Type VI secretion system of Pseudomonas aeruginosa is associated with biofilm formation but not environmental adaptation

Dopamine, an exogenous quorum sensing signaling molecule or a modulating factor in Pseudomonas aeruginosa?

Pseudomonas aeruginosa is recognized globally as an opportunistic pathogen of considerable concern due to its high virulence and pathogenicity, especially in immunocompromised individuals. While research has identified several endogenous quorum sensing (QS) signaling molecules that enhance the virulence and pathogenicity of P. aeruginosa, investigations on exogenous QS signaling molecules or modulating factors remain limited. This study found that dopamine serves as an exogenous QS signaling molecule or modulating factor of P. aeruginosa PAO1, enhancing the production of virulence factors and biofilms. Compared to the control group, treatment with 40 μM dopamine resulted in a 33.1 % increase in biofilm formation, 68.1 % increase in swimming mobility, 63.1 % increase in swarming mobility, 147.2 % increase in the signaling molecule 3-oxo-C12-HSL, and 50.5 %, 28.5 %, 27.0 %, and 33.2 % increases in the virulence factors alginate, rhamnolipids, protease, and pyocyanin, respectively. This study further explored the mechanism of dopamine regulating the biofilm formation and virulence of P. aeruginosa PAO1 through transcriptome and metabolome. Transcriptomic analysis showed that dopamine promoted the expression of virulence genes psl, alg, lasA, rhlABC, rml, and phz in P. aeruginosa PAO1. Metabolomic analysis revealed changes in the concentrations of tryptophan, pyruvate, ethanolamine, glycine, 3-hydroxybutyric acid, and alizarin. Furthermore, KEGG enrichment analysis of altered genes and metabolites indicated that dopamine enhanced phenylalanine, tyrosine, and tryptophan in P. aeruginosa PAO1. The results of this study will contribute to the development of novel exogenous QS signaling molecules or modulating factors and advance our understanding of the interactions between P. aeruginosa and the host environment.

Initiation of H1-T6SS dueling between Pseudomonas aeruginosa

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

Impact of multidrug resistance on the virulence and fitness of Pseudomonas aeruginosa: a microbiological and clinical perspective

Pseudomonas aeruginosa is one of the most common nosocomial pathogens and part of the top emergent species associated with antimicrobial resistance that has become one of the greatest threat to public health in the twenty-first century. This bacterium is provided with a wide set of virulence factors that contribute to pathogenesis in acute and chronic infections. This review aims to summarize the impact of multidrug resistance on the virulence and fitness of P. aeruginosa. Although it is generally assumed that acquisition of resistant determinants is associated with a fitness cost, several studies support that resistance mutations may not be associated with a decrease in virulence and/or that certain compensatory mutations may allow multidrug resistance strains to recover their initial fitness. We discuss the interplay between resistance profiles and virulence from a microbiological perspective but also the clinical consequences in outcomes and the economic impact.

Unveiling the microevolution of antimicrobial resistance in selected Pseudomonas aeruginosa isolates from Egyptian healthcare settings: A genomic approach

The incidence of Pseudomonas aeruginosa infections in healthcare environments, particularly in low-and middle-income countries, is on the rise. The purpose of this study was to provide comprehensive genomic insights into thirteen P. aeruginosa isolates obtained from Egyptian healthcare settings. Phenotypic analysis of the antimicrobial resistance profile and biofilm formation were performed using minimum inhibitory concentration and microtiter plate assay, respectively. Whole genome sequencing was employed to identify sequence typing, resistome, virulome, and mobile genetic elements. Our findings indicate that 92.3% of the isolates were classified as extensively drug-resistant, with 53.85% of these demonstrating strong biofilm production capabilities. The predominant clone observed in the study was ST773, followed by ST235, both of which were associated with the O11 serotype. Core genome multi-locus sequence typing comparison of these clones with global isolates suggested their potential global expansion and adaptation. A significant portion of the isolates harbored Col plasmids and various MGEs, all of which were linked to antimicrobial resistance genes. Single nucleotide polymorphisms in different genes were associated with the development of antimicrobial resistance in these isolates. In conclusion, this pilot study underscores the prevalence of extensively drug-resistant P. aeruginosa isolates and emphasizes the role of horizontal gene transfer facilitated by a diverse array of mobile genetic elements within various clones. Furthermore, specific insertion sequences and mutations were found to be associated with antibiotic resistance.

Molecular mechanisms of DNase inhibition of early biofilm formation Pseudomonas aeruginosa or Staphylococcus aureus: A transcriptome analysis

In vitro studies show that DNase can inhibit Pseudomonas aeruginosa and Staphylococcus aureus biofilm formation. However, the underlying molecular mechanisms remain poorly understood. This study used an RNA-sequencing transcriptomic approach to investigate the mechanism by which DNase I inhibits early P. aeruginosa and S. aureus biofilm formation on a transcriptional level, respectively. A total of 1171 differentially expressed genes (DEGs) in P. aeruginosa and 1016 DEGs in S. aureus enriched in a variety of biological processes and pathways were identified, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that the DEGs were primarily involved in P. aeruginosa two-component system, biofilm formation, and flagellar assembly and in S. aureus biosynthesis of secondary metabolites, microbial metabolism in diverse environments, and biosynthesis of amino acids, respectively. The transcriptional data were validated using quantitative real-time polymerase chain reaction (RT-qPCR), and the expression profiles of 22 major genes remained consistent. These findings suggested that DNase I may inhibit early biofilm formation by downregulating the expression of P. aeruginosa genes associated with flagellar assembly and the type VI secretion system, and by downregulating S. aureus capsular polysaccharide and amino acids metabolism gene expression, respectively. This study offers insights into the mechanisms of DNase treatment-based inhibition of early P. aeruginosa and S. aureus biofilm formation.

Characterization of a hemolytic and antibiotic-resistant Pseudomonas aeruginosa strain S3 pathogenic to fish isolated from Mahananda River in India

Virulent strain Pseudomonas aeruginosa isolated from Mahananda River exhibited the highest hemolytic activity and virulence factors and was pathogenic to fish as clinical signs of hemorrhagic spots, loss of scales, and fin erosions were found. S3 was cytotoxic to the human liver cell line (WRL-68) in the trypan blue dye exclusion assay. Genotype characterization using whole genome analysis showed that S3 was similar to P. aeruginosa PAO1. The draft genome sequence had an estimated length of 62,69,783 bp, a GC content of 66.3%, and contained 5916 coding sequences. Eight genes across the genome were predicted to be related to hemolysin action. Antibiotic resistance genes such as class C and class D beta-lactamases, fosA, APH, and catB were detected, along with the strong presence of multiple efflux system genes. This study shows that river water is contaminated by pathogenic P. aeruginosa harboring an array of virulence and antibiotic resistance genes which warrants periodic monitoring to prevent disease outbreaks.

