Type VI secretion system translocates a phage tail spike-like protein into target cells where it cross-links actin

Established Microbial Colonies Can Survive Type VI Secretion Assault

Type VI secretion (T6S) is a cell-to-cell injection system that can be used as a microbial weapon. T6S kills vulnerable cells, and is present in close to 25% of sequenced Gram-negative bacteria. To examine the ecological role of T6S among bacteria, we competed self-immune T6S+ cells and T6S-sensitive cells in simulated range expansions. As killing takes place only at the interface between sensitive and T6S+ strains, while growth takes place everywhere, sufficiently large domains of sensitive cells can achieve net growth in the face of attack. Indeed T6S-sensitive cells can often outgrow their T6S+ competitors. We validated these findings through in vivo competition experiments between T6S+ Vibrio cholerae and T6S-sensitive Escherichia coli. We found that E. coli can survive and even dominate so long as they have an adequate opportunity to form microcolonies at the outset of the competition. Finally, in simulated competitions between two equivalent and mutually sensitive T6S+ strains, the more numerous strain has an advantage that increases with the T6S attack rate. We conclude that sufficiently large domains of T6S-sensitive individuals can survive attack and potentially outcompete self-immune T6S+ bacteria.

Protective efficacy of recombinant hemolysin co-regulated protein (Hcp) of Aeromonas hydrophila in common carp (Cyprinus carpio)

Motile aeromonad septicemia (MAS) caused by Aeromonas hydrophila is one of the common bacterial causes of fish mortalities. Prophylactic vaccination against this and other diseases is essential for continued growth of aquaculture. The type VI secretion system (T6SS) plays a crucial role in the virulence of A. hydrophila. The hemolysin co-regulated protein (Hcp) is an integral component of the T6SS apparatus and is considered a hallmark of T6SS function. Here, the T6SS effector Hcp was expressed and characterized, and its immunogenicity and protective efficacy were evaluated in common carp (Cyprinus carpio). Hcp secretion was found to be strongly induced by low temperature in A. hydrophila. Immunoblot analysis demonstrated that Hcp is conserved among A. hydrophila strains of different origins. The vaccination with recombinant Hcp resulted in an increased survival (46.67%) in common carp during a 10-day challenge time compared to non-vaccinated fish (7.14%). The vaccinated fish also showed the significantly increased levels of IgM antibody in serum and cytokines such as inerleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in kidney, spleen and gills. The recombinant Hcp shows promise as a vaccine candidate against A. hydrophila.

Gene Expression of Type VI Secretion System Associated with Environmental Survival in Acidovorax avenae subsp. avenae by Principle Component Analysis

Valine glycine repeat G (VgrG) proteins are regarded as one of two effectors of Type VI secretion system (T6SS) which is a complex multi-component secretion system. In this study, potential biological roles of T6SS structural and VgrG genes in a rice bacterial pathogen, Acidovorax avenae subsp. avenae (Aaa) RS-1, were evaluated under seven stress conditions using principle component analysis of gene expression. The results showed that growth of the pathogen was reduced by H₂O₂ and paraquat-induced oxidative stress, high salt, low temperature, and vgrG mutation, compared to the control. However, pathogen growth was unaffected by co-culture with a rice rhizobacterium Burkholderia seminalis R456. In addition, expression of 14 T6SS structural and eight vgrG genes was significantly changed under seven conditions. Among different stress conditions, high salt, and low temperature showed a higher effect on the expression of T6SS gene compared with host infection and other environmental conditions. As a first report, this study revealed an association of T6SS gene expression of the pathogen with the host infection, gene mutation, and some common environmental stresses. The results of this research can increase understanding of the biological function of T6SS in this economically-important pathogen of rice.

Competence-induced type VI secretion might foster intestinal colonization by Vibrio cholerae: Intestinal interbacterial killing by competence-induced V. cholerae

The human pathogen Vibrio cholerae exhibits two distinct lifestyles: one in the aquatic environment where it often associates with chitinous surfaces and the other as the causative agent of the disease cholera. While much of the research on V. cholerae has focused on the host-pathogen interaction, knowledge about the environmental lifestyle of the pathogen remains limited. We recently showed that the polymer chitin, which is extremely abundant in aquatic environments, induces natural competence as a mode of horizontal gene transfer and that this competence regulon also includes the type VI secretion system (T6SS), a molecular killing device. Here, I discuss the putative consequences that chitin-induced T6SS activation could have on intestinal colonization and how the transmission route might influence disease outcome. Moreover, I propose that common infant animal models for cholera might not sufficiently take into account T6SS-mediated interbacterial warfare between V. cholerae and the intestinal microbiota.

Bacterial Secretome Analysis in Hunt for Novel Bacteriocins with Ability to Control Xanthomonas citri subsp. Citri

BACKGROUND

Xanthomonas citri subsp. citri (Xcc), the causative agent of bacterial citrus canker, has affected citriculture worldwide. Varieties of means have been used to minimize its devastating effects, but no attention has been given to bacteriocins.

OBJECTIVES

Here and for the first time, we report the isolation and characterization of two novel bacteriocins.

MATERIALS AND METHODS

Secretome containing bacteriocins of isolated bacteria was separated via SDS-PAGE. Each isolated protein band was characterized and checked for its efficacy in controlling two pathogenic isolates of Xcc via disk diffusion assay. The effects of varieties of carbon, nitrogen and phosphate sources were evaluated on both bacterial growth and bacteriocin production via Taguchi orthogonal method.

RESULTS

The two bacteriocins showed an activity up to 55ºC that were sensitive to proteases suggesting being protein in nature. Analysis of SDS-PAGE purified protein bands of bacterial secretomes with demonstrated potency against Xcc revealed the presence of peptides with relative molecular masses of 16.9 and 17 kDa for Cronobacter and Enterobacter, respectively. Sequence analysis of peptides revealed an HCP1 family VI secretion system homologue for Cronobacter (YP_001439956) and pilin FimA homologue for Enterobacter (CBK85798.1). A Taguchi orthogonal array was also implemented to determine the effect of temperature and eight other chemical factors on bacteriocin production for each bacterium.

CONCLUSIONS

Two peptides with novel antibacterial activities effective against Xcc were isolated, characterized and conditions were optimized for their higher production.

Bile Salts Modulate the Mucin-Activated Type VI Secretion System of Pandemic Vibrio cholerae

The causative agent of cholera, Vibrio cholerae, regulates its diverse virulence factors to thrive in the human small intestine and environmental reservoirs. Among this pathogen’s arsenal of virulence factors is the tightly regulated type VI secretion system (T6SS). This system acts as an inverted bacteriophage to inject toxins into competing bacteria and eukaryotic phagocytes. V. cholerae strains responsible for the current 7^th pandemic activate their T6SS within the host. We established that T6SS-mediated competition occurs upon T6SS activation in the infant mouse, and that this system is functional under anaerobic conditions. When investigating the intestinal host factors mucins (a glycoprotein component of mucus) and bile for potential regulatory roles in controlling the T6SS, we discovered that once mucins activate the T6SS, bile acids can further modulate T6SS activity. Microbiota modify bile acids to inhibit T6SS-mediated killing of commensal bacteria. This interplay is a novel interaction between commensal bacteria, host factors, and the V. cholerae T6SS, showing an active host role in infection.

The type six-secretion system (T6SS) is a molecular syringe that many Gram-negative pathogens use to kill other bacteria, including commensal bacteria of the human gut. We investigated how the environment of the intestine, specifically commensal bacteria, the mucus lining, and bile affect the T6SS of the bacterial pathogen Vibrio cholerae. First, we showed that the mucins, a family of proteins ubiquitously found in the intestine, activate the T6SS thereby allowing V. cholerae to kill other bacteria. Second, we showed that the magnitude of killing is regulated by bile acids. Certain bile acids produced by the host decrease the killing of bacteria by the V. cholerae T6SS. Last, we demonstrated that prominent members of the host microbiota metabolize these bile acids that enhance bacterial killing by V. cholerae into bile acids that diminish the bacterial killing effects of the T6SS. Our study suggests that the gut microbiota is an important first line of defense against bacterial pathogens, and that this line of defense may be impaired in individuals in poor health. Promoting a healthy microbial environment in the gut could play a role in counteracting cholera by reducing the ability of Vibrio cholerae to compete in the gut.

