Van Rillaer, T. | bioRxiv Science

bioRxiv2025

Vitamin B2 Production by Vaginal Lactobacilli Promotes Symbiosis

The human vaginal microbiome, particularly with lactobacilli as the main inhabitants, plays a key role in maintaining womens 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 metabolomics analysis, we detected a high prevalence with strong interpersonal differences 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 sp. 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 model, we showed that microbially derived riboflavin and its pathway intermediates are transported across the vaginal epithelium and modulate host redox balance, cytokine production, and activation of mucosal-associated invariant T (MAIT) cells via induction of MR1 (Major histocompatibility complex, class I-related protein receptor), revealing a potential immunometabolic interface between the vaginal microbiota and its host.

microbiology↗

bioRxiv2025

From Diversity to Dominance: How Salt and CO2 Shape LAB-dominated Ecosystems in Vegetable Fermentations

Research on microbial ecosystems is often challenging due to high diversity of microbial taxa present and the complexity of controlling environmental variables. To address these challenges, fermented foods are simpler and more reproducible model ecosystems, where both community composition and environmental factors can be more precisely controlled and manipulated. In this study, we focused on fermented vegetables which are typically dominated by lactic acid bacteria (LAB). It is not completely understood why lactic acid bacteria (LAB) consistently drive the spontaneous fermentation of vegetables such as cabbage and carrots and how variables such as vegetable substrates, salt addition, and carbon dioxide levels can impact microbial community dynamics. Here, we explored the temporal microbial dynamics in standardized fermentations of 11 different vegetables (including beetroot, bell pepper, cabbage, carrot, cucumber, fennel, green asparagus, leek, parsnip, sunroot, and tomato), revealing a consistent dominance of Leuconostoc and other LAB. Additionally, we investigated the impact of varying salt concentrations, demonstrating that lower salt levels resulted in a delayed appearance of the typically dominant LAB community, while simultaneously revealing a higher abundance of Weissella and various Enterobacterales taxa. These effects imposed by reduced salt concentrations were mitigated by CO2 injection, which reverted the enhanced Enterobacterales levels and increased the overall abundance of Lactobacillales. This study demonstrates how targeted manipulation of environmental parameters, such as salinity and gas composition, can be used to uncover ecological principles governing microbial succession and community assembly in reproducible fermentation-based model ecosystems. ImportanceUnderstanding the ecological principles that shape microbial community assembly is essential for advancing our knowledge of microbial ecosystems. Fermented vegetables, increasingly popular among the general population, provide tractable and reproducible model systems to study microbial succession under controlled environmental conditions. By systematically manipulating variables such as vegetable type, salinity and gas composition, we uncovered the effect of these factors on the microbial dynamics throughout the fermentation. These insights not only contribute to a better understanding of the microbial ecology of these man-made food systems but also suggest directions for novel strategies to optimize fermentation processes for producing faster, safer, and more flavorful foods.

microbiology↗