动物营养学报    2021, Vol. 33 Issue (4): 2199-2212    PDF    
酵母水解物对日本沼虾幼虾生长、抗氧化、免疫和肠道菌群的影响
熊云凤 , 吴佳文 *, 周东生 , 郑金仙 , 邱恬 , 李秋琳 , 孔有琴 , 邵仙萍 , 叶金云 , 丁志丽     
湖州师范学院生命科学学院, 中国水产科学研究院水生动物繁育与营养重点实验室, 浙江省水生生物资源养护与开发技术研究重点实验室, 湖州 313000
摘要: 本试验旨在研究酵母水解物添加水平对日本沼虾幼虾生长、抗氧化、免疫和肠道菌群的影响。在蛋白质水平为37%、脂肪水平为8%的基础饲料中分别添加0(YH1)、0.5%(YH2)、1.0%(YH3)、2.0%(YH4)和4.0%(YH5)的酵母水解物,获得5种试验饲料。将平均体重为(0.104±0.003)g的1 000尾日本沼虾幼虾随机分为5组,每组4个平行(50尾/平行),进行为期8周的饲养试验。结果表明:1)日本沼虾成活率不受饲料中酵母水解物添加水平的影响(P>0.05),但当饲料中酵母水解物添加水平达到1.0%时,增重率和特定生长率最高,且显著高于YH1组(P < 0.05)。2)相比YH1和YH5组,YH3和YH4组肝胰腺丙二醛(MDA)含量显著降低(P < 0.05);YH2、YH3和YH4组肝胰腺总抗氧化力(T-AOC)显著高于YH1和YH5组(P < 0.05);YH4组肝胰腺超氧化物歧化酶(SOD)活性显著高于YH1、YH2和YH5组(P < 0.05);YH5组肝胰腺谷胱甘肽过氧化物酶(GSH-Px)活性显著低于其余各组(P < 0.05);同时,YH5组肝胰腺免疫性能指标一氧化氮(NO)含量和一氧化氮合酶(NOS)活性显著高于其余各组(P < 0.05)。3)日本沼虾肠道菌群从门水平上分析,主要以8种优势菌门为主,分别为变形菌门(Proteobacteria)、厚壁菌门(Firmicutes)、软壁菌门(Tenericutes)、拟杆菌门(Bacteroidetes)、蓝藻门(Cyanobacteria)、放线菌门(Actinobacteria)、衣原体门(Chlamydiae)和浮霉菌门(Planctomycetes);从属水平上分析,主要以11种优势菌属为主,分别为假单胞菌属(Pseudomonas)、希瓦氏菌属(Shewanella)、气单胞菌属(Aeromonas)、雷诺氏菌属(Reyranella)、几丁质杆菌属(Chitinibacter)、不动杆菌属(Acinetobacter)、黄杆菌属(Flavobacterium)、分枝杆菌属(Mycobacterium)、红杆菌属(Rhodobacter)、寡养单胞菌属(Stenotrophomonas)和军团菌属(Legionella);属水平上的有益菌未因酵母水解物的添加而显著增多(P < 0.05),但添加一定量的酵母水解物可抑制希瓦氏菌属、气单胞菌属、不动杆菌属和军团菌属等致病菌和潜在致病菌的生长。由此可见,在饲料中添加1.0%的酵母水解物可促进日本沼虾的生长;当添加水平达到1.0%~2.0%时,可提高虾的抗氧化能力;当添加水平达到4.0%时,不仅可以显著改善虾的肠道菌群,还可以提高虾的非特异性免疫力。根据生长性能和抗氧化能力,建议日本沼虾幼虾饲料中酵母水解物添加水平为1.0%;根据免疫性能分析,建议日本沼虾幼虾饲料中酵母水解物添加水平为4.0%。
关键词: 酵母水解物    日本沼虾    生长性能    抗氧化    免疫    肠道菌群    
Effects of Yeast Hydrolyzate on Growth, Antioxidant, Immune and Intestinal Flora of Juvenile Macrobrachium nipponense
XIONG Yunfeng , WU Jiawen *, ZHOU Dongsheng , ZHENG Jinxian , QIU Tian , LI Qiulin , KONG Youqin , SHAO Xianping , YE Jinyun , DING Zhili     
Zhejiang Provincial Key Laboratory of Aquatic Resources Conservation and Development, Key Laboratory of Aquatic Animal Genetic Breeding and Nutrition of Chinese Academy of Fishery Sciences, College of Life Science, Huzhou University, Huzhou 313000, China
Abstract: The purpose of this study was to study the effects of yeast hydrolyzate (YH) on growth, antioxidation, immunity and intestinal flora of Juvenile Macrobrachium nipponense. Five experimental diets were obtained by adding 0 (YH1), 0.5% (YH2), 1.0% (YH3), 2.0% (YH4) and 4.0% (YH5) YH to the basal diet with protein level of 37% and lipid level of 8%. A total of 1 000 Juvenile M. nipponense with an average weight of (0.104±0.003) g was randomly divided into 5 