A chromosomal location of the mupA gene in Staphylococcus aureus expressing high-level mupirocin resistance

Objectives: To investigate the genetic location of the mupA gene in high-level mupirocin-resistant Staphylococcus aureus isolates. Materials and methods: Antibiotic resistance was detected by disc diffusion. The Etest was used to determine mupirocin MIC. The presence of mupA was detected by PCR using specific primers. Curing, transfer experiments, pulsed-field gel electrophoresis (PFGE) and DNA hybridization were used to study the genetic location of mupA. Results: The isolates had mupirocin MICs > 1024 mg/L and were resistant to methicillin, gentamicin, kanamycin, streptomycin, erythromycin, tetracycline, ciprofloxacin, cadmium acetate, propamidine isethionate and ethidium bromide. They carried two plasmids of similar to26 and 2.8 kb. Curing and transfer experiments demonstrated that the 26 kb plasmid encoded resistance to cadmium acetate, propamidine isethionate and ethidium bromide. Loss of mupirocin resistance corresponded to the loss of a 40 kb DNA fragment from a 175 kb Smal chromosomal fragment. The mupA gene was detected only in the genomic DNA of the mupirocin-resistant strains and in their derivatives cured of the 26 kb plasmid. A labelled mupA probe hybridized to the 175 kb Smal fragment only in the mupirocin-resistant isolates. Conclusion: The absence of mupA on any of the plasmids and its detection only in the chromosomal DNA of the parents and in their derivatives cured of the 26 kb plasmid strongly supports a chromosomal location for mupA in these isolates.