Babies build their gut microbiota mainly from their mothers and families, and their microbial communities mature into a more adult-like state over the first few years. Researchers have now found that babies’ interactions in nurseries can have a major impact on their gut microbiotas—sometimes even more than families—within just a few months.

The findings, published in Nature, suggest that early social interactions, especially among nursery peers, help to build and diversify the infant gut microbiota.

Previous studies showed that microbes are shared between peers, suggesting that nurseries may shape infant microbiotas through baby-to-baby transfer. However, not much is known about how babies’ microbiotas change once they begin interacting with other children.

Liviana Ricci at the University of Trento in Italy and her colleagues followed 43 babies during their first year of nursery and tracked how their gut microbiotas changed over time.

Similar microbiotas

The researchers collected and analyzed more than 1,000 stool samples from babies, family members, caregivers, and household pets. The team confirmed expected differences between adult and baby microbiotas, and showed that babies’ microbial diversity increased over time, especially as they attended nursery. 

Babies shared many microbial strains with their mothers at the start of the study, and people living in the same family also shared more strains with each other than with unrelated individuals. 

Babies with siblings tended to have more diverse microbiotas, and factors such as birth method or antibiotics at birth no longer had a strong effect on gut microbes by about 10 months of age. As babies spent time together in nurseries, their microbiotas became more similar to each other, the researchers found.

Building microbiotas

Using genetic tracking, the researchers were able to reconstruct bacterial strains passing from a baby to another baby in the nursery and then spreading to that child’s parents at home. They also found a few examples of microbes being shared between babies and pets.

Within a few months, bacteria acquired from nursery peers made up a larger share of babies’ microbiotas than those from family members. Even after long breaks, such as summer vacation, babies continued to share more microbes with former nursery peers than with children from other nurseries, the researchers found.

“Overall, our results reveal the centrality of social factors in shaping the infant microbiome via inter-individual microbial transmission, thus rebalancing social interactions as key to building a healthy microbiome,” they say.