Pseudomonas entomophila and Pseudomonas mendocina: potential models for studying the bacterial type VI secretion system

Immune priming modulates Galleria mellonella and Pseudomonas entomophila interaction. Antimicrobial properties of Kazal peptide Pr13a

Galleria mellonella larvae repeatedly infected with Pseudomonas entomophila bacteria re-induced their immune response. Its parameters, i.e. the defence activities of cell-free hemolymph, the presence and activity of antimicrobial peptides, and the expression of immune-relevant genes were modulated after the re-challenge in comparison to non-primed infected larvae, resulting in better protection. No enhanced resistance was observed when the larvae were initially infected with other microorganisms, and larvae pre-infected with P. entomophila were not more resistant to further infection with other pathogens. Then, the peptide profiles of hemolymph from primed- and non-primed larvae infected with P. entomophila were compared by quantitative RP-HPLC (Reverse Phase - High Performance Liquid Chromatography). The level of carbonic anhydrase, anionic peptide-1, proline peptide-2, and finally, unknown so far, putative Kazal peptide Pr13a was higher in the primed infected animals than in the larvae infected with P. entomophila for the first time. The expression of the Pr13a gene increased two-fold after the infection, but only in the primed animals. To check whether the enhanced level of Pr13a could have physiological significance, the peptide was purified to homogeneity and checked for its defence properties. In fact, it had antibacterial activity: at the concentration of 15 µM and 7.5 µM it reduced the number of P. entomophila and Bacillus thuringiensis CFU, respectively, to about 40%. The antibacterial activity of Pr13a was correlated with changes observed on the surface of the peptide-treated bacteria, e.g. surface roughness and adhesion force. The presented results bring us closer to finding hemolymph constituents responsible for the effect of priming on the immune response in re-infected insects.

Pseudomonas virulence factor controls expression of virulence genes in Pseudomonas entomophila

Quorum sensing is a communication strategy that bacteria use to collectively alter gene expression in response to cell density. Pathogens use quorum sensing systems to control activities vital to infection, such as the production of virulence factors and biofilm formation. The Pseudomonas virulence factor (pvf) gene cluster encodes a signaling system (Pvf) that is present in over 500 strains of proteobacteria, including strains that infect a variety of plant and human hosts. We have shown that Pvf regulates the production of secreted proteins and small molecules in the insect pathogen Pseudomonas entomophila L48. Here, we identified genes that are likely regulated by Pvf using the model strain P. entomophila L48 which does not contain other known quorum sensing systems. Pvf regulated genes were identified through comparing the transcriptomes of wildtype P. entomophila and a pvf deletion mutant (ΔpvfA-D). We found that deletion of pvfA-D affected the expression of approximately 300 genes involved in virulence, the type VI secretion system, siderophore transport, and branched chain amino acid biosynthesis. Additionally, we identified seven putative biosynthetic gene clusters with reduced expression in ΔpvfA-D. Our results indicate that Pvf controls multiple virulence mechanisms in P. entomophila L48. Characterizing genes regulated by Pvf will aid understanding of host-pathogen interactions and development of anti-virulence strategies against P. entomophila and other pvf-containing strains.

MicroRNAs reshape the immunity of insects in response to bacterial infection

The interaction between bacteria and insects can significantly impact a wide range of different areas because bacteria and insects are widely distributed around the globe. The bacterial-insect interactions have the potential to directly affect human health since insects are vectors for disease transmission, and their interactions can also have economic consequences. In addition, they have been linked to high mortality rates in economically important insects, resulting in substantial economic losses. MicroRNAs (miRNAs) are types of non-coding RNAs involved in regulating gene expression post-transcriptionally. The length of miRNAs ranges from 19 to 22 nucleotides. MiRNAs, in addition to their ability to exhibit dynamic expression patterns, have a diverse range of targets. This enables them to govern various physiological activities in insects, like innate immune responses. Increasing evidence suggests that miRNAs have a crucial biological role in bacterial infection by influencing immune responses and other mechanisms for resistance. This review focuses on some of the most recent and exciting discoveries made in recent years, including the correlation between the dysregulation of miRNA expression in the context of bacterial infection and the progression of the infection. Furthermore, it describes how they profoundly impact the immune responses of the host by targeting the Toll, IMD, and JNK signaling pathways. It also emphasizes the biological function of miRNAs in regulating immune responses in insects. Finally, it also discusses current knowledge gaps about the function of miRNAs in insect immunity, in addition to areas that require more research in the future.

