微塑料影響抗性基因的傳播與水平基因轉(zhuǎn)移

吳效儉,施國靜,王瑩瑩*

微塑料影響抗性基因的傳播與水平基因轉(zhuǎn)移

吳效儉1,2,3,施國靜1,2,3,王瑩瑩1,2,3*

(1.南開大學(xué)環(huán)境科學(xué)與工程學(xué)院,天津 300350；2.環(huán)境污染過程與基準(zhǔn)教育部重點實驗室,天津 300350；3.天津市城市生態(tài)環(huán)境修復(fù)與污染防治重點實驗室,天津 300350)

微塑料作為一種新興污染物,在多種環(huán)境介質(zhì)(大氣、土壤、水體、沉積物)中均有廣泛檢出.微塑料能夠通過表面形成的生物膜結(jié)構(gòu)攜帶抗性基因,對人體健康造成的潛在風(fēng)險已經(jīng)引起了國內(nèi)外學(xué)者的廣泛關(guān)注.但是,微塑料在抗性基因的長距離傳播以及水平轉(zhuǎn)移方面所發(fā)揮的功能,以及微塑料傳播耐藥性所導(dǎo)致的生態(tài)與環(huán)境效應(yīng)還尚不明晰.本文重點對不同環(huán)境中的微塑料與抗性基因的污染現(xiàn)狀進行了綜述討論,對微塑料傳播抗性基因的主要方式與過程進行了對比分析,最后概述了微塑料參與調(diào)控抗性基因水平轉(zhuǎn)移的機制,并對未來相關(guān)研究的重點方向進行了展望,以促進微塑料污染問題的科學(xué)解決.

微塑料；塑料；抗生素抗性基因；塑料際；污染特征

塑料作為一種人工合成或半人工合成的高聚物,被廣泛應(yīng)用于工業(yè)生產(chǎn)與日常生活中.2015年的塑料全球年產(chǎn)量已達到4.07億t[1].大量塑料制品的使用會導(dǎo)致塑料廢棄物的產(chǎn)生,預(yù)計在2050年,產(chǎn)生的塑料廢棄物會突破2′107t[2].塑料廢棄物進入環(huán)境后,經(jīng)過逐級裂解,形成的粒徑在5mm以內(nèi)的塑料碎片被稱為微塑料[3].在各類環(huán)境,如土壤[4-5]、大氣[6-7]、水體與沉積物中[3,8-9],都有微塑料的廣泛分布.由于尺寸較小,部分微塑料會進入浮游動物體內(nèi),這些含有微塑料的浮游動物被處于更高營養(yǎng)級的魚、蝦等水生動物捕食[10-11],進而在食物鏈中逐級積累,發(fā)生富集作用[12-14].當(dāng)人類食用這些海鮮與水產(chǎn)品時,微塑料隨食物進入人體消化系統(tǒng),對人體健康具有潛在威脅[15].

環(huán)境中的微塑料表面會被微生物所定殖,形成生物膜結(jié)構(gòu),即不同種類的微生物構(gòu)成的群落[16-17].通過生物膜,微塑料可以攜帶耐藥細(xì)菌(ARB).由于耐藥細(xì)菌擁有抗生素抗性基因(ARGs),因此能夠?qū)Σ煌N類的藥品產(chǎn)生抗性[18-20].攜帶耐藥細(xì)菌的微塑料可以在不同類型的環(huán)境中遷移,進而產(chǎn)生諸多問題,其中抗性基因的水平基因轉(zhuǎn)移(HGT)是尤為嚴(yán)重的一個方面.水平基因轉(zhuǎn)移被認(rèn)為是細(xì)菌耐藥性傳播的主要驅(qū)動因素[21],抗性基因可以通過質(zhì)粒,整合子,轉(zhuǎn)座子等可移動元件(MGEs)進行遷移[22],造成細(xì)菌耐藥性的傳播與擴散[23-24].因此揭示微塑料對抗性基因水平轉(zhuǎn)移的機理,將有助于進一步明晰抗性基因的攜帶與傳播作用,以及微塑料參與調(diào)控不同環(huán)境中微塑料的遷移過程對環(huán)境安全與人體健康的影響.

本文主要從細(xì)菌耐藥性的角度,重點討論了不同環(huán)境中微塑料與抗性基因的相關(guān)研究進展,旨在分析以耐藥細(xì)菌為代表的微生物與微塑料污染之間的聯(lián)系,以及兩者作為一個整體所產(chǎn)生的環(huán)境生態(tài)效應(yīng).

1 環(huán)境中微塑料的賦存與抗性基因污染

微塑料與抗生素抗性基因被認(rèn)為是近年來的環(huán)境新興污染物[25-26].攜帶抗性基因的細(xì)菌可以從多個方面影響人體健康,例如通過飲用水、空氣、開放水域等傳播導(dǎo)致人體感染[27].微塑料作為一種可供微生物定殖的微生境,也具有攜帶并傳播抗性基因與人體致病菌的功能.一些人體條件致病菌,例如,等會在微塑料表面富集[17,20].由于微塑料的難降解性,其在環(huán)境中有較長的賦存時間,進而增加人體對抗性基因的暴露風(fēng)險.在不同環(huán)境介質(zhì)(土壤環(huán)境、大氣環(huán)境、水體與沉積物環(huán)境)中,由微塑料所攜帶的抗性基因的種類與豐度信息見表1.

