Type IV secretion systems and their effectors in bacterial pathogenesis

Take it and release it: The use of the Rab1 small GTPase at a bacterium's will

Successful pathogens are equipped to exploit the signaling pathways of their host cell to establish a niche conducive for their survival and proliferation. One emerging example is the modulation of the small GTPase Rab1 by virulence factors of the intracellular pathogen Legionella pneumophila. Besides proteins that mimic host regulatory factors involved in controlling Rab1 activity, this bacterium temporally locks this small GTPase in its active form by AMPylation. Efficient release of Rab1 from the bacterial phagosome requires deAMPylation prior to being inactivated by the bacterial GAP protein LepB. Whether Rab activity is similarly regulated under native condition is unknown, but it is clear that virulence factors from pathogens can be invaluable tools in dissecting the intricacy of host cellular processes.

In search of Brucella abortus type IV secretion substrates: screening and identification of four proteins translocated into host cells through VirB system

Type IV secretion systems (T4SS) are specialized protein complexes used by many bacterial pathogens for the delivery of effector molecules that subvert varied host cellular processes. Brucella spp. are facultative intracellular pathogens capable of survival and replication inside mammalian cells. Brucella T4SS (VirB) is essential to subvert lysosome fusion and to create an organelle permissive for replication. One possible role for VirB is to translocate effector proteins that modulate host cellular functions for the biogenesis of the replicative organelle. We hypothesized that proteins with eukaryotic domains or protein-protein interaction domains, among others, would be good candidates for modulation of host cell functions. To identify these candidates, we performed an in silico screen looking for proteins with distinctive features. Translocation of 84 potential substrates was assayed using adenylate cyclase reporter. By this approach, we identified six proteins that are delivered to the eukaryotic cytoplasm upon infection of macrophage-like cells and we could determine that four of them, encoded by genes BAB1_1043, BAB1_2005, BAB1_1275 and BAB2_0123, require a functional T4SS for their delivery. We confirmed VirB-mediated translocation of one of the substrates by immunofluorescence confocal microscopy, and we found that the N-terminal 25 amino acids are required for its delivery into cells.

A component of the Xanthomonadaceae type IV secretion system combines a VirB7 motif with a N0 domain found in outer membrane transport proteins

Type IV secretion systems (T4SS) are used by Gram-negative bacteria to translocate protein and DNA substrates across the cell envelope and into target cells. Translocation across the outer membrane is achieved via a ringed tetradecameric outer membrane complex made up of a small VirB7 lipoprotein (normally 30 to 45 residues in the mature form) and the C-terminal domains of the VirB9 and VirB10 subunits. Several species from the genera of Xanthomonas phytopathogens possess an uncharacterized type IV secretion system with some distinguishing features, one of which is an unusually large VirB7 subunit (118 residues in the mature form). Here, we report the NMR and 1.0 Å X-ray structures of the VirB7 subunit from Xanthomonas citri subsp. citri (VirB7_XAC2622) and its interaction with VirB9. NMR solution studies show that residues 27–41 of the disordered flexible N-terminal region of VirB7_XAC2622 interact specifically with the VirB9 C-terminal domain, resulting in a significant reduction in the conformational freedom of both regions. VirB7_XAC2622 has a unique C-terminal domain whose topology is strikingly similar to that of N0 domains found in proteins from different systems involved in transport across the bacterial outer membrane. We show that VirB7_XAC2622 oligomerizes through interactions involving conserved residues in the N0 domain and residues 42–49 within the flexible N-terminal region and that these homotropic interactions can persist in the presence of heterotropic interactions with VirB9. Finally, we propose that VirB7_XAC2622 oligomerization is compatible with the core complex structure in a manner such that the N0 domains form an extra layer on the perimeter of the tetradecameric ring.

Many aspects of bacterial life require that they translocate proteins to the cell exterior. To do this, different macromolecular secretion systems of varying complexity have evolved (Type I–VI secretion systems). These secretion systems are often at the front lines of pathogen-host interactions and are important for the development of disease. In this work, we have determined the structure and studied the interactions of an unusually large VirB7 subunit (VirB7_XAC2622) of the outer membrane pore of the Type IV secretion system found in the Xanthomonas genera of phytopathogens. Its mosaic structure combines a canonical VirB7 N-terminal region with a C-terminal globular domain whose topology is observed in a relatively limited set of proteins, all involved in molecular transport across outer membranes. Our results lead to the hypothesis that the VirB7_XAC2622 globular domains can form an extra ring around the perimeter of the outer membrane pore and reveal deeper structural and evolutionary relationships among bacterial macromolecular secretion systems that have evolved to adopt a variety of functions, including structural modules in outer membrane pores (secretins from Type II, III and IV secretion systems, Type IV pili and filamentous phages), signal-transduction modules in TonB-dependent receptors and membrane-penetrating devices in T6SS and long-tailed bacteriophages.

Molecular cross-talk between Helicobacter pylori and human gastric mucosa

Helicobacter pylori (H. pylori) has co-evolved with humans to be transmitted from person to person and to colonize the stomach persistently. A well-choreographed equilibrium between the bacterial effectors and host responses permits microbial persistence and health of the host, but confers a risk for serious diseases including gastric cancer. During its long coexistence with humans, H. pylori has developed complex strategies to limit the degree and extent of gastric mucosal damage and inflammation, as well as immune effector activity. The present editorial thus aims to introduce and comment on major advances in the rapidly developing area of H. pylori/human gastric mucosa interaction (and its pathological sequelae), which is the result of millennia of co-evolution of, and thus of reciprocal knowledge between, the pathogen and its human host.

Genomes of the most dangerous epidemic bacteria have a virulence repertoire characterized by fewer genes but more toxin-antitoxin modules

BACKGROUND

We conducted a comparative genomic study based on a neutral approach to identify genome specificities associated with the virulence capacity of pathogenic bacteria. We also determined whether virulence is dictated by rules, or if it is the result of individual evolutionary histories. We systematically compared the genomes of the 12 most dangerous pandemic bacteria for humans ("bad bugs") to their closest non-epidemic related species ("controls").

METHODOLOGY/PRINCIPAL FINDINGS

We found several significantly different features in the "bad bugs", one of which was a smaller genome that likely resulted from a degraded recombination and repair system. The 10 Cluster of Orthologous Group (COG) functional categories revealed a significantly smaller number of genes in the "bad bugs", which lacked mostly transcription, signal transduction mechanisms, cell motility, energy production and conversion, and metabolic and regulatory functions. A few genes were identified as virulence factors, including secretion system proteins. Five "bad bugs" showed a greater number of poly (A) tails compared to the controls, whereas an elevated number of poly (A) tails was found to be strongly correlated to a low GC% content. The "bad bugs" had fewer tandem repeat sequences compared to controls. Moreover, the results obtained from a principal component analysis (PCA) showed that the "bad bugs" had surprisingly more toxin-antitoxin modules than did the controls.

CONCLUSIONS/SIGNIFICANCE

We conclude that pathogenic capacity is not the result of "virulence factors" but is the outcome of a virulent gene repertoire resulting from reduced genome repertoires. Toxin-antitoxin systems could participate in the virulence repertoire, but they may have developed independently of selfish evolution.