groups, each with 4 parallels (50 fish/parallel), and the feeding experiment lasted for 8 weeks. The results showed as follows: 1) the survival rate of M. nipponense was not affected by the different YH levels in the diet (P>0.05), but when the YH level in the diet was 1.0%, the weight gain rate and specific growth rate of M. nipponense were significantly higher than those in YH1 group (P < 0.05) and reached the highest. 2) Compared with YH1 and YH5 groups, the content of malondialdehyde (MDA) in hepatopancreas in YH3 and YH4 groups decreased significantly (P < 0.05). The total antioxidant capacity (T-AOC) in hepatopancreas in YH2, YH3 and YH4 groups was significantly higher than that in YH1 and YH5 groups (P < 0.05). The activity of superoxide dismutase (SOD) in YH4 group was significantly higher than that in YH1, YH2 and YH5 groups (P < 0.05). Glutathione peroxidase (GSH-Px) activity in YH5 group was significantly lower than that in other groups (P < 0.05). Nitric oxide (NO) content and nitric oxide synthase (NOS) activity in YH5 group were significantly higher than those in other groups (P < 0.05). 3) At the phylum level, there were 8 dominant bacteria in the intestinal tract, which were Proteobacteria, Firmicutes, Tenericutes, Bacteroidetes, Cyanobacteria, Actinobacteria, Chlamydiae and Planctomycetes. At the genus level, there were 11 dominant bacteria in the intestinal tract, which were Pseudomonas, Shewanella, Aeromonas, Reyranella, Chitinibacter, Acinetobacter, Flavobacterium, Mycobacterium, Rhodobacter, Stenotrophomonas and Legionella. No beneficial bacteria at the genus level increased due to the addition of YH (P < 0.05), but adding a certain amount of YH could inhibit the growth of pathogenic bacteria and potential pathogenic bacteria such as Shewanella, Aeromonas, Acinetobacter and Legionella. It can be seen that adding 1.0% YH to the diet can promote the growth of M. nipponense. When the addition level reaches 1.0% to 2.0%, the antioxidant capacity of prawns can be improved. When the addition level reaches 4.0%, it can significantly change the intestinal flora of prawns and improve the nonspecific immunity of prawns. According to growth performance and antioxidant capacity, the addition amount of YH in the diet is suggested to be 1.0%. According to the immune performance, the amount of YH in the diet is suggested to be 4.0%.
Key words: yeast hydrolyzate    Macrobrachium nipponense    growth performance    antioxidant    immune    intestinal flora    

