Production of cytolethal distending toxins by pathogenic Escherichia coli strains isolated from human and animal sources: establishment of the existence of a new cdt variant (Type IV)

Protection of rabbits against enteropathogenic Escherichia coli (EPEC) using an intimin null mutant

Background

Diarrhea and mortality resulting from infections with enteropathogenic Escherichia coli (EPEC) are of major economic importance in the rabbit meat industry. There is a growing need for an effective vaccine to cope with these problems and to reduce the use of antibiotics. EPEC are characterized by an attaching and effacing virulence mechanism. This is partly mediated by the intimate binding between an adhesin, called intimin, and a translocated receptor (Tir) of prokaryote origin. We constructed an intimin deletion mutant of the rabbit EPEC (REPEC) wild-type strain 97/241.6 (bio-/serogroup 3-/O15) and examined its protective capacity.

Results

After verifying its complete loss of virulence, we used the attenuated strain in vaccination-challenge experiments in which complete protection against a homologous, but virulent, strain was observed. The attenuated strain was able to persist in the intestinal lumen, where it elicited an immune response against EPEC-related virulence proteins, as was shown using an EspB-specific ELISA. Despite the priming of an immune response and the generation of specific antibodies, the intimin mutant was not able to fully protect rabbits against challenges with REPEC strains of other bio-/serogroups.

Conclusion

These data indicate that protection against REPEC infections is at least partly bio-/serogroup dependent and a multivalent vaccine may be needed for protection against the full range of REPEC types. Such a combination vaccine may be developed using intimin null mutants, as the latter were clearly shown to be safe and effective against homologous infections.

Pediatric urinary tract infections

Pediatric urinary tract infections are common. These infections have been recognized as a source of acute morbidity and long-term medical consequences in adulthood. There are various risk factors and clinical presentations in children with urinary tract infections. The main objectives in management include prompt diagnosis, appropriate antimicrobial therapy, identification of anatomic anomalies, and, in select patients, long-term follow-up.

Comparison of polymerase chain reaction systems for detection of different cdt genes in Escherichia coli strains

AIMS

Cytolethal distending toxins (CDT) are tripartite toxins encoded by three adjacent or overlapping genes (cdtA, cdtB, cdtC) and found in multiple pathogens. The present knowledge regarding heterogeneity of cdt genes and our previous study revealed that the available polymerase chain reaction (PCR) systems lack adequate specificity. The detection of various cdt genes present in Escherichia coli strains, from different geographical regions demands further assays for wide-range coverage. On the basis of these observations, we were prompted to undertake the present study; hence the specificity of existing PCR systems was addressed using E. coli prototype strains with known cdt gene sequences.

METHODS AND RESULTS

A multiplex PCR designed for the detection of E. coli cdt genes was found to be sensitive and specific enough for initial screening. However, for subtyping, the PCR systems yielded nonspecific products upon amplification. These primers are usually designed for sequences of the cdtB locus (the most conserved region of the gene), and since CDT-producing E. coli strains carry different cdt genes, none of the systems are really type specific. Furthermore, PCR systems with type-specific primers for other regions of the gene, i.e. ORF A or ORF C are found to be strain specific and their applications in different geographical regions have limitations.

CONCLUSIONS

In conclusion, based on our observations, using these available primers, it seems that the existing PCR systems are not sufficiently accurate to differentiate between different types of cdt genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained from this study revealed that so-far reported PCR systems are short in specificity. These PCR protocols were not found to be specific enough to detect various cdt genes and have a limited range of application. Moreover, due to similarities in cdt genes the cross-reaction between different sets of primers exists. Hence for epidemiological studies, some additional PCR protocols are required for screening clinical isolates for cdt genes.

