Chemotaxis of Spirochaeta aurantia: involvement of membrane potential in chemosensory signal transduction.

The effects of valinomycin and nigericin on sugar chemotaxis in Spirochaeta aurantia were investigated by using a quantitative capillary assay, and the fluorescent cation, 3,3'-dipropyl-2,2'-thiodicarbocyanine iodide was used as a probe to study effects of chemoattractants on membrane potential. Addition of a chemoattractant, D-xylose, to cells in either potassium or sodium phosphate buffer resulted in a transient membrane depolarization. In the presence of valinomycin, the membrane potential of cells in potassium phosphate buffer was reduced, and the transient membrane depolarization that resulted from the addition of D-xylose was eliminated. Although there was no detectable effect of valinomycin on motility, D-xylose taxis of cells in potassium phosphate buffer was completely inhibited by valinomycin. In sodium phosphate buffer, valinomycin had little effect on membrane potential or D-xylose taxis. Nigericin is known to dissipate the transmembrane pH gradient of S. aurantia in potassium phosphate buffer. This compound did not dissipate the membrane potential or the transient membrane depolarization observed upon addition of D-xylose to cells in either potassium or sodium phosphate buffer. Nigericin did not inhibit D-xylose taxis in either potassium or sodium phosphate buffer. This study indicates that the membrane potential but not the transmembrane pH gradient of S. aurantia is somehow involved in chemosensory signal transduction.