Pellet feed adsorbed with the recombinant Lactococcus lactis BFE920 expressing SiMA antigen induced strong recall vaccine effects against Streptococcus iniae infection in olive flounder (Paralichthys olivaceus)

Advances in Vaccine Adjuvants for Teleost Fish: Implications for Aquatic Welfare and the Potential of Nanoparticle-Based Formulations

Vaccine adjuvants are crucial for reinforcing the immunogenicity of vaccines. Therefore, they are widely used in the aquaculture sector as vaccine components, facilitating the efficient prevention of infectious diseases and promoting sustainable teleost fish growth. Despite their benefits, there has been a growing concern about the potential adverse effects of vaccine adjuvants in teleost fish, connoting a valid impact on their overall health and welfare. Among the adjuvants used in aquaculture vaccinology, nanoparticle-based formulations have given rise to a promising new alternative to traditional options, such as oil-based emulsions and aluminum compounds, offering the benefit of minimizing relevant side effects. The aim of this paper was to review the current status of the adjuvants used in aquaculture, provide a description and an evaluation of their mode of action and side effects, and explore the potential use of nanoparticle formulations as adjuvants to improve the efficacy of aquaculture vaccines. By demonstrating and assessing the equilibrium between teleost fish welfare and immunological efficacy, this review presents a collective perspective that will assist in establishing a framework for the utilization of effective species-specific practices around adjuvant use in aquaculture, while also addressing the challenges of welfare-friendly immunization.

Lactococcus lactis, a bacterium with probiotic functions and pathogenicity

Lactococcus lactis (L. lactis) is the primary organism for lactic acid bacteria (LAB) and is a globally recognized safe microorganism for the regulation of the intestinal micro-ecological balance of animals and improving the immune performance of the host. L. lactis is known to play a commercially important role in feed fortification, milk fermentation, and vaccine production, but pathogenic L. lactis has been isolated from many clinical cases in recent years, such as the brain of silver carp with Lactococcosis, the liver and spleen of diseased waterfowl, milk samples and padding materials with cow mastitis, and blood and urine from human patients with endocarditis. In dairy farming, where L. lactis has been used as a probiotic in the past, however, some studies have found that L. lactis can cause mastitis in cows, but the lack of understanding of the pathogenesis of mastitis in cows caused by L. lactis has become a new problem. The main objective of this review is to analyze the increasingly serious clinical mastitis caused by L. lactis and combined with the wide application of L. lactis as probiotics, to comprehensively discuss the characteristics and diversity of L. lactis.

Microencapsulation of Piscirickettsia salmonis Antigens for Fish Oral Immunization: Optimization and Stability Studies

The development of fish oral vaccines is of great interest to the aquaculture industry due to the possibility of rapid vaccination of a large number of animals at reduced cost. In a previous study, we evaluated the effect of alginate-encapsulated Piscirickettsia salmonis antigens (AEPSA) incorporated in feed, effectively enhancing the immune response in Atlantic salmon (Salmo salar). In this study, we seek to characterize AEPSA produced by ionic gelation using an aerodynamically assisted jetting (AAJ) system, to optimize microencapsulation efficiency (EE%), to assess microparticle stability against environmental (pH, salinity and temperature) and gastrointestinal conditions, and to evaluate microparticle incorporation in fish feed pellets through micro-CT-scanning. The AAJ system was effective in obtaining small microparticles (d < 20 μm) with a high EE% (97.92%). Environmental conditions (pH, salinity and temperature) generated instability in the microparticles, triggering protein release. 62.42% of the protein content was delivered at the intestinal level after in vitro digestion. Finally, micro-CT-scanning images confirmed microparticle incorporation in fish feed pellets. In conclusion, the AAJ system is effective at encapsulating P. salmonis antigens in alginate with a high EE% and a size small enough to be incorporated in fish feed and produce an oral vaccine.

