Global transcriptional response to mammalian temperature provides new insight into Francisella tularensis pathogenesis

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.

Role of dipA and pilD in Francisella tularensis Susceptibility to Resazurin

The phenoxazine dye resazurin exhibits bactericidal activity against the Gram-negative pathogens Francisella tularensis and Neisseria gonorrhoeae. One resazurin derivative, resorufin pentyl ether, significantly reduces vaginal colonization by Neisseria gonorrhoeae in a mouse model of infection. The narrow spectrum of bacteria susceptible to resazurin and its derivatives suggests these compounds have a novel mode of action. To identify potential targets of resazurin and mechanisms of resistance, we isolated mutants of F. tularensis subsp. holarctica live vaccine strain (LVS) exhibiting reduced susceptibility to resazurin and performed whole genome sequencing. The genes pilD (FTL_0959) and dipA (FTL_1306) were mutated in half of the 46 resazurin-resistant (RZR) strains sequenced. Complementation of select RZR LVS isolates with wild-type dipA or pilD partially restored sensitivity to resazurin. To further characterize the role of dipA and pilD in resazurin susceptibility, a dipA deletion mutant, ΔdipA, and pilD disruption mutant, FTL_0959d, were generated. Both mutants were less sensitive to killing by resazurin compared to wild-type LVS with phenotypes similar to the spontaneous resazurin-resistant mutants. This study identified a novel role for two genes dipA and pilD in F. tularensis susceptibility to resazurin.

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.

Mechanisms Affecting the Acquisition, Persistence and Transmission of Francisella tularensis in Ticks

Over 600,000 vector-borne disease cases were reported in the United States (U.S.) in the past 13 years, of which more than three-quarters were tick-borne diseases. Although Lyme disease accounts for the majority of tick-borne disease cases in the U.S., tularemia cases have been increasing over the past decade, with >220 cases reported yearly. However, when comparing Borrelia burgdorferi (causative agent of Lyme disease) and Francisella tularensis (causative agent of tularemia), the low infectious dose (<10 bacteria), high morbidity and mortality rates, and potential transmission of tularemia by multiple tick vectors have raised national concerns about future tularemia outbreaks. Despite these concerns, little is known about how F. tularensis is acquired by, persists in, or is transmitted by ticks. Moreover, the role of one or more tick vectors in transmitting F. tularensis to humans remains a major question. Finally, virtually no studies have examined how F. tularensis adapts to life in the tick (vs. the mammalian host), how tick endosymbionts affect F. tularensis infections, or whether other factors (e.g., tick immunity) impact the ability of F. tularensis to infect ticks. This review will assess our current understanding of each of these issues and will offer a framework for future studies, which could help us better understand tularemia and other tick-borne diseases.

In silico Prediction of New Drug Candidates Against the Multidrug-Resistant and Potentially Zoonotic Fish Pathogen Serotype III Streptococcus agalactiae

Streptococcus agalactiae is an invasive multi-host pathogen that causes invasive diseases mainly in newborns, elderly, and individuals with underlying health complications. In fish, S. agalactiae causes streptococcosis, which is characterized by septicemia and neurological signs, and leads to great economic losses to the fish farming industry worldwide. These bacteria can be classified into different serotypes based on capsular antigens, and into different sequence types (ST) based on multilocus sequence typing (MLST). In 2015, serotype III ST283 was identified to be associated with a foodborne invasive disease in non-pregnant immunocompetent humans in Singapore, and the infection was related to raw fish consumption. In addition, a serotype III strain isolated from tilapia in Brazil has been reported to be resistant to five antibiotic classes. This specific serotype can serve as a reservoir of resistance genes and pose a serious threat to public health. Thus, new approaches for the control and treatment of S. agalactiae infections are needed. In the present study, 24 S. agalactiae serotype III complete genomes, isolated from human and fish hosts, were compared. The core genome was identified, and, using bioinformatics tools and subtractive criteria, five proteins were identified as potential drug targets. Furthermore, 5,008 drug-like natural compounds were virtually screened against the identified targets. The ligands with the best binding properties are suggested for further in vitro and in vivo analysis.

OpiA, a Type Six Secretion System Substrate, Localizes to the Cell Pole and Plays a Role in Bacterial Growth and Viability 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 a number of host-pathogen interactions, including phagolysosomal escape and invasion of erythrocytes. One known effector of the T6SS, OpiA, has recently been shown to be a phosphatidylinositol-3 kinase. To investigate the role of OpiA in erythrocyte invasion, we constructed an opiA-null mutant in the live vaccine strain, F. tularensis LVS. OpiA was not required for erythrocyte invasion; however, deletion of opiA affected growth of F. tularensis LVS in broth cultures in a medium-dependent manner. We also found that opiA influenced cell size, gentamicin sensitivity, bacterial viability, and the lipid content of F. tularensis A fluorescently tagged OpiA (OpiA-emerald-green fluorescent protein [EmGFP]) accumulated at the cell poles of F. tularensis, which is consistent with the location of the T6SS. However, OpiA-EmGFP also exhibited a highly dynamic localization, and this fusion protein was detected in erythrocytes and THP-1 cells in vitro, further supporting that OpiA is secreted. Similar to previous reports with F. novicida, our data demonstrated that opiA had a minimal effect on intracellular replication of F. tularensis in host immune cells in vitro However, THP-1 cells infected with the opiA mutant produced modestly (but significantly) higher levels of the proinflammatory cytokine tumor necrosis factor alpha compared to these host cells infected with wild-type bacteria. We conclude that, in addition to its role in host-pathogen interactions, our results reveal that the function of opiA is central to the biology of F. tularensis bacteria.IMPORTANCE F. tularensis is a pathogenic intracellular pathogen that is of importance for public health and strategic defense. This study characterizes the opiA gene of F. tularensis LVS, an attenuated strain that has been used as a live vaccine but that also shares significant genetic similarity to related Francisella strains that cause human disease. The data presented here provide the first evidence of a T6SS effector protein that affects the physiology of F. tularensis, namely, the growth, cell size, viability, and aminoglycoside resistance of F. tularensis LVS. This study also adds insight into our understanding of OpiA as a determinant of virulence. Finally, the fluorescence fusion constructs presented here will be useful tools for dissecting the role of OpiA in infection.

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.

