Beechwood xylan-derived dietary fibers: the production, structural properties, and mechanisms to modulate human gut microbiota

• Main Author: Zhao, Sainan
• Other Authors: Raymond Lau Wai Man
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Agricultural Sciences; Engineering; Medicine, Health and Life Sciences
• Online Access: https://hdl.handle.net/10356/173835
• Institution: Nanyang Technological University

The intricate interplay between dietary fiber, gut microbiota, and human health has prompted significant interest among researchers. Xylan, a major component of hemicelluloses present in plant cell walls, has great potential in influencing gut microbiota composition and function. However, the lack of systematic information about the influence of structural variations of xylan-derived fibers on human gut microbiota poses obstacles to developing prebiotic products with precision function. Therefore, the overall objective of this thesis research was to determine the digestive outcome after consuming xylan-derived fibers with variant structural properties through examining changes in human gut microbial communities and the production of fiber-derived metabolites. In the first investigation, we employed five autohydrolysis conditions to partially hydrolyze beechwood xylan (BWX), resulting in five BWX-derived fiber samples (BWX0, BWX5, BWX10, BWX20, and BWX30). Structural characterization showed that the reduction in the molecular size and substitutions of BWX was dependent on the severity of autohydrolysis. Subsequent in vitro human gut fecal fermentation revealed that different BWX hydrolyzates induced distinct alterations in microbial community structures. Specifically, BWX0, characterized by high molecular weights, selectively enriched the genus Bacteroides, whereas BWX5 and BWX10 with medium molecular weights favored the growth of the genus Fusicatenibacter. In contrast, BWX20 and BWX30 with low molecular weights showed specific enrichment of the genera Bifidobacterium and Megasphaera. Furthermore, the production of propionate and butyrate exhibited notable substrate preferences, with BWX0 leading to higher propionate production and BWX30 resulting in higher butyrate production. This study provides insights into the connections among autohydrolysis conditions, structural properties of xylan-derived fibers, and colonic fermentation properties (Chapter 3). To explore whether variations in fine structural differences of xylan-derived fibers could alter the fate of colonic fermentation, five BWX fractions (BWXFs) with degree of polymerization (DP) ranges of >40, 20–40, 10–20, 5–10, and 2–4 were prepared and tested on human gut microbiota. Extracellular xylo-oligosaccharides (XOS) degradation was observed for molecules with a DP exceeding 5. BWXF treatments altered the microbial community structures, and substrate size-dependent effects on the microbial composition and metabolic outputs were observed. Bacteroidaceae were specifically enriched by xylan. Lachnospiraceae were particularly stimulated by XOS with a DP of 20–40 and 2–4. Bifidobacteriaceae were notably enriched by XOS with a DP of 5–20. High butyrate yields were obtained from cultures containing long-chain BWXFs. Microbiota responses differed with xylan DP composition changes, and microbial competition with XOS with a DP of 2–4 requires further exploration. These findings highlighted the impact of minor variations in xylan chain length on the colonic microbiota response during short-term fermentation trials (Chapter 4). To delve deeper into the longitudinal succession dynamics of gut microbial communities influenced by xylan fibers with varying chain lengths, we conducted a sequential batch fermentation. We observed that variations in the chain length of BWX substrates influenced succession dynamics during dilution perturbations, leading to distinct microbial community structures at the end of the intervention. Cultures treated by BWX (the polysaccharide control) maintained a relatively stable community structure as revealed by indices of alpha and beta diversities. In contrast, cultures treated with BWX substrates (F3 and F4) with shorter chain lengths resulted in larger deviations from the initial microbial community. As for microbial taxonomy, the consortia derived from BWX treatments maintained a high abundance of Firmicutes, and the consortia derived from F0, F1, and F2 treatments exhibited balanced distributions of Bacteroidetes, Firmicutes, and Actinobacteria, whereas the consortia derived from F3 and F4 treatments were predominantly Actinobacteria. Microbial interactions illustrated by the pairwise analyses revealed that the BWX substrates with shorter chain lengths led to more competition interactions in contrast to substrates with longer chain lengths. This study highlights the influence of xylan chain length in shaping the gut microbial consortia and the dynamics of community evolving during sequential passages (Chapter 5). Fermenting BWX in the colon leads to the formation of butyrate, which serves as a main microbial metabolite with beneficial implications for human health. In Chapter 3, we observed that the butyrogenic effect of BWX30 positively correlated with the increasing Bifidobacterium population and an unidentified Megasphaera species. We attributed the butyrogenic mechanism to microbial cross-feeding interactions. To validate this hypothesis, we isolated 3 bacterial species (B. longum, B. pseudocatenulatum, and Megasphaera indica) from the cultures treated with BWX30 and conducted a co-culture experiment utilizing genome characterization, cell counting, metabolite analysis, and transcriptome profiling to identify the mechanism of interaction. The results revealed that two Bifidobacterium isolates harbored distinct gene clusters for xylan utilization and exhibited varying substrate preferences. Conversely, M. indica was unable to proliferate on xylose-based substrates but presented significant cell growth when cocultured with bifidobacteria. Coculturing led to lactate depletion and increased butyrate production, as indicated by the upregulation of M. indica genes involved in lactate utilization and butyrate synthesis pathways. The findings revealed a metabolic cross-feeding interaction between Bifidobacterium species and M. indica, wherein Bifidobacterium assimilated xylose-based substrates, and M. indica utilized lactate produced by Bifidobacterium and produced butyrate (Chapter 6). In this thesis research, we conducted a systematic assessment of the effects of structural characteristics on the colonic fermentation responses of xylan fibers. The severity of autohydrolysis could be utilized to manipulate the structures of xylan fibers, thereby influencing their fermentation outcomes. Even minor variations in chain length could induce substrate-specific enrichment of distinct microorganisms and drive distinctive microbial succession dynamics. Furthermore, we have identified a novel butyrate-producing gut commensal and illuminated its cross-feeding mechanism with bifidobacteria in the context of feeding BWX substrates. We believe these findings contribute to understanding how BWX is consumed in the human colon and potentially modulating the gut microbiota through manipulation of dietary fiber structural properties and microbial interactions applicable to achieving precision health goals.