A visual assay to monitor T6SS-mediated bacterial competition

Characterization of a novel root-associated diazotrophic rare PGPR taxa, Aquabacter pokkalii sp. nov., isolated from pokkali rice: new insights into the plant-associated lifestyle and brackish adaptation

Salinity impacts crop growth and productivity and lowers the activities of rhizosphere microbiota. The identification and utilization of habitat-specific salinity-adapted plant growth-promoting rhizobacteria (PGPR) are considered alternative strategies to improve the growth and yields of crops in salinity-affected coastal agricultural fields. In this study, we characterize strain L1I39T, the first Aquabacter species with PGPR traits isolated from a salt-tolerant pokkali rice cultivated in brackish environments. L1I39T is positive for 1-aminocyclopropane-1-carboxylate deaminase activity and nitrogen fixation and can promote pokkali rice growth by supplying fixed nitrogen under a nitrogen-deficient seawater condition. Importantly, enhanced plant growth and efficient root colonization were evident in L1I39T-inoculated plants grown under 20% seawater but not in zero-seawater conditions, identifying brackish conditions as a key local environmental factor critical for L1I39T-pokkali rice symbiosis. Detailed physiological studies revealed that L1I39T is well-adapted to brackish environments. In-depth genome analysis of L1I39T identified multiple gene systems contributing to its plant-associated lifestyle and brackish adaptations. The 16S rRNA-based metagenomic study identified L1I39T as an important rare PGPR taxon. Based on the polyphasic taxonomy analysis, we established strain L1I39T as a novel Aquabacter species and proposed Aquabacter pokkalii sp nov. Overall, this study provides a better understanding of a marine-adapted PGPR strain L1I39T that may perform a substantial role in host growth and health in nitrogen-poor brackish environments.

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.

Transcriptional organization and regulation of the Pseudomonas putida K1 type VI secretion system gene cluster

The type VI secretion system (T6SS) is an antimicrobial molecular weapon that is widespread in Proteobacteria and offers competitive advantages to T6SS-positive micro-organisms. Three T6SSs have recently been described in Pseudomonas putida KT2440 and it has been shown that one, K1-T6SS, is used to outcompete a wide range of phytopathogens, protecting plants from pathogen infections. Given the relevance of this system as a powerful and innovative mechanism of biological control, it is critical to understand the processes that govern its expression. Here, we experimentally defined two transcriptional units in the K1-T6SS cluster. One encodes the structural components of the system and is transcribed from two adjacent promoters. The other encodes two hypothetical proteins, the tip of the system and the associated adapters, and effectors and cognate immunity proteins, and it is also transcribed from two adjacent promoters. The four identified promoters contain the typical features of σ70-dependent promoters. We have studied the expression of the system under different conditions and in a number of mutants lacking global regulators. P. putida K1-T6SS expression is induced in the stationary phase, but its transcription does not depend on the stationary σ factor RpoS. In fact, the expression of the system is indirectly repressed by RpoS. Furthermore, it is also repressed by RpoN and the transcriptional regulator FleQ, an enhancer-binding protein typically acting in conjunction with RpoN. Importantly, expression of the K1-T6SS gene cluster is positively regulated by the GacS-GacA two-component regulatory system (TCS) and repressed by the RetS sensor kinase, which inhibits this TCS. Our findings identified a complex regulatory network that governs T6SS expression in general and P. putida K1-T6SS in particular, with implications for controlling and manipulating a bacterial agent that is highly relevant in biological control.

