An onboard checking mechanism ensures effector delivery of the type VI secretion system in Vibrio cholerae

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.

A coordinated attack by a bacterial secretion system and a small molecule drives prey specificity

Vibrio species are recognized for their role in food- and water-borne diseases in humans, fish, and aquatic invertebrates. We screened bacterial strains isolated from raw food shrimp for those that are bactericidal to Vibrio strains. Here we identify and characterize Aeromonas dhakensis strain A603 which shows robust bactericidal activity specifically towards Vibrio and related taxa but less potency toward other Gram-negative species. Using the A603 genome and genetic analysis, we show that two antibacterial mechanisms account for its vibriocidal activity -- a highly potent Type Six Secretion System (T6SS) and biosynthesis of a vibriocidal phenazine-like small molecule, named here as Ad-Phen. Further analysis indicates coregulation between Ad-Phen and a pore-forming T6SS effector TseC, which potentiates V. cholerae to killing by Ad-Phen.

Functionality of chimeric TssA proteins in the type VI secretion system reveals sheath docking specificity within their N-terminal domains

The genome of Pseudomonas aeruginosa encodes three type VI secretion systems, each comprising a dozen distinct proteins, which deliver toxins upon T6SS sheath contraction. The least conserved T6SS component, TssA, has variations in size which influence domain organisation and structure. Here we show that the TssA Nt1 domain interacts directly with the sheath in a specific manner, while the C-terminus is essential for oligomerisation. We built chimeric TssA proteins by swapping C-termini and showed that these can be functional even when made of domains from different TssA sub-groups. Functional specificity requires the Nt1 domain, while the origin of the C-terminal domain is more permissive for T6SS function. We identify two regions in short TssA proteins, loop and hairpin, that contribute to sheath binding. We propose a docking mechanism of TssA proteins with the sheath, and a model for how sheath assembly is coordinated by TssA proteins from this position.

Constitutive expression of the Type VI Secretion System carries no measurable fitness cost in Vibrio cholerae

The Type VI Secretion System (T6SS) is a widespread and highly effective mechanism of microbial warfare; it confers the ability to efficiently kill susceptible cells within close proximity. Due to its large physical size, complexity, and ballistic basis for intoxication, it has widely been assumed to incur significant growth costs in the absence of improved competitive outcomes. In this study, we precisely examine the fitness costs of constitutive T6SS firing in the bacterium Vibrio cholerae. We find that, contrary to expectations, constitutive expression of the T6SS has a negligible impact on growth, reducing growth fitness by 0.025 ± 0.5% (95% CI) relative to a T6SS- control. Mathematical modeling of microbial populations demonstrates that, due to clonal interference, constitutive expression of the T6SS will often be neutral, with little impact on evolutionary outcomes. Our findings underscore the importance of precisely measuring the fitness costs of microbial social behaviors and help explain the prevalence of the T6SS across Gram-negative bacteria.

Soil bacterium manipulates antifungal weapons by sensing intracellular type IVA secretion system effectors of a competitor

Soil beneficial bacteria can effectively inhibit bacterial pathogens by assembling contact-dependent killing weapons, such as the type IVA secretion system (T4ASS). It's not clear whether these antibacterial weapons are involved in biotrophic microbial interactions in soil. Here we showed that an antifungal antibiotic 2,4-DAPG production of the soil bacterium, Pseudomonas protegens can be triggered by another soil bacterium, Lysobacter enzymogenes, via T4ASS by co-culturing on agar plates to mimic cell-to-cell contact. We demonstrated that the induced 2,4-DAPG production of P. protegens is achieved by intracellular detection of the T4ASS effector protein Le1519 translocated from L. enzymogenes. We defined Le1519 as LtaE (Lysobacter T4E triggering antifungal effects), which specifically stimulates the expression of 2,4-DAPG biosynthesis genes in P. protegens, thereby protecting soybean seedlings from infection by the fungus Rhizoctonia solani. We further found that LtaE directly bound to PhlF, a pathway-specific transcriptional repressor of the 2,4-DAPG biosynthesis, then activated the 2,4-DAPG production. Our results highlight a novel pattern of microbial interspecies and interkingdom interactions, providing a unique case for expanding the diversity of soil microbial interactions.

Acinetobacter type VI secretion system comprises a non-canonical membrane complex

A. baumannii can rapidly acquire new resistance mechanisms and persist on abiotic surface, enabling the colonization of asymptomatic human host. In Acinetobacter the type VI secretion system (T6SS) is involved in twitching, surface motility and is used for interbacterial competition allowing the bacteria to uptake DNA. A. baumannii possesses a T6SS that has been well studied for its regulation and specific activity, but little is known concerning its assembly and architecture. The T6SS nanomachine is built from three architectural sub-complexes. Unlike the baseplate (BP) and the tail-tube complex (TTC), which are inherited from bacteriophages, the membrane complex (MC) originates from bacteria. The MC is the most external part of the T6SS and, as such, is subjected to evolution and adaptation. One unanswered question on the MC is how such a gigantesque molecular edifice is inserted and crosses the bacterial cell envelope. The A. baumannii MC lacks an essential component, the TssJ lipoprotein, which anchors the MC to the outer membrane. In this work, we studied how A. baumannii compensates the absence of a TssJ. We have characterized for the first time the A. baumannii's specific T6SS MC, its unique characteristic, its membrane localization, and assembly dynamics. We also defined its composition, demonstrating that its biogenesis employs three Acinetobacter-specific envelope-associated proteins that define an intricate network leading to the assembly of a five-proteins membrane super-complex. Our data suggest that A. baumannii has divided the function of TssJ by (1) co-opting a new protein TsmK that stabilizes the MC and by (2) evolving a new domain in TssM for homo-oligomerization, a prerequisite to build the T6SS channel. We believe that the atypical species-specific features we report in this study will have profound implication in our understanding of the assembly and evolutionary diversity of different T6SSs, that warrants future investigation.

