fslE is necessary for siderophore-mediated iron acquisition in Francisella tularensis Schu S4

Control of Francisella tularensis Virulence at Gene Level: Network of Transcription Factors

Regulation of gene transcription is the initial step in the complex process that controls gene expression within bacteria. Transcriptional control involves the joint effort of RNA polymerases and numerous other regulatory factors. Whether global or local, positive or negative, regulators play an essential role in the bacterial cell. For instance, some regulators specifically modify the transcription of virulence genes, thereby being indispensable to pathogenic bacteria. Here, we provide a comprehensive overview of important transcription factors and DNA-binding proteins described for the virulent bacterium Francisella tularensis, the causative agent of tularemia. This is an unexplored research area, and the poorly described networks of transcription factors merit additional experimental studies to help elucidate the molecular mechanisms of pathogenesis in this bacterium, and how they contribute to disease.

Francisella tularensis: FupA mutation contributes to fluoroquinolone resistance by increasing vesicle secretion and biofilm formation

Francisella tularensis is the causative agent in tularemia for which the high prevalence of treatment failure and relapse is a major concern. Directed-evolution experiments revealed that acquisition of fluoroquinolone (FQ) resistance was linked to factors in addition to mutations in DNA gyrase. Here, using F. tularensis live vaccine strain (LVS) as a model, we demonstrated that FupA/B (Fer-Utilization Protein) expression is linked to FQ susceptibility, and that the virulent strain F. tularensis subsp. tularensis SCHU S4 deleted for the homologous FupA protein exhibited even higher FQ resistance. In addition to an increased FQ minimal inhibitory concentration, LVSΔfupA/B displayed tolerance toward bactericidal compounds including ciprofloxacin and gentamicin. Interestingly, the FupA/B deletion was found to promote increased secretion of outer membrane vesicles (OMVs). Mass spectrometry-based quantitative proteomic characterization of vesicles from LVS and LVS∆fupA/B identified 801 proteins, including a subset of 23 proteins exhibiting differential abundance between both strains which may therefore contribute to the reduced antibiotic susceptibility of the FupA/B-deleted strain. We also demonstrated that OMVs are key structural elements of LVSΔfupA/B biofilms providing protection against FQ. These results provide a new basis for understanding and tackling antibiotic resistance and/or persistence of Francisella and other pathogenic members of the Thiotrichales class.

Citryl Ornithine Is an Intermediate in a Three-Step Biosynthetic Pathway for Rhizoferrin in Francisella

The Gram-negative bacterium Francisella tularensis secretes the siderophore rhizoferrin to scavenge necessary iron from the environment. Rhizoferrin, also produced by a variety of fungi and bacteria, comprises two citrate molecules linked by amide bonds to a central putrescine (diaminobutane) moiety. Genetic analysis has determined that rhizoferrin production in F. tularensis requires two enzymes: FslA, a siderophore synthetase of the nonribosomal peptide synthetase-independent siderophore synthetase (NIS) family, and FslC, a pyridoxal-phosphate-dependent decarboxylase. To discern the steps in the biosynthetic pathway, we tested F. tularensis strain LVS and its ΔfslA and ΔfslC mutants for the ability to incorporate potential precursors into rhizoferrin. Unlike putrescine supplementation, supplementation with ornithine greatly enhanced siderophore production by LVS. Radioactivity from L-[U-¹⁴C] ornithine, but not from L-[1-¹⁴C] ornithine, was efficiently incorporated into rhizoferrin by LVS. Although neither the ΔfslA nor the ΔfslC mutant produced rhizoferrin, a putative siderophore intermediate labeled by both [U-¹⁴C] ornithine and [1-¹⁴C] ornithine was secreted by the ΔfslC mutant. Rhizoferrin was identified by liquid chromatography and mass spectrometry in LVS culture supernatants, while citryl-ornithine was detected as the siderophore intermediate in the culture supernatant of the ΔfslC mutant. Our findings support a three-step pathway for rhizoferrin production in Francisella; unlike the fungus Rhizopus delemar, where putrescine functions as a primary precursor for rhizoferrin, biosynthesis in Francisella preferentially starts with ornithine as the substrate for FslA-mediated condensation with citrate. Decarboxylation of this citryl ornithine intermediate by FslC is necessary for a second condensation reaction with citrate to produce rhizoferrin.

Structural and functional studies of the metalloregulator Fur identify a promoter-binding mechanism and its role in Francisella tularensis virulence

Francisella tularensis is a Gram-negative bacterium causing tularaemia. Classified as possible bioterrorism agent, it may be transmitted to humans via animal infection or inhalation leading to severe pneumonia. Its virulence is related to iron homeostasis involving siderophore biosynthesis directly controlled at the transcription level by the ferric uptake regulator Fur, as presented here together with the first crystal structure of the tetrameric F. tularensis Fur in the presence of its physiological cofactor, Fe²⁺. Through structural, biophysical, biochemical and modelling studies, we show that promoter sequences of F. tularensis containing Fur boxes enable this tetrameric protein to bind them by splitting it into two dimers. Furthermore, the critical role of F. tularensis Fur in virulence and pathogenesis is demonstrated with a fur-deleted mutant showing an attenuated virulence in macrophage-like cells and mice. Together, our study suggests that Fur is an attractive target of new antibiotics that attenuate the virulence of F. tularensis.

Pérard et al. report the structure of Francisella tularensis Fur (FtFur) with its physiological cofactor Fe²⁺, and show that FtFur is important for virulence. This study identifies a promoter-driven tetramer splitting mechanism that may provide insight into future antibiotics development.

The Ability to Acquire Iron Is Inversely Related to Virulence and the Protective Efficacy of Francisella tularensis Live Vaccine Strain

Francisella tularensis is a highly infectious bacterial pathogen that causes the potentially fatal disease tularemia. The Live Vaccine Strain (LVS) of F. tularensis subsp. holarctica, while no longer licensed as a vaccine, is used as a model organism for identifying correlates of immunity and bacterial factors that mediate a productive immune response against F. tularensis. Recently, it was reported that two biovars of LVS differed in their virulence and vaccine efficacy. Genetic analysis showed that they differ in ferrous iron homeostasis; lower Fe2+ levels contributed to increased resistance to hydrogen peroxide in the vaccine efficacious LVS biovar. This also correlated with resistance to the bactericidal activity of interferon γ-stimulated murine bone marrow-derived macrophages. We have extended these findings further by showing that a mutant lacking bacterioferritin stimulates poor protection against Schu S4 challenge in a mouse model of tularemia. Together these results suggest that the efficacious biovar of LVS stimulates productive immunity by a mechanism that is dependent on its ability to limit the toxic effects of oxidative stress by maintaining optimally low levels of intracellular Fe2+.

