Differential Metabolism of Exopolysaccharides from Probiotic Lactobacilli by the Human Gut Symbiont Bacteroides thetaiotaomicron

Abstract

bProbiotic microorganisms are ingested as food or supplements and impart positive health benefits to consumers. Previous studies have indicated that probiotics transiently reside in the gastrointestinal tract and, in addition to modulating commensal species diversity, increase the expression of genes for carbohydrate metabolism in resident commensal bacterial species. In this study, it is demonstrated that the human gut commensal species Bacteroides thetaiotaomicron efficiently metabolizes fructan exopolysaccharide (EPS) synthesized by probiotic Lactobacillus reuteri strain 121 while only partially degrading reuteran and isomalto/malto-polysaccharide (IMMP) ␣-glucan EPS polymers. B. thetaiotaomicron metabolized these EPS molecules via the activation of enzymes and transport systems encoded by dedicated polysaccharide utilization loci specific for ␤-fructans and ␣-glucans. Reduced metabolism of reuteran and IMMP ␣-glucan EPS molecules may be due to reduced substrate binding by components of the starch utilization system (sus). This study reveals that microbial EPS substrates activate genes for carbohydrate metabolism in B. thetaiotaomicron and suggests that microbially derived carbohydrates provide a carbohydrate-rich reservoir for B. thetaiotaomicron nutrient acquisition in the gastrointestinal tract.T rillions of bacteria inhabit the gut, imparting symbiotic effects that benefit the overall health and well-being of individuals (1). While the human genome has approximately 30,000 genes, the human microbiome contributes an additional 3 to 9 million gene products (termed the gut metagenome) that contribute to functionalities in human lifestyle (2). One prominent example is the human utilization of carbohydrates. Carbohydrates provide a substantial proportion of the daily energy intake of an individual. However, humans have the ability to metabolize only very few dietary carbohydrate compounds, such as lactose, starch, and sucrose, by enzymes either present in saliva (amylases) or anchored to the epithelial wall of the small intestine with glycosylphosphatidylinositol (GPI) anchors (such as invertases and lactases). All other carbohydrates that traverse the gastrointestinal (GI) tract have the potential to be metabolized by commensal gut bacteria, where it is estimated that gut microbiota provide an additional 30,000 enzymes which facilitate the breakdown of carbohydrates in the gut (3).Approximately 30% of resident human gut microbiota are from the Bacteriodetes phylum. These Gram-negative, obligate anaerobes are well armed with a repertoire of carbohydrate-degrading enzymes for harvesting carbohydrate nutritional resources. One species, Bacteroides thetaiotaomicron, devotes approximately 18% of its genome to carbohydrate foraging (4). Discrete polysaccharide utilization loci (PULs) within its genome are upregulated in response to carbohydrates dependent on the source. Most carbohydrate sources for B. thetaiotaomicron nutrient acquisition originate from dietary sources (such as nondigestible plant polysaccharides)...

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

“…In order to determine which polysaccharide utilization loci (PULs) within the B. thetaiotaomicron genome are induced to be expressed an produced as proteins in response to GOS, we took the culture supernatants after growth and analyzed for proteins and enzymes using mass spectrometry as has been previously described30. Previously it was found that B. thetaiotaomicron consumes hMOS using a distinct set of pathways attributed to mucin-O-glycan utilization15 (Table 2).…”

“…Higher DP GOS compounds are metabolized by an endo-galactanase BtGH53, while small DP2 and branched GOS trisaccharides acted as potential inducers for the expression of mucin glycan degradation PUL-encoded systems in B. thetaiotaomicron . This was indicated by the presence of SusC and SusD homologs in B. thetaiotaomicron culture supernatants which correlated with the bacterium expressing transporters that are involved in the uptake of specific products of glycan degradation1430.…”

The Exiguobacteriumsibiricum 255-15 GtfC Enzyme Represents a Novel Glycoside Hydrolase 70 Subfamily of 4,6-α-Glucanotransferase Enzymes

“…Lactic acid bacteria employ glucansucrase enzymes (e.g., GtfA) to synthesize ␣-glucans with various glycosidic linkages from sucrose or GtfB enzymes to convert starch into IMMPs (13). These bacterial polysaccharides are clearly more difficult to digest by hydrolytic enzymes of environmental competitors (33), thus providing a competitive advantage. Also, the carbohydrate products of the GtfC enzymes from non-lactic-acid bacteria may have such roles, but a function in protecting E. sibiricum and E. antarcticum cells against the freezing temperatures of their habitats cannot be excluded either.…”

Characterization of the starch-acting MaAmyB enzyme from Microbacterium aurum B8.A representing the novel subfamily GH13_42 with an unusual, multi-domain organization

“…Additional samples were taken after 1, 2, 3, 4, 5, and 6 h for soluble substrates, and 0, 24, 48, 72, 96, and 120 h for granular substrates. Samples of 1 μl were analyzed on TLC (Merck, Darmstadt, Germany) and run overnight using a butanol, ethanol, water (5:5:3 v/v/v) running buffer, and stained using 40% sulfuric acid and 5% Orcinol (9,10-dihydrophenanthrene) in methanol5354.…”