Bacterial Extracellular Vesicles: New Hype or Hope to Explain Reproductive Host-Microbiota Interactions.

Rapid advances in microbiome research are transforming our understanding of human health and disease, with growing focus on the female reproductive tract as a critical but understudied niche. Evidence for a local microbiome largely derives from bacterial nucleic acid detection; however, the biological relevance of these signals remains debated, with whether they reflect viable microbial communities, transient colonisation, or mere microbial remnants with immunomodulatory effects. Bacterial extracellular vesicles (BEVs) have emerged as pivotal mediators of host-microbiota crosstalk. Their small size enables them to traverse tissue barriers, enter systemic circulation and access barrier-protected anatomical sites like the placenta, thereby extending their biological reach beyond the site of origin. Although BEVs have been extensively characterised in the context of gastrointestinal, respiratory and systemic diseases, their relevance within the reproductive tract remains insufficiently defined. Understanding whether BEVs contribute to processes such as endometrial receptivity, gamete interaction, embryo implantation and immune tolerance in early pregnancy may reveal novel mechanisms of reproductive physiology and pathology. Furthermore, unravelling the role of BEVs could help resolve the ongoing debate regarding the existence of a functional upper reproductive tract (URT) microbiota, reframing it in terms of microbial activity rather than microbial presence alone. This review synthesises the limited but growing body of evidence on BEVs in the reproductive tract, with a particular emphasis on their potential influence on female fertility and early pregnancy outcomes. We also outline the major methodological challenges, including the discrimination of BEVs from host-derived extracellular vesicles (EVs), the technical limitations of current detection approaches and the risk of contamination in low-biomass environments that complicate research in this field. Finally, we highlight conceptual frameworks and future research directions needed to establish BEVs as important players in reproductive biology and to harness their diagnostic and therapeutic potential in reproductive medicine.