Background and Aims: Responses of the proton motive force (the driving force for protons) in Helicobacter pylori to varying medium pH may explain gastric colonization. The aim of this study was to determine the effect of external pH (pH(out)) on the proton motive force, the sum of the pH gradient, and the potential difference across the bacterial membrane. Methods: Intracellular pH (PH(in)) was measured by bis-carboxyethyl- carboxyfluorescein fluorescence and transmembrane potential difference (PD) by fluorescent quenching of 3,3'-dipropyl thiadicarbocyanine iodide at differing pH(out) and was correlated with survival. Results: PD was -131 ± 0.36 mV (n = 3), and ph(in) was about 8.4 at loading PH(out) 7.0. PD increased as PH(out) was increased from 4.0 to 8.0, giving a constant proton motive force of about -220 mV. Outside these limits, PD collapsed irreversibly to zero. Addition of 5 mmol/L urea to weak buffer at pH 3.0 or 3.5 prevented irreversible collapse of PD by elevation of PH(out) caused by NH3 production. Urea addition to weak buffer at pH 7.0 collapsed the PD as urease activity increased the pH(out) to about 8.4. Survival was also limited to this range of pH(out). Conclusions: H. pylori survives over the range of PH(out) where it maintains a proton motive force. The effect of urease activity on PH(out), while allowing gastric survival in acidic media, may limit survival in nonacidic media.
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