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

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.

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.

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.

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.

Evidence remains predominantly observational and heterogeneous, with limited direct mechanistic validation and sparse longitudinal data.

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.