The cyclic AMP (cAMP) phosphodiesterase CpdA required for growth, biofilm formation, motility and pathogenicity of Edwardsiella piscicida

Edwardsiella piscicida is a severe fish pathogen with wide host range, causing the huge economic losses in the aquaculture industry. Cyclic adenosine monophosphate (cAMP) as an important second messenger regulates the physiological and behavioral responses to environmental cues in eukaryotic and prokaryotic. The intracellular level of cAMP for effective activity is tightly controlled by the synthesis of adenylate cyclase, excretion and degradation of phosphodiesterase. In this study, we identified and characterized a class III cAMP phosphodiesterase, named as CpdA, in the E. piscicida. To investigate the role of CpdA in the physiology and pathogenicity, we constructed the in-frame deletion mutant of cpdA of E. piscicida, TX01ΔcpdA. The results showed that TX01ΔcpdA accumulated the higher intracellular cAMP concentration than TX01, indicating that CpdA exerted the hydrolysis of cAMP. In addition, compared to the TX01, the TX01ΔcpdA slowed growth rate, diminished biofilm formation and lost motility. More importantly, pathogenicity analysis confirmed that TX01ΔcpdA significantly impaired the ability of invading the epithelial cells, reproduction in macrophages, tissues dissemination and lethality for healthy tilapias. The most of lost properties of TX01ΔcpdA were restored partially or fully by the introduction of cpdA gene. These results suggest that cpdA is required for regulation of the physiology and virulence of E. piscicida.

Effect of polystyrene nanoplastics exposure on gene expression and pathogenesis of zoonotic pathogen, Edwardsiella piscicida

A surge in the number of anthropogenic pollutants has been caused by increasing industrial activities. Nanoplastics are spotlighted as a new aquatic pollutant that are a threat to microbes and larger organisms. Our previous study showed that the subinhibitory concentrations of aquatic pollutants such as phenol and formalin act as signaling molecules and modulate global gene expression and metabolism. In this study, we aimed to investigate the impact of a new type of anthropogenic contaminant, polystyrene (PS) nanoplastics, on the expression of key virulence factors in zoonotic pathogen Edwardsiella piscicida and the assessment of potential changes in the susceptibility of zebrafish as a model host. The TEM data indicated a noticeable change in the cell membrane indicating that PS particles were possibly entering the bacterial cells. Transcriptome analyses performed to identify the differentially expressed genes upon PS exposure revealed that the genes involved in major virulence factor type VI secretion system (T6SS) were down-regulated. However, the expression of T6SS-related genes was recovered from the PS adapted E. piscicida when nanoplastics are free. This demonstrated the hypervirulence of pathogen in infection assays with both cell lines and in vivo zebrafish model. Therefore, this study provides experimental evidence elucidating the direct regulatory impact of nanoplastics influx into aquatic ecosystems on fish pathogenic bacteria, notably influencing the expression of virulence factors.

The QseE-QseF two-component system: A key mediator of epinephrine-regulated virulence in the marine pathogen Edwardsiella piscicida

Edwardsiella piscicida is a widespread pathogen that infects various fish species and causes massive hemorrhagic septicemia, resulting in significant property damage to the global aquaculture industry. Type III and VI secretion systems (T3/T6SS), controlled by the master regulator EsrB, are important virulence factors of E. piscicida that enable bacterial colonization and evasion from host immune clearance. In this study, we demonstrate that the QseE-QseF two-component system negatively regulated esrB expression by reanalysis of Tn-seq data. Moreover, the response regulator QseF directly bound to esrB promoter and inhibited the expression of T3/T6SS genes, especially in the presence of epinephrine. Furthermore, in response to the prompt increasing of epinephrine level, the host immune genes were delayed repressed and QseE-QseF timely inhibited the expression of T3/T6SS genes to evade immune clearance. In summary, this study enhances our understanding and knowledge of the conditional pathogenesis mechanism and virulence regulation network of E. piscicida.