Bacterial Competition Influences the Ability of Symbiotic Bacteria to Colonize Western Flower Thrips

Symbiont mediated RNAi (SMR) is a promising method for precision control of pest insect species such as Western Flower Thrips (WFT). Two species of bacteria are known to be dominant symbiotic bacteria in WFT, namely BFo1 and BFo2 (Bacteria from Frankliniella occidentalis 1 and 2), as we here confirm by analysis of next-generation sequence data derived to obtain a reference WFT genome sequence. Our first demonstration of SMR in WFT used BFo2, related to Pantoea, isolated from a domesticated Dutch thrips population. However, for successful use of SMR as a thrips control measure, these bacteria need to successfully colonize different environmental thrips populations. Here, we describe a United Kingdom thrips population that does not harbour BFo2, but does contain BFo1, a species related to Erwinia. Attempts to introduce BFo2 indicate that this bacterium is unable to establish itself in the United Kingdom thrips, in contrast to successful colonization by a strain of BFo1 expressing green fluorescent protein. Fluorescence microscopy indicates that BFo1 occupies similar regions of the thrips posterior midgut and hindgut as BFo2. Bacterial competition assays revealed that a barrier to BFo2 establishing itself in thrips is the identity of the resident BFo1; BFo1 isolated from the United Kingdom thrips suppresses growth of BFo2 to a greater extent than BFo1 from the Dutch thrips that is permissive for BFo2 colonization. The ability of the latter strain of BFo1 to colonize the United Kingdom thrips is also likely attributable to its ability to out-compete the resident BFo1. Lastly, we observed that United Kingdom thrips pre-exposed to the Dutch BFo1 could then be successfully colonized by BFo2. These results indicate, for the first time, that microbial competition and strain differences can have a large influence on how symbiotic bacteria can colonize different populations of an insect species.

Putative RNA Ligase RtcB Affects the Switch between T6SS and T3SS in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa is a significant cause of infection in immunocompromised individuals, cystic fibrosis patients, and burn victims. To benefit its survival, the bacterium adapt to either a motile or sessile lifestyle when infecting the host. The motile bacterium has an often activated type III secretion system (T3SS), which is virulent to the host, whereas the sessile bacterium harbors an active T6SS and lives in biofilms. Regulatory pathways involving Gac-Rsm or secondary messengers such as c-di-GMP determine which lifestyle is favorable for P. aeruginosa. Here, we introduce the RNA binding protein RtcB as a modulator of the switch between motile and sessile bacterial lifestyles. Using the wild-type P. aeruginosa PAO1, and a retS mutant PAO1(∆retS) in which T3SS is repressed and T6SS active, we show that deleting rtcB led to simultaneous expression of T3SS and T6SS in both PAO1(∆rtcB) and PAO1(∆rtcB∆retS). The deletion of rtcB also increased biofilm formation in PAO1(∆rtcB) and restored the motility of PAO1(∆rtcB∆retS). RNA-sequencing data suggested RtcB as a global modulator affecting multiple virulence factors, including bacterial secretion systems. Competitive killing and infection assays showed that the three T6SS systems (H1, H2, and H3) in PAO1(∆rtcB) were activated into a functional syringe, and could compete with Escherichia coli and effectively infect lettuce. Western blotting and RT-PCR results showed that RtcB probably exerted its function through RsmA in PAO1(∆rtcB∆retS). Quantification of c-di-GMP showed an elevated intracellular levels in PAO1(∆rtcB), which likely drove the switch between T6SS and T3SS, and contributed to the altered phenotypes and characteristics observed. Our data demonstrate a pivotal role of RtcB in the virulence of P. aeruginosa by controlling multiple virulence determinants, such as biofilm formation, motility, pyocyanin production, T3SS, and T6SS secretion systems towards eukaryotic and prokaryotic cells. These findings suggest RtcB as a potential target for controlling P. aeruginosa colonization, establishment, and pathogenicity.

Type II Antitoxin HigA Is a Key Virulence Regulator in Pseudomonas aeruginosa

Bacterial type II toxin-antitoxin (TA) systems are abundant genetic elements and are involved in a diverse array of physiological processes. These systems encode an antitoxin protein that directly binds and effectively neutralizes the protein toxin. Recent studies have highlighted the key roles of type II TA modules in bacterial virulence and pathogenesis, but the underlying mechanisms remain unclear. Here, we investigated the antitoxin HigA in Pseudomonas aeruginosa infection. Proteomic analysis of the higA deletion strain revealed an enhanced expression of pathogenic proteins. We further verified that HigA negatively controlled T3SS and T6SS expression by directly interacting with the promoter regions of the regulators amrZ and exsA, respectively. In other words, the reversal of HigA-mediated transcriptional inhibition on stress stimulation could induce virulence genes. These findings confirm the crucial roles of the type II antitoxin in bacterial infection, which highlights the potential of the HigBA TA system as an antibacterial treatment target.

