Poisons, ruffles and rockets: bacterial pathogens and the host cell cytoskeleton

Analysis of the Zonula occludens Toxin Found in the Genome of the Chilean Non-toxigenic Vibrio parahaemolyticus Strain PMC53.7

Vibrio parahaemolyticus non-toxigenic strains are responsible for about 10% of acute gastroenteritis associated with this species, suggesting they harbor unique virulence factors. Zonula occludens toxin (Zot), firstly described in Vibrio cholerae, is a secreted toxin that increases intestinal permeability. Recently, we identified Zot-encoding genes in the genomes of highly cytotoxic Chilean V. parahaemolyticus strains, including the non-toxigenic clinical strain PMC53.7. To gain insights into a possible role of Zot in V. parahaemolyticus, we analyzed whether it could be responsible for cytotoxicity. However, we observed a barely positive correlation between Caco-2 cell membrane damage and Zot mRNA expression during PMC53.7 infection and non-cytotoxicity induction in response to purified PMC53.7-Zot. Unusually, we observed a particular actin disturbance on cells infected with PMC53.7. Based on this observation, we decided to compare the sequence of PMC53.7-Zot with Zot of human pathogenic species such as V. cholerae, Campylobacter concisus, Neisseria meningitidis, and other V. parahaemolyticus strains, using computational tools. The PMC53.7-Zot was compared with other toxins and identified as an endotoxin with conserved motifs in the N-terminus and a variable C-terminal region and without FCIGRL peptide. Notably, the C-terminal diversity among Zots meant that not all of them could be identified as toxins. Structurally, PMC53.7-Zot was modeled as a transmembrane protein. Our results suggested that it has partial 3D structure similarity with V. cholerae-Zot. Probably, the PMC53.7-Zot would affect the actin cytoskeletal, but, in the absence of FCIGRL, the mechanisms of actions must be elucidated.

Quantitative insights into actin rearrangements and bacterial target site selection from Salmonella Typhimurium infection of micropatterned cells

Reorganization of the host cell actin cytoskeleton is crucial during pathogen invasion. We established micropatterned cells as a standardized infection model for cell invasion to quantitatively study actin rearrangements triggered by Salmonella Typhimurium (S. Tm). Micropatterns of extracellular matrix proteins force cells to adopt a reproducible shape avoiding strong cell-to-cell variations, a major limitation in classical cell culture conditions. S. Tm induced F-actin-rich ruffles and invaded micropatterned cells similar to unconstrained cells. Yet, standardized conditions allowed fast and unbiased comparison of cellular changes triggered by the SipA and SopE bacterial effector proteins. Intensity measurements in defined regions revealed that the content of pre-existing F-actin remained unchanged during infection, suggesting that newly polymerized F-actin in bacteria-triggered ruffles originates from the G-actin pool. Analysing bacterial target sites, we found that bacteria did not show any preferences for the local actin cytoskeleton specificities. Rather, invasion was constrained to a specific 'cell height', due to flagella-mediated near-surface swimming. We found that invasion sites were similar to bacterial binding sites, indicating that S. Tm can induce a permissive invasion site wherever it binds. As micropatterned cells can be infected by many different pathogens they represent a valuable new tool for quantitative analysis of host-pathogen interactions.

Endocytosis at the nanoscale

Endocytosis is a fundamental process in which eukaryotic cells internalise molecules and macromolecules via deformation of the membrane and generation of membrane-bound carriers. Functional aspects are not only limited to uptake of nutrients, but also play a primary role in evolutionary conserved processes such as the regulation of plasma membrane protein activity (i.e. signal-transducing receptors, small-molecule transporters and ion channels), cell motility and mitosis. The macromolecular nature of the material transported by endocytosis makes this route one of the most important targets for nanomedicine. Indeed, many nanoparticle formulations have been customised to enter cells through endocytosis and deliver the cargo within the cell. In this critical review, we present an overview of the biology of endocytosis and discuss its implications in cell internalisation of nanoparticles. We discuss how nanoparticle size, shape and surface chemistry can control this process effectively. Finally, we discuss different drug delivery strategies on how to evade lysosomal degradation to promote effective release of the cargo (376 references).

