  The bacterial flagellar motor (BFM) is responsible for driving bacterial
locomotion and chemotaxis, fundamental processes in pathogenesis and biofilm
formation. In the BFM, torque is generated at the interface between
transmembrane proteins (stators) and a rotor. It is well-established that the
passage of ions down a transmembrane gradient through the stator complex
provides the energy needed for torque generation. However, the physics involved
in this energy conversion remain poorly understood. Here we propose a
mechanically specific model for torque generation in the BFM. In particular, we
identify two fundamental forces involved in torque generation: electrostatic
and steric. We propose that electrostatic forces serve to position the stator,
while steric forces comprise the actual 'power stroke'. Specifically, we
predict that ion-induced conformational changes about a proline 'hinge' residue
in an $\alpha$-helix of the stator are directly responsible for generating the
power stroke. Our model predictions fit well with recent experiments on a
single-stator motor. Furthermore, we propose several experiments to elucidate
the torque-speed relationship in motors where the number of stators may not be
constant. The proposed model provides a mechanical explanation for several
fundamental features of the flagellar motor, including: torque-speed and
speed-ion motive force relationships, backstepping, variation in step sizes,
and the puzzle of swarming experiments.
