Development of live attenuated Streptococcus agalactiae vaccine for tilapia via continuous passage in vitro

Recent Advances in Oral Vaccines for Animals

Compared to traditional injected vaccines, oral vaccines offer significant advantages for the immunization of livestock and wildlife due to their ease of use, high compliance, improved safety, and potential to stimulate mucosal immune responses and induce systemic immunity against pathogens. This review provides an overview of the delivery methods for oral vaccines, and the factors that influence their immunogenicity. We also highlight the global progress and achievements in the development and use of oral vaccines for animals, shedding light on potential future applications in this field.

Duration of Protection and Humoral Immune Response in Nile Tilapia (Oreochromis niloticus L.) Vaccinated against Streptococcus agalactiae

Streptococcosis caused by Streptococcus agalactiae (S. agalactiae) is a major bacterial disease affecting the production of Nile tilapia (Oreochromis niloticus L.), causing significant economic losses due to mortality in the growing phase. Vaccination is the most effective method for preventing streptococcosis on Nile tilapia farms. In Brazil, the major tilapia-producing regions have long production cycles (6-10 months) and harvest tilapias weighing over 900 g for fillet production. Thus, data on the duration of the humoral immune response and protection in farmed tilapia have not been reported or are poorly described. Furthermore, the efficiency of serological testing for the long-term monitoring of immune responses induced by vaccination against S. agalactiae has never been addressed. This study evaluated the duration of protection and humoral immune response induced in Nile tilapia vaccinated against S. agalactiae until 300 days post-vaccination (dpv). The immunization trial was composed of two groups: vaccinated (Vac), vaccinated intraperitoneally with a commercial vaccine, and unvaccinated (NonVac) group, injected fish with sterile saline solution. At 15, 30, 150, 180, 210, and 300 dpv, blood sampling was conducted to detect anti-S. agalactiae IgM antibodies using indirect Enzyme-Linked Immunosorbent Assay (ELISA), and the fish were challenged with pathogenic S. agalactiae to determine the duration of vaccine protection through relative percentage survival (RPS). Spearman's rank correlation was performed between the ELISA optical density (OD) of vaccinated tilapia and the duration of vaccine protection (RPS). The mean cumulative mortality in NonVac and Vac groups ranged from 65 to 90% and less than 35%, respectively. The average RPS was 71, 93, 94, 70, 86, and 67% at 15, 30, 150, 180, 210, and 300 dpv, respectively. RPS revealed that the vaccine provided protection from 15 to 300 dpv. The specific anti-S. agalactiae IgM antibody levels were significantly higher in the Vac group than that non-Vac group up to 180 dpv. The vaccinated fish exhibited significant protection for up to 10 months after vaccination. There was a positive correlation between the antibody response and RPS. This study revealed that a single dose of commercial vaccine administered to Nile tilapia can confer long-term protection against S. agalactiae and that indirect ELISA can monitor the duration of the humoral immune response for up to six months following vaccination. Finally, vaccine protection over six months can be associated with other components of the fish immune system beyond the humoral immune response by IgM antibodies.

In silico and experimental analysis of the repeated domains in BvaP, a protein important for GBS vaginal colonization

Streptococcus agalactiae (group B strep, GBS) infections in neonates are often fatal and strongly associated with maternal GBS vaginal colonization. Previously, we highlighted the importance of a formerly uncharacterized protein, BvaP, in GBS vaginal colonization. BvaP is highly conserved across GBS and is made up of repeated domains, with a variable number of repeats between strains. Here, we evaluate the prevalence of BvaP repeated domains and their relevance in phenotypes previously associated with vaginal colonization. Using in silico analysis, we found that the number of repeats in the BvaP protein does not generally appear to be associated with serotype, isolation site, or host. Using BvaP truncations in GBS strain A909, we determined that a smaller number of repeats was correlated with decreased bacterial chain length, but adherence to vaginal epithelial cells was complemented using BvaP containing one, two, three, or five repeats. Future research will be geared toward understanding the host immune response to BvaP in vivo and whether vaginal carriage or host response is dependent on the BvaP repeated domains.

Correlation of Streptococcus agalactiae concentration on immune system and effective dose of inactivated vaccine for Chitralada 3 strain Nile tilapia (Oreochromis niloticus) in Thailand

The main pathogen in the Nile tilapia (Oreochromis niloticus) culture, Streptococcus agalactiae, causes economic harm. Infected fish's immune systems worked to eliminate of the infection. This study demonstrated the effect of different bacterial concentrations on tilapia immunity and optimal vaccine concentration to induce immunity in Nile tilapia. The experiment was performed at 102, 104, 106, 108, and 1010 CFU/fish of S. agalactiae compared with the control (PBS) through intraperitoneal injection for 72 h. Fish that survived employed to gather blood, and immune responses were assessed through measures of the survival rate include blood smears, antibody titers, and immunoglobulin gene expression. The vaccine experiment investigated formalin-inactivated S. agalactiae vaccination and administered S. agalactiae injections for 14 days. The statistic revealed a significant difference (p < 0.05) in the 108 and 1010 CFU/fish injections with high survival rates (62.22% and 53.33%, respectively). Immunoglobulin gene expression was highly represented in the 1010 CFU/fish injection; antibody titers were significantly improved from the control group, and antibody levels were high in the 1010 CFU/fish injection. The analysis of blood cell types using the blood smear method revealed a progressive increase in leucocytes, particularly lymphocytes, neutrophils, and monocytes, in the treatment group compared to the control group. Moreover, the erythrocyte/leucocyte ratio decreased significantly in response to the high bacterial injection, indicating an increase in leucocytes. Conversely, the erythrocyte level stayed ed within at the 7.03-9.70 × 102 cell/ml and shown no significant difference (p > 0.05). The lymphocytes were almost two-fold in 1010 CFU/fish compared to 108 CFU/fish. As depicted in the lowest concentration of 106 CFU/fish, the vaccine performance had a high relative percent survival (RPS) at 86.67%. This research suggested that the tilapia infected with high S. agalactiae concentrations did not affect the mortality of the tilapia, and vaccine concentration was effective in 106 CFU/fish.

An Introduction to Relevant Immunology Principles with Respect to Oral Vaccines in Aquaculture

Vaccines continue to play an enormous role in the progression of aquaculture industries worldwide. Though preventable diseases cause massive economic losses, injection-based vaccine delivery is cost-prohibitive or otherwise impractical for many producers. Most oral vaccines, which are much cheaper to administer, do not provide adequate protection relative to traditional injection or even immersion formulas. Research has focused on determining why there appears to be a lack of protection afforded by oral vaccines. Here, we review the basic immunological principles associated with oral vaccination before discussing the recent progress and current status of oral vaccine research. This knowledge is critical for the development and advancement of efficacious oral vaccines for the aquaculture industry.

First Investigation of the Optimal Timing of Vaccination of Nile Tilapia (Oreochromis niloticus) Larvae against Streptococcus agalactiae

To investigate early immune responses and explore the optimal vaccination periods, Nile tilapia at 1, 7, 14, 21, 28, 35, and 42 days after yolk sac collapse (DAYC) were immersed in formalin-killed Streptococcus agalactiae vaccine (FKV-SA). A specific IgM was first detected via ELISA in the 21 DAYC larvae (0.108 g) at 336 h after vaccination (hav), whereas in the 28-42 DAYC larvae (0.330-0.580 g), the specific IgM could be initially detected at 24 hav. qRT-PCR analysis of the TCRβ, CD4, MHCIIα, IgHM, IgHT, and IgHD genes in 21-42 DAYC larvae immunized with the FKV-SA immersion route for 24, 168, and 336 hav revealed that the levels of most immune-related genes were significantly higher in the vaccinated larvae at all DAYCs than in the control larvae (p < 0.05) at 336 hav. Immunohistochemistry demonstrated stronger IgM signals in the gills, head kidney, and intestine tissues at 21, 28, and 35 DAYC in all vaccinated larvae compared with the control. Interestingly, at all DAYCs, FKV-SA larvae exhibited significantly higher survival rates and an increased relative percent survival (RPS) than the control after challenge with viable S. agalactiae, particularly in larvae that were immunized with FKV-SA at 168 and 336 hav (p < 0.05).

