Ferric Uptake Regulator (FurA) is Required for Acidovorax citrulli Virulence on Watermelon

Comparative phenotype and transcriptome analysis revealed the role of ferric uptake regulator (Fur) in the virulence of Vibrio harveyi isolated from diseased American eel (Anguilla rostrata)

Vibrio harveyi is commonly found in salt and brackish water and is recognized as a serious bacterial pathogen in aquaculture worldwide. In this study, we cloned the ferric uptake regulator (fur) gene from V. harveyi wild-type strain HA_1, which was isolated from diseased American eels (Anguilla rostrata) and has a length of 450 bp, encoding 149 amino acids. Then, a mutant strain, HA_1-Δfur, was constructed through homologous recombination of a suicide plasmid (pCVD442). The HA_1-Δfur mutant exhibited weaker biofilm formation and swarming motility, and 18-fold decrease (5.5%) in virulence to the American eels; compared to the wild-type strain, the mutant strain showed time and diameter differences in growth and haemolysis, respectively. Additionally, the adhesion ability of the mutant strain was significantly decreased. Moreover, there were 15 different biochemical indicators observed between the two strains. Transcriptome analysis revealed that 875 genes were differentially expressed in the Δfur mutant, with 385 up-regulated and 490 down-regulated DEGs. GO and KEGG enrichment analysis revealed that, compared to the wild-type strain, the type II and type VI secretion systems (T2SS and T6SS), amino acid synthesis and transport and energy metabolism pathways were significantly down-regulated, but the ABC transporters and biosynthesis of siderophore group non-ribosomal peptides pathways were up-regulated in the Δfur strain. The qRT-PCR results further confirmed that DEGs responsible for amino acid transport and energy metabolism were positively regulated, but DEGs involved in iron acquisition were negatively regulated in the Δfur strain. These findings suggest that the virulence of the Δfur strain was significantly decreased, which is closely related to phenotype changing and gene transcript regulation.

The Mutation of the DNA-Binding Domain of Fur Protein Enhances the Pathogenicity of Edwardsiella piscicida via Inducing Overpowering Pyroptosis

Edwardsiella piscicida is an important fish pathogen with a broad host that causes substantial economic losses in the aquaculture industry. Ferric uptake regulator (Fur) is a global transcriptional regulator and contains two typical domains, the DNA-binding domain and dimerization domain. In a previous study, we obtained a mutant strain of full-length fur of E. piscicida, TX01Δfur, which displayed increased siderophore production and stress resistance factors and decreased pathogenicity. To further reveal the regulatory mechanism of Fur, the DNA-binding domain (N-terminal) of Fur was knocked out in this study and the mutant was named TX01Δfur2. We found that TX01Δfur2 displayed increased siderophore production and enhanced adversity tolerance, including a low pH, manganese, and high temperature stress, which was consistent with the phenotype of TX01Δfur. Contrary to TX01Δfur, whose virulence was weakened, TX01Δfur2 displayed an ascended invasion of nonphagocytic cells and enhanced destruction of phagocytes via inducing overpowering or uncontrollable pyroptosis, which was confirmed by the fact that TX01Δfur2 induced higher levels of cytotoxicity, IL-1β, and p10 in macrophages than TX01. More importantly, TX01Δfur2 displayed an increased global virulence to the host, which was confirmed by the result that TX01Δfur2 caused higher lethality outcomes for healthy tilapias than TX01. These results demonstrate that the mutation of the Fur N-terminal domain augments the resistance level against the stress and pathogenicity of E. piscicida, which is not dependent on the bacterial number in host cells or host tissues, although the capabilities of biofilm formation and the motility of TX01Δfur2 decline. These interesting findings provide a new insight into the functional analysis of Fur concerning the regulation of virulence in E. piscicida and prompt us to explore the subtle regulation mechanism of Fur in the future.

