Small intestinal microbial dysbiosis underlies symptoms associated with functional gastrointestinal disorders.

Small intestinal bacterial overgrowth (SIBO) has been implicated in symptoms associated with functional gastrointestinal disorders (FGIDs), though mechanisms remain poorly defined and treatment involves non-specific antibiotics. Here we show that SIBO based on duodenal aspirate culture reflects an overgrowth of anaerobes, does not correspond with patient symptoms, and may be a result of dietary preferences. Small intestinal microbial composition, on the other hand, is significantly altered in symptomatic patients and does not correspond with aspirate culture results. In a pilot interventional study we found that switching from a high fiber diet to a low fiber, high simple sugar diet triggered FGID-related symptoms and decreased small intestinal microbial diversity while increasing small intestinal permeability. Our findings demonstrate that characterizing small intestinal microbiomes in patients with gastrointestinal symptoms may allow a more targeted antibacterial or a diet-based approach to treatment.

Gut Microbiota-Produced Tryptamine Activates an Epithelial G-Protein-Coupled Receptor to Increase Colonic Secretion.

Tryptamine, a tryptophan-derived monoamine similar to 5-hydroxytryptamine (5-HT), is produced by gut bacteria and is abundant in human and rodent feces. However, the physiologic effect of tryptamine in the gastrointestinal (GI) tract remains unknown. Here, we show that the biological effects of tryptamine are mediated through the 5-HT4 receptor (5-HT4R), a G-protein-coupled receptor (GPCR) uniquely expressed in the colonic epithelium. Tryptamine increases both ionic flux across the colonic epithelium and fluid secretion in colonoids from germ-free (GF) and humanized (ex-GF colonized with human stool) mice, consistent with increased intestinal secretion. The secretory effect of tryptamine is dependent on 5-HT4R activation and is blocked by 5-HT4R antagonist and absent in 5-HT4R-/- mice. GF mice colonized by Bacteroides thetaiotaomicron engineered to produce tryptamine exhibit accelerated GI transit. Our study demonstrates an aspect of host physiology under control of a bacterial metabolite that can be exploited as a therapeutic modality. VIDEO ABSTRACT.

Clostridioides difficile uses amino acids associated with gut microbial dysbiosis in a subset of patients with diarrhea.

The gut microbiota plays a critical role in pathogen defense. Studies using antibiotic-treated mice reveal mechanisms that increase susceptibility to Clostridioides difficile infection (CDI), but risk factors associated with CDI in humans extend beyond antibiotic use. Here, we studied the dysbiotic gut microbiota of a subset of patients with diarrhea and modeled the gut microbiota of these patients by fecal transplantation into germ-free mice. When challenged with C. difficile, the germ-free mice transplanted with fecal samples from patients with dysbiotic microbial communities showed increased gut amino acid concentrations and greater susceptibility to CDI. A C. difficile mutant that was unable to use proline as an energy source was unable to robustly infect germ-free mice transplanted with a dysbiotic or healthy human gut microbiota. Prophylactic dietary intervention using a low-proline or low-protein diet in germ-free mice colonized by a dysbiotic human gut microbiota resulted in decreased expansion of wild-type C. difficile after challenge, suggesting that amino acid availability might be important for CDI. Furthermore, a prophylactic fecal microbiota transplant in mice with dysbiosis reduced proline availability and protected the mice from CDI. Last, we identified clinical risk factors that could potentially predict gut microbial dysbiosis and thus greater susceptibility to CDI in a retrospective cohort of patients with diarrhea. Identifying at-risk individuals and reducing their susceptibility to CDI through gut microbiota-targeted therapies could be a new approach to preventing C. difficile infection in susceptible patients.

Genetic and environmental influences on plasma vitamin D binding protein concentrations.

Recent studies suggest that low vitamin D-binding protein (VDBP aka group-specific complement or Gc) concentrations may be linked with inflammatory-mediated conditions, including asthma, chronic obstructive pulmonary disease, and cancer. However, these studies may be confounded by substantial racial and ethnic or genetic differences. The purpose of this study was to test the hypothesis that circulating VDBP concentrations are significantly associated with genetic ancestry. We used a validated high-performance liquid chromatography tandem mass spectrometry assay of 25-hydroxyvitamin D3 and its downstream metabolite 24,25-dihydroxyvitamin D3. VDBP concentrations (milligrams per liter) were measured in duplicate using a commercial enzyme-linked immunosorbent assay among healthy African American (n = 56) and Caucasian American (n = 60) participants. Ancestry informative markers across the genome were used to estimate individual genetic ancestry proportions, designed to robustly distinguish between West African and European ancestry. Genotype-defined Gc isoforms were defined using rs7041 and rs4588 combination groups. VDBP concentration was correlated with both Gc isoform (r = 0.93, P < 0.001) and West African genetic ancestry (r = -0.66, P < 0.001). In the final model, Gc isoform, the catabolic ratio of serum vitamin D, oral contraceptive use, and body mass index remained significantly associated with VDBP concentration, after adjustment for genetic ancestry. Failure to adjust for Gc isoform may lead to spurious associations in studies of VDBP concentration and disease risk, particularly when the condition of interest may also be associated with genetic ancestry. The higher circulating VDBP concentrations and higher vitamin D catabolic rate among Caucasian Americans observed here appear to be consistent with lower bone mineral density and racial and ethnic differences in vitamin D-inducing cytokines.