A Francisella tularensis subspecies novicida purF mutant, but not a purA mutant, induces protective immunity to tularemia in mice

Lactate promotes the biofilm-to-invasive-planktonic transition in Salmonella enterica serovar Typhimurium via the de novo purine pathway

Salmonella enterica serovar Typhimurium (S. Typhimurium) infection triggers an inflammatory response that changes the concentration of metabolites in the gut impacting the luminal environment. Some of these environmental adjustments are conducive to S. Typhimurium growth, such as the increased concentrations of nitrate and tetrathionate or the reduced levels of Clostridia-produced butyrate. We recently demonstrated that S. Typhimurium can form biofilms within the host environment and respond to nitrate as a signaling molecule, enabling it to transition between sessile and planktonic states. To investigate whether S. Typhimurium utilizes additional metabolites to regulate its behavior, our study delved into the impact of inflammatory metabolites on biofilm formation. The results revealed that lactate, the most prevalent metabolite in the inflammatory environment, impedes biofilm development by reducing intracellular c-di-GMP levels, suppressing the expression of curli and cellulose, and increasing the expression of flagellar genes. A transcriptomic analysis determined that the expression of the de novo purine pathway increases during high lactate conditions, and a transposon mutagenesis genetic screen identified that PurA and PurG, in particular, play a significant role in the inhibition of curli expression and biofilm formation. Lactate also increases the transcription of the type III secretion system genes involved in tissue invasion. Finally, we show that the pyruvate-modulated two-component system BtsSR is activated in the presence of high lactate, which suggests that lactate-derived pyruvate activates BtsSR system after being exported from the cytosol. All these findings propose that lactate is an important inflammatory metabolite used by S. Typhimurium to transition from a biofilm to a motile state and fine-tune its virulence.IMPORTANCEWhen colonizing the gut, Salmonella enterica serovar Typhimurium (S. Typhimurium) adopts a dynamic lifestyle that alternates between a virulent planktonic state and a multicellular biofilm state. The coexistence of biofilm formers and planktonic S. Typhimurium in the gut suggests the presence of regulatory mechanisms that control planktonic-to-sessile transition. The signals triggering the transition of S. Typhimurium between these two lifestyles are not fully explored. In this work, we demonstrated that in the presence of lactate, the most dominant host-derived metabolite in the inflamed gut, there is a reduction of c-di-GMP in S. Typhimurium, which subsequently inhibits biofilm formation and induces the expression of its invasion machinery, motility genes, and de novo purine metabolic pathway genes. Furthermore, high levels of lactate activate the BtsSR two-component system. Collectively, this work presents new insights toward the comprehension of host metabolism and gut microenvironment roles in the regulation of S. Typhimurium biology during infection.

Development, Strategies, and Challenges for Tularemia Vaccine

Francisella tularensis is a facultative intracellular bacterial pathogen that affects both humans and animals. It was developed into a biological warfare weapon as a result. In this article, the current status of tularemia vaccine development is presented. A live-attenuated vaccine that was designed over 50 years ago using the less virulent F. tularensis subspecies holarctica is the only prophylactic currently available, but it has not been approved for use in humans or animals. Other promising live, killed, and subunit vaccine candidates have recently been developed and tested in animal models. This study will investigate some possible vaccines and the challenges they face during development.

Nucleotide biosynthesis: the base of bacterial pathogenesis

Most free-living organisms require the synthesis and/or acquisition of purines and pyrimidines, which form the basis of nucleotides, to survive. In most bacteria, the nucleotides are synthesized de novo and the products are used in many cell functions, including DNA replication, energy storage, and as signaling molecules. Due to their central role in the metabolism of bacteria, both nucleotide biosynthesis pathways have strong links with the virulence of opportunistic and bona fide bacterial pathogens. Recent findings have established a new, shared link in the control of nucleotide biosynthesis and the production of virulence factors. Furthermore, targeting of these pathways forms the basis of interspecies competition and can provide an open source for new antimicrobial compounds. Here, we highlight the contribution of nucleotide biosynthesis to bacterial pathogenesis in a plethora of different diseases and speculate on how they can be targeted by intervention strategies.

