Clostridial gas gangrene. I. Cellular and molecular mechanisms of microvascular dysfunction induced by exotoxins of Clostridium perfringens

Real-time identification of life-threatening necrotizing soft-tissue infections using indocyanine green fluorescence imaging

Significance

Necrotizing soft-tissue infections (NSTIs) are life-threatening infections with a cumulative case fatality rate of 21%. The initial presentation of an NSTI is non-specific, frequently leading to misdiagnosis and delays in care. No current strategies yield an accurate, real-time diagnosis of an NSTI.

Aim

A first-in-kind, observational, clinical pilot study tested the hypothesis that measurable fluorescence signal voids occur in NSTI-affected tissues following intravenous administration and imaging of perfusion-based indocyanine green (ICG) fluorescence. This hypothesis is based on the established knowledge that NSTI is associated with local microvascular thrombosis.

Approach

Adult patients presenting to the Emergency Department of a tertiary care medical center at high risk for NSTI were prospectively enrolled and imaged with a commercial fluorescence imager. Single-frame fluorescence snapshot and first-pass perfusion kinetic parameters-ingress slope (IS), time-to-peak (TTP) intensity, and maximum fluorescence intensity (IMAX)-were quantified using a dynamic contrast-enhanced fluorescence imaging technique. Clinical variables (comorbidities, blood laboratory values), fluorescence parameters, and fluorescence signal-to-background ratios (SBRs) were compared to final infection diagnosis.

Results

Fourteen patients were enrolled and imaged (six NSTI, six cellulitis, one diabetes mellitus-associated gangrene, and one osteomyelitis). Clinical variables demonstrated no statistically significant differences between NSTI and non-NSTI patient groups (p -value ≥ 0.22 ). All NSTI cases exhibited prominent fluorescence signal voids in affected tissues, including tissue features not visible to the naked eye. All cellulitis cases exhibited a hyperemic response with increased fluorescence and no distinct signal voids. Median lesion-to-background tissue SBRs based on snapshot, IS, TTP, and IMAX parameter maps ranged from 3.2 to 9.1, 2.2 to 33.8, 1.0 to 7.5, and 1.5 to 12.7, respectively, for the NSTI patient group. All fluorescence parameters except TTP demonstrated statistically significant differences between NSTI and cellulitis patient groups (p -value < 0.05 ).

Conclusions

Real-time, accurate discrimination of NSTIs compared with non-necrotizing infections may be possible with perfusion-based ICG fluorescence imaging.

Anaerobes in diabetic foot infections: pathophysiology, epidemiology, virulence, and management

SUMMARYDiabetic foot infections (DFI) are a public health problem worldwide. DFI are polymicrobial, biofilm-associated infections involving complex bacterial communities organized in functional equivalent pathogroups, all including anaerobes. Indeed, multiple pathophysiological factors favor the growth of anaerobes in this context. However, the prevalence, role, and contribution of anaerobes in wound evolution remain poorly characterized due to their challenging detection. Studies based on culture reviewed herein showed a weighted average of 17% of patients with anaerobes. Comparatively, the weighted average of patients with anaerobes identified by 16S rRNA gene sequencing was 83.8%. Culture largely underestimated not only the presence but also the diversity of anaerobes compared with cultivation-independent approaches but both methods showed that anaerobic Gram-negative bacilli and Gram-positive cocci were the most commonly identified in DFI. Anaerobes were more present in deeper lesions, and their detection was associated with fever, malodorous lesions, and ulcer depth and duration. More specifically, initial abundance of Peptoniphilus spp. was associated with ulcer-impaired healing, Fusobacterium spp. detection was significantly correlated with the duration of DFI, and the presence of Bacteroides spp. was significantly associated with amputation. Antimicrobial resistance of anaerobes in DFI remains slightly studied and warrants more consideration in the context of increasing resistance of the most frequently identified anaerobes in DFI. The high rate of patients with DFI-involving anaerobes, the increased knowledge on the species identified, their virulence factors, and their potential role in wound evolution support recommendations combining debridement and antibiotic therapy effective on anaerobes in moderate and severe DFI.

Clostridium perfringens Empyema: Anaerobic Invasion in an Uncommon Location

Clostridium perfringens bacteremia arises due to skin inoculation from the external environment or translocation from the gastrointestinal tract. In the event of bacteremia, it tends to colonize in anaerobic environments due to its obligatory anaerobic nature. Its inoculation in the lung, albeit rare, can occur if an anaerobic nidus is created. In the presented case, the patient developed C. perfringens bacteremia andempyema in the area of lung necrosis caused by acute pulmonary embolism. He did not have any history of chest trauma, and the source of bacteremia was deemed to be via gut translocation. The patient was noted to have multiple gastric ulcers on endoscopy and jejunal wall thickening, which likely led to the bacterial translocation into the bloodstream. He underwent video-assisted thoracoscopic surgery-assisted decortication and intravenous antibiotics, eventually leading to clinical improvement. To identify the source of Clostridium in the absence of penetrating trauma, a thorough gastrointestinal evaluation, including a colonoscopy, is warranted to identify the pathology leading to the gastrointestinal translocation.

Clostridium perfringens phospholipase C, an archetypal bacterial virulence factor, induces the formation of extracellular traps by human neutrophils

Neutrophil extracellular traps (NETs) are networks of DNA and various microbicidal proteins released to kill invading microorganisms and prevent their dissemination. However, a NETs excess is detrimental to the host and involved in the pathogenesis of various inflammatory and immunothrombotic diseases. Clostridium perfringens is a widely distributed pathogen associated with several animal and human diseases, that produces many exotoxins, including the phospholipase C (CpPLC), the main virulence factor in gas gangrene. During this disease, CpPLC generates the formation of neutrophil/platelet aggregates within the vasculature, favoring an anaerobic environment for C. perfringens growth. This work demonstrates that CpPLC induces NETosis in human neutrophils. Antibodies against CpPLC completely abrogate the NETosis-inducing activity of recombinant CpPLC and C. perfringens secretome. CpPLC induces suicidal NETosis through a mechanism that requires calcium release from inositol trisphosphate receptor (IP3) sensitive stores, activation of protein kinase C (PKC), and the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) pathways, as well as the production of reactive oxygen species (ROS) by the metabolism of arachidonic acid. Proteomic analysis of the C. perfringens secretome identified 40 proteins, including a DNAse and two 5´-nucleotidases homologous to virulence factors that could be relevant in evading NETs. We suggested that in gas gangrene this pathogen benefits from having access to the metabolic resources of the tissue injured by a dysregulated intravascular NETosis and then escapes and spreads to deeper tissues. Understanding the role of NETs in gas gangrene could help develop novel therapeutic strategies to reduce mortality, improve muscle regeneration, and prevent deleterious patient outcomes.

