Modulation of Gut Microbiota Composition and Microbial Phenolic Catabolism of Phenolic Compounds from <i>Achillea millefolium</i> L. and <i>Origanum majorana</i> L.

The impact of the nonbioaccessible fraction of two phenolic-rich extracts from <i>Achillea millefolium</i> L. (yarrow) and <i>Origanum majorana</i> L. (marjoram) on the modulation of the human gut microbiota was investigated <i>in vitro</i>. Microbial metabolism of the phenolic compounds was also addressed. In general, phenolic acids or <i>O</i>-glycosidic flavones quickly disappeared, in contrast to methoxy- or <i>C</i>-glycosidic flavonoids. This colonic metabolism yielded phloroglucinol, 3,4-dimethoxyphenylacetic acid, 3-(4-hydroxyphenyl)-propionic acid, and 4-hydroxybenzoic acid as the main metabolites of the microbial catabolism of rosmarinic acid or caffeoylquinic acids, among others. The 16S rRNA gene sequencing showed that the most promising modulatory effect was related to the increase in <i>Bifidobacterium</i> spp., <i>Collinsella</i> spp., <i>Romboutsia</i>, and <i>Akkermansia muciniphila</i> for both plant extracts, along with <i>Blautia</i> spp. and <i>Dialister</i> for yarrow extract. This beneficial modulation was accompanied by the increase in butyric acid production, highlighting the potential prebiotic-like effect on the gut microbiota of these two previously unstudied edible plants.