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We demonstrate that bacterial cells enhance the taxis performance by collective migration originating from the cellular alignment interaction without apparent clustering. We confineSalmonellacells in a shallow channel and evaluate the thermotaxis response to local heating and diffusion. By combining cell tracking analysis and numerical simulation based on simple modeling, we show that the alignment interaction suppresses orientation fluctuation, strengthens migration bias, and also prevents the dispersion of accumulated population. The results show a prominent example of how a collective motion of active matter implements a biological function.

Bacterial chemotaxis allows cells to swim toward a more favorable environment. Capillary assays are a major method for exploring bacterial responses to attractive and repellent chemicals, but the accumulation process obtained using a capillary containing chemicals has not been investigated fully. In this study, we quantitatively analyzed the response of Salmonella cells to serine as an attractant diffusing from a capillary placed in a cell suspension. Video microscopy showed that cells gradually accumulated near the tip of the capillary and thereafter directed flows were generated. Flow analysis using microspheres as tracers showed that the flow comprised millimeter-scale convection, which originated at the point source where serine was supplied by the capillary. The generation of convection was attributable to cell accumulation and gravitational force, thereby suggesting that it is a variant of bioconvection. We recorded the time courses of the changes in cell numbers and the convection flow speed at different positions near the capillary, which showed that the number of cells increased initially until an almost saturated level, and the convection flow speed then accelerated as the cell accumulation area increased in size. This result indicates that cell accumulation at the stimulation source and enlargement of the accumulation area were essential for generating the convection.

The bacterial flagellar motor is composed of a rotor and a dozen stators and converts the ion flux through the stator into torque. Each stator unit alternates in its attachment to and detachment from the rotor even during rotation. In some species, stator assembly depends on the input energy, but it remains unclear how an electrochemical potential across the membrane (e.g., proton motive force [PMF]) or ion flux is involved in stator assembly dynamics. Here, we focused on pH dependence of a slow motile MotA(M206I) mutant of Salmonella. The MotA(M206I) motor produces torque comparable to that of the wild-type motor near stall, but its rotation rate is considerably decreased as the external load is reduced. Rotation assays of flagella labeled with 1-μm beads showed that the rotation rate of the MotA(M206I) motor is increased by lowering the external pH whereas that of the wild-type motor is not. Measurements of the speed produced by a single stator unit using 1-μm beads showed that the unit speed of the MotA(M206I) is about 60% of that of the wild-type and that a decrease in external pH did not affect the MotA(M206I) unit speed. Analysis of the subcellular stator localization revealed that the number of functional stators is restored by lowering the external pH. The pH-dependent improvement of stator assembly was observed even when the PMF was collapsed and proton transfer was inhibited. These results suggest that MotA-Met206 is responsible for not only load-dependent energy coupling between the proton influx and rotation but also pH-dependent stator assembly. IMPORTANCE The bacterial flagellar motor is a rotary nanomachine driven by the electrochemical transmembrane potential (ion motive force). About 10 stators (MotA/MotB complexes) are docked around a rotor, and the stator recruitment depends on the load, ion motive force, and coupling ion flux. The MotA(M206I) mutation slows motor rotation and decreases the number of docked stators in Salmonella. We show that lowering the external pH improves the assembly of the mutant stators. Neither the collapse of the ion motive force nor a mutation mimicking the proton-binding state inhibited stator localization to the motor. These results suggest that MotA-Met206 is involved in torque generation and proton translocation and that stator assembly is stabilized by protonation of the stator.

