Enteropathogenic Escherichia coli inhibits phagocytosis

EspH interacts with the host active Bcr related (ABR) protein to suppress RhoGTPases

Enteropathogenic Escherichia coli are bacterial pathogens that colonize the gut and cause severe diarrhea in humans. Upon intimate attachment to the intestinal epithelium, these pathogens translocate via a type III secretion system virulent proteins, termed effectors, into the host cells. These effectors manipulate diverse host cell organelles and functions for the pathogen's benefit. However, the precise mechanisms underlying their activities are not fully understood despite intensive research. EspH, a critical effector protein, has been previously reported to disrupt the host cell actin cytoskeleton by suppressing RhoGTPase guanine exchange factors. However, native host proteins targeted by EspH to mediate these activities remained unknown. Here, we identified the active Bcr related (ABR), a protein previously characterized to possess dual Rho guanine nucleotide exchange factor and GTPase activating protein (GAP) domains, as a native EspH interacting partner. These interactions are mediated by the effector protein's C-terminal 38 amino acid segment. The effector primarily targets the GAP domain of ABR to suppress Rac1 and Cdc42, host cell cytotoxicity, bacterial invasion, and filopodium formation at infection sites. Knockdown of ABR expression abolished the ability of EspH to suppress Rac1, Cdc42. Our studies unravel a novel mechanism by which host RhoGTPases are hijacked by bacterial effectors.

The interaction of Escherichia coli O157 :H7 and Salmonella Typhimurium flagella with host cell membranes and cytoskeletal components

Bacterial flagella have many established roles beyond swimming motility. Despite clear evidence of flagella-dependent adherence, the specificity of the ligands and mechanisms of binding are still debated. In this study, the molecular basis of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium flagella binding to epithelial cell cultures was investigated. Flagella interactions with host cell surfaces were intimate and crossed cellular boundaries as demarcated by actin and membrane labelling. Scanning electron microscopy revealed flagella disappearing into cellular surfaces and transmission electron microscopy of S. Typhiumurium indicated host membrane deformation and disruption in proximity to flagella. Motor mutants of E. coli O157:H7 and S. Typhimurium caused reduced haemolysis compared to wild-type, indicating that membrane disruption was in part due to flagella rotation. Flagella from E. coli O157 (H7), EPEC O127 (H6) and S. Typhimurium (P1 and P2 flagella) were shown to bind to purified intracellular components of the actin cytoskeleton and directly increase in vitro actin polymerization rates. We propose that flagella interactions with host cell membranes and cytoskeletal components may help prime intimate attachment and invasion for E. coli O157:H7 and S. Typhimurium, respectively.

In Vivo Effects of A Pro-PO System Inhibitor on the Phagocytosis of Xenorhabdus Nematophila in Galleria Mellonella Larvae

Xenorhabdus nematophila is a Gram-negative bacterium symbiont of the entomopathogen nematode Steinernema carpocapsae whose immunosuppressive properties over host's immune response have been thoroughly investigated. In particular, live X. nematophila actively impairs phagocytosis in host's hemocytes through the secretion of inhibitors of eicosanoids synthesis. In this article we have investigated the cell surface structural features of X. nematophila responsible for the elusion from phagocytosis. To this end we have studied the uptake of heat-killed (hk), fluorescein isothiocyanate (FITC)-labeled X. nematophila by phagocytes from both a host insect and a mammalian species. In vitro dead X. nematophila passively resists engulfment by insect hemocytes without impairing the phagocytosis machinery whereas, unexpectedly, in vivo a significant phagocytosis of dead X. nematophila was observed. X. nematophila in vivo phagocytosis was increased by the co-injection of the specific inhibitor of pro-phenoloxidase (PO) system phenylthiourea (PTU), even if these effects were not observed in in vitro tests. Furthermore, biochemical modifications of X. nematophila cell wall implement in vivo phagocytosis, suggesting that this bacterium avoid phagocytosis because the ligand of phagocytic receptors is somehow buried or disguised in the cell wall. Finally, dead X. nematophila escapes engulfment even by human phagocytes suggesting that X. nematophila could be a useful model to investigate escape from phagocytosis by mammalian macrophages.

Enteropathogenic E. coli relies on collaboration between the formin mDia1 and the Arp2/3 complex for actin pedestal biogenesis and maintenance

Enteropathogenic and enterohemorrhagic E. coli (EPEC and EHEC) are closely related extracellular pathogens that reorganize host cell actin into “pedestals” beneath the tightly adherent bacteria. This pedestal-forming activity is both a critical step in pathogenesis, and it makes EPEC and EHEC useful models for studying the actin rearrangements that underlie membrane protrusions. To generate pedestals, EPEC relies on the tyrosine phosphorylated bacterial effector protein Tir to bind host adaptor proteins that recruit N-WASP, a nucleation-promoting factor that activates the Arp2/3 complex to drive actin polymerization. In contrast, EHEC depends on the effector EspF_U to multimerize N-WASP and promote Arp2/3 activation. Although these core pathways of pedestal assembly are well-characterized, the contributions of additional actin nucleation factors are unknown. We investigated potential cooperation between the Arp2/3 complex and other classes of nucleators using chemical inhibitors, siRNAs, and knockout cell lines. We found that inhibition of formins impairs actin pedestal assembly, motility, and cellular colonization for bacteria using the EPEC, but not the EHEC, pathway of actin polymerization. We also identified mDia1 as the formin contributing to EPEC pedestal assembly, as its expression level positively correlates with the efficiency of pedestal formation, and it localizes to the base of pedestals both during their initiation and once they have reached steady state. Collectively, our data suggest that mDia1 enhances EPEC pedestal biogenesis and maintenance by generating seed filaments to be used by the N-WASP-Arp2/3-dependent actin nucleation machinery and by sustaining Src-mediated phosphorylation of Tir.

Microbial pathogens that rearrange the host actin cytoskeleton have made valuable contributions to our understanding of cell signaling and movement. The assembly and organization of the actin cytoskeleton is driven by proteins called nucleators, which can be manipulated by bacteria including enteropathogenic Escherichia coli (EPEC), a frequent cause of pediatric diarrhea in developing countries. After ingestion, EPEC adhere tightly to cells of the intestine and hijack the underlying cytoskeleton to create protrusions called actin pedestals. While mechanisms of pedestal assembly involving a nucleator called the Arp2/3 complex have been defined for EPEC, the contribution of additional host nucleators has not been determined. We assessed the roles of several actin nucleators in EPEC pedestals and found that in addition to Arp2/3 complex-mediated nucleation, the formin mDia1 is a key contributor to actin assembly. These findings highlight the importance of nucleator collaboration in pathogenesis, and also advance our understanding of the molecular and cellular basis of EPEC infection, which is ultimately important for the discovery of new drug targets.

Clever Cooperation: Interactions Between EspF and Host Proteins

EspF is a central effector protein of enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), and Citrobacter rodentium (CR) that is secreted through the type III secretion system to host cells. The interaction between EspF and host proteins plays an important role in bacterial pathogenesis. EspF protein binds to host SNX9 and N-WASP proteins to promote the colonization of pathogenic bacteria in intestinal epithelial cells; combines with cytokeratin 18, actin, 14-3-3ζ, Arp2/3, profilin, and ZO-1 proteins to intervene in the redistribution of intermediate filaments, the rearrangement of actin, and the disruption of tight junctions; acts together with Abcf2 to boost host cell intrinsic apoptosis; and collaborates with Anxa6 protein to inhibit phagocytosis. The interaction between EspF and host proteins is key to the pathogenic mechanism of EHEC and EPEC. Here, we review how EspF protein functions through interactions with these 10 host proteins and contributes to the pathogenicity of EHEC/EPEC.

