The Microbiome Signature of the Placenta and its Role in Spontaneous Preterm Birth: A Systematic Review and 16S rRNA Re‐Analysis

PROBLEM: The advent of high-throughput 16S rRNA sequencing has enabled deeper insights into microbial communities associated with adverse pregnancy outcomes, including spontaneous preterm birth (sPTB). While microbial dysbiosis in the cervicovaginal and oral-gut microbiomes has been implicated in sPTB, the existence of a placental microbiome remains contentious. Traditional paradigms of a "sterile womb" have been challenged by studies suggesting a low-biomass microbial community in the placenta, though recent evidence disputes this claim, attributing findings to contamination or transient microbial DNA signals. METHOD: This study systematically reviewed placental microbiome studies employing 16S rRNA sequencing and re-analyzed publicly available datasets to determine microbial signatures in term and preterm placentas. Following a comprehensive search of three databases and stringent inclusion criteria, seven studies were included. The risk of bias was assessed using a modified Joanna-Briggs tool, revealing moderate-to-low risk across studies. Methodological heterogeneity, including differences in contamination controls, sequencing regions, and analytical platforms, was a significant limitation. RESULTS: A re-analysis of sequencing data showed no consistent microbiome signature distinguishing the term from preterm placentas. Beta diversity analysis revealed no group clustering, while alpha diversity indices showed comparable species richness. Bacterial DNA in placental tissues was primarily attributed to contamination from the urogenital tract or laboratory processes. CONCLUSION: Findings underscore the importance of robust contamination control and standardized protocols in low-biomass microbiome research. Future studies should employ advanced techniques, such as metagenomics and fluorescence in situ hybridization, to evaluate the functional relevance of microbial communities in the placenta, as well as rule out microbial DNA deposited in the placenta through circulating bacterial extracellular vesicles (EVs).