And that in turn may influence their future health, including things like whether they will develop asthma, allergies or other medical conditions.
The study, conducted by researchers at the University of Alberta, is published in this week's issue of the Canadian Medical Association Journal.
Senior author Anita Kozyrskyj says it is important that medical teams and parents understand that the choices made about mode of delivery and method of feeding have an impact.
She and colleagues studied the gut microbiomes of 24 infants; that's the term for the diverse collection of bacteria people carry in their intestinal tracts.
They found distinct differences in the numbers and types of bacteria present when they compared babies born by caesarean section to those born by vaginal birth, as well as babies breastfed and fed with formula.
Kozyrskyj says the study was done because too little is known about the possible implications of delivering by C-section or feeding by bottle.
"There are a lot of things we don't know about the impact of health-care interventions such as caesarean section delivery on the developing infant. To date most of the evidence informing physicians and I guess families too is focused on the mother," says Kozyrskyj, who is a research chair and associate professor in the department of pediatrics at the University of Alberta.
It's long been known that people carry an enormous number of bacteria in and on their bodies, most of which are benign or even helpful. For instance, friendly bacteria in the gut help protect against invaders that are ingested with contaminated food and which could cause illness.
It's said that bacteria in the gut outnumber the cells in the respiratory tract by a ratio of 10 to one.
In recent years, though, it has become increasingly clear that the wrong balance of bacteria can have negative health implications.
So Kozyrskyj and colleagues used a DNA sequencing technique to identify the bacteria in 24 babies' guts by studying a stool sample from each. The samples were taken when the babies were four months old.
They found a greater variety of bacteria in the children born vaginally as compared to those that were born by planned C-section — though babies born by emergency C-section had even higher variety of bacteria than those born vaginally.
Kozyrskyj says she can only speculate on why that is, but it might reflect the fact that vaginal birth started before the need for a C-section interrupted the process.
Infants who were exclusively breastfed had a less varied collection of gut bacteria than those who were bottle fed. But in this case, more isn't necessarily better, Kozyrskyj says. Some of the bacteria found in the stools of bottle-fed babies were the types that could cause illness, such as Clostridium difficile, which can cause diarrhea.
A commentary published with the study says mammals like humans get an early start on the development of their immune systems as they pass through their mothers' birth canals. And breast milk further assists that process, creating conditions in which beneficial bacteria can set up shop while harmful ones are kept at bay, say the authors, from the University of Colorado and the University of Puerto Rico.
"Although we do not yet fully understand how changes in microbial diversity or composition can affect microbiome function, certain types of disruption during development may have lifelong effects on the baby's microbiome and thus the individual," says the commentary. Rob Knight, a Howard Hughes Medical Institute Early Career Scientist at the University of Colorado, Boulder, is the senior author of the commentary.
Kozyrskyj says it's not currently known what an ideal microbiome is, or how critical it is for babies to acquire certain types of bacteria at particular points in their development.
She says the group responsible for this work hopes to conduct a larger study, analyzing the microbiomes of 200 infants and comparing results from an early stool sample to one taken after one year.
Some C-section births are medically necessary, to save the mother, the baby or both. But the rate of elective C-sections has been on the rise in Canada and elsewhere, Kozyrskyj says, and is a source of concern.
The study says more than one in four babies in Canada is born by caesarean section and less than 15 per cent of babies are breastfed exclusively for the first six months, as recommended.
It notes that babies delivered by C-section are more likely to develop asthma and Type 1 diabetes, and are at higher risk of being obese later in life.
"Although caesarean section delivery has been shown to be relatively risk free, there are some risks," Kozyrskyj says.
"I think we should be mindful of the impact on the infant. Even though there might not be obvious symptoms or changes that you see in the infant, that there are some changes to biological processes. Normal physiologic processes."
Note to readers: This is a corrected story. A previous version misidentified the Canadian Medical Association Journal
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