Integrin-mediated type IV secretion by Helicobacter: what makes it tick?

Helicobacter pylori (Hp) employs a multi-component type IV secretion system (T4SS) to secrete the effector protein CagA into the cytosol of infected host cells. A longstanding challenge has been to identify the host cell receptor(s) involved. Two recent studies have independently unveiled human β(1) integrin as the receptor but are divided over which T4SS proteins bind to β(1) integrin. Here we revisit the two models in light of previous findings and recent progress in the field. More concerted efforts are required to fully understand the complex T4SS mechanisms that underpin Hp pathogenesis.

Bacterial contact-dependent delivery systems

Bacteria have developed remarkable systems that sense neighboring target cells upon contact and initiate a series of events that enhance their survival and growth at the expense of the target cells. Four main classes of bacterial cell surface structures have been identified that interact with prokaryotic or eukaryotic target cells to deliver DNA or protein effectors. Type III secretion systems (T3SS) use a flagellum-like tube to deliver protein effectors into eukaryotic host cells, whereas Type IV systems use a pilus-based system to mediate DNA or protein transfer into recipient cells. The contact-dependent growth inhibition system (CDI) is a Type V system, using a long β-helical cell surface protein to contact receptors in target cells and deliver a growth inhibitory signal. Type VI systems utilize a phage-like tube and cell puncturing device to secrete effector proteins into both eukaryotic and prokaryotic target cells.

The Coxiella burnetii cryptic plasmid is enriched in genes encoding type IV secretion system substrates

The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a phagolysosome-like parasitophorous vacuole (PV), in which it replicates. The organism encodes a Dot/Icm type IV secretion system (T4SS) predicted to deliver to the host cytosol effector proteins that mediate PV formation and other cellular events. All C. burnetii isolates carry a large, autonomously replicating plasmid or have chromosomally integrated plasmid-like sequences (IPS), suggesting that plasmid and IPS genes are critical for infection. Bioinformatic analyses revealed two candidate Dot/Icm substrates with eukaryotic-like motifs uniquely encoded by the QpH1 plasmid from the Nine Mile reference isolate. CpeC, containing an F-box domain, and CpeD, possessing kinesin-related and coiled-coil regions, were secreted by the closely related Legionella pneumophila Dot/Icm T4SS. An additional QpH1-specific gene, cpeE, situated in a predicted operon with cpeD, also encoded a secreted effector. Further screening revealed that three hypothetical proteins (CpeA, CpeB, and CpeF) encoded by all C. burnetii plasmids and IPS are Dot/Icm substrates. By use of new genetic tools, secretion of plasmid effectors by C. burnetii during host cell infection was confirmed using β-lactamase and adenylate cyclase translocation assays, and a C-terminal secretion signal was identified. When ectopically expressed in HeLa cells, plasmid effectors trafficked to different subcellular sites, including autophagosomes (CpeB), ubiquitin-rich compartments (CpeC), and the endoplasmic reticulum (CpeD). Collectively, these results suggest that C. burnetii plasmid-encoded T4SS substrates play important roles in subversion of host cell functions, providing a plausible explanation for the absolute maintenance of plasmid genes by this pathogen.

Abl family of tyrosine kinases and microbial pathogenesis

Abl nonreceptor tyrosine kinases are activated by multiple stimuli and regulate cytoskeletal reorganization, cell proliferation, survival, and stress responses. Several downstream pathways have direct impact on physiological processes, including development and maintenance of the nervous and immune systems and epithelial morphogenesis. Recent studies also indicated that numerous viral and bacterial pathogens highjack Abl signaling for different purposes. Abl kinases are activated to reorganize the host actin cytoskeleton and promote the direct tyrosine phosphorylation of viral surface proteins and injected bacterial type-III and type-IV effector molecules. However, Abl kinases also play other roles in infectious processes of bacteria, viruses, and prions. These activities have crucial impact on microbial invasion and release from host cells, actin-based motility, pedestal formation, as well as cell-cell dissociation involved in epithelial barrier disruption and other responses. Thus, Abl kinases exhibit important functions in pathological signaling during microbial infections. Here, we discuss the different signaling pathways activated by pathogens and highlight possible therapeutic intervention strategies.

An in vivo high-throughput screening approach targeting the type IV secretion system component VirB8 identified inhibitors of Brucella abortus 2308 proliferation

As bacterial pathogens develop resistance against most currently used antibiotics, novel alternatives for treatment of microbial infectious diseases are urgently needed. Targeting bacterial virulence functions in order to disarm pathogens represents a promising alternative to classical antibiotic therapy. Type IV secretion systems, which are multiprotein complexes in the cell envelope that translocate effectors into host cells, are critical bacterial virulence factors in many pathogens and excellent targets for such "antivirulence" drugs. The VirB8 protein from the mammalian pathogen Brucella was chosen as a specific target, since it is an essential type IV secretion system component, it participates in multiple protein-protein interactions, and it is essential for the assembly of this translocation machinery. The bacterial two-hybrid system was adapted to assay VirB8 interactions, and a high-throughput screen identified specific small-molecule inhibitors. VirB8 interaction inhibitors also reduced the levels of VirB8 and of other VirB proteins, and many of them inhibited virB gene transcription in Brucella abortus 2308, suggesting that targeting of the secretion system has complex regulatory effects in vivo. One compound strongly inhibited the intracellular proliferation of B. abortus 2308 in a J774 macrophage infection model. The results presented here show that in vivo screens with the bacterial two-hybrid assay are suited to the identification of inhibitors of Brucella type IV secretion system function.

Two distinct regions in the model protein Peb1 are critical for its heterologous transport out of Escherichia coli

Background

Escherichia coli is frequently the first-choice host organism in expression of heterologous recombinant proteins in basic research as well as in production of commercial, therapeutic polypeptides. Especially the secretion of proteins into the culture medium of E. coli is advantageous compared to intracellular production due to the ease in recovery of the recombinant protein. Since E. coli naturally is a poor secretor of proteins, a few strategies for optimization of extracellular secretion have been described. We have previously reported efficient secretion of the diagnostically interesting model protein Peb1 of Campylobacter jejuni into the growth medium of Escherichia coli strain MKS12 (ΔfliCfliD). To generate a more detailed understanding of the molecular mechanisms behind this interesting heterologous secretion system with biotechnological implications, we here analyzed further the transport of Peb1 in the E. coli host.

Results

When mature Peb1 was expressed without its SecA-YEG -dependent signal sequence and without the putative signal peptidase II recognition sequence in E. coli MKS111ΔHBB lacking the flagellar secretion complex, the protein was found in the periplasm and growth medium which indicated a flagellum-independent translocation. We assessed the Peb1 secretion proficiency by an exhaustive search for transport-affecting regions using a transposition-based scanning mutagenesis strategy. Strikingly, insertion mutagenesis of only two segments, called TAR1 (residues 42 and 43) and TAR2 (residues 173 to 180), prevented Peb1 secretion individually. We confirmed the importance of TAR regions by subsequent site-specific mutagenesis and verified that the secretion deficiency of Peb1 mutants was not due to insolubility or aggregation of the proteins in the cytoplasm. We found by cell fractionation that the mutant proteins were present in the periplasm as well as in the cytoplasm of MKS12. Hence, mutagenesis of TAR regions did not affect export of Peb1 across the cytoplasmic membrane, whereas its export over the outer membrane was markedly impaired.

