Emergence of tet(X4) and blaNDM−5 in clinical isolates of Salmonella Typhimurium 1,4,[5],12:i:- from Jinhua, China: epidemiological and genomic characterization

Background The monophasic variant of Salmonella enterica serovar Typhimurium (1,4,[5],12:i:-) has emerged as a major multidrug-resistant (MDR) clone worldwide. This study aimed to characterize the genomic epidemiology and the occurrence of critical antimicrobial resistance genes in S . 1,4,[5],12:i:- ST34 isolates from diarrheal patients in Jinhua, China. Methods A total of 51 clinical isolates were collected from diarrheal patients in Jinhua, Zhejiang Province, China, between 2022 and 2024. Antimicrobial susceptibility testing was performed according to CLSI 2023 guidelines, and whole-genome sequencing was used to determine sequence types, resistance genes, and plasmid structures. Results Most isolates carried a conserved MDR backbone including bla TEM − 1B , sul2, tet(B), floR , and aac(6')-Iaa , conferring resistance to ampicillin, tetracycline, chloramphenicol, and trimethoprim-sulfamethoxazole, while all remained susceptible to polymyxin B. Phylogenomic analysis confirmed all belonged to ST34, showing diversification of accessory resistomes over time. Two isolates harbored last-line resistance determinants: tet(X4) on a mosaic IncHI1/FIA plasmid and bla NDM−5 on an IncHI2 plasmid with markedly increased conjugation efficiency at 26 °C, suggesting environmental facilitation of plasmid transfer. Conclusions This study demonstrates the persistence of a stable MDR core within regional S . 1,4,[5],12:i:- ST34 populations and the sporadic emergence of high-risk plasmids carrying tet(X4) and bla NDM−5 . These findings underscore the urgent need for integrated One Health surveillance to limit further dissemination of clinically important resistance in Salmonella .