A novel toxin homologous to large clostridial cytotoxins found in culture supernatant of Clostridium perfringens type C

Identification of a hemorrhagic determinant in Clostridioides difficile TcdA and Paeniclostridium sordellii TcsH

Paeniclostridium sordellii hemorrhagic toxin (TcsH) and Clostridioides difficile toxin A (TcdA) are two major members of the large clostridial toxin (LCT) family. These two toxins share ~87% similarity and are known to cause severe hemorrhagic pathology in animals. Yet, the pathogenesis of their hemorrhagic toxicity has been mysterious for decades. Here, we examined the liver injury after systemic exposure to different LCTs and found that only TcsH and TcdA induce overt hepatic hemorrhage. By investigating the chimeric and truncated toxins, we demonstrated that the enzymatic domain of TcsH alone is not sufficient to determine its potent hepatic hemorrhagic toxicity in mice. Likewise, the combined repetitive oligopeptide (CROP) domain of TcsH/TcdA alone also failed to explain their strong hemorrhagic activity in mice. Lastly, we showed that disrupting the first two short repeats of CROPs in TcsH and TcdA impaired hemorrhagic toxicity without causing overt changes in cytotoxicity and lethality. These findings lead to a deeper understanding of toxin-induced hemorrhage and the pathogenesis of LCTs and could be insightful in developing therapeutic avenues against clostridial infections.

IMPORTANCE

Paeniclostridium sordellii and Clostridioides difficile infections often cause hemorrhage in the affected tissues and organs, which is mainly attributed to their hemorrhagic toxins, TcsH and TcdA. In this study, we demonstrate that TcsH and TcdA, but not other related toxins. including Clostridioides difficile toxin B and TcsL, induce severe hepatic hemorrhage in mice. We further determine that a small region in TcsH and TcdA is critical for the hemorrhagic toxicity but not cytotoxicity or lethality of these toxins. Based on these results, we propose that the hemorrhagic toxicity of TcsH and TcdA is due to an uncharacterized mechanism, such as the presence of an unknown receptor, and future studies to identify the interactive host factors are warranted.

Tyrosine-modifying glycosylation by Yersinia effectors

Mono-O-glycosylation of target proteins by bacterial toxins or effector proteins is a well-known mechanism by which bacteria interfere with essential functions of host cells. The respective glycosyltransferases are important virulence factors such as the Clostridioides difficile toxins A and B. Here, we describe two glycosyltransferases of Yersinia species that have a high sequence identity: YeGT from the zoonotic pathogen Yersinia enterocolitica and YkGT from the murine pathogen Yersinia kristensenii. We show that both modify Rho family proteins by attachment of GlcNAc at tyrosine residues (Tyr-34 in RhoA). Notably, the enzymes differed in their target protein specificity. While YeGT modified RhoA, B, and C, YkGT possessed a broader substrate spectrum and glycosylated not only Rho but also Rac and Cdc42 subfamily proteins. Mutagenesis studies indicated that residue 177 is important for this broader target spectrum. We determined the crystal structure of YeGT shortened by 16 residues N terminally (sYeGT) in the ligand-free state and bound to UDP, the product of substrate hydrolysis. The structure assigns sYeGT to the GT-A family. It shares high structural similarity to glycosyltransferase domains from toxins. We also demonstrated that the 16 most N-terminal residues of YeGT and YkGT are important for the mediated translocation into the host cell using the pore-forming protective antigen of anthrax toxin. Mediated introduction into HeLa cells or ectopic expression of YeGT and YkGT caused morphological changes and redistribution of the actin cytoskeleton. The data suggest that YeGT and YkGT are likely bacterial effectors belonging to the family of tyrosine glycosylating bacterial glycosyltransferases.

An Updated View on the Cellular Uptake and Mode-of-Action of Clostridioides difficile Toxins

Research on the human gut pathogen Clostridioides (C.) difficile and its toxins continues to attract much attention as a consequence of the threat to human health posed by hypervirulent strains. Toxin A (TcdA) and Toxin B (TcdB) are the two major virulence determinants of C. difficile. Both are single-chain proteins with a similar multidomain architecture. Certain hypervirulent C. difficile strains also produce a third toxin, namely binary toxin CDT (C. difficile transferase). C. difficile toxins are the causative agents of C. difficile-associated diseases (CDADs), such as antibiotics-associated diarrhea and pseudomembranous colitis. For that reason, considerable efforts have been expended to unravel their molecular mode-of-action and the cellular mechanisms responsible for their uptake. Many of these studies have been conducted in European laboratories. Here, we provide an update on our previous review (Papatheodorou et al. Adv Exp Med Biol, 2018) on important advances in C. difficile toxins research.

From Foodborne Disease Outbreak (FBDO) to Investigation: The Plant Toxin Trap, Brittany, France, 2018

On 6 July 2018, the Center for Epidemiology and Public Health of the French Armed Forces was informed of an outbreak of acute gastroenteritis among customers of a dining facility at a military base in Brittany, France. A total of 200 patients were reported out of a population of 1700 (attack rate: 12%). The symptoms were mainly lower digestive tract disorders and occurred rapidly after lunch on 5 July (median incubation period: 3.3 h), suggesting a toxin-like pathogenic process. A case-control survey was carried out (92 cases and 113 controls). Statistical analysis pointed to the chili con carne served at lunch on 5 July as the very likely source of poisoning. Phytohaemagglutinin, a plant lectin, was found in the chili con carne at a concentration above the potentially toxic dose (400 HAU/gram). The raw kidney beans incorporated in the chili con carne presented a high haemagglutination activity (66,667 HAU/gram). They were undercooked, and the phytohaemagglutinin was not completely destroyed. FBDOs due to PHA are poorly documented. This study highlights the need to develop methods for routine testing of plant toxins in food matrices. Improved diagnostic capabilities would likely lead to better documentation, epidemiology, and prevention of food-borne illnesses caused by plant toxins.

Overexpression of SIRT1 alleviates oxidative damage and barrier dysfunction in CPB2 toxin-infected IPEC-J2 cells

Clostridium perfringens (C. perfringens) beta2 (CPB2) toxin may induce necrotizing enteritis (NE) in pigs. Sirtuin1 (SIRT1) is involved in inflammatory intestinal diseases and affects intestinal barrier function. However, the effects of SIRT1 on piglet intestinal disease caused by CPB2 toxin are unclear. This study revealed the role of pig SIRT1 in CPB2 toxin-exposed intestinal porcine epithelial cells (IPEC-J2). Herein, we manifested that SIRT1 was dramatically decreased in IPEC-J2 cells infected with CPB2 toxin. Subsequently, we silenced and overexpressed SIRT1 using siRNA and a overexpression vector in CPB2 toxin-treated IPEC-J2 cells. The results indicated that overexpression of SIRT1 suppressed reactive oxygen species (ROS) generates, the expression tumor necrosis factor-α (TNF-α), interleukin (IL)-6 and Bax, nuclear factor-kappa B (NF-κB p65), phospho (p)-NF-kB p65 and lactate dehydrogenase (LDH) activity and apoptosis in CPB2 toxin-treated IPEC-J2 cells, and increased IL-10, mitochondrial membrane potential (ΔΨm), Bcl-2, Claudin1 and Occludin levels and cell viability. These results indicated that SIRT1 protects IPEC-J2 cells against CPB2 toxin-induced oxidative damage and tight junction (TJ) disruption, which provides a theoretical basis for further study of the molecular regulatory mechanism of SIRT1 in C. perfringens-infected NE in piglets.