Time-Resolved Proteome Analysis of Listeria monocytogenes during Infection Reveals the Role of the AAA+ Chaperone ClpC for Host Cell Adaptation

The cellular proteome comprises all proteins expressed at a given time and defines an organism's phenotype under specific growth conditions. The proteome is shaped and remodeled by both protein synthesis and protein degradation. Here, we developed a new method which combines metabolic and chemical isobaric peptide labeling to simultaneously determine the time-resolved protein decay and de novo synthesis in an intracellular human pathogen. We showcase this method by investigating the Listeria monocytogenes proteome in the presence and absence of the AAA+ chaperone protein ClpC. ClpC associates with the peptidase ClpP to form an ATP-dependent protease complex and has been shown to play a role in virulence development in L. monocytogenes. However, the mechanism by which ClpC is involved in the survival and proliferation of intracellular L. monocytogenes remains elusive. Employing this new method, we observed extensive proteome remodeling in L. monocytogenes upon interaction with the host, supporting the hypothesis that ClpC-dependent protein degradation is required to initiate bacterial adaptation mechanisms. We identified more than 100 putative ClpC target proteins through their stabilization in a clpC deletion strain. Beyond the identification of direct targets, we also observed indirect effects of the clpC deletion on the protein abundance in diverse cellular and metabolic pathways, such as iron acquisition and flagellar assembly. Overall, our data highlight the crucial role of ClpC for L. monocytogenes adaptation to the host environment through proteome remodeling. IMPORTANCE Survival and proliferation of pathogenic bacteria inside the host depend on their ability to adapt to the changing environment. Profiling the underlying changes on the bacterial proteome level during the infection process is important to gain a better understanding of the pathogenesis and the host-dependent adaptation processes. The cellular protein abundance is governed by the interplay between protein synthesis and decay. The direct readout of these events during infection can be accomplished using pulsed stable-isotope labeling by amino acids in cell culture (SILAC). Combining this approach with tandem-mass-tag (TMT) labeling enabled multiplexed and time-resolved bacterial proteome quantification during infection. Here, we applied this integrated approach to investigate protein turnover during the temporal progression of adaptation of the human pathogen L. monocytogenes to its host on a system-wide scale. Our experimental approach can easily be transferred to probe the proteome remodeling in other bacteria under a variety of perturbations.

De Novo Purine Biosynthesis Is Required for Intracellular Growth of Staphylococcus aureus and for the Hypervirulence Phenotype of a purR Mutant

Staphylococcus aureus is a noted human and animal pathogen. Despite decades of research on this important bacterium, there are still many unanswered questions regarding the pathogenic mechanisms it uses to infect the mammalian host. This can be attributed to it possessing a plethora of virulence factors and complex virulence factor and metabolic regulation. PurR, the purine biosynthesis regulator, was recently also shown to regulate virulence factors in S. aureus, and mutations in purR result in derepression of fibronectin binding proteins (FnBPs) and extracellular toxins, required for a so-called hypervirulent phenotype.

Staphylococcus aureus is a noted human and animal pathogen. Despite decades of research on this important bacterium, there are still many unanswered questions regarding the pathogenic mechanisms it uses to infect the mammalian host. This can be attributed to it possessing a plethora of virulence factors and complex virulence factor and metabolic regulation. PurR, the purine biosynthesis regulator, was recently also shown to regulate virulence factors in S. aureus, and mutations in purR result in derepression of fibronectin binding proteins (FnBPs) and extracellular toxins, required for a so-called hypervirulent phenotype. Here, we show that hypervirulent strains containing purR mutations can be attenuated with the addition of purine biosynthesis mutations, implicating the necessity for de novo purine biosynthesis in this phenotype and indicating that S. aureus in the mammalian host experiences purine limitation. Using cell culture, we showed that while purR mutants are not altered in epithelial cell binding, compared to that of wild-type (WT) S. aureus, purR mutants have enhanced invasion of these nonprofessional phagocytes, consistent with the requirement of FnBPs for invasion of these cells. This correlates with purR mutants having increased transcription of fnb genes, resulting in higher levels of surface-exposed FnBPs to promote invasion. These data provide important contributions to our understanding of how the pathogenesis of S. aureus is affected by sensing of purine levels during infection of the mammalian host.

Live Attenuated Tularemia Vaccines for Protection Against Respiratory Challenge With Virulent F. tularensis subsp. tularensis

