Skin

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The skin microbiome
Elizabeth A. Grice and Julia A. Segre

Abstract | 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 exogenousenvironmental 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 togain insight into microbial involvement in human skin disorders and to enable
novel promicrobial and antimicrobial therapeutic approaches for their treatment.
Keratinocyte
The predominant cell type of
the epidermis. Keratinocytes
produce keratin as they
terminally differentiate into the
squames of the stratum
corneum.

Squame
An enucleated, dead,
squamous keratinocyte that is
shed fromthe stratum
corneum.

Genetics and Molecular
Biology Branch, National
Human Genome Research
Institute, National Institutes of
Health, Bethesda, Maryland,
20892–4442, USA.
Correspondence to J.A.S. 
e‑mail: jsegre@nhgri.nih.gov
doi:10.1038/nrmicro2537

The skin is an ecosystem composed of 1.8 m2 of diverse
habitats with an abundance of folds, invaginations and
specialized niches thatsupport a wide range of micro­
organisms. The primary role of the skin is to serve as
a physical barrier, protecting our bodies from potential
assault by foreign organisms or toxic substances. The skin
is also an interface with the outside environment and,
as such, is colonized by a diverse collection of micro­
organisms — including bacteria, fungi and viruses — as
well as mites1–7(FIG. 1). As we describe, many of these
microorganisms are harmless and in some cases provide
vital functions that the human genome has not evolved.
Symbiotic microorganisms occupy a wide range of skin
niches and protect against invasion by more pathogenic
or harmful organisms. These microorganisms may also
have a role in educating the billions of T cells that are
found in the skin, priming them torespond to similarly
marked pathogenic cousins.
The perception of the skin as an ecosystem —
composed of living biological and physical components
occupying diverse habitats — can advance our under­
standing of the delicate balance between host and micro­
organism. Disruptions in the balance on either side of
the equation can result in skin disorders or infections.
Perturbations affectingthe host–microorganism rela­
tionship can be endogenous (for example, genetic vari­
ation that selects for a specific microbial community)
or exogenous (for example, hand washing). To further
our understanding of health, disease and infection of the
skin, microbiologists, immunologists and dermatolo­
gists have partnered with genomic scientists to develop
a more complete characterizationof the skin microbiota
and how it interacts with the host (FIG. 2).

The habitat of the skin defined
The physical and chemical features of the skin select for
unique sets of microorganisms that are adapted to the
niche they inhabit. In general, the skin is cool, acidic
and desiccated, but distinct habitats are determined
by skin thickness, folds and the density of hair follicles
andglands8. Structurally, the epidermis is a formidable
physical barrier, resisting penetration by microorgan­
isms and potential toxins while retaining moisture and
nutrients inside the body 9–11. The top layer of the epi­
dermis, the stratum corneum (FIG. 1), is composed of
terminally differentiated, enucleated keratinocytes, that
are known as squames. Squames consist of keratin fibrils
and...