表1 不同環(huán)境介質(zhì)中微塑料攜帶的抗性基因種類與豐度

注: -為無豐度數(shù)據(jù).

1.1 大氣環(huán)境中的微塑料與抗性基因

在城市區(qū)域,空氣、道路揚塵、室內(nèi)灰塵中分布有大量微塑料和抗性基因[2,36].室內(nèi)的裝飾品、織物等會釋放出大量微塑料,在室內(nèi)空氣中漂浮,據(jù)研究,城市室內(nèi)環(huán)境的微塑料濃度可達16個/m3[37].隨著門窗開啟與空氣流通,室內(nèi)環(huán)境中的微塑料會擴散到室外空氣中.汽車輪胎經(jīng)過長期磨損后會釋放出微塑料,部分進入空氣中,這種現(xiàn)象在車流量較大的城市主干道路附近更加明顯.漂浮在空氣中的微塑料易吸附空氣中漂浮的抗生素抗性基因,與其共同發(fā)生遷移.

由于尺寸較小和重量較輕,微塑料很容易在風(fēng)力作用下向較偏遠的區(qū)域發(fā)生遷移,在一些受人為活動影響較小區(qū)域的空氣中,也發(fā)現(xiàn)了氣載微塑料的存在[38-39].一項針對法國山脈附近的氣載微塑料的研究發(fā)現(xiàn),微塑料通過大氣傳輸?shù)木嚯x能夠達到95km[40].抗性基因在大氣環(huán)境中的分布方面存在一定規(guī)律,全球尺度的大氣環(huán)境中,β-內(nèi)酰胺類抗生素抗性基因bla,以及喹諾酮類抗性基因具有較高的豐度[39].在空氣重度污染地區(qū),碳?xì)涿瓜╊惪剐缘幕?i>bla-1占有大氣總抗性基因豐度的30%[41].

漂浮在空氣中的微塑料與抗性基因極易被人體吸入[42],導(dǎo)致呼吸系統(tǒng)的疾病[43],損害人體健康.每個成人每日吸入的微塑料顆粒數(shù)約為20個[44].在人體肺部組織中探測到了合成纖維的存在[45],且嬰兒和兒童的微塑料吸入量遠高于成人[6].空氣氣溶膠中存在著各類攜帶抗性基因的人體條件致病菌,如,等[39,46].當(dāng)這些致病菌以及抗性基因黏附到微塑料表面進入人體肺部后,容易造成肺部感染等疾病.由于目前關(guān)于大氣環(huán)境中微塑料所攜帶的抗性基因方面的研究還較少,因此對于評估抗性基因與微塑料的聯(lián)合毒性效應(yīng)還存在較大難度,同樣地,對于抗性基因隨氣溶膠中的微塑料進行長距離遷移的潛力也有待進一步探究.

1.2 土壤環(huán)境中的微塑料與抗性基因

土壤環(huán)境,尤其是農(nóng)田土壤中,存在大量微塑料,這是由于耕作過程中廣泛使用的地膜等塑料制品在光照、風(fēng)蝕、撞擊等作用下,物理化學(xué)性質(zhì)發(fā)生改變,逐漸裂解形成微塑料.特定的土壤微生物能夠在微塑料表面形成細(xì)菌群落,導(dǎo)致微塑料攜帶的細(xì)菌種類和抗性基因組成與周圍環(huán)境中的細(xì)菌群落均存在差異[47-48].土壤環(huán)境具有區(qū)別于大氣與水環(huán)境的獨特性,土壤的團粒結(jié)構(gòu)組成成分相對復(fù)雜,其中各類殺蟲劑、農(nóng)藥和重金屬的存在導(dǎo)致土壤細(xì)菌同時受到多種污染物的影響,當(dāng)細(xì)菌對外源刺激做出響應(yīng)與反應(yīng),會進一步影響土壤細(xì)菌群落中抗性基因的豐度[49].土壤中的微塑料能夠吸附土壤中的各類污染物,如溴代阻燃劑[50]、重金屬[51]、抗生素[52],是一個獨特的微生境.具有編碼抗生素與重金屬耐性基因的細(xì)菌能夠通過外排泵[53]、降低膜通透性[54]、轉(zhuǎn)化或降解污染物[55]等方式,更好地適應(yīng)生境,從而定殖到微塑料表面.因此,土壤中的微塑料會攜帶并富集抗性基因,進而影響土壤環(huán)境中的抗性基因組成[30,34].

在污水處理廠中,匯聚了大量不同來源的生活、醫(yī)療、農(nóng)業(yè)養(yǎng)殖污水,存在不同類型的微塑料與抗性基因[56].攜帶抗性基因的微塑料進入土壤的另一個重要來源是污水處理廠的活性污泥[57-58].經(jīng)過絮凝與沉淀過程,污水中的微塑料進入活性污泥中[59],之后隨污泥一同被填埋、焚燒、作為農(nóng)田肥料,從而進入到土壤中[57],構(gòu)成了土壤環(huán)境中抗性基因的重要點源污染.研究顯示,污水處理廠污泥中的PVC微塑料是傳播致病菌以及抗性基因的媒介[60].一項針對好氧顆粒污泥系統(tǒng)的研究顯示,進入污泥中的微塑料會使體系內(nèi)攜帶抗性基因的細(xì)菌分裂速率發(fā)生變化,對于賦存于污泥中[32]、滲濾液中[61]和接收污水處理廠經(jīng)過處理后的廢水的河流中[62]抗性基因的增殖情況均有明顯影響.

