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养殖水域抗生素抗性基因污染的研究概况与展望

2017-02-02李云莉高权新张晨捷施兆鸿彭士明王建钢

海洋渔业 2017年3期
关键词:水生抗性耐药性

李云莉,高权新,张晨捷,施兆鸿,彭士明,王建钢

(1.中国水产科学研究院东海水产研究所,上海 200090;2.上海海洋大学水产与生命学院,上海 201306)

·综述·

养殖水域抗生素抗性基因污染的研究概况与展望

李云莉1,2,高权新1,张晨捷1,施兆鸿1,彭士明1,王建钢1

(1.中国水产科学研究院东海水产研究所,上海 200090;2.上海海洋大学水产与生命学院,上海 201306)

在水产养殖中,抗生素是用来治疗细菌性疾病最有用的药物。但近些年的研究发现,过度使用抗生素,反而诱导产生了一系列带有抗性基因的致病菌,严重制约了水产养殖业的发展。本文以近15年来国内外相关研究的文献为依据,概括介绍了抗生素抗性基因(ARGs)的产生及其传播途径、ARGs污染的危害性、国内外ARGs污染研究现状及加强ARGs污染研究的必要性等4个方面的研究进展,围绕抗性基因的检测、ARGs的传播、扩散及作用机制和控制、消除ARGs的方法等方面进行了后续研究重点的展望,以期为我国水产养殖行业的可持续发展提供依据。

抗生素;抗生素抗性基因;水产养殖

抗生素可以用来治疗各种细菌感染或者抑制病原微生物感染,是防治动物病害的主要药物。在促进动物生长以及节约营养成分等方面,抗生素也发挥了作用,因此一直以来在人类的生活与生产中,抗生素都起到了至关重要的作用[1]。虽然抗生素具有较强的杀菌、促进生长等作用,但长期重复使用一种抗生素不仅会降低抗生素的药效,还有可能诱导动物体内产生抗生素抗性基因(ARGs)。ARGs可以在各种环境介质(比如土壤、河水、地下水)中进行迁移、转化,继而整合到质粒、转座子、整合子等可移动基因元件,之后再进入到微生物环境中,在细菌之间利用基因的横向转移进行传播,使原本没有抗生素抗性的细菌获得耐药性[2],最终通过食物链进入到动物体内或者人体内,对养殖生物及人类的健康构成潜在威胁[3]。抗生素的大量使用直接导致了抗性基因的产生和散播,现今,抗生素及ARGs污染已经成为一个全球性的环境问题,由此而产生的潜在生态风险也日益引起各国政府和研究者的广泛关注。本文以近年来国内外相关研究为基础,对养殖水域抗生素抗性基因污染的研究进展进行了较全面的概括总结,主要包括抗生素抗性基因(ARGs)的产生及其传播途径、ARGs污染的危害性、国内外ARGs污染研究现状及加强ARGs污染研究的必要性等4个方面,并对抗性基因的检测、ARGs的传播、扩散及作用机制和控制、消除ARGs的方法等方面进行了展望,以期为我国水产养殖行业的可持续发展提供参考。

1 ARGs的产生及传播途径

ARGs在自然界中普遍存在,有研究发现,抗生素的过量使用与其大量产生与扩散密不可分[4]。抗生素具有较强的杀菌、促进生长等作用,在人们日常的生产和生活中起着非常重要的作用,但长期重复使用一种抗生素可能会导致致病菌产生抗药性,进而使抗生素无法有效地控制或抑制细菌的生长,同时也有可能诱导产生具有抗药性的致病菌。目前ARGs主要有两个来源,一个是环境中细菌自身的内在抗性,还有一个是外源输入[5]。内在抗性是指存在于细菌基因组上的抗性基因的原型、准抗性基因或者平时没有表达的抗性基因[6],细菌可以通过随机突变或表达潜在的抗性基因而获得抗性。而外源输入主要是指抗性细菌通过人或者动物的粪便随肠道细菌排出体外,继而进入到环境中。起初,大部分的抗生素主要是用于医疗,因此在医疗废水中发现抗性基因的检出率较高,而且可以检测到多重耐药菌株[7-8]。在我国农业生产中,经常会利用动物粪便对农田进行施肥,这种方法既环保又省钱,然而有研究显示,通过对施有猪粪的土壤进行检测,发现土壤中含有高水平的抗性基因和抗性质粒[9],并且可以分离筛选出大量具有较强抗性的菌株[10],这说明利用动物粪便施肥是动物体内的抗性基因进入到土壤环境中的主要途径之一。

