Detection of CTX-M-1 extended-spectrum beta-lactamase among ceftiofur-resistant clinical isolates of poultry.

resistance to extended-spectrum cephalosporins (ESC) conferred by cefotaximases () is a growing concern in the United States. Among food-producing animals, poultry are a major reservoir of ESC-resistant . A retrospective study was carried out to further characterize 38 ceftiofur-resistant clinical isolates obtained from poultry during 2007-2018. Of the isolates tested, 31 displayed resistance to ceftriaxone and harbored , whereas 7 isolates demonstrated resistance or reduced susceptibility to cefepime in addition to ceftriaxone resistance. These 7 isolates displayed extended-spectrum β-lactamase activity, harbored , and were recovered only from recent poultry diagnostic submissions made in 2011-2018 as opposed to the 31 isolates that were recovered in 2007-2018. Further characterization of the gene determined that it was located on conjugative IncN/ST1 and IncI1/ST87 plasmids in the isolates from commercial turkeys and broilers, respectively. These plasmids have been responsible for extensive spread of in livestock, poultry, and humans in Europe. Potential transfer of IncN and IncI1 plasmids and/or nontyphoidal carrying these plasmids through the food chain, or by other means to humans, may result in treatment failures. Our study demonstrates the importance of further characterization of ceftiofur-resistant isolates detected by veterinary diagnostic laboratories to identify the sources of and to mitigate the spread of ESC-resistant in the poultry production pyramid.