Prevalence of aminoglycoside-modifying enzymes genes among isolates of Enterococcus faecalis and Enterococcus faecium in Iran.

Disks containing 120 microg of gentamicin were used to detect high-level gentamicin-resistant phenotype (HLGR) among isolates of Enterococcus faecalis (n = 79) and E. faecium (n = 35). These isolates were collected from three hospitals in Tehran during 2002-2004. The macrobroth dilution assay was then used to determine the minimum inhibitory concentration (MIC) of gentamicin. The susceptibility of isolates against amikacin, netilmicin, tobramycin, and kanamycin were also determined by Kirby-Bauer method. All isolates were subjected to polymerase chain reaction (PCR) assays targeting aminoglycoside modifying enzyme (AMEs) genes including aac(6 ')-aph(2 "), aph(2 ")-Ib, aph(2 ")-Ic, aph(2 ")-Ia, aph(2 ")-Id, aph(3 ')-IIIa, and ant(4 ')-Ia. Fifty-nine isolates (52%) showed HLGR phenotype. All isolates with HLGR phenotype and those showing 64 < MIC < 500 microg/ml contained aac(6 ')-aph(2 "). The aph(3 ')-IIIa was found in 61% of the isolates with HLGR phenotypes and in 65% of isolates with MIC < 500. Coexistence of aac(6 ')-aph(2 ") and aph(3 ')-IIIa gene among HLGR isolates of E. faecalis and E. faecium were 60% and 65%, respectively. The gene aph(2 ")-Ic was amplified in two isolates of E. faecium. The results of PCR for aph(2 ")-Id, ant(4 ')-Ia and aph(2 ")-Ib genes were negative. The aac(6 ')-aph(2 ") was the most frequent gene encoding resistance to gentamicin and other aminoglycosides followed by aph(3 ')-IIIa. Isolates lacking these genes were susceptible to all aminoglyocosides tested in this study.