How a Newborn's Skin Builds Its First Microbiome in 24 Hours

A baby emerges from the womb with skin that is, for all practical purposes, sterile. Twenty-four hours later, that same skin hosts millions of bacteria, fungi, and other microbes—a full ecosystem assembled in less time than it takes to file a birth certificate. The speed is startling. The sources are even more surprising.

Where do the first microbes come from?

The answer depends entirely on how the baby is born. Infants delivered vaginally are bathed in their mother's vaginal and fecal microbiota during passage through the birth canal. Within minutes, Lactobacillus species—dominant in the vaginal environment—colonize the newborn's skin, mouth, and gut. It's an efficient microbial handoff, millions of years in the making.

Babies born via cesarean section skip this step entirely. Instead, their first microbial colonizers come from the surrounding environment: the operating room air, the hands of medical staff, the fabric of hospital linens. A landmark study found that C-section babies initially harbor more skin-associated bacteria like Staphylococcus and Corynebacterium—microbes typical of adult skin—rather than the vaginal and gut species seen in vaginally delivered infants. The difference is measurable for months.

What happens in the first 24 hours?

The skin is not passively colonized. It's selective. Newborn skin has a higher pH than adult skin—closer to neutral—and produces less sebum, creating an environment that favors certain pioneer species over others. Staphylococcus epidermidis, a commensal bacterium that will remain a dominant skin resident for life, begins to establish itself within hours. So does Cutibacterium acnes, though in far smaller numbers than it will reach during puberty (for more on that shift, see what puberty does to face bacteria).

Feeding accelerates the process. Breastfed babies ingest maternal skin bacteria from the areola and breast, which then colonize the infant's oral and gut microbiomes—and, through drool and regurgitation, re-seed the facial skin. Formula-fed infants show different early microbial profiles, though the differences narrow over time.

Why the delivery method matters long-term

The initial colonization event has consequences that ripple outward for years. Some research suggests that C-section babies face higher rates of asthma, allergies, and certain autoimmune conditions—outcomes potentially linked to altered early microbial exposure. The immune system is being trained in these first hours and days, learning to distinguish harmless commensals from pathogens. A delayed or skewed introduction may shift that education.

This has prompted some hospitals to experiment with "vaginal seeding," where C-section newborns are swabbed with maternal vaginal fluids immediately after birth. The practice remains controversial—there's limited evidence of long-term benefit, and some risk of pathogen transfer—but it underscores how seriously researchers now take those first microbial encounters (the broader principle of shared microbiomes is explored in how households share skin microbes).

Why this matters for your skin

The microbial foundation laid in infancy doesn't determine your skin's fate, but it does set a baseline. Adults with a history of early antibiotic exposure or C-section delivery sometimes show less microbial diversity on their skin, which can correlate with sensitivity or barrier dysfunction. Understanding that your skin's ecosystem has a developmental history—not just a genetic one—helps explain why some people's skin seems more resilient than others, even with similar routines. For more on how S. epidermidis continues to shape skin health throughout life, see our full species profile.

References

• Dominguez-Bello MG, Costello EK, Contreras M, et al. Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns. Proc Natl Acad Sci U S A. 2010.
• Byrd AL, Belkaid Y, Segre JA. The human skin microbiome. Nat Rev Microbiol. 2018.
• Scharschmidt TC, Vasquez KS, Truong HA, et al. A wave of regulatory T cells into neonatal skin mediates tolerance to commensal microbes. Immunity. 2015.
• Prescott SL, Larcombe DL, Logan AC, et al. The skin microbiome: impact of modern environments on skin ecology, barrier integrity, and systemic immune programming. World Allergy Organ J. 2017.