Phenotypic and genetic characterization of enteropathogenic Escherichia coli (EPEC) and entero-aggregative E. coli (EAEC) from diarrhoeal and non-diarrhoeal children in Libya

A total of 50 Escherichia coli strains isolated in a Libyan hospital (20 from children with diarrhoea and 30 from healthy children) were investigated for their pathotypes and virulence traits. Altogether nine eae-positive (enteropathogenic E. coli, EPEC) and nine aggR-positive (entero-aggregative E. coli, EAEC) strains were identified. Significantly (P=0.001) more EPEC strains were identified from diarrhoeal patients (n=8) than from healthy controls (n=1), while six EAEC strains were identified from diarrhoeal and three from healthy children. Typical (eae(+), EAF(+), bfp(+)) EPEC strains (n=6) belonged to classical EPEC serogroups O55, O114, O127 and showed localized adherence on Hela cells. EAEC strains revealed genetic heterogeneity but uniformly adhered to HeLa cultures in an entero-aggregative adherence pattern. Antibiotic resistance frequently, characterized the strains. Sixty-eight percentage of the strains were resistant against at least one antibiotic and 30% harbored a class 1 integron independently of their clinical background. This is the first report from North Africa demonstrating the significance of EPEC and EAEC.

Characteristics of cytotoxic necrotizing factor and cytolethal distending toxin producing Escherichia coli strains isolated from meat samples in Northern Ireland

Swabs collected from pig, lamb and beef carcasses and samples of pork, lamb and beef mince were cultured for Escherichia coli strains. Strains harbouring cytotoxic necrotizing factors (CNF1 and 2) and cytolethal distending toxins (CDT-I,-II,-III and -IV) were identified in plate cultures of the isolates by colony hybridization with labelled probes and multiplex PCR assays. Simplex and multiplex PCR assays were used to further characterize the isolates to determine the presence of P, S and F17 fimbriae as well as afimbrial adhesins and haemolysin. The serotype was also determined where possible. Thirty strains with the capacity to code for CNF (4), CDT (24) or both (2) were isolated and characterized, and a wide range of associated factor patterns was observed. The methods utilized were successful in demonstrating the detection of viable strains with potentially significant pathogenic factors from human food sources.

Sequence analysis of the Escherichia coli O15 antigen gene cluster and development of a PCR assay for rapid detection of intestinal and extraintestinal pathogenic E. coli O15 strains

A collection of 33 Escherichia coli serogroup O15 strains was studied with regard to O:H serotypes and virulence markers and for detection of the O-antigen-specific genes wzx and wzy. The strains were from nine different countries, originated from healthy or diseased humans and animals and from food, and were isolated between 1941 and 2003. On the basis of virulence markers and clinical data the strains could be split into different pathogroups, such as uropathogenic E. coli, enteropathogenic E. coli, Shiga toxin-producing E. coli, and enteroaggregative E. coli. H serotyping and genotyping of the flagellin (fliC) gene revealed 11 different H types and a close association between certain H types, virulence markers, and pathogroups was found. Nucleotide sequence analysis of the O-antigen gene cluster revealed putative genes for biosynthesis of O15 antigen. PCR assays were developed for sensitive and specific detection of the O15-antigen-specific genes wzx and wzy. The high pathotype diversity found in the collection of 33 O15 strains contrasted with the high level of similarity found in the genes specific to the O15 antigen. This might indicate that the O15 determinant has been spread by horizontal gene transfer to a number of genetically unrelated strains of E. coli.

Characterization of cytolethal distending toxin of campylobacter species isolated from captive macaque monkeys

An association between certain Campylobacter species and enterocolitis in humans and nonhuman primates is well established, but the association between cytolethal distending toxin and disease is incompletely understood. The purpose of the present study was to examine Campylobacter species isolated from captive conventionally raised macaque monkeys for the presence of the cdtB gene and for cytolethal distending toxin activity. The identity of each isolate was confirmed on the basis of phenotypic and genotypic analyses. The presence of cytolethal distending toxin was confirmed on the basis of characteristic morphological changes in HeLa cells incubated with filter-sterilized whole-cell lysates of reference and monkey Campylobacter isolates and examinations by light microscopy, confocal microscopy, and flow cytometry. Although cdtB gene sequences were found in both Campylobacter jejuni and Campylobacter coli, the production of cytolethal distending toxin correlated positively (P < 0.0001) only with C. jejuni. We concluded that cytolethal distending toxin activity is a characteristic of C. jejuni. Our C. jejuni cdtB gene-specific PCR assay might be of assistance for differentiating toxigenic C. jejuni from C. coli in clinical laboratories.

