Escherichia coli cytotoxins and enterotoxins.

Vero cell cytotoxins and cytotonic enterotoxins produced by E. coli are toxic proteins, which have been implicated in a number of specific diseases in humans and animals. Nomenclature for these toxins is complicated by the existence of different names for the same toxin. The Vero cell cytotoxins are called verotoxins because they are lethal for Vero cells in culture; they are also known as Shiga-like toxins (SLTs) because they are clearly related to Shiga toxin in structure, amino acid sequence, mechanism of action, and biological activity. SLTs belong to two classes. SLT-I is identical with Shiga toxin and is in a class by itself (class I). The other SLTs are closely related to each other and form a second class (class II). Class II SLTs include SLT-II, SLT-IIv, SLT-IIvha, SLT-IIvhb, and SLT-IIva. All SLTs that have been investigated are A-B subunit protein toxins, whose A subunits possess N-glycosidase activity against 28S rRNA and cause inhibition of protein synthesis in eukaryotic cells. These toxins are enterotoxic as well as cytotoxic. SLTs produced in the intestine are absorbed into the blood stream and affect vascular endothelial cells in target organs. They may also have a direct toxic effect on enterocytes. Diseases in which E. coli SLTs have been implicated include diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome in humans and edema disease in pigs. Variation in receptor specificities among SLTs may be the reason for different disease syndromes in different host species. The E. coli enterotoxins belong to three distinct classes: heat-labile enterotoxin (LT), heat-stable enterotoxin type I or type a (STI, STa), and heat-stable enterotoxin type II or type b (STII, STb). There is clear evidence that these cytotonic enterotoxins play an essential role in diarrheal disease. LT is an A-B subunit protein toxin, closely related to cholera toxin. Following binding of LT to receptors in enterocytes the A subunit is internalized. The enzymatically active A subunit transfers ADP-ribose from NAD to a GTP-dependent adenylate cyclase regulatory protein, thereby elevating intracellular levels of adenylate cyclase. The increased levels of cyclic AMP cause stimulation of A kinase and lead to hypersecretion of electrolytes and fluid. STI is a small peptide of 18 or 19 amino acids. It binds to receptors in enterocytes and stimulates particulate guanyl cyclase. Elevated intracellular cyclic GMP stimulates G kinase, resulting in increased Cl- secretion and impaired absorption of Na+Cl-. STII is a peptide toxin whose mechanism of action is unknown.(ABSTRACT TRUNCATED AT 400 WORDS)     

Functional analysis of the Shiga toxin and Shiga-like toxin type II variant binding subunits by using site-directed mutagenesis.

The B subunit of Shiga toxin and the Shiga-like toxins (SLTs) mediates receptor binding, cytotoxic specificity, and extracellular localization of the holotoxin. While the functional receptor for Shiga toxin, SLT type I (SLT-I), and SLT-II is the glycolipid designated Gb3, SLT-II variant (SLT-IIv) may use a different glycolipid receptor. To identify the domains responsible for receptor binding, localization in Escherichia coli, and recognition by neutralizing monoclonal antibodies, oligonucleotide-directed site-specific mutagenesis was used to alter amino acid residues in the B subunits of Shiga toxin and SLT-IIv. Mutagenesis of a well-conserved hydrophilic region near the amino terminus of the Shiga toxin B subunit rendered the molecule nontoxic but did not affect immunoreactivity or holotoxin assembly. In addition, elimination of one cysteine residue, as well as truncation of the B polypeptide by 5 amino acids, caused a total loss of activity. Changing a glutamate to a glutamine at the carboxyl terminus of the Shiga toxin B subunit resulted in the loss of receptor binding and immunoreactivity. However, the corresponding mutation in the SLT-IIv B subunit (glutamine to glutamate) did not reduce the levels of cytotoxicity but did affect extracellular localization of the holotoxin in E. coli.     

