm aggregates. Collectively, these findings demonstrate that bacteria can suppress their bioenergetic state by the production of endogenous toxins in a manner that bolsters stress resilience.

Elevated Membrane Potential as a Tetracycline Resistance Mechanism in Escherichia coli.

The metabolic environment is responsible for antibiotic resistance, which highlights the way in which the antibiotic resistance mechanism works. Here, GC-MS-based metabolomics with iTRAQ-based proteomics was used to characterize a metabolic state in tetracycline-resistant Escherichia coli K12 (E. coli-RTET) compared with tetracycline-sensitive E. coli K12. The repressed pyruvate cycle against the elevation of the proton motive force (PMF) and ATP constructed the most characteristic feature as a consequence of tetracycline resistance. To understand the role of the elevated PMF in tetracycline resistance, PMF inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and the pH gradient were used to investigate how the elevation influences bacterial viability and intracellular antibiotic concentration. A strong synergy was detected between CCCP and tetracycline to the viability, which was consistent with increasing intracellular drug and decreasing external pH. Furthermore, E. coli-RTET and E. coli-RGEN with high and low PMF concentrations were susceptible to gentamicin and tetracycline, respectively. The elevated PMF in E. coli-RTET was attributed to the activation of other metabolic pathways, except for the pyruvate cycle, including a malate-oxaloacetate-phosphoenolpyruvate-pyruvate-malate cycle. These results not only revealed a PMF-dependent mechanism for tetracycline resistance but also provided a solution to tetracycline-resistant pathogens by aminoglycosides and aminoglycoside-resistant bacteria by tetracyclines.

Elevated proton motive force is a tetracycline resistance mechanism that leads to the sensitivity to gentamicin in Edwardsiella tarda.

Tetracycline is a commonly used human and veterinary antibiotic that is mostly discharged into environment and thereby tetracycline-resistant bacteria are widely isolated. To combat these resistant bacteria, further understanding for tetracycline resistance mechanisms is needed. Here, GC-MS based untargeted metabolomics with biochemistry and molecular biology techniques was used to explore tetracycline resistance mechanisms of Edwardsiella tarda. Tetracycline-resistant E. tarda (LTB4-RTET ) exhibited a globally repressed metabolism against elevated proton motive force (PMF) as the most characteristic feature. The elevated PMF contributed to the resistance, which was supported by the three results: (i) viability was decreased with increasing PMF inhibitor carbonylcyanide-3-chlorophenylhydrazone; (ii) survival is related to PMF regulated by pH; (iii) LTB4-RTET were sensitive to gentamicin, an antibiotic that is dependent upon PMF to kill bacteria. Meanwhile, gentamicin-resistant E. tarda with low PMF are sensitive to tetracycline is also demonstrated. These results together indicate that the combination of tetracycline with gentamycin will effectively kill both gentamycin and tetracycline resistant bacteria. Therefore, the present study reveals a PMF-enhanced tetracycline resistance mechanism in LTB4-RTET and provides an effective approach to combat resistant bacteria.

Heterogeneous Distribution of Proton Motive Force in Nonheritable Antibiotic Resistance.

Bacterial infections that are difficult to eradicate are often treated by sequentially exposing the bacteria to different antibiotics. Although effective, this approach can give rise to epigenetic or other phenomena that may help some cells adapt to and tolerate the antibiotics. Characteristics of such adapted cells are dormancy and low energy levels, which promote survival without lending long-term genetic resistance against antibiotics. In this work, we quantified motility in cells of Escherichia coli that adapted and survived sequential exposure to lethal doses of antibiotics. In populations that adapted to transcriptional inhibition by rifampicin, we observed that ~1 of 3 cells continued swimming for several hours in the presence of lethal concentrations of ampicillin. As motility is powered by proton motive force (PMF), our results suggested that many adapted cells retained a high PMF. Single-cell growth assays revealed that the high-PMF cells resuscitated and divided upon the removal of ampicillin, just as the low-PMF cells did, a behavior reminiscent of persister cells. Our results are consistent with the notion that cells in a clonal population may employ multiple different mechanisms to adapt to antibiotic stresses. Variable PMF is likely a feature of a bet-hedging strategy: a fraction of the adapted cell population lies dormant while the other fraction retains high PMF to be able to swim out of the deleterious environment. IMPORTANCE Bacterial cells with low PMF may survive antibiotic stress due to dormancy, which favors nonheritable resistance without genetic mutations or acquisitions. On the other hand, cells with high PMF are less tolerant, as PMF helps in the uptake of certain antibiotics. Here, we quantified flagellar motility as an indirect measure of the PMF in cells of Escherichia coli that had adapted to ampicillin. Despite the disadvantage of maintaining a high PMF in the presence of antibiotics, we observed high PMF in ~30% of the cells, as evidenced by their ability to swim rapidly for several hours. These and other results were consistent with the idea that antibiotic tolerance can arise via different mechanisms in a clonal population.

