EmrA1 membrane fusion protein of Francisella tularensis LVS is required for resistance to oxidative stress, intramacrophage survival and virulence in mice

TolC and EmrA1 contribute to Francisella novicida multidrug resistance and modulation of host cell death

Francisella spp. are Gram-negative, facultative intracellular pathogens. Francisella tularensis causes the human disease tularemia and is considered a biological threat agent due to its high infectivity and virulence. A central aspect of Francisella virulence is its ability to dampen host immune responses. We previously identified the outer membrane channel (OMC) protein TolC as a critical F. tularensis virulence factor required for suppression of apoptotic and proinflammatory responses during macrophage infection. TolC functions as part of multidrug efflux systems and the type I secretion pathway that exports bacterial effector proteins. In these systems, TolC forms tripartite complexes together with an inner membrane transporter and periplasmic membrane fusion protein (MFP). To advance understanding of TolC function in Francisella, we analyzed OMC and MFP homologs in Francisella novicida, a widely used model species that causes a tularemia-like disease in mice. In agreement with the previous F. tularensis studies, all three OMCs present in F. novicida contributed to multidrug resistance, but only TolC was important for suppressing macrophage cell death. In addition, we identified the EmrA1 MFP as important for resisting antimicrobial compounds and dampening host cell death. In contrast to results obtained with F. tularensis, the cell death triggered during infection with the F. novicida tolC and emrA1 mutants was dominated by pyroptosis rather than apoptosis. These data expand our understanding of TolC function in Francisella and underscore both conserved and differential aspects of F. novicida and F. tularensis.

IMPORTANCE

Francisella tularensis is a Gram-negative intracellular bacterial pathogen and causative agent of tularemia. We previously identified the outer membrane channel protein TolC as contributing to antimicrobial resistance and subversion of host responses by F. tularensis. To advance understanding of TolC function in Francisella and to identify components that might work together with TolC, we took advantage of a transposon mutant library in F. novicida, a model species that causes a tularemia-like disease in mice. Our findings identify TolC and the membrane fusion protein EmrA1 as important for both antimicrobial resistance and suppression of macrophage cell death. This study also revealed differences in cell death pathways triggered by F. novicida versus F. tularensis infection that may relate to differences in virulence.

Stress responses in an Arctic microalga (Pelagophyceae) following sudden salinity change revealed by gene expression analysis

Marine microbes that have for eons been adapted to stable salinity regimes are confronted with sudden decreases in salinity in the Arctic Ocean. The episodic freshening is increasing due to climate change with melting multi-year sea-ice and glaciers, greater inflows from rivers, and increased precipitation. To investigate algal responses to lowered salinity, we analyzed the responses and acclimatation over 24 h in a non-model Arctic marine alga (pelagophyte CCMP2097) following transfer to realistic lower salinities. Using RNA-seq transcriptomics, here we show rapid differentially expressed genes related to stress oxidative responses, proteins involved in the photosystem and circadian clock, and those affecting lipids and inorganic ions. After 24 h the pelagophyte adjusted to the lower salinity seen in the overexpression of genes associated with freezing resistance, cold adaptation, and salt tolerance. Overall, a suite of ancient widespread pathways is recruited enabling the species to adjust to the stress of rapid salinity change.

Tularemia treatment: experimental and clinical data

Tularemia is a zoonosis caused by the Gram negative, facultative intracellular bacterium Francisella tularensis. This disease has multiple clinical presentations according to the route of infection, the virulence of the infecting bacterial strain, and the underlying medical condition of infected persons. Systemic infections (e.g., pneumonic and typhoidal form) and complications are rare but may be life threatening. Most people suffer from local infection (e.g., skin ulcer, conjunctivitis, or pharyngitis) with regional lymphadenopathy, which evolve to suppuration in about 30% of patients and a chronic course of infection. Current treatment recommendations have been established to manage acute infections in the context of a biological threat and do not consider the great variability of clinical situations. This review summarizes literature data on antibiotic efficacy against F. tularensis in vitro, in animal models, and in humans. Empirical treatment with beta-lactams, most macrolides, or anti-tuberculosis agents is usually ineffective. The aminoglycosides gentamicin and streptomycin remain the gold standard for severe infections, and the fluoroquinolones and doxycycline for infections of mild severity, although current data indicate the former are usually more effective. However, the antibiotic treatments reported in the literature are highly variable in their composition and duration depending on the clinical manifestations, the age and health status of the patient, the presence of complications, and the evolution of the disease. Many patients received several antibiotics in combination or successively. Whatever the antibiotic treatment administered, variable but high rates of treatment failures and relapses are still observed, especially in patients treated more then 2-3 weeks after disease onset. In these patients, surgical treatment is often necessary for cure, including drainage or removal of suppurative lymph nodes or other infectious foci. It is currently difficult to establish therapeutic recommendations, particularly due to lack of comparative randomized studies. However, we have attempted to summarize current knowledge through proposals for improving tularemia treatment which will have to be discussed by a group of experts. A major factor in improving the prognosis of patients with tularemia is the early administration of appropriate treatment, which requires better medical knowledge and diagnostic strategy of this disease.

