細(xì)菌對(duì)有機(jī)污染物的趨化性及其對(duì)降解的影響

王慧，胡金星，秦智慧，徐新華，沈超峰*

工農(nóng)業(yè)生產(chǎn)的迅速發(fā)展給人類(lèi)生產(chǎn)和生活帶來(lái)了許多方便，同時(shí)，也使各種污染物被釋放到環(huán)境中。尤其是大量使用的環(huán)境異生物質(zhì)，因其潛在毒性強(qiáng)，具有生物富集、生物放大等特點(diǎn)，給全球生態(tài)環(huán)境和人類(lèi)健康帶來(lái)嚴(yán)重威脅[1]。對(duì)異生物質(zhì)污染治理刻不容緩，其治理手段主要有物理、化學(xué)和生物修復(fù)等。微生物修復(fù)方法是最具有前景的修復(fù)手段之一，具有安全、經(jīng)濟(jì)、二次污染少等特點(diǎn)。研究表明，環(huán)境中的大部分有機(jī)污染物可被微生物降解[2]。污染物的生物降解效率除了與降解菌本身的降解能力有關(guān)之外，還依賴(lài)于污染物的生物可利用性[3]。土壤污染物的生物可利用性除受土壤介質(zhì)、污染物性質(zhì)影響外，也受土壤微生物移動(dòng)性的影響[4]。大多數(shù)具有移動(dòng)性的細(xì)菌能通過(guò)趨化過(guò)程感知和尋找污染物[5-6]，具有運(yùn)動(dòng)和降解能力的細(xì)菌主動(dòng)移向吸附態(tài)污染物，可以提高污染物的生物可利用性和生物降解效率[4,7-9]。因此，研究者開(kāi)始關(guān)注細(xì)菌對(duì)環(huán)境污染物的趨化性。本文對(duì)細(xì)菌趨化性及趨化信號(hào)傳導(dǎo)機(jī)制、細(xì)菌對(duì)典型有機(jī)污染物的趨化、細(xì)菌趨化對(duì)降解的影響以及趨化與降解的分子關(guān)聯(lián)進(jìn)行了歸納、總結(jié)，以期為進(jìn)一步的研究提供思路。

1 細(xì)菌趨化性

趨化性是指具有運(yùn)動(dòng)能力的細(xì)菌對(duì)物質(zhì)化學(xué)濃度梯度做出的響應(yīng)，順濃度梯度遷移叫正趨化，逆濃度梯度遷移叫負(fù)趨化[10]。最早報(bào)道細(xì)菌趨化現(xiàn)象的是ENGELMANN和PFEFFER，隨后，ADLER[11]和HAZELBAUER[12]深入研究了大腸桿菌對(duì)氨基酸及糖類(lèi)物質(zhì)的趨化性機(jī)制。近年來(lái)，越來(lái)越多的研究者開(kāi)始從事細(xì)菌趨化性研究。

細(xì)菌能夠?qū)﹄S時(shí)改變的化學(xué)濃度梯度做出反應(yīng)，是因?yàn)榧?xì)菌細(xì)胞膜表面有專(zhuān)一的化學(xué)受體蛋白，細(xì)菌能夠利用這些受體蛋白感知外界刺激物信號(hào)[13]。以大腸桿菌為例（圖1），趨化反應(yīng)通過(guò)處于細(xì)胞兩極的受體復(fù)合體和隨機(jī)分布于細(xì)胞四周、埋于細(xì)胞膜中的鞭毛-馬達(dá)復(fù)合體調(diào)節(jié)。甲基趨化受體蛋白（methylaccepting chemotaxis proteins,MCPs）感知刺激物信號(hào)，信號(hào)通過(guò)趨化性組氨酸激酶A（chemotaxis histidine kinase A,CheA）和CheY傳遞給鞭毛馬達(dá)。CheA可發(fā)生自身磷酸化，從而使調(diào)節(jié)蛋白CheY磷酸化，磷酸化的CheY易與鞭毛馬達(dá)蛋白結(jié)合，調(diào)節(jié)鞭毛的旋轉(zhuǎn)方向。磷酸化的CheA同時(shí)調(diào)節(jié)甲基酯酶CheB的磷酸化，磷酸化的CheB和甲基轉(zhuǎn)移酶CheR分別調(diào)節(jié)MCPs的去甲基化和甲基化，使細(xì)菌適應(yīng)化學(xué)物質(zhì)濃度梯度，做出運(yùn)動(dòng)改變[13-15]。

