A high percentage of subjects (77%) carrying a mix of metronidazole-susceptible/resistant strains have been reported . Mixed infections with metronidazole-resistant strains may not be eradicated by metronidazole-based therapy [36–38]. Additionally, patients with mixed infections in the corpus showed a significantly higher rate of intestinal metaplasia in the antrum . Our study showed that patients with duodenal ulcer have higher rates of mixed infections than of chronic gastritis. These findings support those of previous studies indicating that mixed infections facilitate interstrain gene transfer and genetic diversity, enhancing H. pylori survival in the harsh environment of the stomach where disease progression occurs [39, 40].