Hidden Carbapenem Resistance in OXA-48 and Extended-Spectrum β-Lactamase-Positive Escherichia coli.

The purpose of this study was to report the identification OXA-48 carbapenemase in seven extended-spectrum β-lactamase (ESBL)-positive Escherichia coli clinical isolates, fully susceptible to all carbapenems by disk diffusion and E-test methods, but with borderline minimal inhibitory concentration (MIC) values of ertapenem. This report points to the necessity for determination of carbapenem MICs in ESBL-positive E. coli isolates and additional phenotypic testing for carbapenemases in all isolates with borderline ertapenem MIC defined by EUCAST. The isolates showed a high level of resistance to expanded-spectrum cephalosporins because of the production of an additional ESBL belonging to CTX-M family. All isolates and their respective tranconjugants were found to possess L plasmid. Pulsed-field gel electrophoresis analysis revealed two clusters containing highly related isolates. The global spread of multidrug-resistant E. coli should be monitored closely because of the ability of isolates to rapidly obtain additional antibiotic resistance traits such as plasmid-mediated OXA-48 genes.