Francisella tularensis ssp. holarctica is able to colonize natural aquatic ex vivo biofilms

Francisella tularensis (Ft) is an intracellular pathogen causing tularemia in a variety of hosts including humans and rodents. Fth is more frequently associated with aquatic habitats and is widely distributed throughout Eurasia. F. tularensis maintains viability in cold water for long periods of time. In (aquatic) natural environments, biofilm formation is known to increase the survival of bacteria.

The objective of this study was to investigate the survival of a wild type strain of Francisella tularensis ssp. holarctica (Fth) in natural aquatic ex vivo biofilms. To that purpose, we allowed Fth to produce its own biofilm on solid surfaces. The survival rate of biofilam and planctonic bacteria were compared. We also analyzed the ability of Fth to colonize naturally formed biofilms. Light- and electron microscopy showed that Fth WT is able to form a complex, matrix-associated biofilm. Francisella in its own biofilm showed longer cultivabilety in natural water when compared to planktonic bacteria. The same was true for Fth infecting an existing ex vivo biofilm. In addition, Francisella was also able to establish microcolonies and areas with their own exclusive biofilm architecture within the ex vivo biofilm. For the first time, using a Fth wild type strain, we can demonstrate the ability of Francisella to successfully colonize an aquatic multi-species ex-vivo biofilm. It can be assumed that Fth becomes more persistent in the environment when it forms its own biofilm or becomes a part of an existing one. This may have also impact on the long-term survival of Francisella in aquatic habitats and the infection cycles in nature.