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Over the last decade, as more expecting mothers have re-embraced births with limited medical intervention, eating the placenta has also become a growing trend among new parents.
But a recent study from Cambridge University has touched off a debate that has deeply divided researchers in recent years — is the placenta germ-free (and by extension, safe to eat raw, cooked or in pill form)?
The placenta is a temporary organ, generated in the womb of a pregnant woman. It’s thought to act like a police checkpoint at the foot of a bridge — allowing certain foods and nutrients to pass between a mother’s blood and the amniotic sac protecting her fetus. Health websites float the idea that people who consume this organ — placentophagers as they’re called — have better energy levels, improved breast milk production and faster recovery from pregnancy. (Those sites also falsely claim the practice hails from indigenous communities.)
Placenta academics have fallen into two camps: “yes, placentas have a microbiome” — a community of bacteria, viruses, fungi or parasites — and “no, they do not.”
The new study, published July 31 in Nature, seems to offer a comprehensive look at the genetics of the placenta and its native microbes. Through rigorous process of elimination, it offers strong support for the position that the womb and its placenta are sterile, pushing back against findings from a community of researchers who say they’ve indeed discovered a microbiome in placentas.
Each camp, as you might imagine, has unflattering views of each other. There’s enough drama to rival a finale of “The Bachelorette,” and this new study didn’t make things better. During my interviews, one side accused the other of being “naive and ignorant” about the basics of biology. The other side suggested the first was trying to pull off an “Elizabeth-Holmes-Theranos” style con.
Finding two scientists who disagree on an unsettled topic can be as easy as finding two pennies in a fountain. But the reason the debate is so important is that it can help explain how a newborn immune system develops. Does a fetus start as a fresh slate that picks up beneficial or malevolent germs upon birth? Does a C-section disadvantage a newborn because it doesn’t encounter the microbes in the birth canal?
Some of the findings could also help explain why mothers and babies sometimes undergo serious conditions like preeclampsia or premature birth.
Let’s start by winding back the clock.
Doctors and scientists, by and large, had assumed since 1885 that the womb and its placenta were sterile, thanks to a study by German-Austrian pediatrician Theodor Escherich on meconium — the name for an infant’s first stool. Newborns make this poo while in the womb and release it, usually, after birth. When Escherich tossed this stool under a microscope, he couldn’t find any viable bacteria.
For more than a century afterward, microbiologists tried to grow bacteria from placenta and other material in the womb (like the amniotic fluid), and more than 90 percent of the time, they couldn’t find the microbes that would allow that to happen. Adding to the sterile theory, doctors were dealing with separate medical cases of severe combined immunodeficiency — “bubble boy” or “bubble baby” disease — which require that children be raised in sterile environments from birth.
Doctors assumed the placenta or womb must be germ-free, otherwise their utero “bubbles” wouldn’t remain sterile. Given these cases and similar research in rodents, many scientists argued that the placenta serves as a shield — a buffer zone that keeps bacteria in a mother’s bloodstream from filtering into the womb.
Basic diagram of a fetus and its placenta. Image by vecton/via Adobe Stock
But those sentiments changed in 2014, when a team at the Baylor College of Medicine and Texas Children’s Hospital published a study entitled “The Placenta Harbors a Unique Microbiome.”
The study, which examined the placentas of 320 pregnant mothers after birth, used a technique called metagenomics. Metagenomics is a new-wave approach that allows a geneticist to comb through the species that might live in a location based on the scraps of DNA found there.
Metagenomics is most commonly used to study what’s living in a person’s gut, but it can also reveal other microbial communities — like those in polluted waterways or those in the diets of marine mammals when they drop feces in seawater.
Metagenomics is thought to be a cutting-edge and highly sensitive way to spot bacteria because it can identify species using mere fragments of DNA. When the Baylor team ran the placentas through their metagenomic analysis, they found consistent — though not very abundant — signs of bacteria.
Dozens of studies from other labs followed suit, and thus the “yes, the placenta has a microbiome” camp grew larger.
“This is a vibrant field. This isn’t one lab in Texas who figured this out,” said Dr. Kjersti Aagaard, an obstetrician and geneticist who led the 2014 Baylor study. She said there were “hundreds of experiments where people thought diligently about controls,” though a search in Pubmed for “placenta microbiome” suggests the number of studies published since 2014 is around 100.
