By virtue of possessing alcohol dehydrogenase activity, cytosol prepared from Helicobacter pylori produces toxic acetaldehyde from ethanol in vitro. To approach the in vivo situation in the stomach, we have now investigation whether intact H. pylori--without addition of exogenous nicotinamide adenine dinucleotide--also forms acetaldehyde. Furthermore, to assess the energy metabolism of H. pylori, we determined whether the alcohol dehydrogenase-catalyzed reaction can run in the opposite direction with ethanol as the end-product and thereby yield energy for the organism. Intact H. pylori formed acetaldehyde already at low ethanol concentrations (at 0.5% ethanol, acetaldehyde, 64 +/- 21 and 75 +/- 9 mumol/l (mean +/- SEM) for strains NCTC 11637 and NCTC 11638, respectively). H. pylori produced ethanol in concentrations that can be significant for the energy metabolism of the organism. Acetaldehyde production by H. pylori may be an important factor in the pathogenesis of gastroduodenal diseases associated with the organism. The primary function of H. pylori alcohol dehydrogenase may, however, be alcoholic fermentation and consequent energy production under microaerobic conditions.