Immunological enhancement action of endotoxin-free tilapia heat shock protein 70 against Streptococcus iniae

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

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

Effects of juvenile thermal preconditioning on the heat-shock, immune, and stress responses of rainbow trout upon a secondary thermal challenge

Higher water temperatures and pathogens are both significant factors that negatively affect the welfare of teleost fish. In aquaculture, compared to natural populations, these problems are especially exacerbated, as the animals have relatively limited mobility, and the higher density promotes faster spread of infectious diseases. Because of the potential harm these stressors can inflict, methods that can limit the damage of these stressors are particularly valuable. As a method of interest, early-life thermal preconditioning of animals demonstrated some potential for effective improvements in thermotolerance. However, the potential effects of the method on the immune system via the heat-stress model have not been explored. In this experiment, juvenile-stage thermal preconditioned rainbow trout (Oncorhynchus mykiss) were subjected to a secondary thermal challenge, animals were collected and sampled at the time of lost equilibrium. The effects of preconditioning on the general stress response was assessed by measuring the plasma cortisol levels. In addition, we also examined hsp70 and hsc70 mRNA levels in the spleen and gill tissues, as well as IL-1β, IL-6, TNF-α, IFN-1, β2m, and MH class I transcripts via qRT-PCR. No changes in CTmax were observed between the preconditioned and control cohorts upon the second challenge. IL-1β and IL-6 transcripts were generally upregulated with increased temperature of the secondary thermal challenge, whereas IFN-1 transcripts were upregulated in the spleen, but downregulated in the gills, along with MH class I. The juvenile thermal preconditioning produced a series of changes in transcript levels for IL-1β, TNF-α, IFN-1, and hsp70 but the dynamics of these differences were inconsistent. Finally, analysis of plasma cortisol levels presented significantly lower cortisol levels in the pre-conditioned animals compared to the non-pre-conditioned control cohort.

Single molecule real-time sequencing and RNA-seq unravel the role of long non-coding and circular RNA in the regulatory network during Nile tilapia (Oreochromis niloticus) infection with Streptococcus agalactiae

BACKGROUND

The tilapia aquaculture industry is facing heavy economic losses due to Streptococcus agalactiae (S. agalactiae) infections. While progress has been made in past years, the lack of a high-quality tilapia genome and transcript annotations makes systematic and comprehensive exploration for a non-coding RNA regulatory network associated with the infection process unfeasible, and it stunts further research focused on disease defense and treatment. Herein, single molecular real time sequencing (SMRT-Seq) and RNA-seq data were utilized to generate a high-quality transcript annotation. In addition, Changes in mRNA and non-coding RNA expression were also analyzed during a S. agalactiae infection in tilapia.

FINDINGS

In total, 16.79 Gb of clean data were obtained by sequencing on six SMRT cells, with 712,294 inserts (326,645 full-length non-chimeric reads and 354,188 non-full-length reads). A total of 197,952 consensus transcripts were obtained. Additionally, 55,857 transcript sequences were acquired, with 12,297 previously annotated and 43,560 newly identified transcripts. To further examine the immune response in Oreochromis niloticus following a S. agalactiae infection, a total of 470.62 Gb of clean data was generated by sequencing a library containing 18 S. agalactiae infected tilapia samples. Of the identified genes, 9911 were newly exploited, of which 7102 were functional annotated. Furthermore, 7874 mRNAs, 1281 long non-coding RNAs (out of 21,860 long non-coding RNAs), and 61 circular RNAs (out of 1026 circular RNAs) were found to be differentially expressed during infection, with the 1026 circRNAs not previously identified in tilapia. Moreover, k-means clustering and WGCNA analyses revealed that the immune response of tilapia to a S. agalactiae infection can be divided into three stages: cytokines driven rapid immune response, energy metabolism promotion, and the production of lysosomes and phagosomes. During this response, the head kidney and spleen have synergistic effects, while maintaining independent characteristics. Finally, lncRNA-mRNA (trans and cis), lncRNA-miRNA-mRNA, and circRNA-miRNA-mRNA regulatory networks were constructed and revealed that non-coding RNA is involved in the regulation of immune-related genes.

