Proton chemical potential, proton electrical potential and bacterial motility

Na+-driven bacterial flagellar motors

Bacterial flagellar motors are the reversible rotary engine which propels the cell by rotating a helical flagellar filament as a screw propeller. The motors are embedded in the cytoplasmic membrane, and the energy for rotation is supplied by the electrochemical potential of specific ions across the membrane. Thus, the analysis of motor rotation at the molecular level is linked to an understanding of how the living system converts chemical energy into mechanical work. Based on the coupling ions, the motors are divided into two types; one is the H+-driven type found in neutrophiles such as Bacillus subtilis and Escherichia coli and the other is the Na+-driven type found in alkalophilic Bacillus and marine Vibrio. In this review, we summarize the current status of research on the rotation mechanism of the Na+-driven flagellar motors, which introduces several new aspects in the analysis.

A model for bacterial flagellar motor: free energy transduction and self-organization of rotational motion

A bacterial flagellar motor is an energy transducing molecular machine which shows some attractive characteristics. First, this motor is driven by a protonmotive force (PMF) across the membrane, two components of which, electric potential delta psi and chemical potential -(2.3RT/F)delta pH, are equivalently transduced to the mechanical work of the motor rotation. Second, a PMF threshold for rotation is observed. Third, this motor can rotate reversibly either counterclockwise (CCW) or clockwise (CW) at almost the same speed. To clarify the osmomechanical coupling of this motor, these characteristics must be explained consistently at the molecular level. In this paper, in order to allow quantitative analyses of the above characteristics, a theoretical model of a bacterial flagellar motor is constructed assuming that the torque generating sites are electrodes which can be charged by protons and that the electrostatic interaction between the electrodes generates the rotation torque. Electrode reaction reasonably derives the equivalence of delta psi and -(2.3RT/F)delta pH. In this model, rates of charging and discharging of protons are influenced by the motor rotation rate, so that the torque generating sites co-operatively work through the motor rotation. We named this kind of co-operativity among them "dynamic co-operativity" in torque generation. This co-operativity causes autocatalytic generation of motor torque and the existence of the rotation threshold. In this model, the appearance of the stable rotational states can be described by phase transition caused by the dynamic co-operativity among torque generating sites. According to this model, the flagellar motor has two stable rotational states corresponding to CCW and CW, which show the same torques. The motor selects one direction from them to rotate, and that is self-organization of rotational motion. Interpretation of the transition between the two stable rotational states as the chemotactic reversals of the flagellar motor is also discussed.

Dextransucrase secretion in Leuconostoc mesenteroides depends on the presence of a transmembrane proton gradient

The relationship between proton motive force and the secretion of dextransucrase in Leuconostoc mesenteroides was investigated. L. mesenteroides was able to maintain a constant proton motive force of -130 mV when grown in batch fermentors at pH values 5.8 to 7.0. The contribution of the membrane potential and the transmembrane pH gradient varied depending on the pH of the growth medium. The differential rate of dextransucrase secretion was relatively constant at 1,040 delta mU/delta mg (dry weight) when cells were grown at pH 6.0 to 6.7. Over this pH range, the internal pH was alkaline with respect to the external pH. When cells were grown at alkaline pH values, dextransucrase secretion was severely inhibited. This inhibition was accompanied by an inversion of the pH gradient as the internal pH became more acidic than the external pH. Addition of nigericin to cells at alkaline pH partially dissipated the inverted pH gradient and produced a fourfold stimulation of dextransucrase secretion. Treatment of cells with the lipophilic cation methyltriphenylphosphonium had no effect on the rate of dextransucrase secretion at pH 5.5 but inhibited secretion by 95% at pH 7.0. The reduced rate of secretion correlated with the dissipation of the proton motive force by this compound. Values of proton motive force greater than -90 mV were required for maximal rates of dextransucrase secretion. The results of this study indicate that dextransucrase secretion in L. mesenteroides is dependent on the presence of a proton gradient across the cytoplasmic membrane that is directed into the cell.

Analysis of bacterial flagellar rotation

Bacterial flagella have rotary motors at their base; embedded in the cytoplasmic membrane and powered by transmembrane ion gradients instead of ATP. Assays have been developed to measure the torque output of individual motors over a wide regime of load, to correlate the energizing proton flux with rotation speed and relate through genetic analysis motor structure to function. These assays promise substantial advances in understanding mechanochemical coupling in these motors. Here, I summarize the present status of our understanding of energy transduction in bacterial flagella and compare this with the case for muscle.

