Hemolysin-positive enteroaggregative and cell-detaching Escherichia coli strains cause oncosis of human monocyte-derived macrophages and apoptosis of murine J774 cells

Klebsiella spp. cause severe and fatal disease in Mozambican children: antimicrobial resistance profile and molecular characterization

Background

Klebsiella spp. are important pathogens associated with bacteremia among admitted children and is among the leading cause of death in children < 5 years in postmortem studies, supporting a larger role than previously considered in childhood mortality. Herein, we compared the antimicrobial susceptibility, mechanisms of resistance, and the virulence profile of Klebsiella spp. from admitted and postmortem children.

Methods

Antimicrobial susceptibility and virulence factors of Klebsiella spp. recovered from blood samples collected upon admission to the hospital (n = 88) and postmortem blood (n = 23) from children < 5 years were assessed by disk diffusion and multiplex PCR.

Results

Klebsiella isolates from postmortem blood were likely to be ceftriaxone resistant (69.6%, 16/23 vs. 48.9%, 43/88, p = 0.045) or extended-spectrum β-lactamase (ESBL) producers (60.9%, 14/23 vs. 25%, 22/88, p = 0.001) compared to those from admitted children. bla_CTX-M-15 was the most frequent ESBL gene: 65.3%, 9/14 in postmortem isolates and 22.7% (5/22) from admitted children. We found higher frequency of genes associated with hypermucoviscosity phenotype and invasin in postmortem isolates than those from admitted children: rmpA (30.4%; 7/23 vs. 9.1%, 8/88, p = 0.011), wzi-K1 (34.7%; 8/23 vs. 8%; 7/88, p = 0.002) and traT (60.8%; 14/23 vs. 10.2%; 9/88, p < 0.0001), respectively. Additionally, serine protease auto-transporters of Enterobacteriaceae were detected from 1.8% (pic) to 12.6% (pet) among all isolates. Klebsiella case fatality rate was 30.7% (23/75).

Conclusion

Multidrug resistant Klebsiella spp. harboring genes associated with hypermucoviscosity phenotype has emerged in Mozambique causing invasive fatal disease in children; highlighting the urgent need for prompt diagnosis, appropriate treatment and effective preventive measures for infection control.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-021-06245-x.

GtxA is a virulence factor that promotes a Th2-like response during Gallibacterium anatis infection in laying hens

GtxA, a leukotoxic RTX-toxin, has been proposed a main virulence factor of Gallibacterium anatis. To evaluate the impact of GtxA during infection, we experimentally infected laying hens with a G. anatis wild-type (WT) strain and its isogenic gtxA deletion mutant (ΔgtxA), respectively, and monitored the birds during a 6 day period. Birds inoculated with ΔgtxA had significantly reduced gross lesions and microscopic changes compared to the birds inoculated with the WT strain. To assess the host response further, we quantified the expression of pro-inflammatory cytokines and apoptosis genes by RT-qPCR. In the ovarian tissue, the expression levels of IL-4 and TNF-α were significantly lower in the ΔgtxA group compared to the WT group, while IL-6 and IL-10 levels appeared similar in the two groups. In the spleen tissue of ΔgtxA infected chickens, IL-4 expression was also lower compared to the WT infected chickens. The results indicated that GtxA plays a key role in an acute cytokine-mediated Th2-like response against G. anatis infection in the ovary tissue. The pro-inflammatory response in the ovary tissue of birds inoculated with ΔgtxA mutant was thus significantly lower than the wild-type response. This was, at least partly, supported by the apoptosis gene expression levels, which were significantly higher in the ΔgtxA mutant compared to the wild-type infected chickens. In conclusion, GtxA clearly plays an important role in the pathogenesis of G. anatis infections in laying hens. Further investigations into the specific factors regulating the host response is however needed to provide a more complete understanding of the bacteria-host interaction.

Protein- and zinc-deficient diets modulate the murine microbiome and metabolic phenotype

BACKGROUND

Environmental enteropathy, which is linked to undernutrition and chronic infections, affects the physical and mental growth of children in developing areas worldwide. Key to understanding how these factors combine to shape developmental outcomes is to first understand the effects of nutritional deficiencies on the mammalian system including the effect on the gut microbiota.

OBJECTIVE

We dissected the nutritional components of environmental enteropathy by analyzing the specific metabolic and gut-microbiota changes that occur in weaned-mouse models of zinc or protein deficiency compared with well-nourished controls.

DESIGN

With the use of a ¹H nuclear magnetic resonance spectroscopy-based metabolic profiling approach with matching 16S microbiota analyses, the metabolic consequences and specific effects on the fecal microbiota of protein and zinc deficiency were probed independently in a murine model.

RESULTS

We showed considerable shifts within the intestinal microbiota 14-24 d postweaning in mice that were maintained on a normal diet (including increases in Proteobacteria and striking decreases in Bacterioidetes). Although the zinc-deficient microbiota were comparable to the age-matched, well-nourished profile, the protein-restricted microbiota remained closer in composition to the weaned enterotype with retention of Bacteroidetes. Striking increases in Verrucomicrobia (predominantly Akkermansia muciniphila) were observed in both well-nourished and protein-deficient mice 14 d postweaning. We showed that protein malnutrition impaired growth and had major metabolic consequences (much more than with zinc deficiency) that included altered energy, polyamine, and purine and pyrimidine metabolism. Consistent with major changes in the gut microbiota, reductions in microbial proteolysis and increases in microbial dietary choline processing were observed.

CONCLUSIONS

These findings are consistent with metabolic alterations that we previously observed in malnourished children. The results show that we can model the metabolic consequences of malnutrition in the mouse to help dissect relevant pathways involved in the effects of undernutrition and their contribution to environmental enteric dysfunction.

Modulation of the Inflammasome Signaling Pathway by Enteropathogenic and Enterohemorrhagic Escherichia coli

Innate immunity is an essential component in the protection of a host against pathogens. Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC, respectively) are known to modulate the innate immune responses of infected cells. The interference is dependent on their type III secretion system (T3SS) and T3SS-dependent effector proteins. Furthermore, these cytosolically injected effectors have been demonstrated to engage multiple immune signaling pathways, including the IFN/STAT, MAPK, NF-κB, and inflammasome pathways. In this review, recent work describing the interaction between EPEC/EHEC and the inflammasome pathway will be discussed.

