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

Karen K Ho; Austin C Conley; Robert A Kennis; Terri L Hathcock; Dawn M Boothe; Amelia G White

doi:10.1111/vde.12653

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

Vet Dermatol. 2018 May 29. doi: 10.1111/vde.12653. Online ahead of print.

Authors

Karen K Ho¹, Austin C Conley², Robert A Kennis¹, Terri L Hathcock³, Dawn M Boothe², Amelia G White¹

Affiliations

¹ Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, 1220 Wire Road, Auburn, AL, 36849, USA.
² Department of Anatomy, Physiology, and Pharmacology, College of Veterinary Medicine, Auburn University, 109 Greene Hall, Auburn, AL, 36849, USA.
³ Department of Pathobiology, College of Veterinary Medicine, Auburn University, 264 Greene Hall, Auburn, AL, 36849, USA.

PMID: 29845664
DOI: 10.1111/vde.12653

Abstract

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.