The brain is the most highly protected organ in the human body. It has a layer of cells covering it that act as a relentless guard that regulates the passage and exchange of nutrients and molecules between the bloodstream and the brain parenchyma, the nervous tissue in the brain. This barrier – the so-called blood-brain barrier – is essential for the health of both the brain and the nervous system and their proper functioning.

Now, new research carried out with rodents has suggested for the first time that gut microbiota could play an essential role in the integrity and development of this brain barrier. In a study published in Science Translational Medicine, researchers from the Swedish Karolinska Institutet compared what happened in the blood-brain barrier of two groups of mice: the first ones were born from germ-free mothers (kept in a sterile environment without any bacteria), whereas the second were born from a normal healthy group of female rodents and exposed to normal bacteria.

The researchers found that pups from the first group had a defective blood-brain barrier. To test this, they injected dye molecules and traced them throughout the body; in normal healthy animals, these substances would never pass through the barrier and would be excluded from entering the nervous system. But in the mice from the first group, the substances ended up appearing in their brain. Karolinska researchers therefore concluded that the absence of gut microbiota is fundamentally linked to greater permeability of the blood-brain barrier with regards to possibly harmful molecules.

Viorica Braniste, one of the authors of the study, stated in a press release by Karolinksa Institutet, “The results showed that the presence of the maternal gut microbiota during late pregnancy blocked the passage of labelled antibodies from the circulation into the brain parenchyma of the growing foetus. In contrast, in age matched foetuses from germ-free group mothers, these labelled antibodies easily crossed the blood-brain barrier and were detected within the brain parenchyma.”

The researchers also wanted to know whether the effects of the absence of gut microbiota were permanent. To do this, they transplanted gut microbiota from healthy mice into uncolonised rodents and saw how this led to a restoration of the barrier function.

For the study’s principal researcher, Professor Sven Pettersson of the Department of Microbiology, Tumor and Cell Biology at Karolinska Institutet, “Given that the microbiota’s composition and diversity change over time, it is tempting to speculate that the blood brain-barrier’s integrity may also fluctuate depending on the microbiota.” For now, the researchers do not know how these effects are achieved, what the rest of the signals are, or what bacteria are the main players. The study has been shown to work in rodents, but needs to be confirmed in humans. If so, that would mean that not only a pregnant woman’s diet, but also whether or not she is exposed to antibiotics could influence the development of the baby’s blood-brain barrier and nervous system.

“These findings further underscore the importance of maternal microbes during early life and that our bacteria are an integrated component of our body physiology,” considered Pettersson.