Abstract
Lactic acid (LA) has several applications in the food, cosmetics and pharmaceutical industries, as well as in the production of biodegradable plastic polymers, namely polylactides. Industrial production of LA is essentially based on microbial fermentation. Recent reports have shown the potential of the cellulolytic bacterium Clostridium thermocellum for direct LA production from inexpensive lignocellulosic biomass. However, C. thermocellum is highly sensitive to acids and does not grow at pH < 6.0. Improvement of LA tolerance of this microorganism is pivotal for its application in cost-efficient production of LA. In the present study, the LA tolerance of C. thermocellum strains LL345 (wild-type fermentation profile) and LL1111 (high LA yield) was increased by adaptive laboratory evolution. At large inoculum size (10 %), the maximum tolerated LA concentration of strain LL1111 was more than doubled, from 15 g/L to 35 g/L, while subcultures evolved from LL345 showed 50–85 % faster growth in medium containing 45 g/L LA. Gene mutations (pyruvate phosphate dikinase, histidine protein kinase/phosphorylase) possibly affecting carbohydrate and/or phosphate metabolism have been detected in most LA-adapted populations. Although improvement of LA tolerance may sometimes also enable higher LA production in microorganisms, C. thermocellum LA-adapted cultures showed a yield of LA, and generally of other organic acids, similar to or lower than parental strains. Based on its improved LA tolerance and LA titer similar to its parent strain (LL1111), mixed adapted culture LL1630 showed the highest performing phenotype and could serve as a framework for improving LA production by further metabolic engineering.
Highlights
IntroductionApart from traditional applications in the food industry (e.g. as preservative or acidifier) and the production of cosmetics and pharmaceuticals, lactic acid (LA) is used for the synthesis of biodegradable plastic polyesters, namely pol ylactides (PLAs)
The global market for lactic acid (LA) is rapidly expanding [1]
The C. thermocellum strain LL1111 was engineered by disrupting adhE encoding its main alcohol/ aldehyde dehydrogenase, and in which LA was the main fermentation product with a yield of 40 % of the maximum theoretical value [7]
Summary
Apart from traditional applications in the food industry (e.g. as preservative or acidifier) and the production of cosmetics and pharmaceuticals, LA is used for the synthesis of biodegradable plastic polyesters, namely pol ylactides (PLAs). Industrial production of LA is largely based on fermentation of expensive food crops, such as corn [3,4]. Both ethical and economic motivations have stimulated research on alternative feedstocks for LA fermentation, with significant attention on lignocellulose [5,6]. Since microorganisms that naturally produce high amounts of LA, such as lactic acid bacteria (LAB), several bacilli and fungi belonging to Rhizopus sp., cannot directly ferment lignocellulose, metabolic engineering has been used to develop strains combining both lignocellulose fermentation and efficient LA production. Improving LA production in native cellulolytic microorganisms is a promising alternative strategy because (i) gene tools are available for several microbial models such as Clostridium thermocellum, C. cellulolyticum, C. cellulovorans and Caldicellulosiruptor bescii, and (ii) these strategies should not face hurdles linked to the expression of heterologous cellulases [6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
