The cutaneous microbiota as a determinant of skin barrier function: Molecular interactions and therapeutic opportunities

Abstract

As the largest organ of the human body, skin provides the first barrier against environmental insults, including invading pathogens. Many studies have defined commensal skin bacteria; more recent metagenomic studies have extended characterization of the microbiota to resident fungi and viruses. The skin is dominated by members of Actinobacteria, Firmicutes, Proteobacteria, Bacteroidetes, Malassezia spp., bacteriophages and human viruses. Defining the microbiota of both healthy and affected skin provides insight into the influence of the cutaneous microbiota on immune responses and disease states. Crosstalk between commensal microbiota and the innate immune system facilitates proper response and healing. Commensal bacteria appear to protect from pathogens directly by releasing antibacterial products and indirectly by stimulating innate immune responses. Skin pathologies such as atopic dermatitis, rosacea, psoriasis and acne are characterized by disruptions of certain immune pathways and imbalances of skin microbiota. Additionally, susceptibility to skin infection appears to be influenced by the microbial community present on the skin, while infection and the resultant immune response alters the skin microbiota. Understanding the role of the skin microbiota in skin disorders and infection may lead to novel therapies that aim to restore the balance of commensal skin microbes.