Healthy skin carries its own defense force: friendly bacteria that make natural antimicrobials against a common troublemaker, Staphylococcus aureus. On eczema-prone skin, those protective strains are scarce.
The skin microbiome · Science Translational Medicine, 2017 · 5-minute read
Why we're sharing this
When I started Eleusia, one question kept coming back: what if skincare worked with the skin's living ecosystem instead of scrubbing it away?
This is one of the studies that answered it for me. It shows that the bacteria on healthy skin are not just tolerated guests; some of them actively guard it. That idea shapes everything we formulate, from respecting the skin's natural acidity to choosing ingredients that support its barrier rather than strip it.
"Your skin is not sterile. And that's the point."
— Liz, Founder of Eleusia
The study in three sentences
- Researchers at UC San Diego tested thousands of bacteria from the skin of healthy people and people with eczema.
- On healthy skin, most of these bacteria produced natural antimicrobials that stopped Staphylococcus aureus; on eczema-prone skin, those protective strains were rare.
- When protective bacteria were returned to the skin of five people with eczema, S. aureus dropped within 24 hours.
Inside the experiment
A census of the skin. The team swabbed the forearms of 30 healthy adults and 49 people with atopic dermatitis. From those swabs they grew 7,724 individual bacterial colonies and tested each one: could it stop S. aureus from growing?
Finding the weapons. The most active strains, mostly Staphylococcus epidermidis and Staphylococcus hominis, were studied in the lab to identify exactly what they produced. They were then applied to pig skin and to mice carrying S. aureus, side by side with strains that produce nothing.
Back to people. Five people with eczema had their own protective bacteria grown from their skin, mixed into a cream and applied to one forearm. The other forearm got the same cream without bacteria. Because each person was their own comparison, differences between people could not explain the result.
Five things it taught us
1. Healthy skin defends itself. Most bacterial colonies from healthy skin could stop S. aureus. On eczema-prone skin they were far rarer, and rarest where S. aureus had already settled in.
2. It's about balance, not just who's there. People with eczema still had S. epidermidis and S. hominis. What they were missing were the strains that actively protect. Scientists call this imbalance dysbiosis. In fact, no one carrying S. aureus had protective activity in more than half of their skin's colonies.
3. The defense is precise. The bacteria made previously unknown antimicrobial peptides that targeted S. aureus and left other skin bacteria alone. A harsh cleanser does the opposite: it removes everything at once.
4. Skin and bacteria work as a team. These bacterial peptides worked alongside LL-37, an antimicrobial the skin produces on its own. The body's defenses and its resident bacteria reinforce each other.
5. Restoring balance had a measurable effect. After one application, the arm that received protective bacteria had significantly less S. aureus than the arm with plain cream (p = 0.04). Each person's level of protective bacteria also stayed steady over 14 days, which suggests this is a stable trait of the skin, not a passing state.
Read with care
Good science tells you its limits. Here are this study's:
- Small and short. The human test involved five people, one application and one check at 24 hours. It shows the effect happens on real skin, not how long it lasts.
- Bacteria, not symptoms. The researchers measured S. aureus on the skin, not itch, redness or dryness.
- A specific method. The protective bacteria were living strains taken from each person's own skin. The results apply to that approach, not to skincare in general.
- Disclosed interests. Two authors are co-inventors of related university technology, and one co-founded a company developing bacteria-based skin therapies. The paper states this openly.
Keep exploring
- The big picture. An NIH review describes how skin bacteria help train the immune system, and links several skin conditions to microbial imbalance. The human skin microbiome, Nature Reviews Microbiology, 2018
- Why pH matters. The skin's acidic surface (pH 4.5 to 5.5) helps keep its bacterial community balanced; when pH rises, harmful bacteria grow more easily. Choi & Kang, 2024
- Beyond live bacteria. How postbiotics, ingredients made from bacteria but containing no live cells, are being studied for skin health. Microbiome and Postbiotics in Skin Health, Biomedicines, 2025
Sources
Nakatsuji T, Chen TH, Narala S, Chun KA, Two AM, Yun T, et al. Antimicrobials from human skin commensal bacteria protect against Staphylococcus aureus and are deficient in atopic dermatitis. Science Translational Medicine. 2017;9(378):eaah4680.
Read the full study, free · PubMed · Publisher
A note on how we use research: we summarise published science so you can see the thinking behind our formulas. Eleusia had no involvement in this study, and its results are about the research method described, not about our products. This article is for education only and is not medical advice; if you have a skin condition, please see a dermatologist.