Skin microbiome and skin disease: the example of rosacea.

The imbalance and/or the perturbation of the microbial populations that colonize the skin and that contribute to its defense may represent one of the causes of the development of noninfectious skin diseases. Atopic dermatitis, psoriasis, acne, and rosacea can be listed among these kinds of pathologies. In particular, considering that microbes have been long addressed as having a role in rosacea, this common dermatosis can be an interesting model to evaluate the correlation between microbiome alterations and the occurrence of clinical manifestations. Different microorganisms have been suggested to have a role in rosacea, but no direct correlation with the incidence of the pathology has been clearly defined. Skin microbiome composition is crucial for the correct skin immune functions and recent findings indicate an abnormal activation of innate immune system associated with the rosacea. The enhanced expression of toll-like receptor 2 in the epidermis of rosacea patients can represent a possible explanation for the amplified inflammatory response to external stimuli observed during the disease. In addition, significantly higher small intestinal bacterial overgrowth prevalence in rosacea subjects has been found and its eradication has been associated with a regression of the skin lesions. In conclusion, both skin and gut microbiome seem to have a role, even if synergistic with other factors, in the pathogenesis of rosacea. A deeper knowledge of human microbiome composition and microbe-host interactions will contribute to clarify the mechanism of development of rosacea and possibly will provide innovative therapeutic approaches.