Bacterial flagellar motor is one of the most complex and sophisticated nano machineries in nature. A duty ratio $D$ is a fraction of time that the stator and the rotor interact and is a fundamental property to characterize the motor but remains to be determined. It is known that the stator units of the motor bind to and dissociate from the motor dynamically to control the motor torque depending on the load on the motor. At low load where the kinetics such as a proton translocation speed limits the rotation rate, the dependency of the rotation rate on the number of stator units $N$ infers $D$; the dependency becomes larger for smaller $D$. Contradicting observations supporting both the small and large $D$ have been reported. A dilemma is that it is difficult to explore a broad range of $N$ at low load because the stator units easily dissociate, and $N$ is limited to one or two at vanishing load. Here, we develop an electrorotation method to dynamically control the load on the flagellar motor of {\it Salmonella} with a calibrated magnitude of the torque. By instantly reducing the load for keeping $N$ high, we observed that the speed at low load depends on $N$, implying a small duty ratio. We recovered the torque-speed curves of individual motors and evaluated the duty ratio to be $0.14 \pm 0.04$ from the correlation between the torque at high load and the rotation rate at low load., 12 pages, 6 figures. Title was changed from the previous version

The spirochete Leptospira has a coiled cell body and two periplasmic flagella (PFs) that reside beneath the outer sheath. PFs extend from each end of the cell body and are attached to the right-handed spiral protoplasmic cylinder (PC) via a connection with the flagellar motor embedded in the inner membrane. PFs bend each end of the cell body into left-handed spiral (S) or planar hook (H) shapes, allowing leptospiral cells to swim using combined anterior S-end and posterior H-end gyrations with PC rotations. As a plausible mechanism for motility, S- and H-end gyrations by PFs and PC rotations by PF countertorque imply mutual influences among the three parts. Here we show a correlation between H-end gyration and PC rotation from the time records of rotation rates and rotational directions of individual swimming cells. We then qualitatively explain the observed correlation using a simple rotation model based on the measurements of motility and intracellular arrangements of PFs revealed by cryo-electron microscopy and electron cryotomography.

Chemotaxis allows bacterial cells to migrate towards or away from chemical compounds. In the present study, we developed a microscopic agar-drop assay (MAA) to investigate the chemotactic behaviour of a coiled spirochete, Leptospira biflexa . An agar drop containing a putative attractant or repellent was placed around the centre of a flow chamber and the behaviour of free-swimming cells was analysed under a microscope. MAA showed that L. biflexa cells gradually accumulated around an agar drop that contained an attractant such as glucose. Leptospira cells often spin without migration by transformation of their cell body. The frequency at which cells showed no net displacement decreased with a higher glucose concentration, suggesting that sensing an attractive chemical allows these cells to swim more smoothly. Investigation of the chemotactic behaviour of these cells in response to different types of sugars showed that fructose and mannitol induced negative chemotactic responses, whereas xylose and lactose were non-chemotactic for L. biflexa . The MAA developed in this study can be used to investigate other chemoattractants and repellents.

The bacterial flagellar motor generates torque by converting the energy of proton translocation through the transmembrane proton channel of the stator complex formed by MotA and MotB. The MotA/B complex is thought to be anchored to the peptidoglycan (PG) layer through the PG-binding domain of MotB to act as the stator. The stator units dynamically associate with and dissociate from the motor during flagellar motor rotation, and an electrostatic interaction between MotA and a rotor protein FliG is required for efficient stator assembly. However, the association and dissociation mechanism of the stator units still remains unclear. In this study, we analyzed the speed fluctuation of the flagellar motor of Salmonella enterica wild-type cells carrying a plasmid encoding a nonfunctional stator complex, MotA/B(D33N), which lost the proton conductivity. The wild-type motor rotated stably but the motor speed fluctuated considerably when the expression level of MotA/B(D33N) was relatively high compared to MotA/B. Rapid accelerations and decelerations were frequently observed. A quantitative analysis of the speed fluctuation and a model simulation suggested that the MotA/B(D33N) stator retains the ability to associate with the motor at a low affinity but dissociates more rapidly than the MotA/B stator. We propose that the stator dissociation process depends on proton translocation through the proton channel.

