## 1 Introduction  
The occurrence and spread of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) is a serious health protection problem worldwide [1]. According to the World Health Organization, the occurrence of antibiotic resistance among bacterial populations is regarded as one of the major hazards and challenges to public health in the 21 st century [2], since it has serious economic consequences as well as risks to the health and lives of both humans and animals [3]. A major problem in controlling this phenomenon is the lack of national or international legal regulations controlling the spread of ARB and ARGs in the environment [4]. The existing reports and definitions of drug resistant bacteria refer only to clinical strains, while similar characteristics for environmental strains are still missing [5]. The control of the aquatic environment appears particularly important, as it provides the main spread route of ARB and ARGs [6]. Even though the European Union (EU) Water Framework Directive contains a provision on ensuring the good quality of waters according to specific standards, it does not refer to the phenomenon of antibiotic resistance. The genes that are responsible for resistance to antibiotics are regarded as environmental pollutants [7] that are capable of spreading among bacteria in natural environments and drinking water resources [8; 9]. This is due to the fact that non-pathogenic bacterial species with antibiotic resistance genes may serve as a source of antibiotic resistance genes for pathogenic bacteria [10]. ARGs that are found among bacterial pathogens largely originate from the environment, which receives treated or untreated wastewater generated by animal farms, aquaculture, and industry [9; 11]. One of the main reservoirs of ARB and ARG pollutants and the source of their spread in the natural environment [12; 13; 14] are wastewater treatment plants (WWTPs) and treated wastewater discharged to surface water bodies [5; 6; 15; 16; 17]. WWTPs receiving high concentrations of microbial contaminants with wastewater from hospitals, agriculture, and industry stimulate the transfer of genetic information between pathogenic and environmental microorganisms. In addition, the conditions prevailing in wastewater treatment plants, such as a high content of microorganism populations, the relative abundance of nutrients, and the presence of sub-threshold levels of antibiotic substances in wastewater [16], provide an environment favourable for the survival of ARB and the transfer of ARGs. The increase in microbial diversity and the number of mobile genetic elements [18] and ARGs [19] is significantly affected by the activated sludge. Microorganisms forming particles of sludge are responsible for the accumulation and release of bacterial plasmids carrying ARGs [20], which facilitate the exchange of these genes between bacteria. Another factor contributing to the development of antibiotic resistance phenomenon during the wastewater treatment process is the presence of antibiotics, which, even at low concentrations, may induce genetic responses leading to adaptations and mutations among the microbial population [21]. Therefore, conventional activated sludge-based wastewater treatment methods provide ideal conditions for the transfer of these genes [22], which was confirmed by many authors [23; 24; 25]. The results of these studies show that the bacteria released with the discharged treated wastewater may be capable of active transmission of resistance genes among environmental microorganisms. [PERSON] et al. [26] demonstrated that the number of bacteria decreased following the wastewater treatment processes, while the abundance and diversity of genes determining antibiotic resistance did not change significantly. Moreover, a comparison of treated and untreated wastewater samples demonstrated that certain resistance genes, e.g., carbapenemase OXA-48 and the count of mobile genetic elements, were also not significantly reduced [6; 26; 27].