Torque and rotation rate of the bacterial flagellar motor

This paper describes an analysis of microscopic models for the coupling between ion flow and rotation of bacterial flagella. In model I it is assumed that intersecting half-channels exist on the rotor and the stator and that the driving ion is constrained to move together with the intersection site. Model II is based on the assumption that ion flow drives a cycle of conformational transitions in a channel-like stator subunit that are coupled to the motion of the rotor. Analysis of both mechanisms yields closed expressions relating the torque M generated by the flagellar motor to the rotation rate v. Model I (and also, under certain assumptions, model II) accounts for the experimentally observed linear relationship between M and v. The theoretical equations lead to predictions on the relationship between rotation rate and driving force which can be tested experimentally.

Sodium ion transport decarboxylases and other aspects of sodium ion cycling in bacteria

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The stall torque of the bacterial flagellar motor

The bacterial flagellar motor couples the flow of protons across the cytoplasmic membrane to the rotation of a helical flagellar filament. Using tethered cells, we have measured the stall torque required to block this rotation and compared it with the torque of the running motor over a wide range of values of proton-motive force and pH. The stall torque and the running torque vary identically: both appear to saturate at large values of the proton-motive force and both decrease at low or high pH. This suggests that up to speeds of approximately 5 Hz the operation of the motor is not limited by the mobility of its internal components or the rates of proton transfer reactions coupled to flagellar rotation.

Homeoviscous adaptation, growth rate, and morphogenesis in bacteria

Fluorescence polarization, P, of 1,6-diphenyl-1,3,5-hexatriene was studied in Escherichia coli B/r. Modification of nutritional conditions was not compensated by homeoviscous adaptation, demonstrated to exist for temperature variations. Cell diameter, which is known also to vary with nutrition but not with temperature, was found to be positively correlated with 1/P, and may therefore be regulated by membrane lipid order and fluidity.

Role of proton motive force in phototactic and aerotactic responses of Rhodopseudomonas sphaeroides

Rhodopseudomonas sphaeroides grown under nonrigorous anaerobic conditions in the light developed components of a branched respiratory electron transfer chain, and a photosynthetic electron transfer chain. Both respiratory pathways were sensitive to rotenone and high concentrations of cyanide, but oxygen uptake was only partially inhibited by the addition of low concentrations of cyanide or antimycin A. When incubated anaerobically in the dark, R. sphaeroides responded positively to an oxygen gradient in the absence of rotenone. In the presence of rotenone, aerotaxis only occurred when the antimycin A-sensitive branch of the pathway was functioning, although both branches still reduced oxygen. Although there was electron movement along the respiratory chain, aerotaxis only occurred in response to a change in proton motive force. When incubated anaerobically in the light, the movement of R. sphaeroides up a light gradient depended on photosynthetic electron transport. When incubated aerobically, high-intensity actinic illumination inhibited oxygen uptake and aerotaxis. In a low-intensity light gradient the phototactic response was inhibited by oxygen. These results are discussed in relation to the interaction of the electron transfer chains and their roles in controlling tactic responses in R. sphaeroides.

Bacterial chemotaxis: biochemistry of behavior in a single cell

Bacterial chemotaxis is a primitive behavioral system that shows great promise for being amenable to a description of its molecular mechanism. In Gram-negatives like Escherichia coli, addition of amino acid attractant begins a series of events, starting with binding to certain intrinsic membrane proteins, the MCPs, and ending with a period of smooth swimming. Immediately, methyl-esterification of these MCPs begins and continues during this period. By contrast in the Gram-positive Bacillus subtilis, demethylation of MCPs occurs during the same period. At least two other mechanisms for mediating chemotaxis toward the attractants oxygen and phosphotransferase sugars exist in E. coli, and in these, changes in methylation of MCPs plays no role. Moreover, chemotaxis away from many repellents by B. subtilis appears to involve different mechanisms. Many of the repellents include drugs and toxicants, many of them man-made, so that chemoreceptors could not have specifically evolved; yet the bacteria are often exquisitely sensitive to them. Indeed, the B. subtilis membrane seems to act like a generalized antenna for noxious membrane-active substances.

