Abstract Background Infections caused by carbapenem-resistant Escherichia coli (CREC) pose a serious global public health threat due to limited therapeutic options. This study aimed to investigate the molecular epidemiology and the susceptibility to novel β-lactam/β-lactamase inhibitor combinations in clinical CREC isolates. Methods Whole-genome sequencing analysis including multilocus sequence types, antimicrobial resistance genes, serotypes and phylogeny was conducted. Antimicrobial susceptibility testing was performed by the broth microdilution method. Filter mating assays were performed to evaluate the conjugation of the plasmid. Results We identified an emerging extended-spectrum β-lactamase-encoding gene, blaCTX-M-199, in 8.9% (26/291) of CREC strains isolated in South China. Notably, 96.2% (25/26) of blaCTX-M-199-positive CREC strains exhibited aztreonam/avibactam (ATM/AVI) nonsusceptibility, with 38.5% (10/26) being fully resistant. All 26 blaCTX-M-199-positive strains harbored blaNDM-5 and YRIK/YRIN insertions of penicillin-binding protein 3, belonging to ST410 (n = 10) and ST167 (n = 16). Whole-genome sequencing revealed that blaCTX-M-199 can be located on the chromosome or on transferable plasmids. Transconjugants exhibited a 2- to 4-fold increase in the minimum inhibitory concentration of ATM/AVI. Conclusions These findings suggest that blaCTX-M-199 contributes to reduced ATM/AVI susceptibility and, in synergy with YRIK/YRIN insertions, likely confers significant ATM/AVI resistance. Notably, CREC strains coharboring blaCTX-M-199 and YRIK/YRIN insertions are emerging within international high-risk clones ST410 and ST167. Therefore, our study highlights the importance of monitoring the prevalence of blaCTX-M-199-harboring CREC and implementing its detection in clinical practice.
Wang et al. (2026) studied this question.