The bacterial flagellar motor of Escherichia coli is a nanoscale rotary engine essential for bacterial propulsion. Studies on the power output of single motors rely on the measurement of motor torque and rotation under external load. Here, we investigate the use of magnetic tweezers, which in principle allow the application and active control of a calibrated load torque, to study single flagellar motors in Escherichia coli. We manipulate the external load on the motor by adjusting the magnetic field experienced by a magnetic bead linked to the motor, and we probe the motor's response. A simple model describes the average motor speed over the entire range of applied fields. We extract the motor torque at stall and find it to be similar to the motor torque at drag-limited speed. In addition, use of the magnetic tweezers allows us to force motor rotation in both forward and backward directions. We monitor the motor's performance before and after periods of forced rotation and observe no destructive effects on the motor. Our experiments show how magnetic tweezers can provide active and fast control of the external load while also exposing remaining challenges in calibration. Through their non-invasive character and straightforward parallelization, magnetic tweezers provide an attractive platform to study nanoscale rotary motors at the single-motor level.
Bibliographical noteFor Corrigendum to this article (published on 18 May 2018) see 10.1038/srep46980: The Acknowledgements section in this Article is incomplete and should e read: “We thank Seungkyu Ha, Yera Ye. Ussembayev, Richard Janissen, and Hubertus J. E. Beaumont for discussions, Richard M. Berry and Ren Lim for providing the strains, and Theo van Laar for the remaining help with the bacteria. FP was supported by the European Research Council under the European Union’s Seventh Framework Program (FP/2007–2013)/ERC Grant 306475. This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners, and by the Foundation for Fundamental
Research on Matter (FOM).”