Flagellin in blood and Bifidobacterium pseudocatenulatum in gut are associated with live birth upon IVF-frozen embryo transfer.

The role of gut microbiota in live birth attainment upon in vitro fertilization (IVF) remains unclear. We recruited 67 women, evaluated bacterial constituents in the serum, and analyzed the gut microbiota composition and functions prior to an IVF-frozen embryo transfer (FET). A higher serum flagellin level, residues from flagellated bacteria, was observed in women without live birth after FET. Twelve species showed significant differences in abundance between with and without live birth groups, of which Roseburia inulinivorans and Bifidobacterium pseudocatenulatum were the most important to predict live birth outcome. R. inulinivorans abundances were higher among women with high flagellin levels. The cystathionine β-synthase activity in B. pseudocatenulatum, which may play roles in gut integrity, was a critical factor in the negative correlation with serum flagellin and MCP1 levels. The presence of bacterial residues in the circulation may elicit systemic inflammation and decrease the chances of attaining live birth after FET.