The effect of beta-galactosides on the protonmotive force and growth of Escherichia coli.

The effect of three beta-galactosides on the components membrane potential (delta psi) and pH gradient (delta pH) of protonmotive force and growth of Escherichia coli has been examined. A good correlation between the reduction of the protonmotive force and growth inhibition was observed. Thus some galactosides had little effect on either the protonmotive force or growth while lactose diminished the protonmotive force and caused growth inhibition. This effect of lactose was dependent on the ionic composition of the growth media. In Medium A (77 mM-Na+, 85 mM-K+) lactose diminished delta psi but had no effect on delta pH. Growth inhibition was transient at an external pH 6.0 but complete at pH 7.5. In medium KA (approximately 1 mM-Na+, 162 mM-K+) delta pH was diminished and delta psi was not affected and consequently growth inhibition was complete at pH 6.0. In medium NA (163 mM-Na+, 20 mM-K+) lactose had little effect on delta psi, delta pH or growth. These data support Skulachev's hypothesis of buffering of the protonmotive force by K+ and Na+ gradients.