Francisella tularensis subsp. holarctica Releases Differentially Loaded Outer Membrane Vesicles Under Various Stress Conditions

Francisella tularensis is a Gram-negative, facultative intracellular bacterium, causing a severe disease called tularemia. It secretes unusually shaped nanotubular outer membrane vesicles (OMV) loaded with a number of virulence factors and immunoreactive proteins. In the present study, the vesicles were purified from a clinical isolate of subsp. holarctica strain FSC200. We here provide a comprehensive proteomic characterization of OMV using a novel approach in which a comparison of OMV and membrane fraction is performed in order to find proteins selectively enriched in OMV vs. membrane. Only these proteins were further considered to be really involved in the OMV function and/or their exceptional structure. OMV were also isolated from bacteria cultured under various cultivation conditions simulating the diverse environments of F. tularensis life cycle. These included conditions mimicking the milieu inside the mammalian host during inflammation: oxidative stress, low pH, and high temperature (42°C); and in contrast, low temperature (25°C). We observed several-fold increase in vesiculation rate and significant protein cargo changes for high temperature and low pH. Further proteomic characterization of stress-derived OMV gave us an insight how the bacterium responds to the hostile environment of a mammalian host through the release of differentially loaded OMV. Among the proteins preferentially and selectively packed into OMV during stressful cultivations, the previously described virulence factors connected to the unique intracellular trafficking of Francisella were detected. Considerable changes were also observed in a number of proteins involved in the biosynthesis and metabolism of the bacterial envelope components like O-antigen, lipid A, phospholipids, and fatty acids. Data are available via ProteomeXchange with identifier PXD013074.

Development, Characterization, and Standardization of a Nose-Only Inhalation Exposure System for Exposure of Rabbits to Small-Particle Aerosols Containing Francisella tularensis

Inhalation of Francisella tularensis causes pneumonic tularemia in humans, a severe disease with a 30 to 60% mortality rate. The reproducible delivery of aerosolized virulent bacteria in relevant animal models is essential for evaluating medical countermeasures. Here we developed optimized protocols for infecting New Zealand White (NZW) rabbits with aerosols containing F. tularensis We evaluated the relative humidity, aerosol exposure technique, and bacterial culture conditions to optimize the spray factor (SF), a central metric of aerosolization. This optimization reduced both inter- and intraday variability and was applicable to multiple isolates of F. tularensis Further improvements in the accuracy and precision of the inhaled pathogen dose were achieved through enhanced correlation of the bacterial culture optical density and the number of CFU. Plethysmograph data collected during exposures found that respiratory function varied considerably between rabbits, was not a function of weight, and did not improve with acclimation to the system. Live vaccine strain (LVS)-vaccinated rabbits were challenged via aerosol with human-virulent F. tularensis SCHU S4 that had been cultivated in either Mueller-Hinton broth (MHB) or brain heart infusion (BHI) broth. LVS-vaccinated animals challenged with SCHU S4 that had been cultivated in MHB experienced short febrile periods (median, 3.2 days), limited weight loss (<5%), and longer median survival times (∼18 days) that were significantly different from those for unvaccinated controls. In contrast, LVS-vaccinated rabbits challenged with SCHU S4 that had been cultivated in BHI experienced longer febrile periods (median, 5.5 days) and greater weight loss (>10%) than the unvaccinated controls and median survival times that were not significantly different from those for the unvaccinated controls. These studies highlight the importance of careful characterization and optimization of protocols for aerosol challenge with pathogenic agents.

Blowing epithelial cell bubbles with GumB: ShlA-family pore-forming toxins induce blebbing and rapid cellular death in corneal epithelial cells

Medical devices, such as contact lenses, bring bacteria in direct contact with human cells. Consequences of these host-pathogen interactions include the alteration of mammalian cell surface architecture and induction of cellular death that renders tissues more susceptible to infection. Gram-negative bacteria known to induce cellular blebbing by mammalian cells, Pseudomonas and Vibrio species, do so through a type III secretion system-dependent mechanism. This study demonstrates that a subset of bacteria from the Enterobacteriaceae bacterial family induce cellular death and membrane blebs in a variety of cell types via a type V secretion-system dependent mechanism. Here, we report that ShlA-family cytolysins from Proteus mirabilis and Serratia marcescens were required to induce membrane blebbling and cell death. Blebbing and cellular death were blocked by an antioxidant and RIP-1 and MLKL inhibitors, implicating necroptosis in the observed phenotypes. Additional genetic studies determined that an IgaA family stress-response protein, GumB, was necessary to induce blebs. Data supported a model where GumB and shlBA are in a regulatory circuit through the Rcs stress response phosphorelay system required for bleb formation and pathogenesis in an invertebrate model of infection and proliferation in a phagocytic cell line. This study introduces GumB as a regulator of S. marcescens host-pathogen interactions and demonstrates a common type V secretion system-dependent mechanism by which bacteria elicit surface morphological changes on mammalian cells. This type V secretion-system mechanism likely contributes bacterial damage to the corneal epithelial layer, and enables access to deeper parts of the tissue that are more susceptible to infection.

Effects of temperature changes in the transcriptional profile of the emerging fish pathogen Francisella noatunensis subsp. orientalis

One of the major challenges in Nile tilapia (Oreochromis niloticus L.) farming is the occurrence of bacterial infections, and the Francisella noatunensis subsp. orientalis (FNO) is an important pathogen that has emerged in last decades. Francisellosis outbreaks have been reported in the literature as occurring seasonally when water temperature is below 24 °C. The aim of this study was to quantify the median lethal doses (LD₅₀) of FNO in experimental challenges at 28 °C and 22 °C, and to investigate the impact of temperature changes in whole genome expression using microarray technology. The LD₅₀ for Nile tilapia at 28 °C was ∼10^5.7, whereas at 22 °C, the LD₅₀ was ∼10^2.2, showing that the decrease in temperature enhanced disease outcome. Out of 1917 genes screened, a total of 31 and 19 genes were down- and up-regulated at 22 °C, respectively. These genes were grouped by orthology into functional categories of: amino acid, inorganic ion, and carbohydrate transport and metabolism; transcription; and posttranslational modification, protein turnover, and chaperones. Expression of genes related to metabolism, oxidative stress, and thermal shock were regulated by temperature changes, reflecting an ability of FNO to adapt to the environment. Expression of virulence genes usually required for the Francisella genus was not changed between tested temperatures, including that of genes located on the Francisella Pathogenicity Island.

Gene expression of putative type VI secretion system (T6SS) genes in the emergent fish pathogen Francisella noatunensis subsp. orientalis in different physiochemical conditions

Background

Francisella noatunensis subsp. orientalis (Fno) is an emergent fish pathogen and the etiologic agent of piscine francisellosis. Besides persisting in the environment in both biofilm and planktonic forms, Fno is known to infect and replicate inside tilapia macrophages and endothelial-derived cells. However, the mechanism used by this emergent bacterium for intracellular survival is unknown. Additionally, the basis of virulence for Fno is still poorly understood. Several potential virulence determinants have been identified in Fno, including homologues of the recently described F. tularensis Type VI Secretion System (T6SS). In order to gain a better understanding of the role the putative Fno T6SS might play in the pathogenesis of piscine francisellosis, we performed transcriptional analysis of Fno T6SS gene-homologues under temperature, acidic, and oxidative stress conditions.

Results

Few transcriptional differences were observed at different temperatures, growth stages and pHs; however, a trend towards higher expression of Fno T6SS-homologue genes at 25 °C and under oxidative stress was detected when compared to those quantified at 30 °C and under no H₂O₂ (p < 0.05).

