As the body’s largest organ, your skin provides a physical barrier to help protect you from harmful pathogens, UV rays, toxins and other potential environmental threats. But it also provides a home to millions of bacteria, fungi, viruses and other microbes that, together with their genetic material, make up the skin microbiome. These microbes play a critical role in maintaining a healthy skin barrier.
Key members of the skin microbiome
Microorganisms are found all over the surface of the skin, but the specific types will vary depending on where you look. This is because different parts of the body have a different pH, sun exposure, temperature and moisture content (among other factors) that support the growth of different types of microbes.
In general, moist environments like the bends of the elbow or back of the knee are colonised by bacteria that enjoy humid environments, like Staphylococcus and Corynebacterium species. Oily parts of the body such as the forehead or back are often dominated by Cutibacterium species; these lipophilic (fat-loving) bacteria feed on the fatty components found in the oily parts of your skin. Finally, dry parts of the body like your forearm or palm of your hand are usually home to a more diverse community of microbes since these areas are more frequently exposed to changing environments.
While majority of your skin microbiome is made up of bacteria, you also have different types of fungi, viruses and archaea at each of these body sites, which work together with the bacteria and with your own cells to maintain a healthy, balanced skin barrier.
Colonisation of the skin microbiome
Similar to the gut microbiome, microbes begin to colonise the skin at birth. This is largely influenced by your mode of birth—babies born naturally are first exposed to microbes from their mother’s vaginal microbiome, while babies born via Caesarean section are first exposed to skin-associated microbes1. As you grow, other factors like age, where you live and personal hygiene (e.g., skincare products or other health practices) can contribute to the composition of your skin microbiome. However, unlike the gut microbiome, which can change in a matter of days in response to a new diet, the adult skin microbiome is thought to remain relatively stable over time, even when it is constantly exposed to different environments2.
While bacteria in the gut are lucky enough to get a steady supply of nutrients from what you eat, microbes on the skin have developed unique metabolic characteristics that allow them to survive in this environment. Many members of the skin microbiome can break down the nutrients found in sweat, sebum (an oily substance produced to protect and hydrate your skin barrier) and the stratum corneum (the outermost layer of the skin). For example, some bacteria use the urea and amino acids found in sweat as a fuel source, while others produce enzymes that help release amino acids from proteins on the skin, which they can then use to grow3. Others have evolved to use things like fatty acids, ammonia or lactate as their fuel of choice3.
Working together to fight off pathogens
The primary role of the skin microbiome is to enhance our skin’s physical barrier to protect us against potential threats in the environment4. In a healthy ecosystem, beneficial bacteria compete with pathogens for nutrients and space, and some members produce antimicrobial compounds that inhibit the growth of harmful organisms.
Take for example Staphylococcus aureus, a type of bacteria found on the skin of many individuals but can cause serious infections if it enters the bloodstream. S. aureus is also a common cause of antibiotic resistant infections. Common members of the skin microbiome, such as S. epidermidis, can produce antibiotics or other molecules to either kill S. aureus directly or to inhibit their ability to colonise the skin5. This protective effect is crucial in preventing infections and maintaining skin health.
The skin microbiome is also involved in modulating the immune system6. Beginning in early life, the microbiome helps train the immune system to distinguish between beneficial bacteria and harmful organisms. When cells in our skin (called keratinocytes) do detect danger, they signal to the immune system to produce antimicrobial peptides that can rapidly kill potential pathogens while sparing the beneficial members of the microbiome. This interaction is essential for preventing unnecessary inflammatory reactions that can lead to skin disorders.
What happens when the skin microbiome is disrupted?
While a healthy skin microbiome helps regulate pH levels, retain moisture for a healthy skin barrier, and helps educate and regulate the immune system, an imbalance in the skin microbiome (known as dysbiosis) has been linked to several skin conditions.
One common example of this is acne vulgaris, a chronic inflammatory condition of the skin that pops up predominately in teenagers and young adults. The skin microbiome undergoes a shift during puberty, when our skin starts to produce more oil thanks to a wave of hormonal changes. This increased oil production causes an increase in fat-loving bacteria like Cutibacterium acnes, which is the most abundant species on the skin of healthy adults. However, the increase in certain types of C. acnes is in part to blame for the acne that often accompanies puberty7.
In addition to acne vulgaris, imbalances in the skin microbiome have been linked to other inflammatory skin conditions, such as atopic dermatitis (eczema) and psoriasis. However, more research is needed to determine if an altered skin microbiome is a cause or consequence of these conditions. The hope for the future is that a better understanding of the skin microbiome will lead to new ways to prevent or treat these and other skin conditions.
The take-away
The skin microbiome has a major influence on the integrity of your skin barrier, helping to keep moisture in and pathogens or other irritants out. You can help take care of your skin barrier by staying hydrated, eating a diet rich in anti-inflammatory foods, and avoiding harsh skincare products that can damage the barrier or harm beneficial bacteria.
While a lot of research is still needed, a better understanding of the skin microbiome holds potential for innovative disease treatments and skincare products. We hope that the AHMB will help make significant strides towards a better understanding of the skin microbiome.
References
- Dominguez-Bello, M. G. et al. Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns. Proc Natl Acad Sci U S A 107, 11971-11975, doi:10.1073/pnas.1002601107 (2010).
- Oh, J., Byrd, A. L., Park, M., Kong, H. H. & Segre, J. A. Temporal Stability of the Human Skin Microbiome. Cell 165, 854-866, doi:10.1016/j.cell.2016.04.008 (2016).
- Scharschmidt, T. C. & Fischbach, M. A. What Lives On Our Skin: Ecology, Genomics and Therapeutic Opportunities Of the Skin Microbiome. Drug Discov Today Dis Mech 10, doi:10.1016/j.ddmec.2012.12.003 (2013).
- Byrd, A. L., Belkaid, Y. & Segre, J. A. The human skin microbiome. Nature Reviews Microbiology 16, 143-155, doi:10.1038/nrmicro.2017.157 (2018).
- Nakatsuji, T. et al. Antimicrobials from human skin commensal bacteria protect against Staphylococcus aureus and are deficient in atopic dermatitis. Sci Transl Med 9, doi:10.1126/scitranslmed.aah4680 (2017).
- Grice, E. A. & Segre, J. A. The skin microbiome. Nat Rev Microbiol 9, 244-253, doi:10.1038/nrmicro2537 (2011).
- McLaughlin, J. et al. Propionibacterium acnes and Acne Vulgaris: New Insights from the Integration of Population Genetic, Multi-Omic, Biochemical and Host-Microbe Studies. Microorganisms 7, doi:10.3390/microorganisms7050128 (2019).
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