Subversion of innate immune responses by bacterial hindrance of NF-κB pathway

Bacterial infections cause substantial mortality and burden of disease globally. Induction of a strong innate inflammatory response is the first common host mechanism required for elimination of the invading pathogens. The host transcription factor, nuclear factor kappa B (NF-κB) is essential for immune activation. Conversely, bacterial pathogens have evolved strategies to interfere directly with host cell signalling by regulating or mimicking host proteins. Given the key role of NF-κB in the host inflammatory response, bacteria have expectedly developed virulence effectors interfering with NF-κB signalling pathways. In this review, we explore the bacterial mechanisms utilized to prevent effective NF-κB signalling, which in turn usurp the host inflammatory response.

The winter ulcer bacterium Moritella viscosa demonstrates adhesion and cytotoxicity in a fish cell model

Moritella viscosa is considered the main aetiological agent of 'winter ulcer' disease in farmed salmonid fish. To further understand the pathogenesis of this disease, M. viscosa interaction with fish cells was studied using a Chinook salmon embryo cell line (CHSE-214). As winter ulcer appears exclusively at temperatures below 7-8 degrees C, we attempted to identify if this connection is explained by temperature regulated bacterial virulence. Therefore, infection studies were performed at a temperature range from 4 to 15 degrees C. At all temperatures, M. viscosa caused CHSE cells to retract and round up, lose their attachment abilities and finally disintegrate. The bacterium adhered to CHSE cells and caused changes to the cytoskeleton, however, it did not invade the cells. Increased adherence was demonstrated at 4 degrees C compared to adherence at higher temperatures. Extracellular proteins exerted rapid pore formation and lysis of CHSE cells at a temperature range from 4 to 22 degrees C. Furthermore, only small differences were found comparing extracellular proteomes of M. viscosa from 4 and 15 degrees C. We propose that the pathogenic mechanisms exerted by M. viscosa on CHSE cells are disruption of the cytoskeleton which affects cell rigidity and structure, followed by pore formation and lysis caused by secreted products from the bacterium. These processes can also occur at temperatures above those experienced from winter ulcer outbreaks. However, the adhesion mechanisms appear to be temperature regulated and may contribute to temperature dependent disease outbreaks.

A translocated protein of Bartonella henselae interferes with endocytic uptake of individual bacteria and triggers uptake of large bacterial aggregates via the invasome

Bartonella henselae enters human endothelial cells (ECs) by two alternative routes: either by endocytosis, giving rise to Bartonella-containing vacuoles or by invasome-mediated internalization. Only the latter process depends on the type IV secretion system VirB/VirD4 and involves the formation of cell surface-associated bacterial aggregates, which get engulfed by EC membranes in an F-actin-dependent manner, eventually resulting in their complete internalization. Here, we report that among the VirB/VirD4-translocated effector proteins BepA-BepG only BepG is required for triggering invasome-mediated internalization. Expression of BepG in the Bep-deficient DeltabepA-G mutant restored invasome-mediated internalization. Likewise, ectopic expression of BepG in ECs also restored invasome-mediated internalization of the DeltabepA-G mutant, while no discernable cytoskeletal rearrangements were triggered in uninfected cells. Rather, BepG inhibited endocytic uptake of B. henselae into Bartonella-containing vacuoles and other endocytic processes, that is, invasin-mediated uptake of Yersinia enterocolitica and uptake of inert microspheres. BepG thus triggers invasome-mediated internalization primarily by inhibiting bacterial endocytosis. Bacteria accumulating on the cell surface then induce locally the F-actin rearrangements characteristic for the invasome. These cytoskeletal changes encompass both the rearrangement of pre-existing F-actin fibres and the de novo polymerization of cortical F-actin in the periphery of the invasome by Rac1/Scar1/WAVE- and Cdc42/WASP-dependent pathways that involve the recruitment of the Arp2/3 complex.

Beyond good and evil in the oral cavity: insights into host-microbe relationships derived from transcriptional profiling of gingival cells

