Biology and molecular epidemiology of diphtheria toxin and the tox gene

First Report on the Draft Genome Sequences of Corynebacterium diphtheriae Isolates from India

We report here the draft genome sequences of five Corynebacterium diphtheriae isolates of Indian origin. The C. diphtheriae isolates TH1141, TH510, TH1526, TH1337, and TH2031 belong to sequence type ST-50, ST-295, ST-377, ST-405, and ST-405, with an average genome size of 2.5 Mbp.

Recent Advances in the Development of Antineoplastic Agents Derived from Natural Products

Through years of evolutionary selection pressures, organisms have developed potent toxins that coincidentally have marked antineoplastic activity. These natural products have been vital for the development of multiagent treatment regimens currently employed in cancer chemotherapy, and are used in the treatment of a variety of malignancies. Therefore, this review catalogs recent advances in natural product-based drug discovery via the examination of mechanisms of action and available clinical data to highlight the utility of these novel compounds in the burgeoning age of precision medicine. The review also highlights the recent development of antibody-drug conjugates and other immunotoxins, which are capable of delivering highly cytotoxic agents previously deemed too toxic to elicit therapeutic benefit preferentially to neoplastic cells. Finally, the review examines natural products not currently used in the clinic that have novel mechanisms of action, and may serve to supplement current chemotherapeutic protocols.

The ChrSA and HrrSA Two-Component Systems Are Required for Transcriptional Regulation of the hemA Promoter in Corynebacterium diphtheriae

Corynebacterium diphtheriae utilizes heme and hemoglobin (Hb) as iron sources for growth in low-iron environments. In C. diphtheriae, the two-component signal transduction systems (TCSs) ChrSA and HrrSA are responsive to Hb levels and regulate the transcription of promoters for hmuO, hrtAB, and hemA ChrSA and HrrSA activate transcription at the hmuO promoter and repress transcription at hemA in an Hb-dependent manner. In this study, we show that HrrSA is the predominant repressor at hemA and that its activity results in transcriptional repression in the presence and absence of Hb, whereas repression of hemA by ChrSA is primarily responsive to Hb. DNA binding studies showed that both ChrA and HrrA bind to the hemA promoter region at virtually identical sequences. ChrA binding was enhanced by phosphorylation, while binding to DNA by HrrA was independent of its phosphorylation state. ChrA and HrrA are phosphorylated in vitro by the sensor kinase ChrS, whereas no kinase activity was observed with HrrS in vitro Phosphorylated ChrA was not observed in vivo, even in the presence of Hb, which is likely due to the instability of the phosphate moiety on ChrA. However, phosphorylation of HrrA was observed in vivo regardless of the presence of the Hb inducer, and genetic analysis indicates that ChrS is responsible for most of the phosphorylation of HrrA in vivo Phosphorylation studies strongly suggest that HrrS functions primarily as a phosphatase and has only minimal kinase activity. These findings collectively show a complex mechanism of regulation at the hemA promoter, where both two-component systems act in concert to optimize expression of heme biosynthetic enzymes.

IMPORTANCE

Understanding the mechanism by which two-component signal transduction systems function to respond to environmental stimuli is critical to the study of bacterial pathogenesis. The current study expands on the previous analyses of the ChrSA and HrrSA TCSs in the human pathogen C. diphtheriae The findings here underscore the complex interactions between the ChrSA and HrrSA systems in the regulation of the hemA promoter and demonstrate how the two systems complement one another to refine and control transcription in the presence and absence of Hb.

Tunable Lipidoid-Telodendrimer Hybrid Nanoparticles for Intracellular Protein Delivery in Brain Tumor Treatment

A strategy to precisely engineer lipidoid-telodendrimer binary hybrid nanoparticles that offer enhanced cell membrane permeability for therapeutic proteins to reach the intracellular targets is established. The highly controllable biochemical and physical properties of the nanoparticles make them promising for protein-based brain cancer treatment with the assistance of convection-enhanced delivery.

Microbial Ecosystem Analysis in Root Canal Infections Refractory to Endodontic Treatment

INTRODUCTION

The purpose of this study was to combine multiple displacement amplification and checkerboard DNA-DNA hybridization to qualitatively and quantitatively evaluate the microbiota present in infections refractory to endodontic treatment.

METHODS

The subjects of this study were 40 patients presenting with periapical lesions refractory to endodontic treatment. Samples were taken by scraping or filing root canal walls with a #10 K-type hand file. Sample DNA was amplified by multiple displacement amplification, and the levels of 107 bacterial taxa were analyzed by checkerboard DNA-DNA hybridization. The taxa were divided into 3 distinct microbial populations depending on their mean proportion in samples (% DNA probe counts ± standard error of the mean) as follows: dominant (≥3.0%), subdominant (>1.6%-3.0%), and residual (≤1.6%) populations. The significance of differences was determined using the Mann-Whitney test.

RESULTS

The taxa present with the highest mean proportions (constituting the dominant population) were Corynebacterium diphtheriae (8.03 ± 0.98), Porphyromonas gingivalis (5.42 ± 2.09), Streptococcus sobrinus (5.33 ± 0.69), and Stenotrophomonas maltophilia (4.72 ± 1.73). Among the subdominant population were Eubacterium saphenum (3.85 ± 1.06), Helicobacter pylori (3.16 ± 0.62), Dialister pneumosintes (3.12 ± 1.1), Clostridium difficile (2.74 ± 0.41), Enterobacter agglomerans (2.64 ± 0.54), Salmonella enterica (2.51 ± 0.52), Mobiluncus mulieris (2.44 ± 0.6), and Klebsiella oxytoca (2.32 ± 0.66). In the population of bacteria present at the lowest mean proportions (the residual population), Bacteroides ureolyticus (0.04 ± 0.01), Haemophilus influenzae (0.04 ± 0.02), and Prevotella oris (0.01 ± 0.01) were found at the lowest mean proportions. Enterococcus faecalis was detected in the residual population (0.52 ± 0.26).

