Nonlinear dependency of the bacterial flagellar motor speed on proton motive force and its consequences for swimming

The bacterial flagellar motor enables bacteria to swim by rotating helical flagellar filaments that form a bundle at the back of the cell. Escherichia colis motor uses the energy stored in the electrochemical gradient of protons, the proton motive force (PMF), to generate the torque driving this rotation. Until now, motor speed was thought to be proportional to the PMF, irrespective of the viscous load on the motor, and across the physiological range of PMF values. Here, we show that the PMF-speed relationship is non-linear in the high-torque regime. Because saturation in the relationship occurs in the physiologically relevant range, it challenges all current models of motor function that assume a tight coupling between the motor rotation and PMF across the entire physiological range. Furthermore, we experimentally determine the load on the motor experienced by swimming cells, and show that free swimming occurs close to or within the saturation regime, making the observed limiting torque evolutionary relevant.