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Soilborne Ralstonia solanacearum species complex (RSSC) pathogens disrupt microbial communities as they invade roots and fatally wilt plants. RSSC pathogens secrete antimicrobial toxins using a type VI secretion system (T6SS). To investigate how evolution and ecology have shaped the T6SS of these bacterial pathogens, we analyzed the T6SS gene content and architecture across the RSSC and their evolutionary relatives. Our analysis reveals that two ecologically similar Burkholderiaceae taxa, xylem-pathogenic RSSC and Paracidovorax, have convergently evolved to wield large arsenals of T6SS toxins. To understand the mechanisms underlying genomic enrichment of T6SS toxins, we compiled an atlas of 1,066 auxiliary T6SS toxin clusters (aux clusters) across 99 high-quality RSSC genomes. We classified 25 types of aux clusters with toxins that predominantly target lipids, nucleic acids, or unknown cellular substrates. The aux clusters were located in diverse genetic neighborhoods and had complex phylogenetic distributions, suggesting frequent horizontal gene flow. Phages and other mobile genetic elements account for most of the aux cluster acquisition on the chromosome but very little on the megaplasmid. Nevertheless, RSSC genomes were more enriched in aux clusters on the megaplasmid. Although the single, ancestral T6SS was broadly conserved in the RSSC, the T6SS has been convergently lost in atypical, non-soilborne lineages. Overall, our data suggest dynamic interplay between the lifestyle of RSSC lineages and the evolution of T6SSes with robust arsenals of toxins. This pangenomic atlas poises the RSSC as an emerging, tractable model to understand the role of the T6SS in shaping pathogen populations.IMPORTANCEWe explored the eco-evolutionary dynamics that shape the inter-microbial warfare mechanisms of a globally significant plant pathogen, the Ralstonia solanacearum species complex. We discovered that most Ralstonia wilt pathogens have evolved extensive and diverse repertoires of type VI secretion system-associated antimicrobial toxins. These expansive toxin arsenals potentially enhance the ability of Ralstonia pathogens to invade plant microbiomes, enabling them to rapidly colonize and kill their host plants. We devised a classification system to categorize the Ralstonia toxins. Interestingly, many of the toxin gene clusters are encoded on mobile genetic elements, including prophages, which may be mutualistic symbionts that enhance the inter-microbial competitiveness of Ralstonia wilt pathogens. Moreover, our findings suggest that the convergent loss of this multi-gene trait contributes to genome reduction in two vector-transmitted lineages of Ralstonia pathogens. Our findings demonstrate that the interplay between microbial ecology and pathogen lifestyle shapes the evolution of a genetically complex antimicrobial weapon.

The haemolysin co-regulatory protein (Hcp) plays a significant role in the pathogenicity of avian pathogenic Escherichia coli (APEC) as an effector protein of the type VI secretion system (T6SS) to the host. Meanwhile, mitochondria in host have become the target of effector proteins of various secretion systems. Here, we wanted to explore the effects of APEC effector Hcp2b on the mitochondria of DF-1 cells, and found that Hcp2b results in a damage in mitochondria. Next, 68 target proteins in DF-1 cell lysates were identified that interacted with Hcp2b by streptavidin-biotin pull-down assay combined with LC-MS/MS, among which, ADP/ATP transporter carrier (SLC25A4) is a mitochondria-associated protein and protein docking analysis showed that Hcp2b binds well to SLC25A4. Therefore, we hypothesize that the Hcp2b contribute to mitochondrial damage through interaction with the SLC25A4 in DF-1 cells. [ABSTRACT FROM AUTHOR]

The Type VI Secretion System (T6SS) is a sophisticated mechanism utilized by gram-negative bacteria to deliver toxic effector proteins into target cells, influencing microbial community dynamics and host interactions. In this study, we investigated the role of T6SSs in Snodgrassella alvi wkB2, a core bacterial symbiont of the honey bee gut microbiota. We generated single-and double-knockout mutants targeting essential genes (tssD and tssE) in both T6SS-1 and T6SS-2 and assessed their colonization and competition capabilities in vivo. Our results indicate that T6SSs are nonessential for colonization of the bee gut, although T6SS-2 mutant strains exhibited significantly lower colonization levels compared to the wild-type (WT) strain. Further, a defined community experiment showed that S. alvi wkB2 T6SSs do not significantly impact interspecific competition among core gut bacteria. However, cocolonization experiments with closely related S. alvi strains demonstrated that T6SS-1 plays a role in mediating intraspecific competition. Transcriptomic analysis of bee guts monocolonized by WT or T6SS mutants revealed differential expression of host immunity-related genes relative to microbiota-deprived bees, such as upregulation of the antimicrobial peptide apidaecin in the presence of WT S. alvi and the antimicrobial peptide defensin in the presence of T6SS-2 mutant S. alvi, suggesting that T6SSs contribute to shaping host immune responses. These findings provide insight into the ecological roles of T6SSs in the honey bee gut microbiota, emphasizing their importance in maintaining competitive dynamics and influencing host-bacterial interactions. [ABSTRACT FROM AUTHOR]

Species of the Bacteroidales order are among the most abundant and stable bacterial members of the human gut microbiome, with diverse impacts on human health. We cultured and sequenced the genomes of 408 Bacteroidales isolates from healthy human donors representing nine genera and 35 species and performed comparative genomic, gene-specific, metabolomic, and horizontal gene transfer analyses. Families, genera, and species could be grouped based on many distinctive features. We also observed extensive DNA transfer between diverse families, allowing for shared traits and strain evolution. Inter- and intra-species diversity is also apparent in the metabolomic profiling studies. This highly characterized and diverse Bacteroidales culture collection with strain-resolved genomic and metabolomic analyses represents a valuable resource to facilitate informed selection of strains for microbiome reconstitution. [Display omitted] • In-depth analyses of 408 Bacteroidales isolated from the gut of healthy humans • Pan-order genomic analyses reveal distinct features of families, genera, and species • Metabolomic diversity of Bacteroidales correlates with their genomic diversity • Documentation of extensive DNA transfer and strain evolution in the human gut Bacteroidales are the most abundant and stable order of bacteria in the healthy human gut microbiota. Comprehensive, in-depth analysis of a large human gut Bacteroidales culture collection by Zhang et al. reveals extensive strain-level genomic and metabolomic diversity and extensive inter-species DNA transfer and strain evolution in the human gut. [ABSTRACT FROM AUTHOR]

Previous studies revealed that the type VI secretion system (T6SS) has an essential role in bacterial competition and virulence in many gram-negative bacteria. However, the role of T6SS in virulence in Pectobacterium atrosepticum remains controversial. We examined a closely related strain, PccS1, and discovered that its T6SS comprises a single-copy cluster of 17 core genes with a higher identity to homologs from P. atrosepticum. Through extensive phenotypic and functional analyses of over 220 derivatives of PccS1, we found that three of the five VgrGs could be classified into group I VgrGs. These VgrGs interacted with corresponding DUF4123 domain proteins, which were secreted outside of the membrane and were dependent on either the T6SS or type IV secretion system (T4SS). This interaction directly governed virulence and competition. Meanwhile, supernatant proteomic analyses with strains defective in the T6SS and/or T4SS confirmed that effectors, such as FhaB, were secreted redundantly to control the virulence and suppress host callose deposition in the course of infection. Notably, this redundant secretion mechanism between the T6SS and T4SS is believed to be the first of its kind in bacteria. [ABSTRACT FROM AUTHOR]

