The skin microbiota

The skin is thin. Its outermost layer, the stratum corneum, is only a few hundredths of a millimetre thick – yet it plays a crucial role in protecting the body from the outside world. This small barrier is anything but passive. The outermost cells are biologically active and involved in immune defence. An important part of this defence is made up of the skin microbiota – bacteria, fungi and other microorganisms – found across the entire skin surface. But the microbiota is not passive either.
 
"The skin produces natural substances, antimicrobial peptides, that protect against harmful microorganisms. It’s an important part of our first line of defence against bacteria, viruses and fungi," says Andreas Sonesson, researcher at Lund University and dermatologist at Skåne University Hospital.

A struggle for space


In normal, healthy skin there is diversity, a range of different bacteria in which the benign ones predominate. But there is a constant struggle for space and nutrients.
 
"As long as we have an intact skin barrier and favourable conditions for the benign bacteria, they will protect us from the more harmful ones."
 
Healthy skin is quite resilient in itself. It adapts to heat, cold and other stresses. Problems arise when the barrier is damaged – by disease, chemicals or dryness. The balance between microorganisms is crucial – but also fragile. Some bacteria can inhibit others by producing substances that limit their growth. When the skin barrier is damaged, disease-causing bacteria may gain the upper hand and better conditions for attaching themselves.

 One of the most important things in atopic eczema is to restore the skin barrier

"They can then penetrate deeper into the skin and gain access to more nutrients. The amount of bacteria then increases, and more disease-causing species take over," says Andreas Sonesson.
 
At the same time, microorganisms can influence the immune system.
 
"There are bacteria that can trigger inflammation in the skin, especially if the barrier is already damaged. They may not be the cause of the disease, but they can amplify the inflammation."
 
Researchers are now trying to understand these connections better.
 
"In our research group, we are looking at, among other things, how the microbiota changes with different degrees of eczema, and whether the bacteria can be used as a marker of how damaged the skin barrier is and how active the inflammation is."
 
One of the diseases in which the link between the skin barrier and the microbiome is clear is atopic eczema. It is a chronic inflammatory disease that affects up to 20 per cent of all children, and also many people in adulthood.

Impaired protection


In atopic eczema, the skin’s protection is impaired, partly due to hereditary factors. This means the barrier functions less effectively.
 
"What you see in eczematous skin is that the microbial flora becomes fairly uniform. Often Staphylococcus aureus dominates, and the other, more benign bacteria are pushed aside. If the skin breaks, the damaged barrier will favour the bacteria, which in turn then worsen the damage."

Staphylococcus aureus can further break down the skin barrier by secreting substances and contribute to inflammation. It can also form what is known as a biofilm, which is like a sticky “city” where bacteria live together and build a protective wall around themselves, so that they are not so easily killed by, for example, antibiotics or the body’s immune system.
 
One perhaps unexpected area of research concerns treatments that have long been used without our fully understanding why they work. Baths with potassium permanganate, a bacteria-killing agent, have been used for many years for various weeping, infected or intensely itchy skin conditions, such as atopic eczema.
 
"We have shown that these baths can kill disease-causing bacteria present in the skin in atopic eczema," says Andreas Sonesson.
 
Similar results have been found for bleach baths, which are used internationally.
 
"They can be effective against biofilm-producing bacteria, which are otherwise difficult to treat."
 
But the treatments do more than just kill the bacteria; they can probably in several ways help the skin barrier recover and reduce inflammation.
 
"One of the most important things in atopic eczema is to restore the skin barrier, because then the bacteria cannot attach in the same way."
 
When the skin recovers, a more normal skin flora returns.
 
"We have seen that if eczema is well controlled over a longer period, there is a smaller amount of Staphylococcus aureus. It becomes clear that there is a link between the condition of the skin and the bacteria found there," says Andreas Sonesson.
 
Text and photo: Tove Smeds
The article was previously published in the journal Vetenskap & Hälsa, 2026.