Francisella tularensis is the causative agent of tularemia and a Tier I bioterrorism agent. In the 1900s, several vaccines were developed against tularemia including the killed "Foshay" vaccine, subunit vaccines comprising F. tularensis protein(s) or lipoproteins(s) in an adjuvant formulation, and the F. tularensis Live Vaccine Strain (LVS); none were licensed in the U.S.A. or European Union. The LVS vaccine retains toxicity in humans and animals-especially mice-but has demonstrated efficacy in humans, and thus serves as the current gold standard for vaccine efficacy studies. The U.S.A. 2001 anthrax bioterrorism attack spawned renewed interest in vaccines against potential biowarfare agents including F. tularensis. Since live attenuated-but not killed or subunit-vaccines have shown promising efficacy and since vaccine efficacy against respiratory challenge with less virulent subspecies holarctica or F. novicida, or against non-respiratory challenge with virulent subsp. tularensis (Type A) does not reliably predict vaccine efficacy against respiratory challenge with virulent subsp. tularensis, the route of transmission and species of greatest concern in a bioterrorist attack, in this review, we focus on live attenuated tularemia vaccine candidates tested against respiratory challenge with virulent Type A strains, including homologous vaccines derived from mutants of subsp. holarctica, F. novicida, and subsp. tularensis, and heterologous vaccines developed using viral or bacterial vectors to express F. tularensis immunoprotective antigens. We compare the virulence and efficacy of these vaccine candidates with that of LVS and discuss factors that can significantly impact the development and evaluation of live attenuated tularemia vaccines. Several vaccines meet what we would consider the minimum criteria for vaccines to go forward into clinical development-safety greater than LVS and efficacy at least as great as LVS, and of these, several meet the higher standard of having efficacy ≥LVS in the demanding mouse model of tularemia. These latter include LVS with deletions in purMCD, sodBFt , capB or wzy; LVS ΔcapB that also overexpresses Type VI Secretion System (T6SS) proteins; FSC200 with a deletion in clpB; the single deletional purMCD mutant of F. tularensis SCHU S4, and a heterologous prime-boost vaccine comprising LVS ΔcapB and Listeria monocytogenes expressing T6SS proteins.

Murine survival of infection with Francisella novicida and protection against secondary challenge is critically dependent on B lymphocytes

Respiratory infection of mice with Francisella novicida has recently been used as a model for the highly virulent human pathogen Francisella tularensis. Similar to F. tularensis, even small doses of F. novicida administered by respiratory routes are lethal for inbred laboratory mice. This feature obviously limits study of infection-induced immunity. Parenteral sublethal infections of mice with F. novicida are feasible, but the resulting immune responses are incompletely characterized. Here we use parenteral intradermal (i.d.) and intraperitoneal (i.p.) F. novicida infections of C57BL/6J mice to determine the role of B cells in controlling primary and secondary F. novicida infections. Despite developing comparable levels of F. novicida-primed T cells, B cell knockout mice were much more susceptible to both primary i.d. infection and secondary i.p. challenge than wild type (normal) C57BL/6J mice. Transfer of F. novicida-immune sera to either wild type C57BL/6J mice or to B cell knockout mice did not appreciably impact survival of subsequent lethal F. novicida challenge. However, F. novicida-immune mice that were depleted of T cells after priming but just before challenge survived and cleared secondary i.p. F. novicida challenge. Collectively these results indicate that B cells, if not serum antibodies, play a major role in controlling F. novicida infections in mice.

Tularemia vaccines

Francisella tularensis is the causative agent of the potentially lethal disease tularemia. Due to a low infectious dose and ease of airborne transmission, Francisella is classified as a category A biological agent. Despite the possible risk to public health, there is no safe and fully licensed vaccine. A potential vaccine candidate, an attenuated live vaccine strain, does not fulfil the criteria for general use. In this review, we will summarize existing and new candidates for live attenuated and subunit vaccines.

Identification of Francisella novicida mutants that fail to induce prostaglandin E(2) synthesis by infected macrophages

Francisella tularensis is the causative agent of tularemia. We have previously shown that infection with F. tularensis Live Vaccine Strain (LVS) induces macrophages to synthesize prostaglandin E₂ (PGE₂). Synthesis of PGE₂ by F. tularensis infected macrophages results in decreased T cell proliferation in vitro and increased bacterial survival in vivo. Although we understand some of the biological consequences of F. tularensis induced PGE₂ synthesis by macrophages, we do not understand the cellular pathways (neither host nor bacterial) that result in up-regulation of the PGE₂ biosynthetic pathway in F. tularensis infected macrophages. We took a genetic approach to begin to understand the molecular mechanisms of bacterial induction of PGE₂ synthesis from infected macrophages. To identify F. tularensis genes necessary for the induction of PGE₂ in primary macrophages, we infected cells with individual mutants from the closely related strain F. tularensis subspecies novicida U112 (U112) two allele mutant library. Twenty genes were identified that when disrupted resulted in U112 mutant strains unable to induce the synthesis of PGE₂ by infected macrophages. Fourteen of the genes identified are located within the Francisella pathogenicity island (FPI). Genes in the FPI are required for F. tularensis to escape from the phagosome and replicate in the cytosol, which might account for the failure of U112 with transposon insertions within the FPI to induce PGE₂. This implies that U112 mutant strains that do not grow intracellularly would also not induce PGE₂. We found that U112 clpB::Tn grows within macrophages yet fails to induce PGE₂, while U112 pdpA::Tn does not grow yet does induce PGE₂. We also found that U112 iglC::Tn neither grows nor induces PGE₂. These findings indicate that there is dissociation between intracellular growth and the ability of F. tularensis to induce PGE₂ synthesis. These mutants provide a critical entrée into the pathways used in the host for PGE₂ induction.

