The Skin is our Eco System
The skin is an ecosystem composed of 1.8 m2 of diverse habitats with an abundance of folds and specialized niches that support a wide range of microorganisms. The primary role of the skin is to serve as a physical barrier, protecting our bodies from potential assault by foreign organisms or toxic substances. The skin also interacts with the outside environment and, as such, is colonized by a diverse collection of microorganisms — including bacteria, fungi and viruses — as well as mites. Many of these microorganisms are harmless and in some cases provide vital functions. Symbiotic microorganisms occupy a wide range of skin areas and protect against invasion by more harmful organisms. These microorganisms may also have a role in educating the billions of T cells that are found in the skin, priming them to respond to their less beneficial cousins.
The perception of the skin as an ecosystem — composed of living biological and physical components occupying diverse habitats — can further our understanding of the delicate balance between host and microorganism. Disruptions in the balance on either side of the equation can result in skin disorders or infections. For example what may affect the host–microorganism relationship can be for example, genetic variation, or even hand washing. To further our understanding of health, disease and infection of the skin, microbiologists, immunologists and dermatologists have partnered with genomic scientists to develop a more complete characterization of the skin microbiota and how it interacts with the host .
The habitat of the skin defined
The physical and chemical features of the skin select for unique sets of microorganisms that are adapted to the area they inhabit. In general, the skin is cool, acidic and desiccated, but distinct habitats are determined by skin thickness, folds and the density of hair follicles and glands. Structurally, the epidermis is a formidable physical barrier, resisting penetration by microorganisms and potential toxins while retaining moisture and nutrients inside the body. The top layer of the epidermis, is composed of terminally differentiated, keratinocytes that are known as sqames. Squames form the ‘bricks and mortar’ of the epidermis. The skin is a continuously self-renewing organ, and squames are constantly shed from the skin surface as the final stage, having begun their migration from the lowest layer 4 weeks earlier.
Cutaneous areas, including sweat glands, sebaceous glands and hair follicles, are likely to be associated with their own unique microbiota. Other glands, which are more abundant are found on virtually all skin surfaces and continuously bathe the skin surface with their secretion, which is composed mainly of water and salt. The primary role of eccrine sweat is thermoregulation through the release of latent heat from the evaporation of water. Additional functions of those glands include excretion of water and electrolytes, and acidification of the skin, which prevents the colonization and growth of microorganisms. Apocrine glands, which are located in the armpit, nipple and genitoanal regions, respond to adrenaline by producing milky, viscous, odourless secretions. Such secretions have long been postulated to contain molecules that trigger certain behaviours (for example, sexual or alarm) in the receiving individual. The stereotypical odour associated with sweat derives from bacterial processing and utilization of gland secretions.
Sebaceous glands are connected to the hair follicle, forming the pilosebaceous unit, and secrete the lipid-rich substance sebum. Sebum is a coating that protects and lubricates the skin and hair and provides an antibacterial shield. Sebaceous glands support the growth of facultative anaerobes such as Propionibacterium acnes, a common skin bacterium. Free fatty acids also contribute to the acidic pH (~5) of the skin surface. Many common pathogens, such as Staphylococcus aureus andStreptococcus pyogenes, are inhibited by an acidic pH, thus the growth of negative staphylococci and corynebacteria is favoured. However, skin occlusion results in an elevated pH, which favours the growth of S. aureus and S. pyogenes.
The skin surface varies topographically owing to regional differences in skin anatomy and, according to culture-based studies, these regions are known to support distinct sets of microorganisms. Some regions of the skin are partially occluded, such as the groin, axillary vault and toe web. These regions are higher in temperature and humidity, which encourages the growth of microorganisms that thrive in moist conditions (for example, Gram-negative bacilli, coryneforms and S. aureus). The density of sebaceous glands is another factor that influences the skin microbiota, depending on the region. Areas with a high density of sebaceous glands, such as the face, chest and back, encourage the growth of lipophilic microorganisms (for example, Propionibacterium spp. and Malassezia spp). Compared with other skin sites, arm and leg skin is relatively desiccated and experiences large fluctuations in surface temperature. Using culture-based methods, these areas were found to harbour quantitatively fewer organisms than moist areas of the skin surface.
Factors specific to the host, such as age, location and sex, contribute to the variability seen in the microbial flora of the skin. Age has a great effect on the microenvironment of the skin and, thus, on the colonizing microbiota. In the uterus, fetal skin is sterile, but colonization occurs immediately after birth, either during vaginal delivery or in the minutes following birth by caesarian section. One area for future research is to explore how the microbial communities of the skin and other sites are established and stabilized during the first years of life, as a newborn baby explores its environment and matures its immune system. During puberty, changes in sebum production parallel the levels of lipophilic bacteria on the skin. Physiological and anatomical differences between male and female cutaneous environments such as sweat, sebum and hormone production, partially account for the microbial differences seen between the genders.
Environmental factors specific to the individual, such as occupation, clothing choice and antibiotic usage, may modulate colonization by the skin microbiota. The effect of antibiotic treatment on the gut microbiota has been examined using molecular methods. Cosmetics, soaps, hygienic products and moisturizers are also potential factors contributing to the variation of skin microbiota. Cultures demonstrated that high-temperature and high-humidity are associated with increased quantities of bacteria on the back, axillary vaults and feet as compared with high-temperature low-humidity conditions. In the same study, high humidity and low temperature conditions were associated with a higher frequency of Gram-negative bacteria on the back and feet. Ultraviolet (UV) light is a well-documented bactericidal treatment.