CLDN1 and ulcerative colitis: In addition, Wang et al., 2021 showed that B. fragilis NTCC 9343 increased the expression of the tight-junction proteins ZO-1, occludin and claudin-1 in the colon against DSS-induced ulcerative colitis, whereas the B. fragilis strain FJSWX11BF played no protective role (Wang et al., 2021).