Exploitation of host cells by enteropathogenic Escherichia coli

Small, Dynamic Clusters of Tir-Intimin Seed Actin Polymerization

The understanding of actin pedestal formation by enteropathogenic Escherichia coli (EPEC) relies mainly on static ensemble information obtained from cell lysates. Here, the dynamic nature of signaling components on the subsecond timescale, which resemble phase condensates, is demonstrated. Unlike in vitro phase condensates, transfected intimin receptor (Tir) and downstream component form clusters 200 nm in diameter that are spaced ≈500 nm on average, indicating cellular regulation. On supported lipid bilayers with diffusive intimin, Tir-expressing fibroblasts formed Tir-intimin clusters even without Tir tyrosines, although Tir tyrosine phosphorylation is necessary for actin polymerization from clusters. Single-molecule tracking showed that Tir is diffusive in the clusters and exchanges with Tir in the plasma membrane. Further, Nck and N-WASP bind to the clusters and exchange with cytoplasmic molecules. Tir has a similar cluster lifetime to Nck, but longer than that of N-WASP. Actin polymerization from the clusters requires N-WASP binding, involved Arp2/3 activation, and stabilized N-WASP clusters. These dynamic properties are distinct from larger in vitro systems and do not depend significantly upon crosslinking. Thus, Tir-intimin clusters in the plasma membrane are limited in size by exchange and enhance signaling needed for actin polymerization that enables strong and stable bacterial attachment to host cells.

The Diversity of Escherichia coli Pathotypes and Vaccination Strategies against This Versatile Bacterial Pathogen

Escherichia coli (E. coli) is a gram-negative bacillus and resident of the normal intestinal microbiota. However, some E. coli strains can cause diseases in humans, other mammals and birds ranging from intestinal infections, for example, diarrhea and dysentery, to extraintestinal infections, such as urinary tract infections, respiratory tract infections, meningitis, and sepsis. In terms of morbidity and mortality, pathogenic E. coli has a great impact on public health, with an economic cost of several billion dollars annually worldwide. Antibiotics are not usually used as first-line treatment for diarrheal illness caused by E. coli and in the case of bloody diarrhea, antibiotics are avoided due to the increased risk of hemolytic uremic syndrome. On the other hand, extraintestinal infections are treated with various antibiotics depending on the site of infection and susceptibility testing. Several alarming papers concerning the rising antibiotic resistance rates in E. coli strains have been published. The silent pandemic of multidrug-resistant bacteria including pathogenic E. coli that have become more difficult to treat favored prophylactic approaches such as E. coli vaccines. This review provides an overview of the pathogenesis of different pathotypes of E. coli, the virulence factors involved and updates on the major aspects of vaccine development against different E. coli pathotypes.

Pathophysiology of Enteropathogenic Escherichia coli-induced Diarrhea

Enteropathogenic Escherichia coli (EPEC) are important diarrheal pathogens of infants and young children. Since the availability of molecular diagnosis methods, we now have new insights into the incidence and prevalence of these infections. Recent epidemiological studies indicate that atypical EPEC (aEPEC) are seen more frequently than typical EPEC (tEPEC) worldwide, including in both endemic diarrhea and diarrhea outbreaks. Therefore, it is important to further characterize the pathogenicity of these emerging strains. The virulence mechanisms and pathophysiology of the attaching and effacing lesion (A/E) and the type-three-secretion-system (T3SS) are complex but well-studied. A/E strains use their pool of locus of enterocyte effacement (LEE)-encoded and non-LEE-encoded effector proteins to subvert and modulate cellular and barrier properties of the host. However, the exact mechanisms of diarrhea in EPEC infection are not completely understood. From the clinical perspective, there is a need for fast, easy, and inexpensive diagnostic methods to define optimal treatment and prevention for children in endemic areas. In this article, we present a review of the classification of EPEC, epidemiology, pathogenesis of the disease caused by these bacteria, determinants of virulence, alterations in signaling, determinants of colonization vs. those of disease, and the limited information we have on the pathophysiology of EPEC-induced diarrhea. This article combines peer-reviewed evidence from our own studies and the results of an extensive literature search in the databases PubMed, EMBASE, and Scopus.

Nck adaptors at a glance

The non-catalytic region of tyrosine kinase (Nck) family of adaptors, consisting of Nck1 and Nck2, contributes to selectivity and specificity in the flow of cellular information by recruiting components of signaling networks. Known to play key roles in cytoskeletal remodeling, Nck adaptors modulate host cell-pathogen interactions, immune cell receptor activation, cell adhesion and motility, and intercellular junctions in kidney podocytes. Genetic inactivation of both members of the Nck family results in embryonic lethality; however, viability of mice lacking either one of these adaptors suggests partial functional redundancy. In this Cell Science at a Glance and the accompanying poster, we highlight the molecular organization and functions of the Nck family, focusing on key interactions and pathways, regulation of cellular processes, development, homeostasis and pathogenesis, as well as emerging and non-redundant functions of Nck1 compared to those of Nck2. This article thus aims to provide a timely perspective on the biology of Nck adaptors and their potential as therapeutic targets.

Escherichia coli O15 : H7 from Food of Animal Origin in Arsi: Occurrence at Catering Establishments and Antimicrobial Susceptibility Profile

Escherichia coli O157 : H7 (E. coli O157 : H7) has been found to be the major cause of food-borne diseases and a serious public health problem in the world, with an increasing concern for the emergence and spread of antimicrobial-resistant strains. Hitherto, little is known about the carriage of E. coli O157 : H7 and its antimicrobial susceptibility profile in the food of animal origin in Ethiopia. This study aimed to determine the occurrence and multidrug resistance profile of E. coli O157 : H7 from food of animal origin at different catering establishments in the selected study settings of Arsi Zone. One hundred ninety-two animal origin food items, namely, raw/minced meat (locally known as “Kitfo,” “Kurt,” and “Dulet”), raw milk, egg sandwich, and cream cake samples were collected and processed for microbiological detection of E. coli O157 : H7. Out of 192 samples, 2.1% (4/192) were positive for E. coli O157 : H7. Two E. coli O157 : H7 isolates were obtained from “Dulet” (6.3%) followed by “Kurt” (3.1%, 1/32) and raw milk (3.1%, 1/32), whereas no isolate was obtained from “Kitfo,” egg sandwich, and cream cake samples. Of the 4 E. coli O157 : H7 isolates subjected to 10 panels of antimicrobial discs, 3 (75%) were highly resistant to kanamycin, streptomycin, and nitrofurantoin. Besides, all the isolates displayed multidrug resistance phenotypes, 3 to 5 antimicrobial resistance, amid kanamycin, streptomycin, nitrofurantoin, tetracycline, and chloramphenicol. The occurrence of multidrug-resistant E. coli O157 : H7 isolates from foods of animal origin sampled from different catering establishments reveals that the general sanitary condition of the catering establishments, utensils used, and personnel hygienic practices did not comply with the recommended standards. Thus, this finding calls for urgent attention toward appropriate controls and good hygienic practices in different catering establishments dealing with consuming raw/undercooked foods of animal origin.

