1 | SKIN AS A BOUNDARY BETWEEN OUR INTERNAL AND EXTERNAL WORLD
It's not surprising that the skin, our interface with the world, supports the body's most diverse population of bacteria. More than 1,000 species of bacteria are found on the skin, as well as millions of other microbes including fungi and microorganisms. Fortunately, most of these bacteria aren't harmful, and they often act as protective agents. As they live among dead skin cells that cover the surface of our skin, they protect us from disease as they defend their own territory from other microbes. The bacterial microbiome on the skin has been compared to a fingerprint: Unique to each person. Although no two people have the same microbial composition, we do share some similarities from layer to layer.
2 | THE THREE LAYERS OF OUR SKIN
There are three layers of the skin.
THE EPIDERMIS constitutes the topmost layer of our skin, which is exposed to the environment. This layer is the protective barrier that keeps harmful pollutants out and locks nutrients/moisture inside. The epidermis contains cells that produce pigment and protect the immune system. It is heavily colonized by commensal bacterial organisms that help protect us against pathogenic bacteria. The highly regulated and dynamic interaction between the epidermis and commensals involves the host’s production of nutritional factors promoting bacterial growth together with chemical and immunological bacterial inhibitors. On the skin surface, popular bacterial colonies include rod and round bacteria — such as Proteobacteria and Staphylococcus spp., respectively — which form communities that are deeply intertwined among themselves and other microorganisms. Other bacterial colonies that populate the epidermis include P. acnes (which can lead to acne when the microbiome is imbalances), S. aureus (which can result in atopic dermatitis, also known as eczema, during periods of imbalance), and S. epidermidis.
THE DERMIS is the middle layer of the skin, and where most of the skin’s primary functions take place. It contains nerve endings, oil, sweat glands, and hair follicles. The absence of oxygen within this layer, in between hair follicles, creates a unique environment to produce localized individual microbiota. The dermis shares many similar bacterial species with the epidermis, though research has shown that it has far fewer colonies being that it’s not exposed to environmental factors on the surface.
THE SUBCUTANEOUS TISSUE, the innermost layer of our skin, is made up of fat, connective tissue, and blood vessels. Its main purpose is to keep the body insulated and cushioned. Proteobacteria are found to colonize deep, cutaneous compartments, where they may regulate skin pH and regulate host-environment interactions, allowing us to maintain homeostasis. Other popular colonies include Corynebacterium, Staphylococcus, and Streptococcus bacterium.
3 | THE IMPORTANCE OF MICROBIAL BALANCE
Local conditions, such as density of glands or hair follicles, as well as chemical factors, such as pH, moisture, and temperature, all influence bacterial composition. An imbalance in either of these factors can cause alterations in our skin’s microbial communities, characterized by an overgrowth of commensal bacteria or an invasion of harmful microorganisms. This in turn can lead to a state of dysbiosis and potential skin inflammation, thus demonstrating a causal link between skin barrier alterations, dermatitis and the microbiome.