Postpartum clostridial gangrenous metritis in 12 dairy goats in France

Clostridial infections in goats have been associated frequently with enteric diseases or gas gangrene but very rarely with the reproductive system. We describe here 12 cases of fatal postpartum gangrenous metritis in does associated with infection by several clostridial species. Clinically, these cases were characterized by rapid onset of hyperthermia followed by death after kidding. On postmortem examination, the uteri appeared to be necrotic and were hemorrhagic and edematous. Microscopically, the uteri had diffuse coagulative necrosis, edema, hemorrhage, and fibrinous thrombi with intralesional gram-positive rods. Clostridium perfringens was isolated from 7 of 9 uterine samples cultured, and C. perfringens, C. septicum, C. novyi, or C. chauvoei were demonstrated by immunohistochemistry (IHC) in the 5 cases examined. IHC for Paeniclostridium sordellii was negative in all 5 cases. PCR performed on 3 of the C. perfringens isolates was positive for alpha toxin and perfringolysin, identifying these isolates as type A. Clostridial infection should be considered in cases of postpartum gangrenous metritis of does.

Efficacy of Two Probiotic Products Fed Daily to Reduce Clostridium perfringens-Based Adverse Health and Performance Effects in Dairy Calves

Clostridium perfringens is a spore-forming, anaerobic bacterium which produces toxins and exoenzymes that cause disease in calves, especially necro-hemorrhagic enteritis-associated diarrhea often resulting in death. Clostridium infections are currently being treated with antibiotics, but even with the prudent administration of antibiotics, there are significant rates of recurrence. Probiotics, an alternative to antibiotics, are commonly employed to prevent clostridial infections. The objectives of our study were to demonstrate that two commercially available products, when used as daily, direct-fed microbials, are effective in reducing adverse effects of an experimentally induced C. perfringens infection in dairy calves. We conducted a single site efficacy study with masking using a randomized design comprising 10 calves allocated to 3 treatment groups (probiotic 1, probiotic 2, and control). The procedures such as general health scores, body weight, blood samples, and fecal sample collections were done followed by experimental challenge of calves with C. perfringens. Daily feeding of L. animalis LA51 and P. freudenreichii PF24 without or with Bacillus lichenformis CH200 and Bacillus subtilis CH201, before, during and after an oral challenge of C. perfringens significantly reduced the incidence and severity of diarrhea while improving general impression and appearance scores of calves. Most notably, survival of calves in the two probiotic-fed groups was significantly higher than for control calves and further substantiates the potential economic and health benefits of feeding effective probiotics.

Clostridium perfringens-Induced Necrotic Diseases: An Overview

Clostridium perfringens, a prevalent Gram-positive bacterium, causes necrotic diseases associated with abundant life loss and economic burdens of billions of USD. The mechanism of C. perfringens-induced necrotic diseases remains largely unknown, in part, because of the lack of effective animal models and the presence of a large array of exotoxins and diverse disease manifestations from the skin and deep tissues to the gastrointestinal tract. In the light of the advancement of medical and veterinary research, a large body of knowledge is accumulating on the factors influencing C. perfringens-induced necrotic disease onset, development, and outcomes. Here, we present an overview of the key virulence factors of C. perfringens exotoxins. Subsequently, we focus on comprehensively reviewing C. perfringens-induced necrotic diseases such as myonecrosis, acute watery diarrhea, enteritis necroticans, preterm infant necrotizing enterocolitis, and chicken necrotic enteritis. We then review the current understanding on the mechanisms of myonecrosis and enteritis in relation to the immune system and intestinal microbiome. Based on these discussions, we then review current preventions and treatments of the necrotic diseases and propose potential new intervention options. The purpose of this review is to provide an updated and comprehensive knowledge on the role of the host–microbe interaction to develop new interventions against C. perfringens-induced necrotic diseases.

Bacterial phospholipases C with dual activity: phosphatidylcholinesterase and sphingomyelinase

Bacterial phospholipases and sphingomyelinases are lipolytic esterases that are structurally and evolutionarily heterogeneous. These enzymes play crucial roles as virulence factors in several human and animal infectious diseases. Some bacterial phospholipases C (PLCs) have both phosphatidylcholinesterase and sphingomyelinase C activities. Among them, Listeria  monocytogenes PlcB, Clostridium perfringens PLC, and Pseudomonas aeruginosa PlcH are the most deeply understood. In silico predictions of substrates docking with these three bacterial enzymes provide evidence that they interact with different substrates at the same active site. This review discusses structural aspects, substrate specificity, and the mechanism of action of those bacterial enzymes on target cells and animal infection models to shed light on their roles in pathogenesis.

Non-C. difficile Clostridioides Bacteremia in Intensive Care Patients, France

Article Summary: This multicenter study focusing on critically ill patients showed a strong relationship between hemolysis and mortality.

Usually responsible for soft tissue infections, Clostridioides species can also cause bacteremia, life-threatening infections often requiring intensive care unit (ICU) admission. We conducted a multicenter retrospective study to investigate Clostridioides bacteremia in ICUs to describe the clinical and biologic characteristics and outcomes in critically ill patients. We identified 135 patients with Clostridioides bacteremia, which occurred almost exclusively (96%) in patients with underlying conditions. Septic shock and digestive symptoms were the hallmarks of Clostridioides bacteremia in the ICU. We identified 16 different species of Clostridioides, among which C. perfringens accounted for 31% of cases. Despite the high sensitivity of Clostridioides to common antimicrobial drugs, mortality rates were high: 52% for ICU patients and 71% overall at 3 months. In multivariate analysis, the most important factor associated with increased risk for death was the presence of hemolysis. Clostridioides bacteremia often leads to multiple organ failures, which have high mortality rates.