Conclusions

Results from this study suggest that several of the F. tularensis T6SS-homologues may play an important role in the virulence of Fno, particularly when the bacterium is exposed to low temperatures and oxidative stress.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1389-7) contains supplementary material, which is available to authorized users.

Francisella tularensis novicida infection competence differs in cell lines derived from United States populations of Dermacentor andersoni and Ixodes scapularis

In the United States, Dermacentor spp. are common vectors of Francisella tularensis subspecies (ssp.), while Ixodes scapularis is not, though the geographic distribution and host range of pathogen and tick overlap. To examine if differences in infection competence at the cellular level underpin these ecological differences, we evaluated the competence of D. andersoni (DAE100) and I. scapularis (ISE6) cell lines to support F. tularensis ssp. novicida (F. novicida) infection. Importantly, D. andersoni is a vector for both F. tularensis spp. tularensis, and F. novicida. We hypothesized F. novicida infection would be more productive in D. andersoni than in I. scapularis cells. Specifically, we determined if there are differences in F. novicida i) invasion, ii) replication, or iii) tick cell viability between DAE100 and ISE6 cells. We further examined the influence of temperature on infection kinetics. Both cell lines were permissive to F. novicida infection; however, there were significantly higher bacterial levels and mortality in DAE100 compared to ISE6 cells. Infection at environmental temperatures prolonged the time bacteria were maintained at high levels and reduced tick cell mortality in both cell lines. Identifying cellular determinants of vector competence is essential in understanding tick-borne disease ecology and designing effective intervention strategies.

The orange spotted cockroach (Blaptica dubia, Serville 1839) is a permissive experimental host for Francisella tularensis

Francisella tularensis is a zoonotic bacterial pathogen that causes severe disease in a wide range of host animals, including humans. Well-developed murine models of F. tularensis pathogenesis are available, but they do not meet the needs of all investigators. However, researchers are increasingly turning to insect host systems as a cost-effective alternative that allows greater increased experimental throughput without the regulatory requirements associated with the use of mammals in biomedical research. Unfortunately, the utility of previously-described insect hosts is limited because of temperature restriction, short lifespans, and concerns about the immunological status of insects mass-produced for other purposes. Here, we present a novel host species, the orange spotted (OS) cockroach (Blaptica dubia), that overcomes these limitations and is readily infected by F. tularensis. Intrahemocoel inoculation was accomplished using standard laboratory equipment and lethality was directly proportional to the number of bacteria injected. Progression of infection differed in insects housed at low and high temperatures and F. tularensis mutants lacking key virulence components were attenuated in OS cockroaches. Finally, antibiotics were delivered to infected OS cockroaches by systemic injection and controlled feeding; in the latter case, protection correlated with oral bioavailability in mammals. Collectively, these results demonstrate that this new host system provides investigators with a new tool capable of interrogating F. tularensis virulence and immune evasion in situations where mammalian models are not available or appropriate, such as undirected screens of large mutant libraries.

Complement C3 as a Prompt for Human Macrophage Death during Infection with Francisella tularensis Strain SCHU S4

Tularemia is caused by the Gram-negative bacterial pathogen Francisella tularensis Infection of macrophages and their subsequent death are believed to play important roles in the progression of disease. Because complement is a particularly effective opsonin for Francisella, we asked whether complement-dependent uptake of F. tularensis strain SCHU S4 affects the survival of primary human macrophages during infection. Complement component C3 was found to be an essential opsonin in human serum not only for greatly increased uptake of SCHU S4 but also for the induction of macrophage death. Single-cell analysis also revealed that macrophage death did not require a high intracellular bacterial burden. In the presence of C3, macrophage death was observed at 24 h postinfection in a quarter of the macrophages that contained only 1 to 5 bacterial cells. Macrophages infected in the absence of C3 rarely underwent cell death, even when they contained large numbers of bacteria. The need for C3, but not extensive replication of the pathogen, was confirmed by infections with SCHU S4 ΔpurMCD, a mutant capable of phagosome escape but of only limited cytosolic replication. C3-dependent Francisella uptake alone was insufficient to induce macrophage death, as evidenced by the failure of the phagosome escape-deficient mutant SCHU S4 ΔfevR to induce cell death despite opsonization with C3. Together, these findings indicate that recognition of C3-opsonized F. tularensis, but not extensive cytosolic replication, plays an important role in regulating macrophage viability during intracellular infections with type A F. tularensis.

Characterization of a Francisella tularensis-Caenorhabditis elegans Pathosystem for the Evaluation of Therapeutic Compounds

Francisella tularensis is a highly infectious Gram-negative intracellular pathogen that causes tularemia. Because of its potential as a bioterrorism agent, there is a need for new therapeutic agents. We therefore developed a whole-animal Caenorhabditis elegans-F. tularensis pathosystem for high-throughput screening to identify and characterize potential therapeutic compounds. We found that the C. elegans p38 mitogen-activate protein (MAP) kinase cascade is involved in the immune response to F. tularensis, and we developed a robust F. tularensis-mediated C. elegans killing assay with a Z' factor consistently of >0.5, which was then utilized to screen a library of FDA-approved compounds that included 1,760 small molecules. In addition to clinically used antibiotics, five FDA-approved drugs were also identified as potential hits, including the anti-inflammatory drug diflunisal that showed anti-F. tularensis activity in vitro Moreover, the nonsteroidal anti-inflammatory drug (NSAID) diflunisal, at 4× MIC, blocked the replication of an F. tularensis live vaccine strain (LVS) in primary human macrophages and nonphagocytic cells. Diflunisal was nontoxic to human erythrocytes and HepG2 human liver cells at concentrations of ≥32 μg/ml. Finally, diflunisal exhibited synergetic activity with the antibiotic ciprofloxacin in both a checkerboard assay and a macrophage infection assay. In conclusion, the liquid C. elegans-F. tularensis LVS assay described here allows screening for anti-F. tularensis compounds and suggests that diflunisal could potentially be repurposed for the management of tularemia.

