Genome mapping of Clostridium perfringens strains with I-CeuI shows many virulence genes to be plasmid-borne

Virulence Plasmids of the Pathogenic Clostridia

The clostridia cause a spectrum of diseases in humans and animals ranging from life-threatening tetanus and botulism, uterine infections, histotoxic infections and enteric diseases, including antibiotic-associated diarrhea, and food poisoning. The symptoms of all these diseases are the result of potent protein toxins produced by these organisms. These toxins are diverse, ranging from a multitude of pore-forming toxins to phospholipases, metalloproteases, ADP-ribosyltransferases and large glycosyltransferases. The location of the toxin genes is the unifying theme of this review because with one or two exceptions they are all located on plasmids or on bacteriophage that replicate using a plasmid-like intermediate. Some of these plasmids are distantly related whilst others share little or no similarity. Many of these toxin plasmids have been shown to be conjugative. The mobile nature of these toxin genes gives a ready explanation of how clostridial toxin genes have been so widely disseminated both within the clostridial genera as well as in the wider bacterial community.

Recent insights into Clostridium perfringens beta-toxin

Clostridium perfringens beta-toxin is a key mediator of necrotizing enterocolitis and enterotoxemia. It is a pore-forming toxin (PFT) that exerts cytotoxic effect. Experimental investigation using piglet and rabbit intestinal loop models and a mouse infection model apparently showed that beta-toxin is the important pathogenic factor of the organisms. The toxin caused the swelling and disruption of HL-60 cells and formed a functional pore in the lipid raft microdomains of sensitive cells. These findings represent significant progress in the characterization of the toxin with knowledge on its biological features, mechanism of action and structure-function having been accumulated. Our aims here are to review the current progresses in our comprehension of the virulence of C. perfringens type C and the character, biological feature and structure-function of beta-toxin.

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.

Identification of accessory genome regions in poultry Clostridium perfringens isolates carrying the netB plasmid

Necrotic enteritis (NE) is an economically important disease of poultry caused by certain Clostridium perfringens type A strains. NE pathogenesis involves the NetB toxin, which is encoded on a large conjugative plasmid within a 42-kb pathogenicity locus. Recent multilocus sequence type (MLST) studies have identified two predominant NE-associated clonal groups, suggesting that host genes are also involved in NE pathogenesis. We used microarray comparative genomic hybridization (CGH) to assess the gene content of 54 poultry isolates from birds that were healthy or that suffered from NE. A total of 400 genes were variably present among the poultry isolates and nine nonpoultry strains, many of which had putative functions related to nutrient uptake and metabolism and cell wall and capsule biosynthesis. The variable genes were organized into 142 genomic regions, 49 of which contained genes significantly associated with netB-positive isolates. These regions included three previously identified NE-associated loci as well as several apparent fitness-related loci, such as a carbohydrate ABC transporter, a ferric-iron siderophore uptake system, and an adhesion locus. Additional loci were related to plasmid maintenance. Cluster analysis of the CGH data grouped all of the netB-positive poultry isolates into two major groups, separated according to two prevalent clonal groups based on MLST analysis. This study identifies chromosomal loci associated with netB-positive poultry strains, suggesting that the chromosomal background can confer a selective advantage to NE-causing strains, possibly through mechanisms involving iron acquisition, carbohydrate metabolism, and plasmid maintenance.

Necrotic enteritis-derived Clostridium perfringens strain with three closely related independently conjugative toxin and antibiotic resistance plasmids

The pathogenesis of avian necrotic enteritis involves NetB, a pore-forming toxin produced by virulent avian isolates of Clostridium perfringens type A. To determine the location and mobility of the netB structural gene, we examined a derivative of the tetracycline-resistant necrotic enteritis strain EHE-NE18, in which netB was insertionally inactivated by the chloramphenicol and thiamphenicol resistance gene catP. Both tetracycline and thiamphenicol resistance could be transferred either together or separately to a recipient strain in plate matings. The separate transconjugants could act as donors in subsequent matings, which demonstrated that the tetracycline resistance determinant and the netB gene were present on different conjugative elements. Large plasmids were isolated from the transconjugants and analyzed by high-throughput sequencing. Analysis of the resultant data indicated that there were actually three large conjugative plasmids present in the original strain, each with its own toxin or antibiotic resistance locus. Each plasmid contained a highly conserved 40-kb region that included plasmid replication and transfer regions that were closely related to the 47-kb conjugative tetracycline resistance plasmid pCW3 from C. perfringens. The plasmids were as follows: (i) a conjugative 49-kb tetracycline resistance plasmid that was very similar to pCW3, (ii) a conjugative 82-kb plasmid that contained the netB gene and other potential virulence genes, and (iii) a 70-kb plasmid that carried the cpb2 gene, which encodes a different pore-forming toxin, beta2 toxin.