Conclusions

We propose that the localization of the model protein Peb1 in the growth medium of E. coli is due to active secretion by a still unknown pathway of E. coli. The secretion apparently is a two-step process involving a periplasmic step and the TAR regions.

Complexomics study of two Helicobacter pylori strains of two pathological origins: potential targets for vaccine development and new insight in bacteria metabolism

Helicobacter pylori infection plays a causal role in the development of gastric mucosa-associated lymphoid tissue (MALT) lymphoma (LG-MALT) and duodenal ulcer (DU). Although many virulence factors have been associated with DU, many questions remain unanswered regarding the evolution of the infection toward this exceptional event, LG-MALT. The present study describes and compares the complexome of two H. pylori strains, strain J99 associated with DU and strain B38 associated with LG-MALT, using the two-dimensional blue native/SDS-PAGE method. It was possible to identify 90 different complexes (49 and 41 in the B38 and J99 strains, respectively); 12 of these complexes were common to both strains (seven and five in the membrane and cytoplasm, respectively), reflecting the variability of H. pylori strains. The 44 membrane complexes included numerous outer membrane proteins, such as the major adhesins BabA and SabA retrieved from a complex in the B38 strain, and also proteins from the hor family rarely studied. BabA and BabB adhesins were found to interact independently with HopM/N in the B38 and J99 strains, respectively. The 46 cytosolic complexes essentially comprised proteins involved in H. pylori physiology. Some orphan proteins were retrieved from heterooligomeric complexes, and a function could be proposed for a number of them via the identification of their partners, such as JHP0119, which may be involved in the flagellar function. Overall, this study gave new insights into the membrane and cytoplasm structure, and those which could help in the design of molecules for vaccine and/or antimicrobial agent development are highlighted.

Effector prediction in host-pathogen interaction based on a Markov model of a ubiquitous EPIYA motif

Background

Effector secretion is a common strategy of pathogen in mediating host-pathogen interaction. Eight EPIYA-motif containing effectors have recently been discovered in six pathogens. Once these effectors enter host cells through type III/IV secretion systems (T3SS/T4SS), tyrosine in the EPIYA motif is phosphorylated, which triggers effectors binding other proteins to manipulate host-cell functions. The objectives of this study are to evaluate the distribution pattern of EPIYA motif in broad biological species, to predict potential effectors with EPIYA motif, and to suggest roles and biological functions of potential effectors in host-pathogen interactions.

Results

A hidden Markov model (HMM) of five amino acids was built for the EPIYA-motif based on the eight known effectors. Using this HMM to search the non-redundant protein database containing 9,216,047 sequences, we obtained 107,231 sequences with at least one EPIYA motif occurrence and 3115 sequences with multiple repeats of the EPIYA motif. Although the EPIYA motif exists among broad species, it is significantly over-represented in some particular groups of species. For those proteins containing at least four copies of EPIYA motif, most of them are from intracellular bacteria, extracellular bacteria with T3SS or T4SS or intracellular protozoan parasites. By combining the EPIYA motif and the adjacent SH2 binding motifs (KK, R4, Tarp and Tir), we built HMMs of nine amino acids and predicted many potential effectors in bacteria and protista by the HMMs. Some potential effectors for pathogens (such as Lawsonia intracellularis, Plasmodium falciparum and Leishmania major) are suggested.

Conclusions

Our study indicates that the EPIYA motif may be a ubiquitous functional site for effectors that play an important pathogenicity role in mediating host-pathogen interactions. We suggest that some intracellular protozoan parasites could secrete EPIYA-motif containing effectors through secretion systems similar to the T3SS/T4SS in bacteria. Our predicted effectors provide useful hypotheses for further studies.

Interbacterial macromolecular transfer by the Campylobacter fetus subsp. venerealis type IV secretion system

We report here the first demonstration of intra- and interspecies conjugative plasmid DNA transfer for Campylobacter fetus. Gene regions carried by a Campylobacter coli plasmid were identified that are sufficient for conjugative mobilization to Escherichia coli and C. fetus recipients. A broader functional range is predicted. Efficient DNA transfer involves the virB9 and virD4 genes of the type IV bacterial secretion system encoded by a pathogenicity island of C. fetus subsp. venerealis. Complementation of these phenotypes from expression constructions based on the promoter of the C. fetus surface antigen protein (sap) locus was temperature dependent, and a temperature regulation of the sap promoter was subsequently confirmed under laboratory conditions. Gene transfer was sensitive to surface or entry exclusion functions in potential recipient cells carrying IncPα plasmid RP4 implying functional relatedness to C. fetus proteins. The virB/virD4 locus is also known to be involved in bacterial invasion and killing of cultured human cells in vitro. Whether specifically secreted effector proteins contribute to host colonization and infection activities is currently unknown. Two putative effector proteins carrying an FIC domain conserved in a few bacterial type III and type IV secreted proteins of pathogens were analyzed for secretion by the C. fetus or heterologous conjugative systems. No evidence for interbacterial translocation of the Fic proteins was found.

Modulation of host cell function by Legionella pneumophila type IV effectors

Macrophages and protozoa ingest bacteria by phagocytosis and destroy these microbes using a conserved pathway that mediates fusion of the phagosome with lysosomes. To survive within phagocytic host cells, bacterial pathogens have evolved a variety of strategies to avoid fusion with lysosomes. A virulence strategy used by the intracellular pathogen Legionella pneumophila is to manipulate host cellular processes using bacterial proteins that are delivered into the cytosolic compartment of the host cell by a specialized secretion system called Dot/Icm. The proteins delivered by the Dot/Icm system target host factors that play evolutionarily conserved roles in controlling membrane transport in eukaryotic cells, which enables L. pneumophila to create an endoplasmic reticulum-like vacuole that supports intracellular replication in both protozoan and mammalian host cells. This review focuses on intracellular trafficking of L. pneumophila and describes how bacterial proteins contribute to modulation of host processes required for survival within host cells.

Temperature induced bacterial virulence and bleaching disease in a chemically defended marine macroalga

Host-pathogen interactions have been widely studied in humans and terrestrial plants, but are much less well explored in marine systems. Here we show that a marine macroalga, Delisea pulchra, utilizes a chemical defence - furanones - to inhibit colonization and infection by a novel bacterial pathogen, Ruegeria sp. R11, and that infection by R11 is temperature dependent. Ruegeria sp. R11 formed biofilms, invaded and bleached furanone-free, but not furanone-producing D. pulchra thalli, at high (24°C) but not low (19°C) temperatures. Bleaching is commonly observed in natural populations of D. pulchra near Sydney, Australia, during the austral summer when ocean temperatures are at their peak and the chemical defences of the alga are reduced. Furanones, produced by D. pulchra as a chemical defence, inhibit quorum sensing (QS) in bacteria, and this may play a role in furanone inhibition of R11 infection of furanone-free thalli as R11 produces QS signals. This interplay between temperature, an algal chemical defence mechanism and bacterial virulence demonstrates the complex impact environmental change can have on an ecosystem.

