Purpose: The objective of this study was to assess the transcriptome of important foodborne enteric pathogens, especially virulence gene expression in response to bile, mimicking stress conditions in the host intestinal environment.
Methods: Outbreak associated isolates of E. coli O157:H7, E. coli O104:H4 and Salmonella enterica serovar Montevideo were selected. E. coli MG1655 was used as a non-pathogenic control. Cells were grown in DMEM, with and without complex bile acids and short chain fatty acids to simulate the intestinal environment. RNA was harvested at mid-log phase and hybridized to custom high density gene expression (FDA-ECSG and FDA-SEEC, respectively, for E. coli and Salmonella) microarrays. The arrays contain probe sets derived from chromosomal and plasmid sequences representing the pan-genomes of these pathogens.
Results: Bile supplemented media triggers expression of iron stress response, oxidative stress response and membrane efflux in pathogens, indicating a survival mechanism in an iron-scarce environment similar to previous reports. Additionally, our data show that bile suppresses expression of major virulence factors by at least two-fold, including genes for adherence, type three and four secretion system proteins, Shiga toxins (stx2 in E. coli O157:H7), and those found in pathogenicity islands.
Significance: The findings indicate that some foodborne enteric pathogens react to bile through wide-ranging changes in gene transcription. We identify genes that help pathogens adapt to the changing environmental conditions and host defenses encountered during human infection. Comparing these genes expressed in the host to those expressed on food as reported by others, will help identify functional genomic key stress response markers that can be used for rapid detection of pathogens on food.