Comparative in vitro activity and the inoculum effect of ertapenem against Enterobacteriaceae resistant to extended-spectrum cephalosporins

The in vitro activity and the inoculum effect of ertapenem were evaluated against a total of 70 Enterobacteriaceae isolates resistant to extended-spectrum cephalosporins. The extended-spectrum β-lactamase phenotypic confirmatory disk diffusion test was performed and AmpC-inducible species were detected using cefoxitin/cefotaxime disk antagonism tests. β-Lactamases were characterised by isoelectric focusing and TEM-specific polymerase chain reaction. Minimum inhibitory concentrations (MICs) were determined by the National Committee for Clinical Laboratory Standards agar dilution method. Ertapenem showed excellent activity against almost all isolates tested, with MIC50 and MIC90 values of 0.03 mg/L and 0.12 mg/L, respectively. When the inoculum was increased 100-fold, susceptibility decreased from 98.6% to 65.7% for cefepime and from 75.7% to 54.3% for piperacillin/tazobactam, without changing for ertapenem. The data from this study suggest that this new carbapenem may be useful for treating mixed infections involving Enterobacteriaceae isolates resistant to third-generation cephalosporins.