Francisella tularensis universal stress protein contributes to persistence during growth arrest and paraquat-induced superoxide stress

Francisella tularensis is one of the most virulent bacterial pathogens known and causes the disease tularemia, which can be fatal if untreated. This zoonotic and intracellular pathogen is exposed to diverse environmental and host stress factors that require an appropriate response to survive. However, the stress tolerance mechanisms used by F. tularensis to persist are not fully understood. To address this aspect, we evaluated the highly conserved universal stress protein (Usp) that is encoded by a single-copy gene in F. tularensis, unlike the majority of other bacterial pathogens that produce several to many Usp homologs. We determined that the F. tularensis Usp transcript is unusually stable with a half-life of over 30 minutes, and that usp transcript and protein levels remained abundant when exposed to low pH, nutrient deprivation, hydrogen peroxide, and paraquat. Of these and other stress conditions evaluated, the F. tularensis Delta usp mutant only exhibited reduced survival relative to the wild type during stationary phase and exposure to paraquat, a highly toxic compound that generates superoxide anions and other free radicals. Comparison of transcript levels in untreated and paraquat-treated F. tularensis wild type and Delta usp indicated that Usp contributes to enhanced expression of antioxidant defense genes, oxyR and katG. In summary, the high abundance and stability of Usp provide prompt protection during extended periods of growth arrest and free radical exposure, promoting F. tularensis persistence. We propose that F. tularensis Usp contributes to an adaptive response that prolongs viability and increases the longevity of this zoonotic pathogen in the environment.