Voltage clamp effects on bacterial chemotaxis

To examine whether or not sensory signaling in bacteria is by way of fluctuations in membrane potential, we studied the effect of clamping the potential on bacterial chemotaxis. The potential was clamped by valinomycin, a K+ -specific ionophore, in the presence of K+. Despite the clamped potential, sensory signaling did occur: both Escherichia coli and Bacillus subtilis cells were still excitable and adaptable under these conditions. It is concluded that signaling in the excitation and adaptation steps of chemotaxis is not by way of fluctuations in the membrane potential.

Unique developmental characteristics of the swarm and short cells of Proteus vulgaris and Proteus mirabilis

Swarming cells of Proteus mirabilis and Proteus vulgaris could be distinguished from their short-cell counterparts by virtue of their synthesis (or lack of synthesis) of certain enzymes and outer membrane proteins. Urease synthesis was constitutive in swarm cells and uninducible in short cells. In contrast, phenylalanine deaminase was inducible in both short and swarm cells, demonstrating that transcriptional and translational processes were functional. During swarm cell development, the amount of one outer membrane protein (45 kilodaltons) fell and the amounts of two others (50 and 28.3 kilodaltons) rose significantly, the level of cytochrome b decreased, and the synthesis of cytochromes a and d were repressed. Respiratory activities of swarm cells were greatly diminished, suggesting that energy for swarming came from fermentation rather than from respiration. Widespread changes in the pattern of enzyme activities, in cytochrome composition, and in the composition and type of outer membrane proteins suggest that they are due to transcriptional regulation.

Minimal requirements for rotation of bacterial flagella

An in vitro system of cell envelopes from Salmonella typhimurium with functional flagella was used to determine the minimal requirements for flagellar rotation. Rotation in the absence of cytoplasmic constituents could be driven either by respiration or by an artificially imposed chemical gradient of protons. No specific ionic requirements other than protons (or hydroxyls) were found for the motor function.

Direction of flagellar rotation in bacterial cell envelopes

Cell envelopes with functional flagella, isolated from wild-type strains of Escherichia coli and Salmonella typhimurium by formation of spheroplasts with penicillin and subsequent osmotic lysis, demonstrate counterclockwise (CCW)-biased rotation when energized with an electron donor for respiration, DL-lactate. Since the direction of flagellar rotation in bacteria is central to the expression of chemotaxis, we studied the cause of this bias. Our main observations were: (i) spheroplasts acquired a clockwise (CW) bias if instead of being lysed they were further incubated with penicillin; (ii) repellents temporarily caused CW rotation of tethered bacteria and spheroplasts but not of their derived cell envelopes; (iii) deenergizing CW-rotating cheV bacteria by KCN or arsenate treatment caused CCW bias; (iv) cell envelopes isolated from CW-rotating cheC and cheV mutants retained the CW bias, unlike envelopes isolated from cheB and cheZ mutants, which upon cytoplasmic release lost this bias and acquired CCW bias; and (v) an inwardly directed, artificially induced proton current rotated tethered envelopes in CCW direction, but an outwardly directed current was unable to rotate the envelopes. It is concluded that (i) a cytoplasmic constituent is required for the expression of CW rotation (or repression of CCW rotation) in strains which are not defective in the switch; (ii) in the absence of this cytoplasmic constituent, the motor is not reversible in such strains, and it probably is mechanically constricted so as to permit CCW sense of rotation only; (iii) the requirement of CW rotation for ATP is not at the level of the motor or the switch but at one of the preceding functional steps of the chemotaxis machinery; (iv) the cheC and cheV gene products are associated with the cytoplasmic membrane; and (v) direct interaction between the switch-motor system and the repellent sensors is improbable.

Utilization of energy stored in the form of Na+ and K+ ion gradients by bacterial cells