Type VI Secretion System Toxins Horizontally Shared between Marine Bacteria

The type VI secretion system (T6SS) is a widespread protein secretion apparatus used by Gram-negative bacteria to deliver toxic effector proteins into adjacent bacterial or host cells. Here, we uncovered a role in interbacterial competition for the two T6SSs encoded by the marine pathogen Vibrio alginolyticus. Using comparative proteomics and genetics, we identified their effector repertoires. In addition to the previously described effector V12G01_02265, we identified three new effectors secreted by T6SS1, indicating that the T6SS1 secretes at least four antibacterial effectors, of which three are members of the MIX-effector class. We also showed that the T6SS2 secretes at least three antibacterial effectors. Our findings revealed that many MIX-effectors belonging to clan V are “orphan” effectors that neighbor mobile elements and are shared between marine bacteria via horizontal gene transfer. We demonstrated that a MIX V-effector from V. alginolyticus is a functional T6SS effector when ectopically expressed in another Vibrio species. We propose that mobile MIX V-effectors serve as an environmental reservoir of T6SS effectors that are shared and used to diversify antibacterial toxin repertoires in marine bacteria, resulting in enhanced competitive fitness.

The bacterial type VI secretion system (T6SS) is a contact-dependent protein secretion apparatus that is emerging as a major component of interbacterial competition in the environment. The bacterium Vibrio alginolyticus is a pathogen of marine animals and a causal agent of wound infections, otitis, and gastroenteritis in humans. In this study, we provide a comprehensive characterization of the environmental regulation, antibacterial activities, and secreted effector repertoires of the two T6SSs found in this pathogen. We also identify a subset of T6SS effectors that appear to be mobile and shared between marine bacteria that can interact with each other in aquatic environments. Our findings suggest that bacteria can incorporate T6SS effectors from competitors in the environment. These newly acquired toxins may be used to expand and diversify T6SS effector repertoires and enhance bacterial fitness.

ACTIN-DIRECTED TOXIN. ACD toxin-produced actin oligomers poison formin-controlled actin polymerization

The actin cross-linking domain (ACD) is an actin-specific toxin produced by several pathogens, including life-threatening spp. of Vibrio cholerae, Vibrio vulnificus, and Aeromonas hydrophila. Actin cross-linking by ACD is thought to lead to slow cytoskeleton failure owing to a gradual sequestration of actin in the form of nonfunctional oligomers. Here, we found that ACD converted cytoplasmic actin into highly toxic oligomers that potently "poisoned" the ability of major actin assembly proteins, formins, to sustain actin polymerization. Thus, ACD can target the most abundant cellular protein by using actin oligomers as secondary toxins to efficiently subvert cellular functions of actin while functioning at very low doses.

Chimeric adaptor proteins translocate diverse type VI secretion system effectors in Vibrio cholerae

Vibrio cholerae is a diverse species of Gram-negative bacteria, commonly found in the aquatic environment and the causative agent of the potentially deadly disease cholera. These bacteria employ a type VI secretion system (T6SS) when they encounter prokaryotic and eukaryotic competitors. This contractile puncturing device translocates a set of effector proteins into neighboring cells. Translocated effectors are toxic unless the targeted cell produces immunity proteins that bind and deactivate incoming effectors. Comparison of multiple V. cholerae strains indicates that effectors are encoded in T6SS effector modules on mobile genetic elements. We identified a diverse group of chimeric T6SS adaptor proteins required for the translocation of diverse effectors encoded in modules. An example for a T6SS effector that requires T6SS adaptor protein 1 (Tap-1) is TseL found in pandemic V. cholerae O1 serogroup strains and other clinical isolates. We propose a model in which Tap-1 is required for loading TseL onto the secretion apparatus. After T6SS-mediated TseL export is completed, Tap-1 is retained in the bacterial cell to load other T6SS machines.

Biogenesis and structure of a type VI secretion membrane core complex

Bacteria share their ecological niches with other microbes. The bacterial type VI secretion system is one of the key players in microbial competition, as well as being an important virulence determinant during bacterial infections. It assembles a nano-crossbow-like structure in the cytoplasm of the attacker cell that propels an arrow made of a haemolysin co-regulated protein (Hcp) tube and a valine-glycine repeat protein G (VgrG) spike and punctures the prey's cell wall. The nano-crossbow is stably anchored to the cell envelope of the attacker by a membrane core complex. Here we show that this complex is assembled by the sequential addition of three type VI subunits (Tss)-TssJ, TssM and TssL-and present a structure of the fully assembled complex at 11.6 Å resolution, determined by negative-stain electron microscopy. With overall C5 symmetry, this 1.7-megadalton complex comprises a large base in the cytoplasm. It extends in the periplasm via ten arches to form a double-ring structure containing the carboxy-terminal domain of TssM (TssMct) and TssJ that is anchored in the outer membrane. The crystal structure of the TssMct-TssJ complex coupled to whole-cell accessibility studies suggest that large conformational changes induce transient pore formation in the outer membrane, allowing passage of the attacking Hcp tube/VgrG spike.

Tyrosine glycosylation of Rho by Yersinia toxin impairs blastomere cell behaviour in zebrafish embryos

Yersinia species cause zoonotic infections, including enterocolitis and plague. Here we studied Yersinia ruckeri antifeeding prophage 18 (Afp18), the toxin component of the phage tail-derived protein translocation system Afp, which causes enteric redmouth disease in salmonid fish species. Here we show that microinjection of the glycosyltransferase domain Afp18^G into zebrafish embryos blocks cytokinesis, actin-dependent motility and cell blebbing, eventually abrogating gastrulation. In zebrafish ZF4 cells, Afp18^G depolymerizes actin stress fibres by mono-O-GlcNAcylation of RhoA at tyrosine-34; thereby Afp18^G inhibits RhoA activation by guanine nucleotide exchange factors, and blocks RhoA, but not Rac and Cdc42 downstream signalling. The crystal structure of tyrosine-GlcNAcylated RhoA reveals an open conformation of the effector loop distinct from recently described structures of GDP- or GTP-bound RhoA. Unravelling of the molecular mechanism of the toxin component Afp18 as glycosyltransferase opens new perspectives in studies of phage tail-derived protein translocation systems, which are preserved from archaea to human pathogenic prokaryotes.

Yersinia ruckeri is the source of redmouth disease in fish. Here the authors analysed the Yersinia toxin Afp18 and show that it acts to inhibit RhoA activation by glycosylating a distinct tyrosine residue inducing a signalling incompetent structural conformation.

Identification of divergent type VI secretion effectors using a conserved chaperone domain

The type VI secretion system (T6SS) is a lethal weapon used by many bacteria to kill eukaryotic predators or prokaryotic competitors. Killing by the T6SS results from repetitive delivery of toxic effectors. Despite their importance in dictating bacterial fitness, systematic prediction of T6SS effectors remains challenging due to high effector diversity and the absence of a conserved signature sequence. Here, we report a class of T6SS effector chaperone (TEC) proteins that are required for effector delivery through binding to VgrG and effector proteins. The TEC proteins share a highly conserved domain (DUF4123) and are genetically encoded upstream of their cognate effector genes. Using the conserved TEC domain sequence, we identified a large family of TEC genes coupled to putative T6SS effectors in Gram-negative bacteria. We validated this approach by verifying a predicted effector TseC in Aeromonas hydrophila. We show that TseC is a T6SS-secreted antibacterial effector and that the downstream gene tsiC encodes the cognate immunity protein. Further, we demonstrate that TseC secretion requires its cognate TEC protein and an associated VgrG protein. Distinct from previous effector-dependent bioinformatic analyses, our approach using the conserved TEC domain will facilitate the discovery and functional characterization of new T6SS effectors in Gram-negative bacteria.