How cells engulf: a review of theoretical approaches to phagocytosis

Phagocytosis is a fascinating process whereby a cell surrounds and engulfs particles such as bacteria and dead cells. This is crucial both for single-cell organisms (as a way of acquiring nutrients) and as part of the immune system (to destroy foreign invaders). This whole process is hugely complex and involves multiple coordinated events such as membrane remodelling, receptor motion, cytoskeleton reorganisation and intracellular signalling. Because of this, phagocytosis is an excellent system for theoretical study, benefiting from biophysical approaches combined with mathematical modelling. Here, we review these theoretical approaches and discuss the recent mathematical and computational models, including models based on receptors, models focusing on the forces involved, and models employing energetic considerations. Along the way, we highlight a beautiful connection to the physics of phase transitions, consider the role of stochasticity, and examine links between phagocytosis and other types of endocytosis. We cover the recently discovered multistage nature of phagocytosis, showing that the size of the phagocytic cup grows in distinct stages, with an initial slow stage followed by a much quicker second stage starting around half engulfment. We also address the issue of target shape dependence, which is relevant to both pathogen infection and drug delivery, covering both one-dimensional and two-dimensional results. Throughout, we pay particular attention to recent experimental techniques that continue to inform the theoretical studies and provide a means to test model predictions. Finally, we discuss population models, connections to other biological processes, and how physics and modelling will continue to play a key role in future work in this area.

Extracellular motility and cell-to-cell transmission of enterohemorrhagic E. coli is driven by EspFU-mediated actin assembly

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) are closely-related pathogens that attach tightly to intestinal epithelial cells, efface microvilli, and promote cytoskeletal rearrangements into protrusions called actin pedestals. To trigger pedestal formation, EPEC employs the tyrosine phosphorylated transmembrane receptor Tir, while EHEC relies on the multivalent scaffolding protein EspFU. The ability to generate these structures correlates with bacterial colonization in several animal models, but the precise function of pedestals in infection remains unclear. To address this uncertainty, we characterized the colonization properties of EPEC and EHEC during infection of polarized epithelial cells. We found that EPEC and EHEC both formed distinct bacterial communities, or "macrocolonies," that encompassed multiple host cells. Tir and EspFU, as well as the host Arp2/3 complex, were all critical for the expansion of macrocolonies over time. Unexpectedly, EspFU accelerated the formation of larger macrocolonies compared to EPEC Tir, as EspFU-mediated actin assembly drove faster bacterial motility to cell junctions, where bacteria formed a secondary pedestal on a neighboring cell and divided, allowing one of the daughters to disengage and infect the second cell. Collectively, these data reveal that EspFU enhances epithelial colonization by increasing actin-based motility and promoting an efficient method of cell-to-cell transmission.

Survival of the Fittest: How Bacterial Pathogens Utilize Bile To Enhance Infection

Bacterial pathogens have coevolved with humans in order to efficiently infect, replicate within, and be transmitted to new hosts to ensure survival and a continual infection cycle. For enteric pathogens, the ability to adapt to numerous host factors under the harsh conditions of the gastrointestinal tract is critical for establishing infection. One such host factor readily encountered by enteric bacteria is bile, an innately antimicrobial detergent-like compound essential for digestion and nutrient absorption. Not only have enteric pathogens evolved to resist the bactericidal conditions of bile, but these bacteria also utilize bile as a signal to enhance virulence regulation for efficient infection. This review provides a comprehensive and up-to-date analysis of bile-related research with enteric pathogens. From common responses to the unique expression of specific virulence factors, each pathogen has overcome significant challenges to establish infection in the gastrointestinal tract. Utilization of bile as a signal to modulate virulence factor expression has led to important insights for our understanding of virulence mechanisms for many pathogens. Further research on enteric pathogens exposed to this in vivo signal will benefit therapeutic and vaccine development and ultimately enhance our success at combating such elite pathogens.

Inhibition of WAVE Regulatory Complex Activation by a Bacterial Virulence Effector Counteracts Pathogen Phagocytosis

To establish pathogenicity, bacteria must evade phagocytosis directed by remodeling of the actin cytoskeleton. We show that macrophages facilitate pathogen phagocytosis through actin polymerization mediated by the WAVE regulatory complex (WRC), small GTPases Arf and Rac1, and the Arf1 activator ARNO. To establish extracellular infections, enteropathogenic (EPEC) and enterohaemorrhagic (EHEC) Escherichia coli hijack the actin cytoskeleton by injecting virulence effectors into the host cell. Here, we find that the virulence effector EspG counteracts WRC-dependent phagocytosis, enabling EPEC and EHEC to remain extracellular. By reconstituting membrane-associated actin polymerization, we find that EspG disabled WRC activation through two mechanisms: EspG interaction with Arf6 blocked signaling to ARNO while EspG binding of Arf1 impeded collaboration with Rac1, thereby inhibiting WRC recruitment and activation. Investigating the mode of EspG interference revealed sites in Arf1 required for WRC activation and a mechanism facilitating pathogen evasion of innate host defenses.

Novel Host Proteins and Signaling Pathways in Enteropathogenic E. coli Pathogenesis Identified by Global Phosphoproteome Analysis

Enteropathogenic Escherichia coli (EPEC) uses a type III secretion system (T3SS) to directly translocate effector proteins into host cells where they play a pivotal role in subverting host cell signaling needed for disease. However, our knowledge of how EPEC affects host protein phosphorylation is limited to a few individual protein studies. We employed a quantitative proteomics approach to globally map alterations in the host phosphoproteome during EPEC infection. By characterizing host phosphorylation events at various time points throughout infection, we examined how EPEC dynamically impacts the host phosphoproteome over time. This experimental setup also enabled identification of T3SS-dependent and -independent changes in host phosphorylation. Specifically, T3SS-regulated events affected various cellular processes that are known EPEC targets, including cytoskeletal organization, immune signaling, and intracellular trafficking. However, the involvement of phosphorylation in these events has thus far been poorly studied. We confirmed the MAPK family as an established key host player, showed its central role in signal transduction during EPEC infection, and extended the repertoire of known signaling hubs with previously unrecognized proteins, including TPD52, CIN85, EPHA2, and HSP27. We identified altered phosphorylation of known EPEC targets, such as cofilin, where the involvement of phosphorylation has so far been undefined, thus providing novel mechanistic insights into the roles of these proteins in EPEC infection. An overlap of regulated proteins, especially those that are cytoskeleton-associated, was observed when compared with the phosphoproteome of Shigella-infected cells. We determined the biological relevance of the phosphorylation of a novel protein in EPEC pathogenesis, septin-9 (SEPT9). Both siRNA knockdown and a phosphorylation-impaired SEPT9 mutant decreased bacterial adherence and EPEC-mediated cell death. In contrast, a phosphorylation-mimicking SEPT9 mutant rescued these effects. Collectively, this study provides the first global analysis of phosphorylation-mediated processes during infection with an extracellular, diarrheagenic bacterial pathogen.