Impact of Homologous Recombination on Core Genome Evolution and Host Adaptation of Pectobacterium parmentieri

Homologous recombination is a major force mechanism driving bacterial evolution, host adaptability, and acquisition of novel virulence traits. Pectobacterium parmentieri is a plant bacterial pathogen distributed worldwide, primarily affecting potatoes, by causing soft rot and blackleg diseases. The goal of this investigation was to understand the impact of homologous recombination on the genomic evolution of P. parmentieri. Analysis of P. parmentieri genomes using Roary revealed a dynamic pan-genome with 3,742 core genes and over 55% accessory genome variability. Bayesian population structure analysis identified 7 lineages, indicating species heterogeneity. ClonalFrameML analysis displayed 5,125 recombination events, with the lineage 4 exhibiting the highest events. fastGEAR analysis identified 486 ancestral and 941 recent recombination events ranging from 43 bp to 119 kb and 36 bp to 13.96 kb, respectively, suggesting ongoing adaptation. Notably, 11% (412 genes) of the core genome underwent recent recombination, with lineage 1 as the main donor. The prevalence of recent recombination (double compared to ancient) events implies continuous adaptation, possibly driven by global potato trade. Recombination events were found in genes involved in vital cellular processes (DNA replication, DNA repair, RNA processing, homeostasis, and metabolism), pathogenicity determinants (type secretion systems, cell-wall degrading enzymes, iron scavengers, lipopolysaccharides (LPS), flagellum, etc.), antimicrobial compounds (phenazine and colicin) and even CRISPR-Cas genes. Overall, these results emphasize the potential role of homologous recombination in P. parmentieri's evolutionary dynamics, influencing host colonization, pathogenicity, adaptive immunity, and ecological fitness.

The role of the type VI secretion system in the stress resistance of plant-associated bacteria

The type VI secretion system (T6SS) is a powerful bacterial molecular weapon that can inject effector proteins into prokaryotic or eukaryotic cells, thereby participating in the competition between bacteria and improving bacterial environmental adaptability. Although most current studies of the T6SS have focused on animal bacteria, this system is also significant for the adaptation of plant-associated bacteria. This paper briefly introduces the structure and biological functions of the T6SS. We summarize the role of plant-associated bacterial T6SS in adaptability to host plants and the external environment, including resistance to biotic stresses such as host defenses and competition from other bacteria. We review the role of the T6SS in response to abiotic factors such as acid stress, oxidation stress, and osmotic stress. This review provides an important reference for exploring the functions of the T6SS in plant-associated bacteria. In addition, characterizing these anti-stress functions of the T6SS may provide new pathways toward eliminating plant pathogens and controlling agricultural losses.

Genetic Insights into Biofilm Formation by a Pathogenic Strain of Vibrio harveyi

The Vibrio genus includes bacteria widely distributed in aquatic habitats and the infections caused by these bacteria can affect a wide range of hosts. They are able to adhere to numerous surfaces, which can result in biofilm formation that helps maintain them in the environment. The involvement of the biofilm lifestyle in the virulence of Vibrio pathogens of aquatic organisms remains to be investigated. Vibrio harveyi ORM4 is a pathogen responsible for an outbreak in European abalone Haliotis tuberculata populations. In the present study, we used a dynamic biofilm culture technique coupled with laser scanning microscopy to characterize the biofilm formed by V. harveyi ORM4. We furthermore used RNA-seq analysis to examine the global changes in gene expression in biofilm cells compared to planktonic bacteria, and to identify biofilm- and virulence-related genes showing altered expression. A total of 1565 genes were differentially expressed, including genes associated with motility, polysaccharide synthesis, and quorum sensing. The up-regulation of 18 genes associated with the synthesis of the type III secretion system suggests that this virulence factor is induced in V. harveyi ORM4 biofilms, providing indirect evidence of a relationship between biofilm and virulence.

A computational and secretome analysis approach reveals exclusive and shared candidate type six secretion system substrates in Pectobacterium brasiliense 1692

The type 6 secretion system (T6SS) of Gram-negative bacteria (GNB) has implications for bacterial competition, virulence, and survival. For the broad host range pathogen, Pectobacterium brasiliense 1692, T6SS-mediated competition occurs in a tissue-specific manner. However, no other roles have been investigated. The aim of this study was to identify T6SS-associated proteins under virulence inducing conditions. We used Bastion tools to predict 1479 Pbr1692 secreted proteins. Sixteen percent of these overlap between type 1-4 secretion systems (T1SS-T4SS) and T6SS. Using label-free quantitative mass spectrometry of Pbr1692 T6SS active and T6SS inactive strains' secretomes cultured in minimal media supplemented with host extract, 49 T6SS-associated proteins with varied gene ontology predicted functions were identified. We report 19 and 30 T6SS primary substrates and differentially secreted proteins, respectively, in T6SS mutants versus wild type strains. Of the total 49 T6SS-associated proteins presented in this study, 25 were also predicted using the BastionX platform as T6SS exclusive and shared substrates with T1SS-T4SS. This work provides a list of Pbr1692 T6SS secreted effector candidates. These include a potential antibacterial toxin HNH endonuclease and several predicted virulence proteins, including plant cell wall degrading enzymes. A preliminary basis for potential crosstalk between GNB secretion systems is also highlighted.

