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

Siglec7 functions as an inhibitory receptor of non-specific cytotoxic cells and can regulate the innate immune responses in a primitive vertebrate (Oreochromis niloticus)

In mammals, siglec7, an integral component of the siglecs, is principally found on the surface of natural killer (NK) cells, macrophages, and monocytes, where it interacts with various pathogens to perform immunological regulatory activities. Nonetheless, the immune defense and mechanism of siglec7 in early vertebrates remain unknown. In this study, we identified siglec7 from Oreochromis niloticus (OnSiglec7) and revealed its immune functions. Specifically, OnSiglec7 was abundantly expressed in immune-related tissues of healthy tilapia and its transcription level was strongly activated after being challenged with A. hydrophila, S. agalactiae, and Poly: IC. Meanwhile, OnSiglec7 protein was purified and analyzed, which could recognize multiple pathogens through binding and agglutinating activity. Moreover, OnSiglec7-positive cells were mainly distributed in non-specific cytotoxic cells (NCC) of tilapia HKLs and showed cell membrane localization. Furthermore, OnSiglec7 blockage affected multiple innate immune responses (inflammation, apoptosis, and pyroptosis process) by regulating the activation of MAPK, NF-κB, TLR, and JAK-STAT pathways. Finally, OnSiglec7 blockage also greatly enhanced the cytotoxic effect of tilapia NCC. Summarily, this study uncovers immune functions and mechanisms of siglec7 in primitive vertebrates, thereby enhancing our understanding of the systemic evolution and ancient functions of other siglecs within the host's innate immune system (to our knowledge).

The effects of kelp powder and fucoidan on the intestinal digestive capacity, immune response, and bacterial community structure composition of large yellow croakers (Larimichthys crocea)

Feed terrestrial components can induce intestinal stress in fish, affecting their overall health and growth. Recent studies suggest that seaweed products may improve fish intestinal health. In this experiment, three types of feed were prepared: a basic diet (C group), a diet with 0.2 % fucoidan (F group), and a diet with 3 % kelp powder (K group). These diets were fed to large yellow croaker (Larimichthys crocea) over an 8-week period. Each feed was randomly assigned to three seawater cages (4.0 m × 4.0 m × 5.0 m) containing 700 fish per cage. The study assessed changes in growth and intestinal health, including intestinal tissue morphology, digestive enzyme activities, expression of immune-related genes, and bacterial community structure. Results showed that incorporating seaweed products into the diet improved the growth and quality traits of large yellow croakers and significantly enhanced their intestinal digestive capacity (P < 0.05). Specifically, the 0.2 % fucoidan diet significantly increased the intestinal villus length and the activities of digestive enzymes such as trypsin, lipase, and α-amylase (P < 0.05). The 3 % kelp powder diet significantly enhanced the intestinal crypt depth and the activities of trypsin and lipase (P < 0.05). Both seaweed additives significantly enhanced intestinal health by mitigating inflammatory factors. Notably, the control group's biomarkers indicated a high presence of potential pathogenic bacteria, such as Streptococcus, Pseudomonas, Enterococcus, Herbaspirillum, Neisseria, Haemophilus, and Stenotrophomonas. After the addition of seaweed additives, these bacteria were no longer the indicator bacteria, while the abundance of beneficial bacteria like Ligilactobacillus and Lactobacillus increased. Significant reductions in the expression of inflammatory factors (e.g., il-6, tnf-α, ifn-γ in the fucoidan group and il-8 in the kelp powder group) further supported these findings. Our findings suggested that both seaweed additives helped balance intestinal microbial communities and reduce bacterial antigen load. Considering the effects, costs, manufacturing, and nutrition, adding 3 % kelp powder to the feed of large yellow croaker might be preferable. This study substantiated the beneficial effects of seaweed on the aquaculture of large yellow croaker, particularly in improving intestinal health. These findings advocated for its wider and more scientifically validated use in fish farming practices.

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.