A novel double-ribonuclease toxin-antitoxin system linked to the stress response and survival of Acidovorax citrulli

IMPORTANCE

Bacterial fruit blotch (BFB), which is caused by the seed-borne bacterium Acidovorax citrulli, is a devastating disease affecting cucurbit crops throughout the world. Although seed fermentation and treatment with disinfectants can provide effective management of BFB, they cannot completely guarantee pathogen-free seedstock, which suggests that A. citrulli is a highly stress-resistant pathogen. Toxin-antitoxin (TA) systems are common among a diverse range of bacteria and have been reported to play a role in bacterial stress response. However, there is currently much debate about the relationship between TA systems and stress response in bacteria. The current study characterized a novel TA system (Aave_1720-Aave_1719) from A. citrulli that affects both biofilm formation and survival in response to sodium hypochlorite stress. The mechanism of neutralization differed from typical TA systems as two separate mechanisms were associated with the antitoxin, which exhibited characteristics of both type II and type V TA systems. The Aave_1720-Aave_1719 system described here also constitutes the first known report of a double-ribonuclease TA system in bacteria, which expands our understanding of the range of regulatory mechanisms utilized by bacterial TA systems, providing new insight into the survival of A. citrulli in response to stress.

XanFur, a novel Fur protein induced by H2O2, positively regulated by the global transcriptional regulator Clp and required for the full virulence of Xanthomonas oryzae pv. oryzae in rice

IMPORTANCE

Although Xanthomonas oryzae pv. oryzae (Xoo) has been found to be a bacterial pathogen causing bacterial leaf blight in rice for many years, the molecular mechanisms of the rice-Xoo interaction has not been fully understood. In this study, we found that XanFur of Xoo is a novel ferric uptake regulator (Fur) protein conserved among major pathogenic Xanthomonas species. XanFur is required for the virulence of Xoo in rice, and likely involved in regulating the virulence determinants of Xoo. The expression of xanfur is induced by H2O2, and positively regulated by the global transcriptional regulator Clp. Our results reveal the function and regulation of the novel virulence-related Fur protein XanFur in Xoo, providing new insights into the interaction mechanisms of rice-Xoo.

A putative glucose 6-phosphate isomerase has pleiotropic functions on virulence and other mechanisms in Acidovorax citrulli

Acidovorax citrulli (Ac) is a causal agent of watermelon bacterial fruit blotch (BFB) disease. Because resistance cultivars/lines have not yet been developed, it is imperative to elucidate Ac's virulence factors and their mechanisms to develop resistant cultivars/lines in different crops, including watermelon. The glucose-6-phosphate isomerase (GPI) is a reversible enzyme in both glycolysis and gluconeogenesis pathways in living organisms. However, the functions of GPI are not characterized in Ac. In this study, we determined the roles of GpiAc (GPI in Ac) by proteomic and phenotypic analyses of the mutant lacking GPI. The mutant displayed significantly reduced virulence to watermelon in two different virulence assays. The mutant's growth patterns were comparable to the wild-type strain in rich medium and M9 with glucose but not with fructose. The comparative proteome analysis markedly identified proteins related to virulence, motility, and cell wall/membrane/envelope. In the mutant, biofilm formation and twitching halo production were reduced. We further demonstrated that the mutant was less tolerant to osmotic stress and lysozyme treatment than the wild-type strain. Interestingly, the tolerance to alkali conditions was remarkably enhanced in the mutant. These results reveal that GpiAc is involved not only in virulence and glycolysis/gluconeogenesis but also in biofilm formation, twitching motility, and tolerance to diverse external stresses suggesting the pleiotropic roles of GpiAc in Ac. Our study provides fundamental and valuable information on the functions of previously uncharacterized glucose 6-phosphate isomerase and its virulence mechanism in Ac.

Differential plant cell responses to Acidovorax citrulli T3SS and T6SS reveal an effective strategy for controlling plant-associated pathogens