In many instances, the encounter between host and microbial cells, through a long-standing evolutionary association, can be a balanced interaction whereby both cell types co-exist and inflict a minimal degree of harm on each other. In the oral cavity, despite the presence of large numbers of diverse organisms, health is the most frequent status. Disease will ensue only when the host-microbe balance is disrupted on a cellular and molecular level. With the advent of microarrays, it is now possible to monitor the responses of host cells to bacterial challenge on a global scale. However, microarray data are known to be inherently noisy, which is caused in part by their great sensitivity. Hence, we will address several important general considerations required to maximize the significance of microarray analysis in depicting relevant host-microbe interactions faithfully. Several advantages and limitations of microarray analysis that may have a direct impact on the significance of array data are highlighted and discussed. Further, this review revisits and contextualizes recent transcriptional profiles that were originally generated for the specific study of intricate cellular interactions between gingival cells and 4 important plaque micro-organisms. To our knowledge, this is the first report that systematically investigates the cellular responses of a cell line to challenge by 4 different micro-organisms. Of particular relevance to the oral cavity, the model bacteria span the entire spectrum of documented pathogenic potential, from commensal to opportunistic to overtly pathogenic. These studies provide a molecular basis for the complex and dynamic interaction between the oral microflora and its host, which may lead, ultimately, to the development of novel, rational, and practical therapeutic, prophylactic, and diagnostic applications.

Characterization of the enzymatic activity of the actin cross-linking domain from the Vibrio cholerae MARTX Vc toxin

Vibrio cholerae is a Gram-negative bacterial pathogen that exports enterotoxins, which alter host cells through a number of mechanisms resulting in diarrheal disease. Among the secreted toxins is the multifunctional, autoprocessing RTX toxin (MARTX(Vc)), which disrupts actin cytoskeleton by covalently cross-linking actin monomers into oligomers. The region of the toxin responsible for cross-linking activity is the actin cross-linking domain (ACD). In this study, we demonstrate unambiguously that ACD utilizes G- and not F-actin as a substrate for the cross-linking reaction and hydrolyzes one molecule of ATP per cross-linking event. Furthermore, major actin-binding proteins that regulate actin cytoskeleton in vivo do not block the cross-linking reaction in vitro. Cofilin inhibits the cross-linking of G- and F-actin, at a high mole ratio to actin but accelerates F-actin cross-linking at low mole ratios. DNase I completely blocks the cross-linking of actin, likely due to steric hindrance with one of the cross-linking sites on actin. In the context of the holotoxin, the inhibition of Rho by the Rho-inactivating domain of MARTX(Vc) (Sheahan, K. L., and Satchell, K. J. F. (2007) Cell. Microbiol. 9, 1324-1335) would accelerate F-actin depolymerization and provide G-actin, alone or in complex with actin-binding proteins, for cross-linking by ACD, ultimately leading to the observed rapid cell rounding.

Activation of MAPK in fibroblasts by Treponema denticola major outer sheath protein

The major outer sheath protein (Msp) of Treponema denticola induces Ca(2+) entry and actin reorganization in cultured fibroblasts, but the pathways by which Msp mediates these responses are not yet defined. We considered that Msp may activate protein kinases as a stress response that precedes actin remodelling. Phospho-kinase screens showed that Msp induced phosphorylation of multiple kinases in pathways that respond to extracellular agonists and regulate actin assembly. 34 kinases were significantly activated, including p38 and ERK 1/2. Accordingly, the expression and phosphorylation of p38 and ERK 1/2 in whole cell lysates were measured by immunoblotting and densitometry. Both kinases responded in a dose- and time-dependent manner to Msp exposure, were inhibited by SB202190 and U1026, respectively, and were unaffected by extracellular Ca(2+). These data indicate that T. denticola Msp may exert transient stress on fibroblasts through activation of MAP kinase pathways.

Leishmania donovani promastigotes induce periphagosomal F-actin accumulation through retention of the GTPase Cdc42

Leishmania donovani promastigotes inhibit phagosome maturation and induce the accumulation of periphagosomal F-actin during the establishment of infection within macrophages. These events are mediated by the surface glycolipid lipophosphoglycan (LPG), but the underlying mechanisms remain to be elucidated. In this study, we addressed the role of the Rho-family GTPases RhoA, Rac1 and Cdc42 in the uptake of L. donovani promastigotes and in the accumulation of periphagosomal F-actin. Confocal microscopy analyses revealed that association of both Rac1 and RhoA to phagosomes containing L. donovani promastigotes was independent of the presence of LPG. In contrast, Cdc42 and proteins required for F-actin assembly (Arp2/3, WASP, Myosin, alpha-actinin) were retained on phagosomes in a LPG-dependent manner. Expression of the RhoA inhibitor C3-transferase blocked the internalization of complement-opsonized promastigotes, whereas the dominant-negative Rac1N17 blocked the uptake of unopsonized promastigotes. The dominant-negative Cdc42N17 inhibited LPG-mediated phagosomal accumulation of F-actin and retention of Arp2/3 and Myosin. Thus, our data suggest that the effect of LPG on the accumulation of periphagosomal F-actin is the consequence of an abnormal retention or activation of Cdc42 at the phagosome.

