Transcriptional profiles of multiple genes in the anterior kidney of channel catfish vaccinated with an attenuated Aeromonas hydrophila

A Comparative Study on Antimicrobial Resistance in Escherichia coli Isolated from Channel Catfish and Related Freshwater Fish Species

Antimicrobial resistance (AMR) trends in 114 generic Escherichia coli isolated from channel catfish and related fish species were investigated in this study. Of these, 45 isolates were from commercial-sized channel catfish harvested from fishponds in Alabama, while 69 isolates were from Siluriformes products, accessed from the U.S. Department of Agriculture Food Safety and Inspection Service' (FSIS) National Antimicrobial Resistance Monitoring System (NARMS) program. Antibiotic susceptibility testing and whole genome sequencing were performed using the GenomeTrakr protocol. Upon analysis, the fishpond isolates showed resistance to ampicillin (44%), meropenem (7%) and azithromycin (4%). The FSIS NARMS isolates showed resistance to tetracycline (31.9%), chloramphenicol (20.3%), sulfisoxazole (17.4%), ampicillin (5.8%) and trimethoprim-sulfamethoxazole, nalidixic acid, amoxicillin-clavulanic acid, azithromycin and cefoxitin below 5% each. There was no correlation between genotypic and phenotypic resistance in the fishpond isolates, however, there was in NARMS isolates for folate pathway antagonists: Sulfisoxazole vs. sul1 and sul2 (p = 0.0042 and p < 0.0001, respectively) and trimethoprim-sulfamethoxazole vs. dfrA16 and sul1 (p = 0.0290 and p = 0.013, respectively). Furthermore, correlations were found for tetracyclines: Tetracycline vs. tet(A) and tet(B) (p < 0.0001 each), macrolides: Azithromycin vs. mph(E) and msr(E) (p = 0.0145 each), phenicols: Chloramphenicol vs. mdtM (p < 0.0001), quinolones: Nalidixic acid vs. gyrA_S83L=POINT (p = 0.0004), and β-lactams: Ampicillin vs. blaTEM-1 (p < 0.0001). Overall, we recorded differences in antimicrobial susceptibility testing profiles, phenotypic-genotypic concordance, and resistance to critically important antimicrobials, which may be a public health concern.

A Review of Bacterial Co-Infections in Farmed Catfish: Components, Diagnostics, and Treatment Directions

Simple Summary

Catfish aquaculture is a prominent agricultural sector for foodfish production in the Southern United States. Catfish producers often experience high-level mortality events due to bacterial pathogens. In many instances, co-infections caused by multiple bacterial fish pathogens are isolated during diagnostic cases. These bacterial–bacterial interactions may alter the infection dynamics, and many of these mechanisms and interactions remain unclear. Furthermore, these co-infections may complicate disease management plans and treatment strategies. The current review provides an overview of the prevalent bacterial pathogens in catfish culture and previously reported instances of co-infections in catfish and other production fish species.

Abstract

Catfish production is a major aquaculture industry in the United States and is the largest sector of food fish production. As producers aim to optimize production yields, diseases caused by bacterial pathogens are responsible for high pond mortality rates and economic losses. The major bacterial pathogens responsible are Edwardsiella ictaluri, Aeromonas spp., and Flavobacterium columnare. Given the outdoor pond culture environments and ubiquitous nature of these aquatic pathogens, there have been many reports of co-infective bacterial infections within this aquaculture sector. Co-infections may be responsible for altering disease infection mechanics, increasing mortality rates, and creating difficulties for disease management plans. Furthermore, proper diagnoses of primary and secondary pathogens are essential in ensuring the correct treatment approaches for antimicrobials and chemical applications. A thorough understanding of the interactions and infectivity dynamics for these warm water bacterial pathogens will allow for the adoption of new prevention and control methods, particularly in vaccine development. This review aims to provide an overview of co-infective pathogens in catfish culture and present diagnostic case data from Mississippi and Alabama to define prevalence for these multiple-species infections better.

