Cytolethal distending toxin demonstrates genotoxic activity in a yeast model

Cytolethal distending toxins (CDTs) are multisubunit proteins produced by a variety of bacterial pathogens that cause enlargement, cell cycle arrest, and apoptosis in mammalian cells. While their function remains uncertain, recent studies suggest that they can act as intracellular DNases in mammalian cells. Here we establish a novel yeast model for understanding CDT-associated disease. Expression of the CdtB subunit in yeast causes a G2/M arrest, as seen in mammalian cells. CdtB toxicity is not circumvented in yeast genetically altered to lack DNA damage checkpoint control or that constitutively promote cell cycle progression via mutant Cdk1, because CdtB causes a permanent type of damage that results in loss of viability. Finally, we establish that CDTs are likely to be potent genotoxins, as indicated by in vivo degradation of chromosomal DNA associated with expression of CdtB-suggesting that the varied distribution of CDT in bacteria implicates many human pathogens as possessors of genotoxic activity.

The cytolethal distending toxin family

Cytolethal distending toxins are produced by a small but diverse group of bacterial pathogens. This newly discovered toxin family can cause a variety of mammalian cells to become irreversibly blocked in the G2 phase of the cell cycle. How this novel effect is accomplished is unknown but the study of these fascinating toxins promises to reveal new methods of host-pathogen interaction.

Cytolethal distending toxin genes in Campylobacter jejuni and Campylobacter coli isolates: detection and analysis by PCR

Campylobacter jejuni produces a toxin called cytolethal distending toxin (CDT). Knowledge of the prevalence and homogeneity of Campylobacter sp. cdt genes is incomplete. In this work, we identified four PCR primer pairs that collectively amplified cdt genes in all of the C. jejuni and Campylobacter coli strains tested. Restriction analyses of the cdt PCR products showed clear differences between the cdt genes of these two species, yet there were few heterogeneities noted between members of the same species. Consequently, it may be possible to speciate C. jejuni and C. coli isolates on the basis of restriction patterns within their cdt genes.

Detection of cytolethal distending toxin activity and cdt genes in Campylobacter spp. isolated from chicken carcasses

This study was designed to determine whether isolates from chicken carcasses, the primary source of Campylobacter jejuni and Campylobacter coli in human infections, commonly carry the cdt genes and also whether active cytolethal distending toxin (CDT) is produced by these isolates. Campylobacter spp. were isolated from all 91 fresh chicken carcasses purchased from local supermarkets. Campylobacter spp. were identified on the basis of both biochemical and PCR tests. Of the 105 isolates, 70 (67%) were identified as C. jejuni, and 35 (33%) were identified as C. coli. PCR tests amplified portions of the cdt genes from all 105 isolates. Restriction analysis of PCR products indicated that there appeared to be species-specific differences between the C. jejuni and C. coli cdt genes, but that the restriction patterns of the cdt genes within strains of the same species were almost invariant. Quantitation of active CDT levels produced by the isolates indicated that all C. jejuni strains except four (94%) had mean CDT titers greater than 100. Only one C. jejuni strain appeared to produce no active CDT. C. coli isolates produced little or no toxin. These results confirm the high rate of Campylobacter sp. contamination of fresh chicken carcasses and indicate that cdt genes may be universally present in C. jejuni and C. coli isolates from chicken carcasses.

Campylobacter jejuni cytolethal distending toxin causes a G2-phase cell cycle block

Cytolethal distending toxin (CDT) from the diarrheagenic bacterium Campylobacter jejuni was shown to cause a rapid and specific cell cycle arrest in HeLa and Caco-2 cells. Within 24 h of treatment, CDT caused HeLa cells to arrest with a 4N DNA content, indicative of cells in G2 or early M phase. Immunofluorescence studies indicated that the arrested cells had not entered M phase, since no evidence of tubulin reorganization or chromatin condensation was visible. CDT treatment was also shown to cause HeLa cells to accumulate the inactive, tyrosine-phosphorylated form of CDC2. These results indicated that CDT treatment results in a failure to activate CDC2, which leads to cell cycle arrest in G2. This mechanism of action is novel for a bacterial toxin and provides a model for the generation of diarrheal disease by C. jejuni and other diarrheagenic bacteria that produce CDT.

