Development of an Aeromonas hydrophila recombinant extracellular protease vaccine

Lonicera japonica protects Pelodiscus sinensis by inhibiting the biofilm formation of Aeromonas hydrophila

Aquaculture has suffered significant financial losses as a result of the infection of zoonotic Aeromonas hydrophila, which has a high level of resistance to classic antibiotics. In this study, we isolated an A. hydrophila strain B3 from diseased soft-shelled turtle (Pelodiscus sinensis), which is one of the most commercially significant freshwater farmed reptiles in East Asia, and found that A. hydrophila was its dominant pathogen. To better understand the inhibition effect and action mechanism of Chinese herbs on A. hydrophila, we conducted Chinese herbs screening and found that Lonicera japonica had a significant antibacterial effect on A. hydrophila B3. Experimental therapeutics of L. japonica on soft-shelled turtle showed that the supplement of 1% L. japonica to diet could significantly upregulate the immunity-related gene expression of soft-shelled turtle and protect soft-shelled turtle against A. hydrophila infection. Histopathological section results validated the protective effect of L. japonica. As the major effective component of L. japonica, chlorogenic acid demonstrated significant inhibitory effect on the growth of A. hydrophila with MIC at 6.4 mg/mL. The in vitro assay suggested that chlorogenic acid could inhibit the hemolysin/protease production and biofilm formation of A. hydrophila and significantly decrease the expression of quorum sensing, biofilm formation, and hemolysin-related genes in A. hydrophila. Our results showed that the Chinese herb L. japonica would be a promising candidate for the treatment of A. hydrophila infections in aquaculture, and it not only improves the immune response of aquatic animals but also inhibits the virulence factor (such as biofilm formation) expression of A. hydrophila. KEY POINTS: • A. hydrophila was the dominant pathogen of the diseased soft-shelled turtle. • L. japonica can protect soft-shelled turtle against A. hydrophila infection. • Chlorogenic acid inhibits the growth and biofilm formation of A. hydrophila.

Effects of glycyrrhetinic acid β on growth and virulence of Aeromonas hydrophila

Aeromonas hydrophila is a significant pathogen to freshwater farmed animals, and antibiotics are usually used to control the bacterial septicemia caused by A. hydrophila. Due to the severe situation of development and spread of antibiotic resistance, there are stricter restrictions on antibiotics used in aquaculture. To evaluate the feasibility of glycyrrhetinic acid β (GA) as an alternative therapy against bacterial infection, in this study, an A. hydrophila isolated from diseased fish is used to test the antibacterial, anti-virulence activity and therapeutic effect of GA in vitro and in vivo, respectively. Results showed that GA did not affect the growth of A. hydrophila in vitro, while it could down-regulate (p < 0.05) the mRNA expression of the hemolysis-related genes hly and aerA, and significantly inhibited (p < 0.05) hemolytic activity of A. hydrophila. In addition, in vivo test showed that oral administration of GA was ineffective in controlling acute infections caused by A. hydrophila. In conclusion, these findings suggested that GA was a potential anti-virulence candidate against A. hydrophila, but the application of GA for the prevention and treatment of A. hydrophila-related diseases was still a long way.

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.

Identification of a novel protective antigen, 3-oxoacyl-[acyl-carrier-protein] synthase II of Streptococcus equi ssp. zooepidemicus which confers protective effects

Streptococcus equi ssp. zooepidemicus (SEZ) is an important swine pathogen and responsible for a wide variety of infections in many animal species. FabF was a novel protein identified in the previous study. However, its protective efficacy remained to be evaluated. In this study, recombinant fabF of SEZ was expressed and showed a strong immunoreactivity with mini-pig convalescent sera. Study in mice revealed that the recombinant protein induced a marked antibody response and protected 80% of mice against SEZ infection. The hyperimmune sera against fabF could efficiently kill the bacteria in the phagocytosis test. In addition, it was also found that anti- fabF antibodies can significantly inhibit the formation of SEZ biofilm. These study suggest that fabF may represent immunogens of interest for vaccine development against SEZ infection.

