The microbes on skin are:
Human skin hosts a variety of commensal and pathogenic microbes, including bacteria, fungi, and viruses. The most common methods to study skin microbes are traditionally based on cultivation techniques, especially to help diagnose infections. Koch’s postulates for linking a causative microbial agent to a disease represent a familiar and fundamental paradigm for understanding host–microbial interactions. Beyond the classical roles of infection-causing microbes, studies continue to examine how microbes may interact with the host as commensal symbionts and potentially contribute as pathobionts to noninfectious inflammatory diseases, such as atopic dermatitis or psoriasis, and in neoplasms, such as Merkel cell carcinoma. Ongoing research investigating host–microbial interactions increasingly uses multiple different concepts and methods to provide insights into human skin microbes.
One basic approach to exploring the symbionts and pathobionts among skin microbes is to initially determine the composition of the microbes that reside on human skin. A challenge of relying solely on cultivation methods is the difficulty in selecting the optimal culturing methods to capture the entire microbial community of human skin; some microbes may flourish under generic culture conditions, yet other microbes can require specific nutrients or special conditions. The advances in scientific methods and genomic sequencing technologies have enabled a more global culture-independent examination of the skin’s complex communities of microbes.1 Deeper study of the composition and the associated functions of the multitude of microbes residing on human skin may help uncover the roles of skin microbes in health and disease.
The constant exposure of human skin to surrounding environments presents innumerable potential interactions with such things as dust particles, humidity, clothing, emollients, other people, and pets. As a result, a rapidly growing number of studies are examining the microbiome of living organisms, the built environment, and inanimate objects that come into contact with human skin or skin debris, as well as the many potential interventions (eg, antibiotics, hygiene products, probiotics) that may alter the human skin microbiome.2-18 This chapter introduces the concept of the skin microbiome, the major constituents of the cutaneous microbiome, and some of the interactions between skin microbes and the host, primarily highlighting culture-independent studies of the resident microbiome of human skin in healthy individuals and dermatologic disorders.
DEFINING SKIN MICROBIOME STUDIES
FROM CULTURING TO GENOMICS
Based on standard culturing methods, it is well-known that human skin harbors various microbes. Because many skin diseases are associated with microbes, the field of dermatology has been instrumental in investigating the composition of cultured bacteria and fungi on human skin and in exploring host–microbial interactions.19-23 These culture-based studies have defined some of the basic tenets of skin microbes that continue to hold true decades later, including the presence of Cutibacterium (formerly Propionibacterium) and Malassezia (formerly Pityrosporum) on many individuals. An early example is Mary ...