Recent genomic characterisation of translocating Escherichia coli HMLN-1 isolated from mesenteric lymph nodes (MLNs) and blood of a patient with a fatal case of pancreatitis revealed the presence of a type 6 secretion system (T6SS) that was not present in non-translocating E. coli strains. This strain was also genomically similar to adherent-invasive E. coli (AIEC) LF82 pathotype. We aimed to identify the role of T6SS-1 in the pathogenesis of this strain and other pathogenic E. coli. The HMLN-1 strain was initially tested for the presence of six virulence genes (VGs) associated with AIEC strains and an iron sequestering system. Additionally, HMLN-1's interaction with a co-culture of Caco-2:HT29-MTX cells and its intra-macrophagic survival was evaluated. We subsequently screened a collection of 319 pathogenic E. coli strains isolated from patients with urinary tract infection (UTI), diarrhoea, inflammatory bowel disease (IBD) and septicaemia for the presence of T6SS-1 and its expression related to adhesion, invasion and translocation via the above co-culture of the intestinal cell lines. The results showed that HMLN-1 harboured four of the AIEC-associated VGs (dsbA, htrA, ompC and afaC). Screening of the pathogenic E. coli collection detected the presence of the T6SS-1 genes in septicaemic and UTI E. coli strains at a significantly higher level than diarrhoea and IBD strains (p < 0.0001). The high expression of T6SS-1 in E. coli HMLN-1 upon adhesion and invasion, as well as its high prevalence among extra-intestinal E. coli strains, suggests a role for T6SS-1 in the pathogenesis of translocating E. coli. [ABSTRACT FROM AUTHOR]

Currently, it is widely accepted that the type III secretion system (T3SS) serves as the transport platform for bacterial virulence factors, while flagella act as propulsion motors. However, there remains a noticeable dearth of comparative studies elucidating the functional disparities between these two mechanisms. Entomopathogenic nematode symbiotic bacteria (ENS), including Xenorhabdus and Photorhabdus, are Gram-negative bacteria transported into insect hosts by Steinernema or Heterorhabdus. Flagella are conserved in ENS, but the T3SS is only encoded in Photorhabdus. There are few reports on the function of flagella and the T3SS in ENS, and it is not known what role they play in the infection of ENS. Here, we clarified the function of the T3SS and flagella in ENS infection based on flagellar inactivation in X. stockiae (flhDC deletion), T3SS inactivation in P. luminescens (sctV deletion), and the heterologous synthesis of the T3SS of P. luminescens in X. stockiae. Consistent with the previous results, the swarming movement of the ENS and the formation of biofilms are dominated by the flagella. Both the T3SS and flagella facilitate ENS invasion and colonization within host cells, with minimal impact on secondary metabolite formation and secretion. Unexpectedly, a proteomic analysis reveals a negative feedback loop between the flagella/T3SS assembly and the type VI secretion system (T6SS). RT-PCR testing demonstrates the T3SS's inhibition of flagellar assembly, while flagellin expression promotes T3SS assembly. Furthermore, T3SS expression stimulates ribosome-associated protein expression. [ABSTRACT FROM AUTHOR]

Plesiomonas shigelloides, a Gram-negative bacillus, is the only member of the Enterobacteriaceae family able to produce polar and lateral flagella and cause gastrointestinal and extraintestinal illnesses in humans. The flagellar transcriptional hierarchy of P. shigelloides is currently unknown. In this study, we identified FlaK, FlaM, FliA, and FliAL as the four regulators responsible for polar and lateral flagellar regulation in P. shigelloides. To determine the flagellar transcription hierarchy of P. shigelloides, the transcriptomes of the WT and ΔflaK, ΔflaM, ΔfliA, and ΔfliAL were carried out for comparison in this study. Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) and luminescence screening assays were used to validate the RNA-seq results, and the Electrophoretic Mobility Shift Assay (EMSA) results revealed that FlaK can directly bind to the promoters of fliK, fliE, flhA, and cheY, while the FlaM protein can bind directly to the promoters of flgO, flgT, and flgA. Meanwhile, we also observed type VI secretion system (T6SS) and type II secretion system 2 (T2SS-2) genes downregulated in the transcriptome profiles, and the killing assay revealed lower killing abilities for ΔflaK, ΔflaM, ΔfliA, and ΔfliAL compared to the WT, indicating that there was a cross-talk between the flagellar hierarchy system and bacterial secretion system. Invasion assays also showed that ΔflaK, ΔflaM, ΔfliA, and ΔfliAL were less effective in infecting Caco-2 cells than the WT. Additionally, we also found that the loss of flagellar regulators causes the differential expression of some of the physiological metabolic genes of P. shigelloides. Overall, this study aims to reveal the transcriptional hierarchy that controls flagellar gene expression in P. shigelloides, as well as the cross-talk between motility, virulence, and physiological and metabolic activity, laying the groundwork for future research into P. shigelloides' coordinated survival in the natural environment and the mechanisms that infect the host. [ABSTRACT FROM AUTHOR]

Most bacteria live in communities, often with closely related strains and species with whom they must compete for space and resources. Consequently, bacteria have acquired or evolved mechanisms to antagonize competitors through the production of antibacterial toxins. Similar to bacterial systems that combat phage infection and mechanisms to thwart antibiotics, bacteria have also acquired and evolved features to protect themselves from antibacterial toxins. Just as there is a large body of research identifying and characterizing antibacterial proteins and toxin delivery systems, studies of bacterial mechanisms to resist and survive assault from competitors' weapons have also expanded tremendously. Emerging data are beginning to reveal protective processes and mechanisms that are as diverse as the toxins themselves. Protection against antibacterial toxins can be acquired by horizontal gene transfer, receptor or target alteration, induction of protective functions, physical barriers, and other diverse processes. Here, we review recent studies in this rapidly expanding field. This article outlines recent literature regarding the various mechanisms that bacteria use to protect themselves against bacterially produced toxins; including acquired immunity as well as defensive and protective mechanisms to antibacterial diffusible peptide and proteins toxins or to those delivered by T6SSs. [ABSTRACT FROM AUTHOR]

Here, we report the comprehensive genome analysis of Enterobacter cloacae SBP-8 and its pan-genome comparison to other Enterobacter species. Genome comparison was performed by the BioRing Image Generator Tool, which predicted its genome similarity to other closest strains. Detailed annotations of the genome revealed different families of carbohydrate-degrading enzymes (CAZymes) that might be involved in the degradation of decaying plant materials with industrial significance for biomass research and the food industry. Moreover, the microbial secretome contains a number of proteins that generally interact with other microbes, environmental space, and the host. The secretion of proteins is performed by the secretory system present in the bacterial cell, however, only a few secretome proteins have been studied in limited bacterial strains. Therefore, we explored the secretome analysis of E. cloacae SBP-8 by nano LC-MS/MS and identified 776 proteins that provide insight into the role of these proteins in bacterial adaptation to different environments. The secretome proteins were classified into molecular, biological, and cellular functions using Blast2GO, whereas the subcellular localization was predicted by the PSORB tool. Many of the secretome proteins belonged to virulence proteins including Type VI secretion system (T6SS) proteins, chemotaxis & fimbriae formation proteins, outer membrane proteins (OMPs), and exotoxin secretion. Moreover, STRING analysis showed the interaction among the identified proteins. The detailed investigations of secretome proteins may provide an understanding of the possible pathogenicity of test organism E. cloacae SBP-8, and is also clinically important for the development of biomarkers and therapeutic approaches. [ABSTRACT FROM AUTHOR]