Characterization of Collagen Binding Activity of Clostridium perfringens Strains Isolated from Broiler Chickens

Clostridium perfringens is the etiological agent for necrotic enteritis (NE) in broiler chickens, which causes a substantial economic loss of an estimated USD 6 billion annually in the global poultry industry. Collagen adhesion is involved in the NE pathogenesis in poultry. In this study, the binding capabilities of chicken C. perfringens isolates of various genetic backgrounds (netB^-tpeL^-, netB⁺tpeL^-, netB⁺tpeL⁺) to collagen types I-V and gelatin were examined, and the putative adhesin protein cnaA gene was investigated at the genomic level. In total, 28 C. perfringens strains from healthy and NE-inflicted sick chickens were examined. The results on collagen adhesin-encoding gene cnaA by the quantitative-PCR results indicated that netB^-tpeL^- isolates had much lower copies of the detectable cnaA gene than netB⁺ isolates (10 netB⁺tpeL^- isolates, 5 netB⁺tpeL⁺ isolates). Most of the virulent C. perfringens isolates demonstrated collagen-binding abilities to types I-II and IV-V, while some strains showed weak or no binding to collagen type III and gelatin. However, the netB⁺tpeL⁺ isolates showed significantly higher binding capabilities to collagen III than netB^-tpeL^- and netB⁺tpeL^- isolates. The data in this study suggest that the collagen-binding capability of clinical C. perfringens isolates correlates well with their NE pathogenicity levels, especially for C. perfringens isolates carrying genes encoding crucial virulence factors and virulence-associated factors such as netB, cnaA, and tpeL. These results indicate that the presence of the cnaA gene may be correlated with C. perfringens virulence (particularly for netB⁺ isolates).

The genome and antigen proteome analysis of Spiroplasma mirum

Spiroplasma mirum, small motile wall-less bacteria, was originally isolated from a rabbit tick and had the ability to infect newborn mice and caused cataracts. In this study, the whole genome and antigen proteins of S. mirum were comparative analyzed and investigated. Glycolysis, pentose phosphate pathway, arginine metabolism, nucleotide biosynthesis, and citrate fermentation were found in S. mirum, while trichloroacetic acid, fatty acids metabolism, phospholipid biosynthesis, terpenoid biosynthesis, lactose-specific PTS, and cofactors synthesis were completely absent. The Sec systems of S. mirum consist of SecA, SecE, SecDF, SecG, SecY, and YidC. Signal peptidase II was identified in S. mirum, but no signal peptidase I. The relative gene order in S. mirum is largely conserved. Genome analysis of available species in Mollicutes revealed that they shared only 84 proteins. S. mirum genome has 381 pseudogenes, accounting for 31.6% of total protein-coding genes. This is the evidence that spiroplasma genome is under an ongoing genome reduction. Immunoproteomics, a new scientific technique combining proteomics and immunological analytical methods, provided the direction of our research on S. mirum. We identified 49 proteins and 11 proteins (9 proteins in common) in S. mirum by anti-S. mirum serum and negative serum, respectively. Forty proteins in S. mirum were identified in relation to the virulence. All these proteins may play key roles in the pathogeny and can be used in the future for diagnoses and prevention.

Necrotizing Hepatitis Associated with Clostridium perfringens in Broiler Chicks

In this retrospective study we describe unusual cases of clostridial hepatitis associated with high mortality in young broiler chicks. Eleven cases of necrotizing hepatitis in broiler chicks from four companies were submitted to the Poultry Diagnostic and Research Center or the Georgia Poultry Laboratory Network between 2017 and 2020. In most flocks, increased 3-day mortality was followed by an elevated 7-day mortality. Gross lesions included green to dark brown discoloration of the liver, congested lungs, serosanguineous fluid in the caudoventral aspect of the abdomen, and emphysema in the yolk sacs. In birds older than a week of age, disease with neurologic signs became evident and consisted of tremors, stargazing, and incoordination. Histopathologic evaluation revealed multifocal to coalescing fibrinoheterophilic and necrotizing hepatitis associated with gram-positive, long, rod-shaped bacteria. Formalin-fixed liver samples from six cases out of eight cases tested were positive for Clostridium perfringens by immunohistochemistry. Liver samples from two cases were culture positive for Clostridium spp., and C. perfringens was isolated from one sample. Toxinotyping by PCR performed in seven samples revealed the presence of the genes that code for alpha toxin phospholipase C (cpa or plc) and necrotic enteritis toxin B-like (netB) in six samples and as well as C. perfringens large cytotoxin (tpeL) in one sample. Broiler breeders are the suspected source of the infection, and testing revealed C. perfringens in hatchery samples and among broiler breeder flocks. Antimicrobial therapy was coupled with enhanced sanitation at the farm and hatchery in that company, markedly decreasing the mortality and clinical signs. This is the first comprehensive evaluation of clostridial necrotizing hepatitis in newly hatched chicks, and the second ever reported in the literature.

Necrotic Enteritis in Broiler Chickens: A Review on the Pathogen, Pathogenesis, and Prevention

Clostridium perfringens type A and C are the primary etiological agents associated with necrotic enteritis (NE) in poultry. The predisposing factors implicated in the incidence of NE changes the physical properties of the gut, immunological status of birds, and disrupt the gut microbial homeostasis, causing an over-proliferation of C. perfringens. The principal virulence factors contributing to the pathogenesis of NE are the α-toxin, β-toxin, and NetB toxin. The immune response to NE in poultry is mediated by the Th1 pathway or cytotoxic T-lymphocytes. C. perfringens type A and C are also pathogenic in humans, and hence are of public health significance. C. perfringens intoxications are the third most common bacterial foodborne disease after Salmonella and Campylobacter. The restrictions on the use of antibiotics led to an increased incidence of NE in poultry. Hence, it is essential to develop alternative strategies to keep the prevalence of NE under check. The control strategies rely principally on the positive modulation of host immune response, nutritional manipulation, and pathogen reduction. Current knowledge on the etiology, pathogenesis, predisposing factors, immune response, effect on the gut microbial homeostasis, and preventative strategies of NE in this post-antibiotic era is addressed in this review.

Analysis of the complete genome sequences of Clostridium perfringens strains harbouring the binary enterotoxin BEC gene and comparative genomics of pCP13-like family plasmids

Background

BEC-producing Clostridium perfringens is a causative agent of foodborne gastroenteritis. It was first reported in 2014, and since then, several isolates have been identified in Japan and the United Kingdom. The novel binary ADP-ribosylating toxin BEC, which consists of two components (BECa and BECb), is encoded on a plasmid that is similar to pCP13 and harbours a conjugation locus, called Pcp, encoding homologous proteins of the type 4 secretion system. Despite the high in vitro conjugation frequency of pCP13, its dissemination and that of related plasmids, including bec-harbouring plasmids, in the natural environment have not been characterised. This lack of knowledge has limited our understanding of the genomic epidemiology of bec-harbouring C. perfringens strains.