CONCLUSIONS

The microbial climax community in teeth refractory to endodontic treatment not only harbors medically important species but also contains distinct microbial consortia present with different population levels.

CD11c.DTR mice develop a fatal fulminant myocarditis after local or systemic treatment with diphtheria toxin

To assess the role of alveolar macrophages (AMs) during a pulmonary Aspergillus fumigatus infection AMs were depleted by intratracheal application of diphtheria toxin (DTX) to transgenic CD11c.DTR mice prior to fungal infection. Unexpectedly, all CD11c.DTR mice treated with DTX died within 4-5 days, whether being infected with A. fumigatus or not. Despite measurable impact of DTX on lung functional parameters, these constrictions could not explain the high mortality rate. Instead, DTX-treated CD11c.DTR animals developed fulminant myocarditis (FM) characterized by massive leukocyte infiltration and myocardial cell destruction, including central parts of the heart's stimulus transmission system. In fact, standard limb lead ECG recordings of diseased but not healthy mice showed a "Brugada"-like pattern with an abnormally high ST segment pointing to enhanced susceptibility for potential lethal arrhythmias. While CD11c.DTR mice are extensively used for the characterization of CD11c(+) cells, including dendritic cells, several studies have already mentioned adverse side effects following DTX treatment. Our results demonstrate that this limitation is based on severe myocarditis but not on the expected lung constrictions, and has to be taken into consideration if this animal model is used. Based on these properties, however, the CD11c.DTR mouse might serve as useful animal model for FM.

Inside the Pan-genome - Methods and Software Overview

The number of genomes that have been deposited in databases has increased exponentially after the advent of Next-Generation Sequencing (NGS), which produces high-throughput sequence data; this circumstance has demanded the development of new bioinformatics software and the creation of new areas, such as comparative genomics. In comparative genomics, the genetic content of an organism is compared against other organisms, which helps in the prediction of gene function and coding region sequences, identification of evolutionary events and determination of phylogenetic relationships. However, expanding comparative genomics to a large number of related bacteria, we can infer their lifestyles, gene repertoires and minimal genome size. In this context, a powerful approach called Pan-genome has been initiated and developed. This approach involves the genomic comparison of different strains of the same species, or even genus. Its main goal is to establish the total number of non-redundant genes that are present in a determined dataset. Pan-genome consists of three parts: core genome; accessory or dispensable genome; and species-specific or strain-specific genes. Furthermore, pan-genome is considered to be “open” as long as new genes are added significantly to the total repertoire for each new additional genome and “closed” when the newly added genomes cannot be inferred to significantly increase the total repertoire of the genes. To perform all of the required calculations, a substantial amount of software has been developed, based on orthologous and paralogous gene identification.

Taxonomy, Physiology, and Natural Products of Actinobacteria

Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.). Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities. This review presents an update on the biology of this important bacterial phylum.

Heme Binding by Corynebacterium diphtheriae HmuT: Function and Heme Environment

The heme uptake pathway (hmu) of Corynebacterium diphtheriae utilizes multiple proteins to bind and transport heme into the cell. One of these proteins, HmuT, delivers heme to the ABC transporter HmuUV. In this study, the axial ligation of the heme in ferric HmuT is probed by examination of wild-type (WT) HmuT and a series of conserved heme pocket residue mutants, H136A, Y235A, and M292A. Characterization by UV-visible, resonance Raman, and magnetic circular dichroism spectroscopies indicates that H136 and Y235 are the axial ligands in ferric HmuT. Consistent with this assignment of axial ligands, ferric WT and H136A HmuT are difficult to reduce while Y235A is reduced readily in the presence of dithionite. The FeCO Raman shifts in WT, H136A, and Y235A HmuT-CO complexes provide further evidence of the axial ligand assignments. Additionally, these frequencies provide insight into the nonbonding environment of the heme pocket. Ferrous Y235A and the Y235A-CO complex reveal that the imidazole of H136 exists in two forms, one neutral and one with imidazolate character, consistent with a hydrogen bond acceptor on the H136 side of the heme. The ferric fluoride complex of Y235A reveals the presence of at least one hydrogen bond donor on the Y235 side of the heme. Hemoglobin utilization assays showed that the axial Y235 ligand is required for heme uptake in HmuT.

Adherence and invasive properties of Corynebacterium diphtheriae strains correlates with the predicted membrane-associated and secreted proteome

BACKGROUND

Non-toxigenic Corynebacterium diphtheriae strains are emerging as a major cause of severe pharyngitis and tonsillitis as well as invasive diseases such as endocarditis, septic arthritis, splenic abscesses and osteomyelitis. C. diphtheriae strains have been reported to vary in their ability to adhere and invade different cell lines. To identify the genetic basis of variation in the degrees of pathogenicity, we sequenced the genomes of four strains of C. diphtheriae (ISS 3319, ISS 4060, ISS 4746 and ISS 4749) that are well characterised in terms of their ability to adhere and invade mammalian cells.

RESULTS

Comparative analyses of 20 C. diphtheriae genome sequences, including 16 publicly available genomes, revealed a pan-genome comprising 3,989 protein coding sequences that include 1,625 core genes and 2,364 accessory genes. Most of the genomic variation between these strains relates to uncharacterised genes encoding hypothetical proteins or transposases. Further analyses of protein sequences using an array of bioinformatic tools predicted most of the accessory proteome to be located in the cytoplasm. The membrane-associated and secreted proteins are generally involved in adhesion and virulence characteristics. The genes encoding membrane-associated proteins, especially the number and organisation of the pilus gene clusters (spa) including the number of genes encoding surface proteins with LPXTG motifs differed between different strains. Other variations were among the genes encoding extracellular proteins, especially substrate binding proteins of different functional classes of ABC transport systems and 'non-classical' secreted proteins.

