Rapid rotation of flagellar bundles in swimming bacteria

A bacterial ftagellum is driven by a reversible rotary motor. The power input is determined by protonmotive force and proton flux, the power output by torque and speed: interrelationships between these parameters provide important clues to motor mechanisms. Here we describe the relationship between torque and speed at constant protonmotive force. The measurements are analogous to those that could be made by plugging an electric motor into a constant-voltage outlet, varying the external load, and determining the torque delivered at different speeds. We used suspensions of metabolizing cells of a motile Streptococcus, varied the extemal load by changing the viscosity of the medium, determined motor speed from the frequency of vibration of the cell body, and inferred motor torque from the rate of body rotation. The flagellar bundles rotate more rapidly than formerly supposed, at rates that increase linearly with temperature. The torque delivered by the flagellar motor drops linearly with speed. At high speed, the torque-generating cycle associated with the transfer of one proton appears to dissipate free energy in a series of small steps.