Implication of the σE Regulon Members OmpO and σN in the ΔompA299-356-Mediated Decrease of Oxidative Stress Tolerance in Stenotrophomonas maltophilia

Outer membrane protein A (OmpA) is the most abundant porin in bacterial outer membranes. KJΔOmpA299-356, an ompA C-terminal in-frame deletion mutant of Stenotrophomonas maltophilia KJ, exhibits pleiotropic defects, including decreased tolerance to menadione (MD)-mediated oxidative stress. Here, we elucidated the underlying mechanism of the decreased MD tolerance mediated by ΔompA299-356. The transcriptomes of wild-type S. maltophilia and the KJΔOmpA299-356 mutant strain were compared, focusing on 27 genes known to be associated with oxidative stress alleviation; however, no significant differences were identified. OmpO was the most downregulated gene in KJΔOmpA299-356. KJΔOmpA299-356 complementation with the chromosomally integrated ompO gene restored MD tolerance to the wild-type level, indicating the role of OmpO in MD tolerance. To further clarify the possible regulatory circuit involved in ompA defects and ompO downregulation, σ factor expression levels were examined based on the transcriptome results. The expression levels of three σ factors were significantly different (downregulated levels of rpoN and upregulated levels of rpoP and rpoE) in KJΔOmpA299-356. Next, the involvement of the three σ factors in the ΔompA299-356-mediated decrease in MD tolerance was evaluated using mutant strains and complementation assays. rpoN downregulation and rpoE upregulation contributed to the ΔompA299-356-mediated decrease in MD tolerance. OmpA C-terminal domain loss induced an envelope stress response. Activated σE decreased rpoN and ompO expression levels, in turn decreasing swimming motility and oxidative stress tolerance. Finally, we revealed both the ΔompA299-356-rpoE-ompO regulatory circuit and rpoE-rpoN cross regulation. IMPORTANCE The cell envelope is a morphological hallmark of Gram-negative bacteria. It consists of an inner membrane, a peptidoglycan layer, and an outer membrane. OmpA, an outer membrane protein, is characterized by an N-terminal β-barrel domain that is embedded in the outer membrane and a C-terminal globular domain that is suspended in the periplasmic space and connected to the peptidoglycan layer. OmpA is crucial for the maintenance of envelope integrity. Stress resulting from the destruction of envelope integrity is sensed by extracytoplasmic function (ECF) σ factors, which induce responses to various stressors. In this study, we revealed that loss of the OmpA-peptidoglycan (PG) interaction causes peptidoglycan and envelope stress while simultaneously upregulating σP and σE expression levels. The outcomes of σP and σE activation are different and are linked to β-lactam and oxidative stress tolerance, respectively. These findings establish that outer membrane proteins (OMPs) play a critical role in envelope integrity and stress tolerance.

ThioredoxinA1 Controls the Oxidative Stress Response of Francisella tularensis Live Vaccine Strain (LVS)

