Part 2: Micro-organisms on the Skin
Please see the other parts of the article; once they are completed the links will be active:
Part 1: anatomy of the human microbiome
Part 3: micro-organisms in the mouth
Part 4: micro-organisms in the gut
Part 5: implications for modern medicine
On your epidermis (skin), there is an estimated 1 trillion micro-organisms known as skin flora, or skin microbiota. The word microbiome actually refers to their genomes, and there are about one thousand different species. Most live on the top-most layers of the skin and on the top of your hair follicles.
Skin flora are usually commensal (non-harmful) or mutualistic (beneficial). A primary benefit from these micro organisms is protecting against transient pathogens (traveling from another host) by competing for nutrients, secreting chemicals, or stimulating the skin’s immune system. Resident microbes (native to your skin) can cause skin diseases and enter the bloodstream creating illness and disease, particularly in people with weakened immune systems.
There are three different categorizations for the ecology of skin flora: sebaceous (oily or fatty), moist, and dry. sebaceous areas tend to have the greatest richness of bacterial species. The spaces between fingers and toes and inside of the belly button are the most diverse, and the nostrils and back tend to be the most similar between people. Interestingly, one study has shown that the area behind your heel has the greatest fungal diversity of the body. It also seems that as you go up the body from the ground (feet to head) there is a decrease in diversity of fungus, which intuitively makes sense because the ground has a huge amount of bacterial diversity that your body probably wants to protect against. Your feet have lots of fungus and the oily places on your body have lots of bacteria. Interestingly, most of the species of bacteria studied occasionally have pathogenic capabilities. It seems that there is a balance on the body, that when disrupted, can lead to many of the micro-organisms to become pathogenic. Let’s look into that a little bit more.
Flora can be beneficial, pathogenic, or commensal (non-harmful). Often, they can be all three, depending on the strength of the hosts immune system. I recommend reading this study on the balance of flora on the skin and how they interact, because it is extremely complex and very interesting. Essentially, the work in balance to keep each other from becoming too numerous and killing one type of bacteria can lead to growth of fungus.
Odor, interestingly, is not caused by sweat. It is caused by bacteria consuming it and creating byproducts that we consider smelly and unattractive.
Lets get back to the idea that the bacteria on your skin are actually mutualistic and beneficial when in balance. The skin creates antimicrobial peptides that control the growth of skin microbes. One example is cathelicidins which in addition to directly controlling bacterial populations, secretes Cytokine, which induces inflammation, skin regrowth, and blood vessel regrowth. Atopical dermatitis is linked to suppression of cathelicidin production; a major factor contributing to its production is Vitamin D3 (which actually isn’t a vitamin because the body produces it instead of needing to ingest it).
Your skin is slightly acidic when healthy. It has a PH of 4-4.5 due to lactic acid in sweat and produced by skin bacteria. Antimicrobial substances secreted by the skin are enhanced in acidic conditions and in alkaline conditions are more easily shed. The shedding of skin is one way that your body manages the buildup of flora on its surface. The immune system can produce cell mediated immunity against microbes, but some fungi have evolved to limit the immune response against them.
Micro-organisms also play a role in non-infectious skin diseases, like acne, atopic dermatitis, rosacea, and psoriasis. Damaged skin can cause bacteria to become pathogenic. P acnes is a particular bacteria that causes acne, which can be healthy in some people and pathogenic in others. Probiotics are being used to balance the imbalance of skin bacteria that can cause acne.
Atpoical dermatitis is linked with low bacterial diversity; low gut microbial diversity in babies have been associated with increased risk in dermatitis. Other diseases will likely have probiotic cures in the future, used again to balance the populations of bacteria on the skin. These bacteria can also build immunities; the use of bacterial and fungicidal soaps will inevitably lead to bacterial and fungal populations which are resistant to the chemicals employed.
Skin flora also do not readily pass between people. This doesn’t mean you shouldn’t wash your hands after the bathroom (fecal bacteria are different from skin bacteria and make up close to 50% of your feces). The most effective (60 to 80% reduction) antimicrobial washing is with ethanol, isopropanol, and n-propanol. Viruses are most affected by high (95%) concentrations of ethanol, while bacteria are more affected by n-propanol. Unmedicated soaps are largely ineffective at controlling bacterial populations.
Over washing your skin can lead to damage through loss of water creating dryness. There are lipids in the skin that can be removed by detergents and alcohols and wearing gloves can exacerbate the problems of already irritated skin. Damaged skin can lead to normally mutualistic or commensal bacteria to become pathogenic.
Skin flora is less diverse than gut flora. Both are less diverse than soil flora. Next time, we’ll talk about oral bacteria and how your mouth has an ecosystem all its own. Stayed tuned for part 3: Oral microbiology.
I hope you’ve enjoyed this article. Please feel free to ask any questions in the comments, or to add anything you’ve found in your own research. Always happy to start a conversation.
<3 from Vietnam, Elliot