Microbial Cell Factories in Bioprocessing Engineering: Metabolic Engineering for Sustainable Biomanufacturing

Microbial cell factories—microorganisms reprogrammed to convert renewable feedstocks into value-added molecules—have become central to modern bioprocessing engineering. Advances in systems and synthetic biology, genome editing and high-throughput screening now permit the rational design of strains that manufacture fuels, bulk and fine chemicals, pharmaceuticals and materials that were previously accessible only through petrochemical routes. This review outlines the conceptual framework and enabling technologies of microbial cell factory development, including host (chassis) selection, pathway construction and balancing, flux optimisation through metabolic and regulatory engineering, and the iterative design–build–test–learn cycle. Landmark achievements are highlighted, among them the semi-synthetic production of the antimalarial precursor artemisinic acid in Saccharomyces cerevisiae, the commercial fermentative manufacture of 1,3-propanediol and 1,4-butanediol, and gas fermentation of carbon-monoxideand carbon-dioxide-rich streams by acetogenic bacteria. The expanding toolbox of CRISPR-based genome editing, dynamic biosensor-driven control, genome-scale metabolic modelling and machine learning is discussed as a driver of predictable strain engineering. Attention is also given to nonconventional hosts such as Yarrowia lipolytica and Pichia pastoris, and to the bioprocess considerations—medium design, fermentation mode, oxygen transfer, scale-up and downstream recovery—that decide whether a promising strain becomes an economically viable process. Persistent challenges, including metabolic burden, redox and cofactor imbalance, modest product titres, and the gap between laboratory and industrial performance, are examined alongside emerging solutions. Microbial cell factories are positioned as a cornerstone technology for a sustainable, circular bioeconomy.