Twenty-four month longitudinal study suggests little to no horizontal gene transfer in situ between third-generation cephalosporin-resistant Salmonella and third-generation cephalosporin-resistant Escherichia coli in a beef cattle feedyard.

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 since they are more frequently isolated than 3GC-resistant Salmonella at beef cattle feedyards. During each of 24 consecutive months 4 samples of pen surface material were obtained from 5 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 susceptibilities to 14 antimicrobial agents were determined for 121 3GC-susceptible Salmonella , 121 3GC-resistant Salmonella , and 203 3GC-resistant E. coli isolates. 3GC-susceptible Salmonella isolates were predominantly Muenchen (70.2%) and Montevideo Clade 1 (23.1%). 3GC-resistant Salmonella isolates were predominantly Montevideo Clade 2 (84.3%). One bla gene type ( bla CMY-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 . The 3GC-resistant E. coli were distributed across 42 multilocus sequence types. The bla CMY-2 gene and IncC plasmid replicon were present in 37.9% and 9.9% of the 3GC-resistant E . coli , respectively. These results suggested that 3GC resistance in Salmonella was primarily due the persistence of Montevideo Clade 2 with very minimal or no contribution from 3GC-resistant E. coli via horizontal gene transfer, suggesting that 3GC-resistant E. coli may not be a useful indicator for 3GC-resistant Salmonella in beef cattle production environments.