在目前的水产养殖发展中,集约化养殖因其占地少、工业化、效益高等优势,受到越来越多养殖户的喜爱。但由于在集约化养殖中水产动物的营养需求基本依靠投喂人工饲料来满足,所以水产养殖业对饲料的营养需求也随水产养殖业的发展而提高。如果饲料营养不合理,再加上集约化养殖的高密度胁迫[1],必然会对水产动物的生长发育和非特异性免疫等方面带来不利影响,从而降低水产动物的质量[2]。因此,寻求优质的功能性饲料添加剂对维持水产动物的生长、健康和正常的生理功能十分重要。

酵母水解物(yeast hydrolyzate,YH)是酵母细胞经酸性酶解或其他水解方法得到的产物[3],因其含有的大量的β-葡聚糖、核酸、核苷酸、小肽、游离氨基酸和丰富的B族维生素[4-5],可作为水产动物一种重要的功能性添加剂。研究发现,在水产动物饲料中添加酵母水解物,有利于提高水产动物生长性能,调节机体的免疫力[6-9]。Jin等[6]对南美白对虾(Litopenaeus vannamei)的研究表明,饲料中添加10 g/kg的酵母水解物可以促进虾的生长,提高非特异性免疫性能,增强对氨氮的抵抗力。Mohsen等[7]对尼罗罗非鱼(Oreochromis niloticus)的研究表明,饲料中添加1、2和5 g/kg酵母产品可以提高鱼的生长、蛋白质沉积,增强对嗜水气单胞菌的抵抗力。Zhang等[8]对中华绒螯蟹(Eriocheir sinensis)的研究表明,饲料中添加5 g/kg的酵母水解物可以增加可食内脏指数,提高抗氧化能力和非特异免疫性能。柳茜等[9]对大菱鲆(Scophthalmus maximus)幼鱼的研究表明,饲料中添加2%酵母水解物可提高其机体免疫能力、肝脏抗氧化性能和抗应激能力。

众所周知,肠道不仅是营养物质的吸收场所,而且对机体的免疫系统具有重要作用[10],而稳定的肠道菌群对宿主的健康至关重要[11]。在鱼类的研究中发现,肠道微生物能够帮助鱼类抵抗养殖密度和摄食的变化[12],这一发现为高密度集约化养殖提供了突破点。而含有酵母水解物的饲料pH一般偏低,能够抑制有害微生物的生长,促进动物肠道有益菌群的生长,从而提高饲料的利用率和消化率[10-11, 13]。目前,有关酵母水解物所含成分对肠道菌群的作用已有相关研究。如在对塞内加尔鳎米鱼(Solea senegalensis)的研究中发现,单独使用酵母β-葡聚糖可以调节鱼的免疫性能,控制肠道内弧菌的增殖[13]。但有关酵母水解物本身对水产动物肠道菌群的研究报道较少,因此加强酵母水解物对经济水产动物肠道菌群的作用具有重要意义。

日本沼虾(Macrobrachium nipponense)由于味道鲜美、营养丰富、养殖成本相对较低,深受广大养殖户和消费者的喜爱,是一种重要的经济水产养殖物种。但日本沼虾对养殖环境的应激极为敏感[14-16],通过营养来提高其抗应激能力简单易行[17]。鉴于酵母水解物在提高水产动物生长和调节免疫等方面具有重要的生理功能,本研究拟在日本沼虾基础饲料中添加不同水平的酵母水解物,分析酵母水解物对日本沼虾幼虾生长性能、抗氧化能力、非特异性免疫和肠道菌群的影响。

1 材料与方法 1.1 试验材料及饲料

试验所用的酵母水解物源自浙江某生物科技股份有限公司的啤酒酵母泥,其粗蛋白质、粗脂肪、粗纤维、粗灰分和水分的含量分别为45.80%、2.88%、0.74%、6.60%和4.50%,无氮浸出物含量为39.48%,氨基酸态氮含量为2.10%,酸溶蛋白质含量为32.80%,β-葡聚糖含量为22.6%,甘露聚糖含量为9.87%,游离氨基酸含量总和为12.30%。

以豆粕、菜籽粕和鱼粉为蛋白质源,鱼油和豆油为脂肪源配制基础饲料,其组成及营养水平见表 1。在基础饲料(YH1)之上,分别添加0.5%、1.0%、2.0%和4.0%的酵母水解物配制4种试验饲料,并分别命名为YH2、YH3、YH4和YH5。将表 1中的固态原料粉碎后过60目筛,按配方逐一准确称量(鱼油和豆油最后加入),混合均匀后,使用F-26双螺旋挤条机(华南理工大学科技实业总厂制造)制成直径1.5 mm的颗粒饲料,用烘箱在温度不超过50 ℃条件下烘干,常温冷却,装袋标注后-20 ℃冰箱保存备用。

表 1 基础饲料组成及营养水平(风干基础) Table 1 Composition and nutrient levels of the basal diet (air-dry basis) 
1.2 试验动物和饲养管理

