Characterisation of genes encoding aminoglycoside-modifying enzymes among meticillin-resistant Staphylococcus aureus isolated from two hospitals in Tehran, Iran.

Meticillin-resistant Staphylococcus aureus (MRSA) is a major hospital pathogen and typically shows resistance to multiple antimicrobial agents. The susceptibility patterns of 109 MRSA isolates to aminoglycoside antibiotics were determined by the disk diffusion method. Genes encoding the aminoglycoside-modifying enzymes (AMEs) were targeted by polymerase chain reaction (PCR) assays. All isolates were also subjected to multiplex PCR to determine the distribution of staphylococcal cassette chromosome mec (SCCmec) types in the study population. The rates of resistance to various antibiotics were as follows: kanamycin, 97%; tobramycin, 96%; gentamicin, 87%; amikacin, 93%; and netilmicin, 80%. The most prevalent AME genes were aac(6')-Ie-aph(2'') (83%) followed by aph(3')-IIIa (71%). Coexistence of three AME genes was detected in 21% of isolates. The ant(4')-Ia gene was the least frequent AME gene among MRSA isolates (26%). Of the 109 isolates, 106 (97%) were identified as SCCmec type III or IIIA and 3 (3%) as SCCmec type IV. The majority of MRSA isolates belonged to SCCmec III or IIIA and carried the aac(6')-Ie-aph(2'') gene, which is consistent with results of susceptibility testing of these isolates against aminoglycosides.