Type VI Secretion System Transports Zn2+ to Combat Multiple Stresses and Host Immunity

Type VI secretion systems (T6SSs) are widespread multi-component machineries that translocate effectors into either eukaryotic or prokaryotic cells, for virulence or for interbacterial competition. Herein, we report that the T6SS-4 from Yersinia pseudotuberculosis displays an unexpected function in the transportation of Zn²⁺ to combat diverse stresses and host immunity. Environmental insults such as oxidative stress induce the expression of T6SS-4 via OxyR, the transcriptional factor that also regulates many oxidative response genes. Zinc transportation is achieved by T6SS-4-mediated translocation of a novel Zn²⁺-binding protein substrate YezP (YPK_3549), which has the capacity to rescue the sensitivity to oxidative stress exhibited by T6SS-4 mutants when added to extracellular milieu. Disruption of the classic zinc transporter ZnuABC together with T6SS-4 or yezP results in mutants that almost completely lost virulence against mice, further highlighting the importance of T6SS-4 in resistance to host immunity. These results assigned an unconventional role to T6SSs, which will lay the foundation for studying novel mechanisms of metal ion uptake by bacteria and the role of this process in their resistance to host immunity and survival in harmful environments.

One unique feature of type VI secretion system is the presence of multiple distinct systems in certain bacterial species. It is well established that some of these systems function to compete for their living niches among diverse bacterial species, whilst the activity of many such transporters remains unknown. Because metal ions are essential components to virtually all forms of life including bacteria, eukaryotic hosts have evolved complicated strategies to sequester metal ions, which constitute a major branch of their nutritional immunity. Therefore the ability to acquire metal ions is critical for bacterial virulence. This study reveals that the T6SS-4 of Yersinia pseudotuberculosis (Yptb) functions to import Zn²⁺ from the environment to mitigate the detrimental effects such as hydroxyl radicals induced by diverse stresses. Expression of the transporter is activated by multiple regulatory proteins, including OxyR and OmpR that sense diverse environmental cues. Zinc ion acquisition is achieved by translocating a Zn²⁺-binding substrate YezP, which is co-regulated with T6SS-4 by OxyR. Our results reveal a novel role for type VI secretion system, which is important in the study of the mechanism of metal ion acquisition by bacteria and the role of this process in bacterial pathogenesis and survival in detrimental environments.

Two Proteins Form a Heteromeric Bacterial Self-Recognition Complex in Which Variable Subdomains Determine Allele-Restricted Binding

Self- versus nonself-recognition in bacteria has been described recently through genetic analyses in multiple systems; however, understanding of the biochemical properties and mechanisms of recognition-determinant proteins remains limited. Here we extend the molecular and biochemical understanding of two recognition-determinant proteins in bacteria. We have found that a heterotypic complex is formed between two bacterial self-recognition proteins, IdsD and IdsE, the genes of which have been shown to genetically encode the determinants for strain-specific identity in the opportunistic bacterial pathogen Proteus mirabilis. This IdsD-IdsE complex forms independently of other P. mirabilis-encoded self-recognition proteins. We have also shown that the binding between IdsD and IdsE is strain- and allele-specific. The specificity for interactions is encoded within a predicted membrane-spanning subdomain within each protein that contains stretches of unique amino acids in each P. mirabilis variant. Finally, we have demonstrated that this in vitro IdsD-IdsE binding interaction correlates to in vivo population identity, suggesting that the binding interactions between IdsD and IdsE are part of a cellular pathway that underpins self-recognition behavior in P. mirabilis and drives bacterial population sociality.

IMPORTANCE

Here we demonstrate that two proteins, the genes of which were genetically shown to encode determinants of self-identity in bacteria, bind in vitro in an allele-restricted interaction, suggesting that molecular recognition between these two proteins is a mechanism underpinning self-recognition behaviors in P. mirabilis. Binding specificity in each protein is encapsulated in a variable region subdomain that is predicted to span the membrane, suggesting that the interaction occurs in the cell envelope. Furthermore, conversion of binding affinities in vitro correlates with conversion of self-identity in vivo, suggesting that this molecular recognition might help to drive population behaviors.

Internalization of Pseudomonas aeruginosa Strain PAO1 into Epithelial Cells Is Promoted by Interaction of a T6SS Effector with the Microtubule Network

Invasion of nonphagocytic cells through rearrangement of the actin cytoskeleton is a common immune evasion mechanism used by most intracellular bacteria. However, some pathogens modulate host microtubules as well by a still poorly understood mechanism. In this study, we aim at deciphering the mechanisms by which the opportunistic bacterial pathogen Pseudomonas aeruginosa invades nonphagocytic cells, although it is considered mainly an extracellular bacterium. Using confocal microscopy and immunofluorescence, we show that the evolved VgrG2b effector of P. aeruginosa strain PAO1 is delivered into epithelial cells by a type VI secretion system, called H2-T6SS, involving the VgrG2a component. An in vivo interactome of VgrG2b in host cells allows the identification of microtubule components, including the γ-tubulin ring complex (γTuRC), a multiprotein complex catalyzing microtubule nucleation, as the major host target of VgrG2b. This interaction promotes a microtubule-dependent internalization of the bacterium since colchicine and nocodazole, two microtubule-destabilizing drugs, prevent VgrG2b-mediated P. aeruginosa entry even if the invasion still requires actin. We further validate our findings by demonstrating that the type VI injection step can be bypassed by ectopic production of VgrG2b inside target cells prior to infection. Moreover, such uncoupling between VgrG2b injection and bacterial internalization also reveals that they constitute two independent steps. With VgrG2b, we provide the first example of a bacterial protein interacting with the γTuRC. Our study offers key insight into the mechanism of self-promoting invasion of P. aeruginosa into human cells via a directed and specific effector-host protein interaction.

Innate immunity and specifically professional phagocytic cells are key determinants in the ability of the host to control P. aeruginosa infection. However, among various virulence strategies, including attack, this opportunistic bacterial pathogen is able to avoid host clearance by triggering its own internalization in nonphagocytic cells. We previously showed that a protein secretion/injection machinery, called the H2 type VI secretion system (H2-T6SS), promotes P. aeruginosa uptake by epithelial cells. Here we investigate which H2-T6SS effector enables P. aeruginosa to enter nonphagocytic cells. We show that VgrG2b is delivered by the H2-T6SS machinery into epithelial cells, where it interacts with microtubules and, more particularly, with the γ-tubulin ring complex (γTuRC) known as the microtubule-nucleating center. This interaction precedes a microtubule- and actin-dependent internalization of P. aeruginosa. We thus discovered an unprecedented target for a bacterial virulence factor since VgrG2b constitutes, to our knowledge, the first example of a bacterial protein interacting with the γTuRC.

Vibrio cholerae Response Regulator VxrB Controls Colonization and Regulates the Type VI Secretion System

Two-component signal transduction systems (TCS) are used by bacteria to sense and respond to their environment. TCS are typically composed of a sensor histidine kinase (HK) and a response regulator (RR). The Vibrio cholerae genome encodes 52 RR, but the role of these RRs in V. cholerae pathogenesis is largely unknown. To identify RRs that control V. cholerae colonization, in-frame deletions of each RR were generated and the resulting mutants analyzed using an infant mouse intestine colonization assay. We found that 12 of the 52 RR were involved in intestinal colonization. Mutants lacking one previously uncharacterized RR, VCA0566 (renamed VxrB), displayed a significant colonization defect. Further experiments showed that VxrB phosphorylation state on the predicted conserved aspartate contributes to intestine colonization. The VxrB regulon was determined using whole genome expression analysis. It consists of several genes, including those genes that create the type VI secretion system (T6SS). We determined that VxrB is required for T6SS expression using several in vitro assays and bacterial killing assays, and furthermore that the T6SS is required for intestinal colonization. vxrB is encoded in a four gene operon and the other vxr operon members also modulate intestinal colonization. Lastly, though ΔvxrB exhibited a defect in single-strain intestinal colonization, the ΔvxrB strain did not show any in vitro growth defect. Overall, our work revealed that a small set of RRs is required for intestinal colonization and one of these regulators, VxrB affects colonization at least in part through its regulation of T6SS genes.

