The virulence and immune protection of Edwardsiella ictaluri HemR mutants in catfish

Pathological and Ultrastructural Characterization of an Edwardsiella ictaluri Triple hemR Mutant

Enteric septicemia of catfish, which is caused by Edwardsiella ictaluri, is detrimental to farmed Channel Catfish Ictalurus punctatus. The hemin receptor HemR is involved in binding and uptake of heme into bacteria. Here, we explored pathological and ultrastructural changes in catfish fry that were immunized with a triple hemR mutant of E. ictaluri and challenged with wild-type E. ictaluri (EiWT) 28 d after immunization. Following immunization, pathological changes in the triple hemR-immunized fry were less severe compared to the EiWT-exposed control fry. Widely disseminated bacteria and severe necrosis in most organs, especially the kidney and spleen, were detected in both groups at days 4, 5, and 6. Multifocal granulomatous encephalitis with bacteria was seen in hemR-immunized fry at days 21 and 28 and in EiWT-exposed control fry at day 14. Phagocytic cells in the kidney and spleen of EiWT-exposed control fry contained more replicating bacteria compared to hemR-immunized fry. During the EiWT challenge of immunized fry, a robust immune response was observed in the triple hemR-immunized fry compared to the sham-vaccinated group. Many activated phagocytic cells were detected in the kidney and spleen with fragmented or no bacteria in the triple hemR-immunized fry. Our data suggested that virulence of triple hemR was lower and the onset of the lesions was delayed compared to EiWT. Additionally, triple hemR-immunized fry could mount an immune response and had milder lesions compared to the sham control after EiWT exposure.

The Regulatory Protein ChuP Connects Heme and Siderophore-Mediated Iron Acquisition Systems Required for Chromobacterium violaceum Virulence

Chromobacterium violaceum is an environmental Gram-negative beta-proteobacterium that causes systemic infections in humans. C. violaceum uses siderophore-based iron acquisition systems to overcome the host-imposed iron limitation, but its capacity to use other iron sources is unknown. In this work, we characterized ChuPRSTUV as a heme utilization system employed by C. violaceum to explore an important iron reservoir in mammalian hosts, free heme and hemoproteins. We demonstrate that the chuPRSTUV genes comprise a Fur-repressed operon that is expressed under iron limitation. The chu operon potentially encodes a small regulatory protein (ChuP), an outer membrane TonB-dependent receptor (ChuR), a heme degradation enzyme (ChuS), and an inner membrane ABC transporter (ChuTUV). Our nutrition growth experiments using C. violaceum chu deletion mutants revealed that, with the exception of chuS, all genes of the chu operon are required for heme and hemoglobin utilization in C. violaceum. The mutant strains without chuP displayed increased siderophore halos on CAS plate assays. Significantly, we demonstrate that ChuP connects heme and siderophore utilization by acting as a positive regulator of chuR and vbuA, which encode the TonB-dependent receptors for the uptake of heme (ChuR) and the siderophore viobactin (VbuA). Our data favor a model of ChuP as a heme-binding post-transcriptional regulator. Moreover, our virulence data in a mice model of acute infection demonstrate that C. violaceum uses both heme and siderophore for iron acquisition during infection, with a preference for siderophores over the Chu heme utilization system.

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.

Development of fish vaccine in Southeast Asia: A challenge for the sustainability of SE Asia aquaculture

Southeast (SE) Asia plays an important role in global food security as this region has been regarded as one of the major producers of aquaculture product and, to date, freshwater fish accounted for one-third of the total aquaculture in SE Asia. The intensification of freshwater farming corresponding to increase of consumer demands has inevitably led to the emergence and re-emergence of diseases causing tremendous economic loss in the region. Nile tilapia (Oreochromis niloticus) and striped catfish (Pangasianodon hypophthalmus), the major freshwater fish species of SE Asia, have been reported susceptible to several bacterial pathogens, e.g. Streptococcus agalactiae, Edwardsiella ictalurid and Flavobacterium columnare. Since only a limited number of vaccines being registered and marketed, these pathogenic organisms still represent a severe threat to aquaculture industry in SE Asia. However, there is profound advancement in the understanding of disease epidemiology, pathogenic mechanisms, teleost mucosal immunity and vaccine delivery system over the last few years. This review aimed to summarize those recent findings which hopefully can provide novel insight into the future development of suitable vaccine and vaccination regime against bacterial infection in SE Asia region.