Bacterial toxins in musculoskeletal infections

Musculoskeletal infections (MSKIs) remain a major health burden in orthopaedics. Bacterial toxins are foundational to pathogenesis in MSKI, but poorly understood by the community of providers that care for patients with MSKI, inducing an international group of microbiologists, infectious diseases specialists, orthopaedic surgeons and biofilm scientists to review the literature in this field to identify key topics and compile the current knowledge on the role of toxins in MSKI, with the goal of illuminating potential impact on biofilm formation and dispersal as well as therapeutic strategies. The group concluded that further research is needed to maximize our understanding of the effect of toxins on MSKIs, including: (i) further research to identify the roles of bacterial toxins in MSKIs, (ii) establish the understanding of the importance of environmental and host factors and in vivo expression of toxins throughout the course of an infection, (iii) establish the principles of drug-ability of antitoxins as antimicrobial agents in MSKIs, (iv) have well-defined metrics of success for antitoxins as antiinfective drugs, (v) design a cocktail of antitoxins against specific pathogens to (a) inhibit biofilm formation and (b) inhibit toxin release. The applicability of antitoxins as potential antimicrobials in the era of rising antibiotic resistance could meet the needs of day-to-day clinicians.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

The Agr-Like Quorum-Sensing System Is Important for Clostridium perfringens Type A Strain ATCC 3624 To Cause Gas Gangrene in a Mouse Model

Clostridium perfringens type A is involved in gas gangrene in humans and animals. Following a traumatic injury, rapid bacterial proliferation and exotoxin production result in severe myonecrosis. C. perfringens alpha toxin (CPA) and perfringolysin (PFO) are the main virulence factors responsible for the disease. Recent in vitro studies have identified an Agr-like quorum-sensing (QS) system in C. perfringens that regulates the production of both toxins. The system is composed of an AgrB membrane transporter and an AgrD peptide that interacts with a two-component regulatory system in response to fluctuations in the cell population density. In addition, a synthetic peptide named 6-R has been shown to interfere with this signaling mechanism, affecting the function of the Agr-like QS system in vitro In the present study, C. perfringens type A strain ATCC 3624 and an isogenic agrB-null mutant were tested in a mouse model of gas gangrene. When mice were intramuscularly challenged with 10⁶ CFU of wild-type ATCC 3624, severe myonecrosis and leukocyte aggregation occurred by 4 h. Similar numbers of an agrB-null mutant strain produced significantly less severe changes in the skeletal muscle of challenged mice. Complementation of the mutant to regain agrB expression restored virulence to wild-type levels. The burdens of all three C. perfringens strains in infected muscle were similar. In addition, animals injected intramuscularly with wild-type ATCC 3624 coincubated with the 6-R peptide developed less severe microscopic changes. This study provides the first in vivo evidence that the Agr-like QS system is important for C. perfringens type A-mediated gas gangrene.IMPORTANCE Clostridium perfringens type A strains produce toxins that are responsible for clostridial myonecrosis, also known as gas gangrene. Toxin production is regulated by an Agr-like quorum-sensing (QS) system that responds to changes in cell population density. In this study, we investigated the importance of this QS system in a mouse model of gas gangrene. Mice challenged with a C. perfringens strain with a nonfunctional regulatory system developed less severe changes in the injected skeletal muscle compared to animals receiving the wild-type strain. In addition, a synthetic peptide was able to decrease the effects of the QS in this disease model. These studies provide new understanding of the pathogenesis of gas gangrene and identified a potential therapeutic target to prevent the disease.

Inflammasome Activation Induced by Perfringolysin O of Clostridium perfringens and Its Involvement in the Progression of Gas Gangrene

Clostridium perfringens (C. perfringens) is Gram-positive anaerobic, spore-forming rod-shaped bacterial pathogen that is widely distributed in nature. This bacterium is known as the causative agent of a foodborne illness and of gas gangrene. While the major virulence factors are the α-toxin and perfringolysin O (PFO) produced by type A strains of C. perfringens, the precise mechanisms of how these toxins induce the development of gas gangrene are still not well understood. In this study, we analyzed the host responses to these toxins, including inflammasome activation, using mouse bone marrow-derived macrophages (BMDMs). Our results demonstrated, for the first time, that C. perfringens triggers the activation of caspase-1 and release of IL-1β through PFO-mediated inflammasome activation via a receptor of the Nod-like receptor (NLR) family, pyrin-domain containing 3 protein (NLRP3). The PFO-mediated inflammasome activation was not induced in the cultured myocytes. We further analyzed the functional roles of the toxins in inducing myonecrosis in a mouse model of gas gangrene. Although the myonecrosis was found to be largely dependent on the α-toxin, PFO also induced myonecrosis to a lesser extent, again through the mediation of NLRP3. These results suggest that C. perfringens triggers inflammatory responses via PFO-mediated inflammasome activation via NLRP3, and that this axis contributes in part to the progression of gas gangrene. Our findings provide a novel insight into the molecular mechanisms underlying the pathogenesis of gas gangrene caused by C. perfringens.

Histotoxic Clostridial Infections

The pathogenesis of clostridial myonecrosis or gas gangrene involves an interruption to the blood supply to the infected tissues, often via a traumatic wound, anaerobic growth of the infecting clostridial cells, the production of extracellular toxins, and toxin-mediated cell and tissue damage. This review focuses on host-pathogen interactions in Clostridium perfringens -mediated and Clostridium septicum -mediated myonecrosis. The major toxins involved are C. perfringens α-toxin, which has phospholipase C and sphingomyelinase activity, and C. septicum α-toxin, a β-pore-forming toxin that belongs to the aerolysin family. Although these toxins are cytotoxic, their effects on host cells are quite complex, with a range of intracellular cell signaling pathways induced by their action on host cell membranes.

Concurrent Host-Pathogen Transcriptional Responses in a Clostridium perfringens Murine Myonecrosis Infection

To obtain an insight into host-pathogen interactions in clostridial myonecrosis, we carried out comparative transcriptome analysis of both the bacterium and the host in a murine Clostridium perfringens infection model, which is the first time that such an investigation has been conducted. Analysis of the host transcriptome from infected muscle tissues indicated that many genes were upregulated compared to the results seen with mock-infected mice. These genes were enriched for host defense pathways, including Toll-like receptor (TLR) and Nod-like receptor (NLR) signaling components. Real-time PCR confirmed that host TLR2 and NLRP3 inflammasome genes were induced in response to C. perfringens infection. Comparison of the transcriptome of C. perfringens cells from the infected tissues with that from broth cultures showed that host selective pressure induced a global change in C. perfringens gene expression. A total of 33% (923) of C. perfringens genes were differentially regulated, including 10 potential virulence genes that were upregulated relative to their expression in vitro These genes encoded putative proteins that may be involved in the synthesis of cell wall-associated macromolecules, in adhesion to host cells, or in protection from host cationic antimicrobial peptides. This report presents the first successful expression profiling of coregulated transcriptomes of bacterial and host genes during a clostridial myonecrosis infection and provides new insights into disease pathogenesis and host-pathogen interactions.IMPORTANCEClostridium perfringens is the causative agent of traumatic clostridial myonecrosis, or gas gangrene. In this study, we carried out transcriptional analysis of both the host and the bacterial pathogen in a mouse myonecrosis infection. The results showed that in comparison to mock-infected control tissues, muscle tissues from C. perfringens-infected mice had a significantly altered gene expression profile. In particular, the expression of many genes involved in the innate immune system was upregulated. Comparison of the expression profiles of C. perfringens cells isolated from the infected tissues with those from equivalent broth cultures identified many potential virulence genes that were significantly upregulated in vivo These studies have provided a new understanding of the range of factors involved in host-pathogen interactions in a myonecrosis infection.

