Detection of meticillin resistance in Staphylococcus aureus isolates continues to be a challenge. Clinical specimens obtained from abscesses from two epidemiologically unrelated outpatients were positive for meticillin-resistant S. aureus (MRSA) by a commercial PCR assay, but colonies obtained by culture were susceptible to oxacillin by an automated testing method. The colonies were also negative using a penicillin-binding protein 2a (PBP2a) latex agglutination test. Because of the discrepancy between the genotypic and phenotypic results, both isolates were re-tested by PCR, disc diffusion, VITEK® 2 and MicroScan® and were plated on chromogenic agar. Both isolates also underwent cefoxitin induction for additional susceptibility testing studies. Following overnight induction with cefoxitin, both isolates demonstrated resistance to oxacillin and cefoxitin by the two automated methods and by disc diffusion, and were positive using PBP2a latex agglutination tests. Population analysis failed to identify heteroresistant subpopulations in uninduced isolates. Identifying the presence of MRSA by PCR directly in the specimens was critical for determining the appropriate course of antimicrobial therapy for the patients. Both infections resolved with non-β-lactam therapy.
Keywords: Cefoxitin screen; Meticillin resistance; Oxacillin; Staphylococcus aureus; mecA.
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