Imagine a remote island, recently formed by volcanic activity, in the middle of the ocean. At first, it is lifeless, but a growing variety of plants take hold, providing food for pioneering animal species, until eventually there is a diverse and flourishing ecosystem.
This is a useful way to think about how our gut ecosystems develop. “Your microbiome goes on a journey,” says at the University of Aberdeen, UK. “When you’re born, some bugs get in and then, when you start eating solid foods, other bugs replace them. There’s a dynamic process where your microbiome changes until you get to mid-to-late childhood. Then, through adult life, you’ve got a reasonably stable microbial community.”
Does a C-section affect a baby’s microbiome?
The first individuals that colonise an island can have long-lasting influences on its ecosystem, an idea known as the founder effect. Until recently, the thinking went that if the founder bacteria in a baby’s gut were unusual – because the baby was born by Caesarean section, for instance – this might disrupt their bacterial ecosystems. This idea has led some parents to take radical steps to get their children’s microbiomes back on the right track. But the science behind these ideas is far from settled.
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One of the first to document the differences between the gut bacteria of babies born by C-sections and those born vaginally was at Rutgers University in New Jersey. Her research team found that when babies born by C-section were a day old, their faeces had and more of the bacteria that usually colonise the skin. “Babies are like sponges and they get whatever they’re exposed to,” she says.
Previous studies have found that C-section babies go on to have slightly higher rates of certain health conditions, including obesity, allergies and asthma. As disturbed gut bacteria have been claimed to contribute to these conditions, the concern is that C-sections could be causing lasting harm to children’s health.
To counter this, DomĂnguez-Bello’s group helped pioneer the practice of “vaginal seeding”, where a baby’s face and mouth are wiped with a piece of gauze that was previously placed in their mother’s vagina. Crucially, this is only done after tests to check there are no harmful microrganisms in the mother’s vagina.
Some researchers are even giving C-section babies small oral doses of their mothers’ faecal bacteria (again, they are screened for harmful microbes). But the trials done so far have had small numbers of participants, so this isn’t yet considered of proven medical benefit.
The microbiome and obesity
Besides, even the basic premises behind the idea are up for debate. The health conditions of interest, such as obesity and allergies, haven’t been shown to be caused by altered gut bacteria – only to correlate with them. It could be something else, such as diet or lifestyle, causing both the medical condition and the microbiome differences. In fact, a co-authored by Walker concluded that, so far, no reliable microbial signatures of obesity have been found (see “Does the gut microbiome influence body weight?”).
Most importantly, it is questionable if a child’s mode of birth has a lasting effect on their microbiome. A 2018 study found that, in vaginally born babies, bacteria from the mother’s vagina are found in the baby’s gut for only a few days, while the mother’s gut bacteria continued colonising the baby’s gut for months after the birth.
Does breastfeeding affect a child’s microbiome?
A much bigger influence comes from whether or not babies are breastfed, as breast milk contains some sugar-like compounds – absent in most types of baby formula – that promote the growth of Bifidobacterium bacteria. This is thought to be beneficial for several reasons, including . Then, as babies are weaned onto solid foods in their first year, their microbiome starts becoming more like that of an adult.
As children grow, the biggest influence on their gut bacteria is the people they live with (see “How your microbiome is shaped by your friends, family, lovers and pets”). “When people go to the toilet, there will be bugs floating around the house that are derived from stools,” says Walker. One study of a family with six children found that, by a few years of age, the children shared about the same number of gut bacteria with their father as with their mother.
Much is still unclear about how our gut ecosystems bloom into their full complexity and individually unique make-up – even differing between identical twins. “A lot of it seems to be quite random and what you’re exposed to,” says Walker. “Certainly, in adults, there’s no way to tell if someone was breastfed or was born by C-section. The big question is how much that window where it was different really matters. There’s lots to be learned.”
Speak to your doctor before seeking new treatments for medical conditions
This article is part of a special series on the microbiome, in which we explore:
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