Type VI secretion and bacteriophage tail tubes share a common assembly pathway

Initiation of H1-T6SS dueling between Pseudomonas aeruginosa

The Type VI secretion system (T6SS) is a multicomponent apparatus, present in many Gram-negative bacteria, which can inhibit bacterial prey in various ecological niches. Pseudomonas aeruginosa assembles one of its three T6SS (H1-T6SS) to respond to attacks from adjacent competing bacteria. Surprisingly, repeated assemblies of the H1-T6SS, termed dueling, were described in a monoculture in the absence of an attacker strain; however, the underlying mechanism was unknown. Here, we explored the role of H2-T6SS of P. aeruginosa in triggering H1-T6SS assembly. We show that H2-T6SS inactivation in P. aeruginosa causes a significant reduction in H1-T6SS dueling and that H2-T6SS activity directly triggers retaliation by the H1-T6SS. Intraspecific competition experiments revealed that elimination of H2-T6SS in non-immune prey cells conferred protection from H1-T6SS. Moreover, we show that the H1-T6SS response is triggered independently of the characterized lipase effectors of the H2-T6SS, as well as those of Acinetobacter baylyi and Vibrio cholerae. Our results suggest that H1-T6SS response to H2-T6SS in P. aeruginosa can impact intraspecific competition, particularly when the H1-T6SS effector-immunity pairs differ between strains, and could determine the outcome of multistrain colonization.IMPORTANCEThe opportunistic pathogen Pseudomonas aeruginosa harbors three different Type VI secretion systems (H1, H2, and H3-T6SS), which can translocate toxins that can inhibit bacterial competitors or inflict damage to eukaryotic host cells. Unlike the unregulated T6SS assembly in other Gram-negative bacteria, the H1-T6SS in P. aeruginosa is precisely assembled as a response to various cell damaging attacks from neighboring bacterial cells. Surprisingly, it was observed that neighboring P. aeruginosa cells repeatedly assemble their H1-T6SS toward each other. Mechanisms triggering this "dueling" behavior between sister cells were unknown. In this report, we used a combination of microscopy, genetic and intraspecific competition experiments to show that H2-T6SS initiates H1-T6SS dueling. Our study highlights the interplay between different T6SS clusters in P. aeruginosa, which may influence the outcomes of multistrain competition in various ecological settings such as biofilm formation and colonization of cystic fibrosis lungs.

Hemolysin Coregulated Protein (HCP) from Vibrio Cholerae Interacts with the Host Cell Actin Cytoskeleton

Vibrio cholerae (V. cholerae), the etiological agent of cholera, employs various virulence factors to adapt and thrive within both aquatic and human host environments. Among these factors, the type VI secretion system (T6SS) stands out as one of the crucial determinants of its pathogenicity. Valine glycine repeat protein G1 (VgrG1) and hemolysin coregulated protein (HCP) are considered major effector molecules of T6SS. Previous studies have highlighted that VgrG1 interacts with HCP proteins. Additionally, it has been shown that VgrG1 possesses an actin cross-linking domain (ACD) with actin-binding activity. Interestingly, it was reported that purified HCP protein treatment increased the stress fibers within cells. Therefore, we hypothesize that HCP may interact with host cell actin, potentially playing a role in the cytoskeletal rearrangement during V. cholerae infection. To test this hypothesis, we characterized HCP from the V. cholerae O139 serotype and demonstrated its interaction with actin monomers. In silico analysis and experimental validation revealed the presence of an actin-binding site within HCP. Furthermore, overexpression of HCP resulted in its colocalization with actin stress fibers in host cells. Our findings establish HCP as an effector molecule for potent host cell actin cytoskeleton remodeling during V. cholerae infection, providing new insights into bacterial pathogenicity mechanisms. Understanding the interplay between bacterial effectors and host cell components is crucial for developing targeted therapeutic interventions against cholera and related infectious diseases.

Insights into membrane interactions and their therapeutic potential

Recent research into membrane interactions has uncovered a diverse range of therapeutic opportunities through the bioengineering of human and non-human macromolecules. Although the majority of this research is focussed on fundamental developments, emerging studies are showcasing promising new technologies to combat conditions such as cancer, Alzheimer's and inflammatory and immune-based disease, utilising the alteration of bacteriophage, adenovirus, bacterial toxins, type 6 secretion systems, annexins, mitochondrial antiviral signalling proteins and bacterial nano-syringes. To advance the field further, each of these opportunities need to be better understood, and the therapeutic models need to be further optimised. Here, we summarise the knowledge and insights into several membrane interactions and detail their current and potential uses therapeutically.

Burkholderia pseudomallei Complex Subunit and Glycoconjugate Vaccines and Their Potential to Elicit Cross-Protection to Burkholderia cepacia Complex

Burkholderia are a group of Gram-negative bacteria that can cause a variety of diseases in at-risk populations. B. pseudomallei and B. mallei, the etiological agents of melioidosis and glanders, respectively, are the two clinically relevant members of the B. pseudomallei complex (Bpc). The development of vaccines against Bpc species has been accelerated in recent years, resulting in numerous promising subunits and glycoconjugate vaccines incorporating a variety of antigens. However, a second group of pathogenic Burkholderia species exists known as the Burkholderia cepacia complex (Bcc), a group of opportunistic bacteria which tend to affect individuals with weakened immunity or cystic fibrosis. To date, there have been few attempts to develop vaccines to Bcc species. Therefore, the primary goal of this review is to provide a broad overview of the various subunit antigens that have been tested in Bpc species, their protective efficacy, study limitations, and known or suspected mechanisms of protection. Then, we assess the reviewed Bpc antigens for their amino acid sequence conservation to homologous proteins found in Bcc species. We propose that protective Bpc antigens with a high degree of Bpc-to-Bcc sequence conservation could serve as components of a pan-Burkholderia vaccine capable of protecting against both disease-causing groups.

Role of type VI secretion system protein TssJ-3 in virulence and intracellular survival of Burkholderia pseudomallei

TssJ-3 is an outer-membrane lipoprotein and is one of the key components of the type VI secretion system in Burkholderia pseudomallei. TssJ translocates effector proteins to target cells to induce innate immune response in the host. However, the tssJ gene has not been identified in B. pseudomallei and its function in this bacterium has not yet been characterized. tssJ-3 knockout and tssJ-3-complemented B. pseudomallei strains were constructed to determine the effects of tssJ-3 on bacterial growth, biofilm formation, flagellum synthesis, motility, host cell infection, and gene expression in B. pseudomallei. We found that the ΔtssJ-3 mutant strain of B. pseudomallei showed significantly suppressed biofilm formation, flagellum synthesis, bacterial growth, motility, and bacterial invasion into host cells (A549 cells). Furthermore, the ΔtssJ-3 mutation downregulated multiple key genes, including biofilm and flagellum-related genes in B. pseudomallei and induced interleukin-8 gene expression in host cells. These results suggest that tssJ-3, an important gene controlling TssJ-3 protein expression, has regulatory effects on biofilm formation and flagellum synthesis in B. pseudomallei. In addition, B. pseudomallei-derived tssJ-3 contributes to cell infiltration and intracellular replication. This study provides a molecular basis of tssJ-3 for developing therapeutic strategies against B. pseudomallei infections.

Qualitative and Quantitative Methods to Measure Antibacterial Activity Resulting from Bacterial Competition

In the environment, bacteria compete for niche occupancy and resources; they have, therefore, evolved a broad variety of antibacterial weapons to destroy competitors. Current laboratory techniques to evaluate antibacterial activity are usually labor intensive, low throughput, costly, and time consuming. Typical assays rely on the outgrowth of colonies of prey cells on selective solid media after competition. Here, we present fast, inexpensive, and complementary optimized protocols to qualitatively and quantitively measure antibacterial activity. The first method is based on the degradation of a cell-impermeable chromogenic substrate of the β-galactosidase, a cytoplasmic enzyme released during lysis of the attacked reporter strain. The second method relies on the lag time required for the attacked cells to reach a defined optical density after the competition, which is directly dependent on the initial number of surviving cells. Key features First method utilizes the release of β-galactosidase as a proxy for bacterial lysis. Second method is based on the growth timing of surviving cells. Combination of two methods discriminates between cell death and lysis, cell death without lysis, or survival to quasi-lysis. Methods optimized to various bacterial species such as Escherichia coli, Pseudomonas aeruginosa, and Myxococcus xanthus. Graphical overview.

