Abstract
Many bacterial species swim using flagella. The flagellar motor couples ion flow across the cytoplasmic membrane to rotation. Ion flow is driven by both a membrane potential ( V m) and a transmembrane concentration gradient. To investigate their relation to bacterial flagellar motor function we developed a fluorescence technique to measure V m in single cells, using the dye tetramethyl rhodamine methyl ester. We used a convolution model to determine the relationship between fluorescence intensity in images of cells and intracellular dye concentration, and calculated V m using the ratio of intracellular/extracellular dye concentration. We found V m = −140 ± 14 mV in Escherichia coli at external pH 7.0 ( pH ex), decreasing to −85 ± 10 mV at pH ex 5.0. We also estimated the sodium-motive force (SMF) by combining single-cell measurements of V m and intracellular sodium concentration. We were able to vary the SMF between −187 ± 15 mV and −53 ± 15 mV by varying pH ex in the range 7.0–5.0 and extracellular sodium concentration in the range 1–85 mM. Rotation rates for 0.35- μm- and 1- μm-diameter beads attached to Na +-driven chimeric flagellar motors varied linearly with V m. For the larger beads, the two components of the SMF were equivalent, whereas for smaller beads at a given SMF, the speed increased with sodium gradient and external sodium concentration.
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