Pyoverdine-Mediated Killing of Caenorhabditis elegans by Pseudomonas syringae MB03 and the Role of Iron in Its Pathogenicity

Bacterivorous nematodes decipher microbial iron siderophores as prey cue in predator-prey interactions

The minimal levels of biological-available iron in the environment impose growth limitation on all living organisms. Microbes often secrete high iron-binding-affinity siderophores at high concentrations for scavenging iron from the iron-limited habitats. However, the high prevalence of siderophores released by bacteria into the environment raises an intriguing question whether this chemical cue can be detected by bacterivorous predators. Here, we show that the bacterivorous Caenorhabditis elegans nematode could employ its chemosensory receptor Odr-10 to detect pyoverdine, an iron siderophore secreted by an environmental bacterium, Pseudomonas aeruginosa. This enabled the nematode predator to migrate toward the prey. Our soil microcosm study showed that the detection of pyoverdine and subsequent feeding of P. aeruginosa prey by C. elegans could lead to the expansion of its population. These results showed that siderophores are a prey chemical cue by predators, with key implications in predator-prey interactions.

Hydrocarbon degradation strategy and pyoverdine production using the salt tolerant Antarctic bacterium Marinomonas sp. ef1

One of the most concerning environmental problems is represented by petroleum and its derivatives causing contamination of aquatic and underground environments. In this work, the degradation treatment of diesel using Antarctic bacteria is proposed. Marinomonas sp. ef1 is a bacterial strain isolated from a consortium associated with the Antarctic marine ciliate Euplotes focardii. Its potential in the degradation of hydrocarbons commonly present in diesel oil were studied. The bacterial growth was evaluated in culturing conditions that resembled the marine environment with 1% (v/v) of either diesel or biodiesel added; in both cases, Marinomonas sp. ef1 was able to grow. The chemical oxygen demand measured after the incubation of bacteria with diesel decreased, demonstrating the ability of bacteria to use diesel hydrocarbons as a carbon source and degrade them. The metabolic potential of Marinomonas to degrade aromatic compounds was supported by the identification in the genome of sequences encoding various enzymes involved in benzene and naphthalene degradation. Moreover, in the presence of biodiesel, a fluorescent yellow pigment was produced; this was isolated, purified and characterized by UV-vis and fluorescence spectroscopy, leading to its identification as a pyoverdine. These results suggest that Marinomonas sp. ef1 can be used in hydrocarbon bioremediation and in the transformation of these pollutants in molecules of interest.

Insight into the Global Negative Regulation of Iron Scavenger 7-HT Biosynthesis by the SigW/RsiW System in Pseudomonas donghuensis HYS

7-Hydroxytropolone (7-HT) is a unique iron scavenger synthesized by Pseudomonas donghuensis HYS that has various biological activities in addition to functioning as a siderophore. P. donghuensis HYS is more pathogenic than P. aeruginosa toward Caenorhabditis elegans, an observation that is closely linked to the biosynthesis of 7-HT. The nonfluorescent siderophore (nfs) gene cluster is responsible for the orderly biosynthesis of 7-HT and represents a competitive advantage that contributes to the increased survival of P. donghuensis HYS; however, the regulatory mechanisms of 7-HT biosynthesis remain unclear. This study is the first to propose that the ECF σ factor has a regulatory effect on 7-HT biosynthesis. In total, 20 ECF σ factors were identified through genome-wide scanning, and their responses to extracellular ferrous ions were characterized. We found that SigW was both significantly upregulated under high-iron conditions and repressed by an adjacent anti-σ factor. RNA-Seq results suggest that the SigW/RsiW system is involved in iron metabolism and 7-HT biosynthesis. Combined with the siderophore phenotype, we also found that SigW could inhibit siderophore synthesis, and this inhibition can be relieved by RsiW. EMSA assays proved that SigW, when highly expressed, can directly bind to the promoter region of five operons of the nfs cluster to inhibit the transcription of the corresponding genes and consequently suppress 7-HT biosynthesis. In addition, SigW not only directly negatively regulates structural genes related to 7-HT synthesis but also inhibits the transcription of regulatory proteins, including of the Gac/Rsm cascade system. Taken together, our results highlight that the biosynthesis of 7-HT is negatively regulated by SigW and that the SigW/RsiW system is involved in mechanisms for the regulation of iron homeostasis in P. donghuensis HYS. As a result of this work, we identified a novel mechanism for the global negative regulation of 7-HT biosynthesis, complementing our understanding of the function of ECF σ factors in Pseudomonas.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Pathogenesis in Pseudomonas aeruginosa PAO1 Biofilm-Associated Is Dependent on the Pyoverdine and Pyocyanin Siderophores by Quorum Sensing Modulation