Results

In this study, we determined the complete genome sequences of five bec-harbouring C. perfringens strains isolated from 2009 to 2019. Each isolate contains a ~ 3.36 Mbp chromosome and 1–3 plasmids of either the pCW3-like family, pCP13-like family, or an unknown family, and the bec-encoding region in all five isolates was located on a ~ 54 kbp pCP13-like plasmid. Phylogenetic and SNP analyses of these complete genome sequences and the 211 assembled C. perfringens genomes in GenBank showed that although these bec-harbouring strains were split into two phylogenetic clades, the sequences of the bec-encoding plasmids were nearly identical (>99.81%), with a significantly smaller SNP accumulation rate than that of their chromosomes. Given that the Pcp locus is conserved in these pCP13-like plasmids, we propose a mechanism in which the plasmids were disseminated by horizontal gene transfer. Data mining showed that strains carrying pCP13-like family plasmids were unexpectedly common (58/216 strains) and widely disseminated among the various C. perfringens clades. Although these plasmids possess a conserved Pcp locus, their ‘accessory regions’ can accommodate a wide variety of genes, including virulence-associated genes, such as becA/becB and cbp2. These results suggest that this family of plasmids can integrate various foreign genes and is transmissible among C. perfringens strains.

Conclusion

This study demonstrates the potential significance of pCP13-like plasmids, including bec-encoding plasmids, for the characterisation and monitoring of the dissemination of pathogenic C. perfringens strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08453-4.

A Highly Specific Holin-Mediated Mechanism Facilitates the Secretion of Lethal Toxin TcsL in Paeniclostridium sordellii

Protein secretion is generally mediated by a series of distinct pathways in bacteria. Recently, evidence of a novel bacterial secretion pathway involving a bacteriophage-related protein has emerged. TcdE, a holin-like protein encoded by toxigenic isolates of Clostridioides difficile, mediates the release of the large clostridial glucosylating toxins (LCGTs), TcdA and TcdB, and TpeL from C. perfringens uses another holin-like protein, TpeE, for its secretion; however, it is not yet known if TcdE or TpeE secretion is specific to these proteins. It is also unknown if other members of the LCGT-producing clostridia, including Paeniclostridium sordellii (previously Clostridium sordellii), use a similar toxin-release mechanism. Here, we confirm that each of the LCGT-producing clostridia encode functional holin-like proteins in close proximity to the toxin genes. To characterise the respective roles of these holin-like proteins in the release of the LCGTs, P. sordellii and its lethal toxin, TcsL, were used as a model. Construction and analysis of mutants of the P. sordellii tcsE (holin-like) gene demonstrated that TcsE plays a significant role in TcsL release. Proteomic analysis of the secretome from the tcsE mutant confirmed that TcsE is required for efficient TcsL secretion. Unexpectedly, comparative sample analysis showed that TcsL was the only protein significantly altered in its release, suggesting that this holin-like protein has specifically evolved to function in the release of this important virulence factor. This specificity has, to our knowledge, not been previously shown and suggests that this protein may function as part of a specific mechanism for the release of all LCGTs.

Glycosylating Effectors of Legionella pneumophila: Finding the Sweet Spots for Host Cell Subversion

Work over the past two decades clearly defined a significant role of glycosyltransferase effectors in the infection strategy of the Gram-negative, respiratory pathogen Legionella pneumophila. Identification of the glucosyltransferase effectors Lgt1-3, specifically modifying elongation factor eEF1A, disclosed a novel mechanism of host protein synthesis manipulation by pathogens and illuminated its impact on the physiological state of the target cell, in particular cell cycle progression and immune and stress responses. Recent characterization of SetA as a general O-glucosyltransferase with a wide range of targets including the proteins Rab1 and Snx1, mediators of membrane transport processes, and the discovery of new types of glycosyltransferases such as LtpM and SidI indicate that the vast effector arsenal might still hold more so-far unrecognized family members with new catalytic features and substrates. In this article, we review our current knowledge regarding these fascinating biomolecules and discuss their role in introducing new or overriding endogenous post-translational regulatory mechanisms enabling the subversion of eukaryotic cells by L. pneumophila.

Navarro M, Li J, Shrestha A, Uzal F, A. McClane B. Pathogenicity and virulence of Clostridium perfringens

Clostridium perfringens is an extremely versatile pathogen of humans and livestock, causing wound infections like gas gangrene (clostridial myonecrosis), enteritis/enterocolitis (including one of the most common human food-borne illnesses), and enterotoxemia (where toxins produced in the intestine are absorbed and damage distant organs such as the brain). The virulence of this Gram-positive, spore-forming, anaerobe is largely attributable to its copious toxin production; the diverse actions and roles in infection of these toxins are now becoming established. Most C. perfringens toxin genes are encoded on conjugative plasmids, including the pCW3-like and the recently discovered pCP13-like plasmid families. Production of C. perfringens toxins is highly regulated via processes involving two-component regulatory systems, quorum sensing and/or sporulation-related alternative sigma factors. Non-toxin factors, such as degradative enzymes like sialidases, are also now being implicated in the pathogenicity of this bacterium. These factors can promote toxin action in vitro and, perhaps in vivo, and also enhance C. perfringens intestinal colonization, e.g. NanI sialidase increases C. perfringens adherence to intestinal tissue and generates nutrients for its growth, at least in vitro. The possible virulence contributions of many other factors, such as adhesins, the capsule and biofilms, largely await future study.

Characterization of Paeniclostridium sordellii Metalloproteinase-1 in vitro and in an experimental model of infection

OBJECTIVE

Paeniclostridium sordellii is a pathogen that causes rapidly fatal infections characterized by severe edema, extreme leukemoid reaction and lack of an innate immune response. We recently identified a metalloproteinase of P. sordellii-1 (Mcs1) that cleaves human vascular cell adhesion molecule 1, an adhesion molecule important to hematopoietic precursor retention and leukocyte diapedesis. In the current study, we further characterize Mcs1 activity and investigate its role in pathogenesis.

METHODS

Mcs1 peptide cleavage sequence and activity conditions were identified using a semi-quantitative fluorescence-based reporter assay. Additional host targets for Mcs1 protease activity were tested and confirmed by gel electrophoreses and western blots. Finally, Mcs1 knock out (ΔMcs1) and complemented (cMcs1) strains were developed for assessment in our animal model of myonecrosis.

RESULTS

Data show that Mcs1 prefers aliphatic amino acid residues, I or L, especially when adjacent to negatively charged or noncharged-polar residues. In vitro, Mcs1 cleaved or partially cleaved human cell adhesion molecules, E-selectin and intracellular adhesion molecule-1 (ICAM-1), and mediators of innate immune infection defense, complement protein-3 and antimicrobial peptide LL-37. In vivo, infection with the ΔMcs1 P. sordellii strain had little effect on animal survival, tissue destruction or circulating white blood cell counts compared to wild type and cMcs1 strains.

CONCLUSIONS

Similar to proteolytic virulence factors from other pathogens, Mcs1 is a promiscuous protease that cleaves multiple human-host factors. Despite minimal impact of Mcs1 on the murine model of P. sordellii infection, it is worth considering its role in humans and other animal models.

Translocation expands the scope of the large clostridial toxin family

Large clostridial toxins (LCTs) are a family of six homologous disease-causing proteins characterised by their large size (>200 kDa) and conserved multidomain architectures. Using their central translocation and receptor-binding domain (T domain), LCTs bind host cell receptors and translocate their upstream glycosyltransferase and cysteine protease domain across the endosomal membrane and into the cytosol. The recent discovery of hundreds of LCT-like T domains in diverse genomic contexts and domain architectures from bacteria other than clostridia has provided significant new insights into the enigmatic process of LCT translocation, but also has put the definition of what constitutes an LCT into question. In this opinion article, we discuss how these findings have expanded our understanding of LCT translocation and reshaped the scope of the LCT family.

