Immunogenicity of extracellular products from an inactivated vaccine against Aeromonas veronii TH0426 in koi, Cyprinus carpio

Evaluation of virulence of Aeromonas veronii strain GZ21-2 and development of a highly effective vaccine for grass carp with the potential for industrial application

Bacterial septicemia represents a significant disease affecting cultured grass carp culture, with the primary etiological agent identified as the Gram-negative bacterium Aeromonas veronii. In response to an outbreak of septicemia in Guangzhou, we developed a formaldehyde-inactivated vaccine against an A. veronii strain designated AV-GZ21-2. This strain exhibited high pathogenicity in experimental infections across at all developmental stages of grass carp. Mortality rates for grass carp weighing 15 ± 5 g ranged from 16 % to 92 % at exposure temperatures of 19 °C-34 °C, respectively. The median lethal dose (LD50) for grass carp groups weighing 15 ± 5 g, 60 ± 10 g, 150 ± 30 g and 500 ± 50 g were determined to be 1.43, 2.52, 4.65 and 7.12 × 107(CFU/mL), respectively. We investigated the inactivated vaccine in conbination with aluminum hydroxide gel (AV-AHG), Montanide ISA201VG (AV-201VG), and white oil (AV-WO) adjuvants. This study aimed to optimize inactivation conditions and identify the adjuvant that elicits the most robust immune response. The AV-GZ21-2 inactivated bacterial solution (AV),when combined with various adjuvants, was capable of inducing a strong specific immune response in grass carp. The relative percent survival (RPS) following a lethal challenge with AV-GZ21-2 were 94 % for AV-AHG, 88 % for AV-201VG, 84 % for AV-WO and 78 % for AV alone. The minimum immunization dose of the AV-AHG vaccine was determined to be 6.0 × 107 CFU per fish, providing immunity for a duration of six months with an immune protection level exceeding 75 %. Furthermore, the AV-AHG vaccine demonstrated significant protective efficacy against various epidemic isolates of A. veronii. Consequently, we developed an inactivated vaccine targeting a highly pathogenic strain of A. veronii, incorporating an aluminum hydroxide gel adjuvant, which resulted in high immune protection and a duration of immunity exceeding six months. These findings suggest that the AV-AHG vaccine holds substantial potential for industrial application.

Pichia pastoris composition expressed aerolysin mutant of Aeromonas veronii as an oral vaccine evaluated in zebrafish (Danio rerio)

Vaccines are one of the most practical means to stop the spreading of Aeromonas veronii in aquaculture. In this study, virulence factor aerolysin mutant NTaer which has lost its hemolytic activity was used as a target antigen. Pichia pastoris constitutive secretory expression NTaer (GS115-NTaer) was used as a potential safe oral vaccine to evaluate its effectiveness on zebrafish immunity. The result shows that vaccination of GS115- NTaer for four weeks did not affect the growth performance of the host, while eliciting an effective immune protective response. Compared with the control group, the GS115-NTaer could significantly up-regulate the relative expression level of the intestinal tight junction protein 1α (TJP1α) gene, and significantly increased the contents of lysozyme (LYZ), complement C3 and C4 in the gut, indicating that the innate immune response of the fish was activated. The relative gene expression levels of macrophage-expressed gene 1 (MPEG1) and T cell receptor (TCR-α) in the gut, and MPEG1, CD4, CD8, TCR-α, GATA3, and T-bet in the spleen were all increased significantly, indicating that the cellular immune response of the fish was activated. Furthermore, the contents of serum IgM and intestinal mucosa IgZ antibodies were significantly increased, which showed that humoral immunity was also activated. Moreover, inoculation with GS115-NTaer significantly changed the structure of gut microbiota. In particular, the relative ratio of (Firmicutes + Fusobacteriota + Bacteroidota)/Proteobacteria was significantly higher than that of the control and GS115 groups. Lastly, the vaccinated fish were challenged with A. veronii, and the relative percent survival of GS115 and the GS115-NTear groups was 14.28% and 33.43%. This improvement of immunity was not only due to the specific immune response but also attributed to the improvement of innate immunity and the gut microbiota which was demonstrated by the germ-free zebrafish model. Collectively, this study provides information on the effectiveness of GS115-NTear as an oral vaccine for the green prevention and control of A. veronii infection in fish aquaculture.

