Proton chemical potential, proton electrical potential and bacterial motility

Abstract

Bacillus subtilis, maintained at an external pH of 5·5, was found to have a transmembrane proton chemical potential difference of 80mV (inside alkaline) as the only contribution to protonmotive force under these conditions; the electrical potential difference was negligible. (Escherichia coli, in contrast, maintains an appreciable electrical potential over a wide range of pH conditions ( Padan et al., 1976 ; Ramos & Kaback, 1977a ; this paper).) B. subtilis cells at pH 5·5, energized by proton chemical potential alone, were vigorously and indefinitely motile. The mean swimming speed of 40 μm per second was the same as at external pH 7·5, where motility was driven by electrical potential alone. Measurements of motility as a function of protonmotive force at pH 7·5 and 5·5 indicated that (1) a threshold of 30 mV was needed for motility, (2) a further increase to 60 mV yielded a rapid increase in swimming speed, and (3) the motors appeared to be saturated above 60 mV, since no further increase in swimming speed occurred. These characteristics applied to motility driven either by proton chemical potential or by proton electrical potential, although slightly higher potentials were needed in the former case. The implications of a device that converts diffusion potential into sustained mechanical work are discussed.