Francisella tularensis Schu S4 lipopolysaccharide core sugar and O-antigen mutants are attenuated in a mouse model of tularemia

Mutation of wbtJ, a N-formyltransferase involved in O-antigen synthesis, results in biofilm formation, phase variation and attenuation in Francisella tularensis

Two clinically important subspecies, Francisella tularensis subsp. tularensis (type A) and F. tularensis subsp. holarctica (type B) are responsible for most tularaemia cases, but these isolates typically form a weak biofilm under in vitro conditions. Phase variation of the F. tularensis lipopolysaccharide (LPS) has been reported in these subspecies, but the role of variation is unclear as LPS is crucial for virulence. We previously demonstrated that a subpopulation of LPS variants can constitutively form a robust biofilm in vitro, but it is unclear whether virulence was affected. In this study, we show that biofilm-forming variants of both fully virulent F. tularensis subspecies were highly attenuated in the murine tularaemia model by multiple challenge routes. Genomic sequencing was performed on these strains, which revealed that all biofilm-forming variants contained a lesion within the wbtJ gene, a formyltransferase involved in O-antigen synthesis. A ΔwbtJ deletion mutant recapitulated the biofilm, O-antigen and virulence phenotypes observed in natural variants and could be rescued through complementation with a functional wbtJ gene. Since the spontaneously derived biofilm-forming isolates in this study were a subpopulation of natural variants, reversion events to the wbtJ gene were detected that eliminated the phenotypes associated with biofilm variants and restored virulence. These results demonstrate a role for WbtJ in biofilm formation, LPS variation and virulence of F. tularensis.

Intact O-antigen is critical structure for the exceptional tubular shape of outer membrane vesicles in Francisella tularensis

Francisella tularensis is a highly infectious Gram-negative coccobacillus which causes the disease tularemia. The potential for its misuse as a biological weapon has led disease control and prevention centers to classify this bacterium as a category A agent. Bacterial outer membrane vesicles (OMVs) are spherical particles 20-250 nm in size produced by all Gram-negative bacteria and constitute one of the major secretory pathways. Bacteria use them in interacting with both other bacterial cells and eukaryotic (host) cells. OMVs of Francisella contain number of its so far described virulence factors and immunomodulatory proteins. Their role in host-pathogen interactions can therefore be presumed, and the possibility exists also for their potential use in a subunit vaccine. Moreover, Francisella microbes produce both usual spherical and unusual tubular OMVs. Because OMVs emerge from the outermost surface of the bacterial cell, we focused on the secretion of OMVs in several mutant Francisella strains with disrupted surface structures (namely the O-antigen). O-antigen in Francisella is not only the structural component of LPS but also forms another important virulence factor: the O-antigen polysaccharide capsule. Mutant strain phenotypes were evaluated by growth curves, vesiculation rates, their sensitivity to the complement contained in serum, and proliferation inside murine bone marrow macrophages. Morphologies of both OMVs and the bacteria were visualized by electron microscopy. The O-antigen mutant strains were considerably attenuated in serum resistance and intracellular proliferation. All the strains showed lower ability to form the tubular OMVs. Some strains formed tubular protrusions from their outer membrane but their stability was weak. Some hypervesiculating strains were revealed that will serve as source of OMVs for further studies of their protective potential. Our results suggest the presence of LPS and the O-antigen capsule on the surface of Francisella to be critical not only for its virulence but also for the exceptional tubular shape of its OMVs.

Why vary what's working? Phase variation and biofilm formation in Francisella tularensis

The notoriety of high-consequence human pathogens has increased in recent years and, rightfully, research efforts have focused on understanding host-pathogen interactions. Francisella tularensis has been detected in an impressively broad range of vertebrate hosts as well as numerous arthropod vectors and single-celled organisms. Two clinically important subspecies, F. tularensis subsp. tularensis (Type A) and F. tularensis subsp. holarctica (Type B), are responsible for the majority of tularemia cases in humans. The success of this bacterium in mammalian hosts can be at least partly attributed to a unique LPS molecule that allows the bacterium to avoid detection by the host immune system. Curiously, phase variation of the O-antigen incorporated into LPS has been documented in these subspecies of F. tularensis, and these variants often display some level of attenuation in infection models. While the role of phase variation in F. tularensis biology is unclear, it has been suggested that this phenomenon can aid in environmental survival and persistence. Biofilms have been established as the predominant lifestyle of many bacteria in the environment, though, it was previously thought that Type A and B isolates of F. tularensis typically form poor biofilms. Recent studies question this ideology as it was shown that alteration of the O-antigen allows robust biofilm formation in both Type A and B isolates. This review aims to explore the link between phase variation of the O-antigen, biofilm formation, and environmental persistence with an emphasis on clinically relevant subspecies and how understanding these poorly studied mechanisms could lead to new medical countermeasures to combat tularemia.

