Comparative in vitro antimicrobial susceptibility and synergistic activity of antimicrobial combinations against Helicobacter pylori isolates in Taiwan

BackgroundAntimicrobial resistance is a major determinant of Helicobacter pylori treatment failures. We conducted a population-based survey to monitor changing antimicrobial susceptibility of H. pylori isolates in Taiwan, with a focus on combinatorial effects of synergism and the influence of acidity.MethodsH. pylori isolates from endoscopic specimens taken from patients enrolled at two medical centers were obtained between January 2008 and December 2009. Minimum inhibitory concentrations (MICs) were determined by agar dilution and Etest methods. Agar media of varying pH (pH 7.3, 6.0, or 5.0) were used to assess whether acidity influences the bactericidal effects of the agents tested. Time-kill assays were used to assess for synergism between different drug combinations.ResultsA total of 176 non-duplicate H. pylori isolates from endoscopic specimens were tested. The following MIC90 (mg/L) (susceptible) results were obtained at neutral pH 7.3: amoxicillin, 0.25 (100%); tetracycline, 0.5 (100%); metronidazole, 32 (67.6%); clarithromycin, 0.25 (90.3%); ciprofloxacin, 1 (92.0%); gemifloxacin, 0.5 (94.9%); levofloxacin, 1 (93.2%); and moxifloxacin, 1 (91.5%). A decrease in pH from 6.0 to 5.0 significantly decreased the antimicrobial activity of levofloxacin and moxifloxacin against H. pylori. For clarithromycin-susceptible isolates, levofloxacin combined with clarithromycin provided both synergistic and bactericidal effects. For clarithromycin-resistant isolates with amoxicillin hypersusceptibility (MIC <0.01 mg/L), levofloxacin with amoxicillin or minocycline had at best additive effect but no bactericidal effects.ConclusionAmoxicillin, tetracycline, clarithromycin, and fluoroquinolones, but not metronidazole, showed good in vitro anti-H. pylori activity (>90% susceptible). Synergism was only observed for clarithromycin-susceptible isolates. Acidity adversely influenced the antimicrobial activity of levofloxacin against H. pylori.