消化道是动物体内最复杂的微生物“库”。消化道微生物不仅能辅助动物体消化利用饲料,还在消化道形态结构和免疫系统发育过程中扮演重要角色。微生物通过刺激杯状细胞分泌黏蛋白[1-2]和活化树突状细胞[3],调节免疫细胞分化[4-6],实现其对宿主免疫系统的调节;通过介导缺氧[7],激活Janus激酶-信号转导和转录激活因子(JAK-STAT)和氨基末端激酶(JNK)途径[8]或胰岛素/胰岛素样生长因子信号途径[9],调节肠道干细胞分化,实现其对宿主消化道发育的调节。因此消化道微生物群落的平衡与多样性意味着消化道的健康,保障了动物生产性能的最大化。但消化道微生物群落多样性的形成和维持是一个非常复杂的过程。它不仅取决于消化道结构类型,而且受宿主基因型、饲养管理和环境等多种因素的影响。反刍动物因其复胃结构及饲粮组成的特殊性,其消化道微生物演替有别于单胃动物,且断奶前后幼龄反刍动物的反刍功能逐渐形成,此阶段瘤胃及其微生物的发育状态直接影响成年后的生产性能。为此,本文综述了幼龄反刍动物消化道发育的特征,消化道微生物演替与消化道发育的关联及其调控方式,以期为反刍动物健康高效养殖提供新的有力抓手。
1 幼龄反刍动物消化道发育的特征新生反刍动物瘤胃、网胃和瓣胃尚未发育,体积小且壁薄,此时的反刍动物与单胃动物类似,以皱胃的酶消化为主。随着机体营养物质来源逐渐从液体饲料(乳或者代乳粉)转变为固体饲料(开食料和饲草),消化道的形态也随之发生一系列变化。瘤胃和网胃快速发育、重量占比增加,后肠重量占比降低,微绒毛变短,为营养物质吸收利用方式的转变做好形态学的准备[10]。消化道中乳糖酶活性降低,而其他水解酶随着年龄的增长在瘤胃内活性逐渐升高至相对稳定的水平[11]。瘤胃微生物功能预测结果显示,果糖和甘露糖代谢、糖酵解、氨基糖和核糖代谢的基因丰度都随着年龄的增加而逐渐增长[12]。宏基因组结果显示,犊牛瘤胃中饲粮多糖降解相关的SusD、TonB细胞膜外蛋白等基因的丰度在42日龄高于14日龄[13]。酶系的变化使饲粮的消化从后肠转移至瘤胃,饲粮在瘤胃内降解产生挥发性脂肪酸,且随着开食料采食量的增加,瘤胃中乙酸比例逐渐下降,丙酸比例逐渐提升,乙丙比渐渐降低至成年动物水平[14]。饲粮消化主体的转移,使门静脉血中的葡萄糖含量逐渐减少,乳酸和丙酸等糖异生前体物质含量逐渐增加[15]。肝脏中的单羧酸转运蛋白1(MCT1)主要参与C2~C5的单羧酸分子的转运,成年牛肝脏中MCT1基因的表达量是反刍前犊牛的8~9倍[16]。瘤胃中丙酸等生酮物质产量的增加标志着瘤胃发育的成熟,常通过血浆中β-羟丁酸含量间接指征瘤胃发育的状态。
2 幼龄反刍动物消化道微生物演替与消化道发育的关联反刍动物消化道微生物演替贯穿动物的整个生长过程,但以幼龄期变化最为剧烈。不同功能瘤胃微生物变化以形成瘤胃特有的微生态,成就瘤胃发育及其强大的微生物发酵功能。
2.1 生态位抢占和修饰主导了瘤胃微生物演替反刍动物出生后的数分钟,每克瘤胃内容物中就可以检测到高达1.9×108 16S rRNA基因拷贝数的活细菌[17]。这些微生物来源于母畜阴道、口腔、皮肤、粪便、初乳及饲养环境[18]。然而这些以好氧菌和兼性厌氧菌为主的微生物类群通过消耗瘤胃多余氧气,完成瘤胃生态环境修饰后,逐渐为执行饲粮中蛋白质和碳水化合物降解的功能性厌氧微生物所取代[19]。这些功能性厌氧微生物在瘤胃内的定植先后顺序及丰富度取决于相同功能微生物对同一可利用底物的竞争,即瘤胃“生态位抢占”,也取决于先定植微生物对瘤胃环境的改变,即先定植的微生物通过“生态位修饰”为后续微生物创造适宜定植的环境条件。生态位是指生态群落中某一物种的时空特异性,及其与其他物种间的相互关系[20]。瘤胃微生物按其代谢产物可分为产乙酸、产丙酸、产丁酸、产乳酸/产延胡索酸/产琥珀酸、产甲烷和产氨微生物6个功能类群(表 1)。新生反刍动物瘤胃内产乙酸菌占主导生态位,而随着年龄增长,产甲烷菌取代了产乙酸菌成为瘤胃中主导的氢利用菌,这是因为新生反刍动物瘤胃内氢分压较高,满足产乙酸菌利用H2和CO2生成乙酸的热力学要求。产乙酸菌能与其他氢利用微生物有效竞争,抑制其他氢利用微生物类群生长[30-31]。而随着瘤胃内微生物的增长,瘤胃内氢分压逐渐降低,不再满足产乙酸菌利用氢的条件,从而被氢分压要求不高的产甲烷菌取代,使氢更多用于生成甲烷[32]。产乙酸、产丁酸、产乳酸/产琥珀酸/产延胡索酸的微生物共同利用糖酵解生成的丙酮酸,存在生态位抢占关系,处于优势地位的微生物类群主导瘤胃发酵模式。而不同的发酵模式(乙酸型/丙酸型)下瘤胃内氢分压不同,影响产甲烷菌的数量,优势地位菌群与产甲烷菌间构成生态位修饰关系。六大瘤胃微生物功能类群的生态位抢占和修饰关系(表 1)主导了瘤胃微生态的“演变”和“成熟”。
