Background: Gastrointestinal infections with pathogenic Helicobacter species are commonly treated with combination therapies, which often include amoxicillin. Although this treatment is effective for eradication of Helicobacter pylori, the few existing reports are less clear about antibiotic susceptibility of other Helicobacter species. In this study we have determined the susceptibility of gastric and enterohepatic Helicobacter species to amoxicillin, and have investigated the mechanism of amoxicillin resistance in Helicobacter hepaticus.
Materials and methods: The minimal inhibitory concentration (MIC) of antimicrobial compounds was determined by E-test and agar/broth dilution assays. The hefA gene of H. hepaticus was inactivated by insertion of a chloramphenicol resistance gene. Transcription was measured by quantitative real-time polymerase chain reaction.
Results: Three gastric Helicobacter species (H. pylori, H. mustelae, and H. acinonychis) were susceptible to amoxicillin (MIC < 0.25 mg/L). In contrast, three enterohepatic Helicobacter species (H. rappini, H. bilis, and H. hepaticus) were resistant to amoxicillin (MIC of 8, 16, and 6–64 mg/L, respectively). There was no detectable β-lactamase activity in H. hepaticus, and inhibition of β-lactamases did not change the MIC of amoxicillin of H. hepaticus. A H. hepaticus hefA (hh0224) mutant, encoding a TolC-component of a putative efflux system, resulted in loss of amoxicillin resistance (MIC 0.25 mg/L), and also resulted in increased sensitivity to bile acids. Finally, transcription of the hefA gene was not responsive to amoxicillin, but induced by bile acids.
Conclusions: Rodents are frequently colonized by a variety of enterohepatic Helicobacter species, and this may affect their global health status and intestinal inflammatory responses. Animal facilities should have treatment strategies for Helicobacter infections, and hence resistance of enterohepatic Helicobacter species to amoxicillin should be considered when designing eradication programs.