The anaerobic bacterium Clostridium perfringens can cause an avian gastrointestinal disease known as necrotic enteritis. Disease pathogenesis is not well understood, although the plasmid-encoded pore-forming toxin NetB, is an important virulence factor. In this work, we have shown that the plasmid that carries the netB gene is conjugative and has a 40-kb region that is very similar to replication and transfer regions found within each of the sequenced conjugative plasmids from C. perfringens. We also showed that this strain contained two additional large plasmids that were also conjugative and carried a similar 40-kb region. One of these plasmids encoded beta2 toxin, and the other encoded tetracycline resistance. To our knowledge, this is the first report of a bacterial strain that carries three closely related but different independently conjugative plasmids. These results have significant implications for our understanding of the transmission of virulence and antibiotic resistance genes in pathogenic bacteria.

Recent progress in understanding the pathogenesis of Clostridium perfringens type C infections

Clostridium perfringens type C causes necrotizing enteritis in humans and several other animal species. Type C isolates must produce at least beta toxin (CPB) and alpha toxin (CPA) and most strains produce several other toxins including perfringolysin O (PFO) and TpeL. However, current evidence indicates that CPB is the main virulence factor for type C infections. Most of this evidence is based upon the loss of virulence shown by isogenic type C CPB knock out mutants on cells, and also in rabbit intestinal loops and in mouse models. This virulence is regained when these mutants are complemented with the wild-type cpb gene. Many type C isolates respond to close contact with enterocyte-like Caco-2 cells by producing all toxins, except TpeL, much more rapidly than occurs during in vitro growth. This in vivo effect involves rapid transcriptional upregulation of the cpb, cpb2, pfoA and plc toxin genes. Rapid Caco-2 cell-induced upregulation of CPB and PFO production involves the VirS/VirR two-component system, since upregulated in vivo transcription of the pfoA and cpb genes was blocked by inactivating the virR gene and was reversible by complementation to restore VirR expression.

Characterization of toxin plasmids in Clostridium perfringens type C isolates

Clostridium perfringens type C isolates cause enteritis necroticans in humans or necrotizing enteritis and enterotoxemia in domestic animals. Type C isolates always produce alpha toxin and beta toxin but often produce additional toxins, e.g., beta2 toxin or enterotoxin. Since plasmid carriage of toxin-encoding genes has not been systematically investigated for type C isolates, the current study used Southern blot hybridization of pulsed-field gels to test whether several toxin genes are plasmid borne among a collection of type C isolates. Those analyses revealed that the surveyed type C isolates carry their beta toxin-encoding gene (cpb) on plasmids ranging in size from ∼65 to ∼110 kb. When present in these type C isolates, the beta2 toxin gene localized to plasmids distinct from the cpb plasmid. However, some enterotoxin-positive type C isolates appeared to carry their enterotoxin-encoding cpe gene on a cpb plasmid. The tpeL gene encoding the large clostridial cytotoxin was localized to the cpb plasmids of some cpe-negative type C isolates. The cpb plasmids in most surveyed isolates were found to carry both IS1151 sequences and the tcp genes, which can mediate conjugative C. perfringens plasmid transfer. A dcm gene, which is often present near C. perfringens plasmid-borne toxin genes, was identified upstream of the cpb gene in many type C isolates. Overlapping PCR analyses suggested that the toxin-encoding plasmids of the surveyed type C isolates differ from the cpe plasmids of type A isolates. These findings provide new insight into plasmids of proven or potential importance for type C virulence.

NetB, a pore-forming toxin from necrotic enteritis strains of Clostridium perfringens

The Clostridium perfringens necrotic enteritis B-like toxin (NetB) is a recently discovered member of the β-barrel pore-forming toxin family and is produced by a subset of avian C. perfringens type A strains. NetB is cytotoxic for avian cells and is associated with avian necrotic enteritis. This review examines the current state of knowledge of NetB: its role in pathogenesis, its distribution and expression in C. perfringens and its vaccine potential.

