Fig. 2.
Elimination of the urea/ammonium gradient. (A) A schematic cross section of the uninfected gastric mucosa. Under normal conditions urea diffuses from the arterial plasma into the mucus and is transported with it into the lumen. Thus the concentration of urea is constant throughout the mucus layer. (B) The build-up of a gradient in the Helicobacter-infected mucosa. The bacterial urease converts the urea into ammonia. The ammonia immediately reacts to ammonium, neutralizing the bacterial intracellular pH. Most of the urea entering the mucus layer is converted to ammonium, thus creating a gradient, urea at the tissue side and ammonium at the luminal side. (C) To invert these concentrations, we used dialysis to apply ammonium and reduce the plasma urea concentration. High flow of a luminal solution containing urea washed away the luminal ammonium. This inversion of urea and ammonium concentrations should eliminate the gradient, but it failed to affect bacterial orientation. Urea/ammonium is therefore not the critical gradient that Helicobacter uses to orient itself.