Variations in TcdB activity and the hypervirulence of emerging strains of Clostridium difficile

'Get in Early'; Biofilm and Wax Moth (Galleria mellonella) Models Reveal New Insights into the Therapeutic Potential of Clostridium difficile Bacteriophages

Clostridium difficile infection (CDI) is a global health threat associated with high rates of morbidity and mortality. Conventional antibiotic CDI therapy can result in treatment failure and recurrent infection. C. difficile produces biofilms which contribute to its virulence and impair antimicrobial activity. Some bacteriophages (phages) can penetrate biofilms and thus could be developed to either replace or supplement antibiotics. Here, we determined the impact of a previously optimized 4-phage cocktail on C. difficile ribotype 014/020 biofilms, and additionally as adjunct to vancomycin treatment in Galleria mellonella larva CDI model. The phages were applied before or after biofilm establishment in vitro, and the impact was analyzed according to turbidity, viability counts and topography as observed using scanning electron and confocal microscopy. The infectivity profiles and efficacies of orally administered phages and/or vancomycin were ascertained by monitoring colonization levels and larval survival rates. Phages prevented biofilm formation, and penetrated established biofilms. A single phage application reduced colonization causing extended longevity in the remedial treatment and prevented disease in the prophylaxis group. Multiple phage doses significantly improved the larval remedial regimen, and this treatment is comparable to vancomycin and the combined treatments. Taken together, our data suggest that the phages significantly reduce C. difficile biofilms, and prevent colonization in the G. mellonella model when used alone or in combination with vancomycin. The phages appear to be highly promising therapeutics in the targeted eradication of CDI and the use of these models has revealed that prophylactic use could be a propitious therapeutic option.

Coordination between T helper cells, iNKT cells, and their follicular helper subsets in the humoral immune response against Clostridium difficile toxin B

Activation of iNKT cells with the CD1d-binding glycolipid adjuvant α-galactosylceramide (α-GC) enhances humoral immunity specific for coadministered T-dependent Ag. However, the relationship between the iNKT cell and the classic T helper (Th) or T follicular helper (Tfh) function following this immunization modality remains unclear. We show that immunization with the C-terminal domain (CTD) of Clostridium difficile toxin B (TcdB), accompanied by activation of iNKT cells with α-GC, led to enhanced production of CTD-specific IgG, which was CD1d- and iNKT cell-dependent and associated with increased neutralization of active TcdB. Immunization with CTD plus α-GC followed by NP hapten-linked CTD increased NP-specific IgG1 titers in an NKT-dependent manner, suggesting that iNKT activation could enhance Th or Tfh function or that iNKT and iNKTfh cells could provide supplemental, yet independent, B cell help. Th, Tfh, iNKT, and iNKTfh cells were, therefore, examined quantitatively, phenotypically, and functionally following immunization with CTD or with CTD plus α-GC. Our results demonstrated that α-GC-activated iNKT cells had no direct effect on the numbers, phenotype, or function of Th or Tfh cells. However, CD4⁺ T cell-specific ablation of the Bcl6 transcription factor demonstrated that Tfh and iNKTfh cells both contributed to B cell help. This work extends our understanding of the immune response to vaccination and demonstrates an important contribution by NKTfh cells to humoral immunity.

Novel Clostridium difficile Anti-Toxin (TcdA and TcdB) Humanized Monoclonal Antibodies Demonstrate In Vitro Neutralization across a Broad Spectrum of Clinical Strains and In Vivo Potency in a Hamster Spore Challenge Model

Clostridium difficile (C. difficile) infection (CDI) is the main cause of nosocomial antibiotic-associated colitis and increased incidence of community-associated diarrhea in industrialized countries. At present, the primary treatment of CDI is antibiotic administration, which is effective but often associated with recurrence, especially in the elderly. Pathogenic strains produce enterotoxin, toxin A (TcdA), and cytotoxin, toxin B (TcdB), which are necessary for C. difficile induced diarrhea and gut pathological changes. Administration of anti-toxin antibodies provides an alternative approach to treat CDI, and has shown promising results in preclinical and clinical studies. In the current study, several humanized anti-TcdA and anti-TcdB monoclonal antibodies were generated and their protective potency was characterized in a hamster infection model. The humanized anti-TcdA (CANmAbA4) and anti-TcdB (CANmAbB4 and CANmAbB1) antibodies showed broad spectrum in vitro neutralization of toxins from clinical strains and neutralization in a mouse toxin challenge model. Moreover, co-administration of humanized antibodies (CANmAbA4 and CANmAbB4 cocktail) provided a high level of protection in a dose dependent manner (85% versus 57% survival at day 22 for 50 mg/kg and 20 mg/kg doses, respectively) in a hamster gastrointestinal infection (GI) model. This study describes the protective effects conferred by novel neutralizing anti-toxin monoclonal antibodies against C. difficile toxins and their potential as therapeutic agents in treating CDI.

A MLST Clade 2 Clostridium difficile strain with a variant TcdB induces severe inflammatory and oxidative response associated with mucosal disruption

The epidemiology of Clostridium difficile infections is highly dynamic as new strains continue to emerge worldwide. Here we present a detailed analysis of a new C. difficile strain (ICC-45) recovered from a cancer patient in Brazil that died from severe diarrhea. A polyphasic approach assigned a new PCR-ribotype and PFGE macrorestriction pattern to strain ICC-45, which is toxigenic (tcdA(+), tcdB(+) and ctdB(+)) and classified as ST41 from MLST Clade 2 and toxinotype IXb. Strain ICC-45 encodes for a variant TcdB that induces a distinct CPE in agreement with its toxinotype. Unlike epidemic NAP1/027 strains, which are also classified to MLST Clade 2, strain ICC-45 is susceptible to fluoroquinolones and does not overproduce toxins TcdA and TcdB. However, supernatants from strain ICC-45 and a NAP1/027 strain produced similar expression of pro-inflammatory cytokines, epithelial damage, and oxidative stress response in the mouse ileal loop model. These results highlight inflammation and oxidative stress as common features in the pathogenesis of C. difficile Clade 2 strains. Finally, this work contributes to the description of differences in virulence among various C. difficile strains.

Clostridium difficile Infection

Clostridium difficile is an anaerobic, Gram-positive, spore-forming, toxin-secreting bacillus that has long been recognized to be the most common etiologic pathogen of antibiotic-associated diarrhea. C. difficile infection (CDI) is now the most common cause of health care-associated infections in the United States and accounts for 12% of these infections (Magill SS et al., N Engl J Med370:1198-1208, 2014). Among emerging pathogens of public health importance in the United States, CDI has the highest population-based incidence, estimated at 147 per 100,000 (Lessa FC et al., N Engl J Med372:825-834, 2015). In a report on antimicrobial resistance, C. difficile has been categorized by the Centers for Disease Control and Prevention as one of three "urgent" threats (http://www.cdc.gov/drugresistance/threat-report-2013/). Although C. difficile was first described in the late 1970s, the past decade has seen the emergence of hypertoxigenic strains that have caused increased morbidity and mortality worldwide. Pathogenic strains, host susceptibility, and other regional factors vary and may influence the clinical manifestation and approach to intervention. In this article, we describe the global epidemiology of CDI featuring the different strains in circulation outside of North America and Europe where strain NAP1/027/BI/III had originally gained prominence. The elderly population in health care settings has been disproportionately affected, but emergence of CDI in children and healthy young adults in community settings has, likewise, been reported. New approaches in management, including fecal microbiota transplantation, are discussed.

