Studying the Bacterial Flagellar Motor using an Optical Torque Wrench

At the biomolecular level, the physical quantity torque manifests itself in a number of ways e.g. in the conformational changes of biopolymers such as DNA and in actions of both linear and rotary molecular motors. The optical torque wrench, an optical tweezers setup with the capability of applying and measuring both force and torque, has been developed as a versatile setup in tackling biologically relevant issues at high spatial and temporal resolution. Torque control in this optical tweezers setup relies on the manipulation and readout of the polarization state of light used to trap nano-fabricated birefringent cylinders. The flagellar motor of Escherichia coli is a well-know rotary motor of only about 45 nm embedded in the cellular membrane, but besides its protein content the exact functioning of this intriguing motor remains unknown. The rotary motor consists of a rotor attached to a flagellum and of stators ‘pushing’ the rotor around. Stators diffuse in the cytoplasmic membrane upon engaging in the motor complex. The temporal resolution of our setup allows to investigate fast stator dynamics. We are studying the response of the motor at stall torque, forward rotation and backward rotation by optically adjusting the load torque on the motor, on which we present preliminary results. Deploying our optical setup we are trying to unravel the mechanism by which this molecular motor works to propel bacteria.