Rho GTPase-activating bacterial toxins: from bacterial virulence regulation to eukaryotic cell biology

Urinary Tract Infections Caused by Uropathogenic Escherichia coli: Mechanisms of Infection and Treatment Options

Urinary tract infections (UTIs) are common bacterial infections that represent a severe public health problem. They are often caused by Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumonia), Proteus mirabilis (P. mirabilis), Enterococcus faecalis (E. faecalis), and Staphylococcus saprophyticus (S. saprophyticus). Among these, uropathogenic E. coli (UPEC) are the most common causative agent in both uncomplicated and complicated UTIs. The adaptive evolution of UPEC has been observed in several ways, including changes in colonization, attachment, invasion, and intracellular replication to invade the urothelium and survive intracellularly. While antibiotic therapy has historically been very successful in controlling UTIs, high recurrence rates and increasing antimicrobial resistance among uropathogens threaten to greatly reduce the efficacy of these treatments. Furthermore, the gradual global emergence of multidrug-resistant UPEC has highlighted the need to further explore its pathogenesis and seek alternative therapeutic and preventative strategies. Therefore, a thorough understanding of the clinical status and pathogenesis of UTIs and the advantages and disadvantages of antibiotics as a conventional treatment option could spark a surge in the search for alternative treatment options, especially vaccines and medicinal plants. Such options targeting multiple pathogenic mechanisms of UPEC are expected to be a focus of UTI management in the future to help combat antibiotic resistance.

Enzymatic Specificity of Conserved Rho GTPase Deamidases Promotes Invasion of Vibrio parahaemolyticus at the Expense of Infection

Vibrio parahaemolyticus is among the leading causes of bacterial seafood-borne acute gastroenteritis. Like many intracellular pathogens, V. parahaemolyticus invades host cells during infection by deamidating host small Rho GTPases. The Rho GTPase deamidating activity of VopC, a type 3 secretion system (T3SS) translocated effector, drives V. parahaemolyticus invasion. The intracellular pathogen uropathogenic Escherichia coli (UPEC) invades host cells by secreting a VopC homolog, the secreted toxin cytotoxic necrotizing factor 1 (CNF1). Because of the homology between VopC and CNF1, we hypothesized that topical application of CNF1 during V. parahaemolyticus infection could supplement VopC activity. Here, we demonstrate that CNF1 improves the efficiency of V. parahaemolyticus invasion, a bottleneck in V. parahaemolyticus infection, across a range of doses. CNF1 increases V. parahaemolyticus invasion independent of both VopC and the T3SS altogether but leaves a disproportionate fraction of intracellular bacteria unable to escape the endosome and complete their infection cycle. This phenomenon holds true in the presence or absence of VopC but is particularly pronounced in the absence of a T3SS. The native VopC, by contrast, promotes a far less efficient invasion but permits the majority of internalized bacteria to escape the endosome and complete their infection cycle. These studies highlight the significance of enzymatic specificity during infection, as virulence factors (VopC and CNF1 in this instance) with similarities in function (bacterial uptake), catalytic activity (deamidation), and substrates (Rho GTPases) are not sufficiently interchangeable for mediating a successful invasion for neighboring bacterial pathogens. IMPORTANCE Many species of intracellular bacterial pathogens target host small Rho GTPases to initiate invasion, including the human pathogens Vibrio parahaemolyticus and uropathogenic Escherichia coli (UPEC). The type three secretion system (T3SS) effector VopC of V. parahaemolyticus promotes invasion through the deamidation of Rac1 and CDC42 in the host, whereas the secreted toxin cytotoxic necrotizing factor 1 (CNF1) drives UPEC's internalization through the deamidation of Rac1, CDC42, and RhoA. Despite these similarities in the catalytic activity of CNF1 and VopC, we observed that the two enzymes were not interchangeable. Although CNF1 increased V. parahaemolyticus endosomal invasion, most intracellular V. parahaemolyticus aborted their infection cycle and remained trapped in endosomes. Our findings illuminate how the precise biochemical fine-tuning of T3SS effectors is essential for efficacious pathogenesis. Moreover, they pave the way for future investigations into the biochemical mechanisms underpinning V. parahaemolyticus endosomal escape and, more broadly, the regulation of successful pathogenesis.

Health and environment from adaptation to adaptivity: a situated relational account

The definitions and conceptualizations of health, and the management of healthcare have been challenged by the current global scenarios (e.g., new diseases, new geographical distribution of diseases, effects of climate change on health, etc.) and by the ongoing scholarship in humanities and science. In this paper we question the mainstream definition of health adopted by the WHO-'a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity' (WHO in Preamble to the constitution of the World Health Organization as adopted by the international health conference, The World Health Organization, 1948)-and its role in providing tools to understand what health is in the contemporary context. More specifically, we argue that this context requires to take into account the role of the environment both in medical theory and in the healthcare practice. To do so, we analyse WHO documents dated 1984 and 1986 which define health as 'coping with the environment'. We develop the idea of 'coping with the environment', by focusing on two cardinal concepts: adaptation in public health and adaptivity in philosophy of biology. We argue that the notions of adaptation and adaptivity can be of major benefit for the characterization of health, and have practical implications. We explore some of these implications by discussing two recent case studies of adaptivity in public health, which can be valuable to further develop adaptive strategies in the current pandemic scenario: community-centred care and microbiologically healthier buildings.

Molecular and Genomic Characterization of the Pseudomonas syringae Phylogroup 4: An Emerging Pathogen of Arabidopsis thaliana and Nicotiana benthamiana

Environmental fluctuations such as increased temperature, water availability, and air CO2 concentration triggered by climate change influence plant disease dynamics by affecting hosts, pathogens, and their interactions. Here, we describe a newly discovered Pseudomonas syringae strain found in a natural population of Arabidopsis thaliana collected from the southwest of France. This strain, called Psy RAYR-BL, is highly virulent on natural Arabidopsis accessions, Arabidopsis model accession Columbia 0, and tobacco plants. Despite the severe disease phenotype caused by the Psy RAYR-BL strain, we identified a reduced repertoire of putative Type III virulence effectors by genomic sequencing compared to P. syringae pv tomato (Pst) DC3000. Furthermore, hopBJ1Psy is found exclusively on the Psy RAYR-BL genome but not in the Pst DC3000 genome. The plant expression of HopBJ1Psy induces ROS accumulation and cell death. In addition, HopBJ1Psy participates as a virulence factor in this plant-pathogen interaction, likely explaining the severity of the disease symptoms. This research describes the characterization of a newly discovered plant pathogen strain and possible virulence mechanisms underlying the infection process shaped by natural and changing environmental conditions.

