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Microbes of the Skin

Recently I was asked about the human skin microbiome (skin microbial communities) and whether the things we do frequently (e.g. use soap and shampoo, go swimming in a pool) has an effect on our skin microbiome. As I've posted earlier, human skin microbes include bacteria, fungi, viruses, and  archaea. Most of these microbes are harmless or beneficial, but when the microbial communities are out of whack (dysbiosis), then there are diseases or skin disorders (such as acne, psoriasis, and eczema). The human skin acts as a physical barrier, a first line of defense, to pathogens (microbes that can cause disease). Studies have found that using soaps, lotions, make-up, our diet and lifestyle all have some effect on skin microbial communities. Even living with someone results in some microbial exchange. Spending more time outdoors, owning pets, and drinking less alcohol (or none) are all associated with higher levels of microbial skin diversity.

But then I came across a small study from 2016 (National Human Genome Research Institute, NIH, Bethesda, MD) in Cell - Temporal Stability of the Human Skin Microbiome. The researchers found that skin microbial communities are "surprisingly stable over time" (the study lasted 2 years), even though the humans were typically exposed to things daily that could disrupt their skin microbial communities (other people, clothing, the environments). But some individuals had more stable communities than others, and stability varied from site to site (the feet had the least stable microbial communities). Also, they found that bacterial, fungal, and viral communities not only show a strong preference for inhabiting specific skin sites, but also serve as "microbial fingerprints" that are highly unique to individuals. They did point out that "immunosuppression, illness, or the occurrence of disease have been shown to cause major shifts in skin communities".

Then there is a recent 2018 review article - but behind a paywall even though the researchers worked for NIH, thus paid for with our tax dollars (!!).They also discussed all the microbes living on the skin, and how when the microbial communities are out of whack (dysbiosis), then there is disease (whether acne, or eczema, etc.). Microbes that are beneficial in healthy people can become pathogenic, e.g. when the person has a disease. It also pointed out that only with modern genetic sequencing methods (rather than old style "cultures") can one really see what makes up the skin microbial communities. And that using these methods we can compare the skin microbes of healthy persons with those with a disease. And yes, there then is also the possibility of finding protective, beneficial microorganisms which are in healthy persons, but absent or under-represented in those with a disease. Sounds  like probiotics for the skin! Excerpts from Nature Reviews Microbiology:

The human skin microbiome

Functioning as the exterior interface of the human body with the environment, skin acts as a physical barrier to prevent the invasion of foreign pathogens while providing a home to the commensal microbiota. The harsh physical landscape of skin, particularly the desiccated, nutrient-poor, acidic environment, also contributes to the adversity that pathogens face when colonizing human skin. Despite this, the skin is colonized by a diverse microbiota.

Our skin is home to millions of bacteria, fungi and viruses that compose the skin microbiota. Similar to those in our gut, skin microorganisms have essential
roles in the protection against invading pathogens, the education of our immune system and the breakdown of natural products. As the largest organ of the human
body, skin is colonized by beneficial microorganisms and serves as a physical barrier to prevent the invasion of pathogens. In circumstances where the barrier is
broken or when the balance between commensals and pathogens is disturbed, skin disease or even systemic disease can result. Human skin sites can be categorized by their physiological characteristics, that is, whether they are sebaceous (oily), moist or dry. Studying the composition of the microbiota at different sites is valuable for elucidating the aetiology of common skin disorders, which often have a preference for specific skin sites, such as eczema inside the elbow, and psoriasis on the outside of the elbow.

Initial colonization and population shifts: In newborn babies, initial colonization of the skin is dependent on delivery mode; neonates born vaginally acquire bacteria that colonize the vagina, whereas neonates born via Caesarian section acquire microorganisms that are associated with the skin. The long-term effects of these initial skin colonization modes in neonates remain unknown.

In the gut, microbial communities stabilize around 3 years of age. ... By contrast,
the relative abundance of skin microbial species is restructured during puberty, a time when increased levels of hormones stimulate the sebaceous glands to produce additional sebum.

The skin microbiome in disease: Interactions between members of the microbiota both shape the resident microbial community and prevent colonization by pathogenic bacteria in a process termed ‘colonization resistance’ . However, in certain contexts, bacteria that are ordinarily beneficial to their hosts can become pathogenic. Many common skin diseases are associated with changes in the microbiota, termed dysbiosis. This dysbiosis is often driven by common commensal species, as described below for acne, eczema and chronic wounds. Both rare and common skin disorders are thought to have underlying contributions both from individual species and from alterations to the microbial community.

Microorganisms associated with common acne: The prevalent teenage condition acne vulgaris is a chronic inflammatory skin condition that is associated with
the bacterium P. acnes, the most abundant organism in the microbiota of healthy adults.

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