Molecular characterization of netB and tpeL virulence factors and antimicrobial resistance genes of Clostridium perfringens isolated from herbs and spices

The present study aimed to determine some virulence-associated genes and antimicrobial multidrug resistance of Clostridium perfringens recovered from herbs and spices widely distributed in the Egyptian market. C. perfringens virulence and resistance factors were determined using PCR targeting the netB, tpeL, ermB, bla and tetK genes. Thirty three out of 392 samples (8.42%) from herbs and spices submitted to our laboratory for bacteriological screening were positive for presence C. perfringens. PCR results for the tpeL gene in isolated C. perfringens revealed 9 out of 33 (27.3 %) of isolates, while netB was not detected. The isolates were resistant to Clindamycin, Vancomycin, tetracycline, and erythromycin with inhibition zones of 6.28 ± 0.63, 8.78 ± 0.41, 9.63 ± 0.63, and 9.84 ± 0.66 mm, respectively. The genes mentioned above were selected to correspond to the ineffective antimicrobials; ermB for erythromycin, tetK for tetracycline, and bla for the remainder. PCR results for antibacterial resistant genes in isolated C. perfringens revealed their presence. From 33 isolates, bla gene was detected in 21 (63.4 %), tetK in 13 (39.4 %) and ermB in only one isolate (3.03 %). Sequencing analysis was done for the bla gene as an example for the detected genes as detected at the highest incidence (63.4%). No cross-relationship was detected upon comparing incidence data of both studied virulence genes and those of antimicrobial resistance. The present findings may explain the resistance of C. perfringens to the examined antibacterials and recommend avoiding the application of them to control the microbe. In addition, the authors recommend following strict hygienic procedures during the industry of herbs and spices to ensure their clearance from Clostridium perfringens before distributing the products as food additives into the markets.

Low-density lipoprotein receptor-related protein 1 is a CROPs-associated receptor for Clostridioides difficile toxin B

As the leading cause of worldwide hospital-acquired infection, Clostridioides difficile (C. difficile) infection has caused heavy economic and hospitalized burden, while its pathogenesis is not fully understood. Toxin B (TcdB) is one of the major virulent factors of C. difficile. Recently, CSPG4 and FZD2 were reported to be the receptors that mediate TcdB cellular entry. However, genetic ablation of genes encoding these receptors failed to completely block TcdB entry, implicating the existence of alternative receptor(s) for this toxin. Here, by employing the CRISPR-Cas9 screen in CSPG4-deficient HeLa cells, we identified LDL receptor-related protein-1 (LRP1) as a novel receptor for TcdB. Knockout of LRP1 in both CSPG4-deficient HeLa cells and colonic epithelium Caco2 cells conferred cells with increased TcdB resistance, while LRP1 overexpression sensitized cells to TcdB at a low concentration. Co-immunoprecipitation assay showed that LRP1 interacts with full-length TcdB. Moreover, CROPs domain, which is dispensable for TcdB's interaction with CSPG4 and FZD2, is sufficient for binding to LRP1. As such, our study provided evidence for a novel mechanism of TcdB entry and suggested potential therapeutic targets for treating C. diff.

Large Clostridial Toxins: Mechanisms and Roles in Disease

Large clostridial toxins (LCTs) are a family of bacterial exotoxins that infiltrate and destroy target cells. Members of the LCT family include Clostridioides difficile toxins TcdA and TcdB, Paeniclostridium sordellii toxins TcsL and TcsH, Clostridium novyi toxin TcnA, and Clostridium perfringens toxin TpeL. Since the 19th century, LCT-secreting bacteria have been isolated from the blood, organs, and wounds of diseased individuals, and LCTs have been implicated as the primary virulence factors in a variety of infections, including C. difficile infection and some cases of wound-associated gas gangrene. Clostridia express and secrete LCTs in response to various physiological signals. LCTs invade host cells by binding specific cell surface receptors, ultimately leading to internalization into acidified vesicles. Acidic pH promotes conformational changes within LCTs, which culminates in translocation of the N-terminal glycosyltransferase and cysteine protease domain across the endosomal membrane and into the cytosol, leading first to cytopathic effects and later to cytotoxic effects. The focus of this review is on the role of LCTs in infection and disease, the mechanism of LCT intoxication, with emphasis on recent structural work and toxin subtyping analysis, and the genomic discovery and characterization of LCT homologues. We provide a comprehensive review of these topics and offer our perspective on emerging questions and future research directions for this enigmatic family of toxins.

Evaluation of different antigenic preparations against necrotic enteritis in broiler birds using a novel Clostridium perfringens type G strain

OBJECTIVE

Keeping in view, the constraints faced by the Indian broiler industry with lack of a suitable vaccine against Necrotic Enteritis (NE), a study has been proposed to explore the prevalence and detail characterization of C. perfringens type G in NE suspected broiler chicken in the process of suitable vaccine development.

METHODS

Intestinal scrapings/faecal contents of NE suspected broiler chickens were screened to establish the prevalence of C.perfringens type G in broiler birds. A most pathogenic, highly resistant type G isolate of C. perfringens, bearing both tpeL and gapC gene was selected for preparation of three different vaccine formulations, and to evaluate their immunogenic potential in broiler birds.

RESULTS

Screening of clinical samples of NE suspected broiler birds revealed C. perfringens type G, bearing gapC gene in 51.22% samples, of which 47.62% revealed tpeL gene. Seven of the tpeL^pos type G isolates were comparatively more pathogenic for mice, of which, one exhibited multidrug resistance towards ciprofloxacin, norfloxacin, tetracycline and levofloxacin. The sonicated supernatant (SS) prepared from the selected tpeL and gapC positive isolate could maintain a significantly higher protective IgG response than toxoid and bacterin preparation from the 21st to 28^thday of age in immunized birds.

CONCLUSION

The additional TpeL toxin in C. perfringens type G has been proved to be an additional key biological factor in the pathogenesis of NE in broiler chickens. Considering the release of more immunogenic proteins, the SS proved to be a better immunogenic preparation against NE with a multiple immunization dose.

Vaccination against pathogenic clostridia in animals: a review

Clostridium is a Gram-positive, rod-shaped, anaerobic, and spore-forming bacterium, which is found in the surrounding environments throughout the world. Clostridium species cause botulism, tetanus, enterotoxaemia, gas gangrene, necrotic enteritis, pseudomembranous colitis, blackleg, and black disease. Clostridium infection causes severe economic losses in livestock and poultry industries. Vaccination seems to be an effective way to control Clostridial diseases. This review discusses the toxins and vaccine development of the most common pathogenic Clostridium species in animals, including Clostridium perfringens, Clostridium novyi, Clostridium chauvoei, and Clostridium septicum. In this comprehensive study, we will review different kinds of clostridial toxins and the vaccines that are experimentally or practically available and will give a short description on each vaccine focusing on its applications, advantages, and disadvantages.