The peptidoglycan-associated lipoprotein gene mutant elicits robust immunological defense in mice against Salmonella enteritidis

Background

Salmonella enteritidis (S. enteritidis), a zoonotic pathogen with a broad host range, presents a substantial threat to global public health safety. Vaccination stands as an effective strategy for the prevention and control of S. enteritidis infection, highlighting an immediate clinical need for the creation of safe and efficient attenuated live vaccines.

Methods

In this study, a S. enteritidis peptidoglycan-associated lipoprotein (pal) gene deletion strain (Δpal), was constructed. To assess its virulence, we conducted experiments on biofilm formation capability, motility, as well as cell and mouse infection. Subsequently, we evaluated the immune-protective effect of Δpal.

Results

It was discovered that deletion of the pal gene reduced the biofilm formation capability and motility of S. enteritidis. Cell infection experiments revealed that the Δpal strain exhibited significantly decreased abilities in invasion, adhesion, and intracellular survival, with downregulation of virulence gene expression, including mgtC, invH, spvB, sipA, sipB, ssaV, csgA, and pipB. Mouse infection experiments showed that the LD50 of Δpal increased by 104 times, and its colonization ability in mouse tissue organs was significantly reduced. The results indicated that the pal gene severely affected the virulence of S. enteritidis. Further, immunogenicity evaluation of Δpal showed a significant enhancement in the lymphocyte transformation proliferation capability of immunized mice, producing high titers of specific IgG and IgA, suggesting that Δpal possesses good immunogenicity. Challenge protection tests demonstrated that the strain could provide 100% immune protection against wild-type strains in mice.

Discussion

This study proves that the pal gene influences the virulence of S. enteritidis, and Δpal could serve as a candidate strain for attenuated live vaccines, laying the foundation for the development of attenuated live vaccines against Salmonella.

Evaluate the potential use of TonB-dependent receptor protein as a subunit vaccine against Aeromonas veronii infection in Nile tilapia (Oreochromis niloticus)

Aeromonas veronii is an emerging bacterial pathogen that causes serious systemic infections in cultured Nile tilapia (Oreochromis niloticus), leading to massive deaths. Therefore, there is an urgent need to identify effective vaccine candidates to control the spread of this emerging disease. TonB-dependent receptor (Tdr) of A. veronii, which plays a role in the virulence factor of the organism, could be useful in terms of protective antigens for vaccine development. This study aims to evaluate the potential use of Tdr protein as a novel subunit vaccine against A. veronii infection in Nile tilapia. The Tdr gene from A. veronii was cloned into the pET28b expression vector, and the recombinant protein was subsequently produced in Escherichia coli strain BL21 (DE3). Tdr was expressed as an insoluble protein and purified by affinity chromatography. Antigenicity test indicated that this protein was recognized by serum from A. veronii infected fish. When Nile tilapia were immunized with the Tdr protein, specific antibody levels increased significantly (p-value <0.05) at 7 days post-immunization (dpi), and peaked at 21 dpi compared to antibody levels at 0 dpi. Furthermore, bacterial agglutination activity was observed in the fish serum immunized with the Tdr protein, indicating that specific antibodies in the serum can detect Tdr on the bacterial cell surface. These results suggest that Tdr protein has potential as a vaccine candidate. However, challenging tests with A.veronii in Nile tilapia needs to be investigated to thoroughly evaluate its protective efficacy for future applications.

Pathogenicity and inactivated vaccine treatment of Aeromonas veronii JW-4 on crucian carp