Two Lactobacillus Species Inhibit the Growth and α-Toxin Production of Clostridium perfringens and Induced Proinflammatory Factors in Chicken Intestinal Epithelial Cells in Vitro

Clostridium perfringens is the causative pathogen of avian necrotic enteritis. Lactobacillus spp. are well-characterized probiotics with anti-microbial and immune-modulatory activities. In the present study, we investigated the effects of L. acidophilus and L. fermentum on the growth, α-toxin production and inflammatory responses of C. perfringens. In in vitro culture experiments, both lactobacilli inhibited the growth of C. perfringens (P < 0.01), accompanied with a decrease in pH (P < 0.01). Supernatants from lactobacilli cultures also suppressed the growth of C. perfringens during 24 h of incubation (P < 0.01), but this inhibitory effect disappeared after 48 h. Both lactobacilli decreased the α-toxin production of C. perfringens (P < 0.01) without influencing its biomass, and even degraded the established α-toxin (P < 0.01). Lower environmental pH reduced the α-toxin production as well (P < 0.01). Preincubation with L. acidophilus decreased the attachment of C. perfringens to cells (P < 0.01) with the cell cytotoxicity being unaffected. Both lactobacilli pretreatment reduced the up-regulation of proinflammatory factors, peptidoglycan (PGN) receptors and nuclear factor kappa B (NF-κB) p65 in C. perfringens-challenged chicken intestinal epithelial cells (P < 0.05). In conclusion, L. acidophilus and L. fermentum inhibited the pathological effects of C. perfringens in vitro conditions.

Bacterial Sphingomyelinases and Phospholipases as Virulence Factors

Bacterial sphingomyelinases and phospholipases are a heterogeneous group of esterases which are usually surface associated or secreted by a wide variety of Gram-positive and Gram-negative bacteria. These enzymes hydrolyze sphingomyelin and glycerophospholipids, respectively, generating products identical to the ones produced by eukaryotic enzymes which play crucial roles in distinct physiological processes, including membrane dynamics, cellular signaling, migration, growth, and death. Several bacterial sphingomyelinases and phospholipases are essential for virulence of extracellular, facultative, or obligate intracellular pathogens, as these enzymes contribute to phagosomal escape or phagosomal maturation avoidance, favoring tissue colonization, infection establishment and progression, or immune response evasion. This work presents a classification proposal for bacterial sphingomyelinases and phospholipases that considers not only their enzymatic activities but also their structural aspects. An overview of the main physiopathological activities is provided for each enzyme type, as are examples in which inactivation of a sphingomyelinase- or a phospholipase-encoding gene impairs the virulence of a pathogen. The identification of sphingomyelinases and phospholipases important for bacterial pathogenesis and the development of inhibitors for these enzymes could generate candidate vaccines and therapeutic agents, which will diminish the impacts of the associated human and animal diseases.

Membrane-Binding Mechanism of Clostridium perfringens Alpha-Toxin

Clostridium perfringens alpha-toxin is a key mediator of gas gangrene, which is a life-threatening infection that manifests as fever, pain, edema, myonecrosis, and gas production. Alpha-toxin possesses phospholipase C and sphingomyelinase activities. The toxin is composed of an N-terminal domain (1-250 aa, N-domain), which is the catalytic site, and a C-terminal domain (251-370 aa, C-domain), which is the membrane-binding site. Immunization of mice with the C-domain of alpha-toxin prevents the gas gangrene caused by C. perfringens, whereas immunization with the N-domain has no effect. The central loop domain (55-93 aa), especially H….SW(84)Y(85)….G, plays an important role in the interaction with ganglioside GM1a. The toxin binds to lipid rafts in the presence of a GM1a/TrkA complex, and metabolites from phosphatidylcholine to diacylglycerol through the enzymatic activity of alpha-toxin itself. These membrane dynamics leads to the activation of endogenous PLCγ-1 via TrkA. In addition, treatment with alpha-toxin leads to the formation of diacylglycerol at membrane rafts in ganglioside-deficient DonQ cells; this in turn triggers endocytosis and cell death. This article summarizes the current the membrane-binding mechanism of alpha-toxin in detail.

Perfringolysin O: The Underrated Clostridium perfringens Toxin?

The anaerobic bacterium Clostridium perfringens expresses multiple toxins that promote disease development in both humans and animals. One such toxin is perfringolysin O (PFO, classically referred to as θ toxin), a pore-forming cholesterol-dependent cytolysin (CDC). PFO is secreted as a water-soluble monomer that recognizes and binds membranes via cholesterol. Membrane-bound monomers undergo structural changes that culminate in the formation of an oligomerized prepore complex on the membrane surface. The prepore then undergoes conversion into the bilayer-spanning pore measuring approximately 250–300 Å in diameter. PFO is expressed in nearly all identified C. perfringens strains and harbors interesting traits that suggest a potential undefined role for PFO in disease development. Research has demonstrated a role for PFO in gas gangrene progression and bovine necrohemorrhagic enteritis, but there is limited data available to determine if PFO also functions in additional disease presentations caused by C. perfringens. This review summarizes the known structural and functional characteristics of PFO, while highlighting recent insights into the potential contributions of PFO to disease pathogenesis.

Platelet interaction with bacterial toxins and secreted products

Bacteria that enter the bloodstream will encounter components of the cellular and soluble immune response. Platelets contribute to this response and have emerged as an important target for bacterial pathogens. Bacteria produce diverse extracellular proteins and toxins that have been reported to modulate platelet function. These interactions can result in complete or incomplete platelet activation or inhibition of platelet activation, depending on the bacteria and bacterial product. The nature of the platelet response may be highly relevant to disease pathogenesis.

