7 The Skin Microbiome
The Skin Microbiome
The microorganisms that inhabit the largest human organ are important in a variety of ways in regard to host health. The skin microbiome defends against pathogens, educates the immune system, helps wound healing, and moderates progression of diseases, and in return receives nutrients and real estate for colonization, creating a symbiotic establishment (Byrd et al., 2018). As the primary external interface to the environment, the skin serves as the initial physical barrier against invasion of potential pathogens. The territory of the skin itself is harsh for most microbes; its cool, acidic, dry, and considered nutrient poor, and so only those organisms that have adapted to these conditions can successfully colonize it. A majority of these microbes rely on obtaining nutrients from sweat, sebum, and dead skin cells by using proteases and lipases to break apart various compounds to liberate usable resources (Byrd et al., 2018). Though, there are adverse conditions and a general lack of food for microbes on the skin, there is still quite a diverse community which is unique to certain individuals and body locations.
Better characterization of the skin microbiome and its site-specific diversity can allow for greater understanding of skin diseases such as atopic dermatitis, acne, rosacea, and psoriasis that are associated with dysbiosis (Grice and Segre, 2011). The role of resident and transient microorganisms are important in the onset and progression of these types of diseases, and their study may help in diagnosis and treatment protocols.
Composition and Stability
Within a single square centimeter on the skin there can be up to one billion microorganisms, and this mixed community of bacteria, viruses, protozoa, fungi, and mites can be both good and bad for host health (Grice and Segre, 2011, Weyrich et al., 2015). Overall, there are four main bacterial phyla that constitute the human skin microbiome: Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes (in descending order of abundance) (Grice et al., 2009). Microbial composition primarily depends on the specific location of skin site, and particularly whether it is dry, moist, or sebaceous (i.e. oily). Moist areas like the bend of the elbow or the feet harbor bacteria like Staphylococcus or Corynebacterium species and tend to have higher species diversity, while sebaceous locations promote the growth of Propionibacterium species and are lower in diversity (Grice et al., 2009, Oh et al., 2016, Byrd et al., 2018).
The composition of fungi on the skin also depends on location and physiological properties. Fungal members of the skin microbiome primarily consist of species from the genus Malassezia, with some from Penicillium and Aspergillus, and less of a few other genera. Specifically, the community of fungi on the feet have a high amount of diversity and it tends to change more over time as compared with other locations on the body. This may be due to