Drop a one-year-old off at nursery and you assume they will come home with a runny nose. What you probably do not picture is a fresh cargo of gut bacteria picked up from the toddler at the next table. A new study in Nature shows that this is exactly what happens. Babies who spend their days together are steadily seeding each other's guts, and by the end of the first term the microbes a child collects from classmates can match, in sheer quantity, what they inherited from their own family.
The work comes from a team led by Liviana Ricci at the University of Trento. Researchers have long known that a mother hands off much of the starter microbiome around birth. What happens next, once an infant starts leaving the house and mixing with strangers, has been far murkier. This study followed the thread into the nursery room.
Reading a bacterial paper trail
The team tracked 134 people: babies in their first year of nursery, the educators who cared for them, and family members at home. Across three different facilities, they collected 1,013 faecal samples over the year, sampling densely enough to watch individual bacterial strains move from one gut to another. This is the key methodological trick. It is not enough to note that two children carry the same species. The researchers resolved microbes down to the strain level, so a shared strain becomes a fingerprint that betrays who caught what from whom.
The transmission showed up fast. After just one month of nursery attendance, babies within the same group were already carrying strains traceable to their peers. By the close of the first term, nursery-acquired strains made up a share of a child's gut microbiome comparable to the share coming from their own family. The classroom, in other words, had become a microbial commons on par with the household.
An expanding web of tiny transfers
It did not stop there. Baby-to-baby transmission kept growing across the nursery year, and the pattern of who shared with whom grew more tangled over time. In some classes a single strain spread widely, hopping across many children like a rumor moving through a room. Different babies picked up microbes at different rates, and some bacterial species proved far more transmissible than others. The result was less a simple hand-off and more an evolving network of exchange, redrawn each month.
Two factors stood out as levers on this process. Children who had siblings at home showed higher microbiome diversity and, notably, acquired fewer new strains from their nursery peers. A gut already stocked with a rich community from brothers and sisters appears to leave less open ground for classmates to colonize. Antibiotic treatment pushed in the opposite direction. Of all the conditions the team examined, antibiotics did the most to increase the influx of outside strains, consistent with the idea that clearing out resident bacteria opens the door to newcomers.
What the study can and cannot say
This is an observational picture, not an experiment, so it maps where microbes travel rather than proving what that travel does to a child's health. The cohort spans three facilities and 134 individuals, a solid design for tracing transmission but still a specific set of nurseries in a specific place, and the patterns may look different elsewhere. The paper also does not claim that peer-acquired strains are good or bad. A shared microbe could be a helpful early colonizer or an unwelcome one, and sorting that out will take follow-up work that links particular strains to particular outcomes.
What the study does establish is a shift in how we should think about where the infant microbiome comes from. The standard story centers on mother, birth mode, and breastfeeding. Those still matter. But this data adds a social dimension that has been largely invisible: the other babies in the room. Early friendships, it turns out, are also early microbial exchanges, and the ordinary act of putting small children together may be one of the more powerful forces shaping the communities that will live in their guts for years.
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