The influence of seminal microbiota on human testicular steroidogenesis: a prospective study.

Objective: Preclinical evidence has demonstrated that gut microbiota composition can influence steroid hormone biosynthesis and spermatogenesis. This study aims to investigate the association of seminal microbiota and testicular steroidogenesis.

Patients and methods: One hundred adult eugonadal men were consecutively enrolled. The seminal concentration of Lactobacilli, anaerobic and facultative bacteria, as well as serum levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and total testosterone (TT) were evaluated. Unadjusted and adjusted multi-regression models were built to evaluate the relationship between seminal Lactobacilli, anaerobic and facultative bacteria, and Lactobacilli/total bacteria ratio, and serum LH, FSH, and TT. The concentrations of seminal Lactobacilli, anaerobic, and facultative bacteria predictive of serum TT values in the lowest quartile (< 3.8 ng/mL) were calculated.

Results: TT levels were weakly and positively correlated with seminal Lactobacillus concentration (r = 0.33; p = 0.001), with seminal Lactobacilli/total bacteria ratio (r = 0.89; p < 0.001), and negatively with anaerobic and facultative bacteria (r = - 0.69; p < 0.001). Opposite correlations were found for gonadotropin concentrations. These data persisted after adjustment for confounding factors. Seminal concentration of Lactobacilli ≤ 0.1 × 10⁶/mL (AUC 0.917, 95% CI: 0.845 to 0.963), of anaerobic and facultative bacteria > 2 × 10⁴/mL (AUC 0.924, 95% CI: 0.853 to 0.967), or a Lactobacilli/total bacteria ratio ≤ 90% (AUC 0.910, 95% CI: 0.837 to 0.958) were found to predict serum TT level < 3.8 ng/mL with a sensitivity of 92.0% and a specificity of 88.0%.

Conclusion: A relationship between the composition of the seminal microbiota and testicular steroidogenesis seems to exist. The mechanisms underlying this association are still unknown.