Tularensis LVS Research Articles

Role of LpnA in Francisella tularensis Susceptibility to Resazurin

Francisella tularensis is classified by the Centers for Disease Control and Prevention as a Category A bioterrorism agent due to its low infectious dose, ease of aerosolization, and high mortality rate. Given there is no licensed tularemia vaccine and the possibility of antibiotic-resistant F. tularensis strains, there is an urgent need to develop new treatments against this bacterium. Resazurin (Rz) has bactericidal activity against F. tularensis and other gram-negative bacteria including the human pathogens Neisseria gonorrhoeae and Helicobacter pylori. However, the mechanism of action for Rz has yet to be determined. To identify targets of Rz, we screened for Rz-resistant (Rzr) mutants of F. tularensis LVS. Whole genome sequencing was performed on 42 Rzr isolates and all contained mutations within the coding regions of FTL_0421 and FTL_1504. FTL_0421 encodes for LpnA, the major lipoprotein in F. tularensis. The FTL_0421 mutation in Rzr1 resulted in loss of LpnA expression which correlated with increased sensitivity to osmotic stress. To investigate the individual role of lpnA in Rz-resistance, a deletion mutant (ΔlpnA) was generated using standard molecular genetics techniques. An agar dilution assay was then performed to determine the Rz susceptibility of the ΔlpnA strain. A minimum inhibitory concentration (MIC) of 2.75μg/mL was achieved for the ΔlpnA and the LVS wild-type strain. This data suggests mutation of lpnA alone is not sufficient to confer resistance to resazurin. In the future, we will investigate whether lpnA in combination with other genes like FTL_1504 play a role in resazurin susceptibility in F. tularensis. (Supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence & the NASA Space Grant Consortium)

A molecular biology approach to identify Francisella tularensis proteins that interact with Band 3 of human erythrocytes.

Francisella tularensis is a pathogenic gram-negative bacterium that causes the zoonotic disease Tularemia. In addition, F. tularensis is classified as a Class A Bioterrorism agent by the CDC due to the ease of aerosolization and the ability of this bacterium to cause fatal infection in low doses. Previous studies suggest that invasion of mammalian erythrocytes by F. tularensis helps to increase colonization of ticks, an arthropod vector of this pathogen. Our laboratory previously found that the erythrocyte membrane glycoprotein, Band 3 is required by F. tularensis for invasion of red blood cells. Therefore, we predict that bacterial proteins interact with Band 3 to facilitate invasion. To identify these proteins, we will express codon-optimized Band 3 linked to a Glutathione-S transferase tag in F. tularensis LVS. Subsequent pull-down assays may reveal F. tularensis protein binding partners to Band 3 responsible for facilitating erythrocyte invasion.

Peptidoglycan modulation by Francisella tularensis

HAYDEN HESS, STUART CANTLAY, JOSEPH HORZEMPA, Department of Biomedical Sciences, West Liberty University, West Liberty, WV 26074 and MELANIE SAL, Department of Biology, West Virginia Wesleyan College, Buckhannon, WV 26201. Peptidoglycan modulation by Francisella tularensis. Peptidoglycan, a crucial component of bacterial cell walls, plays a fundamental role in maintaining cell shape, integrity, and resistance to osmotic pressure. Mycoplasmas are the only bacteria that do not produce peptidoglycan cell walls. Other peptidoglycan-producing bacteria are capable of transiently existing without a peptidoglycan as an “L-form”, but only on solid media. Preliminary observations made by our laboratory suggested that Francisella tularensis bacteria do not produce detectible levels of peptidoglycan while growing in liquid media. However, increasing salt concentration induces the production of peptidoglycan in all bacteria. In this study we confirmed and extended these observations by incubating bacteria with fluorescent D-alanine (which integrates into the peptidoglycan during synthesis). Fluorescence microscopy revealed that very few F. tularensis bacteria produce detectable peptidoglycan in low salt media, while seemingly all individual bacteria produce this layer in high salt media. Interestingly, the genome of F. tularensis encodes two distinct beta-lactamases. Here we show that both the wild-type F. tularensis LVS and beta-lactamase null mutant were both resistant to high levels of beta-lactam antibiotics in low salt liquid media. However, only the wild type and complemented mutant strains were resistant to beta-lactam antibiotics in high salt media while the beta lactamase null mutant was susceptible. These data further support the conclusion that F. tularensis does not produce, nor need, peptidoglycan in low salt liquid media. Future studies will investigate whether peptidoglycan modulation is required to avoid detection by the innate immune system.