Pseudomonas aeruginosa is an opportunistic pathogenic bacterium for humans, animals, and plants, through producing different molecular factors such as biofilm, siderophores, and other virulence factors which favor bacterial establishment and infection in the host. In P. aeruginosa PAO1, the production of these factors is regulated by the bacterial quorum sensing (QS) mechanisms. From them, siderophores are involved in iron acquisition, transport, and homeostasis. They are also considered some of the main virulence factors in P. aeruginosa; however, detailed mechanisms to induce bacterial pathogenesis are poorly understood. In this work, through reverse genetics, we evaluated the function of bacterial pathogenesis in the pvd cluster genes, which are required for synthesizing the siderophore pyoverdine (PVD). Single pvdI, pvdJ, pvdL, and double mutant strains were analyzed, and contrary to expected, the pvdL and pvdI mutations increased the concentration of PVD and other phenazines, such as pyocyanin (PYO) and phenazine-1-carboxylic acid (PCA) and also an increased biofilm production and morphology depending on the autoinducer 2-alkyl-4-quinolone (PQS) and the QS molecules acyl-homoserine lactones. Consequently, in the in vivo pathogenicity model of Caenorhabditis elegans, the mutations in pvdI, pvdJ, and pvdL increased the survival of the worms exposed to supernatants or biofilms of the bacterial cultures. However, the double mutant pvdI/pvdJ increased its toxicity in agreeing with the biofilm production, PVD, PYO, and PCA. The findings indicate that the mutations in pvd genes encode non-ribosomal peptide synthetases impacted the biofilm's structure, but suppressively also of the phenazines, confirming that the siderophores contribute to the bacterial establishment and pathogenicity of P. aeruginosa PAO1.

RsmA3 modulates RpoS through the RetS-Gac-Rsm signaling pathway in response to H2 O2 stress in the phytopathogen Pseudomonas syringae

Reactive oxygen species are a fatal challenge to the plant pathogenic bacterium Pseudomonas syringae. In this study, we reveal that the global regulatory protein RsmA3 from the RetS-Gac/Rsm signaling pathway modulates RpoS in the early-log growth phase in the P. syringae wild-type strain MB03, thereby regulating oxidative tolerance to H₂ O₂ and ultimately affecting pathogenicity to the host plant. Following increased H₂ O₂ by external addition or endogenous induction by menadione, the resistance of the mutant strain ΔretS to H₂ O₂ is significantly enhanced due to rapid increases in the transcription of Rsm-related non-coding small RNAs (nc sRNAs), a sigma factor RpoS, and H₂ O₂ -detoxifying enzymes. Moreover, the ΔretS mutant is significantly less pathogenic in cucumber leaves. Seven Rsm-related nc sRNAs (namely, rsmZ, rsmY, and rsmX_1-5 ) show functional redundancy in the RetS-Gac-Rsm signaling pathway. External addition of H₂ O₂ stimulates increases in the transcription of both rsmY and rsmZ. Thus, we propose a regulatory model of the RetS-Gac-Rsm signaling pathway in P. syringae MB03 for the regulation of H₂ O₂ tolerance and phytopathogenicity in the host plant.

Nematicidal effects of a peptidase (NlpC/P60) from the phytopathogen Pseudomonas syringae MB03 on the model nematode Caenorhabditis elegans and plant-parasitic Meloidogyne incognita

BACKGROUND

Plant-parasitic nematodes (PPNs) are severe threats to agricultural yields and continue to be challenging to treat in several crops worldwide. Microbial-based control has been suggested as a better alternative to chemical control. In this study, we aimed to identify and characterize nematicidal virulence factors of a common phytopathogenic bacterium, Pseudomonas syringae, mainly focusing on the nematicidal and suppressive activities of an NlpC/P60 family peptidase, namely, Peptidase03, against the model nematode Caenorhabditis elegans and an agriculturally important PPN, Meloidogyne incognita.

