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Evaluation of susceptibility test breakpoints used to predict mecA-mediated resistance in Staphylococcus pseudintermedius isolated from dogs

Correspondence: ¹Corresponding Author: David A. Bemis, Department of Comparative Medicine, College of Veterinary Medicine, The University of Tennessee, Knoxville, TN 37901-1071. bemis{at}utk.edu

Clinical and Laboratory Standards Institute interpretive breakpoints for in vitro susceptibility tests that predict mecA-mediated oxacillin resistance in Staphylococcus pseudintermedius isolates from animals have been changed twice in the past decade. Moreover, there are no counterpart recommendations for human isolates of S. pseudintermedius. Individual medical and veterinary laboratories variably use interpretive breakpoints identical to those recommended for use with Staphylococcus aureus or identical to those recommended for use with coagulase-negative staphylococci. The purpose of the current study was to examine correlations between oxacillin disk diffusion, oxacillin gradient diffusion, oxacillin microbroth dilution, and cefoxitin disk diffusion tests used to predict mecA-mediated resistance in S. pseudintermedius and to retrospectively estimate, from disk diffusion zone diameter measurements, the prevalence and rate of increase of oxacillin resistance among canine S. pseudintermedius isolates submitted to a veterinary teaching hospital laboratory. Oxacillin disk diffusion zone diameters of 17 mm and oxacillin minimum inhibitory concentrations of 0.5 µg/ml were highly correlated with detection of mecA in canine S. pseudintermedius isolates by polymerase chain reaction. MecA-mediated resistance among S. pseudintermedius isolates from dogs increased from less than 5% in 2001 to near 30% in 2007. More than 90% of the methicillin-resistant S. pseudintermedius isolates in 2006 and 2007 were also resistant to representatives of 4 additional antimicrobial drug classes. Cefoxitin disk diffusion with the resistance breakpoint set at 24 mm significantly underestimated the presence of mecA in S. pseudintermedius.

Key Words: Breakpoints • cefoxitin • dogs • mecA • oxacillin • Staphylococcus pseudintermedius

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