The placenta is not as sterile as once thought. New research shows that the placenta harbors a unique, low abundant microbiome, said researchers from Baylor College of Medicine and Texas Children’s Hospital.
The findings, in the current issue of the journal Science Translational Medicine, provide important new insights on the structure of the placental microbial community, the organisms present, and how they might be capable of impacting a pregnancy.
“After we completed our studies of the vaginal microbiome in pregnancy, we noted that the most abundant microbes in the mom’s vagina were not what populated the baby’s intestinal microbiome,” said Dr. Kjersti Aagaard, associate professor of obstetrics and gynecology in the section of maternal fetal medicine at Baylor and the Texas Children’s Pavilion for Women, and the lead and corresponding author on the report. “We reasoned that there must be another source ‘seeding’ the infant’s gut at birth, so we sought to examine the placenta.”
The microbiome is the population of microbes – bacteria, viruses and fungi – that cohabit with human cells and help cells complete their tasks. Understanding what characterizes the microbiome communities is essential for understanding human development, Aagaard said.
Aagaard and her colleagues are key members in the collaborative National Institutes of Health funded Human Microbiome Project, which seeks to further characterize these communities and how they relate to health and healing human disease.
In this study, the first and largest study to focus on the placental microbiome, 320 human subjects’ samples were analyzed comprehensively following a process called shotgun metagenomic sequencing. This technology enables microbiologists to uniquely evaluate bacterial diversity and detect the abundance of specific microbes and all their genetic pathways in a community.
Implications for oral health
The researchers showed that the placenta is not sterile (free from bacteria or other living organisms), but rather harbors a diverse and very unique microbiome.
“The placental microbiome is low in terms of microbial abundance but not as sterile as we previously thought,” said Aagaard.
Of the samples, Escherichia coli (E. coli) was the species with the highest abundance in most individuals, a bacteria that lives in the intestines of most healthy individuals. Prevotella tannerae (gingival crevices) and non-pathogenic Neisseria species (mucosal special surfaces), both species of the oral cavity, were also detected in highest relative abundance.
“Interestingly, when we looked very thoroughly at the placenta in relation to many other sites of the body, we found that the placental microbiome does not bear many similarities to microbiomes closest in terms of anatomic location. Specifically, it is not much like the vaginal or intestinal microbiome, but rather is most similar to the oral microbiome,” said Aagaard.
The finding has important implications on the likely importance of oral health during pregnancy, she said. “It reinforces long-standing data relating periodontal disease to risk of preterm birth.”
Microbiome and preterm birth
Additionally, the researchers observed differences in the placental microbiome based on a remote history of infection during the pregnancy (most commonly urinary tract infections from many months ago that were treated successfully with antibiotics).
The placental microbiome is also unique based on whether a preterm birth occurred, the researchers found.
“Exposure of the fetus to a placental microbiome may have fundamental implications for early human development and the physiology of pregnancy,” said Dr. James Versalovic, co-author on the report, professor of pathology at Baylor and head of pathology at Texas Children’s Hospital.
A larger study is currently underway to expand these findings to describe the placental and microbiome profiles across pregnancy and in relation with preterm birth.
“The hope is that we will get a clearer picture of how several of the microbial communities in women and their placentas change over the course of the entire pregnancy among those at risk for preterm birth. These discoveries could lead to rapid breakthroughs in not only identifying women at risk for preterm birth, but developing new and worthwhile strategies to prevent preterm birth,” said Aagaard. “As we catch glimmers of the microbial biology of pregnancy, we can start to see a not too distant future where we will prevent preterm birth (or its complications in newborns) with truly novel approaches aimed at enhancing the healthy microbes of not just the vagina, but the mouth and gut. As we unravel the mysteries of pregnancy, we are learning that our microbes may be as much friend as foe. That is fantastic news for our moms and their babies.”
Additional authors include Jun Ma, Dr. Kathleen M. Antony, Dr. Radhika Ganu and Dr. Joseph Petrosino all of Baylor.
Funding for this work was provided by the National Institutes of Health Director’s New Innovator Award (to Aagaard); the Burroughs Welcome Fund Preterm Birth Initiative (to Aagaard and Versalovic); the National Institutes of Health’s National Human Genome Research Institute as well as the Director’s Common Fund (to Aagaard, Versalovic, and Petrosino); and the Alkek Center of Metagenomics and Microbiome Research (led by Petrosino with funding awards to Ganu and Aagaard).