Multifaceted roles of extracellular DNA in bacterial physiology

In textbooks, DNA is generally defined as the universal storage material for genetic information in all branches of life. Beyond this important intracellular role, DNA can also be present outside of living cells and is an abundant biopolymer in aquatic and terrestrial ecosystems. The origin of extracellular DNA in such ecological niches is diverse: it can be actively secreted or released by prokaryotic and eukaryotic cells by means of autolysis, apoptosis, necrosis, bacterial secretion systems or found in association with extracellular bacterial membrane vesicles. Especially for bacteria, extracellular DNA represents a significant and convenient element that can be enzymatically modulated and utilized for multiple purposes. Herein, we discuss briefly the main origins of extracellular DNA and the most relevant roles for the bacterial physiology, such as biofilm formation, nutrient source, antimicrobial means and horizontal gene transfer.

Rifaximin-mediated changes to the epithelial cell proteome: 2-D gel analysis

Rifaximin is a semi-synthetic rifamycin derivative that is used to treat different conditions including bacterial diarrhea and hepatic encephalopathy. Rifaximin is of particular interest because it is poorly adsorbed in the intestines and has minimal effect on colonic microflora. We previously demonstrated that rifaximin affected epithelial cell physiology by altering infectivity by enteric pathogens and baseline inflammation suggesting that rifaximin conferred cytoprotection against colonization and infection. Effects of rifaximin on epithelial cells were further examined by comparing the protein expression profile of cells pretreated with rifaximin, rifampin (control antibiotic), or media (untreated). Two-dimensional (2-D) gel electrophoresis identified 36 protein spots that were up- or down-regulated by over 1.7-fold in rifaximin treated cells compared to controls. 15 of these spots were down-regulated, including annexin A5, intestinal-type alkaline phosphatase, histone H4, and histone-binding protein RbbP4. 21 spots were up-regulated, including heat shock protein (HSP) 90α and fascin. Many of the identified proteins are associated with cell structure and cytoskeleton, transcription and translation, and cellular metabolism. These data suggested that in addition to its antimicrobial properties, rifaximin may alter host cell physiology that provides cytoprotective effects against bacterial pathogens.

Role of pore-forming toxins in neonatal sepsis

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

Enterohemorrhagic Escherichia coli specific enterohemolysin induced IL-1β in human macrophages and EHEC-induced IL-1β required activation of NLRP3 inflammasome

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a major foodborne pathogen causing hemorrhagic colitis and hemolytic-uremic syndrome. The role of EHEC O157:H7-enterohemolysin (Ehx) in the pathogenesis of infections remains poorly defined. In this study, we used gene deletion and complement methods to confirm its putative functions. Results demonstrated that, in THP-1 cells, EHEC O157:H7-Ehx is associated with greater production of extracellular interleukin (IL)-1β than other cytokines. The data also showed that EHEC O157:H7-Ehx contributed to cytotoxicity in THP-1 cells, causing the release of lactate dehydrogenase (LDH). Although we observed a positive correlation between IL-1β production and cytotoxicity in THP-1 cells infected with different EHEC O157:H7 strains, our immunoblot results showed that the majority of IL-1β in the supernatant was mature IL-1β and not the pro-IL-1β that can be released after cell death. However, EHEC O157:H7-Ehx had no detectable effect on biologically inactive pro-IL-1β at the mRNA or protein synthesis levels. Neither did it affect the expression of apoptosis-associated speck-like protein containing a CARD (ASC), caspase-1, or NOD-like receptor family pyrin domain containing 3 (NLRP3). RNA interference experiments showed that EHEC O157:H7-induced IL-1β production required the involvement of ASC, caspase-1, and NLRP3 expression in THP-1 cells. Our results demonstrate that Ehx plays a crucial role in EHEC O157:H7-induced IL-1β production and its cytotoxicity to THP-1 cells. NLRP3 inflammasome activation is also involved in EHEC O157:H7-stimulated IL-1β release.

Oncosis: an important non-apoptotic mode of cell death

It is now increasingly accepted that apoptosis may not be the only form of cell death seen in vitro and in vivo; hence there is a need to study novel forms of cell death. The explosion of cell death research that followed the recognition of apoptosis by Kerr and colleagues in the late 1960s completely obscured the fact that apoptosis is not the only form of cell death. Apoptosis manifests itself by cell shrinkage followed by breakup; another form (oncosis) is almost the opposite: it involves cell swelling and coagulation of the cytoplasm. The name oncosis was chosen over a century ago by von Recklinghausen, a top collaborator of Rudolph Virchow and thereby one of the founders of cellular pathology. Nevertheless, oncosis was forgotten, largely because a satisfactory technique for preparing tissue sections did not exist at the time. Also confusion developed regarding the distinction between oncosis as a mode of cell injury and cell death, and necrosis as a degradation process following cell death. In this review we have described the many characteristics of oncosis from a morphological and biochemical standpoint, and we briefly examine the application of oncosis in disease processes.

Multi-locus sequence typing of enteroaggregative Escherichia coli isolates from Nigerian children uncovers multiple lineages

BACKGROUND

Enteroaggregative Escherichia coli (EAEC) are defined by their stacked-brick adherence pattern to human epithelial cells. There is no all-encompassing genetic marker for EAEC. The category is commonly implicated in diarrhea but research is hampered by perplexing heterogeneity.

METHODOLOGY/PRINCIPAL FINDINGS

To identify key EAEC lineages, we applied multilocus sequence typing to 126 E. coli isolates from a Nigerian case-control study that showed aggregative adherence in the HEp-2 adherence assay, and 24 other EAEC strains from diverse locations. EAEC largely belonged to the A, B1 and D phylogenetic groups and only 7 (4.6%) isolates were in the B2 cluster. As many as 96 sequence types (STs) were identified but 60 (40%) of the EAEC strains belong to or are double locus variants of STs 10, 31, and 394. The remainder did not belong to predominant complexes. The most common ST complex, with predicted ancestor ST10, included 32 (21.3%) of the isolates. Significant age-related distribution suggests that weaned children in Nigeria are at risk for diarrhea from of ST10-complex EAEC. Phylogenetic group D EAEC strains, predominantly from ST31- and ST394 complexes, represented 38 (25.3%) of all isolates, include genome-sequenced strain 042, and possessed conserved chromosomal loci.

CONCLUSIONS/SIGNIFICANCE

We have developed a molecular phylogenetic framework, which demonstrates that although grouped by a shared phenotype, the category of 'EAEC' encompasses multiple pathogenic lineages. Principal among isolates from Nigeria were ST10-complex EAEC that were associated with diarrhea in children over one year and ECOR D strains that share horizontally acquired loci.

