Development of attenuated erythromycin-resistant Streptococcus agalactiae vaccine for tilapia (Oreochromis niloticus) culture

The HSP70 and IL-1β of Nile tilapia as molecular adjuvants can enhance the immune protection of DNA vaccine against Streptococcus agalactiae infection

Globally, streptococcal disease caused by Streptococcus agalactiae is known for its high mortality rate, which severely limits the development of the tilapia breeding industry. As a third-generation vaccine, DNA vaccines have shown great application prospects in the prevention and control of aquatic diseases, but their low immunogenicity limits their development. The combination of DNA vaccines and molecular adjuvants proved to be an effective method for inducing protective immunity. This study constructed recombinant plasmids encoding tilapia HSP70 and IL-1β genes (pcHSP70 and pcIL-1β) to verify their effectiveness as molecular adjuvants for S. agalactiae DNA vaccine (pcSIP) in the immunized tilapia model. The results revealed that serum-specific IgM production, enzyme activities, and immune-related gene expression in tilapia immunized with pcSIP plus pcHSP70 or pcIL-1β were significantly higher than those in tilapia immunized with pcSIP alone. It is worth noting that combination with molecular adjuvants improved the immune protection of DNA vaccines, with a relative percentage survival (RPS) of 51.72% (pcSIP plus pcHSP70) and 44.83% (pcSIP plus pcIL-1β), respectively, compared with that of pcSIP alone (24.14%). Thus, our study indicated that HSP70 and IL-1β in tilapia are promising molecular adjuvants of the DNA vaccine in controlling S. agalactiae infection.

Collectin-K1 Plays a Role in the Clearance of Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus)

Collectin-K1 (CL-K1) is a multifunctional C-type lectin that has been identified as playing a crucial role in innate immunity. It can bind to carbohydrates on pathogens, leading to direct neutralization, agglutination, and/or opsonization, thereby inhibiting pathogenic infection. In this study, we investigated a homolog of CL-K1 (OnCL-K1) in Nile tilapia (Oreochromis niloticus) and its role in promoting the clearance of the pathogen Streptococcus agalactiae (S. agalactiae) and enhancing the antibacterial ability of the fish. Our analysis of bacterial load displayed that OnCL-K1 substantially reduced the amount of S. agalactiae in tissues of the liver, spleen, anterior kidney, and brain in Nile tilapia. Furthermore, examination of tissue sections revealed that OnCL-K1 effectively alleviated tissue damage and inflammatory response in the liver, anterior kidney, spleen, and brain tissue of tilapia following S. agalactiae infection. Additionally, OnCL-K1 was found to decrease the expression of the pro-inflammatory factor IL-6 and migration inhibitor MIF, while increasing the expression of anti-inflammatory factor IL-10 and chemokine IL-8 in the spleen, anterior kidney, and brain tissues of tilapia. Moreover, statistical analysis of survival rates demonstrated that OnCL-K1 significantly improved the survival rate of tilapia after infection, with a survival rate of 90%. Collectively, our findings suggest that OnCL-K1 plays a vital role in the innate immune defense of resisting bacterial infection in Nile tilapia. It promotes the removal of bacterial pathogens from the host, inhibits pathogen proliferation in vivo, reduces damage to host tissues caused by pathogens, and improves the survival rate of the host.

Construction of an alanine dehydrogenase gene deletion strain for vaccine development against Nocardia seriolae in hybrid snakehead (Channa maculata ♀ × Channa argus ♂)

Nocardia seriolae is the main pathogen of fish nocardiosis. In our previous study, alanine dehydrogenase was identified as a potential virulence factor of N. seriolae. On the basis of this fact, the alanine dehydrogenase gene of N. seriolae (NsAld) was knocked out to establish the strain ΔNsAld for vaccine development against fish nocardiosis in this study. The LD₅₀ of strain ΔNsAld was 3.90 × 10⁵ CFU/fish, higher than that of wild strain (5.28 × 10⁴ CFU/fish) significantly (p < 0.05). When the strain ΔNsAld was used as a live vaccine to immunize hybrid snakehead (Channa maculata ♀ × Channa argus ♂) at 2.47 × 10⁵ CFU/fish by intraperitoneal injection, the non-specific immune indexes (LZM, CAT, AKP, ACP and SOD activities), specific antibody (IgM) titers and several immune-related genes (CD4, CD8α, IL-1β, MHCIα, MHCIIα and TNFα) were up-regulated in different tissues, indicating that this vaccine could induce humoral and cell-mediated immune responses. Furthermore, the relative percentage survival (RPS) of ΔNsAld vaccine was calculated as 76.48% after wild N. seriolae challenge. All these results suggest that the strain ΔNsAld could be a potential candidate for live vaccine development to control fish nocardiosis in aquaculture.