The new study from Cambridge University argues that all of those experiments didn’t find a placenta microbiome. They argue the previously discovered bacteria in the placenta came from other sources, namely contamination. And the Cambridge team set out to track the bacteria’s origins to those sources.
The Cambridge study, simply put, attempted to replicate the research conducted by Baylor College of Medicine and other labs with additional layers of rigor.
“We already had a large study underway here, in which we collected samples of tissue and blood from about 4,500 women throughout pregnancy,” said Steve Charnock-Jones, a reproductive biologist at Cambridge University.
They expanded this initiative to start looking for bacteria in placentas of 478 women. About half of the women experienced a birth complication — either pre-eclampsia or newborns who were born small for their gestational age. The other half acted as a control group — they had healthy pregnancies, but in categories like age, weight and behavioral habits like smoking, they matched the women in the group who had had complications.
Like the Baylor group, the Cambridge team relied on two types of metagenomics — “deep metagenomic sequencing of total DNA” and “16S rRNA gene amplicon sequencing.” The first, as its name suggests, thoroughly combs through the placenta DNA. It literally reads each DNA sample hundreds of millions of times to look for any species it can find. The second method — 16S rRNA — conducts thousands of “reads” and only searches for signs of bacteria in a sample.
All of those maneuvers are standard, but the Cambridge team threw an unconventional twist into their experiments to test whether the placenta was germ-free. They added bacteria to their placenta samples to get a better idea of just how much truly existed (or didn’t).
Charnock-Jones said the team added “about 1,000 identical colonies of a known bacteria species” to each sample. Those colonies acted as a reference point.
Here’s how that works. Let’s say each bacteria species colony is named Ted. When added to the placenta, the sample gives a metagenomic readout that says “1,000 Teds are here.” The team can now compare the size of this readout against any of bacteria naturally present in the placenta samples (or the lack thereof).
Using the example above, if the readout for E. coli was half the number of the Teds, then the team could estimate about 500 E. coli colonies were in the placenta sample.
In early trials, Charnock-Jones and his colleagues detected various types of bacteria in the placenta samples — but then they noticed something odd.
They found E. coli in 64 separate placentas — but all of the readouts seemed to come from a single strain of the gut bacteria. There are hundreds if not thousands of known strains of E. coli, so the odds that 64 pregnancies would all have the same strain are very small.
So what happened?
“The key word here would be contamination,” Charnock-Jones said. Thanks to a careful accounting of the placenta’s surroundings and laboratory equipment, the team discovered that once a woman’s labor and delivery starts, the placenta begins to collect bacteria from its surrounding environment, but not from the womb itself.
Microbes in the vagina likely infiltrate the placenta during delivery, the team found, because vaginal births across their cohort of 478 pregnancies had a larger abundance of these germs versus C-section births. So they worked backwards to try to find other sources.
They learned that merely prepping the placentas for examination in the lab introduced environmental microbes like Deinococcus geothermalis, a germ commonly found on metal surfaces. When the team examined the chemicals they used to conduct DNA analysis, they discovered microbes as well — meaning that commercially available lab kits come unintentionally pre-contaminated with bacteria that had gotten into their placenta samples.
“It was really a very impressive bunch of detective work,” said Frederic Bushman, a microbiologist at University of Pennsylvania’s Perelman School of Medicine, who wasn’t involved in the study. “They were absolutely artistic at figuring out the sources of different kinds of contamination.”
When they were done, only one species — Streptococcus agalactiae — qualified as a microbe that was naturally present in the placenta prior to birth. If you’ve been pregnant and tested for strep B, Streptococcus agalactiae is the reason why. Streptococcus agalactiae has been implicated in sepsis in newborns, which occurs in 1 to 2 infants per 1,000 births.
“Now that’s a real pathogen, but we only saw it in about 5 percent of the [478 placenta] samples, and we only detected very low numbers of this bacteria,” Charnock-Jones said.
The takeaway from the Cambridge study is that the placenta appears to keep zero microbes — except in rare occasions where it has a small abundance of germs, which is dubbed as being “low biomass.”
“This new British paper is just extremely definitive,” Bushman said. “The evidence is pretty strong for the sterile womb hypothesis.”
