Bacteria in Sperm Could Affect the Success of Fertility Treatment

When it comes to infertility, attention often turns first to the woman. Tests focus, for example, on the egg reserve, ovulation, the uterus, or hormone levels. But the male partner also plays a crucial role. New research is bringing a previously overlooked factor into focus: the bacterial community in semen.

A recent study suggests that certain changes in the so-called semen microbiome may be linked to the success rates of fertility treatments. Among couples undergoing artificial insemination, a specific bacterial pattern in the man’s semen was associated with a lower probability of a live birth and a higher risk of pregnancy loss. The findings offer a new perspective on male fertility—but at the same time raise important questions.

The Male Microbiome Takes Center Stage

The human body harbors an enormous number of microorganisms. Bacteria, viruses, and fungi live in the gut, on the skin, and in the genital tract, among other places. Even seminal fluid is not entirely free of microorganisms. For a long time, however, the so-called semen microbiome received relatively little attention in fertility medicine.

That is increasingly changing. Researchers are interested in which microorganisms are present in the male genital tract and whether their composition can provide insights into sperm quality or the success of fertilization.

The new study therefore examined not only the women but also the microbial communities of both partners. The study included couples undergoing in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI), as well as women with recurrent pregnancy loss. In total, more than 350 women and just under 200 men were examined in two independent groups.

A Specific Bacterial Pattern Was Associated With Fewer Live Births

Of particular interest was a bacterial pattern that the researchers termed “semen dysbiosis.” In this condition, the composition of microorganisms in the semen was altered and, in some cases, resembled bacterial patterns also observed in bacterial vaginosis.

Among couples undergoing IVF, a clear difference emerged: When the male partner exhibited this unfavorable bacterial pattern, approximately 57 percent of the couples in the study group had a live birth. Without this pattern, the rate was around 85 percent. In both study groups, the altered semen microbiome was also associated with an increased risk of pregnancy loss.

These figures are noteworthy but do not mean that bacteria in semen automatically cause infertility. The study was observational in design. Thus, while the researchers were able to identify a correlation, they could not prove that the bacteria themselves were responsible for the poorer treatment outcomes.

Why Might Bacteria in Semen Play a Role?

The exact biological explanation remains unclear. One possibility is that certain microorganisms influence the properties of sperm. Previous studies have already described associations between the composition of the semen microbiome and various sperm characteristics, such as motility or quality. Other studies have found links between certain bacterial groups and the quality of embryos following IVF.

Another possibility involves the interaction between partners. Microorganisms from the male genital tract can come into contact with the female genital tract during sexual intercourse. Researchers therefore suspect that the semen microbiome might also influence the environment in which fertilization and early pregnancy take place.

The immune system could play an important role in this process. Certain bacterial compositions could promote inflammatory processes or alter the local immune response. Theoretically, this could in turn affect embryo implantation or the maintenance of early pregnancy. However, there is not yet sufficient evidence for these mechanisms.

Not Every Bacterium Is Harmful

It is important not to view the semen microbiome as inherently negative. The body is not sterile, and the presence of bacteria does not automatically mean an infection or a disease.

Rather, it apparently depends on the composition of the entire microbial community. Certain bacteria may be beneficial under specific conditions, while others are associated with inflammation or poorer reproductive outcomes.

Previous studies, for example, have found evidence that various bacterial groups are linked to different sperm and embryo characteristics. In studies of the semen microbiome, Lactobacillus, Prevotella, Staphylococcus, and various other bacterial groups were detected, among others. However, the results are not always consistent.

This makes the research particularly complex: it is probably not enough to look for “good” or “bad” bacteria. Rather, the balance of the entire microbiome could be the decisive factor.

The Gut Microbiome Could also Play a Role

The current study was not limited to the male semen microbiome. The researchers also examined the women’s gut microbiome. In doing so, they found evidence suggesting that certain changes in the gut flora could also be linked to a longer time to conception and a lower likelihood of conception.

This is interesting because the gut microbiome can influence numerous processes. These include, among other things, metabolism, immune responses, and inflammatory processes. These, in turn, may be related to reproductive function.

However, the same applies here: The study initially shows only statistical correlations. It is not yet known whether a change in the gut microbiome actually affects fertility or whether other factors underlie both observations.

The Microbiome of Both Partners Could be Important

A distinctive feature of the current research is therefore its focus on both partners. In fertility treatment, attempts are often made to attribute the causes as clearly as possible to either the man or the woman. However, the interaction between the two reproductive systems could be more complex.

Researchers suspect that the partners’ microbiomes may influence each other to some extent. Microorganisms can be transmitted between people, and the microbial environments of the male and female genital tracts may be more closely interconnected than previously assumed.

This does not mean that one partner’s microbiome automatically determines the other’s. However, it does suggest that fertility may depend not only on individual organs or lab values, but also on complex biological interactions between both individuals.

Could Treating the Microbiome Improve Fertility?

This is one of the most exciting questions for the future. If certain bacterial patterns do indeed contribute to poorer treatment outcomes, it could theoretically be possible to specifically modify the microbiome before fertility treatment.

Possible approaches could include, for example, individually selected probiotics, dietary changes, or other microbiome-based therapies. Diagnostic tests could also play a role in the future in identifying specific microbial patterns prior to IVF or ICSI.

However, medicine is still a long way from such applications. The researchers themselves emphasize that it must first be clarified whether a change in the microbiome actually leads to better pregnancy or live birth rates.

Antibiotics Are Not a Simple Solution

Precisely for this reason, one should not conclude from the new findings that men should take antibiotics as a precaution before fertility treatment.

That would not be scientifically justified at this time. Previous studies also show that treating bacteria in the semen of men without symptoms does not automatically lead to better IVF outcomes. An older prospective study even concluded that routine treatment of asymptomatic men with antibiotics may be unnecessary and could potentially have disadvantages.

Furthermore, antibiotics not only affect potentially pathogenic bacteria but can also influence beneficial components of the microbiome. Therefore, targeted treatment would only make sense if it were clear which microorganisms are actually problematic and whether their removal or alteration improves the success of fertility treatment.

A New Perspective on Male Fertility

Research on the semen microbiome is still in its relatively early stages. Nevertheless, it is already changing our understanding of which factors might play a role in male fertility.

Traditionally, examinations of men have focused primarily on sperm count, motility, and morphology. These parameters remain important. However, the microbiome could represent an additional layer that explains why some men have difficulty conceiving despite seemingly normal semen parameters, or why fertility treatments are not always successful.

Previous studies also show that microorganisms in semen may be linked to the quality of sperm and embryos. At the same time, the results of various studies are sometimes contradictory, indicating that much more research is needed.

What This Research Could Mean for Fertility Medicine

If the current findings are confirmed in larger, independent studies, the semen microbiome could become another component of fertility diagnostics in the future. Before IVF or ICSI, it might then be important to consider not only how many sperm are present and how well they move, but also what microbial environment is associated with them.

In the long term, this could lead to more personalized fertility medicine. Instead of following the same protocol for all patients, doctors could take different biological factors into account and select treatments more specifically.

Until then, however, caution is warranted. Current research provides an important clue, but does not yet offer a new standard of care. It must first be clarified which bacteria are actually relevant, what mechanisms underlie this, and whether a targeted modification of the microbiome can indeed increase the likelihood of pregnancy or live birth.

One thing is already clear, however: the microbiome is increasingly taking center stage in research on male fertility. And the new findings suggest that when it comes to the success of fertility treatments, it may be necessary to consider not only the sperm themselves but also the microbial environment in which they are produced and with which they come into contact.