Fangqin Liu, Lu Zhang, Qi Zhao, Min Cheng, Zitong Xu, Ting Li, Shouzhen Xu, Qingchun Shen, Hejia Wang, Chunping Zhang
This study is the first to untangle the differences in animal distribution and serotype associations of third-generation cephalosporin-resistant Salmonella over the past decade and to elucidate, at the genomic level, the formation mechanisms and transmission pathways underlying different resistance phenotypes.
OBJECTIVE: To elucidate the drug resistance characteristics, epidemiological distribution, and molecular mechanisms of third-generation cephalosporin-resistant Salmonella from animal sources in China during 2016-2024.
METHODS: Antimicrobial susceptibility testing, serotyping, and whole-genome sequencing (WGS) were employed.
RESULTS: Salmonella exhibited the highest resistance rate to ampicillin (91.9%), followed by sulfisoxazole (87.4%) and tetracycline (83.1%). Among these, strains producing extended-spectrum β-lactamases (ESBLs) accounted for 67.1% and were widely prevalent in chickens and ducks; their dominant resistance gene, blaCTX-M-55, is closely associated with IncI2 and is co-driven by ISEcp1, ISKpn26, IS150, and IS103. 6.2% of the strains carried cephalosporinases (AmpC), primarily from chickens, with blaCMY-59 associated with ISEcp1 as the predominant genotype. An additional 27.1% carried other β-lactamases, mostly from pigs, with the predominant genotype being blaTEM-1 associated with IS406. Notably, the carbapenemase gene blaNDM-1/5 was detected only in strains producing other β-lactamases and was associated with ISSbol and ISRor2. Serotype distribution showed that S. Kentucky predominantly carried ESBLs and AmpC, while S. Enteritidis was dominated by other β-lactamases. Phylogenetic analysis revealed that serotype is the primary factor determining the structure of Salmonella clonal groups, and the acquisition of resistance to third-generation cephalosporins in Salmonella may depend on both clonal transmission and horizontal gene transfer.
CONCLUSION: This study is the first to untangle the differences in animal distribution and serotype associations of third-generation cephalosporin-resistant Salmonella over the past decade and to elucidate, at the genomic level, the formation mechanisms and transmission pathways underlying different resistance phenotypes.