Clostridium difficile Toxins A and B: Insights into Pathogenic Properties and Extraintestinal Effects

Clostridium difficile infection (CDI) has significant clinical impact especially on the elderly and/or immunocompromised patients. The pathogenicity of Clostridium difficile is mainly mediated by two exotoxins: toxin A (TcdA) and toxin B (TcdB). These toxins primarily disrupt the cytoskeletal structure and the tight junctions of target cells causing cell rounding and ultimately cell death. Detectable C. difficile toxemia is strongly associated with fulminant disease. However, besides the well-known intestinal damage, recent animal and in vitro studies have suggested a more far-reaching role for these toxins activity including cardiac, renal, and neurologic impairment. The creation of C. difficile strains with mutations in the genes encoding toxin A and B indicate that toxin B plays a major role in overall CDI pathogenesis. Novel insights, such as the role of a regulator protein (TcdE) on toxin production and binding interactions between albumin and C. difficile toxins, have recently been discovered and will be described. Our review focuses on the toxin-mediated pathogenic processes of CDI with an emphasis on recent studies.

Zebrafish Sensitivity to Botulinum Neurotoxins

Botulinum neurotoxins (BoNT) are the most potent known toxins. The mouse LD₅₀ assay is the gold standard for testing BoNT potency, but is not sensitive enough to detect the extremely low levels of neurotoxin that may be present in the serum of sensitive animal species that are showing the effects of BoNT toxicity, such as channel catfish affected by visceral toxicosis of catfish. Since zebrafish are an important animal model for diverse biomedical and basic research, they are readily available and have defined genetic lines that facilitate reproducibility. This makes them attractive for use as an alternative bioassay organism. The utility of zebrafish as a bioassay model organism for BoNT was investigated. The 96 h median immobilizing doses of BoNT/A, BoNT/C, BoNT/E, and BoNT/F for adult male Tübingen strain zebrafish (0.32 g mean weight) at 25 °C were 16.31, 124.6, 4.7, and 0.61 picograms (pg)/fish, respectively. These findings support the use of the zebrafish-based bioassays for evaluating the presence of BoNT/A, BoNT/E, and BoNT/F. Evaluating the basis of the relatively high resistance of zebrafish to BoNT/C and the extreme sensitivity to BoNT/F may reveal unique functional patterns to the action of these neurotoxins.

Sensitive assays enable detection of serum IgG antibodies against Clostridium difficile toxin A and toxin B in healthy subjects and patients with Clostridium difficile infection

BACKGROUND

Pathogenic Clostridium difficile produces two proinflammatory exotoxins, toxin A and toxin B. Low level of serum antitoxin IgG antibodies is a risk factor for the development of primary and recurrent C. difficile infection (CDI).

RESULTS

We developed and validated two sensitive, titer-based electrochemiluminescence assays for the detection of serum antibody levels against C. difficile toxins A and B. These assays demonstrated excellent precision. The sensitivity of the assays allowed the detection of antitoxin A and antitoxin B IgG antibodies in all tested serum samples during assay validation.

CONCLUSION

The validated titer-based assays enable assessment of antitoxin A and antitoxin B IgG antibodies as potential biomarkers to identify patients with CDI at increased risk for CDI recurrence.

Analysis of TcdB Proteins within the Hypervirulent Clade 2 Reveals an Impact of RhoA Glucosylation on Clostridium difficile Proinflammatory Activities

Clostridium difficile strains within the hypervirulent clade 2 are responsible for nosocomial outbreaks worldwide. The increased pathogenic potential of these strains has been attributed to several factors but is still poorly understood. During a C. difficile outbreak, a strain from this clade was found to induce a variant cytopathic effect (CPE), different from the canonical arborizing CPE. This strain (NAP1V) belongs to the NAP1 genotype but to a ribotype different from the epidemic NAP1/RT027 strain. NAP1V and NAP1 share some properties, including the overproduction of toxins, the binary toxin, and mutations in tcdC. NAP1V is not resistant to fluoroquinolones, however. A comparative analysis of TcdB proteins from NAP1/RT027 and NAP1V strains indicated that both target Rac, Cdc42, Rap, and R-Ras but only the former glucosylates RhoA. Thus, TcdB from hypervirulent clade 2 strains possesses an extended substrate profile, and RhoA is crucial for the type of CPE induced. Sequence comparison and structural modeling revealed that TcdBNAP1 and TcdBNAP1V share the receptor-binding and autoprocessing activities but vary in the glucosyltransferase domain, consistent with the different substrate profile. Whereas the two toxins displayed identical cytotoxic potencies, TcdBNAP1 induced a stronger proinflammatory response than TcdBNAP1V as determined in ex vivo experiments and animal models. Since immune activation at the level of intestinal mucosa is a hallmark of C. difficile-induced infections, we propose that the panel of substrates targeted by TcdB is a determining factor in the pathogenesis of this pathogen and in the differential virulence potential seen among C. difficile strains.

Memory B Cells Encode Neutralizing Antibody Specific for Toxin B from the Clostridium difficile Strains VPI 10463 and NAP1/BI/027 but with Superior Neutralization of VPI 10463 Toxin B

Secreted toxin B (TcdB) substantially contributes to the pathology observed during Clostridium difficile infection. To be successfully incorporated into a vaccine, TcdB-based immunogens must stimulate the production of neutralizing antibody (Ab)-encoding memory B cells (Bmem cells). Despite numerous investigations, a clear analysis of Bmem cellular responses following vaccination against TcdB is lacking. B6 mice were therefore used to test the ability of a nontoxigenic C-terminal domain (CTD) fragment of TcdB to induce Bmem cells that encode TcdB-neutralizing antibody. CTD was produced from the historical VPI 10463 strain (CTD1) and from the hypervirulent strain NAP1/BI/027 (CTD2). It was then demonstrated that CTD1 induced strong recall IgG antibody titers, and this led to the development of functional Bmem cells that could be adoptively transferred to naive recipients. Bmem cell-driven neutralizing Ab responses conferred protection against lethal challenge with TcdB1. Further experiments revealed that an experimental adjuvant (Imject) and a clinical adjuvant (Alhydrogel) were compatible with Bmem cell induction. Reactivity of human Bmem cells to CTD1 was also evident in human peripheral blood mononuclear cells (PBMCs), suggesting that CTD1 could be a good vaccine immunogen. However, CTD2 induced strong Bmem cell-driven antibody titers, and the CTD2 antibody was neutralizing in vitro, but its protection against lethal challenge with TcdB2 was limited to delaying time to death. Therefore, CTD from different C. difficile strains may be a good immunogen for stimulating B cell memory that encodes in vitro neutralizing Ab but may be limited by variable protection against intoxication in vivo.