污水處理廠的活性污泥是微塑料進入土壤環(huán)境的重要來源,但目前各反應(yīng)階段(如沉淀、絮凝、生化處理、消毒等)對微塑料以及表面抗性基因的種類與豐度變化的影響還了解甚少.因此,有必要針對關(guān)鍵的分離與轉(zhuǎn)化步驟對微塑料的去除效率,以及表面生物膜中抗性基因的賦存情況進行研究,為評估特定步驟的物理化學(xué)條件對微塑料傳播抗性基因的能力的影響.

1.3 水環(huán)境中的微塑料與抗性基因

海洋匯聚了河流、湖泊以及來自大氣沉降的微塑料,被認(rèn)為是一個重要的微塑料的匯[28,63].目前,多項研究證實了各個大洋、河流、內(nèi)陸湖、集水區(qū)等水域[64-67]中均有微塑料的存在,甚至一些受人為活動影響較小的地區(qū),如格陵蘭島附近的海域[68]也有微塑料的檢出.微塑料能夠攜帶并富集水體中的抗性基因[57,69-72].多項關(guān)于水體中微塑料表面生物膜的研究顯示,無論是抗性基因的種類還是豐度,微塑料攜帶抗性基因的能力均高于水體,其表面形成的生物膜結(jié)構(gòu)對抗性基因具有富集作用[17,19,57,69,71-72].一些抗性基因的細(xì)菌宿主(,與)只在微塑料表面被發(fā)現(xiàn),而沒有出現(xiàn)在其他自然介質(zhì)表面(如巖石和葉片中),是微塑料所具有的材料特性所導(dǎo)致的[17].

目前的大部分研究關(guān)注的是水介質(zhì)中的微塑料表面抗性基因的賦存情況,而對于沉積物的研究還較少,這可能是受到采樣設(shè)備與條件的限制,以及沉積物環(huán)境中微塑料表面生物膜較難收集導(dǎo)致[73].由于微塑料在老化、生物污損等因素的作用下,會在水體環(huán)境中發(fā)生垂直遷移,從而進入沉積物中.沉積物環(huán)境中生活著多種底棲動物,微塑料所攜帶的抗性基因與這些生物的相互作用也有待進一步研究.

1.4 大氣、土壤、水體環(huán)境中微塑料表面抗性基因污染的異同點

在水環(huán)境、土壤環(huán)境以及空氣環(huán)境中的微塑料均被發(fā)現(xiàn)能夠攜帶抗性基因,但存在于不同環(huán)境介質(zhì)中微塑料表面形成的生物膜的群落組成與生物量存在差異,導(dǎo)致了其攜帶抗性基因能力的不同.其中,空氣環(huán)境中微塑料攜帶抗性基因的情況與土壤、水體環(huán)境中存在很大差異,主要表現(xiàn)在微塑料表面生物膜的生物量較低,攜帶的抗性基因種類較少,豐度較低.這主要是單位體積空氣中細(xì)菌的豐度小于單位體積的水或者土壤所導(dǎo)致的.

而土壤與水環(huán)境中蘊含著較豐富的營養(yǎng)物與多種細(xì)胞代謝所需的礦物質(zhì)元素,能夠支持存在于水、土介質(zhì)中具有不同生理特性的微生物對微塑料表面的定殖過程,一些適應(yīng)微塑料生境的細(xì)菌在微塑料表面逐漸形成生物量高、物種數(shù)量繁多、結(jié)構(gòu)致密的生物膜系統(tǒng),微塑料是眾多細(xì)菌以群落為單位進行繁殖和演替的微型生境.微塑料表面以生物膜形式存在的細(xì)菌群落對環(huán)境因子的劇烈變化具有更強的抵抗能力.不同細(xì)菌有各自精細(xì)的分工,專營特定的生理反應(yīng)與代謝活動,產(chǎn)生的各類次生代謝物可以釋放到胞外,進入生物膜基質(zhì)中,供給群落中的其他細(xì)菌成員使用.細(xì)菌群落作為整體,具有更加多樣化的功能.部分細(xì)菌通過攜帶特定基因片段,可以利用轉(zhuǎn)化、外排泵等方式將藥物對細(xì)菌菌體的殺傷力降至最小,從而發(fā)揮出耐藥性.在水體與土壤環(huán)境中的微塑料攜帶的抗性基因具有更高的多樣性以及豐度.研究證實,微塑料表面生物膜中耐藥細(xì)菌以及抗性基因的豐度比水體環(huán)境中至少高幾個數(shù)量級[69,74].