Cross-immunity in Nile tilapia vaccinated with Streptococcus agalactiae and Streptococcus iniae vaccines

Streptococcus agalactiae and Streptococcus iniae are major bacterial pathogens of tilapia that can cause high mortality concomitant with large economic losses to aquaculture. Although development of vaccines using formalin-killed bacteria to control these diseases has been attempted, the mechanism of immunity against streptococcal infections and the cross-protective ability of these two bacteria remains unclear. To explore the immunological role of these vaccines, we compared the immune responses of tilapia after immunization with both vaccines and compared the relative percent survival (RPS) and cross-immunization protection of tilapia after separate infection with S. agalactiae and S. iniae. All results revealed that vaccinated fish had significantly higher (P < 0.05) levels of specific antibodies than control fish 14 days post secondary vaccination (PSV) and 7 days post challenge. In vaccinated fish, the mRNA expression of interleukin-8 (IL-8), interleukin-12 (IL-12), caspase-3 (C-3), tumour necrosis factor (TNF), and interferon (IFN) was significantly up regulated (P < 0.05) in the head kidney after immunized; similar results were found for IL-8, TNF and IFN in the posterior kidney, meanwhile the expression levels of C-3 and IFN were significantly increased (P < 0.05) in the spleen of vaccinated fish. Additionally, the levels of acid phosphatase (ACP), alkaline phosphatase (AKP), superoxide dismutase (SOD), and lysozyme (LZM) in vaccinated fish were improved at different degree when compared to the control fish. These results showed that vaccination with formalin-killed cells (FKCs) of either S. agalactiae or S. iniae conferred protection against infection by the corresponding pathogen in Nile tilapia, resulting in RPS values of 92.3% and 91.7%, respectively. Furthermore, cross-protection was observed, as the S. agalactiae FKC vaccine protected fish from S. iniae infection, and vice versa. These results suggested that the S. agalactiae and S. iniae FKC vaccines can induce immune responses and generate excellent protective effects in Nile tilapia.

Digital gene expression analysis in the liver of ScpB-vaccinated and Streptococcus agalactiae-challenged Nile tilapia

In recent years, streptococcal diseases have severely threatened the development of tilapia aquaculture, but effective prevention and control methods have not yet been established. To understand the immune responses of vaccinated Nile tilapia (Oreochromis niloticus), digital gene expression (DGE) technology was applied in this study to detect the gene expression profile of the Nile tilapia (O. niloticus) liver in response to ScpB (Streptococcal C5a peptidase from group B Streptococcus, ScpB) vaccination and a Streptococcus agalactiae-challenge. The control and the ScpB-vaccinated Nile tilapia yielded a total of 25,788,734 and 27,088,598 clean reads, respectively. A total of 1234 significant differentially expressed unigenes were detected (P < 0.05), of which 236 were significantly up-regulated, and 269 were significantly down-regulated (P < 0.05, |fold|>2, FDR<0.05). Of the differentially expressed gene, the identified genes which were enriched using databases of GO and KEGG could be categorized into a total of 67 functional groups and were mapped to 153 signaling pathways including 15 immune-related pathways. The differentially expressed genes (TLR1, TLR2, TLR3, TLR5, TLR9, MyD88, C3, IL-1β, IL-10) were detected in the expression profiles, and this was subsequently verified via quantitative real-time PCR (qPCR). The results of this study can serve as a basis for future research not only on the molecular mechanism of S. agalactiae invasion, but also on the anti-S. agalactiae mechanism in targeted tissues of Nile tilapia.

GWAS Analysis Indicated Importance of NF-κB Signaling Pathway in Host Resistance Against Motile Aeromonas Septicemia Disease in Catfish

Motile Aeromonas septicemia (MAS) disease caused by a bacterial pathogen, Aeromonas hydrophila, is an emerging but severe disease of catfish. Genetic enhancement of disease resistance is considered to be effective to control the disease. To provide an insight into the genomic basis of MAS disease resistance, in this study, we conducted a genome-wide association study (GWAS) to identify quantitative trait loci (QTL). A total of 1820 interspecific backcross catfish of 7 families were challenged with A. hydrophila, and 382 phenotypic extremes were selected for genotyping with the catfish 690 K SNP arrays. Three QTL on linkage group (LG) 2, 26 and 29 were identified to be significantly associated with MAS resistance. Within these regions, a total of 24 genes had known functions in immunity, 10 of which were involved in NF-κB signaling pathway, suggesting the importance of NF-κB signaling pathway in MAS resistance. In addition, three suggestively significant QTL were identified on LG 11, 17, and 20. The limited numbers of QTL involved in MAS resistance suggests that marker-assisted selection may be a viable approach for catfish breeding.