Holin-Dependent Secretion of the Large Clostridial Toxin TpeL by Clostridium perfringens

Large clostridial toxins (LCTs) are secreted virulence factors found in several species, including Clostridioides difficile, Clostridium perfringens, Paeniclostridium sordellii, and Clostridium novyi LCTs are large toxins that lack a secretion signal sequence, and studies by others have shown that the LCTs of C. difficile, TcdA and TcdB, require a holin-like protein, TcdE, for secretion. The TcdE gene is located on the pathogenicity locus (PaLoc) of C. difficile, and holin-encoding genes are also present in the LCT-encoded PaLocs from P. sordellii and C. perfringens However, the holin (TpeE) associated with the C. perfringens LCT TpeL has no homology and a different membrane topology than TcdE. In addition, TpeE has a membrane topology identical to that of the TatA protein, which is the core of the twin-arginine translocation (Tat) secretion system. To determine if TpeE was necessary and sufficient to secrete TpeL, the genes from a type C strain of C. perfringens were expressed in a type A strain of C. perfringens, HN13, and secretion was measured using Western blot methods. We found that TpeE was required for TpeL secretion and that secretion was not due to cell lysis. Mutant forms of TpeE lacking an amphipathic helix and a charged C-terminal domain failed to secrete TpeL, and mutations that deleted conserved LCT domains in TpeL indicated that only the full-length protein could be secreted. In summary, we have identified a novel family of holin-like proteins that can function, in some cases, as a system of protein secretion for proteins that need to fold in the cytoplasm.IMPORTANCE Little is known about the mechanism by which LCTs are secreted. Since LCTs are major virulence factors in clostridial pathogens, we wanted to define the mechanism by which an LCT in C. perfringens, TpeL, is secreted by a protein (TpeE) lacking homology to previously described secretion-associated holins. We discovered that TpeE is a member of a widely dispersed class of holin proteins, and TpeE is necessary for the secretion of TpeL. TpeE bears a high degree of similarity in membrane topology to TatA proteins, which form the pore through which Tat secretion substrates pass through the cytoplasmic membrane. Thus, the TpeE-TpeL secretion system may be a model for understanding not only holin-dependent secretion but also how TatA proteins function in the secretion process.

Bacterial intracellularly active toxins: Membrane localisation of the active domain

Numerous bacterial toxins exert their activity by inactivating or modulating a specific intracellular host target. For this purpose, these toxins have developed efficient strategies to overcome the different host cell defences including specific binding to cell surface, internalisation, passage through the endosome or plasma membrane, exploiting intracellular trafficking and addressing to intracellular targets. Several intracellularly active toxins deliver an active domain into the cytosol that interacts with a target localised to the inner face of the plasma membrane. Thus, the large clostridial glucosylating toxins (LCGTs) target Rho/Ras-GTPases, certain virulence factors of Gram negative bacteria, Rho-GTPases, while Pasteurella multocida toxin (PMT) targets trimeric G-proteins. Others such as botulinum neurotoxins and tetanus neurotoxin have their substrate on synaptic vesicle membrane. LCGTs, PMT, and certain virulence factors from Vibrio sp. show a particular structure constituted of a four-helix bundle membrane (4HBM) protruding from the catalytic site that specifically binds to the membrane phospholipids and then trap the catalytic domain at the proximity of the membrane anchored substrate. Structural and functional analysis indicate that the 4HBM tip of the Clostridium sordellii lethal toxin (TcsL) from the LCGT family contain two loops forming a cavity that mediates the binding to phospholipids and more specifically to phosphatidylserine.

Characterization of Virulent netB+/tpeL+ Clostridium perfringens Strains from Necrotic Enteritis-Affected Broiler Chicken Farms

Clostridium perfringens (CP) type A and newly created type G strains are the key etiological factors in the induction of necrotic enteritis (NE), an important enteric disease that is responsible for the annual loss of $6 billion in the worldwide poultry industry. Several CP toxin genes were found to be critical in NE pathogenesis in chickens, but limited information is available on the CP lethal toxin tpeL gene. In this study, 19 CP strains isolated from NE-affected chicken farms were characterized microbiologically and molecularly and evaluated for their pathogenicity in commercial broiler chickens. Toxin typing by PCR revealed that all strains tested were positive for the netB toxin gene, but only five strains were positive for the tpeL toxin gene (LLY-TpeL 13, -TpeL 15, -TpeL 17, -TpeL 18, and -TpeL 19, simplified as TpeL 13, TpeL 15, TpeL 17, TpeL 18, and TpeL 19). High levels of TpeL proteins were detected in the concentrated culture supernatant from strains TpeL 13, 15, 17, and 19 by western blotting. Quantitative PCR showed that strains TpeL 13, 15, 17, 18, and 19 harbored a high number of copies of tpeL genes, while TpeL 18 had the highest number of copies of the tpeL gene among all CP strains tested when normalized with copy numbers of 16S rRNA gene as a housekeeping gene marker. The in vivo NE challenge test using multiple oral CP inoculations combined with a high-protein diet showed that TpeL 17 was the most virulent in inducing typical NE lesions, followed by TpeL 19 as the next most virulent, when tested in commercial broiler chickens. Infection with TpeL 17 reduced the growth rate significantly, as shown by reduced relative body weight gain percentage at day 5 postinfection. Availability of the virulent netB⁺tpeL⁺ CP strains is essential for the development of a CP-alone NE challenge model that could provide significant tools for understanding CP pathogenesis and for development of alternative to antibiotics.

Intestinal bile acids directly modulate the structure and function of C. difficile TcdB toxin

Intestinal bile acids are known to modulate the germination and growth of Clostridioides difficile Here we describe a role for intestinal bile acids in directly binding and neutralizing TcdB toxin, the primary determinant of C. difficile disease. We show that individual primary and secondary bile acids reversibly bind and inhibit TcdB to varying degrees through a mechanism that requires the combined oligopeptide repeats region to which no function has previously been ascribed. We find that bile acids induce TcdB into a compact "balled up" conformation that is no longer able to bind cell surface receptors. Lastly, through a high-throughput screen designed to identify bile acid mimetics we uncovered nonsteroidal small molecule scaffolds that bind and inhibit TcdB through a bile acid-like mechanism. In addition to suggesting a role for bile acids in C. difficile pathogenesis, these findings provide a framework for development of a mechanistic class of C. difficile antitoxins.

Revisiting the Evolution and Taxonomy of Clostridia, a Phylogenomic Update

Clostridium is a large genus of obligate anaerobes belonging to the Firmicutes phylum of bacteria, most of which have a Gram-positive cell wall structure. The genus includes significant human and animal pathogens, causative of potentially deadly diseases such as tetanus and botulism. Despite their relevance and many studies suggesting that they are not a monophyletic group, the taxonomy of the group has largely been neglected. Currently, species belonging to the genus are placed in the unnatural order defined as Clostridiales, which includes the class Clostridia. Here, we used genomic data from 779 strains to study the taxonomy and evolution of the group. This analysis allowed us to 1) confirm that the group is composed of more than one genus, 2) detect major differences between pathogens classified as a single species within the group of authentic Clostridium spp. (sensu stricto), 3) identify inconsistencies between taxonomy and toxin evolution that reflect on the pervasive misclassification of strains, and 4) identify differential traits within central metabolism of members of what has been defined earlier and confirmed by us as cluster I. Our analysis shows that the current taxonomic classification of Clostridium species hinders the prediction of functions and traits, suggests a new classification for this fascinating class of bacteria, and highlights the importance of phylogenomics for taxonomic studies.

The C. difficile toxin B membrane translocation machinery is an evolutionarily conserved protein delivery apparatus

Large Clostridial Toxins (LCTs) are a family of six homologous protein toxins that are implicated in severe disease. LCTs infiltrate host cells using a translocation domain (LCT-T) that contains both cell-surface receptor binding sites and a membrane translocation apparatus. Despite much effort, LCT translocation remains poorly understood. Here we report the identification of 1104 LCT-T homologs, with 769 proteins from bacteria outside of clostridia. Sequences are widely distributed in pathogenic and host-associated species, in a variety of contexts and architectures. Consistent with these homologs being functional toxins, we show that a distant LCT-T homolog from Serratia marcescens acts as a pH-dependent translocase to deliver its effector into host cells. Based on evolutionary footprinting of LCT-T homologs, we further define an evolutionarily conserved translocase region that we show is an autonomous translocase capable of delivering heterologous cargo into host cells. Our work uncovers a broad class of translocating toxins and provides insights into LCT translocation.