Single and Multi-Strain Probiotics Supplementation in Commercially Prominent Finfish Aquaculture: Review of the Current Knowledge

The Nile tilapia Oreochromis niloticus, Atlantic salmon Salmo salar, rainbow trout Oncorhynchus mykiss, olive flounder Paralichthys olivaceus, common carp Cyprinus carpio, grass carp Ctenopharyngodon idella and rohu carp Labeo rohita are farmed commercially worldwide. Production of these important finfishes is rapidly expanding, and intensive culture practices can lead to stress in fish, often reducing resistance to infectious diseases. Antibiotics and other drugs are routinely used for the treatment of diseases and sometimes applied preventatively to combat microbial pathogens. This strategy is responsible for the emergence and spread of antimicrobial resistance, mass killing of environmental/beneficial bacteria, and residual effects in humans. As an alternative, the administration of probiotics has gained acceptance for disease control in aquaculture. Probiotics have been found to improve growth, feed utilization, immunological status, disease resistance, and to promote transcriptomic profiles and internal microbial balance of host organisms. The present review discusses the effects of single and multi-strain probiotics on growth, immunity, heamato-biochemical parameters, and disease resistance of the above-mentioned finfishes. The application and outcome of probiotics in the field or open pond system, gaps in existing knowledge, and issues worthy of further research are also highlighted.

Biofunctionalization of Endolysins with Oligosacharides: Formulation of Therapeutic Agents to Combat Multi-Resistant Bacteria and Potential Strategies for Their Application

In the aquaculture sector, the biofunctionalization of biomaterials is discussed using materials from algae and analyzed as a possible potential strategy to overcome the challenges that hinder the future development of the application of endolysins in this field. Derived from years of analysis, endolysins have recently been considered as potential alternative therapeutic antibacterial agents, due to their attributes and ability to combat multi-resistant bacterial cells when applied externally. On the other hand, although the aquaculture sector has been characterized by its high production rates, serious infectious diseases have led to significant economic losses that persist to this day. Although there are currently interesting data from studies under in vitro conditions on the application of endolysins in this sector, there is little or no information on in vivo studies. This lack of analysis can be attributed to the relatively low stability of endolysins in marine conditions and to the complex gastrointestinal conditions of the organisms. This review provides updated information regarding the application of endolysins against multi-resistant bacteria of clinical and nutritional interest, previously addressing their important characteristics (structure, properties and stability). In addition, regarding the aquaculture sector, the biofunctionalization of biomaterials is discussed using materials from algae and analyzed as a possible potential strategy to overcome the challenges that hinder the future development of the application of endolysins in this field.

Feeding olive flounder (Paralichthys olivaceus) with Lactococcus lactis BFE920 expressing the fusion antigen of Vibrio OmpK and FlaB provides protection against multiple Vibrio pathogens: A universal vaccine effect

Vibriosis, an illness caused by the Vibrio bacteria species, results in significant economic loss in olive flounder farms. Here we present a novel anti-Vibrio feed vaccine protecting multiple strains of Vibrio pathogens, a universal vaccine effect. The vaccine was generated by engineering Lactococcus lactis BFE920 to express the fusion antigens of Vibrio outer membrane protein K (OmpK) and flagellin B subunit (FlaB). These antigen genes are highly conserved among Vibrio species. Olive flounder (7.1 ± 0.8 g and 140 ± 10 g) were fed the vaccine adsorbed to a regular feed (1 × 10⁷ CFU/g) for one week with a 1-week interval, repeating three times (a triple boost). The vaccinated fish increased the significant levels of antigen-specific antibodies, T cell numbers (CD4-1, CD4-2, and CD8α), cytokine production (T-bet and IFN-γ), and innate immune responses (TLR5M, IL-1β, and IL-12p40). Also, the survival rates of adult and juvenile fish fed the vaccine were significantly elevated when challenged with V. anguillarum, V. alginolyticus, and V. harveyi. In addition, weight gain rate and feed conversion ratio were improved in vaccinated fish. The feed vaccine protected multiple Vibrio pathogens, a universal vaccine effect, by activating innate and adaptive immune responses. This oral vaccine may be developed as an anti-Vibrio vaccine to protect against a broad spectrum of Vibrio pathogens.

