Babies born by Caesarean section have dramatically different gut bacteria to those born vaginally, according to the largest study in the field.
The UK scientists say these early encounters with microbes may act as a “thermostat” for the immune system.
And they may help explain why Caesarean babies are more likely to have some health problems later in life.
The researchers stress women should not swab babies with their vaginal fluids – known as “vaginal seeding”.
How important are gut bacteria?
Our bodies are not entirely human – instead we are an ecosystem with around half our body’s cells made up of microbes such as bacteria, viruses and fungi.
Most of them live in our gut and are collectively known as our microbiome.
The microbiome is linked to diseases including allergy, obesity, inflammatory bowel disease, Parkinson’s, whether cancer drugs work and even depression and autism.
This study – by Wellcome Sanger Institute, UCL, and the University of Birmingham – assessed how the microbiome forms when we leave our mother’s sterile womb and enter a world full of bugs.
Regular samples were taken from the nappies of nearly 600 babies for the first month of life, and some provided faecal samples for up to a year.
The study, published in the journal Nature, showed vaginally born babies got most of their early bacteria from their mother.
But Caesarean-section babies had high levels of hospital bugs such as Klebsiellaand Pseudomonas.
“What surprised me and scared me was the amount of healthcare bugs showing up in these children,” Dr Trevor Lawley, from the Wellcome Sanger Institute, told the BBC.
“It could be 30% of their total microbiome.
“But what excites me is we have an amazing body of data now that we can build on, to think about how to properly establish the human ecosystem, starting at birth.”