CONCLUSIONS

This study generated a greatly-improved transcript annotation for tilapia using long-read PacBio sequencing technology, and revealed the presence of a regulatory network comprised of non-coding RNAs in Nile tilapia infected with S. agalactiae.

Effects of dietary manipulation on compensatory growth of juvenile genetically improved farmed tilapia (Oreochromis niloticus)

A 40-day feeding trial was conducted to investigate whether feeding a low-protein diet (25%) once daily for either 10 (L10H30) or 20 (L20H20) days then re-feeding a high-protein diet (35%) thrice daily elicit compensatory growth (CG) in genetically improved farmed tilapia (GIFT), Oreochromis niloticus (11.02 ± 0.05 g). Fish on the control treatment were fed 35% protein diet over 40 days (H40). Fish were stocked into nine 100-L tanks (30 fish per tank) with 3 replicate tanks for each group. Growth performance, feed utilization, proximate composition of body compartment, serum biochemical parameters, and hepatopancreatic histology and expressions of some genes related to inflammatory cytokine were evaluated every 10 days. Growth of L10H30 fish were similar to the control, whereas the weight of L20H20 fish were lower (P < 0.05) at day 20, but this significant difference disappeared at the end of the experiment. During 20-30 days, specific growth rate and feed intake were significantly higher (P < 0.05) and feed efficiency was lower (P < 0.05) in L20H20 fish than those in H40 fish. Dietary manipulations did not affect (P > 0.05) viscerosomatic and hepatosomatic indices, condition factors, serum biochemical parameters, and hepatopancreatic histology. Significant differences (P < 0.05) in proximate composition were observed only in viscera and muscle between L20H20 fish and H40 fish at day 20. The mRNA expressions of heat shock protein 70 kDa, tumor necrosis factor-α and interleukin (IL)-1β were higher (P < 0.05) in L10H30 and L20H20 fish at day 10, while IL-1β mRNA expression was lower (P < 0.05) in L20H20 fish at day 30 than those in H40 fish. Our results indicated that L20H20 fish elicited a complete CG and induced reversible physiological variations in juvenile GIFT.

The role of regulator FucP in Edwardsiella tarda pathogenesis and the inflammatory cytokine response in tilapia

The animal intestine provides a competitive environment for the microbiota. Successful colonization by pathogens requires sensing chemical signals to regulate the expression of virulence genes. Some bacteria rely on a two-component chemical signal transduction system, named FusKR, to regulate virulence genes expression by intestinal fucose. Here we construct the fucP gene deletion strain prove FucP regulation of the T3SS in E. tarda. The result showed that the mutant strain had down-regulated significantly the gene expression of FusKR and T3SS compared to the wild-type strain (P < 0.05). This mutant strain significantly increased LD₅₀ in zebrafish compared to the wild-type strain (P < 0.05), and significantly decreased penetration and motility in mucin than the wild-type strain (P < 0.05). Meanwhile, tilapia infected with mutant strain show significantly reduced E. tarda adherence and colonization than those infected with the wild-type strain (P < 0.05). Fish infected with EIB202 and ΔfucP showed significantly higher (P < 0.05) gene expression of IL-1β, TNF-α, IFN-γ, TGF-β and HSP-70 in head kidney than fish treated with PBS in the whole observed period; however CPP-3 did not show significant differences (P > 0.05) in all groups. Fish infected with EIB202 showed significantly higher (P < 0.05) gene expression of TGF-β in head kidney than fish treated with ΔfucP in the whole observed period; however other cytokines did not show significant differences (P > 0.05) in the whole observed period. In addition, Fish infected with EIB202 showed significantly higher (P < 0.05) gene expression of IL-1β, TNF-α and TGF-β in spleen than fish treated with ΔfucP in the whole observed period, however IFN-γ, CPP-3, and HSP-70 did not show significant differences (P > 0.05) in the whole observed period. Although the gene expression of cytokines was induced similarly by both strains, all results indicate that the fucP gene deletion down-regulates the key gene expression of FucKR and T3SS, reduces the pathogenicity of E. tarda in fish, particularly decreases inducing the gene expression of TGF-β in the head kidney and IL-1β, TNF-α and TGF-β in the spleen.