Zinc Acquisition Mechanisms Differ between Environmental and Virulent Francisella Species

Zinc is an essential nutrient for bacterial growth. Because host cells can restrict pathogen access to zinc as an antimicrobial defense mechanism, intracellular pathogens such as Francisella must sense their environment and acquire zinc in response. In many bacteria, the conserved transcription factor Zur is a key regulator of zinc acquisition. To identify mechanisms of zinc uptake in Francisella novicida U112, transcriptome sequencing of wild-type and putative zur mutant bacteria was performed. Only three genes were confirmed as directly regulated by Zur and zinc limitation by quantitative reverse transcription-PCR. One of these genes, FTN_0879, is predicted to encode a protein with similarity to the zupT family of zinc transporters, which are not typically regulated by Zur. While a putative znuACB operon encoding a high-affinity zinc transporter was identified in U112, expression of this operon was not controlled by Zur or zinc concentration. Disruption of zupT but not znuA in U112 impaired growth under zinc limitation, suggesting that ZupT is the primary mechanism of zinc acquisition under these conditions. In the virulent Francisella tularensis subsp. tularensis Schu S4 strain, zupT is a pseudogene, and attempts to delete znuA were unsuccessful, suggesting that it is essential in this strain. A reverse TetR repression system was used to knock down the expression of znuA in Schu S4, revealing that znuA is required for growth under zinc limitation and contributes to intracellular growth within macrophages. Overall, this work identifies genes necessary for adaptation to zinc limitation and highlights nutritional differences between environmental and virulent Francisella strains.

IMPORTANCEFrancisella tularensis is a tier 1 select agent with a high potential for lethality and no approved vaccine. A better understanding of Francisella virulence factors is required for the development of therapeutics. While acquisition of zinc has been shown to be required for the virulence of numerous intracellular pathogens, zinc uptake has not been characterized in Francisella. This work characterizes the Zur regulon in F. novicida and identifies two transporters that contribute to bacterial growth under zinc limitation. In addition, these data identify differences in mechanisms of zinc uptake and tolerance to zinc limitation between F. tularensis and F. novicida, highlighting the role of znuA in the growth of Schu S4 under zinc limitation.

The Multiple Localized Glyceraldehyde-3-Phosphate Dehydrogenase Contributes to the Attenuation of the Francisella tularensis dsbA Deletion Mutant

The DsbA homolog of Francisella tularensis was previously demonstrated to be required for intracellular replication and animal death. Disruption of the dsbA gene leads to a pleiotropic phenotype that could indirectly affect a number of different cellular pathways. To reveal the broad effects of DsbA, we compared fractions enriched in membrane proteins of the wild-type FSC200 strain with the dsbA deletion strain using a SILAC-based quantitative proteomic analysis. This analysis enabled identification of 63 proteins with significantly altered amounts in the dsbA mutant strain compared to the wild-type strain. These proteins comprise a quite heterogeneous group including hypothetical proteins, proteins associated with membrane structures, and potential secreted proteins. Many of them are known to be associated with F. tularensis virulence. Several proteins were selected for further studies focused on their potential role in tularemia's pathogenesis. Of them, only the gene encoding glyceraldehyde-3-phosphate dehydrogenase, an enzyme of glycolytic pathway, was found to be important for full virulence manifestations both in vivo and in vitro. We next created a viable mutant strain with deleted gapA gene and analyzed its phenotype. The gapA mutant is characterized by reduced virulence in mice, defective replication inside macrophages, and its ability to induce a protective immune response against systemic challenge with parental wild-type strain. We also demonstrate the multiple localization sites of this protein: In addition to within the cytosol, it was found on the cell surface, outside the cells, and in the culture medium. Recombinant GapA was successfully obtained, and it was shown that it binds host extracellular serum proteins like plasminogen, fibrinogen, and fibronectin.

Differential Growth of Francisella tularensis, Which Alters Expression of Virulence Factors, Dominant Antigens, and Surface-Carbohydrate Synthases, Governs the Apparent Virulence of Ft SchuS4 to Immunized Animals

The gram-negative bacterium Francisella tularensis (Ft) is both a potential biological weapon and a naturally occurring microbe that survives in arthropods, fresh water amoeba, and mammals with distinct phenotypes in various environments. Previously, we used a number of measurements to characterize Ft grown in Brain-Heart Infusion (BHI) broth as (1) more similar to infection-derived bacteria, and (2) slightly more virulent in naïve animals, compared to Ft grown in Mueller Hinton Broth (MHB). In these studies we observed that the free amino acids in MHB repress expression of select Ft virulence factors by an unknown mechanism. Here, we tested the hypotheses that Ft grown in BHI (BHI-Ft) accurately displays a full protein composition more similar to that reported for infection-derived Ft and that this similarity would make BHI-Ft more susceptible to pre-existing, vaccine-induced immunity than MHB-Ft. We performed comprehensive proteomic analysis of Ft grown in MHB, BHI, and BHI supplemented with casamino acids (BCA) and compared our findings to published “omics” data derived from Ft grown in vivo. Based on the abundance of ~1,000 proteins, the fingerprint of BHI-Ft is one of nutrient-deprived bacteria that—through induction of a stringent-starvation-like response—have induced the FevR regulon for expression of the bacterium's virulence factors, immuno-dominant antigens, and surface-carbohydrate synthases. To test the notion that increased abundance of dominant antigens expressed by BHI-Ft would render these bacteria more susceptible to pre-existing, vaccine-induced immunity, we employed a battery of LVS-vaccination and S4-challenge protocols using MHB- and BHI-grown Ft S4. Contrary to our hypothesis, these experiments reveal that LVS-immunization provides a barrier to infection that is significantly more effective against an MHB-S4 challenge than a BHI-S4 challenge. The differences in apparent virulence to immunized mice are profoundly greater than those observed with primary infection of naïve mice. Our findings suggest that tularemia vaccination studies should be critically evaluated in regard to the growth conditions of the challenge agent.

Iron and Virulence in Francisella tularensis

Francisella tularensis, the causative agent of tularemia, is a Gram-negative bacterium that infects a variety of cell types including macrophages, and propagates with great efficiency in the cytoplasm. Iron, essential for key enzymatic and redox reactions, is among the nutrients required to support this pathogenic lifestyle and the bacterium relies on specialized mechanisms to acquire iron within the host environment. Two distinct pathways for iron acquisition are encoded by the F. tularensis genome- a siderophore-dependent ferric iron uptake system and a ferrous iron transport system. Genes of the Fur-regulated fslABCDEF operon direct the production and transport of the siderophore rhizoferrin. Siderophore biosynthesis involves enzymes FslA and FslC, while export across the inner membrane is mediated by FslB. Uptake of the rhizoferrin- ferric iron complex is effected by the siderophore receptor FslE in the outer membrane in a TonB-independent process, and FslD is responsible for uptake across the inner membrane. Ferrous iron uptake relies largely on high affinity transport by FupA in the outer membrane, while the Fur-regulated FeoB protein mediates transport across the inner membrane. FslE and FupA are paralogous proteins, sharing sequence similarity and possibly sharing structural features as well. This review summarizes current knowledge of iron acquisition in this organism and the critical role of these uptake systems in bacterial pathogenicity.