Francisella tularensis is an intracellular, Gram-negative bacterium known for causing a disease known as tularemia in the Northern Hemisphere. F. tularensis is classified as a category A select agent by the CDC based on its possible use as a bioterror agent. F. tularensis overcomes oxidative stress encountered during its growth in the environment or host macrophages by encoding antioxidant enzymes such as superoxide dismutases, catalase, and alkylhydroperoxy reductase. These antioxidant enzymes are regulated by the oxidative stress response regulator, OxyR. In addition to these antioxidant enzymes, F. tularensis also encodes two thioredoxins, TrxA1 (FTL_0611) and TrxA2 (FTL_1224); however, their role in the oxidative stress response of F. tularensis is not known. This study investigated the role of thioredoxins of F. tularensis in the oxidative stress response and intracellular survival. Our results demonstrate that TrxA1 but not TrxA2 plays a major role in the oxidative stress response of F. tularensis. Most importantly, this study elucidates a novel mechanism through which the TrxA1 of F. tularensis controls the oxidative stress response by regulating the expression of the master regulator, oxyR. Further, TrxA1 is required for the intramacrophage survival and growth of Francisella. Overall, this study describes a novel role of thioredoxin, TrxA1, in regulating the oxidative stress response of F. tularensis. IMPORTANCE The role of thioredoxins in the oxidative stress response of F. tularensis is not known. This study demonstrates that of the two thioredoxins, TrxA1 is vital to counter the oxidative stress in F. tularensis live vaccine strain (LVS). Furthermore, this study shows differences in the well-studied thioredoxins of Escherichia coli. First, the expression of TrxA1 of F. tularensis is independent of the oxidative stress response regulator, OxyR. Second and most importantly, TrxA1 regulates the expression of oxyR and, therefore, the OxyR-dependent oxidative stress response of F. tularensis. Overall, this study reports a novel regulatory role of TrxA1 of F. tularensis in the oxidative stress response.

An AraC/XylS Family Transcriptional Regulator Modulates the Oxidative Stress Response of Francisella tularensis

Francisella tularensis is a Gram-negative bacterium that causes a fatal human disease known as tularemia. The Centers for Disease Control and Prevention have classified F. tularensis as a category A tier 1 select agent. The virulence mechanisms of Francisella are not entirely understood. Francisella possesses very few transcription regulators, and most of these regulate the expression of genes involved in intracellular survival and virulence. The F. tularensis genome sequence analysis reveals an AraC (FTL_0689) transcriptional regulator homologous to the AraC/XylS family of transcriptional regulators. In Gram-negative bacteria, AraC activates genes required for l-arabinose utilization and catabolism. The role of the FTL_0689 regulator in F. tularensis is not known. In this study, we characterized the role of FTL_0689 in the gene regulation of F. tularensis and investigated its contribution to intracellular survival and virulence. The results demonstrate that FTL_0689 in Francisella is not required for l-arabinose utilization. Instead, FTL_0689 specifically regulates the expression of the oxidative and global stress response, virulence, metabolism, and other key pathways genes required by Francisella when exposed to oxidative stress. The FTL_0689 mutant is attenuated for intramacrophage growth and virulence in mice. Based on the deletion mutant phenotype, FTL_0689 was termed osrR (oxidative stress response regulator). Altogether, this study elucidates the role of the osrR transcriptional regulator in tularemia pathogenesis. IMPORTANCE The virulence mechanisms of category A select agent Francisella tularensis, the causative agent of a fatal human disease known as tularemia, remain largely undefined. The present study investigated the role of a transcriptional regulator and its overall contribution to the oxidative stress resistance of F. tularensis. The results provide an insight into a novel gene regulatory mechanism, especially when Francisella is exposed to oxidative stress conditions. Understanding such Francisella- specific regulatory mechanisms will help identify potential targets for developing effective therapies and vaccines to prevent tularemia.

Aim2 and Nlrp3 Are Dispensable for Vaccine-Induced Immunity against Francisella tularensis Live Vaccine Strain

Francisella tularensis is a facultative, intracellular, Gram-negative bacterium that causes a fatal disease known as tularemia. Due to its extremely high virulence, ease of spread by aerosolization, and potential to be used as a bioterror agent, F. tularensis is classified by the CDC as a tier 1 category A select agent. Previous studies have demonstrated the roles of the inflammasome sensors absent in melanoma 2 (AIM2) and NLRP3 in the generation of innate immune responses to F. tularensis infection. However, contributions of both the AIM2 and NLRP3 to the development of vaccine-induced adaptive immune responses against F. tularensis are not known. This study determined the contributions of Aim2 and Nlrp3 inflammasome sensors to vaccine-induced immune responses in a mouse model of respiratory tularemia. We developed a model to vaccinate Aim2- and Nlrp3-deficient (Aim2^-/- and Nlrp3^-/-) mice using the emrA1 mutant of the F. tularensis live vaccine strain (LVS). The results demonstrate that the innate immune responses in Aim2^-/- and Nlrp3^-/- mice vaccinated with the emrA1 mutant differ from those of their wild-type counterparts. However, despite these differences in the innate immune responses, both Aim2^-/- and Nlrp3^-/- mice are fully protected against an intranasal lethal challenge dose of F. tularensis LVS. Moreover, the lack of both Aim2 and Nlrp3 inflammasome sensors does not affect the production of vaccination-induced antibody and cell-mediated responses. Overall, this study reports a novel finding that both Aim2 and Nlrp3 are dispensable for vaccination-induced immunity against respiratory tularemia caused by F. tularensis.

