Relationship Between Seminal Microbiota and Sperm Parameters in Humans: A Prospective Cross-Sectional Study

Emerging evidence suggests that the seminal microbiota may play a role in male reproductive health, yet its relationship with sperm parameters remains poorly understood.

This study explores the link between seminal microbiota composition and sperm parameters to understand their impact on the male reproductive function.

This prospective cross-sectional study included 100 eugonadal adult men evaluated at a university-affiliated Endocrinology and Andrology Division. Seminal concentrations of lactobacilli, anerobic, and facultative bacteria, along with serum luteinizing hormone, follicle-stimulating hormone, and testosterone, were assessed alongside conventional sperm parameters. Conventional sperm parameters and seminal leukocyte counts were analyzed. Regression models were used to explore associations, and predictive thresholds for sperm concentration >16 mil/mL and sperm progressive motility >30% were determined.

Seminal lactobacillus concentration positively correlated with sperm concentration (r = 0.42, p < 0.001). Seminal leukocyte concentration and percentage of immature germ cells (spermatids) identified at semen analysis showed negative correlations (r = -0.35, p = 0.002; r = -0.37, p < 0.001, respectively). Anerobic and facultative bacteria negatively correlated with sperm progressive motility (r = -0.77, p < 0.001) and positively correlated with percentage of spermatids (r = 0.72, p < 0.001). The lactobacilli-to-total bacteria ratio negatively correlated with leukocyte concentration (r = -0.96, p < 0.001) and percentage of spermatids (r = -0.89, p <0.001), and positively with sperm progressive motility (r = 0.95, p < 0.001). All of these correlations remained significant after adjustments. Concentrations of lactobacilli, anerobic/facultative bacteria, and the lactobacilli/total bacteria ratio were strong predictors of sperm concentration and motility, demonstrating high sensitivity and specificity.

Our findings suggest that the composition of the seminal microbiota, particularly the relative abundance of lactobacilli versus anerobic/facultative bacteria, may influence key sperm parameters such as concentration and motility. This highlights the potential clinical relevance of microbiota profiling in the male fertility assessment.

A higher abundance of seminal lactobacilli is associated with more favorable sperm parameters, while anerobic and facultative bacteria are linked to poorer outcomes. Microbiota analysis may offer additional insights into male infertility diagnostics, though further studies are needed to confirm causality.