Aeromonas veronii is a common bacterium found in a variety of aquatic environments, capable of causing a diverse array of diseases in both aquatic animals and humans. Therefore, evaluating the pathogenicity of A. veronii and implementing measures to control its spread are essential. In this study, a strain JW-4, identified as A. veronii, was isolated from diseased Scaphesthes macrolepis, a grade Ⅱ protected animal in China. To investigate the pathogenicity of the strain, fish were fed with serial levels JW-4 supplemented diet or basal diet (control group 1, CG1) for 28 days (d). Results showed that JW-4 stimulated an immune response, evidenced by an increase in immune-related enzyme activities (GOT and GPT) of serum and liver and an upregulation of genes expression levels (TNF-α and IFN-γ) of liver and spleen, and these effects gradually decreased over time. Histopathological examination revealed that JW-4 could alter the tissue structure of immune organs, such as liver and kidney. These changes were accompanied by vacuolar degeneration, nuclear dissolution, and an increased lymphocyte count. To assess protective effects of a vaccine against this strain, fish were injected with an inactivated vaccine (immunization group, IG) or 0.85% sterile saline (control group 2, CG2) for 28-day observation period, then challenged with JW-4 on the 28th day. The inactivated vaccine enhanced total and specific IgM to A. veronii levels of the fish, resulting in a relative percentage survival of 75% in IG. These findings provide a foundation for identifying pathogenic bacteria and developing more effective prophylactic strategies in aquaculture.

Aeromonas hydrophila induction method in adult zebrafish (Danio rerio) as animal infection models

Background and Aim: Zebrafish are frequently used as model organisms in scientific research as their genes mirror those of humans. Aeromonas hydrophila bacteria can infect humans and animals, mainly fish. This study aimed to identify the concentration and route of A. hydrophila infection in adult zebrafish. Zebrafish had been used as a challenge test by analyzing their hematological profiles, blood glucose levels, and survival rates. Materials and Methods: Induction of cell supernatant free (CSF) from A. hydrophila bacteria in adult zebrafish was carried out via bath immersion (BI), intraperitoneal injection (IPI), intramuscular injection (IMI), and healthy zebrafish as a control (C). The bacterial concentrations were 107, 109, and 1011 colony-forming units (CFU)/mL. At 24 h post-infection, the outcomes of infection were evaluated based on survival rates, hematological profiles, and blood glucose levels. A one-way analysis of variance with a confidence level of 95% was employed to examine the data. Results: In the BI, IPI, and IMI treatment groups, the survival rate of the fish reached a peak of 100%, 22%–100%, and 16%–63%, respectively, compared with the injection technique. In the IMI2 group, a 109 CFU/mL bacterial concentration was determined to correspond to the lethal dosage 50. All infection groups had lower erythrocyte and hemoglobin counts but higher leukocyte counts than the control group. The blood sugar levels of the healthy and infected groups were not significantly different. Conclusion: The route of A. hydrophila infection through Intramuscular injection with a concentration of 109 CFU/mL indicated a high performance compared to other techniques. This method could be developed as a reproducible challenge test.

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.

Chitosan-polymer based nanovaccine as promising immersion vaccine against Aeromonas veronii challenge in red tilapia (Oreochromis sp.)

Red tilapia (Oreochromis sp.), one of the important freshwater fish species in fish farming in Thailand, has for long been suffering from a serious bacterial disease named epizootic ulcerative syndrome and hemorrhagic septicemia. The disease is mainly caused by Aeromonas veronii. Vaccine is proposed to be a major impact tool for sustainable control and prevention strategies. Vaccination by immersion has many benefits over injection. However, the conventional immersion method suffers from a low potency due to the inefficient uptake of antigens across mucosal tissue. Here, we developed a chitosan-polymer based nanovaccine together with an efficient delivery vehicle to enhance the immunogenicity of immersion vaccination, increasing bioavailability and inducing local immune responses during transit to mucosal inductive immune sites. The physiochemical properties of nanovaccine, which was modified on surface particle by using a mucoadhesive polymer, were assessed for size, zeta potential, and particle distribution. Our study demonstrated by SEM image and microscopic fluorescence image that nanovaccine greatly increased the binding and penetrating ability into gills when compared with formalin killed vaccine. The nano-sized particles were well dispersed in water and trapped in core nanoparticle as confirmed by TEM image. The efficacy of vaccine was performed by immersion challenge with virulent A.veronii after 30 days post vaccination in tilapia. The result revealed a high level of mortality in the control, empty-polymeric nanovaccine and formalin killed bacterin vaccine groups. A high relative percentage survival (RPS) of vaccinated fish was noted with chitosan-polymer based nanovaccine. Our studies indicated that this chitosan-polymer based nanovaccine derived from cell fragments and supernatant was the improved version of the conventional formalin killed vaccine. The chitosan polymer based particle could increase the efficacy of nanovaccine toward the target mucosal membrane and enhance protection against A. veronii infection in red tilapia.

