Department of Food Science and Technology, Food Innovation Center, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
Nebraska Food for Health Center, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
Appl Environ Microbiol. 2022 Oct 26;88(20):e0129922. doi: 10.1128/aem.01299-22. Epub 2022 Oct 6.
Xylans, a family of xylose-based polysaccharides, are dietary fibers resistant to digestion. They therefore reach the large intestine intact; there, they are utilized by members of the gut microbiota. They are initially broken down by primary degraders that utilize extracellular xylanases to cleave xylan into smaller oligomers. The resulting xylooligosaccharides (XOS) can either be further metabolized directly by primary degraders or cross-feed secondary consumers, including . While several species have metabolic systems for XOS, most grow poorly on longer-chain XOS and xylan substrates. In this study, we isolated strains of Bifidobacterium pseudocatenulatum and observed that some, including B. pseudocatenulatum ED02, displayed growth on XOS with a high degree of polymerization (DP) and straight-chain xylan, suggesting a primary degrader phenotype that is rare in . analyses revealed that only the genomes of these xylan-fermenting (xylan) strains contained an extracellular GH10 endo-β-1.4 xylanase, a key enzyme for primary degradation of xylan. The presence of an extracellular xylanase was confirmed by the appearance of xylan hydrolysis products in cell-free supernatants. Extracellular xylanolytic activity was only detected in xylan strains, as indicated by the production of XOS fragments with a DP of 2 to 6, identified by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). Additionally, fecal fermentations revealed that strains with a xylan phenotype can persist with xylan supplementation. These results indicate that xylan B. pseudocatenulatum strains may have a competitive advantage in the complex environment of the gastrointestinal tract, due to their ability to act as primary degraders of xylan through extracellular enzymatic degradation. The beneficial health effects of dietary fiber are now well established. Moreover, low fiber consumption is associated with increased risks of metabolic and systemic diseases. This so-called "fiber gap" also has a profound impact on the composition of the gut microbiome, leading to a disrupted or dysbiotic microbiota. Therefore, understanding the mechanisms by which keystone bacterial species in the gut utilize xylans and other dietary fibers may provide a basis for developing strategies to restore gut microbiome function. The results described here provide biochemical and genetic evidence for primary xylan utilization by human-derived Bifidobacterium pseudocatenulatum and show also that cooperative utilization of xylans occurs among other members of this species.
木聚糖是一种基于木糖的多糖类膳食纤维，具有抗消化特性。因此，它们可以完整地到达大肠；在那里，它们被肠道微生物群的成员利用。它们首先被主要降解者分解，这些降解者利用细胞外木聚糖酶将木聚糖分解成较小的低聚物。得到的木寡糖（XOS）可以被主要降解者进一步直接代谢，也可以交叉喂养包括 在内的二级消费者。虽然有几种 物种具有 XOS 的代谢系统，但大多数在较长链的 XOS 和木聚糖底物上生长不良。在这项研究中，我们分离了双歧杆菌假链状亚种的菌株，并观察到其中一些，包括双歧杆菌假链状亚种 ED02，在具有高聚合度（DP）和直链木聚糖的 XOS 上生长，这表明其具有在 中很少见的主要降解者表型。分析表明，只有这些能发酵木聚糖（xylan）的菌株的基因组中含有细胞外 GH10 内切-β-1.4 木聚糖酶，这是木聚糖主要降解的关键酶。细胞外木聚糖酶的存在通过细胞外上清液中木聚糖水解产物的出现得到证实。细胞外木聚糖酶活性仅在木聚糖菌株中检测到，这表明通过薄层色谱（TLC）和高效液相色谱（HPLC）鉴定出的 DP 为 2 至 6 的 XOS 片段的产生表明了这一点。此外，粪便发酵表明具有木聚糖表型的菌株可以通过木聚糖补充剂持续存在。这些结果表明，由于其通过细胞外酶降解来充当木聚糖的主要降解者的能力，双歧杆菌假链状亚种菌株可能在胃肠道的复杂环境中具有竞争优势。膳食纤维的有益健康效果现在已得到充分证实。此外，低纤维消耗与代谢和系统性疾病风险增加有关。这种所谓的“纤维缺口”也对肠道微生物组的组成产生深远影响，导致微生物失调或失调。因此，了解肠道中关键细菌物种利用木聚糖和其他膳食纤维的机制可能为开发恢复肠道微生物组功能的策略提供依据。这里描述的结果提供了双歧杆菌假链状亚种的生化和遗传证据，表明其能够进行木聚糖的初级利用，并且还表明其他双歧杆菌假链状亚种的成员之间存在木聚糖的协同利用。