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.

Diversity of the type VI secretion systems in the Neisseria spp

Complete Type VI Secretion Systems were identified in the genome sequence data of Neisseria subflava isolates sourced from throat swabs of human volunteers. The previous report was the first to describe two complete Type VI Secretion Systems in these isolates, both of which were distinct in terms of their gene organization and sequence homology. Since publication of the first report, Type VI Secretion System subtypes have been identified in Neisseria spp. The characteristics of each type in N. subflava are further investigated here and in the context of the other Neisseria spp., including identification of the lineages containing the different types and subtypes. Type VI Secretion Systems use VgrG for delivery of toxin effector proteins; several copies of vgrG and associated effector / immunity pairs are present in Neisseria spp. Based on sequence similarity between strains and species, these core Type VI Secretion System genes, vgrG, and effector / immunity genes may diversify via horizontal gene transfer, an instrument for gene acquisition and repair in Neisseria spp.

VgrG Spike Dictates PAAR Requirement for the Assembly of the Type VI Secretion System

Widely employed by Gram-negative pathogens for competition and pathogenesis, the type six protein secretion system (T6SS) can inject toxic effectors into neighboring cells through the penetration of a spear-like structure comprising a long Hcp tube and a VgrG-PAAR spike complex. The cone-shaped PAAR is believed to sharpen the T6SS spear for penetration but it remains unclear why PAAR is required for T6SS functions in some bacteria but dispensable in others. Here, we report the conditional requirement of PAAR for T6SS functions in Aeromonas dhakensis, an emerging human pathogen that may cause severe bacteremia. By deleting the two PAAR paralogs, we show that PAAR is not required for T6SS secretion, bacterial killing, or specific effector delivery in A. dhakensis. By constructing combinatorial PAAR and vgrG deletions, we demonstrate that deletion of individual PAAR moderately reduced T6SS functions but double or triple deletions of PAAR in the vgrG deletion mutants severely impaired T6SS functions. Notably, the auxiliary-cluster-encoded PAAR2 and VgrG3 are less critical than the main-cluster-encoded PAAR1 and VgrG1&2 proteins to T6SS functions. In addition, PAAR1 but not PAAR2 contributes to antieukaryotic virulence in amoeba. Our data suggest that, for a multi-PAAR T6SS, the variable role of PAAR paralogs correlates with the VgrG-spike composition that collectively dictates T6SS assembly. IMPORTANCE Gram-negative bacteria often encode multiple paralogs of the cone-shaped PAAR that sits atop the VgrG-spike and is thought to sharpen the spear-like T6SS puncturing device. However, it is unclear why PAAR is required for the assembly of some but not all T6SSs and why there are multiple PAARs if they are not required. Our data delineate a VgrG-mediated conditional requirement for PAAR and suggest a core-auxiliary relationship among different PAAR-VgrG modules that may have been acquired sequentially by the T6SS during evolution.

A Dueling-Competent Signal-Sensing Module Guides Precise Delivery of Cargo Proteins into Target Cells by Engineered Pseudomonas aeruginosa

To recognize and manipulate a specific microbe of a crowded community is a highly challenging task in synthetic biology. Here we introduce a highly selective protein delivery platform, termed DUEC, which responds to direct contact of attacking cells by engineering the tit-for-tat/dueling response of H1-T6SS (type VI secretion system) in Pseudomonas aeruginosa. Using a Cre-recombinase-dependent reporter, we screened H1-T6SS-secreted substrates and developed Tse6^N as the most effective secretion tag for Cre delivery. DUEC cells can discriminately deliver the Tse6^N-Cre cargo into the cytosol of T6SS⁺ but not T6SS^- Vibrio cholerae cells. DUEC could also deliver a nuclease cargo, Tse6^N-NucSe1, to selectively kill provoking cells in a mixed community. These data demonstrate that the DUEC cell not only is a prototypical physical-contact sensor and delivery platform but also may be coupled with recombination-based circuits with the potential for complex tasks in mixed microbial communities.

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.

Pesticin-Like Effector VgrG3cp Targeting Peptidoglycan Delivered by the Type VI Secretion System Contributes to Vibrio cholerae Interbacterial Competition

Vibrio cholerae can utilize a type VI secretion system (T6SS) to increase its intra- and interspecies competition. However, much still remains to be understood about the underlying mechanism of this intraspecies competition. In this study, we isolated an environmental V. cholerae strain E1 that lacked the typical virulence factors toxin-coregulated pilus and cholera toxin and that encoded a functional T6SS. We identified an evolved VgrG3 variant with a predicted C-terminal pesticin-like domain in V. cholerae E1, designated VgrG3cp. Using heterologous expression, protein secretion, and peptidoglycan-degrading assays, we demonstrated that VgrG3cp is a T6SS-dependent effector harboring cell wall muramidase activity and that its toxicity can be neutralized by cognate immunity protein TsiV3cp. Site-directed mutagenesis proved that the aspartic acid residue at position 867 is crucial for VgrG3cp-mediated antibacterial activity. Bioinformatic analysis showed that genes encoding VgrG3cp-like homologs are distributed in Vibrio species, are linked with T6SS structural genes and auxiliary genes, and the vgrG3cp-tsiV3cp gene pair of V. cholerae probably evolved from Vibrio anguillarum and Vibrio fluvialis via homologous recombination. Through a time-lapse microscopy assay, we directly determined that cells accumulating VgrG3cp disrupted bacterial division, while the cells continued to increase in size until the loss of membrane potential and cell wall breakage and finally burst. The results of the competitive killing assay showed that VgrG3cp contributes to V. cholerae interspecies competition. Collectively, our study revealed a novel T6SS E-I pair representing a new T6SS toxin family which allows V. cholerae to gain dominance within polymicrobial communities by T6SS. IMPORTANCE The type VI secretion system used by a broad range of Gram-negative bacteria delivers toxic proteins to target adjacent eukaryotic and prokaryotic cells. Diversification of effector proteins determines the complex bacterium-bacterium interactions and impacts the health of hosts and environmental ecosystems in which bacteria reside. This work uncovered an evolved valine-glycine repeat protein G3, carrying a C-terminal pesticin-like domain (VgrG3cp), which has been suggested to harbor cell wall hydrolase activity and is able to affect cell division and the integrity of cell wall structure. Pesticin-like homologs constitute a family of T6SS-associated effectors targeting bacterial peptidoglycan which are distributed in Vibrio species, and genetic loci of them are linked with T6SS structural genes and auxiliary genes. T6SS-delivered VgrG3cp mediated broad-spectrum antibacterial activity for several microorganisms tested, indicating that VgrG3cp-mediated antimicrobial activity is capable of conferring bacteria a competitive advantage over competitors in the same niches.