Genetic Determinants of Antibiotic Resistance in Francisella

Tularemia, caused by Francisella tularensis, is endemic to the northern hemisphere. This zoonotic organism has historically been developed into a biological weapon. For this Tier 1, Category A select agent, it is important to expand our understanding of its mechanisms of antibiotic resistance (AMR). Francisella is unlike many Gram-negative organisms in that it does not have significant plasmid mobility, and does not express AMR mechanisms on plasmids; thus plasmid-mediated resistance does not occur naturally. It is possible to artificially introduce plasmids with AMR markers for cloning and gene expression purposes. In this review, we survey both the experimental research on AMR in Francisella and bioinformatic databases which contain genomic and proteomic data. We explore both the genetic determinants of intrinsic AMR and naturally acquired or engineered antimicrobial resistance as well as phenotypic resistance in Francisella. Herein we survey resistance to beta-lactams, monobactams, carbapenems, aminoglycosides, tetracycline, polymyxins, macrolides, rifampin, fosmidomycin, and fluoroquinolones. We also highlight research about the phenotypic AMR difference between planktonic and biofilm Francisella. We discuss newly developed methods of testing antibiotics against Francisella which involve the intracellular nature of Francisella infection and may better reflect the eventual clinical outcomes for new antibiotic compounds. Understanding the genetically encoded determinants of AMR in Francisella is key to optimizing the treatment of patients and potentially developing new antimicrobials for this dangerous intracellular pathogen.

Structural and functional analysis of the Francisella lysine decarboxylase as a key actor in oxidative stress resistance

Francisella tularensis is one of the most virulent pathogenic bacteria causing the acute human respiratory disease tularemia. While the mechanisms underlying F. tularensis pathogenesis are largely unknown, previous studies have shown that a F. novicida transposon mutant with insertions in a gene coding for a putative lysine decarboxylase was attenuated in mouse spleen, suggesting a possible role of its protein product as a virulence factor. Therefore, we set out to structurally and functionally characterize the F. novicida lysine decarboxylase, which we termed LdcF. Here, we investigate the genetic environment of ldcF as well as its evolutionary relationships with other basic AAT-fold amino acid decarboxylase superfamily members, known as key actors in bacterial adaptative stress response and polyamine biosynthesis. We determine the crystal structure of LdcF and compare it with the most thoroughly studied lysine decarboxylase, E. coli LdcI. We analyze the influence of ldcF deletion on bacterial growth under different stress conditions in dedicated growth media, as well as in infected macrophages, and demonstrate its involvement in oxidative stress resistance. Finally, our mass spectrometry-based quantitative proteomic analysis enables identification of 80 proteins with expression levels significantly affected by ldcF deletion, including several DNA repair proteins potentially involved in the diminished capacity of the F. novicida mutant to deal with oxidative stress. Taken together, we uncover an important role of LdcF in F. novicida survival in host cells through participation in oxidative stress response, thereby singling out this previously uncharacterized protein as a potential drug target.

Stringent response governs the oxidative stress resistance and virulence of Francisella tularensis