Application of reverse vaccinology to design a multi-epitope subunit vaccine against a new strain of Aeromonas veronii

BACKGROUND

Aeromonas veronii is one of the most common pathogens of freshwater fishes that cause sepsis and ulcers. There are increasing numbers of cases showing that it is a significant zoonotic and aquatic agent. Epidemiological studies have shown that A. veronii virulence and drug tolerance have both increased over the last few years as a result of epidemiological investigations. Cadaverine reverse transporter (CadB) and maltoporin (LamB protein) contribute to the virulence of A. veronii TH0426. TH0426 strain is currently showing severe cases on fish species, and its resistance against therapeutic has been increasing. Despite these devastating complications, there is still no effective cure or vaccine for this strain of A.veronii.

RESULTS

In this regard, an immunoinformatic method was used to generate an epitope-based vaccine against this pathogen. The immunodominant epitopes were identified using the CadB and LamB protein of A. veronii. The final constructed vaccine sequence was developed to be immunogenic, non-allergenic as well as have better solubility. Molecular dynamic simulation revealed significant binding stability and structural compactness. Finally, using Escherichia coli K12 as a model, codon optimization yielded ideal GC content and a higher CAI value, which was then included in the cloning vector pET2+ (a).

CONCLUSION

Altogether, our outcomes imply that the proposed peptide vaccine might be a good option for A. veronii TH0426 prophylaxis.

Aeromonas veronii infection remarkably increases expression of lysozymes in grass carp (Ctenopharyngodon idellus) and injection of lysozyme expression cassette along with QCDC adjuvant significantly upregulates immune factors and decreases cumulative mortality

Aeromonas veronii AvX005 is a pathogenic bacterium with high toxicity to grass carp (Ctenopharyngodon idellus). The expression levels of g-type (goose-type lysozyme, Lys-g) and c-type lysozyme (chicken-type lysozyme, Lys-c) in the spleen of grass carp infected with AvX005 were significantly increased by approximately 4.5 times and 27 times, respectively. The recombinant proteins rLys-g and rLys-c produced in a recombinant expression system of Escherichia coli showed significant antibacterial activity against the pathogenic bacteria AvX005. A challenge test was conducted after rLys-g and rLys-c were expressed in grass carp L8824 liver cells, and compared with the survival rate of the control cells (46.3%), the survival rate of the experimental cells (77.6% for rLys-g and 68.6% for rLys-c) was significantly increased. Grass carp were infected with AvX005 on the second day after delivering pcDNA3.1-lys-g and pcDNA-lys-c with the Quil A/cholesterol/DDA/Carbopol (QCDC) adjuvant, and both pcDNA3.1-lys-g and pcDNA-lys-c provided 70% relative protection for grass carp. The activity of lysozyme and alkaline phosphatase in the serum of grass carp was significantly increased after injection of DNA. The expression of the immune factors IgM, C3 and IL8 in the kidney was upregulated to varying degrees for pcDNA3.1-lys-g and immune factors C3 and IgM was upregulated for pcDNA-lys-c. The results indicated that pcDNA3.1-lys-g and pcDNA-lys-c may be used as immunostimulants to protect grass carp from the pathogenic bacterium AvX005.

Development of autogenous vaccines for farmed European seabass against Aeromonas veronii using zebrafish as a model for efficacy assessment

Aeromonas veronii bv. sobria is an emerging pathogen for the European seabass cultured in the Aegean Sea (Mediterranean) causing significant problems in the Greek and Turkish aquaculture industry since no licensed vaccine is currently available for the disease. A bivalent vaccine was developed based on two phenotypically distinct strains of the pathogen, PDB (motile, pigment-producing strain) and NS (non-motile, non-pigment-producing). The two strains comprising the bivalent vaccine were evaluated as monovalent products in zebrafish before the seabass trials. Challenges using the homologous or the heterologous strain showed that both vaccines were protective with RPS values ranging between 66 and 100% in zebrafish. The bivalent vaccine was then tested in European seabass following dip or intraperitoneal administration. Efficacy was evaluated separately against both strains comprising the bivalent vaccine. Dip vaccination applied to juvenile seabass of 2.5 g average weight provided protection following challenge tests 30 days post vaccination only in one of the two strains tested (strain PDB, RPS: 88%). This was also the case in the injection vaccination of adult seabass of 60 g average weight where the vaccine was effective only against the PDB strain (RPS: 63%). High antibody titers against both strains were found at 30 and 60 days after intraperitoneal vaccination in the adult seabass. The use of zebrafish as a model for vaccine development for aquaculture species is discussed.

