Construction and evaluation of an Edwardsiella ictaluri fhuC mutant

Trans-Cinnamaldehyde Primes More Robust Channel Catfish Immune Responses to Edwardsiella ictaluri Infection

Infection with Edwardsiella ictaluri, a causative agent of enteric septicemia of catfish, threatens profitable catfish production through inventory losses. We previously demonstrated that trans-cinnamaldehyde (TC) enhances the survival of catfish following E. ictaluri infection. The present study was conducted to investigate catfish immune responses to TC feeding and E. ictaluri infection. The expression of 13 proinflammatory, innate, and adaptive immune-related genes was evaluated over time in two sets of experiments using real-time polymerase chain reaction (PCR). In the first experiment, catfish were fed a basal diet with or without TC supplementation, while in the second they were fed a TC-supplemented or normal diet followed by infection with E. ictaluri. The catfish group infected with E. ictaluri and fed a TC-diet showed significant changes in the expression of innate and adaptive immune-related genes compared to control group. At 21 and 28 days post-infection, recovered fish showed significant increases in the expression of IgM in the anterior kidney and spleen. These results suggest that the supplemental dietary intake of TC can improve the immune status of catfish via engaging innate and adaptive immune responses and the production of memory cells in immunocompetent tissues. Together, this study provides an important foundation for the potential application of TC as an antimicrobial alternative in aquaculture.

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.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Virulence and live vaccine potential of Edwardsiella piscicida phoP and phoQ mutants in catfish against edwardsiellosis

Edwardsiella piscicida is a Gram-negative facultative intracellular bacterium causing edwardsiellosis in catfish, the largest aquaculture industry in the United States. A safe and effective vaccine is an urgent need to avoid economic losses associated with E. piscicida outbreaks. PhoP/PhoQ is a two-component signal transduction system (TCS) that plays an important role in bacterial pathogenesis through sense and response to environmental and host stress signals. This study aimed to explore the contribution of PhoQ/PhoP in E. piscicida virulence and develop live attenuated vaccines against E. piscicida infection in channel catfish (Ictalurus punctatus) and hybrid catfish (channel catfish ♀ × blue catfish (I. furcatus) ♂). In the current study, two in-frame deletion mutants were constructed by deleting phoP (ETAC_09785) and phoQ (ETAC_09790) genes in E. piscicida strain C07-087, and the virulence and protection efficacy of the constructed strains were evaluated in catfish following intraperitoneal injection. Both EpΔphoP and EpΔphoQ strains had a delayed adaptation to oxidative stress (0.2% H₂ O₂ ) compared to E. piscicida wild type. The EpΔphoP and EpΔphoQ mutants produced significantly less biofilm compared to wild-type E. piscicida. Notably, EpΔphoP and EpΔphoQ mutants were significantly attenuated in channel catfish compared with wild-type E. piscicida (6.63% and 4.17% versus 49.16% mortalities), and channel catfish vaccinated with EpΔphoP and EpΔphoQ were significantly protected (95.65% and 97.92% survival) against E. piscicida infection at 21 days post-vaccination. In hybrid catfish, EpΔphoP was significantly more attenuated than EpΔphoQ, but EpΔphoQ provided significantly better protection than EpΔphoP. EpΔphoP and EpΔphoQ strains both induced specific antibodies in channel catfish against E. piscicida at 14 and 21 days post-vaccination. This result indicated that EpΔphoP and EpΔphoQ mutants were safe and protective in channel catfish fingerlings, while EpΔphoP was safe in hybrid catfish. Our findings show that PhoP and PhoQ are required for adaptation to oxidative stress and biofilm formation and may help E. piscicida face tough environmental challenges; thus, functional PhoP and PhoQ are critical for a successful infection.

Adaptive immune responses in channel catfish exposed to Edwardsiella ictaluri live attenuated vaccine and wild type strains through the specific gene expression profiles

We extend the previous findings on the differential activity of immune-related genes in the lymphoid organs of channel catfish in the 7 days post-challenge (dpc) with E. ictaluri live attenuated vaccines (LAVs) and wild type (WT) strains by assessing the expression of these genes in the 21 dpc. The expression of T and B cell-specific genes were significantly elevated in the spleen at 14 dpc and in the AK at 21 dpc in catfish treated with E. ictaluri WT and LAV strains compared to a non-treated control group. The gene expression of IFN-γ correlated with adaptive immunity genes in the lymphoid tissues of catfish. These data indicate that two novel LAVs were able to trigger the activation of T helper1 polarization cytokine IFN-γ gene and specific lymphocyte genes in the spleen followed by their activation in the AK of catfish without causing inflammation, thus providing protective immunity in E. ictaluri infection.