Bringing down the host: enteropathogenic and enterohaemorrhagic Escherichia coli effector-mediated subversion of host innate immune pathways

Enteric bacterial pathogens commonly use a type III secretion system (T3SS) to successfully infect intestinal epithelial cells and survive and proliferate in the host. Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC; EHEC) colonize the human intestinal mucosa, form characteristic histological lesions on the infected epithelium and require the T3SS for full virulence. T3SS effectors injected into host cells subvert cellular pathways to execute a variety of functions within infected host cells. The EPEC and EHEC effectors that subvert innate immune pathways--specifically those involved in phagocytosis, host cell survival, apoptotic cell death and inflammatory signalling--are all required to cause disease. These processes are reviewed within, with a focus on recent work that has provided insights into the functions and host cell targets of these effectors.

Enteropathogenic Escherichia coli dynamically regulates EGFR signaling in intestinal epithelial cells

The diarrheagenic pathogen enteropathogenic Escherichia coli (EPEC) dynamically modulates the survival of infected host intestinal epithelial cells. In the initial stages of infection, several prosurvival signaling events are activated in host cells. These include the phosphorylation of epidermal growth factor receptor (EGFR) and the consequent activation of the phosphatidylinositol-3 kinase/Akt pathway. While studying this pathway in infected epithelial cells, we observed EGFR depletion at later stages of infection, followed subsequently by a decrease in phospho-EGFR. EGFR loss was not dependent on receptor phosphorylation, or on canonical proteasome- and lysosome-dependent processes. Although a type III secretion mutant (ΔescN) stimulated EGFR phosphorylation, it failed to induce receptor degradation. To identify the specific EPEC effector molecule(s) that influenced EGFR stability, epithelial cells infected with isogenic mutant EPEC strains were examined. An EPEC ΔespF strain failed to induce EGFR degradation, whereas EPEC ΔespZ accentuated receptor loss in infected cells. Given the known and contrasting effects of EspF and EspZ on caspase activation, and the known role of proteases in cleaving EGFR, we explored the effect of caspase inhibitors on infection-dependent EGFR loss. The pan-caspase inhibitor Q-VD-OPh blocked EPEC-induced EGFR cleavage in a dose-dependent manner. Taken together, our data suggest that EPEC EspF stimulates caspase-dependent EGFR cleavage and loss, whereas EspZ inhibits this process. Whereas EGFR phosphorylation contributes to the survival of host cells early in infection, EspF-driven caspase activation and consequent EGFR loss likely induce a precipitous increase in host cell death later in the infectious process.

In vitro and in vivo model systems for studying enteropathogenic Escherichia coli infections

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) belong to a group of bacteria known as attaching and effacing (A/E) pathogens that cause disease by adhering to the lumenal surfaces of their host's intestinal epithelium. EPEC and EHEC are major causes of infectious diarrhea that result in significant childhood morbidity and mortality worldwide. Recent advances in in vitro and in vivo modeling of these pathogens have contributed to our knowledge of how EPEC and EHEC attach to host cells and subvert host-cell signaling pathways to promote infection and cause disease. A more detailed understanding of how these pathogenic microbes infect their hosts and how the host responds to infection could ultimately lead to new therapeutic strategies to help control these significant enteric pathogens.

Enteropathogenic Escherichia coli inhibits ileal sodium-dependent bile acid transporter ASBT

Apical sodium-dependent bile acid transporter (ASBT) is responsible for the absorption of bile acids from the intestine. A decrease in ASBT function and expression has been implicated in diarrhea associated with intestinal inflammation. Whether infection with pathogenic microorganisms such as the enteropathogenic Escherichia coli (EPEC) affect ASBT activity is not known. EPEC is a food-borne enteric pathogen that translocates bacterial effector molecules via type three secretion system (TTSS) into host cells and is a major cause of infantile diarrhea. We investigated the effects of EPEC infection on ileal ASBT function utilizing human intestinal Caco2 cells and HEK-293 cells stably transfected with ASBT-V5 fusion protein (2BT cells). ASBT activity was significantly inhibited following 60 min infection with EPEC but not with nonpathogenic E. coli. Mutations in bacterial escN, espA, espB, and espD, the genes encoding for the elements of bacterial TTSS, ablated EPEC inhibitory effect on ASBT function. Furthermore, mutation in the bacterial BFP gene encoding for bundle-forming pili abrogated the inhibition of ASBT by EPEC, indicating the essential role for bacterial aggregation and the early attachment. The inhibition by EPEC was associated with a significant decrease in the V(max) of the transporter and a reduction in the level of ASBT on the plasma membrane. The inhibition of ASBT by EPEC was blocked in the presence of protein tyrosine phosphatase inhibitors. Our studies provide novel evidence for the alterations in the activity of ASBT by EPEC infection and suggest a possible effect for EPEC in influencing intestinal bile acid homeostasis.

Role of EscU auto-cleavage in promoting type III effector translocation into host cells by enteropathogenic Escherichia coli

Background

Type III secretion systems (T3SS) of bacterial pathogens coordinate effector protein injection into eukaryotic cells. The YscU/FlhB group of proteins comprises members associated with T3SS which undergo a specific auto-cleavage event at a conserved NPTH amino acid sequence. The crystal structure of the C-terminal portion of EscU from enteropathogenic Escherichia coli (EPEC) suggests this auto-cleaving protein provides an interface for substrate interactions involved in type III secretion events.

Results

We demonstrate EscU must be auto-cleaved for bacteria to efficiently deliver type III effectors into infected cells. A non-cleaving EscU(N262A) variant supported very low levels of in vitro effector secretion. These effector proteins were not able to support EPEC infection of cultured HeLa cells. In contrast, EscU(P263A) was demonstrated to be partially auto-cleaved and moderately restored effector translocation and functionality during EPEC infection, revealing an intermediate phenotype. EscU auto-cleavage was not required for inner membrane association of the T3SS ATPase EscN or the ring forming protein EscJ. In contrast, in the absence of EscU auto-cleavage, inner membrane association of the multicargo type III secretion chaperone CesT was altered suggesting that EscU auto-cleavage supports docking of chaperone-effector complexes at the inner membrane. In support of this interpretation, evidence of novel effector protein breakdown products in secretion assays were linked to the non-cleaved status of EscU(N262A).

Conclusions

These data provide new insight into the role of EscU auto-cleavage in EPEC. The experimental data suggests that EscU auto-cleavage results in a suitable binding interface at the inner membrane that accommodates protein complexes during type III secretion events. The results also demonstrate that altered EPEC genetic backgrounds that display intermediate levels of effector secretion and translocation can be isolated and studied. These genetic backgrounds should be valuable in deciphering sequential and temporal events involved in EPEC type III secretion.

Invasive potential of gut mucosa-derived Fusobacterium nucleatum positively correlates with IBD status of the host

BACKGROUND

Fusobacterium nucleatum is a heterogeneous oral pathogen that is also a common resident of the human gut mucosa. Given that some strains of F. nucleatum are known to be invasive and proinflammatory in the oral mucosa, we compared strains isolated from patients with inflammatory bowel disease (IBD) with strains isolated from healthy controls to determine 1) whether this species was more commonly associated with IBD patients; and 2) whether gut-derived F. nucleatum strains from IBD patients showed an increased capacity for invasion.