因为抗性基因能够增强细菌对抗生素的耐受力,所以当抗生素进入到水生环境后,携带有抗性基因的细菌就会迅速繁殖。水生菌的基因组复杂多样,所具有的遗传元件和基因都能够产生和散播抗性基因。水生菌可移动的遗传元件携带着遗传转移因子,遗传转移因子调控的水平基因转移对于海洋菌落结构具有非常重要的作用[11]。在水生环境中,噬菌体能够携带抗性基因,由噬菌体裂解而产生的裸露DNA和鱼类肠道的细菌质粒都可以参与调控抗生素诱发的水平基因转移,而为了能够在含有抗生素的水体环境中生存,水生菌(如鳗弧菌)会竞争吸收、整合这些含有抗性基因的裸露DNA[12]。在水生环境以及鱼类肠道内,细菌的数量是非常庞大的,这为在压迫环境下产生耐药性突变菌创造了条件,之后由突变产生的耐药性菌株会在新环境中大量繁殖,从而发展成为优势菌[13]。在高密度水产养殖模式下,为有效控制细菌的繁衍与疾病的发生,通常会加大抗生素的使用量。在这一过程中,所产生的具有多重耐药性的突变菌对环境的适应能力更强,能够同时耐受多种抗生素,所以在群体竞争中更有优势,能够成为优势菌。此外,长期处于含有抗生素的环境中,水生菌的SOS系统(由细菌DNA损伤引发的修补反应)可以被激活。这一系统可以通过产生氧离子而增加突变几率[14]。抗生素(如喹诺酮、β-内酰胺酶等)激活的SOS应激反应不仅可以促使水平基因位移,而且能够活化整合酶,从而促使整合子进行重组[15]。水生菌中整合子的出现几率容易受到人类活动(如水产养殖)的影响。鱼类肠道及水生环境中耐药性突变菌的突变基因可以被整合子捕获和整合,转座子和质粒可以调动这些抗性基因,从而产生新的抗性基因[16]。这些耐药性细菌抗性基因大多是由质粒和可移动的遗传元件参与、调节产生的,许多耐药性细菌(爱德华氏菌、气单胞菌、链球菌等)能够引发人、鱼共患传染病[17]。向水生环境中添加抗生素,可能会导致由质粒调节的喹诺酮抗性基因在水生致病菌间(比如嗜冷黄杆菌、杀蛙气单胞菌、鲁克氏耶尔森氏菌等)以水平基因转移的(HGT)方式进行传播[18],这使得喹诺酮抗性基因在环境中的浓度不断增加,同时细菌DNA变异的机率会降低。天然存在的质粒R1具有耐受氯霉素、卡那霉素、链霉素、磺胺、氨苄青霉素的抗性基因,5个抗性基因组合成一个抗性决定子,抗性决定子也可从水生环境或鱼贝类肠道中的微生物中获得。因此,抗菌药物的添加,会促使细菌的水平基因转移和突变,致使水生环境以及鱼贝类肠道中的抗性基因密度增多。

2 ARGs污染的危害性

随着全球水产养殖业的迅速发展,抗生素在水产养殖中的应用也越来越广泛,目前常用的抗生素主要有酰胺醇类、四环素类、磺胺类等[19]。其中,由于四环素类抗生素具有广谱性和低毒性的特点,在治疗和抵抗细菌疾病方面发挥了重要的作用,但大量使用四环素类抗生素也使得水体环境中四环素抗性病原菌的数量暴增,并且此类抗性基因在环境中的检出率较之其它类抗生素也是最高。目前,已报道的四环素类抗性基因就有40多种[20]。

