Babies catch critical intestinal bacteria from their mothers at birth, says placenta study | Science

During pregnancy, the placenta provides the fetus with everything it needs to grow: oxygen, food, waste disposal and even the mother's immune system antibody. But no microbes. A new study reveals that the placenta is devoid of bacteria, reaffirming the idea that babies acquire a microbiome at birth.

This study is the latest salvo in a heated debate about when humans first acquire the microbes that shape their nutrient processing and their ability to fight disease later in life.

The idea of ​​a sterile uterus has been around for more than a century. However, in 2014, a study found that the placenta harbors bacteria similar to those present in our mouth – suggesting that some microbes can pbad from the mother's mouth into her blood, then into the fetus. Since then, scientists have wondered whether bacterial signals from the placenta are real or contaminants. The researchers hope to settle this debate with the largest study ever conducted, an badysis of placentas recovered from more than 500 women shortly after giving birth. Posted today in NatureThe study reports that healthy placentas do not contain bacteria.

"It's almost as clear as you could ask," said Frederic Bushman, a microbiologist at the University of Pennsylvania, who did not participate in the study. He says the researchers are "very adept with the detective work they've done to track down multiple sources of contamination."

To identify the weak signals of a potentially small microbial population in the placenta and not background signals, the researchers used two methods of sequencing the DNA. One approach targeted parts of genes common to all bacteria. The other approach was broader, combining all the DNAs present and mapping them for specific bacterial and animal species, says co-author Stephen Charnock-Jones, a reproductive biologist at the University of Cambridge in the UK.

Researchers were careful to include a number of controls. For example, they enriched samples with a pathogenic microbe found in lizards. By including a known amount of bacteria that would not be present in humans, researchers were able to measure the abundance of other bacteria. They also compared the bacterial signals with what they expected from contamination by sources such as DNA sequencing kits or the birth process itself. They found that some microbes are more common when women give birth badlly rather than by cesarean section, suggesting that these bacteria enter the placenta when delivered instead of living there before birth.

After eliminating the sources of contamination, the researchers found only one type of bacteria: Streptococcus agalactiae. This strain, present in only 5% of samples, is a sign of infection and can cause fatal sepsis in the newborn. Taken together, the results show that there is no microbiome in the placenta unless it is infected, Charnock-Jones explains.

The negative result is important because it demonstrates the challenges that research on the microbiome faces, says Eran Elinav, immunologist and microbiome specialist at the Weizmann Scientific Institute in Rehovot, Israel. Research on the microbiome has begun and focuses on dense microbial communities in the intestine, harboring billions of bacteria. However, when researchers are looking at smaller ecosystems, it becomes difficult to distinguish between truly indigenous colonies and contamination. "I really appreciate the efforts of researchers to use the best available tools we have today to answer this very basic question," Elinav said.

But Kjersti Aagaard, a specialist in maternal fetal medicine at Baylor College of Medicine and Texas Children's Hospital in Houston, was not impressed by the latest result. Aagaard, who led the 2014 study on the discovery of a placental microbiome, says many of the signals that the experiment called "contamination" are actually evidence of the placental microbiome. For example, it does not accept the researchers' choice to ignore some microbes found on the placenta simply because they overlap those found in the bad.

Aagaard continues to look for ways to detect these bacterial signals and understand the role that microbes might play in the placenta. According to her, one of the hypotheses is that placental microbes prevent the colonization of the fetus by dangerous bacteria.

Although Nicola Segata, a computer biologist from the University of Trento in Italy, finds the latest convincing negative results, it will be difficult to collect definitive evidence of a sterile uterus. Collection of the placenta or amniotic fluid after birth provides a window into the possible formation of the baby's microbiome, whereas direct observation methods – testing the gut of the fetus itself – would be both contrary to ethics and techniques.