陳華美 劉四新 李從發(fā)
摘 要 細(xì)菌纖維素是一種由微生物合成的新型納米材料,具有化學(xué)純度高、持水性好、楊氏模量高、良好的生物相容性和可降解性等特性,廣泛應(yīng)用于食品、醫(yī)療、化工等領(lǐng)域。但發(fā)酵工藝落后、產(chǎn)量低、生產(chǎn)成本高一直是限制其應(yīng)用于高附加值產(chǎn)品生產(chǎn)的瓶頸。本文概述纖維素產(chǎn)生菌及分離改良、發(fā)酵培養(yǎng)基優(yōu)化、發(fā)酵條件對纖維素合成和產(chǎn)量的影響,以及發(fā)酵方式對BC產(chǎn)量和性能影響等方面的研究進(jìn)展,并對BC在生產(chǎn)中的不足和未來應(yīng)用前景進(jìn)行展望,為開展更深入的研究和實(shí)際生產(chǎn)提供一定借鑒。
關(guān)鍵詞 細(xì)菌纖維素;生物合成;發(fā)酵;研究進(jìn)展
中圖分類號(hào) O636.11 文獻(xiàn)標(biāo)識(shí)碼 A
Abstract Bacterial cellulose(BC)is a new type of nano material synthesized by some microorganisms, with high chemical purity, good water holding capacity, high Youngs modulus, good biocompatibility and biodegradability and so on, widely used in food, medical, chemical and other fields. But the backward fermentation technology, low the yield of BC and high production costs have been to limit its application in the production of high value-added products. The research progress of bacteria producing cellulose and isolation, improved strains, optimization of fermentation culture medium, the influence of fermentation conditions on cellulose synthesis and yield, and the effect of fermentation on the yield and properties of BC were summarized, and the shortage of BC in production and its application prospect in the future are forecasted, which will provide some references for the further research and practical production.
Key words Bacterial cellulose; Biosynthesis; Fermentation; Advances
doi 10.3969/j.issn.1000-2561.2016.08.031
細(xì)菌纖維素(Bacterial cellulose,BC),是一種由微生物產(chǎn)生的高純度三維網(wǎng)狀多聚物。因其持水性好、楊氏模量高[1],被廣泛應(yīng)用于食品、化妝品、造紙、音響和光學(xué)等方面[2-7]。改良后的BC具有無毒、親水性強(qiáng)[8]、吸附性好、生物可降解性[9]等特性,應(yīng)用于醫(yī)療行業(yè)。研究發(fā)現(xiàn),其在腳手架組織和替代皮膚組織(如軟骨、骨軟組織血管和角膜)等方面應(yīng)用時(shí),可吸收藥物并控制釋放[10-11]。