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Interplay of efflux, impermeability, and AmpC activity contributes to cefuroxime resistance in clinical, non-ESBL-producing isolates of Escherichia coli.

Cefuroxime resistance in Escherichia coli strains susceptible to extended-spectrum cephalosporins is not uncommon, but the resistance mechanisms have so far not been elucidated. Therefore, 14 clinical non-extended-spectrum beta-lactamase isolates of E. coli were examined, 11 of which were cefuroxime resistant. Quantitative RT-PCR was used to examine the transcription levels of the genes acrA (encoding AcrA, part of the AcrAB-TolC efflux pump system) and ompF (encoding the porin OmpF). Isoelectric focusing was used for detection of beta-lactamases, and a spectrophotometric assay was used to measure AmpC activity. Among the 11 cefuroxime-resistant isolates, 7 had increased acrA transcription (from 2.4 to 38 times the ATCC strain), 3 isolates had very low ompF transcription levels (<or=0.01 times the ATCC strain), and 2 isolates showed increased AmpC activity (confirmed by 3-aminophenylboronic acid inhibition). We suggest that efflux, impermeability, and increased AmpC activity all contribute to cefuroxime resistance in E. coli.