Role of the rdxA and frxA genes in oxygen-dependent metronidazole resistance of Helicobacter pylori

1 Department of Gastroenterology and Hepatology, Erasmus MC-University Medical Center, Rotterdam, The Netherlands 2 Department of Internal Medicine, Bethesda Hospital, Hoogeveen, The Netherlands 3 Regional Public Health Laboratory Groningen/Drenthe, Groningen, The Netherlands Correspondence Arnoud H. M. van Vliet a.h.m.vanvliet{at}erasmusmc.nl Received April 15, 2004 Accepted July 23, 2004 Almost 50 % of all Helicobacter pylori isolates are resistant to metronidazole, which reduces the efficacy of metronidazole-containing regimens, but does not make them completely ineffective. This discrepancy between in vitro metronidazole resistance and treatment outcome may partially be explained by changes in oxygen pressure in the gastric environment, as metronidazole-resistant (Mtz R ) H. pylori isolates become metronidazole-susceptible (Mtz S ) under low oxygen conditions in vitro . In H. pylori the rdxA and frxA genes encode reductases which are required for the activation of metronidazole, and inactivation of these genes results in metronidazole resistance. Here the role of inactivating mutations in these genes on the reversibility of metronidazole resistance under low oxygen conditions is established. Clinical H. pylori isolates containing mutations resulting in a truncated RdxA and/or FrxA protein were selected and incubated under anaerobic conditions, and the effect of these conditions on the MICs of metronidazole, amoxycillin, clarithromycin and tetracycline, and cell viability were determined. While anaerobiosis had no effect on amoxycillin, clarithromycin and tetracycline resistance, all isolates lost their metronidazole resistance when cultured under anaerobic conditions. This loss of metronidazole resistance also occurred in the presence of the protein synthesis inhibitor chloramphenicol. Thus, factor(s) that activate metronidazole under low oxygen tension are not specifically induced by low oxygen conditions, but are already present under microaerophilic conditions. As there were no significant differences in cell viability between the clinical isolates, it is likely that neither the rdxA nor the frxA gene participates in the reversibility of metronidazole resistance. Abbreviations: Mtz R , metronidazole-resistant; RAPD, random amplified polymorphic DNA. The GenBank accession numbers for the rdxA and frxA gene sequences of seven metronidazole-resistant H. pylori strains are AY568322–AY568328 and AY568330–AY568336.