Location: Meat Safety and QualityTitle: 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...
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2021
Publication Date: N/A
Interpretive Summary: It has been theorized that Salmonella present at cattle feedyards may become resistant to the primary antibiotic used to treat serious human Salmonella infections by acquiring the genes responsible for resistance from non-pathogenic resistant E. coli. This theory has not been tested because although this gene transfer can be shown in artificial laboratory conditions, it has not been conclusively demonstrated in real world conditions. This study characterized resistance genes in 242 Salmonella and 203 E. coli isolated from a beef feedyard over two years. The primary resistance gene in resistant Salmonella was present in only 37.9% of the resistant E. coli. Likewise, the primary resistance gene in most resistant E. coli was at low levels in resistant Salmonella. These results demonstrated that resistant Salmonella at this feedlot were primarily due the persistence of a well-adapted Salmonella sub-population with very minimal or no contribution of resistance genes from E. coli.
Technical Abstract: 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 (blaCMY-2) and the IncC plasmid replicon were present in 100% and 97.5% of the 3GC-resistant Salmonella, respectively. Eleven bla genes were detected in the 3GC-resistant E. coli isolates. The 3GC-resistant E. coli 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 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, indicating that 3GC-resistant E. coli is not a useful indicator for 3GC-resistant Salmonella.