The cutaneous microbiota

Cutaneous microbiota is extremely diverse. Its composition varies according to the cutaneous zone and between individuals, and its imbalance is associated with skin diseases.

Cutaneous microbiota is particularly complex, with extremely significant inter- and intra-individual variations¹. It is composed of a set of microorganisms, bacteria, yeasts, viruses, and parasites. At least 19 major families of bacteria have been identified: the primary ones are Actinobacteria (Corynebacterium, etc.), Firmicutes (Staphylococcus, Streptococcus, etc.) and Proteobacteria (Acinetobacter, etc.)^1,2. Among the bacterial strains present on healthy skin, the primary ones found are Corynebacterium jeikeium, Pseudomonas aeruginosa, Staphylococcus aureus and Staphylococcus epidermidis³. Microbiota composition varies based on the location, ranging from 100 microorganisms per cm² on the back or the tips of the fingers to 106 on the forehead or in the armpits².

This microbiota develops from birth, as a result of the contact with the vaginal flora after a vaginal birth or with the stomach flora in the case of a caesarean section. It then changes with age⁵ . It also varies according to gender⁴ and is influenced by an individual’s health status and lifestyle: immune status, hygiene, use of antibiotics, cosmetics, type of clothing, occupation, climate, geographic location, or exposure to UV^1,2,6 .

In adults, the surface of the skin represents between 1.5 and 2 square meters, so the cutaneous microbiota plays a fundamental defensive role against surrounding infectious agents. S. epidermidis, for example, produces antimicrobial peptides and Propionibacterium acnes contributes to the skin’s acidity² . These two strains are also involved in topical immunity⁷. Beyond that, this microbiota may modulate the systemic immune system, in the same way as the intestinal microbiota^8,9.

Interactions between these microorganisms are innumerable and disruptions in this equilibrium are associated with skin problems like acne, diseases such as psoriasis or atopic dermatitis, or even abnormal wound healing² .

Sources
1- Catherine Dunyach-Remy et al. Le microbiote cutané: étude de la diversité microbienne et de son role dans la pathogénicité. Revue Francophone des Laboratoires, Février 2015 – N°469 : 51-58. http://linkinghub.elsevier.com/retrieve/pii/S1773035X15728212?via=sd
2- A. Schwiertz (Ed.) Microbiota of the Human Body. Advances in Experimental Medicine and Biology 902: 61-81. http://link.springer.com/book/10.1007%2F978-3-319-31248-4
3- Chiller K et al. Skin microflora and bacterial infections of the skin. J Investing Dermatol Symp Proc 2001; 6: 170-4. https://www.ncbi.nlm.nih.gov/pubmed/11924823
4- Fierer N. et al. The influence of sex, handedness and washing on te diversity of hand surface bacteria. Proc Natl Acad Sci USA 2008; 105:17994-9. http://www.pnas.org/content/105/46/17994.abstract
5- Somerville DA. The normal flora of the skin in different age groups. Br J Dermatol 1969;81:248-58. https://www.ncbi.nlm.nih.gov/pubmed/5778713
6- McBride ME et al. The environment and the microbial ecology of human skin. Appl Environ Microbiol 1977; 33:603-8. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC170732/
7- Christensen GJM et al. Bacterial skin commensals and their role as host guardians. Benefic microbes 2014 ; 5 : 201-215. https://www.ncbi.nlm.nih.gov/pubmed/24322878
8- Belkaid Y et al. Dialogue between skin microbiota and immunity. Science 2014 ; 346 :954-959. https://www.ncbi.nlm.nih.gov/pubmed/25414304
9- Nakamizo S et al. Commensal bacteria and cutanueous immunity. Semin Immunopathol 2015 ; 37 ;73-80. https://www.ncbi.nlm.nih.gov/pubmed/25326105