Colonization of the gut by the human microbiome was thought to begin at birth, but some scientists have reported that a mother-to-child transfer of commensal bacteria through the placental barrier may occur. The maternal oral microbiome has been suggested as the source of the first bacteria that colonize infants.
Delivery mode has a major influence on the gut microbiota in early life. When born vaginally, an infant is colonized by the mother’s gut and vaginal microbiota first. In contrast, when the baby is delivered via Caesarean section, it is exposed to the skin and hospital environment microbiota first. Recent data also suggests rectal mother-to-child bacterial transmission.
As such, in early life, the infant born vaginally acquires bacteria resembling its own mother’s vaginal and fecal microbiota, showing a low level of diversity and depending mainly on the baby’s diet.
Afterwards, mode of feeding—breast milk versus formula—has a major impact on the composition of the infant’s microbiota. Human breast milk not only contains live bacteria, but also a great variety of complex carbohydrates (named human milk oligosaccharides) that cannot be digested by babies. In turn, these complex carbohydrates act as prebiotics by influencing the composition of the bacteria in the infant’s gut. While the gut microbiota of breastfed infants is dominated by Lactobacillus and Bifidobacterium, formula-fed infants develop a microbiota resembling that of an adult, with higher overall bacterial diversity.
The transition from breastfeeding to solid foods high in protein and fiber leads to an increase in gut bacteria diversity. Scientists consider that by the time a child is between 2 and 3 years old, their gut microbiota resembles that of an adult and will remain stable during their adult life.