Recombinant ATPase of Virulent Aeromonas hydrophila Protects Channel Catfish Against Motile Aeromonas Septicemia

Channel catfish farming dominates the aquaculture industry in the United States. However, epidemic outbreaks of motile Aeromonas septicemia (MAS), caused by virulent Aeromonas hydrophila (vAh), have become a prominent problem in the catfish industry. Although vaccination is an effective preventive method, there is no vaccine available against MAS. Recombinant proteins could induce protective immunity. Thus, in this work, vAh ATPase protein was expressed, and its protective capability was evaluated in catfish. The purified recombinant ATPase protein was injected into catfish, followed by experimental infection with A. hydrophila strain ML09-119 after 21 days. Results showed catfish immunized with ATPase exhibited 89.16% relative percent survival after challenge with A. hydrophila strain ML09-119. Bacterial concentrations in liver, spleen, and anterior kidney were significantly lower in vaccinated fish compared with the non-vaccinated sham group at 48 h post-infection (p < 0.05). Catfish immunized with ATPase showed a significant (p < 0.05) higher antibody response compared to the non-vaccinated groups. Overall, ATPase recombinant protein has demonstrated potential to stimulate protective immunity in catfish against virulent A. hydrophila infection.

OmpW expressed by recombinant Lactobacillus casei elicits protective immunity against Aeromonas veronii in common carp

Aeromonas veronii is an opportunistic pathogen that is capable of infecting both aquatic livestock and mammals. Natural infection in fishes results in irreparable damage to the aquaculture industry. In this study, we sought to investigate whether recombinant Lactobacillus casei expressing the outer membrane protein W (OmpW) of A.veronii could elicit protective immunity against A.veronii infections. We generated two recombinant Lactobacillus casei (L.casei) strains expressing the OmpW of A.veronii (surface-displayed or secreted) and evaluated the effect on immune responses in a fish model. A 600-bp gene fragment was subcloned into the L.casei expression plasmids pPG-1 (surface-displayed) and pPG-2 (secreted). Expression of the recombinant OmpW protein was also confirmed by Western blot and immunofluorescence assays. Common carp immunized with Lc-pPG-1- OmpW and Lc-pPG-2- OmpW via oral administration elicited high serum specific antibody titers and high LZM, ACP, and SOD activities. High levels of the IL-10, IL-β, IFN-γ, and TNF-α genes in different organs indicated that the inflammatory response and cell immune response were triggered. Additionally, when immunized fish were challenged with A.veronii, Lc-pPG1-OmpW and Lc-pPG2-OmpW demonstrated 40% and 50% protective efficacy. These data indicate that the combination of OmpW delivery and the lactic acid bacteria (LAB) approach may be a promising mucosal therapeutic strategy for treatment of A.veronii.

Analysis of agglutinants elicited by antiserum of channel catfish immunized with extracellular proteins of virulent Aeromonas hydrophila

Motile Aeromonas septicemia (MAS), caused by new virulent Aeromonas hydrophila (vAh) strains, has been one of the major diseases in channel catfish in recent years. Previous studies showed that channel catfish developed immunity against vAh infection after immunization with the pathogen's extracellular proteins (ECP). To understand the mechanisms associated with the immunity, anti-ECP fish serum (antiserum) was analyzed in this study. Our results revealed that the antiserum elicited agglutination of both ECP and cells of vAh. Five fish proteins were identified in ECP agglutinants, including two innate immunity associated proteins (serotransferrin and rhamnose-binding lectin), two immunoglobulin M (IgM) molecules (IgM heavy chain and light chain) and a constitutively-produced protein (warm temperature acclimation protein). More than 68 vAh proteins in ECP were recognized and caused to aggregate by IgM in the antiserum. IgM was isolated from vAh cell agglutinants and the native IgM was shown to form a tetramer that was responsible for bacterial agglutination. Immunoblotting analysis indicated that the isolated native IgM was able to recognize some proteins in ECP, such as aerolysin and hemolysin (in the form of a high molecular weight heterologous polymer). Gene expression analysis by quantitative PCR showed that fish immunized with vAh ECP had more transcripts of genes coding for IgM, serotransferrin and rhamnose binding lectin than mock-immunized fish. Both innate and antibody-mediated immune responses in serum and expressed genes contributed to fish immunity upon immunization with ECP. Results of this study shed light on the versatility of vAh antigens and catfish IgM, which would help identify specific antigens for vaccine development and antigen specific antibodies in catfish.

