The role of Salmonella Genomic Island 4 in metal tolerance of Salmonella enterica serovar I 4,[5],12:i:- pork outbreak isolate USDA15WA-1

Authors: Bearson, Bradley - Brad; Trachsel, Julian; Shippy, Daniel; SIVASANKARAN, SATHESH - Iowa State University; Kerr, Brian; Loving, Crystal; Brunelle, Brian; CURRY, SHELBY - Orise Fellow; GABLER, NICHOLAS - Iowa State University; Bearson, Shawn

Submitted to: Genes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2020
Publication Date: 10/30/2020
Citation: Bearson, B.L., Trachsel, J.M., Shippy, D.C., Sivasankaran, S.K., Kerr, B.J., Loving, C.L., Brunelle, B.W., Curry, S.M., Gabler, N.K., Bearson, S.M. 2020. The role of Salmonella Genomic Island 4 in metal tolerance of Salmonella enterica serovar I 4,[5],12:i:- pork outbreak isolate USDA15WA-1. Genes. 11(11). Article 1291. https://doi.org/10.3390/genes11111291.
DOI: https://doi.org/10.3390/genes11111291

Interpretive Summary: Salmonella is the most common bacterial causative agent of human foodborne illness in the U.S. with an estimated 1.3 million infections and 420 deaths annually. There are >2,600 subtypes of Salmonella called serovars and the prevalence of a specific serovar called I 4,[5],12:i:- has increased globally over the last 2 decades. In 2015, a multistate foodborne outbreak occurred due to consumption of pork contaminated with antimicrobial-resistant Salmonella serovar I 4,[5],12:i:-. Because our genomic analysis of a Salmonella serovar I 4,[5],12:i:- isolate from the 2015 pork outbreak previously revealed the presence of numerous antimicrobial resistance and metal tolerance genes, studies were performed to evaluate the role of the acquired genes in metal tolerance including copper exposure. Analyses confirmed that serovar I 4,[5],12:i:- has increased tolerance to copper, arsenic, and antimony, and that exposure to copper altered Salmonella gene expression, including the increased expression of multiple metal tolerance genes (copper, arsenic, silver, and mercury). Furthermore, administering elevated levels of copper and zinc as an antimicrobial in the diet of pigs did not reduce fecal shedding or tissue colonization of Salmonella serovar I 4,[5],12:i:- in the pigs. Altogether, the data corroborate current concerns in the literature that acquisition of metal tolerance genes by Salmonella may facilitate their survival in diverse metal environments and select for antimicrobial resistance genes co-located on the genetic cassettes encoding the induced metal tolerance genes (i.e. co-selection of heavy-metal-tolerant and antimicrobial-resistant Salmonella). Swine producers, veterinarians, and nutritionists will be interested to know that inclusion of elevated levels of metals (as antimicrobials) in pig diets may have an unintended consequence of selecting for metal-tolerant bacterial pathogens.

Technical Abstract: Multidrug-resistant (MDR; resistance to >3 antimicrobial classes) Salmonella enterica serovar I 4,[5],12:i:- strains were linked to a 2015 foodborne outbreak from pork. Strain USDA15WA-1, associated with the outbreak, harbors an MDR module and the metal tolerance element Salmonella Genomic Island 4 (SGI-4). Characterization of SGI-4 revealed that conjugational transfer of SGI-4 resulted in the mobile genetic element (MGE) replicating as a plasmid or integrating into the chromosome. Tolerance to copper, arsenic, and antimony compounds was increased in Salmonella strains containing SGI-4 compared to strains lacking the MGE. Following Salmonella exposure to copper, RNA-seq transcriptional analysis demonstrated significant differential expression of diverse genes and pathways, including induction of numerous metal tolerance genes (copper, arsenic, silver, and mercury). Evaluation of swine administered elevated levels of zinc oxide (2,000 mg/kg) and copper chloride (200 mg/kg) as an antimicrobial feed additive (Zn+Cu) in their diet for 4 weeks prior to and 3 weeks post-inoculation with serovar I 4,[5],12:i:- indicated that Salmonella shedding levels declined at a significantly slower rate in pigs receiving in-feed Zn+Cu compared to control pigs (no Zn+Cu). The presence of metal tolerance genes in MDR Salmonella serovar I 4,[5],12:i:- may provide benefits for environmental survival or swine colonization in metal-containing settings.