Acanthamoeba castellanii strongly increases the number of Legionella pneumophila in model tap water biofilms.

Recently the authors demonstrated through several uptake experiments that some bacterial species do not act as competitors on Legionella pneumophila uptake by Acanthamoeba castellanii, but are able to influence intracellular Legionella replication. Biofilm experiments were performed at 35°C in a rotating annular reactor. Experiments were repeated at least three times and always consisted of two parts: during the first part, without present A. castellanii (days 0 to 36), the authors investigated if L. pneumophila were able to attach and eventually grow in the biofilm. During the second part, A. castellanii trophozoites were added to the system and the evolution of biofilm-associated L. pneumophila was investigated. Addition of A. castellanii on day 36 resulted within 48 h in a significant decrease of biofilm-associated non-Legionella bacteria due to predation, while biofilm-associated L. pneumophila increased with 1.5 log units due to intracellular replication in the present A. castellanii trophozoites. The association between Legionnaires’ disease and the presence of high numbers of human pathogenic L. pneumophila bacteria in man-made water supplies has already been confirmed frequently. Therefore, the results stress the importance of further obtaining data concerning interactions between L. pneumophila and potential amoeba host populations associated with biofilms.