Requirement for the Rac GTPase in Chlamydia trachomatis invasion of non-phagocytic cells

Chlamydiae are gram-negative obligate intracellular pathogens to which access to an intracellular environment is paramount to their survival and replication. To this end, chlamydiae have evolved extremely efficient means of invading nonphagocytic cells. To elucidate the host cell machinery utilized by Chlamydia trachomatis in invasion, we examined the roles of the Rho GTPase family members in the internalization of chlamydial elementary bodies. Upon binding of elementary bodies on the cell surface, actin is rapidly recruited to the sites of internalization. Members of the Rho GTPase family are frequently involved in localized recruitment of actin. Clostridial Toxin B, which is a known enzymatic inhibitor of Rac, Cdc42 and Rho GTPases, significantly reduced chlamydial invasion of HeLa cells. Expression of dominant negative constructs in HeLa cells revealed that chlamydial uptake was dependent on Rac, but not on Cdc42 or RhoA. Rac but not Cdc42 was found to be activated by chlamydial attachment. The effect of dominant negative Rac expression on chlamydial uptake is manifested through the inhibition of actin recruitment to the sites of chlamydial entry. Studies utilizing Green Fluorescent Protein fusion constructs of Rac, Cdc42 and RhoA, showed Rac to be the sole member of the Rho GTPase family recruited to the site of chlamydial entry.

Recognition of Bacteria in the Cytosol of Mammalian Cells by the Ubiquitin System

Recent studies have suggested the existence of innate host surveillance systems for the detection of bacteria in the cytosol of mammalian cells. The molecular details of how bacteria are recognized in the cytosol, however, remain unclear. Here we examined the fate of Salmonella typhimurium, a gram-negative bacterial pathogen that can infect a variety of hosts, in the cytosol of mammalian cells. These bacteria typically occupy a membrane bound compartment, the Salmonella-containing vacuole (SCV), in host cells. We show that some wild-type bacteria escape invasion vacuoles and are released into the cytosol. Subsequently, polyubiquitinated proteins accumulate on the bacterial surface, a response that was witnessed in several cell types. In macrophages but not epithelial cells, the proteasome was observed to undergo a dramatic subcellular relocalization and become associated with the surface of bacteria in the cytosol. Proteasome inhibition promoted replication of S. typhimurium in the cytosol of both cell types, in part through destabilization of the SCV. Surprisingly, the cytosol-adapted pathogen Listeria monocytogenes avoided recognition by the ubiquitin system by using actin-based motility. Our findings indicate that the ubiquitin system plays a major role in the recognition of bacterial pathogens in the cytosol of mammalian cells.

Cytoskeletal rearrangements in gastric epithelial cells in response to Helicobacter pylori infection

Helicobacter pylori causes host epithelial cell cytoskeletal rearrangements mediated by the translocation and tyrosine phosphorylation of an outer-membrane protein, CagA, and by the vacuolating cytotoxin, VacA. However, the mechanisms by which H. pylori mediates cytoskeletal rearrangements in infected host cells need to be more clearly defined. The aim of this study was to determine the effects of H. pylori isolates from children on the architecture of host gastric epithelial cells. Gastric epithelial (AGS) cells were infected with type I (cagE(+), cagA(+), VacA(+)) H. pylori, a type II H. pylori strain (cagE(-), cagA(-), VacA(-)) or a cagE isogenic mutant. Double-labelled immune fluorescence was used to detect adherent H. pylori and the distribution of F-actin, alpha-actinin and Arp3. Both type I and type II H. pylori strains induced stress fibres in gastric epithelial cells that were not observed in uninfected cells. Type I H. pylori also induced cell elongation (hummingbird phenotype) after 4 h of infection, whereas the type II H. pylori strain did not. Less elongation occurred when AGS cells were exposed to a cagE isogenic mutant, compared with the parental strain. Confocal microscopy showed Arp3 accumulation in AGS cells infected with wild-type H. pylori, but not in response to infection with the cagE mutant. These findings indicate that type I H. pylori induce a stress fibre-like phenotype in infected gastric epithelia by a mechanism that is different from the induction of host-cell elongation. In addition to CagA and VacA, cagE also impacts on the morphology of infected gastric epithelial cells.

