Occurrence of antibiotic resistant bacteria in urban sewage and role of wastewater treatment plants in environmental dispersion

The presence of antibiotic resistant bacteria (ARB) in wastewater was investigated and the role of wastewater treatment plants (WWTPs) in promoting or limiting antibiotic resistance was assessed. Total heterotrophic bacteria (THB) resistance to ampicillin, chloramphenicol and tetracycline was monitored in 3 WWTPs located in Milan urban area (Italy). The effluent was collected at three sampling points along the treatment sequence (WWTP influent, biological effluent, WWTP effluent), being the three WWTPs characterized by different operating parameters in biological process and disinfectants (peracetic acid, UV radiation, sodium hypochlorite). Antibiotic resistance to ampicillin was observed for THB, being the influent to the three WWTPs characterized by different contents of ARB (17-25%). The biological process affected the composition of the bacterial population, determining a change in ampicillin-resistant bacteria presence, although no relations with the operating parameters were identified. PAA was effective in selecting ARB, unlike UV radiation and sodium hypochlorite. No significant resistance was observed for chloramphenicol and tetracycline.