Synthetic auxotrophs accelerate cell factory development through growth-coupled models

The engineering of microbial cell factories for the production of high-value chemicals from renewable resources presents several challenges, including the optimization of key enzymes, pathway fluxes and metabolic networks. Addressing these challenges involves the development of synthetic auxotrophs, a strategy that links cell growth with enzyme properties or biosynthetic pathways. This linkage allows for the improvement of enzyme properties by in vivo directed enzyme evolution, the enhancement of metabolic pathway fluxes under growth pressure, and remodeling of metabolic networks through directed strain evolution. The advantage of employing synthetic auxotrophs lies in the power of growth-coupled selection, which is not only high-throughput but also labor-saving, greatly simplifying the development of both strains and enzymes. Synthetic auxotrophs play a pivotal role in advancing microbial cell factories, offering benefits from enzyme optimization to the manipulation of metabolic networks within single microbes. Furthermore, this strategy extends to coculture systems, enabling collaboration within microbial communities. This review highlights the recently developed applications of synthetic auxotrophs as microbial cell factories, and discusses future perspectives, aiming to provide a practical guide for growth-coupled models to produce value-added chemicals as part of a sustainable biorefinery.