Differential Growth of Francisella tularensis, Which Alters Expression of Virulence Factors, Dominant Antigens, and Surface-Carbohydrate Synthases, Governs the Apparent Virulence of Ft SchuS4 to Immunized Animals

The gram-negative bacterium Francisella tularensis (Ft) is both a potential biological weapon and a naturally occurring microbe that survives in arthropods, fresh water amoeba, and mammals with distinct phenotypes in various environments. Previously, we used a number of measurements to characterize Ft grown in Brain-Heart Infusion (BHI) broth as (1) more similar to infection-derived bacteria, and (2) slightly more virulent in naïve animals, compared to Ft grown in Mueller Hinton Broth (MHB). In these studies we observed that the free amino acids in MHB repress expression of select Ft virulence factors by an unknown mechanism. Here, we tested the hypotheses that Ft grown in BHI (BHI-Ft) accurately displays a full protein composition more similar to that reported for infection-derived Ft and that this similarity would make BHI-Ft more susceptible to pre-existing, vaccine-induced immunity than MHB-Ft. We performed comprehensive proteomic analysis of Ft grown in MHB, BHI, and BHI supplemented with casamino acids (BCA) and compared our findings to published “omics” data derived from Ft grown in vivo. Based on the abundance of ~1,000 proteins, the fingerprint of BHI-Ft is one of nutrient-deprived bacteria that—through induction of a stringent-starvation-like response—have induced the FevR regulon for expression of the bacterium's virulence factors, immuno-dominant antigens, and surface-carbohydrate synthases. To test the notion that increased abundance of dominant antigens expressed by BHI-Ft would render these bacteria more susceptible to pre-existing, vaccine-induced immunity, we employed a battery of LVS-vaccination and S4-challenge protocols using MHB- and BHI-grown Ft S4. Contrary to our hypothesis, these experiments reveal that LVS-immunization provides a barrier to infection that is significantly more effective against an MHB-S4 challenge than a BHI-S4 challenge. The differences in apparent virulence to immunized mice are profoundly greater than those observed with primary infection of naïve mice. Our findings suggest that tularemia vaccination studies should be critically evaluated in regard to the growth conditions of the challenge agent.

The potential for flower nectar to allow mosquito to mosquito transmission of Francisella tularensis

Francisella tularensis is disseminated in nature by biting arthropods such as mosquitoes. The relationship between mosquitoes and F. tularensis in nature is highly ambiguous, due in part to the fact that mosquitoes have caused significant tularemia outbreaks despite being classified as a mechanical vector of F. tularensis. One possible explanation for mosquitoes being a prominent, yet mechanical vector is that these insects feed on flower nectar between blood meals, allowing for transmission of F. tularensis between mosquitoes. Here, we aimed to assess whether F. tularensis could survive in flower nectar. Moreover, we examined if mosquitoes could interact with or ingest and transmit F. tularensis from one source of nectar to another. F. tularensis exhibited robust survivability in flower nectar with concentrations of viable bacteria remaining consistent with the rich growth medium. Furthermore, F. tularensis was able to survive (albeit to a lesser extent) in 30% sucrose (a nectar surrogate) over a period of time consistent with that of a typical flower bloom. Although we observed diminished bacterial survival in the nectar surrogate, mosquitoes that fed on this material became colonized with F. tularensis. Finally, colonized mosquitoes were capable of transferring F. tularensis to a sterile nectar surrogate. These data suggest that flower nectar may be capable of serving as a temporary source of F. tularensis that could contribute to the amplification of outbreaks. Mosquitoes that feed on an infected mammalian host and subsequently feed on flower nectar could deposit some F. tularensis bacteria into the nectar in the process. Mosquitoes subsequently feeding on this nectar source could potentially become colonized by F. tularensis. Thus, the possibility exists that flower nectar may allow for vector-vector transmission of F. tularensis.

The Role and Mechanism of Erythrocyte Invasion by Francisella tularensis

Francisella tularensis is an extremely virulent bacterium that can be transmitted naturally by blood sucking arthropods. During mammalian infection, F. tularensis infects numerous types of host cells, including erythrocytes. As erythrocytes do not undergo phagocytosis or endocytosis, it remains unknown how F. tularensis invades these cells. Furthermore, the consequence of inhabiting the intracellular space of red blood cells (RBCs) has not been determined. Here, we provide evidence indicating that residing within an erythrocyte enhances the ability of F. tularensis to colonize ticks following a blood meal. Erythrocyte residence protected F. tularensis from a low pH environment similar to that of gut cells of a feeding tick. Mechanistic studies revealed that the F. tularensis type VI secretion system (T6SS) was required for erythrocyte invasion as mutation of mglA (a transcriptional regulator of T6SS genes), dotU, or iglC (two genes encoding T6SS machinery) severely diminished bacterial entry into RBCs. Invasion was also inhibited upon treatment of erythrocytes with venom from the Blue-bellied black snake (Pseudechis guttatus), which aggregates spectrin in the cytoskeleton, but not inhibitors of actin polymerization and depolymerization. These data suggest that erythrocyte invasion by F. tularensis is dependent on spectrin utilization which is likely mediated by effectors delivered through the T6SS. Our results begin to elucidate the mechanism of a unique biological process facilitated by F. tularensis to invade erythrocytes, allowing for enhanced colonization of ticks.

Detection of Francisella tularensis and analysis of bacterial growth in ticks in Japan

Francisella tularensis is distributed in the Northern hemisphere and it is the bacterial agent responsible for tularaemia, a zoonotic disease. We collected 4 527 samples of DNA from ticks in Japan, which were then analysed by real-time PCR and nested PCR. Francisella DNA was detected by real-time PCR in 2·15% (45/2 093) of Ixodes ovatus, 0·66% (14/2 107) of I. persulcatus, 8·22% (6/73) of I. monospinosus and 0·72% (1/138) of Haemaphysalis flava specimens. Finally, Francisella DNA was detected by nested PCR in 42 and five samples I. ovatus and I. persulcatus, respectively, which were positive according to real-time PCR. Phylogenetic analysis showed that the sequence from I. ovatus and I. persulcatus were clustered with F. tularensis type B strains distributed in Eurasia. Microinjected live F. tularensis persisted in ticks, whereas heat-killed F. tularensis decreased. Microinjected F. tularensis hlyD mutant decreased in ticks significantly compared to parent strain, thereby suggesting that HlyD in F. tularensis contributes to the adaptation or survive of bacterial infection in ticks.

SIGNIFICANCE AND IMPACTS OF THE STUDY

Francisella tularensis has been detected in ticks, suggesting that it is a tick-borne pathogen. However, F. tularensis has not been detected in ticks in Japan since 1991. In this study, we performed a large-scale analysis of DNA isolated from ticks in Japan and detected F. tularensis by real-time polymerase chain reaction (PCR) and nested PCR. We found that F. tularensis could survive in ticks based on an experimental tick-infection model. We also identified a bacterial factor that contributes to survival in ticks. Our results suggest that ticks are candidate vectors that mediate F. tularensis infection in Japan.

Genetic engineering of Francisella tularensis LVS for use as a novel live vaccine platform against Pseudomonas aeruginosa infections

Francisella tularensis LVS (Live Vaccine Strain) is an attenuated bacterium that has been used as a live vaccine. Patients immunized with this organism show a very long-term memory response (over 30 years post vaccination) evidenced by the presence of indicators of robust cell-mediated immunity. Because F. tularensis LVS is such a potent vaccine, we hypothesized that this organism would be an effective vaccine platform. First, we sought to determine if we could genetically modify this strain to produce protective antigens of a heterologous pathogen. Currently, there is not a licensed vaccine against the important opportunistic bacterial pathogen, Pseudomonas aeruginosa. Because many P. aeruginosa strains are also drug resistant, the need for effective vaccines is magnified. Here, F. tularensis LVS was genetically modified to express surface proteins PilA_Pa, OprF_Pa, and FliC_Pa of P. aeruginosa. Immunization of mice with LVS expressing the recombinant FliC_Pa led to a significant production of antibodies specific for P. aeruginosa. However, mice that had been immunized with LVS expressing PilA_Pa or OprF_Pa did not produce high levels of antibodies specific for P. aerugionsa. Therefore, the recombinant LVS strain engineered to produce FliC_Pa may be able to provide immune protection against a P. aeruginosa challenge. However for future use of this vaccine platform, selection of the appropriate recombinant antigen is critical as not all recombinant antigens expressed in this strain were immunogenic.

