Prevalence, plasmid transmission, and chromosomal integration of blaCTX-M genes in Salmonella isolated from retail chicken and pork meats in China.

Extended-spectrum β-lactamase (ESBL)-producing Salmonella poses a growing threat to food safety, yet the transmission of blaCTX-M genes in foodborne Salmonella remains incompletely understood. This study investigated the prevalence, antimicrobial resistance profiles, horizontal transferability, and genetic characteristics of blaCTX-Ms in 950 Salmonella isolates recovered from retail chicken and pork in China. A total of 103 (10.8%) blaCTX-M-positive isolates were identified, with a significantly higher prevalence in chicken (20.7%, 96/464) than in pork (1.4%, 7/486). Geographically, blaCTX-M-positive isolates were more prevalent in the 3 northern provinces (19.93%, 59/296) than in the 5 southern provinces (6.73%, 44/654). These isolates represented 11 "sequence type (ST)-serotype" combinations, predominantly ST26 Salmonella enterica serovar Thompson (S. Thompson) (36.9%, 38/103), ST198 S. Kentucky (31.1%, 32/103), and ST17 S.Indiana (13.8%, 18/103). Nine blaCTX-M subtypes were identified, dominated by blaCTX-M-55 (33.0%, 34/103) and blaCTX-M-65 (33.0%, 34/103). Overall, 78.6% (81/103) of blaCTX-M-positive isolates failed to yield detectable transconjugants in Escherichia coli C600, with no transconjugants detected in ST198 S. Kentucky or ST17 S.Indiana. The blaCTX-Ms were mainly carried by IncHI2-HI2A plasmids, which exhibited significantly lower conjugation frequencies than blaCTX-M-positive IncN and IncFII-X1 plasmids. Chromosomal integration of blaCTX-Ms was detected in ST198 S. Kentucky, ST26 S. Thompson, ST17 S.Indiana, and ST13 S. Agona, characterized by signature direct repeats and mediated by ISEcp1, IS15, and IS26. Consistent with these findings, further analysis of 418 blaCTX-M-positive complete Salmonella genomes from the NCBI database showed that 310 plasmids carried blaCTX-Ms, mainly on IncHI2-HI2A plasmids (33.9%, 105/310), whereas 117 isolates carried chromosomal blaCTX-Ms, dominated by blaCTX-M-55 (76.1%, 89/117) and most frequently occurring in ST413 S. Mbandaka, ST198 S. Kentucky, and ST13 S. Agona. Our findings highlight that blaCTX-Ms disseminate in Salmonella through plasmid-mediated transfer and chromosomal integration, providing a mechanistic basis for the long-term persistence of ESBL-producing Salmonella and associated food safety risk.