Characterization of virulence plasmid diversity among Clostridium perfringens type B isolates

The important veterinary pathogen Clostridium perfringens type B is unique for producing the two most lethal C. perfringens toxins, i.e., epsilon-toxin and beta-toxin. Our recent study (K. Miyamoto, J. Li, S. Sayeed, S. Akimoto, and B. A. McClane, J. Bacteriol. 190:7178-7188, 2008) showed that most, if not all, type B isolates carry a 65-kb epsilon-toxin-encoding plasmid. However, this epsilon-toxin plasmid did not possess the cpb gene encoding beta-toxin, suggesting that type B isolates carry at least one additional virulence plasmid. Therefore, the current study used Southern blotting of pulsed-field gels to localize the cpb gene to approximately 90-kb plasmids in most type B isolates, although a few isolates carried a approximately 65-kb cpb plasmid distinct from their etx plasmid. Overlapping PCR analysis then showed that the gene encoding the recently discovered TpeL toxin is located approximately 3 kb downstream of the plasmid-borne cpb gene. As shown earlier for their epsilon-toxin-encoding plasmids, the beta-toxin-encoding plasmids of type B isolates were found to carry a tcp locus, suggesting that they are conjugative. Additionally, IS1151-like sequences were identified upstream of the cpb gene in type B isolates. These IS1151-like sequences may mobilize the cpb gene based upon detection of possible cpb-containing circular transposition intermediates. Most type B isolates also possessed a third virulence plasmid that carries genes encoding urease and lambda-toxin. Collectively, these findings suggest that type B isolates are among the most plasmid dependent of all C. perfringens isolates for virulence, as they usually carry three potential virulence plasmids.

Genetic variation and evolution of the pathogenicity island of Enterococcus faecalis

Enterococcus faecalis is a leading cause of nosocomial infections and is known for its ability to acquire and transfer virulence and antibiotic resistance determinants from other organisms. A 150-kb pathogenicity island (PAI) encoding several genes that contribute to pathogenesis was identified among antibiotic-resistant clinical isolates. In the current study, we examined the structure of the PAI in a collection of isolates from diverse sources in order to gain insight into its genesis and dynamics. Using multilocus sequence typing to assess relatedness at the level of strain background and microarray analysis to identify variations in the PAI, we determined the extent to which structural variations occur within the PAI and also the extent to which these variations occur independently of the chromosome. Our findings provide evidence for a modular gain of defined gene clusters by the PAI. These results support horizontal transfer as the mechanism for accretion of genes into the PAI and highlight a likely role for mobile elements in the evolution of the E. faecalis PAI.

Sequencing and diversity analyses reveal extensive similarities between some epsilon-toxin-encoding plasmids and the pCPF5603 Clostridium perfringens enterotoxin plasmid

Clostridium perfringens type B and D isolates produce epsilon-toxin, the third most potent clostridial toxin. The epsilon-toxin gene (etx) is plasmid borne in type D isolates, but etx genetics have been poorly studied in type B isolates. This study reports the first sequencing of any etx plasmid, i.e., pCP8533etx, from type B strain NCTC8533. This etx plasmid is 64.7 kb, carries tcp conjugative transfer genes, and encodes additional potential virulence factors including beta2-toxin, sortase, and collagen adhesin but not beta-toxin. Interestingly, nearly 80% of pCP8533etx open reading frames (ORFs) are also present on pCPF5603, an enterotoxin-encoding plasmid from type A isolate F5603. Pulsed-field gel electrophoresis and overlapping PCR indicated that a pCP8533etx-like etx plasmid is also present in most, if not all, other type B isolates and some beta2-toxin-positive, cpe-negative type D isolates, while other type D isolates carry different etx plasmids. Sequences upstream of the etx gene vary between type B isolates and some type D isolates that do not carry a pCP8533etx-like etx plasmid. However, nearly all type B and D isolates have an etx locus with an upstream IS1151, and those etx loci typically reside near a dcm ORF. These results suggest that pCPF5603 and pCP8533etx evolved from insertion of mobile genetic elements carrying enterotoxin or etx genes, respectively, onto a common progenitor plasmid.

Virulence plasmid diversity in Clostridium perfringens type D isolates

Clostridium perfringens type D isolates are important in biodefense and also cause natural enterotoxemias in sheep, goats, and occasionally cattle. In these isolates, the gene (etx) encoding epsilon-toxin is thought to reside on poorly characterized large plasmids. Type D isolates sometimes also produce other potentially plasmid-encoded toxins, including C. perfringens enterotoxin and beta2 toxin, encoded by the cpe and cbp2 genes, respectively. In the current study we demonstrated that the etx, cpe, and cpb2 genes are carried on plasmids in type D isolates and characterized the toxin-encoding plasmids to obtain insight into their genetic organization, potential transferability, and diversity. Southern blotting of pulsed-field gels showed that the etx gene of type D isolates can be present on at least five different plasmids, whose sizes range from 48 to 110 kb. The etx plasmids also typically carried IS1151 and tcp open reading frames (ORFs) known to mediate conjugative transfer of C. perfringens plasmid pCW3. PCR studies revealed that other than their tcp ORFs, etx plasmids of type D isolates do not carry substantial portions of the conserved or variable regions in the cpe plasmids of type A isolates. Southern blotting also demonstrated that in type D isolates the cpe and cpb2 genes are sometimes present on the etx plasmid. Collectively, these findings confirmed that the virulence of type D isolates is heavily plasmid dependent and indicated that (i) a single type D isolate can carry multiple virulence plasmids, (ii) a single type D virulence plasmid can carry up to three different toxin genes, and (iii) many etx plasmids should be capable of conjugative transfer.