Leptospira are spirochete bacteria distinguished by a short-pitch coiled body and intracellular flagella. Leptospira cells swim in liquid with an asymmetric morphology of the cell body; the anterior end has a long-pitch spiral shape (S-end) and the posterior end is hook-shaped (H-end). Although the S-end and the coiled cell body called the protoplasmic cylinder are thought to be responsible for propulsion together, most observations on the motion mechanism have remained qualitative. In this study, we analyzed the swimming speed and rotation rate of the S-end, protoplasmic cylinder, and H-end of individual Leptospira cells by one-sided dark-field microscopy. At various viscosities of media containing different concentrations of Ficoll, the rotation rate of the S-end and protoplasmic cylinder showed a clear correlation with the swimming speed, suggesting that these two helical parts play a central role in the motion of Leptospira. In contrast, the H-end rotation rate was unstable and showed much less correlation with the swimming speed. Forces produced by the rotation of the S-end and protoplasmic cylinder showed that these two helical parts contribute to propulsion at nearly equal magnitude. Torque generated by each part, also obtained from experimental motion parameters, indicated that the flagellar motor can generate torque >4000 pN nm, twice as large as that of Escherichia coli. Furthermore, the S-end torque was found to show a markedly larger fluctuation than the protoplasmic cylinder torque, suggesting that the unstable H-end rotation might be mechanically related to changes in the S-end rotation rate for torque balance of the entire cell. Variations in torque at the anterior and posterior ends of the Leptospira cell body could be transmitted from one end to the other through the cell body to coordinate the morphological transformations of the two ends for a rapid change in the swimming direction.

The motility of bacteria is an important factor in their infectivity. In this study, the motility of Leptospira, a member of the spirochete family that causes a zoonotic disease known as leptospirosis, was analyzed in different viscous or osmotic conditions. Motility assays revealed that both pathogenic and saprophytic strains increase their swimming speeds with increasing viscosity. However, only pathogenic Leptospira interrogans maintained vigorous motility near physiological osmotic conditions. This suggests that active motility in physiological conditions is advantageous when Leptospira enters hosts and when it migrates toward target tissues.

The bacterial flagellar motor is made of a rotor and stators. In Salmonella it is thought that about a dozen MotA/B complexes are anchored to the peptidoglycan layer around the motor through the C-terminal peptidoglycan-binding domain of MotB to become active stators as well as proton channels. MotB consists of 309 residues, forming a single transmembrane helix (30-50), a stalk (51-100) and a C-terminal peptidoglycan-binding domain (101-309). Although the stalk is dispensable for torque generation by the motor, it is required for efficient motor performance. Residues 51 to 72 prevent premature proton leakage through the proton channel prior to stator assembly into the motor. However, the role of residues 72-100 remains unknown. Here, we analyzed the torque-speed relationship of the MotB(Δ72-100) motor. At a low speed near stall, this mutant motor produced torque at the wild-type level. Unlike the wild-type motor, however, torque dropped off drastically by slight decrease in external load and then showed a slow exponential decay over a wide range of load by its further reduction. Since it is known that the stator is a mechano-sensor and that the number of active stators changes in a load-dependent manner, we interpreted this unusual torque-speed relationship as anomaly in load-dependent control of the number of active stators. The results suggest that residues 72-100 of MotB is required for proper load-dependent control of the number of active stators around the rotor.

Bacterial chemotaxis is induced by sensing chemical stimuli via chemoreceptors embedded in the cytoplasmic membrane, enabling the cells to migrate toward nutrients or away from toxins. The chemoreceptors of Escherichia coli and Salmonella spp. have been well studied and are functionally classified on the basis of detectable substrates. The spirochete Leptospira possesses more than ten chemoreceptors and shows attractive or repellent responses against some sugars, amino acids, and fatty acids. However, the roles of these chemoreceptors have not been investigated. In this study, we conducted a chemotaxis assay called microscopic agar drop assay in combination with competition experiments, determining whether two kinds of attractants are recognized by the same type of chemoreceptor in the saprophytic Leptospira strain, Leptospira biflexa. Analyzing the competition effect observed between several pairs of chemicals, we found that L. biflexa senses sugars via chemoreceptors different from those that sense amino acids and fatty acids.