Comparative Extracellular Proteomics of Aeromonas hydrophila Reveals Iron-Regulated Secreted Proteins as Potential Vaccine Candidates

In our previous study, several iron-related outer membrane proteins in Aeromonas hydrophila, a serious pathogen of farmed fish, conferred high immunoprotectivity to fish, and were proposed as potential vaccine candidates. However, the protective efficacy of these extracellular proteins against A. hydrophila remains largely unknown. Here, we identified secreted proteins that were differentially expressed in A. hydrophila LP-2 in response to iron starvation using an iTRAQ-based quantitative proteomics method. We identified 341 proteins, of which 9 were upregulated in response to iron starvation and 24 were downregulated. Many of the differently expressed proteins were associated with protease activity. We confirmed our proteomics results with Western blotting and qPCR. We constructed three mutants by knocking out three genes encoding differentially expressed proteins (Δorf01830, Δorf01609, and Δorf03641). The physiological characteristics of these mutants were investigated. In all these mutant strains, protease activity decreased, and Δorf01609, and Δorf01830 were less virulent in zebrafish. This indicated that the proteins encoded by these genes may play important roles in bacterial infection. We next evaluated the immune response provoked by the six iron-related recombinant proteins (ORF01609, ORF01830, ORF01839, ORF02943, ORF03355, and ORF03641) in zebrafish as well as the immunization efficacy of these proteins. Immunization with these proteins significantly increased the zebrafish immune response. In addition, the relative percent survival (RPS) of the immunized zebrafish was 50-80% when challenged with three virulent A. hydrophila strains, respectively. Thus, these extracellular secreted proteins might be effective vaccine candidates against A. hydrophila infection in fish.

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

Aeromonas veronii is a type of human-livestock-aquatic animal pathogen; it is widely found in nature and causes many deaths among aquatic animals. Extracellular products (ECPs) are secreted by the pathogen during growth and reproduction. These products are considered effective protective antigens that can induce the host to produce an immune response. In this study, the ECPs of A.veronii TH0426 were prepared by ultrafiltration, and then the pathogenicity and enzymatic activity of the ECPs were determined. All the groups were injected intraperitoneally, as follows: group one: ECP protein with an equal volume of Freund's adjuvant; group two: ECPs and formalin-killed cells (FKC) of A.veronii combined with an equal volume of Freund's adjuvant (FKC + ECPs); group three: formalin-killed cells (FKC) of A.veronii combined with an equal volume of Freund's adjuvant (FKC); and, group four: sterile PBS as the control group. The expression levels of IgM, IL-1β, and TNF-α and the lysozyme activity in blood were examined at 7, 14, and 21 days after the immunizations. The results show that the ECPs can produce protease, lipase, amylase and hemolyase, and there was no lecithinase, urease, or gelatinase activity. The results indicate that the ECPs were clearly pathogenic to koi fish, and the LD₅₀ dose was 391.6 μg/fish. Throughout this study, the RPS of the three experimental groups were 75%, 50%, and 70%. This study indicates that the ECPs of A.veronii can effectively enhance the ability of kio fish to resist bacterial invasion.

Development and validation of glycoprotein-based native-subunit vaccine for fish against Aeromonas hydrophila

Aeromonas hydrophila is known to be causative agent of an infection named as Bacterial haemorrhagic septicaemia or red pest in freshwater fish. The aim of this study was to develop and validate the glycoprotein-based fish vaccine against Aeromonas hydrophila. For this aim, after identification and characterization of A. hydrophila isolates from fish farms, one A. hydrophila isolate was selected as vaccine strain. Antigenic glycoproteins of this vaccine strain were determined by Western blotting and glycan detection kit. The connection types of these glycoproteins were examined by glycoprotein differentiation kit. Two glycoproteins, molecular weights of 19 and 38 kDa, with SNA connection type were selected for use in vaccination trials. After their purification by SNA-specific lectin and size-exclusion chromatography, protection studies with purified proteins were performed. For challenge trials, four experimental fish groups were designated: Group I (with montanide), Group II (with montanide and ginseng), Group III [with Al(OH)3 ] and Group IV [with Al(OH)3 and ginseng]. The survival ratings of fish were determined, and protection was calculated as 21.56%, 29.41%, 69.83% and 78.88% in groups I, II, III and IV, respectively. In conclusion, A. hydrophila glycoproteins with Al(OH)3 and ginseng could be used as a safe and effective vaccine for fish.