ABSTRACT Soilborne Ralstonia solanacearum species complex (RSSC) pathogens disrupt microbial communities as they invade roots and fatally wilt plants. RSSC pathogens secrete antimicrobial toxins using a type VI secretion system (T6SS). To investigate how evolution and ecology have shaped the T6SS of these bacterial pathogens, we analyzed the T6SS gene content and architecture across the RSSC and their evolutionary relatives. Our analysis reveals that two ecologically similar Burkholderiaceae taxa, xylem-pathogenic RSSC and Paracidovorax, have convergently evolved to wield large arsenals of T6SS toxins. To understand the mechanisms underlying genomic enrichment of T6SS toxins, we compiled an atlas of 1,066 auxiliary T6SS toxin clusters (“aux” clusters) across 99 high-quality RSSC genomes. We classified 25 types of aux clusters with toxins that predominantly target lipids, nucleic acids, or unknown cellular substrates. The aux clusters were located in diverse genetic neighborhoods and had complex phylogenetic distributions, suggesting frequent horizontal gene flow. Phages and other mobile genetic elements account for most of the aux cluster acquisition on the chromosome but very little on the megaplasmid. Nevertheless, RSSC genomes were more enriched in aux clusters on the megaplasmid. Although the single, ancestral T6SS was broadly conserved in the RSSC, the T6SS has been convergently lost in atypical, non-soilborne lineages. Overall, our data suggest dynamic interplay between the lifestyle of RSSC lineages and the evolution of T6SSes with robust arsenals of toxins. This pangenomic atlas poises the RSSC as an emerging, tractable model to understand the role of the T6SS in shaping pathogen populations.IMPORTANCEWe explored the eco-evolutionary dynamics that shape the inter-microbial warfare mechanisms of a globally significant plant pathogen, the Ralstonia solanacearum species complex. We discovered that most Ralstonia wilt pathogens have evolved extensive and diverse repertoires of type VI secretion system-associated antimicrobial toxins. These expansive toxin arsenals potentially enhance the ability of Ralstonia pathogens to invade plant microbiomes, enabling them to rapidly colonize and kill their host plants. We devised a classification system to categorize the Ralstonia toxins. Interestingly, many of the toxin gene clusters are encoded on mobile genetic elements, including prophages, which may be mutualistic symbionts that enhance the inter-microbial competitiveness of Ralstonia wilt pathogens. Moreover, our findings suggest that the convergent loss of this multi-gene trait contributes to genome reduction in two vector-transmitted lineages of Ralstonia pathogens. Our findings demonstrate that the interplay between microbial ecology and pathogen lifestyle shapes the evolution of a genetically complex antimicrobial weapon.

Pathogens of the Burkholderia genus are causing diseases in a diverse variety of hosts. After the discovery of T6SS, it was found to play a pivotal role in virulence and other pathogenicity factors in different pathogenic Burkholderia species. For this study, three strains of Burkholderia cenocepacia were selected from different ecological niches; J2315 from humans, MC0-3 from the rhizosphere of maize, and YG-3 from the Populus tree. The sequenced genomes were retrieved from PATRIC. It was found that B. cenocepacia J2315 and MC0-3 strains had only 1 cluster of T6SS in their genomes while the YG-3 strain had 3 clusters. The circular genomic map and phylogenetic tree suggested major differences in T6SS clusters 2 and 3 of the YG-3 strain from other clusters. From the results obtained in the study and reviewing the literature, it was concluded that all 3 strains harbor T6SS-1 type cluster that is involved in causing virulence in eukaryotic organisms and several bacterial species. This factor of causing virulence in the bacteria species might be helpful for B. cenocepacia strains J2315, MC0-3 and YG-3 in survival and niche adaptation.

Down-flow hanging sponge (DHS) reactors, employed in domestic wastewater treatment, have demonstrated efficacy in eliminating Escherichia coli and other potentially pathogenic bacteria. The aim of this study was to elucidate the mechanism of removal of E. coli by employing a cube-shaped polyurethane sponge carrier within a compact hanging reactor. An E. coli removal experiment was conducted on this prepared sponge. Escherichia. coli level was found to decrease by more than 2 logs after passing through five nutrient-restricted DHS sponges. Conversely, a newly introduced sponge did not exhibit a comparable reduction in E. coli level. Furthermore, under conditions of optimal nutritional status, the reduction in E. coli level was limited to 0.5 logs, underscoring the crucial role of nutrient restriction in achieving effective elimination. Analysis of the sponge-associated bacterial community revealed the presence of a type VI secretion system (T6SS), a competitive mechanism observed in bacteria. This finding suggests that T6SS might play a pivotal role in contributing to the observed decline in E. coli level. [ABSTRACT FROM AUTHOR]

Bacterial secretion systems, such as the type 3, 4, and 6 are multiprotein nanomachines expressed at the surface of pathogens with Gram‐negative like envelopes. They are known to be crucial for virulence and to translocate bacteria‐encoded effector proteins into host cells to manipulate cellular functions. This facilitates either pathogen attachment or invasion of the targeted cell. Effector proteins also promote evasion of host immune recognition. Imaging by cryo‐electron microscopy in combination with structure determination has become a powerful approach to understand how these nanomachines work. Still, questions on their assembly, the precise secretion mechanisms, and their direct involvement in pathogenicity remain unsolved. Here, we present an overview of the recent developments in in situ cryo‐electron microscopy. We discuss its potential for the investigation of the role of bacterial secretion systems during the host‐bacterial crosstalk at the molecular level. These in situ studies open new perspectives for our understanding of secretion system structure and function. [ABSTRACT FROM AUTHOR]

The type VI secretion system (T6SS) of many gram‐negative bacteria injects toxic effectors into adjacent cells to manipulate host cells during pathogenesis or to kill competing bacteria. However, the identification and function of the T6SS effectors remains only partly known. Pantoea ananatis, a gram‐negative bacterium, is commonly found in various plants and natural environments, including water and soil. In the current study, genomic analysis of P. ananatis DZ‐12 causing brown stalk rot on maize demonstrated that it carries three T6SS gene clusters, namely, T6SS‐1, T6SS‐2, and T6SS‐3. Interestingly, only T6SS‐1 secretion systems are involved in pathogenicity and bacterial competition. The study also investigated the T6SS‐1 system in detail and identified an unknown T6SS‐1‐secreted effector TseG by using the upstream T6SS effector chaperone TecG containing a conserved domain of DUF2169. TseG can directly interact with the chaperone TecG for delivery and with a downstream immunity protein TsiG for protection from its toxicity. TseG, highly conserved in the Pantoea genus, is involved in virulence in maize, potato, and onion. Additionally, P. ananatis uses TseG to target Escherichia coli, gaining a competitive advantage. This study provides the first report on the T6SS‐1‐secreted effector from P. ananatis, thereby enriching our understanding of the various types and functions of type VI effector proteins. [ABSTRACT FROM AUTHOR]

[Objective] To investigate the effect and mechanism of Bacteroides fragilis type VI protein secretion system(T6SS)on the intestinal barrier. [Method] Suicide vector was used to construct B. fragilis T6SS mutant strain. The dextran sulfate sodium(DSS)salt-induced colitis mouse model was constructed, and supplemented mice with PBS, B. fragilis WT, and B. fragilis ΔT6SS, respectively. The disease characters and intestinal barrier integrity were compared among the three groups. Quantitative PCR and immunohistochemistry were used to detect the expressions of tight junction proteins. Untargeted metabolomics was used to compare gut differential metabolites in each group of mice.[Result] The deletion of T6SS did not affect bio-viability of B. fragilis. B. fragilis WT showed the improvements in body weight loss and colon length compared to PBS controls, demonstrating the protection to DSS-induced colitis; while this protection was lost with the absence of T6SS. The fluorescein isothiocyanate dextran concentration in the mouse serum and histological examination in intestinal pathological sections indicated that B. fragilis T6SS improved the integrity of the intestinal barrier in mice. T6SS mutation affected intestinal tight junction protein expression, confirmed by quantitative PCR and immunohistochemistry. Untargeted metabolomics analysis revealed 96 up-regulated differentially expressed metabolites in the B. fragilis WT group, compared to the B. fragilis ΔT6SS group, with multiple metabolites enriched in cholinergic synaptic metabolism and glycerophospholipid metabolism-related pathways. [Conclusion] B. fragilis T6SS alters intestinal metabolome and increases the expression of tight junction protein in intestinal cells, thus improving the permeability of intestinal barrier, being involved in the protection of B. fragilis to the intestinal barrier. [ABSTRACT FROM AUTHOR]

ABSTRACT 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.

