Helicobacter spp. represent a proportionately small but significant component of the normal intestinal microflora of animal hosts. Several of these intestinal Helicobacter spp. are known to induce colitis in mouse models, yet the mechanisms by which these bacteria induce intestinal inflammation are poorly understood. To address this question, we performed in vitro co-culture experiments with mouse and human epithelial cell lines stimulated with a selection of Helicobacter spp., including known pathogenic species as well as ones for which the pathogenic potential is less clear. Strikingly, a member of the normal microflora of rodents, Helicobacter muridarum, was found to be a particularly strong inducer of CXC chemokine (Cxcl1/KC, Cxcl2/MIP-2) responses in a murine intestinal epithelial cell line. Time-course studies revealed a biphasic pattern of chemokine responses in these cells, with H. muridarum lipopolysaccharide (LPS) mediating early (24-48 h) responses and live bacteria seeming to provoke later (48-72 h) responses. H. muridarum LPS per se was shown to induce CXC chemokine production in HEK293 cells stably expressing Toll-like receptor 2 (TLR2), but not in those expressing TLR4. In contrast, live H. muridarum bacteria were able to induce NF-kappaB reporter activity and CXC chemokine responses in TLR2-deficient HEK293 and in AGS epithelial cells. These responses were attenuated by transient transfection with a dominant negative construct to NOD1, and by stable expression of NOD1 siRNA, respectively. Thus, the data suggest that both TLR2 and NOD1 may be involved in innate immune sensing of H. muridarum by epithelial cells. This work identifies H. muridarum as a commensal bacterium with pathogenic potential and underscores the potential roles of ill-defined members of the normal flora in the initiation of inflammation in animal hosts. We suggest that H. muridarum may act as a confounding factor in colitis model studies in rodents.