Induction of attenuated Nocardia seriolae and their use as live vaccine trials against fish nocardiosis

Nocardia seriolae, a Gram-positive facultative intercellular pathogen, has been identified as the causative agent of fish nocardiosis, causing substantial mortality and morbidity of a wide range of fish species. Looking into that fact, the effective vaccine against this pathogen is urgently needed to control the significant losses in aquaculture practices. In order to induct attenuated strains for developing the potential live vaccines, the mutagenic N. seriolae strain S-250 and U-20 were obtained from wild-type strain ZJ0503 through continuous passaging and ultraviolet (UV) irradiation, respectively. Additionally, the biological characteristic, virulence, stability, mediating immune response and supplying protective efficacy to hybrid snakehead of the S-250 and U-20 strains were determined in the present study. The results showed that U-20 strain displayed dramatic changes in morphological characteristic and significant decreased in the virulence to hybrid snakehead, while that of S-250 strain had no obvious different in comparison to ZJ0503 strain. When hybrid snakehead were intraperitoneally injected with ZJ0503, S-250 and U-20 strains at their respective sub-clinical dosage, the non-specific immunity parameters (serum LYZ, POD, ACP, AKP and SOD activities), specific antibody (IgM) titers production and immune-related genes (CC1, CC2, IL-1β, IL-8, TNFα, IFNγ, MHCIα, MHCIIα, CD4, CD8α, TCRα and TCRβ) expression were up-regulated, indicating that they were able to trigger humoral and cell-mediated immune responses. Furthermore, the protective efficacy in hybrid snakehead after vaccination with ZJ0503, S-250 and U-20 strains, in terms of relative percentage survival (RPS), were 28.85%, 56.89% and 89.65% respectively. Taken together, two attenuated N. seriolae strains S-250 and U-20 were obtained successfully and they could elicit strong immune response and supply protective efficacy to hybrid snakehead against N. seriolae, which suggested that these two attenuated strains were the potential candidates for live vaccine development to control fish nocardiosis in aquaculture.

Attenuated Streptococcus agalactiae WC1535 ∆Sia perturbs the gut microbiota of Oreochromis niloticus, massively colonizes the intestine, and induces intestinal mucosal immunity after intraperitoneal inoculation

We previously developed and assessed the effectiveness of the attenuated Streptococcus agalactiae (Group B Streptococcus, GBS) strain WC1535 ∆Sia (with neuA-D gene cluster deletion) vaccine in tilapia (Oreochromis niloticus). In this study, we characterized the bacterial communities of the tilapia intestines by 16S rRNA high-throughput sequencing and assessed the serum antibody response, expression of immune-related genes, and histological changes following formalin-killed GBS vaccine (FKV) and the live attenuated vaccine ∆Sia (LAV). Results showed that FKV and LAV induced robust systemic and intestinal mucosal immune responses in tilapia without causing obvious pathological changes in the hindgut, spleen, and head kidney but exerted different effects on intestinal bacterial communities. The richness or diversity of the intestinal bacterial community of FKV tilapia showed no significant changes compared with that of the control fish (p &gt; 0.05) at either day 21 post-initial vaccination (21 dpiv) or day 35 (day 14 after the second immunization) (35 dpiv). The community composition of FKV tilapia and controls was significantly similar, although the relative abundance of some genera was significantly altered. Relative to control fish, the gut ecosystem of LAV tilapia was significantly disturbed with a substantial increase in community diversity at 21 dpiv (p &lt; 0.05) and a significant decrease at 35 dpiv in fish with high serum antibody response (ΔSia35H) (p &lt; 0.05). However, there was no significant difference between ΔSia35H and ΔSia35L (low serum antibody response) fish (p &gt; 0.05). Moreover, the community composition of LAV tilapia at 21 dpiv or 35 dpiv was considerably different from that of the controls. Particularly, GBS ∆Sia was found to be abundant in the intestine at 21 and 35 dpiv. This result suggested that the parenteral administration of the LAV (∆Sia) may also have the effect of oral vaccination in addition to the immune effect of injection vaccination. In addition, a significant correlation was found between the expression of immune-related genes and certain bacterial species in the intestinal mucosal flora. Our findings will contribute to a better understanding of the effects of inactivated and attenuated vaccines on gut microbiota and their relationship with the immune response.

