It has long been suspected that the way we are born plays a key role in determining which microorganisms would colonize our guts. Now, these suspicions have been confirmed by the largest ever study of newborns’ gut microbiota and the impact of delivery methods.

According to the new research, babies born by Caesarean section have different gut bacteria to those born vaginally, although those differences tend to disappear by the time the children reach between 6 and 9 months of age.

Whether those early differences have further implications for health in later life is, for the moment, unknown, although C-section delivery had previously been associated with an increased risk of developing asthma and allergies, and autoimmune diseases.

In the new study, published in Nature, researchers from the Wellcome Sanger Institute, University College London (UCL) and the University of Birmingham, analyzed 1,679 gut microbiota samples from almost 600 newborns and 175 mothers. They took fecal samples from healthy babies aged 4, 7 and 21 days born in UK hospitals, 314 of whom were delivered vaginally and 282 via C-section. In addition, the study continued to follow a small subset of infants until they were a year old.

Scientists confirmed that the babies born vaginally had received most of their gut microbes from their mothers, whereas C-section babies presented bacteria usually associated with hospital environments.

The researchers also found that the babies’ gut microbiota did not seem to come from their mothers’ vaginas but, rather, from their guts. These results call into question the controversial practice of vaginal seeding, which has become increasingly popular in recent years.

Along with the delivery mode, researchers also saw that antibiotics given systematically to women before a C-section can also play a role in early gut microbiota composition in babies.

By the time the infants were weaned (between 6 and 9 months of age), the researchers observed that the differences in gut microbiota composition had mostly disappeared; the only exception was the commensal bacteria Bacteroides, which have been linked to a positive influence on the immune system and help with suppressing inflammation. Infants born by C-section present very low levels of these bacteria or no level at all. And it continues to be that way 9 months later for most of the children. At that age, even though they had been breastfed for at least 6 months, 6 out of 10 babies still had very few or no Bacteroides in their microbiota.

For the moment, the authors do not know if there are any long-term consequences to these findings. In this regard, the researchers highlight that in many cases, a Caesarean is a life-saving procedure both for a woman and her baby. More in-depth studies need to be done to get a better insight, thus the results of this work should not deter future mothers from having a C-section, with more research needed to improve understanding of this area.

The study is part of the project Baby Biome Study, which aims to follow thousands more newborns into childhood to understand the role birth mode plays in gut colonization by microbes. In the near future, the study will also consider the impact that others factors, such as antibiotics and breastfeeding, have on gut microbiota and, as a consequence, in global health during life.

Further studies will help us understand the role of gut microbiota in early life and could help us develop therapeutics to build a healthy microbiome”, said the Baby Biome Study’s principal investigator, Professor Peter Brocklehurst from the University of Birmingham.

Reference study

Shao Y, Forster SC, Tsaliki E et al. Stunted gut microbiota and increased pathogen colonisation associated with caesarean birth. Nature, 2019 DOI: 10.1038/s41586-019-1560-1

Cristina Sáez
Cristina Sáez
Cristina Saez is a freelance science journalist. She works for several media, for instance the Spanish newspaper La Vanguardia, where she coordinates the science section, Big Vang; as well as research centres and scientific societies. She has been awarded for her journalistic work, among others, with the Boehringer Ingelheim Award in Medical Journalism 2015. Follow Cristina on Twitter @saez_cristina