Powering the Escherichia coli flagellar motor with an external voltage source

any bacteria swim by rotating thin, helical flagella at rates that can exceed 250 Hz. These flagella are driven by reversible motors that span the cytoplasmic membrane and the cell wall. In Escherichia coli, the energy for this rotation is provided by a proton flux through the flagellar motor. The electrochemical driving force for protons is called the protonmotive force, and together with the proton flux it determines the energy input into the flagellar motor.;In order to study the energetics of the flagellar motor, a technique has been developed that allows the protonmotive force of individual cells of Escherichia coli to be controlled, while the output of the motor is observed. Cephalexin-induced filamentous cells are pulled part-way into glass micropipettes containing the antibiotic gramicidin S. This permeabilizes the end of the cell inside the micropipette and collapses the cell's endogenous protonmotive force. With electrical access to the cytoplasm thus obtained, a protonmotive force is generated using an external voltage source. Prior to the development of this technique, it was not possible to generate large protonmotive forces of known magnitude in bacteria.;Using this technique, it was discovered that the torque generators in the motor are inactivated when the protonmotive force is dissipated. When a negative protonmotive force is reimposed, these torque generators are activated one at a time, giving rise to step increases in speed. This may explain the time lags in starting up seen previously when artificial energization methods were used. It was not possible to sustain rotation with a positive protonmotive force in Escherichia coli.;The torque-protonmotive force relationship for the motor at high load was also measured and found to be linear up to a protonmotive force value of at least...