F. novicida-Infected A. castellanii Does Not Enhance Bacterial Virulence in Mice

Abstract

Francisella tularensis is a facultative intracellular bacterium that causes tularemia in humans and animals. Epidemiology of tularemia worldwide is often associated with water-borne transmission, which includes mosquitoes and amoebae as the potential host reservoirs of the bacteria in water environment. In vitro studies showed intracellular replication of F. tularensis within Acanthamoeba castellanii and Hartmanella vermiformis cells. While infection of amoeba by Legionella pneumophila has been shown to enhance infectivity of L. pneumophila the role of F. tularensis-infected protozoa in the pathogenesis of tularemia is not known. We used 6 h coculture of A. castellanii and F. novicida for investigation of the effect of inhaled amoeba on the pathogenesis of tularemia on in vivo model. Balb/c mice were infected intratracheally with F. novicida or with F. novicida-infected A. castellanii. Surprisingly, infection with F. novicida-infected A. castellanii did not lead to bronchopneumonia in Balb/c mice, and Francisella did not disseminate into the liver and spleen. Upon inhalation, F. novicida infects a variety of host cells, though neutrophils are the predominant cells early during infection in the lung infiltrates of pulmonary tularemia. The numbers of neutrophils in the lungs of Balb/c mice were significantly lower in the infection of mice with F. novicida-infected A. castellanii in comparison to group of mice infected only with F. novicida. These results demonstrate that following inoculation of mice with F. novicida-infected A. castellanii, mice did not develop tularemia.

Tularemia as a waterborne disease: a review

“…tularensis in A. castellannii 24 h post-infection [150]. F. novicida and F. philomiragia were also able to multiply within Hartmanella vermiformis and A. castellanii [142,146,148,150,151]. F. noatunensis was also able to infect and replicate within the amoeba Dictostelium discoideum [149].…”

“…holarctica was also shown to enter in a VBNC state [147]. In contrast to L. pneumophila, passage through amoebae did not increase Francisella novicida virulence [148]. Despite some disagreements about interaction mechanisms, all these reports argue that Francisella spp.…”

Assessing the impact, genomics and evolution of type II secretion across a large, medically important genus: the Legionella type II secretion paradigm

“…NttF has only a single homologue found in the genome of Piscirickettsia litoralis . Finally, NttG has only a single homologue, and that related protein is encoded by aquatic Francisella halioticida , a member of a genus that, like Legionella , is pathogenic for both amoebae and humans [208, 209]. Structural analysis suggests that NttG is a VirK-like protein, yet its activity and role of infection remain undefined [154].…”

Increased Sensitivity of Amoeba-Grown Francisella Species to Disinfectants

“…F. tularensis subsp. tularensis , holarctica , and F. novicida can enter and multiply within A. castellanii [ 15 ], H. vermiformis cells [ 16 , 17 ] and amoebal cysts [ 18 ], suggesting a role of amoebae as a natural reservoir for Francisella . For most bacterial pathogens, it has been proposed that amoebae serve as an environmental reservoir where bacteria develop and refine their virulence strategies to infect mammalian hosts with similar cellular defense mechanisms.…”

“…For most bacterial pathogens, it has been proposed that amoebae serve as an environmental reservoir where bacteria develop and refine their virulence strategies to infect mammalian hosts with similar cellular defense mechanisms. However, our previous studies showed no beneficial effect of F. novicida grown in A. castellanii on bacterial virulence in mice, which might result from differing life cycles and virulence strategies in macrophages and amoebae [ 15 ].…”