Novel PI 3-kinase-dependent mechanisms of trypanosome invasion and vacuole maturation

Mammalian cell invasion by the protozoan parasite, Trypanosoma cruzi, is facilitated by the activation of host cell phosphatidylinositol 3 (PI 3)-kinases. We demonstrate that the well-characterized Ca2+-regulated lysosome-mediated parasite entry pathway is abolished by wortmannin pretreatment. In addition, we have characterized a novel route of T. cruzi invasion unexpectedly revealed in the course of this study. For over a decade, targeted exocytosis of lysosomes at the host cell plasma membrane was considered as the primary mechanism for T. cruzi entry into non-professional phagocytic cells. We now provide evidence that a significant fraction (50% or greater) of invading T. cruzi trypomastigotes exploit an alternate actin-independent entry pathway that involves formation of a tightly associated host cell plasma membrane-derived vacuole enriched in the lipid products of class I PI 3-kinases, PtdInsP3/PtdIns(3,4)P2. Initially devoid of lysosomal markers, the resultant parasite-containing vacuoles gradually acquire lysosome associated membrane protein 1 (lamp-1) and fluid phase endocytic tracer from the lysosomal compartment. In striking contrast to latex bead phagosomes, few T. cruzi vacuoles associate with the early endosomal marker, EEA1 and the 'maturation' process becomes refractory to PI 3-kinase inhibition immediately following parasite internalization. Jointly, these data provide a new paradigm for T. cruzi invasion of non-professional phagocytic cells and reveal a novel vacuole maturation process that appears to bypass the requirement for EEA1.

Control of vesicular trafficking by Rho GTPases

Although vesicular trafficking is essential for a large variety of cellular processes, the regulation of vesicular trafficking is still poorly understood. Members of the Rho family of small GTPases have recently emerged as important control elements of many stages of vesicular trafficking, providing new insight into the regulation of these events. We will discuss the diverse roles played by Rho proteins in membrane trafficking and focus on the biological implications of these functions.

Microbial pathogenesis and cytoskeletal function

Pathogenic microbes subvert normal host-cell processes to create a specialized niche, which enhances their survival. A common and recurring target of pathogens is the host cell's cytoskeleton, which is utilized by these microbes for purposes that include attachment, entry into cells, movement within and between cells, vacuole formation and remodelling, and avoidance of phagocytosis. Our increased understanding of these processes in recent years has not only contributed to a greater comprehension of the molecular causes of infectious diseases, but has also revealed fundamental insights into normal functions of the cytoskeleton. From the use of bacterial toxins to investigate Rho family GTPases to in vitro studies of actin polymerization using Listeria and Shigella, the study of pathogenesis has provided important tools to probe cytoskeletal function.

Regulated portals of entry into the cell

The plasma membrane is the interface between cells and their harsh environment. Uptake of nutrients and all communication among cells and between cells and their environment occurs through this interface. 'Endocytosis' encompasses several diverse mechanisms by which cells internalize macromolecules and particles into transport vesicles derived from the plasma membrane. It controls entry into the cell and has a crucial role in development, the immune response, neurotransmission, intercellular communication, signal transduction, and cellular and organismal homeostasis. As the complexity of molecular interactions governing endocytosis are revealed, it has become increasingly clear that it is tightly coordinated and coupled with overall cell physiology and thus, must be viewed in a broader context than simple vesicular trafficking.

Hijacking Rho GTPases by protein toxins and apoptosis: molecular strategies of pathogenic bacteria

Certain bacterial toxins and type-III-translocated virulence factors have a peculiar property: they exert part of their actions by modulating Rho GTPases. These toxins target the actin cytoskeleton of host cells and reorganize it to their own advantage, either to facilitate macropinocytosis, which is required for invasive bacteria to enter cells, or to block pathogen sequestration by macrophages. In addition, by acting on Rho GTPases, bacteria may also interfere with the fate of host cells, favoring survival or death depending on their needs. Rho GTPases control the activation of NF-kappaB, which is involved in the expression of antiapoptotic proteins and mediates immunological responses as well. Here, we give a perspective on how NF-kappaB may participate in linking Rho-acting toxins and apoptosis.

