Inflammatory bowel diseases (IBD), including Crohn's disease and ulcerative colitis, are characterised by an abnormal activation of the gut-associated immune system resulting in a chronic inflammation of the digestive tract (DT). IBD patients show flares of remission and relapses with symptoms of bloody diarrhea, abdominal pain and rectal bleeding. Although the etiology of IBD remains unclear, it is generally considered that a combination of several factors including genetic predisposition, immune disorders and environmental factors could be involved. The role of commensal and pathogenic bacteria in the induction of these diseases is particularly well documented [1-7].
Recent studies linked intestinal oxidative stress to the observed epithelial damage. In small quantities, reactive oxygen species (ROS) are involved in certain signalization or regulation pathways. During normal inflammation, ROS toxicity leads to the elimination of infectious agents: ROS are generated inside the DT during oxidative burst by activated phagocyte cells which possess ROS-producing enzymes, such as NADPH oxidase , NO synthase  and myeloperoxidase  and they infiltrate the lamina propria. However, in the case of IBD, excessive amounts of ROS accumulate and lead to oxidative epithelial damage. Several studies have shown a correlation between the increase in ROS production and disease activity in inflamed biopsies of IBD patients [11-14]. The measured effects on antioxidative systems diverge between studies: antioxidative enzyme activity was either increased [15,16] or decreased  in IBD biopsies, depending on the nature of the enzyme and the state of disease activity. Moreover, several studies provide direct evidence of in vivo oxidative injury in inflamed epithelial cells of IBD patients [12,17,18]. Recently, several antioxidative strategies have been evaluated using animal colitis models and appear to be efficient in the reduction of inflammatory damage [19-23]. These antioxidative strategies could be based on the activity of two enzymes: superoxide dismutase (SOD) which reduced superoxides and catalases which catabolized in H2O + O2 the hydrogen peroxide resulting from the reduction of superoxides. Here, we proposed to evaluate the efficiency of in situ delivery of one catalase by the lactic acid bacterium (LAB) Lactobacillus casei. In parallel to their traditional use for the production of fermented products, LAB are also studied for their probiotic properties. Some strains could improve IBD patients' health, as it observed, for example, with the administration of the VSL#3 probiotic cocktail which delayed the relapse into pouchitis after surgical resection [24,25]. Natural anti-inflammatory effects were recently shown for Lb. salivarius , Bifidobacterium and Lb. plantarum [27,28] and Lb. casei Shirota [29,30] using experimental colitis models. New recombinant strategies are also in progress to engineer LAB strains for in situ delivery of heterologous therapeutic proteins. This strategy has been first applied to IBD, where the intragastric administration of a genetically engineered L. lactis strain producing an anti-inflammatory cytokine, the interleukin-10 (IL-10), caused a significant reduction in colitis in mice treated with DSS .
We previously showed that the improvement of the antioxidative capacities of Lactococcus lactis and Lb. casei by the introduction of a catalase gene led to an increase of bacterial survival in the presence of hydrogen peroxide and under aerated conditions. We evaluated the in vitro catalase activity of Lb. casei BL23 producing the manganese-dependant catalase (hereafter called MnKat) from Lb. plantarum ATCC14431  at 16 μmol of H2O2/min/mg of protein [33,34]. In this study, the preventive effects of two isogenic Lb. casei strains producing or not MnKat on the development of the inflammation were analyzed using a DSS-induced intestinal colitis model in BALB/c mice. We tested in vivo whether the MnKat-producing Lb. casei strain could reduce epithelial damage of the digestive tract.