Enteropathogenic Escherichia coli—A Summary of the Literature

Diarrheal disease is still a major public health concern, as it is still considered an important cause of death in children under five years of age. A few decades ago, the detection of enteropathogenic E. coli was made by detecting the O, H, and K antigens, mostly by agglutination. The recent protocols recommend the molecular methods for diagnosing EPEC, as they can distinguish between typical and atypical EPEC by identifying the presence/absence of specific virulence factors. EPEC are defined as diarrheagenic strains of E. coli that can produce attaching and effacing lesions on the intestinal epithelium while being incapable of producing Shiga toxins and heat-labile or heat-stable enterotoxins. The ability of these strains to produce attaching and effacing lesions enable them to cause localized lesions by attaching tightly to the surface of the intestinal epithelial cells, disrupting the surfaces of the cells, thus leading to the effacement of the microvilli. EPEC are classified on typical and atypical isolates, based on the presence or absence of E. coli adherence factor plasmids. All the EPEC strains are eae positive; typical EPEC strains are eae+, bfpA+, while atypical strains are eae+, bfpA−. No vaccines are currently available to prevent EPEC infections.

β-arrestin 2 quenches TLR signaling to facilitate the immune evasion of EPEC

The protein translocated intimin receptor (Tir) from enteropathogenic Escherichia coli shares sequence similarity with the host cellular immunoreceptor tyrosine-based inhibition motifs (ITIMs). The ITIMs of Tir are required for Tir-mediated immune inhibition and evasion of host immune responses. However, the underlying molecular mechanism by which Tir regulates immune inhibition remains unclear. Here we demonstrated that β-arrestin 2, which is involved in the G-protein-coupled receptor (GPCR) signal pathway, interacted with Tir in an ITIM-dependent manner. For the molecular mechanism, we found that β-arrestin 2 enhanced the recruitment of SHP-1 to Tir. The recruited SHP-1 inhibited K63-linked ubiquitination of TRAF6 by dephosphorylating TRAF6 at Tyr288, and inhibited K63-linked ubiquitination and phosphorylation of TAK1 by dephosphorylating TAK1 at Tyr206, which cut off the downstream signal transduction and subsequent cytokine production. Moreover, the inhibitory effect of Tir on immune responses was diminished in β-arrestin 2-deficient mice and macrophages. These findings suggest that β-arrestin 2 is a key regulator in Tir-mediated immune evasion, which could serve as a new therapeutic target for bacterial infectious diseases.

Occurrence, virulence genes, and antimicrobial profiles of Escherichia coli O157 isolated from ruminants slaughtered in Al Ain, United Arab Emirates

BACKGROUND

Shiga toxin-producing Escherichia coli (STEC) is a major source of food-borne illness around the world. E. coli O157 has been widely reported as the most common STEC serogroup and has emerged as an important enteric pathogen. Cattle, in particular have been identified as a major E. coli O157:H7 reservoir of human infections; however, the prevalence of this organism in camels, sheep, and goats is less understood. The aim of this study was to evaluate the occurrence and concentration of E. coli serotype O157 in the feces of healthy camels (n = 140), cattle (n = 137), sheep (n = 141) and goats (n = 150) slaughtered in United Arab Emirates (UAE) for meat consumption between September 2017 and August 2018. We used immunomagnetic separation coupled with a culture-plating method to detect E. coli O157. Non-sorbitol fermenting colonies were assessed via latex-agglutination testing, and positive cultures were analyzed by performing polymerase chain reactions to detect genes encoding attaching and effacing protein (eaeA), hemolysin A (hlyA, also known as ehxA) and Shiga toxin (stx1 and stx2), and E. coli O157:H7 specific genes (rfb O157, uidA, and fliC). All E. coli O157 isolates were analyzed for their susceptibility to 20 selected antimicrobials.

RESULTS

E. coli O157 was observed in camels, goats, and cattle fecal samples at abundances of 4.3, 2, and 1.46%, respectively, but it was undetectable in sheep feces. The most prevalent E. coli O157 gene in all STEC isolates was stx₂;_, whereas, stx₁ was not detected in any of the samples. The fecal samples from camels, goats, and cattle harbored E. coli O157 isolates that were 100% susceptible to cefotaxime, chloramphenicol, ciprofloxacin, norfloxacin, and polymyxin B.

CONCLUSION

To our knowledge, this is the first report on the occurrence of E. coli O157 in slaughter animals in the UAE. Our results clearly demonstrate the presence of E. coli O157 in slaughtered animals, which could possibly contaminate meat products intended for human consumption.

Formyl peptide receptor 2 orchestrates mucosal protection against Citrobacter rodentium infection

Citrobacter rodentium is an attaching and effacing intestinal murine pathogen which shares similar virulence strategies with the human pathogens enteropathogenic- and enterohemorrhagic Escherichia coli to infect their host. C. rodentium is spontaneously cleared by healthy wild-type (WT) mice whereas mice lacking Muc2 or specific immune regulatory genes demonstrate an impaired ability to combat the pathogen. Here we demonstrate that apical formyl peptide receptor 2 (Fpr2) expression increases in colonic epithelial cells during C. rodentium infection. Using a conventional inoculum dose of C. rodentium, both WT and Fpr2^−/− mice were infected and displayed similar signs of disease, although Fpr2^−/− mice recovered more slowly than WT mice. However, Fpr2^−/− mice exhibited increased susceptibility to C. rodentium colonization in response to low dose infection: 100% of the Fpr2^−/− and 30% of the WT mice became colonized and Fpr2^−/− mice developed more severe colitis and more C. rodentium were in contact with the colonic epithelial cells. In line with the larger amount of C. rodentium detected in the spleen in Fpr2^−/− mice, more C. rodentium and enteropathogenic Escherichia coli translocated across an in vitro mucosal surface to the basolateral compartment following FPR2 inhibitor treatment. Fpr2^−/− mice also lacked the striated inner mucus layer that was present in WT mice. Fpr2^−/− mice had decreased mucus production and different mucin O-glycosylation in the colon compared to WT mice, which may contribute to their defect inner mucus layer. Thus, Fpr2 contributes to protection against infection and influence mucus production, secretion and organization.

Complete Genome Sequence of Escherichia coli O157:H7 Strain Al Ain, Isolated from Camel Feces in the United Arab Emirates

Escherichia coli O157:H7 is a common food pathogen which has a serious effect on human health. We report here the complete genome sequence of Escherichia coli O157:H7 strain Al Ain, isolated from camel feces in the United Arab Emirates.