Mucosal immunization with live attenuated Francisella novicida U112ΔiglB protects against pulmonary F. tularensis SCHU S4 in the Fischer 344 rat model

The need for an efficacious vaccine against Francisella tularensis is a consequence of its low infectious dose and high mortality rate if left untreated. This study sought to characterize a live attenuated subspecies novicida-based vaccine strain (U112ΔiglB) in an established second rodent model of pulmonary tularemia, namely the Fischer 344 rat using two distinct routes of vaccination (intratracheal [i.t.] and oral). Attenuation was verified by comparing replication of U112ΔiglB with wild type parental strain U112 in F344 primary alveolar macrophages. U112ΔiglB exhibited an LD₅₀>10⁷ CFU compared to the wild type (LD₅₀ = 5×10⁶ CFU i.t.). Immunization with 10⁷ CFU U112ΔiglB by i.t. and oral routes induced antigen-specific IFN-γ and potent humoral responses both systemically (IgG2a>IgG1 in serum) and at the site of mucosal vaccination (respiratory/intestinal compartment). Importantly, vaccination with U112ΔiglB by either i.t. or oral routes provided equivalent levels of protection (50% survival) in F344 rats against a subsequent pulmonary challenge with ∼25 LD₅₀ (1.25×10⁴ CFU) of the highly human virulent strain SCHU S4. Collectively, these results provide further evidence on the utility of a mucosal vaccination platform with a defined subsp. novicida U112ΔiglB vaccine strain in conferring protective immunity against pulmonary tularemia.

Tularemia vaccines: recent developments and remaining hurdles

Francisella tularensis subsp. tularensis is a facultative intracellular bacterial pathogen of humans and other mammals. Its inhaled infectious dose is very low and can result in very high mortality. Historically, subsp. tularensis was developed as a biological weapon and there are now concerns about its abuse as such by terrorists. A live attenuated vaccine developed pragmatically more than half a century ago from the less virulent holarctica subsp. is the sole prophylactic available, but it remains unlicensed. In recent years several other potential live, killed and subunit vaccine candidates have been developed and tested in mice for their efficacy against respiratory challenge with subsp. tularensis. This article will review these vaccine candidates and the development hurdles they face.

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.

A Francisella tularensis SCHU S4 mutant deficient in γ-glutamyltransferase activity induces protective immunity: characterization of an attenuated vaccine candidate

Francisella tularensis is an intracellular pathogen which causes tularaemia. There is no licensed vaccine currently available for prophylaxis. The γ-glutamyl transpeptidase (GGT) encoded by the ggt gene has been shown to be important for the intracellular survival of F. tularensis. In this study we have constructed a ggt deletion mutant in the highly virulent F. tularensis strain SCHU S4. Characterization of the mutant strain confirmed the function of ggt, and confirmed the role of GGT in cysteine acquisition. The mutant strain was highly attenuated both in vitro and in vivo using murine models of infection. Moreover, we have demonstrated that the attenuated mutant is able to induce protective immunity against an F. tularensis SCHU S4 challenge, and thus may be a candidate for the development of an attenuated vaccine.

Glycosylation of DsbA in Francisella tularensis subsp. tularensis

In Francisella tularensis subsp. tularensis, DsbA has been shown to be an essential virulence factor and has been observed to migrate to multiple protein spots on two-dimensional electrophoresis gels. In this work, we show that the protein is modified with a 1,156-Da glycan moiety in O-linkage. The results of mass spectrometry studies suggest that the glycan is a hexasaccharide, comprised of N-acetylhexosamines, hexoses, and an unknown monosaccharide. Disruption of two genes within the FTT0789-FTT0800 putative polysaccharide locus, including a galE homologue (FTT0791) and a putative glycosyltransferase (FTT0798), resulted in loss of glycan modification of DsbA. The F. tularensis subsp. tularensis ΔFTT0798 and ΔFTT0791::Cm mutants remained virulent in the murine model of subcutaneous tularemia. This indicates that glycosylation of DsbA does not play a major role in virulence under these conditions. This is the first report of the detailed characterization of the DsbA glycan and putative role of the FTT0789-FTT0800 gene cluster in glycan biosynthesis.

