Physical and biochemical characterization of the qacA gene encoding antiseptic and disinfectant resistance in Staphylococcus aureus.

We have previously cloned a 3.5 kb fragment from the Staphylococcus aureus multiresistance plasmid pSK1 which carries the qacA determinant responsible for linked resistance to acriflavine (Acr), ethidium bromide (Ebr), quaternary ammonium compounds (Qar), propamidine isethionate (Pir), and diamidinodiphenylamine dihydrochloride (Ddr). This report presents a biochemical and physical analysis of qacA and shows the widespread carriage of this gene on S. aureus resistance plasmids. Tn5 insertion mutagenesis defined the extent of qacA to within 2.40 kb of pSK1 DNA. Examination of the expression of insertion and deletion mutants of the cloned qacA sequences in both maxicells and minicells led to the association of a 50 kDa protein, designated QacA, with the AcrEbrQarPirDdr phenotype. Based on fluorimetric and isotopic assays used to determine the extent of accumulation of ethidium bromide by S. aureus strains harbouring pSK1, we propose that the basis of AcrEbrQarPirDdr in S. aureus is a qacA-mediated efflux system.