Background: The interaction of commensal bacteria with the intestinal immune system is an essential factor in the development of inflammatory bowel disease (IBD). The study of isolated commensal bacteria's effects on the mucosal immune response might be relevant for a better understanding of pathophysiological mechanisms in IBD. Methods: We investigated the immune responses to signals from the commensal Escherichia coli ATCC 35345 and the probiotic Lactobacillus casei DN-114 001 in Crohn's disease (CD) mucosa. Ileal specimens were obtained during surgery from CD patients. Mucosal explants were incubated with L. casei or its genomic DNA; TNF-α, IFN-γ, IL-2, IL-6, IL-8, and CXCL1 were measured in the supernatant. Second, tissue expression of key proinflammatory cytokines (IL-6, TGF-β, IL-23p19, IL-12p35, IL-17F), and chemokines (IL-8, CXCL1, CXCL2) was evaluated after incubation with L. casei or E. coli. Finally, combination experiments were carried out by incubating both strains with mucosal explants at different timepoints. Results: Live L. casei significantly decreased secretion of TNF-α, IFN-γ, IL-2, IL-6, IL-8, and CXCL1 by CD mucosa, but the effect was not reproduced by L. casei DNA. Second, live L. casei down- regulated expression of IL-8, IL-6, and CXCL1 and did not modify expression of IL-23p19, IL-12p35, and IL-17F. In contrast, E. coli significantly upregulated expression of all these cytokines. Interestingly, combination experiments revealed the ability of L. casei to prevent and counteract the proinflammatory effects of E. coli. Conclusions: Live L. casei can counteract the proinflammatory effects of E. coli on CD inflamed mucosa by specific downregulation of key proinflammatory mediators. Copyright © 2008 Crohn's & Colitis Foundation of America, Inc.