CN101283101A - 培养微生物的方法 - Google Patents
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Abstract
本发明为一种用于生产适合于乙醇生产的嗜热微生物的方法,包括:(i)在需氧或厌氧条件下、在适合的培养基中培养嗜热微生物;和(ii)将适量的乙醇掺入所述培养基以诱导乙醇耐受性。
Description
技术领域
本发明涉及生产适合于产生作为细菌发酵产物的乙醇的微生物。
背景技术
根据细菌种类和环境条件,细菌代谢可通过各种不同机制发生。异养菌(包括所有的病原体)通过氧化有机化合物获得能量,碳水化合物(特别是葡萄糖)、脂质和蛋白质是最常见的被氧化的化合物。利用细菌对这些有机化合物进行的生物氧化最终会合成作为化学能来源的ATP。该过程也会生成细菌细胞进行生物合成反应所需的更加简单的有机化合物(前体分子)。细菌代谢适合底物的一般过程是糖酵解,糖酵解是一系列将葡萄糖转化为丙酮酸同时生成ATP的反应。生成代谢能的过程中丙酮酸的去向会根据微生物和环境条件而变化。主要的丙酮酸反应有三种。
第一,在需氧条件下,许多微生物会在丙酮酸脱氢酶(PDH)的催化下利用柠檬酸循环和将丙酮酸转化为乙酰辅酶A来产生能量。
第二,在厌氧条件下,某些产乙醇生物可通过在丙酮酸脱羧酶(PDC)的催化下将丙酮酸脱羧成为乙醛,并且随后在醇脱氢酶(ADH)的催化下利用NADH将乙醛还原为乙醇来进行乙醇发酵。
第三个过程是通过乳酸脱氢酶(LDH)催化,将丙酮酸转化为乳酸。
人们对使用微生物——使用天然进行厌氧发酵的微生物或使用引入了乙醇生产中所涉及基因的重组微生物——来生产乙醇很感兴趣。尽管在利用这些微生物生产乙醇方面获得了一定的成功,但是通常乙醇浓度增加会使发酵受损,特别是当微生物的乙醇耐受性水平较低时。
已经有人提出使用嗜热细菌进行乙醇生产,使用嗜热细菌的优势在于可以在较高温度下进行发酵,这使得可以在50℃以上的温度下将所产生的乙醇以蒸气的形式移出;这也使得可以利用高糖浓度进行发酵。然而,问题在于能否找到可以有效生产乙醇的合适嗜热细菌。
WO01/49865公开了一种被转化了编码丙酮酸脱羧酶的异源基因、并且具有天然的醇脱氢酶功能的革兰氏阳性细菌用于乙醇生产。所述细菌为嗜热性的杆菌(Bacillus),可通过插入转座子使乳酸脱氢酶基因失活来对该细菌进行修饰。WO01/49865中公开的细菌全部来源于杆菌属菌株LLD-R,它是一种自发地从培养物中长出的孢子形成缺陷菌株,且在该菌株中ldh基因已经通过自发突变或化学诱变失活。所公开的菌株LN和TN是LLD-R菌株的改良过的衍生菌株。然而,全部的菌株都含有Hae III型限制性系统,所述限制性系统妨碍质粒转化并因此阻止非甲基化DNA中的转化。
WO01/85966公开了通过体内甲基化以克服限制性问题而制备的微生物。这需要将Hae III甲基转移酶从埃及嗜血杆菌(Haemophilusaegyptius)转化到LLD-R、LN和TN菌株中。然而,LLD-R、LN和TN菌株是不稳定的突变体并且会自发地回复为产乳酸的野生型菌株,特别是在低pH和高糖浓度的环境中。这会导致发酵产物从乙醇变为乳酸,使得所述菌株不适合用于乙醇生产。
WO02/29030公开了LLD-R菌株及其衍生菌株在ldh基因编码区包括一个天然存在的插入元件(IE)。将该插入元件转入(和转出)ldh基因的转座以及随后的基因失活都是不稳定的,从而导致回复。对此给出的解决方案是将质粒DNA整合到所述IE序列中。
因此,在本领域中,用于乙醇生产的微生物的产生依赖于对实验室产生的化学诱变过的杆菌属微生物进行修饰、用体内甲基化方法处理这些微生物和进一步修饰所述微生物以便将质粒DNA整合到所述IE序列中。该方法复杂并且不确定,且也存在如何使用所述菌株的管理问题。