CONCLUSIONS

The structure and organisation of the spa gene clusters correlates with differences in the ability of C. diphtheriae strains to adhere and invade the host cells. Furthermore, differences in the number of genes encoding membrane-associated proteins, e.g., additional proteins with LPXTG motifs could also result in variation in the adhesive properties between different strains. The variation in the secreted proteome may be associated with the degree of pathogenesis. While the role of the 'non-classical' secretome in virulence remains unclear, differences in the substrate binding proteins of various ABC transport systems and cytoplasmic proteins potentially suggest strain variation in nutritional requirements or a differential ability to utilize various carbon sources.

Corynebacterium ulcerans cutaneous diphtheria

We describe the case of a patient with cutaneous diphtheria caused by toxigenic Corynebacterium ulcerans who developed a right hand flexor sheath infection and symptoms of sepsis such as fever, tachycardia, and elevated C-reactive protein, after contact with domestic cats and dogs, and a fox. We summarise the epidemiology, clinical presentation, microbiology, diagnosis, therapy, and public health aspects of this disease, with emphasis on improving recognition. In many European countries, C ulcerans has become the organism commonly associated with cutaneous diphtheria, usually seen as an imported tropical disease or resulting from contact with domestic and agricultural animals. Diagnosis relies on bacterial culture and confirmation of toxin production, with management requiring appropriate antimicrobial therapy and prompt administration of antitoxin, if necessary. Early diagnosis is essential for implementation of control measures and clear guidelines are needed to assist clinicians in managing clinical diphtheria. This case was a catalyst to the redrafting of the 2014 national UK interim guidelines for the public health management of diphtheria, released as final guidelines in March, 2015.

Non-toxigenic tox gene-bearing Corynebacterium ulcerans in a traumatic ulcer from a human case and his asymptomatic dog

A non-toxigenic tox gene-bearing (NTTB) Corynebacterium ulcerans was grown from the wound of a 61-year-old gardener and in a nasal specimen from the patient's asymptomatic dog. The two isolates were similar in terms of antibiogram, multilocus sequence typing (ST341), virulence genes, and only three SNPs were found to differentiate the two NTTB C. ulcerans isolates supporting a zoonotic transmission to or between the patient and his dog. Of interest, we found that the two C. ulcerans isolates, although not expressing the diphtheria toxin tox, possessed 13 out of 14 recently described virulence candidate genes.

Next generation sequencing analysis of nine Corynebacterium ulcerans isolates reveals zoonotic transmission and a novel putative diphtheria toxin-encoding pathogenicity island

BACKGROUND

Toxigenic Corynebacterium ulcerans can cause a diphtheria-like illness in humans and have been found in domestic animals, which were suspected to serve as reservoirs for a zoonotic transmission. Additionally, toxigenic C. ulcerans were reported to take over the leading role in causing diphtheria in the last years in many industrialized countries.

METHODS

To gain deeper insights into the tox gene locus and to understand the transmission pathway in detail, we analyzed nine isolates derived from human patients and their domestic animals applying next generation sequencing and comparative genomics.

RESULTS

We provide molecular evidence for zoonotic transmission of C. ulcerans in four cases and demonstrate the superior resolution of next generation sequencing compared to multi-locus sequence typing for epidemiologic research. Additionally, we provide evidence that the virulence of C. ulcerans can change rapidly by acquisition of novel virulence genes. This mechanism is exemplified by an isolate which acquired a prophage not present in the corresponding isolate from the domestic animal. This prophage contains a putative novel virulence factor, which shares high identity with the RhuM virulence factor from Salmonella enterica but which is unknown in Corynebacteria so far. Furthermore, we identified a putative pathogenicity island for C. ulcerans bearing a diphtheria toxin gene.

CONCLUSION

The novel putative diphtheria toxin pathogenicity island could provide a new and alternative pathway for Corynebacteria to acquire a functional diphtheria toxin-encoding gene by horizontal gene transfer, distinct from the previously well characterized phage infection model. The novel transmission pathway might explain the unexpectedly high number of toxigenic C. ulcerans.

Multilocus sequence typing of Corynebacterium ulcerans provides evidence for zoonotic transmission and for increased prevalence of certain sequence types among toxigenic strains

Human-to-human-transmitted Corynebacterium diphtheriae was historically the main pathogen causing diphtheria and has therefore been studied extensively in the past. More recently, diphtheria caused by toxigenic Corynebacterium ulcerans is an emerging disease in several industrial countries, including the United Kingdom, the United States, France, and Germany. However, toxigenic C. ulcerans has so far been almost neglected in the development of epidemiologic tools. One of the most important tools in modern epidemiology to understand transmission pathways is sequence typing of pathogens. Here, we provide a protocol for multilocus sequence typing (MLST) to type C. ulcerans strains rapidly and relatively cost-effectively. Applying MLST to C. ulcerans for the first time, we show that related sequence types (STs) might be associated with the presence of the diphtheria toxin gene, which encodes diphtheria toxin (DT), the most important diphtheria-causing virulence factor. Interestingly, we found only two very closely related STs in the isolates derived from six dogs. Additionally, our data show that all STs derived from animals which were at least twice present in our analysis were found in humans as well. This finding is congruent with zoonotic transmission of C. ulcerans.

