Hidden in Plain Sight: The Skin Microbiome and Gut-Brain-Skin Axis

When you hear “microbiome,” you may think of gut bacteria first. Cutting-edge research, however, shows that the skin biome is another vital population of microorganisms that influences human health in surprising ways.

From 2009 to 2019, the annual number of peer-reviewed papers examining the skin biome increased more than ten-fold. Scientific interest in the topic is growing exponentially as new sequencing techniques allow the identification of previously undiscovered species, as well as novel insights into the roles they play in wellness and disease.

Figure: Skin microbial communities are shaped by physiological characteristics and the individual. 

In the past, scientists believed skin microbes mainly represented a source of infection. But as a seminal 2018 paper from Nature Reviews Microbiology summarizes, “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.”

At 2 square meters of surface area, the skin is the most sizable organ of the human body. However, when taking into account the surface area of hair follicles and sweat ducts, the number grows to 25 square meters of surface area–or the same square footage as a small studio apartment.

Figure: Schematic of skin histology viewed in cross-section with microorganisms and skin appendages.

Therefore, human skin is easily the largest epithelial surface for interaction with microbes, particularly when taking into account the interfollicular areas. We now know that microbes on the skin surface can also influence the behavior of cells below the epidermis and communicate with host tissues in areas previously believed to be sterile.

How far does this influence extend, really? No one knows yet, but new data suggests the skin microbiome could influence the risk of various conditions including aging skin, allergies, autoimmunity, and even some cancers.

The gut-brain-skin axis refers to an emerging perspective that gut function and the gut microbiome can influence brain function, including stress levels, which in turn affect skin health and inflammation.

Is the gut-brain-skin axis a two-way street? It would seem to be a distinct possibility, but no one has formally ventured an answer to that question in the literature…yet. It’s one of many intriguing areas within this emerging field of study that we can hope to learn more about in the coming decade.

Figure: Gut-brain-skin axis.

New evidence demonstrates that, similar to the gut, higher microbial diversity on the skin relates to greater health, with loss of species diversity showing adverse effects. And fascinatingly, greater exposure to natural environments also correlates to higher skin biome diversity and better health outcomes.

The skin biome is not only a new frontier in understanding key aspects of our physiology, but some research also suggests that modern hygiene habits and other chemical exposures are harmful to symbiotic bacteria, fungi, viruses, and archaea that serve helpful roles in skin health and beyond.

According to a 2004 survey from the Environmental Working Group, adult women apply 168 unique chemicals to their skin each day on average. For men, the average number was lower, but still impressive: 85 unique daily chemicals.

Modern “skincare” products may damage the epithelial barrier of skin, resulting in increased pH, moisture loss, and greater permeability to pathogenic species. Ultimately, the result could be bacterial dysbiosis, an imbalance giving rise to harmful microbiota.

Figure: Damaged skin barrier with dysbiosis. 

In other words, modern Homo sapiens may be doing skincare (soaps, shampoos, moisturizers, and cosmetic products) all wrong. Clearly, none of these conventional products are built around the understanding that human skin contains its own vital ecosystem, let alone optimized to be safe for the skin microbiome.

Is it possible that people would have healthier, more beautiful skin, and perhaps even better general wellness, if they allowed natural microbes from dirt and forests to colonize their skin?

A recent study investigating the effects of a bacterial culture isolated from soil represents the first scientific research into this question.

In a blinded, placebo-controlled trial, volunteers were assigned to a group that received a suspension of Ammonia-Oxidizing Bacteria (AOB for short) isolated from organic soil, or a placebo control. The active group received instructions to apply the bacterial suspension to their face and scalp for a week. Neither group washed their face or hair for the following two weeks.

The study results were impressive: the AOB group reported significant improvements in skin clarity, tone, smoothness, and other cosmetic measures, while the placebo group noted little or no improvement. (Note: While the results of the trial did not appear in a peer-reviewed journal, they were presented at the fifth annual American Society of Microbiology Conference in Washington, DC.)

And now, the same live probiotic strain is available to support a healthy skin biome and restore balance to skin.

A skincare company founded by an MIT-trained biochemical engineer, Mother Dirt’s product line revolves around the Ammonia-Oxidizing Bacteria called Nitrosomonas eutropha.

The science team at Mother Dirt explains that N. eutropha thrives in nitrogen-rich ecosystems, especially soil.

And it was once abundant on human skin, where it can oxidize and break down ammonia into nitrites, but modern hygiene practices and a lack of outdoor time appear to have wiped out native skin populations of nitrifying bacteria.

N. eutropha acts as a keystone or “peacekeeper” species, meaning it can rebalance any ecosystem of which it’s a part–including the skin biome.

Figure: Nitrification and denitrification cycle.

The company hypothesizes that without AOB, which evolved alongside humans for millions of years, our skin chemistry is incomplete. Although research into other facets is ongoing, their main focus is on beauty and daily cosmetic skincare.

And now, the same live probiotic strain is available to support a healthy skin biome and restore balance to skin. It’s suitable for all skin types.

For some people, the hardest part of adopting a Biome-Friendly regimen is giving up all the harsh chemicals they rely on to feel (and smell) clean.

But according to Mother Dirt, there’s no need to worry–ammonia-oxidizing bacteria “eat” the ammonia found in sweat, helping to lower the pH of skin and keep body odor in check naturally.

Whether your complexion is oily, dry, flaky, or red and irritated, an AOB-based skincare regimen can hydrate and smooth skin and deliver clearer, visibly healthier skin in just four weeks.

The Skin Recovery Kit from Mother Dirt is an easy way to get started. It’s a complete rescue regimen to save your skin from modern chemicals and reduce your dependence on moisturizers, deodorants, and harsh cleansers.

And it’s now 20% off for a limited time for readers of Dr. Fitzgerald’s FxMed Blog. Use code DERM20 at checkout to receive the discount.