Membrane potential in stationary phase cells exposed to copper. 48 h MT or MT + P cells of the indicated strains were resuspended in T buffer and diluted in 5 mM HEPES buffer pH 7.5 to an OD560nm = 0.1. Fluorescence as Arbitrary Units (AU) was measured after addition of the specific dye DisC3. After dye stabilization 0.1 mM Cu2+ was added. ΔΨCu was the difference between the fluorescence value after 5 min incubation with Cu2+ (ΔΨf) and initial stabilization value (ΔΨi). Data are expressed as average ± SD of seven independent experiments. Different letters indicate significant differences according to Tukey’s test with a p-value of 0.05.