Elucidation of Proteus mirabilis as a Key Bacterium in Crohn's Disease Inflammation

Background & aims: Proteus spp., Gram-negative facultative anaerobic bacilli, have recently been associated with Crohn's disease (CD) recurrence after intestinal resection. We investigated the genomic and functional role of Proteus as a gut pathogen in CD.

Methods: Proteus spp. abundance was assessed by ure gene-specific PCR in 54 pairs of faecal samples and 101 intestinal biopsies from CD patients and healthy controls. The adherence, invasion and intra-cellular presence of two distinct isolates of P. mirabilis in epithelial cells were evaluated using immunofluorescence and electron microscopy. Intracellular gene expression profiles and regulated pathways were analyzed by RNA seq and KEGG pathway analysis. Biological functions of two isolates of P. mirabilis were determined by in vitro cell culture, and in vivo using conventional mice and germ-free mice.

Results: Proteus spp. were significantly more prevalent and abundant in faecal samples and colonic tissue of patients with CD than controls. A greater abundance of the genus Fusobacterium and a lesser abundance of the genus Faecalibacterium were seen in CD patients with a high Proteus spp. abundance. All 24 Proteus-monoclones isolated from CD patients belonged to members of P. mirabilis lineages and two isolates, recovered from stool or mucosa, were used in further studies. Mice gavaged with either P. mirabilis strain had more severe colonic inflammation. Co-culture of the isolates with epithelial cell lines showed bacterial adherence, invasion, increased production of pro-inflammatory cytokines IL-18 and IL-1α, and cell necrosis. Both isolates induced key pro-inflammatory pathways including NOD-like receptor signaling, Jak-STAT signaling and MAPK signaling, and induced pro-inflammatory genes and activated inflammation-related pathways in gnotobiotic mice.

Conclusion: P. mirabilis in the gut is associated with CD and can induce inflammation in cells and animal models of colitis. P. mirabilis may act as a pathobiont and play a crucial role in the pathogenesis of CD.

Keywords: Crohn’s disease; Inflammation; P. mirabilis; Pathogen.