## 4 Discussion
The results that were obtained by [PERSON] et al. [35] also indicated the dominant occurrence of _bla_TEM gene in relation to the _bla_SHIV and _bla_OXA genes in the DNA of isolates originating from wastewater treatment plants. However, this study found that WWTPs that received only municipal wastewater proved to exhibit a much better performance in reducing the analysed genes responsible for antibiotic resistance than wastewater treatment plants also receiving wastewater from the food industry and hospitals. Following the wastewater treatment process, an increase or a slight reduction in the concentration of studied ARGs was observed in the DNA of the total bacterial population and in the DNA of the _E. coli_ population, mainly in WWTPs with a mechanical and biological system (group B). Moreover, an increase in the concentration of all the studied genes was observed in the DNA of the total bacterial population from WWTP No. II and in the DNA of the _E. coli_ population isolated from WWTP No. IV. An increase in the ARG concentration following the wastewater treatment process was also found in the DNA samples from both studied populations isolated from WWTP No. V with a mechanical and biological system. Moreover, an over 90% decrease in all ARGs in the DNA of the total bacterial population was noted in WWTP No. VI, which used a mechanical and biological wastewater treatment system. At the same time, an increase in the number of _tel_M and _bla_TEM gene copies and the presence of _tet_A gene was observed in the DNA of the _E. coli_ populations that were isolated from the same plant. Interestingly enough, the latter gene was not detected in the untreated wastewater received by the plant. The highest reduction in the studied ARGs was recorded for the WWTP No. XII (group D) with an increased nutrient removal mechanical and biological system in the DNA samples isolated from the total bacterial populations and for the WWTPs No. VIII and IX with sequencing batch reactors (group C) in the DNA samples that were obtained from the _E. coli_ populations. [PERSON] et al. [31] also found differences in the concentrations of genes in treated wastewater from various WWTPs, including an increase in the concentration of genes in wastewater from a treatment plant which receives municipal and hospital wastewater. The differences in ARG concentrations between wastewater samples from various treatment plants may be due to the differences in microbiological composition of wastewater incoming to particular treatment plants and the diversity of antibiotics (concentration and type) present in wastewater [17]. [PERSON] et al. [36] found that ARGs from chromosomal mutations, under the great selective pressure of antibiotics, decreased along with the degradation of total antibiotics in final effluents, instead of proliferating through biological treatment stages. [PERSON] et al. [35] demonstrated that the ARB count and the ARG number were statistically significantly positively correlated with the COD and the contaminant load, and negatively correlated with the oxygen content and the wastewater temperature. Moreover, [PERSON] et al. [37] observed that the absolute concentration of ARGs decreased by one order of magnitude in the DNA of samples that were collected in winter as compared with the DNA samples collected in summer, which could be explained by the fact that low temperatures may cause microbial enzyme deactivation, thus having a direct effect on ARG removal effectiveness. These correlations were not confirmed by [PERSON] et al. [25], who suggest that varied temperatures throughout different seasons does not have a significant effect on the ARG concentration. These differences may be explained by regional diversification of antibiotic doses or by other factors that are connected with seasonal temperature fluctuations. A factor with a significant effect on the ARG concentrations may also be the composition of the wastewater reaching WWTPs.