現(xiàn)有的研究顯示,土壤與水環(huán)境中微塑料表面攜帶的抗性基因的情況有所差異,這主要是由于土壤與水體環(huán)境中占有優(yōu)勢主導(dǎo)地位的細(xì)菌群落不同所導(dǎo)致的.具有不同分類學(xué)地位的細(xì)菌宿主會編碼不同的抗性基因序列.水體與土壤中微塑料具有各自獨特的表面性質(zhì)特征,對表面的細(xì)菌群落產(chǎn)生特異性選擇作用.例如,多項研究顯示水體環(huán)境中微塑料對特定類群的細(xì)菌具有富集作用[75-76]另一方面,水體與土壤環(huán)境的多項環(huán)境因子均具有差異性,導(dǎo)致微塑料中含有的添加劑的析出程度不同,這也是造成水與土中微塑料攜帶抗性基因類別具有差異的重要原因.

污水處理廠、耕地、人體呼吸道等生境中,均存在核心抗性基因的現(xiàn)象[77-79].例如,研究收集了來自種植玉米、花生和大豆耕地的105份土壤樣品,發(fā)現(xiàn)即使這些土壤中所生長的植物種類、土壤養(yǎng)分條件以及地理位置(樣本之間相距距離最長2200km)不同,有一些基因在超過50%的樣本中均被檢測到,例如多重耐藥型抗性基因,-05,以及-04等[80].微塑料是賦存于大氣、土壤與水體環(huán)境中一類高度分化的生態(tài)位,但是目前對其中賦存的核心抗性基因的組成方面的了解還較少.多數(shù)研究區(qū)域面積較小,且種類單一,例如湖泊的近岸區(qū)、農(nóng)田土壤的局部,得到的規(guī)律性結(jié)論在推廣到其他環(huán)境時存在一定的困難.另外,多數(shù)研究受到微塑料生物膜生物量以及采樣方法等條件的限制,所涵蓋的樣本量通常較少,對于解析抗性基因的豐度關(guān)系方面存在一定困難.因此,未來有必要在多環(huán)境介質(zhì)、較大區(qū)域尺度范圍內(nèi)進行聯(lián)合研究,以此為基礎(chǔ)探討微塑料對富集核心抗性基因的現(xiàn)象以及其中包含的規(guī)律.

2 微塑料表面生物膜的成因與影響條件

在自然環(huán)境中,多數(shù)土壤介質(zhì)以及幾乎所有水體中的固體表面均能夠支持生物膜的生長[81].微塑料表面生物膜的形成條件主要包括以下幾個方面:

2.1 固體介質(zhì)表面性質(zhì)

在微塑料生物膜形成的最初始階段,需要固體介質(zhì)表面為環(huán)境中存在的種類繁多的細(xì)菌提供附著與定殖的空間.只有細(xì)菌成功定殖到微塑料表面后,才能啟動生物膜后續(xù)形成過程(圖1).因此,固體介質(zhì)微塑料的存在,是生物膜形成的基礎(chǔ).由于部分塑料材質(zhì)的醫(yī)療用品,如導(dǎo)尿管,在制作加工時表面使用了聚兩性離子涂層,具有抑菌效果[82].為了減少致病菌在導(dǎo)尿管上形成生物膜,部分導(dǎo)尿管的塑料材質(zhì)表面還經(jīng)過納米尺度的結(jié)構(gòu)修飾,細(xì)菌無法很好地黏附到管壁上,也就無法形成生物膜,從而降低尿路感染引起的炎癥效應(yīng)[83].相比于未經(jīng)物理與化學(xué)改性修飾的普通微塑料,表面覆蓋有特殊涂層,經(jīng)過修飾的塑料制品經(jīng)過裂解后形成的微塑料更加難以被細(xì)菌定殖,形成生物膜的能力較弱,攜帶抗性基因的能力較差.

環(huán)境中通過鞭毛等結(jié)構(gòu)進行自由移動的細(xì)菌接觸到微塑料表面后,松散地吸附到材料上.這個階段的黏附是可逆的,也就是說,一部分細(xì)菌還會離開微塑料表面,重新回歸為浮游狀態(tài).而部分細(xì)菌通過開始表達與黏性蛋白相關(guān)的基因[84],合成相關(guān)黏附素,將菌體緊密地固定到微塑料表面,同時鞭毛等主管移動的附件結(jié)構(gòu)的活動性也發(fā)生變化,細(xì)菌對表面的吸附進入不可逆的狀態(tài)[85],至此,細(xì)菌完成對微塑料表面的定殖過程,這些細(xì)菌被稱作先鋒種.先鋒種細(xì)菌在微塑料表面進行細(xì)胞分裂與繁殖,改變了介質(zhì)的表面性質(zhì),為后續(xù)其他種類細(xì)菌在微塑料表面形成細(xì)菌群落提供了先導(dǎo)條件.

2.2 營養(yǎng)與水文條件

在經(jīng)歷了先鋒種的定殖后,逐漸有更多種類的細(xì)菌開始在微塑料表面穩(wěn)定存在,由于微塑料作為一類微型生境,能夠提供的空間與營養(yǎng)是有限的,因此周圍環(huán)境中介質(zhì)的更新速度決定了細(xì)菌獲取營養(yǎng)以支撐群落發(fā)展的速率以及規(guī)模,也對生物膜的形貌特征具有決定性作用[86].有研究顯示,在水體環(huán)境中,液體的流速影響了固體介質(zhì)表面生物膜的物理結(jié)構(gòu),主要表現(xiàn)為在一定范圍內(nèi),流速越高,生物膜的生物量越大[87].這主要是水流能夠為生物膜帶來更高濃度的營養(yǎng)物質(zhì),同時將產(chǎn)生的多余代謝物帶走,不斷進行局部環(huán)境的更新.但是,當(dāng)水流流速過高時,生物膜結(jié)構(gòu)無法承受水流剪切力作用,表面的細(xì)菌會從生物膜中剝離,導(dǎo)致生物量的減少[88-89].在特定的河流、湖泊等環(huán)境中,水流的速度大致在一定范圍內(nèi)進行波動,其中微塑料表面生物膜的生物量以及群落中細(xì)菌總量穩(wěn)定在特定水平,其攜帶抗性基因的能力是相對穩(wěn)定的.

