A pectic polysaccharide isolated from Achyranthes bidentata is metabolized by human gut Bacteroides spp.

Achyranthes bidentata (A. bidentata) is a famous traditional Chinese medicine (TGM) for treatment osteoporosis. Polysaccharides, a major factor for shaping the gut microbiota, are the primary ingredients of A. bidentata. However, bioactivity of A. bidentata polysaccharide on human gut microbiota (HGM) remains unknown. Here, a homogeneous pectic polysaccharide A23-1 with average molecular weight of 93.085 kDa was extracted and purified from A. bidentata. And A23-1 was compsed of rhamnose, glucuronic acid, galacturonic acid, glucose, galactose and arabinose in a molar ratio of 7.26: 0.76: 5.12: 2.54: 23.51: 60.81. GC-MS, partial acid hydrolysis and NMR results indicated the backbone of A23-1 was composed of 1, 2, 4-Rhap and 1, 4-GlapA, while the branches were composed of galactose, arabinose, glucose and glucuronic acid. Further, A23-1 was found to be degraded into monosaccharides and fragments. Taking Bacteroides thetaiotaomicron (BT) as a model, we suggested three polysaccharide utilization loci (PULs) might be involved in the A23-1 degradation. Degraded products generated by BO might not support the growth of probiotics. Besides, acetate and propionate as the main end products were generated by Bacteroides spp. and probiotics utilizing A23-1. These findings suggested A23-1 was possible one of food sources of human gut Bacteroides spp.

Diet-derived galacturonic acid regulates virulence and intestinal colonization in enterohaemorrhagic Escherichia coli and Citrobacter rodentium.

Enteric pathogens sense the complex chemistry within the gastrointestinal tract to efficiently compete with the resident microbiota and establish a colonization niche. Here, we show that enterohaemorrhagic Escherichia coli and Citrobacter rodentium, its surrogate in a mouse infection model, sense galacturonic acid to initiate a multi-layered program towards successful mammalian infection. Galacturonic acid utilization as a carbon source aids the initial pathogen expansion. The main source of galacturonic acid is dietary pectin, which is converted to galacturonic acid by the prominent member of the microbiota, Bacteroides thetaiotamicron. This is regulated by the ExuR transcription factor. However, galacturonic acid is also sensed as a signal through ExuR to modulate the expression of the genes encoding a molecular syringe known as a type III secretion system, leading to infectious colitis and inflammation. Galacturonic acid acts as both a nutrient and a signal directing the exquisite microbiota-pathogen relationships within the gastrointestinal tract. This work highlights that differential dietary sugar availability influences the relationship between the microbiota and enteric pathogens, as well as disease outcomes.

Provitamin D3 modulation through prebiotics supplementation: simulation based assessment.

Vitamin D is important in multiple health conditions. Vitamin D deficiency is prevalent globally even with exposure to adequate sunlight. Reduction in provitamin D3 (7-dehydrocholesterol, 7-DHC) is an important cause of vitamin D3 deficiency. Vitamin supplementation, food fortification, and use of probiotics are some approaches to reduce vitamin D3 deficiency. This study investigates plausibility of 7-DHC biosynthesis through dietary prebiotics supplementation. Furthermore, it reports mechanistic details and constraints for the biosynthesis using flux balance analysis (FBA) simulations. The FBA simulations using co-metabolism models comprising human host and a resident bacterium (Faecalibacterium prausnitzii or Bacteroides thetaiotamicron) indicated increased flux of 7-DHC with short-chain fructooligosaccharide (scFOS) or inulin supplementation. We observed around 2-fold increase in flux compared to the baseline. Biosynthesis of 7-DHC was primarily modulated through acetate, pyruvate and lactate secreted by the bacterium. We observed diverse mechanisms and dose dependent responses. We extended this assessment to 119 resident bacteria and investigated the metabolites profiles with prebiotics supplementation. In summary, the current study suggests the potential use of applying prebiotics in enhancing 7-DHC biosynthesis. Furthermore, performance of the different gut bacteria with prebiotic supplementation for secreted metabolites profile is reported. These results may be useful to design future clinical studies.

