In Vitro Susceptibility and Synergistic Effect of Bismuth Against Helicobacter pylori

<b>Background/objectives:</b> Bismuth is commonly used in <i>Helicobacter pylori</i> (<i>H. pylori</i>) eradication therapy. However, few studies have examined the in vitro susceptibility of <i>H. pylori</i> to bismuth. Moreover, the exact mechanism of action of bismuth on <i>H. pylori</i> remains unclear. The aim of this study was to identify the anti-bacterial effect of bismuth as well as to evaluate potential synergistic effects between bismuth and various antibiotics. <b>Methods:</b> The minimum inhibitory concentrations (MICs) of three bismuth preparations, bismuth subsalicylate, bismuth potassium citrate, and colloidal bismuth subcitrate (CBS, De-Nol) were determined for <i>H. pylori</i> strains using the agar dilution technique. Agar plates of varying pH values from 5.0 to 8.0 were used to investigate whether acidity influences the anti-bacterial effect of bismuth. A checkerboard assay was performed to assess the synergism between CBS and antibiotics (amoxicillin, clarithromycin, and metronidazole). <b>Results:</b> Twelve <i>H. pylori</i> strains, including three reference strains (<i>H. pylori</i> 26695, J99, and ATCC 43504), and nine clinically isolated strains were tested. The MICs for bismuth subsalicylate, bismuth potassium citrate, and CBS ranged from 4 to 32 μg/mL, 2 to 16 μg/mL, and 1 to 8 μg/mL, respectively. The bismuth MICs for the reference strains were similar at pH 5-8. In the checkerboard assay, no interactions between CBS and any of the antibiotics were observed in the reference <i>H. pylori</i> strains. <b>Conclusions:</b> Bismuth showed in vitro susceptibility against <i>H. pylori</i>. The enhanced eradication efficacy of bismuth-containing regimens appears to be due to mechanisms other than direct synergy with antibiotics.