Comparison Between Broth Microdilution and an Innovative Disk Diffusion Method Supplemented With Aztreonam Solution for Detecting Synergism With Ceftazidime/Avibactam in Enterobacterales Producing NDM or NDM and KPC

Since the aztreonam/avibactam combination has not yet been approved for use in Brazil, the treatment of infections caused by Enterobacterales producing New Delhi metallo-beta-lactamase (NDM), either alone or in co-production with the serine-carbapenemase Klebsiella pneumoniae carbapenemase (KPC), is empirically performed using ceftazidime/avibactam (CZA) combined with aztreonam (ATM). The objective of this study was to compare the performance of an innovative method (IM) with the gold standard method for the in vitro detection of synergism between ATM and CZA in Enterobacterales producing NDM or NDM and KPC. Sixty Enterobacterales isolates producing NDM or NDM and KPC were selected from a bacterial isolate bank with reduced susceptibility to meropenem. Half were NDM producers only, and the other half were coproducers of NDM and KPC. The type of carbapenemase was determined by high-resolution melting PCR (HRM-qPCR) and/or NG-Test® Carba 5 (NG Biotech®, France). For isolates producing NDM as the only carbapenemase, a phenotypic test for Extended-Spectrum Beta-Lactamase (ESBL) was performed, which was positive in 17 isolates. Synergism between CZA and ATM was detected by broth microdilution (gold standard), defined as a reduction of at least two concentrations compared to each agent tested alone. The IM consists of inoculating a solid culture medium with the bacterial suspension, followed by placing a CZA disk and adding 10 µL of ATM solution onto it. A synergistic effect is observed when the inhibition zone increases by more than 5 mm compared with each disk tested alone. Synergism between CZA and ATM was detected by both methods in all coproducing NDM and KPC isolates and in NDM producers that also produced ESBL (78%). Among the isolates producing only NDM, seven showed synergism, four did not, and two could not be evaluated due to limitations of the gold standard method. The IM detected synergism in only one isolate producing NDM alone; the others could not be evaluated due to technical limitations. The IM showed 100% concordance with the gold standard method, demonstrating its feasibility for incorporation into routine laboratory practice, offering lower cost and easier execution.