Genetic analysis of methicillin-resistant Staphylococcus aureus expressing high- and low-level mupirocin resistance

Clinical strains of methicillin-resistant Staphylococcus aureus (MRSA) expressing high- and low-level mupirocin resistance were studied to determine the genetic location of mupirocin and other resistance determinants. Mupirocin resistance was confirmed by MIC determination with E-test strips. Curing and transfer experiments were used to establish the genetic location of the resistance determinants and the PCR with mupA-specific primers was used to detect the presence of mupA genes. High-level mupirocin-resistant isolates had MICs >1024 mg/L, whereas the low-level resistant isolates had MICs of 32-128 mg/L. The isolates carried plasmids ranging from 2.8 to 38 kb in size. All of them carried 26- and 3.0-kb plasmids, but only the high-level mupirocin-resistant isolates carried a 38-kb plasmid. Curing and transfer experiments revealed that the 26-kb plasmid encoded resistance to cadmium, mercuric chloride, propamidine isethionate and ethidium bromide and the 38-kb plasmid was a conjugative plasmid encoding high-level mupirocin resistance. One isolate, IBN287, carried both plasmid-borne high-level and chromosomal low-level mupirocin resistance. The mupA gene was detected on the 38-kb plasmid DNA but not in the genomic DNA of the low-level mupirocin-resistant isolates. The genomic DNA of strain IBN287 cured of the 38-kb mupirocin resistance plasmid did not contain mupA. The results suggest that different genes encoded low- and high-level mupirocin resistance in these isolates.