Essential oils and extracts of plants as biocides against microorganisms isolated from the ruins of the Roman city of Conímbriga (Portugal)

Abstract

Biodeterioration of monumental complexes is in large part due to the proliferation of various microorganisms that attack the physical–chemical structures of support materials. Various conservation and restoration interventions use commercial biocides of synthetic origin, which exhibit some human and environmental toxicity and sometimes side effects on support materials. The main objective of this work is the assessment of new biocides obtained from endemic Mediterranean plants, to be used in the preservation of cultural heritage with the goal of contributing to the sustainable use of ecosystems and to the development of Mediterranean local communities. The biocidal potential of essential oils (EOs) and solvent extracts (SEs) (ethanol and n-hexane) obtained from four plants were evaluated: Thymus mastichina (Tm), Mentha pulegium (Mp), Foeniculum vulgare (Fv), and Lavandula viridis (Lv). Microorganisms collected at an emblematic site of Portuguese cultural heritage, the ruins of the Roman city of Conímbriga, were used to evaluate the biocidal activity of the EOs and SEs. It can be concluded that (i) SEs did not exhibit fungicidal nor bactericidal activity, except for one fungus specie; (ii) biocidal activity of EOs depends on the microorganism specie. The EOs showed a relative average biocidal activity (when compared to the commercial biocide Biotin T (1% v/v)) of 64%, 32%, 30%, and 25% for Mp, Fv, Lv, and Tm. On carbonate rocks, the application of Fv and Mp EOs up to 3 layers do not promote significant color/tonality changes in the surface of the rock. And the application of three layers of Lv and four layers of Fv, Mp, and Lv OEs only promote the occurrence of blurs or stains (variation of tonality) on rocks that presents very low porosity. It can also be noted that the EO of Mp has the broadest spectrum of activity. The results allow considering the use of Mp, Fv, Lv, and Tm EOs as valid alternatives to commercial biocides, providing a prospective application in the field of green conservation of building heritage.