A Francisella tularensis locus required for spermine responsiveness is necessary for virulence

Unveiling a Virulence-Regulating Mechanism in Aeromonas hydrophila: a Quantitative Exoproteomic Analysis of an AraC-Like Protein

Bacterial AraC is a transcription factor family that initiates transcription by recruiting RNA polymerase to the promoter and directly regulating various bacterial phenotypes. It also directly regulates various bacterial phenotypes. However, how this transcription factor regulates bacterial virulence and affects host immunity is still largely unknown. In this study, deleting the orf02889 (AraC-like transcription factor) gene in virulent Aeromonas hydrophila LP-2 affected several important phenotypes, such as increasing biofilm formation and siderophore production abilities. Moreover, Δorf02889 also significantly decreased the virulence of A. hydrophila and has promising attenuated vaccine potential. To better understand the effects of orf02889 on biological functions, a data independent acquisition (DIA)-based quantitative proteomics method was performed to compare the differentially expressed proteins between Δorf02889 and the wild-type strain in extracellular fractions. The following bioinformatics analysis suggested that ORF02889 may regulate various metabolic pathways, such as quorum sensing and ATP binding cassette (ABC) transporter metabolism. Moreover, 10 selected genes from the top 10 decreasing abundances in proteomics data were deleted, and their virulence to zebrafish was evaluated, respectively. The results showed that ΔcorC, Δorf00906, and Δorf04042 significantly reduced bacterial virulence. Finally, the following chromatin immunoprecipitation and polymerase chain reaction (ChIP-PCR) assay validated that the promoter of corC was directly regulated by ORF02889. Overall, these results provide insight into the biological function of ORF02889 and demonstrate its inherent regulatory mechanism for the virulence of A. hydrophila.

Arginine Catabolism and Polyamine Biosynthesis Pathway Disparities Within Francisella tularensis Subpopulations

Francisella tularensis is a highly infectious zoonotic pathogen with as few as 10 organisms causing tularemia, a disease that is fatal if untreated. Although F. tularensis subspecies tularensis (type A) and subspecies holarctica (type B) share over 99.5% average nucleotide identity, notable differences exist in genomic organization and pathogenicity. The type A clade has been further divided into subtypes A.I and A.II, with A.I strains being recognized as some of the most virulent bacterial pathogens known. In this study, we report on major disparities that exist between the F. tularensis subpopulations in arginine catabolism and subsequent polyamine biosynthesis. The genes involved in these pathways include the speHEA and aguAB operons, along with metK. In the hypervirulent F. tularensis A.I clade, such as the A.I prototype strain SCHU S4, these genes were found to be intact and highly transcribed. In contrast, both subtype A.II and type B strains have a truncated speA gene, while the type B clade also has a disrupted aguA and truncated aguB. Ablation of the chromosomal speE gene that encodes a spermidine synthase reduced subtype A.I SCHU S4 growth rate, whereas the growth rate of type B LVS was enhanced. These results demonstrate that spermine synthase SpeE promotes faster replication in the F. tularensis A.I clade, whereas type B strains do not rely on this enzyme for in vitro fitness. Our ongoing studies on amino acid and polyamine flux within hypervirulent A.I strains should provide a better understanding of the factors that contribute to F. tularensis pathogenicity.

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

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

Polyamines in the virulence of bacterial pathogens of respiratory tract

Polyamines are positively charged hydrocarbons that are essential for the growth and cellular maintenance in prokaryotes and eukaryotes. Polyamines have been demonstrated to play a role in bacterial pathogenicity and biofilm formation. However, the role of extracellular polyamines as a signaling molecule in the regulation of virulence is not investigated in detail. The bacterial pathogens residing in the respiratory tract remain asymptomatic for an extended period; however, the factors that lead to symptomatic behavior are poorly understood. Further investigation to understand the relation between the host-secreted factors and virulence of pathogenic bacteria in the respiratory tract may provide insights into the pathogenesis of respiratory tract infections. Polyamines produced within the bacterial cell are generally sequestered. Therefore, the pool of extracellular polyamines formed by secretion of the commensals and the host may be one of the signaling molecules that might contribute toward the alterations in the expression of virulence factors in bacterial pathogens. Besides, convergent mechanisms of polyamine biosynthesis do exist across the border of species and genus level. Also, several novel polyamine transporters in the host and bacteria remain yet to be identified. The review focuses on the role of polyamines in the expression of virulence phenotypes and biofilm formation of the respiratory tract pathogens.

