Babies in the womb are not as sheltered from the outside world as you might think. The placenta harbours a unique ecosystem of bacteria which may have a surprising origin 鈥� the mother鈥檚 mouth.
Disturbances of the placenta鈥檚 bacterial community may explain why some women give birth prematurely, and could also be one of the ways that a woman鈥檚 diet affects her offspring鈥檚 gut bacteria, and as a result the child鈥檚 disease risk. 鈥淒ifferent nutrients [in the mother鈥檚 diet] are a huge determinant of which microbes take up residence in the placenta,鈥� says of Texas Children鈥檚 Hospital and Baylor College of Medicine in Houston, who led the study.
In the past decade there has been growing awareness of the important role of the human microbiome 鈥� all the bacteria, viruses and fungi that live on and in our bodies. Disturbances to the gut microbiome have been linked with conditions ranging from obesity to .
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Until recently it was generally thought that babies are born with a sterile gut and that they pick up microbes on their journey through their mother鈥檚 vagina which migrate to colonise the gut. Additional microbes are collected from their environment over the first few years of life. This theory was challenged when bacteria were found in the meconium, a baby鈥檚 first stool passed within hours of their birth.
We now have a clue to where these bugs are coming from. Aagaard and her colleagues genetically sequenced the bacteria of the placenta, the organ that transfers nutrients and oxygen to the fetus from the mother鈥檚 blood. They took samples from the placentas of 320 women after they had given birth, taking tissue from inside the placenta to avoid any contamination by vaginal bacteria.
Jaw-dropping origins
The team found a broad range of bacteria present, including those necessary for metabolising some of the vitamins and nutrients needed by the fetus. The first surprise was that the bacterial species were most similar to those normally found in the adult mouth, as opposed to the vagina or gut. 鈥淭he placenta has its own ecology and these were not the bacteria we were expecting,鈥� says James Kinross, a colon surgeon at Imperial College London who researches gut bacteria and was not involved in the new work. 鈥淢ost people would have expected it to be a vaginal flora,鈥� he says, because of its close proximity.
The fact that it was most similar to the bacterial community found in our mouths suggests that these bacteria are somehow finding their way through the bloodstream to the placenta. Having got that far, they could then reach the baby either by crossing into the baby鈥檚 blood vessels within the placenta or by passing into amniotic fluid, which is swallowed by the baby, suggests Aagaard.
The team also found that some bacterial species were more common and others less common in the women who had given birth prematurely 鈥� before 37 weeks of pregnancy 鈥� than the typical bacterial profile of the women who went to full term. Intriguingly, previous studies have found that . Aagaard speculates that if oral bacteria do reach the placenta through the blood, then it is possible that diseased and bleeding gums could allow harmful bacteria to reach and colonise the placenta, potentially triggering premature birth.
In a separate study in macaques, Aagaard鈥檚 team has shown that giving . Many previous studies have shown that a person鈥檚 risk of obesity and heart disease is affected by their mother鈥檚 diet, but it was thought this was passed on through epigenetic mechanisms 鈥� chemical changes that switch the offspring鈥檚 geness on or off. 鈥淏ut layered on top of that are variations in the microbiome,鈥� says Aagaard.
Journal reference: Science of Translational Medicine, DOI:
Clarification, 22 May 2014: Kjersti Aagaard is affiliated to Baylor College of Medicine as well as Texas Children鈥檚 Hospital. We have updated the article accordingly.