Characterization of Non-O157 Escherichia coli from Cattle Faecal Samples in the North-West Province of South Africa

Escherichia coli are commensal bacteria in the gastrointestinal tract of mammals, but some strains have acquired Shiga-toxins and can cause enterohemorrhagic diarrhoea and kidney failure in humans. Shiga-toxigenic E. coli (STEC) strains such as E. coli O157:H7 and some non-O157 strains also contain other virulence traits, some of which contribute to their ability to form biofilms. This study characterized non-O157 E. coli from South African cattle faecal samples for their virulence potential, antimicrobial resistance (AMR), biofilm-forming ability, and genetic relatedness using culture-based methods, pulsed-field gel electrophoresis (PFGE), and whole genome sequencing (WGS). Of 80 isolates screened, 77.5% (62/80) possessed Shiga-toxins genes. Of 18 antimicrobials tested, phenotypic resistance was detected against seven antimicrobials. Resistance ranged from 1.3% (1/80) for ampicillin-sulbactam to 20% (16/80) for tetracycline. Antimicrobial resistance genes were infrequently detected except for tetA, which was found in 31.3% (25/80) and tetB detected in 11.3% (9/80) of isolates. Eight biofilm-forming associated genes were detected in STEC isolates (n = 62) and two non-STEC strains. Prevalence of biofilm genes ranged from 31.3% (20/64) for ehaA^β passenger to 100% for curli structural subunit (csgA) and curli regulators (csgA and crl). Of the 64 STEC and multi-drug resistant isolates, 70.3% (45/64) and 37.5% (24/64) formed strong biofilms on polystyrene at 22 and 37 °C, respectively. Of 59 isolates screened by PFGE, 37 showed unique patterns and the remaining isolates were grouped into five clusters with a ≥90% relatedness. In silico serotyping following WGS on a subset of 24 non-O157 STEC isolates predicted 20 serotypes comprising three novel serotypes, indicating their diversity as potential pathogens. These findings show that North West South African cattle harbour genetically diverse, virulent, antimicrobial-resistant and biofilm-forming non-O157 E. coli. Biofilm-forming ability may increase the likelihood of persistence of these pathogens in the environment and facilitate their dissemination, increasing the risk of cross contamination or establishment of infections in hosts.

Taming the Triskelion: Bacterial Manipulation of Clathrin

The entry of pathogens into nonphagocytic host cells has received much attention in the past three decades, revealing a vast array of strategies employed by bacteria and viruses. A method of internalization that has been extensively studied in the context of viral infections is the use of the clathrin-mediated pathway. More recently, a role for clathrin in the entry of some intracellular bacterial pathogens was discovered. Classically, clathrin-mediated endocytosis was thought to accommodate internalization only of particles smaller than 150 nm; however, this was challenged upon the discovery that Listeria monocytogenes requires clathrin to enter eukaryotic cells. Now, with discoveries that clathrin is required during other stages of some bacterial infections, another paradigm shift is occurring. There is a more diverse impact of clathrin during infection than previously thought. Much of the recent data describing clathrin utilization in processes such as bacterial attachment, cell-to-cell spread and intracellular growth may be due to newly discovered divergent roles of clathrin in the cell. Not only does clathrin act to facilitate endocytosis from the plasma membrane, but it also participates in budding from endosomes and the Golgi apparatus and in mitosis. Here, the manipulation of clathrin processes by bacterial pathogens, including its traditional role during invasion and alternative ways in which clathrin supports bacterial infection, is discussed. Researching clathrin in the context of bacterial infections will reveal new insights that inform our understanding of host-pathogen interactions and allow researchers to fully appreciate the diverse roles of clathrin in the eukaryotic cell.

Tricellular Tight Junction Protein Tricellulin Is Targeted by the Enteropathogenic Escherichia coli Effector EspG1, Leading to Epithelial Barrier Disruption

Enteropathogenic Escherichia coli (EPEC)-induced diarrhea is often associated with disruption of intestinal epithelial tight junctions. Although studies have shown alterations in the expression and localization of bicellular tight junction proteins during EPEC infections, little is known about whether tricellular tight junction proteins (tTJs) are affected. Using Caco-2 cell monolayers, we investigated if EPEC is capable of targeting the tTJ protein tricellulin. Our results demonstrated that at 4 h postinfection, EPEC induced a significant reduction in tricellulin levels, accompanied by a significant loss of transepithelial resistance (TEER) and a corresponding increase in paracellular permeability. Conversely, cells overexpressing tricellulin were highly resistant to EPEC-induced barrier disruption. Confocal microscopy revealed the distribution of tricellulin into the plasma membrane of infected epithelial cells and confirmed the localization of EPEC aggregates in close proximity to tTJs. Moreover, infections with EPEC strains lacking genes encoding specific type III secreted effector proteins demonstrated a crucial role for the effector EspG1 in modulating tricellulin expression. Complementation studies suggest that the EspG-induced depletion of tricellulin is microtubule dependent. Overall, our results show that EPEC-induced epithelial barrier dysfunction is mediated in part by EspG1-induced microtubule-dependent depletion of tricellulin.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

Molecular characterization of Escherichia coli O157:H7 recovered from meat and meat products relevant to human health in Riyadh, Saudi Arabia

Raw meat can harbor pathogenic bacteria, potentially harmful to humans such as Escherichia coli O157:H7 causing diarrhea and hemolytic-uremic syndrome (HS). Therefore, the current study was carried out to evaluate the prevalence and the molecular detection characterization of E. coli serotype O157:H7 recovered from raw meat and meat products collected from Saudi Arabia. During the period of 25th January 2013 to 25th March 2014, 370 meat samples were collected from abattoirs and markets located in Riyadh, Saudi Arabia "200 raw meat samples and 170 meat products". Bacteriological analysis of the meat samples and serotyping of the isolated E. coli revealed the isolation of 11 (2.97%) strains of E. coli O157:H7. Isolation of E. coli O157:H7 in raw beef, chicken and mutton were 2%, 2.5%, and 2.5%, respectively, however, there was no occurrence in raw turkey. The incidences of E. coli O157:H7 in ground beef, beef burgers, beef sausage, ground chicken and chicken burgers were 5%, 10%, 0.0%, 5% and 0.0%, respectively. The multiplex PCR assay revealed that 3 (27.27%) out of 11 E. coli O157:H7 isolates from raw beef, chicken and mutton had stx1, stx2, and eae while 5 (45.45%) E. coli O157:H7 isolates from ground beef, ground chicken, and raw beef had both stx1 and stx2. However, from beef burgers, only one E. coli O157:H7 isolate had stx1 while two were positive for hlyA gene. These results call for urgent attention toward appropriate controls and good hygienic practices in dealing with raw meat.

