Background. Bacterial DNA has been detected in the placenta in healthy term pregnancies as well as in pregnancy complications. We recently demonstrated that the composition of the placental microbiota closely resembles that of the maternal oral microbiota at higher taxonomic levels but that the overlap decreases substantially at family and genus level. Bacterial DNA is however no conclusive evidence for the presence of bacteria in the placenta. The aim of this study is to study the presence of bacteria in placental tissue from healthy pregnancies and pregnancy complications.
Methods. Human placental samples selected from a cohort of women with and without pregnancy complications were stained for bacteria with a Gram tissue stain. DNA was isolated from the placental biopsies and the presence of 16S rRNA was examined with PCR and Q-PCR.
Results. Gram-positive and Gram-negative bacteria were detected in all placentas independent of clinical status. Bacteria were present in villous tissue and membranes. The bacteria were localized to the syncytiotrophoblasts, the villous parenchyma and blood vessels. Only a small number of placental samples were stained (n=5 healthy controls, n=5 late-onset pre-eclampsia and n=4 gestational diabetes), and no differences in the number or pattern of bacteria were detected. Bacterial 16S rRNA expression was detected in all placental samples and did not differ between the groups.
Conclusion. The placenta is colonised by bacteria in both villi and the membranes. All placentas showed presence of bacteria. The number and pattern of bacterial colonisation was not substantially altered by pregnancy complications but could differ in bacterial species type.