ABSTRACT Within the realm of Gram-negative bacteria, bacteriocins are secreted almost everywhere, and the most representative are colicin and pyocin, which are secreted by Escherichia coli and Pseudomonas aeruginosa, respectively. Signal peptides at the amino terminus of bacteriocins or ABC transporters can secrete bacteriocins, which then enter bacteria through cell membrane receptors and exert toxicity. In general, the bactericidal spectrum is usually narrow, killing only the kin or closely related species. Our previous research indicates that YPK_0952 is an effector of the third Type VI secretion system (T6SS-3) in Yersinia pseudotuberculosis. Next, we sought to determine its identity and characterize its toxicity. We found that YPK_0952 (a pyocin-like effector) can achieve intra-species and inter-species competitive advantages through both contact-dependent and contact-independent mechanisms mediated by the T6SS-3 while enhancing the intestinal colonization capacity of Y. pseudotuberculosis. We further identified YPK_0952 as a DNase dependent on Mg2+, Ni2+, Mn2+, and Co2+ bivalent metal ions, and the homologous immune protein YPK_0953 can inhibit its activity. In summary, YPK_0952 exerts toxicity by degrading nucleic acids from competing cells, and YPK_0953 prevents self-attack in Y. pseudotuberculosis.IMPORTANCEBacteriocins secreted by Gram-negative bacteria generally enter cells through specific interactions on the cell surface, resulting in a narrow bactericidal spectrum. First, we identified a new pyocin-like effector protein, YPK_0952, in the third Type VI secretion system (T6SS-3) of Yersinia pseudotuberculosis. YPK_0952 is secreted by T6SS-3 and can exert DNase activity through contact-dependent and contact-independent entry into nearby cells of the same and other species (e.g., Escherichia coli) to help Y. pseudotuberculosis to exert a competitive advantage and promote intestinal colonization. This discovery lays the foundation for an in-depth study of the different effector protein types within the T6SS and their complexity in competing interactions. At the same time, this study provides a new development for the toolbox of toxin/immune pairs for studying Gram-negative bacteriocin translocation.

Recent genomic characterisation of translocating Escherichia coli HMLN-1 isolated from mesenteric lymph nodes (MLNs) and blood of a patient with a fatal case of pancreatitis revealed the presence of a type 6 secretion system (T6SS) that was not present in non-translocating E. coli strains. This strain was also genomically similar to adherent-invasive E. coli (AIEC) LF82 pathotype. We aimed to identify the role of T6SS-1 in the pathogenesis of this strain and other pathogenic E. coli. The HMLN-1 strain was initially tested for the presence of six virulence genes (VGs) associated with AIEC strains and an iron sequestering system. Additionally, HMLN-1’s interaction with a co-culture of Caco-2:HT29-MTX cells and its intra-macrophagic survival was evaluated. We subsequently screened a collection of 319 pathogenic E. coli strains isolated from patients with urinary tract infection (UTI), diarrhoea, inflammatory bowel disease (IBD) and septicaemia for the presence of T6SS-1 and its expression related to adhesion, invasion and translocation via the above co-culture of the intestinal cell lines. The results showed that HMLN-1 harboured four of the AIEC-associated VGs (dsbA, htrA, ompC and afaC). Screening of the pathogenic E. coli collection detected the presence of the T6SS-1 genes in septicaemic and UTI E. coli strains at a significantly higher level than diarrhoea and IBD strains (p < 0.0001). The high expression of T6SS-1 in E. coli HMLN-1 upon adhesion and invasion, as well as its high prevalence among extra-intestinal E. coli strains, suggests a role for T6SS-1 in the pathogenesis of translocating E. coli.

Introduction: The type VI secretion system (T6SS) is a crucial virulence factor in the nosocomial pathogen Acinetobacter baumannii. However, its association with drug resistance is less well known. Notably, the roles that different T6SS components play in the process of antimicrobial resistance, as well as in virulence, have not been systematically revealed. Methods: The importance of three representative T6SS core genes involved in the drug resistance and virulence of A. baumannii, namely, ΔtssB, ΔtssD (hcp), and tssM was elucidated. Results: A higher ratio of the three core genes was detected in drug-resistant strains than in susceptible strains among our 114 A. baumannii clinical isolates. Upon deletion of tssB in AB795639, increased antimicrobial resistance to cefuroxime and ceftriaxone was observed, alongside reduced resistance to gentamicin. The ΔtssD mutant showed decreased resistance to ciprofloxacin, norfloxacin, ofloxacin, tetracycline, and doxycycline, but increased resistance to tobramycin and streptomycin. The tssM-lacking mutant showed an increased sensitivity to ofloxacin, polymyxin B, and furazolidone. In addition, a significant reduction in biofilm formation was observed only with the DtssM mutant. Moreover, the ΔtssM strain, followed by the ΔtssD mutant, showed decreased survival in human serum, with attenuated competition with Escherichia coli and impaired lethality in Galleria mellonella. Discussion: The above results suggest that T6SS plays an important role, participating in the antibiotic resistance of A. baumannii, especially in terms of intrinsic resistance. Meanwhile, ΔtssM and ΔtssD contribute to bacterial virulence to a greater degree, with ΔtssM being associated with greater importance. [ABSTRACT FROM AUTHOR]

Pathogens of the Burkholderia genus are causing diseases in a diverse variety of hosts. After the discovery of T6SS, it was found to play a pivotal role in virulence and other pathogenicity factors in different pathogenic Burkholderia species. For this study, three strains of Burkholderia cenocepacia were selected from different ecological niches; J2315 from humans, MC0-3 from the rhizosphere of maize, and YG-3 from the Populus tree. The sequenced genomes were retrieved from PATRIC. It was found that B. cenocepacia J2315 and MC0-3 strains had only 1 cluster of T6SS in their genomes while the YG-3 strain had 3 clusters. The circular genomic map and phylogenetic tree suggested major differences in T6SS clusters 2 and 3 of the YG-3 strain from other clusters. From the results obtained in the study and reviewing the literature, it was concluded that all 3 strains harbor T6SS-1 type cluster that is involved in causing virulence in eukaryotic organisms and several bacterial species. This factor of causing virulence in the bacteria species might be helpful for B. cenocepacia strains J2315, MC0-3 and YG-3 in survival and niche adaptation. [ABSTRACT FROM AUTHOR]

Melioidosis, a severe tropical illness caused by Burkholderia pseudomallei, poses significant treatment challenges due to limited therapeutic options and the absence of effective vaccines. The pathogen's intrinsic resistance to numerous antibiotics and propensity to induce sepsis during acute infections further complicate management strategies. Thus, exploring alternative methods for prevention and treatment is crucial. Monoclonal antibodies (mAbs) have emerged as a promising strategy for the prevention and treatment of infectious diseases. This study focused on generating three mAbs (13F1, 14G11, and 15D9) targeting hemolysin-coregulated protein 1 (Hcp1), a protein involved in the type VI secretion system cluster 1 (T6SS1) of B. pseudomallei. Notably, pretreatment with 13F1 mAb significantly reduced the intracellular survival of B. pseudomallei and inhibited the formation of macrophage-derived multinucleated giant cells (MNGCs). This protective effect was also observed in vivo. We identified a sequence of amino acids (Asp95-Leu114) within Hcp1 as the likely binding site for 13F1 mAb. In summary, our findings reveal that 13F1 mAb counteracts infection by targeting Hcp1, offering potential new targets and insights for melioidosis prevention. [ABSTRACT FROM AUTHOR]

