Local pH elevation mediated by the intrabacterial urease of Helicobacter pylori cocultured with gastric cells

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Abstract
Helicobacter pylori resists gastric acidity by modulating the proton-gated urea channel UreI, allowing for pHout-dependent regulation of urea access to intrabacterial urease. We employed pH- and Ca2+-sensitive fluorescent dyes and confocal microscopy to determine the location, rate, and magnitude of pH changes in an H. pylori-AGS cell coculture model, comparing wild-type bacteria with nonpolar ureI-deletion strains (ureI-ve). Addition of urea at pH 5.5 to the coculture resulted first in elevation of bacterial periplasmic pH, followed by an increase of medium pH and then pH in AGS cells. No change in periplasmic pH occurred in ureI-deletion mutants, which also induced a slower increase in the pH of the medium. Pretreatment of the mutant bacteria with the detergent C12E8 before adding urea resulted in rapid elevation of bacterial cytoplasmic pH and medium pH. UreI-dependent NH3 generation by intrabacterial urease buffers the bacterial periplasm, enabling acid resistance at the low urea concentrations found in gastric juice. Perfusion of AGS cells with urea-containing medium from coculture at pH 5.5 did not elevate pHin or [Ca2+]in, unless the conditioned medium was first neutralized to elevate the NH3/NH4+ ratio. Therefore, cellular effects of intrabacterial ammonia generation under acidic conditions are indirect and not through a type IV secretory complex. The pHin and [Ca2+]in elevation that causes the NH3/NH4+ ratio to increase after neutralization of infected gastric juice may contribute to the gastritis seen with H. pylori infection.