Characterization of Complex Regulatory Circuits in Enterohemorrhagic Escherichia coli O157:H7

Pathogens have complex networks of overlapping and integrated signaling pathways that provide highly adapted responses to different external stimuli. Pathogens utilize external stimuli to sense the host environment and regulate the expression of virulence traits according to location. For gastrointestinal pathogens, this equates to pathogens sensing the host gastrointestinal tract and colonizing. Previous studies have demonstrated that the signaling molecule ethanolamine (EA) plays an extensive role in the regulation of virulence traits in EHEC. We hypothesized that EA is involved in the regulation of virulence traits through multiple signaling cascades. We determined that the EA-utilization operon encoded regulatory protein EutR directly binds to the promoter regions of ler, the master regulator of the locus of enterocyte effacement (LEE), to regulate transcription. We further explored signaling cascades regulating the LEE by investigating the function of previously uncharacterized open-reading frame, etrB. We determined that expression of EtrB is directly regulated by QseA, and that EtrB functions as a direct regulator of the LEE and influences the expression of other virulence traits, including non-LEE encoded effectors and fimbrial locus 11. Previous studies demonstrated that EA promotes the expression of fimbrial genes in EHEC. Data from our lab suggests that the fimbrial loci Erf1 and Erf2, while not important for mediating early attachment events, are necessary for the expression of virulence traits important for later stages of infection, including AE lesion formation and Shiga toxin expression. We determined that this phenotype of fimbriae influencing AE lesion formation is not conferred to all fimbrial loci as a deletion in fimbrial locus 3 has no effect on AE lesion formation to HeLa cells. Furthermore, we determined that expression of Erf1 or Erf2 as surface structures is not necessary to modulate LEE expression and AE lesion formation. Overall, this work has demonstrated how pathogens, such as EHEC, utilize widely conserved transcriptional regulators to coordinate virulence gene expression in response to the host environment. Many pathogens encode the EA-utilization operon, the ETT2, and fimbriae. Therefore, our studies may describe a general mechanism used by pathogens to coordinate the expression of virulence traits.