Diversity and Evolution in the Genome of Clostridium difficile

Clostridium difficile infection (CDI) is the leading cause of antimicrobial and health care-associated diarrhea in humans, presenting a significant burden to global health care systems. In the last 2 decades, PCR- and sequence-based techniques, particularly whole-genome sequencing (WGS), have significantly furthered our knowledge of the genetic diversity, evolution, epidemiology, and pathogenicity of this once enigmatic pathogen. C. difficile is taxonomically distinct from many other well-known clostridia, with a diverse population structure comprising hundreds of strain types spread across at least 6 phylogenetic clades. The C. difficile species is defined by a large diverse pangenome with extreme levels of evolutionary plasticity that has been shaped over long time periods by gene flux and recombination, often between divergent lineages. These evolutionary events are in response to environmental and anthropogenic activities and have led to the rapid emergence and worldwide dissemination of virulent clonal lineages. Moreover, genome analysis of large clinically relevant data sets has improved our understanding of CDI outbreaks, transmission, and recurrence. The epidemiology of CDI has changed dramatically over the last 15 years, and CDI may have a foodborne or zoonotic etiology. The WGS era promises to continue to redefine our view of this significant pathogen.

Toxicity assessment of Clostridium difficile toxins in rodent models and protection of vaccination

Clostridium difficile is the leading cause of hospital-acquired diarrhea, also known as C. difficile associated diarrhea. The two major toxins, toxin A and toxin B are produced by most C. difficile bacteria, but some strains, such as BI/NAP1/027 isolates, produce a third toxin called binary toxin. The precise biological role of binary toxin is not clear but it has been shown to be a cytotoxin for Vero cells. We evaluated the toxicity of these toxins in mice and hamsters and found that binary toxin causes death in both animals similar to toxins A and B. Furthermore, immunization of mice with mutant toxoids of all three toxins provided protection upon challenge with native toxins. These results support the concept that binary toxin contributes to the pathogenicity of C. difficile and provide a method for monitoring the toxicity of binary toxin components in vaccines.

Novel therapeutic strategies for Clostridium difficile infections

INTRODUCTION

In recent years, Clostridium difficile has become the primary cause of antibiotic-associated diarrhea and pseudomembranous colitis, resulting in long and complicated hospital stays that represent a serious burden for patients as well as health care systems. Currently, conservative treatment of C. difficile infection (CDI) relies on the antibiotics vancomycin, metronidazole or fidaxomicin, or in case of multiple recurrences, fecal microbiota transplantation (FMT).

AREAS COVERED

The fast-spreading, epidemic nature of this pathogen urgently necessitates the search for alternative treatment strategies as well as antibiotic targets. Accordingly, in this review, we highlight the recent findings regarding virulence associated traits of C. difficile, evaluate their potential as alternative drug targets, and present current efforts in designing inhibitory compounds, with the aim of pointing out possibilities for future treatment strategies.

EXPERT OPINION

Increased attention on systematic analysis of the virulence mechanisms of C. difficile has already led to the identification of several alternative drug targets. In the future, applying state of the art 'omics' and the development of novel infection models that mimic the human gut, a highly complex ecological niche, will unveil the genomic and metabolic plasticity of this pathogen and will certainly help dealing with future challenges.

Pathogenesis of Clostridium difficile Infection and Its Potential Role in Inflammatory Bowel Disease

Colonization with toxigenic Clostridium difficile may be associated with a wide spectrum of clinical presentation ranging from asymptomatic carriage to mild diarrhea to life-threatening colitis. Over the last 15 years, there has been a marked increase in the incidence of C. difficile infection, which predominantly affects elderly patients on antibiotics. More recently, there has been significant interest in the association between inflammatory bowel disease (IBD) and C. difficile infection. This review article discusses in some detail current knowledge of the mechanisms by which C. difficile toxins may mediate mucosal inflammation, together with the role of cell wall components of the microorganism in disease pathogenesis. Innate and adaptive host responses to C. difficile toxins and other components are described and include consideration of the potential role of known mucosal changes in IBD that may lead to an enhanced inflammatory response in the presence of C. difficile infection. Recent studies, which have characterized resident microbiota that may mediate protection against colonization by C. difficile, including their mechanisms of action, are also discussed. This includes the role of bile acids and 7α-dehydroxylase-expressing bacteria, such as Clostridium scindens. Recent studies suggest a higher carriage rate of C. difficile in patients with IBD. It is anticipated that future studies will determine the role of dysbiosis in IBD in predisposing to colonization with C. difficile.

Mechanisms of hypervirulent Clostridium difficile ribotype 027 displacement of endemic strains: an epidemiological model

Following rapid, global clonal dominance of hypervirulent ribotypes, Clostridium difficile now constitutes the primary infectious cause of nosocomial diarrhoea. Evidence indicates at least three possible mechanisms of hypervirulence that facilitates the successful invasion of these atypical strains: 1) increased infectiousness relative to endemic strains; 2) increased symptomatic disease rate relative to endemic strains; and 3) an ability to outcompete endemic strains in the host’s gut. Stochastic simulations of an infection transmission model demonstrate clear differences between the invasion potentials of C. difficile strains utilising the alternative hypervirulence mechanisms, and provide new evidence that favours certain mechanisms (1 and 2) more than others (3). Additionally, simulations illustrate that direct competition between strains (inside the host’s gut) is not a prerequisite for the sudden switching that has been observed in prevailing ribotypes; previously dominant C. difficile strains can be excluded by hypervirulent ribotypes through indirect (exploitative) competition.

A chimeric protein comprising the glucosyltransferase and cysteine proteinase domains of toxin B and the receptor binding domain of toxin A induces protective immunity against Clostridium difficile infection in mice and hamsters

Clostridium difficile is the major cause of hospital-acquired infectious diarrhea and colitis in developed countries. The pathogenicity of C. difficile is mainly mediated by the release of 2 large potent exotoxins, toxin A (TcdA) and toxin B (TcdB), both of which require neutralization to prevent disease occurrence. We have generated a novel chimeric protein, designated mTcd138, comprised of the glucosyltransferase and cysteine proteinase domains of TcdB and the receptor binding domain of TcdA and expressed it in Bacillus megaterium. To ensure that mTcd138 is atoxic, 2 point mutations were introduced to the glucosyltransferase domain of TcdB, which essentially eliminates toxicity of mTcd138. Parenteral immunizations of mice and hamsters with mTcd138 induced protective antibodies to both toxins and provided protection against infection with the hyper-virulent C. difficile strain UK6.

The prospect for vaccines to prevent Clostridium difficile infection

Clostridium difficile is a spore-forming anaerobic gram-positive organism that is the leading cause of antibiotic-associated nosocomial infectious diarrhea in the Western world. This article describes the evolving epidemiology of C difficile infection (CDI) in the twenty-first century, evaluates the importance of vaccines against the disease, and defines the roles of both innate and adaptive host immune responses in CDI. The effects of passive immunotherapy and active vaccination against CDI in both humans and animals are also discussed.