因此需要改良过的用于乙醇生产的微生物。
发明内容
根据本发明的第一方面,一种用于生产适合于乙醇生产的嗜热微生物的方法,包括:
(i)在需氧或厌氧条件下、在适合的培养基中培养嗜热微生物;和
(ii)将用量逐渐增加的乙醇掺入所述培养基以诱导乙醇耐受性。
具体实施方式
本发明是基于处理嗜热微生物以使所述微生物对乙醇更加耐受,并因此可以更好地生产乙醇。增加所述微生物的乙醇耐受性使得所述微生物可以在其发酵过程中对所产生的乙醇更加耐受。这使得可对发酵进行改良。
所述用于嗜热微生物生产的方法包括在需氧或厌氧条件下、在适合的培养基中培养嗜热微生物并且将适量的乙醇掺入所述培养基以诱导乙醇耐受性。在一个实施方案中,向所述培养基中增加乙醇是随时间并且通常以增量的方式进行的,以使得所述微生物可以适应培养基中增加的乙醇。优选将乙醇掺入到最高达3%w/v的终浓度,然后将乙醇浓度增加0.5%w/v或更少直至培养基中的乙醇浓度至少为6%w/v,更优选至少7.5%。在一个实施方案中,初始的培养基含有3%w/v的乙醇,然后将其以0.5%的增量增加到6%w/v,然后以0.25%的增量增加到7.5%w/v。
在该步骤中,可以监测细胞密度以确保细胞持续生长。优选地,如果细胞密度(通过OD600nm测定)下降超过25%并且随乙醇浓度增加而持续下降,则将乙醇浓度降低至上一个最高浓度,并且在继续用乙醇处理前恢复培养基。
一种备选的用于具有较高乙醇耐受性的嗜热微生物产生的方法包括在需氧或厌氧条件下、在适合的培养基中培养所述嗜热微生物。一旦培养物达到稳态——此时,所述生物的生长已经达到了恒定速率(通过OD600nm确定)——则向所述培养基中一次加入适量的乙醇以使乙醇达到一个特定的所需浓度,例如所述培养基设定工作体积的10%或20%w/v。以低稀释速率(优选0.08-0.15h-1)继续该连续培养,使得乙醇浓度缓慢降低直至所述嗜热生物的最初生长速率得以恢复(通过OD600nm测定)。此时,再向所述培养基中一次加入适量(与第一次的加入量相同)的乙醇,以使得乙醇达到一个特定的所需浓度。重复所述的使所述培养物恢复最初生长速率的过程,并且进一步分批加入乙醇直至发现所述培养物迅速恢复,该过程花费不到24小时。此时会出现两种结果之一。通过在所需的乙醇浓度(优选高于7.5%w/v)下传代培养来从该培养物中筛选乙醇耐受菌株,或者将更大量的乙醇加入到所述培养物中并且重复加入乙醇之后生长速率恢复的过程。
在55℃-65℃下,使所述微生物在含限碳底物的限定培养基中生长,pH为6.0-7.5(优选6.3-7.2),稀释速率为0.08-0.5。
在分批培养中,可以实施一种类似于上文所述方法的方法,在含有过量碳的任何适合的培养基中生长并且在对数生长期早期加入乙醇。然后使用初始培养物对数生长期末期的细胞来接种新的烧瓶,并再次在对数期早期加入增量的乙醇。重复该步骤,同时重复在对数期早期向所述培养基中加入增量的乙醇。
可以对本发明中所用的嗜热微生物进行修饰以中断或增强与乙醇生物合成相关的生物化学途径中所涉及的基因的表达,例如中断乳酸脱氢酶基因。这会引导丙酮酸代谢背离生产乳酸的方向而朝向生产乙醇的方向,并且在乳酸阴性突变体中观察到乙醇水平升高。
使乳酸脱氢酶基因失活有助于防止丙酮酸降解成乳酸,并且因此利用丙酮酸脱羧酶和醇脱氢酶促进(在适合条件下)丙酮酸降解成乙醇。
所述野生型微生物可以是任何嗜热微生物,但是所述微生物优选地为杆菌属种(Bacillus spp)。所述微生物特别优选地为地芽孢杆菌属种(Geobacillus species),特别是热葡糖苷酶地芽孢杆菌(Geobacillusthermoglucosidasius)。
