Colonic mucus attenuates Bacteroides fragilis virulence in vitro

OBJECTIVES: Enterotoxigenic Bacteroides fragilis (ETBF) is a subspecies of B. fragilis that is considered to contribute to colorectal carcinogenesis through the activity of the B. fragilis toxin (BFT). However, colonic carriage of ETBF is also evident in a subset of asymptomatic individuals. In vitro studies using the HT29 cell line have linked BFT to the disruption of cell-cell adhesion and inflammation. This cell line, however, fails to consider the role of the mucus barrier in protecting the colonic epithelium in healthy individuals. This study aimed to investigate the role of a cell-associated mucus layer in modulating host responses to ETBF. METHODS: Mucus-producing HT29 MTX-E12 cells and non-mucus-producing HT29 cells were co-cultured with ETBF to evaluate the impact of the mucus layer on bacterial-host interactions. This was assessed by quantifying changes in mucus depth, mucin protein levels, E-cadherin expression, IL-8 secretion, and bft transcription. RESULTS: Both ETBF and nontoxigenic B. fragilis (NTBF) demonstrated the ability to infiltrate the mucus layer and were associated with decreased levels of the MUC2 mucin protein. The presence of a mucus layer, however, prevented the loss of E-cadherin and reduced IL-8 secretion, markers of cell-cell adhesion and inflammation, respectively. Downregulation of bft transcription following co-culture with mucus-producing cells directly linked the mucus layer to attenuated toxin production. CONCLUSIONS: These findings demonstrate that the colonic mucus layer functions not only as a physical barrier but also as an active regulator of bacterial virulence, potentially contributing to the asymptomatic carriage of ETBF in healthy individuals.