Contribution of sucrose transporters to phloem unloading within Sorghum bicolor stem internodes

ABSTRACTSucrose produced in source leaves is loaded into collection phloem, transported to sinks and unloaded for utilization or storage. In the context of long distance transport, sucrose transporters (SUTs) can function to load sucrose into collection phloem, retrieve leaked sucrose during long distance transport, and load sucrose into sink cells. SUTs have also been proposed to efflux sucrose under conditions of low proton motive force and low extracellular sucrose. The involvement of sucrose transporters in phloem unloading in a representative monocot stem, Sorghum bicolor, was evaluated during different stages of internode development. Transcript levels and functional properties of selected key transporters were measured, with both cellular and subcellular localization determined.

Mechanism of formate-nitrite transporters by dielectric shift of substrate acidity.

Bacterial formate-nitrite transporters (FNTs) regulate the metabolic flow of small, weak mono-acids. Recently, the eukaryotic PfFNT was identified as the malaria parasite's lactate transporter and novel drug target. Despite crystal data, central mechanisms of FNT gating and transport remained unclear. Here, we show elucidation of the FNT transport mechanism by single-step substrate protonation involving an invariant lysine in the periplasmic vestibule. Opposing earlier gating hypotheses and electrophysiology reports, quantification of total uptake by radiolabeled substrate indicates a permanently open conformation of the bacterial formate transporter, FocA, irrespective of the pH Site-directed mutagenesis, heavy water effects, mathematical modeling, and simulations of solvation imply a general, proton motive force-driven FNT transport mechanism: Electrostatic attraction of the acid anion into a hydrophobic vestibule decreases substrate acidity and facilitates protonation by the bulk solvent. We define substrate neutralization by proton transfer for transport via a hydrophobic transport path as a general theme of the Amt/Mep/Rh ammonium and formate-nitrite transporters.

Editorial: Enzymes from Extreme Environments.

Editorial: Enzymes from Extreme Environments.

Probing the Interaction of the Diarylquinoline TMC207 with Its Target Mycobacterial ATP Synthase

Infections with Mycobacterium tuberculosis are substantially increasing on a worldwide scale and new antibiotics are urgently needed to combat concomitantly emerging drug-resistant mycobacterial strains. The diarylquinoline TMC207 is a highly promising drug candidate for treatment of tuberculosis. This compound kills M. tuberculosis by binding to a new target, mycobacterial ATP synthase. In this study we used biochemical assays and binding studies to characterize the interaction between TMC207 and ATP synthase. We show that TMC207 acts independent of the proton motive force and does not compete with protons for a common binding site. The drug is active on mycobacterial ATP synthesis at neutral and acidic pH with no significant change in affinity between pH 5.25 and pH 7.5, indicating that the protonated form of TMC207 is the active drug entity. The interaction of TMC207 with ATP synthase can be explained by a one-site binding mechanism, the drug molecule thus binds to a defined binding site on ATP synthase. TMC207 affinity for its target decreases with increasing ionic strength, suggesting that electrostatic forces play a significant role in drug binding. Our results are consistent with previous docking studies and provide experimental support for a predicted function of TMC207 in mimicking key residues in the proton transfer chain and blocking rotary movement of subunit c during catalysis. Furthermore, the high affinity of TMC207 at low proton motive force and low pH values may in part explain the exceptional ability of this compound to efficiently kill mycobacteria in different microenvironments.

Adaptations of the ATP synthase a-subunit to support synthesis at low protonmotive force at both pH 7.5 and 10.5 may underpin the more stringent requirement for lysine-180 in TMH-4 by alkaliphilic Bacillus pseudofirmus OF4 than by more modestly alkaliphilic thermoalkaliphile Bacillus sp. TA2.A1

Adaptations of the ATP synthase a-subunit to support synthesis at low protonmotive force at both pH 7.5 and 10.5 may underpin the more stringent requirement for lysine-180 in TMH-4 by alkaliphilic Bacillus pseudofirmus OF4 than by more modestly alkaliphilic thermoalkaliphile Bacillus sp. TA2.A1

Interaction between Cytochrome caa3 and F1F0-ATP Synthase of Alkaliphilic Bacillus pseudofirmus OF4 Is Demonstrated by Saturation Transfer Electron Paramagnetic Resonance and Differential Scanning Calorimetry Assays

