Phenotypic and genotypic determinants of mupirocin resistance among Staphylococcus aureus isolates recovered from clinical samples of children: an Iranian hospital-based study.

Backgrounds: The aim of this study was to evaluate both phenotypic and genotypic determinants of mupirocin resistance among methicillin-resistant Staphylococcus aureus (MRSA) and methicillin susceptible S. aureus (MSSA) strains recovered from different clinical samples of children who were admitted to the Children's Medical Center (CMC) Hospital, Tehran, Iran.

Materials and methods: A total of 120 clinical isolates of S. aureus were collected from the microbiology laboratory of CMC Hospital. Antimicrobial susceptibility of the isolates to different antimicrobial agents was determined by disk diffusion method. The methicillin resistance phenotype (MRSA) was identified using a 30 µg cefoxitin disk. The minimum inhibitory concentration (MIC) of mupirocin was determined by broth microdilution method. Strains with mupirocin MIC between 8 and 256 µg/mL were considered as low-level mupirocin resistant (LLMR), and strains with an MIC≥512 µg/mL were considered as high-level mupirocin resistant (HLMR). The presence of genes encoding HLMR (ie, mup A and mup B genes) was evaluated by PCR method.

Results: Four out of 120 isolates (3%) had mupirocin MIC≥512 µg/mL and were HLMR; however, no LLMR isolate was detected. Fifty-two isolates (43%) were MRSA, and there were no differences in the distribution of mupirocin resistance among MRSA and MSSA isolates ( P >0.05). The PCR method identified mup A gene in two out of four HLMR isolates, and mup B gene was not detected in any HLMR isolates.

Conclusion: Because of discrepancies between the phenotypic and genotypic patterns of mupirocin resistance and due to the avoidance of false-negative results, it is better to determine the mupirocin resistance by both antibiotic susceptibility tests and PCR method. Considering the increasing need of mupirocin for the control of S. aureus infections, continuous checking of its susceptibility status is necessary.