Skin: Basic Structure and Function
The skin serves many critical functions (MacNeal 2016a). Not only is it a barrier against the external environment, it is also an active part of the immune system (Jafferany 2016), regulates body temperature, helps maintain water and electrolyte levels, shields against solar radiation, and makes vitamin D with adequate sun exposure (MacNeal 2016a).
Skin is made up of three layers: the epidermis (superficial/outer layer), dermis (middle layer), and subcutaneous tissue (deep layer). The epidermal structural protein, keratin, and the oily secretion, sebum, give skin a tough surface that is relatively waterproof, protects against injury, and prevents entry of foreign substances. The dermis houses sweat and sebaceous glands, hair follicles, nerve endings, and capillaries. Its fibrous and elastic tissues give skin strength and flexibility. The subcutaneous layer, made up of fat cells and fibrous tissue, provides insulation, padding, and energy storage (MacNeal 2016b).
The Skin Microbiome
An estimated one million bacteria from hundreds of species reside on every square centimeter of skin surface (Egert 2016; Dreno 2016; Barnard 2017). Collectively, these bacteria comprise the skin microbiome, which plays an important role in skin immune function (Chen 2013). Alterations in the composition of the skin microbiome are linked to both inflammatory and infectious skin disorders (Biedermann 2015; Pasparakis 2014; Chen 2013).
The recognition of the important role bacteria play in skin integrity and immune function has led to intriguing new research avenues. For instance, we now have evidence that certain probiotic strains can reduce colonization by pathogenic microorganisms such as methicillin-resistant Staphylococcus aureus (MRSA) and other antibiotic-resistant bacteria (Hafez 2013; Sikorska 2013). Probiotics are also a promising treatment option for atopic dermatitis (Seite 2017).