Dynamic velocity response of E. coli powered by proteorhodopsin

Escherichia coli swimming motility is powered by the flagellar motor, a rotary nanomachine driven by inward proton flux through its torque-generating stators. How these proton currents arise from proton motive force is often described using a simple circuit model, in which the membrane acts as a capacitor discharging through the flagellar motors and other resistive proton channels. By monitoring the swimming activity of E. coli expressing a light-driven outward proton pump, we probe the dynamical response of the system under tunable optical driving and test the limits of simplified circuit-based description. Our results show that the flagellar motors are not the main sink for proton motive force discharge. Instead, other membrane channels carry a larger proton current and exhibit a nonlinear resistive behavior. Using the same experimental approach, we directly quantify proteorhodopsin pumping activity as a function of illumination wavelength and compare it with previously reported absorption spectra.