Genomic and phenotypic comparison of two variants of multidrug-resistant Salmonella enterica serovar Heidelberg isolated during the 2015-2017 multi-state outbreak in cattle

Authors: BURCIAGA, SELMA - Oak Ridge Institute For Science And Education (ORISE); Trachsel, Julian; SOCKETT, DONALD - University Of Wisconsin; AULIK, NICOLE - University Of Wisconsin; Monson, Melissa; Anderson, Christopher; Bearson, Shawn

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2023
Publication Date: 10/20/2023
Citation: Burciaga, S., Trachsel, J.M., Sockett, D.C., Aulik, N.A., Monson, M.S., Anderson, C.L., Bearson, S.M. 2023. Genomic and phenotypic comparison of two variants of multidrug-resistant Salmonella enterica serovar Heidelberg isolated during the 2015-2017 multi-state outbreak in cattle. Frontiers in Microbiology. 14: Article 1282832. https://doi.org/10.3389/fmicb.2023.1282832.
DOI: https://doi.org/10.3389/fmicb.2023.1282832

Interpretive Summary: Salmonella is one of the top five foodborne pathogens and one of the leading causes of foodborne illness in humans in the United States and worldwide. Over the years, multidrug-resistant Salmonella Heidelberg has been implicated in six multistate outbreaks in the United States, and five of the outbreaks were associated with consumption of chicken- or turkey-related products. However, the 2015-2017 Salmonella Heidelberg outbreak was linked to human contact with dairy beef calves. Typically, foodborne isolates of Salmonella cause disease in humans but the carrier food animal does not display any signs of disease. The 2015-2017 Heidelberg outbreak was unique because the isolates associated with the outbreak were recovered from ill humans as well as sick dairy calves. Two main variants of Salmonella Heidelberg were isolated from the intestines of calves during the outbreak and because greater morbidity and mortality in calves was associated with one variant compared to the other, the two isolates were deeply characterized to explore factors likely contributing to their contrasting disease severity. The Salmonella Heidelberg isolate with greater morbidity and mortality in dairy beef calves also had elevated expression of virulence genes and higher invasion of human and bovine epithelial cells, potentially enhancing this isolate’s pathogenicity, and contributing to the ecological success in dairy calves.

Technical Abstract: Salmonella enterica subspecies enterica serovar Heidelberg (Salmonella Heidelberg) has caused several multistate foodborne outbreaks in the United States, largely associated with the consumption of poultry. However, a 2015-2017 multidrug-resistant (MDR) Salmonella Heidelberg outbreak was linked to contact with dairy beef calves. Traceback investigations revealed calves infected with outbreak strains of Salmonella Heidelberg exhibited symptoms of disease frequently followed by death from septicemia. To investigate virulence characteristics of Salmonella Heidelberg as a pathogen in bovine, two variants with distinct pulse-field gel electrophoresis (PFGE) patterns that differed in morbidity and mortality received during the multistate outbreak were genotypically and phenotypically characterized and compared. Strain SX 245 with PFGE pattern JF6X01.0523 was identified as a dominant and highly pathogenic variant causing high morbidity and mortality in affected calves, whereas strain SX 244 with PFGE pattern JF6X01.0590 was classified as a low pathogenic variant causing less morbidity and mortality. Comparison of whole-genome sequences determined SX 245 lacked ~200 genes present in SX 244, including genes associated with the IncI1 plasmid and phages; SX 244 lacked eight genes present in SX 245 including the YdiV Anti-FlhC(2)FlhD(4) factor, a lysin motif domain containing protein, and a pentapeptide repeat protein. RNA-sequencing revealed fimbriae-related, flagella-related, and chemotaxis genes had increased expression in SX 245 compared to SX 244. Furthermore, SX 245 displayed higher invasion of human and bovine epithelial cells than SX 244. These data suggest the presence and up-regulation of genes involved in type 1 fimbriae production, flagella regulation and biogenesis, and chemotaxis may play a role in the increased pathogenicity and host range expansion of the Salmonella Heidelberg isolates involved in the bovine-related outbreak.