Examining mechanisms of virulence gene regulation and the early host interactions in Francisella tularenisis

Francisella tularensis is a facultative intracellular pathogen and is the etiological agent of tularemia. One key aspect to the success of Francisella as a pathogen is ability of the organism to establish infection with a low inoculum, as few as 10 colony forming units (cfu). Essential to this process is the Francisella pathogenicity island (FPI). Several studies have been performed to understand how the FPI is regulated; however, the working model is not complete, as the signals important for regulation are unknown. Additionally, the mechanisms of the proteins MigR, TrmE, and CphA, which are important for activation of the FPI, are unknown. I initiated the study of this regulatory system by measuring the ability of various cellular stresses to activate an iglA-lacZ reporter. I identified that amino acid starvation and growth in basic pH activated expression of the reporter in both LVS and Schu S4. By combining these two stresses I was able to induce iglA-lacZ reporter expression in an additive manner. As it was previously demonstrated that ppGpp is important for stabilization of the regulatory complex that transcribes FPI genes, I demonstrated by TLC that both amino acid starvation and basic pH effected iglA-lacZ expression by increasing ppGpp. Due to the importance of ppGpp in FPI expression and because MigR, TrmE, and CphA each appear to be involved in a metabolic process: fatty acid metabolism (migR) t-RNA modification (trmE) and amino acid storage (cphA), I had hypothesized that the effect on these mutations were due to decreased levels of the small alarmone ppGpp. I compared ppGpp accumulation of LVS mutants in migR, trmE, and cphA to the parent strain and observed that loss of these genes resulted in reduced ppGpp. To better understand the importance of ppGpp synthesis in F. tularensis pathogenesis, I compared the phenotypes of these strains in primary human macrophages and two immortalized epithelial cell lines. These experiments demonstrated that although each of these strains had reduced ppGpp, there were cell line specific growth phenotypes. Mice infected with these strains survived suggesting tight regulation of the FPI is required for virulence. When similar mutations were characterized in the Schu S4 background these mutations retained their regulatory role; however, mutation of migR did not significantly decrease virulence in mice. As my data demonstrated that there are different challenges that Francisella must overcome to successfully replicate within cells, I developed an in vitro model to study the interactions of F. tularensis with human alveolar type II cells (AT-II). Interestingly, Schu S4 internalizes and replicates in these recently immortalized human AT-II cells whereas, LVS internalizes, but replicates poorly within these cells. Finally, to better understand the role of AT-II cells in vivo, I performed Transmission Electron Microscopy (TEM) of infected mice. These data confirmed that Schu S4 infected both alveolar macrophages and AT-II cells. Together, this work contributes to the understanding of how Francisella adapts to various environments by modulating virulence gene expression and highlights differences between virulent Schu S4 and LVS, which may partially contribute to virulence differences observed between strains.