Impacts of astaxanthin-enriched Paracoccus carotinifaciens on growth, immune responses, and reproduction performance of broodstock Nile tilapia during winter season

The growth, immune response, and reproductive performance of broodstock of Nile tilapia (Oreochromis niloticus) under winter stress conditions were investigated the effects of supplementary diets with astaxanthin-enriched Paracoccus carotinifaciens. Throughout an eight-week period in the winter season, male and female tilapia were fed with diets containing different levels of P. carotinifaciens dietary supplementation: 0 g/kg (T1; control), 5 g/kg (T2), 10 g/kg (T3), and 20 g/kg (T4). Subsequently, a four-week mating system was implemented during the winter stress period. The results revealed that there were no significant differences observed in growth, hematological indices, and blood chemical profiles among all treatment groups for both male and female tilapia. However, a significant increase in cholesterol content was noted in both male and female tilapia fed with the T4 diet (p<0.05). The total carotenoid content in the muscle was evaluated, and significantly higher values were found in both male and female tilapia that fed T4 supplementation (p<0.05). Moreover, immunological parameters such as myeloperoxidase and antioxidant parameters in the liver including superoxide dismutase activity and catalase enzyme activity showed significant increases in tilapia fed with the T4 diet. The impact of P. carotinifaciens supplementation on broodstock tilapia indicated a significant increase in spermatozoa concentration in males and increased egg production in females after consumption of the T4 diet (p<0.05). Thus, this study highlighted that the presence of astaxanthin-enriched P. carotinifaciens in the diet of broodstock Nile tilapia can lead to the accumulation of carotenoids in their muscle tissue, improvement in antioxidant status, enhancement of immune function, and potential enhancement of reproductive capabilities, even under overwintering conditions.

Comparative transcriptomes reveal different tolerance mechanisms to Streptococcus agalactiae in hybrid tilapia, nile tilapia, and blue tilapia

Tilapia is one of the most economically important freshwater fish farmed in China. Streptococcosis outbreaks have been extensively documented in farmed tilapia species. Hybrid tilapia (Oreochromis niloticus ♀ × O. aureus ♂) exhibit greater disease resistance than Nile tilapia (O. niloticus) and blue tilapia (O. aureus). However, the molecular mechanism underlying the enhanced tolerance of hybrid tilapia is still poorly understood. In this study, comparative transcriptome analysis was performed to reveal the different tolerance mechanisms to Streptococcus agalactiae in the three tilapia lines. In total, 1982, 2355, and 2076 differentially expressed genes were identified at 48 h post-infection in hybrid tilapia, Nile tilapia, and blue tilapia, respectively. Functional enrichment analysis indicated that numerous metabolic and immune-related pathways were activated in all three tilapia lines. The differential expression of specific genes associated with phagosome, focal adhesion, cytokine-cytokine receptor interaction, and toll-like receptor signaling pathways contributed to the resistance of hybrid tilapia. Notably, immune response genes in hybrid tilapia, such as P38, TLR5, CXCR3, CXCL12, PSTPIP1, and TFR, were generally suppressed under normal conditions but selectively induced following pathogen challenge. These results expand our knowledge of the molecular mechanisms underlying S. agalactiae tolerance in hybrid tilapia and provide valuable insights for tilapia breeding programs.

Interleukin-8 involved in Nile Tilapia (Oreochromis niloticus) against bacterial infection

Interleukin 8 (IL8) is vital in promoting inflammation and is a crucial mediator in various physiopathological processes while influencing immunological function. The effect of IL8 on the immunological response to acute bacterial infections in Nile tilapia (Oreochromis niloticus) remains unknown. This work found an IL8 gene from Nile tilapia (On-IL8). It includes a 285 bp open reading frame and codes for 94 amino acids. The transcript levels of On-IL8 were highest in the head-kidney tissue and sharply induced by Streptococcus agalactiae and Aeromonas hydrophila. Besides, in vitro experiments revealed that On-IL8 regulated a variety of immunological processes and promoted inflammatory responses. Moreover, On-IL8 suppressed the NF-κB signaling pathway, consistent with in vitro results. These significant findings serve as the basis for further investigation into how IL8 confers protection to bony fish in opposition to bacterial infections.

Probiotics enhance resistance to Streptococcus iniae in Nile tilapia (Oreochromis niloticus) reared in biofloc systems