The pore-forming α-toxin from clostridium septicum activates the MAPK pathway in a Ras-c-Raf-dependent and independent manner

Clostridium septicum is the causative agent of atraumatic gas gangrene, with α-toxin, an extracellular pore-forming toxin, essential for disease. How C. septicum modulates the host’s innate immune response is poorly defined, although α-toxin-intoxicated muscle cells undergo cellular oncosis, characterised by mitochondrial dysfunction and release of reactive oxygen species. Nonetheless, the signalling events that occur prior to the initiation of oncosis are poorly characterised. Our aims were to characterise the ability of α-toxin to activate the host mitogen activated protein kinase (MAPK) signalling pathway both in vitro and in vivo. Treatment of Vero cells with purified α-toxin activated the extracellular-signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 arms of the MAPK pathway and stimulated the release of TNF-α in a dose-dependent manner. Studies using inhibitors of all three MAPK components suggested that activation of ERK occurred in a Ras-c-Raf dependent manner, whereas activation of JNK and p38 occurred by a Ras-independent mechanism. Toxin-mediated activation was dependent on efficient receptor binding and pore formation and on an influx of extracellular calcium ions. In the mouse myonecrosis model we showed that the MAPK pathway was activated in tissues of infected mice, implying that it has an important role in the disease process.

Membrane vesicles of Clostridium perfringens type A strains induce innate and adaptive immunity

Vesicle shedding from bacteria is a universal process in most Gram-negative bacteria and a few Gram-positive bacteria. In this report, we isolate extracellular membrane vesicles (MVs) from the supernatants of Gram-positive pathogen Clostridium perfringens (C. perfringens). We demonstrated vesicle production in a variety of virulent and nonvirulent type A strains. MVs did not contain alpha-toxin and NetB toxin demonstrated by negative reaction to specific antibody and absence of specific proteins identified by LC-MS/MS. C. perfringens MVs contained DNA components such as 16S ribosomal RNA gene (16S rRNA), alpha-toxin gene (plc) and the perfringolysin O gene (pfoA) demonstrated by PCR. We also identified a total of 431 proteins in vesicles by 1-D gel separation and LC-MS/MS analysis. In vitro studies demonstrated that vesicles could be internalized into murine macrophage RAW264.7 cells without direct cytotoxicity effects, causing release of inflammation cytokines including granulocyte colony stimulating factor (G-CSF), tumor necrosis factor-alpha (TNF-α) and interleukin-1 (IL-1), which could also be detected in mice injected with MVs through intraperitoneal (i.p.) route. Mice immunized with C. perfringens MVs produced high titer IgG, especially IgG1, antibodies against C. perfringens membrane proteins. However, this kind of antibody could not provide protection in mice following challenge, though it could slightly postpone the time of death. Our results indicate that release of MVs from C. perfringens could provide a previously unknown mechanism to induce release of inflammatory cytokines, especially TNF-α, these findings may contribute to a better understanding of the pathogenesis of C. perfringens infection.

Toxin plasmids of Clostridium perfringens

In both humans and animals, Clostridium perfringens is an important cause of histotoxic infections and diseases originating in the intestines, such as enteritis and enterotoxemia. The virulence of this Gram-positive, anaerobic bacterium is heavily dependent upon its prolific toxin-producing ability. Many of the ∼16 toxins produced by C. perfringens are encoded by large plasmids that range in size from ∼45 kb to ∼140 kb. These plasmid-encoded toxins are often closely associated with mobile elements. A C. perfringens strain can carry up to three different toxin plasmids, with a single plasmid carrying up to three distinct toxin genes. Molecular Koch's postulate analyses have established the importance of several plasmid-encoded toxins when C. perfringens disease strains cause enteritis or enterotoxemias. Many toxin plasmids are closely related, suggesting a common evolutionary origin. In particular, most toxin plasmids and some antibiotic resistance plasmids of C. perfringens share an ∼35-kb region containing a Tn916-related conjugation locus named tcp (transfer of clostridial plasmids). This tcp locus can mediate highly efficient conjugative transfer of these toxin or resistance plasmids. For example, conjugative transfer of a toxin plasmid from an infecting strain to C. perfringens normal intestinal flora strains may help to amplify and prolong an infection. Therefore, the presence of toxin genes on conjugative plasmids, particularly in association with insertion sequences that may mobilize these toxin genes, likely provides C. perfringens with considerable virulence plasticity and adaptability when it causes diseases originating in the gastrointestinal tract.

BEC, a novel enterotoxin of Clostridium perfringens found in human clinical isolates from acute gastroenteritis outbreaks

Clostridium perfringens is a causative agent of food-borne gastroenteritis for which C. perfringens enterotoxin (CPE) has been considered an essential factor. Recently, we experienced two outbreaks of food-borne gastroenteritis in which non-CPE producers of C. perfringens were strongly suspected to be the cause. Here, we report a novel enterotoxin produced by C. perfringens isolates, BEC (binary enterotoxin of C. perfringens). Culture supernatants of the C. perfringens strains showed fluid-accumulating activity in rabbit ileal loop and suckling mouse assays. Purification of the enterotoxic substance in the supernatants and high-throughput sequencing of genomic DNA of the strains revealed BEC, composed of BECa and BECb. BECa and BECb displayed limited amino acid sequence similarity to other binary toxin family members, such as the C. perfringens iota toxin. The becAB genes were located on 54.5-kb pCP13-like plasmids. Recombinant BECb (rBECb) alone had fluid-accumulating activity in the suckling mouse assay. Although rBECa alone did not show enterotoxic activity, rBECa enhanced the enterotoxicity of rBECb when simultaneously administered in suckling mice. The entertoxicity of the mutant in which the becB gene was disrupted was dramatically decreased compared to that of the parental strain. rBECa showed an ADP-ribosylating activity on purified actin. Although we have not directly evaluated whether BECb delivers BECa into cells, rounding of Vero cells occurred only when cells were treated with both rBECa and rBECb. These results suggest that BEC is a novel enterotoxin of C. perfringens distinct from CPE, and that BEC-producing C. perfringens strains can be causative agents of acute gastroenteritis in humans. Additionally, the presence of becAB on nearly identical plasmids in distinct lineages of C. perfringens isolates suggests the involvement of horizontal gene transfer in the acquisition of the toxin genes.

Infections of the Liver

The portal vein carries blood from the gastrointestinal tract to the liver and in so doing carries microbes as well. The liver may therefore be involved in infections with a myriad number of microbial organisms. While some of these infections most commonly occur in the immunocompromised host, others affect the immune competence. Hepatic infections may be primary in nature or secondary, as part of systemic or contagious disease. The purpose of this chapter is to provide a brief overview of the various infections of the liver in the pediatric patient.