The Burkholderia pseudomallei hmqA-G Locus Mediates Competitive Fitness against Environmental Gram-Positive Bacteria

Burkholderia pseudomallei is an opportunistic pathogen that is responsible for the disease melioidosis in humans and animals. The microbe is a tier 1 select agent because it is highly infectious by the aerosol route, it is inherently resistant to multiple antibiotics, and no licensed vaccine currently exists. Naturally acquired infections result from contact with contaminated soil or water sources in regions of endemicity. There have been few reports investigating the molecular mechanism(s) utilized by B. pseudomallei to survive and persist in ecological niches harboring microbial competitors. Here, we report the isolation of Gram-positive bacteria from multiple environmental sources and show that ∼45% of these isolates are inhibited by B. pseudomallei in head-to-head competition assays. Two competition-deficient B. pseudomallei transposon mutants were identified that contained insertion mutations in the hmqA-G operon. This large biosynthetic gene cluster encodes the enzymes that produce a family of secondary metabolites called 4-hydroxy-3-methyl-2-alkylquinolines (HMAQs). Liquid chromatography and mass spectrometry conducted on filter-sterilized culture supernatants revealed five HMAQs and N-oxide derivatives that were produced by the parental strain but were absent in an isogenic hmqD deletion mutant. The results demonstrate that B. pseudomallei inhibits the growth of environmental Gram-positive bacteria in a contact-independent manner via the production of HMAQs by the hmqA-G operon. IMPORTANCE Burkholderia pseudomallei naturally resides in water, soil, and the rhizosphere and its success as an opportunistic pathogen is dependent on the ability to persist in these harsh habitats long enough to come into contact with a susceptible host. In addition to adapting to limiting nutrients and diverse chemical and physical challenges, B. pseudomallei also has to interact with a variety of microbial competitors. Our research shows that one of the ways in which B. pseudomallei competes with Gram-positive environmental bacteria is by exporting a diverse array of closely related antimicrobial secondary metabolites.

Pseudomonas fluorescens F113 type VI secretion systems mediate bacterial killing and adaption to the rhizosphere microbiome

The genome of Pseudomonas fluorescens F113, a model rhizobacterium and a plant growth-promoting agent, encodes three putative type VI secretion systems (T6SSs); F1-, F2- and F3-T6SS. Bioinformatic analysis of the F113 T6SSs has revealed that they belong to group 3, group 1.1, and group 4a, respectively, similar to those previously described in Pseudomonas aeruginosa. In addition, in silico analyses allowed us to identify genes encoding a total of five orphan VgrG proteins and eight putative effectors (Tfe), some with their cognate immunity protein (Tfi) pairs. Genes encoding Tfe and Tfi are found in the proximity of P. fluorescens F113 vgrG, hcp, eagR and tap genes. RNA-Seq analyses in liquid culture and rhizosphere have revealed that F1- and F3-T6SS are expressed under all conditions, indicating that they are active systems, while F2-T6SS did not show any relevant expression under the tested conditions. The analysis of structural mutants in the three T6SSs has shown that the active F1- and F3-T6SSs are involved in interbacterial killing while F2 is not active in these conditions and its role is still unknown.. A rhizosphere colonization analysis of the double mutant affected in the F1- and F3-T6SS clusters showed that the double mutant was severely impaired in persistence in the rhizosphere microbiome, revealing the importance of these two systems for rhizosphere adaption.

Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

Bacterial communities are governed by a wide variety of social interactions, some of which are antagonistic with potential significance for bacterial warfare. Several antagonistic mechanisms, such as killing via the type VI secretion system (T6SS), require killer cells to directly contact target cells. The T6SS is hypothesized to be a highly potent weapon, capable of facilitating the invasion and defence of bacterial populations. However, we find that the efficacy of contact killing is severely limited by the material consequences of cell death. Through experiments with Vibrio cholerae strains that kill via the T6SS, we show that dead cell debris quickly accumulates at the interface that forms between competing strains, preventing physical contact and thus preventing killing. While previous experiments have shown that T6SS killing can reduce a population of target cells by as much as 106-fold, we find that, as a result of the formation of dead cell debris barriers, the impact of contact killing depends sensitively on the initial concentration of killer cells. Killer cells are incapable of invading or eliminating competitors on a community level. Instead, bacterial warfare itself can facilitate coexistence between nominally antagonistic strains. While a variety of defensive strategies against microbial warfare exist, the material consequences of cell death provide target cells with their first line of defence.

Pleiotropic effects of RsmA and RsmE proteins in Pseudomonas fluorescens 2P24

Background

Pseudomonas fluorescens 2P24 is a rhizosphere bacterium that produces 2,4-diacetyphloroglucinol (2,4-DAPG) as the decisive secondary metabolite to suppress soilborne plant diseases. The biosynthesis of 2,4-DAPG is strictly regulated by the RsmA family proteins RsmA and RsmE. However, mutation of both of rsmA and rsmE genes results in reduced bacterial growth.

Results

In this study, we showed that overproduction of 2,4-DAPG in the rsmA rsmE double mutant influenced the growth of strain 2P24. This delay of growth could be partially reversal when the phlD gene was deleted or overexpression of the phlG gene encoding the 2,4-DAPG hydrolase in the rsmA rsmE double mutant. RNA-seq analysis of the rsmA rsmE double mutant revealed that a substantial portion of the P. fluorescens genome was regulated by RsmA family proteins. These genes are involved in the regulation of 2,4-DAPG production, cell motility, carbon metabolism, and type six secretion system.

Conclusions

These results suggest that RsmA and RsmE are the important regulators of genes involved in the plant-associated strain 2P24 ecologic fitness and operate a sophisticated mechanism for fine-tuning the concentration of 2,4-DAPG in the cells.

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 opportunistic pathogen Stenotrophomonas maltophilia utilizes a type IV secretion system for interbacterial killing

Bacterial type IV secretion systems (T4SS) are a highly diversified but evolutionarily related family of macromolecule transporters that can secrete proteins and DNA into the extracellular medium or into target cells. It was recently shown that a subtype of T4SS harboured by the plant pathogen Xanthomonas citri transfers toxins into target cells. Here, we show that a similar T4SS from the multi-drug-resistant opportunistic pathogen Stenotrophomonas maltophilia is proficient in killing competitor bacterial species. T4SS-dependent duelling between S. maltophilia and X. citri was observed by time-lapse fluorescence microscopy. A bioinformatic search of the S. maltophilia K279a genome for proteins containing a C-terminal domain conserved in X. citri T4SS effectors (XVIPCD) identified twelve putative effectors and their cognate immunity proteins. We selected a putative S. maltophilia effector with unknown function (Smlt3024) for further characterization and confirmed that it is indeed secreted in a T4SS-dependent manner. Expression of Smlt3024 in the periplasm of E. coli or its contact-dependent delivery via T4SS into E. coli by X. citri resulted in reduced growth rates, which could be counteracted by expression of its cognate inhibitor Smlt3025 in the target cell. Furthermore, expression of the VirD4 coupling protein of X. citri can restore the function of S. maltophilia ΔvirD4, demonstrating that effectors from one species can be recognized for transfer by T4SSs from another species. Interestingly, Smlt3024 is homologous to the N-terminal domain of large Ca2+-binding RTX proteins and the crystal structure of Smlt3025 revealed a topology similar to the iron-regulated protein FrpD from Neisseria meningitidis which has been shown to interact with the RTX protein FrpC. This work expands our current knowledge about the function of bacteria-killing T4SSs and increases the panel of effectors known to be involved in T4SS-mediated interbacterial competition, which possibly contribute to the establishment of S. maltophilia in clinical and environmental settings.

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.