Alginate microencapsulation of an attenuated O-antigen mutant of Francisella tularensis LVS as a model for a vaccine delivery vehicle

Francisella tularensis is the etiologic agent of tularemia and a Tier I Select Agent. Subspecies tularensis (Type A) is the most virulent of the four subspecies and inhalation of as few as 10 cells can cause severe disease in humans. Due to its niche as a facultative intracellular pathogen, a successful tularemia vaccine must induce a robust cellular immune response, which is best achieved by a live, attenuated strain. F. tularensis strains lacking lipopolysaccharide (LPS) O-antigen are highly attenuated, but do not persist in the host long enough to induce protective immunity. Increasing the persistence of an O-antigen mutant may help stimulate protective immunity. Alginate encapsulation is frequently used with probiotics to increase persistence of bacteria within the gastrointestinal system, and was used to encapsulate the highly attenuated LVS O-antigen mutant WbtI_G191V. Encapsulation with alginate followed by a poly-L-lysine/alginate coating increased survival of WbtI_G191V in complement-active serum. In addition, BALB/c mice immunized intraperitoneally with encapsulated WbtI_G191V combined with purified LPS survived longer than mock-immunized mice following intranasal challenge. Alginate encapsulation of the bacteria also increased antibody titers compared to non-encapsulated bacteria. These data suggest that alginate encapsulation provides a slow-release vehicle for bacterial deposits, as evidenced by the increased antibody titer and increased persistence in serum compared to freely suspended cells. Survival of mice against high-dose intranasal challenge with the LVS wildtype was similar between mice immunized within alginate capsules or with LVS, possibly due to the low number of animals used, but bacterial loads in the liver and spleen were the lowest in mice immunized with WbtI_G191V and LPS in beads. However, an analysis of the immune response of surviving mice indicated that those vaccinated with the alginate vehicle upregulated cell-mediated immune pathways to a lesser extent than LVS-vaccinated mice. In summary, this vehicle, as formulated, may be more effective for pathogens that require predominately antibody-mediated immunity.

Phase Variation of LPS and Capsule Is Responsible for Stochastic Biofilm Formation in Francisella tularensis

Biofilms have been established as an important lifestyle for bacteria in nature as these structured communities often enable survivability and persistence in a multitude of environments. Francisella tularensis is a facultative intracellular Gram-negative bacterium found throughout much of the northern hemisphere. However, biofilm formation remains understudied and poorly understood in F. tularensis as non-substantial biofilms are typically observed in vitro by the clinically relevant subspecies F. tularensis subsp. tularensis and F. tularensis subsp. holarctica (Type A and B, respectively). Herein, we report conditions under which robust biofilm development was observed in a stochastic, but reproducible manner in Type A and B isolates. The frequency at which biofilm was observed increased temporally and appeared switch-like as progeny from the initial biofilm quickly formed biofilm in a predictable manner regardless of time or propagation with fresh media. The Type B isolates used for this study were found to more readily switch on biofilm formation than Type A isolates. Additionally, pH was found to function as an environmental checkpoint for biofilm initiation independently of the heritable cellular switch. Multiple colony morphologies were observed in biofilm positive cultures leading to the identification of a particular subset of grey variants that constitutively produce biofilm. Further, we found that constitutive biofilm forming isolates delay the onset of a viable non-culturable state. In this study, we demonstrate that a robust biofilm can be developed by clinically relevant F. tularensis isolates, provide a mechanism for biofilm initiation and examine the potential role of biofilm formation.

