# Biosynthesis of Edible Terpenoids: Hosts and Applications
## 2 Microbial Chassis as Cell Factories for Edible Terpenoids  
Traditionally, terpenoids are extracted from various sources, including plants, animals, microorganisms, and marine organisms [14, 15]. However, due to the complex mixture of compounds present in cells, these extracts often contain a diverse array of components. When these extracts or essential oils are used directly as food additives, they may introduce non-target components, potentially leading to unexpected flavors or functions. For instance, steviol glycosides (SGs) extracted from _Stevia rebuadiuna_ are rich in stevioside, which gives SGs a bitter taste. Furthermore, the yield of active substances obtained from natural materials is often low, failing to meet the high demand. Therefore, chemists continually strive to develop chemical synthesis routes based on terpenoid structures. However, chemical synthesis often fails to fully replicate the intricate configuration of the active ingredients. For example, \(\beta\)-carotene synthesized by chemical methods exhibits an all-trans configuration, which differs from its natural counterpart, and high-dose intake indicates carcinogenic activity [16].  
Microorganisms, noted for their rapid reproduction rate, short growth cycle, easily controlled cultivation conditions, mature fermentation processes, and independence from geographical climate influences, have been employed in the synthesis of a variety of compounds. In fact, some microorganisms in nature are capable of producing nutritious and healthy microbial foods via fermentation. However, the variety of terpenoids in wild strains is restricted and insufficient to satisfy diverse human needs. Advancement of omics research has led to the identification of terpenoid pathways and key synthesis enzymes, including cyclases, from various sources and functions. This has provided convenient conditions for the heterologous synthesis of terpenoids [17], thus bringing the microbial synthesis of natural terpenoids into public awareness. Over recent decades, scientists have successfully integrated terpenoid synthesis pathways into both typical and non-typical microbial chassis, achieving the biosynthesis of an array of terpenoids through synthetic biology strategies [18, 19].  
In 2003, [PERSON] and colleagues introduced the yeast MVA pathway and amorpha-4,11-diene synthase into _E. coli_, leading to the synthesis of sesquiterpene olefin precursor to artemisinin, namely amorphadiene [20]. A decade later, the cytochrome \(b_{5}\) cDNA sequence and dehydrogenase gene were introduced into yeast, resulting in a high-yield engineered strain for producing artemisinin acid, a precursor of artemisinin. This patented technology was subsequently employed by Sanofi to produce tens of tons of artemisinin acid annually. When combined with chemical methods, this achievement becomes a significant milestone in the successful application of synthetic biology [21]. In terms of isoprene, [PERSON] assessed various factors, including materials, energy efficiency, economic feasibility, and land use, and revealed that the _E. coli_ cell factory is economically viable compared to conventional petrochemical alternatives [22].