Different types of cell death induced by enterotoxins

The infection of bacterial organisms generally causes cell death to facilitate microbial invasion and immune escape, both of which are involved in the pathogenesis of infectious diseases. In addition to the intercellular infectious processes, pathogen-produced/secreted enterotoxins (mostly exotoxins) are the major weapons that kill host cells and cause diseases by inducing different types of cell death, particularly apoptosis and necrosis. Blocking these enterotoxins with synthetic drugs and vaccines is important for treating patients with infectious diseases. Studies of enterotoxin-induced apoptotic and necrotic mechanisms have helped us to create efficient strategies to use against these well-characterized cytopathic toxins. In this article, we review the induction of the different types of cell death from various bacterial enterotoxins, such as staphylococcal enterotoxin B, staphylococcal alpha-toxin, Panton-Valentine leukocidin, alpha-hemolysin of Escherichia coli, Shiga toxins, cytotoxic necrotizing factor 1, heat-labile enterotoxins, and the cholera toxin, Vibrio cholerae. In addition, necrosis caused by pore-forming toxins, apoptotic signaling through cross-talk pathways involving mitochondrial damage, endoplasmic reticulum stress, and lysosomal injury is discussed.

Afa/Dr-expressing, diffusely adhering Escherichia coli strain C1845 triggers F1845 fimbria-dependent phosphatidylserine externalization on neutrophil-like differentiated PLB-985 cells through an apoptosis-independent mechanism

The enterovirulent Escherichia coli strains potentially involved in inflammatory bowel diseases include diffusely adherent strains expressing Afa/Dr fimbriae (Afa/Dr DAEC). We have previously observed type 1 pilus-mediated interleukin-8 (IL-8) hyperproduction in infected neutrophils. As pathogen induction of host cell death programs and clearance of apoptotic infected cells are crucial for innate immune system homeostasis and host integrity, we examined modulation of neutrophil cell death by Afa/Dr DAEC. Using the human PLB-985 cell line differentiated into fully mature neutrophils, we found that the wild-type enterovirulent E. coli strain C1845 and the recombinant strain DH5alpha/pF1845 (expressing the fimbrial adhesin F1845) similarly induced time-dependent phosphatidylserine (PS) externalization, suggesting a major specific role of this virulence factor. Using small interfering RNA (siRNA) decay-accelerating factor (DAF)-transfected PLB-985 cells, we then showed that this PS externalization was triggered in part by glycosylphosphatidylinositol (GPI)-anchored DAF receptor engagement (leading to tyrosine kinase and protein kinase C activation) and that it required cytoskeleton and lipid raft architectural integrity. PS externalization under these conditions was not dependent on caspases, mitochondria, lysosomes, or reactive oxygen or nitrogen species. F1845-mediated PS externalization was sufficient to enable macrophage engulfment of infected differentiated PLB-985 cells. These findings provide new insights into the neutrophil response to Afa/Dr DAEC infection and highlight a new role for F1845 fimbriae. Interestingly, although apoptosis pathways were not engaged, C1845-infected PLB-985 cells displayed enhanced removal by macrophages, a process that may participate in the resolution of Afa/Dr DAEC infection and related inflammation.

Programmed cellular necrosis mediated by the pore-forming alpha-toxin from Clostridium septicum

Programmed necrosis is a mechanism of cell death that has been described for neuronal excitotoxicity and ischemia/reperfusion injury, but has not been extensively studied in the context of exposure to bacterial exotoxins. The α-toxin of Clostridium septicum is a β-barrel pore-forming toxin and a potent cytotoxin; however, the mechanism by which it induces cell death has not been elucidated in detail. We report that α-toxin formed Ca²⁺-permeable pores in murine myoblast cells, leading to an increase in intracellular Ca²⁺ levels. This Ca²⁺ influx did not induce apoptosis, as has been described for other small pore-forming toxins, but a cascade of events consistent with programmed necrosis. Ca²⁺ influx was associated with calpain activation and release of cathepsins from lysosomes. We also observed deregulation of mitochondrial activity, leading to increased ROS levels, and dramatically reduced levels of ATP. Finally, the immunostimulatory histone binding protein HMGB1 was found to be released from the nuclei of α-toxin-treated cells. Collectively, these data show that α-toxin initiates a multifaceted necrotic cell death response that is consistent with its essential role in C. septicum-mediated myonecrosis and sepsis. We postulate that cellular intoxication with pore-forming toxins may be a major mechanism by which programmed necrosis is induced.

Clostridium septicum is a highly virulent pathogen that causes spontaneous gas gangrene or clostridial myonecrosis. The essential virulence factor of C. septicum is a β-barrel toxin, α-toxin, that forms small pores in host cell membranes. This toxin is frequently described as a hemolysin, because the formation of these pores causes lysis of red blood cell cells due to membrane disruption. However, this description does not recognize additional effects that may be observed in nucleated host cells, which are more sensitive to α-toxin. We investigated how nucleated cells responded to α-toxin by treating a physiologically relevant muscle cell line with purified toxin and monitoring the response using various assays. We observed α-toxin-mediated programmed cellular necrosis that culminated in the release of the immunostimulatory molecule, HMGB1. This mechanism of cell death induction is consistent with the extensive necrosis that is evident in C. septicum-mediated myonecrosis and with the overwhelming sepsis that frequently contributes to the high mortality rate. These results represent an important advance in the understanding of the toxicity of β-barrel pore-forming toxins and how they may contribute to necrotic and systemic disease pathology.

Neutrophil apoptosis and the resolution of infection

Polymorphonuclear leukocytes (PMNs) are the most abundant white cell in humans and an essential component of the innate immune system. PMNs are typically the first type of leukocyte recruited to sites of infection or areas of inflammation. Ingestion of microorganisms triggers production of reactive oxygen species and fusion of cytoplasmic granules with forming phagosomes, leading to effective killing of ingested microbes. Phagocytosis of bacteria typically accelerates neutrophil apoptosis, which ultimately promotes the resolution of infection. However, some bacterial pathogens alter PMN apoptosis to survive and thereby cause disease. Herein, we review PMN apoptosis and the ability of microorganisms to alter this important process.

