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In the bacterial chemotaxis network, receptor clusters process input, and flagellar motors generate output. Receptor and motor complexes are coupled by the diffusible protein CheY-P. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell. However, the motor is ultrasensitive, with a narrow operating range of CheY-P concentrations. How the match between receptor output and motor input might be optimized is unclear. Here we show that the motor can shift its operating range by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. This shift in sensitivity explains the slow partial adaptation observed in mutants that lack the receptor methyltransferase and methylesterase and why motors show signal-dependent FliM turnover. Adaptive remodelling is likely to be a common feature in the operation of many molecular machines.

The motion of peritrichously flagellated bacteria close to surfaces is relevant to understanding the early stages of biofilm formation and of pathogenic infection. This motion differs from the random-walk trajectories of cells in free solution. Individual Escherichia coli cells swim in clockwise, circular trajectories near planar glass surfaces. On a semi-solid agar substrate, cells differentiate into an elongated, hyperflagellated phenotype and migrate cooperatively over the surface, a phenomenon called swarming. We have developed a technique for observing isolated E. coli swarmer cells moving on an agar substrate and confined in shallow, oxidized poly(dimethylsiloxane) (PDMS) microchannels. Here we show that cells in these microchannels preferentially 'drive on the right', swimming preferentially along the right wall of the microchannel (viewed from behind the moving cell, with the agar on the bottom). We propose that when cells are confined between two interfaces--one an agar gel and the second PDMS--they swim closer to the agar surface than to the PDMS surface (and for much longer periods of time), leading to the preferential movement on the right of the microchannel. Thus, the choice of materials guides the motion of cells in microchannels.

Bacterial chemotaxis is a model system for signal transduction, noted for its relative simplicity, high sensitivity, wide dynamic range and robustness. Changes in ligand concentrations are sensed by a protein assembly consisting of transmembrane receptors, a coupling protein (CheW) and a histidine kinase (CheA). In Escherichia coli, these components are organized at the cell poles in tight clusters that contain several thousand copies of each protein. Here we studied the effects of variation in the composition of clusters on the activity of the kinase and its sensitivity to attractant stimuli, monitoring responses in vivo using fluorescence resonance energy transfer. Our results indicate that assemblies of bacterial chemoreceptors work in a highly cooperative manner, mimicking the behaviour of allosteric proteins. Conditions that favour steep responses to attractants in mutants with homogeneous receptor populations also enhance the sensitivity of the response in wild-type cells. This is consistent with a number of models that assume long-range cooperative interactions between receptors as a general mechanism for signal integration and amplification.

Rotation of the bacterial flagellar motor is driven by an ensemble of torque-generating units containing the proteins MotA and MotB. Here, by inducing expression of MotA in motA- cells under conditions of low viscous load, we show that the limiting speed of the motor is independent of the number of units: at vanishing load, one unit turns the motor as rapidly as many. This result indicates that each unit may remain attached to the rotor for most of its mechanochemical cycle, that is, that it has a high duty ratio. Thus, torque generators behave more like kinesin, the protein that moves vesicles along microtubules, than myosin, the protein that powers muscle. However, their translation rates, stepping frequencies and power outputs are much higher, being greater than 30 microm s(-1), 12 kHz and 1.5 x 10(5) pN nm s(-1), respectively.

Motile cells of Escherichia coli aggregate to form stable patterns of remarkable regularity when grown from a single point on certain substrates. Central to this self-organization is chemotaxis, the motion of bacteria along gradients of a chemical attractant that the cells themselves excrete. Here we show how these complex patterns develop. The long-range spatial order arises from interactions between two multicellular aggregate structures: a 'swarm ring' that expands radially, and focal aggregates that have lower mobility. Patterning occurs through alternating domination by these two sources of excreted attractant (which we identify here as aspartate). The pattern geometries vary in a systematic way, depending on how long an aggregate remains active; this depends, in turn, on the initial concentration of substrate (here, succinate).

Rotary motors of bacterial flagella are driven by ions that move across the cytoplasmic membrane down an electrochemical gradient. For Escherichia coli, the ions are protons, and the maximum work per unit charge that they can do is the protonmotive force. To test whether motor efficiency is limited by proton leakage or mechanical nonlinearities, we measured torque as a function of protonmotive force. Filamentous cells were drawn into micropipettes and energized with an external voltage source. Torque was proportional to protonmotive force up to -150 mV, twice the span accessible by earlier techniques. This is consistent with a mechanism in which a fixed number of protons, working at unit efficiency, carry the motor through each revolution. We also found that individual torque-generating elements inactivate at low potentials or potentials of reverse sign. When normal potentials are restored, they reactivate sequentially.

When chemotactic strains of the bacterium Escherichia coli are inoculated on semi-solid agar containing mixtures of amino acids or sugars, the cells swarm outwards in a series of concentric rings: they respond to spatial gradients of attractants generated by uptake and catabolism. Cells also drift up gradients generated artificially, for example by diffusion from the tip of a capillary tube or by mixing. Here we describe conditions under which cells aggregate in response to gradients of attractant which they excrete themselves. When cells are grown in semi-solid agar on intermediates of the tricarboxylic acid cycle, they form symmetrical arrays of spots or stripes that arise sequentially. When cells in a thin layer of liquid culture are exposed to these compounds, spots appear synchronously, more randomly arrayed. In either case, the patterns are stationary. The attractant is a chemical sensed by the aspartate receptor. Its excretion can be triggered by oxidative stress. As oxygen is limiting at high cell densities, aggregation might serve as a mechanism for collective defence.

Mot mutants of Escherichia coli are paralysed: their flagella appear to be intact but do not rotate. The motA and motB gene products are found in the cytoplasmic membrane; they do not co-purify with flagellar basal bodies isolated in neutral detergents. Silverman et al. found that mot mutants could be ' resurrected ' through protein synthesis directed by lambda transducing phages carrying the wild-type genes. Here, we have studied this activation at the level of a single flagellar motor. Cells of a motB strain carrying plasmids in which transcription of the wild-type motB gene was controlled by the lac promoter were tethered to a glass surface by a single flagellum. These cells began to spin within several minutes after the addition of a lac inducer, and their rotational speed changed in a series of equally spaced steps. As many as 7 steps were seen in individual cells and, from the final speeds attained, as many as 16 steps could be inferred. These experiments show that each flagellar motor contains several independent force-generating units comprised, at least in part, of motB protein.

The development of the gradient force optical particle trap ('optical tweezers') has made it possible to manipulate biological materials using a single beam of laser light. Optical traps can produce forces in the microdyne range on intact cells without causing overt damage: such forces are sufficient to arrest actively swimming bacteria and can overcome torque generated by the flagellar motor of a bacterium tethered to a glass surface by a flagellar filament. By calibrating the trapping force against Stokes' drag and measuring the twist that is sustained by this force, we determined the torsional compliance of flagella in tethered Escherichia coli and a motile Streptococcus. Flagella behaved as linear torsion springs for roughly half a revolution, but became much more rigid when turned beyond this point in either direction.

Bacteria swim by rotating their flagella. They back up or choose new directions at random by changing the direction of the rotation. The probability of such changes is biased by sensory reception. The bias depends on the way in which the intensity ofthe stimulus changes with time, so that the bacteria tend to swim up a gradient of attractant and down a gradient of repellent chemicals.