Identification and characterization of a phospholipase A1 activity type three secreted protein, PP_ExoU from Pseudomonas plecoglossicida NB2011, the causative agent of visceral granulomas disease in large yellow croaker (Larimichthys crocea)

The bacterial pathogen Pseudomonas plecoglossicida, its epidemiology, virulence factors, vaccine development, and host-pathogen interactions

OBJECTIVE

Pseudomoans plecoglossicida has been identified as a fish pathogen since 2000 and has caused serious infections in cultured Large Yellow Croakers Larimiththys crocea in coastal eastern China during recent years.

METHODS

Published literatures of this pathogen have been reviewed.

RESULT

Several strains with high genomic similarity have been isolated and identified; the bacteria induce natural infection at lower water temperatures (12.0-25.5°C) and induce numerous granulomas and nodules in the visceral organs of croakers. Researchers have investigated the epidemiology of P. plecoglossicida infection, identified major virulence factors, searched for pathogenic genes, analyzed host-pathogen interactions, and endeavored to develop efficient vaccines.

CONCLUSION

This paper provides an overview of these research advances to elucidate the virulence mechanisms of the pathogen and to promote vaccine development against infection.

Comparative secretome analysis reveals cross-talk between type III secretion system and flagella assembly in Pseudomonas plecoglossicida

The Gram-negative bacterium Pseudomonas plecoglossicida has caused visceral granulomas disease in several farmed fish species, including large yellow croaker (Larimichthys crocea), which results in severe economic losses. Type III secretion systems (T3SS) are protein secretion and translocation nanomachines widely employed by many Gram-negative bacterial pathogens for infection and pathogenicity. However, the exact role of T3SS in the pathogenesis of P. plecoglossicida infection is still unclear. In this study, a T3SS translocators deletion strain (△popBD) of P. plecoglossicida was constructed to investigate the function of T3SS. Then comparative secretome analysis of the P. plecoglossicida wild-type (WT) and △popBD mutant strains was conducted by label-free quantitation (LFQ) mass spectrometry. The results show that knockout of T3SS translocators popB and popD has an adverse effect on the effector protein ExoU secretion, flagella assembly, and biofilm formation. Further experimental validations also confirmed that popB-popD deletion could affect the P. plecoglossicida flagella morphology/formation, adherence, mobility, and biofilm formation. These data indicate that a cross-talk exists between the P. plecoglossicida T3SS and the flagella system. Our results, therefore, will facilitate the further under-standing of the pathogenic mechanisms leading to visceral granulomas disease caused by P. plecoglossicida.

Design combinations of evolved phage and antibiotic for antibacterial guided by analyzing the phage resistance of poorly antimicrobial phage

Although antibiotics are the primary method against bacterial infections, the rapid emergence of antibiotic resistance has forced interest in alternative antimicrobial strategies. Phage has been considered a new biological antimicrobial agent due to its high effectiveness in treating bacterial infections. However, the applications of phage therapy have been limited by the quick development of phage-resistant bacteria. Therefore, more effective phage treatment strategies need to be explored guided by characterizing phage-resistant mutants. In this study, Pseudomonas plecoglossicida phage vB_PpS_SYP was isolated from the sewage but exhibited weak antibacterial activity caused by phage-resistant bacteria. Phage-resistant mutants were isolated and their whole genomes were analyzed for differences. The results showed that mutations in glycosyltransferase family 1 (GT-1) and hypothetical outer membrane protein (homP) led to bacterial phage resistance. The GT-1 mutants had lower biofilm biomass and higher antibiotic sensitivity than wild-type strain. Phage SYP evolved a broader host range and improved antimicrobial efficacy to infect homP mutants. Therefore, we designed a strategy for combined antibiotic and evolved phage inhibition driven by the two phage-resistant mutants. The results showed that the combination was more effective against bacteria than either antibiotics or phage alone. Our findings presented a novel approach to utilizing poorly antimicrobial phages by characterizing their phage-resistant mutants, with the potential to be expanded to include phage therapy for a variety of pathogens. IMPORTANCE The rapid emergence of antibiotic resistance renews interest in phage therapy. However, the lack of efficient phages against bacteria and the emergence of phage resistance impaired the efficiency of phage therapy. In this study, the isolated Pseudomonas plecoglossicida phage exhibited poor antibacterial capacity and was not available for phage therapy. Analysis of phage-resistant mutants guided the design of antibacterial strategies for the combination of antibiotics with evolved phages. The combination has a good antibacterial effect compared to the original phage. Our findings facilitate ideas for the development of antimicrobial-incapable phage, which have the potential to be applied to the phage treatment of other pathogens.