F 1 -ATPase is a nanosized biological energy transducer working as part of F o F 1 -ATP synthase. Its rotary machinery transduces energy between chemical free energy and mechanical work and plays a central role in the cellular energy transduction by synthesizing most ATP in virtually all organisms. However, information about its energetics is limited compared to that of the reaction scheme. Actually, fundamental questions such as how efficiently F 1 -ATPase transduces free energy remain unanswered. Here, we demonstrated reversible rotations of isolated F 1 -ATPase in discrete 120° steps by precisely controlling both the external torque and the chemical potential of ATP hydrolysis as a model system of F o F 1 -ATP synthase. We found that the maximum work performed by F 1 -ATPase per 120° step is nearly equal to the thermodynamical maximum work that can be extracted from a single ATP hydrolysis under a broad range of conditions. Our results suggested a 100% free-energy transduction efficiency and a tight mechanochemical coupling of F 1 -ATPase.

F(1)-ATPase is a rotary molecular motor powered by the torque generated by another rotary motor F(0) to synthesize ATP in vivo. Therefore elucidation of the behavior of F(1) under external torque is very important. Here, we applied controlled external torque by electrorotation and investigated the ATP-driven rotation for the first time. The rotation was accelerated by assisting torque and decelerated by hindering torque, but F(1) rarely showed rotations in the ATP synthesis direction. This is consistent with the prediction by models based on the assumption that the rotation is tightly coupled to ATP hydrolysis and synthesis. At low ATP concentrations (2 and 5 microM), 120 degrees stepwise rotation was observed. Due to the temperature rise during experiment, quantitative interpretation of the data is difficult, but we found that the apparent rate constant of ATP binding clearly decreased by hindering torque and increased by assisting torque.

バクテリアは，らせん形をしたべん毛繊維を回転させて水中を遊泳する．べん毛の働きは，スクリューに当たる．べん毛の正確な形状を観測することは，その微小なサイズと高速回転故に容易ではなかった．本論文では，レーザー暗視野顕微鏡を用いて，べん毛のらせんパラメータを計測する方法を提案し，サルモネラ菌へ適用した結果を示す．らせん形のべん毛を，らせん軸に対して45°または90°の方向からレーザービームで一方向照明すると，べん毛の像が一周期に1個または2個の輝点の列となる．そのような像を，電子シャッターを備えた高感度CCDカメラを使って記録した．記録した輝点の間隔を読み取ることで，菌体から脱落したべん毛（回転していない）のらせんピッチと半径を求めることができた．同様にして菌体上で高速回転するべん毛に関しても，ピッチの値が求まり，その微小な変化を捉えることもできた．

Leptospira is a spirochete possessing intracellular flagella. Each Leptospira flagellar filament is linked with a flagellar motor composed of a rotor and a dozen stators. For many bacterial species, it is known that the stator functions as an ion channel and that the ion flux through the stator is coupled with flagellar rotation. The coupling ion varies depending on the species; for example, H(+) is used in Escherichia coli, and Na(+) is used in Vibrio spp. to drive a polar flagellum. Although genetic and structural studies illustrated that the Leptospira flagellar motor also contains a stator, the coupling ion for flagellar rotation remains unknown. In the present study, we analyzed the motility of Leptospira under various pH values and salt concentrations. Leptospira cells displayed motility in acidic to alkaline pH. In the presence of a protonophore, the cells completely lost motility in acidic to neutral pH but displayed extremely slow movement under alkaline conditions. This result suggests that H(+) is a major coupling ion for flagellar rotation over a wide pH range; however, we also observed that the motility of Leptospira was significantly enhanced by the addition of Na(+), though it vigorously moved even under Na(+)-free conditions. These results suggest that H(+) is preferentially used and that Na(+) is secondarily involved in flagellar rotation in Leptospira. The flexible ion selectivity in the flagellar system could be advantageous for Leptospira to survive in a wide range of environment.

