Microbial biodeterioration of 18 ^th -century oil paintings in Orosi, Costa Rica, and in vitro evaluation of volatile essential oil components as antimic

Microbial colonization is a major cause of deterioration in paintings, leading to discoloration, pigment degradation, and loss of structural integrity. While biodeterioration of artworks has been studied in temperate climates, tropical environments remain underexplored despite their high humidity and temperature, which can promote microbial growth. This study assessed the microbiological deterioration of two 18 ^th -century oil paintings, La Muerte de San José and Virgen de Guadalupe, located in Orosi’s Colonial Church and Religious Art Museum, Costa Rica. Microorganisms were isolated and identified using VITEK 2, microscopy, and matrix-assisted laser desorption ionization–time of flight mass spectrometry analysis, and their biofilm-forming capacity was evaluated. Additionally, the antimicrobial activity of six essential oil components was tested using direct and indirect contact assays. Seventeen bacterial taxa and 15 fungal genera were identified, with Bacillus , Staphylococcus , Cladosporium , and Aspergillus among the most common. Based on the fungal isolate-frequency distributions, Virgen de Guadalupe had a higher Shannon–Wiener diversity index than La Muerte de San José. Several isolates displayed strong biofilm formation, particularly Bacillus subtilis / amyloliquefaciens / vallismortis , and Staphylococcus saprophyticus . Linalool exhibited the strongest inhibitory activity, achieving complete bacterial growth inhibition in non-contact assays. Short-term environmental monitoring documented daily variation in temperature, relative humidity, and CO ₂ concentration during the 8-day measurement period. Together, these results characterize the culturable microorganisms recovered from two tropical heritage paintings and identify volatile essential oil components as candidates for further evaluation in preventive conservation.