Distinction between changes in membrane potential and surface charge upon chemotactic stimulation of Escherichia coli

Galactose and other chemotactic attractants have been shown to trigger an apparent hyperpolarization in Escherichia coli (Eisenbach, M., 1982, Biochemistry, 21:6818–6825). The probe used to measure membrane potential in that study, tetraphenylphosphonium (TPP+), may respond also to surface-charge changes in the membrane. The distinction between true changes in membrane potential and changes in the surface charge of the membrane is crucial for the study of this phenomenon in bacterial chemotaxis. To distinguish between these parameters, we compared the response to galactose with different techniques: K+ distribution in the presence of valinomycin (measured with a K+-selective electrode), TPP+ distribution (measured with a TPP+-selective electrode) at different ionic strengths, absorbance changes of bis(3-phenyl-5-oxoisoxazol-4-yl)pentamethineoxonol (oxonol V), and fluorescence changes of three probes with different mechanisms of response. All the techniques revealed stimulation by galactose of transient hyperpolarization, of comparable magnitude. This indicates the involvement of ion currents rather than alterations of local surface properties.