The giant acceleration (GA) of diffusion is a universal phenomenon predicted by the theoretical analysis given by Reimann et al. [Phys. Rev. Lett. 87, 010602 (2001)]. Here we apply the theory of the GA of diffusion to a single-molecule experiment on a rotary motor protein, ${\mathrm{F}}_{1}$, which is a component of ${\mathrm{F}}_{\mathrm{o}}{\mathrm{F}}_{1}$ adenosine triphosphate synthase. We discuss the energetic properties of ${\mathrm{F}}_{1}$ and identify a high energy barrier of the rotary potential to be $20{k}_{B}T$, with the condition that the adenosine diphosphates are tightly bound to the ${\mathrm{F}}_{1}$ catalytic sites. To conclude, the GA of diffusion is useful for measuring energy barriers in nonequilibrium and single-molecule experiments.

Many strains of lactic acid bacteria have been used for the production of probiotics. Some metabolites produced by lactic acid bacteria impair the motilities of pathogenic bacteria. Because bacterial motility is strongly associated with virulence, the metabolic activities of lactic acid bacteria are effective for suppressing bacterial infections. Here we show that lactose fermentation by Lactococcus lactis subsp. lactis inhibits the motility of Salmonella enterica serovar Typhimurium. A single-cell tracking and rotation assay for a single flagellum showed that the swimming behaviour of Salmonella was severely but transiently impaired through disruption of flagellar rotation on exposure to media cultivated with Lac. lactis. Using a pH-sensitive fluorescent protein, we observed that the intracellular pH of Salmonella was decreased because of some fermentation products of Lac. lactis. We identified acetate as the lactose fermentation product of Lac. lactis triggering the paralysis of Salmonella flagella. The motilities of Pseudomonas, Vibrio and Leptospira strains were also severely disrupted by lactose utilization by Lac. lactis. These results highlight the potential use of Lac. lactis for preventing infections by multiple bacterial species.

The deformation of a flagellum of Vibrio alginolyticus, single-flagellate bacteria, is analyzed theoretically assuming the shape of the flagellum to be a circular helix. The viscous force exerted on the flagellum in aqueous fluid is estimated applying the resistive-force theory based on the Stokes flow. The moment of force in the flagellum are described in analytical expressions and also the curvature and the torsion of the deformed flagellum are expressed analytically according to the Kirchhoff rod model. The deformation of the flagellum is obtained numerically solving evolution equations which determine a space curve from the curvature and the torsion. Comparing variations of the pitch of helical flagella between the numerical solutions and the results of measurement, the flexural rigidity or the elastic bending coefficient for the flagellum of Vibrio alginolyticus is estimated.

The forward and backward swimming speeds and periods of a Vibrio alginolyticus strain that has a single polar flagellum were measured. The backward swimming speeds were 1.5 times greater than the forward ones on average and the average period of backward swimming was shorter than forward swimming. However, the swimming speed and period were not correlated. Similar results were obtained for a mutant that has a 1.6 times longer flagellum on average.

There are some cases where the swimming speed of a bacterial cell in a polymer solution increases with the viscosity. This phenomenon is not yet explained by the traditional theories, and such phenomena never appear under macroscopic conditions. We showed it could be explained by the hypothesis in which the anisotropic effect of polymer was introduced into a traditional theory. To evaluate the hypothesis, it is necessary to measure the hydrodynamic force acting on an ultra small needle in both the directions parallel and perpendicular to its axis. Tobacco mosaic virus (TMV; needle-shaped, 18 nm in width, 400 nm in length) is a suitable sample for this purpose. It is, however, very difficult to measure TMV Brownian motion and estimate the force acting on it because TMV can not be observed by typical optical microscopy. In this paper, we introduce our recent study about the measurement of TMV Brownian motion.

