## 3 Results and Discussion  
The results of the cluster analysis did not show a similarity between the results for samples collected in the same season. The samples collected in different seasons from the same sampling sites were often grouped in one cluster on the dendrogram. It can be concluded that the sampling season probably had no effect on the presence of ARGs in the analyzed samples (Figure S1). However, the prevalence of ARGs differed across the types of genes and the stages of wastewater treatment. Raw sewage was a significant source of ARGs and contained all of the analyzed genes (Table 3). Influent wastewater is an important reservoir of ARGs. Wastewater treatment plants offer supportive conditions for the spread of ARGs by horizontal gene transfer (HGT) [45; 46; 47]. The low effectiveness of treatment processes can contribute to the transmission of ARGs from treated effluents to surface water [2; 19; 48]. Antibiotic resistance genes are localized on MGEs, which can be exchanged by both closely related and phylogenetically distant bacterial species. As a result, antibiotic resistance can be spread between bacteria of anthropogenic origin and bacterial communities in the natural environment [6; 42]. ARGs in the environment stays in two forms: intracellular ARGs (iARGs) and extracellular ARGs (eARGs). The forms of ARGs that occur in the environment determine the character of its further transmission. iARGs support the spreading of antibiotic resistance via conjugation and transduction, while eARGs can be uptaken and integrated by the competent non-resistant bacteria via natural transformation. Of the mentioned HGT mechanisms, conjugation is deemed to have the biggest impact on the dissemination of ARGs, while transformation and transduction are considered less important [49]. present study, the \(su1\) gene was identified in all wastewater and sewage sludge samples collected in both WWTPs. This gene was also present in samples of river water collected upstream from the effluent discharge point. The above findings indicate that the \(su1\) gene is widely spread in the environment. The analyzed gene was not eliminated in successive stages of treatment, which may increase the concentration of this gene in the water and bottom sediments of effluent-receiving rivers. Surface waters are highly contaminated with the \(su1\) gene [15; 16; 18; 52]. The \(su1\) gene has been also identified in samples of municipal wastewater [53], activated sludge [47] and wastewater containing animal waste [54]. Hospital sewage appears to be the major source of the \(su1\) gene in the environment. According to [PERSON] et al. [2], the prevalence of \(su1\) is considerably higher in hospital wastewater than in other types of sewage. Similar results were reported by [PERSON] et al. [14], who analyzed hospital sewage in China. In other studies, hospital wastewater was characterized by the highest concentration of the \(su1\) gene relative to other ARGs [13; 55]. The ubiquitous character of \(su1\) could result from a close relationship between \(su1\) and class 1 integrins that are widespread in the environment [56]. According to [PERSON] et al. [57], \(su1\) occurs in the variable region (gene cassette) of class 1 integrins. In the current study, hospital sewage accounted for 2-6% of the wastewater processed by the WWTPs. WM-WWTP treats wastewater from three hospitals, and S-WWTP treats wastewater from one hospital. Hospital sewage could be a major source of \(su1\) in the analyzed WWTPs, which could explain the high prevalence of this gene in all samples. The \(su2\) gene occurred less frequently than the \(su1\) gene. The \(su2\) gene was not identified in river water upstream from the effluent discharge point (excluding site S8 in fall). This gene was present in all samples of raw wastewater, but it was eliminated in successive stages of treatment. These observations could also suggest that \(su2\) was less abundant in raw sewage that \(su1\). The presence of the \(su1\) gene in river water was noted only in WM-WWTP in summer.