陳華美 劉四新 李從發
摘 要 細菌纖維素是一種由微生物合成的新型納米材料,具有化學純度高、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,廣泛應用于食品、醫療、化工等領域。但發酵工藝落后、產量低、生產成本高一直是限制其應用于高附加值產品生產的瓶頸。本文概述纖維素產生菌及分離改良、發酵培養基優化、發酵條件對纖維素合成和產量的影響,以及發酵方式對BC產量和性能影響等方面的研究進展,并對BC在生產中的不足和未來應用前景進行展望,為開展更深入的研究和實際生產提供一定借鑒。
關鍵詞 細菌纖維素;生物合成;發酵;研究進展
中圖分類號 O636.11 文獻標識碼 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
細菌纖維素(Bacterial cellulose,BC),是一種由微生物產生的高純度三維網狀多聚物。因其持水性好、楊氏模量高[1],被廣泛應用于食品、化妝品、造紙、音響和光學等方面[2-7]。改良后的BC具有無毒、親水性強[8]、吸附性好、生物可降解性[9]等特性,應用于醫療行業。研究發現,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)等方面應用時,可吸收藥物并控制釋放[10-11]。因其流變學特性,可形成水凝膠,廣泛用于食品工業,稱為Nata,可作為甜點[12];食物和飲料中,作增稠劑、穩定劑和紋理改良劑控制多功能食品的性質[13];還因可食性和生物降解特性,BC可用作食品包裝材料[14]。并且高強度紙制品、聲音振動膜、人工皮膚、創口貼、繃帶、紗布、面膜、貢丸等BC產品已經用于商品化生產,在其他許多方面也具有廣泛的商業化應用潛力。其生產不依賴于地理氣候條件,控制生產要素就能提高產率;其純度高,不含木質素或其他污染物,可避免凈化的能耗[15];可通過改良菌株的基因來生產具有所需特性的纖維素;農業和工業廢物可以用作培養原料以節約經濟成本。然而,產量低、成本高,仍是BC實現生產規模化、產業化的一個瓶頸。
基于此現狀,將BC合成及發酵方面的研究報道進行分析,擬通過從BC的產生菌及分離改良、發酵培養基優化、發酵條件對BC合成和產量影響以及發酵方式對BC產量和性能影響等方面進行綜述,旨在為開展更進一步的研究做好鋪墊,為實際生產提供一定借鑒。
1 細菌纖維素的產生菌
早在1886年,英國科學家Brown就發現,酸醋桿菌靜置培養時,發酵液的氣-液表面會形成一層白色的凝膠狀薄膜,經化學分析,確定其成分是纖維素[16-17]。隨后,許多微生物被報道能產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個屬[18-20],以駒形氏桿菌屬的報道最多,也最深入[21-22]。截止目前,已報道該屬中有纖維素合成能力的菌種14個[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合成機理、代謝調控等理論研究的模式菌種,也是商業化生產和應用開發最常用的菌種[26]。
2 菌種的分離和改良
盡管能夠產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BC的能力不高,遠不能滿足實際生產應用的需要。目前,對K. xylinus的研究比較全面,但一般的菌株產量較低、多次傳代后極易衰退而且生產性能不穩定。因此,選育穩定高產的BC產生菌株非常有必要。現主要從篩選野生穩定高產菌株、物理化學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需要又能降低生產成本的優良菌株。
2.1 野生高產菌株的分離
BC的產生菌株主要從天然資源中分離得到,然后通過傳統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個樣品中分離到26株,其中醋酸桿菌屬(Acetobacter pasteutinus subsp. xylinum)產量最高可達14 g/L,還有一株A. hansenii的BC產量更高(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,不但產量高(靜態產量為10.99 g/L)而且生產性能穩定。
另一些學者從傳統生產中分離出產BC的菌株。馮勁等[29]從紅茶菌液里篩選出一株BC產生菌株,經鑒定是中間葡糖酸醋桿菌(G. intermedius),產生纖維素I型晶體,其純度達91.32%,濕膜含水率達99.16%,每克干膜能吸28.59 g水。蘇俊霞等[30]在傳統固態發酵食醋醋醅中分離得到5株產BC的菌株,經鑒定均屬于G. intermedius,只有一株BC產量較高,通過對其培養條件(溫度、培養時間、碳源、初始pH)優化,BC的產量可從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BC的菌株,在其最適生長pH(4~5、5~6)時,BC產量分別是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BC能力較差,為獲得穩定高產的菌株,通常采用一系列傳統手段及基因工程方法對其進行改良。
