The microbiome can be defined as all micro-organisms found within a living organism (both inside and outside). It is a symbiosis between the human body and micro-organisms. Micro-organisms enrich and influence the life of their host. They play an important part in digestion, but also in protecting the organism against pathogens through a protective barrier on the skin. This barrier is established by the superficial layer of the skin which is called Stratum corneum (horny layer of epidermis). This layer is composed of corneocytes (dead cells) and a hydrolipidic film. 
Fig 1: Representation of the different layers of the epidermis
These two elements play a significant role in the protection of the skin. This is due to a low pH (between 4 and 5.5 ), and a small amount of water (waterproof role) . Most bacteria (neutrophils) require a pH between 5.5 and 8  and many bacteria have a water activity (Aw) superior to 0.91. That is why they cannot easily colonize new surfaces.
|aw=0,91 ... 0,95||Many bacteria|
Fig 2: Summary table of different values of water activity in relation to various micro-organisms 
Corneocytes are rich in keratin, a hard and insoluble protein that reinforces the skin’s barrier role. Moreover, the hydrolipidic film forms a sort of “cement” between corneocytes, which makes it impermeable to foreign bodies . Thanks to its components, this film also has antibacterial properties. As a matter of fact, it is formed of sweat, sebum, lipids and antimicrobial peptides, which are secreted by the sweat glands. However, the Stratum corneum is not a sterile medium, on the contrary, many bacteria are found in its extracellular matrix.
It is estimated that more than 1 000 bacterial species can be found on our skin, mainly in the upper layers of the epidermis and on hair follicles. These epidermal bacteria are continually renewed and eliminated by desquamation—a natural phenomenon that causes the stratum corneum to peal in the form of flaking skin .
Unlike pathogenic bacteria, certain bacteria are commensal, which means that they naturally interact with the host in a safe manner. They are beneficial to our organism and it is necessary to protect their balance. Each individual is different and possesses a unique microbiome, their own personal “bacterial fauna.”
“A balanced flora leads to a skin that is less attacked, that regenerates better and that ages less.”
The main objective of the microbiome study is to isolate one or more “good bacteria” that is strong enough to fight “bad bacteria,” without affecting the skin’s natural balance. If we disrupt this balance, undesirable effects may arise, such as dandruff formation due to scalp sensitization.
This practice, consisting of enhancing beneficial bacteria, is known as the science of probiotics, which means conducive to life. In fact, probiotics are the opposite of antibiotics ‑ rather than removing bacteria, they contribute to increasing the number of beneficial bacteria.
After extensive research, 3 bacterial families stood out by their unique characteristics and potential: Propionibacterium, Staphylococcus (negative coagulase) and Corynebacterium. These bacteria play the role of “cleaners”—they feed on debris from corneocytes or sebum cells and prevent colonization by pathogenic bacteria . In more familiar terms, they are known as “decomposers.”
However, as with all bacteria, certain species show less potential, even among these promising families. It was therefore necessary to study these bacteria in close detail, in order to learn their good and bad characteristics.
- Propionibacterium acnes
This bacterium is responsible for acne. It generates enzymes that deteriorate the skin as well as certain immunogenic proteins. In other words, it causes an inflammatory reaction . These bacteria are responsible for acne related inflammation and it is therefore necessary to reduce the activity of this species .
Staphylococci are the most abundant bacteria colonizing the skin. The good bacteria, which are beneficial to the skin, are non-aureus, i.e. coagulase-negative Staphylococcus.
- Staphylococcus epidermidis
This species represents 44% of Staphylococcus present on the skin. This micro-organism inhibits pathogen adhesion, especially S. aureus, and allows the creation of biofilms . S. epidermidis also produces antimicrobial peptides and amplifies the immune response of keratinocytes against pathogens (90% of the skin’s superficial layer cells are subsequently differentiated in corneocytes). Like all bacteria, S. epidermidis has pathogenic properties which can cause diseases or skin infections, but only in the case of immunodepressed individuals.
- Staphylococcus hominis
This species represents 22% staphylococcus found on the skin. S. Hominis produces thioalcool, which is responsible for body odours. Thioalcool is an alcohol in which an oxygen atom has been replaced by a sulfur atom, hence generating a strong odour. This bacterium must therefore be inhibited, but only partially, since it is an integral part of the skin flora and thus of the proper functioning in the skin’s barrier role.
- Corynebacterium minutissimum
This bacterial species is responsible for benign skin infections, especially in areas of folds (armpits…), where it can cause red spots. It is necessary to inhibit its development.
- Corynebacterium tenuis
These bacteria must also be inhibited, since it can lead to a chronic infection of body hair called Trichomycosis pubis, or axillaris.
Other micro-organisms are also interested in colonizing the skin. This is the case of Demodes folliculorum, more commonly known as a mite. Often found inside houses, these micro-organisms are present on the skin of many individuals. These mites may cause hair follicle disturbances. This can lead to a transient or chronic immune deficiency or hypersecretion of sebum.
Fig 3: Comparison of a healthy and infected hair follicle 
Moreover, according to a study carried out at the National University of Ireland, these mites could cause rosacea, a disease responsible for cheek redness that can eventually become permanent. Indeed, these micro-organisms feed on sebum, especially when the host is stressed, because it then releases substances that are consumed by mites. 
- https://books.google.ca/books?id=MRC81rjQO5IC&pg=PA6&dq=propionibacterium+acnes+et+epiderme&hl=fr&sa=X&ved=0ahUKEwiUrr3blZ3UAhWs64MKHYGFBnEQ6AEIJDAA#v=onepage&q=propionibacterium%20acnes&f=false page 18