因其流變學(xué)特性,可形成水凝膠,廣泛用于食品工業(yè),稱為Nata,可作為甜點(diǎn)[12];食物和飲料中,作增稠劑、穩(wěn)定劑和紋理改良劑控制多功能食品的性質(zhì)[13];還因可食性和生物降解特性,BC可用作食品包裝材料[14]。并且高強(qiáng)度紙制品、聲音振動(dòng)膜、人工皮膚、創(chuàng)口貼、繃帶、紗布、面膜、貢丸等BC產(chǎn)品已經(jīng)用于商品化生產(chǎn),在其他許多方面也具有廣泛的商業(yè)化應(yīng)用潛力。其生產(chǎn)不依賴于地理氣候條件,控制生產(chǎn)要素就能提高產(chǎn)率;其純度高,不含木質(zhì)素或其他污染物,可避免凈化的能耗[15];可通過改良菌株的基因來生產(chǎn)具有所需特性的纖維素;農(nóng)業(yè)和工業(yè)廢物可以用作培養(yǎng)原料以節(jié)約經(jīng)濟(jì)成本。然而,產(chǎn)量低、成本高,仍是BC實(shí)現(xiàn)生產(chǎn)規(guī)模化、產(chǎn)業(yè)化的一個(gè)瓶頸。
基于此現(xiàn)狀,將BC合成及發(fā)酵方面的研究報(bào)道進(jìn)行分析,擬通過從BC的產(chǎn)生菌及分離改良、發(fā)酵培養(yǎng)基優(yōu)化、發(fā)酵條件對BC合成和產(chǎn)量影響以及發(fā)酵方式對BC產(chǎn)量和性能影響等方面進(jìn)行綜述,旨在為開展更進(jìn)一步的研究做好鋪墊,為實(shí)際生產(chǎn)提供一定借鑒。
1 細(xì)菌纖維素的產(chǎn)生菌
早在1886年,英國科學(xué)家Brown就發(fā)現(xiàn),酸醋桿菌靜置培養(yǎng)時(shí),發(fā)酵液的氣-液表面會(huì)形成一層白色的凝膠狀薄膜,經(jīng)化學(xué)分析,確定其成分是纖維素[16-17]。隨后,許多微生物被報(bào)道能產(chǎn)BC,主要涉及醋桿菌屬(Acetobacter)、土壤桿菌屬(Agrobaeterium)、無色桿菌屬(Achromobacrer)、沙門氏菌屬(Salmonella)、腸桿菌屬(Enterobacter)、埃希氏菌屬(Escherichia)、假單胞菌屬(Pseudomonas)、葡糖醋桿菌屬(Glucoacetobacter)、駒形氏桿菌屬(Komagataeibacter)等17個(gè)屬[18-20],以駒形氏桿菌屬的報(bào)道最多,也最深入[21-22]。截止目前,已報(bào)道該屬中有纖維素合成能力的菌種14個(gè)[23],如K. xylinus、K. nataicola、K. rhaeticus、K. europaeus、K. swingsii、K. hansenii等[24-25]。其中,K. xylinus是最早作為BC合成機(jī)理、代謝調(diào)控等理論研究的模式菌種,也是商業(yè)化生產(chǎn)和應(yīng)用開發(fā)最常用的菌種[26]。
2 菌種的分離和改良
盡管能夠產(chǎn)BC的種屬和菌株很多,但自身合成BC能力差異很大,總體來說產(chǎn)BC的能力不高,遠(yuǎn)不能滿足實(shí)際生產(chǎn)應(yīng)用的需要。目前,對K. xylinus的研究比較全面,但一般的菌株產(chǎn)量較低、多次傳代后極易衰退而且生產(chǎn)性能不穩(wěn)定。因此,選育穩(wěn)定高產(chǎn)的BC產(chǎn)生菌株非常有必要?,F(xiàn)主要從篩選野生穩(wěn)定高產(chǎn)菌株、物理化學(xué)生物等方法誘變育種、基因工程法改良育種三方面研究,以獲得既能滿足生產(chǎn)需要又能降低生產(chǎn)成本的優(yōu)良菌株。
2.1 野生高產(chǎn)菌株的分離
BC的產(chǎn)生菌株主要從天然資源中分離得到,然后通過傳統(tǒng)的馴化方法將其改良。Kim等[27]從果蔬、酸菜、酸米酒以及菜園土等150個(gè)樣品中分離到26株,其中醋酸桿菌屬(Acetobacter pasteutinus subsp. xylinum)產(chǎn)量最高可達(dá)14 g/L,還有一株A. hansenii的BC產(chǎn)量更高(16 g/L)。周伶俐等[28]從殘次水果中篩選出A. xylinum NUST4通過紫外燈照射的物理方法誘變育種,不但產(chǎn)量高(靜態(tài)產(chǎn)量為10.99 g/L)而且生產(chǎn)性能穩(wěn)定。