2.2.1 傳統方法改良 對于纖維素生產菌株而言,現已報道的傳統改良方法主要有物理、化學等三方面誘變育種。
物理:搖瓶培養時,通常使纖維素合成陰性菌株(Cel-)產生致BC產量減少;換回靜態培養時,突變體產BC的能力亦可恢復[32]。Ayd1n等[33]發現,攪拌型反應器中能產生突變體,其BC產率和產量增加到3.25 g/L和17.20%。靜水高壓處理也可得到突變體[34],Feng等[35]用該法將G. hansenii產BC的量增加到7.02 g/L。另外,研究紫外線誘變的學者也比較多,但通常將其與化學試劑進行復合誘變。Hungund等[36]用紫外輻射和甲基磺酸乙酯(EMS)對G. xylinus NCIM 2526進行改良,經紫外輻射后獲得3株BC產量較高的突變體,其中GHUV4產量最高(3.92 g/L),比野生型提高了30%;對其進行甲基磺酸乙酯(EMS)處理,得到的突變體(GHEM4)產BC的量(5.96 g/L)比親本和野生型產量分別多50%、98%。
化學:用亞硝基胍(NTG)、硫酸二乙酯(DS)、甲基磺酸乙酯(EMS)等化學物質對BC產生菌進行誘變。Premjet等[37]將野生型菌體(NU4)經亞硝基胍誘變得到突變體(NU4-NTG30-51),BC產量分別是對照菌(A. xylinum ATCC 10245)和親本菌株的54.68%和43.69%;再將NU4-NTG30-51進行紫外誘變得到突變體(NU4-UV40-07),產量分別是其50.59%和39.60%,且后者的結晶度比前者低。鄧毛程等[38]將紫外線和硫酸二乙酯對木葡糖酸醋桿菌進行復合誘變,得到一株遺傳性穩定的突變菌株,BC產量達15.6 g/L,比親本菌株產量提高44.4%。
2.2.2 基因工程法改良 Deng等[39]利用Tn5轉座子對G. hansenii ATCC 23769進行插入突變研究,得到6個不產纖維素的突變菌株。同時,可利用轉座子對細菌的一些代謝旁路進行突變,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)基因獲得不產葡萄糖酸的突變體,以增加BC的產量。John等[41]在K. hansenii細胞內提取異源二聚體-acsb ACSA,蛋白經純化、翻譯后加工,形成活性異源二聚體-acsb ACSA。最后經誘變作用,研究CSC酶 AcsC、AcsD和CcAx三個亞基的作用。但目前用基因工程法改良纖維素產生菌還未獲得真正的基因構建菌[42-43]。
3 發酵培養基的優化
培養基類型和組成對微生物生長和代謝產物的積累影響很大。對于BC生產,不同菌株的營養要求不盡相同,但對已經或能夠用于BC大規模生產的菌種而言,有一些基本的規律可尋。
3.1 碳源優化
BC的合成是一個受多種酶共同調控、耗能的復雜代謝途徑,培養基組成的不同對生產菌株生長、BC合成、副產物積累有較大影響。由于碳源直接影響BC的合成,因此研究報道較多。
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,合成BC的產量最高,達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HS培養基中的碳源培養G. xylinus ATCC 53524發現,其較偏愛于蔗糖和甘油,產量分別可達3.83和3.75 g/L。同時還發現,碳源種類不能影響BC的結構特性。Mohammadkazemi等[46]利用糖漿、葡萄糖、蔗糖、食品級蔗糖等替換掉HS、Yamanaka(Y)、Zhou(Z)三種培養基中的碳源,發現糖漿和食品級蔗糖不適合K. xylinus PTCC 1734生長,且合成BC的量也較差。同時,Dayal[47]、Santos[48]、Liu[49]等也對培養基的組成進行了報道。
實際生產中,單一碳源或培養基糖轉換率低、生產成本較高。因此,不少學者還對尋找廉價的原料作為BC的生產培養基進行研究,如使用農業[50-51]和工業廢物[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]已報道。Algar等[67]使用工業菠蘿殘渣來發酵G. medellinensis產BC。Gomes等[68]使用橄欖油殘渣來培養G. sacchari。Huang等先后利用脂質廢水[69]、玉米芯酸水解液[66]培養K. xylinus CH001,Bilgi使用工業角豆和扁豆廢液培養K. xylinus產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生產中最常用的氮源,因為他們能夠為生產菌株提供氮源和生長因子。因其成本較高,由此一些學者正努力尋找合適的替代物。Noro等[71]發現玉米漿(Corn steep liquor,CSL)是最有效的培養基,并在CSL中添加乳酸和蛋氨酸效果會更好,CSL可以對pH有緩沖作用,可將生產過程中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BC。與HS培養基相比,BC產量從1.53 g/L提高至3.12 g/L。李飛等[55]將玉米漿作氮源時,BC的產量為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行了調查,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、谷氨酸等會影響菌株產BC的能力[74]。