Beatriz Dörrie, Laurine Wehmeier, Markus Petzold, Olga Brovkina, Regina Nogueira
The environmental source of Legionella infections, which can cause severe pneumonia, often remains unidentified. Increasing global incidence highlight the need to explore additional infection sources. Wastewater treatment plants (WWTPs) have been linked to Legionella outbreaks. However, it is unclear how their communities change temporally within these systems and which Legionella spp. occur in WWTPs. We sampled five industrial aerobic activated sludge (AS) systems, including influent entering the AS system, AS and treated wastewater samples, from industrial WWTPs with conditions favourable for Legionella occurrence, each on five occasions. Legionella diversity was assessed by sequencing the V3 and V4 regions of their 16S rRNA gene and calculating the alpha-, beta-, and zeta-diversity. Concentrations were measured using quantitative polymerase chain reaction and culture. Mass balance analysis was conducted, assuming no growth and complete retention in AS. Significant variation was found in Legionella community composition across WWTPs (p < 0.05), with transiently occurring amplicon sequence variants (ASVs) being predominant. Legionella richness decreased with increasing of wastewater temperature and influent protein concentration but increased with the number of aerated treatment processes. Mass balance analysis suggested that Legionella may grow in AS systems. Among the identified Legionella-ASVs, human diseases associated species were found, with L. pneumophila and L. feeleii/L. donaldsonii being the most frequently identified. Additionally, elevated concentrations of culturable Legionella were found in some AS and treated wastewater samples (up to 6.6 log CFU/mL). Our findings highlight the epidemiological relevance of industrial WWTPs with aerated AS systems receiving warm wastewater as potential legionellosis source and emphasize the need to integrate such WWTPs into epidemiological studies.