Live attenuated Edwardsiella ictaluri vaccines enhance the protective innate immune responses of channel catfish B cells

Edwardsiella ictaluri causes enteric septicemia of catfish. Our group developed two E. ictaluri live attenuated vaccines (LAVs). However, their effects on the innate functions of catfish B cells are still unexplored. We evaluated phagocytosis and killing of wild-type (WT) E. ictaluri opsonized with sera from vaccinated fish and the survival of B cells exposed to E. ictaluri strains. We assessed phagocytosis of the opsonized WT at 30 °C and 4 °C. B cells killed the internalized E. ictaluri opsonized with sera from vaccinated fish with LAVs more efficiently than other groups at 30 °C. However, catfish B cells were unable to destroy E. ictaluri at 4 °C. Furthermore, E. ictaluri opsonized with serum from fish exposed to WT induce apoptosis and decreased live B cells numbers. Results indicate that opsonization of E. ictaluri with sera from vaccinated fish enhanced phagocytosis and killing activity in B cells and inhibited apoptotic changes in the infected B cells.

An Edwardsiella piscicida esaS mutant reveals contribution to virulence and vaccine potential

Edwardsiella piscicida is a Gram-negative pathogen that causes disease in diverse aquatic organisms. The disease leads to extensive losses in commercial aquaculture species, including farmed U.S. catfish. The type III secretion system (T3SS) often contributes to virulence of Gram-negative bacteria. The E. piscicida esaS gene encodes a predicted T3SS export apparatus protein. In the current study, an E. piscicida esaS mutant was constructed and characterized to increase our understanding of the role of T3SS in E. piscicida virulence. Deletion of esaS did not significantly affect biofilm formation and hemolytic activity of E. piscicida, but it had significant effects on expression of hemolysis and T3SS effector genes during biofilm growth. EpΔesaS showed significantly (P < 0.05) reduced virulence in catfish compared to the parent strain. No mortalities occurred in fish infected with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU/fish compared to 26% mortality in fish infected with wild-type E. piscicida at 7.5 × 10⁵ CFU/fish. Bioluminescence imaging indicated that EpΔesaS invades catfish and colonizes for a short period in the organs. Furthermore, catfish immunized with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU provided 47% and 87% relative percent survival, respectively. These findings demonstrated that esaS plays a role in E. piscicida virulence, and the deletion mutant has vaccine potential for protection against wild-type E. piscicida infection.

Transposon mutagenesis and identification of mutated genes in growth-delayed Edwardsiella ictaluri

Background

Edwardsiella ictaluri is a Gram-negative facultative intracellular anaerobe and the etiologic agent of enteric septicemia of channel catfish (ESC). To the catfish industry, ESC is a devastating disease due to production losses and treatment costs. Identification of virulence mechanisms of E. ictaluri is critical to developing novel therapeutic approaches for the disease. Here, we report construction of a transposon insertion library and identification of mutated genes in growth-delayed E. ictaluri colonies. We also provide safety and efficacy of transposon insertion mutants in catfish.

Results

An E. ictaluri transposon insertion library with 45,000 transposants and saturating 30.92% of the TA locations present in the E. ictaluri genome was constructed. Transposon end mapping of 250 growth-delayed E. ictaluri colonies and bioinformatic analysis of sequences revealed 56 unique E. ictaluri genes interrupted by the MAR2xT7 transposon, which are involved in metabolic and cellular processes and mostly localized in the cytoplasm or cytoplasmic membrane. Of the 56 genes, 30 were associated with bacterial virulence. Safety and vaccine efficacy testing of 19 mutants showed that mutants containing transposon insertions in hypothetical protein (Eis::004), and Fe-S cluster assembly protein (IscX, Eis::039), sulfurtransferase (TusA, Eis::158), and universal stress protein A (UspA, Eis::194) were safe and provided significant protection (p < 0.05) against wild-type E. ictaluri.

