Prevalence of aac (6′)-Ie-aph (2′′)-Ia resistance gene and its linkage to Tn5281 in Enterococcus faecalis and Enterococcus faecium isolates from Tabriz hospitals

High-level gentamicin resistance (HLGR: MIC ≥ 500 μg/ml) in Enterococci is mediated by aminoglycoside modifying enzymes which is mainly encoded by aac(6′)-Ie-aph(2′′)-Ia gene. The aim of this study was to evaluate the frequency of aac(6′)-Ie-aph(2′′)-Ia gene in clinical isolates of Enterococcus facium and Enterococcus faecalis collected from hospitals in northwest of Iran. In the present study a total of 111 enterococcus isolates were collected from 4 hospitals during a two year period (July 2009-August 2011). Bacterial identification and species determination were carried out by standard biochemical tests. Antimicrobial susceptibility was evaluated by Kirby Bauer disc diffusion method. MICs were determined by agar dilution method. The frequency of aac(6′)Ie-aph(2″)Ia gene in the isolates was determined by PCR. The carriage of resistance gene on Tn5281 transposon was identified by long PCR and dot-blot hybridization methods. Antibiotic susceptibility tests revealed that the highest resistance was against streptomycin (74.77%) and erythromycin (67.58%) whereas the highest susceptibility was observed to vancomycin (81.1%). 36 isolates (32.43%) were identified as HLGR, 34(94.44%) of them had resistant gene in their genome. Long PCR studies revealed that 88 % of HLGR clinical isolates harboured Tn5281. The aac(6′)Ie-aph(2″)Ia resistance gene was present on Tn5281 transposon in all 32 isolates according to dot blot hybridization test. The results of this study indicated that aac(6′)Ie-aph(2″)Ia resistance gene is highly prevalent in gentamicin resistant isolates. Carriage of aac(6′)Ie-aph(2″)Ia resistance gene on Tn5281 transposable element suggests possible contribution of this transposone on dissemination of resistance gene among enterococcus isolates.