The effect of intermittent hypoxia under different temperature on the immunomodulation in Streptococcus agalactiae vaccinated Nile tilapia (Oreochromis niloticus)

Dissolved oxygen (DO) and temperature are the potential immunomodulators in fish and play the important roles in regulating immunity. We studied the effect of intermittent hypoxia under different temperature on the immunomodulation in vaccinated Nile tilapia (Oreochromis niloticus). The expression of immune-related genes, enzymatic activities, histology, cumulative mortality, and S. agalactiae clearance were assessed. Study conditions were intermittently hypoxic (4.0 ± 1.0 mg/L DO) at 30 ± 0.5 °C or 35 ± 0.5 °C. Interleukin-1beta (IL-1β), tumor necrosis factor alpha (TNF-α) and gamma interferon (IFN-γ) mRNA expression in spleen and head kidney were significantly lower in vaccinated hypoxic fish compared to the vaccinated normoxic fish. Levels of heat shock protein 70 (HSP70) in tissues showed an opposite tendency. Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities were significantly lower in vaccinated hypoxic fish. Malondialdehyde levels were significantly greater under hypoxic conditions. In vitro studies evaluated the effects of intermittent hypoxia at different temperatures on cells of vaccinated O. niloticus. Phagocytic activity of peripheral blood leucocytes (PBLs) and intracellular reactive oxygen species (ROS) production in head kidney cells were significantly decreased by intermittent hypoxia at either 30 °C or 35 °C, while nitric oxide levels in tissues cells increased significantly under hypoxic conditions. These changes were well reflected by the further suppression modulation on S. agalactiae clearance in vaccinated O. niloticus and higher cumulative mortality by intermittent hypoxia. Taken together, intermittent hypoxia at either 30 °C or 35 °C could suppress immunomodulation in vaccinated Nile tilapia.

Basal polarization of the immune responses to Streptococcus agalactiae susceptible and resistant tilapia (Oreochromis niloticus)

One of the highest priority areas for improvement is the development of effective strategies for decreasing disease mortality levels in aquaculture production, a better understanding of the components of the fish immune system and their functions in the context of pathogen invasion is needed. Tilapia is the most common fish in South China, and Streptococcus agalactiae has become the most serious disease problem for tilapia industry in China. Here, we profiled gene expression differences between tilapia differing in their susceptibility to S. agalactiae both basally (before infection) and at three early timepoints post-infection (5 h, 50 h, and 7 d). Between group comparisons revealed 5756 unique genes differentially expressed greater than 2-fold at one or more timepoints. And the resistant fish showed much more strong ability in pathogen recognition, antigen presentation, immune activation, while the susceptible fish showed fast activation of apoptosis. Taken together, the immune profiles expand our knowledge for molecular mechanisms for disease resistance, as well as provide solid molecular resources for further identification of the candidate markers for disease-resistant selection and evaluation of disease prevention and treatment options for tilapia industry.

LncRNA and mRNA profiling during activation of tilapia macrophages by HSP70 and Streptococcus agalactiae antigen

Objectives

To investigate the lncRNA profiling during tilapia peritoneal macrophages (TPMs) activation and discuss the relationship between lncRNA and mRNA.

Materials and Methods

RNA sequencing was used to investigate the lncRNA and mRNA profiles of TPMs activation following stimulation with Streptococcus agalactiae (Sa) antigen, heat shock protein 70 (HSP70) and HSP70+Sa. The expressions of lncRNA and mRNA were confirmed by qPCR. 356 lncRNA, 10173 mRNA and 1782 transcripts of uncertain coding potential (TUCP) were differentially expressed by pairwise comparison. These lncRNAs were shorter in length, fewer in exon number and higher in expression levels as compared with mRNAs. 683 lncRNAs and 4320 mRNAs were co-located, while 316 lncRNAs and 9997 mRNAs were in co-expression networks. Seven mRNAs (ANKRD34A, FMODA, GJA3, CNTN5, BMP10, BAI2 and HS3ST6) were involved in both networks of LNC_00035 and LNC_000466. Differentially expressed genes were involved in signaling pathways, such as "phosphorylation", "cytokine-cytokine receptor interaction", "endocytosis" and "MHC protein complex". LNC_000792, LNC_000215, LNC_000035 and LNC_000310, with cis and/or trans relationships with mRNAs, were also involved in ceRNA network.