Generation of a MDCK cell line with constitutive expression of the Enteropathogenic E. coli effector protein Map as an in vitro model of pathogenesis

Enteropathogenic E. coli (EPEC) cause diarrhea and are the major cause of mortality in developing countries. EPEC use a type III secretion system to deliver effector proteins into the host epithelial cells. To understand the functions of these effectors, majority of studies on EPEC pathogenesis have relied on infections of animals or cell lines with wild type strains of EPEC or mutant strains deficient in one or more effectors. While these studies have provided valuable data, it can be difficult to assess functions of an individual effector in the presence of other EPEC effectors. Recent studies have reported the use of transient transfections with plasmids encoding various EPEC effectors into different cell lines. However, variable transfection efficiencies and expression levels of the effector proteins coupled with their expression for relatively short periods of time pose a problem if the long term effects of these effectors need to be examined. We have generated a MDCK cell line with constitutive expression of the EPEC effector Map (Mitochondrial associated protein) for efficient stable expression of EGFP-tagged Map. We observed that the constitutive expression of Map increased the permeability of charged and non-charged molecules. We also generated polyclonal antibodies against Map and checked for their specificity in MDCK-Map expressing cells. Map has been reported to contribute to the onset of diarrhea but the underlying mechanism is yet to be identified. The MDCK-Map cell line and the anti-Map antibodies generated by us can be used for in vitro studies to examine the role of Map in EPEC pathogenesis.

Multicopy Single-Stranded DNA Directs Intestinal Colonization of Enteric Pathogens

Multicopy single-stranded DNAs (msDNAs) are hybrid RNA-DNA molecules encoded on retroelements called retrons and produced by the action of retron reverse transcriptases. Retrons are widespread in bacteria but the natural function of msDNA has remained elusive despite 30 years of study. The major roadblock to elucidation of the function of these unique molecules has been the lack of any identifiable phenotypes for mutants unable to make msDNA. We report that msDNA of the zoonotic pathogen Salmonella Typhimurium is necessary for colonization of the intestine. Similarly, we observed a defect in intestinal persistence in an enteropathogenic E. coli mutant lacking its retron reverse transcriptase. Under anaerobic conditions in the absence of msDNA, proteins of central anaerobic metabolism needed for Salmonella colonization of the intestine are dysregulated. We show that the msDNA-deficient mutant can utilize nitrate, but not other alternate electron acceptors in anaerobic conditions. Consistent with the availability of nitrate in the inflamed gut, a neutrophilic inflammatory response partially rescued the ability of a mutant lacking msDNA to colonize the intestine. These findings together indicate that the mechanistic basis of msDNA function during Salmonella colonization of the intestine is proper production of proteins needed for anaerobic metabolism. We further conclude that a natural function of msDNA is to regulate protein abundance, the first attributable function for any msDNA. Our data provide novel insight into the function of this mysterious molecule that likely represents a new class of regulatory molecules.

Plasma membrane-associated superstructure: Have we overlooked a new type of organelle in eukaryotic cells?

A variety of intriguing plasma membrane-associated regions, including focal adhesions, adherens junctions, tight junctions, immunological synapses, neuromuscular junctions and the primary cilia, among many others, have been described in eukaryotic cells. Emphasizing their importance, alteration in their molecular structures induces or correlates with different pathologies. These regions display surface proteins connected to intracellular molecules, including cytoskeletal component, which maintain their cytoarchitecture, and signalling proteins, which regulate their organization and functions. Based on the molecular similarities and other common features observed, we suggest that, despite differences in external appearances, all these regions are just the same superstructure that appears in different locations and cells. We hypothesize that this superstructure represents an overlooked new type of organelle that we call plasma membrane-associated superstructure (PMAS). Therefore, we suggest that eukaryotic cells include classical organelles (e.g. mitochondria, Golgi and others) and also PMAS. We speculate that this new type of organelle might be an innovation associated to the emergence of eukaryotes. Finally we discuss the implications of the hypothesis proposed.

Enteropathogenic E. coli: breaking the intestinal tight junction barrier

Enteropathogenic E. coli (EPEC) causes acute intestinal infections in infants in the developing world. Infection typically spreads through contaminated food and water and leads to severe, watery diarrhea. EPEC attaches to the intestinal epithelial cells and directly injects virulence factors which modulate multiple signaling pathways leading to host cell dysfunction. However, the molecular mechanisms that regulate the onset of diarrhea are poorly defined. A major target of EPEC is the host cell tight junction complex which acts as a barrier and regulates the passage of water and solutes through the paracellular space. In this review, we focus on the EPEC effectors that target the epithelial barrier, alter its functions and contribute to leakage through the tight junctions.

Enteropathogenic E. coli: breaking the intestinal tight junction barrier

Enteropathogenic E. coli (EPEC) causes acute intestinal infections in infants in the developing world. Infection typically spreads through contaminated food and water and leads to severe, watery diarrhea. EPEC attaches to the intestinal epithelial cells and directly injects virulence factors which modulate multiple signaling pathways leading to host cell dysfunction. However, the molecular mechanisms that regulate the onset of diarrhea are poorly defined. A major target of EPEC is the host cell tight junction complex which acts as a barrier and regulates the passage of water and solutes through the paracellular space. In this review, we focus on the EPEC effectors that target the epithelial barrier, alter its functions and contribute to leakage through the tight junctions.

Molecular and serotyping characterization of shiga toxogenic Escherichia coli associated with food collected from Saudi Arabia

Shiga toxin-producing Escherichia coli (STEC) strains are considered as one of the major food-borne disease agents in humans worldwide. STEC strains, also called verotoxin-producing E. coli strains. The objectives of the present study were serotyping and molecular characterization of shiga toxigenic E. coli associated with raw meat and milk samples collected from Riyadh, Saudi Arabia. A total of 540 milk samples were collected from 5 dairy farms and 150 raw meat samples were collected from different abattoirs located in Riyadh, Saudi Arabia. E. coli were recovered from 86 milk samples (15.93%), serotyping of E. coli isolates revealed, 26 (4.81%) strains O157: H7, 23 (4.26%) strains O111, 20 (3.70%) strains O113: H21, 10 (1.85%) strains O22: H8 and 7 (1.3%) strains O172: H21. Meanwhile, 17 (11.33%) strains of E. coli were recovered from raw meat samples, serotyping of E. coli isolates revealed, 6 (4%) strains O157: H7, 5 (3.33%) strains O111 and 4 (2.67%) strains O174: H2 and only two (1.33%) strains were identified as O22: H8. Shiga toxin2 was detected in 58 (67.44%) serotypes of E. coli recovered from milk samples and 16 (94.12%) serotypes of E. coli recovered from meat samples, while intimin gene was detected in 38 (44.186%) serotypes of E. coli recovered from milk samples and in 10 (58.82%) serotypes of E. coli recovered from meat samples. The results of this study revealed the efficiency of combination between serotyping and molecular typing of E. coli isolates recovered from food of animal origin for rapid detection and characterization of STEC.

