The components of the transmembrane electrochemical proton gradient, or proton motive force (PMF, delta p), were determined in cells of Streptococcus mutans Ingbritt and Streptococcus sobrinus ATCC 27352 growing in continuous culture under conditions of changing glucose concentration, growth rate and growth pH. The pH gradient (delta pH) and membrane electrical potential (delta psi) were assayed with the weak acid, salicyclic acid, and the lipophilic cation, methyltriphenylphosphonium iodide, respectively. S. mutans Ingbritt growing in continuous culture (pH 7.0, dilution rate (D) = 0.1 h-1) at 8 glucose concentrations ranging from 2.8 to 288 mM maintained a relatively constant delta p of 58.3 mV (SD +/- 5.8) in spite of a transition from glucose to nitrogen-limited growth and significant changes in cell physiology. Changes included a decreasing yield constant, increasing glucose uptake rates in the chemostat, repression of Ellglc of the PEP phosphotransferase sugar transport system and decreasing glycolytic capacity of the cells as the medium glucose concentration increased. Changes in the dilution or growth rate of S. mutans Ingbritt from 0.1 to 1.0 h-1 and S. sobrinus from 0.1 to 0.8 h-1, when growing at pH 7.0 with limited glucose and lactose, respectively, resulted in significantly lower delta p values due to the dissipation of the delta psi. When the cells of S. mutans Ingbritt were grown with excess glucose (nitrogen limitation), lower delta p values were observed at pH 5.5, but not at pH 7.0.(ABSTRACT TRUNCATED AT 250 WORDS)