The Effect of Lactococcus lactis subsp. lactis PTCC 1403 on the Growth Performance, Digestive Enzymes Activity, Antioxidative Status, Immune Response, and Disease Resistance of Rainbow Trout (Oncorhynchus mykiss)

The effect of Lactococcus lactis subsp. lactis strain PTCC 1403 as a potential probiotic was investigated on the growth, hematobiochemical, immune responses, and resistance to Yersinia ruckeri infection in rainbow trout. A total of 240 fish were distributed into 12 fiberglass tanks representing four groups (× 3 replicates). Each tank was stocked with 20 fish (average initial weight: 11.81 ± 0.32 g) and fed L. lactis subsp. lactis PTCC 1403 at 0 (control, T0), 1 × 10⁹ (T1), 2 × 10⁹ (T2), and 3 × 10⁹ (T3) CFU/g feed for 8 weeks. The results showed enhanced protein efficiency ratio and reduced feed conversion ratio in the fish-fed T2 diet. Further, fish-fed T2 and T3 diets showed a significantly higher survival rate than the control (p < 0.05). Trypsin, lipase, and protease activities were increased in fish-fed L. lactis subsp. lactis PTCC 1403 compared to the control (p < 0.05). Fish fed with a T2 diet showed significantly (p < 0.05) lower glucose content than other groups. The blood lysozyme activity and IgM showed significantly (p < 0.05) higher values in fish-fed T2 and T3 diets than in other groups. The antioxidative responses were increased in fish-fed T2 and T3 diets (p < 0.05). After 7 days post-Y. ruckeri challenge, the cumulative mortality rate showed the lowest value in fish fed with T1 and T2 diets, while the highest value was recorded in the control group. In conclusion, the results revealed beneficial effects of L. lactis subsp. lactis PTCC 1403 on the feed efficiency, immune response, and resistance to Y. ruckeri infection in rainbow trout.

Plasmid Replicons for the Production of Pharmaceutical-Grade pDNA, Proteins and Antigens by Lactococcus lactis Cell Factories

The Lactococcus lactis bacterium found in different natural environments is traditionally associated with the fermented food industry. But recently, its applications have been spreading to the pharmaceutical industry, which has exploited its probiotic characteristics and is moving towards its use as cell factories for the production of added-value recombinant proteins and plasmid DNA (pDNA) for DNA vaccination, as a safer and industrially profitable alternative to the traditional Escherichia coli host. Additionally, due to its food-grade and generally recognized safe status, there have been an increasing number of studies about its use in live mucosal vaccination. In this review, we critically systematize the plasmid replicons available for the production of pharmaceutical-grade pDNA and recombinant proteins by L. lactis. A plasmid vector is an easily customized component when the goal is to engineer bacteria in order to produce a heterologous compound in industrially significant amounts, as an alternative to genomic DNA modifications. The additional burden to the cell depends on plasmid copy number and on the expression level, targeting location and type of protein expressed. For live mucosal vaccination applications, besides the presence of the necessary regulatory sequences, it is imperative that cells produce the antigen of interest in sufficient yields. The cell wall anchored antigens had shown more promising results in live mucosal vaccination studies, when compared with intracellular or secreted antigens. On the other side, engineering L. lactis to express membrane proteins, especially if they have a eukaryotic background, increases the overall cellular burden. The different alternative replicons for live mucosal vaccination, using L. lactis as the DNA vaccine carrier or the antigen producer, are critically reviewed, as a starting platform to choose or engineer the best vector for each application.

Oral immunization of trout fry with recombinant Lactococcus lactis NZ3900 expressing G gene of viral hemorrhagic septicaemia virus (VHSV)

This study has investigated the ability of Lactococcus lactic (NZ3900) carried G gene of viral haemorrhagic septicaemia virus (VHSV) under nisin-controlled gene expression (NICE) system in rainbow trout (O.Mykiss). Two groups of trout fry (7 ± 0.65 g) were immunized with 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g recombinant L. lactis NZ3900, two groups of fish were fed 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g L. lactis vector free, and one group was fed by the basal diet as a control. Oral immunization was done on days 1-7 and boosting was performed on days 15-21. The relative expression of IFN-1 and MX-1 genes significantly increased in head kidney of vaccinated fish depend on vaccine dosage compared to the control group. Fish in vaccinated group also showed elevated VHSV-specific antibody levels compared to the control groups. Relative percent survival (RPS), under virulent isolate VHSV challenge were estimated 62%, 78% for 10⁸ cfu/g 10¹⁰ cfu/g feed vaccinated groups 21 days post-vaccination, while groups fed similar doses of L. lactis vector free illustrated 22% and 27% RPSs, respectively. The significant reduction of viral loads (transcript levels of N gene) were detected in the immunized groups. Increased weight gain and decreased feed consumption in vaccinated group attributed to the probiotic effect were also observed. In conclusion, our results demonstrate the ability of recombinant L. lactis as oral vaccine against VHS in rainbow trout, which can be considered as effective method against different fish pathogens.