Pathogenic bacteria experience varying conditions during infection of human hosts and often use two-component signal transduction systems (TCSs) to monitor their environment. TCS consists of a histidine kinase (HK), which senses environmental signals, and a corresponding response regulator (RR), which mediates a cellular response. The genome of the human pathogen V. cholerae contains a multitude of genes encoding HKs and RRs proteins. In the present study, we systematically analyzed the role of each V. cholerae RR for its role in pathogenesis. We identified a previously uncharacterized RR, VxrB, as a new virulence factor. We demonstrated that VxrB controls expression of the type VI secretion system (T6SS), a virulence nanomachine that directly translocates effectors into bacterial or host cells, thereby facilitating colonization by competing with sister cells and intestinal microbiota. This study represents the first systematic analysis of the role of all RRs in V. cholerae pathogenesis and provides a foundation for understanding the signal transduction pathways controlling V. cholerae pathogenesis.

New players in the toxin field: polymorphic toxin systems in bacteria

Bacteria have evolved numerous strategies to increase their competitiveness and fight against each other. Indeed, a large arsenal of antibacterial weapons is available in order to inhibit the proliferation of competitor cells. Polymorphic toxin systems (PTS), recently identified by bioinformatics in all major bacterial lineages, correspond to such a system primarily involved in conflict between related bacterial strains. They are typically composed of a secreted multidomain toxin, a protective immunity protein, and multiple cassettes encoding alternative toxic domains. The C-terminal domains of polymorphic toxins carry the toxic activity, whereas the N-terminal domains are related to the trafficking mode. In silico analysis of PTS identified over 150 distinct toxin domains, including putative nuclease, deaminase, or peptidase domains. Immunity genes found immediately downstream of the toxin genes encode small proteins that protect bacteria against their own toxins or against toxins secreted by neighboring cells. PTS encompass well-known colicins and pyocins, contact-dependent growth inhibition systems which include CdiA and Rhs toxins and some effectors of type VI secretion systems. We have recently characterized the MafB toxins, a new family of PTS deployed by pathogenic Neisseria spp. Many other putative PTS have been identified by in silico predictions but have yet to be characterized experimentally. However, the high number of these systems suggests that PTS have a fundamental role in bacterial biology that is likely to extend beyond interbacterial competition.

Structure of the type VI secretion system contractile sheath

Bacteria use rapid contraction of a long sheath of the type VI secretion system (T6SS) to deliver effectors into a target cell. Here, we present an atomic-resolution structure of a native contracted Vibrio cholerae sheath determined by cryo-electron microscopy. The sheath subunits, composed of tightly interacting proteins VipA and VipB, assemble into a six-start helix. The helix is stabilized by a core domain assembled from four β strands donated by one VipA and two VipB molecules. The fold of inner and middle layers is conserved between T6SS and phage sheaths. However, the structure of the outer layer is distinct and suggests a mechanism of interaction of the bacterial sheath with an accessory ATPase, ClpV, that facilitates multiple rounds of effector delivery. Our results provide a mechanistic insight into assembly of contractile nanomachines that bacteria and phages use to translocate macromolecules across membranes.

H-NS Silencing of the Salmonella Pathogenicity Island 6-Encoded Type VI Secretion System Limits Salmonella enterica Serovar Typhimurium Interbacterial Killing

The secretion of bacterial toxin proteins is achieved by dedicated machineries called secretion systems. The type VI secretion system (T6SS) is a widespread versatile machine used for the delivery of protein toxins to both prokaryotic and eukaryotic cells. In Salmonella enterica serovar Typhimurium, the expression of the T6SS genes is activated during macrophage or mouse infection. Here, we show that the T6SS gene cluster is silenced by the histone-like nucleoid structuring H-NS protein using a combination of reporter fusions, electrophoretic mobility shift assays, DNase footprinting, and fluorescence microscopy. We further demonstrate that derepression of the S. Typhimurium T6SS genes induces T6SS-dependent intoxication of competing bacteria. Our results suggest that relieving T6SS H-NS silencing may be used as a sense-and-kill mechanism that will help S. Typhimurium to homogenize and synchronize the microbial population to gain efficiency during infection.

T346Hunter: a novel web-based tool for the prediction of type III, type IV and type VI secretion systems in bacterial genomes

T346Hunter (Type Three, Four and Six secretion system Hunter) is a web-based tool for the identification and localisation of type III, type IV and type VI secretion systems (T3SS, T4SS and T6SS, respectively) clusters in bacterial genomes. Non-flagellar T3SS (NF-T3SS) and T6SS are complex molecular machines that deliver effector proteins from bacterial cells into the environment or into other eukaryotic or prokaryotic cells, with significant implications for pathogenesis of the strains encoding them. Meanwhile, T4SS is a more functionally diverse system, which is involved in not only effector translocation but also conjugation and DNA uptake/release. Development of control strategies against bacterial-mediated diseases requires genomic identification of the virulence arsenal of pathogenic bacteria, with T3SS, T4SS and T6SS being major determinants in this regard. Therefore, computational methods for systematic identification of these specialised machines are of particular interest. With the aim of facilitating this task, T346Hunter provides a user-friendly web-based tool for the prediction of T3SS, T4SS and T6SS clusters in newly sequenced bacterial genomes. After inspection of the available scientific literature, we constructed a database of hidden Markov model (HMM) protein profiles and sequences representing the various components of T3SS, T4SS and T6SS. T346Hunter performs searches of such a database against user-supplied bacterial sequences and localises enriched regions in any of these three types of secretion systems. Moreover, through the T346Hunter server, users can visualise the predicted clusters obtained for approximately 1700 bacterial chromosomes and plasmids. T346Hunter offers great help to researchers in advancing their understanding of the biological mechanisms in which these sophisticated molecular machines are involved. T346Hunter is freely available at http://bacterial-virulence-factors.cbgp.upm.es/T346Hunter.

Pantoea ananatis Utilizes a Type VI Secretion System for Pathogenesis and Bacterial Competition

Type VI secretion systems (T6SSs) are a class of macromolecular machines that are recognized as an important virulence mechanism in several gram-negative bacteria. The genome of Pantoea ananatis LMG 2665(T), a pathogen of pineapple fruit and onion plants, carries two gene clusters whose predicted products have homology with T6SS-associated gene products from other bacteria. Nothing is known regarding the role of these T6SS-1 and T6SS-3 gene clusters in the biology of P. ananatis. Here, we present evidence that T6SS-1 plays an important role in the pathogenicity of P. ananatis LMG 2665(T) in onion plants, while a strain lacking T6SS-3 remains as pathogenic as the wild-type strain. We also investigated the role of the T6SS-1 system in bacterial competition, the results of which indicated that several bacteria compete less efficiently against wild-type LMG 2665(T) than a strain lacking T6SS-1. Additionally, we demonstrated that these phenotypes of strain LMG 2665(T) were reliant on the core T6SS products TssA and TssD (Hcp), thus indicating that the T6SS-1 gene cluster encodes a functioning T6SS. Collectively, our data provide the first evidence demonstrating that the T6SS-1 system is a virulence determinant of P. ananatis LMG 2665(T) and plays a role in bacterial competition.