Occurrence of virulence genes associated with diarrheagenic Escherichia coli isolated from raw cow's milk from two commercial dairy farms in the Eastern Cape Province, South Africa

Escherichia coli remains a public health concern worldwide as an organism that causes diarrhea and its reservoir in raw milk may play an important role in the survival and transport of pathogenic strains. Diarrheagenic E. coli strains are diverse food-borne pathogens and causes diarrhea with varying virulence in humans. We investigated the prevalence of pathogenic E. coli in raw milk from two commercial dairy farms. Four hundred raw milk samples, 200 from each dairy farm, were screened for the presence of fliCH7, eagR, ial, eagg, lt, and papC genes. In dairy farm A, 100 E. coli were identified based on culture, oxidase and Gram staining, while 88 isolates from dairy farm B were identified in the same manner. Gene detection showed fliCH7 27 (54%) to be the highest gene detected from farm A and lt 2 (4%) to be the lowest. The highest gene detected in dairy farm B was fliCH7 16 (43.2%) and papC 1 (2.7%) was the least. The amplification of pathogenic genes associated with diarrheagenic E. coli from cows' raw milk demonstrates that potentially virulent E. coli strains are widely distributed in raw milk and may be a cause of concern for human health.

Invasion of differentiated intestinal Caco-2 cells is a sporadic property among atypical enteropathogenic Escherichia coli strains carrying common intimin subtypes

Atypical enteropathogenic Escherichia coli (aEPEC) strains produce attaching-effacing (AE) lesions on enterocytes due to the interaction of the adhesin intimin with its translocated receptor. aEPEC strain 1551-2 was previously shown to invade HeLa and T84 cells by means of the uncommon intimin subtype omicron. Other aEPEC strains carrying uncommon intimin subtypes have also been shown to invade differentiated T84 intestinal cells. In this study, seven aEPEC strains carrying the most common EPEC intimin subtypes (alpha, beta, and gamma) were evaluated regarding the ability to invade differentiated intestinal Caco-2 cells. Although all strains adhered to and promoted AE lesions, the numbers of cell-associated bacteria varied significantly between the different strains regardless of the intimin subtype (P < 0.05). Gentamicin protection assay and transmission electron microscopy analyses showed that in comparison with the invasive strain 1551-2, only one strain (aEPEC EC423/03, intimin beta) was invasive (P = 0.05). Although both strains persisted intracellularly until 48 h, the number of viable bacteria of EC423/03 decreased, whereas that of 1551-2 increased significantly up to 24 h and then decreased. In conclusion, invasiveness is a sporadic property among aEPEC strains carrying some common intimin subtypes.

Identification of EnterohemorrhagicEscherichia coliO157-Specific DNA Sequence Obtained from Amplified Fragment Length Polymorphism Analysis

An approximately 1.1 kbp fragment that was commonly observed only in the enterohemorrhagic Escherichia coli (EHEC) O157 strains in an analysis of amplified fragment length polymorphism was found to be a partial gene sequence encoding the locus of toxB and a useful molecular marker for the identification of EHEC O157.

Real-time sensing of enteropathogenic E. coli-induced effects on epithelial host cell height, cell-substrate interactions, and endocytic processes by infrared surface plasmon spectroscopy

Enteropathogenic Escherichia coli (EPEC) is an important, generally non-invasive, bacterial pathogen that causes diarrhea in humans. The microbe infects mainly the enterocytes of the small intestine. Here we have applied our newly developed infrared surface plasmon resonance (IR-SPR) spectroscopy approach to study how EPEC infection affects epithelial host cells. The IR-SPR experiments showed that EPEC infection results in a robust reduction in the refractive index of the infected cells. Assisted by confocal and total internal reflection microscopy, we discovered that the microbe dilates the intercellular gaps and induces the appearance of fluid-phase-filled pinocytic vesicles in the lower basolateral regions of the host epithelial cells. Partial cell detachment from the underlying substratum was also observed. Finally, the waveguide mode observed by our IR-SPR analyses showed that EPEC infection decreases the host cell's height to some extent. Together, these observations reveal novel impacts of the pathogen on the host cell architecture and endocytic functions. We suggest that these changes may induce the infiltration of a watery environment into the host cell, and potentially lead to failure of the epithelium barrier functions. Our findings also indicate the great potential of the label-free IR-SPR approach to study the dynamics of host-pathogen interactions with high spatiotemporal sensitivity.

Escherichia coli O157:H7-Clinical aspects and novel treatment approaches

Escherichia coli O157:H7 is a notorious pathogen often contracted by intake of contaminated water or food. Infection with this agent is associated with a broad spectrum of illness ranging from mild diarrhea and hemorrhagic colitis to the potentially fatal hemolytic uremic syndrome (HUS). Treating E. coli O157:H7 infection with antimicrobial agents is associated with an increased risk of severe sequelae such as HUS. The difficulty in treating this bacterium using conventional modalities of antimicrobial agent administration has sparked an interest in investigating new therapeutic approaches to this bacterium. These approaches have included the use of probiotic agents and natural products with variable success rates. In addition, novel modalities and regimen of antimicrobial agent administration have been assessed in an attempt at decreasing their association with aggravating infection outcomes.

Characterization of a novel EAST-negative enteropathogenic E. coli strain implicated in a food-borne outbreak of diarrhoea in adults

Enteropathogenic Escherichia coli (EPEC) is usually associated with outbreaks and sporadic cases of severe infantile diarrhoea in the developing world, and less commonly with sporadic cases in developed countries. Very little evidence indicates that EPEC is a food-borne pathogen for adults. In a previous study, two groups of adult travellers became ill, and eae(+) E. coli of serogroup O111 was isolated from affected individuals and epidemiologically linked to food consumption. Here the strain responsible was further investigated and characterized as an unusual atypical EPEC. PCR analysis of the designated type isolate showed the presence of the rorf1 and espB genes of the LEE pathogenicity island, which was inserted at the chromosomal selC locus. The isolate was negative for the enteroaggregative E. coli EAST-1 toxin present in other strains of EPEC associated with food-borne outbreaks. The strain adhered sparsely to HEp-2 cell monolayers in a diffuse manner, but fluorescent actin staining demonstrated that it was capable of inducing polymerization of actin at the sites of bacterial attachment. Strain P2583 is the first EAST-negative EPEC to be confirmed as a cause of outbreaks of infection in adults following the consumption of contaminated food or water.

Molecular analysis of typical and atypical enteropathogenic Escherichia coli (EPEC) isolated from children with diarrhoea

Diarrhoea continues to be one of the most common causes of morbidity and mortality among infants and children in developing countries. To investigate the incidence, antimicrobial resistance and genetic relationships of enteropathogenic Escherichia coli (EPEC) in children with diarrhoea, a total of 612 stool specimens were collected in Tehran, Iran, and cultured to isolate strains of EPEC. The disc diffusion method was used to determine the susceptibility of the isolates according to the Clinical and Laboratory Standards Institute (CLSI) guidelines. The presence of eae, stx and bfp-A genes was determined by PCR. The genetic relationships between EPEC isolates were determined by pulsed-field gel electrophoresis (PFGE). Out of the 412 strains of E. coli obtained from 612 diarrhoeal stool specimens, 23 (5.6 %) were identified as EPEC, of which seven (30.4 %) were classified as typical strains of EPEC and 16 (69.6 %) were classified as atypical. Out of the 23 EPEC isolates, 69.5 % were resistant to ampicillin, 39.1 % were resistant to tetracycline and cotrimoxazole, 30.4 % were resistant to cefpodoxime, ceftazidime, ceftriaxone and aztreonam, and 26.1 % were resistant to imipenem. The isolates were classified into 21 pulsotypes by PFGE profiles. The present study shows that typical and atypical EPEC isolates displayed considerable heterogeneity in PFGE profiles and EPEC infections were only sporadic in Tehran. Overall 69 % of isolates were resistant to at least one of the antibiotics tested.