Single cell fluorescence imaging of glycan uptake by intestinal bacteria.

Microbes in the intestines of mammals degrade dietary glycans for energy and growth. The pathways required for polysaccharide utilization are functionally diverse; moreover, they are unequally dispersed between bacterial genomes. Hence, assigning metabolic phenotypes to genotypes remains a challenge in microbiome research. Here we demonstrate that glycan uptake in gut bacteria can be visualized with fluorescent glycan conjugates (FGCs) using epifluorescence microscopy. Yeast α-mannan and rhamnogalacturonan-II, two structurally distinct glycans from the cell walls of yeast and plants, respectively, were fluorescently labeled and fed to Bacteroides thetaiotaomicron VPI-5482. Wild-type cells rapidly consumed the FGCs and became fluorescent; whereas, strains that had deleted pathways for glycan degradation and transport were non-fluorescent. Uptake of FGCs, therefore, is direct evidence of genetic function and provides a direct method to assess specific glycan metabolism in intestinal bacteria at the single cell level.

Nonmicrobicidal Small Molecule Inhibition of Polysaccharide Metabolism in Human Gut Microbes: A Potential Therapeutic Avenue.

A new approach for the nonmicrobicidal phenotypic manipulation of prominent gastrointestinal microbes is presented. Low micromolar concentrations of a chemical probe, acarbose, can selectively inhibit the Starch Utilization System and ablate the ability of Bacteroides thetaiotaomicron and B. fragilis strains to metabolize potato starch and pullulan. This strategy has potential therapeutic relevance for the selective modulation of the GI microbiota in a nonmicrobicidal manner.

Complex pectin metabolism by gut bacteria reveals novel catalytic functions.

The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiota. It is unclear, however, whether bacterial consortia or single organisms are required to depolymerize highly complex glycans. Here we show that the gut bacterium Bacteroides thetaiotaomicron uses the most structurally complex glycan known: the plant pectic polysaccharide rhamnogalacturonan-II, cleaving all but 1 of its 21 distinct glycosidic linkages. The deconstruction of rhamnogalacturonan-II side chains and backbone are coordinated to overcome steric constraints, and the degradation involves previously undiscovered enzyme families and catalytic activities. The degradation system informs revision of the current structural model of rhamnogalacturonan-II and highlights how individual gut bacteria orchestrate manifold enzymes to metabolize the most challenging glycan in the human diet.

Prebiotic galactooligosaccharides activate mucin and pectic galactan utilization pathways in the human gut symbiont Bacteroides thetaiotaomicron.

Galactooligosaccharides (GOS) are prebiotic carbohydrates that impart changes in the gut bacterial composition of formula-fed infants to more closely resemble that of breast-fed infants. Consuming human milk oligosaccharides (HMOs) provides specific bacterial strains with an advantage for colonizing the infant intestine. These same effects are seen in infants after GOS consumption, however GOS are very complex mixtures and the underlying molecular mechanisms of how GOS mimic HMOs are relatively unknown. Here we studied the effects of GOS utilization on a prominent gut symbiont, Bacteroides thetaiotaomicron, which has been previously shown to consume HMOs via mucin O-glycan degradation pathways. We show that several pathways for targeting O-mucin glycans are activated in B. thetaiotaomicron by GOS, as well as the galactan utilization sytem. Characterization of the endo-galactanase from this system identified activity on various longer GOS substrates while a subset of GOS compounds were identified as potential activators of mucin glycan metabolism in B. thetaiotaomicron. Our results show that GOS functions as an inducer of mucin-glycan pathways while providing a nutrient source in the form of ß-(1 → 4)-galactan. These metabolic features of GOS mixtures may serve to explain the beneficial effects that are seen for GOS supplemented infant formula.