METHODS AND RESULTS

Genome-wide virulence factor prediction of the P. syringae wild-type strain MB03 revealed numerous nematode pathogenic determinants. We selected 11 predicted nematicidal genes for cloning and induced expression in an Escherichia coli expression system and then performed comparative nematicidal bioassays on the model nematode C. elegans. The recombinant strain expressing Peptidase03 showed the highest level of toxicity against C. elegans, with 75.9% mortality, compared to the other tested strains. Purified Peptidase03 showed significant toxicity against C. elegans and M. incognita, with half lethal concentration (LC50) values of 147.9 µg/mL and 211.50 µg/mL, respectively. We also demonstrated that Peptidase03 could damage the intestinal tissues of C. elegans and exhibit detrimental effects on its growth, brood size, and locomotion.

CONCLUSIONS

The Peptidase03 protein from P. syringae MB03 had significant nematicidal and suppressive activities against C. elegans and M. incognita, thereby showing potential for the development of an effective PPN-controlling agent for use in agricultural practice.

Characterization of Nematicidal Activity and Nematode-Toxic Metabolites of a Soilborne Brevundimonas bullata Isolate

The increasing prevalence of crop-threatening root-knot nematodes (RKNs) has stimulated extensive research to discover effective nematicides. A highly focused strategy for accomplishing this is the development of biocontrol agents by a variety of soilborne microorganisms, as different bacterial metabolites have demonstrated promising nematicidal activities. In this study, we characterized the nematicidal and suppressive activity of a bacterial isolate against the agriculturally important RKN Meloidogyne incognita and the model nematode Caenorhabditis elegans, and the main M. incognita-toxic metabolite of the strain. After a preliminary screening of 22 bacterial isolates with a corrected mortality (CM) of whole-cell culture greater than 50% against C. elegans from different RKN-incident soils in China, a total of 14 isolates with CM of the supernatant of culture suspension (SCS) higher than 50% against both M. incognita and C. elegans were rescreened. An isolate with the highest CM of 86.1% and 95.0% for M. incognita and C. elegans, respectively, was further identified as the species Brevundimonas bullata via morphological examination, physiological and biochemical assays and alignment analysis of 16S rRNA gene sequences. The SCS of this strain, namely, B. bullata MB756, exhibited synchronous M. incognita killing activity along with significant detrimental effects on the growth, brood size, and locomotion of C. elegans. The effects of heat treatment, pH, inoculations, and protease K proteolysis on the CM of MB756 SCS were evaluated. A major M. incognita-toxic substance in the MB756 SCS was assayed and identified using thin-layer chromatography, column chromatography and high-performance liquid chromatography with a mass spectrometer, and it was preliminarily identified as 2-ethylhexan-1-ol, with a molecular formula of C₈H₁₈O and a molecular weight of 130.3 Da.

Identification of the Genes of the Plant Pathogen Pseudomonas syringae MB03 Required for the Nematicidal Activity Against Caenorhabditis elegans Through an Integrated Approach

Nematicidal potential of the common plant pathogen Pseudomonas syringae has been recently identified against Caenorhabditis elegans. The current study was designed to investigate the detailed genetic mechanism of the bacterial pathogenicity by applying comparative genomics, transcriptomics, mutant library screening, and protein expression. Results showed that P. syringae strain MB03 could kill C. elegans in the liquid assay by gut colonization. The genome of P. syringae MB03 was sequenced and comparative analysis including multi locus sequence typing, and genome-to-genome distance placed MB03 in phylogroup II of P. syringae. Furthermore, comparative genomics of MB03 with nematicidal strains of Pseudomonas aeruginosa (PAO1 and PA14) predicted 115 potential virulence factors in MB03. However, genes for previously reported nematicidal metabolites, such as phenazine, pyochelin, and pyrrolnitrin, were found absent in the MB03 genome. Transcriptomics analysis showed that the growth phase of the pathogen considerably affected the expression of virulence factors, as genes for the flagellum, glutamate ABC transporter, phoP/phoQ, fleS/fleR, type VI secretion system, and serralysin were highly up-regulated when stationary phase MB03 cells interacted with C. elegans. Additionally, screening of a transposon insertion mutant library led to the identification of other nematicidal genes such as acnA, gltP, oprD, and zapE. Finally, the nematicidal activity of selected proteins was confirmed by heterologous expression in Escherichia coli.

