# Antibiotic Resistance in Filters from Car Air Conditioning Systems
## 3 Results and Discussion
In addition, antibiotic-resistant actinomycetes were also found in the filter material (Figure 9); however, it was not possible to obtain pure strains, so further studies focused on isolates of _Pseudomonas_ sp. It should be pointed out that the environment of a cabin filter is very specific, due to highly variable operating conditions, sometimes a significant degree of air pollution and air flow--both fresh and recycled. This creates very specific conditions for the growth of microorganisms, particularly limiting their viability, especially in the case of pathogenic microorganisms, which usually live in more stable conditions. Therefore, over the course of the study, we were able to isolate and purify only a few strains, as most of them were passable with very low efficiency. Furthermore, only a few of them were able to be  
Figure 8: Percentage of antibiotic-resistant bacteria in the total number of bacteria inhabiting cabin filters. maintained in good enough condition to obtain meaningful results from biochemical tests and to keep pure isolates as a source of DNA for sequencing. In our research, we mainly focused on the environmental context and the possibility of the presence of antibiotic-resistant bacteria in car cabin filters. However, it should be noted that in our research, the bacteria of the genus _Pseudomonas_ appeared to be dominant and most resistant not only to antibiotics but also to environmental conditions. This is of particular importance from the point of view of health risks, since it is among the representatives of this genus that there are numerous microorganisms which are opportunistic pathogens of humans, animals, and plants. Moreover, they are also characterized by high genetic potential and adaptability to different environments, and, importantly from the point of view of their role in the spread of antibiotic resistance--they present great ease when it comes to horizontal gene transfer [40]. Selected isolates of antibiotic-resistant bacteria belonging to the genus _Pseudomonas_ were used to estimate the values of concentrations of tested antibiotics that cause inhibition of the growth of these microorganisms. Minimal inhibition concentrations were determined, and the results are presented in Table 3. The results clearly demonstrate that the strains capable of colonizing filter materials were able to grow in the presence of antibiotic concentrations even higher than those used for their isolation in the culture tests (100 mg/L), particularly with respect to resistance to penicillin and nitrofurantoin. A strain obtained from a Subaru-derived filter proved to be more resistant, especially towards rifampicin. The obtained values were also much higher than the minimum inhibitory concentrations of antibiotics for _Pseudomonas_ spp. strains according to EUCAST, which are in the range of 2-8 mg/L for penicillin, 0.5-2 mg/L for fluoroquinolones, 2-16 for aminoglycosoides. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Bacteria**} & \multicolumn{5}{c}{**Minimal Inhibition Concentration (mg/L)**} \\ \cline{2-6}  & **Penicillin** & **Nitrofurantoin** & **Rifampicin** & **Doxycycline** & **Gentamicin** \\ \hline _Pseudomonas_ sp. & 125 & 125 & 15 & \textless{}5 & \textless{}5 \\ (filter from Toyota) & 225 & 150 & 125 & 15 & \textless{}5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Antibiotic concentrations inhibiting the growth of selected bacteria extracted from cabin filters. Figure 9: Antibiotic-resistant actinomycetes isolated from cabin filters. The use of filters to treat the air inside the air conditioning system is currently one of the simplest solutions when it comes to ensuring adequate air quality in the internal environment of the car cabin [36]. Cabin filters should be characterized by high efficiency in eliminating contaminants present in the air, low pressure drop, low sensitivity to changes in temperature and humidity [41].