Tl1a-overexpression in mice is followed by ileitis and fibrosis.133 When germ-free wild-type and Tl1a-transgenic mice were inoculated intragastrically with stool from specific pathogen–free (SPF) mice and a healthy human donor, the reconstitution with SPF, but not human microbiota, resulted in increased intestinal collagen deposition and fibroblast activation in the Tl1a-transgenic strain.133 Microbiota strains that were associated with increased fibrosis in this mouse model were groups of mucolytic bacteria such as Mucispirillum schaedleri and Ruminococcus. This evidence concerns the gene TNFSF15 and Crohn ileitis.