Legionella Survives and Elongates in Algal Consortia Containing Bacteria in Alkaline Oligotrophic Conditions

Cooling towers are a major source of Legionella, which causes Legionnaires' disease. These bacteria grow in predatory organisms; however, the impact of non-predatory organisms in cooling towers on Legionella survival and growth remains unclear. Therefore, we investigated the effects of photosynthetic algae, the primary component of biofilms in open cooling water systems, on Legionella. We cultivated Legionella with algae collected from the towers or with pure algal strain under alkaline conditions and revealed that the Legionella 16S rRNA copy number was higher than that of Legionella alone. We also exami-ned Legionella using an in situ hybridization chain reaction and found that some were elongated and exhibited a filamentous morphology on algal cells. Furthermore, Legionella was more active when co-cultured with pure algal strain plus Serratia spp. than when co-cultured with pure alga alone. 18S rRNA gene sequencing revealed that the algae collected had not previously been reported to coexist with Legionella. This result suggests that diverse algae in the environment support the growth of Legionella. This is the first study to experimentally demonstrate that algae promote Legionella elongation, and also that the coexistence of bacteria furthers this phenomenon. These results provide a new perspective on the ecology of Legionella and the role of non-predatory organisms.