Biofloc technology is a rearing technique that maintains desired water quality by manipulating carbon and nitrogen and their inherent mixture of organic matter and microbes. Beneficial microorganisms in biofloc systems produce bioactive metabolites that may deter the growth of pathogenic microbes. As little is known about the interaction of biofloc systems and the addition of probiotics, this study focused on this integration to manipulate the microbial community and its interactions within biofloc systems. The present study evaluated two probiotics (B. velezensis AP193 and BiOWiSH FeedBuilder Syn 3) for use in Nile tilapia (Oreochromis niloticus) culture in a biofloc system. Nine independent 3785 L circular tanks were stocked with 120 juveniles (71.4 ± 4.4 g). Tilapia were fed for 16 weeks and randomly assigned three diets: a commercial control diet or a commercial diet top-coated with either AP193 or BiOWiSH FeedBuilder Syn3. At 14 weeks, the fish were challenged with a low dose of Streptococcus iniae (ARS-98-60, 7.2 × 107  CFU mL-1 , via intraperitoneal injection) in a common garden experimental design. At 16 weeks, the fish were challenged with a high dose of S. iniae (6.6 × 108  CFU mL-1 ) in the same manner. At the end of each challenge trial, cumulative per cent mortality, lysozyme activity and expression of 4 genes (il-1β, il6, il8 and tnfα) from the spleen were measured. In both challenges, the mortalities of the probiotic-fed groups were significantly lower (p < .05) than in the control diet. Although there were some strong trends, probiotic applications did not result in significant immune gene expression changes related to diet during the pre-trial period and following exposure to S. iniae. Nonetheless, overall il6 expression was lower in fish challenged with a high dose of ARS-98-60, while tnfα expression was lower in fish subjected to a lower pathogen dose. Study findings demonstrate the applicability of probiotics as a dietary supplement for tilapia reared in biofloc systems.

Genomics-driven prophylactic measures to increase streptococcosis resistance in tilapia

Streptococcosis caused by Streptococcus agalactiae and S. iniae is a significant problem that affects the success of tilapia aquaculture industries worldwide. In this critical review, we summarize the applicable practical strategies which may effectively enhance the world tilapia aquaculture development. Recently, the effect of vaccination and selective breeding programmes has been recognized as valuable tools to control the target disease and other consequent negative impacts caused by chemical and drug application. Advances in sequencing and molecular technologies are vital helpful factors with which to develop robust vaccines and increase the selective breeding programme's precision against streptococcosis. The genomic selection for streptococcosis-resistant tilapia strains and crucial genomic application for genomics' contribution to the development of novel Streptococcus vaccine, comparative genomics approach identifying vaccine candidates by reverse vaccinology, and next-generation vaccine design were described. Information from our review is encouraging for practical implementation of the development of vaccination and genomic selection in tilapia for streptococcosis resistance, which may be vital factors to sustain the world tilapia aquaculture industry effectively.

Chitosan nanoparticles and green synthesized silver nanoparticles as novel alternatives to antibiotics for preventing A.hydrophila subsp. hydrophila infection in Nile tilapia, Oreochromis niloticus

Recently, nanoparticles have attracted attention as a preventive tool for certain infectious diseases affecting fish in aquaculture. Furthermore, freshwater fishes are frequently vulnerable to summer mass morality caused by Aeromonas bacteria. In this regard, we focused on the evaluation of the in vitro and in vivo antimicrobial activity of chitosan (CNPs) and silver (AgNPs) nanoparticles against Aeromonas hydrophila subsp. hydrophila. CNPs and AgNPs were prepared at a mean particle size of 9.03 and 12.8 nm and a charge equalled+36.4 and -19.3 mV for CNPs and AgNPs, respectively. A. hydrophila subsp. hydrophila, Aeromonas caviae, and Aeromonas punctata were retrieved and identified by traditional and molecular techniques. The sensitivity of the obtained bacteria to eight different antibiotic discs was also tested. The antibiotic sensitivity studies revealed the presence of multidrug-resistant (MDR) Aeromonas species (spp.). The bacterium that showed the highest multidrug resistance against the tested antibiotic discs was Aeromonas hydrophila subsp. hydrophila. Therefore, CNPs and AgNPs were in vitro tested against the isolated bacterium and exhibited inhibition zones of 15 and 25 mm, respectively. TEM images also showed that CNPs and AgNPs had an antagonistic action against the same bacterium causing loss of architecture and bacterial death.

Effect of an organic acid blend in Nile tilapia growth performance, immunity, gut microbiota, and resistance to challenge against francisellosis

Organic acids (OAs) are a class of feed additives that have prophylactic and inhibitory properties against pathogenic bacteria. In this study, we investigated growth performance, innate immune response, gut microbiota, and disease resistance against Francisella orientalis F1 in Nile tilapia (Oreochromis niloticus) fed different doses of Bacti-nil®Aqua, a blend of short- and medium-chain OAs. For 21 days, 680 juvenile tilapias were fed a control diet or diets supplemented with a 0.3% (D3) or 0.5% (D5) OA blend. The feed conversion rate of fish fed the 0.5% enriched diet was considerably lower (p < 0.05) than that of the fish fed the basal diet. Lysozyme and serum bactericidal activities were significantly elevated following OA administration. After infection, no differences in the diversity and composition of gut microbiota were observed between the groups. After the bacterial challenge, the mortality was significantly lower in group D5 (p < 0.01). The diet supplemented with Bacti-nil®Aqua (Adisseo) improved the immune response and resistance of tilapia juveniles against F. orientalis infection. Thus, this OA blend could serve as a feed additive with good activity against F. orientalis.