Emergence and molecular characterisation of non-toxigenic tox gene-bearing Corynebacterium diphtheriae biovar mitis in the United Kingdom, 2003–2012

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The Trojan Horse of the microbiological arms race: phage-encoded toxins as a defence against eukaryotic predators

Phage-encoded Shiga toxin (Stx) acts as a bacterial defence against the eukaryotic predator Tetrahymena. To function as an effective bacterial anti-predator defence, Stx must kill a broad spectrum of predators. Consistent with that assertion, we show here that bacterially encoded Stx efficiently kills the bacteriovore Acanthamoeba castellanii in co-culture. We also show that, in addition to Stx, the phage-encoded exotoxin, diphtheria toxin (Dtx) expressed by Corynebacterium diphtheriae also can function as part of an anti-predator strategy; it kills Acanthamoeba in co-culture. Interestingly, only exotoxins produced by bacteria internalized by the Acanthamoeba predator are cytolethal; the presence of purified Dtx or Stx in culture medium has no effect on predator viability. This finding is consistent with our results indicating that intoxication of Acanthamoeba by these exotoxins does not require a receptor. Thus bacteria, in the disguise of a food source, function as a 'Trojan Horse', carrying genes encoding an exotoxin into target organisms. This 'Trojan Horse' mechanism of exotoxin delivery into predator cells allows intoxication of predators that lack a cell surface receptor for the particular toxin, allowing bacteria-bearing exotoxins to kill a broader spectrum of predators, increasing the fitness of the otherwise 'defenceless' prey bacteria.

Identification of a human monoclonal antibody to replace equine diphtheria antitoxin for treatment of diphtheria intoxication

Diphtheria antitoxin (DAT) has been the cornerstone of the treatment of Corynebacterium diphtheriae infection for more than 100 years. Although the global incidence of diphtheria has declined steadily over the last quarter of the 20th century, the disease remains endemic in many parts of the world, and significant outbreaks still occur. DAT is an equine polyclonal antibody that is not commercially available in the United States and is in short supply globally. A safer, more readily available alternative to DAT would be desirable. In the current study, we obtained human monoclonal antibodies (hMAbs) directly from antibody-secreting cells in the circulation of immunized human volunteers. We isolated a panel of diverse hMAbs that recognized diphtheria toxoid, as well as a variety of recombinant protein fragments of diphtheria toxin. Forty-five unique hMAbs were tested for neutralization of diphtheria toxin in in vitro cytotoxicity assays with a 50% effective concentration of 0.65 ng/ml for the lead candidate hMAb, 315C4. In addition, 25 μg of 315C4 completely protected guinea pigs from intoxication in an in vivo lethality model, yielding an estimated relative potency of 64 IU/mg. In comparison, 1.6 IU of DAT was necessary for full protection from morbidity and mortality in this model. We further established that our lead candidate hMAb binds to the receptor-binding domain of diphtheria toxin and physically blocks the toxin from binding to the putative receptor, heparin-binding epidermal growth factor-like growth factor. The discovery of a specific and potent human neutralizing antibody against diphtheria toxin holds promise as a potential therapeutic.

Pilus gene pool variation and the virulence of Corynebacterium diphtheriae clinical isolates during infection of a nematode

Toxigenic Corynebacterium diphtheriae strains cause diphtheria in humans. The toxigenic C. diphtheriae isolate NCTC13129 produces three distinct heterotrimeric pili that contain SpaA, SpaD, and SpaH, making up the shaft structure. The SpaA pili are known to mediate bacterial adherence to pharyngeal epithelial cells. However, to date little is known about the expression of different pili in various clinical isolates and their importance in bacterial pathogenesis. Here, we characterized a large collection of C. diphtheriae clinical isolates for their pilin gene pool by PCR and for the expression of the respective pilins by immunoblotting with antibodies against Spa pilins. Consistent with the role of a virulence factor, the SpaA-type pili were found to be prevalent among the isolates, and most significantly, corynebacterial adherence to pharyngeal epithelial cells was strictly correlated with isolates that were positive for the SpaA pili. By comparison, the isolates were heterogeneous for the presence of SpaD- and SpaH-type pili. Importantly, using Caenorhabditis elegans as a model host for infection, we show here that strain NCTC13129 rapidly killed the nematodes, the phenotype similar to isolates that were positive for toxin and all pilus types. In contrast, isogenic mutants of NCTC13129 lacking SpaA-type pili or devoid of toxin and SpaA pili exhibited delayed killing of nematodes with similar kinetics. Consistently, nontoxigenic or toxigenic isolates that lack one, two, or all three pilus types were also attenuated in virulence. This work signifies the important role of pili in corynebacterial pathogenesis and provides a simple host model to identify additional virulence factors.

The pan-genome of the animal pathogen Corynebacterium pseudotuberculosis reveals differences in genome plasticity between the biovar ovis and equi strains

Corynebacterium pseudotuberculosis is a facultative intracellular pathogen and the causative agent of several infectious and contagious chronic diseases, including caseous lymphadenitis, ulcerative lymphangitis, mastitis, and edematous skin disease, in a broad spectrum of hosts. In addition, Corynebacterium pseudotuberculosis infections pose a rising worldwide economic problem in ruminants. The complete genome sequences of 15 C. pseudotuberculosis strains isolated from different hosts and countries were comparatively analyzed using a pan-genomic strategy. Phylogenomic, pan-genomic, core genomic, and singleton analyses revealed close relationships among pathogenic corynebacteria, the clonal-like behavior of C. pseudotuberculosis and slow increases in the sizes of pan-genomes. According to extrapolations based on the pan-genomes, core genomes and singletons, the C. pseudotuberculosis biovar ovis shows a more clonal-like behavior than the C. pseudotuberculosis biovar equi. Most of the variable genes of the biovar ovis strains were acquired in a block through horizontal gene transfer and are highly conserved, whereas the biovar equi strains contain great variability, both intra- and inter-biovar, in the 16 detected pathogenicity islands (PAIs). With respect to the gene content of the PAIs, the most interesting finding is the high similarity of the pilus genes in the biovar ovis strains compared with the great variability of these genes in the biovar equi strains. Concluding, the polymerization of complete pilus structures in biovar ovis could be responsible for a remarkable ability of these strains to spread throughout host tissues and penetrate cells to live intracellularly, in contrast with the biovar equi, which rarely attacks visceral organs. Intracellularly, the biovar ovis strains are expected to have less contact with other organisms than the biovar equi strains, thereby explaining the significant clonal-like behavior of the biovar ovis strains.

