Genomic and in vitro pharmacodynamic analysis of rifampicin resistance in multidrug-resistant canine Staphylococcus pseudintermedius isolates.

Abstract

Antimicrobial resistance is a growing concern in canine Staphylococcus pseudintermedius dermatitis. Treatment with rifampicin (RFP) is considered only in meticillin-resistant and multidrug-resistant S.pseudintermedius (MDR-MRSP). To determine an optimal RFP dosing for MDR-MRSP treatment without induction of RFP resistance and identify causal mutations for antimicrobial resistance. Time-kill assays were performed in a control isolate and three MDR-MRSP isolates at six clinically relevant concentrations [32 to 1,024×MIC (the minimum inhibitory concentration)]. Whole-genome resequencing and bioinformatic analysis were performed in the resistant strains developed in this assay. The genomic analysis identified nine antimicrobial resistance genes (ARGs) in MDR-MRSP isolates, which are responsible for resistance to seven classes of antibiotics. RFP activity against all four isolates was consistent with a time-dependent and bacteriostatic response. RFP resistance was observed in six of the 28 time-kill assays, including concentrations 64×MIC in MDR-MRSP1 isolates at 24h, 32×MIC in MDR-MRSP2 at 48h, 32×MIC in MDR-MRSP3 at 48h and 256×MIC in MDR-MRSP3 at 24h. Genome-wide mutation analyses in these RFP-resistant strains discovered the causal mutations in the coding region of the rpoB gene. A study has shown that 6mg/kg per os results in plasma concentrations of 600-1,000×MIC of S.pseudintermedius. Based on our data, this dose should achieve the minimum MIC (×512) to prevent RFP resistance development; therefore, we recommend a minimum daily dose of 6mg/kg for MDR-MRSP pyoderma treatment when limited antibiotic options are available.