Efficacy of polyvalent vaccine on immune response and disease resistance against streptococcosis/lactococcosis and yersiniosis in rainbow trout (Oncorhynchus mykiss)

Diseases are the most significant challenge in the development and stability of aquaculture. In this study, the immunogenic efficiency of polyvalent streptococcosis/lactococcosis and yersiniosis vaccines was evaluated by injection and immersion methods in rainbow trout.. The 450 fish with an average weight of 50 ± 5 g were divided into three treatments and three replications as follows: injection vaccine treatment, immersion vaccine treatment and control group without vaccine administration. Fish were kept for 74 days and sampling was done on days 20, 40 and 60. Then, from the 60th to the 74th day, the immunized groups were challenged with three bacteria Streptococcus iniae (S. iniae), Lactococcus garvieae (L. garvieae) and Yersinia ruckeri (Y. ruckeri) separately. A significant difference was observed in the weight gained (WG) in the immunized groups compared to the control group (P < 0.05). The relative survival percentage (RPS) after 14 days of challenge with S. iniae, L. garvieae and Y. ruckeri in the injection group compared to the control group increased respectively (60%, 60% and 70%), (P < 0.05). Also, RPS in the immersion group had an increase respectively (30%, 40% and 50%) after the challenge with S. iniae, L.garvieae and Y. ruckeri compared to the control group. Immune indicators such as antibody titer, complement and lysozyme activity significantly increased in comparison to the control group (P < 0.05). In general, it can be concluded that applying three vaccines by injection and immersion method has significant effects on immune protection and survival rate. However, the injection method is more effective and more suitable than the immersion method.

Comparative study of the effects of Montanide™ ISA 763A VG and ISA 763B VG adjuvants on the immune response against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is regarded as a major bacterial pathogen of farmed fish, with outbreaks in Nile tilapia causing significant losses. Vaccination is considered the most suitable method for disease control in aquaculture, with the potential to prevent such outbreaks if highly efficacious vaccines are available for use. Several vaccines have been produced to protect against S. agalactiae infection in tilapia, including inactivated vaccines, live attenuated vaccines, and subunit vaccines, with variable levels of protection seen. Two commercial adjuvants, Montanide™ ISA 763A VG and ISA 763B VG, have been developed recently and designed to improve the safety and efficacy of oil-based emulsions delivered by intraperitoneal injection. In particular, their mode of action may help identify and stimulate particular immunological pathways linked to the intended protective response, which is an important tool for future vaccine development. Therefore, this study aimed to characterize the potential of two adjuvanted-bacterial vaccines against S. agalactiae (SAIV) comparatively, to determine their usefulness for improving protection and to analyse the immune mechanisms involved. Nile tilapia were divided into four groups: 1) fish injected with PBS as a control, 2) fish injected with the SAIV alone, 3) fish injected with the SAIV + Montanide™ ISA 763A VG, and 4) fish injected with the SAIV + Montanide™ ISA 763B VG. Following immunization selected innate immune parameters were analysed, including serum lysozyme, myeloperoxidase, and bactericidal activity, with significantly increased levels seen after immunization. Cytokines associated with innate and adaptive immunity were also studied, with expression levels of several genes showing significant up-regulation, indicating good induction of cell-mediated immune responses. Additionally, the specific IgM antibody response against S. agalactiae was determined and found to be significantly induced post-vaccination, with higher levels seen in the presence of the adjuvants. In comparison to the protection seen with the unadjuvanted vaccine (61.29% RPS), both Montanide™ ISA 763A VG and Montanide™ ISA 763B VG improved the RPS, to 77.42% and 74.19% respectively. In conclusion, Montanide™ ISA 763A VG and Montanide™ ISA 763B VG have shown potential for use as adjuvants for fish vaccines against streptococcosis, as evidenced by the enhanced immunoprotection seen when given in combination with the SAIV vaccine employed in this study.

Supervivencia observada en tres familias de tilapia del Nilo (Oreochromis niloticus) infectadas con Streptococcus agalactiae

La estreptococosis es una de las principales enfermedades en los peces de agua dulce que causa altas tasas de mortalidad. El objetivo de este estudio fue evaluar la respuesta en la supervivencia a la infección por Streptococcus agalactiae en tres familias de tilapia. El experimento se llevó a cabo en el Laboratorio de Enfermedades de los Peces de la Universidad Federal de Lavras. Se utilizaron peces con un peso de 93,7 ± 5,4 g de tres familias diferentes (FA, FB y FC). Se utilizaron 36 peces en cada unidad experimental, inoculados intraperitonealmente con 107 UFC/mL de Streptococcus agalactiae por peces y un grupo control por familia con 9 peces con 1 mL de caldo BHI (Infusión Cerebro Corazón) evaluados durante 15 días. No hubo mortalidad del grupo control. Se observó la presencia de exoftalmia, coloración oscura en todo el cuerpo, letargo y dilatación abdo­minal antes de la muerte en las tres familias evaluadas expuestas al patógeno. El estimador no paramétrico de Kaplan-Meier se utilizó para observar las curvas de supervivencia. Durante los 15 días del desafío, el tiempo promedio de supervivencia de un individuo en las familias FA, FB y FC fue de 9,4; 6,90 y 8,14 días, respectivamente. Pruebas de Log-rank y Peto & Peto para evaluar la diferencia entre las curvas de supervivencia arrojaron que no hubo diferencias significativas entre las familias evaluadas (P=0,08 y P= 0,09), respectivamente.

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.

Marine Based Natural Products: Exploring the Recent Developments in the Identification of Antimicrobial Agents

The marine ecosystem is the less explored, biologically diverse, and vastest resource to discover novel antimicrobial agents. In recent decades' antimicrobial drugs are losing their effectiveness due to the growing resistance among pathogens, which causes diseases to have considerable death rates across the globe. Therefore, there is a need for the discovery of new antibacterials that can reach the market. There is a gradual growth of compounds from marine sources which are entering the clinical trials. Thus, the prominence of marine natural products in the field of drug design and discovery across the academia and pharmaceutical industry is gaining attention. Herein,  the present review covers nearly 200 marine based antimicrobial agents of 11 structural classes discovered from the year 2010 to 2022. All the discussed compounds have exhibited medium to high antimicrobial activity in inhibiting various microorganisms.

A Novel Efficient Piscine Oral Nano-Vaccine Delivery System: Modified Halloysite Nanotubes (HNTs) Preventing Streptococcosis Disease in Tilapia (Oreochromis sp.)

Generally, the injection method is recommended as the best efficient method for vaccine applications in fish. However, labor-intensive and difficult injection for certain fish sizes is always considered as a limitation to aquatic animals. To demonstrate the effectiveness of a novel oral delivery system for the piscine vaccine with nano-delivery made from nano clay, halloysite nanotubes (HNTs) and their modified forms were loaded with killed vaccines, and we determined the ability of the system in releasing vaccines in a mimic digestive system. The efficaciousness of the oral piscine vaccine nano-delivery system was evaluated for its level of antibody production and for the level of disease prevention in tilapia. Herein, unmodified HNTs (H) and modified HNTs [HNT-Chitosan (HC), HNT-APTES (HA) and HNT-APTES-Chitosan (HAC)] successfully harbored streptococcal bivalent vaccine with inactivated S. agalactiae, designated as HF, HAF, HCF and HACF. The releasing of the loading antigens in the mimic digestive tract demonstrated a diverse pattern of protein releasing depending on the types of HNTs. Remarkably, HCF could properly release loading antigens with relevance to the increasing pH buffer. The oral vaccines revealed the greatest elevation of specific antibodies to S. agalactiae serotype Ia in HCF orally administered fish and to some extent in serotype III. The efficacy of streptococcal disease protection was determined by continually feeding with HF-, HAF-, HCF- and HACF-coated feed pellets for 7 days in the 1st and 3rd week. HCF showed significant RPS (75.00 ± 10.83%) among the other tested groups. Interestingly, the HCF-treated group exhibited noticeable efficacy similar to the bivalent-vaccine-injected group (RPS 81.25 ± 0.00%). This novel nano-delivery system for the fish vaccine was successfully developed and exhibited appropriated immune stimulation and promised disease prevention through oral administration. This delivery system can greatly support animals' immune stimulation, which conquers the limitation in vaccine applications in aquaculture systems. Moreover, this delivery system can be applied to carrying diverse types of biologics, including DNA, RNA and subunit protein vaccines.