Exposure of neutralizing epitopes in the carboxyl-terminal domain of TcdB is altered by a proximal hypervariable region

The sequence, activity, and antigenicity of TcdB varies between different strains of Clostridium difficile. As a result, ribotype-specific forms of TcdB exhibit different toxicities and are not strongly cross-neutralized. Using a combination of biochemical and immunological approaches, we compared two important variants of TcdB (TcdB012 and TcdB027) to identify the mechanisms through which sequence differences alter epitopes and activity of the toxin. These analyses led to the discovery of a critical variation in the 1753-1851 (B2') region of TcdB, which affects the exposure of neutralizing epitopes in the toxin. Sequence comparisons found that the B2' region exhibits only 77% identity and is the most variable sequence between the two forms of TcdB. A combination of biochemical, analytical, and mutagenesis experiments revealed that the B2' region promotes protein-protein interactions. These interactions appear to shield neutralizing epitopes that would otherwise be exposed in the toxin, an event found to be less prominent in TcdB012 due to sequence differences in the 1773-1780 and 1791-1798 regions of the B2' domain. When the carboxyl-terminal domains of TcdB012 and TcdB027 are swapped, neutralization experiments suggest that the amino terminus of TcdB interacts with the B2' region and impacts the exposure of neutralizing epitopes in the carboxyl terminus. Collectively, these data suggest that variations in the B2' region affect protein-protein interactions within TcdB and that these interactions influence the exposure of neutralizing epitopes.

Clostridium difficile virulence factors: Insights into an anaerobic spore-forming pathogen

The worldwide emergence of epidemic strains of Clostridium difficile linked to increased disease severity and mortality has resulted in greater research efforts toward determining the virulence factors and pathogenesis mechanisms used by this organism to cause disease. C. difficile is an opportunist pathogen that employs many factors to infect and damage the host, often with devastating consequences. This review will focus on the role of the 2 major virulence factors, toxin A (TcdA) and toxin B (TcdB), as well as the role of other putative virulence factors, such as binary toxin, in C. difficile-mediated infection. Consideration is given to the importance of spores in both the initiation of disease and disease recurrence and also to the role that surface proteins play in host interactions.

Clostridium difficile infection: molecular pathogenesis and novel therapeutics

The Gram-positive anaerobic bacterium Clostridium difficile produces toxins A and B, which can cause a spectrum of diseases from pseudomembranous colitis to C. difficile-associated diarrhea. A limited number of C. difficile strains also produce a binary toxin that exhibits ADP ribosyltransferase activity. Here, the structure and the mechanism of action of these toxins as well as their role in disease are reviewed. Nosocomial C. difficile infection is often contracted in hospital when patients treated with antibiotics suffer a disturbance in normal gut microflora. C. difficile spores can persist on dry, inanimate surface for months. Metronidazole and oral vancomycin are clinically used for treatment of C. difficile infection but clinical failure and concern about promotion of resistance are motivating the search for novel non-antibiotic therapeutics. Methods for controlling both toxins and spores, replacing gut microflora by probiotics or fecal transplant, and killing bacteria in the anaerobic gut by photodynamic therapy are discussed.

Clostridium difficile infection: prevention, treatment, and surgical management

Clostridium difficile is increasing in both incidence and severity. Although metronidazole and vancomycin remain the gold standard for medical management, and surgical colectomy the gold standard for surgical management, new treatment alternatives, including the creation of a diverting loop ileostomy along with colonic lavage and vancomycin enemas, are being investigated that may lead to changes in the current treatment algorithms. The most exciting development in the treatment options for C difficile infection, however, is likely to be novel immunologic agents, which hold the potential to reduce the incidence, mortality, and costs associated with C difficile.

The roles of host and pathogen factors and the innate immune response in the pathogenesis of Clostridium difficile infection

Clostridium difficile (C. difficile) is the most common cause of nosocomial antibiotic-associated diarrhea and the etiologic agent of pseudomembranous colitis. The clinical manifestation of C. difficile infection (CDI) is highly variable, from asymptomatic carriage, to mild self-limiting diarrhea, to the more severe pseudomembranous colitis. Furthermore, in extreme cases, colonic inflammation and tissue damage can lead to toxic megacolon, a condition requiring surgical intervention. C. difficile expresses two key virulence factors; the exotoxins, toxin A (TcdA) and toxin B (TcdB), which are glucosyltransferases that target host-cell monomeric GTPases. In addition, some hypervirulent strains produce a third toxin, binary toxin or C. difficile transferase (CDT), which may contribute to the pathogenesis of CDI. More recently, other factors such as surface layer proteins (SLPs) and flagellin have also been linked to the inflammatory responses observed in CDI. Although the adaptive immune response can influence the severity of CDI, the innate immune responses to C. difficile and its toxins play crucial roles in CDI onset, progression, and overall prognosis. Despite this, the innate immune responses in CDI have drawn relatively little attention from clinical researchers. Targeting these responses may prove useful clinically as adjuvant therapies, especially in refractory and/or recurrent CDI. This review will focus on recent advances in our understanding of how C. difficile and its toxins modulate innate immune responses that contribute to CDI pathogenesis.

Clostridium difficile Infection: Clinical Challenges and Management Strategies

Clostridium difficile has become the leading cause of nosocomial diarrhea in adults. A substantial increase has occurred in morbidity and mortality associated with disease caused by C difficile and in the identification of new hypervirulent strains, warranting a high clinical index of suspicion for infections due to this organism. Prevention of infection requires a multidisciplinary approach, including early recognition of disease, effective contact isolation precautions, adherence to disinfectant policies, and judicious use of antibiotics. Current treatment approaches are based on the severity of illness. As hypervirulent strains evolve, unsuccessful treatments are more common. Complicated colitis caused by C difficile may benefit from surgical intervention. Subtotal colectomy and end ileostomy have been the procedures of choice, but are associated with a high mortality rate because of late surgical consultation and use of surgery as a salvage therapy. A promising surgical alternative is creation of a diverting loop ileostomy with colonic lavage.

Variations in virulence and molecular biology among emerging strains of Clostridium difficile

Clostridium difficile is a Gram-positive, spore-forming organism which infects and colonizes the large intestine, produces potent toxins, triggers inflammation, and causes significant systemic complications. Treating C. difficile infection (CDI) has always been difficult, because the disease is both caused and resolved by antibiotic treatment. For three and a half decades, C. difficile has presented a treatment challenge to clinicians, and the situation took a turn for the worse about 10 years ago. An increase in epidemic outbreaks related to CDI was first noticed around 2003, and these outbreaks correlated with a sudden increase in the mortality rate of this illness. Further studies discovered that these changes in CDI epidemiology were associated with the rapid emergence of hypervirulent strains of C. difficile, now collectively referred to as NAP1/BI/027 strains. The discovery of new epidemic strains of C. difficile has provided a unique opportunity for retrospective and prospective studies that have sought to understand how these strains have essentially replaced more historical strains as a major cause of CDI. Moreover, detailed studies on the pathogenesis of NAP1/BI/027 strains are leading to new hypotheses on how this emerging strain causes severe disease and is more commonly associated with epidemics. In this review, we provide an overview of CDI, discuss critical mechanisms of C. difficile virulence, and explain how differences in virulence-associated factors between historical and newly emerging strains might explain the hypervirulence exhibited by this pathogen during the past decade.

