Fermentation of Alginate and Its Oligosaccharides by the Human Gut Microbiota: Structure–Property Relationships and New Findings Focusing on Bacteroides xylanisolvens

<b>Background/Objectives:</b> Alginate and its oligosaccharides (AOS) are widely used in the food industry all over the world. However, how they are fermented by the human gut microbiota has not been fully elucidated. Here, we aim to explore the structure-property relationships of the fermentation of these carbohydrates by the human gut microbiota. <b>Methods:</b> High-performance liquid chromatography, 16S rRNA gene amplicon high-throughput sequencing, whole genome sequencing, and metabolome analysis were used to study the fermentation of alginate and AOS by the human gut microbiota. <b>Results and Conclusions:</b> Low-molecular-weight alginate and AOS were more fermentable than alginate. Moreover, fermentation of AOS with a molecular weight (Mw) of 0.8 kDa produced higher amounts of acetate and butyrate than that with a Mw of 0.3 kDa. <i>B. xylanisolvens</i> was a keystone species responsible for the fermentation. Additionally, each <i>B. xylanisolvens</i> strain was characterized with a unique capability for AOS fermentation. Specifically, <i>B. xylanisolvens</i> P19-10, a bacterium isolated from healthy human colon, exhibited the best fermentation capacity. Genomic analysis suggested that <i>B. xylanisolvens</i> P19-10 was armed with a plethora of carbohydrate-active enzymes. Additionally, the polysaccharide lyase family 6_1 was identified as a candidate enzyme responsible for the utilization of AOS. Moreover, fermentation of AOS by <i>B. xylanisolvens</i> P19-10 was associated with significant changes in bacterial metabolites and metabolic pathways. <b>Future perspectives:</b> Our study provides novel mechanistic insights into the fermentation of alginate and AOS by human gut microbiota, which has applications for the development of new carbohydrate-based nutraceuticals and foods.