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.

Widespread presence of a highly virulent Edwardsiella ictaluri strain in farmed tilapia, Oreochromis spp

Edwardsiella ictaluri is an emerging bacterial pathogen that affects farmed tilapia (Oreochromis spp.). This study reports the widespread presence of E. ictaluri in farmed tilapia in Vietnam. Among 26 disease outbreaks from nine provinces in Northern Vietnam during 2019-2021, 19 outbreaks originated from imported seeds, while outbreaks in seven farms were from domestic sources. Clinically sick fish showed the appearance of numerous white spots in visceral organs, and accumulative mortality reached 30%-65%. A total of 26 representative bacterial isolates recovered from 26 disease outbreaks were identified as E. ictaluri based on a combination of phenotypic tests, genus- and species-specific polymerase chain reaction assays, 16S rRNA and gyrB sequencing, and phylogenetic analysis. All isolates harbored the same virulence gene profiles esrC⁺ , evpC⁺ , ureA-C⁺ , eseI^- , escD^- , and virD4^- . Antimicrobial susceptibility tests revealed that 80.8%-100% of isolates were multidrug resistant, with resistance to 4-8 antimicrobials in the groups of penicillin, macrolides, sulfonamides, amphenicols, and glycopeptides. The experimental challenge successfully induced disease that mimicked natural infection. The median lethal doses (LD₅₀ ) of the tested isolates (n = 4) were 42-61 colony forming units/fish, indicating their extremely high virulence. This emerging pathogen is established and has spread to various geographical locations, causing serious impacts on farmed tilapia in northern Vietnam. It is likely that this pathogen will continue to spread through contaminated stocks (both imported and domestic sources) and persist. Thus, increased awareness, combined with biosecurity measures and emergent vaccination programs is essential to mitigate the negative impact of this emerging disease on the tilapia farming industry. This article is protected by copyright. All rights reserved.

Fundamentals of Fish Vaccination

Fish health management has become a critical component of disease control and is invaluable for improved harvests and sustainable aquaculture. Vaccination is generally accepted as the most effective prophylactic measure for fish disease prevention, on environmental, social, and economic grounds. Although the historical approach for developing fish vaccines was based on the principle of Louis Pasteur's "isolate, inactivate and inject," but their weak immunogenicity and low efficacies in many cases, have shifted the focus of fish vaccine development from traditional to next-generation technologies. However, before any fish vaccine can be successfully commercialized, several hurdles need to be overcome regarding the production cost, immunogenicity, effectiveness, mode of administration, environmental safety, and associated regulatory concerns. In this context, the chapter summarises the basic aspects of fish vaccination such as type of vaccine, modalities of vaccine delivery, the immunological basis of fish immunization as well as different challenges associated with the development process and future opportunities.

Prevention and Control of Streptococcosis in Tilapia Culture: A Systematic Review

Tilapia culture is a very promising industry within the aquaculture sector. However, disease outbreaks have continued to threaten the industry, causing serious economic losses among the producers. Streptococcosis has become the major bacterial disease affecting tilapia production in most regions of the world. To combat the disease and minimize its economic impact on fish producers, numerous preventive and control measures have been developed and reported over the years. This paper aims to systematically review the measures that could be used to manage the disease outbreaks and maintain fish health based on previously published scientific studies. Although numerous measures currently available have been highlighted, it is far better for the producers to maximize the preventive measures for management to be economically feasible. Among the currently available preventive measures, the use of vaccines has been shown to have the most promise, while the use of herbs has been demonstrated to be a more sustainable and economically affordable control measure. However, there are still a number of important gaps in existing literature that require further investigation. Overall, significant progress has been made in preventing and controlling streptococcosis in tilapia although, no single effective measure has been identified. Therefore, a combination of these measures may provide a more effective result.

Immunogenicity in Oreochromis niloticus vaccinated with sonicated antigens against streptococcosis

Streptococcosis causes great economic losses in intensive culture of tilapia. Vaccination is the most effective and safest way to tackle infectious diseases. Thus, this study sought the more effective and safer antigenic fraction after sonication of Streptococcus agalactiae to elaborate a vaccine against streptococcosis in Nile tilapia. For this, twenty-one days after vaccination with different fractions (soluble and insoluble) of S. agalactiae, the fish were challenged with the homologous strain (LD50). Then, samples were taken at zero, 14, 28, 60 and 90 days post-vaccination (DPV, n = 7). Blood and organs (cranial kidney, spleen and liver) were collected from vaccinated and unvaccinated fish. Finally, insoluble fraction vaccine presented the best effect, resulting in a 100% relative percent of survival (RPS) and without clinical manifestations. In view of the results, it was to evaluate the role of the insoluble fraction of the antigen in the protective immunity against streptococcosis. The results indicate that the spleen might be the main organ in the vaccine response in Nile tilapia due to the great morphological and immunological differences in vaccinated fish, evidenced by the greater of melanomacrophage centers (MMC) and IgM + lymphocytes in relation to the non-vaccinated fish. At 60 DPV, it was observed the peak of the protective immunity related to the maximum concentration of proteins, circulating leukocytes, antibody titers in the serum and tissue changes with greater expression of IgM + and MMC number in the spleen and kidney of Oreochromis niloticus. Vaccination with insoluble fraction of S. agalactiae was safe and provided effective protection against streptococcosis with maximum protective response at 60 DPV.

Impacts of pineapple peel powder on growth performance, innate immunity, disease resistance, and relative immune gene expression of Nile tilapia, Oreochromis niloticus

An 8-week growth trial was conducted to examine the efficacy of pineapple peel powder (PAPP) on growth rate and immunity of Nile tilapia, O. niloticus. Three hundred Nile tilapia (20.91 ± 0.11 g) were fed five diets containing different levels of PAPP at 0, 10, 20, 30 and 40 g kg^-1 PAPP, respectively. After four and eight weeks of the feeding trial, growth rates, and immune responses were tested. A challenge test using Streptococcus agalactiae and relative immune gene expression were performed after eight weeks of PAPP feeding. It was found that skin mucus and serum lysozyme, skin mucus and serum peroxidase, alternative complement, phagocytosis, and respiratory burst activities were significantly increased with the addition of PAPP. The maximum (P ≤ 0.05) innate immune values were noted in fish fed 10 g kg^-1 PAPP. Similarly, the up-regulation of IL1, IL8, and LBP gene expressions were also detected in fish fed PAPP diets, with the maximum value was found in 10 g kg^-1 PAPP fed fish. The relative percentage of survival (RPS) of Oreochromis niloticus after the challenge test were (56.00%, 72.00%, 60.00%, and 44.00%) for the 5, 10, 20 and 40 g kg^-1 PAPP diets, respectively. Fish fed the 10 g kg^-1 PAPP supplemented diet achieved the highest (P < 0.05) survival rate against S. agalactiae. Growth and feed efficiency were outstandingly (P < 0.05) enhanced in the PAPP groups. In conclusion, PAPP can be potentially used as a feed additive in Nile tilapia culture under Biofloc system.

Systemic and Mucosal B and T Cell Responses Upon Mucosal Vaccination of Teleost Fish

The development of mucosal vaccines against pathogens is currently a highly explored area of research in both humans and animals. This is due to the fact that mucosal vaccines have the potential to best elicit protective responses at these mucosal surfaces, which represent the frontline of host defense, thus blocking the pathogen at its initial replication sites. However, in order to provide an efficient long-lasting protection, these mucosal vaccines have to be capable of eliciting an adequate systemic immune response in addition to local responses. In aquaculture, the need for mucosal vaccines has further practical implications, as these vaccines would avoid the individual manipulation of fish out of the water, being beneficial from both an economic and animal welfare point of view. However, how B and T cells are organized in teleost fish within these mucosal sites and how they respond to mucosally delivered antigens varies greatly when compared to mammals. For this reason, it is important to establish which mucosally delivered antigens have the capacity to induce strong and long-lasting B and T cell responses. Hence, in this review, we have summarized what is currently known regarding the adaptive immune mechanisms that are induced both locally and systemically in fish after mucosal immunization through different routes of administration including oral and nasal vaccination, anal intubation and immersion vaccination. Finally, based on the data presented, we discuss how mucosal vaccination strategies could be improved to reach significant protection levels in these species.