Humans as reservoir for enterotoxin gene--carrying Clostridium perfringens type A

Humans may play a role in the transmission of gastrointestinal diseases caused by C. perfringens.

We found a prevalence of 18% for enterotoxin gene–carrying (cpe+) Clostridium perfringens in the feces of healthy food handlers by PCR and isolated the organism from 11 of 23 PCR-positive persons by using hydrophobic grid membrane filter-colony hybridization. Several different cpe genotypes were recovered. The prevalence was 3.7% for plasmidial IS1151-cpe, 2.9% for plasmidial IS1470-like-cpe, 0.7% for chromosomal IS1470-cpe, and 1.5% for unknown cpe genotype. Lateral spread of cpe between C. perfringens strains was evident because strains from the same person carried IS1470-like cpe but shared no genetic relatedness according to pulsed-field gel electrophoresis analysis. Our findings suggest that healthy humans serve as a rich reservoir for cpe+ C. perfringens type A and may play a role in the etiology of gastrointestinal diseases caused by this organism. The results also indicate that humans should be considered a risk factor for spread of C. perfringens type A food poisoning and that they are a possible source of contamination for C. perfringens type A food poisoning.

Comparison of virulence plasmids among Clostridium perfringens type E isolates

Clostridium perfringens type E isolates produce iota-toxin, which is encoded by iap and ibp genes. Using Southern blot analyses, the current study identified iap/ibp plasmids of approximately 97 or approximately 135 kb among eight type E isolates. For most of these isolates, their iap/ibp plasmid also encoded urease and lambda-toxin. However, the beta2-toxin gene, if present, was on a different plasmid from the iap/ibp plasmid. For all isolates, the iap/ibp plasmid carried a tcp locus, strongly suggesting that these plasmids are conjugative. Overlapping PCR analyses demonstrated some similarity between the iap/ibp plasmids and enterotoxin-encoding plasmids of type A isolates. Additional PCR analyses demonstrated that the iap/ibp locus is located near dcm sequences, an apparent plasmid hot spot for toxin gene insertion, and that two IS1151-related sequences are present in the iap/ibp locus. To begin testing whether those IS1151-like sequences can mobilize iap/ibp genes, a PCR assay was performed that amplifies a product only from circular DNA forms that could represent transposition intermediates. This PCR assay detected circular forms containing iap/ibp genes and silent enterotoxin gene sequences, with or without an IS1151-like sequence. Collectively, these results suggest that a mobile genetic element carrying iap/ibp has inserted onto a tcp-carrying enterotoxin plasmid in a type A isolate, creating a progenitor iap/ibp plasmid. That plasmid then spread via conjugation to other isolates, converting them to type E. Further iap/ibp plasmid diversity occurred when either the iap/ibp genes later remobilized and inserted onto other conjugative plasmids or some iap/ibp plasmids acquired additional DNA sequences.

Comparison of the levels of heat resistance of wild-type, cpe knockout, and cpe plasmid-cured Clostridium perfringens type A strains

An enterotoxin (cpe) plasmid was cured from a Clostridium perfringens non-food-borne gastrointestinal disease (NFBGID) isolate, and the heat resistance levels of wild-type, cpe knockout, and cpe plasmid-cured strains were compared. Our results demonstrated that (i) wild-type cpe has no influence in mediating high-level heat resistance in C. perfringens and (ii) the cpe plasmid does not confer heat sensitivity on NFBGID isolates.

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.

