Francisella tularensis does not manifest virulence in viable but non-culturable state

Francisella tularensis is a small Gram-negative bacterium that causes tularemia in animals and man. The disease can be transmitted by handling of infected animals, by contaminated dust, by insect vectors, or by drinking contaminated water. In the present study cells of F. tularensis were subjected to extended storage in cold water devoid of carbon sources. Total cell counts remained constant throughout a 70-day period and beyond, while plate counts decreased to an undetectable level after 70 days. Attempts to resuscitate the cells were unsuccessful. Quantitative PCR targeting the 16S rDNA of F. tularensis showed an increase in variability after 25 days and the signal was lost after 45 days. Metabolic activity, measured by accumulation of rhodamine 123, declined to approximately 35% after a 140-day period. Analyses of substrate responsiveness of cells stored for 140 days in cold water showed that approximately 30% of the population increased in size after incubation in rich medium in the presence of nalidixic acid. Approximately 105 of these cells were injected intraperitoneally into mice. No signs or symptoms of tularemia were observed during 3 weeks. In addition, there was no evidence of stimulation of lymphocytes with F. tularensis as recall antigen. In conclusion, viable but non-culturable cells of F. tularensis are avirulent in mice, giving new insight into the ecological niche of this bacterium.