Infection by shiga toxin-producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC), especially of serotype O157:H7, cause a zoonotic food or waterborne enteric illness that is often associated with large epidemic outbreaks as well as the hemolytic uremic syndrome (HUS), the leading cause of acute renal failure in children. After ingestion, STEC colonize enterocytes of the large bowel with a characteristic attaching and effacing pathology, which is mediated by components of a type III secretion apparatus encoded by the LEE pathogenicity island. Shiga toxins are translocated from the bowel to the circularoty system and transported by leukocytes to capillary endothelial cells in renal glomeruli and other organs. After binding to the receptor globotriaosylceramide on target cells, the toxin is internalized by receptor-mediated endocytosis and interacts with the subcellular machinery to inhibit protein synthesis. This leads to pathophysiological changes that result in HUS. Specific therapeutic or preventive strategies are presently not available. The recent sequencing of genomes of two epidemic E. coli O157 strains has revealed novel pathogenicity islands which will likely provide new insights into the virulence of these bacteria.     

Mutational analysis of the Shiga toxin and Shiga-like toxin II enzymatic subunits.

The A-subunit polypeptides of Shiga toxin, the Shiga-like toxins (SLTs), and the plant lectin ricin inactivate eucaryotic ribosomes by enzymatically depurinating 28S rRNA. Comparison of the amino acid sequences of the members of the Shiga toxin family and ricin revealed two regions of significant homology that lie within a proposed active-site cleft of the ricin A chain. In previous studies, these conserved sequences of the SLT-I and ricin A subunits have been implicated as active sites. To establish the importance of these regions of homology, we used site-directed mutagenesis to alter the A-subunit sequences of two members of the Shiga toxin family. Substitution of an aspartic acid for glutamic acid 166 of the Slt-IIA subunit decreased the capacity of the polypeptides to inhibit protein synthesis at least 100-fold in a cell-free translation system. However, this mutation did not prevent the expression of immunoreactive, full-length Slt-IIA. In addition, SLT-II holotoxin containing the mutated A subunit was 1,000-fold less toxic to Vero cells. Finally, site-directed mutagenesis was used to delete sequences encoding amino acids 202 through 213 of the Shiga toxin A subunit. Although this deletion did not prevent holotoxin assembly, it abolished cytotoxic activity.     

Change in conformation with reduction of alpha-helix content causes loss of neutrophil binding activity in fully cytotoxic Shiga toxin 1

Shiga toxins (Stx) play an important role in the pathogenesis of hemolytic uremic syndrome, a life-threatening renal sequela of human intestinal infection caused by specific Escherichia coli strains. Stx target a restricted subset of human endothelial cells that possess the globotriaosylceramide receptor, like that in renal glomeruli. The toxins, composed of five B chains and a single enzymatic A chain, by removing adenines from ribosomes and DNA, trigger apoptosis and the production of pro-inflammatory cytokines in target cells. Because bacteria are confined to the gut, the toxins move to the kidney through the circulation. Polymorphonuclear leukocytes (PMN) have been indicated as the carriers that "piggyback" shuttle toxins to the kidney. However, there is no consensus on this topic, because not all laboratories have been able to reproduce the Stx/PMN interaction. Here, we demonstrate that conformational changes of Shiga toxin 1, with reduction of α-helix content and exposition to solvent of hydrophobic tryptophan residues, cause a loss of PMN binding activity. The partially unfolded toxin was found to express both enzymatic and globotriaosylceramide binding activities being fully active in intoxicating human endothelial cells; this suggests the presence of a distinct PMN-binding domain. By reviewing functional and structural data, we suggest that A chain moieties close to Trp-203 are recognized by PMN. Our findings could help explain the conflicting results regarding Stx/PMN interactions, especially as the groups reporting positive results obtained Stx by single-step affinity chromatography, which could have preserved the correct folding of Stx with respect to more complicated multi-step purification methods.     

Latrunculin B facilitates Shiga toxin 1 transcellular transcytosis across T84 intestinal epithelial cells

Shiga toxins (Stx), released into the intestinal lumen by enterohemorrhagic Escherichia coli (EHEC), are major virulence factors responsible for gastrointestinal and systemic illnesses. These pathologies are believed to be due to the action of the toxins on endothelial cells, which express the Stx receptor, the glycosphingolipid Gb3. To reach the endothelial cells, Stx must translocate across the intestinal epithelial monolayer. This process is poorly understood. We investigated Stx1 movement across the intestinal epithelial T84 cell model and the role of actin turnover in this transcytosis. We showed that changes in the actin cytoskeleton due to latrunculin B, but not cytochalasin D or jasplakinolide, significantly facilitate toxin transcytosis across T84 monolayers. This trafficking is transcellular and completely inhibited by tannic acid, a cell impermeable plasma membrane fixative. This indicates that actin turnover could play an important role in Stx1 transcellular transcytosis across intestinal epithelium in vitro. Since EHEC attachment to epithelial cells causes an actin rearrangement, this finding may be highly relevant to Stx-induced disease.     

