Dietary Fiber Utilization in the Gut: The Role of Human Gut Microbes in the Degradation and Consumption of Xylose-Based Carbohydrates

Xylans are a family of xylose-based polysaccharides naturally present in fruits, vegetables, and cereal grains. Resistant to digestion by host enzymes, xylans reach the large intestine intact, where they are utilized by members of the gut microbiome. They are initially hydrolyzed by primary degraders that utilize extracellular xylanases to cleave xylan into smaller oligomers. These xylooligosaccharides (XOS) can then either be further hydrolyzed by primary degraders or can cross-feed secondary consumers, including Bifidobacterium. While several Bifidobacterium species have metabolic systems for XOS, studies have shown these species grow poorly on longer XOS and xylan substrates. In this thesis, we assessed the ability of Bifidobacterium pseudocatenulatum strains to grow on XOS and xylan. Two distinct phenotypes were observed: a xylan+ phenotype, where strains displayed growth on xylan and XOS regardless of chain length, and a xylan- phenotype, where strains showed no growth on xylan and a preference for shorter XOS fractions. In-silico analysis revealed XOS-active gene clusters, with the presence or absence of key genes correlating to observed phenotypes. Importantly, only xylan+ strains contained an extracellular GH10 endo-1,4-beta-xylanase, a key component to primary degradation of xylan. The presence of an extracellular xylanase was confirmed by detection of xylan hydrolysates via TLC and HPLC analysis. Furthermore, xylan- strains showed improved growth on xylan when combined with a xylan+ strain supernatant, indicating crossfeeding capabilities. These results indicate some strains of Bifidobacterium pseudocatenulatum act as primary degraders of xylan through extracellular enzymatic degradation, a novel trait within the genus Bifidobacterium. Advisor: Robert W. Hutkin