Francisella tularensis is a Gram-negative bacterium responsible for causing tularemia in the northern hemisphere. F. tularensis has long been developed as a biological weapon due to its ability to cause severe illness upon inhalation of as few as ten organisms and, based on its potential to be used as a bioterror agent is now classified as a Tier 1 Category A select agent by the CDC. The stringent response facilitates bacterial survival under nutritionally challenging starvation conditions. The hallmark of stringent response is the accumulation of the effector molecules ppGpp and (p)ppGpp known as stress alarmones. The relA and spoT gene products generate alarmones in several Gram-negative bacterial pathogens. RelA is a ribosome-associated ppGpp synthetase that gets activated under amino acid starvation conditions whereas, SpoT is a bifunctional enzyme with both ppGpp synthetase and ppGpp hydrolase activities. Francisella encodes a monofunctional RelA and a bifunctional SpoT enzyme. Previous studies have demonstrated that stringent response under nutritional stresses increases expression of virulence-associated genes encoded on Francisella Pathogenicity Island. This study investigated how stringent response governs the oxidative stress response of F. tularensis. We demonstrate that RelA/SpoT-mediated ppGpp production alters global gene transcriptional profile of F. tularensis in the presence of oxidative stress. The lack of stringent response in relA/spoT gene deletion mutants of F. tularensis makes bacteria more susceptible to oxidants, attenuates survival in macrophages, and virulence in mice. This work is an important step forward towards understanding the complex regulatory network underlying the oxidative stress response of F. tularensis.

Contributions of TolC Orthologs to Francisella tularensis Schu S4 Multidrug Resistance, Modulation of Host Cell Responses, and Virulence

Francisella tularensis is a Gram-negative, facultative intracellular pathogen and the causative agent of tularemia. Previous studies with the attenuated live vaccine strain (LVS) identified a role for the outer membrane protein TolC in modulation of host cell responses during infection and virulence in the mouse model of tularemia. TolC is an integral part of efflux pumps that export small molecules and type I secretion systems that export a range of bacterial virulence factors. In this study, we analyzed TolC and its two orthologs, FtlC and SilC, present in the fully virulent F. tularensis Schu S4 strain for their contributions to multidrug efflux, suppression of innate immune responses, and virulence. We found that each TolC ortholog participated in multidrug efflux, with overlapping substrate specificities for TolC and FtlC and a distinct substrate profile for SilC. In contrast to their shared roles in drug efflux, only TolC functioned in the modulation of macrophage apoptotic and proinflammatory responses to Schu S4 infection, consistent with a role in virulence factor delivery to host cells. In agreement with previous results with the LVS, the Schu S4 ΔtolC mutant was highly attenuated for virulence in mice by both the intranasal and intradermal routes of infection. Unexpectedly, FtlC was also critical for Schu S4 virulence, but only by the intradermal route. Our data demonstrate a conserved and critical role for TolC in modulation of host immune responses and Francisella virulence and also highlight strain- and route-dependent differences in the pathogenesis of tularemia.

Role of peroxiredoxin of the AhpC/TSA family in antioxidant defense mechanisms of Francisella tularensis

Francisella tularensis is a Gram-negative, facultative intracellular pathogen and the causative agent of a lethal human disease known as tularemia. Due to its extremely high virulence and potential to be used as a bioterror agent, F. tularensis is classified by the CDC as a Category A Select Agent. As an intracellular pathogen, F. tularensis during its intracellular residence encounters a number of oxidative and nitrosative stresses. The roles of the primary antioxidant enzymes SodB, SodC and KatG in oxidative stress resistance and virulence of F. tularensis live vaccine strain (LVS) have been characterized in previous studies. However, very fragmentary information is available regarding the role of peroxiredoxin of the AhpC/TSA family (annotated as AhpC) of F. tularensis SchuS4; whereas the role of AhpC of F. tularensis LVS in tularemia pathogenesis is not known. This study was undertaken to exhaustively investigate the role of AhpC in oxidative stress resistance of F. tularensis LVS and SchuS4. We report that AhpC of F. tularensis LVS confers resistance against a wide range of reactive oxygen and nitrogen species, and serves as a virulence factor. In highly virulent F. tularensis SchuS4 strain, AhpC serves as a key antioxidant enzyme and contributes to its robust oxidative and nitrosative stress resistance, and intramacrophage survival. We also demonstrate that there is functional redundancy among primary antioxidant enzymes AhpC, SodC, and KatG of F. tularensis SchuS4. Collectively, this study highlights the differences in antioxidant defense mechanisms of F. tularensis LVS and SchuS4.

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.

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+.

