Summary: Microbes present in the placenta may influence the developing fetal immune system.
Source: Baylor College of Medicine
Researchers at Baylor College of Medicine previously reported that the placenta contains a sparse but detectable community of microorganisms. They and other investigators hypothesize these microbes could contribute to important pregnancy functions, including immune development.
“There has been ongoing discussion about our findings and those of other groups regarding microbes in the placenta,” said senior author Dr. Kjersti Aagaard, professor and the Henry and Emma Meyer Chair of Obstetrics and Gynecology at Baylor. “Because this is a low-biomass community, a reasonable question is how much of the signal we detect truly represents bacterial presence versus environmental contamination or maternal blood.”
Visual confirmation
“Earlier studies identified bacterial signatures using metagenomic sequencing. In this study, we validated that signal visually by labeling bacterial RNA with a fluorescent probe and directly observing it,” said Dr. Maxim Seferovic, instructor in Obstetrics and Gynecology at Baylor and lead author. “By applying a sensitive new imaging approach, we increased specificity for bacterial rRNA and could see microbes within the placental microarchitecture.”
Investigators examined placentas from term and preterm deliveries using a signal-amplified 16S universal in situ hybridization probe targeting bacterial rRNA, together with established histologic methods. The study was carefully designed to minimize contamination so that the small number of bacteria observed could be confidently assigned to specific placental sites.
“We did not find differences in the overall abundance or numerical presence of microbes between preterm and full-term births, nor did we see them concentrated in entirely different tissue layers,” Aagaard explained. “However, the types of bacteria present varied between preterm and term placentas, which aligns with our earlier findings and those from other groups.”
A sparse, distinct community
The team aimed to confirm whether previous reports accurately identified a low-biomass microbial community that could be distinguished from background contamination. Their combined imaging and sequencing evidence, together with results from other laboratories, strengthens confidence that microbes can be both sequenced and visually localized to consistent placental locations.
Seferovic and Aagaard note that these observations encourage researchers to investigate how intrauterine microbes might shape fetal immune development, and whether factors such as maternal diet or preterm birth influence that process.
“Humans eventually acquire trillions of microbes without triggering a destructive immune response,” Aagaard said. “We speculate that these low-abundance communities in the placenta may help educate the fetal immune system — teaching it which microbes are benign or beneficial and which could be harmful.”
Both investigators acknowledge that much work remains to understand the developing microbiome and its role in early immune programming. They hope the imaging approaches and tools refined in this study will support other researchers working with difficult, low-biomass samples.
Contributors to the study include Dr. Ryan M. Pace, Dr. Matthew Carroll, Benjamin Belfort, Angela M. Major, Dr. Derrick M. Chu, Dr. Diana A. Racusin, Dr. Eumenia C. C. Castro, Dr. Kenneth L. Muldrew and Dr. James Versalovic, all from Baylor College of Medicine. Aagaard also holds professorships in the Departments of Molecular and Human Genetics, Molecular and Cellular Biology, and Molecular Physiology and Biophysics.
Funding: This work was supported by the March of Dimes Preterm Birth Research Initiative, the Burroughs Wellcome Fund Preterm Birth Initiative, and the NIH (grants 1R01NR014792, 6R01DK089201, R01HD091731, NICHD N01-HD-80020 NCS Formative Research).
Source:
Baylor College of Medicine
Media Contacts:
Graciela Gutierrez – Baylor College of Medicine
Image Source:
The image is in the public domain.
Original Research: Closed access
“Visualization of microbes by 16S in situ hybridization in term and preterm placentae without intraamniotic infection.” Kjersti Aagaard et al. American Journal of Obstetrics & Gynecology. doi: 10.1016/j.ajog.2019.04.036
Abstract
Visualization of microbes by 16S in situ hybridization in term and preterm placentae without intraamniotic infection
Background
Histologic studies have reported bacteria in placental membranes and the basal plate decidua without associated inflammation, and metagenomic analyses have reported altered taxonomic profiles linked to spontaneous preterm birth. This study aimed to localize intact bacterial cells using molecular histology within placental microanatomy.
Objective
The study used a signal-amplified 16S in situ hybridization probe set for bacterial rRNA alongside Warthin–Starry and Gram stains and clinical culture methods. It also compared 16S sequencing profiles against germ-free mouse samples and extraction and amplicon contamination controls.
Study Design
Placentas were collected from 53 subjects: term labored (n = 4), term unlabored cesarean (n = 22), preterm vaginal (n = 18), and preterm cesarean (n = 8). One placenta with clinical chorioamnionitis served as a positive control. The preterm group included spontaneous preterm births with and without premature rupture of membranes, and medically indicated preterm births. In situ hybridization targeted conserved regions of the bacterial 16S ribosome with branched DNA probes to amplify signal. Extracted nucleic acids were sequenced (16S V4) with environmental and kit contaminant controls; a subset of term, unlabored cesarean samples also underwent clinical culture.
Results
Molecular in situ hybridization allowed visualization and localization of low-abundance microbes after systematic high-power scanning. Even without clinical or histologic chorioamnionitis in 52 of 53 subjects, 16S rRNA signal was observed in many spontaneous preterm placentas (13 of 16) and in term unlabored cesarean placentas (18 of 22), a difference that was not statistically significant. Signals localized mainly to the villous parenchyma and syncytiotrophoblast, and less often to the chorion and maternal intervillous space. Visualization occurred despite negative results by routine culture, hematoxylin–eosin, and Gram stains in most cases. Taxonomic analyses distinguished placental 16S profiles from negative and contamination controls, and contaminant-filtered data revealed distinct taxonomic compositions between term and preterm cohorts.
Conclusion
Intact placental microbes can be visualized as low-abundance, sparse populations within the placenta regardless of gestational age or delivery mode. Their taxonomic profiles differ from contamination controls, supporting previous metagenomic findings of low-biomass placental microbial communities.