MECHANICAL LIMITS OF BACTERIAL FLAGELLAR MOTORS PROBED BY ELECTROROTATION

We used the technique of electrorotation to apply steadily increasing external torque to tethered cells of the bacterium Escherichia coli wh ile continuously recording the speed of cell rotation. We found that t he bacterial flagellar motor generates constant torque when rotating f orward at low speeds and constant but considerably higher torque when rotating backward. At intermediate torques, the motor stalls. The torq ue-speed relationship is the same in both directional modes of switchi ng motors. Motors forced backward usually break, either suddenly and i rreversibly or progressively. Motors broken progressively rotate predo minantly at integral multiples of a unitary speed during the course of both breaking and subsequent recovery, as expected if progressive bre aking affects individual torque-generating units. Torque is reduced by the same factor at all speeds in partially broken motors, implying th at the torque-speed relationship is a property of the individual torqu e-generating units.