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.

Trial Evaluation of Protection and Immunogenicity of Piscine Bivalent Streptococcal Vaccine: From the Lab to the Farms

Streptococcosis is one of the major diseases that causes devastation to farmed fish, leading to significant economic losses all around the world. Currently, two serotypes of Streptococcus agalactiae, serotype Ia and III, have been identified as virulent strains and major causative agents of the disease in farmed Nile tilapia (Oreochromis niloticus Linn.) in Thailand. Upon inactivated vaccine development, monovalent inactivated whole-cell vaccines demonstrated high specific antibody production against homologous serotypes and limited production with heterologous serotypes. However, for higher efficacy, a bivalent streptococcal vaccine was designed to maximize protective immunity to both serotypes. Interestingly, our bivalent vaccine could successfully induce specific antibody production against both serotypes with similar levels, and the response could extend over the 8 weeks of the experimental period. Evaluation of vaccines in the laboratory scale revealed relative percent survival (RPS) of vaccinated tilapia to serotype Ia (81.2 ± 9.4%) and serotype III (72.2 ± 4.8%), respectively. The efficacy of the bivalent vaccine showed significant RPS higher than the monovalent vaccine (p < 0.05) at 30 days, and the protection of all those vaccines was reduced thereafter. Evaluation of the vaccine in a farm trial in different locations in Thailand revealed the efficacy of the bivalent vaccine in increasing the production yield by greater than 80% in all tested farms in 2015 and 2021. Taken together, this study affirms the efficacy of the bivalent streptococcal vaccine in the prevention of streptococcus disease in Nile tilapia, which could be used in different areas. This vaccine development could be effectively applied in the tilapia culture industry.

Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

AIMS

This study aimed to develop a live attenuated vaccine as an effective approach to prevent streptococcosis in tilapia (Oreochromis niloticus).

METHODS AND RESULTS

We eliminated the virulence factor, sialic acid (Sia) encoded by the neuA-D gene cluster from the Group B Streptococcus (Streptococcus agalactiae, GBS) strain WC1535, to construct Sia-deficient S. agalactiae (ΔSia) mutant by homologous recombination. Results showed that the ΔSia mutant had higher adherence to HEp-2 cells and lower resistance to RAW264.7 cell phagocytosis than the wild-type S. agalactiae. The virulence of the ΔSia mutant to tilapia dramatically decreased with no virulence recovery. The relative percent survivals (RPSs) were 50.00% and 54.50% at 30 days when challenged at the wild-type WC1535 doses of 1.0 × 10⁷ and 5.0 × 10⁷ CFU fish^-1 , respectively, via intraperitoneal (IP) injection. The tilapia vaccinated via IP injection with the ΔSia mutant induced strong antibody agglutination titers. The expression of IL-1β, TNF-α, MHC-Iα, and MHC-IIβ could be enhanced in the intestine, spleen, and head kidney for tilapia administered with the ΔSia mutant.

CONCLUSIONS

GBS Sia plays a critical role in adherence to HEp-2 cells and resistance to the immune clearance of RAW264.7 cells. Moreover, the ΔSia mutant is a safe, stable, and immunogenic live attenuated vaccine candidate to protect tilapia against GBS infection.

SIGNIFICANCE AND IMPACT OF STUDY

The results offer more evidence of the importance of Sia in GBS and may be instructive in the control of tilapia streptococcosis.

Natural component plumbagin as a potential antibacterial agent against Streptococcus agalactiae infection

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), can infect humans, terrestrial animals and fish. The emergence of bacterial resistance of S. agalactiae to antibiotics leads to an urgent need of exploration of new antimicrobial agents. In the study, the antibacterial activity of natural component plumbagin (PLB) against S. agalactiae was investigated in vitro and in vivo. The results showed that the minimal inhibitory concentration (MIC) of PLB against S. agalactiae was 8 mg/L. The growth curve assay revealed that PLB could inhibit the growth of S. agalactiae. In addition, the time-killing curve showed that S. agalactiae was killed almost completely by 2-fold MIC of PLB within 12 h. Transmission electron microscopy results showed obvious severe morphological destruction and abnormal cells of S. agalactiae after treated with PLB. The pathogenicity of S. agalactiae to zebrafish was significantly decreased after preincubation with PLB for 2 h in vitro, further indicating the bactericidal activity of PLB. Interestingly, PLB could kill S. agalactiae without inducing resistance development. Furthermore, pretreatment and post-treatment assays suggested that PLB also exhibited the antibacterial activity against S. agalactiae infection in vivo by effectively reducing the bacterial load and improving the survival rate of S. agalactiae-infected zebrafish. In summary, PLB had potent antibacterial activity against S. agalactiae in vitro and in vivo, and it could be an excellent antimicrobial candidate to prevent and control S. agalactiae infection.