Conclusions

The results indicate that random transposon mutagenesis causing growth-delayed phenotype results in identification bacterial virulence genes, and attenuated strains with transposon interrupted virulence genes could be used as vaccine to activate fish immune system.

Assessment of the Live Attenuated and Wild-Type Edwardsiella ictaluri-Induced Immune Gene Expression and Langerhans-Like Cell Profiles in the Immune-Related Organs of Catfish

Edwardsiella ictaluri is a Gram-negative intracellular pathogen that causes enteric septicemia of catfish (ESC). Successful vaccination against intracellular pathogens requires T cell priming by antigen presenting cells (APCs) that bridge innate and adaptive immunity. However, the evidence on immunological mechanisms that underscore E. ictaluri pathogenesis and the protective role of live attenuated vaccines (LAVs) is scarce. We assessed the expression of immune genes related to antigen presentation by real-time PCR and the distribution patterns of Langerhans-like (L/CD207⁺) cells by immunohistochemistry in the immune-related tissues of channel catfish challenged with two novel E. ictaluri LAVs, EiΔevpB, and ESC-NDKL1 and wild type (WT) strain. Our results indicated significantly elevated expression of IFN-γ gene in the anterior kidney (AK) and spleen of vaccinated catfish at the early stages of exposure, which correlated with increased numbers of L/CD207⁺ cells. In general, the ESC-NDKL1-induced IFN-γ gene expression patterns in the AK resembled that of the patterns induced by EiΔevpB. However the MHCII gene expression patterns differed between the strains with significant increases at 6 h post-challenge (pc) with the EiΔevpB and at 7 d pc with the ESC-NDKL1 strains, respectively. Significant increases in activity of T helper type polarization genes such as IFN-γ and T cell co-receptors after exposure to ESC-NDKL1, in combination with elevated numbers of L/CD207⁺ cells at 7 d pc with both LAVs compared to uninfected and the WT-exposed counterparts, were documented in the spleen. The dominant pro-inflammatory environment with dramatically overexpressed inflammatory genes in the AK and 7 d pc in the spleen in response to E. ictaluri was found in exposed catfish. In general, the pro-inflammatory gene expression profiles in the ESC-NDKL1 pc showed more similarities to the WT strain-induced gene profiles compared to the EiΔevpB counterpart. In addition, E. ictaluri WT significantly decreased the numbers of Langerhans-like L/CD207⁺ cells in the AK and spleen at 3 and 7 days pc. In conclusion, we report the differential framework of initiation of innate and adaptive immune responses between E. ictaluri strains with both LAVs having a potential of satisfying the stringent requirements for successful vaccines.

Comparative Genomics of Aeromonas hydrophila Secretion Systems and Mutational Analysis of hcp1 and vgrG1 Genes From T6SS

Virulent Aeromonas hydrophila causes severe motile Aeromonas septicemia in warmwater fishes. In recent years, channel catfish farming in the U.S.A. and carp farming in China have been affected by virulent A. hydrophila, and genome comparisons revealed that these virulent A. hydrophila strains belong to the same clonal group. Bacterial secretion systems are often important virulence factors; in the current study, we investigated whether secretion systems contribute to the virulent phenotype of these strains. Thus, we conducted comparative secretion system analysis using 55 A. hydrophila genomes, including virulent A. hydrophila strains from U.S.A. and China. Interestingly, tight adherence (TaD) system is consistently encoded in all the vAh strains. The majority of U.S.A. isolates do not possess a complete type VI secretion system, but three core elements [tssD (hcp), tssH, and tssI (vgrG)] are encoded. On the other hand, Chinese isolates have a complete type VI secretion system operon. None of the virulent A. hydrophila isolates have a type III secretion system. Deletion of two genes encoding type VI secretion system proteins (hcp1 and vgrG1) from virulent A. hydrophila isolate ML09-119 reduced virulence 2.24-fold in catfish fingerlings compared to the parent strain ML09-119. By determining the distribution of genes encoding secretion systems in A. hydrophila strains, our study clarifies which systems may contribute to core A. hydrophila functions and which may contribute to more specialized adaptations such as virulence. Our study also clarifies the role of type VI secretion system in A. hydrophila virulence.

