Outer membrane vesicles secreted by Bacteroides fragilis inhibit CFTR chloride secretion by human colon organoids

The goals of this study were to develop a model to study host-pathogen interactions in primary human colon organoids and to test the hypothesis that Bacteroides fragilis toxin (BFT-2) secreted in outer membrane vesicles (OMVs) modulates mucosal immunity and CFTR Cl⁻ secretion. Since Bacteroides species reside in mucus, OMVs are likely to represent a mechanism of communication between Bacteroides and the host. Two strains of Bacteroides were studied: enterotoxigenic Bacteroides fragilis (ETBF), which produces BFT-2, and the non-toxigenic Bacteroides fragilis strain NCTC 9343 (NTBF), which does not produce BFT-2. We also utilized two additional strains of Bacteroides fragilis: one in which bft-2 was knocked out (ETBF Δbft) and one that was engineered to contain bft-2 (NTBF+bft). We report that Bacteroides fragilis OMVs reduced CFTR Cl⁻ secretion, but had no effect on tight junction or cell adhesion proteins, transepithelial resistance (TER), or cytokine secretion by primary human colon organoids. NTBF OMVs containing BFT-2 were more effective in reducing CFTR Cl^- secretion than NTBF lacking BFT-2. By contrast, deletion of bft in ETBF did not have a significant effect on CFTR Cl⁻ secretion compared to ETBF. We conclude that OMVs secreted by Bacteroides can be an important mechanism of host-pathogen interactions in the colon by reducing CFTR Cl⁻ secretion, and that the effect of BFT-2 on CFTR Cl⁻ secretion is dependent on the strain of Bacteroides fragilis.