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

Extended-spectrum beta-lactamase (ESBL)-producing <i>Salmonella enterica</i> 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 <i>S</i>. Enteritidis isolates. In this study, we identified a hybrid transposon, Tn<i>1721</i>/Tn<i>21</i>, embedded within IncF plasmids and linked to <i>bla</i>_CTX-M-15 in <i>S</i>. Enteritidis isolates. This association between Tn<i>1721</i>/Tn<i>21</i> and <i>bla</i>_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.