The widespread of different NDM variants in clinical Enterobacterales isolates poses a serious public health concern, which requires continuous monitoring. In this study, three E. coli strains carrying two novel blaNDM variants of blaNDM-36, -37 were identified from a patient with refractory urinary tract infection (UTI) in China. We conducted antimicrobial susceptibility testing (AST), enzyme kinetics analysis, conjugation experiment, whole-genome sequencing (WGS), and bioinformatics analysis to characterize the blaNDM-36, -37 enzymes and their carrying strains. The blaNDM-36, -37 harboring E. coli isolates belonged to ST227, O9:H10 serotype and exhibited intermediate or resistance to all β-lactams tested except aztreonam and aztreonam/avibactam. The genes of blaNDM-36, -37 were located on a conjugative IncHI2-type plasmid. NDM-37 differed from NDM-5 by a single amino acid substitution (His261Tyr). NDM-36 differed from NDM-37 by an additional missense mutation (Ala233Val). NDM-36 had increased hydrolytic activity toward ampicillin and cefotaxime relative to NDM-37 and NDM-5, while NDM-37 and NDM-36 had lower catalytic activity toward imipenem but higher activity against meropenem in comparison to NDM-5. This is the first report of co-occurrence of two novel blaNDM variants in E. coli isolated from the same patient. The work provides insights into the enzymatic function and demonstrates the ongoing evolution of NDM enzymes.

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Background Salmonella enterica is a significant public health concern due to its ability to cause foodborne illnesses worldwide. To gain insights into the epidemiology and characteristics of Salmonella strains in China, a total of 20 Salmonella isolates were subjected to phenotypic and molecular analyses. Methods The 20 Salmonella strains were characterized through serotype identification, antimicrobial susceptibility testing (AST), biofilm formation assessment and plasmid transmission studies. Molecular characteristics and relatedness were determined using pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS). Additionally, SNP-based phylogenetic analysis and core genome multilocus sequence typing (cgMLST) were employed to contextualize the strains among previously sequenced S. enterica isolates in China and worldwide. Results The predominant serovar identified was Salmonella enterica serovar Enteritidis (S. Enteritidis), followed by S. Mbandaka, Thompson, Livingston, Alachua, and Infantis. The SNP-based phylogenetic analysis revealed substantial diversity among S. Enteritidis isolates, with a minimum of 32 SNPs between strains. Conversely, S. Mbandaka isolates were suspected to have originated from a single clonal expansion, displaying a difference within 6 SNPs. SNP-based phylogenetic analysis suggested S. Mbandaka clone in this study was likely to have been derived from a single source that persisted over 5 years in China. S. Mbandaka, Infantis, Livingston, and Alachua exhibited susceptibility to the tested antibiotics, while strains of S. Thompson and Enteritidis demonstrated resistance to ampicillin, quinolone, and sulfamethoxazole/trimethoprim (SMZ/TMP). Particularly concerning was the identification of one S. Enteritidis isolate carrying the highly transferable IncB/O/K/Z plasmid with blaCTX−M15, conferring cephalosporin resistance. Another isolate, harboring the mrkABCDF operon in an IncX1 plasmid, isolated from cutaneous lesions, demonstrated robust biofilm formation. Conclusions Our study emphasizes the importance of SNP-based surveillance and prompt response to Salmonella infections to protect public health. The dissemination of blaCTX−M15-harboring IncB/O/K/Z plasmid and the spread of virulent mrkABCDF operon among S. enterica in China and other global regions warrant close monitoring.

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