engleski

Helicobacter pylori : In vitro induction of resistance to azithromycin

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 MIC was determined using an agar dilution method (Mueller-Hinton agar with 7.5% unlysed horse blood, pH=7.2, at 350C, during 72h in a humid microaerobic athmosphere). Each strain was first cultivated at a half of MIC concentration, then in doubling concentrations until showing no growth. All experiments for induction of resistance were performed on the same media, incubation temperature, atmosphere and time of MIC determination. Out of 73 strains, five died during experiments, and in the remaining 68 strains serial passage on increasing azithromycin concentrations resulted in development of resistance in 19 (26.9%) strains. Two strains had MIC of 16 mg/L azithromycin. Thirty-three (48.5%) strains remained in the same or double MIC, 16 (23.5%) strains had 4 to 16- fold MIC but still remainded sensitive, 2 resistant strains had 128-fold MIC and 17 resistant strains had MIC increased by more than 128 times. Seventeen highly resistant strains (MIC>128mg/l) were kept frozen at -70 0C for 3 months in 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 clear impact on choosing the right combinations of macrolides with other agents as well as dosage of macrolide antibiotics.

Helicobacter pylori; azithromycin; acquired resistance; resistance induction

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