rfaG-dependent cell envelope biogenesis supports type 1 fimbrial expression and intestinal colonization in adherent-invasive Escherichia coli.

Increased intestinal colonization by adherent-invasive Escherichia coli (AIEC) has been implicated in Crohn's disease, an inflammatory bowel disease characterized by abnormal immune responses and chronic inflammation in the gastrointestinal tract. LPS-Toll-like receptor 4 signalling may contribute to AIEC-associated intestinal inflammation. Here, we show that disruption of rfaG-dependent cell envelope biogenesis is associated with reduced type 1 fimbrial expression, epithelial invasion and intestinal colonization in the AIEC strain LF82. Deletion of rfaG, a gene involved in cell envelope biogenesis, resulted in reduced type 1 fimbrial expression and activity, accompanied by reduced transcription of fim genes. The rfaG mutant also exhibited impaired invasion of epithelial cells compared with the parental LF82 strain. Finally, the rfaG mutant showed reduced competitive colonization in the murine intestinal tract, whereas chromosomal complementation with rfaG restored this colonization defect. Inducible expression of type 1 fimbriae also partially reversed the reduced colonization at the early stage of infection, raising the possibility that additional rfaG-dependent mechanisms, independently of type 1 fimbrial expression, contribute to intestinal colonization. These findings suggest that rfaG-dependent cell envelope biogenesis contributes to the coordinated expression of surface-associated virulence factors associated with intestinal colonization in AIEC.