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The components of the transmembrane electrochemical proton gradient, or proton motive force (PMF, Δp), 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 (ΔpH) and membrane electrical potential (ΔΨ) were assayed with the weak acid, salicylic 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 Δp 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 ehemostat, repression of Ell glc 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 Δp values due to the dissipation of the ΔΨ. When the cells of S. mutans Ingbritt were grown with excess glucose (nitrogen limitation), lower Δp values were observed at pH 5.5, but not at pH 7.0. These results indicate that the generation of proton motive force is maintained by S. mutans and S. sobrinus during growth in continuous culture, except when the organisms are stressed at high growth rates and with excess glucose at low pH. Furthermore, the data indicate that, unlike other bacteria, these oral streptococci do not maintain significant pH gradients (i.e., < 0.74) even when growing in continuous culture at pH values as low as 5.0.
Ian R. Hamilton (Mon,) studied this question.