Placental Microbiome: a Paradigm Shift or Flaw in the Methodology?

Abstract

Rapid development of high-throughput DNA sequencing technologies and bioinformatics methods, together with a substantial reduction of their cost, have provided tremendous opportunities for studying the human microbiome. In recent years, much attention has been paid to studies of the microbiome of the upper reproductive tract of woman and the fetoplacental system, which have traditionally been considered sterile. Obtaining irrefutable evidence of the existence of the placental microbiome would enable us to believe with a high degree of certainty that microorganisms colonize the fetus already in the womb, which would have far-reaching consequences not only for medicine, but also for basic biology. This issue triggered a heated discussion among microbiologists, molecular biologists, obstetricians, and neonatologists. In the past few years, a number of studies have been published, both refuting and confirming the dogma, accepted for many decades, that the placenta and fetus are sterile during a healthy pregnancy. This literature review is a critical analysis of the results of studies into the placental microbiome. It provides arguments both for supporters of the hypothesis of the resident microbiota of the placenta and their opponents. Particular attention is paid to the methodological requirements for molecular studies of biological material with low microbial biomass, compliance with which is crucial for obtaining reliable results.