Antibiotic resistance in microorganisms poses a global hazard to human and animal health. To combat antibiotic resistance, a deeper understanding of the mechanisms behind the spread of antibiotic resistance genes (ARGs) in bacteria is crucial. Plasmids are major vehicles for the spreading of ARGs, so their conjugation-mediated transfer is examined in this thesis. Escherichia coli (E. coli) was isolated from a wastewater treatment plant (WWTP) and clean water in Stockholm, Sweden and the isolates were used as donors of mobile plasmids and a lab strain of E. coli (CV601) was used as recipient. Transfer efficiency was analysed by calculating conjugation frequencies (CF), after testing conjugation by fast conjugation and filter mating method. The resulting transconjugants were analysed for antibiotic resistance pattern by the disc diffusion method. Additionally, sequence analysis was used to analyse the transferred plasmid(s) and ARGs. The qPCR was performed to check the expression of transfer genes (TraN and TraJ) during conjugation of IncN plasmids. The results showed that several isolates from WWTP were multi-resistant and successfully transferred plasmids to CV601 and donor strain 18 with a mobile IncN plasmid had the highest CF that depended mainly on the type of pili. The ARGs qnrS1, dfra14, and blaCTX-M-15 were transferred to transconjugants mainly by IncN plasmids and qPCR findings indicated that the level of gene expression of transfer genes in donor strains affected CF. The findings of this study expand our understanding of antibiotic resistance dissemination and knowledge of plasmids found in WWTPs and clean water.