The new engineered bacterial strain, functioning as a complete microbial catalyst, has facilitated the industrialization process of lignocellulosic bioconversion.

How to achieve the high-value utilization of this low-value raw material, lignocellulosic biomass, has always been a research hotspot both at home and abroad. The Metabolomics Team of Qingdao Institute of Bioenergy and Bioprocess Technology of the Chinese Academy of Sciences aimed at breaking the foreign technological monopoly and overcoming the bottleneck of lignocellulose saccharification technology, has been dedicated to the genetic modification and metabolic engineering research of cellulose-degrading bacteria such as Thermotoga maritima for a long time. By utilizing a series of gene manipulation tools developed by the team (J Microbiol Methods, 2012, 89: 201-8.; PloS One 2013, 8: e69032; Appl Microbiol Biotechnol, 2014, 98: 313-23; Biotechnol Biofuels, 2015, 8: 36.), they have conducted research on Thermotoga maritima and its cellulose-degrading enzyme system. — The directional modification of fibril bodies has resulted in the construction of a new type of engineered strain, which can serve as a whole-cell catalyst to achieve efficient conversion of lignocellulosic substrates to fermentable sugars. This has significantly promoted the industrialization process of lignocellulosic bioconversion. The related research results were published online on May 12th in Biotechnology for Biofuels [Zhang J, et al., 2017, 10(1): 124]. Among them, doctoral student Zhang J was the first author of this paper, researcher Cui Qiu and associate researcher Liu Yajun were the corresponding authors of this paper. Lignocellulosic biomass has attracted attention due to its reserves and renewability, but the unreasonable disposal of agricultural and forestry waste will greatly increase environmental pressure and cause serious environmental pollution problems such as water pollution and haze from incineration. Therefore, the efficient utilization of non-food lignocellulose is an urgent global issue that has important strategic significance for achieving sustainable economic development. However, the industrialization, large-scale application and commercialization of lignocellulosic biomass have not truly started yet. The main reason lies in the failure to break through the bottleneck step of efficient and low-cost conversion of lignocellulose to fermentable sugars. Fibril bodies are currently one of the most efficient cellulose-degrading molecular machines known in nature. As a typical cellulose-producing fibril body strain, Clostridium thermocellum has the natural characteristic of efficient degradation of cellulose substrates. Therefore, it is considered to be the most promising strain that can achieve efficient biocatalytic conversion of lignocellulosic biomass through the integration of bioprocessing technology. However, the existing wild strains and their fibril bodies have shortcomings such as substrate hydrolysis activity being inhibited by the feedback of enzyme catalytic products, which cannot meet the requirements of industrialization. In response to this research situation, the metabolomics team conducted targeted directional modification of Clostridium thermocellum and its fibril bodies. By establishing a scarless genome editing system, the β-glucosidase CaBglA from an extreme thermophilic bacterium was fused with the key fibril body enzyme Cel48S and expressed and assembled onto the extracellular fibril body. Using this recombinant strain as a whole-cell catalyst for saccharification reactions, it was found that when 100 g/L microcrystalline cellulose was used as the substrate, the reducing sugar yield reached 489 mM (converted to approximately 88 g/L based on the molecular weight of glucose). The ability of this bacterium to efficiently degrade cellulose and produce fermentable sugars has preliminarily proved the feasibility of the lignocellulosic whole-cell catalytic saccharification strategy in industrial applications. This research expands the new perspective of lignocellulose saccharification and strongly promotes the development of the industrial fermentation field where cellulose sugar is used as a carbon source to replace starch sugar.