Acidovorax citrulli is a gram-negative plant pathogen that employs the type Ⅲ secretion system (T3SS) to infect cucurbit crops and cause bacterial fruit blotch. This bacterium also possesses an active type Ⅵ secretion system (T6SS) with strong antibacterial and antifungal activities. However, how plant cells respond to these two secretion systems and whether there is any cross talk between T3SS and T6SS during infection remain unknown. Here, we employ transcriptomic analysis to compare cellular responses to the T3SS and the T6SS during in planta infection and report distinctive effects on multiple pathways. The T3SS-mediated differentially expressed genes were enriched in the pathways of phenylpropanoid biosynthesis, plant-pathogen interaction, MAPK signaling pathway, and glutathione metabolism, while the T6SS uniquely affected genes were related to photosynthesis. The T6SS does not contribute to the in planta virulence of A. citrulli but is critical for the survival of the bacterium when mixed with watermelon phyllosphere bacteria. In addition, T3SS-mediated virulence is independent of the T6SS, and the inactivation of the T3SS does not affect the T6SS-mediated competition against a diverse set of bacterial pathogens that commonly contaminate edible plants or directly infect plants. A T6SS-active T3SS-null mutant (Acav) could inhibit the growth of Xanthomonas oryzae pv. oryzae significantly both in vitro and in vivo and also reduce symptoms of rice bacterial blight. In conclusion, our data demonstrate the T6SS in A. citrulli is nonpathogenic to the plant host and can be harnessed as a pathogen killer against plant-associated bacteria. IMPORTANCE Chemical pesticides are widely used to protect crops from various pathogens. Still, their extensive use has led to severe consequences, including drug resistance and environmental contamination. Here, we show that an engineered T6SS-active, but avirulent mutant of Acidovorax citrulli has strong inhibition capabilities against several pathogenic bacteria, demonstrating an effective strategy that is an alternative to chemical pesticides for sustainable agricultural practices.

Vibrio splendidus Fur regulates virulence gene expression, swarming motility, and biofilm formation, affecting its pathogenicity in Apostichopus japonicus

Vibrio splendidus is an opportunistic pathogen that causes skin ulcer syndrome and results in huge losses to the Apostichopus japonicus breeding industry. Ferric uptake regulator (Fur) is a global transcription factor that affects varieties of virulence-related functions in pathogenic bacteria. However, the role of the V. splendidus fur (Vsfur) gene in the pathogenesis of V. splendidus remains unclear. Hence, we constructed a Vsfur knock-down mutant of the V. splendidus strain (MTVs) to investigate the role of the gene in the effect of biofilm, swarming motility, and virulence on A. japonicus. The result showed that the growth curves of the wild-type V. splendidus strain (WTVs) and MTVs were almost consistent. Compared with WTVs, the significant increases in the transcription of the virulence-related gene Vshppd mRNA were 3.54- and 7.33-fold in MTVs at the OD₆₀₀ of 1.0 and 1.5, respectively. Similarly, compared with WTVs, the significant increases in the transcription of Vsm mRNA were 2.10- and 15.92-fold in MTVs at the OD₆₀₀ of 1.0 and 1.5, respectively. On the contrary, the mRNA level of the flagellum assembly gene Vsflic was downregulated 0.56-fold in MTVs at the OD₆₀₀ of 1.0 compared with the WTVs. MTVs caused delayed disease onset time and reduced A. japonicus mortality. The median lethal doses of WTVs and MTVs were 9.116 × 10⁶ and 1.658 × 10¹¹ CFU·ml^-1, respectively. Compared with WTVs, the colonization abilities of MTVs to the muscle, intestine, tentacle, and coelomic fluid of A. japonicus were significantly reduced. Correspondingly, the swarming motility and biofilm formation in normal and iron-replete conditions were remarkably decreased compared with those of WTVs. Overall, these results demonstrate that Vsfur contributes to the pathogenesis of V. splendidus by regulating virulence-related gene expression and affecting its swarming and biofilm formation abilities.

Visible-Light-Driven Zinc Oxide Quantum Dots for the Management of Bacterial Fruit Blotch Disease and the Improvement of Melon Seedlings Growth

Bacterial fruit blotch is one of the most destructing diseases of melon producing-regions. Here, zinc oxide quantum dots (ZnO QDs) were synthesized, and their antibacterial activity against Acidovorax citrulli was investigated. The results indicated that the obtained ZnO QDs displayed 5.7-fold higher antibacterial activity than a commercial Zn-based bactericide (zinc thiazole). Interestingly, the antibacterial activity of ZnO QDs irradiated with light was 1.8 times higher than that of the dark-treated group. It was because ZnO QDs could induce the generation of hydroxyl radicals and then up-regulate the expression of oxidative stress-related genes, finally leading to the loss of cell membrane integrity. A pot experiment demonstrated that foliar application of ZnO QDs significantly reduced the bacterial fruit blotch disease incidence (32.0%). Furthermore, the supply of ZnO QDs could improve the growth of infected melon seedlings by activating the antioxidant defense system. This work provides a promising light-activated quantum-bactericide for the management of pathogenic bacterial infections in melon crop protection.