METHODS

Biopsy material was obtained from 56 adult patients undergoing colonoscopy for colon cancer screening purposes or assessment of irritable bowel syndrome status (34 patients), or to assess for presence of gastrointestinal disease (i.e., IBD or indeterminate colitis, 22 patients). We enumerated Fusobacterium spp. strains isolated from human gut biopsy material in a blinded fashion, and then compared the virulence potential of a subset of F. nucleatum strains using an invasion assay in a Caco-2 model system.

RESULTS

Fusobacterium spp. were isolated from 63.6% of patients with gastrointestinal disease compared to 26.5% of healthy controls (P = 0.01). In total, 69% of all Fusobacterium spp. recovered from patients were identified as F. nucleatum. F. nucleatum strains originating from inflamed biopsy tissue from IBD patients were significantly more invasive in a Caco-2 cell invasion assay than strains that were isolated from healthy tissue from either IBD patients or control patients (P < 0.05 to 0.001).

CONCLUSIONS

This study indicates that colonization of the intestinal mucosa by highly invasive strains of F. nucleatum may be a useful biomarker for gastrointestinal disease.

The RACK1 signaling scaffold protein selectively interacts with Yersinia pseudotuberculosis virulence function

Many gram-negative bacteria use type III secretion systems to translocate effector proteins into host cells. These effectors interfere with cellular functions in a highly regulated manner resulting in effects that are beneficial for the bacteria. The pathogen Yersinia can resist phagocytosis by eukaryotic cells by translocating Yop effectors into the target cell cytoplasm. This is called antiphagocytosis, and constitutes an important virulence feature of this pathogen since it allows survival in immune cell rich lymphoid organs. We show here that the virulence protein YopK has a role in orchestrating effector translocation necessary for productive antiphagocytosis. We present data showing that YopK influences Yop effector translocation by modulating the ratio of the pore-forming proteins YopB and YopD in the target cell membrane. Further, we show that YopK that can interact with the translocators, is exposed inside target cells and binds to the eukaryotic signaling protein RACK1. This protein is engaged upon Y. pseudotuberculosis-mediated β1-integrin activation and localizes to phagocytic cups. Cells with downregulated RACK1 levels are protected from antiphagocytosis. This resistance is not due to altered levels of translocated antiphagocytic effectors, and cells with reduced levels of RACK1 are still sensitive to the later occurring cytotoxic effect caused by the Yop effectors. Further, a yopK mutant unable to bind RACK1 shows an avirulent phenotype during mouse infection, suggesting that RACK1 targeting by YopK is a requirement for virulence. Together, our data imply that the local event of Yersinia-mediated antiphagocytosis involves a step where YopK, by binding RACK1, ensures an immediate specific spatial delivery of antiphagocytic effectors leading to productive inhibition of phagocytosis.

Eating the enemy in Crohn's disease: an old theory revisited

Several old and new observations suggest the existence in Crohn's disease of a phagocytic disorder of macrophages related to impaired bactericidal activity of host cells or to the presence of invasive bacteria that have developed strategies to counteract macrophage killing. It was recently reported that disordered macrophage cytokine secretion underlies impaired acute inflammation and bacterial clearance in Crohn's disease. Secretion of proinflammatory cytokines by CD macrophages was impaired in response to E. coli or specific Toll-like receptor agonists. In addition, major advances in the etiology of Crohn's disease came from the existence of polymorphism in NOD2 and autophagy-related susceptibility genes (ATG16L1 and IRGM) in patients and from the identification of the presence of adherent-invasive E. coli (AIEC) colonizing the CD ileal mucosa and able to resist to macrophage killing. The role of impaired autophagy in Crohn's disease patients has been recently reinforced by the observation that the peptidoglycan receptor NOD2, in addition to sense intracellular bacteria, can induce autophagy by recruiting the critical autophagy protein ATG16L1 to the plasma membrane during bacterial internalization. Defects in autophagy might be the key element of the pathogenic pathway that lead to defective microbial killing, increased exposure to commensal and pathogenic intestinal bacteria and T cell activation. Defects in Paneth cells secreting lysozyme and antimicrobial peptides are observed in patients with ATG16L1 risk allele. Thus, the induction of autophagy or administration of preparations that mirrors the secretion of Paneth cells or both may be regarded as new therapeutic avenues for the treatment of Crohn's disease.

A bacterial effector targets host DH-PH domain RhoGEFs and antagonizes macrophage phagocytosis

Bacterial pathogens often harbour a type III secretion system (TTSS) that injects effector proteins into eukaryotic cells to manipulate host processes and cause diseases. Identification of host targets of bacterial effectors and revealing their mechanism of actions are crucial for understating bacterial virulence. We show that EspH, a type III effector conserved in enteric bacterial pathogens including enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli and Citrobacter rodentium, markedly disrupts actin cytoskeleton structure and induces cell rounding up when ectopically expressed or delivered into HeLa cells by the bacterial TTSS. EspH inactivates host Rho GTPase signalling pathway at the level of RhoGEF. EspH directly binds the DH-PH domain in multiple RhoGEFs, which prevents their binding to Rho and thereby inhibits nucleotide exchange-mediated Rho activation. Consistently, infection of mouse macrophages with EPEC harbouring EspH attenuates phagocytosis of the bacteria as well as FcgammaR-mediated phagocytosis. EspH represents the first example of targeting RhoGEFs by bacterial effectors, and our results also reveal an unprecedented mechanism used by enteric pathogens to counteract the host defence system.

Subversion of myosin function by E. coli

Enteropathogenic E. coli establish close contact with host cells by nucleating localized actin rearrangements and directly evading phagocytosis. Iizumi et al. now show in a recent issue of Cell Host and Microbe that the type III secretion effector EspB, initially thought to be involved in the translocation of other bacterial effectors, mediates antiphagocytosis and microvilli lesions by inhibiting myosin function.

The enteropathogenic E. coli effector EspB facilitates microvillus effacing and antiphagocytosis by inhibiting myosin function

Enteropathogenic Escherichia coli (EPEC) destroys intestinal microvilli and suppresses phagocytosis by injecting effectors into infected cells through a type III secretion system (TTSS). EspB, a component of the TTSS, is also injected into the cytoplasm of host cells. However, the physiological functions of EspB within the host cell cytoplasm remain unclear. We show that EspB binds to myosins, which are a superfamily of proteins that interact with actin filaments and mediate essential cellular processes, including microvillus formation and phagocytosis. EspB inhibits the interaction of myosins with actin, and an EspB mutant that lacks the myosin-binding region maintained its TTSS function but could not induce microvillus effacing or suppress phagocytosis. Moreover, the myosin-binding region of EspB is essential for Citrobacter rodentium, an EPEC-related murine pathogen, to efficiently infect mice. These results suggest that EspB inhibits myosin functions and thereby facilitates efficient infection by EPEC.

