Morphological analysis of Francisella novicida epithelial cell infections in the absence of functional FipA

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.