The human gastrointestinal (GI) tract contains communities of microbes (bacteria, fungi, viruses) that vary by anatomic location and impact human health. Microbial communities differ in composition ...based on age, diet, and location in the gastrointestinal tract. Differences in microbial composition have been associated with chronic disease states. In terms of function, microbial metabolites provide key signals that help maintain healthy human physiology. Alterations of the healthy gastrointestinal microbiome have been linked to the development of various disease states including inflammatory bowel disease, diabetes, and colorectal cancer. While the definition of a healthy GI microbiome cannot be precisely identified, features of a healthy gut microbiome include relatively greater biodiversity and relative abundances of specific phyla and genera. Microbes with desirable functional profiles for the human host have been identified, in addition to specific metabolic features of the microbiome. This article reviews the composition and function of the healthy human GI microbiome, including the relative abundances of different bacterial taxa and the specific metabolic pathways and classes of microbial metabolites contributing to human health and disease prevention.
Much remains unknown about how the intestinal microbiome interfaces with the protective intestinal mucus layer.
species colonize the intestinal mucus layer and can modulate mucus production by goblet ...cells. However, select
strains can also degrade protective glycans on mucin proteins. We hypothesized that the human-derived species
would increase intestinal mucus synthesis and expulsion, without extensive degradation of mucin glycans.
data revealed that
lacked the enzymes necessary to extensively degrade mucin glycans. This finding was confirmed by demonstrating that
could not use naive mucin glycans as primary carbon sources
To examine
mucus modulation
, Swiss Webster germfree mice were monoassociated with live or heat-killed
Live
-monoassociated mice exhibited increased colonic expression of goblet cell markers
(
),
(
),
-β,
, and several glycosyltransferases compared to both heat-killed
and germfree counterparts. Likewise, live
-monoassociated colon had increased acidic mucin-filled goblet cells, as denoted by Periodic Acid-Schiff-Alcian Blue (PAS-AB) staining and MUC2 immunostaining.
,
-secreted products, including acetate, were able to increase MUC2 levels in T84 cells. We also identified that
-secreted products, such as γ-aminobutyric acid (GABA), stimulated autophagy-mediated calcium signaling and MUC2 release. This work illustrates that
is capable of enhancing the intestinal mucus layer and goblet cell function via upregulation of gene expression and autophagy signaling pathways, with a net increase in mucin production.
Microbe-host interactions in the intestine occur along the mucus-covered epithelium. In the gastrointestinal tract, mucus is composed of glycan-covered proteins, or mucins, which are secreted by goblet cells to form a protective gel-like structure above the epithelium. Low levels of mucin or alterations in mucin glycans are associated with inflammation and colitis in mice and humans. Although current literature links microbes to the modulation of goblet cells and mucins, the molecular pathways involved are not yet fully understood. Using a combination of gnotobiotic mice and mucus-secreting cell lines, we have identified a human-derived microbe,
, which adheres to intestinal mucus and secretes metabolites that upregulate the major mucin MUC2 and modulate goblet cell function. Unlike other
species,
does not extensively degrade mucin glycans and cannot grow on mucin alone. This work points to the potential of using
and similar mucin-friendly microbes as therapeutic agents for intestinal disorders with disruptions in the mucus barrier.
Endoplasmic reticulum (ER) stress compromises the secretion of MUC2 from goblet cells and has been linked with inflammatory bowel disease (IBD). Although Bifidobacterium can beneficially modulate ...mucin production, little work has been done investigating the effects of Bifidobacterium on goblet cell ER stress. We hypothesized that secreted factors from Bifidobacterium dentium downregulate ER stress genes and modulates the unfolded protein response (UPR) to promote MUC2 secretion. We identified by mass spectrometry that B. dentium secretes the antioxidant γ-glutamylcysteine, which we speculate dampens ER stress-mediated ROS and minimizes ER stress phenotypes. B. dentium cell-free supernatant and γ-glutamylcysteine were taken up by human colonic T84 cells, increased glutathione levels, and reduced ROS generated by the ER-stressors thapsigargin and tunicamycin. Moreover, B. dentium supernatant and γ-glutamylcysteine were able to suppress NF-kB activation and IL-8 secretion. We found that B. dentium supernatant, γ-glutamylcysteine, and the positive control IL-10 attenuated the induction of UPR genes GRP78, CHOP, and sXBP1. To examine ER stress in vivo, we first examined mono-association of B. dentium in germ-free mice which increased MUC2 and IL-10 levels compared to germ-free controls. However, no changes were observed in ER stress-related genes, indicating that B. dentium can promote mucus secretion without inducing ER stress. In a TNBS-mediated ER stress model, we observed increased levels of UPR genes and pro-inflammatory cytokines in TNBS treated mice, which were reduced with addition of live B. dentium or γ-glutamylcysteine. We also observed increased colonic and serum levels of IL-10 in B. dentium- and γ-glutamylcysteine-treated mice compared to vehicle control. Immunostaining revealed retention of goblet cells and mucus secretion in both B. dentium- and γ-glutamylcysteine-treated animals. Collectively, these data demonstrate positive modulation of the UPR and MUC2 production by B. dentium-secreted compounds.
