Calibrated electrorotation for characterizing the torque generation by individual flagellar motors

Bacterial flagellar motor is one of the most complex and sophisticated nano machineries in nature. A torque-speed curve indicates the performance of a motor and contains fruitful information about its torque generation mechanism. However, a precise measurement of the torque-speed characteristic of a single flagellar motor is still challenging. With the conventional methods based on the beads assay, the characteristics of individual motors are lost due to the averaging of multiple torque-speed curves. This limits the detailed modelling of the rotation mechanism. Here, we demonstrate to measure the torque-speed relationship of individual flagellar motors of Salmonella under dynamically-controlled external load by a calibrated electrorotation method. The obtained torque-speed curves have a steep rise near the stall, indicating the presence of a barrier to reverse the rotation.