Antibodies for treatment of Clostridium difficile infection

Antibodies for the treatment of Clostridium difficile infection (CDI) have been demonstrated to be effective in the research and clinical environments. Early uncertainties about molecular and treatment modalities now appear to have converged upon the systemic dosing of mixtures of human IgG1. Although multiple examples of high-potency monoclonal antibodies (MAbs) exist, significant difficulties were initially encountered in their discovery. This minireview describes historical and contemporary MAbs and highlights differences between the most potent MAbs, which may offer insight into the pathogenesis and treatment of CDI.

The role of the humoral immune response to Clostridium difficile toxins A and B in susceptibility to C. difficile infection: a case-control study

Antibody levels to Clostridium difficile toxin A (TcdA), but not toxin B (TcdB), have been found to determine risk of C. difficile infection (CDI). Historically, TcdA was thought to be the key virulence factor; however the importance of TcdB in disease is now established. We re-evaluated the role of antibodies to TcdA and TcdB in determining patient susceptibility to CDI in two separate patient cohorts. In contrast to earlier studies, we find that CDI patients have lower pre-existing IgA titres to TcdB, but not TcdA, when compared to control patients. Our findings suggest that mucosal immunity to TcdB may be important in the early stages of infection and identifies a possible target for preventing CDI progression.

A novel multivalent, single-domain antibody targeting TcdA and TcdB prevents fulminant Clostridium difficile infection in mice

The incidence of Clostridium difficile infection (CDI) and associated mortality have increased rapidly worldwide in recent years. Therefore, it is critical to develop new therapies for CDI. In this study, we generated a novel, potently neutralizing, tetravalent, and bispecific antibody composed of 2 heavy-chain-only VH (VHH) binding domains against both TcdA and TcdB (designated "ABA") that reverses fulminant CDI in mice infected with an epidemic 027 strain after a single injection of the antibody. We demonstrated that ABA bound to both toxins simultaneously and displayed a significantly enhanced neutralizing activity both in vitro and in vivo. Additionally, ABA was able to broadly neutralize toxins from clinical C. difficile isolates that express both TcdA and TcdB but failed to neutralize the toxin from TcdA(-)TcdB(+) C. difficile strains. This study thus provides a rationale for the development of multivalent VHHs that target both toxins and are broadly neutralizing for treating severe CDI.

Translocation domain mutations affecting cellular toxicity identify the Clostridium difficile toxin B pore

Disease associated with Clostridium difficile infection is caused by the actions of the homologous toxins TcdA and TcdB on colonic epithelial cells. Binding to target cells triggers toxin internalization into acidified vesicles, whereupon cryptic segments from within the 1,050-aa translocation domain unfurl and insert into the bounding membrane, creating a transmembrane passageway to the cytosol. Our current understanding of the mechanisms underlying pore formation and the subsequent translocation of the upstream cytotoxic domain to the cytosol is limited by the lack of information available regarding the identity and architecture of the transmembrane pore. Here, through systematic perturbation of conserved sites within predicted membrane-insertion elements of the translocation domain, we uncovered highly sensitive residues--clustered between amino acids 1,035 and 1,107--that when individually mutated, reduced cellular toxicity by as much as >1,000-fold. We demonstrate that defective variants are defined by impaired pore formation in planar lipid bilayers and biological membranes, resulting in an inability to intoxicate cells through either apoptotic or necrotic pathways. These findings along with the unexpected similarities uncovered between the pore-forming "hotspots" of TcdB and the well-characterized α-helical diphtheria toxin translocation domain provide insights into the structure and mechanism of formation of the translocation pore for this important class of pathogenic toxins.

Zebrafish (Danio rerio) bioassay for visceral toxicosis of catfish and botulinum neurotoxin serotype E

Visceral toxicosis of catfish (VTC), a sporadic disease of cultured channel catfish ( Ictalurus punctatus) often with high mortality, is caused by botulinum neurotoxin serotype E (BoNT/E). Presumptive diagnosis of VTC is based on characteristic clinical signs and lesions, and the production of these signs and mortality after sera from affected fish is administered to sentinel catfish. The diagnosis is confirmed if the toxicity is neutralized with BoNT/E antitoxin. Because small catfish are often unavailable, the utility of adult zebrafish ( Danio rerio) was evaluated in BoNT/E and VTC bioassays. Channel catfish and zebrafish susceptibilities were compared using trypsin-activated BoNT/E in a 96-hr trial by intracoelomically administering 0, 1.87, 3.7, 7.5, 15, or 30 pg of toxin per gram of body weight (g-bw) of fish. All of the zebrafish died at the 7.5 pg/g-bw and higher, while the catfish died at the 15 pg/g-bw dose and higher. To test the bioassay, sera from VTC-affected fish or control sera were intracoelomically injected at a dose of 10 µl per zebrafish and 20 µl/g-bw for channel catfish. At 96 hr post-injection, 78% of the zebrafish and 50% of the catfish receiving VTC sera died, while no control fish died. When the VTC sera were preincubated with BoNT/E antitoxin, they became nontoxic to zebrafish. Histology of zebrafish injected with either VTC serum or BoNT/E demonstrated renal necrosis. Normal catfish serum was toxic to larval zebrafish in immersion exposures, abrogating their utility in VTC bioassays. The results demonstrate bioassays using adult zebrafish for detecting BoNT/E and VTC are sensitive and practical.

Diagnosis of Clostridium difficile infection: an ongoing conundrum for clinicians and for clinical laboratories

Clostridium difficile is a formidable nosocomial and community-acquired pathogen, causing clinical presentations ranging from asymptomatic colonization to self-limiting diarrhea to toxic megacolon and fulminant colitis. Since the early 2000s, the incidence of C. difficile disease has increased dramatically, and this is thought to be due to the emergence of new strain types. For many years, the mainstay of C. difficile disease diagnosis was enzyme immunoassays for detection of the C. difficile toxin(s), although it is now generally accepted that these assays lack sensitivity. A number of molecular assays are commercially available for the detection of C. difficile. This review covers the history and biology of C. difficile and provides an in-depth discussion of the laboratory methods used for the diagnosis of C. difficile infection (CDI). In addition, strain typing methods for C. difficile and the evolving epidemiology of colonization and infection with this organism are discussed. Finally, considerations for diagnosing C. difficile disease in special patient populations, such as children, oncology patients, transplant patients, and patients with inflammatory bowel disease, are described. As detection of C. difficile in clinical specimens does not always equate with disease, the diagnosis of C. difficile infection continues to be a challenge for both laboratories and clinicians.

Small animal models for the study of Clostridium difficile disease pathogenesis

Clostridium difficile is the leading cause of bacterial antibiotic-associated diarrhoea in hospitals in the developed world. Despite this notoriety, the complex mechanisms employed by this pathogen to overcome innate host defences and induce fulminant disease are poorly understood. Various animal models have been used extensively for C. difficile research to study disease pathogenesis. Until recently, the most commonly used C. difficile disease model has utilised hamsters; however, mouse and pig models have now been developed that unravel different aspects of C. difficile pathology. This review summarises key aspects of the small animal models currently used in C. difficile studies with a specific focus on major differences between them. Furthermore, this review highlights the advantages and disadvantages of each model and illustrates that careful consideration is required when selecting models for use in C. difficile research.