2 細(xì)菌對(duì)典型有機(jī)污染物的趨化性

雖然細(xì)菌的趨化性在大腸桿菌中研究的比較深入，但是大腸桿菌主要對(duì)簡(jiǎn)單的氨基酸、糖類(lèi)等物質(zhì)具有趨化性。近幾十年的研究發(fā)現(xiàn)，細(xì)菌對(duì)很多化合物都具有趨化性（表1），趨化物主要包括萘[16]、甲苯[17-18]、聯(lián)苯[19-20]、多氯聯(lián)苯[19]、苯甲酸[19]、氯代苯甲酸[21]、硝基芳香化合物[22]、甲基對(duì)硫磷[23]、阿特拉津[24]、2，4-D[5]、呋喃類(lèi)化合物[25]等。涉及的細(xì)菌種屬主要有假單胞菌（Pseudomonas sp.）、羅爾斯通菌（Ralstonia sp.）、固氮螺菌（Azospirillum sp.）、根瘤菌（Rhizobium sp.）、伯克氏菌（Burkholderia sp.）和節(jié)桿菌（Arthrobacter sp.）等。細(xì)菌能以大部分趨化物為碳源和能源，在碳源和能源相對(duì)缺乏的環(huán)境中，細(xì)菌對(duì)環(huán)境異生物質(zhì)的趨化性無(wú)疑是有益于其生存的行為。

圖1 大腸桿菌趨化信號(hào)傳導(dǎo)機(jī)制Fig.1 Signal transduction of chemotaxis in Escherichia coli

表1 對(duì)環(huán)境污染物有趨化性的細(xì)菌Table 1 Chemotaxis to pollutants by bacteria

表1 (續(xù)) Continuation of Table 1

3 細(xì)菌趨化性對(duì)污染物降解的影響

有機(jī)污染物的生物可利用性被認(rèn)為是影響生物降解效率的主要限制因子之一。環(huán)境中的一些有機(jī)污染物水溶性較低，常常被吸附到固體顆粒表面。尤其在土壤環(huán)境里，高疏水性的有機(jī)污染物通常被吸附到土壤顆?；蛘叻撬嘁后w（nonaqueousphase liquid,NAPL）里，細(xì)菌難以接觸到污染物，生物可利用性低。趨化作用使降解菌有效感應(yīng)并靠近污染物，增強(qiáng)污染物的生物可利用性，提高生物降解效率，在污染物的微生物降解過(guò)程中發(fā)揮著重要作用[7]。

大量研究表明，細(xì)菌趨化性能通過(guò)各種機(jī)制提高污染物的生物可利用性，其中，惡臭假單胞菌（Pseudomonas putida）G7對(duì)萘的趨化性及其對(duì)降解的影響研究比較深入。早在1999年，GRIMM等[26]研究發(fā)現(xiàn)萘的受體蛋白NahY由代謝質(zhì)粒編碼時(shí)，就猜測(cè)細(xì)菌的趨化可能提高生物降解效率。MARX等[40]的研究證明了這一猜想，他們通過(guò)比較野生菌株P(guān)seudomonas putida G7、趨化突變株和移動(dòng)突變株對(duì)萘的降解，發(fā)現(xiàn)野生菌株的降解速率最快，如果要達(dá)到相同的降解效果，突變株需要更長(zhǎng)的降解時(shí)間。LAW等[41]在非水相液體2，2，4，4，6，8，8-七甲基壬烷中也證明了野生型菌株P(guān)seudomonas putida G7的趨化性能明顯提高萘的降解效率。

