Background: Trinitrobenzene sulphonic acid (TNBS) induces chronic transmural inflammatory lesions in the rat colon. Injury is facilitated by barrier disruption and invasion of commensal bacteria. However, certain bacteria have shown anti-inflammatory properties in in vitro models. Aim: To investigate in vivo the anti-inflammatory effect of Lactobacillus casei DN-114 001. Methods: Rats with a colonic segment excluded from faecal transit were surgically prepared. After washing the lumen with antibiotics, the excluded segment was recolonised (control group: standard flora of rat origin; test group: standard flora and L casei). Microbial colonisation was confirmed by culture of segment washing, and colitis was then induced by instillation of TNBS. One day after, intestinal lesions were blindly graded by macro- and microscopic scores, and myeloperoxidase activity measured in tissue homogenates. Translocation of bacteria to mesenteric lymph nodes, spleen and liver was investigated. Results: Test rats showed a smaller area of mucosal injury than control rats (p<0.05). Maximum depth lesion scores were similar in both groups but myeloperoxidase activity was lower in test than in control rats (p<0.05). Remarkably, bacterial translocation was quantitatively lower (p<0.01) and less frequent (p<0.05) in test than in control rats. Conclusion: In rats colonised with L casei, mucosal injury, inflammatory response, and barrier disruption after TNBS challenge were attenuated. Bacterial communities colonising the mucosa can modify inflammatory responses to luminal challenges.