Qac genes genes and biocide tolerance in clinical veterinary methicillin-resistant and methicillin-susceptible Staphylococcus aureus and Staphylococcus pseudintermedius

Qac genes are associated with increased tolerance to quaternary ammonium compounds and other cationic biocides such as chlorhexidine. This study aimed to determine whether qac genes and increased biocide tolerance were present in 125 clinical methicillin-resistant and susceptible veterinary staphylococci. A total of 125 methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant and-susceptible Staphylococcus pseudintermedius (MRSP and MSSP) from three archived Australian veterinary staphylococci collections underwent whole genome sequencing, multilocus sequence typing and qac gene screening. Two MRSA isolates (12%) harboured qacA/B genes; both isolates were ST8 from horses. QacJ, qacG and smr genes were identified in 28/90 (31%) MRSP and 1/18 (6%) MSSP isolates. ST71 MRSP was significantly more likely to harbour qac genes than other MRSP clones (p < 0.05). A random subset of 31 isolates underwent minimum bactericidal concentration (MBC) testing against F10SC (TM) (benzalkonium chloride and biguanide), and Hexacon (TM) (chlorhexidine gluconate), with and without the addition of bovine serum albumin (BSA) as an in vitro substitute for organic matter contamination. Qac genes were not associated with increased phenotypic biocide tolerance but biocide efficacy was significantly affected by the presence of BSA. In the absence of BSA, all MBC values were well below the recommended usage concentration. When BSA was present, regardless of qac gene presence, 50% of MRSA and 43% of MRSP had an F10SC (TM) MBC above the recommended concentration for general disinfection. Qac genes did not confer increased in vitro biocide tolerance to veterinary staphylococci. Organic matter contamination must be minimized to ensure the efficacy of biocides against MRSA and MRSP.