试验中所用日本沼虾购于当地某苗种培育场。在养殖试验之前,对养殖使用的所有器具进行清洗和消毒处理,再在每个白水槽(塑胶材质,300 L)中放置一定量的蓝色水管和网片作为虾的隐藏地,以减少虾蜕壳时遭到攻击和争夺饲料而产生的伤亡。先将虾放置于暂养桶中以基础饲料喂养1周,使其适应试验条件。随后选择健康、大小均匀、初始体重为(0.104±0.003) g的1 000尾日本沼虾幼虾,随机放入20个白水槽中,每组4个平行,每平行(箱)50尾虾。每天换水1次,换水量为水体的1/3;每日分2次(08:00和17:00)定时饱食投喂,以刚好吃完为准;每日16:00观察试验虾状态并统计死虾数目。在自然光周期下,水温保持在25~28 ℃,水体中溶解氧含量>6.5 mg/L、氨和硝酸盐含量<0.1 mg/L,pH 7.6~8.1,静水养殖8周。

1.3 样品采集

在养殖试验结束后,饥饿24 h。捞取每箱全部的虾,分别记录尾数并称重。在无菌条件下,从YH1、YH3和YH5组的每个平行取10只左右的日本沼虾肠道,用于肠道菌群分析。取出5只虾完整的肝胰腺,将肝胰腺与肠道分别装管标记后保存于-80 ℃超低温冰箱,以供后续相关指标的测定及分析。

1.4 检测指标及方法 1.4.1 饲料营养成分的测定

水分含量采用失重法(105 ℃烘干至恒重)测定;粗蛋白质含量采用凯氏定氮法(Kjeltec-8200, FOSS, 丹麦)测定;粗脂肪含量使用索氏抽提法测定,乙醚(化学纯)为抽提液。

1.4.2 生长性能的测定与计算
1.4.3 免疫指标和抗氧化指标的测定

通过测定虾肝胰腺组织中一氧化氮(NO)含量、一氧化氮合酶(NOS)活性,以确定酵母水解物对日本沼虾免疫性能的影响;同时通过测定肝胰腺中丙二醛(MDA)含量、总抗氧化能力(T-AOC)、超氧化物歧化酶(SOD)以及谷胱甘肽过氧化物酶(GSH-Px)活性,以判定酵母水解物对日本沼虾抗氧化能力的影响。免疫指标和抗氧化指标的测定均采用南京建成生物工程研究所的试剂盒经全波长酶标仪(Thermo, Multiskan GO 1510)进行测定。

1.4.4 肠道菌群的测定

采集3组(YH1、YH3和YH5组,每组3个平行)肠道及其内容物,经提取DNA后,送至上海元莘公司,采用Illumina高通量测序和16S核糖体RNA基因的测序结果对日本沼虾的肠道菌群进行分析。具体过程参照郑纯纯等[18]方法进行。测定结果已上传至NCBI,SRA登录号为SRR12400126-12400134。

1.5 数据统计

试验数据用SPSS 25.0软件进行单因素方差分析(one-way ANOVA),若组间差异显著,再用Tukey氏法进行多重比较,以P<0.05为差异显著,结果采用平均值±标准差表示。

2 结果与分析 2.1 酵母水解物对日本沼虾生长性能的影响

表 2可知,成活率不受饲料中酵母水解物添加水平的影响(P>0.05),但呈现出先上升后降低的趋势,在YH4组中成活率达到最高;当饲料中酵母水解物添加水平为1.0%(YH3组)时,日本沼虾的增重率和特定生长率达到最高,且显著高于YH1组(P<0.05)。

表 2 酵母水解物对日本沼虾生长性能的影响 Table 2 Effects of yeast hydrolyzate on growth performance of Macrobrachium nipponense
2.2 酵母水解物对日本沼虾肝胰腺抗氧化指标的影响

表 3可知,相比YH1组,YH3、YH4和YH5组的肝胰腺MDA含量显著降低(P<0.05),并在YH3组达到最低;与MDA含量结果相应,肝胰腺T-AOC结果显示,YH2、YH3和YH4组显著高于YH1和YH5组(P<0.05),在YH3组达到最高;肝胰腺SOD活性随着酵母水解物的添加水平提高呈现出先上升后下降的趋势,YH2、YH3和YH4组的肝胰腺SOD活性也高于其余2组,其中,YH4组显著高于YH1组(P<0.05);YH5组的肝胰腺GSH-Px活性显著低于其余各组(P<0.05)。

