Discerning the role of Bacteroides fragilis in celiac disease pathogenesis

Celiac disease (CD) is associated with intestinal dysbiosis, which can theoretically lead to dysfunctions in host-microbe interactions and contribute to the disease. In the present study, possible differences in Bacteroides spp. and their pathogenic features between CD patients and controls were investigated. Bacteroides clones (n = 274) were isolated, identified, and screened for the presence of the virulence genes (bft and mpII) coding for metalloproteases. The proteolytic activity of selected Bacteroides fragilis strains was evaluated by zymography and, after gastrointestinal digestion of gliadin, by high-pressure liquid chromatography/electrospray ionization/tandem mass spectrometry. The effects of B. fragilis strains on Caco-2 cell culture permeability and inflammatory response to digested gliadin were determined. B. fragilis was more frequently identified in CD patients than in healthy controls, in contrast to Bacteroides ovatus. B. fragilis clones carrying virulence genes coding for metalloproteases were more abundant in CD patients than in controls. B. fragilis strains, representing the isolated clones and carrying metalloprotease genes, showed gelatinase activity and exerted the strongest adverse effects on the integrity of the Caco-2 cell monolayer. All B. fragilis strains also showed gliadin-hydrolyzing activity, and some of them generated immunogenic peptides that preserved or increased inflammatory cytokine production (tumor necrosis factor alpha) and showed increased ability to permeate through Caco-2 cell cultures. These findings suggest that increased abundance of B. fragilis strains with metalloprotease activities could play a role in CD pathogenesis, although further in vivo studies are required to support this hypothesis.