所述微生物可以是“野生型”的,即它们不是实验室产生的突变体。所述微生物分离自预计含嗜热生物的环境样品。所分离的野生型微生物具有生产乙醇的能力,但是由于未经修饰,主要的发酵产物可能是乳酸。还根据分离株在高温下利用己糖和/或戊糖生长的能力来对它们进行筛选。也可以使用非野生型突变体微生物。
优选地,本发明的微生物具有某些能使得所述微生物被用于发酵过程的所需特征。所述微生物应优选地不具有限制性系统,因此避免了体内甲基化的需要。此外,所述微生物应具有利用C5和C6糖(包括纤维二糖和淀粉)作为底物的能力。优选地,所述微生物可以以高频率转化。此外,所述微生物在连续培养中的生长率应在0.3小时-1以上。
所述微生物可为嗜热生物并且可在40℃-85℃的温度范围内生长。优选地,所述微生物在50℃-70℃的温度范围内生长。此外,所述微生物在pH 6.5或更低的条件下生长,特别是pH 6.5-pH 4.5的条件下生长是理想的。
乳酸脱氢酶的核酸序列现在是已知的。使用这一序列,本领域技术人员能靶向乳酸脱氢酶基因以通过不同机制来实现该基因的失活。优选地,乳酸脱氢酶基因是通过插入转座子来失活的或者优选地是通过缺失该基因序列或该基因序列的一部分来失活的。优选通过缺失,因为缺失避免了基因序列再活化的难题,所述基因序列再活化这一难题在采用转座子失活法时会经常遇到。在一个优选的实施方案中,所述乳酸脱氢酶基因是通过整合一个温度敏感质粒(质粒pUB190-ldh)来失活的,这实现了质粒与微生物染色体之间的天然同源重组或整合。根据染色体的整合体对抗菌剂(卡那霉素)的抗性来对其进行筛选。乳酸脱氢酶基因内的整合可通过单次交换重组事件或通过两次(或多次)交换重组事件来实现。
在一个优选的实施方案中,所述微生物含有异源的醇脱氢酶基因和异源的丙酮酸脱羧酶基因。这些异源基因的表达会导致这样一些酶的产生,所述酶会将所述代谢重新定向以使得乙醇成为主要的发酵产物。这些基因获自通常进行需氧发酵的微生物,包括酵单胞菌属(zymomonas)种,包括运动发酵单孢菌(zymomonas mobilis)。
用于制备和将这些基因引入微生物中的方法是已知的,例如Ingram et al,Biotech & BioEng,1998;58(2+3):204-214和US 5916787,其内容均通过引用的方式纳入本说明书。本领域技术人员应理解的是,所述基因可以用质粒引入或是被整合到染色体中。
根据所选择的嗜热微生物,可在常规的培养条件下培养本发明的微生物。根据已知的培养要求来选择底物、温度、pH和其它生长条件,例如参见WO01/49865和WO01/85966。表1、2和3中示出了适合的培养和发酵条件:
表1
化学试剂 | 体积/L | 终浓度 |
NaH2PO4·2H2O | 25mM | |
K2SO4 | 10mM |
柠檬酸·H2O | 2mM | |
MgSO4·7H2O | 1.25mM | |
CaCl2·2H2O | 0.02mM | |
硫酸盐微量元素溶液 | 5ml | 见下表 |
Na2MoO4·2H2O | 1.65μM | |
酵母提取物 | 10g | |
消泡剂 | 0.5ml | |
高压灭菌后的添加物 | ||
4M尿素 | 25ml | 100mM |
1%生物素 | 300μl | 12μM |
20%葡萄糖2 | 50ml | 1% |
表2
硫酸盐微量元素储液
化学试剂 | gl-1(ml) | gl-1(ml) | 培养基浓度 |
浓硫酸 | 5ml | 50ml | |
ZnSO4·7H2O | 1.44 | 14.4 | 25μM |
FeSO4·7H2O | 5.56 | 55.6 | 100μM |
MnSO4·H2O | 1.69 | 16.9 | 50μM |
CuSO4·5H2O | 0.25 | 2.5 | 5μM |
CoSO4·7H2O | 0.562 | 5.62 | 10μM |