Pathways followed by ricin and Shiga toxin into cells

The plant toxin ricin and the bacterial toxin Shiga toxin belong to a group of protein toxins that inhibit protein synthesis in cells enzymatically after entry into the cytosol. Ricin and Shiga toxin, which both have an enzymatically active moiety that inactivates ribosomes and a moiety that binds to cell surface receptors, enter the cytosol after binding to the cell surface, endocytosis by different mechanisms, and retrograde transport to the Golgi apparatus and the endoplasmic reticulum (ER). The toxins can be used to investigate the various transport steps involved, both the endocytic mechanisms as well as pathways for retrograde transport to the ER. Recent studies show that not only do several endocytic mechanisms exist in the same cell, but they are not equally sensitive to removal of cholesterol. New data have revealed that there is also more than one pathway leading from endosomes to the Golgi apparatus and retrogradely from the Golgi to the ER. Trafficking of protein toxins along these pathways will be discussed in the present article.     

Protein toxins from plants and bacteria: Probes for intracellular transport and tools in medicine

A number of protein toxins produced by bacteria and plants enter eukaryotic cells and inhibit protein synthesis enzymatically. These toxins include the plant toxin ricin and the bacterial toxin Shiga toxin, which we will focus on in this article. Although a threat to human health, toxins are valuable tools to discover and characterize cellular processes such as endocytosis and intracellular transport. Bacterial infections associated with toxin production are a problem worldwide. Increased knowledge about toxins is important to prevent and treat these diseases in an optimal way. Interestingly, toxins can be used for diagnosis and treatment of cancer.     

Facing Inward from Compartment Shores: How Many Pathways were we Looking For?

Protein toxins of the Shiga family have become potent tools in studying a number of intracellular transport events such as endocytosis, the communication between endosomes and the biosynthetic/secretory pathway, and retrograde transport from the Golgi apparatus to the endoplasmic reticulum. It seems clear today that most of these transport events can be explained from the toxins' interactions with cellular factors. This review will primarily focus on the discussion of recent data obtained on Shiga toxin and related toxins. We will point out to what extent the study of these proteins has opened new avenues for the development of intracellular targeting tools.     

Retrograde transport of protein toxins through the Golgi apparatus

A number of protein toxins from plants and bacteria take advantage of transport through the Golgi apparatus to gain entry into the cytosol where they exert their action. These toxins include the plant toxin ricin, the bacterial Shiga toxins, and cholera toxin. Such toxins bind to lipids or proteins at the cell surface, and they are endocytosed both by clathrin-dependent and clathrin-independent mechanisms. Sorting to the Golgi and retrograde transport to the endoplasmic reticulum (ER) are common to these toxins, but the exact mechanisms turn out to be toxin and cell-type dependent. In the ER, the enzymatically active part is released and then transported into the cytosol, exploiting components of the ER-associated degradation system. In this review, we will discuss transport of different protein toxins, but we will focus on factors involved in entry and sorting of ricin and Shiga toxin into and through the Golgi apparatus.     

Shiga toxin binding in normal and inflamed human intestinal mucosa

Shiga toxins are associated with haemolytic uraemic syndrome but human intestinal epithelium does not express the Gb3 receptor. We describe Gb3 expression and Shiga toxin binding in histologically normal intestine and demonstrate that the pattern is unaltered in inflammatory disease states. Gb3 expression and Shiga toxin binding were identified in Paneth cells in both normal and inflamed mucosae.     

