Structure-dependent stimulation of gut bacteria by arabinoxylo-oligosaccharides (AXOS): a review

Abstract

Arabinoxylo-oligosaccharides (AXOS) are non-digestible dietary fibers that potentially confer a health benefit by stimulating beneficial bacteria in the gut. Still, a detailed overview of the diversity of gut bacteria and their specificity to utilize structurally different AXOS has not been provided to date and was aimed for in this study. Moreover, we assessed the genetic information of summarized bacteria, and we extracted genes expected to encode for enzymes that are involved in AXOS hydrolysis (based on the CAZy database). The taxa involved in AXOS fermentation in the gut display a large variety of AXOS-active enzymes in their genome and consequently utilize AXOS to a highly different extent. Clostridia and Bacteroidales are generalists that consume many structurally diverse AXOS, whereas Bifidobacterium are specialists that specifically consume AXOS with a low degree of polymerization. Further complexity is evident from the fact that the exact bacterial species, and in some cases even the bacterial strains (e.g. in Bifidobacterium longum) that are stimulated, highly depend on the specific AXOS molecular structure. Furthermore, certain species in Bifidobacterium and Lactobacillaceae are active as cross-feeders and consume monosaccharides and unbranched short xylo-oligosaccharides released from AXOS. Our review highlights the possibility that (enzymatic) fine-tuning of specific AXOS structures leads to improved precision in targeting growth of specific beneficial bacterial species and strains in the gut.

Keywords: Arabinoxylo-oligosaccharides (AXOS); Bacteroides; Bifidobacterium; Clostridia; Prevotella; Short-chain fatty acids (SCFAs); arabinoxylan; cross-feeding; metabolism; microbiome; prebiotics; xylanase.

Figure 1.

Structural features of arabinoxylan from wheat pericarp cells.^, a) methylated glucuronic acid, [4-O-methyl-α-d-GlcpA-(1→2)]-β-d-Xylp. b) glucuronic acid, [α-d-GlcpA-(1→2)]-β-d-Xylp. c) acetic acid, 3-O-acetyl-β-d-Xylp. d) acetic acid, 2-O-acetyl-β-d-Xylp. e) arabinose, [α-l-Araf-(1→2)]-β-d-Xylp. f) arabinose, [α-l-Araf-(1→2)][α-l-Araf-(1→3)]-β-d-Xylp. g) arabinose, [α-l-Araf-(1→3)]-β-d-Xylp. h) ferulic acid, [5-O-feruloyl-α-l-Araf-(1→3)]-β-d-Xylp.

Figure 2.

Potential hydrolysis sites in a schematic wheat AX structure by GH5, GH8, GH10, and GH11 endoxylanases and ABF^m2,3 and ABF^d3 arabinofuranosidases. The arabinoxylan structure was created with DrawGlycan-SNFG.

Figure 3.

Illustration showing the fermentation of AXOS and associated (simplified) cross-feeding mechanisms in the gut. a) high DP AXOS (blue) are degraded by Bacteroidales and clostridia, and the released hydrolysis products can be utilized by either bifidobacterium or Lactobacillaceae. Low DP AXOS (red) are utilized by Bifidobacterium, Bacteroidales, and clostridia. Cross-feeding clostridia utilize the metabolites acetate and lactate to produce butyrate, and negativicutes utilize metabolites with varying metabolisms. b) the AXOS utilization by B. longum is strain-dependent (may not be limited to this genus and species), as different strains possess different enzymes and specific transporters. Furthermore, some B. longum strains produce additional acetate, formate, and potentially ethanol instead of lactate when fermenting AXOS. Created with BioRender.com.