A recombinant carboxy-terminal domain of alpha-toxin protects mice against Clostridium perfringens

Clostridium perfringens alpha-toxin (CP, 370 residues) is one of the main agents involved in the development of gas gangrene. In this study, the immunogenicity and protective efficacy of the C-terminal domain (CP251-370) of the toxin and phospholipase C (PLC; CB, 372 residues) of Clostridum bifermentans isolated from cases of clostridium necrosis were examined. The recombinant proteins were expressed as glutathione S-transferase (GST) fusion proteins. Antibodies that cross-reacted with alpha-toxin were produced after immunization with recombinant proteins including GST-CP251-370, GST-CP281-370, GST-CP311-370, CB1-372 and GST-CB251-372. Anti-GST-CP251-370, anti-GST-CP281-370 and anti-GST-CP311-370 sera neutralized both the PLC and hemolytic activities of alpha-toxin, whereas anti-CB1-372 and anti-GST-CB251-372 weakly neutralized these activities. Immunization with GST-CP251-370 and GST-CP281-370 provided protection against the lethal effects of the toxin and C. perfringens type A NCTC8237. Partial protection from the toxin and C. perfringens was elicited by immunization with GST-CP311-370 and CB1-372. GST-CP251-370 and GST-CP281-370 are promising candidates for vaccines for clostridial-induced gas gangrene.

Staphylococcus aureus α-hemolysin promotes platelet-neutrophil aggregate formation

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) causes severe hemorrhagic necrotizing pneumonia associated with high mortality. Exotoxins have been implicated in the pathogenesis of this infection; however, the cellular mechanisms responsible remain largely undefined. Because platelet-neutrophil aggregates (PNAs) can dysregulate inflammatory responses and contribute to tissue destruction, we investigated whether exotoxins from MRSA could stimulate formation of PNAs in human whole blood. Strong PNA formation was stimulated by toxins from stationary phase but not log phase CA-MRSA, and α-hemolysin was singularly identified as the mediator of this activity. MRSA exotoxins also caused neutrophil (polymorphonuclear leukocyte) activation, as measured by increased CD11b expression, although platelet binding was not driven by this mechanism; rather, α-hemolysin-induced PNA formation was solely platelet P-selectin dependent. These findings suggest a role for S. aureus α-hemolysin-induced PNA formation in alveolar capillary destruction in hemorrhagic/necrotizing pneumonia caused by CA-MRSA and offer novel targets for intervention.

Necrotizing soft-tissue infections

NSTI is a life-threatening, surgical, and medical emergency. Clinical presentation, at least in the initial phase, can be misleading. Various studies have shown that delay in surgical debridement is associated with increased mortality. A high index of suspicion is important in early recognition and in instituting prompt therapy without delay. Early diagnosis, aggressive surgical debridement, aggressive supportive care, and optimal presumptive antibiotic therapy significantly improve morbidity and mortality associated with NSTIs.

Role of sphingomyelinase in infectious diseases caused by Bacillus cereus

Bacillus cereus (B. cereus) is a pathogen in opportunistic infections. Here we show that Bacillus cereus sphingomyelinase (Bc-SMase) is a virulence factor for septicemia. Clinical isolates produced large amounts of Bc-SMase, grew in vivo, and caused death among mice, but ATCC strains isolated from soil did not. A transformant of the ATCC strain carrying a recombinant plasmid containing the Bc-SMase gene grew in vivo, but that with the gene for E53A, which has little enzymatic activity, did not. Administration of an anti-Bc-SMase antibody and immunization against Bc-SMase prevented death caused by the clinical isolates, showing that Bc-SMase plays an important role in the diseases caused by B. cereus. Treatment of mouse macrophages with Bc-SMase resulted in a reduction in the generation of H₂O₂ and phagocytosis of macrophages induced by peptidoglycan (PGN), but no effect on the release of TNF-α and little release of LDH under our experimental conditions. Confocal laser microscopy showed that the treatment of mouse macrophages with Bc-SMase resulted in the formation of ceramide-rich domains. A photobleaching analysis suggested that the cells treated with Bc-SMase exhibited a reduction in membrane fluidity. The results suggest that Bc-SMase is essential for the hydrolysis of SM in membranes, leading to a reduction in phagocytosis.

Life-threatening clostridial infections

Life-threatening soft tissue infections caused by Clostridium species have been described in the medical literature for hundreds of years largely because of their fulminant nature, distinctive clinical presentations and complex management issues. The Clostridium species perfringens, septicum and histolyticum are the principal causes of trauma-associated gas gangrene and their incidence increases dramatically in times of war, hurricanes, earthquakes and other mass casualty conditions. Recently, there has also been an increased incidence of spontaneous gas gangrene caused by Clostridium septicum in association with gastrointestinal abnormalities and neutropenia. Similarly, over the last 15 years there has been increased recognition of a toxic shock-like syndrome associated with Clostridium sordellii in individuals skin-popping black tar heroin, in women undergoing childbirth or other gynecologic procedures including medically-induced abortion. Like their cousins Clostridium tetanus and Clostridium botulinum, the pathogenesis of these clostridial infections is largely the consequence of potent exotoxin production. Strategies to inhibit toxin production, neutralize circulating toxins and prevent their interaction with cells of the innate immune response are sorely needed. Recent studies have elucidated novel targets that may hold promise for newer therapeutic modalities.

Necrotizing soft-tissue infections

OBJECTIVE

To provide a contemporary review of the diagnosis and management of necrotizing soft-tissue infections.

DATA SOURCES

Scientific literature and internet sources.

STUDY SELECTION

Major articles of importance in this area.

CONCLUSIONS

The mortality for necrotizing soft-tissue infections appears to be decreasing, possibly due to improved recognition and earlier delivery of more effective therapy. Establishing a diagnosis and initiating treatment as soon as possible provides the best opportunity for a good outcome.

The cysteine protease α-clostripain is not essential for the pathogenesis of Clostridium perfringens-mediated myonecrosis

Clostridium perfringens is the causative agent of clostridial myonecrosis or gas gangrene and produces many different extracellular toxins and enzymes, including the cysteine protease α-clostripain. Mutation of the α-clostripain structural gene, ccp, alters the turnover of secreted extracellular proteins in C. perfringens, but the role of α-clostripain in disease pathogenesis is not known. We insertionally inactivated the ccp gene C. perfringens strain 13 using TargeTron technology, constructing a strain that was no longer proteolytic on skim milk agar. Quantitative protease assays confirmed the absence of extracellular protease activity, which was restored by complementation with the wild-type ccp gene. The role of α-clostripain in virulence was assessed by analysing the isogenic wild-type, mutant and complemented strains in a mouse myonecrosis model. The results showed that although α-clostripain was the major extracellular protease, mutation of the ccp gene did not alter either the progression or the development of disease. These results do not rule out the possibility that this extracellular enzyme may still have a role in the early stages of the disease process.

Clostridial myonecrosis: new insights in pathogenesis and management

Clostridial myonecrosis remains an important cause of human morbidity and mortality worldwide. Although traumatic gas gangrene can be readily diagnosed from clinical findings and widely available technologies, spontaneous gas gangrene is more insidious, and gynecologic infections due to Clostridium sordellii progress so rapidly that death often precedes diagnosis. In each case, extensive tissue destruction and the subsequent systemic manifestations are mediated directly and indirectly by potent bacterial exotoxins. The management triumvirate of timely diagnosis, thorough surgical removal of necrotic tissue, and treatment with antibiotics that inhibit toxin synthesis remains the gold standard of care. Yet, despite these measures, mortality remains 30% to 100% and survivors often must cope with life-altering amputations. Recent insights regarding the genetic regulation of toxin production, the molecular mechanisms of toxin-induced host cell dysfunction, and the roles of newly described toxins in pathogenesis suggest that novel prevention, diagnostic, and treatment modalities may be on the horizon for these devastating infections.

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

Molecular and cellular basis of microvascular perfusion deficits induced by Clostridium perfringens and Clostridium septicum

Reduced tissue perfusion leading to tissue ischemia is a central component of the pathogenesis of myonecrosis caused by Clostridium perfringens. The C. perfringens α-toxin has been shown capable of inducing these changes, but its potential synergy with perfringolysin O (θ-toxin) is less well understood. Similarly, Clostridium septicum is a highly virulent causative agent of spontaneous gas gangrene, but its effect on the microcirculation has not been examined. Therefore, the aim of this study was to use intravital microscopy to examine the effects of C. perfringens and C. septicum on the functional microcirculation, coupled with the use of isogenic toxin mutants to elucidate the role of particular toxins in the resultant microvascular perfusion deficits. This study represents the first time this integrated approach has been used in the analysis of the pathological response to clostridial toxins. Culture supernatants from wild-type C. perfringens induced extensive cell death within 30 min, as assessed by in vivo uptake of propidium iodide. Furthermore, significant reductions in capillary perfusion were observed within 60 min. Depletion of either platelets or neutrophils reduced the alteration in perfusion, consistent with a role for these blood-borne cells in obstructing perfusion. In addition, mutation of either the α-toxin or perfringolysin O structural genes attenuated the reduction in perfusion, a process that was reversed by genetic complementation. C. septicum also induced a marked reduction in perfusion, with the degree of microvascular compromise correlating with the level of the C. septicum α-toxin. Together, these data indicate that as a result of its ability to produce α-toxin and perfringolysin O, C. perfringens rapidly induces irreversible cellular injury and a marked reduction in microvascular perfusion. Since C. septicum induces a similar reduction in microvascular perfusion, it is postulated that this function is central to the pathogenesis of clostridial myonecrosis, irrespective of the causative bacterium.

Clostridial myonecrosis is a life-threatening process induced by infection with species such as C. perfringens and C. septicum. The associated pathology includes muscle death and a characteristic disruption in tissue perfusion. Exotoxins produced by these species have been implicated in the reduction in perfusion. However, how these toxins function in tandem remains unclear. In this study we used intravital microscopy to study microvascular blood flow in a muscle exposed to products of C. perfringens and C. septicum. C. perfringens supernatants induced cellular injury and a progressive reduction in blood flow. Removal of blood-borne platelets and neutrophils from the circulation reduced the alteration in blood flow. In addition, this response was reduced by genetic deletion of either the α-toxin or perfringolysin O, providing the first indication that each of these exotoxins contributes to the reduction in blood supply to affected tissues. Using a similar approach, we observed that C. septicum supernatant induced a comparable reduction in perfusion, which was mediated in part via the C. septicum α-toxin. These results indicate that platelets, neutrophils and multiple clostridial toxins contribute to reduced blood supply and oxygen delivery associated with clostridial infection and suggest that the dominant component of the pathology is toxin-induced cellular injury and death.

Fournier's gangrene progressing from the buttocks to the scrotum following a perianal abscess

We describe a case of gas-producing infection following a perianal abscess. A 61-year-old man was admitted to our hospital complaining of perineal pain and was found to have a perianal abscess. He was diabetic but had not received treatment for the disease. Although the perianal abscess was drained and antibiotic treatment started, severe swelling of the scrotum, with crepitation, redness, and partial necrosis progressed rapidly. Computed tomography revealed subcutaneous gas formation in the scrotum. A culture study revealed Clostridium, Enterococcus, and numerous other types of bacteria. The patient was diagnosed with Fournier's gangrene caused by infection with Clostridium in combination with other species of bacteria. The infection was refractory to drainage and antibiotic therapy. Thus, repeated extensive debridement of all necrotic tissue in the scrotum was required until healthy granulation was present in the wound. Our case shows that, in patients with Fournier's gangrene caused by infection with Clostridium in combination with other species of bacteria, the mainstay of treatment should be open drainage and aggressive surgical debridement of all necrotic tissue, followed by broad-spectrum antibiotic therapy.

Spontaneous Clostridium perfringens myonecrosis of the shoulder: a case report

Clostridium perfringens is a rare, life-threatening infection. We present an atypical evolution of spontaneous Clostridium perfringens myonecrosis in a monitored healthy patient. After a head injury, the hemodynamically stable patient became febrile and grew cultures of Clostridium perfringens with no identifiable focus. Antibiotics were initiated. Seventy-two hours after culture results, the patient complained of shoulder pain and imaging revealed periscapular subcutaneous emphysema. Multiple surgical débridements of the necrotic supraspinatus muscle were required to eradicate the infection. The unique features of this patient include the opportunity to (1) follow the progression of clostridial myonecrosis in a healthy patient in a monitored setting and (2) study the extended period of time between positive cultures and the onset of clinical myonecrosis, a feature at odds with the classic rapid progression.

The role of neutrophils and monocytic cells in controlling the initiation of Clostridium perfringens gas gangrene

Clostridium perfringens is a common cause of the fatal disease gas gangrene (myonecrosis). Established gas gangrene is notable for a profound absence of neutrophils and monocytic cells (phagocytes), and it has been suggested that the bactericidal activities of these cells play an insignificant role in controlling the progression of the infection. However, large inocula of bacteria are needed to establish an infection in experimental animals, suggesting phagocytes may play a role in inhibiting the initiation of gangrene. Examination of tissue sections of mice infected with a lethal (1 x 10(9)) or sublethal (1 x 10(6)) inoculum of C. perfringens revealed that phagocyte infiltration in the first 3 h postinfection was inhibited with a lethal dose but not with a sublethal dose, indicating that exclusion of phagocytes begins very early in the infection cycle. Experiments in which mice were depleted of either circulating monocytes or neutrophils before infection with C. perfringens showed that monocytes play a role in inhibiting the onset of gas gangrene at intermediate inocula but, although neutrophils can slow the onset of the infection, they are not protective. These results suggest that treatments designed to increase monocyte infiltration and activate macrophages may lead to increased resistance to the initiation of gas gangrene.

Clostridium perfringens phospholipase C-induced platelet/leukocyte interactions impede neutrophil diapedesis

Clostridium perfringens gas gangrene is a fulminant necrotizing infection in which inflammatory cells are notably absent from infected tissues but are often massed within adjacent vessels. It has been shown that C. perfringens phospholipase C (PLC) stimulates formation of large intravascular platelet/leukocyte complexes and that PLC-induced activation of platelet gpIIbIIIa plays a major role. In vivo, such aggregates contribute to microvascular thrombosis and ischaemic necrosis of tissue. However, the effects of adherent platelets on neutrophil diapedesis have not been established. The present work investigated (1) the contribution of platelet P-selectin (CD62P) to PLC-induced cellular complex formation and (2) the effects of platelet adhesion on neutrophil diapedesis. The effects of anti-gpIIbIIIa and anti-CD62P strategies on PLC-induced complex formation were measured by flow cytometry and followed by light microscopy. Both platelet gpIIbIIIa and CD62P contributed to the formation of platelet/leukocyte complexes. Specifically, gpIIbIIIa mediated the formation of large platelet/platelet aggregates that were tethered to the leukocyte principally via CD62P. Neutrophil diapedesis, quantified by a transendothelial cell migration assay and visualized by electron microscopy, was significantly reduced (>60%) by the adherence of large platelet aggregates. It was concluded that the absence of a tissue inflammatory response in C. perfringens gas gangrene is due, in part, to impaired neutrophil mobility caused by large aggregates of adherent platelets induced by PLC. Further, an adjunctive immunotherapeutic strategy targeting both gpIIbIIIa and CD62P may improve the tissue inflammatory response, prevent vascular occlusion, maintain tissue viability, and reduce the need for radical amputation in patients with clostridial gas gangrene.

The alpha-toxin of Clostridium septicum is essential for virulence

Clostridium septicum is the causative agent of spontaneous gas gangrene or atraumatic myonecrosis, a sudden and frequently fatal infection that is increasingly associated with malignancy of the colon. Little is known about the disease process although the focus of virulence studies has been the alpha-toxin, a pore-forming cytolysin that is encoded by the csa gene and secreted as an inactive protoxin. Until now a lack of techniques for the genetic manipulation of C. septicum has hindered the use of molecular approaches to understand pathogenesis. By introducing plasmids by conjugation from Escherichia coli, we have developed methods for the genetic manipulation of C. septicum and constructed a chromosomal csa mutant by allelic exchange. Virulence testing of an isogenic series of strains consisting of the wild type, the csa mutant, and a csa mutant complemented with the wild-type csa gene revealed that the development of fulminant myonecrosis in mice was dependent on the ability to produce a functional haemolytic alpha-toxin. Furthermore, the inhibition of leukocyte influx into the lesion, which is very typical of clostridial myonecrosis, was also dependent on the ability to produce alpha-toxin. This study represents the first definitive identification of a virulence factor in this organism and opens the way for further studies that will delineate the role of other putative virulence factors in this significant pathogen.

Effects of Clostridium perfringens alpha-toxin (PLC) and perfringolysin O (PFO) on cytotoxicity to macrophages, on escape from the phagosomes of macrophages, and on persistence of C. perfringens in host tissues

Clostridium perfringens is the most common cause of clostridial myonecrosis (gas gangrene). Polymorphonuclear cells (PMNs) appear to play only a minor role in preventing the onset of myonecrosis in a mouse animal model of the disease (unpublished results). However, the importance of macrophages in the host defense against C. perfringens infections is still unknown. Two membrane-active toxins produced by the anaerobic C. perfringens, alpha-toxin (PLC) and perfringolysin O (PFO), are thought to be important in the pathogenesis of gas gangrene and the lack of phagocytic cells at the site of infection. Therefore, C. perfringens mutants lacking PFO and PLC were examined for their relative cytotoxic effects on macrophages, their ability to escape the phagosome of macrophages, and their persistence in mouse tissues. C. perfringens survival in the presence of mouse peritoneal macrophages was dependent on both PFO and PLC. PFO was shown to be the primary mediator of C. perfringens-dependent cytotoxicity to macrophages. Escape of C. perfringens cells from phagosomes of macrophage-like J774-33 cells and mouse peritoneal macrophages was mediated by either PFO or PLC, although PFO seemed to play a more important role in escape from the phagosome in peritoneal macrophages. At lethal doses (10(9)) of bacteria only PLC was necessary for the onset of myonecrosis, while at sublethal doses (10(6)) both PFO and PLC were necessary for survival of C. perfringens in mouse muscle tissue. These results suggest PFO-mediated cytotoxicity toward macrophages and the ability to escape macrophage phagosomes may be important factors in the ability of C. perfringens to survive in host tissues when bacterial numbers are low relative to those of phagocytic cells, e.g., early in an infection.

Role of Clostridium perfringens phospholipase C in the pathogenesis of gas gangrene

Gas gangrene is an acute and devastating infection most frequently caused by Clostridium perfringens and characterized by severe myonecrosis, intravascular leukocyte accumulation, and significant thrombosis. Several lines of evidence indicate that C. perfringens phospholipase C (Cp-PLC), also called alpha-toxin, is the major virulence factor in this disease. This toxin is a Zn2+ metalloenzyme with lecithinase and sphingomyelinase activities. Its three dimensional structure shows two domains, an N-terminal domain which contains the active site, and a C-terminal domain required for the Ca2+dependent interaction with membranes. Cp-PLC displays several biological activities: it increases capillary permeability, induces platelet aggregation, hemolysis, myonecrosis, decreases cardiac contractility, and is lethal. Experiments with genetically engineered Cp-PLC variants have revealed that the sphingomyelinase activity and the C-terminal domain are required for toxicity. The myotoxicity of Cp-PLC is largely dependent on its membrane damaging effect. In addition, it has been suggested that the alterations in the blood flow induced by this toxin also contribute to muscle damage. In gas gangrene, Cp-PLC dysregulates transduction pathways in endothelial cells, platelets and neutrophils leading to the uncontrolled production of several intercellular mediators and adhesion molecules. Thus, Cp-PLC alters the traffic of neutrophils to the infected tissue and promotes thrombotic events, enhancing the conditions for anaerobic growth.