A putative 2,3-bisphosphoglycerate-dependent phosphoglycerate mutase is involved in the virulence, carbohydrate metabolism, biofilm formation, twitching halo, and osmotic tolerance in Acidovorax citrulli

Acidovorax citrulli (Ac) is a gram-negative bacterium that causes bacterial fruit blotch (BFB) disease in cucurbit crops including watermelon. However, despite the great economic losses caused by this disease worldwide, Ac-resistant watermelon cultivars have not been developed. Therefore, characterizing the virulence factors/mechanisms of Ac would enable the development of effective control strategies against BFB disease. The 2,3-bisphosphoglycerate-dependent phosphoglycerate mutase (BdpM) is known to participate in the glycolysis and gluconeogenesis pathways. However, the roles of the protein have not been characterized in Ac. To elucidate the functions of BdpmAc (Bdpm in Ac), comparative proteomic analysis and diverse phenotypic assays were conducted using a bdpmAc knockout mutant (bdpmAc:Tn) and a wild-type strain. The virulence of the mutant to watermelon was remarkably reduced in both germinated seed inoculation and leaf infiltration assays. Moreover, the mutant could not grow with fructose or pyruvate as a sole carbon source. However, the growth of the mutant was restored to levels similar to those of the wild-type strain in the presence of both fructose and pyruvate. Comparative proteomic analyses revealed that diverse proteins involved in motility and wall/membrane/envelop biogenesis were differentially abundant. Furthermore, the mutant exhibited decreased biofilm formation and twitching halo size. Interestingly, the mutant exhibited a higher tolerance against osmotic stress. Overall, our findings suggest that BdpmAc affects the virulence, glycolysis/gluconeogenesis, biofilm formation, twitching halo size, and osmotic tolerance of Ac, suggesting that this protein has pleiotropic properties. Collectively, our findings provide fundamental insights into the functions of a previously uncharacterized phosphoglycerate mutase in Ac.

OxyR contributes to virulence of Acidovorax citrulli by regulating anti-oxidative stress and expression of flagellin FliC and type IV pili PilA

In many bacteria, OxyR acts as a transcriptional regulator that facilitates infection via degrading hydrogen peroxide (H₂O₂) generated by the host defense response. Previous studies showed that OxyR also plays an important role in regulating biofilm formation, cell motility, pili relate-genes expression, and surface polysaccharide production. However, the role of OxyR has not been determined in Acidovorax citrulli strain xjl12. In the current study, the qRT-PCR and western blot assays revealed that the expression level of oxyR was significantly induced by H₂O₂. The oxyR deletion mutant of A. citrulli was significantly impaired bacterial tolerance to oxidative stress and reduced catalase (CAT) activity. In addition, oxyR mutant resulted in reduced swimming motility, twitching motility, biofilm formation, virulence, and bacterial growth in planta by significantly affecting flagellin and type IV pili-related gene (fliC and pilA) expression. The qRT-PCR assays and western blot revealed that OxyR positively regulated the expression of fliC and pilA. Furthermore, bacterial one-hybrid assay demonstrated that OxyR directly affected pilA and fliC promoter. Through bacterial two-hybrid assay, it was found that OxyR can directly interact with PilA and FliC. These results suggest that OxyR plays a major role in the regulating of a variety of virulence traits, and provide a foundation for future research on the global effects of OxyR in A. citrulli.

Hcp of the Type VI Secretion System (T6SS) in Acidovorax citrulli Group II Strain Aac5 Has a Dual Role as a Core Structural Protein and an Effector Protein in Colonization, Growth Ability, Competition, Biofilm Formation, and Ferric Iron Absorption