Interaction between the cytochrome caa3 respiratory chain complex and F1F0-ATP synthase from extremely alkaliphilic Bacillus pseudofirmus OF4 has been hypothesized to be required for robust ATP synthesis by this alkaliphile under conditions of very low protonmotive force. Here, such an interaction was probed by differential scanning calorimetry (DSC) and by saturation transfer electron paramagnetic resonance (STEPR). When the two purified complexes were embedded in phospholipid vesicles individually [(caa3)PL, (F1F0)PL)] or in combination [(caa3 + F1F0)PL] and subjected to DSC analysis, they underwent exothermic thermodenaturation with transition temperatures at 69, 57, and 46/75 degrees C, respectively. The enthalpy change, deltaH (-8.8 kcal/mmol), of protein-phospholipid vesicles containing both cytochrome caa3 and F1F0 was smaller than that (-12.4 kcal/mmol) of a mixture of protein-phospholipid vesicles formed from each individual electron transfer complex [(caa3)PL + (F1F0)PL]. The rotational correlation time of spin-labeled caa3 (65 micros) in STEPR studies increased significantly when the complex was mixed with F1F0 prior to being embedded in phospholipid vesicles (270 micros). When the complexes were reconstituted separately and then mixed together, or either mitochondrial cytochrome bc1 or F1F0 was substituted for the alkaliphile F1F0, the correlation time was unchanged (65-70 micros). Varying the ratio of the two alkaliphile complexes in both the DSC and STEPR experiments indicated that the optimal stoichiometry is 1:1. These results demonstrate a physical interaction between the cytochrome caa3 and F1F0-ATP synthase from B. pseudofirmus OF4 in a reconstituted system. They support the suggestion that such an interaction between these complexes may contribute to sequestered proton transfers during alkaliphile oxidative phosphorylation at high pH.

Bacillus subtilis YqkI Is a Novel Malic/Na+-Lactate Antiporter That Enhances Growth on Malate at Low Protonmotive Force

Bacillus subtilis yheL encodes a Na(+)/H(+) antiporter, whereas its paralogue, yqkI, encodes a novel antiporter that achieves a simultaneous Na(+)/H(+) and malolactate antiport. B. subtilis yufR, a control in some experiments, encodes a Na(+)/malate symporter. YqkI complemented a malate transport mutant of Escherichia coli if Na(+) and lactate were present. YheL conferred Na(+) uptake capacity on everted membrane vesicles from an antiporter-deficient E. coli mutant that was consistent with a secondary Na(+)/H(+) antiport, but YqkI-dependent Na(+) uptake depended on intravesicular malate and extravesicular lactate. YqkI-dependent lactate uptake depended on intravesicular malate and extravesicular Na(+). YqkI mediated an electroneutral exchange, which is proposed to be a malic(-2)-2H(+) (or fully protonated malate)/Na(+)-lactate(-1) antiport. Because the composite YqkI-mediated exchanges could be driven by gradients of the malate-lactate pair, this transporter could play a role in growth of B. subtilis on malate at low protonmotive force. A mutant with a disruption of yqkI exhibited an abrupt arrest in the mid-logarithmic phase of growth on malate when low concentrations of protonophore were present. Thus growth of B. subtilis to high density on a putatively nonfermentative dicarboxylic acid substrate depends on a malolactate exchange at suboptimal protonmotive force.

Redundancy of aerobic respiratory chains in bacteria? Routes, reasons and regulation

Bacteria are the most remarkable organisms in the biosphere, surviving and growing in environments that support no other life forms. Underlying this ability is a flexible metabolism controlled by a multitude of environmental sensors and regulators of gene expression. It is not surprising, therefore, that bacterial respiration is complex and highly adaptable: virtually all bacteria have multiple, branched pathways for electron transfer from numerous low-potential reductants to several terminal electron acceptors. Such pathways, particularly those involved in anaerobic respiration, may involve periplasmic components, but the respiratory apparatus is largely membrane-bound and organized such that electron flow is coupled to proton (or sodium ion) transport, generating a protonmotive force. It has long been supposed that the multiplicity of pathways serves to provide flexibility in the face of environmental stresses, but the existence of apparently redundant pathways for electrons to a single acceptor, say dioxygen, is harder to explain. Clues have come from studying the expression of oxidases in response to growth conditions, the phenotypes of mutants lacking one or more oxidases, and biochemical characterization of individual oxidases. Terminal oxidases that share the essential properties of substrate (cytochrome c or quinol) oxidation, dioxygen reduction and, in some cases, proton translocation, differ in subunit architecture and complement of redox centres. Perhaps more significantly, they differ in their affinities for oxidant and reductant, mode of regulation, and inhibitor sensitivity; these differences to some extent rationalize the presence of multiple oxidases. However, intriguing requirements for particular functions in certain physiological functions remain unexplained. For example, a large body of evidence demonstrates that cytochrome bd is essential for growth and survival under certain conditions. In this review, the physiological basis of the many phenotypes of Cyd-mutants is explored, particularly the requirement for this oxidase in diazotrophy, growth at low protonmotive force, survival in the stationary phase, and resistance to oxidative stress and Fe(III) chelators.

