## 3 Results and Discussion  
Assessment of Bacterial Contamination of Air-Quantitative Analysis Using the Classical Microbiology Approach  
There are no special protocols for assessing bacterial air contamination in places of cultural heritage significance. Moreover, the European standard for microbiological air contamination in cleanrooms, which are facilities that are required to maintain extremely low levels of particulates (ISO 14698-1:2003), does not recommend any specific culture media for microbiological assessments. The only requirement established in these norms is that a non-selective medium is used. In this study, seven different growth media (LB, NA, BHI, YEA, BA, FA, and R2A) were tested for their suitability for use in the screening of microbiological air contamination. We focused on airborne mesophilic bacteria, as this group provides the best estimate for contamination with human pathogens [30]. Figure 2 presents the overall air contamination that was measured using classical microbiology methods. Pairwise statistical analysis between yields from different media highlighted the YEA medium as having the lowest global recovery rate. The pairwise Wilcox rank-sum tests with YEA were always statistically significant, with \(p\)-values \(<10^{-6}\) (Table S1). This highlighted the fact that the YEA medium enabled the formation of significantly fewer colonies (mean CFU/m\({}^{3}\) equaled \(3.0\times 10^{2}\)) compared with the other investigated growth media (mean CFU/m\({}^{3}\) ranged between \(5.2\times 10^{2}\) and \(9.3\times 10^{2}\)). Figure 2: Box plots showing the measured CFU/m\({}^{3}\) distributions with a logarithmic scale. (**a**) Graph presenting the measured CFU/m\({}^{3}\) counts for each investigated medium. (**b**) Graph showing the CFU/m\({}^{3}\) counts by sampled rooms. The bold, central horizontal bars are the medians. The lower and upper limits of the box are the first and third quartiles, respectively. Points above and below the whiskers’ upper and lower bounds are outliers. NA—nutrient agar, LB—lysogeny broth, BHI—brain heart infusion agar, BA—Blackfeldt agar, FA—Frazier agar, R2A—Reasoner’s 2A agar, YEA—yeast extract agar. The R2A, FA, and LB media showed the highest global recovery rates, with average CFU/m\({}^{3}\) values equal to \(9.3\times 10^{2}\), \(7.1\times 10^{2}\), and \(7.0\times 10^{2}\), respectively. It is worth mentioning that the highest average CFU/m\({}^{3}\) was obtained for the R2A medium, which was designed to promote the growth of slow-growing heterotrophic bacteria. This observation is in line with the findings of other studies, highlighting the potential for the application of R2A medium in air contamination assessment protocols. In this case, the R2A medium could replace commonly used rich media, such as NA or tryptone soya agar (TSA) [31; 32; 33]. The benefit of using the R2A medium relies on its specific features, as a wide range of cultivable bacteria can be obtained due to the promoted growth of slow-growing species. In contrast, the diversity of cultivable isolates observed on rich media was lower, as the promoted growth of fast-growing bacteria suppressed the appearance of slow-growing ones. The results obtained for R2A, FA, and LB media were in line with other works describing indoor environments. [PERSON] et al. investigated indoor air pollution in human dwellings in Upper Silesia, Poland. According to their case study, the average microbiological air pollution in human dwellings was \(9.6\times 10^{2}\) CFU/m\({}^{3}\)[34]. A more comprehensive analysis of microbiological air pollution in office buildings conducted in another study led by [PERSON] reported recovery rates of \(1.4\times 10^{3}\) CFU/m\({}^{3}\) in naturally ventilated offices and \(9.7\times 10^{2}\) CFU/m\({}^{3}\) in offices with air conditioning [35]. A study by [PERSON] et al. investigated microbiological air contamination in different rooms of a university library.