FmvB: A Francisella tularensis Magnesium-Responsive Outer Membrane Protein that Plays a Role in Virulence

Francisella tularensis is the causative agent of the lethal disease tularemia. Despite decades of research, little is understood about why F. tularensis is so virulent. Bacterial outer membrane proteins (OMPs) are involved in various virulence processes, including protein secretion, host cell attachment, and intracellular survival. Many pathogenic bacteria require metals for intracellular survival and OMPs often play important roles in metal uptake. Previous studies identified three F. tularensis OMPs that play roles in iron acquisition. In this study, we examined two previously uncharacterized proteins, FTT0267 (named fmvA, for Francisellametal and virulence) and FTT0602c (fmvB), which are homologs of the previously studied F. tularensis iron acquisition genes and are predicted OMPs. To study the potential roles of FmvA and FmvB in metal acquisition and virulence, we first examined fmvA and fmvB expression following pulmonary infection of mice, finding that fmvB was upregulated up to 5-fold during F. tularensis infection of mice. Despite sequence homology to previously-characterized iron-acquisition genes, FmvA and FmvB do not appear to be involved iron uptake, as neither fmvA nor fmvB were upregulated in iron-limiting media and neither ΔfmvA nor ΔfmvB exhibited growth defects in iron limitation. However, when other metals were examined in this study, magnesium-limitation significantly induced fmvB expression, ΔfmvB was found to express significantly higher levels of lipopolysaccharide (LPS) in magnesium-limiting medium, and increased numbers of surface protrusions were observed on ΔfmvB in magnesium-limiting medium, compared to wild-type F. tularensis grown in magnesium-limiting medium. RNA sequencing analysis of ΔfmvB revealed the potential mechanism for increased LPS expression, as LPS synthesis genes kdtA and wbtA were significantly upregulated in ΔfmvB, compared with wild-type F. tularensis. To provide further evidence for the potential role of FmvB in magnesium uptake, we demonstrated that FmvB was outer membrane-localized. Finally, ΔfmvB was found to be attenuated in mice and cytokine analyses revealed that ΔfmvB-infected mice produced lower levels of pro-inflammatory cytokines, including GM-CSF, IL-3, and IL-10, compared with mice infected with wild-type F. tularensis. Taken together, although the function of FmvA remains unknown, FmvB appears to play a role in magnesium uptake and F. tularensis virulence. These results may provide new insights into the importance of magnesium for intracellular pathogens.

Elucidation of a mechanism of oxidative stress regulation in Francisella tularensis live vaccine strain

Francisella tularensis causes a lethal human disease known as tularemia. As an intracellular pathogen, Francisella survives and replicates in phagocytic cells, such as macrophages. However, to establish an intracellular niche, Francisella must overcome the oxidative stress posed by the reactive oxygen species (ROS) produced by the infected macrophages. OxyR and SoxR/S are two well-characterized transcriptional regulators of oxidative stress responses in several bacterial pathogens. Only the OxyR homolog is present in F. tularensis, while the SoxR homologs are absent. The functional role of OxyR has not been established in F. tularensis. We demonstrate that OxyR regulates oxidative stress responses and provides resistance against ROS, thereby contributing to the survival of the F. tularensis subsp. holarctica live vaccine strain (LVS) in macrophages and epithelial cells and contributing to virulence in mice. Proteomic analysis reveals the differential production of 128 proteins in the oxyR gene deletion mutant, indicating its global regulatory role in the oxidative stress response of F. tularensis. Moreover, OxyR regulates the transcription of the primary antioxidant enzyme genes by binding directly to their putative promoter regions. This study demonstrates that OxyR is an important virulence factor and transcriptional regulator of the oxidative stress response of the F. tularensis LVS.

Two parallel pathways for ferric and ferrous iron acquisition support growth and virulence of the intracellular pathogen Francisella tularensis Schu S4

Iron acquisition mechanisms in Francisella tularensis, the causative agent of tularemia, include the Francisella siderophore locus (fsl) siderophore operon and a ferrous iron–transport system comprising outer‐membrane protein FupA and inner‐membrane transporter FeoB. To characterize these mechanisms and to identify any additional iron uptake systems in the virulent subspecies tularensis, single and double deletions were generated in the fsl and feo iron acquisition systems of the strain Schu S4. Deletion of the entire fsl operon caused loss of siderophore production that could be restored by complementation with the biosynthetic genes fslA and fslC and Major Facilitator Superfamily (MFS) transporter gene fslB. ⁵⁵Fe‐transport assays demonstrated that siderophore‐iron uptake required the receptor FslE and MFS transporter FslD. A ΔfeoB′ mutation resulted in loss of ability to transport ferrous iron (⁵⁵Fe²⁺). A ΔfeoB′ ΔfslA mutant that required added exogenous siderophore for growth in vitro was unable to grow within tissue culture cells and was avirulent in mice, indicating that no compensatory cryptic iron uptake systems were induced in vivo. These studies demonstrate that the fsl and feo pathways function independently and operate in parallel to effectively support virulence of F. tularensis.

From the Outside-In: The Francisella tularensis Envelope and Virulence

Francisella tularensis is a highly-infectious bacterium that causes the rapid, and often lethal disease, tularemia. Many studies have been performed to identify and characterize the virulence factors that F. tularensis uses to infect a wide variety of hosts and host cell types, evade immune defenses, and induce severe disease and death. This review focuses on the virulence factors that are present in the F. tularensis envelope, including capsule, LPS, outer membrane, periplasm, inner membrane, secretion systems, and various molecules in each of aforementioned sub-compartments. Whereas, no single bacterial molecule or molecular complex single-handedly controls F. tularensis virulence, we review here how diverse bacterial systems work in conjunction to subvert the immune system, attach to and invade host cells, alter phagosome/lysosome maturation pathways, replicate in host cells without being detected, inhibit apoptosis, and induce host cell death for bacterial release and infection of adjacent cells. Given that the F. tularensis envelope is the outermost layer of the bacterium, we highlight herein how many of these molecules directly interact with the host to promote infection and disease. These and future envelope studies are important to advance our collective understanding of F. tularensis virulence mechanisms and offer targets for future vaccine development efforts.

Burkholderia Diffusible Signal Factor Signals to Francisella novicida To Disperse Biofilm and Increase Siderophore Production

In many bacteria, the ability to modulate biofilm production relies on specific signaling molecules that are either self-produced or made by neighboring microbes within the ecological niche. We analyzed the potential interspecies signaling effect of the Burkholderia diffusible signal factor (BDSF) on Francisella novicida, a model organism for Francisella tularensis, and demonstrated that BDSF both inhibits the formation and causes the dispersion of Francisella biofilm. Specificity was demonstrated for the cis versus the trans form of BDSF. Using transcriptome sequencing, quantitative reverse transcription-PCR, and activity assays, we found that BDSF altered the expression of many F. novicida genes, including genes involved in biofilm formation, such as chitinases. Using a chitinase inhibitor, the antibiofilm activity of BDSF was also shown to be chitinase dependent. In addition, BDSF caused an increase in RelA expression and increased levels of (p)ppGpp, leading to decreased biofilm production. These results support our observation that exposure of F. novicida to BDSF causes biofilm dispersal. Furthermore, BDSF upregulated the genes involved in iron acquisition (figABCD), increasing siderophore production. Thus, this study provides evidence for a potential role and mechanism of diffusible signal factor (DSF) signaling in the genus Francisella and suggests the possibility of interspecies signaling between Francisella and other bacteria. Overall, this study suggests that in response to the interspecies DSF signal, F. novicida can alter its gene expression and regulate its biofilm formation.

