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 × 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 × MIC in MDR‐MRSP1 isolates at 24 h, 32 × MIC in MDR‐MRSP2 at 48 h, 32 × MIC in MDR‐MRSP3 at 48 h and 256 × 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 × 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 6 mg/kg for MDR‐MRSP pyoderma treatment when limited antibiotic options are available. Résumé Contexte Les résistances aux antibiotiques sont de plus en plus importantes pour les dermatites canines à Staphylococcus pseudintermedius. Le traitement à la rifampicine (RFP) est envisagé seulement pour les MDR‐MRSP (S. pseudintermedius résistant à la méticilline et multi résistant). Hypothèses/Objectifs Déterminer une dose optimale de RFP pour le traitement des MDR‐MRSP sans induire de résistance à RFP et identifier les mutations en cause pour la résistance antimicrobienne. Matériels et methods Le temps d’élimination a été réalisé pour une souche contrôle et trois MDR‐MRSP à six concentrations cliniquement importantes [32 à 1,024 × MIC (minimum inhibitory concentration)]. Le séquençage de tout le génome et l’analy