Effects of Escherichia Coli Subtilase Cytotoxin and Shiga Toxin 2 on Primary Cultures of Human Renal Tubular Epithelial Cells

Shiga toxin (Stx)-producing Escherichia coli (STEC) cause post-diarrhea Hemolytic Uremic Syndrome (HUS), which is the most common cause of acute renal failure in children in many parts of the world. Several non-O157 STEC strains also produce Subtilase cytotoxin (SubAB) that may contribute to HUS pathogenesis. The aim of the present work was to examine the cytotoxic effects of SubAB on primary cultures of human cortical renal tubular epithelial cells (HRTEC) and compare its effects with those produced by Shiga toxin type 2 (Stx2), in order to evaluate their contribution to renal injury in HUS. For this purpose, cell viability, proliferation rate, and apoptosis were assayed on HRTEC incubated with SubAB and/or Stx2 toxins. SubAB significantly reduced cell viability and cell proliferation rate, as well as stimulating cell apoptosis in HRTEC cultures in a time dependent manner. However, HRTEC cultures were significantly more sensitive to the cytotoxic effects of Stx2 than those produced by SubAB. No synergism was observed when HRTEC were co-incubated with both SubAB and Stx2. When HRTEC were incubated with the inactive SubA_A272B toxin, results were similar to those in untreated control cells. Similar stimulation of apoptosis was observed in Vero cells incubated with SubAB or/and Stx2, compared to HRTEC. In conclusion, primary cultures of HRTEC are significantly sensitive to the cytotoxic effects of SubAB, although, in a lesser extent compared to Stx2.     

Serine Protease EspP from Enterohemorrhagic Escherichia Coli Is Sufficient to Induce Shiga Toxin Macropinocytosis in Intestinal Epithelium

Life-threatening intestinal and systemic effects of the Shiga toxins produced by enterohemorrhagic Escherichia coli (EHEC) require toxin uptake and transcytosis across intestinal epithelial cells. We have recently demonstrated that EHEC infection of intestinal epithelial cells stimulates toxin macropinocytosis, an actin-dependent endocytic pathway. Host actin rearrangement necessary for EHEC attachment to enterocytes is mediated by the type 3 secretion system which functions as a molecular syringe to translocate bacterial effector proteins directly into host cells. Actin-dependent EHEC attachment also requires the outer membrane protein intimin, a major EHEC adhesin. Here, we investigate the role of type 3 secretion in actin turnover occurring during toxin macropinocytosis. Toxin macropinocytosis is independent of EHEC type 3 secretion and intimin attachment. EHEC soluble factors are sufficient to stimulate macropinocytosis and deliver toxin into enterocytes in vitro and in vivo; intact bacteria are not required. Intimin-negative enteroaggregative Escherichia coli (EAEC) O104:H4 robustly stimulate Shiga toxin macropinocytosis into intestinal epithelial cells. The apical macropinosomes formed in intestinal epithelial cells move through the cells and release their cargo at these cells’ basolateral sides. Further analysis of EHEC secreted proteins shows that a serine protease EspP alone is able to stimulate host actin remodeling and toxin macropinocytosis. The observation that soluble factors, possibly serine proteases including EspP, from each of two genetically distinct toxin-producing strains, can stimulate Shiga toxin macropinocytosis and transcellular transcytosis alters current ideas concerning mechanisms whereby Shiga toxin interacts with human enterocytes. Mechanisms important for this macropinocytic pathway could suggest new potential therapeutic targets for Shiga toxin-induced disease.     

Lipid requirements for entry of protein toxins into cells

The plant toxin ricin and the bacterial toxin Shiga toxin both belong to a group of protein toxins having one moiety that binds to the cell surface, and another, enzymatically active moiety, that enters the cytosol and inhibits protein synthesis by inactivating ribosomes. Both toxins travel all the way from the cell surface to endosomes, the Golgi apparatus and the ER before the ribosome-inactivating moiety enters the cytosol. Shiga toxin binds to the neutral glycosphingolipid Gb3 at the cell surface and is therefore dependent on this lipid for transport into the cells, whereas ricin binds both glycoproteins and glycolipids with terminal galactose. The different steps of transport used by these toxins have specific requirements for lipid species, and with the recent developments in mass spectrometry analysis of lipids and microscopical and biochemical dissection of transport in cells, we are starting to see the complexity of endocytosis and intracellular transport. In this article we describe lipid requirements and the consequences of lipid changes for the entry and intoxication with ricin and Shiga toxin. These toxins can be a threat to human health, but can also be exploited for diagnosis and therapy, and have proven valuable as tools to study intracellular transport.     