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.

Isolation and molecular characterization of streptococcal species recovered from clinical infections in farmed Nile tilapia (Oreochromis niloticus) in the Philippines

Streptococcosis cause severe losses for global tilapia farming, especially in developing countries. The aim of this study was to identify and characterize streptococci recovered from Nile tilapia farmed in the Philippines. Moribund and apparently healthy fish were sampled from grow-out cages, ponds and hatcheries. Clinical signs observed included exophthalmia, eye opacity, ascites, lethargy, erratic swimming and haemorrhages. Results showed that both Streptococcus iniae and Streptococcus agalactiae were associated with disease in these sites. Consistent with global reports, including those from South-East Asia, S. agalactiae was more widespread than S. iniae. Molecular serotyping of the S. agalactiae isolates identified the serotype Ia and serotype Ib. Histopathological findings were meningitis, meningoencephalitis and septicaemia. Identical virulence profiles were found for all strains of S. iniae, while S. agalactiae strains were separated into virulence profile I and profile II. All strains were susceptible to the tested antibiotics and resistant to oxolinic acid. Only S. agalactiae serotype Ib showed resistance to sulphamethoxazole-trimethoprim. This is the first study from the Philippines to characterize the streptococci involved in disease outbreaks in tilapia aquaculture. Outputs from this study will promote the development of efficacious disease control strategies in tilapia farming for the Philippines and South-East Asia.

Genome-wide analysis revealed the virulence attenuation mechanism of the fish-derived oral attenuated Streptococcus iniae vaccine strain YM011

Streptococcus iniae has become one the most serious aquatic pathogens causing invasive diseases in farmed marine and freshwater fish worldwide, and orally attenuated vaccine is still the best option in protecting these invasive diseases. In this study, the safety, stability, immunogenicity of the S. iniae attenuated strain YM011 were evaluated, and comprehensively analyzed its virulence weakening mechanism at whole genome level. The results shown that attenuated S. iniae strain YM011 completely lost its pathogenicity to tilapia and had good immunogenicity with relative percent survival being 93.25% at 15 days and 90.31% at 30 days via IP injection, respectively, and 76.81% at 15 days and 56.69% at 30 days via oral gavage, respectively. Back-passage safety assay indicated that YM011 did not cause diseases or death in tilapia after 100 generations of serial passaging. Comparative genome-wide sequencing shown that YM011 had a 0.4 M large inversion fragment compared with its parental strain virulent strain GX005, which encoded 372 genes including drug resistance genes pbp2A and tet, as well as known virulence factors including hemolysin transport system gene, recA, and mutator family transposase. The attenuated S. iniae strain YM011 is an ideal attenuated oral vaccine candidate with good immunogenicity, safety and stability. Abnormal expression of important drug resistance genes as well as known virulence factors due to inversion of a 0.4 M large fragment is the leading mechanism underlying its attenuated virulence.

Development of fish vaccine in Southeast Asia: A challenge for the sustainability of SE Asia aquaculture

Southeast (SE) Asia plays an important role in global food security as this region has been regarded as one of the major producers of aquaculture product and, to date, freshwater fish accounted for one-third of the total aquaculture in SE Asia. The intensification of freshwater farming corresponding to increase of consumer demands has inevitably led to the emergence and re-emergence of diseases causing tremendous economic loss in the region. Nile tilapia (Oreochromis niloticus) and striped catfish (Pangasianodon hypophthalmus), the major freshwater fish species of SE Asia, have been reported susceptible to several bacterial pathogens, e.g. Streptococcus agalactiae, Edwardsiella ictalurid and Flavobacterium columnare. Since only a limited number of vaccines being registered and marketed, these pathogenic organisms still represent a severe threat to aquaculture industry in SE Asia. However, there is profound advancement in the understanding of disease epidemiology, pathogenic mechanisms, teleost mucosal immunity and vaccine delivery system over the last few years. This review aimed to summarize those recent findings which hopefully can provide novel insight into the future development of suitable vaccine and vaccination regime against bacterial infection in SE Asia region.

Molecular typing of Streptococcus agalactiae isolates of serotype Ia from tilapia in southern China

Streptococcus agalactiae is an important pathogen of tilapia causing enormous economic losses worldwide. In this study, multilocus sequence typing indicated that 75 S. agalactiae isolates from tilapia in southern China belonged to sequence type-7, as well as belonging to serotype Ia, as confirmed by multiplex PCR assay. The putative-virulence gene profiles and genetic variation of these strains were determined by three sets of multiplex PCR and multi-virulence locus sequencing typing (MVLST), respectively. Analysis of putative-virulence gene profiles showed that each strain harbored 18 putative-virulence genes but lacked lmb and scpB. Three putative-virulence genes (srr-1, bibA and fbsA) were further selected for MVLST analysis. Our data showed that the strains had 14 MVLST types (1-14) and clustered in three groups (Groups I-Ⅲ). The period of time during 2013 and 2014 was an important turning point for the differentiation of the putative-virulence genes of S. agalactiae, as type 1 within Group Ⅱ became the predominant MVLST type. There were significant differences in MVLST types of S. agalactiae isolated from different tilapia farming regions. MVLST assay may improve the discriminatory power and is suitable for understanding the epidemiology of S. agalactiae serotype Ia and screening multivalent vaccine candidate strains.

Vaccination of Tilapia against Motile Aeromonas Septicemia: A Review

The production of tilapia Oreochromis spp. is rapidly growing throughout the world, but atypical motile aeromonad septicemia (MAS) is a current threat to the tilapia farming industry. The etiological agent of this disease is usually Aeromonas hydrophila. Mortality rates due to MAS are frequently high, resulting in a devastating negative impact on this industry worldwide; therefore, proper control measures regarding both prevention and treatment are necessary. Although vaccines against MAS for tilapia are available, their effectiveness is entirely dependent on the specific strain of problematic bacteria. Until now, whole-cell inactivated A. hydrophila vaccines for tilapia have exhibited the highest level of protection over live attenuated and recombinant vaccines. Among the various vaccine administration systems, only intraperitoneal (i.p.) injections of the A. hydrophila vaccine into tilapia were found to provide prominent immune protection. Vaccine efficacy was primarily measured by using the i.p. injection challenge model and estimating the relative percent survival of the immunized tilapia. Freund's incomplete adjuvant showed to be the most effective for tilapia MAS vaccines. In this review, multiple factors that directly or indirectly influence the efficacy of MAS vaccines for tilapia (adjuvants, challenge models, immunization doses and duration, and size of vaccinated fish) are discussed.

Large-scale profiling of the proteome and dual transcriptome in Nile tilapia (Oreochromis niloticus) challenged with low- and high-virulence strains of Streptococcus agalactiae

Streptococcus agalactiae is a common pathogen in aquatic animals, especially tilapia, that hinders aquaculture development and leads to serious economic losses. Previously, a S. agalactiae strain named HN016 was identified from infected tilapia, and the attenuated strain YM001 was subsequently obtained by continuous passaging in Tryptic Soy Broth (TSB) medium. YM001 has been demonstrated as a safe vaccine for S. agalactiae infection in tilapia. To understand the molecular bases of the virulence of these two strains, we performed proteomic and transcriptomic analysis to reveal the protein and gene expression changes in the liver and intestine during the infection process. HN016 significantly decreased the contents of white blood cells (WBCs), neutrophils (NEUs), red blood cells (RBCs) and hematocrit (HCT) and increased the levels of total protein (TP), albumin (ALB) and globulin (GLO), while no such significant differences were observed when comparing the control with YM001. During the infection process, pathogenic peptidoglycan hydrolase, CSPA and membrane proteins were significantly differentially expressed between YM001 and HN016. Furthermore, both proteome and transcriptome data showed that the complement and coagulation cascades pathway and the antigen processing and presentation pathway were stimulated in the liver and intestine, respectively, by YM001 infection compared to HN016 infection. The interaction network analysis of key virulence genes from pathogens suggested that CSPA, as a key node, affects the expression of DOLPP1, MIPEP, PA2G4, OCIAD1, G3BP1 and CLIC5 with a positive correlation. The present evidence suggests that during the infection process, CSPA was the key genes contributing to low virulence in YM001.

