Distinct ESBL dissemination mechanism associated with the hybrid transposon Tn 1721 /Tn 21 in bla CTX-M-15 -carrying Salmonella Enteritidis from poultry in South Korea

ABSTRACT Extended-spectrum beta-lactamase (ESBL)-producing Salmonella enterica serovar Enteritidis ( S . Enteritidis) is emerging as a significant threat to food safety via its limitation of therapeutic options and potential transmission through poultry products. However, the structural and genetic characteristics of mobile genetic elements (MGEs) associated with horizontal transfer of the ESBL gene in S . Enteritidis isolates from poultry remain insufficiently characterized. The present study aimed to identify and characterize the ESBL gene and its associated MGEs and to assess their distribution. Whole-genome sequencing was applied to ESBL-producing and non-ESBL-producing isolates in combination with pan-genome analysis, conjugation assays, and comparative genomics using publicly available genomes. Among 17 isolates, 9 were ESBL-producing and all carried bla CTX-M-15 . We observed co-transfer of bla CTX-M-15 , tetA , and the IncF plasmid at relatively high frequencies (2.0–5.3 × 10⁻²) in ESBL-producing isolates. Moreover, we identified a hybrid transposon (Tn 1721 /Tn 21 ) inserted into IncF plasmids that comprised elements of Tn 21 ( merRTPCADE , tniA , and urf2 ) and Tn 1721 ( tetA , tetR , a DMT-family efflux gene, and a partial tnpA ), with IS Ecp1 and bla CTX-M-15 adjacent to the hybrid transposon. Tn 1721 /Tn 21 was prevalent among bla CTX-M-15 -carrying S . Enteritidis isolates from South Korea (19/20) but absent in those from other countries ( n = 9), suggesting geographical variation. This study identified a unique hybrid Tn 1721 /Tn 21 transposon as the dominant MGE in bla CTX-M-15 -carrying S . Enteritidis from South Korean poultry, highlighting its potential role in the regional dissemination of antimicrobial resistance. Continued surveillance and targeted intervention in poultry production are warranted to mitigate the spread of ESBL-producing S . Enteritidis. IMPORTANCE Extended-spectrum beta-lactamase (ESBL)-producing Salmonella enterica serovar Enteritidis from poultry represents a growing public health threat due to limited treatment options and the potential for transmission through the food chain. Despite this concern, the mobile genetic elements underlying ESBL gene dissemination remain insufficiently characterized in South Korean poultry-associated S . Enteritidis isolates. In this study, we identified a hybrid transposon, Tn 1721 /Tn 21 , embedded within IncF plasmids and linked to bla CTX-M-15 in S . Enteritidis isolates. This association between Tn 1721 /Tn 21 and bla CTX-M-15 suggests a region-specific mechanism of resistance dissemination that may reflect antimicrobial selective pressure within poultry production systems. These findings highlight the importance of integrated One Health surveillance to mitigate the emergence and spread of antimicrobial resistance across animal and human populations.