Generating Isogenic Deletions (Knockouts) in Francisella tularensis, a Highly-infectious and Fastidious Gram-negative Bacterium

Generating bacterial gene deletion mutants, also known as knockouts (KOs), is a powerful tool to investigate individual gene functions. However, fastidious bacteria such as Francisella tularensis (F. tularensis) often are difficult to genetically manipulate. Indeed, many different approaches have been tested to generate F. tularensis mutants. First, Tn5-based EZ::TN transposons have been successfully used to generate transposon libraries in F. tularensis (Qin and Mann, 2006; Weiss et al., 2007). However, creating a comprehensive transposon library with saturating mutations can be laborious, screening for gene disruption requires high-throughput assays where known phenotypes can be measured, and transposons may not completely inactivate the gene of interest or may alter downstream gene expression. Second, group II introns (also referred to as Targetron) have been used to inactivate F. tularensis genes of interest (Rodriguez et al., 2008; Rodriguez et al., 2009). Targetron functions by forming a complex between plasmid-encoded RNA and chromosomal DNA, followed by group II intron insertion into the gene of interest. The main advantage of Targetron is that it does not require an antibiotic resistance marker. However, as noted for transposons, targetron gene insertions may not eliminate all gene functions or may affect downstream gene expression. Third, homologous recombination can be used to completely replace the chromosomal target gene with a selectable marker, such as an antibiotic resistance marker. This classical genetic technique has been used in many F. tularensis studies (Ramakrishnan et al., 2008; Ren et al., 2014; Mohapatra et al., 2008; Robertson et al., 2013). To accomplish this, a suicide plasmid is engineered to include a selectable marker flanked by regions upstream and downstream of the gene of interest. This KO plasmid can be delivered into host bacteria by many methods, including electroporation, chemical transformation, or conjugation. Here, we describe an optimized procedure to generate KO plasmid constructs, use E. coli to conjugatively transfer the plasmid into F. tularensis, select for F. tularensis KOs using a series of kanamycin-, hygromycin-, and sucrose-resistance steps, and confirm that the gene of interest has been deleted (general overview of the knockout protocol diagramed in Figure 1). This optimized procedure is relatively simple, rapid, and, more importantly, includes a series of both positive and negative selection steps to increase the chances of deleting a target gene from F. tularensis.

The Yersinia pestis siderophore, yersiniabactin, and the ZnuABC system both contribute to zinc acquisition and the development of lethal septicaemic plague in mice

Bacterial pathogens must overcome host sequestration of zinc (Zn(2+) ), an essential micronutrient, during the infectious disease process. While the mechanisms to acquire chelated Zn(2+) by bacteria are largely undefined, many pathogens rely upon the ZnuABC family of ABC transporters. Here we show that in Yersinia pestis, irp2, a gene encoding the synthetase (HMWP2) for the siderophore yersiniabactin (Ybt) is required for growth under Zn(2+) -deficient conditions in a strain lacking ZnuABC. Moreover, growth stimulation with exogenous, purified apo-Ybt provides evidence that Ybt may serve as a zincophore for Zn(2+) acquisition. Studies with the Zn(2+) -dependent transcriptional reporter znuA::lacZ indicate that the ability to synthesize Ybt affects the levels of intracellular Zn(2+) . However, the outer membrane receptor Psn and TonB as well as the inner membrane (IM) ABC transporter YbtPQ, which are required for Fe(3+) acquisition by Ybt, are not needed for Ybt-dependent Zn(2+) uptake. In contrast, the predicted IM protein YbtX, a member of the Major Facilitator Superfamily, was essential for Ybt-dependent Zn(2+) uptake. Finally, we show that the ZnuABC system and the Ybt synthetase HMWP2, presumably by Ybt synthesis, both contribute to the development of a lethal infection in a septicaemic plague mouse model.

The reduced genome of the Francisella tularensis live vaccine strain (LVS) encodes two iron acquisition systems essential for optimal growth and virulence

Bacterial pathogens require multiple iron-specific acquisition systems for survival within the iron-limiting environment of the host. Francisella tularensis is a virulent intracellular pathogen that can replicate in multiple cell-types. To study the interrelationship of iron acquisition capability and virulence potential of this organism, we generated single and double deletion mutants within the ferrous iron (feo) and ferric-siderophore (fsl) uptake systems of the live vaccine strain (LVS). The Feo system was disrupted by a partial deletion of the feoB gene (ΔfeoB′), which led to a growth defect on iron-limited modified Muller Hinton agar plates. ⁵⁵Fe uptake assays verified that the ΔfeoB′ mutant had lost the capacity for ferrous iron uptake but was still competent for ⁵⁵Fe-siderophore-mediated ferric iron acquisition. Neither the ΔfeoB′ nor the siderophore-deficient ΔfslA mutant was defective for replication within J774A.1 murine macrophage-like cells, thus demonstrating the ability of LVS to survive using either ferrous or ferric sources of intracellular iron. A LVS ΔfslA ΔfeoB′ mutant defective for both ferrous iron uptake and siderophore production was isolated in the presence of exogenous F. tularensis siderophore. In contrast to the single deletion mutants, the ΔfslA ΔfeoB′ mutant was unable to replicate within J774A.1 cells and was attenuated in virulence following intraperitoneal infection of C57BL/6 mice. These studies demonstrate that the siderophore and feoB-mediated ferrous uptake systems are the only significant iron acquisition systems in LVS and that they operate independently. While one system can compensate for loss of the other, both are required for optimal growth and virulence.

The FupA/B protein uniquely facilitates transport of ferrous iron and siderophore-associated ferric iron across the outer membrane of Francisella tularensis live vaccine strain

Francisella tularensis is a highly infectious Gram-negative pathogen that replicates intracellularly within the mammalian host. One of the factors associated with virulence of F. tularensis is the protein FupA that mediates high-affinity transport of ferrous iron across the outer membrane. Together with its paralogue FslE, a siderophore-ferric iron transporter, FupA supports survival of the pathogen in the host by providing access to the essential nutrient iron. The FupA orthologue in the attenuated live vaccine strain (LVS) is encoded by the hybrid gene fupA/B, the product of an intergenic recombination event that significantly contributes to attenuation of the strain. We used (55)Fe transport assays with mutant strains complemented with the different paralogues to show that the FupA/B protein of LVS retains the capacity for high-affinity transport of ferrous iron, albeit less efficiently than FupA of virulent strain Schu S4. (55)Fe transport assays using purified siderophore and siderophore-dependent growth assays on iron-limiting agar confirmed previous findings that FupA/B also contributes to siderophore-mediated ferric iron uptake. These assays further demonstrated that the LVS FslE protein is a weaker siderophore-ferric iron transporter than the orthologue from Schu S4, and may be a result of the sequence variation between the two proteins. Our results indicate that iron-uptake mechanisms in LVS differ from those in Schu S4 and that functional differences in the outer membrane iron transporters have distinct effects on growth under iron limitation.

FeoB-mediated uptake of iron by Francisella tularensis

Francisella tularensis, the bacterial cause of tularemia, infects the liver and replicates in hepatocytes in vivo and in vitro. However, the factors that govern adaptation of F. tularensis to the intrahepatocytic niche have not been identified. Using cDNA microarrays, we determined the transcriptional profile of the live vaccine strain (LVS) of F. tularensis grown in the FL83B murine hepatocytic cell line compared to that of F. tularensis cultured in broth. The fslC gene of the fsl operon was the most highly upregulated. Deletion of fslC eliminated the ability of the LVS to produce siderophore, which is involved in uptake of ferric iron, but it did not impair its growth in hepatocytes, A549 epithelial cells, or macrophages. Therefore, we sought an alternative means by which F. tularensis might obtain iron. Deletion of feoB, which encodes a putative ferrous iron transporter, retarded replication of the LVS in iron-restricted media, reduced its growth in hepatocytic and epithelial cells, and impaired its acquisition of iron. Survival of mice infected intradermally with a lethal dose of the LVS was slightly improved by deletion of fslC but was not altered by loss of feoB. However, the ΔfeoB mutant showed diminished ability to colonize the lungs, liver, and spleen of mice that received sublethal inocula. Thus, FeoB represents a previously unidentified mechanism for uptake of iron by F. tularensis. Moreover, failure to produce a mutant strain lacking both feoB and fslC suggests that FeoB and the proteins of the fsl operon are the only major means by which F. tularensis acquires iron.