除了液相體系，在土壤等高異質(zhì)性介質(zhì)里，細(xì)菌的趨化性在生物降解中也發(fā)揮著重要作用。Pseudomonas putida DLL-1是甲基對(duì)硫磷的高效降解菌株[42]，該菌株對(duì)甲基對(duì)硫磷也具有趨化性[23]。通過(guò)基因打靶使Pseudomonas putida DLL-1染色體上單拷貝的cheA基因失活，成功獲得與野生菌株生長(zhǎng)沒(méi)有顯著差異的趨化突變株P(guān)seudomonas putida DAK。cheA基因的失活中斷了整個(gè)趨化信號(hào)傳導(dǎo)，使菌株DAK喪失了對(duì)甲基對(duì)硫磷的趨化性。在搖瓶實(shí)驗(yàn)中，野生和突變株對(duì)甲基對(duì)硫磷的降解效率幾乎沒(méi)有差異，說(shuō)明在液態(tài)環(huán)境中，菌體與污染物充分接觸對(duì)降解影響不大。但是，在土培實(shí)驗(yàn)中，趨化突變株DAK對(duì)甲基對(duì)硫磷的降解效率比野生菌株DLL-1的降解效率低約20%～30%[23]，揭示出在原位條件下，趨化性在生物降解過(guò)程中具有重要作用。PAUL等[43]以土壤為介質(zhì)，采用能降解對(duì)硝基苯酚但無(wú)趨化性的洋蔥伯克霍爾德菌（Burkholderia cepacia）RKJ200作為陰性對(duì)照，發(fā)現(xiàn)Ralstonia sp.SJ98對(duì)對(duì)硝基酚具有趨化性，并且有明顯的降解現(xiàn)象，而B(niǎo)urkholderia cepacia RKJ200則無(wú)明顯的降解現(xiàn)象。他們的研究均證明了在土壤介質(zhì)里細(xì)菌對(duì)污染物的趨化，說(shuō)明細(xì)菌趨化性確實(shí)能提高土壤和沉積物里污染物的生物降解效率。

生物膜是細(xì)菌細(xì)胞附著在非生物或者生物機(jī)體表面、并由自身產(chǎn)生的胞外多聚物（extracellular polymeric substance,EPS）包裹所形成的多細(xì)胞聚合體[44]，是細(xì)菌在自然界中常見(jiàn)的存在形式，使細(xì)菌在不利環(huán)境中生存，抵抗外界不利條件。生物膜在污水處理、原位污染治理與生態(tài)修復(fù)等過(guò)程中發(fā)揮重要作用[45-46]。研究表明，趨化性在生物膜的形成過(guò)程中發(fā)揮重要作用[47-49]。細(xì)菌通過(guò)趨化作用感知環(huán)境中的營(yíng)養(yǎng)物質(zhì)，通過(guò)鞭毛、菌毛黏附在附著物表面，趨化運(yùn)動(dòng)使細(xì)菌沿著附著物表面生長(zhǎng)，膜不斷擴(kuò)散開(kāi)并加厚，最后成熟脫落，再形成新的生物膜[45,50-51]。因此，趨化性可能通過(guò)影響生物膜的形成，進(jìn)而影響污染物的生物降解效率。

大量研究證明，趨化性可以提高污染物的生物可利用性，在污染物的降解，尤其是在原位修復(fù)過(guò)程中發(fā)揮重要作用。細(xì)菌趨化性對(duì)降解的影響表明，趨化性可能是降解的重要屬性之一。

4 細(xì)菌趨化與降解的內(nèi)在關(guān)聯(lián)

從以上分析可以看出，趨化可促進(jìn)降解，反過(guò)來(lái)，降解對(duì)趨化也有影響。研究降解對(duì)趨化的影響有助于進(jìn)一步揭示趨化與降解的內(nèi)在聯(lián)系。從細(xì)菌對(duì)污染物趨化性的表觀現(xiàn)象可以發(fā)現(xiàn)，很多細(xì)菌只對(duì)能降解的底物具有趨化性，而且有些趨化需要底物誘導(dǎo)。Ralstonia sp.SJ98能降解某些硝基芳香族化合物（nitroaromatic compounds,NACs）；且該菌株只對(duì)能降解的硝基芳香族化合物有趨化性，而對(duì)不能降解的硝基芳香族化合物則無(wú)趨化性[22]。Pseudomonas sp.JHN也只對(duì)能代謝的4-氯-2-硝基酚具有趨化性，對(duì)不能代謝的底物沒(méi)有趨化性[33]。氯代苯甲酸是多氯聯(lián)苯在好氧降解過(guò)程中易積累的中間代謝產(chǎn)物，該類(lèi)化合物能通過(guò)抑制降解菌的生長(zhǎng)而影響多氯聯(lián)苯的降解效率[52-53]。GORDILLO等[19]的研究表明，經(jīng)聯(lián)苯或苯甲酸誘導(dǎo)的聯(lián)苯降解菌Pseudomonas sp.B4對(duì)苯甲酸具有趨化作用，經(jīng)苯甲酸誘導(dǎo)的B4對(duì)4-氯代苯甲酸具有趨化作用，說(shuō)明Pseudomonas sp.B4對(duì)這些底物的趨化屬于誘導(dǎo)趨化。TREMAROLI等[21]的研究也表明，類(lèi)產(chǎn)堿假單胞菌（Pseudomonas pseudoalcaligenes）KF707對(duì)苯甲酸及氯代苯甲酸的趨化也屬于誘導(dǎo)趨化。

