Nonlinear dependency of the bacterial flagellar motor speed on proton motive force and its consequences for swimming
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The bacterial flagellar motor enables bacteria to swim by rotating helical flagellar filaments that form a bundle at the back of the cell. Escherichia coli ’s motor uses the energy stored in 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. Here, we show that the PMF-speed proportionality saturates at high load and high PMF, and, thus, the PMF-speed relationship is nonlinear in the high torque regime. Furthermore, we show that free swimming occurs close to or within this saturated regime, making the observed limiting torque evolutionary relevant.