Prevalence of cytolethal distending toxin production in Campylobacter jejuni and relatedness of Campylobacter sp. cdtB gene

Campylobacter jejuni produces a toxin called cytolethal distending toxin (CDT). The genes encoding this toxin in C. jejuni 81-176 were cloned and sequenced. The nucleotide sequence of the genes revealed that there are three genes, cdtA, cdtB, and cdtC, encoding proteins with predicted sizes of 30,11-6, 28,989, and 21,157 Da, respectively. All three proteins were found to be related to the Escherichia coli CDT proteins, yet the amino acid sequences have diverged significantly. All three genes were required for toxic activity in a HeLa cell assay. HeLa cell assays of a variety of C. jejuni and C. coli strains suggested that most C. jejuni strains produce significantly higher CDT titers than do C. coli strains. Southern hybridization experiments demonstrated that the cdtB gene is present on a 6.0-kb ClaI fragment in all but one of the C. jejuni strains tested; the cdtB gene was on a 6.9-kb ClaI fragment in one strain. The C. jejuni 81-176 cdtB probe hybridized weakly to DNAs from C. coli strains. The C. jejuni 81-176 cdtB probe did not hybridize to DNAs from representative C. fetus, C. lari, C. "upsaliensis," and C. hyointestinalis strains, although the HeLa cell assay indicated that these strains make CDT. PCR experiments indicated the probable presence of cdtB sequences in all of these Campylobacter species.

Genetic organization of the region upstream from the Campylobacter jejuni flagellar gene flhA

Campylobacter jejuni (Cj) is a Gram-bacterium that causes a diarrheal disease in humans. A Cj homolog of the LcrD/FlhA family of proteins was recently described [Miller et al., Infect. Immun. 61 (1993) 2930-2936]. This family includes proteins that are involved in flagellar biogenesis, such as the Cj FlhA protein, but also includes proteins found in invasive pathogens, such as the Yersinia pestis LcrD protein, that play a role in the regulation and/or secretion of virulence-related proteins. Hybridization studies indicated that both the flhA gene and upstream DNA are present in several bacterial species closely related to Cj, including C. fetus, C. lari, C. upsaliensis and C. hyointestinalis. The presence of a second flhA/lcrD homolog was not detected in Cj, indicating that a a separate homolog involved in secretion of virulence proteins may not be present. The 4-kb region immediately upstream from Cj flhA was analyzed. Three open reading frames (ORFs) were found: a 408-nucleotide (nt) gene encoding a homolog of proteins present in Escherichia coli and Desulfovibrio vulgaris, but of unknown function, a 266-nt rpsO gene and a 2823-nt gene encoding a homolog of the Bacillus subtilis SpoIIIE protein. The Cj SpoIIIE homolog had 53% similar or identical amino acids when compared to the B. subtilis protein, and like the B. subtilis protein contained a nt-binding domain and potential transmembrane (TM) regions. All three ORFs were expressed in E. coli minicells, apparently from their own promoters.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic, enzymatic, and pathogenic studies of the iron superoxide dismutase of Campylobacter jejuni