Inoculation of Freund's adjuvant in European eel (Anguilla anguilla) revealed key KEGG pathways and DEGs of host anti-Edwardsiella anguillarum infection

Freund's complete (FCA) and incomplete adjuvants (FIA), generally applied in subunit fishery vaccine, have not been explored on the molecular mechanism of the nonspecific immune enhancement. In this study, we examined the RNA-seq in the spleen of European eel (Anguilla anguilla) inoculated with FCA and FIA (FCIA group) to elucidate the key KEGG pathways and differential expressed genes (DEGs) in the process of Edwardsiella anguillarum infection and A. anguilla anti-E. anguillarum infection using genome-wide transcriptome. After eels were challenged by E. anguillarum at 28 d post the first inoculation (dpi), compared to the control uninfected eels (Con group), the control infected eels (Con_inf group) showed severe pathological changes in the liver, kidney and spleen, although infected eels post the inoculation of FCIA (FCIA_inf group) also formed slight bleeding. Compared to the FCIA_inf group, there was more than 10 times colony forming unit (cfu) in the Con_inf group per 100 μg spleen, kidney or blood, and the relative percent survival (RPS) of eels was 44.4% in FCIA_inf vs Con_inf. Compared to the Con group, the SOD activity in the FCIA group increased significantly in the liver and spleen. Using high-throughput transcriptomics, DEGs were identified and 29 genes were verified using fluorescence real-time polymerase chain reaction (qRT-PCR). The result of DEGs clustering showed 9 samples in 3 groups of Con, FCIA and FCIA_inf were similar, contrast to distinct differences of 3 samples in the Con_inf group. We found 3795 up and 3548 down regulated DEGs in the compare of FCIA_inf vs Con_inf, of which 5 enriched KEGG pathways of "Lysosome", "Autophagy", "Apoptosis", "C-type lectin receptor signaling" and "Insulin signaling" were ascertained, and 26 of 30 top GO terms in the compare were significantly enriched. Finally, protein-protein interactions between the DEGs of the 5 KEGG pathways and other DEGs were explored using Cytoscape 3.9.1. The compare of FCIA_inf vs Con_inf showed 110 DEGs from the 5 pathways and 718 DEGs from other pathways formed total of 9747° in a network, of which 9 hub DEGs play vital roles in anti-infection or apoptosis. Together, the interaction networks revealed that 9 DEGs involved in the 5 pathways underlies the key process of A. anguilla anti-E. anguillarum infection or host cell apoptosis.

Comparative genomics of Edwardsiella anguillarum and Edwardsiella piscicida isolated in Taiwan enables the identification of distinctive features and potential virulence factors using Oxford-Nanopore MinION® sequencing

Edwardsiella tarda (ET) and Edwardsiella anguillarum (EA) are the most harmful bacterial fish pathogens in Taiwan. However, there is confusion regarding the genotypic identification of E. tarda and E. piscicida (EP). Therefore, we used a novel Nanopore MinION MK1C platform to sequence and compare the complete genomes of E. piscicida and E. anguillarum. The number of coding genes, rRNA, and tRNA recorded for E. anguillarum and E. piscicida were 8322, 25, and 98, and 5458, 25, and 98, respectively. Ribosomal multilocus sequence typing (rMLST) for E. piscicida indicated 35 rps. The shared clusters between E. anguillarum and E. piscicida indicated several unique clusters for the individual genomes. The phylogenetic tree analysis for all complete genomes indicated that E. anguillarum and E. piscicida were placed into two species-specific genotypes. Distribution of subsystems for annotated genomes found that genes related to virulence, defence, and disease for E. anguillarum were 103 and those for E. piscicida were 60 and pathogenic islands (PI) were 498 and 225, respectively. Vaccine candidates were identified in silico from the core genes using high antigenic, solubility, and secretion probabilities. Altogether, the genome data revealed distinctive features between E. anguillarum and E. piscicida, which suggest different pathogenicity and thus the need for separate preventive strategies.

Evaluation of Edwardsiella piscicida basS and basR mutants as vaccine candidates in catfish against edwardsiellosis