An oral pH-responsive Streptococcus agalactiae vaccine formulation provides protective immunity to pathogen challenge in tilapia: A proof-of-concept study

Intensive tilapia farming has contributed significantly to food security as well as to the emergence of novel pathogens. This includes Streptococcus agalactiae or Group B Streptococcus (GBS) sequence type (ST) 283, which caused the first known outbreak of foodborne GBS illness in humans. An oral, easy-to-administer fish vaccine is needed to reduce losses in fish production and the risk of zoonotic transmission associated with GBS. We conducted a proof-of-concept study to develop an oral vaccine formulation that would only release its vaccine cargo at the site of action, i.e., in the fish gastrointestinal tract, and to evaluate whether it provided protection from experimental challenge with GBS. Formalin-inactivated S. agalactiae ST283, was entrapped within microparticles of Eudragit® E100 polymer using a double-emulsification solvent evaporation method. Exposure to an acidic medium simulating the environment in tilapia stomach showed that the size of the vaccine-loaded microparticles decreased rapidly, reflecting microparticle erosion and release of the vaccine cargo. In vivo studies in tilapia showed that oral administration of vaccine-loaded microparticles to fish provided significant protection from subsequent homologous pathogen challenge with GBS ST283 by immersion compared to the control groups which received blank microparticles or buffer, reducing mortality from 70% to 20%. The high efficacy shows the promise of the vaccine platform developed herein, which might be adapted for other bacterial pathogens and other fish species.

Vasoactive Intestinal Peptide (VIP) Protects Nile Tilapia (Oreochromis niloticus) against Streptococcus agalatiae Infection

Vasoactive intestinal peptide (VIP), a member of secretin/glucagon family, is involved in a variety of biological activities such as gut motility, immune responses, and carcinogenesis. In this study, the VIP precursor gene (On-VIP) and its receptor gene VIPR1 (On-VIPR1) were identified from Nile tilapia (Oreochromis niloticus), and the functions of On-VIP in the immunomodulation of Nile tilapia against bacterial infection were investigated and characterized. On-VIP and On-VIPR1 contain a 450 bp and a 1326 bp open reading frame encoding deduced protein of 149 and 441 amino acids, respectively. Simultaneously, the transcript of both On-VIP and On-VIPR1 were highly expressed in the intestine and sharply induced by Streptococcus agalatiae. Moreover, the positive signals of On-VIP and On-VIPR1 were detected in the longitudinal muscle layer and mucosal epithelium of intestine, respectively. Furthermore, both in vitro and in vivo experiments indicated several immune functions of On-VIP, including reduction of P65, P38, MyD88, STAT3, and AP1, upregulation of CREB and CBP, and suppression of inflammation. Additionally, in vivo experiments proved that On-VIP could protect Nile tilapia from bacterial infection and promote apoptosis and pyroptosis. These data lay a theoretical basis for further understanding of the mechanism of VIP guarding bony fish against bacterial infection.

Blood biomarkers as diagnostic tools: An overview of climate-driven stress responses in fish

Global climate change due to anthropogenic activities affects the dynamics of aquatic communities by altering the adaptive capacities of their inhabitants. Analysis of blood provides valuable insights in the form of a comprehensive representation of the physiological and functional status of fish under various environmental and treatment conditions. This review synthesizes currently available information about blood biomarkers used in climate change induced stress responses in fish. Alterations in informative blood-based indicators are used to monitor the physiological fitness of individual fishes or entire populations. Specific characteristics of fish blood, such as serum and plasma metabolites, cell composition, cellular abnormalities, cellular and antioxidant enzymes necessitate adapted protocols, as well as careful attention to experimental designs and meticulous interpretation of patterns of data. Moreover, the sampling technique, transportation, type of culture system, acclimation procedure, and water quality must all be considered for valid interpretation of hemato-biochemical parameters. Besides, blood collection, handling, and storage time of blood samples can all have significant impacts on the results of a hematological analysis, so it is optimal to perform hemato-biochemical evaluations immediately after blood collection because long-term storage can alter the results of the analyses, at least in part as a result of storage-related degenerative changes that may occur. However, the scarcity of high-throughput sophisticated approaches makes fish blood examination studies promising for climate-driven stress responses in fish.