Campylobacter jejuni is a microaerobic bacterium that produces an acute, self-limiting, watery or bloody diarrhea in humans. Little is known about how C. jejuni causes disease or even what specific capabilities it requires for survival in vivo. The enzyme, superoxide dismutase (SOD), which catalyzes the breakdown of superoxide radicals to hydrogen peroxide and dioxygen is one of the bacterial cell's major defense mechanisms against oxidative damage. A PCR-based search for sod genes in C. jejuni 81-176 revealed that this bacterium contained at least one sod gene. We cloned and sequenced a sod gene from 81-176 and determined that its predicted protein product was most similar to that of FeSODs (sodB genes). Transcriptional analysis indicated that this gene is monocistronic and may be transcribed from a sigma 70-like promoter. Nondenaturing polyacrylamide gels stained to reveal SOD activities, accompanied by inhibition studies, demonstrated that C. jejuni produces five electrophoretically distinct bands of SOD activity, all of which appeared to be FeSODs. Analysis of an 81-176 sodB strain revealed that all of these FeSOD activities may be products of the one sodB gene that we cloned. The expression and enzymatic activity of the respective sodB and FeSOD produced by both C. jejuni and Helicobacter pylori were examined in Escherichia coli. Both genes were expressed in E. coli, and the proteins produced were enzymatically active. Finally, the ability of the 81-176 sodB strain to survive INT407 cell invasion was found to be significantly decreased (12-fold) compared with that of the parent, suggesting a potential role for SodB in C. jejuni intracellular survival.

Genetic organization and enzymatic activity of a superoxide dismutase from the microaerophilic human pathogen, Helicobacter pylori

Helicobacter pylori (Hp) is a microaerobic human pathogen that has been implicated as a factor in the development of chronic type-B gastritis, gastric ulcers and gastric carcinoma. The enzyme superoxide dismutase (SOD), a major defense mechanism against oxidative damage, catalyzes the breakdown of superoxide radicals to hydrogen peroxide and dioxygen. A search for sod genes in Hp, employing PCR, revealed that this bacterium contained at least one sod gene. We cloned and sequenced a sod from this organism and determined that the deduced protein encoded by this gene was most similar to an iron SOD (FeSOD). Northern blot and primer extension analysis of Hp RNA showed that the cloned gene is monocistronic and is probably transcribed from a sigma 70-like promoter. Assays for SOD activities, accompanied by inhibition studies, demonstrated that Hp produces an FeSOD. No other SOD activities were seen.

Cloning, sequencing, and expression of the Escherichia coli cytolethal distending toxin genes

A limited number of Escherichia coli isolates which produce an apparently novel toxin, termed cytolethal distending toxin (CDT), have been reported. The toxic activity produced by these strains causes certain cultured cell lines to become slowly distended and then disintegrate. DNA was isolated from the CDT-producing E. coli strain, 9142-88, and cloned into a cosmid vector. Plasmid DNA from a toxin-positive transductant was further subcloned until a plasmid with a 4-kb insert which still encoded the toxin activity was obtained. Nucleotide sequencing of a portion of this insert revealed the presence of three adjacent open reading frames. Further subcloning and deletion analysis suggested that the products of all three open reading frames may be required for toxin activity. Minicell experiments identified the products of all three open reading frames. The three proteins had predicted sizes of 27,753,29,531, and 19,938 Da, and all three appeared to have strong consensus leader sequences. None of the three predicted proteins had significant homology to known proteins.

A Campylobacter jejuni homolog of the LcrD/FlbF family of proteins is necessary for flagellar biogenesis

A Campylobacter jejuni homolog of the lcrD/flbF family of genes was cloned and sequenced. The nucleotide sequence of the gene, called flbA, predicted a protein of 78,864 Da, with significant homology to a group of related proteins including the Yersinia pestis LcrD, Salmonella typhimurium InvA, and Caulobacter crescentus FlbF proteins. The greatest homology was seen with the C. crescentus FlbF protein, with an overall amino acid sequence homology of 57%. An insertion mutation in the C. jejuni 81-176 flbA gene was constructed. The resultant strain did not synthesize flagellin and was nonmotile.