Inhibition of TLR signaling by a bacterial protein containing immunoreceptor tyrosine-based inhibitory motifs

The protein Tir (translocated intimin receptor) in enteric bacteria shares sequence similarity with the host cellular immunoreceptor tyrosine-based inhibition motifs (ITIMs). Despite the importance of Tir in pedestal formation, relatively little is known about the role of Tir and its ITIMs in the regulation of the host immune response. Here we demonstrate that Tir from enteropathogenic Escherichia coli (EPEC) interacted with the host cellular tyrosine phosphatase SHP-1 in an ITIM phosphorylation-dependent manner. The association of Tir with SHP-1 facilitated the recruitment of SHP-1 to the adaptor TRAF6 and inhibited the ubiquitination of TRAF6. Moreover, the ITIMs of Tir suppressed EPEC-stimulated expression of proinflammatory cytokines and inhibited intestinal immunity to infection with Citrobacter rodentium. Our findings identify a previously unknown mechanism by which bacterial ITIM-containing proteins can inhibit innate immune responses.

Redistribution of tight junction proteins during EPEC infection in vivo

Enteropathogenic Escherichia coli (EPEC) is a leading cause of diarrhea among infants. Tight junction plays a vital role in intestinal paracellular permeability by forming physical intercellular barriers in epithelial cells. However, the impact of this enteric pathogen on tight junctions in vivo has not been fully investigated. In the present study, the alterations in tight junctions following EPEC infection in vivo were investigated. Western blot analysis revealed that the tight junction proteins, occludin and claudin-1, were displaced from tight junction membrane microdomains to Triton X-100 soluble fractions after EPEC infection. Changes in intestinal paracellular permeability were determined using the molecular tracer biotin, which was observed to penetrate the epithelia and extended into the lamina propria, indicating disruption in tight junction barrier function. Our results suggested that redistribution of tight junction proteins plays an important role in the disruption of epithelial barrier function induced by EPEC infection, which may provide new insight into the pathogenesis of diarrhea caused by EPEC.

Toxofilin upregulates the host cortical actin cytoskeleton dynamics, facilitating Toxoplasma invasion

Toxoplasma gondii, a human pathogen and a model apicomplexan parasite, actively and rapidly invades host cells. To initiate invasion, the parasite induces the formation of a parasite-cell junction, and progressively propels itself through the junction, inside a newly formed vacuole that encloses the entering parasite. Little is known about how a parasite that is a few microns in diameter overcomes the host cell cortical actin barrier to achieve the remarkably rapid process of internalization (less than a few seconds). Using correlative light and electron microscopy in conjunction with electron tomography and three-dimensional image analysis we identified that toxofilin, an actin-binding protein, secreted by invading parasites correlates with localized sites of disassembly of the host cell actin meshwork. Moreover, quantitative fluorescence speckle microscopy of cells expressing toxofilin showed that toxofilin regulates actin filament disassembly and turnover. Furthermore, Toxoplasma tachyzoites lacking toxofilin, were found to be impaired in cortical actin disassembly and exhibited delayed invasion kinetics. We propose that toxofilin locally upregulates actin turnover thus increasing depolymerization events at the site of entry that in turn loosens the local host cell actin meshwork, facilitating parasite internalization and vacuole folding.

Enterocyte microvillus-derived vesicles detoxify bacterial products and regulate epithelial-microbial interactions

The continuous monolayer of intestinal epithelial cells (IECs) lining the gut lumen functions as the site of nutrient absorption and as a physical barrier to prevent the translocation of microbes and associated toxic compounds into the peripheral vasculature. IECs also express host defense proteins such as intestinal alkaline phosphatase (IAP), which detoxify bacterial products and prevent intestinal inflammation. Our laboratory recently showed that IAP is enriched on vesicles that are released from the tips of IEC microvilli and accumulate in the intestinal lumen. Here, we show that these native "lumenal vesicles" (LVs) (1) contain catalytically active IAP that can dephosphorylate lipopolysaccharide (LPS), (2) cluster on the surface of native lumenal bacteria, (3) prevent the adherence of enteropathogenic E. coli (EPEC) to epithelial monolayers, and (4) limit bacterial population growth. We also find that IECs upregulate LV production in response to EPEC and other Gram-negative pathogens. Together, these results suggest that microvillar vesicle shedding represents a novel mechanism for distributing host defense machinery into the intestinal lumen and that microvillus-derived LVs modulate epithelial-microbial interactions.

SNAREs, HOPS and regulatory lipids control the dynamics of vacuolar actin during homotypic fusion in S. cerevisiae

Homotypic vacuole fusion requires SNAREs, the Rab Ypt7p, the tethering complex HOPS, regulatory lipids and actin. In Saccharomyces cerevisiae, actin functions at two stages of vacuole fusion. Pre-existing actin filaments are depolymerized to allow docking and assembly of the vertex ring (a microdomain enriched in proteins and lipids that mediate fusion). Actin is then polymerized late in the pathway to aid fusion. Here, we report that the fusion machinery regulates the accumulation of actin at the vertex ring. Using Cy3-labeled yeast actin to track its dynamics, we found that its vertex enrichment was abolished when actin monomers were stabilized by latrunculin-B, independent of the extent of incorporation. By contrast, stabilization of filamentous actin with jasplakinolide markedly augmented actin vertex enrichment. Importantly, agents that inhibit SNAREs, Ypt7p and HOPS inhibited the vertex enrichment of actin, demonstrating that the cytoskeleton and the fusion machinery are interdependently regulated. Actin mobilization was also inhibited by ligating ergosterol and PtdIns(3)P, whereas the ligation or modification of PtdIns(4,5)P(2) augmented the vertex enrichment of actin. The proteins and lipids that regulated actin mobilization to the vertex did not affect the total incorporation of Cy3-actin, indicating that actin mobilization and polymerization activities can be dissociated during membrane fusion.

Ribotyping of EPEC isolates from diarrheal and asymptomatic patients in Iran

AIM

We intended to find out the diversity of EPEC isolates among asymptomatic or diarrheal children in Iran using ribotyping.

BACKGROUND

Enteropathogenic Escherichia coli (EPEC) is responsible for gastroenteritis especially in young children.

PATIENTS AND METHODS

A total of 39 EPEC collected strains were serotyped and the presence of virulence genes as well as EAF plasmid among the strains was studied. Adherence assay was also performed. Clonal diversity of the isolates was investigated using ribotyping.

