The branching ratio of enzymatically synthesized <i>α</i>-glucans impacts microbiome and metabolic outcomes of <i>in vitro </i>fecal fermentation

The aim of this study was to evaluate the impacts of enzymatically synthesized alpha-glucans possessing alpha-1,4- and alpha-1,6-glucose linkages, and varying in branching ratio, on colonic microbiota composition and metabolic function. Four different alpha-glucans varying in branching ratio were synthesized by amylosucrase from Neisseria polysaccharea and glycogen branching enzyme from Rhodothermus obamensis. The branching ratios were found to range from 0 % to 2.8 % using GC/MS. In vitro fecal fermentation analyses (n = 8) revealed that the branching ratio dictates the short-chain fatty acid (SCFA) generation by fecal microbiota. Specifically, slightly branched (0.49 %) alpha-glucan resulted in generation of significantly (P < 0.05) higher amounts of propionate, compared to more-branched counterparts. In addition, the amount of butyrate generated from this alpha-glucan was statistically (P > 0.05) indistinguishable than those observed in resistant starches. 16S rRNA sequencing revealed that enzymatically synthesized alpha-glucans stimulated Lachnospiraceae and Ruminococcus related OTUs. Overall, the results demonstrated metabolic function of colonic microbiota can be manipulated by altering the branching ratio of enzymatically synthesized alpha-glucans, providing insights into specific structure-function relationships between dietary fibers and the colonic microbiome. Furthermore, the slightly branched alpha-glucans could be used as functional carbohydrates to stimulate the beneficial microbiota and SCFAs in the colon.