Genetic analysis of enteropathogenic and enterohemorrhagic Escherichia coli serogroup O103 strains by molecular typing of virulence and housekeeping genes and pulsed-field gel electrophoresis

We investigated the genetic relationships of 54 Escherichia coli O103 strains from humans, animals, and meat by molecular typing of housekeeping and virulence genes and by pulsed-field gel electrophoresis (PFGE). Multilocus sequence typing (MLST) of seven housekeeping genes revealed seven profiles, I through VII. MLST profiles I plus III cover 45 Shiga toxin-producing E. coli (STEC) O103:H2 strains from Australia, Canada, France, Germany, and Northern Ireland that are characterized by the intimin (eae) epsilon gene and carry enterohemorrhagic E. coli (EHEC) virulence plasmids. MLST profile II groups five human and animal enteropathogenic E. coli (EPEC) O103:H2 strains that were positive for intimin (eae) beta. Although strains belonging to MLST groups II and I plus III are closely related to each other (92.6% identity), major differences were found in the housekeeping icdA gene and in the virulence-associated genes eae and escD. E. coli O103 strains with MLST patterns IV to VII are genetically distant from MLST I, II, and III strains, as are the non-O103 E. coli strains EDL933 (O157), MG1655 (K-12), and CFT073 (O6). Comparison of MLST results with those of PFGE and virulence typing demonstrated that E. coli O103 STEC and EPEC have recently acquired different virulence genes and DNA rearrangements, causing alterations in their PFGE patterns. PFGE typing was very useful for identification of genetically closely related subgroups among MLST I strains, such as Stx2-producing STEC O103 strains from patients with hemolytic uremic syndrome. Analysis of virulence genes contributed to grouping of E. coli O103 strains into EPEC and STEC. Novel virulence markers, such as efa (EHEC factor for adherence), paa (porcine adherence factor), and cif (cell cycle-inhibiting factor), were found widely associated with E. coli O103 EPEC and STEC strains.

Cytolethal distending toxin generates cell death by inducing a bottleneck in the cell cycle

Cytolethal distending toxin (CDT) is a bacterial protein that is widely distributed among gram-negative bacteria including Escherichia coli, Campylobacter spp., enterohepatic Helicobacter spp., Actinobacillus actinomycetemcomitans and Haemophilus ducreyi. In vitro studies demonstrated that it is able to stop proliferation of various cell lines. The toxin is composed of three subunits designated CDTs A, B and C. The B subunit targets the eukaryotic DNA and triggers a signalling pathway involving different protein kinases which results in a cell block before entering into mitosis. To date, the individual role of the A and C subunits has not been totally elucidated. There are indications that the CDT is also produced in vivo. Its exact role in pathogenesis is not yet clear, but possible actions include inhibition of epithelial cell proliferation, apoptosis of immune cells and inhibition of a fibrotic response.

Predominance of afr2 and ral fimbrial genes related to those encoding the K88 and CS31A fimbrial adhesins in enteropathogenic Escherichia coli isolates from rabbits with postweaning diarrhea in Central Europe

PCR tests designed in these studies identified three rabbit adhesive factor genes among 43 enteropathogenic E. coli (EPEC) strains: afr1 (2 strains), the F4(K88)/CS31A-related afr2 (10 strains), and ral (15 strains). Several EPEC strains (i.e., O153:H7 and O157:H2) lacked these genes but did adhere to HeLa cells and produced attaching and effacing lesions in rabbits.