Iron acquisition and hemolysin production by Campylobacter jejuni

Campylobacter jejuni strains were tested for their ability to acquire iron from various iron sources present in humans. Hemin, hemoglobin, hemin-hemopexin, and hemoglobin-haptoglobin stimulated the growth of C. jejuni strains in low-iron medium. Transferrin, lactoferrin, and ferritin were unable to provide iron to the strains tested. Derivatives of the naturally transformable C. jejuni strain 81-176 were isolated on the basis of their inability to use hemin as an iron source. These mutants were also unable to use hemoglobin, hemin-hemopexin, or hemoglobin-haptoglobin as iron sources. Some mutants lacked a 71,000-Da iron-regulated outer membrane protein, while others appeared to retain all of their outer membrane proteins. Growth curves and a recombination experiment that exploited natural transformation were used to further characterize the mutants. A hemolytic activity was shown to be produced by several C. jejuni strains, but it did not appear to be iron regulated.

Cloning, nucleotide sequence, and hybridization studies of the type IIb heat-labile enterotoxin gene of Escherichia coli

Type IIb heat-labile enterotoxin (LT-IIb) is produced by Escherichia coli 41. Restriction fragments of total cell DNA from strain 41 were cloned into a cosmid vector, and one cosmid clone that encoded LT-IIb was identified. The genes for LT-IIb were subcloned into a variety of plasmids, expressed in minicells, sequenced, and compared with the structural genes for other members of the Vibrio cholerae-E. coli enterotoxin family. The A subunits of these toxins all have similar ADP-ribosyltransferase activity. The A genes of LT-IIa and LT-IIb exhibited 71% DNA sequence homology with each other and 55 to 57% homology with the A genes of cholera toxin (CT) and the type I enterotoxins of E. coli (LTh-I and LTp-I). The A subunits of the heat-labile enterotoxins also have limited homology with other ADP-ribosylating toxins, including pertussis toxin, diphtheria toxin, and Pseudomonas aeruginosa exotoxin A. The B subunits of LT-IIa and LT-IIb differ from each other and from type I enterotoxins in their carbohydrate-binding specificities. The B genes of LT-IIa and LT-IIb were 66% homologous, but neither had significant homology with the B genes of CT, LTh-I, and LTp-I. The A subunit genes for the type I and type II enterotoxins represent distinct branches of an evolutionary tree, and the divergence between the A subunit genes of LT-IIa and LT-IIb is greater than that between CT and LT-I. In contrast, it has not yet been possible to demonstrate an evolutionary relationship between the B subunits of type I and type II heat-labile enterotoxins. Hybridization studies with DNA from independently isolated LT-II producing strains of E. coli also suggested that additional variants of LT-II exist.

Genetics of type IIa heat-labile enterotoxin of Escherichia coli: operon fusions, nucleotide sequence, and hybridization studies

Operon fusions for the Escherichia coli heat-labile enterotoxin type IIa (LT-IIa) operon were isolated and characterized. The LT-IIa genes are organized in a transcriptional unit similar to those of cholera toxin (CT) and the closely related E. coli heat-labile toxin type I (LT-I, with subtypes LTh-I and LTp-I). The nucleotide sequence of the LT-IIa genes was determined and compared with the sequences of LTh-I and CT. The A subunit gene of LT-IIa was found to be 57% homologous with the A subunit gene of LTh-I and 55% homologous with the A gene of CT. Most of the homology derived from the region of the A gene which encodes the A1 fragment. The B gene of LT-IIa was not homologous with the B gene of LTh-I or CT. DNA probes containing various portions of the LT-IIa genes and adjacent sequences were used for hybridization studies with restriction endonuclease fragments of DNA from a collection of LT-II-producing strains. These studies showed that a probe containing much of the A subunit gene hybridized well to DNA from the various strains, but a probe for the B subunit gene did not.

Production of type II heat-labile enterotoxin by Escherichia coli isolated from food and human feces

Escherichia coli strains isolated in Sao Paulo, Brazil, from feces of patients with diarrhea and from food samples produced toxin(s) that was shown to be related both immunologically and genetically to the recently characterized type II heat-labile enterotoxin of E. coli. The new isolates of type II heat-labile enterotoxin-producing E. coli belonged to five different serotypes and did not represent a single clone.

