Aquatic long-term persistence ofFrancisella tularensisssp.holarcticais driven by water temperature and transition to a viable but non-culturable state

AbstractFrancisella tularensisis a highly virulent bacterium causing tularemia zoonosis. An increasing proportion of infections occur through contaminated hydro-telluric sources, especially for the subspeciesholarctica(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 virulentFthstrain 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 ofFththrough a transition of the wholeFthpopulation in a viable but non-culturable (VBNC) state. Low temperature of water (≤18°C) favors the persistence of the bacteria in a VBNC state, while a temperature above 30°C kills culturable and VBNCFthbacteria. These findings provide new insights into the environmental cycle ofFrancisella tularensisthat suggest that the yet unidentified primary reservoir of the subspeciesholarcticamay be the aquatic environment itself in which the bacteria could persist for months or years without the need for a host.