Fabrication of Model Plant Cell Wall Materials to Probe Gut Microbiota Use of Dietary Fiber
thesisposted on 2022-01-31, 15:33 authored by Nuseybe BulutNuseybe Bulut
The cell wall provides a complex and rigid structure to the plant for support, protection from environmental factors, and transport. It is mainly composed of polysaccharides, proteins, and lignin. Arabinoxylan (AX), pectin (P), and cellulose (C) are the main components of cereal cell walls and are particularly concentrated in the bran portion of the grain. Cereal arabinoxylans create networks in plant cell walls in which other cell wall polysaccharides are imbedded forming complex matrices. These networks give an insolubility profile to plant cell wall. A previous study in our lab showed that soluble crosslinked arabinoxylan with relatively high residual ferulic acid from corn bran provided advantageous in vitro human fecal fermentation products and promoted butyrogenic gut bacteria. In the present work, arabinoxylan was isolated from corn bran with a mild sodium hydroxide concentration to keep most of its ferulic acid content. Highly ferulated corn bran arabinoxylan was crosslinked to create an insoluble network to mimic the cereal grain cell wall matrices. Firstly, arabinoxylan film (Cax-F), pectin film (P-F), the film produced by embedding pectin into arabinoxylan networks (CaxP-F), and cellulose embedding arabinoxylan matrices (CaxC-F), and embedding the mixture of cellulose and pectin into arabinoxylan networks (CaxCP-F) were fabricated into simulated plant cell wall materials. Water solubility of films in terms of monosaccharide content was examined and revealed that Cax-F was insoluble, and P-F was partially insoluble, and nanosized pectin and cellulose were partially entrapped inside the crosslinked-arabinoxylan matrices. In a further study, these films were used in an in vitro human fecal fermentation assay to understand how gut microbiota access and utilize the different simulated plant cell walls to highlight the role of each plant cell wall component during colonic fermentation. In vitro fecal samples, obtained from three healthy donors were used to ferment the films (Cax-F, P-F, CaxP-F, CaxC-F, and CaxCP-F) and controls (free form of cell wall components -Cax, P and C). The fabricated films that were compositionally similar to cell walls were fermented more slowly than the free polysaccharides (Cax and P). Besides, CaxP-F produced the highest short chain fatty acids (SCFA) amount among the films after 24 hour in vitro fecal fermentation. Regarding specific SCFA, butyrate molar ratio of all films was significantly higher than the free, soluble Cax and P. 16S rRNA gene sequencing explained the differences of the butyrate proportion derived from specific butyrogenic bacteria. Particularly, some bacteria, especially in a butyrogenic genera from Clostridium cluster XIVa, were increased in arabinoxylan films forms compared to the native free arabinoxylan polysaccharide. However, no changes were observed between P and P-F in terms of both end products (SCFA) and microbiota compositions. Moreover, CaxP-F promoted the butyrogenic bacteria in fecal samples compared with pectin alone, arabinoxylan alone, and the arabinoxylan film. Differences in matrix insolubility of the film, which was high for the covalently linked arabinoxylan films, but low for the non-covalent ionic-linked pectin film, appears to play an important role in targeting Clostridial bacterial groups. Overall, the cell wall-like films were useful to understand which bacteria degrade them related to their physical form and location of the fiber polymers. This study showed how fabricated model plant cell wall films influence specificity and competitiveness of some gut bacteria and suggest that fabricated materials using natural fibers might be used for targeted support of certain gut bacteria and bacterial groups.