The hypothesis that Na+ and K+ gradients have an energy storing function [V. P. Skulachev (1978) FEBS Lett. 87, 171-176] has been tested in experiments with Escherichia coli, the marine bacterium Vibrio harveyi, an extremely halophilic Halobacterium halobium and a fresh-water cyanobacterium Phormidium uncinatum from Lake Baikal living at an extremely low salt concentration. The capability of these microorganisms to maintain delta microH was compared using motility as a delta microH-supported function. It was found that in all cases the gradient of monovalent cations is competent to prolong the period of active motility after other energy sources are exhausted. Maximal prolongation was found in H. halobium, which in a Na+ medium was still motile when light was switched off for 9 h under anaerobic conditions. In V. harveyi the motility was maintained for 1 h, in E. coli for about 10 min and in Ph. uncinatum for about 2 min. Thus the delta microH buffer capacity of the monovalent cation gradient is proportional to the content of these cations in the habitat. It was also found that in Ph. uncinatum only delta pK is effective, whereas in E. coli and V. harveyi both delta pK and delta pNa are. In E. coli when the K+ release is completed and the cells become motionless, motility can be temporarily restored by adding NaCl which initiates an H+ efflux. Under conditions of exhaustion of energy sources, the Na+ and K+ gradient was shown to stabilize potential in H. halobium cells, measured with a tetraphenylphosphonium probe. In H. halobium and E. coli, the anaerobic ATP level was found to stabilize when the Na+ and K+ gradients were present. Addition of N,N'-dicyclohexylcarbodiimide destabilized this level, which indicated that Na+ and K+ gradients could support de novo ATP synthesis. It is concluded that the data obtained are in agreement with the concept of the energy storing by the Na+ and K+ gradients. Other functions of these gradients and the mechanisms of their formation are discussed.

The use of valinomycin, nigericin and trichlorocarbanilide in control of the protonmotive force in Escherichia coli cells

Valinomycin, nigericin and trichlorocarbanilide were assessed for their ability to control the protonmotive force in Escherichia coli cells. Valinomycin, at high K+ concentrations, was found to decrease the membrane potential delta phi and indirectly to decrease the pH gradient delta pH. Nigericin was found to have two modes of action. At low concentrations (0.05-2 microM) it carried out K+/H+ exchange and decreased delta pH. At higher concentrations (50 microM) it carried out a K+-dependent transfer of H+, decreasing both delta phi and delta pH. In EDTA-treated cells only the latter mode of action was evident, whereas in a mutant sensitive to deoxycholate both types of effect were observed. Trichlorocarbanilide is proposed as an alternative to nigericin for the specific control of delta pH, and it can be used in cells not treated with EDTA.

Proton electrochemical gradients in washed cells of Nitrosomonas europaea and Nitrobacter agilis

The components of the proton motive force (Deltap), namely, membrane potential (Deltapsi) and transmembrane pH gradient (DeltapH), were determined in the nitrifying bacteria Nitrosomonas europaea and Nitrobacter agilis. In these bacteria both Deltapsi and DeltapH were dependent on external pH. Thus at pH 8.0, Nitrosomonas europaea and Nitrobacter agilis had Deltapsi values of 173 mV and 125 mV (inside negative), respectively, as determined by the distribution of the lipophilic cation [(3)H]tetraphenyl phosphonium. Intracellular pH was determined by the distribution of two weak acids, (14)C-benzoic and (14)C-acetyl salicylic, and the weak base [(14)C]methylamine. Nitrosomonas europaea accumulated (14)C-benzoic acid and (14)C-acetyl salicylic acid when the external pH was below 7.0 and [(14)C]methylamine at alkaline pH. Similarly, Nitrobacter agilis accumulated the two weak acids below an external pH of about 7.5 and [(14)C]methylamine above this pH. As these bacteria grow best between pH 7.5 and 8.0, they do not appear to have a DeltapH (inside alkaline). Thus, above pH 7.0 for Nitrosomonas europaea and pH 7.5 for Nitrobacter agilis, Deltapsi only contributed to Deltap. In Nitrosomonas europaea the total Deltap remained almost constant (145 to 135 mV) when the external pH was varied from 6 to 8.5. In Nitrobacter agilis, Deltap decreased from 178 mV (inside negative) at pH 6.0 to 95 mV at pH 8.5. Intracellular pH in Nitrosomonas europaea varied from 6.3 at an external pH of 6.0 to 7.8 at external pH 8.5. In Nitrobacter agilis, however, intracellular pH was relatively constant (7.3 to 7.8) over an external pH range of 6 to 8.5. In Nitrosomonas europaea, Deltap and its components (Deltapsi and DeltapH) remained constant in cells at various stages of growth, so that the metabolic state of cells did not affect Deltap. Such an experiment was not possible with Nitrobacter agilis because of low cell yields. The effects of protonophores and ATPase inhibitors on DeltapH and Deltapsi in the two nitrifying bacteria are considered.

