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Competitive Fitness of Multi-Drug Resistant Pathogenic Salmonella Enterica Serotype Newport Isolates from Cattle


The overall goal of this research is to obtain fundamental information on the competitive fitness of multi-drug resistant, pathogenic Salmonella enterica serotype Newport strains in vitro and in vivo. <P>Such information should aid in understanding the mechanism underlying the rapid emergence of S. serotype Newport strains among cattle and lead to management regimes that reduce the dissemination and emergence of new multi-drug resistant Salmonella strains.

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Findings: Ninety-nine Salmonella enterica serotype Newport isolates were obtained from the laboratory of Dr. Mohammad Koohmaraie (USDA/ARS/MARC). Eighty-six of these Salmonella Newport strains were resistant to multiple antibiotics. Thirteen were susceptible to all antibiotics tested (pansusceptible). Eleven of the pansusceptible isolates originated from beef cattle carcasses. This represented all of the beef cattle isolates we obtained. Genetic analysis of the beef strains showed that there were only two clones. In order to establish that the multi-drug resistant Salmonella Newport strains are primarily coming from dairy cattle as opposed to beef cattle a greater representation of strains from beef cattle will need to be characterized. The fact that we can see this current trend probably means we are observing an early snapshot of the Newport serotype evolution and dissemination.

<P>The results presented here represent the first known identification of Salmonella field isolates carrying the IncA/C incompatibility type plasmid. Very little has ever been published in the scientific literature on IncA/C. An expanded characterization of Salmonella serogroups may aid in determining the origin and dissemination of the IncA/C plasmid acquired by the Newport isolates characterized in this study.

<P>Overall the possession of multi-drug resistance did not incur a significant fitness cost as compared to the pansusceptible Salmonella Newport isolates and the resistance genes were also transferable to E. coli. Other authors have had difficulty transferring resistance from Salmonella Newport isolates; however, we used an overnight broth technique that allowed us to detect very low levels of transfer. This indicated the plasmid was mobile; therefore, it is likely that plasmid transfer could occur between different species of bacteria in the bovine gastrointestinal tract.

<P> For complete projects details, view the <a href="…; target="_blank">Project Summary. </a>

Poole, Toni
USDA - Agricultural Research Service
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