Copyright C 1992, American Society for Microbiology Further Evidence of the Toxic Effect of Ammonia Produced by

Former studies have shown that Helicobacter pylori can induce vacuolation of epithelial cells in vitro and possibly in vivo, either by direct action of a cytotoxin or by the action of its strong urease, which breaks down the urea physiologically present in the stomach into cytotoxic ammonia. We have developed a test using HEp2 cells with adherent H. pylori bacteria in order to compare the effects of an H. pylori urease-negative variant with those of its urease-positive parent strain in the presence of 10 mM urea. The level of ammonia production as well as cell vacuolation and viability were monitored for 72 h. The ammonia produced (20 mM) was found to be the essential determinant of the degree of cell vacuolation and viability of HEp2 cells. However, the addition of acetohydroxamic acid (200 mg/liter), a potent urease inhibitor which inhibits ammonia production, did not completely restore cell growth, suggesting the difficulty of neutralizing the ammonia in the vicinity of the cells. Antibodies directed against H. pylori did not neutralize the urease activity. When H. mustelae was tested in the same manner, the detrimental effects were not observed because a lower quantity of ammonia (5 mM) was generated. This was due to a lower urease activity, although the adherence properties of H. mustelae were different from those of H. pyloni both quantitatively (greater adherence) and qualitatively (localized instead of diffuse adherence). We conclude that H. pylori-induced ammonia is an essential determinant of its cell toxicity as well as its adherence properties, which allow a high concentration of ammonia at the cellular level