The pathogenesis, detection, and prevention of Vibrio parahaemolyticus

Vibrio parahaemolyticus, a Gram-negative motile bacterium that inhabits marine and estuarine environments throughout the world, is a major food-borne pathogen that causes life-threatening diseases in humans after the consumption of raw or undercooked seafood. The global occurrence of V. parahaemolyticus accentuates the importance of investigating its virulence factors and their effects on the human host. This review describes the virulence factors of V. parahaemolyticus reported to date, including hemolysin, urease, two type III secretion systems and two type VI secretion systems, which both cause both cytotoxicity in cultured cells and enterotoxicity in animal models. We describe various types of detection methods, based on virulence factors, that are used for quantitative detection of V. parahaemolyticus in seafood. We also discuss some useful preventive measures and therapeutic strategies for the diseases mediated by V. parahaemolyticus, which can reduce, to some extent, the damage to humans and aquatic animals attributable to V. parahaemolyticus. This review extends our understanding of the pathogenic mechanisms of V. parahaemolyticus mediated by virulence factors and the diseases it causes in its human host. It should provide new insights for the diagnosis, treatment, and prevention of V. parahaemolyticus infection.

VgrG2 of type VI secretion system 2 of Vibrio parahaemolyticus induces autophagy in macrophages

Type VI secretion system (T6SS) is a macromolecular transenvelope machine encoded within the genomes of several proteobacteria species. Vibrio parahaemolyticus contains two putative T6SS systems, VpT6SS1 and VpT6SS2, both contributing to adherence to Caco-2 and/or HeLa cells. However, it remains unknown if these systems are involved in cellular responses. In order to exclude the effects of other virulence factors known to induce cytotoxicity or autophagy, a triple deletion mutant dTTT (with deletion of tdh, and T3SS1 and T3SS2 structural protein genes) was used as the parent strain to construct deletion mutants of T6SS genes. The mutant dTTT-ΔicmF2, but not dTTT-ΔicmF1, reduced autophagic response upon 4 h of infection of the macrophage. Further attempt was made to search for the possible effector proteins that might be responsible for direct induction of autophagy by deletion of the genes encoding Hcp2 and VgrG2, two putative translocons of T6SS2 of V. parahaemolyticus. Deletion of either hcp2 or vgrG2 did reduce the autophagic response. However, increased LC3-II lipidation was seen only in the macrophage cells transfected with pVgrG2, but not with pHcp2. Chloroquinine treatment increased accumulation of LC3-II, suggesting that VgrG2 enhanced autophagic flux. The fact that vgrG2 deletion led to reduced level of intracellular cAMP suggests a possible role of cAMP signaling in autophagic responses to the bacterium. We conclude that VgrG2 of V. parahaemolyticus induces autophagy in macrophages.

Production, crystallization and X-ray diffraction analysis of a complex between a fragment of the TssM T6SS protein and a camelid nanobody

The type VI secretion system (T6SS) is a machine evolved by Gram-negative bacteria to deliver toxin effectors into target bacterial or eukaryotic cells. The T6SS is functionally and structurally similar to the contractile tail of the Myoviridae family of bacteriophages and can be viewed as a syringe anchored to the bacterial membrane by a transenvelope complex. The membrane complex is composed of three proteins: the TssM and TssL inner membrane components and the TssJ outer membrane lipoprotein. The TssM protein is central as it interacts with both TssL and TssJ, therefore linking the membranes. Using controlled trypsinolysis, a 32.4 kDa C-terminal fragment of enteroaggregative Escherichia coli TssM (TssM32Ct) was purified. A nanobody obtained from llama immunization, nb25, exhibited subnanomolar affinity for TssM32Ct. Crystals of the TssM32Ct-nb25 complex were obtained and diffracted to 1.9 Å resolution. The crystals belonged to space group P64, with unit-cell parameters a = b = 95.23, c = 172.95 Å. Molecular replacement with a model nanobody indicated the presence of a dimer of TssM32Ct-nb25 in the asymmetric unit.

Extended loop region of Hcp1 is critical for the assembly and function of type VI secretion system in Burkholderia pseudomallei

The Type VI Secretion System cluster 1 (T6SS1) is essential for the pathogenesis of Burkholderia pseudomallei, the causative agent of melioidosis, a disease endemic in the tropics. Inside host cells, B. pseudomallei escapes into the cytosol and through T6SS1, induces multinucleated giant cell (MNGC) formation that is thought to be important for bacterial cell to cell spread. The hemolysin-coregulated protein (Hcp) is both a T6SS substrate, as well as postulated to form part of the T6SS secretion tube. Our structural study reveals that Hcp1 forms hexameric rings similar to the other Hcp homologs but has an extended loop (Asp40-Arg56) that deviates significantly in position compared to other Hcp structures and may act as a key contact point between adjacent hexameric rings. When two residues within the loop were mutated, the mutant proteins were unable to stack as dodecamers, suggesting defective tube assembly. Moreover, infection with a bacterial mutant containing in situ substitution of these hcp1 residues abolishes Hcp1 secretion inside infected cells and MNGC formation. We further show that Hcp has the ability to preferentially bind to the surface of antigen-presenting cells, which may contribute to its immunogenicity in inducing high titers of antibodies seen in melioidosis patients.

Transcriptome analysis of Burkholderia pseudomallei T6SS identifies Hcp1 as a potential serodiagnostic marker

Burkholderia pseudomallei, the causative agent of melioidosis, is able to survive extreme environments and utilizes various virulence factors for survival and pathogenicity. To compete and survive within these different ecological niches, B. pseudomallei has evolved specialized pathways, including the Type VI secretion systems (T6SSs), that have a role in pathogenesis as well as interbacterial interactions. We examined the expression profile of B. pseudomallei T6SS six gene clusters during infection of U937 macrophage cells. T6SS-5 was robustly transcribed while the other five clusters were not significantly regulated proposing the utility of T6SS-5 as a potential biomarker of exposure to B. pseudomallei. Transcription of T6SS regulators VirAG and BprB was also not significant during infection when compared to bacteria grown in culture. Guided by these findings, three highly expressed T6SS genes, tssJ-4, hcp1 and tssE-5, were expressed as recombinant proteins and screened against melioidosis patient sera by western analysis and ELISA. Only Hcp1 was reactive by both types of analysis. The recombinant Hcp1 protein was further evaluated against a cohort of melioidosis patients (n = 32) and non-melioidosis individuals (n = 20) sera and the data clearly indicates a higher sensitivity (93.7%) and specificity (100%) for Hcp1 compared to bacterial lysate. The detection of anti-Hcp1 antibodies in patients' sera indicating the presence of B. pseudomallei highlights the potential of Hcp1 to be further developed as a serodiagnostic marker for melioidosis.

Composition, function, and regulation of T6SS in Pseudomonas aeruginosa

Bacterial cells can communicate with their surrounding environment through secretion systems. Type VI secretion system (T6SS) is one of the most recently discovered secretion systems, which is distributed widely in Gram-negative bacteria such as Pseudomonas aeruginosa (P. aeruginosa), an important opportunistic pathogen. This protein secretion system shares similarity with the puncturing device of bacteriophages in structure. P. aeruginosa is an important opportunistic pathogen and distributes widely in diverse environment. T6SS is beneficial to survival advantage of P. aeruginosa by delivering toxins to its neighboring pathogens and translocating protein effectors into the host cells. T6SS is also the virulence factor and takes part in biofilm formation of P. aeruginosa. The functions of T6SS in P. aeruginosa are regulated at transcriptional, posttranscriptional and posttranslational levels by diverse mechanisms. This article reviews the latest progress in the structure, effector proteins, biological function, and regulation mechanisms of P. aeruginosa T6SS.

The type VI protein secretion system contributes to biofilm formation and seed-to-seedling transmission of Acidovorax citrulli on melon

The type VI protein secretion system (T6SS) is essential for the virulence of several Gram-negative bacteria. In this study, we identified a T6SS gene cluster in Acidovorax citrulli, a plant-pathogenic bacterium that causes bacterial fruit blotch (BFB) of cucurbits. One T6SS cluster, of approximately 25 kb in length and comprising 17 genes, was found in the A. citrulli AAC00-1 genome. Seventeen A. citrulli mutants were generated, each with a deletion of a single T6SS core gene. There were significant differences in BFB seed-to-seedling transmission between wild-type A. citrulli strain, xjl12, and ΔvasD, ΔimpK, ΔimpJ and ΔimpF mutants (71.71%, 9.83%, 8.41%, 7.15% and 5.99% BFB disease index, respectively). In addition, we observed that these four mutants were reduced in melon seed colonization and biofilm formation; however, they were not affected in virulence when infiltrated into melon seedling tissues. There were no significant differences in BFB seed-to-seedling transmission, melon tissue colonization and biofilm formation between xjl12 and the other 13 T6SS mutants. Overall, our results indicate that T6SS plays a role in seed-to-seedling transmission of BFB on melon.

