Babies who attend daycare share

Socializing from a young age helps develop greater diversity in children’s microbiomes, analysis of baby-to-baby transmission of gut bacteria suggests

By Chris Sims & Nature magazine

Much of a baby’s developing microbiota comes from peers at daycare, even after just a month of attendance, an analysis suggests.

The study, published today in Natureanalyzed the gut microbiomes of infants during their first year of nursery. The amount of microorganisms passed between babies has increased throughout the year. After four months, babies in a nursery already shared 15 to 20 percent of their microbial species.

“It was higher than the proportion of all the microbes they had acquired from birth to that point in the family,” says Nicola Segata, a microbiologist at the University of Trento in Italy.

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Some of the changes in children’s microbiomes will be due to the diet they had at nursery, but the study shows that the transmission of microbial strains between babies is important during the first year of nursery, and highlights that social interactions at this stage are essential for build a diverse and healthy microbiomeadds Segata.

Bug reporting

Even though the fetus is still in the womb, its microbiome is considered non-existent during a healthy pregnancy, but it begins to develop rapidly after birth, mainly due to microbial transmission from mother. Research has shown that people who live together begin to share microbial strains. But how the developing microbiota changes during the first years of life has not been well studied.

To fill the knowledge gap, Segata and colleagues examined the microbiomes of 43 babies with a median age of 10 months at the start of the study. They followed them before, during and after their first year of nursery in Trento, Italy.

“We registered babies who were meeting each other for the first time, on the first day of daycare,” Segata explains. “This is a time when their gut is much more likely to acquire strains from other babies and adults because the immune system is not yet well trained.”

The team analyzed stool samples from the babies, as well as 10 nursery staff and people living in the same homes as the children: 39 mothers, 30 fathers, 7 siblings, 3 dogs and 2 cats.

Once the infants started day care, the researchers continued to collect samples every week until the Christmas holiday, and for some infants, this continued until July. All participants had follow-up samples collected in July and one year after the start of the study.

Microbial richness

The analysis revealed widespread baby-to-baby microbiome transmission just one month after starting nursery, which continued to grow over the course of the nursery year. If a baby had a sibling, they received more microbes from that sibling than from their parents, they tended to have a more diverse overall microbiota, and they acquired fewer bacterial strains from their peers in nursery.

The study also mapped the transmission of individual microbial species between individuals. Segata gives an example of what happened with a strain of bacteria called Akkermansia muciniphila. “We have an example of a strain that passes from mother to child. The baby in daycare then passed it on to another baby, who passed it on to both parents.”

There were even signs that pets and infants exchanged bacterial strains. “Interestingly, it was only for babies and not adults. So maybe there are more ‘intimate’ interactions with babies and pets,” says Segata.

However, the most drastic effect on babies’ microbiota comes from the use of antibiotics. Antibiotic treatment during the first year of life significantly reduced the number of bacterial strains in the infants’ gut microbiota, but this was followed by rapid recovery facilitated by a large influx of fresh strains.

“For me it was a surprise to see how mothers also contracted bacteria from other families through their children,” says María Carmen Collado, a food biotechnologist at the Institute of Agrochemistry and Food Technology in Valencia, Spain.

“I think this is really good work that fills a gap in knowledge about microbiome transmission,” says Collado. “I think this will open up new possibilities, not only with the microbiome, but also with our understanding of how pathogens spread. »

THE long-term health impacts whether exposure to other microbial strains in day care on infants’ gut microbiomes is not known. It’s probably a combination of diet and lifestyle which maintains the diversity of bacteria in the gut microbiota later in life, Segata explains. Since the newly acquired strains were still there at the end of the year, it is possible that they could persist into adulthood.

“Maybe in 20 years we’ll find that people still need to thank their friends at daycare for the germs they picked up while they were there,” he says.

This article is reproduced with permission and has been published for the first time January 21, 2026.