Detection of Oxacillin Resistance in Coagulase Negaitve Staphylococci with Phoenix

� CNS belong to the normal flora of the human skin and mucous membrane and are amongst the 5 most frequent causes of nosocomial infections. Despite the low virulence, the past years have shown a dramatic increase of incidence of nosocomial septicemia and foreign body associated infections due to CNS. Antibiotic therapy of CNS is increasingly problematic. The majority of clinically relevant CNS strains carry the mecA gene which codes for an additional penicillin binding protein, the PBP2a. PBP2a is crucial for the phenotypic expression of the methicillin resistance due to its low affinity for all s-lactam antibiotics. It's well known that phenotypic detection of methicillin resistance in CNS is difficult due to the heterogenic mecA expression and due to the detections system’s focus for S. aureus. Tests like the oxacillin agar screen, agar diffusion or MIC by microdilution are based on modified culture conditions in order to increase expression of resistance. Disadvantage of phenotypic methods is the long TTR, low sensitivity or a high workload. PCR detection of mecA is highly sensitive and seen as the gold standard. However, except for reference centers this technique is very laborous, expensive and not feasable. The Phoenix, a new automated system for ID and AST, was evaluated for its performance of the detection of this important resistance mechanism. MATERIALS AND METHODS The results of the mecA PCR as gold standard for detection of methicillin resistance were compared with the oxacillin MIC of the Phoenix Gram positive combo panels (<=0.25 thru 4 µg/mL). In addition we analyzed the time to detection for the oxacillin MIC. We tested 200 CNS, mainly isolated from blood cultures and infected catheters. S. aureus ATCC 43300 (mecApositive) and ATCC 29213 (mecA-negative) was used as QC for each run. ID was done with ID 32 Staph and conventional methods. The mecA PCR was done with the primers. S. epidermidis 1457 served as negative control and S. epidermidis 1057 and RP62A as positive control. PBP2a was done with a latex agglutination test (Denka). The oxacillin screen agar test (6 µg oxacillin, 2% NaCl) was incubated at 30°C for 48 hours.