Model of the bacterial flagellar motor: response to varying viscous load.

A model of bacterial flagellar drive by cytomembrane streaming is described and applied to experiments of bacterial propulsion under varying viscous load. The theory predicts a linear dependence of the reciprocal propulsion velocity on the viscosity of the suspension medium, if the velocity of cytomembrane streaming far from the basal body of the flagella is assumed independent of the external viscosity. The theoretical predictions are in good agreement with experiments on free-swimming and on tethered bacteria. From comparison of the theory with experiments, the surface viscosity of the bacterial cytomembrane is evaluated for different bacterial species and turns out to be in the range observed experimentally on lipid monolayers.