As soon as a baby is born it receives its initial microbiome. If a baby is born via a vaginal birth, the bacteria from the mother’s vagina is passed onto the baby. If the baby is born by way of a cesarean section, the baby is exposed to other types of bacteria from the mother’s skin. In this case, the optimal situation is a vaginal birth where the baby is exposed to bacteria called bifidobacterium. This is the exact kind of bacteria that a baby needs since the mother's breast milk contains food especially for them.
Breast milk contains special types of oligosaccharides that the baby cannot digest without the bifidobacterium. On the other hand, with c-section babies, streptococci bacteria are colonized in the gut. These type of bacteria are unable to digest breast milk int he same way, therefore it is thought that the body is able to find a way to compensate and allow the "right" bacteria to reach the baby's intestines. This is why breast milk is ideal, since it contains both desired bacteria and the food that suits them. The baby's food - breast milk or formula - affects how the microbiome develops. During the first year of a baby's life, the microbiome undergoes the greatest changes. By the age of 3, the microbiome is similar to that of an adult.