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 type 2 secretion Pseudopilin, gspJ, is required for multihost pathogenicity of Burkholderia cenocepacia AU1054

Burkholderia cenocepacia AU1054 is an opportunistic pathogen isolated from the blood of a person with cystic fibrosis. AU1054 is a multihost pathogen causing rapid pathogenicity to Caenorhabditis elegans nematodes. Within 24 h, AU1054 causes greater than 50% mortality, reduced growth, emaciated body, distended intestinal lumen, rectal swelling, and prolific infection of the nematode intestine. To determine virulence mechanisms, 3,000 transposon mutants were screened for attenuated virulence in nematodes. Fourteen virulence-attenuated mutants were isolated, and the mutant genes were identified. These genes included paaA, previously identified as being required for full virulence of B. cenocepacia K56-2. Six mutants were restored in virulence by complementation with their respective wild-type gene. One of these contained an insertion in gspJ, predicted to encode a pseudopilin component of the type 2 secretion system (T2SS). Nematodes infected with AU1054 gspJ had fewer bacteria present in the intestine than those infected with the wild type but still showed rectal swelling. The gspJ mutant was also defective in pathogenicity to onion and in degradation of polygalacturonic acid and casein. This result differs from previous studies where no or little role was found for T2SS in Burkholderia virulence, although virulence factors such as zinc metalloproteases and polygalacturonase are known to be secreted by the T2SS. This study highlights strain specific differences in B. cenocepacia virulence mechanisms important for understanding what enables environmental microbes to function as opportunistic pathogens.

Francisella virulence: significant advances, ongoing challenges and unmet needs

Francisella tularensis, the causative agent of tularemia, is an organism of concern as a potential biowarfare agent. Progress towards understanding the molecular basis of pathogenicity has been hampered by a lack of tools with which to manipulate the pathogen. However, the availability of genome sequence data for a range of strains and the development of a range of plasmids and mutagenesis protocols for use in Francisella has resulted in a huge advance in understanding. No licensed vaccine is yet available. Various approaches towards a new vaccine are being evaluated, but novel adjuvants and delivery systems are needed to induce the complex response required for immunity. Better animal models to more accurately represent human responses to infection are also required.

Vaccines against tularemia

Francisella tularensis is a Category A select agent for which vaccine and countermeasure development are a priority. In the past eight years, renewed interest in this pathogen has led to the generation of an enormous amount of new data on both the pathogen itself and its interaction with host cells. This information has fostered the development of various vaccine candidates including acellular subunit, killed whole cell and live attenuated. This review summarizes the progress and promise of these various candidates.

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.

Francisella tularensis vaccines

Francisella tularensis has attracted attention historically as a biological weapon, due to its high infectivity in aerosols, and the severity of disease in humans. There is no licensed vaccine currently available, although an attenuated live vaccine strain (LVS) was identified in the middle of the last century and has been successfully used to protect humans. Efforts are underway to determine the basis of attenuation of LVS, and to understand the immunity required for protection. Alternative approaches to produce subunit vaccines and defined attenuated strains are also in progress. However, the limitations of animal models may make licensing a candidate vaccine challenging.

Induction of protective immunity against Burkholderia pseudomallei using attenuated mutants with defects in the intracellular life cycle

Melioidosis is a severe infectious disease caused by the Gram-negative rod Burkholderia pseudomallei. There is currently no vaccine available. We recently generated and characterized several highly attenuated transposon mutants with defects in the intracellular life cycle of B. pseudomallei. In the present study we examined the protective effects of six of these mutants: four harbouring knockouts in genes involved in several biosynthetic pathways (purN(-), purM(-), hisF(-), pabB(-)); a putative lipoate-protein ligase B; and a hypothetical protein. All live mutants conferred protection to some degree against wild-type challenge in susceptible BALB/c mice. Two mutants defective in distinct steps of the purine biosynthetic pathway were selected for further studies. Mutant 30:93 with a defect in the purN gene provided better protection against intraperitoneal challenge than mutant 56:65, which harboured a nonfunctional purM gene. Although mutant 30:93 conferred significant protection against acute fatal disease after intranasal and intraperitoneal challenge with B. pseudomallei, vaccination did not confer protection against chronic forms of melioidosis. Moreover, no protective effect could be seen against intravenous challenge. Further studies are required to analyze the precise nature of the immune response induced by the various live attenuated vaccines with different protective potential.

RelA regulates virulence and intracellular survival of Francisella novicida

Analysis of the genome of Francisella tularensis has revealed few regulatory systems, and how the organism adapts to conditions in different niches is poorly understood. The stringent response is a global stress response mediated by (p)ppGpp. The enzyme RelA has been shown to be involved in generation of this signal molecule in a range of bacterial species. We investigated the effect of inactivation of the relA gene in Francisella by generating a mutant in Francisella novicida. Under amino acid starvation conditions, the relA mutant was defective for (p)ppGpp production. Characterization showed the mutant to grow similarly to the wild-type, except that it entered stationary phase later than wild-type cultures, resulting in higher cell yields. The relA mutant showed increased biofilm formation, which may be linked to the delay in entering stationary phase, which in turn would result in higher cell numbers present in the biofilm and reduced resistance to in vitro stress. The mutant was attenuated in the J774A macrophage cell line and was shown to be attenuated in the mouse model of tularaemia, but was able to induce a protective immune response. Therefore, (p)ppGpp appears to be an important intracellular signal, integral to the pathogenesis of F. novicida.