The Sensor Kinase QseC Regulates the Unlinked PmrA Response Regulator and Downstream Gene Expression in Francisella

The facultative intracellular bacterial pathogen Francisella tularensis is the causative agent of tularemia in humans and animals. Gram-negative bacteria utilize two-component regulatory systems (TCS) to sense and respond to their changing environment. No classical, tandemly arranged sensor kinase and response regulator TCS genes exist in the human virulent Francisella tularensis subsp. tularensis, but orphaned members are present. PmrA is an orphan response regulator responsible for intramacrophage growth and virulence; however, the regulation of PmrA activity is not understood. We and others have shown that PmrA represses the expression of priM, described to encode an antivirulence determinant. By screening a mutant library for increased priM promoter activity, we identified the sensor kinase homolog QseC as an upstream regulator of priM expression, and this regulation is in part dependent upon the aspartate phosphorylation site of PmrA (D51). Several examined environmental signals, including epinephrine, which is reported to activate QseC in other bacteria, do not affect priM expression in a manner dependent on PmrA. Intramacrophage survival assays also question the finding that PriM is an antivirulence factor. Thus, these data suggest that the PmrA-regulated gene priM is modulated by the QseC-PmrA (QseB) TCS in Francisella IMPORTANCE The disease tularemia is caused by the highly infectious Gram-negative pathogen Francisella tularensis This bacterium encodes few regulatory factors (e.g., two-component systems [TCS]). PmrA, required for intramacrophage survival and virulence in the mouse model, is encoded by an orphan TCS response regulator gene. It is unclear how PmrA is responsive to environmental signals to regulate loci, including the PmrA-repressed gene priM We identify an orphan sensor kinase (QseC) that is required for priM repression and further explore both environmental signals that might regulate the QseC-PmrA TCS and the function of PriM.

Polyamine depletion has global effects on stress and virulence gene expression and affects HilA translation in Salmonella enterica serovar typhimurium

Polyamines are small cationic amines required for modulating multiple cell process, including cell growth and DNA and RNA stability. In Salmonella polyamines are primarily synthesized from L-arginine or L-ornithine. Based on a previous study, which demonstrated that polyamines affect the expression of virulence gene in S. Typhimurium, we investigated the role of polyamines in the global gene and protein expression in S. Typhimurium. The depletion of polyamine biosynthesis led to down-regulation of genes encoding structural components of the Type Three Secretion system 1 (TTSS1) and its secreted effectors. Interestingly, Expression of HilA, which is the master regulator of Salmonella Pathogenicity Island 1 (SPI1), was only reduced at the post-transcriptional in the polyamine mutant. Enzymes related to biosynthesis and/or transport of several amino acids were up-regulated, just as the Mg²⁺-transport systems were three to six-fold up-regulated at both the transcriptional and protein levels. Furthermore, in the polyamine depletion mutant, proteins related to stress response (IbpA, Dps, SodB), were 2-5 fold up-regulated. Together our data provide strong evidence that polyamine depletion affects expression of proteins linked with virulence and stress response of S. Typhimurium. Furthermore, polyamines positively affected translation of HilA, the major regulator of SPI1.

A predicted Francisella tularensis DXD-motif glycosyltransferase blocks immune activation

Pathogens enhance their survival during infections by manipulating host defenses. Francisella tularensis evades innate immune responses, which we have found to be dependent on an understudied gene ybeX (FTL_0883/FTT_0615c). To understand the function of YbeX, we sought protein interactors in F. tularensis subsp. holarctica live vaccine strain (LVS). An unstudied Francisella protein co-immunoprecipitated with recombinant YbeX, which is a predicted glycosyltransferase with a DXD-motif. There are up to four genomic copies of this gene with identical sequence in strains of F. tularensis pathogenic to humans, despite ongoing genome decay. Disruption mutations were generated by intron insertion into all three copies of this glycosyltransferase domain containing gene in LVS, gdcA1-3. The resulting strains stimulated more cytokines from macrophages in vitro than wild-type LVS and were attenuated in two in vivo infection models. GdcA was released from LVS during culture and was sufficient to block NF-κB activation when expressed in eukaryotic cells. When co-expressed in zebrafish, GdcA and YbeX were synergistically lethal to embryo development. Glycosyltransferases with DXD-motifs are found in a variety of pathogens including NleB, an Escherichia coli type-III secretion system effector that inhibits NF-κB by antagonizing death receptor signaling. To our knowledge, GdcA is the first DXD-motif glycosyltransferase that inhibits NF-κB in immune cells. Together, these findings suggest DXD-motif glycosyltransferases may be a conserved virulence mechanism used by pathogenic bacteria to remodel host defenses.

Putrescine biosynthesis and export genes are essential for normal growth of avian pathogenic Escherichia coli

BACKGROUND

Avian pathogenic Escherichia coli (APEC) is the infectious agent of a wide variety of avian diseases, which causes substantial economic losses to the poultry industry worldwide. Polyamines contribute to the optimal synthesis of nucleic acids and proteins in bacteria. The objectives of this study were to investigate; i) whether APEC E. coli encodes the same systems for biosynthesis and uptake as described for E. coli K12 and ii) the role of polyamines during in vitro growth of an avian pathogenic E. coli strain (WT-ST117- O83:H4T).

