Here, There, Everywhere: The Skin Microbiome and Your Health

If you’re a germaphobe, it’s probably your worst nightmare to read that your skin is crawling with millions of bacteria, viruses and fungi all the time. The network of tiny microorganisms that live on our skin is called the skin microbiome. You may be relieved, however, to learn that some of these microorganisms are necessary to keep you healthy. The skin microbiome directly influences the immune system and can have substantial effects on your health.

The Skin Microbiome

Different areas of your body house distinct collections of microorganisms that thrive in various bodily conditions. While the bacteria in your gut, for example, have evolved to survive in stomach acid, the bacteria on your skin have adapted to the sweaty, salty, nutrient-scarce environment of your skin. Most of these microorganisms are harmless. Some are even beneficial, helping your immune system defend you from disease-causing organisms known as pathogens. In exchange for shelter, these microbes help your immune system fight infection.

Healthy human skin has one of the largest stores of immune cells in your body, housing about 20 billion of them! The skin microbiome interacts with these cells to significantly impact both your innate and adaptive immune responses. Innate immunity is your body’s first line of defense to protect itself from harm. It is the protection you were born with and that is immediately activated at the first sign of threat from an invading pathogen. When your innate immune system is not enough to control an infection, however, adaptive immunity comes into play. Adaptive immunity is known as “acquired” immunity because it develops throughout your lifetime as you are exposed to various pathogens. Unlike the innate immune system, the adaptive immune system retains memory of foreign invaders so that when you face a pathogen you’ve seen before, you don’t get as sick or don’t stay ill as for as long. These two systems work together to keep you healthy.

The Skin Microbiome & Immunity

The innate immune system deploys both physical and biological defenses to prevent and fight infection. For example, your skin provides a physical barrier to pathogens trying to access your body. But also, the bacteria that live on your skin secrete compounds that can regulate our innate immune response. One microbe called S. epidermidis, for instance, produces a molecule that can promote wound healing and reduce inflammation. The skin microbiota also activates a cascade of activity that promotes the body’s ability to clear out pathogens. For example, one species of skin microbe called Propionibacterium controls the production of antimicrobial peptide (AMP), a small protein that kills or inactivates various pathogens.

 Studies have found that mice lacking skin microbes have lower levels of these proteins, making them more susceptible to infection. Other skin microbes have been found to increase the production of a type of proteins called interleukins that help regulate immune responses.

The adaptive immune system functions as an extension of the innate immune system. Therefore, when the skin microbiota stimulates the innate immune system, they also enhance our adaptive immune response. For example, when skin bacteria stimulate production of a specific interleukin protein called IL-1, this protein then promotes inflammation and fever and prompts your adaptive immune cells. According to one study, in mice that don’t have adaptive immune cells, their skin microbiota aren’t able to control pathogens, allowing them to spread. This shows us that by regulating and promoting your adaptive and innate immune responses, your skin microbiota prevents and contains infection.

An Unbalanced Microbiome Could Seriously Impact Your Health

Because the skin microbiome plays such an important role in regulating your immune responses, any irregularities in skin microbiota could wreak havoc on your health. The key to maintaining healthy skin is keeping the right types and abundance of bacteria. Inflammatory skin diseases such as eczema, atopic dermatitis, and psoriasis have been associated with bacterial imbalances. Additionally, one study found that plaque psoriasis patients have abnormal levels of the skin microbe P. acnes.

The bacteria that make up the skin microbiome have defined roles that help regulate the immune system through a variety of mechanisms. Compared to the workings of the gut microbiome, these mechanisms are not yet well characterized, yet scientists have already implicated the skin microbiome in more aspects of disease than ever before. Probiotics—which promote the formation of healthy bacteria—when applied to the skin, help treat conditions like acne, atopic dermatitis, and rosacea. In fact, some cosmetics and other topical products already include probiotics. As we learn more, maybe we’ll realize that the answer to better health is already among us. Or in this case, on us.

References

• Belkaid, Y., Segre, J.A. Dialouge between skin microbiota and immunity. Science. 2014.
• R. L. Gallo, L. V. Hooper, Nat. Rev. Immunol. 12, 503–516 (2012).
• Y. Lai et al., Nat. Med. 15, 1377–1382 (2009).
• S. Naik et al., Science 337, 1115–1119 (2012).
• Y. Lai, R. L. Gallo, Trends Immunol. 30, 131–141 (2009).