title: Helicobacter pylori: in vitro induction of resistance to azithromycin.
abstract: Helicobacter pylori resistance to macrolides is possibly an important factor for the failure of macrolide therapy for H. pylori infection. The aim of this study was to assess the propensity of H. pylori to develop in vitro resistance to azithromycin. In 73 clinical isolates taken from patients before starting antimicrobial therapy of H. pylori infection, MIC was determined using an agar dilution method (Müller-Hinton agar with 7.5% unlysed horse blood, pH = 7.2, at 35 degrees C, during 72 h in a humid microaerobic atmosphere). Each strain was first cultivated at half minimal inhibitory concentration (MIC) then in doubling concentrations until growth arrest. All experiments for induction of resistance were performed on the same media, incubation temperature, atmosphere and time of MIC determination. MIC interpretative standards for sensitivity, intermediate sensitivity and resistance of H. pylori to azithromycin were < or = 2, 4 and > or = 8 mg/l, respectively. Of 73 strains, 5 died during the experiments, and in the remaining 68 strains, serial passage with increasing azithromycin concentrations resulted in the development of resistance in 19 (26.9%) strains. Two strains had an MIC of 16 mg/l azithromycin. Thirty-three (48.5%) strains kept the same MIC or doubled their MIC, 16 (23.5%) strains had 4- to 16-fold MIC but still remained sensitive, 2 resistant strains had 128-fold MICs and 17 resistant strains had increased their MICs more than 128 times. Seventeen highly resistant strains (MIC > 128 mg/l) were kept frozen at -70 degrees C for 3 months in a brain-heart infusion broth with 15% glycerol. MIC was assessed again to determine the stability of resistance. Eleven strains kept MICs > 128 mg/l, 2 became sensitive and 1 intermediate, but reverted easily, after only 2 passages, to an MIC of > 128 mg/l azithromycin. Although macrolides are very active against H. pylori, the propensity to develop resistance in a high proportion of strains has a clear impact on the choice of the right combinations of macrolides with other agents as well as the dosage of the macrolide antibiotics.
authors.affiliation: Clinical Hospital Centre Zagreb, Department of Clinical and Molecular Microbiology, Croatia. skalenic@mamef.mef.hr
authors.forename: S
authors.initials: S
authors.lastname: Kalenić
authors.forename: V
authors.initials: V
authors.lastname: Plecko
authors.forename: S
authors.initials: S
authors.lastname: Kresić
authors.forename: V
authors.initials: V
authors.lastname: Presecki
authors.forename: V
authors.initials: V
authors.lastname: Tripković
authors.forename: L
authors.initials: L
authors.lastname: Zele-Stracević
authors.forename: M
authors.initials: M
authors.lastname: Katicić
authors.forename: M
authors.initials: M
authors.lastname: Dominis
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D017963:Azithromycin / Q000494:pharmacology*; D004352:Drug Resistance, Microbial; D016480:Helicobacter pylori / Q000187:drug effects*; D006801:Humans; D008826:Microbial Sensitivity Tests
chemical_list: D000900:Anti-Bacterial Agents; D017963:Azithromycin