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Is Semen Really Sterile?
There is virtually no part of the human body that is completely sterile — and this includes semen. As scientific technology has advanced, new culture-independent techniques (that don’t rely on being able to grow bacteria in a lab to detect their presence) have been used to investigate the male reproductive tract. Previously considered to be completely free of any microorganisms, semen does in fact have a microbiome, and knowledge about the role of these microorganisms in health and disease is increasing.
Rather than focusing on infertility — where much of the existing research is concentrated — this post will focus on two key questions: what do we already know about the seminal microbiome in healthy people, and how does it differ from and interact with the vaginal microbiome?
What Does the Healthy Seminal Microbiome Look Like?
Most studies looking at the healthy seminal microbiome have found that, like the gut or vaginal microbiome, the seminal microbiome is highly individualised and person-specific. The main bacterial genera identified in semen include Lactobacillus, Finegoldia, Prevotella, Corynebacterium, Staphylococcus and Streptococcus. One study that looked at almost 100 semen samples reported that the seminal microbiome can be categorised into one of three groups: Lactobacillus-predominant, Pseudomonas-predominant, or Prevotella-predominant.
Interestingly, research also suggests that the seminal vesicles, prostate and testes all have their own unique microbiome composition. Even with this ever-expanding knowledge, a lot more work is needed — particularly to understand whether microbes are transient or able to persist within and colonise components of the male reproductive tract.
How Does the Seminal Microbiome Differ from the Vaginal Microbiome?
Traditionally, the vaginal and seminal microbiota have been studied independently of each other, and there is considerably more research available for the vaginal microbiome than for the seminal microbiome. One of the most important differences between the two is diversity: the vaginal microbiome is far less diverse than the seminal microbiome, and is instead largely dominated by Lactobacillus spp., which play a critical role in maintaining vaginal pH.
The seminal microbiome, by contrast, contains a broader range of bacterial genera — including several that have been linked to bacterial vaginosis (BV), such as Prevotella, Porphyromonas, Peptoniphilus, Finegoldia, Campylobacter, and Mobiluncus. This raises the question of whether semen could act as a reservoir for these bacteria.
The Seminal Microbiome and Bacterial Vaginosis: A Couples Issue?
Very few studies have looked at interacting vaginal and seminal microbiomes, however there is some evidence suggesting a link between the two — especially in long-term intimate partners. There is also evidence suggesting that the influence of the male genital microbiome on the vaginal microbiota is likely minor in healthy individuals. However, in the context of bacterial vaginosis, this is not the case.
A recent clinical trial looked at couples where the female partner had recurrent BV, comparing treating just the female partner versus treating both partners. The results were striking — male-partner treatment was so much more effective at preventing recurrence that the trial had to be modified mid-way through, as it was deemed unethical to continue the female-only treatment arm. This finding has significant implications for how recurrent BV is understood and managed.
The Seminal Microbiome and Vulvovaginal Candidiasis (VVC)
Vulvovaginal candidiasis (VVC) is another common infection strongly linked to microbiome composition. Caused by the overgrowth of Candida spp. — most commonly Candida albicans — up to 75% of women will experience VVC at least once in their lifetime.
While recurrent VVC has long been suspected to be linked to partner microbiome composition, direct scientific evidence is limited for obvious ethical reasons. What research does show is that C. albicans can survive and grow in semen — though interestingly, more viscous semen appears to inhibit its growth. Because Candida spp. are considered a normal part of the human microbiome and may even offer some benefits, current World Health Organisation guidelines do not recommend treating male partners for recurrent VVC unless they also display symptoms. However, as research progresses, this may change.
What the Seminal Microbiome Means for Women’s Health
Overall, while research in this area is still in its early stages, it is becoming increasingly clear that the seminal microbiome is biologically important — both in its own right and in how it may interact with the vaginal microbiome. As couple-based research expands, our understanding of these intimate microbial relationships and their role in health and disease will continue to grow.
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