Clinical Emergence and Genomic Characterization of Aztreonam-Avibactam-Resistant Escherichia coli ST410 Isolates in China.

BACKGROUND: Aztreonam-avibactam (ATM-AVI) is an important therapeutic option for infections caused by metallo-β-lactamase-producing Enterobacterales. However, the emergence of resistance may limit its clinical effectiveness. Here, we report two high-level ATM-AVI-resistant Escherichia coli isolates recovered from critically ill patients in a tertiary hospital in Guangzhou, both of whom had prior exposure to broad-spectrum antimicrobial agents. METHODS: Antimicrobial susceptibility testing was performed by broth microdilution. Whole-genome sequencing (WGS) was conducted using the Illumina NovaSeq and Oxford Nanopore Technologies platforms for hybrid assembly. Resistome analysis, multilocus sequence typing (MLST), plasmid replicon typing, and phylogenomic analysis were performed. Comparative genomics with global ST410 isolates was used to investigate the evolutionary origin. RESULTS: Antimicrobial susceptibility testing revealed high-level resistance to ATM-AVI. Hybrid whole-genome sequencing showed that both isolates belonged to the globally disseminated high-risk ST410 lineage and carried blaNDM-5, CTX-M-type extended-spectrum β-lactamase genes, and an identical four-amino-acid insertion in penicillin-binding protein 3 (ftsI_I334IYRIK). Both isolates exhibited highly conserved chromosomal backbones, differed by only 29 core-genome single-nucleotide polymorphisms, and clustered within a China-associated ST410 clade. Notably, their key resistance determinants were located on distinct plasmid backgrounds: blaNDM-5 was found on a predicted conjugative IncFIB/IncFIC plasmid in one isolate and on an IncX1 element with potential mobilization in the other. CONCLUSIONS: These findings provide important clinical evidence of high-level ATM-AVI resistance in Escherichia coli ST410 isolates in China and highlight the emergence of ATM-AVI resistance-associated determinants within a high-risk genomic background. Active phenotypic and genomic surveillance is warranted as ATM-AVI enters broader clinical use.