The skin harbours a resident flora that has adapted to its environment: a dry, nutrient-poor and acidic surface. When the pH of this surface rises, the adherence and the composition of this flora change.
Today we speak of the skin microbiome. Its study has progressed considerably in twenty years. It invites us to see the skin as an inhabited landscape.
An inhabited landscape
The skin surface is colonised by a great diversity of bacteria, yeasts and other microorganisms. According to a reference review, most are harmless to their host, and some are even useful to it (Grice and Segre, 2011).
This population varies from one area to another, according to the local ecology. Oily areas such as the forehead or the back, moist areas such as the skin folds, and dry areas such as the forearm host different communities. Each reflects the conditions of the terrain: moisture, sebum, temperature, pH.
These communities are also remarkably stable. Repeated sampling over several months and years has shown that each person largely retains their own strains (Oh et al., 2016).
A dry, poor and acidic environment
Researchers describe the skin surface as a “desiccated, nutrient-poor and acidic” environment (Byrd et al., 2018). These three characteristics select the organisms capable of living there.
Acidity is one of these filters. Measured without recent washing, the skin surface has an estimated average pH of 4.7 (Lambers et al., 2006). The resident species have adapted to it over a long cohabitation with human beings.
What the measurements show
Two studies provide direct data on the link between pH and resident flora.
Adherence. Lambers' team studied how resident bacteria remain attached to the skin. At an acidic pH, between 4 and 4.5, the flora remained attached to the surface. At an alkaline pH, between 8 and 9, it tended to detach and disperse (Lambers et al., 2006). The title of the article summarises the authors' conclusion: an average pH below 5 is favourable to the resident flora.
Composition. In an eight-week crossover trial, ten healthy volunteers washed their forehead and forearm twice a day, first with soap and then with a synthetic cleanser, or in the reverse order. During the soap period, the skin pH was higher and the number of propionibacteria, a family of resident bacteria, was clearly greater. On the forehead, the number of bacteria was correlated with the skin pH (Korting et al., 1987).
The authors draw a cautious conclusion. The skin pH can change durably according to washing habits, and this change can influence the composition of the resident flora.
A two-way relationship
The skin pH and its flora are linked. Surface conditions select the microorganisms. In return, some of them take part in the transformation of surface lipids, for example by releasing fatty acids from sebum (Grice and Segre, 2011). The landscape and its inhabitants evolve together.
This ecological vision is still young. The studies cited here describe associations and mechanisms observed under specific conditions. They do not make it possible to predict the effect of an isolated action on the microbiome of a given person.
What can be taken from this in everyday life
Above all, this knowledge invites moderation.
- Wash without excess. Each wash temporarily modifies the pH and the surface film.
- Take the pH of cleansers into account. Classic soaps are alkaline. Other cleansers are formulated at a pH close to that of the skin.
- Rinse thoroughly so as not to leave residues.
- Respect the areas. The face, the skin folds and the dry areas do not have the same needs.
The skin microbiome remains an active field of research. For any question about the condition of your skin, a dermatologist remains the appropriate contact.
This article is part of the PHYTO 5 Library.
Sources
- Byrd AL, Belkaid Y, Segre JA. The human skin microbiome. Nature Reviews Microbiology. 2018;16(3):143-155. DOI : 10.1038/nrmicro.2017.157
- Grice EA, Segre JA. The skin microbiome. Nature Reviews Microbiology. 2011;9(4):244-253. DOI : 10.1038/nrmicro2537
- Korting HC, Kober M, Mueller M, Braun-Falco O. Influence of repeated washings with soap and synthetic detergents on pH and resident flora of the skin of forehead and forearm. Results of a cross-over trial in health probationers. Acta Dermato-Venereologica. 1987;67(1):41-47. DOI : 10.2340/00015555674147
- Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. 2006;28(5):359-370. DOI : 10.1111/j.1467-2494.2006.00344.x
- Oh J, Byrd AL, Park M, NISC Comparative Sequencing Program, Kong HH, Segre JA. Temporal stability of the human skin microbiome. Cell. 2016;165(4):854-866. DOI : 10.1016/j.cell.2016.04.008