Enterotoxigenicity and genetic relatedness of Clostridium perfringens isolates from retail foods in the United States

Clostridium perfringens is a leading cause of bacterial food-borne illness in countries where consumption of meat and poultry is high. For example, each year in the United States, this organism is the second or third most common cause of confirmed cases of food-borne illness. Surveys of the incidence of this organism in retail foods were done in the 1960s without regard to whether isolates were enterotoxigenic. It is now known that not all strains of this organism possess the enterotoxin gene responsible for illness. We examined the incidence of this organism in 131 food samples from retail food stores in an area of the northeastern United States. Forty isolates were obtained by using the iron milk method at 45 degrees C, with confirmation by use of motility nitrate and lactose gelatin media. The presence of the C. perfringens enterotoxin (cpe) and alpha toxin (cpa) genes was determined by PCR using previously published primer sequences. All isolates possessed cpa. None of the isolates were identified as carrying the cpe gene by this method or by another method using a digoxigenin-labeled gene probe. Consistent with these results, none of the sporulating-cell extracts contained enterotoxin as determined by reverse passive latex hemagglutination. Pulsed-field gel electrophoresis was used to determine the genetic relatedness of the isolates. About 5% of the isolates were considered to be closely related (2- to 3-band difference). The others were considered to be unrelated to one another. The results demonstrate the rarity of cpe(+) strains in retail foods and the genetic diversity among nonoutbreak strains.

Molecular epidemiology of Clostridium perfringens related to food-borne outbreaks of disease in Finland from 1984 to 1999

From 1975 to 1999, Clostridium perfringens caused 238 food-borne disease outbreaks in Finland, which is 20% of all such reported outbreaks during these years. The fact that C. perfringens is commonly found in human and animal stools and that it is also widespread in the environment is a disadvantage when one is searching for the specific cause of a food-borne infection by traditional methods. In order to strengthen the evidence-based diagnostics of food poisonings suspected to be caused by C. perfringens, we retrospectively investigated 47 C. perfringens isolates by PCR for the cpe gene, which encodes enterotoxin; by reversed passive latex agglutination to detect the enterotoxin production; and by pulsed-field gel electrophoresis (PFGE) to compare their genotypes after restriction of DNA by the enzymes SmaI and ApaI. The strains were isolated during 1984 to 1999 from nine food-borne outbreaks of disease originally reported as having been caused by C. perfringens. In seven of the nine outbreaks our results supported the fact that the cause was C. perfringens. Our findings emphasize the importance of a more detailed characterization of C. perfringens isolates than mere identification to the species level in order to verify the cause of an outbreak. Also, to increase the probability of finding the significant cpe-positive C. perfringens strains, it is very important to isolate and investigate more than one colony from the fecal culture of a patient and screen all these isolates for the presence of the cpe gene before further laboratory work is done.

Clostridium perfringens and foodborne infections

Clostridium perfringens type A food poisoning is one of the more common in the industrialised world. This bacterium is also responsible for the rare but severe food borne necrotic enteritis. C. perfringens enterotoxin (CPE) has been shown to be the virulence factor responsible for causing the symptoms of C. perfringens type A food poisoning. CPE is a single polypeptide chain with a molecular weight of 3.5 kDa that binds to receptors on the target epithelial cells. Through a unique four-step membrane action it finally causes a breakdown in normal plasma membrane permeability properties. Genetic studies of cpe have shown that cpe can be either chromosomal or plasmid-borne and that only a small minority of the global C. perfringens population is cpe positive. CPE expression appears to be transcriptionally regulated during sporulation, at least in part, by regulatory factors that are common to all C. perfringens isolates.

Gene transfer between Salmonella enterica serovar Typhimurium inside epithelial cells

Virulence and antibiotic resistance genes transfer between bacteria by bacterial conjugation. Conjugation also mediates gene transfer from bacteria to eukaryotic organisms, including yeast and human cells. Predicting when and where genes transfer by conjugation could enhance our understanding of the risks involved in the release of genetically modified organisms, including those being developed for use as vaccines. We report here that Salmonella enterica serovar Typhimurium conjugated inside cultured human cells. The DNA transfer from donor to recipient bacteria was proportional to the probability that the two types of bacteria occupied the same cell, which was dependent on viable and invasive bacteria and on plasmid tra genes. Based on the high frequencies of gene transfer between bacteria inside human cells, we suggest that such gene transfers occur in situ. The implications of gene transfer between bacteria inside human cells, particularly in the context of antibiotic resistance, are discussed.

Clostridial hydrolytic enzymes degrading extracellular components

Bacteria belonging to the genus Clostridium, both glycolytic and proteolytic, and both pathogenic and non-pathogenic, produce a battery of hydrolytic enzymes to obtain nutrients from various biopolymers. The clostridial hydrolytic enzymes are diverse, and are used or are potentially useful for fundamental and applied research purposes. Among them, enzymes degrading the major components in the extracellular matrix or on the cell surface in vertebrates are herein reviewed with special emphasis on recent knowledge gained through molecular biology of clostridial collagenases, sialidases and hyaluronidases. This paper also reviews some literature on the biotechnological approach to the designing of new molecular tools and drug delivery systems involving clostridial hydrolytic enzymes.

