In Vitro Activity of Cefepime and Other Broad-Spectrum β-Lactams Tested Against 129 mec A-negative Staphylococcus spp. Isolates: A Multicenter Sample

Abstract The in vitro activity of cefepime was compared to that of penicillin, piperacillin/tazobactam, ceftazidime, ceftriaxone, imipenem, vancomycin, and teicoplanin by using the broth microdilution method against 129 isolates of Staphylococcus [50 S. aureus and 79 coagulase-negative staphylococci (CoNS)], selected for their lack of the mec A gene as determined by the polymerase chain reaction. These isolates were obtained from a recent (1995–1996) surveillance of nearly 5000 nosocomial blood stream isolates from more than 40 geographically diverse U.S. medical centers. These results were compared to CoNS results from the same collection selected for their phenotypic susceptibility to oxacillin (OS; MIC ≤ 2 μg/mL) regardless of their mec A genotype. Cefepime, as well as piperacillin/tazobactam, ceftriaxone, and imipenem, showed 100% susceptibility against OS and mec A -negative staphylococci. Ceftazidime showed relative resistance (30.2% resistant) against CoNS classified as OS based on phenotypic characteristics (MIC ≤ 2 μg/mL) as compared to strains of mec A -negative CoNS (5.1% resistant). Accurate phenotypic detection of mec A -positive staphylococci by simple standardized in vitro susceptibility tests becomes very important to guide empirical use of β-lactams for therapy. Furthermore, previously published MIC 90 and range data for broad-spectrum β-lactams versus OS have been falsely elevated by the presence of mec A -positive strains. The greater use of these potent β-lactams against true mec A -negative staphylococci should enhance clinical outcomes and reduce the need for vancomycin.