Steps, shot noise and diffusion in the bacterial flagellar motor

Many bacteria like {\em E. coli} swim by virtue of small rotary motors that drive rotation of helical flagella. Each motor is powered by a transmembrane proton flux passing through the motor. This flux is converted into torque with near-perfect efficiency by a mechanism whose details remain largely unknown. First I will describe the important biophysical properties of the motor, as measured in experiments, including the recent observation of a stepping behaviour at low speeds. I will then present a simple physical model that allows us to explain most of these data, but also to make new predictions. In particular, I will show how steps can be interpreted as barrier-crossing events in a corrugated energy landscape. Then I will show how to use our model to calculate the effect of shot noise (due to the discrete nature of the energy source--the protons) on motor diffusivity, and thus propose experiments to measure the proton cooperativity in the torque generation process.