Rationally designed tularemia vaccines

Tularemia, caused by the Gram-negative bacterium Francisella tularensis, can be contracted by the bite of an arthropod vector or by inhalation. This disease occurs relatively infrequently but can be severe and even life-threatening if untreated. Until recently, there were few laboratories studying this organism; however, concerns over its potential use as a biological weapon have led to renewed attention to F. tularensis research, particularly in the area of vaccine development. Advances in the ability to genetically manipulate F. tularensis, along with knowledge gained from the creation and refinement of attenuated bacterial vaccines for other diseases, continue to foster significant progress in the development of live-attenuated bacterial vaccines, as well as defined antigen and subunit vaccines.

Description of two new plasmids isolated from Francisella philomiragia strains and construction of shuttle vectors for the study of Francisella tularensis

Francisella tularensis is the causative agent of tularemia, a zoonotic disease often transmitted to humans by infected animals. The lack of useful specific genetic tools has long hampered the study of F. tularensis subspecies. We identified and characterized two new plasmids, pF242 and pF243, isolated from Francisella philomiragia strains ATCC 25016 and ATCC 25017, respectively. Sequence analysis revealed that pF242 and pF243 are closely related to pC194 and pFNL10 plasmids, respectively. Two generations of pF242- and pF243-based shuttle vectors, harboring several antibiotic resistance markers, were developed. We used the first generation to compare transformation efficiencies in two virulent F. tularensis subspecies. We found that electroporation was more efficient than cryotransformation: almost all vectors tested were successfully introduced by electroporation into Francisella strains with a high level of efficiency. The second generation of shuttle vectors, containing a multiple cloning site and/or gfp gene downstream of Francisella groES promotor, was used for GFP production in F. tularensis. The development of new shuttle vectors offers new perspectives in the genetic manipulation of F. tularensis, helping to elucidate the mechanisms underlying its virulence.

Identification of genes contributing to the virulence of Francisella tularensis SCHU S4 in a mouse intradermal infection model

Background

Francisella tularensis is a highly virulent human pathogen. The most virulent strains belong to subspecies tularensis and these strains cause a sometimes fatal disease. Despite an intense recent research effort, there is very limited information available that explains the unique features of subspecies tularensis strains that distinguish them from other F. tularensis strains and that explain their high virulence. Here we report the use of targeted mutagenesis to investigate the roles of various genes or pathways for the virulence of strain SCHU S4, the type strain of subspecies tularensis.

Methodology/Principal Findings

The virulence of SCHU S4 mutants was assessed by following the outcome of infection after intradermal administration of graded doses of bacteria. By this route, the LD₅₀ of the SCHU S4 strain is one CFU. The virulence of 20 in-frame deletion mutants and 37 transposon mutants was assessed. A majority of the mutants did not show increased prolonged time to death, among them notably ΔpyrB and ΔrecA. Of the remaining, mutations in six unique targets, tolC, rep, FTT0609, FTT1149c, ahpC, and hfq resulted in significantly prolonged time to death and mutations in nine targets, rplA, wbtI, iglB, iglD, purL, purF, ggt, kdtA, and glpX, led to marked attenuation with an LD₅₀ of >10³ CFU. In fact, the latter seven mutants showed very marked attenuation with an LD₅₀ of ≥10⁷ CFU.

Conclusions/Significance

The results demonstrate that the characterization of targeted mutants yielded important information about essential virulence determinants that will help to identify the so far little understood extreme virulence of F. tularensis subspecies tularensis.

Recombinant attenuated Listeria monocytogenes vaccine expressing Francisella tularensis IglC induces protection in mice against aerosolized Type A F. tularensis

Fransicella tularensis, the causative agent of tularemia, is in the top category (Category A) of potential agents of bioterrorism. To develop a safer vaccine against aerosolized F. tularensis, we have employed an attenuated Listeria monocytogenes, which shares with F. tularensis an intracellular and extraphagosomal lifestyle, as a delivery vehicle for F. tularensis antigens. We constructed recombinant L. monocytogenes (rLm) vaccines stably expressing seven F. tularensis proteins including IglC (rLm/iglC), and tested their immunogenicity and protective efficacy against lethal F. tularensis challenge in mice. Mice immunized intradermally with rLm/iglC developed significant cellular immune responses to F. tularensis IglC as evidenced by lymphocyte proliferation and CD4+ and CD8+ T-cell intracellular expression of interferon gamma. Moreover, mice immunized with rLm/iglC were protected against lethal challenge with F. tularensis LVS administered by the intranasal route, a route chosen to mimic airborne infection, and, most importantly, against aerosol challenge with the highly virulent Type A F. tularensis SchuS4 strain.