Global transcriptional analysis of Geobacter sulfurreducens gsu1771 mutant biofilm grown on two different support structures

Electroactive biofilms formation by the metal-reducing bacterium Geobacter sulfurreducens is a step crucial for bioelectricity generation and bioremediation. The transcriptional regulator GSU1771 controls the expression of essential genes involved in electron transfer and biofilm formation in G. sulfurreducens, with GSU1771-deficient producing thicker and more electroactive biofilms. Here, RNA-seq analyses were conducted to compare the global gene expression patterns of wild-type and Δgsu1771 mutant biofilms grown on non-conductive (glass) and conductive (graphite electrode) materials. The Δgsu1771 biofilm grown on the glass surface exhibited 467 differentially expressed (DE) genes (167 upregulated and 300 downregulated) versus the wild-type biofilm. In contrast, the Δgsu1771 biofilm grown on the graphite electrode exhibited 119 DE genes (79 upregulated and 40 downregulated) versus the wild-type biofilm. Among these DE genes, 67 were also differentially expressed in the Δgsu1771 biofilm grown on glass (56 with the same regulation and 11 exhibiting counter-regulation). Among the upregulated genes in the Δgsu1771 biofilms, we identified potential target genes involved in exopolysaccharide synthesis (gsu1961-63, gsu1959, gsu1972-73, gsu1976-77). RT-qPCR analyses were then conducted to confirm the differential expression of a selection of genes of interest. DNA-protein binding assays demonstrated the direct binding of the GSU1771 regulator to the promoter region of pgcA, pulF, relA, and gsu3356. Furthermore, heme-staining and western blotting revealed an increase in c-type cytochromes including OmcS and OmcZ in Δgsu1771 biofilms. Collectively, our findings demonstrated that GSU1771 is a global regulator that controls extracellular electron transfer and exopolysaccharide synthesis in G. sulfurreducens, which is crucial for electroconductive biofilm development.

Transcriptomic Profiles of Vibrio parahaemolyticus During Biofilm Formation

Vibrio parahaemolyticus, the leading cause of bacterial seafood-associated gastroenteritis, can form biofilms. In this work, the gene expression profiles of V. parahaemolyticus during biofilm formation were investigated by transcriptome sequencing. A total of 183, 503, and 729 genes were significantly differentially expressed in the bacterial cells at 12, 24 and 48 h, respectively, compared with that at 6 h. Of these, 92 genes were consistently activated or repressed from 6 to 48 h. The genes involved in polar flagellum, chemotaxis, mannose-sensitive haemagglutinin type IV pili, capsular polysaccharide, type III secretion system 1 (T3SS1), T3SS2, thermostable direct hemolysin (TDH), type VI secretion system 1 (T6SS1) and T6SS2 were downregulated, whereas those involved in V. parahaemolyticus pathogenicity island (Vp-PAI) (except for T3SS2 and TDH) and membrane fusion proteins were upregulated. Three extracellular protease genes (vppC, prtA and VPA1071) and a dozen of outer membrane protein encoding genes were also significantly differentially expressed during biofilm formation. In addition, five putative c-di-GMP metabolism-associated genes were significantly differentially expressed, which may account for the drop in c-di-GMP levels after the beginning of biofilm formation. Moreover, many putative regulatory genes were significantly differentially expressed, and more than 1000 putative small non-coding RNAs were detected, suggesting that biofilm formation was tightly regulated by complex regulatory networks. The data provided a global view of gene expression profiles during biofilm formation, showing that the significantly differentially expressed genes were involved in multiple cellular pathways, including virulence, biofilm formation, metabolism, and regulation.

Paraburkholderia sabiae Uses One Type VI Secretion System (T6SS-1) as a Powerful Weapon against Notorious Plant Pathogens

Paraburkholderia sabiae LMG24235 is a nitrogen-fixing betaproteobacterium originally isolated from a root nodule of Mimosa caesalpiniifolia in Brazil. We show here that this strain effectively kills strains from several bacterial families (Burkholderiaceae, Pseudomonadaceae, Enterobacteriaceae) which include important plant pathogens in a contact-dependent manner. De novo assembly of the first complete genome of P. sabiae using long sequencing reads and subsequent annotation revealed two gene clusters predicted to encode type VI secretion systems (T6SS), which we named T6SS-1 and T6SS-3 according to previous classification methods (G. Shalom, J. G. Shaw, and M. S. Thomas, Microbiology, 153:2689-2699, 2007, https://doi.org/10.1099/mic.0.2007/006585-0). We created P. sabiae with mutations in each of the two T6SS gene clusters that abrogated their function, and the T6SS-1 mutant was no longer able to outcompete other strains in a contact-dependent manner. Notably, our analysis revealed that T6SS-1 is essential for competition against several important plant pathogens in vitro, including Burkholderia plantarii, Ralstonia solanacearum, Pseudomonas syringae, and Pectobacterium carotovorum. The 9-log reduction in P. syringae cells in the presence of P. sabiae was particularly remarkable. Importantly, in an in vivo assay, P. sabiae was able to protect potato tubers from bacterial soft rot disease caused by P. carotovorum, and this protection was partly dependent on T6SS-1. IMPORTANCE Rhizobia often display additional beneficial traits such as the production of plant hormones and the acquisition of limited essential nutrients that improve plant growth and enhance plant yields. Here, we show that the rhizobial strain P. sabiae antagonizes important phytopathogens such as P. carotovorum, P. syringae, and R. solanacearum and that this effect is due to contact-dependent killing mediated by one of two T6SS systems identified in the complete, de novo assembled genome sequence of P. sabiae. Importantly, co-inoculation of Solanum tuberosum tubers with P. sabiae also resulted in a drastic reduction of soft rot caused by P. carotovorum in an in vivo model system. This result highlights the protective potential of P. sabiae against important bacterial plant diseases, which makes it a valuable candidate for application as a biocontrol agent. It also emphasizes the particular potential of rhizobial inoculants that combine several beneficial effects such as plant growth promotion and biocontrol for sustainable agriculture.

Regulation of type VI secretion systems at the transcriptional, posttranscriptional and posttranslational level

Bacteria live in complex polymicrobial communities and are constantly competing for resources. The type VI secretion system (T6SS) is a widespread antagonistic mechanism used by Gram-negative bacteria to gain an advantage over competitors. T6SSs translocate toxic effector proteins inside target prokaryotic cells in a contact-dependent manner. In addition, some T6SS effectors can be secreted extracellularly and contribute to the scavenging scarce metal ions. Bacteria deploy their T6SSs in different situations, categorizing these systems into offensive, defensive and exploitative. The great variety of bacterial species and environments occupied by such species reflect the complexity of regulatory signals and networks that control the expression and activation of the T6SSs. Such regulation is tightly controlled at the transcriptional, posttranscriptional and posttranslational level by abiotic (e.g. pH, iron) or biotic (e.g. quorum-sensing) cues. In this review, we provide an update on the current knowledge about the regulatory networks that modulate the expression and activity of T6SSs across several species, focusing on systems used for interbacterial competition.

