The first strain of Clostridium perfringens isolated from an avian source has an alpha-toxin with divergent structural and kinetic properties

Structural Basis of the Pore-Forming Toxin/Membrane Interaction

With the rapid growth of antibiotic-resistant bacteria, it is urgent to develop alternative therapeutic strategies. Pore-forming toxins (PFTs) belong to the largest family of virulence factors of many pathogenic bacteria and constitute the most characterized classes of pore-forming proteins (PFPs). Recent studies revealed the structural basis of several PFTs, both as soluble monomers, and transmembrane oligomers. Upon interacting with host cells, the soluble monomer of bacterial PFTs assembles into transmembrane oligomeric complexes that insert into membranes and affect target cell-membrane permeability, leading to diverse cellular responses and outcomes. Herein we have reviewed the structural basis of pore formation and interaction of PFTs with the host cell membrane, which could add valuable contributions in comprehensive understanding of PFTs and searching for novel therapeutic strategies targeting PFTs and interaction with host receptors in the fight of bacterial antibiotic-resistance.

Phylogenetic and genomic analysis reveals high genomic openness and genetic diversity of Clostridium perfringens

Clostridium perfringens is associated with a variety of diseases in both humans and animals. Recent advances in genomic sequencing make it timely to re-visit this important pathogen. Although the genome sequence of C. perfringens was first determined in 2002, large-scale comparative genomics with isolates of different origins is still lacking. In this study, we used whole-genome sequencing of 45 C. perfringens isolates with isolation time spanning an 80-year period and performed comparative analysis of 173 genomes from worldwide strains. We also conducted phylogenetic lineage analysis and introduced an openness index (OI) to evaluate the openness of bacterial genomes. We classified all these genomes into five lineages and hypothesized that the origin of C. perfringens dates back to ~80 000 years ago. We showed that the pangenome of the 173 C. perfringens strains contained a total of 26 954 genes, while the core genome comprised 1020 genes, accounting for about a third of the genome of each isolate. We demonstrated that C. perfringens had the highest OI compared with 51 other bacterial species. Intact prophage sequences were found in nearly 70.0 % of C. perfringens genomes, while CRISPR sequences were found only in ~40.0 %. Plasmids were prevalent in C. perfringens isolates, and half of the virulence genes and antibiotic resistance genes (ARGs) identified in all the isolates could be found in plasmids. ARG-sharing network analysis showed that C. perfringens shared its 11 ARGs with 55 different bacterial species, and a high frequency of ARG transfer may have occurred between C. perfringens and species in the genera Streptococcus and Staphylococcus. Correlation analysis showed that the ARG number in C. perfringens strains increased with time, while the virulence gene number was relative stable. Our results, taken together with previous studies, revealed the high genome openness and genetic diversity of C. perfringens and provide a comprehensive view of the phylogeny, genomic features, virulence gene and ARG profiles of worldwide strains.

Immunization with a nontoxic naturally occurring Clostridium perfringens alpha toxin induces neutralizing antibodies in rabbits

Clostridium perfringens alpha toxin, encoded by plc gene, has been implicated in gas gangrene, a life threatening infection. Vaccination is considered one of the best solutions against Clostridium infections. Although studies have identified many low quality clostridial vaccines, the use of recombinant proteins has been considered a promising alternative. Previously, a naturally occurring alpha toxin isoform (αAV1b) was identified with a mutation at residue 11 (His/Tyr), which can affect its enzymatic activity. The aim of the present study was to evaluate whether the mutation in the αAV1b isoform could result in an inactive toxin and was able to induce protection against the native alpha toxin. We used recombinant protein techniques to determine whether this mutation in αAV1b could result in an inactive toxin compared to the active isoform, αZ23. Rabbits were immunized with the recombinant toxins (αAV1b and αZ23) and with native alpha toxin. αAV1b showed no enzymatic and hemolytic activities. ELISA titration assays showed a high titer of both anti-recombinant toxin (anti-rec-αAV1b and anti-rec-αZ23) antibodies against the native alpha toxin. The alpha antitoxin titer detected in the rabbits' serum pool was 24.0 IU/mL for both recombinant toxins. These results demonstrate that the inactive naturally mutated αAV1b is able to induce an immune response, and suggest it can be considered as a target for the development of a commercial vaccine against C. perfringens alpha toxin.

