自然感染无乳链球菌罗非鱼的比较病理学及毒力基因谱分析

在自然感染无乳链球菌(Streptococcus agalactiae)的罗非鱼(Oreochromis niloticus)成鱼、稚鱼和自然携带无乳链球菌的罗非鱼体内分别获得14株、4株和2株无乳链球菌。临床和组织病理学分析显示,罗非鱼成鱼出现无规则游动,脑、眼眶、鳃和鳍条充血,眼球突出、白浊,内脏器官肿大、充血,以肾小管玻璃样变性、脑膜炎和心外膜炎等组织病理学变化为特征;罗非鱼稚鱼体表无明显症状,但部分内脏器官呈现肿大、充血现象,以脾脏血管区出血、肾小管上皮细胞变性、脑组织炎症反应较轻为其主要组织病理学特征。此外,罗非鱼胃固有层内及稚鱼肝脏组织中有大量的嗜酸性粒细胞浸润,可观察到无乳链球菌在成鱼的脑、心脏以及稚鱼肝脏中增殖;自然携带无乳链球菌的罗非鱼临床症状和组织学病变均不明显。PCR检测发现,各无乳链球菌毒力基因谱相同,但自然感染无乳链球菌的罗非鱼成鱼、稚鱼和自然携带无乳链球菌的罗非鱼的病理学损伤差异显著。     

BALB/c小鼠用于评价尼罗罗非鱼无乳链球菌毒力的研究

实验采用BALB/c小鼠作为实验动物,旨在建立尼罗罗非鱼无乳链球菌毒力测定的BALB/c小鼠模型。BALB/c小鼠经腹腔注射尼罗罗非鱼源无乳链球菌建立感染模型,比较了尼罗罗非鱼源无乳链球菌分别感染尼罗罗非鱼和小鼠的LD_(50)差异,分别测定了不同毒力尼罗罗非鱼无乳链球菌对尼罗罗非鱼和小鼠的毒力。结果显示,小鼠经腹腔注射无乳链球菌,在24 h内出现死亡现象,且对小鼠脑、肝脏、脾脏、肾脏等组织造成损伤。3次测定尼罗罗非鱼无乳链球菌TFJ0901对尼罗罗非鱼和小鼠LD_(50)分别为7.7×10~7、2.2×10~8、3.5×10~9 CFU/mL和405、361、419 CFU/只。将无乳链球菌TFJ0901和THN0901感染尼罗罗非鱼(1.0×10~7 CFU/mL)和小鼠(100 CFU/只),尼罗罗非鱼和小鼠存活率分别为100%、6.7%±5.8%和100%、0,其存活率都具有显著性差异。将无乳链球菌TFJ0901和TFJ-F感染尼罗罗非鱼(3.0×10~8 CFU/mL)和小鼠(2 500 CFU/只),尼罗罗非鱼的存活率分别为73.3%±11.5%和80.0%±10.0%,存活率差异不显著,小鼠存活率分别为13.3%±11.5%和100.0%,存活率具有显著性差异。研究表明,本实验成功建立了BALB/c小鼠作为尼罗罗非鱼源无乳链球菌毒力测定的稳定模型,测定不同毒力的尼罗罗非鱼源无乳链球菌对小鼠毒力与对尼罗罗非鱼毒力一致,且该模型能够区分尼罗罗非鱼模型难以区分的毒力相近的无乳链球菌。     