More than just antibodies: Protective mechanisms of a mucosal vaccine against fish pathogen Flavobacterium columnare

A recently developed attenuated vaccine for Flavobacterium columnare has been demonstrated to provide superior protection for channel catfish, Ictalurus punctatus, against genetically diverse columnaris isolates. We were interested in examining the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated juvenile catfish. Accordingly, 58 day old fingerling catfish (28 days post-vaccination or unvaccinated control) were challenged with a highly virulent F. columnare isolate (BGSF-27) and gill tissues collected pre-challenge (0 h), and 1 h and 2 h post infection, time points previously demonstrated to be critical in early host-pathogen interactions. Following RNA-sequencing and transcriptome assembly, differential expression (DE) analysis within and between treatments revealed several patterns and pathways potentially underlying improved survival of vaccinated fish. Most striking was a pattern of dramatically higher basal expression of an array of neuropeptides (e.g. somatostatin), hormones, complement factors, and proteases at 0 h in vaccinated fish. Previous studies indicate these are likely the preformed mediators of neuroendocrine cells and/or eosinophilic granular (mast-like) cells within the fish gill. Following challenge, these elements fell to almost undetectable levels (>100-fold downregulated) by 1 h in vaccinated fish, suggesting their rapid release and/or cessation of synthesis following degranulation. Concomitantly, levels of pro-inflammatory cytokines (IL-1b, IL-8, IL-17) were induced in unvaccinated fish. In contrast, in vaccinated catfish, we observed widespread induction of genes needed for collagen deposition and tissue remodeling. Taken together, our results indicate an important component of vaccine protection in fish mucosal tissues may be the sensitization, proliferation and arming of resident secretory cells in the period between primary and secondary challenge.

Expression analysis of nine Toll-like receptors in yellow catfish (Pelteobagrus fulvidraco) responding to Aeromonas hydrophila challenge

Toll-like receptors (TLRs) are important components of pattern recognition receptors (PRRs), which play significant roles in innate immunity to defense against pathogen invasion. Many TLRs have been found in teleosts, but there are no reports about cloning and expression of TLR genes in yellow catfish (Pelteobagrus fulvidraco). In this study, we analyzed the sequence characters and the relative mRNA expression levels of nine TLRs (TLR1, TLR2, TLR3, TLR4-1, TLR5, TLR7, TLR8-2, TLR9 and TLR22) in different tissues of yellow catfish. The results showed that all nine TLR genes are highly expressed in head kidney, trunk kidney, spleen and liver, all of which are related to host immunity. Subsequently we used Aeromonas hydrophila as a stimulating agent to detect the expression profiles of these nine TLRs in the liver, spleen, trunk kidney and head kidney of yellow catfish at different time points after injection with killed Aeromonas hydrophila. All nine TLRs responded to A. hydrophila challenge with tissue-specific patterns in different immune tissues. The kinetics of up- or down-regulation of these nine TLRs exhibited a similar trend, rising to an elevated level at first and then falling to the basal level, but the peak value differed at different time points in different tissues. The expression levels of the TLR3, TLR4-1, TLR9 and TLR22 genes were significantly up-regulated after bacterial challenge in the liver, spleen, head kidney and trunk kidney. The relatively high expression of TLR genes in the immune tissues in response to the A. hydrophila challenge indicated that TLRs may play important roles in the innate immune response against gram-negative bacteria in yellow catfish.