A Dynamic Mathematical Model To Clarify Signaling Circuitry Underlying Programmed Cell Death Control in Arabidopsis Disease Resistance

Plant cells undergo programmed cell death in response to invading pathogens. This cell death limits the spread of the infection and triggers whole plant antimicrobial and immune responses. The signaling network connecting molecular recognition of pathogens to these responses is a prime target for manipulation in genetic engineering strategies designed to improve crop plant disease resistance. Moreover, as alterations to metabolism can be misinterpreted as pathogen infection, successful plant metabolic engineering will ultimately depend on controlling these signaling pathways to avoid inadvertent activation of cell death. Programmed cell death resulting from infection of Arabidopsis thaliana with Pseudomonas syringae bacterial pathogens was chosen as a model system. Signaling circuitry hypotheses in this model system were tested by construction of a differential-equations-based mathematical model. Model-based simulations of time evolution of signaling components matched experimental measurements of programmed cell death and associated signaling components obtained in a companion study. Simulation of systems-level consequences of mutations used in laboratory studies led to two major improvements in understanding of signaling circuitry: (1) Simulations supported experimental evidence that a negative feedback loop in salicylic acid biosynthesis postulated by others does not exist. (2) Simulations showed that a second negative regulatory circuit for which there was strong experimental support did not affect one of two pathways leading to programmed cell death. Simulations also generated testable predictions to guide future experiments. Additional testable hypotheses were generated by results of individually varying each model parameter over 2 orders of magnitude that predicted biologically important changes to system dynamics. These predictions will be tested in future laboratory studies designed to further elucidate the signaling network control structure.

Detection of inhibition of antimicrobial activity by mycobacterial lysates in human monocytes infected with Legionella pneumophila

Antimicrobial activity in human monocytes infected with Mycobacterium tuberculosis has been difficult to demonstrate in vitro, and the molecular mechanisms allowing the bacteria to survive intracellularly are unknown. As a means to test the influence of bacterial products in the microbicidal activity of monocytes we have developed an infection model with Legionella pneumophila, which is killed by interferon gamma activated cells. We demonstrate that this model is useful because M. tuberculosis lysates inhibit one hundred fold the interferon gamma induced activity against L. pneumophila. Comparable degrees of inhibition are also detected when we use lysates from the less pathogenic Mycobacterium gordonae and the pathogenic Staphylococcus aureus, suggesting the participation of a common mechanism. This hypothesis is supported by the fact that the pattern of cytokine secretion is similar in all cases. A significant difference is, however, observed when we used lysates from the non-pathogenic Escherichia coli, which resulted in the recovery of low numbers of bacteria, probably because they induce the cell death of infected monocytes.

Strain-dependent induction of epithelial cell oncosis by Campylobacter jejuni is correlated with invasion ability and is independent of cytolethal distending toxin

Induction of host cell death is thought to play an important role in bacterial pathogenesis. Campylobacter jejuni is a prevalent cause of bacterial enteritis; however, its effects on enterocytes remain unclear. The present study indicates for the first time that C. jejuni induces oncotic, rather than apoptotic death of T84 enterocytes. C. jejuni-treated enterocytes exhibited extensive cytoplasmic vacuolation, rapid (3–6 h) loss of plasma membrane integrity (‘cytotoxicity’), loss of mitochondrial transmembrane potential, and ATP depletion. Enterocytes also exhibited increased oligonucleosomal DNA fragmentation, a feature characteristic of apoptosis. However, consistent with a non-apoptotic process, DNA fragmentation and cytotoxicity were not caspase dependent. During apoptosis, caspases mediate cleavage of poly(ADP-ribose) polymerase; however, cleavage was not observed in C. jejuni-treated monolayers. Cytotoxicity, ATP depletion and DNA fragmentation were not prevented by the deletion of the cytolethal distending toxin (CDT) gene, indicating that C. jejuni causes enterocyte oncosis via a mechanism that is CDT independent. The ability to cause oncosis was significantly decreased in a FlaAFlaB mutant (CDT⁺) that was defective in the ability to adhere and invade enterocytes. Analysis of clinical isolates revealed that oncosis was strain dependent and correlated with increased invasive ability. These observations offer new insights into the pathogenesis of C. jejuni infection.

Occurrence and characteristics of cytotoxic necrotizing factors, cytolethal distending toxins and other virulence factors in Escherichia coli from human blood and faecal samples

Escherichia coli isolates from human blood (n=266) and faecal (n=237) samples were examined for cytotoxic necrotizing factors 1 and 2 (CNF 1 and 2), cytolethal distending toxin (CDT), and putative virulence factors that have been associated with disease conditions in humans and animals. PCR showed that the chromosomally encoded, Rho-activating, CNF1 (68/544, 12.5%) was more common than the transmissible plasmid-borne CNF2 (3/544, 0.6%). The relative risk of having either CNF or CDT toxin genes in blood compared to faecal isolates was 3.88 (95% CI 2.36-6.38). This was highly significant (P<0.0001) and demonstrates the importance of these factors in bloodstream infections. Fifty-one of 65 (78%) E. coli bearing CNF1 and 11 of 21 (52%) of E. coli bearing CDT also carried the pyelonephritis-associated pilus gene, papG. The S fimbrial adhesin gene, sfa, was found in 57 blood (21%) and eight faecal samples (3%). The F17 fimbrial adhesin gene and afimbrial adhesin gene afa did not occur frequently. Haemolysin (hly) was found in all of the isolates tested. Further studies must be designed to identify the clinical significance of these genes and their role in pathogenesis.

The ESAT6 protein of Mycobacterium tuberculosis induces apoptosis of macrophages by activating caspase expression

The secreted Mycobacterium tuberculosis protein, ESAT6, has been studied extensively in pathogenicity and vaccine experiments. Despite these studies little is known about the function of this protein. In this report, we demonstrate that ESAT6 induces apoptosis in THP-1 human macrophages using fluorescein isothiocyanate-Annexin V and intracellular caspase staining. We show that the induction of apoptosis by ESAT6 is dependent on the dose of the protein and the expression of caspase genes. Using real-time RT-PCR, we found that expression of caspase-1, -3, -5, -7 and -8 genes was upregulated in cells treated with ESAT6 relative to untreated cells. Furthermore, we show that while infection of THP-1 cells with wild-type M. tuberculosis strain H37Rv resulted in significant apoptosis 48 h post infection, a deletion mutant that does not express ESAT6 failed to induce significant apoptosis. Finally, experimental results using a cell impermeable fluorescent stain suggests that the formation of membrane pores may be a primary mechanism by which ESAT6 evokes an apoptotic response.

