Claims of bacteria in babies before birth rejected in UCC-led study

Experts find flaws in studies suggesting existence of foetal microbiome

Scientific claims that babies harbour live bacteria while still in the womb are inaccurate and may have impeded progress in medical research, according to an investigation led by scientists at University College Cork.

Their verdict was published on Wednesday in the leading science journal Nature.

Earlier claims that the human placenta and amniotic fluid are normally colonised by bacteria would, if true, have serious implications for clinical medicine and paediatrics and would undermine established principles in immunology and reproductive biology, according to Prof Jens Walter, principal investigator at APC Microbiome Ireland, based at UCC.

He assembled a trans-disciplinary team from around the world of 46 leading specialists in reproductive biology, microbiome science and immunology to evaluate the evidence for microbes in human foetuses.


The team unanimously refuted the concept of a foetal microbiome and concluded the detection of microbiomes in foetal tissues was due to contamination of samples drawn from the womb. Contamination occurred during vaginal delivery, clinical procedures or during laboratory analysis, they concluded.

If neonates were proven to harbour live microbes, “it would require a radical revision of our understanding of the development of the immune system and other systems in early life”, their paper notes.

The human microbiome is a vast network of hundreds if not thousands of species of microbes that live in the body, notably in the gut.

Knowing that the foetus is in a sterile environment confirms that colonisation by bacteria happens during birth and in early postnatal life

—  Prof Jens Walter, APC Microbiome Ireland

There is increasing recognition that interactions between these bacterial and viral species may contribute not only to physical health – or illnesses – but also to mental wellbeing.

In their report in Nature, the experts encourage researchers to focus their studies on the microbiomes of mothers and their newborn infants, and on the microbial metabolites crossing the placenta which prepare the foetus for postnatal life in a microbial world.

Prof Walter added: “This consensus provides guidance for the field to move forward, to concentrate research efforts where they will be most effective. Knowing that the foetus is in a sterile environment confirms that colonisation by bacteria happens during birth and in early postnatal life, which is where therapeutic research on modulation of the microbiome should be focused.”

The authors also provide guidance on how scientists in the future can avoid pitfalls of contamination in the analysis of other samples where microbes are expected to be absent or present at low levels, such as internal organs and tissues within the human body.

APC Microbiome Ireland is a Science Foundation Ireland research centre which explores the role the microbiome plays in health and disease. The microbiome is a target for the prevention and treatment of disease, and is also a source of functional food ingredients, new drugs and disease biomarkers.

Kevin O'Sullivan

Kevin O'Sullivan

Kevin O'Sullivan is Environment and Science Editor and former editor of The Irish Times