It is quite well-known nowadays that our body cavities such as the gut, mouth, nose, lungs, vagina, or our skin surface, shelter millions of micro-organisms: these communities are called microbiomes. In recent years, more confidential microbiomes have been described: it is the case for the semen microbiome, which is the object of several systematic reviews and meta-analysis (1,2). Like others, it arouses great interest about its potential implication in physio-pathological mechanisms, and more specifically those of male infertility.
Usually, doctors seek for the presence of bacteria in sperm as an indicator of infection, deemed abnormal. The prevalence of this “bacteriospermia” greatly varies among published studies, from 6 to 68% of infertile males, depending on the techniques of isolation and analysis. So does the composition of the microbiome. Indeed, studies using next-generation sequencing methods, also known as the 16S metabarcoding technique, described Lactobacillus and Prevotella as dominant bacterial generaspecies. Other studies, using culture-based methods to isolate bacteria found mostly Escherichia, Staphylococcus, Streptococcus and Enterococcus. Using PCR techniques, others identified Ureaplasma, Chlamydia trachomatis and Mycoplasma. Hard to find out… but quite logical that culture-based analyses reveal more aerobic species than NGS, or that PCR identify poorly abundant but pre-specified anaerobic pathogens.
So, apart from infection from the outside that could contaminate the semen, where does those microbes come from? And, could “dysbiosis”, or imbalanced semen microbiome, explain disorders such as infertility? The proximity between urine and semen microbiomes suggest that both are fueled by organs located upstream the common ureteral pipe: seminal vesicle, prostate and/or testis. The authors of one review pointed out the low number of case-control studies able to compare fertile and infertile men (1). With NGS, this comparison brought no huge difference as regards to the semen microbiome. However, the presence of Lactobacillus or Pseudomonas was associated with a good sperm quality, conversely to Prevotella (1,2). In addition, the meta-analysis of data from culture- or PCR_based analyses showed that the sperm of infertile men was about twice more likely to contain Ureaplasma urealyticum than that of fertile ones. Bacteriospermia was also associated with a reduced mean concentration and motility of spermatozoa, with a high DNA fragmentation index and protamine deficiency, which are indicators of poor quality of sperm.
Even if those data argue for a causative role, the mechanisms that would link bacteria and fertility (or infertility) are still to be elucidated. These systematic reviews ultimately underline more questions than answers. Should bacteriospermia be considered deleterious, or does a rich and diverse semen microbiota indicate a healthy environment helpful to control acute infections? Does the semen microbiome vary throughout life, from childhood to old age, through puberty and the adult sexually active period? Does it differ around the world, according to geography, ethnic groups or lifestyle? Is there a relationship between urine, gut and semen microbiomes of an individual, and vaginal microbiome of her partner? The research has only just begun, and the field to be cleared is immense…
1. Farahani L, Tharakan T, Yap T, Ramsay JW, Jayasena CN, Minhas S. The semen microbiome and its impact on sperm function and male fertility: A systematic review and meta-analysis. Andrology. janv 2021;9(1):115‑44.
2. Venneri MA, Franceschini E, Sciarra F, Rosato E, D’Ettorre G, Lenzi A. Human genital tracts microbiota: dysbiosis crucial for infertility. J Endocrinol Invest. juin 2022;45(6):1151‑60.
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