For several years, researchers have been arguing about when our body is colonized by viruses, bacteria and other microorganisms. For a long time it was thought that this only occurs during childbirth, but more recent studies have reported that amniotic fluid and placenta samples already contained such microorganisms. This would suggest that fetuses already have a so-called microbiome. An international team now clearly contradicts this: Based on a multidisciplinary analysis published in the journal “Nature”, it found that the womb is usually sterile after all.

Our microbiome, i.e. the entirety of all microorganisms that are in and on the skin, mucous membranes and organs, plays a central role in our health and our immune system. Newborns already have this immune system – however, its development still poses questions for scientists, and in particular when a fetus first comes into contact with bacteria, viruses and other microbes.

Scientific rethinking since 2010

For a long time it was believed that the unborn child and the uterus in which it grows, including the placenta (placenta) and amniotic fluid are sterile in a healthy pregnancy. However, since 2010, several research teams have reported finding bacteria in samples of the placenta and amniotic fluid, concluding the presence of a fetal microbiome, which would require a complete re-evaluation of the conventional notion of the development of the unborn child’s immune system.

A consortium of 46 experts from reproductive biology, microbiome science and immunology now vehemently contradicts the thesis of the fetal microbiome. The researchers, led by Jens Walter from University College Cork, reviewed the analyzes of the studies discussed from their respective specialist perspectives and came to the unanimous conclusion that the detection of microbiomes in the unborn child is due to contamination of samples.

For example, amniotic fluid samples differed significantly in caesarean and natural births, which should not be the case for a general fetal microbiome. “We are aware that our position is at odds with dozens of publications providing evidence of microbial populations in the womb, but we believe in the validity of our multi-pronged approach,” the authors conclude.

guidance for the future

As study leader Walter adds in a name contribution, a fetal microbiome would contradict the knowledge of human biology: “For example, we know that the placenta is full of anatomical and immunological barriers that prevent microbes from entering and colonizing it.” In fact, very few microorganisms can cross the placenta and then infect the fetus, including the herpes virus that causes cytomegalovirus infection, rubella and chickenpox viruses, and listeria.

The authors now hope that their consensus will provide guidance for future research. “It provides a solid scientific basis to focus research efforts where they are most effective,” said Walter. Knowing that the fetus is in a sterile environment confirms that bacterial colonization takes place during birth and in the early postnatal period – this has also been confirmed by recent studies showing that babies develop the first microbes during the birth process and more obtained through breast milk.

It is still important to find out how the immune system of the fetus develops. With the published consensus, however, the focus of research should shift away from living microbes towards the cell components of microbes and the chemicals they produce, so-called metabolites, says Walter: “It has been shown that such compounds penetrate the placenta and damage the immune system of the human body Preparing the fetus for life in a germ-ridden world.”