Asynchronous switching of flagellar motors on a single bacterial cell

Salmonella possesses several flagella, each capable of counterclockwise and clockwise rotation. Counterclockwise rotation produces swimming, clockwise rotation produces tumbling. Switching between senses occurs stochastically. The rotational sense of individual flagella on a single cell could be monitored under special conditions (partially de-energized cells of cheC and cheZ mutants). Switching was totally asynchronous, indicating that the stochastic process operates at the level of the individual organelle. Coordinated rotation in the flagellar bundle during swimming may therefore derive simply from a high counterclockwise probability enhanced by mechanical interactions, and not from a synchronizing switch mechanism. Different flagella on a given cell had different switching probabilities, on a time scale (greater than 2 min) spanning many switching events. This heterogeneity may reflect permanent structural differences, or slow fluctuations in some regulatory process.

Flagellated ectosymbiotic bacteria propel a eucaryotic cell

A devescovinid flagellate from termites exhibits rapid gliding movements only when in close contact with other cells or with a substrate. Locomotion is powered not by the cell's own flagella nor by its remarkable rotary axostyle, but by the flagella of thousands of rod bacteria which live on its surface. That the ectosymbiotic bacteria actually propel the protozoan was shown by the following: (a) the bacteria, which lie in specialized pockets of the host membrane, bear typical procaryotic flagella on their exposed surface; (b) gliding continues when the devescovinid's own flagella and rotary axostyle are inactivated; (c) agents which inhibit bacterial flagellar motility, but not the protozoan's motile systems, stop gliding movements; (d) isolated vesicles derived from the surface of the devescovinid rotate at speeds dependent on the number of rod bacteria still attached; (e) individual rod bacteria can move independently over the surface of compressed cells; and (f) wave propagation by the flagellar bundles of the ectosymbiotic bacteria is visualized directly by video-enhanced polarization microscopy. Proximity to solid boundaries may be required to align the flagellar bundles of adjacent bacteria in the same direction, and/or to increase their propulsive efficiency (wall effect). This motility-linked symbiosis resembles the association of locomotory spirochetes with the Australian termite flagellate Mixotricha (Cleveland, L. R., and A. V. Grimstone, 1964, Proc. R. Soc. Lond. B Biol. Sci., 159:668-686), except that in our case propulsion is provided by bacterial flagella themselves. Since bacterial flagella rotate, an additional novelty of this system is that the surface bearing the procaryotic rotary motors is turned by the eucaryotic rotary motor within.

Role of proton motive force in genetic transformation of Bacillus subtilis

This study explored the role of the proton motive force in the processes of DNA binding and DNA transport of genetic transformation of Bacillus subtilis 168 strain 8G-5 (trpC2). Transformation was severely inhibited by the ionophores valinomycin, nigericin, and 3,5-di-tert-4-hydroxybenzylidenemalononitrite (SF-6847) and by tetraphenylphosphonium. The ionophores valinomycin and nigericin also severely inhibited binding of transforming DNA to the cell envelope, whereas SF-6847 and carbonylcyanide-p-trifluoromethoxyphenylhydrazone hardly affected binding. The proton motive force, therefore, does not contribute to the process of DNA binding, and valinomycin and nigericin interact directly with the DNA binding sites at the cell envelope. The effects of ionophores, weak acids, and tetraphenylphosphonium on the components of the proton motive force and on the entry of transforming DNA after binding to the cell envelope was investigated. DNA entry, as measured by the amount of DNase I-resistant cell-associated [3H]DNA and by the formation of DNA breakdown products, was severely inhibited under conditions of a small proton motive force and also under conditions of a small delta pH and a high electrical potential. These results suggest that the proton motive force and especially the delta pH component functions as a driving force for DNA uptake in transformation.

Chemotaxis of Spirochaeta aurantia: involvement of membrane potential in chemosensory signal transduction