Comparative Genomic Analysis of a Novel Vibrio sp. Isolated from an Ulcer Disease Event in Atlantic Salmon (Salmo salar)

Ulcer diseases are a recalcitrant issue at Atlantic salmon (Salmo salar) aquaculture cage-sites across the North Atlantic region. Classical ulcerative outbreaks (also called winter ulcer disease) refer to a skin infection caused by Moritella viscosa. However, several bacterial species are frequently isolated from ulcer disease events, and it is unclear if other undescribed pathogens are implicated in ulcer disease in Atlantic salmon. Although different polyvalent vaccines are used against M. viscosa, ulcerative outbreaks are continuously reported in Atlantic salmon in Canada. This study analyzed the phenotypical and genomic characteristics of Vibrio sp. J383 isolated from internal organs of vaccinated farmed Atlantic salmon displaying clinical signs of ulcer disease. Infection assays conducted on vaccinated farmed Atlantic salmon and revealed that Vibrio sp. J383 causes a low level of mortalities when administered intracelomic at doses ranging from 10⁷-10⁸ CFU/dose. Vibrio sp. J383 persisted in the blood of infected fish for at least 8 weeks at 10 and 12 °C. Clinical signs of this disease were greatest 12 °C, but no mortality and bacteremia were observed at 16 °C. The Vibrio sp. J383 genome (5,902,734 bp) has two chromosomes of 3,633,265 bp and 2,068,312 bp, respectively, and one large plasmid of 201,166 bp. Phylogenetic and comparative analyses indicated that Vibrio sp. J383 is related to V. splendidus, with 93% identity. Furthermore, the phenotypic analysis showed that there were significant differences between Vibrio sp. J383 and other Vibrio spp, suggesting J383 is a novel Vibrio species adapted to cold temperatures.

Oncogenic potential of Campylobacter infection in the gastrointestinal tract: narrative review

BACKGROUND

Campylobacter jejuni is the leading cause of zoonotic gastroenteritis. The other emerging group of Campylobacters spp. are part of human oral commensal, represented by C. concisus (CC), which has been recently linked to non-oral conditions. Although long-term gastrointestinal (GI) complications from these two groups of Campylobacters have been previously reviewed individually, overall impact of Campylobacter infection on GI carcinogenesis and their inflammatory precursor lesions has not been assessed collectively.

AIMS

To evaluate the available evidence concerning the association between Campylobacter infection/colonization and inflammatory bowel disease (IBD), reflux esophagitis/metaplasia colorectal cancer (CRC) and esophageal cancer (EC).

METHODS

We performed a comprehensive literature search of PubMed for relevant original publications and systematic reviews/meta-analyses of epidemiological and clinical studies. In addition, we gathered additional information concerning microbiological data, animal models and mechanistic data from in vitro studies.

RESULTS

Both retrospective and prospective studies on IBD showed relatively consistent increased risk associated with Campylobacter infection. Despite lack of supporting prospective studies, retrospective studies based on tissue/fecal microbiome revealed consistent enrichment of Campylobacter in CRC samples. Studies on EC precursor lesions (esophagitis and metaplasia) were generally supportive for the association with Campylobacter, while inconsistent observations on EC. Studies on both IBD and EC precursors suggested the predominant role of CC, but studies on CRC were not informative of species.

CONCLUSIONS

There is sufficient evidence calling for concerted effort in unveiling direct and indirect connection of this organism to colorectal and esophageal cancer in humans.

From salty to thriving: plant growth promoting bacteria as nature's allies in overcoming salinity stress in plants

Soil salinity is one of the main problems that affects global crop yield. Researchers have attempted to alleviate the effects of salt stress on plant growth using a variety of approaches, including genetic modification of salt-tolerant plants, screening the higher salt-tolerant genotypes, and the inoculation of beneficial plant microbiome, such as plant growth-promoting bacteria (PGPB). PGPB mainly exists in the rhizosphere soil, plant tissues and on the surfaces of leaves or stems, and can promote plant growth and increase plant tolerance to abiotic stress. Many halophytes recruit salt-resistant microorganisms, and therefore endophytic bacteria isolated from halophytes can help enhance plant stress responses. Beneficial plant-microbe interactions are widespread in nature, and microbial communities provide an opportunity to understand these beneficial interactions. In this study, we provide a brief overview of the current state of plant microbiomes and give particular emphasis on its influence factors and discuss various mechanisms used by PGPB in alleviating salt stress for plants. Then, we also describe the relationship between bacterial Type VI secretion system and plant growth promotion.

Molecular epidemiology and clinical characteristics of the type VI secretion system in Klebsiella pneumoniae causing abscesses

Purpose

The type VI system (T6SS) has the potential to be a new virulence factor for hypervirulent Klebsiella pneumoniae (hvKp) strains. This study aimed to characterize the molecular and clinical features of T6SS-positive and T6SS-negative K. pneumoniae isolates that cause abscesses.

Patients and methods

A total of 169 non-duplicate K. pneumoniae strains were isolated from patients with abscesses in a tertiary hospital in China from January 2018 to June 2022, and clinical data were collected. For all isolates, capsular serotypes, T6SS genes, virulence, and drug resistance genes, antimicrobial susceptibility testing, and biofilm formation assays were assessed. Multilocus sequence typing was used to analyze the genotypes of hvKp. T6SS-positive hvKp, T6SS-negative hvKp, T6SS-positive cKP, and T6SS-negative cKP (n = 4 strains for each group) were chosen for the in vivo Galleria mellonella infection model and in vitro competition experiments to further explore the microbiological characteristics of T6SS-positive K. pneumoniae isolates.

Results

The positive detection rate for T6SS was 36.1%. The rates of hvKp, seven virulence genes, K1 capsular serotype, and ST23 in T6SS-positive strains were all higher than those in T6SS-negative strains (p < 0.05). Multivariate logistic regression analysis indicated that the carriage of aerobactin (OR 0.01) and wcaG (OR 33.53) were independent risk factors for T6SS-positive strains (p < 0.05). The T6SS-positive strains had a stronger biofilm-forming ability than T6SS-negative strains (p < 0.05). The T6SS-positive and T6SS-negative strains showed no significant differences in competitive ability (p = 0.06). In the in vivo G. mellonella infection model, the T6SS(+)/hvKP group had the worst prognosis. Except for cefazolin and tegacyclin, T6SS-positive isolates displayed a lower rate of antimicrobial resistance to other drugs (p < 0.05). The T6SS-positive isolates were more likely to be acquired from community infections (p < 0.05).

