Presence of microbial pathogens and genetic diversity of Listeria monocytogenes in a constructed wetland system

The potential of constructed wetlands (CWs) as a low technology for wastewater treatment is timely but there is a need to understand the route of pathogenic bacteria (Listeria monocytogenes and Salmonella spp.) and indicator organisms (Enterobacteriaceae and Escherichia coli) present in wastewater within those systems and to what extent they can compromise the further use of the plants (enabling in-house ornamental use) and water reuse (irrigation purposes). In the present study the fate of potential pathogenic bacteria within a CW, with ornamental plants, and an outlet pond established in a tourism house for wastewater treatment was assessed. The fate of pathogenic bacteria in lettuce, both plant tissues and soil, subject to irrigation with water contaminated either with L. monocytogenes or E. coli (indicator organism) was investigated. The uptake of indicator organisms by the CW ornamental plants was low and there was a decrease, from the CW wastewater inlet to the outlet, of up to 3 log for Enterobacteriaceae and up to 2 log for E. coli. Concerning Salmonella spp. no detection was registered in the water, plant tissues or substrate within the CW. On the other hand, several L. monocytogenes isolates were retrieved from water, root and substrate samples, found to have a high susceptibility to a battery of antibiotics. The irrigation of lettuce with water containing pathogenic bacteria (L. monocytogenes) and indicator organisms (E. coli) must be considered with caution since both bacteria were detected in the soil growing media and internalization of L. monocytogenes in roots was observed. These issues are poorly investigated and to our knowledge not addressed in the sphere of CWs.