Minimum inhibitory concentration and killing properties of rifampicin against canine Staphylococcus pseudintermedius isolates from dogs in the southeast USA

Background Meticillin‐resistant (MR) staphylococcal pyoderma in dogs has led to increased use of alternate antibiotics such as rifampicin (RFP). However, little information exists regarding its pharmacodynamics in MR Staphylococcus pseudintermedius. Hypothesis/objectives To determine the minimum inhibitory concentration (MIC) and killing properties of RFP for canine Staphylococcus pseudintermedius isolates. Methods The MIC of RFP was determined using the ETEST® for 50 meticillin‐susceptible (MS) and 50 MR S. pseudintermedius isolates collected from dogs. From these isolates, two MS isolates (RFP MIC of 0.003 and 0.008 μg/mL, respectively) and two MR isolates (RFP MIC of 0.003 and 0.012 μg/mL, respectively) were subjected to time–kill studies. Mueller–Hinton broth was supplemented with RFP at 0, 0.5, 1, 2, 4, 8, 16 and 32 times the MIC for 0, 2, 4, 10, 16 and 24 h. The number of viable colony forming units in each sample was determined using a commercial luciferase assay kit. Results The MIC50 and MIC90 were the same for MS and MR isolates, at 0.004 μg/mL and 0.008 μg/mL, respectively. Rifampicin kill curves were not indicative of concentration‐dependency, suggesting time‐dependent activity. Two isolates (MS 0.003 and 0.008 μg/mL) exhibited bacteriostatic activity, whereas two others (MR 0.003 and 0.012 μg/mL) exhibited bactericidal activity. Conclusions and clinical importance This study demonstrated that MS and MR S. pseudintermedius isolates were equally susceptible to rifampicin and that dosing intervals should be designed for time‐dependent efficacy. These data can support pharmacokinetic studies of RFP in dogs with susceptible infections caused by S. pseudintermedius. Résumé Contexte Les pyodermites canines liées à des staphylocoques résistants à la méticiline (MR) mènent à une utilisation croissante d'antibiotiques alternatifs tels que la rifampicine (RFP). Cependant, peu de données existent sur leur pharmacodynamique face à Staphylococcus pseudintermedius MR. Hypothèses/Objectifs Déterminer la concentration minimale inhibitrice (MIC) et les propriétés bactéricides de RFP pour les souches canines de Staphylococcus pseudintermedius. Méthodes La MIC de RFP a été déterminée par ETEST® pour 50 souches de S. pseudintermedius sensible à la méticiline (MS) et 50 souches MR prélevées sur des chiens. De ces souches, deux MS (RFP MIC de 0.003 et 0.008 μg/mL, respectivement) et deux souches MR (RFP MIC de 0.003 et 0.012 μg/mL, respectivement) ont été étudiées s