另一些學(xué)者從傳統(tǒng)生產(chǎn)中分離出產(chǎn)BC的菌株。馮勁等[29]從紅茶菌液里篩選出一株BC產(chǎn)生菌株,經(jīng)鑒定是中間葡糖酸醋桿菌(G. intermedius),產(chǎn)生纖維素I型晶體,其純度達(dá)91.32%,濕膜含水率達(dá)99.16%,每克干膜能吸28.59 g水。蘇俊霞等[30]在傳統(tǒng)固態(tài)發(fā)酵食醋醋醅中分離得到5株產(chǎn)BC的菌株,經(jīng)鑒定均屬于G. intermedius,只有一株BC產(chǎn)量較高,通過對其培養(yǎng)條件(溫度、培養(yǎng)時(shí)間、碳源、初始pH)優(yōu)化,BC的產(chǎn)量可從3.90 g/L增加至7.90 g/L。王雪奇等[31]從黃酒和市售紅茶菌中篩選和鑒定,得到2株產(chǎn)BC的菌株,在其最適生長pH(4~5、5~6)時(shí),BC產(chǎn)量分別是2.0和1.7 g/L。
2.2 菌株的改良
自然界中篩選的菌株,往往產(chǎn)BC能力較差,為獲得穩(wěn)定高產(chǎn)的菌株,通常采用一系列傳統(tǒng)手段及基因工程方法對其進(jìn)行改良。
2.2.1 傳統(tǒng)方法改良 對于纖維素生產(chǎn)菌株而言,現(xiàn)已報(bào)道的傳統(tǒng)改良方法主要有物理、化學(xué)等三方面誘變育種。
物理:搖瓶培養(yǎng)時(shí),通常使纖維素合成陰性菌株(Cel-)產(chǎn)生致BC產(chǎn)量減少;換回靜態(tài)培養(yǎng)時(shí),突變體產(chǎn)BC的能力亦可恢復(fù)[32]。Ayd1n等[33]發(fā)現(xiàn),攪拌型反應(yīng)器中能產(chǎn)生突變體,其BC產(chǎn)率和產(chǎn)量增加到3.25 g/L和17.20%。靜水高壓處理也可得到突變體[34],F(xiàn)eng等[35]用該法將G. hansenii產(chǎn)BC的量增加到7.02 g/L。另外,研究紫外線誘變的學(xué)者也比較多,但通常將其與化學(xué)試劑進(jìn)行復(fù)合誘變。Hungund等[36]用紫外輻射和甲基磺酸乙酯(EMS)對G. xylinus NCIM 2526進(jìn)行改良,經(jīng)紫外輻射后獲得3株BC產(chǎn)量較高的突變體,其中GHUV4產(chǎn)量最高(3.92 g/L),比野生型提高了30%;對其進(jìn)行甲基磺酸乙酯(EMS)處理,得到的突變體(GHEM4)產(chǎn)BC的量(5.96 g/L)比親本和野生型產(chǎn)量分別多50%、98%。
化學(xué):用亞硝基胍(NTG)、硫酸二乙酯(DS)、甲基磺酸乙酯(EMS)等化學(xué)物質(zhì)對BC產(chǎn)生菌進(jìn)行誘變。Premjet等[37]將野生型菌體(NU4)經(jīng)亞硝基胍誘變得到突變體(NU4-NTG30-51),BC產(chǎn)量分別是對照菌(A. xylinum ATCC 10245)和親本菌株的54.68%和43.69%;再將NU4-NTG30-51進(jìn)行紫外誘變得到突變體(NU4-UV40-07),產(chǎn)量分別是其50.59%和39.60%,且后者的結(jié)晶度比前者低。鄧毛程等[38]將紫外線和硫酸二乙酯對木葡糖酸醋桿菌進(jìn)行復(fù)合誘變,得到一株遺傳性穩(wěn)定的突變菌株,BC產(chǎn)量達(dá)15.6 g/L,比親本菌株產(chǎn)量提高44.4%。