Large Clostridial toxins infiltrate host cells using a translocation domain (LCT-T). Here, using a genomics-driven approach and functional assays, the authors uncover the presence of distant LCT-T homologs in bacteria outside clostridia and provide evidence for a toxic effector function in the gammaproteobacterium Serratia marcescens.

Clostridium perfringens type C necrotic enteritis in pigs: diagnosis, pathogenesis, and prevention

Clostridium perfringens type C causes severe and lethal necrotic enteritis (NE) in newborn piglets. NE is diagnosed through a combination of pathology and bacteriologic investigations. The hallmark lesion of NE is deep, segmental mucosal necrosis with marked hemorrhage of the small intestine. C. perfringens can be isolated from intestinal samples in acute cases but it is more challenging to identify pathogenic strains in subacute-to-chronic cases. Toxinotyping or genotyping is required to differentiate C. perfringens type C from commensal type A strains. Recent research has extended our knowledge about the pathogenesis of the disease, although important aspects remain to be determined. The pathogenesis involves rapid overgrowth of C. perfringens type C in the small intestine, inhibition of beta-toxin (CPB) degradation by trypsin inhibitors in the colostrum of sows, and most likely initial damage to the small intestinal epithelial barrier. CPB itself acts primarily on vascular endothelial cells in the mucosa and can also inhibit platelet function. Prevention of the disease is achieved by immunization of pregnant sows with C. perfringens type C toxoid vaccines, combined with proper sanitation on farms. For the implementation of prevention strategies, it is important to differentiate between disease-free and pathogen-free status of a herd. The latter is more challenging to maintain, given that C. perfringens type C can persist for a long time in the environment and in the intestinal tract of adult animals and thus can be distributed via clinically and bacteriologically inapparent carrier animals.

Sialidases From Clostridium perfringens and Their Inhibitors

Clostridium perfringens is an important human and animal pathogen that is the primary causative agent of necrotizing enteritis and enterotoxemia in many types of animals; it causes traumatic gas gangrene in humans and animals and is associated with cases of food poisoning in humans. C. perfringens produces a variety of toxins as well as many enzymes, including three sialidases, NanH, NanI, and NanJ. Sialidases could be important virulence factors that promote the pathogenesis of C. perfringens. Among them, NanI promotes the colonization of C. perfringens in the intestinal tract and enhances the cytotoxic activity and association of several major C. perfringens toxins with host cells. In recent years, studies on the structure and functions of sialidases have yielded interesting results, and the functions of sialic acid and sialidases in bacterial pathogenesis have become a hot research topic. An in-depth understanding and additional studies of sialidases will further elucidate mechanisms of C. perfringens pathogenesis and could promote the development and clinical applications of sialidase inhibitors. This article reviews the structural characteristics, expression regulation, roles of sialidases in C. perfringens pathogenesis, and effects of their inhibitors.

The prevalence of plasmid-coded cpe enterotoxin, β2 toxin, tpeL toxin, and tetracycline resistance in Clostridium perfringens strains isolated from different sources

Clostridium perfringens, an anaerobic, spore-forming bacterium, causes infections in humans and animals by producing several toxins encoded by genes found either on the chromosomes or on diverse plasmids. The plasmids may code for more than one toxin gene or antimicrobial-resistance gene. In this study, the prevalence of the β2, cpe and tpeL toxin genes and the tetA(P), tetB(P) and tetM tetracycline-resistance genes, in 56 strains of C. perfringens type A isolated from diseased domestic animals and 15 strains isolated from chickens, was compared with that in 74 strains isolated from other sources. The frequency of chromosome-associated cpe enterotoxin genes was higher in strains not isolated from diseased domestic animals; however, plasmid-associated cpe genes were found in strains from some animal sources more than others. Enterotoxin production was detected in some strains that had chromosomal or plasmid cpe genes, but not in all. The percentages of strains carrying β2 toxin genes among chicken, swine, human patient and soil isolates were higher than those among bovine, canine and food isolates. The incidence of the tpeL toxin gene was lower than that of the β2 gene. Phenotypic resistance to tetracycline was found in more than 50% of the porcine, bovine, and canine isolates, which carried a wide range of plasmids of 2-100 kb size, most of which had the tcpH clostridial transfer gene. PCR amplified tetA(P) and tetB(P) genes from most isolates from diseased animals. Some strains that carried <40 kb plasmids and had the tcpH gene also had one or more toxin genes or tetracycline-resistance gene. This study shows that the prevalence of plasmid-borne toxins and antimicrobial resistance genes varied among C. perfringens strains isolated from different sources. Plasmids of smaller size than those previously reported in strains of C. perfringens type A may also harbor toxin genes and antimicrobial-resistance genes.

Targeting oncogenic Ras by the Clostridium perfringens toxin TpeL

Clostridium perfringens toxin TpeL belongs to the family of large clostridial glycosylating toxins. The toxin causes N-acetylglucosaminylation of Ras proteins at threonine35 thereby inactivating the small GTPases. Here, we show that all main types of oncogenic Ras proteins (H-Ras, K-Ras and N-Ras) are modified by the toxin in vitro and in vivo. Toxin-catalyzed modification of Ras was accompanied by inhibition of the MAP kinase pathway. Importantly, TpeL inhibited the paradoxical activation of the MAP kinase pathway induced by the BRAF inhibitor Vemurafenib in the human melanoma cell line SBCL2. The toxin also blocked Ras signaling in a zebrafish embryo model expressing oncogenic H-Ras^G12V, resulting in a reduction of melanocyte number. By using the binding and translocation component of anthrax toxin (protective antigen), the glucosyltransferase domain of TpeL was effectively introduced into target cells that were not sensitive to native TpeL toxin. To reach a higher specificity towards cancer cells, a chimeric TpeL toxin was engineered that possessed the knob region of adenovirus serotype 35 fiber, which interacts with CD46 of target cells frequently overexpressed in cancer cells. The chimeric TpeL fusion toxin efficiently inhibited Ras and MAP kinases in human pancreatic cancer Capan-2 cells, which were insensitive to the wild-type toxin. The data reveal that TpeL and TpeL-related immunotoxins provide a new toolset as Ras-inactivating agents.

Clostridium sordellii Pathogenicity Locus Plasmid pCS1-1 Encodes a Novel Clostridial Conjugation Locus

A major virulence factor in Clostridium sordellii-mediated infection is the toxin TcsL, which is encoded within a region of the genome called the pathogenicity locus (PaLoc). C. sordellii isolates carry the PaLoc on the pCS1 family of plasmids, of which there are four characterized members. Here, we determined the potential mobility of pCS1 plasmids and characterized a fifth unique pCS1 member. Using a derivative of the pCS1-1 plasmid from strain ATCC 9714 which had been marked with the ermB erythromycin resistance gene, conjugative transfer into a recipient C. sordellii isolate, R28058, was demonstrated. Bioinformatic analysis of pCS1-1 identified a novel conjugation gene cluster defined as the C. sordellii transfer (cst) locus. Interruption of genes within the cst locus resulted in loss of pCS1-1 transfer, which was restored upon complementation in trans. These studies provided clear evidence that genes within the cst locus are essential for the conjugative transfer of pCS1-1. The cst locus is present on all pCS1 subtypes, and homologous loci were identified on toxin-encoding plasmids from Clostridium perfringens and Clostridium botulinum and also carried within genomes of Clostridium difficile isolates, indicating that it is a widespread clostridial conjugation locus. The results of this study have broad implications for the dissemination of toxin genes and, potentially, antibiotic resistance genes among members of a diverse range of clostridial pathogens, providing these microorganisms with a survival advantage within the infected host.