Baseplate Component TssK and Spatio-Temporal Assembly of T6SS in Pseudomonas aeruginosa

The Gram-negative bacteria use the contractile multi-molecular structure, called the Type VI Secretion System (T6SS) to inject toxic products into eukaryotic and prokaryotic cells. In this study, we use fluorescent protein fusions and time-lapse microscopy imaging to study the assembly dynamics of the baseplate protein TssK in Pseudomonas aeruginosa T6SS. TssK formed transient higher-order structures that correlated with dynamics of sheath component TssB. Assembly of peri-membrane TssK structures occurred de novo upon contact with competing bacteria. We show that this assembly required presence of TagQ-TagR envelope sensors, activity of PpkA kinase and anchoring to the inner membrane via TssM. Disassembly and repositioning of TssK component was dependent on PppA phosphatase and indispensable for repositioning and deployment of the entire contractile apparatus toward a new target cell. We also show that TssE is necessary for correct elongation and stability of TssB-sheath, but not for TssK assembly. Therefore, in P. aeruginosa, assembly of the TssK-containing structure relays on the post-translational regulatory envelope module and acts as spatio-temporal marker for further recruitment and subsequent assembly of the contractile apparatus.

Genome-wide mapping of the RNA targets of the Pseudomonas aeruginosa riboregulatory protein RsmN

Pseudomonads typically carry multiple non-identical alleles of the post-transcriptional regulator rsmA. In Pseudomonas aeruginosa, RsmN is notable in that its structural rearrangement confers distinct and overlapping functions with RsmA. However, little is known about the specificities of RsmN for its target RNAs and overall impact on the biology of this pathogen. We purified and mapped 503 transcripts directly bound by RsmN in P. aeruginosa. About 200 of the mRNAs identified encode proteins of demonstrated function including some determining acute and chronic virulence traits. For example, RsmN reduces biofilm development both directly and indirectly via multiple pathways, involving control of Pel exopolysaccharide biosynthesis and c-di-GMP levels. The RsmN targets identified are also shared with RsmA, although deletion of rsmN generally results in less pronounced phenotypes than those observed for ΔrsmA or ΔrsmArsmNind mutants, probably as a consequence of different binding affinities. Targets newly identified for the Rsm system include the small non-coding RNA CrcZ involved in carbon catabolite repression, for which differential binding of RsmN and RsmA to specific CrcZ regions is demonstrated. The results presented here provide new insights into the intricacy of riboregulatory networks involving multiple but distinct RsmA homologues.

Contribution of Cyclic di-GMP in the Control of Type III and Type VI Secretion in Pseudomonas aeruginosa

Bacteria produce toxins to enhance their competitiveness in the colonization of an environment as well as during an infection. The delivery of toxins into target cells is mediated by several types of secretion systems, among them our focus is Type III and Type VI Secretion Systems (T3SS and T6SS, respectively). A thorough methodology is provided detailing how to identify if cyclic di-GMP signaling plays a role in the P. aeruginosa toxin delivery mediated by T3SS or T6SS. This includes in vitro preparation of the samples for Western blot analysis aiming at detecting possible c-di-GMP-dependent T3SS/T6SS switch, as well as in vivo analysis using the model organism Galleria mellonella to demonstrate the ecological and pathogenic consequence of the switch between these two secretion systems.

TagF-mediated repression of bacterial type VI secretion systems involves a direct interaction with the cytoplasmic protein Fha