Transcriptional frameshifting rescues Citrobacter rodentium type VI secretion by the production of two length variants from the prematurely interrupted tssM gene

The Type VI secretion system (T6SS) mediates toxin delivery into both eukaryotic and prokaryotic cells. It is composed of a cytoplasmic structure resembling the tail of contractile bacteriophages anchored to the cell envelope through a membrane complex composed of the TssL and TssM inner membrane proteins and of the TssJ outer membrane lipoprotein. The C-terminal domain of TssM is required for its interaction with TssJ, and for the function of the T6SS. In Citrobacter rodentium, the tssM1 gene does not encode the C-terminal domain. However, the stop codon is preceded by a run of 11 consecutive adenosines. In this study, we demonstrate that this poly-A tract is a transcriptional slippery site that induces the incorporation of additional adenosines, leading to frameshifting, and hence the production of two TssM1 variants, including a full-length canonical protein. We show that both forms of TssM1, and the ratio between these two forms, are required for the function of the T6SS in C. rodentium. Finally, we demonstrate that the tssM gene associated with the Yersinia pseudotuberculosis T6SS-3 gene cluster is also subjected to transcriptional frameshifting.

Nonstandard decoding mechanisms lead to the synthesis of different protein variants from a single DNA sequence. These mechanisms are particularly important when the genome length has to be limited such as viral genomes, limited by the available space in the capsid, or to synthesize two different polypeptides that have distinct functional properties. Here, we report that tssM, a gene encoded within the Citrobacter rodentium Type VI secretion (T6S) gene cluster, is interrupted by a premature stop codon; however, the stop codon is preceded by a slippery site constituted by 11 consecutive adenosines. Reiterative transcription leads to the incorporation of additional nucleotides in the mRNA and therefore restores the original framing. As a consequence, two different TssM variants are created by transcriptional frameshifting, including a full-length 130-kDa protein and an 88-kDa truncated variant. We further show that both forms, and the ratio between these two forms, are required for the function of the transport apparatus. Interestingly, a similar mechanism regulates the synthesis of two TssM variants in Yersinia pseudotuberculosis.

Comparative analysis of surface-exposed virulence factors of Acinetobacter baumannii

Background

Acinetobacter baumannii is a significant hospital pathogen, particularly due to the dissemination of highly multidrug resistant isolates. Genome data have revealed that A. baumannii is highly genetically diverse, which correlates with major variations seen at the phenotypic level. Thus far, comparative genomic studies have been aimed at identifying resistance determinants in A. baumannii. In this study, we extend and expand on these analyses to gain greater insight into the virulence factors across eight A. baumannii strains which are clonally, temporally and geographically distinct, and includes an isolate considered non-pathogenic and a community-acquired A. baumannii.

Results

We have identified a large number of genes in the A. baumannii genomes that are known to play a role in virulence in other pathogens, such as the recently studied proline-alanine-alanine-arginine (PAAR)-repeat domains of the type VI secretion systems. Not surprising, many virulence candidates appear to be part of the A. baumannii core genome of virulent isolates but were often found to be insertionally disrupted in the avirulent A. baumannii strain SDF. Our study also reveals that many known or putative virulence determinants are restricted to specific clonal lineages, which suggests that these virulence determinants may be crucial for the success of these widespread common clones. It has previously been suggested that the high level of intrinsic and adaptive resistance has enabled the widespread presence of A. baumannii in the hospital environment. This appears to have facilitated the expansion of its repertoire of virulence traits, as in general, the nosocomial strains in this study possess more virulence genes compared to the community-acquired isolate.

Conclusions

Major genetic variation in known or putative virulence factors was seen across the eight strains included in this study, suggesting that virulence mechanisms are complex and multifaceted in A. baumannii. Overall, these analyses increase our understanding of A. baumannii pathogenicity and will assist in future studies determining the significance of virulence factors within clonal lineages and/or across the species.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1020) contains supplementary material, which is available to authorized users.

The structural basis of the Tle4–Tli4 complex reveals the self-protection mechanism of H2-T6SS inPseudomonas aeruginosa

The type VI secretion system (T6SS) has recently been demonstrated to mediate interbacterial competition and to discriminate between self and nonself. T6SS+bacteria employ toxic effectors to inhibit rival cells and concurrently use effector cognate immunity proteins to protect their sibling cells. The effector and immunity pairs (E–I pairs) endow the bacteria with a great advantage in niche competition. Tle4–Tli4 (PA1510–PA1509) is a newly identified E–I pair that is controlled by H2-T6SS inPseudomonas aeruginosa. Tle4 exhibits phospholipase activity, which destroys the cell membrane of rival cells, and the periplasm-located Tli4 in donor cells eliminates this toxic effect of Tle4. In this paper, the structure of the Tle4–Tli4 complex is reported at 1.75 Å resolution. Tle4 consists of two domains: a conserved α/β-hydrolase domain and an unusual cap domain in which two lid regions (lid1 and lid2) display a closed conformation that buries the catalytic triad in a deep funnel. Tli4 also displays a two-domain structure, in which a large lobe and a small lobe form a crab claw-like conformation. Tli4 uses this crab claw to grasp the cap domain of Tle4, especially the lid2 region, which prevents the interfacial activation of Tle4 and thus causes enzymatic dysfunction of Tle4 in sister cells.

A disordered region in the EvpP protein from the type VI secretion system of Edwardsiella tarda is essential for EvpC binding

The type VI secretion system (T6SS) of pathogenic bacteria plays important roles in both virulence and inter-bacterial competitions. The effectors of T6SS are presumed to be transported either by attaching to the tip protein or by interacting with HcpI (haemolysin corregulated protein 1). In Edwardsiella tarda PPD130/91, the T6SS secreted protein EvpP (E. tardavirulent protein P) is found to be essential for virulence and directly interacts with EvpC (Hcp-like), suggesting that it could be a potential effector. Using limited protease digestion, nuclear magnetic resonance heteronuclear Nuclear Overhauser Effects, and hydrogen-deuterium exchange mass spectrometry, we confirmed that the dimeric EvpP (40 kDa) contains a substantial proportion (40%) of disordered regions but still maintains an ordered and folded core domain. We show that an N-terminal, 10-kDa, protease-resistant fragment in EvpP connects to a shorter, 4-kDa protease-resistant fragment through a highly flexible region, which is followed by another disordered region at the C-terminus. Within this C-terminal disordered region, residues Pro143 to Ile168 are essential for its interaction with EvpC. Unlike the highly unfolded T3SS effector, which has a lower molecular weight and is maintained in an unfolded conformation with a dedicated chaperone, the T6SS effector seems to be relatively larger, folded but partially disordered and uses HcpI as a chaperone.

Bacteria in Ostreococcus tauri cultures - friends, foes or hitchhikers?

Marine phytoplankton produce half of the oxygen we breathe and their astounding diversity is just starting to be unraveled. Many microbial phytoplankton are thought to be phototrophic, depending solely on inorganic sources of carbon and minerals for growth rather than preying on other planktonic cells. However, there is increasing evidence that symbiotic associations, to a large extent with bacteria, are required for vitamin or nutrient uptake for many eukaryotic microalgae. Here, we use in silico approaches to look for putative symbiotic interactions by analysing the gene content of microbial communities associated with 13 different Ostreococcus tauri (Chlorophyta, Mamilleophyceae) cultures sampled from the Mediterranean Sea. While we find evidence for bacteria in all cultures, there is no ubiquitous bacterial group, and the most prevalent group, Flavobacteria, is present in 10 out of 13 cultures. Among seven of the microbiomes, we detected genes predicted to encode type 3 secretion systems (T3SS, in 6/7 microbiomes) and/or putative type 6 secretion systems (T6SS, in 4/7 microbiomes). Phylogenetic analyses show that the corresponding genes are closely related to genes of systems identified in bacterial-plant interactions, suggesting that these T3SS might be involved in cell-to-cell interactions with O. tauri.

