Characterization of Extended-Spectrum Cephalosporin (ESC) Resistance in Salmonella Isolated from Chicken and Identification of High Frequency Transfer of bla CMY-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 integrated chicken production companies throughout Korea were collected. Among the 17 ESC-resistant Salmonella ; bla _CTX-M-15 was the most prevalent gene and two strains carried bla _TEM-1 / bla _CTX-M-15 and bla _CMY-2 , respectively. The transferable bla _CTX-M-15 gene was carried by IncFII plasmid in three isolates and the bla _CMY-2 gene carried by IncI1 plasmid in one isolate. bla _CMY-2 gene-harboring strain was selected as the donor based on the high frequency of bla _CMY-2 gene transfer in vitro and its transfer frequencies were determined at 10 ^-3 transconjugants per recipient. The transfer of bla _CMY-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 pressure. From the competition experiment; bla _CMY-2 gene-harboring transconjugant showed variable fitness burden depends on the parent strains. Our study demonstrated direct evidence that the bla _CMY-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.