The human microbiota begins developing at birth and undergoes continual change as we age. Infancy marks a critical period of microbial sensitivity, offering a “window of opportunity” during which beneficial microbes help mature the immune system. By age three, children typically develop a more stable and diverse microbial community. Newborns acquire microbes from their immediate environment; vaginal delivery favors maternal vaginal microbes, while cesarean births favor microbes from the skin and surroundings. Current studies are investigating the potential benefits of exposing C-section newborns to maternal vaginal microbes to compensate for this difference.
Breast milk plays a selective role in nurturing beneficial bacteria such as Bifidobacterium by providing oligosaccharides that these microbes can metabolize. Their fermentation byproducts—acetate and lactate—not only nourish the infant but also acidify the gut, impeding pathogenic growth. Transitioning to formula or solid foods leads to a decline in Bifidobacteria and a rise in other gut microbes like Bacteroidota and Clostridia. During the first year, the infant gut transitions from a relatively simple to a more complex microbiome, enriched in genes for vitamin synthesis and energy production.
Delivery method and early diet significantly affect microbial colonization. Vaginally delivered infants share a larger proportion of microbes with their mothers, including beneficial genera like Bifidobacterium and Lactobacillus. Infants born by C-section show greater colonization by skin and environmental microbes. Breastfeeding further promotes beneficial species and microbial diversity, on the other hand to formula feeding, which is associated with potentially pathogenic species like Clostridium difficile. By 12 months, the microbial differences between delivery types narrows, though C-section infants still show more heterogeneous communities. Cessation of breastfeeding—not the introduction of solids—is what most significantly triggers the formation of an adult-like microbiota.
In adulthood, the gut microbiome remains relatively stable over time, with species belonging to Bacteroidota and Actinobacteria persisting the longest. Aging alters this balance, often decreasing Bacillota (previously Firmicutes) and microbial diversity—especially among those in institutional care. Reduced diversity in the elderly is linked to increased frailty, suggesting that maintaining a varied microbiota may promote longevity and overall health.
The development of human microbiota begins at the time of birth.
Newborns get microbes from the mother. Vaginal delivery is associated with colonization by Bifidobacterium and Bacteroides, while cesarean delivery is linked to colonization by Staphylococcus and Corynebacterium.
In infants, breast milk promotes the growth of Bifidobacterium species by providing oligosaccharides that specifically serve as nutrients for these bacteria.
As the infant's diet transitions to include solid foods, microbial diversity increases, and adult-associated taxa such as Bacteroidota and Bacillota become more prominent.
By the age of three, the gut microbiota stabilizes into a more adult-like composition.
In adulthood, the gut microbiota remains relatively stable but varies greatly between individuals and is influenced by diet, lifestyle, and major life events.
With aging, microbial diversity often decreases, with a notable shift in the balance of Bacillota and Bacteroidota, and reduced levels of beneficial Bifidobacteria.
Maintaining a diverse, balanced gut microbiota supports metabolic health, immune function, and may influence longevity.
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