Molecular recognition determinants for type IV secretion of diverse families of conjugative relaxases

In preparation for transfer conjugative type IV secretion systems (T4SS) produce a nucleoprotein adduct containing a relaxase enzyme covalently linked to the 5' end of single-stranded plasmid DNA. The bound relaxase is expected to present features necessary for selective recognition by the type IV coupling protein (T4CP), which controls substrate entry to the envelope spanning secretion machinery. We prove that the IncF plasmid R1 relaxase TraI is translocated to the recipient cells. Using a Cre recombinase assay (CRAfT) we mapped two internally positioned translocation signals (TS) on F-like TraI proteins that independently mediate efficient recognition and secretion. Tertiary structure predictions for the TS matched best helicase RecD2 from Deinococcus radiodurans. The TS is widely conserved in MOB(F) and MOB(Q) families of relaxases. Structure/function relationships within the TS were identified by mutation. A key residue in specific recognition by T4CP TraD was revealed by a fidelity switch phenotype for an F to plasmid R1 exchange L626H mutation. Finally, we show that physical linkage of the relaxase catalytic domain to a TraI TS is necessary for efficient conjugative transfer.

Legionella pneumophila strain 130b possesses a unique combination of type IV secretion systems and novel Dot/Icm secretion system effector proteins

Legionella pneumophila is a ubiquitous inhabitant of environmental water reservoirs. The bacteria infect a wide variety of protozoa and, after accidental inhalation, human alveolar macrophages, which can lead to severe pneumonia. The capability to thrive in phagocytic hosts is dependent on the Dot/Icm type IV secretion system (T4SS), which translocates multiple effector proteins into the host cell. In this study, we determined the draft genome sequence of L. pneumophila strain 130b (Wadsworth). We found that the 130b genome encodes a unique set of T4SSs, namely, the Dot/Icm T4SS, a Trb-1-like T4SS, and two Lvh T4SS gene clusters. Sequence analysis substantiated that a core set of 107 Dot/Icm T4SS effectors was conserved among the sequenced L. pneumophila strains Philadelphia-1, Lens, Paris, Corby, Alcoy, and 130b. We also identified new effector candidates and validated the translocation of 10 novel Dot/Icm T4SS effectors that are not present in L. pneumophila strain Philadelphia-1. We examined the prevalence of the new effector genes among 87 environmental and clinical L. pneumophila isolates. Five of the new effectors were identified in 34 to 62% of the isolates, while less than 15% of the strains tested positive for the other five genes. Collectively, our data show that the core set of conserved Dot/Icm T4SS effector proteins is supplemented by a variable repertoire of accessory effectors that may partly account for differences in the virulences and prevalences of particular L. pneumophila strains.

Interactions between bacterial pathogens and mitochondrial cell death pathways

The modulation of host cell death pathways by bacteria has been recognized as a major pathogenicity mechanism. Among other strategies, bacterial pathogens can hijack the cell death machinery of host cells by influencing the signalling pathways that converge on the mitochondria. In particular, many bacterial proteins have evolved to interact in a highly specific manner with host mitochondria, thereby modulating the decision between cell life and death. In this Review, we explore the intimate interactions between bacterial pathogens and mitochondrial cell death pathways.

A small fibronectin-mimicking protein from bacteria induces cell spreading and focal adhesion formation

Fibronectin, a 250-kDa eukaryotic extracellular matrix protein containing an RGD motif plays crucial roles in cell-cell communication, development, tissue homeostasis, and disease development. The highly complex fibrillar fibronectin meshwork orchestrates the functions of other extracellular matrix proteins, promoting cell adhesion, migration, and intracellular signaling. Here, we demonstrate that CagL, a 26-kDa protein of the gastric pathogen and type I carcinogen Helicobacter pylori, mimics fibronectin in various cellular functions. Like fibronectin, CagL contains a RGD motif and is located on the surface of the bacterial type IV secretion pili as previously shown. CagL binds to the integrin receptor alpha(5)beta(1) and mediates the injection of virulence factors into host target cells. We show that purified CagL alone can directly trigger intracellular signaling pathways upon contact with mammalian cells and can complement the spreading defect of fibronectin(-/-) knock-out cells in vitro. During interaction with various human and mouse cell lines, CagL mimics fibronectin in triggering cell spreading, focal adhesion formation, and activation of several tyrosine kinases in an RGD-dependent manner. Among the activated factors are the nonreceptor tyrosine kinases focal adhesion kinase and Src but also the epidermal growth factor receptor and epidermal growth factor receptor family member Her3/ErbB3. Interestingly, fibronectin activates a similar range of tyrosine kinases but not Her3/ErbB3. These findings suggest that the bacterial protein CagL not only exhibits functional mimicry with fibronectin but is also capable of activating fibronectin-independent signaling events. We thus postulate that CagL may contribute directly to H. pylori pathogenesis by promoting aberrant signaling cross-talk within host cells.

Type IV secretion systems: versatility and diversity in function

Type IV secretion systems (T4SSs) are large protein complexes which traverse the cell envelope of many bacteria. They contain a channel through which proteins or protein–DNA complexes can be translocated. This translocation is driven by a number of cytoplasmic ATPases which might energize large conformational changes in the translocation complex. The family of T4SSs is very versatile, shown by the great variety of functions among family members. Some T4SSs are used by pathogenic Gram-negative bacteria to translocate a wide variety of virulence factors into the host cell. Other T4SSs are utilized to mediate horizontal gene transfer, an event that greatly facilitates the adaptation to environmental changes and is the basis for the spread of antibiotic resistance among bacteria. Here we review the recent advances in the characterization of the architecture and mechanism of substrate transfer in a few representative T4SSs with a particular focus on their diversity of structure and function.

Molecular architecture of bacterial type IV secretion systems

In Gram-negative bacteria, type IV secretion (T4S) systems form ATP-powered complexes that span the entire cellular envelope and secrete a wide variety of substrates from single proteins to protein-protein and protein-DNA complexes. Recent structural data, namely the electron microscopy structure of the T4S core complex and the atomic-resolution structure of its outer-membrane pore, have profoundly altered our understanding of T4S architecture and mechanisms.