2.3 其他環(huán)境因子

多種環(huán)境因子,例如,光照條件、溫度、pH值、培養(yǎng)時間等均會從不同方面對生物膜中細(xì)菌的生長造成影響,從而改變生物膜的群落特征.研究顯示,在黑暗條件下,蓄水庫中生物膜的厚度在30μm以下,在有光照的條件下,蓄水庫中生物膜的厚度有了大幅度的增加,約為50～130μm[90].溫度對生物膜的生長速率有十分重要的影響.當(dāng)水體溫度過低時,細(xì)菌的細(xì)胞進入休眠狀態(tài),只進行最基本的代謝活動,生物膜的組成不再發(fā)生大的變化[91].研究顯示,當(dāng)微塑料在污水處理廠污泥中的存在時間超過60d時,微塑料所攜帶的抗性基因的豐度有顯著增加[60,92],這表明微塑料在特定生境中的賦存時間對攜帶抗性基因的細(xì)菌在介質(zhì)表面的增殖以及向周圍環(huán)境中的擴散產(chǎn)生的影響是直接的,進而間接影響該生境中抗性基因的組成與豐度.

圖1 微塑料表面生物膜的形成條件

在微塑料所處的特定生境中,所有環(huán)境因子共同對生物膜群落的變化與演替施加影響.經(jīng)過各個細(xì)菌類群的比例動態(tài)變化的階段后,逐漸達到群落結(jié)構(gòu)相對穩(wěn)定的狀態(tài),主要表現(xiàn)為養(yǎng)分分配的平衡,對外界環(huán)境因子的劇烈變化有一定的抵抗能力.這個穩(wěn)定的狀態(tài)并非是各個種屬的細(xì)菌比例保持一成不變,而是生物膜群體能夠隨著外界環(huán)境條件隨時調(diào)整自身結(jié)構(gòu)形成的動態(tài)平衡.但是,目前的研究多關(guān)注于微塑料表面經(jīng)過長時間培養(yǎng)后形成的生物膜結(jié)構(gòu),對于其動態(tài)形成過程中群落組成的變化以及抗性基因的豐度波動還需要更加詳細(xì)的探究.

3 微塑料影響抗性基因傳播的途徑

3.1 微塑料的理化性質(zhì)影響抗性基因傳播

通過野外調(diào)查與實驗室模擬,多項研究表明,抗性基因在微塑料表面生物膜中的賦存與傳播受到微塑料理化性質(zhì)的影響,主要包括介質(zhì)的化學(xué)組成(材質(zhì))、粒徑和老化程度等方面.

3.1.1 微塑料的化學(xué)組成影響抗性基因的傳播 一項針對滲濾液中微塑料的研究顯示,聚氯乙烯(PVC)、聚丙烯(PP)、聚苯乙烯(PS)所富集的抗性基因種類與豐度具有差異[92],這可能是由于不同的化學(xué)組成影響了微塑料表面所定殖的菌群的結(jié)構(gòu),從而導(dǎo)致所攜帶的抗性基因的差異.

塑料高聚物的種類也同樣影響微塑料對于抗性基因的豐度富集效果.例如,河口環(huán)境與滲濾液微宇宙系統(tǒng)中的聚乙烯(PE)微塑料生物膜中的抗性基因的豐度均高于PP微塑料[92-93].不同化學(xué)組成的微塑料中所含有的添加劑成分也不同,對于不同細(xì)菌類群在微塑料表面形成生物膜的影響具有差異,其促進或抑制效應(yīng)需要針對具體細(xì)菌類群的情況進行分析.

3.1.2 微塑料粒徑與老化程度影響抗性基因傳播 有研究通過批次培養(yǎng)實驗,對不同粒徑范圍的微塑料表面生物膜群落組成進行了探究,分別對比了粒徑范圍在106～125μm與355～425μm的PS微塑料,粒徑范圍在125～250μm與250～500μm的PE微塑料,兩組實驗結(jié)果顯示,粒徑范圍并未顯著影響微塑料的表面微生物群落結(jié)構(gòu),其攜帶的抗性基因的差異也較小[94].

微塑料進入環(huán)境后,經(jīng)歷一系列物理、化學(xué)、生物過程導(dǎo)致其表面性質(zhì)發(fā)生改變的現(xiàn)象稱為老化.物理方面,在機械磨損作用下會發(fā)生分子鏈的斷裂;化學(xué)方面,在紫外線照射下,微塑料表面發(fā)生光氧化,涉及官能團的改變;微生物作用下,其表面的粗糙度、電荷含量以及表面自由能變得不同,含有的添加劑更容易析出,且更容易吸附來自外部的污染物,如抗生素類的藥品以及重金屬等.微塑料老化后,材料的疏水性發(fā)生變化,表面形成的生物膜中更容易積累重金屬、有機污染物、抗生素等物質(zhì)[49,95].這些物質(zhì)對細(xì)菌具有定向選擇壓力,不僅會導(dǎo)致耐藥性細(xì)菌的增加,也會直接影響耐藥細(xì)菌中質(zhì)粒攜帶的抗性基因發(fā)生轉(zhuǎn)移的速率(圖2).