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.

Global gene expression by Bacillus anthracis during growth in mammalian blood

During the late stages of systemic anthrax, Bacillus anthracis grows rapidly in the host bloodstream. To identify potential genes necessary for this observed rapid growth, we defined the transcriptional profile of B. anthracis during in vitro growth in bovine blood. Genome-wide transcriptome analysis indicated that B. anthracis undergoes significant changes in its transcriptome profile during growth in blood, including the differential regulation of genes associated both with metabolism and known virulence factors. Collectively, these data provide a framework for future studies identifying specific B. anthracis factors required for growth in the mammalian bloodstream.

Francisella tularensis: No Evidence for Transovarial Transmission in the Tularemia Tick Vectors Dermacentor reticulatus and Ixodes ricinus

Background

Tularemia is a zoonosis caused by the Francisella tularensis, a highly infectious Gram-negative coccobacillus. Due to easy dissemination, multiple routes of infection, high environmental contamination and morbidity and mortality rates, Francisella is considered a potential bioterrorism threat and classified as a category A select agent by the CDC. Tick bites are among the most prevalent modes of transmission, and ticks have been indicated as a possible reservoir, although their reservoir competence has yet to be defined. Tick-borne transmission of F. tularensis was recognized in 1923, and transstadial transmission has been demonstrated in several tick species. Studies on transovarial transmission, however, have reported conflicting results.

Objective

The aim of this study was to evaluate the role of ticks as reservoirs for Francisella, assessing the transovarial transmission of F. tularensis subsp. holarctica in ticks, using experimentally-infected females of Dermacentor reticulatus and Ixodes ricinus.

Results

Transmission electron microscopy and fluorescence in situ hybridization showed F. tularensis within oocytes. However, cultures and bioassays of eggs and larvae were negative; in addition, microscopy techniques revealed bacterial degeneration/death in the oocytes.

Conclusions

These results suggest that bacterial death might occur in oocytes, preventing the transovarial transmission of Francisella. We can speculate that Francisella does not have a defined reservoir, but that rather various biological niches (e.g. ticks, rodents), that allow the bacterium to persist in the environment. Our results, suggesting that ticks are not competent for the bacterium vertical transmission, are congruent with this view.

RNA structures are involved in the thermoregulation of bacterial virulence-associated traits

Pathogenic bacteria are exposed to temperature changes during colonization of the human body and during exposure to environmental conditions. Virulence-associated traits are mainly expressed by pathogenic bacteria at 37°C. We review different cases of post-transcriptional regulation of virulence-associated proteins through RNA structures (called RNA thermometers or RNATs) that modulate the translation of mRNAs. The analysis of RNATs in pathogenic bacteria has started to produce a comprehensive picture of the structures involved, and of the genes regulated by this mechanism. However, we are still not able to predict the functionality of putative RNATs predicted by bioinformatics methods, and there is not a global approach to measure the effect of these RNA structures in gene regulation during bacterial infections.

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.

Gene expression profiling of Clostridium botulinum under heat shock stress

During growth, C. botulinum is always exposed to different environmental changes, such as temperature increase, nutrient deprivation, and pH change; however, its corresponding global transcriptional profile is uncharacterized. This study is the first description of the genome-wide gene expression profile of C. botulinum in response to heat shock stress. Under heat stress (temperature shift from 37°C to 45°C over a period of 15 min), 176 C. botulinum ATCC 3502 genes were differentially expressed. The response included overexpression of heat shock protein genes (dnaK operon, groESL, hsp20, and htpG) and downregulation of aminoacyl-tRNA synthetase genes (valS, queA, tyrR, and gatAB) and ribosomal and cell division protein genes (ftsZ and ftsH). In parallel, several transcriptional regulators (marR, merR, and ompR families) were induced, suggesting their involvement in reshuffling of the gene expression profile. In addition, many ABC transporters (oligopeptide transport system), energy production and conversion related genes (glpA and hupL), cell wall and membrane biogenesis related genes (fabZ, fabF, and fabG), flagella-associated genes (flhA, flhM, flhJ, flhS, and motAB), and hypothetical genes also showed changed expression patterns, indicating that they may play important roles in survival under high temperatures.

Comparative proteomic analysis reveals new components of the PhoP regulon and highlights a role for PhoP in the regulation of genes encoding the F1F0 ATP synthase in Edwardsiella tarda

Edwardsiella tarda is an important cause of haemorrhagic septicaemia in fish and also of gastro- and extra-intestinal infections in humans. We have recently demonstrated that the PhoP-PhoQ two-component regulatory system plays important roles in both virulence and stress tolerance in E. tarda. In this study, the proteomes of the WT and phoP mutant strains were compared to define components of the PhoP regulon in E. tarda EIB202. Overall, 18 proteins whose expression levels exhibited a twofold or greater change were identified; 13 of these proteins were found to require the presence of PhoP for full expression, while five were expressed at a higher level in the phoP mutant background. Identified proteins represented diverse functional categories, including energy production, amino acid metabolism and oxidative stress defence. Quantitative real-time PCR analysis of the mRNA levels for the identified proteins confirmed the proteomics data. Interestingly, the β subunit of the F1F0 ATP synthase, playing an important role in growth and virulence of E. tarda, was listed as one of the proteins whose expression was greatly dependent on PhoP. The F1F0 ATP synthase was encoded in a gene cluster (atpIBEFHAGDC) and the nine genes were transcribed as an operon. PhoP positively regulated the transcription of the nine ATP synthase genes and exerted this effect through direct binding to the promoter of atpI. Overall, the results provide new insights into the PhoP regulon and unravel a novel role for PhoP in the regulation of the F1F0 ATP synthase.

Bartonella quintana deploys host and vector temperature-specific transcriptomes

The bacterial pathogen Bartonella quintana is passed between humans by body lice. B. quintana has adapted to both the human host and body louse vector niches, producing persistent infection with high titer bacterial loads in both the host (up to 10⁵ colony-forming units [CFU]/ml) and vector (more than 10⁸ CFU/ml). Using a novel custom microarray platform, we analyzed bacterial transcription at temperatures corresponding to the host (37°C) and vector (28°C), to probe for temperature-specific and growth phase-specific transcriptomes. We observed that transcription of 7% (93 genes) of the B. quintana genome is modified in response to change in growth phase, and that 5% (68 genes) of the genome is temperature-responsive. Among these transcriptional changes in response to temperature shift and growth phase was the induction of known B. quintana virulence genes and several previously unannotated genes. Hemin binding proteins, secretion systems, response regulators, and genes for invasion and cell attachment were prominent among the differentially-regulated B. quintana genes. This study represents the first analysis of global transcriptional responses by B. quintana. In addition, the in vivo experiments provide novel insight into the B. quintana transcriptional program within the body louse environment. These data and approaches will facilitate study of the adaptation mechanisms employed by Bartonella during the transition between human host and arthropod vector.