表 3 酵母水解物对日本沼虾肝胰腺抗氧化指标的影响 Table 3 Effects of yeast hydrolyzate on antioxidant indexes in hepatopancreas of Macrobrachium nipponense
2.3 酵母水解物对日本沼虾肝胰腺免疫指标的影响

表 4可知,酵母水解物添加水平可影响肝胰腺NO含量和NOS活性,在YH5组显著高于其余各组(P<0.05)。

表 4 酵母水解物对日本沼虾肝胰腺免疫指标的影响 Table 4 Effects of yeast hydrolyzate on immune indexes in hepatopancreas of Macrobrachium nipponense
2.4 酵母水解物对日本沼虾肠道菌群多样性的影响

表 5可知,3个组的高质量序列总数为254 739条,平均每个样本有84 913条序列。每组的覆盖率超过99.9%,表明这些群体中的16S rRNA基因序列代表了本研究样本中存在的大多数细菌。各组间序列数(Reads)、操作分类单元(OTUs)、菌群丰度(Chao指数)和多样性(Simpson指数)都无显著差异(P>0.05)。但Shannon指数组间出现显著差异,YH3组显著低于YH1组(P<0.05),继续增加酵母水解物添加水平,Shannon指数又得到回升。

表 5 酵母水解物对日本沼虾肠道菌群的α多样性的影响 Table 5 Effects of yeast hydrolyzate on α diversity of intestinal flora of Macrobrachium nipponense

图 1中根据unweight-UniFrac距离,用主坐标分析(PCoA)图显示在不同酵母水解物添加水平下日本沼虾肠道菌群组成。从图可知,在主成分3(PC3)水平上各组未因酵母水解物添加水平不同而出现分离;但在主成分1(PC1)水平上添加了酵母水解物的YH3、YH5组与YH1组之间出现了明显分离。

YH1-1、YH1-2和YH1-3表示YH1组;YH3-1、YH3-2和YH3-3表示YH3组;YH5-1、YH5-2和YH5-3表示YH5组。图 3同。 YH1-1, YH1-2 and YH1-3 indicated the YH1 group, YH3-1, YH3-2 and YH3-3 indicated the YH3 group, and YH5-1, YH5-2 and YH5-3 indicated the YH5 group. The same as Fig. 3. 图 1 摄食不同水平酵母水解物后日本沼虾肠道菌群的主坐标分析 Fig. 1 Principal co-ordinates analysis of intestinal bacteria in Macrobrachium nipponense fed different yeast hydrolyzate levels

图 2所示,抽平后3组共留下1 391个OTUs,其中各组日本沼虾肠道菌群共同具有342个OTUs。YH1、YH3、YH5组的OTUs数分别为404、153和209个。

YH1表示YH1组;YH3表示YH3组;YH5表示YH5组。 YH1 indicated the YH1 group, YH3 indicated the YH3 group, and YH5 indicated the YH5 group. 图 2 摄食不同水平酵母水解物后日本沼虾肠道菌群的OTUs分布 Fig. 2 OUTs distribution of intestinal bacteria composition in Macrobrachium nipponense fed different yeast hydrolyzate levels

图 3以门为水平分类,发现了8种优势菌门,即:变形菌门(Proteobacteria)、厚壁菌门(Firmicutes)、软壁菌门(Tenericutes)、拟杆菌门(Bacteroidetes)、蓝藻门(Cyanobacteria)、放线菌门(Actinobacteria)、衣原体门(Chlamydiae)和浮霉菌门(Planctomycetes)。在这3组中,各组这8种优势菌门的相对丰度分别为:YH1组68.20%、3.18%、0.28%、5.05%、9.48%、4.73%、0.71%、1.78%;YH3组66.34%、24.09%、0.18%、4.19%、0.43%、1.78%、1.90%、0.43%;YH5组49.15%、6.60%、21.90%、5.52%、2.12%、4.71%、4.16%、3.31%。

