Potential bioterrorism agent Francisella tularensis subspecies tularensis (F. tularensis) causes tularemia; a disease that can cause up to 60% mortality. An essential step to developing tularemia is the ability of F. tularensis to escape the vacuole that it initially occupies to replicate in the cytosol of epithelial cells. We hypothesize F. tularensis possesses virulence factors (VFs) that facilitate this process. In this study, we identified a crucial VF for tularemia development. To identify this VF, we screened a 3,050 Francisella mutant library for microbes deficient in bacterial replication once inside host cells. By using subsequent bioinformatics analysis we identified proteins in a subset of mutated bacteria that fit traits of VFs and we tested their ability to cause tularemia‐induced death in mice. Mice infected with bacteria inactivated in 1 novel gene appeared uninfected whereas wild‐type infected mice died within 2 days. Electron and immunofluorescence micrographs of infected hepatocytes showed that this gene aids the bacteria in exiting lysosome‐associated membrane protein‐1 (LAMP‐1) positive membrane vacuoles leading to bacterial replication in the cytosol. Our findings demonstrate this novel VF enables F. tularensis to escape into the cytosol and ultimately cause disease.

Francisella novicida is a surrogate pathogen commonly used to study infections by the potential bioterrorism agent, Francisella tularensis. One of the primary sites of Francisella infections is the liver where >90% of infected cells are hepatocytes. It is known that once Francisella enter cells it occupies a membrane-bound compartment, the Francisella-containing vacuole (FCV), from which it rapidly escapes to replicate in the cytosol. Recent work examining the Francisella disulfide bond formation (Dsb) proteins, FipA and FipB, have demonstrated that these proteins are important during the Francisella infection process; however, details as to how the infections are altered in epithelial cells have remained elusive. To identify the stage of the infections where these Dsbs might act during epithelial infections, we exploited a hepatocyte F. novicida infection model that we recently developed. We found that F. novicida ΔfipA-infected hepatocytes contained bacteria clustered within lysosome-associated membrane protein 1-positive FCVs, suggesting that FipA is involved in the escape of F. novicida from its vacuole. Our morphological evidence provides a tangible link as to how Dsb FipA can influence Francisella infections.

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