The effect of BCG on iron metabolism in the early neonatal period: A controlled trial in Gambian neonates

Bacillus Calmette-Guerin (BCG) vaccination has been reported to protect neonates from non-tuberculous pathogens, but no biological mechanism to explain such effects is known. We hypothesised that BCG produces broad-spectrum anti-microbial protection via a hepcidin-mediated hypoferraemia, limiting iron availability for pathogens.

To test this we conducted a trial in 120 Gambian neonates comparing iron status in the first 5-days of life after allocation to: (1) All routine vaccinations at birth (BCG/Oral Polio Vaccine (OPV)/Hepatitis B Vaccine (HBV)); (2) BCG delayed until after the study period (at day 5); and (3) All routine vaccinations delayed until after the study period.

Vaccine regime at birth did not significantly impact on any measured parameter of iron metabolism. However, the ability to detect an effect of BCG on iron metabolism may have been limited by short follow-up time and high activation of the inflammatory-iron axis in the study population.

Comparative renal gene expression in response to abrupt hypoosmotic shock in spotted scat (Scatophagus argus)

Scatophagus argus, a euryhaline fish, is notable for its ability to tolerate a wide range of environmental salinities and especially for its tolerance to a rapid, marked reduction in salinity. Therefore, S. argus is a good model for studying the molecular mechanisms mediating abrupt hyperosmoregulation. The serum osmotic pressure decreased steeply within one hour after transferring S. argus from seawater (SW) to freshwater (FW) and remained at new balance throughout the duration of one week. To explain this phenomenon and understand the molecular responses to an abrupt hypoosmotic shock, hypoosmotic stress responsive genes were identified by constructing two suppression subtractive hybridization (SSH) cDNA libraries from the kidneys of S. argus that had been transferred from SW to FW. After trimming and blasting, 52 ESTs were picked out from the subtractive library. Among them, 11 genes were significantly up-regulated (p < 0.05). The kinetics studies of gene expression levels were conducted for 1 week after the transfer using quantitative real-time PCR. A significant variation in the expression of these genes occurred within 12h after the hypoosmotic shock, except for growth hormone (GH) and polyadenylate binding protein 1 (PBP1), which were significantly up-regulated 2 days post-transfer. Our results suggest different functional roles for these genes in response to hypoosmotic stress during the stress response phase (1 hpt-12 hpt) and stable phase (12 hpt-7 dpt). Furthermore, the plasma growth hormone level was detected to be significantly elevated at 1 hpt and 24 hpt following abrupt hypoosmotic shock. Meanwhile, several hematological parameters, hemoglobin (HGB), red blood cell (RBC) and mean cellular hemoglobin concentration (MCHC), were observed to be significantly increased at 12 hpt and 2 dpt compared with that of control group. Our results provide a solid basis from which to conduct future studies on the osmoregulatory mechanisms in the euryhaline fish.

Humoral immune response and TLR9 gene expression in Pacific red snapper (Lutjanus peru) experimentally exposed to Aeromonas veronii

Aquaculture production of Pacific red snapper Lutjanus peru is growing rapidly in Mexico, especially in Gulf of California. As it is a relatively new aquaculture species there are few reports evaluating its immune response to pathogens. The Gram-negative bacteria Aeromonas veronii is a heterogeneous organism that causes the disease known as motile aeromonad septicemia, which is responsible for serious economic loss in seabream culture due to bacterial infections. For the purpose of this study, juvenile Pacific red snapper specimens were intraperitoneally injected with low doses of A. veronii (1 × 10(6) CFU ml(-1)). Changes in humoral immune parameters (total protein, myeloperoxidase, lisozyme and antiprotease activities and IgM levels), as well as superoxide dismutase and catalase activities, and TLR9 gene expression were evaluated 24 and 48 h after injection. Overall, the results showed an enhanced in humoral immune parameters and SOD and CAT activities in fish infected with A. veronii compared with control group at 24 or 48 h. By real time PCR assays, the basal mRNA transcripts of TLR9 showed that were highly expressed in intestine and leucocytes compared to skin, head kidney, liver and gill. Then, the mRNA expression levels of TLR9 in head kidney, skin, liver and intestine were analyzed in non-infected and experimentally infected fish 24 and 48 h after injection. A. veronii up-regulated the expression of TLR9 at 24 or 48 h of exposure in all samples analyzed except in liver. Interestingly, intestine produced the greatest increase in transcript levels upon exposure (48 h) to A. veronii. Taken together, our results suggest that low doses of A. veronii infection inducing humoral immune system and TLR9 immune gene in Pacific red snapper that can be useful in the health control of this species.