RESULTS

Following whole genome sequencing, polyamine biosynthesis and export genes present in E. coli MG1655 (K-12) were found to be identical in WT-ST117. Defined mutants were constructed in putrescine and spermidine biosynthesis pathways (ΔspeB, ΔspeC, ΔspeF, ΔspeB/C and ΔspeD/E), and in polyamines transport systems (ΔpotE, ΔyeeF, ΔpotABCD and ΔpotFGHI). Contrary to what was observed for MG1655, the ΔpotE-ST117 mutant was growth attenuated, regardless of putrescine supplementation. The addition of spermidine or orthinine restored the growth to the level of WT-ST117. Growth attenuation after induction of membrane stress by SDS suggested that PotE is involved in protection against this stress. The ΔspeB/C-ST117 mutant was also growth attenuated in minimal medium. The addition of putrescine or spermidine to the media restored growth rate to the wild type level. The remaining biosynthesis and transport mutants showed a growth similar to that of WT-ST117. Analysis by Ultra-High Performance Liquid Chromatography revealed that the ΔspeB/C mutant was putrescine-deficient, despite that the gene speF, which is also involved in the synthesis of putrescine, was expressed.

CONCLUSIONS

Deletion of the putrescine transport system, PotE, or the putrescine biosynthesis pathway genes speB/C affected in vitro growth of APEC (ST117- O83:H4) strain, but not E. coli MG1655, despite the high similarity of the genetic make-up of biosynthesis and transport genes. Therefore, blocking these metabolic reactions may be a suitable way to prevent APEC growth in the host without disturbing the commensal E. coli population.

Transcription Elongation Factor GreA Plays a Key Role in Cellular Invasion and Virulence of Francisella tularensis subsp. novicida

Francisella tularensis is a facultative intracellular Gram-negative bacterium that causes the zoonotic disease tularemia. We identified the transcription elongation factor GreA as a virulence factor in our previous study, but its role was not defined. Here, we investigate the effects of the inactivation of the greA gene, generating a greA mutant of F. tularensis subsp. novicida. Inactivation of greA impaired the bacterial invasion into and growth within host cells, and subsequently virulence in mouse infection model. A transcriptomic analysis (RNA-Seq) showed that the loss of GreA caused the differential expression of 196 bacterial genes, 77 of which were identified as virulence factors in previous studies. To confirm that GreA regulates the expression of virulence factors involved in cell invasion by Francisella, FTN_1186 (pepO) and FTN_1551 (ampD) gene mutants were generated. The ampD deletion mutant showed reduced invasiveness into host cells. These results strongly suggest that GreA plays an important role in the pathogenesis of Francisella by affecting the expression of virulence genes and provide new insights into the complex regulation of Francisella infection.

Deletion of the Major Facilitator Superfamily Transporter fptB Alters Host Cell Interactions and Attenuates Virulence of Type A Francisella tularensis

Francisella tularensis is a Gram-negative, facultative, intracellular coccobacillus that can infect a wide variety of hosts. In humans, F. tularensis causes the zoonosis tularemia following insect bites, ingestion, inhalation, and the handling of infected animals. The fact that a very small inoculum delivered by the aerosol route can cause severe disease, coupled with the possibility of its use as an aerosolized bioweapon, has led to the classification of Francisella tularensis as a category A select agent and has renewed interest in the formulation of a vaccine. To this end, we engineered a type A strain SchuS4 derivative containing a targeted deletion of the major facilitator superfamily (MFS) transporter fptB. Based on the attenuating capacity of this deletion in the F. tularensis LVS background, we hypothesized that the deletion of this transporter would alter the intracellular replication and cytokine induction of the type A strain and attenuate virulence in the stringent C57BL/6J mouse model. Here we demonstrate that the deletion of fptB significantly alters the intracellular life cycle of F. tularensis, attenuating intracellular replication in both cell line-derived and primary macrophages and inducing a novel cytosolic escape delay. Additionally, we observed prominent differences in the in vitro cytokine profiles in human macrophage-like cells. The mutant was highly attenuated in the C57BL/6J mouse model and provided partial protection against virulent type A F. tularensis challenge. These results indicate a fundamental necessity for this nutrient transporter in the timely progression of F. tularensis through its replication cycle and in pathogenesis.

The membrane transporter PotE is required for virulence in avian pathogenic Escherichia coli (APEC)

Over the last few years, polyamines have been described as key-signal of virulence in pathogenic bacteria. In the current study, we investigated whether the knockout of genes related to polyamine biosynthesis and putrescine transport affected the virulence of an avian pathogenic E. coli (APEC) strain. One-week-old White Leghorn chickens were infected intratracheally with mutants in polyamine biosynthesis (ΔspeB/C and ΔspeD/E) and transport genes (ΔpotE) of a well-characterized APEC strain of ST117 (O83: H4). All polyamine mutants and the wild-type strain were able to infect chicken; however, we observed significantly fewer lesions in the lungs of the chickens infected with the polyamine mutants in comparison with chicken infected with the wild-type. Results derived from histology of infected lungs detected significantly fewer lesions in the lung of birds infected within particular the putrescine transport mutant (ΔpotE). A decrease in colonization levels was observed in the liver and spleen of birds infected with the putrescine biosynthesis mutant ΔspeB/C, and likewise, a decrease of the colonization levels of all organs from birds infected with the ΔpotE was detected. Together, our data demonstrate that the deletion of polyamine genes, and in particular the PotE membrane protein, attenuates the virulence of APEC during infection of chickens.

