Identification of major sequence types among aminoglycoside resistant Staphylococcus aureus and Staphylococcus epidermidis strains isolated from clinical samples

Aminoglycosides have been widely used for treating severe staphylococcal infections. Production aminoglycoside modifying enzymes (AMEs) is the main mechanism of resistance to this antibiotic. The aim of this study was to determine the prevalence of AME genes and molecular characterization of aminoglycoside-resistant Staphylococcus aureus and Staphylococcus epidermidis strains isolated from clinical specimens in Iran.

A total of 42 clinical isolates of Gram-positive cocci (20 S. aureus and 22 S. epidermidis) with resistance to gentamicin were tested for antimicrobial resistance and differentiated by multilocus sequence typing (MLST).

All 42 isolates were resistant to methicillin, kanamycin,and most of them were also resistant to amikacin (98%), tobramycin (98%) and netilmycin (78.5%). Overall, aac(6’)-Ie-aph(2’’)-Ia was the dominant AME gene found in 100% of isolates, followed by aph(3')IIIa found in 90% of isolates. MLST classified S. aureus and S. epidermidis into 5 and 9 distinct sequence types (ST), respectively. The majority of the strains belonged to ST239 (50%) for S. aureus and ST2 (36%) for S. epidermidis.

The resistance to aminoglycosides was mainly due to the presence of the aac(6’)-Ie-aph(2’’)-Ia and aph(3')IIIa genes as well as the ST239 for S. aureus and ST2 for S. epidermidis have become the predominant clones in the selected university hospital of Tehran, Iran. Thus, it is critical that clinicians and healthcare workers are aware of the population of S. aureus and S. epidermidis present in order to make decisions for appropriate treatment and infection control practices.