Lipidomic analysis by LC-MS/MS of wildtype, mutant and VBNC strains of Francisella tularensis LVS.

Francisella tularensis is one of the many bacteria that can enter a viable but nonculturable state (VBNC). The VBNC state acts as an adaptive strategy that allows for long-term survival of bacteria in unfavorable environments. A defining characteristic of VBNC bacteria is a loss of culturability, making VBNC bacteria difficult to detect in the environment using standard plating assays. Failure to identify VBNC bacteria in the food industry or water distribution centers poses a serious risk to human health. Our preliminary data suggests that lipid metabolism plays a role in VBNC adaptation in F. tularensis. To investigate further, genetic deletion mutants have been constructed from two predicted lipid metabolism genes, FTL_1569 (phosphoglycolate phosphatase) and FTL_1570 (phospholipase D family protein). Lipid extracts were collected from wildtype, mutant and VBNC strains of F. tularensis to determine any lipid species associated with the VBNC state. A single-phase lipid extraction method was used to obtain lipids from F. tularensis cultures grown for 24 hours (culturable) and 336 hours (VBNC). Liquid chromatography with tandem mass spectrometry (LC-MS-MS) and the ThermoFisher Compound Discoverer software were utilized for lipid identification, and comparative analyses. Differences in the lipid profile of wildtype, mutant and VBNC strains of F. tularensis have been identified. (Supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence)

Phenotypic and transcriptional characterization of F. tularensis LVS during transition into a viable but non-culturable state.

Francisella tularensis is a gram-negative, intracellular pathogen which can cause serious, potentially fatal, illness in humans. Species of F. tularensis are found across the Northern Hemisphere and can infect a broad range of host species, including humans. Factors affecting the persistence of F. tularensis in the environment and its epidemiology are not well understood, however, the ability of F. tularensis to enter a viable but non-culturable state (VBNC) may be important. A broad range of bacteria, including many pathogens, have been observed to enter the VBNC state in response to stressful environmental conditions, such as nutrient limitation, osmotic or oxidative stress or low temperature. To investigate the transition into the VBNC state for F. tularensis, we analyzed the attenuated live vaccine strain, F. tularensis LVS grown under standard laboratory conditions. We found that F. tularensis LVS rapidly and spontaneously enters a VBNC state in broth culture at 37°C and that this transition coincides with morphological differentiation of the cells. The VBNC bacteria retained an ability to interact with both murine macrophages and human erythrocytes in in vitro assays and were insensitive to treatment with gentamicin. Finally, we present the first transcriptomic analysis of VBNC F. tularensis, which revealed clear differences in gene expression, and we identify sets of differentially regulated genes which are specific to the VBNC state. Identification of these VBNC specific genes will pave the way for future research aimed at dissecting the molecular mechanisms driving entry into the VBNC state.

Chronically hypertensive transgenic mice expressing human AT1R haplotype-I exhibit increased susceptibility to Francisella tularensis.

