Physical and Biochemical Characterization of the qacA Gene Encoding Antiseptic and Disinfectant Resistance in Staphylococcus aureus

1 Department of Microbiology, Monash University, Clayton, Victoria 3168, Australia 2 Department of Biochemistry, University of Hull, Hull HU6 7RX, UK ABSTRACT 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 (Ac r ), ethidium bromide (Eb r ), quaternary ammonium compounds (Qa r ), propamidinc isethionate (Pi r ), and diamidinodiphenylamine dihydrochloride (Dd r ). This report presents a biochemical and physical analysis of qacA and shows the widespread carriage of this gene on S. aureus resistance plasmids. Tn 5 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 Ac r Eb r Qa r Pi r Dd r 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 Ac r Eb r Qa r Pi r Dd r in S. aureus is a qacA -mediated efflux system. Present address: Department of Microbiology, University of Umeå, S-901 87, Umeå, Sweden. Present address: CSIRO Division of Plant Industry, Canberra, ACT 2601, Australia.