Type IV secretion machinery promotes ton-independent intracellular survival of Neisseria gonorrhoeae within cervical epithelial cells

Survival of Neisseria gonorrhoeae within host epithelial cells is expected to be important in the pathogenesis of gonococcal disease. We previously demonstrated that strain FA1090 derives iron from a host cell in a process that requires the Ton complex and a putative TonB-dependent transporter, TdfF. FA1090, however, lacks the gonococcal genetic island (GGI) that is present in the majority of strains. The GGI in strain MS11 has been partially characterized, and it encodes a type IV secretion system (T4SS) involved in DNA release. In this study we investigated the role of iron acquisition and GGI-encoded gene products in gonococcal survival within cervical epithelial cells. We demonstrated that intracellular survival of MS11 was dependent on acquisition of iron from the host cell, but unlike the findings for FA1090, expression of the Ton complex was not required. Survival was not dependent on a putative TonB-like protein encoded in the GGI but instead was directly linked to T4SS structural components in a manner independent of the ability to release or internalize DNA. These data suggest that expression of selected GGI-encoded open reading frames confers an advantage during cervical cell infection. This study provides the first link between expression of the T4SS apparatus and intracellular survival of gonococci.

Proteomic screening for possible effector molecules secreted by the obligate intracellular pathogen Coxiella burnetii

Coxiella burnetii is a Gram-negative, gamma-proteobacteria with nearly worldwide distribution, and it is the pathogenic agent of Q-fever in man. It is an obligate intracellular parasite that is highly adapted to reside within the eukaryotic phagolysosome. In fact, it is the only known intracellular bacterium that manages to survive and replicate within a fully formed, acidic phagolysosome. C. burnetti possesses a functional Type 4 Secretion System (T4SS), similar to the Dot/Icm system of Legionella pneumophila. Up to date there have been no reports for the effector molecules secreted by Coxiella's T4SS. These are speculated to have quite different roles than the effectors of other intracellular pathogens, since there is no need for phagosomal arrest or escape in the case of Coxiella. In this study, we have investigated the cytoplasm of Vero cells infected with C. burnetti strain Nine Mile Phase II. We have identified by mass spectrometry (ESI-MS/MS) several C. burnetti proteins that bear typical characteristics of effector molecules. Most of the identified proteins were also very alkaline, something which is supportive for a protective strategy that has evolved in this bizarre pathogen against acidic environments.

The versatility of Helicobacter pylori CagA effector protein functions: The master key hypothesis

Several bacterial pathogens inject virulence proteins into host target cells that are substrates of eukaryotic tyrosine kinases. One of the key examples is the Helicobacter pylori CagA effector protein which is translocated by a type-IV secretion system. Injected CagA becomes tyrosine-phosphorylated on EPIYA sequence motifs by Src and Abl family kinases. CagA then binds to and activates/inactivates multiple signaling proteins in a phosphorylation-dependent and phosphorylation-independent manner. A recent proteomic screen systematically identified eukaryotic binding partners of the EPIYA phosphorylation sites of CagA and similar sites in other bacterial effectors by high-resolution mass spectrometry. Individual phosphorylation sites recruited a surprisingly high number of interaction partners suggesting that each phosphorylation site can interfere with many downstream pathways. We now count 20 reported cellular binding partners of CagA, which represents the highest quantitiy among all yet known virulence-associated effector proteins in the microbial world. This complexity generates a highly remarkable and puzzling scenario. In addition, the first crystal structure of CagA provided us with new information on the function of this important virulence determinant. Here we review the recent advances in characterizing the multiple binding signaling activities of CagA. Injected CagA can act as a 'master key' that evolved the ability to highjack multiple host cell signalling cascades, which include the induction of membrane dynamics, actin-cytoskeletal rearrangements and the disruption of cell-to-cell junctions as well as proliferative, pro-inflammatory and anti-apoptotic nuclear responses. The discovery that different pathogens use this common strategy to subvert host cell functions suggests that more examples will emerge soon.

Novel insights into the genomic basis of citrus canker based on the genome sequences of two strains of Xanthomonas fuscans subsp. aurantifolii

BACKGROUND

Citrus canker is a disease that has severe economic impact on the citrus industry worldwide. There are three types of canker, called A, B, and C. The three types have different phenotypes and affect different citrus species. The causative agent for type A is Xanthomonas citri subsp. citri, whose genome sequence was made available in 2002. Xanthomonas fuscans subsp. aurantifolii strain B causes canker B and Xanthomonas fuscans subsp. aurantifolii strain C causes canker C.

RESULTS

We have sequenced the genomes of strains B and C to draft status. We have compared their genomic content to X. citri subsp. citri and to other Xanthomonas genomes, with special emphasis on type III secreted effector repertoires. In addition to pthA, already known to be present in all three citrus canker strains, two additional effector genes, xopE3 and xopAI, are also present in all three strains and are both located on the same putative genomic island. These two effector genes, along with one other effector-like gene in the same region, are thus good candidates for being pathogenicity factors on citrus. Numerous gene content differences also exist between the three cankers strains, which can be correlated with their different virulence and host range. Particular attention was placed on the analysis of genes involved in biofilm formation and quorum sensing, type IV secretion, flagellum synthesis and motility, lipopolysacharide synthesis, and on the gene xacPNP, which codes for a natriuretic protein.

CONCLUSION

We have uncovered numerous commonalities and differences in gene content between the genomes of the pathogenic agents causing citrus canker A, B, and C and other Xanthomonas genomes. Molecular genetics can now be employed to determine the role of these genes in plant-microbe interactions. The gained knowledge will be instrumental for improving citrus canker control.

Characterization of peptidoglycan hydrolase in Cag pathogenicity island of Helicobacter pylori

The Cag Type IV secretion apparatus proteins in Helicobacter pylori can mediate the injection of effector CagA protein into eukaryotic target cells. Although this apparatus forms an important pathway for bacterium-host interaction, its assembly process in vivo is poorly understood, and the proteins which contribute to break the bacterial cell walls in Cag-PAI have not yet been identified. The cagγ gene in Cag-PAI is a unique member that contains a conserved SLT catalysis domain, which makes it an attracting question whether cagy gene has the capacity to digest the bacterial cell wall. In the current study, therefore, the cagγ gene was cloned from the H. pylori NCTC 11637 and expressed in Escherichia coli, and its lytic effect on cell walls in vitro was observed. Results indicated that Cagγ protein has a lytic activity against bacterial cell walls. An allelic-exchange mutant (Δcagγ) was further constructed to investigate the relationship between Cagγ and effector CagA translocation. These results suggested that Cagγ contributed to the assembly of Cag Type IV secretion apparatus by digesting the peptidoglycan meshwork of bacterial cell walls.

Increased expression of the type IV secretion system in piliated Neisseria gonorrhoeae variants

Neisseria gonorrhoeae produces a type IV secretion system that secretes chromosomal DNA. The secreted DNA is active in the transformation of other gonococci in the population and may act to transfer antibiotic resistance genes and variant alleles for surface antigens, as well as other genes. We observed that gonococcal variants that produced type IV pili secreted more DNA than variants that were nonpiliated, suggesting that the process may be regulated. Using microarray analysis, we found that a piliated strain showed increased expression of the gene for the putative type IV secretion coupling protein TraD, whereas a nonpiliated variant showed increased expression of genes for transcriptional and translational machinery, consistent with its higher growth rate compared to that of the piliated strain. These results suggested that type IV secretion might be controlled by either traD expression or growth rate. A mutant with a deletion in traD was found to be deficient in DNA secretion. Further mutation and complementation analysis indicated that traD is transcriptionally and translationally coupled to traI, which encodes the type IV secretion relaxase. We were able to increase DNA secretion in a nonpiliated strain by inserting a gene cassette with a strong promoter to drive the expression of the putative operon containing traI and traD. Together, these data suggest a model in which the type IV secretion system apparatus is made constitutively, while its activity is controlled through regulation of traD and traI.

