Role of membrane potential and calcium in chemotactic sensing by bacteria

Changes in Escherichia coli membrane potential in response to a wide variety of chemotactic stimuli were measured using a permeant, lipophilic cation (tetraphenylphosphonium chloride) and a fluorescent dye (3-propyl-2-(5-(3-propyl-2(3H)-benzothiazolylidene)-1,3-pentadienyl)iodide). Some attractants and repellants caused permanent, monotonic depolarizations or hyperpolarizations. Not all chemoeffectors, however, produced potential changes, and the direction of change did not correlate with physiological responses to these compounds. Moreover, changes were observed in a number of chemotactic mutants. From these results, we conclude that perturbations in membrane potential effected by chemical stimuli are not related to chemotactic sensing. These findings, and the close correlation between cytoplasmic ionic conditions and membrane potential, led us to examine the role of calcium in chemotaxis. By growing cells in the presence of a calcium chelator, we were able to lower cellular calcium levels over tenfold, with no change in behavior. These results indicate that sensory transduction in these cells is not mediated by this cation.