Identification and characterization of a Streptococcus equi ssp. zooepidemicus immunogenic GroEL protein involved in biofilm formation

Streptococcus equi ssp. zooepidemicus (S. equi spp. zooepidemicus) is an opportunistic pathogen that causes major economic losses in the swine industry in China and is also a threat for human health. Biofilm formation by this bacterium has been previously reported. In this study, we used an immunoproteomic approach to search for immunogenic proteins expressed by biofilm-grown S. equi spp. zooepidemicus. Seventeen immunoreactive proteins were found, of which nine common immunoreactive proteins were identified in planktonic and biofilm-grown bacteria. The immunogenicity and protective efficacy of the S. equi spp. zooepidemicus immunoreactive GroEL chaperone protein was further investigated in mice. The protein was expressed in vivo and elicited high antibody titers following S. equi spp. zooepidemicus infections of mice. An animal challenge experiment with S. equi spp. zooepidemicus showed that 75% of mice immunized with the GroEL protein were protected. Using in vitro biofilm inhibition assays, evidence was obtained that the chaperonin GroEL may represent a promising target for the prevention and treatment of persistent S. equi spp. zooepidemicus biofilm infections. In summary, our results suggest that the recombinant GroEL protein, which is involved in biofilm formation, may efficiently stimulate an immune response, which protects against S. equi spp. zooepidemicus infections. It may therefore be a candidate of interest to be included in vaccines against S. equi spp. zooepidemicus infections.

The effect of an adding histidine on biological activity and stability of Pc-pis from Pseudosciaena crocea

Pc-pis is a novel piscidin-like antimicrobial polypeptide that was identified in Pseudosciaena crocea. Although active against most bacteria tested, Pc-pis was inactive against Aeromonas hydrophila and Pseudomonas aeruginosa. The Pc-pis analogue Pc-pis-His was designed by adding a histidine residue at the carboxyl terminal. Pc-pis-His demonstrated a more broad-spectrum and stronger antimicrobial activity against a representative set of microorganisms and more potent antiparasitic activity against Cryptocaryon irritans trophonts than Pc-pis. The stability assay revealed that Pc-pis-His was active against Staphylococcus aureus not only in acidic (pH 5.5-7.3) and relatively low concentration monovalent cation (0-160 mM NaCl) environments but also in alkaline (pH 7.5-9.5), divalent cation (1.25-160 mM MgCl2 and 1.25-40 mM CaCl2) and high concentration monovalent cation (320-2560 mM NaCl) environments, which indicates that the added histidine residue conferred better salt-, acid- and alkali-tolerance to Pc-pis-His. Pc-pis-His also possessed the desired heat-tolerance, which was reflected by the antimicrobial activity of the peptide after being boiled for 10-60 minutes. Hemolytic activity analysis revealed that Pc-pis-His at concentrations up to 6 µM exhibited no hemolysis against human erythrocytes, with 6 µM being a concentration that is highly active against most of the microorganisms tested, although the hemolytic activity of Pc-pis-His was enhanced compared to Pc-pis. These results provide a unique, reasonable basis for designing novel piscidins with potent, broad-spectrum and stable antimicrobial activity and new insight into the future development of piscidins as potential therapeutic agents against microbial and external protozoan parasite infections.

Identification of protective immunogens from extracellular secretome of Edwardsiella tarda

Edwardsiella tarda is an opportunistic pathogen that causes a great loss in aquaculture. Identification of immune protective immunogens is a key step for development of subunit vaccines and control of the infectious diseases caused by the bacterium. This study aims to identify the protective antigens from extracellular secretory proteome of E. tarda. Out of 38 extracellular secretory proteins predicted by PSORTb, 20 genes were randomly cloned and their recombinant proteins were expressed in Escherichia coli BL21 and purified by either affinity chromatography or inclusion body washing. The purified recombinant proteins were used for investigation of immune protection in zebrafish model using active immunization approach. Half of them had significant immune protection compared with the control. Out of them, four, EseC, ETAE_2088, FlgD and ETAE_2130, showed approximately 60% relative percent survivals as a result of the highly protective antigens identified. Except for FlgD, the other three were first reported here. Moreover, the present study identified EseC and ETAE_2088 in bacterial extracellular fraction. These results indicate that secretory proteome is an interesting pool used for identification of immune protective antigens, and the four highly protective antigens identified provide useful candidates for development of subunit vaccines.