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Compound produced by bacteria protects the skin

The study shows how Staphylococcus epidermidis bacteria, shown here in green, can help protect the skin.NIAID

The human skin is home to about a thousand species of bacteria and other microbes, collectively called the skin microbiome. Many of these are important for health.

Beneficial skin bacteria can prevent the growth of other, potentially harmful microbes. Studies have also shown that some skin bacteria interact with the immune system to help fight infection. But the role that bacteria may play in the normal maintenance of healthy skin hasn’t been well understood.

The growth and protection of the skin’s protective outer layer is a complex process. It involves a mix of cells and the fatty insulation they produce. Together, these prevent water from escaping the skin and pathogens from getting in.

Ceramides are one type of protective fatty molecule found in the outer skin. Low ceramide levels result in dry skin and are associated with aging and some skin disorders.

In a new study, researchers led by Dr. Michael Otto from NIH’s National Institute of Allergy and Infectious Diseases (NIAID) examined the contribution of a common skin bacterium called Staphylococcus epidermidis to skin protection. Results were published on February 1, 2022, in Cell Host & Microbe.

When the researchers applied S. epidermidis to the skin of mice that had been exposed to common irritants, water loss through the animals’ skin was reduced. This showed that the bacteria were somehow helping maintain the health of the skin’s outer layer.

The team suspected that a type of enzyme called a sphingomyelinase (Sph) might be responsible. Sph is produced by some bacteria. It breaks sphingomyelin, a fat found on the surface of cells, into ceramides and phosphocholine (PC), a nutrient source for the bacteria.

The researchers found the gene that encodes for Sph in 98% of a collection of S. epidermidis isolated from human skin samples. Laboratory experiments didn’t find that Sph production promoted any potentially dangerous behavior of the bacteria that might harm the host.

Additional experiments in cultured cells confirmed that S. epidermidis uses Sph to acquire PC from the sphingomyelin found on skin cells. PC production helped the bacteria survive in high-salt conditions, such as that found in the skin’s outer layer.

In mice exposed to normal drying conditions, as well as those with an itchy skin condition associated with depleted ceramide, application of S. epidermidis reduced water loss though the skin. This protective effect was dependent on the bacteria’s ability to produce Sph. The process appeared to be symbiotic—that is, it benefitted both microbe and host. The breakdown of sphingomyelin helped the microbes survive while creating ceramides to protect the hosts’ skin.

“Our study highlights the potential for S. epidermidis to be used as a probiotic treatment to promote skin health during aging or in people suffering from skin diseases,” Otto says.

The research team is now hoping to test the approach in clinical trials in people.