Investigation of the effects of enzymatically synthesized α-glucans on large intestine microbiota

In recent years, resistant starch and starch-like structures have been of great interest in food, nutrition and health due to their ability to promote butyrate production and beneficial microorganisms in the colon. The ability of starch-like structures to promote butyrate-producing bacteria is mainly attributed to their unique linkage composition, particularly their α-1,4 and α-1,6 glucose linkages. However, the effects of changes in the ratio of α-1,4 and α-1,6 glucose linkages on colonic microbiota are still unknown. The aim of this study was to determine the effects of non-digestible α-glucans synthesized by biocatalytic methods with different ratios of α-1,4 and α-1,6 glycosidic linkages on colonic microbiota and microbial metabolites. In this study, a total of four structurally different α-glucans (with different branching points) synthesized using amylosucrase from Neisseria polysaccharea and glycogen branching enzymes from Rhodothermus obamensis were used. The samples were treated with upper gastrointestinal tract digestion enzymes and subsequently their branching densities and molecular sizes were determined using gas chromatography/mass spectrometry (GC/MS) and high-performance size exclusion chromatography, respectively. The effects of α-glucans on colonic microbiota composition and function were investigated by in vitro fecal fermentation using fecal microbiota from eight different individuals and changes in colonic microbiota composition and microbial metabolites (short-chain fatty acids - SCFAs) were determined by 16S rRNA sequencing technology and GC, respectively. Our results revealed that the branching densities of enzymatically synthesized α-glucans treated with upper gastrointestinal digestive enzymes ranged from 0.10 to 2.80%. SCFA analysis revealed that butyrate formation varied according to their structural features. In general, α-glucans with lower branching ratios resulted in formations of higher microbial butyrate. Interestingly, the SCFA analysis suggests that all α-glucans tested in this study fermented at a slower rate compared to the resistant starch controls. Amplicon sequencing revealed that although there were inter-individual differences in colonic microbiota composition, α-glucans generally promoted the development of Lachnospiraceae and Ruminococcus bromii related OTUs, which are known to be beneficial microbial groups in the colon. In contrast, resistant starch controls (consisting of α-1,4 and α-1,6 glucose linkages) were found to promote the growth of Blautia related OTUs. This suggests that in addition to the type of glycosidic linkage (chemical structure), other structural characteristics (e.g. morphological and physical) also play important roles in determining the ability of dietary fibers to promote specific bacterial groups in the colon. In conclusion, the results obtained in this study reveal that non-digestible α-glucans can promote beneficial microorganisms and microbial metabolites in the colon, but the degree of promotion depends on the structure.