Outer membrane vesicle-associated lipase FtlA enhances cellular invasion and virulence in Francisella tularensis LVS

Francisella tularensis is a highly infectious intracellular pathogen that infects a wide range of host species and causes fatal pneumonic tularemia in humans. ftlA was identified as a potential virulence determinant of the F. tularensis live vaccine strain (LVS) in our previous transposon screen, but its function remained undefined. Here, we show that an unmarked deletion mutant of ftlA was avirulent in a pneumonia mouse model with a severely impaired capacity to infect host cells. Consistent with its sequence homology with GDSL lipase/esterase family proteins, the FtlA protein displayed lipolytic activity in both E. coli and F. tularensis with a preference for relatively short carbon-chain substrates. FtlA thus represents the first F. tularensis lipase to promote bacterial infection of host cells and in vivo fitness. As a cytoplasmic protein, we found that FtlA was secreted into the extracellular environment as a component of outer membrane vesicles (OMVs). Further confocal microscopy analysis revealed that the FtlA-containing OMVs isolated from F. tularensis LVS attached to the host cell membrane. Finally, the OMV-associated FtlA protein complemented the genetic deficiency of the ΔftlA mutant in terms of host cell infection when OMVs purified from the parent strain were co-incubated with the mutant bacteria. These lines of evidence strongly suggest that the FtlA lipase promotes F. tularensis adhesion and internalization by modifying bacterial and/or host molecule(s) when it is secreted as a component of OMVs.

Glycosylation of a Capsule-Like Complex (CLC) by Francisella novicida Is Required for Virulence and Partial Protective Immunity in Mice

Francisella tularensis is a Gram-negative bacterium and the etiologic agent of tularemia. F. tularensis may appear encapsulated when examined by transmission electron microscopy (TEM), which is due to production of an extracellular capsule-like complex (CLC) when the bacterium is grown under specific environmental conditions. Deletion of two glycosylation genes in the live vaccine strain (LVS) results in loss of apparent CLC and attenuation of LVS in mice. In contrast, F. novicida, which is also highly virulent for mice, is reported to be non-encapsulated. However, the F. novicida genome contains a putative polysaccharide locus with homology to the CLC glycosylation locus in F. tularensis. Following daily subculture of F. novicida in Chamberlain's defined medium, an electron dense material surrounding F. novicida, similar to the F. tularensis CLC, was evident. Extraction with urea effectively removed the CLC, and compositional analysis indicated the extract contained galactose, glucose, mannose, and multiple proteins, similar to those found in the F. tularensis CLC. The same glycosylation genes deleted in LVS were targeted for deletion in F. novicida by allelic exchange using the same mutagenesis vector used for mutagenesis of LVS. In contrast, this mutation also resulted in the loss of five additional genes immediately upstream of the targeted mutation (all within the glycosylation locus), resulting in strain F. novicida Δ1212-1218. The subcultured mutant F. novicida Δ1212-1218 was CLC-deficient and the CLC contained significantly less carbohydrate than the subcultured parent strain. The mutant was severely attenuated in BALB/c mice inoculated intranasally, as determined by the lower number of F. novicida Δ1212-1218 recovered in tissues compared to the parent, and by clearance of the mutant by 10-14 days post-challenge. Mice immunized intranasally with F. novicida Δ1212-1218 were partially protected against challenge with the parent, produced significantly reduced levels of inflammatory cytokines, and their spleens contained only areas of lymphoid hyperplasia, whereas control mice challenged with the parent exhibited hypercytokinemia and splenic necrosis. Therefore, F. novicida is capable of producing a CLC similar to that of F. tularensis, and glycosylation of the CLC contributed to F. novicida virulence and immunoprotection.

Multiple roles of putrescine and spermidine in stress resistance and virulence of Salmonella enterica serovar Typhimurium

Polyamines (putrescine and spermidine) are small-cationic amines ubiquitous in nature and present in most living cells. In recent years they have been linked to virulence of several human pathogens including Shigella spp and Salmonella enterica serovar Typhimurium (S. Typhimurium). Central to S. Typhimurium virulence is the ability to survive and replicate inside macrophages and resisting the antimicrobial attacks in the form of oxidative and nitrosative stress elicited from these cells. In the present study, we have investigated the role of polyamines in intracellular survival and systemic infections of mice. Using a S. Typhimurium mutant defective for putrescine and spermidine biosynthesis, we show that polyamines are essential for coping with reactive nitrogen species, possibly linking polyamines to increased intracellular stress resistance. However, using a mouse model defective for nitric oxide production, we find that polyamines are required for systemic infections independently of host-produced reactive nitrogen species. To distinguish between the physiological roles of putrescine and spermidine, we constructed a strain deficient for spermidine biosynthesis and uptake, but with retained ability to produce and import putrescine. Interestingly, in this mutant we observe a strong attenuation of virulence during infection of mice proficient and deficient for nitric oxide production suggesting that spermidine, specifically, is essential for virulence of S. Typhimurium.