消化道微生物不仅能辅助动物体消化利用饲粮,其代谢产生的短链脂肪酸、维生素、氨基酸衍生物、胆汁酸等产物还能调节消化道的发育和功能。碳水化合物在消化道中迅速发酵产生短链脂肪酸。短链脂肪酸是消化道上皮细胞的主要供能物质,并作为活性信号因子调控消化道上皮的增殖和分化。其中以丁酸的作用效果最优,丙酸次之。在瘤胃中,80%的丁酸转化为酮体,为瘤胃上皮的生长提供80%以上的能量[33];在肠道中,丁酸通过β-氧化为肠上皮细胞提供70%的能量[34]。另外,短链脂肪酸是组蛋白去乙酰化酶(HDACs)的抑制剂和G蛋白偶联受体(GPCRs)的配体,因此能够通过激活基因转录和GPCRs信号途径,调节丝裂原活化蛋白激酶(MAPK)、鞘脂类、胰岛素、催产素、WNT、钙离子等信号通路,调控细胞增殖和凋亡[35-36]。在培养的瘤胃上皮细胞中,丁酸增加细胞周期相关基因表达、减少细胞凋亡相关基因的表达,从而调控瘤胃上皮细胞的增殖[37];在结肠上皮细胞中,丁酸诱导组蛋白H4的高度乙酰化,减少细胞周期蛋白(cyclin) D1转录水平的表达,增加cyclin D3和p21蛋白的表达,使细胞周期停滞在G1期,进而抑制细胞增殖,促进细胞分化[38]。丁酸还能通过调控内分泌系统,促进胰岛素样生长因子-1(IGF-1)的分泌,通过上调IGF-1受体的表达,加速细胞有丝分裂,进一步促进上皮细胞增殖[39-40]。
3 幼龄反刍动物消化道微生物定植与消化道发育的调控宿主基因型、饲粮结构、饲养管理策略等均会影响幼龄反刍动物消化道微生物的定植。近年来研究者通过营养与微生物直接干预的方式调控了反刍动物消化道微生物的定植过程,提高了动物的生产性能,并有一定的长期影响。
3.1 微生物介导营养素调控消化道发育饲粮作为微生物发酵的底物,其物理性质和营养组成直接影响微生物的定植过程。液体饲粮饲喂的犊牛瘤胃中乳杆菌(Lactobacillus)、拟杆菌(Bacteroides)和Parabacteroides等菌属的丰度较高,这些微生物能够利用乳中的营养成分。补饲固体饲粮后瘤胃中与淀粉降解相关的巨型球菌(Megasphaera)、沙棘菌(Sharpea)和琥珀酸弧菌(Succinivribrio)等菌属的丰度增加[41],普雷沃氏菌属(Prevotella)等成年瘤胃中的主要微生物快速增长[42],纤维杆菌(Fibrobacter)、氨基酸球菌(Acidaminococcus)、琥珀酸菌(Succiniclasticum)等菌属的丰度显著增加[43];结肠上皮黏附微生物中S24-7、颤杆菌(Oscillibacter)、Prevotella、Parabacteroides、双歧杆菌(Bifidobacterium)、反刍杆菌(Ruminobacter)和Succinivibrio菌属的丰度显著升高,疣微菌科(Ruminococcaceae)、RC9 gut group、Blautia、Phocaeicola、考拉杆菌属(Phascolarctobacterium)、BS11 gut group、Family ⅩⅢ和弯曲杆菌属(Campylobacter)丰度显著下降[44]。开食料中添加亚麻籽油,能够促进瘤胃中琥珀酸弧菌科(Succinivibrionaceae)和韦荣氏菌科(Veillonellaceae)的定植,该作用效果在停喂亚麻籽油后仍得以保持[45];混合添加大蒜油和亚麻籽油促进普雷沃氏菌科(Prevotellaceae)的定植,降低毛螺菌科(Lachnospiraceae)、Fibrobacteriaceae和Bacteroidales RF16 group的相对丰度[46];添加亮氨酸可以提高瘤胃中Prevotella的相对丰度,降低梭菌属(Clostridium)和Megasphaera的相对丰度[47]。
