Jesús A. Espinoza-Valverde

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. IMPORTANCE: Understanding the microbiological deterioration of cultural heritage in tropical environments is crucial for designing sustainable conservation strategies. While microbial colonization of artworks has been widely studied in temperate regions, data from tropical heritage settings remain limited. This study integrates microbiological, environmental, and physicochemical analyses to characterize culturable microorganisms recovered from 18 ^th -century oil paintings in Orosi, Costa Rica. By combining microbial identification, biofilm quantification, and essential oil biocide testing, it bridges applied microbiology and cultural heritage conservation. The in vitro inhibition of biofilm-forming bacteria by volatile compounds such as linalool in a non-contact assay supports further investigation of their potential in conservation, including their efficacy and compatibility with artwork materials. These findings provide a basis for future studies of microbial deterioration in tropical heritage settings and for evaluating conservation applications of volatile compounds.

Eligibility propagation (e-prop) is a biologically plausible learning rule for recurrent spiking neural networks (SNNs). It approximates gradients computed by backpropagation through time using online updates based on eligibility traces and learning signals. We reformulate the originally time-driven gradient calculations used to compute synaptic weight updates in an event-driven form and show that the resulting formulation accurately reproduces the time-driven results on the benchmark tasks of the original study. We further incorporate additional biological features and show on the event-camera dataset neuromorphic MNIST that they do not degrade learning performance. Enabled by the event-driven formulation, we implement mechanisms that reduce gradient computations, particularly in sparse settings: exactly for the original e-prop model and approximately for the biologically extended model. We assess runtime and memory scaling of both event-driven e-prop models up to millions of neurons, with flat weak-scaling curves suggesting feasibility beyond this range. By providing a reference implementation in the NEST simulation code, an established open-source framework optimized for large-scale SNN simulation, this work supports reproducible studies of e-prop-like learning rules and their prototyping before deployment on neuromorphic hardware.

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 promote microbial growth. This study assessed the microbiological deterioration of two eighteenth-century oil paintings, La Muerte de San Jose 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 MALDI-ToF 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. Twenty-three bacterial species and fifteen fungal genera were identified, with Bacillus, Staphylococcus, Cladosporium, and Aspergillus among the most common. Notably, La Virgen de Guadalupe displayed the highest microbial diversity, reflected in a high Shannon index, indicative of a more complex microbial community. 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. Environmental monitoring revealed persistently elevated relative humidity and CO2 levels during the study period. Together, these results reveal the complex microbial ecology of tropical heritage paintings and demonstrate that volatile essential oil components can serve as candidates for low-impact antimicrobial strategies in preventive conservation.