Advances, challenges, and future directions towards a cellulolytic Escherichia coli

Escherichia coli has been widely engineered as a microbial chassis for the biosynthesis of a broad spectrum of products ranging from basic building blocks and biocommodities to high-value fine chemicals and drugs. However, most current bioprocesses still rely on conventional feedstocks such as corn-derived glucose and sugarcane-derived sucrose, which are also demanded by established food and industrial supply chains. Consequently, allocating these sugars to the production of biocommodities, which are required in large quantities and at low cost, may exacerbate concerns about food security and undermine the long-term sustainability of large-scale biomanufacturing relative to low-cost petrochemical routes, particularly for bulk chemicals.

Lignocellulosic biomass, derived from agricultural residues and urban waste, presents a low-cost, renewable, and abundant alternative source of fermentable sugars. Composed of an intricate matrix of cellulose, hemicellulose, and lignin, lignocellulose exhibits a high degree of structural recalcitrance. To access its fermentable components, biomass typically needs physicochemical pretreatment and enzymatic saccharification using commercial cellulases. Unfortunately, these enzymes are expensive, and their optimal catalytic properties are typically achieved under conditions incompatible with those required by conventional E. coli production strains, thereby complicating the integration of saccharification and fermentation processes. To overcome this barrier, significant efforts have been made to engineer recombinant cellulolytic E. coli strains capable of degrading cellulose. Strategies explored include intracellular expression of cellulases followed by induced lysis, enzyme secretion via signal peptides, and surface display of cellulolytic enzymes, among others.

Key advances toward achieving saccharolytic E. coli have been achieved. Proof-of-concept studies have demonstrated simultaneous saccharification and fermentation using pretreated lignocellulosic biomass, and high-throughput platforms for evolving cellulolytic enzymes have been established. Despite substantial progress, knowledge in this field remains fragmented across diverse studies. This review consolidates key advancements in the development of cellulolytic E. coli strains, emphasizing the molecular, metabolic, and physiological engineering strategies employed. It also highlights key challenges and future directions for integrating lignocellulose utilization into industrial E. coli-based bioprocesses.