Abilities of the mCP Agar method and CRENAME alpha toxin-specific real-time PCR assay to detect Clostridium perfringens spores in drinking water

We first determined the analytical specificity and ubiquity (i.e., the ability to detect all or most strains) of a Clostridium perfringens-specific real-time PCR (rtPCR) assay based on the cpa gene (cpa rtPCR) by using a bacterial strain panel composed of C. perfringens and non-C. perfringens Clostridium strains. All non-C. perfringens Clostridium strains tested negative, whereas all C. perfringens strains tested positive with the cpa rtPCR, for an analytical specificity and ubiquity of 100%. The cpa rtPCR assay was then used to confirm the identity of 116 putative C. perfringens isolates recovered after filtration of water samples and culture on mCP agar. Colonies presenting discordant results between the phenotype on mCP agar and cpa rtPCR were identified by sequencing the 16S rRNA and cpa genes. Four mCP(-)/rtPCR(+) colonies were identified as C. perfringens, whereas 3 mCP(+)/rtPCR(-) colonies were identified as non-C. perfringens. The cpa rtPCR was negative with all 51 non-C. perfringens strains and positive with 64 of 65 C. perfringens strains. Finally, we compared mCP agar and a CRENAME (concentration and recovery of microbial particles, extraction of nucleic acids, and molecular enrichment) procedure plus cpa rtPCR (CRENAME + cpa rtPCR) for their abilities to detect C. perfringens spores in drinking water. CRENAME + cpa rtPCR detected as few as one C. perfringens CFU per 100 ml of drinking water sample in less than 5 h, whereas mCP agar took at least 25 h to deliver results. CRENAME + cpa rtPCR also allows the simultaneous and sensitive detection of Escherichia coli and C. perfringens from the same potable water sample. In itself, it could be used to assess the public health risk posed by drinking water potentially contaminated with pathogens more resistant to disinfection.

Role of pore-forming toxins in neonatal sepsis

Protein toxins are important virulence factors contributing to neonatal sepsis. The major pathogens of neonatal sepsis, group B Streptococci, Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus, secrete toxins of different molecular nature, which are key for defining the disease. Amongst these toxins are pore-forming exotoxins that are expressed as soluble monomers prior to engagement of the target cell membrane with subsequent formation of an aqueous membrane pore. Membrane pore formation is not only a means for immediate lysis of the targeted cell but also a general mechanism that contributes to penetration of epithelial barriers and evasion of the immune system, thus creating survival niches for the pathogens. Pore-forming toxins, however, can also contribute to the induction of inflammation and hence to the manifestation of sepsis. Clearly, pore-forming toxins are not the sole factors that drive sepsis progression, but they often act in concert with other bacterial effectors, especially in the initial stages of neonatal sepsis manifestation.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

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.

Sequence variation in the alpha-toxin encoding plc gene of Clostridium perfringens strains isolated from diseased and healthy chickens

The aim of the present study was to analyse the genetic diversity of the alpha-toxin encoding plc gene and the variation in alpha-toxin production of Clostridium perfringens type A strains isolated from presumably healthy chickens and chickens suffering from either necrotic enteritis (NE) or cholangio-hepatitis. The alpha-toxin encoding plc genes from 60 different pulsed-field gel electrophoresis (PFGE) types (strains) of C. perfringens were sequenced and translated in silico to amino acid sequences and the alpha-toxin production was investigated in batch cultures of 45 of the strains using an enzyme-linked immunosorbent assay (ELISA) approach. Overall, the truncated amino acid sequences showed close similarity (>98% at the amino acid level) to previously reported sequences from chicken-derived C. perfringens isolates. Variations were however observed in 23 out of 379 aa positions leading to the definition of 26 different alpha-toxin sequence types among the 60 strains. Moreover, a type II intron of 834 non-coding nucleotides was identified in the plc gene of three of the investigated strains. The in vitro alpha-toxin production investigated in 45 of the strains, including the three harbouring the intron, revealed no correlation between PFGE type, alpha-toxin sequence type, health status of the host chickens and level of alpha-toxin production. It is therefore concluded that neither plc gene type nor alpha-toxin production level seems to correlate to origin (healthy or diseased chicken) of the C. perfringens strains.

Analysis of protection afforded by a Clostridium perfringens α-toxoid against heterologous clostridial phospholipases C

The major virulence determinant in clostridial myonecrosis caused by Clostridium perfringens is a phospholipase C (PLC), the alpha-toxin. Previously, mice have been protected against challenge with heterologous alpha-toxin or Clostridium perfringens spores by immunisation with the C-domain (known as Cpa(247-370) or alpha-toxoid) of the alpha-toxin. In this study, we have determined the ability of the alpha-toxoid to protect against the lethal effects of a divergent C. perfringens alpha-toxin and against the PLCs of C. absonum or C. bifermentans, species which have been isolated from cases of clostridial myonecrosis. Protection against the C. perfringens alpha-toxin variant, the C. absonum alpha-toxin or the C. bifermentans PLC was elicited by immunisation with the alpha-toxoid in vivo.

