U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Date of this Version


Document Type



SCHMIDT ET AL. Journal of Food Protection, Vol. 85, No. 2, 2022, Pages 323–335 https://doi.org/10.4315/JFP-21-371


Published 2022 by the International Association for Food Protection Not subject to U.S. Copyright


Third-generation cephalosporins (3GCs) are preferred treatments for serious human Salmonella enterica infections. Beef cattle are suspected to contribute to human 3GC-resistant Salmonella infections. Commensal 3GC-resistant Escherichia coli are thought to act as reservoirs of 3GC resistance because these strains are isolated more frequently than are 3GC-resistant Salmonella strains at beef cattle feedyards. During each of 24 consecutive months, four samples of pen surface material were obtained from five pens (N = 480) at a Nebraska feedyard to determine to the contribution of 3GC-resistant E. coli to the occurrence of 3GC-resistant Salmonella. Illumina whole genome sequencing was performed, and susceptibility to 14 antimicrobial agents was determined for 121 3GC-susceptible Salmonella, 121 3GC-resistant Salmonella, and 203 3GCresistant E. coli isolates. 3GC-susceptible Salmonella isolates were predominantly from serotypes Muenchen (70.2%) and Montevideo clade 1 (23.1%). 3GC-resistant Salmonella isolates were predominantly from serotypes Montevideo clade 2 (84.3%). One bla gene type (blaCMY-2) and the IncC plasmid replicon were present in 100 and 97.5% of the 3GC-resistant Salmonella, respectively. Eleven bla gene types were detected in the 3GC-resistant E. coli, which were distributed across 42 multilocus sequence types. The blaCMY-2 gene and IncC plasmid replicon were present in 37.9 and 9.9% of the 3GC-resistant E. coli, respectively. These results suggest that 3GC resistance in Salmonella was primarily due the persistence of Salmonella Montevideo clade 2 with very minimal or no contribution from 3GC-resistant E. coli via horizontal gene transfer and that 3GCresistant E. coli may not be a useful indicator for 3GC-resistant Salmonella in beef cattle production environments.