INTRODUCTION
Thermal water has been used for its medicinal benefits since Roman times. It has been reported to benefit a variety of diseases across dermatology, pulmonology, hematology and gastroenterology.1-3 La Roche-Posay Thermal Spring water (LRP-TSW) from France has been useful in treating skin diseases including atopic dermatitis and psoriasis.1,2 The LRP-TSW Thermal Center serves as a hospital staffed by 8 Dermatologists treating over 8,000 patients per year, 25% of whom are children. Patients at the Center are treated with therapeutic baths, known as balneotherapy (BPT). The regimen consists of an 18-day treatment with a daily high-pressure filiform shower (15 bars for 3 minutes) using crude thermal spring water.This review discusses the role of TSW as a probiotic and prebiotic therapy in enhancing the diversity of the skin microbiota in inflammatory skin diseases as well as the clinical improvements in signs of these diseases using LRP-TSW balneotherapy.Mineral and Microbial Composition of Thermal Spring Water and Its Chemical and Physical PropertiesLRP-TSW is comprised of specific minerals and non-pathogenic microbes.1,2 The low mineral and silicate concentration gives LRP-TSW its name in French, “L’eau de velours” or velvet water. LRP-TSW is oligotrophic meaning that it contains a low concentration of nutrients (<1 g/L). However, the presence of elements like selenium (Se) and strontium (Sr) play an important role in its biological activities.2 Selenium salts are necessary for cellular functions including enzyme activity like glutathione peroxidase, thioredoxin (a class of small redox proteins) reductase, and deiodinases. Those elements also influence bacterial growth and can impact the body’s global microbiota.3 Since decades the reactions catalyzed by formate dehydrogenase of bacteria, or glycine reductase of clostridia, have been described.3 The common denominator of these selenium-dependent processes is that they are all oxidation-reduction reactions.3 The antioxidant properties of LRP-TSW may help improve skin condition by reducing skin dryness and pruritus of patients affected by chronic dermatoses such as atopic dermatitis and psoriasis.2 The effect of selenium on lipid peroxidation has been studied in cultured human skin fibroblasts.2 A reduction of thiobarbituric acid reactive substances (TBARS), an index of lipid peroxidation and oxidative stress, was noticed if the cells were cultured in a medium supplemented with Se or with LRP-TSW compared to the control medium with demineralized water.2 Together, selenium-dependent glutathione peroxidase (Se-GSH.Px) activity and cell viability were significantly increased.2LRP-TSW is rich in non-pathogenic micro-organisms that play a major role treating various skin diseases. The microbial composition of LRP-TSW has been characterized with a metagenomics approach using an Illumina next-generation sequencing (NGS) of the V1–V3 hypervariable regions of the 16S rRNA genes. A global bacterial picture of LRP-TSW was determined2 and is shown in Figure 1. The main bacterial characteristics of crude LRP-TSW are:
