Comparative transcriptomic analyses of brain-liver-muscle in channel catfish (Ictalurus punctatus) with differential growth rate

Growth is an important economically trait for aquatic animals. The popularity of farmed channel catfish (Ictalurus punctatus) in China has recently surged, prompting a need for research into the genetic mechanisms that drive growth and development to expedite the selection of fast-growing variants. In this study, the brain, liver and muscle transcriptomes of channel catfish between fast-growing and slow-growing groups were analyzed using RNA-Seq. Totally, 63, 110 and 86 differentially expressed genes (DEGs) were from brain, liver and muscle tissues. DEGs are primarily involved in growth, development, metabolism and immunity, which are related to the growth regulation of channel catfish, such as growth hormone receptor b (ghrb), fibroblast growth factor receptor 4 (fgfr4), bone morphogenetic protein 1a (bmp1a), insulin-like growth factor 2a (igf2a), collagen, type I, alpha 1a (col1a1a), acyl-CoA synthetase long chain family member 2 (acsl2) and caveolin 1 (cav1). This study advances our knowledge of the genetic mechanisms accounting for differences in growth rate and offers crucial gene resources for future growth-related molecular breeding programs in channel catfish.

Effect of Agaricus bisporus Polysaccharides (ABPs) on anti-CCV immune response of channel catfish

Herein, the effects of Agaricus bisporus Polysaccharides (ABPs) on anti-channel catfish virus (CCV) infections to promote their application in channel catfish culture were explored. Transcriptome and metabolome analyses were conducted on the spleen of a CCV-infected channel catfish model fed with or without ABPs. CCV infections upregulated many immune and apoptosis-related genes, such as IL-6, IFN-α3, IFN-γ1, IL-26, Casp3, Casp8, and IL-10, and activated specific immunity mediated by B cells. However, after adding ABPs, the expression of inflammation-related genes decreased in CCV-infected channel catfish, and the inflammatory inhibitors NLRC3 were upregulated. Meanwhile, the expression of apoptosis-related genes was reduced, indicating that ABPs can more rapidly and strongly enhance the immunity of channel catfish to resist viral infection. Moreover, the metabonomic analysis showed that channel catfish had a high energy requirement during CCV infection, and ABPs could enhance the immune function of channel catfish. In conclusion, ABPs can enhance the antiviral ability of channel catfish by enhancing immune response and regulating inflammation. Thus, these findings provided new insights into the antiviral response effects of ABPs, which might support their application in aquaculture.

Effects of diets rich in Agaricus bisporus polysaccharides on the growth, antioxidant, immunity, and resistance to Yersinia ruckeri in channel catfish

To promote the application of Agaricus bisporus polysaccharides (ABPs) in channel catfish (Ictalurus punctatus) culture, we evaluated the effects of ABPs on the growth, immunity, antioxidant, and antibacterial activity of channel catfish. When the amount of ABPs was 250 mg/kg, channel catfish's weight gain and specific growth rates increased significantly while the feed coefficient decreased. We also found that adding ABPs in the feed effectively increased the activities of ACP, MDA, T-SOD, AKP, T-AOC, GSH, and CAT enzymes and immune-related genes such as IL-1β, Hsp70, and IgM in the head kidney of channel catfish. Besides, long-term addition will not cause pathological damage to the head kidney. When the amount of ABPs was over 125 mg/kg, the protection rate of channel catfish was more than 60%. According to the intestinal transcriptome analysis, the addition of ABPs promoted the expression of intestinal immunity genes and growth metabolism-related genes and enriched multiple related KEEG pathways. When challenged by Yersinia ruckeri infection, the immune response of channel catfish fed with ABPs was intenser and quicker. Additionally, the 16S rRNA gene sequencing analysis showed that the composition of the intestinal microbial community of channel catfish treated with ABPs significantly changed, and the abundance of microorganisms beneficial to channel catfish growth, such as Firmicutes and Bacteroidota increased. In conclusion, feeding channel catfish with ABPs promoted growth, enhanced immunity and antioxidant, and improved resistance to bacterial infections. Our current results might promote the use of ABPs in channel catfish and even other aquacultured fish species.

