New Study Casts Light on the Mystery of Placental Bacteria

It is only recently that we have begun the process of understanding the beneficial role of healthy bacteria in human health and development. However, microbes have gone from being so small as to be practically invisible to us to being a staple of the wellness industry – think probiotic yoghurts, kefir, kombucha, kimchi and countless supplements – a pioneering area of research into mental illness and a key component to our understanding of the development of the human immune system.

One further new frontier is the understanding of the role of health-giving microbes during pregnancy, childbirth and the early years. Previous research appeared to confirm that the placenta played a vital role in providing newborns with the healthy bacteria they need. After all, it would be a natural place to start; we already know that the placenta provides many of life’s essential building blocks: food, oxygen and antibodies.

However, a new study has found that the placenta most likely has little if anything to do with equipping baby with the microbes it needs for its development. In fact, the study, which was carried out by scientists at the University of Cambridge and reported in the journal Nature, found that the placenta hadn’t any bacteria at all.

The study seems to disprove a 2014 study by Kjersti Aagaard from Baylor College of Medicine which claimed to document a “unique placental microbiome”; a confident assertion that now seems to have stemmed from that most basic of scientific errors: laboratory contamination. Many specialists now agree that the bacterial DNA Aagaard recorded came from microbes living in the wider world, which had contaminated equipment and samples. Four subsequent studies that tried to account for these contaminants, in the end, found zero evidence of a placental microbiome.

The latest study seems to have legs: it is the largest of its kind yet and analysed the placentas of 500 women shortly after childbirth. It was unambiguous in its finding: healthy placentas have no bacteria.

The study was meticulous in its approach, looking at placental microbial populations using two DNA sequencing methods.

Although the healthy placentas did show signs of microbes, these were more diverse and more populous in women who gave birth vaginally rather than by caesarean section. This led to the obvious conclusion: the bacteria are acquired vaginally, rather than rather pre-existing in the placenta before birth.

There was a strain of bacterium present in some placentas pre-birth, but the particular strain – streptococcus agalactiae – was in just 5% of the samples and is a sign of infection and can result in sepsis and neonatal death.

Kjersti Aagaard, the scientist from Baylor College of Medicine and Texas Children’s Hospital in Houston who led the 2014 study, said there were a number of problems with the latest study. For example, she claimed that it ignored many microbes found on the placenta simply because they are the same as those found in the vagina.

The Cambridge University team, however, stand by their results. “We spent a very long time thinking about how to remove and identify every source of contamination,” commented Julian Parkhill, one of the study’s leaders. “When we did that, we realized there was nothing left.”

Furthermore, the Cambridge researchers have received the endorsement of many of their peers. “This study, in my view, settles the matter,” said Marie-Claire Arrieta from the University of Calgary.