[ODP-023] Paramecium bursaria as a novel host model for Francisella novicida
Francisella novicida is a facultative intracellular pathogen and the causative agent of tularemia. Although cases of infection caused by exposure to contaminated water have been reported, its natural host and ecology in the environment are largely unclear. In this study, we investigated the possibility that an aquatic protist, Paramecium bursaria, may be a useful tool as a protist host model of F. novicida. Experimental infection with F. novicida resulted in a stable intracellular relationship within P. bursaria. This symbiotic intracellular relationship was not observed in experimental infections with other Francisella species and Legionella pneumophila. We also found that F. novicida showed similar behavior to that of the eukaryotic endosymbiont of P. bursaria, the green algae Chlorella, in the internalization process in P. bursaria. In addition, stable intracellular localization of F. novicida was possible only when Chlorella was not present. Although we investigated the type VI secretion system of F. novicida as a candidate for the bacterial factor, we found that it was not involved in the establishment of an intracellular relationship with P. bursaria. These results suggested that P. bursaria is potentially a protist host model for F. novicida and may be a useful tool for understanding the relationship between protist hosts and their symbionts.