Quantification and surface localization of the hemolysin A type 1 secretion system at the endogenous level and under conditions of overexpression

Secretion systems are essential for Gram-negative bacteria as these nanomachineries allow a communication with the outside world by exporting proteins into the extracellular space or directly into the cytosol of a host cell. For example, type one secretion systems (T1SS) secrete a broad range of substrates across both membranes into the extracellular space. One well-known example is the hemolysin A (HlyA) T1SS from Escherichia coli (E. coli), which consists of an ABC transporter (HlyB), a membrane fusion protein (HlyD), the outer membrane protein TolC and the substrate HlyA, a member of the family of RTX (repeats in toxins) toxins. Here, we determined the amount of TolC at the endogenous level (parental strain, UTI89) and under conditions of overexpression (T7 expression system, BL21(DE3)-BD). The overall amount of TolC was not influenced by the overexpression of the HlyBD complex. Moving one step further, we determined the localization of the HlyA T1SS by super-resolution microscopy. In contrast to other bacterial secretion systems, no polarization was observed with respect to endogenous or overexpression levels. Additionally, the cell growth and division cycle did not influence the polarization. Most importantly, the size of the observed T1SS clusters did not correlate with the recently proposed outer membrane islands. These data indicate that T1SS cluster at the outer membrane generating domains of so far not described identity. Importance Uropathogenic Escherichia coli (UPEC) strains cause about 110 million urinary tract infections each year worldwide representing a global burden to the healthcare system. UPEC secrete many virulence factors among these the TX toxin hemolysin A via a cognate T1SS into the extracellular space. In this study, we determined the endogenous copy number of the HlyA T1SS in UTI89 and analyzed the surface localization in BL21(DE3)-BD and UTI89, respectively. With approximately 800 copies of the T1SS in UTI89, this is one of the highest expressed bacterial secretion systems. Furthermore and in clear contrast to other secretion systems, no polarized surface localization was detected. Finally, quantitative analysis of the super-resolution data revealed that clusters of the HlyA T1SS are not related to the recently identified outer membrane protein islands. These data provide insights into the quantitative molecular architecture of the HlyA T1SS.

Escherichia coli and colorectal cancer: Unfolding the enigmatic relationship

Colorectal cancer (CRC) is one of the deadliest cancers in the world. Specific strains of intestinal Escherichia coli (E. coli) may influence the initiation and development of CRC by exploiting virulence factors and inflammatory pathways. Mucosa-associated E. coli strains are more prevalent in CRC biopsies in comparison to healthy controls. Moreover, these strains can survive and replicate within macrophages and induce a pro-inflammatory response. Chronic exposure to inflammatory mediators can lead to increased cell proliferation and cancer. Production of colobactin toxin by the majority of mucosa-associated E. coli isolated from CRC patients is another notable finding. Colibactin-producing E. coli strains, in particular, induce double-strand DNA breaks, stop the cell cycle, involve in chromosomal rearrangements of mammalian cells and are implicated in carcinogenic effects in animal models. Moreover, some enteropathogenic E. coli (EPEC) strains are able to survive and replicate in colon cells as chronic intracellular pathogens and may promote susceptibility to CRC by downregulation of DNA Mismatch Repair (MMR) proteins. In this review, we discuss current evidence and focus on the mechanisms by which E. coli can influence the development of CRC.

Urinary tract infection: Pathogenicity, antibiotic resistance and development of effective vaccines against Uropathogenic Escherichia coli

Urinary tract infections (UTIs) are recognized as one of the most common infectious diseases in the world that can be divided to different types. Uropathogenic Escherichia coli (UPEC) strains are the most prevalent causative agent of UTIs that applied different virulence factors such as fimbriae, capsule, iron scavenger receptors, flagella, toxins, and lipopolysaccharide for their pathogenicity in the urinary tract. Despite the high pathogenicity of UPEC strains, host utilizes different immune systems such as innate and adaptive immunity for eradication of them from the urinary tract. The routine therapy of UTIs is based on the use of antibiotics such as β-lactams, trimethoprim, nitrofurantoin and quinolones in many countries. Unfortunately, the widespread and misuse of these antibiotics resulted in the increasing rate of resistance to them in the societies. Increasing antibiotic resistance and their side effects on human body show the need to develop alternative strategies such as vaccine against UTIs. Developing a vaccine against UTI pathogens will have an important role in reduction the mortality rate as well as reducing economic costs. Different vaccines based on the whole cells (killed or live-attenuated vaccines) and antigens (subunits, toxins and conjugatedvaccines) have been evaluated against UTIs pathogens. Furthermore, other therapeutic strategies such as the use of probiotics and antimicrobial peptides are considered against UTIs. Despite the extensive efforts, limited success has been achieved and more studies are needed to reach an alternative of antibiotics for treatment of UTIs.

The pyrin inflammasome: from sensing RhoA GTPases-inhibiting toxins to triggering autoinflammatory syndromes

Numerous pathogens including Clostridium difficile and Yersinia pestis have evolved toxins or effectors targeting GTPases from the RhoA subfamily (RhoA/B/C) to inhibit or hijack the host cytoskeleton dynamics. The resulting impairment of RhoA GTPases activity is sensed by the host via an innate immune complex termed the pyrin inflammasome in which caspase-1 is activated. The cascade leading to activation of the pyrin inflammasome has been recently uncovered. In this review, following a brief presentation of RhoA GTPases-modulating toxins, we present the pyrin inflammasome and its regulatory mechanisms. Furthermore, we discuss how some pathogens have developed strategies to escape detection by the pyrin inflammasome. Finally, we present five monogenic autoinflammatory diseases associated with pyrin inflammasome deregulation. The molecular insights provided by the study of these diseases and the corresponding mutations on pyrin inflammasome regulation and activation are presented.

Bacterial Toxins and Targeted Brain Therapy: New Insights from Cytotoxic Necrotizing Factor 1 (CNF1)

Pathogenic bacteria produce toxins to promote host invasion and, therefore, their survival. The extreme potency and specificity of these toxins confer to this category of proteins an exceptionally strong potential for therapeutic exploitation. In this review, we deal with cytotoxic necrotizing factor (CNF1), a cytotoxin produced by Escherichia coli affecting fundamental cellular processes, including cytoskeletal dynamics, cell cycle progression, transcriptional regulation, cell survival and migration. First, we provide an overview of the mechanisms of action of CNF1 in target cells. Next, we focus on the potential use of CNF1 as a pharmacological treatment in central nervous system’s diseases. CNF1 appears to impact neuronal morphology, physiology, and plasticity and displays an antineoplastic activity on brain tumors. The ability to preserve neural functionality and, at the same time, to trigger senescence and death of proliferating glioma cells, makes CNF1 an encouraging new strategy for the treatment of brain tumors.

Cytotoxic Escherichia coli strains encoding colibactin and cytotoxic necrotizing factor (CNF) colonize laboratory macaques

Background

Many Escherichia coli strains are considered to be a component of the normal flora found in the human and animal intestinal tracts. While most E. coli strains are commensal, some strains encode virulence factors that enable the bacteria to cause intestinal and extra-intestinal clinically-relevant infections. Colibactin, encoded by a genomic island (pks island), and cytotoxic necrotizing factor (CNF), encoded by the cnf gene, are genotoxic and can modulate cellular differentiation, apoptosis and proliferation. Some commensal and pathogenic pks+ and cnf+ E. coli strains have been associated with inflammation and cancer in humans and animals.

