Commensal bacterial were screened for their ability to modulate inflammatory gene expression in epithelial cells. The influence of intestinal pathogens on cytokine responses was also investigated using co-culture models. It was demonstrated that Bacteroids thetaiotaomicron and Bacteroides vulgatus, which are both dominant members of the human colonic microflora, had the ability differentially to modulate inflammatory responses in intestinal epithelial cells. These observations were confirmed in vivo using a minimal flora rat model. The human pathogen Salmonella enteritidis, when incubated with the Caco-2 human intestinal epithelial cell line, upregulated mRNA expression of the pro-inflammatory cytokines TNF-, MIP-2 and IL-8, but down regulated mRNA expression of the anti-inflammatory cytokine TGF-. In contrast, co-culture of Salmonella enteritidis with Bacteroides thetaiotaomicron, resulted in attenuation of TNF- , MIP-2 and IL-8 mRNA expression and restoration of TGF- to control levels. The viability, growth, attachment and invasion characteristics of the bacteria were not allowed during the time course of the experiments. The attenuating effect of Bacteroides thetaiotaomicron on the expression of pro-inflammatory genes, in particular IL-8, was shown significantly to reduce polymorphonuclear neutrophil (PMN) recruitment in vitro, using an epithelial cell/neutrophil transwell co-culture system. These results were also confirmed in vivo using minimal flora (isolator-reared) rats. PMN recruitment both in vitro and in vivo was assessed using myeloperoxidase activity. The immunosuppressive activity of B. thetaiotaomicron was found to involve down regulation of the nuclear translocation of the transcription factors NF-B/p65 and phosphorylation of AP-1 sub-unit proteins, both of which regulate the expression of TNF- and IL-8 mRNA in intestinal epithelial cells. This effect required viable Bacteroides thetaiotaomicron. The influence of Bacteroides vulgatus, which augmented expression of pro-inflammatory cytokines TNF- and IL-8, when compared to Salmonella alone was functionally distinct from that of Bacteroides thetaiotaomicron. Furthermore, the presence of Bacteroides vulgatus increased the nuclear translocation of the NF-B/p65 subunit, when compared to Salmonella alone.