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.

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.

Novel Chimeric Multiepitope Vaccine for Streptococcosis Disease in Nile Tilapia (Oreochromis niloticus Linn.)

Streptococcus agalactiae is a causative agent of streptococcosis disease in various fish species, including Nile tilapia (Oreochromis niloticus Linn.). Vaccination is an effective disease prevention and control method, but limitations remain for protecting against catastrophic mortality of fish infected with different strains of streptococci. Immunoproteomics analysis of S. agalactiae was used to identify antigenic proteins and construct a chimeric multiepitope vaccine. Epitopes from five antigenic proteins were shuffled in five helices of a flavodoxin backbone, and in silico analysis predicted a suitable RNA and protein structure for protein expression. 45F2 and 42E2 were identified as the best candidates for a chimeric multiepitope vaccine. Recombinant plasmids were constructed to produce a recombinant protein vaccine and DNA vaccine system. Overexpressed proteins were determined to be 30 kDa and 25 kDa in the E. coli and TK1 systems, respectively. The efficacy of the chimeric multiepitope construct as a recombinant protein vaccine and DNA vaccine was evaluated in Nile tilapia, followed by S. agalactiae challenge at 1 × 107 CFU/mL. Relative percentage survival (RPS) and cumulative mortality were recorded at approximately 57-76% and 17-30%, respectively. These chimeric multiepitope vaccines should be applied in streptococcosis disease control and developed into a multivalent vaccine to control multiple diseases.

Current use and development of fish vaccines in China

In the past decades, the aquaculture industry made great progress in China, which contributes more than 70% yield of the world's farmed fish. Along with the rapid growth of fish production, increased emergence and outbreak of numbers of diseases pose harm to the aquaculture industry and food safety. From the efficient, safe, environmental and ethical aspects, vaccines is definitely the most appropriate and focused method to control different kinds of fish diseases. In China, researchers have done huge works on the fish vaccines, and so far six domestic aquatic vaccine products along with one imported aquatic vaccine have obtained the national veterinary medicine certificate. More critically, some new vaccines have also entered the field experiment stage and showed broad market prospects. In the present review, authors summarize seven aquatic vaccines, including the live vaccine against grass carp hemorrhagic disease, the inactivated vaccine against Aeromonas hydrophila sepsis in fish, the live vaccine against Edwardsiella tarda in turbot, the anti-idiotypic antibody vaccine against Vibrio alginolyticus, V. parahaemolyticus, and E. tarda in Japanese flounder, the cell-cultured inactivated vaccine against grass carp hemorrhagic disease, the inactivated vaccine against fish infectious spleen and kidney necrosis virus (ISKNV), and the genetically engineered live vaccine against V. anguillarum in turbot. Moreover, different delivery routes of fish vaccines are also compared in this review, along with differential fish immune response after vaccination. All these efforts will ultimately benefit the healthy and sustainable development of aquaculture industry in China.

Multiomics analyses reveal that NOD-like signaling pathway plays an important role against Streptococcus agalactiae in the spleen of tilapia

Streptococcus aglactiae(GBS) infection in tilapia is a serious global disease that causes significant production loss. Here, we studied the role of GBS in the spleen and the spleen's response against the pathogen through dual RNA-seq and proteome technology. Animals were divided into three groups: control, virulent treated (HN016), and attenuated treated (YM001). Spleen samples were collected and analysis when a disease outbreak. Dual RNA-seq result showed the virulence factor genes of GBS, included CAMP factor, PGK, OCT, enolase, scpB, Sip, bca, were upregulation. downregulation of GapA, cylE, OCT, scpB, C5AP, rlmB, hly, FBP, in HN016 and YM001. But for proteomic, OCT and bca were downregulation, the others were upregulation. For host transcriptome KEGG analysis showed, the NOD-like receptor signaling pathway (NLRs) and TOLL-like receptor signaling pathway (TLRs) were upreguoation in HN016 infected fish than the control fish; But for proteome KEGG, only the NLRS was up, the TLRS was not change. Compared with YM001 infected fishes, for transcriptome, NLRs and TLRs in infected HN016 fishes were significance rise (p < 0.01); for proteome, the NLRs was up (p < 0.05), but TLRs was no change.Analysis of pathogen-host interaction showed that the peptidoglycan (PNG), CD2, LCK, and host's Zap70 were involved in the regulation of NLRs; PNG, LCK, and ZAP70 were involved in the regulation of TRLs. Conclusion: the virulent strain HN016 and attenuated strainYM001 differed in the quantity of virulence factors. In tilapia's innate immune system, NLRs was the main defense factors, but bacteria avoided the host defense through TLRs.

The Interaction between Phagocytes and Streptococcus agalactiae (GBS) Mediated by the Activated Complement System is the Key to GBS Inducing Acute Bacterial Meningitis of Tilapia

Simple Summary

Streptococcus agalactiae (GBS) is a serious threat to farmed tilapia, which results in high mortality and seriously hinders tilapia farming development. The pathogenic mechanism of tilapia infected with GBS which die rapidly in production remains unknown. We provided a comprehensive comparative analysis of the tilapias infected with fish-derived GBS attenuated strain YM001 and its parental virulent strain HN016. The present study indicates that the interaction between phagocytes and GBS mediated by the activated complement system is key to GBS inducing tilapia acute bacterial meningitis. The low survival ability caused by reduced β-lactam antibiotics resistance is one of the important reasons YM001 lost its pathogenicity to tilapia. Our study provided a comprehensive cognition of the mechanism of acute bacterial meningitis caused by GBS.

Abstract

Streptococcus agalactiae is an important pathogen for tilapia meningitis. Most of the infected tilapia die rapidly in production, when the way to study the pathogenic mechanism of bacteria on host through chronic infection in laboratory is not comprehensive and accurate enough to elucidate the real pathogenic mechanism. The objective of this study was to investigate the mechanism of acute bacterial meningitis of tilapia caused by Streptococcus agalactiae (GBS), and provide a theoretical basis for its prevention and treatment. Duel RNA-seq, proteome analysis, histopathological analysis, plasma biochemical indexes, and blood routine examination were performed on tilapias infected with fish-derived GBS attenuated strain YM001 and its parental virulent strain HN016. The results showed that the contents of white blood cell (WBC), monocytes (MON), and neutrophil (NEU) were significantly lower in the HN016 group compared to that in the YM001 group (p < 0.05). Histopathological examination showed that there were partially lesions in the examined tissues of tilapia infected by HN016, while no obvious histopathological changes occurred in the YM001 group. The differential expressed genes (DEGs) and differential expressed proteins (DEPs) between YM001 and HN016 were mainly enriched in the beta-lactam resistance pathway (oppA1, oppA2, oppB, oppC, oppD, oppF, and mrcA). The DEGs DEPs between YM001-brain and HN016-brain were mainly enriched in the complement and coagulation cascades signaling pathway (C2a, c4b, c3b, c7, CD59, ITGB2, and ITGAX). The present study indicates that the interaction between phagocytes and GBS mediated by the activated complement system is the key to GBS inducing tilapia acute bacterial meningitis. The low survival ability caused by reduced β-lactam antibiotics resistance is one of the important reasons for why YM001 lost its pathogenicity to tilapia.

Development of an attenuated oral vaccine strain of tilapia Group B Streptococci serotype Ia by gene knockout technology

Our previous studies demonstrated that the deletion of D2 fragment in tilapia Streptococcus agalactiae(GBS) attenuated strain YM001 is the main reason for the loss of virulence to tilapia. In this study, a Δ2 mutant that deletion of D2 fragment in parental virulent strain HN016 was constructed, and the safety, stability, immunogenicity, and growth characteristics, as well as the virulence mechanism of Δ2 mutant were evaluated. The results showed that Δ2 mutant was not pathogenic to tilapia, and the virulent revertants were not observed after 50 generations of passage. The RPS reached 96.11% at 15 days and 93.05% at 30 days, respectively, after intraperitoneal injection, while RPS reached 74.80% at 15 days and 53.16% at 30 days, respectively, after oral immunization. The growth of Δ2 mutant was significantly faster than YM001, and genes that were enriched in the nitrogen metabolism and arginine biosynthesis signaling pathway (arc, glnA, and gdhA) were identified as important candidate genes responsible for growth rate of S. agalactiae. The absence of D2 fragment affected the expression of Sip, therefore influencing the bacterial virulence. Altogether, this study demonstrated that deletion of D2 fragment in HN016 causes the loss of virulence to tilapia, and Δ2 mutant is a promising, better attenuated oral vaccine strain of S. agalactiae compared to YM001.

