Published February 5, 2024
Human skin is a sophisticated organ, and one of its main functions is to protect our other organs from external stresses. The skin microbiota is a major contributor to this role. In this article, we'll look at what it's made of, how it lives, changes, and protects us, what can damage it, and how to look after it.
WHAT IS A MICROBIOTA?
A microbiota is a community of microorganisms (fungi, bacteria, viruses, yeasts, and even parasites like mites) that live in symbiosis with their host. The word 'symbiosis' is important here, because it means that the microorganisms cause no harm to their host, and at the same time, the host doesn’t try to get rid of them.
A microbiota is found on every part of the body that comes into contact with the outside world. Humans can therefore be considered to have 4 different microbiota:
- The gut microbiota, present in the stomach and intestines. This is also often called gut flora.
- The skin microbiota.
- The respiratory tract microbiota, found in the mouth, pharynx, and lungs.
- The urogenital microbiota.
Bacteria are the most common microorganisms found in humans. We tend to call them commensal bacteria or normal flora, to set them apart from pathogenic bacteria. They constantly maintain our natural defense systems, which in turn boosts our immunity.
So far, a little over 1,000 different “good” bacteria have been identified.
WHAT’S THE DIFFERENCE BETWEEN SKIN AND GUT FLORA?
Our gut flora is the most well understood community of microbial and bacteriological organisms found in the human body. It never comes into contact with the outside world and performs a number of functions:
- Developing and cultivating the immune system, which is essential for the body to fight off infections
- Metabolizing nutrients, including the fermentation of dietary fiber and the absorption of carbohydrates, proteins, and lipids.
- Protecting against external pathogens.
- Maintaining the integrity of the intestinal barrier, by strengthening the intestinal walls.
- Producing certain vitamins, like B vitamins and vitamin K.
- Producing neurotransmitters—substances that activate the brain and play a part in the communication between the brain and the intestine (it's not a coincidence that the digestive tract is often nicknamed our “second brain”).
Interest in gut flora has grown steadily over the last decade, and for good reason. Many illnesses or physical complaints could be linked to an imbalance (or, to use the scientific term, dysbiosis) within this community of microorganisms. Its health is highly dependent on what we eat, physical activity, stress levels, sleep quality and quantity, and the medication (particularly antibiotics) we take.
Unlike gut flora, skin flora comes into direct contact with our environment. Obviously, it doesn't play any role in digestion, but it is vital to the health of the skin barrier. We’ll come back to this later on.
Lastly, it's worth noting that the microbiota found throughout the body are unique to each individual, which means that different people have different needs.
THE ANATOMY AND COMPOSITION OF THE SKIN MICROBIOTA
Research into the structure of the skin microbiota and its influence on skin health is a flourishing field, and new findings are constantly emerging.
We currently know that a single cm2 of human skin contains up to several tens of millions of microorganisms. They are present on the skin's surface, as well as within the deeper layers of the epidermis and the dermis.
Skin bacteria belong to four main strains, which vary depending on where they are found on the body. This is because sweat, sebum, pH, moisture, and skin temperature all combine to create a variety of skin microenvironments, which in turn affects the types of bacteria able to thrive in each ecosystem.
Four microenvironments have been identified:
- A moist environment, including areas like the armpits, elbow crease, between the toes, or groin.
- A sebaceous region including the forehead and back.
- A dry region, like the top of the forearms or the upper part of the buttocks.
- As yet uncategorised regions, like those located in the sweat glands or hair follicles.
So, the skin is host to a variety of different “growing environments” that suit different microorganisms.
A key thing to remember is that skin moisture has a major influence on the growth of microorganisms, and this varies considerably between the main regions mentioned above. For example, dry, dehydrated skin inhibits the growth of commensal bacteria and is conducive to the development of the pathogenic Staphylococcus aureus bacteria that causes skin infections.
Keeping skin in a well-moisturized state helps to maintain the microbiota's ideal profile and, simultaneously, the skin barrier function.
But the proliferation of pathogenic bacteria on the skin's surface is not the only risk associated with compromised skin flora.
WHAT ARE THE EFFECTS OF AN IMBALANCED SKIN MICROBIOTA ON THE IMMUNE SYSTEM?
To fully understand the implications of skin dysbiosis on health, it's important to understand the role of the skin microbiota.