A quantitative electrochemiluminescence assay for Clostridium perfringens alpha toxin

Described is a rapid direct sandwich format electrochemiluminescence assay for identifying and assaying Clostridium perfringens alpha toxin. Biotinylated antibodies to C. perfringens alpha toxin bound to streptavidin paramagnetic beads specifically immunoadsorbed soluble sample alpha toxin which subsequently selectively immunoadsorbed ruthenium (Ru)-labeled detection antibodies. The ruthenium chelate of detection antibodies chemically reacted in the presence of tripropylamine and upon electronic stimulation emitted photons (electrochemiluminescence) that were detected by the photodiode of the detector. Elevated toxin concentrations increased toxin immunoadsorption and the specific immunoadsorption of Ru-labeled antibodies to alpha toxin, which resulted in increased dose-dependent electrochemiluminescent signals. The standardized assay was rapid (single 2.5-h coincubation of all reagents), required no wash steps, and had a sensitivity of about 1 ng/ml of toxin. The assay had excellent accuracy and precision and was validated in buffer, serum, and urine with no apparent matrix effects.

Cloning of α-β fusion gene from Clostridium perfringens and its expression

AIM: To study the cloning of α-β fusion gene from Clostridium perfringens and the immunogenicity of α-β fusion expression.

METHODS: Cloning was accomplished after PCR amplification from strains NCTC64609 and C58-1 of the protective antigen genes of α-toxin and β-toxin. The fragment of the gene was cloned using plasmid pZCPAB. This fragment coded for the gene with the stable expression of α-β fusion gene binding. In order to verify the exact location of the α-β fusion gene, domain plasmids were constructed. The two genes were fused into expression vector pBV221. The expressed α-β fusion protein was identified by ELISA, SDS-PAGE, Western blotting and neutralization assay.

RESULTS: The protective α-toxin gene (cpa906) and the β-toxin gene (cpb930) were obtained. The recombinant plasmid pZCPAB carrying α-β fusion gene was constructed and transformed into BL21(DE3). The recombinant strain BL21(DE3)(pZCPAB) was obtained. After the recombinant strain BL21(DE3)(pZCPAB) was induced by 42°C，its expressed product was about 22.14% of total cellular protein at SDS-PAGE and thin-layer gel scanning analysis. Neutralization assay indicated that the antibody induced by immunization with α-β fusion protein could neutralize the toxicity of α-toxin and β-toxin.

CONCLUSION: The obtained α-toxin and β-toxin genes are correct. The recombinant strain BL21(DE3)(pZCPAB) could produce α-β fusion protein. This protein can be used for immunization and is immunogenic. The antibody induced by immunization with α-β fusion protein could neutralize the toxicity of α-toxin and β-toxin.

Clostridium perfringens: insight into virulence evolution and population structure

Clostridium perfringens is an important pathogen in veterinary and medical fields. Diseases caused by this organism are in many cases life threatening or fatal. At the same time, it is part of the ecological community of the intestinal tract of man and animals. Virulence in this species is not fully understood and it does seem that there is erratic distribution of the toxin/enzyme genes within C. perfringens population. We used the recently developed multiple-locus variable-number tandem repeat analysis (MLVA) scheme to investigate the evolution of virulence and population structure of this species. Analysis of the phylogenetic signal indicates that acquisition of the major toxin genes as well as other plasmid-borne toxin genes is a recent evolutionary event and their maintenance is essentially a function of the selective advantage they confer in certain niches under different conditions. In addition, it indicates the ability of virulent strains to cause disease in different host species. More interestingly, there is evidence that certain normal flora strains are virulent when they gain access to a different host species. Analysis of the population structure indicates that recombination events are the major tool that shapes the population and this panmixia is interrupted by frequent clonal expansion that mostly corresponds to disease processes. The signature of positive selection was detected in alpha toxin gene, suggesting the possibility of adaptive alleles on the other chromosomally encoded determinants. Finally, C. perfringens proved to have a dynamic population and availability of more genome sequences and use of comparative proteomics and animal modeling would provide more insight into the virulence of this organism.