C. sordellii is a bacterial pathogen that causes severe infections in humans and animals, with high mortality rates. While the pathogenesis of C. sordellii infections is not well understood, it is known that the toxin TcsL is an important virulence factor. Here, we have shown the ability of a plasmid carrying the tcsL gene to undergo conjugative transfer between distantly related strains of C. sordellii, which has far-reaching implications for the ability of C. sordellii to acquire the capacity to cause disease. Plasmids that carry tcsL encode a previously uncharacterized conjugation locus, and individual genes within this locus were shown to be required for conjugative transfer. Furthermore, homologues on toxin plasmids from other clostridial species were identified, indicating that this region represents a novel clostridial conjugation locus. The results of this study have broad implications for the dissemination of virulence genes among members of a diverse range of clostridial pathogens.

Supplemental thymol and carvacrol increases ileum Lactobacillus population and reduces effect of necrotic enteritis caused by Clostridium perfringes in chickens

Necrotic enteritis (NE) caused by Clostridium perfringens is one of the most detrimental infectious diseases in poultry. This study examined the effect of blends of essential oils (BEOs) (25% thymol and 25% carvacrol) on NE and bacterial dynamics and functions in chicks challenged with C. perfringens. Chicks were assigned to a Control diet and BEOs diet (Control diet + 120 mg/kg BEOs), were challenged with C. perfringens from days 14 to 20 and were killed on day 21 for assessment. Supplementation with BEOs decreased the mortality, alleviated gut lesions, and decreased the virulence factors of pathogenic bacteria (VF 0073-ClpE, VF0124-LPS, and VF0350-BSH). Lack of supplementation also changed the nutrient and immunological dynamics of host microbiota in responding to C. perfringens infection. Adding BEOs changed the host ileum microbial population by increasing the numbers of Lactobacillus crispatus and Lactobacillus agilis, and decreasing Lactobacillus salivarius and Lactobacillus johnsonii. The functional roles of these changing host bacterial populations coupled with the putative reduced pathogenicity of C. perfringens by BEOs contributed to the reduction in gut lesions and mortality in infected chickens. It suggests that dietary supplementation with BEOs could significantly reduce the impact of NE caused by C. perfringens on broilers.

Adhesion and invasion of Clostridium perfringens type A into epithelial cells

Clostridium perfringens is the causative agent for necrotic enteritis. It secretes the major virulence factors, and α- and NetB-toxins that are responsible for intestinal lesions. The TpeL toxin affects cell morphology by producing myonecrosis, but its role in the pathogenesis of necrotic enteritis is unclear. In this study, the presence of netB and tpeL genes in C. perfringens type A strains isolated from chickens with necrotic enteritis, their cytotoxic effects and role in adhesion and invasion of epithelial cells were evaluated. Six (27.3%) of the 22 C. perfringens type A strains were harboring the tpeL gene and produced morphological alterations in Vero cells after 6 h of incubation. Strains tpeL (−) induced strong cell rounding after 6 h of incubation and produced cell enlargement. None of the 22 strains harbored netB gene. All the six tpeL (+) gene strains were able to adhere to HEp-2 cells; however, only four of them (66.6%) were invasive. Thus, these results suggest that the presence of tpeL gene or TpeL toxin might be required for the adherence of bacteria to HEp-2 cells; however, it could not have any role in the invasion process.

Clostridium perfringens Sporulation and Sporulation-Associated Toxin Production

The ability of Clostridium perfringens to form spores plays a key role during the transmission of this Gram-positive bacterium to cause disease. Of particular note, the spores produced by food poisoning strains are often exceptionally resistant to food environment stresses such as heat, cold, and preservatives, which likely facilitates their survival in temperature-abused foods. The exceptional resistance properties of spores made by most type A food poisoning strains and some type C foodborne disease strains involve their production of a variant small acid-soluble protein-4 that binds more tightly to spore DNA than to the small acid-soluble protein-4 made by most other C. perfringens strains. Sporulation and germination by C. perfringens and Bacillus spp. share both similarities and differences. Finally, sporulation is essential for production of C. perfringens enterotoxin, which is responsible for the symptoms of C. perfringens type A food poisoning, the second most common bacterial foodborne disease in the United States. During this foodborne disease, C. perfringens is ingested with food and then, by using sporulation-specific alternate sigma factors, this bacterium sporulates and produces the enterotoxin in the intestines.

Clostridium difficile Toxin Biology

Clostridium difficile is the cause of antibiotics-associated diarrhea and pseudomembranous colitis. The pathogen produces three protein toxins: C. difficile toxins A (TcdA) and B (TcdB), and C. difficile transferase toxin (CDT). The single-chain toxins TcdA and TcdB are the main virulence factors. They bind to cell membrane receptors and are internalized. The N-terminal glucosyltransferase and autoprotease domains of the toxins translocate from low-pH endosomes into the cytosol. After activation by inositol hexakisphosphate (InsP6), the autoprotease cleaves and releases the glucosyltransferase domain into the cytosol, where GTP-binding proteins of the Rho/Ras family are mono-O-glucosylated and, thereby, inactivated. Inactivation of Rho proteins disturbs the organization of the cytoskeleton and affects multiple Rho-dependent cellular processes, including loss of epithelial barrier functions, induction of apoptosis, and inflammation. CDT, the third C. difficile toxin, is a binary actin-ADP-ribosylating toxin that causes depolymerization of actin, thereby inducing formation of the microtubule-based protrusions. Recent progress in understanding of the toxins' actions include insights into the toxin structures, their interaction with host cells, and functional consequences of their actions.

Evidence for dual receptor-binding sites in Clostridium difficile toxin A

TcdA (308 kDa) and TcdB (270 kDa) disrupt the integrity of the intestinal epithelial barrier and provide an environment favorable for Clostridium difficile colonization. Recent evidence suggests that entry of TcdA into cells is mediated by at least two domains. Here, we report the characterization of a second receptor-binding domain (RBD2) for TcdA. While both the isolated combined repetitive oligopeptides (CROPs) and RBD2 fragments are rapidly internalized into cells under physiologic conditions, only the CROPs domain appreciably accumulates at the cell surface. Once internalized, CROPs and RBD2 are trafficked to late endosomal compartments. An internal deletion of RBD2 from TcdA holotoxin ablated toxicity in HT29 cells. These data are consistent with the recently proposed dual receptor model of cellular entry.

