Accumulation of thallous ions (Tl+) as a measure of the electrical potential difference across the cytoplasmic membrane of bacteria.

The accumulation of thallous ions (204Tl+) by intact bacteria was investigated. I conclude that Tl+ is a permeant cation, and that it therefore accumulates in response to the electrical potential difference (delta psi) across the cytoplasmic membrane (interior negative). A comparison with other methods shows that the distribution ratio of 204Tl+ serves as a reasonably satisfactory method for measuring the membrane potential of Streptococcus faecalis. Glycolyzing cells of this organism develop membrane potentials of up to 180 mV. Preliminary experiments with Escherichia coli, especially those with a mutant defective in the proton-translocating ATPase, indicate that the Tl+ distribution also serves as a measure of the membrane potential in this organism. The particular advantage of Tl+ over other indicators of the membrane potential is that the cells need not be pretreated in any way. By use of the Tl+ distribution, it was calculated that respiring cells of E. coli develop a membrane potential of 160 mV with D-lactate and 180 mV with glucose as a substrate, respectively.