The type VI secretion system (T6SS) is a large secretory device, widely found in Gram-negative bacteria, which plays important roles in virulence, bacterial competition, and environmental adaptation. Vibrio alginolyticus (V. alginolyticus) is an opportunistic pathogen that causes vibriosis in aquaculture animals. V. alginolyticus possesses two type VI secretion systems (named the T6SS1 and T6SS2), but their functions remain largely unclear. In this paper, the roles of the core component of the T6SS2 cluster of V. alginolyticus HY9901, hemolysin-coregulated protein2 coding gene hcp2, are reported. Deletion of hcp2 clearly impaired the swarming motility, adhesive capacity, and pathogenicity of V. alginolyticus against zebrafish. Furthermore, transmission electron microscopy (TEM) found that the abnormal morphology of flagellum filament in the hcp2 mutant strain could be partially restored by hcp2 complementarity. By proteomic and RT-qPCR analysis, we confirmed that the expression levels of flagellar flagellin and assembly-associated proteins were remarkably decreased in an hcp2 mutant strain, compared with the wild-type strain, and could be partially restored with a supply of hcp2. Accordingly, hcp2 had a positive influence on the transcription of flagellar regulons rpoN, rpoS, and fliA; this was verified by RT-qPCR. Taken together, these results suggested that hcp2 was involved in mediating the motility, adhesion, and pathogenicity of Vibrio alginolyticus through positively impacting its flagellar system. [ABSTRACT FROM AUTHOR]

ABSTRACT Enterobacter cloacae is an emerging pathogen isolated in healthcare-associated infections. A major virulence factor of this bacterium is the type VI secretion system (T6SS). The genome of E. cloacae harbors two T6SS gene clusters (T6SS-1 and T6SS-2), and the functional characterization of both systems showed that these two T6SSs are not expressed under the same conditions. Here, we report that the major histone-like protein HU positively regulates the expression of both T6SSs and, therefore, the function that each T6SS exerts in E. cloacae. Single deletions of the genes encoding the HU subunits (hupA and hupB) decreased mRNA levels of both T6SS. In contrast, the hupA hupB double mutant dramatically affected the T6SS expression, diminishing its transcription. The direct binding of HU to the promoter regions of T6SS-1 and T6SS-2 was confirmed by electrophoretic mobility shift assay. In addition, single and double mutations in the hup genes affected the ability of inter-bacterial killing, biofilm formation, adherence to epithelial cells, and intestinal colonization, but these phenotypes were restored when such mutants were trans-complemented. Our data broaden our understanding of the regulation of HU-mediated T6SS in these pathogenic bacteria.IMPORTANCET6SS is a nanomachine that functions as a weapon of bacterial destruction crucial for successful colonization in a specific niche. Enterobacter cloacae expresses two T6SSs required for bacterial competition, adherence, biofilm formation, and intestinal colonization. Expression of T6SS genes in pathogenic bacteria is controlled by multiple regulatory systems, including two-component systems, global regulators, and nucleoid proteins. Here, we reported that the HU nucleoid protein directly activates both T6SSs in E. cloacae, affecting the T6SS-related phenotypes. Our data describe HU as a new regulator involved in the transcriptional regulation of T6SS and its impact on E. cloacae pathogenesis.

Wenke Liu,1,&ast; Min Li,1,&ast; Shiwen Cao,1 Hafiz Muhammad Ishaq,2 Huajie Zhao,1 Fan Yang,1 Liang Liu1 1Department of Pathogenic Biology, School of Basic Medical Sciences, Xinxiang Medical University, Xinxiang, People’s Republic of China; 2Faculty of Veterinary and Animal Sciences, Muhammad Nawaz Shareef University of Agriculture, Multan, Pakistan&ast;These authors contributed equally to this workCorrespondence: Liang Liu, Email robin2371@163.comAbstract: Bacteria communicate with their surroundings through diverse secretory systems, and the recently discovered Type VI Secretion System (T6SS) has gained significant attention. Klebsiella pneumoniae (K. pneumoniae), an opportunistic pathogen known for causing severe infections in both hospital and animal settings, possesses this intriguing T6SS. This system equips K. pneumoniae with a formidable armory of protein-based weaponry, enabling the delivery of toxins into neighboring cells, thus granting a substantial competitive advantage. Remarkably, the T6SS has also been associated with K. pneumoniae’s ability to form biofilms and acquire resistance against antibiotics. However, the precise effects of the T6SS on K. pneumoniae’s functions remain inadequately studied, despite research efforts to understand the intricacies of these mechanisms. This comprehensive review aims to provide an overview of the current knowledge regarding the biological functions and regulatory mechanisms of the T6SS in K. pneumoniae.Keywords: Klebsiella pneumoniae, T6SS, competition, drug resistance, virulence

The type VI secretion system (T6SS) present in many Gram-negative bacteria is a contact-dependent apparatus that can directly deliver secreted effectors or toxins into diverse neighboring cellular targets including both prokaryotic and eukaryotic organisms. Recent reverse genetics studies with T6 core gene loci have indicated the importance of functional T6SS toward overall competitive fitness in various pathogenic Xanthomonas spp. To understand the contribution of T6SS toward ecology and evolution of Xanthomonas spp., we explored the distribution of the three distinguishable T6SS clusters, i3*, i3***, and i4, in approximately 1,740 Xanthomonas genomes, along with their conservation, genetic organization, and their evolutionary patterns in this genus. Screening genomes for core genes of each T6 cluster indicated that 40% of the sequenced strains possess two T6 clusters, with combinations of i3*** and i3* or i3*** and i4. A few strains of Xanthomonas citri, Xanthomonas phaseoli, and Xanthomonas cissicola were the exception, possessing a unique combination of i3* and i4. The findings also indicated clade-specific distribution of T6SS clusters. Phylogenetic analysis demonstrated that T6SS clusters i3* and i3*** were probably acquired by the ancestor of the genus Xanthomonas, followed by gain or loss of individual clusters upon diversification into subsequent clades. T6 i4 cluster has been acquired in recent independent events by group 2 xanthomonads followed by its spread via horizontal dissemination across distinct clades across groups 1 and 2 xanthomonads. We also noted reshuffling of the entire core T6 loci, as well as T6SS spike complex components, hcp and vgrG, among different species. Our findings indicate that gain or loss events of specific T6SS clusters across Xanthomonas phylogeny have not been random.

Salmonella genus harbors five Type VI Secretion System (T6SS) gene clusters. The T6SS encoded in SPI-6 (T6SSSPI-6) contributes to Salmonella Typhimurium colonization of chickens and mice, while the T6SS encoded in SPI-19 (T6SSSPI-19) of Salmonella Gallinarum contributes to chicken colonization. Interestingly, the T6SSSPI-19 of Salmonella Gallinarum complemented the defect in chicken colonization of a Salmonella Typhimurium strain that lacks the T6SSSPI-6, suggesting that both T6SSs are interchangeable. Here we show that the transfer of Salmonella Gallinarum T6SSSPI-19 complemented the defect in mice colonization of a Salmonella Typhimurium ΔT6SSSPI-6 strain, indicating that both T6SSs are functionally redundant during host colonization.