研究顯示,微塑料表面吸附的污染物對于微塑料表面抗性基因的豐度具有促進作用[33].在甲氧芐氨嘧啶的作用下,抗性基因水平轉(zhuǎn)移與垂直轉(zhuǎn)移速率均顯著增加[96];重金屬與多環(huán)芳烴的存在,也會對抗性基因的轉(zhuǎn)移效率產(chǎn)生影響[97].從塑料中分離出的細(xì)菌具有更強的制造胞外多聚物的能力,因此,當(dāng)具有這些能力的細(xì)菌定殖于塑料表面時,有助于形成致密的生物膜結(jié)構(gòu)[98-99].塑料防污涂層中含有的銅元素與鋅元素能夠使微塑料富集更高豐度的大環(huán)內(nèi)酯類-林可胺類-鏈霉素類(MLS)抗性基因[100].在種植蔬菜的農(nóng)田土壤中經(jīng)過自然老化的微塑料吸附的抗生素與重金屬含量更高,也增加了可移動性遺傳元件的豐度,這對于微塑料表面抗性基因的傳播有較多貢獻[101].

圖2 微塑料表面生物膜影響抗性基因的傳播

3.2 微塑料在環(huán)境介質(zhì)中的遷移影響抗性基因傳播

3.2.1 微塑料在環(huán)境介質(zhì)內(nèi)部的遷移 在水體環(huán)境內(nèi)部,微塑料起初多數(shù)存在于表層水體,之后在表面生物膜的影響下向深層水體以及沉積物中遷移.當(dāng)微塑料表面的生物膜被部分以細(xì)菌為食的浮游生物攝取后,微塑料的密度發(fā)生變化,可能會重新懸浮到水體表層.該過程不斷循環(huán),伴隨著微塑料的沉降與再懸浮,所攜帶的抗性基因也隨之進入水體環(huán)境的各個組分中[102].土壤中微塑料的移動主要是依靠土壤動物翻動土壤,微塑料隨著土壤顆粒共同運動,其運動路徑大體遵循由淺層土壤進入深層土壤的趨勢.大氣環(huán)境中存在的常常是密度較輕的微塑料,其運動受到風(fēng)力、降水等因素的影響較多[103-104].

3.2.2 微塑料在多環(huán)境介質(zhì)之間的遷移 河流是微塑料從陸源向水體環(huán)境中遷移的重要媒介.污水處理廠接收的廢水中含有的大量微塑料無法被完全去除,依舊存在于水介質(zhì)中,隨著末端排放進入河流中.河流中攜帶著大量微塑料匯入海洋.據(jù)統(tǒng)計,全球90%的微塑料是通過河流輸送到海洋、內(nèi)陸湖等環(huán)境中的[105].

另一方面,大氣的全球循環(huán)是水體、空氣、土壤中的微塑料向其他介質(zhì)中遷移的重要渠道.在風(fēng)力與氣流的作用下,微塑料進入空中,之后在沉降作用下進入陸地與海洋環(huán)境[103-104],但是目前氣載微塑料的流通過程與通量的計量還需要結(jié)合模型進行解析與評估.

以生物膜形式存在的細(xì)菌,其生活方式區(qū)別于浮游態(tài)的細(xì)菌.胞外多聚物結(jié)構(gòu)對于細(xì)菌菌體的黏附聚集、形成特定的空間結(jié)構(gòu)、抵抗外界環(huán)境因子波動以及細(xì)菌群落內(nèi)部的交流和耐藥性方面,都具有十分重要的作用.致密的胞外多聚物可以加強微塑料表面細(xì)菌之間的聯(lián)系,能夠增加包括質(zhì)粒、噬菌體、基因島、轉(zhuǎn)座子和插入序列等在內(nèi)的可移動遺傳元件的豐度,促進不同種屬的細(xì)菌之間抗性基因的交流.已有研究顯示,水環(huán)境中微塑料攜帶的耐藥性細(xì)菌豐度是水體中的100倍[106],微塑料生物膜中的細(xì)菌獲得質(zhì)粒的頻率約為浮游態(tài)細(xì)菌的兩倍[107],質(zhì)粒攜帶抗性基因的序列在環(huán)境微生物與耐藥性細(xì)菌之間轉(zhuǎn)移,增加了普通細(xì)菌獲得耐藥性的可能性.其他實驗數(shù)據(jù)也表明微塑料攜帶的抗性基因具有較高的基因轉(zhuǎn)移速率[69,106].微塑料作為一個匯集了耐藥性細(xì)菌、污染物、抗生素的微生境,細(xì)菌之間的聯(lián)系與交流都強于環(huán)境中浮游狀態(tài)的細(xì)菌,對環(huán)境抗性基因組也會產(chǎn)生較大的影響.微塑料通過在多種介質(zhì)之間的移動,會導(dǎo)致攜帶的抗性基因在表面生物膜與所經(jīng)過的環(huán)境微生物之間發(fā)生交換.這種抗性基因的傳播效應(yīng)可以在長距離尺度發(fā)生[49,108].

