Revisiting the Cutaneous Epithelium: Insights from a Nontraditional Model System

In the last 10 years, biology has made revolutionary advances from century-old debates about the relative importance of non-pathogenic bacteria. New applications of sequencing technologies are transforming our understanding of the biology of the skin. Similar to all other epithelia, the skin is colonized by an assemblage of microorganisms which, for the most part, peacefully coexist with their hosts. Alterations to microbial communities are associated with, and likely contribute to, a number of cutaneous disorders. This review focuses on the host factors that shape and maintain skin microbial communities, and the reciprocal role of microbes in pathogen defense and tissue homeostasis. We highlight how work in the non-traditional model system Hydra is uncovering new mechanisms to old cell biological problems that provide a foundation for understanding, preventing and in the long-term even treating skin diseases. We demonstrate that the cutaneous epithelium is an ecosystem hosting a complex microbiome and that components of the innate immune system as well as transcriptional regulators of stem cells are involved in maintaining homeostasis between epithelia and their resident microbiota. We conclude that beneficial bacterial-host interactions should be considered an integral part of skin biology and that nontraditional model systems can provide a holistic understanding of the cutaneous epithelium.