ABSTRACT Type VI secretion system (T6SS) is a versatile, contact-dependent contractile nano-weapon in Gram-negative bacteria that fires proteinaceous effector molecules directly into prokaryotic and eukaryotic cells aiding in manipulation of the host and killing of competitors in complex niches. In plant pathogenic xanthomonads, T6SS has been demonstrated to play these diverse roles in individual pathosystems. However, the molecular network underlying the regulation of T6SS is still elusive in Xanthomonas spp. To bridge this knowledge gap, we conducted an in vitro transcriptome screen using plant apoplast mimicking minimal medium, XVM2 medium, to decipher the effect of tssM deletion, a core gene belonging to T6SS-cluster i3*, on the regulation of gene expression in Xanthomonas perforans strain AL65. Transcriptomic data revealed that a total of 277 and 525 genes were upregulated, while 307 and 392 genes were downregulated in the mutant strain after 8 and 16 hours of growth in XVM2 medium. The transcript abundance of several genes associated with flagellum and pilus biogenesis as well as type III secretion system was downregulated in the mutant strain. Deletion of tssM of cluster-i3* resulted in upregulation of several T6SS genes belonging to cluster-i3*** and genes involved in biofilm and cell wall biogenesis. Similarly, transcription regulators like rpoN, Pho regulon, rpoE, and csrA were identified to be upregulated in the mutant strain. Our results suggest that T6SS modulates the expression of global regulators like csrA, rpoN, and pho regulons, triggering a signaling cascade, and co-ordinates the expression of suite of virulence factors, stress response genes, and metabolic genes. IMPORTANCE T6SS has received attention due to its significance in mediating interorganismal competition through contact-dependent release of effector molecules into prokaryotic and eukaryotic cells. Reverse-genetic studies have indicated the role of T6SS in virulence in a variety of plant pathogenic bacteria, including the one studied here, Xanthomonas. However, it is not clear whether such effect on virulence is merely due to a shift in the microbiome-mediated protection or if T6SS is involved in a complex virulence regulatory network. In this study, we conducted in vitro transcriptome profiling in minimal medium to decipher the signaling pathways regulated by tssM-i3* in X. perforans AL65. We show that TssM-i3* regulates the expression of a suite of genes associated with virulence and metabolism either directly or indirectly by altering the transcription of several regulators. These findings further expand our knowledge on the intricate molecular circuits regulated by T6SS in phytopathogenic bacteria.

ABSTRACTTularaemia is a zoonotic disease caused by the Gram-negative bacterium, Francisella tularensis. Depending on its entry route into the organism, F. tularensis causes different diseases, ranging from life-threatening pneumonia to less severe ulceroglandular tularaemia. Various strains with different geographical distributions exhibit different levels of virulence. F. tularensis is an intracellular bacterium that replicates primarily in the cytosol of the phagocytes. The main virulence attribute of F. tularensis is the type 6 secretion system (T6SS) and its effectors that promote escape from the phagosome. In addition, F. tularensis has evolved a peculiar envelope that allows it to escape detection by the immune system. In this review, we cover tularaemia, different Francisella strains, and their pathogenicity. We particularly emphasize the intracellular life cycle, associated virulence factors, and metabolic adaptations. Finally, we present how F. tularensis largely escapes immune detection to be one of the most infectious and lethal bacterial pathogens.

ABSTRACTVibrio 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.

The model rhizobacterium Pseudomonas ogarae F113, a relevant plant growth-promoting bacterium, encodes three different Type VI secretion systems (T6SS) in its genome. In silico analysis of its genome revealed the presence of a genetic auxiliary module containing a gene encoding an orphan VgrG protein (VgrG5a) that is not genetically linked to any T6SS structural cluster, but is associated with genes encoding putative T6SS-related proteins: a possible adaptor Tap protein, followed by a putative effector, Tfe8, and its putative cognate immunity protein, Tfi8. The bioinformatic analysis of the VgrG5a auxiliary module has revealed that this cluster is only present in several subgroups of the P. fluorescens complex of species. An analysis of the mutants affecting the vgrG5a and tfe8 genes has shown that the module is involved in bacterial killing. To test whether Tfe8/Tfi8 constitute an effector–immunity pair, the genes encoding Tfe8 and Tfi8 were cloned and expressed in E. coli, showing that the ectopic expression of tfe8 affected growth. The growth defect was suppressed by tfi8 ectopic expression. These results indicate that Tfe8 is a bacterial killing effector, while Tfi8 is its cognate immunity protein. The Tfe8 protein sequence presents homology to the proteins of the MATE family involved in drug extrusion. The Tfe8 effector is a membrane protein with 10 to 12 transmembrane domains that could destabilize the membranes of target cells by the formation of pores, revealing the importance of these effectors for bacterial interaction. Tfe8 represents a novel type of a T6SS effector present in pseudomonads. [ABSTRACT FROM AUTHOR]

Enterobacter species are considered to be an opportunistic human pathogen owing to the existence of antibiotic-resistant strains and drug resides; however, the detailed analysis of the antibiotic resistance and virulence features in environmental isolates is poorly characterized. Here, in the study, we characterized the biochemical characteristics, and genome, pan-genome, and comparative genome analyses of an environmental isolate Enterobacter sp. S-16. The strain was identified as Enterobacter spp. by using 16S rRNA gene sequencing. To unravel genomic features, whole genome of Enterobacter sp. S-16 was sequenced using a hybrid assembly approach and genome assembly was performed using the Unicycler tool. The assembled genome contained the single conting size 5.3 Mbp, GC content 55.43%, and 4500 protein-coding genes. The genome analysis revealed the various gene clusters associated with virulence, antibiotic resistance, type VI secretion system (T6SS), and many stress tolerant genes, which may provide important insight for adapting to changing environment conditions. Moreover, different metabolic pathways were identified that potentially contribute to environmental survival. Various hydrolytic enzymes and motility functions equipped the strain S-16 as an active colonizer. The genome analysis confirms the presence of carbohydrate-active enzymes (CAZymes), and non-enzymatic carbohydrate-binding modules (CBMs) involved in the hydrolysis of complex carbohydrate polymers. Moreover, the pan-genome analysis provides detailed information about the core genes and shared genes with the closest related Enterobacter species. The present study is the first report showing the presence of YdhE/NorM in Enterobacter spp. Thus, the elucidation of genome sequencing will increase our understanding of the pathogenic nature of environmental isolate, supporting the One Health Concept. [ABSTRACT FROM AUTHOR]

Bacterial injectisomes are crucial nanomachines responsible for the direct secretion into host cells of a variety of substrates, ranging from proteins to DNAs. Prior to being injected, these substrates must be recruited and sometimes processed by large complexes called substrate recruitment platforms. These platforms are characterized by: • their ability to bind effectors and effector/chaperone complexes; • their ability to target the substrates to the injectisome channel for subsequent secretion; • their diversity and complexity, some made of one component, others forming large complexes; • their diverse modes of action, some requiring an ATPase, others where the core components of the injectisome act as the payload carrier. Bacteria use a wide arsenal of macromolecular substrates (DNA and proteins) to interact with or infect prokaryotic and eukaryotic cells. To do so, they utilize substrate-injecting secretion systems or injectisomes. However, prior to secretion, substrates must be recruited to specialized recruitment platforms and then handed over to the secretion apparatus for secretion. In this review, we provide an update on recent advances in substrate recruitment and delivery by gram-negative bacterial recruitment platforms associated with Type III, IV, and VI secretion systems. [ABSTRACT FROM AUTHOR]

