AIMS To determine the role of the iglC gene in intracellular life of F. tularensis subsp. novicida within human macrophages and amoeba through growth kinetics, modulation of apoptosis, biogenesis of the phagosome and ultrastructural organisation of the cells after the infection. METHODS The cells of A. castellanii, H. vermiformis and human macrophages were infected with F. tularensis subsp. novicida and its mutant iglC. In certain time periods after infection the cells were processed in order to determine colony forming units by growth kinetics, to examine the level of phagosome acidification by confocal microscopy and ultrastructural organisation of the cell was determined by electron microscopy. VII Resistance to apoptosis and activation of caspase-1 and caspase-3 in human macrophages infected with F. tularensis subsp. novicida or iglC mutant was determined with immunoflourescence staining by confocal microscopy. RESULTS F. tularensis triggers nuclear translocation of the p65 subunit of NF-B in hMDMs, but sustained nuclear translocation of NF-B is defective in the iglC mutant. Although caspase-1 and caspase-3 are triggered within F. tularensis-infected hMDMs during early stages of infection, cell death is delayed, which is correlated with simultaneous activation of NF-B. In vitro studies showed intracellular replication of F. tularensis subsp. novicida within A. castellanii and H. vermiformis, but iglC mutant is defective for survival and replication in these amoeba. CONCLUSIONS This study, for the first time showed simultaneous modulation of caspase-1, caspase-3 and the anti-apoptosis nuclear transcription factor NF-B and sensitive balance among them in order to preserve capability for survival of the infected cells. Francisella is capable to grow and survive in amoeba, and iglC is necessary for intracellular growth in A. castellanii and H. vermiformis. In contrast to mammalian cells where bacteria replicate in the cytosol in amoeba F. tularensis subsp. novicida replicates within pahgosomes.