The bacterial type VI secretion system (T6SS) delivers effectors into eukaryotic host cells or toxins into bacterial competitor for survival and fitness. The T6SS is positively regulated by the threonine phosphorylation pathway (TPP) and negatively by the T6SS-accessory protein TagF. Here, we studied the mechanisms underlying TagF-mediated T6SS repression in two distinct bacterial pathogens, Agrobacterium tumefaciens and Pseudomonas aeruginosa. We found that in A. tumefaciens, T6SS toxin secretion and T6SS-dependent antibacterial activity are suppressed by a two-domain chimeric protein consisting of TagF and PppA, a putative phosphatase. Remarkably, this TagF domain is sufficient to post-translationally repress the T6SS, and this inhibition is independent of TPP. This repression requires interaction with a cytoplasmic protein, Fha, critical for activating T6SS assembly. In P. aeruginosa, PppA and TagF are two distinct proteins that repress T6SS in TPP-dependent and -independent pathways, respectively. P. aeruginosa TagF interacts with Fha1, suggesting that formation of this complex represents a conserved TagF-mediated regulatory mechanism. Using TagF variants with substitutions of conserved amino acid residues at predicted protein–protein interaction interfaces, we uncovered evidence that the TagF–Fha interaction is critical for TagF-mediated T6SS repression in both bacteria. TagF inhibits T6SS without affecting T6SS protein abundance in A. tumefaciens, but TagF overexpression reduces the protein levels of all analyzed T6SS components in P. aeruginosa. Our results indicate that TagF interacts with Fha, which in turn could impact different stages of T6SS assembly in different bacteria, possibly reflecting an evolutionary divergence in T6SS control.

The role of the type VI secretion system vgrG gene in the virulence and antimicrobial resistance of Acinetobacter baumannii ATCC 19606

The Type VI Secretion System (T6SS) is an important virulence system that exists in many bacterial pathogens, and has emerged as a potent mediator of pathogenicity in Acinetobacter baumannii. In this study, we inactivated one of the T6SS components vgrG (valine–glycine repeat G) gene in A. baumannii ATCC 19606 and constructed a complementation strain. BEAS-2b human alveolar epithelial cells was adopted to assess bacterial adhesion, and wild female BALB/c mice were used for in vivo experiments to assess the bacterial killing ability to host. Upon deletion of the vgrG gene, increased antimicrobial resistance to ampicillin/sulbactam, but reduced resistance to chloramphenicol were observed. The vgrG mutant strain showed lower growth rate, reduced eukaryotic cell adherence and impaired lethality in mice. However, the vgrG mutant strain is not implicated in biofilm formation. Our study suggests that the Type VI Secretion System core component VgrG contributes to both virulence and antimicrobial resistance in A. baumannii ATCC 19606.

The Type VI Secretion System: A Dynamic System for Bacterial Communication?

Numerous studies in Gram-negative bacteria have focused on the Type VISecretion Systems (T6SSs), Quorum Sensing (QS), and social behavior, such as in biofilms. These interconnected mechanisms are important for bacterial survival; T6SSs allow bacteria to battle other cells, QS is devoted to the perception of bacterial cell density, and biofilm formation is essentially controlled by QS. Here, we review data concerning T6SS dynamics and T6SS–QS cross-talk that suggest the existence of inter-bacterial communication via T6SSs.

Helicobacter pullorum: An Emerging Zoonotic Pathogen

Helicobacter pullorum (H.pullorum) commonly colonizes the gastrointestinal tract of poultry causing gastroenteritis. The bacterium may be transmitted to humans through contaminated meat where it has been associated with colitis and hepatitis. Despite the high prevalence of H. pullorum observed in poultry, little is known about the mechanisms by which this bacterium establishes infection in host and its virulence determinants. In this article we aim to provide an overview of this emerging zoonotic pathogen; its general characteristics, hosts, prevalence, and transmission as well as its pathogenic potential. We also discuss possible control strategies and risk of disease emergence.

The Pseudomonas putida T6SS is a plant warden against phytopathogens

Bacterial type VI secretion systems (T6SSs) are molecular weapons designed to deliver toxic effectors into prey cells. These nanomachines have an important role in inter-bacterial competition and provide advantages to T6SS active strains in polymicrobial environments. Here we analyze the genome of the biocontrol agent Pseudomonas putida KT2440 and identify three T6SS gene clusters (K1-, K2- and K3-T6SS). Besides, 10 T6SS effector-immunity pairs were found, including putative nucleases and pore-forming colicins. We show that the K1-T6SS is a potent antibacterial device, which secretes a toxic Rhs-type effector Tke2. Remarkably, P. putida eradicates a broad range of bacteria in a K1-T6SS-dependent manner, including resilient phytopathogens, which demonstrates that the T6SS is instrumental to empower P. putida to fight against competitors. Furthermore, we observed a drastically reduced necrosis on the leaves of Nicotiana benthamiana during co-infection with P. putida and Xanthomonas campestris. Such protection is dependent on the activity of the P. putida T6SS. Many routes have been explored to develop biocontrol agents capable of manipulating the microbial composition of the rhizosphere and phyllosphere. Here we unveil a novel mechanism for plant biocontrol, which needs to be considered for the selection of plant wardens whose mission is to prevent phytopathogen infections.

