The human microbiota refers to all the micro-organisms (viruses, bacteria and fungi) living throughout our body. In fact, we have more than 100,000 billion bacteria, including more than 500 different species. But rest assured, each of these micro-organisms is an integral part of who we are and what we are. They are naturally present in our body, live in total symbiosis and are essential for the proper functioning of our body.
This microbiota is distributed over the different areas where the human body is in contact with the outside world : the digestive tract (mouth, oesophagus, stomach, digestive tract), the respiratory tract, the urogenital tract and the skin. Each has specific characteristics such as temperature, pH, fat and protein content. All of these constitute a defined environment that will be specifically favourable to certain species. Thus, from one area to another one will not find quantitatively and qualitatively the same bacteria, viruses and fungi.
Any dysbiosis or imbalance in this environment will lead to the proliferation of external pathogens. This leads to various consequences such as dermatitis, vaginal mycosis or digestive disorders.
It comes mainly from the mother. Indeed, at the prenatal stage our skin is sterile, it is at the time of our birth that it settles. Either by direct contact with its vaginal flora during a vaginal delivery or by its digestive flora if caesarean section.
It evolves and diversifies thereafter throughout our lives. In an intrinsic way, it will be in greater quantity during puberty because of the hormonal peak and it will decrease naturally as we age. But also extrinsically via our daily environment (our place of residence, our transport, our activities, our state of health, etc.).
It's important to dissociate residential and transient skin flora. Indeed, the composition of these microbial communities, their living environments and their implications are different.
The resident cutaneous flora
It corresponds to the set of micro-organisms implanted for life and in a stable way on our skin (on its surface, in each sub-layer, in the glands and in the hair follicles). They are not pathogenic, on the contrary, their presence is indispensable for the proper functioning of the skin because they mainly participate in its defence against the various bacteria, viruses and fungi in the environment but also against UV radiation or pollution for example.
It is composed of different bacteria, the three most predominant genera of which are Straphylococcus, Propionibacterium and Cornynebacterium. There are also yeasts of the Malassezia genus, mites of the Dermodex genus and certain viruses.
The transient cutaneous flora
Unlike the previous one, this one is made up of micro-organisms from the external environment, contaminated environments or other flora (digestive or urogenital).
Not being adapted to the cutaneous environment, their residence time on the skin surface is short. This flora is therefore unstable, it is easily transmitted from individual to individual and generally varies during the day depending on the activities carried out or our external environment.
However, if this environment were to change (change in pH, temperature, excess sebum, a wound, etc.) it could become favourable to them. They will be able to compete with the resident skin flora, colonize the skin and create various skin pathologies such as acne or psoriasis.
It is mainly composed of bacteria of the genus Pseudomonas, Enterococcus, Staphylococcus, Streptococcus and Bacillus but also of yeast of the Candida genus.
What is intra-individual variability?
Intra-individual variability means that on our entire body surface we do not have the same bacteria, viruses and fungi, or at least in different areas, in different quantities. This distribution is specific to each individual and constitutes our "identity card".
Remember, each microorganism has its own specific environment. The environment of the skin flora is defined by body temperature, pH (skin acidity), the level of sebum, sweat and protein, hair growth, the light received, etc.
On these parameters, we can therefore dissociate three skin areas where the composition and distribution of these micro-organisms will be different:
As described above, we are unique. Thus, from one individual to another we do not have the same skin flora.
This is defined by our body identity according to :
But also by certain external parameters such as :
Black, matt and mixed skins have different characteristics from Caucasian skins. Indeed, the skin on their faces is more acidic, more dehydrated and more oily. They renew themselves and eliminate their dead cells more slowly, which creates an irregular skin texture.