Evaluation of reference genes for reverse transcription quantitative PCR analyses of fish-pathogenic Francisella strains exposed to different growth conditions

Background

Reverse transcription quantitative PCR has become a powerful technique to monitor mRNA transcription in response to different environmental conditions in many bacterial species. However, correct evaluation of data requires accurate and reliable use of reference genes whose transcription does not change during the course of the experiment. In the present study exposure to different growth conditions was used to validate the transcription stability of eight reference gene candidates in three strains from two subspecies of Francisella noatunensis, a pathogen causing disease in both warm and cold water fish species.

Results

Relative transcription levels for genes encoding DNA gyrase (gyrA), RNA polymerase beta subunit (rpoB), DNA polymerase I (polA), cell division protein (ftsZ), outer membrane protein (fopA), riboflavin biosynthesis protein (ribC), 16S ribosomal RNA (16S rRNA) and DNA helicases (uvrD) were quantified under exponential, stationary and iron-restricted growth conditions. The suitability of selected reference genes for reliable interpretation of gene expression data was tested using the virulence-associated intracellular growth locus subunit C (iglC) gene.

Conclusion

Although the transcription stability of the reference genes was slightly different in the three strains studied, fopA, ftsZ and polA proved to be the most stable and suitable for normalization of gene transcription in Francisella noatunensis ssp.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

PanG, a new ketopantoate reductase involved in pantothenate synthesis

Pantothenate, commonly referred to as vitamin B(5), is an essential molecule in the metabolism of living organisms and forms the core of coenzyme A. Unlike humans, some bacteria and plants are capable of de novo biosynthesis of pantothenate, making this pathway a potential target for drug development. Francisella tularensis subsp. tularensis Schu S4 is a zoonotic bacterial pathogen that is able to synthesize pantothenate but is lacking the known ketopantoate reductase (KPR) genes, panE and ilvC, found in the canonical Escherichia coli pathway. Described herein is a gene encoding a novel KPR, for which we propose the name panG (FTT1388), which is conserved in all sequenced Francisella species and is the sole KPR in Schu S4. Homologs of this KPR are present in other pathogenic bacteria such as Enterococcus faecalis, Coxiella burnetii, and Clostridium difficile. Both the homologous gene from E. faecalis V583 (EF1861) and E. coli panE functionally complemented Francisella novicida lacking any KPR. Furthermore, panG from F. novicida can complement an E. coli KPR double mutant. A Schu S4 ΔpanG strain is a pantothenate auxotroph and was genetically and chemically complemented with panG in trans or with the addition of pantolactone. There was no virulence defect in the Schu S4 ΔpanG strain compared to the wild type in a mouse model of pneumonic tularemia. In summary, we characterized the pantothenate pathway in Francisella novicida and F. tularensis and identified an unknown and previously uncharacterized KPR that can convert 2-dehydropantoate to pantoate, PanG.

Identification of a novel small RNA modulating Francisella tularensis pathogenicity

Francisella tularensis is a highly virulent bacterium responsible for the zoonotic disease tularemia. It is a facultative intracellular pathogen that replicates in the cytoplasm of host cells, particularly in macrophages. Here we show that F. tularensis live vaccine strain (LVS) expresses a novel small RNA (sRNA), which modulates the virulence capacities of the bacterium. When this sRNA, designated FtrC (for Francisella tularensisRNA C), is expressed at high levels, F. tularensis replicates in macrophages less efficiently than the wild-type parent strain. Similarly, high expression of FtrC reduces the number of viable bacteria recovered from the spleen and liver of infected mice. Our data demonstrate that expression of gene FTL_1293 is regulated by FtrC. Furthermore, we show by in vitro gel shift assays that FtrC interacts specifically with FTL_1293 mRNA and that this happens independently of the RNA chaperone Hfq. Remarkably, FtrC interacts only with full-length FTL_1293 mRNA. These results, combined with a bioinformatic analysis, indicate that FtrC interacts with the central region of the mRNA and hence does not act by sterically hindering access of the ribosome to the mRNA. We further show that gene FTL_1293 is not required for F. tularensis virulence in vitro or in vivo, which indicates that another unidentified FtrC target modulates the virulence capacity of the bacterium.

TetR-based gene regulation systems for Francisella tularensis

There are a number of genetic tools available for studying Francisella tularensis, the etiological agent of tularemia; however, there is no effective inducible or repressible gene expression system. Here, we describe inducible and repressible gene expression systems for F. tularensis based on the Tet repressor, TetR. For the inducible system, a tet operator sequence was cloned into a modified F. tularensis groESL promoter sequence and carried in a plasmid that constitutively expressed TetR. To monitor regulation the luminescence operon, luxCDABE, was cloned under the hybrid Francisella tetracycline-regulated promoter (FTRp), and transcription was initiated with addition of anhydrotetracycline (ATc), which binds TetR and alleviates TetR association with tetO. Expression levels measured by luminescence correlated with ATc inducer concentrations ranging from 20 to 250 ng ml(-1). In the absence of ATc, luminescence was below the level of detection. The inducible system was also functional during the infection of J774A.1 macrophages, as determined by both luminescence and rescue of a mutant strain with an intracellular growth defect. The repressible system consists of FTRp regulated by a reverse TetR mutant (revTetR), TetR r1.7. Using this system with the lux reporter, the addition of ATc resulted in decreased luminescence, while in the absence of ATc the level of luminescence was not significantly different from that of a construct lacking TetR r1.7. Utilizing both systems, the essentiality of SecA, the protein translocase ATPase, was confirmed, establishing that they can effectively regulate gene expression. These two systems will be invaluable in exploring F. tularensis protein function.

LPS remodeling is an evolved survival strategy for bacteria

Maintenance of membrane function is essential and regulated at the genomic, transcriptional, and translational levels. Bacterial pathogens have a variety of mechanisms to adapt their membrane in response to transmission between environment, vector, and human host. Using a well-characterized model of lipid A diversification (Francisella), we demonstrate temperature-regulated membrane remodeling directed by multiple alleles of the lipid A-modifying N-acyltransferase enzyme, LpxD. Structural analysis of the lipid A at environmental and host temperatures revealed that the LpxD1 enzyme added a 3-OH C18 acyl group at 37 °C (host), whereas the LpxD2 enzyme added a 3-OH C16 acyl group at 18 °C (environment). Mutational analysis of either of the individual Francisella lpxD genes altered outer membrane (OM) permeability, antimicrobial peptide, and antibiotic susceptibility, whereas only the lpxD1-null mutant was attenuated in mice and subsequently exhibited protection against a lethal WT challenge. Additionally, growth-temperature analysis revealed transcriptional control of the lpxD genes and posttranslational control of the LpxD1 and LpxD2 enzymatic activities. These results suggest a direct mechanism for LPS/lipid A-level modifications resulting in alterations of membrane fluidity, as well as integrity and may represent a general paradigm for bacterial membrane adaptation and virulence-state adaptation.