Iron starvation regulates the type III secretion system in Bordetella bronchiseptica

The type III secretion system (T3SS) plays a key role in the exertion of full virulence by Bordetella bronchiseptica. However, little is known about the environmental stimuli that induce expression of T3SS genes. Here, it is reported that iron starvation is a signal for T3SS gene expression in B. bronchiseptica. It was found that, when B. bronchiseptica is cultured under iron-depleted conditions, secretion of type III secreted proteins is greater than that in bacteria grown under iron-replete conditions. Furthermore, it was confirmed that induction of T3SS-dependent host cell cytotoxicity and hemolytic activity is greatly enhanced by infection with iron-depleted Bordetella. In contrast, production of filamentous hemagglutinin is reduced in iron-depleted Bordetella. Thus, B. bronchiseptica controls the expression of virulence genes in response to iron starvation.

Diphtheria in Europe: current problems and new challenges

Diphtheria, caused by toxigenic strains of Corynebacterium diphtheriae, is an ancient disease with high incidence and mortality that has always been characterized by epidemic waves of occurrence. Whilst towards the beginning of the 1980s, many European countries were progressing towards the elimination of diphtheria, an epidemic re-emergence of diphtheria in the Russian Federation and the Newly Independent States of the former Soviet Union demonstrated a continuous threat of the disease into the 1990s. At present, the epidemic is under control and only sporadic cases are observed in Europe. However, the circulation of toxigenic strains is still observed in all parts of the world, posing a constant threat to the population with low levels of seroprotection. More recently, Corynebacterium ulcerans has been increasingly isolated as emerging zoonotic agent of diphtheria from companion animals such as cats or dogs, indicating the enduring threat of this thought-to-be controlled disease.

Pangenomic study of Corynebacterium diphtheriae that provides insights into the genomic diversity of pathogenic isolates from cases of classical diphtheria, endocarditis, and pneumonia

Corynebacterium diphtheriae is one of the most prominent human pathogens and the causative agent of the communicable disease diphtheria. The genomes of 12 strains isolated from patients with classical diphtheria, endocarditis, and pneumonia were completely sequenced and annotated. Including the genome of C. diphtheriae NCTC 13129, we herewith present a comprehensive comparative analysis of 13 strains and the first characterization of the pangenome of the species C. diphtheriae. Comparative genomics showed extensive synteny and revealed a core genome consisting of 1,632 conserved genes. The pangenome currently comprises 4,786 protein-coding regions and increases at an average of 65 unique genes per newly sequenced strain. Analysis of prophages carrying the diphtheria toxin gene tox revealed that the toxoid vaccine producer C. diphtheriae Park-Williams no. 8 has been lysogenized by two copies of the ω(tox)(+) phage, whereas C. diphtheriae 31A harbors a hitherto-unknown tox(+) corynephage. DNA binding sites of the tox-controlling regulator DtxR were detected by genome-wide motif searches. Comparative content analysis showed that the DtxR regulons exhibit marked differences due to gene gain, gene loss, partial gene deletion, and DtxR binding site depletion. Most predicted pathogenicity islands of C. diphtheriae revealed characteristics of horizontal gene transfer. The majority of these islands encode subunits of adhesive pili, which can play important roles in adhesion of C. diphtheriae to different host tissues. All sequenced isolates contain at least two pilus gene clusters. It appears that variation in the distributed genome is a common strategy of C. diphtheriae to establish differences in host-pathogen interactions.

Characterization of OxyR as a negative transcriptional regulator that represses catalase production in Corynebacterium diphtheriae

Corynebacterium diphtheriae and Corynebacterium glutamicum each have one gene (cat) encoding catalase. In-frame Δcat mutants of C. diphtheriae and C. glutamicum were hyper-sensitive to growth inhibition and killing by H(2)O(2). In C. diphtheriae C7(β), both catalase activity and cat transcription decreased ~2-fold during transition from exponential growth to early stationary phase. Prototypic OxyR in Escherichia coli senses oxidative stress and it activates katG transcription and catalase production in response to H(2)O(2). In contrast, exposure of C. diphtheriae C7(β) to H(2)O(2) did not stimulate transcription of cat. OxyR from C. diphtheriae and C. glutamicum have 52% similarity with E. coli OxyR and contain homologs of the two cysteine residues involved in H(2)O(2) sensing by E. coli OxyR. In-frame ΔoxyR deletion mutants of C. diphtheriae C7(β), C. diphtheriae NCTC13129, and C. glutamicum were much more resistant than their parental wild type strains to growth inhibition by H(2)O(2). In the C. diphtheriae C7(β) ΔoxyR mutant, cat transcripts were about 8-fold more abundant and catalase activity was about 20-fold greater than in the C7(β) wild type strain. The oxyR gene from C. diphtheriae or C. glutamicum, but not from E. coli, complemented the defect in ΔoxyR mutants of C. diphtheriae and C. glutamicum and decreased their H(2)O(2) resistance to the level of their parental strains. Gel-mobility shift, DNaseI footprint, and primer extension assays showed that purified OxyR from C. diphtheriae C7(β) bound, in the presence or absence of DTT, to a sequence in the cat promoter region that extends from nucleotide position -55 to -10 with respect to the +1 nucleotide in the cat ORF. These results demonstrate that OxyR from C. diphtheriae or C. glutamicum functions as a transcriptional repressor of the cat gene by a mechanism that is independent of oxidative stress induced by H(2)O(2).

