Bacterial vaginosis (BV) is a polymicrobial syndrome that modifies vaginal secretions and increases risks of reproductive complications and most sexually transmissible infections. This syndrome is associated with a shift in vaginal microbiota composition from lactic-acid producing bacteria to specific bacterial communities. Concomitant with such shift, sialidases can be detected in vaginal secretions where they release sialic acids (SA), sugar molecules that some bacteria can use as a nutrient source and to escape host’s immunity. While the presence of sialidases is used as a BV diagnosis method, we still know little about SA metabolism in the species associated with BV, and even less about SA-related interactions in the community. The proposed project will directly address this question with the ultimate aim of understanding the role of SA metabolism in the transition from health to disease. I will focus my research on the three most prevalent BV-associated bacteria: Gardnerella vaginalis, Prevotella bivia and Atopobium vaginae. First, I will investigate the genomic organization and distribution of the genes involved in SA metabolism. Second, I will link these genotypes to phenotypes by experimentally assessing the expression of those genes in clinical samples from BV-positive and negative women. Third, I will assess the evolutionary stability of SA metabolism genes and bring insight into the respective roles of SA availability and host’s immune system in the evolution of bacterial SA metabolism. Finally, I will create ex-vivo minimal model communities of BV bacteria and examine the ecological interactions between species. By going beyond a taxonomic description to better understand eco-evolutionary functioning, this project will expand our view of the BV microbiome both at the species level, examining in particular intraspecies diversity in genetic composition and expression, and at the community level through the investigation of SA-related interactions.