Cutaneous microbiota: Role in skin diseases

The skin is the largest human organ and represents a multiple barrier to external agents. Still, it is also a complex ecosystem inhabited by diverse microbiota, which plays a key role in maintaining skin homeostasis and protecting the host from pathogens. The skin microbiota is the second largest in humans, after the gut microbiota, and includes about 40 trillion microorganisms, with the following dominant phyla: - Bacteria: Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes - Fungi: dominated by Malassezia - Viruses: mainly bacteriophages, but also human viruses, such as human papillomavirus - Archaea: dominated by the class Nitrososphaeria. The cutaneous microbiota is less diverse than the intestinal microbiota, yet it is extremely complex. Its composition is influenced by numerous endogenous (age, gender, variations in the genome, and immune response) and exogenous factors (geographic and climatic factors, differences in lifestyle, and medication use). Due to its topographic organization with different microclimatic zones, there are different areas (“niches”) on the skin that have a specific microbiota structure. Among the most important environmental factors that influence such composition of the microbiota are: the amount of sebum in that specific zone, temperature fluctuations, humidity level, the presence of skin folds, and the composition of antimicrobial lipids and peptides. In addition to its proven role in preserving the skin barrier, the skin microbiota, through its influence on the immune system, metabolic activity, and mutual regulation with other microbiomes, additionally contributes to maintaining the health of the skin and the whole organism. Disturbance of microbiota homeostasis with a disruption of diversity in the skin ecosystem (dysbiosis) is considered an important factor in the pathogenesis of various skin diseases, namely atopic and seborrheic dermatitis, psoriasis, hidradenitis suppurativa, acne, and rosacea. Also, understanding the influence of microbiota on the development of various skin diseases led to the emergence of the first therapeutic modalities based on modulation of the profile of microorganisms, such as microbiota transplantation and bacteriophage therapy.