A variety of organisms including bacteria, fungi, and plants, produce small molecule secondary metabolites also known as natural products, which play a role in cell signaling, pigmentation and in defense against predation. Secondary metabolites are a rich source of biologically active compounds and hence are often used as research tools and leads for drug discovery. Examples of secondary metabolites include:
* Alkaloids
* Glycosides
* Lipids
* Nonribosomal peptides, such as actinomycin-D
* Phenazines
* Natural phenols (including flavonoids)
* Polyketide
* Terpenes and terpenoids, including steroids
* Tetrapyrroles.