Compared to its female counterpart, the microbiota of the male genital tract has
not been studied extensively. With this study, we aimed to evaluate the bacterial
composition of seminal fluid and its impact on sperm parameters. We hypothesized
that a dysbiotic microbiota composition may have an influence on sperm quality.
Semen samples of 26 men with normal spermiogram and 68 men with at least one
abnormal spermiogram parameter were included in the study. Samples were stratified
based on total sperm count, spermatozoa concentration, progressive motility, total
motility and spermatozoa morphology. Microbiota profiling was performed using 16S
rRNA gene amplicons sequencing and total bacterial load was determined using a
panbacterial quantitative PCR. Semen samples broadly clustered into three microbiota
profiles: Prevotella-enriched, Lactobacillus-enriched, and polymicrobial. Prevotellaenriched samples had the highest bacterial load (p < 0.05). Network analysis identified
three main co-occurrence modules, among which two contained bacteria commonly
found in the vaginal flora. Genera from the same module displayed similar oxygen
requirements, arguing for the presence of different ecological niches for bacteria that
colonize semen through the passage. Contrary to our hypothesis, shifts in overall
microbiota composition (beta-diversity) did not correlate with spermiogram parameters.
Similarly, we did not find any difference in microbial richness or diversity (alphadiversity). Differential abundance testing, however, revealed three specific genera that
were significantly enriched or depleted in some of the sperm quality groups (p < 0.05).
Prevotella relative abundance was increased in samples with defective sperm motility
while Staphylococcus was increased in the corresponding control group. In addition,
we observed an increased relative abundance of Lactobacillus in samples with normal
sperm morphology. Our study indicates that overall bacterial content of sperm might not
play a major role in male infertility. Although no major shifts in microbiota composition or
diversity were found, the differential abundance of specific bacterial genera in the sperm
suggests that a small subset of microbes might impact the spermatozoal physiology
during sperm transition, more specifically motility and morphology. Further studies are
required to challenge this finding and develop potential strategies to induce the formation
of a healthy seminal microbiota.