2.2.2 基因工程法改良 Deng等[39]利用Tn5轉(zhuǎn)座子對G. hansenii ATCC 23769進(jìn)行插入突變研究,得到6個(gè)不產(chǎn)纖維素的突變菌株。同時(shí),可利用轉(zhuǎn)座子對細(xì)菌的一些代謝旁路進(jìn)行突變,比如葡萄糖酸和Acetan的代謝途徑等,這也有效提高BC產(chǎn)量的方法。Kuo等[40]敲出K. xylinus體中的葡萄糖脫氫酶(GDH)基因獲得不產(chǎn)葡萄糖酸的突變體,以增加BC的產(chǎn)量。John等[41]在K. hansenii細(xì)胞內(nèi)提取異源二聚體-acsb ACSA,蛋白經(jīng)純化、翻譯后加工,形成活性異源二聚體-acsb ACSA。最后經(jīng)誘變作用,研究CSC酶 AcsC、AcsD和CcAx三個(gè)亞基的作用。但目前用基因工程法改良纖維素產(chǎn)生菌還未獲得真正的基因構(gòu)建菌[42-43]。
3 發(fā)酵培養(yǎng)基的優(yōu)化
培養(yǎng)基類型和組成對微生物生長和代謝產(chǎn)物的積累影響很大。對于BC生產(chǎn),不同菌株的營養(yǎng)要求不盡相同,但對已經(jīng)或能夠用于BC大規(guī)模生產(chǎn)的菌種而言,有一些基本的規(guī)律可尋。
3.1 碳源優(yōu)化
BC的合成是一個(gè)受多種酶共同調(diào)控、耗能的復(fù)雜代謝途徑,培養(yǎng)基組成的不同對生產(chǎn)菌株生長、BC合成、副產(chǎn)物積累有較大影響。由于碳源直接影響B(tài)C的合成,因此研究報(bào)道較多。
Chao等[44]考察了A. xylinum BPR2001在不同濃度果糖中合成BC的能力,得到果糖濃度60~70 g/L時(shí),合成BC的產(chǎn)量最高,達(dá)10.4 g/L。Mckenna等[45]用葡萄糖、甘露醇、甘油、果糖、蔗糖、半乳糖充當(dāng)HS培養(yǎng)基中的碳源培養(yǎng)G. xylinus ATCC 53524發(fā)現(xiàn),其較偏愛于蔗糖和甘油,產(chǎn)量分別可達(dá)3.83和3.75 g/L。同時(shí)還發(fā)現(xiàn),碳源種類不能影響B(tài)C的結(jié)構(gòu)特性。Mohammadkazemi等[46]利用糖漿、葡萄糖、蔗糖、食品級(jí)蔗糖等替換掉HS、Yamanaka(Y)、Zhou(Z)三種培養(yǎng)基中的碳源,發(fā)現(xiàn)糖漿和食品級(jí)蔗糖不適合K. xylinus PTCC 1734生長,且合成BC的量也較差。同時(shí),Dayal[47]、Santos[48]、Liu[49]等也對培養(yǎng)基的組成進(jìn)行了報(bào)道。
實(shí)際生產(chǎn)中,單一碳源或培養(yǎng)基糖轉(zhuǎn)換率低、生產(chǎn)成本較高。因此,不少學(xué)者還對尋找廉價(jià)的原料作為BC的生產(chǎn)培養(yǎng)基進(jìn)行研究,如使用農(nóng)業(yè)[50-51]和工業(yè)廢物[52-53]為原料,其中玉米漿[54-55]、糖蜜[56-58]、醋[59-60]、果皮[61-62]、果汁[63-64]、小麥秸稈酸水解[65]和玉米芯水解液[66]已報(bào)道。Algar等[67]使用工業(yè)菠蘿殘?jiān)鼇戆l(fā)酵G. medellinensis產(chǎn)BC。Gomes等[68]使用橄欖油殘?jiān)鼇砼囵B(yǎng)G. sacchari。Huang等先后利用脂質(zhì)廢水[69]、玉米芯酸水解液[66]培養(yǎng)K. xylinus CH001,Bilgi使用工業(yè)角豆和扁豆廢液培養(yǎng)K. xylinus產(chǎn)BC[70]。
3.2 氮源
菌株的生長需要特定的氮源以滿足菌體繁殖對核苷酸、氨基酸等的需求。酵母提取物和蛋白胨是BC生產(chǎn)中最常用的氮源,因?yàn)樗麄兡軌驗(yàn)樯a(chǎn)菌株提供氮源和生長因子。因其成本較高,由此一些學(xué)者正努力尋找合適的替代物。Noro等[71]發(fā)現(xiàn)玉米漿(Corn steep liquor,CSL)是最有效的培養(yǎng)基,并在CSL中添加乳酸和蛋氨酸效果會(huì)更好,CSL可以對pH有緩沖作用,可將生產(chǎn)過程中pH控制在最佳范圍。Jung等[72]利用糖蜜做碳源和玉米漿做氮源來生產(chǎn)BC。與HS培養(yǎng)基相比,BC產(chǎn)量從1.53 g/L提高至3.12 g/L。李飛等[55]將玉米漿作氮源時(shí),BC的產(chǎn)量為9.2 g/L,其成本只是對照組的15%。