DotU expression is highly induced during in vivo infection and responsible for virulence and Hcp1 secretion in avian pathogenic Escherichia coli

Type VI secretion systems (T6SSs) contribute to pathogenicity in many pathogenic bacteria. Three distinguishable T6SS loci have been discovered in avian pathogenic Escherichia coli (APEC). The sequence of APEC T6SS2 locus is highly similar to the sequence of the newborn meningitis Escherichia coli (NMEC) RS218 T6SS locus, which might contribute to meningitis pathogenesis. However, little is known about the function of APEC T6SS2. We showed that the APEC T6SS2 component organelle trafficking protein (DotU) could elicit antibodies in infected ducks, suggesting that DotU might be involved in APEC pathogenicity. To investigate DotU in APEC pathogenesis, mutant and complemented strains were constructed and characterized. Inactivation of the APEC dotU gene attenuated virulence in ducks, diminished resistance to normal duck serum, and reduced survival in macrophage cells and ducks. Furthermore, deletion of the dotU gene abolished hemolysin-coregulated protein (Hcp) 1 secretion, leading to decreased interleukin (IL)-6 and IL-8 gene expression in HD-11 chicken macrophages. These functions were restored for the complementation strain. Our results demonstrated that DotU plays key roles in the APEC pathogenesis, Hcp1 secretion, and intracellular host response modulation.

A gold nanoparticle-linked glycoconjugate vaccine against Burkholderia mallei

Burkholderia mallei are Gram-negative bacteria, responsible for the disease glanders. B. mallei has recently been classified as a Tier 1 agent owing to the fact that this bacterial species can be weaponised for aerosol release, has a high mortality rate and demonstrates multi-drug resistance. Furthermore, there is no licensed vaccine available against this pathogen. Lipopolysaccharide (LPS) has previously been identified as playing an important role in generating host protection against Burkholderia infection. In this study, we present gold nanoparticles (AuNPs) functionalised with a glycoconjugate vaccine against glanders. AuNPs were covalently coupled with one of three different protein carriers (TetHc, Hcp1 and FliC) followed by conjugation to LPS purified from a non-virulent clonal relative, B. thailandensis. Glycoconjugated LPS generated significantly higher antibody titres compared with LPS alone. Further, they improved protection against a lethal inhalation challenge of B. mallei in the murine model of infection.

FROM THE CLINICAL EDITOR

Burkholderia mallei is associated with multi-drug resistance, high mortality and potentials for weaponization through aerosol inhalation. The authors of this study present gold nanoparticles (AuNPs) functionalized with a glycoconjugate vaccine against this Gram negative bacterium demonstrating promising results in a murine model even with the aerosolized form of B. Mallei.

Biochemical analysis of TssK, a core component of the bacterial Type VI secretion system, reveals distinct oligomeric states of TssK and identifies a TssK-TssFG subcomplex

Gram-negative bacteria use the Type VI secretion system (T6SS) to inject toxic proteins into rival bacteria or eukaryotic cells. However, the mechanism of the T6SS is incompletely understood. In the present study, we investigated a conserved component of the T6SS, TssK, using the antibacterial T6SS of Serratia marcescens as a model system. TssK was confirmed to be essential for effector secretion by the T6SS. The native protein, although not an integral membrane protein, appeared to localize to the inner membrane, consistent with its presence within a membrane-anchored assembly. Recombinant TssK purified from S. marcescens was found to exist in several stable oligomeric forms, namely trimer, hexamer and higher-order species. Native-level purification of TssK identified TssF and TssG as interacting proteins. TssF and TssG, conserved T6SS components of unknown function, were required for T6SS activity, but not for correct localization of TssK. A complex containing TssK, TssF and TssG was subsequently purified in vitro, confirming that these three proteins form a new subcomplex within the T6SS. Our findings provide new insight into the T6SS assembly, allowing us to propose a model whereby TssK recruits TssFG into the membrane-associated T6SS complex and different oligomeric states of TssK may contribute to the dynamic mechanism of the system.

Genetic and biochemical analysis of TssK, an essential component of the bacterial Type VI secretion system, revealed that it forms a new subcomplex with two other core components, TssF and TssG, and displays several oligomerization states in vitro.

Historical account on gaining insights on the mechanism of crown gall tumorigenesis induced by Agrobacterium tumefaciens

The plant tumor disease known as crown gall was not called by that name until more recent times. Galls on plants were described by Malpighi (1679) who believed that these extraordinary growth are spontaneously produced. Agrobacterium was first isolated from tumors in 1897 by Fridiano Cavara in Napoli, Italy. After this bacterium was recognized to be the cause of crown gall disease, questions were raised on the mechanism by which it caused tumors on a variety of plants. Numerous very detailed studies led to the identification of Agrobacterium tumefaciens as the causal bacterium that cleverly transferred a genetic principle to plant host cells and integrated it into their chromosomes. Such studies have led to a variety of sophisticated mechanisms used by this organism to aid in its survival against competing microorganisms. Knowledge gained from these fundamental discoveries has opened many avenues for researchers to examine their primary organisms of study for similar mechanisms of pathogenesis in both plants and animals. These discoveries also advanced the genetic engineering of domesticated plants for improved food and fiber.

A type VI secretion-related pathway in Bacteroidetes mediates interbacterial antagonism

Bacteroidetes are a phylum of Gram-negative bacteria abundant in mammalian-associated polymicrobial communities, where they impact digestion, immunity, and resistance to infection. Despite the extensive competition at high cell density that occurs in these settings, cell contact-dependent mechanisms of interbacterial antagonism, such as the type VI secretion system (T6SS), have not been defined in this group of organisms. Herein we report the bioinformatic and functional characterization of a T6SS-like pathway in diverse Bacteroidetes. Using prominent human gut commensal and soil-associated species, we demonstrate that these systems localize dynamically within the cell, export antibacterial proteins, and target competitor bacteria. The Bacteroidetes system is a distinct pathway with marked differences in gene content and high evolutionary divergence from the canonical T6S pathway. Our findings offer a potential molecular explanation for the abundance of Bacteroidetes in polymicrobial environments, the observed stability of Bacteroidetes in healthy humans, and the barrier presented by the microbiota against pathogens.

Tn-Seq analysis of Vibrio cholerae intestinal colonization reveals a role for T6SS-mediated antibacterial activity in the host

Analysis of genes required for host infection will provide clues to the drivers of evolutionary fitness of pathogens like Vibrio cholerae, a mounting threat to global heath. We used transposon insertion site sequencing (Tn-seq) to comprehensively assess the contribution of nearly all V. cholerae genes toward growth in the infant rabbit intestine. Four hundred genes were identified as critical to V. cholerae in vivo fitness. These included most known colonization factors and several new genes affecting the bacterium's metabolic properties, resistance to bile, and ability to synthesize cyclic AMP-GMP. Notably, a mutant carrying an insertion in tsiV3, encoding immunity to a bacteriocidal type VI secretion system (T6SS) effector VgrG3, exhibited a colonization defect. The reduced in vivo fitness of tsiV3 mutants depends on their cocolonization with bacterial cells carrying an intact T6SS locus and VgrG3 gene, suggesting that the V. cholerae T6SS is functional and mediates antagonistic interbacterial interactions during infection.