Rotation of the Na+-driven flagellar motor ofVibrio alginolyticuswas investigated under the influence of inhibitors specific to the motor, amiloride and phenamil. The rotation rate of a single flagellum on a cell stuck to a glass slide was examined using laser dark-field microscopy. In the presence of 50 mM NaCl, the average rotation rate (ω) was about 600 r.p.s. with a standard deviation (σω) of 9% of ω. When ω was decreased to about 200 r.p.s. by the presence of 1.5 mM amiloride, σωincreased to 15% of ω. On the other hand, when ω was decreased to about 200 r.p.s. by the addition of 0.6 μM phenamil, a large increase in σωup to 50% of ω, was observed. Similarly large fluctuations were observed at other concen trations of phenamil. These observations suggest that dissociation of phenamil from the motor was much slower than that of amiloride. A very low concentration of phenamil caused a transient but substantial reduction in rotation rate. This might suggest that binding of only a single molecule of phenamil strongly inhibits the torque generation in the flagellar motor.

The motility of bacteria is an important factor in their infectivity. In this study, the motility of Leptospira, a member of the spirochete family that causes a zoonotic disease known as leptospirosis, was analyzed in different viscous or osmotic conditions. Motility assays revealed that both pathogenic and saprophytic strains increase their swimming speeds with increasing viscosity. However, only pathogenic Leptospira interrogans maintained vigorous motility near physiological osmotic conditions. This suggests that active motility in physiological conditions is advantageous when Leptospira enters hosts and when it migrates toward target tissues.

The Na(+)-driven flagellar motor in Vibrio alginolyticus rotates very fast. Rotation of a single flagellum on a stuck cell was measured by laser darkfield microscopy with submillisecond temporal resolution. The rotation rate increased with increasing external concentration of NaCl, and reached 1000 r.p.s. at 300 mM NaCl. The Na+ influx through the motor should determine the rotation period (tau) and affect the speed stability. Fluctuation of the rotation period was analyzed at various rotation rates (from approximately 50 r.p.s. to approximately 1000 r.p.s.), which were changed by changing the external concentration of NaCl and the addition of a protonophore or a specific inhibitor. At high rotation rates (over 400 r.p.s.), the observed rotation was stable, and the standard deviation of tau (sigma tau) ranged from 7% to 16% of the average rotation period (tau). At low rotation rates (under 100 r.p.s), the rotation period tended to fluctuate, and the distributions of tau were non-Gaussian. The value of sigma tau ranged from 10 to 30% oftau. However, the observed minimum value of sigma tau at various rotation rates was approximately equal to the calculated standard deviation due to the rotational diffusion of the flagellar filament. These results suggest that the torque was stably generated at various Na+ influxes through the motor. We observed large fluctuations that cannot be explained by rotational diffusion. We discuss the factors that induce the large fluctuation.

Leptospires are a group of bacteria with a unique ultrastructure and a fascinating swimming behavior that cause a number of emerging and re-emerging diseases worldwide called leptospirosis. The unusual form of motility is thought to play a critical role in the infection process. However, the inhibition mechanism of antiserum on the motility of Leptospira to attenuate the infection efficiency is unknown. In this study, effect of antiserum on motility was quantitatively investigated by swimming speed. Relatively low concentration of antiserum was found to inhibit leptospiral motility, suggesting that the basic immunization can affect the infection efficiency. Recovery of motility a few hours later after the addition of antiserum was observed. This raises a hypothesis that Leptospira carries surface molecules bound with antibodies toward the cell end to escape and recovers the motility.

This paper gives an overview of consecutive studies on the asymmetrical motion of Vibrio alginolyticus cells, which possess a single polar flagellum. Inertial forces are negligible because of the cell size and the motion is expected to be symmetrical. However, asymmetrical characteristics between forward and backward motions were observed. The asymmetry observed in trajectory, swimming speed, and residence time appears only when a cell swims close to a surface. In backward motion, a cell traces circular path, while in forward motion the cell moves in a straight line. The backward swimming speed is faster than the forward speed. Backward swimming cells tend to stay close to a surface longer than forward swimming cells do. An explanation for these asymmetrical characteristics is given based on the results of boundary element analyses of creeping flow around a cell model that consists of a cell body and a rotating flagellum. According to the explanation, the attitude of a cell relative to a surface produces the asymmetry. The studies presented here indicate that the fluid-dynamic interaction between bacterial cells and a surface produces the unexpected asymmetrical motion. This asymmetry may help cells search for preferable states on a surface or to attach to the surface.

