Biochemical Basis of Xylooligosaccharide Utilisation by Gut Bacteria.

Xylan is one of the major structural components of the plant cell wall. Xylan present in the human diet reaches the large intestine undigested and becomes a substrate to species of the gut microbiota. Here, we characterised the capacity of and strains to utilise xylan derivatives. We showed that ATCC 53608 and ATCC 27340 produced β-D-xylosidases, enabling growth on xylooligosaccharide (XOS). The recombinant enzymes were highly active on artificial (-nitrophenyl β-D-xylopyranoside) and natural (xylobiose, xylotriose, and xylotetraose) substrates, and showed transxylosylation activity and tolerance to xylose inhibition. The enzymes belong to glycoside hydrolase family 120 with Asp as nucleophile and Glu as proton donor, as shown by homology modelling and confirmed by site-directed mutagenesis. In silico analysis revealed that these enzymes were part of a gene cluster in but not in strains, and quantitative proteomics identified other enzymes and transporters involved in XOS utilisation. Based on these findings, we proposed a model for an XOS metabolism pathway in and strains. Together with phylogenetic analyses, the data also revealed the extended xylanolytic potential of the gut microbiota.