We describe an unusual clinical strain of catalase-negative methicillin-resistant Staphylococcus aureus sensu stricto. Sequence analysis of its catalase gene showed 99.60% identities to the catalase genes of the reference strains. A 5-base deletion, however, led to a shift of the nucleotide reading frame and a loss of the enzymatic activity.

Penicillin-binding protein (PBP) 2a latex agglutination was compared with conventional susceptibility testing and mecA real-time PCR for the detection of oxacillin resistance in Staphylococcus aureus. Inoculum volume and induction with oxacillin were PBP 2a testing variables. For coagulase-negative Staphylococcus, an increased inoculum volume of 10 l greatly reduced the number of isolates requiring induction.Staphylococcus aureus and coagulase-negative Staphylococcus (CoNS) are common isolates from positive blood and sterile body fluids (1). At our institution, oxacillin resistance in these organisms is relatively high; 47% of S. aureus and 71% of CoNS isolates are resistant to oxacillin by conventional (phenotypebased) antimicrobial susceptibility testing (AST) (2004 antibiogram; University of North Carolina Hospitals, McLendon Clinical Laboratories). The mecA product, penicillin-binding protein (PBP) 2a, mediates oxacillin resistance. PBP 2a has a lower affinity for beta-lactam antibiotics than do endogenous PBPs of staphylococci, thus allowing peptidoglycan synthesis in the presence of lethal doses of beta-lactams. Due to high rates of oxacillin resistance, empirical vancomycin therapy is rampant. To reduce the unnecessary use of vancomycin, the clinical microbiology laboratory must be able to provide accurate oxacillin susceptibility results with rapid turnaround times. However, the heteroresistant nature of oxacillin resistance in staphylococcal isolates can make detection problematic (3). Disk diffusion and oxacillin screening plate results are occasionally discordant for S. aureus isolates, and resistance detection in CoNS at our institution relies solely on disk diffusion. To improve the performance of phenotype-based susceptibility tests, we sought to evaluate the use of PBP 2a detection in staphylococcal isolates by using mecA real-time PCR as the reference method (11). In addition, the data in the literature vary regarding the necessities of inducing CoNS with oxacillin and of increasing the inoculum volume for the detection of PBP 2a (6-10, 13, 20, 21, 23). This study aims to determine a standard protocol for PBP 2a detection in CoNS that minimizes the time to detection of oxacillin resistance.(A preliminary report of this work has been presented previously [M. B.

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