Cyanine Dye as Monitor of Membrane Potentials in Escherichia coli Cells and Membrane Vesicles

Abstract

The fluorescence response of a positively charged cyanine dye: 3,3''-dimethylindodicarbocyanine iodide can be specifically related to the generation in E. coli cells and E. coli membrane vesicles of an electrical membrane potential [.DELTA..psi.] induced either by substrate oxidation or by an artifically imposed K diffusion gradient. The energy-dependent quenching of the dye fluorescence correlates well with the known effect on .DELTA..psi. of: oxidation of various energy sources, external pH and solute accumulation. Thus, in the vesicles, the fluorescence quenching of the dye increases from succinate to D-lactate, to ascorbate/phenazine methosulfate and parallels the increasing ability of these electron donors to generate a .DELTA..psi.. In the vesicles, .DELTA..psi. is only weakly dependent on external pH, whereas in the cells, .DELTA..psi. increases with increasing external pH. Lactose accumulation in the vesicles results in the partial utilization of .DELTA..psi.. A calibration of the dye fluorescence in terms of .DELTA..psi. was determined using valinomycin-induced potassium diffusion potential.