Age-related illnesses, including hypertension and accompanying metabolic disorders, compromise immunity and exacerbate infection-associated fatalities. Renin-angiotensin system (RAS) is the key mechanism that controls blood pressure. Upregulation of RAS through angiotensin receptor type 1 (AT1R), a G-protein coupled receptor, contributes to the pathophysiological consequences leading to vascular remodeling, hypertension, and end-organ damage. Genetic variations that increase the expression of human AT1R may cause the above pathological outcomes associated with hypertension. Previously we have shown that our chronically hypertensive transgenic (TG) mice containing the haplotype-I variant (Hap-I, hypertensive genotype) of human AT1R (hAT1R) gene are more prone to develop the metabolic syndrome-related disorders as compared to the TG mice containing the haplotype-II variant (Hap-II, normotensive genotype). Since aging and an increased risk of hypertension can impact multiple organ systems in a complex manner, including susceptibility to various infections, the current study investigated the susceptibility and potential effect of acute bacterial infection using a Gram-negative intracellular bacterial pathogen, Francisella tularensis in our hAT1R TG mice. Our results show that compared to Hap-II, F. tularensis-infected aged Hap-I TG mice have significantly higher mortality post-infection, higher bacterial load and lung pathology, elevated inflammatory cytokines and altered gene expression profile favoring hypertension and inflammation. Consistent with worsened phenotype in aged Hap-I mice post-Francisella infection, gene expression profiles from their lungs revealed significantly altered expression of more than 1,400 genes. Furthermore, bioinformatics analysis identified genes associated with RAS and IFN-γ pathways regulating blood pressure and inflammation. These studies demonstrate that haplotype-dependent over-expression of the hAT1R gene leads to enhanced susceptibility and lethality due to F. tularensis LVS infection, which gets aggravated in aged animals. Clinically, these findings will help in exploring the role of AT1R-induced hypertension and enhanced susceptibility to infection-related respiratory diseases.

Applying RNA-Seq to investigate the transition into a Viable But Non Culturable State (VBNC) for the intracellular pathogen, Francisella tularensis LVS.

Many species of bacteria, under conditions of stress or nutrient limitation, enter a state of dormancy referred to as viable but non-culturable (VBNC). VBNC bacteria persist in the environment, are difficult to detect and identify by many standard laboratory methods and can be altered in their susceptibility to antibiotics. Entry into the VBNC state is often accompanied by morphological changes; however, the mechanisms underlying this are poorly understood. Francisella transitions rapidly and spontaneously to a VBNC state, and therefore has the potential to be an excellent model organism for the study of this phenomenon. To investigate the transcriptome of VBNC F. tularensis LVS we extracted RNA from culturable and VBNC cells and carried out an RNA-Seq analysis using both Long-read Nanopore and Illumina Sequencing. Differentially expressed genes (DEGs) were identified using a DE-Seq pipeline. Over 300 genes were significantly upregulated and ~100 genes were down regulated in VBNC cells. Amongst the upregulated genes were some involved in the transport of metals or small molecules, and we have also identified putative transcriptional regulators that may be master controllers of the VBNC process. Our data represents the first transcriptomic analysis of F. tularensis LVS as it transitions into the VBNC state. Identifying genes that are involved in this transition is a critical first step in understanding the mechanisms that drive F. tularensis LVS into the VBNC state and will help us identify factors that allow the resuscitation of these bacteria. This will have important implications for understanding both environmental persistence and pathogenicity of Francisella species. (Supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence)

Role of KatG in Francisella tularensis Susceptibility to Resazurin.

The CDC classifies Francisella tularensis as a Category A bioterrorism agent. Due to the risk of potential release of antibiotic-resistant F. tularensis strains, new therapeutics against F. tularensis must be developed. Resazomycins are resazurin (Rz)-based compounds that exhibit antimicrobial activity against F. tularensis and other gram-negative bacteria. The mode of action of resazomycins is not understood, but potential targets of the antibiotic were identified in a high throughput screen for Rz-resistant isolates. The FTL_1504 gene was identified as being mutated in all of the Rz-resistant (Rzr) strains sequenced. FTL_1504 encodes for the catalase KatG which catalyzes the decomposition of hydrogen peroxide to water and oxygen. Moreover, KatG has also been shown to play an essential role in the activity of a different antibiotic, isonazid. Four different coding mutations were characterized in katG that included single base pair substitutions and a deletion with all resulting in the introduction of a premature stop codon. All Rzr mutants tested had reduced catalase activity compared to wild-type F. tularensis LVS with three of the four mutations resulting in loss of KatG expression. Based on this data, we hypothesize KatG plays a role in the susceptibility of F. tularensis to resazurin. To address this, we are generating a katG deletion mutant in F. tularensis LVS and measuring its Rz susceptibility via agar dilution assays. Investigation of the contribution of KatG may unveil a unique role for the oxidative stress response in the mode of action of this family of antibiotics