Cytolethal distending toxin: creating a gap in the cell cycle

Cytolethal distending toxin (CDT) is a novel bacterial toxin that is produced by a variety of pathogenic bacteria. The mechanism of cytotoxicity of CDT is unique in that it enters into eukaryotic cells and breaks double-stranded DNA. This initiates the cell's own DNA damage-response mechanisms, resulting in the arrest of the cell cycle at the G2/M boundary. Affected cells enlarge until they finally undergo programmed cell death. This review encompasses recent work on CDT and focuses on the molecular mechanisms used by this toxin to block cell-cycle progression, the benefit to the bacterium of possession of this toxin and the clinical relevance of intoxication.

Evolutionary genetics of a new pathogenic Escherichia species: Escherichia albertii and related Shigella boydii strains

A bacterium originally described as Hafnia alvei induces diarrhea in rabbits and causes epithelial damage similar to the attachment and effacement associated with enteropathogenic Escherichia coli. Subsequent studies identified similar H. alvei-like strains that are positive for an intimin gene (eae) probe and, based on DNA relatedness, are classified as a distinct Escherichia species, Escherichia albertii. We determined sequences for multiple housekeeping genes in five E. albertii strains and compared these sequences to those of strains representing the major groups of pathogenic E. coli and Shigella. A comparison of 2,484 codon positions in 14 genes revealed that E. albertii strains differ, on average, at approximately 7.4% of the nucleotide sites from pathogenic E. coli strains and at 15.7% from Salmonella enterica serotype Typhimurium. Interestingly, E. albertii strains were found to be closely related to strains of Shigella boydii serotype 13 (Shigella B13), a distant relative of E. coli representing a divergent lineage in the genus Escherichia. Analysis of homologues of intimin (eae) revealed that the central conserved domains are similar in E. albertii and Shigella B13 and distinct from those of eae variants found in pathogenic E. coli. Sequence analysis of the cytolethal distending toxin gene cluster (cdt) also disclosed three allelic groups corresponding to E. albertii, Shigella B13, and a nontypeable isolate serologically related to S. boydii serotype 7. Based on the synonymous substitution rate, the E. albertii-Shigella B13 lineage is estimated to have split from an E. coli-like ancestor approximately 28 million years ago and formed a distinct evolutionary branch of enteric pathogens that has radiated into groups with distinct virulence properties.

Cytolethal distending toxin from Shiga toxin-producing Escherichia coli O157 causes irreversible G2/M arrest, inhibition of proliferation, and death of human endothelial cells

Recently, cytolethal distending toxin V (CDT-V), a new member of the CDT family, was identified in Shiga toxin-producing Escherichia coli (STEC) O157 and particular non-O157 serotypes. Here we investigated the biological effects of CDT-V from STEC O157:H(-) (strain 493/89) on human endothelial cells, which are believed to be major pathogenetic targets in severe STEC-mediated diseases. CDT-V caused dose-dependent G(2)/M cell cycle arrest leading to distension, inhibition of proliferation, and death in primary human umbilical vein endothelial cells (HUVEC) and two endothelial cell lines, EA.hy 926 cells (HUVEC derived) and human brain microvascular endothelial cells (HBMEC). The cell cycle effects of CDT-V were cell type specific. In HUVEC and EA.hy 926 cells, CDT-V caused a slowly developing but persistent G(2)/M block which resulted in delayed nonapoptotic cell death. In contrast, in HBMEC, CDT-V induced a rapidly evolving but transient G(2)/M block which was followed by progressive, mostly apoptotic cell death. In both HBMEC and EA.hy 926 cells, G(2)/M arrest was preceded by the early accumulation of a phosphorylated inactive form of cdc2 kinase. Significant G(2)/M arrest and inhibition of proliferation in both HUVEC and each of the endothelial cell lines were induced by 2 to 15 min of exposure to CDT-V, indicating that the effects of the toxin are irreversible. CDT-V-treated HBMEC and EA.hy 926 cells displayed fragmented nuclei and expressed phosphorylated histone protein H2AX, indicative of DNA damage followed by a DNA repair response. Our data demonstrate that CDT-V causes irreversible damage to human endothelial cells and thus may contribute to the pathogenesis of STEC-mediated diseases.

