Abstract
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.
Highlights
The bacterial flagellar motor (BFM) is a rotary motor at the heart of propulsion in many species of swimming bacteria[1]
If the magnetic torque is larger than the motor torque, we can control the orientation of the bead and, indirectly, the orientation of the motor by rotating the magnets
To illustrate the capabilities of our assay, we gradually reduce the average speed of the motors by increasing the external magnetic field strength until the motors stall
Summary
The bacterial flagellar motor (BFM) is a rotary motor at the heart of propulsion in many species of swimming bacteria[1]. In the tethered-bead assay[12,13], the cell body is fixed to a surface, and a bead is attached to the hook or flagellar stub such that motor rotation results in rotation of the bead In both techniques, the load torque on the motor is based on drag, which limits torque control to either adjusting the viscosity of the surrounding medium or using beads of different sizes. The load torque on the motor is based on drag, which limits torque control to either adjusting the viscosity of the surrounding medium or using beads of different sizes Extensions to these techniques include electrorotation[14,15,16] and the use of lever-arm configurations in optical tweezers[17,18,19,20], which provide more active control over the torque in terms of magnitude and direction.
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