Results

In the present study, E. coli strains encoding colibactin and CNF were identified in macaque samples. We performed bacterial cultures utilizing rectal swabs and extra-intestinal samples from clinically normal macaques. A total of 239 E. coli strains were isolated from 266 macaques. The strains were identified biochemically and selected isolates were serotyped as O88:H4, O25:H4, O7:H7, OM:H14, and OM:H16. Specific PCR for pks and cnf1 gene amplification, and phylogenetic group identification were performed on all E. coli strains. Among the 239 isolates, 41 (17.2%) were pks+/cnf1−, 19 (7.9%) were pks−/cnf1+, and 31 (13.0%) were pks+/cnf1+. One hundred forty-eight (61.9%) E. coli isolates were negative for both genes (pks−/cnf1−). In total, 72 (30.1%) were positive for pks genes, and 50 (20.9%) were positive for cnf1. No cnf2+ isolates were detected. Both pks+ and cnf1+ E. coli strains belonged mainly to phylogenetic group B2, including B2₁. Colibactin and CNF cytotoxic activities were observed using a HeLa cell cytotoxicity assay in representative isolates. Whole genome sequencing of 10 representative E. coli strains confirmed the presence of virulence factors and antibiotic resistance genes in rhesus macaque E. coli isolates.

Conclusions

Our findings indicate that colibactin- and CNF-encoding E. coli colonize laboratory macaques and can potentially cause clinical and subclinical diseases that impact macaque models.

Electronic supplementary material

The online version of this article (10.1186/s13099-017-0220-y) contains supplementary material, which is available to authorized users.

Coordinated delivery and function of bacterial MARTX toxin effectors

Bacteria often coordinate virulence factors to fine-tune the host response during infection. These coordinated events can include toxins counteracting or amplifying effects of another toxin or though regulating the stability of virulence factors to remove their function once it is no longer needed. Multifunctional autoprocessing repeats-in toxin (MARTX) toxins are effector delivery toxins that form a pore into the plasma membrane of a eukaryotic cell to deliver multiple effector proteins into the cytosol of the target cell. The function of these proteins includes manipulating actin cytoskeletal dynamics, regulating signal transduction pathways and inhibiting host secretory pathways. Investigations into the molecular mechanisms of these effector domains are providing insight into how the function of some effectors overlap and regulate one another during infection. Coordinated crosstalk of effector function suggests that MARTX toxins are not simply a sum of all their parts. Instead, modulation of cell function by effector domains may depend on which other effector domain are co-delivered. Future studies will elucidate how these effectors interact with each other to modulate the bacterial host interaction.

High yield purification and first structural characterization of the full-length bacterial toxin CNF1

The Cytotoxic Necrotizing Factor 1 (CNF1) is a bacterial toxin secreted by certain Escherichia coli strains causing severe pathologies, making it a protein of pivotal interest in toxicology. In parallel, the CNF1 capability to influence important neuronal processes, like neuronal arborization, astrocytic support, and efficient ATP production, has been efficiently used in the treatment of neurological diseases, making it a promising candidate for therapy. Nonetheless, there are still some unsolved issues about the CNF1 mechanism of action and structuration probably caused by the difficulty to achieve sufficient amounts of the full-length protein for further studies. Here, we propose an efficient strategy for the production and purification of this toxin as a his-tagged recombinant protein from E. coli extracts (CNF1-H8). CNF1-H8 was expressed at the low temperature of 15°C to diminish its characteristic degradation. Then, its purification was achieved using an immobilized metal affinity chromatography (IMAC) and a size exclusion chromatography so as to collect up to 8 mg of protein per liter of culture in a highly pure form. Routine dynamic light scattering (DLS) experiments showed that the recombinant protein preparations were homogeneous and preserved this state for a long time. Furthermore, CNF1-H8 functionality was confirmed by testing its activity on purified RhoA and on HEp-2 cultured cells. Finally, a first structural characterization of the full-length toxin in terms of secondary structure and thermal stability was performed by circular dichroism (CD). These studies demonstrate that our system can be used to produce high quantities of pure recombinant protein for a detailed structural analysis. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 34:150-159, 2018.

Type VI secretion system MIX-effectors carry both antibacterial and anti-eukaryotic activities

Most type VI secretion systems (T6SSs) described to date are protein delivery apparatuses that mediate bactericidal activities. Several T6SSs were also reported to mediate virulence activities, although only few anti-eukaryotic effectors have been described. Here, we identify three T6SSs in the marine bacterium Vibrio proteolyticus and show that T6SS1 mediates bactericidal activities under warm marine-like conditions. Using comparative proteomics, we find nine potential T6SS1 effectors, five of which belong to the polymorphic MIX-effector class. Remarkably, in addition to six predicted bactericidal effectors, the T6SS1 secretome includes three putative anti-eukaryotic effectors. One of these is a MIX-effector containing a cytotoxic necrotizing factor 1 domain. We demonstrate that T6SS1 can use this MIX-effector to target phagocytic cells, resulting in morphological changes and actin cytoskeleton rearrangements. In conclusion, the V. proteolyticus T6SS1, a system homologous to one found in pathogenic vibrios, uses a suite of polymorphic effectors that target both bacteria and eukaryotic neighbors.

Heterogeneous Family of Cyclomodulins: Smart Weapons That Allow Bacteria to Hijack the Eukaryotic Cell Cycle and Promote Infections

Some bacterial pathogens modulate signaling pathways of eukaryotic cells in order to subvert the host response for their own benefit, leading to successful colonization and invasion. Pathogenic bacteria produce multiple compounds that generate favorable conditions to their survival and growth during infection in eukaryotic hosts. Many bacterial toxins can alter the cell cycle progression of host cells, impairing essential cellular functions and impeding host cell division. This review summarizes current knowledge regarding cyclomodulins, a heterogeneous family of bacterial effectors that induce eukaryotic cell cycle alterations. We discuss the mechanisms of actions of cyclomodulins according to their biochemical properties, providing examples of various cyclomodulins such as cycle inhibiting factor, γ-glutamyltranspeptidase, cytolethal distending toxins, shiga toxin, subtilase toxin, anthrax toxin, cholera toxin, adenylate cyclase toxins, vacuolating cytotoxin, cytotoxic necrotizing factor, Panton-Valentine leukocidin, phenol soluble modulins, and mycolactone. Special attention is paid to the benefit provided by cyclomodulins to bacteria during colonization of the host.

RNA-based mechanisms of virulence control in Enterobacteriaceae

Enteric pathogens of the family Enterobacteriaceae colonize various niches within animals and humans in which they compete with intestinal commensals and are attacked by the host immune system. To survive these hostile environments they possess complex, multilayer regulatory networks that coordinate the control of virulence factors, host-adapted metabolic functions and stress resistance. An important part of these intricate control networks are RNA-based control systems that enable the pathogen to fine-tune its responses. Recent next-generation sequencing approaches revealed a large repertoire of conserved and species-specific riboregulators, including numerous cis- and trans-acting non-coding RNAs, sensory RNA elements (RNA thermometers, riboswitches), regulatory RNA-binding proteins and RNA degrading enzymes which regulate colonization factors, toxins, host defense processes and virulence-relevant physiological and metabolic processes. All of which are important cues for pathogens to sense and respond to fluctuating conditions during the infection. This review covers infection-relevant riboregulators of E. coli, Salmonella, Shigella and Yersinia, highlights their versatile regulatory mechanisms, complex target regulons and functions, and discusses emerging topics and future challenges to fully understand and exploit RNA-based control to combat bacterial infections.

