Laboratory of Veterinary Public Health, Joint Faculty of Veterinary Medicine, Yamaguchi University, Yamaguchi, Japan.
National BioResource Project Paramecium, Joint Faculty of Veterinary Medicine, Yamaguchi University, Yamaguchi, Japan.
Environ Microbiol Rep. 2022 Feb;14(1):50-59. doi: 10.1111/1758-2229.13029. Epub 2021 Nov 29.
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 remain unclear. In this study, we investigated in vitro the possibility that 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 found that F. novicida showed similar behaviour to that of the eukaryotic endosymbiont of P. bursaria, the green algae Chlorella, in the internalization process. 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.
弗氏柠檬酸杆菌是一种兼性细胞内病原体，也是土拉热的病原体。尽管有报道称接触污染水会导致感染病例，但它在自然环境中的天然宿主和生态学仍不清楚。在本研究中，我们研究了草履虫是否可能成为弗氏柠檬酸杆菌的有用原生动物宿主模型的体外可能性。用弗氏柠檬酸杆菌进行实验感染导致其与草履虫之间形成稳定的细胞内关系。在其他弗朗西斯氏菌和嗜肺军团菌的实验感染中未观察到这种共生的细胞内关系。我们发现，在内化过程中，弗氏柠檬酸杆菌的行为类似于草履虫的真核内共生体——绿藻小球藻。此外，只有在不存在小球藻的情况下，才能实现弗氏柠檬酸杆菌在细胞内的稳定定位。虽然我们研究了弗氏柠檬酸杆菌的 VI 型分泌系统作为候选细菌因子，但发现它不参与与草履虫建立细胞内关系。这些结果表明，草履虫可能是弗氏柠檬酸杆菌的潜在原生动物宿主模型，并且可能是理解原生动物宿主与其共生体之间关系的有用工具。