Purification and characterization of type II heat-labile enterotoxin of Escherichia coli

Type II heat-labile enterotoxin (LT-II) of Escherichia coli has several biologic activities similar to cholera toxin (CT) and E. coli type I heat-labile enterotoxin (LT-I), but it is not neutralized by antiserum prepared against CT or LT-I. LT-II was purified from E. coli SA53 and from E. coli HB101(pCP3837), a strain that contains the cloned LT-II genes in a hybrid plasmid and produces up to 600 times more LT-II than does SA53. Purification involved sonic disruption of bacterial cells, ammonium sulfate fractionation, chromatography on Affi-Gel Blue, chromatofocusing, and gel filtration on Sephadex G-100. The LT-II purified to apparent homogeneity from HB101(pCP3837) had an isoelectric point of 6.8, induced increased vascular permeability in rabbit intracutaneous tests, caused rounding of cultured Y1 adrenal cells accompanied by increased intracellular cyclic AMP, and was 25 to 50 times more potent than CT or LT-I in the Y1 adrenal-cell assay. In contrast, purified LT-II did not cause secretion in ligated rabbit ileal segments at doses corresponding to CT controls that gave strongly positive reactions. LT-II was composed of two different polypeptides with MrS of 28,000 (A) and 11,800 (B); treatment of LT-II with trypsin cleaved the A polypeptide to fragments A1 (Mr, 21,000) and A2 (Mr, 7,000). The activity of LT-II was not blocked by ganglioside GM1 at concentrations that inactivated LT-I or CT. Antiserum against the LT-II from E. coli HB101(pCP3837) completely neutralized purified LT-II and the LT-II in crude extracts of SA53, but it did not neutralize purified LT-I or CT.

Cloning of genes that encode a new heat-labile enterotoxin of Escherichia coli

The genes for a new enterotoxin were cloned from Escherichia coli SA53. The new toxin was heat labile and activated adenylate cyclase but was not neutralized by antisera against cholera toxin or E. coli heat-labile enterotoxin. Subcloning and minicell experiments indicated that the toxin is composed of two polypeptide subunits that are encoded by two genes. The two toxin subunits exhibited mobilities on polyacrylamide gels that are similar to those of cholera toxin and E. coli heat-labile enterotoxin subunits. A 0.8-kilobase DNA probe for the new enterotoxin failed to hybridize with the cloned structural genes for E. coli heat-labile enterotoxin.

Two fep genes are required for ferrienterochelin uptake in Escherichia coli K-12

Escherichia coli mutants defective in the assimilation of iron from ferrienterochelin were isolated and characterized. One mutant was able to bind ferrienterochelin to its outer membrane but could not transport it into the cell. Complementation tests with lambda hybrid phage were employed to distinguish the defective gene, which we term fepB, from fepA, the structural gene for the outer membrane ferrienterochelin receptor protein. These same physiological and genetic tests were employed to tentatively classify several previously described fep mutants as carrying either fepA or fepB. The data demonstrate the existence of fepB and provide an explanation for previous difficulties in identifying fepB mutants.

Iron uptake in pseudorevertants of Escherichia coli K-12 mutants with multiple defects in the enterochelin system

Iron uptake in pseudorevertants of Escherichia coli K-12 strains which lack the ability to synthesize enterochelin, 2,3 dihydroxybenzoate, and the ferrienterochelin receptor protein was characterized. In four independent pseudorevertants, the suppressor mutations which permitted growth in iron-poor environments appeared to be located in omp B, the regulatory locus for the porin proteins. Unlike wild-type cells, the pseudorevertants were unable to utilize ferrienterochelin and could acquire iron from citrate without induction by prior growth in citrate. The energy requirements of the pseudorevertant system appeared to be identical to those of the enterochelin system. Evidence that loss of the porin proteins results in the secretion by the pseudorevertants of a molecule with siderophore activity is presented; this siderophore is able to remove iron from the non-biological iron chelators nitrilotriacetic acid and alpha, alpha'-dipyridyl but not fom the siderophores ferrichrome and enterochelin.