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In vitro activity of cefepime and other broad-spectrum beta-lactams tested against 129 mec A-negative Staphylococcus
S A Marshall1, W W Wilke, Y S Chen
1Department of Pathology, University of Iowa College of Medicine, Iowa City 52242, USA.
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 < or = 2 micrograms/ml) regardless of their med 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 < or = 2 micrograms/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 beta-lactams for therapy. Furthermore, previously published MIC90 and range data for broad-spectrum beta-lactams versus OS have been falsely elevated by the presence of mec A-positive strains. The greater use of these potent beta-lactams against true mec A-negative staphylococci should enhance clinical outcomes and reduce the need for vancomycin.
Insights
Cefepime and other beta-lactams show 100% effectiveness against mec A-negative staphylococci. Accurate testing is crucial for guiding antibiotic therapy and improving patient outcomes.
Area of Science:
- Microbiology
- Clinical Pharmacy
- Infectious Diseases
Background:
- Staphylococcus species are a common cause of nosocomial bloodstream infections.
- The mec A gene confers resistance to beta-lactam antibiotics, including cefepime.
- Accurate identification of mec A-negative staphylococci is essential for effective treatment.
Purpose of the Study:
- To compare the in vitro activity of cefepime and other antibiotics against Staphylococcus isolates.
- To evaluate the susceptibility patterns of mec A-negative and oxacillin-susceptible staphylococci.
- To highlight the importance of accurate phenotypic detection of antibiotic resistance.
Main Methods:
- Broth microdilution method was used to test antibiotic susceptibility.
- 129 Staphylococcus isolates (50 S. aureus, 79 CoNS) lacking the mec A gene were analyzed.
- Isolates were identified using polymerase chain reaction (PCR) and phenotypic oxacillin susceptibility testing.
Main Results:
- Cefepime, piperacillin/tazobactam, ceftriaxone, and imipenem demonstrated 100% susceptibility against mec A-negative and oxacillin-susceptible staphylococci.
- Ceftazidime showed higher resistance rates against oxacillin-susceptible CoNS (30.2%) compared to mec A-negative CoNS (5.1%).
- Presence of mec A-positive strains may falsely elevate reported resistance data for broad-spectrum beta-lactams.
Conclusions:
- Cefepime and other potent beta-lactams are highly effective against mec A-negative staphylococci.
- Accurate in vitro susceptibility testing is critical for guiding empirical beta-lactam therapy.
- Improved detection of mec A-negative strains can enhance clinical outcomes and reduce vancomycin use.