No evidence for a placental microbiome in human pregnancies at term.

American journal of obstetrics and gynecology (2020-09-02)
Irene Sterpu, Emma Fransson, Luisa W Hugerth, Juan Du, Marcela Pereira, Liqin Cheng, Sebastian Alexandru Radu, Lorena Calderón-Pérez, Yinghua Zha, Pia Angelidou, Alexandra Pennhag, Fredrik Boulund, Annika Scheynius, Lars Engstrand, Eva Wiberg-Itzel, Ina Schuppe-Koistinen
ABSTRACT

The placenta plays 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. This study aimed to investigate the existence and origin of a placental microbiome. A cross-sectional study comparing samples (3 layers of placental tissue, amniotic fluid, vernix caseosa, and saliva, vaginal, and rectal samples) from 2 groups of full-term births: 50 women not in labor with elective cesarean deliveries and 26 with vaginal deliveries. The comparisons were performed using polymerase chain reaction amplification and DNA sequencing techniques and bacterial culture experiments. There were no significant differences regarding background characteristics between women who delivered by elective cesarean and those who delivered vaginally. Quantitative measurements of bacterial content in all 3 placental layers (quantitative polymerase chain reaction of the 16S ribosomal RNA gene) did not show any significant difference among any of the sample types and the negative controls. Here, 16S ribosomal RNA gene sequencing of the maternal side of the placenta could not differentiate between bacteria in the placental tissue and contamination of the laboratory reagents with bacterial DNA. Probe-specific quantitative polymerase chain reaction for bacterial taxa suspected to be present in the placenta could not detect any statistically significant difference between the 2 groups. In bacterial cultures, substantially more bacteria were observed in the placenta layers from vaginal deliveries than those from cesarean deliveries. In addition, 16S ribosomal RNA gene sequencing of 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. We found no evidence to support the existence of a placental microbiome in our study of 76 term pregnancies, which used polymerase chain reaction 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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Water, Nuclease-Free Water, for Molecular Biology
Sigma-Aldrich
LB Broth (Miller), Highly-referenced nutrient-rich microbial growth powder medium, suitable for regular E.coli culture
Sigma-Aldrich
Gentamicin solution, 10 mg/mL in deionized water, liquid, 0.1 μm filtered, BioReagent, suitable for cell culture
Sigma-Aldrich
Fluorescein (free acid), Dye content 95 %