Whole genome evaluation of horizontal transfers in the pathogenic fungus Aspergillus fumigatus

Background

Numerous cases of horizontal transfers (HTs) have been described for eukaryote genomes, but in contrast to prokaryote genomes, no whole genome evaluation of HTs has been carried out. This is mainly due to a lack of parametric methods specially designed to take the intrinsic heterogeneity of eukaryote genomes into account. We applied a simple and tested method based on local variations of genomic signatures to analyze the genome of the pathogenic fungus Aspergillus fumigatus.

Results

We detected 189 atypical regions containing 214 genes, accounting for about 1 Mb of DNA sequences. However, the fraction of atypical DNA detected was smaller than the average amount detected in the same conditions in prokaryote genomes (3.1% vs 5.6%). It appeared that about one third of these regions contained no annotated genes, a proportion far greater than in prokaryote genomes. When analyzing the origin of these HTs by comparing their signatures to a home made database of species signatures, 3 groups of donor species emerged: bacteria (40%), fungi (25%), and viruses (22%). It is to be noticed that though inter-domain exchanges are confirmed, we only put in evidence very few exchanges between eukaryotic kingdoms.

Conclusions

In conclusion, we demonstrated that HTs are not negligible in eukaryote genomes, bearing in mind that in our stringent conditions this amount is a floor value, though of a lesser extent than in prokaryote genomes. The biological mechanisms underlying those transfers remain to be elucidated as well as the biological functions of the transferred genes.

the versatility of the Helicobacter pylori vacuolating cytotoxin vacA in signal transduction and molecular crosstalk

By modulating important properties of eukaryotic cells, many bacterial protein toxins highjack host signalling pathways to create a suitable niche for the pathogen to colonize and persist. Helicobacter pylori VacA is paradigm of pore-forming toxins which contributes to the pathogenesis of peptic ulceration. Several cellular receptors have been described for VacA, which exert different effects on epithelial and immune cells. The crystal structure of VacA p55 subunit might be important for elucidating details of receptor interaction and pore formation. Here we discuss the multiple signalling activities of this important toxin and the molecular crosstalk between VacA and other virulence factors.

Anaplasma phagocytophilum and Ehrlichia chaffeensis type IV secretion and Ank proteins

The obligatory intracellular bacterial pathogens Anaplasma and Ehrlichia infect leukocytes by hijacking host-cell components and processes. The type IV secretion system is up-regulated during infection. Among type IV secretion candidate substrates, an ankyrin repeat protein of Anaplasma phagocytophilum, AnkA, is delivered into the host cytoplasm via a complex that includes VirD4. AnkA is highly tyrosine phosphorylated and binds to the Abl interactor 1, SHP-1, and nuclear DNA fragments. Ehrlichia chaffeensis AnkA was recently reported to be translocated into host-cell nucleus. The recent discovery of several ankyrin repeat proteins secreted via the type IV secretion system of different intracellular bacteria suggests that a common strategy evolved to subvert host-cell functions.

Toxins from bacteria

Bacterial toxins damage the host at the site of bacterial infection or distant from the site. Bacterial toxins can be single proteins or oligomeric protein complexes that are organized with distinct AB structure-function properties. The A domain encodes a catalytic activity. ADP ribosylation of host proteins is the earliest post-translational modification determined to be performed by bacterial toxins; other modifications include glucosylation and proteolysis. Bacterial toxins also catalyze the non-covalent modification of host protein function or can modify host cell properties through direct protein-protein interactions. The B domain includes two functional domains: a receptor-binding domain, which defines the tropism of a toxin for a cell and a translocation domain that delivers the A domain across a lipid bilayer, either on the plasma membrane or the endosome. Bacterial toxins are often characterized based upon the secretion mechanism that delivers the toxin out of the bacterium, termed types I-VII. This review summarizes the major families of bacterial toxins and also describes the specific structure-function properties of the botulinum neurotoxins.

Subversion of innate immune responses by Brucella through the targeted degradation of the TLR signaling adapter, MAL

Gram-negative bacteria belonging to the Brucella species cause chronic infections that can result in undulant fever, arthritis, and osteomyelitis in humans. Remarkably, Brucella sp. genomes encode a protein, named TcpB, that bears significant homology with mammalian Toll/IL-1 receptor domains and whose expression causes degradation of the phosphorylated, signal competent form of the adapter MyD88-adapter-like (MAL). This effect of TcpB is mediated through its box 1 region and has no effect on other TLR adapter proteins such as MyD88 or TIR-domain containing adapter protein-inducing IFNbeta. TcpB also does not affect a mutant, signal-incompetent form of MAL that cannot be phosphorylated. Interestingly, the presence of TcpB leads to enhanced polyubiquitination of MAL, which is likely responsible for its accelerated degradation. A Brucella abortus mutant lacking TcpB fails to reduce levels of MAL in infected macrophages. Therefore, TcpB represents a unique pathogen-derived molecule that suppresses host innate-immune responses by specifically targeting an individual adapter molecule in the TLR signaling pathway for degradation.

Virulence factors encoded by Legionella longbeachae identified on the basis of the genome sequence analysis of clinical isolate D-4968

Legionella longbeachae causes most cases of legionellosis in Australia and may be underreported worldwide due to the lack of L. longbeachae-specific diagnostic tests. L. longbeachae displays distinctive differences in intracellular trafficking, caspase 1 activation, and infection in mouse models compared to Legionella pneumophila, yet these two species have indistinguishable clinical presentations in humans. Unlike other legionellae, which inhabit freshwater systems, L. longbeachae is found predominantly in moist soil. In this study, we sequenced and annotated the genome of an L. longbeachae clinical isolate from Oregon, isolate D-4968, and compared it to the previously published genomes of L. pneumophila. The results revealed that the D-4968 genome is larger than the L. pneumophila genome and has a gene order that is different from that of the L. pneumophila genome. Genes encoding structural components of type II, type IV Lvh, and type IV Icm/Dot secretion systems are conserved. In contrast, only 42/140 homologs of genes encoding L. pneumophila Icm/Dot substrates have been found in the D-4968 genome. L. longbeachae encodes numerous proteins with eukaryotic motifs and eukaryote-like proteins unique to this species, including 16 ankyrin repeat-containing proteins and a novel U-box protein. We predict that these proteins are secreted by the L. longbeachae Icm/Dot secretion system. In contrast to the L. pneumophila genome, the L. longbeachae D-4968 genome does not contain flagellar biosynthesis genes, yet it contains a chemotaxis operon. The lack of a flagellum explains the failure of L. longbeachae to activate caspase 1 and trigger pyroptosis in murine macrophages. These unique features of L. longbeachae may reflect adaptation of this species to life in soil.