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

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

The VgrG proteins are "à la carte" delivery systems for bacterial type VI effectors

The bacterial type VI secretion system (T6SS) is a supra-molecular complex akin to bacteriophage tails, with VgrG proteins acting as a puncturing device. The Pseudomonas aeruginosa H1-T6SS has been extensively characterized. It is involved in bacterial killing and in the delivery of three toxins, Tse1–3. Here, we demonstrate the independent contribution of the three H1-T6SS co-regulated vgrG genes, vgrG1abc, to bacterial killing. A putative toxin is encoded in the vicinity of each vgrG gene, supporting the concept of specific VgrG/toxin couples. In this respect, VgrG1c is involved in the delivery of an Rhs protein, RhsP1. The RhsP1 C terminus carries a toxic activity, from which the producing bacterium is protected by a cognate immunity. Similarly, VgrG1a-dependent toxicity is associated with the PA0093 gene encoding a two-domain protein with a putative toxin domain (Toxin_61) at the C terminus. Finally, VgrG1b-dependent killing is detectable upon complementation of a triple vgrG1abc mutant. The VgrG1b-dependent killing is mediated by PA0099, which presents the characteristics of the superfamily nuclease 2 toxin members. Overall, these data develop the concept that VgrGs are indispensable components for the specific delivery of effectors. Several additional vgrG genes are encoded on the P. aeruginosa genome and are not linked genetically to other T6SS genes. A closer inspection of these clusters reveals that they also encode putative toxins. Overall, these associations further support the notion of an original form of secretion system, in which VgrG acts as the carrier.

Advances in understanding Pseudomonas

Pseudomonas aeruginosa, the type species of pseudomonads, is an opportunistic pathogen that colonizes a wide range of niches. Current genome sequencing projects are producing previously inconceivable detail about the population biology and evolution of P. aeruginosa. Its pan-genome has a larger genetic repertoire than the human genome, which explains the broad metabolic capabilities of P. aeruginosa and its ubiquitous distribution in aquatic habitats. P. aeruginosa may persist in the airways of individuals with cystic fibrosis for decades. The ongoing whole-genome analyses of serial isolates from cystic fibrosis patients provide the so far singular opportunity to monitor the microevolution of a bacterial pathogen during chronic infection over thousands of generations. Although the evolution in cystic fibrosis lungs is neutral overall, some pathoadaptive mutations are selected during the within-host evolutionary process. Even a single mutation may be sufficient to generate novel complex traits provided that predisposing mutational events have previously occurred in the clonal lineage.

Subinhibitory concentration of kanamycin induces the Pseudomonas aeruginosa type VI secretion system

Pseudomonas aeruginosa is a Gram-negative bacterium found in natural environments including plants, soils and warm moist surfaces. This organism is also in the top ten of nosocomial pathogens, and prevalent in cystic fibrosis (CF) lung infections. The ability of P. aeruginosa to colonize a wide variety of environments in a lasting manner is associated with the formation of a resistant biofilm and the capacity to efficiently outcompete other microorganisms. Here we demonstrate that sub-inhibitory concentration of kanamycin not only induces biofilm formation but also induces expression of the type VI secretion genes in the H1-T6SS cluster. The H1-T6SS is known for its role in toxin production and bacterial competition. We show that the antibiotic induction of the H1-T6SS only occurs when a functional Gac/Rsm pathway is present. These observations may contribute to understand how P. aeruginosa responds to antibiotic producing competitors. It also suggests that improper antibiotic therapy may enhance P. aeruginosa colonization, including in the airways of CF patients.