Intermediate filaments enable pathogen docking to trigger type 3 effector translocation

Type 3 secretion systems (T3SSs) of bacterial pathogens translocate bacterial effector proteins that mediate disease into the eukaryotic cytosol. Effectors traverse the plasma membrane through a translocon pore formed by T3SS proteins. In a genome-wide selection, we identified the intermediate filament vimentin as required for infection by the T3SS-dependent pathogen S. flexneri. We found that vimentin is required for efficient T3SS translocation of effectors by S. flexneri and other pathogens that use T3SS, Salmonella enterica serovar Typhimurium and Yersinia pseudotuberculosis. Vimentin and the intestinal epithelial intermediate filament keratin 18 interact with the C-terminus of the Shigella translocon pore protein IpaC. Vimentin and its interaction with IpaC are dispensable for pore formation, but are required for stable docking of S. flexneri to cells; moreover, stable docking triggers effector secretion. These findings establish that stable docking of the bacterium specifically requires intermediate filaments, is a process distinct from pore formation, and is a prerequisite for effector secretion.

Effects of norspermidine and spermidine on biofilm formation by potentially pathogenic Escherichia coli and Salmonella enterica wild-type strains

Polyamines are present in all living cells. In bacteria, polyamines are involved in a variety of functions, including biofilm formation, thus indicating that polyamines may have potential in the control of unwanted biofilm. In the present study, the effects of the polyamines norspermidine and spermidine on biofilms of 10 potentially pathogenic wild-type strains of Escherichia coli serotype O103:H2, Salmonella enterica subsp. enterica serovar Typhimurium, and S. enterica serovar Agona were investigated. We found that exogenously supplied norspermidine and spermidine did not mediate disassembly of preformed biofilm of any of the E. coli and S. enterica strains. However, the polyamines did affect biofilm production. Interestingly, the two species reacted differently to the polyamines. Both polyamines reduced the amount of biofilm formed by E. coli but tended to increase biofilm formation by S. enterica. Whether the effects observed were due to the polyamines specifically targeting biofilm formation, being toxic for the cells, or maybe a combination of the two, is not known. However, there were no indications that the effect was mediated through binding to exopolysaccharides, as earlier suggested for E. coli. Our results indicate that norspermidine and spermidine do not have potential as inhibitors of S. enterica biofilm. Furthermore, we found that the commercial polyamines used contributed to the higher pH of the test medium. Failure to acknowledge and control this important phenomenon may lead to misinterpretation of the results.

Role of pathogenicity determinant protein C (PdpC) in determining the virulence of the Francisella tularensis subspecies tularensis SCHU

Francisella tularensis subspecies tularensis, the etiological agent of tularemia, is highly pathogenic to humans and animals. However, the SCHU strain of F. tularensis SCHU P0 maintained by passaging in artificial media has been found to be attenuated. To better understand the molecular mechanisms behind the pathogenicity of F. tularensis SCHU, we attempted to isolate virulent bacteria by serial passages in mice. SCHU P5 obtained after 5th passages in mice remained avirulent, while SCHU P9 obtained after 9th passages was completely virulent in mice. Moreover, SCHU P9 grew more efficiently in J774.1 murine macrophages compared with that in the less pathogenic SCHU P0 and P5. Comparison of the nucleotide sequences of the whole genomes of SCHU P0, P5, and P9 revealed only 1 nucleotide difference among P0, P5 and P9 in 1 of the 2 copies of pathogenicity determinant protein C (pdpC) gene. An adenine residue deletion was observed in the pdpC1 gene of SCHU P0, P5, and P9 and in the pdpC2 gene of SCHU P0, and P5, while P9 was characterized by the wild type pdpC2 gene. Thus, SCHU P0 and P5 expressed only truncated forms of PdpC protein, while SCHU P9 expressed both wild type and truncated versions. To validate the pathogenicity of PdpC, both copies of the pdpC gene in SCHU P9 have been inactivated by Targetron mutagenesis. SCHU P9 mutants with inactivated pdpC gene showed low intracellular growth in J774.1 cells and did not induce severe disease in experimentally infected mice, while virulence of the mutants was restored by complementation with expression of the intact PdpC. These results demonstrate that PdpC is crucial in determining the virulence of F. tularensis SCHU.