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.

Invasion of erythrocytes by Francisella tularensis

Francisella tularensis is the causative agent of tularemia and is classified as a category A biodefense agent by the Centers for Disease Control and Prevention because of its highly infectious nature. F. tularensis infects leukocytes and exhibits an extracellular phase in the blood of the host. It is unknown, however, whether F. tularensis can infect erythrocytes; thus, we examined this possibility in vivo and in vitro. In the murine model of pulmonary type A tularemia, we showed the presence of intraerythrocytic bacteria by double-immunofluorescence microscopy and ex vivo gentamicin protection of the purified erythrocyte fraction. In vitro, F. tularensis invaded human erythrocytes, as shown in the gentamicin protection assays, double-immunofluorescence microscopy, flow cytometry, scanning electron microscopy, and transmission electron microscopy with immunogold labeling of the bacteria. Additional in vitro tests indicated that serum complement-dependent and complement-independent mechanisms contribute to erythrocyte invasion. Our results reveal a novel intraerythrocytic phase during F. tularensis infection.

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

Tularemia is a debilitating febrile illness caused by the category A biodefense agent Francisella tularensis. This pathogen infects over 250 different hosts, has a low infectious dose, and causes high morbidity and mortality. Our understanding of the mechanisms by which F. tularensis senses and adapts to host environments is incomplete. Polyamines, including spermine, regulate the interactions of F. tularensis with host cells. However, it is not known whether responsiveness to polyamines is necessary for the virulence of the organism. Through transposon mutagenesis of F. tularensis subsp. holarctica live vaccine strain (LVS), we identified FTL_0883 as a gene important for spermine responsiveness. In-frame deletion mutants of FTL_0883 and FTT_0615c, the homologue of FTL_0883 in F. tularensis subsp. tularensis Schu S4 (Schu S4), elicited higher levels of cytokines from human and murine macrophages compared to wild-type strains. Although deletion of FTL_0883 attenuated LVS replication within macrophages in vitro, the Schu S4 mutant with a deletion in FTT_0615c replicated similarly to wild-type Schu S4. Nevertheless, both the LVS and the Schu S4 mutants were significantly attenuated in vivo. Growth and dissemination of the Schu S4 mutant was severely reduced in the murine model of pneumonic tularemia. This attenuation depended on host responses to elevated levels of proinflammatory cytokines. These data associate responsiveness to polyamines with tularemia pathogenesis and define FTL_0883/FTT_0615c as an F. tularensis gene important for virulence and evasion of the host immune response.

Biochemical characterization of the O-linked glycosylation pathway in Neisseria gonorrhoeae responsible for biosynthesis of protein glycans containing N,N'-diacetylbacillosamine

The O-linked protein glycosylation pathway in Neisseria gonorrhoeae is responsible for the synthesis of a complex oligosaccharide on undecaprenyl diphosphate and subsequent en bloc transfer of the glycan to serine residues of select periplasmic proteins. Protein glycosylation (pgl) genes have been annotated on the basis of bioinformatics and top-down mass spectrometry analysis of protein modifications in pgl-null strains [Aas, F. E., et al. (2007) Mol. Microbiol. 65, 607-624; Vik, A., et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 4447-4452], but relatively little biochemical analysis has been performed to date. In this report, we present the expression, purification, and functional characterization of seven Pgl enzymes. Specifically, the enzymes studied are responsible for synthesis of an uncommon uridine diphosphate (UDP)-sugar (PglD, PglC, and PglB-acetyltransferase domain), glycan assembly (PglB-phospho-glycosyltransferase domain, PglA, PglE, and PglH), and final oligosaccharide transfer (PglO). UDP-2,4-diacetamido-2,4,6-trideoxy-α-d-hexose (DATDH), which is the first sugar in glycan biosynthesis, was produced enzymatically, and the stereochemistry was assigned as uridine diphosphate N'-diacetylbacillosamine (UDP-diNAcBac) by nuclear magnetic resonance characterization. In addition, the substrate specificities of the phospho-glycosyltransferase, glycosyltransferases, and oligosaccharyltransferase (OTase) were analyzed in vitro, and in most cases, these enzymes exhibited strong preferences for the native substrates relative to closely related glycans. In particular, PglO, the O-linked OTase, and PglB(Cj), the N-linked OTase from Campylobacter jejuni, preferred the native N. gonorrhoeae and C. jejuni substrates, respectively. This study represents the first comprehensive biochemical characterization of this important O-linked glycosylation pathway and provides the basis for further investigations of these enzymes as antibacterial targets.

New vector tools with a hygromycin resistance marker for use with opportunistic pathogens

The ability of many bacterial strains to tolerate antibiotics can limit the number of molecular tools available for research of these organisms. To help address this problem, we have modified a diverse set of vectors to include a broadly expressed hygromycin resistance (HmR) marker. Hygromycin B is an aminoglycoside antibiotic not used to treat infections in humans and has antimicrobial activity against a wide range of microorganisms. Vectors with four replication origins are represented, with potential applications including general cloning, allelic replacement, and transcriptional analysis. We show that vectors with the broad host range pBBR1-replicon conferred HmR to Achromobacter xylosoxidans, Acinetobacter baumannii, Pseudomonas aeruginosa, and Serratia marcescens, and a pC194-based vector was able to confer HmR to Francisella tularensis. We also used a subset of these plasmids to manipulate the genome of S. marcescens. Each vector has an origin of transfer for conjugation, and is also able to replicate in Saccharomyces cerevisiae to take advantage of the powerful yeast recombineering system.

Development of transcriptional fusions to assess Leptospira interrogans promoter activity

Background

Leptospirosis is a zoonotic infectious disease that affects both humans and animals. The existing genetic tools for Leptospira spp. have improved our understanding of the biology of this spirochete as well as the interaction of pathogenic leptospires with the mammalian host. However, new tools are necessary to provide novel and useful information to the field.

Methodology and Principal Findings

A series of promoter-probe vectors carrying a reporter gene encoding green fluorescent protein (GFP) were constructed for use in L. biflexa. They were tested by constructing transcriptional fusions between the lipL41, Leptospiral Immunoglobulin-like A (ligA) and Sphingomielynase 2 (sph2) promoters from L. interrogans and the reporter gene. ligA and sph2 promoters were the most active, in comparison to the lipL41 promoter and the non-induced controls. The results obtained are in agreement with LigA expression from the L. interrogans Fiocruz L1-130 strain.