PFAS levels in fish species in the Po River (Italy): New generation PFAS, fish ecological traits and parasitism in the foreground

Per- and polyfluoroalkyl substances (PFAS) are resistant to breakdown and are now considered ubiquitous and concerning contaminants. Although scientific and legislative interest in these compounds has greatly increased in recent decades, our knowledge about their environmental fate and their effects on organisms is still incomplete, especially those of the new generation PFAS. In this study, we analysed the level of PFAS contamination in the fish fauna of the Po River, the most important waterway in Italy, to evaluate the influence of different factors (such as fish ecological traits and parasitism) on the accumulation of 17 PFAS. After solvent extraction and purification, hepatic or intestinal tissues from forty specimens of bleak, channel catfish, and barbel were analysed by liquid chromatography coupled with mass spectrometry (LOQ = 2.5 ng/g w.w.). The prevalent PFAS were perfluorooctane sulfonate (PFOS), present in all samples at the highest concentration (reaching a maximum of 126.4 ng/g and 114.4 ng/g in bleak and channel catfish, respectively), and long-chain perfluoroalkyl carboxylic acids (PFDA and PFUnDA). Perfluorooctanoic acid and new generation PFAS (Gen X and C6O4) were not detected. Comparison of the hepatic contamination between the benthic channel catfish and the pelagic bleak showed similar concentrations of PFOS (p > 0.05) but significantly higher concentrations of other individual PFAS and of the sum of all measured PFAS (p < 0.05) in bleak. No correlation was found between the hepatic level of PFAS and fish size in channel catfish. For the first time, PFAS partitioning in a parasite-fish system was studied: intestinal acanthocephalans accumulated PFOS at lower levels than the intestinal tissue of their host (barbel), in contrast to what has been reported for other pollutants (e.g., metals). The infection state did not significantly alter the level of PFAS accumulation in fish, and acanthocephalans do not appear to be a good bioindicator of PFAS pollution.

Combined exposure to polystyrene nanoplastics and bisphenol A induces hepato- and intestinal-toxicity and disturbs gut microbiota in channel catfish (Ictalurus punctatus)

The widespread consumption of nanoplastics (NPs) and bisphenol A (BPA) affected the aquatic ecosystem and imposed risks to the safety of aquatic organisms. This study was aimed at assessing the ecotoxicological effects of single and combined exposure to BPA and polystyrene nanoplastics (PSNPs) on the channel catfish (Ictalurus punctatus). A total of 120 channel catfish were separated into four groups with triplicate (each contains 10 fish) and exposed to chlorinated tap water (control group), PSNP single exposure (0.3 mg/L), BPA single exposure (500 μg/L) and PSNPs (0.3 mg/L) + BPA (500 μg/L) co-exposure for 7 days. Our results showed a relatively higher intestinal accumulation of PSNPs in co-exposure group, compared to PSNP single exposure group. Histopathological analysis showed that single exposure to PSNPs and BPA caused breakage of intestinal villi and swelling of hepatocytes in channel catfish, while the co-exposure exacerbated the histopathological damage. In addition, co-exposure significantly increased SOD, CAT activities and MDA contents in the intestine and liver, inducing oxidative stress. In terms of immune function, the activities of ACP and AKP were significantly decreased. The expressions of immune-related genes such as IL-1β, TLR3, TLR5, hepcidin and β-defensin were significantly up-regulated, and the expression of IL-10 was down-regulated. Additionally, the co-exposure significantly altered the composition of the intestinal microbiota, leading to an increase in the Shannon index and a decrease in the Simpson index. In summary, this study revealed that mixture exposure to PSNPs and BPA exacerbated toxic effects on histopathology, oxidative stress, immune function and intestinal microbiota in channel catfish. It emphasized the threat of NPs and BPA to the health of aquatic organisms and human food safety, with a call for effective ways to regulate the consumption of these anthropogenic chemicals.

Development of Bioluminescent Virulent Aeromonas hydrophila for Understanding Pathogenicity

