Shaping of cutaneous function by encounters with commensals

Abstract

The skin is the largest organ in the human body and provides the first line of defence against environmental attack and pathogen invasion. It harbor multiple commensal microbial communities at different body sites, which play important roles in sensing the environment, protecting against colonization and infection of pathogens, and guiding the host immune system in response to foreign invasions. The skin microbiome is largely variable between individuals and body sites, with several core commensal members commonly shared among individuals at the healthy state. These microbial commensals are essential to skin health and can potentially lead to disease when their abundances and activities change due to alterations in the environment or in the host. While recent advances in sequencing technologies have enabled a large number of studies to characterize the taxonomic composition of the skin microbiome at various body sites and under different physiological conditions, we have limited understanding of the microbiome composition and dynamics at the strain level, which is highly important to many microbe-related diseases. Functional studies of the skin microbial communities and the interactions among community members and with the host are currently scant, warranting future investigations. In this review, we summarize the recent findings on the skin microbiome, highlighting the roles of the major commensals, including bacteria, fungi and bacteriophages, in modulating skin functions in health and disease. Functional studies of the skin microbiota at the metatranscriptomic and proteomic levels are also included to illustrate the interactions between the microbiota and the host skin.

Figure 1. The composition and function of the human skin microbiota

Driven by physiological properties of the skin, the microbial communities at dry, moist and sebaceous skin sites are largely different with various species dominating at each site. Staphylococcus epidermidis, Propionibacterium acnes and Malassezia spp. metabolize skin nutrients, produce physiologically important molecules, such as antimicrobial peptides, bacteriocins and short chain fatty acids, and play a role in defending against pathogen colonization and infection.

Figure 2. Schematic diagram of biofilm formation in impaired wound healing

Commensal skin microorganisms colonize the wound site and adhere to exposed substratum tissues of the wound surface. Secretions of extracellular polymeric substances facilitate biofilm maturation, leading to the formation of polymicrobial biofilms.