Deletion of FTL_1199 to determine the role of this gene in erythrocyte invasion by Francisella tularensis.

Francisella tularensis is a bacterium that induces the zoonotic disease tularemia. In the course of infection, F. tularensis bacteria invade erythrocytes, a phenomenon that heightens the colonization of ticks after a blood meal. To better understand the mechanism of erythrocyte invasion, we hypothesized that transcription of bacterial genes significant in erythrocyte invasion would be upregulated upon exposure to these host cells. An RNA-seq unveiled that transcription of 7% of F. tularensis genes augment when in erythrocyte presence. Of these, we pinpointed a putative transcriptional regulator, FTL_1199. The goal was to delete FTL_1199 in F. tularensis LVS. SOE PCR amplified and duplicated the up and downstream regions of the target gene in tandem into a shuttle vector that is insecure within F. tularensis. This newly generated plasmid, pDEL1199, was mobilized inside of F. tularensis by conjugation. Merodiploid strains generated by homologous recombination were isolated and transformed with pGUTS. Expression of I-Sce1 within the merodiploid produces a double-stranded break. This breakage resulted in a second recombination that either ensued to wild-type or deletion of FTL_1199 deduced through a PCR. Finally, in DFTL_1199 strains, pGUTS was cured by successive cultivation in the absence of selection followed by replica-plating on chocolate II agar ± kanamycin. Gentamicin protection assays showed reduced levels of erythrocyte invasion for F. tularensis DFTL_1199 compared to wild type bacteria. However, complementation of FTL_1199 to the deletion mutant restored this strain’s ability to invade red blood cells. These findings demonstrate that FTL_1199 is important for erythrocyte invasion by F. tularensis.
 
 (Supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence, R15HL14735 from NHLBI, and funds from the NASA West Virginia Space Grant Consortium).

Characterization of two predicted lipid metabolism genes (FTL_1569 and FTL_1570) in the intracellular pathogen, Francisella tularensis LVS.

Many species of bacteria enter a viable but non-culturable (VBNC) state when exposed to stressful conditions. The VBNC state acts as an adaptive strategy that allows for long-term survival of bacteria in unfavorable environments. VBNC bacteria are difficult to detect in the environment due to an inability to grow on nutrient agar. Although the ability to enter the VBNC state may be advantageous for bacteria, it poses a risk to human health. Francisella tularensis, transitions rapidly and spontaneously to the VBNC state, giving it the potential to be an excellent model organism for the study of this phenomenon. Microscopic observations reveal that changes in cell morphology are coincident with the VBNC state. To characterize these morphological changes, fluorescence microscopy imaging using membrane lipid staining has been used in the live vaccine strain of F. tularensis. Microscopy imaging suggested that lipid metabolism may be important for the transition into the VBNC state, and the change in morphology. To investigate further, genetic deletion mutants have been constructed from two predicted lipid metabolism genes, FTL_1569 (phosphoglycolate phosphatase) and FTL_1570 (phospholipase D family protein). Gentamicin protection assays with immune cells and red blood cells have been conducted to investigate the role of these genes in virulence of F. tularensis. Our preliminary results suggest that lipid metabolism may play roles in both persistence and in invasion in F. tularensis. (This work was supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence).