Virulent Aeromonas hydrophila (vAh) strains that cause motile Aeromonas septicemia (MAS) in farmed channel catfish (Ictalurus punctatus) have been an important problem for more than a decade. However, the routes of infection of vAh in catfish are not well understood. Therefore, it is critical to study the pathogenicity of vAh in catfish. To this goal, a new bioluminescence expression plasmid (pAKgfplux3) with the chloramphenicol acetyltransferase (cat) gene was constructed and mobilized into vAh strain ML09-119, yielding bioluminescent vAh (BvAh). After determining optimal chloramphenicol concentration, plasmid stability, bacteria number-bioluminescence relationship, and growth kinetics, the catfish were challenged with BvAh, and bioluminescent imaging (BLI) was conducted. Results showed that 5 to 10 µg/mL chloramphenicol was suitable for stable bioluminescence expression in vAh, with some growth reduction. In the absence of chloramphenicol, vAh could not maintain pAKgfplux3 stably, with the half-life being 16 h. Intraperitoneal injection, immersion, and modified immersion (adipose fin clipping) challenges of catfish with BvAh and BLI showed that MAS progressed faster in the injection group, followed by the modified immersion and immersion groups. BvAh was detected around the anterior mouth, barbels, fin bases, fin epithelia, injured skin areas, and gills after experimental challenges. BLI revealed that skin breaks and gills are potential attachment and entry portals for vAh. Once vAh breaches the skin or epithelial surfaces, it can cause a systemic infection rapidly, spreading to all internal organs. To our best knowledge, this is the first study that reports the development of a bioluminescent vAh and provides visual evidence for catfish-vAh interactions. Findings are expected to provide a better understanding of vAh pathogenicity in catfish.

Improved Growth and High Inheritance of Melanocortin-4 Receptor (mc4r) Mutation in CRISPR/Cas-9 Gene-Edited Channel Catfish, Ictalurus punctatus

Effects of CRISPR/Cas9 knockout of the melanocortin-4 receptor (mc4r) gene in channel catfish, Ictalurus punctatus, were investigated. Three sgRNAs targeting the channel catfish mc4r gene in conjunction with Cas9 protein were microinjected in embryos and mutation rate, inheritance, and growth were studied. Efficient mutagenesis was achieved as demonstrated by PCR, Surveyor® assay, and DNA sequencing. An overall mutation rate of 33% and 33% homozygosity/bi-allelism was achieved in 2017. Approximately 71% of progeny inherited the mutation. Growth was generally higher in MC4R mutants than controls (CNTRL) at all life stages and in both pond and tank environments. There was a positive relationship between zygosity and growth, with F₁ homozygous/bi-allelic mutants reaching market size 30% faster than F₁ heterozygotes in earthen ponds (p = 0.022). At the stocker stage (~ 50 g), MC4R × MC4R mutants generated in 2019 were 40% larger than the mean of combined CNTRL × CNTRL families (p = 0.005) and 54% larger than F₁ MC4R × CNTRL mutants (p = 0.001) indicating mutation may be recessive. With a high mutation rate and inheritance of the mutation as well as improved growth, the use of gene-edited MC4R channel catfish appears to be beneficial for application on commercial farms.

Transcriptome Analysis Reveals Key Gene Expression Changes in Blue Catfish Sperm in Response to Cryopreservation

The hybrids of female channel catfish (Ictalurus punctatus) and male blue catfish (I. furcatus) account for >50% of US catfish production due to superior growth, feed conversion, and disease resistance compared to both parental species. However, these hybrids can rarely be naturally spawned. Sperm collection is a lethal procedure, and sperm samples are now cryopreserved for fertilization needs. Previous studies showed that variation in sperm quality causes variable embryo hatch rates, which is the limiting factor in hybrid catfish breeding. Biomarkers as indicators for sperm quality and reproductive success are currently lacking. To address this, we investigated expression changes caused by cryopreservation using transcriptome profiles of fresh and cryopreserved sperm. Sperm quality measurements revealed that cryopreservation significantly increased oxidative stress levels and DNA fragmentation, and reduced sperm kinematic parameters. The present RNA-seq study identified 849 upregulated genes after cryopreservation, including members of all five complexes in the mitochondrial electron transport chain, suggesting a boost in oxidative phosphorylation activities, which often lead to excessive production of reactive oxygen species (ROS) associated with cell death. Interestingly, functional enrichment analyses revealed compensatory changes in gene expression after cryopreservation to offset detrimental effects of ultra-cold storage: MnSOD was induced to control ROS production; chaperones and ubiquitin ligases were upregulated to correct misfolded proteins or direct them to degradation; negative regulators of apoptosis, amide biosynthesis, and cilium-related functions were also enriched. Our study provides insight into underlying molecular mechanisms of sperm cryoinjury and lays a foundation to further explore molecular biomarkers on cryo-survival and gamete quality.