Assessing the Immune Response of Atlantic Salmon (Salmo salar) after the Oral Intake of Alginate-Encapsulated Piscirickettsia salmonis Antigens

Salmon rickettsial septicaemia (SRS) is the infectious disease that produces the highest losses in the Chilean salmon industry. As a new strategy for the control of SRS outbreaks, in this study we evaluated the effect of alginate-encapsulated Piscirickettsia salmonis antigens (AEPSA) incorporated in the feed as an oral vaccine to induce the immune response in Atlantic salmon (Salmo salar). Fish were distributed into three vaccination groups (injectable, oral high dose, oral low dose). Feed intake and fish growth were recorded during the trial. The P. salmonis-specific IgM levels in blood plasma were measured by ELISA. Alginate microparticles containing the antigen were effectively incorporated in fish feed to produce the oral vaccine. Incorporation of AEPSA did not affect the palatability of the feed or the fish appetite. Furthermore, the oral vaccine did not have a negative effect on fish growth. Finally, the oral vaccine (high and low dose) produced an acquired immune response (IgM) similar to the injectable vaccine, generating a statistically significant increase in the IgM levels at 840-degree days for both experimental groups. These findings suggest that AEPSA incorporated in the feed can be an effective alternative to boost the immune response in Atlantic salmon (S. salar).

Immunostimulation by starch hydrogel-based oral vaccine using formalin-killed cells against edwardsiellosis in Japanese eel, Anguilla japonica

Edwardsiellosis outbreaks cause significant losses in Japanese eel aquaculture. The causative agent, Edwardsiella tarda, is an intracellular pathogen, and the use of antibiotics has a limited effectiveness. As Japanese eels are sensitive to stress, injection vaccines are not recommended for treatment; immersion methods are less stressful, but not cost-effective. Alternatively, oral vaccination methods are more promising. The aim of this study was to develop a starch hydrogel-based oral (SHO) vaccine against edwardsiellosis in Japanese eel, using formalin-killed cells. To assess the protective effect, we compared SHO vaccine with the conventional formalin-killed cell (FKC) vaccine. A bacterial agglutination test showed that agglutination titers in SHO-vaccinated group were higher than in the FKC-vaccinated group. Japanese eel survival rate (%) was monitored after challenge by E. tarda at four weeks post-vaccination. Survival rates in the FKC group (60%, first trial; 70%, second trial) were lower than in SHO groups. Percentage survival rates in three SHO groups (first and second trials, respectively) were as follows: 70% and 80% in the group vaccinated once per day for one day; and 80% and 90% in both groups vaccinated for four and eight days. Additionally, a boost SHO vaccination at 46 days prompted a similar or even higher protection against edwardsiellosis than after the initial vaccination. Both FKC and SHO vaccination upregulated levels of pro-inflammatory cytokines (interleukin (IL)-6, tumor necrosis factor (TNF)-α), and host defense cytokine (interferon (IFN)-α) in all immunized groups of fish when compared with the control. These results reveal the immunostimulation effect of SHO vaccine in Japanese eel, emphasizing its potential as an oral vaccine in aquaculture.