Screening putative transcriptional regulators in Francisella tularensis for attenuation using a multifaceted approach.

Francisella tularensis is a gram-negative intracellular pathogen that produces a severe infection known as Tularemia. RNA-Seq data revealed that in the presence of erythrocytes, several genes encoding putative transcriptional regulators were modulated: FTL_0671, FTL_1199, and FTL_1665. Gene deletion strains were constructed using Francisella tularensis LVS. The objective of this project was to screen these gene deletion strains for attenuation. A multifaceted approach was utilized to determine the level of replication within macrophages and overall attenuation in vivo. Transforming the bacteria with green fluorescent protein allowed a plate reader to visualize and quantify intracellular growth in macrophages. However, inconsistencies in the data from these experiments led to the utilization of a gentamicin protection assay. This protocol provided a more accurate and reliable method of determining intracellular replication. The results of this experiment revealed a significant increase in the replication of FTL_1665 within macrophages. We sought to determine if these results would translate to hypervirulence in a live model. A chicken embryo infection model confirmed that FTL_1665 was significantly hypervirulent in vivo. In the future, we plan to experiment with the upregulation of the target gene to produce an attenuated strain. This gene may also serve as a potential drug target.

Role of FTL_0445 in Francisella tularensis Susceptibility to Resazurin

Tularemia is a potentially fatal disease caused by the Category A bioterrorism agent Francisella tularensis. Aminoglycosides, fluoroquinolones, and tetracyclines can be used to treat tularemia; however, there is a high incidence of relapse and treatment failures when using these drugs. Furthermore, there is no tularemia vaccine licensed for use in the United States. Therefore, new antibiotics that target F. tularensis are being investigated. A novel family of resazurin-based antibiotics called resazomycins exhibit antimicrobial activity against F. tularensis and other Gram-negative pathogens including Neisseria gonorrhoeae. The mode of action of resazomycins has yet to be determined. To elucidate potential targets of resazurin (Rz), we screened for spontaneous Rz-resistant (Rzr) F. tularensis LVS mutants. Through the screen, 93% of all Rzr mutants sequenced contained mutations within the coding regions of FTL_0421, FTL_0895, and FTL_1504. In addition, 100% of all Rzr mutants sequenced contained a mutation approximately 50bp upstream of the gene FTL_0445. To understand the effect the mutation has on transcription of FTL_0445, RNA was isolated from wild-type LVS and an Rzr mutant (Rzr1). Quantitative reverse transcription PCR revealed FTL_0445 was upregulated in Rzr1 compared to wild-type LVS. To determine the role of FTL_0445 in Rz resistance, a FTL_0445 null deletion mutant is being generated in Rzr1 using standard molecular genetic techniques. Upon completion, the Rz sensitivity of the deletion mutant will be assessed using agar dilution assays. Understanding the role of FTL_0445 in Rz susceptibility would facilitate further development of these compounds as potential treatments for tularemia and gonorrhea.

Improvement of Approaches to the Verification of the Vaccine Strain <i>Francisella tularensis</i> 15 NIIEG during Long-Term Storage

The aim of the study was to improve the methods for verifying the vaccine strain Francisella tularensis 15 NIIEG during long-term storage under current conditions.Materials and methods. The paper summarizes the results of studying the phenotypic and genetic properties of lyophilized cultures of the vaccine strain F. tularensis 15 NIIEG (1953, 1966, 1969, 1987, 1990, 2003, 2012 and 2013) stored at SCEMAP for a period of one to 60 years.Results and discussion. Previous studies have revealed that freeze-dried cultures of F. tularensis 15 NIIEG generally had the characteristics of the vaccine strain, with the exception of deviations from the regulatory requirements for residual virulence and specific safety. The stability of preservation of deletions in the pilA and pilE genes (the region of differentiation RD19) and the genes encoding lpp lipoprotein (RD18) in the vaccine strain, which was stored for various periods of time in a lyophilized state, has been confirmed. The vaccine-strain-specific mutation C178404T (by the genome of F. tularensis LVS strain, GenBank NCBI no. CP009694) has been identified, and an approach to determine it has been developed. The data obtained are promising as regards using the above deletions in the RD18/RD19 regions in combination with the C178404T mutation to assess the authenticity of the vaccine strain using molecular genetic methods. Thus, the conducted retrospective analysis of the data on the cultures of tularemia microbe vaccine strain from the 1940s to 2013 and the gathered experimental data, made it possible to supplement the uniform requirements for the manufacture, study, maintenance, storage and movement of F. tularensis 15 NIIEG vaccine strain with new evidence. Based on the results obtained, the authors have drawn a draft methodological recommendations of the federal level “Vaccinal strain Francisella tularensis 15 NIIEG: order of handling”.