Lin等[75]用啤酒廢酵母液考察K. hansenii CGMCC 3917生產纖維素的能力。常冬妹等[76]研究不同濃度煙酸和生物素對A. xylinum靜態培養產BC的影響,發現加煙酸濃度為1 mg/L時,產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+可以促進二鳥苷酸環化酶活性,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,Mg2+、Mn2+和Co2+可以促進其催化活性,但Ca2+、Fe2+和Ni+起抑制作用,間接影響纖維素合酶的活性。此外,細菌細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影響BC產率[79],因此,培養過程中通氣很重要[80]。
3.5 其他添加物
除原料外,不少學者還探索通過在培養基中添加其它物質來增加BC產量,如乙醇[81-82],VC[83]、有機酸[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、促進效果也明顯。乙醇可以從多方面影響BC生物合成,可為連續發酵合成BC過程中提供能量[85];還可抑制磷酸轉乙酰酶活性,使碳流經TCA循環通量和Pta-Ack途徑流量減少[86]。Keshka等[83]報道了VC對4株G. xylinus產BC的量和晶體結構的影響。因VC具有抗氧化性,可降低菌株產葡萄糖酸的能力,因此當其濃度為0.5%(w/w)時所有菌株產BC的量都增加,平均為0.016 g/mL,是對照組的2倍。
4 發酵條件對纖維素合成和產量的影響
培養基的初始pH值、溫度、氧濃度[87]、菌種狀態(種齡、接種量等)及轉速等發酵參數都能影響BC產量和結構性質。Jagannath等[88]指出,以椰子水為原料靜態發酵產BC時pH對其厚度有影響。pH3.5時,即使培養20 d也無明顯的BC膜出現;pH4.0時,可得最大厚度10.2 mm的BC膜。同時,許多研究報道表明,菌株產BC的最適溫度范圍為28~30 ℃,其不僅影響BC產量,還對BC性質(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%)。
種子液培養條件應與BC生產發酵相對應,若用靜態發酵方式生產,二級種子通過靜態培養獲得,而動態發酵方式生產BC,二級種子相應為動態培養。Hu等[89]研究了不同轉速對A. xylinum JCM 9730(ATCC 700178)對產BC形貌的影響。150 mL的三角瓶盛100 mL HS培養基,當轉速為150 r/min時,能產生球形BC顆粒,直徑大小為10 mm;轉速為200 r/min時,產生的顆粒直徑大小為7~8 mm。
5 發酵方式對纖維素產量和性能的影響
BC的培養方式主要有靜置法和動態法(搖瓶、攪拌或者氣升式),各具特色。靜置培養能更好地保持BC的正常形態[90],能產出均勻光滑的BC產品。這種方法需要更多的生產空間和勞動力,規模化生產成本高。動態培養主要有振蕩器、搖床、攪拌式發酵罐和氣升式發酵罐等,產生BC的形貌差異較大,有絲狀、球形、星狀、絮狀或團塊狀等。但攪拌時需空間和勞動力少,因此減少了工業生產成本。然而,一些傳統的方法如震動玻璃瓶和攪拌棒的利用,可以誘導產突變菌株,使BC產量降低[91]。同時,BC產品易吸附于反應器的軸上,使其很難收集,并且不易清理[92]。為緩解此問題,近日,一些學者在動態培養條件下探索設計更有效的BC生產反應器,其中,以球泡罩塔型生物反應器[93]、氣升式反應器[94]和改良后氣升式反應器罐[87]尤為著名。Cheng等[92]使用羧甲基纖維素(CMC)塑料復合生物膜反應器用于BC生產,以便隨時采樣和連續生產。
另一方面,有的反應裝置可在“相對靜止”條件下發酵生產片狀和膜狀的BC,如旋轉圓盤反應器、旋轉生物膜反應器、氣溶膠生物反應器、膜生物反應器和水平式提升反應器[95]。相比于傳統靜態發酵,使用旋轉圓盤反應器可以提高BC產率。事實上,據Kim等[96]報道,使用旋轉下生物膜接觸器優化培養條件,BC產量可以達到6.17 g/L。雖然使用氣溶膠生物反應器生產BC的聚合度(DP)較低,但可改善其膜學性能[97]。Kralisch等[98]利用水平提升反應器(HoLiR)生產BC,盡管不能使其產量增加較多,仍然在5~15 g/L,但可生產有效控制長度和可調厚度的平面纖維素薄膜。盡管如此,進一步增加BC產量和降低商業化大規模生產成本,是一項具有挑戰性的目標。
6 展望
雖然BC在食品、醫療、化工等領域的應用前景十分看好,但尚未充分利用。目前還有許多困難需要克服:(1)產量和品質的進一步提高,商業化規模生產成本的降低;(2)探索新的方法和技術,如高靜水壓技術、分子結構,擴大其生產應用。因此,進一步選育或構建纖維素高產菌株、優化培養基組成、改善發酵條件、設計高效的BC生產反應器以提高BC的產率、增加底物轉化率及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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