Clostridium perfringens Sialidases: Potential Contributors to Intestinal Pathogenesis and Therapeutic Targets

Clostridium perfringens is a major cause of histotoxic and intestinal infections of humans and other animals. This Gram-positive anaerobic bacterium can produce up to three sialidases named NanH, NanI, and NanJ. The role of sialidases in histotoxic infections, such as gas gangrene (clostridial myonecrosis), remains equivocal. However, recent in vitro studies suggest that NanI may contribute to intestinal virulence by upregulating production of some toxins associated with intestinal infection, increasing the binding and activity of some of those toxins, and enhancing adherence of C. perfringens to intestinal cells. Possible contributions of NanI to intestinal colonization are further supported by observations that the C. perfringens strains causing acute food poisoning in humans often lack the nanI gene, while other C. perfringens strains causing chronic intestinal infections in humans usually carry a nanI gene. Certain sialidase inhibitors have been shown to block NanI activity and reduce C. perfringens adherence to cultured enterocyte-like cells, opening the possibility that sialidase inhibitors could be useful therapeutics against C. perfringens intestinal infections. These initial in vitro observations should be tested for their in vivo significance using animal models of intestinal infections.

Clostridium perfringens TpeL Induces Formation of Stress Fibers via Activation of RhoA-ROCK Signaling Pathway

Clostridium perfringens TpeL belongs to a family of large clostridial glucosylating cytotoxins. TpeL modifies Rac1 and Ras subfamily proteins. Herein we report TpeL-induced formation of stress fibers via RhoA-Rho kinase (ROCK) signaling. A recombinant protein (TpeL1-525) derived from the TpeL N-terminal catalytic domain in the presence of streptolysin O (SLO) induced the formation of actin stress fibers in Madin-Darby canine kidney (MDCK) cells in a dose-dependent manner. The RhoA/ROCK pathway is known to control the formation of stress fibers. We examined the role of the RhoA/ROCK pathway in TpeL-induced formation of stress fibers. TpeL1-525-induced formation of stress fibers was inhibited by the ROCK inhibitor, Y27632 and Rho protein inhibitor, C3 transferase. TpeL1-525 activated RhoA and ROCK in a dose-dependent manner. C3 transferase blocked TpeL1-525-induced activation of RhoA and ROCK whereas Y27632 inhibited TpeL-induced activation of ROCK. These results demonstrate for the first time that TpeL induces the formation of stress fibers by activating the RhoA/ROCK signaling pathway.

The CpAL system regulates changes of the trans-epithelial resistance of human enterocytes during Clostridium perfringens type C infection

Clostridium perfringens type C strains produce severe disease in humans and animals including enterotoxaemia and hemorrhagic diarrhea. Type C disease is mediated by production of toxins that damage the site of infection inducing loss of bloody fluids. Production of type C toxins, such as CPA, PFO, and, CPB is regulated by the C. perfringens Agr-like (CpAL) quorum sensing (QS) system. The CpAL system is also required to recapitulate, in vivo, intestinal signs of C. perfringens type C-induced disease, including hemorrhagic diarrhea and accumulation of fluids. The intestinal epithelium forms a physical barrier, made up of a series of intercellular junctions including tight junctions (TJs), adherens junctions (AJs) and desmosomes (DMs). This selective barrier regulates important physiological processes, including paracellular movement of ions and solutes, which, if altered, results in loss of fluids into the intestinal lumen. In this work, the effects of C. perfringens infection on the barrier function of intestinal epithelial cells was evaluated by measuring trans-epithelial resistance (TEER). Our studies demonstrate that infection of human enterocytes with C. perfringens type C strain CN3685 induced a significant drop on TEER. Changes in TEER were mediated by the CpAL system as a CN3685ΔagrB mutant did not induce such a drop. Physical contact between bacteria and enterocytes produced more pronounced changes in TEER and this phenomenon appeared also to be mediated by the CpAL system. Finally, immunofluorescence studies demonstrate that C. perfringens type C infection redistribute TJs protein occludin, and Claudin-3, and DMs protein desmoglein-2, but did not affect the AJs protein E-cadherin.

The Tip of the Four N-Terminal α-Helices of Clostridium sordellii Lethal Toxin Contains the Interaction Site with Membrane Phosphatidylserine Facilitating Small GTPases Glucosylation

Clostridium sordellii lethal toxin (TcsL) is a powerful virulence factor responsible for severe toxic shock in man and animals. TcsL belongs to the large clostridial glucosylating toxin (LCGT) family which inactivates small GTPases by glucosylation with uridine-diphosphate (UDP)-glucose as a cofactor. Notably, TcsL modifies Rac and Ras GTPases, leading to drastic alteration of the actin cytoskeleton and cell viability. TcsL enters cells via receptor-mediated endocytosis and delivers the N-terminal glucosylating domain (TcsL-cat) into the cytosol. TcsL-cat was found to preferentially bind to phosphatidylserine (PS)-containing membranes and to increase the glucosylation of Rac anchored to the lipid membrane. We have previously reported that the N-terminal four helical bundle structure (1–93 domain) recognizes a broad range of lipids, but that TcsL-cat specifically binds to PS and phosphatidic acid. Here, we show using mutagenesis that the PS binding site is localized on the tip of the four-helix bundle which is rich in positively-charged amino acids. Residues Y14, V15, F17, and R18 on loop 1, between helices 1 and 2, in coordination with R68 from loop 3, between helices 3 and 4, form a pocket which accommodates L-serine. The functional PS-binding site is required for TcsL-cat binding to the plasma membrane and subsequent cytotoxicity. TcsL-cat binding to PS facilitates a high enzymatic activity towards membrane-anchored Ras by about three orders of magnitude as compared to Ras in solution. The PS-binding site is conserved in LCGTs, which likely retain a common mechanism of binding to the membrane for their full activity towards membrane-bound GTPases.

Optimization of the Production of Inactivated Clostridium novyi Type B Vaccine Using Computational Intelligence Techniques

Clostridium novyi causes necrotic hepatitis in sheep and cattle, as well as gas gangrene. The microorganism is strictly anaerobic, fastidious, and difficult to cultivate in industrial scale. C. novyi type B produces alpha and beta toxins, with the alpha toxin being linked to the presence of specific bacteriophages. The main strategy to combat diseases caused by C. novyi is vaccination, employing vaccines produced with toxoids or with toxoids and bacterins. In order to identify culture medium components and concentrations that maximized cell density and alpha toxin production, a neuro-fuzzy algorithm was applied to predict the yields of the fermentation process for production of C. novyi type B, within a global search procedure using the simulated annealing technique. Maximizing cell density and toxin production is a multi-objective optimization problem and could be treated by a Pareto approach. Nevertheless, the approach chosen here was a step-by-step one. The optimum values obtained with this approach were validated in laboratory scale, and the results were used to reload the data matrix for re-parameterization of the neuro-fuzzy model, which was implemented for a final optimization step with regards to the alpha toxin productivity. With this methodology, a threefold increase of alpha toxin could be achieved.

Receptors and Binding Structures for Clostridium difficile Toxins A and B

Two characteristics of toxins A and B from C. difficile (TcdA, TcdB) are important for the understanding of the pathogenic effect of these homologous toxins. First, these toxins are huge single-chain but multidomain proteins that display their action intracellularly within the cytosol of host cells. And second, albeit various cell types highly differ in their sensitivity toward these toxins, no toxin-resistant cell type has been described yet. Investigation of receptor-mediated uptake of these toxins is very ambitious. It demands discrimination between cell surface binding, interaction with more than one functional receptor responsible for uptake as well as other functional receptors that recognize bacterial pathogens and are not necessarily related with endocytosis. The current understanding of a complex uptake process is that TcdB interacts with at least two facultative receptors that mediate entry into host cells by redundant endocytotic pathways. Although both homologous toxins do obviously not share the same receptors, this principle of redundant binding domains found for TcdB does also account for TcdA.