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Natural derivatives, such as essential oils, are presented as an alternative to classical biocides to the treatment of biocolonization. Thus, in this work, the cleaning and biocidal potential of some natural derivatives towards two natural biofilms’ growth on the same granite wall, with different microbial composition, was evaluated. For this purpose, three essential oils (EOs) (from Origanum vulgare, Thymus vulgaris and Calamintha nepeta) and their main active principles (APs) (carvacrol, thymol and R-(+)-pulegone, respectively) were embedded in a hydrogel matrix, with different combinations of EOs and APs, in order to evaluate the synergistic action of different actives. For comparative purposes, pure hydrogel and a mechanical method (brushing) were also used. Colorimetric measurements and chlorophyll a fluorescence analyses were performed to evaluate the cleaning action of the treatments on the biofilms. Overall, the EOs and APs present in the hydrogel proved to be reliable treatments to limit natural biocolonization, with O. vulgare being one of the most effective treatments in combination with other compounds, due to the majority presence of carvacrol. Moreover, the effect of the different treatments strictly depended on the biofilm in question, as well as its ability to adhere to the substrate.

Travertine is a sedimentary carbonate rock widely used in Roman arts and frequently subjected to biodeterioration. In this preliminary multi-analytical study, three essential oils (EOs) from T. vulgaris, O. vulgare, C. nepeta and their respective phenolic major components (thymol, carvacrol, pulegone) were selected to evaluate their biocidal potential against a multi-species biofilm grown on a travertine test wall located inside Sapienza University Campus in Rome. A preliminary characterization of the biofilm occurred through the employment of genomic methodologies. It has been confirmed the presence of microorganisms belonging to fungi, plant and bacteria kingdoms amplifying the rDNA in the characteristic regions (ITS, 18S, 16S) by performing PCR amplification with specific primers. For this purpose, two different biofilm sampling procedures, one completely non-invasive (swab in physiologic saline solution) and one micro-invasive (adhesive tape) have been employed: the adhesive tape performed better than swab in relation to DNA purity and quantity. After the biofilm characterization, the application of the phytochemicals occurred. For the realization of formulations, the EOs and their major components have been incorporated inside an innovative hydrogel matrix conceived for the cleaning of cultural heritage. The biocidal and cleaning action was evaluated with colorimetric measurements before and after the application of the products and, in the case of the surfaces treated with the essential oils, these measurements have been repeated 6 weeks after the end of the treatment. It emerged that the formulations containing the active phenolic compounds have, in most cases, a better efficacy against the multispecies biofilm. Anyway, it was verified that the biocidal action of the essential oils lasts in time, since colour variation towards the original colour of the surface have been determined 6 weeks after the end of the procedure.