Modulation of transcriptomic profile in aquatic animals: Probiotics, prebiotics and synbiotics scenarios

Aquaculture and fisheries have provided protein sources for human consumption for a long time, but diseases have induced declines in product benefits and raised concerns, resulting in great losses to these industries in many countries. The overuse of antibiotics for the treatment of diseases has increased the chemical concentrations in culture systems and weakened the natural immunity of aquatic organisms. Concerns regarding the detrimental effects of antibiotics on the environment and human health due to residual antibiotic-related issues encourage the development of reliable, environmental and health safety methods, such as vaccines, probiotics, prebiotics, synbiotics and phytobiotics, for protection against disease and for reducing and possibly eliminating disease occurrence. Immunity has been effectively enhanced by pro-, pre-, and synbiotics, which confer strong protection and reduce the risks associated with stressors and disease outbreaks in culture systems. These agents confer several benefits, including enhancing both host growth and immune responses against pathogens, while sustaining health and environmental stability, and their use is thus widely accepted. Alterations in gene expression in individual cells could serve as an indicator of the immunity and growth rate of aquatic animals after pro-, pre- and synbiotic feeding. This review addresses the potential use of pro, pre- and synbiotics as immunostimulants for improved aquaculture management and environmental health and chronicles the recent insights regarding the application of pro-, pre- and synbiotics with special emphasis on their immunomodulatory and antioxidative responses based on gene expression changes. Furthermore, the current review describes the research gaps and other issues that merit further investigation.

Lactic Acid Bacteria in Finfish-An Update

A complex and dynamic community of microorganisms, play important roles within the fish gastrointestinal (GI) tract. Of the bacteria colonizing the GI tract, are lactic acid bacteria (LAB) generally considered as favorable microorganism due to their abilities to stimulating host GI development, digestive function, mucosal tolerance, stimulating immune response, and improved disease resistance. In early finfish studies, were culture-dependent methods used to enumerate bacterial population levels within the GI tract. However, due to limitations by using culture methods, culture-independent techniques have been used during the last decade. These investigations have revealed the presence of Lactobacillus, Lactococcus, Leuconostoc, Enterococcus, Streptococcus, Carnobacterium, Weissella, and Pediococcus as indigenous species. Numerous strains of LAB isolated from finfish are able to produce antibacterial substances toward different potential fish pathogenic bacteria as well as human pathogens. LAB are revealed be the most promising bacterial genera as probiotic in aquaculture. During the decade numerous investigations are performed on evaluation of probiotic properties of different genus and species of LAB. Except limited contradictory reports, most of administered strains displayed beneficial effects on both, growth-and reproductive performance, immune responses and disease resistance of finfish. This eventually led to industrial scale up and introduction LAB-based commercial probiotics. Pathogenic LAB belonging to the genera Streptococcus, Enterococcus, Lactobacillus, Carnobacterium, and Lactococcus have been detected from ascites, kidney, liver, heart, and spleen of several finfish species. These pathogenic bacteria will be addressed in present review which includes their impacts on finfish aquaculture, possible routes for treatment. Finfish share many common structures and functions of the immune system with warm-blooded animals, although apparent differences exist. This similarity in the immune system may result in many shared LAB effects between finfish and land animals. LAB-fed fish show an increase in innate immune activities leading to disease resistances: neutrophil activity, lysozyme secretion, phagocytosis, and production of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, and TNF-α). However, some LAB strains preferentially induces IL-10 instead, a potent anti-inflammatory cytokine. These results indicate that LAB may vary in their immunological effects depending on the species and hosts. So far, the immunological studies using LAB have been focused on their effects on innate immunity. However, these studies need to be further extended by investigating their involvement in the modulation of adaptive immunity. The present review paper focuses on recent findings in the field of isolation and detection of LAB, their administration as probiotic in aquaculture and their interaction with fish immune responses. Furthermore, the mode of action of probiotics on finfish are discussed.

CCD and RSM optimization approach for antioxidative activity and immune regulation in head kidney of yellow catfish (Pelteobagrus fulvidraco) based on different lipid levels and temperatures

