Differences in antimicrobial susceptibility testing complicating management of IMP carbapenemase-producing Enterobacterales infection

C. Hickey, S. Nguyen, J. Anes, D. Hurley, O. Donoghue, S. Fanning, K. Schaffer

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OBJECTIVES: IMP-type carbapenemases are rarely detected in Europe and limited information is available to guide the treatment of infections caused by carbapenemase-producing Enterobacterales (CPE) producing these carbapenemases. Accurate antimicrobial susceptibility testing (AST) results are essential for optimal antibiotic management. Here we report discrepancies in AST of IMP-producing Enterobacterales (IMP-CPE) complicating the management of severe sepsis. 

METHODS: Antimicrobial susceptibilities were analysed by in-house VITEK® 2, Etest and broth microdilution (BMD). Carbapenemase-encoding genes were detected by PCR. Whole-genome sequencing (WGS) was performed using an Illumina MiSeq platform. 

RESULTS: Minimum inhibitory concentrations (MICs) determined by VITEK® 2 for Enterobacter hormaechei and Klebsiella oxytoca blood culture isolates were ≥16 mg/L for meropenem and ≤0.5 mg/L for ertapenem. In contrast, Etest analysis and BMD returned MICs of 2 mg/L and 1 mg/L, respectively. Both isolates tested positive for IMP carbapenemase-encoding genes by PCR. WGS revealed that both isolates carried the same blaIMP-4 gene. Based on VITEK® 2 susceptibilities, initial treatment was with tigecycline and amikacin. After subsequent deterioration, the patient was successfully treated with ertapenem and amikacin.

CONCLUSION: This case highlights that automated AST by VITEK® 2 can over-report meropenem resistance for IMP carbapenemase-producers compared with Etest and BMD. Clinicians need to be cautious deciding against carbapenem treatment based on VITEK® 2 susceptibility testing results for IMP-positive Enterobacterales. Tigecycline was inferior to carbapenem treatment for pyelonephritis caused by isolates expressing IMP carbapenemases, however specific evidence guiding the treatment of these infections is lacking.

Original languageEnglish
Pages (from-to)284-288
Number of pages5
JournalJournal of global antimicrobial resistance
Early online date24 Nov 2021
Publication statusPublished - 01 Dec 2021
Externally publishedYes

Bibliographical note

Publisher Copyright:
Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.


  • Antimicrobial susceptibility testing
  • Carbapenemase-producing Enterobacterales
  • CPE
  • IMP carbapenemase

ASJC Scopus subject areas

  • Microbiology
  • Immunology and Allergy
  • Immunology
  • Microbiology (medical)


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