Evaluation of Oxacillin and Cefoxitin Disk and MIC Breakpoints for Prediction of Methicillin Resistance in Human and Veterinary Isolates of Staphylococcus intermedius Group

Abstract

Staphylococcus pseudintermedius is a coagulase-positive species that colonizes the nares and anal mucosa of healthy dogs and cats. Human infections with S. pseudintermedius range in severity from bite wounds and rhinosinusitis to endocarditis; historically, these infections were thought to be uncommon, but new laboratory methods suggest that their true incidence is underreported. Oxacillin and cefoxitin disk and MIC tests were evaluated for the detection of mecA-or mecC-mediated methicillin resistance in 115 human and animal isolates of the Staphylococcus intermedius group (SIG), including 111 Staphylococcus pseudintermediusand 4 Staphylococcus delphini isolates, 37 of which were mecA positive. The disk and MIC breakpoints evaluated included the Clinical and Laboratory Standards Institute (CLSI) M100-S25 Staphylococcus aureus/Staphylococcus lugdunensis oxacillin MIC breakpoints and cefoxitin disk and MIC breakpoints, the CLSI M100-S25 coagulase-negative Staphylococcus (CoNS) oxacillin MIC breakpoint and cefoxitin disk breakpoint, the CLSI VET01-S2 S. pseudintermedius oxacillin MIC and disk breakpoints, and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) S. pseudintermedius cefoxitin disk breakpoint. The oxacillin results interpreted by the VET01-S2 (disk and MIC) and M100-S25 CoNS (MIC) breakpoints agreed with the results of mecA/mecC PCR for all isolates, with the exception of one false-resistant result (1.3% of mecA/ mecC PCR-negative isolates). In contrast, cefoxitin tests performed poorly, ranging from 3 to 89% false susceptibility (very major errors) and 0 to 48% false resistance (major errors). BD Phoenix, bioMérieux Vitek 2, and Beckman Coulter MicroScan commercial automated susceptibility test panel oxacillin MIC results were also evaluated and demonstrated >95% categorical agreement with mecA/mecC PCR results if interpreted by using the M100-S25 CoNS breakpoint. The Alere penicillin-binding protein 2a test accurately detected all mecA-positive isolates, although for four isolates, cefoxitin induction was required prior to testing. These data demonstrate that the cefoxitin surrogate test does not reliably detect the presence of mecA in S. pseudintermedius isolates and that laboratories should perform oxacillin disk or MIC tests of these isolates when they are encountered. Staphylococcus pseudintermedius is a coagulase-positive, hemolytic species that colonizes the nares and anal mucosa of healthy dogs and cats (1-4). Clinically, S. pseudintermedius is the most common cause of canine pyoderma and occasionally causes urinary tract and joint infections in dogs and cats (5-8). Phenotypically, S. pseudintermedius is difficult to differentiate from Staphylococcus intermedius and Staphylococcus delphini, which are also coagulase-positive veterinary staphylococci. Collectively, these three species are referred to as the S. intermedius group (SIG). Differentiation of the members of this group requires molecular analysis (9), although matrix-assisted laser desorption ionization-time of flig...

Multicenter Evaluation of a Modified Cefoxitin Disk Diffusion Method and PBP2a Testing To Predict mecA -Mediated Oxacillin Resistance in Atypical Staphylococcus aureus

“…If the colony morphotypes observed at F1 bred true to the F2 plate, these were tested independently. Each colony morphotype was evaluated for the presence of mecA by PCR in one laboratory, using a method described previously (16).…”

Diagnostic microbiology in veterinary dermatology: present and future

“…44 Although the cefoxitin disk test is generally recognized as reliable for MRSA detection, a recent study has shown that cefoxitin may not be a good surrogate for MRSP detection by disk diffusion. 47 In the absence of an internationally recognized cefoxitin breakpoint clearly differentiating mecA-positive from mecA-negative isolates of S. pseudintermedius, we recommend that laboratories use oxacillin disk or MIC tests for detection of meticillin resistance in this and other staphylococcal species, other than S. aureus.…”

The Brief Case: Staphylococcus intermedius Group—Look What the Dog Dragged In

“…As in S. aureus, methicillin resistance in SIG species is conferred by alternative penicillin binding proteins (PBPs) through the acquisition of the mecA gene, which encodes PBP2a (10). Unlike in S. aureus and the coagulase-negative staphylococci, cefoxitin has proven to be a poor surrogate indicator of methicillin resistance in SIG isolates; compared to the presence of mecA as determined by PCR, the use of cefoxitin as a surrogate indicator of methicillin resistance resulted in both major and very major errors (5). Rather, oxacillin is the appropriate surrogate for methicillin resistance in SIG isolates (Fig.…”

“…It is recognized that this testing can be problematic for some clinical laboratories, since automated identification and susceptibility systems (such as the BD Phoenix and bioMérieux Vitek 2 systems) may not routinely report an oxacillin MIC result (5) and since some clinical laboratories do not use oxacillin for antimicrobial susceptibility testing. For this reason, immunochromatographic testing for the presence of PBP2a has been studied in SIG isolates and was shown to have 100% sensitivity and 100% specificity for the detection of methicillin resistance in a comparison with mecA PCR (10), particularly when induced PBP2a testing with cefoxitin was used (5). Isolates of the SIG can also grow on chromogenic media for the detection of methicillin-resistant S. aureus (MRSA), such as Spectra MRSA agar (Remel Microbiology Products, Lenexa, KS).…”

“…Rather, oxacillin is the appropriate surrogate for methicillin resistance in SIG isolates (Fig. 2D) (5,10). Oxacillin susceptibility breakpoints for SIG members have been added to CLSI supplement M100 for this purpose (1).…”