Multidrug-Resistant Salmonella Strains Emerging in United States

pigs in a farm
pigs in a farm
Analysis demonstrated that the differentiation into clades correlated more closely with the presence of an ASSuT genotype and certain MLST sequence types than with the geographic origin of the isolates.

Salmonella 4,[5],12:i:-, a variant of S Typhimurium that lacks the 2nd phase flagellar antigen and which are currently circulating in swine from the US Midwest, are part of an emerging clade with multidrug-resistant phenotypes and are similar to S typhimurium from Europe, according to the results of recent research published in Clinical Infectious Diseases.

In this study of 659 S 4,[5],12:i:- samples and 325 S typhimurium samples from the United States and Europe, whole genome sequencing was used to evaluate the genetic heterogeneity of the S 4,[5],12:i:- strains from the US Midwest. Resistance genes and other virulence factors were also evaluated in 50 livestock isolates and 22 human isolates.

The researchers found that 84% of the S 4,[5],12:i:- isolates from the US Midwest collected between 2014 and 2016 were part of an emerging clade. Genotypic antimicrobial resistance against ampicillin, streptomycin, sulfonamides, and tetracycline was common. Phenotypic antimicrobial resistance to enrofloxacin (n=11/50) and ceftiofur (n=9/50) were present with plasmid-mediated resistance genes.

When S 4,[5],12:i:- isolates from the United States were compared with S typhimurium based on average pairwise single-nucleotide polymorphisms, US S 4,[5],12:i:- were more similar to European S typhimurium than to the US S typhimurium (P <.001).

In an interview with Infectious Disease Advisor, Ehud Elnekave, DVM, PhD, postdoctoral research associate in the Department of Veterinary Population Medicine at the University of Minnesota, explained that “the presence of plasmid-mediated genes, which acquire resistance to important antibiotic classes such as third-generation cephalosporins and fluoroquinolones in Salmonella samples of swine origin, is an important finding, as these can potentially be transmitted to other pathogens.”

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Based on these findings, he concluded that “antimicrobial stewardship in both veterinary and human practice is recommended.”


Elnekave E, Hong S, Mather AE, et al. Salmonella enterica serotype 4,[5],12:i:- in swine in the United States Midwest: an emerging multidrug resistant clone [published online October 23, 2017]. Clin Infect Dis. doi: 10.1093/cid/cix909