Conclusion

Klebsiella pneumoniae isolates causing abscesses have a high prevalence of T6SS genes. T6SS-positive K. pneumoniae isolates are associated with virulence, and the T6SS genes may be involved in the hvKp virulence mechanism.

Diversity of the type VI secretion systems in the Neisseria spp

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

Pseudomonas aeruginosa and Staphylococcus aureus Display Differential Proteomic Responses to the Silver(I) Compound, SBC3

The urgent need to combat antibiotic resistance and develop novel antimicrobial therapies has triggered studies on novel metal-based formulations. N-heterocyclic carbene (NHC) complexes coordinate transition metals to generate a broad range of anticancer and/or antimicrobial agents, with ongoing efforts being made to enhance the lipophilicity and drug stability. The lead silver(I) acetate complex, 1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene (NHC*) (SBC3), has previously demonstrated promising growth and biofilm-inhibiting properties. In this work, the responses of two structurally different bacteria to SBC3 using label-free quantitative proteomics were characterised. Multidrug-resistant Pseudomonas aeruginosa (Gram-negative) and Staphylococcus aureus (Gram-positive) are associated with cystic fibrosis lung colonisation and chronic wound infections, respectively. SBC3 increased the abundance of alginate biosynthesis, the secretion system and drug detoxification proteins in P. aeruginosa, whilst a variety of pathways, including anaerobic respiration, twitching motility and ABC transport, were decreased in abundance. This contrasted the affected pathways in S. aureus, where increased DNA replication/repair and cell redox homeostasis and decreased protein synthesis, lipoylation and glucose metabolism were observed. Increased abundance of cell wall/membrane proteins was indicative of the structural damage induced by SBC3 in both bacteria. These findings show the potential broad applications of SBC3 in treating Gram-positive and Gram-negative bacteria.

Pseudomonas aeruginosa H3-T6SS Combats H2O2 Stress by Diminishing the Amount of Intracellular Unincorporated Iron in a Dps-Dependent Manner and Inhibiting the Synthesis of PQS

The type VI secretion system (T6SS), a protein translocation nanomachine, is widely distributed in Gram-negative bacteria and delivers effectors directly into target cells or the extracellular environment to help the bacteria gain a competitive fitness advantage and promote bacterial survival in harmful environments. In this study, we demonstrated that the synthesis of the Pseudomonas quinolone signal (PQS) in Pseudomonas aeruginosa PAO1 was inhibited by the H3-T6SS gene cluster under iron-rich conditions, and that this inhibition was relieved under iron starvation conditions. Conversely, PQS differentially regulated the expression of the H3-T6SS structural genes and the effector protein gene tseF. The expression of tseF was inhibited by PQS, while the expressions of the H3-T6SS structural genes were positively regulated by PQS. Further studies showed that the H3-T6SS was involved in the resistance of P. aeruginosa to oxidative stress caused by hydrogen peroxide (H₂O₂). Interestingly, H3-T6SS expression was neither induced by H₂O₂ stress nor regulated by OxyR (a global anti-oxidative transcriptional regulator) but was positively regulated by RpoS (a major transcription regulator of the stress response). In addition, we found that the clpV3 (a structural gene of H3-T6SS) mutation resulted in upregulation of two proteins related to PQS synthesis and many proteins related to oxidative stress resistance, while the expression of some iron storage proteins, especially Dps, were significantly downregulated. Furthermore, the clpV3 mutation led to an increase in the intracellular free Fe²⁺ content of P. aeruginosa. Further studies showed that both the PQS deficient mutation and overexpression of dps effectively restored the H₂O₂ sensitive phenotype of the H3-T6SS mutant. Finally, we proposed the following model of H3-T6SS-mediated resistance to H₂O₂ stress in P. aeruginosa. H3-T6SS not only reduces the intracellular free Fe²⁺ level by upregulating the expression of ferritin Dps, but also inhibits the synthesis of PQS to mediate the resistance of P. aeruginosa to H₂O₂ stress. This study highlights the important role of H3-T6SS in the ability of P. aeruginosa to combat H₂O₂ stress and provides a perspective for understanding the stress response mechanism of bacteria.

Transcriptional profiling of Pseudomonas aeruginosa mature single- and dual-species biofilms in response to meropenem

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen frequently isolated from chronic infections of the cystic fibrosis lung and burn wounds, and is a major cause of antimicrobial-resistant nosocomial infections. P. aeruginosa is frequently co-isolated with the opportunistic fungal pathogen Candida albicans, with the presence of C. albicans in dual-species biofilms promoting tolerance to meropenem. Here, transcription profiling of mature P. aeruginosa single- or dual-species biofilms was carried out to understand the molecular mechanism(s) by which C. albicans enhances meropenem tolerance. C. albicans appeared to have a mild impact on the transcriptome of P. aeruginosa mature biofilms, with most differentially regulated genes being involved in interkingdom interactions (i.e. quorum sensing and phenazine biosynthesis). The addition of meropenem to mature single- or dual-species biofilms resulted in a significant bacterial transcriptional response, including the induction of the beta-lactamase, ampC, genes involved in biofilm formation. P. aeruginosa elicited a similar transcriptional response to meropenem in the presence of C. albicans, but C. albicans promoted the expression of additional efflux pumps, which could play roles in increasing the tolerance of P. aeruginosa to meropenem.

H3-T6SS of Pseudomonas aeruginosa PA14 contributes to environmental adaptation via secretion of a biofilm-promoting effector

Microbial species often occur in complex communities and exhibit intricate synergistic and antagonistic interactions. To avoid predation and compete for favorable niches, bacteria have evolved specialized protein secretion systems. The type VI secretion system (T6SS) is a versatile secretion system widely distributed among Gram-negative bacteria that translocates effectors into target cells or the extracellular milieu via various physiological processes. Pseudomonas aeruginosa is an opportunistic pathogen responsible for many diseases, and it has three independent T6SSs (H1-, H2-, and H3-T6SS). In this study, we found that the H3-T6SS of highly virulent P. aeruginosa PA14 is negatively regulated by OxyR and OmpR, which are global regulatory proteins of bacterial oxidative and acid stress. In addition, we identified a H3-T6SS effector PA14_33970, which is located upstream of VgrG3. PA14_33970 interacted directly with VgrG3 and translocated into host cells. Moreover, we found that H3-T6SS and PA14_33970 play crucial roles in oxidative, acid, and osmotic stress resistance, as well as in motility and biofilm formation. PA14_33970 was identified as a new T6SS effector promoting biofilm formation and thus named TepB. Furthermore, we found that TepB contributes to the virulence of P. aeruginosa PA14 toward Caenorhabditis elegans. Overall, our study indicates that H3-T6SS and its biofilm-promoting effector TepB are regulated by OxyR and OmpR, both of which are important for adaptation of P. aeruginosa PA14 to multiple stressors, providing insights into the regulatory mechanisms and roles of T6SSs in P. aeruginosa.