E. colivirulence factor hemolysin induces neutrophil apoptosis and necrosis/lysis in vitro and necrosis/lysis and lung injury in a rat pneumonia model

Enteric gram-negative bacilli, such as Escherichia coli are the most common cause of nosocomial pneumonia. In this study a wild-type extraintestinal pathogenic strain of E. coli (ExPEC)(CP9) and isogenic derivatives deficient in hemolysin (Hly) and cytotoxic necrotizing factor (CNF) were assessed in vitro and in a rat model of gram-negative pneumonia to test the hypothesis that these virulence factors induce neutrophil apoptosis and/or necrosis/lysis. As ascertained by in vitro caspase-3/7 and LDH activities and neutrophil morphology, Hly mediated neutrophil apoptosis at lower E. coli titers (1 × 105–6cfu) and necrosis/lysis at higher titers (≥1 × 107cfu). Data suggest that CNF promotes apoptosis but not necrosis or lysis. We also demonstrate that annexin V/7-amino-actinomycin D staining was an unreliable assessment of apoptosis using live E. coli. The use of caspase-3/7 and LDH activities and neutrophil morphology supported the notion that necrosis, not apoptosis, was the primary mechanism by which neutrophils were affected in our in vivo gram-negative pneumonia model using live E. coli. In addition, in vivo studies demonstrated that Hly mediates lung injury. Neutrophil necrosis was not observed when animals were challenged with purified lipopolysaccharide, demonstrating the importance of using live bacteria. These findings establish that Hly contributes to ExPEC virulence by mediating neutrophil toxicity, with necrosis/lysis being the dominant effect of Hly on neutrophils in vivo and by lung injury. Whether Hly-mediated lung injury is due to neutrophil necrosis, a direct effect of Hly, or both is unclear.

Pathogenesis of Afa/Dr diffusely adhering Escherichia coli

Over the last few years, dramatic increases in our knowledge about diffusely adhering Escherichia coli (DAEC) pathogenesis have taken place. The typical class of DAEC includes E. coli strains harboring AfaE-I, AfaE-II, AfaE-III, AfaE-V, Dr, Dr-II, F1845, and NFA-I adhesins (Afa/Dr DAEC); these strains (i) have an identical genetic organization and (ii) allow binding to human decay-accelerating factor (DAF) (Afa/Dr(DAF) subclass) or carcinoembryonic antigen (CEA) (Afa/Dr(CEA) subclass). The atypical class of DAEC includes two subclasses of strains; the atypical subclass 1 includes E. coli strains that express AfaE-VII, AfaE-VIII, AAF-I, AAF-II, and AAF-III adhesins, which (i) have an identical genetic organization and (ii) do not bind to human DAF, and the atypical subclass 2 includes E. coli strains that harbor Afa/Dr adhesins or others adhesins promoting diffuse adhesion, together with pathogenicity islands such as the LEE pathogenicity island (DA-EPEC). In this review, the focus is on Afa/Dr DAEC strains that have been found to be associated with urinary tract infections and with enteric infection. The review aims to provide a broad overview and update of the virulence aspects of these intriguing pathogens. Epidemiological studies, diagnostic techniques, characteristic molecular features of Afa/Dr operons, and the respective role of Afa/Dr adhesins and invasins in pathogenesis are described. Following the recognition of membrane-bound receptors, including type IV collagen, DAF, CEACAM1, CEA, and CEACAM6, by Afa/Dr adhesins, activation of signal transduction pathways leads to structural and functional injuries at brush border and junctional domains and to proinflammatory responses in polarized intestinal cells. In addition, uropathogenic Afa/Dr DAEC strains, following recognition of beta(1) integrin as a receptor, enter epithelial cells by a zipper-like, raft- and microtubule-dependent mechanism. Finally, the presence of other, unknown virulence factors and the way that an Afa/Dr DAEC strain emerges from the human intestinal microbiota as a "silent pathogen" are discussed.

Mechanisms underlying Mannheimia haemolytica leukotoxin-induced oncosis and apoptosis of bovine alveolar macrophages

Mannheimia (Pasteurella) haemolytica leukotoxin (LktA) binds to the bovine beta2 integrins (such as LFA-1-CD11a/CD18) and leads to subsequent cellular effects in a dose dependent manner. The objectives of this study were to delineate the mechanisms that underlie LktA-induced oncosis and apoptosis and to examine the role of LktA/LFA-1 interaction in these events. The results demonstrate that LktA-induced oncosis proceeds through a LFA-1 and caspase-1 dependent pathway referred to as 'pyrotosis', as well as through a LFA-1- and caspase-1-independent pathway. LktA-induced apoptosis in alveolar macrophages involves activation of caspase-3 and engages the extrinsic and intrinsic pathways of apoptosis, with the extrinsic pathway being dependent on LFA-1 signaling and TNFalpha.

A novel caspase-1/toll-like receptor 4-independent pathway of cell death induced by cytosolic Shigella in infected macrophages

Shigella-induced macrophage cell death is an important step in the induction of acute inflammatory responses that ultimately lead to bacillary dysentery. Cell death was previously reported to be dependent upon the activation of caspase-1 via interaction with IpaB secreted by intracellular Shigella, but in this study, we show that Shigella infection of macrophages can also induce cell death independent of caspase-1 or IpaB activity. Time-lapse imaging and electron microscopic analyses indicated that caspase-1-dependent and -independent cell death is morphologically indistinguishable and that both resemble necrosis. Analyses of Shigella mutants or Escherichia coli using co-infection with Listeria suggested that a component common to Gram-negative bacteria is involved in inducing caspase-1-independent cell death. Further studies revealed that translocation of bacterial lipid A into the cytosol of macrophages potentially mediates cell death. Notably, cell death induced by cytosolic bacteria was TLR4-independent. These results identify a novel cell death pathway induced by intracellular Gram-negative bacteria that may play a role in microbial-host interactions and inflammatory responses.

Increased rate of apoptosis and diminished phagocytic ability of human neutrophils infected with Afa/Dr diffusely adhering Escherichia coli strains

The proinflammatory effect of Afa/Dr diffusely adhering Escherichia coli (Afa/Dr DAEC) strains have been recently demonstrated in vitro by showing that polymorphonuclear leukocyte (PMN) transepithelial migration is induced after bacterial colonization of apical intestinal monolayers. The effect of Afa/Dr DAEC-PMN interaction on PMN behavior has been not investigated. Because of the putative virulence mechanism of PMN apoptosis during infectious diseases and taking into account the high level of expression of the decay-accelerating factor (DAF, or CD55), the receptor of Afa/Dr DAEC on PMNs, we sought to determine whether infection of PMNs by Afa/Dr DAEC strains could promote cell apoptosis. We looked at the behavior of PMNs incubated with Afa/Dr DAEC strains once they had transmigrated across polarized monolayers of intestinal (T84) cells. Infection of PMNs by Afa/Dr DAEC strains induced PMN apoptosis characterized by morphological nuclear changes, DNA fragmentation, caspase activation, and a high level of annexin V expression. However, transmigrated and nontransmigrated PMNs incubated with Afa/Dr DAEC strains showed similar elevated global caspase activities. PMN apoptosis depended on their agglutination, induced by Afa/Dr DAEC, and was still observed after preincubation of PMNs with anti-CD55 and/or anti-CD66 antibodies. Low levels of phagocytosis of Afa/Dr DAEC strains were observed both in nontransmigrated and in transmigrated PMNs compared to that observed with the control E. coli DH5alpha strain. Taken together, these data strongly suggest that interaction of Afa/Dr DAEC with PMNs may increase the bacterial virulence both by inducing PMN apoptosis through an agglutination process and by diminishing their phagocytic capacity.

