动物营养学报    2021, Vol. 33 Issue (5): 2439-2451    PDF    
植物精油对畜禽肠道健康、免疫调节和肉品质的研究进展
张嘉琦1,2 , 张会艳1,2 , 赵青余1,2 , 张军民1,2     
1. 中国农业科学院北京畜牧兽医研究所, 动物营养学国家重点实验室, 北京 100193;
2. 中国农业科学院北京畜牧兽医研究所, 农业农村部华北动物遗传资源与营养科学观测实验站, 北京 100193
摘要: 近年来抗生素在畜禽养殖业中的滥用加重了病原菌的耐药性,增加了抗生素在农副产品中残留以及环境污染等问题。植物精油(PEO)是从植物中提取纯化的天然挥发性芳香物质,已经有大量研究证明其具有良好的抗菌、抗寄生虫、抗氧化和增强免疫的作用,并且毒性小、在动物体内几乎无残留。因此,使用PEO作为抗生素替代品引起了人们的关注。本文归纳了多种PEO的主要成分,总结了PEO对畜禽肠道健康、免疫调节和肉品质的影响及其作用机理,为PEO在畜牧业中的应用提供参考。
关键词: 植物精油    肠道健康    免疫调节    肉品质    应用前景    
Research Progress of Plant Essential Oil on Intestinal Health, Immune Regulation and Meat Quality of Livestock and Poultry
ZHANG Jiaqi1,2 , ZHANG Huiyan1,2 , ZHAO Qingyu1,2 , ZHANG Junmin1,2     
1. State Key Laboratory of Animal Nutrition, Institute of Animal Sciences of Chinese Academy of Agricultural Sciences, Beijing 100193, China;
2. Scientific Observing and Experiment Station of Animal Genetic Resources and Nutrition in North China of Ministry of Agriculture and Rural Affairs, Institute of Animal Sciences of Chinese Academy of Agricultural Sciences, Beijing 100193, China
Abstract: In recent years, the abuse of antibiotics in livestock and poultry farming industry has aggravated the resistance of pathogens, the residue of antibiotics in agricultural and by-products, and environmental pollution. Plant essential oil (PEO) is a naturally volatile aromatic substance extracted from plants and has been shown to have good antibacterial, anti-parasite, antioxidant and immune-enhancing properties, as well as low toxicity and virtually no residue in animals, so the use of PEO as an antibiotic substitute has attracted attention. This paper summarized the main components of PEO, the effects of PEO on intestinal health, immune system and meat quality of livestock and poultry as well as the mechanism of action. It provided a theoretical basis for the application of PEO in animal husbandry.
Key words: plant essential oil    intestinal health    immune function    meat quality    application prospect    

中国是世界上最大的抗生素生产和使用国,同时也是抗生素滥用和细菌耐药性的重灾区。在畜禽生产中,抗生素可以改善动物生长性能、保障动物健康、减少疾病的发生和降低饲养成本。然而,抗生素的滥用导致大量抗生素被排放到了水土环境中,致使环境中细菌耐药性的日益严重。环境中的抗生素及耐药细菌一旦进入人体的肠道,将会破坏人体肠道的正常菌群稳态,对人体的肠道健康造成了严重的威胁;而残存于环境中的污染物,则加剧了细菌耐药对人体健康的影响。因此,欧盟、日本、韩国等国家或组织分别于2006年、2008年和2011年禁止在饲料中使用抗生素作为促生长类药物饲料添加剂;我国农业农村部于2019年发布的194号公告称“自2020年7月1日起,饲料生产企业停止生产含有促生长类药物饲料添加剂(中药类除外)的商品饲料”[1]。因此,寻找新的更加安全、有效的抗生素替代品成为了畜牧业当前的研究热点。

植物精油(PEO)是从植物的各个部分,包括种子、根、茎、叶和果实[2-3],通过蒸汽蒸馏或溶剂萃取等方法获得的挥发性芳香物质[4-5]。目前,PEO广泛应用在香料、化妆品、肥皂和药品中。同时,许多草本植物和香料精油也是受欢迎的食品调味品,如从大蒜、辣椒、肉桂和小茴香中提取的PEO,具有增强食品风味、激发人们的进食欲望的作用。实际上,PEO也是良好的抗生素替代品[6],因为PEO及其成分具有良好的抗菌[7-8]、降血脂[9]、抗氧化[10]、抗病毒[11-12]、抗虫[13-14]和抗炎作用[15],这些特性与抗生素在畜禽养殖业的作用相似。一些研究人员发现,在畜禽生产中,许多草本植物和PEO能够有效抑制有害病原微生物的生长繁殖、保护动物呼吸道及肠道健康、促进动物生长和减少应激[16-17]。PEO与抗生素相比,具有安全性高、副作用小、成本低廉且资源丰富的优势[18]。因此,使用PEO作为新的抗生素替代品引起了人们的关注[19]。本文阐述了PEO对畜禽肠道健康、免疫调节和肉品质的作用,并提出了PEO未来的研究方向。