Hemolysin Co-regulated Family Proteins Hcp1 and Hcp2 Contribute to Edwardsiella ictaluri Pathogenesis

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of catfish (ESC), a devastating disease resulting in significant economic losses in the U.S. catfish industry. Bacterial secretion systems are involved in many bacteria's virulence, and Type VI Secretion System (T6SS) is a critical apparatus utilized by several pathogenic Gram-negative bacteria. E. ictaluri strain 93-146 genome has a complete T6SS operon with 16 genes, but the roles of these genes are still not explored. In this research, we aimed to understand the roles of two hemolysin co-regulated family proteins, Hcp1 (EvpC) and Hcp2. To achieve this goal, single and double E. ictaluri mutants (EiΔevpC, EiΔhcp2, and EiΔevpCΔhcp2) were generated and characterized. Catfish peritoneal macrophages were able to kill EiΔhcp2 better than EiΔevpC, EiΔevpCΔhcp2, and E. ictaluri wild-type (EiWT). The attachment of EiΔhcp2 and EiΔevpCΔhcp2 to ovary cells significantly decreased compared to EiWT whereas the cell invasion rates of these mutants were the same as that of EiWT. Mutants exposed to normal catfish serum in vitro showed serum resistance. The fish challenges demonstrated that EiΔevpC and EiΔevpCΔhcp2 were attenuated completely and provided excellent protection against EiWT infection in catfish fingerlings. Interestingly, EiΔhcp2 caused higher mortality than that of EiWT in catfish fingerlings, and severe clinical signs were observed. Although fry were more susceptible to vaccination with EiΔevpC and EiΔevpCΔhcp2, their attenuation and protection were significantly higher compared to EiWT and sham groups, respectively. Taken together, our data indicated that evpC (hcp1) is involved in E. ictaluri virulence in catfish while hcp2 is involved in adhesion to epithelial cells and survival inside catfish macrophages.

Amino acid profile of food fishes with potential to diversify fish farming activity

In Brazil, 47% of the fish farming correspond to the tilapia species, and the main producers are the south and northeast regions. This work aimed at characterizing the amino acid profile of three fishes with the potential for rearing and diversification in Brazil and worldwide. The fishes grass carp (Ctenopharyngodon idella), pacu (Piaractus mesopotamicus), and catfish (Ictalurus punctatus) were obtained from a rural property located in the city of Pato Branco, PR, Brazil, and analyzed regarding the amino acid composition through High-Performance Liquid Chromatography. The amino acid profile showed the presence of glutamic acid, followed by lysine, aspartic acid, and leucine as the most prominent amino acids for the three fishes. Among the studied species, the grass carp presented the highest content of essential amino acids, registering all values superior to the minimum recommendation standards determined by the Food and Agriculture Organization of the United Nations (FAO). The disclosure of the significant nutritional value presented by these species consolidate their protein quality and expand possibilities to improve the fish farming and development of derivative products.

The potential for salt toxicity: Can the trans-epithelial potential (TEP) across the gills serve as a metric for major ion toxicity in fish?

An emerging Multi-Ion Toxicity (MIT) model for assessment of environmental salt pollution is based on the premise that major ion toxicity to aquatic organisms is related to a critical disturbance of the trans-epithelial potential across the gills (ΔTEP), which can be predicted by electrochemical theory. However, the model has never been evaluated physiologically. We directly tested key assumptions by examining the individual effects of eight different salts (NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl, K₂SO₄, CaCl₂, and CaSO₄) on measured TEP in three different fish species (fathead minnow, Pimephales promelas = FHM; channel catfish, Ictalurus punctatus = CC; bluegill, Lepomis macrochirus = BG). A geometric concentration series based on previously reported 96-h LC50 values for FHM was used. All salts caused concentration-dependent increases in TEP to less negative/more positive values in a pattern well-described by the Michaelis-Menten equation. The ΔTEP responses for different salts were similar to one another within each species when concentrations were expressed as a percentage of the FHM LC50. A plateau was reached at or before 100 % of the LC50 where the ΔTEP values were remarkably consistent, with only 1.4 to 2.2-fold variation. This relative uniformity in the ΔTEP responses contrasts with 28-fold variation in salt concentration (in mmol L^-1), 9.6-fold in total dissolved solids, and 7.9-fold in conductivity at the LC50. The Michaelis-Menten Km values (salt concentrations causing 50 % of the ΔTEP_max) were positively related to the 96-h LC50 values. ΔTEP responses were not a direct effect of osmolarity in all species and were related to specific cation rather than specific anion concentrations in FHM. These responses were stable for up to 24 h in CC. The results provide strong physiological support for the assumptions of the MIT model, are coherent with electrochemical theory, and point to areas for future research.