Hype or hypervirulence: a reflection on problematic C. difficile strains

Clostridium difficile infections (CDI) have emerged as a major cause of healthcare associated disease, and recent epidemiological evidence also suggests an important role in community-acquired diarrhea. This increase is associated with specific types, especially PCR ribotypes 027 and 078, which are sometimes referred to as “hypervirulent”. Over the past years major advances have been made in our understanding of C. difficile pathogenicity, with the identification and characterization of the major clostridial toxins TcdA and TcdB. However, the relation between the toxins, their regulation, and “hypervirulence” remain unclear. Here I review our current understanding of C. difficile pathogenicity and argue that “hypervirulent” is an inadequate term to describe PCR ribotypes 027 and 078, that the ability of C. difficile to cause problematic infections is a consequence of a multifactorial process that extends beyond toxins, sporulation, and antimicrobial resistance, and that vigilance is in order toward types that are closely related to ribotypes 027 and 078, but are currently not considered problematic.

Applications of small molecule probes in dissecting mechanisms of bacterial virulence and host responses

Elucidating the molecular and biochemical details of bacterial infections can be challenging because of the many complex interactions that exist between a pathogen and its host. Consequently, many tools have been developed to aid the study of bacterial pathogenesis. Small molecules are a valuable complement to traditional genetic techniques because they can be used to rapidly perturb genetically intractable systems and to monitor post-translationally regulated processes. Activity-based probes are a subset of small molecules that covalently label an enzyme of interest based on its catalytic mechanism. These tools allow monitoring of enzyme activation within the context of a native biological system and can be used to dissect the biochemical details of enzyme function. This review describes the development and application of activity-based probes for examining aspects of bacterial infection on both sides of the host-pathogen interface.

Clostridium difficile 027/BI/NAP1 encodes a hypertoxic and antigenically variable form of TcdB

The Clostridium difficile exotoxin, TcdB, which is a major virulence factor, varies between strains of this pathogen. Herein, we show that TcdB from the epidemic BI/NAP1/027 strain of C. difficile is more lethal, causes more extensive brain hemorrhage, and is antigenically variable from TcdB produced by previously studied strains of this pathogen (TcdB₀₀₃). In mouse intoxication assays, TcdB from a ribotype 027 strain (TcdB₀₂₇) was at least four fold more lethal than TcdB₀₀₃. TcdB₀₂₇ caused a previously undescribed brain hemorrhage in mice and this correlated with a heightened sensitivity of brain microvascular endothelial cells to the toxin. TcdB₀₀₃ and TcdB₀₂₇ also differed in their antigenic profiles and did not share cross-neutralizing epitopes in a major immunogenic region of the protein. Solid phase humoral mapping of epitopes in the carboxy-terminal domains (CTD) of TcdB₀₂₇ and TcdB₀₀₃ identified 11 reactive epitopes that varied between the two forms of TcdB, and 13 epitopes that were shared or overlapping. Despite the epitope differences and absence of neutralizing epitopes in the CTD of TcdB₀₂₇, a toxoid form of this toxin primed a strong protective response. These findings indicate TcdB₀₂₇ is a more potent toxin than TcdB₀₀₃ as measured by lethality assays and pathology, moreover the sequence differences between the two forms of TcdB alter antigenic epitopes and reduce cross-neutralization by antibodies targeting the CTD.

During the past decade, the C. difficile BI/NAP1/027 strain has emerged and in some settings predominated as the cause of C. difficile infection. Moreover, in some reports C. difficile BI/NAP1/027 has been associated with more severe disease. The reasons for association of this strain with more severe disease and relapse are poorly understood. We compared the toxicity and antigenic profiles of the major C. difficile virulence factor, TcdB, from a previously studied reference strain and a BI/NAP1/027 strain. The results indicate TcdB₀₂₇, the toxin from the BI/NAP1/027 strain, is more lethal and causes more extensive brain hemorrhaging than TcdB₀₀₃, the toxin produced by a reference strain of C. difficile. Furthermore, the results show that the antigenic carboxy-terminal domain (CTD) encodes at least 11 epitopes that differ between the two forms of TcdB. In line with this, experiments demonstrate that antiserum against the CTD does not cross-neutralize TcdB₀₀₃ and TcdB₀₂₇ toxicity against CHO cells, and TcdB₀₂₇ appears to be devoid of neutralizing epitopes in this domain. These findings indicate differences in TcdB₀₀₃ and TcdB₀₂₇ contribute to increased virulence of C. difficile BI/NAP1/027 and reduce the likelihood of acquired immunity providing cross-protection against infection by these strains.

Caspase activation as a versatile assay platform for detection of cytotoxic bacterial toxins

Pathogenic bacteria produce several virulence factors that help them establish infection in permissive hosts. Bacterial toxins are a major class of virulence factors and hence are attractive therapeutic targets for vaccine development. Here, we describe the development of a rapid, sensitive, and high-throughput assay that can be used as a versatile platform to measure the activities of bacterial toxins. We have exploited the ability of these toxins to cause cell death via apoptosis of sensitive cultured cell lines as a readout for measuring toxin activity. Caspases (cysteine-aspartic proteases) are induced early in the apoptotic pathway, and so we used their induction to measure the activities of Clostridium difficile toxins A (TcdA) and B (TcdB) and binary toxin (CDTa-CDTb), Corynebacterium diphtheriae toxin (DT), and Pseudomonas aeruginosa exotoxin A (PEA). Caspase induction in the cell lines, upon exposure to toxins, was optimized by toxin concentration and intoxication time, and the specificity of caspase activity was established using a genetically mutated toxin and a pan-caspase inhibitor. In addition, we demonstrate the utility of the caspase assay for measuring toxin potency, as well as neutralizing antibody (NAb) activity against C. difficile toxins. Furthermore, the caspase assay showed excellent correlation with the filamentous actin (F-actin) polymerization assay for measuring TcdA and TcdB neutralization titers upon vaccination of hamsters. These results demonstrate that the detection of caspase induction due to toxin exposure using a chemiluminescence readout can support potency and clinical immunogenicity testing for bacterial toxin vaccine candidates in development.