Characterization of a Unique Outer Membrane Protein Required for Oxidative Stress Resistance and Virulence of Francisella tularensis

Francisella tularensis, the causative agent of tularemia, lacks typical bacterial virulence factors and toxins but still exhibits extreme virulence. The bacterial multidrug efflux systems consist of an inner membrane, a transmembrane membrane fusion protein, and an outer membrane (OM) component that form a contiguous channel for the secretion of a multitude of bacterial products. Francisella contains three orthologs of the OM proteins; two of these, termed TolC and FtlC, are important for tularemia pathogenesis. The third OM protein, SilC, is homologous to the silver cation efflux protein of other bacterial pathogens. The silC gene (FTL_0686) is located on an operon encoding an Emr-type multidrug efflux pump of F. tularensis The role of SilC in tularemia pathogenesis is not known. In this study, we investigated the role of SilC in secretion and virulence of F. tularensis by generating a silC gene deletion (ΔsilC) mutant and its transcomplemented strain. Our results demonstrate that the ΔsilC mutant exhibits increased sensitivity to antibiotics, oxidants, silver, diminished intramacrophage growth, and attenuated virulence in mice compared to wild-type F. tularensis However, the secretion of antioxidant enzymes of F. tularensis is not impaired in the ΔsilC mutant. The virulence of the ΔsilC mutant is restored in NADPH oxidase-deficient mice, indicating that SilC resists oxidative stress in vivo Collectively, this study demonstrates that the OM component SilC serves a specialized role in virulence of F. tularensis by conferring resistance against oxidative stress and silver.IMPORTANCEFrancisella tularensis, the causative agent of a fatal human disease known as tularemia, is a category A select agent and a potential bioterror agent. The virulence mechanisms of Francisella are not completely understood. This study investigated the role of a unique outer membrane protein, SilC, of a multidrug efflux pump in the virulence of F. tularensis This is the first report demonstrating that the OM component SilC plays an important role in efflux of silver and contributes to the virulence of F. tularensis primarily by providing resistance against oxidative stress. Characterization of these unique virulence mechanisms will provide an understanding of the pathogenesis of tularemia and identification of potential targets for the development of effective therapeutics and prophylactics for protection from this lethal disease.

Amino acid deprivation and central carbon metabolism regulate the production of outer membrane vesicles and tubes by Francisella

Francisella tularensis is a highly virulent Gram-negative bacterial pathogen that causes the zoonotic disease tularemia. F. novicida, a model tularemia strain, produces spherical outer membrane vesicles (OMV), as well as novel tubular vesicles and extensions of the cell surface. These OMV and tubes (OMV/T) are produced in a regulated manner and contain known virulence factors. Mechanisms by which bacterial vesicles are produced and regulated are not well understood. We performed a genetic screen in F. novicida to decipher the molecular basis for regulated OMV/T formation, and identified both hypo- and hyper-vesiculating mutants. Mutations in fumA and tktA, involved in central carbon metabolism, and in FTN_0908 and FTN_1037, of unknown function, resulted in severe defects in OMV/T production. Cysteine deprivation was identified as the signal that triggers OMV/T formation in F. novicida during growth in rich medium. We also found that fully virulent F. tularensis produces OMV/T in a similarly regulated manner. Further analysis revealed that OMV/T production is responsive to deprivation of essential amino acids in addition to cysteine, and that the hypo-vesiculating mutants are defective in responding to this signal. Thus, amino acid starvation, such as encountered by Francisella during host cell invasion, regulates the production of membrane-derived structures.

Comparative Transcriptional Analyses of Francisella tularensis and Francisella novicida

Francisella tularensis is composed of a number of subspecies with varied geographic distribution, host ranges, and virulence. In view of these marked differences, comparative functional genomics may elucidate some of the molecular mechanism(s) behind these differences. In this study a shared probe microarray was designed that could be used to compare the transcriptomes of Francisella tularensis subsp. tularensis Schu S4 (Ftt), Francisella tularensis subsp. holarctica OR960246 (Fth), Francisella tularensis subsp. holarctica LVS (LVS), and Francisella novicida U112 (Fn). To gain insight into expression differences that may be related to the differences in virulence of these subspecies, transcriptomes were measured from each strain grown in vitro under identical conditions, utilizing a shared probe microarray. The human avirulent Fn strain exhibited high levels of transcription of genes involved in general metabolism, which are pseudogenes in the human virulent Ftt and Fth strains, consistent with the process of genome decay in the virulent strains. Genes encoding an efflux system (emrA2 cluster of genes), siderophore (fsl operon), acid phosphatase, LPS synthesis, polyamine synthesis, and citrulline ureidase were all highly expressed in Ftt when compared to Fn, suggesting that some of these may contribute to the relative high virulence of Ftt. Genes expressed at a higher level in Ftt when compared to the relatively less virulent Fth included genes encoding isochorismatases, cholylglycine hydrolase, polyamine synthesis, citrulline ureidase, Type IV pilus subunit, and the Francisella Pathogenicity Island protein PdpD. Fth and LVS had very few expression differences, consistent with the derivation of LVS from Fth. This study demonstrated that a shared probe microarray designed to detect transcripts in multiple species/subspecies of Francisella enabled comparative transcriptional analyses that may highlight critical differences that underlie the relative pathogenesis of these strains for humans. This strategy could be extended to other closely-related bacterial species for inter-strain and inter-species analyses.