Zhang等[48]研究发现,断奶前饲喂高水平的代乳粉能降低断奶时的炎症反应,但代乳粉饲喂水平对瘤胃微生物区系几乎没有影响。Lin等[28]基于瘤胃微生物宏基因组及瘤胃上皮转录组分析揭示,开食料提高瘤胃中乙酸产生菌(光岗菌)、乳酸产生菌、乳酸利用菌(Megasphaera)和内毛虫等的丰度,增加糖苷水解酶家族13的丰度,它们的代谢产物可能通过上调瘤胃上皮细胞生长调节因子丝裂原活化蛋白激酶1(MAPK1)、PI3激酶β亚基(PIK3CB)、肿瘤坏死因子超家族10(TNFSF10)、整合素6α(ITGA6)、锌指蛋白(SNAI2)、salvador家族蛋白(SAV1)、膜相关鸟苷酸激酶大支架蛋白(DLG)和下调促细胞死亡因子(BAD)转录水平的表达,影响瘤胃的形态和发育。本课题组以羔羊为研究对象,研究发现补饲苜蓿在断奶前增加瘤胃中unclassified Lachnospiraceae、密螺旋体属(Treponema)和unclassified Ruminococcaceae的相对丰度;上调瘤胃组织中小窝蛋白3(CAV3)、钙离子电压门控通道蛋白α亚基1D(CACNA1D)、SH3和半胱氨酸丰富域(STAC)、缝隙连接蛋白1(GJA1)、钾离子电压门控通道互作蛋白2(KCNIP2)、RAR相关孤儿受体A(RORA)、PPARG共激活剂1α(PPARGC1A)、一氧化氮合酶2(NOS2)、蛋白激酶Cβ(PRKCB)转录水平的表达量,下调高尔基相关分泌途径激酶(FAM20C)转录水平的表达量,加速瘤胃肌肉细胞的钙内流,促进瘤胃组织发育;微生物的变化与瘤胃发育间存在显著相关关系,补饲苜蓿使羔羊的瘤胃及其微生物在断奶前就能更好地适应断奶后饲粮的转换,减缓断奶应激[49-50]。
3.2 外源微生物通过改变原生瘤胃微生物群落组成改善动物生长性能微生态制剂是能够促进动物消化道微生态平衡,调节动物机体健康的活性微生物[51]。适用于反刍动物的微生态制剂可以是瘤胃源的微生物,也可以是非瘤胃源的微生物。反刍动物断奶前后,通过直接饲喂含有活菌的微生态制剂,能够改变原生瘤胃微生物在瘤胃中的定植过程,发挥促生长或缓解腹泻的作用(表 2)。
粪菌移植(FMT)是指将健康供体的粪便微生物移植到患病受体的消化道内重塑受体肠道菌群结构来治疗特定疾病的方法。粪菌是微生态制剂的一类特殊形式。理论上,相比于外源微生物,来自于瘤胃中的微生物更适合在瘤胃中定植。因此与粪菌移植相似,瘤胃微生物移植(RMT)被认为是一种重塑瘤胃菌群结构,提高反刍动物生产性能的方法。Belanche等[70]给出生后的山羊连续灌注成年羊的新鲜瘤胃液11周,促进了原虫在瘤胃中的定植,提高了粗饲料的采食量、瘤胃中氨态氮、挥发性脂肪酸和血浆中β-羟丁酸含量,并且瘤胃液取自高粗料饲喂的成年羊的效果要优于高精料饲喂的成年羊。给母羊饲喂椰子油或过瘤胃保护脂肪后,将其瘤胃液移植给羔羊,可提高羔羊瘤胃中Veillonellaceae和Prevotellacea的丰度[71]。本课题组给成年羊饲喂羔羊的开食料后将其瘤胃微生物冻干粉饲喂给羔羊,改善了羔羊全肠道消化性,提高了瘤胃淀粉酶活性,增加了瘤胃中丙酸的供应量[72]。并且研究发现断奶前移植更有助于瘤胃微生物“成熟”[73]。但Bu等[74]通过断奶前连续灌注瘤胃液的方式给犊牛移植成年奶牛瘤胃微生物,却未改变断奶27 d后犊牛的瘤胃细菌结构,移植没有改善采食量、日增重和料重比,仅有效降低了腹泻率。
4 小结调控幼龄反刍动物消化道微生物定植的文章相对较多,除营养和微生物干预外,也有研究者考虑母子一体化策略,但这些变化的微生物与消化道发育调节之间的关联仍然停留在相关分析层面。瘤胃微生物通过何种途径实现复胃及肠道发育的调节仍需细胞、组织层面直接的因果研究。生态位理论能够帮助研究者更好地解读瘤胃内微生物的整体功能和演替规律。因此针对幼龄反刍动物的靶向微生物调控破坏原有的生态位抢占状态或针对性添加修饰瘤胃环境的微生物制剂等方法能够重新定向瘤胃微生物的演替过程,实现瘤胃的高效调控。
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