Proteins involved in Francisella tularensis survival and replication inside macrophages

Francisella tularensis, the etiological agent of tularemia, is a member of the γ-proteobacteria class of Gram-negative bacteria. This highly virulent bacterium can infect a large range of mammalian species and has been recognized as a human pathogen for a century. F. tularensis is able to survive in vitro in a variety of cell types. In vivo, the bacterium replicates mainly in infected macrophages, using the cytoplasmic compartment as a replicative niche. To successfully adapt to this stressful environment, F. tularensis must simultaneously: produce and regulate the expression of a series of dedicated virulence factors; adapt its metabolic needs to the nutritional context of the host cytosol; and control the innate immune cytosolic surveillance pathways to avoid premature cell death. We will focus here on the secretion or release of bacterial proteins in the host, as well as on the envelope proteins, involved in bacterial survival inside macrophages.

Evaluation of reference genes for reverse transcription quantitative PCR analyses of fish-pathogenic Francisella strains exposed to different growth conditions

Background

Reverse transcription quantitative PCR has become a powerful technique to monitor mRNA transcription in response to different environmental conditions in many bacterial species. However, correct evaluation of data requires accurate and reliable use of reference genes whose transcription does not change during the course of the experiment. In the present study exposure to different growth conditions was used to validate the transcription stability of eight reference gene candidates in three strains from two subspecies of Francisella noatunensis, a pathogen causing disease in both warm and cold water fish species.

Results

Relative transcription levels for genes encoding DNA gyrase (gyrA), RNA polymerase beta subunit (rpoB), DNA polymerase I (polA), cell division protein (ftsZ), outer membrane protein (fopA), riboflavin biosynthesis protein (ribC), 16S ribosomal RNA (16S rRNA) and DNA helicases (uvrD) were quantified under exponential, stationary and iron-restricted growth conditions. The suitability of selected reference genes for reliable interpretation of gene expression data was tested using the virulence-associated intracellular growth locus subunit C (iglC) gene.

Conclusion

Although the transcription stability of the reference genes was slightly different in the three strains studied, fopA, ftsZ and polA proved to be the most stable and suitable for normalization of gene transcription in Francisella noatunensis ssp.

Subversion of host recognition and defense systems by Francisella spp

Francisella tularensis is a gram-negative intracellular pathogen and the causative agent of the disease tularemia. Inhalation of as few as 10 bacteria is sufficient to cause severe disease, making F. tularensis one of the most highly virulent bacterial pathogens. The initial stage of infection is characterized by the "silent" replication of bacteria in the absence of a significant inflammatory response. Francisella achieves this difficult task using several strategies: (i) strong integrity of the bacterial surface to resist host killing mechanisms and the release of inflammatory bacterial components (pathogen-associated molecular patterns [PAMPs]), (ii) modification of PAMPs to prevent activation of inflammatory pathways, and (iii) active modulation of the host response by escaping the phagosome and directly suppressing inflammatory pathways. We review the specific mechanisms by which Francisella achieves these goals to subvert host defenses and promote pathogenesis, highlighting as-yet-unanswered questions and important areas for future study.

NaxD is a deacetylase required for lipid A modification and Francisella pathogenesis

Modification of specific Gram-negative bacterial cell envelope components, such as capsule, O-antigen and lipid A, are often essential for the successful establishment of infection. Francisella species express lipid A molecules with unique characteristics involved in circumventing host defences, which significantly contribute to their virulence. In this study, we show that NaxD, a member of the highly conserved YdjC superfamily, is a deacetylase required for an important modification of the outer membrane component lipid A in Francisella. Mass spectrometry analysis revealed that NaxD is essential for the modification of a lipid A phosphate with galactosamine in Francisella novicida, a model organism for the study of highly virulent Francisella tularensis. Significantly, enzymatic assays confirmed that this protein is necessary for deacetylation of its substrate. In addition, NaxD was involved in resistance to the antimicrobial peptide polymyxin B and critical for replication in macrophages and in vivo virulence. Importantly, this protein is also required for lipid A modification in F. tularensis as well as Bordetella bronchiseptica. Since NaxD homologues are conserved among many Gram-negative pathogens, this work has broad implications for our understanding of host subversion mechanisms of other virulent bacteria.

Genetic diversity within the genus Francisella as revealed by comparative analyses of the genomes of two North American isolates from environmental sources

Background

Francisella tularensis is an intracellular pathogen that causes tularemia in humans and the public health importance of this bacterium has been well documented in recent history. Francisella philomiragia, a distant relative of F. tularensis, is thought to constitute an environmental lineage along with Francisella novicida. Nevertheless, both F. philomiragia and F. novicida have been associated with human disease, primarily in immune-compromised individuals. To understand the genetic relationships and evolutionary contexts among different lineages within the genus Francisella, the genome of Francisella spp. strain TX07-7308 was sequenced and compared to the genomes of F. philomiragia strains ATCC 25017 and 25015, F. novicida strain U112, and F. tularensis strain Schu S4.

Results

The size of strain ATCC 25017 chromosome was 2,045,775 bp and contained 1,983 protein-coding genes. The size of strain TX07-7308 chromosome was 2,035,931 bp and contained 1,980 protein-coding genes. Pairwise BLAST comparisons indicated that strains TX07-7308 and ATCC 25017 contained 1,700 protein coding genes in common. NUCmer analyses revealed that the chromosomes of strains TX07-7308 and ATCC 25017 were mostly collinear except for a few gaps, translocations, and/or inversions. Using the genome sequence data and comparative analyses with other members of the genus Francisella (e.g., F. novicida strain U112 and F. tularensis strain Schu S4), several strain-specific genes were identified. Strains TX07-7308 and ATCC 25017 contained an operon with six open reading frames encoding proteins related to enzymes involved in thiamine biosynthesis that was absent in F. novicida strain U112 and F. tularensis strain Schu S4. Strain ATCC 25017 contained an operon putatively involved in lactose metabolism that was absent in strain TX07-7308, F. novicida strain U112, and F. tularensis strain Schu S4. In contrast, strain TX07-7308 contained an operon putatively involved in glucuronate metabolism that was absent in the genomes of strain ATCC 25017, F. novicida strain U112, and F. tularensis strain Schu S4. The polymorphic nature of polysaccharide biosynthesis/modification gene clusters among different Francisella strains was also evident from genome analyses.

Conclusions

From genome comparisons, it appeared that genes encoding novel functions have contributed to the metabolic enrichment of the environmental lineages within the genus Francisella. The inability to acquire new genes coupled with the loss of ancestral traits and the consequent reductive evolution may be a cause for, as well as an effect of, niche selection of F. tularensis. Sequencing and comparison of the genomes of more isolates are required to obtain further insights into the ecology and evolution of different species within the genus Francisella.