Probiotic Bacteria with High Alpha-Gal Content Protect Zebrafish against Mycobacteriosis

Mycobacteriosis affects wild fish and aquaculture worldwide, and alternatives to antibiotics are needed for an effective and environmentally sound control of infectious diseases. Probiotics have shown beneficial effects on fish growth, nutrient metabolism, immune responses, disease prevention and control, and gut microbiota with higher water quality. However, the identification and characterization of the molecules and mechanisms associated with probiotics is a challenge that requires investigation. To address this challenge, herein we used the zebrafish model for the study of the efficacy and mechanisms of probiotic interventions against tuberculosis. First, bacteria from fish gut microbiota were identified with high content of the surface glycotope Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal) that has been shown to induce protective immune responses. The results showed that probiotics of selected bacteria with high α-Gal content, namely Aeromonas veronii and Pseudomonas entomophila, were biosafe and effective for the control of Mycobacterium marinum. Protective mechanisms regulating immunity and metabolism activated in response to α-Gal and probiotics with high α-Gal content included modification of gut microbiota composition, B-cell maturation, anti-α-Gal antibodies-mediated control of mycobacteria, induced innate immune responses, beneficial effects on nutrient metabolism and reduced oxidative stress. These results support the potential of probiotics with high -Gal content for the control of fish mycobacteriosis and suggested the possibility of exploring the development of combined probiotic treatments alone and in combination with -Gal for the control of infectious diseases.

Transcriptome analysis of the Macrobrachium nipponense hepatopancreas provides insights into immunoregulation under Aeromonas veronii infection

The oriental river prawn Macrobrachium nipponense is a commercially important freshwater shrimp that is widely farmed in China. Aeromonas veronii is a conditional pathogen of farmed shrimp, which has caused huge economic losses to the industry. Therefore, there is urgency to study the host-pathogen interactions between M. nipponense and A. veronii to screen individuals with antimicrobial resistance. In this study, we examined the hepatopancreas of moribund M. nipponense infected with A. veronii and healthy individuals at both the histopathological and transcriptomic levels. We showed that A. veronii infection resulted in tubular necrosis of the M. nipponense hepatopancreas. Such changes likely affect assimilation, storage, and excretion by the hepatopancreas, which could ultimately affect the survival and growth of infected individuals. Among the 61,345 unigenes obtained through RNA sequencing and de novo transcriptome assembly, 232 were differentially expressed between the two groups. KEGG and GO analyses revealed that these differentially expressed genes were implicated in pathways, including PPAR, PI3K/AKT, and AMPK signaling. The results of this study will contribute to an analysis of the immune response of M. nipponense to A. veronii infection at the transcriptomic level. Furthermore, the RNA-seq data generated here provide an important genomic resource for research on M. nipponense in the absence of a reference genome.

Dual Antibacterial Activities and Biofilm Eradication of a Marine Peptide-N6NH2 and Its Analogs against Multidrug-Resistant Aeromonas veronii

Aeromonas veronii is one of the main pathogens causing various diseases in humans and animals. It is currently difficult to eradicate drug-resistant A. veronii due to the biofilm formation by conventional antibiotic treatments. In this study, a marine peptide-N6NH2 and its analogs were generated by introducing Orn or replacing with D-amino acids, Val and Pro; their enzymic stability and antibacterial/antibiofilm ability against multi-drug resistant (MDR) A. veronii ACCC61732 were detected in vitro and in vivo, respectively. The results showed that DN6NH2 more rapidly killed A. veronii ACCC61732 and had higher stability in trypsin, simulated gastric/intestinal fluid, proteinase K, and mouse serum than the parent peptide-N6NH2. DN6NH2 and other analogs significantly improved the ability of N6NH2 to penetrate the outer membrane of A. veronii ACCC61732. DN6NH2, N6PNH2 and V112N6NH2 protected mice from catheter-associated biofilm infection with MDR A. veronii ACCC61732, superior to N6NH2 and CIP. DN6NH2 had more potent efficacy at a dose of 5 μmol/kg (100% survival) in a mouse peritonitis model than other analogs (50-66.67%) and CIP (83.33%), and it inhibited the bacterial translocation, downregulated pro-inflammatory cytokines, upregulated the anti-inflammatory cytokine, and ameliorated multiple-organ injuries (including the liver, spleen, lung, and kidney). These data suggest that the analogs of N6NH2 may be a candidate for novel antimicrobial and antibiofilm agents against MDR A. veronii infections.