PdpC, a secreted effector protein of the type six secretion system, is required for erythrocyte invasion by Francisella tularensis LVS.

Francisella tularensis is a gram negative, intracellular pathogen that is the causative agent of the potentially fatal disease, tularemia. During infection, F. tularensis is engulfed by and replicates within host macrophages. Additionally, this bacterium has also been shown to invade human erythrocytes and, in both cases, the Type Six Secretion System (T6SS) is required for these host-pathogen interaction. One T6SS effector protein, PdpC, is important for macrophage infection, playing a role in phagolysosomal escape and intracellular replication. To determine if PdpC also plays a role in erythrocyte invasion, we constructed a pdpC-null mutant in the live vaccine strain, F. tularensis LVS. We show that PdpC is required for invasion of human and sheep erythrocytes during in vitro assays and that reintroduction of a copy of pdpC, in trans, rescues this phenotype. The interaction with human erythrocytes was further characterized using double-immunofluorescence microscopy to show that PdpC is required for attachment of F. tularensis LVS to erythrocytes as well as invasion. To learn more about the role of PdpC in erythrocyte invasion we generated a strain of F. tularensis LVS expressing pdpC-emgfp. PdpC-EmGFP localizes as discrete foci in a subset of F. tularensis LVS cells grown in broth culture and accumulates in erythrocytes during invasion assays. Our results are the first example of a secreted effector protein of the T6SS shown to be involved in erythrocyte invasion and indicate that PdpC is secreted into erythrocytes during invasion.

PdpC, a Type Six Secretion System Substrate, is Required for Erythrocyte Invasion in Francisella tularensis LVS

Francisella tularensis is an intracellular pathogen and the causative agent of tularemia. The F. tularensis type six secretion system (T6SS) is required for phagolysosomal escape and invasion of erythrocytes. An effector of the T6SS, PdpC, is required for phagosomal escape and we wanted to test if PdpC was also required for erythrocyte invasion. We constructed a pdpC-null mutant in the live vaccine strain, F. tularensis LVS. The pdpC-null strain is required for invasion of both human and sheep erythrocytes and reintroduction of a copy of pdpC, in trans, rescues this phenotype. Differential Immuno-Fluorescence Microscopy (DIFM) showed that the pdpC-null strain is affected in attachment as well as invasion. Further, a fluorescently labelled pdpC-null strain of F. tularensis LVS was unable to proliferate in THP-1 human peripheral blood monocyte cells. Finally, we constructed a fluorescent fusion of pdpC to emgfp and the resulting PdpC-EmGFP fusion protein localizes as discrete foci in a subset of broth cultured F. tularensis LVS cells. Our results confirm previous observations that PdpC is required for infection and virulence in phagocytic host cells and are the first description of an effector of the Type Six Secretion System that is required for erythrocyte invasion. The pdpC-emgfp strain will be a useful tool to further characterize the role of PdpC in both macrophage infection and red blood cell invasion. (This research was made possible by NASA West Virginia Space Grant Consortium Training Grant #NNX15A101H and by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence).