RESULTS

Of 39 studied strains of E. coli, 6 serogroups of EPEC were represented. The presence of the stx gene was ascertained in 7 isolates and the eaeA, eaeB and bfpA genes were harbored by 5, 3 and 1 strains, respectively. Ribotyping yielded 9 different clusters.

CONCLUSION

According to our results there was not a significant correlation between the results of serotyping and those of ribotyping. However, different serotypes of E. coli may belong to the same ribotype clusters and vice versa.

Decrease in Shiga toxin expression using a minimal inhibitory concentration of rifampicin followed by bactericidal gentamicin treatment enhances survival of Escherichia coli O157:H7-infected BALB/c mice

Background

Treatment of Escherichia coli O157:H7 infections with antimicrobial agents is controversial due to an association with potentially fatal sequelae. The production of Shiga toxins is believed to be central to the pathogenesis of this organism. Therefore, decreasing the expression of these toxins prior to bacterial eradication may provide a safer course of therapy.

Methods

The utility of decreasing Shiga toxin gene expression in E. coli O157:H7 with rifampicin prior to bacterial eradication with gentamicin was evaluated in vitro using real-time reverse-transcription polymerase chain reaction. Toxin release from treated bacterial cells was assayed for with reverse passive latex agglutination. The effect of this treatment on the survival of E. coli O157:H7-infected BALB/c mice was also monitored.

Results

Transcription of Shiga toxin-encoding genes was considerably decreased as an effect of treating E. coli O157:H7 in vitro with the minimum inhibitory concentration (MIC) of rifampicin followed by the minimum bactericidal concentration (MBC) of gentamicin (> 99% decrease) compared to treatment with gentamicin alone (50-75% decrease). The release of Shiga toxins from E. coli O157:H7 incubated with the MIC of rifampicin followed by addition of the MBC of gentamicin was decreased as well. On the other hand, the highest survival rate in BALB/c mice infected with E. coli O157:H7 was observed in those treated with the in vivo MIC equivalent dose of rifampicin followed by the in vivo MBC equivalent dose of gentamicin compared to mice treated with gentamicin or rifampicin alone.

Conclusions

The use of non-lethal expression-inhibitory doses of antimicrobial agents prior to bactericidal ones in treating E. coli O157:H7 infection is effective and may be potentially useful in human infections with this agent in addition to other Shiga toxin producing E. coli strains.

Modulation of connexin signaling by bacterial pathogens and their toxins

Inherent to their pivotal tasks in the maintenance of cellular homeostasis, gap junctions, connexin hemichannels, and pannexin hemichannels are frequently involved in the dysregulation of this critical balance. The present paper specifically focuses on their roles in bacterial infection and disease. In particular, the reported biological outcome of clinically important bacteria including Escherichia coli, Shigella flexneri, Yersinia enterocolitica, Helicobacter pylori, Bordetella pertussis, Aggregatibacter actinomycetemcomitans, Pseudomonas aeruginosa, Citrobacter rodentium, Clostridium species, Streptococcus pneumoniae, and Staphylococcus aureus and their toxic products on connexin- and pannexin-related signaling in host cells is reviewed. Particular attention is paid to the underlying molecular mechanisms of these effects as well as to the actual biological relevance of these findings.

Colonic microbiota alters host susceptibility to infectious colitis by modulating inflammation, redox status, and ion transporter gene expression

Individuals vary in their resistance to enteric infections. The role of the intestinal microbiota in altering susceptibility to enteric infection is relatively unknown. Previous studies have identified that C3H/HeOuJ mice suffer 100% mortality during Citrobacter rodentium-induced colitis, whereas C57BL/6 mice recover from infection. The basis for their differences in susceptibility is unclear and has been mainly attributed to differences in host genetics. This study investigated the role of the intestinal microbiota in altering susceptibility to C. rodentium-induced colitis. When the feces of C57BL/6 mice were gavaged into antibiotic treated C3H/HeOuJ mice, the C57BL/6 microflora led to a complete reversal in mortality patterns where 100% of the C3H/HeOuJ mice survived infection. This protection corresponded with reduced colonic pathology and less systemic pathogen load and was associated with increased inflammatory and redox responses with reduced epithelial cell death. C3H/HeOuJ mice are normally susceptible to infection-induced dehydration due to defective expression of colonic ion transporters such as Dra, CA IV, and CA I; expression of these genes was normalized when C3H/HeOuJ mice were colonized with the C57BL/6 microflora. Together, these data reveal that the colonic microbiota play a critical role in protecting against intestinal infection by inducing proinflammatory and prooxidant responses that control pathogen load as well as ion transporter gene expression previously shown to prevent fatal dehydration. Protection of mice from lethal colitis was associated with higher levels of bacteria from Bacteroidetes. This study reveals that the microbiota is sufficient to overcome inherent genetic susceptibility patterns in C3H/HeOuJ mice that cause mortality during C. rodentium infection.

Molecular and phenotypic characterization of atypical enteropathogenic Escherichia coli serotypes isolated from children with and without diarrhea

BACKGROUND

We characterized 36 atypical enteropathogenic Escherichia coli (EPEC) serotypes isolated from children with and without diarrhea in Iran. Because the identification of atypical EPEC based on biochemical features is rather difficult and time consuming, we used a combination of three approaches, including a polymerase chain reaction-based method, culture adherence assay, and the restriction analysis of fliC gene (fliC-restriction fragment length polymorphism), to identify E coli serotypes.

METHODS

To distinguish typical and atypical EPEC strains, the presence of EPEC attaching effacing A gene (eaeA) gene and EPEC-attaching factor (EAF) plasmid were analyzed. All E coli strains were identified based on the detection of the eaeA(+), bundle-forming pili A gene (bfpA(-)), EAF(-) or eaeA(-), bfpA(+), EAF(-) profiles and the absence of stx (encoded for shiga toxin) gene as atypical EPEC.

RESULTS

All strains studied belonged to 5 atypical EPEC serogroups and 15 serotypes based on the virulence profiles. Of 36 atypical EPEC serotypes, 22 (61.2%) and 14 (38.8%) strains isolated from diarrheal and healthy cases, respectively. O142:H48 (19.5%) and O111:H21 (11.1%) serotypes were the most prevalent isolates, followed by serotypes O111: H(-) and O86:H48 (5.6% each).

CONCLUSIONS

The characteristics of the atypical EPEC serotypes from children with diarrhea were significantly different from those without diarrhea. The compilation of data on atypical EPEC strains presented here indicates the importance of a combined approach of conventional and molecular tests to study the virulence and epidemiology of EPEC serotypes in human subjects.

