Riboflavin produced in the vagina by lactobacilli as symbiotic factor

The human vaginal microbiome, particularly its symbiosis with lactobacilli, plays a key role in maintaining women’s health. While lactic acid-mediated pathogen exclusion is well known, broader metabolic functions of vaginal lactobacilli remain underexplored. In this study, we analyzed the vaginal microbiome and metabolome of 258 healthy women from the Isala program. Using targeted HILIC-QTOF MS, we detected a high prevalence of most B-vitamins, their precursors, and vitamin A in the vaginal microenvironment. Riboflavin (B2) and biotin (B7) showed strong associations with Lactobacillus crispatus and Limosilactobacillus. Comparative genomics, phenotypic assays, and in vivo metatranscriptomic data (VIRGO2) collectively confirmed riboflavin biosynthesis by these taxa. Using a riboflavin overproducing Lim. reuteri as a functional probe, we show that microbially derived riboflavin and its intermediates are transported across the vaginal epithelium and modulate host redox balance, cytokine production, and activation of mucosal-associated invariant T (MAIT) cells via MR1 receptor induction, revealing a potential immunometabolic interface between host and microbiota.