Immunoadjuvant Properties of the Rho Activating Factor CNF1 in Prophylactic and Curative Vaccination against Leishmania infantum

There is a need to develop new effective immunoadjuvants for prophylactic or therapeutic vaccines against intracellular pathogens. The activation of Rho GTPases by bacterial cytotoxic necrotizing factor 1 (CNF1) elicits humoral protective responses against protein antigens. Here, we set out to investigate whether CNF1 activity initiates humoral immunity against co-administered parasite antigens and anti-microbial immune signaling. We report that co-administration of wild-type (WT) CNF1 with Leishmania (L.) promastigote antigens at the nasal mucosa triggered prophylactic and curative vaccine responses against this parasite. Vaccination of the mucosa with promastigote lysate antigens combined with WT CNF1 conferred protection against high inoculum L. infantum infection, which reached 82% in the spleen. Immune parameter analysis by antigen recall indicated robust T-helper (Th)1 polarization of immune memory cells, with high IL-2 and IFN-γ production combined with decreased IL-4 production. Additionally, we explored the curative effect of WT CNF1 on previously infected animals. We observed that PL combined with WT CNF1, but not the inactive C866S mutant CNF1 (mCNF1), induced a 58% decrease in the parasite burden in the spleen.

Switching Rho GTPase activation into effective antibacterial defenses requires the caspase-1/IL-1beta signaling axis

The monitoring of the activation state of Rho GTPases has emerged as a potent innate immune mechanism for detecting pathogens. In the March issue of PLOS Pathogens, we show that the activation of Rho GTPases by the CNF1 toxin during E. coli-triggered bacteremia leads to a GR1(+)cell-mediated efficient bacterial clearing and improves host survival. Host alarm requires the Caspase-1/IL-1beta signaling axis. Furthermore, we discover that pathogenic bacteria have the capacity to block immune responses via the expression of the α-hemolysin pore-forming toxin. In this commentary, we will comment on these findings and highlight the questions raised by this example of attack-defense mechanisms used alternatively by the pathogen and the host during blood infection.

Effector triggered immunity

Pathogenic bacteria produce virulence factors called effectors, which are important components of the infection process. Effectors aid in pathogenesis by facilitating bacterial attachment, pathogen entry into or exit from the host cell, immunoevasion, and immunosuppression. Effectors also have the ability to subvert host cellular processes, such as hijacking cytoskeletal machinery or blocking protein translation. However, host cells possess an evolutionarily conserved innate immune response that can sense the pathogen through the activity of its effectors and mount a robust immune response. This “effector triggered immunity” (ETI) was first discovered in plants but recent evidence suggest that the process is also well conserved in metazoans. We will discuss salient points of the mechanism of ETI in metazoans from recent studies done in mammalian cells and invertebrate model hosts.

Antibiotic Resistance, Virulence, and Genetic Background of Community-Acquired Uropathogenic Escherichia coli from Algeria

The aim of the study was to investigate antibiotic resistance mechanisms, virulence traits, and genetic background of 150 nonrepetitive community-acquired uropathogenic Escherichia coli (CA-UPEC) from Algeria. A rate of 46.7% of isolates was multidrug resistant. bla genes detected were blaTEM (96.8% of amoxicillin-resistant isolates), blaCTX-M-15 (4%), overexpressed blaAmpC (4%), blaSHV-2a, blaTEM-4, blaTEM-31, and blaTEM-35 (0.7%). All tetracycline-resistant isolates (51.3%) had tetA and/or tetB genes. Sulfonamides and trimethoprim resistance genes were sul2 (60.8%), sul1 (45.9%), sul3 (6.7%), dfrA14 (25.4%), dfrA1 (18.2%), dfrA12 (16.3%), and dfrA25 (5.4%). High-level fluoroquinolone resistance (22.7%) was mediated by mutations in gyrA (S83L-D87N) and parC (S80I-E84G/V or S80I) genes. qnrB5, qnrS1, and aac(6')-Ib-cr were rare (5.3%). Class 1 and/or class 2 integrons were detected (40.7%). Isolates belonged to phylogroups B2+D (50%), A+B1 (36%), and F+C+Clade I (13%). Most of D (72.2%) and 38.6% of B2 isolates were multidrug resistant; they belong to 14 different sequence types, including international successful ST131, ST73, and ST69, reported for the first time in the community in Algeria and new ST4494 and ST4529 described in this study. Besides multidrug resistance, B2 and D isolates possessed virulence factors of colonization, invasion, and long-term persistence. The study highlighted multidrug-resistant CA-UPEC with high virulence traits and an epidemic genetic background.

Characterization of trh2 harbouring Vibrio parahaemolyticus strains isolated in Germany

BACKGROUND

Vibrio parahaemolyticus is a recognized human enteropathogen. Thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) as well as the type III secretion system 2 (T3SS2) are considered as major virulence factors. As tdh positive strains are not detected in coastal waters of Germany, we focused on the characterization of trh positive strains, which were isolated from mussels, seawater and patients in Germany.

RESULTS

Ten trh harbouring V. parahaemolyticus strains from Germany were compared to twenty-one trh positive strains from other countries. The complete trh sequences revealed clustering into three different types: trh1 and trh2 genes and a pseudogene Ψtrh. All German isolates possessed alleles of the trh2 gene. MLST analysis indicated a close relationship to Norwegian isolates suggesting that these strains belong to the autochthonous microflora of Northern Europe seawaters. Strains carrying the pseudogene Ψtrh were negative for T3SS2β effector vopC. Transcription of trh and vopC genes was analyzed under different growth conditions. Trh2 gene expression was not altered by bile while trh1 genes were inducible. VopC could be induced by urea in trh2 bearing strains. Most trh1 carrying strains were hemolytic against sheep erythrocytes while all trh2 positive strains did not show any hemolytic activity. TRH variants were synthesized in a prokaryotic cell-free system and their hemolytic activity was analyzed. TRH1 was active against sheep erythrocytes while TRH2 variants were not active at all.

CONCLUSION

Our study reveals a high diversity among trh positive V. parahaemolyticus strains. The function of TRH2 hemolysins and the role of the pseudogene Ψtrh as pathogenicity factors are questionable. To assess the pathogenic potential of V. parahaemolyticus strains a differentiation of trh variants and the detection of T3SS2β components like vopC would improve the V. parahaemolyticus diagnostics and could lead to a refinement of the risk assessment in food analyses and clinical diagnostics.

Escherichia coli α-hemolysin counteracts the anti-virulence innate immune response triggered by the Rho GTPase activating toxin CNF1 during bacteremia

The detection of the activities of pathogen-encoded virulence factors by the innate immune system has emerged as a new paradigm of pathogen recognition. Much remains to be determined with regard to the molecular and cellular components contributing to this defense mechanism in mammals and importance during infection. Here, we reveal the central role of the IL-1β signaling axis and Gr1+ cells in controlling the Escherichia coli burden in the blood in response to the sensing of the Rho GTPase-activating toxin CNF1. Consistently, this innate immune response is abrogated in caspase-1/11-impaired mice or following the treatment of infected mice with an IL-1β antagonist. In vitro experiments further revealed the synergistic effects of CNF1 and LPS in promoting the maturation/secretion of IL-1β and establishing the roles of Rac, ASC and caspase-1 in this pathway. Furthermore, we found that the α-hemolysin toxin inhibits IL-1β secretion without affecting the recruitment of Gr1+ cells. Here, we report the first example of anti-virulence-triggered immunity counteracted by a pore-forming toxin during bacteremia.