Catfish farming is the largest aquaculture industry in the United States and an important economic driver in several southeastern states. Edwardsiella piscicida is a Gram-negative pathogen associated with significant losses in catfish aquaculture. Several Gram-negative bacteria use the BasS/BasR two-component system (TCS) to adapt to environmental changes and the host immune system. Currently, the role of BasS/BasR system in E. piscicida virulence has not been characterized. In the present study, two mutants were constructed by deleting the basS and basR genes in E. piscicida strain C07-087. Both mutant strains were characterized for virulence and immune protection in catfish hosts. The EpΔbasS and EpΔbasR mutants were more sensitive to acidic environments and produced significantly less biofilm than the wild-type. In vivo studies in channel catfish (Ictalurus punctatus) revealed that both EpΔbasS and EpΔbasR were significantly attenuated compared with the parental wild-type (3.57% and 4.17% vs. 49.16% mortalities). Moreover, there was significant protection, 95.2% and 92.3% relative percent survival (RPS), in channel catfish vaccinated with EpΔbasS and EpΔbasR against E. piscicida infection. Protection in channel catfish was associated with a significantly higher level of antibodies and upregulation of immune-related genes (IgM, IL-8 and CD8-α) in channel catfish vaccinated with EpΔbasS and EpΔbasR strains compared with non-vaccinated fish. Hybrid catfish (channel catfish ♀ × blue catfish ♂) challenges demonstrated long-term protection against subsequent challenges with E. piscicida and E. ictaluri. Our findings demonstrate BasS and BasR contribute to acid tolerance and biofilm formation, which may facilitate E. piscicida survival in harsh environments. Further, our results show that EpΔbasS and EpΔbasR mutants were safe and protective in channel catfish fingerlings, although their virulence and efficacy in hybrid catfish warrant further investigation. These data provide information regarding an important mechanism of E. piscicida virulence, and it suggests EpΔbasS and EpΔbasR strains have potential as vaccines against this emergent catfish pathogen.

Pathology and virulence of Edwardsiella tarda, Edwardsiella piscicida, and Edwardsiella anguillarum in channel (Ictalurus punctatus), blue (Ictalurus furcatus), and channel × blue hybrid catfish

In the mid-2010s, Edwardsiella tarda was reaffiliated into three discrete taxa (E. anguillarum, E. piscicida, and E. tarda), obscuring previous descriptions of E. tarda-induced pathology in fish. To clarify ambiguity regarding the pathology of E. tarda, E. piscicida, and E. anguillarum infections in US farm-raised catfish, channel catfish (Ictalurus punctatus), blue catfish (I. furcatus), and channel × blue catfish hybrids were challenged with comparable doses of each bacterium. The most severe pathology and mortality occurred in fish challenged with E. piscicida, supporting previous reports of increased pathogenicity in commercially important ictalurids, while E. anguillarum and E. tarda warrant only minimal concern. Acute pathologic lesions among bacterial species were predominantly necrotizing and characteristic of gram-negative sepsis but became progressively granulomatous over time. After 100 days, survivors were exposed to the approximate median lethal doses of E. piscicida and E. ictaluri, revealing some cross-protective effects among E. piscicida, E. anguillarum, and E. ictaluri. In contrast, no fish that survived E. tarda challenge demonstrated any protection against E. piscicida or E. ictaluri. This work supports reports of increased susceptibility of channel, blue, and hybrid catfish to E. piscicida, while highlighting potential cross-protective affects among fish associated Edwardsiella spp.

Comprehensive Relationship Analysis of the Long Noncoding RNAs (lncRNAs) and the Target mRNAs in Response to the Infection of Edwardsiella anguillarum in European eel (Anguilla anguilla) Inoculated with Freund's Adjuvant

Freund's complete adjuvant (FCA) and incomplete adjuvant (FIA), generally applied in subunit fishery vaccine, have not been explored on the molecular mechanism of the non-specific immune enhancement. As long noncoding RNAs (lncRNAs) play vital regulating roles in various biological activities, in this study, we examined the genome-wide expression of transcripts in the liver of European eel (Anguilla anguilla, Aa) inoculated with FCA and FIA (FCIA) to elucidate the regulators of lncRNAs in the process of Edwardsiella anguillarum (Ea) infection and Aa anti-Ea infection using strand-specific RNA-seq. After eels were challenged by Ea at 28 days post the first inoculation (dpi), compared to the control uninfected eels (Li group), the control infected eels (Con_Li group) showed severe bleeding, hepatocyte atrophy, and thrombi formed in the hepatic vessels of the liver, although eels inoculated with FCIA (FCIA_Li group) also formed slight thrombi in the hepatic vessels. Compared to the FCIA_Li group, there was about 10 times colony-forming unit (cfu) in the Con_Li group per 100 μg liver tissue, and the relative percent survival (RPS) of eels was 50% in FCIA_Li vs Con_Li. Using high-throughput transcriptomics, differential expressed genes (DEGs) and transcripts were identified and the results were verified using fluorescence real-time polymerase chain reaction (qRT-PCR). Interactions between the differential expressed lncRNAs (DE-lncRNAs) and the target DEGs were explored using Cytoscape according to their co-expression and co-location relationship. We found 13,499 lncRNAs (10,176 annotated and 3423 novel lncRNAs) between 3 comparisons of Con_Li vs Li, FCIA_Li vs Li, and FCIA_Li vs Con_Li, of which 111, 110, and 129 DE-lncRNAs were ascertained. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DEGs targeted by DE-lncRNAs revealed these DEGs mainly involved in single-organism cellular process in BP, membrane in CC and binding in MF, and KEGG pathways showed that the target DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 118 DE-lncRNAs target 1161 DEGs were involved in an interaction network of 8474 co-expression and 333 co-location-related links, of which 16 DE-lncRNAs play vital roles in anti-Ea infection. Taken together, the interaction networks revealed that DE-lncRNAs underlies the process of Ea infection and Aa anti-Ea infection.

