Abstract1,2-Propanediol is an important building block as a component used in the manufacture of unsaturated polyester resin, antifreeze, biofuel, nonionic detergent, etc. Commercial production of 1,2-propanediol through microbial biosynthesis is limited by low efficiency, and chemical production of 1,2-propanediol requires petrochemically derived routes involving wasteful power consumption and high pollution emissions. With the development of various strategies based on metabolic engineering, a series of obstacles are expected to be overcome. This review provides an extensive overview of the progress in the microbial production of 1,2-propanediol, particularly the different micro-organisms used for 1,2-propanediol biosynthesis and microbial production pathways. In addition, outstanding challenges associated with microbial biosynthesis and feasible metabolic engineering strategies, as well as perspectives on the future microbial production of 1,2-propanediol, are discussed.

Background Lignocellulosic biomass is an attractive and sustainable alternative to petroleum-based feedstock for the production of a range of biochemicals, and pretreatment is generally regarded as indispensable for its biorefinery. However, various inhibitors that severely hinder the growth and fermentation of microorganisms are inevitably produced during the pretreatment of lignocellulose. Presently, there are few reports on a single microorganism that can detoxify or tolerate toxic mixtures of pretreated lignocellulose hydrolysate while effectively transforming sugar components into valuable compounds. Alternatively, microbial coculture provides a simpler and more efficacious way to realize this goal by distributing metabolic functions among different specialized strains. Results In this study, a novel synthetic microbial consortium, which is composed of a responsible for detoxification bacterium engineered Pseudomonas putida KT2440 and a lactic acid production specialist Bacillus coagulans NL01, was developed to directly produce lactic acid from highly toxic lignocellulosic hydrolysate. The engineered P. putida with deletion of the sugar metabolism pathway was unable to consume the major fermentable sugars of lignocellulosic hydrolysate but exhibited great tolerance to 10 g/L sodium acetate, 5 g/L levulinic acid, 10 mM furfural and HMF as well as 2 g/L monophenol compound. In addition, the engineered strain rapidly removed diverse inhibitors of real hydrolysate. The degradation rate of organic acids (acetate, levulinic acid) and the conversion rate of furan aldehyde were both 100%, and the removal rate of most monoaromatic compounds remained at approximately 90%. With detoxification using engineered P. putida for 24 h, the 30% (v/v) hydrolysate was fermented to 35.8 g/L lactic acid by B. coagulans with a lactic acid yield of 0.8 g/g total sugars. Compared with that of the single culture of B. coagulans without lactic acid production, the fermentation performance of microbial coculture was significantly improved. Conclusions The microbial coculture system constructed in this study demonstrated the strong potential of the process for the biosynthesis of valuable products from lignocellulosic hydrolysates containing high concentrations of complex inhibitors by specifically recruiting consortia of robust microorganisms with desirable characteristics and also provided a feasible and attractive method for the bioconversion of lignocellulosic biomass to other value-added biochemicals.

Background The conversion of lignin-derived aromatic monomers into valuable chemicals has promising potential to improve the economic competitiveness of biomass biorefineries. Pinosylvin is an attractive pharmaceutical with multiple promising biological activities. Results Herein, Escherichia coli was engineered to convert the lignin-derived standard model monomer cinnamic acid into pinosylvin by introducing two novel enzymes from the wood plant: stilbene synthase from Pinus pinea (PpSTS) and 4-Coumarate-CoA ligase from Populus trichocarpa (Ptr4CL4). The expression of Ptr4CL4 drastically improved the production of pinosylvin (42.5 ± 1.1 mg/L), achieving values 15.7-fold higher than that of Ptr4CL5 (another 4-Coumarate-CoA ligase from Populus trichocarpa) in the absence of cerulenin. By adjusting the expression strategy, the optimized engineered strain produced pinosylvin at 153.7 ± 2.2 mg/L with an extremely high yield of 1.20 ± 0.02 mg/mg cinnamic acid in the presence of cerulenin, which is 83.9% ± 1.17 of the theoretical yield. This is the highest reported pinosylvin yield directly from cinnamic acid to date. Conclusion Our work highlights the feasibility of microbial production of pinosylvin from cinnamic acid and paves the way for converting lignin-related aromatics to valuable chemicals.

Background Lignocellulosic biomass is an attractive and sustainable alternative to petroleum-based feedstock for the production a range of biochemicals, and a pretreatment is generally regarded to be indispensable for its bio-refinery. Nevertheless, various inhibitors that severely hindered the growth and fermentation of microorganisms were produced inevitably during the pretreatment of lignocellulose. Presently, a single microorganism that can tolerate toxic mixtures of pretreatment hydrolysate while effectively transforming sugar components into valuable compound is less well reported. Alternatively, microbial co-culture provides a simpler and more efficacious way to realize this goal via distributing metabolic tasks among proper strains. Results In this study, a novel synthetic microbial consortia, which is composed of a responsible for detoxification bacterium engineered Pseudomonas putida KT2440 and a lactic acid production specialist Bacillus coagulans NL01, was developed to directly produce lactic acid from high-toxic lignocellulosic hydrolysate. The engineered P. putida with deletion of sugar metabolism pathway was suggested to be unable to consume the major fermentable sugars of lignocellulosic hydrolysate, but can rapidly remove inhibitors in hydrolysate. With detoxification using engineered P. putida for 24 h, the pretreated hydrolysate was fermented into 35.8 g/L of lactic acid by B. coagulans with a yield of 90%. The fermentation performance of microbial co-culture was significantly improved than that single culture of B. coagulans without lactic acid production. Conclusions The microbial coculture system constructed by this study demonstrated strong potential of the process for biosynthesis of valuable product from lignocellulosic hydrolysate containing high concentration of complex inhibitors by specifically recruited consortia of robust microorganisms with desirable characteristics and also provided a feasible and attractive method for bioconversion of lignocellulosic biomass to other value-added biochemicals.