Proposed Changes to Clinical Laboratory Standards Institute Interpretive Criteria for Methicillin-Resistant Staphylococcus Pseudintermedius Isolated from Dogs

Thank you for publishing the recent paper by Schissler and colleagues 3 in which they proposed changes to the Clinical Laboratory Standards Institute (CLSI) interpretive criteria for methicillin-resistant Staphylococcus pseudintermedius isolated from dogs. The paper presented some important data and recommendations regarding the minimum inhibitory concentration (MIC) breakpoints and zone sizes. The findings of these authors were in agreement with another recent paper 1 on the identification of methicillin-resistant S. pseudintermedius from dogs. These authors concluded that the current CLSI interpretive criteria for S. pseudintermedius may not accurately identify some isolates that carry the mecA gene, which determines resistance mediated by PBP2a.

The current CLSI document 2 lists the oxacillin MIC breakpoint for resistant Staphylococcus aureus and other coagulase-positive staphylococci as ≥4 mg/ml, and the zone size for resistant isolates as ≤10 mm. As reported in the papers cited above, 1,3 these MIC and zone size interpretive criteria may not identify some methicillin-resistant S. pseudintermedius isolates. The Veterinary Antimicrobial Susceptibility Testing (VAST) subcommittee of the CLSI considered this issue at its 2009 meetings and reached a decision that was consistent with those subsequently expressed in these papers. 1,3 The subcommittee voted unanimously to revise the current standard to indicate that oxacillin disk diffusion (R ≥ 17 mm) and MIC breakpoints (R ≥ 0.5 μg/ml) applicable to Staphylococcus spp. are accurate indicators of mecA-mediated resistance in S. pseudintermedius. The VAST subcommittee agrees with the conclusions of Schissler et al. 3 and Bemis et al. 1 that the interpretive criteria applicable to S. aureus failed to detect a considerable number of mecA-positive S. pseudintermedius strains.

The revised standard for interpretation of oxacillin MIC breakpoints and disk diffusion zones will appear in Table 2 of CLSI document M31-M31, which is scheduled to be published in 2010. Until then, the CLSI VAST subcommittee recommends that diagnostic laboratories use the interpretive criteria suggested by the previously cited papers 1,3 for oxacillin disk diffusion (R ≤ 17 mm) and MIC breakpoints (R ≥ 0.5 μg/ml) to indicate mecA-mediated resistance in S. pseudintermedius.

It was also concluded in the papers cited 1,3 that the cefoxitin test is not appropriate for detecting methicillin resistance in S. pseudintermedius isolated from dogs. The CLSI VAST subcommittee agreed with these conclusions and also voted unanimously at the aforementioned meeting to remove Table 9D from M31 and to add a footnote to Table 2 stating that cefoxitin breakpoints are not predictive of mecA-mediated resistance to methicillin/oxacillin in S. pseudintermedius.

Once again, we thank the Journal of Veterinary Diagnostic Investigation for publishing these important papers. The CLSI VAST subcommittee, as always, welcomes input from veterinary diagnostic laboratories for suggestions to improve the standards published by our committee.

Sincerely,

Mark G. Papich, Vice Chair, on behalf of the Clinical

Laboratory Standards Institute Veterinary Antimicrobial

Susceptibility Testing (VAST) subcommittee

Professor of Clinical Pharmacology

Department of Molecular Biomedical Sciences

College of Veterinary Medicine

4700 Hillsborough Street

Raleigh, NC 27606

Phone: [phone]

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