Agrobacterium tumefaciens deploys a superfamily of type VI secretion DNase effectors as weapons for interbacterial competition in planta

The type VI secretion system (T6SS) is a widespread molecular weapon deployed by many Proteobacteria to target effectors/toxins into both eukaryotic and prokaryotic cells. We report that Agrobacterium tumefaciens, a soil bacterium that triggers tumorigenesis in plants, produces a family of type VI DNase effectors (Tde) that are distinct from previously known polymorphic toxins and nucleases. Tde exhibits an antibacterial DNase activity that relies on a conserved HxxD motif and can be counteracted by a cognate immunity protein, Tdi. In vitro, A. tumefaciens T6SS could kill Escherichia coli but triggered a lethal counterattack by Pseudomonas aeruginosa upon injection of the Tde toxins. However, in an in planta coinfection assay, A. tumefaciens used Tde effectors to attack both siblings cells and P. aeruginosa to ultimately gain a competitive advantage. Such acquired T6SS-dependent fitness in vivo and conservation of Tde-Tdi couples in bacteria highlights a widespread antibacterial weapon beneficial for niche colonization.

Nitrogen-fixing rhizobial strains isolated from common bean seeds: phylogeny, physiology, and genome analysis

Rhizobial bacteria are commonly found in soil but also establish symbiotic relationships with legumes, inhabiting the root nodules, where they fix nitrogen. Endophytic rhizobia have also been reported in the roots and stems of legumes and other plants. We isolated several rhizobial strains from the nodules of noninoculated bean plants and looked for their provenance in the interiors of the seeds. Nine isolates were obtained, covering most known bean symbiont species, which belong to the Rhizobium and Sinorhizobium groups. The strains showed several large plasmids, except for a Sinorhizobium americanum isolate. Two strains, one Rhizobium phaseoli and one S. americanum strain, were thoroughly characterized. Optimal symbiotic performance was observed for both of these strains. The S. americanum strain showed biotin prototrophy when subcultured, as well as high pyruvate dehydrogenase (PDH) activity, both of which are key factors in maintaining optimal growth. The R. phaseoli strain was a biotin auxotroph, did not grow when subcultured, accumulated a large amount of poly-β-hydroxybutyrate, and exhibited low PDH activity. The physiology and genomes of these strains showed features that may have resulted from their lifestyle inside the seeds: stress sensitivity, a ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) complex, a homocitrate synthase (usually present only in free-living diazotrophs), a hydrogenase uptake cluster, and the presence of prophages. We propose that colonization by rhizobia and their presence in Phaseolus seeds may be part of a persistence mechanism that helps to retain and disperse rhizobial strains.

Ribosomally encoded antibacterial proteins and peptides from Pseudomonas

Members of the Pseudomonas genus produce diverse secondary metabolites affecting other bacteria, fungi or predating nematodes and protozoa but are also equipped with the capacity to secrete different types of ribosomally encoded toxic peptides and proteins, ranging from small microcins to large tailocins. Studies with the human pathogen Pseudomonas aeruginosa have revealed that effector proteins of type VI secretion systems are part of the antibacterial armamentarium deployed by pseudomonads. A novel class of antibacterial proteins with structural similarity to plant lectins was discovered by studying antagonism among plant-associated Pseudomonas strains. A genomic perspective on pseudomonad bacteriocinogeny shows that the modular architecture of S pyocins of P. aeruginosa is retained in a large diversified group of bacteriocins, most of which target DNA or RNA. Similar modularity is present in as yet poorly characterized Rhs (recombination hot spot) proteins and CDI (contact-dependent inhibition) proteins. Well-delimited domains for receptor recognition or cytotoxicity enable the design of chimeric toxins with novel functionalities, which has been applied successfully for S and R pyocins. Little is known regarding how these antibacterials are released and ultimately reach their targets. Other remaining issues concern the identification of environmental triggers activating these systems and assessment of their ecological impact in niches populated by pseudomonads.

Two functional type VI secretion systems in avian pathogenic Escherichia coli are involved in different pathogenic pathways

Type VI secretion systems (T6SSs) are involved in the pathogenicity of several Gram-negative bacteria. The VgrG protein, a core component and effector of T6SS, has been demonstrated to perform diverse functions. The N-terminal domain of VgrG protein is a homologue of tail fiber protein gp27 of phage T4, which performs a receptor binding function and determines the host specificity. Based on sequence analysis, we found that two putative T6SS loci exist in the genome of the avian pathogenic Escherichia coli (APEC) strain TW-XM. To assess the contribution of these two T6SSs to TW-XM pathogenesis, the crucial clpV clusters of these two T6SS loci and their vgrG genes were deleted to generate a series of mutants. Consequently, T6SS1-associated mutants presented diminished adherence to and invasion of several host cell lines cultured in vitro, decreased pathogenicity in duck and mouse infection models in vivo, and decreased biofilm formation and bacterial competitive advantage. In contrast, T6SS2-associated mutants presented a significant decrease only in the adherence to and invasion of mouse brain microvascular endothelial cell (BMEC) line bEnd.3 and brain tissue of the duck infection model. These results suggested that T6SS1 was involved in the proliferation of APEC in systemic infection, whereas VgrG-T6SS2 was responsible only for cerebral infection. Further study demonstrated that VgrG-T6SS2 was able to bind to the surface of bEnd.3 cells, whereas it did not bind to DF-1 (chicken embryo fibroblast) cells, which further proved the interaction of VgrG-T6SS2 with the surface of BMECs.

TssB is essential for virulence and required for type VI secretion system in Ralstonia solanacearum

The type VI secretion system (T6SS) is recently discovered machinery in Gram-negative bacteria for translocation of proteins and also is required for full virulence. TssB is a highly conserved protein among the T6SSs, and indispensable for composition and function of T6S. The plant pathogenic bacterium Ralstonia solanacearum also harbours T6SS gene clusters, and a homologue of TssB, hereafter designated as TssBRS, but up to date its characterization and function remain unclear. In this study, we showed that TssBRS of R. solanacearum was required for secretion of Hcp, the haemolysin coregulated protein and a hallmark of T6S pathway. Deletion of tssBRS in R. solanacearum GMI1000 strain resulted in defect of biofilm formation, and the expression of the flagella operon is decreased, leading to decreased motility. More importantly, tssBRS mutant strain had significantly attenuated its virulence on tomato plants. TssB is essential for virulence and required for type VI secretion system in R. solanacearum.

Structure of the VipA/B type VI secretion complex suggests a contraction-state-specific recycling mechanism

The bacterial type VI secretion system is a multicomponent molecular machine directed against eukaryotic host cells and competing bacteria. An intracellular contractile tubular structure that bears functional homology with bacteriophage tails is pivotal for ejection of pathogenic effectors. Here, we present the 6 Å cryoelectron microscopy structure of the contracted Vibrio cholerae tubule consisting of the proteins VipA and VipB. We localized VipA and VipB in the protomer and identified structural homology between the C-terminal segment of VipB and the tail-sheath protein of T4 phages. We propose that homologous segments in VipB and T4 phages mediate tubule contraction. We show that in type VI secretion, contraction leads to exposure of the ClpV recognition motif, which is embedded in the type VI-specific four-helix-bundle N-domain of VipB. Disaggregation of the tubules by the AAA+ protein ClpV and recycling of the VipA/B subunits are thereby limited to the contracted state.

Marker for type VI secretion system effectors

Bacteria use diverse mechanisms to kill, manipulate, and compete with other cells. The recently discovered type VI secretion system (T6SS) is widespread in bacterial pathogens and used to deliver virulence effector proteins into target cells. Using comparative proteomics, we identified two previously unidentified T6SS effectors that contained a conserved motif. Bioinformatic analyses revealed that this N-terminal motif, named MIX (marker for type six effectors), is found in numerous polymorphic bacterial proteins that are primarily located in the T6SS genome neighborhood. We demonstrate that several MIX-containing proteins are T6SS effectors and that they are not required for T6SS activity. Thus, we propose that MIX-containing proteins are T6SS effectors. Our findings allow for the identification of numerous uncharacterized T6SS effectors that will undoubtedly lead to the discovery of new biological mechanisms.