The effects of valinomycin and nigericin on sugar chemotaxis in Spirochaeta aurantia were investigated by using a quantitative capillary assay, and the fluorescent cation, 3,3'-dipropyl-2,2'-thiodicarbocyanine iodide was used as a probe to study effects of chemoattractants on membrane potential. Addition of a chemoattractant, D-xylose, to cells in either potassium or sodium phosphate buffer resulted in a transient membrane depolarization. In the presence of valinomycin, the membrane potential of cells in potassium phosphate buffer was reduced, and the transient membrane depolarization that resulted from the addition of D-xylose was eliminated. Although there was no detectable effect of valinomycin on motility, D-xylose taxis of cells in potassium phosphate buffer was completely inhibited by valinomycin. In sodium phosphate buffer, valinomycin had little effect on membrane potential or D-xylose taxis. Nigericin is known to dissipate the transmembrane pH gradient of S. aurantia in potassium phosphate buffer. This compound did not dissipate the membrane potential or the transient membrane depolarization observed upon addition of D-xylose to cells in either potassium or sodium phosphate buffer. Nigericin did not inhibit D-xylose taxis in either potassium or sodium phosphate buffer. This study indicates that the membrane potential but not the transmembrane pH gradient of S. aurantia is somehow involved in chemosensory signal transduction.

Effects of lipophilic cations on motility and other physiological properties of Bacillus subtilis

Lipophilic cations (tetraphenylarsonium, tetraphenylphosphonium, and triphenylmethylphosphonium) caused a number of major changes in the physiology of Bacillus subtilis. Macromolecular synthesis was inhibited, adenosine 5'-triphosphate concentration increased, swimming speed was reduced, tumbling was suppressed, and the capacity to take up the cations was greatly enhanced; respiration was not significantly altered. The effects occurred at lipophilic cation concentrations in the range commonly employed for measurement of membrane potential. Neither the enhancement of cation uptake nor the motility inhibition was a consequence of alteration of membrane potential, since both effects were still seen in the presence of valinomycin, with the extent of 86Rb+ uptake indicating a constant potential. Because suppression of tumbling accompanied speed reduction, as has also been found when protonmotive force is reduced, it is likely that lipophilic cations are perturbing the process of conversion of proton energy into work, rather than simply causing structural damage.

Genetic and biochemical requirements for chemotaxis to L-proline in Escherichia coli

Chemotaxis to L-proline was examined by the capillary assay, using a set of Escherichia coli strains bearing well-defined defects in the enzymes of proline transport and utilization. Aspartate taxis was measured as a constitutive, control activity whose receptor and transducer requirements are known. Proline chemotaxis showed a pattern of induction more analogous to that of proline dehydrogenase than of that of proline transport, but chemotaxis to proline was eliminated by mutations eliminating either or both of these activities. No response to proline was observed in the absence of a proline concentration gradient or when succinate was provided as an oxidizable carbon source. These data suggest that the chemotactic response to proline results from a direct impact of proline oxidation on the energy metabolism of the cell.

31P nuclear magnetic resonance studies of energy transduction in Rhodopseudomonas sphaeroides

31P nuclear magnetic resonance spectra of th phototrophic bacterium Rhodopseudomonas sphaeroides reveal the presence of inorganic phosphate, sugar phosphates and two non-identified P,P1-diesterified pyrophosphate compounds. Due to the presence of paramagnetic cations the resonances of these compounds can only be detected after repeated washing of the bacterial cells with a buffer, containing EDTA plus excess Mg2+. Washing with Mg2+-free EDTA buffer deteriorates the structural integrity of the membranes of Rps. sphaeroides. This is indicated by the appearance of an extra resonance peak in the spectra of these cells in a region where the phospholipids absorb and by a fivefold increase in proton permeability of the cytoplasmic membrane of Rps. sphaeroides under these conditions. Upon illumination of the cell suspension in the NMR tube the generation of a transmembrane pH gradient can be inferred from the shift in the resonances of extracellular and intracellular inorganic phosphate. Intracellular inorganic phosphate shows one homogeneous resonance peak upon illumination. This demonstrates that the mixing system, which has been developed for this application, functions efficiently. The magnitude of the light-dependent pH difference is 0.8 at the external pH 6. The width at half height of the internal inorganic phosphate peak is essentially independent of internal pH from pH 5--8, remains unchanged upon addition of uncoupler and is inversely proportional to the number of EDTA washings applied. These observations indicate that the inorganic phosphate NMR peak width is predominantly determined by the presence of a residual amount of paramagnetic cations, rather than by a broad distribution of internal pH values over the cells. Ionophores have an effect on the light-dependent pH-gradient in accordance with the chemiosmotic theory: valinomycin increases, and carbonylcyanide p-trifluoromethoxyphenylhydrazone decreases, the magnitude of this gradient.