EspJ of enteropathogenic and enterohaemorrhagic Escherichia coli inhibits opsono-phagocytosis

A key strategy in microbial pathogenesis is the subversion of the first line of cellular immune defences presented by professional phagocytes. Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC respectively) remain extracellular while colonizing the gut mucosa by attaching and effacing mechanism. EPEC use the type three secretion system effector protein EspF to prevent their own uptake into macrophages. EPEC can also block in trans the internalization of IgG-opsonized particles. In this study, we show that EspJ is the type three secretion system effector protein responsible for trans-inhibition of macrophage opsono-phagocytosis by both EPEC and EHEC. While EspF plays no role in trans-inhibition of opsono-phagocytosis, espJ mutants of EPEC or EHEC are unable to block uptake of opsonized sheep red blood cells (RBC), a phenotype that is rescued upon complementation with the espJ gene. Importantly, ectopic expression of EspJ(EHEC) in phagocytes is sufficient to inhibit internalization of both IgG- and C3bi-opsonized RBC. These results suggest that EspJ targets a basic mechanism common to these two unrelated phagocytic receptors. Moreover, EspF and EspJ target independent aspects of the phagocytic function of mammalian macrophages in vitro.

Survival and replication of Escherichia coli O157:H7 inside the mice peritoneal macrophages

The replication of Escherichia coli O157:H7 on the resident peritoneal macrophages of four mice strains (BALB/c, CD1, C57BL, and Swiss) has been investigated. Macrophagial bactericidal killing activity was estimated via studying their ability to internalize (gentamicin-protected) E. coli during 2, 4, 24, and 48 h assays. Host genetic background has been found to show no significant effect on the ability of resident peritoneal macrophages to kill E. coli O157:H7.

Translocation of enteropathogenic Escherichia coli across an in vitro M cell model is regulated by its type III secretion system

Enteropathogenic Escherichia coli (EPEC) is an extracellular pathogen that utilizes a type III secretion system (TTSS) to modulate diverse host cell processes including cytoskeletal dynamics, tight junction permeability and macrophage phagocytosis. Some EPEC strains exhibit selective tropism for the specialized follicle-associated epithelium (FAE) overlying lymphoid follicles in the gut, which is a major site of uptake of inert particulates and pathogens, but do not translocate from the intestinal lumen in significant numbers. We have investigated the interaction of EPEC with FAE using an established in vitro model of the specialized FAE in which polarized enterocyte-like Caco-2 cells cocultured with the Raji B cell line undergo a phenotypic switch to a form that morphologically and functionally resembles the specialized antigen-transporting M cells found within FAE. Having confirmed that coculture with Raji B cells induces brush border reorganization and enhances particle transport across Caco-2 cells, we investigated translocation of bacteria across the M cell model. While Salmonella translocation was markedly upregulated by Raji coculture, transport of wild-type EPEC occurred at similarly low levels across both native Caco-2 and Caco-2/Raji-cocultured layers. Translocation rates were markedly higher for EPEC strains lacking either functional TTSS or the effector protein EspF. These observations resemble previously reported data on the inhibition of macrophage phagocytosis by EPEC, which has also been reported to be dependent on TTSS and EspF. Furthermore, as with macrophage phagocytosis, enhanced translocation of a TTSS mutant was blocked by wortmannin, implicating inhibition of phosphatidyl inositol 3-kinase-mediated signalling in the regulation of M cell translocation by EPEC.

Pathogenicity of Edwardsiella tarda to olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

The LD50 and cytotoxic and enzymatic activities of both cells and extracellular products (ECPs) of eight Edwardsiella tarda strains were determined and their bacterial superoxide dismutase gene (sodB) and catalase gene (katB) were sequenced. Strains were also examined for their ability to resist the immune responses of olive flounder, Paralichthys olivaceus. LD50 values of strains (FSW910410, KE1, 2, 3, 4, 5 and 6) in olive flounder ranged between 10(2.5) and 10(5.3) cfu (colony forming units) per fish. Unlike the avirulent strain SU100 (LD50>or=10(7)), all pathogenic strains were able to survive in flounder serum and head kidney leucocytes (except for KE2). The virulent strains possessed type I sodB and katB, whereas SU100 had type II sodB but not katB. However, there was no difference between avirulent and virulent strains in haemolytic and cytotoxic activities. The results of this study demonstrated that the ability of E. tarda to resist complement activity and phagocytosis is conferred by its superoxide dismutase and catalase, which thus play an essential role in the pathogenicity of this bacterium. In addition genotyping of sodB and kat B proved to be a very useful tool to distinguish virulent from avirulent strains.

Modulation of phosphoinositide metabolism by pathogenic bacteria

Phosphoinositide metabolism plays a pivotal role in the regulation of receptor-mediated signal transduction, actin remodelling and membrane dynamics. Phosphoinositides co-ordinate these processes by recruiting protein effectors to distinct cellular membranes in a time- and organelle-dependent manner. Intracellular bacterial pathogens interfere with phosphoinositide metabolism to direct their entry into eukaryotic cells, form replication-permissive vacuoles, modulate apoptosis, or trigger fluid secretion. Gram-negative pathogens such as Legionella pneumophila, Shigella flexneri, or Salmonella enterica employ secretion systems to invade host cells by 'pathogen-triggered phagocytosis' and thereby bypass a requirement for phosphatidylinositol 3-kinases [PI(3)Ks]. Contrarily, 'receptor-mediated phagocytosis' of Yersinia spp., Listeria monocytogenes and other pathogenic bacteria depends on PI(3)Ks. Secreted effector proteins have been found to directly bind to and modify host cell phosphoinositides, thus modulating phagocytosis and intracellular survival of the pathogens. These effectors include L. pneumophila proteins that specifically attach to phosphatidylinositol 4-phosphate [PI(4)P] on the Legionella-containing vacuole, and phosphoinositide phosphatases produced by S. flexneri, S. enterica or Mycobacterium tuberculosis. This review covers current knowledge about subversion of host cell phosphoinositide metabolism by intracellular bacterial pathogens with an emphasis on recently identified secreted effector proteins directly engaging phosphoinositides.

The enteropathogenic Escherichia coli EspF effector molecule inhibits PI-3 kinase-mediated uptake independently of mitochondrial targeting

Delivery of effector molecules into LMme(v) macrophages by enteropathogenic Escherichia coli, via its type three secretion system (T3SS), inhibits bacterial uptake by a phosphatidylinositol-3 (PI-3) kinase-dependent pathway. The T3SS system, encoded by the locus of enterocyte effacement (LEE) pathogenicity island, delivers LEE- and non-LEE-encoded effector proteins into host cells. Previous studies discounted essential roles for the LEE-encoded Map, EspF, Tir or Intimin proteins in this process but correlated it with loss of phosphorylation of the PI-3 kinase substrate, Akt (Celli et al., 2001, EMBO J 20: 1245-1258). Given the more recent finding that these bacterial proteins are multifunctional and can act together to subvert host cellular processes, we generated a quadruple deletion mutant (Map, Tir, EspF and Intimin deficient) to unearth any cooperativity in inhibiting uptake. The quadruple mutant was as defective as the T3SS-defective strain at preventing bacterial uptake with further studies revealing a surprising dependence on EspF but not Map, Tir or Intimin. Subversive activities previously associated with EspF are disruption of epithelial barrier function and programmed cell death, with the latter linked to EspF targeting mitochondria. Interestingly, the C-terminal domain possesses a polyproline motif associated with protein-protein interactions. We demonstrate that EspF-mediated inhibition of PI-3 kinase-dependent uptake: (i) is independent of mitochondrial targeting, (ii) requires the N-terminal domain with and (iii) the C-terminal domain is sufficient to disrupt barrier function but not inhibition of bacterial uptake. Moreover, loss of PI-3 kinase-dependent phosphorylation of Akt and gross changes in host phosphotyrosine protein profiles could not be linked to inhibition of the PI-3 kinase-dependent uptake process.