De novo assembly of the grass carp Ctenopharyngodon idella transcriptome to identify miRNA targets associated with motile aeromonad septicemia

Background

De novo transcriptome sequencing is a robust method of predicting miRNA target genes, especially for organisms without reference genomes. Differentially expressed miRNAs had been identified previously in kidney samples collected from susceptible and resistant grass carp (Ctenopharyngodon idella) affected by Aeromonas hydrophila. Target identification for these differentially expressed miRNAs poses a major challenge in this non-model organism.

Results

Two cDNA libraries constructed from mRNAs of susceptible and resistant C. idella were sequenced by Illumina Hiseq 2000 technology. A total of more than 100 million reads were generated and de novo assembled into 199,593 transcripts which were further extensively annotated by comparing their sequences to different protein databases. Biochemical pathways were predicted from these transcript sequences. A BLASTx analysis against a non-redundant protein database revealed that 61,373 unigenes coded for 28,311 annotated proteins. Two cDNA libraries from susceptible and resistant samples showed that 721 unigenes were expressed at significantly different levels; 475 were significantly up-regulated and 246 were significantly down-regulated in the SG samples compared to the RG samples. The computational prediction of miRNA targets from these differentially expressed genes identified 188 unigenes as the targets of 5 conserved and 4 putative novel miRNA families.

Conclusion

This study demonstrates the feasibility of identifying miRNA targets by transcriptome analysis. The transcriptome assembly data represent a substantial increase in the genomic resources available for C. idella and will provide insights into the gene expression profile analysis and the miRNA function annotations in further studies.

Immune responses and protective efficacy of recombinant outer membrane protein R (rOmpR)-based vaccine of Aeromonas hydrophila with a modified adjuvant formulation in rohu (Labeo rohita)

Despite the importance and success of developing a candidate vaccine against Aeromonas hydrophila infection in fish, little is known about the molecular mechanisms of the vaccine-induced immunoprotection in Indian major carp, Labeo rohita, primarily due to lack of information on most of the immune related genes of the species. In this study, a novel candidate antigen recombinant outer membrane protein R (rOmpR) of A. hydrophila was evaluated as a vaccine candidate along with a modified adjuvant formulation. Protective efficacy of the rOmpR immunization was assessed in terms of survival against A. hydrophila challenge as well as modulation of immune response in vaccinated fish after 1, 3, 6, 12, 24, 72 h and 10 days post-injection (using immune gene expression analysis) and 10, 28, 56 and 140 days post-injection (serum immune parameter analysis). The generated immune response was compared with a formalin-killed A. hydrophila antigen preparation using mineral oil only and modified adjuvant alone. We report a variable up-regulation of the immune-related genes viz., lysozyme G, complement factor 4, immunoglobulin M, β2-microglobulin, major histocompatibility complex I and II, and interleukin-1β in anterior kidney and spleen tissues at early time points post-immunization in all the groups, when compared to the control fish. The vaccinated fish also showed an increase in serum natural hemolysin titer, lysozyme and myeloperoxidase activities, and antibody titer irrespective of vaccine formulations as compared to control fish on days 10, 28 and 56. However, the increase in the serum parameters was more pronounced on day 140 in rOmpR-modified adjuvant injected group, indicating the modulatory role of this new vaccine formulation. Upon challenge with live A. hydrophila on days 56 and 140 post-immunization, significantly reduced percent mortality was noted in the group immunized with modified adjuvant based rOmpR vaccine formulation. Taken together, our results suggest that rOmpR along with modified adjuvant could potentially be used as a vaccine formulation to handle A. hydrophila infection on a long-term basis.

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.

