They are mum’s first gift to her newborn baby on the day of its zeroeth birthday – bacteria, fresh from her vagina. Vaginal bacteria are among the trillions of microscopic hitchhikers that share our bodies with us. Collectively known as the ‘microbiota’, these passengers outnumber our own cells by ten to one. Children partly inherit their microbiota from their mothers. During birth, they pass from the largely bacteria-free conditions of the womb through the microbe-laden vagina into the equally bacterial outside world.

Being slathered in vaginal microbes might not seem like much of a treat from our adult perspective, but to a newborn, it’s a key event. The microbiota are important partners, influencing our physiology and our risk of disease. Now, Maria Dominguez-Bello from the University of Puerto Rico found that the way we enter the world determines the identities of our first bacterial colonisers. Babies delivered by Caesarean section end up with a very different portfolio to those who are born naturally.

To characterise baby’s first bacteria, Dominguez-Bello compared the microbiota of 10 mothers and their newborn babies, four of who had been delivered naturally and six of who were born through C-sections. She sampled the mothers’ skins, mouths and vaginas an hour before delivery, and the babies’ skins, mouths and noses less than five minutes afterwards.

She found that all the infants had the same bacteria all over their bodies. That’s very different to the situation in the mothers (and indeed, other adults), where different parts of the body are as varied in their resident species as deserts are to rainforests. In adult bodies, certain species of bacteria dominate certain habitats, so that your butt microbes have more in common with mine that they do with your elbow bacteria. Again, this wasn’t the case in babies.

Dominguez-Bello also found that the membership of the babies’ bacterial clubs was mainly influenced by their route of delivery. Those who were delivered naturally harboured bacteria all over their bodies that resembled those in their mother’s vagina – mainly Lactobacillus, which help us to digest milk.

But babies who were delivered by C-section were colonised by bacteria that are more commonly found on the skin, including Staphylococcus. These colonists didn’t necessarily come from mum either. The skin bacteria of C-section mothers were no more similar to those of their own babies than to those of other infants delivered in the same way. Instead, it seems that C-section babies pick their first bacteria from the hospital environment.

These early differences could directly affect a baby’s health. Staphylococcus is mostly benign but they can also go rogue, causing a wide variety of infections from spots to pneumonia. Antibiotic-resistant strains such as MRSA are particularly problematic and it’s perhaps unsurprising that in a Californian study, between 64-82% of newborns with MRSA infections were delivered through C-sections.

Other scientists have found similar trends for the bacteria in a baby’s gut, which also differ depending on how the child is delivered. Dominguez-Bello’s new study extends this knowledge to other parts of the body, but it’s more of a starting point for further research (and perhaps a narrow one at that, given that it only considered ten babies) rather than a conclusion in itself. We still need to understand the shifts that take place as children grow and the members of their microbiota rise, fall and take up residence in different body parts.

Dominguez-Bello suspects that all of these events hinge on the moment of birth. She thinks that the bacterial heirlooms that babies inherit from their mothers might act as a shield, preventing more dangerous microbes like MRSA from setting up shop. In this way, early colonisers can influence the direction of late successions. By changing baby’s first bacteria, C-sections could alter the make-up of their later communities, leading to long-term effects on health and nutrition.

There is some evidence to support this idea, but to date, it’s pretty circumstantial. Some studies have suggested that babies delivered through C-section might be more susceptible to allergies, although the increase in risk is pretty small. Nonetheless, in one study, probiotic drinks containing Lactobacillus could help to counteract this increased risk, at least for some types of allergies.

Results like these are easy to politicise, especially since a quarter of babies in the UK are delivered by Caesarean section. But this study is no condemnation of C-section deliveries, which are often necessary. Opinion pieces often decry women who opt for elective C-sections because they are allegedly “too posh to push”, but the reality is very different to the ones these straw (wo)men caricatures present. Official records show that such births only account for 1.5% of the total. And last year, a UK study showed that just 3% of women would opt for a C-section if they didn’t have a medical need for one.

It’s also important to note that this new study merely documents how the method of delivery affects the bacteria that babies inherit. The effect that this has on our health is still largely speculative (contrary to what some papers would have you believe). And whether those effects have any significant importance in the grand scheme of things also remains to be seen. Dominguez-Bello’s paper ends with a call for more research and that’s exactly what is now needed.

Reference: PNAS http://dx.doi.org/10.1073/pnas.1002601107

More on the microbiota:

• Gut bacteria in Japanese people borrowed sushi-digesting genes from ocean bacteria
• The bacterial zoo in your bowel
• The bacterial zoo living on your skin
• Gut bacteria – fat or thin, family or friends, shared or unique
• Human gut bacteria linked to obesity