## 4 Discussion
Moreover, the study demonstrated that, with an increase in TC concentration, the concentration of genes that are mainly responsible for the efflux pump (_tet_A, _tet_B, _tet_C) increased, while the concentration of genes responsible for the abundance of ribosomal genes of protective proteins (_tet_M, _tet_O, _tet_S) was lower when compared with the number of the efflux pump genes and that this number changed slightly with an increase in TC concentration. The present study also noted the highest concentrations of _tet_A and _bla_OXA genes in the DNA of _E. coli_ populations, while the occurrence of _tet_M and _bla_OXA genes was not noted in all the studied samples of the DNA of the _E. coli_ populations from wastewater treatment plants. As a result of the intake of antibiotics, antibiotic resistance genes can be modified and acquired by the microbiota inhabiting the gastrointestinal tract, which may be released to the environment with faeces via drainage systems [23]. [PERSON] et al. [28] found that, following the wastewater treatment process, the frequency of the occurrence of antibiotic resistance genes increased among _E. coli_ bacteria. The authors also noted an increase in the percentage of multi-drug resistant _E. coli_ as compared to untreated wastewater. [PERSON] and [PERSON] [32] suggest that wastewater treatment plants are ineffective in eliminating bacteria, including faecal contamination indicator bacteria, even though studies with opposite results have been reported [30]. These great differences in the efficiency of processed wastewater treatment may result from the wastewater treatment plant capacity and/or the type of wastewater [33]. [PERSON] et al. [4] also found a slight reduction in the concentration of ARGs during standard wastewater treatment at seven wastewater treatment plants of different capacities. The conducted study indicated concentrations of _uid_A gene that enable the determination of _E. coli_ of an order up to \(10^{-1}\) copies/\(16S\)_r_RNA gene copies. In most cases, no significant reductions in the number of copies of this gene following the wastewater treatment process were noted. [PERSON] et al. [31] demonstrated that the number of copies of bacterial 16S _r_RNA was up to 5.84x10\({}^{8}\) copies/mL of sample, and there were slight differences (\(p\) > 0.05) between the number of gene copies in untreated and treated wastewater. The results of the studies of those authors suggest that the applied wastewater treatment processes were not effective in reducing the bacterial load, which is also true for wastewater treatment plants applying UV disinfection following the conventional treatment process. Even though a study by [PERSON] and [PERSON] [32] demonstrated that wastewater treatment plants may decrease the load of nutrients and faecal coliforms from treated wastewater, [PERSON] et al. [31] indicated that the total abundance of bacteria (estimated based on the number of copies of the bacterial gene 16S rRNA) was not reduced following the treatment process, irrespective of the process applied. Therefore, biological wastewater treatment plants may provide an appropriate environment for the spread of ARGs, as both the conditions prevailing during wastewater treatment, and the great diversity of microorganisms facilitate the transmission of genes through, _inter alia_, horizontal gene transfer (HGT) [6]. For most WWTPs receiving only municipal wastewater, the _bla_OXA gene was not detected in the DNA obtained from the total population of bacteria isolated on TSA medium or in the DNA of the _E. coli_ population. In the DNA of populations that were isolated from wastewater in other treatment plants, the concentration of this gene was dominant. Another quantitatively dominant gene responsible for resistance to beta-lactams was the _bla_TEM gene. It is believed that the group of TEM-type beta-lactamases may be of paramount significance in the development of resistance to beta-lactam antibiotics due to its great diversity (over 220 types of genes belong to this group) [34].