Polyphenols, provided by the consumption of fruits and vegetables, have a positive effect on human health due to their antioxidant properties and their ability to modulate the redox signaling pathways. These compounds are biotransformed by the gut microbiota into low molecular weight metabolites with higher bioavaibility in systemic than the parent polyphenol, which may lead to a higher bioactivity [1]. Thus, we have studied the biotransformation of 5-O-caffeoylquinic acid (5-CQA), a dietary polyphenol by Lactobacillus reuteri, Bacteroides fragilis and Bifidobacterium longum, belonging to the dominant bacterial phyla of the gut microbiota. The biotransformed extracts were analysed by LC-MS/MS and the data were subject to MzMine 2.53 and GNPS treatment for subsequent molecular networking analysis. L. reuteri was only able to bioconverse 5-CQA into several metabolites, including caffeic acid (CA) and a known natural compound esculetin [2]. This study put in the spotlight, for the first time, an interesting oxidative pathway carried out by gut bacteria. Considering that 5-CQA reach the colon at μM concentration and its microbial metabolites such as CA and esculetin could be close to the gut epithelium when produced, we have investigated whether these compounds could have a direct impact on epithelial barrier integrity using a quadruple in vitro model [3]. Different measurements were evaluated in presence of the tested polyphenols: cell viability, transepithelial electrical resistance (TEER) and gene expression such as occludin and ZO-1. Esculetin exhibited finally a beneficial effect on this integrity compared to the parent polyphenol (5-CQA) and one of the other biotransformed metabolites (CA). [1] Zhang H. & Tsao R., Curr. Opin. Food Sci., 8, 33-42, 2016. [2] Balaj G., Tamanai-Shacoori Z., Olivier-Jimenez D., Sauvager A., Faustin M., Bousarghin L., David Le Gall S., Guyot S., Nebija D., Tomasi S. & Abasq M.L., Food Funct., 13, 6195-6204, 2022. [3] Gautier T., Fahet N., Tamanai-Shacoori Z., Oliviero N., Blot M., Sauvager A., Burel A., David-Le Gall S., Tomasi S., Blat S. & Bousarghin L., Microorganisms., 10, 2263, https://doi.org/10.3390/microorganisms10112263, 2022.