The phospholipase effector Tle1Vc promotes Vibrio cholerae virulence by killing competitors and impacting gene expression

Vibrio cholerae utilizes the Type VI secretion system (T6SS) to gain an advantage in interbacterial competition by delivering anti-prokaryotic effectors in a contact-dependent manner. However, the impact of T6SS and its secreted effectors on physiological behavior remains poorly understood. In this study, we present Tle1Vc, a phospholipase effector in atypical pathogenic V. cholerae E1 that is secreted by T6SS via its interaction with VgrG1E1. Tle1Vc contains a DUF2235 domain and belongs to the Tle1 (type VI lipase effector) family. Bacterial toxicity assays, lipase activity assays and site-directed mutagenesis revealed that Tle1Vc possessed phospholipase A1 activity and phospholipase A2 activity, and that Tle1Vc-induced toxicity required a serine residue (S356) and two aspartic acid residues (D417 and D496). Cells intoxication with Tle1Vc lead to membrane depolarization and alter membrane permeability. Tli1tox-, a cognate immunity protein, directly interacts with Tle1Vc to neutralize its toxicity. Moreover, Tle1Vc can kill multiple microorganisms by T6SS and promote in vivo fitness of V. cholerae through mediating antibacterial activity. Tle1Vc induces bacterial motility by increasing the expression of flagellar-related genes independently of functional T6SS and the tit-for-tat (TFT) response, where Pseudomonas aeruginosa uses its T6SS-H1 cluster to counterattack other offensive attackers. Our study also demonstrated that the physical puncture of E1 T6SS can induce a moderate TFT response, which is essential to the Tle1Vc-mediated strong TFT response, maximizing effector functions. Overall, our study characterized the antibacterial mechanism of phospholipase effector Tle1Vc and its multiple physiological significance.

Heterologous Assembly of the Type VI Secretion System Empowers Laboratory Escherichia coli with Antimicrobial and Cell Penetration Capabilities

The synthetic biology toolbox has amassed a vast number of diverse functional modules, but protein translocation modules for cell penetration and cytosol-to-cytosol delivery remain relatively scarce. The type VI secretion system (T6SS), commonly found in many Gram-negative pathogens, functions as a contractile device to translocate protein toxins to prokaryotic and eukaryotic cells. Here, we have assembled the T6SS of Aeromonas dhakensis, an opportunistic waterborne pathogen, in the common laboratory strain Escherichia coli BL21(DE3). We constructed a series of plasmids (pT6S) carrying the T6SS structural and effector genes under native or tetracycline-inducible promoters, the latter for controlled expression. Using fluorescence microscopy and biochemical analyses, we demonstrate a functional T6SS in E. coli capable of secreting proteins directly into the cytosol of neighboring bacteria and outcompeting a number of drug-resistant pathogens. The heterologous assembly of T6SS not only confers the lab workhorse E. coli with the cytosol-to-cytosol protein delivery capability but also demonstrates the potential for harnessing the T6SS of various pathogens for general protein delivery and antibacterial applications. IMPORTANCE The T6SS is a powerful and versatile protein delivery system. However, the complexity of its macromolecular structure and gene regulation makes it not a trivial task to reconstitute the T6SSs of pathogens in a nonpathogenic host. In this study, we have assembled an inducible T6SS in E. coli BL21(DE3) and demonstrated its functions in protein delivery and antimicrobial activities. The engineered T6SS empowers E. coli to deliver protein cargos into a wide range of prokaryotic and eukaryotic cells.

Abiotic factors modulate interspecies competition mediated by the type VI secretion system effectors in Vibrio cholerae

Vibrio cholerae, the etiological pathogen of cholera, employs its type VI secretion system (T6SS) as an effective weapon to survive in highly competitive communities. Antibacterial and anti-eukaryotic functions of the T6SS depend on its secreted effectors that target multiple cellular processes. However, the mechanisms that account for effector diversity and different effectiveness during interspecies competition remain elusive. Here we report that environmental cations and temperature play a key role in dictating cellular response and effector effectiveness during interspecies competition mediated by the T6SS of V. cholerae. We found that V. cholerae could employ its cell-wall-targeting effector TseH to outcompete the otherwise resistant Escherichia coli and the V. cholerae immunity deletion mutant ∆tsiH when Mg²⁺ or Ca²⁺ was supplemented. Transcriptome and genetic analyses demonstrate that the metal-sensing PhoPQ two-component system is important for Mg²⁺-dependent sensitivity. Competition analysis in infant mice shows that TseH was active under in vivo conditions. Using a panel of V. cholerae single-effector active mutants, we further show that E. coli also exhibited variable susceptibilities to other T6SS effectors depending on cations and temperatures, respectively. Lastly, V. cholerae effector VasX could sensitize Pseudomonas aeruginosa to its intrinsically resistant antibiotic irgasan in a temperature-dependent manner. Collectively, these findings suggest that abiotic factors, that V. cholerae frequently encounters in natural and host environments, could modulate cellular responses and dictate the competitive fitness conferred by the T6SS effectors in complex multispecies communities.