Conclusions

The novel vectors facilitated the in vitro evaluation of L. interrogans promoter activity under defined growth conditions which simulate the mammalian host environment. The fluorescence and rt-PCR data obtained closely reflected transcriptional regulation of the promoters, thus demonstrating the suitability of these vectors for assessing promoter activity in L. biflexa.

Bacillus cereus phosphopentomutase is an alkaline phosphatase family member that exhibits an altered entry point into the catalytic cycle

Bacterial phosphopentomutases (PPMs) are alkaline phosphatase superfamily members that interconvert α-D-ribose 5-phosphate (ribose 5-phosphate) and α-D-ribose 1-phosphate (ribose 1-phosphate). We investigated the reaction mechanism of Bacillus cereus PPM using a combination of structural and biochemical studies. Four high resolution crystal structures of B. cereus PPM revealed the active site architecture, identified binding sites for the substrate ribose 5-phosphate and the activator α-D-glucose 1,6-bisphosphate (glucose 1,6-bisphosphate), and demonstrated that glucose 1,6-bisphosphate increased phosphorylation of the active site residue Thr-85. The phosphorylation of Thr-85 was confirmed by Western and mass spectroscopic analyses. Biochemical assays identified Mn(2+)-dependent enzyme turnover and demonstrated that glucose 1,6-bisphosphate treatment increases enzyme activity. These results suggest that protein phosphorylation activates the enzyme, which supports an intermolecular transferase mechanism. We confirmed intermolecular phosphoryl transfer using an isotope relay assay in which PPM reactions containing mixtures of ribose 5-[(18)O(3)]phosphate and [U-(13)C(5)]ribose 5-phosphate were analyzed by mass spectrometry. This intermolecular phosphoryl transfer is seemingly counter to what is anticipated from phosphomutases employing a general alkaline phosphatase reaction mechanism, which are reported to catalyze intramolecular phosphoryl transfer. However, the two mechanisms may be reconciled if substrate encounters the enzyme at a different point in the catalytic cycle.

Francisella-arthropod vector interaction and its role in patho-adaptation to infect mammals

Francisella tularensis is a Gram-negative, intracellular, zoonotic bacterium, and is the causative agent of tularemia with a broad host range. Arthropods such as ticks, mosquitoes, and flies maintain F. tularensis in nature by transmitting the bacteria among small mammals. While the tick is largely believed to be a biological vector of F. tularensis, transmission by mosquitoes and flies is largely believed to be mechanical on the mouthpart through interrupted feedings. However, the mechanism of infection of the vectors by F. tularensis is not well understood. Since F. tularensis has not been localized in the salivary gland of the primary human biting ticks, it is thought that bacterial transmission by ticks is through mechanical inoculation of tick feces containing F. tularensis into the skin wound. Drosophila melanogaster is an established good arthropod model for arthropod vectors of tularemia, where F. tularensis infects hemocytes, and is found in hemolymph, as seen in ticks. In addition, phagosome biogenesis and robust intracellular proliferation of F. tularensis in arthropod-derived cells are similar to that in mammalian macrophages. Furthermore, bacterial factors required for infectivity of mammals are often required for infectivity of the fly by F. tularensis. Several host factors that contribute to F. tularensis intracellular pathogenesis in D. melanogaster have been identified, and F. tularensis targets some of the evolutionarily conserved eukaryotic processes to enable intracellular survival and proliferation in evolutionarily distant hosts.

Regulation of francisella tularensis virulence

Francisella tularensis is one of the most virulent bacteria known and a Centers for Disease Control and Prevention Category A select agent. It is able to infect a variety of animals and insects and can persist in the environment, thus Francisella spp. must be able to survive in diverse environmental niches. However, F. tularensis has a surprising dearth of sensory and regulatory factors. Recent advancements in the field have identified new functions of encoded transcription factors and greatly expanded our understanding of virulence gene regulation. Here we review the current knowledge of environmental adaptation by F. tularensis, its transcriptional regulators and their relationship to animal virulence.

Genetic manipulation of francisella tularensis

Francisella tularensis is a facultative intracellular pathogen that causes the disease tularemia. F. tularensis subsp. tularensis causes the most severe disease in humans and has been classified as a Category A select agent and potential bioweapon. There is currently no vaccine approved for human use, making genetic manipulation of this organism critical to unraveling the genetic basis of pathogenesis and developing countermeasures against tularemia. The development of genetic techniques applicable to F. tularensis have lagged behind those routinely used for other bacteria, primarily due to lack of research and the restricted nature of the biocontainment required for studying this pathogen. However, in recent years, genetic techniques, such as transposon mutagenesis and targeted gene disruption, have been developed, that have had a dramatic impact on our understanding of the genetic basis of F. tularensis virulence. In this review, we describe some of the methods developed for genetic manipulation of F. tularensis.

Francisella philomiragia biofilm formation and interaction with the aquatic protist Acanthamoeba castellanii

The bacterium Francisella philomiragia has been isolated from environmental samples originating from around the globe. F. philomiragia-related strains cause francisellosis of both farmed and wild fish. In addition, occasional human infections caused by F. philomiragia are found in victims of near-drowning and patients with chronic granulomatous disease. We have shown that F. philomiragia forms in vitro biofilms with increased formation at 25 °C over 37 °C conditions. We found that F. philomiragia can form a biofilm in a co-culture with live Acanthamoeba castellanii, an aquatic amoeba. Interestingly, amoeba-conditioned supernatant has an inhibitory effect on production of biofilm by F. philomiragia, whereas Francisella-conditioned supernatant has no effect on growth of amoebae. We have shown that F. philomiragia can infect A. castellanii after only 5 days of co-incubation and that it infects A. castellanii more quickly than the related species F. novicida does. Our studies point to a potentially overlooked interaction between F. philomiragia and Acanthamoeba. This relationship in the marine lifecycle of F. philomiragia may support the persistence of the bacterium in waterways and its ability to infect fish. An understanding of the persistence of this organism in aquatic systems through biofilm formation and its interaction with Acanthamoeba will be important in developing prevention strategies for this pathogen.

Crystallization and preliminary X-ray analysis of a phosphopentomutase from Bacillus cereus

Phosphopentomutases (PPMs) interconvert D-ribose 5-phosphate and alpha-D-ribose 1-phosphate to link glucose and nucleotide metabolism. PPM from Bacillus cereus was overexpressed in Escherichia coli, purified to homogeneity and crystallized. Bacterial PPMs are predicted to contain a di-metal reaction center, but the catalytically relevant metal has not previously been identified. Sparse-matrix crystallization screening was performed in the presence or absence of 50 mM MnCl(2). This strategy resulted in the formation of two crystal forms from two chemically distinct conditions. The crystals that formed with 50 mM MnCl(2) were more easily manipulated and diffracted to higher resolution. These results suggest that even if the catalytically relevant metal is not known, the crystallization of putative metalloproteins may still benefit from supplementation of the crystallization screens with potential catalytic metals.