Generation and characterization of an attenuated mutant in a response-regulator gene of Francisella tularensis live vaccine strain (LVS)

Francisella tularensis is a zoonotic bacterium that must exist in diverse environments ranging from arthropod vectors to mammalian hosts. To better understand how genes are regulated in these different environments, a transcriptional response- regulator gene (genome locus FTL0552) was deleted in F. tularensis live vaccine strain (LVS). The FTL0552 deletion mutant exhibited slightly reduced rates of extracellular growth but was unable to replicate or survive in mouse macrophages and was avirulent in the mouse model using either BALB/c or C57BL/6 mice. Mice infected with the FTL0552 mutant produced reduced levels of inflammatory cytokines, exhibited reduced histopathology and cleared the bacteria quicker than mice infected with LVS. Mice that survived infection with the FTL0552 mutant were afforded partial protection when challenged with a lethal dose of the virulent Schu S4 strain (4 of 10 survivors, day 21 post infection) when compared to naïve mice (0 of 10 survivors by day 7 post infection). Microarray experiments indicate that 148 genes are regulated in the FTL0552 mutant. Most of the genes are down regulated, indicating that FTL0552 controls transcription of genes in a positive manner. The list of down regulated genes includes genes located within the Francisella Pathogenicity Island (FPI) that are essential for intracellular survival and virulence of Francisella tularensis. Furthermore, a mutant in FTL0552 or the comparable locus in Schu S4 (FTT1557c) may be an alternative candidate vaccine for tularemia