An intracellularly inducible gene involved in virulence and polyphosphate production in Francisella

Francisella tularensis is an intracellular pathogen capable of multiplying to high levels in macrophages. By protein analysis, only a few proteins have been shown previously to be expressed at high levels in macrophages relative to bacteria grown in culture media. To identify additional genes that show increased expression during intracellular growth, we developed a plasmid for use in Francisella based on the induction of expression of green fluorescent protein. Clones of F. tularensis subsp. novicida were identified that were fluorescent only intracellularly and not when grown in vitro. Sequencing identified a range of genes comprising some such as dnaK that are already known to be expressed intracellularly and some novel targets. One of these newly identified regulated genes, FTN1472/FTT1564, was selected for further study. Isogenic mutants were generated in F. tularensis subsp. novicida and subsp. tularensis by allelic replacement. Inactivation of the gene resulted in abolition of polyphosphate production by F. novicida, strongly supporting the bioinformatic analysis, which had suggested that the gene may encode a polyphosphate kinase. The mutants exhibited defects for intracellular growth in macrophages and were attenuated in mice, indicating a key role for the putative polyphosphate kinase in the virulence of Francisella.

Identification of an essential Francisella tularensis subsp. tularensis virulence factor

Francisella tularensis, the highly virulent etiologic agent of tularemia, is a low-dose intracellular pathogen that is able to escape from the phagosome and replicate in the cytosol. Although there has been progress in identifying loci involved in the pathogenicity of this organism, analysis of the genome sequence has revealed few obvious virulence factors. We previously reported isolation of an F. tularensis subsp. tularensis strain Schu S4 transposon insertion mutant with a mutation in a predicted hypothetical lipoprotein, FTT1103, that was deficient in intracellular replication in HepG2 cells. In this study, a mutant with a defined nonpolar deletion in FTT1103 was created, and its phenotype, virulence, and vaccine potential were characterized. A phagosomal integrity assay and lysosome-associated membrane protein 1 colocalization revealed that DeltaFTT1103 mutant bacteria were defective in phagosomal escape. FTT1103 mutant bacteria were maximally attenuated in the mouse model; mice survived, without visible signs of illness, challenge by more than 10(10) CFU when the intranasal route was used and challenge by 10(6) CFU when the intraperitoneal, subcutaneous, or intravenous route was used. The FTT1103 mutant bacteria exhibited dissemination defects. Mice that were infected by the intranasal route had low levels of bacteria in their livers and spleens, and these bacteria were cleared by 3 days postinfection. Mutant bacteria inoculated by the subcutaneous route failed to disseminate to the lungs. BALB/c or C57BL/6 mice that were intranasally vaccinated with 10(8) CFU of FTT1103 mutant bacteria were protected against subsequent challenge with wild-type strain Schu S4. These experiments identified the FTT1103 protein as an essential virulence factor and also demonstrated the feasibility of creating defined attenuated vaccines based on a type A strain.

Genome-wide screen in Francisella novicida for genes required for pulmonary and systemic infection in mice

Francisella tularensis is a gram-negative, highly infectious, aerosolizable facultative intracellular pathogen that causes the potentially life-threatening disease tularemia. To date there is no approved vaccine available, and little is known about the molecular mechanisms important for infection, survival, and dissemination at different times of infection. We report the first whole-genome screen using an inhalation mouse model to monitor infection in the lung and dissemination to the liver and spleen. We queried a comprehensive library of 2,998 sequence-defined transposon insertion mutants in Francisella novicida strain U112 using a microarray-based negative-selection screen. We were able to track the behavior of 1,029 annotated genes, equivalent to a detection rate of 75% and corresponding to approximately 57% of the entire F. novicida genome. As expected, most transposon mutants retained the ability to colonize, but 125 candidate virulence genes (12%) could not be detected in at least one of the three organs. They fell into a variety of functional categories, with one-third having no annotated function and a statistically significant enrichment of genes involved in transcription. Based on the observation that behavior during complex pool infections correlated with the degree of attenuation during single-strain infection we identified nine genes expected to strongly contribute to infection. These included two genes, those for ATP synthase C (FTN_1645) and thioredoxin (FTN_1415), that when mutated allowed increased host survival and conferred protection in vaccination experiments.