Comparison of binding and effects of Escherichia coli Shiga toxin 1 on bovine and ovine granulocytes

Granulocytes play a pivotal role in the pathogenesis of Shiga toxin (Stx)-producing Escherichia coli (STEC) related diseases in humans. Granulocytes are attracted and activated by Stxs in the enteric mucosa and are believed to thereby contribute to the intestinal inflammation. Mature ruminants, the main reservoir hosts of STEC, do not develop pathological changes that can be attributed to the Stxs. To prove whether the latter phenomenon correlates with the inability of the Stxs to affect granulocytes of ruminants, we investigated the ability of Stx1 to bind to granulocytes of cattle and sheep and analysed the effects of Stx1 on viability, phagocytosis, and oxidative burst activity. Bovine granulocytes from blood and milk did not express Stx1-binding sites even after activation of the cells and also were resistant to Stx1. In contrast to bovine granulocytes, granulocytes of sheep constitutively expressed Stx1-receptors of the Gb(3)/CD77 type ex vivo and bound the recombinant B-subunit of Stx1 (rStxB1). Stx1 holotoxin induced apoptosis in ovine granulocytes after prolonged incubation (18h) but Stx1 only slightly altered the phagocytosis and oxidative burst activities. The rStxB1 had no effect on granulocytes of either species. While arguing in favour of our initial hypothesis, that granulocytes of both, cattle and sheep are not activated by Stxs, the results of our study are the first evidences for differences in the cellular distribution of Stx-receptors in species equally regarded as STEC carriers.

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.

Encapsulated Streptococcus suis inhibits activation of signaling pathways involved in phagocytosis

Streptococcus suis capsular type 2 is an important zoonotic agent of meningitis. Previous studies reported that, in contrast to nonencapsulated mutants, encapsulated S. suis is able to resist phagocytosis. However, the mechanisms by which S. suis avoids phagocytosis are unknown. To elucidate the signaling pathway(s) involved in S. suis antiphagocytosis, we compared the ability of an encapsulated strain and its nonencapsulated mutant to induce the activation of Akt and protein kinase C (PKC), which are downstream kinases of the phosphatidylinositol 3-kinase (PI-3K) pathway, known to be involved in the phagocytosis processes. The results demonstrated high levels of Akt and PKCalpha phosphorylation after infection of J774 macrophages with the nonencapsulated mutant, whereas the encapsulated strain showed reduced activation of PI-3K/Akt/PKCalpha signaling pathway, as well as several protein tyrosine events. These results correlated with the number of intracellular bacteria. Macrophages pretreated with specific PI-3K or PKC inhibitors showed reduced levels of Akt and PKCalpha phosphorylation, resulting in 50% reduction of phagocytosis. The role of phosphatases in the antiphagocytic mechanisms was evaluated by using phosphatase inhibitors, as well as SHP-1-deficient macrophages. Only in the absence of SHP-1 did the phagocytosis of encapsulated S. suis significantly increase, leading to Akt phosphorylation levels similar to those observed with the nonencapsulated strain, indicating activation of this important SH2 domain-containing tyrosine phosphatase by encapsulated S. suis. Finally, when purified S. suis capsular polysaccharide (CPS) was added to macrophages, no phosphorylation events were observed. In addition, CPS and encapsulated S. suis were able to inhibit the uptake of the nonencapsulated mutant. These results suggest the importance of CPS in the mechanisms, whereby S. suis downmodulates phagocytosis.

Identification and characterization of NleA, a non-LEE-encoded type III translocated virulence factor of enterohaemorrhagic Escherichia coli O157:H7

Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 uses a specialized protein translocation apparatus, the type III secretion system (TTSS), to deliver bacterial effector proteins into host cells. These effectors interfere with host cytoskeletal pathways and signalling cascades to facilitate bacterial survival and replication and promote disease. The genes encoding the TTSS and all known type III secreted effectors in EHEC are localized in a single pathogenicity island on the bacterial chromosome known as the locus for enterocyte effacement (LEE). In this study, we performed a proteomic analysis of proteins secreted by the LEE-encoded TTSS of EHEC. In addition to known LEE-encoded type III secreted proteins, such as EspA, EspB and Tir, a novel protein, NleA (non-LEE-encoded effector A), was identified. NleA is encoded in a prophage-associated pathogenicity island within the EHEC genome, distinct from the LEE. The LEE-encoded TTSS directs translocation of NleA into host cells, where it localizes to the Golgi apparatus. In a panel of strains examined by Southern blot and database analyses, nleA was found to be present in all other LEE-containing pathogens examined, including enteropathogenic E. coli and Citrobacter rodentium, and was absent from non-pathogenic strains of E. coli and non-LEE-containing pathogens. NleA was determined to play a key role in virulence of C. rodentium in a mouse infection model.

Role of lipid-mediated signal transduction in bacterial internalization

Receptor-mediated phagocytosis normally represents an important first line of immune defence. Invading microbes are internalized into phagosomes and are typically killed by exposure to a battery of microbicidal agents. To some intracellular pathogens, however, receptor-mediated phagocytosis represents an opportunity to access a protected niche within the host cell. Another type of intracellular pathogen, including Salmonella enterica serovar Typhimurium and Shigella flexneri, invade host cells in a more direct manner. These pathogens deliver effectors into the host cell via a type III secretion apparatus, initiating a ruffling response that leads to their uptake into intracellular vacuoles. Recent studies have demonstrated the importance of lipid signal transduction events in the uptake of pathogenic bacteria by both receptor-mediated phagocytosis and type III secretion-mediated invasion. In this review we highlight some of these discoveries, with a focus on phospholipid-dependent signalling events.

Modulation of inducible nitric oxide synthase expression by the attaching and effacing bacterial pathogen citrobacter rodentium in infected mice

Citrobacter rodentium belongs to the attaching and effacing family of enteric bacterial pathogens that includes both enteropathogenic and enterohemorrhagic Escherichia coli. These bacteria infect their hosts by colonizing the intestinal mucosal surface and intimately attaching to underlying epithelial cells. The abilities of these pathogens to exploit the cytoskeleton and signaling pathways of host cells are well documented, but their interactions with the host's antimicrobial defenses, such as inducible nitric oxide synthase (iNOS), are poorly understood. To address this issue, we infected mice with C. rodentium and found that iNOS mRNA expression in the colon significantly increased during infection. Immunostaining identified epithelial cells as the major source for immunoreactive iNOS. Finding that nitric oxide (NO) donors were bacteriostatic for C. rodentium in vitro, we examined whether iNOS expression contributed to host defense by infecting iNOS-deficient mice. Loss of iNOS expression caused a small but significant delay in bacterial clearance without affecting tissue pathology. Finally, immunofluorescence staining was used to determine if iNOS expression was localized to infected cells by staining for the C. rodentium virulence factor, translocated intimin receptor (Tir), as well as iNOS. Interestingly, while more than 85% of uninfected epithelial cells expressed iNOS, fewer than 15% of infected (Tir-positive) cells expressed detectable iNOS. These results demonstrate that both iNOS and intestinal epithelial cells play an active role in host defense during C. rodentium infection. However, the selective expression of iNOS by uninfected but not infected cells suggests that this pathogen has developed mechanisms to locally limit its exposure to host-derived NO.

