A Francisella tularensis Locus Required for Spermine Responsiveness Is Necessary for Virulence
ABSTRACTTularemia is a debilitating febrile illness caused by the category A biodefense agentFrancisella tularensis. This pathogen infects over 250 different hosts, has a low infectious dose, and causes high morbidity and mortality. Our understanding of the mechanisms by whichF. tularensissenses and adapts to host environments is incomplete. Polyamines, including spermine, regulate the interactions ofF. tularensiswith host cells. However, it is not known whether responsiveness to polyamines is necessary for the virulence of the organism. Through transposon mutagenesis ofF. tularensissubsp.holarcticalive vaccine strain (LVS), we identified FTL_0883 as a gene important for spermine responsiveness. In-frame deletion mutants of FTL_0883 and FTT_0615c, the homologue of FTL_0883 inF. tularensissubsp.tularensisSchu S4 (Schu S4), elicited higher levels of cytokines from human and murine macrophages compared to wild-type strains. Although deletion of FTL_0883 attenuated LVS replication within macrophagesin vitro, the Schu S4 mutant with a deletion in FTT_0615c replicated similarly to wild-type Schu S4. Nevertheless, both the LVS and the Schu S4 mutants were significantly attenuatedin vivo. Growth and dissemination of the Schu S4 mutant was severely reduced in the murine model of pneumonic tularemia. This attenuation depended on host responses to elevated levels of proinflammatory cytokines. These data associate responsiveness to polyamines with tularemia pathogenesis and define FTL_0883/FTT_0615c as anF. tularensisgene important for virulence and evasion of the host immune response.