Conclusions

These results might represent the first identified expression profile of lncRNAs and mRNAs in tilapia macrophages activated by HSP70 and Sa.

RNA-Seq Analysis of the Antioxidant Status and Immune Response of Portunus trituberculatus Following Aerial Exposure

Desiccation tolerance has been long considered as an important trait for the life survival under acute environmental stress. One of the biggest problems for modern commercial crab farming is desiccation during transportation; high mortality could occur following the aerial exposure. In this regard, here, we utilized RNA-seq-based transcriptome profiling to characterize the molecular responses of swimming crab in response to aerial exposure. In present study, following aerial exposure, the gill samples were sequenced at 0, 6, 12, and 18 h. And the sequenced reads were assembled into 274,594 contigs, with average length of 735.59 bp and N50 size of 1262 bp. After differential expression analysis, a total of 1572 genes were captured significantly differentially expressed, and were categorized into antioxidant/oxidative stress response, chaperones/heat shock proteins, immune alteration, cell proliferation/apoptosis, and cytoskeletal. Our analysis revealed the dramatic tissue oxidant stress and the alteration of the tissue epithelial integrity, especially many genes that have not been reported in crab species. With the limited functional information in crab, further studies are needed and underway in our lab to further characterize the key cellular actors governing the crab tolerance to aerial exposure. Taken together, our results provide molecular resources for further identification of key genes for desiccation tolerance, and to facilitate the molecular selection and breeding of desiccation tolerant strain and family.

Toxic effects in juvenile sea cucumber Apostichopus japonicas (Selenka) exposure to benzo[a]pyrene

To understand the physiological response of sea cucumber, Apostichopus japonicas, were exposed to different concentration of benzo[a]pyrene (B[a]P), and the growth, survival, antioxidant enzyme (SOD and T-AOC) activities were tested. Meanwhile, the quantitative real-time PCR technology was utilized to quantize the expression of immune related genes (i.e. innate immune genes, HSP genes and anti-oxidative genes). In our result, the SOD activity and T-AOC activity were induced at lower level of B[a]P (0.03 μg/L), however, a reduction of SOD activity and T-AOC activity were observed at relatively high B[a]P concentration (3 and 9 μg/L) for A. japonicas. Furthermore, the distinct expression patterns of selected immune-related genes were detected among different concentrations, and a general trend of down-regulation was observed at higher concentration. Especially, lysozyme almost showed the highest down-regulation at all concentrations, followed by NOS. Collectively, the growth, survival and expression signatures of immune related genes reflected an overall suppression of innate immunity in sea cucumber following exposure. Future functional studies should be carried out to characterize the detailed roles of immune genes and their related responses under B[a]P toxicity. Additionally, better understanding of the molecular indicators governing the healthy status under environmental toxicity would facilitate a healthy and sustainable culture program.

The effects of Natucin C-Natucin P mixture on blood biochemical parameters, antioxidant activity and non-specific immune responses in tilapia (Oreochromis niloticus)

