Identification of chromosomal location of mupA gene, encoding low-level mupirocin resistance in staphylococcal isolates

Low- and high-level mupirocin resistance have been reported in Staphylococcus aureus. The expression of plasmid-encoded mupA is responsible for high-level mupirocin resistance. Low-level mupirocin-resistant strains do not contain plasmid-encoded mupA, and a chromosomal location for this gene has not previously been reported. We examined high- and low-level mupirocin-resistant S. aureus strains to determine if mupA was present on the chromosome of low-level-resistant isolates. Southern blot analysis of DNA from four mupirocin-resistant strains identified mupA in both high- and low-level mupirocin-resistant strains. Low-level mupirocin-resistant strains contained a copy of mupA on the chromosome, while the high-level mupirocin-resistant isolate contained a copy of the gene on the plasmid. PCR amplification of genomic DNA from each mupirocin-resistant strain resulted in a 1.65-kb fragment, the predicted product from the intragenic mupA primers. This is the first report of a chromosomal location for the mupA gene conferring low-level mupirocin resistance.