生物膜作為一個具有高度自組織結(jié)構(gòu)的整體,部分細(xì)菌類群數(shù)量始終處于不斷變化中,這樣能夠更好地適應(yīng)不斷變化的外界環(huán)境.處于不同演替階段的微塑料表面生物膜在傳播抗性基因方面所發(fā)揮的功能有何種不同還有待進一步探索.另一方面,土壤固體團粒結(jié)構(gòu)以及水流流體具有不同的介質(zhì)特點,介質(zhì)中存在的污染物的種類以及可遷移性方面也會影響抗性基因的傳播,相關(guān)的具體過程以及內(nèi)部調(diào)控機制也尚未明晰.最后,針對微塑料在不同生態(tài)系統(tǒng)中發(fā)生老化的過程,對于攜帶抗性基因的能力的影響的評估也十分重要,能夠為評估微塑料的環(huán)境行為以及生態(tài)風(fēng)險評估提供理論支持.

4 微塑料影響抗性基因水平轉(zhuǎn)移的過程與機制

微塑料所攜帶的可移動遺傳元件為抗性基因的水平基因轉(zhuǎn)移提供了基礎(chǔ)[109].整合子能夠捕獲并整合外源性基因,并將其轉(zhuǎn)變?yōu)楣δ苄曰虻谋磉_單位,與轉(zhuǎn)座子或者接合性質(zhì)粒共同使抗性基因在不同細(xì)菌之間發(fā)生水平傳播[110].例如,抗性基因1位于I類整合子的3’-保守末端(3’-CS),當(dāng)整合子在不同細(xì)菌之間移動時,所攜帶的1也會隨之轉(zhuǎn)移[111].此外研究報道在微塑料表面有部分可移動遺傳元件能夠同時攜帶重金屬抗性基因(MRGs)和抗生素抗性基因,且兩者的豐度具有顯著相關(guān)關(guān)系[70].目前,對微塑料如何調(diào)控抗性基因水平轉(zhuǎn)移方面的機制還有待進一步了解.對近5年報道的幾類主要水平基因轉(zhuǎn)移方式進行總結(jié)分析,微塑料在這些過程中所發(fā)揮的功能與調(diào)控作用見圖3.

圖3 微塑料影響抗性基因轉(zhuǎn)移的機制

4.1 微塑料影響細(xì)菌的接合過程

與浮游態(tài)的細(xì)菌相比,微塑料表面的細(xì)菌具有更高的基因轉(zhuǎn)移頻率[109].抗性基因能夠通過接合的方式發(fā)生水平轉(zhuǎn)移.接合(Conjugation)是供體菌通過性菌毛將DNA傳遞給受體菌,從而將抗性基因整合到受體菌的基因組中,使其表達出相應(yīng)的抗性性狀.接合方式介導(dǎo)的基因轉(zhuǎn)移以供體菌通過性菌毛與受體菌發(fā)生直接接觸為基礎(chǔ)[23],微塑料會對接合過程的發(fā)生頻率產(chǎn)生影響.聚苯乙烯微塑料會極大程度地促進MG1655的pKJK5質(zhì)粒向sp.細(xì)菌以及天然湖泊中細(xì)菌的接合轉(zhuǎn)移,這表明微塑料不僅會影響模式細(xì)菌在純培養(yǎng)體系中的接合頻率,也會影響自然環(huán)境中的細(xì)菌的接合轉(zhuǎn)移[107].微塑料表面可以吸附其他類型的污染物,例如納米材料(ENMs)[112]以及藥物和個人護理品(PPCPs)[113]等.這些吸附的污染物(如ZnO、Al2O3、GO、Ag、CuO、TiO2、NaNO2、TCS等)已被證實能夠不同程度地提高細(xì)菌的接合效率[114-119].

4.2 微塑料影響活性氧簇調(diào)控胞外多聚物形成過程

微塑料表面附著的生物膜具有胞外多聚物結(jié)構(gòu),含有的多糖、核酸、蛋白質(zhì)、脂質(zhì)以及其他小分子,對細(xì)菌相互作用與信息交流具有重要作用[120].當(dāng)胞外多聚物的含量較高時,會促進細(xì)菌的黏附作用,而胞外多聚物含量較低時,則會通過靜電排斥力的作用抑制細(xì)胞黏附過程[121].由于胞外多聚物的形成過程受到胞內(nèi)活性氧簇(ROS)濃度的影響[122],活性氧簇的濃度又受到污染物暴露的影響[123-124].微塑料表面吸附的污染物(如重金屬離子等),會通過影響細(xì)菌胞內(nèi)活性氧簇濃度水平的方式,間接影響影響表面生物膜的形成.由于適度的活性氧簇暴露會引發(fā)細(xì)胞一系列保護性措施,也有利于增加水平基因轉(zhuǎn)移發(fā)生的頻率[125];但是,當(dāng)所接觸的污染物含量較高時,過量的活性氧簇也會導(dǎo)致細(xì)胞損傷甚至死亡,導(dǎo)致接合頻率的下降[126].

