Comparison of antibiotic resistance in the influent and effluent of two wastewater treatment plants

The antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are considered new classes of water contaminants due to their potential adverse effects on aquatic ecosystems and human health. This paper describes the susceptibility evaluation of Escherichia coli, isolated from the influent and effluent of the biological reactors in two wastewater treatment plants (WWTP1 and WWTP2), and the evaluation of the disinfection efficiency of the UV radiation system in WWTP2. The exposure to different families of antibiotics was evaluated, namely, macrolides (erythromycin, azithromycin, clarithromycin), quinolones (ofloxacin, ciprofloxacin), nitroimidazoles (metronidazole), sulfanilamides (sulfamethoxazole) and trimethoprim, the latter is used in combination with sulfamethoxazole due to its synergistic effect. The results of the antimicrobial susceptibility tests, using disc diffusion (Kirby–Bauer) method, showed the occurrence of E. coli strains resistance. The analysis performed indicated an overall resistance (considering both influents and effluents) in WWTP1 and WWTP2, of respectively: 33.3% and 37.5% to erythromycin; 0.0% and 4.0% to azithromycin; 25.0% and 29.2% to clarithromycin; 12.5% and 4.2% to ofloxacin; 16.7% and 4.2% to ciprofloxacin; 29.2% and 12.5% to trimethoprim; 41.7% and 12.5% to sulfamethoxazole. A variability of resistance was observed along the studied period, from WWTP1 and WWTP2, and from influent to effluent. Disinfection by UV demonstrated good performance, achieving in some samples 100% removal of E. coli that has growth in TBX agar. However, a proper supervision is needed in order to achieve the allowed limits concerning the number of bacteria. The results from this work contribute to a better awareness of ARB dissemination from wastewater treatment plants to the aquatic environment.