Gut microbes can synthesize multiple neuro-active metabolites. We profiled neuro-active compounds produced by the gut commensal Bacteroides ovatus in vitro and in vivo by LC-MS/MS. We found that ...B. ovatus generates acetic acid, propionic acid, isobutyric acid, and isovaleric acid. In vitro, B. ovatus consumed tryptophan and glutamate and synthesized the neuro-active compounds glutamine and GABA. Consistent with our LC-MS/MS-based in vitro data, we observed elevated levels of acetic acid, propionic acid, isobutyric acid, and isovaleric acid in the intestines of B. ovatus mono-associated mice compared with germ-free controls. B. ovatus mono-association also increased the concentrations of intestinal GABA and decreased the concentrations of tryptophan and glutamine compared with germ-free controls. Computational network analysis revealed unique links between SCFAs, neuro-active compounds, and colonization status. These results highlight connections between microbial colonization and intestinal neurotransmitter concentrations, suggesting that B. ovatus selectively influences the presence of intestinal neurotransmitters.
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•Bacteroides ovatus modulates key neuro-active compounds in vivo in gnotobiotic animals
Neuroscience; Microbiology; Microbiome
Background
Lactic acid bacteria are commensal members of the gut microbiota and are postulated to promote host health. Secreted factors and cell surface components from Lactobacillus species have ...been shown to modulate the host immune system. However, the precise role of L. reuteri secreted factors and surface proteins in influencing dendritic cells (DCs) remains uncharacterized.
Hypothesis
We hypothesize that L. reuteri secreted factors will promote DC maturation, skewing cells toward an anti‐inflammatory phenotype. In acute colitis, we speculate that L. reuteri promotes IL‐10 and dampens pro‐inflammatory cytokine production, thereby improving colitis.
Methods & Results
Mouse bone marrow‐derived DCs were differentiated into immature dendritic cells (iDCs) via IL‐4 and GM‐CSF stimulation. iDCs exposed to L. reuteri secreted factors or UV‐irradiated bacteria exhibited greater expression of DC maturation markers CD83 and CD86 by flow cytometry. Additionally, L. reuteri stimulated DCs exhibited phenotypic maturation as denoted by cytokine production, including anti‐inflammatory IL‐10. Using mouse colonic organoids, we found that the microinjection of L. reuteri secreted metabolites and UV‐irradiated bacteria was able to promote IL‐10 production by DCs, indicating potential epithelial‐immune cross‐talk. In a TNBS‐model of acute colitis, L. reuteri administration significantly improved histological scoring, colonic cytokine mRNA, serum cytokines, and bolstered IL‐10 production.
Conclusions
Overall these data demonstrate that both L. reuteri secreted factors and its bacterial components are able to promote DC maturation. This work points to the specific role of L. reuteri in modulating intestinal DCs.
New & Noteworthy
Lactobacillus reuteri colonizes the mammalian gastrointestinal tract and exerts beneficial effects on host health. However, the mechanisms behind these effects have not been fully explored. In this article, we identified that L. reuteri ATTC PTA 6475 metabolites and surface components promote dendritic cell maturation and IL‐10 production. In acute colitis, we also demonstrate that L. reuteri can promote IL‐10 and suppress inflammation. These findings may represent a crucial mechanism for maintaining intestinal immune homeostasis.
L. reuteri secreted factors and its bacterial components are able to promote DC maturation and IL‐10 production. Additionally, L. reuteri is able to elevate IL‐10 and suppress inflammation in a TNBS model of colitis.
Background: Accumulating evidence indicates that the gut microbiota can synthesize neurotransmitters as well as impact host-derived neurotransmitter levels. In the past, it has been challenging to ...decipher which microbes influence neurotransmitters due to the complexity of the gut microbiota. Methods: To address whether a single microbe, Bifidobacterium dentium, could regulate important neurotransmitters, we examined Bifidobacteria genomes and explored neurotransmitter pathways in secreted cell-free supernatant using LC-MS/MS. To determine if B. dentium could impact neurotransmitters in vivo, we mono-associated germ-free mice with B. dentium ATCC 27678 and examined fecal and brain neurotransmitter concentrations. Results: We found that B. dentium possessed the enzymatic machinery to generate γ-aminobutyric acid (GABA) from glutamate, glutamine, and succinate. Consistent with the genome analysis, we found that B. dentium secreted GABA in a fully defined microbial media and elevated fecal GABA in B. dentium mono-associated mice compared to germ-free controls. We also examined the tyrosine/dopamine pathway and found that B. dentium could synthesize tyrosine, but could not generate L-dopa, dopamine, norepinephrine, or epinephrine. In vivo, we found that B. dentium mono-associated mice had elevated levels of tyrosine in the feces and brain. Conclusions: These data indicate that B. dentium can contribute to in vivo neurotransmitter regulation.
The purpose of our experiment was to explore how stochastic (inter‐individual variation) gut microbiome composition may link to inflammatory bowel disease (IBD) susceptibility and guide the ...development of a perinatal preventative probiotic. Dextran sodium sulfate (DSS) was introduced to C57BL/BJ mice to induce acute colitis as a model of IBD. Potentially protective bacteria were identified using a discovery‐validation cohort approach toward stochastic DSS susceptibility. Lactobacilli (two different cocktails of L. reuteri and L. johnsonii strains) or control media were supplemented by mouth to dams prior to delivery and during lactation (i.e., perinatal probiotic). The pups were evaluated for DSS susceptibility at young adulthood. Fecal Lactobacillus was increased in the DSS‐resistant mice in both the discovery and validation cohorts. Maternal supplementation of female offspring with an L. reuteri cocktail (strains 6798‐1, 6798‐jm, and 6798‐cm) induced progressive microbiome separation and protection against colitis by young adulthood. Maternal supplementation of L. reuteri could confer protection against DSS colitis in young adult female mice. This work is the first to exploit stochastic mammalian microbiome variation to guide microbial therapeutic identification. Our findings underscore neonatal microbiome plasticity and set the stage for the potential development of perinatally deliverable protective probiotics against human IBD.
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