RpoN/Sfa2-dependent activation of the Pseudomonas aeruginosa H2-T6SS and its cognate arsenal of antibacterial toxins

Pseudomonas aeruginosa uses three type six secretion systems (H1-, H2- and H3-T6SS) to manipulate its environment, subvert host cells and for microbial competition. These T6SS machines are loaded with a variety of effectors/toxins, many being associated with a specific VgrG. How P. aeruginosa transcriptionally coordinates the main T6SS clusters and the multiple vgrG islands spread through the genome is unknown. Here we show an unprecedented level of control with RsmA repressing most known T6SS-related genes. Moreover, each of the H2- and H3-T6SS clusters encodes a sigma factor activator (SFA) protein called, Sfa2 and Sfa3, respectively. SFA proteins are enhancer binding proteins necessary for the sigma factor RpoN. Using a combination of RNA-seq, ChIP-seq and molecular biology approaches, we demonstrate that RpoN coordinates the T6SSs of P. aeruginosa by activating the H2-T6SS but repressing the H1- and H3-T6SS. Furthermore, RpoN and Sfa2 control the expression of the H2-T6SS-linked VgrGs and their effector arsenal to enable very effective interbacterial killing. Sfa2 is specific as Sfa3 from the H3-T6SS cannot complement loss of Sfa2. Our study further delineates the regulatory mechanisms that modulate the deployment of an arsenal of T6SS effectors likely enabling P. aeruginosa to adapt to a range of environmental conditions.

More Than Just a Spearhead: Diverse Functions of PAAR for Assembly and Delivery of Toxins of the Contractile Injection Systems

The type VI secretion system (T6SS) belongs to the evolutionarily related group of contractile injection systems that employ a contractile outer sheath to inject a rigid spear-like inner tube into target bacterial and eukaryotic cells. The tip of the rigid tube is often decorated by a PAAR-repeat protein as a key structural component. Many members of the PAAR protein family can also have additional and diverse functions by serving as toxins for those with extended domains or as carriers for interacting toxins. A plethora of toxin modules or modules of unknown functions have been bioinformatically predicted to be associated with PAAR either as a fused domain or as an interacting partner, and yet only a small number of PAAR proteins have been studied, highlighting the exciting and dire need for future research to better understand the diverse PAAR-mediated functions.

Unlocking the bacterial contact-dependent antibacterial activity to engineer a biocontrol alliance of two species from natural incompatibility to artificial compatibility

Plant growth-promoting rhizobacteria (PGPR) contain various biocontrol bacteria with broad-spectrum antimicrobial activity, and their single species has been extensively applied to control crop diseases. The development of complex biocontrol community by mixing two or more PGPR members together is a promising strategy to enlarge the efficacy and scope of biocontrol. However, an effective method to assess the natural compatibility of PGPR members has not yet been established to date. Here, we developed such a tool by using the bacterial contact-dependent antibacterial activity (CDAA) as a probe. We showed that the CDAA events are common in two-species interactions in the four selected representative PGPRs, represented by the incompatible interaction of Lysobacter enzymogenes strain OH11 (OH11) and Lysobacter antibioticus strain OH13 (OH13). We further showed that the CDAA between OH11 and OH13 is jointly controlled by a contact-dependent killing device, called the type IV secretion system (T4SS). By deleting the respective T4SS synthesis genes, the T4SS in both strains was co-inactivated and this step unlocked  their natural CDAA, resulting in an engineered, compatible mutant alliance that co-displayed antibacterial and antifungal activity. Therefore, this study reveals that releasing bacterial CDAA is effective to rationally engineer the biocontrol community.

VgrG-dependent effectors and chaperones modulate the assembly of the type VI secretion system

The type VI secretion system (T6SS) is a spear-like nanomachine found in gram-negative pathogens for delivery of toxic effectors to neighboring bacterial and host cells. Its assembly requires a tip spike complex consisting of a VgrG-trimer, a PAAR protein, and the interacting effectors. However, how the spike controls T6SS assembly remains elusive. Here we investigated the role of three VgrG-effector pairs in Aeromonas dhakensis strain SSU, a clinical isolate with a constitutively active T6SS. By swapping VgrG tail sequences, we demonstrate that the C-terminal ~30 amino-acid tail dictates effector specificity. Double deletion of vgrG1&2 genes (VgrG3⁺) abolished T6SS secretion, which can be rescued by ectopically expressing chimeric VgrG3 with a VgrG1/2-tail but not the wild type VgrG3. In addition, deletion of effector-specific chaperones also severely impaired T6SS secretion, despite the presence of intact VgrG and effector proteins, in both SSU and Vibrio cholerae V52. We further show that SSU could deliver a V. cholerae effector VasX when expressing a plasmid-borne chimeric VgrG with VasX-specific VgrG tail and chaperone sequences. Pull-down analyses show that two SSU effectors, TseP and TseC, could interact with their cognate VgrGs, the baseplate protein TssK, and the key assembly chaperone TssA. Effectors TseL and VasX could interact with TssF, TssK and TssA in V. cholerae. Collectively, we demonstrate that chimeric VgrG-effector pairs could bypass the requirement of heterologous VgrG complex and propose that effector-stuffing inside the baseplate complex, facilitated by chaperones and the interaction with structural proteins, serves as a crucial structural determinant for T6SS assembly.

