Patrick L.J.M. Zeeuwen; Michiel Kleerebezem; Harro M. Timmerman; Joost Schalkwijk

Disclosures

Curr Opin Allergy Clin Immunol. 2013;13(5):514-520. 

In This Article

The Human Skin Microbiome

The microbiota present on the skin can be transient or resident; the latter are considered to be members of the 'normal' commensal skin microbiota that is in homeostasis with the host, whereas the former are microbes from the environment that temporarily live on the skin.[11] The diversity of these transient and permanent microbes on our skin is dependent on the topographical regions of the body that have distinctive characteristics (pH, moisture, salinity, and sebum content), and may also vary due to intrinsic factors (e.g. genotype, age, and sex) and extrinsic individual-dependent factors such as occupation, lifestyle, geographical location, and use of antibiotics, or cosmetics.[12] The human skin microbiota controls colonization by potentially pathogenic microorganisms,[13,14] can modulate the cutaneous immune system,[15,16,17] and is necessary for optimal skin immune fitness.[18] Altogether, this indicates that maintaining our skin microbiota is beneficial for our health.[19]

Recent culture-independent studies of the cutaneous microbiota revealed a considerably greater diversity of organisms than presumed from culture-based methods.[20] These studies[21,22] showed that bacterial diversity largely depends on the topographical location on the body, and that it remains relatively stable over time. These large-scale studies of the composition of microbial communities in healthy volunteers have revealed that most of the resident skin bacteria are categorized into four different phyla: Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes. Furthermore, it was demonstrated that specific bacteria were associated with moist, dry, and sebaceous microenvironments, respectively. For example, Staphylococcus spp. and Corynebacterium spp. are organisms that abundantly colonize moist areas such as the armpit, the inner elbow, and behind the knee, whereas Propionibacterium spp. are found in large quantities in sebaceous areas such as the forehead, the back, and behind the ear.[21,22] The dry areas of the skin, such as the forearm, buttock, and parts of the hand, are colonized by a microbiota of the highest diversity, but lowest absolute numbers. In general, the human skin microbiome has a high degree of variation between persons. Within subjects, site-specific differences exist (e.g. moist versus dry regions); the variability of the skin microbiota within a subject was lower, however, when comparing symmetrical sites (e.g. left and right antecubital creases).[8,21,22] Importantly, all these studies were performed under nonchallenged, static conditions in healthy volunteers. Recently, the dynamics of recolonization of skin microbiota was investigated following skin barrier disruption by tapestripping as a model for superficial injury.[23] It was shown that bacteria are not uniformly distributed in the stratum corneum. The highest bacterial density is, not surprisingly, found in the surface layers, whereas the stratum corneum adjacent to the stratum granulosum (the first living cell layer) contains very few bacteria. A recent publication showed that the microbiome may even extend to the dermal compartmen.[24] The developing neo-microbiome at 2 weeks after skin barrier disruption was more similar to the deeper stratum corneum layers than to the initial surface microbiome, which suggested that the bacteria found in the deeper layers of the stratum corneum predominate during the recolonization process of injured skin.[23] In addition, considerable interindividual variation in epidermal antimicrobial protein expression following superficial injury was observed. The individual-specific levels of constitutive and induced expression of innate immunity functions are likely to account for the host-specific microbiota present on normal and challenged or wounded skin.

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