Energetics of a rotary molecular motor F1-ATPase was studied by applying recent developments in nonequilibrium physics. Since molecular motors are engines that transduce chemical energy to mechanical motions, it is essential to focus on their energetics. Here, for a single F1-ATPase molecule, we have evaluated the amount of heat dissipation Qrot through its rotational degree of freedom as well as the work W against external load. Qrot was estimated using a new nonequilibrium equality connecting the heat dissipation to the violation of the fluctuation dissipation theorem. External torque was applied using the electrorotation method. We found a nontrivial energy balance relation that W+Qrot per 120° rotation was almost equal to the free energy change in a single ATP hydrolysis ∈″∈1/4 under various conditions. This implies that F1-ATPase focuses the free energy consumption toward rotations with an efficiency of nearly 100%. Moreover, we found that under a sufficiently strong torque in the opposite direction of ATP hydrolytic rotations, it rotated in the opposite direction, or the ATP synthetic direction, in a stepwise manner. The torque necessary for rotations in the synthetic direction times 120° was nearly equal to ∈″∈1/4 under various conditions except for conditions at sufficiently low ADP concentrations.

The bacterial flagellar motor is the only molecular rotary machine found in living organisms, converting the protonmotive force, i.e., the membrane voltage and proton gradients across the cell membrane, into the mechanical force of rotation (torque). We have developed a method for holding a bacterial cell at the tip of a glass micropipette and applying electric pulses through the micropipette. This method has enabled us to observe the dynamical responses of flagellar rotation to electric pulses that change the membrane voltage transiently and repeatedly. We have observed that acceleration and deceleration of motor rotation are induced by application of these electric pulses. The change in the rotation rate occurred within 5 ms after pulse application.

Bacterial flagella responsible for motility are driven by rotary motors powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. The stator of proton-driven flagellar motor converts proton influx into mechanical work. However, the energy conversion mechanism remains unclear. Here, we show that the motor is sensitive to intracellular proton concentration for high-speed rotation at low load, which was considerably impaired by lowering intracellular pH, while zero-speed torque was not affected. The change in extracellular pH did not show any effect. These results suggest that a high intracellular proton concentration decreases the rate of proton translocation and therefore that of the mechanochemical reaction cycle of the motor but not the actual torque generation step within the cycle by the stator-rotor interactions.

BACTERIA such as Escherichia coli and Salmonella typhlmurium swim by rotating their flagella1,2, each of which consists of an external helical filament and a rotary motor embedded in the cell surface (see ref. 3 for a review). The function of the flagellar motor has been examined mainly by tethering the flagellar filament to a glass slide and observing the resultant rotation of the cell body2. But under these conditions the motor operates at a very low speed (about lOr.p.s.) owing to the unnaturally high load conditions inherent in this technique. Lowe et al.4 analysed the frequency of light scattered from swimming cells to estimate the average rotation speed of flagellar bundles of E. coli as about 270 r.p.s. To analyse motor function in more detail, however, measurement of high-speed rotation of a single flagellum (at low load) with a temporal resolution better than 1 ms is needed. We have now developed a new method—laser dark-field microscopy—which fulfils these requirements. We find that although the average rotation speed of S. typhimurium flagella is rather stable, there are occasional abrupt slowdowns, pauses and reversals (accomplished within 1 ms). These changes were frequently observed in mutants defective in one of the motor components (called the switch complex), suggesting that this component is important not only in switching rotational direction but also in torque generation or regulation.

