Development of Human Microbiota

Vaginal delivery exposes newborns to maternal vaginal microbes, promoting colonization by Bifidobacterium and Bacteroides. Cesarean delivery results in colonization by skin and environmental microbes like Staphylococcus and Corynebacterium. By 12 months, microbial differences between delivery types narrow, though C-section infants retain more heterogeneous communities.

Breast milk provides oligosaccharides that selectively promote Bifidobacterium growth. These bacteria ferment oligosaccharides into acetate and lactate, nourishing the infant and acidifying the gut to inhibit pathogens. Breastfeeding increases microbial diversity and beneficial species, while formula feeding is associated with potentially pathogenic species like Clostridium difficile.

By age three, the gut microbiota stabilizes into a more adult-like composition with increased microbial diversity. The transition from breast milk to solid foods triggers shifts toward adult-associated taxa like Bacteroidota and Bacillota. Cessation of breastfeeding, rather than introduction of solids, most significantly drives formation of adult-like microbiota.

In adulthood, the gut microbiota remains relatively stable, with species from Bacteroidota and Actinobacteria persisting longest. However, it varies greatly between individuals and is influenced by diet, lifestyle, and major life events. This stability provides a foundation for metabolic health and immune function throughout adult life.

With aging, microbial diversity often decreases, with notable shifts in the balance of Bacillota and Bacteroidota and reduced beneficial Bifidobacteria levels. Reduced diversity in elderly individuals, especially those in institutional care, is linked to increased frailty. Maintaining a diverse, balanced microbiota may support longevity and overall health.

Infancy marks a critical period of microbial sensitivity offering a window of opportunity during which beneficial microbes help mature the immune system. Early microbial exposures establish foundational communities that influence long-term health outcomes. Delivery method and early diet during this period have lasting effects on microbiota composition and diversity.

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. Microbial diversity increases as diet expands from breast milk or formula to solid foods. Adult-associated taxa become more prominent, establishing the foundation for stable adult microbiota composition.