Effectors of bacterial secretion systems are generally considered as secreted proteins for interspecies interactions rather than components of the secretion apparatus. Our results reveal the complex interactions of effectors, chaperones, and structural proteins are crucial for T6SS assembly, suggesting an integral role of effectors as parts of the apparatus and distinctive from other secretion systems.

Diversification of the Type VI Secretion System in Agrobacteria

The type VI secretion system (T6SS) is used by many Gram-negative bacteria to deploy toxic effectors for interbacterial competition. This system provides a competitive advantage in planta to agrobacteria, a diverse group with phytopathogenic members capable of genetically transforming plants. To inform on the ecology and evolution of agrobacteria, we revealed processes that diversify their effector gene collections. From genome sequences of diverse strains, we identified T6SS loci, functionally validated associated effector genes for toxicity, and predicted genes homologous to those that encode proteins known to interact with effectors. The gene loci were analyzed in a phylogenetic framework, and results show that strains of some species-level groups have different patterns of T6SS expression and are enriched in specific sets of T6SS loci. Findings also demonstrate that the modularity of T6SS loci and their associated genes engenders dynamicity, promoting reshuffling of entire loci, fragments therein, and domains to swap toxic effector genes across species. However, diversification is constrained by the need to maintain specific combinations of gene subtypes, congruent with observations that certain genes function together to regulate T6SS loading and activation. Data are consistent with a scenario where species can acquire unique T6SS loci that are then reshuffled across the genus in a restricted manner to generate new combinations of effector genes. IMPORTANCE The T6SS is used by several taxa of Gram-negative bacteria to secrete toxic effector proteins to attack others. Diversification of effector collections shapes bacterial interactions and impacts the health of hosts and ecosystems in which bacteria reside. We uncovered the diversity of T6SS loci across a genus of plant-associated bacteria and show that diversification is driven by the acquisition of new loci and reshuffling among species. However, linkages between specific subtypes of genes need to be maintained to ensure that proteins whose interactions are necessary to activate the T6SS remain together. Results reveal how organization of gene loci and domain structure of genes provides flexibility to diversify under the constraints imposed by the system. Findings inform on the evolution of a mechanism that influences bacterial communities.

Defending against the Type Six Secretion System: beyond Immunity Genes

The bacterial type six secretion system (T6SS) delivers toxic effector proteins into neighboring cells, but bacteria must protect themselves against their own T6SS. Immunity genes are the best-characterized defenses, protecting against specific cognate effectors. However, the prevalence of the T6SS and the coexistence of species with heterologous T6SSs suggest evolutionary pressure selecting for additional defenses against it. Here we review defenses against the T6SS beyond self-associated immunity genes, such as diverse stress responses that can recognize T6SS-inflicted damage and coordinate induction of molecular armor, repair pathways, and overall survival. Some of these stress responses are required for full survival even in the presence of immunity genes. Finally, we propose that immunity gene-independent protection is, mechanistically, bacterial innate immunity and that such defenses and the T6SS have co-evolved and continue to shape one another in polymicrobial communities.

A New Contact Killing Toxin Permeabilizes Cells and Belongs to a Broadly Distributed Protein Family

Vibrio cholerae is an aquatic Gram-negative bacterium that causes severe diarrheal cholera disease when ingested by humans. To eliminate competitor cells in both the external environment and inside hosts, V. cholerae uses the type VI secretion system (T6SS). The T6SS is a macromolecular contact-dependent weapon employed by many Gram-negative bacteria to deliver cytotoxic proteins into adjacent cells. In addition to canonical T6SS gene clusters encoded by all sequenced V. cholerae isolates, strain BGT49 encodes another locus, which we named auxiliary (Aux) cluster 4. The Aux 4 cluster is located on a mobile genetic element and can be used by killer cells to eliminate both V. cholerae and Escherichia coli cells in a T6SS-dependent manner. A putative toxin encoded in the cluster, which we name TpeV (type VI permeabilizing effector Vibrio), shares no homology to known proteins and does not contain motifs or domains indicative of function. Ectopic expression of TpeV in the periplasm of E. coli permeabilizes cells and disrupts the membrane potential. Using confocal microscopy, we confirm that susceptible target cells become permeabilized when competed with killer cells harboring the Aux 4 cluster. We also determine that tpiV, the gene located immediately downstream of tpeV, encodes an immunity protein that neutralizes the toxicity of TpeV. Finally, we show that TpeV homologs are broadly distributed across important human, animal, and plant pathogens and are localized in proximity to other T6SS genes. Our results suggest that TpeV is a toxin that belongs to a large family of T6SS proteins. IMPORTANCE Bacteria live in polymicrobial communities where competition for resources and space is essential for survival. Proteobacteria use the T6SS to eliminate neighboring cells and cause disease. However, the mechanisms by which many T6SS toxins kill or inhibit susceptible target cells are poorly understood. The sequence of the TpeV toxin that we describe here is unlike any previously described protein. We demonstrate that it has antimicrobial activity by permeabilizing cells, eliminating membrane potentials, and causing severe cytotoxicity. TpeV homologs are found near known T6SS genes in human, animal, and plant bacterial pathogens, indicating that the toxin is a representative member of a broadly distributed protein family. We propose that TpeV-like toxins contribute to the fitness of many bacteria. Finally, since antibiotic resistance is a critical global health threat, the discovery of new antimicrobial mechanisms could lead to the development of new treatments against resistant strains.