Pseudomonas Virulence Factor Pathway Synthesizes Autoinducers That Regulate the Secretome of a Pathogen

Cell-to-cell communication via chemical signals is an essential mechanism that pathogenic bacteria use to coordinate group behaviors and promote virulence. The Pseudomonas virulence factor (pvf) gene cluster is distributed in more than 500 strains of proteobacteria including both plant and human pathogens. The pvf cluster has been implicated in the production of signaling molecules important for virulence; however, the regulatory impact of these signaling molecules on virulence had not been elucidated. Using the insect pathogen Pseudomonas entomophila L48 as a model, we demonstrated that pvf-encoded biosynthetic enzymes produce PVF autoinducers that regulate the expression of pvf genes and a gene encoding the toxin monalysin via quorum sensing. In addition, PVF autoinducers regulate the expression of nearly 200 secreted and membrane proteins, including toxins, motility proteins, and components of the type VI secretion system, which play key roles in bacterial virulence, colonization, and competition with other microbes. Deletion of pvf also altered the secondary metabolome. Six major compounds upregulated by PVF autoinducers were isolated and structurally characterized, including three insecticidal 3-indolyl oxazoles, the labradorins, and three antimicrobial pyrrolizidine alkaloids, the pyreudiones. The signaling properties of PVF autoinducers and their wide-ranging regulatory effects indicate multifaceted roles of PVF in controlling cell physiology and promoting virulence. The broad genome distribution of pvf suggests that PVF-mediated signaling is relevant to many bacteria of agricultural and biomedical significance.

Bactericidal Effect of Entomopathogenic Bacterium Pseudomonas entomophila Against Xanthomonas citri Reduces Citrus Canker Disease Severity

The bacterium Pseudomonas entomophila has been recognized as an exceptional species within the Pseudomonas genus, capable of naturally infecting and killing insects from at least three different orders. P. entomophila ingestion leads to irreversible gut damage resulting from a global blockage of translation, which impairs both immune and tissue repair systems in the insect intestine. In this study we isolated a P. entomophila bacterial strain from soil samples which displayed a strong activity against Xanthomonas citri subsp, citri (Xcc), the etiological agent of citrus canker disease. The antagonism potential of isolated bacteria against Xcc and its ability to reduce citrus canker severity was assessed both ex planta and in planta. Our findings show that pathogenicity assays in Citrus x limonia by pressure infiltration and spray with a mixture of P. entomophila and Xcc leaded to a significant reduction in the number of canker lesions in high susceptible citrus leaves, at 21 days post-infection. To the best of our knowledge this is the first report of antibacterial activity of P. entomophila against a phytopathogenic bacterium. Collective action of P. entomophila factors such as diketopiperazine production and the type 6 secretion system (T6SS) may be involved in this type of biological control of citrus canker. The results suggest that the P. entomophila strain could be a promising biocontrol agent acting directly against Xcc.

Secretion Systems and Secreted Proteins in Gram-Negative Entomopathogenic Bacteria: Their Roles in Insect Virulence and Beyond

Many Gram-negative bacteria have evolved insect pathogenic lifestyles. In all cases, the ability to cause disease in insects involves specific bacterial proteins exported either to the surface, the extracellular environment, or the cytoplasm of the host cell. They also have several distinct mechanisms for secreting such proteins. In this review, we summarize the major protein secretion systems and discuss examples of secreted proteins that contribute to the virulence of a variety of Gram-negative entomopathogenic bacteria, including Photorhabdus, Xenorhabdus, Serratia, Yersinia, and Pseudomonas species. We also briefly summarize two classes of exported protein complexes, the PVC-like elements, and the Tc toxin complexes that were first described in entomopathogenic bacteria.

A Microfluidics and Agent-Based Modeling Framework for Investigating Spatial Organization in Bacterial Colonies: The Case of Pseudomonas Aeruginosa and H1-Type VI Secretion Interactions

The factors leading to changes in the organization of microbial assemblages at fine spatial scales are not well characterized or understood. However, they are expected to guide the succession of community development and function toward specific outcomes that could impact human health and the environment. In this study, we put forward a combined experimental and agent-based modeling framework and use it to interpret unique spatial organization patterns of H1-Type VI secretion system (T6SS) mutants of P. aeruginosa under spatial confinement. We find that key parameters, such as T6SS-mediated cell contact and lysis, spatial localization, relative species abundance, cell density and local concentrations of growth substrates and metabolites are influenced by spatial confinement. The model, written in the accessible programming language NetLogo, can be adapted to a variety of biological systems of interest and used to simulate experiments across a broad parameter space. It was implemented and run in a high-throughput mode by deploying it across multiple CPUs, with each simulation representing an individual well within a high-throughput microwell array experimental platform. The microfluidics and agent-based modeling framework we present in this paper provides an effective means by which to connect experimental studies in microbiology to model development. The work demonstrates progress in coupling experimental results to simulation while also highlighting potential sources of discrepancies between real-world experiments and idealized models.

Plasmids from Shiga Toxin-Producing Escherichia coli Strains with Rare Enterohemolysin Gene (ehxA) Subtypes Reveal Pathogenicity Potential and Display a Novel Evolutionary Path

Most Shiga toxin-producing Escherichia coli (STEC) strains associated with severe disease, such as hemolytic-uremic syndrome (HUS), carry large enterohemolysin-encoding (ehxA) plasmids, e.g., pO157 and pO103, that contribute to STEC clinical manifestations. Six ehxA subtypes (A through F) exist that phylogenetically cluster into eae-positive (B, C, F), a mix of eae-positive (E) and eae-negative (A), and a third, more distantly related, cluster of eae-negative (D) STEC strains. While subtype B, C, and F plasmids share a number of virulence traits that are distinct from those of subtype A, sequence data have not been available for subtype D and E plasmids. Here, we determined and compared the genetic composition of four subtype D and two subtype E plasmids to establish their evolutionary relatedness among ehxA subtypes and define their potential role in pathogenicity. We found that subtype D strains carry one exceptionally large plasmid (>200 kbp) that carries a variety of virulence genes that are associated with enterotoxigenic and enterohemorrhagic E. coli, which, quite possibly, enables these strains to cause disease despite being food isolates. Our data offer further support for the hypothesis that this subtype D plasmid represents a novel virulence plasmid, sharing very few genetic features with other plasmids; we conclude that these plasmids have evolved from a different evolutionary lineage than the plasmids carrying the other ehxA subtypes. In contrast, the 50-kbp plasmids of subtype E (pO145), although isolated from HUS outbreak strains, carried only few virulence-associated determinants, suggesting that the clinical presentation of subtype E strains is largely a result of chromosomally encoded virulence factors.

IMPORTANCE

Bacterial plasmids are known to be key agents of change in microbial populations, promoting the dissemination of various traits, such as drug resistance and virulence. This study determined the genetic makeup of virulence plasmids from rare enterohemolysin subtype D and E Shiga toxin-producing E. coli strains. We demonstrated that ehxA subtype D plasmids represent a novel E. coli virulence plasmid, and although subtype D plasmids were derived from nonclinical isolates, they encoded a variety of virulence determinants that are associated with pathogenic E. coli In contrast, subtype E plasmids, isolated from strains recovered from severely ill patients, carry only a few virulence determinants. The results of this study reemphasize the plasticity and vast diversity among E. coli plasmids. This work demonstrates that, although E. coli strains of certain serogroups may not be frequently associated with disease, they should not be underestimated in protecting human health and food safety.

Comparative genomic analysis of multiple strains of two unusual plant pathogens: Pseudomonas corrugata and Pseudomonas mediterranea

The non-fluorescent pseudomonads, Pseudomonas corrugata (Pcor) and P. mediterranea (Pmed), are closely related species that cause pith necrosis, a disease of tomato that causes severe crop losses. However, they also show strong antagonistic effects against economically important pathogens, demonstrating their potential for utilization as biological control agents. In addition, their metabolic versatility makes them attractive for the production of commercial biomolecules and bioremediation. An extensive comparative genomics study is required to dissect the mechanisms that Pcor and Pmed employ to cause disease, prevent disease caused by other pathogens, and to mine their genomes for genes that encode proteins involved in commercially important chemical pathways. Here, we present the draft genomes of nine Pcor and Pmed strains from different geographical locations. This analysis covered significant genetic heterogeneity and allowed in-depth genomic comparison. All examined strains were able to trigger symptoms in tomato plants but not all induced a hypersensitive-like response in Nicotiana benthamiana. Genome-mining revealed the absence of type III secretion system and known type III effector-encoding genes from all examined Pcor and Pmed strains. The lack of a type III secretion system appears to be unique among the plant pathogenic pseudomonads. Several gene clusters coding for type VI secretion system were detected in all genomes. Genome-mining also revealed the presence of gene clusters for biosynthesis of siderophores, polyketides, non-ribosomal peptides, and hydrogen cyanide. A highly conserved quorum sensing system was detected in all strains, although species specific differences were observed. Our study provides the basis for in-depth investigations regarding the molecular mechanisms underlying virulence strategies in the battle between plants and microbes.