Francisella novicida Mutant XWK4 Triggers Robust Inflammasome Activation Favoring Infection

Bacterial infection tendentiously triggers inflammasome activation, whereas the roles of inflammasome activation in host defense against diverse infections remain unclear. Here, we identified that an ASC-dependent inflammasome activation played opposite roles in host defense against Francisella novicida wild-type (WT) U112 and mutant strain XWK4. Comparing with U112, XWK4 infection induced robust cytokine production, ASC-dependent inflammasome activation, and pyroptosis. Both AIM2 and NLRP3 were involved and played independent roles in XWK4-induced inflammasome activation. Type II interferon was partially required for XWK4-triggered inflammasome activation, which was different from type I interferon dependency in U112-induced inflammasome activation. Distinct from F. novicida U112 and Acinetobacter baumannii infection, Asc^-/- mice were more resistant than WT mice response to XWK4 infection by limiting bacterial burden in vivo. The excessive inflammasome activation triggered by XWK4 infection caused dramatical cell death and pathological damage. Our study offers novel insights into mechanisms of inflammasome activation in host defense and provides potential therapeutic approach against bacterial infections and inflammatory diseases.

Development, Phenotypic Characterization and Genomic Analysis of a Francisella tularensis Panel for Tularemia Vaccine Testing

Francisella tularensis is one of several biothreat agents for which a licensed vaccine is needed to protect against this pathogen. To aid in the development of a vaccine protective against pneumonic tularemia, we generated and characterized a panel of F. tularensis isolates that can be used as challenge strains to assess vaccine efficacy. Our panel consists of both historical and contemporary isolates derived from clinical and environmental sources, including human, tick, and rabbit isolates. Whole genome sequencing was performed to assess the genetic diversity in comparison to the reference genome F. tularensis Schu S4. Average nucleotide identity analysis showed >99% genomic similarity across the strains in our panel, and pan-genome analysis revealed a core genome of 1,707 genes, and an accessory genome of 233 genes. Three of the strains in our panel, FRAN254 (tick-derived), FRAN255 (a type B strain), and FRAN256 (a human isolate) exhibited variation from the other strains. Moreover, we identified several unique mutations within the Francisella Pathogenicity Island across multiple strains in our panel, revealing unexpected diversity in this region. Notably, FRAN031 (Scherm) completely lacked the second pathogenicity island but retained virulence in mice. In contrast, FRAN037 (Coll) was attenuated in a murine pneumonic tularemia model and had mutations in pdpB and iglA which likely led to attenuation. All of the strains, except FRAN037, retained full virulence, indicating their effectiveness as challenge strains for future vaccine testing. Overall, we provide a well-characterized panel of virulent F. tularensis strains that can be utilized in ongoing efforts to develop an effective vaccine against pneumonic tularemia to ensure protection is achieved across a range F. tularensis strains.

Francisella FlmX broadly affects lipopolysaccharide modification and virulence

The outer membrane protects Gram-negative bacteria from the host environment. Lipopolysaccharide (LPS), a major outer membrane constituent, has distinct components (lipid A, core, O-antigen) generated by specialized pathways. In this study, we describe the surprising convergence of these pathways through FlmX, an uncharacterized protein in the intracellular pathogen Francisella. FlmX is in the flippase family, which includes proteins that traffic lipid-linked envelope components across membranes. flmX deficiency causes defects in lipid A modification, core remodeling, and O-antigen addition. We find that an F. tularensis mutant lacking flmX is >1,000,000-fold attenuated. Furthermore, FlmX is required to resist the innate antimicrobial LL-37 and the antibiotic polymyxin. Given FlmX's central role in LPS modification and its conservation in intracellular pathogens Brucella, Coxiella, and Legionella, FlmX may represent a novel drug target whose inhibition could cripple bacterial virulence and sensitize bacteria to innate antimicrobials and antibiotics.

The Francisella tularensis Polysaccharides: What Is the Real Capsule?

Francisella tularensis is a tier 1 select agent responsible for tularemia in humans and a wide variety of animal species. Extensive research into understanding the virulence factors of the bacterium has been ongoing to develop an efficacious vaccine. At least two such virulence factors are described as capsules of F. tularensis: the O-antigen capsule and the capsule-like complex (CLC). These two separate entities aid in avoiding host immune defenses but have not been clearly differentiated. These components are distinct and differ in composition and genetic basis. The O-antigen capsule consists of a polysaccharide nearly identical to the lipopolysaccharide (LPS) O antigen, whereas the CLC is a heterogeneous complex of glycoproteins, proteins, and possibly outer membrane vesicles and tubes (OMV/Ts). In this review, the current understanding of these two capsules is summarized, and the historical references to "capsules" of F. tularensis are clarified. A significant amount of research has been invested into the composition of each capsule and the genes involved in synthesis of the polysaccharide portion of each capsule. Areas of future research include further exploration into the molecular regulation and pathways responsible for expression of each capsule and further elucidating the role that each capsule plays in virulence.