Enterotoxin plasmid from Clostridium perfringens is conjugative

Clostridium perfringens enterotoxin is the major virulence factor involved in the pathogenesis of C. perfringens type A food poisoning and several non-food-borne human gastrointestinal illnesses. The enterotoxin gene, cpe, is located on the chromosome of food-poisoning isolates but is found on a large plasmid in non-food-borne gastrointestinal disease isolates and in veterinary isolates. To evaluate whether the cpe plasmid encodes its own conjugative transfer, a C. perfringens strain carrying pMRS4969, a plasmid in which a 0.4-kb segment internal to the cpe gene had been replaced by the chloramphenicol resistance gene catP, was used as a donor in matings with several cpe-negative C. perfringens isolates. Chloramphenicol resistance was transferred at frequencies ranging from 2.0 x 10(-2) to 4.6 x 10(-4) transconjugants per donor cell. The transconjugants were characterized by PCR, pulsed-field gel electrophoresis, and Southern hybridization analyses. The results demonstrated that the entire pMRS4969 plasmid had been transferred to the recipient strain. Plasmid transfer required cell-to-cell contact and was DNase resistant, indicating that transfer occurred by a conjugation mechanism. In addition, several fragments of the prototype C. perfringens tetracycline resistance plasmid, pCW3, hybridized with pMRS4969, suggesting that pCW3 shares some similarity to pMRS4969. The clinical significance of these findings is that if conjugative transfer of the cpe plasmid occurred in vivo, it would have the potential to convert cpe-negative C. perfringens strains in normal intestinal flora into strains capable of causing gastrointestinal disease.

Genotyping of enterotoxigenic Clostridium perfringens fecal isolates associated with antibiotic-associated diarrhea and food poisoning in North America

Clostridium perfringens type A isolates producing enterotoxin (CPE) are an important cause of food poisoning and non-food-borne human gastrointestinal (GI) diseases, including antibiotic-associated diarrhea (AAD). Recent studies suggest that C. perfringens type A food poisoning is caused by C. perfringens isolates carrying a chromosomal cpe gene, while CPE-associated non-food-borne GI diseases, such as AAD, are caused by plasmid cpe isolates. Those putative relationships, obtained predominantly with European isolates, were tested in the current study by examining 34 cpe-positive, C. perfringens fecal isolates from North American cases of food poisoning or AAD. These North American disease isolates were all classified as type A using a multiplex PCR assay. Furthermore, restriction fragment length polymorphism and pulsed-field gel electrophoresis genotyping analyses showed the North American AAD isolates included in this collection all have a plasmid cpe gene, but the North American food poisoning isolates all carry a chromosomal cpe gene. Western blotting demonstrated CPE expression by nearly all of these disease isolates, confirming their virulence potential. These findings with North American isolates provide important new evidence that, regardless of geographic origin or date of isolation, plasmid cpe isolates cause most CPE-associated AAD cases and chromosomal cpe isolates cause most C. perfringens type A food poisoning cases. These findings hold importance for the development of assays for distinguishing cases of CPE-associated food-borne and non-food-borne human GI illnesses and also identify potential epidemiologic tools for determining the reservoirs for these illnesses.

Comparative experiments to examine the effects of heating on vegetative cells and spores of Clostridium perfringens isolates carrying plasmid genes versus chromosomal enterotoxin genes

Clostridium perfringens enterotoxin (CPE) is an important virulence factor for both C. perfringens type A food poisoning and several non-food-borne human gastrointestinal diseases. Recent studies have indicated that C. perfringens isolates associated with food poisoning carry a chromosomal cpe gene, while non-food-borne human gastrointestinal disease isolates carry a plasmid cpe gene. However, no explanation has been provided for the strong associations between certain cpe genotypes and particular CPE-associated diseases. Since C. perfringens food poisoning usually involves cooked meat products, we hypothesized that chromosomal cpe isolates are so strongly associated with food poisoning because (i) they are more heat resistant than plasmid cpe isolates, (ii) heating induces loss of the cpe plasmid, or (iii) heating induces migration of the plasmid cpe gene to the chromosome. When we tested these hypotheses, vegetative cells of chromosomal cpe isolates were found to exhibit, on average approximately twofold-higher decimal reduction values (D values) at 55 degrees C than vegetative cells of plasmid cpe isolates exhibited. Furthermore, the spores of chromosomal cpe isolates had, on average, approximately 60-fold-higher D values at 100 degrees C than the spores of plasmid cpe isolates had. Southern hybridization and CPE Western blot analyses demonstrated that all survivors of heating retained their cpe gene in its original plasmid or chromosomal location and could still express CPE. These results suggest that chromosomal cpe isolates are strongly associated with food poisoning, at least in part, because their cells and spores possess a high degree of heat resistance, which should enhance their survival in incompletely cooked or inadequately warmed foods.