NiSO4·6H2O | 0.886 | 8.86 | 16.85μM |
H3BO3 | 0.08 | ||
去离子水(终) | 1000ml | 10升 |
表3
发酵罐条件
接种物 | 10%v/v |
体积 | 1000ml |
温度 | 60℃ |
PH | 7.0(用NaOH调节) |
充气 | 0.4vvm |
N2流 | 0.05lpm |
搅拌 | 400rpm |
培养基 | 用于发酵罐的尿素硫酸盐CDM |
糖供给 | 100ml 50%的葡萄糖 |
消泡剂 |
参照附图通过下述实施例对本发明进行说明。
实施例
制备了下述培养基:
SAM2-perL
酵母提取物 1.0g
胰蛋白胨 0.5g
NH4Cl 1.0g
NaH2PO4 0.5g
MgSO4·7H2O 0.2g
KCl 0.2g
MnCl2·4H2O 3mg(加100μL 30mg/mL的储
液)
CaCl2·2H2O 5mg(加100μL 50mg/mL的储
液)
PIPES缓冲液 12.096g
溶于蒸馏水后的总体积:950mL,用NaOH或H2SO4将pH调至7.0。
高压灭菌。
冷却后,加入2.5mL硫酸盐微量元素储液(参见表2),以及50mL20%w/v过滤灭菌的糖溶液。
改良的US(脲盐)培养基(USM)
葡萄糖 10.0g/L
酵母提取物 0.8g/L
柠檬酸 0.42g/L
MgSO4 0.31g/L
NaH2PO4 3.1g/L
K2SO4 3.5g/L
尿素 3.0g/L
CaCl2 2mg/L
Na2MoO4 4mg/L
微量元素溶液(表2) 5.0ml/L
对于固体培养基,加入20.0g/L细菌培养用琼脂。
在灭菌前用3M NaOH校正至pH 7。
TGP培养基
细菌培养用胰蛋白胨 17.0g/L
大豆蛋白胨 3.0g/L
NaCl 5.0g/L
K2HPO4 2.5g/L
丙酮酸钠 4.0/L
甘油 4.0mL/L
对于固体培养基,加入20.0g/L细菌培养用琼脂。在灭菌前用3MNaOH将培养基校正至pH 7。
测试了一种野生型生物(NCIMB 11955)的乙醇耐受性以确定起点。使该生物生长过夜(LB琼脂平板,60℃)并使用一个菌落来接种过夜培养物(100mL USM,1%葡萄糖,60℃,250rpm)。然后使用这一培养物来接种含有0、1、2、3或4%乙醇的分别设三个重复的烧瓶,然后使其生长36小时,之后测量生长(50mL USM,1%葡萄糖,60℃,250rpm)。结果在图1中示出,结果表明NCIMB 11955不会耐受高于4%的乙醇。
利用这一结果作为比较的基础,进行实验以将突变体TM89的乙醇耐受性增加至8%v/v的乙醇。
发酵方法
微生物: Gt TM-89
接种物: 50ml 2xYT培养物(7%v/v)
设备: LH玻璃发酵罐(700ml工作体积)
使用Anglicon控制系统控制温度
和pH
使用磁力搅拌器混合
设定值: 温度:60℃
pH:6.80
空气:0.2-0.4vvm
搅拌速度:225rpm
使用Watson Marlow泵(0-100ml/
小时)控制流速
培养基: SAM2 2%葡萄糖、0.05%有机消泡
剂(参见培养基表)
使用10%NaOH调节PH
乙醇加入量(spike):75ml或150ml乙醇(加入
10-20%)
采用的策略为:
1)达到稳态并且测量产物产率/糖利用率;和
2)将乙醇添加至培养物,使培养物恢复(随时间不断洗掉乙醇),取出样品并制备甘油储液,根据需要重复步骤(2)。
为评估乙醇耐受性,开发了下述方案。
1.将5ml TGP肉汤加入到两个无菌通用试管中。
2.将100μl的TM-89和TM89-1甘油储液分别加入到每个试管中。
3.在60℃下振荡培养5-6小时(因此活化了细胞,此时它们应处于对数期)。
4.测量每份肉汤的OD600(由此可知初始的OD600)。