Development and Characterization of a Novel Live Attenuated Vaccine Against Enteric Septicemia of Catfish

Edwardsiella ictaluri is a Gram-negative intracellular pathogen causing enteric septicemia of channel catfish (ESC). Type six secretion system (T6SS) is a sophisticated nanomachine that delivers effector proteins into eukaryotic host cells as well as other bacteria. In the current work, we in-frame deleted the E. ictalurievpB gene located in the T6SS operon by allelic exchange. The safety and efficacy of EiΔevpB as well as Aquavac-ESC, a commercial vaccine manufactured by Intervet/Merck Animal Health, were evaluated in channel catfish (Ictalurus punctatus) fingerlings and fry by immersion exposure. Our results showed that the EiΔevpB strain was avirulent and fully protective in catfish fingerlings. The EiΔevpB strain was also safe in catfish fry, and immersion vaccination with EiΔevpB at doses 10⁶ and 10⁷ CFU/ml in water resulted in 34.24 and 80.34% survival after wild-type immersion challenge compared to sham-vaccinated fry (1.79% survival). Catfish fry vaccinated with EiΔevpB at doses 10⁶, 10⁷, and 10⁸ CFU/ml in water exhibited dose-dependent protection. When compared with Aquavac-ESC, EiΔevpB provided significantly higher protection in catfish fingerlings and fry (p < 0.05). Results indicate that the EiΔevpB strain is safe and can be used to protect catfish fingerlings and fry against E. ictaluri.

Construction and evaluation of type III secretion system mutants of the catfish pathogen Edwardsiella piscicida

Catfish is the largest aquaculture industry in the United States. Edwardsiellosis is considered one of the most significant problems affecting this industry. Edwardsiella piscicida is a newly described species within the genus Edwardsiella, and it was previously classified as Edwardsiella tarda. It causes gastrointestinal septicaemia, primarily in summer months, in farmed channel catfish in the south-eastern United States. In the current study, we adapted gene deletion methods used for Edwardsiella to E. piscicida strain C07-087, which was isolated from a disease outbreak in a catfish production pond. Four genes encoding structural proteins in the type III secretion system (T3SS) apparatus of E. piscicida were deleted by homologous recombination and allelic exchange to produce in-frame deletion mutants (EpΔssaV, EpΔesaM, EpΔyscR and EpΔescT). The mutants were phenotypically characterized, and virulence and vaccine efficacy were evaluated. Three of the mutants, EpΔssaV, EpΔyscR and EpΔesaM, were significantly attenuated compared to the parent strain (p < .05), but EpΔescT strain was not. Vaccination of catfish with the four mutant strains (EpΔssaV, EpΔesaM, EpΔyscR and EpΔescT) provided significant protection when subsequently challenged with wild-type strain. In conclusion, we report methods for gene deletion in E. piscicida and development of vaccine candidates derived from a virulent catfish isolate.

Phagocytic and Bactericidal Properties of Channel Catfish Peritoneal Macrophages Exposed to Edwardsiella ictaluri Live Attenuated Vaccine and Wild-Type Strains

