High-level resistance to ethidium bromide and antiseptics in Staphylococcus aureus

A gene encoding high-level resistance to ethidium bromide (EB) and antiseptics was isolated from a transferable plasmid, pTZ22, in Staphylococcus aureus. DNA sequencing revealed that the plasmid has two copies of the ebr gene that normally mediates low-level resistance to EB and antiseptics. The efflux rate for EB of strains with duplicated ebr genes was twice the rate of strains with a single ebr gene. It was concluded that the duplication of ebr is responsible for the high-level resistance to EB and antiseptics.