THE MAJOR FUNCTIONS OF THE SKIN MICROBIOTA
The skin is under constant attack from the environment, and its microbiota is the first to suffer. It protects the whole body thanks to:
THE SKIN BARRIER
We briefly touched on this above. The good bacteria found on the skin act as a first line of defense against external stresses. They compete with external threats for food resources and space in which to reproduce. This prevents opportunistic bacteria from colonizing and proliferating.
The skin microbiota affects the skin barrier function by secreting enzymes and proteins that (among other things) inhibit the growth of pathogens. For example, Cutibacterium acnes bacteria help to make the skin hostile to “bad” bacteria like Staphylococcus aureus and Streptococcus pyogenes, and allow commensal bacteria like Staphylococcus epidermidis to thrive.
THE IMMUNE SYSTEM
Commensal and pathogenic bacteria are constantly interacting. If “bad” bacteria come into contact with “good” bacteria, the microbiota provides proactive resistance. Keratinocytes (the predominant cells in the outermost layer of the epidermis) are constantly assessing the condition of our skin. They can detect potential inflammation in advance and call in immune cells if the microbiota should be unable to fend off invaders on its own.
Working alongside the skin's immune system, the skin microbiota can therefore contain a number of infections.
HOW SKIN DYSBIOSIS IS EXPRESSED
Skin dysbiosis is less of a problem than gut dysbiosis, but it is still the cause of a number of complaints that can affect quality of life. At present, two dermatological ailments have been attributed to it. It's possible that there are others (like rosacea or psoriasis), but specialists need more data to confirm this with certainty.
Atopic dermatitis is a chronic, inflammatory disorder that causes severe itching. It develops in childhood and is often accompanied by asthma and allergic rhinitis. Although it may not be the only cause, people with atopic dermatitis tend to exhibit an impaired skin barrier marked by reduced microbial diversity compared with individuals without the condition.
While the skin microbiota plays a subordinate role in causing this condition (as it is not itself the cause), it does seem to contribute to eczema flare-ups.
Acne is a chronic inflammatory condition that occurs on areas where the skin is oily (face, neck, chest, and back). While it is most common in teenagers because of increased production of sex hormones, other forms can develop in adulthood.
Changes in the production and composition of sebum due to acne have a direct effect on the skin microbiota. But it should be noted that the bacteria that thrive on the oiliest areas of skin either promote or, conversely, reduce the risk of inflammation. It is highly dependent on the physiology of the person affected.
HOW CAN WE PROTECT OUR SKIN MICROBIOTA?
The epidermis constantly renews itself. This in turn means that the skin microbiota must do the same, given its presence on the skin's surface. Also, frequent washing means a greater need for regeneration, as washing your skin also eliminates protective bacteria.
We've already discussed how moisturizing the skin is a good way of maintaining a healthy skin microbiota, as is maintaining optimal microbial and bacterial diversity (meaning that, to avoid dysbiosis, one species should not overpower another).
For this reason, skin experts recommend following a few simple rules:
- Do not over-cleanse your skin. Overly frequent cleansing can damage the hydrolipidic film on the skin's surface and compromise the healthy growth of the skin's microbial flora.
- Avoid cleansers with a high pH. Such products can damage the existing microbiota or encourage the growth of harmful bacteria. For example, the Staphylococcus aureus bacteria linked to atopic dermatitis thrive at a pH above 7. So, try to use products with a pH comparable to that of the skin, which tends to be around 5.5
- Make sure to thoroughly dry the parts of the body where the skin has the most folds, as these are likely to trap moisture.
- Use cosmetics and deodorants that have been dermatologically tested, again with the aim of not adversely affecting the skin's microbial communities.
As we've seen, the skin is home to a whole host of microorganisms that help provide protection against invasive pathogens. So, any impairment of the skin microbiota can result in inflammation or lead to infections. It is therefore essential to avoid upsetting the bacterial equilibrium of the microbiota, both by using appropriate cosmetics and by not cleansing more than necessary.
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Baldwin HE, Bhatia ND, Friedman A, Eng RM, Seite S. The Role of Cutaneous Microbiota Harmony in Maintaining a Functional Skin Barrier. J Drugs Dermatol. 2017 Jan 1;16(1):12-18. PMID: 28095528.