Polyamines are essential for virulence in Salmonella enterica serovar Gallinarum despite evolutionary decay of polyamine biosynthesis genes

Serovars of Salmonella enterica exhibit different host-specificities where some have broad host-ranges and others, like S. Gallinarum and S. Typhi, are host-specific for poultry and humans, respectively. With the recent availability of whole genome sequences it has been reported that host-specificity coincides with accumulation of pseudogenes, indicating adaptation of host-restricted serovars to their narrow niches. Polyamines are small cationic amines and in Salmonella they can be synthesized through two alternative pathways directly from l-ornithine to putrescine and from l-arginine via agmatine to putrescine. The first pathway is not active in S. Gallinarum and S. Typhi, and this prompted us to investigate the importance of polyamines for virulence in S. Gallinarum. Bioinformatic analysis of all sequenced genomes of Salmonella revealed that pseudogene formation of the speC gene was exclusive for S. Typhi and S. Gallinarum and happened through independent events. The remaining polyamine biosynthesis pathway was found to be essential for oral infection with S. Gallinarum since single and double mutants in speB and speE, encoding the pathways from agmatine to putrescine and from putrescine to spermidine, were attenuated. In contrast, speB was dispensable after intraperitoneal challenge, suggesting that putrescine was less important for the systemic phase of the disease. In support of this hypothesis, a ΔspeE;ΔpotCD mutant, unable to synthesize and import spermidine, but with retained ability to import and synthesize putrescine, was attenuated after intraperitoneal infection. We therefore conclude that polyamines are essential for virulence of S. Gallinarum. Furthermore, our results point to distinct roles for putrescine and spermidine during systemic infection.

Stress and polyamine metabolism in fungi

Fungi, as well as the rest of living organisms must deal with environmental challenges such as stressful stimuli. Fungi are excellent models to study the general mechanisms of the response to stress, because of their simple, but conserved, signal-transduction and metabolic pathways that are often equivalent to those present in other eukaryotic systems. A factor that has been demonstrated to be involved in these responses is polyamine metabolism, essentially of the three most common polyamines: putrescine, spermidine and spermine. The gathered evidences on this subject suggest that polyamines are able to control cellular signal transduction, as well as to modulate protein-protein interactions. In the present review, we will address the recent advances on the study of fungal metabolism of polyamines, ranging from mutant characterization to potential mechanism of action during different kinds of stress in selected fungal models.

The contribution of the glycine cleavage system to the pathogenesis of Francisella tularensis

Biosynthesis and acquisition of nutrients during infection are integral to pathogenesis. Members of a metabolic pathway, the glycine cleavage system, have been identified in virulence screens of the intracellular bacterium Francisella tularensis but their role in pathogenesis remains unknown. This system generates 5,10-methylenetetrahydrofolate, a precursor of amino acid and DNA synthesis, from glycine degradation. To characterize this pathway, deletion of the gcvT homolog, an essential member of this system, was performed in attenuated and virulent F. tularensis strains. Deletion mutants were auxotrophic for serine but behaved similar to wild-type strains with respect to host cell invasion, intracellular replication, and stimulation of TNF-α. Unexpectedly, the glycine cleavage system was required for the pathogenesis of virulent F. tularensis in a murine model. Deletion of the gcvT homolog delayed mortality and lowered bacterial burden, particularly in the liver and bloodstream. To reconcile differences between the cell culture model and animal model, minimal tissue culture media was employed to mimic the nutritionally limiting environment of the host. This reevaluation demonstrated that the glycine cleavage system contributes to the intracellular replication of virulent F. tularensis in serine limiting environments. Thus, the glycine cleavage system is the serine biosynthetic pathway of F. tularensis and contributes to pathogenesis in vivo.

The use of resazurin as a novel antimicrobial agent against Francisella tularensis

The highly infectious and deadly pathogen, Francisella tularensis, is classified by the CDC as a Category A bioterrorism agent. Inhalation of a single bacterium results in an acute pneumonia with a 30-60% mortality rate without treatment. Due to the prevalence of antibiotic resistance, there is a strong need for new types of antibacterial drugs. Resazurin is commonly used to measure bacterial and eukaryotic cell viability through its reduction to the fluorescent product resorufin. When tested on various bacterial taxa at the recommended concentration of 44 μM, a potent bactericidal effect was observed against various Francisella and Neisseria species, including the human pathogens type A F. tularensis (Schu S4) and N. gonorrhoeae. As low as 4.4 μM resazurin was sufficient for a 10-fold reduction in F. tularensis growth. In broth culture, resazurin was reduced to resorufin by F. tularensis. Resorufin also suppressed the growth of F. tularensis suggesting that this compound is the biologically active form responsible for decreasing the viability of F. tularensis LVS bacteria. Replication of F. tularensis in primary human macrophages and non-phagocytic cells was abolished following treatment with 44 μM resazurin indicating this compound could be an effective therapy for tularemia in vivo.