Proteobacteria:变形菌门;Firmicutes:厚壁菌门;Tenericutes:软壁菌门;Bacteroidetes:拟杆菌门;Cyanobacteria: 蓝藻门;Actinobacteria:放线菌门;Chlamydiae:衣原体门;Planctomycetes:浮霉菌门;Bcteria_Unclassified:无法归类的细菌;Others: 其他。 图 3 在门水平下摄食不同水平酵母水解物后日本沼虾肠道微生物分布直方图 Fig. 3 Histogram of intestinal microbial distribution in Macrobrachium nipponense fed different yeast hydrolyzate levels at phylum level

以相对丰度高于1%为标准,对门和属水平下日本沼虾肠道主要菌群进行常规方差统计分析(表 6)。由表可知,在门水平,主要为变形菌门、厚壁菌门、软壁菌门、拟杆菌门、蓝藻门、放线菌门、衣原体门和浮霉菌门,且仅在YH5组中软壁菌门相对丰度显著高于YH1和YH3组;在属水平,主要为假单胞菌属(Pseudomonas)、希瓦氏菌属(Shewanella)、气单胞菌属(Aeromonas)、雷诺氏菌属(Reyranella)、几丁质杆菌属(Chitinibacter)、不动杆菌属(Acinetobacter)、黄杆菌属(Flavobacterium)、分枝杆菌属(Mycobacterium)、红杆菌属(Rhodobacter)、寡养单胞菌属(Stenotrophomonas)和军团菌属(Legionella),其中添加1.0%(YH3组)的酵母水解物可抑制不动杆菌属和希瓦氏菌属的相对丰度(P<0.05);当添加4.0%(YH5组)酵母水解物时,气单胞菌属、不动杆菌属和军团菌属相对丰度显著低于YH1组(P<0.05)。

表 6 门和属水平下摄食不同水平酵母水解物后日本沼虾肠道菌群组成 Table 6 Composition of gut microbiota in Macrobrachium nipponense fed different yeast hydrolyzate levels at genus and phylum levels 
3 讨论 3.1 酵母水解物对日本沼虾生长性能的影响

在本试验条件下,添加了酵母水解物对日本沼虾成活率有所提高,但并未因酵母水解物添加水平不同而与YH1组产生显著差异。但对增重率和特定生长率的分析发现,添加1.0%酵母水解物时,日本沼虾增重率和特定生长率显著高于YH1组。这一结果说明,酵母水解物对日本沼虾生长性能有一定的促进作用。这与在尼罗罗非鱼[6]和凡纳滨对虾[19]中的研究结果相符。饲喂含1%酵母水解物的饲料可显著提高尼罗罗非鱼[6]和凡纳滨对虾[19]的增重率、特定生长率和蛋白质效率,并且具有较低的饲料转化率。然而,研究发现酵母水解物对水产动物提高生长性能的作用在不同物种、甚至同一物种中也有一定的差异。在异育银鲫的研究中发现酵母水解物对其存活率、蛋白质沉积率并没有显著影响,只对特定生长率和饲料系数有显著影响[20-21]。在齐口裂腹鱼(Schizothorax prenanti)中也发现酵母水解物仅降低饲料系数,对该鱼的增重率和特定生长率没有显著影响[22]。曾本和等[23]在加州鲈(Micropterus salmoides)饲料中添加1.3%的酵母水解物可提高该鱼的增重率、特定生长率和蛋白质效率,而Gong等[24]在加州鲈的研究中发现添加1.5%的酵母水解物才可显著提高该鱼的增重率、特定生长率。基于酵母水解物对提高水产动物性能的差异性,发现这极可能与酵母所含成分(如β-葡聚糖、小肽和核苷酸)的含量不同有关。研究发现,酵母核苷酸具有较好的诱食作用,在饲料中添加0.8%酵母核苷酸能显著促进凡纳滨对虾摄食,提高其生长性能[25];在饲料中使用β-1, 3-葡聚糖饲喂大鳞副泥鳅(Paramisgurnus dabryanus)60 d后,泥鳅的各项生长指标均显著高于对照组[26];在饲料中添加适量小肽也可有效促进星斑川鲽(Platichthys stellatus)幼鱼[27]和石斑鱼(Epinephelus coioides)[29]的生长。由此可见,酵母水解物提高水产动物生长性能的差异性不仅与物种、试验条件和试验动物的生长阶段有关,还可能与酵母水解物所含成分含量相关[30-32]