Exosomes released from Shiga toxin 2a–treated human macrophages modulate inflammatory responses and induce cell death in toxin receptor expressing human cells

Shiga toxins (Stxs) produced by Stx‐producing Escherichia coli are the primarily virulence factors of hemolytic uremic syndrome and central nervous system (CNS) impairment. Although the precise mechanisms of toxin dissemination remain unclear, Stxs bind to extracellular vesicles (EVs). Exosomes, a subset of EVs, may play a key role in Stx‐mediated renal injury. To test this hypothesis, we isolated exosomes from monocyte‐derived macrophages in the presence of Stx2a or Stx2 toxoids. Macrophage‐like differentiated THP‐1 cells treated with Stxs secreted Stx‐associated exosomes (Stx‐Exo) of 90–130 nm in diameter, which induced cytotoxicity in recipient cells in a toxin receptor globotriaosylceramide (Gb₃)‐dependent manner. Stx2‐Exo engulfed by Gb₃‐positive cells were translocated to the endoplasmic reticulum in the human proximal tubule epithelial cell line HK‐2. Stx2‐Exo contained pro‐inflammatory cytokine mRNAs and proteins and induced more severe inflammation than purified Stx2a accompanied by greater death of target cells such as human renal or retinal pigment epithelial cells. Blockade of exosome biogenesis using the pharmacological inhibitor GW4869 reduced Stx2‐Exo‐mediated human renal cell death. Stx2‐Exo isolated from human primary monocyte–derived macrophages activated caspase 3/7 and resulted in significant cell death in primary human renal cortical epithelial cells. Based on these results, we speculate that Stx‐containing exosomes derived from macrophages may exacerbate cytotoxicity and inflammation and trigger cell death in toxin‐sensitive cells. Therapeutic interventions targeting Stx‐containing exosomes may prevent or ameliorate Stx‐mediated acute vascular dysfunction.     

Macropinocytosis in Shiga toxin 1 uptake by human intestinal epithelial cells and transcellular transcytosis

Shiga toxin 1 and 2 production is a cardinal virulence trait of enterohemorrhagic Escherichia coli infection that causes a spectrum of intestinal and systemic pathology. However, intestinal sites of enterohemorrhagic E. coli colonization during the human infection and how the Shiga toxins are taken up and cross the globotriaosylceramide (Gb3) receptor-negative intestinal epithelial cells remain largely uncharacterized. We used samples of human intestinal tissue from patients with E. coli O157:H7 infection to detect the intestinal sites of bacterial colonization and characterize the distribution of Shiga toxins. We further used a model of largely Gb3-negative T84 intestinal epithelial monolayers treated with B-subunit of Shiga toxin 1 to determine the mechanisms of non-receptor-mediated toxin uptake. We now report that E. coli O157:H7 were found at the apical surface of epithelial cells only in the ileocecal valve area and that both toxins were present in large amounts inside surface and crypt epithelial cells in all tested intestinal samples. Our in vitro data suggest that macropinocytosis mediated through Src activation significantly increases toxin endocytosis by intestinal epithelial cells and also stimulates toxin transcellular transcytosis. We conclude that Shiga toxin is taken up by human intestinal epithelial cells during E. coli O157:H7 infection regardless of the presence of bacterial colonies. Macropinocytosis might be responsible for toxin uptake by Gb3-free intestinal epithelial cells and transcytosis. These observations provide new insights into the understanding of Shiga toxin contribution to enterohemorrhagic E. coli-related intestinal and systemic diseases.     

Effects of human intravenous immune globulin on diarrhea caused by Shiga-like toxin I and Shiga-like toxin II in infant rabbits.

Shiga toxin and the related Shiga-like toxins (SLT), produced by Escherichia coli, can cause hemorrhagic colitis and hemolytic uremic syndrome (HUS). Human intravenous immune globulin (HIVIg) blocks the cytotoxicity of some SLTs in vitro. To examine the ability of HIVIg to modify disease caused by Shiga-like toxin I or Shiga-like toxin II (SLT-I or SLT-II), we injected 3-day-old rabbits intraperitoneally with SLT-containing cell-free supernatants from Escherichia coli O157: H7. A subset of rabbits was treated with subcutaneous HIVIg. All rabbits given 10(4) CD50 of SLT-I developed severe diarrhea, and 5 died. When HIVIg 500 mg/kg was given in addition to SLT-I, only 6 of 18 rabbits (33.3%) developed diarrhea (P < 0.0001), and 1 died. HIVIg 500 mg/kg or 1,000 mg/kg protected against diarrhea when given one hour prior to toxin. HIVIg 1,000 mg/kg was protective when administered one hour after toxin, but not at 6 or 24 hr. Seventeen of 18 rabbits given 10(6) CD50 of SLT-II developed severe diarrhea, and 4 died. In contrast to SLT-I-associated disease, HIVIg had no effect on diarrhea in rabbits given SLT-II. We conclude that HIVIg protects infant rabbits from diarrhea and death caused by intraperitoneally administered SLT-I, but does not affect the course of SLT-II-associated illness.     

