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

Int J Antimicrob Agents. 2008 Oct;32(4):333-8. doi: 10.1016/j.ijantimicag.2008.04.023. Epub 2008 Aug 12.


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 beta-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 MIC(50) and MIC(90) (MIC for 50% and 90% of the organisms, respectively) for the antimicrobial agents tested were, respectively: piperacillin/tazobactam (TZP), 32mg/L and >256mg/L; ciprofloxacin, 16mg/L and 32mg/L; gentamicin, 8mg/L and 128mg/L; amikacin, 2mg/L and 16mg/L; ertapenem, 0.03mg/L and 0.12mg/L; imipenem, 0.03mg/L and 0.12mg/L; meropenem, 0.03mg/L and 0.03mg/L; and tigecycline 0.12mg/L and 0.5mg/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.

Publication types

  • Evaluation Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Automation
  • Deoxyribonucleases, Type II Site-Specific / metabolism
  • Drug Resistance, Multiple, Bacterial*
  • Electrophoresis, Gel, Pulsed-Field
  • Escherichia coli / drug effects*
  • Escherichia coli / enzymology
  • Escherichia coli / isolation & purification
  • Escherichia coli Infections / microbiology
  • Escherichia coli Proteins / genetics
  • Escherichia coli Proteins / metabolism
  • Humans
  • Isoelectric Focusing
  • Microbial Sensitivity Tests / methods
  • Microbial Sensitivity Tests / standards
  • Penicillanic Acid / analogs & derivatives*
  • Penicillanic Acid / pharmacology
  • Piperacillin / pharmacology*
  • Polymerase Chain Reaction / methods
  • Sequence Analysis, DNA
  • Tazobactam
  • beta-Lactamases / genetics
  • beta-Lactamases / metabolism*


  • Anti-Bacterial Agents
  • Escherichia coli Proteins
  • Penicillanic Acid
  • endodeoxyribonuclease XBAI
  • Deoxyribonucleases, Type II Site-Specific
  • beta-Lactamases
  • Tazobactam
  • Piperacillin