Tetracyclines are a group of broad-spectrum antibiotic compounds that have a common basic structure and are either isolated directly from several species of Streptomyces bacteria or produced semi-synthetically from those isolated compounds. Tetracycline molecules comprise a linear fused tetracyclic nucleus (rings designated A, B, C and D) to which a variety of functional groups are attached. Tetracyclines are named after their four ("tetra-") hydrocarbon rings ("-cycl-") derivation ("-ine"). They are defined as a subclass of polyketides, having an octahydrotetracene-2-carboxamide skeleton and are known as derivatives of polycyclic naphthacene carboxamide.While all tetracyclines have a common structure, they differ from each other by the presence of chloro, methyl, and hydroxyl groups. These modifications do not change their broad antibacterial activity, but do affect pharmacological properties such as half-life and binding to proteins in serum.
Tetracyclines were discovered in the 1940s and exhibited activity against a wide range of microorganisms including gram-positive and gram-negative bacteria, chlamydiota, mycoplasmatota, rickettsiae, and protozoan parasites. Tetracycline itself was discovered later than chlortetracycline and oxytetracycline but is still considered as the parent compound for nomenclature purposes.Tetracyclines are among the cheapest classes of antibiotics available and have been used extensively in prophylaxis and in treatment of human and animal infections, as well as at subtherapeutic levels in animal feed as growth promoters.