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

Abstract

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. To determine the minimum inhibitory concentration (MIC) and killing properties of RFP for canine Staphylococcus pseudintermedius isolates. 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 0.008μg/mL, respectively) and two MR isolates (RFP MIC of 0.003 and 0.012μg/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 24h. The number of viable colony forming units in each sample was determined using a commercial luciferase assay kit. The MIC50 and MIC90 were the same for MS and MR isolates, at 0.004μg/mL and 0.008μg/mL, respectively. Rifampicin kill curves were not indicative of concentration-dependency, suggesting time-dependent activity. Two isolates (MS 0.003 and 0.008μg/mL) exhibited bacteriostatic activity, whereas two others (MR 0.003 and 0.012μg/mL) exhibited bactericidal activity. 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.