Influence of the structural complexity of cereal arabinoxylans on human fecal fermentation and their degradation mechanism by gut bacteria .

摘要

Cereal arabinoxylans from different sources have been found to possess a large structural heterogeneity and generate different fermentation profiles. Among them, corn arabinoxylan is a relatively homogeneous polymer group and has an initial slow fermentation property. A high level of complex branches containing terminal xylose and terminal galactose, other than the commonly existing terminal arabinose, have been identified in corn arabinoxylan and correlated to its slow fermentation property. Structural models of corn arabinoxylan relating to fermentation were previously proposed, but without considering possible distribution patterns of the various branches that may play an important role in determining its fermentation property. Therefore, the first objective of this study was to establish a more accurate structural model for corn arabinoxylan to explain its slow fermentation property. A highly organized structural feature of multiple layers was revealed for corn arabinoxylan for the first time, in which the complex branches assemble to form regions that are connected by simply-branched parts, and the complex-branched regions containing further sub-layers. The structural subunits containing high levels of complex branches were found to act as the functional parts of corn arabinoxylan with different slow fermentation properties. Based on the new structural model, a range of corn arabinoxylan-based fiber molecules (14 fractions) were produced by different enzymatic treatments and were fermented using human fecal microbiota. Differences in fermentation rates revealed a sensitive response of gut microbiota to subtly different structural features within one fiber polymer. To investigate the involved mechanisms of digestion, an idealized experimental model was designed using Bacteroides pure strains. Specific molecular regions of dietary fibers were found to differentiate xylanolytic Bacteroides growth and influence their competition patterns. While a most complex corn arabinoxylan structural region made one strain of B. cellulosylitcus (DSM 14830) outcompete a strain of B. ovatus (3-1-23), a more generalized lightly branched structure favored the latter strain. It is speculated that each bacteria type in the colon may have a substrate structure specific for itself, whereby it can be utilized to specifically favor its growth in the competitive environment of the colon. Moreover, different degradation mechanisms were found within one Bacteroides strain using four substrates by monitoring the targeted gene expression profiles and structural changes of the remaining substrate and were based on the structural composition of fiber molecules. In conclusion, the slow fermentation property of corn arabinoxylan was caused by molecular regions consisting of high structural complexity. A subtle manipulation of the structural complexity of fiber molecules resulted in a considerable change in human colon fermentation property. Gut bacteria compete for the different regions of fiber molecules and are equipped with different enzyme degrading systems that target unique structural features, which were revealed in different degradation mechanisms for fiber molecules according to their structural compositions. This work furthers our knowledge of substrate specificity for gut bacteria, and suggests the possibility for more specific manipulation of the colon microbiota composition and fermentation property in a designed way.