Killing in the name of: T6SS structure and effector diversity

The life of bacteria is challenging, to endure bacteria employ a range of mechanisms to optimize their environment, including deploying the type VI secretion system (T6SS). Acting as a bacterial crossbow, this system delivers effectors responsible for subverting host cells, killing competitors and facilitating general secretion to access common goods. Due to its importance, this lethal machine has been evolutionarily maintained, disseminated and specialized to fulfil these vital functions. In fact, T6SS structural clusters are present in over 25 % of Gram-negative bacteria, varying in number from one to six different genetic clusters per organism. Since its discovery in 2006, research on the T6SS has rapidly progressed, yielding remarkable breakthroughs. The identification and characterization of novel components of the T6SS, combined with biochemical and structural studies, have revealed fascinating mechanisms governing its assembly, loading, firing and disassembly processes. Recent findings have also demonstrated the efficacy of this system against fungal and Gram-positive cells, expanding its scope. Ongoing research continues to uncover an extensive and expanding repertoire of T6SS effectors, the genuine mediators of T6SS function. These studies are shedding light on new aspects of the biology of prokaryotic and eukaryotic organisms. This review provides a comprehensive overview of the T6SS, highlighting recent discoveries of its structure and the diversity of its effectors. Additionally, it injects a personal perspective on avenues for future research, aiming to deepen our understanding of this combative system.

Sporulation Activated via σW Protects Bacillus from a Tse1 Peptidoglycan Hydrolase Type VI Secretion System Effector

Within bacterial communities, community members engage in interactions employing diverse offensive and defensive tools to reach coexistence. Extracellular-matrix production and sporulation are defensive mechanisms used by Bacillus subtilis cells when they interact with Pseudomonas chlororaphis strains expressing a type VI secretion system (T6SS). Here, we define Tse1 as the main toxin mobilized by the Pseudomonas chlororaphis T6SS that triggers sporulation in Bacillus subtilis. We characterize Tse1 as a peptidoglycan hydrolase that indirectly alters the dynamics and functionality of the Bacillus cell membrane. We also delineate the response of Bacillus cells to Tse1, which through the coordinated actions of the extracellular sigma factor σW and the cytoplasmic histidine kinases KinA and KinB, culminates in activation of the sporulation cascade. We propose that this cellular developmental response permits bacilli to defend against the toxicity of T6SS-mobilized Tse1 effector. IMPORTANCE The study of bacterial interactions is helping to define species-specific strategies used to modulate the competition dynamics underlying the development of community compositions. In this study, we deciphered the role of Pseudomonas T6SS when competing with Bacillus and the mechanism by which a T6SS-toxin modifies Bacillus physiology. We found that Pseudomonas triggers Bacillus sporulation by injecting through T6SS a toxin that we called Tse1. We found that Tse1 is a hydrolase that degrades Bacillus peptidoglycan and indirectly damages Bacillus membrane functionality. In addition, we demonstrated the mechanism by which Bacillus cells increase the sporulation rate upon recognition of the presence of Tse1. Interestingly, asporogenic Bacillus cells are more sensitive to T6SS activity, which led us to propose sporulation as a last resort of bacilli to overcome this family of toxins.

Bacterial type VI secretion system (T6SS): an evolved molecular weapon with diverse functionality

Bacterial secretion systems are nanomolecular complexes that release a diverse set of virulence factors/or proteins into its surrounding or translocate to their target host cells. Among these systems, type VI secretion system 'T6SS' is a recently discovered molecular secretion system which is widely distributed in Gram-negative (-ve) bacteria, and shares structural similarity with the puncturing device of bacteriophages. The presence of T6SS is an advantage to many bacteria as it delivers toxins to its neighbour pathogens for competitive survival, and also translocates protein effectors to the host cells, leading to disruption of lipid membranes, cell walls, and cytoskeletons etc. Recent studies have characterized both anti-prokaryotic and anti-eukaryotic effectors, where T6SS is involved in diverse cellular functions including favouring colonization, enhancing the survival, adhesive modifications, internalization, and evasion of the immune system. With the evolution of advanced genomics and proteomics tools, there has been an increase in the number of characterized T6SS effector arsenals and also more clear information about the adaptive significance of this complex system. The functions of T6SS are generally regulated at the transcription, post-transcription and post-translational levels through diverse mechanisms. In the present review, we aimed to provide information about the distribution of T6SS in diverse bacteria, any structural similarity/or dissimilarity, effectors proteins, functional significance, and regulatory mechanisms. We also tried to provide information about the diverse roles played by T6SS in its natural environments and hosts, and further any changes in the microbiome.

Type VI Secretion Systems: Environmental and Intra-host Competition of Vibrio cholerae

The Vibrio Type VI Secretion System (T6SS) is a harpoon-like nanomachine that serves as a defense system and is encoded by approximately 25% of all gram-negative bacteria. In this chapter, we describe the structure of the T6SS in different Vibrio species and outline how the use of different T6SS effector and immunity proteins control kin selection. We summarize the genetic loci that encode the structural elements that make up the Vibrio T6SSs and how these gene clusters are regulated. Finally, we provide insights into T6SS-based competitive dynamics, the role of T6SS genetic exchange in those competitive dynamics, and roles for the Vibrio T6SS in virulence.

Coevolution-Guided Mapping of the Type VI Secretion Membrane Complex-Baseplate Interface

The type VI secretion system (T6SS) is a multiprotein weapon evolved by Gram-negative bacteria to deliver effectors into eukaryotic cells or bacterial rivals. The T6SS uses a contractile mechanism to propel an effector-loaded needle into its target. The contractile tail is built on an assembly platform, the baseplate, which is anchored to a membrane complex. Baseplate-membrane complex interactions are mainly mediated by contacts between the C-terminal domain of the TssK baseplate component and the cytoplasmic domain of the TssL inner membrane protein. Currently, the structural details of this interaction are unknown due to the marginal stability of the TssK-TssL complex. Here we conducted a mutagenesis study based on putative TssK-TssL contact pairs identified by co-evolution analyses. We then evaluated the impact of these mutations on T6SS activity, TssK-TssL interaction and sheath assembly and dynamics in enteroaggregative Escherichia coli. Finally, we probed the TssK-TssL interface by disulfide cross-linking, allowing to propose a model for the baseplate-membrane complex interface.

Label-free relative quantitative proteomics reveals extracellular vesicles as a vehicle for Salmonella effector protein delivery

Extracellular vesicles are small vesicles with a diameter of 30-150 nm that are actively secreted by eukaryotic cells and play important roles in intercellular communication, immune responses, and tumorigenesis. Previous studies have shown that extracellular vesicles are involved in the process of Salmonella enterica serovar Typhimurium (S. Typhimurium) infection. However, changes in the protein content of extracellular vesicles elicited by S. Typhimurium infection have not been determined. Here, we extracted the extracellular vesicles with high purity from S. Typhimurium-infected Henle-407 cells, a kind of human intestinal epithelial cells, by ultracentrifugation combined with an extracellular vesicles purification kit, and analyzed their protein composition using label-free relative quantitative proteomics. The extracted extracellular vesicles exhibited an oval vesicular structure under electron microscopy, with a mean diameter of 140.4 ± 32.4 nm. The exosomal marker proteins CD9, CD63, and HSP70 were specifically detected. Compared with the uninfected group, nearly 1,234 specifically loaded proteins were uncovered in S. Typhimurium-infected Henle-407 cells. Among them were 409 S. Typhimurium-derived specific proteins, indicating a significant alteration in protein composition of extracellular vesicles by S. Typhimurium infection. Notably, these proteins included 75 secretory proteins and over 300 non-secretory proteins of S. Typhimurium, implicating novel pathways for bacterial protein delivery, although it remains unclear if their loading into extracellular vesicles is active or passive. To investigate the roles of these extracellular proteins, we exemplified the function of SopB, a well-known T3SS effector protein, and showed that the extracellular SopB could be taken up by RAW264.7 macrophages, activating the phosphorylation of Akt. This study provides new insights into the mechanism of Salmonella infection through extracellular vesicles that transport virulence proteins to uninfected neighboring cells to facilitate further infection.

TssI2-TsiI2 of Vibrio fluvialis VflT6SS2 delivers pesticin domain-containing periplasmic toxin and cognate immunity that modulates bacterial competitiveness

Vibrio fluvialis is a halophilic Gram-negative bacterium regarded as an emerging unusual enteric pathogen of increasing public health concern. Our previous work has identified two type VI secretion systems (T6SSs) in V. fluvialis, VflT6SS1, and VflT6SS2, and the latter is functional in mediating interbacterial competitiveness. However, its antibacterial effectors remain to be clarified. In this work, we focused on a new potential effector/immunity pair TssI2/TsiI2. Bioinformatics analysis revealed that the C-terminal domain of TssI2 belongs to a widespread family of pesticin, and its antibacterial toxicity and corresponding protection by TsiI2 were proved via bacterial killing assays, and their action sites were localized to the periplasm of bacterial cells. The interaction of TssI2 and TsiI2 was demonstrated by the bacterial adenylate cyclase two-hybrid, protein pull-down and isothermal titration calorimetry assays. Site-directed mutagenesis demonstrated that, in addition to Glu-844, Thr-863, and Asp-869, which correspond to three reported residues in pesticin of Yersinia pestis, additional residues including Phe-837, Gly-845, Tyr-851, Gly-867, Gln-963, Trp-975, and Arg-1000 were also proved to be crucial to the bactericidal activity of TssI2. Muramidase/lysozyme-related peptidoglycan (PG) hydrolase activities of TssI2 and its variants were validated with permeabilized Escherichia coli cells and purified PG substrate. Based on sequence homologies at C-terminals in various V. fluvialis isolates, TssI2 was subdivided into five clusters (12-22% identity among them), and the antibacterial activities of representative effectors from other four Clusters were also confirmed through periplasmic over-expression in E. coli host. Two selected cognate immunities were proved to confer protection against the toxicities of their effectors. Additionally, TsiI2, which belongs to Cluster I, exhibited cross-protection to effector from Cluster V. Together, current findings expand our knowledge of the diversity and consistency of evolved VgrG effectors in V. fluvialis and on how VflT6SS2 mediates a competitive advantage to gain a better survival.

Vibrio ostreae sp. nov., a novel gut bacterium isolated from a Yellow Sea oyster

A Gram-stain-negative, oxidase- and catalase-positive, facultative anaerobic motile bacterium, designated strain OG9-811^T, was isolated from the gut of an oyster collected in the Yellow Sea, Republic of Korea. The strain grew at 10-37 °C, pH 6.0-9.0 and with 0.5-10% (w/v) NaCl. Phylogenetic analysis based on the 16S rRNA gene sequences revealed that strain OG9-811^T affiliated with the genus Vibrio, with the highest sequence similarity of 98.2% to Vibrio coralliilyticus ATCC BAA-450^T followed by Vibrio variabilis R-40492^T (98.0 %), Vibrio hepatarius LMG 20362^T (97.7 %) and Vibrio neptunius LMG 20536^T (97.6 %); other relatives were Vibrio tritonius JCM 16456^T (97.4 %), Vibrio fluvialis NBRC 103150^T (97.0 %) and Vibrio furnissii CIP 102972^T (97.0 %). The complete genome of strain OG9-811^T comprised two chromosomes of a total 4 807 684 bp and the G+C content was 50.2 %. Results of analysis based on the whole genome sequence showed the distinctiveness of strain OG9-811^T. The average nucleotide identity (ANI) values between strain OG9-811^T and the closest strains V. coralliilyticus ATCC BAA-450^T, V. variabilis R-40492^T, V. hepatarius LMG 20362^T, V. neptunius KCTC 12702^T , V. tritonius JCM 16456^T, V. fluvialis ATCC 33809^T and V. furnissi CIP 102972^T were 73.0, 72.6, 73.3, 73.0, 72.7, 78.5 and 77.8 %, respectively, while the digital DNA-DNA hybridization values between strain OG9-811^T and the above closely related strains were 20.8, 21.2, 20.8, 21.7, 20.7, 23.2 and 22.4 %, respectively. The major fatty acids of strain OG9-811^T were summed feature 3 (C_16:1 ω7c and/or C_16:1 ω6c), summed feature 8 (C_18:1 ω6c and/or C_18:1 ω7c) and C_16:0. The polar lipids contained phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. Strain OG9-811^T contained Q-8 as a quinone. On the basis of polyphasic taxonomic characteristics, strain OG9-811^T is considered to represent a novel species, for which the name Vibrio ostreae sp. nov. is proposed. The type strain is OG9-811^T (=KCTC 72623^T=GDMCC 1.2610^T).

Evaluation of two different vaccine platforms for immunization against melioidosis and glanders

Burkholderia pseudomallei and the closely related species, Burkholderia mallei, produce similar multifaceted diseases which range from rapidly fatal to protracted and chronic, and are a major cause of mortality in endemic regions. Besides causing natural infections, both microbes are Tier 1 potential biothreat agents. Antibiotic treatment is prolonged with variable results, hence effective vaccines are urgently needed. The purpose of our studies was to compare candidate vaccines that target both melioidosis and glanders to identify the most efficacious one(s) and define residual requirements for their transition to the non-human primate aerosol model. Studies were conducted in the C57BL/6 mouse model to evaluate the humoral and cell-mediated immune response and protective efficacy of three Burkholderia vaccine candidates against lethal aerosol challenges with B. pseudomallei K96243, B. pseudomallei MSHR5855, and B. mallei FMH. The recombinant vaccines generated significant immune responses to the vaccine antigens, and the live attenuated vaccine generated a greater immune response to OPS and the whole bacterial cells. Regardless of the candidate vaccine evaluated, the protection of mice was associated with a dampened cytokine response within the lungs after exposure to aerosolized bacteria. Despite being delivered by two different platforms and generating distinct immune responses, two experimental vaccines, a capsule conjugate + Hcp1 subunit vaccine and the live B. pseudomallei 668 ΔilvI strain, provided significant protection and were down-selected for further investigation and advanced development.

Identification of Antigenic Properties of Acinetobacter baumannii Proteins as Novel Putative Vaccine Candidates Using Reverse Vaccinology Approach

Multidrug-resistant Acinetobacter baumannii (A. baumannii) infections are becoming more prevalent all over the world. As a cost-effective and preventative method, vaccination seems to be required against this bacterium. In the present study, subtractive proteomics along with reverse vaccinology approaches was used to predict suitable therapeutics against A. baumannii. Using the Vaxign online tool, we studied over 35 genomes of A. baumannii strains and chose outer membrane and secreted proteins of A. baumannii 1656-2 as possible vaccine candidates. Then, investigations were performed on the immunogenicity, antigenic characteristics, physicochemical properties, B-cell and MHC class I, and MHC class II molecules epitope densities of proteins. After optimizing the codon of the proteins, the pcDNA3.1( +) expression construct was designed and the immunogenicity, allergenicity, and physicochemical properties of the vaccine construct were predicted. Hcp and OmpC proteins were predicted as extracellular and outer membrane proteins, respectively. These proteins interact with 10 other proteins to form a network of protein interactions with virulence properties. Immunoassays of Hcp and OmpC proteins showed antigenicity of 0.88 and 0.79, respectively. These proteins have 5 structural cell epitope points and 5 linear B epitope points. They are also able to bind to different HLA alleles of MCH class I/class II as selected immunogenic proteins and designed non-allergenic structures with solubility of 0.650 and immunogenicity score of 0.91. The results of this "in silico" study indicate high specificity and the development of a significant humoral and cellular immune response. It can be concluded that the Hcp and OmpC dual vaccine construct is one of the promising candidates against A. baumannii. The findings of this "in silico" study show excellent specificity and the emergence of a substantial humoral and cellular immune response. This is a computer-based study that needs to be tested in vitro and in vivo to corroborate the conclusions of the vaccine design procedures.