Dual Mechanisms of Tricarboxylate Transport and Catabolism by Acidaminococcus fermentans

Acidaminococcus fermentans utilized citrate or the citrate analog aconitate as an energy source for growth, and these tricarboxylates were used simultaneously. Citrate utilization and uptake showed biphasic kinetics. High-affinity citrate uptake had a K(t) of 40 muM, but the V(max) was only 25 nmol/mg of protein per min. Low-affinity citrate utilization had a 10-fold higher V(max), but the K(s) was greater than 1.0 mM. Aconitate was a competitive inhibitor (K(i) = 34muM) of high-affinity citrate uptake, but low-affinity aconitate utilization had a 10-fold-lower requirement for sodium than did low-affinity citrate utilization. On the basis of this large difference in sodium requirements, it appeared that A. fermentans probably has two systems of tricarboxylate uptake: (i) a citrate/aconitate carrier with a low affinity for sodium and (ii) an aconitate carrier with a high affinity for sodium. Citrate was catabolized by a pathway involving a biotin-requiring, avidin-sensitive, sodium-dependent, membrane-bound oxaloacetate decarboxylase. The cells also had aconitase, but this enzyme was unable to convert citrate to isocitrate. Since cell-free extracts converted either aconitate or glutamate to 2-oxoglutarate, it appeared that aconitate was being catabolized by the glutaconyl-CoA decarboxylase pathway. Exponentially growing cultures on citrate or citrate plus aconitate were inhibited by the sodium/proton antiporter, monensin. Because monensin had no effect on cultures growing with aconitate alone, it appeared that citrate metabolism was acting as an inducer of monensin sensitivity. A. fermentans cells always had a low proton motive force (<50 mV), and cells treated with the protonophore TCS (3,3',4',5-tetrachlorosalicylanide) grew even though the proton motive force was less than 20 mV. On the basis of these results, it appeared that A. fermentans was depending almost exclusively on a sodium motive force for its membrane energetics.

Molar growth yields and bioenergetic parameters of extremely alkaliphilic Bacillus species in batch cultures, and growth in a chemostat at pH 10.5

Alkaliphilic Bacillus species that grow at pH 10.5 must cope with a low protonmotive force (-50 mV) due to a reversed transmembrane pH gradient at least 2 pH units more acid inside. Here we demonstrate that strictly alkaliphilic B. firmus RAB and two strains of B. alcalophilus (ATCC 27467 and DSM 485) grow exponentially in batch cultures with a doubling time of less than 1 h in 100 mM buffered medium, while the actual medium pH remains above 10.2. The ATCC strain continued to grow rapidly for at least 7 h, but the growth rate of the DSM strain declined dramatically after 3 h. However, both the B. alcalophilus strains, B. firmus RAB and facultatively alkaliphilic B. firmus OF4 were readily maintained for at least 24 h between pH 10.4 and 10.6 in a chemostat where nutrients were constantly replenished. A critical nutrient may be limiting in batch cultures of the DSM strain of B. alcalophilus. The facultative alkaliphile grew equally well in batch cultures at an initial pH of 7.5 or 10.5. Its molar growth yield (23 mg dry wt mmol-1) on malate (Ymal) was the same at the two pH values and was comparable to Ymal for B. subtilis grown at neutral pH. B. firmus RAB and B. alcalophilus ATCC 27467 grown at pH 10.5 also showed Ymal values at least as high as the neutralphile, indicating efficient use of the energy source even at low protonmotive force. Moreover, the phosphorylation potential of B. firmus OF4 grown at pH 7.5 (45.2 kJ mol-1) or pH 10.5 (46 kJ mol-1) was in a conventional range for bacteria.