Preliminary studies on the different roles of T6SSs in pathogenicity of Pseudomonas plecoglossicida NB2011

Pseudomonas plecoglossicida, the causative agent of visceral granulomas in the large yellow croaker (Larimichthys crocea) in China, encodes three sets of type Ⅵ secretion systems (T6SS1-3). The purpose of this study was to characterize the different roles of T6SSs involved in infection. In-frame deletion of T6SSs was constructed, which resulted in 8 mutants. Competition against E. coli DH5α, virulence against the croaker and in vivo survival ability of the mutants were tested. The expression and secretion of Hcp by P. plecoglossicida NB2011 were investigated. The results showed T6SS2 mutant failed to inhibit the growth of E. coli, which is an indication of T6SS2 acting against environmental bacteria. The LD₅₀ value of T6SS1 mutant strongly increased; T6SS2 and T6SS3 mutants were similar to that of the wild type; and the virulence of double deletion or triple deletion mutant was drastically alleviated, indicating that T6SS1 being one of the major virulence factors, and T6SS2 and T6SS3 directly or indirectly being involved in the pathogenicity. T6SS1 mutant disappeared in the fish spleen in 3 days, while other strains kept increasing, indicating the T6SS1 stimulation bacteria replication in vivo. Hcp1 secreted at 12-28°C and Hcp2 secreted at 12-35°C, while Hcp3 secretion not detected in vitro. This study has thrown some insights on the understanding of pathogenicity mechanisms of this pathogen.

Characterization of a highly lethal barramundi (Lates calcarifer) model of Pseudomonas plecoglossicida infection

Pseudomonas plecoglossicida is a highly lethal causative agent associated with severe economic losses in aquaculture industry. P. plecoglossicida has been documented as a highly alarming pathogen in a wide variety of freshwater cultured fish including ayu (Plecoglossus altivelis), rainbow trout (Oncorhynchus mykiss) and pejerrey (Odontesthes bonariensis), and marine cultured fish such as large yellow croaker (Larimichthys crocea) and orange-spotted grouper (Epinephelus coioides) etc. Fish infected with P. plecoglossicida usually exhibited various symptoms, including lethargy, inappetence, disorientation, abdominal swelling with severe ascites and numerous white spots covered on the surface of spleen tissue. In present study, barramundi, zebrafish, spotted seabass and mandarinfish were investigated as potential hosts of P. plecoglossicida. Among them, barramundi was confirmed the most sensitive host fish species for P. plecoglossicida infection. Dynamic histopathology revealed that P. plecoglossicida caused various histopathological effects to barramundi: a) spleen: granulomas appeared at 2 days post infection (dpi) and matured at 4 dpi; b) liver: steatosis at 1 dpi and fat necrosis over time, and damaged the most compared to spleens and metanephros; c) metanephros: Bowman capsule space became larger and glomerulus shrank were even collapsed at 1 dpi; d) ascites: either bacterium or melanin were wrapped in cells from ascites. All these results indicated that P. plecoglossicida could cause systemic diseases with typical clinical sighs to barramundi and would be an alarming pathogen to barramundi industry.

Dual RNA-seq provides novel insight into the roles of dksA from Pseudomonas plecoglossicida in pathogen-host interactions with large yellow croakers ( Larimichthys crocea)

Pseudomonas plecoglossicida is a rod-shaped, gram-negative bacterium with flagella. It causes visceral white spot disease and high mortality in Larimichthys crocea during culture, resulting in serious economic loss. Analysis of transcriptome and quantitative real-time polymerase chain reaction (PCR) data showed that dksA gene expression was significantly up-regulated after 48 h of infection with Epinephelus coioides (log 2FC=3.12, P<0.001). RNAi of five shRNAs significantly reduced the expression of dksA in P. plecoglossicida, and the optimal silencing efficiency was 96.23%. Compared with wild-type strains, the symptoms of visceral white spot disease in L. crocea infected with RNAi strains were reduced, with time of death delayed by 48 h and mortality reduced by 25%. The dksA silencing led to a substantial down-regulation in cellular component-, flagellum-, and ribosome assembly-related genes in P. plecoglossicida, and the significant up-regulation of fliC may be a way in which virulence is maintained in P. plecoglossicida. The GO and KEGG results showed that RNAi strain infection in L. crocea led to the down-regulation of inflammatory factor genes in immune-related pathways, which were associated with multiple immune response processes. Results also showed that dksA was a virulence gene in P. plecoglossicida. Compared with the wild-type strains, RNAi strain infection induced a weaker immune response in L. crocea.