Environmentally relevant concentrations of triclosan exposure promote the horizontal transfer of antibiotic resistance genes mediated by Edwardsiella piscicida

Aquaculture pathogen and antibiotic resistance genes (ARGs) co-occur in the aquatic environment. Accumulated evidence suggests that aquaculture pathogens can facilitate the horizontal transfer of plasmid-mediated ARGs. However, the role of Edwardsiella piscicida (E. piscicida) in ARG dissemination is still not fully understood. In addition, the potential impact of triclosan (TCS) on the spread of ARGs mediated by E. piscicida is still unknown, so a mating model system was established to investigate the transfer process of ARGs. The results showed that E. piscicida disseminated ARGs on RP4 by horizontal gene transfer (HGT). Furthermore, TCS exposure promoted this process. The conjugative transfer frequencies were enhanced approximately 1.2-1.4-fold by TCS at concentrations from 2 to 20 μg/L, when compared with the control. TCS promoted the HGT of ARGs by stimulating reactive oxygen species (ROS) production, increasing cell membrane permeability, and altering expressions of conjugative transfer-associated genes. Together, the results suggested that aquaculture pathogens spread ARGs and that the emerging contaminant TCS enhanced the transfer of ARGs between bacteria.

Cross-protective efficacy of a live-attenuated Edwardsiella ictaluri vaccine against heterologous Edwardsiella piscicida isolates in channel and channel × blue catfish hybrids

Edwardsiella piscicida is a growing problem for catfish aquaculture in the southeastern United States, particularly in channel (Ictalurus punctatus) x blue (I. furcatus) catfish hybrids. Research has shown E. piscicida isolates recovered from farmed catfish in Mississippi form at least five discrete phyletic groups, with no apparent differences in virulence in channel and hybrid catfish. Laboratory trials have shown a live-attenuated E. ictaluri vaccine (340X2) cross-protects against at least one E. piscicida isolate (S11-285) in channel and hybrid catfish, although it is unknown if this protection exists for other E. piscicida variants. To this end, channel and hybrid catfish were immunized by immersion with E. ictaluri 340X2. Thirty days later, fish were challenged by intracoelomic injection with representative E. piscicida variants from each phyletic group. Relative percent survival (RPS) for hybrids ranged from 54.7% to 77.8%, while RPS in channels ranged from 80.5% to 100%. A second study investigated whether channel and hybrid catfish exposed to heterologous E. piscicida isolates were similarly protected against wild-type E. ictaluri. Fish were exposed by bath immersion to representative E. piscicida isolates from each phyletic group. Thirty days post-immunization, fish were challenged by immersion with wild-type E. ictaluri isolate S97-773. Regardless of variant, previous exposure to heterologous E. piscicida isolates significantly improved survival following E. ictaluri challenge. These findings suggest the presence of shared and conserved antigens among E. piscicida and E. ictaluri that could be exploited by application of polyvalent or cross-protective vaccines.