Susceptibility of hamsters to Clostridium difficile isolates of differing toxinotype

Clostridium difficile is the most commonly associated cause of antibiotic associated disease (AAD), which caused ∼21,000 cases of AAD in 2011 in the U.K. alone. The golden Syrian hamster model of CDI is an acute model displaying many of the clinical features of C. difficile disease. Using this model we characterised three clinical strains of C. difficile, all differing in toxinotype; CD1342 (PaLoc negative), M68 (toxinotype VIII) & BI-7 (toxinotype III). The naturally occurring non-toxic strain colonised all hamsters within 1-day post challenge (d.p.c.) with high-levels of spores being shed in the faeces of animals that appeared well throughout the entire experiment. However, some changes including increased neutrophil influx and unclotted red blood cells were observed at early time points despite the fact that the known C. difficile toxins (TcdA, TcdB and CDT) are absent from the genome. In contrast, hamsters challenged with strain M68 resulted in a 45% mortality rate, with those that survived challenge remaining highly colonised. It is currently unclear why some hamsters survive infection, as bacterial & toxin levels and histology scores were similar to those culled at a similar time-point. Hamsters challenged with strain BI-7 resulted in a rapid fatal infection in 100% of the hamsters approximately 26 hr post challenge. Severe caecal pathology, including transmural neutrophil infiltrates and extensive submucosal damage correlated with high levels of toxin measured in gut filtrates ex vivo. These data describes the infection kinetics and disease outcomes of 3 clinical C. difficile isolates differing in toxin carriage and provides additional insights to the role of each toxin in disease progression.

Detection of Clostridium difficile toxins A, B and binary toxin with slow off-rate modified aptamers

Rapid and accurate diagnostic tests for Clostridium difficile infections (CDI) are crucial for management of patients with suspected CDI and for infection control. Enzyme immunoassays for detection of the toxins are routinely used but lack adequate sensitivity. We generated slow off-rate modified aptamers (SOMAmer™ reagents) via in vitro selection (SELEX) that bind toxins A, B and binary toxin with high affinity and specificity. Using SOMAmers alone or in conjunction with antibodies, we have developed toxin assays with a 1 pmol/L (300 pg/mL) limit of detection and a 3 log dynamic range. SOMAmers proved useful as capture or detection agents in equilibrium solution binding radioassays, pull-down capture assays, dot blots, and plate- or membrane-based sandwich assays, thus represent a promising alternative to antibodies in diagnostic applications. SOMAmers detected toxins A, B and binary toxin in culture supernatants from toxigenic C. difficile, including a BI/NAP1 strain and historic strains.

A segment of 97 amino acids within the translocation domain of Clostridium difficile toxin B is essential for toxicity

Clostridium difficile toxin B (TcdB) intoxicates target cells by glucosylating Rho GTPases. TcdB (269 kDa) consists of at least 4 functional domains including a glucosyltransferase domain (GTD), a cysteine protease domain (CPD), a translocation domain (TD), and a receptor binding domain (RBD). The function and molecular mode of action of the TD, which is the largest segment of TcdB and comprises nearly 50% of the protein, remain largely unknown. Here we show that a 97-amino-acid segment (AA1756 - 1852, designated as ?97 or D97), located in the C-terminus of the TD and adjacent to the RBD, is essential for the cellular activity of TcdB. Deletion of this segment in TcdB (designated as TxB-D97), did not adversely alter toxin enzymatic activities or its cellular binding and uptake capacity. TxB-D97 bound to and entered cells in a manner similar to TcdB holotoxin. Both wild type and mutant toxins released their GTDs similarly in the presence of inositol hexakisphosphate (InsP6), and showed a similar glucosyltransferase activity in a cell-free glucosylating assay. Despite these similarities, the cytotoxic activity of TxB-D97 was reduced by more than 5 logs compared to wild type toxin, supported by the inability of TxB-D97 to glucosylate Rac1 of target cells. Moreover, the mutant toxin failed to elicit tumor necrosis factor alpha (TNF-α) in macrophages, a process dependent on the glucosyltransferase activity of the toxin. Cellular fractionation of toxin-exposed cells revealed that TxB-D97 was unable to efficiently release the GTD into cytosol. Thereby, we conclude the 97-amino-acid region of the TD C-terminus of TcdB adjacent to the RBD, is essential for the toxicity of TcdB.

Using phenotype microarrays to determine culture conditions that induce or repress toxin production by Clostridium difficile and other microorganisms

Toxin production is a central issue in the pathogenesis of Clostridium difficile and many other pathogenic microorganisms. Toxin synthesis is influenced by a variety of known and unknown factors of genetics, physiology, and environment. To facilitate the study of toxin production by C. difficile, we have developed a new, reliable, quantitative, and robust cell-based cytotoxicity assay. Then we combined this new assay with Phenotype MicroArrays (PM) technology which provides high throughput testing of culture conditions. This allowed us to quantitatively measure toxin production by C. difficile type strain ATCC 9689 under 768 culture conditions. The culture conditions include different carbon, nitrogen, phosphorus, and sulfur sources. Among these, 89 conditions produced strong toxin induction and 31 produced strong toxin repression. Strong toxin inducers included adenine, guanosine, arginine dipeptides, γ-D-Glu-Gly, methylamine, and others. Some leucine dipeptides and the triple-leucine tripeptide were among the strongest toxin repressors. While some results are consistent with previous observations, others are new observations that provide insights into toxin regulation and pathogenesis of C. difficile. Additionally, we have demonstrated that this combined assay technology can be applied broadly to a wide range of toxin producing microorganisms. This study is the first demonstration of simultaneous assessment of a large number of culture conditions influencing bacterial toxin production. The new functional cytotoxin quantitation method developed provides a valuable tool for studying toxigenic microorganisms and may also find applications in clinical and epidemiological research.

Cytotoxicity of Clostridium difficile toxin B does not require cysteine protease-mediated autocleavage and release of the glucosyltransferase domain into the host cell cytosol

Clostridium difficile virulence requires secretion of two exotoxins: TcdA and TcdB. The precise mechanism of toxin uptake and delivery is undefined, but current models predict that the cysteine protease domain (CPD)-mediated autocleavage and release of glucosyltransferase domain (GTD) are crucial for intoxication. To determine the importance of CPD-mediated cleavage to TcdB cytotoxicity, we generated two mutant toxins--TcdB-C698S and TcdB-H653A--and assayed their abilities to intoxicate cells. The CPD mutants include an intact GTD but lack the cysteine protease activity. The mutants had reduced potency in that their effect on cells was delayed and required higher concentrations than wild-type TcdB. They did eventually cause cell rounding, glucosylation of Rho GTPases, and apoptosis that was indistinguishable from that caused by TcdB. Although the mutant toxins caused a complete cell rounding, they failed to release their GTD into cytosol, whereas wild-type TcdB displayed significant autocleavage and release of GTD. We conclude that the cysteine protease-mediated autocleavage and release of GTD is not a prerequisite for the cytotoxic activity of TcdB, but rather limits the potency and speed of Rho GTPase glucosylation. Our findings revise and refine the current model for the mode of the action and cellular trafficking of TcdB.

Fidaxomicin inhibits toxin production in Clostridium difficile

OBJECTIVES

Fidaxomicin, which was recently approved for the treatment of Clostridium difficile-associated diarrhoea, demonstrates narrow-spectrum bactericidal activity via inhibition of RNA polymerase. In this study we evaluated its inhibitory activity versus C. difficile toxin gene expression and toxin production by quantifying toxin mRNA and protein.