Paralogous outer membrane proteins mediate uptake of different forms of iron and synergistically govern virulence in Francisella tularensis tularensis

Francisella tularensis subsp. tularensis is a highly infectious bacterium causing acute disease in mammalian hosts. Mechanisms for the acquisition of iron within the iron-limiting host environment are likely to be critical for survival of this intracellular pathogen. FslE (FTT0025) and FupA (FTT0918) are paralogous proteins that are predicted to form β-barrels in the outer membrane of virulent strain Schu S4 and are unique to Francisella species. Previous studies have implicated both FupA, initially identified as a virulence factor and FslE, encoded by the siderophore biosynthetic operon, in iron acquisition. Using single and double mutants, we demonstrated that these paralogs function in concert to promote growth under iron limitation. We used a (55)Fe transport assay to demonstrate that FslE is involved in siderophore-mediated ferric iron uptake, whereas FupA facilitates high affinity ferrous iron uptake. Optimal replication within J774A.1 macrophage-like cells required at least one of these uptake systems to be functional. In a mouse model of tularemia, the ΔfupA mutant was attenuated, but the ΔfslE ΔfupA mutant was significantly more attenuated, implying that the two systems of iron acquisition function synergistically to promote virulence. These studies highlight the importance of specific iron acquisition functions, particularly that of ferrous iron, for virulence of F. tularensis in the mammalian host.

The role of MglA for adaptation to oxidative stress of Francisella tularensis LVS

BACKGROUND

The Francisella tularensis protein MglA performs complex regulatory functions since it influences the expression of more than 100 genes and proteins in F. tularensis. Besides regulating the igl operon, it has been suggested that it also regulates several factors such as SspA, Hfq, CspC, and UspA, all important to stress adaptation. Therefore, it can be hypothesized that MglA plays an important role for Francisella stress responses in general and for the oxidative stress response specifically.

RESULTS

We investigated the oxidative stress response of the ΔmglA mutant of the live vaccine strain (LVS) of F. tularensis and found that it showed markedly diminished growth and contained more oxidized proteins than the parental LVS strain when grown in an aerobic milieu but not when grown microaerobically. Moreover, the ΔmglA mutant exhibited an increased catalase activity and reduced expression of the fsl operon and feoB in the aerobic milieu. The mutant was also found to be less susceptible to H(2)O(2). The aberrant catalase activity and gene expression was partially normalized when the ΔmglA mutant was grown in a microaerobic milieu.

CONCLUSIONS

Altogether the results show that the ΔmglA mutant exhibits all the hallmarks of a bacterium subjected to oxidative stress under aerobic conditions, indicating that MglA is required for normal adaptation of F. tularensis to oxidative stress and oxygen-rich environments.

Macrophage replication screen identifies a novel Francisella hydroperoxide resistance protein involved in virulence

Francisella tularensis is a Gram-negative facultative intracellular pathogen and the causative agent of tularemia. Recently, genome-wide screens have identified Francisella genes required for virulence in mice. However, the mechanisms by which most of the corresponding proteins contribute to pathogenesis are still largely unknown. To further elucidate the roles of these virulence determinants in Francisella pathogenesis, we tested whether each gene was required for replication of the model pathogen F. novicida within macrophages, an important virulence trait. Fifty-three of the 224 genes tested were involved in intracellular replication, including many of those within the Francisella pathogenicity island (FPI), validating our results. Interestingly, over one third of the genes identified are annotated as hypothetical, indicating that F. novicida likely utilizes novel virulence factors for intracellular replication. To further characterize these virulence determinants, we selected two hypothetical genes to study in more detail. As predicted by our screen, deletion mutants of FTN_0096 and FTN_1133 were attenuated for replication in macrophages. The mutants displayed differing levels of attenuation in vivo, with the FTN_1133 mutant being the most attenuated. FTN_1133 has sequence similarity to the organic hydroperoxide resistance protein Ohr, an enzyme involved in the bacterial response to oxidative stress. We show that FTN_1133 is required for F. novicida resistance to, and degradation of, organic hydroperoxides as well as resistance to the action of the NADPH oxidase both in macrophages and mice. Furthermore, we demonstrate that F. holarctica LVS, a strain derived from a highly virulent human pathogenic species of Francisella, also requires this protein for organic hydroperoxide resistance as well as replication in macrophages and mice. This study expands our knowledge of Francisella's largely uncharacterized intracellular lifecycle and demonstrates that FTN_1133 is an important novel mediator of oxidative stress resistance.

Impact of Francisella tularensis pilin homologs on pilus formation and virulence

Francisella tularensis is a facultative intracellular bacterium and the causative agent of tularemia. Virulence factors for this bacterium, particularly those that facilitate host cell interaction, remain largely uncharacterized. However, genes homologous to those involved in type IV pilus structure and assembly, including six genes encoding putative major pilin subunit proteins, are present in the genome of the highly virulent Schu S4 strain. To analyze the roles of three putative pilin genes in pili structure and function we constructed individual pilE4, pilE5, and pilE6 deletion mutants in both the F. tularensis tularensis strain Schu S4 and the Live Vaccine Strain (LVS), an attenuated derivative strain of F. tularensis holarctica. Transmission electron microscopy (TEM) of Schu S4 and LVS wild-type and deletion strains confirmed that pilE4 was essential for the expression of type IV pilus-like fibers by both subspecies. By the same method, pilE5 and pilE6 were dispensable for pilus production. In vitro adherence assays with J774A.1 cells revealed that LVS pilE4, pilE5, and pilE6 deletion mutants displayed increased attachment compared to wild-type LVS. However, in the Schu S4 background, similar deletion mutants displayed adherence levels similar to wild-type. In vivo, LVS pilE5 and pilE6 deletion mutants were significantly attenuated compared to wild-type LVS by intradermal and subcutaneous murine infection, while no Schu S4 deletion mutant was significantly attenuated compared to wild-type Schu S4. While pilE4 was essential for fiber expression on both Schu S4 and LVS, neither its protein product nor the assembled fibers contributed significantly to virulence in mice. Absent a role in pilus formation, we speculate PilE5 and PilE6 are pseudopilin homologs that comprise, or are associated with, a novel type II-related secretion system in Schu S4 and LVS.

Common ancestry and novel genetic traits of Francisella novicida-like isolates from North America and Australia as revealed by comparative genomic analyses

Francisella novicida is a close relative of Francisella tularensis, the causative agent of tularemia. The genomes of F. novicida-like clinical isolates 3523 (Australian strain) and Fx1 (Texas strain) were sequenced and compared to F. novicida strain U112 and F. tularensis strain Schu S4. The strain 3523 chromosome is 1,945,310 bp and contains 1,854 protein-coding genes. The strain Fx1 chromosome is 1,913,619 bp and contains 1,819 protein-coding genes. NUCmer analyses revealed that the genomes of strains Fx1 and U112 are mostly colinear, whereas the genome of strain 3523 has gaps, translocations, and/or inversions compared to genomes of strains Fx1 and U112. Using the genome sequence data and comparative analyses with other members of the genus Francisella, several strain-specific genes that encode putative proteins involved in RTX toxin production, polysaccharide biosynthesis/modification, thiamine biosynthesis, glucuronate utilization, and polyamine biosynthesis were identified. The RTX toxin synthesis and secretion operon of strain 3523 contains four open reading frames (ORFs) and was named rtxCABD. Based on the alignment of conserved sequences upstream of operons involved in thiamine biosynthesis from various bacteria, a putative THI box was identified in strain 3523. The glucuronate catabolism loci of strains 3523 and Fx1 contain a cluster of nine ORFs oriented in the same direction that appear to constitute an operon. Strains U112 and Schu S4 appeared to have lost the loci for RTX toxin production, thiamine biosynthesis, and glucuronate utilization as a consequence of host adaptation and reductive evolution. In conclusion, comparative analyses provided insights into the common ancestry and novel genetic traits of these strains.

