Genomic And In Vitro Pharmacodynamic Analysis Of Rifampicin Resistance In Multidrug-Resistant Canine Staphylococcus Pseudintermedius Isolates

Background 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).Hypothesis/Objectives To determine an optimal RFP dosing for MDR-MRSP treatment without induction of RFP resistance and identify causal mutations for antimicrobial resistance.Methods and materials Time-kill assays were performed in a control isolate and three MDR-MRSP isolates at six clinically relevant concentrations [32 to 1,024 x MIC (the minimum inhibitory concentration)]. Whole-genome resequencing and bioinformatic analysis were performed in the resistant strains developed in this assay.Results 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 x MIC in MDR-MRSP1 isolates at 24 h, 32 x MIC in MDR-MRSP2 at 48 h, 32 x MIC in MDR-MRSP3 at 48 h and 256 x MIC in MDR-MRSP3 at 24 h. Genome-wide mutation analyses in these RFP-resistant strains discovered the causal mutations in the coding region of the rpoB gene.Conclusions and clinical relevance A study has shown that 6 mg/kg per os results in plasma concentrations of 600-1,000 x MIC of S. pseudintermedius. Based on our data, this dose should achieve the minimum MIC (x512) to prevent RFP resistance development; therefore, we recommend a minimum daily dose of 6 mg/kg for MDR-MRSP pyoderma treatment when limited antibiotic options are available.