Do antibiotics maintain antibiotic resistance?

Important human pathogens resistant to antibiotics result from the human use of antibiotics. Does this imply that reducing their usage or removing antibiotics from medicine and agriculture will restore the effectiveness of these drugs? The authors argue that resistance evolution and susceptibility evolution are not, in a sense, just different sides of the same coin. Resistance genes acquire new functions and the initial costs of resistance can evolve into advantages. Decreasing drug use might not replace a fundamental change in drug design to avoid the evolution of resistant, and encourage the evolution of susceptible, microorganisms.

Sequence and organization of pXO1, the large Bacillus anthracis plasmid harboring the anthrax toxin genes

The Bacillus anthracis Sterne plasmid pXO1 was sequenced by random, "shotgun" cloning. A circular sequence of 181,654 bp was generated. One hundred forty-three open reading frames (ORFs) were predicted using GeneMark and GeneMark.hmm, comprising only 61% (110,817 bp) of the pXO1 DNA sequence. The overall guanine-plus-cytosine content of the plasmid is 32.5%. The most recognizable feature of the plasmid is a "pathogenicity island," defined by a 44.8-kb region that is bordered by inverted IS1627 elements at each end. This region contains the three toxin genes (cya, lef, and pagA), regulatory elements controlling the toxin genes, three germination response genes, and 19 additional ORFs. Nearly 70% of the ORFs on pXO1 do not have significant similarity to sequences available in open databases. Absent from the pXO1 sequence are homologs to genes that are typically required to drive theta replication and to maintain stability of large plasmids in Bacillus spp. Among the ORFs with a high degree of similarity to known sequences are a collection of putative transposases, resolvases, and integrases, suggesting an evolution involving lateral movement of DNA among species. Among the remaining ORFs, there are three sequences that may encode enzymes responsible for the synthesis of a polysaccharide capsule usually associated with serotype-specific virulent streptococci.

Gene duplication and multiplicity of collagenases in Clostridium histolyticum

Clostridium histolyticum collagenase contains a number of different active components. Previously we have shown that colH encodes a 116-kDa collagenase (ColH) and a 98-kDa gelatinase. We purified a different 116-kDa collagenase (ColG) from the culture supernatant and sequenced its gene (colG). We also identified four other gelatinases (105, 82, 78, and 67 kDa) and determined their N-terminal amino acid sequences, all of which coincided with that of either ColG or ColH. Hybridization experiments showed that each gene is present in a single copy and each gene is transcribed into a single mRNA. These results suggest that all the gelatinases are produced from the respective full-length collagenase by the proteolytic removal of C-terminal fragments. The substrate specificities of the enzymes suggest that colG and colH encode class I and class II enzymes, respectively. Analysis of their DNA locations by pulsed-field gel electrophoresis and nucleotide sequencing of their surrounding regions revealed that the two genes are located in different sites on the chromosome. C. histolyticum colG is more similar to C. perfringens colA than to colH in terms of domain structure. Both colG and colA have a homologous gene, mscL, at their 3' ends. These results suggest that gene duplication and segment duplication have occurred in an ancestor cell common to C. histolyticum and C. perfringens and that further divergence of the parent gene produced colG and colA.

Clostridium perfringens type E animal enteritis isolates with highly conserved, silent enterotoxin gene sequences

Several Clostridium perfringens genotype E isolates, all associated with hemorrhagic enteritis of neonatal calves, were identified by multiplex PCR. These genotype E isolates were demonstrated to express alpha and iota toxins, but, despite carrying sequences for the gene (cpe) encoding C. perfringens enterotoxin (CPE), were unable to express CPE. These silent cpe sequences were shown to be highly conserved among type E isolates. However, relative to the functional cpe gene of type A isolates, these silent type E cpe sequences were found to contain nine nonsense and two frameshift mutations and to lack the initiation codon, promoters, and ribosome binding site. The type E animal enteritis isolates carrying these silent cpe sequences do not appear to be clonally related, and their silent type E cpe sequences are always located, near the iota toxin genes, on episomal DNA. These findings suggest that the highly conserved, silent cpe sequences present in most or all type E isolates may have resulted from the recent horizontal transfer of an episome, which also carries iota toxin genes, to several different type A C. perfringens isolates.