A type VI secretion system (T6SS) gene cluster has been reported in Acidovorax citrulli. Research on the activation conditions, functions, and the interactions between key elements in A. citrulli T6SS is lacking. Hcp (Hemolysin co-regulated protein) is both a structural protein and a secretion protein of T6SS, which makes it a special element. The aims of this study were to determine the role of Hcp and its activated conditions to reveal the functions of T6SS. In virulence and colonization assays of hcp deletion mutant strain Δhcp, tssm (type VI secretion system membrane subunit) deletion mutant strain Δtssm and double mutant ΔhcpΔtssm, population growth was affected but not virulence after injection of cotyledons and seed-to-seedling transmission on watermelon. The population growth of Δhcp and Δtssm were lower than A. citrulli wild type strain Aac5 of A. citrulli group II at early stage but higher at a later stage. Deletion of hcp also affected growth ability in different culture media, and the decline stage of Δhcp was delayed in KB medium. Biofilm formation ability of Δhcp, Δtssm and ΔhcpΔtssm was lower than Aac5 with competition by prey bacteria but higher in KB and M9-Fe³⁺ medium. Deletion of hcp reduced the competition and survival ability of Aac5. Based on the results of Western blotting and qRT-PCR analyses, Hcp is activated by cell density, competition, ferric irons, and the host plant. The expression levels of genes related to bacterial secretion systems, protein export, and several other pathways, were significantly changed in the Δhcp mutant compared to Aac5 when T6SS was activated at high cell density. Based on transcriptome data, we found that a few candidate effectors need further identification. The phenotypes, activated conditions and transcriptome data all supported the conclusion that although there is only one T6SS gene cluster present in the A. citrulli group II strain Aac5, it related to multiple biological processes, including colonization, growth ability, competition and biofilm formation.

yggS Encoding Pyridoxal 5'-Phosphate Binding Protein Is Required for Acidovorax citrulli Virulence

Bacterial fruit blotch, caused by seed-borne pathogen Acidovorax citrulli, poses a serious threat to the production of cucurbits globally. Although the disease can cause substantial economic losses, limited information is available about the molecular mechanisms of virulence. This study identified that, a random transposon insertion mutant impaired in the ability to elicit a hypersensitive response on tobacco. The disrupted gene in this mutant was determined to be Aave_0638, which is predicted to encode a YggS family pyridoxal phosphate-dependent enzyme. YggS is a highly conserved protein among multiple organisms, and is responsible for maintaining the homeostasis of pyridoxal 5′-phosphate and amino acids in cells. yggS deletion mutant of A. citrulli strain XjL12 displayed attenuated virulence, delayed hypersensitive response, less tolerance to H₂O₂ and pyridoxine, increased sensitivity to antibiotic β-chloro-D-alanine, and reduced swimming. In addition, RNA-Seq analysis demonstrated that yggS was involved in regulating the expression of certain pathogenicity-associated genes related to secretion, motility, quorum sensing and oxidative stress response. Importantly, YggS significantly affected type III secretion system and its effectors in vitro. Collectively, our results suggest that YggS is indispensable for A.citrulli virulence and expands the role of YggS in the biological processes.

Phenazine-1-carboxylic Acid Produced by Pseudomonas chlororaphis YL-1 Is Effective against Acidovorax citrulli

The bacterial pathogen Acidovorax citrulli causes the destructive fruit blotch (BFB) on cucurbit plants. Pseudomonas chlororaphis YL-1 is a bacterial strain isolated from Mississippi soil and its genome harbors some antimicrobial-related gene clusters, such as phenazine, pyrrolnitrin, and pyoverdine. Here, we evaluated the antimicrobial activity of strain YL-1 as compared with its deficient mutants of antimicrobial-related genes, which were obtained using a sacB-based site-specific mutagenesis strategy. We found that only phenazine-deficient mutants ΔphzE and ΔphzF almost lost the inhibitory effects against A. citrulli in LB plates compared with the wild-type strain YL-1, and that the main antibacterial compound produced by strain YL-1 in LB medium was phenazine-1-carboxylic acid (PCA) based on the liquid chromatography-mass spectrometry (LC-MS) analysis. Gene expression analyses revealed that PCA enhanced the accumulation of reactive oxygen species (ROS) and increased the activity of catalase (CAT) in A. citrulli. The inhibition effect of PCA against A. citrulli was lowered by adding exogenous CAT. PCA significantly upregulated the transcript level of katB from 6 to 10 h, which encodes CAT that helps to protect the bacteria against oxidative stress. Collectively, the findings of this research suggest PCA is one of the key antimicrobial metabolites of bacterial strain YL-1, a promising biocontrol agent for disease management of BFB of cucurbit plants.