异源无乳链球菌胞外产物灭活疫苗对罗非鱼的免疫效果

为了研究异源无乳链球菌胞外产物灭活疫苗免疫罗非鱼后的免疫效果, 本研究采用 10%甲醛溶液灭活法制备了无乳链球菌 HN0901 和 GX1101 胞外产物灭活疫苗, 通过腹腔注射途径免疫健康奥尼罗非鱼(Oreochromis niloticus♀×O.aureus♂), 在免疫后 28 d 用同/异源无乳链球菌攻毒, 测定了免疫后和攻毒后罗非鱼的免疫应答反应和血清抗体效价, 并比较了罗非鱼在腹腔注射 1×108 CFU/尾无乳链球菌后的相对免疫保护率和交叉免疫保护率。 结果显示, 免疫鱼谷丙转氨酶(ALT)和谷草转氨酶(AST)在免疫后 28 d 显著高于对照组, 但在攻毒后显著低于对照组; 与对照鱼相比, 免疫鱼的超氧化物歧化酶(SOD)和过氧化氢酶(CAT)水平在免疫和攻毒后均有不同程度的提高, 丙二醛(MDA)显著降低(P＜0.05)。实时定量 PCR 结果显示, 炎性细胞因子基因 TNF-α、IL-1β 和 TGF-β, 体液免疫相关基因 HSP70, MHC Ⅱ和 IgM 在免疫和攻毒后也呈现显著升高趋势, 并显著高于对照组(P＜0.05)。血清抗体效价于免疫后第 28 天达到 1∶6400, 显著高于对照组(P＜0.05)。攻毒实验结果显示, 免疫组在同源和异源无乳链球菌攻毒后相对的免疫保护率在 31.6%~47.4%。结果表明, 研制的灭活疫苗免疫罗非鱼后, 能够显著诱导罗非鱼体内的免疫应答, 产生中等疫苗效力, 对同/异源无乳链球菌都具有免疫保护能力。本研究结果为深入研究罗非鱼链球菌病的免疫预防技术和研制二价疫苗奠定了基础。     

无乳链球菌和海豚链球菌早期预警分子检测

为提高罗非鱼链球菌病发前的早期预警,根据海豚链球菌的16S rRNA基因的部分序列和无乳链球菌的Sip基因特异性序列,分别合成特异性检测引物,经过对反应体系和反应条件的优化,建立检测较低含量的无乳链球菌和海豚链球菌的分子技术。试验结果显示,所建立的双重PCR可扩增出870 bp无乳链球菌和614 bp海豚链球菌的特异性片段,而迟钝爱德华氏菌、维氏气单胞菌、创伤弧菌、温和气单胞菌、溶藻弧菌、大肠杆菌均为阴性;无乳链球菌和海豚链球菌基因组DNA最低检测量分别为9.84×10~(-5) ng/μL和9.30×10~(-5) ng/μL,菌液最低检测量分别为2.76×10~3 cfu/mL和2.51×10~3 cfu/mL,远低于其半致死密度10~7 cfu/mL;对无乳链球菌和海豚链球菌混合感染的模拟临床样品的检出率为100%。研究结果表明,该分子技术的特异性较强、灵敏度较高、检出率较高,可用于较低含量的无乳链球菌和海豚链球菌的快速鉴别,为罗非鱼链球菌的早期预警分子检测提供技术支持。     

罗非鱼源无乳链球菌的间接ELISA快速检测方法

以罗非鱼源无乳链球菌为抗原,免疫新西兰兔获得高效价的多克隆抗体;以此多克隆抗体作为一抗,羊抗兔Ig G-HRP作为酶标二抗,建立无乳链球菌的间接ELISA快速检测法。采用棋盘滴定法确定抗原与一抗的最佳工作浓度分别为106 CFU/m L和1∶10 000;酶标二抗的最适工作浓度为1∶1000。病原菌检测灵敏度为每孔103 CFU。该方法标准化后具有快速、灵敏等特性,与海豚链球菌等其他常见水产病原菌无交叉反应;阻断实验中的阻断率达72.02%;交叉反应和阻断实验的结果显示该方法具有较高的特异性。将该方法标准化后检测了44株2007—2013年分离的罗非鱼无乳链球菌,阳性检测率为100%;对人工感染后死亡的30尾罗非鱼的脑组织进行检测,阳性检测率为100%;对人工感染后未死亡的20尾罗非鱼的脑,肝和脾组织进行检测,脑的阳性检测率为0%,肝的阳性检测率为25%,脾的阳性检测率为50%;表明该技术不仅能够检测已发病的罗非鱼,而且能够检测无症状的带菌罗非鱼,该方法的建立有助于快速准确地诊断由无乳链球菌引起的罗非鱼链球菌病。     