Comparative multi-omics systems analysis reveal the glycolysis / gluconeogenesis signal pathway play an important role in virulence attenuation in fish-derived GBS YM001

Streptococcus agalactiae(GBS) is a seriously threat to the farmed tilapia, and oral vaccination was considered to be the most desirable means which requires deep understanding of virulence mechanism of the fish-derived GBS. Our previous genome study of the fish-derived attenuated strain YM001 showed that there were two large deletions in YM001 compared to its parental virulent strain HN016. In this study, a combined transcriptomic and proteomic analysis was performed on YM001 and HN016 strains, and the important genes were verified by RT-qPCR in bacteria strains and infected-tilapia tissues. Overall, we have shown that a total of 958 genes and 331 proteins were significantly differential expressed between YM001 and HN016. By functional annotation of these DEGs and DEPs, genes that were enriched in pentose phosphate pathway(pgm, ptsG, pgi pfkA, fbaA and FBP3) and pyruvate metabolism pathway(pdhA, pdhB, pdhC and pdhD) were identifed as important candidate genes for leads low growth ability in attenuated strain, which may be an important reasons leading virulence attenuation in the end. The expression levels the candidate genes in pentose phosphate pathway and pyruvate metabolism pathway were significant differential expressed in tilapia’ brain and spleen when infected with YM001 and HN016. Our study indicated that the pentose phosphate pathway and pyruvate metabolism pathway that affecting the growth of the strain may be one of the important reasons for the virulence attenuation in HN016.

Efficacy of live attenuated vaccine derived from the Streptococcus agalactiae on the immune responses of Oreochromis niloticus

Streptococcus agalactiae species have been recognized as the main pathogen causing high mortality in fish leading to significant worldwide economical losses to the aquaculture industries. Vaccine development has become a priority in combating multidrug resistance in bacteria; however, there is a lack of commercial live attenuated vaccine (LAV) against S. agalactiae in Malaysia. The aim of this study is to compare two methods using attenuated bacteria as live vaccine and to evaluate the efficacy of selected LAV on the immune responses and resistance of Oreochromis niloticus (tilapia) against S. agalactiae. The LAV derived from S. agalactiae had been weakened using the chemical agent Acriflavine dye (LAV1), whereas the second vaccine was weakened using serial passages of bacteria on broth media (LAV2). Initial immunization was carried out only on day one, given twice-in the morning and evening, for the 42 day period. Serum samples were collected to determine the systemic antibody (IgM) responses and lysozymal (LSZ) activity using ELISA. On day 43 after immunization, the fish were injected intraperitoneally (i.p) with 0.1 mL of S. agalactiae at LD₅₀ = 1.5 × 10⁵ (CFU)/fish. Fish were monitored daily for 10 days. Clinical signs, mortality and the relative percent of survival (RPS) were recorded. Trial 1 results showed a significant increased (P < 0.05) in serum IgM titers and LSZ activity as compared to LAV2 and the control group (unvaccinated fish). The efficacy of LAV1 was proven effective as determined by the RPS values, LAV1 at 81.58% as compared to LAV2 at 65.79%. Trial 2 of LAV1 and control group were further determined by administering primary and booster doses revealed a RPS value for LAV1 of 82.05%, with the significant enhancement on the immune responses of tilapia as compared to control group. In conclusion, LAV revealed to elevate antibody IgM levels, LSZ activity and provide long-term protection when added to feed. LAV is a low-cost vaccine shown to rapidly increase the immune response of fish and increase survival rates of fish against S. agalactiae infection.

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

Streptococcus agalactiae is an important pathogen in fish, causing great losses of intensive tilapia farming. To develop a potential live attenuated vaccine, a re-attenuated S. agalactiae (named TFJ-ery) was developed from a natural low-virulence S. agalactiae strain TFJ0901 through selection of resistance to erythromycin. The biological characteristics, virulence, stability and the immunization protective efficacy to tilapia of TFJ-ery were determined. The results indicated that TFJ-ery grew at a slower rate than TFJ0901. The capsule thickness of TFJ-ery was significantly less (p < 0.05) than TFJ0901. When Nile tilapia were intraperitoneally (IP) injected with TFJ-ery, the mortality of fish was decreased than that injected with TFJ0901. The RPS of fish immunized with TFJ-ery at a dose of 5.0 × 10⁷ CFU was 95.00%, 93.02% and 100.00% at 4, 8 and 16 weeks post-vaccination, respectively. ELISA results showed that the vaccinated fish produced significantly higher (p < 0.05) antibody titres compared to those of control at 2 or 4 weeks post-vaccination. Taken together, our results suggest that erythromycin could be used to attenuate S. agalactiae, and TFJ-ery is a potent attenuated vaccine candidate to protect tilapia against S. agalactiae infections.

Spatiotemporal distribution of Streptococcus agalactiae attenuated vaccine strain YM001 in the intestinal tract of tilapia and its effect on mucosal associated immune cells

In this study, the tilapia was orally vaccinated by the attenuated Streptococcus agalactiae(S. agalactiae) strain YM001, and the distribution and the pathological effect of strain YM001 in different intestinal segments of tilapia were evaluated by real-time PCR(qPCR), immunohistochemistry(IHC) and histomorphology. The qPCR results showed that the number of bacteria was the highest in the intestinal tracts at 12 h post oral gavage in the YM001 group, then began to decrease sharply and eliminated at 7 d. And the number of bacteria was highest in the foregut, hindgut, and rectum at 12 h, 24 h, and 3 d, respectively. IHC indicated that bacteria mainly distributed in the margin epithelium and the goblet cells at 12 h - 24 h, and in the submucosa and muscle layer in the YM001 group in 3 d post gavage, then almost disappeared at 7 d. Histological examination of intestines post gavage displayed that an inflammation was observed at 7 d in the YM001 group and the intestinal structure was fully recovered at 15 d. and the intestinal structure was fully recovered at 15 d. Conclusion: The attenuated S. agalactiae vaccine strain YM001 could enter the intestinal tissue after oral gavage and had a strong spatial and temporal selectivity in the intestinal tract, which could cause obvious mucosal immune response and mild pathological reaction, but the pathological change could be gradually repaired with the extinction of bacteria in the body.

Correlates of protective immunity for fish vaccines

Vaccination is one of the most effective disease control strategies that has contributed to the significant reduction of disease outbreaks and antibiotics usage in salmonid aquaculture. To date, licensing of fish vaccines is to a limited extent based on in vitro correlates of protection, as done for many mammalian vaccines. This is because the immunological mechanisms of vaccine protection have not been clearly elucidated for most fish vaccines. Herein, we provide an overview of the different steps required to establish correlates of protective immunity required to serve as benchmarks in optimizing vaccine production in aquaculture. We highlight the importance of optimizing challenge models needed to generate consistent results used during vaccine development as a basis for establishing immune correlates of protection. Data generated this far shows that antibodies are potentially the most reliable correlates of protective immunity for fish vaccines. Our findings also show that antigen dose can be optimized to serve as a correlate of protection for fish vaccines. Further, there is need to establish signatures of T-cell protective immunity when antibodies fail to serve as proxies of immune protection, particularly for vaccines against intracellular pathogens. We can anticipate that documentation of efficacy for future vaccines in aquaculture, particularly batch testing will be based on in vitro correlates of protective immunity.