Enterohemorrhagic Escherichia coli infection stimulates Shiga toxin 1 macropinocytosis and transcytosis across intestinal epithelial cells

Gastrointestinal infection with Shiga toxins producing enterohemorrhagic Escherichia coli causes the spectrum of gastrointestinal and systemic complications, including hemorrhagic colitis and hemolytic uremic syndrome, which is fatal in ～10% of patients. However, the molecular mechanisms of Stx endocytosis by enterocytes and the toxins cross the intestinal epithelium are largely uncharacterized. We have studied Shiga toxin 1 entry into enterohemorrhagic E. coli-infected intestinal epithelial cells and found that bacteria stimulate Shiga toxin 1 macropinocytosis through actin remodeling. This enterohemorrhagic E. coli-caused macropinocytosis occurs through a nonmuscle myosin II and cell division control 42 (Cdc42)-dependent mechanism. Macropinocytosis of Shiga toxin 1 is followed by its transcytosis to the basolateral environment, a step that is necessary for its systemic spread. Inhibition of Shiga toxin 1 macropinocytosis significantly decreases toxin uptake by intestinal epithelial cells and in this way provides an attractive, antibiotic-independent strategy for prevention of the harmful consequences of enterohemorrhagic E. coli infection.     

Cloning and sequencing of a Shiga-like toxin type II variant from Escherichia coli strain responsible for edema disease of swine.

A Shiga-like toxin type II variant (SLT-IIv) is produced by strains of Escherichia coli responsible for edema disease of swine and is antigenically related to Shiga-like toxin type II (SLT-II) of enterohemorrhagic E. coli. However, SLT-IIv is only active against Vero cells, whereas SLT-II is active against both Vero and HeLa cells. The structural genes for SLT-IIv were cloned from E. coli S1191, and the nucleotide sequence was determined and compared with those of other members of the Shiga toxin family. The A subunit genes for SLT-IIv and SLT-II were highly homologous (94%), whereas the B subunit genes were less homologous (79%). The SLT-IIv genes were more distantly related (55 to 60% overall homology) to the genes for Shiga toxin of Shigella dysenteriae type 1 and the nearly identical Shiga-like toxin type I (SLT-I) of enterohemorrhagic E. coli. (These toxins are referred to together as Shiga toxin/SLT-I.) The A subunit of SLT-IIv, like those of other members of this toxin family, had regions of homology with the plant lectin ricin. SLT-IIv did not bind to galactose-alpha 1-4-galactose conjugated to bovine serum albumin, which is an analog of the eucaryotic cell receptor for Shiga toxin/SLT-I and SLT-II. These findings support the hypothesis that SLT-IIv binds to a different cellular receptor than do other members of the Shiga toxin family but has a similar mode of intracellular action. The organization of the SLT-IIv operon was similar to that of other members of the Shiga toxin family. Iron did not suppress SLT-IIv or SLT-II production, in contrast with its effect on Shiga toxin/SLT-I. Therefore, the regulation of synthesis of SLT-IIv and SLT-II differs from that of Shiga toxin/SLT-I.     

Epithelial cell signaling responses to enterohemorrhagic Escherichia coli infection

Enterohemorrhagic Escherichia coli, including the serotype O157:H7 that is most commonly identified with human disease, cause both sporadic cases and outbreaks of non-bloody diarrhea and hemorrhagic colitis. In about 10% of infected subjects, the hemolytic uremic syndrome (hemolytic anemic, thrombocytopenia, and acute renal failure) develops, likely as a consequence of systemic spread of bacterial-derived toxins variously referred to as Shiga-like toxin, Shiga toxin, and Verotoxin. Increasing evidence points to a complex interplay between bacterial products--for example, adhesins and toxins--and host signal transduction pathways in mediating responses to infection. Identification of critical signaling pathways could result in the development of novel strategies for intervention to both prevent and treat this microbial infection in humans.