Extended-spectrum cephalosporin resistance in Escherichia coli from broiler chickens raised with or without antibiotics in Ontario, Canada

Extended-spectrum cephalosporin (ESC)-resistant Escherichia coli were isolated from fecal samples enriched in ESC-containing broth over two years from broiler chickens raised with and without therapeutic antimicrobials. Pooled samples were obtained from 13 different farms in Canada over two rearing cycles each. Resistant isolates were screened for blaCMY, blaCTX-M, and blaSHV by PCR, and several isolates were sequenced and assembled using Oxford Nanopore MinION technology. Flocks raised with or without antimicrobials contained similar ESCresistant E. coli. Some plasmids were found in isolates from both farm types and many shared replicon types (IncI1, F, C, and B/O/K/Z) and resistance determinants. Although the use of cephalosporins has stopped in poultry production in Canada, the prevalence of ESC-resistant E. coli found in this study remained high; therefore, further studies are required to determine routes of transmission and persistence.