Engineered Type Six Secretion Systems Deliver Active Exogenous Effectors and Cre Recombinase

Genetic editing has revolutionized biotechnology, but delivery of endonuclease genes as DNA can lead to aberrant integration or overexpression, leading to off-target effects. Here, we develop a mechanism to deliver Cre recombinase as a protein by engineering the bacterial type six secretion system (T6SS). Using multiple T6SS fusion proteins, Aeromonas dhakensis or attenuated Vibrio cholerae donor strains, and a gain-of-function cassette for detecting Cre recombination, we demonstrate successful delivery of active Cre directly into recipient cells. The most efficient transfer was achieved using a truncated version of PAAR2 from V. cholerae, resulting in a relatively small (118-amino-acid) delivery tag. We further demonstrate the versatility of this system by delivering an exogenous effector, TseC, enabling V. cholerae to kill Pseudomonas aeruginosa. This implies that P. aeruginosa is naturally resistant to all native effectors of V. cholerae and that the TseC chaperone protein is not required for its activity. Moreover, it demonstrates that the engineered system can improve T6SS efficacy against specific pathogens, proposing future application in microbiome manipulation or as a next-generation antimicrobial. Inexpensive and easy to produce, this protein delivery system has many potential applications, ranging from studying T6SS effectors to genetic editing.

Antifungal weapons of Lysobacter, a mighty biocontrol agent

Bacteria interact with fungi in a variety of ways to inhibit fungal growth, while the underlying mechanisms remain only partially characterized. The plant-beneficial Bacillus and Pseudomonas species are well-known antifungal biocontrol agents, whereas Lysobacter are far less studied. Members of Lysobacter are easy to grow in fermenters and are safe to humans, animals and plants. These environmentally ubiquitous bacteria use a diverse arsenal of weapons to prey on other microorganisms, including fungi and oomycetes. The small molecular toxins secreted by Lysobacter represent long-range weapons effective against filamentous fungi. The secreted hydrolytic enzymes act as intermediate-range weapons against non-filamentous fungi. The contact-dependent killing devices are proposed to work as short-range weapons. We describe here the structure, biosynthetic pathway, action mode and applications of one of the best-characterized long-range weapons, the heat-stable antifungal factor (HSAF) produced by Lysobacter enzymogenes. We discuss how the flagellar type III secretion system has evolved into an enzyme secretion machine for the intermediate-range antifungal weapons. We highlight an intricate mechanism coordinating the production of the long-range weapon, HSAF and the proposed contact-dependent killing device, type VI secretion system. We also overview the regulatory mechanisms of HSAF production involving specific transcription factors and the bacterial second messenger c-di-GMP.

Lysobacter enzymogenes antagonizes soilborne bacteria using the type IV secretion system

Soil microbiome comprises numerous microbial species that continuously interact with each other. Among the modes of diverse interactions, cell-cell killing may play a key role in shaping the microbiome composition. Bacteria deploy various secretion systems to fend off other microorganisms and Type IV Secretion System (T4SS) in pathogenic bacteria was shown to function as a contact-dependent, inter-bacterial killing system only recently. The present study investigated the role played by T4SS in the killing behaviour of the soilborne biocontrol bacterium Lysobacter enzymogenes OH11. Results showed that L. enzymogenes OH11 genome encompasses genes encoding all the components of T4SS and effectors potentially involved in inter-bacterial killing system. Generation of knock-out mutants revealed that L. enzymogenes OH11 uses T4SS as the main contact-dependent weapon against other soilborne bacteria. The T4SS-mediated killing behaviour of L. enzymogenes OH11 decreased the antibacterial and antifungal activity of two Pseudomonas spp. but at the same time, protected carrot from infection by Pectobacterium carotovorum. Overall, this study showed for the first time the involvement of T4SS in the killing behaviour of L. enzymogenes and its impact on the multiple interactions occurring in the soil microbiome.

Clp is a "busy" transcription factor in the bacterial warrior, Lysobacter enzymogenes

Cyclic AMP receptor protein (CRP) is a well-characterized group of global transcription factors in bacteria. They are known to regulate numerous cellular processes by binding DNA and/or cAMP (a ligand called bacterial second messenger) to control target gene expression. Gram-negative Lysobacter enzymogenes is a soilborne, plant-beneficial bacterium without flagella that can fight against filamentous fungi and oomycete. Driven by the type IV pilus (T4P) system, this bacterium moves to nearby pathogens and uses a “mobile-attack” antifungal strategy to kill them via heat-stable antifungal factor (HSAF) and abundant lyases. This strategy is controlled by a unique “busy” transcription factor Clp, which is a CRP-like protein that is inactivated by binding of c-di-GMP, another ubiquitous second messenger of bacteria. In this review, we summarize the current progress in how Clp initiates a “mobile-attack” strategy through a series of previously uncharacterized mechanisms, including binding to DNA in a unique pattern, directly interacting with or responding to various small molecules, and interacting specifically with proteins adopting distinct structure. Together, these characteristics highlight the multifunctional roles of Clp in L. enzymogenes, a powerful bacterial warrior against fungal 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.

