A Sterile Start — or Is It?
The traditional view held that the foetal environment is sterile and microbial colonisation begins at birth. While some studies have reported detecting bacterial DNA in placenta, amniotic fluid, and meconium — suggesting prenatal colonisation — these findings remain contested, with contamination during sample processing being a major confound. What is clear is that birth itself represents the first massive microbial exposure.
Birth Mode Matters
Vaginal delivery exposes the infant to maternal vaginal and intestinal microbiota — primarily Lactobacillus, Prevotella, and Bacteroides species. Caesarean section, by contrast, results in initial colonisation with skin-associated and hospital-environment bacteria — Staphylococcus, Corynebacterium, and Propionibacterium species. These differences in early colonisation patterns are detectable for months to years after birth, and epidemiological studies associate caesarean delivery with modestly increased risks of asthma, allergic disease, obesity, and type 1 diabetes — though confounding factors make causal claims premature.
Breast Milk: More Than Nutrition
Breast milk is not sterile. It contains approximately 10⁵ to 10⁷ bacteria per millilitre, including Bifidobacterium, Lactobacillus, Staphylococcus, and Streptococcus species — delivered directly to the infant gut. Additionally, breast milk contains human milk oligosaccharides (HMOs) — complex sugars that the infant cannot digest but that selectively nourish Bifidobacterium species. This elegant system ensures that the infant gut is seeded with beneficial bacteria and provided with their preferred food source simultaneously. Formula-fed infants develop a more diverse but less Bifidobacterium-dominated microbiome.
The First 1000 Days
The microbiome undergoes rapid evolution during the first three years of life, gradually converging toward an adult-like configuration. During this developmental window, microbial diversity increases with the introduction of solid foods, exposure to siblings and pets, and environmental contact. By age 3, the core community structure is largely established, though fine-tuning continues into later childhood.
Antibiotics in Early Life
Early-life antibiotic exposure — particularly in the first year — is associated with lasting microbiome perturbations. Studies link early antibiotic courses with increased risks of childhood obesity, asthma, and allergic disease, potentially by disrupting immune education during a critical developmental window. This does not mean antibiotics should be withheld when medically necessary — rather, it underscores the importance of appropriate prescribing and considering narrow-spectrum options.
Clinical Relevance
Understanding the infant microbiome has practical implications: supporting vaginal birth where safely possible, promoting breastfeeding, avoiding unnecessary early-life antibiotics, and allowing environmental microbial exposure (pets, outdoor play, siblings) rather than pursuing sterility. These simple measures support microbial diversity during the most influential period of microbiome development.