Are we really born sterile?
A controversial discussion is running among microbiome scientists. Is the womb sterile? For more than a century we have assumed that the placenta is sterile.
Since 2011, this dogma was questioned by different scientists around the globe.
Indira Mysorekar found bacteria in one third of 200 placenta samples. The bacteria were even inside the cells, no immune cells near them meaning that there was no inflammation to come. The bacteria were not only found in women who gave birth early, but also in women who had normal healthy pregnancies. They found bacteria in the placenta, the amniotic fluid and in the meconium, the first stool that the baby forms in utero. This might be a hint to a fetal microbiome.
A team led by Kjersti Aagaard found bacterial DNA in placental tissue.
“Babies were supposed to get the bacteria that will become their microbiome in the birth canal, but she saw a mismatch between the bacteria present in the vaginas of pregnant women and those present in infants in their first week of life. That might make sense, she thought, if the microbiome gets seeded before birth.”
They analysed the placentas of 320 women, some of them gave birth early. Not every placenta contained bacterial DNA, but many did. They found bacterial communities dominated by E. coli and some other groups. The communities found matched best those that are typically found in the mouth. The question is, how did the bacteria get from the mouth to the placenta? Aagard argues that the bacteria found in placentas of women who gave early birth differs from bacteria found in women who gave normal, healthy birth. This might be one hint that indeed there is some bacteria in the placenta which do not induce inflammation, thus belonging to the fetal microbiome.
However, a number of researchers highly doubt the thesis of a fetal microbiome.
They argue that the microbial traces found in the placenta are contaminations of the laboratory equipment. Samuel Parry's team in Philadelphia actually found these contaminations in swabs, DNA-purification kits and other equipment when they were preparing for their own studies of the placental microbiome. With the DNA contaminations in the background they couldn't find any bacterial DNA in the placentas.
Another argument against the fetal microbiome thesis comes from Maria Dominguez-Bello who studies the transfer of the mothers’ vaginal bacteria to their babies which were delivered via caesarean birth. She argues that sterility is broken when the amniotic sac breaks, which leaves plenty of time for bacteria to make their way into the infant’s gut.
“Labour takes hours, during which the baby is swallowing and rubbing against the walls of the birth canal,” she adds. "Even if a baby is born by caesarean section, it might take hours or even days for the infant to pass its first stool — a window during which it might acquire bacteria outside the womb.”
The most convincing evidence that the fetal microbiome doesn’t exist, is the existence of laboratory mice which are sterile. These mice were delivered surgically from mice with normal microbiomes and then raised under sterile conditions. If a fetal microbiome would exist, these sterile mice wouldn´t already be used successfully for 70 years.
We might have an answer soon in this controversial discussion. A team of Samuel Parry and the obstetrician Robert Romero at the National Institute of Child Health and Human Development in Detroit, Michigan, are planning a multi-centre study to examine the question in even more placentas. They have invited Aagaard who is willing to participate. They are planning to have answers as soon as next year – “this controversy can be solved”, Romero says.
For more details follow this link: https://www.nature.com/articles/d41586-018-00664-8
