Characterization of extended-spectrum cephalosporin (Esc) resistance in salmonella isolated from chicken and identification of high frequency transfer of blacmy-2 gene harboring plasmid in vitro and in vivo

A total of 136 Salmonella isolates from chicken feces and meat samples of the top 12 inte-grated chicken production companies throughout Korea were collected. Among the 17 ESC-re-sistant Salmonella; blaCTX-M-15 was the most prevalent gene and two strains carried blaTEM-1/blaCTX-M-15 and blaCMY-2, respectively. The transferable blaCTX-M-15 gene was carried by IncFII plasmid in three isolates and the blaCMY-2 gene carried by IncI1 plasmid in one isolate. blaCMY-2 gene-harboring strain was selected as the donor based on the high frequency of blaCMY-2 gene transfer in vitro and its transfer frequencies were determined at 10⁻³ transconjugants per recipient. The transfer of blaCMY-2 gene-harboring plasmid derived from chicken isolate into a human pathogen; enteroinvasive Escherichia coli (EIEC), presented in mouse intestine with about 10⁻¹ transfer frequency without selective pres-sure. From the competition experiment; blaCMY-2 gene-harboring transconjugant showed variable fitness burden depends on the parent strains. Our study demonstrated direct evidence that the blaCMY-2 gene harboring Salmonella from chicken could frequently transfer its ESC-resistant gene to E. coli in a mouse intestine without antimicrobial pressure; resulting in the emergence of multidrug resistance in potentially virulent EIEC isolates of significance to human health; which can increase the risk of therapeutic inadequacy or failures.