Progress on production of malic acid and succinic acid by industrially-important engineered microorganisms

Organic acids are widely used as additives in the food, pharmaceutical, chemical, and plastic industries. Currently, the industrial production methods of organic acids mainly include plant extraction and chemical synthesis. The latter mainly uses petroleum-based compounds as raw materials to synthesize organic acids through a series of chemical reactions. All of these methods have problems such as environmental pollution, high cost, and unsustainability. By contrast, microbial fermentation can effectively utilize a variety of carbon sources. Due to its low production cost, environmental friendliness, and high product purity, microbial fermentation has received increasing attention in recent years. However, the low yield and long fermentation cycle of microbial fermentation limits its industrial application. With the development of genomics, transcriptomics, and other omics technologies, the metabolic pathways of various strains producing organic acids have gradually been elucidated. Based on this, new technologies such as synthetic biology and high-throughput screening have also been extensively studied. This review summarizes the latest research progress in improving organic acid biosynthesis through metabolic engineering, focusing on L-malic acid (L-MA) and succinic acid (SA). Finally, we also discuss the challenges and future prospects of this field. This review has important reference value in the fields of food, pharmaceuticals, and chemicals, providing a theoretical basis for the study of organic acid biosynthesis.