Helicobacter pylori CagA upregulation of CIP2A is dependent on the Src and MEK/ERK pathways

Helicobacter pylori is classified as a class I carcinogenic factor and its persistent colonization in the stomach induces gastric cancer. Cancerous Inhibitor of PP2A (CIP2A) is a newly identified oncoprotein overexpressed in gastric cancer. Serving as a key oncoprotein, CIP2A also participates in regulation of senescence and proliferation of gastric cells. The combination of aberrant CIP2A expression inducing unlimited cell proliferation, and H. pylori infection eliciting aberrant expression of some key proteins, results in the onset of gastric tumorigenesis. However, the relationship between H. pylori infection and CIP2A expression still remains undefined. The aim of our study was to verify the effect of H. pylori infection on CIP2A expression levels and identify H. pylori signalling molecules and corresponding pathways influencing CIP2A expression. Following plasmid-mediated expression of CagA in human gastric cell lines, the cells were infected with H. pylori and CIP2A expression levels were examined by immunoblotting. Signal inhibitors were used to verify which signal pathways were involved. We also performed CIP2A depletion and H. pylori infection after depletion in AGS cells. H. pylori infection-induced CIP2A expression was dependent on cagA gene expression and CagA phosphorylation. Bacterial oncoprotein CagA upregulated CIP2A expression and this upregulation effect was dependent on Src and Ras/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathways. H. pylori infection-induced Myc stabilization was partially attenuated by CIP2A depletion. The results of our study provide further information for understanding the mechanism of H. pylori carcinogenesis.

Two type IV secretion systems with different functions in Burkholderia cenocepacia K56-2

Bacterial type IV secretion systems (T4SS) perform two fundamental functions related to pathogenesis: the delivery of effector molecules to eukaryotic target cells, and genetic exchange. Two T4SSs have been identified in Burkholderia cenocepacia K56-2, a representative of the ET12 lineage of the B. cepacia complex (Bcc). The plant tissue watersoaking (Ptw) T4SS encoded on a resident 92 kb plasmid is a chimera composed of VirB/D4 and F-specific subunits, and is responsible for the translocation of effector(s) that have been linked to the Ptw phenotype. The bc-VirB/D4 system located on chromosome II displays homology to the VirB/D4 T4SS of Agrobacterium tumefaciens. In contrast to the Ptw T4SS, the bc-VirB/D4 T4SS was found to be dispensable for Ptw effector(s) secretion, but was found to be involved in plasmid mobilization. The fertility inhibitor Osa did not affect the secretion of Ptw effector(s) via the Ptw system, but did disrupt the mobilization of a RSF1010 derivative plasmid.

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

A genomic island defines subspecies-specific virulence features of the host-adapted pathogen Campylobacter fetus subsp. venerealis

The pathogen Campylobacter fetus comprises two subspecies, C. fetus subsp. fetus and C. fetus subsp. venerealis. Although these taxa are highly related on the genome level, they are adapted to distinct hosts and tissues. C. fetus subsp. fetus infects a diversity of hosts, including humans, and colonizes the gastrointestinal tract. In contrast, C. fetus subsp. venerealis is largely restricted to the bovine genital tract, causing epidemic abortion in these animals. In light of their close genetic relatedness, the specific niche preferences make the C. fetus subspecies an ideal model system to investigate the molecular basis of host adaptation. In this study, a subtractive-hybridization approach was applied to the genomes of the subspecies to identify different genes potentially underlying this specificity. The comparison revealed a genomic island uniquely present in C. fetus subsp. venerealis that harbors several genes indicative of horizontal transfer and that encodes the core components necessary for bacterial type IV secretion. Macromolecular transporters of this type deliver effector molecules to host cells, thereby contributing to virulence in various pathogens. Mutational inactivation of the putative secretion system confirmed its involvement in the pathogenicity of C. fetus subsp. venerealis.

Genome sequence of the versatile fish pathogen Edwardsiella tarda provides insights into its adaptation to broad host ranges and intracellular niches

BACKGROUND

Edwardsiella tarda is the etiologic agent of edwardsiellosis, a devastating fish disease prevailing in worldwide aquaculture industries. Here we describe the complete genome of E. tarda, EIB202, a highly virulent and multi-drug resistant isolate in China.

METHODOLOGY/PRINCIPAL FINDINGS

E. tarda EIB202 possesses a single chromosome of 3,760,463 base pairs containing 3,486 predicted protein coding sequences, 8 ribosomal rRNA operons, and 95 tRNA genes, and a 43,703 bp conjugative plasmid harboring multi-drug resistant determinants and encoding type IV A secretion system components. We identified a full spectrum of genetic properties related to its genome plasticity such as repeated sequences, insertion sequences, phage-like proteins, integrases, recombinases and genomic islands. In addition, analysis also indicated that a substantial proportion of the E. tarda genome might be devoted to the growth and survival under diverse conditions including intracellular niches, with a large number of aerobic or anaerobic respiration-associated proteins, signal transduction proteins as well as proteins involved in various stress adaptations. A pool of genes for secretion systems, pili formation, nonfimbrial adhesions, invasions and hemagglutinins, chondroitinases, hemolysins, iron scavenging systems as well as the incomplete flagellar biogenesis might feature its surface structures and pathogenesis in a fish body.

CONCLUSION/SIGNIFICANCE

Genomic analysis of the bacterium offered insights into the phylogeny, metabolism, drug-resistance, stress adaptation, and virulence characteristics of this versatile pathogen, which constitutes an important first step in understanding the pathogenesis of E. tarda to facilitate construction of a practical effective vaccine used for combating fish edwardsiellosis.

Helicobacter pylori dupA gene is not associated with clinical outcomes in the Japanese population

The dupA gene of Helicobacter pylori was suggested to be a risk factor for duodenal ulcer but protective against gastric cancer. The present study aimed to re-examine the role of dupA in H. pylori-infected Japanese patients. We found that dupA status was not associated with any gastroduodenal disease, histological score of chronic gastritis or with the extent of interleukin-8 production from gastric cell lines. These results indicate that dupA is unlikely to be a virulence factor of H. pylori in the Japanese population.

Helicobacter pylori counteracts the apoptotic action of its VacA toxin by injecting the CagA protein into gastric epithelial cells

Infection with Helicobacter pylori is responsible for gastritis and gastroduodenal ulcers but is also a high risk factor for the development of gastric adenocarcinoma and lymphoma. The most pathogenic H. pylori strains (i.e., the so-called type I strains) associate the CagA virulence protein with an active VacA cytotoxin but the rationale for this association is unknown. CagA, directly injected by the bacterium into colonized epithelium via a type IV secretion system, leads to cellular morphological, anti-apoptotic and proinflammatory effects responsible in the long-term (years or decades) for ulcer and cancer. VacA, via pinocytosis and intracellular trafficking, induces epithelial cell apoptosis and vacuolation. Using human gastric epithelial cells in culture transfected with cDNA encoding for either the wild-type 38 kDa C-terminal signaling domain of CagA or its non-tyrosine-phosphorylatable mutant form, we found that, depending on tyrosine-phosphorylation by host kinases, CagA inhibited VacA-induced apoptosis by two complementary mechanisms. Tyrosine-phosphorylated CagA prevented pinocytosed VacA to reach its target intracellular compartments. Unphosphorylated CagA triggered an anti-apoptotic activity blocking VacA-induced apoptosis at the mitochondrial level without affecting the intracellular trafficking of the toxin. Assaying the level of apoptosis of gastric epithelial cells infected with wild-type CagA⁺/VacA⁺H. pylori or isogenic mutants lacking of either CagA or VacA, we confirmed the results obtained in cells transfected with the CagA C-ter constructions showing that CagA antagonizes VacA-induced apoptosis. VacA toxin plays a role during H. pylori stomach colonization. However, once bacteria have colonized the gastric niche, the apoptotic action of VacA might be detrimental for the survival of H. pylori adherent to the mucosa. CagA association with VacA is thus a novel, highly ingenious microbial strategy to locally protect its ecological niche against a bacterial virulence factor, with however detrimental consequences for the human host.