RipA, a cytoplasmic membrane protein conserved among Francisella species, is required for intracellular survival

Francisella tularensis is a highly virulent bacterial pathogen that invades and replicates within numerous host cell types, including macrophages and epithelial cells. In an effort to better understand this process, we screened a transposon insertion library of the F. tularensis live vaccine strain (LVS) for mutant strains that invaded but failed to replicate within alveolar epithelial cell lines. One such strain isolated from this screen contained an insertion in the gene FTL_1914, which is conserved among all sequenced Francisella species yet lacks significant homology to any gene with known function. A deletion strain lacking FTL_1914 was constructed. This strain did not replicate in either epithelial or macrophage-like cells, and intracellular replication was restored by the wild-type allele in trans. Based on the deletion mutant phenotype, FTL_1914 was termed ripA (required for intracellular proliferation, factor A). Following uptake by J774.A1 cells, F. tularensis LVS Delta ripA colocalized with LAMP-1 then escaped the phagosome at the same rate and frequency as wild-type LVS-infected cells. Electron micrographs of the F. tularensis LVS Delta ripA mutant demonstrated the reentry of the mutant bacteria into double membrane vacuoles characteristic of autophagosomes in a process that was not dependent on replication. The F. tularensis LVS Delta ripA mutant was significantly impaired in its ability to persist in the lung and in its capacity to disseminate and colonize the liver and spleen in a mouse model of pulmonary tularemia. The RipA protein was expressed during growth in laboratory media and localized to the cytoplasmic membrane. Thus, RipA is a cytoplasmic membrane protein conserved among Francisella species that is required for intracellular replication within the host cell cytoplasm as well as disease progression, dissemination, and virulence.

An improved vaccine for prevention of respiratory tularemia caused by Francisella tularensis SchuS4 strain

Vaccination of mice with Francisella tularensis live vaccine strain (LVS) mutants described so far have failed to induce protection in C57BL/6 mice against challenge with the virulent strain F. tularensis SchuS4. We have previously reported that a mutant of F. tularensis LVS deficient in iron superoxide dismutase (sodB(Ft)) is hypersensitive to oxidative stress and attenuated for virulence in mice. Herein, we evaluated the efficacy of this mutant as a vaccine candidate against respiratory tularemia caused by F. tularensis SchuS4. C57BL/6 mice were vaccinated intranasally (i.n.) with the sodB(Ft) mutant and challenged i.n. with lethal doses of F. tularensis SchuS4. The level of protection against SchuS4 challenge was higher in sodB(Ft) vaccinated group as compared to the LVS vaccinated mice. sodB(Ft) vaccinated mice following SchuS4 challenge exhibited significantly reduced bacterial burden in lungs, liver and spleen, regulated production of pro-inflammatory cytokines and less severe histopathological lesions compared to the LVS vaccinated mice. The sodB(Ft) vaccination induced a potent humoral immune response and protection against SchuS4 required both CD4 and CD8 T cells in the vaccinated mice. sodB(Ft) mutants revealed upregulated levels of chaperonine proteins DnaK, GroEL and Bfr that have been shown to be important for generation of a potent immune response against Francisella infection. Collectively, this study describes an improved live vaccine candidate against respiratory tularemia that has an attenuated virulence and enhanced protective efficacy than the LVS.

Identification of Francisella tularensis Himar1-based transposon mutants defective for replication in macrophages

Francisella tularensis, the etiologic agent of tularemia in humans, is a potential biological threat due to its low infectious dose and multiple routes of entry. F. tularensis replicates within several cell types, eventually causing cell death by inducing apoptosis. In this study, a modified Himar1 transposon (HimarFT) was used to mutagenize F. tularensis LVS. Approximately 7,000 Km(r) clones were screened using J774A.1 macrophages for reduction in cytopathogenicity based on retention of the cell monolayer. A total of 441 candidates with significant host cell retention compared to the parent were identified following screening in a high-throughput format. Retesting at a defined multiplicity of infection followed by in vitro growth analyses resulted in identification of approximately 70 candidates representing 26 unique loci involved in macrophage replication and/or cytotoxicity. Mutants carrying insertions in seven hypothetical genes were screened in a mouse model of infection, and all strains tested appeared to be attenuated, which validated the initial in vitro results obtained with cultured macrophages. Complementation and reverse transcription-PCR experiments suggested that the expression of genes adjacent to the HimarFT insertion may be affected depending on the orientation of the constitutive groEL promoter region used to ensure transcription of the selective marker in the transposon. A hypothetical gene, FTL_0706, postulated to be important for lipopolysaccharide biosynthesis, was confirmed to be a gene involved in O-antigen expression in F. tularensis LVS and Schu S4. These and other studies demonstrate that therapeutic targets, vaccine candidates, or virulence-related genes may be discovered utilizing classical genetic approaches in Francisella.