Characterization and pathogenicity of Aeromonas veronii associated with mortality in cage farmed grass carp, Ctenopharyngodon idella (Valenciennes, 1844) from the Central Himalayan region of India

In the study, Aeromonas strains (n = 12) were isolated from moribund grass carp fry reared in the cage culture unit from the Central Himalayan region of India. They were identified as Aeromonas veronii, by biochemically and 16S rRNA analysis. The experimental bath infection of grass carp fry was performed using A. veronii GCAFBLC 228, one of the 12 isolates at cell concentrations 10⁶ and 10⁸ CFU mL^-1. The infected fry showed varied behavioural characteristics followed by tail rot, black pigmentation and hemorrhage in the body 48-96 h post infection. The post bath challenged demonstrated maximum mortality (23%) at cell concentration 10⁸ CFU mL^-1 during 10^th and 12th day. Histopathology revealed hypertrophy, hyperplasia, fusion of gill lamellae, detachment and epithelial cell detachment in gill, swelling of hepatocytes, granular deposition in liver and tubular degeneration and yellow pigmented macrophage aggregates in the kidney. The in vitro assays for virulence traits recorded that A. veronii GCAFBLC 228 was β-haemolytic having strong cell surface hydrophobicity (CHS) characteristic (> 50%), precipitated after boiling, produced slime, non-suicidal and bound to crystal violet. The antibiogram showed that the strain was susceptible to ciprofloxacin (5 μg), cefotaxime (30 μg), ceftazidime (30 μg), cefoxitin (30 μg), ceftriaxone (30 μg), chloramphenicol (30 μg) and tetracycline (30 μg). Negative staining transmission electron microscopy revealed presence of the lateral flagellum-like structure and cell adherence possibly could be correlated with the pathogenicity of A. veronii GCAFBLC 228. The further investigation is warranted to study the transmission, pathogenesis and epidemiology of A. veronii GCAFBLC 228 to develop the best health management practice for cage farmed fish.

An effective live attenuated vaccine against Aeromonas veronii infection in the loach (Misgurnus anguillicaudatus)

Aeromonas veronii is a major pathogenic bacterium in humans and animals. When it causes outbreaks, there are enormous economic losses to the aquaculture industry. An effective live attenuated vaccine strain, ΔhisJ, was obtained in our previous studies by gene knockout in Aeromonas veronii TH0426 using the suicide vector pRE112. Here, we evaluated whether the live attenuated vaccine ΔhisJ was suitable for prevention of Aeromonas veronii infection by injection and immersion in loaches. Compared with that of the TH0426 wild-type strain, the virulence of the live vaccine was significantly weakened. Vaccine safety assessment results also indicated that 1 × 10⁷ CFU/mL live vaccine was safe and did not induce clinical symptoms or obvious pathological changes. Additionally, after challenging loaches with Aeromonas veronii TH0426, the relative percent survival of the IN3 injection group was 65.66%, and that of the IM group was 50.78%. Our data show that the live attenuated vaccine ΔhisJ can improve the immune protection rate of loaches. Furthermore, increased enzyme activity parameters (SOD, LZM, ACP, and AKP) in the skin mucus, increased enzyme activity parameters (SOD, LZM, ACP, AKP, and GPx) in the serum, increased specific IgM antibodies and cytokine IL-1β contents in the serum, and increased cytokine (IL-15, pIgR, IL-1β, and TNF-α) expression in the liver and spleen were observed. These data are the first to indicate that the live attenuated vaccine ΔhisJ is suitable for the development of a safe and effective vaccine against Aeromonas veronii infection in loach aquaculture.