3.2 酵母水解物对日本沼虾抗氧化指标的影响

MDA是脂质过氧化的最终产物,而SOD、GSH-Px等酶可消除机体内氧自由基达到抗氧化作用[33-34],因此,MDA含量、SOD、GSH-Px活性以及T-AOC常被用于反映机体抗氧化能力。在本试验中发现,酵母水解物的添加可使虾体内MDA含量显著下降,同时T-AOC、SOD、GSH-Px活性等抗氧化指标随着酵母水解物添加水平的增加显示出先升高后降低的趋势。在本试验中,添加4.0%酵母水解物会显著降低虾肝胰腺GSH-Px活性。该结果说明,添加一定水平的酵母水解物对日本沼虾有增强抗氧化能力的作用。这一结果与在其他水产动物中的研究结果类似。在加州鲈饲料中添加1.5%和3.0%酵母水解物可显著提高其抗氧化能力[24];在尼罗罗非鱼饲料中添加1%~3%的酵母水解物能显著提高其血清抗氧化酶活性[6];在齐口裂腹鱼[22]、青鱼(Mylopharyngodon piceus)[35]和草鱼(Ctenopharyngodon idellus)[36]中也发现酵母水解物可提高鱼体的抗氧化能力;在异育银鲫[20-21]中还发现添加酵母水解物产品可修护氧化应激所产生的损伤。此外,有研究发现酵母水解物的抗氧化性能与其所含成分(如酵母β-葡聚糖和小肽物质)的抗氧化作用密切相关[37]。饲料中添加0.75%~1.50%小肽可显著提高星斑川鲽幼鱼的抗氧化能力[27];添加400~800 mg/kg的β-葡聚糖能显著降低花鲈(Lateolabrax japonicas)体内的MDA含量,起到抗氧化作用[38];与花鲈研究结果类似,在凡纳滨对虾的研究中发现添加0.02%和0.04% β-葡聚糖时可显著提升虾的抗氧化能力,缓解低盐应激反应[39]。因此,在本试验中酵母水解物的添加提高了日本沼虾的抗氧化性能不排除与产品中的各成分含量有关。

3.3 酵母水解物对日本沼虾免疫性能的影响

研究表明,NO可非特异性的抑制或杀灭细菌、真菌、病毒和寄生虫等对生物体免疫功能产生影响[40-41],是机体内激活巨噬细胞发挥作用的一种有效分子[42]。在沙门氏菌影响大菱鲆免疫结果中发现,NO可消除细菌病原体[43]。又因NO在体内的生物合成由NOS催化,NOS比NO稳定性高,通常用NOS活性反映NO含量[40],故本试验采用NO和NOS作为免疫指标。试验结果显示,在本试验酵母水解物的添加水平下,NOS活性提高,且在添加水平为4.0%(YH5)时,肝胰腺NO含量和NOS活性都显著高于YH1组,说明酵母水解物可提高日本沼虾的非特异性免疫性能。此结果与在加州鲈[23]、团头鲂(Megalobrama amblycephala)[43]、大菱鲆[9, 44]和尼罗罗非鱼[7, 45]等水产动物中的研究结果类似,但最佳免疫性能要求的酵母水解物的最适添加水平各有不同。例如,在加州鲈体内试验中,发现添加1.5%和3.0%的酵母水解物可显著提高其免疫相关酶活性,增强免疫反应[6];在体外培养团头鲂肝细胞,发现添加酵母水解物可明显提高其免疫酶活性和白蛋白含量,同时免疫相关基因的表达通过补充酵母水解物也显著上调[43];在尼罗罗非鱼的研究中,Andriamialinirina等[45]发现添加1%酵母水解物能显著提高鱼体免疫相关基因的表达量,从而提高其免疫性能。这一差异极可能与酵母水解物中β-葡聚糖含量的高低有关[46-48]。研究表明,酵母水解物中的β-葡聚糖具有改善生长性能、抗氧化、抗感染、促进消化酶分泌等作用,常在水产养殖中作免疫增强剂使用[49],所以酵母水解物增强免疫的高低应与其含β-葡聚糖含量的高低密切相关。异育银鲫摄食酵母β-葡聚糖发现,β-葡聚糖不仅具有一定的促进生长和改善肝功能的作用,而且可以增强鱼体抵抗嗜水气单胞菌人工感染的能力[46]。β-1, 3/1, 6-葡聚糖可刺激湖拟鲤(Rutilus rutilus lacustris)的非特异性抵制机制,从而改善其吞噬细胞的胞饮活性,增加其肠道免疫细胞的数量[47]。甚至在育种中发现β-葡聚糖还可促进后代的非特异性免疫[48]