Genomic diversity in flavobacterial pathogens of aquatic origin

Flavobacterium species are considered important fish pathogens in wild and cultured fish throughout the world. They can cause acute, subacute, and chronic infections, which are mainly characterized by gill damage, skin lesions, and deep necrotic ulcerations. Primarily, three Flavobacterium species, F. branchiophilum, F. columnare, and F. psychrophilum, have been reported to cause substantial losses to freshwater fish. In this study, we evaluated genomes of 86 Flavobacterium species isolated from aquatic hosts (mainly fish) to identify their unique and shared genome features. Our results showed that F. columnare genomes cluster into four different genetic groups. In silico secretion system analysis identified that all genomes carry type I (T1SS) and type IX (T9SS) secretion systems, but the number of type I secretion system genes shows diversity between species. F. branchiophilum, F. araucananum, F. chilense, F. spartansii, and F. tructae genomes have full type VI secretion system (T6SS). F. columnare, F. hydatis, and F. plurextorum carry partial T6SS with some of the T6SS genes missing. F. columnare, F. araucananum, F. chilense, F. spartansii, F. araucananum, F. tructae, Flavobacterium sp., F. crassostreae, F. succinicans, F. hydatis, and F. plurextorum carry most of the type IV secretion system genes (T4SS). F. columnare genetic groups 1 and 2, Flavobacterium sp., and F. crassostreae encode the least number of antibiotic resistance elements. F. hydatis, F. chilense, and F. plurextorum encode the greatest number of antibiotic resistance genes. Additionally, F. spartansii, F. araucananum, and chilense encode the greatest number of virulence genes while Flavobacterium sp. and F. crassostreae encode the least number of virulence genes. In conclusion, comparative genomics of Flavobacterium species of aquatic origin will help our understanding of Flavobacterium pathogenesis.

The D-alanyl-d-alanine carboxypeptidase enzyme is essential for virulence in the Schu S4 strain of Francisella tularensis and a dacD mutant is able to provide protection against a pneumonic challenge

Low molecular mass penicillin binding proteins (LMM PBP) are bacterial enzymes involved in the final steps of peptidoglycan biosynthesis. In Escherichia coli, most LMM PBP exhibit dd-carboxypeptidase activity, are not essential for growth in routine laboratory media, and contributions to virulent phenotypes remain largely unknown. The Francisella tularensis Schu S4 genome harbors the dacD gene (FTT_1029), which encodes a LMM PBP with homology to PBP6b of E. coli. Disruption of this locus in the fully virulent Schu S4 strain resulted in a mutant that could not grow in Chamberlain's Defined Medium and exhibited severe morphological defects. Further characterization studies demonstrated that the growth defects of the dacD mutant were pH-dependent, and could be partially restored by growth at neutral pH or fully restored by genetic complementation. Infection of murine macrophage-like cells showed that the Schu S4 dacD mutant is capable of intracellular replication. However, this mutant was attenuated in BALB/c mice following intranasal challenge (LD₅₀ = 603 CFU) as compared to mice challenged with the parent (LD₅₀ = 1 CFU) or complemented strain (LD₅₀ = 1 CFU). Additionally, mice that survived infection with the dacD mutant showed significant protection against subsequent challenge with the parent strain. Collectively, these results indicate that the DacD protein of F. tularensis is essential for growth in low pH environments and virulence in vivo. These results also suggest that a PBP mutant could serve as the basis of a novel, live attenuated vaccine strain.