Proteomic and Phenotypic Analyses of a Putative Glycerol-3-Phosphate Dehydrogenase Required for Virulence in Acidovorax citrulli

Acidovorax citrulli (Ac) is the causal agent of bacterial fruit blotch (BFB) in watermelon, a disease that poses a serious threat to watermelon production. Because of the lack of resistant cultivars against BFB, virulence factors or mechanisms need to be elucidated to control the disease. Glycerol-3-phosphate dehydrogenase is the enzyme involved in glycerol production from glucose during glycolysis. In this study, we report the functions of a putative glycerol-3-phosphate dehydrogenase in Ac (GlpdAc) using comparative proteomic analysis and phenotypic observation. A glpdAc knockout mutant, AcΔglpdAc(EV), lost virulence against watermelon in two pathogenicity tests. The putative 3D structure and amino acid sequence of GlpdAc showed high similarity with glycerol-3-phosphate dehydrogenases from other bacteria. Comparative proteomic analysis revealed that many proteins related to various metabolic pathways, including carbohydrate metabolism, were affected by GlpdAc. Although AcΔglpdAc(EV) could not use glucose as a sole carbon source, it showed growth in the presence of glycerol, indicating that GlpdAc is involved in glycolysis. AcΔglpdAc(EV) also displayed higher cell-to-cell aggregation than the wild-type bacteria, and tolerance to osmotic stress and ciprofloxacin was reduced and enhanced in the mutant, respectively. These results indicate that GlpdAc is involved in glycerol metabolism and other mechanisms, including virulence, demonstrating that the protein has pleiotropic effects. Our study expands the understanding of the functions of proteins associated with virulence in Ac.

Biochemical characterization of ferric uptake regulator (Fur) from Aliivibrio salmonicida. Mapping the DNA sequence specificity through binding studies and structural modelling

Iron is an essential nutrient for bacteria, however its propensity to form toxic hydroxyl radicals at high intracellular concentrations, requires its acquisition to be tightly regulated. Ferric uptake regulator (Fur) is a metal-dependent DNA-binding protein that acts as a transcriptional regulator in maintaining iron metabolism in bacteria and is a highly interesting target in the design of new antibacterial drugs. Fur mutants have been shown to exhibit decreased virulence in infection models. The protein interacts specifically with DNA at binding sites designated as 'Fur boxes'. In the present study, we have investigated the interaction between Fur from the fish pathogen Aliivibrio salmonicida (AsFur) and its target DNA using a combination of biochemical and in silico methods. A series of target DNA oligomers were designed based on analyses of Fur boxes from other species, and affinities assessed using electrophoretic mobility shift assay. Binding strengths were interpreted in the context of homology models of AsFur to gain molecular-level insight into binding specificity.

Putative Bifunctional Chorismate Mutase/Prephenate Dehydratase Contributes to the Virulence of Acidovorax citrulli

Acidovorax citrulli (Ac) is a plant pathogenic bacterium that causes bacterial fruit blotch (BFB) in cucurbit crops. Despite its importance in the cucurbit industry, resistant cultivars/lines against BFB have not yet been identified. Therefore, there is a need to characterize the virulence factors/mechanisms in Ac to control the disease. Chorismate mutase, a key enzyme in the shikimate pathway, produces aromatic amino acids. Here, we report the functions of putative bifunctional chorismate mutase/prephenate dehydratase in Ac (CmpAc) determined by proteomic analysis and phenotypic assays. Ac strain lacking CmpAc, AcΔcmpAc(EV), were significantly less virulent on watermelon in the germinated-seed inoculation and leaf infiltration assays. Sequence analysis revealed that CmpAc possesses two distinct domains: chorismate mutase and prephenate dehydratase, indicating that CmpAc is a bifunctional protein. Auxotrophic assays demonstrated that CmpAc is required for the biosynthesis of phenylalanine, but not tyrosine. The comparative proteomic analysis revealed that CmpAc is mostly involved in cell wall/membrane/envelop biogenesis. Furthermore, AcΔcmpAc(EV) showed reduced twitching halo production and enhanced biofilm formation. In addition, AcΔcmpAc(EV) was less tolerant to osmotic stress but more tolerant to antibiotics (polymyxin B). Thus, our study provides new insights into the functions of a putative bifunctional protein related to virulence in Ac.