1 PEO的生物活性成分

目前用于提取PEO的植物主要包括牛至、薰衣草、生姜、薄荷、丁香、肉桂、小茴香、迷迭香、鼠尾草、猫爪草和桉树等,提取方法主要包括蒸馏提取法、溶剂萃取法、过饱和法、树脂萃取法、蜡包埋法和冷压法[20]等。PEO主要成分通常利用色谱分离,借助质谱进行鉴定,其组成成分主要包括萜烯类化合物(大部分为单萜和倍半萜)、芳香族化合物(主要为萜源衍生物和苯丙烷类衍生物)、脂肪族化合物(主要为烯类、烷烃类和醇类)和含氮含硫化合物(具有刺激性气味,含量较少)。PEO组成成分中的大多数物质具有芳香气味,其中萜类化合物是中草药中的有效成分,具有多种药理活性,如祛痰、止咳、镇痛、消炎等[16-17],同时也是一类重要的天然香料,在畜禽养殖中发挥着替代抗生素促生长的作用。PEO的各种成分含量受多种因素的影响,包括品种、环境条件、栽培方法、采摘季节时间、储存、加工工艺、提取方法和条件等[21-23]。几种常见PEO的主要成分可见表 1

表 1 常见PEO及其主要成分 Table 1 Common PEO and main components
2 PEO对肠道健康的调控方式

肠道菌群的基因数量约是宿主基因的100倍,在宿主的整个生长阶段发挥着至关重要的作用,因此有报道将其描述为“被遗忘的器官”[37]。肠道菌群的类型和数量始终处于动态变化中,饮食是影响肠道菌群组成和活性最重要的因素之一[38]。PEO中含有大量的酚、萜烯、醛和酮类物质,具有良好的抗菌活性,能够抑制革兰氏阴性菌、阳性菌和部分真菌的生长,尤其对革兰氏阳性菌的抑制效果更佳[39-41],而PEO的选择性抗菌会调节肠道菌群的平衡,改变肠道菌群多样性和相对丰度。目前有关PEO的抗菌机理尚不明确,不同类型的PEO可能通过不同途径来抑制微生物的生长。大蒜精油能够降低金黄色葡萄球菌生物膜的厚度,进而破坏细菌生物膜完整性,并导致细胞膜通透性增加,使细胞中ATP浓度和pH降低,细胞成分流失,最终导致细菌死亡,从而发挥抗菌作用[42]。而茶树精油则通过干扰遗传信息处理相关的基因表达,包括DNA复制、转录和修复,破坏这些基本功能,最终抑制细菌生长[43]

PEO可通过介导肠道菌群的平衡进而调节肠道菌群的代谢物,而肠道菌群的代谢物与肠道健康之间有着密切的联系[44-45]。肠道菌群能将未消化的食物转化为功能性代谢产物,如短链脂肪酸和吲哚类物质,帮助宿主消化吸收;或者产生有害代谢产物,如细菌外毒素,抑制肠道上皮细胞的增殖,影响宿主健康[46-47]。短链脂肪酸是主要的细菌代谢产物之一,可以作为能量直接被肠道上皮细胞利用,并改善肠道免疫功能[48],动物流行病学研究显示,细菌代谢产物短链脂肪酸能够减轻各种顽固性疾病症状,例如自闭症、溃疡性结肠炎和克罗恩病[49-51],尤其是丁酸脂,具有免疫调节、诱导细胞凋亡和抗癌等作用。从肉鸡盲肠中提取微生物并进行了培养,发现牛至精油能够增加粪肠球菌和乳酸杆菌的比例,并减少链球菌属相对丰度,菌群的改变增加了有益短链脂肪酸(尤其是乙酸和丁酸)的含量,进而改善肠道健康[52]。PEO对动物肠道微生物及代谢产物的影响见表 2

表 2 PEO对动物肠道微生物及代谢产物的影响 Table 2 Effects of PEO on intestinal microflora and metabolites in animals