环境中的绝大部分抗生素最终都会进入到水环境中,因此对水环境的影响最为严重,也是最受关注的环境污染问题[21]。在水产养殖业中,由于养殖生物粪便可以直接排放到水环境中,导致在全球不同养殖水域的水体和底泥中都可以检测到多种ARGs,其中以磺胺类和四环素类ARGs为主[22-24]。国外学者SCHMIDT等[25]针对丹麦虹鳟(Oncorhynchusmykiss)鱼场进行检测,结果发现37%的气单胞菌具有抗土霉素与磺胺嘧啶/假氧苄氨嘧啶基因的联合抗性。之后他们又发现了抗恶喹酸、羟氨苄青霉素、氟苯尼考与磺胺嘧啶/假氧苄氨嘧啶的致病菌鲁氏耶尔森菌、黄杆菌和杀鲑气单胞菌[26]。虽然气单胞菌并不是鱼类和人类的致病菌,但是通过水平基因转移可以将抗性基因转移到致病菌上,最终引起鱼类的死亡,并对人体健康产生很大威胁。

养殖业是我国水产业的重要组成部分,在一些地区更是支柱型产业。养殖水域中低浓度的抗生素可以通过消化系统进入水生生物体内,在肠道内诱导出抗性细菌,同时水中的ARGs也可以通过水生细菌的水平基因转移进入鱼、贝类等生物体[27]。之后经过生物排泄进入到水环境中,这不仅仅对养殖区域、周围的农业环境以及人居环境构成基因污染,还会对公共健康和食品、饮用水安全构成威胁。有研究发现,ARGs可以通过食物链传递给高营养级的生物,人类通过食用鱼类等海产品会将抗生素抗性转移到人体内[28],进而影响人类健康。此外,我国的海水养殖点重点分布于沿海地区,抗性菌株和抗性基因很容易扩散进入海湾和公海水域。抗性基因一旦传播给水生生物,将对整个水生生态系统造成不可逆的破坏,甚至对相邻国家造成抗性基因的污染,这将直接影响我国在国际上进出口贸易的国际形象和经济收益[27]。

3 国内外ARGs污染研究现状

现有的研究表明,抗性基因的存在导致细菌存在多重耐药性[29-31]。目前,已有的研究证实,水生生态系统是多种抗生素抗性基因的富集库,外源性抗生素对水生生态系统的污染是诱导ARGs富集、散播的重要因素[32-34]。最新的研究报道指出,全球多个地区都检测到了多种ARGs的存在[35],这表明水环境中ARGs污染问题已经成为全球性的问题。这不仅仅是一个地区或者一个国家的问题,世界各国都面临着严峻的考验。众所周知,水产养殖中抗生素的使用量通常比较大,并且养殖水体一般都不会经过特殊的处理而是直接排放到江河湖泊之中,因此,由水产养殖业所引起的水环境中的ARGs污染则更为严重。