Long-Lived Plasma Cells Secrete High-Affinity Antibodies Responding to a T-Dependent Immunization in a Teleost Fish

The recent discovery of long-lived plasma cells (LLPCs) in mammals, which provide a constant expression of specific high-affinity antibodies that mediate humoral memory, has caused a dramatic paradigm shift in the study of immunity and vaccine development. In teleost fish, there are few studies regarding the association between LLPCs and antibody production, and the affinity of the antibodies secreted by the LLPCs is poorly understood. In the present study, channel catfish (Ictalurus punctatus) were immunized with trinitrophenylated-keyhole limpet hemocyanin (TNP-KLH) to examine TNP-specific antibody titers, affinities, antibody-secreting cell (ASC) dynamic changes, and especially the affinity of secreted antibodies by LLPCs post-immunization. We demonstrated that TNP-specific LLPCs were generated starting at week 4 post-immunization, achieved a maximal number at week 8, and maintained a comparable level throughout the 18-week post-immunization period, which was correlated with the dynamics of serum antibody titers and affinity maturation in the response. The LLPCs appeared to mostly reside within, or migrate to, the anterior kidney (bone marrow-like tissue in mammals), but a small portion was also located in the spleen and peripheral blood. The antibodies produced by the LLPCs possessed high affinities, indicating that the generation and development of LLPCs were driven by affinity selection in teleosts. Collectively, the results of this study provide insights toward the evolutionary understanding of the affinity-dependent mechanism of LLPC generation and development.

Edwardsiella ictaluri type III secretion system (T3SS) effector EseN is a phosphothreonine lyase that inactivates ERK1/2

EseN is a type III secretion system (T3SS) effector that is encoded on the Edwardsiella ictaluri chromosome and is homologous to a family of T3SS effector proteins with phosphothreonine lyase (PTL) activity, including OspF from Shigella and SpvC from Salmonella. A yeast-2-hybrid system was used to identify the major vault protein (MVP) as a specific host-cell binding partner for EseN, and the proximity ligation assay (PLA) confirmed the interaction. Similar to other pathogens, E. ictaluri invasion activates extracellular signal-regulated kinases 1 and 2 (ERK1/2) early in the infection, which are subsequently inactivated by EseN. Structurally, EseN contains a highly conserved docking motif that is required for specific binding to mitogen-activated protein kinases, such as ERK1/2, and a motif that is essential for PTL activity. Immunoblotting and immunofluorescence analyses indicate that EseN inactivates ERK1/2 by dephosphorylation in vivo in the head kidney of infected fish and ex vivo in head kidney derived macrophages. Interaction of EseN with phosphorylated ERK1/2 (pERK1/2) was also confirmed using PLA, suggesting that MVP serves as a signaling scaffold for ERK1/2 and EseN. Channel catfish Ictalurus punctatus infected with E. ictaluri strains lacking the eseN gene had reduced numbers of E. ictaluri in the tissues following infection and reduced mortality compared to fish infected with the wild-type. Our results indicate that eseN encodes a PTL domain that interacts with MVP as a possible scaffold protein and inactivates pERK1/2 to ERK1/2, resulting in increased proliferation of E. ictaluri and, ultimately, death of the host.

Potential of Double-crested Cormorants ( Phalacrocorax auritus), American White Pelicans ( Pelecanus erythrorhynchos), and Wood Storks ( Mycteria americana) to Transmit a Hypervirulent Strain of Aeromonas hydrophila between Channel Catfish Culture Ponds

Aeromonas hydrophila is a Gram-negative bacterium ubiquitous to freshwater and brackish aquatic environments that can cause disease in fish, humans, reptiles, and birds. Recent severe outbreaks of disease in commercial channel catfish ( Ictalurus punctatus) aquaculture ponds have been associated with a hypervirulent Aeromonas hydrophila strain (VAH) that is genetically distinct from less virulent strains. The epidemiology of this disease has not been determined. Given that research has shown that Great Egrets ( Ardea alba) can shed viable hypervirulent A. hydrophila after consuming diseased fish, we hypothesized that Double-crested Cormorants ( Phalacrocorax auritus), American White Pelicans ( Pelecanus erythrorhynchos), and Wood Storks ( Mycteria americana) could also serve as a reservoir for VAH and spread the pathogen during predation of fish in uninfected catfish ponds. All three species, when fed VAH-infected catfish, shed viable VAH in their feces, demonstrating their potential to spread VAH.

