Evaluation of gradient strip diffusion for susceptibility testing of aztreonam-avibactam in metallo-β-lactamase-producing Enterobacterales

Abstract

The emergence of metallo-β-lactamase (MBL)-producing Enterobacterales presents unique clinical treatment challenges. Recently developed β-lactam/ β-lactamase inhibitor combination agents, while effective against other carbapenemase-producing organisms, are notably ineffective against MBL producers. While MBLs do not hydrolyze monobactams (aztreonam), many MBL-producing organisms are resistant to aztreonam through alternate mechanisms, leaving cefiderocol as the sole monotherapy treatment option recommended for MBL producers. Recent guidelines for the treatment of MBL-harboring organisms have added combination therapy with aztreonam and ceftazidime-avibactam, using ceftazidime-avibactam as a source of the β-lactamase inhibitor avibactam. Current laboratory testing options for the combination of aztreonam-avibactam are limited to broth microdilution (BMD) and broth disk elution (BDE) methods, which are not practical in most clinical laboratories. In this study, we evaluated the performance of aztreonam/avibactam gradient strips on 103 MBL-producing Enterobacterales patient isolates as well as an additional 31 isolates from the CDC AR Bank. All MBL Enterobacterales patient isolates included in this study harbored a New Delhi metallo-β-lactamase (bla_NDM) gene. Essential agreement of gradient strip minimal inhibitory concentrations (MICs) for patient isolates compared to BMD was 93.2%. While there are no established breakpoints for aztreonam-avibactam, category agreement (CA) for patient isolates was 97.1% when using the CLSI aztreonam breakpoints. There were no major or very major errors observed. There were three minor errors. Precision for aztreonam-avibactam gradient strip diffusion was 100%. These data demonstrate that the use of gradient strip diffusion for aztreonam-avibactam MIC determination in MBL-producing Enterobacterales is a viable option for clinical laboratories.

Keywords: NDM; antimicrobial susceptibility testing; aztreonam–avibactam; gradient strip diffusion; metallo-beta-lactamase.

Fig 1

Distribution of aztreonam–avibactam gradient strip diffusion MIC compared to broth microdilution for clinical isolates. The distribution of aztreonam–avibactam MIC as tested by gradient strip diffusion (GSD) compared to the MIC measured by the comparator method (broth microdilution, BMD) for the clinical (n = 103) isolates included in this study is shown. Isolates with perfect MIC correlation are shaded in gray, and majority of isolates (63/103, 61.2%) were in this category. Isolates with a GSD MIC that measured two or greater doubling-dilutions different from the comparator BMD MIC are highlighted in red, and a minority of isolates (7/103, 6.8%) were in this category. There are currently no published interpretive breakpoints for aztreonam–avibactam. Blue lines represent the Clinical and Laboratory Standards Institute (CLSI) breakpoints for aztreonam as a surrogate for proposed aztreonam–avibactam breakpoints. Using these surrogate breakpoints, there were three isolates that would have incorrect interpretations by GSD. All three would be categorized as minor errors.