Identification of Type VI Secretion Systems Effector Proteins That Contribute to Interbacterial Competition in Salmonella Dublin

The Type VI Secretion System (T6SS) is a multiprotein device that has emerged as an important fitness and virulence factor for many Gram-negative bacteria through the injection of effector proteins into prokaryotic or eukaryotic cells via a contractile mechanism. While some effector proteins specifically target bacterial or eukaryotic cells, others can target both types of cells (trans-kingdom effectors). In Salmonella, five T6SS gene clusters have been identified within pathogenicity islands SPI-6, SPI-19, SPI-20, SPI-21, and SPI-22, which are differentially distributed among serotypes. Salmonella enterica serotype Dublin (S. Dublin) is a cattle-adapted pathogen that harbors both T6SS_SPI-6 and T6SS_SPI-19. Interestingly, while both systems have been linked to virulence and host colonization in S. Dublin, an antibacterial activity has not been detected for T6SS_SPI-6 in this serotype. In addition, there is limited information regarding the repertoire of effector proteins encoded within T6SS_SPI-6 and T6SS_SPI-19 gene clusters in S. Dublin. In the present study, we demonstrate that T6SS_SPI-6 and T6SS_SPI-19 of S. Dublin CT_02021853 contribute to interbacterial competition. Bioinformatic and comparative genomic analyses allowed us to identify genes encoding three candidate antibacterial effectors located within SPI-6 and two candidate effectors located within SPI-19. Each antibacterial effector gene is located upstream of a gene encoding a hypothetic immunity protein, thus conforming an effector/immunity (E/I) module. Of note, the genes encoding these effectors and immunity proteins are widely distributed in Salmonella genomes, suggesting a relevant role in interbacterial competition and virulence. Finally, we demonstrate that E/I modules SED_RS01930/SED_RS01935 (encoded in SPI-6), SED_RS06235/SED_RS06230, and SED_RS06335/SED_RS06340 (both encoded in SPI-19) contribute to interbacterial competition in S. Dublin CT_02021853.

Bacterial Antagonism of Chromobacterium haemolyticum and Characterization of its Putative Type VI Secretion System

This study reports the isolation of a new Chromobacterium haemolyticum strain named WI5 from a hydroponic farming facility. WI5 exhibited remarkable bacterial antagonistic properties, eliminating Salmonella, Escherichia coli, Listeria monocytogenes and Staphylococcus aureus (initial inoculum load ∼10⁵ CFU/ml) in dual-species co-culture biofilms. Antagonism was strictly contact-dependent and highly influenced by nutrient availability. Next, we identified a complete suite of putative Type VI secretion system (T6SS) genes in the WI5 genome, annotated the gene locus architecture, and determined the crystal structure of hallmark T6SS tube protein Hcp1, which revealed a hexameric ring structure with an outer and inner diameter of 77 and 45Å, respectively. Structural comparison with homologs showed differences in the key loops connecting the β-strands in which the conserved residues are located, suggesting a role of these residues in the protein function. The T6SS is well-known to facilitate interbacterial competition, and the putative T6SS characterized herein might be responsible for the remarkable antagonism by C. haemolyticum WI5. Collectively, these findings shed light on the nature of bacterial antagonism and a putative key virulence determinant of C. haemolyticum, which might aid in further understanding its potential ecological role in natural habitats.

Pangenomic Analysis of Dickeya dianthicola Strains Related to the Outbreak of Blackleg and Soft Rot of Potato in the United States

Dickeya dianthicola has caused an outbreak of blackleg and soft rot of potato in the eastern half of the United States since 2015. To investigate genetic diversity of the pathogen, a comparative analysis was conducted on genomes of D. dianthicola strains. Whole genomes of 16 strains from the United States outbreak were assembled and compared with 16 previously sequenced genomes of D. dianthicola isolated from potato or carnation. Among the 32 strains, eight distinct clades were distinguished based on phylogenomic analysis. The outbreak strains were grouped into three clades, with the majority of the strains in clade I. Clade I strains were unique and homogeneous, suggesting a recent incursion of this strain into potato production from alternative hosts or environmental sources. The pangenome of the 32 strains contained 6,693 genes, 3,377 of which were core genes. By screening primary protein subunits associated with virulence from all U.S. strains, we found that many virulence-related gene clusters, such as plant cell wall degrading enzyme genes, flagellar and chemotaxis related genes, two-component regulatory genes, and type I/II/III secretion system genes, were highly conserved but that type IV and type VI secretion system genes varied. The clade I strains encoded two clusters of type IV secretion systems, whereas the clade II and III strains encoded only one cluster. Clade I and II strains encoded one more VgrG/PAAR spike protein than did clade III. Thus, we predicted that the presence of additional virulence-related genes may have enabled the unique clade I strain to become predominant in the U.S. outbreak.

Bacterial proteomics and its application for pathogenesis studies

Bacteria build their structures by implementing several macromolecules such as proteins, polysaccharides, phospholipids, and nucleic acids, which leads to preserve their lives and play an essential role in their pathogenesis. There are two genomic and proteomic methods to study various macromolecules of bacteria, which are complementary methods and provide comprehensive information. Proteomic approaches are used to identify proteins and their cell applications. Furthermore, to study bacterial proteins, macromolecules are involved in the bacteria's structures and functions. These protein-based methods provide comprehensive information about the cells, such as the external structures, internal compositions, post-translational modifications, and mechanisms of particular actions such as biofilm formation, antibiotic resistance, and adaptation to the environment, which are helpful in promoting bacterial pathogenesis. These methods use various devices such as MALDI-TOF MS, LC-MS, and two-dimensional electrophoresis, which are valuable tools for studying different structural and functional proteins of the bacteria and their mechanisms of pathogenesis that causes rapid, easy, and accurate diagnosis of the infections.

Inhibiting Type VI Secretion System Activity with a Biomimetic Peptide Designed To Target the Baseplate Wedge Complex

Human health is threatened by bacterial infections that are increasingly resistant to multiple drugs. A recently emerged strategy consists of disarming pathogenic bacteria by targeting and blocking their virulence factors. The type VI secretion system (T6SS) is a widespread secretion nanomachine encoded and employed by pathogenic strains to establish their virulence process during host invasion. Given the conservation of T6SS in several human bacterial pathogens, the discovery of an effective broad-spectrum T6SS virulence blocker represents an attractive target for development of antivirulence therapies. Here, we identified and validated a protein-protein interaction interface, TssK-TssG, as a key factor in the assembly of the T6SS baseplate (BP) complex in the pathogen enteroaggregative Escherichia coli (EAEC). In silico and biochemical studies revealed that the determinants of the interface are broadly conserved among pathogenic species, suggesting a role for this interface as a target for T6SS inhibition. Based on the high-resolution structure of the TssKFGE wedge complex, we rationally designed a biomimetic cyclic peptide (BCP) that blocks the assembly of the EAEC BP complex and inhibits the function of T6SS in bacterial cultures. Our BCP is the first compound completely designed from prior structural knowledge with anti-T6SS activity that can be used as a model to target human pathogens.

Sensing of intracellular Hcp levels controls T6SS expression in Vibrio cholerae

The type 6 secretion system (T6SS) is a bacterial weapon broadly distributed in gram-negative bacteria and used to kill competitors and predators. Featuring a long and double-tubular structure, this molecular machine is energetically costly to produce and thus is likely subject to diverse regulation strategies that are largely ill defined. In this study, we report a quantity-sensing control of the T6SS that down-regulates the expression of secreted components when they accumulate in the cytosol due to T6SS inactivation. Using Vibrio cholerae strains that constitutively express an active T6SS, we demonstrate that mRNA levels of secreted components, including the inner-tube protein component Hcp, were down-regulated in T6SS structural gene mutants while expression of the main structural genes remained unchanged. Deletion of both hcp gene copies restored expression from their promoters, while Hcp overexpression negatively impacted expression. We show that Hcp directly interacts with the RpoN-dependent T6SS regulator VasH, and deleting the N-terminal regulator domain of VasH abolishes this interaction as well as the expression difference of hcp operons between T6SS-active and inactive strains. We find that negative regulation of hcp also occurs in other V. cholerae strains and the pathogens Aeromonas dhakensis and Pseudomonas aeruginosa This Hcp-dependent sensing control is likely an important energy-conserving mechanism that enables T6SS-encoding organisms to quickly adjust T6SS expression and prevent wasteful build-up of its major secreted components in the absence of their efficient export out of the bacterial cell.