Essential functions of chaperones and adaptors of protein secretion systems in Gram-negative bacteria

Equipped with a plethora of secreted toxic effectors, protein secretion systems are essential for bacteria to interact with and manipulate their neighboring environment to survive in host microbiota and other highly competitive communities. While effectors have received spotlight attention in secretion system studies, many require accessory chaperone and adaptor proteins for proper folding/unfolding and stability throughout the secretion process. Here, we review the functions of chaperones and adaptors of three protein secretions systems, type 3 secretion system (T3SS), type 4 secretion system (T4SS), and type 6 secretion system (T6SS), which are employed by many Gram-negative bacterial pathogens to deliver toxins to bacterial, plant, and mammalian host cells through direct contact. Since chaperone and adaptor functions of the T3SS and the T4SS are relatively well studied, we discuss in detail the methods of chaperone-facilitated effector secretion by the T6SS and highlight commonalities between the effector chaperone/adaptor proteins of these diverse secretion systems. While the chaperones and adaptors are generally referred to as accessory proteins as they are not directly involved in toxicities to target cells, they are nonetheless vital for the biological functions of the secretion systems. Future research on biochemical and structural properties of these chaperones will not only elucidate the mechanisms of chaperone-effector binding and release process but also facilitate custom design of cargo effectors to be translocated by these widespread secretion systems for biotechnological applications.

Differential Cellular Response to Translocated Toxic Effectors and Physical Penetration by the Type VI Secretion System

The type VI secretion system (T6SS) is a lethal microbial weapon that injects a large needle-like structure carrying toxic effectors into recipient cells through physical penetration. How recipients respond to physical force and effectors remains elusive. Here, we use a series of effector mutants of Vibrio cholerae to determine how T6SS elicits response in Pseudomonas aeruginosa and Escherichia coli. We show that TseL, but no other effectors or physical puncture, triggers the tit-for-tat response of P. aeruginosa H1-T6SS. Although E. coli is sensitive to all periplasmically expressed effectors, P. aeruginosa is most sensitive to TseL alone. We identify a number of stress response pathways that confer protection against TseL. Physical puncture of T6SS has a moderate inhibitory effect only on envelope-impaired tolB and rseA mutants. Our data reveal that recipient cells primarily respond to effector toxicity but not to physical contact, and they rely on the stress response for immunity-independent protection.

Characterization of Lysozyme-Like Effector TseP Reveals the Dependence of Type VI Secretion System (T6SS) Secretion on Effectors in Aeromonas dhakensis Strain SSU

The type VI secretion system (T6SS) is a widespread weapon employed by Gram-negative bacteria for interspecies interaction in complex communities. Analogous to a contractile phage tail, the double-tubular T6SS injects toxic effectors into prokaryotic and eukaryotic neighboring cells. Although effectors dictate T6SS functions, their identities remain elusive in many pathogens. Here, we report the lysozyme-like effector TseP in Aeromonas dhakensis, a waterborne pathogen that can cause severe gastroenteritis and systemic infection. Using secretion, competition, and enzymatic assays, we demonstrate that TseP is a T6SS-dependent effector with cell wall-lysing activities, and TsiP is its cognate immunity protein. Triple deletion of tseP and two known effector genes, tseI and tseC, abolished T6SS-mediated secretion, while complementation with any single effector gene partially restored bacterial killing and Hcp secretion. In contrast to whole-gene deletions, the triple-effector inactivation in the 3eff_c mutant abolished antibacterial killing but not T6SS secretion. We further demonstrate that the 3eff_c mutation abolished T6SS-mediated toxicity of SSU to Dictyostelium discoideum amoebae, suggesting that the T6SS physical puncture is nontoxic to eukaryotic cells. These data highlight not only the necessity of possessing functionally diverse effectors for survival in multispecies communities but also that effector inactivation would be an efficient strategy to detoxify the T6SS while preserving its delivery efficiency, converting the T6SS to a platform for protein delivery to a variety of recipient cells. IMPORTANCE Delivery of cargo proteins via protein secretion systems has been shown to be a promising tool in various applications. However, secretion systems are often used by pathogens to cause disease. Thus, strategies are needed to detoxify secretion systems while preserving their efficiency. The T6SS can translocate proteins through physical puncture of target cells without specific surface receptors and can target a broad range of recipients. In this study, we identified a cell wall-lysing effector, and by inactivating it and the other two known effectors, we have built a detoxified T6SS-active strain that may be used for protein delivery to prokaryotic and eukaryotic recipient cells.

The β-encapsulation cage of rearrangement hotspot (Rhs) effectors is required for type VI secretion

Bacteria deploy rearrangement hotspot (Rhs) proteins as toxic effectors against both prokaryotic and eukaryotic target cells. Rhs proteins are characterized by YD-peptide repeats, which fold into a large β-cage structure that encapsulates the C-terminal toxin domain. Here, we show that Rhs effectors are essential for type VI secretion system (T6SS) activity in Enterobacter cloacae (ECL). ECL rhs ^- mutants do not kill Escherichia coli target bacteria and are defective for T6SS-dependent export of hemolysin-coregulated protein (Hcp). The RhsA and RhsB effectors of ECL both contain Pro-Ala-Ala-Arg (PAAR) repeat domains, which bind the β-spike of trimeric valine-glycine repeat protein G (VgrG) and are important for T6SS activity in other bacteria. Truncated RhsA that retains the PAAR domain is capable of forming higher-order, thermostable complexes with VgrG, yet these assemblies fail to restore secretion activity to ∆rhsA ∆rhsB mutants. Full T6SS-1 activity requires Rhs that contains N-terminal transmembrane helices, the PAAR domain, and an intact β-cage. Although ∆rhsA ∆rhsB mutants do not kill target bacteria, time-lapse microscopy reveals that they assemble and fire T6SS contractile sheaths at ∼6% of the frequency of rhs ⁺ cells. Therefore, Rhs proteins are not strictly required for T6SS assembly, although they greatly increase secretion efficiency. We propose that PAAR and the β-cage provide distinct structures that promote secretion. PAAR is clearly sufficient to stabilize trimeric VgrG, but efficient assembly of T6SS-1 also depends on an intact β-cage. Together, these domains enforce a quality control checkpoint to ensure that VgrG is loaded with toxic cargo before assembling the secretion apparatus.