The membrane-active polyaminoisoprenyl compound NV716 re-sensitizes Pseudomonas aeruginosa to antibiotics and reduces bacterial virulence

Pseudomonas aeruginosa is intrinsically resistant to many antibiotics due to the impermeability of its outer membrane and to the constitutive expression of efflux pumps. Here, we show that the polyaminoisoprenyl compound NV716 at sub-MIC concentrations re-sensitizes P. aeruginosa to abandoned antibiotics by binding to the lipopolysaccharides (LPS) of the outer membrane, permeabilizing this membrane and increasing antibiotic accumulation inside the bacteria. It also prevents selection of resistance to antibiotics and increases their activity against biofilms. No stable resistance could be selected to NV716-itself after serial passages with subinhibitory concentrations, but the transcriptome of the resulting daughter cells shows an upregulation of genes involved in the synthesis of lipid A and LPS, and a downregulation of quorum sensing-related genes. Accordingly, NV716 also reduces motility, virulence factors production, and biofilm formation. NV716 shows a unique and highly promising profile of activity when used alone or in combination with antibiotics against P. aeruginosa, combining in a single molecule anti-virulence and potentiator effects. Additional work is required to more thoroughly understand the various functions of NV716.

The polyaminoisoprenyl compound NV716 re-sensitizes Pseudomonas aeruginosa to antibiotics through permeabilizing the outer membrane and increases the activity of antibiotics on biofilms.

Characterization of 4 deletion mutants of Pseudomonas plecoglossicida and their potential for live attenuated vaccines in large yellow croaker (Larimichthys crocea)

To search for live attenuated vaccines (LAV) candidates against Pseudomonas plecoglossicida, the causative agent of the visceral granulomas disease in farmed large yellow croaker (Larimichthys crocea), two type Ⅵ secretion systems (T6SS) and a predicted α/β fold family hydrolase encoding gene, ORF4885 were targeted to construct deletion mutants. The biological profiles of 4 mutants were characterized; LD50 to the croakers detected, in vivo survival post-infection investigated， relative percent of survival (RPS) of the croakers 28d post-vaccination determined, and transcription of five immunity-related genes of the treated fish was quantified. On comparison to the WT, the mutants revealed similar growth curves in 11h; swarming motility of Δ4885 declined significantly at 72h post-incubation (P < 0.05); ΔS1Δ4885 showed significantly poor biofilm formation and weak resistance to fish serum bactericidal activity (P < 0.05). LD50 of the mutants were much higher than the WT, indication of strong virulence attenuation; in vivo survival test showed the mutant ΔS1Δ4885 and ΔS1ΔS3 were eliminated by the host 10d post-infection, demonstration of the safety and potentiality to be LAV candidates. Immunization with the mutant ΔS1Δ4885 provided higher RPS than ΔS1ΔS3. Transcription of IgT was significant in all immunized groups while IgM increased only in intraperitoneally injected groups. This study successfully searched a quite safe and strong immunogenic LAV candidate to defeat P. plecoglossicida infection.

Pseudomonas aeruginosa: pathogenesis, virulence factors, antibiotic resistance, interaction with host, technology advances and emerging therapeutics

Pseudomonas aeruginosa (P. aeruginosa) is a Gram-negative opportunistic pathogen that infects patients with cystic fibrosis, burn wounds, immunodeficiency, chronic obstructive pulmonary disorder (COPD), cancer, and severe infection requiring ventilation, such as COVID-19. P. aeruginosa is also a widely-used model bacterium for all biological areas. In addition to continued, intense efforts in understanding bacterial pathogenesis of P. aeruginosa including virulence factors (LPS, quorum sensing, two-component systems, 6 type secretion systems, outer membrane vesicles (OMVs), CRISPR-Cas and their regulation), rapid progress has been made in further studying host-pathogen interaction, particularly host immune networks involving autophagy, inflammasome, non-coding RNAs, cGAS, etc. Furthermore, numerous technologic advances, such as bioinformatics, metabolomics, scRNA-seq, nanoparticles, drug screening, and phage therapy, have been used to improve our understanding of P. aeruginosa pathogenesis and host defense. Nevertheless, much remains to be uncovered about interactions between P. aeruginosa and host immune responses, including mechanisms of drug resistance by known or unannotated bacterial virulence factors as well as mammalian cell signaling pathways. The widespread use of antibiotics and the slow development of effective antimicrobials present daunting challenges and necessitate new theoretical and practical platforms to screen and develop mechanism-tested novel drugs to treat intractable infections, especially those caused by multi-drug resistance strains. Benefited from has advancing in research tools and technology, dissecting this pathogen's feature has entered into molecular and mechanistic details as well as dynamic and holistic views. Herein, we comprehensively review the progress and discuss the current status of P. aeruginosa biophysical traits, behaviors, virulence factors, invasive regulators, and host defense patterns against its infection, which point out new directions for future investigation and add to the design of novel and/or alternative therapeutics to combat this clinically significant pathogen.

