Thermal fluctuations assist high-speed rotation of the bacterial flagellar motor at low load

The bacterial flagellar motor is a proton-driven rotary nanomachine that converts ion flow into mechanical motion. Proton-coupled rotation of the MotA5-MotB2 stator complex generates torque, yet how this process drives high-speed rotation remains unclear. Here we use single-molecule nanophotometry to resolve stepwise rotation under low-load conditions. Lowering intracellular pH selectively prolongs dwell times without affecting step durations, indicating that proton dissociation triggers torque generation. Structural analysis further suggests that the rotor disengages from the stator before completing the elementary step angle (~11 degree), implying that the power stroke alone is insufficient. Notably, the rotational diffusion coefficient of stator-less motors closely matches that inferred from wild-type motors at low load. These findings support a model in which intrinsic thermal fluctuations compensate for the limited reach of the power stroke, enabling rapid rotation. Our results reveal a hybrid mechanism in which ion-driven conformational changes bias stochastic motion to achieve efficient energy transduction.