title: Ciliate Paramecium is a natural reservoir of Legionella pneumophila.
abstract: Legionella pneumophila, the causative agent of Legionnaires' disease, replicates within alveolar macrophages and free-living amoebae. However, the lifestyle of L. pneumophila in the environment remains largely unknown. Here we established a novel natural host model of L. pneumophila endosymbiosis using the ciliate Paramecium caudatum. We also identified Legionella endosymbiosis-modulating factor A (LefA), which contributes to the change in life stage from endosymbiosis to host lysis, enabling escape to the environment. We isolated L. pneumophila strains from the environment, and they exhibited cytotoxicity toward P. caudatum and induced host lysis. Acidification of the Legionella-containing vacuole (LCV) was inhibited, and enlarged LCVs including numerous bacteria were observed in P. caudatum infected with L. pneumophila. An isogenic L. pneumophila lefA mutant exhibited decreased cytotoxicity toward P. caudatum and impaired the modification of LCVs, resulting in the establishment of endosymbiosis between them. Our results suggest that L. pneumophila may have a mechanism to switch their endosymbiosis in protistan hosts in the environment.
authors.affiliation: The United Graduate School of Veterinary Science, Yamaguchi University, Yamaguchi, Japan.|Joint Faculty of Veterinary Medicine, Laboratory of Veterinary Public Health, Yamaguchi University, Yamaguchi, Japan.
authors.forename: Kenta
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: The Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Nakao
authors.affiliation: Department of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi, Japan.|National BioResource Project of Japan Agency for Medical Research and Development, Chiyoda-ku, Tokyo, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Fujishima
authors.affiliation: Division of Biomedical Food Research, National Institute of Health Sciences, Setagaya-ku, Tokyo, Japan.
authors.forename: Masato
authors.initials: M
authors.lastname: Tachibana
authors.affiliation: The United Graduate School of Veterinary Science, Yamaguchi University, Yamaguchi, Japan.|Joint Faculty of Veterinary Medicine, Laboratory of Veterinary Public Health, Yamaguchi University, Yamaguchi, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Shimizu
authors.affiliation: The United Graduate School of Veterinary Science, Yamaguchi University, Yamaguchi, Japan.|Joint Faculty of Veterinary Medicine, Laboratory of Veterinary Public Health, Yamaguchi University, Yamaguchi, Japan.
authors.forename: Masahisa
authors.initials: M
authors.lastname: Watarai
mesh_terms: D002460:Cell Line; D004197:Disease Reservoirs; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D054884:Host-Pathogen Interactions; D006801:Humans; D016952:Legionella pneumophila / Q000502:physiology; D007877:Legionnaires' Disease / Q000382:microbiology / Q000635:transmission; D008264:Macrophages / Q000382:microbiology; D009154:Mutation; D010247:Paramecium / Q000382:microbiology*; D013559:Symbiosis