Thèse de doctorat en Biologie
Sous la direction de Philippe Pochart.
Soutenue en 2008
à Paris, CNAM .
Mucosa-associated microflora of the intestine and inflammation
The etiology of Crohn’s disease (CD) is still unknown, but the intestinal microbiota seems to play an important role in the onset and recurrence of the disease. The mucosa-associated microflora of the intestine (MAMI) should play a crucial role because of its proximity to the mucosa and system immune cells. In most studies, the MAMI differed between subjects with CD and healthy controls. CD is characterized by chonic and often patchy mucosal lesions. We hypothesized that this could be due to a localized change in the microflora and these local changes might favour inflammatory lesions. Metronidazole (MTZ), an antibiotic decreased the early recurrence of Crohn’s disease lesions after ileal resection and acts as an antioxidant in colonic tissue of healthy rats. MTZ effects on intestinal microflora remain unclear. The first part of this work has investigated local changes in the MAMI that might explain the patchy distribution of the lesions in CD and postoperative recurrence. The second part has analysed MTZ impact on luminal microflora and MAMI of rats. The MAMI of CD patients differed between subjects. A predominance of Bacteroidetes or Proteobacteria phyla or Clostridium coccoides group (Firmicute phylum) was observed in CD patients. In CD, specific patterns of the MAMI in inflamed parts that could explain the patchy distribution of the lesions have not been observed. However, the composition of the MAMI of CD patients was analysed 6 months after surgical resection and we found that a reduction of a major member of Firmicutes, Faecalibacterium prausnitzii, is associated with a higher risk of postoperative recurrence of CD. This bacteria exhibited anti-inflammatory effects on cellular model and in induced colitic animal. This effect was partly due to secreted metabolites able to block NF-κB activation and IL-8 production. In the colonic tissue of healthy rat, the antioxidant effect of MTZ could be partly explained by an increase of the mucus layer thickness and consequently of the distance between bacteria and mucosa thus preventing contact with potentially damaging bacteria. Moreover, the composition of intestinal microbiota (luminal and MAMI) was modified by MTZ treatment and particularly the bifidobacterial population which increased in MTZ-treated rats. There was a dominant Bifidobacterium strain. It has been isolated and, based on partial sequence of 16S rRNA gene, corresponded to the species Bifidobacterium pseudolongum. This strain has been named B. Pseudolongum strain Patronus. The potential ability of bifidobacteria to exert antioxidant properties has been investigated. This is the first evidence for B. Pseudolongum strain Patronus to exert an effect on a biomarker of oxidative damage either in the colon or in the small bowel. B. Pseudolongum strain Patronus and F. Prausnitzii as candidate probiotic agents appears a promising strategy in CD treatment.