Exocytotic secretion of toxins from macrophages infected with Escherichia coli O157

This study examined whether macrophages are involved in the development of pathogenicity in Shiga-like toxin (SLT)-producing enterohemorrhagic Escherichia coil (EHEC) O157:H7. Macrophages were infected with the bacteria, after which the macrophage culture medium showed a clear increase in toxicity in rats in vivo as well as in rat aortic endothelial cells in vitro. The increased toxicity resulted mainly from a rapid increase in the concentrations of SLT type I (SLT-I) and type II (SLT-II) and partly from an increase in concentrations of the proinflammatory cytokines, tumor necrosis factor alpha (TNFalpha) and interleukin-1 (IL-1), in the culture medium. Most of the EHEC O157 added to the macrophage culture were quickly incorporated to form phagosomes, which then fused with lysosomes to become phagolysosomes. During this intracellular digestion process, the EHEC O157 remained alive for about 15 min, and continued synthesizing and secreting the toxins SLT-1 and SLT-II. The bacteria were then killed and digested in the phagolysosomes with significant amounts of the toxins retained. Subsequently, the contents of the phagolysosomes were exocytotically secreted from the macrophage cell membrane into the surrounding culture medium. Such a sequence of events in macrophages may occur in vivo, suggesting the active involvement of macrophages in the rapid increase in pathogenicity, such as seen in the onset of hemolytic-uremic syndrome (HUS) in patients infected with EHEC O157. The exocytotic secretion is considered to be one of the most basic cellular functions in macrophages.

Molecular epidemiology of the iron utilization genes of enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) strains are etiologic agents of acute and persistent diarrhea. In this study, the results of phenotypic assays suggested that EAEC strains possess specialized iron acquisition systems. Genes required for the synthesis (iucA) or transport (fepC) of siderophores, and genes encoding siderophore (fyuA, ireA, and iroN) or heme transport (chu) receptors or hemoglobin proteases (pic and hbp), were sought in EAEC strains which have been characterized with respect to known virulence genes and phylogeny. The chuA, iucA, fyuA, fepC, and pic genes were detected in 33, 76.2, 85.7, 33, and 61.9% of these EAEC strains, respectively, and the other genes were absent. The majority of EAEC strains possessed genes encoding multiple iron transport systems, and there was no phylogenetic correlation in the distribution of the majority of these loci, as is typical for EAEC. The notable exceptions were chuA and fepC (which is associated with the prrA-modA-fepC pathogenicity island); these genes were restricted to the EAEC2 and DAEC2 phylogenetic groups, which could represent pathogenic subsets. When collections of EAEC strains isolated during case-control studies in Nigeria and Brazil were examined, no association of the presence of either chuA or iucA alone with diarrhea was seen, but both genes together were present in significantly more strains from cases than from controls in the Nigerian collection (P < 0.05). It is possible that the presence of both genes marks at least some virulent strains. The data also demonstrate geographical variation in the association of iron utilization genes with disease in EAEC.

Interactions between Brucella melitensis and human phagocytes: bacterial surface O-Polysaccharide inhibits phagocytosis, bacterial killing, and subsequent host cell apoptosis

Brucellae are gram-negative intracellular pathogens that survive and multiply within host phagocytic cells. Smooth organisms present O-polysaccharides (OPS) on their surface. The wboA gene, which codes for the enzyme glycosyl transferase, is essential for the assembly of O-chain in Brucella. Deletion of wboA in smooth, virulent B. melitensis 16M results in a rough mutant designated WRR51. Unlike B. abortus, both smooth and rough strains of B. melitensis are resistant to complement-mediated killing. To determine the role of surface OPS in the interactions of B. melitensis with monocytes/macrophages (M/M), 16M and WRR51 were transformed with the plasmid pBBR1MCS-6y encoding green fluorescent protein, and the transformants were used to infect human mononuclear phagocytes with and without fresh human serum as a source of complement. Human monocytes were cultured in the presence of macrophage colony-stimulating factor to allow their differentiation into macrophages during the course of infection. Intracellular bacteria were easily visualized using fluorescence microscopy. Infection in M/M, identified by surface staining and fate of infected phagocytes, was quantitated by flow cytometry. Rough bacteria were internalized, with no requirement for opsonization by serum, at a higher rate than smooth organisms. Smooth B. melitensis survived and multiplied for at least 6 days inside M/M, but rough organisms were eliminated by death of the infected cells. In human monocytes cultured for 1 day without serum in order to trigger the apoptotic pathway, infection by rough brucellae accelerated phagocyte death; smooth brucellae inhibited apoptosis. This study suggests that the presence of surface OPS on live B. melitensis benefits the bacterium by preventing the death of macrophages, Brucella's preferred target for intracellular replication.

The Dot/Icm type IV secretion system of Legionella pneumophila is essential for the induction of apoptosis in human macrophages

We have previously shown that Legionella pneumophila induces caspase 3-dependent apoptosis in mammalian cells during early stages of infection. In this report, we show that nine L. pneumophila strains with mutations in the dotA, dotDCB, icmT, icmGCD, and icmJB loci are completely defective in the induction of apoptosis, in addition to their severe defects in intracellular replication and pore formation-mediated cytotoxicity. Importantly, all nine dot/icm mutants were complemented for all their defective phenotypes with the respective wild-type loci. We show that the role of the Dot/Icm type IV secretion system in the induction of apoptosis is independent of the RtxA toxin, the dot/icm-regulated pore-forming toxin, and the type II secretion system. However, the pore-forming toxin, which is triggered upon entry into the postexponential growth phase, enhances the ability of L. pneumophila to induce apoptosis. Our data provide the first example of the role of a type IV secretion system of a bacterial pathogen in the induction of apoptosis in the host cell.

Enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) are an increasingly important cause of diarrhoea. E. coli belonging to this category cause watery diarrhoea, which is often persistent and can be inflammatory. EAEC have been implicated in sporadic diarrhoea in children and adults, in both developing and developed countries, and have been identified as the cause of several outbreaks worldwide. EAEC are defined by their ability to adhere to epithelial cells in a characteristic "stacked-brick" pattern but are otherwise highly heterogeneous. Genes that could contribute to the pathogenicity of EAEC encode adhesins, toxins, and other factors, all of which are only partially conserved. Practicable tools are needed to improve diagnosis and identify risk factors. EAEC-infected individuals can be treated with fluoroquinolones but there is a need to examine alternative treatment protocols.

