Biocide Treatments on Stone Materials from Pompeii: Microbial Selection, Efficacy and Emerging Risks

At the archeological site of Pompeii, the deterioration of exposed structures is frequently associated with the combined action of microbial colonization and soluble salts, both recognized as major agents of decay affecting ancient surfaces. Although biocides are commonly applied during cleaning procedures to reduce microbial biomass, their incorporation into restoration-oriented formulations for the protection of porous stone substrates requires careful assessment of efficacy, microbiological risks, and sustainability. This study evaluated the performance of 2,4,5,6-tetrachloroisophthalonitrile (chlorothalonil) and iodopropynyl butylcarbamate (IPBC) as candidate active ingredients for conservation applications in activated new mortars. Yellow tuff, gray tuff, and brick samples collected from different sectors of Pompeii were investigated through culture-based analyses, ATP quantification, and metabolic profiling. Biocidal treatments were subsequently tested under laboratory conditions. The investigated substrates exhibited variable microbial counts and metabolic activity, generally reflecting different degrees of deterioration. Chlorothalonil showed negligible inhibitory effects, whereas IPBC reduced fungal growth in a dose-dependent manner. However, the highest IPBC concentration induced a red chromatic alteration associated with the selection of a bacterial strain preliminarily identified as Micrococcus roseus. Phenotype microarray analyses revealed broad chemical tolerance. Overall, biocidal treatments may alter microbial communities, favor tolerant microorganisms, and produce undesirable aesthetic effects. Finally, the study also assessed the environmental impact associated with laboratory and field activities, highlighting potential mitigation strategies to support more sustainable conservation research practices.