Live attenuated vaccine-based control of necrotic enteritis of broiler chickens

A vaccine for necrotic enteritis (NE) of chickens would reduce the current need to prevent or treat the disease in broiler chickens with antimicrobial drugs. The objective of this study was to understand aspects of immunity to the disease. The first experiment examined the virulence of six strains of Clostridium perfringens isolated from cases of NE in broiler chickens. Using a 5-day experimental oral infection of 2-week-old broiler chickens, four of the six strains were found to be virulent. Pulsed-field gel electrophoresis and PCR showed that virulence was not associated with a plasmid encoding the beta2 toxin gene, cpb2, since this was present in virulent and one of the two avirulent strains. In the second experiment, two virulent and one avirulent strains were tested for their ability to immunize ("infection-immunization") chickens through the oral route. The procedure used experimental infection for 5 days followed by bacitracin treatment for 9 days, and then re-challenge 2 days later with a virulent strain, CP4. Infection-immunization with the virulent isolates protected chickens from subsequent virulent challenge, whereas the infection-immunization with the avirulent isolate did not. In a third experiment, two of four alpha-toxin-negative mutants of CP4 protected birds from experimental NE after oral immunization. These two mutants were also attenuated for virulence. We conclude that it is possible to immunize chickens successfully against NE and that immunogen(s) other than alpha-toxin are important in protective immunity against oral infection.

Evidence of the protective immunogenicity of native and recombinant Clostridium haemolyticum phospholipase C (beta toxin) in guinea pigs

The immunogenic effects of the beta toxin of Clostridium haemolyticum were investigated in guinea pigs. Guinea pigs passively immunized with toxin-neutralizing monoclonal antibodies were protected from a 100 LD(50) spore challenge that was lethal to nonvaccinated controls. Guinea pigs actively immunized with varying doses of immunoaffinity-purified native beta toxin were similarly protected. In a third experiment, a recombinant toxoid was prepared from E. coli expressing the beta toxin gene. Guinea pigs immunized three times with recombinant toxoid also were protected against challenge. In each experiment, protection was correlated to the presence of anti-beta toxin antibodies in the serum. Taken together, these results indicate that a neutralizing antibody response to the beta toxin is a key component of protective immunity to C. haemolyticum in guinea pigs.

Highly conserved alpha-toxin sequences of avian isolates of Clostridium perfringens

Clostridium perfringens causes necrotic enteritis in chickens, and alpha-toxin has been suggested to be a key virulence determinant. Analysis of the alpha-toxin of 25 chicken-derived C. perfringens strains demonstrated high homology to mammal-derived strains rather than to the only avian-derived C. perfringens alpha-toxin sequence reported previously.

Maternal vaccination against subclinical necrotic enteritis in broilers

The inclusion of antibacterial feed additives has until now been the major strategy for controlling Clostridium perfringens-associated necrotic enteritis in broilers. In the present study, the effect of maternal immunization against the disease was examined. Broiler breeder hens were injected intramuscularly with candidate vaccines based on C. perfringens type A and type C toxoids adjuvanted with aluminium hydroxide. Vaccination resulted in a strong serum immunoglobulin G response to C. perfringens alpha-toxin in parent hens, and specific antibodies were transferred to their progeny. Subclinical necrotic enteritis in broilers was induced under field conditions or in a disease model, and the occurrence of specific enteric and hepatic lesions was evaluated in randomly selected birds. In three experiments, estimates of odds ratio for developing such lesions were 0.23, 0.33 and 0.56 in maternally toxoid C-immunized broilers compared with non-immunized controls. In toxoid A-immunized birds, odds ratios were estimated at 0.41, 0.61 and 0.63. From these results, immunoprophylaxis seems to be an interesting alternative for the control of necrotic enteritis in broilers.

Clostridium absonum alpha-toxin: new insights into clostridial phospholipase C substrate binding and specificity

Clostridium absonum phospholipase C (Caa) is a 42.7 kDa protein, which shows 60% amino acid sequence identity with the Clostridium perfringens phospholipase C, or alpha-toxin (Cpa), and has been isolated from patients suffering from gas gangrene. We report the cloning and sequencing, purification, characterisation and crystal structure of the Caa enzyme. Caa had twice the phospholipid-hydrolysing (lecithinase) activity, 1.5 times the haemolytic activity and over seven times the activity towards phosphatidylcholine-based liposomes when compared with Cpa. However, the Caa enzyme had a lower activity than Cpa to the free (i.e. not in lipid bilayer) substrate para-nitrophenylphosphorylcholine, towards sphingomyelin-based liposomes and showed half the cytotoxicity. The lethal dose (LD(50)) of Caa in mice was approximately twice that of Cpa. The crystal structure of Caa shows that the 72-93 residue loop is in a conformation different from those of previously determined open-form alpha-toxin structures. This conformational change suggests a role for W84 in membrane binding and a possible route of entry into the active site along a hydrophobic channel created by the re-arrangement of this loop. Overall, the properties of Caa are compatible with a role as a virulence-determinant in gas gangrene caused by C.absonum.