The Breadth and Molecular Basis of Hcp-Driven Type VI Secretion System Effector Delivery

The type VI secretion system (T6SS) is a bacterial nanoscale weapon that delivers toxins into prey ranging from bacteria and fungi to animal hosts. The cytosolic contractile sheath of the system wraps around stacked hexameric rings of Hcp proteins, which form an inner tube. At the tip of this tube is a puncturing device comprising a trimeric VgrG topped by a monomeric PAAR protein. The number of toxins a single system delivers per firing event remains unknown, since effectors can be loaded on diverse sites of the T6SS apparatus, notably the inner tube and the puncturing device. Each VgrG or PAAR can bind one effector, and additional effector cargoes can be carried in the Hcp ring lumen. While many VgrG- and PAAR-bound toxins have been characterized, to date, very few Hcp-bound effectors are known. Here, we used 3 known Pseudomonas aeruginosa Hcp proteins (Hcp1 to -3), each of which associates with one of the three T6SSs in this organism (H1-T6SS, H2-T6SS, and H3-T6SS), to perform in vivo pulldown assays. We confirmed the known interactions of Hcp1 with Tse1 to -4, further copurified a Hcp1-Tse4 complex, and identified potential novel Hcp1-bound effectors. Moreover, we demonstrated that Hcp2 and Hcp3 can shuttle T6SS cargoes toxic to Escherichia coli. Finally, we used a Tse1-Bla chimera to probe the loading strategy for Hcp passengers and found that while large effectors can be loaded onto Hcp, the formed complex jams the system, abrogating T6SS function.

Roles of the Hcp family proteins in the pathogenicity of Salmonella typhimurium 14028s

The type VI secretion system (T6SS) is a new secretion system that is widely distributed among Gram-negative bacteria. The core component hemolysin-coregulated protein (Hcp) can be used as both its structural protein and secretory protein or chaperone protein. Studies on Hcp are important to elucidate the overall virulence mechanism of T6SS. Salmonella typhimurium is an important foodborne pathogen. There are three copies of hcp genes identified in S. Typhimurium 14028s. This study aimed to characterize the functions of the three Hcp family proteins and to elucidate the interactions among them. The hcp gene deletion mutants were constructed by λ Red-based recombination system. Effects of hcp mutation on the pathogenicity of 14028s were studied by bacterial competition assays, Dictyostelium discoideum assays and mouse model. The three Hcp family proteins were found to play different roles. Hcp1 can affect the transcription of rpoS and type 2 flagellar gene and influence the motility of 14028s. It is also involved in the intracellular survival of 14028s in Dictyostelium discoideum; Hcp2 is involved in the early proliferative capacity of 14028s in mice and can prevent its excessive proliferation; Hcp3 did not show direct functions in these assays. Hcp1 can interact with Hcp2 and Hcp3. Deletion of one hcp gene can result in a transcription level variation in the other two hcp genes. Our findings elucidated the functions of the three Hcp family proteins in S.Typhimurium and illustrated that there are interactions between different Hcp proteins. This study will be helpful to fully understand how T6SS actions in an organism.

An Overview of Anti-Eukaryotic T6SS Effectors

The type VI secretion system (T6SS) is a transmembrane multiprotein nanomachine employed by many Gram-negative bacterial species to translocate, in a contact-dependent manner, effector proteins into adjacent prokaryotic or eukaryotic cells. Typically, the T6SS gene cluster encodes at least 13 conserved core components for the apparatus assembly and other less conserved accessory proteins and effectors. It functions as a contractile tail machine comprising a TssB/C sheath and an expelled puncturing device consisting of an Hcp tube topped by a spike complex of VgrG and PAAR proteins. Contraction of the sheath propels the tube out of the bacterial cell into a target cell and leads to the injection of toxic proteins. Different bacteria use the T6SS for specific roles according to the niche and versatility of the organism. Effectors are present both as cargo (by non-covalent interactions with one of the core components) or specialized domains (fused to structural components). Although several anti-prokaryotic effectors T6SSs have been studied, recent studies have led to a substantial increase in the number of characterized anti-eukaryotic effectors. Against eukaryotic cells, the T6SS is involved in modifying and manipulating diverse cellular processes that allows bacteria to colonize, survive and disseminate, including adhesion modification, stimulating internalization, cytoskeletal rearrangements and evasion of host innate immune responses.

Two Type VI Secretion Systems of Enterobacter cloacae Are Required for Bacterial Competition, Cell Adherence, and Intestinal Colonization

Enterobacter cloacae has emerged as an opportunistic pathogen in healthcare-associated infections. Analysis of the genomic sequences of several E. cloacae strains revealed the presence of genes that code for expression of at least one type VI secretion system (T6SS). Here, we report that E. cloacae strain ATCC 13047 codes for two functional T6SS named T6SS-1 and T6SS-2. T6SS-1 and T6SS-2 were preferentially expressed in tryptic soy broth and tissue culture medium (DMEM), respectively. Mutants in T6SS-1-associated genes clpV1 and hcp1 significantly affected their ability of inter- and intra-bacterial killing indicating that T6SS-1 is required for bacterial competition. In addition, the Hcp effector protein was detected in supernatants of E. cloacae cultures and a functional T6SS-1 was required for the secretion of this protein. A clpV2 mutant was impaired in both biofilm formation and adherence to epithelial cells, supporting the notion that these phenotypes are T6SS-2 dependent. In vivo data strongly suggest that both T6SSs are required for intestinal colonization because single and double mutants in clpV1 and clpV2 genes were defective in gut colonization in mice. We conclude that the two T6SSs are involved in the pathogenesis scheme of E. cloacae with specialized functions in the interaction with other bacteria and with host cells.

Engineering Protein Nanoparticles Out from Components of the Human Microbiome

Nanoscale protein materials are highly convenient as vehicles for targeted drug delivery because of their structural and functional versatility. Selective binding to specific cell surface receptors and penetration into target cells require the use of targeting peptides. Such homing stretches should be incorporated to larger proteins that do not interact with body components, to prevent undesired drug release into nontarget organs. Because of their low interactivity with human body components and their tolerated immunogenicity, proteins derived from the human microbiome are appealing and fully biocompatible building blocks for the biofabrication of nonreactive, inert protein materials within the nanoscale. Several phage and phage-like bacterial proteins with natural structural roles are produced in Escherichia coli as polyhistidine-tagged recombinant proteins, looking for their organization as discrete, nanoscale particulate materials. While all of them self-assemble in a variety of sizes, the stability of the resulting constructs at 37 °C is found to be severely compromised. However, the fine adjustment of temperature and Zn²⁺ concentration allows the formation of robust nanomaterials, fully stable in complex media and under physiological conditions. Then, microbiome-derived proteins show promise for the regulatable construction of scaffold protein nanomaterials, which can be tailored and strengthened by simple physicochemical approaches.

Solving the Puzzle: Connecting a Heterologous Agrobacterium tumefaciens T6SS Effector to a Pseudomonas aeruginosa Spike Complex

The type VI secretion system (T6SS) is a contractile injection apparatus that translocates a spike loaded with various effectors directly into eukaryotic and prokaryotic target cells. Such T6SS spike consists of a needle-shaped trimer of VgrG proteins topped by a conical and sharp PAAR protein that facilitates puncturing of the target membrane. T6SS-delivered effector proteins can be either fused to one of the two spike proteins or interact with either in a highly specific manner. In Agrobacterium tumefaciens the T6SS effector Tde1 is targeted to its cognate VgrG1 protein. Here, we attempted to use a VgrG shuttle to deliver a heterologous T6SS effector by directing Tde1 onto a T6SS spike in Pseudomonas aeruginosa. For this, we designed chimeras between VgrG1 from A. tumefaciens and VgrG1a from P. aeruginosa and showed that modification of the spike protein hampered T6SS functionality in the presence of the Tde1 effector complex. We provide evidence suggesting that Tde1 specifically binds to the VgrG spike in the heterologous environment and propose that there are additional requirements to allow proper effector delivery and translocation. Our work sheds light on complex aspects of the molecular mechanisms of T6SS delivery and highlights some limitations on how effectors can be translocated using this nanomachine.

Bacterial-induced cell fusion is a danger signal triggering cGAS-STING pathway via micronuclei formation

Burkholderia pseudomallei is a bacterial pathogen that causes melioidosis, an infectious disease in the tropics with high morbidity and mortality. It has a unique property among bacteria: to fuse infected host cells. We found that our immune system detects bacterial- or chemical-induced host cell fusion as a danger signal. Abnormal cell fusion leads to genomic instability and formation of micronuclei. This triggers the host to activate a signaling pathway leading to a form of cell death known as autophagic death, which likely serves to limit abnormal cellular transformation.

Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease in the tropics and subtropics with high morbidity and mortality. The facultative intracellular bacterium induces host cell fusion through its type VI secretion system 5 (T6SS5) as an important part of its pathogenesis in mammalian hosts. This allows it to spread intercellularly without encountering extracellular host defenses. We report that bacterial T6SS5-dependent cell fusion triggers type I IFN gene expression in the host and leads to activation of the cGAMP synthase–stimulator of IFN genes (cGAS–STING) pathway, independent of bacterial ligands. Aberrant and abortive mitotic events result in the formation of micronuclei colocalizing with cGAS, which is activated by double-stranded DNA. Surprisingly, cGAS–STING activation leads to type I IFN transcription but not its production. Instead, the activation of cGAS and STING results in autophagic cell death. We also observed type I IFN gene expression, micronuclei formation, and death of chemically induced cell fusions. Therefore, we propose that the cGAS–STING pathway senses unnatural cell fusion through micronuclei formation as a danger signal, and consequently limits aberrant cell division and potential cellular transformation through autophagic death induction.

Manipulating the type VI secretion system spike to shuttle passenger proteins

The type VI secretion system (T6SS) is a contractile injection apparatus that translocates a spike loaded with various effectors directly into eukaryotic or prokaryotic target cells. Pseudomonas aeruginosa can load either one of its three T6SSs with a variety of toxic bullets using different but specific modes. The T6SS spike, which punctures the bacterial cell envelope allowing effector transport, consists of a torch-like VgrG trimer on which sits a PAAR protein sharpening the VgrG tip. VgrG itself sits on the Hcp tube and all elements, packed into a T6SS sheath, are propelled out of the cell and into target cells. On occasion, effectors are covalent extensions of VgrG, PAAR or Hcp proteins, which are then coined "evolved" components as opposed to canonical. Here, we show how various passenger domains could be fused to the C terminus of a canonical VgrG, VgrG1a from P. aeruginosa, and be sent into the bacterial culture supernatant. There is no restriction on the passenger type, although the efficacy may vary greatly, since we used either an unrelated T6SS protein, β-lactamase, a covalent extension of an "evolved" VgrG, VgrG2b, or a Hcp-dependent T6SS toxin, Tse2. Our data further highlights an exceptional modularity/flexibility for loading the T6SS nano-weapon. Refining the parameters to optimize delivery of passenger proteins of interest would have attractive medical and industrial applications. This may for example involve engineering the T6SS as a delivery system to shuttle toxins into either bacterial pathogens or tumour cells which would be an original approach in the fight against antimicrobial resistant bacteria or cancer.

The Tip of the VgrG Spike Is Essential to Functional Type VI Secretion System Assembly in Acinetobacter baumannii

The type VI secretion system (T6SS) is a critical weapon in bacterial warfare between Gram-negative bacteria. Although invaluable for niche establishment, this machine represents an energetic burden to its host bacterium. Acinetobacter baumannii is an opportunistic pathogen that poses a serious threat to public health due to its high rates of multidrug resistance. In some A. baumannii strains, the T6SS is transcriptionally downregulated by large multidrug resistance plasmids. Other strains, such as the clinical isolate AbCAN2, express T6SS-related genes but lack T6SS activity under laboratory conditions, despite not harboring these plasmids. This suggests that alternative mechanisms exist to repress the T6SS. Here, we used a transposon mutagenesis approach in AbCAN2 to identify novel T6SS repressors. Our screen revealed that the T6SS of this strain is inhibited by a homolog of VgrG, an essential structural component of all T6SSs reported to date. We named this protein inhibitory VgrG (VgrGi). Biochemical and in silico analyses demonstrated that the unprecedented inhibitory capability of VgrGi is due to a single amino acid mutation in a widely conserved C-terminal domain of unknown function, DUF2345. We also show that unlike in other bacteria, the C terminus of VgrG is essential for functional T6SS assembly in A. baumannii Our study provides insight into the architectural requirements underlying functional assembly of the T6SS of A. baumannii We propose that T6SS-inactivating point mutations are beneficial to the host bacterium, since they eliminate the energy cost associated with maintaining a functional T6SS, which appears to be unnecessary for A. baumannii virulence.IMPORTANCE Despite the clinical relevance of A. baumannii, little is known about its fundamental biology. Here, we show that a single amino acid mutation in VgrG, a critical T6SS structural protein, abrogates T6SS function. Given that this mutation was found in a clinical isolate, we propose that the T6SS of A. baumannii is probably not involved in virulence; this idea is supported by multiple genomic analyses showing that the majority of clinical A. baumannii strains lack proteins essential to the T6SS. We also show that, unlike in other species, the C terminus of VgrG is a unique architectural requirement for functional T6SS assembly in A. baumannii, suggesting that over evolutionary time, bacteria have developed changes to their T6SS architecture, leading to specialized systems.

Crosstalk Between Type VI Secretion System and Mobile Genetic Elements

Many bacterial processes require cell-cell contacts. Such are the cases of bacterial conjugation, one of the main horizontal gene transfer mechanisms that physically spreads DNA, and the type VI secretion systems (T6SSs), which deploy antibacterial activity. Bacteria depend on conjugation to adapt to changing environments, while T6SS killing activity could pose a threat to mating partners. Here we review the experimental evidences of overlapping and interaction between the T6SSs, bacterial conjugation, and conjugative genetic elements.

The role of Acinetobacter baumannii response regulator BfmR in pellicle formation and competitiveness via contact-dependent inhibition system

Background

Acinetobacter baumannii is one of the most important opportunistic pathogens responsible for hospital acquired infections. It displays multi-drug resistance profile and has the ability to colonize surfaces and persist under harsh conditions. A. baumannii two-component signal transduction system BfmRS, consisting of response regulator BfmR and sensor kinase BfmS, has been implicated in the control of various virulence-related traits and has been suggested to act as a global modulator of A. baumannii physiology.

Results

Here, we assessed the role of BfmR regulator in pellicle formation and bacterial competition, features important for the establishment of A. baumannii in clinical environment. We show that BfmR is required for the pellicle formation of A. baumannii, as ΔbfmRS mutant lacked this phenotype. The loss of bfmRS also greatly reduced the secretion of A. baumannii Hcp protein, which is a component of T6SS secretion system. However, T6SS-mediated killing phenotype was not impaired in ΔbfmRS mutant. On the contrary, the same mutation resulted in the transcriptional activation of contact-dependent inhibition (CDI) system, which A. baumannii used to inhibit the growth of another clinical A. baumannii strain and a closely related species Acinetobacter baylyi.

Conclusions

The obtained results indicate that BfmR is not only required for the pellicle phenotype induction in A. baumannii, but also, due to the down-regulation of a CDI system, could allow the incorporation of other A. baumannii strains or related species, possibly increasing the likelihood of the pathogens’ survival.

Type VI Secretion System in Pathogenic Escherichia coli: Structure, Role in Virulence, and Acquisition

Bacterial pathogens utilize a myriad of mechanisms to invade mammalian hosts, damage tissue sites, and evade the immune system. One essential strategy of Gram-negative bacteria is the secretion of virulence factors through both inner and outer membranes to reach a potential target. Most secretion systems are harbored in mobile elements including transposons, plasmids, pathogenicity islands, and phages, and Escherichia coli is one of the more versatile bacteria adopting this genetic information by horizontal gene transfer. Additionally, E. coli is a bacterial species with members of the commensal intestinal microbiota and pathogens associated with numerous types of infections such as intestinal, urinary, and systemic in humans and other animals. T6SS cluster plasticity suggests evolutionarily divergent systems were acquired horizontally. T6SS is a secretion nanomachine that is extended through the bacterial double membrane; from this apparatus, substrates are conveyed straight from the cytoplasm of the bacterium into a target cell or to the extracellular space. This nanomachine consists of three main complexes: proteins in the inner membrane that are T4SS component-like, the baseplate complex, and the tail complex, which are formed by components evolutionarily related to contractile bacteriophage tails. Advances in the T6SS understanding include the functional and structural characterization of at least 13 subunits (so-called core components), which are thought to comprise the minimal apparatus. So far, the main role of T6SS is on bacterial competition by using it to kill neighboring non-immune bacteria for which antibacterial proteins are secreted directly into the periplasm of the bacterial target after cell-cell contact. Interestingly, a few T6SSs have been associated directly to pathogenesis, e.g., roles in biofilm formation and macrophage survival. Here, we focus on the advances on T6SS from the perspective of E. coli pathotypes with emphasis in the secretion apparatus architecture, the mechanisms of pathogenicity of effector proteins, and the events of lateral gene transfer that led to its spread.

