Stator pool size shapes the mechanosensitivity of the bacterial flagellar motor

The bacterial flagellar motor adapts to mechanical load by adding or removing stator units, which can either be inactive in the inner membrane (the stator pool) or attached to the motor in a torque-generating configuration. This adaptation is well understood, but what sets the load range over which the motor responds remains unclear. Building on a previous two-state model of stator binding, we derive how the on-rate depends on the diffusing stator pool and find that it increases in proportion to pool size. The model predicts that pool size governs motor occupancy at low load but not at high load, where the motor saturates regardless of how many stators are available. We confirmed this prediction in Escherichia coli: increasing motAB induction increased swimming speed and single-motor rotation rate at low load. This dependence on induction weakened as load rose and was undetectable at high load. The model further predicts that larger pools make the motor mechanosensitive at lower loads, which our measurements of load-induced occupancy change support. Mechanosensitivity is therefore not fixed by motor architecture alone, but also determined by stator abundance, which cells can set through expression to position their mechanosensory range.