Seminal Microbiota and Pregnancy Outcomes: Mechanistic Insights, Clinical Implications, and Future Directions

BACKGROUND: Seminal fluid contains a low-biomass microbial community that may influence male reproductive health and pregnancy outcomes, but its biological and clinical significance remains uncertain. OBJECTIVES: To critically synthesise evidence linking seminal microbiota with sperm quality, reproductive tract inflammation, natural conception, and assisted reproductive technology outcomes, and to evaluate proposed mechanisms and clinical implications. MATERIALS AND METHODS: This review integrates human observational studies with relevant in vitro, animal, and female reproductive tract evidence, with particular attention to 16S rRNA gene sequencing, low-biomass contamination, and methodological heterogeneity. RESULTS: Genus-enriched and compositionally distinct seminal profiles have been reported, but no reproducible community types have been established across populations. Lactobacillus-enriched profiles are associated with favourable semen parameters in some cohorts, whereas infection-associated or opportunistic taxa are more often linked to impaired semen characteristics. Proposed pathways include immune activation, oxidative stress, spermmicrobe interactions, and guttestis signalling. DISCUSSION: Evidence remains predominantly observational and heterogeneous, with limited direct mechanistic validation and sparse longitudinal data. CONCLUSION: Standardised, contamination-aware studies with independent validation are needed. Current evidence does not support routine seminal microbiome testing or microbiota-directed interventions in infertility care.