Analysis of virulence and immunogenic factors in Aeromonas hydrophila: Towards the development of live vaccines

Aeromonas hydrophila, a bacterium that is widespread in aquatic environments, is responsible for causing haemorrhagic disease in both aquatic and terrestrial species. With the purpose of developing a live vaccine, herein we have investigated nine strains of A. hydrophila (Ah-01 to Ah-09) isolated from diseased fish. A study of virulence factors that contribute to pathogenicity and immunogenicity in the host Cyprinus carpio suggests that the presence of β-hly, act and fla genes contribute to pathogenesis: strains Ah-01, Ah-02 and Ah-03 (β-hly⁺ /act⁺ /fla⁺ genotype) were highly pathogenic to C. carpio, whereas Ah-05 and Ah-06 (β-hly^- /act^- /fla^- genotype) showed weak pathogenicity. Accordingly, Ah-02 and Ah-03 were selected to prepare inactivated vaccines, whereas Ah-05 and Ah-06 were chosen as live vaccines. Ah-06 live vaccine was found to have the best protective efficacy, with a protective rate of about 85%, whereas rates of other vaccines were significantly lower, in the range 37%-59%. In addition, DNA vaccines based on genes altA, aha and omp showed immune protection rates of 25%, 37.5% and 75%, respectively. Our data demonstrate that the β-hly^- /act^- /fla^- /altA⁺ /aha⁺ /omp⁺ genotype has weak pathogenicity and high immunogenicity, and provide a simple and effective way to screen for live A. hydrophila vaccines.

Alteration of the gut microbiome and immune factors of grass carp infected with Aeromonas veronii and screening of an antagonistic bacterial strain (Streptomyces flavotricini)

Aeromonas veronii is a widely distributed novel pathogen that can affect humans and animals, it can cause sepsis in fish with high mortality and serious economic losses to aquaculture. In the study, the gut microbiome of the infected and uninfected grass carp with Aeromonas veronii were analyzed probiotics and pathogenic bacteria by the Miseq high-throughput sequencing, the results showed that the infected fish were significantly higher in Proteobacteria, Firmicutes, Fusobacteria, and the immune factors in liver and kidney were up-regulated by qRT-PCR. In order to effectively inhibit the pathogen, we screened an actinomycete strain and had good antibacterial effect on Aeromonas veronii. The new antagonistic bacteria was named as Streptomyces flavotricini X101, the whole genome sequencing revealed that the metabolic process was most active. After grass carp was inoculated with the minimum inhibitory concentration of 900 μg/mL of the strain's fermentation supernatant, then Aeromonas veronii was injected, we found that the pathological symptoms such as body surface, anus and abdominal congestion were alleviated by H&E staining. Cellular experiments showed that it wasn't toxic to liver cells of grass carp. Overall, this is the first study of changes in intestinal flora, phenotype, and immune factors in grass crap infected with Aeromonas veronii, it had important theoretical significance and application value for immunization and prevention.

Cadaverine reverse transporter (CadB protein) contributes to the virulence of Aeromonas veronii TH0426

Aeromonas veronii is one of the main pathogens causing sepsis and ulcer syndrome in freshwater fish. Analysis of the results of epidemiological investigations in recent years has revealed that the virulence of A. veronii and its tolerance to drugs have been increasing year by year. Currently, most of the research on A. veronii focuses on its isolation, identification, and drug susceptibility, whereas research on its virulence factors and pathogenesis mechanisms is relatively rare. In this study, we identified and obtained the highly expressed TH0426 cadaverine reverse transporter (CadB) of A. veronii. We used efficient suicide plasmid-mediated homologous recombination to delete the cadB gene in TH0426 and constructed a cadB deletion strain. The LD50 of ΔcadB was 93.2 times higher than that of TH0426 in zebrafish, the toxicity of ΔcadB was 9.5 times less than that of TH0426 in EPC cells, and the biofilm formation ability of ΔcadB was 5.6-fold greater than that of TH0426. In addition, motility detection results indicated that ΔcadB had lost its swimming ability. The results of flagellar staining and TEM demonstrated that ΔcadB shed the flagella. In summary, the virulence and adhesion of A. veronii TH0426 were significantly decreased by the deletion of cadB, which might provide a theoretical basis for research into A. veronii virulence factors.

