## 3 Results and Discussion
### Assessment of Bacterial Contamination of Air-Qualitative Analysis Using a Metagenomic Approach  
PCR amplicons were obtained using bacterial material collected from 1050 Petri dishes. Metagenomic analysis of seven samples (with each sample being the sum of bacteria cultured on a specific medium) revealed 130 bacterial genera. The number of bacterial genera obtained from each medium is presented in Table 1. Sequences that could not be classified at the genus level constituted between 1.3% (YEA medium) and 4.4% (BHI medium) of sequences obtained from each medium type and were assigned to the \"others\" group (Figure 3). The relative abundance analysis revealed that all used media reported similar dominant genera, i.e., _Acinetobacter_, _Bacillus_, _Enhydrobacter_, _Micrococcus_, and _Staphylococcus_ (Figure 3). Amongst them, _Acinetobacter_, _Enhydrobacter_, and _Staphylococcus_ contain mainly species associated with common human skin microbiota, including several pathogens [37], whereas _Bacillus_ and _Micrococcus_ can be included in the large group of so-called environmental bacteria. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Taxonomic** & **NA** & **LB** & **BHI** & **BA** & **FA** & **R2A** & **YEA** \\
**Rank** & & & & & & & & \\ \hline Genera & 54 & 55 & 45 & 49 & 71 & 63 & 27 \\ Families & 30 & 30 & 29 & 30 & 38 & 38 & 21 \\ Orders & 17 & 16 & 15 & 16 & 20 & 21 & 12 \\ Classes & 6 & 5 & 5 & 6 & 5 & 7 & 5 \\ \hline \hline \end{tabular}  
NA—nutrient agar, LB—lysogeny broth, BHI—brain heart infusion agar, BA—Blickfeldt agar, FA—Frazier agar, R2A—Reasoner’s 2A agar, YEA—yeast extract agar.  
\end{table} 
Table 1: Number of bacterial taxa recovered on each growth medium. While the dominant genera were similar on all media, the metabarcoding analysis showed that the total recovered bacterial taxa were different, depending on the type of medium. [PERSON]'s biodiversity and [PERSON]'s evenness indexes were similar for all media (Figure 3). On the other hand, [PERSON]'s phylogenetic diversity index indicated significant differences between the tested samples. The lowest Faith's phylogenetic diversity index was obtained for the YEA medium (14.46), while the R2A medium showed the highest value (23.45) (Figure 3). This result further supported the finding that rich media should not be used when trying to recover a high diversity of environmental microbiomes. Figure 3: Cultivable bacteria taxonomy and their relative abundances. Heatmap with a dendrogram, depicting genera detected on the investigated media, and the following alpha-diversity indexes for each medium: [PERSON]’s index, [PERSON]’s phylogenetic diversity, and [PERSON]’s evenness index. Only the genera that constituted at least 0.5%, abundance-wise, of all genera on at least one medium are shown on the heatmap. Genera accounting for less than 0.5% across all samples were assigned to the “others” group. NA—nutrient agar, LB—lysogeny broth, BHI—brain heart infusion agar, BA—Blickfeldt agar, FA—Frazier agar, R2A—Reasoner’s 2A agar, YEA—yeast extract agar. Based on the 16S rDNA amplicon analysis, we computed media combinations that would allow for the recovery of the highest bacterial diversity when sampling cultural heritage objects (Table 2). It was revealed that the FA medium enabled the growth of the most diverse community. It recovered 64.62% of cultivable bacterial genera detected on all tested media (Table 2). The addition of the R2A medium raised this value to 80.77% (by adding 21 new genera) (Table 2). Therefore, the combination of these two media could be used as a basis for microbiological assessments. The addition of other media subsequently increased the recovery rate, as shown in Table 2.  
This result can be used as a guideline for selecting optimal growth media for microbiological air pollution assessments. Usually, due to budget and human resources limitations, it is impossible