A Mutant of Francisella tularensisStrain SCHU S4 Lacking the Ability To Express a 58-Kilodalton Protein Is Attenuated for Virulence and Is an Effective Live Vaccine

ABSTRACTFrancisella tularensissubsp. tularensis(type A) strain SCHU S4 is a prototypic strain of the pathogen that is highly virulent for humans and other mammals. Its intradermal (i.d.) 50% lethal dose (LD50) for mice is <10 CFU. We discovered a spontaneous mutant, designated FSC043, of SCHU S4 with an i.d. LD50of >108CFU. FSC043 effectively vaccinated mice against challenge with a highly virulent type A strain, and the protective efficacy was at least as good as that of F. tularensisLVS, an empirically attenuated strain which has been used as an efficacious human vaccine. Comparative proteomics was used to identify two proteins of unknown function that were identified as defective in LVS and FSC043, and deletion mutants of SCHU S4 were created for each of the two encoding genes. One mutant, the ΔFTT0918 strain, failed to express a 58-kDa protein, had an i.d. LD50of ∼105CFU, and was found to be less capable than SCHU S4 of growing in peritoneal mouse macrophages. Mice that recovered from sublethal infection with the ΔFTT0918 mutant survived when challenged 2 months later with >100 LD50s of the highly virulent type A strain FSC033. This is the first report of the generation of defined mutants of F. tularensissubsp. tularensisand their use as live vaccines.