Are the gut bacteria of children different from those of adults?

From birth, the baby gut is a completely different environment from that of an adult. Bacteria from the mother are the first to arrive, but not all of them take hold equally well: bacteria from the mother's skin and vagina are present only temporarily, while the mother's gut bacteria 'fit' the baby gut better and stay longer. In the first days of life a strong selection process therefore takes place, after which the community gradually becomes more stable.

What a baby is fed makes a big difference to the composition of those gut bacteria. With breastfeeding, the unique sugar molecules in breast milk determine which bacteria thrive, resulting in a relatively simple but well-balanced community. Formula-fed babies, by contrast, harbour a greater variety of species. In adults, dietary pattern also plays a role, but such a direct and pronounced influence on composition is not seen.

Other factors surrounding birth also leave a clear mark: whether a baby is born vaginally or by caesarean section is one of the strongest influencing factors. In addition, the mother's diet during pregnancy, obesity, smoking and antibiotic use all count. These kinds of early circumstances shape the baby gut in a way that has no direct equivalent in adults.

The differences are also visible in disease. Research into children and adults with the same serious liver condition (related to obesity) showed that their gut microbiota are so far apart that a computer model can distinguish between them with 97% accuracy. Even in the presence of the same disease, the childhood microbiota therefore remains clearly distinguishable.

This unique composition in babies has health consequences. Newborns with lower levels of Bifidobacterium and certain protective bacteria in the gut are more likely to have allergies at age 2 and asthma at age 4. In short, the infant gut is not a smaller version of the adult gut, but a distinct, age-specific ecosystem.