Novel Plasmid-borne Gene QacJ Mediates Resistance to Quaternary Ammonium Compounds in Equine Staphylococcus Aureus, Staphylococcus Simulans, and Staphylococcus Intermedius

We identified a novel plasmid-borne gene (designated qacJ) encoding resistance to quaternary ammonium compounds (QACs) in three staphylococcal species associated with chronic infections in four horses. qacJ was located on a 2,650-bp plasmid (designated pNVH01), a new member of the pC194 family of rolling-circle replication plasmids. The 107-amino-acid protein, QacJ, showed similarities to known proteins of the small multidrug resistance family: Smr/QacC (72.5%), QacG (82.6%), and QacH (73.4%). The benzalkonium chloride MIC for a qacJ-containing recombinant was higher than those for otherwise isogenic recombinants expressing Smr, QacG, or QacH. Molecular epidemiological analyses by pulsed-field gel electrophoresis suggested both the clonal spread of a qacJ-harboring Staphylococcus aureus strain and the horizontal transfer of pNVH01 within and between different equine staphylococcal species. The presence of pNVH01 of identical nucleotide sequence in different staphylococcal species suggests that recent transfer has occurred. In three of the horses, a skin preparation containing cetyltrimethylammonium bromide had been used extensively for several years; this might explain the selection of staphylococci harboring the novel QAC resistance gene.