The pathogenic potentials of most microbes depend on a repertoire of virulence factors. Despite major progress in the understanding of the molecular mechanisms underlying the activities of bacterial effectors, little is known about how they cooperate during infection to overcome host immune defenses and promote microbial persistence. Here, we investigated the roles of two uropathogenic Escherichia coli (UPEC) effectors that are co-ordinately expressed, α-hemolysin (HlyA) and cytotoxic necrotizing factor 1 (CNF1). We demonstrated that the HlyA toxin is critical for bacterial stability in the blood and showed that one important role of HlyA is to inhibit the CNF1-induced host response. Collectively, these findings reveal why the coordinated activities of HlyA and CNF1 are necessary for the full virulence of UPEC. Moreover, they unravel a HlyA-driven counter-defense mechanism used by bacteria to facilitate their survival.

Streptococcus pneumoniae ClpL modulates adherence to A549 human lung cells through Rap1/Rac1 activation

Caseinolytic protease L (ClpL) is a member of the HSP100/Clp chaperone family, which is found mainly in Gram-positive bacteria. ClpL is highly expressed during infection for refolding of stress-induced denatured proteins, some of which are important for adherence. However, the role of ClpL in modulating pneumococcal virulence is poorly understood. Here, we show that ClpL impairs pneumococcal adherence to A549 lung cells by inducing and activating Rap1 and Rac1, thus increasing phosphorylation of cofilin (inactive form). Moreover, infection with a clpL mutant (ΔclpL) causes a greater degree of filopodium formation than D39 wild-type (WT) infection. Inhibition of Rap1 and Rac1 impairs filopodium formation and pneumococcal adherence. Therefore, ClpL can reduce pneumococcal adherence to A549 cells, likely via modulation of Rap1- and Rac1-mediated filopodium formation. These results demonstrate a potential role for ClpL in pneumococcal resistance to host cell adherence during infection. This study provides insight into further understanding the interactions between hosts and pathogens.

Bacterial factors exploit eukaryotic Rho GTPase signaling cascades to promote invasion and proliferation within their host

Actin cytoskeleton is a main target of many bacterial pathogens. Among the multiple regulation steps of the actin cytoskeleton, bacterial factors interact preferentially with RhoGTPases. Pathogens secrete either toxins which diffuse in the surrounding environment, or directly inject virulence factors into target cells. Bacterial toxins, which interfere with RhoGTPases, and to some extent with RasGTPases, catalyze a covalent modification (ADPribosylation, glucosylation, deamidation, adenylation, proteolysis) blocking these molecules in their active or inactive state, resulting in alteration of epithelial and/or endothelial barriers, which contributes to dissemination of bacteria in the host. Injected bacterial virulence factors preferentially manipulate the RhoGTPase signaling cascade by mimicry of eukaryotic regulatory proteins leading to local actin cytoskeleton rearrangement, which mediates bacterial entry into host cells or in contrast escape to phagocytosis and immune defense. Invasive bacteria can also manipulate RhoGTPase signaling through recognition and stimulation of cell surface receptor(s). Changes in RhoGTPase activation state is sensed by the innate immunity pathways and allows the host cell to adapt an appropriate defense response.

Calcineurin/NFAT signaling and innate host defence: a role for NOD1-mediated phagocytic functions

The calcineurin/nuclear factor of activated T cells (NFATs) signaling pathway plays a central role in T cell mediated adaptive immune responses, but a number of recent studies demonstrated that calcineurin/NFAT signaling also plays a key role in the control of the innate immune response by myeloid cells. Calcineurin inhibitors, such as cyclosporine A (CsA) and tacrolimus (FK506), are commonly used in organ transplantation to prevent graft rejection and in a variety of immune diseases. These immunosuppressive drugs have adverse effects and significantly increase host's susceptibility towards bacterial or fungal infections. Recent studies highlighted the role of NFAT signaling in fungal infection and in the control of the pattern recognition receptor nucleotide-binding oligomerization domain-containing protein 1 (NOD1), which predominantly senses invasive Gram-negative bacteria and mediates neutrophil phagocytic functions. This review summarises some of the current knowledge concerning the role of NFAT signaling in the innate immune response and the recent advances on NFAT-dependent inhibition of NOD1-mediated innate immune response caused by CsA, which may contribute to sensitizing transplant recipients to bacterial infection.

Recent Advances in Understanding the Pathogenesis of Lawsonia intracellularis Infections

Proliferative enteropathy is an infectious disease caused by an obligate intracellular bacterium, Lawsonia intracellularis, and characterized by thickening of the intestinal epithelium due to enterocyte proliferation. The disease is endemic in swine herds and has been occasionally reported in various other species. Furthermore, outbreaks among foals began to be reported on breeding farms worldwide within the past 5 years. Cell proliferation is directly associated with bacterial infection and replication in the intestinal epithelium. As a result, mild to severe diarrhea is the major clinical sign described in infected animals. The dynamics of L. intracellularis infection in vitro and in vivo have been well characterized, but little is known about the genetic basis for the pathogenesis or ecology of this organism. The present review focuses on the recent advances regarding the pathogenesis and host-pathogen interaction of L. intracellularis infections.

The Vibrio parahaemolyticus effector VopC mediates Cdc42-dependent invasion of cultured cells but is not required for pathogenicity in an animal model of infection

Vibrio parahaemolyticus is a Gram-negative marine bacterium that causes acute gastroenteritis in humans. The virulence of V. parahaemolyticus is dependent upon a type III secretion system (T3SS2). One effector for T3SS2, VopC, is a homologue of the catalytic domain of cytotoxic necrotizing factor (CNF), and was recently reported to be a Rho family GTPase activator and to be linked to internalization of V. parahaemolyticus by non-phagocytic cultured cells. Here, we provide direct evidence that VopC deamidates Rac1 and CDC42, but not RhoA, in vivo. Our results alsosuggest that VopC, through its activation of Rac1, contributes to formation of actin stress fibres in infected cells. Invasion of host cells, which occurs at a low frequency, does not seem linked to Rac1 activation, but instead appears to require CDC42. Finally, using an infant rabbit model of V. parahaemolyticus infection, we show that the virulence of V. parahaemolyticus is not dependent upon VopC-mediated invasion. Genetic inactivation of VopC did not impair intestinal colonization nor reduce signs of disease, including fluid accumulation, diarrhoea and tissue destruction. Thus, although VopC can promote host cell invasion, such internalization is not a critical step of the disease process, consistent with the traditional view of V. parahaemolyticus as an extracellular pathogen.