IglE is an outer membrane-associated lipoprotein essential for intracellular survival and murine virulence of type A Francisella tularensis

IglE is a small, hypothetical protein encoded by the duplicated Francisella pathogenicity island (FPI). Inactivation of both copies of iglE rendered Francisella tularensis subsp. tularensis Schu S4 avirulent and incapable of intracellular replication, owing to an inability to escape the phagosome. This defect was fully reversed following single-copy expression of iglE in trans from attTn7 under the control of the Francisella rpsL promoter, thereby establishing that the loss of iglE, and not polar effects on downstream vgrG gene expression, was responsible for the defect. IglE is exported to the Francisella outer membrane as an ∼13.9-kDa lipoprotein, determined on the basis of a combination of selective Triton X-114 solubilization, radiolabeling with [(3)H]palmitic acid, and sucrose density gradient membrane partitioning studies. Lastly, a genetic screen using the iglE-null live vaccine strain resulted in the identification of key regions in the carboxyl terminus of IglE that are required for intracellular replication of Francisella tularensis in J774A.1 macrophages. Thus, IglE is essential for Francisella tularensis virulence. Our data support a model that likely includes protein-protein interactions at or near the bacterial cell surface that are unknown at present.

MyD88-dependent signaling prolongs survival and reduces bacterial burden during pulmonary infection with virulent Francisella tularensis

Francisella tularensis is the causative agent of the debilitating febrile illness tularemia. The severe morbidity associated with F. tularensis infections is attributed to its ability to evade the host immune response. Innate immune activation is undetectable until more than 48 hours after infection. The ensuing inflammatory response is considered pathological, eliciting a septic-like state characterized by hypercytokinemia and cell death. To investigate potential pathological consequences of the innate immune response, mice deficient in a key innate immune signaling molecule, MyD88, were studied. MyD88 knockout (KO) mice were infected with the prototypical virulent F. tularensis strain, Schu S4. MyD88 KO mice succumbed to infection more rapidly than wild-type mice. The enhanced pathogenicity of Schu S4 in MyD88 KO mice was associated with greater bacterial burdens in lungs and distal organs, and the absence of IFN-γ in the lungs, spleens, and sera. Cellular infiltrates were not observed on histological evaluation of the lungs, livers, or spleens of MyD88 KO mice, the first KO mouse described with this phenotype to our knowledge. Despite the absence of cellular infiltration, there was more cell death in the lungs of MyD88 KO mice. Thus, the host proinflammatory response is beneficial, and MyD88 signaling is required to limit bacterial burden and prolong survival during pulmonary infection by virulent F. tularensis.

Polyamines: emerging players in bacteria-host interactions

Polyamines are small polycationic molecules found in almost all cells and associated with a wide variety of physiological processes. In recent years it has become increasingly clear that, in addition to core physiological functions, polyamines play a crucial role in bacterial pathogenesis. Considerable evidence has built up that bacteria have evolved mechanisms to turn these molecules to their own advantage and a novel standpoint to look at host-bacterium interactions emerges from the interplay among polyamines, host cells and infecting bacteria. In this review, we highlight how human bacterial pathogens have developed their own resourceful strategies to exploit polyamines or manipulate polyamine-related processes to optimize their fitness within the host. Besides contributing to a better understanding of the complex relationship between a pathogen and its host, acquisitions in this field have a significant potential towards the development of novel antibacterial therapeutic approaches.

Live attenuated tularemia vaccines: recent developments and future goals

In the aftermath of the 2001 anthrax attacks in the U.S., numerous efforts were made to increase the level of preparedness against a biological attack both in the US and worldwide. As a result, there has been an increase in research interest in the development of vaccines and other countermeasures against a number of agents with the potential to be used as biological weapons. One such agent, Francisella tularensis, has been the subject of a surge in the level of research being performed, leading to a substantial increase in knowledge of the pathogenic mechanisms of the organism and the induced immune responses. This information has facilitated the development of multiple new Francisella vaccine candidates. Herein we review the latest live attenuated F. tularensis vaccine efforts. Historically, live attenuated vaccines have demonstrated the greatest degree of success in protection against tularemia and the greatest promise in recent efforts to develop of a fully protective vaccine. This review summarizes recent live attenuated Francisella vaccine candidates and the lessons learned from those studies, with the goal of collating known characteristics associated with successful attenuation, immunogenicity, and protection.

Effects of polyamines on Vibrio cholerae virulence properties

Vibrio cholerae is the causative agent of the severe enteric disease cholera. To cause cholera the bacterium must be able to synthesize both cholera toxin (CT) and toxin-coregulated pilus (TCP) which mediates autoagglutination and is required for colonization of the small intestine. Only a few environmental signals have been shown to regulate V. cholerae virulence gene expression. Polyamines, which are ubiquitous in nature, and have been implicated in regulating virulence gene expression in other bacteria, have not been extensively studied for their effect on V. cholerae virulence properties. The objective of this study was to test the effect of several polyamines that are abundant in the human intestine on V. cholerae virulence properties. All of the polyamines tested inhibited autoagglutination of V. cholerae O1 classical strain in a concentration dependent manner. Putrescine and cadaverine decreased the synthesis of the major pilin subunit, TcpA, spermidine increased its production, and spermine had no effect. Putrescine and spermidine led to a decrease and increase, respectively, on the relative abundance of TCP found on the cell surface. Spermine led to a small reduction in cholera toxin synthesis whereas none of the other polyamines had an effect. The polyamines did not affect pili bundling morphology, but caused a small reduction in CTXφ transduction, indicating that the TCP present on the cell surface may not be fully functional. We hypothesize the inhibition of autoagglutination is likely to be caused by the positively charged amine groups on the polyamines electrostatically disrupting the pili-pili interactions which mediate autoagglutination. Our results implicate that polyamines may have a protective function against colonization of the small intestine by V. cholerae.