Campylobacter fetus adheres to and enters INT 407 cells

Campylobacter fetus is a Gram-negative bacterial pathogen of humans and ungulates and is normally transmitted via ingestion of contaminated food or water with infection resulting in mild to severe enteritis. However, despite clinical evidence that C. fetus infection often involves transient bacteremic states from which systemic infection may develop and the frequent isolation of C. fetus from extra-intestinal sites, this organism displays very poor invasiveness in in vitro models of infection. In this study, immunofluorescence microscopy and gentamicin protection assays were used to investigate the ability of six clinical isolates and one reference strain of C. fetus to adhere to and invade the human intestinal epithelial cell line, INT 407. During an initial 4-h infection period, all C. fetus strains were detected intracellularly using both techniques, though adherence and internalization levels were very low when determined from gentamicin protection assays. Microscopy results indicated that during a 4-h infection period, four of the five clinical strains tested were adherent to 41.3-87.3% of INT 407 cells observed and that 25.2-34.6% of INT 407 cells contained intracellular C. fetus. The C. fetus reference strain displayed the lowest levels of adherence and internalization. A modified infection assay revealed that C. fetus adherence did not necessarily culminate in internalization. Despite the large percentage of INT 407 cells with adherent bacteria, the percentage of INT 407 cells with intracellular bacteria remained unchanged when incubation was extended from 4 h to 20 h. However, microscopy of INT 407 cells 24 h postinfection (p.i.) revealed that infected host cells contained clusters of densely packed C. fetus cells. Gentamicin protection assays revealed that intracellular C. fetus cells were not only viable 24 h p.i. but also that C. fetus had increased in number approximately three- to fourfold between 4 and 24 h p.i., indicative of intracellular replication. Investigation of the role of the host cell cytoskeleton revealed that pretreatment of host cells with cytochalasin D, colchicine, vinblastine, taxol, or dimethyl sulfoxide (DMSO) did not impact upon C. fetus adherence or internalization of INT 407 cells. Microscopy indicated neither rearrangement nor colocalization of either microtubules or microfilaments in INT 407 cells in response to C. fetus adherence or internalization. Together, these data indicate that clinical isolates of C. fetus are capable of adhering, entering, and surviving within the nonphagocytic epithelial cell line, INT 407.

Efa1 influences colonization of the bovine intestine by shiga toxin-producing Escherichia coli serotypes O5 and O111

Shiga toxin-producing Escherichia coli (STEC) comprises a broad group of bacteria, some of which cause attaching and effacing (AE) lesions and enteritis in animals and humans. Non-O157 STEC serotypes contain a gene (efa1) that mediates attachment to cultured epithelial cells. An almost-identical gene in enteropathogenic E. coli (lifA) encodes lymphostatin, which inhibits the proliferation of mitogen-activated lymphocytes and the synthesis of proinflammatory cytokines. We have investigated the role of the efa1 gene in colonization of 4- and 11-day-old conventional calves by STEC serotypes O5 and O111. Our findings show that Efa1 is required for efficient colonization of the bovine intestinal tract by STEC, since efa1 deletion and insertion mutants were shed in the feces in significantly lower numbers. In addition, efa1 mutations dramatically reduced the number of bacteria associated with the intestinal epithelium. Expression and secretion of locus for enterocyte effacement-encoded type III secreted proteins that are required for adhesion and AE-lesion formation were impaired by mutation of efa1 in STEC but not by mutation of lifA in enteropathogenic E. coli. However, STEC efa1 mutants retain the ability to nucleate filamentous actin under sites of bacterial attachment to cultured eukaryotic cells. Efa1 is only the second STEC factor shown to influence carriage of the bacteria in the bovine intestine. Our data may have implications for strategies to reduce the prevalence of STEC in cattle.

Bacterial infection of a model insect: Photorhabdus luminescens and Manduca sexta

Invertebrates, including insects, are being developed as model systems for the study of bacterial virulence. However, we understand little of the interaction between bacteria and specific invertebrate tissues or the immune system. To establish an infection model for Photorhabdus, which is released directly into the insect blood system by its nematode symbiont, we document the number and location of recoverable bacteria found during infection of Manduca sexta. After injection into the insect larva, P. luminescens multiplies in both the midgut and haemolymph, only later colonizing the fat body and the remaining tissues of the cadaver. Bacteria persist by suppressing haemocyte-mediated phagocytosis and culture supernatants grown in vitro, as well as plasma from infected insects, suppress phagocytosis of P. luminescens. Using GFP-labelled bacteria, we show that colonization of the gut begins at the anterior of the midgut and proceeds posteriorly. Within the midgut, P. luminescens occupies a specific niche between the extracellular matrix and basal membrane (lamina) of the folded midgut epithelium. Here, the bacteria express the gut-active Toxin complex A (Tca) and an RTX-like metalloprotease PrtA. This close association of the bacteria with the gut, and the production of toxins and protease, triggers a massive programmed cell death of the midgut epithelium.

Bacterial avoidance of phagocytosis

Phagocytosis constitutes the primary line of host innate and adaptive defence against incoming microbial pathogens, providing an efficient means for their removal and destruction. However, several virulent bacteria that do not function as intracellular pathogens have evolved mechanisms to avoid and prevent phagocytosis that constitute an essential part of their pathogenic capacity. Some of these mechanisms include preventing recognition by phagocytic receptors or blocking uptake by professional phagocytes. Recently, the molecular mechanisms of such antiphagocytic properties have been elucidated for some pathogens. Such mechanisms illustrate the diversity of mechanisms bacterial pathogens use to avoid phagocytic uptake.

Mice lacking T and B lymphocytes develop transient colitis and crypt hyperplasia yet suffer impaired bacterial clearance during Citrobacter rodentium infection

The bacterial pathogen Citrobacter rodentium belongs to a family of gastrointestinal pathogens that includes enteropathogenic and enterohemorrhagic Escherichia coli and is the causative agent of transmissible colonic hyperplasia in mice. The molecular mechanisms used by these pathogens to colonize host epithelial surfaces and form attaching and effacing (A/E) lesions have undergone intense study. In contrast, little is known about the host's immune response to these infections and its importance in tissue pathology and bacterial clearance. To address these issues, wild-type mice and mice lacking T and B lymphocytes (RAG1 knockout [KO]) were infected with C. rodentium. By day 10 postinfection (p.i.), both wild-type and RAG1 KO mice developed colitis and crypt hyperplasia, and these responses became more exaggerated in wild-type mice over the next 2 weeks, as they cleared the infection. By day 24 p.i., bacterial clearance was complete, and the colitis had subsided; however, crypt heights remained increased. In contrast, inflammatory and crypt hyperplastic responses in the RAG1 KO mice were transient, subsiding after 2 weeks. By day 24 p.i., RAG1 KO mice showed no signs of bacterial clearance and infection was often fatal. Surprisingly, despite remaining heavily infected, tissues from RAG1 KO mice surviving the acute colitis showed few signs of disease. These results thus emphasize the important contribution of the host immune response during infection by A/E bacterial pathogens. While T and/or B lymphocytes are essential for host defense against C. rodentium, they also mediate much of the tissue pathology and disease symptoms that occur during infection.