Aryl hydrocarbon receptor is required for immune response in Epinephelus coioides and Danio rerio infected by Pseudomonas plecoglossicida

Aryl hydrocarbon receptor (AhR), a ligand-dependent transcriptional factor that responds to environmental chemicals, has been recently found to be closely associated with immune response in mammals. Pseudomonas plecoglossicida (P. plecoglossicida) is a temperature-dependent bacterial pathogen of visceral white spot disease in fish. Using dual RNA-seq, we previously evaluated the expression levels of ahr1a, ahr1b, ahr2 and cyp1a in the spleen of Epinephelus coioides at different time points after infection with P. plecoglossicida. In the present study, the expression levels of ahr1a, ahr1b, ahr2 and cyp1a in different organs of E. coioides and Danio rerio showed similar trends after being infected by P. plecoglossicida. It also was noted that liver, intestine, spleen, and heart were the most obviously affected organs, and ahr2 particularly showed a dramatically increase in the spleen. Subsequently, macrophages of E. coioides were isolated, and then infected by P. plecoglossicida, followed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay, which revealed that the expression level of ahr1a in macrophages was significantly down-regulated, while expression levels of ahr1b, ahr2 and cyp1a were noticeably up-regulated. Eventually, it was noted that ahr1b and ahr2 were knocked-down in macrophages, and intracellular survival rate and immune escape rate of P. plecoglossicida were markedly improved. Taken together, ahr1a, ahr1b, ahr2 and cyp1a participate in the immune response to P. plecoglossicida in different organs of fish, while ahr1b and ahr2 may play pivotal roles in the immune response of spleen and macrophages.

Dual RNA-Seq Unveils the Role of the Pseudomonas plecoglossicida fliA Gene in Pathogen-Host Interaction with Larimichthys crocea

In the present study, Larimichthys crocea and Pseudomonas plecoglossicida were selected as a host-pathogen interaction model for teleosts and prokaryotic pathogens. Five shRNAs were designed and synthesized to silence the fliA gene, all of which resulted in pronounced reductions in fliA mRNA; the mutant strain with the best silencing efficiency of 92.16% was chosen for subsequent analysis. A significant decrease in motility, intracellular survival and escape was observed for the fliA-RNAi strain of P. plecoglossicida, whereby silencing of the fliA gene led to a 30% decrease in mortality and a four-day delay in the onset of infection in L. crocea. Moreover, silencing of P. plecoglossicida fliA resulted in a significant change in both the pathogen and host transcriptome in the spleens of infected L. crocea. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of pathogen transcriptome data showed that silencing fliA resulted in downregulation of 18 flagellum-related genes; KEGG analysis of host transcriptome data revealed that infection with the fliA-RNAi strain caused upregulation of 47 and downregulation of 106 immune-related genes. These pathogen-host interactions might facilitate clearance of P. plecoglossicida by L. crocea, with a significant decrease in fliA-RNAi P. plecoglossicida strain virulence in L. crocea.

Mechanistic insight into the roles of Pseudomonas plecoglossicida clpV gene in host-pathogen interactions with Larimichthys crocea by dual RNA-seq

Large yellow croaker (Larimichthys crocea) is an economical important farmed fish in China. "Visceral White Spot Disease" caused by Pseudomonas plecoglossicida is a disease with a high mortality rate in cage-cultured L. crocea in recent years and resulted in heavy economy lossess. The dual RNA-seq results of previous study showed that the expression of clpV gene in P. plecoglossicida was significantly up-regulated during infection. RNAi significantly reduced the expression of clpV in P. plecoglossicida with maximum silencing efficiency of 96.1%. Compared with the wild type strain, infection of clpV-RNAi strain resulted in a delayed onset time and a 25% reduction in mortality of L. crocea, as well as lessening the symptoms of the spleen. The results of dual RNA-seq of L. crocea infected by clpV-RNAi strain of P. plecoglossicida changed considerably, compared with the counterpart infected with the wild strain. The KEGG enrichment analysis showed that Cytokine-cytokine receptor interaction, Toll-like receptor signaling pathway, C-type lectin receptor signaling pathway and MAPK signaling pathway of L. crocea were most affected by the silence of clpV in P. plecoglossicida. RNAi of clpV resulted in the downregulation of genes in flagella assembly pathway and a weaker immune response of host.

