Prevalence of Listeria monocytogenes in the river receiving the effluent of municipal wastewater treatment plant

 The objective of this study was to evaluate the prevalence of Listeria spp. in the river water before and after discharge of the effluent of the municipal wastewater treatment plant (WWTP) in Isfahan, Iran. 

 A total of 66 samples were collected bi-weekly over 4 months from eleven discrete sampling locations in Zayandehrood River, Iran. Three sampling sites were located above the discharge point and five sites were located after the discharge point of WWTP. Samples were also collected from the influent and the effluent of WWTP. Listeria spp. were isolated using a selective enrichment procedure and a subculture onto polymyxin-acriflavine-lithium chloride-ceftazidime-esculin-mannitol Agar. All isolates were subjected to standard biochemical tests. 

 L. monocytogenes was isolated from influent (83%), effluent (50%) and (18.5%) river water. Listeria spp. was not found before the discharge point in river water. However, L. monocytogenes was isolated in samples collected from 200 m (33%), 500 m (33%), 2 km (16.5%), 5 km (16.5%) and 10 km (16.5%) downstream from the WWTP. Listeria innocua (9%) and Listeria seeligeri (10%) were the second most frequently isolated species. 

 During the wastewater treatment, Listeria spp. is not removed completely. L. monocytogenes is widely distributed in the Zayandehrood river. L. monocytogenes released into surface water demonstrates a potential risk for public health. These results indicate the need for appropriate water management in order to reduce human and animal exposure to such pathogens.