Adherent invasive Escherichia coli strains from patients with Crohn's disease survive and replicate within macrophages without inducing host cell death

Escherichia coli strains recovered from Crohn's disease (CD) lesions are able to adhere to and invade cultured intestinal epithelial cells. We analyzed the behavior within macrophages of adherent invasive E. coli (AIEC) strains isolated from patients with CD. All the 15 AIEC strains tested were able to replicate extensively within J774-A1 cells: the numbers of intracellular bacteria increased 2.2- to 74.2-fold at 48 h over that at 1 h postinfection. By use of murine peritoneal macrophages and human monocyte-derived-macrophages, the reference AIEC strain LF82 was confirmed to be able to survive intracellularly. Transmission electron micrographs of AIEC LF82-infected macrophages showed that at 24 h postinfection, infected cells harbored large vacuoles containing numerous bacteria, as a result of the fusion of several vacuoles occurring after 8 h postinfection. No lactate dehydrogenase (LDH) release, no sign of DNA fragmentation or degradation, and no binding to fluorescein isothlocyanate-labeled annexin V were observed with LF82-infected J774-A1 cells, even after 24 h postinfection. LF82-infected J774-A1 cells secreted 2.7-fold more tumor necrosis factor alpha (TNF-alpha) than cells stimulated with 1 microg of lipopolysaccharide (LPS)/ml. No release of interleukin-1beta was observed with LPS-prestimulated J774-A1 cells infected with AIEC LF82. These findings showed that (i) AIEC strains are able to survive and to replicate within macrophages, (ii) AIEC LF82 replication does not induce any cell death of the infected cells, and (iii) LF82-infected J774-A1 cells release high levels of TNF-alpha. These properties could be related to some features of CD and particularly to granuloma formation, one of the hallmarks of CD lesions.

Pyelonephritogenic diffusely adhering Escherichia coli EC7372 harboring Dr-II adhesin carries classical uropathogenic virulence genes and promotes cell lysis and apoptosis in polarized epithelial caco-2/TC7 cells

Diffusely adhering Escherichia coli (DAEC) strains expressing adhesins of the Afa/Dr family bind to epithelial cells in a diffuse adherence pattern by recognizing a common receptor, the decay-accelerating factor (CD55). Recently, a novel CD55-binding adhesin, named Dr-II, was identified from the pyelonephritogenic strain EC7372. In this report, we show that despite the low level of sequence identity between Dr-II and other members of the Afa/Dr family, EC7372 induces pathophysiological effects similar to those induced by other Afa/Dr DAEC strains on the polarized epithelial cell line Caco-2/TC7. Specifically, the Dr-II adhesin was sufficient to promote CD55 and CD66e clustering around adhering bacteria and apical cytoskeleton rearrangements. Unlike other Afa/Dr DAEC strains, EC7372 expresses a functional hemolysin that promotes a rapid cellular lysis. In addition, cell death by apoptosis or necrosis was observed in EC7372-infected Caco-2/TC7 cells, depending on infection time. Our results indicate that EC7372 harbors a pathogenicity island (PAI) similar to the one described for the pyelonephritogenic strain CFT073, which carries both hly and pap operons. Cumulatively, our findings indicate that strain EC7372 can be considered a prototype of a subclass of Afa/Dr DAEC isolates that have acquired a PAI harboring several classical uropathogenic virulence genes.

CD95/CD95 ligand interactions on epithelial cells in host defense to Pseudomonas aeruginosa

Pseudomonas aeruginosa causes severe infections, particularly of the lung, that are life threatening. Here, we show that P. aeruginosa infection induces apoptosis of lung epithelial cells by activation of the endogenous CD95/CD95 ligand system. Deficiency of CD95 or CD95 ligand on epithelial cells prevented apoptosis of lung epithelial cells in vivo as well as in vitro. The importance of CD95/CD95 ligand-mediated lung epithelial cell apoptosis was demonstrated by the rapid development of sepsis in CD95- or CD95 ligand-deficient mice, but not in normal mice, after P. aeruginosa infection.

Cytocidal and apoptotic effects of the ClyA protein from Escherichia coli on primary and cultured monocytes and macrophages

Cytolysin A (ClyA) is a newly discovered cytolytic protein of Escherichia coli K-12 that mediates a hemolytic phenotype. We show here that highly purified ClyA and ClyA-expressing E. coli were cytotoxic and apoptogenic to fresh as well as cultured human and murine monocytes/macrophages.

Shigella flexneri IpaH(7.8) facilitates escape of virulent bacteria from the endocytic vacuoles of mouse and human macrophages

The behavior of Shigella flexneri ipaH mutants was studied in human monocyte-derived macrophages (HMDM), in 1-day-old human monocytes, and in J774 mouse macrophage cell line. In HMDM, strain pWR700, an ipaH(7.8) deletion mutant of S. flexneri 2a strain 2457T, behaved like the wild-type strain 2457T. This strain caused rapid host cell death by oncosis, and few bacterial CFU were recovered after incubation in the presence of gentamicin as previously described for 2457T-infected HMDM. However, analysis of bacterial compartmentalization within endocytic vacuoles with gentamicin and chloroquine indicated that more pWR700 than 2457T was present within the endocytic vacuoles of HMDM, suggesting that ipaH(7.8) deletion mutant transited more slowly from the vacuoles to the cytoplasm. In contrast to findings with HMDM, CFU recovered from pWR700-infected mouse J774 cells were 2 to 3 logs higher than CFU from 2457T-infected J774 cells. These values exceeded CFU recovered after infection of J774 cells with plasmid-cured avirulent strain M4243A1. Incubation with gentamicin and chloroquine clearly showed that pWR700 within J774 cells was mostly present within the endocytic vacuoles. This distribution pattern was similar to that seen with M4243A1 and contrasted with the pattern seen with 2457T. Complementation of pWR700 with a recombinant clone expressing ipaH(7. 8) restored the intracellular distribution of bacteria to that seen with the wild-type strain. Strains with deletions in ipaH(4.5) or ipaH(9.8), however, behaved like 2457T in both HMDM and J774 cells. The distribution profile of pWR700 in 1-day-old monocytes was similar to that seen in J774 cells. Like infected J774 cells, 1-day-old human monocytes demonstrated apoptosis upon infection with virulent Shigella. These results suggest that a role of the ipaH(7. 8) gene product is to facilitate the escape of the virulent bacteria from the phagocytic vacuole of monocytes and macrophages.

