Energetics of flagellar rotation in bacteria

Abstract

We have measured the rotation rates of tethered cells of Streptococcus strain V4051 as a function of dynamic load and protonmotive force. The cells do not spin in the absence of an exogenous energy source, but they start spinning 20 to 30 seconds after exposure to glucose. The angular velocity of metabolizing cells is inversely proportional to the viscosity of the medium. Starved cells spin within ten seconds after valinomycin is added to induce a potassium diffusion potential (cell interior negative) or after the medium is acidified to generate a transmembrane pH gradient (cell interior alkaline). In either case, the angular velocity of the cells is a linear function of the protonmotive force. These results imply that the passage of a fixed number of protons carries the flagellar motor through one revolution. The threshold protonmotive force for rotation, although not determined directly, appears to lie close to 0 mV. Starved cells also spin when protons move out of the cells in response to a diffusion potential or upon alkalinization of the medium. Thus, flagellar rotation can be driven by a protonmotive force of either sign.