Antimicrobial susceptibility of well-characterised multiresistant CTX-M-producing Escherichia coli: failure of automated systems to detect resistance to piperacillin/tazobactam

This study was designed to determine the in vitro activities of several antimicrobial agents against well-characterised CTX-M-producing Escherichia coli strains isolated from clinical specimens. Minimum inhibitory concentrations (MICs) were determined for 202 extended-spectrum β-lactamase (ESBL)-producing E. coli using microbroth dilution and Vitek methods according to Clinical and Laboratory Standards Institute criteria. Molecular characterisation was performed using isoelectric focusing and polymerase chain reaction (PCR) with sequencing, whilst strain relatedness was determined by pulsed-field gel electrophoresis (PFGE) using XbaI. Of the 202 ESBL-producing E. coli, 2 produced VEB-1, 12 produced TEM-52, 32 produced SHV types (including SHV-2 and -12) and 156 produced CTX-M types (including CTX-M-2, -3, -14, -15, -24, -27 and -30). The MIC50 and MIC90 (MIC for 50% and 90% of the organisms, respectively) for the antimicrobial agents tested were, respectively: piperacillin/tazobactam (TZP), 32 mg/L and >256 mg/L; ciprofloxacin, 16 mg/L and 32 mg/L; gentamicin, 8 mg/L and 128 mg/L; amikacin, 2 mg/L and 16 mg/L; ertapenem, 0.03 mg/L and 0.12 mg/L; imipenem, 0.03 mg/L and 0.12 mg/L; meropenem, 0.03 mg/L and 0.03 mg/L; and tigecycline 0.12 mg/L and 0.5 mg/L. Vitek Legacy and Vitek 2 failed to detect TZP resistance in 91 (90%) and 75 (74%) of 101 TZP-resistant ESBL-producing strains, respectively, especially CTX-M-15-producing isolates that co-produced OXA-1. The carbapenems, amikacin and tigecycline had good in vitro activities against multiresistant CTX-M-producing E. coli. We recommend that laboratories using Vitek should employ alternative susceptibility testing methods for TZP before reporting the activity of this agent against ESBL-producing E. coli.