G-protein coupled receptor 18 (GPR18) in channel catfish: expression analysis and efficacy as immunostimulant against Aeromonas hydrophila infection

The objectives of this study were: 1) to determine the transcriptional profiles of G-protein coupled receptor 18 (GPR18) in channel catfish after infection with Aeromonas hydrophila compared to that in healthy catfish; 2) to determine whether over-expression of GPR18 in catfish gill cells will offer protection against infection of A. hydrophila; 3) to determine whether recombinant pcDNA-GPR18 could be used as an immunostimulant to protect channel catfish against A. hydrophila infection. Quantitative PCR revealed that the transcription levels of GPR18 in all tissues of infected catfish were significantly (P < 0.05) induced except in the intestine. When pcDNA3.2-vectored recombinant GPR18 was transfected in catfish gill cells G1B, the over-expression of pcDNA-GPR18 offered significant (P < 0.05) protection to G1B cells against A. hydrophila infection. When channel catfish were intraperitoneally injected with QCDCR adjuvant formulated pcDNA-GPR18 and challenged with a highly virulent A. hydrophila strain at 1-, 2-, 14-, and 28-days post treatment, pcDNA-GPR18 offered 50%, 100%, 57%, and 55% protection to channel catfish, respectively. Macrophages of fish treated with pcDNA-GPR18 produced significantly (P < 0.05) higher amounts of reactive oxygen species and nitric oxide than that of fish treated with pcDNA vector alone. In addition, serum lysozyme activity of catfish injected with pcDNA-GPR18 was significantly (P < 0.08) increased. Taken together, our results suggest that pcDNA-GPR18 could be used as a novel immunostimulant to provide immediate protection to channel catfish against A. hydrophila infection.

Apolipoprotein A1 in channel catfish: transcriptional analysis, antimicrobial activity, and efficacy as plasmid DNA immunostimulant against Aeromonas hydrophila infection

The objectives of this study were to: 1) determine transcriptional profiles of apolipoprotein A1 (ApoA1) in collected channel catfish tissues after infection with Aeromonas hydrophila by bath immersion; 2) investigate whether recombinant channel catfish apolipoprotein A1 produced in Escherichia coli expression system possesses any antimicrobial activity against A. hydrophila; 3) evaulate whether recombinant channel catfish apolipoprotein A1 plasmid DNA could be used as immunostimulant to protect fish against A. hydrophila infection. Quantitative PCR revealed that the transcription levels of ApoA1 in infected catfish were significantly (P < 0.05) more induced in the anterior kidney. Recombinant apoA1 produced in E. coli expression system exhibited lytic activity against Gram-positive Micrococcus lysodeikticus and Gram-negative A. hydrophila. When pcDNA3.2-vectored recombinant apoA1 was transfected in channel catfish gill cells G1B, the over-expression of pcDNA-ApoA1 offered significant (P < 0.05) protection to G1B cells against A. hydrophila infection. When channel catfish were intraperitoneally injected with QCDCR adjuvant formulated pcDNA-ApoA1 and challenged with a highly virulent A. hydrophila strain AL-09-71 at two days post injection, pcDNA-ApoA1 injection offered 100% protection to channel catfish. Macrophages of fish injected with pcDNA-ApoA1 produced significantly (P < 0.05) higher amounts of reactive oxygen species and nitric oxide than that of fish injected with pcDNA vector alone. Our results suggest that pcDNA-ApoA1 could be used as a novel immunostimulant to offer immediate protection to catfish against A. hydrophila infection.

