Occurrence of Uncultured Legionella spp. in Treated Wastewater Effluent and Its Impact on Human Health (SCA.Re.S Project)

Wastewater treatment plants (WWTPs) provide optimal conditions for the environmental spread of <i>Legionella</i>. As part of the Evaluation of Sanitary Risk Related to the Discharge of Wastewater to the Ground (SCA.Re.S) project, this study was conducted to evaluate the presence of <i>Legionella</i> in WWTP effluent and in groundwater samples collected from two wells located downstream from the plant. The samples were analyzed to determine the concentrations of <i>Legionella</i> spp using the standard culture-based method and molecular techniques, followed by genomic sequencing analysis. <i>Legionella</i> was detected only with the molecular methods (except in one sample of effluent positive for <i>L. pneumophila</i> serogroup 6), which showed viable <i>Legionella pneumophila</i> and <i>L.</i> non-<i>pneumophila</i> through the use of free DNA removal solution in both the effluent and groundwater, with concentrations that progressively decreased downstream from the plant. Viable <i>L. pneumophila</i> appeared to be slightly more concentrated in warm months. However, no significant differences (<i>p</i> ≥ 0.05) in concentrations between cold and warm months were observed. A genotypic analysis characterized the species present in the samples and found that uncultured <i>Legionella</i> spp, as yet undefined, constituted the prevalent species in all the samples (range 77.15-83.17%). WWTPs play an important role in the hygienic and sanitary quality of groundwater for different uses. The application of <i>Legionella</i> control systems during the purification of effluents is warranted to prevent possible outbreaks of legionellosis.