Legionella pneumophila LbtU acts as a novel, TonB-independent receptor for the legiobactin siderophore

Gram-negative Legionella pneumophila produces a siderophore (legiobactin) that promotes lung infection. We previously determined that lbtA and lbtB are required for the synthesis and secretion of legiobactin. DNA sequence and reverse transcription-PCR (RT-PCR) analyses now reveal the presence of an iron-repressed gene (lbtU) directly upstream of the lbtAB-containing operon. In silico analysis predicted that LbtU is an outer membrane protein consisting of a 16-stranded transmembrane β-barrel, multiple extracellular domains, and short periplasmic tails. Immunoblot analysis of cell fractions confirmed an outer membrane location for LbtU. Although replicating normally in standard media, lbtU mutants, like lbtA mutants, were impaired for growth on iron-depleted agar media. While producing typical levels of legiobactin, lbtU mutants were unable to use supplied legiobactin to stimulate growth on iron-depleted media and displayed an inability to take up iron. Complemented lbtU mutants behaved as the wild type did. The lbtU mutants were also impaired for infection in a legiobactin-dependent manner. Together, these data indicate that LbtU is involved in the uptake of legiobactin and, based upon its location, is most likely the Legionella siderophore receptor. The sequence and predicted two-dimensional (2D) and 3D structures of LbtU were distinct from those of all known siderophore receptors, which generally contain a 22-stranded β-barrel and an extended N terminus that binds TonB in order to transduce energy from the inner membrane. This observation coupled with the fact that L. pneumophila does not encode TonB suggests that LbtU is a new type of receptor that participates in a form of iron uptake that is mechanistically distinct from the existing paradigm.

Regulation of francisella tularensis virulence

Francisella tularensis is one of the most virulent bacteria known and a Centers for Disease Control and Prevention Category A select agent. It is able to infect a variety of animals and insects and can persist in the environment, thus Francisella spp. must be able to survive in diverse environmental niches. However, F. tularensis has a surprising dearth of sensory and regulatory factors. Recent advancements in the field have identified new functions of encoded transcription factors and greatly expanded our understanding of virulence gene regulation. Here we review the current knowledge of environmental adaptation by F. tularensis, its transcriptional regulators and their relationship to animal virulence.

Genetic manipulation of francisella tularensis

Francisella tularensis is a facultative intracellular pathogen that causes the disease tularemia. F. tularensis subsp. tularensis causes the most severe disease in humans and has been classified as a Category A select agent and potential bioweapon. There is currently no vaccine approved for human use, making genetic manipulation of this organism critical to unraveling the genetic basis of pathogenesis and developing countermeasures against tularemia. The development of genetic techniques applicable to F. tularensis have lagged behind those routinely used for other bacteria, primarily due to lack of research and the restricted nature of the biocontainment required for studying this pathogen. However, in recent years, genetic techniques, such as transposon mutagenesis and targeted gene disruption, have been developed, that have had a dramatic impact on our understanding of the genetic basis of F. tularensis virulence. In this review, we describe some of the methods developed for genetic manipulation of F. tularensis.

Iron content differs between Francisella tularensis subspecies tularensis and subspecies holarctica strains and correlates to their susceptibility to H(2)O(2)-induced killing

Francisella tularensis, the causative agent of tularemia, is one of the most infectious bacterial pathogens known and is classified as a category A select agent and a facultative intracellular bacterium. Why F. tularensis subsp. tularensis causes a more severe form of tularemia than F. tularensis subsp. holarctica does is not known. In this study, we have identified prominent phenotypic differences between the subspecies, since we found that F. tularensis subsp. tularensis strains contained less iron than F. tularensis subsp. holarctica strains. Moreover, strain SCHU S4 of F. tularensis subsp. tularensis was less susceptible than FSC200 and the live vaccine strain (LVS) of F. tularensis subsp. holarctica to H(2)O(2)-induced killing. The activity of the H(2)O(2)-degrading enzyme catalase was similar between the strains, whereas the iron content affected their susceptibility to H(2)O(2), since iron starvation rendered F. tularensis subsp. holarctica strains more resistant to H(2)O(2). Complementing LVS with fupA, which encodes an important virulence factor that regulates iron uptake, reduced its iron content and increased the resistance to H(2)O(2)-mediated killing. By real-time PCR, it was demonstrated that FSC200 and LVS expressed higher levels of gene transcripts related to iron uptake and storage than SCHU S4 did, and this likely explained their high iron content. Together, the results suggest that F. tularensis subsp. tularensis strains have restricted iron uptake and storage, which is beneficial for their resistance to H(2)O(2)-induced killing. This may be an important factor for the higher virulence of this subspecies of F. tularensis, as reactive oxygen species, such as H(2)O(2), are important bactericidal components during tularemia.

Francisella tularensis metabolism and its relation to virulence

Francisella tularensis is a Gram-negative bacterium capable of causing the zoonotic disease tularaemia in a large number of mammalian species and in arthropods. F. tularensis is a facultative intracellular bacterium that infects and replicates in vivo mainly inside macrophages. During its systemic dissemination, F. tularensis must cope with very different life conditions (such as survival in different target organs or tissues and/or survival in the blood stream…) and may thus encounter a broad variety of carbon substrates, nitrogen, phosphor, and sulfur sources, as well as very low concentrations of essential ions. The development of recent genome-wide genetic screens have led to the identification of hundreds of genes participating to variable extents to Francisella virulence. Remarkably, an important proportion of the genes identified are related to metabolic and nutritional functions. However, the relationship between nutrition and the in vivo life cycle of F. tularensis is yet poorly understood. In this review, we will address the importance of metabolism and nutrition for F. tularensis pathogenesis, focusing specifically on amino acid and carbohydrate requirements.

The fslE homolog, FTL_0439 (fupA/B), mediates siderophore-dependent iron uptake in Francisella tularensis LVS

The Gram-negative pathogen Francisella tularensis secretes a siderophore to obtain essential iron by a TonB-independent mechanism. The fslABCDE locus, encoding siderophore-related functions, is conserved among different Francisella strains. In the virulent strain Schu S4, fslE is essential for siderophore utilization and for growth under conditions of iron limitation. In contrast, we found that deletion of fslE did not affect siderophore utilization by the attenuated live vaccine strain (LVS). We found that one of the fslE paralogs encoded in the LVS genome, FTL_0439 (fupA/B), was able to partially complement a Schu S4 ΔfslE mutant for siderophore utilization. We generated a deletion of fupA/B in LVS and in the LVS ΔfslE background. The ΔfupA/B mutant showed reduced growth under conditions of iron limitation. It was able to secrete but was unable to utilize siderophore. Mutation of both fupA/B and fslE resulted in a growth defect of greater severity. The ΔfupA/B mutants showed a replication defect in J774.1A cells and decreased virulence following intraperitoneal infection in mice. Complementation of the ΔfupA/B mutation in cis restored the ability to utilize siderophore and concomitantly restored virulence. Our results indicate that fupA/B plays a significant role in the siderophore-mediated iron uptake mechanism of LVS whereas fslE appears to play a secondary role. Variation in iron acquisition mechanisms may contribute to virulence differences between the strains.