Machine-learning from Pseudomonas putida KT2440 transcriptomes reveals its transcriptional regulatory network

Bacterial gene expression is orchestrated by numerous transcription factors (TFs). Elucidating how gene expression is regulated is fundamental to understanding bacterial physiology and engineering it for practical use. In this study, a machine-learning approach was applied to uncover the genome-scale transcriptional regulatory network (TRN) in Pseudomonas putida KT2440, an important organism for bioproduction. We performed independent component analysis of a compendium of 321 high-quality gene expression profiles, which were previously published or newly generated in this study. We identified 84 groups of independently modulated genes (iModulons) that explain 75.7% of the total variance in the compendium. With these iModulons, we (i) expand our understanding of the regulatory functions of 39 iModulon associated TFs (e.g., HexR, Zur) by systematic comparison with 1993 previously reported TF-gene interactions; (ii) outline transcriptional changes after the transition from the exponential growth to stationary phases; (iii) capture group of genes required for utilizing diverse carbon sources and increased stationary response with slower growth rates; (iv) unveil multiple evolutionary strategies of transcriptome reallocation to achieve fast growth rates; and (v) define an osmotic stimulon, which includes the Type VI secretion system, as coordination of multiple iModulon activity changes. Taken together, this study provides the first quantitative genome-scale TRN for P. putida KT2440 and a basis for a comprehensive understanding of its complex transcriptome changes in a variety of physiological states.

Acylase enzymes disrupting quorum sensing alter the transcriptome and phenotype of Pseudomonas aeruginosa, and the composition of bacterial biofilms from wastewater treatment plants

Biofilms represent an essential way of life and colonization of new environments for microorganisms. This feature is regulated by quorum sensing (QS), a microbial communication system based on autoinducer molecules, such as N-acyl-homoserine lactones (AHLs) in Gram negative bacteria. In artificial ecosystems, like Wastewater Treatment Plants (WWTPs), biofilm attachment in filtration membranes produces biofouling. In this environment, the microbial communities are mostly composed of Gram-negative phyla. Thus, we used two AHLs-degrading enzymes, obtained from Actinoplanes utahensis (namely AuAAC and AuAHLA) to determine the effects of degradation of QS signals in the biofilm formation, among other virulence factors, of a Pseudomonas aeruginosa strain isolated from a WWTP, assessing molecular mechanisms through transcriptomics. Besides, we studied the possible effects on community composition in biofilms from activated sludge samples. Although the studied enzymes only degraded the AHLs involved in one of the four QS systems of P. aeruginosa, these activities produced the deregulation of the complete QS network. In fact, AuAAC -the enzyme with higher catalytic efficiency- deregulated all the four QS systems. However, both enzymes reduced the biofilm formation and pyocyanin and protease production. The transcriptomic response of P. aeruginosa affected QS related genes, moreover, transcriptomic response to AuAAC affected mainly to QS related genes. Regarding community composition of biofilms, as expected, the abundance of Gram-negative phyla was significantly decreased after enzymatic treatment. These results support the potential use of such AHLs-degrading enzymes as a method to reduce biofilm formation in WWTP membranes and ameliorate bacterial virulence.

Impacts of Ser/Thr Protein Kinase Stk1 on the Proteome, Twitching Motility, and Competitive Advantage in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous gram-negative bacterium in the environment and a leading cause of nosocomial infections worldwide. Therefore, it is listed by the WHO as a human pathogen that urgently needs the development of new antibacterial drugs. Recent findings have demonstrated that eukaryote-type Ser/Thr protein kinases play a vital role in regulating various bacterial physiological processes by catalyzing protein phosphorylation. Stk1 has proven to be a Ser/Thr protein kinase in P. aeruginosa. However, the regulatory roles of Stk1 have not yet been revealed. Thus, we constructed a stk1 knockout mutant (∆stk1) from the P. aeruginosa PAO1 strain and employed a Tandem Mass Tag (TMT) labeling-based quantitative proteomic strategy to characterize proteome-wide changes in response to the stk1 knockout. In total, 620 differentially expressed proteins, among which 288 proteins were upregulated and 332 proteins were downregulated, were identified in ∆stk1 compared with P. aeruginosa PAO1. A detailed bioinformatics analysis of these differentially expressed proteins was performed, including GO annotation, protein domain profile, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, subcellular localization and enrichment analysis. Notably, the downregulation of type IV pilus-related proteins and upregulation of T6SS-H1-related proteins were found in the ∆stk1 strain, and the results were corroborated by quantitative PCR at the mRNA level. Further experiments confirmed that the loss of stk1 weakens bacterial twitching motility and promotes a growth competition advantage, which are, respectively, mediated by type IV pilus-related proteins and T6SS-H1-related proteins. These findings contribute to a better understanding of the physiological role of Stk1, and proteomic data will help further investigations of the roles and mechanisms of Stk1 in P. aeruginosa, although the detailed regulation and mechanism of Stk1 still need to be revealed.

Beyond dueling: roles of the type VI secretion system in microbiome modulation, pathogenesis and stress resistance

Bacteria inhabit diverse and dynamic environments, where nutrients may be limited and toxic chemicals can be prevalent. To adapt to these stressful conditions, bacteria have evolved specialized protein secretion systems, such as the type VI secretion system (T6SS) to facilitate their survival. As a molecular syringe, the T6SS expels various effectors into neighboring bacterial cells, eukaryotic cells, or the extracellular environment. These effectors improve the competitive fitness and environmental adaption of bacterial cells. Although primarily recognized as antibacterial weapons, recent studies have demonstrated that T6SSs have functions beyond interspecies competition. Here, we summarize recent research on the role of T6SSs in microbiome modulation, pathogenesis, and stress resistance.

Let-7b-5p in vesicles secreted by human airway cells reduces biofilm formation and increases antibiotic sensitivity of P. aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that forms antibiotic-resistant biofilms, which facilitate chronic infections in immunocompromised hosts. We have previously shown that P. aeruginosa secretes outer-membrane vesicles that deliver a small RNA to human airway epithelial cells (AECs), in which it suppresses the innate immune response. Here, we demonstrate that interdomain communication through small RNA-containing membrane vesicles is bidirectional and that microRNAs (miRNAs) in extracellular vesicles (EVs) secreted by human AECs regulate protein expression, antibiotic sensitivity, and biofilm formation by P. aeruginosa Specifically, human EVs deliver miRNA let-7b-5p to P. aeruginosa, which systematically decreases the abundance of proteins essential for biofilm formation, including PpkA and ClpV1-3, and increases the ability of beta-lactam antibiotics to reduce biofilm formation by targeting the beta-lactamase AmpC. Let-7b-5p is bioinformatically predicted to target not only PpkA, ClpV1, and AmpC in P. aeruginosa but also the corresponding orthologs in Burkholderia cenocepacia, another notorious opportunistic lung pathogen, suggesting that the ability of let-7b-5p to reduce biofilm formation and increase beta-lactam sensitivity is not limited to P. aeruginosa Here, we provide direct evidence for transfer of miRNAs in EVs secreted by eukaryotic cells to a prokaryote, resulting in subsequent phenotypic alterations in the prokaryote as a result of this interdomain communication. Since let-7-family miRNAs are in clinical trials to reduce inflammation and because chronic P. aeruginosa lung infections are associated with a hyperinflammatory state, treatment with let-7b-5p and a beta-lactam antibiotic in nanoparticles or EVs may benefit patients with antibiotic-resistant P. aeruginosa infections.