尼罗罗非鱼无乳链球菌Sip蛋白乳酸菌活载体口服疫苗的研制及其免疫效果

链球菌病是威胁我国罗非鱼养殖产业健康发展的重要病害之一。为研制出免疫效果好、操作简便的罗非鱼链球菌病疫苗,本研究构建重组表达无乳链球菌Sip蛋白的穿梭质粒pNZ8124-Sip,通过酶切和测序验证后电转化乳酸乳球菌NZ9000,获得能够诱导重组表达无乳链球菌Sip蛋白的乳酸菌活菌载体疫苗。采用SPS-PAGE电泳摸索最佳诱导浓度和诱导时间以获得最大表达量,通过镍柱纯化目的蛋白并进行Western blot检测;利用不同浓度的重组乳酸菌活载体疫苗灌胃口服免疫尼罗罗非鱼,采用间接ELISA法测定免疫后血清抗体水平变化,通过人工腹腔注射感染无乳链球菌获得相对免疫保护率。研究结果显示,构建的重组乳酸乳球菌可通过nisin诱导表达大小为48 ku特异性蛋白,与目的蛋白大小一致;PAGE电泳显示,重组蛋白主要以可溶蛋白和包涵体2种形式存在,其中胞内可溶性蛋白浓度达7.65 mg/mL;诱导表达的最佳条件为100 ng/mL nisin诱导6 h;Western blot检测结果显示,诱导蛋白可与鼠抗His标签抗体特异性结合。口服免疫结果显示,中浓度组(2.24×10~(10) CFU/mL)和低浓度组(2.24×10~9 CFU/mL)免疫2次能够显著提高尼罗罗非鱼的血清抗体水平和抗无乳链球菌感染能力,中浓度免疫组的相对免疫保护率最高为41.0%。本研究可为罗非鱼链球菌病口服疫苗的研究奠定基础,具有广阔的应用前景。     

温度对尼罗罗非鱼无乳链球菌毒力的影响

为了解温度对鱼源无乳链球菌毒力的影响机制,本实验研究了温度对无乳链球菌毒力相关参数(生长、粘附、入侵、毒力基因表达以及对罗非鱼致死率)的影响。结果发现,不同培养温度下(25~40 ℃)无乳链球菌的生长速度不同,37 ℃为其最适生长温度,25 ℃时生长速度最慢;25~34 ℃内无乳链球菌粘附在惰性基质上的菌体对应的吸光值(OD_590nm)之间无显著差异(P>0.05),37 ℃时显著增加,40 ℃时又急剧下降;罗非鱼在人工感染无乳链球菌后各时间点(6、12、24和48 h),鱼体脑组织中菌量随注射菌体培养温度的增加呈先上升后下降的趋势,且与不同培养温度下的无乳链球菌对罗非鱼的致死率呈正相关;无乳链球菌毒力基因的表达也与培养温度相关,hly和cfb基因的表达量随菌体培养温度的升高先增加后降低,分别在34和37 ℃处达到峰值,不同培养温度下无乳链球菌sip基因表达的变化幅度较小,而scpB基因的表达则随培养温度增加而下降;无乳链球菌对罗非鱼的致死率随其培养温度的升高呈先升高后降低趋势,低温(25和28 ℃)培养条件下的菌体对罗非鱼致死率较低(<20%),37 ℃时致死率最高66.67%±6.67%。以上结果表明温度参与无乳链球菌生长、粘附、转移、入侵和部分毒力基因表达的调控,它们共同影响无乳链球菌对罗非鱼的毒力。     

12种药物对罗非鱼无乳链球菌的抑菌试验

无乳链球菌(Streptococcus agalactiae)普遍存在于养殖水体中,可产生外毒素,对动物具有致死性,由无乳链球菌和海豚链球菌(Streptococcus inia)引起的罗非鱼链球菌病被认为是对罗非鱼危害最严重的细菌性疾病,而且链球菌还可随伤口感染人类.近几年来,因罗非鱼链球菌病的暴发流行,给罗非鱼养殖业造成极大的危害,甚至威胁到罗非鱼养殖产业的稳定发展.2011年5月～10月,广西多地暴发罗非鱼链球菌病,危害各种规格的养殖罗非鱼,共造成7127吨鱼死亡,经济损失8610.8万元,已严重制约了广西罗非鱼养殖业的健康发展.研究结果表明,近两年广西养殖罗非鱼链球菌病的病原菌为无乳链球菌,且大多表现为多重耐药菌株.而生产实际中,药物防治仍是目前养殖户针对罗非鱼链球菌病普遍采用的主要控制措施.本试验选取12种常用国标渔药,研究其对无乳链球菌的抑菌作用,旨在为罗非鱼无乳链球菌病的药物防治提供参考.     