Efeito protetor das frações proteicas do soro de leite em camundongos Balb/C infectados por Escherichia coli O157: H7

Avaliou-se o efeito protetor das frações proteicas do soro do leite sobre as vilosidades intestinais de camundongos Balb/C, fêmeas, infectadas por Escherichia coli O157:H7. Foram utilizados 48 animais, distribuídos aleatoriamente em oito grupos de seis fêmeas cada um. Os animais dos grupos 1 e 2 (controles) receberam dieta AIN93G padrão; os dos grupos 3 e 4, AIN93G + alfalactalbumina; os dos grupos 5 e 6, AIN93G + betalactoglobulina e os dos grupos 7 e 8, AIN93G + concentrado proteico total e água ad libitum por sete dias. No dia zero, os animais dos grupos 2, 4, 6 e 8 foram inoculados, por meio de cânula de gavagem, com 0,5mL de E. coli O157:H7, na concentração de 7 x 10(10)UFC/mL. Os animais foram acompanhados clinicamente e sacrificados, no oitavo dia experimental. Verificou-se, por meio de exames histológicos e da morfometria, que as frações betalactoglobulina e alfalactalbumina exerceram efeito protetor sobre as vilosidades intestinais do jejuno distal e do íleo (P<0,05), respectivamente. O concentrado proteico total não demonstrou efeito protetor sobre as vilosidades intestinais.

ppGpp conjures bacterial virulence

Like for all microbes, the goal of every pathogen is to survive and replicate. However, to overcome the formidable defenses of their hosts, pathogens are also endowed with traits commonly associated with virulence, such as surface attachment, cell or tissue invasion, and transmission. Numerous pathogens couple their specific virulence pathways with more general adaptations, like stress resistance, by integrating dedicated regulators with global signaling networks. In particular, many of nature's most dreaded bacteria rely on nucleotide alarmones to cue metabolic disturbances and coordinate survival and virulence programs. Here we discuss how components of the stringent response contribute to the virulence of a wide variety of pathogenic bacteria.

Muc2 protects against lethal infectious colitis by disassociating pathogenic and commensal bacteria from the colonic mucosa

Despite recent advances in our understanding of the pathogenesis of attaching and effacing (A/E) Escherichia coli infections, the mechanisms by which the host defends against these microbes are unclear. The goal of this study was to determine the role of goblet cell-derived Muc2, the major intestinal secretory mucin and primary component of the mucus layer, in host protection against A/E pathogens. To assess the role of Muc2 during A/E bacterial infections, we inoculated Muc2 deficient (Muc2^−/−) mice with Citrobacter rodentium, a murine A/E pathogen related to diarrheagenic A/E E. coli. Unlike wildtype (WT) mice, infected Muc2^−/− mice exhibited rapid weight loss and suffered up to 90% mortality. Stool plating demonstrated 10–100 fold greater C. rodentium burdens in Muc2^−/− vs. WT mice, most of which were found to be loosely adherent to the colonic mucosa. Histology of Muc2^−/− mice revealed ulceration in the colon amid focal bacterial microcolonies. Metabolic labeling of secreted mucins in the large intestine demonstrated that mucin secretion was markedly increased in WT mice during infection compared to uninfected controls, suggesting that the host uses increased mucin release to flush pathogens from the mucosal surface. Muc2 also impacted host-commensal interactions during infection, as FISH analysis revealed C. rodentium microcolonies contained numerous commensal microbes, which was not observed in WT mice. Orally administered FITC-Dextran and FISH staining showed significantly worsened intestinal barrier disruption in Muc2^−/− vs. WT mice, with overt pathogen and commensal translocation into the Muc2^−/− colonic mucosa. Interestingly, commensal depletion enhanced C. rodentium colonization of Muc2^−/− mice, although colonic pathology was not significantly altered. In conclusion, Muc2 production is critical for host protection during A/E bacterial infections, by limiting overall pathogen and commensal numbers associated with the colonic mucosal surface. Such actions limit tissue damage and translocation of pathogenic and commensal bacteria across the epithelium.

Enteropathogenic E. coli (EPEC) and Enterohemorrhagic E. coli (EHEC) are important causes of diarrheal disease and other serious complications worldwide. Despite many studies addressing the pathogenic strategies used by these microbes, how the host protects itself from these pathogens is poorly understood. A critical question we address here is whether the thick mucus layer that overlies the intestinal surface plays a role in host protection. Since EPEC and EHEC do not infect mice efficiently, we used a related mouse pathogen called Citrobacter rodentium to infect and compare responses between wildtype mice and Muc2-deficient mice, which are defective in mucus production. We show that Muc2-deficient mice are extremely susceptible to C. rodentium infection-induced mortality and disease. Muc2-deficient mice were also colonized faster and had higher pathogen burdens throughout the experiment. Resident (non-pathogenic) bacteria were found to interact with C. rodentium and host tissues in Muc2-deficient mice, indicating Muc2 regulates all forms of intestinal microbiota at the gut surface. Deficiency in mucus production also contributed to increased leakiness of the gut, which allowed microbes to enter mucosal tissues. Our study shows that Muc2-dependent mucus production is critical for effective management of both pathogenic and non-pathogenic bacteria during infection by an EPEC/EHEC-like pathogen.

The path to Crohn's disease: is mucosal pathology a secondary event?

Current models of Crohn's disease (CD) invoke an initial disturbance of the epithelial interface between the gut mucosa and intestinal microbiota. This "outside-in" paradigm, mirroring the pathophysiology of acute gastroenteritis, suggests that mucosal damage by luminal bacteria is an early, initiating factor in the etiopathogenesis of disease. However, a number of features of CD argue against a primary mucosal process, including phenotypic studies of CD patients that point to a macrophage defect and genetic studies that predict impaired innate immunity to intracellular bacteria. Intracellular pathogens, such as Listeria, Salmonella, and Mycobacteria, invade via the gastrointestinal tract with minimal or no acute mucosal pathology. These organisms then infect and persist in lymphatic tissues before inducing pathology, in the gut or elsewhere, as a secondary process. In a disease resulting from impaired macrophage responses to intracellular pathogens, mucosal damage could instead represent a terminal event in the pathogenesis of disease. Such an "inside-out" model is also compatible with observations on postoperative disease relapses where subepithelial pathology precedes ulceration. This alternative disease paradigm suggests that clinical and experimental research efforts should be directed at deeper processes in the gut wall and attached mesentery to understand how intracellular bacteria could initiate or exacerbate this chronic inflammatory disease.

Initial adherence of EPEC, EHEC and VTEC to host cells

Initial adherence to host cells is the first step of the infection of enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic Escherichia coli (EHEC) and verotoxigenic Escherichia coli (VTEC) strains. The importance of this step in the infection resides in the fact that (1) adherence is the first contact between bacteria and intestinal cells without which the other steps cannot occur and (2) adherence is the basis of host specificity for a lot of pathogens. This review describes the initial adhesins of the EPEC, EHEC and VTEC strains. During the last few years, several new adhesins and putative colonisation factors have been described, especially in EHEC strains. Only a few adhesins (BfpA, AF/R1, AF/R2, Ral, F18 adhesins) appear to be host and pathotype specific. The others are found in more than one species and/or pathotype (EPEC, EHEC, VTEC). Initial adherence of EPEC, EHEC and VTEC strains to host cells is probably mediated by multiple mechanisms.