Identification of Francisella tularensis Live Vaccine Strain CuZn Superoxide Dismutase as Critical for Resistance to Extracellularly Generated Reactive Oxygen Species

Francisella tularensis is an intracellular pathogen whose survival is in part dependent on its ability to resist the microbicidal activity of host-generated reactive oxygen species (ROS) and reactive nitrogen species (RNS). In numerous bacterial pathogens, CuZn-containing superoxide dismutases (SodC) are important virulence factors, localizing to the periplasm to offer protection from host-derived superoxide radicals (O(2)(-)). In the present study, mutants of F. tularensis live vaccine strain (LVS) deficient in superoxide dismutases (SODs) were used to examine their role in defense against ROS/RNS-mediated microbicidal activity of infected macrophages. An in-frame deletion F. tularensis mutant of sodC (DeltasodC) and a F. tularensis DeltasodC mutant with attenuated Fe-superoxide dismutase (sodB) gene expression (sodB DeltasodC) were constructed and evaluated for susceptibility to ROS and RNS in gamma interferon (IFN-gamma)-activated macrophages and a mouse model of respiratory tularemia. The F. tularensis DeltasodC and sodB DeltasodC mutants showed attenuated intramacrophage survival in IFN-gamma-activated macrophages compared to the wild-type F. tularensis LVS. Transcomplementing the sodC gene in the DeltasodC mutant or inhibiting the IFN-gamma-dependent production of O(2)(-) or nitric oxide (NO) enhanced intramacrophage survival of the sod mutants. The DeltasodC and sodB DeltasodC mutants were also significantly attenuated for virulence in intranasally challenged C57BL/6 mice compared to the wild-type F. tularensis LVS. As observed for macrophages, the virulence of the DeltasodC mutant was restored in ifn-gamma(-/-), inos(-/-), and phox(-/-) mice, indicating that SodC is required for resisting host-generated ROS. To conclude, this study demonstrates that SodB and SodC act to confer protection against host-derived oxidants and contribute to intramacrophage survival and virulence of F. tularensis in mice.