Effects of Attenuated S. agalactiae Strain YM001 on Intestinal Microbiota of Tilapia Are Recoverable

Previously, we constructed and characterized the vaccine efficacy of attenuated S. agalactiae strain YM001 in tilapia. In this study, the potential impacts of YM001 on the tilapia intestinal microbiota were assessed by qPCR and 16S rRNA sequencing methods. The results showed that YM001 distributed unevenly in different parts of intestine, peaked in the intestine at 12 h after oral administration, and then declined gradually. YM001 caused 0% mortality of fish during the entire experimental period, while the referent strain HN016 caused 100% mortality at 3 d after oral administration. However, the intestinal microbiota could be changed by YM001, the diversity of intestinal microbiota decreased first and gradually recovered after oral administration. The diversity of intestinal microbiota of tilapia was negatively correlated with the content of HN016 in the intestinal tract. The oral YM001 mainly changed the abundance of Streptococcus, Cetobacterium, Akkermansia, Romboutsia, Bacteroides, Brevinema, Lachnospiraceae_NK4A136-group, coprothermobactter, presiomonas, and Roseburia in intestine. The present study indicate that oral administration of YM001 altered the diversity and composition of intestinal microbiota in tilapia, but these change were only temporary, non-lethal, and recoverable. The results provide a more comprehensive experimental basis for the safety of oral YM001 vaccines.

Generation of a novel Streptococcus agalactiae ghost vaccine and examination of its immunogenicity against virulent challenge in tilapia

Streptococcus agalactiae (S. agalactiae) is a gram-positive pathogen that causes a wide range of infections in fish and other animals including humans. Bacterial ghosts (BGs) are nonliving, empty cell envelopes and are well represented as novel vaccine candidates. In this study, we examined the immunogenicity and protective efficacy of S. agalactiae ghosts (SAG) against a virulent challenge in tilapia. Nonliving SAG was generated by a culture with Penicillin and Streptolysin, and then treated with the MIC of sodium hydroxide. The formation of a transmembrane lysis tunnel structure in SAG was visualized by electron microscopy. To investigate the SAG as a vaccine candidate, fish were divided into three groups, A (SAG immunized), B [Formalin-inactivated S. agalactiae (FSA) immunized] and C (phosphate-buffered saline, PBS-immunized control). The IgM antibody responses were significantly stronger in the SAG-immunized group than in FSA-immunized group, which was higher than in the non-immunized control group (P < 0.05). Moreover, phagocytic activity (percent phagocytes, PP) was significantly higher (p < 0.05) in the SAG-immunized group than in FSA-immunized group, which was higher than in the non-immunized control group (P < 0.05). In addition, non-specific immune immunity, such as lysozyme and superoxide dismutase activities, in the SAG-immunized fish showed significantly higher activities than FSA-immunized fish and the control group fish (P < 0.05). Also, fish immunized with SAG and FSA showed significantly higher (p < 0.05) gene expression of IL-1β, TNF-α, IFN-γ and TGF-β in the head kidney and spleen than fish treated with PBS during the whole observed period. In addition, fish immunized with SAG showed significantly higher gene expression of L-1β, TNF-α, and TGF-β in the spleen than in the FSA-immunized fish. Although there was no significant (P > 0.05) difference of survival rate (SR) or relative percent survival (RPS) between SAG and FSA immunized groups, they were all significantly more protected against the S. agalactiae challenge (SR: 86.67%, RPS: 76.395) and (SR: 80.00%, RPS: 67.50%) respectively, compared to the PBS-treated group (SR: 33.33%). These results suggest that immunization with SAG induces immune responses and provides protection against a virulent S. agalactiae challenge.

Helvolic Acid Derivatives with Antibacterial Activities against Streptococcus agalactiae from the Marine-Derived Fungus Aspergillus fumigatus HNMF0047

Streptococcus agalactiae is a hazardous pathogen that can cause great harm to humans and fish. In the present study, the known fungal metabolite helvolic acid (10), seven new helvolic acid derivatives named 16- O-deacetylhelvolic acid 21,16-lactone (2), 6- O-propionyl-6,16- O-dideacetylhelvolic acid 21,16-lactone (3), 1,2-dihydro-6,16- O-dideacetylhelvolic acid 21,16-lactone (4), 1,2-dihydro-16- O-deacetylhelvolic acid 21,16-lactone (5), 16- O-propionyl-16- O-deacetylhelvolic acid (6), 6- O-propionyl-6- O-deacetylhelvolic acid (7), and 24- epi-6β,16β-diacetoxy-25-hydroxy-3,7-dioxo-29-nordammara-1,17(20)-diene-21,24-lactone (9), and two known ones (1 and 8) were isolated from the marine-derived fungus Aspergillus fumigatus HNMF0047 obtained from an unidentified sponge from Wenchang Beach, Hainan Province, China. The structures and the absolute configurations of the new compounds were unambiguously elucidated by spectroscopic data and electronic circular dichroism (ECD) spectroscopic analyses along with quantum ECD calculations. In addition, the spectroscopic data of compound 1 are reported here for the first time, the configuration of C-24 of known compound 8 was revised based on comparison of its ROESY data with its C-24 epimer 9, and the absolute configuration of 8 was also determined for the first time. Compounds 6, 7, and 10 showed stronger antibacterial activity than a tobramycin control against S. agalactiae with MIC values of 16, 2, and 8 μg/mL, respectively.

Functional annotation and distribution overview of RNA families in 27 Streptococcus agalactiae genomes

Background

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a Gram-positive bacterium that colonizes the gastrointestinal and genitourinary tract of humans. This bacterium has also been isolated from various animals, such as fish and cattle. Non-coding RNAs (ncRNAs) can act as regulators of gene expression in bacteria, such as Streptococcus pneumoniae and Streptococcus pyogenes. However, little is known about the genomic distribution of ncRNAs and RNA families in S. agalactiae.

Results

Comparative genome analysis of 27 S. agalactiae strains showed more than 5 thousand genomic regions identified and classified as Core, Exclusive, and Shared genome sequences. We identified 27 to 89 RNA families per genome distributed over these regions, from these, 25 were in Core regions while Shared and Exclusive regions showed variations amongst strains. We propose that the amount and type of ncRNA present in each genome can provide a pattern to contribute in the identification of the clonal types.

Conclusions

The identification of RNA families provides an insight over ncRNAs, sRNAs and ribozymes function, that can be further explored as targets for antibiotic development or studied in gene regulation of cellular processes. RNA families could be considered as markers to determine infection capabilities of different strains. Lastly, pan-genome analysis of GBS including the full range of functional transcripts provides a broader approach in the understanding of this pathogen.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4951-z) contains supplementary material, which is available to authorized users.

Current Challenges of Streptococcus Infection and Effective Molecular, Cellular, and Environmental Control Methods in Aquaculture

Several bacterial etiological agents of streptococcal disease have been associated with fish mortality and serious global economic loss. Bacterial identification based on biochemical, molecular, and phenotypic methods has been routinely used, along with assessment of morphological analyses. Among these, the molecular method of 16S rRNA sequencing is reliable, but presently, advanced genomics are preferred over other traditional identification methodologies. This review highlights the geographical variation in strains, their relatedness, as well as the complexity of diagnosis, pathogenesis, and various control methods of streptococcal infections. Several limitations, from diagnosis to control, have been reported, which make prevention and containment of streptococcal disease difficult. In this review, we discuss the challenges in diagnosis, pathogenesis, and control methods and suggest appropriate molecular (comparative genomics), cellular, and environmental solutions from among the best available possibilities.

Transcriptome analysis of hybrid tilapia (Oreochromis spp.) with Streptococcus agalactiae infection identifies Toll-like receptor pathway-mediated induction of NADPH oxidase complex and piscidins as primary immune-related responses

Streptococcus agalactiae infection is one of the most significant bacterial diseases in tilapia aquaculture. Identification of immune-related genes associated with Streptococcus agalactiae infection may provide a basis for breeding selection or therapeutics to augment disease resistance. Therefore, we utilized transcriptome profiling to study the host response in tilapia following Streptococcus agalactiae infection. Based on GO and KEGG enrichment analyses, we found that differentially expressed genes are widely involved in immune-related pathways, including the induction of antimicrobial peptides. Moreover, the main components of two immune-related pathways (Toll-like receptor signaling and leukocyte transendothelial migration) and four environmental information processing pathways (TNF, PI3K-Akt, Jak-STAT and MAPK) were identified. Finally, a time-course expression profile for several of the identified transcripts including tilapia piscidin 3 (TP3), tilapia piscidin 4 (TP4), TLR2, TLR5, MyD88, TRAF6, p38, and interleukin components was performed by qRT-PCR. Collectively, these results provide a starting point to study molecular mechanisms of tilapia immune response to Streptococcus agalactiae infection and may be applied as a basis for developing disease resistant strains by breeding selection.

