No evidence for a placental microbiome in human pregnancies at term

Abstract BACKGROUND The placenta has an important role in the modulation of pregnancy immunity; however, there is no consensus regarding the existence of a placental microbiome in healthy full-term pregnancies. OBJECTIVE To investigate the existence and origin of a placental microbiome. STUDY DESIGN A cross-sectional study comparing samples (three layers of placental tissue, amniotic fluid, vernix as well as saliva, vaginal and rectal samples) from two groups of full-term births: 50 women not in labor with elective cesarean deliveries and 26 with vaginal deliveries. The comparisons were performed using PCR amplification and DNA sequencing techniques and bacterial culture experiments. RESULTS There were no significant differences regarding background characteristics between the women who delivered by elective cesarean and those who delivered vaginally. Quantitative measurements of bacterial content in all three placental layers (qPCR of the 16S rRNA gene) did not show any significant difference between any of the sample types and the negative controls. 16S rRNA gene sequencing of the maternal side of the placenta could not differentiate between bacteria in the placental tissue and contamination of laboratory reagents with bacterial DNA. Probe-specific qPCR for bacterial taxa suspected to be present in the placenta could not detect any statistically significant difference between the two groups. In bacterial cultures, significantly more bacteria were observed in placenta layers from vaginal deliveries than cesarean deliveries. 16S rRNA gene sequencing of the bacterial colonies revealed that most of the bacteria that grew on the plates were genera typically found in human skin; moreover, it revealed that placentas delivered vaginally contained a high prevalence of common vaginal bacteria. Bacterial growth inhibition experiments indicated that placental tissue may facilitate the inhibition of bacterial growth. CONCLUSIONS We found no evidence to support the existence of a placental microbiome in our study of 76 term pregnancies, which used PCR amplification and sequencing techniques and bacterial culture experiments. Incidental findings of bacterial species could be due to contamination or to low-grade bacterial presence in some locations; such bacteria do not represent a placental microbiome per se.