The ChrA response regulator in Corynebacterium diphtheriae controls hemin-regulated gene expression through binding to the hmuO and hrtAB promoter regions

Corynebacterium diphtheriae, the etiologic agent of diphtheria, utilizes heme and hemoglobin (Hb) as iron sources for growth. Heme-iron utilization involves HmuO, a heme oxygenase that degrades cytosolic heme, resulting in the release of heme-associated iron. Expression of the hmuO promoter is under dual regulation, in which transcription is repressed by DtxR and iron and activated by a heme source, such as hemin or Hb. Hemin-dependent activation is mediated primarily by the ChrAS two-component system, in which ChrS is a putative heme-responsive sensor kinase while ChrA is proposed to serve as a response regulator that activates transcription. It was recently shown that the ChrAS system similarly regulates the hrtAB genes, which encode an ABC transporter involved in the protection of C. diphtheriae from hemin toxicity. In this study, we characterized the phosphorelay mechanism in the ChrAS system and provide evidence for the direct regulation of the hmuO and hrtAB promoters by ChrA. A fluorescence staining method was used to show that ChrS undergoes autophosphorylation and that the phosphate moiety is subsequently transferred to ChrA. Promoter fusion studies identified regions upstream of the hmuO and hrtAB promoters that are critical for the heme-dependent regulation by ChrA. Electrophoretic mobility shift assays revealed that ChrA specifically binds at the hmuO and hrtAB promoter regions and that binding is phosphorylation dependent. A phosphorylation-defective mutant of ChrA [ChrA(D50A)] exhibited significantly diminished binding to the hmuO promoter region relative to that of wild-type ChrA. DNase I footprint analysis further defined the sequences in the hmuO and hrtAB promoters that are involved in ChrA binding, and this analysis revealed that the DtxR binding site at the hmuO promoter partially overlaps the binding site for ChrA. DNase I protection studies as well as promoter fusion analysis suggest that ChrA and DtxR compete for binding at the hmuO promoter. Collectively, these data demonstrate that the ChrA response regulator directly controls the expression of hmuO and the hrtAB genes and the binding activity of ChrA is dependent on phosphorylation by its cognate sensor kinase ChrS.

Comparative analysis of two complete Corynebacterium ulcerans genomes and detection of candidate virulence factors

Background

Corynebacterium ulcerans has been detected as a commensal in domestic and wild animals that may serve as reservoirs for zoonotic infections. During the last decade, the frequency and severity of human infections associated with C. ulcerans appear to be increasing in various countries. As the knowledge of genes contributing to the virulence of this bacterium was very limited, the complete genome sequences of two C. ulcerans strains detected in the metropolitan area of Rio de Janeiro were determined and characterized by comparative genomics: C. ulcerans 809 was initially isolated from an elderly woman with fatal pulmonary infection and C. ulcerans BR-AD22 was recovered from a nasal sample of an asymptomatic dog.

Results

The circular chromosome of C. ulcerans 809 has a total size of 2,502,095 bp and encodes 2,182 predicted proteins, whereas the genome of C. ulcerans BR-AD22 is 104,279 bp larger and comprises 2,338 protein-coding regions. The minor difference in size of the two genomes is mainly caused by additional prophage-like elements in the C. ulcerans BR-AD22 chromosome. Both genomes show a highly similar order of orthologous coding regions; and both strains share a common set of 2,076 genes, demonstrating their very close relationship. A screening for prominent virulence factors revealed the presence of phospholipase D (Pld), neuraminidase H (NanH), endoglycosidase E (EndoE), and subunits of adhesive pili of the SpaDEF type that are encoded in both C. ulcerans genomes. The rbp gene coding for a putative ribosome-binding protein with striking structural similarity to Shiga-like toxins was additionally detected in the genome of the human isolate C. ulcerans 809.

Conclusions

The molecular data deduced from the complete genome sequences provides considerable knowledge of virulence factors in C. ulcerans that is increasingly recognized as an emerging pathogen. This bacterium is apparently equipped with a broad and varying set of virulence factors, including a novel type of a ribosome-binding protein. Whether the respective protein contributes to the severity of human infections (and a fatal outcome) remains to be elucidated by genetic experiments with defined bacterial mutants and host model systems.

Defining dendritic cells by conditional and constitutive cell ablation

Recent years have seen a major advance in our understanding of the organization of the dendritic cell (DC) compartment. Particularly rewarding in this respect have been studies investigating DC origins, based on the identification of transcription factor and growth factor requirements, as well as direct demonstrations of precursor/progeny relationships by adoptive cell transfers. However, to fully understand the organization of the DC compartment, functional definitions of DC subsets must be provided and potential task divisions revealed that distinguish DC from other immune cells, including the closely related mononuclear phagocytes, such as macrophages. In fact, functional definitions might eventually replace the current distinction between DC and macrophages, which is in part based on arbitrary historic considerations, i.e. mononuclear phagocytes identified before the advent of DC in the mid 1970s generally termed macrophages. In this article, we review recent insight in the functions of classical DC in the mouse, focusing on our own work involving conditional and constitutive cell ablation.

Contractile function of the heart and the state of some stages of lipid metabolism during acute diphtheritic intoxication

Acute diphtheritic intoxication was modeled in rabbits by intravenous administration of native diphtherin (0.3 minimal lethal dose per 1 kg body weight). The contractile function of the left and right ventricles (peak systolic pressure under conditions of basal hemodynamics and during 5-sec occlusion of the aorta and pulmonary artery, respectively) was estimated 1, 3 and 5 days after the start of the pathological process, the intensity of lipid peroxidation was evaluated by measuring the content of TBA- reactive products in the myocardium. Impairment of the contractile function of both ventricles was observed in the course of intoxication. The level of TBA-reactive products in the left ventricle significantly decreased on day 1, but then returned to normal. Thus, impairment of the contractile function of the left ventricle at the early stages of diphtheritic intoxication is not mediated by activation of lipid peroxidation in cardiomyocytes.