The gram-negative bacterium Helicobacter pylori is the main causative agent of peptic ulcer and gastric cancer in humans. Our work sheds light on a new molecular mechanism by which H. pylori would exert its highly efficient colonization strategy of the human host. In this paper, we show that the H. pylori CagA protein counteracts, by two distinct non-overlapping mechanisms, the apoptotic activity of the H. pylori VacA toxin on human gastric epithelial cells so as to allow a protection of the bacterium niche against VacA, giving a rationale for the association of these two virulence factors in the most pathogenic H. pylori strains. This is a new, highly ingenious mechanism by which a bacterium locally protects its ecological niche against the action of one of its own virulence factors. However, while exerting a beneficial role for survival and growth of the bacterium by counteracting VacA toxin, CagA injection in the gastric epithelial cells triggers proinflammatory and anti-apoptotic responses which are detrimental for the human host in the long-term and favor the development of ulcer and cancer.

The structural biology of type IV secretion systems

Type IV secretion systems (T4SSs) are versatile secretion systems that are found in both Gram-negative and Gram-positive bacteria and secrete a wide range of substrates, from single proteins to protein-protein and protein-DNA complexes. They usually consist of 12 components that are organized into ATP-powered, double-membrane-spanning complexes. The structures of single soluble components or domains have been solved, but an understanding of how these structures come together has only recently begun to emerge. This Review focuses on the structural advances that have been made over the past 10 years and how the corresponding structural insights have helped to elucidate many of the details of the mechanism of type IV secretion.

Interactions of Burkholderia cenocepacia and other Burkholderia cepacia complex bacteria with epithelial and phagocytic cells

Burkholderia cenocepacia is a member of the B. cepacia complex (Bcc), a group of opportunistic bacteria that infect the airways of patients with cystic fibrosis (CF) and are extraordinarily resistant to almost all clinically useful antibiotics. Infections in CF patients with Bcc bacteria generally lead to a more rapid decline in lung function, and in some cases to the ‘cepacia syndrome’, a virtually deadly exacerbation of the lung infection with systemic manifestations. These characteristics of Bcc bacteria contribute to higher morbidity and mortality in infected CF patients. In the last 10 years considerable progress has been made in understanding the interactions between Bcc bacteria and mammalian host cells. Bcc isolates can survive either intracellularly within eukaryotic cells or extracellularly in host tissues. They survive within phagocytes and respiratory epithelial cells, and they have the ability to breach the respiratory epithelium layer. Survival and persistence of Bcc bacteria within host cells and tissues are believed to play a key role in pulmonary infection and to contribute to the persistent inflammation observed in patients with CF. This review summarizes recent findings concerning the interaction between Bcc bacteria and epithelial and phagocytic cells.

Advances in research on Helicobacter pylori virulence factors

Helicobacter pylori (H. pylori) infection is a common chronic infectious disease in the world. It can lead to several divergent clinical diseases, such as chronic gastritis, duodenal ulcer, mucosa-associated lymphoid tissue lymphoma and gastric adenocarcinoma. These divergent clinical diseases are caused through complex mechanisms involving interaction between the bacterium and host. Recent investigations of virulence pathogenic factors have provided more information to reveal the pathogenic mechanism of H. pylori infection. Here, we will review the recent advances in research on various H. pylori virulence factors, such as CagA, VacA, BabA, SabA, OipA and DupA.

Analysis of complete genome sequence of Neorickettsia risticii: causative agent of Potomac horse fever

Neorickettsia risticii is an obligate intracellular bacterium of the trematodes and mammals. Horses develop Potomac horse fever (PHF) when they ingest aquatic insects containing encysted N. risticii-infected trematodes. The complete genome sequence of N. risticii Illinois consists of a single circular chromosome of 879 977 bp and encodes 38 RNA species and 898 proteins. Although N. risticii has limited ability to synthesize amino acids and lacks many metabolic pathways, it is capable of making major vitamins, cofactors and nucleotides. Comparison with its closely related human pathogen N. sennetsu showed that 758 (88.2%) of protein-coding genes are conserved between N. risticii and N. sennetsu. Four-way comparison of genes among N. risticii and other Anaplasmataceae showed that most genes are either shared among Anaplasmataceae (525 orthologs that generally associated with housekeeping functions), or specific to each genome (>200 genes that are mostly hypothetical proteins). Genes potentially involved in the pathogenesis of N. risticii were identified, including those encoding putative outer membrane proteins, two-component systems and a type IV secretion system (T4SS). The bipolar localization of T4SS pilus protein VirB2 on the bacterial surface was demonstrated for the first time in obligate intracellular bacteria. These data provide insights toward genomic potential of N. risticii and intracellular parasitism, and facilitate our understanding of PHF pathogenesis.

Type IV secretion system of Anaplasma phagocytophilum and Ehrlichia chaffeensis

The intracellular bacterial pathogens Ehrlichia chaffeensis and Anaplasma phagocytophilum have evolved to infect leukocytes and hijack biological compounds and processes of these host defensive cells. Bacterial type IV secretion (T4S) system transports macromolecules across the membrane in an ATP-dependent manner and is increasingly recognized as a virulence factor delivery mechanism that allows pathogens to modulate eukaryotic cell functions for their own benefit. Genes encoding T4S system homologous to those of a plant pathogen Agrobacterium tumefaciens have been identified in E. chaffeensis and A. phagocytophilum. Upon interaction with new host cells, E. chaffeensis and A. phagocytophilum genes encoding the T4S apparatus are upregulated. The delivered macromolecules are referred to as T4S substrates, or effectors, because they affect and alter basic host cellular processes, resulting in disease development. Recently, A. phagocytophilum 160-kDa AnkA protein was to be delivered by T4S system into the host cytoplasm. Thus, dynamic signal transduction events are likely induced by T4S substrates in the host cells for successful establishment of intracellular infection. Further studies on Ehrlichia and Anaplasma T4S effectors cognate host cell molecules will undoubtedly advance our understanding of the complex interplay between obligatory intracellular pathogens and their hosts. Such data can be applied toward treatment, diagnosis, and control of ehrlichiosis and anaplasmosis.