3.4 酵母水解物对日本沼虾肠道菌群的影响

研究发现,水产动物肠道微生物对机体的营养消化吸收、机体免疫力及抵制疾病等方面有重要作用[50]。本试验使用16S rRNA Illumina高通量测序数据来揭示饲料中酵母水解物对日本沼虾肠道菌群的作用,结果显示,在门水平中,虾肠道中的优势菌门共8种,在优势菌群中变形菌门占绝对优势,YH5组的软壁菌门相对丰度显著高于YH3和YH1组。这与在尼罗罗非鱼、大西洋鲑鱼(Salmo salar)等鱼类的研究结果[51-54]类似。有研究指出,变形菌门和软壁菌门可作为鱼体健康指标[51],增加变形菌数量可以改善大西洋鲑鱼的健康状况[54];软壁菌门对鱼体生长和抑制致病细菌有积极作用[53-54]。在甲壳动物研究上,还未明确指出变形菌门和软壁菌门的作用,但在对克氏原螯虾(Procambarus clarkii)的研究中发现变形菌门和软壁菌门是健康虾肠道菌群中的优势菌群[55]。在属水平上,优势菌属共11种,其中益生菌为几丁质杆菌属[56];有害菌和潜在有害菌为假单胞菌属[57]、气单胞菌属[58]、希瓦氏菌属[59]、黄杆菌属[57]、分枝杆菌属[60]、不动杆菌属[57]和军团菌属[61]。本试验中虽未检测到属水平上的有益菌相对丰度因酵母水解物的添加水平增加而增多,但在希瓦氏菌属、气单胞菌属、不动杆菌属、军团菌属等致病菌和潜在致病菌中发现,添加一定量的酵母水解物可有效抑制其生长。由此可知,使用酵母水解物可抑制日本沼虾肠道中部分有害菌和潜在有害菌的生长。这一结果与在大口鲈鱼[62]中的研究结果类似,但其在优势菌群种类和促进部分有益菌群生长方面与本研究结果存在差异。这可能与肠道微生物本身易受环境、饲料、动物自身生长发育等因素影响所致[57, 63-65],特别是摄食对水产动物肠道微生物的影响巨大[64-66]

此外,研究发现肠道菌群可反映动物的免疫性能[50, 67],并且证明了酵母水解物中所具有的增强免疫性能的物质(如甘露寡糖、β-葡聚糖、小肽和核苷酸等)都具有促进动物体肠道发育、平衡肠道微生物菌群、加强有益菌的增殖和促进肠道自我修复的功能[67-69]。例如,摄食酵母水解物有助于大口黑鲈肠道中一些益生菌的生长并抑制有害菌的增殖,从而减少潜在的病原体[62];而使用酵母β-葡聚糖也可以调节塞内加尔鳎(Solea senegalensis)的免疫性能,控制其肠道内弧菌的增殖[13];另外,在饲料中添加200~400 mg/kg甘露寡糖就可促进泥鳅(Misgurnus anguillicaudatus)肠道发育,优化肠道微生态环境[70]。在本试验中也发现添加4.0%酵母水解物时,日本沼虾的免疫性能最强,抑制致病菌的种类最多。因此可推测,酵母水解物对肠道菌群的改善作用与这些免疫增强物质密切相关。

4 结论

综上所述,酵母水解物能显著提高日本沼虾的生长性能、抗氧化能力和免疫性能;酵母水解物对日本沼虾肠道菌中希瓦氏菌属、气单胞菌属、不动杆菌属和军团菌属等致病菌和潜在致病菌有抑制作用。根据生长性能和抗氧化能力,建议日本沼虾幼虾饲料中酵母水解物添加水平为1.0%;根据免疫性能分析,建议日本沼虾幼虾饲料中酵母水解物添加水平为4.0%。

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