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.
The skin is the human body's largest organ, colonized by a diverse milieu of microorganisms, most of which are harmless or even beneficial to their host. Colonization is driven by the ecology of the skin surface, which is highly variable depending on topographical location, endogenous host factors and exogenous environmental factors. The cutaneous innate and adaptive immune responses can modulate the skin microbiota, but the microbiota also functions in educating the immune system. The development of molecular methods to identify microorganisms has led to an emerging view of the resident skin bacteria as highly diverse and variable. An enhanced understanding of the skin microbiome is necessary to gain insight into microbial involvement in human skin disorders and to enable novel promicrobial and antimicrobial therapeutic approaches for their treatment.
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We thank H. Kong and E. Hobbs for critical reading of the manuscript and J. Fekecs and D. Leja for graphical assistance. E.A.G. is supported by a Pharmacology Research Associate Training Fellowship, US National Institute of General Medical Sciences. This work was supported by the US National Human Genome Research Institute Intramural Research Program and the US National Institutes of Health Common Fund AR057504.
The authors declare no competing financial interests.
The predominant cell type of the epidermis. Keratinocytes produce keratin as they terminally differentiate into the squames of the stratum corneum.
An enucleated, dead, squamous keratinocyte that is shed from the stratum corneum.
The oily, lipid-containing substance that is secreted by the sebaceous glands of the skin. Sebaceous glands are connected to the hair follicle and form the pilosebaceous unit. Sebum protects and emolliates the skin and hair.
- 16S ribosomal RNA metagenomic sequencing
Genomic analysis of 16S ribosomal RNA phylotypes from DNA that is extracted directly from bacterial communities in clinical or environmental samples, a process that circumvents culturing.
All of the genetic material of a microbial community sequenced together.
A taxon-neutral way to describe organisms based on their phylogenetic relationships to other organisms. Phylotypes are determined by comparing 16S ribosomal RNA gene sequences. A common threshold used to define species-level phylotypes is 97% sequence identity of the 16S rRNA gene sequence.
- Whole-genome shotgun metagenomic sequencing
Genomic analysis of DNA that is extracted directly from a clinical or environmental sample and whole-genome shotgun (WGS) sequenced to represent the full microbiome.
- Pattern recognition receptor
(PRR). A receptor present on the surface of keratinocytes and other cells of the innate immune system that recognizes microorganism-specific molecules (for example, lipopolysaccharide and flagellin).
- Pathogen-associated molecular pattern
(PAMP). A molecule that is associated with a pathogen and recognized by a pathogen recognition receptor. Examples include lipopolysaccharide, flagellin, lipoteichoic acid, double-stranded RNA, peptidoglycan and unmethylated CpG motifs.
- Atopic dermatitis
(AD). A type of eczema characterized by red, flaky, itchy skin, typically affecting the inner elbows and behind the knees. It is often associated with other atopic diseases such as allergic rhinitis, hay fever and asthma
- Seborrhoeic dermatitis
An inflammatory, hyperproliferative skin condition characterized by red, flaky, skin often affecting sebaceous areas of the face, scalp and trunk. Commonly known as dandruff.
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