The composition of the gut microbiota changes, not only between one individual and another, but also over time. Its formation begins immediately after birth and develops over a period of about two years. At the end of this period, its composition tends to stabilize until adulthood.


The way in which a child is born and lives its first months can right away constitute a strong discriminant in promoting, or not, the creation of a healthy adult microbiota.
A naturally born, adequately breastfed baby’s bacterial flora is enriched by bacteria, such as Lactobacilli, living on the vaginal epithelium and by bacteria, like Bifidobacteria, which, because they are somewhat “oxygen-averse,” are transmitted only through the mother’s milk. On the contrary, cesarean delivery or the lack of breastfeeding can affect the bacterial population of the newborn, modifying its composition in terms of both quality and quantity.


Starting from the first days of life, the bulk of the microbial mass of a newborn essentially consists of the extremely important Bifidobacteria. The formation of a mature microbiota begins with the introduction of solid food. Bacteria, like Firmicutes (which also includes the genus Bifidobacterium) and Bacteroidetes, which make up the phyla prevalent in adulthood, begin to multiply. At around two and a half years, the child’s microbiota develops to become that which is typical of adults.


While the composition of the microbiota remains substantially unchanged during youth, maturity and adulthood, in the elderly it is possible to find a significant difference when compared to their previous, younger microbiota. A large number of studies—the Irish ELDERMET Project, for example—have observed that some typical frailties in older individuals could be correlated to a distinct trend towards a general diminution of the microbiota.