Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan 430068, PR China.
Microb Cell Fact. 2013 Jun 7;12:57. doi: 10.1186/1475-2859-12-57.
Polylactic acid (PLA), a biodegradable polymer, has the potential to replace (at least partially) traditional petroleum-based plastics, minimizing "white pollution". However, cost-effective production of optically pure L-lactic acid is needed to achieve the full potential of PLA. Currently, starch-based glucose is used for L-lactic acid fermentation by lactic acid bacteria. Due to its competition with food resources, an alternative non-food substrate such as cellulosic biomass is needed for L-lactic acid fermentation. Nevertheless, the substrate (sugar stream) derived from cellulosic biomass contains significant amounts of xylose, which is unfermentable by most lactic acid bacteria. However, the microorganisms that do ferment xylose usually carry out heterolactic acid fermentation. As a result, an alternative strain should be developed for homofermentative production of optically pure L-lactic acid using cellulosic biomass.
In this study, an ethanologenic Escherichia coli strain, SZ470 (ΔfrdBC ΔldhA ΔackA ΔpflB ΔpdhR ::pflBp6-acEF-lpd ΔmgsA), was reengineered for homofermentative production of L-lactic acid from xylose (1.2 mole xylose = > 2 mole L-lactic acid), by deleting the alcohol dehydrogenase gene (adhE) and integrating the L-lactate dehydrogenase gene (ldhL) of Pediococcus acidilactici. The resulting strain, WL203, was metabolically evolved further through serial transfers in screw-cap tubes containing xylose, resulting in the strain WL204 with improved anaerobic cell growth. When tested in 70 g L-1 xylose fermentation (complex medium), WL204 produced 62 g L-1 L-lactic acid, with a maximum production rate of 1.631 g L-1 h-1 and a yield of 97% based on xylose metabolized. HPLC analysis using a chiral column showed that an L-lactic acid optical purity of 99.5% was achieved by WL204.
These results demonstrated that WL204 has the potential for homofermentative production of L-lactic acid using cellulosic biomass derived substrates, which contain a significant amount of xylose.
聚乳酸（PLA）是一种可生物降解的聚合物，具有替代（至少部分替代）传统石油基塑料的潜力，从而最大限度地减少“白色污染”。然而，需要具有成本效益的光学纯 L-乳酸生产工艺，才能充分发挥 PLA 的潜力。目前，乳酸细菌发酵 L-乳酸采用基于淀粉的葡萄糖。由于其与食物资源的竞争，L-乳酸发酵需要替代非食物基质，如纤维素生物质。然而，来自纤维素生物质的基质（糖流）含有大量木糖，大多数乳酸细菌无法发酵利用。然而，能够发酵木糖的微生物通常进行异型乳酸发酵。因此，需要开发替代菌株，利用纤维素生物质进行同型发酵生产光学纯 L-乳酸。
在这项研究中，对产乙醇大肠杆菌 SZ470（ΔfrdBCΔldhAΔackAΔpflBΔpdhR ::pflBp6-acEF-lpdΔmgsA）进行了重新设计，使其能够从木糖（1.2 摩尔木糖=＞2 摩尔 L-乳酸）同型发酵生产 L-乳酸，方法是删除乙醇脱氢酶基因（adhE）并整合乳球菌（Pediococcus acidilactici）的 L-乳酸脱氢酶基因（ldhL）。所得菌株 WL203 通过在含有木糖的螺旋盖管中进行连续传代进一步进行代谢进化，得到了一株厌氧细胞生长能力得到改善的菌株 WL204。在 70 g L-1 木糖发酵（复杂培养基）中测试时，WL204 生产了 62 g L-1 L-乳酸，最大生产速率为 1.631 g L-1 h-1，基于消耗的木糖计算的产率为 97%。使用手性柱的 HPLC 分析表明，WL204 实现了 99.5%的 L-乳酸光学纯度。
这些结果表明，WL204 具有利用含有大量木糖的纤维素生物质衍生底物进行同型发酵生产 L-乳酸的潜力。