Deletion of FTL_1199 to determine the role of this gene in erythrocyte invasion by Francisella tularensis.

Francisella tularensis is a bacterium that induces the zoonotic disease tularemia. In the course of infection, F. tularensis bacteria invade erythrocytes, a phenomenon that heightens the colonization of ticks after a blood meal. To better understand the mechanism of erythrocyte invasion, we hypothesized that transcription of bacterial genes significant in erythrocyte invasion would be upregulated upon exposure to these host cells. An RNA-seq unveiled that transcription of 7% of F. tularensis genes augment when in erythrocyte presence. Of these, we pinpointed three putative transcriptional regulators, namely FTL_0671, FTL_1199, and FTL_1665. The goal was to delete FTL_1199 in F. tularensis LVS. Splicing by overlap extension PCR amplified and duplicated the up and downstream (~500 bp each) regions of the target gene in tandem into a shuttle vector that is insecure within F. tularensis. This newly generated plasmid, pDEL1199, was mobilized inside of F. tularensis by conjugation. Merodiploid strains generated by homologous recombination were isolated and transformed with pGUTS – a stable plasmid that encodes a homing endonuclease (I-SceI) and a kanamycin resistance cassette. Expression of I-SceI within the merodiploid produces a double-stranded break in pDEL1199 that had previously integrated in the chromosome. This breakage resulted in a second recombination that either ensued to wild-type or deletion of FTL_1199 deduced through a PCR. Finally, in DFTL_1199 strains, pGUTS was cured by successive cultivation in the absence of selection followed by replica-plating on chocolate II agar ± kanamycin. Gentamicin protection assays involving F. tularensis DFTL_1199 suggest that FTL_1199 is important in erythrocyte invasion.
 
 (Supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence, R15HL14735 from NHLBI, and funds from the NASA West Virginia Space Grant Consortium).

Deletion of FTL_0671 to determine the role of this gene in erythrocyte invasion by Francisella tularensis

Francisella tularensis is a bacterium that causes the zoonotic disease tularemia. During infection, F. tularensis bacteria invade erythrocytes, a phenomenon that enhances the colonization of ticks after a blood meal. To gain insight into the mechanism of erythrocyte invasion, we hypothesized that transcription of bacterial genes important in red blood cell invasion would increase upon exposure to these host cells. An RNA-seq analysis revealed that transcription of 7% of F. tularensis genes increased when in the presence of erythrocytes. Of these, we identified three putative transcriptional regulators, namely FTL_0671, FTL_1199, and FTL_1665. The goal of this work was to determine the role of FTL_0671 in erythrocyte invasion by F. tularensis LVS. After successfully generating a DFTL_0671 strain by homologous recombination, we tested the ability of this mutant to invade erythrocytes using a gentamicin protection assay. As seemingly more DFTL_0671 bacteria invaded red blood cells than wild-type LVS, this preliminary experiment suggested that FTL_0671 may restrict erythrocyte invasion. In addition to being a potential transcriptional regulator, FTL_0671 is homologous to PanK, a protein required for CoA biosynthesis in some bacteria. To determine whether FTL_0671 functioned in this capacity, LVS or DFTL_0671 was cultured in a chemically defined medium, devoid of CoA. In this experiment, DFTL_0671 did not exhibit diminished growth, suggesting that either FTL_0671 does not encode an enzyme required for CoA biosynthesis, or that F. tularensis can utilize an alternative pathway to synthesize this critical coenzyme.

