The role of Bifidobacterium pseudocatenulatum in the degradation and consumption of xylan-derived carbohydrates

Xylans, a family of xylose-based polysaccharides, are dietary fibers resistant to digestion. They therefore reach the large intestine intact, where they are utilized by members of the gut microbiome. They are initially broken down by primary degraders that utilize extracellular xylanases to cleave xylan into smaller oligomers. The resulting xylooligosaccharides (XOS) can either be further hydrolyzed by primary degraders or cross feed secondary consumers, including Bifidobacterium. While several Bifidobacterium species have metabolic systems for XOS, most grow poorly on longer chain XOS and xylan substrates. Here, we isolated and observed that some strains of Bifidobacterium pseudocatenulatum, including B. pseudocatenulatum ED02, displayed growth on longer chain XOS (LXOS) and straight chain xylan, suggesting a primary degrader phenotype that is rare in Bifidobacterium. In-silico analyses revealed only the genomes of these xylan fermenting (Xylan+) strains contained an extracellular GH10 endo-beta-1.4 xylanase, a key component for primary degradation of xylan. The presence of an extracellular xylanase was confirmed by the appearance of xylan hydrolysis products in cell-free supernatants. Extracellular xylanolytic activity was only detected in Xylan+ strains, as indicated by the production of XOS fragments with a degree of polymerization (DP) of 2-6, identified by TLC and HPLC. Additionally, in-vitro fecal fermentations revealed that strains with Xylan+ phenotype can persist with xylan supplementation. These results indicate that Xylan+ B. pseudocatenulatum may have a competitive advantage in the complex environment of the gastrointestinal tract, due to their ability to act as primary degraders of xylan through extracellular enzymatic degradation.