Salmonella enterica subsp. Serovar Heidelberg Food Isolates Associated with a Salmonellosis Outbreak Have Enhanced Stress Tolerance Capabilities.

serovar Heidelberg is currently the 12th most common serovar of causing salmonellosis in the United States and results in twice the average incidence of blood infections caused by nontyphoidal salmonellae. Multiple outbreaks of salmonellosis caused by Heidelberg resulted from the same poultry processor, which infected 634 people during 2013 and 2014. The hospitalization and invasive illness rates were 38% and 15%, respectively. We hypothesized that the outbreak strains of Heidelberg had enhanced stress tolerance and virulence capabilities. We sourced nine food isolates collected during the outbreak investigation and three reference isolates to assess their tolerance to heat and sanitizers, ability to attach to abiotic surfaces, and invasiveness We performed RNA sequencing on three isolates (two outbreak-associated isolates and a reference Heidelberg strain) with various levels of heat tolerance to gain insight into the mechanism behind the isolates' enhanced heat tolerance. We also performed genomic analyses to determine the genetic relationships among the outbreak isolates. Ultimately, we determined that (i) six Heidelberg isolates associated with the foodborne outbreak had enhanced heat tolerance, (ii) one outbreak isolate with enhanced heat tolerance also had an enhanced biofilm-forming ability under stressful conditions, (iii) exposure to heat stress increased the expression of Heidelberg multidrug efflux and virulence genes, and (iv) outbreak-associated isolates were likely transcriptionally primed to better survive processing stresses and, potentially, to cause illness. This study provides a deep analysis of the intrinsic stress tolerance and virulence capabilities of Heidelberg that may have contributed to the length and severity of a recent salmonellosis outbreak. Additionally, this study provides a comprehensive analysis of the transcriptomic response of strains to heat stress conditions and compares baseline stationary-phase gene expression among outbreak- and non-outbreak-associated Heidelberg isolates. These data can be used in assay development to screen isolates for stress tolerance and subsequent survival. This study adds to our understanding of the strains associated with the outbreak and informs ongoing regulatory discussions on in poultry.