Flavonoid-modifying capabilities of the gut microbiome – an in silico study

Abstract Flavonoids are a major group of dietary plant polyphenols and have a positive health impact, but their modification and degradation in the human gut is still widely unknown. Due to the rise of human gut metagenome data and the assembly of hundreds of thousands of bacterial metagenome-assembled genomes (MAGs), large-scale screening for potential flavonoid-modifying enzymes is now feasible. With sequences from characterized flavonoid-transforming enzymes as queries, the Unified Human Gastrointestinal Protein catalog was analyzed and quantification of putative flavonoid-modifying enzymes was carried out. The results revealed that flavonoid-modifying enzymes are often highly abundant in bacteria hitherto not considered as flavonoid-modifying gut bacteria. The enzymes for the physiologically important daidzein to equol conversion, well studied in Slackia isoflavoniconvertens, were encoded only to a low extent in Slackia MAGs, but more abundant in Adlercreutzia equolifaciens and an uncharacterizedEggerthellaceae species. In addition, a high abundance of genes with a similarity of only about 35% in uncultivated Collinsella species suggest a hitherto uncharacterized Daidzein-to-equol potential in these bacteria. Of all potential flavonoid modification steps, O-deglycosylation (including derhamnosylation) was by far the most abundant in this analysis. In contrast, enzymes putatively involved in C-deglycosylation were detected less often in human gut bacteria and mainly found in Agathobacter faecis (formerly Roseburia faecis). Phloretin hydrolase, flavanonol/flavanone-cleaving reductase and flavone reductase (all three most abundant in Flavonifractor plautii) and O-demethylase (Intestinibacter bartlettii) homologs were of intermediate prevalence (several hundreds of MAGs). This first comprehensive insight into the black box of flavonoid modification in the human gut highlights many hitherto overlooked and uncultured bacterial genera and species as key organisms in flavonoid modification by the human gut microbiota. This could lead to a significant contribution to future biochemical-microbiological investigations on gut bacterial flavonoid transformation. In addition, our results are important for individual nutritional recommendations and for biotechnological applications which rely on novel enzymes catalyzing potentially useful flavonoid modification reactions.