## 3 Results and Discussion  
### Microbial Toxicity Concentration  
In general, in accordance with the methodological recommendations published by [PERSON] [14], the MARA assay measurements were performed on samples diluted with water. The data obtained from MARA can be expressed as the percentage of microbial growth, the percentage of relative inhibition, and microbial toxicity concentration (MTC). The result of the MARA tests is summarized in Table 2.  
In addition, MTC values for each strain are represented in Figure 4. Generally, based on the MARA result, all samples were categorized by internal system software as nontoxic or as having an extremely low toxicity. Samples 1 and 4 have a similar result, with the minimum MTC of both samples being 100%. It indicates that the samples were extremely low toxicity. The growth of all microorganisms was not affected by inducing those samples to the wells. Sample 2 significantly influenced the growth of microorganism nos. 2, 4, 6, 7, 9, and 10. The most sensitive microorganism was _Delftia acidovoran_ with the lowest MTC value of \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Average MTC (\%)** & **Minimum MTC (\%)** & **Most Sensitive Organism** & **Overall Effect** \\ \hline
1 & \(>\)max & 100 & - & No or very low toxicity \\
2 & 43 & 10 & No. 6 _Delftia acidovorans_ & No or very low toxicity \\
3 & 85 & 44 & No. 7 _Kurthia gibsonii_ & No or very low toxicity \\
4 & \(>\)max & 100 & - & No or very low toxicity \\
5 & \(>\)max & 81 & No. 6 _Delftia acidovorans_ & No or very low toxicity \\
6 & 75 & 20 & No. 6 _Delftia acidovorans_ & No or very low toxicity \\ \hline \hline \end{tabular}
\end{table} 
Table 2: MARA test results summary. Figure 4: MTC value of six mushrooms tested for each strain, strain with y-axis 100%, indicates the MTC value above maximum concentration (description of 1–11—see Table 1). 10%, which corresponds to 10 mg mL\({}^{-1}\) of mushroom extract. On average, Sample 2 had the lowest MTC values among other samples (43% 43 mg mL\({}^{-1}\) of extract). Sample 3 had an average MTC value of 85%, which slowed down the growth of microorganisms no. 1, 2, 7, 9, 10. The lowest MTC value was 44%, which refers to the most sensitive microorganism (_Kurthia gibsonii_). Sample 5 did not interrupt almost all the microbes' growth. It very slightly reduced the pellet size in the cases of organism no. 4 (_Conamondas testosteroneii_) and no. 6 (_Delftia acidovorans_) with MTC values of 93% and 81%, respectively. It corresponds to at least 93 mg mL\({}^{-1}\) and 81 mg mL\({}^{-1}\) of mushroom concentration needed to inhibit the growth of microbes nos. 4 and 6 accordingly. Sample 6 had no adverse effect on organisms no. 3, 5, 7, 9, 10, 11. However, it influenced the growth of organisms no. 8 (_Staphylococcus warneri_), no. 1 (_Microbacterium species_), no. 2 (_Brevundimonas diminuta_), no. 4 (_Camomonas testosteroneii_) and no. 6 (_Delftia acidovorans_) with the decreasing value of MTC ranging from 83% (83 mg mL\({}^{-1}\)) to 20% (20 mg mL\({}^{-1}\)). Mushroom toxicity is a global problem encountered in many countries. For instance, [PERSON] et al. [28] reported that there were 6307 cases caused by mushroom exposure in Switzerland. According to its report, _Boletus edulis_ was the species with the most frequent cases (305). Our study aimed to evaluate whether the MARA assay may be a reliable tool to determine the overall toxicity of a mushroom. For this purpose, samples of King bolete were collected in several regions and sites characterized by different risks of mushroom contamination. According to a new report by the European Environment Agency (EEA) [29], Poland is the most polluted country in Europe, and the cities of Warsaw and Lodz and their surrounding areas, in particular, see high levels of particulate pollution. The MARA system has been developed to determine toxicity based on multi-species assay with many advantages: Cost-effectiveness, simplicity, and ease of use [12].