Delivery of the Pseudomonas aeruginosa Phospholipase Effectors PldA and PldB in a VgrG- and H2-T6SS-Dependent Manner

The bacterial pathogen Pseudomonas aeruginosa uses three type VI secretion systems (T6SSs) to drive a multitude of effector proteins into eukaryotic or prokaryotic target cells. The T6SS is a supramolecular nanomachine, involving a set of 13 core proteins, which resembles the contractile tail of bacteriophages and whose tip is considered as a puncturing device helping to cross membranes. Effectors can attach directly to the T6SS spike which is composed of a VgrG (valine-glycine-rich proteins) trimer, of which P. aeruginosa produces several. We have previously shown that the master regulator RsmA controls the expression of all three T6SS gene clusters (H1-, H2- and H3-T6SS) and a range of remote vgrG and effector genes. We also demonstrated that specific interactions between VgrGs and various T6SS effectors are prerequisite for effector delivery in a process we called "à la carte delivery." Here, we provide an in-depth description on how the two H2-T6SS-dependent effectors PldA and PldB are delivered via their cognate VgrGs, VgrG4b and VgrG5, respectively. We show that specific recognition of the VgrG C terminus is required and effector specificity can be swapped by exchanging these C-terminal domains. Importantly, we established that effector recognition by a cognate VgrG is not always sufficient to achieve successful secretion, but it is crucial to provide effector stability. This study highlights the complexity of effector adaptation to the T6SS nanomachine and shows how the VgrG tip can possibly be manipulated to achieve effector delivery.

Environmental and cellular factors affecting the localization of T6SS proteins in Burkholderia thailandensis

The type VI secretion system (T6SS) injects effector proteins into neighboring bacteria and host cells. Effector translocation is driven by contraction of a tubular sheath in the cytoplasm that expels an inner needle across the cell envelope. The AAA + ATPase ClpV disassembles and recycles the contracted sheath. While ClpV-1-GFP of the Burkholderia T6SS-1, which targets prokaryotic cells, assembles into randomly localized foci, ClpV-5-GFP of the virulence-associated T6SS-5 displays a polar distribution. The mechanisms underlying the localization of T6SSs to a particular site in the bacterial cell are currently unknown. We recently showed that ClpV-5-GFP retains its polar localization in the absence of all T6SS-5 components during infection of host cells. Herein, we set out to identify factors involved in the distribution of ClpV-5 and ClpV-1 in Burkholderia thailandensis. We show that focal assembly and polar localization of ClpV-5-GFP is not dependent on the intracellular host cell environment, known to contain the signal to induce T6SS-5 gene expression. In contrast to ClpV-5-GFP, localization of ClpV-1-GFP was dependent on the cognate T6SS. Foci formation of both ClpV5-GFP and ClpV-1-GFP was decreased by D cycloserine-mediated inhibition of peptidoglycan synthesis while treatment of B. thailandensis with A22 blocking the cytoskeletal protein MreB did not affect assembly of ClpV-5 and ClpV-1 into single discrete foci. Furthermore, we found that surface contact promotes but is not essential for localization of ClpV-5-GFP to the pole whereas expression of clpV-1-gfp appears to be induced by surface contact. In summary, the study provides novel insights into the localization of ClpV ATPases of T6SSs targeting prokaryotic and eukaryotic cells.

Structure and Activity of the Type VI Secretion System

The type VI secretion system (T6SS) is a multiprotein machine that uses a spring-like mechanism to inject effectors into target cells. The injection apparatus is composed of a baseplate on which is built a contractile tail tube/sheath complex. The inner tube, topped by the spike complex, is propelled outside of the cell by the contraction of the sheath. The injection system is anchored to the cell envelope and oriented towards the cell exterior by a trans-envelope complex. Effectors delivered by the T6SS are loaded within the inner tube or on the spike complex and can target prokaryotic and/or eukaryotic cells. Here we summarize the structure, assembly, and mechanism of action of the T6SS. We also review the function of effectors and their mode of recruitment and delivery.

Role and Recruitment of the TagL Peptidoglycan-Binding Protein during Type VI Secretion System Biogenesis

The type VI secretion system (T6SS) is an injection apparatus that uses a springlike mechanism for effector delivery. The contractile tail is composed of a needle tipped by a sharpened spike and wrapped by the sheath that polymerizes in an extended conformation on the assembly platform, or baseplate. Contraction of the sheath propels the needle and effectors associated with it into target cells. The passage of the needle through the cell envelope of the attacker is ensured by a dedicated trans-envelope channel complex. This membrane complex (MC) comprises the TssJ lipoprotein and the TssL and TssM inner membrane proteins. MC assembly is a hierarchized mechanism in which the different subunits are recruited in a specific order: TssJ, TssM, and then TssL. Once assembled, the MC serves as a docking station for the baseplate. In enteroaggregative Escherichia coli, the MC is accessorized by TagL, a peptidoglycan-binding (PGB) inner membrane-anchored protein. Here, we show that the PGB domain is the only functional domain of TagL and that the N-terminal transmembrane region mediates contact with the TssL transmembrane helix. Finally, we conduct fluorescence microscopy experiments to position TagL in the T6SS biogenesis pathway, demonstrating that TagL is recruited to the membrane complex downstream of TssL and is not required for baseplate docking.IMPORTANCE Bacteria use weapons to deliver effectors into target cells. One of these weapons, called the type VI secretion system (T6SS), could be compared to a nano-spear gun using a springlike mechanism for effector injection. By targeting bacteria and eukaryotic cells, the T6SS reshapes bacterial communities and hijacks host cell defenses. In enteroaggregative Escherichia coli, the T6SS is a multiprotein machine that comprises a cytoplasmic tail and a peptidoglycan-anchored trans-envelope channel. In this work, we show that TagL comprises an N-terminal domain that mediates contact with the channel and a peptidoglycan-binding domain that binds the cell wall. We then determine at which stage of T6SS biogenesis TagL is recruited and how TagL absence impacts the assembly pathway.

In situ and high-resolution cryo-EM structure of a bacterial type VI secretion system membrane complex

Bacteria have evolved macromolecular machineries that secrete effectors and toxins to survive and thrive in diverse environments. The type VI secretion system (T6SS) is a contractile machine that is related to Myoviridae phages. It is composed of a phage tail-like structure inserted in the bacterial cell envelope by a membrane complex (MC) comprising the TssJ, TssL and TssM proteins. We previously reported the low-resolution negative-stain electron microscopy structure of the enteroaggregative Escherichia coli MC and proposed a rotational 5-fold symmetry with a TssJ:TssL:TssM stoichiometry of 2:2:2. Here, cryo-electron tomography analyses of the T6SS MC confirm the 5-fold symmetry in situ and identify the regions of the structure that insert into the bacterial membranes. A high-resolution model obtained by single-particle cryo-electron microscopy highlights new features: five additional copies of TssJ, yielding a TssJ:TssL:TssM stoichiometry of 3:2:2, an 11-residue loop in TssM, protruding inside the lumen of the MC and constituting a functionally important periplasmic gate, and hinge regions. Based on these data, we propose an updated model on MC structure and dynamics during T6SS assembly and function.

