Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler’s diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (enteropathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.

Diarrheagenic Escherichia coli

Campylobacter jejuni is a foodborne bacterial pathogen that is common in the developed world. However, we know less about its biology and pathogenicity than we do about other less prevalent pathogens. Interest in C. jejuni has increased in recent years as a result of the growing appreciation of its importance as a pathogen and the availability of new model systems and genetic and genomic technologies. C. jejuni establishes persistent, benign infections in chickens and is rapidly cleared by many strains of laboratory mouse, but causes significant inflammation and enteritis in humans. Comparing the different host responses to C. jejuni colonization should increase our understanding of this organism.

Campylobacter jejuni : molecular biology and pathogenesis

Superoxide, O2•–, is formed in all living organisms that come in contact with air, and, depending upon its biological context, it may act as a signaling agent, a toxic species, or a harmless intermediate that decomposes spontaneously Its levels are limited in vivo by two different types of enzymes, superoxide reductase (SOR) and superoxide dismutase (SOD) Although superoxide has long been an important factor in evolution, it was not so when life first emerged on Earth at least 35 billion years ago At that time, the early biosphere was highly reducing and lacking in any significant concentrations of dioxygen (O2), very different from what it is today Consequently, there was little or no O2•– and therefore no reason for SOR or SOD enzymes to evolve Instead, the history of biological O2•– probably commences somewhere around 24 billion years ago, when the biosphere started to experience what has been termed the “Great Oxidation Event”, a transformation driven by the increase in O2 levels, formed by cyanobacteria as a product of oxygenic photosynthesis1 The rise of O2 on Earth caused a reshaping of existing metabolic pathways, and it triggered the development of new ones2 Its appearance led to the formation of the so-called “reactive oxygen species” (ROS), for example, superoxide, hydrogen peroxide, and hydroxyl radical, and to a need for antioxidant enzymes and other antioxidant systems to protect against the growing levels of oxidative damage to living systems


Dioxygen is a powerful four-electron oxidizing agent, and the product of this reduction is water




1


When O2 is reduced in four sequential one-electron steps, the intermediates formed are the three major ROS, that is, O2•–, H2O2, and HO•




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3





4





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Each of these intermediates is a potent oxidizing agent The consequences of their presence to early life must have been an enormous evolutionary challenge In the case of superoxide, we find the SOD and SOR enzymes to be widely distributed throughout current living organisms, both aerobic and anaerobic, suggesting that, from the start of the rise of O2 on Earth, the chemistry of superoxide has been an important factor during evolution


The SORs and three very different types of SOD enzymes are redox-active metalloenzymes that have evolved entirely independently from one another for the purpose of lowering superoxide concentrations SORs catalyze the one-electron reduction of O2•– to give H2O2, a reaction requiring two protons per superoxide reacted as well as an external reductant to provide the electron (eq 6) SODs catalyze the disproportionation of superoxide to give O2 and H2O2, a reaction requiring one proton per superoxide reacted, but no external reductant (eq 7)




6





7



All of the SOR enzymes contain only iron, while the three types of SODs are the nickel-containing SODs (NiSOD), the iron- or manganese-containing SODs (FeSOD and MnSOD), and the copper- and zinc-containing SODs (CuZnSOD) Although the structures and other properties of these four types of metalloenzymes are quite different, they all share several characteristics, including the ability to react rapidly and selectively with the small anionic substrate O2•– Consequently, there are some striking similarities between these otherwise dissimilar enzymes, many of which can be explained by considering the nature of the chemical reactivity of O2•– (see below)

Numerous valuable reviews describing the SOD and SOR enzymes have appeared over the years, but few have covered and compared all four classes of these enzymes, as we attempt to do here Thus, the purpose of this Review is to describe, compare, and contrast the properties of the SOR and the four SOD enzymes; to summarize what is known about their evolutionary pathways; and to analyze the properties of these enzymes in light of what is known of the inherent chemical reactivity of superoxide

Superoxide dismutases and superoxide reductases

Campylobacter jejuni binds intestinal H(O) antigen (Fuc alpha 1, 2Gal beta 1, 4GlcNAc), and fucosyloligosaccharides of human milk inhibit its binding and infection.

Helicobacter pylori (Hp) vacuolating cytotoxin VacA induces cellular vacuolation in epithelial cells. We found that VacA could efficiently block proliferation of T cells by inducing a G1/S cell cycle arrest. It interfered with the T cell receptor/interleukin-2 (IL-2) signaling pathway at the level of the Ca2+-calmodulin-dependent phosphatase calcineurin. Nuclear translocation of nuclear factor of activated T cells (NFAT), a transcription factor acting as a global regulator of immune response genes, was abrogated, resulting in down-regulation of IL-2 transcription. VacA partially mimicked the activity of the immunosuppressive drug FK506 by possibly inducing a local immune suppression, explaining the extraordinary chronicity of Hp infections.

https://science.sciencemag.org/content/sci/301/5636/1099.full.pdf

Helicobacter pylori vacuolating cytotoxin inhibits T lymphocyte activation.

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.

https://iai.asm.org/content/iai/64/6/2070.full.pdf

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

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.

Campylobacter jejuni Cytolethal Distending Toxin Causes a G2-Phase Cell Cycle Block

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.

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

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

Campylobacter jejuni produces a toxin, called cytolethal distending toxin (CDT), which causes direct DNA damage leading to invocation of DNA damage checkpoint pathways. The affected cells arrest in G1 or G2 and eventually die. CDT consists of three protein subunits, CdtA, CdtB, and CdtC, with CdtB recently identified as a nuclease. However, little is known about the functions of CdtA or CdtC. In this work, enzyme-linked immunosorbent assay-based experiments were used to show, for the first time, that both CdtA and CdtC bound with specificity to the surface of HeLa cells, whereas CdtB did not. Varying the order of the addition of subunits for reconstitution of the holotoxin had no effect on activity. In addition, mutants containing deletions of conserved regions of CdtA and CdtC were able to bind to the surface of HeLa cells but were not able to participate in holotoxin assembly. Finally, both Cdt mutant subunits were able to effectively compete with CDT holotoxin in the HeLa cell binding assay.

D L Cottle

Papers

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

Campylobacter jejuni Cytolethal Distending Toxin Causes a G2-Phase Cell Cycle Block

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

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

Interactions of Campylobacter jejuni Cytolethal Distending Toxin Subunits CdtA and CdtC with HeLa Cells