Background: The topical agent mupirocin plays a crucial role in strategies designed to control outbreaks of methicillin-resistant Staphylococcus aureus. The rate of high- and low-level mupirocin resistance among S. aureus strains from Iranian hospitals is not known.
Material and Method: Two hundred twenty-two nonduplicate S. aureus strains consecutively isolated in four university hospitals in Tehran, Iran, were tested for mupirocin susceptibility by disc diffusion agar method and minimum inhibitory concentration (MIC) determination by the E-test. Susceptibility to 16 other antimicrobial agents was also determined.
Results: With the disc diffusion agar method, the majority of strains (97.3%) were susceptible to mupirocin and only 2.7% were resistant. The S. aureus strains showed high resistance (>50%) to most antibiotics, including penicillin G, ampicillin-sulbactam, oxacillin, cefoxitin, ciprofloxacin, erythromycin, tetracycline, clindamycin, gentamicin, and rifampicin, but resistance to linezolid, chloramphenicol, cotrimoxazole, and quinupristin/dalfopristin was low and no isolate was resistant to vancomycin. In the E-test, six strains had MICs of >4 mg/l, i.e. five strains had MICs of 8-256 mg/l (low-level mupirocin resistance) and one strain had 1024 mg/l (high-level mupirocin resistance). One strain was resistant to mupirocin in the disc diffusion agar method but showed sensitivity in the E-test (MIC: 0.94 mg/l). The mupirocin-resistant S. aureus isolates were all methicillin resistant and more resistant to the other antimicrobial agents compared with the mupirocin-susceptible strains.
Conclusions: This study is the first report about mupirocin resistance of S. aureus in Iranian hospitals.

Keywords: Mupirocin - pharmacology, Microbial Sensitivity Tests, Methicillin-Resistant Staphylococcus aureus - physiology, Iran, Methicillin Resistance - physiology, Drug Resistance, Multiple, Bacterial - physiology, Anti-Bacterial Agents - pharmacology, Adult