Yellow catfish (Pelteobagrus fulvidraco) is an important economic cultured fish in China. Here we report antioxidative activity and immune regulation in head kidney using a central composite design based on water temperature (20-34 °C) and dietary lipid (2-17%). Response values were optimized using response surface methodology to maximize the immune response and relieve oxidative stress. The experiment was conducted under laboratory conditions and lasted for seven weeks. The results showed that the linear effects of lipid level on superoxide dismutase (SOD, and lysozyme (LYZ) activity, and malondialdehyde (MDA) content in head kidney, respiratory burst activity (RBA) of head kidney macrophages, and cumulative mortality of fish infected by Streptococcus iniae (S. iniae) were significant (P < 0.05). Similarly, the linear effects of water temperature on SOD activity, MDA content, and cumulative mortality were significant (P < 0.05). In addition, the quadratic effects of water temperature and lipid level on all experimental response values were significant (P < 0.05), and no interactive effect was found between water temperature and lipid level (P > 0.05). High water temperature and high lipid diet significantly reduced the antioxidative activity and immune response in head kidney, and increased MDA content, which caused increased mortality of the S. iniae-infected fish. The adjusted R² values for SOD activity, MDA content, LYZ activity, RBA, phagocytic activity, and cumulative mortality regression models were 0.76, 0.85, 0.87, 0.79, 0.64, and 0.87, respectively. The optimal combination of water temperature and lipid level was 26.9 °C and 7.7%, at which good antioxidative activity and immune regulation were achieved, with reliability of 0.878. This combination was close to the optimal combination of water temperature and lipid level for growth performance (27.5 °C and 9.2%) reported previously. Thus, the optimal combination may not only promote growth, but also enhance antioxidant and immune levels.

A Lactococcus lactis BFE920 feed vaccine expressing a fusion protein composed of the OmpA and FlgD antigens from Edwardsiella tarda was significantly better at protecting olive flounder (Paralichthys olivaceus) from edwardsiellosis than single antigen vaccines

Edwardsiellosis is a major fish disease that causes a significant economic damage in the aquaculture industry. Here, we assessed vaccine efficacy after feeding oral vaccines to olive flounder (Paralichthys olivaceus), either L. lactis BFE920 expressing Edwardsiella tarda outer membrane protein A (OmpA), flagellar hook protein D (FlgD), or a fusion antigen of the two. Feed vaccination was done twice with a one-week interval. Fish were fed regular feed adsorbed with the vaccines. Feed vaccination was given over the course of one week to maximize the interaction between the feed vaccines and the fish intestine. Flounder fed the vaccine containing the fusion antigen had significantly elevated levels T cell genes (CD4-1, CD4-2, and CD8α), type 1 helper T cell (Th1) subset indicator genes (T-bet and IFN-γ), and antigen-specific antibodies compared to the groups fed the single antigen-expressing vaccines. Furthermore, the superiority of the fusion vaccine was also observed in survival rates when fish were challenged with E. tarda: OmpA-FlgD-expressing vaccine (82.5% survival); FlgD-vaccine (55.0%); OmpA-vaccine (50%); WT L. lactis BFE920 (37.5%); Ctrl (10%). In addition, vaccine-fed fish exhibited increased weight gain (∼20%) and a decreased feed conversion ratio (∼20%) during the four week vaccination period. Flounder fed the FlgD-expressing vaccine, either the single or the fusion form, had significantly increased expression of TLR5M, IL-1β, and IL-12p40, suggesting that the FlgD may be a ligand of olive flounder TLR5M receptor or closely related to the TLR5M pathway. In conclusion, the present study demonstrated that olive flounder fed L. lactis BFE920 expressing a fusion antigen composed of E. tarda OmpA and FlgD showed a strong protective effect against edwardsiellosis indicating this may be developed as an E. tarda feed vaccine.

A review on Lactococcus lactis: from food to factory

Lactococcus lactis has progressed a long way since its discovery and initial use in dairy product fermentation, to its present biotechnological applications in genetic engineering for the production of various recombinant proteins and metabolites that transcends the heterologous species barrier. Key desirable features of this gram-positive lactic acid non-colonizing gut bacteria include its generally recognized as safe (GRAS) status, probiotic properties, the absence of inclusion bodies and endotoxins, surface display and extracellular secretion technology, and a diverse selection of cloning and inducible expression vectors. This have made L. lactis a desirable and promising host on par with other well established model bacterial or yeast systems such as Escherichia coli, Saccharomyces [corrected] cerevisiae and Bacillus subtilis. In this article, we review recent technological advancements, challenges, future prospects and current diversified examples on the use of L. lactis as a microbial cell factory. Additionally, we will also highlight latest medical-based applications involving whole-cell L. lactis as a live delivery vector for the administration of therapeutics against both communicable and non-communicable diseases.