罗非鱼链球菌病防治策略新思考

罗非鱼链球菌病有2种病原，即海豚链球菌和无乳链球菌。在5年前，罗非鱼链球菌病的主要病原是海豚链球菌，且疾病的发生多呈局部性的零星暴发流行。但随着罗非鱼养殖密度增大、养殖环境恶化、抗病性能退化，罗非鱼链球菌病呈现大面积、高死亡的急性流行暴发，且病原主要是无乳链球菌。近3年来，罗非鱼链球菌病的广泛暴发，给罗非鱼养殖业造成极...     

中草药复方制剂对罗非鱼无乳链球菌病药效研究

<正>感染罗非鱼的链球菌主要有无乳链球菌、海豚链球菌、副乳房链球菌、格氏乳球菌等，其中较常见的是无乳链球菌(柯剑等，2010)。目前，链球菌病的防治以抗生素为主，但抗生素对无乳链球菌或不敏感，或多为农业限用、禁用品种，亟须寻找替代抗生素防治链球菌病的方法。     

Differential pathogenicity of five Streptococcus agalactiae isolates of diverse geographic origin in Nile tilapia (Oreochromis niloticus L.)

Streptococcus agalactiae is an emerging pathogen of fish and has caused significant morbidity and mortality worldwide. The main objective of this study is to assess whether pathogenic differences exist among isolates from different geographic locations. Nile tilapia (Oreochromis niloticus L.) were administered an intraperitoneal injection of suspension containing USA, Brazil, Honduras, Israel, or Kuwait S. agalactiae isolates at concentrations ranging from 10² to 10⁷ cfu mL^?1. The LD₅₀ values 7 days after challenge were as follows: USA (1.0 × 10² cfu mL^?1), Brazil (1.5 × 10³ cfu mL^?1), Honduras (6.8 × 10³ cfu mL^?1), Israel (1.0 × 10⁴ cfu mL^?1) and Kuwait (7.2 × 10⁵ cfu mL^?1). Fish from all groups exhibited lethargy, anorexia, exophthalmia, corneal opacity, erratic swimming, petechiae and mortality. Opercular clearing and ascites were only found after infection with certain geographic isolates. The findings in this study indicate that S. agalactiae isolates of different geographic origin can cause significant mortalities after experimental challenge and can have different pathogenic capacities. Isolates from the Americas (USA, Brazil and Honduras) were more pathogenic to Nile tilapia than isolates from the Middle East/Asia (Israel and Kuwait).     

Efficacy of an experimentally inactivated Streptococcus agalactiae vaccine in Nile tilapia (Oreochromis niloticus) reared in Brazil

Tilapia aquaculture is one of the fastest‐growing segments of fish production in Brazil. Nile tilapia (Oreochromis niloticus) is largely cultivated in the state of Parana, where Streptococcus agalactiae is the cause of severe disease outbreaks. The objective of this paper was to evaluate an inactivated S. agalactiae vaccine in tilapia for the control of streptococcal disease outbreaks. Tilapia, weighing approximately 20 g each, were intraperitoneally (i.p.) inoculated with 0.1 mL of the vaccine at a dose of 2.0 × 10⁸ colony‐forming unit (CFU) mL^?1. One group of tilapia (treatment 1) received one vaccine dose, and the other group of tilapia (treatment 2) received two doses, with an interval of 21 days. The control group was i.p. inoculated with 0.1 mL tryptic soy broth fish^?1. Immunized and control tilapia were i.p. challenged with 0.1 mL of 3.0 × 10⁷ CFU mL^?1 at 30 days post vaccination. The fish were monitored daily for disease signs and for mortality for 16 days post challenge. A statistically significant difference (P=0.0045) was found between the mortality of treatments 1 and 2. The value of relative per cent of survival of 83.6% and 96.4%, respectively, indicate that this vaccine was efficient in Nile tilapia.     

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.     