There are a few things to consider when weighing both teams’ findings.
“‘Microbiome’ usually means a population of bacteria that are alive and interacting with their environment,” Bushman said. But neither the Cambridge team nor the Baylor team side actually extracted the bacteria from their placenta samples to see what was alive and capable of growth.
Aagaard from Baylor — whose 2014 study opened this debate — said her research never used the term “placental microbiome.” (The title of Aagaard’s 2014 paper is “The Placenta Harbors a Unique Microbiome.”) She said her team was merely surveying all the genetic material that could be found in a placenta, and was not necessarily identifying what was colonized there.
There’s also a distinction in what we’re actually calling the “placenta,” Aagaard said. The Cambridge research didn’t examine the entire placenta. Instead, the British group looked at a portion of the organ called the terminal villi. These are the closest points of contact between the fetus and the mother’s blood system and tissue — which the researchers wrote “would be the most plausible site for bacteria to be found.”
That’s “sampling a subfraction of what the rest of us have done,” to study the placenta as a whole, Aagaard said.
Collecting a more comprehensive range of the placenta, however, increases the odds of extracting tissue and microbes from adjacent organs like the uterus. The Cambridge team would argue those uterine microbes contaminate the placenta after birth, but Aagaard said the two organs intermingle too closely to ever distinguish them.
The uterus “is a physically adjacent intimate environment that the placenta embeds into,” Aagaard said. “So why would we disregard all of those microbes and say they must have come as a ‘contamination’ from vaginal delivery?”
It’s hard to say based on the evidence presented in both studies.
There are some questions in human biology that are incredibly difficult to answer without being able to hop into a micro-sized submarine and directly observe what happens at the scene. Alzheimer’s disease, for example, likely involves hundreds of different things going wrong over a period of decades.
The Cambridge study suggests the placenta has essentially no microbiome — at least in healthy women or those who experience pre-eclampsia and babies with low birth weights. Aagaard’s work has focused on women who had infections during pregnancy — like urinary tract infections — or had a premature birth. Maybe this difference in subjects drove the difference in their findings.
If the placenta and womb harbored microbes, as Aagaard’s study suggests, then kids who are severely immunocompromised probably wouldn’t be able to survive pregnancy. Along those lines, sterile placentas are found in other mammals — and scientists use tissue from microbeless rodent wombs to study immunodeficiency disease.
Aagaard said those rodent models use chemical sterilizing agents that strip the womb of microbes, and wouldn’t comment on specific instances of bubble baby disease, given one of the most prominent cases was born at Texas Children’s Hospital where she works.
According to Aagaard, other scientists had research to support her side on the placenta microbiome and her assertions about the rodent models, namely microbiologists Andrew Onderdonk at Brigham and Women’s Hospital and James Fox at MIT. Onderdonk did not respond to interview requests, and Fox declined.
A human placenta and umbilical cord are steamed prior to dehydration February 9, 2015, in Washington, DC. The placenta is dehydrated and ground to a powder which is then put into a pill capsule for a new mother to consume, hoping to increase their milk production or reduce the postpartum depression. Photo by Raphaelle Picard/AFP/Getty Images
So at the end of the day, consuming the placenta remains somewhat a matter of personal preference. If you feel stronger postpartum after noshing on placenta, then by all means do what you want.
But as of 2017, there was no evidence that placentophagers — the name for people who choose to eat their placenta — gain a clinical or health advantage from doing so. Also remember, the Cambridge study found 5 percent of placentas contain Streptococcus agalactiae, which is symbiotic most of the time with humans.
“Many people carry it, and they’re unaffected,” Charnock-Jones said. But there is at least one case of an infant becoming severely ill with Streptococcus agalactiae after their mother consumed placenta pills. It is also worth noting that cooking and prepping the placenta for pills may not neutralize viruses like HIV, hepatitis and Zika.
A placenta from a C-section will be much closer to being sterile, Charnock-Jones said. And as the Cambridge study tries to make clear, “if you go with a vaginal delivery, there are bacteria from the vaginal microbiome in the placenta.”
Nsikan Akpan is the digital science producer for PBS NewsHour and co-creator of the award-winning, NewsHour digital series ScienceScope.
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