Sperm Quality

The Bacteria Living Alongside Sperm

The research here is early and the studies are small, but the measurement is specific rather than vague. Semen was assumed sterile unless infected. It isn't. It carries its own bacterial community, and the composition of that community tracks with how well sperm perform. Between 30% and 50% of male infertility has no identified cause, which is why this is being looked at closely.

What was actually measured

Researchers analysed semen from 73 men, using genetic sequencing rather than culture.

Twenty-seven were being investigated for fertility problems. Forty-six were attending for vasectomy, having already fathered children, and served as the comparison group.

The headline finding concerned one species. Among men with abnormal sperm motility, meaning sperm that don't swim well, Lactobacillus iners made up an average of 9.4% of the bacteria present. Among men with normal motility it was 2.6%.

The detail that makes this interesting is what didn't differ. Overall bacterial variety was similar between the groups.

So this doesn't look like general disruption of the community. It looks like the abundance of one particular species mattering.

Source: Semen microbiota are dramatically altered in men with abnormal sperm parameters

Why that species specifically

There's a plausible mechanism, and it comes from an unexpected direction.

Lactobacillus species are generally regarded as beneficial, and in semen a Lactobacillus-dominant community is associated with better quality. More than 80% of normal semen samples cluster into that group.

Lactobacillus iners behaves differently from its relatives. It produces predominantly one form of lactic acid, and that form is associated with a more inflammatory local environment.

Sperm are unusually vulnerable to inflammation and to the oxidative stress that comes with it. Their membranes are rich in fats that damage easily, and they carry very little machinery to repair themselves.

The same species has been studied far more in the vaginal microbiome, where it's consistently associated with worse outcomes than its close relatives. This was the first work linking it to male fertility.

Other organisms, and an inconsistency worth noting

The picture beyond that one species is less tidy.

Three Pseudomonas species were examined in the same study. Two, Pseudomonas fluorescens and Pseudomonas stutzeri, were more common in men with abnormal sperm concentration. A third, Pseudomonas putida, was less common.

That's a useful corrective. Closely related organisms don't necessarily behave alike, so grouping bacteria by genus can obscure what's happening.

Different research groups also disagree. One high-quality study comparing fertile and infertile men did find differences in overall bacterial variety, which the study above did not. The likeliest explanations are different patient populations and different analysis methods.

Larger work has since clustered semen communities into groups dominated by Lactobacillus, Prevotella or Streptococcus, with the Lactobacillus-dominant group associated with the best semen quality.

Source: Male infertility and the human microbiome

Where the gut fits in

The connection between gut bacteria and semen bacteria hasn't been directly established, and it's worth being clear about that.

What is established is that the gut influences the conditions sperm are made in. Oxidative stress and inflammation are among the best-documented contributors to poor sperm quality, and gut bacteria are among the strongest influences on background inflammatory signalling.

The barrier protecting developing sperm from the immune system is also affected by systemic inflammation.

Early trials have tested probiotics directly. In a small pilot of nine men with poor sperm motility, six weeks of specific strains improved motility and reduced DNA damage in sperm. Nine men is far too few to conclude anything, and larger trials are needed.

Introducing bacteria that way also only works if they're fed afterwards. A strain arriving in a gut with little fibre and few plant compounds reaching it won't establish, and whatever it was producing stops with it.

What this means for you

Difficulty conceiving after a year of trying warrants a semen analysis and assessment for both partners, since male factors contribute to roughly half of cases and several causes are treatable. In addition, the research above found one bacterial species substantially more abundant in men with poor sperm motility, in a community where overall variety looked otherwise normal. A varied, plant-rich diet supports the same gut bacteria this research ties to lower inflammation and oxidative stress, which are among the best-documented contributors to poor sperm quality. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.