3.3 生長因子
除碳源、氮源等主要營養(yǎng)成分外,生長因子對BC合成也有較大影響。
早在1980年,Gosselé等[73]就對95株Gluconobacter sp.所需生長因子的狀況進(jìn)行了調(diào)查,表明有58%只缺泛酸,28%缺泛酸和煙酸,6%的菌株除了缺泛酸和煙酸外,還缺維生素B1。一些成分如膽堿衍生物、甜菜堿、脂肪酸(鹽和酯類)以及一些氨基酸如蛋氨酸、谷氨酸等會(huì)影響菌株產(chǎn)BC的能力[74]。Lin等[75]用啤酒廢酵母液考察K. hansenii CGMCC 3917生產(chǎn)纖維素的能力。常冬妹等[76]研究不同濃度煙酸和生物素對A. xylinum靜態(tài)培養(yǎng)產(chǎn)BC的影響,發(fā)現(xiàn)加煙酸濃度為1 mg/L時(shí),產(chǎn)BC的量為2.842 g/L,是對照的1.88倍;而添加25 mg/L的生物素,BC產(chǎn)量(3.118 g/L)為對照的2.06倍。
3.4 金屬離子
Christen和Julien等[77-78]指出,金屬離子對BC的合成有也影響。Mg2+可以促進(jìn)二鳥苷酸環(huán)化酶活性,間接影響纖維素合酶的活性。磷酸二酯酶催化c-di-GMP分解為pGpG的反應(yīng),Mg2+、Mn2+和Co2+可以促進(jìn)其催化活性,但Ca2+、Fe2+和Ni+起抑制作用,間接影響纖維素合酶的活性。此外,細(xì)菌細(xì)胞的新陳代謝還取決于對氧的利用,氧氣、二氧化碳分壓同樣會(huì)影響B(tài)C產(chǎn)率[79],因此,培養(yǎng)過程中通氣很重要[80]。
3.5 其他添加物
除原料外,不少學(xué)者還探索通過在培養(yǎng)基中添加其它物質(zhì)來增加BC產(chǎn)量,如乙醇[81-82],VC[83]、有機(jī)酸[74]、水溶性多糖[72]和木素磺酸鹽[84]等。其中,以添加乙醇的研究最多、促進(jìn)效果也明顯。乙醇可以從多方面影響B(tài)C生物合成,可為連續(xù)發(fā)酵合成BC過程中提供能量[85];還可抑制磷酸轉(zhuǎn)乙酰酶活性,使碳流經(jīng)TCA循環(huán)通量和Pta-Ack途徑流量減少[86]。Keshka等[83]報(bào)道了VC對4株G. xylinus產(chǎn)BC的量和晶體結(jié)構(gòu)的影響。因VC具有抗氧化性,可降低菌株產(chǎn)葡萄糖酸的能力,因此當(dāng)其濃度為0.5%(w/w)時(shí)所有菌株產(chǎn)BC的量都增加,平均為0.016 g/mL,是對照組的2倍。
4 發(fā)酵條件對纖維素合成和產(chǎn)量的影響
培養(yǎng)基的初始pH值、溫度、氧濃度[87]、菌種狀態(tài)(種齡、接種量等)及轉(zhuǎn)速等發(fā)酵參數(shù)都能影響B(tài)C產(chǎn)量和結(jié)構(gòu)性質(zhì)。Jagannath等[88]指出,以椰子水為原料靜態(tài)發(fā)酵產(chǎn)BC時(shí)pH對其厚度有影響。pH3.5時(shí),即使培養(yǎng)20 d也無明顯的BC膜出現(xiàn);pH4.0時(shí),可得最大厚度10.2 mm的BC膜。同時(shí),許多研究報(bào)道表明,菌株產(chǎn)BC的最適溫度范圍為28~30 ℃,其不僅影響B(tài)C產(chǎn)量,還對BC性質(zhì)(聚合度和親水能力)有影響。30 ℃條件下的BC比25 ℃和35 ℃的聚合度低(約10 000)和親水能力高(約164%)。
種子液培養(yǎng)條件應(yīng)與BC生產(chǎn)發(fā)酵相對應(yīng),若用靜態(tài)發(fā)酵方式生產(chǎn),二級(jí)種子通過靜態(tài)培養(yǎng)獲得,而動(dòng)態(tài)發(fā)酵方式生產(chǎn)BC,二級(jí)種子相應(yīng)為動(dòng)態(tài)培養(yǎng)。Hu等[89]研究了不同轉(zhuǎn)速對A. xylinum JCM 9730(ATCC 700178)對產(chǎn)BC形貌的影響。150 mL的三角瓶盛100 mL HS培養(yǎng)基,當(dāng)轉(zhuǎn)速為150 r/min時(shí),能產(chǎn)生球形BC顆粒,直徑大小為10 mm;轉(zhuǎn)速為200 r/min時(shí),產(chǎn)生的顆粒直徑大小為7~8 mm。