粪肠球菌对吉富罗非鱼的生长、体组成、消化酶活性及血液生理生化指标的影响

为研究饲料中添加不同浓度的粪肠球菌对吉富罗非鱼生长、体组成、消化能力及血液生理生化指标的影响,实验选用初始体质量为(50.59±0.59)g的吉富罗非鱼300尾,随机分成5组,每组设3个重复,每个重复20尾鱼,养殖实验期60 d。分别投喂实测含1.3×10~2(对照组)、1.4×10~5、1.7×10~6、1.5×10~7和1.8×10~8 CFU/g粪肠球菌的5种等氮(36%)等脂(6.75%)的实验饲料。结果显示:(1)罗非鱼的末均重(FBW)、增重率(WGR)、特定生长率(SGR)和尾均摄食量(FI)均在1.5×10~7 CFU/g组达到最大且显著高于对照组,而饲料系数(FCR)显著低于对照组。以FBW、WGR和SGR为评价指标,通过二次回归分析得出,罗非鱼饲料的粪肠球菌适宜添加浓度范围为7.5×10~7~1.1×10~8 CFU/g。(2)各添加组的全鱼粗蛋白含量均显著高于对照组,1.5×10~7 CFU/g组的粗脂肪含量显著高于对照组,各组间全鱼水分和粗灰分含量无显著差异。1.5×10~7 CFU/g组的干物质、蛋白质和脂肪沉积率均达到最大值且显著高于对照组。(3)1.5×10~7和1.8×10~8 CFU/g组的肠脂肪酶活性均显著高于对照组,而1.8×10~8 CFU/g组的肠道蛋白酶活性显著低于对照组;各组间肠道淀粉酶活性无显著差异。(4)1.5×10~7 CFU/g组的平均红细胞体积、血红蛋白浓度和血小板数均显著低于对照组,各组间红细胞数无显著差异。(5)1.8×10~8 CFU/g组的血清中胆固醇、葡萄糖和丙二醛含量显著低于对照组;1.5×10~7 CFU/g组血清中碱性磷酸酶活性显著高于对照组,而谷丙转氨酶和谷草转氨酶活性显著低于对照组。综上所述,吉富罗非鱼(50~210 g)饲料中粪肠球菌的适宜添加浓度范围为7.5×10~7~1.1×10~8 CFU/g,但添加粪肠球菌对血液载氧能力有负面影响。     

Genomic comparison of virulent and non‐virulent Streptococcus agalactiae in fish

Streptococcus agalactiae infections in fish are predominantly caused by beta‐haemolytic strains of clonal complex (CC) 7, notably its namesake sequence type (ST) 7, or by non‐haemolytic strains of CC552, including the globally distributed ST260. In contrast, CC23, including its namesake ST23, has been associated with a wide homeothermic and poikilothermic host range, but never with fish. The aim of this study was to determine whether ST23 is virulent in fish and to identify genomic markers of fish adaptation of S. agalactiae. Intraperitoneal challenge of Nile tilapia, Oreochromis niloticus (Linnaeus), showed that ST260 is lethal at doses down to 10² cfu per fish, whereas ST23 does not cause disease at 10⁷ cfu per fish. Comparison of the genome sequence of ST260 and ST23 with those of strains derived from fish, cattle and humans revealed the presence of genomic elements that are unique to subpopulations of S. agalactiae that have the ability to infect fish (CC7 and CC552). These loci occurred in clusters exhibiting typical signatures of mobile genetic elements. PCR‐based screening of a collection of isolates from multiple host species confirmed the association of selected genes with fish‐derived strains. Several fish‐associated genes encode proteins that potentially provide fitness in the aquatic environment.     

Antibacterial activity of oxytetracycline on microbial ecology of Nile tilapia (Oreochromis niloticus) gastrointestinal tract under laboratory condition

