Causal Effects of Gut Microbial Genera on Abnormal Spermatozoa Revealed Through Integrated Mendelian Randomization and Mediation Analysis

PURPOSE: Sperm quality is a key contributing factor in infertility. This study aimed to investigate the causal relationship between gut microbiota genera and abnormal spermatozoa (defects in sperm count, motility, or morphology) and to evaluate potential mediating factors. PATIENTS AND METHODS: Summary statistics for gut microbiota genera, abnormal spermatozoa, microbiota metabolic pathways, immune cell traits, and serum bioavailable testosterone (BioT) levels were extracted from publicly available GWAS databases. A fixed-effects meta-analysis was performed on gut microbiota genera from the MiBioGen and DMP cohorts, followed by two-sample Mendelian randomization (MR) analysis to evaluate causal associations. To strengthen reliability, a triangulation framework incorporating MR analyses, HyPrColoc colocalization analysis, and Steiger directionality tests was employed. Mediation analysis focused on microbiota metabolic pathways, immune cell traits, and BioT levels as potential mediators. RESULTS: Eight gut microbiota genera showed significant causal associations with abnormal spermatozoa: four risk genera (Lachnospiraceae UCG001.id.11321, Coprococcus 1.id.11301, Barnesiella, Dorea) and four protective genera (Clostridium sensu stricto 1.id.1873, Ruminiclostridium 6.id.11356, Lachnospiraceae_noname, Veillonella). The triangulation framework validated these causal relationships. Although two genera affected BioT levels, formal mediation analysis did not confirm BioT, immune cells, or metabolic pathways as mediators, suggesting novel gut-testis axis mechanisms. CONCLUSION: This large-scale MR analysis establishes a causal connection between specific gut microbiota genera and abnormal spermatozoa. The "causation without colocalization" pattern and null mediation findings suggest distant regulatory mechanisms and novel gut-testis axes, offering microbiota-targeted therapeutic strategies for male infertility.