An association between Legionella contamination of river water used as tap water sources and outbreaks of Legionnaires’ disease in the surrounding area has been reported in the United States; however, the status of Legionella contamination of river water in Japan is unknown. While there is an association between Legionella and amoebae in bathtub water and cooling tower water, the association is not clear in the river waters. In this study, we investigated the association between legionellae and amoebae and their habitats in river water used as a tap water sources. Although Legionella was not detected by the culture method, the qPCR and amoeba co-culture-qPCR methods revealed that difficult-to-culture Legionella different from L. pneumophila inhabited in the river waters. It was also suggested that amoebae inhabiting the river waters provide suitable sites for the growth of Legionella and increase the bacterial number of Legionella .

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