Temperature-driven growth of Legionella in lab-scale activated sludge systems and interaction with protozoa

The occurrence of Legionella pneumophila in activated sludge systems has been reported in the literature. However, the factors triggering its growth are not yet well understood. This knowledge is needed to develop strategies to minimize the risk of the spread of Legionnaires' disease that originates in these systems. In the present study, L. pneumophila multiplied exponentially at 35 °C in activated sludge, but lower temperatures (24 °C and 15 °C) did not favour the growth of the pathogen despite the presence of its protozoan hosts (free-living amoeba and ciliates). L. non-pneumophila species, which are adapted to low temperature, prevailed at 15 °C. Legionella was found dispersed in the activated sludge, forming clusters of different sizes and also inside protozoa. This study shows that temperature is a key parameter triggering the growth of L. pneumophila in activated sludge. The detection of infected protozoa suggests that these are important vehicles for the multiplication of Legionella. However, in this study it was not possible to exclude the growth of Legionella directly on the substrate or on the activated sludge matrix.