Pseudomonas aeruginosa: An Audacious Pathogen with an Adaptable Arsenal of Virulence Factors

Pseudomonas aeruginosa is a dominant pathogen in people with cystic fibrosis (CF) contributing to morbidity and mortality. Its tremendous ability to adapt greatly facilitates its capacity to cause chronic infections. The adaptability and flexibility of the pathogen are afforded by the extensive number of virulence factors it has at its disposal, providing P. aeruginosa with the facility to tailor its response against the different stressors in the environment. A deep understanding of these virulence mechanisms is crucial for the design of therapeutic strategies and vaccines against this multi-resistant pathogen. Therefore, this review describes the main virulence factors of P. aeruginosa and the adaptations it undergoes to persist in hostile environments such as the CF respiratory tract. The very large P. aeruginosa genome (5 to 7 MB) contributes considerably to its adaptive capacity; consequently, genomic studies have provided significant insights into elucidating P. aeruginosa evolution and its interactions with the host throughout the course of infection.

Differential Expression of Paraburkholderia phymatum Type VI Secretion Systems (T6SS) Suggests a Role of T6SS-b in Early Symbiotic Interaction

Paraburkholderia phymatum STM815, a rhizobial strain of the Burkholderiaceae family, is able to nodulate a broad range of legumes including the agriculturally important Phaseolus vulgaris (common bean). P. phymatum harbors two type VI Secretion Systems (T6SS-b and T6SS-3) in its genome that contribute to its high interbacterial competitiveness in vitro and in infecting the roots of several legumes. In this study, we show that P. phymatum T6SS-b is found in the genomes of several soil-dwelling plant symbionts and that its expression is induced by the presence of citrate and is higher at 20/28°C compared to 37°C. Conversely, T6SS-3 shows homologies to T6SS clusters found in several pathogenic Burkholderia strains, is more prominently expressed with succinate during stationary phase and at 37°C. In addition, T6SS-b expression was activated in the presence of germinated seeds as well as in P. vulgaris and Mimosa pudica root nodules. Phenotypic analysis of selected deletion mutant strains suggested a role of T6SS-b in motility but not at later stages of the interaction with legumes. In contrast, the T6SS-3 mutant was not affected in any of the free-living and symbiotic phenotypes examined. Thus, P. phymatum T6SS-b is potentially important for the early infection step in the symbiosis with legumes.

ClpV3 of the H3-Type VI Secretion System (H3-T6SS) Affects Multiple Virulence Factors in Pseudomonas aeruginosa

The type VI secretion system (T6SS) is a toxic effector delivery apparatus widely distributed in Gram-negative bacteria. The opportunistic pathogen Pseudomonas aeruginosa encodes three T6SSs, namely H1-, H2-, and H3-T6SS. Each T6SS possesses its own effectors and their roles are not yet fully understood. Here, we report that an H3-T6SS deletion mutant PAO1(ΔclpV3) significantly affected the virulence-related phenotypes including pyocyanin production, biofilm formation, proteolytic activity, and motilities. Most interestingly, the expression of T3SS genes was markedly affected, indicating a link between H3-T6SS and T3SS. RNA-Sequencing was performed to globally identify the genes differentially expressed when H3-T6SS was inactivated and the results obtained correlated well with the observed phenotypes. Interestingly, the expressions of T2SS, T3SS, H2-T6SS, and H3-T6SS were all significantly decreased, while H1-T6SS was increased in the PAO1(ΔclpV3) strain. We also observed that the intracellular concentration of secondary messenger cAMP was reduced in PAO1(ΔclpV3), and the c-di-GMP level was also decreased as indicated by the decreased cdrA reporter activity. Finally, by using a Galleria mellonella infection model, we show that H3-T6SS plays a key role in the pathogenicity of P. aeruginosa in vivo. Overall, our study highlights the unique connection of H3-T6SS in P. aeruginosa with T3SS, pyocyanin production, biofilm formation and in vivo pathogenicity.

Type VI secretion system of Pseudomonas aeruginosa is associated with biofilm formation but not environmental adaptation

Pseudomonas aeruginosa encodes three type VI secretion systems (T6SSs), namely H1‐, H2‐, and H3‐T6SS. P. aeruginosa hemolysin‐coregulated protein (Hcp) is the effector protein and the hallmark of T6SS. Although T6SS is ubiquitous and affects ecology and human health, its general mechanism and physiological role are still not fully understood. Therefore, in this study, we investigated the impact of the P. aeruginosa T6SS on biofilm formation and environmental adaptation. To this end, we collected P. aeruginosa clinical isolates, divided them into strong biofilm formation (SBF) and nonbiofilm formation (NBF) groups based on their biofilm‐forming ability, and compared their associated clinical characteristics. The duration of hospitalization was longer in patients infected with SBF than those infected with NBF strains. The expression levels of T6SS‐related genes (hcp1 and hcp3) and a quorum‐sensing gene (lasR) were higher in the SBF group as compared to those in the NBF group. In addition, the expression level of lasR was negatively associated with that of hcp1, but was positively associated with those of hcp2 and hcp3. Moreover, we evaluated the expression of T6SS‐ and biofilm‐associated genes in planktonic and biofilm cells of the P. aeruginosa strain PAO1, and constructed strain PAO1△clpV1 to study the adaptation characteristics of H1‐T6SS. The expression levels of hcp1, hcp2, hcp3, lasR, and other biofilm‐associated genes were significantly higher in PAO1 biofilm cells as compared to those of planktonic cells. However, except for swarming ability as a vital feature for biofilm formation, there were no significant differences in the biofilm‐forming ability and expression of biofilm‐associated genes, adherence ability, growth characteristics, resistance to acid and osmotic pressure, surface structure, and morphology between the PAO1△clpV1 and PAO1 wild‐type strains. Collectively, our results suggest that T6SS might play a role in biofilm formation and that H1‐T6SS does not contribute to environmental adaptation in P. aeruginosa.