Edwardsiella ictaluri (E. ictaluri), a Gram-negative, intracellular, facultative bacterium, is the causative agent of enteric septicemia of catfish (ESC), which is one of the most significant diseases of farmed channel catfish. Macrophages have a critical role in major defense mechanisms against bacterial infections by migrating to the site of infection, engulfing and killing pathogens, and priming adaptive immune responses. Vaccination of catfish with E. ictaluri live attenuated vaccine (LAV) strains increased the efficiency of phagocytosis and bacterial killing in catfish peritoneal macrophages compared in vitro with macrophages from non-vaccinated fish. Recently, our group developed several protective LAV strains from E. ictaluri. However, their effects on the antigen uptake and bacterial killing in catfish macrophages have not been evaluated. In this study, we assessed the phagocytic and bactericidal activity of peritoneal macrophages in the uptake of E. ictaluri wild-type (WT) and two LAV strains. We found that phagocytosis of LAV strains was significantly higher compared to their WT counterpart in peritoneal macrophages. Moreover, the uptake of E. ictaluri opsonized with sera from vaccinated catfish was more efficient than when opsonized with sera from sham-vaccinated fish. Notably, catfish macrophages did not lose their phagocytic properties at 4°C, as described previously in mammalian and zebrafish models. Also, opsonization of E. ictaluri with inactivated sera from vaccinated and sham-vaccinated catfish decreased significantly phagocytic uptake of bacteria at 32°C, and virtually suppressed endocytosis at 4°C, suggesting the important role of complement-dependent mechanisms in catfish macrophage phagocytosis. In conclusion, our data on enhanced phagocytic capacity and effective killing ability in macrophages of vaccine strains suggested the LAVs’ advantage if processed and presented in the form of peptides to specific lymphocytes of an adaptive immune system and emphasize the importance of macrophage-mediated immunity against ESC. Furthermore, we showed the role of complement-dependent mechanisms in the phagocytic uptakes of E. ictaluri in catfish peritoneal macrophages at 4 and 32°C. Finally, LAV vaccine-induced bacterial phagocytosis and killing properties of peritoneal macrophages emphasized the importance of the innate immune responses in ESC.

The Role of TonB Gene in Edwardsiella ictaluri Virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen that causes enteric septicemia in catfish (ESC). Stress factors including poor water quality, poor diet, rough handling, overcrowding, and water temperature fluctuations increase fish susceptibility to ESC. The TonB energy transducing system (TonB-ExbB-ExbD) and TonB-dependent transporters of Gram-negative bacteria support active transport of scarce resources including iron, an essential micronutrient for bacterial virulence. Deletion of the tonB gene attenuates virulence in several pathogenic bacteria. In the current study, the role of TonB (NT01EI_RS07425) in iron acquisition and E. ictaluri virulence were investigated. To accomplish this, the E. ictaluri tonB gene was in-frame deleted. Growth kinetics, iron utilization, and virulence of the EiΔtonB mutant were determined. Loss of TonB caused a significant reduction in bacterial growth in iron-depleted medium (p > 0.05). The EiΔtonB mutant grew similarly to wild-type E. ictaluri when ferric iron was added to the iron-depleted medium. The EiΔtonB mutant was significantly attenuated in catfish compared with the parent strain (21.69 vs. 46.91% mortality). Catfish surviving infection with EiΔtonB had significant protection against ESC compared with naïve fish (100 vs. 40.47% survival). These findings indicate that TonB participates in pathogenesis of ESC and is an important E. ictaluri virulence factor.

Ferric hydroxamate uptake system contributes to Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia in fish, particularly in channel catfish. Ferric iron is an essential micronutrient for bacterial survival, and some bacterial pathogens use secreted hydroxamate-type siderophores to chelate iron in host tissues. Siderophore-iron complexes are taken up by these bacteria via the ferric hydroxamate uptake (Fhu) system. In E. ictaluri, the Fhu system consists of fhuC, fhuD, fhuB, and fhuA genes. However, the importance of the Fhu system in E. ictaluri virulence has not been investigated completely. Here, we present construction of E. ictaluri fhuD and fhuB mutants (EiΔfhuD and EiΔfhuB) by in-frame gene deletion and evaluation of the mutants' virulence and immunogenicity in channel catfish fingerlings and fry. Immersion challenges showed that EiΔfhuD was not significantly attenuated (p < 0.05) in catfish fingerlings, whereas EiΔfhuB was significantly attenuated (p < 0.01). Catfish fingerlings immunized with EiΔfhuD and EiΔfhuB showed 100% and 97.62% survival, respectively. Fry immersion challenges indicated EiΔfhuB was also significantly attenuated (p < 0.05) in two-week old fry compared to the wild-type (48.96% vs. 82.14% mortalities). The survival rate in the fry vaccinated with EiΔfhuB was significantly higher (p < 0.05) than that of non-vaccinated fry (96.77% vs. 21.42% survival). Our data indicates that the fhuB gene, but not the fhuD gene, contributes to E. ictaluri virulence.