Oral Administration of Lactobacillus Casei Expressing Flagellin A Protein Confers Effective Protection against Aeromonas Veronii in Common Carp, Cyprinus Carpio

Aeromonas veronii is a pathogen capable of infecting humans, livestock and aquatic animals, resulting in serious economic losses. In this study, two recombinant Lactobacillus casei expressing flagellin A (FlaA) of A. veronii, Lc-pPG-1-FlaA (surface-displayed) and Lc-pPG-2-FlaA (secretory) were constructed. The immune responses in fish administered with recombinant L. casei were evaluated. The two recombinant L. casei were orally administered to common carp, which stimulated high serum IgM and induced higher ACP, AKP, SOD and LYZ activity. Using qRT-PCR, the expression of IL-10, IL-8, IL-1β, TNF-α and IFN-γ in the tissue of fish immunized with recombinant L. casei was significantly (p < 0.05) upregulated, which indicated that recombinant L. casei could activate the innate immune system to trigger the cell immune response and inflammatory response. Furthermore, recombinant L. casei was able to survive the intestinal environment and colonize in intestine mucosal. The study showed that after being challenged by A. veronii, fish administered with Lc-pPG-1-FlaA (70%) and Lc-pPG-2-FlaA (50%) had higher survival rates compared to Lc-pPG and PBS, indicating that recombinant L. casei might prevent A. veronii infection by activating the immune system to trigger immune responses. We demonstrated that flagellin as an antigen of vaccine, is acceptable for preventing A. veronii infection in fish. The recombinant L. casei expressing FlaA may be a novel mucosal vaccine for treating and controlling A. veronii.

Oral immunization with recombinant Lactobacillus casei expressing flaB confers protection against Aeromonas veronii challenge in common carp, Cyprinus carpio

Aeromonas veronii is an important type of gram-negative pathogen of human-livestock-aquatic animal and causes great economic losses in the aquaculture industry. Vaccination is an effective method of defence against A. veronii. There are many factors that restrict the use of vaccination, and the development of new oral vaccines is urgently needed. The selection of suitable antigens is of great significance for the development of aquaculture vaccines. Bacterial flagellin can specifically bind to TLR5 and induce the release of cytokines from the organism, which could be used in the development of vaccines. In this study, we constructed two recombinant Lactobacillus casei (L. casei) (surface-displayed or secretory) expressing the flaB of A. veronii and evaluated the effect of immune responses in common carp. The flaB gene (900 bp) of A. veronii was subcloned into the L. casei expression plasmids pPG-1 (surface-displayed) and pPG-2 (secretory). Western blot and immunofluorescence assays confirmed the expression of the recombinant flaB protein. Common carp immunized with Lc-pPG-1-flaB and Lc-pPG-2-flaB via oral administration route exhibited induction of antibody expression and innate immune responses. The results indicated that Lc-pPG-1-flaB and Lc-pPG-2-flaB can induce high levels of IgM, ACP, AKP, LZM and SOD activity in organisms, and Lc-pPG-1-flaB can induce even higher levels. The recombinant L. casei may effectively induce humoral immunity and increase the serum immunological index. Furthermore, leukocytes phagocytosis percentage and index of the recombinant L. casei were enhanced. The results of qRT-PCR showed that recombinant L. casei can significantly increase the expression of IL-10, IL-β, IFN-γ and TNF-α in the tissues of immunized common carp, compared with control groups. Viable recombinant L. casei strains, which were delivered directly survived throughout the intestinal tract. Common carp that received Lc-pPG-1-flaB (66.7%) and Lc-pPG-2-flaB (53.3%) exhibited higher survival rates than the controls after challenge with the pathogen A. veronii. Our work indicated that Lc-pPG-1-flaB and Lc-pPG-2-flaB had beneficial effects on immune response and enhanced the disease resistance of common carp against A. veronii infection. The combination of flaB delivery and the Lactic acid bacteria (LAB) approach may be a promising method for the development of oral vaccines for treating A. veronii. In future research, we will focus on the colonization ability of LAB in the intestines and on the impact of these bacteria on intestinal flora.