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“Background Francisella tularensis is a facultative intracellular, gram-negative coccobacillus, which causes the potentially lethal disease tularemia. This zoonotic disease is transmitted via vectors such as ticks and mosquitoes and infects predominantly mammals such as small rodents, hares and rabbits [1]. The subspecies tularensis and holarctica also give rise to human infections. The pathogen is highly contagious,

requiring Luminespib as few as 10 bacteria to cause human infection, and subspecies tularensis causes a very aggressive disease with high mortality in humans if untreated [2]. The high virulence, ease of spread, and potentially high mortality of tularemia has led to the classification of F. tularensis as one of six category A select agents, i.e., the agents most likely to be used for bioterrorism [3]. In experimental infections, F.

novicida and F. tularensis LVS are often used since both show significant virulence in small rodents but still are classified as BSL2 pathogens. The former species very rarely causes human infections and the latter is a human EGFR inhibition vaccine strain of subspecies holarctica origin [4]. An important virulence trait of F. tularensis is its ability to survive and multiply in an array of different cell types including hepatocytes and professional phagocytes [5]. The intracellular lifestyle relies on escape from the phagosome and the subsequent proliferation in the cytoplasm [6]. The mechanism of escape from the phagosome GSK2126458 mw is not known but requires expression of the global regulator MglA (macrophage growth locus) Olopatadine [7]. This is most likely through its positive regulation of

the genes belonging to the intracellular growth locus (igl) and other genes of the Francisella pathogenicity island. MglA together with an ortholog, SspA, forms a complex that directly interacts with the RNA polymerase [8] conferring a complex regulatory role that leads to the control of more than 100 genes and proteins in F. tularensis [9, 10]. Besides the igl operon, it has been suggested that the activities of several stress-regulated factors, such as SspA, Hfq, CspC, and UspA, are linked to the MglA-dependent regulation [10]. Thereby, it plays an important role for the intracellular growth and stress responses in general and for the adaptation to oxidative stress response specifically. Iron is essential for the survival of almost all living organisms. Limiting the amount of iron accessible to pathogens is therefore an important part of the host defence system [11]. Thus, it is essential for successful pathogens to circumvent this and they have evolved various strategies, such as the usage of siderophores, which are high affinity iron chelators synthesized in response to iron starvation [12]. Siderophore production in Francisella is dependent on proteins encoded in the fsl operon (Francisella siderophore locus) [13–15].