A Review of Fish Vaccine Development Strategies: Conventional Methods and Modern Biotechnological Approaches

Fish immunization has been carried out for over 50 years and is generally accepted as an effective method for preventing a wide range of bacterial and viral diseases. Vaccination efforts contribute to environmental, social, and economic sustainability in global aquaculture. Most licensed fish vaccines have traditionally been inactivated microorganisms that were formulated with adjuvants and delivered through immersion or injection routes. Live vaccines are more efficacious, as they mimic natural pathogen infection and generate a strong antibody response, thus having a greater potential to be administered via oral or immersion routes. Modern vaccine technology has targeted specific pathogen components, and vaccines developed using such approaches may include subunit, or recombinant, DNA/RNA particle vaccines. These advanced technologies have been developed globally and appear to induce greater levels of immunity than traditional fish vaccines. Advanced technologies have shown great promise for the future of aquaculture vaccines and will provide health benefits and enhanced economic potential for producers. This review describes the use of conventional aquaculture vaccines and provides an overview of current molecular approaches and strategies that are promising for new aquaculture vaccine development.

Effects of Live Attenuated Vaccine and Wild Type Strains of Edwardsiella ictaluri on Phagocytosis, Bacterial Killing, and Survival of Catfish B Cells

Edwardsiella ictaluri, a Gram-negative facultative intracellular pathogen, is the causative agent of enteric septicemia of catfish (ESC). The innate functions of B cells have been demonstrated in several teleost fish, including zebrafish, rainbow trout, and channel catfish. Recently, our group has developed several protective E. ictaluri live attenuated vaccines (LAVs). However, the innate role of catfish B cells to phagocytose and destroy E. ictaluri wild-type (WT) and live attenuated vaccine (LAV) strains has not been evaluated. In this study, we assessed the efficacy of E. ictaluri WT and two LAVs on phagocytosis, microbial killing, and survival of catfish anterior kidney (AK) B cells. Initially, we documented active uptake of E. ictaluri WT and two LAVs in B cells by flow cytometry and light microscopy. Then, we observed the E. ictaluri strains-induced phagosome and/or phagolysosome formation in the cytoplasm of catfish magnetically sorted IgM+ B cells. Furthermore, we demonstrated that AK B cells were able to destroy the internalized E. ictaluri WT and LAV strains efficiently. Finally, we documented early and late apoptotic/necrotic manifestations induced by E. ictaluri in catfish AK B cells. In conclusion, our results suggest that both LAVs and WT strain initiate similar innate immune responses such as active phagocytic uptake, induced bactericidal activity as well as promote early and late apoptotic changes in catfish B cells. Our data suggest that phagocytic and microbicidal B cells may serve as professional APCs in initiation of protective adaptive immune responses against ESC in channel catfish.

A Review of Molecular Responses of Catfish to Bacterial Diseases and Abiotic Stresses

Catfish is one of the major aquaculture species in the United States. However, the catfish industry is threatened by several bacterial diseases such as enteric septicemia of catfish (ESC), columnaris disease and Aeromonas disease, as well as by abiotic stresses such as high temperature and low oxygen. Research has been conducted for several decades to understand the host responses to these diseases and abiotic stresses. With the development of sequencing technologies, and the application of genome-wide association studies in aquaculture species, significant progress has been made. This review article summarizes recent progress in understanding the molecular responses of catfish after bacterial infection and stress challenges, and in understanding of genomic and genetic basis for disease resistance and stress tolerance.