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.

Components of the type six secretion system are substrates of Francisella tularensis Schu S4 DsbA-like FipB protein

FipB, an essential virulence factor in the highly virulent Schu S4 strain of F. tularensis subsp. tularensis, shares sequence similarity with Disulfide Bond formation (Dsb) proteins, which can have oxidoreductase, isomerase, or chaperone activity. To further explore FipB's role in virulence potential substrates were identified by co-purification and 2D gel electrophoresis, followed by protein sequencing using mass spectrometry. A total of 119 potential substrates were identified. Proteins with predicted enzymatic activity were prevalent, and there were 19 proteins that had been previously identified as impacting virulence. Among the potential substrates were IglC, IglB, and PdpB, three components of the Francisella Type Six Secretion System (T6SS), which is also essential for virulence. T6SS are widespread in Gram-negative pathogens, but have not been reported to be dependent on Dsb-like proteins for assembly or function. The presented results suggest that FipB affects IglB and IglC substrates differently. In a fipB mutant there were differences in free sulfhydryl accessibility of IglC, but not IglB, when compared to wild-type bacteria. However, for both proteins FipB appears to act as a chaperone that facilitates proper folding and conformation. Understanding the role FipB plays the assembly and structure in this T6SS may reveal critical aspects of assembly that are common and novel among this widely distributed class of secretion systems.

Temperature-Dependent Gentamicin Resistance of Francisella tularensis is Mediated by Uptake Modulation

Gentamicin (Gm) is an aminoglycoside commonly used to treat bacterial infections such as tularemia – the disease caused by Francisella tularensis. In addition to being pathogenic, F. tularensis is found in environmental niches such as soil where this bacterium likely encounters Gm producers (Micromonospora sp.). Here we show that F. tularensis exhibits increased resistance to Gm at ambient temperature (26°C) compared to mammalian body temperature (37°C). To evaluate whether F. tularensis was less permeable to Gm at 26°C, a fluorescent marker [Texas Red (Tr)] was conjugated with Gm, yielding Tr-Gm. Bacteria incubated at 26°C showed reduced fluorescence compared to those at 37°C when exposed to Tr-Gm suggesting that uptake of Gm was reduced at 26°C. Unconjugated Gm competitively inhibited uptake of Tr-Gm, demonstrating that this fluorescent compound was taken up similarly to unconjugated Gm. Lysates of F. tularensis bacteria incubated with Gm at 37°C inhibited the growth of Escherichia coli significantly more than lysates from bacteria incubated at 26°C, further indicating reduced uptake at this lower temperature. Other facultative pathogens (Listeria monocytogenes and Klebsiella pneumoniae) exhibited increased resistance to Gm at 26°C suggesting that the results generated using F. tularensis may be generalizable to diverse bacteria. Regulation of the uptake of antibiotics provides a mechanism by which facultative pathogens survive alongside antibiotic-producing microbes in nature.

Antioxidant Defenses of Francisella tularensis Modulate Macrophage Function and Production of Proinflammatory Cytokines

Francisella tularensis, the causative agent of a fatal human disease known as tularemia, has been used in the bioweapon programs of several countries in the past, and now it is considered a potential bioterror agent. Extreme infectivity and virulence of F. tularensis is due to its ability to evade immune detection and to suppress the host's innate immune responses. However, Francisella-encoded factors and mechanisms responsible for causing immune suppression are not completely understood. Macrophages and neutrophils generate reactive oxygen species (ROS)/reactive nitrogen species as a defense mechanism for the clearance of phagocytosed microorganisms. ROS serve a dual role; at high concentrations they act as microbicidal effector molecules that destroy intracellular pathogens, and at low concentrations they serve as secondary signaling messengers that regulate the expression of various inflammatory mediators. We hypothesized that the antioxidant defenses of F. tularensis maintain redox homeostasis in infected macrophages to prevent activation of redox-sensitive signaling components that ultimately result in suppression of pro-inflammatory cytokine production and macrophage microbicidal activity. We demonstrate that antioxidant enzymes of F. tularensis prevent the activation of redox-sensitive MAPK signaling components, NF-κB signaling, and the production of pro-inflammatory cytokines by inhibiting the accumulation of ROS in infected macrophages. We also report that F. tularensis inhibits ROS-dependent autophagy to promote its intramacrophage survival. Collectively, this study reveals novel pathogenic mechanisms adopted by F. tularensis to modulate macrophage innate immune functions to create an environment permissive for its intracellular survival and growth.

