The skin is a physical barrier against invasion by pathogenic organisms and foreign substances. The skin is also an ecosystem, host to a microbial milieu that, for the most part, is harmless.The habitat of the skin varies topographically and is likely to be associated with variation in the colonizing microbiota. Factors contributing to variation in the skin microbiota include the density of hair follicles and glands (sweat or sebaceous), host factors (such as age and sex) and environmental factors (such as occupation, climate and hygiene).Analysing skin bacterial microbiota by sequencing of 16S ribosomal RNA genes reveals a greater diversity of organisms than has been found by culture-based methods.The microenvironment of the skin site sampled determines to a large extent the colonization by the predominant species, the temporal variation and the interpersonal variation. Propionibacterium spp. predominate in sebaceous areas, Corynebacterium and Staphylococcus spp. predominate in moist areas, and dry areas exhibit the greatest amount of diversity.Compared with other mucosal microbiomes, the skin microbiome shows the greatest variability over time and harbours the greatest phylogenetic diversity.The cutaneous immune system modulates colonization by the microbiota and is also vital during infection and wounding. Dysregulation of the skin immune response is evident in several skin disorders.A wide range of skin disorders are postulated to arise in part owing to a microbial component. These disorders include atopic dermatitis, acne, seborrhoeic dermatitis and chronic wounds. Additionally, commensal bacteria (for example, Staphylococcus epidermidis) can become pathogenic and cause invasive infection.
