Prevalence of ant(4′)-Ia gene among clinical isolates of methicillin-resistant Staphylococcus aureus using Multiplex-PCR method

Methicillin-resistant Staphylococcus aureus is a major cause of nosocomial and community-acquired infections. Aminoglycosides are potent bactericidal agents that are often used in combination with either a β-lactam or a glycopeptide, especially in the treatment of staphylococcal endocarditis. The main mechanism of aminoglycoside resistance in staphylococci is drug inactivation by cellular aminoglycoside-modifying enzymes. The main aim of the present study is determining the prevalence of ant(4′)-Ia gene encoding one of the most important aminoglycoside-modifying enzymes and simultaneous detection of mecA gene responsible for methicillin resistance in clinical isolates of Staphylococcus aureus by Multiplex-PCR method. A total of 100 clinical S. aureus isolates were collected from Shariati and Baqiatollah hospitals in Tehran, then antibiotic susceptibility pattern of strains were determined by disk diffusion method using penicillin, oxacillin, vancomycin, tetracycline, erythromycin, gentamicin, tobramycin, amikacin, netilmicin and kanamycin disks, considering CLSI principles. Using agar dilution method the MIC for oxacillin, gentamicin, tobramycin and amikacin were also determined. In order to detect resistance genes, ant(4′)-Ia and mecA, two pairs of specific primers were used and their prevalence was determined by using a Multiplex-PCR method. All strains were resistant to penicillin (100%) and after that the highest rate of resistance was observed against kanamycin (68%), tetracycline (61%), erythromycin (56%), tobramycin (53%), gentamicin (52%), amikacin and oxacillin (48%) and netilmicin (22%), respectively. All of the strains were also susceptible to vancomycin. In agar dilution method 50% of strains were oxacillin resistant and 49%, 45% and 51% of the strains showed resistance toward gentamicin, amikacin and tobramycin, respectively. Thirty-seven percent of the strains also showed high-level gentamicin resistance with MIC of ≥128µg/ml. In Multiplex-PCR method 53% of the strains possessed mecA gene and 58% of the strains were ant(4′)-Ia positive. Results obtained by phenotypic and genotypic antibiotic susceptibility determination tests show that there is a statistically meaningful relationship between methicillin resistance and aminoglycoside resistance in MRSA strains (P<0.05).