Bordetella pertussis and vaccination: the persistence of a genetically monomorphic pathogen

Before childhood vaccination was introduced in the 1950s, pertussis or whooping cough was a major cause of infant death worldwide. Widespread vaccination of children was successful in significantly reducing morbidity and mortality. However, despite vaccination, pertussis has persisted and, in the 1990s, resurged in a number of countries with highly vaccinated populations. Indeed, pertussis has become the most prevalent vaccine-preventable disease in developed countries with estimated infection frequencies of 1-6%. Recently vaccinated children are well protected against pertussis disease and its increase is mainly seen in adolescents and adults in which disease symptoms are often mild. The etiologic agent of pertussis, Bordetella pertussis, is extremely monomorphic and its ability to persist in the face of intensive vaccination is intriguing. Numerous studies have shown that B. pertussis populations changed after the introduction of vaccination suggesting adaptation. These adaptations did not involve the acquisition of novel genes but small genetic changes, mainly SNPs, and occurred in successive steps in a period of 40 years. The earliest adaptations resulted in antigenic divergence with vaccine strains. More recently, strains emerged with increased pertussis toxin (Ptx) production. Here I argue that the resurgence of pertussis is the compound effect of pathogen adaptation and waning immunity. I propose that the removal by vaccination of naïve infants as the major source for transmission was the crucial event which has driven the changes in B. pertussis populations. This has selected for strains which are more efficiently transmitted by primed hosts in which immunity has waned. The adaptation of B. pertussis to primed hosts involved delaying an effective immune response by antigenic divergence with vaccine strains and by increasing immune suppression through higher levels of Ptx production. Higher levels of Ptx may also benefit transmission by enhancing clinical symptoms. The study of B. pertussis populations has not only increased our understanding of pathogen evolution, but also suggests way to improve pertussis vaccines, underlining the public health significance of population-based studies of pathogens.

A review of the international issues surrounding the availability and demand for diphtheria antitoxin for therapeutic use

Diphtheria treatment requires early administration of diphtheria antitoxin (DAT), an immunoglobulin preparation that neutralises circulating diphtheria toxin. Here, we review issues relating to the supply and use of DAT and assess its availability by means of an international survey. Results showed that several countries do not currently hold DAT stockpiles due to low prevalence, and hence perceived risk of diphtheria, and/or difficulties in obtaining DAT supplies. The potential for importation of cases into any country exists globally, since diphtheria remains endemic in many regions. It is therefore important that DAT be readily available - particularly since waning diphtheria immunity has been observed among adult populations in countries with good vaccination coverage. Options for diphtheria therapy are discussed.

At the crossroads of bacterial metabolism and virulence factor synthesis in Staphylococci

Bacteria live in environments that are subject to rapid changes in the availability of the nutrients that are necessary to provide energy and biosynthetic intermediates for the synthesis of macromolecules. Consequently, bacterial survival depends on the ability of bacteria to regulate the expression of genes coding for enzymes required for growth in the altered environment. In pathogenic bacteria, adaptation to an altered environment often includes activating the transcription of virulence genes; hence, many virulence genes are regulated by environmental and nutritional signals. Consistent with this observation, the regulation of most, if not all, virulence determinants in staphylococci is mediated by environmental and nutritional signals. Some of these external signals can be directly transduced into a regulatory response by two-component regulators such as SrrAB; however, other external signals require transduction into intracellular signals. Many of the external environmental and nutritional signals that regulate virulence determinant expression can also alter bacterial metabolic status (e.g., iron limitation). Altering the metabolic status results in the transduction of external signals into intracellular metabolic signals that can be "sensed" by regulatory proteins (e.g., CodY, Rex, and GlnR). This review uses information derived primarily using Bacillus subtilis and Escherichia coli to articulate how gram-positive pathogens, with emphasis on Staphylococcus aureus and Staphylococcus epidermidis, regulate virulence determinant expression in response to a changing environment.

EGFR-targeted diphtheria toxin stimulates TRAIL killing of glioblastoma cells by depleting anti-apoptotic proteins

Current treatments for Glioblastoma multiforme (GBM) involve surgery, radiotherapy, and cytotoxic chemotherapy; however, these treatments are not effective and there is an urgent need for better treatments. We investigated GBM cell killing by a novel drug combination involving DT-EGF, an Epidermal Growth Factor Receptor-targeted bacterial toxin, and Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL) or antibodies that activate the TRAIL receptors DR4 and DR5. DT-EGF kills GBM cells by a non apoptotic mechanism whereas TRAIL kills by inducing apoptosis. GBM cells treated with DT-EGF and TRAIL were killed in a synergistic fashion in vitro and the combination was more effective than either treatment alone in vivo. Tumor cell death with the combination occurred by caspase activation and apoptosis due to DT-EGF positively regulating TRAIL killing by depleting FLIP, a selective inhibitor of TRAIL receptor-induced apoptosis. These data provide a mechanism-based rationale for combining targeted toxins and TRAIL receptor agonists to treat GBM.

Regulation and activity of a zinc uptake regulator, Zur, in Corynebacterium diphtheriae

Regulation of metal ion homeostasis is essential to bacterial cell survival, and in most species it is controlled by metal-dependent transcriptional regulators. In this study, we describe a Corynebacterium diphtheriae ferric uptake regulator-family protein, Zur, that controls expression of genes involved in zinc uptake. By measuring promoter activities and mRNA levels, we demonstrate that Zur represses transcription of three genes (zrg, cmrA, and troA) in zinc-replete conditions. All three of these genes have similarity to genes involved in zinc uptake. Transcription of zrg and cmrA was also shown to be regulated in response to iron and manganese, respectively, by mechanisms that are independent of Zur. We demonstrate that the activity of the zur promoter is slightly decreased under low zinc conditions in a process that is dependent on Zur itself. This regulation of zur transcription is distinctive and has not yet been described for any other zur. An adjacent gene, predicted to encode a metal-dependent transcriptional regulator in the ArsR/SmtB family, is transcribed from a separate promoter whose activity is unaffected by Zur. A C. diphtheriae zur mutant was more sensitive to peroxide stress, which suggests that zur has a role in protecting the bacterium from oxidative damage. Our studies provide the first evidence of a zinc specific transcriptional regulator in C. diphtheriae and give new insights into the intricate regulatory network responsible for regulating metal ion concentrations in this toxigenic human pathogen.

