Objective To retrospectively analyze the drug-resistant phenotypes and genetic characteristics of Salmonella typhimurium in China from 2020 to 2023, and reveal the genetic relationship among strains and possible epidemiological patterns.

Methods Antibiotic susceptibility testing to 17 agents were performed for 1874 strains of S. typhimurium isolated in China during 2020−2023. Whole genome sequencing was conducted for all the strains, then drug resistance genes were identified using the ResFinder database, and a phylogenetic tree was constructed based on single nucleotide polymorphisms (SNPs) in the core genome for genetic evolutionary analysis.

Results The 1874 strains of S. typhimurium showed different degrees of resistance to 17 antibiotics, with an overall resistance rate of 96.80%. The resistance rate to ampicillin was highest (82.44%), followed by that to cefotaxime (22.2%) and to ceftazidime (11.66%), and the annual resistance rates to chloramphenicol, streptomycin and ampicillin increased with year. The multi-drug resistance rate was as high as 87.14%, and the predominant resistance pattern was tetracycline-streptomycin-ampicillin resistance. A total of 113 resistance genes to 11 classes of antibiotics, such as aminoglycosides, macrolides and tetracyclines, were detected, of which β-lactams accounted for the highest proportion (77.37%), and the carriage rates of the most resistance genes increased with year. The concordance was observed between most resistance genes and the corresponding phenotypes. Most S. typhimurium strains (61.79%) belonged to sequence type (ST) 34. The phylogenetic tree showed that S. typhimurium in China was divided into 3 major clades with different STs and resistance gene carriage in each clade. Three case clusters were found, of which two were confirmed to be the outbreaks of S. typhimurium infection by epidemiological investigation. Moreover, it was found that 14 isolates (0.75%) exhibited resistance to meropenem.

Conclusion The results demonstrated that S. typhimurium exhibited high genetic diversity in China, and ST34, which carried multi drug resistance genes and showed multi-durg resistance, was predominant. Given the increase of resistance genes by year and emerging of carbapenem resistance, it is necessary to strengthen antibiotic resistance surveillance and investigate the related risk factors to prevent the emergence and spread of novel resistant or pan-resistant clones of S. typhimurium.