The widely accepted dogma of intrauterine sterility and initial colonisation of the newborn during birth has been blurred by recent observations of microbial presence in meconium, placenta and amniotic fluid. Given the importance of a maternal-derived in utero infant seeding, it is crucial to exclude potential environmental or procedural contaminations, and to assess foetal colonisation before parturition. To ascertain antenatal microbial colonisation in mammals, we analysed sterilely collected intestinal tissues from rodent foetuses in parallel with experimental controls, and tissues from autoptic human foetuses. Next generation sequencing (NGS) showed the presence of pioneer microbes in both rat and human intestines, as well as in rodent placentas and amniotic fluids. Live microbes were isolated from culture-dependent analyses from homogenized rat foetal intestines. Microbial communities showed foetus- and dam-dependent clustering, confirming the high inter-individual variability of microbiota even in the antenatal period. Fluorescent in situ hybridisation analysis confirmed the microbes existence in the lumen of the developing gut. These findings have vast implications for an emerging field of enhancing the management of healthy pregnancies, and for understanding how the infant microbiome starts and it is thus shaped.

Antenatal microbial colonisation of mammalian gut