Assessment of chlorohexidine and benzalkonium chloride activity against Staphylococcus pseudintermedius from skin and soft-tissue infections in companion animals

Introduction: Staphylococcus pseudintermedius is the main bacterial pathogen responsible for skin and soft-tissue infections (SSTIs) in companion animals and is increasingly associated with antimicrobial resistance (AMR). Objective: This study analyzed the effectiveness of the biocides chlorhexidine (CHX) and benzalkonium chloride (BAC) against S. pseudintermedius causing SSTIs in companion animals. Methodology: Time-kill assays were performed according to the European Standard EN 1040 for CHX and BAC for the reference strain S. pseudintermedius DSM 21284T and, for CHX for two clinical isolates with representative AMR traits (methicillin-resistant and multidrug-resistant) and clonal lineages (ST71/ST118) among a collection of 155 S. pseudintermedius. The strains were exposed for 24h to the biocides at concentrations varying from sub-inhibitory concentrations (1/2 MIC) up to the in-use concentration. The killing effect of the biocides was evaluated at several time points. Results: CHX showed effectiveness (no bacterial growth) against the three S. pseudintermedius strains studied at the recommended in-use concentration and time of exposure (10 min). However, at this time point, a lower efficacy (reduction up to ≈4 log CFU/mL) was observed for lower concentrations (≤ 10x MIC). After 24h of exposure, no bacterial growth was observed for all concentrations, except for one isolate exposed to the lowest concentration studied (1/2 MIC). BAC demonstrated effectiveness against S. pseudintermedius DSM 21284T at the recommended use conditions, but bacterial growth was observed for lower concentrations. At 24h of exposure, no growth was detected at all concentrations. Conclusions: This undergoing study suggests that the CHX and BAC effect is immediate when used at the recommended in-use concentration and exposure time. However, lower biocide concentrations showed a lower efficacy and, in some cases, allowed bacterial growth at 24h of exposure. These preliminary data indicate that the inappropriate use of biocides could potentially select strains with lower susceptibility to biocides and antibiotics that share the same resistance mechanisms and thus may promote AMR dissemination in this relevant veterinary pathogen.publishe