Evidence that the enterotoxin gene can be episomal in Clostridium perfringens isolates associated with non-food-borne human gastrointestinal diseases

Clostridium perfringens enterotoxin (CPE) is responsible for the diarrheal and cramping symptoms of human C. perfringens type A food poisoning. CPE-producing C. perfringens isolates have also recently been associated with several non-food-borne human gastrointestinal (GI) illnesses, including antibiotic-associated diarrhea and sporadic diarrhea. The current study has used restriction fragment length polymorphism (RFLP) and pulsed-field gel electrophoresis (PFGE) analyses to compare the genotypes of 43 cpe-positive C. perfringens isolates obtained from diverse sources. All North American and European food-poisoning isolates examined in this study were found to carry a chromosomal cpe, while all non-food-borne human GI disease isolates characterized in this study were determined to carry their cpe on an episome. Collectively, these results provide the first evidence that distinct subpopulations of cpe-positive C. perfringens isolates may be responsible for C. perfringens type A food poisoning versus CPE-associated non-food-borne human GI diseases. If these putative associations are confirmed in additional surveys, cpe RFLP and PFGE genotyping assays may facilitate the differential diagnosis of food-borne versus non-food-borne CPE-associated human GI illnesses and may also be useful epidemiologic tools for identifying reservoirs or transmission mechanisms for the subpopulations of cpe-positive isolates specifically responsible for CPE-associated food-borne versus non-food-borne human GI diseases.

Beta2 toxin, a novel toxin produced by Clostridium perfringens

A novel toxin (Beta2) and its gene were characterized from a Clostridium perfringens strain isolated from a piglet with necrotic enteritis. At the amino-acid level, Beta2 toxin (27670 Da) has no significant homology with the previously identified Beta toxin (called Beta1) (34861 kDa) from C. perfringens type B NCTC8533 ( Hunter, S.E.C., Brown, J.E., Oyston, P.C.F., Sakurai, J., Titball, R.W., 1993. Molecular genetic analysis of beta-toxin of Clostridium perfringens reveals sequence homology with alpha-toxin, gamma-toxin, and leukocidin of Staphylococcus aureus. Infect. Immun. 61, 3958-3965). Both Beta1 and Beta2 toxins were lethal for mice and cytotoxic for the cell line 1407, inducing cell rounding and lysis without affecting the actin cytoskeleton. The genes encoding Beta1 and Beta2 toxins have been localized in unlinked loci in large plasmids of C. perfringens. In addition, Beta2 toxin-producing C. perfringens strains were found to be associated with animal diseases such as necrotic enteritis in piglets and enterocolitis in horses.

Evolutionarily conserved and functionally important residues in the I-CeuI homing endonuclease

Two approaches were used to discern critical amino acid residues for the function of the I- Ceu I homing endonuclease: sequence comparison of subfamilies of homologous proteins and genetic selection. The first approach revealed residues potentially involved in catalysis and DNA recognition. Because I- Ceu I is lethal in Escherichia coli , enzyme variants not perturbing cell viability were readily selected from an expression library. A collection of 49 variants with single amino acid substitutions at 37 positions was assembled. Most of these positions are clustered within or around the LAGLI-DADG dodecapeptide and the TQH sequence, two motifs found in all protein subfamilies examined. The Km and kcat values of the wild-type and nine variant enzymes synthesized in vitro were determined. Three variants, including one showing a substitution of the glutamine residue in the TQH motif, revealed no detectable endonuclease activity; five others showed reduced activity compared to the wild-type enzyme; whereas the remaining variant cleaved the top strand about three times more efficiently than the wild-type. Our results not only confirm recent reports indicating that amino acids in the LAGLI-DADG dodecapeptide are functionally critical, but they also suggest that some residues outside this motif directly participate in catalysis.

Clostridium perfringens urease genes are plasmid borne

Although many bacteria are ureolytic, and in some cases urease acts as a virulence factor, the urease phenotype has not been analyzed in the anaerobic pathogen Clostridium perfringens. In this study, approximately 2% of C. perfringens strains, representing the principal biotypes, were found to harbor the urease structural genes, ureABC, and these were localized on large plasmids that often encode, in addition, the lethal epsilon or iota toxins or the enterotoxin. This represents the first report of a plasmid-encoded urease in a gram-positive bacterium. The C. perfringens enzyme was highly similar to the ureases of other bacteria and cross-reacted with antibodies raised against the urease purified from Helicobacter pylori. Urease production was inhibited by urea and induced under growth conditions where the availability of nitrogen sources was limiting. To date, this form of regulation has been observed only for chromosomal ureABC genes.