Enterohemorrhagic Escherichia coli induces apoptosis which augments bacterial binding and phosphatidylethanolamine exposure on the plasma membrane outer leaflet

Enterohemorrhagic Escherichia coli (EHEC) is a gastrointestinal pathogen that causes watery diarrhea and hemorrhagic colitis and can lead to serious and even fatal complications such as hemolytic uremic syndrome. We investigated the ability of EHEC to kill host cells using three human epithelial cell lines. Analysis of phosphatidylserine expression, internucleosomal cleavage of host cell DNA and morphological changes detected by electron microscopy changes revealed evidence of apoptotic cell death. The rates and extents of cell death were similar for both verotoxin-producing and nonproducing strains of EHEC as well as for a related gastrointestinal pathogen, enteropathogenic E. coli (EPEC). The induction of apoptosis by bacterial attachment was independent of verotoxin production and greater than that produced by a similar treatment with verotoxin alone. Expression of phosphatidylethanolamine, previously reported to bind EHEC and EPEC, was also increased on apoptotic cells but with little correlation to phosphatidylserine expression. Phosphatidylethanolamine levels but not phosphatidylserine levels on dying cells correlated with EHEC binding. Cells treated with phosphatidylethanolamine-containing liposomes also showed increased EHEC binding. These results suggest that bacterial induction of apoptosis offers an advantage for bacterial attachment by augmenting outer leaflet levels of the phosphatidylethanolamine receptor.

Pseudomonas aeruginosa cystic fibrosis isolates induce rapid, type III secretion-dependent, but ExoU-independent, oncosis of macrophages and polymorphonuclear neutrophils

Pseudomonas aeruginosa, an opportunistic pathogen responsible most notably for severe infections in cystic fibrosis (CF) patients, utilizes the type III secretion system for eukaryotic cell intoxication. The CF clinical isolate CHA shows toxicity towards human polymorphonuclear neutrophils (PMNs) which is dependent on the type III secretion system but independent of the cytotoxin ExoU. In the present study, the cytotoxicity of this strain toward human and murine macrophages was demonstrated. In low-multiplicity infections (multiplicity of infection, 10), approximately 40% of the cells die within 60 min. Analysis of CHA-infected cells by transmission electron microscopy, DNA fragmentation assay, and Hoechst staining revealed the hallmarks of oncosis: cellular and nuclear swelling, disintegration of the plasma membrane, and absence of DNA fragmentation. A panel of 29 P. aeruginosa CF isolates was screened for type III system genotype, protein secretion profile, and cytotoxicity toward PMNs and macrophages. This study showed that six CF isolates were able to induce rapid ExoU-independent oncosis on phagocyte cells.

Bordetella bronchiseptica-mediated cytotoxicity to macrophages is dependent on bvg-regulated factors, including pertactin

The effect of Bordetella bronchiseptica infection on the viability of murine macrophage-like cells and on primary porcine alveolar macrophages was investigated. The bacterium was shown to be cytotoxic for both cell types, particularly where tight cell-to-cell contacts were established. In addition, bvg mutants were poorly cytotoxic for the eukaryotic cells, while a prn mutant was significantly less toxic than wild-type bacteria. B. bronchiseptica-mediated cytotoxicity was inhibited in the presence of cytochalasin D or cycloheximide, an inhibitor of microfilament-dependent phagocytosis or de novo eukaryotic protein synthesis, respectively. The mechanism of eukaryotic cell death was examined, and cell death was found to occur primarily through a necrotic pathway, although a small proportion of the population underwent apoptosis.

Host cell death due to enteropathogenic Escherichia coli has features of apoptosis

Enteropathogenic Escherichia coli (EPEC) is a cause of prolonged watery diarrhea in children in developing countries. The ability of EPEC to kill host cells was investigated in vitro in assays using two human cultured cell lines, HeLa (cervical) and T84 (colonic). EPEC killed epithelial cells as assessed by permeability to the vital dyes trypan blue and propidium iodide. In addition, EPEC triggered changes in the host cell, suggesting apoptosis as the mode of death; such changes included early expression of phosphatidylserine on the host cell surface and internucleosomal cleavage of host cell DNA. Genistein, an inhibitor of tyrosine kinases, and wortmannin, an inhibitor of host phosphatidylinositol 3-kinase, markedly increased EPEC-induced cell death and enhanced the features of apoptosis. EPEC-induced cell death was contact dependent and required adherence of live bacteria to the host cell. A quantitative assay for EPEC-induced cell death was developed by using the propidium iodide uptake method adapted to a fluorescence plate reader. With EPEC, the rate and extent of host cell death were less that what has been reported for Salmonella, Shigella, and Yersinia, three other genera of enteric bacteria known to cause apoptosis. However, rapid apoptosis of the host cell may not favor the pathogenic strategy of EPEC, a mucosa-adhering, noninvasive pathogen.

An apoptotic response by J774 macrophage cells is common upon infection with diarrheagenic Escherichia coli

Representative strains of the different diarrheagenic Escherichia coli virotypes were tested for their potential cytotoxicity in the J774 macrophage cell line. All the seven virotypes of E. coli were cytotoxic to J774 macrophages, and in most cases the bacteria induced an apoptotic response. With the exception of the enterotoxigenic E. coli (ETEC) strain, all the other six virotypes caused induction of apoptosis as evidenced by quantitative analysis of the characteristic DNA fragmentation at the individual cell level. These results suggest that apoptosis could be one of the mechanisms contributing to the diarrheal disease development.

Identification of a vibrio cholerae RTX toxin gene cluster that is tightly linked to the cholera toxin prophage

We identify and characterize a gene cluster in El Tor Vibrio cholerae that encodes a cytotoxic activity for HEp-2 cells in vitro. This gene cluster contains four genes and is physically linked to the cholera toxin (CTX) element in the V. cholerae genome. We demonstrate by using insertional mutagenesis that this gene cluster is required for the cytotoxic activity. The toxin, RtxA, resembles members of the RTX (repeats in toxin) toxin family in that it contains a GD-rich repeated motif. Like other RTX toxins, its activity depends on an activator, RtxC, and an associated ABC transporter system, RtxB and RtxD. In V. cholerae strains of the classical biotype, a deletion within the gene cluster removes rtxC and eliminates cytotoxic activity. Other strains, including those of the current cholera pandemic, contain a functional gene cluster and display cytotoxic activity. Thus, the RTX gene cluster in El Tor O1 and O139 strains might have contributed significantly to their emergence. Furthermore, the RTX toxin of V. cholerae may be associated with residual adverse properties displayed by certain live, attenuated cholera vaccines.