Acute myeloid leukemia-targeted toxin activates both apoptotic and necroptotic death mechanisms

Background

Acute myelogenous leukemia (AML) is the second most common leukemia with approximately 13,410 new cases and 8,990 deaths annually in the United States. A novel fusion toxin treatment, diphtheria toxin GM-CSF (DT-GMCSF) has been shown to selectively eliminate leukemic repopulating cells that are critical for the formation of AML. We previously showed that DT-GMCSF treatment of U937 cells, an AML cell line, causes activation of caspases and the induction of apoptosis.

Methods and Findings

In this study we further investigate the mechanisms of cell death induced by DT-GMCSF and show that, in addition to the activation of caspase-dependent apoptosis, DT-GMCSF also kills AML cells by simultaneously activating caspase-independent necroptosis. These mechanisms depend on the ability of the targeted toxin to inhibit protein synthesis, and are not affected by the receptor that is targeted or the mechanism through which protein synthesis is blocked.

Conclusions

We conclude that fusion toxin proteins may be effective for treating AML cells whether or not they are defective in apoptosis.

Lack of conventional dendritic cells is compatible with normal development and T cell homeostasis, but causes myeloid proliferative syndrome

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.

Probing in vivo dendritic cell functions by conditional cell ablation

Dendritic cells (DCs) play a central role in T-cell activation and the control of the inherent autoreactivity of the T-cell compartment. Pleiotropic DC functions are likely associated with discrete DC subsets. However, the latter remain largely defined by phenotype and unique anatomic location, rather than function. The investigation of DC involvement in complex phenomena that rely on multicellular interactions, such as immuno-stimulation and tolerization calls for an assessment of DC functions within physiological context. Given the highly dynamic DC compartment, the method of choice to study in vivo DC functions is their conditional ablation in the intact organism. Here, we summarize the recent progress in this field highlighting pitfalls and prospects of the approach.

Corynebacterium diphtheriae: identification and characterization of a channel-forming protein in the cell wall

The cell wall fraction of the gram-positive, nontoxic Corynebacterium diphtheriae strain C8r(-) Tox- (=ATCC 11913) contained a channel-forming protein, as judged from reconstitution experiments with artificial lipid bilayer experiments. The channel-forming protein was present in detergent-treated cell walls and in extracts of whole cells obtained using organic solvents. The protein had an apparent molecular mass of about 66 kDa as determined on Tricine-containing sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and consisted of subunits having a molecular mass of about 5 kDa. Single-channel experiments with the purified protein suggested that the protein formed channels with a single-channel conductance of 2.25 nS in 1 M KCl. Further single-channel analysis suggested that the cell wall channel is wide and water filled because it has only slight selectivity for cations over anions and its conductance followed the mobility sequence of cations and anions in the aqueous phase. Antibodies raised against PorA, the subunit of the cell wall channel of Corynebacterium glutamicum, detected both monomers and oligomers of the isolated protein, suggesting that there are highly conserved epitopes in the cell wall channels of C. diphtheriae and PorA. Localization of the protein on the cell surface was confirmed by an enzyme-linked immunosorbent assay. The prospective homology of PorA with the cell wall channel of C. diphtheriae was used to identify the cell wall channel gene, cdporA, in the known genome of C. diphtheriae. The gene and its flanking regions were cloned and sequenced. CdporA is a protein that is 43 amino acids long and does not have a leader sequence. cdporA was expressed in a C. glutamicum strain that lacked the major outer membrane channels PorA and PorH. Organic solvent extracts of the transformed cells formed in lipid bilayer membranes the same channels as the purified CdporA protein of C. diphtheriae formed, suggesting that the expressed protein is able to complement the PorA and PorH deficiency of the C. glutamicum strain. The study is the first report of a cell wall channel in a pathogenic Corynebacterium strain.

Interplay between DtxR and nitric oxide reductase activities: a functional genomics approach indicating involvement of homologous protein domains in bacterial pathogenesis

Corynebacterium diphtheriae pathogenesis depends on the production of toxin (Dtx), which in turn depends on a micromolar concentration of nitric oxide (NO)-mediated deactivation of DtxR (an iron-dependent regulator). Inside a host, the pathogen often encounters excess of NO that acts as an oxidative toxicant. Therefore a critical level of NO needs to be maintained by the pathogen. This necessitates reduction of excess NO by the presence of a reductase, namely nitric oxide reductase (NOR). Similar to the expression of toxin, the expression of NOR is possibly regulated by another regulator NorR, as has been found in other gram positive and gram-negative bacteria. Therefore, a correlation between concentration of NO on the deactivation of DtxR and transactivation of NorR becomes apparent. However, unlike many other pathogens the presence of NOR and NorR in C. diphtheriae has not been established. We applied a combination of bioinformatics and comparative genomics approach on C. diphtheriae genome using Escherichia coli as a model organism to find some structural and functional homologoues for the two genes in question. The various domain characteristics for the two proteins (NOR and NorR) have been taken into account in this analysis. Through extensive genome and proteome search we have been able to identify key regulatory genes, which are possibly involved in coordination and control of NO stress in C. diphtheriae. Our finding will progress the understanding of the complete regulatory mechanism for evasion and maintenance of pathogenesis by this and other pathogenic organisms.