In addition to its protective function, the skin, the largest organ of the human body, can be considered a complex ecosystem, composed of a vast diversity of microorganisms distributed among the different niches that make it up. These organisms can be bacteria, fungi, viruses, and symbiotic mites, which can help protect us against pathogenic organisms. And, as in any other ecosystem, imbalance can lead to the emergence of diseases and infections.
The skin of mammals contains appendages such as sebaceous glands, sweat glands, and hair follicles, each with its own microbiota. For example, the bacteria that inhabit the sweat glands are responsible for the characteristic smell when we sweat. Additionally, the skin can also be divided by regions, according to the anatomy and physiology of the body, with different types of microorganisms present in each region. Symbiotic microorganisms can be one of the greatest partners of human health, allowing for a greater understanding of the interaction with the host and its relationship with the emergence of diseases.
Much of what is currently known about the skin microbiota has been made possible through culture methods for these microorganisms. Previously, knowledge was restricted to what could be cultivated, leaving aside a whole diversity that did not grow in the laboratory. However, with advances in technology, methods based on large-scale sequencing have revealed a much greater diversity of microorganisms present on the skin than those previously seen by traditional methods.
A study published in 2009 in the journal Science illustrates the potential of large-scale sequencing. Samples from 20 different regions of the skin of 10 healthy patients were used. The analysis through sequencing of the 16S ribosomal RNA (rRNA) gene showed that the bacteria present on our skin mainly belong to four different phyla: Actinobacteria, Firmicutes, Bacteroidetes, and Proteobacteria; and that the distribution of members of these four groups occurs according to the region and physiology of the skin. Different regions contain distinct compositions of the four groups.
The analysis through metagenomics has revealed, for example, that species of Staphylococcus and Corynebacterium spp are the main colonizers of moist regions of the skin (such as the soles of the feet). In drier regions with a higher number of sebaceous glands (such as the skin of the face, chest, and back), the presence of the lipophilic bacterium Propionibacterium spp increases. Dry areas are the most diverse, with representatives from all four phyla.
Other factors can also influence the diversity of the skin microbiota. An individual's lifestyle (diet or type of clothing) as well as age, sex, and place of residence can influence the variability of the present microorganisms. In fact, a baby in its mother's womb is in a sterile environment, and during birth, the newborn's skin is colonized by microorganisms present in the vaginal canal or on the mother's skin (in the case of a cesarean section). During puberty, the skin microbiome can be significantly altered by the increase of lipophilic bacteria, which proliferate due to increased sebum production on the skin's surface, caused by changes in the sex hormones characteristic of this phase.
The use of antibiotics and other factors also causes changes in the microbiota. Continuous-use agents such as soaps, creams, personal hygiene products, and cosmetics also bear significant responsibility for altering this microbiota. However, to date, no study has examined the changes in this ecosystem in the presence of diseases at the molecular details.
In the United States, in 2007, the NIH (National Institutes of Health) initiated a project called the Human Microbiome Project, with investments of approximately $140 million over 5 years, aimed at characterizing the microbiota of various parts of the human body, such as nasal cavities, oral cavities, skin, gastrointestinal tract, and urogenital areas, and investigating its relationship with human health.
This type of initiative, along with the use of modern molecular techniques and metagenomic analysis, can be of great utility for the development of new products and medications that can assist in curing diseases as well as improving the quality of life and well-being of human beings. Perhaps, in a few years, we will have cosmetic products designed not only for certain skin types but also planned to favor symbiotic microorganisms, just as we currently consume probiotic foods.
References:
Grice EA and Segre JA. The skin microbiome. Nature Reviews Microbiology 9, 244-253 (2011).
Grice EA et al. Topographical and temporal diversity of the human skin microbiome. Science 324, 1190–1192 (2009).