The cytotoxic necrotizing factor of Yersinia pseudotuberculosis (CNFY) enhances inflammation and Yop delivery during infection by activation of Rho GTPases

Some isolates of Yersinia pseudotuberculosis produce the cytotoxic necrotizing factor (CNF_Y), but the functional consequences of this toxin for host-pathogen interactions during the infection are unknown. In the present study we show that CNF_Y has a strong influence on virulence. We demonstrate that the CNF_Y toxin is thermo-regulated and highly expressed in all colonized lymphatic tissues and organs of orally infected mice. Most strikingly, we found that a cnfY knock-out variant of a naturally toxin-expressing Y. pseudotuberculosis isolate is strongly impaired in its ability to disseminate into the mesenteric lymph nodes, liver and spleen, and has fully lost its lethality. The CNF_Y toxin contributes significantly to the induction of acute inflammatory responses and to the formation of necrotic areas in infected tissues. The analysis of the host immune response demonstrated that presence of CNF_Y leads to a strong reduction of professional phagocytes and natural killer cells in particular in the spleen, whereas loss of the toxin allows efficient tissue infiltration of these immune cells and rapid killing of the pathogen. Addition of purified CNF_Y triggers formation of actin-rich membrane ruffles and filopodia, which correlates with the activation of the Rho GTPases, RhoA, Rac1 and Cdc42. The analysis of type III effector delivery into epithelial and immune cells in vitro and during the course of the infection further demonstrated that CNF_Y enhances the Yop translocation process and supports a role for the toxin in the suppression of the antibacterial host response. In summary, we highlight the importance of CNF_Y for pathogenicity by showing that this toxin modulates inflammatory responses, protects the bacteria from attacks of innate immune effectors and enhances the severity of a Yersinia infection.

Various toxins and effector proteins of bacterial pathogens have been found to manipulate eukaryotic cell machineries to promote persistence and proliferation within their hosts. Many of these virulence factors target small Rho GTPases, but their role in pathogenesis is often unknown. Here, we addressed the expression and functional consequences of the CNF_Y toxin found in some isolates of Y. pseudotuberculosis. We found that CNF_Y besides modulating the cell cytoskeleton by activation of the GTPases RhoA, Rac1 and Cdc42, contributes to increased inflammation and tissue damage. Moreover, CNF_Y increases the ability of Yersinia to prevent the attack of the immune system, by enhancing the delivery of antiphagocytic and cytotoxic effectors into professional phagocytes. Our findings provide the first insights into the multi-functional action and severe consequences of the CNF_Y toxin on the inflammatory response and disease-associated tissue damage during the natural course of the infection.

Laser microdissection coupled with RNA-seq analysis of porcine enterocytes infected with an obligate intracellular pathogen (Lawsonia intracellularis)

BACKGROUND

Lawsonia intracellularis is an obligate intracellular bacterium and the etiologic agent of proliferative enteropathy. The disease is endemic in pigs, emerging in horses and has been described in various other species including nonhuman primates. Cell proliferation is associated with bacterial replication in enterocyte cytoplasm, but the molecular basis of the host-pathogen interaction is unknown. We used laser capture microdissection coupled with RNA-seq technology to characterize the transcriptional responses of infected enterocytes and the host-pathogen interaction.

RESULTS

Proliferative enterocytes was associated with activation of transcription, protein biosynthesis and genes acting on the G1 phase of the host cell cycle (Rho family). The lack of differentiation in infected enterocytes was demonstrated by the repression of membrane transporters related to nutrient acquisition. The activation of the copper uptake transporter by infected enterocytes was associated with high expression of the Zn/Cu superoxide dismutase by L. intracellularis. This suggests that the intracellular bacteria incorporate intracytoplasmic copper and express a sophisticated mechanism to cope with oxidative stress.

CONCLUSIONS

The feasibility of coupling microdissection and RNA-seq was demonstrated by characterizing the host-bacterial interactions from a specific cell type in a heterogeneous tissue. High expression of L. intracellularis genes encoding hypothetical proteins and activation of host Rho genes infers the role of unrecognized bacterial cyclomodulins in the pathogenesis of proliferative enteropathy.

Hijacking of Rho GTPases during bacterial infection

Highly pathogenic bacteria, including Yersinia, Salmonella, E. coli and Clostridia, produce an amazing array of virulence factors that target Rho proteins. These pathogens exploit and/or impair many aspects of Rho protein activities by activating or inhibiting these key molecular switches. Here, we describe examples illustrating how modulation of Rho protein activity is the underlying molecular mechanism used by pathogens to disrupt host epithelial/endothelial barriers, paralyze immune cell migration and phagocytic functions, invade epithelial cells, replicate, and form reservoirs or disseminate in epithelia. Remarkably, emerging evidence points to the capacity of target cells to not only perceive the imbalance of Rho activity induced by virulence factors but also to respond by stimulating the production of anti-microbial responses that alert the host to the pathogenic threat. Furthermore, toxins that activate Rho proteins have been extremely useful in revealing the exquisite cellular regulations of these GTPases, notably by the ubiquitin and proteasome system. Finally, a number of studies indicate that toxins targeting Rho proteins have great potential in the development of new therapeutic tools.

Ubiquitylation of active Rac1 by the E3 ubiquitin-ligase HACE1

Rho GTPases undergo ubiquitylation and degradation via the ubiquitin-proteasome pathway. We now report in the November issue of Developmental Cell that the E3 ubiquitin-ligase HACE1 catalyzes the ubiquitylation of GTP-bound Rac1. Depletion of HACE1 leads to an increase of Rac1 activity. We have proposed that HACE1 limits Rac1 activity in cells, a regulation that is usurped by some pathogenic bacteria for efficient invasion of host cell monolayers. We here review these findings in parallel with the regulation of RhoA by the ubiquitin and proteasome system (UPS) and discuss the impact of these regulations on the capacity of Rho GTPases to signal.

IAPs as E3 ligases of Rac1: shaping the move

Inhibitors of Apoptosis Proteins (IAPs) are well-studied E3 ubiquitin ligases predominantly known for regulation of apoptosis. We uncovered that IAPs can function as a direct E3 ubiquitin ligase of RhoGTPase Rac1. cIAP1 and XIAP directly conjugate polyubiquitin chains to Lysine 147 of activated Rac1 and target it for proteasomal degradation. Consistently, loss of these IAPs by various strategies led to stabilization of Rac1 and mesenchymal mode of migration in tumor cells. IAPs also regulate Rac1 degradation upon RhoGDI1 depletion and CNF1 toxin treatment. Our observations revealed an evolutionarily conserved role of IAPs in regulating Rac1 stability shedding light on to the mechanisms behind ubiquitination–dependent inactivation of Rac1 signaling.

Substrate specificity of clostridial glucosylating toxins and their function on colonocytes analyzed by proteomics techniques

Clostridium difficile is the major cause of intestinal infections in hospitals. The major virulence factors are toxin A (TcdA) and toxin B (TcdB), which belong to the group of clostridial glucosylating toxins (CGT) that inactivate small GTPases. After a 24 h incubation period with TcdA or a glucosyltransferase-deficient mutant TcdA (gdTcdA), quantitative changes in the proteome of colonic cells (Caco-2) were analyzed using high-resolution LC-MS/MS and the SILAC technique. The changes in abundance of more than 5100 proteins were quantified. Nearly 800 toxin-responsive proteins were identified that were involved in cell cycle, cell structure, and adhesion as well as metabolic processes. Several proteins localized to mitochondria or involved in lipid metabolism were consistently of higher abundance after TcdA treatment. All changes of protein abundance depended on the glucosyltransferase activity of TcdA. Glucosylation of the known targets of TcdA such as RhoA, RhoC, RhoG was detected by LC-MS/MS. In addition, an almost complete glucosylation of Rap1(A/B), Rap2(A/B/C) and a partial glucosylation of Ral(A/B) and (H/K/N)Ras were detected. The glucosylation pattern of TcdA was compared to that of other CGT like TcdB, the variant TcdB from C. difficile strain VPI 1470 (TcdBF), and lethal toxin from C. sordellii (TcsL).

