Deep microbial analysis of multiple placentas shows no evidence for a placental microbiome

Objectives: To resolve the controversy regarding the presence of a microbiota in the placenta. Design: Classical and molecular microbiological study. Setting: All samples were collected during caesarean section. Population: A total of 28 human placentas and six murine placentas. Methods: All 28 human placentas were checked for 16S rRNA gene amplification products. Three locations from four selected human placentas and three ‘environmental controls’ for each placenta were placed in seven culture media. The four selected human placentas were further analysed using Gram stain, immunohistochemistry for bacteria, electron microscopy, and TaqMan RT-qPCR. Six placentas from three SPF mice were cut into four pieces each, and further analysed for 16S rRNA gene amplification. Main outcome measures: Microbiological and molecular evidence of bacteria. Results: None of the placental cultures used for the full analysis, or their environmental cultures, was positive for bacterial growth. None of the other methods showed any evidence of bacteria. Immunohistochemistry showed negligible bacterial counts. None of the murine placentas showed evidence of 16S rRNA gene amplification. Conclusions: Our results support that the fetal environment in the womb is sterile. Based on the immunohistochemistry and the limit of detection of the other methods used, if a placental microbiome exists, it is of extreme low biomass, and thus its effect on clinical phenotypes is probably minor, if it exists at all. Tweetable abstract: Using several microbiological and molecular methods in parallel, we found no compelling evidence of bacteria in human and mouse placentas.