Formation of the Francisella tularensis Biofilm is Affected by Cell Surface Glycosylation, Growth Medium, and a Glucan Exopolysaccharide

Biofilms are matrix-associated communities that enable bacteria to colonise environments unsuitable for free-living bacteria. The facultative intracellular pathogen Francisella tularensis can persist in water, amoebae, and arthropods, as well as within mammalian macrophages. F. tularensis Types A and B form poor biofilms, but F. tularensis mutants lacking lipopolysaccharide O-antigen, O-antigen capsule, and capsule-like complex formed up to 15-fold more biofilm than fully glycosylated cells. The Type B live vaccine strain was also 50% less capable of initiating surface attachment than mutants deficient in O-antigen and capsule-like complex. However, the growth medium of all strains tested also influenced the formation of biofilm, which contained a novel exopolysaccharide consisting of an amylose-like glucan. In addition, the surface polysaccharide composition of the bacterium affected the protein:DNA:polysaccharide composition of the biofilm matrix. In contrast, F. novicida attached to surfaces more efficiently and made a more robust biofilm than Type A or B strains, but loss of O-antigen or capsule-like complex did not significantly affect F. novicida biofilm formation. These results indicated that suppression of surface polysaccharides may promote biofilm formation by F. tularensis Types A and B. Whether biofilm formation enhances survival of F. tularensis in aquatic or other environmental niches has yet to be determined.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

A spontaneous mutation in kdsD, a biosynthesis gene for 3 Deoxy-D-manno-Octulosonic Acid, occurred in a ciprofloxacin resistant strain of Francisella tularensis and caused a high level of attenuation in murine models of tularemia

Francisella tularensis, a gram-negative facultative intracellular bacterial pathogen, is the causative agent of tularemia and able to infect many mammalian species, including humans. Because of its ability to cause a lethal infection, low infectious dose, and aerosolizable nature, F. tularensis subspecies tularensis is considered a potential biowarfare agent. Due to its in vitro efficacy, ciprofloxacin is one of the antibiotics recommended for post-exposure prophylaxis of tularemia. In order to identify therapeutics that will be efficacious against infections caused by drug resistant select-agents and to better understand the threat, we sought to characterize an existing ciprofloxacin resistant (CipR) mutant in the Schu S4 strain of F. tularensis by determining its phenotypic characteristics and sequencing the chromosome to identify additional genetic alterations that may have occurred during the selection process. In addition to the previously described genetic alterations, the sequence of the CipR mutant strain revealed several additional mutations. Of particular interest was a frameshift mutation within kdsD which encodes for an enzyme necessary for the production of 3-Deoxy-D-manno-Octulosonic Acid (KDO), an integral component of the lipopolysaccharide (LPS). A kdsD mutant was constructed in the Schu S4 strain. Although it was not resistant to ciprofloxacin, the kdsD mutant shared many phenotypic characteristics with the CipR mutant, including growth defects under different conditions, sensitivity to hydrophobic agents, altered LPS profiles, and attenuation in multiple models of murine tularemia. This study demonstrates that the KdsD enzyme is essential for Francisella virulence and may be an attractive therapeutic target for developing novel medical countermeasures.

Multifaceted effects of Francisella tularensis on human neutrophil function and lifespan

Francisella tularensis in an intracellular bacterial pathogen that causes a potentially lethal disease called tularemia. Studies performed nearly 100 years ago revealed that neutrophil accumulation in infected tissues correlates directly with the extent of necrotic damage during F. tularensis infection. However, the dynamics and details of bacteria-neutrophil interactions have only recently been studied in detail. Herein, we review current understanding regarding the mechanisms that recruit neutrophils to F. tularensis-infected lungs, opsonization and phagocytosis, evasion and inhibition of neutrophil defense mechanisms, as well as the ability of F. tularensis to prolong neutrophil lifespan. In addition, we discuss distinctive features of the bacterium, including its ability to act at a distance to alter overall neutrophil responsiveness to exogenous stimuli, and the evidence which suggests that macrophages and neutrophils play distinct roles in tularemia pathogenesis, such that macrophages are major vehicles for intracellular growth and dissemination, whereas neutrophils drive tissue destruction by dysregulation of the inflammatory response.

Outer membrane vesicles displaying engineered glycotopes elicit protective antibodies