Ribosomally encoded antibacterial proteins and peptides from Pseudomonas

Members of the Pseudomonas genus produce diverse secondary metabolites affecting other bacteria, fungi or predating nematodes and protozoa but are also equipped with the capacity to secrete different types of ribosomally encoded toxic peptides and proteins, ranging from small microcins to large tailocins. Studies with the human pathogen Pseudomonas aeruginosa have revealed that effector proteins of type VI secretion systems are part of the antibacterial armamentarium deployed by pseudomonads. A novel class of antibacterial proteins with structural similarity to plant lectins was discovered by studying antagonism among plant-associated Pseudomonas strains. A genomic perspective on pseudomonad bacteriocinogeny shows that the modular architecture of S pyocins of P. aeruginosa is retained in a large diversified group of bacteriocins, most of which target DNA or RNA. Similar modularity is present in as yet poorly characterized Rhs (recombination hot spot) proteins and CDI (contact-dependent inhibition) proteins. Well-delimited domains for receptor recognition or cytotoxicity enable the design of chimeric toxins with novel functionalities, which has been applied successfully for S and R pyocins. Little is known regarding how these antibacterials are released and ultimately reach their targets. Other remaining issues concern the identification of environmental triggers activating these systems and assessment of their ecological impact in niches populated by pseudomonads.

Comparative genomics of multiple strains of Pseudomonas cannabina pv. alisalensis, a potential model pathogen of both monocots and dicots

Comparative genomics of closely related pathogens that differ in host range can provide insights into mechanisms of host-pathogen interactions and host adaptation. Furthermore, sequencing of multiple strains with the same host range reveals information concerning pathogen diversity and the molecular basis of virulence. Here we present a comparative analysis of draft genome sequences for four strains of Pseudomonas cannabina pathovar alisalensis (Pcal), which is pathogenic on a range of monocotyledonous and dicotyledonous plants. These draft genome sequences provide a foundation for understanding host range evolution across the monocot-dicot divide. Like other phytopathogenic pseudomonads, Pcal strains harboured a hrp/hrc gene cluster that codes for a type III secretion system. Phylogenetic analysis based on the hrp/hrc cluster genes/proteins, suggests localized recombination and functional divergence within the hrp/hrc cluster. Despite significant conservation of overall genetic content across Pcal genomes, comparison of type III effector repertoires reinforced previous molecular data suggesting the existence of two distinct lineages within this pathovar. Furthermore, all Pcal strains analyzed harbored two distinct genomic islands predicted to code for type VI secretion systems (T6SSs). While one of these systems was orthologous to known P. syringae T6SSs, the other more closely resembled a T6SS found within P. aeruginosa. In summary, our study provides a foundation to unravel Pcal adaptation to both monocot and dicot hosts and provides genetic insights into the mechanisms underlying pathogenicity.

Type VI secretion system regulation as a consequence of evolutionary pressure

The type VI secretion system (T6SS) is a mechanism evolved by Gram-negative bacteria to negotiate interactions with eukaryotic and prokaryotic competitors. T6SSs are encoded by a diverse array of bacteria and include plant, animal, human and fish pathogens, as well as environmental isolates. As such, the regulatory mechanisms governing T6SS gene expression vary widely from species to species, and even from strain to strain within a given species. This review concentrates on the four bacterial genera that the majority of recent T6SS regulatory studies have been focused on: Vibrio, Pseudomonas, Burkholderia and Edwardsiella.

Pseudomonas viridiflava, a multi host plant pathogen with significant genetic variation at the molecular level

The pectinolytic species Pseudomonas viridiflava has a wide host range among plants, causing foliar and stem necrotic lesions and basal stem and root rots. However, little is known about the molecular evolution of this species. In this study we investigated the intraspecies genetic variation of P. viridiflava amongst local (Cretan), as well as international isolates of the pathogen. The genetic and phenotypic variability were investigated by molecular fingerprinting (rep-PCR) and partial sequencing of three housekeeping genes (gyrB, rpoD and rpoB), and by biochemical and pathogenicity profiling. The biochemical tests and pathogenicity profiling did not reveal any variability among the isolates studied. However, the molecular fingerprinting patterns and housekeeping gene sequences clearly differentiated them. In a broader phylogenetic comparison of housekeeping gene sequences deposited in GenBank, significant genetic variability at the molecular level was found between isolates of P. viridiflava originated from different host species as well as among isolates from the same host. Our results provide a basis for more comprehensive understanding of the biology, sources and shifts in genetic diversity and evolution of P. viridiflava populations and should support the development of molecular identification tools and epidemiological studies in diseases caused by this species.