Depending on the origin and function, metabolites can be divided into two major categories; namely, Primary and Secondary metabolites.
Primary metabolites are microbial products produced continuously during the exponential phase of growth and are involved in primary metabolic processes such as respiration and photosynthesis. They include intermediates and end-products of anabolic metabolism, which are used by the cell as building blocks for essential macromolecules (e.g. amino acids, nucleotides) or are converted to coenzymes (e.g. vitamins). Industrially, the most significant primary metabolites are amino acids, nucleotides, vitamins, solvents and organic acids. Many of these metabolites are manufactured by microbial fermentation and isolation instead of chemical synthesis because the fermentations are economically competitive and produce biologically useful isomeric forms.
Microbial secondary metabolites are compounds produced by an organism that are not required for primary metabolic processes, although they can have important ecologic and other functions. Streptomycetes and related actinomycetes continue to be prolific sources of novel secondary metabolites. Microbial secondary metabolites include antibiotics, pigments, toxins, effectors of ecological competition and symbiosis, pheromones, enzyme inhibitors, immunomodulating agents, receptor antagonists and agonists, pesticides, antitumor agents and growth promoters of animals and plants. Secondary metabolites are derived by pathways in which primary metabolites involve. Therefore, secondary metabolites are considered as the end products of primary metabolites.