PEO影响肠道黏膜免疫反应、上皮屏障功能、氧化应激和炎症反应,这可能是直接或间接地通过肠道菌群代谢物的差异表达来保护肠道屏障[61-62]。广藿香精油的添加会明显增加小鼠肠道中产短链脂肪酸细菌的丰度,如丁酸厌氧菌(Anaerostipes butyraticus)和乳杆菌(Lactobacillus lactis)等,明显降低病原菌丰度,如萨特菌属(Sutterlla spp.)、死亡梭杆菌(Fusobacterium mortiferum)和幽门螺杆菌(Helicobacter spp.)。肠道上皮细胞中短链脂肪酸的关键受体G蛋白受体(GPR)41、GPR 43和GPR 109a被显著刺激,进一步促使肠道上皮中杯状细胞黏蛋白2(Muc 2)基因表达,并且促进潘氏细胞分泌溶菌酶和防御素,使E-钙黏蛋白/N-钙黏蛋白的比例升高(在溃疡性结肠炎和克罗恩疾病的患者中通常会检测到E-钙黏蛋白的下调和N-钙黏蛋白的上调)[63-64]。此外,紧密连接蛋白-1(ZO-1)和闭合蛋白(occludin)的表达量增加;参与炎症反应的Ⅰ型血管细胞黏附蛋白(VCAM-1)和细胞间黏附因子-1(ICAM-1)的表达均显著下调,VCAM-1和ICAM-1主要在内皮细胞中表达,并负责聚集白细胞以激活炎症反应,这可能进一步刺激巨噬细胞由M1型转变为M2型,进而减少诱导型一氧化氮合酶(iNOS)的活性缓解机体炎症水平,提高机体的抗炎能力[57]。在仔猪肠道组织的研究中,发现PEO能够改善肠道形态(绒毛高度升高,绒毛高度和隐窝深度的比值升高),提高蔗糖酶和乳糖酶的活性,促进与营养物质运输相关的关键蛋白的表达,如葡萄糖转运蛋白2(GLUT2)和钠-葡萄糖协同转运蛋白1(SGLT1),以及紧密连接蛋白(occludin)的表达[65-66]。这些结果表明,PEO可以通过调节紧密连接蛋白的表达,提高消化酶的活性,增强肠道屏障,并改善肠道形态(图 1)。

图 1 PEO对肠道健康和免疫的影响 Fig. 1 Effects of PEO on gut health and immunity[46-47, 53, 57, 74]
3 PEO在动物机体中的免疫调节作用

炎症是机体对不同类型有害因子的复杂免疫过程,包括病原微生物、刺激性化合物和有害物质等,它们会诱导急性炎症反应,反应一般持续时间较短,这对宿主健康是有益的。但是如果炎症持续存在或消退不充分,易引发宿主多种疾病,例如癌症、心血管、神经系统和免疫系统疾病等[67]。动物机体的免疫通常处于动态平衡,当动物机体的免疫系统遭受攻击时,如肠道菌群中过量的病原菌分泌的内毒素(LPS)或应激情况下导致的活性氧(ROS)失衡,会加速抑制蛋白(IκB)与核因子-κB(NF-κB)的解离,刺激NF-κB转移至细胞核内,转录因子NF-κB诱导大量的促炎蛋白表达,包括参与机体炎症、细胞凋亡和线粒体凋亡的细胞因子、趋化因子和酶等。同时,细菌LPS和应激均会传递信号给具有氧化应激感应能力的Kelch样环氧氯丙烷相关蛋白1(Keap1)蛋白,接收信号的Keap1蛋白将与核因子E2相关因子(Nrf2)解离,并通过活化其下游基因产生抗氧化作用,有助于细胞清除过量的ROS,抵抗外来有毒物质(图 2)。

图 2 PEO对免疫的影响 Fig. 2 Effects of PEO on immune[68-69]

近年来,由于抗生素的使用受到了限制,PEO作为抗生素替代品已成为人们关注的一种可能的天然添加剂。PEO可促进动物对营养物质的消化吸收,减少肠道中病原菌的数量,调节免疫系统[70-71]。同时PEO主要成分中的萜烯类物质具有免疫调节特性,可在细胞和分子水平上对免疫系统的各个部分产生影响,包括在细胞免疫中调节T细胞和其他免疫效应细胞活性、在体液免疫中促进抗炎细胞因子和抗体的产生等[72]。当Aβ淀粉样蛋白处理PC12细胞时,桉树油主要功能性成分1, 8-桉叶素可以通过减缓NF-κB通路的激活,减少促炎细胞因子白细胞介素-6(IL-6)和白细胞介素-1β(IL-1β)的分泌,并抑制促炎通路相关的酶如环氧合酶-2(COX-2)和一氧化氮合酶(NOS)的转录表达,导致一氧化氮(NO)信号强度减弱,进而缓解线粒体损伤和细胞炎症水平[68]。大肠杆菌攻毒肉鸭后补充PEO,能够缓解机体炎症反应,降低血清总蛋白和球蛋白含量,减少机体器官损伤以及通过减轻大肠杆菌对空肠Toll样受体4(TLR4)的刺激而减少肿瘤坏死因子-α(TNF-α)、炎症细胞因子IL-1β和IL-6的分泌,增加免疫球蛋白G(IgG)和免疫球蛋白M(IgM)的含量,提高机体免疫功能[69]。PEO对动物免疫系统的影响见表 3