Immunoelectron microscopic localization of the F 1F 0 ATPase (ATP synthase) on the cytoplasmic membrane of alkalophilic Bacillus firmus RAB

Evidence that the F 1F 0 ATPase (ATP synthase) of alkalophilic Bacillus firmus RAB is localized exclusively on the cytoplasmic membrane was obtained by immunogold electron microscopy using a highly specific polyclonal antibody against the β subunit of Escherichia coli F 1F 0 ATPase. The energetic problem faced by cells of B. firmus RAB growing oxidatively at pH 10.5 despite a low protonmotive force across the cytoplasmic membrane cannot, therefore, be circumvented by localization of energy transducing functions on hypothetical internal membranes.

Effects of K+ on the proton motive force of Bradyrhizobium sp. strain 32H1.

In previous studies, respiring Bradyrhizobium sp. strain 32H1 cells grown under 0.2% O2, conditions that derepress N2 fixation, were found to have a low proton motive force of less than -121 mV, because of a low membrane potential (delta psi). In contrast, cells grown under 21% O2, which do not fix N2, had high proton motive force values of -175 mV or more, which are typical of respiring bacteria, because of high delta psi values. In the present study, we found that a delta psi of 0 mV in respiring cells requires growth in relatively high-[K+] media (8 mM), low O2 tension, and high internal [K+]. When low-[O2], high-[K+]-grown cells were partially depleted of K+, the delta psi was high. When cells were grown under 21% O2 or in media low in K+ (50 microM K+), the delta psi was again high. The transmembrane pH gradient was affected only slightly by varying the growth or assay conditions. In addition, low-[O2], high-[K+]-grown cells had a greater proton permeability than did high-[O2]-grown cells. To explain these findings, we postulate that cells grown under conditions that derepress N2 fixation contain an electrogenic K+/H+ antiporter that is responsible for the dissipation of the delta psi. The consequence of this alteration in K+ cycling is rerouting of proton circuits so that the putative antiporter becomes the major pathway for H+ influx, rather than the H+-ATP synthase.

The puzzle of high phosphorylation potential and low protonmotive force generated by oxidative phosphorylation in Paracoccus denitrificans membrane vesicles in the presence of nitrate

The puzzle of high phosphorylation potential and low protonmotive force generated by oxidative phosphorylation in <i>Paracoccus denitrificans</i> membrane vesicles in the presence of nitrate

The steady-state counterclockwise/clockwise ratio of bacterial flagellar motors is regulated by protonmotive force

A new aspect of the regulation of the bacterial flagellar motor is described. Protonmotive force, i.e. proton electrochemical potential across the cell membrane, not only drives motility, but was found to exert a regulatory effect on the switching of the motor between counterclockwise and clockwise senses. When motor speed was reduced by reducing protonmotive force (but not when speed was reduced by increasing external load), the motor spent a higher fraction of time in counter-clockwise rotation, although the speed in both senses remained equal; at approximately 80% of the maximum motor speed under any given external load condition, no detectable clockwise rotation remained in most cells, i.e. the motor no longer reversed and so cells swam without tumbling. The altered state of the motor remained as long as the state of low protonmotive force remained. Therefore the phenomenon was not a transient tactic response, although it did modify the response to chemotactic stimulation. The regulation of counter-clockwise xxx clockwise switching by protonmotive force appears to be rather general, having been demonstrated in Salmonella, Escherichia coli and Bacillus subtilis. A moderate reduction in protonmotive force from its normal level in actively metabolizing cells did not affect motor reversibility. Motor speed was also unaffected under these conditions (Khan & Macnab, following paper). The existence of a saturation regime with regard to both speed and reversibility suggests that the regulation of motor switching may involve the actual protons driving the motor. All general chemotaxis mutants tested, including those known to be defective in the chemotactic methylation system, were susceptible to protonmotive force regulation in a manner consistent with their normal steady-state counter-clockwise/clockwise ratio. Even mutants that appear “counterclockwise only” could be made more so upon lowering protonmotive force, in that their capacity for clockwise tactic responses was impaired. Since responses to attractants and repellents do not involve an alteration in speed, we conclude that protonmotive force and chemotactic regulatory inputs to the motor operate in parallel. the clockwise/counterclockwise ratio showed an increasing exponential dependence on motor speed. This was a result of both an increased counterclockwise→clockwise switching probability and a decreased clockwise→counterclockwise switching probability. These probabilities were found to be inversely related, so that their product was roughly independent of motor speed. An examination of the pattern of clockwise/counterclockwise switching in tactically stimulated cells and in chemotactic mutants suggests that this is a fundamental property of the motor, which may indicate that switching events in both directions are under the same controlling mechanism. We suggest that suppression of tumbling in circumstances of metabolic deprivation might have survival value by permitting a more rapid dispersal of a population from a region of scarcity of food or oxygen, even when no local gradient is present.