Detection of the cytolethal distending toxin locus cdtB among diarrheagenic Escherichia coli isolates from humans in Iran

Cytolethal distending toxin (CDT) represents an emerging family of newly described bacterial products that are produced by a number of pathogens. Genes encoding this toxin were identified as a cluster of three adjacent genes, cdt-A, cdt-B and cdt-C. Five cdt genetic variants, designated cdt-I, cdt-II, cdt-III, cdt-IV and cdt-V, have been identified so far. To determine the presence of cdt genes among Escherichia coli isolates, a PCR assay was employed. Using multiplex primers designed for detection of E. coli cdt genes, PCR analysis indicated the presence of cdt genes in 37.5% of these isolates. While specific primers were located in the cdt-B locus for detection of cdt-I, cdt-II, cdt-III and cdt-IV, in multiplex PCR positive isolates indicated that a cdt-I-like gene was present in 45.3% of these isolates. However, in 52% of the isolates, the cdt-III-like gene could be detected. It should be mentioned that in 30.8% of these isolates, cdt-II and -III were detected simultaneously. In 2.6% of the isolates, the cdt-IV gene was present. The cnf-1 gene was detected in 29.4% of strains carrying the cdt-I gene; however, the cnf-2 gene was detected only in 23.1% of the cdt-III-like strains. Furthermore, the data obtained by PCR analysis indicated the presence of cdt-like genes among our E. coli isolates, although in the CHO cell assay, all isolates showed a cytopathic effect characteristic of CDT.

Serotyping and Virulence Genes Detection in Escherichia coli Isolated from Fertile and Infertile Eggs, Dead-in-Shell Embryos, and Chickens with Yolk Sac Infection

Escherichia coli is a common avian pathogen mainly associated with extraintestinal infections such as yolk sac infection (YSI). The aim of this study was to determine the serotypes and the presence of some virulence genes of E. coli strains isolated from different samples in a vertically integrated poultry operation in Mexico. Two hundred sixty-seven E. coli isolates from different samples were serotyped using rabbit serum against the 175 somatic (O) and 56 flagellar (H) antigens of the typing schema. Virulence genes were determined by colony blot hybridization, using DNA probes for st, eae, agg1, agg2, bfp, lt, cdt, slt, and ipaH diarrhea-associated virulence factors. The serogroup of 85% of the strains was determined; O19 (12%), 084 (9%), 08 (6%), and 078 (5%) were the most common. Using the complete antigenic formula (O and H), O19:NM (n = 31) was the serotype most frequently isolated from dead-in-shell embryos and in broilers that had died on the fourth, fifth, sixth, and seventh days after hatch. One hundred ten strains (41.2%) hybridized with one or more of the used probes. Of these, ipaH (72%), eae (30%), and cdt (27%) were the most common. Considering the origin of the respective isolates, 40% of the broiler farm strains were positive for at least one probe. Results show that some avian E. coli strains isolated in Mexico are included in avian pathogenic E. coli serotypes not previously reported, suggesting that they could be specific for this geographic area. The wide distribution of the ipaH gene among nonmotile strains suggests that this invasiveness trait could be important in YSI pathogenesis. On the other hand, some other genes could contribute to E. coli virulence during YSI.