The O-antigen polysaccharide (O-PS) component of lipopolysaccharides on the surface of gram-negative bacteria is both a virulence factor and a B-cell antigen. Antibodies elicited by O-PS often confer protection against infection; therefore, O-PS glycoconjugate vaccines have proven useful against a number of different pathogenic bacteria. However, conventional methods for natural extraction or chemical synthesis of O-PS are technically demanding, inefficient, and expensive. Here, we describe an alternative methodology for producing glycoconjugate vaccines whereby recombinant O-PS biosynthesis is coordinated with vesiculation in laboratory strains of Escherichia coli to yield glycosylated outer membrane vesicles (glycOMVs) decorated with pathogen-mimetic glycotopes. Using this approach, glycOMVs corresponding to eight different pathogenic bacteria were generated. For example, expression of a 17-kb O-PS gene cluster from the highly virulent Francisella tularensis subsp. tularensis (type A) strain Schu S4 in hypervesiculating E. coli cells yielded glycOMVs that displayed F. tularensis O-PS. Immunization of BALB/c mice with glycOMVs elicited significant titers of O-PS-specific serum IgG antibodies as well as vaginal and bronchoalveolar IgA antibodies. Importantly, glycOMVs significantly prolonged survival upon subsequent challenge with F. tularensis Schu S4 and provided complete protection against challenge with two different F. tularensis subsp. holarctica (type B) live vaccine strains, thereby demonstrating the vaccine potential of glycOMVs. Given the ease with which recombinant glycotopes can be expressed on OMVs, the strategy described here could be readily adapted for developing vaccines against many other bacterial pathogens.

Evidence Suggesting That Francisella tularensis O-Antigen Capsule Contains a Lipid A-Like Molecule That Is Structurally Distinct from the More Abundant Free Lipid A

Francisella tularensis, the Gram-negative bacterium that causes tularemia, produces a high molecular weight capsule that is immunologically distinct from Francisella lipopolysaccharide but contains the same O-antigen tetrasaccharide. To pursue the possibility that the capsule of Francisella live vaccine strain (LVS) has a structurally unique lipid anchor, we have metabolically labeled Francisella with [¹⁴C]acetate to facilitate highly sensitive compositional analysis of capsule-associated lipids. Capsule was purified by two independent methods and yielded similar results. Autoradiographic and immunologic analysis confirmed that this purified material was largely devoid of low molecular weight LPS and of the copious amounts of free lipid A that the Francisellae accumulate. Chemical hydrolysis yielded [¹⁴C]-labeled free fatty acids characteristic of Francisella lipid A but with a different molar ratio of 3-OH C18:0 to 3-OH C16:0 and different composition of non-hydroxylated fatty acids (mainly C14:0 rather than C16:0) than that of free Francisella lipid A. Mild acid hydrolysis to induce selective cleavage of KDO-lipid A linkage yielded a [¹⁴C]-labeled product that partitioned during Bligh/Dyer extraction and migrated during thin-layer chromatography like lipid A. These findings suggest that the O-antigen capsule of Francisella contains a covalently linked and structurally distinct lipid A species. The presence of a discrete lipid A-like molecule associated with capsule raises the possibility that Francisella selectively exploits lipid A structural heterogeneity to regulate synthesis, transport, and stable bacterial surface association of the O-antigen capsular layer.

Metabolic Reprogramming of Host Cells by Virulent Francisella tularensis for Optimal Replication and Modulation of Inflammation

A shift in macrophage metabolism from oxidative phosphorylation to aerobic glycolysis is a requirement for activation to effectively combat invading pathogens. Francisella tularensis is a facultative intracellular bacterium that causes an acute, fatal disease called tularemia. Its primary mechanism of virulence is its ability to evade and suppress inflammatory responses while replicating in the cytosol of macrophages. The means by which F. tularensis modulates macrophage activation are not fully elucidated. In this study, we demonstrate that virulent F. tularensis impairs production of inflammatory cytokines in primary macrophages by preventing their shift to aerobic glycolysis, as evidenced by the downregulation of hypoxia inducible factor 1α and failure to upregulate pfkfb3 We also show that Francisella capsule is required for this process. In addition to modulating inflammatory responses, inhibition of glycolysis in host cells is also required for early replication of virulent Francisella Taken together, our data demonstrate that metabolic reprogramming of host cells by F. tularensis is a key component of both inhibition of host defense mechanisms and replication of the bacterium.

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.