Magnolol protects channel catfish from Aeromonas hydrophila infection via inhibiting the expression of aerolysin

Aeromonas hydrophila is a common zoonotic pathogen which can cause several infections both in human and animals, particular aquatic animals. Antibiotics have been widely used in the treatment of A. hydrophila infections, however, the development of resistance has limited the treatment for these infections. There is an urgent need for novel agents and strategies against these infections. Aerolysin, a pore-forming toxin secreted by most pathogenic A. hydrophila, is known to contribute to the pathogenesis of A. hydrophila infections. Therefore, aerolysin has been identified as a potential target for drug discovery. In this paper, we found that magnolol, a natural compound without anti -A. hydrophila activity, could significantly inhibit the hemolytic activity of A. hydrophila culture supernatants by inhibiting the transcription of the aerolysin encoding gene aerA at low concentrations. Furthermore, the survival assay showed that magnolol could significantly reduce the mortality induced by A. hydrophila infection in channel catfish (Ictalurus punctatus). Taken together, these findings provide a potent agent against A. hydrophila infections.

Editing of the Luteinizing Hormone Gene to Sterilize Channel Catfish, Ictalurus punctatus, Using a Modified Zinc Finger Nuclease Technology with Electroporation

Channel catfish (Ictalurus punctatus) is the most important freshwater aquaculture species in the USA. Genetically enhanced fish that are sterile could both profit the catfish industry and reduce potential environmental and ecological risks. As the first step to generate sterile channel catfish, three sets of zinc finger nuclease (ZFN) plasmids targeting the luteinizing hormone (LH) gene were designed and electroporated into one-cell embryos, different concentrations were introduced, and the Cel-I assay was conducted to detect mutations. Channel catfish carrying the mutated LH gene were sterile, as confirmed by DNA sequencing and mating experiments. The overall mutation rate was 19.7 % for 66 channel catfish, and the best treatment was ZFN set 1 at the concentration 25 μg/ml. To our knowledge, this is the first instance of gene editing of fish via plasmid introduction instead of mRNA microinjection. The introduction of the ZFN plasmids may have reduced mosaicism, as mutated individuals were gene edited in every tissue evaluated. Apparently, the plasmids were eventually degraded without integration, as they were not detectable in mutated individuals using PCR. Carp pituitary extract failed to induce spawning and restoration of fertility, indicating the need for developing other hormone therapies to achieve reversal of sterility upon demand. This is the first sterilization achieved using ZFN technology in an aquaculture species and the first successful gene editing of channel catfish. Our results will help understand the roles of the LH gene, purposeful sterilization of teleost fishes, and is a step towards control of domestic, hybrid, exotic, invasive, and transgenic fishes.

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.

Effect of multiple mutations in tricarboxylic acid cycle and one-carbon metabolism pathways on Edwardsiella ictaluri pathogenesis

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of catfish (ESC). We have shown recently that tricarboxylic acid cycle (TCA) and one-carbon (C1) metabolism are involved in E. ictaluri pathogenesis. However, the effect of multiple mutations in these pathways is unknown. Here, we report four novel E. ictaluri mutants carrying double gene mutations in TCA cycle (EiΔmdhΔsdhC, EiΔfrdAΔsdhC), C1 metabolism (EiΔglyAΔgcvP), and both TCA and C1 metabolism pathways (EiΔgcvPΔsdhC). In-frame gene deletions were constructed by allelic exchange and mutants' virulence and vaccine efficacy were evaluated using in vivo bioluminescence imaging (BLI) as well as end point mortality counts in catfish fingerlings. Results indicated that all the double gene mutants were attenuated compared to wild-type (wt) E. ictaluri. There was a 1.39-fold average reduction in bioluminescence, and hence bacterial numbers, from all the mutants except for EiΔfrdAΔsdhC at 144 h post-infection. Vaccination with mutants was very effective in protecting channel catfish against subsequent infection with virulent E. ictaluri 93-146 strain. In particular, immersion vaccination resulted in complete protection. Our results provide further evidence on the importance of TCA and C1 metabolism pathways in bacterial pathogenesis.