Dual RNA-Seq uncovers the function of an ABC transporter gene in the host-pathogen interaction between Epinephelus coioides and Pseudomonas plecoglossicida

As an important pathogen in aquaculture, Pseudomonas plecoglossicida has caused heavy losses. The expression of an ABC transporter gene-L321_23611 of P. plecoglossicida at 18 °C was found significant higher than those at 28 °C by RNA-seq and qRT-PCR. RNAi significantly reduced the content of L321_23611 mRNA in P. plecoglossicida with a maximal decrease of 89.2%. Compared with the wild type strain, the infection of L321_23611-RNAi strain resulted in the reduction in mortality and the onset time delay of a kind of marine teleosts, Epinephelus coioides. The results of dual RNA-seq showed that the RNAi of L321_23611 resulted in a significant change in both pathogen and host transcriptome in the spleens of infected E. coioides. The result of GO and KEGG analysis from dual RNA-seq data showed both host genes of chemokine signaling pathway, coagulation and complement system, hematopoietic cell lineage pathway as well as hemoglobin complex GO term and pathogenic genes of bacterial-type flagellum-dependent cell mortality GO term and flagellar assembly, biosynthesis of amino acids and lysine biosynthesis systems pathways were mainly affected by L321_23611 gene of P. plecoglossicida. The results indicated that: 1. ABC transporter gene-L321_23611 was a virulent gene of P. plecoglossicida. 2. Both the activation of the host immune pathways and depression of pathogenic virulence-related pathways facilitated E. coioides to remove L321_23611-RNAi strain than the wild type strain of P. plecoglossicida.

clpV is a key virulence gene during in vivo Pseudomonas plecoglossicida infection

Interaction between bacterial pathogen and aquatic animal host is exceedingly complex, which involves large dynamic changes in gene expression during different stages of the disease. However, research on identifying key virulence genes based on the dynamics of gene expression changes of a one-sided bacterial pathogen in tissue has not been reported so far across different stages of infectious disease. The clpV for the T6SS of Pseudomonas plecoglossicida was identified for a candidate for key virulence gene based on dynamic changes of gene expression. For the Epinephelus coioides infected using clpV-RNAi strain, no deaths were observed up to 20 dpi. The spleens, kidneys and livers of all the E. coioides that received clpV-RNAi strain failed to develop visible nodules at 5-8 dpi, with the swelling gradually disappearing. The burdens of clpV-RNAi strain in the spleen and blood were greatly reduced at most of the time points after injection, and the burdens of clpV-RNAi strain in the head kidneys and trunk kidneys also had a sharp reduction from 72 to 120 hpi. This paper provides a new insight into the discovery of key virulence genes of pathogens in infected tissue systems.

Dual RNA-Seq Unveils Pseudomonas plecoglossicida htpG Gene Functions During Host-Pathogen Interactions With Epinephelus coioides

Pseudomonas plecoglossicida is a temperature-dependent opportunistic pathogen which is associated with a variety of diseases in fish. During the development of "white nodules" disease, the expression of htpG in P. plecoglossicida was found to be significantly up-regulated at its virulent temperature of 18°C. The infection of htpG-RNAi strain resulted in the onset time delay, reduction in mortality and infection symptoms in spleen of Epinephelus coioides, and affected the bacterial tissue colonization. In order to reveal the effect of htpG silencing of P. plecoglossicida on the virulence regulation in P. plecoglossicida and immune response in E. coioides, dual RNA-seq was performed and a pathogen-host integration network was constructed. Our results showed that infection induced the expression of host genes related to immune response, but attenuated the expression of bacterial virulence genes. Novel integration was found between host immune genes and bacterial virulence genes, while IL6, IL1R2, IL1B, and TLR5 played key roles in the network. Further analysis with GeneMANIA indicated that flgD and rplF might play key roles during the htpG-dependent virulence regulation, which was in accordance with the reduced biofilm production, motility and virulence in htpG-RNAi strain. Meanwhile, IL6 and IL1B were found to play key roles during the defense against P. plecoglossicida, while CELA2, TRY, CPA1, CPA2, and CPB1 were important targets for P. plecoglossicida attacking to the host.