5.准备11个含有10ml TGP肉汤的无菌通用试管,所述TGP肉汤分别含有浓度为0%、1%、2%、3%、4%、5%、6%、7%、8%、9%、10%(v/v)的乙醇,每个TM-89菌株设两个重复(即44个试管)。
6.用100μl细胞接种每个试管,所述细胞取自处于1+2阶段的对应的5ml TGP肉汤。
7.在60℃下振荡培养过夜。
8.从每个试管中取出1ml,按1∶5稀释并且以H2O为空白测量OD600。
结果分析如下:
(1)使用Jencons分光光度计测量培养物的光密度[按照A600x0.3计算细胞浓度(g/L)]。
(2)使用血糖计(Roche)测量葡萄糖浓度。
(3)使用基于酶的测定试剂盒(由R-Biopharm提供)测量乙醇浓度。
结果在图2中示出。与用TGP和含有一系列浓度乙醇的TGP培养的初始TM89菌株相比,发酵结束时分离的菌株一直显示出较高的OD值。乙醇为5%时的生长的显著差异表明了改良的乙醇耐受性。
Claims (12)
1.一种生产适合于乙醇生产的嗜热微生物的方法,包括:
(i)在需氧或厌氧条件下、在适合的培养基中培养嗜热微生物;和
(ii)将适量的乙醇掺入所述培养基以诱导乙醇耐受性。
2.权利要求1的方法,其中向所述培养基中逐渐增量地加入乙醇,并在后一次增量地加入乙醇之前使所述微生物适应已加入的乙醇。
3.权利要求1或2的方法,其中将乙醇掺入到终浓度至少为3%w/v。
4.前述任一项权利要求的方法,其中将乙醇掺入到终浓度至少为6%w/v。
5.前述任一项权利要求的方法,其中将乙醇掺入到终浓度至少为7.5%w/v。
6.前述任一项权利要求的方法,其中乙醇是以0.5%w/v或更少的增量来掺入的。
7.前述任一项权利要求的方法,其中乙醇是在对数期早期掺入到所述培养基中的。
8.前述任一项权利要求的方法,其中所述微生物具有失活的乳酸脱氢酶基因。
9.前述任一项权利要求的方法,其中所述微生物不含有限制性系统。
10.前述任一项权利要求的方法,其中所述微生物为热葡糖苷酶地芽孢杆菌(Geobacillus thermoglucosidasius)。
11.前述任一项权利要求的方法,其中所述微生物含有非天然的pdc基因。
12.前述任一项权利要求的方法,其中所述微生物含有非天然的adh基因。
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NZ566781A (en) | 2010-07-30 |
US20080305536A1 (en) | 2008-12-11 |
EA013467B1 (ru) | 2010-04-30 |
AU2006298543B2 (en) | 2010-07-22 |
ZA200802933B (en) | 2009-11-25 |
WO2007039753A1 (en) | 2007-04-12 |
KR20080056180A (ko) | 2008-06-20 |
JP2009509520A (ja) | 2009-03-12 |
EP1931793A1 (en) | 2008-06-18 |
GB0520344D0 (en) | 2005-11-16 |
EP1931793B1 (en) | 2013-05-08 |
BRPI0616908A2 (pt) | 2011-07-05 |
NO20082062L (no) | 2008-06-23 |
CA2623364A1 (en) | 2007-04-12 |
EA200800711A1 (ru) | 2008-08-29 |
AU2006298543A1 (en) | 2007-04-12 |
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