4.3 微塑料影響細(xì)胞膜通透性與群體感應(yīng)調(diào)節(jié)系統(tǒng)

與浮游細(xì)菌相比,微塑料表面以生物膜狀態(tài)存在的細(xì)菌,其營養(yǎng)策略、細(xì)胞間的交流、細(xì)胞膜通透性均具有不同的特征,這對抗性基因的轉(zhuǎn)移也會構(gòu)成影響.當(dāng)細(xì)胞膜通透性較大時,疏水的脂質(zhì)屏障被削弱,促進抗性基因的釋放與吸收.暴露于微塑料表面污染物的細(xì)菌細(xì)胞膜容易形成孔洞,使供體菌的性菌毛更容易與受體菌接觸,從而發(fā)生接合作用[127].

群體感應(yīng)調(diào)節(jié)系統(tǒng)(QS)支持了生物膜中細(xì)胞與細(xì)胞之間的通訊,能夠通過信號分子獲取自身以及其它細(xì)菌種群的數(shù)量信息,當(dāng)細(xì)菌的密度達到一定閾值時,會通過啟動相關(guān)基因來調(diào)控細(xì)菌群落的密度水平[128].調(diào)控方式主要是依靠特定信號分子的濃度實現(xiàn)的,如N-?；呓z氨酸內(nèi)酯(AHLs).這些信號分子也參與了生物膜中抗性基因水平轉(zhuǎn)移的過程.但是不同種類的AHLs的調(diào)控作用存在差異,例如,C4-HSL信號分子會通過啟動AHL效應(yīng)器SdiA,抑制部分細(xì)菌的種間接合轉(zhuǎn)移[129];而另一項研究顯示C6-HSL信號分子對細(xì)菌E. coli HB101向E. coli NK5449的屬內(nèi)接合過程產(chǎn)生促進作用[130].因此微塑料生物膜中的群體效應(yīng)的信號分子如何調(diào)控抗性基因傳播的機制還需進一步的研究.

5 總結(jié)與展望

5.1 目前的研究已經(jīng)證實攜帶抗性基因的微生物能夠以微塑料為載體進入到不同營養(yǎng)級的生物體內(nèi),與腸道菌群發(fā)生相互作用,導(dǎo)致抗性基因的傳播、進化.未來還需要重點研究由微塑料攜帶的抗性基因、環(huán)境中的由浮游生物攜帶的抗性基因、以及生物腸道菌群內(nèi)的抗性基因的共有部分以及同源性,以揭示抗性基因在不同介質(zhì)中的擴散潛力以及進化速度.

5.2 目前關(guān)于大氣環(huán)境中微塑料攜帶的抗性基因方面的研究還較少,評估抗性基因與微塑料的聯(lián)合毒性效應(yīng)還存在較大難度,因此抗性基因隨氣溶膠中的微塑料進行長距離遷移的潛力亟需進一步探究,以便全面評估由吸入等途徑進入人體的微塑料所產(chǎn)生的人體健康效應(yīng).

5.3 當(dāng)前研究多關(guān)注抗性基因在微塑料表面的賦存情況,對于微塑料導(dǎo)致抗性基因水平轉(zhuǎn)移的頻率增加的機理與調(diào)控機制還尚未明晰,因此需要研究微塑料參與抗性基因水平轉(zhuǎn)移的各個步驟,從而對抗性基因的傳播、功能以及生態(tài)作用提供全面的概述,全面加深微塑料遷移過程對生態(tài)穩(wěn)定性的影響.

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Effects of microplastics on the spread and horizontal gene transfer of antibiotic resistance genes.

WU Xiao-jian1,2,3,SHI Guo-jing1,2,3,WANG Ying-ying1,2,3*

(1.College of Environmental Science and Engineering,Nankai University,Tianjin 300350,China；2.Key Laboratory Processes and Environmental Criteria (Ministry of Education),Tianjin 300350,China；3.Tianjin Key Laboratory of Environmental Remediation and Pollution Control,Tianjin 300350,China).,2022,42(8)：3957～3968

As an emerging pollutant,microplastics are ubiquitous in different types of environments. The antibiotic resistance genes (ARGs) carried by microplastic biofilms have adverse effects on human health,which has aroused wide concerns. However,the role of microplastics in the process of long migration and horizontal gene transfer of ARGs is still not clear. The environmental impacts of the resistance dissemination caused by microplastics also need further investigation. The current status of microplastic pollutants and the ARGs they carry was summarized. The pathways by which microplastics spread ARGs and the regulation of ARG transfer via microplastics are also compared and discussed. Future research perspectives are proposed to provide possible approaches for solving the problem of antibiotic resistance dissemination by microplastics.

microplastic；plastic；antibiotic resistance genes；plastisphere；pollution characteristics

X171.5

A

1000-6923(2022)08-3957-12

2022-01-11

國家自然科學(xué)基金委面上項目(31870485);天津市自然科學(xué)基金項目(19JCZDJC39600);天津市研究生科研創(chuàng)新項目(2019YJSB052)

* 責(zé)任作者,教授,wangyy@nankai.edu.cn

吳效儉(1995-),女,天津人,南開大學(xué)博士研究生.主要從事抗性基因的宏基因組、宏轉(zhuǎn)錄組檢測與定量分析相關(guān)研究.發(fā)表論文2篇.