某些降解菌的趨化基因位于代謝質(zhì)粒上，趨化和代謝基因共轉(zhuǎn)錄。如：Pseudomonas putida G7對(duì)萘的趨化受體蛋白NahY由代謝質(zhì)粒NAH7編碼，由nahY和降解基因共轉(zhuǎn)錄，且同受轉(zhuǎn)錄調(diào)控因子NahR的調(diào)控[26]。Pseudomonas sp.ZWL73是4-氯硝苯基的完全降解菌，其降解由質(zhì)?？刂?。Pseudomonas putia PaW 340不能利用也不能轉(zhuǎn)化4-氯硝基苯，而菌株ZP8是將ZWL73的質(zhì)粒轉(zhuǎn)入無(wú)質(zhì)粒的Pseudomonas putia PaW 340得到的質(zhì)粒轉(zhuǎn)移接合子，該菌獲得了以4-氯硝基苯為唯一碳、氮源及能源生長(zhǎng)的能力。ZP8和ZWL73對(duì)受試化合物具有相同趨化譜，說(shuō)明ZWL73對(duì)4-氯硝基苯的趨化基因也由其質(zhì)粒編碼[32]。

研究發(fā)現(xiàn)，某些細(xì)菌趨化、轉(zhuǎn)運(yùn)和代謝之間有內(nèi)在聯(lián)系。LUU等[54]證明了Pseudomonas putida F1對(duì)4-羥基苯甲酸的趨化受體基因pcaY的表達(dá)受轉(zhuǎn)錄激活因子PcaR的調(diào)控，同時(shí)需要4-羥基苯甲酸代謝中間產(chǎn)物β-酮己二酸的誘導(dǎo)，并且和代謝基因共轉(zhuǎn)錄。該研究還發(fā)現(xiàn)，超家族轉(zhuǎn)運(yùn)蛋白PcaK能通過(guò)促進(jìn)4-羥基苯甲酸的吸收，累積大量的β-酮己二酸，誘導(dǎo)PcaR對(duì)pcaY的調(diào)控，從而間接影響pcaY的表達(dá)（圖2A）。羅爾斯通菌（Ralstonia eutropha）JMP134含有自主轉(zhuǎn)移質(zhì)粒pJP4，該質(zhì)粒編碼2，4-D的降解基因族tfd。研究發(fā)現(xiàn)，對(duì)質(zhì)粒pJP4的改造導(dǎo)致該菌株喪失了對(duì)2，4-D的趨化性，說(shuō)明菌株JMP134對(duì)2，4-D的趨化基因也由該質(zhì)粒編碼。tfd基因簇同時(shí)編碼2，4-D的轉(zhuǎn)運(yùn)蛋白TfdK，且tfdK缺失突變株對(duì)2，4-D沒(méi)有趨化現(xiàn)象[5]，因此推測(cè)TfdK可能間接影響JMP134對(duì)2，4-D的趨化?？梢?jiàn)，細(xì)菌對(duì)物質(zhì)的轉(zhuǎn)運(yùn)、趨化和降解基因之間的協(xié)調(diào)表達(dá)，可能是細(xì)菌選擇最佳生存環(huán)境的一般機(jī)制，也體現(xiàn)了細(xì)菌趨化和降解的分子關(guān)聯(lián)。