Hospital-acquired pneumonia caused by Acinetobacter baumannii is a major healthcare burden. Type VI Secretion System (T6SS) contributes to both virulence and drug resistance in this bacteria. This study aims to investigate the diagnostic value of hemolysin co-regulated protein (Hcp) gene in A. baumannii pneumonia and further explore the effect of hcp on clinical, pathogenicity and drug resistance. 53 clinical A. baumannii strains from patients' respiratory tract at a teaching hospital were included in this study. Real-time quantitative polymerase chain reaction (qRT-PCR) was carried out to examine the expression of hcp. Recombinant Hcp expression plasmids (pET-28a(+)-hcp) were constructed and his-tagged Hcp were purified to stimulate Tohoku Hospital Pediatrics-1 (THP-1) macrophages. Nuclear Factor Kappa B p65 (NF-κBp65) and Interleukin 8 (IL-8) were detected by qRT-PCR. Antimicrobial susceptibility testing (AST) were examined by an automated instrument system. Hcp gene had 92.6% sensitivity and 75% specificity for distinguishing invasive or colonizing A. baumannii from the respiratory tract. His-tagged Hcp induced NF-κBp65 and IL-8 at gene level in THP-1 macrophages. Additional, high hcp expression isolates showed higher rate of antimicrobial agent exposure (< 30 days) of carbapenems, antibiotic combination therapy and multiple or extensive drug-resistant (MDR/XDR) and exhibited higher resistance rate to clinical commonly-used antimicrobial agents. Hcp gene could serve as a novel diagnostic biomarker to distinguish A. baumannii respiratory tract infection from colonization and participate in eliciting inflammatory responses in vitro. T6SS/hcp may play a role in the development of carbapenem-resistant A. baumannii (CRAB), multiple or extensive drug-resistant A. baumannii (MDRAB/XDRAB). [ABSTRACT FROM AUTHOR]

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

The Gram-negative bacterium Vibrio cholerae is responsible for the severe diarrheal pandemic disease cholera, representing a major global public health concern. This pathogen transitions from aquatic reservoirs into epidemics in human populations, and has evolved numerous mechanisms to sense this transition in order to appropriately regulate its gene expression for infection. At the intersection of pathogen and host in the gastrointestinal tract lies the community of native gut microbes, the gut microbiome. It is increasingly clear that the diversity of species and biochemical activities within the gut microbiome represents a driver of infection outcome, through their ability to manipulate the signals used by V. cholerae to regulate virulence and fitness in vivo. A better mechanistic understanding of how commensal microbial action interacts with V. cholerae pathogenesis may lead to novel prophylactic and therapeutic interventions for cholera. Here, we review a subset of this burgeoning field of research.

Yersinia pestis, the causative agent of plague, is a genetically monomorphic bacterial pathogen that evolved from Yersinia pseudotuberculosis approximately 7,400 years ago. We observed unusually frequent mutations in Y. pestis YPO0623, mostly resulting in protein translation termination, which implies a strong natural selection. These mutations were found in all phylogenetic lineages of Y. pestis, and there was no apparent pattern in the spatial distribution of the mutant strains. Based on these findings, we aimed to investigate the biological function of YPO0623 and the reasons for its frequent mutation in Y. pestis. Our in vitro and in vivo assays revealed that the deletion of YPO0623 enhanced the growth of Y. pestis in nutrient-rich environments and led to increased tolerance to heat and cold shocks. With RNA-seq analysis, we also discovered that the deletion of YPO0623 resulted in the upregulation of genes associated with the type VI secretion system (T6SS) at 26°C, which probably plays a crucial role in the response of Y. pestis to environment fluctuations. Furthermore, bioinformatic analysis showed that YPO0623 has high homology with a PLP-dependent aspartate aminotransferase in Salmonella enterica, and the enzyme activity assays confirmed its aspartate aminotransferase activity. However, the enzyme activity of YPO0623 was significantly lower than that in other bacteria. These observations provide some insights into the underlying reasons for the high-frequency nonsense mutations in YPO0623, and further investigations are needed to determine the exact mechanism.

The type VI secretion system (T6SS) is a contact-dependent contractile multiprotein apparatus widely distributed in Gram-negative bacteria. These systems can deliver different effector proteins into target bacterial and/or eukaryotic cells, contributing to the environmental fitness and virulence of many bacterial pathogens. Salmonella harbors five different T6SSs encoded in different genomic islands. The T6SS encoded in Salmonella Pathogenicity Island 6 (SPI-6) contributes to Salmonella competition with the host microbiota and its interaction with infected host cells. Despite its relevance, information regarding the total number of effector proteins encoded within SPI-6 and its distribution among different Salmonella enterica serotypes is limited. In this work, we performed bioinformatic and comparative genomics analyses of the SPI-6 T6SS gene cluster to expand our knowledge regarding the T6SS effector repertoire and the global distribution of these effectors in Salmonella. The analysis of a curated dataset of 60 Salmonella enterica genomes from the Secret6 database revealed the presence of 23 new putative T6SS effector/immunity protein (E/I) modules. These effectors were concentrated in the variable regions 1 to 3 (VR1-3) of the SPI-6 T6SS gene cluster. VR1-2 were enriched in candidate effectors with predicted peptidoglycan hydrolase activity, while VR3 was enriched in candidate effectors of the Rhs family with C-terminal extensions with predicted DNase, RNase, deaminase, or ADP-ribosyltransferase activity. A global analysis of known and candidate effector proteins in Salmonella enterica genomes from the NCBI database revealed that T6SS effector proteins are differentially distributed among Salmonella serotypes. While some effectors are present in over 200 serotypes, others are found in less than a dozen. A hierarchical clustering analysis identified Salmonella serotypes with distinct profiles of T6SS effectors and candidate effectors, highlighting the diversity of T6SS effector repertoires in Salmonella enterica. The existence of different repertoires of effector proteins suggests that different effector protein combinations may have a differential impact on the environmental fitness and pathogenic potential of these strains. [ABSTRACT FROM AUTHOR]

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Campylobacter is the leading cause of bacterial gastroenteritis worldwide and an emerging and neglected pathogen in South America. This zoonotic pathogen colonizes the gastrointestinal tract of a wide range of mammals and birds, with poultry as the most important reservoir for human infections. Apart from its high morbidity rates, the emergence of resistant strains is of global concern. The aims of this work were to determine genetic diversity, presence of antimicrobial resistance determinants and virulence potential of Campylobacter spp. isolated from patients with acute gastrointestinal disease at 'Clinica Alemana', Santiago de Chile. The study considered the isolation of Campylobacter spp., from stool samples during a 20-month period (January 2020 to September 2021). We sequenced (NextSeq, Illumina) and performed an in-depth analysis of the genome sequences of 88 Campylobacter jejuni and 2 Campylobacter coli strains isolated from clinical samples in Chile. We identified a high genetic diversity among C. jejuni strains and the emergence of prevalent clonal complexes, which were not identified in our previous reports. While ~40% of strains harbored a mutation in the gyrA gene associated with fluoroquinolone resistance, no macrolide-resistance determinants were detected. Interestingly, gene clusters encoding virulence factors such as the T6SS or genes associated with long-term sequelae such as Guillain-Barré syndrome showed lineage-relatedness. In addition, our analysis revealed a high degree of variability regarding the presence of fT3SS and T6SS effector proteins in comparison to type strains 81-176, F38011, and NCTC 11168 and 488. Our study provides important insights into the molecular epidemiology of this emerging foodborne pathogen. In addition, the differences observed regarding the repertoire of fT3SS and T6SS effector proteins could have an impact on the pathogenic potential and transmissibility of these Latin American isolates, posing another challenge in characterizing the infection dynamics of this emergent and neglected bacterial pathogen. [ABSTRACT FROM AUTHOR]