针对水产养殖水环境中抗生素及ARGs污染的问题,国外的研究起步较早。水产养殖中抗生素的大量使用,致使鱼类致病菌产生耐药性,从而导致难以有效的治疗鱼类甚至动物、人类的疾病。LALUMERA等[36]对意大利2个鲑鱼养殖场和3个鲈鱼养殖场的底泥进行检测分析时发现,抗生素的使用,导致环境中ARGs的富集,其中土霉素和氟甲喹的最高浓度分别达到了246.3μg ·kg-1和578.8μg·kg-1,在水产养殖中持续使用抗生素,会造成抗性基因和耐药性细菌的持续性增加。这种状况也会促使多重耐药性基因和携带菌株的产生、增加及传播。经研究发现,水产养殖不仅会导致附近水域富集各种具有多重耐药性的细菌[37],还会使远离水产养殖区域的环境中同样具有非常高的多重耐药性的细菌检出率[38]。由于抗性基因可以在水生菌、陆生菌、人类致病菌之间流动、散播,因此水产养殖产业大的国家和地区,水环境中的细菌会携带大量的抗性基因[39],并且这些抗性基因转移到陆生菌和人类致病菌的可能性会大大增加,抗性基因的密度也会大幅度上升。抗生素会筛选出那些具有抗性基因的细菌,使其数量大量增加,并诱使抗性基因的产生或传播,促使水体中的抗性基因向陆生菌或人类致病菌散播。大量的实验数据及野外证据表明,水生菌可以频繁的通过基因的水平转移[40],使新型的抗性基因成为陆生菌(包括人类致病菌)基因的一部分,从而导致抗生素无法有效的治疗人类疾病。基因的水平移动,使水生菌的遗传基因可以传播和分散,并且遗传片段也会呈现出多样性。比如人类肠道拟杆菌可以从水生菌中获得降解海藻多糖的基因,从而使其自身具有降解海藻多糖的能力[41]。近年来,随着人们对食品安全问题的重视,养殖水环境中抗生素及ARGs污染影响水产品质量安全的问题也日益突出[3]。KANG等[42]在从韩国西部海域贝类样品中分离筛选的24株腐败希瓦氏菌(Shewanella putrefaciens)对16种抗生素具有耐药性。CIZEK等[43]从鲤(Cyprinus carpio)皮肤中也分离培养了出了具有多重抗药的气单胞菌(Aeromonas spp.)。HAMMAD等[44]研究也发现,生鱼片是肠球菌(Enterococcus spp.)的富集库,且携带多重抗性因子。

我国针对水产养殖环境中ARGs污染问题的研究起步较晚,但近年来的研究也取得了一定的进展。我国是世界第一水产养殖大国,养殖产量约占世界总产量的60%以上[45]。然而,产量上的飞速发展与健康养殖的可持续性却存在着严重的矛盾。在沿海的广东[46-47]、河北[48]、上海[49]等许多地区均存在不同程度的ARGs污染。LING等[50]在南、北方的江河中均检测出了磺胺类、四环素类ARGs。我国是抗生素生产和使用大国,抗生素年生产量接近21×104t,其中有9.7 ×104t用于畜牧水产养殖业,占年总产量的46%[51]。在水产养殖业中,大量使用抗生素不仅会使抗生素残留在水产品体内,导致我国水产品在国际市场上面临巨大的绿色贸易壁垒,而且水体及底部沉积物中大量残留的抗生素会诱导产生携带ARGs的抗药菌株。DANG等[52]在中国北方的一个海参和海胆的养殖场内,从海参和海胆中分离出了抗氯霉素菌株,而氯霉素在1999年就被禁止用于海水养殖。在我国天津的6个水产养殖场检测的四环素类和磺胺类抗生素的抗药性基因,均普遍存在磺胺类抗药性基因[53]。此外,陈琳琳等[54]对分离自水产养殖中的四环素耐药菌、ARGs进行检测发现,ARGs与耐药菌的基因无直接关联,表明四环素ARGs可以在水产养殖环境中传播扩散。

4 ARGs污染研究的必要性

随着世界各国经济的不断发展,水产养殖也得到了迅猛的发展,但水产养殖本身对环境和社会公共安全等方面所产生的负面影响,也越来越引起人们的关注,其中最为主要的问题就包括环境抗生素污染问题、水产品中抗生素残留及其耐药菌株的产生[55-56]。抗生素一旦进入水生环境中,整个水体中的菌落结构将发生改变,这种改变会使水生菌对抗生素产生耐受力,整个过程也会伴随着某些益生菌的减少甚至消失,而具有耐受性的细菌就会增加,成为优势菌。水生环境中微生物结构的改变,大多发生在对抗生素敏感和具有耐药性的微生物之间[57]。在水产养殖场周围的水生环境中,能够耐受土霉素、喹诺酮类、磺胺类等药物的微生物往往会大量繁殖[58]。研究发现,抗生素的使用量及使用时间与周围环境中具有相关耐药性的微生物的数量和规模具有极高的相关性[59]。

