Microbes living in human placentas may impact pregnancy

The human placenta is not as sterile as once thought, but rather harbors a small but diverse group of microbes that may influence the course of pregnancy, U.S. researchers said Wednesday. The findings, published in the U.S. journal Science Translational Medicine, hinted at an association between the composition of the placental microbial community known as microbiome and preterm birth, or birth of a baby at less than 37 weeks gestational age. "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," lead author Kjersti Aagaard, associate professor of Baylor College of Medicine in Houston, Texas, said in a statement. "We reasoned that there must be another source 'seeding' the infant's gut at birth, so we sought to examine the placenta." Aagaard and her colleagues characterized the bacteria in more than 300 placentas and found out the long-held notion that the placenta is a sterile environment is wrong. "The placental microbiome is low in terms of microbial abundance but not as sterile as we previously thought," said Aagaard. Of the placentas studied, Escherichia coli (E. coli) was the species with the highest abundance in most individuals, a bacterium 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 ... is not much like the vaginal or intestinal microbiome, but rather is most similar to the oral microbiome," said Aagaard. The researchers suspected that oral microbes might slip into the mother's bloodstream and end up in placenta. 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." The mostly non-pathogenic bacteria living in the placenta have important day-to-day functions, like metabolizing vitamins at healthy levels for a developing fetus, they said. Additionally, the researchers observed differences between the placental microbiome of infants born slightly prematurely and infants born at full term, suggesting a link between bacterial composition in the placenta and preterm birth. "Exposure of the fetus to a placental microbiome may have fundamental implications for early human development and the physiology of pregnancy," said James Versalovic, co-author of the report and professor of pathology at Baylor and head of pathology at Texas Children's Hospital. They plan to further explore the connection in a future study looking at the oral and placental microbiomes of more than 500 women at risk for preterm birth. The knowledge 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.