Link between placental microbiome and pre-term birth

According to a recently published study in Science translation medicine, the placental microbiome might be associated with pre-term birth less than 37 weeks, and a remote history of antenatal infection in the first trimester. In this study, which was based on placental collection from 320 subjects, the investigators analyzed the human placental microbiome and provided details on microorganisms present, their probable functions and overall structure of placental microbial community.

Due to recent advances in sequencing technologies, a large number of studies have looked at the structure, function and diversity of human microbiome across multiple body sites in both healthy and disease states. Studies conducted on newborn babies have shown that the gut microbiome of neonates show complex microbial communities in the first few weeks of life. However, the source of these microbes and the time at which the infants acquire those is not yet known. It has also been shown that gut microbiome of full-term infants is different from that of low birth weight infants during the first few weeks of life, indicating that the infants might acquire these microbes in utero through a common source and that the composition of the microbiome might change with length of gestation. The authors of this study hypothesized that the human placenta could be that source.

The investigators of this study used whole-genome shotgun (WGS), a technique involving sequencing long strands of DNA, to determine the taxonomic classification of placental microbiome. They noted that several species of oral microbiome were detected in the placenta. These included Prevotella tannerae and non-pathogenic Neisseria species. They also found high abundance of E. coli in most individuals. The authors further compared the microbial community observed in placenta with those reported from other body sites (oral, stool, skin, nasal and vaginal) from non-pregnant healthy subjects using publicly available HMP (Human Microbiome Project) data. They observed that placental microbiota showed similarity to oral phyla only. Furthermore, the relative abundance of metabolic pathways was different in placenta when compared to other body sites. For example, the metabolism of co-factors and vitamins showed a greater relative abundance in placental functional gene profile.

The authors found a statistically significantly association between placental microbiome and pre-term birth less than 37 weeks. The researchers also identified various placental taxa whose abundance was enriched or diminished among women who experienced pre-term births. They also observed a statistically significant association between placental microbial community and a remote history of antepartum infection. Streptococcus and Acinetobacter were found to be enriched in women with a remote history of antepartum infection.

In conclusion, Aagaard and colleagues show that in contrast to the widespread believe that intra-uterine environment is sterile, human placenta harbors a variety of non-pathogenic commensal species, and the overall microbiome profile of placenta is most similar to that of non-pregnant human oral cavity. They also observed correlation between placental microbiome and pre-term birth and a history of antenatal infection.

The authors also noted several limitations of the study. The authors compared the placental microbiome with microbiome of other body sites from non-pregnant subjects. Moreover, the authors were not able to characterize the placental microbiome in early preterm gestation in women who had a full term delivery. Still, the study presents some interesting findings and indicates towards an essential role of placental microbiota in human pregnancy.