Neu1 deficiency increases the susceptibility of zebrafish to Edwardsiella piscicida infection via lysosomal dysfunction

Neu1 is a lysosomal glycosidase that catalyzes the removal of sialic acids from glycoconjugates. Although Neu1 sialidase is highly conserved among vertebrates, the role of fish Neu1 is not fully understood because of its unique aquatic living situation. Compared to land animals, fish have a higher chance of bacterial infection, and to understand the role of fish Neu1, the susceptibility of Neu1 knockout zebrafish (Neu1-KO) was evaluated using Edwardsiella piscicida, a fish pathogen. Neu1-KO larvae showed high susceptibility to E. piscicida, despite the activation of macrophages, and presented increased lysosomal signals induced by the accumulation of Sia α2-3 linked oligosaccharides. The accumulation coincided with the signal of the macrophage marker, suggesting that the dysfunction of lysosomes in macrophages would result in a high susceptibility of Neu1-KO to E. piscicida. Chloroquine, an inhibitor of lysosomal degradation, induced high mortality of wild type zebrafish with E. piscicida infection accompanied by increased lysosomal accumulation, similar to Neu1-KO zebrafish. This study revealed that Neu1 sialidase plays a crucial role in the lysosomal degradation of macrophages with a bacterial infection.

FabR senses long-chain unsaturated fatty acids to control virulence in pathogen Edwardsiella piscicida

Long-chain unsaturated fatty acids (UFAs) can serve as nutrient sources or building blocks for bacterial membranes. However, little is known about how UFAs may be incorporated into the virulence programs of pathogens. A previous investigation identified FabR as a positive regulator of virulence gene expression in Edwardsiella piscicida. Here, chromatin immunoprecipitation-sequencing coupled with RNA-seq analyses revealed that 10 genes were under the direct control of FabR, including fabA, fabB, and cfa, which modulate the composition of UFAs. The binding of FabR to its target DNA was facilitated by oleoyl-CoA and inhibited by stearoyl-CoA. In addition, analyses of enzyme mobility shift assay and DNase I footprinting with wild-type and a null mutant (F131A) of FabR demonstrated crucial roles of FabR in binding to the promoters of fabA, fabB, and cfa. Moreover, FabR also binds to the promoter region of the virulence regulator esrB for its activation, facilitating the expression of the type III secretion system (T3SS) in response to UFAs. Furthermore, FabR coordinated with RpoS to modulate the expression of T3SS. Collectively, our results elucidate the molecular machinery of FabR regulating bacterial fatty acid composition and virulence in enteric pathogens, further expanding our knowledge of its crucial role in host-pathogen interactions.

Dysregulation of Cytosolic c-di-GMP in Edwardsiella piscicida Promotes Cellular Non-Canonical Ferroptosis

Programmed cell death plays an important role in modulating host immune defense and pathogen infection. Ferroptosis is a type of inflammatory cell death induced by intracellular iron-dependent accumulation of toxic lipid peroxides. Although ferroptosis has been associated with cancer and other sterile diseases, very little is known about the role of ferroptosis in modulating host-pathogen interactions. We show that accumulation of the secondary messenger bis-(3′,5′)-cyclic dimeric GMP (c-di-GMP) in the pathogenic bacterium Edwardsiella piscicida (E. piscicida) triggers a non-canonical ferroptosis pathway in infected HeLa cells. Moreover, we observed that the dysregulation of c-di-GMP in E. piscicida promotes iron accumulation, mitochondrial dysfunction, and production of reactive oxygen species, all of which that can be blocked by iron chelator. Importantly, unlike classical ferroptosis that is executed via excess lipid peroxidation, no lipid peroxidation was detected in the infected cells. Furthermore, lipoxygenases inhibitors and lipophilic antioxidants are not able to suppress morphological changes and cell death induced by E. piscicida mutant producing excess c-di-GMP, and this c-di-GMP dysregulation attenuates bacterial virulence in vivo. Collectively, our results reveal a novel non-canonical ferroptosis pathway mediated by bacterial c-di-GMP and provide evidence for a role of ferroptosis in the regulation of pathogen infection.