Characterization of multiple type-VI secretion system (T6SS) VgrG proteins in the pathogenicity and antibacterial activity of porcine extra-intestinal pathogenic Escherichia coli

Porcine extra-intestinal pathogenic Escherichia coli (ExPEC) causes great economic losses to the pig industry and poses a serious threat to public health worldwide. Some secreted virulence factors have been reported to be involved in the pathogenicity of the infection caused by ExPEC. Type-VI secretion system (T6SS) is discovered in many Gram-negative bacteria and contributes to the virulence of pathogenic bacteria. Valine-glycine repeat protein G (VgrG) has been reported as an important component of the functional T6SS. In our previous studies, a functional T6SS was identified in porcine ExPEC strain PCN033. Further analysis of the PCN033 genome identified two putative vgrGs genes (vgrG1 and 0248) located inside T6SS cluster and another two (vgrG2 and 1588) outside it. This study determined the function of the four putative VgrG proteins by constructing a series of mutants and complemented strains. In vitro, the VgrG1 protein was observed to be involved in the antibacterial ability and the interactions with cells. The animal model experiment showed that the deletion of vgrG1 significantly led to the decrease in the multiplication capacity of PCN033. However, the deletion of 0248 and/or the deletion of vgrG2 and 1588 had no effect on the pathogenicity of PCN033. The study of four putative VgrGs in PCN033 indicated that only VgrG1 plays an important role in the interaction between PCN033 and other bacteria or host cells. This study can provide a novel perspective to the pathogenesis of PCN033 and lay the foundation for discovering potential T6SS effectors.

Bidirectional contraction of a type six secretion system

Contractile injection systems (CISs) mediate cell-cell interactions by a phage tail-like apparatus. Their conserved mechanism relies on the anchoring of the proximal end of a sheath-tube module to a membrane, followed by contraction of the sheath towards the attachment site and ejection of the inner tube. Here we reveal a major variation of the CIS mechanism in the type six secretion system (T6SS) of enteroaggregative Escherichia coli (EAEC). We show that both ends of the sheath-tube module are attached to opposite sides of the cell, enabling the structure to contract in two opposite directions. The protein TssA1 mediates the interaction of the distal end with the cell envelope, the termination of tail elongation, and non-canonical contraction towards the distal end. We provide a framework for the molecular processes at the T6SS distal end. Further research will address whether bidirectional contraction allows for bidirectional effector secretion. The unrecognized concept of non-canonical contractions could be relevant to biofilms of the human intestine.

In vitro Trypanocidal Activity, Genomic Analysis of Isolates, and in vivo Transcription of Type VI Secretion System of Serratia marcescens Belonging to the Microbiota of Rhodnius prolixus Digestive Tract

Serratia marcescens is a bacterium with the ability to colonize several niches, including some eukaryotic hosts. S. marcescens have been recently found in the gut of hematophagous insects that act as parasite vectors, such as Anopheles, Rhodnius, and Triatoma. While some S. marcescens strains have been reported as symbiotic or pathogenic to other insects, the role of S. marcescens populations from the gut microbiota of Rhodnius prolixus, a vector of Chagas’ disease, remains unknown. Bacterial colonies from R. prolixus gut were isolated on BHI agar. After BOX-PCR fingerprinting, the genomic sequences of two isolates RPA1 and RPH1 were compared to others S. marcescens from the NCBI database in other to estimate their evolutionary divergence. The in vitro trypanolytic activity of these two bacterial isolates against Trypanosoma cruzi (DM28c clone and Y strain) was assessed by microscopy. In addition, the gene expression of type VI secretion system (T6SS) was detected in vivo by RT-PCR. Comparative genomics of RPA1 and RPH1 revealed, besides plasmid presence and genomic islands, genes related to motility, attachment, and quorum sensing in both genomes while genes for urea hydrolysis and type II secretion system (T2SS) were found only in the RPA1 genome. The in vitro trypanolytic activity of both S. marcescens strains was stronger in their stationary phases of growth than in their exponential ones, with 65–70 and 85–90% of epimastigotes (Dm28c clone and Y strain, respectively) being lysed after incubation with RPA1 or RPH1 in stationary phase. Although T6SS transcripts were detected in guts up to 40 days after feeding (DAF), R. prolixus morbidity or mortality did not appear to be affected. In this report, we made available two trypanolytic S. marcescens strains from R. prolixus gut to the scientific community together with their genomic sequences. Here, we describe their genomic features with the purpose of bringing new insights into the S. marcescens adaptations for colonization of the specific niche of triatomine guts. This study provides the basis for a better understanding of the role of S. marcescens in the microbiota of R. prolixus gut as a potential antagonist of T. cruzi in this complex system.

Structural insights into the function of type VI secretion system TssA subunits

The type VI secretion system (T6SS) is a multi-protein complex that injects bacterial effector proteins into target cells. It is composed of a cell membrane complex anchored to a contractile bacteriophage tail-like apparatus consisting of a sharpened tube that is ejected by the contraction of a sheath against a baseplate. We present structural and biochemical studies on TssA subunits from two different T6SSs that reveal radically different quaternary structures in comparison to the dodecameric E. coli TssA that arise from differences in their C-terminal sequences. Despite this, the different TssAs retain equivalent interactions with other components of the complex and position their highly conserved N-terminal ImpA_N domain at the same radius from the centre of the sheath as a result of their distinct domain architectures, which includes additional spacer domains and highly mobile interdomain linkers. Together, these variations allow these distinct TssAs to perform a similar function in the complex.

TssA is an important component of the bacterial type VI secretion system (T6SS). Here, Dix et al. integrate structural, phylogenetic and functional analysis of the TssA subunits, providing new insights into their role in T6SS assembly and function.

Vibrio parahaemolyticus RhsP represents a widespread group of pro-effectors for type VI secretion systems

Type VI secretion systems (T6SSs) translocate effector proteins, such as Rhs toxins, to eukaryotic cells or prokaryotic competitors. All T6SS Rhs-type effectors characterized thus far contain a PAAR motif or a similar structure. Here, we describe a T6SS-dependent delivery mechanism for a subset of Rhs proteins that lack a PAAR motif. We show that the N-terminal Rhs domain of protein RhsP (or VP1517) from Vibrio parahaemolyticus inhibits the activity of the C-terminal DNase domain. Upon auto-proteolysis, the Rhs fragment remains inside the cells, and the C-terminal region interacts with PAAR2 and is secreted by T6SS2; therefore, RhsP acts as a pro-effector. Furthermore, we show that RhsP contributes to the control of certain “social cheaters” (opaR mutants). Genes encoding proteins with similar Rhs and PAAR-interacting domains, but diverse C-terminal regions, are widely distributed among Vibrio species.

It is unclear how Rhs toxins lacking a PAAR motif are secreted by Type VI secretion systems. Here, the authors show for one of these proteins that the mechanism requires removal of an N-terminal fragment by auto-proteolysis, followed by interaction with a PAAR protein and then secretion.

TssA from Burkholderia cenocepacia: expression, purification, crystallization and crystallographic analysis

TssA is a core component of the type VI secretion system, and phylogenetic analysis of TssA subunits from different species has suggested that these proteins fall into three distinct clades. Whilst representatives of two clades, TssA1 and TssA2, have been the subjects of investigation, no members of the third clade (TssA3) have been studied. Constructs of TssA from Burkholderia cenocepacia, a representative of clade 3, were expressed, purified and subjected to crystallization trials. Data were collected from crystals of constructs of the N-terminal and C-terminal domains. Analysis of the data from the crystals of these constructs and preliminary structure determination indicates that the C-terminal domain forms an assembly of 32 subunits in D16 symmetry, whereas the N-terminal domain is not involved in subunit assocation.

TssA from Aeromonas hydrophila: expression, purification and crystallographic studies

TssA is a core subunit of the type VI secretion system, which is a major player in interspecies competition in Gram-negative bacteria. Previous studies on enteroaggregative Escherichia coli TssA suggested that it is comprised of three putative domains: a conserved N-terminal domain, a middle domain and a ring-forming C-terminal domain. X-ray studies of the latter two domains have identified their respective structures. Here, the results of the expression and purification of full-length and domain constructs of TssA from Aeromonas hydrophila are reported, resulting in diffraction-quality crystals for the middle domain (Nt2) and a construct including the middle and C-terminal domains (Nt2-CTD).

Type VI Secretion Systems Present New Insights on Pathogenic Yersinia

The type VI secretion system (T6SS) is a versatile secretion system widely distributed in Gram-negative bacteria that delivers multiple effector proteins into either prokaryotic or eukaryotic cells, or into the extracellular milieu. T6SS participates in various physiological processes including bacterial competition, host infection, and stress response. Three pathogenic Yersinia species, namely Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica, possess different copies of T6SSs with distinct biological functions. This review summarizes the pathogenic, antibacterial, and stress-resistant roles of T6SS in Yersinia and the ion-transporting ability in Y. pseudotuberculosis. In addition, the T6SS-related effectors and regulators identified in Yersinia are discussed.