Michael Eisenstein in Nature:
In contrast to the gut, which offers a near-ideal habitat for the growth of fermentative bacteria, the skin is an inhospitable expanse. Much of the epidermal layer that protects humans from the elements is dry, salty, acidic and nutrient-poor. The exceptions are the oases around lipid-rich hair follicles. Despite this adversity, a diverse and physiologically important array of bacteria, viruses, fungi and archaea make their home on the skin. Typically, a person has around 1,000 species of bacteria on their skin. And, as might be expected from such a large area — roughly two square metres for an average adult — the skin offers a variety of distinct ecosystems, which create conditions that favour different subsets of organisms.
The skin microbiome is seeded at birth. The first microbial colonists help to train the immune system to tolerate commensal organisms (which have a neutral or beneficial impact on their host) while remaining alert to pathogens. These microbial communities continue to grow and diversify until puberty, when hormonal and developmental changes help to sculpt the final composition that is carried throughout adulthood. Over the past decade, researchers have uncovered evidence of extensive communication between bacteria, skin cells and immune cells. These interactions help to reinforce and repair the barrier formed by the skin, bolster the body’s defences against infection and tamp down excess inflammation.