Abstract
ABSTRACT Francisella tularensis, a tier 1 select agent, is the causative bacterium of tularemia, a zoonosis with a large animal reservoir. However, F. tularensis, like many other Francisella species, is assumed to have an aquatic reservoir. The mechanisms of Francisella species persistence in surface water remain poorly characterized. In this study, we deeply investigated the long-term interactions of the tularemia agent F. tularensis subsp. holarctica, F. novicida or F. philomiragia with amoebae of the Acanthamoeba species. In amoeba plate screening tests, all the Francisella species tested resisted the attack by amoebae. In in vitro infection models, intra-amoebic growth of Francisella varied according to the involved bacterial species and strains, but also the amoeba culture medium used. In co-culture models, the amoebae favoured Francisella survival over 16 days, which was likely dependent on direct contact between bacteria and amoebae for F. novicida and on amoeba-excreted compounds for F. novicida and for F. tularensis. In a spring water co-culture model, amoebae again enhanced F. novicida survival and preserved bacterial morphology. Overall, our results demonstrate that amoebae likely promote Francisella survival in aquatic environments, including the tularemia agent F. tularensis. However, bacteria-amoebae interactions are complex and depend on the Francisella species considered.
Highlights
Francisella tularensis is a Gram-negative, facultative intracellular bacterium, causing the potentially life-threatening zoonosis tularemia
Since AMOEBA PLATE TESTS (APT) revealed that Francisella sp. resisted to amoebae and infection in starvation medium (SM) did not suggested intra-amoebic survival of Francisella sp., we developed co-culture models in order to evaluate if amoebae could promote Francisella sp. survival whether intracellular or extracellular
We first evaluated the interactions between Francisella and amoebae using the Amoeba Plate Test (APT)
Summary
Francisella tularensis is a Gram-negative, facultative intracellular bacterium, causing the potentially life-threatening zoonosis tularemia. This microorganism is classified as a category. The mechanisms and conditions of F. tularensis long-term survival in this environment have to be characterized Some intracellular bacteria such as Legionella pneumophila and Mycobacterium avium persist in natural ecosystem by using free living amoebae as a multiplication niche and reservoir 6. Amoebae such as Acanthamoeba and Vermamoeba species are ubiquitous in water and soil 7. A few experimental studies have investigated the relationships between Francisella sp. and amoebae, with contradictory results
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