The piezoelectric composite-bar method was applied to elastic modulus measurement of ceramics (zirconia, alumina, silicon nitride and silicon carbide). The procedure of measurement was simplified by using quartz vibrators with various resonant frequencies and grease as the adhesive. The resonant frequency fs of the specimen was observed to depend on the resonant frequency fq of the vibrator. The value of fs was maximum at fs=fq. The most probable value of fs (fs0) is obtained at fs=fq, and was determined by plotting fs against fq. The experimental error in fs was evaluated by the plot of fs vs. fq (fs/fs0 vs. fq/fs0) and was smaller than 0.1%, when the difference between fq and fs0 was 10%. The value of Q-1 was observed to be minimum at fq/fs0=1.

Elastic modulus measurement of ceramics up to 1000°C was made practicable by improving the piezoelectric composite-bar method. The piezoelectric vibrators were made from an LiNbO3 single crystal with the Curie temperature as high as 1210°C. The effective piezoelectric constants of the LiNbO3 crystal were analyzed, and then the cutting direction which yields pure longitudinal vibration was determined. The present method was applied to measuring the Young's modulus E and the internal friction Q-1 of 2Y-TZP with the dimension of 1×2×12mm, and anomalous behavior in E and Q-1 was observed at ca. 300°C.

Many bacteria swim by rotating their helical flagellar filaments which are driven by flagellar motors embedded in the cell membranes. In mechanical engineering, bacterial swimming is an interesting subtopic of robotics and nano-mechanics since countless nano-machines made of bio-molecules are packed into 1 µm cells. In this paper, we present three exceptionally interesting facts about swimming bacteria, which have been known for the past decade. First, a flagellar motor rotates extremely fast (the maximum recorded is 1,700 rps). This information produces many new questions regarding, for example, the torque generation mechanism and the wear. The second fact concerns the flagellar filament as an intelligent material. It is sufficiently rigid for a use as a propeller and yet can change its helical form to relax the stress when an excessive force acts on it. The mechanism is now being explored at an atomic level. The last fact is that bacterial cells sometimes swim well in viscous environments. This phenomenon contradicts common knowledge but could be explained by a new hypothesis in which the effect of the polymer network on the bacterial motion was expressed mathematically. We were impressed by the acumen of bacteria. (Review)

Swimming speed (v) and flagellar-bundle rotation rate (f) of Salmonella typhimurium, which has peritrichous flagella, were simultaneously measured by laser dark-field microscopy (LDM). Clear periodic changes in the LDM signals from a rotating bundle indicated in-phase rotation of the flagella in the bundle. A roughly linear relation between v and f was observed, though the data points were widely distributed. The ratio of v to f (v-f ratio), which indicates the propulsive distance during one flagellar rotation, was 0.27 microm (11% of the flagellar pitch) on average. The experimental v-f ratio was twice as large as the calculated one on the assumption that a cell had a single flagellum. A flagellar bundle was considered to propel a cell more efficiently than a single flagellum.

The polar flagella of Vibrio alginolyticus are driven by sodium motive force and those motors are specifically and strongly inhibited by phenamil, an amiloride analog that is thought to interact with a sodium channel of the flagellar motor. To study the sodium ion coupling site, we isolated motility mutants resistant to phenamil and named the phenotype Mpa(r) for motility resistant to phenamil. The motility of the wild-type (Mpa(s)) was inhibited by 50 microM phenamil, whereas Mpa(r) strains were still motile in the presence of 200 microM phenamil. The Ki value for phenamil in the Mpa(r) strain was estimated to be five times larger than that in the Mpa(s) strain. However, the sensitivities to amiloride or benzamil, another amiloride analog, were not distinctly changed in the Mpa(r) strain. The rotation rate of the wild-type Na+-driven motor fluctuates greatly in the presence of phenamil, which can be explained in terms of a relatively slow dissociation rate of phenamil from the motor. We therefore studied the stability of the rotation of the Mpa(r) and Mpa(s) motors by phenamil. The speed fluctuations of the Mpa(r) motors were distinctly reduced relative to the Mpas motors. The steadier rotation of the Mpa(r) motors can be explained by an increase in the phenamil dissociation rate from a sodium channel of the motor, which suggests that a phenamil-specific binding site of the motor is mutated in the Mpa(r) strain.