Susceptibility of enterohaemorrhagic Escherichia coli to azithromycin in France and analysis of resistance mechanisms

Abstract

Objectives: Antibiotic therapy is not recommended during enterohaemorrhagic Escherichia coli (EHEC) infection. However, the potential benefit of azithromycin has recently been described. This study was conducted to evaluate the macrolide susceptibility of EHEC isolates from France and to analyse the mechanisms of resistance to azithromycin in EHEC. Methods: Strains from patients with EHEC infections were collected by the associated French National Reference Laboratory for E. coli from 2004 to 2014. Antimicrobial susceptibility testing was performed by disc diffusion and Etest methods. For strains presenting macrolide resistance, antibiotic resistance genes were searched for by PCR. Genetic transfer was performed by conjugation and plasmid analysis was done by Southern-blot hybridization after PFGE. Results: We tested 508 isolates of EHEC. Azithromycin MICs ranged between 0.25 and 16 mg/L (median1/43mg/L), except for two atypical strains, 34396 and 36493, for which MICs were >256 mg/L. Plasmid transferability of macrolide resistance was demonstrated. Strain 34396, of serotype O106:H18, harboured two macrolide resistance genes [mph(A) and erm(B)] and two other antimicrobial resistance genes (blaDHA-1 for β-lactam resistance and qnrB4 for quinolone resistance). mph(A), blaDHA-1 and qnrB4 were localized on the same plasmid. Strain 36493 belonging to the highly virulent O26:H11 EHEC clonal group harboured a plasmid containing mph(A). Conclusions: For the first time we describe plasmid-borne macrolide resistance genes in EHEC strains. Dissemination of such plasmids may threaten the potential benefit of azithromycin during the management of patients infected or colonized with EHEC. Determination of azithromycin MICs should be considered prior to its therapeutic use and 16 mg/L may be used as the epidemiological cut-off value.