Epidemiology, Molecular Characteristics, and Virulence Factors of Carbapenem-Resistant Pseudomonas aeruginosa Isolated from Patients with Urinary Tract Infections

Purpose

Pseudomonas aeruginosa is an important pathogen that causes urinary tract infections. Carbapenems are the drugs of choice for the treatment of P. aeruginosa infections. However, the emergence and spread of carbapenem-resistant P. aeruginosa (CRPA) is a serious global health threat. In this study, we investigated the epidemiology, molecular characteristics, drug resistance, and virulence factors of CRPA isolated from urine samples.

Methods

A total of 124 P. aeruginosa isolates were obtained from urine samples collected between March 2020 and February 2021. Clonal relatedness was evaluated using multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE). We performed antimicrobial susceptibility tests and investigated the presence of carbapenemase genes and virulence factors in CRPA isolates.

Results

The carbapenem resistance rate of the isolated P. aeruginosa was 46.7% (59/124). A total of 54 (91.5%) out of the 59 CRPA isolates were identified as multidrug-resistant. The majority of the CRPA isolates (81.4%, 48/59) harbored carbapenemase genes, such as bla_IMP-6 or bla_NDM-1. In an epidemiological analysis using MLST, 88.1% of CRPA isolates were confirmed to be ST773 (50.8%, 30/59) or ST235 (37.3%, 22/59). The CRPA isolates harboring bla_IMP-6 and bla_NDM-1 belonged to ST235 (PFGE pulsotypes E1-E18, F) and ST773 (PFGE pulsotypes H1-H2, I1-I16) subtypes, respectively. The studied CRPA isolates simultaneously harbored 10 to 14 virulence factors of the 16 virulence factors examined. Nine virulence factor genes (toxA, exoT, plcH, plcN, phzM, phzS, lasB, aprA, and algD) were identified in all CRPA isolates.

Conclusion

Our study shows that P. aeruginosa ST235 harboring bla_IMP-6 and ST773 harboring bla_NDM-1—known internationally as high-risk clones with multiple virulence factors—are widely spread in the study area. These findings suggest that continuous monitoring is necessary to prevent the further spread of carbapenemase-producing CRPA.

Nematicidal effects of 2-methyl-aconitate isomerase from the phytopathogen Pseudomonas syringae MB03 on the model nematode Caenorhabditis elegans

The pathogenicity of a common phytopathogenic bacterium, Pseudomonas syringae, against animal model hosts, such as mice and Caenorhabditis elegans, has been recently revealed. However, most of the virulence determinants associated with pathogenesis remain elusive. In the current study, we performed predictive analysis of virulence factors against C. elegans in the genome of the wild-type P. syringae strain MB03. Nine predicted nematicidal proteins were expressed and purified in recombinant Escherichia coli strains and were evaluated to define their toxicity against C. elegans in liquid killing assays. Next, we focused on one essential 2-methyl citrate cycle protein, PrpF03, which showed the highest lethal activity against C. elegans compared to the other tested proteins with a half lethal concentration (LC₅₀) of 155.3 (123.4-176.6) µg mL^-1 and a half lethal time (LT₅₀) of 3.72 (1.64-4.85) days. Purified PrpF03 also caused adverse effects on the brood size, growth, and motility of C. elegans. Moreover, the PrpF03 protein exhibited pathological activity towards the intestinal tract of C. elegans. We surmise that the PrpF03 protein functions as a virulence factor when it blocks the average circulation of the 2-methyl citrate cycle of C. elegans by accumulating 2-methyl citrate in the gut of C. elegans, which damages and restrains the growth of intestinal tissues that ultimately kill C. elegans. The discovery of specific nematicidal activities of PrpF03 provides a better understanding of the mechanisms of phytopathogenic P. syringae against nematodes and could aid in developing nematode pest-controlling agents in agriculture.