表 3 PEO对动物免疫系统的影响 Table 3 Effects of PEO on animal immune system
4 PEO对肉品质的影响

PEO的某些挥发性化合物成分具有天然抗氧化剂的潜力[79],由于PEO比合成抗氧化剂更容易被人们接受,因此使用PEO来改善肉品质成为了新的研究热点。目前有关PEO对肉品质影响的研究主要分为2个方向:1)通过在畜禽饮食中补充PEO,改变机体脂质代谢来影响其肉品质[80];2)在肉制品中直接添加PEO,利用PEO的抗氧化性保持肉品质的稳定,如使用PEO制作的生物膜包装密封肉制品或在肉制品中使用PEO作为抗氧化剂。

脂质是风味物质的重要前体物质,PEO可能通过调节脂质代谢来影响肉的风味。辣椒素可以激活瞬时受体电位香草酸亚型1(TRPV1)通道,阻止3T3-L1前脂肪细胞和内脏脂肪组织细胞中的脂肪生成[80]。此外,饮食中的姜黄素可以降低肝胆固醇合成,影响脂质代谢过程[81]。Yan等[82]在仔猪饮食中添加了0.01%的混合PEO(包含百里香、迷迭香和牛至精油),发现混合PEO能够增加眼肌面积,并改善猪肉颜色和大理石花纹评分。张云峰[83]发现PEO能够显著提高肉鸭肌肉的亮度值,提高系水率,降低剪切力并提高嫩度。然而,Ranucci等[84]得到了与之矛盾的试验结果,在饲粮中补充迷迭香、大蒜、牛至或生姜的精油(500 mg/kg),结果表明并没有改变猪肉的脂肪酸谱,其他指标如蒸煮损失、滴水损失、剪切力值和化学成分也没有显著变化,但饲喂牛至精油的猪肉却呈现脂质氧化程度减少的趋势。因此,PEO能否改变畜禽肉中的脂质组成有待进一步的研究证明。

人们普遍认为,脂质氧化是造成储存过程中肉品质下降的主要原因,其次蛋白质的氧化也增加了肉的韧性,降低肉品质,因此调控肉制品的氧化过程也是改善肉品质的关键。PEO常用作天然抗氧化剂用于阻止油脂和脂肪食品中脂质的过氧化过程。有关芳香PEO抗氧化活性的研究表明,富含百里香酚和香芹酚的牛至精油对氧化过程具有较大的抑制作用[85-86]。Kulisic等[87]研究发现,牛至精油的抗氧化能力与α-生育酚及合成的抗氧化剂丁基化羟基甲苯(BHT)相当,并发现牛至精油与其他抗氧化剂的协同作用也会增强整体抗氧化能力。在活性食品包装中使用PEO是未来的趋势,PEO可以延长储存期间的食品稳定性,抑制腐败或病原微生物的生长,并防止食品氧化[88-90]。Zhang等[91]在鲤鱼真空包装袋中加入肉桂精油,发现能够降低肉中巨型球菌的相对丰度,抑制了气单胞菌属的生长,而乳酸菌相对丰度相对较高,其保质期延长了约2 d。PEO制成的活性食品包装的生物可降解薄膜,具有较强的防水性,不会对环境造成污染,对食品有很好的保鲜作用,具有广阔的应用前景[89]

5 小结

PEO作为天然安全的抗生素替代品,尽管其抗菌性、抗氧化性和抗炎性已经得到了验证,但是PEO中含有复杂的功能性物质,对动物机体产生影响的机制尚未阐释清楚,例如PEO如何提高动物生长性能等。在畜牧行业中,为了更加安全有效地使用PEO,未来PEO可能的研究方向集中在以下几个方向:

① 生物活性物质的鉴定和标准化。PEO复杂的功能性成分存在着协同和拮抗的作用,明确PEO整体的功能特性与单一化学成分的研究同样重要。单一化学成分的研究有助于确定发挥关键作用的功能性物质,整体研究有助于明确协同和拮抗的最终结果。由于不同PEO的成分及含量差异较大,功能性物质的鉴定和标准化显得尤为重要。

② PEO和其他添加剂的联合使用。PEO与其他添加剂或混合精油的联合使用可能存在着相互作用,这种复杂的相互作用制约着精油的发展应用,合理的利用协同作用将PEO的抗菌性和抗氧化性最大化,降低有效作用时的最低添加水平,在当下畜牧业发展过程中具有重要生产意义。

③ PEO对肠道健康的复杂调控机理。PEO能够改善肠道健康已经被众多学者所证实。然而,PEO对肠道菌群的调节、肠道菌群与宿主肠道健康的相互关系和潜在的分子机理过于复杂,不同PEO的作用机理可能存在差异,这些都有待于进一步研究探索。

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