Transcriptomic analysis using dual RNA sequencing revealed a Pathogen-Host interaction after Edwardsiella anguillarum infection in European eel (Anguilla anguilla)

Many studies have explored differentially expressed genes (DEGs) between some pathogens and hosts, but no study has focused on the interaction of DEGs between Edwardsiella anguillarum (Ea) and Anguilla anguilla (Aa). In this study, we examined the interactions of DEGs during Ea infection and Aa anti-infection processes by dual RNA sequencing. Total RNA from in vitro and in vivo (Aa liver) Ea culture was extracted. Using high-throughput transcriptomics, significant DEGs that were expressed between Ea cultured in vitro versus in vivo and those in the liver of the infected group versus control group were identified. Protein-protein interactions between the pathogen and host were explored using Cytoscape according to the HPIDB 3.0 interaction transcription database. The results showed that the liver in the infection group presented with severe bleeding and a large number of thrombi in the hepatic vessels. We found 490 upregulated and 398 downregulated DEGs of Ea in vivo versus Ea cultured in vitro, and 2177 upregulated and 970 downregulated genes in the liver of the infected eels. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of the pathogen DEGs revealed that the upregulated genes were mainly enriched in migration, colonization, biofilm formation, and significantly enriched in ABC transport and quorum sensing; the downregulated genes were mainly involved in metabolism, information transduction, organelle formation, enzyme catalysis, molecular transport, and binding. GO of the host DEGs showed that metabolic process, catalytic activity, single organism metabolic process, small molecule binding, nucleotide binding, nucleotide phosphate binding, and anion binding were markedly enriched. Finally, we found that 79 Ea and 148 Aa proteins encoded by these DEGs were involved in an interaction network, and some pathogen (DegP, gcvP, infC, carB, rpoC, trpD, sthA, and FhuB) and host proteins (MANBA, STAT1, ETS2, ZEP1, TKT1, NMI and RBPMS) appear to play crucial roles in infection. Thus, determining the interaction networks revealed crucial molecular mechanisms underlying the process of pathogenic infection and host anti-infection.

Transcriptome RNA-seq revealed lncRNAs activated by Edwardsiella anguillarum post the immunization of OmpA protecting European eel (Anguilla anguilla) from being infected

Long noncoding RNAs (lncRNAs) play important roles in various biological activities as vital regulators. However, no study has focused on the lncRNA regulation of Outer membrane protein (OMP) immunization against aquatic bacterial infection. In this study, we examined the genome-wide expression of lncRNAs in the liver of European eel (Anguilla anguilla, Aa) administrated by a recombinant OmpA (rOmpA) from Edwardsiella anguillarum (Ea) to elucidate the functions of lncRNAs in the process of Ea infection and Aa anti-Ea infection using strand specific RNA-seq. Eels were challenged by Ea at 28 d post the immunization (dpi) of OmpA, and the result showed, compared to uninfected livers in the PBS group (Con group), the infected livers in the PBS group (Con_inf group) showed severe bleeding, hepatocyte atrophy and thrombi formed in the hepatic vessels; livers in the OmpA group (OmpA_inf) also formed slight thrombi in the hepatic vessels. The relative percent survival of eels in OmpA_inf vs Con_inf was 78.6%. Using high-throughput transcriptomics, we found 13405 lncRNAs in 3 compares of Con_inf vs Con, OmpA_inf vs Con and OmpA_inf vs Con_inf, of which 111, 129 and 158 DE-lncRNAs were ascertained. GO analysis of the DE-lncRNAs revealed the targeting DEGs were mainly involved in single-organism process, signaling, biological process and response to stimulus in BP, component of membrane in CC and binding in MF; KEGG pathways showed that the targeting DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 54 DE-lncRNAs targeting 1675 DEGs were involved in an interaction network of 21692 co-expression and 483 co-location related links, of which 18 DE-lncRNAs appear to play crucial roles in anti-Ea infection. Thus, the interaction networks revealed crucial DE-lncRNAs underlying the process of Ea infection and Aa anti-Ea infection pre and post the immunization of OmpA.

Effectiveness of Cathelicidin Antimicrobial Peptide against Ictalurid Catfish Bacterial Pathogens