由于各种抗生素的过度使用,细菌对抗生素的抗药性已由单一抗药逐渐发展为多重抗药。细菌具有多重抗药性的主要原因在于各种ARGs可以在环境微生物之间进行连锁传播[60]。因此,ARGs污染已经成为21世纪威胁人类健康的重大挑战之一,由此产生的潜在生态风险也日益引起了人们的广泛关注。有资料显示,2010年,超级细菌新德里金属β内酰胺酶-1(New Delhi metallo-β-lactamase 1,简称NDM-1)的发现,曾一度引发全球恐慌。2011年德国爆发的“毒黄瓜”事件,短期内使得欧洲至少有9个国家受到了疫情的侵袭,确认的死亡人数有33人,超过3 000人受到了感染,其中包括至少470人出现肾功能衰竭并发症。2013年,美国疾病和预防控制中心发布的报告《美国2013年抗生素耐药威胁》显示,美国每年至少有200万人感染耐药菌,其中2.3万人死亡,感染导致250亿美元的医疗支出和350亿生产损失,耐药菌产生的速度远远超过新药研制的速度,若不加以严格控制,未来人类的某些病症将处于无药可救的境地。以上这些均表明抗生素污染及由此导致的耐药菌株的产生已对人类健康构成了严重威胁,应引起人们的高度重视。目前,我国水产养殖环境中ARGs的污染问题尚未引起人们的足够重视,抗生素滥用现象比较普遍,直接导致了环境中ARGs的产生和传播扩散。作为一个水产养殖大国,为实现水产养殖业的健康可持续发展,在当前抗生素与ARGs污染日趋严峻的形势下,加强ARGs这一新型环境污染的研究力度、探清其污染机理、制定相应的控制措施及监管制度已刻不容缓,也责无旁贷。

5 ARGs污染的研究展望

针对水产养殖水体中ARGs污染所导致的环境和食品安全问题,研究制定有效的ARGs污染防控措施已经成为当前急需攻克的难题。我国是世界第一水产养殖大国,细菌性疾病对水产养殖的危害极大,特别是在密集型的养殖系统中,发生的更为频繁[60-61],因此,抗生素滥用现象仍普遍存在。针对这一现状,应系统且具有针对性的开展水产养殖环境中抗生素及ARGs污染的研究。首先,必须建立水产养殖中抗性基因污染的定性和定量的方法。目前ARGs的检测方法主要有两种:一种是传统的微生物抑制法。通过微生物培养法,即基于最小抑制浓度的药敏试验[62]。传统检测法由于对其最适生长温度、pH、营养成分等不十分清楚,因此许多微生物难以进行平板培养。另一种是分子生物学方法。利用PCR技术对样品总DNA进行分析,该方法具有提取DNA率高,简便快捷的优点[63]。而近年来的荧光定量PCR技术,更是实现了从定性到定量的飞跃。其次,分析环境因子(抗生素、温度、pH、溶氧等)对ARGs传播扩散的影响及其机制,揭示ARGs在水产养殖环境中的扩散和传播规律,对于评价ARGs的生态风险十分必要,这将极大地促进我国水产养殖的可持续发展,加快建立我国水产品生态安全体系,快速提升我国水产养殖业的国际市场竞争力。最后,研究相应的控制和去除水产养殖环境中抗生素与ARGs的方法。目前,关于如何降解和消除水体中ARGs污染问题的研究,主要集中在对城市污水的处理[64-66]。在水产养殖业中,如何降解和消除养殖水体中ARGs的污染还未得到足够的重视,进行ARGs对水产动物相关风险性研究,了解ARGs与水产动物以及人类致病菌之间的关系,建立ARGs的生态环境安全评价体系及预警体系,不仅能确保我国水产养殖业的健康可持续发展,而且对于我国的生态安全和经济发展同样具有重要的现实意义。

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Current status and prospect ofantibiotic resistance genes(ARGs)pollution in the aquaculture

LIYun-li1,2,GAO Quan-xin1,ZHANG Chen-jie1,SHIZhao-hong1,PENG Shi-ming1,WANG Jian-gang1
(1.East China Sea Fisheries Research Institute,Chinese Academy of Fishery Sciences,Shanghai 200090,China;2.College of Fisheries and Life Science,Shanghai Ocean University,Shanghai 201306,China)

