<i>Legionella</i> Confer Survival Benefits to <i>Paramecium</i> Hosts by Inhibiting Phagocytosis

Legionella survive in the natural environment by remaining within protist host cells. Many protist species, including Paramecium spp., are potential hosts for Legionella. However, the factors and mechanisms involved in the establishment of this relationship are unknown. The advantages gained by Paramecium spp. when they maintain Legionella are also unclear, and the existence of these relationships has not been confirmed. In the present study, feeding with Legionella increased the number of Paramecium cells over time. However, the growth-promoting effect of Legionella was weaker than that of Klebsiella pneumoniae, which is considered the optimal bacterial feed for Paramecium. Phagocytosis was strongly inhibited in Paramecium cells fed Legionella, indicating that this relationship prevents the uptake of harmful organisms. The inhibition of phagocytosis was also observed when Paramecium cells were treated with the Legionella culture supernatant. Despite the inhibition of phagocytosis, the presence of live Legionella within host cells allowed Paramecium spp. to survive and even increase in number, as observed earlier. This result suggests that Legionella support the survival of Paramecium hosts from a nutritional aspect. Although it is difficult to definitively state whether the relationship between Legionella and Paramecium hosts is completely mutualistic, the present results provide one rationale for defining their relationship.