The Importance of Urease in Acid Protection for the Gastric-colonising BacteriaHelicobacter pyloriandHelicobacter felissp. nov

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Abstract
The urease of the gastric pathogen Helicobacter pylori shares numerous characteristics with the urease of a bacterium isolated from cat gastric mucosa, Helicobacter felis sp. nov. The native enzymes have similar apparent molecular weights and, when subjected to SDSPAGE, disassociate into active subunits of comparable relative mobilities. In contrast, a bacterium (Stl) that colonises rodent ileal mucosa, and is related to Helicobacter spp., expresses a distinct urease from those of the gastric bacteria. Cell-free preparations of H. felis and of four strains of H. pylori had mean apparent Km values of 0.5 ± 0.2 mM and 0.4 ± 0.1 mM urea, respectively. The Km value estimated for cell-free extracts of St 1 was 1.7 ± 0.3 mM urea. Under physiological conditions ([urea] = 3 mM), this difference was unlikely to be important. Of several ureolytic bacteria tested, only H. pylori and H. felis tolerated acid when a physiological level of urea was present. It was concluded that urease is important for the protection of H. felis and H. pylori from lethal gastric acidity. This acid protection may be dependent upon the intrinsic acid stability of the ureases, both enzymes retaining substantial activity between pH 4 and 6. Inhibition of urease activity in vivo, by treatment of mice with acetohydroxamic acid, significantly reduced gastric colonisation by H. felis, whilst treatment of already colonised animals had no effect. This finding confirms the importance of urease in the initial steps of colonisation, prior to entry of bacteria into the neutral mucous layer.