In aquaculture,aquatic products is often affected by bacterial diseases,which leads to the decline in production,while antibiotics are the most effective drug to treat the bacterial diseases.However,recent studies have found that the excessive use of antibiotics did not get to increase the expected economic output.On the contrary,it induced a series of pathogenic bacteria carrying the resistance genes,which seriously restricted the development of aquaculture.Currently,there are two sources for the antibiotic resistance genes(ARGs).One is intrinsic resistance of bacteria in the environment which is the prototype of the resistance gene and quasi resistant gene existing in the bacterial genome or genes not expressed normally.The other is exogenous inputwhich refers to the bacteriawith resistance gene excreting through human or animal feceswith the intestinal bacteria,and then entering the environment.The resistance gene can enhance the resistance of bacteria to antibiotics.So when the antibiotics enter the aquatic environment,the bacteria carrying the resistance gene will quicklymultiply and become the dominant flora.In the water environment,because the genomes of aquatic bacteria are complex and diverse and they contain the genetic elements or genes that are capable of producing and spreading resistance genes.The resistance genes can transfer to the bacteria that do not contain resistance genes by horizontal gene transfer,and then through the food chain enrichment enter the human body eventnally.The majority of antibiotics in the environment will finally enter the water environment,therefore it is the most serious problem for the water environment and has become the most concerned environmental pollution.In some eastern coastal cities of China,aquaculture industry has become the pillar industry.And many breeding sites are located in the tidal flat,so the resistant strains and resistance genes is prone to diffusing into the Gulf and the waters of high seas.Once the resistance genes is transmitted to the aquatic organisms,it will cause irreversible damage to the whole aquatic ecosystem and even the neighboring countries can be polluted by the resistance genes,which will directly affect the our country’s image in the international import and export trade and the economic benefits.Existing studies have indicated that the presence of resistance genes led to themultiple drug resistance in bacteria.Aquatic ecosystem is the enriched library of the multiple antibiotic resistance genes,and the pollution of exogenous antibiotics to aquatic ecosystem is the important factor to induce ARGs’accumulation and dissemination.Researches about the pollution of the antibiotics and ARGs in the aquaculture environment stated relatively earlier abroad.A large number of antibiotics usage in aquaculture resulted in the emergence of drug resistance to the pathogens in fish,which led to the difficulies to be effective in the treatmentof fish and even animals or human diseases.Though China is the largest country in aquaculture in the world,the relevant research still lagged behind.Thus,with the rapid development of China’s aquaculture industry and the continuous expansion of the scale of aquaculture,resistance gene not only led to the continuous decline in aquaculture production,but also had caused great pressures on the environment.Moreover,withmore andmore attention paid on the food security,the possible problems brought by ARGs pollution in the aquaculture have been worried about in recent years.Therefore,it should systematically carry out antibiotics and ARGs pollution research in aquaculture environment,and establishmethods for screening and identifying the resistant strains in aquaculture organismsand their environment,and analyze the environmental factors’(antibiotics,temperature,pH,dissolved oxygen,etc.)influence on ARGs diffusion and the relevant mechanisms,and reveal the diffusion and propagation reguations of ARGs in aquaculture environment,and establish the method for the control and removal of antibiotics and ARGs in aquaculture environment.The paper summarized the research progress of the resistance gene pollution in cultured waters from four aspects,which were production and transmission of ARGs,the formation and harm of ARGs pollution,research status of ARGs pollution at home and abroad and the necessity of strengthening the study of ARGs pollution.The future research keynoteswere also discussed at the end of this paper.

antibiotic;antibiotic resistance gene;aquaculture

S 949

A

1004-2490(2017)03-0351-10

2016-08-18

国家自然基金项目(31202009);中央级公益性科研院所基本科研业务费(东2014Z02)

李云莉,硕士。E-mail:liyunli109@163.com

彭士明,副研究员。E-mail:shiming.peng@163.com

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