High prevalence of mucosa-associated E. coli producing cyclomodulin and genotoxin in colon cancer

Some Escherichia coli strains produce toxins designated cyclomodulins (CMs) which interfere with the eukaryotic cell cycle of host cells, suggesting a possible link between these bacteria and cancers. There are relatively few data available concerning the colonization of colon tumors by cyclomodulin- and genotoxic-producing E. coli. We did a qualitative and phylogenetic analysis of mucosa-associated E. coli harboring cyclomodulin-encoding genes from 38 patients with colorectal cancer (CRC) and 31 with diverticulosis. The functionality of these genes was investigated on cell cultures and the genotoxic activity of strains devoid of known CM-encoding gene was investigated. Results showed a higher prevalence of B2 phylogroup E. coli harboring the colibatin-producing genes in biopsies of patients with CRC (55.3%) than in those of patients with diverticulosis (19.3%), (p<0.01). Likewise, a higher prevalence of B2 E. coli harboring the CNF1-encoding genes in biopsies of patients with CRC (39.5%) than in those of patients with diverticulosis (12.9%), (p = 0.01). Functional analysis revealed that the majority of these genes were functional. Analysis of the ability of E. coli to adhere to intestinal epithelial cells Int-407 indicated that highly adherent E. coli strains mostly belonged to A and D phylogroups, whatever the origin of the strains (CRC or diverticulosis), and that most E. coli strains belonging to B2 phylogroup displayed very low levels of adhesion. In addition, 27.6% (n = 21/76) E. coli strains devoid of known cyclomodulin-encoding genes induced DNA damage in vitro, as assessed by the comet assay. In contrast to cyclomodulin-producing E. coli, these strains mainly belonged to A or D E. coli phylogroups, and exhibited a non significant difference in the distribution of CRC and diverticulosis specimens (22% versus 32.5%, p = 0.91). In conclusion, cyclomodulin-producing E. coli belonging mostly to B2 phylogroup colonize the colonic mucosa of patients with CRC.

Queuosine biosynthesis is required for sinorhizobium meliloti-induced cytoskeletal modifications on HeLa Cells and symbiosis with Medicago truncatula

Rhizobia are symbiotic soil bacteria able to intracellularly colonize legume nodule cells and form nitrogen-fixing symbiosomes therein. How the plant cell cytoskeleton reorganizes in response to rhizobium colonization has remained poorly understood especially because of the lack of an in vitro infection assay. Here, we report on the use of the heterologous HeLa cell model to experimentally tackle this question. We observed that the model rhizobium Sinorhizobium meliloti, and other rhizobia as well, were able to trigger a major reorganization of actin cytoskeleton of cultured HeLa cells in vitro. Cell deformation was associated with an inhibition of the three major small RhoGTPases Cdc42, RhoA and Rac1. Bacterial entry, cytoskeleton rearrangements and modulation of RhoGTPase activity required an intact S. meliloti biosynthetic pathway for queuosine, a hypermodifed nucleoside regulating protein translation through tRNA, and possibly mRNA, modification. We showed that an intact bacterial queuosine biosynthetic pathway was also required for effective nitrogen-fixing symbiosis of S. meliloti with its host plant Medicago truncatula, thus indicating that one or several key symbiotic functions of S. meliloti are under queuosine control. We discuss whether the symbiotic defect of que mutants may originate, at least in part, from an altered capacity to modify plant cell actin cytoskeleton.

General aspects and recent advances on bacterial protein toxins

Bacterial pathogens produce protein toxins to influence host-pathogen interactions and tip the outcome of these encounters toward the benefit of the pathogen. Protein toxins modify host-specific targets through posttranslational modifications (PTMs) or noncovalent interactions that may inhibit or activate host cell physiology to benefit the pathogen. Recent advances have identified new PTMs and host targets for toxin action. Understanding the mechanisms of toxin action provides a basis to develop vaccines and therapies to combat bacterial pathogens and to develop new strategies to use toxin derivatives for the treatment of human disease.

Structure of the catalytic domain of the Salmonella virulence factor SseI

SseI is secreted into host cells by Salmonella and contributes to the establishment of systemic infections. The crystal structure of the C-terminal domain of SseI has been solved to 1.70 Å resolution, revealing it to be a member of the cysteine protease superfamily with a catalytic triad consisting of Cys178, His216 and Asp231 that is critical to its virulence activities. Structure-based analysis revealed that SseI is likely to possess either acyl hydrolase or acyltransferase activity, placing this virulence factor in the rapidly growing class of enzymes of this family utilized by bacterial pathogens inside eukaryotic cells.

Effect of RhoA on transforming growth factor β1-induced rat hepatic stellate cell migration

BACKGROUND

Although the migration of hepatic stellate cells (HSCs) is essential to the hepatic fibrotic response, the intracellular and extracellular signals that regulate their migration are poorly understood.

AIMS

To investigate the role of Rho guanosine triphosphatase (Rho GTPase) signalling, specifically via RhoA, in transforming growth factor β1 (TGFβ1)-induced HSC migration.

METHODS

Both primary rat HSCs and the HSC-T6 rat hepatic stellate cell line were used in this study. Cell migration was evaluated using the Transwell Boyden Chamber assay, whereas cytoskeletal changes were observed using laser confocal microscopy. Western blotting was used to detect the expression of Rho GTPases (RhoA, Rac1 and Cdc42) in HSCs, and their activation was determined using glutathione S-transferase (GST) pull-down assays. Finally, the specific effects of RhoA on TGFβ1-induced cell migration were analysed in HSC-T6 cells stably transfected with constitutively active (CA, Q63L) or dominant-negative (DN, T19N) RhoA mutants.

RESULTS

Transforming growth factor β1 induced cytoskeletal remodelling and migration of rat HSCs following RhoA activation. The level of RhoA activation determined the motility of the HSCs.

CONCLUSIONS

These findings broaden our understanding of the intracellular and extracellular signals that regulate HSC migration. Furthermore, RhoA may be a candidate therapeutic target for hepatic fibrosis.

Increased Cell-Matrix Adhesion upon Constitutive Activation of Rho Proteins by Cytotoxic Necrotizing Factors from E. Coli and Y. Pseudotuberculosis

Cytotoxic necrotizing factors (CNFs) encompass a class of autotransporter toxins produced by uropathogenic E. coli (CNF1) or Y. pseudotuberculosis (CNFy). CNF toxins deamidate and thereby constitutively activate RhoA, Rac1, and Cdc42. In this study, the effects of CNF1 on cell-matrix adhesion are analysed using functional cell-adhesion assays. CNF1 strongly increased cell-matrix binding of suspended Hela cells and decreased the susceptibly of cells to trypsin-induced cell detachment. Increased cell-matrix binding was also observed upon treatment of Hela cells with isomeric CNFy, that specifically deamidates RhoA. Increased cell-matrix binding thus appears to depend on RhoA deamidation. In contrast, increased cell spreading was specifically observed upon CNF1 treatment, suggesting that it rather depended on Rac1/Cdc42 deamidation. Increased cell-matrix adhesion is further presented to result in reduced cell migration of adherent cells. In contrast, migration of suspended cells was not affected upon treatment with CNF1 or CNFy. CNF1 and CNFy thus reduced cell migration specifically under the condition of pre-established cell-matrix adhesion.

