Influence of nutrient status and grazing pressure on the fate of Francisella tularensis in lake water

Abstract

The natural reservoir of Francisella tularensis, the causative agent of tularaemia, is yet to be identified. We investigated the possibility that Francisella persists in natural aquatic ecosystems between outbreaks. It was hypothesized that nutrientrich environments, with strong protozoan predation, favour the occurrence of the tularaemia bacterium. To investigate the differences in adaptation to aquatic environments of the species and subspecies of Francisella, we screened 23 strains for their ability to survive grazing by the ciliate Tetrahymena pyriformis. All the Francisella strains tested were consumed at a low rate, although significant differences between subspecies were found. The survival and virulence of gfp-labelled F. tularensis ssp. holarctica were then studied in a microcosm experiment using natural lake water, with varying food web complexities and nutrient availabilities. High nutrient conditions in combination with high abundances of nanoflagellates were found to favour F. tularensis ssp. holarctica. The bacterium was observed both free-living and within the cells of a nanoflagellate. Francisella tularensis entered a viable but nonculturable state during the microcosm experiment. When studied over a longer period of time, F. tularensis ssp. holarctica survived in the lake water, but loss of virulence was not prevented by either high nutrient availability or the presence of predators.

Contributions ofFrancisella tularensissubsp.novicidaChitinases and Sec Secretion System to Biofilm Formation on Chitin

“…Thelaus et al found that F. tularensis subsp. holarctica had increased resistance to both ciliate and flagellate protozoa compared to that of E. coli (71). Although the role of biofilm formation in resistance to predation was not addressed, this observation suggests that F. tularensis may actively prevent protozoal grazing in nature.…”

Rapid adaptation of predation resistance in bacteria isolated from a seawater microcosm

“…The ciliate was much more efficient in degrading the bacteria than the flagellate, but the same trend in mortality rate of 'edible' and 'inedible' bacteria, was observed, i.e. the degradation ratio between the edible and the inedible bacteria remained constant, regardless of the predator (Thelaus et al 2009). We therefore think that the relative change in edibility would have been similar if another bacterivore had been used.…”

“…In a study by Thelaus et al (2009), the consumption rates of different bacteria by the same ciliate as used in this study (T. pyriformis) and an interception feeding flagellate (Ochromonas danica) were compared. The ciliate was much more efficient in degrading the bacteria than the flagellate, but the same trend in mortality rate of 'edible' and 'inedible' bacteria, was observed, i.e.…”

“…This bacterivorous ciliate has a length of ~40 μm and has been used in previous studies for testing edibility of different bacterial isolates (e.g. Thelaus et al 2008aThelaus et al , 2009). The bacteria had average cell size of ~0.2 to 0.3 um 3 , indicating that they were within the size range for consumption by T. pyriformis (e.g.…”

Fate of Francisella noatunensis, a pathogen of Atlantic cod Gadus morhua, in blue mussels Mytilus edulis

“…It has previously been demonstrated that F. tularensis survives in bacterivorous amoebae, ciliates, and nanoflagellates, suggesting protists may play a role in the environmental survival of Francisella spp. (Abd et al 2003, Thelaus et al 2009). Also, a novel F. noatunensis subspecies, Candidatus F. noatunensis subsp.…”