Location: Meat Safety and QualityTitle: Persistence of third-generation cephalosporin-resistant Salmonella over two years at a beef cattle feedlot
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/16/2019
Publication Date: N/A
Technical Abstract: Objective: Measure and characterize third-generation cephalosporin (3GC)-resistant Escherichia coli, generic Salmonella enterica, and 3GC-resistant Salmonella present at a beef cattle feedyard over two-years to gain insights into the contribution of horizontal gene transfer to the occurrence of 3GC-resistant Salmonella. Experimental Design & Analysis: Pen surface material (defined as the soil-manure mixture present on the pen surface) samples (N = 480) obtained from 5 pens at a cattle feeding operation were cultured to detect and quantify generic E. coli, 3GC-resistant E. coli, generic Salmonella, and 3GC-resistant Salmonella. For each positive sample, between 2 and 4 3GC-resistant Salmonella and 3GC-resistant E. coli colonies were PCR screened to determine the presence of blaCMY-2 and blaCTX-M genes. For each month and pen, blaCMY-2 3GC-resistant E. coli and blaCTX-M 3GC-resistant E. coli colony counts were estimated. For the pens highest and lowest levels of 3GC-resistant Salmonella three isolates of generic Salmonella, 3GC-resistant Salmonella, blaCMY-2 3GC-resistant E. coli, and blaCTX-M 3GC-resistant E. coli were obtained from independent samples for each of the 24 sampled months, if available. Genomic DNA was prepared using the QIAamp DNA Mini Kit. Genomic DNA libraries were sequenced on a NextSeq 550 platform (Illumina) Genomes were assembled using the Shovill version 1.0.4 pipeline. Assembled genome sequences were annotated for serotype, multi-locus sequence type (MLST), plasmid incompatibility (Inc) group, and antimicrobial resistance genes. Key Results: All 3GC-resistant Salmonella PCR screened in this study harbored blaCMY-2 while none harbored blaCTX-M. Conversely, blaCTX-M predominated 3GC-resistant E. coli. This indicates the dominant 3GC-resistant E. coli population was not responsible for horizontal gene transfer of 3GC resistance to Salmonella. However, this result in itself did not eliminate the possibility that the minority blaCMY-2 E. coli population was maintaining 3GC resistance in Salmonella by horizontal gene transfer. WGS sequence from 23 of the 24 generic Salmonella isolates are a single subtype, Muenchen/Virginia ST83. The remaining isolate was a pansusceptible Montevideo ST138. For 3GC-resistant Salmonella, 23 of 24 isolates were a single subtype, Montevideo ST138. The remaining isolate was a Muenchen/Virginia ST83. All of the 3GC-resistant Salmonella Montevideo ST138 harbored an Inc A/C2 plasmid. Since 23 of the 24 3GC-resistant Salmonella were the same subtype (Montevideo ST138), with the same plasmid Inc (A/C2), this strongly suggests that 3GC-resistance is maintained in this pen by persistence of this subtype. WGS is currently being obtained for a further 120 isolates to strengthen the results. How can this information be applied in the industry: Results suggest that adapted Salmonella subtypes are responsible for persistence of 3GC-resistant Salmonella and that horizontal gene transfer (from E. coli or other bacteria) is not a significant contributor. This data will be a critical resource for the effective design of interventions to reduce the occurrence of Salmonella resistant to clinically important antimicrobials at cattle feedyards. This study will also be a resource for providing empirical answers to conjecture pertaining to the factors contributing to the occurrence of 3GC-resistant Salmonella at cattle feedlots.