Virulence potential and genomic mapping of the worldwide clone Escherichia coli ST131

Recently, the worldwide propagation of clonal CTX-M-15-producing Escherichia coli isolates, namely ST131 and O25b:H4, has been reported. Like the majority of extra-intestinal pathogenic E. coli isolates, the pandemic clone ST131 belongs to phylogenetic group B2, and has recently been shown to be highly virulent in a mouse model, even though it lacks several genes encoding key virulence factors (Pap, Cnf1 and HlyA). Using two animal models, Caenorhabditis elegans and zebrafish embryos, we assessed the virulence of three E. coli ST131 strains (2 CTX-M-15- producing urine and 1 non-ESBL-producing faecal isolate), comparing them with five non-ST131 B2 and a group A uropathogenic E. coli (UPEC). In C. elegans, the three ST131 strains showed intermediate virulence between the non virulent group A isolate and the virulent non-ST131 B2 strains. In zebrafish, the CTX-M-15-producing ST131 UPEC isolates were also less virulent than the non-ST131 B2 strains, suggesting that the production of CTX-M-15 is not correlated with enhanced virulence. Amongst the non-ST131 B2 group isolates, variation in pathogenic potential in zebrafish embryos was observed ranging from intermediate to highly virulent. Interestingly, the ST131 strains were equally persistent in surviving embryos as the non-ST131-group B2 strains, suggesting similar mechanisms may account for development of persistent infection. Optical maps of the genome of the ST131 strains were compared with those of 24 reference E. coli strains. Although small differences were seen within the ST131 strains, the tree built on the optical maps showed that these strains belonged to a specific cluster (86% similarity) with only 45% similarity with the other group B2 strains and 25% with strains of group A and D. Thus, the ST131 clone has a genetic composition that differs from other group B2 strains, and appears to be less virulent than previously suspected.

mRNA context dependent regulation of cytotoxic necrotizing factor 1 translation by GidA, a tRNA modification enzyme in Escherichia coli

Cytotoxic necrotizing factor 1 (CNF1), the paradigm of Rho GTPase activating bacterial toxins has been shown to promote E. coli invasion of human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier, but its synthesis and secretion is unclear. In this study, we performed mini Tn5 mutagenesis screen to identify genetic requirements for CNF1 production and secretion. Transposon mutagenesis screen of meningitis-causing E. coli K1 strain RS218 revealed that CNF1 production was markedly decreased in a transposon mutant (NBC-28G9) where transposon insertion occurred in the 5' end of gidA gene. In contrast, total deletion of gidA gene has less drastic effect on the production of CNF1. The N-terminus truncated GidA exhibited dominant negative effect on the production of CNF1. The inhibition of CNF1 production by N-terminus truncated GidA was shown to occur at the translational level. This was supported by our demonstrations that cnf1 mRNA transcription levels did not differ between strains RS218 and NBC-28G9; and the production of recombinant CNF1 under the control of artificial promoter was also repressed by truncated GidA. Progressive deletion of DNA regions in cnf1 gene identified two putative regions that were responsible for translational inhibition mediated by truncated GidA.

Bacterial effectors: learning on the fly

A common defining characteristic of pathogenic bacteria is the expression of a repertoire of effector molecules that have been named virulence factors. These bacterial factors include a -variety of proteins, such as toxins that are internalized by receptors and translocate across endosomal membranes to reach the cytosol, as well as others that are introduced directly into the cell by means of bacterial secretory apparatuses. Given the importance of these effectors for understanding bacterial pathogenicity, significant effort has been made to dissect their molecular mechanisms of action and their respective roles during infection. Herein we will discuss how Drosophila have been used as a model system to study these important microbial effectors, and to understand their contribution to pathogenicity.

Assessing ubiquitylation of Rho GTPases in mammalian cells

Rho GTPases including RhoA, Cdc42, and Rac1 are master regulators of cell cytoskeleton dynamic, thus controlling essential cellular processes notably cell polarity, migration and cytokinesis. These GTPases undergo a spatiotemporal regulation primarily controlled by cellular factors inducing both the exchange of GDP for GTP and the hydrolysis of GTP into GDP. Recent findings have unveiled another layer of complexity in the regulation of Rho proteins consisting in their ubiquitylation followed by their proteasomal degradation. Here, we describe how to assess the level of ubiquitylation of Rho proteins in cells, taking Rac1 as an example.

YgfZ contributes to secretion of cytotoxic necrotizing factor 1 into outer-membrane vesicles in Escherichia coli

Cytotoxic necrotizing factor 1 (CNF1), a Rho GTPase-activating bacterial toxin, has been shown to contribute to invasion by meningitis-causing Escherichia coli K1 of human brain microvascular endothelial cells (HBMEC), which constitute the blood-brain barrier. However, CNF1 is a cytosolic protein and it remains unclear how its secretion occurs, contributing to E. coli invasion of HBMEC. To investigate the genetic requirement for CNF1 secretion in E. coli K1 strain RS218, we performed mini-Tn5 in vitro mutagenesis and constructed a transposon mutant library of strain NBC, in which β-lactamase was fused to the C-terminus of CNF1 in the chromosome of strain RS218. We identified a transposon mutant (NBC-1E6) that exhibited reduced β-lactamase activity in its culture supernatant and had the transposon inserted into the ygfZ gene. When ygfZ was deleted from the genome of strain RS218 (ΔygfZ), the translocation of CNF1 into HBMEC was impaired. Subcellular localization analysis of CNF1 demonstrated that YgfZ, a periplasmic protein, contributes to secretion of CNF1 into outer-membrane vesicles (OMVs). The ΔygfZ mutant was significantly defective in invasion of HBMEC compared to the parent E. coli K1 strain. The defects of the ΔygfZ mutant in CNF1 secretion into OMVs and translocation into HBMEC as well as invasion of HBMEC were abrogated by complementation with ygfZ. Taken together, our findings demonstrate that YgfZ contributes to CNF1 secretion into OMVs in meningitis-causing E. coli K1.

Multifaceted interactions of bacterial toxins with the gastrointestinal mucosa

The digestive tract is one of the ecosystems that harbors the largest number and greatest variety of bacteria. Among them, certain bacteria have developed various strategies, including the synthesis of virulence factors such as toxins, to interact with the intestinal mucosa, and are responsible for various pathologies. A large variety of bacterial toxins of different sizes, structures and modes of action are able to interact with the gastrointestinal mucosa. Some toxins, termed enterotoxins, directly stimulate fluid secretion in enterocytes or cause their death, whereas other toxins pass through the intestinal barrier and disseminate by the general circulation to remote organs or tissues, where they are active. After recognition of a membrane receptor on target cells, toxins can act at the cell membrane by transducing a signal across the membrane in a hormone-like manner, by pore formation or by damaging membrane compounds. Other toxins can enter the cells and modify an intracellular target leading to a disregulation of certain physiological processes or disorganization of some structural architectures and cell death. Toxins are fascinating molecules, which mimic or interfere with eukaryotic physiological processes. Thereby, they have permitted the identification and characterization of new natural hormones or regulatory pathways. Besides use as protective antigens in vaccines, toxins offer multiple possibilities in pharmacology, such as immune modulation or specific delivery of a protein of interest into target cells.