Does environmental water support the survival of Francisella tularensis, the etiological agent of tularemia?

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Hennebique, Aurélie | Brunet, Camille | Peyroux, Julien | Knezevic, Masa | Santic, Marina | Pelloux, Isabelle | Caspar, Yvan | Boisset, Sandrine | Maurin, Max

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International audience. Background Tularemia is a zoonosis caused by the bacterium Francisella tularensis. Humancontamination mainly occurs through contact with wildlife animals or arthropod bites. Tularemiacan also be a water-borne disease. However, F. tularensis mechanisms of survival in aquaticenvironments are poorly characterized.Methods Firstly, using in vitro models, we investigated interactions of F. tularensis subsp.holarctica (the tularemia agent found in the northern hemisphere), and the aquatic species F.novicida and F. philomiragia with amoebae of the Acanthamoeba species.Secondly, weconducted an environmental study by collecting surface water samples in a tularemia endemicregion in France. We investigated the presence of F. tularensis and other Francisella species inwater using a set of qPCRs.Results In amoeba plate screening tests, all the Francisella species tested resisted the attack byamoebae. In infection model, focusing on strictly intra-amoebic growth, none of the Francisellaspecies tested displayed any intra-amoebic multiplication. In co-culture models, focusing both onintra and extra-amoebic bacterial multiplication, the amoebae favoured Francisella survival over16 days. In a spring water co-culture model, amoebae again enhanced F. novicida survival andpreserved bacterial morphology.Two sampling campaigns were conducted in West of France, inJuly 2019 and January 2020, allowing the collection of 87 water samples. Among 57 samples ofthe first campaign, 15 (26.3%) were positive for Francisella sp., nine (15.8%) for F. tularensis,and four (7.0%) for F. tularensis subsp. holarctica. The percentages were 83.3%, 80.0%, and13.3% for the 30 samples of the second campaign. Among the 30 sites sampled during the twocampaigns, nine were positive for Francisella sp. both times, seven for F. tularensis, and one forF. tularensis subsp. holarctica.Conclusions Our in vitro experiments suggest that amoebae promote Francisella sp. survival inaquatic environments, including the tularemia agent F. tularensis.Our environmental studyreveals a high prevalence and occasional persistence of Francisella sp., including F. tularensis,in the studied aquatic environment.Altogether, our results suggest that the aquatic environmentcan be a natural reservoir of F. tularensis, and thus could participate in the maintenance of thispathogen in tularemia endemic areas.

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