Listeria species incidence and characterisation in a river receiving town sewage from a sewage treatment plant

Abstract

This study evaluated the ability of a sewage treatment plant to eliminate Listeria species and the incidence of Listeria species in the river receiving the effluent. The average efficiency of removal of Listeria species in the treatment plant was 92%. Despite the efficiency of the plant being within the limits established by the EC Directive in terms of BOD5 decrease, an increase in the levels of Listeria species was still observed downstream from the plant. The species isolated with greatest frequency, both in the influent and effluent as well as the river water, was L. monocytogenes followed by L. seeligeri and L. welshimeri. L. ivanovii was only found in very low proportions in the river samples. The analysis of the sensitivity of Listeria species to 22 antimicrobial agents demonstrated the presence of a high percentage of bacteria resistant to aztrenam, cefuroxme, cetriaxone and to a leser degree moxalactam and cefotaxime. In contrast, resistance to aminoglycosides and tetracycline was only observed in a low number of isolates.