METHODS

The effects of fidaxomicin, its major metabolite (OP-1118), vancomycin and metronidazole on toxin A and toxin B production were determined by assaying culture supernatants of two C. difficile isolates (ATCC 43255, a high-level toxin-producing strain, and UK-14, a NAP1/027/BI epidemic strain) using a commercial ELISA. The effects of the drugs on toxin gene expression were assessed in stationary-phase cells of C. difficile strain UK-1 (NAP1/027/BI type epidemic strain) and in the closely related non-epidemic strain CD196 by quantitative RT-PCR.

RESULTS

Subinhibitory levels (1/4× MIC) of fidaxomicin or OP-1118 (but not vancomycin or metronidazole) strongly suppressed toxin production in C. difficile (≥ 60%) through at least 1 week of culture. Additionally, transcripts from the pathogenicity loci (tcdR, tcdA and tcdB) were nearly completely inhibited by both fidaxomicin (2× MIC) and OP-1118 (2.5× MIC), but not vancomycin (2.5× MIC).

CONCLUSIONS

Both fidaxomicin and OP-1118 are able to inhibit toxin production in vitro, which may explain prior post-treatment observations of less frequent detectable toxin in fidaxomicin-treated patients (27 subjects) than those treated with vancomycin (8 patients).

Targeted restoration of the intestinal microbiota with a simple, defined bacteriotherapy resolves relapsing Clostridium difficile disease in mice

Relapsing C. difficile disease in humans is linked to a pathological imbalance within the intestinal microbiota, termed dysbiosis, which remains poorly understood. We show that mice infected with epidemic C. difficile (genotype 027/BI) develop highly contagious, chronic intestinal disease and persistent dysbiosis characterized by a distinct, simplified microbiota containing opportunistic pathogens and altered metabolite production. Chronic C. difficile 027/BI infection was refractory to vancomycin treatment leading to relapsing disease. In contrast, treatment of C. difficile 027/BI infected mice with feces from healthy mice rapidly restored a diverse, healthy microbiota and resolved C. difficile disease and contagiousness. We used this model to identify a simple mixture of six phylogenetically diverse intestinal bacteria, including novel species, which can re-establish a health-associated microbiota and clear C. difficile 027/BI infection from mice. Thus, targeting a dysbiotic microbiota with a defined mixture of phylogenetically diverse bacteria can trigger major shifts in the microbial community structure that displaces C. difficile and, as a result, resolves disease and contagiousness. Further, we demonstrate a rational approach to harness the therapeutic potential of health-associated microbial communities to treat C. difficile disease and potentially other forms of intestinal dysbiosis.

Pathological imbalances within the intestinal microbiota, termed dysbiosis, are often associated with chronic Clostridium difficile infections in humans. We show that infection of mice with the healthcare pathogen C. difficile leads to persistent intestinal dysbiosis that is associated with chronic disease and a highly contagious state. Using this model we rationally designed a simple mixture of phylogenetically diverse intestinal bacteria that can disrupt intestinal dysbiosis and as a result resolve disease and contagiousness. Our results validate the microbiota as a viable therapeutic target and open the way to rationally design bacteriotherapy to treat chronic C. difficile infections and potentially other forms of persistent dysbiosis.

Immunization strategies for Clostridium difficile infections

Clostridium difficile infection is a major cause of nosocomial disease in Western countries. The recent emergence of hypervirulent strains resistant to most antibiotics correlates with increasing disease incidence, severity and lethal outcomes. Current treatments rely on metronidazol and vancomycin, but the limited ability of these antibiotics to cure infection and prevent relapse highlights the need for new strategies. A better knowledge of the molecular mechanisms of the disease, the host immune response and identification of key virulence factors of Clostridium difficile now permits the development of new products specifically targeting the pathogen. Immune-based strategies relying on active vaccination or passive administration of antibody products are the focus of intense research and, today, the efficacy of monoclonal antibodies and of two vaccines are evaluated clinically. This review presents recent data, discusses the different strategies and highlights the challenges linked to the development of immunization strategies against this emerging threat.

Protection against Clostridium difficile infection with broadly neutralizing antitoxin monoclonal antibodies

The spore-forming bacterium Clostridium difficile represents the principal cause of hospital-acquired diarrhea and pseudomembranous colitis worldwide. C. difficile infection (CDI) is mediated by 2 bacterial toxins, A and B; neutralizing these toxins with monoclonal antibodies (mAbs) provides a potential nonantibiotic strategy for combating the rising prevalence, severity, and recurrence of CDI. Novel antitoxin mAbs were generated in mice and were humanized. The humanized antitoxin A mAb PA-50 and antitoxin B mAb PA-41 have picomolar potencies in vitro and bind to novel regions of the respective toxins. In a hamster model for CDI, 95% of animals treated with a combination of humanized PA-50 and PA-41 showed long-term survival relative to 0% survival of animals treated with standard antibiotics or comparator mAbs. These humanized mAbs provide insight into C. difficile intoxication and hold promise as potential nonantibiotic agents for improving clinical management of CDI.

A chimeric toxin vaccine protects against primary and recurrent Clostridium difficile infection

The global emergence of Clostridium difficile infection (CDI) has contributed to the recent surge in severe antibiotic-associated diarrhea and colonic inflammation. C. difficile produces two homologous glucosylating exotoxins, TcdA and TcdB, both of which are pathogenic and require neutralization to prevent disease occurrence. However, because of their large size and complex multifunctional domain structures, it has been a challenge to produce native recombinant toxins that may serve as vaccine candidates. Here, we describe a novel chimeric toxin vaccine that retains major neutralizing epitopes from both toxins and confers complete protection against primary and recurrent CDI in mice. Using a nonpathogenic Bacillus megaterium expression system, we generated glucosyltransferase-deficient holotoxins and demonstrated their loss of toxicity. The atoxic holotoxins induced potent antitoxin neutralizing antibodies showing little cross-immunogenicity or protection between TcdA and TcdB. To facilitate simultaneous protection against both toxins, we generated an active clostridial toxin chimera by switching the receptor binding domain of TcdB with that of TcdA. The toxin chimera was fully cytotoxic and showed potent proinflammatory activities. This toxicity was essentially abolished in a glucosyltransferase-deficient toxin chimera, cTxAB. Parenteral immunization of mice or hamsters with cTxAB induced rapid and potent neutralizing antibodies against both toxins. Complete and long-lasting disease protection was conferred by cTxAB vaccinations against both laboratory and hypervirulent C. difficile strains. Finally, prophylactic cTxAB vaccination prevented spore-induced disease relapse, which constitutes one of the most significant clinical issues in CDI. Thus, the rational design of recombinant chimeric toxins provides a novel approach for protecting individuals at high risk of developing CDI.

Toward a structural understanding of Clostridium difficile toxins A and B

Clostridium difficile is a toxin-producing bacterium that is a frequent cause of hospital-acquired and antibiotic-associated diarrhea. The incidence, severity, and costs associated with C. difficile associated disease are substantial and increasing, making C. difficile a significant public health concern. The two primary toxins, TcdA and TcdB, disrupt host cell function by inactivating small GTPases that regulate the actin cytoskeleton. This review will discuss the role of these two toxins in pathogenesis and the structural and molecular mechanisms by which they intoxicate cells. A focus will be placed on recent publications highlighting mechanistic similarities and differences between TcdA, TcdB, and different TcdB variants.