a significant worsened body weight gain concomitant with enhanced colitis score and MPO activity. The colonic cytokine profiles of rat fed with Nissle ΔclbA mutant were also similar to that seen in DSS-treated rats fed with PBS (Fig. 2). The non-genotoxic E. coli Nissle 1917 mutant exacerbates the mortality of colitic mice. We next examined the probiotic properties of the non-genotoxic E. coli Nissle 1917 mutant in a T-cell dependent model of chronic colitis induced by the adoptive transfer of naïve CD4+ CD45RBhigh T cells in immunocompro- mised SCID mice. Oral treatments with PBS, Nissle WT or Nissle ΔclbA were initiated 3 d after T cell trans- fer and were repeated every 3 d initially for 8 weeks. The non-transferred control mice remained healthy during the entire observation period. In contrast, the injection of CD4+ CD45RBhigh T cells in PBS-treated SCID mice resulted in discernible symptoms such as hunched pos- ture and weight loss (Fig. 3A), diarrhea, rectal prolapse and anal bleeding. Colitis severity eventually resulted in death of the animal (6.6%) and an increase of mortal- ity especially in animals fed with Nissle ΔclbA (19%) that led us to abort prematurely the experiment at day 45 (Fig. 3B). Body weight loss concomitant with an earlier onset of colitis were also observed in trans- ferred SCID mice fed with both Nissle WT or Nissle ΔclbA (Fig. 3A), but only the treatment with the non- genotoxic Nissle ΔclbA leads to an increased mortality in comparison with mice fed with Nissle WT (5.3%; Fig. 3B). A maximal colitis score of 8 was attributed to mice died from colitis (Fig. 4A) but no further analysis was possible for those animals, leading to underestima- tion of inflammation assessment particularly for surviv- ing mice fed with the isogenic ΔclbA mutant strain of E. coli Nissle 1917. E. coli Nissle 1917 requires a functional pks island for attenuation of CD4+ CD45RBhigh T cell-induced colitis. Among the CD4+ CD45RBhigh T cells trans- ferred SCID mice that survived, those fed with PBS developed inflammation involving the entire colon and resulted in diarrhea, bloody stool and macroscopic damage such as colon wall thickening and colon length shortening (Fig. 4A–C). Their colon weight/length ratio was significantly increased as a consequence of intestinal inflammation in comparison with control healthy mice (Fig. 4C). Microscopically, colonic sec- tions from PBS-treated mice presented prominent epithelial hyperplasia, submucosal edema and massive infiltration in the lamina propria, mainly with mono- nuclear cells (Fig. 4D). In some diseased mice, crypt abscesses and mucosal ulceration were also observed. All these macroscopic and microscopic changes were accompanied with an increase expression of pro- inflammatory cytokines such as IL-1β and an increase of anti-inflammatory cytokine IL-10 (Fig. 4E). In contrast, oral administration of Nissle WT improved clinical signs of colitis www.landesbioscience.com Gut Microbes 503 Figure 2. E. coli Nissle 1917 requires a functional pks island for prevention of Dss - induced colitis. Rats were fed with Nissle WT or Nissle ΔclbA (109 c FU/day) or with pBs , 7 d before and over the course of Dss treatment (7 d, 5% in drinking water). error bars represent the seM in all cases and data are representative of one experi- ment performed in 10 rats per group. (A–B) Body weight gain [% of initial (A)] and disease severity (individual values and median, B) were evaluated. *p < 0.05, Nissle WT-treated rats vs. Nissle ΔclbA-treated rats, two-factor RM a NOVa analysis, Bonferroni’s multiple comparison test (A), *p < 0.05, Kruskal-Wallis test, Dunn’s multiple comparison test (B). (C–D) Duplicate colonic sections were homogenized for MpO activity determination (C) and cytokine production measurement (D). MpO activity (mean) and tissue concentrations (mean, normalized to total protein) of pro-inflammatory and regulatory cytokines IL-1β and IL-10 were determined. *p < 0.05, one-factor a NOVa analysis, Bonferroni’s multiple comparison test. and decreased colonic damage (Fig. 4A and B). Furthermore, Nissle WT treatment improved the shortening and thickening of