Preclinical testing of a vaccine candidate against tularemia

Tularemia is caused by a gram-negative, intracellular bacterial pathogen, Francisella tularensis (Ft). The history weaponization of Ft in the past has elevated concerns that it could be used as a bioweapon or an agent of bioterrorism. Since the discovery of Ft, three broad approaches adopted for tularemia vaccine development have included inactivated, live attenuated, or subunit vaccines. Shortcomings in each of these approaches have hampered the development of a suitable vaccine for prevention of tularemia. Recently, we reported an oxidant sensitive mutant of Ft LVS in putative EmrA1 (FTL_0687) secretion protein. The emrA1 mutant is highly sensitive to oxidants, attenuated for intramacrophage growth and virulence in mice. We reported that EmrA1 contributes to oxidant resistance by affecting the secretion of antioxidant enzymes SodB and KatG. This study investigated the vaccine potential of the emrA1 mutant in prevention of respiratory tularemia caused by Ft LVS and the virulent SchuS4 strain in C57BL/6 mice. We report that emrA1 mutant is safe and can be used at an intranasal (i. n.) immunization dose as high as 1x106 CFU without causing any adverse effects in immunized mice. The emrA1 mutant is cleared by vaccinated mice by day 14-21 post-immunization, induces minimal histopathological lesions in lungs, liver and spleen and a strong humoral immune response. The emrA1 mutant vaccinated mice are protected against 1000-10,000LD100 doses of i.n. Ft LVS challenge. Such a high degree of protection has not been reported earlier against respiratory challenge with Ft LVS using a single immunization dose with an attenuated mutant generated on Ft LVS background. The emrA1 mutant also provides partial protection against i.n. challenge with virulent Ft SchuS4 strain in vaccinated C57BL/6 mice. Collectively, our results further support the notion that antioxidants of Ft may serve as potential targets for development of effective vaccines for prevention of tularemia.

Roles of reactive oxygen species-degrading enzymes of Francisella tularensis SCHU S4

Francisella tularensis is a facultative intracellular bacterium utilizing macrophages as its primary intracellular habitat and is therefore highly capable of resisting the effects of reactive oxygen species (ROS), potent mediators of the bactericidal activity of macrophages. We investigated the roles of enzymes presumed to be important for protection against ROS. Four mutants of the highly virulent SCHU S4 strain with deletions of the genes encoding catalase (katG), glutathione peroxidase (gpx), a DyP-type peroxidase (FTT0086), or double deletion of FTT0086 and katG showed much increased susceptibility to hydrogen peroxide (H2O2) and slightly increased susceptibility to paraquat but not to peroxynitrite (ONOO(-)) and displayed intact intramacrophage replication. Nevertheless, mice infected with the double deletion mutant showed significantly longer survival than SCHU S4-infected mice. Unlike the aforementioned mutants, deletion of the gene coding for alkyl-hydroperoxide reductase subunit C (ahpC) generated a mutant much more susceptible to paraquat and ONOO(-) but not to H2O2. It showed intact replication in J774 cells but impaired replication in bone marrow-derived macrophages and in internal organs of mice. The live vaccine strain, LVS, is more susceptible than virulent strains to ROS-mediated killing and possesses a truncated form of FTT0086. Expression of the SCHU S4 FTT0086 gene rendered LVS more resistant to H2O2, which demonstrates that the SCHU S4 strain possesses additional detoxifying mechanisms. Collectively, the results demonstrate that SCHU S4 ROS-detoxifying enzymes have overlapping functions, and therefore, deletion of one or the other does not critically impair the intracellular replication or virulence, although AhpC appears to have a unique function.