## 1 Introduction  
Studying the original and restoration materials used in historical monuments, investigating their conservation and planning the appropriate methodology represent sustainable requirements in the field of restoration, and these are permanent concerns that must be taken into account in order to identify biodeterioration and the first stage of microbial colonization and ensure long-term preservation. Physical and chemical factors can act as deteriogens, but they also stimulate microbial growth [1]. Identification of microorganisms on mural paintings does not always imply a change in the composition or physical properties of the materials. Nevertheless, in special environmental conditions, bacteria and fungi act as biodeteriogens that start to grow and colonize surfaces, stimulating or accelerating the deterioration process [2]. Microorganisms are not only biodeteriogens but can also become a risk factor for the health of restorers and visitors [3; 4; 5]. Microbial colonization of either original or restored mural paintings takes place due to these microbes' ability to use organic and inorganic materials as nutrients. As a result, the biodeterioration process becomes visible through aesthetic and structural changes. Microbialcolonization and deterioration processes are enhanced by a wide range of abiotic factors, such as humidity, temperature, nutrients and the chemical composition of the air [6; 7; 8]. Execution techniques in mural painting involve a layered structure comprising the support, ground and pictorial layers. Over time, organic compounds may be added, either through the restoration process or from human activities, such as firing candles in churches [9]. Bacteria, mostly those with low nutritional needs, are the first colonizers [6], but later on, due to the application of consolidants and the accumulation of organic deposits, fungi become the main colonizers producing physical and chemical deterioration phenomena. Mural paintings stand in confined and semi-confined environments [6] with extreme humidity and abundant nutrients, which promote the germination of fungal spores and mycelia growth. Fungal colonization starts on the surface of the consolidants, in the pictorial layer or in cracks. As a consequence of in-depth development of the hyphae, degradation of consolidants, production of exfoliations and loss of the pictorial layer take place [10; 11]. [PERSON] et al. [12] revealed that fungal species adapt to different environmental conditions, which can be reflected both in the rate of growth and the developing life forms that contribute to changes in the substrate. Fungi produce aesthetic damage due to pigment discolorations, mycelia pigmentation and release of organic pigments of different colours [13]. Extracellular enzymes and organic acids are also released in the substrate, causing chemical alterations to the original pigments [14; 15; 16; 17]. Black fungi produce discoloration on both the pictorial layer and the consolidants due to melanised cell walls [18; 19; 20]. Assessment of the biodegradation capacities of fungi, such as production of organic acids, proteseses, esterases and pigments, through rapid screening tests is important for the evaluation of their potential risk to cultural heritage [21]. To minimize and/or prevent microbial growth, contaminated areas are treated with biocides [22; 23; 24; 25; 26; 27; 28] and evaluated from the microbiological point of view due to the high spore resistance [29]. According to [PERSON] et al. [30], to reduce biodeterioration of outdoor surfaces, biocides could be applied directly to stone surfaces or added into coating formulations. After evaluation with FT-IR spectroscopy in ATR mode and Raman spectroscopy, promising results were obtained with two silica nanosystems loaded with 2-mercaptoethanoltriazole2 ole. [PERSON] et al. [31] reported a successful treatment based on a controlled release system using mesoporous silica nanoparticles as carriers loaded with Preventol RI-80.