Aztreonam-avibactam resistance rates and resistance mechanisms of NDM and NDM/OXA48-like dual-carbapenemase-producing Enterobacterales in Singapore.

Aztreonam-avibactam is a treatment option for metallo-β-lactamase carbapenemase-producing Enterobacterales (CPE). This study was performed to determine resistance rates of a collection of CPE against aztreonam-avibactam and characterise associated resistance mechanisms. Minimum inhibitory concentrations (MICs) of a collection of New Delhi metallo-β-lactamase (NDM)- and NDM/OXA-48-like dual-CPE isolates were determined using two methods, the MIC Test Strip (MTS) and broth microdilution (Sensititre). Whole-genome sequencing was also performed for molecular characterisation. A total of 44 NDM-CPE and 11 NDM/OXA-48 dual-CPE isolates from 46 patients were included. The species breakdown of included isolates was as follows: Escherichia coli (n=33), Klebsiella pneumoniae (n=12), Klebsiella oxytoca (n=1), Enterobacter cloacae complex (n=2), Citrobacter amalonaticus (n=2), Citrobacter farmeri (n=1), Citrobacter freundii (n=1), Providencia rettgeri (n=2), and Pantoea calida (n=1). Essential agreement between the two test methods was 96.4%, with 100% categorical agreement. Overall resistance was 13.7% after excluding duplicate isolates. Of note, resistance occurred only in E. coli and P. rettgeri isolates. Resistance rates in E. coli isolates were 18.8% (6/32, all isolates), 8.0% (2/25) for NDM-CPE, and 57.1% (4/7) for NDM/OXA-48 dual-CPE. All resistant isolates were found to have penicillin-binding protein 3 (PBP3) insertions in association with CMY (AmpC-type) β-lactamase; 77.8% (7/9) of E. coli isolates with increased MICs of 0.5-4 mg/L, but still within susceptible breakpoints, also had either PBP3 insertion or AmpC β-lactamase. The six aztreonam-avibactam-resistant E. coli consisted of four ST361 and two ST167 isolates. All three ST361 NDM/OXA-48-like dual-CPE isolates had 0-5 single-nucleotide polymorphisms. Aztreonam-avibactam resistance was detected in E. coli and Providencia spp. Resistance in E. coli isolates was driven by PBP3 mutations in association with AmpC production. Higher resistance rates were seen in NDM-OXA-48-like dual-carbapenemase-producing E. coli, driven by ST361 isolates. Aztreonam-avibactam susceptibility testing should be performed as part of routine diagnostic testing for invasive NDM-CPE infections.