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 (including F. novicida and F. philomiragia), 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. Such enhanced survival 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.

In 2018, the incidence of tularemia increased twofold in the west of France, with many pneumonic forms, suggesting environmental sources of infection. We investigated the presence of Francisella tularensis subsp. holarctica and other Francisella species DNA in the natural aquatic environment of this geographic area. Two sampling campaigns, in July 2019 and January 2020, allowed the collection of 87 water samples. Using a combination of real-time PCR assays, we tested the presence of either Francisella sp., F. tularensis/F. novicida, and F. tularensis subsp. holarctica, the latter being the only tularemia agent in Europe. Among 57 water samples of the first campaign, 15 (26.3%) were positive for Francisella sp., nine (15.8%) for F. tularensis and/or F. novicida, and four (7.0%) for F. tularensis subsp. holarctica. Ratios were 25/30 (83.3%), 24/30 (80.0%), and 4/30 (13.3%) for the second campaign. Among the thirty sites sampled during the two campaigns, nine were positive both times for Francisella sp., seven for F. tularensis and/or F. novicida, and one for F. tularensis subsp. holarctica. Altogether, our study reveals a high prevalence of Francisella sp. DNA (including the tularemia agent) in the studied aquatic environment. This aquatic environment could therefore participate in the endemicity of tularemia in the west of France.

Francisella tularensis is a highly virulent bacterium causing tularemia zoonosis. An increasing proportion of infections occur through contaminated hydro-telluric sources, especially for the subspecies holarctica (Fth). Although this bacterium has been detected in several aquatic environments, the mechanisms of its long-term persistence in water are not yet elucidated. We evaluated the culturability and the viability of a virulent Fth strain in independent microcosms filled with nutrient-poor water. At 37°C, the bacteria remained culturable for only one week, while culturability was extended to 6 weeks at 18°C and up to 11 weeks at 4°C. However, while the viability of the bacteria declined similarly to culturability at 37°C, the viability of the bacteria remained stable overtime at 18°C and 4°C for more than 24 months, long after loss of culturability. We identified water temperature as one of the major factors driving the aquatic survival of Fth through a transition of the whole Fth population in a viable but non-culturable (VBNC) state. Low temperature of water (4 and 18°C) favors the persistence of the bacteria in a VBNC state, while a temperature above 30°C kills culturable and VBNC Fth bacteria. These findings provide new insights into the environmental cycle of Francisella tularensis that suggest that the yet unidentified primary reservoir of the subspecies holarctica may be the aquatic environment itself in which the bacteria could persist for months or years without the need for a host.