Background: Ulcerative colitis (UC) is a prevalent inflammatory disease of the colon, and it associated with lower levels of secondary bile acids and bile salt hydrolase (BSH) activity. Our lab has demonstrated that native bacteria can serve as chassis to functionally manipulate the gut microbiome. Native Escherichia coli engineered to express BSH can perpetually colonize conventionally-raised hosts in non-sterile conditions after a single gavage, retain their function, alter luminal and serum bile acid pool, and affect host physiology without changing the microbiome composition in mice. Others demonstrated that secondary bile acids can restrain T helper 17 (Th17) cells and promote regulatory T cell differentiation, thus potentially affecting inflammation. Our aim was to determine whether microbial bile acid biotransformation with engineered native bacteria ameliorates colitis in mouse models.

Methods: We engineered a murine-isolated native strain of E. coli to express a functional BSH enzyme and assessed its effect on acute and chronic colitis induced by dextran sulfate sodium (DSS) administration (n=5-10 mice/group, 2 independent experiments). Colitis severity was blindly assessed after hematoxylin and eosin staining of colonic tissue by two independent pathologists. Lamina propria T cells were analyzed by flow cytometry. Bioplex was used to quantify cytokines in peripheral blood serum (n=15 mice/group).

Results: Our engineered native E. coli successfully colonized the gastrointestinal tract of mice with colitis after a single gavage and maintained BSH activity throughout the duration of the experiment (>60 days). BSH-expressing bacteria accumulated mainly in the distal colon which is the site of higher colitis activity (LogCFU/cm of tissue= 4.08 vs 5.42, p-value= 0.033). Functional expression of BSH resulted in colitis amelioration and protection from severe disease, in both acute and chronic settings (75% vs 40% of mice, p-value<0.001). Moreover, we found fewer inflammatory CD4 T cell populations, including interleukin-17A (IL-17A), tumor necrosis factor a (TNFa), and interferon g (IFNg) -producing T cells, in the colonic lamina propria of mice receiving BSH bacteria (p-value<0.01). Interestingly, the anti-inflammatory activity of BSH seems to be confined to the gut. Indeed, we did not see modifications to the levels of the inflammatory cytokines in the serum from peripheral blood.

Conclusion: Our findings demonstrate that modulation of bile acids by engineered native bacteria ameliorate colitis in a preclinical model and can serve a strategy for functionally curative treatment of UC. Moreover, these results show the potential contribution of bacterial BA biotransformation to the progression and severity of UC.

Rachel Sawada

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