The human gastro-intestinal (GI) tract is colonized by approximately 10¹⁴ bacterial cells that belong to about 400 different species and co-exist with host cells. However, introduction of pathogens such as enterohemorrhagic E. coli (EHEC) disturbs this homeostasis and rapidly leads to colonization and infection. A characteristic feature of the GI tract is the abundance of signaling molecules, both of prokaryotic and eukaryotic origin, present in the lumen. Both pathogenic and non pathogenic bacteria secrete various quorum sensing and metabolic signals such as AI-2, AI-3, and indole. Similarly, host hormones like norepinephrine, dopamine and serotonin are also synthesized in the GI tract by the enteric nervous system. The close proximity of bacteria and the host cells, as well as the abundance of the signals they secrete, has led to a new signal-centric paradigm wherein the different GI tract signals are considered to be important mediators of homeostasis and infections through intra-kingdom (i.e., recognition of bacterial signals by other bacteria) and/or inter-kingdom (i.e., recognition of host signals by bacteria and vice-versa) signaling and communication [1, 2].

Work in our laboratory has focused on investigating intra- and inter-kingdom signaling in host cells, commensal bacteria, and the pathogen EHEC. Based on the structural similarity between indole and the human hormone melatonin, we hypothesized that indole acts as an inter-kingdom signal in intestinal epithelial cells. Gene expression studies with HCT-8 cells exposed to the bacterial signal indole showed that indole significantly up-regulated the expression of genes involved in apoptosis, immune response, interferons, and interleukins; thereby, suggesting that this commensal bacteria-secreted signal is used by intestinal epithelial cells in host cell inflammation and maintaining homeostasis. This observation was further validated through fluorescence microscopy analysis of the pro-inflammatory transcription factor NF-κB where the presence of indole decreased the induction TNF-α induced inflammatory response. Together, these results clearly demonstrate inter-kingdom recognition of bacterial signals by intestinal epithelial cells.

We also investigated inter-kingdom signaling in pathogenic bacteria by investigating the effect of intestinal epithelial cell secreted factors on EHEC pathogenicity. Gene expression studies with EHEC exposed to spent eukaryotic culture medium significantly increased the expression of 29 virulence genes belonging to LEE pathogenicity island and also of 22 genes involved in cell division and adhesion. Several AI-2 uptake genes and iron related genes were also upregulated. These observations are also corroborated by in vitro attachment assays where spent medium caused a 6-fold increase in EHEC attachment to HCT-8 cells. Together our results provide the basis of an inter-kingdom signaling-based framework where recognition of different signals by different elements underlies the balance between homeostasis and infection in the GI tract.

References

1. Bansal, T., D. Englert, J. Lee, M. Hegde, T. K. Wood, and A. Jayaraman. 2007. Differential Effects of Epinephrine, Norepinephrine, and Indole on Escherichia coli O157:H7 Chemotaxis, Colonization, and Gene Expression. Infect Immun 75:4597-4607.

2. Bansal, T., P. Jesudhasan, S. Pillai, T. K. Wood, and A. Jayaraman. 2008. Temporal Regulation of Enterohemorrhagic Escherichia coli Virulence Mediated by Autoinducer-2. Appl Microbiol Biotechnol 78:811-819.