Effects of Cordyceps militaris spent mushroom substrate on mucosal and serum immune parameters, disease resistance and growth performance of Nile tilapia, (Oreochromis niloticus)

The aim of present study was determination effects of dietary administration of C. militaris spent mushroom substrate (SMS) on mucosal and serum immune parameters, disease resistance, and growth performance of Nile tilapia (Oreochromis niloticus). Two hundred twenty five fish of similar weight (37.28 ± 0.10 g) were assigned to the following diets [0 (T1- Control), 5 (T2), 10 (T3), 20 (T4) and 40 g kg^-1 (T5) SMS]. After 60 days of feeding trial, growth performance, skin mucus lysozyme and peroxidase activities as well as serum innate immune were measured. In addition, survival rate and innate immune responses were calculated after challenge test (15 days) against Streptococcus agalactiae. The results revealed that regardless of inclusion levels, feeding Nile tilapia with SMS supplemented diets significantly increased skin mucus lysozyme and peroxidase activities as well as serum immune parameters (SL, ACH50, PI, RB, and RB) compared control group (P < 0.05). The highest increment of immune parameters was observed in fish fed 10 g kg^-1 SMS which was significantly higher than other treatments (P < 0.05). Also, the relative percent survival (RSP) in T2, T3, T4, and T5 was 61.11%, 88.89%, 66.67, and 55.56%, respectively. Among the supplemented groups, fish fed 10 g kg^-1 SMS showed significant higher RPS and resistance to S. agalactiae than other groups (P < 0.05). Regarding the growth performance, SGR, WG, FW, and FCR were remarkably improved (P < 0.05) in SMS groups; the highest improvement observed in 10 g kg^-1 SMS treatment. According to these finding, administration of 10 g kg^-1 SMS is suggested in tilapia to improve growth performance and health status.

Comprehensive evaluation of immunomodulation by moderate hypoxia in S. agalactiae vaccinated Nile tilapia

Streptococcus agalactiae is a major bacterial pathogen in tilapia aquaculture. Vaccines are known to provide protection but S. agalactiae clearance in tilapia can be reduced by marginal environmental conditions. Therefore, the purpose of this study is to examine S. agalactiae clearance in vaccinated Nile tilapia under moderate hypoxic (55± 5% DO) and normoxic (85 ± 5%DO) conditions. Fish were acclimatized to either moderate hypoxia or normoxia and immunized with formalin-inactivated S. agalactiae. Fish were experimentally challenged with S. agalactiae at 30 days post-vaccination. Serum antibody titer was significantly higher in vaccinated fish kept under normoxic condition compared to the moderate hypoxic condition at fifteen and thirty days post-vaccination. The cumulative mortality following challenge was significantly reduced in vaccinated fish kept under normoxic condition compared to those in moderate hypoxic condition reflecting that pre-challenge antibody titer may correlate with survival of fish. Blood and tissue pathogen burden detection of S. agalactiae studies revealed that culturable S. agalactiae cells could not be detected in the blood of normoxic vaccinated fish at all the sampling points. In contrast, fish vaccinated in moderate hypoxic condition had considerable number of culturable S. agalactiae cells in their blood up to 5 days following challenge. Phagocytosis and intracellular reactive oxygen species (ROS) production were lowered by moderate hypoxia in vitro. Furthermore, presence of specific antibodies and higher specific antibody level in the serum increased phagocytosis, ROS production and lowered intracellular survival of S. agalactiae in head kidney leukocytes. Overall this study has highlighted that S. agalactiae clearance in vaccinated Nile tilapia is modulated by moderate hypoxia. One of the possible explanations for this might be less efficient phagocytic activities due to low oxygen availability and lower specific antibody production in vaccinated fish.

Protective efficacy of cationic-PLGA microspheres loaded with DNA vaccine encoding the sip gene of Streptococcus agalactiae in tilapia

Streptococcus agalactiae (S. agalactiae) is an important fish pathogen, which has received more attention in the past decade due to the increasing economic losses in the tilapia industry worldwide. As existing effective vaccines of S. agalactiae in fish have obvious disadvantage, to select immunoprotective antigens and package materials would undoubtedly contribute to the development of novel oral vaccines. In the present study, surface immunogenic protein (sip) was selected from the S. agalactiae serovar I a genomes as immunogenic protein in DNA vaccine form with cationic chitosan and biodegradable and biocompatible PLGA. The pcSip plasmid in cationic-PLGA was successfully expressed in tissues of immunized tilapia and the immunogenicity was assessed in tilapia challenge model. A significant increase was observed in the cytokine levels of IL-1β, TNF-α, CC1, CC2 in spleen and kidney tissues. Furthermore, immunized tilapia conferred different levels of protection against challenge with a lethal dose of highly virulent serovar I a S. agalactiae. Our results indicated that the pcSip plasmid in cationic-PLGA induced high level of antibodies and protection against S. agalactiae infection, could be effective oral DNA vaccine candidates.

Construction of a Streptococcus agalactiae phoB mutant and evaluation of its potential as an attenuated modified live vaccine in golden pompano, Trachinotus ovatus

Streptococcus agalactiae is a Gram-positive pathogen that can survive inside professional phagocytes and nonphagocytic cells to cause septicemia and meningoencephalitis in freshwater and marine fish. However, vaccines based on extracellular products (ECP) and formalin-killed whole S. agalactiae cells, as well as subunit vaccine are unable to protect fish from infection by variant serotypes S. agalactiae. The search for live attenuated vaccine with highly conserved and virulent-related genes is essential for producing a vaccine to help understand and control streptococcosis In this study, the phoB gene was cloned from pathogenic S. agalactiae TOS01 strain and the mutant strain SAΔphoB was constructed via allelic exchange mutagenesis. The results showed that the deduced amino acid of S. agalactiae TOS01 shares high similarities with other Streptococcus spp. and has high conserved response regulator receiver domain (REC) and DNA-binding effector domain of two-component system response regulators (Trans_reg_C). Cell adherence and invasion assays, challenge experiments and histopathological changes post-vaccination were performed and observed, the results showed that the mutant strain SAΔphoB has a lower adherence and invasion rate and less virulent than the wild type strain in golden pompano, and it doesn't induce clinical symptoms and obvious pathological changes in golden pompano, thereby indicating that the deletion of phoB affects the virulence and infectious capacity of S. agalactiae. Golden pompano vaccinated via intraperitoneal injection SAΔphoB had the relative percent survival value of 93.1% after challenge with TOS01, demonstrating its high potential as an effective attenuated live vaccine candidate. Real-time PCR assays showed that the SAΔphoB was able to enhance the expression of immune-related genes, including MHC-I, MyD88, IL-22 and IL-10 after vaccination, indicating that the SAΔphoB is able to induce humoral and cell-mediated immune response in golden pompano over a long period of time.

Pathological analysis, detection of antigens, FasL expression analysis and leucocytes survival analysis in tilapia (Oreochromis niloticus) after infection with green fluorescent protein labeled Streptococcus agalactiae

The pathogenesis of Streptococcus agalactiae infection in tilapia has not been fully described. To understand this, we investigated the clinic-pathological features of acute experimental septicemia in tilapia (Oreochromis niloticus) after receiving an intra-peritoneal injection with S. agalactiae THN-1901GFP. Immunohistochemistry and sections of pathological tissues were used to estimate the level of damage in the head-kidney, liver, spleen and trunk-kidney. The expression of FasL was analyzed by western blotting in these samples based on their damage levels. Leucocytes were isolated from the head-kidney and incubated with S. agalactiae THN-1901GFP. Then, phagocytosis, programmed cell death and the expression of FasL were analyzed. The infected tissues showed varying degrees of necrosis and histolysis. The serous membrane of the intestine was dissolved by S. agalactiae THN-1901GFP. Antigens of S. agalactiae THN-1901GFP accumulated in different parts of the infected organs. In the head-kidney and spleen, the expression of FasL was up-regulated in parallel with increased tissue damage. After being incubated with S. agalactiae THN-1901GFP, the phagocytic capacity and ability were both very high and the expression of FasL remained high in leucocytes. S. agalactiae THN-1901GFP was able to survive for a long period of time after being engulfed by phagocytic cells. These findings offer insight into the pathogenesis of S. agalactiae infection in tilapia.