For more than a century, doctors have assumed that the fetus and the womb itself is sterile, unless something goes wrong. Not so, according to new studies which find bacteria lurk in the placenta, amniotic fluid and umbilical cord blood, making the womb a germy place. So why are we so afraid of germs for newborns?
Studies seeking to understand how microbes help to shape human health and development have become extremely popular over the past few decades, but some researchers are concerned that a crucial question -- when bacteria first colonize the body -- has not yet been answered.
Many mammals have symbiotic retroviruses which have enabled them to evolve a placenta over many generations. In order to let a fetus mature inside a mother's uterus, an animal needed a way to provide oxygen and nutrients while removing waste and keeping both blood supplies separate.
Bacteria are a normal -- perhaps even crucial -- part of pregnancy, they could have an important role in shaping the developing immune system. Scientists might be able to find ways to shift the microbial composition in the womb and possibly ward off allergies, asthma and other conditions. They might also be able to uncover microbial profiles associated with preterm birth or other complications during pregnancy, which could help to illuminate why they occur.
New studies continue to emerge revealing the role of endogenous retroviruses in the more recent evolution of humans showing that snippets of DNA are helping to blur the boundary between human and virus. Humans are, in a very real sense, part virus and immersed in microrganisms from birth.
Bacteria in and on our bodies outnumber our own cells 10 to 1. As the hunt for bacteria intensifies, researchers are starting to turn up microbes in locales that were once thought to be sacred ground: The brain is teeming with bacteria. Urine is most definitely not sterile and now, a new study, reveals that the placenta, a pancake-shaped organ that nourishes a developing fetus, is awash with microbes.
One paediatrician likens the controversy over the placental microbiome to a scientific "knife fight". But if fetal microbiomes do exist, that could have far-reaching implications not only for medicine, but also for basic biology. "If we start thinking of the placenta as a conduit or facilitator of maternal-fetal communication and not as a barrier, then I think we open ourselves up to very interesting perspectives on how we've interpreted a lot of developmental biology today," says Kjersti Aagaard, an obstetrician at Baylor College of Medicine in Houston, Texas.
The results shatter the idea that the womb, and the fetus nestled inside, is sterile. The old thinking, proposed by French pediatrician Henry Tissier over a century ago, was that the fetus, nestled in the inner sanctum of the uterus, grows germ-free. Born sterile, babies pick up bacteria only as they pass through the birth canal, are held by adoring adults and are breastfed. It also shatters the concept that a baby's immune system goes into action at birth with the assumption that the child only confronts bacteria outside of the womb leading to the flawed concept of vaccination for infants who have no immunity.
Bacteria breaks into the fetus long before the big squeeze into life on the outside. Scientists have spotted bacteria in amniotic fluid, blood in the umbilical cord, the membrane that surrounds the fetus and even babies' first poop. All of those tissues should be sterile if everything else in there is. Babies in these studies were all healthy, suggesting that these bugs aren't harmful. Instead, they're just a part of normal human development.
Bacteria-sharing from mom to youngster isn't limited to people. In cockroaches, bacteria squeeze into eggs just prior to ovulation. Stinkbugs smear their newly laid eggs with bacteria-rich feces. And mouse pups harbor particular kinds of bacteria present in their mom, even when they're born by sterile C-section. Those are just a few examples of the wide range of bacterial transfer described in a 2013 PLOS Biologyarticle by Lisa Funkhouser and Seth Bordenstein of Vanderbilt University in Nashville.
"We can no longer ignore the fact that exposure to microbes in the womb is likely and may even be a universal part of human pregnancy, serving as the first inoculation of beneficial microbes before birth," Funkhouser and Bordenstein write.
Because scientists are just starting to tally up these microbes, it's not yet clear what role they play in a newborn. A tantalizing hint comes from the new placental bacteria study: Certain mixes of bacteria were linked with a higher risk of premature birth. Of course, correlation isn't causation. Those bacteria could just be along for the ride, or a side effect of whatever caused the premature birth in the first place. But it's possible that the microbes aren't just bystanders. They could be influencing how a baby grows, in ways both good and bad.