The microbiome consists of bacteria, fungi, and other microorganisms that live in or on the body. These microbial communities play important roles in human health and disease, including both protective and pathogenic functions.

The microbiome of the skin is one of the most diverse, represented by more than a thousand different bacterial species alone. These species play important roles in maintaining the skin’s typical functions, including fighting against disease-causing pathogens that can cause illness.

The composition of the skin microbiome is not static, though. The abundance of certain microbes can change based on a variety of factors, including age, hormone levels, antibiotic exposure, and skin conditions such as eczema.

This article explores the link between the skin microbiome and eczema, including how each affects the other. It will also examine how a better understanding of the skin microbiome may be leveraged to provide new treatment options for preventing or treating eczema.

Disruption of the skin microbiome is a well-established feature of many types of eczema. One specific group of bacteria — Staphylococcus­ — is often found to be present at higher levels on the skin of people with eczema.

“Patients with flaring atopic dermatitis — a common form of eczema — often have increased Staphylococcus aureus on their skin,” noted Heidi H. Kong, MD, a dermatologist and head of the Cutaneous Microbiome and Inflammation Section at the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). “Some patients may have increased S. epidermidis.”

“When the skin has a rash like eczema, it sets up a special environment that kills off beneficial microbes from the skin and permits the bad bacteria to overgrow,” explained Richard Gallo, MD, PhD, founding chair of the Department of Dermatology at the University of California, San Diego.

“Patients with atopic dermatitis usually have a damaged skin barrier and inflamed skin,” said Kong. “This skin barrier disruption can change what microbes are found on skin. For example, we often see S. aureus on skin when the skin is not intact because S. aureus can bind more easily to a damaged skin barrier.”

Research suggests that the strains of S. aureus found on the skin of people with eczema are better adapted to colonize the skin and evade the immune system.

Overgrowth of S. aureus and other bacterial species can lead to reductions in the amount of commensal (non-harmful) bacterial species and lower microbial diversity. This can lead to a further expansion of pathogenic bacterial species and potentially cause skin infections.

“There hasn’t been enough research in humans to know if the skin microbiome affects atopic dermatitis development,” noted Kong.

Results from some studies have suggested that immune reactions to Staphylococcus overgrowth on the skin may contribute to the development of atopic dermatitis and other forms of eczema, but it is unclear whether this is a driving factor or simply a consequence of other disease processes.

“Some of the microbes that overgrow on the skin of eczema produce toxins and other molecules that make the rash worse and cause the itch,” said Gallo.

“We have seen in a very small study from Ireland that when compared with babies who didn’t have eczema at 1 year of age, babies who had eczema at 1 year had some [initial] differences in their skin microbiome at 2 months of age,” Kong explained. “But with this type of study, it’s not clear what that skin microbiome difference might mean.”

Whether bacterial colonization or overgrowth is a cause or effect (or both) of eczema development, a higher burden of S. aureus has been reported on both skin lesions and nonlesional skin in people with eczema compared with the skin of people without eczema.

In people with eczema, S. aureus density was higher on lesions than on skin without lesions in the same individuals and was found to be significantly linked to the severity of eczema symptoms.

Although it is not clear how bacteria like S. aureus contribute to eczema symptoms, overgrowth of these microbes can still be problematic, causing bothersome and sometimes difficult-to-treat skin infections. Many people therefore want to understand what they can do to cultivate a healthy skin microbiome.

“There isn’t a definition of a ‘healthy skin microbiome,’ though,” said Kong. “In small studies of healthy volunteers, we see that the combination of skin microbes found on one person’s healthy skin is different from what is found on other people’s skin.”

Skin microbiome composition has also been found to vary based on the site in question. For instance, the microbiomes of the arms and hands are different from the microbiomes of the feet or elbows.

“What we generally recommend for people with eczema is using emollients, which is important in repairing the skin barrier, and treating skin inflammation,” Kong explained.

“Skin moisturizers help,” agreed Gallo. “And if the eczema is bad, then several medications are available to help the skin environment improve.”

One study found that improvements in skin symptoms of moderate-to-severe atopic dermatitis seen following treatment are typically followed by a normalization of the microbiome to a more “healthy” state and a decrease in the abundance of S. aureus on the skin.

“Some people find that dilute bleach baths a few times a week can be helpful for their eczema,” added Kong. “But dilute bleach baths may not reduce S. aureus even when eczema and itching improve, which raises questions about whether [these] baths are affecting microbes or possibly decreasing skin inflammation.”

Given the role of the skin microbiome in eczema, some experts have proposed that correcting the changes seen in the microbiome may represent an opportunity to relieve eczema symptoms.

“Researchers are working to develop ways to replace the beneficial bacteria. This is known as ‘bacteriotherapy,’” explained Gallo. “It is a very new approach that is currently only in the clinical trial stage.”

Preliminary results with bacteriotherapy for eczema have been conflicting. In one small study involving 10 adults and 5 children with atopic dermatitis, topical treatment with the commensal bacterium Roseomonas mucosa found that 6 of the adults and 4 of the children had at least a 50% improvement in their eczema symptoms.

However, in a larger, randomized version of this study, there was no difference in treatment response between those who received topical R. mucosa and those who received a placebo treatment.

Kong also noted a 2021 study in which the use of topical Staphylococcus hominis was found to be safe and effective for decreasing levels of S. aureus on the skin, but the treatment did not reduce eczema symptoms or severity.

“Since the skin microbiome is individualized in healthy individuals, it is possible that a personalized treatment approach may be needed,” said Kong. “More studies would be needed to test that.”

Skin microbiome changes are common in eczema, typically characterized by overgrowth of the S. aureus bacterium. Dysregulation of the skin microbiome is more pronounced in people with severe symptoms and may contribute to complications such as skin infections.

Moisturizers and eczema treatments can help restore skin health and potentially correct changes in the skin microbiome. While researchers are interested in understanding how the microbiome could be leveraged to support eczema treatment, this research is still in the early stages, and more studies are needed.