Interaction of cblA/adhesin-positive Burkholderia cepacia with squamous epithelium

A highly transmissible strain of Burkholderia cepacia from genomovar III carries the cable pilin gene, expresses the 22 kDa adhesin (cblA +ve/Adh +ve), binds to cytokeratin 13 (CK13) and is invasive. CK13 is expressed abundantly in the airway epithelia of cystic fibrosis (CF) patients. We have now investigated whether binding of cblA +ve/Adh +ve B. cepacia to CK13 potentiates bacterial invasion and epithelial damage using bronchial epithelial cell cultures differentiated into either squamous (CK13-enriched) or mucociliary (CK13-deficient) epithelia. Three different B. cepacia isolates (cblA +ve/Adh +ve, cblA +ve/Adh -ve and cblA -ve/Adh -ve) showed minimal binding to mucociliary cultures, and did not invade or cause cell damage. In contrast, the cblA +ve/Adh +ve isolate, but not others, bound to CK13-expressing cells in squamous cultures, caused cytotoxicity and stimulated IL-8 release within 2 h. By 24 h, this isolate invaded and migrated across the squamous culture, causing moderate to severe epithelial damage. A specific antiadhesin antibody, which blocked the initial binding of the cblA +ve/Adh +ve isolate to CK13, significantly inhibited all the pathologic effects. Transmission electron microscopy of squamous cultures incubated with the cblA +ve/Adh +ve isolate, revealed bacteria on the surface surrounded by filopodia by 2 h, and within the cells in membrane-bound vesicles by 24 h. Bacteria were also observed free in the cytoplasm, surrounded by intermediate filaments containing CK13. These findings suggest that binding of B. cepacia to CK13 is an important initial event and that it promotes bacterial invasion and epithelial damage.

Membrane dynamics in phagocytosis

Engulfment of particles by phagocytes involves remodeling of the plasma membrane. We review recent work that suggests that focal exocytosis of endomembranes plays an important role in pseudopod extension during phagocytosis.

In vivo covalent cross-linking of cellular actin by the Vibrio cholerae RTX toxin

Enteric pathogens often export toxins that elicit diarrhea as a part of the etiology of disease, including toxins that affect cytoskeletal structure. Recently, we discovered that the intestinal pathogen Vibrio cholerae elicits rounding of epithelial cells that is dependent upon a gene we designated rtxA. Here we investigate the association of rtxA with the cell-rounding effect. We find that V. cholerae exports a large toxin, RTX (repeats-in-toxin) toxin, to culture supernatant fluids and that this toxin is responsible for cell rounding. Furthermore, we find that cell rounding is not due to necrosis, suggesting that RTX toxin is not a typical member of the RTX family of pore-forming toxins. Rather, RTX toxin causes depolymerization of actin stress fibers and covalent cross-linking of cellular actin into dimers, trimers and higher multimers. This RTX toxin-specific cross-linking occurs in cells previously rounded with cytochalasin D, indicating that G-actin is the toxin target. Although several models explain our observations, our simultaneous detection of actin cross-linking and depolymerization points toward a novel mechanism of action for RTX toxin, distinguishing it from all other known toxins.

Exploitation of host cells by enteropathogenic Escherichia coli

Microbial pathogens have evolved many ingenious ways to infect their hosts and cause disease, including the subversion and exploitation of target host cells. One such subversive microbe is enteropathogenic Escherichia coli (EPEC). A major cause of infantile diarrhea in developing countries, EPEC poses a significant health threat to children worldwide. Central to EPEC-mediated disease is its colonization of the intestinal epithelium. After initial adherence, EPEC causes the localized effacement of microvilli and intimately attaches to the host cell surface, forming characteristic attaching and effacing (A/E) lesions. Considered the prototype for a family of A/E lesion-causing bacteria, recent in vitro studies of EPEC have revolutionized our understanding of how these pathogens infect their hosts and cause disease. Intimate attachment requires the type III-mediated secretion of bacterial proteins, several of which are translocated directly into the infected cell, including the bacteria's own receptor (Tir). Binding to this membrane-bound, pathogen-derived protein permits EPEC to intimately attach to mammalian cells. The translocated EPEC proteins also activate signaling pathways within the underlying cell, causing the reorganization of the host actin cytoskeleton and the formation of pedestal-like structures beneath the adherent bacteria. This review explores what is known about EPEC's subversion of mammalian cell functions and how this knowledge has provided novel insights into bacterial pathogenesis and microbe-host interactions. Future studies of A/E pathogens in animal models should provide further insights into how EPEC exploits not only epithelial cells but other host cells, including those of the immune system, to cause diarrheal disease.