Cytoplasmic pH mediates pH taxis and weak-acid repellent taxis of bacteria

Bacteria migrate away from an acid pH and from a number of chemicals, including organic acids such as acetate; the basis for detection of these environmental cues has not been demonstrated. Membrane-permeant weak acids caused prolonged tumbling when added to Salmonella sp. or Escherichia coli cells at pH 5.5. Tethered Salmonella cells went from a prestimulus behavior of 14% clockwise rotation to 80% clockwise rotation when 40 mM acetate was added and remained this way for more than 30 min. A low external pH in the absence of weak acid did not markedly affect steady-state tumbling frequency. Among the weak acids tested, the rank for acidity (salicylate greater than benzoate greater than acetate greater than 5,5-dimethyl-2,4-oxazolidinedione) was the same as the rank for the ability to collapse the transmembrane pH gradient and to cause tumbling. At pH 7.0, the tumbling responses caused by the weak acids were much briefer. Indole, a non-weak-acid repellent, did not cause prolonged tumbling at low pH. Two chemotaxis mutants (a Salmonella mutant defective in the chemotaxis methylesterase and an E. coli mutant defective in the methyl-accepting protein in MCP I) showed inverse responses of enhanced counterclockwise rotation in the first 1 min after acetate addition. The latter mutant had been found previously to be defective in the sensing of gradients of extracellular pH and (at neutral pH) of acetate. We conclude (i) that taxes away from acid pH and membrane-permeant weak acids are both mediated by a pH-sensitive component located either in the cytoplasm or on the cytoplasmic side of the membrane, rather than by an external receptor (as in the case of the attractants), and (ii) that both of these taxes involve components of the chemotaxis methylation system, at least in the early phase of the response.

Aerotaxis in Salmonella typhimurium: role of electron transport

Sensory transduction in aerotaxis required electron transport, in contrast to chemotaxis, which is independent of electron transport. Assays for aerotaxis were developed by employing spatial and temporal oxygen gradients imposed independently of respiration. By varying the step increase in oxygen concentration in the temporal assay, the dose-response relationship was obtained for aerotaxis in Salmonella typhimurium. A half-maximal response at 0.4 microM oxygen and inhibition by 5 mM KCN suggested that the "receptor" for aerotaxis is cytochrome o. The response was independent of adenosine triphosphate formation via oxidative phosphorylation but did correlate with changes in membrane potential monitored with the fluorescent cyanine dye diS-C3-(5). Nitrate and fumarate, which are alternative electron acceptors for the respiratory chain in S. typhimurium, inhibited aerotaxis when nitrate reductase and fumarate reductase were induced. These results support the hypothesis that taxis to oxygen, nitrate, and fumarate is mediated by the electron transport system and by changes in the proton motive force. Aerotaxis was normal in Escherichia coli mutants that were defective in the tsr, tar, or trg genes; in S. typhimurium, oxygen did not stimulate methylation of the products of these genes. A cheC mutant which shows an inverse response to chemoattractants also gave an inverse response to oxygen. Therefore, aerotaxis is transduced by a distinct and unidentified signally protein but is focused into the common chemosensory pathway before the step involving the cheC product. When S. typhimurium became anaerobic, the decreased proton motive force from glycolysis supported slow swimming but not tumbling, indicating that a minimum proton motive force was required for tumbling. The bacteria rapidly adapted to the anaerobic condition and resumed tumbling after about 3 min. The adaptation period was much shorter when the bacteria had been previously grown anaerobically.

Quantitative measurements of proton motive force and motility in Bacillus subtilis

The protein motive force of metabolizing Bacillus subtilis cells was only slightly affected by changes in the external pH between 5 and 8, although the electrical component and the chemical component of the proton motive force contributed differently at different external pH. The electrical component of the proton motive force was very small at pH 5, and the chemical component was almost negligible at pH 7.5. At external pH values between 6 and 7.7, swimming speed of the cells stayed constant. Thus, either the electrical component or the chemical component of the proton motive force could drive the flagellar motor. When the proton motive force of valinomycin-treated cells was quantitatively decreased by increasing the external K+ concentration, the swimming speed of the cells changed in a unique way: the swimming speed was not affected until about--100 mV, then decreased linearly with further decrease in the proton motive force, and was almost zero at about--30 mV. The rotation rate of a flagellum, measured by a tethered cell, showed essentially the same characteristics. Thus, there are a threshold proton motive force and a saturating proton motive force for the rotation of the B. subtilis flagellar motor.