Abstract

Primary metabolites are microbial products made during the exponential phase of growth whose synthesis is an integral part of the normal growth process. They include intermediates and end products of anabolic pathways leading to the formation of monomers, which are used by the cell as building blocks (e.g., amino acids and nucleotides) for the biosynthesis of polymers (e.g., protein and DNA) and coenzymes (e.g., vitamins). On the other hand, primary metabolites of catabolic pathways (e.g., citric acid, acetic acid, and ethanol) are not biosynthetic precursors but are essential for growth as they are related to energy generation, redox balance, and substrate utilizations. Industrially, the most important primary metabolites are amino acids, nucleotides, vitamins, solvents, and organic acids. These are made by a wide range of bacteria and fungi and have numerous applications in the food, chemical, pharmaceutical, and nutraceutical industries. Many of these metabolites are manufactured by microbial fermentation rather than chemical synthesis because the fermentations are economically competitive and produce biologically active isomers. Several other industrially important chemicals could be manufactured via microbial fermentations (e.g., glycerol and other polyhydroxy alcohols) but are presently synthesized cheaply as petroleum by-products. However, a renewed interest in the microbial production of ethanol, organic acids, and solvents has been triggered as a consequence of successive rises in the price of crude oils.