Role of NK cells in host defense against pulmonary type A Francisella tularensis infection

Pneumonic tularemia is a potentially fatal disease caused by the Category A bioterrorism agent Francisella tularensis. Understanding the pulmonary immune response to this bacterium is necessary for developing effective vaccines and therapeutics. In this study, characterization of immune cell populations in the lungs of mice infected with the type A strain Schu S4 revealed a significant loss in natural killer (NK) cells over time. Since this decline in NK cells correlated with morbidity and mortality, we hypothesized these cells contribute to host defense against Schu S4 infection. Depletion of NK cells prior to Schu S4 challenge significantly reduced IFN-γ and granzyme B in the lung but had no effect on bacterial burden or disease progression. Conversely, increasing NK cell numbers with the anti-apoptotic cytokine IL-15 and soluble receptor IL-15Rα had no significant impact on Schu S4 growth in vivo. A modest decrease in median time to death, however, was observed in live vaccine strain (LVS)-vaccinated mice depleted of NK1.1+ cells and challenged with Schu S4. Therefore, NK cells do not appear to contribute to host defense against acute respiratory infection with type A F. tularensis in vivo, but they play a minor role in protection elicited by LVS vaccination.

Subversion of host recognition and defense systems by Francisella spp

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

Polyamines are required for virulence in Salmonella enterica serovar Typhimurium

Sensing and responding to environmental cues is a fundamental characteristic of bacterial physiology and virulence. Here we identify polyamines as novel environmental signals essential for virulence of Salmonella enterica serovar Typhimurium, a major intracellular pathogen and a model organism for studying typhoid fever. Central to its virulence are two major virulence loci Salmonella Pathogenicity Island 1 and 2 (SPI1 and SPI2). SPI1 promotes invasion of epithelial cells, whereas SPI2 enables S. Typhimurium to survive and proliferate within specialized compartments inside host cells. In this study, we show that an S. Typhimurium polyamine mutant is defective for invasion, intracellular survival, killing of the nematode Caenorhabditis elegans and systemic infection of the mouse model of typhoid fever. Virulence of the mutant could be restored by genetic complementation, and invasion and intracellular survival could, as well, be complemented by the addition of exogenous putrescine and spermidine to the bacterial cultures prior to infection. Interestingly, intracellular survival of the polyamine mutant was significantly enhanced above the wild type level by the addition of exogenous putrescine and spermidine to the bacterial cultures prior to infection, indicating that these polyamines function as an environmental signal that primes S. Typhimurium for intracellular survival. Accordingly, experiments addressed at elucidating the roles of these polyamines in infection revealed that expression of genes from both of the major virulence loci SPI1 and SPI2 responded to exogenous polyamines and was reduced in the polyamine mutant. Together our data demonstrate that putrescine and spermidine play a critical role in controlling virulence in S. Typhimurium most likely through stimulation of expression of essential virulence loci. Moreover, our data implicate these polyamines as key signals in S. Typhimurium virulence.

A Francisella tularensis live vaccine strain that improves stimulation of antigen-presenting cells does not enhance vaccine efficacy

Vaccination is a proven strategy to mitigate morbidity and mortality of infectious diseases. The methodology of identifying and testing new vaccine candidates could be improved with rational design and in vitro testing prior to animal experimentation. The tularemia vaccine, Francisella tularensis live vaccine strain (LVS), does not elicit complete protection against lethal challenge with a virulent type A Francisella strain. One factor that may contribute to this poor performance is limited stimulation of antigen-presenting cells. In this study, we examined whether the interaction of genetically modified LVS strains with human antigen-presenting cells correlated with effectiveness as tularemia vaccine candidates. Human dendritic cells infected with wild-type LVS secrete low levels of proinflammatory cytokines, fail to upregulate costimulatory molecules, and activate human T cells poorly in vitro. One LVS mutant, strain 13B47, stimulated higher levels of proinflammatory cytokines from dendritic cells and macrophages and increased costimulatory molecule expression on dendritic cells compared to wild type. Additionally, 13B47-infected dendritic cells activated T cells more efficiently than LVS-infected cells. A deletion allele of the same gene in LVS displayed similar in vitro characteristics, but vaccination with this strain did not improve survival after challenge with a virulent Francisella strain. In vivo, this mutant was attenuated for growth and did not stimulate T cell responses in the lung comparable to wild type. Therefore, stimulation of antigen-presenting cells in vitro was improved by genetic modification of LVS, but did not correlate with efficacy against challenge in vivo within this model system.