The antibacterial effect of oxytetracycline (OTC) on bacterial load, pathogen and possible development of antibiotic resistance in intestinal flora of apparently healthy Nile tilapia (Oreochromis niloticus) was investigated under laboratory condition. OTC, a broad spectrum approved antibiotic widely used in aquaculture, was fed to Nile tilapia through medicated diet at a rate of 2 g/ kg of feed for consecutive 7 days in treatment 1 (T₁) and 30 days in treatment 2 (T₂). Changes in physico‐chemical parameters of water were also recorded where pH and dissolved oxygen influenced bacterial load and coliform count respectively. At the start of the experiment, initial bacterial load in fish intestine was 1.61 ± 2.25 × 10¹⁰ cfu/g, which after OTC treatment decreased significantly (p < .05) to 3.06 ± 2.08 × 10⁸ cfu/g on day 7 and 3.45 ± 4.46 × 10⁷ cfu/g on day 30 when compared with the control group. Among the 10 bacterial genera identified in the intestine, 4 potential public health concern bacteria viz., Salmonella, Escherichia, Enterobacter and Staphylococcaus were predominant. There was a sharp increase in bacterial load on day 18 in T₂ that may be attributed to the development of antibiotic resistance in intestinal bacteria. Antibiotic susceptibility test for isolates against seven antibiotics: amoxycillin, ciprofloxacin, co‐trimoxazole, erythromycin, gentamicin, nitrofurantoin and tetracycline using disc diffusion method revealed significantly increased resistance of Gram‐negative rods (p < .05) that possibly caused increased frequency of OTC‐resistant microorganisms. In conclusion, short‐ and long‐term exposure to OTC treatment affected the distribution of bacterial genera including pathogens in the Nile tilapia gastrointestinal tract and concomitantly influenced their antimicrobial resistance.     

Comparison between the intestinal microbiome of healthy fish and fish experimentally infected with Streptococcus agalactiae

Nile tilapia (Oreochromis niloticus) farming is an economic activity that is soaring in the whole world. Septicemia due to Streptococcus agalactiae is the main disease impacting fish farming. The aim of this study was to compare the gut microbiome of healthy animals and animals experimentally infected with S. agalactiae strain 21171A. The microbiome was established with 16S ribosomal DNA next‐generation sequencing (NGS). One hundred Nile tilapias, with an average weight of 35 g, were distributed into two groups. Fifty fish from the challenged group were orally inoculated with 100 μl of a bacterial solution containing 1.98 × 10³ CFU/ml of S. agalactiae strain 21171A, while 50 controls were orally inoculated with sterile saline. After the experiment, 24 fish from the challenged group and 27 fish from the control group were analysed. For both groups, bacteria attached to the mucosa (M) and present in faeces (F) were analysed. The mean of the number of taxa identified in the infected group (M + F) (45.87 ± 30.13) was lower than in the control (M + F) (67.70 ± 21.10) (p < .01). Nineteen bacterial taxa were more abundant in faecal samples from the infected group when compared with the control group (p < .01). Thirty‐nine taxa were associated with mucosa samples from the challenged group when compared to the control samples (p < .01). No OTU was associated with healthy samples. The results demonstrate that the infection with S. agalactiae reduces the variability of the gut microbiota. Moreover, some bacteria proliferate during the infection.     

Dietary supplementation with Bacillus subtilis LT3‐1 enhance the growth,immunity and disease resistance against Streptococcus agalactiae infection in genetically improved farmed tilapia,Oreochromis niloticus

A feeding trial was conducted to investigate the effect of different levels of Bacillus subtilis LT3‐1 in diets on growth, immune parameters, intestinal morphology and disease resistance in genetically improved farmed tilapia, Oreochromis niloticus. Fish (46.91 ± 0.17 g) were fed with a basal diet supplemented with B. subtilis LT3‐1 at 0 (B0), 3.8 × 10¹⁰ (B1), 7.6 × 10¹⁰ (B2), 1.14 × 10¹¹ (B3) and 1.52 × 10¹¹ (B4) CFU kg^?1 for 6 weeks. The results showed that the weight gain of fish in B1 group was significantly enhanced compared to that in B0 group (p < 0.05). The addition of B. subtilis significantly affected serum biochemical indices (total protein, albumin, aspartate aminotransferase, alkaline phosphatase). Besides, the haematocrit, total counts of red and white blood cells, as well as the serum catalase and lysozyme activities, were increased, whereas the serum malondialdehyde, the serum immunoglobulin M and complement three contents were reduced. Parameters for intestinal morphology suggested a healthier intestine for the fish fed B. subtilis‐supplemented diets than fish fed the control diet. The survival rate after Streptococcus agalactiae challenge increased in tilapia fed with B. subtilis. The present study demonstrated B. subtilis can effectively improve growth, immunological status and resistance against S. agalactiae infection in tilapia farming.