Pseudomonas aeruginosa ExoT acts in vivo as a GTPase-activating protein for RhoA, Rac1, and Cdc42

The Pseudomonas aeruginosa protein ExoT is a bacterial GTPase-activating protein (GAP) that has in vitro activity toward Rho, Rac, and Cdc42 GTPases. Expression of ExoT both inhibits the internalization of strain PA103 by macrophages and epithelial cells and is associated with morphological changes (cell rounding and detachment) of infected cells. We find that expression of ExoT leads to the loss of GTP-bound RhoA, Rac1, and Cdc42 in transfected HeLa cells, demonstrating that ExoT has GAP activity in vivo toward all three GTPases. GAP activity is absolutely dependent on the presence of arginine at position 149 but is not affected by whether ExoT is expressed in the absence or presence of other P. aeruginosa type III secreted proteins. We also demonstrate that expression of ExoT in epithelial cells is sufficient to cause stress fiber disassembly by means of ExoT's GAP activity toward RhoA.

Enteropathogenic Escherichia coli induces modification of the focal adhesions of infected host cells

Enteropathogenic Escherichia coli (EPEC) is a human-specific pathogen that causes severe diarrhoea in young children. The disease involves intimate interaction between the pathogen and the brush border of enterocytes. During infection, EPEC uses a type III secretion system (TTSS) to inject several proteins into the infected cells, and these effector proteins modify specific processes in the host cell. We show that, upon infection, EPEC induces detachment of the infected host cells from the substratum, modification of focal adhesions (FA) in the infected cells and specific dephosphorylation of focal adhesion kinase (FAK). We also show that EPEC-induced cell detachment is dependent on FAK expression by the infected cells. Finally, we demonstrate that cell detachment, FA modification and FAK dephosphorylation are dependent on functional TTSS in the infecting EPEC. These results suggest that EPEC is using its TTSS to inject protein(s) into the infected cells, which can induce FAK dephosphorylation, as well as FAK-dependent FA modification and cell detachment. These processes are specific and probably play an important role in EPEC virulence.

Mechanism of action of EPEC type III effector molecules

Enteropathogenic E. coli (EPEC) is a prototypic member of the family of related 'attaching and effacing (A/E)' pathogens that induce diarrhoeal disease, especially to the young that can be fatal, of a wide range of mammalian species. Disease is correlated with the loss of absorptive gut epithelial microvilli and the reorganisation of host cytoskeletal proteins into pedestal-like structures beneath the adherent bacteria. These phenotypes are dependent on a pathogenicity island (LEE; Locus of Enterocyte Effacement) encoding a type III secretion system, secreted proteins, chaperone molecules, regulatory proteins and the bacterial outer membrane protein intimin. The type III secretion apparatus directs the transfer of specific proteins across the bacterial envelope, with a subset (EPEC secreted proteins - EspA, EspB and EspD) functioning to transfer effector proteins into host cells. These effector molecules subvert cellular processes that undoubtedly benefit the pathogen and contribute to disease. Three LEE-encoded EPEC effector molecules have so far been identified with one, Tir (Translocated intimin receptor), being transferred into host cells where it is modified by host kinases and becomes inserted into the plasma membrane to orchestrate cytoskeletal rearrangements linked to disease. This activity is dependent on its interaction with intimin and on tyrosine phosphorylation, with Tir-intimin interaction essential for virulence. A second effector Map, Mitochondrial-associated protein, is targeted to mitochondria where it has membrane-potential disrupting activity. The third, EspF disrupts intestinal barrier function and can induce host cell death by unknown mechanisms. Recent data relating to the mechanism by which Tir and Map function within host cells is discussed.

Enteric bacteria counteract lipopolysaccharide induction of antimicrobial peptide genes

The humoral immunity of Drosophila involves the production of antimicrobial peptides, which are induced by evolutionary conserved microbial molecules, like LPS. By using Drosophila mbn-2 cells, we found that live bacteria, including E. coli, Salmonella typhimurium, Erwinia carotovora, and Pseudomonas aeruginosa, prevented LPS from inducing antimicrobial peptide genes, while Micrococcus luteus and Streptococcus equi did not. The inhibitory effect was seen at bacterial levels from 20 per mbn-2 cell, while antimicrobial peptides were induced at lower bacterial concentrations (< or =2 bacteria per cell) also in the absence of added LPS. Gel shift experiment suggests that the inhibitory effect is upstream or at the level of the activation of the transcription factor Relish, a member of the NF-kappaB/Rel family. The bacteria have to be in physical contact with the cells, but not phagocytosed, to prevent LPS induction. Interestingly, the inhibiting mechanism is, at least for E. coli, independent of the type III secretion system, indicating that the inhibitory mechanism is unrelated to the one earlier described for YopJ from Yersinia.

Type III secretion-dependent cell cycle block caused in HeLa cells by enteropathogenic Escherichia coli O103

Rabbit enteropathogenic Escherichia coli (EPEC) O103 induces in HeLa cells an irreversible cytopathic effect characterized by the recruitment of focal adhesions, formation of stress fibers, and inhibition of cell proliferation. We have characterized the modalities of the proliferation arrest and investigated its underlying mechanisms. We found that HeLa cells that were exposed to the rabbit EPEC O103 strain E22 progressively accumulated at 4C DNA content and did not enter mitosis. A significant proportion of the cells were able to reinitiate DNA synthesis without division, leading to 8C DNA content. This cell cycle inhibition by E22 was abrogated in mutants lacking EspA, -B, and -D and was restored by transcomplementation. In contrast, intimin and Tir mutants retained the antiproliferative effect. The cell cycle arrest was not a direct consequence of the formation of stress fibers, since their disruption by toxins during exposure to E22 did not reverse the cell cycle inhibition. Likewise, the cell cycle arrest was not dependent on the early tyrosine dephosphorylation events triggered by E22 in the cells. Two key partner effectors controlling entry into mitosis were also investigated: cyclin B1 and the associated cyclin-dependent kinase 1 (Cdk1). Whereas cyclin B1 was not detectably affected in E22-exposed cells, Cdk1 was maintained in a tyrosine-phosphorylated inactive state and lost its affinity for p13(suc1)-agarose beads. This shows that Cdk1 is implicated in the G2/M arrest caused by EPEC strain E22.

Icm/dot-dependent upregulation of phagocytosis by Legionella pneumophila

Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.

Modulation of host cell signalling by enteropathogenic and Shiga toxin-producing Escherichia coli

The majority of Escherichia coli strains are harmless symbionts in the intestinal tract. However, there are several pathogenic forms, which are responsible for various diseases in humans and live stock. In this review we discuss the interactions between Shiga toxin-producing E. coli and enteropathogenic E. coli and their target host cells, describing their strategies to activate specific cellular signalling pathways which lead to subversion of critical physiological functions. We mainly concentrate on those pathogenic mechanisms that are dependent on a functional type III secretion system, but we also briefly discuss additional factors that contribute to the specific pathogenic profiles of Shiga toxin-producing E. coli and enreropathogenic E. coli.

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.