Exposure to metronidazole in vivo readily induces resistance in Helicobacter pylori and reduces the efficacy of eradication therapy in mice

The Helicobacter pylori SS1 mouse model was used to characterize the develo pment of resistance in H. pylori after treatment with metronidazole monothe rapy and to examine the effect of prior exposure to metronidazole on the ef ficacy of a metronidazole-containing eradication regimen. Mice colonized wi th the metronidazole-sensitive H. pylori SS1 strain were treated for 7 days with either peptone trypsin broth or the mouse equivalent of 400 mg of met ronidazole once a day or three times per day (TID). In a separate experimen t, H. pylori-infected mice were administered either peptone trypsin broth o r the mouse equivalent of 400 mg of metronidazole TID for 7 days, followed 1 month later by either peptone trypsin broth or the mouse equivalent of 20 mg of omeprazole, 250 mg of clarithromycin, and 400 mg of metronidazole tw ice a day for 7 days. At least 1 month after the completion of treatment, t he mice were sacrificed and their stomachs were cultured for H. pylori. The susceptibilities of isolates to metronidazole were assessed by agar diluti on determination of the MICs. Mixed populations of metronidazole-resistant and -sensitive strains were isolated from 70% of mice treated with 400 mg o f metronidazole TID. The ratio of resistant to sensitive strains was 1:100, and the MICs for the resistant strains varied from 8 to 64 mu g/ml. In the second experiment, H. pylori was eradicated from 70% of mice treated with eradication therapy alone, compared to 25% of mice pretreated with metronid azole (P < 0.01). Mice still infected after treatment with metronidazole an d eradication therapy contained mixed populations of metronidazole-resistan t and -sensitive isolates in a ratio of 1:25. These results demonstrate tha t H. pylori readily acquires resistance to metronidazole in vivo and that p rior exposure of the organism to metronidazole is associated with failure o f eradication therapy. H. pylori-infected mice provide a suitable model for the study of resistance mechanisms in H. pylori and will be useful in dete rmining optimal regimens for the eradication of resistant strains.