One of the major goals in aquaculture is to protect fish against infectious diseases as disease outbreaks could lead to economic losses if not controlled. Antimicrobial peptides (AMPs), a class of highly conserved peptides known to possess direct antimicrobial activities against invading pathogens, were evaluated for their ability to protect Channel Catfish Ictalurus punctatus and hybrid catfish (female Channel Catfish × male Blue Catfish I. furcatus) against infection caused by the fish pathogen Aeromonas hydrophila ML09-119. To identify effective peptides, the minimum inhibitory concentrations against bacterial pathogens Edwardsiella ictaluri S97-773, Edwardsiella piscicida E22-10, A. hydrophila ML09-119, Aeromonas veronii 03X03876, and Flavobacterium columnare GL-001 were determined in vitro. In general and overall, cathelicidins derived from alligator and sea snake exhibited more potent and rapid antimicrobial activities against the tested catfish pathogens as compared to cecropin and pleurocidin AMPs and ampicillin, the antibiotic control. When the peptides (2.5 µg of peptide/g of fish) were injected into fish and simultaneously challenged with A. hydrophila through immersion, increased survival rates in Channel Catfish and hybrid catfish were observed in both cathelicidin (alligator and sea snake) treatments as compared to other peptides and the infected control (P < 0.001) with alligator cathelicidin being the overall best treatment. Bacterial numbers in the kidney and liver of Channel Catfish and hybrid catfish also decreased (P < 0.05) for cathelicidin-injected groups at 24 and 48 h after challenge infection. These results show the potential of cathelicidin to protect catfish against bacterial infections and suggest that an approach overexpressing the peptide in transgenic fish, which is the long-term goal of this research program, may provide a method of decreasing bacterial disease problems in catfish as delivering the peptides via individual injection or feeding would not be economically feasible.

Immunization of a novel bivalent outer membrane protein simultaneously resisting Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus infection in European eels (Angullia angullia)

In cultivated European eels, Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus are three important bacterial pathogens. In this study, European eels (Anguilla anguilla) were immunized by the bivalent expression products of the outer membrane protein (Omp) gene from A. hydrophila (OmpⅡ) and E. anguillarum (OmpA), and the effects of the bivalent protein (rOmpⅡ-A) on the immune function of the European eel were detected. Three hundred eels were divided average into three groups of PBS, adjuvant and rOmp. Eels of three goups were injected intraperitoneal with 0.2 mL of PBS (0.01 mol/L, pH7.4), PBS + F (PBS mixed equal volume of freund's uncomplete adjuvant) or rOmpⅡ-A (1 mg mL^-1 rOmpⅡ-A mixed equal volume of freund's uncomplete adjuvant). Four immune-related genes expression, proliferation of whole blood cells, serum and skin mucus antibody titer, superoxide dismutase (SOD) activity and the relative percent of survival (RPS) were studied at different days (or hours) post the immunization. The results showed that the igm, lysC, mhc2 and sod gene in the liver, spleen, kidney and intestine tract were significant increased in the Omp group; On the 28 day post the immunization (dpi), blood cell proliferation was increased in the Omp group, and on the 14, 21, 28 and 42 dpi, antibody titers in serum and mucus of the Omp group were significantly higher than that of the PBS and adjuvant group, regardless of coating with bacteria or Omp antigen. The SOD activity of Omp group increased significantly in liver, kidney, skin mucus and serum from 14 to 42 dpi, especially in serum. Eels chanllenged by A. hydrophila, E. anguillarum and V. vulnificus in the bivalent Omp group showed the RPS were 83.33%, 55.56% and 44.44%, respectively. The results of this study showed that immunization of the bivalent Omp could effectively improve the immune function of European eels, and produced effectively protection to A. hydrophila and E. anguillarum infection. Simultaneously, the bivalent Omp also produced distinct cross-protection to the eels challenged by V. vulnificus.

Edwardsiella piscicida: a significant bacterial pathogen of cultured fish

Edwardsiella piscicida, a Gram-negative, facultative aerobic pathogen belonging to the Enterobacteriaceae family, is the etiological agent of edwardsiellosis in fish and a significant problem in global aquaculture. E. piscicida has been reported from a broad geographical range and has been isolated from more than 20 fish host species to date, but this is likely to be an underestimation, because misidentification of E. piscicida as other species within the genus remains to be resolved. Common clinical signs associated with edwardsiellosis include, but are not limited to, exophthalmia, haemorrhages of the skin and in several internal organs, mild to moderate dermal ulcerations, abdominal distension, discoloration in the fish surface, and erratic swimming. Many antibiotics are currently effective against E. piscicida, although legal restrictions and the cost of medicated feeds have encouraged significant research investment in vaccination for the management of edwardsiellosis in commercial aquaculture. Here we summarise the current understanding of E. piscicida and highlight the difficulties with species assignment and the need for further research on epidemiology and strain variability.