State-of-the-Art Vaccine Research for Aquaculture Use: The Case of Three Economically Relevant Fish Species

In the last three decades, the aquaculture sector has experienced a 527% growth, producing 82 million tons for a first sale value estimated at 250 billion USD. Infectious diseases caused by bacteria, viruses, or parasites are the major causes of mortality and economic losses in commercial aquaculture. Some pathologies, especially those of bacterial origin, can be treated with commercially available drugs, while others are poorly managed. In fact, despite having been recognized as a useful preventive measure, no effective vaccination against many economically relevant diseases exist yet, such as for viral and parasitic infections. The objective of the present review is to provide the reader with an updated perspective on the most significant and innovative vaccine research on three key aquaculture commodities. European sea bass (Dicentrarchus labrax), Nile tilapia (Oreochromis niloticus), and Atlantic salmon (Salmo salar) were chosen because of their economic relevance, geographical distinctiveness, and representativeness of different culture systems. Scientific papers about vaccines against bacterial, viral, and parasitic diseases will be objectively presented; their results critically discussed and compared; and suggestions for future directions given.

Characterization of novel antigenic vaccine candidates for nile tilapia (Oreochromis niloticus) against Streptococcus agalactiae infection

Streptococcus agalactiae is one of the important pathogens responsible for high mortality and economic losses of the tilapia industry worldwide. Based on ten serovars of S. agalactiae infection, subunit vaccine with conserved antigens is promising strategy corresponding stimulated long-term immunity and provides protection for animals against different serotypes of S. agalactiae. In the present study, eight proteins (AP, AL, LivK, ESAT6, essA, essB, essC and esaA) were selected from the S. agalactiae serotype Ia genome as immunogenic antigens with bioinformation and immune experiment assays. These recombinant proteins were successfully obtained through expression in Escherichia coli and the immunogenicity was assessed in tilapia challenge model. The results showed that the recombinant proteins caused high-level-specific antibodies production and high lysozyme activities, suggesting that the recombinant proteins induced specific humoral immune response and innate immune response of tilapia. The signficant increase were observed in the cytokines levels of TNF-α, IL-1β, IFN-γ, cc1, cc2 and immune-related genes levels of CD8α and MHC factors in the spleen and head kidney tissues, suggesting that the recombinant proteins induced immune response of tilapia through cytokines signal pathway and activated high cytotoxic T-lymphocyte (CTL) activity of tilapia. Furthermore, vaccinated tilapia conferred high levels of protection against challenge with a lethal dose of highly virulent serovar Ⅰa (highest RPS was 91.60% in AL and essC protein groups). Our results indicated that the eight recombinant proteins induced high level of immune responses and offered protection against S. agalactiae infection, could be potential subunit vaccine candidates.

An effective live attenuated vaccine against Streptococcus agalactiae infection in farmed Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is an important pathogen associated with various aquatic animals, especially tilapia. Streptococcosis has greatly limited the healthy development of tilapia aquaculture in recent times. The development of novel effective vaccines is important for the prevention and control of streptococcosis in fish. We previously constructed a non-encapsulated S. agalactiae strain △cps by the in-frame deletion method. Here, we evaluated whether this mutant △cps is safe for tilapia and suitable for protection against streptococcosis. We observed that the △cps strain was non-pathogenic to tilapia, and there was no reversion of virulence when it was passaged in tilapia. Moreover, the △cps strain survived for at least 11 d in the main immune organs of tilapia. The tilapia vaccinated via intraperitoneal (IP) injection with △cps strain induced a high antibody titer, and the IgM antibody levels were significantly higher in the vaccinated group than in the control group. The vaccination protected tilapia against the S. agalactiae challenge with a relative percent survival of 90.47%. In addition, tilapia immunized with the △cps strain showed significantly higher expression level of IFN-γ, IL-1β, MyD88, IgM, and MHC-Iα in the head kidney than those in the control during the entire observation period. The expression of MHC-IIβ was inhibited during 1-7 d of immunization. These results revealed that the △cps strain is able to induce humoral and cell-mediated immune response in tilapia. Therefore, the strain △cps has a broad application prospect as a target for attenuation in vaccine development.