Characterization by 16S Amplicon Sequencing of Bacterial Communities Overall and During the Maturation Process of Peloids in Two Spas of an Italian Thermal Complex

Peloids are made by mixing clay materials with thermo-mineral waters, enriched with organic substances from microorganisms during maturation. Their beneficial properties may depend on clay minerals, water characteristics, and microbial components, although strong evidence is lacking. Next Generation Sequencing (NGS) allows a comprehensive approach to studying the entire microbial community, including cultivable and uncultivable bacteria. Our study aims to characterize, by NGS, the bacterial community overall and during the maturation process of thermal muds in two spas (A-B) of an Italian thermal complex. Peloids were produced from sulfurous-bromine-iodine thermal water and clay material: natural mud for spa A and sterile clay for spa B. Thermal waters and peloids at different maturation stages (2/4/6 months) were analyzed for microbiome characterization by 16S amplicon sequencing. Biodiversity profiles showed a low level of similarity between peloids and water used for their maturation. Peloids from spa A showed greater microbial richness than those from spa B, suggesting that natural mud with an existing bacterial community leads to greater biodiversity than sterile clay. Genera involved in sulfur metabolism were prevalent in both spas, as expected considering peloids matured in sulfide-rich water. For all three maturation stages, the prevalent genera were Thiobacillus and Pelobacter in spa A and Thiobacillus, Thauera, Pelobacter, and Desulfuromonas in spa B. Richness and diversity indices showed that the community seemed to stabilize after 2-4 months. The 16S amplicon sequencing to study bacterial communities enables the identification of a biological signature that characterizes a specific thermal matrix, defining its therapeutic and cosmetic properties. The bacterial composition of peloids is affected by the thermal water and the type of clay material used in their formulation and maturation.