The effect of particle size on wheat bran fermentation by human gut microbiota

0. ABSTRACT Dietary fibers within whole grains reach the large intestine where they shape the microbial composition. However, the bioavailability of these dietary nutrients to the microbiota is likely limited due to entrapment within the grain particle and requires liberation by microbial enzymes. Here, we used batch fecal fermentation from mixed donors on a range of sizes of wheat particles generated by cyclone milling from a single source to identify bacterial taxa and genomic signatures that are responsive to differences in wheat bran fine structures. We present evidence that different taxa within the same genus colonize wheat bran particles of different sizes. Further, neutral sugar content varied across wheat bran particles despite originating from the same batch, suggesting different polysaccharide structures and nutritional niches. In line with the taxonomic and compositional differences, specific short chain fatty acids varied across particle sizes; in fine wheat bran particle fermentations propionate was high and butyrate low. To identify relevant genomic features implicated in bran colonization, we took a metagenomic approach. From this, we linked genes associated with polysaccharide fermentation to wheat bran particles independent of size, however, within one well-distributed taxon, Lachnospiraceae, genes related to motility were linked to large and medium wheat bran particles. Overall, these results suggest that differences in fine structures and resource availability, as generated through milling, can drive compositional changes in the gut microbiota in an organism-specific manner, mediated through its genomic capacity. IMPORTANCE Cereal brans comprise a large fraction of the dietary fiber consumption. Although it is well-known that dietary fibers influence the metabolic output and taxonomic composition of the gut microbiota, relatively little is known regarding whether the fine structures and resource availability of milled whole grains exert any influence on the microbial makeup. Our data suggest that the sugar content varies across bran milled from a single source to different sizes. These differences in composition may result in colonization differences by related, but unique, taxa, mediated by genes related to polysaccharide fermentation, thus leading to differences in metabolic output. Furthermore, our data suggest that genes related to motility might influence the capacity of microorganisms to colonize particles. Taken together, our data suggest that physical context can influence gut microbiota composition in turn impacting metabolic output. Footnotes Author Note: We have no known conflict of interest to disclose. This research has complied with all institutional and federal policies regarding the use of human subjects.