5 發(fā)酵方式對纖維素產(chǎn)量和性能的影響
BC的培養(yǎng)方式主要有靜置法和動(dòng)態(tài)法(搖瓶、攪拌或者氣升式),各具特色。靜置培養(yǎng)能更好地保持BC的正常形態(tài)[90],能產(chǎn)出均勻光滑的BC產(chǎn)品。這種方法需要更多的生產(chǎn)空間和勞動(dòng)力,規(guī)?;a(chǎn)成本高。動(dòng)態(tài)培養(yǎng)主要有振蕩器、搖床、攪拌式發(fā)酵罐和氣升式發(fā)酵罐等,產(chǎn)生BC的形貌差異較大,有絲狀、球形、星狀、絮狀或團(tuán)塊狀等。但攪拌時(shí)需空間和勞動(dòng)力少,因此減少了工業(yè)生產(chǎn)成本。然而,一些傳統(tǒng)的方法如震動(dòng)玻璃瓶和攪拌棒的利用,可以誘導(dǎo)產(chǎn)突變菌株,使BC產(chǎn)量降低[91]。同時(shí),BC產(chǎn)品易吸附于反應(yīng)器的軸上,使其很難收集,并且不易清理[92]。為緩解此問題,近日,一些學(xué)者在動(dòng)態(tài)培養(yǎng)條件下探索設(shè)計(jì)更有效的BC生產(chǎn)反應(yīng)器,其中,以球泡罩塔型生物反應(yīng)器[93]、氣升式反應(yīng)器[94]和改良后氣升式反應(yīng)器罐[87]尤為著名。Cheng等[92]使用羧甲基纖維素(CMC)塑料復(fù)合生物膜反應(yīng)器用于BC生產(chǎn),以便隨時(shí)采樣和連續(xù)生產(chǎn)。
另一方面,有的反應(yīng)裝置可在“相對靜止”條件下發(fā)酵生產(chǎn)片狀和膜狀的BC,如旋轉(zhuǎn)圓盤反應(yīng)器、旋轉(zhuǎn)生物膜反應(yīng)器、氣溶膠生物反應(yīng)器、膜生物反應(yīng)器和水平式提升反應(yīng)器[95]。相比于傳統(tǒng)靜態(tài)發(fā)酵,使用旋轉(zhuǎn)圓盤反應(yīng)器可以提高BC產(chǎn)率。事實(shí)上,據(jù)Kim等[96]報(bào)道,使用旋轉(zhuǎn)下生物膜接觸器優(yōu)化培養(yǎng)條件,BC產(chǎn)量可以達(dá)到6.17 g/L。雖然使用氣溶膠生物反應(yīng)器生產(chǎn)BC的聚合度(DP)較低,但可改善其膜學(xué)性能[97]。Kralisch等[98]利用水平提升反應(yīng)器(HoLiR)生產(chǎn)BC,盡管不能使其產(chǎn)量增加較多,仍然在5~15 g/L,但可生產(chǎn)有效控制長度和可調(diào)厚度的平面纖維素薄膜。盡管如此,進(jìn)一步增加BC產(chǎn)量和降低商業(yè)化大規(guī)模生產(chǎn)成本,是一項(xiàng)具有挑戰(zhàn)性的目標(biāo)。
6 展望
雖然BC在食品、醫(yī)療、化工等領(lǐng)域的應(yīng)用前景十分看好,但尚未充分利用。目前還有許多困難需要克服:(1)產(chǎn)量和品質(zhì)的進(jìn)一步提高,商業(yè)化規(guī)模生產(chǎn)成本的降低;(2)探索新的方法和技術(shù),如高靜水壓技術(shù)、分子結(jié)構(gòu),擴(kuò)大其生產(chǎn)應(yīng)用。因此,進(jìn)一步選育或構(gòu)建纖維素高產(chǎn)菌株、優(yōu)化培養(yǎng)基組成、改善發(fā)酵條件、設(shè)計(jì)高效的BC生產(chǎn)反應(yīng)器以提高BC的產(chǎn)率、增加底物轉(zhuǎn)化率及BC的生物降解性和安全性,仍是未來研究工作的方向。
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