Selective Proliferation of Antibiotic-Resistant Bacteria in the Biological Treatment Process at a Municipal Wastewater Treatment Plant in India

Abstract

There are mixed opinions on whether municipal wastewater treatment plants (MWTP) act as a significant hub for the selective growth and proliferation of antibiotic-resistant bacteria. In this study, investigations were carried out on the occurrence and fate of fluoroquinolone (FQ) antibiotics and antibiotic-resistant bacteria at different units within an activated sludge-based MWTP in India. The results indicated that the raw wastewater at the inlet of the MWTP had concentrations of ciprofloxacin, norfloxacin, and ofloxacin in the range of 6–17 μg/L, which was significantly higher than the concentrations of these antibiotics reported in wastewater treatment plants in developed countries. The biological treatment unit of the MWTP removed 60%–90% of influent antibiotics. The fractional concentration of FQ-resistant bacteria in the total bacterial population rose from an average value of 0.22 at the inlet to a value of 17 within the unit—an increase of two orders of magnitude. In addition, studies involving 16 other commonly used antibiotics revealed that the average multi-antibiotic resistance (MAR) index value increased in the wastewater treatment plant. The MAR index distribution data also indicated that the treatment plant might contribute to bacteria gaining resistance to more antibiotics classes. Further, 16S rRNA gene sequencing revealed that the antibiotic-resistant bacteria within the MWTP belonged to the family Enterobacteriaceae, of which Escherichia, Salmonella, and Shigella were dominant species. The antibiotic-resistant bacteria even evaded a disinfection step that used chlorination, and showed up in the final discharge from the MWTP into the River Ganges.