The prevailing paradigm in obstetrics has been the sterile womb hypothesis. However, some investigators have recently asserted that the placenta, intra-amniotic environment, and fetus harbor resident microbial communities. The primary focus of the debate is that most studies proposing the existence of placental and fetal microbiota in humans and animal models have relied heavily (if not exclusively) on DNA sequencing techniques, and the bacterial signals identified in these studies may actually be background DNA contaminants from extraction kits, PCR and sequencing reagents, and general laboratory environments.
PRB researchers determined whether the placental and fetal tissues of rhesus macaques obtained from cesarean deliveries without labor harbor bacterial communities by using bacterial culture, quantitative real-time PCR, and 16S rRNA gene sequencing, and by comparing the bacterial profiles of these tissues to those of background technical controls. Cultures of fetal and placental tissues did not yield bacteria. Furthermore, the bacterial burden and profiles of fetal and placental tissues did not exceed or differ from those of background technical controls. By contrast, the bacterial burden and profiles of low microbial biomass positive controls (i.e. urine) did exceed and differ from those of background technical controls. Among the rhesus macaque samples, distinct microbial signals were limited to the uterine wall. Therefore, using multiple modes of microbiologic inquiry, there was no consistent evidence of bacterial communities inhabiting the placental or fetal tissues of rhesus macaques. This study provides further evidence against the in utero colonization hypothesis and the existence of a placental microbiota. If there are intrauterine bacterial communities, they are limited to the uterine wall.