What’s Living on Your Skin?

SkinThe average human body has about 1.2 square meters of skin. Scattered across and nestled in its myriad crevasses and canyons would lie trillions of bacteria, fungi, viruses, and mites. Before you shudder and reach for the bleach and a stiff scouring brush, remember the importance of cultivating and supporting the billions of bacteria living in your gut. Recall the vital roles they play (that we know about) in our health and realize that the skin microbiome isn’t any different. Although research is young, we are learning that the critters living on our skin, who number in the billions per centimeter of skin, are supposed to be there. And even though we don’t know exactly all they’re doing, we know this:

The skin microbiome isn’t an artifact of civilization or agriculture. We’ve always hosted colonies of microbes on our bodies and we’ve evolved in the context of a skin microbiome. Since our current environment is more sterile than any preceding it, our ancestral skin microbiome was almost certainly more diverse. We must exercise caution, then, when trying to extirpate our skin of “germs.”

The composition of our skin microbiome varies between areas of the body. Your left hand, for example, has a markedly different microbial community than your right hand.

Like the gut bacteria, our skin bacteria perform important roles. It’s possible, maybe even probable, that the fungi, viruses, and mites living on our skin are also necessary, but we simply don’t know enough about those critters to say for sure so today we’ll focus mainly on how the bacteria living on our skin impact us:

They regulate the skin-level immune response to allergens. For instance, in adolescents, the presence of Acinetobacter bacteria on the skin was associated with the expression of anti-inflammatory molecules and protection against allergic reactions. In a mouse model portion of the study, those same microbes directly inhibited allergic sensitization.

They control the inflammatory response. By controlling whether or not inflammatory or anti-inflammatory molecules are produced, certain skin bacteria can influence the progression and expression of autoimmune diseases like arthritis, psoriasis. Without the relevant bacteria – or with too many or too few – the inflammatory response can go unchecked.

They protect against incursions by pathogens. Staphylococcus epidermis, a common resident on our skin, produces antibacterial peptides that prevent the formation of pathogenic staphylococcus aureus (the microbe responsible for the infamous staph infection) biofilms. One experiment found that human subjects with S. epidermis in their nasal cavities were totally free of S. aureus infections.

Skin bacterial dysbiosis is involved in many skin diseases. Several lines of evidence suggest a role for skin biome dysbiosis in atopic dermatitis (PDF):

  • Atopic dermatitis flare-ups are characterized by lowered diversity in skin bacteria; successful resolution sees a recovery of diversity.
  • Atopic dermatitis flare-ups often occur in areas of low microbial diversity, like the inner knee or elbow crease.
  • Atopic dermatitis patients tend to have S. aureus living on their skin, crowding out commensal bacteria, taking up space, and reducing microbial diversity. The normally commensal S. epidermis may also grow overrepresented and occupy space that could be used by more diverse bacteria.

It’s not just atopic dermatitis. There are also differences between the skin microbiota in healthy patients and patients with eczema, and reduced microbial diversity is commonly found in patients with psoriasis.

The same bacteria can be helpful or harmful depending on the context (PDF). For instance:

  • S. epidermis is normally a commensal bacteria that produces antimicrobial peptides to protect against pathogens. If it breaches the surface of the skin, however, it can become an opportunistic pathogen. This mostly occurs in hospital settings with immunocompromised subjects, usually after surgery.
  • There’s also the case of Propionibacterium acnes, the bacteria most often associated with and blamed for acne. P. acnes is present on everyone’s skin, even healthy people without any visible acne. It’s often the only species present on the face and shoulders, suggesting that its presence is normal, not pathological. One helpful byproduct of normal P. acnes metabolism are short chain fatty acids which lower the skin pH and inhibit the growth and colonization of pathogenic microbes.

Some skin bacteria attract mosquitoes. Others repel them. Generally, malaria mosquitoes are attracted to people with low microbial diversity on their skin. Human skin with greater microbial diversity seems to be protective. More specifically, odors produced by the common bacterial species Pseudomonas aeruginosa are less attractive to malaria mosquitoes than odors from four other common skin microbes. This suggests a role for the skin biome in determining a person’s vulnerability to malaria, and perhaps other insect or tick-borne diseases.

It might appear as if the skin biome isn’t as “important” as the gut biome. Acne, eczema, and allergic reactions that make you itch and break out are annoying, but we don’t usually consider them life-threatening. I would caution against that position. For one thing, those conditions can really affect a person’s enjoyment of life. They show on the surface but their effects run deeper. Secondly, we have a lot to learn. We still haven’t identified most of the bacterial species living on our skin. And even the term “species” isn’t exactly accurate for bacteria, since horizontal gene transfer between disparate bacteria and other complexities have researchers questioning whether we even know how to classify bacterial species (or if they exist at all!).

But if there’s a general rule that appears to be true, it’s that greater diversity in the skin biome is preferable to less diversity.

What do we do about it, then? In a future post, I’ll explore the available evidence to figure out how to optimize the health and diversity of our skin biome.

Thanks for reading, everyone!

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TAGS:  skin/hair

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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