The study investigates the unique microbiome of the placenta, a physiological niche that has been underexplored despite recent findings of intracellular bacteria with diverse metabolic and immune regulatory functions. Using a population-based cohort of 320 subjects, the researchers conducted 16S ribosomal DNA-based and whole-genome shotgun (WGS) metagenomic studies to characterize the placental microbiome. They found that the placental microbiome is composed of nonpathogenic commensal bacteria from the Firmicutes, Tenericutes, Proteobacteria, Bacteroidetes, and Fusobacteria phyla. The microbiome was most similar to the human oral microbiome, with a Bray-Curtis dissimilarity index of <0.3. The study also revealed associations between the placental microbiome and a remote history of antenatal infection and preterm birth. The findings suggest that the placental microbiome may be established by hematogenous spread from the oral cavity, and that periodontal disease could potentially influence the placental microbiome and increase the risk of preterm birth.The study investigates the unique microbiome of the placenta, a physiological niche that has been underexplored despite recent findings of intracellular bacteria with diverse metabolic and immune regulatory functions. Using a population-based cohort of 320 subjects, the researchers conducted 16S ribosomal DNA-based and whole-genome shotgun (WGS) metagenomic studies to characterize the placental microbiome. They found that the placental microbiome is composed of nonpathogenic commensal bacteria from the Firmicutes, Tenericutes, Proteobacteria, Bacteroidetes, and Fusobacteria phyla. The microbiome was most similar to the human oral microbiome, with a Bray-Curtis dissimilarity index of <0.3. The study also revealed associations between the placental microbiome and a remote history of antenatal infection and preterm birth. The findings suggest that the placental microbiome may be established by hematogenous spread from the oral cavity, and that periodontal disease could potentially influence the placental microbiome and increase the risk of preterm birth.