Recombinant goose-type lysozyme in channel catfish: lysozyme activity and efficacy as plasmid DNA immunostimulant against Aeromonas hydrophila infection

The objectives of this study were: 1) to investigate whether recombinant channel catfish lysozyme-g (CC-Lys-g) produced in Escherichia coli expression system possesses any lysozyme activity; and 2) to evaluate whether channel catfish lysozyme-g plasmid DNA could be used as an immunostimulant to protect channel catfish against Aeromonas hydrophila infection. Recombinant CC-Lys-g produced in E. coli expression system exhibited significant (P < 0.05) lytic activity against Gram-positive Micrococcus lysodeikticus and Gram-negative A. hydrophila. When pcDNA3.2-vectored recombinant channel catfish lysozyme-g (pcDNA-Lys-g) was transfected in channel catfish gill cells G1B, the over-expression of pcDNA-Lys-g offered significant (P < 0.05) protection to G1B cells against A. hydrophila infection. When channel catfish were intraperitoneally injected with pcDNA-Lys-g along with an adjuvant QCDCR, the transcriptional level of Lys-g was significantly (P < 0.05) increased. When pcDNA-Lys-g injected fish was challenged with a highly virulent A. hydrophila strain AL-09-71, pcDNA-Lys-g offered 100% protection to channel catfish at two days post DNA injection. Macrophages of fish injected with pcDNA-Lys-g produced significantly (P < 0.05) higher amounts of reactive oxygen species and nitric oxide than that of fish injected with pcDNA vector alone at two days post DNA injection. Taken together, our results suggest that pcDNA-Lys-g could be used as a novel immunostimulant to offer immediate protection to channel catfish against A. hydrophila infection.

Chicken-type lysozyme in channel catfish: expression analysis, lysozyme activity, and efficacy as immunostimulant against Aeromonas hydrophila infection

To understand whether chicken-type lysozyme (Lys-c) in channel catfish was induced by infection of Aeromonas hydrophila, the transcriptional levels of Lys-c in skin, gut, liver, spleen, posterior kidney, and blood cells in healthy channel catfish was compared to that in channel catfish infected with A. hydrophila by bath immersion. Quantitative PCR revealed that the transcription levels of Lys-c in infected catfish were significantly (P < 0.05) induced in all five tissues tested as well as in blood cells. Recombinant CC-Lys-c produced in Escherichia coli expression system (R-CC-Lys-c) exhibited significant (P < 0.05) lytic activity to Gram-positive Micrococcus lysodeikticus and Gram-negative A. hydrophila. When pcDNA3.2-vectored recombinant channel catfish lysozyme-c (pcDNA-Lys-c) was transfected in channel catfish gill cells G1B, the over-expression of pcDNA-Lys-c offered significant (P < 0.05) protection to G1B against A. hydrophila infection. When channel catfish were intraperitoneally injected with QCDCR adjuvant formulated pcDNA-Lys-c and challenged with a highly virulent A. hydrophila strain AL-09-71 at 1-, 2-, 14-, and 28-days post treatment, pcDNA-Lys-c offered 75%, 100%, 60%, and 77% protection to channel catfish, respectively. Macrophages of fish treated with pcDNA-Lys-c produced significantly (P < 0.05) higher amounts of reactive oxygen species and nitric oxide than that of fish treated with pcDNA vector alone. Taken together, our results suggest that pcDNA-Lys-c could be used as a novel immunostimulant to protect channel catfish against A. hydrophila infection.

Gene expression profiling in live attenuated Edwardsiella tarda vaccine immunized and challenged zebrafish: insights into the basic mechanisms of protection seen in immunized fish

Despite the importance and success of vaccine immunization against bacterial diseases in fish, little is known about the molecular mechanisms of vaccine-induced immune protection in teleost fish. In this study, the live attenuated Edwardsiella tarda vaccine strain WED, which has been shown to evoke efficacious protection against edwardsiellosis and ascites diseases in fish, was extensively evaluated for multiple parameters in a 5-week immunization and challenge experiment in zebrafish. The parameters evaluated included the immunologic potency (relative percent survival, RPS), the specific IgM antibody titers and the expression profiles of multiple immune-related gene markers at multiple time points following immunization and challenge. During the 4-week immunization phase, the toll-like receptor (TLR) 5 signaling pathway, the MHC-I antigen processing pathway and cytotoxic T lymphocyte (CTL) responses were activated in succession. In contrast, the MHC-II antigen processing pathway and the markers of CD4(+) T lymphocyte activation were down-regulated, and IgM transcription and specific IgM antibody titers were not significantly induced following immunization. During the 1-week challenge phase, the induction of MHC-I and CTL responses and the inhibition of MHC-II and CD4(+) T cell responses were similarly observed in immunized zebrafish following challenge with wild E. tarda. With the 5-week immunization and challenge model, our data suggest the basic mechanism that underlying the long-lasting protective immunity elicited by WED in zebrafish. This mechanism involved the induction of the TLR-5 signaling pathway, the MHC-I antigen processing pathway and CTL effector function, and CTL function seems play a major role in the protection against E. tarda infection in zebrafish.

