SignificanceThe bacterial flagellar motor (BFM) is the rotary motor powering swimming of many motile bacteria. Many of the components of this molecular machine are dynamic, a property which allows the cell to optimize its behavior in accordance with the surrounding environment. A prime example is the stator unit, a membrane-bound ion channel that is responsible for applying torque to the rotor. The stator units are mechanosensitive, with the number of engaged units dependent on the viscous load on the motor. We measure the kinetics of the stators as a function of the viscous load and find that the mechanosensitivity of the BFM is governed by a catch bond: a counterintuitive type of bond that becomes stronger under force.

The bacterial flagellar motor (BFM) is the rotary motor which powers the swimming and swarming of many motile bacteria. The torque is provided by stator units, ion motive force powered ion channels known to assemble and disassemble dynamically in the BFM. This turnover is mechano-sensitive, with the number of engaged units dependent upon the viscous load experienced by the motor through the flagellum. However, the molecular mechanism driving BFM mechano-sensitivity is unknown. Here we directly measure the kinetics of arrival and departure of the stator units in individual wild-type motors via analysis of high-resolution recordings of motor speed, while dynamically varying the load on the motor via external magnetic torque. Obtaining the real-time stator stoichiometry before and after periods of forced motor stall, we measure both the number of active stator units at steady-state as a function of the load and the kinetic association and dissociation rates, by fitting the data to a reversible random sequential adsorption model. Our measurements indicate that BFM mechano-sensing relies on the dissociation rate of the stator units, which decreases with increasing load, while their association rate remains constant. This implies that the lifetime of an active stator unit assembled within the BFM increases when a higher force is applied to its anchoring point in the cell wall, providing strong evidence that a catch-bond mechanism can explain the mechano-sensitivity of the BFM.