Madeline Smoot

Introduction. Patients with Inflammatory Bowel Disease (IBD) experience chronic inflammation of the gastrointestinal tract. During bouts of inflammation a concomitant alteration of the gut microbiota is observed (dysbiosis). Inflammation-associated dysbiosis is frequently characterized by an increase in the relative abundance of facultative anaerobes, commonly of the family Enterobacteriaceae.¹ It has been hypothesized that as part of the host inflammatory response, oxygen², formate², and nitrate become available as electron acceptors or donors in the gut lumen. In combination with inflammatory angiogenesis, respiration of host-derived oxygen, formate, and nitrate would allow facultative anaerobic Enterobacteriaceae to outcompete the obligate anaerobic commensals. Additionally, it was hypothesized that streptomycin treatment contributed to dysbiosis. Methods. To test these hypotheses, Escherichia coli was employed as a model organism and analysis of E. coli mutants defective for utilization of oxygen, nitrate, and formate during respiration was completed. To evaluate nitrate contribution, a mutant moaA deficient E.coli was generated that was unable to synthesize molybdenum cofactor, rendering all of the nitrate reductases inactive.³ To analyze oxygen utilization, a competition assay was conducted within a dextran sodium sulfate (DSS) colitis mouse model.² The mouse was inoculated with wild-type (WT) E. coli and a cydAB oxidase deficient E. coli mutant.² Formate oxidation was tested in a similar manner.² The mice were inoculated with equal amounts of WT E. coli and a fdnG fdoG deficient mutant that was unable to utilize formate.² To evaluate the contribution of angiogenesis to dysbiosis, an experimental group of DSS colitis mice were given Saccharomyces boulardii (Sb) via oral gavage.⁴ To test the role of streptomycin treatment in dysbiosis, DSS colitis mice were treated with streptomycin and E. coli variants.⁵ Results. In cases of mice treated with DSS, WT E. coli was recovered at higher numbers than the moaA, cydAB, and fdnG/fdoG mutants, suggesting that nitrate, formate, and oxygen respiration was a growth advantage for the WT E. coli.^2,3 Sb exposure directly caused a reduction of capillary branching indicating reduced angiogenesis.⁴ In DSS + Streptomycin treated mice, there was an increase in nitric oxide synthase, thus fueling an outgrowth of E. coli by nitrate respiration.⁵ Conclusions. Nitrate respiration, oxygen respiration, formate oxidation, streptomycin treatment, and angiogenesis conferred growth advantages for the WT E. coli in the inflamed intestine. All of the mentioned factors contribute to worsening dysbiosis within the intestinal lumen and further research needs to be conducted in order to offer improved treatment options for patients with IBD.

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2. Hughes, Elizabeth R. et al. Microbial Respiration and Formate Oxidation as Metabolic Signatures of Inflammation-Associated Dysbiosis. Cell Host & Microbe, Volume 21, Issue 2, 208 – 219.
3. Winter SE, Winter MG, Xavier MN, et al. Host-derived nitrate boosts growth of E. coli in the inflamed gut. Science (New York, NY). 2013;339(6120):708-711. doi:10.1126/science.1232467.
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