Involvement of tolQ and tolR genes in Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative intracellular facultative pathogen causing enteric septicemia of channel catfish (ESC). The Tol system, consisting of four envelope proteins TolQ, TolR, TolA, and TolB, are required for colicin import and contributes to bacterial virulence in several pathogenic bacteria. However, the Tol system and its importance in E. ictaluri virulence have not been investigated. Here we present construction and evaluation of the E. ictaluri TolQ, TolR and TolQR mutants (EiΔtolQ, EiΔtolR, and EiΔtolQR). The Tol mutants were developed using in-frame gene deletion and their attenuation and vaccine efficacy were determined in catfish fingerlings. The EiΔtolQ, EiΔtolR, and EiΔtolQR mutants showed reduced virulence in catfish (28.93%, 19.70%, and 39.82% mortality, respectively) compared to wild type (46.91% mortality). Further, vaccination with these mutants protected catfish against subsequent wild-type infection. This study suggests that the Tol system contributes to E. ictaluri virulence in catfish.

Succinate dehydrogenase mutant of Listonella anguillarum protects rainbow trout against vibriosis

Listonella anguillarum is a Gram-negative facultative anaerobic rod causing hemorrhagic septicemia in marine and rarely in freshwater fish. Succinate dehydrogenase (SDH) plays an important role in the tricarboxylic acid (TCA) cycle by oxidizing succinate to fumarate while reducing ubiquinone to ubiquinol. Recent studies indicate that central metabolic pathways, including the TCA cycle, contribute to bacterial virulence. However, the role of SDH in L. anguillarum virulence has not been studied. Here, we report in-frame deletion of the succinate dehydrogenase iron-sulfur protein (SDHB) and its role in L. anguillarum virulence in rainbow trout. To accomplish this goal, upstream and downstream regions of the L. anguillarum sdhB gene were amplified in-frame and cloned into a suicide plasmid. The chromosomal sdhB gene of L. anguillarum was deleted by homologous recombination. Virulence and immunogenicity of the L. anguillarum ΔsdhB mutant (LaΔsdhB) were determined in rainbow trout. Results show that LaΔsdhB was highly attenuated in rainbow trout, and fish immunized with LaΔsdhB displayed high relative survival rate after exposure to wild type L. anguillarum. These findings indicate SDH is important in L. anguillarum virulence in rainbow trout, and LaΔsdhB could be used as an immersion, oral, or injection vaccine to protect rainbow trout against vibriosis.

Identification of Differentially Abundant Proteins of Edwardsiella ictaluri during Iron Restriction

Edwardsiella ictaluri is a Gram-negative facultative anaerobe intracellular bacterium that causes enteric septicemia in channel catfish. Iron is an essential inorganic nutrient of bacteria and is crucial for bacterial invasion. Reduced availability of iron by the host may cause significant stress for bacterial pathogens and is considered a signal that leads to significant alteration in virulence gene expression. However, the precise effect of iron-restriction on E. ictaluri protein abundance is unknown. The purpose of this study was to identify differentially abundant proteins of E. ictaluri during in vitro iron-restricted conditions. We applied two-dimensional difference in gel electrophoresis (2D-DIGE) for determining differentially abundant proteins and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF/TOF MS) for protein identification. Gene ontology and pathway-based functional modeling of differentially abundant proteins was also conducted. A total of 50 unique differentially abundant proteins at a minimum of 2-fold (p ≤ 0.05) difference in abundance due to iron-restriction were detected. The numbers of up- and down-regulated proteins were 37 and 13, respectively. We noted several proteins, including EsrB, LamB, MalM, MalE, FdaA, and TonB-dependent heme/hemoglobin receptor family proteins responded to iron restriction in E. ictaluri.

Oral Vaccination of Channel Catfish against Enteric Septicemia of Catfish Using a Live Attenuated Edwardsiella ictaluri Isolate

