Biocide efficacy and consolidant effect on the mycoflora of historical stuccos in indoor environment

Investigations are needed to address and optimize the use of biocides and restoration materials with reference to the fungal diversity, which often characterizes cultural heritage surfaces. This work aimed to examine the diversity of fungi responsible of aesthetic decay on the stuccos of the vault of a religious building in Torino (NW-Italy), and to evaluate the sensitivity of the detected set of species to widely used biocidal products (benzalkonium chloride, isothiazolinones, sulphamide derivatives) and their application solvents. The effect of four commercial consolidants on their potential (re-)colonization following restoration interventions was also assessed. Four different deterioration phenomena were related to the occurrence of Chaetomium murale, Stachybotrys chartarum, Penicillium chrysogenum and Sarocladium kiliense, respectively. Surface receptivity to the different species – identified on morphological and molecular bases – was related to slightly different thermo-hygrometric conditions, the distribution of painted surfaces, salts, and local remnants of cellulose poultice used in past restoration interventions. Specific sensitivity to two solvents and ten different biocide treatments was evaluated in terms of inhibition of mycelial growth from transplanted inocula at 9 (T1) and 27 (T2) days after the incubation. The different solvents and biocide products differently affected growth and/or pigmentation of the four species. Only 40% of the ten performed biocide treatments determined the growth inhibition of all the examined species at both T1 and T2. In other cases, inhibition observed at T1 for C. murorum, S. chartharum and S. kiliense, was followed at T2 by the colonization of inhibition zones. The mycelial growth on an oligotrophic culture medium poured with four commercial consolidants was evaluated one year after the incubation. All the species displayed some growth from the inocula, with a scarce biomass being only observed in negative (water) controls and upon one consolidant treatment. The three other products strongly supported a higher growth of at least two of the examined fungal species with respect to negative controls. In conclusion, different sensitivity of each fungal species for most biocidal treatments and stimulation by consolidants indicate that species-specific assays of products are necessary to calibrate and optimize restoration works. In particular, effective inhibition of fungi by biocides, and potential stimulation by consolidants, should be evaluated after several weeks and months, respectively, since short-term monitoring may be misleading.