  Many swimming bacteria are propelled by flagellar motors that stochastically
switch between the clockwise and counterclockwise rotation direction. While the
switching dynamics are one of the most important characteristics of flagellar
motors, the mechanisms that control switching are poorly understood. We present
a statistical-mechanical model of the flagellar rotary motor, which consists of
a number of stator proteins that drive the rotation of a ring of rotor
proteins, which in turn drives the rotation of a flagellar filament. At the
heart of our model is the assumption that the rotor protein complex can exist
in two conformational states corresponding to the two respective rotation
directions, and that switching between these states depends on interactions
with the stator proteins. This naturally couples the switching dynamics to the
rotation dynamics, making the switch sensitive to torque and speed. Another key
element of our model is that after a switching event, it takes time for the
load to build up, due to polymorphic transitions of the filament. Our model
predicts that this slow relaxation dynamics of the filament, in combination
with the load dependence of the switching frequency, leads to a characteristic
switching time, in agreement with recent observations.