Enteric septicemia of catfish (ESC), caused by Edwardsiella ictaluri, is the most problematic bacterial disease affecting catfish aquaculture in the southeastern United States. Efforts to develop an effective ESC vaccine have had limited industrial success. In commercial settings, ESC vaccines are typically administered by immersion when fry are transferred from the hatchery to rearing ponds. While this approach is a practical method of mass delivery, this strategy administers vaccines to very young fish, which lack a fully developed immune system. To circumvent this limitation, an oral vaccination strategy was evaluated as a means of immunizing catfish at the fingerling stage of production, when fish possess a more complete immune arsenal. A virulent E. ictaluri isolate (S97-773) was attenuated by successive passage on media containing increasing concentrations of rifamycin. In laboratory trials, cultured vaccine was diluted and mixed with feed (100 mL diluted vaccine/454 g feed). This mixture was then fed to Channel Catfish Ictalurus punctatus fingerlings. Two separate dilutions of cultured vaccine (1:10 and 1:100) were used to create the vaccine-feed mixture, equating to estimated doses of 5 × 10⁷ and 5 × 10⁶ CFU/g of feed, respectively. After 30 d, catfish were exposed by immersion (1 × 10⁶ CFU/mL) to the virulent parental strain of E. ictaluri. The target dose (1:100 dilution, ∼5 × 10⁶ CFU/g of feed) offered exceptional protection (relative percent survival = 82.6-100%). In addition, negligible deaths occurred in fish vaccinated at 10 times the target dose (1:10 dilution, ∼5 × 10⁷ CFU/g of feed). In pond trials, antibody production increased 18-fold in orally vaccinated fish. When compared with nonvaccinated controls, vaccination significantly improved survival, feed fed, feed conversion, biomass produced, and total harvest. This research demonstrates Channel Catfish can be successfully immunized in a commercial setting against E. ictaluri with a single dose of an orally delivered, live attenuated, E. ictaluri vaccine.

Tissue persistence and vaccine efficacy of tricarboxylic acid cycle and one-carbon metabolism mutant strains of Edwardsiella ictaluri

Edwardsiella ictaluri causes enteric septicemia in fish. Recently, we reported construction of E. ictaluri mutants with single and double gene deletions in tricarboxylic acid cycle (TCA) and one-carbon (C-1) metabolism. Here, we report the tissue persistence, virulence, and vaccine efficacy of TCA cycle (EiΔsdhC, EiΔfrdA, and EiΔmdh), C-1 metabolism (EiΔgcvP and EiΔglyA), and combination mutants (EiΔfrdAΔsdhC, EiΔgcvPΔsdhC, EiΔmdhΔsdhC, and EiΔgcvPΔglyA) in channel catfish. The tissue persistence study showed that EiΔsdhC, EiΔfrdA, EiΔfrdAΔsdhC, and EiΔgcvPΔsdhC were able to invade catfish and persist until 11 days post-infection. Vaccination of catfish fingerlings with all nine mutants provided significant (P<0.05) protection against subsequent challenge with the virulent parental strain. Vaccinated catfish fingerlings had 100% survival when subsequently challenged by immersion with wild-type E. ictaluri except for EiΔgcvPΔglyA and EiΔgcvP. Mutant EiΔgcvPΔsdhC was found to be very good at protecting catfish fry, as evidenced by 10-fold higher survival compared to non-vaccinated fish.

Tricarboxylic acid cycle and one-carbon metabolism pathways are important in Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of channel catfish (ESC). The disease causes considerable economic losses in the commercial catfish industry in the United States. Although antibiotics are used as feed additive, vaccination is a better alternative for prevention of the disease. Here we report the development and characterization of novel live attenuated E. ictaluri mutants. To accomplish this, several tricarboxylic acid cycle (sdhC, mdh, and frdA) and one-carbon metabolism genes (gcvP and glyA) were deleted in wild type E. ictaluri strain 93-146 by allelic exchange. Following bioluminescence tagging of the E. ictaluri ΔsdhC, Δmdh, ΔfrdA, ΔgcvP, and ΔglyA mutants, their dissemination, attenuation, and vaccine efficacy were determined in catfish fingerlings by in vivo imaging technology. Immunogenicity of each mutant was also determined in catfish fingerlings. Results indicated that all of the E. ictaluri mutants were attenuated significantly in catfish compared to the parent strain as evidenced by 2,265-fold average reduction in bioluminescence signal from all the mutants at 144 h post-infection. Catfish immunized with the E. ictaluri ΔsdhC, Δmdh, ΔfrdA, and ΔglyA mutants had 100% relative percent survival (RPS), while E. ictaluri ΔgcvP vaccinated catfish had 31.23% RPS after re-challenge with the wild type E. ictaluri.