Identification of the Francisella novicida FTN_0096 as a factor involved in intracellular replication and host response.

Francisella tularensis is the causative agent of the zoonotic disease tularemia. We investigated a pathogenic factor of F. tularensis subsp. novicida (F. novicida). Accordingly, we established a novel infection model using HeLa cells. F. novicida usually infects macrophage lineage cells and less frequently epithelial cells. We successfully infected HeLa cells expressing the Fc receptor (HeLa-FcγRII cells) using F. novicida supplemented with mouse serum containing F. novicida antibodies. A total of 2,232 transposon mutants of F. novicida were screened to determine the relatively fewer cytotoxic strains of the HeLa-FcγRII cells, and 13 strains were thus isolated. Sequencing analysis of transposon insertion sites identified 13 genes, including FTN_0096. We focused on FTN_0096. Although the F. novicida wild-type strain proliferated in HeLa-FcγRII and THP-1 cells, the number of intracellular FTN_0096 mutant decreased. FTN_0096 mutant cannot escape from phagolysosomes in the initial phases of infection. Moreover, FTN_0096 mutant was detected in the mitochondria and Golgi complex. These findings indicate the importance of FTN_0096 of F. novicida for intracellular replication in the cells.