Cytolethal distending toxins

The cytolethal distending toxins (CDTs) constitute the most recently discovered family of bacterial protein toxins. CDTs are unique among bacterial toxins as they have the ability to induce DNA double strand breaks (DSBs) in both proliferating and nonproliferating cells, thereby causing irreversible cell cycle arrest or death of the target cells. CDTs are encoded by three linked genes ( cdtA, cdtB and cdtC) which have been identified among a variety of Gram-negative pathogenic bacteria. All three of these gene products are required to constitute the fully active holotoxin, and this is in agreement with the recently determined crystal structure of CDT. The CdtB component has functional homology with mammalian deoxyribonuclease I (DNase I). Mutation of the conserved sites necessary for this catalytic activity prevents the induction of DSBs as well as all subsequent intoxication responses of target cells. CDT is endocytosed via clathrin-coated pits and requires an intact Golgi complex to exert the cytotoxic activity. Several issues remain to be elucidated regarding CDT biology, such as the detailed function(s) of the CdtA and CdtC subunits, the identity of the cell surface receptor(s) for CDT, the final steps in the cellular internalization pathway, and a molecular understanding of how CDT interacts with DNA. Moreover, the role of CDTs in the pathogenesis of diseases still remains unclear.

Characterization of cytolethal distending toxin genes and expression in shiga toxin-producing Escherichia coli strains of non-O157 serogroups

We identified cytolethal distending toxin and its gene (cdt) in 17 of 340 non-O157 Shiga toxin-producing Escherichia coli (STEC) strains (serotypes O73:H18, O91:H21, O113:H21, and O153:H18), all of which were eae negative. cdt is either chromosomal and homologous to cdt-V (serotypes O73:H18, O91:H21, and O113:H21) or plasmidborne and identical to cdt-III (serotype O153:H18). Among eae-negative STEC, cdt was associated with disease (P = 0.003).

Pathogenic Escherichia coli

Few microorganisms are as versatile as Escherichia coli. An important member of the normal intestinal microflora of humans and other mammals, E. coli has also been widely exploited as a cloning host in recombinant DNA technology. But E. coli is more than just a laboratory workhorse or harmless intestinal inhabitant; it can also be a highly versatile, and frequently deadly, pathogen. Several different E. coli strains cause diverse intestinal and extraintestinal diseases by means of virulence factors that affect a wide range of cellular processes.

Multiplex PCRs for identification of necrotoxigenic Escherichia coli

Two multiplex PCRs were developed for the detection of necrotoxigenic Escherichia coli virulence genes. M1 contained the primers for the toxins and the aerobactin, and M2 contained the primers for the adhesins. They were validated by single PCRs performed with reference E. coli strains and by multiplex PCRs with necrotoxigenic E. coli strains isolated from different animal species.

Prevalence and identity of cdt-related sequences in necrotoxigenic Escherichia coli

The cytolethal distending toxins (CDT) are responsible for the mitosis block at G2/M and the cycle arrest of cells in culture. Escherichia coli isolated from humans and animals with intestinal and extra-intestinal diseases can be positive for the production of a CDT-like cytopathic effect or for the presence of cdt-related genes. The purpose of this study was to compare the prevalence and the identity of cdt-related sequences in necrotoxigenic E. coli (NTEC). A collection of 98 bovine type 2 NTEC (NTEC2) and 45 bovine, 20 canine, 3 feline, 65 human and 129 porcine type 1 NTEC (NTEC1) isolates was studied by colony hybridisation and PCR assays specific for the cdtB genes encoding the B sub-unit of the CDT-I, CDT-II, CDT-III and CDT-IV toxins produced by E. coli. cdtB-III sequences were frequent amongst bovine NTEC2, since 83% of these isolates were positive by colony hybridisation and/or PCR, whereas cdtB-related sequences were rare amongst NTEC1, since only 2 bovine (4%), 3 canine (15%), 10 human (15%) and 13 porcine (10%) of these isolates were positive. The 28 probe-positive NTEC1 harboured cdtB-IV sequences (13 isolates), cdtB-I sequences (10 isolates), or still unidentified cdt-related sequences (5 isolates). After comparison with previously published and unpublished results of phenotypic assay on cell cultures, existence of other cdt-related sequences is suggested amongst NTEC1. The differences between NTEC1 and NTEC2 in their CDT profiles may have implication for the pathogenesis of those two classes of pathogenic E. coli.