What is the skin microbiome? What it does and what we don't know

What is the skin microbiome?

It is the community of microbes that live on your skin, plus (depending on who is defining it) their genes and how they interact with their surroundings. Think of it as a set of small ecosystems, not one uniform film.

Your skin covers roughly 1.8 square meters, and it is not one flat habitat. Folds, hair follicles, sebaceous glands and sweat glands each support bacteria, fungi, viruses and mites.

A small vocabulary note, because the words get mixed up. The microbiota is the community of organisms; the microbiome is their collective genome and environmental interactions. Most people, and most brands, use microbiome for both.

The surface itself is a fairly harsh place to live: generally cool, acidic and dry, with a pH around 5. Part of that acidity comes from free fatty acids released from sebum by Cutibacterium (formerly P. acnes). We cover the wider topic in our skin microbiome hub.

What is the skin microbiome made up of?

Mostly bacteria, with fungi, mites and viruses in supporting roles. Sequencing studies find four dominant bacterial groups: Actinobacteria, Firmicutes, Bacteroidetes and Proteobacteria. One review puts rough shares at 36 to 51 percent, 24 to 34 percent, 6 to 9 percent and 11 to 16 percent for Actinobacteria, Firmicutes, Bacteroidetes and Proteobacteria respectively.

• Bacteria: common genera include Staphylococcus, Corynebacterium, Cutibacterium, Micrococcus, Enhydrobacter and Veillonella. Compared with gut and mouth, skin has more Actinobacteria and fewer Firmicutes and Bacteroidetes.
• Fungi: Malassezia is the fungus most commonly isolated, especially on oily sites.
• Mites: Demodex folliculorum and D. brevis live in or near hair follicles, feed on sebum and become more common after puberty.
• Viruses: most are bacteriophages, which infect bacteria rather than you. A 2011 review called commensal skin viruses essentially unstudied, so treat this category as the least mapped.

There is a method caveat here. Old culture-based work missed most organisms, and sequencing of the 16S rRNA gene revealed far more diversity. But 16S sequencing cannot tell living from dead microbes, or residents from passers-by. Culture is still needed to study how organisms actually behave.

Where is the skin microbiome located?

Everywhere on your skin, including inside hair follicles and glands, but the mix depends on the local conditions. Researchers group body sites into three types.

The logic is simple: oily sites favor microbes that thrive on lipids, and dry sites carry a broader mix. The 2011 review also described sebaceous glands as relatively low in oxygen, which suits anaerobes.

Location also changes with age. Infant skin is colonized rapidly after birth, and its composition shifts over the first year with increasing diversity.

What does the skin microbiome do?

Some functions are reasonably established, some are plausible, and the headline-friendly ones are still early. It helps to sort them by evidence.

• Body odor (well established): staphylococci and corynebacteria metabolize apocrine sweat secretions, and that chemistry is a major source of underarm smell.
• Keeping pathogens in check (plausible, partly mechanistic): acidic skin favors coagulase-negative staphylococci and corynebacteria while inhibiting S. aureus and S. pyogenes. Occlusion raises pH and favors the pathogens. This rests on lab and observational work summarized in reviews.
• Immune education (mostly mice and lab tissue): in mouse studies, S. epidermidis helped accumulate regulatory T cells and build tolerance to resident microbes, and it also induced T cells that protect against Candida albicans.

In ex vivo human skin, S. epidermidis activated gamma-delta T cells and raised an antimicrobial protein called perforin-2, linked to better killing of S. aureus inside cells. That is interesting. It is also donated skin in a dish, not a living person.

Why is the normal skin microbiome considered beneficial?

Mostly by inference. The 2011 Grice and Segre review says most skin microbes are harmless or beneficial, possibly protecting against pathogens and helping prime T cells, but that is a summary of a field, not a measured health benefit.

The case rests on three things: mechanistic and animal work (above), disease studies, and a general ecology idea that established residents crowd out invaders.

The disease evidence is where the picture gets thin. In atopic dermatitis, skin tends to show lower diversity and more S. aureus. But one shotgun-sequencing study behind that finding used 10 patients and 5 healthy controls across 14 skin sites. That is a small sample.

Cross-sectional comparisons also cannot tell whether microbial change causes disease or follows from barrier damage. One review frames atopic dermatitis as a self-reinforcing loop of barrier dysfunction, immune dysregulation and dysbiosis, which fits that uncertainty.

What is a healthy skin microbiome?

No one can say precisely. We found no validated, consensus definition, no measurable threshold and no clinical test that tells you whether yours is healthy.

What the literature does offer is a description. Healthy skin is defined by site, not by a single signature. The 2011 review portrayed it as a handful of stable inhabitants (Cutibacterium, Staphylococcus), with rarer or transient species making up the rest.

Diversity is not a simple score either. Dry forearm skin is the most diverse habitat and oily skin the least, so high diversity does not equal healthier. Be skeptical of any product or quiz that treats it that way.

Does everyone have the same skin microbiome?

No, though the body site you pick matters more than the person. In the Grice and Segre data, the inner elbow, back, nostril and heel each resembled the same site on another person more than other sites on the same person.

Still, individual variation is large at the genus and species level. In a six-person study, only 6.6 percent of forearm genera were shared by all subjects, while 68.1 percent turned up in just one person. In a hand study, one person's two hands shared 17 percent of species-level types; hands from different people shared 13 percent.

• Oily sites vary least from person to person.
• Stability over time depends on site. The ear canal, nostril and groin crease were the most stable, and skin overall varied more over time than gut or mouth.
• Drivers include age, sex, sebum, sweat, hormones, occupation, clothing, humidity, temperature, UV and antibiotics. Sex differences may reflect physiology, hygiene or cosmetic use, and the review does not separate these.

How soaps, cosmetics and moisturizers change the microbiota in healthy people was labeled unclear in the 2011 review.

How much of the skin microbiome research is solid versus early?

The mapping is fairly solid. The mechanisms and the treatments are early. Here is how we sort it.

• Which microbes live at which sites, and the oily, moist and dry pattern.
• Skin communities differ from gut and oral ones.
• Strong person-to-person variation and site-dependent stability.
• S. aureus overrepresentation in atopic dermatitis skin, though the study we read was small.

• Immune benefits: mostly mouse, ex vivo and cell data.
• Causation: association studies dominate, so cause and effect stay unclear.
• Methods: 16S sequencing cannot separate live from dead or resident from transient microbes. Shotgun metagenomics does better but is costlier and used in small studies.

On treatments, a 2025 meta-analysis pooled 12 randomized trials (817 people) of probiotics, synbiotics and postbiotics in atopic dermatitis. It found improvement over placebo (SMD −0.72; 95% CI −1.26 to −0.17), but heterogeneity was very high (I² = 85.1%). That limits confidence in how big the effect is, and the mix includes oral products, so it says little about skin-care products.

A separate review of 14 cosmetic trials using defined probiotic or postbiotic strains in healthy volunteers reported better hydration and radiance and less wrinkle depth. The trials are small, and we did not see effect sizes.

As for microbiome-friendly labels, a 2026 review says most conventional cosmetics do not cause major dysbiosis in healthy skin when properly formulated. It also calls evidence for microbiome-targeted cosmetics preliminary, citing varied designs, small samples and no standard criteria for the claim. We found no dedicated safety data for topical live-microbe products, so safety is unestablished, not demonstrated.