Intestine and spleen microbiota composition in healthy and diseased tilapia

Symbiotic bacteria within the gut microbiome of various organisms, including fish, provide the host with several functions that improve the immune system. Although the spleen plays an important role in the modulation of immune responses, the role of spleen microbiota in shaping the immune system is unclear. Our study aimed at understanding the relationship between fish health and microbiota composition in the intestine and spleen. Our model organism was the hybrid tilapia (Oreochromis aureus × Oreochromis niloticus). We sampled intestine and spleen from healthy and diseased adult tilapia and determined their microbiota composition by sequencing the 16S rRNA gene. Significant differences were found between the intestine and the spleen microbiota composition of healthy compared to diseased fish as well as between intestines and spleens of fish with the same health condition. The microbiota diversity of healthy fish compared to diseased fish was significantly different as well. In the intestine of healthy fish, Cetobacterium was the most abundant genus while Mycoplasma was the most abundant genus in the spleen. Vibrio was the most abundant genus in the intestine and spleen of diseased fish. Moreover, it seems that there is a co-infection interaction between Vibrio and Aeromonas, which was reflected in the spleen of diseased fish. While Vibrio, Aeromonas and Streptococcus were the probable pathogens in the diseased fish, the role of Mycoplasma as a pathogen of cultured hybrid tilapia remains uncertain. We conclude that the intestine and spleen microbiota composition is strongly related to the health condition of the fish.

Economic appraisal of using genetics to control Streptococcus agalactiae in Nile tilapia under cage and pond farming system in Malaysia

Disease outbreaks have been seen as the major threat to sustainable aquaculture worldwide. Injectable vaccines have been one of the few strategies available to control the diseases, however, the adoption of this technology globally is limited. Genetic selection for disease resistance has been proposed as the alternative strategy in livestock and aquaculture. Economic analysis for such strategies is lacking and this study assesses the economic worth of using tilapia fingerlings resistant to Streptococcosis in both cage and pond production systems. The paper also assesses the profitability of paying the higher price for such fingerlings. Partial-budgeting was used to develop a stochastic simulation model that considers the benefits and costs associated with the adoption of tilapia fingerlings resistant to Streptococcosis at the farm level, in one production cycle. In both ponds and cage production systems, the use of genetically selected Streptococcus resistant tilapia fingerlings was found to be profitable where Streptococcus infection is prevalent. In the cages and ponds where Streptococcus related mortality was ≥ 10%, the Nile tilapia aquaculture was found to be profitable even if the amount paid for genetically selected Streptococcus resistant tilapia fingerlings was 100% higher than the amount paid for standard fingerlings.

Comparative Transcriptomic Analysis Reveals the Regulated Expression Profiles in Oreochromis niloticus in Response to Coinfection of Streptococcus agalactiae and Streptococcus iniae

Tilapia (Oreochromis sp.) is one of the important economical fishes in the world. Streptococcosis is commonly found in tilapia, causing severe and devastating effects in tilapia cultures. Streptococcus agalactiae and Streptococcus iniae are the predominant pathogens causing tilapia streptococcosis. To understand the molecular mechanisms underlying differential streptococcal infection patterns, Nile tilapias (Oreochromis niloticus) were infected by 1 × 107 CFU/mL S. agalactiae, 1 × 107 CFU/mL S. iniae, and 1 × 107 CFU/mL S. agalactiae and S. iniae (1:1), respectively, and transcriptome analysis was conducted to the intestine samples of Nile tilapia (Oreochromis niloticus) at 6, 12, 24 h, and 7 days post-infection. A total of 6,185 genes that differentially expressed among groups were identified. Eight differentially expressed genes (DEGs) including E3 ubiquitin-protein ligase TRIM39-like, C-X-C motif chemokine 10-like(CXCL 10), C-C motif chemokine 19-like, interleukin-1 beta-like, IgM heavy chain VH region, partial, IgG Fc-binding protein, proteasome subunit beta type-8 (PSMB8), and ATP synthase F(0) complex subunit B1, mitochondrial that involved in the immune system were selected, and their expression levels in the coinfection group were significantly higher than those in either of the single infection groups. These genes were associated with four different KEGG pathways. Additionally, the differential expression of eight DEGs was validated by using the RT-qPCR approach, and their immunological importance was discussed. The results provided insights into the responses of tilapia against S. agalactiae and S. iniae at the transcriptome level, promoting our better understanding of immune responses for aquatic animal against Streptococcus.