Working toward the future: insights into Francisella tularensis pathogenesis and vaccine development

Francisella tularensis is a facultative intracellular gram-negative pathogen and the etiological agent of the zoonotic disease tularemia. Recent advances in the field of Francisella genetics have led to a rapid increase in both the generation and subsequent characterization of mutant strains exhibiting altered growth and/or virulence characteristics within various model systems of infection. In this review, we summarize the major properties of several Francisella species, including F. tularensis and F. novicida, and provide an up-to-date synopsis of the genes necessary for pathogenesis by these organisms and the determinants that are currently being targeted for vaccine development.

Gallium disrupts iron uptake by intracellular and extracellular Francisella strains and exhibits therapeutic efficacy in a murine pulmonary infection model

Francisella tularensis requires iron (Fe) for growth, but the biologic sources of Fe for this organism are largely unknown. We found that Francisella sp. growing in broth culture or within human macrophages can acquire Fe from the two major host Fe-binding proteins, lactoferrin (Lf) and transferrin (Tf). Fe acquisition is a potential target for novel therapies. Gallium (Ga) is a transition metal that interferes with cellular Fe metabolism by competing with Fe for uptake/utilization. Growth of either F. tularensis live vaccine strain (LVS) or Francisella novicida was inhibited by >or=2 microM Ga chelated to Tf or Lf, with GaLf being somewhat more potent. Francisella spp. express two Fe-containing antioxidant enzymes, catalase (KatG) and Fe cofactored superoxide dismutase (FeSOD). Growth of LVS with 10 muM GaTf or GaLf led to a dramatic decrease in bacterial catalase activity and in FeSOD activity that was associated with an increased susceptibility to H(2)O(2). Ga also protected mice from intranasal challenge with F. novicida. Whereas 100% of the F. novicida-infected mice died by day 9, 75% of the mice receiving Ga continued to survive to at least day 15. Thus, a single intranasal dose of Ga followed by daily intraperitoneal Ga at a dose tolerated by the animals resulted in prolonged survival. These data support the potential utility of Ga as a therapy for F. tularensis infection of the lung.

Environmental adaptation of Francisella tularensis

Concerns over weaponizable bacteria have recently prompted considerable interest in Francisella tularensis (Ft). In addition to its potential illicit use, Ft occurs naturally in diverse ecological niches including mammals, arthropods, and fresh water protozoans. Here we review the current knowledge of Ft adaptation which has ramifications for both basic and applied research.

The 58-kilodalton major virulence factor of Francisella tularensis is required for efficient utilization of iron

We investigated the role of the 58-kDa FTT0918 protein in the iron metabolism of Francisella tularensis. The phenotypes of SCHU S4, a prototypic strain of F. tularensis subsp. tularensis, and the Delta FTT0918 and Delta fslA isogenic mutants were analyzed. The gene product missing in the Delta fslA mutant is responsible for synthesis of a siderophore. When grown in broth with various iron concentrations, the two deletion mutants generally reached lower maximal densities than SCHU S4. The Delta FTT0918 mutant, but not the Delta fslA mutant, upregulated the genes of the F. tularensis siderophore locus (fsl) operon even at high iron concentrations. A chrome azurol sulfonate plate assay confirmed siderophore production by all strains except the Delta fslA strain. In a cross-feeding experiment using medium devoid of free iron, SCHU S4 promoted growth of the Delta fslA strain but not of the Delta FTT0918 strain. The sensitivity of SCHU S4 and the Delta FTT0918 and Delta fslA strains to streptonigrin demonstrated that the Delta FTT0918 strain contained a smaller free intracellular iron pool and that the Delta fslA strain contained a larger one than SCHU S4. In contrast to the marked attenuation of the Delta FTT0918 strain, the Delta fslA strain was as virulent as SCHU S4 in a mouse model. Altogether, the data demonstrate that the FTT0918 protein is required for F. tularensis to utilize iron bound to siderophores and that it likely has a role also in siderophore-independent iron acquisition. We suggest that the FTT0918 protein be designated Fe utilization protein A, FupA.

Intracellular biology and virulence determinants of Francisella tularensis revealed by transcriptional profiling inside macrophages

Summary The highly infectious bacterium Francisella tularensis is a facultative intracellular pathogen, whose virulence requires proliferation inside host cells, including macrophages. Here we have performed a global transcriptional profiling of the highly virulent F. tularensis ssp. tularensis Schu S4 strain during its intracellular cycle within primary murine macrophages, to characterize its intracellular biology and identify pathogenic determinants based on their intracellular expression profiles. Phagocytosed bacteria rapidly responded to their intracellular environment and subsequently altered their transcriptional profile. Differential gene expression profiles were revealed that correlated with specific intracellular locale of the bacteria. Upregulation of general and oxidative stress response genes was a hallmark of the early phagosomal and late endosomal stages, while induction of transport and metabolic genes characterized the cytosolic replication stage. Expression of the Francisella Pathogenicity Island (FPI) genes, which are required for intracellular proliferation, increased during the intracellular cycle. Similarly, 27 chromosomal loci encoding putative hypothetical, secreted, outer membrane proteins or transcriptional regulators were identified as upregulated. Among these, deletion of FTT0383, FTT0369c or FTT1676 abolished the ability of Schu S4 to survive or proliferate intracellularly and cause lethality in mice, therefore identifying novel determinants of Francisella virulence from their intracellular expression profile.

Purification of Legiobactin and importance of this siderophore in lung infection by Legionella pneumophila

When cultured in a low-iron medium, Legionella pneumophila secretes a siderophore (legiobactin) that is both reactive in the chrome azurol S (CAS) assay and capable of stimulating the growth of iron-starved legionellae. Using anion-exchange high-pressure liquid chromatography (HPLC), we purified legiobactin from culture supernatants of a virulent strain of L. pneumophila. In the process, we detected the ferrated form of legiobactin as well as other CAS-reactive substances. Purified legiobactin had a yellow-gold color and absorbed primarily from 220 nm and below. In accordance, nuclear magnetic resonance spectroscopy revealed that legiobactin lacks aromatic carbons, and among the 13 aliphatics present, there were 3 carbonyls. When examined by HPLC, supernatants from L. pneumophila mutants inactivated for lbtA and lbtB completely lacked legiobactin, indicating that the LbtA and LbtB proteins are absolutely required for siderophore activity. Independently derived lbtA mutants, but not a complemented derivative, displayed a reduced ability to infect the lungs of A/J mice after intratracheal inoculation, indicating that legiobactin is required for optimal intrapulmonary survival by L. pneumophila. This defect, however, was not evident when the lbtA mutant and its parental strain were coinoculated into the lung, indicating that legiobactin secreted by the wild type can promote growth of the mutant in trans. Legiobactin mutants grew normally in murine lung macrophages and alveolar epithelial cells, suggesting that legiobactin promotes something other than intracellular infection of resident lung cells. Overall, these data represent the first documentation of a role for siderophore expression in the virulence of L. pneumophila.