A developed ecosystem
The skin is the largest organ of the human body. It is the first line of defence against the external environment and what it represents as physical, chemical and biological risks. After a first contact in the genital canal at birth, a flora will then colonise the skin and continue to evolve until death.
An adult’s skin hosts an average population of 1,000 billion bacteria from more than 1,000 different species, between fungi, viruses and arthropods1,2,3. This flora lives on the surface and in the superficial layers of the epidermis, forming a complex ecosystem whose characterisation was based on culture of samples taken from the skin surface or biopsies4.
The body areas richest in bacteria are:
- arms and legs
A unique and variable ecosystem
Resident or transient, the skin microbiota varies quantitatively and qualitatively from one person to another according to age, sex, site, immune system and certain physicochemical factors such as humidity, pH and tempreture1,3,5. So, the microbiota of the scalp is different from that of the face, armpits, chest and genital parts. The skin flora varies also between birth, puberty and aging, according of sweat and sebaceous glands activity. Likewise, a man’s cutaneous flora is different from that of a woman for the same age and body site.
A balanced ecosystem
The composition of skin microbiota is the result of a balance between local conditions and metabolic properties of its microorganisms3. On the one hand, the latters feed on lipids, proteins and other components excreted by the skin itself, including the secretions of sebaceous glands. On the other hand, the biofilm, represented by the skin microbiota, should be constantly renewed, as an adaptation to natural desquamation and other skin erosions, as well as practices of personal hygiene. These must not be excessive in order to preserve the defensive barrier action of the skin, which in turn, controls and maintains naturally its own microbiota through several innate and adaptive cellular mechanisms6,7. Among these mechanisms one can mention the synthesis of antimicrobial peptides and the production of antibodies present in the sweat.
In interaction with the host
Whether the immune system can influence the skin flora, the latter largely contributes into the host’s means of defence. In fact, it is more and more clearly that the microbiota of the skin plays an essential role in the development and function of its immune system. Several mechanisms have been suggested among which the microbial interference phenomena, the bacterial adhesion process to epidermal cells, the production of inhibitors, the creation of unfavourable pH conditions or modification of receptors.
Cutaneous microbiota and cosmetics
The skin microbiota contributes to the olfactory “signature” of human skin.
Perfumes, beauty products or repellents unintentionally or deliberately in contact with the skin can:
- interfere with the skin’s smell and reinforce or modify it
- temporarily destroy the skin hydrolipid film
1Grice EA, Segre JA. The skin microbiome. Nat Rev Microbiol. 2011; 9: 244-253.
2 Findley K, Grice EA. The skin microbiome: a focus on pathogens and their association with skin disease. PLoS Pathog. 2014; 10: e1004436.
3 Mokni M, Abdelhak S. Flore cutanée, microbiote et microbiome. Dermatologie infectieuse. 2014: pp 4.
4 Grice EA, Kong HH, Renaud G, Young AC; NISC Comparative Sequencing Program, Bouffard GG, Blakesley RW, Wolfsberg TG, Turner ML, Segre JA. A diversity profile of the human skin microbiota. Genome Res. 2008; 18: 1043-1050.
5 SanMiguel A, Grice EA. Interactions between host factors and the skin microbiome Cell Mol Life Sci. 2015; 72: 1499-1515.
6 Sanford JA, Gallo RL. Functions of the skin microbiota in health and disease. Semin Immunol. 2013; 25: 370-377.
7 Grice EA. The skin microbiome: potential for novel diagnostic and therapeutic approaches to cutaneous disease. Semin Cutan Med Surg. 2014; 33: 98-103.