Engineering Corynebacterium glutamicum for synthesis of poly(3-hydroxybutyrate) from lignocellulose biomass.

Lignocellulose is the only feasible carbohydrates feedstock for commercial scale and carbon neutral production of poly(3-hydroxybutyrate) (PHB) biopolymer by its great abundance and availability. Microbial cell factories for fermentative PHB synthesis are highly restricted by the growth suppression of inhibitors from lignocellulose pretreatment. This study targeted a potential PHB-producing cell factory Corynebacterium glutamicum owing to its strong inhibitors tolerance. A systematic metabolic engineering was conducted starting with the stable PHB synthesis pathway construction from glucose and xylose, followed by the enhancement of PHB synthesis on PHA synthase activity and stability, cell morphology modification, and growth factors regulation. The relocation of the PHA synthase on the cell membrane guided by secrete signal peptides and cell membrane display motifs increased the PHB content by 2.4 folds. Excessive nitrogen preferentially promoted the PHB synthesis capacity and resulted in the PHB content increased by 13.3 folds. Modification of the genes responsible for cell division changed the cell morphology but the cell size was not enlarged to a PHB accumulation favorable environment. The metabolic engineering of C. glutamicum resulted in a high fermentative production of PHB using wheat straw as feedstock. This study provided an important microbial cell factory choice for PHB production using lignocellulose feedstock.

Enhanced cellulosic d-lactic acid production from sugarcane bagasse by pre-fermentation of water-soluble carbohydrates before acid pretreatment.

After several rounds of milling process for sugars extraction from sugarcane, certain amounts of water-soluble carbohydrates (WSC) still remain in sugarcane bagasse. It is a bottleneck to utilize WSC in sugarcane bagasse biorefinery, since these sugars are easily degraded into inhibitors during pretreatment. Herein, a simple pre-fermentation step before pretreatment was conducted, and 98 % of WSC in bagasse was fermented into d-lactic acid. The obtained d-lactic acid was stably preserved in bagasse and 5-hydroxymethylfurfural (HMF) generation was sharply reduced from 46.0 mg/g to 6.2 mg/g of dry bagasse, after dilute acid pretreatment. Consequently, a higher d-lactic acid titer (57.0 g/L vs 33.2 g/L) was achieved from the whole slurry of the undetoxified and pretreated sugarcane bagasse by one-pot simultaneous saccharification and co-fermentation (SSCF), with the overall yield of 0.58 g/g dry bagasse. This study gave an efficient strategy for enhancing lactic acid production using the lignocellulosic waste from sugar industry.

Adaptive evolution of Gluconobacter oxydans accelerates the conversion rate of non-glucose sugars derived from lignocellulose biomass.

Gluconobacter oxydans is capable of oxidizing various lignocellulose derived sugars into the corresponding sugar acids including glucose, xylose, arabinose, galactose and mannose, but simultaneous utilization of these sugars is difficult. This study attempted an adaptive evolution of G. oxydans by alternate transfer in inhibitors containing hydrolysate and inhibitors free hydrolysate for intensifying sugars simultaneous utilization. After 420 days' continuous culture, the conversion rate of all non-glucose sugars significantly improved by several folds and achieved complete conversion of lignocellulose-derived sugars to the corresponding sugar acids. The significant up-regulation of mGDH gene in the adapted G. oxydans strain (more than 40-fold greater than the parental) was considered as the decisive factor for the improvement of strain performance. This evolution adaptation strategy also could be used to accelerate robust sugars utilization for other fermented strains in lignocellulose biorefinery.