Role of Recipient Susceptibility Factors During Contact-Dependent Interbacterial Competition

Bacteria evolved multiple strategies to survive and develop optimal fitness in their ecological niche. They deployed protein secretion systems for robust and efficient delivery of antibacterial toxins into their target cells, therefore inhibiting their growth or killing them. To maximize antagonism, recipient factors on target cells can be recognized or hijacked to enhance the entry or toxicity of these toxins. To date, knowledge regarding recipient susceptibility (RS) factors and their mode of action is mostly originating from studies on the type Vb secretion system that is also known as the contact-dependent inhibition (CDI) system. Yet, recent studies on the type VI secretion system (T6SS), and the CDI by glycine-zipper protein (Cdz) system, also reported the emerging roles of RS factors in interbacterial competition. Here, we review these RS factors and their mechanistic impact in increasing susceptibility of recipient cells in response to CDI, T6SS, and Cdz. Past and future strategies for identifying novel RS factors are also discussed, which will help in understanding the interplay between attacker and prey upon secretion system-dependent competition. Understanding these mechanisms would also provide insights for developing novel antibacterial strategies to antagonize aggressive bacteria-killing pathogens.

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.

An intrinsic mechanism for coordinated production of the contact-dependent and contact-independent weapon systems in a soil bacterium

Soil bacteria possess multiple weapons to fend off microbial competitors. Currently, we poorly understand the factors guiding bacterial decisions about weapon systems deployment. In this study, we investigated how such decisions are made by the soil bacterium Lysobacter enzymogenes, used in antifungal plant protection. We found that weapons production is guided by environmental cues. In rich media, which likely mimic environments crowded with other microbes, L. enzymogenes produces a contact-dependent weapon, type six secretion system (T6SS). In nutrient-poor media, likely dominated by filamentous oomycetes and fungi, L. enzymogenes synthesizes and secretes a heat-stable antifungal factor (HSAF), a contact-independent weapon. Surprisingly, the T6SS inner tube protein Hcp is accumulated intracellularly even in nutrient-poor media, when the T6SS is not assembled. We found that Hcp interacts with the transcription factor Clp required for activating HSAF biosynthesis operon expression. Hcp protects Clp from binding to c-di-GMP, an intracellular second messenger inhibiting DNA binding. The increased concentration of c-di-GMP-free Clp thus leads to higher gene expression and HSAF production. Therefore, when the contact-dependent weapon, T6SS, is not in use, accumulation of one of its structural components, Hcp, serves as a signal to enhance production of the contact-independent weapon, HSAF. The uncovered environment-dependent and auto-regulatory mechanisms shed light on the processes governing deployment of various weapon systems in environmental bacteria.

Soil bacteria face competition from diverse microbial species. To stay competitive, they deploy a variety of weapons. At present, we know little about factors influencing decisions about which weapons to produce at any given time, and about mechanisms through which these decisions are carried out. In this study, we show that in the soil bacterium, Lysobacter enzymogenes, synthesis of the contact-dependent weapon, known as type six secretion system (T6SS) occurs under different conditions, compared to those conductive to the production of the contact-independent weapon, toxin HSAF. Further, when T6SS is not assembled, one of its structural components, Hcp, coactivates HSAF operon expression and HSAF synthesis. This study reveals that decisions about contact-dependent and contact-independent weapon production in bacteria are governed by both environmental cues and intrinsic coordination mechanisms.

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.

Envelope stress responses defend against type six secretion system attacks independently of immunity proteins

The arms race among microbes is a key driver in the evolution of not only the weapons but also defence mechanisms. Many gram-negative bacteria use the type six secretion system (T6SS) to deliver toxic effectors directly into neighbouring cells. Defence against effectors requires cognate immunity proteins. However, here we show immunity-independent protection mediated by envelope stress responses in Escherichia coli and Vibrio cholerae against a V. cholerae T6SS effector, TseH. We demonstrate that TseH is a PAAR-dependent species-specific effector highly potent against Aeromonas species but not against its V. cholerae immunity mutant or E. coli. Structural analysis reveals TseH is likely a NlpC/P60 family cysteine endopeptidase. We determine that two envelope stress response pathways, Rcs and BaeSR, protect E. coli from TseH toxicity by mechanisms including capsule synthesis. The two-component system WigKR (VxrAB) is critical for protecting V. cholerae from its own T6SS despite expressing immunity genes. WigR also regulates T6SS expression, suggesting a dual role in attack and defence. This deepens our understanding of how bacteria survive T6SS attacks and suggests that defending against the T6SS represents a major selective pressure driving the evolution of species-specific effectors and protective mechanisms mediated by envelope stress responses and capsule synthesis.

Effector loading onto the VgrG carrier activates type VI secretion system assembly

The type VI secretion system (T6SS) is used by many bacteria to engage in social behavior and can affect the health of its host plant or animal. Because activities associated with T6SSs are often costly, T6SSs must be tightly regulated. However, our knowledge regarding how T6SS assembly and contraction are regulated remains limited. Using the plant pathogen Agrobacterium tumefaciens, we show that effectors are not just passengers but also impact on T6SS assembly. The A. tumefaciens strain C58 encodes one T6SS and two Tde DNase toxin effectors used as major weapons for interbacterial competition. Here, we demonstrate that loading of Tde effectors onto their cognate carriers, the VgrG spikes, is required for active T6SS secretion. The assembly of the TssBC contractile sheath occurs only in the presence of Tde effectors. The requirement of effector loading for efficient T6SS secretion was also validated in other A. tumefaciens strains. We propose that such a mechanism is used by bacteria as a strategy for efficacious T6SS firing and to ensure that effectors are loaded onto the T6SS prior to completing its assembly.