Interactions of Francisella tularensis with Alveolar Type II Epithelial Cells and the Murine Respiratory Epithelium

Francisella tularensis is classified as a Tier 1 select agent by the CDC due to its low infectious dose and the possibility that the organism can be used as a bioweapon. The low dose of infection suggests that Francisella is unusually efficient at evading host defenses. Although ~50 cfu are necessary to cause human respiratory infection, the early interactions of virulent Francisella with the lung environment are not well understood. To provide additional insights into these interactions during early Francisella infection of mice, we performed TEM analysis on mouse lungs infected with F. tularensis strains Schu S4, LVS and the O-antigen mutant Schu S4 waaY::TrgTn. For all three strains, the majority of the bacteria that we could detect were observed within alveolar type II epithelial cells at 16 hours post infection. Although there were no detectable differences in the amount of bacteria within an infected cell between the three strains, there was a significant increase in the amount of cellular debris observed in the air spaces of the lungs in the Schu S4 waaY::TrgTn mutant compared to either the Schu S4 or LVS strain. We also studied the interactions of Francisella strains with human AT-II cells in vitro by characterizing the ability of these three strains to invade and replicate within these cells. Gentamicin assay and confocal microscopy both confirmed that F. tularensis Schu S4 replicated robustly within these cells while F. tularensis LVS displayed significantly lower levels of growth over 24 hours, although the strain was able to enter these cells at about the same level as Schu S4 (1 organism per cell), as determined by confocal imaging. The Schu S4 waaY::TrgTn mutant that we have previously described as attenuated for growth in macrophages and mouse virulence displayed interesting properties as well. This mutant induced significant airway inflammation (cell debris) and had an attenuated growth phenotype in the human AT-II cells. These data extend our understanding of early Francisella infection by demonstrating that Francisella enter significant numbers of AT-II cells within the lung and that the capsule and LPS of wild type Schu S4 helps prevent murine lung damage during infection. Furthermore, our data identified that human AT-II cells allow growth of Schu S4, but these same cells supported poor growth of the attenuated LVS strain in vitro. Collectively, these data further our understanding of the role of AT-II cells in Francisella infections.

Characterization of Francisella tularensis Schu S4 mutants identified from a transposon library screened for O-antigen and capsule deficiencies

The lipopolysaccharide (LPS) and O-antigen polysaccharide capsule structures of Francisella tularensis play significant roles in helping these highly virulent bacteria avoid detection within a host. We previously created pools of F. tularensis mutants that we screened to identify strains that were not reactive to a monoclonal antibody to the O-antigen capsule. To follow up previously published work, we characterize further seven of the F. tularensis Schu S4 mutant strains identified by our screen. These F. tularensis strains carry the following transposon mutations: FTT0846::Tn5, hemH::Tn5, wbtA::Tn5, wzy::Tn5, FTT0673p/prsA::Tn5, manB::Tn5, or dnaJ::Tn5. Each of these strains displayed sensitivity to human serum, to varying degrees, when compared to wild-type F. tularensis Schu S4. By Western blot, only FTT0846::Tn5, wbtA::Tn5, wzy::Tn5, and manB::Tn5 strains did not react to the capsule and LPS O-antigen antibody 11B7, although the wzy::Tn5 strain did have a single O-antigen reactive band that was detected by the FB11 monoclonal antibody. Of these strains, manB::Tn5 and FTT0846 appear to have LPS core truncations, whereas wbtA::Tn5 and wzy::Tn5 had LPS core structures that are similar to the parent F. tularensis Schu S4. These strains were also shown to have poor growth within human monocyte derived macrophages (MDMs) and bone marrow derived macrophages (BMDMs). We examined the virulence of these strains in mice, following intranasal challenge, and found that each was attenuated compared to wild type Schu S4. Our results provide additional strong evidence that LPS and/or capsule are F. tularensis virulence factors that most likely function by providing a stealth shield that prevents the host immune system from detecting this potent pathogen.

Uncovering the components of the Francisella tularensis virulence stealth strategy

Over the last decade, studies on the virulence of the highly pathogenic intracellular bacterial pathogen Francisella tularensis have increased dramatically. The organism produces an inert LPS, a capsule, escapes the phagosome to grow in the cytosol (FPI genes mediate phagosomal escape) of a variety of host cell types that include epithelial, endothelial, dendritic, macrophage, and neutrophil. This review focuses on the work that has identified and characterized individual virulence factors of this organism and we hope to highlight how these factors collectively function to produce the pathogenic strategy of this pathogen. In addition, several recent studies have been published characterizing F. tularensis mutants that induce host immune responses not observed in wild type F. tularensis strains that can induce protection against challenge with virulent F. tularensis. As more detailed studies with attenuated strains are performed, it will be possible to see how host models develop acquired immunity to Francisella. Collectively, detailed insights into the mechanisms of virulence of this pathogen are emerging that will allow the design of anti-infective strategies.