Natucin C (NC) and Natucin P (NP) are two kinds of antimicrobial peptides (AMPs). In the present study, the effects of NC-NP mixture on a tilapia species (Oreochromis niloticus) were examined. Animals were fed with either a control diet or one of five AMP-supplemented diets for eight weeks. AMP-supplemented diets contained five increasing levels of NP from G1 to G5 and one level of NC (200 mg/kg). Results showed that fish in the G3, G4 and G5 groups had significantly higher levels of total protein (TP), albumin (ALB) and globulin (GLO) in serum than fish in the control group. Fish fed with G4 and G5 diets exhibited significantly higher high-density lipoprotein cholesterol (HDL-C) levels compared to the control fish. Lipopolysaccharide (LPS) levels in all AMP-supplemented groups were significantly lower than the control. In addition, the total antioxidant capacity (TAOC) and lysozyme (LZM) activities were significantly increased in fish fed with the G3 and G4 diets, respectively compared to the control. The serum malondialdehyde (MDA) levels in fish fed with AMP-supplemented diets were significantly decreased compared to those not supplemented with AMPs. Furthermore, the mRNA expressions of tumor necrosis factor alpha (TNF-α), interleukin-1-beta (IL-1β), gamma interferon (IFN-γ) and heat shock protein 70 (HSP70) in the hepatopancreas, spleen, kidney and gill were measured. Overall, the expression levels were enhanced in an NP dose-dependent and tissue-specific manner. The expressions of four genes in four organs (except IL-1β in spleen, and TNF-α and HSP70 in gill) were significantly upregulated in fish fed with the G5 diet. Fish fed with the G4 diet had increased expression levels of IL-1β in spleen and IFN-γ in kidney. The relative expression levels of TNF-α, IL-1β and HSP70 in the hepatopancreas in fish fed with the G3 diet were significantly upregulated compared to the control. Transcriptional levels of IL-1β and HSP70 in the hepatopancreas, IFN-γ and HSP70 in the kidney and IL-1β in the gills of fish fed with the G2 diet were upregulated. Taken together, our results indicated that the NC-NP mixture can enhance the antioxidant capacity and innate immune ability of O. niloticus, indicating that this mixture might be a potential alternative to antibiotics when used as a feed additive.

CHARACTERIZATION OF ANTERIOR SEGMENT OPHTHALMOLOGIC LESIONS IDENTIFIED IN FREE-RANGING DOLPHINS AND THOSE UNDER HUMAN CARE

Cetaceans in the wild and under human care develop a variety of ocular lesions. Although they have echolocation, cetacean species have good sight, making ocular health an important part of overall health care. The cornea is the primary site of abnormalities in both populations. Typical lesions of cetaceans under human care are characterized in this retrospective review of cases. One hundred eighty animals (n = 360 eyes) were chosen from the author's ophthalmologic examination reports from different geographic areas; they included Atlantic bottlenose dolphins (Tursiops truncatus), Pacific bottle nose dolphins (Tursiopstruncatus gilli), Indopacific bottlenose dolphins (Steno bredanensis), Indopacific humpback dolphins (Sousa chinensis), and roughtooth dolphins (Steno bredanensis). These animals were examined at least once, although most were examined numerous times over many years; lesions were categorized and are described. Seventy-seven eyes from 47 animals were normal. Medial keratopathy was the most common lesion and identified in 180 eyes from 97 animals, with 83 affected bilaterally. Horizontal keratopathy was identified in 69 eyes from 41 animals, with 28 affected bilaterally. Axial keratopathy and nonspecific axial opacities were identified in 67 eyes from 44 animals, with 21 affected bilaterally. Seventy-eight eyes from 50 animals, with 28 affected bilaterally, had more than one type of corneal lesion. Cataracts were identified in 32 eyes from 19 animals, with 13 affected bilaterally. Traumatic injuries were also common and involved eyelids and cornea. Sixteen eyes from 11 animals were blind; five dolphins were blind bilaterally due to phthisis bulbi secondary to corneal perforation or severe trauma. None of the diseases had a sex predisposition; however, medial keratopathy was significantly more common as a bilateral presentation than as a unilateral presentation. Cetaceans under human care with impaired sight can use echolocation; however, ocular health should definitely be a priority in their overall health plan.

Genome-wide identification of Hsp70 genes in channel catfish and their regulated expression after bacterial infection

Heat shock proteins 70/110 (Hsp70/110) are a family of conserved ubiquitously expressed heat shock proteins which are produced by cells in response to exposure to stressful conditions. Besides the chaperone and housekeeping functions, they are also known to be involved in immune response during infection. In this study, we identified 16 Hsp70/110 geness in channel catfish (Ictalurus punctatus) through in silico analysis using RNA-Seq and genome databases. Among them 12 members of Hsp70 (Hspa) family and 4 members of Hsp110 (Hsph) family were identified. Phylogenetic and syntenic analyses provided strong evidence in supporting the orthologies of these HSPs. In addition, we also determined the expression patterns of Hsp70/110 genes after Flavobacterium columnare and Edwardsiella ictaluri infections by meta-analyses, for the first time in channel catfish. Ten out of sixteen genes were significantly up/down-regulated after bacterial challenges. Specifically, nine genes were found significantly expressed in gill after F. columnare infection. Two genes were found significantly expressed in intestine after E. ictaluri infection. Pathogen-specific pattern and tissue-specific pattern were found in the two infections. The significantly regulated expressions of catfish Hsp70 genes after bacterial infections suggested their involvement in immune response in catfish.

