Beta-lactam susceptibility of coagulase-negative staphylococci causing catheter sepsis in pediatric patients

We evaluated standard oxacillin and methicillin disk diffusion (DD) and broth microdilution (MD)-MIC tests with and without 2% NaCl for detecting heteroresistance among 47 blood isolates of coagulase-negative staphylococci (CNS) causing catheter sepsis in pediatric patients. The 24-hr oxacillin DD test detected the greatest number (40) of apparent hetero-resistant isolates, but methicillin DD and oxacillin MD-MIC with 2% NaCl performed equally as well (38 and 37 resistant isolates, respectively). An additional 24-hr incubation did not significantly increase the number of apparent heteroresistant isolates detected by these methods. Discrepant results with the various test methods occurred most commonly among Staphylococcus epidermidis isolates with MD-MIC values near the breakpoint concentrations for interpretation of susceptible and resistant strains. For detection of heteroresistance among the CNS, we encourage use of standard oxacillin DD and MD-MIC tests but would suggest that isolates with MIC values ranging from 1-2 micrograms/ml be interpreted cautiously until clinical studies demonstrate the efficacy of treating patients with infections caused by such strains.