Evasion of mucosal defenses during Aeromonas hydrophila infection of channel catfish (Ictalurus punctatus) skin

The mucosal surfaces of fish serve as the first line of defense against the myriad of aquatic pathogens present in the aquatic environment. The immune repertoire functioning at these interfaces is still poorly understood. The skin, in particular, must process signals from several fronts, sensing and integrating environmental, nutritional, social, and health cues. Pathogen invasion can disrupt this delicate homeostasis with profound impacts on signaling throughout the organism. Here, we investigated the transcriptional effects of virulent Aeromonas hydrophila infection in channel catfish skin, Ictalurus punctatus. We utilized a new 8 × 60 K Agilent microarray for catfish to examine gene expression profiles at critical early timepoints following challenge--2 h, 8 h, and 12 h. Expression of a total of 2,168 unique genes was significantly perturbed during at least one timepoint. We observed dysregulation of genes involved in antioxidant, cytoskeletal, immune, junctional, and nervous system pathways. In particular, A. hydrophila infection rapidly altered a number of potentially critical lectins, chemokines, interleukins, and other mucosal factors in a manner predicted to enhance its ability to adhere to and invade the catfish host.

Early mucosal responses in blue catfish (Ictalurus furcatus) skin to Aeromonas hydrophila infection

Bacterial pathogens are well-equipped to detect, adhere to, and initiate infection in their finfish hosts. The mucosal surfaces of fish, such as the skin, function as the front line of defense against such bacterial insults that are routinely encountered in the aquatic environment. While recent progress has been made, and despite the obvious importance of mucosal surfaces, the precise molecular events that occur soon after encountering bacterial pathogens remain unclear. Indeed, these early events are critical in mounting appropriate responses that ultimately determine host survival or death. In the present study, we investigated the transcriptional consequences of a virulent Aeromonas hydrophila challenge in the skin of blue catfish, Ictalurus furcatus. We utilized an 8×60K Agilent microarray to examine gene expression profiles at key early timepoints following challenge (2 h, 12 h, and 24 h). A total of 1155 unique genes were significantly altered during at least one timepoint. We observed dysregulation in a number of genes involved in diverse pathways including those involved in antioxidant responses, apoptosis, cytoskeletal rearrangement, immunity, and extracellular matrix protein diversity and regulation. Taken together, A. hydrophila coordinately modulates mucosal factors across numerous cellular pathways in a manner predicted to enhance its ability to adhere to and infect the blue catfish host.

Matrix metalloproteinase 2 of grass carp Ctenopharyngodon idella (CiMMP2) is involved in the immune response against bacterial infection

The gene encoding matrix metalloproteinase 2 (MMP2) was cloned from grass carp (Ctenopharyngodon idella), and its expression levels during Aeromonas hydrophila infection and embryonic development stages were evaluated. The complete open reading frame of CiMMP2 was 1974 bp in length, encoding a 658-amino acid polypeptide. The deduced MMP2 protein contained four conserved domain structures, including an N-terminal signal sequence, a propeptide domain, three repeats of fibronectin-type II domain inserted in the catalytic domain and a C-terminal hemopexin-like domain. Phylogenetic analysis of MMP2s grouped grass carp with other teleosts. Detected in all fish tissues examined, CiMMP2 expression increased in the spleen and head kidney at 4 h and was significantly downregulated at 1 d after A. hydrophila infection. CiMMP2 transcripts were present in unfertilized eggs, suggesting its maternal origin. These findings implicate an important role for CiMMP2 in A. hydrophila-related diseases and early embryonic developmental stages of grass carp.