The role of three putative transcriptional regulators of Francisella tularensis during replication within host cells

Francisella tularensis is a pathogenic bacterium capable of replication within host monocytes and macrophages and is the causative agent of Tularemia. We utilized the attenuated live vaccine strain to model the infection and replication of F. tularensis in the mouse macrophage RAW 264.7 cell line. Our lab has previously identified three genes predicted to encode transcriptional regulators that are upregulated in the presence of human blood cells: FTL_0671, FTL_1199, and FTL_1665. The objective of this project is to determine if these genes play a role in the pathogenesis of F. tularensis. 
 Wild type F. tularensis LVS and mutant bacteria with FTL_0671, FTL_1199, and FTL_1665 gene deletions were constructed and engineered to express an extremely bright variant of Emerald Green Fluorescent Protein (encoded in the plasmid, pKHEG). These strains were used to infect RAW 264.7 macrophages and fluorescence was monitored over time using a plate reader. Our results suggest that FTL_1199 and FTL_1665 are attenuated for intracellular replication indicating that the products of the mutated genes may be important for pathogenesis.

Characterization of two predicted lipid metabolism genes (FTL_1569 and FTL_1570) in the intracellular pathogen, Francisella tularensis LVS

Many species of bacteria enter a viable but non-culturable (VBNC) state when exposed to stressful conditions. Since cells in the VBNC state can only be resuscitated under favorable conditions, this acts as an adaptive strategy that allows for long-term survival of bacteria in unfavorable environments. The ability to enter the VBNC state may be advantageous for bacteria, but it poses a risk to human health. Francisella tularensis,transitions rapidly and spontaneously to the VBNC state, giving it the potential to be an excellent model organism for the study of this phenomenon. Although dormant, these cells are still biologically active, and can stimulate cytokine production and interact with host cells. Microscopic observations reveal that changes in cell morphology are coincident with the VBNC state. To characterize these morphological changes, fluorescence microscopy using membrane lipid (acridine orange 10-nonyl bromide) staining has been used and these experiments suggest that lipid metabolism may be important for the transition into the VBNC state. To investigate further, genetic mutants have been constructed of two genes, FTL_1569 (phosphoglycolate phosphatase) and FTL_1570 (Phospholipase D family protein) which are predicted to be involved in lipid metabolism. Gentamicin protection assays with immune cells and red blood cells have been conducted to investigate the role of these genes in virulence of F. tularensis. Our preliminary results suggest that lipid metabolism may play roles in both persistence and in infection of F. tularensis. (This work was supported by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence).

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

Francisella tularensis is the etiologic agent of tularemia and a Tier I Select Agent. Subspecies tularensis (Type A) is the most virulent of the four subspecies and inhalation of as few as 10 cells can cause severe disease in humans. Due to its niche as a facultative intracellular pathogen, a successful tularemia vaccine must induce a robust cellular immune response, which is best achieved by a live, attenuated strain. F. tularensis strains lacking lipopolysaccharide (LPS) O-antigen are highly attenuated, but do not persist in the host long enough to induce protective immunity. Increasing the persistence of an O-antigen mutant may help stimulate protective immunity. Alginate encapsulation is frequently used with probiotics to increase persistence of bacteria within the gastrointestinal system, and was used to encapsulate the highly attenuated LVS O-antigen mutant WbtIG191V. Encapsulation with alginate followed by a poly-L-lysine/alginate coating increased survival of WbtIG191V in complement-active serum. In addition, BALB/c mice immunized intraperitoneally with encapsulated WbtIG191V combined with purified LPS survived longer than mock-immunized mice following intranasal challenge. Alginate encapsulation of the bacteria also increased antibody titers compared to non-encapsulated bacteria. These data suggest that alginate encapsulation provides a slow-release vehicle for bacterial deposits, as evidenced by the increased antibody titer and increased persistence in serum compared to freely suspended cells. Survival of mice against high-dose intranasal challenge with the LVS wildtype was similar between mice immunized within alginate capsules or with LVS, possibly due to the low number of animals used, but bacterial loads in the liver and spleen were the lowest in mice immunized with WbtIG191V and LPS in beads. However, an analysis of the immune response of surviving mice indicated that those vaccinated with the alginate vehicle upregulated cell-mediated immune pathways to a lesser extent than LVS-vaccinated mice. In summary, this vehicle, as formulated, may be more effective for pathogens that require predominately antibody-mediated immunity.