Before childbirth, the baby’s gut is considered sterile (although this hypothesis is questioned, see “gut microbial colonization : and if it began before birth ?).
The acquisition of the intestinal microbiota occurs at birth, when the fetal membranes rupture. The bacteria will then rapidly and extensively colonize the digestive tract of the infant. This is explained by the immune system immaturity of the newborn which makes the digestive tract permissive for the implantation of the bacteria. The diversity and composition of the intestinal microbiota is influenced by many factors, including the mode of delivery, gestational age, maternal microbioma, feeding patterns in early childhood, lifestyle, diet, medication… This explains the idea that each individual has a different intestinal microbiota.
However, some bacterial species are common to all people as Bacteroides uniformis, Faecalibacterium prausnittzii, Roseburia intestinalis and several species of Lactobacilli and Bifidobateria.
The composition of the intestinal microbiota will gradually become more complex with age. The maturity of the intestinal microbiota is only reached at the age of 2-3 years.
Relatively stable over time, its composition can evolve transiently over the course of life according to diet, environment, state of health or even stress. It nevertheless tends to return to its initial state in 1 to 2 months.
Then, the intestinal microbiota undergoes a major change in the elderly: the composition and the number of bacteria is reduced, especially for the Bifidobacteria. This is due to a lower adhesion capacity and/or changes in the chemical composition and structure of the mucus.
Therefore, diversity of Bifidobacteria is practically limited to two species : Bifidobacterium longum and Bifidobacterium adolescentis… Hence the importance of supplementation.
Ottman N, Smidt H, De Vos WM, Belzer C. The function of our microbiota: who is out there and what do they do? Front. Cell. Inf. Microbio. 2012.2(104).