Evaluation of Susceptibility Test Breakpoints Used to Predict mecA-Mediated Resistance in Staphylococcus Pseudintevmedius Isolated from Dogs

Abstract

Abstract. 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 mg/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.

Evaluation of Clinical Laboratory Standards Institute Interpretive Criteria for Methicillin-Resistant Staphylococcus Pseudintermedius Isolated from Dogs

“…Furthermore, the findings presented in the current study demonstrate that an unacceptably higher rate of false-negative results occur with application of the 2008 CLSI criteria, consistent with recent work. 2 Therefore, it is advised that veterinary diagnostic laboratories abstain from using the 2008 CLSI criteria when evaluating methicillin resistance in canine S. pseudintermedius. The 2004 CLSI criteria for oxacillin Kirby-Bauer disk diffusion and oxacillin broth microdilution should be used to evaluate methicillin resistance in canine S. pseudintermedius isolates.…”

“…17,22 Methicillin-resistant S. pseudintermedius (MRSP) has been increasingly reported as a primary pathogen in canine infections. 2,12,13,19,23,24 All methicillin-resistant staphylococci are, by convention, regarded to be resistant to all b-lactam antibiotics in vivo. 5,7 Cephalosporins are the most common antimicrobials used to treat S. pseudintermedius infections in dogs 18 but are not efficacious for the treatment of MRSP infections.…”

Ever-increasing emergence of multi-drug resistant Staphylococcus pseudintermedius in the dog and its zoonotic potentials

“…The use of oxacillin disk diffusion along with the interpretative criteria applied to Staphylococcus aureus has led to a high percentage of false-negative results thus making it rather inappropriate as a screening tool (Bemis et al, 2009). In 2009, the Clinical and Laboratory Standards Institute (CLSI) of Veterinary Antimicrobial Susceptibility Testing (VAST) subcommittee re-evaluated the interpretative criteria, proposing an oxacillin MIC of ≥0.5 mg/L (agar and broth dilution) and a zone diameter of ≤17 mm (disc diffusion) as highly reliable for the detection of mecA in S. pseudintermedius strains (Papich, 2010).…”

Resisting the resistance. Is there progress in maintaining antimicrobial efficacy?

“…For example, in the Netherlands, there was an increase from 0.9% in 2004 to 7% in data published in 2016. [5][6][7][8][9][10][11][12][13] Vanessa Schmidt concluded that MDR strains were common in the population studied. It is increasingly important to obtain bacterial cultures before treatment.…”