Figure 4.

Calibration of DiSC₃(5) assay. (A) Fluorescence intensity of DiSC₃(5) in B. subtilis cell suspensions in media with varying K⁺ concentrations. Upon addition of K⁺ carrier valinomycin (4 μM), membrane potential reaches stable levels pre-determined by the K⁺ gradient across the membrane (see Table 2). The time point of valinomycin addition is highlighted with an arrow. (B) The fluorescence levels obtained from the experiment shown in panel (A) can be used to calibrate the arbitrary DiSC₃(5) fluorescence values. The estimated membrane potential for untreated B. subtilis cells (−110 mV) is indicated with a dashed line. Strain used: B. subtilis 168 (wild type).