Pseudomonas syringae pv. aptata is a member of the sugar beet pathobiome and the causative agent of leaf spot disease. Like many pathogenic bacteria, P. syringae relies on the secretion of toxins, which manipulate host-pathogen interactions, to establish and maintain an infection. This study analyzes the secretome of six pathogenic P. syringae pv. aptata strains with different defined virulence capacities in order to identify common and strain-specific features, and correlate the secretome with disease outcome. All strains show a high type III secretion system (T3SS) and type VI secretion system (T6SS) activity under apoplast-like conditions mimicking the infection. Surprisingly, we found that low pathogenic strains show a higher secretion of most T3SS substrates, whereas a distinct subgroup of four effectors was exclusively secreted in medium and high pathogenic strains. Similarly, we detected two T6SS secretion patterns: while one set of proteins was highly secreted in all strains, another subset consisting of known T6SS substrates and previously uncharacterized proteins was exclusively secreted in medium and high virulence strains. Taken together, our data show that P. syringae pathogenicity is correlated with the repertoire and fine-tuning of effector secretion and indicate distinct strategies for establishing virulence of P. syringae pv. aptata in plants. [ABSTRACT FROM AUTHOR]

Salmonella genus harbors five Type VI Secretion System (T6SS) gene clusters. The T6SS encoded in SPI-6 (T6SSSPI-6) contributes to Salmonella Typhimurium colonization of chickens and mice, while the T6SS encoded in SPI-19 (T6SSSPI-19) of Salmonella Gallinarum contributes to chicken colonization. Interestingly, the T6SSSPI-19 of Salmonella Gallinarum complemented the defect in chicken colonization of a Salmonella Typhimurium strain that lacks the T6SSSPI-6, suggesting that both T6SSs are interchangeable. Here we show that the transfer of Salmonella Gallinarum T6SSSPI-19 complemented the defect in mice colonization of a Salmonella Typhimurium ΔT6SSSPI-6 strain, indicating that both T6SSs are functionally redundant during host colonization. [ABSTRACT FROM AUTHOR]

All bacteria must compete for growth niches and other limited environmental resources. These existential battles are waged at several levels, but one common strategy entails the transfer of growth-inhibitory protein toxins between competing cells. These antibacterial effectors are invariably encoded with immunity proteins that protect cells from intoxication by neighboring siblings. Several effector classes have been described, each designed to breach the cell envelope of target bacteria. Although effector architectures and export pathways tend to be clade specific, phylogenetically distant species often deploy closely related toxin domains. Thus, diverse competition systems are linked through a common reservoir of toxin-immunity pairs that is shared via horizontal gene transfer. These toxin-immunity protein pairs are extraordinarily diverse in sequence, and this polymorphism underpins an important mechanism of self/nonself discrimination in bacteria. This review focuses on the structures, functions, and delivery mechanisms of polymorphic toxin effectors that mediate bacterial 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.

Abstract Background RpoN, also known as σ54, first reported in Escherichia coli, is a subunit of RNA polymerase that strictly controls the expression of different genes by identifying specific promoter elements. RpoN has an important regulatory function in carbon and nitrogen metabolism and participates in the regulation of flagellar synthesis, bacterial motility and virulence. However, little is known about the effect of RpoN in Plesiomonas shigelloides. Results To identify pathways controlled by RpoN, RNA sequencing (RNA-Seq) of the WT and the rpoN deletion strain was carried out for comparison. The RNA-seq results showed that RpoN regulates ~ 13.2% of the P. shigelloides transcriptome, involves amino acid transport and metabolism, glycerophospholipid metabolism, pantothenate and CoA biosynthesis, ribosome biosynthesis, flagellar assembly and bacterial secretion system. Furthermore, we verified the results of RNA-seq using quantitative real-time reverse transcription PCR, which indicated that the absence of rpoN caused downregulation of more than half of the polar and lateral flagella genes in P. shigelloides, and the ΔrpoN mutant was also non-motile and lacked flagella. In the present study, the ability of the ΔrpoN mutant to kill E. coli MG1655 was reduced by 54.6% compared with that of the WT, which was consistent with results in RNA-seq, which showed that the type II secretion system (T2SS-2) genes and the type VI secretion system (T6SS) genes were repressed. By contrast, the expression of type III secretion system genes was largely unchanged in the ΔrpoN mutant transcriptome and the ability of the ΔrpoN mutant to infect Caco-2 cells was also not significantly different compared with the WT. Conclusions We showed that RpoN is required for the motility and contributes to the killing ability of P. shigelloides and positively regulates the T6SS and T2SS-2 genes.

Simple Summary: Rhizobium etli Mim1 (ReMim1) possesses a protein secretion system type VI (T6SS) that is active in free living and symbiosis. The T6SS is a nanosyringe that secretes proteins called effectors to both eukaryotic and prokaryotic target cells. The ReMim1 T6SS gene cluster encodes for a toxic effector (Re78) together with an immunity protein (Re79) as demonstrated when expressed in Escherichia coli. In addition, it was observed that the toxic role of the Re78 protein is outside the cytoplasm since its toxic effect on E. coli only occurred when a signal peptide was added to it. Re79 is found in ReMim1 periplasm and is T6SS-independent for its translocation. Additionally, the Re78/Re79 pair is involved in bacterial competition and nodule occupancy. A better understanding of the role of this secretion system can be very useful to select highly competitive rhizobia for inoculants. The genes of the type VI secretion system (T6SS) from Rhizobium etli Mim1 (ReMim1) that contain possible effectors can be divided into three modules. The mutants in them indicated that they are not required for effective nodulation with beans. To analyze T6SS expression, a putative promoter region between the tssA and tssH genes was fused in both orientations to a reporter gene. Both fusions are expressed more in free living than in symbiosis. When the module-specific genes were studied using RT-qPCR, a low expression was observed in free living and in symbiosis, which was clearly lower than the structural genes. The secretion of Re78 protein from the T6SS gene cluster was dependent on the presence of an active T6SS. Furthermore, the expression of Re78 and Re79 proteins in E. coli without the ReMim1 nanosyringe revealed that these proteins behave as a toxic effector/immunity protein pair (E/I). The harmful action of Re78, whose mechanism is still unknown, would take place in the periplasmic space of the target cell. The deletion of this ReMim1 E/I pair resulted in reduced competitiveness for bean nodule occupancy and in lower survival in the presence of the wild-type strain. [ABSTRACT FROM AUTHOR]

The type VI secretion system (T6SS) is important for interbacterial competition and virulence in Vibrio species. It is generally agreed that T6SS provides a fitness advantage to Vibrios. Some Vibrio species possess one, while others possess two T6SSs. Even within the same Vibrio species, different strains can harbor a variable number of T6SSs. Such is the case in V. fluvialis, an opportunistic human pathogen, that some V. fluvialis strains do not harbor T6SS1. This study found that Amphritea, Marinomonas, Marinobacterium, Vibrio, Photobacterium, and Oceanospirillum species have genes encoding V. fluvialis T6SS1 homologs. The cladogram of T6SS1 genes suggested that these genes appeared to be horizontally acquired by V. fluvialis, V. furnissii, and some other Vibrio species, when compared with the species tree. Codon insertions, codon deletions, nonsense mutations, and the insertion sequence are found in many genes, such as clpV1, tssL1, and tssF1, which encode structure components of T6SS1 in V. furnissii and V. fluvialis. Codon deletion events are more common than codon insertion, insertion sequence disruption, and nonsense mutation events in genes that encode components of T6SS1. Similarly, codon insertions and codon deletions are found in genes relevant to T6SS2, including tssM2, vgrG2 and vasH, in V. furnissii and V. fluvialis. These mutations are likely to disable the functions of T6SSs. Our findings indicate that T6SS may have a fitness disadvantage in V. furnissii and V. fluvialis, and the loss of function in T6SS may help these Vibrio species to survive under certain conditions. [ABSTRACT FROM AUTHOR]