Comparative transcriptional analysis reveals distinct expression patterns of channel catfish genes after the first infection and re-infection with Aeromonas hydrophila

To determine whether transcriptional levels of channel catfish (Ictalurus punctatus) genes are differentially regulated between a first infection with Aeromonas hydrophila and a re-infection, suppression subtractive hybridization (SSH) was performed in this study using anterior kidney cDNA after the re-infection as tester. Of the 96 clones isolated from the SSH library, 28 unique expressed sequence tags (ESTs) were obtained, of which eight were confirmed to be slightly but significantly (P < 0.05) more up-regulated by the re-infection at 6 h post infection (hpi). Expression kinetics studies at 3, 6, 12, 24, and 48 hpi revealed that the eight ESTs were significantly (P = 0.016) more up-regulated by the first infection, with a major peak at 3 hpi. A total of 96 genes reported in literature to be up-regulated by bacterial infections were selected and subjected to expression analysis at 3 hpi. Of the 96 selected genes, 19 were found to be significantly (P < 0.05) induced by A. hydrophila after the first infection and the re-infection. The 19 genes belonged to the following five main categories: 1) toll-like receptor (TLR2, TLR3, TLR5, TLR21); 2) antimicrobial peptide (NK-lysin type 1, NK-lysin type 2, NK-lysin type 3, cathepsin D, transferrin, hepcidin); 3) cytokine or chemokine (interleukin-1β, interleukin-10, tumor necrosis factor α, chemokine CXCL-10); 4) signaling proteins (cadherin EGF LAG seven-pass G-type receptor 1, very large inducible GTPase 1, arginine deiminase type 2, lymphokine-activated killer T-cell originated protein kinase); 5) lysozyme (lysozyme c). Overall, the total 27 genes (8 ESTs plus the 19 selected genes) were significantly (P < 0.001) more induced by the first infection. Peaked expression of lysozyme c and serum lysozyme activity after the first infection were seen at 24 hpi, whereas that after the re-infection were seen at 12 hpi, suggesting that both innate and adaptive immunity were involved in the defense against the re-infection of A. hydrophila.

Expression profiles of toll-like receptors in channel catfish (Ictalurus punctatus) after infection with Ichthyophthirius multifiliis

Toll-like receptors (TLRs) play a crucial role in the innate immune system, but to date the roles of fish TLRs in response to parasitic infection are still poorly understood. In the present study, we used channel catfish (Ictalurus punctatus) and the ciliate parasite Ichthyophthirius multifiliis as a model to investigate whether and which fish TLRs play important roles in the immune response against parasitic pathogens by detecting the expression profiles of a complete set of TLRs in catfish at different time points after infection with I. multifiliis. The expression profiles of TLR1 and TLR2 were similar, and both were significantly up-regulated in the skin and head kidney at most time points after infection. Furthermore, the expression of TLR2 was also up-regulated in the gill and spleen. TLR9 was induced in the skin and gill, whereas TLR21 was induced in the head kidney and spleen after infection. For TLR19, significant up-regulation was observed in the skin and gill, but significant down-regulation was detected in the head kidney and spleen. In contrast to TLR19, TLR25 was significantly up-regulated in the head kidney and spleen at some time points. No significant changes were observed for the rest of the TLRs at most time points. The results indicated that some TLRs may play essential roles in catfish defense against I. multifiliis infection.

Effects of dietary sweet potato leaf meal on the growth, non-specific immune responses, total phenols and antioxidant capacity in channel catfish (Ictalurus punctatus)

BACKGROUND

Traditional energy sources in catfish diets have become costly, and economical alternatives are needed. Sweet potato leaves are underutilised agricultural by-products that provide energy and substantial amounts of phenols, which affect animal and human health. There is little information on the effects of these compounds on catfish, or the capacity of catfish to accumulate dietary phenols. Catfish enriched with phenols have marketing potential as functional foods. This study investigated the effects of diets with sweet potato leaf meal (SPLM) on growth performance, health and total phenolic compounds in catfish.

RESULTS

SPLM was substituted for wheat middlings in three diets fed to groups of juvenile catfish for 10 weeks. Weight gain, feed conversion, survival, alternative complement activity and lysozyme activity were similar among diets. Haematocrit was lower in fish fed diets with SPLM, but within the normal range. Total phenols and antioxidant capacity in the whole body were similar among treatments.

CONCLUSION

SPLM was an effective energy source for catfish up to the maximum level tested (230 g kg(-1) diet). SPLM did not enhance total phenols in catfish, but there were no apparent antinutritional effects of the meal on catfish growth, health or survival.