Trancriptomic profiling revealed the signatures of acute immune response in tilapia (Oreochromis niloticus) following Streptococcus iniae challenge

Streptococcus iniae is the most significant bacterial disease of tilapia throughout the world, and commonly leads to tremendous economic losses. In contrast to other important fish species, our knowledge about the molecular mechanisms of tilapia in response to bacterial infection is still limited. Here, therefore, we utilized RNA-seq to first profiling of host responses in tilapia spleen following S. iniae infection at transcriptome level. A total of 223 million reads were obtained and assembled into 192,884 contigs with average length 844 bp. Gene expression analysis between control and infected samples at 5 h, 50 h, and 7 d revealed 1475 differentially expressed genes. In particular, the differentially expressed gene set was dramatically induced as early as 5 h, and rapidly declined to basal levels at 50 h. Enrichment and pathway analysis of the differentially expressed genes revealed the centrality of the pathogen attachment and recognition, cytoskeletal rearrangement and immune activation/inflammation in the pathogen entry and host inflammatory responses. Understanding of these responses can highlight mechanisms of tilapia host defense, and expand our knowledge of teleost immunology. Our findings will set a foundation of valuable biomarkers for future individual, strain, and family-level studies to evaluate immune effect of vaccine and individual response in host defense mechanisms to S. iniae infection, to select disease resistant families and strains.

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

Fish Streptococcus agalactiae (S. agalactiae) seriously harms the world's aquaculture industry and causes huge economic losses. This study aimed to develop a potential live attenuated vaccine of S. agalactiae. Pre-screened vaccine candidate strain S. agalactiae HN016 was used as starting material to generate an attenuated strain S. agalactiae YM001 by continuous passage in vitro. The biological characteristics, virulence, and stability of YM001 were detected, and the protective efficacy of YM001 immunization in tilapia was also determined. Our results indicated that the growth, staining, characteristics of pulsed-field gel electrophoresis (PFGE) genotype, and virulence of YM001 were changed significantly as compared to the parental strain HN016. High doses of YM001 by intraperitoneal (IP) injection (1.0 × 10(9) CFU/fish) and oral gavage (1.0 × 10(10) CFU/fish) respectively did not cause any mortality and morbidity in tilapia. The relative percent survivals (RPSs) of fishes immunized with YM001 (1.0 × 10(8) CFU/fish, one time) via injection, immersion, and oral administration were 96.88, 67.22, and 71.81%, respectively, at 15 days, and 93.61, 60.56, and 53.16%, respectively, at 30 days. In all tests with 1-3 times of immunization in tilapia, the dosages at 1 × 10(8) and 1 × 10(9) CFU/fish displayed the similar best results, whereas the immunoprotection of the dosages at 1 × 10(6) and 1 × 10(7) CFU/fish declined significantly (P < 0.01), and 1 × 10(5) CFU/fish hardly displayed any protective effect. In addition, the efficacy of 2-3 times of immunization was significantly higher than that of single immunization (P < 0.01) while no significant difference in the efficacy between twice and thrice of immunization was seen (P > 0.05). The level of protective antibody elicited by oral immunization was significantly higher compared to that of the control group (P < 0.01), and the antibody reached their maximum levels 14-21 days after the immunization but decreased significantly after 28 days of vaccination. YM001 bacteria were isolated from the brain, liver, kidney, and spleen tissues of fish after oral immunization and the bacteria existed for the longest time in the spleen (up to 15 days). Taken together, this study obtained a safe, stable, and highly immunogenic attenuated S. agalactiae strain YM001; oral immunization of tilapia with this strain produced a good immune protection.