雖然鞭毛介導(dǎo)的細(xì)菌趨化信號(hào)傳導(dǎo)的核心機(jī)制相對(duì)保守，但是負(fù)責(zé)感受趨化信號(hào)的甲基趨化受體蛋白在不同種屬間差異較大。在某些情況下，受體蛋白不能直接感知趨化物，而是通過(guò)感知代謝中間產(chǎn)物實(shí)現(xiàn)趨化過(guò)程。NI等[39,55]研究發(fā)現(xiàn)，菌株睪丸酮叢毛單胞菌（Comamonas testosteronei）CNB-1的甲基趨化受體蛋白MCP2201和MCP2983不能直接與芳環(huán)類(lèi)化合物結(jié)合，而是與三羧酸循環(huán)中間產(chǎn)物結(jié)合（圖2B）。他們的研究揭示了一類(lèi)新的趨化途徑，即細(xì)菌可以通過(guò)感知三羧酸循環(huán)中間化合物實(shí)現(xiàn)對(duì)芳香類(lèi)化合物的趨化。代謝突變株對(duì)芳環(huán)類(lèi)化合物沒(méi)有趨化性，說(shuō)明CNB-1對(duì)芳環(huán)類(lèi)化合物的趨化屬于代謝依賴(lài)趨化，這更深入地揭示了降解和趨化之間的內(nèi)在關(guān)聯(lián)。

5 結(jié)語(yǔ)

關(guān)于細(xì)菌趨化性與降解性之間關(guān)系的研究備受關(guān)注，目前的研究主要集中在趨化現(xiàn)象的表征上，很多細(xì)菌對(duì)污染物的探測(cè)機(jī)制尚不清楚，包括化學(xué)受體的鑒定、反應(yīng)調(diào)控以及趨化、降解和轉(zhuǎn)運(yùn)之間的關(guān)系等研究相對(duì)較少。深入研究細(xì)菌對(duì)污染物的趨化機(jī)制，闡明趨化、轉(zhuǎn)運(yùn)和降解之間的分子關(guān)聯(lián)，對(duì)于提高污染物的生物降解效率有重要意義。本文從以下幾個(gè)方面提出展望：

1）趨化受體蛋白作為細(xì)菌感應(yīng)外界刺激信號(hào)的關(guān)鍵分子，到目前為止，只有少部分趨化受體蛋白被發(fā)現(xiàn)與細(xì)菌對(duì)污染物的趨化相關(guān)，多數(shù)受體蛋白對(duì)污染物的探測(cè)機(jī)制依然未知。因此，以后的研究工作仍需繼續(xù)鑒別細(xì)菌對(duì)污染物的趨化受體蛋白，以闡明細(xì)菌對(duì)污染物的感應(yīng)機(jī)制。

2）研究者雖然進(jìn)行了大量的趨化研究，但大多是在半固體培養(yǎng)基里進(jìn)行的，在土壤等實(shí)際固體環(huán)境中研究趨化性的報(bào)道鮮見(jiàn)。今后應(yīng)在實(shí)際污染的土壤環(huán)境中進(jìn)行更多的實(shí)驗(yàn)，利用生化與分子手段評(píng)估降解菌是否完全適應(yīng)脅迫環(huán)境。明確細(xì)菌對(duì)污染物的探測(cè)機(jī)制，有目的地調(diào)控細(xì)菌對(duì)污染物的趨化行為，對(duì)污染土壤的生物修復(fù)具有重要意義。

3）目前研究較多的是鞭毛介導(dǎo)的運(yùn)動(dòng)與趨化行為，除此之外，也有細(xì)菌借助菌毛的伸縮在固體表面進(jìn)行蹭行運(yùn)動(dòng)[56-59]及黏性物質(zhì)介導(dǎo)的滑行運(yùn)動(dòng)[60-70]等。但在實(shí)際污染物環(huán)境里，無(wú)鞭毛降解菌占有一定的生境[71-73]，它們?nèi)绾慰朔袡C(jī)污染物的高疏水性和低生物可利用性，如何與污染物分子相互接觸，及其在土壤環(huán)境中的分散機(jī)制等是否與趨化性相關(guān)，目前尚不清楚。因此，有必要進(jìn)行更多降解菌運(yùn)動(dòng)模式與趨化機(jī)制的研究。

圖2 Pseudomonas putida F1(A)和Comamonas testosteronei CNB-1(B)對(duì)4-羥基苯甲酸趨化信號(hào)探測(cè)示意圖Fig.2 Schematic representation of 4-hydroxybenzoate chemotaxis in Pseudomonas putida F1(A)and Comamonas testosteronei CNB-1(B)

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